HIF and the lung: role of hypoxia-inducible factors in pulmonary development and disease

Increased lung volume in infants and toddlers at high compared to low altitude

Children and adults residing at high altitude (HA) compared to low altitude (LA) have larger lung volumes; however, it is unknown whether this response to chronic hypoxia begins early in life. Our objective was to determine whether infants and toddlers at HA have larger lung volumes compared to infants and toddlers at LA. Oxygen saturation (SaO2 ), functional residual capacity (FRC), as well as serum levels of vascular endothelial growth factor (VEGF) and erythropoietin (EPO) were measured in infants and toddlers from HA (N = 50; 3,440 m) and LA (N = 35; 440 m). There were no significant differences in somatic size for HA and LA subjects; however, HA subjects had significantly lower SaO2 (88.5% vs. 96.7%; P < 0.0001). Subjects at HA had significantly greater FRC compared to subjects at LA (group mean: 209 and 157 ml; P < 0.0001), adjusting for body length. Male infants at HA had a significantly greater FRC compared to males at LA (57 ml; P-value < 0.001); however, the increase in FRC for females at HA compared to LA was not significant (20 ml; P-value = 0.101). VEGF and EPO were significantly higher for subjects at HA compared to LA with no gender differences. In summary, infants and toddlers at HA have lower oxygen saturations, higher serum levels of VEGF and EPO, and higher FRC compared to subjects at LA; however, chronic hypoxia appears to generate a more robust response in lung growth in male compared to female infants early in life.

Inhibition of hypoxia-inducible factor prolyl hydroxylase domain oxygen sensors: tricking the body into mounting orchestrated survival and repair responses

Hypoxia-inducible factor (HIF) is an oxygen-sensitive dimeric transcription factor that responds to pathophysiologically low O2 tensions via up-regulation, which leads to an orchestrated biological response to hypoxia. The HIF prolyl hydroxylase domain (PHD) enzymes are non-heme, iron-containing dioxygenases requiring for activity both molecular oxygen and 2-oxoglutarate that, under normoxia, selectively hydroxylate proline residues of HIF, initiating proteosomal degradation of the latter. The dependence of HIF protein levels on the concentration of O2 present, mediated by the PHD enzymes, forms the basis for one of the most significant biological sensor systems of tissue oxygenation in response to ischemic and inflammatory events. Consequently, pharmacological inhibition of PHD enzymes, leading to stabilization of HIF, may be of considerable therapeutic potential in treating conditions of tissue stress and injury. This Perspective reviews the PHDs and small molecule drug discovery efforts. A critical view of this challenging field is offered, which addresses potential concerns and highlights exciting possibilities for the future.

Early lung development: lifelong effect on respiratory health and disease

Interest in the contribution of changes in lung development during early life to subsequent respiratory morbidity is increasing. Most evidence of an association between adverse intrauterine factors and structural effects on the developing lung is from animal studies. Such evidence has been augmented by epidemiological studies showing associations between insults to the developing lung during prenatal and early postnatal life and adult respiratory morbidity or reduced lung function, and by physiological studies that have elucidated mechanisms underlying these associations. The true effect of early insults on subsequent respiratory morbidity can be understood only if the many prenatal and postnatal factors that can affect lung development are taken into account. Adverse factors affecting lung development during fetal life and early childhood reduce the attainment of maximum lung function and accelerate lung function decline in adulthood, initiating or worsening morbidity in susceptible individuals. In this Review, we focus on factors that adversely affect lung development in utero and during the first 5 years after birth, thereby predisposing individuals to reduced lung function and increased respiratory morbidity throughout life. We focus particularly on asthma and COPD.

