Mutation of von Hippel-Lindau tumour suppressor and human cardiopulmonary physiology

Hypoxic pulmonary vasoconstriction does not limit maximal exercise capacity in healthy volunteers breathing 12% oxygen at sea level

Maximal exercise capacity is reduced at altitude or during hypoxia at sea level. It has been suggested that this might reflect increased right ventricular afterload due to hypoxic pulmonary vasoconstriction. We have shown previously that the pulmonary vascular sensitivity to hypoxia is enhanced by sustained isocapnic hypoxia, and inhibited by intravenous iron. In this study, we tested the hypothesis that elevated pulmonary artery pressure contributes to exercise limitation during acute hypoxia. Twelve healthy volunteers performed incremental exercise tests to exhaustion breathing 12% oxygen, before and after sustained (8-h) isocapnic hypoxia at sea level. Intravenous iron sucrose (n = 6) or saline placebo (n = 6) was administered immediately before the sustained hypoxia. In the placebo group, there was a substantial (12.6 ± 1.5 mmHg) rise in systolic pulmonary artery pressure (SPAP) during sustained hypoxia, but no associated fall in maximal exercise capacity breathing 12% oxygen. In the iron group, the rise in SPAP during sustained hypoxia was markedly reduced (3.4 ± 1.0 mmHg). There was a small rise in maximal exercise capacity following sustained hypoxia within the iron group, but no overall effect of iron, compared with saline. These results do not support the hypothesis that elevated SPAP inhibits maximal exercise capacity during acute hypoxia in healthy volunteers.

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.

The anaemia treatment journey of CKD patients: from epoetins to hypoxia-inducible factor-prolyl hydroxylase inhibitors

The discovery and development of erythropoiesis-stimulating agents was a journey lasting more than a century, leading to the cloning and approval of recombinant human erythropoietin (rHuEpo). This was an impressive clinical advance, providing the possibility of correcting the symptoms associated with anaemia in chronic kidney disease. Associated iron use was needed to produce new haemoglobin-containing blood red cells. Partial anaemia correction became the standard of care since trials aiming for near-normal haemoglobin levels showed a higher risk of adverse cardiovascular events. Hoping to reduce the cardiovascular risks, a new category of drugs was developed and tested. Hypoxia-inducible factor prolyl hydroxylase inhibitors (HIF-PHIs) are small molecules than can be formulated into orally active pills. They simulate reduced tissue oxygen pressure, thus stimulating the production of endogenous erythropoietin (Epo) by the kidneys and liver. Clinical trials with these compounds demonstrated that HIF-PHIs are at least as effective as rHuEpo in treating or correcting anaemia in non-dialysis and dialysis patients. Trials with HIF-PHIs did not demonstrate superiority in safety outcomes and in some trials, outcomes were worse. There was also a focus on oral delivery, a possible beneficial iron-sparing effect and the ability to overcome Epo resistance in inflamed patients. A negative effect is possible iron depletion, which may explain adverse outcomes.

Regulation of Erythropoiesis by the Hypoxia-Inducible Factor Pathway: Effects of Genetic and Pharmacological Perturbations

Red blood cells transport O₂ from the lungs to body tissues. Hypoxia stimulates kidney cells to secrete erythropoietin (EPO), which increases red cell mass. Hypoxia-inducible factors (HIFs) mediate EPO gene transcriptional activation. HIF-α subunits are subject to O₂-dependent prolyl hydroxylation and then bound by the von Hippel-Lindau protein (VHL), which triggers their ubiquitination and proteasomal degradation. Mutations in the genes encoding EPO, EPO receptor, HIF-2α, prolyl hydroxylase domain protein 2 (PHD2), or VHL cause familial erythrocytosis. In addition to O₂, α-ketoglutarate is a substrate for PHD2, and analogs of α-ketoglutarate inhibit hydroxylase activity. In phase III clinical trials evaluating the treatment of anemia in chronic kidney disease, HIF prolyl hydroxylase inhibitors were as efficacious as darbepoetin alfa in stimulating erythropoiesis. However, safety concerns have arisen that are focused on thromboembolism, which is also a phenotypic manifestation of VHL or HIF-2α mutation, suggesting that these events are on-target effects of HIF prolyl hydroxylase inhibitors.

Molecular basis and clinical implications of HIFs in cardiovascular diseases

Oxygen maintains the homeostasis of an organism in a delicate balance in different tissues and organs. Under hypoxic conditions, hypoxia-inducible factors (HIFs) are specific and dominant factors in the spatiotemporal regulation of oxygen homeostasis. As the most basic functional unit of the heart at the cellular level, the cardiomyocyte relies on oxygen and nutrients delivered by the microvasculature to keep the heart functioning properly. Under hypoxic stress, HIFs are involved in acute and chronic myocardial pathology because of their spatiotemporal specificity, thus granting them therapeutic potential. Most adult animals lack the ability to regenerate their myocardium entirely following injury, and complete regeneration has long been a goal of clinical treatment for heart failure. The precise manipulation of HIFs (considering their dynamic balance and transformation) and the development of HIF-targeted drugs is therefore an extremely attractive cardioprotective therapy for protecting against myocardial ischemic and hypoxic injury, avoiding myocardial remodeling and heart failure, and promoting recovery of cardiac function.

Cardio-onco-metabolism: metabolic remodelling in cardiovascular disease and cancer

Cardiovascular disease and cancer are the two leading causes of morbidity and mortality in the world. The emerging field of cardio-oncology has revealed that these seemingly disparate disease processes are intertwined, owing to the cardiovascular sequelae of anticancer therapies, shared risk factors that predispose individuals to both cardiovascular disease and cancer, as well the possible potentiation of cancer growth by cardiac dysfunction. As a result, interest has increased in understanding the fundamental biological mechanisms that are central to the relationship between cardiovascular disease and cancer. Metabolism, appropriate regulation of energy, energy substrate utilization, and macromolecular synthesis and breakdown are fundamental processes for cellular and organismal survival. In this Review, we explore the emerging data identifying metabolic dysregulation as an important theme in cardio-oncology. We discuss the growing recognition of metabolic reprogramming in cardiovascular disease and cancer and view the novel area of cardio-oncology through the lens of metabolism.

Early‐onset and severe Pulmonary Arterial Hypertension due to a novel compound heterozygous association of rare VHL mutations: a case report and review of existing data

Very rare cases of pulmonary arterial hypertension (PAH) have been linked to homozygous or compound heterozygous von Hippel–Lindau (VHL) tumor suppressor gene mutations, while heterozygous VHL mutations lead to VHL tumor syndrome. Although those entities are defined, the genotype–phenotype correlation is incompletely understood, and patient management recommendations are lacking. Here, we describe a case of severe early‐onset PAH due to a so‐far unreported compound heterozygous association of VHL mutations and review the existing data.

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.

