The Endothelial Prolyl-4-Hydroxylase Domain 2/Hypoxia-Inducible Factor 2 Axis Regulates Pulmonary Artery Pressure in Mice

Endothelial HIFα/PDGF-B to smooth muscle Beclin1 signaling sustains pathological muscularization in pulmonary hypertension

Mechanisms underlying maintenance of pathological vascular hypermuscularization are poorly delineated. Herein, we investigated retention of smooth muscle cells (SMCs) coating normally unmuscularized distal pulmonary arterioles in pulmonary hypertension (PH) mediated by chronic hypoxia with or without Sugen 5416, and reversal of this pathology. With hypoxia in mice or culture, lung endothelial cells (ECs) upregulated hypoxia-inducible factor 1α (HIF1-α) and HIF2-α, which induce platelet-derived growth factor B (PDGF-B), and these factors were reduced to normoxic levels with re-normoxia. Re-normoxia reversed hypoxia-induced pulmonary vascular remodeling, but with EC HIFα overexpression during re-normoxia, pathological changes persisted. Conversely, after establishment of distal muscularization and PH, EC-specific deletion of Hif1a, Hif2a, or Pdgfb induced reversal. In human idiopathic pulmonary artery hypertension, HIF1-α, HIF2-α, PDGF-B, and autophagy-mediating gene products, including Beclin1, were upregulated in pulmonary artery SMCs and/or lung lysates. Furthermore, in mice, hypoxia-induced EC-derived PDGF-B upregulated Beclin1 in distal arteriole SMCs, and after distal muscularization was established, re-normoxia, EC Pdgfb deletion, or treatment with STI571 (which inhibits PDGF receptors) downregulated SMC Beclin1 and other autophagy products. Finally, SMC-specific Becn1 deletion induced apoptosis, reversing distal muscularization and PH mediated by hypoxia with or without Sugen 5416. Thus, chronic hypoxia induction of the HIFα/PDGF-B axis in ECs is required for non-cell-autonomous Beclin1-mediated survival of pathological distal arteriole SMCs.

Endothelial Function in Pulmonary Arterial Hypertension: From Bench to Bedside

Pulmonary arterial hypertension is a complex pathology whose etiology is still not completely well clarified. The pathogenesis of pulmonary arterial hypertension involves different molecular mechanisms, with endothelial dysfunction playing a central role in disease progression. Both individual genetic predispositions and environmental factors seem to contribute to its onset. To further understand the complex relationship between endothelial and pulmonary hypertension and try to contribute to the development of future therapies, we report a comprehensive and updated review on endothelial function in pulmonary arterial hypertension.

PHD1-3 oxygen sensors in vivo-lessons learned from gene deletions

Oxygen sensors enable cells to adapt to limited oxygen availability (hypoxia), affecting various cellular and tissue responses. Prolyl-4-hydroxylase domain 1-3 (PHD1-3; also called Egln1-3, HIF-P4H 1-3, HIF-PH 1-3) proteins belong to the Fe2+- and 2-oxoglutarate-dependent dioxygenase superfamily and utilise molecular oxygen (O2) alongside 2-oxoglutarate as co-substrate to hydroxylate two proline residues of α subunits of the dimeric hypoxia inducible factor (HIF) transcription factor. PHD1-3-mediated hydroxylation of HIF-α leads to its degradation and inactivation. Recently, various PHD inhibitors (PHI) have entered the clinics for treatment of renal anaemia. Pre-clinical analyses indicate that PHI treatment may also be beneficial in numerous other hypoxia-associated diseases. Nonetheless, the underlying molecular mechanisms of the observed protective effects of PHIs are only partly understood, currently hindering their translation into the clinics. Moreover, the PHI-mediated increase of Epo levels is not beneficial in all hypoxia-associated diseases and PHD-selective inhibition may be advantageous. Here, we summarise the current knowledge about the relevance and function of each of the three PHD isoforms in vivo, based on the deletion or RNA interference-mediated knockdown of each single corresponding gene in rodents. This information is crucial for our understanding of the physiological relevance and function of the PHDs as well as for elucidating their individual impact on hypoxia-associated diseases. Furthermore, this knowledge highlights which diseases may best be targeted by PHD isoform-selective inhibitors in case such pharmacologic substances become available.

Pericytes contribute to pulmonary vascular remodeling via HIF2α signaling

Vascular remodeling is the process of structural alteration and cell rearrangement of blood vessels in response to injury and is the cause of many of the world's most afflicted cardiovascular conditions, including pulmonary arterial hypertension (PAH). Many studies have focused on the effects of vascular endothelial cells and smooth muscle cells (SMCs) during vascular remodeling, but pericytes, an indispensable cell population residing largely in capillaries, are ignored in this maladaptive process. Here, we report that hypoxia-inducible factor 2α (HIF2α) expression is increased in the lung tissues of PAH patients, and HIF2α overexpressed pericytes result in greater contractility and an impaired endothelial-pericyte interaction. Using single-cell RNAseq and hypoxia-induced pulmonary hypertension (PH) models, we show that HIF2α is a major molecular regulator for the transformation of pericytes into SMC-like cells. Pericyte-selective HIF2α overexpression in mice exacerbates PH and right ventricular hypertrophy. Temporal cellular lineage tracing shows that HIF2α overexpressing reporter NG2+ cells (pericyte-selective) relocate from capillaries to arterioles and co-express SMA. This novel insight into the crucial role of NG2+ pericytes in pulmonary vascular remodeling via HIF2α signaling suggests a potential drug target for PH.

A hypothesis: Potential contributions of metals to the pathogenesis of pulmonary artery hypertension

Pulmonary artery hypertension (PAH) is characterized by vasoconstriction and vascular remodeling resulting in both increased pulmonary vascular resistance (PVR) and pulmonary artery pressure (PAP). The chronic and high-pressure stress experienced by endothelial cells can give rise to inflammation, oxidative stress, and infiltration by immune cells. However, there is no clearly defined mechanism for PAH and available treatment options only provide limited symptomatic relief. Due to the far-reaching effects of metal exposures, the interaction between metals and the pulmonary vasculature is of particular interest. This review will briefly introduce the pathophysiology of PAH and then focus on the potential roles of metals, including essential and non-essential metals in the pathogenic process in the pulmonary arteries and right heart, which may be linked to PAH. Based on available data from human studies of occupational or environmental metal exposure, including lead, antimony, iron, and copper, the hypothesis of metals contributing to the pathogenesis of PAH is proposed as potential risk factors and underlying mechanisms for PAH. We propose that metals may initiate or exacerbate the pathogenesis of PAH, by providing potential mechanism by which metals interact with hypoxia-inducible factor and tumor suppressor p53 to modulate their downstream cellular proliferation pathways. These need further investigation. Additionally, we present future research directions on roles of metals in PAH.

Loss of prolyl hydroxylase 1 and 2 in SM22α-expressing cells prevents Hypoxia-Induced pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a disease characterized by increased vasoconstriction and vascular remodeling. Pulmonary artery smooth muscle cells (PASMCs) highly express the transcription factor hypoxia-inducible factor-1α (HIF-1α), yet the role of PASMC HIF-1α in the development of PAH remains controversial. To study the role of SMC HIF-1α in the pulmonary vascular response to acute and chronic hypoxia, we used a gain-of-function strategy to stabilize HIF-1α in PASMC by generating mice lacking prolyl hydroxylase domain (PHD) 1 and 2 in SM22α-expressing cells. This strategy increased HIF-1α expression and transcriptional activity under conditions of normoxia and hypoxia. Acute hypoxia increased right ventricular systolic pressure (RVSP) in control, but not in SM22α-PHD1/2-/- mice. Chronic hypoxia increased RVSP and vascular remodeling more in control SM22α-PHD1/2+/+ than in SM22α-PHD1/2-/- mice. In vitro studies demonstrated increased contractility and myosin light chain phosphorylation in isolated PHD1/2+/+ compared with PHD1/2-/- PASMC under both normoxic and hypoxic conditions. After chronic hypoxia, there was more p27 and less vascular remodeling in SM22α-PHD1/2-/- compared with SM22α-PHD1/2+/+ mice. Hypoxia increased p27 in PASMC isolated from control patients, but not in cells from patients with idiopathic pulmonary arterial hypertension (IPAH). These findings highlight an SM22α-expressing cell-specific role for HIF-1α in the inhibition of pulmonary vasoconstriction and vascular remodeling. Modulating HIF-1α expression in PASMC may represent a promising preventative and therapeutic strategy for patients with PAH.NEW & NOTEWORTHY In a mouse model wherein hypoxia-inducible factor 1 alpha (HIF-1α) is stabilized in vascular smooth muscle cells, we found that HIF-1α regulates vasoconstriction by limiting phosphorylation of myosin light chain and regulates vascular remodeling through p27 induction. These findings highlight a cell-specific role for HIF-1α in the inhibition of pulmonary vasoconstriction and vascular remodeling.

