Is peroxisome proliferator-activated receptor gamma (PPARγ) a therapeutic target for the treatment of pulmonary hypertension?

doi:10.4103/2045-8932.78101

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Year : 2011 | Volume : 1 | Issue : 1 | Page : 33-47

Is peroxisome proliferator-activated receptor gamma (PPARγ) a therapeutic target for the treatment of pulmonary hypertension?

David E Green, Roy L Sutliff, C Michael Hart
Department of Medicine, Emory University, Atlanta Veterans Affairs Medical Center, Decatur, Georgia, USA

Date of Web Publication

16-Mar-2011

Correspondence Address:
C Michael Hart
Atlanta VA Medical Center (151),1670 Clairmont Road, Decatur, Georgia 30033
USA

DOI: 10.4103/2045-8932.78101

PMID: 21547012

Abstract

Pulmonary hypertension (PH), a progressive disorder associated with significant morbidity and mortality, is caused by complex pathways that culminate in structural and functional alterations of the pulmonary circulation and increases in pulmonary vascular resistance and pressure. Diverse genetic, pathological, or environmental triggers stimulate PH pathogenesis culminating in vasoconstriction, cell proliferation, vascular remodeling, and thrombosis. We conducted a thorough literature review by performing MEDLINE searches via PubMed to identify articles pertaining to PPARγ as a therapeutic target for the treatment of PH. This review examines basic and preclinical studies that explore PPARγ and its ability to regulate PH pathogenesis. Despite the current therapies that target specific pathways in PH pathogenesis, including prostacyclin derivatives, endothelin-receptor antagonists, and phosphodiesterase type 5 inhibitors, morbidity and mortality related to PH remain unacceptably high, indicating the need for novel therapeutic approaches. Consequently, therapeutic targets that simultaneously regulate multiple pathways involved in PH pathogenesis have gained attention. This review focuses on peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. While the PPARγ receptor is best known as a master regulator of lipid and glucose metabolism, a growing body of literature demonstrates that activation of PPARγ exerts antiproliferative, antithrombotic, and vasodilatory effects on the vasculature, suggesting its potential efficacy as a PH therapeutic target.

Keywords: Peroxisome proliferator-activated receptor gamma, pulmonary hypertension, therapy

How to cite this article:
Green DE, Sutliff RL, Hart C M. Is peroxisome proliferator-activated receptor gamma (PPARγ) a therapeutic target for the treatment of pulmonary hypertension?. Pulm Circ 2011;1:33-47

How to cite this URL:
Green DE, Sutliff RL, Hart C M. Is peroxisome proliferator-activated receptor gamma (PPARγ) a therapeutic target for the treatment of pulmonary hypertension?. Pulm Circ [serial online] 2011 [cited 2013 May 21];1:33-47. Available from: http://www.pulmonarycirculation.org/text.asp?2011/1/1/33/78101

Introduction

Pulmonary hypertension (PH) is a rare and progressive disorder with a prevalence of 15 cases per million. ^[1] The hemodynamic definition of PH requires elevation of the mean pulmonary artery pressure >25 mm Hg at rest or 30 mm Hg with exercise and a mean pulmonary-capillary wedge pressure or left ventricular end-diastolic pressure ≤15 mm Hg. The World Health Organization (WHO) now classifies PH into five groups with similar disease mechanisms, histopathologic features and responses to treatment. ^[2] Current concepts suggest that PH pathogenesis involves three primary processes: vasoconstriction, cellular proliferation/vascular remodeling, and thrombosis. ^[3] Evolving evidence suggests that peroxisome proliferator-activated receptor gamma (PPARγ), a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors, can favorably modulate cellular proliferation, vascular tone and coagulation. This review provides an overview of PPARγ and considers how PPARγ impacts these primary processes involved in PH pathogenesis.

A thorough literature review was conducted to identify articles pertaining to PPAR as a therapeutic target for the treatment of PH. We performed MEDLINE searches through PubMed in the National Library of Medicine to identify relevant articles. Articles accepted for review included full text journal articles, clinical trials, reviews, guidelines, and randomized controlled trials. Articles not accepted for review included letters, editorials and correspondence, and manuscripts published before 1985.

PPARγ and PH

Overview of PPARg biology

Peroxisome proliferator-activated receptors (PPARs) belong to subfamily 1 of the nuclear hormone receptor superfamily (Nuclear Receptors Nomenclature Committee, 1999). There are three distinct isotypes of PPARs (α, β/δ, and γ) that are expressed throughout the body and possess similar structural and functional features. PPARs are responsive to the lipid status of the cell and act as ligand-activated transcription factors. PPAR isotypes are distinguished by their tissue distribution, ligand specificity and target genes. PPARα is most highly expressed in tissues with high catabolic rates such as the liver, kidney, heart, brown adipose tissue, and the intestine. ^[4],[5] PPARβ/δ has the broadest expression pattern with the expression in many cases dependent on the degree of cell proliferation and differentiation. ^[4],[6],[7] PPARγ exists as two isoforms that differ in their N terminus. PPARγ2 is found in adipose and PPARγ1 is more widely expressed in tissues such as the brain, vascular tissues, small intestine, and lymphatic tissues. ^[4],[8] Each PPAR isotype is preferentially activated by a wide range of naturally occurring or metabolized lipids derived from the diet or from intracellular signaling pathways, which include n-3 and n-6 family polyunsaturated fatty acids (PUFAs) and eicosanoid products of cyclooxygenase and lipoxygenase.

As illustrated in [Figure 1], transcriptional regulation by PPARs requires the formation of heterodimers with retinoid X receptor (RXR) isotypes. PPARs typically form heterodimers with RXRalpha (RXRα). PPAR:RXR heterodimers bind to DNA at sites composed of the hexameric direct repeat sequence, AGGTCA, separated by a single nucleotide. Gene regulation involves ligand-induced conformational changes in the PPAR receptor which mediate interaction with specific coactivator (e.g. steroid receptor coactivator-1 and p300) and corepressor molecules. Coactivator proteins possess either histone acetyltransferase activity or recruit other proteins with this activity to the transcription start site. Acetylation of histone proteins alters chromatin structure, facilitating the binding of RNA polymerase and the initiation of transcription. ^[9] PPARγ can also repress gene expression by interfering with the clearance of corepressors from selected promoters. ^[10] Due to their numerous metabolic and therapeutic actions, PPARs have become pharmacological targets. For example, PPARα agonists such as fibrate medications are used in the treatment of dyslipidemia and may have anti-inflammatory effects. PPARβ/δ selective agonists such as GW0742 augment high density lipoprotein (HDL) cholesterol and have been implicated in promoting tissue repair.^{[9],[10],[11],[12]} On the other hand, PPARγ agonists such as rosiglitazone and pioglitazone, used to treat type 2 diabetes, have beneficial effects in vascular disease in animal models. ^[11],[12] These reports emphasize that ligands for PPAR receptors can exert diverse biological effects in numerous organs and tissues. Among these PPAR receptors, PPARγ is of particular interest in the pulmonary circulation.

Figure 1: Schematic illustration of PPARƒÁ-mediated gene regulation. PPARƒÁ ligands originating from outside or inside the cell bind to the PPARƒÁ receptor, stimulating formation of a heterodimer with the retinoid X receptor (RXR). The PPARƒÁ– RXR heterodimer binds to PPAR response elements (PPRE) comprising hexameric repeats of the sequence, AGGTCA-X-AGGTCA. This active heterodimer recruits coactivators to and/or derecruits corepressor molecules from the transcriptional start site, leading to increased transcription of selected genes. Activation of PPARƒÁ can also inhibit the expression of genes regulated by specifi c proinfl ammatory transcription factors such as NF-kB, AP-1, and STAT

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The role of altered PPARg expression in PH

PPARγ is abundantly expressed in many cell types in the lung, including those of the pulmonary vascular wall such as endothelial and smooth muscle cells. Several studies indicate that expression of PPARγ is reduced by conditions associated with PH. Ameshima and colleagues were the first to demonstrate that compared to normal or chronic obstructive pulmonary disease (COPD) patients, patients with PH had significantly reduced or absent expression of PPARγ in precapillary arteriolar plexiform lesions.^[13] PPARγ expression was markedly reduced in endothelial cells isolated from PH patients when compared to normal patients (unpublished observation). These clinical findings have also been corroborated using in vivo0 experimental models of PH. For example, PPARγ expression was reduced in pulmonary vascular lesions in the rat model of hypoxia-induced PH.^[13],[14] Similarly, using in vitro cell culture models, increased shear stress or hypoxia was demonstrated to directly alter PPARγ expression. Exposure of ECV304 endothelial cells to increased fluid shear stress decreased PPARγ expression. ^[13] Similarly, exposure of endothelial cells to 1% hypoxia decreased expression of PPARγ.^[15] Collectively, these findings suggest that PPARγ expression is reduced in PH and that cells exposed to conditions that promote PH have decreased PPARγ expression. These reductions in PPARγ could contribute to an abnormal, proliferative, and apoptosis-resistant endothelial cell phenotype.

To further examine the role of PPARγ in pulmonary vascular biology, more recent studies have employed PPARγ knockout mice. Because global deletion of PPARγ results in embryonic lethality,^[16] investigators have examined experimental animals with tissue-targeted deletion of PPARγ. For example, Guignabert and co-workers reported that targeted deletion of PPARγ in the vascular endothelium of mice (ePPARγ^-/- ) results in spontaneous PH with right ventricular hypertrophy and muscularization of small distal pulmonary arteries. ^[17] The ePPARγ^-/- mice exposed to chronic hypoxia (10% O ₂ ) for 3 weeks developed a similar degree of PH as wild-type control mice. However, following cessation of hypoxia, PH persisted longer in the ePPARγ^-/- mice compared to wild-type mice exposed to hypoxia, suggesting that reduced endothelial PPARγ signaling is sufficient to cause mild PH and impair recovery from chronic hypoxia exposure.^[17] Targeted deletion of PPARγ from smooth muscle (smPPARγ^-/- ) also resulted in spontaneous PH in mice. ^[18] Microarray analysis of bovine pulmonary artery endothelial cells following treatment with a PPARγ antagonist revealed alterations in the expression of numerous genes including those that might stimulate cell cycle progression and proliferation. ^[19] Taken together, these reports suggest that loss of PPARγ function in pulmonary vascular wall cells stimulates PH pathogenesis.

PPARg activation ameliorates experimental PH

Mounting experimental evidence indicates that PPARγ stimulation ameliorates PH development in animal models of PH. Monocrotaline (MCT)-induced PH and vascular remodeling in the rat were attenuated by treatment with the PPARγ ligands, pioglitazone or troglitazone.^[20] Interestingly, PPARγ ligands also inhibited MCT-induced vascular wall thickening and staining for proliferating cell nuclear antigen, suggesting that PPARγ ligands suppressed cell proliferation and vascular remodeling.^[20] In Wistar-Kyoto rats exposed to continuous hypobaric hypoxia for 3 weeks, treatment with rosiglitazone attenuated hypoxia-induced right ventricular hypertrophy and vascular smooth muscle cell (VSMC) proliferation, as well as pulmonary vascular collagen and elastin deposition, infiltration of c-Kit-positive cells into the adventitia, and matrix metalloproteinase-2 (MMP-2) activity. In this study, rosiglitazone failed to attenuate hypoxia-induced increases in pulmonary artery pressure, an observation attributed to the inability of PPARγ ligands to modulate Rho kinase signaling, a critical mediator of pulmonary vasoconstriction.^[21] Hansmann and colleagues reported that ApoE knockout mice fed high fat diets developed significant increases in right ventricular systolic pressure, pulmonary vascular remodeling and right ventricular hypertrophy and that administration of PPARγ ligands in this model attenuated PH.^[22] An elegant series of experiments in this model provided evidence that PPARγ ligands attenuated PH by inhibiting platelet derived growth factor (PDGF) signaling.