Heritable disorders of oxygen sensing

Hypoxia-inducible factors (HIFs) activate gene transcription in response to reduced O₂ availability and play critical roles in development, physiology, and disease pathogenesis. Mutations that dysregulate HIF activity are the genetic basis for tumor predisposition in the von Hippel-Lindau syndrome and excess red blood cell production in hereditary erythrocytosis.

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.

Oxygen sensing, anaesthesia and critical care: a narrative review

In 2019, the scientists who discovered how cells sense and adapt to oxygen availability were awarded the Nobel Prize. This elegant sensing pathway is conserved throughout evolution, and it underpins the physiology and pathology that we, as clinicians in anaesthesia and critical care, encounter on a daily basis. The purpose of this review is to bring hypoxia-inducible factor, and the oxygen-sensing pathway as a whole, to the wider clinical community. We describe how this unifying mechanism was discovered, and how it orchestrates diverse changes such as erythropoiesis, ventilatory acclimatisation, pulmonary vascular remodelling and altered metabolism. We explore the lessons learnt from genetic disorders of oxygen sensing, and the wider implications in evolution of all animal species, including our own. Finally, we explain how this pathway is relevant to our clinical practice, and how it is being manipulated in new treatments for conditions such as cancer, anaemia and pulmonary hypertension.

Hypoxia Inducible Factors as Central Players in the Pathogenesis and Pathophysiology of Cardiovascular Diseases

Cardiovascular (CV) diseases are the major cause of death in industrialized countries. The main function of the CV system is to deliver nutrients and oxygen to all tissues. During most CV pathologies, oxygen and nutrient delivery is decreased or completely halted. Several mechanisms, including increased oxygen transport and delivery, as well as increased blood flow are triggered to compensate for the hypoxic state. If the compensatory mechanisms fail to sufficiently correct the hypoxia, irreversible damage can occur. Thus, hypoxia plays a central role in the pathogenesis and pathophysiology of CV diseases. Hypoxia inducible factors (HIFs) orchestrate the gene transcription for hundreds of proteins involved in erythropoiesis, glucose transport, angiogenesis, glycolytic metabolism, reactive oxygen species (ROS) handling, cell proliferation and survival, among others. The overall regulation of the expression of HIF-dependent genes depends on the severity, duration, and location of hypoxia. In the present review, common CV diseases were selected to illustrate that HIFs, and proteins derived directly or indirectly from their stabilization and activation, are related to the development and perpetuation of hypoxia in these pathologies. We further classify CV diseases into acute and chronic hypoxic states to better understand the temporal relevance of HIFs in the pathogenesis, disease progression and clinical outcomes of these diseases. We conclude that HIFs and their derived factors are fundamental in the genesis and progression of CV diseases. Understanding these mechanisms will lead to more effective treatment strategies leading to reduced morbidity and mortality.

Breakthrough Science: Hypoxia-Inducible Factors, Oxygen Sensing, and Disorders of Hematopoiesis

Hypoxia-inducible factors (HIF) were discovered as activators of erythropoietin gene transcription in response to reduced O2 availability. O2-dependent hydroxylation of HIFs on proline and asparagine residues regulates protein stability and transcription activity, respectively. Mutations in genes encoding components of the oxygen sensing pathway cause familial erythrocytosis. Several small molecule inhibitors of HIF prolyl hydroxylases are currently in clinical trials as erythropoiesis stimulating agents. HIFs are overexpressed in bone marrow neoplasms, and the development of HIF inhibitors may improve outcome in these disorders.

Genetic Background of Congenital Erythrocytosis

True erythrocytosis is present when the red cell mass is greater than 125% of predicted sex and body mass, which is reflected by elevated hemoglobin and hematocrit. Erythrocytosis can be primary or secondary and congenital or acquired. Congenital defects are often found in those diagnosed at a young age and with a family history of erythrocytosis. Primary congenital defects mainly include mutations in the Erythropoietin receptor gene but SH2B3 has also been implicated. Secondary congenital erythrocytosis can arise through a variety of genetic mechanisms, including mutations in the genes in the oxygen sensing pathway, with high oxygen affinity hemoglobin variants and mutations in other genes such as BPMG, where ultimately the production of erythropoietin is increased, resulting in erythrocytosis. Recently, mutations in PIEZ01 have been associated with erythrocytosis. In many cases, a genetic variant cannot be identified, leaving a group of patients with the label idiopathic erythrocytosis who should be the subject of future investigations. The clinical course in congenital erythrocytosis is hard to evaluate as these are rare cases. However, some of these patients may well present at a young age and with sometimes catastrophic thromboembolic events. There is little evidence to guide the management of congenital erythrocytosis but the use of venesection and low dose aspirin should be considered.

Oxygen sensing, mitochondrial biology and experimental therapeutics for pulmonary hypertension and cancer

The homeostatic oxygen sensing system (HOSS) optimizes systemic oxygen delivery. Specialized tissues utilize a conserved mitochondrial sensor, often involving NDUFS2 in complex I of the mitochondrial electron transport chain, as a site of pO₂-responsive production of reactive oxygen species (ROS). These ROS are converted to a diffusible signaling molecule, hydrogen peroxide (H₂O₂), by superoxide dismutase (SOD2). H₂O₂ exits the mitochondria and regulates ion channels and enzymes, altering plasma membrane potential, intracellular Ca²⁺ and Ca²⁺-sensitization and controlling acute, adaptive, responses to hypoxia that involve changes in ventilation, vascular tone and neurotransmitter release. Subversion of this O₂-sensing pathway creates a pseudohypoxic state that promotes disease progression in pulmonary arterial hypertension (PAH) and cancer. Pseudohypoxia is a state in which biochemical changes, normally associated with hypoxia, occur despite normal pO₂. Epigenetic silencing of SOD2 by DNA methylation alters H₂O₂ production, activating hypoxia-inducible factor 1α, thereby disrupting mitochondrial metabolism and dynamics, accelerating cell proliferation and inhibiting apoptosis. Other epigenetic mechanisms, including dysregulation of microRNAs (miR), increase pyruvate dehydrogenase kinase and pyruvate kinase muscle isoform 2 expression in both diseases, favoring uncoupled aerobic glycolysis. This Warburg metabolic shift also accelerates cell proliferation and impairs apoptosis. Disordered mitochondrial dynamics, usually increased mitotic fission and impaired fusion, promotes disease progression in PAH and cancer. Epigenetic upregulation of dynamin-related protein 1 (Drp1) and its binding partners, MiD49 and MiD51, contributes to the pathogenesis of PAH and cancer. Finally, dysregulation of intramitochondrial Ca²⁺, resulting from impaired mitochondrial calcium uniporter complex (MCUC) function, links abnormal mitochondrial metabolism and dynamics. MiR-mediated decreases in MCUC function reduce intramitochondrial Ca²⁺, promoting Warburg metabolism, whilst increasing cytosolic Ca²⁺, promoting fission. Epigenetically disordered mitochondrial O₂-sensing, metabolism, dynamics, and Ca²⁺ homeostasis offer new therapeutic targets for PAH and cancer. Promoting glucose oxidation, restoring the fission/fusion balance, and restoring mitochondrial calcium regulation are promising experimental therapeutic strategies.