Oxygen-sensing pathways and the pulmonary circulation

The unique property of the pulmonary circulation to constrict in response to hypoxia, rather than dilate, brings advantages in both health and disease. Hypoxic pulmonary vasoconstriction (HPV) acts to optimise ventilation-perfusion matching - this is important clinically both in focal disease (such as pneumonia) and in one-lung ventilation during anaesthesia for thoracic surgery. However, during global hypoxia such as that encountered at high altitude, generalised pulmonary vasoconstriction can lead to pulmonary hypertension. There is now a growing body of evidence that links the hypoxia-inducible factor (HIF) pathway and pulmonary vascular tone - in both acute and chronic settings. Genetic and pharmacological alterations to all key components of this pathway (VHL - von Hippel-Lindau ubiquitin E3 ligase; PHD2 - prolyl hydroxylase domain protein 2; HIF1 and HIF2) have clear effects on the pulmonary circulation, particularly in hypoxia. Furthermore, knowledge of the molecular biology of the prolyl hydroxylase enzymes has led to an extensive and ongoing body of research into the importance of iron in both HPV and pulmonary hypertension. This review will explore these relationships in more detail and discuss future avenues of research.

Post-ischemic inactivation of HIF prolyl hydroxylases in endothelium promotes maladaptive kidney repair by inducing glycolysis

Ischemic acute kidney injury (AKI) is common in hospitalized patients and increases the risk for chronic kidney disease (CKD). Impaired endothelial cell (EC) functions are thought to contribute in AKI to CKD transition, but the underlying mechanisms remain unclear. Here, we identify a critical role for endothelial oxygen sensing prolyl hydroxylase domain (PHD) enzymes 1-3 in regulating post-ischemic kidney repair. In renal endothelium, we observed compartment-specific differences in the expression of the three PHD isoforms in both mice and humans. We found that post-ischemic concurrent inactivation of endothelial PHD1, PHD2, and PHD3 but not PHD2 alone promoted maladaptive kidney repair characterized by exacerbated tissue injury, fibrosis, and inflammation. Single-cell RNA-seq analysis of the post-ischemic endothelial PHD1, PHD2 and PHD3 deficient (PHDTiEC) kidney revealed an endothelial glycolytic transcriptional signature, also observed in human kidneys with severe AKI. This metabolic program was coupled to upregulation of the SLC16A3 gene encoding the lactate exporter monocarboxylate transporter 4 (MCT4). Strikingly, treatment with the MCT4 inhibitor syrosingopine restored adaptive kidney repair in PHDTiEC mice. Mechanistically, MCT4 inhibition suppressed pro-inflammatory EC activation reducing monocyte-endothelial cell interaction. Our findings suggest avenues for halting AKI to CKD transition based on selectively targeting the endothelial hypoxia-driven glycolysis/MCT4 axis.

Hypoxia-Inducible Factor-1α in SM22α-Expressing Cells Modulates Alveolarization

Worldwide, the incidence of both preterm births and chronic lung disease of infancy, or bronchopulmonary dysplasia, remains high. Infants with bronchopulmonary dysplasia have larger and fewer alveoli, a lung pathology that can persist into adulthood. Although recent data point to a role for hypoxia-inducible factor-1α (HIF-1α) in mediating pulmonary angiogenesis and alveolarization, the cell-specific role of HIF-1α remains incompletely understood. Thus, we hypothesized that HIF-1α, in a distinct subset of mesenchymal cells, mediates postnatal alveolarization. To test the hypothesis, we generated mice with a cell-specific deletion of HIF-1α by crossing SM22α promoter-driven Cre mice with HIF-1αflox/flox mice (SM22α-HIF-1α-/-), determined SM-22α-expressing cell identity using single-cell RNA sequencing, and interrogated samples from preterm infants. Deletion of HIF-1α in SM22α-expressing cells had no effect on lung structure at day 3 of life. However, at 8 days, there were fewer and larger alveoli, a difference that persisted into adulthood. Microvascular density, elastin organization, and peripheral branching of the lung vasculature were decreased in SM22α-HIF-1α-/- mice, compared with control mice. Single-cell RNA sequencing demonstrated that three mesenchymal cell subtypes express SM22α: myofibroblasts, airway smooth muscle cells, and vascular smooth muscle cells. Pulmonary vascular smooth muscle cells from SM22α-HIF-1α-/- mice had decreased angiopoietin-2 expression and, in coculture experiments, a diminished capacity to promote angiogenesis that was rescued by angiopoietin-2. Angiopoietin-2 expression in tracheal aspirates of preterm infants was inversely correlated with overall mechanical ventilation time, a marker of disease severity. We conclude that SM22α-specific HIF-1α expression drives peripheral angiogenesis and alveolarization in the lung, perhaps by promoting angiopoietin-2 expression.

A Molecular Troika of Angiogenesis, Coagulopathy and Endothelial Dysfunction in the Pathology of Avascular Necrosis of Femoral Head: A Comprehensive Review

Avascular necrosis of the femoral head (ANFH) is a painful disorder characterized by the cessation of blood supply to the femoral head, leading to its death and subsequent joint collapse. Influenced by several risk factors, including corticosteroid use, excessive alcohol intake, hypercholesterolemia, smoking and some inflammatory disorders, along with cancer, its clinical consequences are thrombus formation due to underlying inflammation and endothelial dysfunction, which collaborates with coagulopathy and impaired angiogenesis. Nonetheless, angiogenesis resolves the obstructed free flow of the blood by providing alternative routes. Clinical manifestations of early stage of ANFH mimic cysts or lesions in subchondral bone, vasculitis and transient osteoporosis of the hip, rendering it difficult to diagnose, complex to understand and complicated to cure. To date, the treatment methods for ANFH are controversial as no foolproof curative strategy is available, and these depend upon different severity levels of the ANFH. From an in-depth understanding of the pathological determinants of ANFH, it is clear that impaired angiogenesis, coagulopathy and endothelial dysfunction contribute significantly. The present review has set two aims, firstly to examine the role and relevance of this molecular triad (impaired angiogenesis, coagulopathy and endothelial dysfunction) in ANFH pathology and secondly to propose some putative therapeutic strategies, delineating the fact that, for the better management of ANFH, a combined strategy to curtail this molecular triangle must be composed rather than focusing on individual contributions.

Targeting Endothelial HIF2α/ARNT Expression for Ischemic Heart Disease Therapy

Ischemic heart disease (IHD) is a major cause of mortality and morbidity worldwide, with novel therapeutic strategies urgently needed. Endothelial dysfunction is a hallmark of IHD, contributing to its development and progression. Hypoxia-inducible factors (HIFs) are transcription factors activated in response to low oxygen levels, playing crucial roles in various pathophysiological processes related to cardiovascular diseases. Among the HIF isoforms, HIF2α is predominantly expressed in cardiac vascular endothelial cells and has a key role in cardiovascular diseases. HIFβ, also known as ARNT, is the obligate binding partner of HIFα subunits and is necessary for HIFα's transcriptional activity. ARNT itself plays an essential role in the development of the cardiovascular system, regulating angiogenesis, limiting inflammatory cytokine production, and protecting against cardiomyopathy. This review provides an overview of the current understanding of HIF2α and ARNT signaling in endothelial cell function and dysfunction and their involvement in IHD pathogenesis. We highlight their roles in inflammation and maintaining the integrity of the endothelial barrier, as well as their potential as therapeutic targets for IHD.