Thioredoxin-1 mediates hypoxia-induced pulmonary artery smooth muscle cell proliferation

Pathological pulmonary artery smooth muscle cell (PASMC) proliferation contributes to pulmonary vascular remodeling in pulmonary hypertensive diseases associated with hypoxia. Both the hypoxia-inducible factor (HIF) and phosphatidylinositol 3-kinase (PI3K)/serine/threonine kinase (Akt) pathways have been implicated in hypoxia-induced PASMC proliferation. Thioredoxin-1 (Trx1) is a ubiquitously expressed protein that is involved in redox-dependent signaling via HIF and PI3K-Akt in cancer. The role of Trx1 in PASMC proliferation has not been elucidated. The present studies tested the hypothesis that Trx1 regulates hypoxia-induced PASMC proliferation via HIF and/or PI3K- and Akt-dependent mechanisms. Following exposure to chronic hypoxia, our data indicate that Trx1 activity is increased in adult murine lungs. Furthermore, hypoxia-induced increases in cellular proliferation are correlated with increased Trx1 expression, HIF activation, and Akt activation in cultured human PASMC. Both small-interfering RNA-mediated knockdown and pharmacological Trx1 inhibition attenuated hypoxia-induced PASMC proliferation, HIF activation, and Akt activation. While Trx1 knockdown suppressed hypoxia-induced PI3K-Akt activation in PASMC, PI3K-Akt inhibition prevented hypoxia-induced proliferation but had no effect on hypoxia-induced increases in Trx1 or HIF activation. Thus, our findings indicate that Trx1 contributes to hypoxia-induced PASMC proliferation by modulating HIF activation and subsequent PI3K-Akt activation. These novel data suggest that Trx1 might represent a novel therapeutic target to prevent hypoxic PASMC proliferation.

Early life influences on the development of chronic obstructive pulmonary disease

There is increasing evidence that chronic obstructive pulmonary disease (COPD) is not simply a disease of old age that is largely restricted to heavy smokers, but may be associated with insults to the developing lung during foetal life and the first few years of postnatal life, when lung growth and development are rapid. A better understanding of the long-term effects of early life factors, such as intrauterine growth restriction, prenatal and postnatal exposure to tobacco smoke and other pollutants, preterm delivery and childhood respiratory illnesses, on the subsequent development of chronic respiratory disease is imperative if appropriate preventive and management strategies to reduce the burden of COPD are to be developed. The extent to which insults to the developing lung are associated with increased risk of COPD in later life depends on the underlying cause, timing and severity of such derangements. Suboptimal conditions in utero result in aberrations of lung development such that affected individuals are born with reduced lung function, which tends to remain diminished throughout life, thereby increasing the risk both of wheezing disorders during childhood and subsequent COPD in genetically susceptible individuals. If the current trend towards the ever-increasing incidence of COPD is to be reversed, it is essential to minimize risks to the developing lung by improvements in antenatal and neonatal care, and to reduce prenatal and postnatal exposures to environmental pollutants, including passive tobacco smoke. Furthermore, adult physicians need to recognize that lung disease is potentially associated with early life insults and provide better education regarding diet, exercise and avoidance of smoking to preserve precious reserves of lung function in susceptible adults. This review focuses on factors that adversely influence lung development in utero and during the first 5 years of life, thereby predisposing to subsequent COPD.

Hypoxia-inducible factor pathway and diseases of the vascular wall

BACKGROUND

Hypoxia may contribute to the pathogenesis of various diseases of the vascular wall. Hypoxia-inducible factors (HIFs) are nuclear transcriptional factors that regulate the transcription of genes that mediate cellular and tissue homeostatic responses to altered oxygenation. This article reviews the published literature on and discusses the role of the HIF pathway in diseases involving the vascular wall, including atherosclerosis, arterial aneurysms, pulmonary hypertension, vascular graft failure, chronic venous diseases, and vascular malformation.

METHODS

PubMed was searched with the terms "hypoxia-inducible factor" or "HIF" and "atherosclerosis," "carotid stenosis," "aneurysm," "pulmonary artery hypertension," "varicose veins," "venous thrombosis," "graft thrombosis," and "vascular malformation."