Male C57Bl/6 mice exposed to chronic hypoxia (10% O ₂) for 3 weeks developed PH that was attenuated by treatment with the PPARγ agonist, rosiglitazone (10 mg/kg/day by gavage) during the final 10 days of hypoxia exposure. ^[15] Rosiglitazone treatment also reduced hypoxia-induced right ventricular hypertrophy and muscularization of small pulmonary arterioles. From a therapeutic perspective, this study also demonstrated that rosiglitazone could reverse the established PH by introducing rosiglitazone treatment only after animals developed PH. ^[15] The mechanisms of these therapeutic effects were attributed to PPARγ-mediated reductions in Nox4 expression, oxidative stress, and PDGF signaling in the lung. Collectively, these reports indicate that PPARγ ligands attenuated pulmonary vascular remodeling and hypertension caused by a variety of stimuli in experimental models. The effect of alterations in PPARγ expression and activation on PH in various experimental models is summarized in [Table 1]. These studies have begun to identify specific pathways modulated by PPARγ in the pathogenesis of PH. Additional evidence, reviewed below, suggests that PPARγ has the potential to regulate a diverse spectrum of pathways and mediators implicated in PH. The ensuing section will consider how targeting PPARγ can potentially modulate additional pathways fundamental to the pathogenesis of PH.

Table 1: Current evidence linking PPARƒ× and pulmonary hypertension

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Aspects of PH Pathogenesis that are Potentially Regulated by PPARγ

Vascular tone in PH

Common mechanisms in the pathogenesis of PH are endothelial dysfunction, reduced endothelial production of vasodilators, and increased production of vasoconstrictors leading to vasoconstriction and increased pulmonary vascular resistance. Recent advances in PH therapeutics have sought to promote vascular function by restoring vasodilator levels. For example, circulating levels of the vasodilator, prostacyclin, are decreased in PH, ^[23] and the administration of prostacyclin or its analogues represents a significant advance in PH therapy. ^[24] Nitric oxide (NO)-mediated vasorelaxation is also impaired in PH. ^[25] Phosphodiesterase type 5 inhibitors which prolong NO-mediated increases in cGMP are now employed in selected patients with PH. ^[24] Conversely, levels of vasoconstrictors such as endothelin-1 (ET-1) and thromboxane (TXA ₂ ) are increased in PH patients. ^[26] By blocking these vasoconstrictive effects, both endothelin receptor antagonists and calcium channel blockers comprise additional strategies in the current PH therapeutic armamentarium. In addition to altering vascular tone, these agents may also modulate structural changes in the pulmonary vasculature and vascular remodeling, sequelae of vascular injury and increased intraluminal pressure or flow. While the current therapies target individual mediators or mechanisms in PH pathogenesis, PPARγ may simultaneously modulate several of these pathways involved in PH pathogenesis as described below.

Prostacyclin

As a critical regulator of pulmonary vascular function, the endothelial-derived mediator, prostacyclin, is a potent vasodilator that inhibits platelet aggregation and exerts anti-inflammatory, anti-thrombotic, and anti-proliferative vascular effects. ^[27] Overexpression of prostacyclin synthase protected mice from chronic hypoxia-induced PH, whereas prostacyclin-receptor deficient mice were sensitized to hypoxia-induced PH. ^[28] Prostacyclin synthase expression was reduced in the pulmonary arteries of patients with severe PH compared to normal subjects, and the vascular endothelium was found to be the major site of lung vascular prostacyclin synthase expression. ^[29] In patients with PH, prostacyclin derivatives decreased urinary isoprostane metabolites, an index of oxidative stress, without altering TXA ₂ . ^[30] Currently, augmenting prostacyclin levels constitutes a therapeutic strategy in PH, but the precise cellular mechanisms responsible for prostacyclin-mediated benefits remain to be defined. The classical signaling pathway activated by prostacyclin involves binding the G-protein coupled cell surface prostacyclin receptor (IP), which when activated, stimulates adenyl cyclase and increases cellular cAMP content. However, prostacyclin and its analogues can also activate PPAR receptors including PPARδ and PPARγ to mediate biological effects. ^[31],[32] Activation of the PPARγ receptor with thiazolidinedione also reduced systemic vascular production of the potent vasoconstrictor, thromboxane, ^[33] and attenuated iNOS and Cox-2 upregulation. ^[34] These reports emphasize that PPARγ can regulate prostanoid production and that additional studies will be required to determine if PPARγ can regulate prostacyclin and its metabolites in the pulmonary circulation.

Nitric oxide

In addition to prostacyclin, NO represents an additional endothelium-derived vasodilator whose bioavailability is reduced in PH. ^[35],[36] Downregulation of the enzyme that produces nitric oxide, endothelial nitric oxide synthase (eNOS), has been described in PH in some studies, ^[37] whereas others report unchanged or increased levels of the enzyme. (More consistent evidence demonstrates that endothelium-derived, NO-mediated vasodilation is impaired in models of PH.) Thus, it is likely that reductions in NO bioavailability in PH are more closely related to post-translational alterations in eNOS regulation and/or enhanced NO degradation rather than reduced eNOS expression. The critical role of NO bioavailability in PH is supported by the evidence that genetic deletion of eNOS enhanced susceptibility to hypoxia-induced PH, ^[38] a defect reversed by adenoviral-mediated transfection of the pulmonary vasculature with eNOS. ^[39] Furthermore, overexpression of eNOS attenuated hypoxia-induced PH. ^[40] In addition, NO inhalation improves pulmonary hemodynamics and quality of life in a subset of patients with PH, ^[41] and recent advances in cell-based eNOS gene transfer to the lung have demonstrated that eNOS can reverse the established PH in animal models and facilitate restoration of the pulmonary microvasculature. ^[42]

NO is produced constitutively in vascular endothelial cells from the amino acid, l-arginine, by the Type III, eNOS isoform. Enzyme activity is largely regulated by intracellular Ca ²⁺ , cofactor availability, and eNOS post-translational modifications including phosphorylation and interaction with other proteins such as caveolin and heat shock protein 90 (hsp90). ^[43] Derangements in eNOS phosphorylation and interactions between eNOS and caveolin have been reported in PH. PPARγ ligands can affect these post-translational pathways.^[44],[45] PPARγ increased endothelial NO release by reducing the inhibitory interaction between eNOS and caveolin, increasing interactions between eNOS and the molecular chaperone, hsp90, and enhancing phosphorylation of eNOS on serine 1177, all post-translational modifications associated with enhanced eNOS activity. ^[46],[47] PPARγ agonists can increase NO bioavailability by blunting the degradation of inducible nitric oxide synthase (iNOS) and by decreasing serum levels of asymmetric dimethylarginine (ADMA), the endogenous inhibitor of NOS. ^{[48],[49],[50]}

Once produced, endothelial-derived NO reduces: 1) vascular tone, ^[51] 2) platelet activation and aggregation, ^[52] 3) stimulated vascular smooth muscle proliferation, ^[53] and 4) leukocyte adherence. ^[54] To exert these biological effects, NO must diffuse into the vascular wall where its activity may be limited by local concentrations of superoxide. Superoxide combines at diffusion-limited rates with NO, forming the potent oxidant, peroxynitrite, thereby reducing the vascular protective effects of NO and enhancing oxidative stress. Peroxynitrite also oxidizes the NOS cofactor, tetrahydrobiopterin. ^[55] Deficiency of tetrahydrobiopterin alters electron flow through eNOS to molecular oxygen rather than arginine, producing superoxide rather than NO, a condition referred to as eNOS uncoupling. Enhanced superoxide production in the vascular wall may, therefore, reduce NO bioavailability through multiple mechanisms. As a result, NO bioavailability can be regulated not only by the rate of NO formation, but also by the rate of NO degradation. Thus, increased superoxide generation constitutes an important mechanism of NO inactivation and endothelial dysfunction in the vascular wall. Current evidence indicates that PPARγ can stimulate endothelial NO release and simultaneously reduce superoxide generation in vascular endothelial cells, suggesting that PPARγ ligands could enhance NO bioavailability and the vascular protective effects of NO to favorably modulate vasoconstriction and PH.^[46],[47]

Endothelin-1

The potent vasoconstricting polypeptide, ET-1, has been implicated in PH pathogenesis. ET-1 receptors are upregulated in the lung in both animal models and patients with PH. ^[25] ET-1, as well as endothelium-derived reactive oxygen species (ROS), attenuated NO-dependent pulmonary vasodilation, following exposure to chronic hypoxia in isolated rat lungs. ^[56] ET-1-induced pulmonary vasoconstriction was markedly reduced by administration of Cu/Zn superoxide dismutase and was completely attenuated in gp91phox deficient mice. ^[57] These findings suggest that NADPH oxidase and superoxide play an important role in pulmonary vascular effects of ET-1. ET-1 receptor antagonists have been employed in patients with PH to improve functional status and other indices of PH related morbidity, ^[56] further suggesting that ET-1 is an important mediator of pulmonary vascular dysregulation. Emerging evidence in several disease states indicates that PPARγ activation attenuates ET-1 signaling. PPAR ligands inhibited ET-1 secretion by vascular endothelial cells in vitro. ^{[57],[58],[59],[60]} Similarly, in non-diabetic patients with metabolic syndrome, treatment with rosiglitazone reduced several markers of vascular inflammation including plasma levels of ET-1 and improved markers of metabolic control, while lowering blood pressure and improving flow-mediated vasodilation. ^[61] PPARγ activation also reduced hypertrophy and anti-apoptotic effects caused by ET-1 in cardiac myocytes in vitro through altered nuclear factor of activated T cells (NFAT) signaling. Treatment with PPARγ ligands in several rat models of hypertension reduced ET-1 expression in cardiac and vascular tissues. ^[62] Collectively, these findings suggest that PPARγ activation can attenuate expression of ET-1 in cardiovascular tissues in response to a variety of stimuli and can attenuate ET-1-mediated signaling in selected models. Potential pathways by which PPARγ regulates vascular tone are illustrated in [Figure 2].

Figure 2: Potential pathways by which PPARγ regulates vascular tone. Activation of PPARƒÁ potentially modulates several pathways involved in the regulation of vascular tone. In addition, prostacyclin or its analogues may activate PPARγ. Arrowheads at the end of lines denote stimulatory effects, whereas perpendicular lines denote inhibitory effects. (ADMA – Asymmetric dimethyl arginine; cAMP – Cyclic adenosine nucleotide monophosphate; cGMP – Cyclic guanosine nucleotide monophosphate; eNOS – Endothelial nitric oxide synthase; ET-1 – Endothelin-1; ET-R – Endothelin-1 receptor; NO – Nitric oxide; IP receptor – Prostacyclin receptor)

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Abnormal vascular remodeling, inflammation, and cell proliferation in PH

Remodeling of the pulmonary vasculature can reduce its cross-sectional diameter and compliance, increase pulmonary vascular resistance, and contribute to sustained PH. Studies examining the molecular mechanisms underlying pulmonary vascular remodeling have implicated growth factor pathways as well as matrix remodeling in the development and progression of PH. ^[63] Inflammation also plays a significant role in altered pulmonary vascular function during the development of PH. ^[64] Inflammatory markers are elevated in PH, and the plexiform lesions that characterize severe PH are surrounded by macrophages, T and B lymphocytes, and dendritic cells. ^[65] These cells may exacerbate PH by releasing growth factors, ROS and additional cytokines. ^[66] Chemokines such as CX3CL1, CCL5 and MCP-1 which recruit inflammatory cells are elevated in PH patients. ^{[67],[68],[69]} Therefore, agents that target the generation of these oxidative and inflammatory stimuli in the pulmonary vascular wall may reduce vascular dysfunction and attenuate the development or progression of PH. Relationships between PPARγ and these complex pathways implicated in PH pathogenesis are illustrated in [Figure 3].