Heritable disorders of oxygen sensing

Hypoxia-inducible factors (HIFs) activate gene transcription in response to reduced O₂ availability and play critical roles in development, physiology, and disease pathogenesis. Mutations that dysregulate HIF activity are the genetic basis for tumor predisposition in the von Hippel-Lindau syndrome and excess red blood cell production in hereditary erythrocytosis.

Profile of Daprodustat in the Treatment of Renal Anemia Due to Chronic Kidney Disease

Anemia is a major complication of chronic kidney disease (CKD), which mainly results from appropriate erythropoietin production impairment. Prolyl hydroxylase domain (PHD) inhibitors are currently being developed and approved in some countries as a new treatment for CKD patients with anemia due to the stabilization of intracellular hypoxia-inducible factor (HIF) 1α and HIF2α by PHD inhibition. Daprodustat is one of the orally administrated small-molecule HIF-PH inhibitors, leading to an increase in erythropoietin production, which is regulated by HIF. Also, daprodustat is expected to improve iron metabolism. Recently, several clinical trials showed its efficacy and safety in both hemodialysis- and non-hemodialysis- dependent CKD patients. In addition, some international Phase 3 studies are underway to confirm these effects and reveal the safety profile. This article summarizes the development process and results of each clinical trial.

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.

Influence of iron manipulation on hypoxic pulmonary vasoconstriction and pulmonary reactivity during ascent and acclimatization to 5050 m

KEY POINTS

Iron acts as a cofactor in the stabilization of the hypoxic-inducible factor family, and plays an influential role in the modulation of hypoxic pulmonary vasoconstriction. It is uncertain whether iron regulation is altered in lowlanders during either (1) ascent to high altitude, or (2) following partial acclimatization, when compared to high-altitude adapted Sherpa. During ascent to 5050 m, the rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders; however, upon arrival to 5050 m, PASP levels were comparable in both groups, but the reduction in iron bioavailability was more prevalent in lowlanders compared to Sherpa. Following partial acclimatization to 5050 m, there were differential influences of iron status manipulation (via iron infusion or chelation) at rest and during exercise between lowlanders and Sherpa on the pulmonary vasculature.

ABSTRACT

To examine the adaptational role of iron bioavailability on the pulmonary vascular responses to acute and chronic hypobaric hypoxia, the haematological and cardiopulmonary profile of lowlanders and Sherpa were determined during: (1) a 9-day ascent to 5050 m (20 lowlanders; 12 Sherpa), and (2) following partial acclimatization (11 ± 4 days) to 5050 m (18 lowlanders; 20 Sherpa), where both groups received an i.v. infusion of either iron (iron (iii)-hydroxide sucrose) or an iron chelator (desferrioxamine). During ascent, there were reductions in iron status in both lowlanders and Sherpa; however, Sherpa appeared to demonstrate a more efficient capacity to mobilize stored iron, compared to lowlanders, when expressed as a Δhepcidin per unit change in either body iron or the soluble transferrin receptor index, between 3400-5050 m (P = 0.016 and P = 0.029, respectively). The rise in pulmonary artery systolic pressure (PASP) was blunted in Sherpa, compared to lowlanders during ascent; however, PASP was comparable in both groups upon arrival to 5050 m. Following partial acclimatization, despite Sherpa demonstrating a blunted hypoxic ventilatory response and greater resting hypoxaemia, they had similar hypoxic pulmonary vasoconstriction when compared to lowlanders at rest. Iron-infusion attenuated PASP in both groups at rest (P = 0.005), while chelation did not exaggerate PASP in either group at rest or during exaggerated hypoxaemia ( P I O 2  = 67 mmHg). During exercise at 25% peak wattage, PASP was only consistently elevated in Sherpa, which persisted following both iron infusion or chelation. These findings provide new evidence on the complex interplay of iron regulation on pulmonary vascular regulation during acclimatization and adaptation to high altitude.

Roles of HIF and 2-Oxoglutarate-Dependent Dioxygenases in Controlling Gene Expression in Hypoxia

Simple Summary

Hypoxia—reduction in oxygen availability—plays key roles in both physiological and pathological processes. Given the importance of oxygen for cell and organism viability, mechanisms to sense and respond to hypoxia are in place. A variety of enzymes utilise molecular oxygen, but of particular importance to oxygen sensing are the 2-oxoglutarate (2-OG) dependent dioxygenases (2-OGDs). Of these, Prolyl-hydroxylases have long been recognised to control the levels and function of Hypoxia Inducible Factor (HIF), a master transcriptional regulator in hypoxia, via their hydroxylase activity. However, recent studies are revealing that such dioxygenases are involved in almost all aspects of gene regulation, including chromatin organisation, transcription and translation.

Abstract

Hypoxia—reduction in oxygen availability—plays key roles in both physiological and pathological processes. Given the importance of oxygen for cell and organism viability, mechanisms to sense and respond to hypoxia are in place. A variety of enzymes utilise molecular oxygen, but of particular importance to oxygen sensing are the 2-oxoglutarate (2-OG) dependent dioxygenases (2-OGDs). Of these, Prolyl-hydroxylases have long been recognised to control the levels and function of Hypoxia Inducible Factor (HIF), a master transcriptional regulator in hypoxia, via their hydroxylase activity. However, recent studies are revealing that dioxygenases are involved in almost all aspects of gene regulation, including chromatin organisation, transcription and translation. We highlight the relevance of HIF and 2-OGDs in the control of gene expression in response to hypoxia and their relevance to human biology and health.

Targeting the HIF2-VEGF axis in renal cell carcinoma

Insights into the role of the tumor suppressor pVHL in oxygen sensing motivated the testing of drugs that target the transcription factor HIF or HIF-responsive growth factors, such as VEGF, for the treatment of cancers caused by VHL inactivation, such as clear-cell renal cell carcinoma (ccRCC). Multiple VEGF inhibitors are now approved for the treatment of ccRCC, and a HIF2α inhibitor has advanced to phase 3 development for this disease. These inhibitors are now also increasingly combined with immune-checkpoint blockers. In this Perspective, we describe the understanding of the mechanisms of oxygen sensing and hypoxia signaling that resulted in the development of HIF2α-targeted therapies for patients with VHL-associated tumors. We also present future directions for extending the use of these therapies to other cancers.

The Genetic Epidemiology of Pediatric Pulmonary Arterial Hypertension

OBJECTIVE

To describe the prevalence of pulmonary arterial hypertension (PAH)-associated gene mutations, and other genetic characteristics in a national cohort of children with PAH from the Dutch National registry and to explore genotype-phenotype associations and outcomes.