RESULTS

In atherosclerotic plaque, HIF-1α was localized in macrophages and smooth muscle cells bordering the necrotic core. Increased HIF-1α may contribute to atherosclerosis through alteration of smooth muscle cell proliferation and migration, angiogenesis, and lipid metabolism. The expression of HIF-1α is significantly elevated in aortic aneurysms compared with nonaneurysmal arteries. In pulmonary hypertension, HIF-1α contributes to the increase of intracellular K(+) and Ca(2+) leading to vasoconstriction of pulmonary smooth muscle cells. Alteration of the HIF pathway may contribute to vascular graft failure through the formation of intimal hyperplasia. In chronic venous disease, HIF pathway dysregulation contributes to formation of varicose veins and venous thromboembolism. However, whether the activation of the HIF pathway is protective or destructive to the venous wall is unclear. Increased activation of the HIF pathway causes aberrant expression of angiogenic factors contributing to the formation and maintenance of vascular malformations.

CONCLUSIONS

Pathologic vascular wall remodelling of many common diseases of the blood vessels has been found to be associated with altered activity of the HIF pathway. Therefore, understanding the role of the HIF pathway in diseases of the vascular wall is important to identify novel therapeutic strategies in the management of these pathologies.

MicroRNA in the Diseased Pulmonary Vasculature: Implications for the Basic Scientist and Clinician

Since the first descriptions of their active functions more than ten years ago, small non-coding RNA species termed microRNA (miRNA) have emerged as essential regulators in a broad range of adaptive and maladaptive cellular processes. With an exceptionally rapid pace of discovery in this field, the dysregulation of many individual miRNAs has been implicated in the development and progression of various cardiovascular diseases. MiRNA are also expected to play crucial regulatory roles in the progression of pulmonary vascular diseases such as pulmonary hypertension (PH), yet direct insights in this field are only just emerging. This review will provide an overview of pulmonary hypertension and its molecular mechanisms, tailored for both basic scientists studying pulmonary vascular biology and physicians who manage PH in their clinical practice. We will describe the pathobiology of pulmonary hypertension and mechanisms of action of miRNA relevant to this disease. Moreover, we will summarize the potential roles of miRNA as biomarkers and therapeutic targets as well as future strategies for defining the cooperative actions of these powerful effectors in pulmonary vascular disease.

Hypoxia inducible factor 3α plays a critical role in alveolarization and distal epithelial cell differentiation during mouse lung development

Lung development occurs under relative hypoxia and the most important oxygen-sensitive response pathway is driven by Hypoxia Inducible Factors (HIF). HIFs are heterodimeric transcription factors of an oxygen-sensitive subunit, HIFα, and a constitutively expressed subunit, HIF1β. HIF1α and HIF2α, encoded by two separate genes, contribute to the activation of hypoxia inducible genes. A third HIFα gene, HIF3α, is subject to alternative promoter usage and splicing, leading to three major isoforms, HIF3α, NEPAS and IPAS. HIF3α gene products add to the complexity of the hypoxia response as they function as dominant negative inhibitors (IPAS) or weak transcriptional activators (HIF3α/NEPAS). Previously, we and others have shown the importance of the Hif1α and Hif2α factors in lung development, and here we investigated the role of Hif3α during pulmonary development. Therefore, HIF3α was conditionally expressed in airway epithelial cells during gestation and although HIF3α transgenic mice were born alive and appeared normal, their lungs showed clear abnormalities, including a post-pseudoglandular branching defect and a decreased number of alveoli. The HIF3α expressing lungs displayed reduced numbers of Clara cells, alveolar epithelial type I and type II cells. As a result of HIF3α expression, the level of Hif2α was reduced, but that of Hif1α was not affected. Two regulatory genes, Rarβ, involved in alveologenesis, and Foxp2, a transcriptional repressor of the Clara cell specific Ccsp gene, were significantly upregulated in the HIF3α expressing lungs. In addition, aberrant basal cells were observed distally as determined by the expression of Sox2 and p63. We show that Hif3α binds a conserved HRE site in the Sox2 promoter and weakly transactivated a reporter construct containing the Sox2 promoter region. Moreover, Hif3α affected the expression of genes not typically involved in the hypoxia response, providing evidence for a novel function of Hif3α beyond the hypoxia response.

