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Peroxisome proliferator-activated receptor gamma (PPARγ) is a type II nuclear receptor, initially recognized in adipose tissue for its role in fatty acid storage and glucose metabolism. It promotes lipid uptake and adipogenesis by increasing insulin sensitivity and adiponectin release. Later, PPARγ was implicated in cardiac development and in critical conditions such as pulmonary arterial hypertension (PAH) and kidney failure. Recently, a cluster of different papers linked PPARγ signaling with another superfamily, the transforming growth factor beta (TGFβ), and its receptors, all of which play a major role in PAH and kidney failure. TGFβ is a multifunctional cytokine that drives inflammation, fibrosis, and cell differentiation while PPARγ activation reverses these adverse events in many models. Such opposite biological effects emphasize the delicate balance and complex crosstalk between PPARγ and TGFβ. Based on solid experimental and clinical evidence, the present review summarizes connections and their implications for PAH and kidney failure, highlighting the similarities and differences between lung and kidney mechanisms as well as discussing the therapeutic potential of PPARγ agonist pioglitazone.

Purpose of Review: Pulmonary arterial hypertension (PAH) is characterized by pulmonary arterial endothelial cell (PAEC) dysfunction and apoptosis, pulmonary arterial smooth muscle cell (PASMC) proliferation, inflammation, vasoconstriction, and metabolic disturbances that include disrupted bone morphogenetic protein receptor (BMPR2)-peroxisome proliferator-activated receptor gamma (PPARγ) axis and DNA damage. Activation of PPARγ improves many of these mechanisms, although erroneous reports on potential adverse effects of thiazolidinedione (TZD)-class PPARγ agonists reduced their clinical use in the past decade. Here, we review recent findings in heart, lung, and kidney research related to the pathobiology of vascular remodeling and tissue fibrosis, and also potential therapeutic effects of the PPARγ agonist pioglitazone., Recent Findings: Independent of its metabolic effects (improved insulin sensitivity and fatty acid handling), PPARγ activation rescues BMPR2 dysfunction, inhibits TGFβ/Smad3/CTGF and TGFβ/pSTAT3/pFoxO1 pathways, and induces the PPARγ/apoE axis, inhibiting vascular remodeling. PPARγ activation dampens mtDNA damage via PPARγ/UBR5/ATM pathway, improves function of endothelial progenitor cells (EPCs), and decrease renal fibrosis by repressing TGFβ/pSTAT3 and TGFβ/EGR1., Summary: Pharmacological PPARγ activation improves many hallmarks of PAH, including dysfunction of BMPR2-PPARγ axis, PAEC, PASMC, EPC, mitochondria/metabolism, and inflammation. Recent randomized controlled trials, including IRIS (Insulin Resistance Intervention After Stroke Trial), emphasize the beneficial effects of PPARγ agonists in PAH patients, leading to recent revival for clinical use.

Translational research is essential to the development of reverse-remodeling strategies for the treatment of pulmonary vascular disease, pulmonary hypertension, and heart failure via mechanistic in vivo studies using animal models resembling human pulmonary arterial hypertension (PAH), cardiovascular remodeling, and progressive right heart failure. Since 2007, peroxisome proliferator-activated receptor γ (PPARγ) agonists have emerged as promising novel, antiproliferative, antiinflammatory, insulin-sensitizing, efficient medications for the treatment of PAH. However, early diabetes study results, their subsequent misinterpretations, errors in published review articles, and rumors regarding potential adverse effects in the literature have dampened enthusiasm for considering pharmacological PPARγ activation for the treatment of cardiovascular diseases, including PAH. Most recently, the thiazolidinedione class PPARγ agonist pioglitazone underwent a clinical revival, especially based on the IRIS (Insulin Resistance Intervention After Stroke) study, a randomized controlled trial in 3,876 patients without diabetes status post-transient ischemic attack/ischemic stroke who were clinically followed for 4.8 years. We discuss preclinical basic translational findings and randomized controlled trials related to the beneficial and adverse effects of PPARγ agonists of the thiazolidinedione class, with a particular focus on the last 5 years. The objective is a data-driven approach to set the preclinical and clinical study record straight. The convincing recent clinical trial data on the lack of significant toxicity in high-risk populations justify the timely conduct of clinical studies to achieve "repurposing" or "repositioning" of pioglitazone for the treatment of clinical PAH.

