Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1alpha

Heterozygous deficiency of hypoxia-inducible factor-2alpha protects mice against pulmonary hypertension and right ventricular dysfunction during prolonged hypoxia

Chronic hypoxia induces pulmonary vascular remodeling, leading to pulmonary hypertension, right ventricular hypertrophy, and heart failure. Heterozygous deficiency of hypoxia-inducible factor-1alpha (HIF-1alpha), which mediates the cellular response to hypoxia by increasing expression of genes involved in erythropoiesis and angiogenesis, has been previously shown to delay hypoxia-induced pulmonary hypertension. HIF-2alpha is a homologue of HIF-1alpha and is abundantly expressed in the lung, but its role in pulmonary hypertension remains unknown. Therefore, we analyzed the pulmonary response of WT and viable heterozygous HIF-2alpha-deficient (Hif2alpha(+/-)) mice after exposure to 10% O(2) for 4 weeks. In contrast to WT mice, Hif2alpha(+/-) mice were fully protected against pulmonary hypertension and right ventricular hypertrophy, unveiling a critical role of HIF-2alpha in hypoxia-induced pulmonary vascular remodeling. Pulmonary expression levels of endothelin-1 and plasma catecholamine levels were increased threefold and 12-fold respectively in WT but not in Hif2alpha(+/-) mice after hypoxia, suggesting that HIF-2alpha-mediated upregulation of these vasoconstrictors contributes to the development of hypoxic pulmonary vascular remodeling.

In vivo gene transfer of the O2-sensitive potassium channel Kv1.5 reduces pulmonary hypertension and restores hypoxic pulmonary vasoconstriction in chronically hypoxic rats

BACKGROUND

Alveolar hypoxia acutely elicits pulmonary vasoconstriction (HPV). Chronic hypoxia (CH), despite attenuating HPV, causes pulmonary hypertension (CH-PHT). HPV results, in part, from inhibition of O2-sensitive, voltage-gated potassium channels (Kv) in pulmonary artery smooth muscle cells (PASMCs). CH decreases Kv channel current/expression and depolarizes and causes Ca2+ overload in PASMCs. We hypothesize that Kv gene transfer would normalize the pulmonary circulation (restore HPV and reduce CH-PHT), despite ongoing hypoxia.

METHODS AND RESULTS

Adult male Sprague-Dawley rats were exposed to normoxia or CH for 3 to 4 weeks and then nebulized orotracheally with saline or adenovirus (Ad5) carrying genes for the reporter, green fluorescent protein reporter+/-human Kv1.5 (cloned from normal PA). HPV was assessed in isolated lungs. Hemodynamics, including Fick and thermodilution cardiac output, were measured in vivo 3 and 14 days after gene therapy by use of micromanometer-tipped catheters. Transgene expression, measured by quantitative RT-PCR, was confined to the lung, persisted for 2 to 3 weeks, and did not alter endogenous Kv1.5 levels. Ad5-Kv1.5 caused no mortality or morbidity, except for sporadic, mild elevation of liver transaminases. Ad5-Kv1.5 restored the O2-sensitive K+ current of PASMCs, normalized HPV, and reduced pulmonary vascular resistance. Pulmonary vascular resistance decreased at day 2 because of increased cardiac output, and remained reduced at day 14, at which time there was concomitant regression of right ventricular hypertrophy and PA medial hypertrophy.

CONCLUSIONS

Kv1.5 is an important O2-sensitive channel and potential therapeutic target in PHT. Kv1.5 gene therapy restores HPV and improves PHT. This is, to the best of our knowledge, the first example of K+ channel gene therapy for a vascular disease.

Hypoxia and High Altitude

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

HIF and oxygen sensing; as important to life as the air we breathe?

Molecular oxygen (O2)is a basic requirement for cellular growth and viability and many aspects of anatomy and physiology are dedicated to achieving reliable distribution. Recent work has identified a specific sensing and response system, centred around a transcription complex called Hypoxia-inducible Factor 1 (HIF-1), which forms the focus of this review. The HIF-system operates in all cell types and modulates a very broad range of cellular pathways, consistent with the broad importance of oxygen. It is implicated in a rapidly expanding range of developmental, physiological and pathological settings, and is potentially relevant to almost all areas of clinical medicine. Excitingly, the pathway can be activated with low molecular weight compounds which should offer therapeutic benefit, especially in diseases where oxygen supply is compromised.

Differential display analysis of presenilin 1-deficient mouse brains

Missense mutations in presenilin 1 (PS1) gene are the most common cause of early onset familial Alzheimer's disease (FAD). AD pathogenic PS1 mutations result in elevated gamma-secretase cleavage of APP and diminished S3-site cleavage of Notch. We have previously described a PS1-hypomorphic mouse line that could survive postnatally with markedly reduced gamma-secretase cleavage of APP and S3-site cleavage of Notch, resulting in a Notch developmental phenotype similar to PS1-null mice. This model was exploited to identify genes whose expression is altered due to the loss of PS1. A global gene expression study by differential display was performed on whole brains of PS1-hypomorphic mice and their wild type siblings. In total, more than 16,000 bands corresponding to cDNAs were compared between the mutant and wild-type brains. This analysis identified 19 cDNAs showing significantly altered expression resulting from PS1 deficiency. Four of the identified cDNAs corresponded to genes that could be associated with AD or presenilin function. Hypoxia inducible factor 1a (Hif1a), NPRAP (delta-catenin) and cell division cycle 10 (CDC10) showed significantly reduced expression in the PS1-hypomorphic compared to wild-type brains, whereas expression of nucleoside diphosphate kinase sub-unit A (NDPK-A) was markedly elevated in the respective brains. Clarification of the possible role of these genes in AD and the basis for their differential expression induced by PS1-deficiency may provide insight into the disease, presenilin function and consequences of its loss, as well as possible deleterious effects of AD therapeutics aimed at inhibiting PS1.

Molecular and cellular basis of pulmonary vascular remodeling in pulmonary hypertension

Clinical pulmonary hypertension is characterized by a sustained elevation in pulmonary arterial pressure. Pulmonary vascular remodeling involves structural changes in the normal architecture of the walls of pulmonary arteries. The process of vascular remodeling can occur as a primary response to injury, or stimulus such as hypoxia, within the resistance vessels of the lung. Alternatively, the changes seen in more proximal vessels may arise secondary to a sustained increase in intravascular pressure. To withstand the chronic increase in intraluminal pressure, the vessel wall becomes thickened and stronger. This "armouring" of the vessel wall with extra-smooth muscle and extracellular matrix leads to a decrease in lumen diameter and reduced capacity for vasodilatation. This maladaptive response results in increased pulmonary vascular resistance and consequently, sustained pulmonary hypertension. The process of pulmonary vascular remodeling involves all layers of the vessel wall and is complicated by the finding that cellular heterogeneity exists within the traditional compartments of the vascular wall: intima, media, and adventitia. In addition, the developmental stage of the organism greatly modifies the response of the pulmonary circulation to injury. This review focuses on the latest advances in our knowledge of these processes as they relate to specific forms of pulmonary hypertension and particularly in the light of recent genetic studies that have identified specific pathways involved in the pathogenesis of severe pulmonary hypertension.