STUDY DESIGN

Children (n = 70) diagnosed with idiopathic PAH, heritable PAH, PAH associated with congenital heart disease with coincidental shunt (PAH-congenital heart disease group 3), PAH after closure of a cardiac shunt (PAH-congenital heart disease group 4), or PAH associated with other noncardiac conditions were enrolled. Targeted next-generation sequencing was performed on PAH-associated genes (BMPR2, ACVRL1, EIF2AK4, CAV1, ENG, KCNK3, SMAD9, and TBX4). Also, children were tested for specific genetic disorders in case of clinical suspicion. Additionally, children were tested for copy number variations.

RESULTS

Nineteen children (27%) had a PAH-associated gene mutation/variant: BMPR2 n = 7, TBX4 n = 8, ACVRL1 n = 1, KCNK3 n = 1, and EIF2AK4 n = 2. Twelve children (17%) had a genetic disorder with an established association with PAH (including trisomy 21 and cobalamin C deficiency). In another 16 children (23%), genetic disorders without an established association with PAH were identified (including Noonan syndrome, Beals syndrome, and various copy number variations). Survival rates differed between groups and was most favorable in TBX4 variant carriers.

CONCLUSIONS

Children with PAH show a high prevalence of genetic disorders, not restricted to established PAH-associated genes. Genetic architecture could play a role in risk-stratified care management in pediatric PAH.

Pharmacologic Targeting of Hypoxia-Inducible Factors

Hypoxia-inducible factors (HIFs) control transcriptional responses to reduced O₂ availability. HIFs are heterodimeric proteins composed of an O₂-regulated HIF-α subunit and a constitutively expressed HIF-1β subunit. HIF-α subunits are subject to prolyl hydroxylation, which targets the proteins for degradation under normoxic conditions. Small molecule prolyl hydroxylase inhibitors, which stabilize the HIF-α subunits and increase HIF-dependent expression of erythropoietin, are in phase III clinical trials for the treatment of anemia in patients with chronic kidney disease. HIFs contribute to the pathogenesis of many cancers, particularly the clear cell type of renal cell carcinoma in which loss of function of the von Hippel-Lindau tumor suppressor blocks HIF-2α degradation. A small molecule inhibitor that binds to HIF-2α and blocks dimerization with HIF-1β is in clinical trials for the treatment of renal cell carcinoma. Targeting HIFs for stabilization or inhibition may improve outcomes in diseases that are common causes of mortality in the US population.

Plasma insulin-like growth factor binding protein 1 in pulmonary arterial hypertension

BACKGROUND

Beside the pulmonary vasoconstriction observed in pulmonary arterial hypertension (PAH), severe proliferative and antiapoptotic cellular phenotypes result in vascular remodelling. Many recent findings indicate similarities between PAH and tumour pathology. For instance, insulin-like growth factor (IGF)-1 signalling, which is known to promote tumour development, is implicated in PAH. Higher circulating IGF binding protein (IGFBP)-1 levels are associated with worse survival in PAH. The present study aimed to investigate the relationship between plasma levels of various tumour-related biomarkers and PAH. Methods: IGFBP-1, -2 and -7, along with other tumour-related biomarkers, were measured in plasma from 48 treatment-naïve PAH patients and 16 healthy controls, using proximity extension assays. Among the PAH patients, 33 were also studied at an early treatment follow-up. Results: Plasma IGFBP-1 (p < .003), IGFBP-2 (p < .001), IGFBP-7 (p < .008), vimentin (p < .001), carbonic anhydrase 9 (p < .001), S100A11 (p < .001), human epididymis protein 4 (p < .001) and folate receptor-α (p < .004) were elevated in PAH, compared to controls. IGFBP-1 exhibited the most interesting correlations to clinical parameters and was selected for further analyses. IGFBP-1 correlated specifically to N-terminal prohormone of brain natriuretic peptide (NT-proBNP) (r = 0.44, p < .002), mean right atrial pressure (r = 0.41, p < .004), venous oxygen saturation (r = -0.43, p < .003), cardiac index (r = -0.32, p < .03) and 6-minute walking distance (r = -0.29, p < .05). Plasma IGFBP-1 also correlated to risk scores based on the European Society of Cardiology/European Respiratory Society (ESC/ERS) PAH guidelines (r = 0.43, p < .003) and the REVEAL model (r = 0.46, p < .001). PAH patients with supra-median baseline IGFBP-1 levels showed a trend for worse overall survival than those with infra-median levels (p = .087). IGFBP-1 was unaltered between baseline and an early treatment follow-up. However, IGFBP-1 changes, between baseline and follow-up, correlated to changes in NT-proBNP (r = 0.48, p < .006). Conclusion: Plasma IGFBP-1 levels at PAH diagnosis show moderate association to NT-proBNP and hemodynamics as well as with ESC/ERS and REVEAL risk scores.

Intravenous iron delivers a sustained (8-week) lowering of pulmonary artery pressure during exercise in healthy older humans

In older individuals, pulmonary artery pressure rises markedly during exercise, probably due in part to increased pulmonary vascular resistance and in part to an increase in left-heart filling pressure. Older individuals also show more marked pulmonary vascular response to hypoxia at rest. Treatment with intravenous iron reduces the rise in pulmonary artery pressure observed during hypoxia. Here, we test the hypothesis that intravenous iron administration may also attenuate the rise in pulmonary artery pressure with exercise in older individuals. In a randomized double-blind placebo-controlled physiology study in 32 healthy participants aged 50-80 years, we explored the hypothesis that iron administration would deliver a fall in systolic pulmonary artery pressure (SPAP) during moderate cycling exercise (20 min duration; increase in heart rate of 30 min^-1 ) and a change in maximal cycling exercise capacity ( V ˙ O 2 m a x ). Participants were studied before, and at 3 h to 8 weeks after, infusion. SPAP was measured using Doppler echocardiography. Iron administration resulted in marked changes in indices of iron homeostasis over 8 weeks, but no significant change in hemoglobin concentration or inflammatory markers. Resting SPAP was also unchanged, but SPAP during exercise was lower by ~3 mmHg in those receiving iron (P < 0.0001). This effect persisted for 8 weeks. Although V ˙ O 2 m a x remained unaffected in the iron-replete healthy participants studied here, this study demonstrates for the first time the ability of intravenous iron supplementation to reduce systolic pulmonary artery pressure during exercise.

The Genomics and Genetics of Oxygen Homeostasis

Human survival is dependent upon the continuous delivery of O₂ to each cell in the body in sufficient amounts to meet metabolic requirements, primarily for ATP generation by oxidative phosphorylation. Hypoxia-inducible factors (HIFs) regulate the transcription of thousands of genes to balance O₂ supply and demand. The HIFs are negatively regulated by O₂-dependent hydrox-ylation and ubiquitination by prolyl hydroxylase domain (PHD) proteins and the von Hippel-Lindau (VHL) protein. Germline mutations in the genes encoding VHL, HIF-2α, and PHD2 cause hereditary erythrocytosis, which is characterized by polycythemia and pulmonary hypertension and is caused by increased HIF activity. Evolutionary adaptation to life at high altitude is associated with unique genetic variants in the genes encoding HIF-2α and PHD2 that blunt the erythropoietic and pulmonary vascular responses to hypoxia.