Figure 3: Potential pathways by which PPARγ regulates vascular remodeling, cell proliferation and hypertrophy in PH. PPARγ has the potential to regulate a complex variety of pathways that are involved in remodeling of the pulmonary vasculature during the development of PH. Arrowheads at the end of lines denote stimulatory effects, whereas perpendicular lines denote inhibitory effects. *Pathway components that are inhibited by PPARγ, +pathway components that are stimulated by PPARγ. (BMP-2 – Bone morphogenetic protein-2; BMPR-2 – BMP-2 receptor; EPC – endothelial progenitor cell; ET-1 – Endothelin – 1; ET-R – Endothelin-1 receptor; ROS – Reactive oxygen species; TF – Infl ammatory transcription factors (e.g. NF-kB); TGF-β – Transforming growth factor beta; TGF946;R - TGF-946; receptor)

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NADPH oxidases

NADPH oxidases, a major source of superoxide production in the vasculature, have been implicated in PH and contribute to endothelial dysfunction and vascular cell proliferation. ^[70],[71] Originally described in phagocytic cells, the gp91phox-based NADPH oxidase is a multicomponent, membrane-associated, enzyme that catalyzes the one electron reduction of oxygen to superoxide, using NADH or NADPH as the electron donor. ^[70] The classical phagocytic NADPH oxidase is composed of several components or subunits including the membrane-bound gp91phox (Nox2) and p22phox subunits as well as the cytosolic p47phox and p67phox subunits that, when stimulated, combine with the small G-protein, rac, and translocate to the membrane to activate the enzyme complex. On the other hand, in nonphagocytic cells, the catalytic moiety of NADPH oxidases is composed of one or more Nox2 homologues, Nox 1, 3, 4, 5, Duox1 or Duox2. ^[72] These Nox homologues associate with the membrane-bound p22phox subunit and are differentially regulated and targeted to distinct subcellular loci, suggesting that these oxidases serve unique roles in cell function. Nox1 and 3 are activated through interactions with rac and the p47phox and p67phox homologues, NOXA1 and NOXO1.

Current evidence indicates that Nox4 expression is increased in hypoxia-induced PH in the mouse and in the pulmonary vasculature of patients with PH. ^[15],[73] Nox4 is highly expressed in vascular wall cells including smooth muscle and endothelial cells where it is constitutively active. ^[74] Furthermore, hypoxia increased Nox4 expression and pulmonary artery smooth muscle cell (PASMC) proliferation, ^[73] and Nox4 stimulated the proliferation of endothelial and smooth muscle cells. ^[75] Hypoxia stimulated Nox4 expression and cell proliferation in the mouse lung in vivo and in human pulmonary artery endothelial cell (HPAEC) and human pulmonary artery smooth muscle cell (HPASMC) in vitro.[15] Treatment with rosiglitazone during the last 10 days of hypoxia exposure reduced Nox4 levels and ROS production and attenuated hypoxia-induced PH, right ventricular hypertrophy and vascular remodeling. Similarly, rosiglitazone attenuated hypoxia-induced Nox4 expression and proliferation in human PAEC and PASMC in vitro. Taken together, these findings suggest that NADPH oxidases are important mediators of cell proliferation and vasoconstriction in PH that can be regulated by PPARγ [summarized in [Figure 4]].

Figure 4: The effects of PPARγ activation on chronic hypoxia-induced PH and PDGF signaling. PPARγ activation decreases oxidative stress and hypoxic vasoconstriction by blunting hypoxia-induced increases NADPH oxidase expression and decreases in NO, respectively. PPARγ stimulated PTEN expression blocks PDGF-mediated vascular smooth muscle cell proliferation. Arrowheads at the end of lines denote stimulatory effects, whereas perpendicular lines denote inhibitory effects

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Although increases in Nox4 mRNA levels were associated with increased Nox4 activity, ^[76] detailed understanding of Nox4 transcriptional regulation remains to be established. Nox4 induction has been reported in response to diverse stimuli including hypoxia in kidney and ischemia in brain. ^[71] In smooth muscle cells, activators of Nox4 transcription include urokinase, plasminogen activator, angiotensin II, transforming growth factor-beta 1 (TGF)-β1, and tumor necrosis factor (TNF)-α.^[77] In contrast, in endothelial cells, oscillatory shear stress ^[78] and PPARγ activation^[79] suppressed Nox4 mRNA levels. However, few studies have examined regulatory elements in the Nox4 promoter. Hypoxia stimulated activation of the Nox4 promoter in part through nuclear factor (NF)-κB-mediated signaling and enhanced p65 binding to the Nox4 promoter which increased Nox4 expression and activity.^[80] Furthermore, treatment with rosiglitazone inhibited hypoxia-induced Nox4 expression and activity, proliferation, and p65-Nox4 promoter interaction. ^[80] HIF-1α and the E2F family transcription factors also activated the Nox4 promoter although their regulation by PPARγ at this site has yet to be confirmed.^[81] Based on evidence that Nox4 stimulates smooth muscle and endothelial cell proliferation, these findings suggest that activation of PPARγ can attenuate the proliferation of pulmonary vascular wall cells that may participate in the pathogenesis of PH.^[82] Coupled with the ability of NADPH oxidases to attenuate NO bioavailability, Nox4, in particular, and NADPH oxidases, in general, may be an important target of PPARγ amelioration of PH.

PDGF signaling

PDGF participates in PH pathogenesis. Two genes (A and B) produce three biologically active forms of PDGF protein (AA, AB, and BB). ^[83],[84] These proteins activate one or more PDGF receptors (αα, αβ, or ββ) to stimulate cell migration and survival. Ligand binding promotes PDGF receptor tyrosine autophosphorylation and subsequent activation of several downstream signaling pathways including Src, phosphatidylinositol 3 kinase (PI3K), phospholipase Cγ, and Ras. These signaling pathways are largely activated by recruitment of these enzymes to PDGF-R SH-2 domains. The composition of the downstream signaling pathways activated by PDGF and their integration into specific cellular responses continue to be defined. ^[85] Although the expression of PDGF and its receptors is limited in the vascular wall at baseline, several pathological stimuli, including alterations in blood pressure and shear stress, induce peptide and receptor expression. ^[84] PDGF receptor expression was increased in the lungs of patients with PH, and the PDGF receptor antagonist, imatinib, reversed MCT- or hypoxia-induced PH in rodents and improved pulmonary vascular resistance and exercise capacity in a patient with severe idiopathic PH. ^{[86],[87],[88]} PPARγ ligands attenuated hypoxia-induced PDGF activation in a mouse model of PH in vivo. ^[15] Coupled with reports that NO inhibits PDGF signaling and that PPARγ ligands inhibit PDGF-stimulated VSMC migration in vitro , ^{[89],[90],[91]} these studies suggest that PPARγ can regulate important proliferative signaling pathways in experimental PH, including those activated by PDGF.

PDGF receptor activity can also be regulated by phosphatase and tensin homologue deleted on chromosome 10 (PTEN), a dual specificity phosphatase that dephosphorylates both lipid and protein substrates. ^[92] PTEN catalyzes the removal of the phosphate moiety from the 3-position of the phosphatidylinositol ring, converting the second messenger, PI(3, 4, 5)P ₃ , to PI(4,5)P ₂ . ^[93] PTEN and PI3K thereby have opposing actions on cellular levels of PI(3, 4, 5)P ₃ . PTEN also dephosphorylates the PDGF receptor. Because PDGF receptor activation mediates cell proliferation and migration in part through stimulation of PI3K, PTEN can inhibit PDGF signaling both at the receptor and through lowering of PI(3, 4, 5)P ₃ levels, thereby lowering the activity of PI3K-related downstream mediators such as the protein kinase B, Akt, which mediates survival, growth, and proliferative signals by inhibiting apoptosis.

PTEN activity is regulated at the transcriptional and post-translational level. At the transcriptional level, pathological stimuli such as ischemia reduced PTEN expression and promoted hypertrophy and remodeling. ^[94],[95] Limited evidence suggests that NF-κB activation may lead to suppression of PTEN expression.^[96],[97] On the other hand, the PTEN promoter contains two PPAR response elements, and several studies demonstrated that PPARγ ligands stimulated PTEN expression.^[98],[99] Furthermore, PTEN overexpression reduced VSMC proliferation and migration and inhibited injury-induced vascular remodeling in vivo. ^[100],[101] PTEN activity is inhibited by ROS which reversibly oxidize cysteine residues in the phosphatase active site. ^[102],[103] In fact, NADPH oxidase-derived ROS facilitated PDGF signaling by inhibiting PTEN. ^[104] These reports suggested that chronic hypoxia caused sustained PI3K/Akt activation, in part, through the generation of NADPH oxidase-derived ROS that stimulated PDGF and inhibited PTEN signaling pathways. A recent report confirmed that chronic hypoxia increased PDGF receptor activation and reduced PTEN expression in the lung. ^[15] Furthermore, treatment with rosiglitazone attenuated hypoxia-induced PDGF receptor activation and restored PTEN levels in hypoxic mice to levels comparable to control animals. The ability of PPARγ ligands to simultaneously stimulate PTEN expression and lower oxidative stress-induced PTEN inactivation while attenuating PH provides a unique strategy to lower PDGF receptor phosphorylation and activation and reduce cellular PI(3, 4, 5)P ₃ levels. These integrated effects may contribute to the ability of PPARγ to attenuate pulmonary VSMC proliferation and hypertrophy as well as vascular remodeling caused by chronic hypoxia. A hypothetical schema depicting relationships between PPARγ, NADPH oxidase, PDGF, and PTEN is provided in [Figure 4].

Altered Bone Morphogenetic Protein (BMP) and TGF-β1 Signaling in PH

BMP and TGF-β1 belong to the TGF beta superfamily of growth factors which are involved in multiple cellular processes including proliferation, differentiation, inflammation, and immunity.^[105],[106] All TGF-β superfamily ligands are generated as inactive dimeric precursor proteins that are subsequently cleaved by proteases, activated and secreted. ^[105] The ligands bind to one of two types of serine/threonine kinase receptors (type I and II) causing Smad substrate recruitment, phosphorylation, transduction of intracellular signals and nuclear translocation. ^{[106],[107],[108]} Approximately 70% of patients with familial pulmonary arterial hypertension and 11-40% with idiopathic pulmonary arterial hypertension (IPAH) have germline mutations in bone morphogenetic protein receptor-2 (BMPR-2), a receptor needed for normal vascular development. ^[109] BMPR-2 expression is decreased in some PH cases without identified BMPR mutation. ^[110] In comparison to unaffected patients, IPAH patients have altered cellular growth responses to TGF-β1 and BMP signals which favor VSMC proliferation.^[106],[111] Specifically, in IPAH patients, TGF-β1 induces a heightened SMC proliferative response and BMP signaling fails to confer an expected antiproliferative and proapoptotic effect. ^[49] TGF-β1 signaling is responsible for pulmonary artery remodeling in rat models as evidenced by the attenuation of SMC migration and muscularization of small pulmonary arteries that occurred upon the inhibition of the TGF-β1 receptor activin like kinase-5 (ALK-5).^[112]

Cell culture experiments reveal that TGF-β1 promotes smooth muscle proliferation via an autocrine induction of PDGF and Nox4.^[113] Mechanistically, Nox4-derived ROS may cause transient oxidative inactivation of counterbalancing phosphatases involved in kinase-based cell growth cascades, effectively promoting cell cycle transition and proliferation. Nox4 and TGF-β1 signaling are closely linked and both increase in response to hypoxia and mediate proliferation of HPASMC in vitro.[15],[114] Specifically, TGF-β1 was found to be the proximal mediator of HPASMC proliferation through a cascade involving sequential signaling of phosphatidylinositol 3-kinase (PI3K) and serine/threonine kinase (Akt) phosphorylation, insulin-like growth factor binding protein-3 (IGFBP-3), and Nox 4. ^[114] Dominant negative mutations of the TGF-β type II receptor ameliorated hypoxia-induced pulmonary vascular remodeling in mice, supporting the role of TGF-β1 in promoting PH through enhanced cellular proliferation. ^[115],[116]

Several lines of evidence suggest that PPARγ activation can modulate TGF-β1 signaling through Smad-dependent and Smad-independent mechanisms. For example, both natural (15d-PGJ₂ ) and synthetic (ciglitazone and rosiglitazone) PPARγ ligands inhibited the profibrotic effects of TGF-β1 on human lung fibroblasts in a Smad-independent manner as evidenced by blunted myofibroblast differentiation and collagen I synthesis.^[117] Adenoviral PPARγ gene overexpression similarly decreased fibroblast to myofibroblast differentiation, collagen I and III production in alkali burned mouse corneas. ^[118] In this study, PPARγ overexpression also reduced TGF-β1mRNA transcription in fibroblasts, cultured macrophages, and epithelial cells. In other studies, PPARγ agonists prevented TGF-β1 induced mesangial and hepatic stellate cell activation and extracellular matrix secretion.^[119],[120] Finally, PPARγ activation inhibited TGF-β1-mediated Smad 3 phosphorylation and induction of connective tissue growth factor expression, a key regulator of extracellular matrix production and neointima formation following vascular injury. ^[121] These findings emphasize that PPARγ modulation of TGF-β1 signaling is cell type and context specific and that PPARγ agonists may blunt abnormal TGF-β1/BMP-2 signaling and resultant pulmonary vascular remodeling in PH. Additional pathways by which PPARγ regulates cellular proliferation and vascular remodeling are summarized in [Figure 4].