Hypoxic pulmonary artery fibroblasts trigger proliferation of vascular smooth muscle cells: role of hypoxia-inducible transcription factors

Chronic lung hypoxia causes vascular remodeling with pulmonary artery smooth muscle cell (SMCPA) hyperplasia, resulting in pulmonary hypertension and cor pulmonale. We investigated SMCPA and pulmonary artery adventitial fibroblasts (FBPA) for their proliferative response to hypoxia. Strong SMCPA growth occurred under hypoxic conditions in SMCPA/FBPA co-cultures, but not in SMCPA monocultures. SMCPA growth was fully reproduced by transferring serum-free supernatant from hypoxic cultured FBPA to normoxic SMCPA. Hypoxia-inducible-transcription-factor subtypes (HIF-1alpha, HIF-2alpha, HIF-3alpha) and its dependent target genes, carrying the hypoxia-responsive-element as regulatory component, were strongly activated in both hypoxic FBPA and SMCPA. HIF-transcription-factor decoy technique, employed to FBPA during hypoxic culturing, blocked the mitogenic activity of FBPA conditioned medium on SMCPA. The data suggest that hypoxia-driven gene regulation in pulmonary artery fibroblasts results in a mitogenic stimulus on adjacent pulmonary artery smooth muscle cells, and HIF-transcription-decoy may offer a new therapeutic approach to suppress these events.

Oxygen-dependent signaling in pulmonary vascular smooth muscle

The pulmonary circulation constricts in response to acute hypoxia, which is reversible on reexposure to oxygen. On exposure to chronic hypoxia, in addition to vasoconstriction, the pulmonary vasculature undergoes remodeling, resulting in a sustained increase in pulmonary vascular resistance that is not immediately reversible. Hypoxic pulmonary vasoconstriction is physiological in the fetus, and there are many mechanisms by which the pulmonary vasculature relaxes at birth, principal among which is the acute increase in oxygen. Oxygen-induced signaling mechanisms, which result in pulmonary vascular relaxation at birth, and the mechanisms by which chronic hypoxia results in pulmonary vascular remodeling in the fetus and adult, are being investigated. Here, the roles of cGMP-dependent protein kinase in oxygen-mediated signaling in fetal pulmonary vascular smooth muscle and the effects of chronic hypoxia on ion channel activity and smooth muscle function such as contraction, growth, and gene expression were discussed.

Multiple sites of oxygen sensing and their contributions to hypoxic pulmonary vasoconstriction

Oxygen sensing by the pulmonary vasculature is important for the regulation of vessel tone and the matching of lung perfusion to ventilation. Airways hypoxia is a major stimulus for vasoconstriction, which diverts blood from hypoxic alveoli to better ventilated areas of the lung. Several hypotheses have emerged to explain how pulmonary arteries sense a decrease in oxygen and mediate hypoxic pulmonary vasoconstriction (HPV). They differ mainly in where they place the main site of HPV: in the endothelial or smooth muscle cells of the artery wall. HPV probably results from synergistic actions on both cell types, but it can proceed in the absence of endothelium, suggesting that the primary oxygen sensor is the smooth muscle cell and endothelium-derived agents modulate the muscle response. Several oxygen-sensing targets have been identified in smooth muscle, including potassium channels, Ca(2+) stores in the sarcoplasmic reticulum (SR) and the Ca(2+) sensitivity of the contractile proteins. The evidence for different oxygen-sensing mechanisms in pulmonary vessels is discussed.

HIF-1 and tumor progression: pathophysiology and therapeutics

Hypoxia-inducible factor 1 (HIF-1) controls oxygen delivery (via angiogenesis) and metabolic adaptation to hypoxia (via glycolysis). HIF-1 consists of a constitutively expressed HIF-1 beta subunit and an oxygen- and growth-factor-regulated HIF-1 alpha subunit. In xenografts, tumor growth and angiogenesis are correlated with HIF-1 expression. In human cancers, HIF-1 alpha is overexpressed as a result of intratumoral hypoxia and genetic alterations affecting key oncogenes and tumor suppressor genes. HIF-1 alpha overexpression in biopsies of brain, breast, cervical, esophageal, oropharyngeal and ovarian cancers is correlated with treatment failure and mortality. Increased HIF-1 activity promotes tumor progression, and inhibition of HIF-1 could represent a novel approach to cancer therapy.

Functional activation of heat shock factor and hypoxia-inducible factor in the kidney

Renal ischemia is the result of a complex series of events, including decreases in oxygen supply (hypoxia) and the availability of cellular energy (ATP depletion). In this study, the functional activation of two stress-responsive transcription factors, i.e., heat shock factor-1 (HSF-1) and hypoxia-inducible factor-1 (HIF-1), in the kidney was assessed. When rats were subjected to 45 min of renal ischemia, electrophoretic mobility shift assays of kidney nuclear extracts revealed rapid activation of both HIF-1 and HSF. Western blot analyses further demonstrated that this activation resulted in increased expression of the HSF and HIF-1 target genes heat shock protein-72 and heme oxygenase-1, respectively. Whether hypoxia or ATP depletion alone could produce similar activation patterns in vitro was then investigated. Renal epithelial LLC-PK(1) cells were subjected to either ATP depletion (0.1 microM antimycin A and glucose deprivation) or hypoxia (1% O(2)). After ATP depletion, HSF was rapidly activated (within 30 min), whereas HIF-1 was unaffected. In contrast, hypoxia led to the activation of HIF-1 but not HSF. Hypoxic activation of HIF-1 was observed within 30 min and persisted for 4 h, whereas no HSF activation was detected even with prolonged periods of hypoxia. HIF-1 was transcriptionally active in LLC-PK(1) cells, as demonstrated by luciferase reporter gene assays using the vascular endothelial growth factor promoter or a synthetic promoter construct containing three hypoxia-inducible elements. Interestingly, intracellular ATP levels were not affected by hypoxia but were significantly reduced by ATP depletion. These findings suggest that HIF-1 is activated specifically by decreased O(2) concentrations and not by reduced ATP levels alone. In contrast, HSF is activated primarily by metabolic stresses associated with ATP depletion and not by isolated O(2) deprivation. In vivo, the two transcription factors are simultaneously activated during renal ischemia, which might account for observed differences between in vivo and in vitro epithelial cell injury and repair. Selective modulation of either pathway might therefore be of potential interest for modification of the response of the kidney to ischemia, as well as the processes involved in recovery from ischemia.

Cellular adaptation to hypoxia: O2-sensing protein hydroxylases, hypoxia-inducible transcription factors, and O2-regulated gene expression

Although it was known for a long time that oxygen deprivation leads to the transcriptional induction of the gene encoding erythropoietin, the molecular mechanisms behind this process remained enigmatic. The cloning of the hypoxia-inducible factors (HIFs), the finding that HIF-1 regulates the expression of many more genes apart from erythropoietin, and the elucidation of the oxygen-dependent mechanisms degrading the HIF alpha subunits recently led to the spectacular discovery of the molecular principles of oxygen sensing. This review aims to summarize our current knowledge of oxygen-regulated gene expression..

