Improving Right Ventricular Function by Increasing BMP Signaling with FK506

3D Imaging Reveals Complex Microvascular Remodeling in the Right Ventricle in Pulmonary Hypertension

BACKGROUND

Pathogenic concepts of right ventricular (RV) failure in pulmonary arterial hypertension focus on a critical loss of microvasculature. However, the methods underpinning prior studies did not take into account the 3-dimensional (3D) aspects of cardiac tissue, making accurate quantification difficult. We applied deep-tissue imaging to the pressure-overloaded RV to uncover the 3D properties of the microvascular network and determine whether deficient microvascular adaptation contributes to RV failure.

METHODS

Heart sections measuring 250-µm-thick were obtained from mice after pulmonary artery banding (PAB) or debanding PAB surgery and properties of the RV microvascular network were assessed using 3D imaging and quantification. Human heart tissues harvested at the time of transplantation from pulmonary arterial hypertension cases were compared with tissues from control cases with normal RV function.

RESULTS

Longitudinal 3D assessment of PAB mouse hearts uncovered complex microvascular remodeling characterized by tortuous, shorter, thicker, highly branched vessels, and overall preserved microvascular density. This remodeling process was reversible in debanding PAB mice in which the RV function recovers over time. The remodeled microvasculature tightly wrapped around the hypertrophied cardiomyocytes to maintain a stable contact surface to cardiomyocytes as an adaptation to RV pressure overload, even in end-stage RV failure. However, microvasculature-cardiomyocyte contact was impaired in areas with interstitial fibrosis where cardiomyocytes displayed signs of hypoxia. Similar to PAB animals, microvascular density in the RV was preserved in patients with end-stage pulmonary arterial hypertension, and microvascular architectural changes appeared to vary by etiology, with patients with pulmonary veno-occlusive disease displaying a lack of microvascular complexity with uniformly short segments.

CONCLUSIONS

3D deep tissue imaging of the failing RV in PAB mice, pulmonary hypertension rats, and patients with pulmonary arterial hypertension reveals complex microvascular changes to preserve the microvascular density and maintain a stable microvascular-cardiomyocyte contact. Our studies provide a novel framework to understand microvascular adaptation in the pressure-overloaded RV that focuses on cell-cell interaction and goes beyond the concept of capillary rarefaction.

Integrative bioinformatics approach for identifying key genes and potential therapeutic targets in the concurrent manifestation of hypertrophic cardiomyopathy and pulmonary hypertension

Background

Hypertrophic cardiomyopathy (HCM), identified as a primary cause of sudden cardiac death (SCD), intertwines with pulmonary hypertension (PH) to amplify cardiovascular morbidity. This complex synergy poses significant therapeutic challenges due to the absence of drugs specifically targeting their concurrent manifestation. This study seeks to unravel the molecular intricacies linking HCM and PH, aiming to lay the groundwork for targeted therapeutic interventions.

Methods

Through the analysis of gene expression profiles from datasets GSE36961 (HCM) and GSE113439 (PH) within the public data repository of Gene Expression Omnibus (GEO), this research systematically identified differentially expressed genes (DEGs), conducted extensive functional annotations, and constructed detailed protein-protein interaction (PPI) networks to uncover crucial hub genes. Further, co-expression analyses, alongside drug prediction and molecular docking simulations, were employed to pinpoint potential therapeutic agents that could ameliorate the combined pathology of HCM and PH.

Results

Our comprehensive analysis unearthed 79 DEGs shared between HCM and PH, highlighting fourteen as pivotal hub genes. Validation across three additional datasets (GSE35229, GSE32453, and GSE53408) from GEO accentuated secreted phosphoprotein 1 (SPP1) as a key gene of interest. Remarkably, the study identified tacrolimus, ponatinib, bosutinib, dasatinib, doxorubicin, and zanubrutinib as promising drugs for addressing the dual challenge of HCM and PH.

Conclusions

The findings of this investigation shed light on the genetic underpinnings of HCM and PH's simultaneous occurrence, emphasizing the central role of SPP1 in their pathogenesis. The identification of six candidate drugs offers a hopeful vista for future therapeutic strategies targeting this complex cardiovascular interplay, marking a significant stride towards mitigating the compounded morbidity of HCM and PH. Future mechanistic and clinical studies are warranted for the investigation of this potential target and therapeutics.