Effects of Germline VHL Deficiency on Growth, Metabolism, and Mitochondria

Mutations in VHL, which encodes von Hippel-Lindau tumor suppressor (VHL), are associated with divergent diseases. We describe a patient with marked erythrocytosis and prominent mitochondrial alterations associated with a severe germline VHL deficiency due to homozygosity for a novel synonymous mutation (c.222C→A, p.V74V). The condition is characterized by early systemic onset and differs from Chuvash polycythemia (c.598C→T) in that it is associated with a strongly reduced growth rate, persistent hypoglycemia, and limited exercise capacity. We report changes in gene expression that reprogram carbohydrate and lipid metabolism, impair muscle mitochondrial respiratory function, and uncouple oxygen consumption from ATP production. Moreover, we identified unusual intermitochondrial connecting ducts. Our findings add unexpected information on the importance of the VHL-hypoxia-inducible factor (HIF) axis to human phenotypes. (Funded by Associazione Italiana Ricerca sul Cancro and others.).

The von Hippel-Lindau Tumor Suppressor Gene: Implications and Therapeutic Opportunities

The discovery of the von Hippel-Lindau (VHL) gene marked a milestone in our understanding of clear cell renal cell carcinoma (ccRCC) pathogenesis. VHL inactivation is not only a defining feature of ccRCC, but also the initiating event. Herein, we discuss canonical and noncanonical pVHL functions, as well as breakthroughs shaping our understanding of ccRCC evolution and evolutionary subtypes. We conclude by presenting evolving strategies to therapeutically exploit effector mechanisms downstream of pVHL.

Suppression of HIF2 signalling attenuates the initiation of hypoxia-induced pulmonary hypertension

Most published studies addressing the role of hypoxia inducible factors (HIFs) in hypoxia-induced pulmonary hypertension development employ models that may not recapitulate the clinical setting, including the use of animals with pre-existing lung/vascular defects secondary to embryonic HIF ablation or activation. Furthermore, critical questions including how and when HIF signalling contributes to hypoxia-induced pulmonary hypertension remain unanswered.Normal adult rodents in which global HIF1 or HIF2 was inhibited by inducible gene deletion or pharmacological inhibition (antisense oligonucleotides (ASO) and small molecule inhibitors) were exposed to short-term (4 days) or chronic (4-5 weeks) hypoxia. Haemodynamic studies were performed, the animals euthanised, and lungs and hearts obtained for pathological and transcriptomic analysis. Cell-type-specific HIF signals for pulmonary hypertension initiation were determined in normal pulmonary vascular cells in vitro and in mice (using cell-type-specific HIF deletion).Global Hif1a deletion in mice did not prevent hypoxia-induced pulmonary hypertension at 5 weeks. Mice with global Hif2a deletion did not survive long-term hypoxia. Partial Hif2a deletion or Hif2-ASO (but not Hif1-ASO) reduced vessel muscularisation, increases in pulmonary arterial pressures and right ventricular hypertrophy in mice exposed to 4-5 weeks of hypoxia. A small molecule HIF2 inhibitor (PT2567) significantly attenuated early events (monocyte recruitment and vascular cell proliferation) in rats exposed to 4 days of hypoxia, as well as vessel muscularisation, tenascin C accumulation and pulmonary hypertension development in rats exposed to 5 weeks of hypoxia. In vitro, HIF2 induced a distinct set of genes in normal human pulmonary vascular endothelial cells, mediating inflammation and proliferation of endothelial cells and smooth muscle cells. Endothelial Hif2a knockout prevented hypoxia-induced pulmonary hypertension in mice.Inhibition of HIF2 (but not HIF1) can provide a therapeutic approach to prevent the development of hypoxia-induced pulmonary hypertension. Future studies are needed to investigate the role of HIFs in pulmonary hypertension progression and reversal.

Cell Autonomous and Non-cell Autonomous Regulation of SMC Progenitors in Pulmonary Hypertension

Pulmonary hypertension is a devastating disease characterized by excessive vascular muscularization. We previously demonstrated primed platelet-derived growth factor receptor β⁺ (PDGFR-β⁺)/smooth muscle cell (SMC) marker⁺ progenitors at the muscular-unmuscular arteriole border in the normal lung, and in hypoxia-induced pulmonary hypertension, a single primed cell migrates distally and expands clonally, giving rise to most of the pathological smooth muscle coating of small arterioles. Little is known regarding the molecular mechanisms underlying this process. Herein, we show that primed cell expression of Kruppel-like factor 4 and hypoxia-inducible factor 1-α(HIF1-α) are required, respectively, for distal migration and smooth muscle expansion in a sequential manner. In addition, the HIF1-α/PDGF-B axis in endothelial cells non-cell autonomously regulates primed cell induction, proliferation, and differentiation. Finally, myeloid cells transdifferentiate into or fuse with distal arteriole SMCs during hypoxia, and Pdgfb deletion in myeloid cells attenuates pathological muscularization. Thus, primed cell autonomous and non-cell autonomous pathways are attractive therapeutic targets for pulmonary hypertension.

Molecular Characteristics and Treatment of Endothelial Dysfunction in Patients with COPD: A Review Article

Patients with chronic obstructive pulmonary disease (COPD) show systemic consequences, such as chronic systemic inflammation leading to changes in the airway, airway penetrability, and endothelial function. Endothelial dysfunction is characterized by a list of alterations of endothelium towards reduced vasodilation, proinflammatory state, detachment and apoptosis of endothelial cells, and development of atherosclerosis. COPD-induced endothelial dysfunction is associated with elevated cardiovascular risk. The increment of physical activities such as pulmonary rehabilitation (PR) training have a significant effect on COPD, thus, PR can be an integrative part of COPD treatment. In this narrative review the focus is on the function of endothelial inflammatory mediators [cytokines, chemokines, and cellular proteases] and pulmonary endothelial cells and endothelial dysfunction in COPD as well as the effects of dysfunction of the endothelium may play in COPD-related pulmonary hypertension. The relationship between smoking and endothelial dysfunction is also discussed. The connection between different pulmonary rehabilitation programs, arterial stiffness and pulse wave velocity (PWV) is presented. Endothelial dysfunction is a significant prognostic factor of COPD, which can be characterized by PWV. We discuss future considerations, like training programs, as an important part of the treatment that has a favorable impact on the endothelial function.