Vascular remodeling in pulmonary hypertension

Pulmonary hypertension is a complex, progressive condition arising from a variety of genetic and pathogenic causes. Patients present with a spectrum of histologic and pathophysiological features, likely reflecting the diversity in underlying pathogenesis. It is widely recognized that structural alterations in the vascular wall contribute to all forms of pulmonary hypertension. Features characteristic of the remodeled vasculature in patients with pulmonary hypertension include increased stiffening of the elastic proximal pulmonary arteries, thickening of the intimal and/or medial layer of muscular arteries, development of vaso-occlusive lesions, and the appearance of cells expressing smooth muscle-specific markers in normally non-muscular small diameter vessels, resulting from proliferation and migration of pulmonary arterial smooth muscle cells and cellular transdifferentiation. The development of several animal models of pulmonary hypertension has provided the means to explore the mechanistic underpinnings of pulmonary vascular remodeling, although none of the experimental models currently used entirely replicates the pulmonary arterial hypertension observed in patients. Herein, we provide an overview of the histological abnormalities observed in humans with pulmonary hypertension and in preclinical models and discuss insights gained regarding several key signaling pathways contributing to the remodeling process. In particular, we will focus on the roles of ion homeostasis, endothelin-1, serotonin, bone morphogenetic proteins, Rho kinase, and hypoxia-inducible factor 1 in pulmonary arterial smooth muscle and endothelial cells, highlighting areas of cross-talk between these pathways and potentials for therapeutic targeting.

Epigenetic regulation of miR-17~92 contributes to the pathogenesis of pulmonary fibrosis

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a disease of progressive lung fibrosis with a high mortality rate. In organ repair and remodeling, epigenetic events are important. MicroRNAs (miRNAs) regulate gene expression post-transcriptionally and can target epigenetic molecules important in DNA methylation. The miR-17~92 miRNA cluster is critical for lung development and lung epithelial cell homeostasis and is predicted to target fibrotic genes and DNA methyltransferase (DNMT)-1 expression.

OBJECTIVES

We investigated the miR-17~92 cluster expression and its role in regulating DNA methylation events in IPF lung tissue.

METHODS

Expression and DNA methylation patterns of miR-17~92 were determined in human IPF lung tissue and fibroblasts and fibrotic mouse lung tissue. The relationship between the miR-17~92 cluster and DNMT-1 expression was examined in vitro. Using a murine model of pulmonary fibrosis, we examined the therapeutic potential of the demethylating agent, 5'-aza-2'-deoxycytidine.

MEASUREMENTS AND MAIN RESULTS

Compared with control samples, miR-17~92 expression was reduced in lung biopsies and lung fibroblasts from patients with IPF, whereas DNMT-1 expression and methylation of the miR-17~92 promoter was increased. Several miRNAs from the miR-17~92 cluster targeted DNMT-1 expression resulting in a negative feedback loop. Similarly, miR-17~92 expression was reduced in the lungs of bleomycin-treated mice. Treatment with 5'-aza-2'-deoxycytidine in a murine bleomycin-induced pulmonary fibrosis model reduced fibrotic gene and DNMT-1 expression, enhanced miR-17~92 cluster expression, and attenuated pulmonary fibrosis.

CONCLUSIONS

This study provides insight into the pathobiology of IPF and identifies a novel epigenetic feedback loop between miR-17~92 and DNMT-1 in lung fibrosis.

Hypoxia-induced inflammation in the lung: a potential therapeutic target in acute lung injury?