Glucose uptake and adenoviral mediated GLUT1 infection decrease hypoxia-induced HIF-1alpha levels in cardiac myocytes

Hypoxia causes a large array of adaptive and physiological responses in all cells including cardiac myocytes. In order to elucidate the molecular effects of increased glucose flux on hypoxic cardiac myocytes we focused on the basic helix-loop-helix transcription factor, hypoxia inducible factor 1 alpha (HIF-1alpha), which is rapidly upregulated in hypoxic cells and elicits a number of responses including augmentation of glucose uptake. Primary cultures of neonatal rat cardiac myocytes as well as embryonic rat heart-derived myogenic H9c2 cells demonstrated a significant upregulation of HIF-1alpha when subjected to hypoxia of 6-8h in the absence of glucose. Re-addition of extracellular glucose to the medium resulted in a decrease of HIF-1alpha levels by almost 50%. This glucose effect was blocked by addition of glycolytic inhibitors. In addition, glucose uptake and glycolysis resulted in substantial decreased levels of p53, which is regulated by HIF-1alpha. Adenoviral infection of cultures of cardiac myocytes with the facilitative glucose transporter, GLUT1 followed by hypoxia of 24h also resulted in a significant reduction in the protein expression of HIF-1alpha compared to control vector-infected cultures. GLUT1 infected cultures also demonstrated fewer apoptotic cells and a reduction in the release of cytochrome c after hypoxia. Inhibition of the ubiquitin-proteasomal pathway by a variety of 26S proteasomal inhibitors increased HIF-1alpha to similar levels under both normoxic and hypoxic conditions and in the presence or absence of glucose. This result suggested that glucose induces HIF-1alpha degradation via a proteasomal pathway. This conclusion was substantiated by immunoprecipitation experiments of total cell extracts, which demonstrated an increase of ubiquitinated HIF-1alpha relative to total HIF-1alpha in the presence of glucose during hypoxia. Thus, glucose as well as GLUT1 overexpression diminishes hypoxia-induced HIF-1alpha protein via an ubiquitin-proteasomal pathway in hypoxic cardiac myocytes. This represents a novel feedback mechanism that may play an important role in adaptation of cardiac myocytes to hypoxia and ischemia.

Hypoxia-inducible factor in brain

HIF-1 is composed of HIF-1alpha and HIF-1beta protein subunits. HIF-1 is induced by hypoxia and binds to promoter/enhancer elements and stimulates the transcription of hypoxia-inducible target genes. Because HIF-1 activation might promote cell survival in hypoxic tissues, we studied the effect of stroke on the expression of HIF-1alpha, HIF-1beta and several HIF-1 target genes in adult rat brain. After focal cerebral ischemia, mRNAs encoding HIF-1alpha, glucose transporter-1 and several glycolytic enzymes including lactate dehydrogenase were up-regulated in the areas around the infarction. HIF and its target genes were induced by 7.5 hours after the onset of ischemia and increased further at 19 and 24 hours. Since hypoxia induces HIF in other tissues, systemic hypoxia (6% O2 for 4.5 h) was also shown to increase HIF-1alpha protein expression in the adult rat brain. It is proposed that decreased blood flow to the penumbra decreases the supply of oxygen and that this induces HIF-1 and its target genes. Because HIF-1 activation may promote cell survival in hypoxic tissues, we studied the effect of hypoxic preconditioning on HIF-1 expression in neonatal rat brain. Hypoxic preconditioning (8% O2/3 hrs), a treatment known to protect the newborn rat brain against hypoxic-ischemic injury, markedly increased HIF-1alpha and HIF-1beta expression. We also studied the effect of two other known HIF-1 inducers, cobalt chloride (CoCl2) and desferrioxamine (DFX), on HIF-1 expression and neuroprotection in newborn brain. HIF-1alpha and HIF-1beta protein levels were markedly increased after i.p. injection of CoCl2 and DFX. Preconditioning with CoCl2 or DFX 24 hours before the stroke decreased infarction by 75% and 56% respectively, compared with vehicle-injected, littermate controls. Thus, HIF-1 activation could contribute to protective brain preconditioning.

Expression of hypoxia-inducible factor 1alpha in tumours of patients with glioblastoma

Angiogenesis is a prominent feature of glioblastomas but the mechanisms involved in the control of this process are poorly understood. We have investigated the potential role of a recently described transcription factor, hypoxia-inducible factor 1 (HIF-1), which initiates the transcription of a number of hypoxia-inducible genes, including those encoding vascular endothelial growth factor and its receptors. HIF-1 protein expression was assessed by immunocytochemistry, using a monoclonal antibody to the alpha subunit (HIF-1alpha). HIF-1 mRNA expression was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR) and the ribonuclease protection assay (RPA). Strong nuclear expression of HIF-1alpha protein was seen in the majority of glioblastomas and anaplastic astrocytomas, particularly surrounding areas of necrosis in glioblastomas. In the majority of these tumours upregulation of HIF-1alpha mRNA was also demonstrated, with a significant increase in glioblastomas compared to lower grade tumours. No correlation was found between the presence of HIF-1alpha protein and immunohistochemical expression of p53 protein. These findings are in keeping with an important role of HIF-1alpha in the vascularization of glioblastomas and suggest that upregulation is at least partly at a transcriptional level.

Desferrioxamine elevates pulmonary vascular resistance in humans: potential for involvement of HIF-1

Hypoxia-inducible factor (HIF)-1 is stabilized by hypoxia and iron chelation. We hypothesized that HIF-1 might be involved in pulmonary vascular regulation and that infusion of desferrioxamine over 8 h would consequently mimic hypoxia and elevate pulmonary vascular resistance. In study A, we characterized the pulmonary vascular response to 4 h of isocapnic hypoxia; in study B, we measured the pulmonary vascular response to 8 h of desferrioxamine infusion. For study A, 11 volunteers undertook two protocols: 1) 4 h of isocapnic hypoxia (end-tidal PO(2) = 50 Torr), followed by 2 h of recovery with isocapnic euoxia (end-tidal PO(2) = 100 Torr), and 2) 6 h of air breathing (control). For study B, nine volunteers undertook two protocols while breathing air: 1) continuous infusion of desferrioxamine (4 g/70 kg) over 8 h and 2) continuous infusion of saline over 8 h (control). In both studies, pulmonary vascular resistance was assessed at 0.5- to 1-h intervals by Doppler echocardiography via the maximum pressure gradient during systole across the tricuspid valve. Results show a progressive rise in pressure gradient over the first 3-4 h with both isocapnic hypoxia (P < 0.001) and desferrioxamine infusion (P < 0.005) to increases of ~16 and 4 Torr, respectively. These results support a role for HIF-regulated gene activation in human hypoxic pulmonary vasoconstriction.