Matrix alterations

PH involves remodeling of pulmonary vessels with muscularization of nonmuscular distal arterioles, proliferation and migration of VSMC, and increased production of extracellular matrix proteins including fibronectin, collagen, and elastin. ^[122],[123] Alterations in matrix composition may be related to increased matrix degradation resulting from an imbalance in the matrix metalloproteinases-tissue inhibitor of metalloproteinases system (MMP-TIMP) with deposition of collagen, elastin, fibronectin, and tenascin-C. ^[122] Inhibition of MMPs and elastase prevented the progression and actually induced regression of vascular remodeling in an experimental model of PH. ^[122] In addition, the elastase inhibitor, elafin, protected mice from chronic hypoxia-induced PH. ^[124] Recent evidence indicates that the PPARγ ligand, rosiglitazone, attenuated and reversed vascular remodeling in rat and mouse models of chronic hypoxia-induced PH.^[15],[21] Rosiglitazone decreased collagen production and elastin deposition and increased MMP-2 activity. Treatment with PPARγ ligands has also been associated with attenuation of matrix deposition in disorders other than PH.^{[125],[126],[127]} Taken together, these reports suggest that therapeutic mechanisms of PPARγ activation in PH could involve attenuation of matrix deposition and remodeling in the pulmonary vasculature.

Inflammation

Several lines of evidence implicate inflammation in the pathobiology of PH. Despite the heterogeneity of disease conditions that lead to PH, similar inflammatory cells types can be found in the plexiform vascular lesions in patients with IPAH and PH associated with connective tissue disease and HIV. Perivascular mononuclear cell infiltrates comprising macrophages, T and B lymphocytes, and dendritic cells are reported in patients with PH. ^{[64],[66],[68]} Circulating markers of inflammation are also increased in patients with PH. ^[128] Proinflammatory cytokines and growth factors such as IL-1, IL-6, PDGF, epidermal growth factor (EGF), and vascular endothelial growth factor (VEGF) are increased and contribute to mitogenic and chemoattractant events in the vascular wall in PH. ^[64] Chemokines that mediate inflammation by recruiting leukocytes, monocytes, and T cells to the vascular wall are increased in patients with PH. Chemokines, including fractalkine (CX3CL1), regulated upon activation, normal T-cell expressed and secreted (RANTES), and monocyte chemotactic protein-1 (MCP-1), also promote pulmonary vascular remodeling in PH. ^[64],[68] Fracktalkine is directly associated with pulmonary artery SMC proliferation in MCT-induced PH in the rat, ^[129] and both RANTES ^[68] and MCP-1 ^[130] may indirectly contribute to mitogenesis and vasoconstriction by inducing endothelin converting enzyme-1 (ECE-1) and ET-1.

The stimulation of these inflammatory pathways and their modulation by PPARγ likely occurs at the level of transcriptional regulation. The transcription factor, NFAT, is upregulated in the pulmonary artery walls and circulating inflammatory cells of PH patients. NFAT, a master activator of T cells, regulates the expression of many inflammatory genes. In PAH, increases in NFAT confer a proliferative and apoptosis resistant phenotype on PASMC by increasing bcl-2, mitochondrial hyperpolarization, and downregulating voltage-gated potassium channels (Kv1.5). ^[131] Common stimuli of PH such as hypoxia cause inflammatory responses in the pulmonary vasculature. PPARγ is expressed in T cells, macrophages, leukocytes, and dendritic cells, suggesting that its activation could modulate inflammation in the pulmonary vasculature that contributes to the development of PH. Studies demonstrate that PPARγ ligands attenuate inflammation in numerous models. ^[132] PPARγ activation reduced macrophage recruitment and inflammatory mediator production, impaired dendritic cell priming of T cells, as well as T lymphocyte proliferation and viability. PPARγ ligands also induced regulatory T cells which downregulate immune responses. ^[133],[134] Current evidence indicates that these anti-inflammatory effects of PPARγ are mediated not through transactivation of specific target genes, rather through physical binding to other proinflammatory transcription factors such as NF-kB, AP-1, and STAT, leading to suppression of these proinflammatory transcriptional pathways. The precise mechanisms of these transrepression effects remain to be completely defined but may involve ligand-dependent SUMOylation of PPARγ which targets the receptor to corepressor complexes on the promoters of inflammatory genes preventing proteosomal degradation of the corepressors and thereby causing inhibition of inflammatory gene expression.^[10]

Apoptosis

Apoptosis is an important process whereby selected cells are programmed for death and removal. ^[135] Apoptosis counterbalances cell proliferation and mitotic division in a cascade of events involving caspase activation and protein cleavage, nuclear DNA fragmentation, cytoplasmic condensation, cell fragment sequestration and formation of apoptotic bodies. ^[136] Ineffective apoptosis contributes to the pathology of diseases including some malignancies, autoimmunity and persistent infections. Deregulated apoptosis, on the other hand, may be implicated in atherosclerosis and other cardiovascular diseases. ^[136] Triggers for apoptosis include, but are not limited to Fas death receptor stimulation by Fas ligand, recognition of cells with DNA damage, abnormal cellular migration and proliferation, ROS, angiotensin type 2, and BMP-2. ^[135]

The pulmonary artery SMCs of patients with PAH demonstrated increased tendency to proliferate and exhibited resistance to normal apoptotic signals such as bone morphogenetic protein-2 (BMP-2). ^[137] Accordingly, germline mutations in BMPR-2 significantly contribute to disease pathogenesis in 70% of patients with familial PAH. ^[109] Impaired apoptosis and proliferation of PASMC may result from increases in the apoptosis inhibitor, survivin, and the sequential occurrences: hyperpolarization of mitochondria, increased HIF-1α transcription and nuclear translocation, and decreased Kv1.5 voltage-gated potassium channel expression.^[131] These derangements in the regulation of apoptosis in pulmonary vascular wall cells likely contribute to the increased cell proliferation and migration characteristic of the remodeled pulmonary vasculature in PH. ^[135] PPARγ agonists promote apoptosis of proliferating VSMCs in an extracellular signal-related kinase 1/2 (ERK 1/2) - independent manner that may involve interferon regulatory factor-1 (IRF-1) or activation of the proapoptotic genes, p53 and Gadd 45. ^[138],[139] PPARγ agonists also induce apoptosis in endothelial cells in a PPARγ-dependent manner.^[140] Similarly, pulmonary artery endothelial cells derived from patients with IPAH displayed a proliferative, apoptosis-resistant phenotype associated with enhanced STAT 3 signaling, and PPAR ligands reduced STAT 3 signaling.^[141],[142] Taken together, these studies indicate that multiple pathways can lead to an apoptosis-resistant phenotype in pulmonary vascular wall cells during the development of PH and that PPARγ may counterbalance these proliferative pathways by stimulating apoptosis.

Progenitor cell recruitment

Endothelial progenitor cells (EPCs) are bone marrow-derived cells that are mobilized into the systemic circulation in response to ischemia or vascular injury. ^[143] The role of EPCs in vascular repair, vascular homeostasis, and formation of new blood vessels continues to be defined. ^[144] Clinical trials suggest that EPCs may serve as markers of vascular dysfunction and cardiovascular disease. ^[145],[146] Because endothelial dysfunction participates in the pathogenesis of PH, EPCs may play a role in pulmonary vascular disease. ^[147] The endogenous erythropoietin system recruits EPCs to the lung in experimental PH in mice. ^[148] Furthermore, mesenchymal stem cells overexpressing eNOS or EPCs expressing adrenomedullin attenuated MCT-induced PH in rats. ^[149] These reports suggest that migration of EPCs to the pulmonary vasculature during experimental PH can exert beneficial effects. PPARγ agonists facilitate the differentiation of angiogenic progenitor cells into EPC.^[150] PPARγ agonists also increased the number and migratory activity of EPC in patients with type 2 diabetes and impaired endothelial function and increased EPC migratory activity and reduced EPC apoptosis in mice. ^[151] Collectively, these reports suggest that PPARγ ligands might stimulate EPC to reduce pulmonary vascular dysfunction and reduce PH.

Thrombosis in PH

Pulmonary artery thrombosis is a common pathologic finding occurring in 48-56% of patients with IPAH. ^[152],[153] Thrombosis can also be found in other forms of PH associated with collagen vascular disease, HIV, portal hypertension, drugs and anorexigens. Thrombotic lesions in IPAH can be eccentric or concentric and usually occur in situ in peripheral muscular arteries where they form lesions that result from mural organization of thrombi. ^[49],[153] Thrombosis is in large part due to abnormal platelet activation and endothelial dysfunction which results in coagulation and loss of counterbalancing antifibrinolytic mechanisms. ^[152],[153] Shear stress from elevated pulmonary pressures may contribute to endothelial injury and release of mediators of coagulation. ^[49] Accordingly, patients with IPAH have increased serum levels of coagulation mediators including plasminogen activator inhibitor type-1 (PAI-1), ^[154] Von Willebrand factor (vWF), ^[155] fibrinopeptide A, ^[156] and factor VIIIc. ^[157] The abnormal platelet activation in PH can be attributed to increases in platelet-derived thromboxane (TXA ₂ ) and decreased levels of endothelial derived prostacyclin (PGI ₂ ) and nitric oxide synthase (eNOS). ^[158] Collectively, these imbalances favor coagulation and platelet aggregation which ultimately predisposes patients with PH to the development of thrombosis.

PPARγ agonists modulate platelets' immunoregulatory and proinflammatory functions and favorably affect vascular patency. Platelets release a variety of proinflammatory mediators and cytokines including prostaglandin E1, TGF-β, IL-1B, PAI-1, and CD 40.^[48] For example, the CD 40 ligand is a transmembrane protein expressed on stimulated CD4+ T cells and platelets, ^[159],[160] whose expression can be reduced by PPARγ ligands.^[48],[161] CD 40 ligand is associated with platelet activation and increased risk for cardiovascular disease. ^{[48],[159],[160]} Upon platelet activation, the CD 40 ligand is released and promotes the expression of vasoactive, inflammatory, and thrombotic mediators including cyclooxygenase-2 (COX-2), prostaglandins, TNF-α, IFN-γ, tissue factor, MMPs, selected interleukins, chemokines, and multiple adhesion molecules. ^{[48],[159],[160],[161]} Thus, CD 40 receptor-ligand binding links platelet activation and vascular inflammation to hemostasis and increased risk for intravascular thrombosis. In vitro, thiazolidinediones decreased platelet CD 40 ligand expression and release. ^[159] In vivo experiments confirmed that diabetic mice treated with pioglitazone had decreased CD 40 expression after platelet activation. ^[162] These studies demonstrate that PPARγ agonists modulate platelet immunoregulatory and hemostatic function by attenuating CD 40 expression.