Acute lung injury (ALI) is a severe form of hypoxic lung disease responsible for a large number of deaths worldwide. Despite recent advances in supportive care, no reduction in mortality has been evident since the introduction of a standard consensus definition almost two decades ago. New strategies are urgently required to help design effective therapies for this condition. A key pathological feature of ALI involves regional alveolar hypoxia. Because alveolar hypoxia in isolation, such as that encountered at high altitude, causes an inflammatory pulmonary phenotype in the absence of any other pathogenic stimuli, these regions may not be passive bystanders but may actually contribute to the pathogenesis and progression of lung injury. Unique transcriptional responses to hypoxia in the lung apparently allow it to express an inflammatory phenotype at levels of hypoxia that would not produce such a response in other organs. We will review recent advances in our understanding of these unique transcriptional responses to moderate levels of alveolar hypoxia, which may provide new insights into the pathogenesis of ALI.

The role of Connexin 46 promoter in lens and other hypoxic tissues

Gap junctions are multimeric membrane protein channels that connect the cytoplasm of one cell to another. Much information about connexins regards electrophysiology and channel function but relatively little information is known about non-channel functions of connexins. Lens connexins, Cx43, Cx46 and Cx50, have been extensively studied for their role in lens homeostasis. Connexins allow the movement of small metabolically relevant molecules and ions between cells and this action in the lens prevents cataract formation. Interruption of Cx46 channel function leads to cataract formation due to dysregulation of lens homeostasis. The loss of Cx46 upregulates Cx43 in lens cell culture and suppresses tumor growth in breast and retinoblastoma tumor xenografts. Upregulation of Cx46 in hypoxic tissues has been noted and may be due in part to the effects of hypoxia and HIF activators. Here, we report that the Cx46 promoter is regulated by hypoxia and also offer speculation about the role of Cx46 in lens differentiation and solid tumor growth.

Identification of genes underlying hypoxia tolerance in Drosophila by a P-element screen

Hypoxia occurs in physiologic conditions (e.g. high altitude) or during pathologic states (e.g. ischemia). Our research is focused on understanding the molecular mechanisms that lead to adaptation and survival or injury to hypoxic stress using Drosophila as a model system. To identify genes involved in hypoxia tolerance, we screened the P-SUP P-element insertion lines available for all the chromosomes of Drosophila. We screened for the eclosion rates of embryos developing under 5% O(2) condition and the number of adult flies surviving one week after eclosion in the same hypoxic environment. Out of 2187 lines (covering ~1870 genes) screened, 44 P-element lines representing 44 individual genes had significantly higher eclosion rates (i.e. >70%) than those of the controls (i.e. ~7-8%) under hypoxia. The molecular function of these candidate genes ranged from cell cycle regulation, DNA or protein binding, GTP binding activity, and transcriptional regulators. In addition, based on pathway analysis, we found these genes are involved in multiple pathways, such as Notch, Wnt, Jnk, and Hedgehog. Particularly, we found that 20 out of the 44 candidate genes are linked to Notch signaling pathway, strongly suggesting that this pathway is essential for hypoxia tolerance in flies. By employing the UAS/RNAi-Gal4 system, we discovered that genes such as osa (linked to Wnt and Notch pathways) and lqf (Notch regulator) play an important role in survival and development under hypoxia in Drosophila. Based on these results and our previous studies, we conclude that hypoxia tolerance is a polygenic trait including the Notch pathway.

MicroRNA-199a-5p is associated with hypoxia-inducible factor-1α expression in lungs from patients with COPD

BACKGROUND

MicroRNAs (miRNAs) are small noncoding RNAs that silence target gene expression posttranscriptionally, and their impact on gene expression has been reported in various diseases. It has been reported that the expression of the hypoxia-inducible factor-1α (HIF-1α) is reduced and that of p53 is increased in lungs from patients with COPD. However, the role of miRNAs associated with these genes in lungs from patients with COPD is unknown.