HIF Activation Against CVD in CKD: Novel Treatment Opportunities

Cardiovascular disease is a common and serious complication in patients with chronic kidney disease (CKD). One of the fundamental functions of the cardiovascular system is oxygen delivery, therefore cardiovascular disease inherently is linked to insufficient tissue oxygenation. Advances in our knowledge of cellular oxygen sensing by a family of prolyl hydroxylases (PHDs) and their role in regulating hypoxia-inducible factors (HIFs) have led to the discovery of PHD inhibitors as HIF stabilizers. Several small-molecule PHD inhibitors are currently in clinical trials for the treatment of anemia in CKD. An additional advantage of PHD inhibition may be found in the potential impact on cardiovascular consequences associated with CKD. Several preclinical studies have suggested a potential benefit of HIF activation in myocardial infarction, cardiac remodeling, atherosclerosis, and peripheral artery disease. Ameliorating glucose and lipid metabolism and lowering blood pressure may also contribute to cardiovascular protection. On the other hand, the broad spectrum of HIF-dependent functions also may include unwanted side effects. Clinical application of PHD inhibitors therefore necessitates careful evaluation of the net systemic effect of HIF activation.

Hif-1α paralogs play a role in the hypoxic ventilatory response of larval and adult zebrafish (Danio rerio)

Hypoxia-inducible factor (Hif) 1α, an extensively studied transcription factor, is involved in the regulation of many biological processes in hypoxia including the hypoxic ventilatory response. In zebrafish, there are two paralogs of Hif-1α (Hif-1A and Hif-1B), but little is known about the specific roles or potential sub-functionalization of the paralogs in response to hypoxia. Using knockout lines of Hif-1α paralogs, we examined their involvement in the hypoxic ventilatory response, measured as ventilation frequency (f _V) in larval and adult zebrafish (Danio rerio). In wild-type zebrafish, f _V increased across developmental time (4, 7, 10 and 15 days post--fertilization, dpf) in response to hypoxia (55 mmHg). In contrast, the Hif-1B knockout fish did not exhibit an increase in hypoxic f _V at 4 dpf. Similar to wild-type, as larvae of all knockout lines developed, the magnitude of f _V increased but to a lesser degree than in the wild-type larvae, until 15 dpf at which point there was no difference among the genotypes. In adult zebrafish, only in Hif-1B knockout fish was there an attenuation in f _V during sustained exposure to 30 mmHg for 1 h but there was no effect when fish were exposed for a shorter duration to progressive hypoxia. The mechanism of action of Hif-1α, in part, may be through its downstream target, nitric oxide synthase, and its product, nitric oxide. Overall, the effect of each Hif-1α paralog on the hypoxic ventilatory response of zebrafish varies over development and is dependent on the type of hypoxic stress.

Therapeutic Targeting of Vascular Remodeling and Right Heart Failure in Pulmonary Arterial Hypertension with a HIF-2α Inhibitor

RATIONALE

Pulmonary arterial hypertension (PAH) is a devastating disease characterized by progressive vasoconstriction and obliterative vascular remodeling that leads to right heart failure (RHF) and death. Current therapies do not target vascular remodeling and RHF, and result in only modest improvement of morbidity and mortality.

OBJECTIVES

To determine whether targeting HIF-2α (hypoxia-inducible factor-2α) with a HIF-2α-selective inhibitor could reverse PAH and RHF in various rodent PAH models.

METHODS

HIF-2α and its downstream genes were evaluated in lung samples and pulmonary arterial endothelial cells and smooth muscle cells from patients with idiopathic PAH as well as various rodent PAH models. A HIF-2α-selective inhibitor was used in human lung microvascular endothelial cells and in Egln1Tie2Cre mice, and in Sugen 5416/hypoxia- or monocrotaline-exposed rats.

MEASUREMENTS AND MAIN RESULTS

Upregulation of HIF-2α and its target genes was observed in lung tissues and isolated pulmonary arterial endothelial cells from patients with idiopathic PAH and three distinct rodent PAH models. Pharmacological inhibition of HIF-2α by the HIF-2α translation inhibitor C76 (compound 76) reduced right ventricular systolic pressure and right ventricular hypertrophy and inhibited RHF and fibrosis as well as obliterative pulmonary vascular remodeling in Egln1Tie2Cre mice and Sugen 5416/hypoxia PAH rats. Treatment of monocrotaline-exposed PAH rats with C76 also reversed right ventricular systolic pressure, right ventricular hypertrophy, and pulmonary vascular remodeling; prevented RHF; and promoted survival.

CONCLUSIONS

These findings demonstrate that pharmacological inhibition of HIF-2α is a promising novel therapeutic strategy for the treatment of severe vascular remodeling and right heart failure in patients with PAH.

Polyglobulia in patients with hemangioblastomas is related to tumor size but not to serum erythropoietin

Background

Hemangioblastomas are associated with elevated hemoglobin (Hb) levels (polyglobulia), which is associated with a higher risk for cerebral stroke, cardiac infarction and pulmonary embolism. The pathomechanism of polyglobulia remains unclear and different theories have been postulated. Among those are elevated serum erythropoietin (EPO) levels caused by secretion of the tumor or associated tumor cyst.

Methods

To elucidate the pathomechanism, we systematically investigated the relation between polyglobulia, serum EPO level, size of the solid tumor and associated cyst in hemangioblastomas. We prospectively evaluated hemoglobin and EPO levels in a series of 33 consecutive patients operated on hemangioblastomas in our center. We measured the size of the solid tumor and associated cyst in magnetic resonance imaging. Statistical evaluations were performed using the Fisher's exact test and student's t-test.

Results

As a result five patients had elevated hemoglobin levels. Only one of these had an elevated serum EPO level. Of 26 patients with normal hemoglobin levels, 4 patients had elevated EPO levels.Patients with low or normal hemoglobin levels (84%) had an average tumor size of 0.8 cm³, which differed significantly from patients with elevated hemoglobin levels (16%), who had an average solid tumor size of 8.0 cm³ (p < 0.05). We did not observe a significant correlation between EPO levels or polyglobulia and associated cysts.

Conclusions

We therefore conclude that in contrast to previous case reports and interpretations, our data show no correlation between polyglobulia and EPO levels or associated cysts in patients with hemangioblastomas. In fact, it is the size of the solid tumor that correlates with polyglobulia.The study was retrospectively registered in the German Clinical Trial Registry on 10 July 2014; Trial registration: DRKS00006310.

Sensors and signals: the role of reactive oxygen species in hypoxic pulmonary vasoconstriction

When lung cells experience hypoxia, the functional response, termed hypoxic pulmonary vasoconstriction, activates a multitude of pathways with the goal of optimizing gas exchange. While previously controversial, overwhelming evidence now suggests that increased reactive oxygen species - produced at complex III of the mitochondrial electron transport chain and released into the intermembrane space - is the cellular oxygen signal responsible for triggering hypoxic pulmonary vasoconstriction. The increased reactive oxygen species (ROS) activate many downstream targets that ultimately lead to increased intracellular ionized calcium concentration and contraction of pulmonary arterial smooth muscle cells. While the specific targets of ROS signals are not completely understood, it is clear that this signalling pathway is critical for development and for normal lung function in newborns and adults.