Since PPARγ belongs to a nuclear receptor superfamily of transcription factors, historically PPARγ expression was thought to be limited to nucleated cells. However, Akbiyik and colleagues reported that platelets contain PPARγ.^[159] Given that platelets have no nucleus and express no PPARγ mRNA, PPARγ agonists may affect platelet function through non-transcriptional pathways such as modulation of intracellular signaling or platelet-protein interactions. For example, two studies reported that pioglitazone delayed the onset of iatrogenic arterial thrombosis in animal models. ^[163],[164] The mechanisms for these PPARγ effects may include attenuation of platelet activation as evidenced by reductions in soluble and platelet bound P-selectin, ^[48],[162] CD 40 ligand, ^[49],[159] and TXA ₂ , ^[49],[159] or to direct effects of PPARγ ligands on vascular endothelium and its production of platelet regulators. PPARγ agonists also decreased adenosine diphosphate and arachidonic acid-induced platelet aggregation,^[159],[163] ATP production, ^[163] and increased nitric oxide-mediated activation of fibrinolysis and inhibition of coagulation. ^[163],[165] Other proposed anti-thrombotic mechanisms involve PPARγ-induced increases in prostacyclin,^[164] thrombomodulin, ^[163] as well as decreases in PAI-1 and fibrinogen. ^[166],[167] In summary, PPARγ ligands likely decrease platelet activation and aggregation through direct effects on platelets and cells of the vascular wall that stimulate endothelial and platelet-derived vasodilatory mediators to maintain vascular patency and blood flow.

Conclusions and Future Directions

The evidence reviewed above suggests that PPARγ can participate in the regulation of numerous pathways implicated in PH pathogenesis. Activation of PPARγ can reduce vasoconstriction, vascular remodeling and inflammation, and thrombosis that contribute to the generation and progression of PH. Studies employing animal models of PH have demonstrated that tissue-targeted deletion of PPARγ in vascular wall cells can promote PH while activation of PPARγ with exogenous ligands attenuated PH or vascular remodeling in MCT- or hypoxia-induced rodent models of PH. ^{[14],[15],[17],[20],[21],[22]} The limitations of these animal models have been recently reviewed emphasizing that the performance of pharmacological tools in the treatment of common experimental models of PH in rodents may not accurately translate to the treatment of human PAH. ^[168] The investigation of PPARγ ligands in recently reported rodent models which more closely reproduce pathological changes seen in the pulmonary arteries of patients with advanced PAH provides an experimental strategy that might circumvent some of the limitations of common existing models. ^[169]

Based on the findings in this review, the next step in the evaluation of PPARγ as a therapeutic target in PH would appear to be clinical trials employing PPARγ ligands in patients with PH. The abundant evidence reviewed above that PPARγ ligands can favorably modulate many pathological pathways and mediators involved in PH would support this position. In addition, the current availability of synthetic thiazolidinedione PPARγ ligands (rosiglitazone and pioglitazone) for the treatment of type 2 diabetes in the United States suggests that clinical trials could be expedited. However, recent findings surrounding potential adverse cardiovascular events in diabetic patients taking rosiglitazone emphasize the need for caution before employing this agent in PH patients with preexisting cardiopulmonary disease. ^[170],[171] In contrast, clinical studies with pioglitazone in diabetic patients have reported a lowered risk for adverse cardiovascular endpoints. ^{[172],[173],[174],[175]} These reports emphasize that individual PPARγ ligands may regulate unique patterns of gene expression that differentially modulate cell and tissue function. Thus, the therapeutic potential of strategies targeting PPARγ will be optimized by future studies that must determine not only the relevant molecular pathways that are altered by PPARγ in a ligand-specific manner, but also the cellular site of action of any ligand and its relative dependence on the PPARγ receptor. Evidence supporting successful therapy of PH with existing PPARγ ligands could also stimulate the development of novel pharmacological PPARγ ligands with enhanced therapeutic efficacy and/or reduced side effects.

Acknowledgments

The authors acknowledge grant support from the Research Service of the Atlanta Veterans Affairs Medical Center and the National Institutes of Health (R01 DK 074518). Dr. Green is supported by an NHLBI T32 training grant (HL076118-06).