METHODS

Lung tissue samples from 55 patients were included in this study. Total RNA, miRNA, and protein were extracted from lung tissues and used for reverse transcriptase polymerase chain reaction and Western blot analysis. Cell culture experiments were performed using cultured human pulmonary microvascular endothelial cells (HPMVECs).

RESULTS

miR-34a and miR-199a-5p expressions were increased, and the phosphorylation of AKT was decreased in the lung tissue samples of patients with COPD. The miR-199a-5p expression was correlated with HIF-1α protein expression in the lungs of patients with COPD. Transfection of HPMVECs with the miR-199a-5p precursor gene decreased HIF-1α protein expression, and transfection with the miR-34a precursor gene increased miR-199a-5p expression.

CONCLUSIONS

These data suggest that miR-34a and miR-199a-5p contribute to the pathogenesis of COPD, and these miRNAs may also affect the HIF-1α-dependent lung structure maintenance program.

Severe pulmonary arterial hypertension induced by SU5416 and ovalbumin immunization

The combination of chronic hypoxia and treatment of rats with the vascular endothelial growth factor (VEGF) receptor blocker, SU5416, induces pulmonary angio-obliteration, resulting in severe pulmonary arterial hypertension (PAH). Inflammation is thought to contribute to the pathology of PAH. Allergic inflammation caused by ovalbumin (OVA) immunization causes muscularization of pulmonary arteries, but not severe PAH. Whether disturbance of the immune system and allergic inflammation in the setting of lung endothelial cell apoptosis causes PAH is unknown. We investigated the effects of OVA-allergic inflammation on the development of PAH initiated by VEGF blockade-induced lung endothelial cell apoptosis. OVA-immunized rats were treated with SU5416 to induce pulmonary vascular endothelial cell apoptosis. The combination of OVA and SU5416 treatment resulted in severe angio-obilterative PAH, accompanied by increased IL-6 expression in the lungs. c-Kit(+) and Sca-1(+) cells were found in and around the lung vascular lesions. Pan-caspase inhibiton, dexamethasone treatment, and depletion of B-lymphocytes using an anti-CD20 antibody suppressed this remodeling. OVA immunization also increased lung tissue hypoxia-induced factor-1α and VEGF expression. Our results also suggest that the increased expression of hypoxia-induced factor-1α and IL-6 induced by the allergic lung inflammation may be a component of the pathogenesis of PAH.

Differential modulation of surfactant protein D under acute and persistent hypoxia in acute lung injury

Hypoxia contributes to the development of fibrosis with epithelial-mesenchymal transition (EMT) via stimulation of hypoxia-inducible factor 1α (HIF-1α) and de novo twist expression. Although hypoxemia is associated with increasing levels of surfactant protein D (SP-D) in acute lung injury (ALI), the longitudinal effects of hypoxia on SP-D expression in lung tissue injury/fibrosis have not been fully evaluated. Here, the involvement of hypoxia and SP-D modulation was evaluated in a model of bleomycin-induced lung injury. We also investigated the molecular mechanisms by which hypoxia might modulate SP-D expression in alveolar cells, by using a doxycycline (Dox)-dependent HIF-1α expression system. Tissue hypoxia and altered SP-D levels were present in bleomycin-induced fibrotic lesions. Acute hypoxia induced SP-D expression, supported by the finding that Dox-induced expression of HIF-1α increased SP-D expression. In contrast, persistent hypoxia repressed SP-D expression coupled with an EMT phenotype and twist expression. Long-term expression of HIF-1α caused SP-D repression with twist expression. Ectopic twist expression repressed SP-D expression. The longitudinal observation of hypoxia and SP-D levels in ALI in vivo was supported by the finding that HIF-1α expression stabilized by acute hypoxia induced increasing SP-D expression in alveolar cells, whereas persistent hypoxia induced de novo twist expression in these cells, causing repression of SP-D and acquisition of an EMT phenotype. Thus this is the first study to demonstrate the molecular mechanisms, in which SP-D expression under acute and persistent hypoxia in acute lung injury might be differentially modulated by stabilized HIF-1α expression and de novo twist expression.