Background: It has been proposed that peroxisome proliferator-activated receptor-γ (PPARγ) agonists might reduce renal fibrosis, however, several studies had contradictory results. Moreover, the possible interaction of TGF-β 1 , PPARγ, and transcription factors in renal fibrosis have not been investigated. We hypothesized that oral pioglitazone treatment would inhibit TGF-β-driven renal fibrosis and its progression, by modulating profibrotic transcription factors in TGF-β 1 transgenic mice., Methods: Male C57Bl/6 J mice (control, CTL, n = 14) and TGF-β overexpressing transgenic mice (TGFβ, n = 14, having elevated plasma TGF-β 1 level) were divided in two sets at 10 weeks of age. Mice in the first set were fed with regular rodent chow (CTL and TGFβ, n = 7/group). Mice in the second set were fed with chow containing pioglitazone (at a dose of 20 mg/kg/day, CTL + Pio and TGFβ+Pio, n = 7/group). After 5 weeks of treatment, blood pressure was assessed and urine samples were collected, and the kidneys were analyzed for histology, mRNA and protein expression., Results: TGF-β 1 induced glomerulosclerosis and tubulointerstitial damage were significantly reduced by pioglitazone. Pioglitazone inhibited renal mRNA expression of all the profibrotic effectors: type-III collagen, TGF-β 1 , CTGF and TIMP-1, and alike transcription factors cFos/cJun and protein expression of EGR-1, and STAT3 protein phosphorylation., Conclusions: Oral administration of PPARγ agonist pioglitazone significantly reduces TGF-β 1 -driven renal fibrosis, via the attenuation of EGR-1, STAT3 and AP-1. This implies that PPARγ agonists might be effective in the treatment of chronic kidney disease patients.

Rationale: Arterial remodeling-a hallmark of many cardiovascular pathologies including pulmonary arterial hypertension (PAH)-is regulated by TGFβ1 (transforming growth factor-β1)-TGFβ receptors and the antagonistic, vasoprotective BMPR2 (bone morphogenetic protein receptor 2)-PPARγ (peroxisome proliferator-activated receptor-γ) axis. However, it is unclear which factors drive detrimental TGFβ1 pathways in the hypertensive pulmonary vasculature., Objective: We hypothesized that LRP1 (low-density lipoprotein receptor-related protein 1) expression is decreased in PAH, leading to enhancement (disinhibition) of TGFβ1 signals and that the PPARγ agonist pioglitazone can restore vascular homeostasis and prevent PAH resulting from LRP1 deletion in vascular smooth muscle cells (SMCs)., Methods and Results: Targeted deletion of LRP1 in vascular SMC (smLRP1 -/- ) in mice disinhibited TGFβ1-CTGF (connective tissue growth factor) signaling, leading to spontaneous PAH and distal pulmonary arterial muscularization as assessed by closed-chest cardiac catheterization and anti-αSMA staining. Pioglitazone inhibited the canonical TGFβ1-CTGF axis in human pulmonary artery SMC and smLRP1 -/- main pulmonary artery (CTGF and NOX4) and reversed PAH in smLRP1 -/- mice. TGFβ1 boosted pSmad3 in PASMC from smLRP1 -/- mice versus controls. Pioglitazone-activated PPARγ binds to Smad3 in human pulmonary artery SMC (coimmunoprecipitation), thereby blocking its phosphorylation and overriding LRP1 deficiency. Finally, mRNA and protein expression of LRP1 was decreased in pulmonary plexiform lesions of patients with end-stage idiopathic PAH (laser capture microdissection, qPCR, and immunohistochemistry). Downregulation of LRP1 protein was also demonstrated in explanted PASMC from patients with PAH and accompanied by enhanced TGFβ1-pSmad3-CTGF signaling and increased TGFβ1-induced PASMC proliferation that was prevented by pioglitazone., Conclusions: Here, we identify LRP1 as an integrator of TGFβ1-mediated mechanisms that regulate vascular remodeling in mice and clinical PAH and PPARγ as a therapeutic target that controls canonical TGFβ1 pathways. Hence, pharmacologic PPARγ activation represents a promising new therapy for patients with PAH who lack the vasoprotective LRP1 in vascular SMC.