High-Altitude Andean H194R HIF2A Allele Is a Hypomorphic Allele

For over 10,000 years, Andeans have resided at high altitude where the partial pressure of oxygen challenges human survival. Recent studies have provided evidence for positive selection acting in Andeans on the HIF2A (also known as EPAS1) locus, which encodes for a central transcription factor of the hypoxia-inducible factor pathway. However, the precise mechanism by which this allele might lead to altitude-adaptive phenotypes, if any, is unknown. By analyzing whole genome sequencing data from 46 high-coverage Peruvian Andean genomes, we confirm evidence for positive selection acting on HIF2A and a unique pattern of variation surrounding the Andean-specific single nucleotide variant (SNV), rs570553380, which encodes for an H194R amino acid substitution in HIF-2α. Genotyping the Andean-associated SNV rs570553380 in a group of 299 Peruvian Andeans from Cerro de Pasco, Peru (4,338 m), reveals a positive association with increased fraction of exhaled nitric oxide, a marker of nitric oxide biosynthesis. In vitro assays show that the H194R mutation impairs binding of HIF-2α to its heterodimeric partner, aryl hydrocarbon receptor nuclear translocator. A knockin mouse model bearing the H194R mutation in the Hif2a gene displays decreased levels of hypoxia-induced pulmonary Endothelin-1 transcripts and protection against hypoxia-induced pulmonary hypertension. We conclude the Andean H194R HIF2A allele is a hypomorphic (partial loss of function) allele.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

No benefit of HIF prolyl hydroxylase inhibition for hypertensive renal damage in renovascular hypertensive rats

Introduction: We previously reported that malignant hypertension is associated with impaired capillary density of target organs. Here, we tested the hypothesis that stabilization of hypoxia-inducible factor (HIF) in a modified "preconditioning" approach prevents the development of malignant hypertension. To stabilize HIF, we employed pharmacological inhibition of HIF prolyl hydroxylases (PHD), that profoundly affect HIF metabolism. Methods: Two-kidney, one-clip renovascular hypertension (2K1C) was induced in rats; controls were sham operated. 2K1C rats received either intermittent injections of the PHD inhibitor ICA (2-(1-chloro-4-hydroxyisoquinoline-3-carboxamido) acetate) or placebo. Thirty-five days after clipping, the frequency of malignant hypertension was assessed (based on weight loss and the occurrence of characteristic vascular lesions). In addition, kidney injury was compared between all ICA treated versus all placebo treated 2K1C, regardless of the occurrence of malignant hypertension. HIF stabilization was evaluated by immunohistochemistry, and HIF target gene expression by RT-PCR. Results: Blood pressure was elevated to the same degree in ICA- and placebo-treated 2K1C compared to control rats. ICA treatment did not affect the frequency of malignant hypertension or the extent of kidney tissue fibrosis, inflammation, or capillary density. There was a trend towards higher mortality and worse kidney function in ICA-treated 2K1C rats. ICA increased the number of HIF-1α-positive renal tubular cell nuclei and induced several HIF-1 target genes. In contrast, expression of HIF-2α protein as well as HIF-2 target genes were markedly enhanced by 2K1C hypertension, irrespective of ICA treatment. Discussion: We conclude that intermittent PHD inhibition did not ameliorate severe renovascular hypertension in rats. We speculate that the unexpected strong renal accumulation of HIF-2α in renovascular hypertension, which could not be further augmented by ICA, may contribute to the lack of a benefit from PHD inhibition.

Transcription factors in the pathogenesis of pulmonary arterial hypertension-Current knowledge and therapeutic potential

Pulmonary arterial hypertension (PAH) is a disease characterized by elevated pulmonary vascular resistance and pulmonary artery pressure. Mortality remains high in severe cases despite significant advances in management and pharmacotherapy. Since currently approved PAH therapies are unable to significantly reverse pathological vessel remodeling, novel disease-modifying, targeted therapeutics are needed. Pathogenetically, PAH is characterized by vessel wall cell dysfunction with consecutive remodeling of the pulmonary vasculature and the right heart. Transcription factors (TFs) regulate the process of transcribing DNA into RNA and, in the pulmonary circulation, control the response of pulmonary vascular cells to macro- and microenvironmental stimuli. Often, TFs form complex protein interaction networks with other TFs or co-factors to allow for fine-tuning of gene expression. Therefore, identification of the underlying molecular mechanisms of TF (dys-)function is essential to develop tailored modulation strategies in PAH. This current review provides a compendium-style overview of TFs and TF complexes associated with PAH pathogenesis and highlights their potential as targets for vasculoregenerative or reverse remodeling therapies.

Updated perspective of EPAS1 and the role in pulmonary hypertension

Pulmonary hypertension (PH) is a group of syndromes characterized by irreversible vascular remodeling and persistent elevation of pulmonary vascular resistance and pressure, leading to ultimately right heart failure and even death. Current therapeutic strategies mainly focus on symptoms alleviation by stimulating pulmonary vessel dilation. Unfortunately, the mechanism and interventional management of vascular remodeling are still yet unrevealed. Hypoxia plays a central role in the pathogenesis of PH and numerous studies have shown the relationship between PH and hypoxia-inducible factors family. EPAS1, known as hypoxia-inducible factor-2 alpha (HIF-2α), functions as a transcription factor participating in various cellular pathways. However, the detailed mechanism of EPAS1 has not been fully and systematically described. This article exhibited a comprehensive summary of EPAS1 including the molecular structure, biological function and regulatory network in PH and other relevant cardiovascular diseases, and furthermore, provided theoretical reference for the potential novel target for future PH intervention.

Pleotropic effects of hypoxia-inducible factor-prolyl hydroxylase domain inhibitors: are they clinically relevant?

Anemia is common in patients with chronic kidney disease (CKD) and is mainly caused by insufficient production of erythropoietin from fibrotic kidney. Because anemia impairs quality of life and overall prognosis, recombinant human erythropoietin-related products (erythropoiesis-stimulating agents, ESAs) have been developed to increase hemoglobin level for decades. However, many safety concerns have been announced regarding the use of ESAs, including an increased occurrence of cardiovascular events, vascular access thrombosis, cancer progression, and recurrence. Hypoxia-inducible factor (HIF) is crucial to erythropoietin production, as a result, prolyl hydroxylase domain (PHD) enzyme inhibitors have been new therapeutic agents for the treatment of anemia in CKD. They can be administered orally, which is a preferred route for patients not undergoing hemodialysis. In clinical trials, PHD inhibitor could induce noninferior effect on erythropoiesis and improve functional iron deficiency compared with ESAs. Although no serious adverse events were reported, safety is still a concern because HIF stabilization induced by PHD inhibitor has pleotropic effects, such as angiogenesis, metabolic change, and cell survival, which might lead to unwanted deleterious effects, including fibrosis, inflammation, cardiovascular risk, and tumor growth. More molecular mechanisms of PHD inhibition and long-term clinical trials are needed to observe these pleotropic effects for the confirmation of safety and efficacy of PHD inhibitors.

Regnase-1 Prevents Pulmonary Arterial Hypertension Through mRNA Degradation of Interleukin-6 and Platelet-Derived Growth Factor in Alveolar Macrophages

BACKGROUND

Pulmonary arterial hypertension (PAH) is a type of pulmonary hypertension (PH) characterized by obliterative pulmonary vascular remodeling, resulting in right-sided heart failure. Although the pathogenesis of PAH is not fully understood, inflammatory responses and cytokines have been shown to be associated with PAH, in particular, with connective tissue disease-PAH. In this sense, Regnase-1, an RNase that regulates mRNAs encoding genes related to immune reactions, was investigated in relation to the pathogenesis of PH.

METHODS

We first examined the expression levels of ZC3H12A (encoding Regnase-1) in peripheral blood mononuclear cells from patients with PH classified under various types of PH, searching for an association between the ZC3H12A expression and clinical features. We then generated mice lacking Regnase-1 in myeloid cells, including alveolar macrophages, and examined right ventricular systolic pressures and histological changes in the lung. We further performed a comprehensive analysis of the transcriptome of alveolar macrophages and pulmonary arteries to identify genes regulated by Regnase-1 in alveolar macrophages.

RESULTS

ZC3H12A expression in peripheral blood mononuclear cells was inversely correlated with the prognosis and severity of disease in patients with PH, in particular, in connective tissue disease-PAH. The critical role of Regnase-1 in controlling PAH was also reinforced by the analysis of mice lacking Regnase-1 in alveolar macrophages. These mice spontaneously developed severe PAH, characterized by the elevated right ventricular systolic pressures and irreversible pulmonary vascular remodeling, which recapitulated the pathology of patients with PAH. Transcriptomic analysis of alveolar macrophages and pulmonary arteries of these PAH mice revealed that Il6, Il1b, and Pdgfa/b are potential targets of Regnase-1 in alveolar macrophages in the regulation of PAH. The inhibition of IL-6 (interleukin-6) by an anti-IL-6 receptor antibody or platelet-derived growth factor by imatinib but not IL-1β (interleukin-1β) by anakinra, ameliorated the pathogenesis of PAH.