Redox regulation of mitochondrial function

Redox-dependent processes influence most cellular functions, such as differentiation, proliferation, and apoptosis. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this review, we examine the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that alter mitochondrial function and the role of these redox modifications on metabolism, mitochondrial biogenesis, receptor-mediated signaling, and apoptotic pathways. We also consider the role of mitochondria in modulating these pathways, and discuss how redox-dependent events may contribute to pathobiology by altering mitochondrial function.

Hypoxia-inducible factors in physiology and medicine

Oxygen homeostasis represents an organizing principle for understanding metazoan evolution, development, physiology, and pathobiology. The hypoxia-inducible factors (HIFs) are transcriptional activators that function as master regulators of oxygen homeostasis in all metazoan species. Rapid progress is being made in elucidating homeostatic roles of HIFs in many physiological systems, determining pathological consequences of HIF dysregulation in chronic diseases, and investigating potential targeting of HIFs for therapeutic purposes.

Targeting energetic metabolism: a new frontier in the pathogenesis and treatment of pulmonary hypertension

This perspective highlights advances in the understanding of the role of cellular metabolism in the pathogenesis of pulmonary hypertension. Insights gained in the past 20 years have revealed several similarities between the cellular processes underlying the pulmonary vascular remodeling in pulmonary hypertension and those seen in cancer processes. In line with these insights, there is increasing recognition that abnormal cellular metabolism, notably of aerobic glycolysis (the "Warburg effect"), the potential involvement of hypoxia-inducible factor in this process, and alterations in mitochondrial function, are key elements in the pathogenesis of this disease. The glycolytic shift may underlie the resistance to apoptosis and increased vascular cell proliferation, which are hallmarks of pulmonary hypertension. These investigations have led to novel approaches in the diagnosis and therapy of pulmonary hypertension.

Digoxin inhibits development of hypoxic pulmonary hypertension in mice

Chronic hypoxia is an inciting factor for the development of pulmonary arterial hypertension. The mechanisms involved in the development of hypoxic pulmonary hypertension (HPH) include hypoxia-inducible factor 1 (HIF-1)-dependent transactivation of genes controlling pulmonary arterial smooth muscle cell (PASMC) intracellular calcium concentration ([Ca(2+)](i)) and pH. Recently, digoxin was shown to inhibit HIF-1 transcriptional activity. In this study, we tested the hypothesis that digoxin could prevent and reverse the development of HPH. Mice were injected daily with saline or digoxin and exposed to room air or ambient hypoxia for 3 wk. Treatment with digoxin attenuated the development of right ventricle (RV) hypertrophy and prevented the pulmonary vascular remodeling and increases in PASMC [Ca(2+)](i), pH, and RV pressure that occur in mice exposed to chronic hypoxia. When started after pulmonary hypertension was established, digoxin attenuated the hypoxia-induced increases in RV pressure and PASMC pH and [Ca(2+)](i). These preclinical data support a role for HIF-1 inhibitors in the treatment of HPH.

YC-1 attenuates hypoxia-induced pulmonary arterial hypertension in mice

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by a progressive increase in pulmonary vascular resistance and elevation of pulmonary arterial pressure, leading to right ventricular failure and eventual death. Currently, no curative therapy for PAH is available, and the overall prognosis is very poor. Recently, direct activators of soluble guanylyl cyclase (sGC) have been tested as a novel therapeutic modality in experimental models of pulmonary arterial hypertension (PAH).

OBJECTIVE

In this study, we used in vitro and in vivo models to evaluate the therapeutic potential of 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), a dual functioning chemical, as a direct activator of guanylyl cyclase and an inhibitor of hypoxia-inducible factor-1.