Hypoxia and carbon monoxide in the vasculature

Hypoxia is sensed by all mammalian cells and elicits a variety of adaptive and pathophysiological responses at the molecular and cellular level. For the pulmonary vasculature, hypoxia causes increased vasoconstriction and vessel-wall remodeling. These responses are mediated by complex intracellular cascades leading to altered gene expression and cell-cell interaction. Hypoxia transiently increases the transcriptional rate of the heme oxygenase-1 (HO-1) gene, resulting in increased production of carbon monoxide (CO) and bilirubin. CO has vasodilatory and antiinflammatory properties in the vasculature, whereas bilirubin is an antioxidant. Both enzymatic products could thus modulate the hypoxic cellular response. Accumulating data suggest that CO inhibits the hypoxic induction of genes encoding vasoconstrictors and smooth muscle cell mitogens in the early hypoxic phase. During chronic hypoxia, low CO levels tilt the balance toward increased production of growth factors and vasoconstrictors that promote vessel-wall remodeling. Mice null in the HO-1 gene manifest decreased tolerance to hypoxia with right ventricular dilatation and infarction, whereas targeted lung overexpression of HO-1 prevents hypoxia-induced inflammatory responses and protects against the development of pulmonary hypertension. Such observations point to CO as a critical modulator of the body's adaptive responses to hypoxia.

Pulmonary hypertension in patients with chronic obstructive pulmonary disease: recent advances in pathophysiology and management

In patients with chronic obstructive pulmonary disease (COPD), pulmonary hypertension (PH) is associated with a worse prognosis. Recently, information has been increasing concerning the cellular and molecular aspects of the pathophysiology of PH in COPD. The most striking finding is the role of vascular endothelial cells and endogenous mediators released by these cells. Endothelial cell-dependent relaxation is impaired in COPD patients with PH. Moreover, vascular remodelling in these patients is mainly responsible for irreversible PH in advanced COPD. Smoking cessation will slow down the progression of the disease process and may prevent the development of PH in COPD. The timing of initiation of long-term oxygen therapy is important for the effective management of PH in COPD. Research on therapeutic agents for the effective treatment of PH is still needed in the management aspect of patients with COPD. This review focuses on the recent advances in our understanding of the pathophysiology and treatment of PH in COPD.

Inhibition of PPAR gamma 2 gene expression by the HIF-1-regulated gene DEC1/Stra13: a mechanism for regulation of adipogenesis by hypoxia

Cellular differentiation involves transcriptional responses to environmental stimuli. Adipocyte differentiation is inhibited under hypoxic conditions, indicating that oxygen (O(2)) is an important physiological regulator of adipogenesis. Hypoxia inhibits PPAR gamma 2 nuclear hormone receptor transcription, and overexpression of PPAR gamma 2 or C/EBP beta stimulates adipogenesis under hypoxia. Mouse embryonic fibroblasts deficient in hypoxia-inducible transcription factor 1 alpha (HIF-1 alpha) are refractory to hypoxia-mediated inhibition of adipogenesis. The HIF-1-regulated gene DEC1/Stra13, a member of the Drosophila hairy/Enhancer of split transcription repressor family, represses PPAR gamma 2 promoter activation and functions as an effector of hypoxia-mediated inhibition of adipogenesis. These data indicate that an O(2)-sensitive signaling mechanism regulates adipogenesis. Thus, agents that regulate HIF-1 activity or O(2) sensing may be used to inhibit adipogenesis and control obesity.

Overexpression of the serine elastase inhibitor elafin protects transgenic mice from hypoxic pulmonary hypertension

BACKGROUND

Increased serine elastase activity has been implicated in the vascular remodeling associated with chronic hypoxia-related pulmonary hypertension in rats.

METHODS AND RESULTS

In this study we determined the time course of hypoxia-induced serine elastase activity in the murine lung and related this to initiation of a proteolytic cascade characterized by an increase in matrix metalloproteinases (MMPs). We then used transgenic mice in which overexpression of the selective serine elastase inhibitor elafin was targeted to the cardiovascular system to determine whether upregulation of a naturally occurring serine elastase inhibitor suppresses MMPs and the hemodynamic and structural response to chronic hypoxia (air at 380 mm Hg). In nontransgenic but not in elafin-transgenic mice, we documented a transient increase in serine elastase activity after 12 hours of hypoxic exposure attributed to a 30-kDa protein as determined by elastin zymography and fluorophosphonate/fluorophosphate-biotin labeling. Two days after hypoxia, the pro-forms of MMP-2 and MMP-9 were induced in the nontransgenic mice, but MMP-9 was suppressed in elafin-transgenic mice. Acute hypoxic vasoconstriction was similar in nontransgenic and elafin-transgenic littermates. Chronic hypoxia for 26 days resulted in >1-fold increase in right ventricular pressure (P<0.004) in nontransgenic compared with control or elafin-transgenic littermates. In the latter mice, normalization of the right ventricular pressure was associated with reduced muscularization and preservation of the number of distal vessels (P<0.04 for both comparisons).

CONCLUSIONS

Modulation of the severity of chronic hypoxia-induced pulmonary vascular disease could be a function of endogenously expressed serine elastase inhibitors.

Defective carotid body function and impaired ventilatory responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1 alpha

To investigate whether the transcriptional activator hypoxia-inducible factor 1 (HIF-1) is required for ventilatory responses to hypoxia, we analyzed mice that were either wild type or heterozygous for a loss-of-function (knockout) allele at the Hif1a locus, which encodes the O(2)-regulated HIF-1 alpha subunit. Although the ventilatory response to acute hypoxia was not impaired in Hif1a(+/-) mice, the response was primarily mediated via vagal afferents, whereas in wild-type mice, carotid body chemoreceptors played a predominant role. When carotid bodies isolated from wild-type mice were exposed to either cyanide or hypoxia, a marked increase in sinus nerve activity was recorded, whereas carotid bodies from Hif1a(+/-) mice responded to cyanide but not to hypoxia. Histologic analysis revealed no abnormalities of carotid body morphology in Hif1a(+/-) mice. Wild-type mice exposed to hypoxia for 3 days manifested an augmented ventilatory response to a subsequent acute hypoxic challenge. In contrast, prior chronic hypoxia resulted in a diminished ventilatory response to acute hypoxia in Hif1a(+/-) mice. Thus partial HIF-1 alpha deficiency has a dramatic effect on carotid body neural activity and ventilatory adaptation to chronic hypoxia.

A model of sleep-disordered breathing in the C57BL/6J mouse

To investigate the pathophysiological sequelae of sleep-disordered breathing (SDB), we have developed a mouse model in which hypoxia was induced during periods of sleep and was removed in response to arousal or wakefulness. An on-line sleep-wake detection system, based on the frequency and amplitude of electroencephalograph and electromyograph recordings, served to trigger intermittent hypoxia during periods of sleep. In adult male C57BL/6J mice (n = 5), the sleep-wake detection system accurately assessed wakefulness (97.2 +/- 1.1%), non-rapid eye movement (NREM) sleep (96.0 +/- 0.9%) and rapid eye movement (REM) sleep (85.6 +/- 5.0%). After 5 consecutive days of SDB, 554 +/- 29 (SE) hypoxic events were recorded over a 24-h period at a rate of 63.6 +/- 2.6 events/h of sleep and with a duration of 28.2 +/- 0.7 s. The mean nadir of fraction of inspired O(2) (FI(O(2))) on day 5 was 13.2 +/- 0.1%, and 137.1 +/- 13.2 of the events had a nadir FI(O(2)) <10% O(2). Arterial blood gases confirmed that hypoxia of this magnitude lead to a significant degree of hypoxemia. Furthermore, 5 days of SDB were associated with decreases in both NREM and REM sleep during the light phase compared with the 24-h postintervention period. We conclude that our murine model of SDB mimics the rate and magnitude of sleep-induced hypoxia, sleep fragmentation, and reduction in total sleep time found in patients with moderate to severe SDB in the clinical setting.