CONCLUSIONS

Regnase-1 maintains lung innate immune homeostasis through the control of IL-6 and platelet-derived growth factor in alveolar macrophages, thereby suppressing the development of PAH in mice. Furthermore, the decreased expression of Regnase-1 in various types of PH implies its involvement in PH pathogenesis and may serve as a disease biomarker, and a therapeutic target for PH as well.

SM22α cell-specific HIF stabilization mitigates hyperoxia-induced neonatal lung injury

Though survival rates for preterm infants are improving, the incidence of chronic lung disease of infancy, or bronchopulmonary dysplasia (BPD), remains high. Histologically, BPD is characterized by larger and fewer alveoli. Hypoxia-inducible factors (HIFs) may be protective in the context of hyperoxia-induced lung injury, but the cell-specific effects of HIF expression in neonatal lung injury remain unknown. Thus, we sought to determine whether HIF stabilization in SM22α-expressing cells can limit hyperoxia-induced neonatal lung injury. We generated SM22α-specific HIF-1α-stabilized mice (SM22α-PHD1/2^-/- mice) by cross-breeding SM22α-promotor-driven Cre recombinase mice with prolyl hydroxylase PHD1^flox/flox and PHD2^flox/flox mice. Neonatal mice were randomized to 21% O₂ (normoxia) or 80% O₂ (hyperoxia) exposure for 14 days. For the hyperoxia recovery studies, neonatal mice were recovered from normoxia for an additional 10 wk. SM22α-specific HIF-1α stabilization mitigated hyperoxia-induced lung injury and preserved microvessel density compared with control mice for both neonates and adults. In SM22α-PHD1/2^-/- mice, pulmonary artery endothelial cells (PAECs) were more proliferative and pulmonary arteries expressed more collagen IV compared with control mice, even under hyperoxic conditions. Angiopoietin-2 (Ang2) mRNA expression in pulmonary artery smooth muscle cells (PASMC) was greater in SM22α-PHD1/2^-/- compared with control mice in both normoxia and hyperoxia. Pulmonary endothelial cells (PECs) cocultured with PASMC isolated from SM22α-PHD1/2^-/- mice formed more tubes and branches with greater tube length compared with PEC cocultured with PASMC isolated from SM22α-PHD1/2^+/+ mice. Addition of Ang2 recombinant protein further augmented tube formation for both PHD1/2^+/+ and PHD1/2^-/- PASMC. Cell-specific deletion of PHD1 and 2 selectively increases HIF-1α expression in SM22α-expressing cells and protects neonatal lung development despite prolonged hyperoxia exposure. HIF stabilization in SM22α-expressing cells preserved endothelial cell proliferation, microvascular density, increased angiopoietin-2 expression, and lung structure, suggesting a role for cell-specific HIF-1α stabilization to prevent neonatal lung injury.

Possible association between a polymorphism of EPAS1 gene and persistent pulmonary hypertension of the newborn: a case-control study

OBJECTIVE

To explore possible genes related to the development of persistent pulmonary hypertension of the newborn (PPHN).

METHODS

The authors identified 285 single nucleotide polymorphisms (SNPs) of 11 candidate genes (BMPR2, EPAS1, PDE3A, VEGFA, ENG, NOTCH3, SOD3, CPS1, ABCA3, ACVRL1, and SMAD9), using an Illumina Asian Screening Array-24 v1.0 BeadChip Array. The FastLmmC and R package was used for statistical analyses. The chi-square test and Cochrane-Armitage trend test were used to compare the allele and genotype frequencies between the groups and to test the genetic models, respectively.

RESULTS

A total of 45 PPHN infants and 294 control subjects were analyzed. The most common cause of PPHN was meconium aspiration syndrome. Among the 285 SNPs, 17 SNPs from 6 candidate genes (BMPR2, EPAS1, PDE3A, VEGFA, ENG, and NOTCH3) were significantly associated with PPHN (P < 0.05). After using the Bonferroni correction (P < 0.00018), only the rs17034984 SNP located in intron 1 of the EPAS1 gene remained significantly different between the PPHN and control subjects (P = 0.00014). The frequency of the TC/TT genotype of rs17034984 in the gene with the dominant model was significant in the patients with PPHN (OR = 5.38, 95% CI: 2.15-13.49). The T allele frequency of rs17034984 in the gene showed a significant difference compared with the control subjects (OR = 4.89, 95% CI: 2.03-11.82).

CONCLUSIONS

The present study suggests that the rs17034984 variant of EPAS1 gene is associated with PPHN.

Hypoxia-inducible factors: roles in cardiovascular disease progression, prevention, and treatment

Hypoxia-inducible factors (HIF)-1 and HIF-2 are master regulators of oxygen homeostasis that regulate the expression of thousands of genes in order to match O2 supply and demand. A large body of experimental data links HIF activity to protection against multiple disorders affecting the cardiovascular system: ischemic cardiovascular disease (including coronary artery disease and peripheral artery disease), through collateral blood vessel formation and preconditioning phenomena; emphysema; lymphedema; and lung transplant rejection. In these disorders, strategies to increase the expression of one or both HIFs may be of therapeutic utility. Conversely, extensive data link HIFs to the pathogenesis of pulmonary arterial hypertension and drugs that inhibit one or both HIFs may be useful in treating this disease.

The Latest in Animal Models of Pulmonary Hypertension and Right Ventricular Failure

Pulmonary hypertension (PH) describes heterogeneous population of patients with a mean pulmonary arterial pressure >20 mm Hg. Rarely, PH presents as a primary disorder but is more commonly part of a complex phenotype associated with comorbidities. Regardless of the cause, PH reduces life expectancy and impacts quality of life. The current clinical classification divides PH into 1 of 5 diagnostic groups to assign treatment. There are currently no pharmacological cures for any form of PH. Animal models are essential to help decipher the molecular mechanisms underlying the disease, to assign genotype-phenotype relationships to help identify new therapeutic targets, and for clinical translation to assess the mechanism of action and putative efficacy of new therapies. However, limitations inherent of all animal models of disease limit the ability of any single model to fully recapitulate complex human disease. Within the PH community, we are often critical of animal models due to the perceived low success upon clinical translation of new drugs. In this review, we describe the characteristics, advantages, and disadvantages of existing animal models developed to gain insight into the molecular and pathological mechanisms and test new therapeutics, focusing on adult forms of PH from groups 1 to 3. We also discuss areas of improvement for animal models with approaches combining several hits to better reflect the clinical situation and elevate their translational value.

The role of hypoxia-inducible factors in cardiovascular diseases

Cardiovascular diseases are the leading cause of death worldwide. During the development of cardiovascular diseases, hypoxia plays a crucial role. Hypoxia-inducible factors (HIFs) are the key transcription factors for adaptive hypoxic responses, which orchestrate the transcription of numerous genes involved in angiogenesis, erythropoiesis, glycolytic metabolism, inflammation, and so on. Recent studies have dissected the precise role of cell-specific HIFs in the pathogenesis of hypertension, atherosclerosis, aortic aneurysms, pulmonary arterial hypertension, and heart failure using tissue-specific HIF-knockout or -overexpressing animal models. More importantly, several compounds developed as HIF inhibitors or activators have been in clinical trials for the treatment of renal cancer or anemia; however, little is known on the therapeutic potential of these inhibitors for cardiovascular diseases. The purpose of this review is to summarize the recent advances on HIFs in the pathogenesis and pathophysiology of cardiovascular diseases and to provide evidence of potential clinical therapeutic targets.

Polydatin Glycosides Improve Monocrotaline-Induced Pulmonary Hypertension Injury by Inhibiting Endothelial-To-Mesenchymal Transition

Objective: To study the effect of polydatin on the injury of pulmonary arterial hypertension (PAH) induced by monocrotaline (MCT).