HIF Oxygen Sensing Pathways in Lung Biology

Cellular responses to oxygen fluctuations are largely mediated by hypoxia-inducible factors (HIFs). Upon inhalation, the first organ inspired oxygen comes into contact with is the lungs, but the understanding of the pulmonary HIF oxygen-sensing pathway is still limited. In this review we will focus on the role of HIF1α and HIF2α isoforms in lung responses to oxygen insufficiency. In particular, we will discuss novel findings regarding their role in the biology of smooth muscle cells and endothelial cells in the context of hypoxia-induced pulmonary vasoconstriction. Moreover, we will also discuss recent studies into HIF-dependent responses in the airway epithelium, which have been even less studied than the HIF-dependent vascular responses in the lungs. In summary, we will review the biological functions executed by HIF1 or HIF2 in the pulmonary vessels and epithelium to control lung responses to oxygen fluctuations as well as their pathological consequences in the hypoxic lung.

Pulmonary Arterial Hypertension: Iron Matters

The interplay between iron and oxygen is longstanding and central to all aerobic life. Tight regulation of these interactions including homeostatic regulation of iron utilization ensures safe usage of this limited resource. However, when control is lost adverse events can ensue, which are known to contribute to an array of disease processes. Recently, associations between disrupted iron homeostasis and pulmonary artery hypertension (PAH) have been described with the suggestion that there is a contributory link with disease. This review provides a background for iron regulation in humans, describes PAH classifications, and discusses emerging literature, which suggests a role for disrupted iron homeostatic control in various sub-types of PAH, including a role for decompartmentalization of hemoglobin. Finally, the potential for therapeutic options to restore iron homeostatic balance in PAH are discussed.

Human hypoxic pulmonary vasoconstriction is unaltered by 8 h of preceding isocapnic hyperoxia

Exposure to sustained hypoxia of 8 h duration increases the sensitivity of the pulmonary vasculature to acute hypoxia, but it is not known whether exposure to sustained hyperoxia affects human pulmonary vascular control. We hypothesized that exposure to 8 h of hyperoxia would diminish the hypoxic pulmonary vasoconstriction (HPV) that occurs in response to a brief exposure to hypoxia. Eleven healthy volunteers were studied in a crossover protocol with randomization of order. Each volunteer was exposed to acute isocapnic hypoxia (end‐tidal PO2 = 50 mmHg for 10 min) before and after 8 h of hyperoxia (end‐tidal PO2 = 420 mmHg) or euoxia (end‐tidal PO2 = 100 mmHg). After at least 3 days, each volunteer returned and was exposed to the other condition. Systolic pulmonary artery pressure (an index of HPV) and cardiac output were measured, using Doppler echocardiography. Eight hours of hyperoxia had no effect on HPV or the response of cardiac output to acute hypoxia.

Phase I Dose-Escalation Trial of PT2385, a First-in-Class Hypoxia-Inducible Factor-2α Antagonist in Patients With Previously Treated Advanced Clear Cell Renal Cell Carcinoma

Purpose The von Hippel-Lindau tumor suppressor is inactivated in the majority of clear cell renal cell carcinomas (ccRCCs), leading to inappropriate stabilization of hypoxia-inducible factor-2α (HIF-2α). PT2385 is a first-in-class HIF-2α antagonist. Objectives of this first-in-human study were to characterize the safety, pharmacokinetics, pharmacodynamics, and efficacy, and to identify the recommended phase II dose (RP2D) of PT2385. Patients and Methods Eligible patients had locally advanced or metastatic ccRCC that had progressed during one or more prior regimens that included a vascular endothelial growth factor inhibitor. PT2385 was administered orally at twice-per-day doses of 100 to 1,800 mg, according to a 3 + 3 dose-escalation design, followed by an expansion phase at the RP2D. Results The dose-escalation and expansion phases enrolled 26 and 25 patients, respectively. Patients were heavily pretreated, with a median of four (range, one to seven) prior therapies. No dose-limiting toxicity was observed at any dose. On the basis of safety, pharmacokinetic, and pharmacodynamic profiling, the RP2D was defined as 800 mg twice per day. PT2385 was well tolerated, with anemia (grade 1 to 2, 35%; grade 3, 10%), peripheral edema (grade 1 to 2, 37%; grade 3, 2%), and fatigue (grade 1 to 2, 37%; no grade 3 or 4) being the most common treatment-emergent adverse events. No patients discontinued treatment because of adverse events. Complete response, partial response, and stable disease as best response were achieved by 2%, 12%, and 52% of patients, respectively. At data cutoff, eight patients remained in the study, with 13 patients in the study for ≥ 1 year. Conclusion PT2385 has a favorable safety profile and is active in patients with heavily pretreated ccRCC, validating direct HIF-2α antagonism for the treatment of patients with ccRCC.

Cardiopulmonary phenotype associated with human PHD2 mutation

Oxygen‐dependent regulation of the erythropoietin gene is mediated by the hypoxia‐inducible factor (HIF) family of transcription factors. When oxygen is plentiful, HIF undergoes hydroxylation by a family of oxygen‐dependent prolyl hydroxylase domain (PHD) proteins, promoting its association with the von Hippel‐Lindau (VHL) ubiquitin E3 ligase and subsequent proteosomal degradation. When oxygen is scarce, the PHD enzymes are inactivated, leading to HIF accumulation and upregulation not only of erythropoietin expression, but also the expression of hundreds of other genes, including those coordinating cardiovascular and ventilatory adaptation to hypoxia. Nevertheless, despite the identification of over 50 mutations in the PHD‐HIF‐VHL pathway in patients with previously unexplained congenital erythrocytosis, there are very few reports of associated cardiopulmonary abnormalities. We now report exaggerated pulmonary vascular and ventilatory responses to acute hypoxia in a 35‐year‐old man with erythrocytosis secondary to heterozygous mutation in PHD2, the most abundant of the PHD isoforms. We compare this phenotype with that reported in patients with the archetypal disorder of cellular oxygen sensing, Chuvash polycythemia, and discuss the possible clinical implications of our findings, particularly in the light of the emerging role for small molecule PHD inhibitors in clinical practice.

Recent advances in oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological reaction, which adapts lung perfusion to regional ventilation and optimizes gas exchange. Impaired HPV may cause systemic hypoxemia, while generalized HPV contributes to the development of pulmonary hypertension. The triggering mechanisms underlying HPV are still not fully elucidated. Several hypotheses are currently under debate, including a possible decrease as well as an increase in reactive oxygen species as a triggering event. Recent findings suggest an increase in the production of reactive oxygen species in pulmonary artery smooth muscle cells by complex III of the mitochondrial electron transport chain and occurrence of oxygen sensing at complex IV. Other essential components are voltage-dependent potassium and possibly L-type, transient receptor potential channel 6, and transient receptor potential vanilloid 4 channels. The release of arachidonic acid metabolites appears also to be involved in HPV regulation. Further investigation of the HPV mechanisms will facilitate the development of novel therapeutic strategies for the treatment of HPV-related disorders.