Gene transfer to induce angiogenesis in myocardial and limb ischaemia

Stimulation of angiogenesis/arteriogenesis by gene transfer methods offers hope for treating patients with myocardial and peripheral limb ischaemia who are not candidates for standard revascularisation procedures. Preclinical studies showed that adenoviral and plasmid vectors encoding various angiogenic cytokines were capable of inducing functionally significant angiogenesis in vitro and in animal models of chronic myocardial ischaemia. Early clinical studies using VEGF121-, FGF-4- and VEGF165-encoding vectors showed a reasonable safety profile with promising results. However, significant advances in vector technology including regulatable and longer-term expression, delivery strategies (local and organ/tissue specific), clinical trial design, and outcome measure development are needed before this investigational treatment becomes reality.

Expression of angiogenesis-related molecules in plexiform lesions in severe pulmonary hypertension: evidence for a process of disordered angiogenesis

Pulmonary arteries of patients with severe pulmonary hypertension (SPH) presenting in an idiopathic form (primary PH-PPH) or associated with congenital heart malformations or collagen vascular diseases show plexiform lesions. It is postulated that in lungs with SPH, endothelial cells in plexiform lesions express genes encoding for proteins involved in angiogenesis, in particular, vascular endothelial growth factor (VEGF) and those involved in VEGF receptor-2 (VEGFR-2) signalling. On immunohistochemistry and in situ hybridization, endothelial cells in the plexiform lesions expressed VEGF mRNA and protein and overexpressed the mRNA and protein of VEGFR-2, and the transcription factor subunits HIF-1alpha and HIF-1beta of hypoxia inducible factor, which are responsible for the hypoxia-dependent induction of VEGF. When compared with normal lungs, SPH lungs showed decreased expression of the kinases PI3 kinase and src, which, together with Akt, relay the signal transduction downstream of VEGFR-2. Because markers of angiogenesis are expressed in plexiform lesions in SPH, it is proposed that these lesions may form by a process of disordered angiogenesis.

The pathobiology of pulmonary hypertension. Endothelium

Dysfunctional endothelial cells have a central and critical role in the initiation and progression of severe pulmonary hypertension. The elucidation of the mechanisms involved in the control of endothelial cell proliferation and cell death in the pulmonary vasculature, therefore, is fundamentally important in the pathogenesis of severe pulmonary hypertension and of great interest for a better understanding of endothelial cell biology. Because the intravascular growth of endothelial cells resulting in tumorlets is unique to severe pulmonary hypertension, this phenomenon can teach researchers about the factors involved in the formation and maintenance of the normal endothelial cell monolayer. Clearly, in severe pulmonary hypertension, the "law of the endothelial cell monolayer" has been broken. The ultimate level of such a control is at the altered gene expression pattern that is conducive to endothelial cell growth and disruption of pulmonary blood flow. Secondary pulmonary hypertension certainly also is associated with proliferated pulmonary endothelial cells and plexiform lesions that are histologically indistinguishable from those in PPH. What is then the difference in the mechanisms of endothelial cell proliferation between primary and secondary pulmonary hypertension? The authors believe that PPH is a disease caused by somatic mutations in key angiogenesis- or apoptosis-related genes such as the TGF-beta receptor-2 and Bax. The loss of these important cell growth control mechanisms allows for the clonal expansion of endothelial cells from a single cell that has acquired a selective growth advantage. On the other hand, the proliferated endothelial cells in secondary pulmonary hypertension are polyclonal. It follows from this finding that local (vascular) factor(s) (such as increased shear stress), rather than mutations, play a major role in triggering endothelial cell proliferation. In PPH and secondary pulmonary hypertension, the researcher can postulate that the pulmonary vascular bed contains progenitor-like cells with the capacity of dysregulated growth. The main difference in the pathogenesis of primary and secondary pulmonary endothelial cell proliferation therefore may be the initial mechanism involved in the recruitment of an endothelial progenitor-like cell. In PPH, anorexigen-associated, and familial PPH, the proliferation of endothelial cells occurs from a mutated single cell, whereas in secondary pulmonary hypertension, several progenitor-like cells would be activated to grow. The abnormal endothelial cells in both forms of severe pulmonary hypertension expand because of the expression of angiogenesis-related molecules such as VEGF, VEGFR-2, HIF-1 alpha, and HIF-beta. Also important for the expansion of these cells is the down-regulation of expression of apoptosis-related mediators such as TGF-beta receptor-2 or Bax. The success of any therapy for severe pulmonary hypertension requires that the underlying process of endothelial cell proliferation could be controlled or reversed.

Hypoxia-inducible factor 1: oxygen homeostasis and disease pathophysiology

Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate adaptive responses to reduced oxygen availability. The HIF-1beta subunit is constitutively expressed, whereas the HIF-1alpha subunit is subject to ubiquitination and proteasomal degradation, a process that is inhibited under hypoxic conditions. Recent data indicate that HIF-1 plays major roles in the prevention of myocardial and cerebral ischemia and in the pathogenesis of pulmonary hypertension and cancer. Modulation of HIF-1 activity by genetic or pharmacological means could provide a novel therapeutic approach to these common causes of mortality.

Chronic intermittent hypercapnic hypoxia increases pulmonary arterial pressure and haematocrit in rats

Sleep-disordered breathing is associated with pulmonary hypertension and raised haematocrit. The multiple episodes of apnoea in this condition cause chronic intermittent hypoxia and hypercapnia but the effects of such blood gas changes on pulmonary pressure or haematocrit are unknown. The present investigation tests the hypothesis that chronic intermittent hypercapnic hypoxia causes increased pulmonary arterial pressure and erythropoiesis. Rats were treated with alternating periods of normoxia and hypercapnic hypoxia every 30 s for 8 h per day for 5 days per week for 5 weeks, as a model of the intermittent blood gas changes which occur in sleep-disordered breathing in humans. Haematocrit, red blood cell count and haemoglobin concentration were measured each week and systemic and pulmonary arterial blood pressure and heart weight were measured after 5 weeks. In relation to control, chronic intermittent hypercapnic hypoxia caused a significant increase in systemic (104.3+/-4.7 mmHg versus 121.0+/-10.4 mmHg) and pulmonary arterial pressure (20.7+/-6.8 mmHg versus 31.3+/-7.2 mmHg), right ventricular weight (expressed as ratios) and haematocrit (45.2+/-1.0% versus 51.5+/-1.5%). It is concluded that the pulmonary hypertension and elevated haematocrit associated with sleep-disordered breathing is caused by chronic intermittent hypercapnic hypoxia.