Right ventricular (RV) heart failure is the leading cause of death in pulmonary arterial hypertension (PAH). Peroxisome proliferator-activated receptor γ (PPARγ) acts as a vasoprotective metabolic regulator in smooth muscle and endothelial cells; however, its role in the heart is unclear. We report that deletion of PPARγ in cardiomyocytes leads to biventricular systolic dysfunction and intramyocellular lipid accumulation in mice. In the SU5416/hypoxia (SuHx) rat model, oral treatment with the PPARγ agonist pioglitazone completely reverses severe PAH and vascular remodeling and prevents RV failure. Failing RV cardiomyocytes exhibited mitochondrial disarray and increased intramyocellular lipids (lipotoxicity) in the SuHx heart, which was prevented by pioglitazone. Unbiased ventricular microRNA (miRNA) arrays, mRNA sequencing, and lipid metabolism studies revealed dysregulation of cardiac hypertrophy, fibrosis, myocardial contractility, fatty acid transport/oxidation (FAO), and transforming growth factor-β signaling in the failing RV. These epigenetic, transcriptional, and metabolic alterations were modulated by pioglitazone through miRNA/mRNA networks previously not associated with PAH/RV dysfunction. Consistently, pre-miR-197 and pre-miR-146b repressed genes that drive FAO ( Cpt1b and Fabp4 ) in primary cardiomyocytes. We recapitulated our major pathogenic findings in human end-stage PAH: (i) in the pressure-overloaded failing RV (miR-197 and miR-146b up-regulated), (ii) in peripheral pulmonary arteries (miR-146b up-regulated, miR-133b down-regulated), and (iii) in plexiform vasculopathy (miR-133b up-regulated, miR-146b down-regulated). Together, PPARγ activation can normalize epigenetic and transcriptional regulation primarily related to disturbed lipid metabolism and mitochondrial morphology/function in the failing RV and the hypertensive pulmonary vasculature, representing a therapeutic approach for PAH and other cardiovascular/pulmonary diseases., (Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

BMP2 and TGFβ1 are functional antagonists of pathological remodeling in the arteries, heart, and lung; however, the mechanisms in VSMCs, and their disturbance in pulmonary arterial hypertension (PAH), are unclear. We found a pro-proliferative TGFβ1-Stat3-FoxO1 axis in VSMCs, and PPARγ as inhibitory regulator of TGFβ1-Stat3-FoxO1 and TGFβ1-Smad3/4, by physically interacting with Stat3 and Smad3. TGFβ1 induces fibrosis-related genes and miR-130a/301b, suppressing PPARγ. Conversely, PPARγ inhibits TGFβ1-induced mitochondrial activation and VSMC proliferation, and regulates two glucose metabolism-related enzymes, platelet isoform of phosphofructokinase (PFKP, a PPARγ target, via miR-331-5p) and protein phosphatase 1 regulatory subunit 3G (PPP1R3G, a Smad3 target). PPARγ knockdown/deletion in VSMCs activates TGFβ1 signaling. The PPARγ agonist pioglitazone reverses PAH and inhibits the TGFβ1-Stat3-FoxO1 axis in TGFβ1-overexpressing mice. We identified PPARγ as a missing link between BMP2 and TGFβ1 pathways in VSMCs. PPARγ activation can be beneficial in TGFβ1-associated diseases, such as PAH, parenchymal lung diseases, and Marfan's syndrome., (Copyright © 2017 Elsevier Inc. All rights reserved.)

The pathobiology of pulmonary arterial hypertension (PAH) involves multiple molecular pathways and environmental modifiers and is characterized by progressive obliteration of pulmonary arterioles, leading to increased pulmonary vascular resistance (PVR), right heart failure, and death in approximately 40 to 60% of patients 5 years after diagnosis. There is emerging evidence that many key genes involved in PAH development are targets of the insulin-sensitizing transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma), and that pharmacological PPARgamma activation would lead to their beneficial induction or repression and subsequent antiproliferative, anti-inflammatory, proapoptotic, and direct vasodilatory effects in the vasculature. PPARgamma acts downstream of bone morphogenetic protein receptor II (BMP-RII), which is the cell surface receptor that is mutated or dysfunctional in many forms of PAH. Because our recent clinical observations indicate that insulin resistance may be an environmental risk factor or disease modifier ("second hit"), we suggest that PPARgamma-activating agents might be beneficial in the future treatment of both insulin-resistant and insulin-sensitive PAH patients with or without BMP-RII mutations.