RV-specific Targeting of Snai1 Rescues Pulmonary Hypertension-induced Right Ventricular Failure by Inhibiting EndMT and Fibrosis via LOXL2 Mediated Mechanism

Background

Pulmonary hypertension (PH)-induced right ventricular (RV) failure (PH-RVF) is a significant prognostic determinant of mortality and is characterized by RV hypertrophy, endothelial-to-mesenchymal transition (EndMT), fibroblast-to-myofibroblast transition (FMT), fibrosis, and extracellular matrix (ECM)-remodeling. Despite the importance of RV function in PH, the mechanistic details of PH-RVF, especially the regulatory control of RV EndMT, FMT, and fibrosis, remain unclear. The action of transcription factor Snai1 is shown to be mediated through LOXL2 recruitment, and their co-translocation to the nucleus, during EndMT progression. We hypothesize that RV EndMT and fibrosis in PH-RVF are governed by the TGFβ1-Snai1-LOXL2 axis. Furthermore, targeting Snai1 could serve as a novel therapeutic strategy for PH-RVF.

Methods

Adult male Sprague Dawley rats (250-300g) received either a single subcutaneous injection of Monocrotaline (MCT, 60mg/kg, n=9; followed for 30-days) or Sugen (SU5416 20mg/kg, n=9; 10% O 2 hypoxia for 3-weeks followed by normoxia for 2-weeks) or PBS (CTRL, n=9). We performed secondary bioinformatics analysis on the RV bulk RNA-Seq data from MCT, SuHx, and PAB rats and human PH-PVF. We validated EndMT and FMT and their association with Snai1 and LOXL2 in the RVs of MCT and SuHx rat models and human PH-RVF using immunofluorescence, qPCR, and Western blots. For in vivo Snai1 knockdown (Snai1-KD), MCT-rats either received Snai1-siRNA (n=7; 5nM/injection every 3-4 days; 4-injections) or scramble (SCRM-KD; n=7) through tail vein from day 14-30 after MCT. Echocardiography and catheterization were performed terminally. Bulk RNASeq and differential expression analysis were performed on Snai1- and SCRM-KD rat RVs. In vitro Snai1-KD was performed on human coronary artery endothelial cells (HCAECs) and human cardiac fibroblasts (HCFs) under hypoxia+TGFβ1 for 72-hrs.

Results

PH-RVF had increased RVSP and Fulton index and decreased RV fractional area change (RVFAC %). RV RNASeq demonstrated EndMT as the common top-upregulated pathway between rat (MCT, SuHx, and PAB) and human PH-RVF. Immunofluorescence using EndMT- and FMT-specific markers demonstrated increased EndMT and FMT in RV of MCT and SuHx rats and PH-RVF patients. Further, RV expression of TGFβ1, Snai1, and LOXL2 was increased in MCT and SuHx. Nuclear co-localization and increased immunoreactivity, transcript, and protein levels of Snai1 and LOXL2 were observed in MCT and SuHx rats and human RVs. MCT rats treated with Snai1-siRNA demonstrated decreased Snai1 expression, RVSP, Fulton index, and increased RVFAC. Snai1-KD resulted in decreased RV-EndMT, FMT, and fibrosis via a LOXL2-dependent manner. Further, Snai1-KD inhibited hypoxia+TGFβ1-induced EndMT in HCAECs and FMT in HCFs in vitro by decreasing perinuclear/nuclear Snai1+LOXL2 expression and co-localization.

Conclusions

RV-specific targeting of Snai1 rescues PH-RVF by inhibiting EndMT and Fibrosis via a LOXL2-mediated mechanism.