Methods: SD rats were induced to develop PAH injury by a single subcutaneous injection of MCT (60 mg/kg). From the second day, rats in the administration group were orally given sildenafil (20 mg/kg) and polydatin (30 or 60 mg/kg) for 3 weeks. At the end of the experiment, right ventricular hypertrophy (RVH) index of SD rats was calculated, pathological damage was assessed by HE staining, transcription levels of target genes were detected by RT-PCR and Elisa, and expression levels of Endothelial-to-mesenchymal transition (EndMT) related proteins were detected by immunohistochemistry (IHC) and immunofluorescence (IF). Finally, molecular docking analysis was used to verify the interaction of polydatin on the main targets.

Results: Polydatin could significantly restore the body function, reduce MCT-induced PAH injury, reduce serum biochemical indices; polydatin could effectively inhibit EndMT process by decreasing the expression of N-cadherin, β-catenin and vimentin; polydatin could down-regulate TAGLN expression and increase PECAM1 expression to reduce pulmonary vascular remodeling. The interaction between polydatin and EndMT target was confirmed by molecular docking operation.

Conclusion: Pharmacological experiments combined with Combining molecular docking was first used to clarify that polydatin can reduce the pulmonary endothelial dysfunction and pulmonary vascular remodeling induced by MCT by inhibiting EndMT. The results of the study provide new ideas for the further treatment of PAH injury.

Signal Transduction during Metabolic and Inflammatory Reprogramming in Pulmonary Vascular Remodeling

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by (mal)adaptive remodeling of the pulmonary vasculature, which is associated with inflammation, fibrosis, thrombosis, and neovascularization. Vascular remodeling in PAH is associated with cellular metabolic and inflammatory reprogramming that induce profound endothelial and smooth muscle cell phenotypic changes. Multiple signaling pathways and regulatory loops act on metabolic and inflammatory mediators which influence cellular behavior and trigger pulmonary vascular remodeling in vivo. This review discusses the role of bioenergetic and inflammatory impairments in PAH development.

PHD2 deletion in endothelial or arterial smooth muscle cells reveals vascular cell type-specific responses in pulmonary hypertension and fibrosis

Hypoxia plays an important regulatory role in the vasculature to adjust blood flow to meet metabolic requirements. At the level of gene transcription, the responses are mediated by hypoxia-inducible factor (HIF) the stability of which is controlled by the HIF prolyl 4-hydroxylase-2 (PHD2). In the lungs hypoxia results in vasoconstriction, however, the pathophysiological relevance of PHD2 in the major arterial cell types; endothelial cells (ECs) and arterial smooth muscle cells (aSMCs) in the adult vasculature is incompletely characterized. Here, we investigated PHD2-dependent vascular homeostasis utilizing inducible deletions of PHD2 either in ECs (Phd2^∆ECi) or in aSMCs (Phd2^∆aSMC). Cardiovascular function and lung pathologies were studied using echocardiography, Doppler ultrasonography, intraventricular pressure measurement, histological, ultrastructural, and transcriptional methods. Cell intrinsic responses were investigated in hypoxia and in conditions mimicking hypertension-induced hemodynamic stress. Phd2^∆ECi resulted in progressive pulmonary disease characterized by a thickened respiratory basement membrane (BM), alveolar fibrosis, increased pulmonary artery pressure, and adaptive hypertrophy of the right ventricle (RV). A low oxygen environment resulted in alterations in cultured ECs similar to those in Phd2^∆ECi mice, involving BM components and vascular tone regulators favoring the contraction of SMCs. In contrast, Phd2^∆aSMC resulted in elevated RV pressure without alterations in vascular tone regulators. Mechanistically, PHD2 inhibition in aSMCs involved  actin polymerization -related tension development via activated cofilin. The results also indicated that hemodynamic stress, rather than PHD2-dependent hypoxia response alone, potentiates structural remodeling of the extracellular matrix in the pulmonary microvasculature and respiratory failure.

Hypoxia-Inducible Factor Signaling in Inflammatory Lung Injury and Repair

Inflammatory lung injury is characterized by lung endothelial cell (LEC) death, alveolar epithelial cell (AEC) death, LEC-LEC junction weakening, and leukocyte infiltration, which together disrupt nutrient and oxygen transport. Subsequently, lung vascular repair is characterized by LEC and AEC regeneration and LEC-LEC junction re-annealing, which restores nutrient and oxygen delivery to the injured tissue. Pulmonary hypoxia is a characteristic feature of several inflammatory lung conditions, including acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and severe coronavirus disease 2019 (COVID-19). The vascular response to hypoxia is controlled primarily by the hypoxia-inducible transcription factors (HIFs) 1 and 2. These transcription factors control the expression of a wide variety of target genes, which in turn mediate key pathophysiological processes including cell survival, differentiation, migration, and proliferation. HIF signaling in pulmonary cell types such as LECs and AECs, as well as infiltrating leukocytes, tightly regulates inflammatory lung injury and repair, in a manner that is dependent upon HIF isoform, cell type, and injury stimulus. The aim of this review is to describe the HIF-dependent regulation of inflammatory lung injury and vascular repair. The review will also discuss potential areas for future study and highlight putative targets for inflammatory lung conditions such as ALI/ARDS and severe COVID-19. In the development of HIF-targeted therapies to reduce inflammatory lung injury and/or enhance pulmonary vascular repair, it will be vital to consider HIF isoform- and cell-specificity, off-target side-effects, and the timing and delivery strategy of the therapeutic intervention.

Single-Cell RNA Sequencing Reveals Novel Genes Regulated by Hypoxia in the Lung Vasculature

Pulmonary arterial hypertension (PAH) is a chronic progressive disease with significant morbidity and mortality. The disease is characterized by vascular remodeling that includes increased muscularization of distal blood vessels and vessel stiffening associated with changes in extracellular matrix deposition. In humans, chronic hypoxia causes PAH, and hypoxia-induced rodent models of PAH have been used for years to study the disease. With the development of single-cell RNA sequencing technology, it is now possible to examine hypoxia-dependent transcriptional changes in vivo at a cell-specific level. In this study, we used single-cell RNA sequencing to compare lungs from wild-type (Wt) mice exposed to hypoxia for 28 days to normoxia-treated control mice. We additionally examined mice deficient for Notch3, a smooth muscle-enriched gene linked to PAH. Data analysis revealed that hypoxia promoted cell number changes in immune and endothelial cell types in the lung, activated the innate immunity pathway, and resulted in specific changes in gene expression in vascular cells. Surprisingly, we found limited differences in lungs from mice deficient for Notch3 compared to Wt controls. These findings provide novel insight into the effects of chronic hypoxia exposure on gene expression and cell phenotypes in vivo and identify unique changes to cells of the vasculature.

Role of Endothelial Prolyl-4-Hydroxylase Domain Protein/Hypoxia-Inducible Factor Axis in Acute Kidney Injury

Ischemia reperfusion injury (IRI) results from a cessation or restriction of blood supply to an organ followed by reestablishment of perfusion and reoxygenation. In the kidney, IRI due to transplantation, cardiac surgery with cardiopulmonary bypass, and other major vascular surgeries contributes to acute kidney injury (AKI), a clinical condition associated with significant morbidity and mortality in hospitalized patients. In the postischemic kidney, endothelial damage promotes inflammatory responses and leads to persistent hypoxia of the renal tubular epithelium. Like other cell types, endothelial cells respond to low oxygen tension by multiple hypoxic signaling mechanisms. Key mediators of adaptation to hypoxia are hypoxia-inducible factors (HIF)-1 and -2, transcription factors whose activity is negatively regulated by prolyl-hydroxylase domain proteins 1 to 3 (PHD1 to PHD3). The PHD/HIF axis controls several processes determining injury outcome, including ATP generation, cell survival, proliferation, and angiogenesis. Here, we discuss recent advances in our understanding of the endothelial-derived PHD/HIF signaling and its effects on postischemic AKI.