References

1.	Humbert M, Sitbon O, Chaouat A, Bertocchi M, Habib G, Gressin V, et al. Pulmonary arterial hypertension in France: Results from a national registry. Am J Respir Crit Care Med 2006;173:1023-30.
2.	Simonneau G, Robbins IM, Beghetti M, Channick RN, Delcroix M, Denton CP, et al. Updated clinical classification of pulmonary hypertension. J Am Coll Cardiol 2009;54:S43-54.
3.	McLaughlin VV, Archer SL, Badesch DB, Barst RJ, Farber HW, Lindner JR, et al. ACCF/AHA 2009 expert consensus document on pulmonary hypertension: A report of the American College of Cardiology Foundation Task Force on Expert Consensus Documents and the American Heart Association: Developed in collaboration with the American College of Chest Physicians, American Thoracic Society, Inc., and the Pulmonary Hypertension Association. Circulation 2009;119:2250-94.
4.	Desvergne B, Wahli W. Peroxisome proliferator-activated receptors: Nuclear control of metabolism. Endocr Rev 1999;20:649-88.
5.	Mandard S, Müller M, Kersten S. Peroxisome proliferator-activated receptor alpha target genes. Cell Mol Life Sci 2004;61:393-416.
6.	Barak Y, Liao D, He W, Ong ES, Nelson MC, Olefsky JM, et al. Effects of peroxisome proliferator-activated receptor delta on placentation, adiposity, and colorectal cancer. Proc Natl Acad Sci U S A 2002;99:303-8.
7.	Peters JM, Lee SS, Li W, Ward JM, Gavrilova O, Everett C, et al. Growth, adipose, brain, and skin alterations resulting from targeted disruption of the mouse peroxisome proliferator-activated receptor beta(delta). Mol Cell Biol 2000;20:5119-28.
8.	Zhu Y, Qi C, Korenberg JR, Chen XN, Noya D, Rao MS, et al. Structural organization of mouse peroxisome proliferator-activated receptor gamma (mPPAR gamma) gene: Alternative promoter use and different splicing yield two mPPAR gamma isoforms. Proc Natl Acad Sci U S A 1995;92:7921-5.
9.	Rosen ED, Sarraf P, Troy AE, Bradwin G, Moore K, Milstone DS, et al. PPAR gamma is required for the differentiation of adipose tissue in vivo and in vitro. Mol Cell 1999;4:611-7.
10.	Pascual G, Fong AL, Ogawa S, Gamliel A, Li AC, Perissi V, et al. A SUMOylation-dependent pathway mediates transrepression of inflammatory response genes by PPAR-gamma. Nature 2005;437:759-63.
11.	Li AC, Brown KK, Silvestre MJ, Willson TM, Palinski W, Glass CK. Peroxisome proliferator-activated receptor gamma ligands inhibit development of atherosclerosis in LDL receptor-deficient mice. J Clin Invest 2000;106:523-31.
12.	Xu HE, Lambert MH, Montana VG, Parks DJ, Blanchard SG, Brown PJ, et al. Molecular recognition of fatty acids by peroxisome proliferator-activated receptors. Mol Cell 1999;3:397-403.
13.	Ameshima S, Golpon H, Cool CD, Chan D, Vandivier RW, Gardai SJ, et al. Peroxisome proliferator-activated receptor gamma (PPARgamma) expression is decreased in pulmonary hypertension and affects endothelial cell growth. Circ Res 2003;92:1162-9.
14.	Kim EK, Lee JH, Oh YM, Lee YS, Lee SD. Rosiglitazone attenuates hypoxia-induced pulmonary arterial hypertension in rats. Respirology 2010;15:659-68.
15.	Nisbet RE, Bland JM, Kleinhenz DJ, Mitchell PO, Walp ER, Sutliff RL, et al. Rosiglitazone attenuates chronic hypoxia-induced pulmonary hypertension in a mouse model. Am J Respir Cell Mol Biol 2010;42:482-90.
16.	Barak Y, Nelson MC, Ong ES, Jones YZ, Ruiz-Lozano P, Chien KR, et al. PPAR gamma is required for placental, cardiac, and adipose tissue development. Mol Cell 1999;4:585-95.
17.	Guignabert C, Alvira CM, Alastalo TP, Sawada H, Hansmann G, Zhao M, et al. Tie2-Mediated loss of peroxisome proliferator-activated receptor-{gamma} in mice causes PDGF-receptor {beta}-dependant pulmonary arterial muscularization. Am J Physiol Lung Cell Mol Physiol 2009;297:L1082-90.
18.	Hansmann L, Groeger S, von Wulffen W, Bein G, Hackstein H. Human monocytes represent a competitive source of interferon-alpha in peripheral blood. Clin Immunol 2008;127:252-64.
19.	Tian J, Smith A, Nechtman J, Podolsky R, Aggarwal S, Snead C, et al. Effect of PPARgamma inhibition on pulmonary endothelial cell gene expression: Gene profiling in pulmonary hypertension. Physiol Genomics 2009;40:48-60.
20.	Matsuda Y, Hoshikawa Y, Ameshima S, Suzuki S, Okada Y, Tabata T, et al. [Effects of peroxisome proliferator-activated receptor gamma ligands on monocrotaline-induced pulmonary hypertension in rats]. Nihon Kokyuki Gakkai Zasshi 2005;43:283-8.
21.	Crossno JT Jr, Garat CV, Reusch JE, Morris KG, Dempsey EC, McMurtry IF, et al. Rosiglitazone attenuates hypoxia-induced pulmonary arterial remodeling. Am J Physiol Lung Cell Mol Physiol 2007;292:L885-97.
22.	Hansmann G, de Jesus Perez VA, Alastalo TP, Alvira CM, Guignabert C, Bekker JM, et al. An antiproliferative BMP-2/PPARgamma/apoE axis in human and murine SMCs and its role in pulmonary hypertension. J Clin Invest 2008;118:1846-57.
23.	Christman BW, McPherson CD, Newman JH, King GA, Bernard GR, Groves BM, et al. An imbalance between the excretion of thromboxane and prostacyclin metabolites in pulmonary hypertension. N Engl J Med 1992;327:70-5.
24.	Farber HW, Loscalzo J. Pulmonary arterial hypertension. N Engl J Med 2004;351:1655-65.
25.	Giaid A, Saleh D. Reduced expression of endothelial nitric oxide synthase in the lungs of patients with pulmonary hypertension. N Engl J Med 1995;333:214-21.
26.	Li H, Chen SJ, Chen YF, Meng QC, Durand J, Oparil S, et al. Enhanced endothelin-1 and endothelin receptor gene expression in chronic hypoxia. J Appl Physiol 1994;77:1451-9.
27.	Olschewski H, Rose F, Schermuly R, Ghofrani HA, Enke B, Olschewski A, et al. Prostacyclin and its analogues in the treatment of pulmonary hypertension. Pharmacol Ther 2004;102:139-53.
28.	Hoshikawa Y, Voelkel NF, Gesell TL, Moore MD, Morris KG, Alger LA, et al., Prostacyclin receptor-dependent modulation of pulmonary vascular remodeling. Am J Respir Crit Care Med 2001;164:314-8.
29.	Tuder RM, Cool CD, Geraci MW, Wang J, Abman SH, Wright L, et al. Prostacyclin synthase expression is decreased in lungs from patients with severe pulmonary hypertension. Am J Respir Crit Care Med 1999;159:1925-32.
30.	Robbins IM, Morrow JD, Christman BW. Oxidant stress but not thromboxane decreases with epoprostenol therapy. Free Radic Biol Med 2005;38:568-74.
31.	Gupta RA, Tan J, Krause WF, Geraci MW, Willson TM, Dey SK, et al. Prostacyclin-mediated activation of peroxisome proliferator-activated receptor delta in colorectal cancer. Proc Natl Acad Sci U S A 2000;97:13275-80.
32.	Lim H, Dey SK. A novel pathway of prostacyclin signaling-hanging out with nuclear receptors. Endocrinology 2002;143:3207-10.
33.	Peredo HA, Mayer MA, Carranza A, Puyó AM. Pioglitazone and losartan modify hemodynamic and metabolic parameters and vascular prostanoids in fructose-overloaded rats. Clin Exp Hypertens 2008;30:159-69.
34.	Tesse A, Al-Massarani G, Wangensteen R, Reitenbach S, Martínez MC, Andriantsitohaina R. Rosiglitazone, a peroxisome proliferator-activated receptor-gamma agonist, prevents microparticle-induced vascular hyporeactivity through the regulation of proinflammatory proteins. J Pharmacol Exp Ther 2008;324:539-47.
35.	Adnot S, Raffestin B, Eddahibi S, Braquet P, Chabrier PE. Loss of endothelium-dependent relaxant activity in the pulmonary circulation of rats exposed to chronic hypoxia. J Clin Invest 1991;87:155-62.
36.	Steinhorn RH, Russell JA, Lakshminrusimha S, Gugino SF, Black SM, Fineman JR. Altered endothelium-dependent relaxations in lambs with high pulmonary blood flow and pulmonary hypertension. Am J Physiol Heart Circ Physiol 2001;280:H311-7.
37.	McQuillan LP, Leung GK, Marsden PA, Kostyk SK, Kourembanas S. Hypoxia inhibits expression of eNOS via transcriptional and posttranscriptional mechanisms. Am J Physiol 1994;267:H1921-7.
38.	Fagan KA, Fouty BW, Tyler RC, Morris KG Jr, Hepler LK, Sato K, et al. The pulmonary circulation of homozygous or heterozygous eNOS-null mice is hyperresponsive to mild hypoxia. J Clin Invest 1999;103:291-9.
39.	Champion HC, Bivalacqua TJ, Greenberg SS, Giles TD, Hyman AL, Kadowitz PJ. Adenoviral gene transfer of endothelial nitric-oxide synthase (eNOS) partially restores normal pulmonary arterial pressure in eNOS-deficient mice. Proc Natl Acad Sci U S A 2002;99:13248-53.
40.	Ozaki M, Kawashima S, Yamashita T, Ohashi Y, Rikitake Y, Inoue N, et al. Reduced hypoxic pulmonary vascular remodeling by nitric oxide from the endothelium. Hypertension 2001;37:322-7.
41.	Channick RN, Newhart JW, Johnson FW, Williams PJ, Auger WR, Fedullo PF, et al. Pulsed delivery of inhaled nitric oxide to patients with primary pulmonary hypertension: An ambulatory delivery system and initial clinical tests. Chest 1996;109:1545-9.
42.	Zhao YD, Courtman DW, Ng DS, Robb MJ, Deng YP, Trogadis J, et al. Microvascular regeneration in established pulmonary hypertension by angiogenic gene transfer. Am J Respir Cell Mol Biol 2006;35:182-9.
43.	Pritchard KA Jr, Ackerman AW, Gross ER, Stepp DW, Shi Y, Fontana JT, et al. Heat shock protein 90 mediates the balance of nitric oxide and superoxide anion from endothelial nitric-oxide synthase. J Biol Chem 2001;276:17621-4.
44.	Konduri GG, Ou J, Shi Y, Pritchard KA Jr. Decreased association of HSP90 impairs endothelial nitric oxide synthase in fetal lambs with persistent pulmonary hypertension. Am J Physiol Heart Circ Physiol 2003;285:H204-11.
45.	Murata T, Sato K, Hori M, Ozaki H, Karaki H. Decreased endothelial nitric-oxide synthase (eNOS) activity resulting from abnormal interaction between eNOS and its regulatory proteins in hypoxia-induced pulmonary hypertension. J Biol Chem 2002;277:44085-92.
46.	Calnek DS, Mazzella L, Roser S, Roman J, Hart CM. Peroxisome proliferator-activated receptor gamma ligands increase release of nitric oxide from endothelial cells. Arterioscler Thromb Vasc Biol 2003;23:52-7.
47.	Polikandriotis JA, Mazzella LJ, Rupnow HL, Hart CM. Peroxisome proliferator-activated receptor gamma ligands stimulate endothelial nitric oxide production through distinct peroxisome proliferator-activated receptor gamma-dependent mechanisms. Arterioscler Thromb Vasc Biol 2005;25:1810-6.
48.	Borchert M, Schöndorf T, Lübben G, Forst T, Pfützner A. Review of the pleiotropic effects of peroxisome proliferator-activated receptor gamma agonists on platelet function. Diabetes Technol Ther 2007;9:410-20.
49.	Morrell NW, Adnot S, Archer SL, Dupuis J, Jones PL, MacLean MR, et al. Cellular and molecular basis of pulmonary arterial hypertension. J Am Coll Cardiol 2009;54:S20-31.
50.	Wakino S, Hayashi K, Tatematsu S, Hasegawa K, Takamatsu I, Kanda T, et al. Pioglitazone lowers systemic asymmetric dimethylarginine by inducing dimethylarginine dimethylaminohydrolase in rats. Hypertens Res 2005;28:255-62.
51.	Palmer RM, Ferrige AG, Moncada S. Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor. Nature 1987;327:524-6.
52.	Azuma H, Ishikawa M, Sekizaki S. Endothelium-dependent inhibition of platelet aggregation. Br J Pharmacol 1986;88:411-5.
53.	Garg UC, Hassid A. Nitric oxide-generating vasodilators and 8-bromo-cyclic guanosine monophosphate inhibit mitogenesis and proliferation of cultured rat vascular smooth muscle cells. J Clin Invest 1989;83:1774-7.
54.	Bath PM, Hassall DG, Gladwin AM, Palmer RM, Martin JF. Nitric oxide and prostacyclin. Divergence of inhibitory effects on monocyte chemotaxis and adhesion to endothelium in vitro. Arterioscler Thromb 1991;11:254-60.
55.	Kuzkaya N, Weissmann N, Harrison DG, Dikalov S. Interactions of peroxynitrite, tetrahydrobiopterin, ascorbic acid, and thiols: Implications for uncoupling endothelial nitric-oxide synthase. J Biol Chem 2003;278:22546-54.
56.	Jernigan NL, Walker BR, Resta TC. Endothelium-derived reactive oxygen species and endothelin-1 attenuate NO-dependent pulmonary vasodilation following chronic hypoxia. Am J Physiol Lung Cell Mol Physiol 2004;287:L801-8.