Pathophysiology of the right ventricle in health and disease: an update

The contribution of the right ventricle (RV) to cardiac output is negligible in normal resting conditions when pressures in the pulmonary circulation are low. However, the RV becomes relevant in healthy subjects during exercise and definitely so in patients with increased pulmonary artery pressures both at rest and during exercise. The adaptation of RV function to loading rests basically on an increased contractility. This is assessed by RV end-systolic elastance (Ees) to match afterload assessed by arterial elastance (Ea). The system has reserve as the Ees/Ea ratio or its imaging surrogate ejection fraction has to decrease by more than half, before the RV undergoes an increase in dimensions with eventual increase in filling pressures and systemic congestion. RV-arterial uncoupling is accompanied by an increase in diastolic elastance. Measurements of RV systolic function but also of diastolic function predict outcome in any cause pulmonary hypertension and heart failure with or without preserved left ventricular ejection fraction. Pathobiological changes in the overloaded RV include a combination of myocardial fibre hypertrophy, fibrosis and capillary rarefaction, a titin phosphorylation-related displacement of myofibril tension-length relationships to higher pressures, a metabolic shift from mitochondrial free fatty acid oxidation to cytoplasmic glycolysis, toxic lipid accumulation, and activation of apoptotic and inflammatory signalling pathways. Treatment of RV failure rests on the relief of excessive loading.

Molecular mechanisms and targets of right ventricular fibrosis in pulmonary hypertension

Right ventricular fibrosis is a stress response, predominantly mediated by cardiac fibroblasts. This cell population is sensitive to increased levels of pro-inflammatory cytokines, pro-fibrotic growth factors and mechanical stimulation. Activation of fibroblasts results in the induction of various molecular signaling pathways, most notably the mitogen-activated protein kinase cassettes, leading to increased synthesis and remodeling of the extracellular matrix. While fibrosis confers structural protection in response to damage induced by ischemia or (pressure and volume) overload, it simultaneously contributes to increased myocardial stiffness and right ventricular dysfunction. Here, we review state-of-the-art knowledge of the development of right ventricular fibrosis in response to pressure overload and provide an overview of all published preclinical and clinical studies in which right ventricular fibrosis was targeted to improve cardiac function.

2023 Taiwan Society of Cardiology (TSOC) and Taiwan College of Rheumatology (TCR) Joint Consensus on Connective Tissue Disease-Associated Pulmonary Arterial Hypertension

Background

Pulmonary arterial hypertension (PAH), defined as the presence of a mean pulmonary artery pressure > 20 mmHg, pulmonary artery wedge pressure ≤ 15 mmHg, and pulmonary vascular resistance (PVR) > 2 Wood units based on expert consensus, is characterized by a progressive and sustained increase in PVR, which may lead to right heart failure and death. PAH is a well-known complication of connective tissue diseases (CTDs), such as systemic sclerosis, systemic lupus erythematosus, Sjogren's syndrome, and other autoimmune conditions. In the past few years, tremendous progress in the understanding of PAH pathogenesis has been made, with various novel diagnostic and screening methods for the early detection of PAH proposed worldwide.

Objectives

This study aimed to obtain a comprehensive understanding and provide recommendations for the management of CTD-PAH in Taiwan, focusing on its clinical importance, prognosis, risk stratification, diagnostic and screening algorithm, and pharmacological treatment.

Methods

The members of the Taiwan Society of Cardiology (TSOC) and Taiwan College of Rheumatology (TCR) reviewed the related literature thoroughly and integrated clinical trial evidence and real-world clinical experience for the development of this consensus.

Conclusions

Early detection by regularly screening at-risk patients with incorporations of relevant autoantibodies and biomarkers may lead to better outcomes of CTD-PAH. This consensus proposed specific screening flowcharts for different types of CTDs, the risk assessment tools applicable to the clinical scenario in Taiwan, and a recommendation of medications in the management of CTD-PAH.

Pathophysiology and new advances in pulmonary hypertension

Pulmonary hypertension is a progressive and often fatal cardiopulmonary condition characterised by increased pulmonary arterial pressure, structural changes in the pulmonary circulation, and the formation of vaso-occlusive lesions. These changes lead to increased right ventricular afterload, which often progresses to maladaptive right ventricular remodelling and eventually death. Pulmonary arterial hypertension represents one of the most severe and best studied types of pulmonary hypertension and is consistently targeted by drug treatments. The underlying molecular pathogenesis of pulmonary hypertension is a complex and multifactorial process, but can be characterised by several hallmarks: inflammation, impaired angiogenesis, metabolic alterations, genetic or epigenetic abnormalities, influence of sex and sex hormones, and abnormalities in the right ventricle. Current treatments for pulmonary arterial hypertension and some other types of pulmonary hypertension target pathways involved in the control of pulmonary vascular tone and proliferation; however, these treatments have limited efficacy on patient outcomes. This review describes key features of pulmonary hypertension, discusses current and emerging therapeutic interventions, and points to future directions for research and patient care. Because most progress in the specialty has been made in pulmonary arterial hypertension, this review focuses on this type of pulmonary hypertension. The review highlights key pathophysiological concepts and emerging therapeutic directions, targeting inflammation, cellular metabolism, genetics and epigenetics, sex hormone signalling, bone morphogenetic protein signalling, and inhibition of tyrosine kinase receptors.