Selected Contribution: Pulmonary hypertension in mice following intermittent hypoxia

Sleep apnea (intermittent periods of hypoxia with or without hypercapnia) is associated with systemic hypertension and increased mortality from cardiovascular disease, but the relationship to pulmonary hypertension is uncertain. Previous studies on intermittent hypoxia (IH) in rats that demonstrated pulmonary hypertension utilized relatively long periods of hypoxia. Recent studies that utilized brief periods of hypoxia have conflicting reports of right ventricular (RV) hypertrophy. In addition, many studies have not measured pulmonary hemodynamics to asses the severity of pulmonary hypertension in vivo. Given the increasing availability of genetically engineered mice and the need to establish a rodent model of IH-induced pulmonary hypertension, we studied the effect of IH (2-min cycles of 10% and 21% O2, 8 h/day, 4 wk) on wild-type mice, correlating in vivo measurements of pulmonary hypertension with RV mass and pulmonary vascular remodeling. RV systolic pressure was increased after IH (36 +/- 0.9 mmHg) compared with normoxia (29.5 +/- 0.6) but was lower than continuous hypoxia (44.2 +/- 3.4). RV mass [RV-to-(left ventricle plus septum) ratio] correlated with pressure measurements (IH = 0.27 +/- 0.02, normoxia = 0.22 +/- 0.01, and continuous hypoxia = 0.34 +/- 0.01). Hematocrits were also elevated after IH and continuous hypoxia (56 +/- 1.6 and 54 +/- 1.1 vs. 44.3 +/- 0.5%). Evidence of neomuscularization of the distal pulmonary circulation was found after IH and continuous hypoxia. We conclude that mice develop pulmonary hypertension following IH, representing a possible animal model of pulmonary hypertension in response to the repetitive hypoxia-reoxygenation of sleep apnea.

Molecular adaptations in human skeletal muscle to endurance training under simulated hypoxic conditions

This study was performed to explore changes in gene expression as a consequence of exercise training at two levels of intensity under normoxic and normobaric hypoxic conditions (corresponding to an altitude of 3,850 m). Four groups of human subjects trained five times a week for a total of 6 wk on a bicycle ergometer. Muscle biopsies were taken, and performance tests were carried out before and after the training period. Similar increases in maximal O(2) uptake (8.3-13.1%) and maximal power output (11.4-20.8%) were found in all groups. RT-PCR revealed elevated mRNA concentrations of the alpha-subunit of hypoxia-inducible factor 1 (HIF-1) after both high- (+82.4%) and low (+78.4%)-intensity training under hypoxic conditions. The mRNA of HIF-1alpha(736), a splice variant of HIF-1alpha newly detected in human skeletal muscle, was shown to be changed in a similar pattern as HIF-1alpha. Increased mRNA contents of myoglobin (+72.2%) and vascular endothelial growth factor (+52.4%) were evoked only after high-intensity training in hypoxia. Augmented mRNA levels of oxidative enzymes, phosphofructokinase, and heat shock protein 70 were found after high-intensity training under both hypoxic and normoxic conditions. Our findings suggest that HIF-1 is specifically involved in the regulation of muscle adaptations after hypoxia training. Fine-tuning of the training response is recognized at the molecular level, and with less sensitivity also at the structural level, but not at global functional responses like maximal O(2) uptake or maximal power output.

Partial HIF-1alpha deficiency impairs pulmonary arterial myocyte electrophysiological responses to hypoxia

Chronic hypoxia depolarizes and reduces K+ current in pulmonary arterial smooth muscle cells (PASMCs). Our laboratory previously demonstrated that hypoxia-inducible factor-1 (HIF-1) contributed to the development of hypoxic pulmonary hypertension. In this study, electrophysiological parameters were measured in PASMCs isolated from intrapulmonary arteries of mice with one null allele at the Hif1a locus encoding HIF-1alpha [Hif1a(+/-)] and from their wild-type [Hif1a(+/+)] littermates after 3 wk in air or 10% O2. Hematocrit and right ventricular wall and left ventricle plus septum weights were measured. Capacitance, K+ current, and membrane potential were measured with whole cell patch clamp. Similar to our laboratory's previous results, hypoxia-induced right ventricular hypertrophy and polycythemia were blunted in Hif1a(+/-) mice. Hypoxia increased PASMC capacitance in Hif1a(+/+) mice but not in Hif1a(+/-) mice. Chronic hypoxia depolarized and reduced K+ current density in PASMCs from Hif1a(+/+) mice. In PASMCs from hypoxic Hif1a(+/-) mice, no reduction in K+ current density was observed, and depolarization was significantly blunted. Thus partial deficiency of HIF-1alpha is sufficient to impair hypoxia-induced depolarization, reduction of K+ current density, and PASMC hypertrophy.

Hypoxia-inducible factor 1: control of oxygen homeostasis in health and disease

Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that mediates changes in gene expression in response to changes in cellular oxygen concentrations. HIF-1 is a heterodimer consisting of an oxygen-regulated HIF-1 alpha subunit and a constitutively expressed HIF-1 beta subunit. In mice, complete HIF-1 alpha deficiency results in embryonic lethality at midgestation because of cardiac and vascular malformations. Analyses of animal and cell culture models as well as human tissue have provided evidence that HIF-1 plays important roles in the pathophysiology of preeclampsia, intrauterine growth retardation, hypoxia-mediated pulmonary hypertension, and cancer. HIF-1 promotes neovascularization in response to myocardial or retinal ischemia by activating transcription of the gene encoding vascular endothelial growth factor. HIF-1 may also mediate the protective response to cerebral ischemia known as late-phase preconditioning.

Gene transfer for angiogenesis in coronary artery disease

Angiogenesis is a promising novel therapeutic strategy to provide new venues for blood flow in patients with severe ischemic heart and peripheral vascular disease, who are not candidates for standard revascularization strategies. We describe the underlying mechanisms involved in physiologic and therapeutic angiogenesis, underscoring the relative importance of vasculogenesis, angiogenesis, and arteriogenesis. We then present the various gene transfer vectors including plasmid, viral, and cell-based vectors, and various delivery modalities. The available preclinical data are presented, followed by a description of preliminary clinical experience, with an emphasis on the preliminary nature of these results, which address safety and not efficacy. Finally, we discuss the promises and pitfalls of clinical angiogenesis and gene transfer studies, stressing the importance of proper design of clinical trials and adequate protection of research subjects.