Peroxisome proliferator-activated receptor (PPAR)-gamma is reduced in pulmonary arteries (PAs) of patients with PA hypertension (PAH), and we reported that deletion of PPARgamma in smooth muscle cells (SMCs) of transgenic mice results in PAH. However, the sequelae of loss of PPARgamma in PA endothelial cells (ECs) are unknown. Therefore, we bred Tie2-Cre mice with PPARgamma(flox/flox) mice to induce EC loss of PPARgamma (Tie2 PPARgamma(-/-)), and we assessed PAH by right ventricular systolic pressure (RVSP), RV hypertrophy (RVH), and muscularized distal PAs in room air (RA), after chronic hypoxia (CH), and after 4 wk of recovery in RA (Rec-RA). The Tie2 PPARgamma(-/-) mice developed spontaneous PAH in RA with increased RVSP, RVH, and muscularized PAs vs. wild type (WT); both genotypes exhibited a similar degree of PAH following chronic hypoxia, but Tie2 PPARgamma(-/-) mice had more residual PAH compared with WT mice after Rec-RA. The Tie2 PPARgamma(-/-) vs. WT mice in RA had increased platelet-derived growth factor receptor-beta (PDGF-Rbeta) expression and signaling, despite an elevation in the PPARgamma target apolipoprotein E, an inhibitor of PDGF signaling. Inhibition of PDGF-Rbeta signaling with imatinib, however, was sufficient to reverse the PAH observed in the Tie2 PPARgamma(-/-) mice. Thus the disruption of PPARgamma signaling in EC is sufficient to cause mild PAH and to impair recovery from CH-induced PAH. Inhibition of heightened PDGF-Rbeta signaling is sufficient to reverse PAH in this genetic model.

Loss-of-function mutations in bone morphogenetic protein receptor II (BMP-RII) are linked to pulmonary arterial hypertension (PAH); the ligand for BMP-RII, BMP-2, is a negative regulator of SMC growth. Here, we report an interplay between PPARgamma and its transcriptional target apoE downstream of BMP-2 signaling. BMP-2/BMP-RII signaling prevented PDGF-BB-induced proliferation of human and murine pulmonary artery SMCs (PASMCs) by decreasing nuclear phospho-ERK and inducing DNA binding of PPARgamma that is independent of Smad1/5/8 phosphorylation. Both BMP-2 and a PPARgamma agonist stimulated production and secretion of apoE by SMCs. Using a variety of methods, including short hairpin RNAi in human PASMCs, PAH patient-derived BMP-RII mutant PASMCs, a PPARgamma antagonist, and PASMCs isolated from PPARgamma- and apoE-deficient mice, we demonstrated that the antiproliferative effect of BMP-2 was BMP-RII, PPARgamma, and apoE dependent. Furthermore, we created mice with targeted deletion of PPARgamma in SMCs and showed that they spontaneously developed PAH, as indicated by elevated RV systolic pressure, RV hypertrophy, and increased muscularization of the distal pulmonary arteries. Thus, PPARgamma-mediated events could protect against PAH, and PPARgamma agonists may reverse PAH in patients with or without BMP-RII dysfunction.

Background: Patients with pulmonary arterial hypertension (PAH) have reduced expression of apolipoprotein E (apoE) and peroxisome proliferator-activated receptor-gamma in lung tissues, and deficiency of both has been linked to insulin resistance. ApoE deficiency leads to enhanced platelet-derived growth factor signaling, which is important in the pathobiology of PAH. We therefore hypothesized that insulin-resistant apoE-deficient (apoE-/-) mice would develop PAH that could be reversed by a peroxisome proliferator-activated receptor-gamma agonist (eg, rosiglitazone)., Methods and Results: We report that apoE-/- mice on a high-fat diet develop PAH as judged by elevated right ventricular systolic pressure. Compared with females, male apoE-/- were insulin resistant, had lower plasma adiponectin, and had higher right ventricular systolic pressure associated with right ventricular hypertrophy and increased peripheral pulmonary artery muscularization. Because male apoE-/- mice were insulin resistant and had more severe PAH than female apoE-/- mice, we treated them with rosiglitazone for 4 and 10 weeks. This treatment resulted in markedly higher plasma adiponectin, improved insulin sensitivity, and complete regression of PAH, right ventricular hypertrophy, and abnormal pulmonary artery muscularization in male apoE-/- mice. We further show that recombinant apoE and adiponectin suppress platelet-derived growth factor-BB-mediated proliferation of pulmonary artery smooth muscle cells harvested from apoE-/- or C57Bl/6 control mice., Conclusions: We have shown that insulin resistance, low plasma adiponectin levels, and deficiency of apoE may be risk factors for PAH and that peroxisome proliferator-activated receptor-gamma activation can reverse PAH in an animal model.