CD226 deficiency promotes glutaminolysis and alleviates mitochondria damage in vascular endothelial cells under hemorrhagic shock

Hemorrhagic shock (HS) is common in clinical emergencies, leading to millions of deaths each year globally. CD226 is a costimulatory adhesion molecule expressed on both immune cells and endothelial cells (ECs) to regulate their metabolic activity and function. As endothelial dysfunction occurs after HS, the roles CD226 plays in vascular EC metabolism were investigated. CD226^fl/fl Tek^cre mice were adopted to achieve vascular EC-specific knockout of CD226, and subjected to HS modelling. Serum levels of crucial intermediate metabolites were evaluated through liquid chromatography-mass spectrometry analysis. Human umbilical vein ECs (HUVECs) were used to study the effects of CD226 under hypoxia in vitro. Seahorse analysis evaluated the cellular glycolysis and mitochondria bioenergetics. Results showed that CD226 deficiency in vascular ECs alleviated HS-induced intestinal damage and inflammatory response in mice. Animal studies indicated an improved energy metabolism when CD226 was knocked out in ECs after HS, as evidenced by enhanced glutamine-glutamate metabolism and decreased lactic acid levels. Glut-1 was upregulated in mouse vascular ECs after HS and HUVECs under hypoxia, combined with decreased CD226. Moreover, HUVECs with CD226 knockdown exhibited relieved mitochondrial damage and early apoptosis under hypoxia, whereas CD226 overexpression showed opposite effects. Seahorse analysis showed that downregulated CD226 significantly increased mitochondrial ATP production and glucose uptake in HUVECs under hypoxia. Additionally, Erk/PHD2 signaling-mediated HIF-1α/Glut-1 and HIF-2α/ASCT2 pathways were involved in CD226 regulation on HUVEC glutaminolysis after hypoxia. Hence, CD226 deficiency promotes bypass energy supply to vascular ECs under ischemic or hypoxic stress, to ameliorate the stress-mediated metabolic disturbance.

Established pulmonary hypertension in rats was reversed by a combination of a HIF-2α antagonist and a p53 agonist

BACKGROUND AND PURPOSE

Recent studies reported therapeutic effects of monotherapy with either tumour suppressor p53 (p53) agonist or hypoxia-inducible factor 2α (HIF-2α) antagonist for pulmonary hypertension (PH). This study investigated whether a combined treatment of p53 agonist, Nutlin3a, and HIF-2α antagonist, PT2385, would be more effective than monotherapy, based on the cell type-divergent regulation of p53 in pulmonary arterial smooth muscle cells (PASMC) and endothelial cells (PAEC) in patients and animals with PH.

EXPERIMENTAL APPROACH

The SU5416/hypoxia-induced PH (SuHx-PH) rat model was used, along with cultured human PASMC and PAEC. Western blot, RT-PCR, siRNA and immunohistochemical methods were used along with echocardiography and studies with isolated pulmonary arteries.

KEY RESULTS

Hypoxia-induced proliferation of PASMC is associated with decreased p53, whereas hypoxia-induced PAEC apoptosis is associated with increased p53, via a HIF-2α-dependent mechanism. Combined treatment with Nutlin3a and PT2385 is more effective by simultaneously inhibiting the hypoxia-induced PASMC proliferation and PAEC apoptosis, overcoming the side-effects of monotherapy. These are (i) Nutlin3a exacerbates hypoxia-induced PAEC apoptosis by inducing p53 in PAEC and (ii) PT2385 inhibits PAEC apoptosis because HIF-2α is predominantly expressed in PAEC but lacks direct effects on the hypoxia-induced PASMC proliferation. In rats, combination treatment is more effective than monotherapy in reversing established SuHx-PH, especially in protecting pulmonary arterial vasculature, by normalizing smooth muscle thickening, protecting against endothelial damage and improving function.

CONCLUSION AND IMPLICATIONS

Combination treatment confers greater therapeutic efficacy against PH through a selective modulation of p53 and HIF-2α in PASMC and PAEC.

Endothelial cells in the pathogenesis of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating disease that involves pulmonary vasoconstriction, small vessel obliteration, large vessel thickening and obstruction, and development of plexiform lesions. PAH vasculopathy leads to progressive increases in pulmonary vascular resistance, right heart failure and, ultimately, premature death. Besides other cell types that are known to be involved in PAH pathogenesis (e.g. smooth muscle cells, fibroblasts and leukocytes), recent studies have demonstrated that endothelial cells (ECs) have a crucial role in the initiation and progression of PAH. The EC-specific role in PAH is multi-faceted and affects numerous pathophysiological processes, including vasoconstriction, inflammation, coagulation, metabolism and oxidative/nitrative stress, as well as cell viability, growth and differentiation. In this review, we describe how EC dysfunction and cell signalling regulate the pathogenesis of PAH. We also highlight areas of research that warrant attention in future studies, and discuss potential molecular signalling pathways in ECs that could be targeted therapeutically in the prevention and treatment of PAH.

EPAS1 (Endothelial PAS Domain Protein 1) Orchestrates Transactivation of Endothelial ICAM1 (Intercellular Adhesion Molecule 1) by Small Nucleolar RNA Host Gene 5 (SNHG5) to Promote Hypoxic Pulmonary Hypertension

[Figure: see text].

Hypoxic preconditioning protects against ischemic kidney injury through the IDO1/kynurenine pathway

Prolonged cellular hypoxia leads to energetic failure and death. However, sublethal hypoxia can trigger an adaptive response called hypoxic preconditioning. While prolyl-hydroxylase (PHD) enzymes and hypoxia-inducible factors (HIFs) have been identified as key elements of oxygen-sensing machinery, the mechanisms by which hypoxic preconditioning protects against insults remain unclear. Here, we perform serum metabolomic profiling to assess alterations induced by two potent cytoprotective approaches, hypoxic preconditioning and pharmacologic PHD inhibition. We discover that both approaches increase serum kynurenine levels and enhance kynurenine biotransformation, leading to preservation of NAD+ in the post-ischemic kidney. Furthermore, we show that indoleamine 2,3-dioxygenase 1 (Ido1) deficiency abolishes the systemic increase of kynurenine and the subsequent renoprotection generated by hypoxic preconditioning and PHD inhibition. Importantly, exogenous administration of kynurenine restores the hypoxic preconditioning in the context of Ido1 deficiency. Collectively, our findings demonstrate a critical role of the IDO1-kynurenine axis in mediating hypoxic preconditioning.

Hypoxia-Inducible Factor 2-Alpha Mediated Gene Sets Differentiate Pulmonary Arterial Hypertension

OBJECTIVES

HIF2α is of vital importance in the regulation of endothelial dysfunction, cell proliferation, migration, and pulmonary vascular remodeling in pulmonary hypertension. Our previous studies demonstrated that conditional and inducible deletion of HIF2α in mouse lung endothelial cells, dramatically protected the mice against vascular remodeling and the development of pulmonary arterial hypertension (PAH). Here, we provide a novel transcriptome insight into the impact of HIF2α in PAH pathogenesis and the potential to use HIF2α-mediated gene sets to differentiate PAH human subjects.

METHODS

Using transcriptome data, we first tapped the value of the difference in gene expression profile between wild type (WT) and Hif2a knockdown (KD) cell lines. We considered the deregulated genes between WT and Hif2a-KD cells as HIF2α influenced genes. By examining the lung tissue transcriptome data set with nine controls and eight PAH patients, we evaluated the HIF2α regulatory network in PAH pathogenesis to further determine the identification ability of HIF2α-mediated gene sets in human PAH subjects. On the other hand, using peripheral blood mononuclear cells (PBMCs) transcriptome data from PAH patients and healthy controls, we further validated the potential of the HIF2α-mediated PBMC gene sets as a possible diagnostic tool for PAH. To verify the ability of HIF2α-mediated gene sets for the identification of PAH, endothelial cell-specific Phd2 knockout mice with spontaneous pulmonary hypertension were used for reverse validation experiments.

RESULTS

19 identified GO biological process terms were significantly correlated with the genes down-regulated in Hif2a-KD cells, all of which are strongly related to the PAH pathogenesis. We further assessed the discriminative power of these HIF2α-mediated gene sets in PAH human subjects. We found that the expression profile of the HIF2α-mediated gene sets in lung tissues and PBMCs were differentiated both between controls and PAH patients. Further, a significant positive correlation was observed between hypoxia and Phd2 deficiency mediated gene set expression profiles. As expected, 7 of the 19 significantly down-regulated GO terms in Hif2a-KD cells were found to overlap with the up-regulated GO gene sets in Phd2 EC-/- mice compared to WT controls, suggesting opposing effects of HIF2α and PHD2 on PAH pathogenesis.

CONCLUSION

HIF2α-mediated gene sets may be used to differentiate pulmonary arterial hypertension.