Chronic hypoxia decreases K(V) channel expression and function in pulmonary artery myocytes

Activity of voltage-gated K+ (KV) channels regulates membrane potential (E(m)) and cytosolic free Ca2+ concentration ([Ca2+](cyt)). A rise in ([Ca2+](cyt))in pulmonary artery (PA) smooth muscle cells (SMCs) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. Chronic hypoxia (PO(2) 30-35 mmHg for 60-72 h) decreased mRNA expression of KV channel alpha-subunits (Kv1.1, Kv1.5, Kv2.1, Kv4.3, and Kv9.3) in PASMCs but not in mesenteric artery (MA) SMCs. Consistently, chronic hypoxia attenuated protein expression of Kv1.1, Kv1.5, and Kv2.1; reduced KV current [I(KV)]; caused E(m) depolarization; and increased ([Ca2+](cyt)) in PASMCs but negligibly affected KV channel expression, increased I(KV), and induced hyperpolarization in MASMCs. These results demonstrate that chronic hypoxia selectively downregulates KV channel expression, reduces I(KV), and induces E(m) depolarization in PASMCs. The subsequent rise in ([Ca2+](cyt)) plays a critical role in the development of pulmonary vasoconstriction and medial hypertrophy. The divergent effects of hypoxia on KV channel alpha-subunit mRNA expression in PASMCs and MASMCs may result from different mechanisms involved in the regulation of KV channel gene expression.

Improved cardiac contractile functions in hypoxia-reoxygenation in rats treated with low concentration Co(2+)

An intracellular mechanism that senses decreases in tissue oxygen level and stimulates hypoxia-related gene expression has been reported in various cell types including the cardiac cell. The mechanism can also be activated by Co(2+) in normoxia. Thus we investigated the effects of prior chronic oral CoCl(2) on mechanical functions of isolated, perfused rat hearts in hypoxia-reoxygenation. In normoxic rats, 43 days of Co(2+) administration increased hematocrit from 45 +/- 0.3% (control, n = 18) to 51 +/- 0.6% (n = 19). In hypoxia and reoxygenation, Co(2+)-pretreated hearts exhibited a significantly higher rate-pressure product (267 and 163%, respectively) and coronary flow (127 and 118%, respectively) and lower end-diastolic pressure (72 and 60%, respectively) compared with the control hearts. Although the oral Co(2+) administration significantly raised myocardial Co(2+) concentration, it did not affect mitochondrial respiration, tissue glycogen concentration, or myocardial tissue histology. The levels of vascular endothelial growth factor, aldolase-A, and glucose transporter-1 mRNA were significantly elevated in the Co(2+)-treated myocardium. We conclude that cardiac contractile functions would gain hypoxic tolerance when the endogenous cellular oxygen-sensing mechanism is activated.

Expression of hypoxia-inducible factor-1alpha in the brain of rats during chronic hypoxia

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates adaptive responses to the lack of oxygen in mammalian cells. HIF-1 consists of two proteins, HIF-1alpha and HIF-1beta. HIF-1alpha accumulates under hypoxic conditions, whereas HIF-1beta is constitutively expressed. HIF-1alpha and HIF-1beta expression were measured during adaptation to hypobaric hypoxia (0.5 atm) in rat cerebral cortex. Western blot analyses indicated that HIF-1alpha rapidly accumulated during the onset of hypoxia and did not fall for 14 days but fell to normal by 21 days despite the continuous low arterial oxygen tension. Immunostaining showed that neurons, astrocytes, ependymal cells, and possibly endothelial cells were the cell types expressing HIF-1alpha. Genes with hypoxia-responsive elements were activated under these conditions, as evidenced by elevated vascular endothelial growth factor and glucose transporter-1 mRNA levels. When 21-day-adapted rats were exposed to a more severe hypoxic challenge (8% oxygen), HIF-1alpha accumulated again. On the basis of these results, we speculate that the vascular remodeling and metabolic changes triggered during prolonged hypoxia are capable of restoring normal tissue oxygen levels.

Regulation of the hypoxia-inducible factor-1 alpha. ARNT is not necessary for hypoxic induction of HIF-1 alpha in the nucleus

Hypoxia-inducible factor-1 (HIF-1) is a master regulator of mammalian oxygen homeostasis. HIF-1 consists of two subunits, HIF-1 alpha and the aryl hydrocarbon receptor nuclear translocator (ARNT). Whereas hypoxia prevents ubiquitination and proteasomal degradation of HIF-1 alpha, ARNT expression is thought to be oxygen-independent. We and others previously showed that ARNT is indispensable for HIF-1 DNA-binding and transactivation function. To examine the requirement of ARNT for accumulation and nuclear translocation of HIF-1 alpha in hypoxia, we used ARNT-mutant mouse hepatoma and ARNT-deficient embryonic stem cells. As shown by immunofluorescence, HIF-1 alpha accumulation in the nucleus of hypoxic cells did not require ARNT, demonstrating that nuclear translocation is intrinsic to HIF-1 alpha. During biochemical separation, both HIF-1 alpha and ARNT tend to leak from the nuclei in the absence of the corresponding subunit, suggesting that heterodimerization is required for stable association within the nuclear compartment. Nuclear stabilization of the heterodimer might also explain the hypoxically increased total cellular ARNT levels observed in some of the cell lines examined.

Hypoxia, HIF-1, and the pathophysiology of common human diseases

Hypoxia plays a fundamental role in the pathophysiology of common causes of mortality, including ischemic heart disease, stroke, cancer, chronic lung disease, and congestive heart failure. In these disease states, hypoxia induces changes in gene expression in target organs that either fail to result in adequate adaptation or directly contribute to disease pathogenesis. Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that is expressed in response to cellular hypoxia and mediates multiple cellular and systemic homeostatic responses to hypoxia. Recent studies have provided evidence that important pathophysiological responses to hypoxia in pulmonary hypertension, myocardial ischemia, and cancer are mediated by HIF-1. Pharmacologic and gene therapy strategies designed to modulate HIF-1 activity may represent a novel and effective therapeutic approach to these common disorders.

O2-sensing after carotid chemodenervation: hypoxic ventilatory responsiveness and upregulation of tyrosine hydroxylase mRNA in brainstem catecholaminergic cells

Ventilatory responses to acute and long-term hypoxia are classically triggered by carotid chemoreceptors. The chemosensory inputs are carried within the carotid sinus nerve to the nucleus tractus solitarius and the brainstem respiratory centres. To investigate whether hypoxia acts directly on brainstem neurons or secondarily via carotid body inputs, we tested the ventilatory responses to acute and long-term hypoxia in rats with bilaterally transected carotid sinus nerves and in sham-operated rats. Because brainstem catecholaminergic neurons are part of the chemoreflex pathway, the ventilatory response to hypoxia was studied in association with the expression of tyrosine hydroxylase (TH). TH mRNA levels were assessed in the brainstem by in situ hybridization and hypoxic ventilatory responses were measured in vivo by plethysmography. After long-term hypoxia, TH mRNA levels in the nucleus tractus solitarius and ventrolateral medulla increased similarly in chemodenervated and sham-operated rats. Ventilatory acclimatization to hypoxia developed in chemodenervated rats, but to a lesser extent than in sham-operated rats. Ventilatory response to acute hypoxia, which was initially low in chemodenervated rats, was fully restored within 21 days in long-term hypoxic rats, as well as in normoxic animals which do not overexpress TH. Therefore, activation of brainstem catecholaminergic neurons and ventilatory adjustments to hypoxia occurred independently of carotid chemosensory inputs. O2-sensing mechanisms unmasked by carotid chemodenervation triggered two ventilatory adjustments: (i) a partial acclimatization to long-term hypoxia associated with TH upregulation; (ii) a complete restoration of acute hypoxic responsivity independent of TH upregulation.