Insights into bone morphogenetic proteins in cardiovascular diseases

Bone morphogenetic proteins (BMPs) are secretory proteins belonging to the transforming growth factor-β (TGF-β) superfamily. These proteins play important roles in embryogenesis, bone morphogenesis, blood vessel remodeling and the development of various organs. In recent years, as research has progressed, BMPs have been found to be closely related to cardiovascular diseases, especially atherosclerosis, vascular calcification, cardiac remodeling, pulmonary arterial hypertension (PAH) and hereditary hemorrhagic telangiectasia (HHT). In this review, we summarized the potential roles and related mechanisms of the BMP family in the cardiovascular system and focused on atherosclerosis and PAH.

PVECs-Derived Exosomal microRNAs Regulate PASMCs via FoxM1 Signaling in IUGR-induced Pulmonary Hypertension

Background Intrauterine growth restriction (IUGR) is closely related to systemic or pulmonary hypertension (PH) in adulthood. Aberrant crosstalk between pulmonary vascular endothelial cells (PVECs) and pulmonary arterial smooth muscle cells (PASMCs) that is mediated by exosomes plays an essential role in the progression of PH. FoxM1 (Forkhead box M1) is a key transcription factor that governs many important biological processes. Methods and Results IUGR-induced PH rat models were established. Transwell plates were used to coculture PVECs and PASMCs. Exosomes were isolated from PVEC-derived medium, and a microRNA (miRNA) screening was proceeded to identify effects of IUGR on small RNAs enclosed within exosomes. Dual-Luciferase assay was performed to validate the predicted binding sites of miRNAs on FoxM1 3' untranslated region. FoxM1 inhibitor thiostrepton was used in IUGR-induced PH rats. In this study, we found that FoxM1 expression was remarkably increased in IUGR-induced PH, and PASMCs were regulated by PVECs through FoxM1 signaling in a non-contact way. An miRNA screening showed that miR-214-3p, miR-326-3p, and miR-125b-2-3p were downregulated in PVEC-derived exosomes of the IUGR group, which were associated with overexpression of FoxM1 and more significant proliferation and migration of PASMCs. Dual-Luciferase assay demonstrated that the 3 miRNAs directly targeted FoxM1 3' untranslated region. FoxM1 inhibition blocked the PVECs-PASMCs crosstalk and reversed the abnormal functions of PASMCs. In vivo, treatment with thiostrepton significantly reduced the severity of PH. Conclusions Transmission of exosomal miRNAs from PVECs regulated the functions of PASMCs via FoxM1 signaling, and FoxM1 may serve as a potential therapeutic target in IUGR-induced PH.

The role of circular RNAs in pulmonary hypertension

Circular RNAs (circRNAs) are endogenous, covalently circularised, non-protein-coding RNAs generated from back-splicing. Most circRNAs are very stable, highly conserved, and expressed in a tissue-, cell- and developmental stage-specific manner. circRNAs play a significant role in various biological processes, such as regulation of gene expression and protein translation via sponging of microRNAs and binding with RNA-binding proteins. circRNAs have become a topic of great interest in research due to their close link with the development of various diseases. Their high stability, conservation and abundance in body fluids make them promising biomarkers for many diseases. A growing body of evidence suggests that aberrant expression of circRNAs and their targets plays a crucial role in pulmonary vascular remodelling and pulmonary arterial hypertension (group 1) as well as other forms (groups 3 and 4) of pulmonary hypertension (PH). Here we discuss the roles and molecular mechanisms of circRNAs in the pathogenesis of pulmonary vascular remodelling and PH. We also highlight the therapeutic and biomarker potential of circRNAs in PH.