Cellular Pathways Promoting Pulmonary Vascular Remodeling by Hypoxia

Exposure to hypoxia increases pulmonary vascular resistance, leading to elevated pulmonary arterial pressure and, potentially, right heart failure. Vascular remodeling is an important contributor to the increased pulmonary vascular resistance. Hyperproliferation of smooth muscle, endothelial cells, and fibroblasts, and deposition of extracellular matrix lead to increased wall thickness, extension of muscle into normally non-muscular arterioles, and vascular stiffening. This review highlights intrinsic and extrinsic modulators contributing to the remodeling process.

Controversies in optimal anemia management: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Conference

In chronic kidney disease, anemia and disordered iron homeostasis are prevalent and associated with significant adverse consequences. In 2012, Kidney Disease: Improving Global Outcomes (KDIGO) issued an anemia guideline for managing the diagnosis, evaluation, and treatment of anemia in chronic kidney disease. Since then, new data have accrued from basic research, epidemiological studies, and randomized trials that warrant a re-examination of previous recommendations. Therefore, in 2019, KDIGO decided to convene 2 Controversies Conferences to review the latest evidence, explore new and ongoing controversies, assess change implications for the current KDIGO anemia guideline, and propose a research agenda. The first conference, described here, focused mainly on iron-related issues, including the contribution of disordered iron homeostasis to the anemia of chronic kidney disease, diagnostic challenges, available and emerging iron therapies, treatment targets, and patient outcomes. The second conference will discuss issues more specifically related to erythropoiesis-stimulating agents, including epoetins, and hypoxia-inducible factor-prolyl hydroxylase inhibitors. Here we provide a concise overview of the consensus points and controversies resulting from the first conference and prioritize key questions that need to be answered by future research.

Therapeutic inhibition of HIF-2α reverses polycythemia and pulmonary hypertension in murine models of human diseases

Polycythemia and pulmonary hypertension are 2 human diseases for which better therapies are needed. Upregulation of hypoxia-inducible factor-2α (HIF-2α) and its target genes, erythropoietin (EPO) and endothelin-1, causes polycythemia and pulmonary hypertension in patients with Chuvash polycythemia who are homozygous for the R200W mutation in the von Hippel Lindau (VHL) gene and in a murine mouse model of Chuvash polycythemia that bears the same homozygous VhlR200W mutation. Moreover, the aged VhlR200W mice developed pulmonary fibrosis, most likely due to the increased expression of Cxcl-12, another Hif-2α target. Patients with mutations in iron regulatory protein 1 (IRP1) also develop polycythemia, and Irp1-knockout (Irp1-KO) mice exhibit polycythemia, pulmonary hypertension, and cardiac fibrosis attributable to translational derepression of Hif-2α, and the resultant high expression of the Hif-2α targets EPO, endothelin-1, and Cxcl-12. In this study, we inactivated Hif-2α with the second-generation allosteric HIF-2α inhibitor MK-6482 in VhlR200W, Irp1-KO, and double-mutant VhlR200W;Irp1-KO mice. MK-6482 treatment decreased EPO production and reversed polycythemia in all 3 mouse models. Drug treatment also decreased right ventricular pressure and mitigated pulmonary hypertension in VhlR200W, Irp1-KO, and VhlR200W;Irp1-KO mice to near normal wild-type levels and normalized the movement of the cardiac interventricular septum in VhlR200Wmice. MK-6482 treatment reduced the increased expression of Cxcl-12, which, in association with CXCR4, mediates fibrocyte influx into the lungs, potentially causing pulmonary fibrosis. Our results suggest that oral intake of MK-6482 could represent a new approach to treatment of patients with polycythemia, pulmonary hypertension, pulmonary fibrosis, and complications caused by elevated expression of HIF-2α.

Hypoxia-inducible factor-prolyl hydroxylase inhibitors in the treatment of anemia of chronic kidney disease

Hypoxia-inducible factor-prolyl hydroxylase domain inhibitors (HIF-PHIs) are a promising new class of orally administered drugs currently in late-stage global clinical development for the treatment of anemia of chronic kidney disease (CKD). HIF-PHIs activate the HIF oxygen-sensing pathway and are efficacious in correcting and maintaining hemoglobin levels in patients with non-dialysis- and dialysis-dependent CKD. In addition to promoting erythropoiesis through the increase in endogenous erythropoietin production, HIF-PHIs reduce hepcidin levels and modulate iron metabolism, providing increases in total iron binding capacity and transferrin levels, and potentially reducing the need for i.v. iron supplementation. Furthermore, HIF-activating drugs are predicted to have effects that extend beyond erythropoiesis. This review summarizes clinical data from current HIF-PHI trials in patients with anemia of CKD, discusses mechanisms of action and pharmacologic properties of HIF-PHIs, and deliberates over safety concerns and potential impact on anemia management in patients with CKD.

Animal models of pulmonary hypertension: Getting to the heart of the problem

Despite recent therapeutic advances, pulmonary hypertension (PH) remains a fatal disease due to the development of right ventricular (RV) failure. At present, no treatments targeted at the right ventricle are available, and RV function is not widely considered in the preclinical assessment of new therapeutics. Several small animal models are used in the study of PH, including the classic models of exposure to either hypoxia or monocrotaline, newer combinational and genetic models, and pulmonary artery banding, a surgical model of pure RV pressure overload. These models reproduce selected features of the structural remodelling and functional decline seen in patients and have provided valuable insight into the pathophysiology of RV failure. However, significant reversal of remodelling and improvement in RV function remains a therapeutic obstacle. Emerging animal models will provide a deeper understanding of the mechanisms governing the transition from adaptive remodelling to a failing right ventricle, aiding the hunt for druggable molecular targets.

Developmental vascular malformations in EPAS1 gain-of-function syndrome

Mutations in EPAS1, encoding hypoxia-inducible factor-2α (HIF-2α), were previously identified in a syndrome of multiple paragangliomas, somatostatinoma, and polycythemia. HIF-2α, when dimerized with HIF-1β, acts as an angiogenic transcription factor. Patients referred to the NIH for new, recurrent, and/or metastatic paraganglioma or pheochromocytoma were confirmed for EPAS1 gain-of-function mutation; imaging was evaluated for vascular malformations. We evaluated the Epas1^A529V transgenic syndrome mouse model, corresponding to the mutation initially detected in the patients (EPAS1^A530V), for vascular malformations via intravital 2-photon microscopy of meningeal vessels, terminal vascular perfusion with Microfil silicate polymer and subsequent intact ex vivo 14T MRI and micro-CT, and histologic sectioning and staining of the brain and identified pathologies. Further, we evaluated retinas from corresponding developmental time points (P7, P14, and P21) and the adult dura via immunofluorescent labeling of vessels and confocal imaging. We identified a spectrum of vascular malformations in all 9 syndromic patients and in all our tested mutant mice. Patient vessels had higher variant allele frequency than adjacent normal tissue. Veins of the murine retina and intracranial dura failed to regress normally at the expected developmental time points. These findings add vascular malformation as a new clinical feature of EPAS1 gain-of-function syndrome.

We discovered vascular malformations due to failure of developmental vascular regression in patients with EPAS1 gain-of-function mutation syndrome and the corresponding transgenic mouse model.

Targeting HIF2α-ARNT hetero-dimerisation as a novel therapeutic strategy for pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a destructive disease of the pulmonary vasculature often leading to right heart failure and death. Current therapeutic intervention strategies only slow disease progression. The role of aberrant hypoxia-inducible factor (HIF)2α stability and function in the initiation and development of pulmonary hypertension (PH) has been an area of intense interest for nearly two decades.Here we determine the effect of a novel HIF2α inhibitor (PT2567) on PH disease initiation and progression, using two pre-clinical models of PH. Haemodynamic measurements were performed, followed by collection of heart, lung and blood for pathological, gene expression and biochemical analysis. Blood outgrowth endothelial cells from idiopathic PAH patients were used to determine the impact of HIF2α-inhibition on endothelial function.Global inhibition of HIF2a reduced pulmonary vascular haemodynamics and pulmonary vascular remodelling in both su5416/hypoxia prevention and intervention models. PT2567 intervention reduced the expression of PH-associated target genes in both lung and cardiac tissues and restored plasma nitrite concentration. Treatment of monocrotaline-exposed rodents with PT2567 reduced the impact on cardiovascular haemodynamics and promoted a survival advantage. In vitro, loss of HIF2α signalling in human pulmonary arterial endothelial cells suppresses target genes associated with inflammation, and PT2567 reduced the hyperproliferative phenotype and overactive arginase activity in blood outgrowth endothelial cells from idiopathic PAH patients. These data suggest that targeting HIF2α hetero-dimerisation with an orally bioavailable compound could offer a new therapeutic approach for PAH. Future studies are required to determine the role of HIF in the heterogeneous PAH population.