57.	Liu JQ, Zelko IN, Erbynn EM, Sham JS, Folz RJ. Hypoxic pulmonary hypertension: Role of superoxide and NADPH oxidase (gp91phox). Am J Physiol Lung Cell Mol Physiol 2006;290:L2-10.
58.	Delerive P, Martin-Nizard F, Chinetti G, Trottein F, Fruchart JC, Najib J, et al. Peroxisome proliferator-activated receptor activators inhibit thrombin-induced endothelin-1 production in human vascular endothelial cells by inhibiting the activator protein-1 signaling pathway. Circ Res 1999;85:394-402.
59.	Martin-Nizard F, Furman C, Delerive P, Kandoussi A, Fruchart JC, Staels B, et al. Peroxisome proliferator-activated receptor activators inhibit oxidized low-density lipoprotein-induced endothelin-1 secretion in endothelial cells. J Cardiovasc Pharmacol 2002;40:822-31.
60.	Fukunaga Y, Itoh H, Doi K, Tanaka T, Yamashita J, Chun TH, et al. Thiazolidinediones, peroxisome proliferator-activated receptor gamma agonists, regulate endothelial cell growth and secretion of vasoactive peptides. Atherosclerosis 2001;158:113-9.
61.	Wang TD, Chen WJ, Cheng WC, Lin JW, Chen MF, Lee YT. Relation of improvement in endothelium-dependent flow-mediated vasodilation after rosiglitazone to changes in asymmetric dimethylarginine, endothelin-1, and C-reactive protein in nondiabetic patients with the metabolic syndrome. Am J Cardiol 2006;98:1057-62.
62.	Iglarz M, Touyz RM, Amiri F, Lavoie MF, Diep QN, Schiffrin EL. Effect of peroxisome proliferator-activated receptor-alpha and -gamma activators on vascular remodeling in endothelin-dependent hypertension. Arterioscler Thromb Vasc Biol 2003;23:45-51.
63.	Jeffery TK, Morrell NW. Molecular and cellular basis of pulmonary vascular remodeling in pulmonary hypertension. Prog Cardiovasc Dis 2002;45:173-202.
64.	Hassoun PM, Mouthon L, Barberà JA, Eddahibi S, Flores SC, Grimminger F, et al. Inflammation, growth factors, and pulmonary vascular remodeling. J Am Coll Cardiol 2009;54:S10-9.
65.	Tuder RM, Groves B, Badesch DB, Voelkel NF. Exuberant endothelial cell growth and elements of inflammation are present in plexiform lesions of pulmonary hypertension. Am J Pathol 1994;144:275-85.
66.	Tuder RM, Voelkel NF. Pulmonary hypertension and inflammation. J Lab Clin Med 1998;132:16-24.
67.	Balabanian K, Foussat A, Dorfmüller P, Durand-Gasselin I, Capel F, Bouchet-Delbos L, et al. CX(3)C chemokine fractalkine in pulmonary arterial hypertension. Am J Respir Crit Care Med 2002;165:1419-25.
68.	Dorfmüller P, Zarka V, Durand-Gasselin I, Monti G, Balabanian K, Garcia G, et al. Chemokine RANTES in severe pulmonary arterial hypertension. Am J Respir Crit Care Med 2002;165:534-9.
69.	Sanchez O, Marcos E, Perros F, Fadel E, Tu L, Humbert M, et al. Role of endothelium-derived CC chemokine ligand 2 in idiopathic pulmonary arterial hypertension. Am J Respir Crit Care Med 2007;176:1041-7.
70.	Griendling KK, Sorescu D, Ushio-Fukai M. NAD(P)H oxidase: Role in cardiovascular biology and disease. Circ Res 2000;86:494-501.
71.	Bedard K, Krause KH. The NOX family of ROS-generating NADPH oxidases: Physiology and pathophysiology. Physiol Rev 2007;87:245-313.
72.	Lambeth JD. Nox/Duox family of nicotinamide adenine dinucleotide (phosphate) oxidases. Curr Opin Hematol 2002;9:11-7.
73.	Mittal M, Roth M, König P, Hofmann S, Dony E, Goyal P, et al. Hypoxia-dependent regulation of nonphagocytic NADPH oxidase subunit NOX4 in the pulmonary vasculature. Circ Res 2007;101:258-67.
74.	Ambasta RK, Kumar P, Griendling KK, Schmidt HH, Busse R, Brandes RP. Direct interaction of the novel Nox proteins with p22phox is required for the formation of a functionally active NADPH oxidase. J Biol Chem 2004;279:45935-41.
75.	Djordjevic T, BelAiba RS, Bonello S, Pfeilschifter J, Hess J, Görlach A. Human urotensin II is a novel activator of NADPH oxidase in human pulmonary artery smooth muscle cells. Arterioscler Thromb Vasc Biol 2005;25:519-25.
76.	Serrander L, Cartier L, Bedard K, Banfi B, Lardy B, Plastre O, et al. NOX4 activity is determined by mRNA levels and reveals a unique pattern of ROS generation. Biochem J 2007;406:105-14.
77.	Lambeth JD, Kawahara T, Diebold B. Regulation of Nox and Duox enzymatic activity and expression. Free Radic Biol Med 2007;43:319-31.
78.	Sorescu GP, Song H, Tressel SL, Hwang J, Dikalov S, Smith DA, et al. Bone morphogenic protein 4 produced in endothelial cells by oscillatory shear stress induces monocyte adhesion by stimulating reactive oxygen species production from a nox1-based NADPH oxidase. Circ Res 2004;95:773-9.
79.	Hwang J, Kleinhenz DJ, Lassègue B, Griendling KK, Dikalov S, Hart CM. Peroxisome proliferator-activated receptor-gamma ligands regulate endothelial membrane superoxide production. Am J Physiol Cell Physiol 2005;288:C899-905.
80.	Lu X, Murphy TC, Nanes MS, Hart CM. PPAR{gamma} regulates hypoxia-induced Nox4 expression in human pulmonary artery smooth muscle cells through NF-{kappa}B. Am J Physiol Lung Cell Mol Physiol 2010;299:L559-66.
81.	Zhang L, Sheppard OR, Shah AM, Brewer AC. Positive regulation of the NADPH oxidase NOX4 promoter in vascular smooth muscle cells by E2F. Free Radic Biol Med 2008;45:679-85.
82.	Chen K, Kirber MT, Xiao H, Yang Y, Keaney JF Jr. Regulation of ROS signal transduction by NADPH oxidase 4 localization. J Cell Biol 2008;181:1129-39.
83.	Fredriksson L, Li H, Eriksson U. The PDGF family: Four gene products form five dimeric isoforms. Cytokine Growth Factor Rev 2004;15:197-204.
84.	Raines EW. PDGF and cardiovascular disease. Cytokine Growth Factor Rev 2004;15:237-54.
85.	Tallquist M, Kazlauskas A. PDGF signaling in cells and mice. Cytokine Growth Factor Rev 2004;15:205-13.
86.	Balasubramaniam V, Le Cras TD, Ivy DD, Grover TR, Kinsella JP, Abman SH. Role of platelet-derived growth factor in vascular remodeling during pulmonary hypertension in the ovine fetus. Am J Physiol Lung Cell Mol Physiol 2003;284:L826-33.
87.	Ghofrani HA, Seeger W, Grimminger F. Imatinib for the treatment of pulmonary arterial hypertension. N Engl J Med 2005;353:1412-3.
88.	Schermuly RT, Dony E, Ghofrani HA, Pullamsetti S, Savai R, Roth M, et al. Reversal of experimental pulmonary hypertension by PDGF inhibition. J Clin Invest 2005;115:2811-21.
89.	Goetze S, Xi XP, Kawano H, Gotlibowski T, Fleck E, Hsueh WA, et al. PPAR gamma-ligands inhibit migration mediated by multiple chemoattractants in vascular smooth muscle cells. J Cardiovasc Pharmacol 1999;33:798-806.
90.	Marx N, Schönbeck U, Lazar MA, Libby P, Plutzky J. Peroxisome proliferator-activated receptor gamma activators inhibit gene expression and migration in human vascular smooth muscle cells. Circ Res 1998;83:1097-103.
91.	Law RE, Goetze S, Xi XP, Jackson S, Kawano Y, Demer L, et al. Expression and function of PPARgamma in rat and human vascular smooth muscle cells. Circulation 2000;101:1311-8.
92.	Gericke A, Munson M, Ross AH. Regulation of the PTEN phosphatase. Gene 2006;374:1-9.
93.	Maehama T, Dixon JE. The tumor suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate. J Biol Chem 1998;273:13375-8.
94.	Schwartzbauer G, Robbins J. The tumor suppressor gene PTEN can regulate cardiac hypertrophy and survival. J Biol Chem 2001;276:35786-93.
95.	Tong H, Chen W, Steenbergen C, Murphy E. Ischemic preconditioning activates phosphatidylinositol-3-kinase upstream of protein kinase C. Circ Res 2000;87:309-15.
96.	Han SW, Roman J. Fibronectin induces cell proliferation and inhibits apoptosis in human bronchial epithelial cells: Pro-oncogenic effects mediated by PI3-kinase and NF-kappa B. Oncogene 2006;25:4341-9.
97.	Wang Q, Zhou Y, Wang X, Chung DH, Evers BM. Regulation of PTEN expression in intestinal epithelial cells by c-Jun NH2-terminal kinase activation and nuclear factor-kappaB inhibition. Cancer Res 2007;67:7773-81.
98.	Farrow B, Evers BM. Activation of PPARgamma increases PTEN expression in pancreatic cancer cells. Biochem Biophys Res Commun 2003;301:50-3.
99.	Patel L, Pass I, Coxon P, Downes CP, Smith SA, Macphee CH. Tumor suppressor and anti-inflammatory actions of PPARgamma agonists are mediated via upregulation of PTEN. Curr Biol 2001;11:764-8.
100.	Huang J, Kontos CD. Inhibition of vascular smooth muscle cell proliferation, migration, and survival by the tumor suppressor protein PTEN. Arterioscler Thromb Vasc Biol 2002;22:745-51.
101.	Huang J, Niu XL, Pippen AM, Annex BH, Kontos CD. Adenovirus-mediated intraarterial delivery of PTEN inhibits neointimal hyperplasia. Arterioscler Thromb Vasc Biol 2005;25:354-8.
102.	Lee SR, Yang KS, Kwon J, Lee C, Jeong W, Rhee SG. Reversible inactivation of the tumor suppressor PTEN by H2O2. J Biol Chem 2002;277:20336-42.
103.	Leslie NR, Bennett D, Lindsay YE, Stewart H, Gray A, Downes CP. Redox regulation of PI 3-kinase signalling via inactivation of PTEN. EMBO J 2003;22:5501-10.
104.	Kwon J, Lee SR, Yang KS, Ahn Y, Kim YJ, Stadtman ER, et al. Reversible oxidation and inactivation of the tumor suppressor PTEN in cells stimulated with peptide growth factors. Proc Natl Acad Sci U S A 2004;101:16419-24.
105.	Goumans MJ, Liu Z, ten Dijke P. TGF-beta signaling in vascular biology and dysfunction. Cell Res 2009;19:116-27.
106.	Upton PD, Morrell NW. TGF-beta and BMPR-II pharmacology--implications for pulmonary vascular diseases. Curr Opin Pharmacol 2009;9:274-80.
107.	Miyazono K. Transforming growth factor-beta signaling in epithelial-mesenchymal transition and progression of cancer. Proc Jpn Acad Ser B Phys Biol Sci 2009;85:314-23.
108.	Richter A, Yeager ME, Zaiman A, Cool CD, Voelkel NF, Tuder RM. Impaired transforming growth factor-beta signaling in idiopathic pulmonary arterial hypertension. Am J Respir Crit Care Med 2004;170:1340-8.
109.	Cogan JD, Vnencak-Jones CL, Phillips JA 3 ^rd , Lane KB, Wheeler LA, Robbins IM, et al. Gross BMPR2 gene rearrangements constitute a new cause for primary pulmonary hypertension. Genet Med 2005;7:169-74.
110.	Du L, Sullivan CC, Chu D, Cho AJ, Kido M, Wolf PL, et al. Signaling molecules in nonfamilial pulmonary hypertension. N Engl J Med 2003;348:500-9.
111.	Humbert M, Morrell NW, Archer SL, Stenmark KR, MacLean MR, Lang IM, et al. Cellular and molecular pathobiology of pulmonary arterial hypertension. J Am Coll Cardiol 2004;43:13S-24S.
112.	Long L, Crosby A, Yang X, Southwood M, Upton PD, Kim DK, et al. Altered bone morphogenetic protein and transforming growth factor-beta signaling in rat models of pulmonary hypertension: Potential for activin receptor-like kinase-5 inhibition in prevention and progression of disease. Circulation 2009;119:566-76.
113.	Sturrock A, Cahill B, Norman K, Huecksteadt TP, Hill K, Sanders K, et al. Transforming growth factor-beta1 induces Nox4 NAD(P)H oxidase and reactive oxygen species-dependent proliferation in human pulmonary artery smooth muscle cells. Am J Physiol Lung Cell Mol Physiol 2006;290:L661-73.
114.	Ismail S, Sturrock A, Wu P, Cahill B, Norman K, Huecksteadt T, et al. NOX4 mediates hypoxia-induced proliferation of human pulmonary artery smooth muscle cells: The role of autocrine production of transforming growth factor-{beta}1 and insulin-like growth factor binding protein-3. Am J Physiol Lung Cell Mol Physiol 2009;296:L489-99.
115.	Ambalavanan N, Nicola T, Hagood J, Bulger A, Serra R, Murphy-Ullrich J, et al. Transforming growth factor-beta signaling mediates hypoxia-induced pulmonary arterial remodeling and inhibition of alveolar development in newborn mouse lung. Am J Physiol Lung Cell Mol Physiol 2008;295:L86-95.
116.	Chen G, Khalil N. TGF-beta1 increases proliferation of airway smooth muscle cells by phosphorylation of map kinases. Respir Res 2006;7:2.
117.	Burgess HA, Daugherty LE, Thatcher TH, Lakatos HF, Ray DM, Redonnet M, et al. PPARgamma agonists inhibit TGF-beta induced pulmonary myofibroblast differentiation and collagen production: Implications for therapy of lung fibrosis. Am J Physiol Lung Cell Mol Physiol 2005;288:L1146-53.