METHODS

We analyzed the effects of YC-1 on cell proliferation and the levels of p21 and p53 in human pulmonary artery smooth muscle cells (HPASMCs) under hypoxia. We also determined the effects of YC-1 on expression of endothelin-1 (ET-1) and phosphorylation status of endothelial nitric oxide synthase (eNOS) at Ser(1179) in human pulmonary artery endothelial cells (HPAECs) under hypoxia. In mice, hypoxic PAH was induced by exposure to normobaric hypoxic conditions for 28 days. To assess preventive or therapeutic effects, randomized mice were subjected to once daily i.p. injections of YC-1 for the entire hypoxic period (5 mg/kg) or for the last seven days of a 28-day hypoxic period (5 and 10 mg/kg). On day 28, we measured the right ventricular systolic pressure (RVSP) and determined the degrees of right ventricular hypertrophy (RVH) and vascular remodeling.

RESULTS

In HPASMCs, YC-1 inhibited hypoxia-induced proliferation and induction of p53 and p21 in a concentration-dependent manner. Also, YC-1 suppressed the hypoxia-induced expression of ET-1 mRNA and dephosphorylation of eNOS at Ser(1179) in HPAECs. In the preventive in vivo model, a daily dose of 5 mg/kg YC-1 significantly prevented the elevation of RVSP, development of RVH, and pulmonary vascular remodeling, which were caused by hypoxic exposure. In the therapeutic model, YC-1 at daily doses of 5 and 10 mg/kg alleviated RVH and pulmonary vascular remodeling but did not prevent the elevation of RVSP.

CONCLUSIONS

Our results indicate that YC-1 prevents the development of hypoxia-induced PAH in a preventive model and alleviates RVH and pulmonary vascular remodeling in a therapeutic model. Therefore, these data imply that YC-1 has therapeutic potential for use in a single or combination therapy for PAH.

COPD/emphysema: The vascular story

In this perspective, we review published data which support the concept that many or most chronic and progressive lung diseases also involve the lung vessels and that microvascular abnormalities and endothelial cell death contribute to the pathobiology of emphysema. Lung vessel maintenance depends on Vascular Endothelial Growth Factor signaling and both are compromised in the emphysematous lung tissue. Although hypoxic pulmonary vasoconstriction has been considered as an important factor contributing to the vascular remodeling in chronic obstructive pulmonary disease (COPD) (COPD/emphysema, it is now clear that inhaled cigarette smoke can damage the lung vessels independent of the lung vascular tone. We propose that a "sick lung circulation" rather than the right heart afterload may better explain the cardiac abnormalities in COPD patients which are usually summarized with the term "cor pulmonale." The mechanisms and causes of pulmonary hypertension are likely complex and include vessel loss, in situ thrombosis, and endothelial cell dysfunction. Assessment of the functional importance of pulmonary hypertension in COPD requires hemodynamic measurements during exercise.

HIF prolyl hydroxylase inhibitors for anemia

INTRODUCTION

The hypoxia-inducible factor (HIF) system mediates the body's response to hypoxia, locally, inducing angiogenesis and a shift to anaerobic metabolism, and systemically, increasing red cell mass in anemia. HIF prolyl hydroxylases (HIF-PH) modify HIF, decreasing its activity. Increasing HIF activity through inhibition of HIF-PH may provide an alternative treatment for anemia and may protect against damage related to ischemia-reperfusion.

AREAS COVERED

The review discusses the basic science underpinnings of the HIF system and the clinical effects of the HIF system and its pharmacologic manipulation.

EXPERT OPINION

Manipulation of the HIF system may improve outcomes in anemia by bypassing the effective iron deficiency found in anemia of chronic disease and by increasing red cell mass without supraphysiologic increases in erythropoietin. HIF-PH may also find a clinical use in the prevention of ischemia-reperfusion damage in strokes, cardiac ischemia, ischemic renal failure, etc.