HIF2A gain-of-function mutation modulates the stiffness of smooth muscle cells and compromises vascular mechanics

Heterozygous gain-of-function (GOF) mutations of hypoxia-inducible factor 2α (HIF2A), a key hypoxia-sensing regulator, are associated with erythrocytosis, thrombosis, and vascular complications that account for morbidity and mortality of patients. We demonstrated that the vascular pathology of HIF2A GOF mutations is independent of erythrocytosis. We generated HIF2A GOF-induced pluripotent stem cells (iPSCs) and differentiated them into endothelial cells (ECs) and smooth muscle cells (SMCs). Unexpectedly, HIF2A-SMCs, but not HIF2A-ECs, were phenotypically aberrant, more contractile, stiffer, and overexpressed endothelin 1 (EDN1), myosin heavy chain, elastin, and fibrillin. EDN1 inhibition and knockdown of EDN1-receptors both reduced HIF2-SMC stiffness. Hif2A GOF heterozygous mice displayed pulmonary hypertension, had SMCs with more disorganized stress fibers and higher stiffness in their pulmonary arterial smooth muscle cells, and had more deformable pulmonary arteries compared with wild-type mice. Our findings suggest that targeting these vascular aberrations could benefit patients with HIF2A GOF and conditions of augmented hypoxia signaling.

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HIF2-SMCs are stiffer than WT-SMCs and differ in contractile SMC marker expression

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HIF2-SMCs and WT-SMCs differ in EDN1 production and ECM composition

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HIF- 2α induces EDN1; EDNI subsequently induces SMC stiffening

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Hif2A GOF mouse arterial SMCs have more disorganized stress fibers and are stiffer

Hypoxia-inducible factor signaling in pulmonary hypertension

Pulmonary hypertension (PH) is characterized by pulmonary artery remodeling that can subsequently culminate in right heart failure and premature death. Emerging evidence suggests that hypoxia-inducible factor (HIF) signaling plays a fundamental and pivotal role in the pathogenesis of PH. This Review summarizes the regulation of HIF isoforms and their impact in various PH subtypes, as well as the elaborate conditional and cell-specific knockout mouse studies that brought the role of this pathway to light. We also discuss the current preclinical status of pan- and isoform-selective HIF inhibitors, and propose new research areas that may facilitate HIF isoform-specific inhibition as a novel therapeutic strategy for PH and right heart failure.

Pulmonary Hypertension in Acute and Chronic High Altitude Maladaptation Disorders

Alveolar hypoxia is the most prominent feature of high altitude environment with well-known consequences for the cardio-pulmonary system, including development of pulmonary hypertension. Pulmonary hypertension due to an exaggerated hypoxic pulmonary vasoconstriction contributes to high altitude pulmonary edema (HAPE), a life-threatening disorder, occurring at high altitudes in non-acclimatized healthy individuals. Despite a strong physiologic rationale for using vasodilators for prevention and treatment of HAPE, no systematic studies of their efficacy have been conducted to date. Calcium-channel blockers are currently recommended for drug prophylaxis in high-risk individuals with a clear history of recurrent HAPE based on the extensive clinical experience with nifedipine in HAPE prevention in susceptible individuals. Chronic exposure to hypoxia induces pulmonary vascular remodeling and development of pulmonary hypertension, which places an increased pressure load on the right ventricle leading to right heart failure. Further, pulmonary hypertension along with excessive erythrocytosis may complicate chronic mountain sickness, another high altitude maladaptation disorder. Importantly, other causes than hypoxia may potentially underlie and/or contribute to pulmonary hypertension at high altitude, such as chronic heart and lung diseases, thrombotic or embolic diseases. Extensive clinical experience with drugs in patients with pulmonary arterial hypertension suggests their potential for treatment of high altitude pulmonary hypertension. Small studies have demonstrated their efficacy in reducing pulmonary artery pressure in high altitude residents. However, no drugs have been approved to date for the therapy of chronic high altitude pulmonary hypertension. This work provides a literature review on the role of pulmonary hypertension in the pathogenesis of acute and chronic high altitude maladaptation disorders and summarizes current knowledge regarding potential treatment options.

Recommendations by the Asian Pacific society of nephrology (APSN) on the appropriate use of HIF-PH inhibitors

Renal anaemia is a common and important complication in patients with chronic kidney disease (CKD). The current standard‐of‐care treatment for renal anaemia in CKD patients involves ensuring adequate iron stores and administration of erythropoietin stimulating agents (ESA). Hypoxia inducible factor (HIF) is a key transcription factor primarily involved in the cellular regulation and efficiency of oxygen delivery. Manipulation of the HIF pathway by the use of HIF‐prolyl hydroxylase inhibitors (HIF‐PHI) has emerged as a novel approach for renal anaemia management. Despite it being approved for clinical use in various Asia‐Pacific countries, its novelty mandates the need for nephrologists and clinicians generally in the region to well understand potential benefits and harms when prescribing this class of drug. The Asian Pacific society of nephrology HIF‐PHI Recommendation Committee, formed by a panel of 11 nephrologists from the Asia‐Pacific region who have clinical experience or have been investigators in HIF‐PHI studies, reviewed and deliberated on the clinical and preclinical data concerning HIF‐PHI. This recommendation summarizes the consensus views of the committee regarding the use of HIF‐PHI, taking into account both available data and expert opinion in areas where evidence remains scarce.

The Asian Pacific society of nephrology HIF‐PHI Recommendation Committee summarizes the consensus views of the committee regarding the use of HIF‐PHI, taking into account both available data and expert opinion in areas where evidence remains scarce.

Vascular homeostasis at high-altitude: role of genetic variants and transcription factors

High-altitude pulmonary edema occurs most frequently in non-acclimatized low landers on exposure to altitude ≥2500 m. High-altitude pulmonary edema is a complex condition that involves perturbation of signaling pathways in vasoconstrictors, vasodilators, anti-diuretics, and vascular growth factors. Genetic variations are instrumental in regulating these pathways and evidence is accumulating for a role of epigenetic modification in hypoxic responses. This review focuses on the crosstalk between high-altitude pulmonary edema-associated genetic variants and transcription factors, comparing high-altitude adapted and high-altitude pulmonary edema-afflicted subjects. This approach might ultimately yield biomarker information both to understand and to design therapies for high-altitude adaptation.

Endothelial HIF-2α as a Key Endogenous Mediator Preventing Emphysema

Rationale: Endothelial injury may provoke emphysema, but molecular pathways of disease development require further discernment. Emphysematous lungs exhibit decreased expression of HIF-2α (hypoxia-inducible factor-2α)-regulated genes, and tobacco smoke decreases pulmonary HIF-2α concentrations. These findings suggest that decreased HIF-2α expression is important in the development of emphysema.Objectives: The objective of this study was to evaluate the roles of endothelial-cell (EC) HIF-2α in the pathogenesis of emphysema in mice.Methods: Mouse lungs were examined for emphysema after either the loss or the overexpression of EC Hif-2α. In addition, SU5416, a VEGFR2 inhibitor, was used to induce emphysema. Lungs were evaluated for HGF (hepatocyte growth factor), a protein involved in alveolar development and homeostasis. Lungs from patients with emphysema were measured for endothelial HIF-2α expression.Measurements and Main Results: EC Hif-2α deletion resulted in emphysema in association with fewer ECs and pericytes. After SU5416 exposure, EC Hif-2α-knockout mice developed more severe emphysema, whereas EC Hif-2α-overexpressing mice were protected. EC Hif-2α-knockout mice demonstrated lower levels of HGF. Human emphysema lung samples exhibited reduced EC HIF-2α expression.Conclusions: Here, we demonstrate a unique protective role for pulmonary endothelial HIF-2α and how decreased expression of this endogenous factor causes emphysema; its pivotal protective function is suggested by its ability to overcome VEGF antagonism. HIF-2α may maintain alveolar architecture by promoting vascular survival and associated HGF production. In summary, HIF-2α may be a key endogenous factor that prevents the development of emphysema, and its upregulation has the potential to foster lung health in at-risk patients.