Puerarin-V prevents the progression of hypoxia- and monocrotaline-induced pulmonary hypertension in rodent models

Pulmonary hypertension (PH) is a cardiopulmonary disease characterized by a progressive increase in pulmonary vascular resistance. One of the initial pathogenic factors of PH is pulmonary arterial remodeling under various stimuli. Current marketed drugs against PH mainly relieve symptoms without significant improvement in overall prognosis. Discovering and developing new therapeutic drugs that interfere with vascular remodeling is in urgent need. Puerarin is an isoflavone compound extracted from the root of Kudzu vine, which is widely used in the treatment of cardiovascular diseases. In the present study, we evaluated the efficacy of puerarin in the treatment of experimental PH. PH was induced in rats by a single injection of MCT (50 mg/kg, sc), and in mice by exposure to hypoxia (10% O2) for 14 days. After MCT injection the rats were administered puerarin (10, 30, 100 mg · kg-1 · d-1, i.g.) for 28 days, whereas hypoxia-treated mice were pre-administered puerarin (60 mg · kg-1 · d-1, i.g.) for 7 days. We showed that puerarin administration exerted significant protective effects in both experimental PH rodent models, evidenced by significantly reduced right ventricular systolic pressure (RVSP) and lung injury, improved pulmonary artery blood flow as well as pulmonary vasodilation and contraction function, inhibited inflammatory responses in lung tissues, improved resistance to apoptosis and abnormal proliferation in lung tissues, attenuated right ventricular injury and remodeling, and maintained normal function of the right ventricle. We revealed that MCT and hypoxia treatment significantly downregulated BMPR2/Smad signaling in the lung tissues and PPARγ/PI3K/Akt signaling in the lung tissues and right ventricles, which were restored by puerarin administration. In addition, we showed that a novel crystal type V (Puer-V) exerted better therapeutic effects than the crude form of puerarin (Puer). Furthermore, Puer-V was more efficient than bosentan (a positive control drug) in alleviating the abnormal structural changes and dysfunction of lung tissues and right ventricles. In conclusion, this study provides experimental evidence for developing Puer-V as a novel therapeutic drug to treat PH.

Genome- and transcriptome-wide association studies show that pulmonary embolism is associated with bone-forming proteins

BACKGROUND

Pulmonary embolism (PE) is a leading cause of death in stroke patients and a severe health burden worldwide. There is a pressing need to understand the mechanisms by which it occurs and to identify at-risk patients efficiently and accurately.

OBJECTIVES

The aim of this paper was to analyze the genetic correlation between PE and human plasma proteins through genome-wide association study (GWAS) with transcriptome-wide association study (TWAS), in combination with mRNA expression profiling at three levels: DNA, RNA, and protein.

METHODS

First, based on data from GWAS in European populations, we performed a linkage disequilibrium score regression (LDSC) analysis of plasma proteins and PE in 3,283 individuals and additionally analyzed the genetic association between PE and fracture. Then, we performed a TWAS on PE GWAS data using skeletal muscle and blood for gene expression references. Finally, we validated the genetic correlation between PE and human plasma proteins by co-matching the genes encoding the identified proteins and those identified using TWAS with the differentially expressed genes obtained from mRNA expression profiling of PE (Figure1).

RESULTS

We identified five plasma proteins associated with PE, including hydroxycarboxylic acid receptor 2, defensin 118, and bone morphogenetic protein (BMP) 7, as well as a relationship between PE and fracture. Comparison of genes encoding these proteins with genes obtained from TWAS and then with differentially expressed genes obtained from PE mRNA expression profiling revealed that PE was highly correlated with the BMP family of genes.

Cardiac Fibrosis in the Pressure Overloaded Left and Right Ventricle as a Therapeutic Target