118.	Saika S, Yamanaka O, Nishikawa-Ishida I, Kitano A, Flanders KC, Okada Y, et al. Effect of Smad7 gene overexpression on transforming growth factor beta-induced retinal pigment fibrosis in a proliferative vitreoretinopathy mouse model. Arch Ophthalmol 2007;125:647-54.
119.	Zou R, Xu G, Liu XC, Han M, Jiang JJ, Huang Q, et al. PPARgamma agonists inhibit TGF-beta-PKA signaling in glomerulosclerosis. Acta Pharmacol Sin 2010;31:43-50.
120.	Zhao C, Chen W, Yang L, Chen L, Stimpson SA, Diehl AM. PPARgamma agonists prevent TGFbeta1/Smad3-signaling in human hepatic stellate cells. Biochem Biophys Res Commun 2006;350:385-91.
121.	Fu M, Zhang J, Zhu X, Myles DE, Willson TM, Liu X, et al. Peroxisome proliferator-activated receptor gamma inhibits transforming growth factor beta-induced connective tissue growth factor expression in human aortic smooth muscle cells by interfering with Smad3. J Biol Chem 2001;276:45888-94.
122.	Cowan KN, Jones PL, Rabinovitch M. Elastase and matrix metalloproteinase inhibitors induce regression, and tenascin-C antisense prevents progression, of vascular disease. J Clin Invest 2000;105:21-34.
123.	Stenmark KR, Fagan KA, Frid MG. Hypoxia-induced pulmonary vascular remodeling: Cellular and molecular mechanisms. Circ Res 2006;99:675-91.
124.	Zaidi SH, You XM, Ciura S, Husain M, Rabinovitch M. Overexpression of the serine elastase inhibitor elafin protects transgenic mice from hypoxic pulmonary hypertension. Circulation 2002;105:516-21.
125.	Hao GH, Niu XL, Gao DF, Wei J, Wang NP. Agonists at PPAR-gamma suppress angiotensin II-induced production of plasminogen activator inhibitor-1 and extracellular matrix in rat cardiac fibroblasts. Br J Pharmacol 2008;153:1409-19.
126.	Makino N, Sugano M, Satoh S, Oyama J, Maeda T. Peroxisome proliferator-activated receptor-gamma ligands attenuate brain natriuretic peptide production and affect remodeling in cardiac fibroblasts in reoxygenation after hypoxia. Cell Biochem Biophys 2006;44:65-71.
127.	Peng Y, Liu H, Liu F, Liu Y, Li J, Chen X. Troglitazone inhibits synthesis of transforming growth factor-beta1 and reduces matrix production in human peritoneal mesothelial cells. Nephrology (Carlton) 2006;11:516-23.
128.	Bakouboula B, Morel O, Faure A, Zobairi F, Jesel L, Trinh A, et al. Procoagulant membrane microparticles correlate with the severity of pulmonary arterial hypertension. Am J Respir Crit Care Med 2008;177:536-43.
129.	Perros F, Dorfmüller P, Souza R, Durand-Gasselin I, Godot V, Capel F, et al. Fractalkine-induced smooth muscle cell proliferation in pulmonary hypertension. Eur Respir J 2007;29:937-43.
130.	Molet S, Furukawa K, Maghazechi A, Hamid Q, Giaid A. Chemokine- and cytokine-induced expression of endothelin 1 and endothelin-converting enzyme 1 in endothelial cells. J Allergy Clin Immunol 2000;105:333-8.
131.	Bonnet S, Rochefort G, Sutendra G, Archer SL, Haromy A, Webster L, et al. The nuclear factor of activated T cells in pulmonary arterial hypertension can be therapeutically targeted. Proc Natl Acad Sci U S A 2007;104:11418-23.
132.	Szanto A, Nagy L. The many faces of PPARgamma: Anti-inflammatory by any means? Immunobiology 2008;213:789-803.
133.	Hontecillas R, Bassaganya-Riera J. Peroxisome proliferator-activated receptor gamma is required for regulatory CD4+ T cell-mediated protection against colitis. J Immunol 2007;178:2940-9.
134.	Wohlfert EA, Nichols FC, Nevius E, Clark RB. Peroxisome proliferator-activated receptor gamma (PPARgamma) and immunoregulation: Enhancement of regulatory T cells through PPARgamma-dependent and -independent mechanisms. J Immunol 2007;178:4129-35.
135.	Gurbanov E, Shiliang X. The key role of apoptosis in the pathogenesis and treatment of pulmonary hypertension. Eur J Cardiothorac Surg 2006;30:499-507.
136.	Haunstetter A, Izumo S. Apoptosis: Basic mechanisms and implications for cardiovascular disease. Circ Res 1998;82:1111-29.
137.	Zhang S, Fantozzi I, Tigno DD, Yi ES, Platoshyn O, Thistlethwaite PA, et al. Bone morphogenetic proteins induce apoptosis in human pulmonary vascular smooth muscle cells. Am J Physiol Lung Cell Mol Physiol 2003;285:L740-54.
138.	Okura T, Nakamura M, Takata Y, Watanabe S, Kitami Y, Hiwada K. Troglitazone induces apoptosis via the p53 and Gadd45 pathway in vascular smooth muscle cells. Eur J Pharmacol 2000;407:227-35.
139.	Aizawa Y, Kawabe J, Hasebe N, Takehara N, Kikuchi K. Pioglitazone enhances cytokine-induced apoptosis in vascular smooth muscle cells and reduces intimal hyperplasia. Circulation 2001;104:455-60.
140.	Bishop-Bailey D, Hla T. Endothelial cell apoptosis induced by the peroxisome proliferator-activated receptor (PPAR) ligand 15-deoxy-Delta12, 14-prostaglandin J2. J Biol Chem 1999;274:17042-8.
141.	Masri FA, Xu W, Comhair SA, Asosingh K, Koo M, Vasanji A, et al. Hyperproliferative apoptosis-resistant endothelial cells in idiopathic pulmonary arterial hypertension. Am J Physiol Lung Cell Mol Physiol 2007;293:L548-54.
142.	Park EJ, Park SY, Joe EH, Jou I. 15d-PGJ2 and rosiglitazone suppress Janus kinase-STAT inflammatory signaling through induction of suppressor of cytokine signaling 1 (SOCS1) and SOCS3 in glia. J Biol Chem 2003;278:14747-52.
143.	Asahara T, Murohara T, Sullivan A, Silver M, van der Zee R, Li T, et al. Isolation of putative progenitor endothelial cells for angiogenesis. Science 1997;275:964-7.
144.	Rosenzweig A. Circulating endothelial progenitors--cells as biomarkers. N Engl J Med 2005;353:1055-7.
145.	Hill JM, Zalos G, Halcox JP, Schenke WH, Waclawiw MA, Quyyumi AA, et al. Circulating endothelial progenitor cells, vascular function, and cardiovascular risk. N Engl J Med 2003;348:593-600.
146.	Werner N, Kosiol S, Schiegl T, Ahlers P, Walenta K, Link A, et al. Circulating endothelial progenitor cells and cardiovascular outcomes. N Engl J Med 2005;353:999-1007.
147.	Sata M. Role of circulating vascular progenitors in angiogenesis, vascular healing, and pulmonary hypertension: Lessons from animal models. Arterioscler Thromb Vasc Biol 2006;26:1008-14.
148.	Satoh K, Kagaya Y, Nakano M, Ito Y, Ohta J, Tada H, et al. Important role of endogenous erythropoietin system in recruitment of endothelial progenitor cells in hypoxia-induced pulmonary hypertension in mice. Circulation 2006;113:1442-50.
149.	Nagaya N, Kangawa K, Kanda M, Uematsu M, Horio T, Fukuyama N, et al. Hybrid cell-gene therapy for pulmonary hypertension based on phagocytosing action of endothelial progenitor cells. Circulation 2003;108:889-95.
150.	Wang CH, Ciliberti N, Li SH, Szmitko PE, Weisel RD, Fedak PW, et al. Rosiglitazone facilitates angiogenic progenitor cell differentiation toward endothelial lineage: A new paradigm in glitazone pleiotropy. Circulation 2004;109:1392-400.
151.	Gensch C, Clever YP, Werner C, Hanhoun M, Böhm M, Laufs U. The PPAR-gamma agonist pioglitazone increases neoangiogenesis and prevents apoptosis of endothelial progenitor cells. Atherosclerosis 2007;192:67-74.
152.	Chaouat A, Weitzenblum E, Higenbottam T. The role of thrombosis in severe pulmonary hypertension. Eur Respir J 1996;9:356-63.
153.	Herve P, Humbert M, Sitbon O, Parent F, Nunes H, Legal C, et al. Pathobiology of pulmonary hypertension. The role of platelets and thrombosis. Clin Chest Med 2001;22:451-8.
154.	Welsh CH, Hassell KL, Badesch DB, Kressin DC, Marlar RA. Coagulation and fibrinolytic profiles in patients with severe pulmonary hypertension. Chest 1996;110:710-7.
155.	Hoeper MM, Sosada M, Fabel H. Plasma coagulation profiles in patients with severe primary pulmonary hypertension. Eur Respir J 1998;12:1446-9.
156.	Eisenberg PR, Lucore C, Kaufman L, Sobel BE, Jaffe AS, Rich S. Fibrinopeptide A levels indicative of pulmonary vascular thrombosis in patients with primary pulmonary hypertension. Circulation 1990;82:841-7.
157.	Altman R, Scazziota A, Rouvier J, Gurfinkel E, Favaloro R, Perrone S, et al. Coagulation and fibrinolytic parameters in patients with pulmonary hypertension. Clin Cardiol 1996;19:549-54.
158.	Tuder RM, Chacon M, Alger L, Wang J, Taraseviciene-Stewart L, Kasahara Y, et al. Expression of angiogenesis-related molecules in plexiform lesions in severe pulmonary hypertension: Evidence for a process of disordered angiogenesis. J Pathol 2001;195:367-74.
159.	Akbiyik F, Ray DM, Gettings KF, Blumberg N, Francis CW, Phipps RP. Human bone marrow megakaryocytes and platelets express PPARgamma, and PPARgamma agonists blunt platelet release of CD40 ligand and thromboxanes. Blood 2004;104:1361-8.
160.	Schönbeck U, Libby P. CD40 signaling and plaque instability. Circ Res 2001;89:1092-103.
161.	Henn V, Slupsky JR, Gräfe M, Anagnostopoulos I, Förster R, Müller-Berghaus G, et al., CD40 ligand on activated platelets triggers an inflammatory reaction of endothelial cells. Nature 1998;391:591-4.
162.	Bodary PF, Vargas FB, King SA, Jongeward KL, Wickenheiser KJ, Eitzman DT. Pioglitazone protects against thrombosis in a mouse model of obesity and insulin resistance. J Thromb Haemost 2005;3:2149-53.
163.	Li D, Chen K, Sinha N, Zhang X, Wang Y, Sinha AK, et al. The effects of PPAR-gamma ligand pioglitazone on platelet aggregation and arterial thrombus formation. Cardiovasc Res 2005;65:907-12.
164.	Smyth SS, Jennings JL. PPARgamma agonists: A new strategy for antithrombotic therapy. J Thromb Haemost 2005;3:2147-8.
165.	Cylwik D, Mogielnicki A, Buczko W. L-arginine and cardiovascular system. Pharmacol Rep 2005;57:14-22.
166.	Fonseca VA, Reynolds T, Hemphill D, Randolph C, Wall J, Valiquet TR, et al. Effect of troglitazone on fibrinolysis and activated coagulation in patients with non-insulin-dependent diabetes mellitus. J Diabetes Complications 1998;12:181-6.
167.	Kato K, Yamada D, Midorikawa S, Sato W, Watanabe T. Improvement by the insulin-sensitizing agent, troglitazone, of abnormal fibrinolysis in type 2 diabetes mellitus. Metabolism 2000;49:662-5.
168.	Stenmark KR, Meyrick B, Galie N, Mooi WJ, McMurtry IF. Animal models of pulmonary arterial hypertension: The hope for etiological discovery and pharmacological cure. Am J Physiol Lung Cell Mol Physiol 2009;297:L1013-32.
169.	Abe K, Toba M, Alzoubi A, Ito M, Fagan KA, Cool CD, et al. Formation of plexiform lesions in experimental severe pulmonary arterial hypertension. Circulation 2010;121:2747-54.
170.	Nissen SE, Wolski K. Effect of rosiglitazone on the risk of myocardial infarction and death from cardiovascular causes. N Engl J Med 2007;356:2457-71.
171.	Rosen CJ. The rosiglitazone story--lessons from an FDA Advisory Committee meeting. N Engl J Med 2007;357:844-6.
172.	Dormandy JA, Charbonnel B, Eckland DJ, Erdmann E, Massi-Benedetti M, Moules IK, et al. Secondary prevention of macrovascular events in patients with type 2 diabetes in the PROactive Study (PROspective pioglitAzone Clinical Trial In macroVascular Events): A randomised controlled trial. Lancet 2005;366:1279-89.
173.	Erdmann E, Dormandy JA, Charbonnel B, Massi-Benedetti M, Moules IK, Skene AM, et al. The effect of pioglitazone on recurrent myocardial infarction in 2,445 patients with type 2 diabetes and previous myocardial infarction: Results from the PROactive (PROactive 05) Study. J Am Coll Cardiol 2007;49:1772-80.
174.	Lincoff AM, Wolski K, Nicholls SJ, Nissen SE. Pioglitazone and risk of cardiovascular events in patients with type 2 diabetes mellitus: A meta-analysis of randomized trials. JAMA 2007;298:1180-8.
175.	Wilcox R, Bousser MG, Betteridge DJ, Schernthaner G, Pirags V, Kupfer S, et al. Effects of pioglitazone in patients with type 2 diabetes with or without previous stroke: Results from PROactive (PROspective pioglitAzone Clinical Trial In macroVascular Events 04). Stroke 2007;38:865-73.

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