Therapeutic targeting of the HIF oxygen-sensing pathway: Lessons learned from clinical studies

The oxygen-sensitive hypoxia-inducible factor (HIF) pathway plays a central role in the control of erythropoiesis and iron metabolism. The discovery of prolyl hydroxylase domain (PHD) proteins as key regulators of HIF activity has led to the development of inhibitory compounds that are now in phase 3 clinical development for the treatment of renal anemia, a condition that is commonly found in patients with advanced chronic kidney disease. This review provides a concise overview of clinical effects associated with pharmacologic PHD inhibition and was written in memory of Professor Lorenz Poellinger.

Systems biology of oxygen homeostasis

Metazoan species maintain oxygen homeostasis through the activity of hypoxia-inducible factors, which are transcriptional activators that regulate the expression of hundreds of genes to match O₂ supply and demand. Here, we review the involvement of hypoxia-inducible factors in the molecular physiology and pathophysiology of cellular O₂ sensing, O₂ delivery, O₂ utilization, and systemic O₂ sensing. WIREs Syst Biol Med 2017, 9:e1382. doi: 10.1002/wsbm.1382 For further resources related to this article, please visit the WIREs website.

Hypoxic Pulmonary Vasoconstriction: From Molecular Mechanisms to Medicine

Hypoxic pulmonary vasoconstriction (HPV) is a homeostatic mechanism that is intrinsic to the pulmonary vasculature. Intrapulmonary arteries constrict in response to alveolar hypoxia, diverting blood to better-oxygenated lung segments, thereby optimizing ventilation/perfusion matching and systemic oxygen delivery. In response to alveolar hypoxia, a mitochondrial sensor dynamically changes reactive oxygen species and redox couples in pulmonary artery smooth muscle cells (PASMC). This inhibits potassium channels, depolarizes PASMC, activates voltage-gated calcium channels, and increases cytosolic calcium, causing vasoconstriction. Sustained hypoxia activates rho kinase, reinforcing vasoconstriction, and hypoxia-inducible factor (HIF)-1α, leading to adverse pulmonary vascular remodeling and pulmonary hypertension (PH). In the nonventilated fetal lung, HPV diverts blood to the systemic vasculature. After birth, HPV commonly occurs as a localized homeostatic response to focal pneumonia or atelectasis, which optimizes systemic Po2 without altering pulmonary artery pressure (PAP). In single-lung anesthesia, HPV reduces blood flow to the nonventilated lung, thereby facilitating thoracic surgery. At altitude, global hypoxia causes diffuse HPV, increases PAP, and initiates PH. Exaggerated or heterogeneous HPV contributes to high-altitude pulmonary edema. Conversely, impaired HPV, whether due to disease (eg, COPD, sepsis) or vasodilator drugs, promotes systemic hypoxemia. Genetic and epigenetic abnormalities of this oxygen-sensing pathway can trigger normoxic activation of HIF-1α and can promote abnormal metabolism and cell proliferation. The resulting pseudohypoxic state underlies the Warburg metabolic shift and contributes to the neoplasia-like phenotype of PH. HPV and oxygen sensing are important in human health and disease.

Discovery of a murine model of clinical PAH: Mission impossible?

Pulmonary arterial hypertension (PAH) is a lung vascular disease characterized with a progressive increase of pulmonary vascular resistance and obliterative pulmonary vascular remodeling resulting in right heart failure and premature death. In this brief review, we document the recent advances in identifying genetically modified murine models of PH, with a focus on the recent discovery of the mouse model of Tie2 Cre-mediated deletion of prolyl hydroxylase 2, which exhibits progressive obliterative vascular remodeling, severe PAH, and right heart failure, thus recapitulating many of the features of clinical PAH. We will also discuss the translational potential of recent findings arising from experimental studies of murine PH models.

Gene panel sequencing improves the diagnostic work-up of patients with idiopathic erythrocytosis and identifies new mutations

Erythrocytosis is a rare disorder characterized by increased red cell mass and elevated hemoglobin concentration and hematocrit. Several genetic variants have been identified as causes for erythrocytosis in genes belonging to different pathways including oxygen sensing, erythropoiesis and oxygen transport. However, despite clinical investigation and screening for these mutations, the cause of disease cannot be found in a considerable number of patients, who are classified as having idiopathic erythrocytosis. In this study, we developed a targeted next-generation sequencing panel encompassing the exonic regions of 21 genes from relevant pathways (~79 Kb) and sequenced 125 patients with idiopathic erythrocytosis. The panel effectively screened 97% of coding regions of these genes, with an average coverage of 450×. It identified 51 different rare variants, all leading to alterations of protein sequence, with 57 out of 125 cases (45.6%) having at least one of these variants. Ten of these were known erythrocytosis-causing variants, which had been missed following existing diagnostic algorithms. Twenty-two were novel variants in erythrocytosis-associated genes (EGLN1, EPAS1, VHL, BPGM, JAK2, SH2B3) and in novel genes included in the panel (e.g. EPO, EGLN2, HIF3A, OS9), some with a high likelihood of functionality, for which future segregation, functional and replication studies will be useful to provide further evidence for causality. The rest were classified as polymorphisms. Overall, these results demonstrate the benefits of using a gene panel rather than existing methods in which focused genetic screening is performed depending on biochemical measurements: the gene panel improves diagnostic accuracy and provides the opportunity for discovery of novel variants.

The Zinc Finger of Prolyl Hydroxylase Domain Protein 2 Is Essential for Efficient Hydroxylation of Hypoxia-Inducible Factor α

Prolyl hydroxylase domain protein 2 (PHD2) (also known as EGLN1) is a key oxygen sensor in mammals that posttranslationally modifies hypoxia-inducible factor α (HIF-α) and targets it for degradation. In addition to its catalytic domain, PHD2 contains an evolutionarily conserved zinc finger domain, which we have previously proposed recruits PHD2 to the HSP90 pathway to promote HIF-α hydroxylation. Here, we provide evidence that this recruitment is critical both in vitro and in vivo We show that in vitro, the zinc finger can function as an autonomous recruitment domain to facilitate interaction with HIF-α. In vivo, ablation of zinc finger function by a C36S/C42S Egln1 knock-in mutation results in upregulation of the erythropoietin gene, erythrocytosis, and augmented hypoxic ventilatory response, all hallmarks of Egln1 loss of function and HIF stabilization. Hence, the zinc finger ordinarily performs a critical positive regulatory function. Intriguingly, the function of this zinc finger is impaired in high-altitude-adapted Tibetans, suggesting that their adaptation to high altitude may, in part, be due to a loss-of-function EGLN1 allele. Thus, these findings have important implications for understanding both the molecular mechanism of the hypoxic response and human adaptation to high altitude.