Myocardial fibrosis is a remodeling process of the extracellular matrix (ECM) following cardiac stress. "Replacement fibrosis" is a term used to describe wound healing in the acute phase of an injury, such as myocardial infarction. In striking contrast, ECM remodeling following chronic pressure overload insidiously develops over time as "reactive fibrosis" leading to diffuse interstitial and perivascular collagen deposition that continuously perturbs the function of the left (L) or the right ventricle (RV). Examples for pressure-overload conditions resulting in reactive fibrosis in the LV are systemic hypertension or aortic stenosis, whereas pulmonary arterial hypertension (PAH) or congenital heart disease with right sided obstructive lesions such as pulmonary stenosis result in RV reactive fibrosis. In-depth phenotyping of cardiac fibrosis has made it increasingly clear that both forms, replacement and reactive fibrosis co-exist in various etiologies of heart failure. While the role of fibrosis in the pathogenesis of RV heart failure needs further assessment, reactive fibrosis in the LV is a pathological hallmark of adverse cardiac remodeling that is correlated with or potentially might even drive both development and progression of heart failure (HF). Further, LV reactive fibrosis predicts adverse outcome in various myocardial diseases and contributes to arrhythmias. The ability to effectively block pathological ECM remodeling of the LV is therefore an important medical need. At a cellular level, the cardiac fibroblast takes center stage in reactive fibrotic remodeling of the heart. Activation and proliferation of endogenous fibroblast populations are the major source of synthesis, secretion, and deposition of collagens in response to a variety of stimuli. Enzymes residing in the ECM are responsible for collagen maturation and cross-linking. Highly cross-linked type I collagen stiffens the ventricles and predominates over more elastic type III collagen in pressure-overloaded conditions. Research has attempted to identify pro-fibrotic drivers causing fibrotic remodeling. Single key factors such as Transforming Growth Factor β (TGFβ) have been described and subsequently targeted to test their usefulness in inhibiting fibrosis in cultured fibroblasts of the ventricles, and in animal models of cardiac fibrosis. More recently, modulation of phenotypic behaviors like inhibition of proliferating fibroblasts has emerged as a strategy to reduce pathogenic cardiac fibroblast numbers in the heart. Some studies targeting LV reactive fibrosis as outlined above have successfully led to improvements of cardiac structure and function in relevant animal models. For the RV, fibrosis research is needed to better understand the evolution and roles of fibrosis in RV failure. RV fibrosis is seen as an integral part of RV remodeling and presents at varying degrees in patients with PAH and animal models replicating the disease of RV afterload. The extent to which ECM remodeling impacts RV function and thus patient survival is less clear. In this review, we describe differences as well as common characteristics and key players in ECM remodeling of the LV vs. the RV in response to pressure overload. We review pre-clinical studies assessing the effect of anti-fibrotic drug candidates on LV and RV function and their premise for clinical testing. Finally, we discuss the mode of action, safety and efficacy of anti-fibrotic drugs currently tested for the treatment of left HF in clinical trials, which might guide development of new approaches to target right heart failure. We touch upon important considerations and knowledge gaps to be addressed for future clinical testing of anti-fibrotic cardiac therapies.

Therapeutic Approaches for Treating Pulmonary Arterial Hypertension by Correcting Imbalanced TGF-β Superfamily Signaling

Pulmonary arterial hypertension (PAH) is a rare disease characterized by high blood pressure in the pulmonary circulation driven by pathological remodeling of distal pulmonary arteries, leading typically to death by right ventricular failure. Available treatments improve physical activity and slow disease progression, but they act primarily as vasodilators and have limited effects on the biological cause of the disease—the uncontrolled proliferation of vascular endothelial and smooth muscle cells. Imbalanced signaling by the transforming growth factor-β (TGF-β) superfamily contributes extensively to dysregulated vascular cell proliferation in PAH, with overactive pro-proliferative SMAD2/3 signaling occurring alongside deficient anti-proliferative SMAD1/5/8 signaling. We review the TGF-β superfamily mechanisms underlying PAH pathogenesis, superfamily interactions with inflammation and mechanobiological forces, and therapeutic strategies under development that aim to restore SMAD signaling balance in the diseased pulmonary arterial vessels. These strategies could potentially reverse pulmonary arterial remodeling in PAH by targeting causative mechanisms and therefore hold significant promise for the PAH patient population.

Fetal Gene Reactivation in Pulmonary Arterial Hypertension: GOOD, BAD, or BOTH?

Pulmonary arterial hypertension is a debilitating chronic disorder marked by the progressive obliteration of the pre-capillary arterioles. This imposes a pressure overload on the right ventricle (RV) pushing the latter to undergo structural and mechanical adaptations that inexorably culminate in RV failure and death. Thanks to the advances in molecular biology, it has been proposed that some aspects of the RV and pulmonary vascular remodeling processes are orchestrated by a subversion of developmental regulatory mechanisms with an upregulation of a suite of genes responsible for the embryo's early growth and normally repressed in adults. In this review, we present relevant background regarding the close relationship between overactivation of fetal genes and cardiopulmonary remodeling, exploring whether the reawakening of developmental factors plays a causative role or constitutes a protective mechanism in the setting of PAH.