Endothelial cell responses to hypoxia: initiation of a cascade of cellular interactions

The origin of several vascular pathologies involves sudden or recurrent oxygen deficiency. In this review, we examine what the biochemical and molecular responses of the endothelial cells to the lack of oxygen are and how these responses may account for the features observed in pathological situations, mainly by modifications of cell-cell interactions. Two major responses of the endothelial cells have been observed depending on the degree and duration of the oxygen deficiency. Firstly, acute hypoxia rapidly activates the endothelial cells to release inflammatory mediators and growth factors. These inflammatory mediators are able to recruit and promote the adherence of neutrophils to the endothelium where they become activated. The synthesis of platelet-activating factor plays a key role in this adherence process. Secondly, longer periods of hypoxia increase the expression of specific genes such as those encoding some cytokines as well as for the growth factors platelet-derived growth factor and vascular endothelial growth factor. The transcriptional induction of these genes is mediated through the activation of several transcription factors, the most important one being hypoxia inducible factor-1. The link between our knowledge of the signalling cascade of the cellular and molecular events initiated by hypoxia and their involvement in several vascular pathological situations, varicose veins, tumor angiogenesis and pulmonary hypertension is discussed briefly.

Pulmonary expression of early growth response-1: biphasic time course and effect of oxygen concentration

Hypoxia induces complex adaptive responses. In this report, induction of early growth response-1 (Egr-1) transcripts in lungs of mice subjected to hypoxia is shown to be dose and time dependent. Within 30 min of hypoxia, Egr-1 transcripts were approximately 20-fold elevated in 6% oxygen, approximately 5.2-fold increased by 10% oxygen, and returned to the normoxic baseline by 12% oxygen. Time course studies up to 48 h showed a biphasic profile with an initial steep rise in Egr-1 transcripts after 0.5 h of hypoxia and a second elevation beginning after 20-24 h. Hypoxic induction of Egr-1 was paralleled by enhanced expression of the downstream target gene tissue factor. Egr-1 and tissue factor antigen were visualized in bronchial and vascular smooth muscle and in alveolar macrophages. Egr-1 has the capacity to modulate expression of genes involved in the remodeling of the extracellular matrix and properties of smooth muscle, thus possibly contributing to the pulmonary response to chronic hypoxia.

Attenuated hypoxic pulmonary hypertension in mice lacking the 5-hydroxytryptamine transporter gene

Hypoxia is a well-recognized stimulus for pulmonary blood vessel remodeling and pulmonary hypertension development. One mechanism that may account for these effects is the direct action of hypoxia on the expression of specific genes involved in vascular smooth muscle cell (SMC) proliferation. Previous studies demonstrated that the serotonin (5-hydroxytryptamine; 5-HT) transporter (5-HTT) mediates the mitogenic activity of 5-HT in pulmonary vascular SMCs and is overexpressed during hypoxia. Thus, 5-HT-related mitogenic activity is increased during hypoxia. Here, we report that mice deficient for 5-HTT (5-HTT(-/-)) developed less hypoxic pulmonary hypertension and vascular remodeling than paired 5-HTT(+/+) controls. When maintained under normoxia, 5-HTT(-/-)-mutant mice had normal hemodynamic parameters, low blood 5-HT levels, deficient platelet 5-HT uptake, and unchanged blood levels of 5-hydroxyindoleacetic acid, a metabolite of 5-HT. After exposure to 10% O(2) for 2 or 5 weeks, the number and medial wall thickness of muscular pulmonary vessels were reduced in hypoxic 5-HTT(-/-) mice as compared with wild-type paired controls. Concomitantly, right ventricular systolic pressure was lower and right ventricle hypertrophy less marked in the mutant mice. This occurred despite potentiation of acute hypoxic pulmonary vasoconstriction in the 5-HTT(-/-) mice. These data further support a key role of 5-HTT in hypoxia-induced pulmonary vascular SMC proliferation and pulmonary hypertension.

Mammalian oxygen sensing, signalling and gene regulation

Oxygen is essential to the life of all aerobic organisms. Virtually every cell type is able to sense a limited oxygen supply (hypoxia) and specifically to induce a set of oxygen-regulated genes. This review summarizes current concepts of mammalian oxygen-sensing and signal-transduction pathways. Since the discovery of the hypoxia-inducible factors (HIFs), a great deal of progress has been made in our comprehension of how hypoxia induces the expression of oxygen-regulated genes. The alpha subunit of the heterodimeric transcription factors HIF-1, 2 and 3 is unstable under normoxia but is rapidly stabilized upon exposure to hypoxic conditions. Following heterodimerization with the constitutively expressed beta subunit, HIFs activate the transcription of an increasing number of genes involved in maintaining oxygen homeostasis at the cellular, local and systemic levels.

HIF-1: mediator of physiological and pathophysiological responses to hypoxia

All organisms can sense O(2) concentration and respond to hypoxia with adaptive changes in gene expression. The large body size of mammals necessitates the development of multiple complex physiological systems to ensure adequate O(2) delivery to all cells under normal conditions. The transcriptional regulator hypoxia-inducible factor 1 (HIF-1) is an essential mediator of O(2) homeostasis. HIF-1 is required for the establishment of key physiological systems during development and their subsequent utilization in fetal and postnatal life. HIF-1 also appears to play a key role in the pathophysiology of cancer, cardiovascular disease, and chronic lung disease, which represent the major causes of mortality among industrialized societies. Genetic or pharmacological modulation of HIF-1 activity in vivo may represent a novel therapeutic approach to these disorders.

Expression of heme oxygenase-1 in the lung in chronic hypoxia

Heme oxygenase (HO)-1 is an oxygen-dependent enzyme that may regulate vascular tone and cell proliferation through the production of carbon monoxide (CO). We tested the hypothesis that HO-1 is upregulated in the lung in chronic hypoxia by exposing male Sprague-Dawley rats to 17,000 feet (395 Torr) for 0, 1, 3, 7, 14, or 21 days. After exposure, blood gases, carboxyhemoglobin (COHb) levels, and hematocrit were measured, and the lungs were either inflation fixed for immunohistochemistry or frozen for later measurement of HO enzyme activity, Western blot for HO-1 protein, and RT-PCR for HO-1 mRNA. The heart was excised and weighed, and the right-to-left heart weight ratio was determined. During hypoxia, the hematocrit increased progressively, reaching significantly higher values than the control value after 3 days. COHb levels increased above the control value after 1 day of hypoxia and increased progressively between 14 and 21 days, whereas arterial PO(2) and arterial PCO(2) did not vary significantly. HO-1 protein determined by Western blot increased for the first 7 days and declined thereafter; however, enzyme activity was elevated only after 1 day. Changes in HO-1 during hypoxia were localized by immunohistochemistry to inflammatory cells (early) and newly muscularized arterioles (later). Lung HO-1 mRNA normalized to glyceraldehyde-3-phosphate dehydrogenase was increased after 1 and 21 days. The data indicate that lung HO-1 protein and activity are upregulated only during early chronic hypoxia, whereas persistent COHb elevations indicate high endogenous CO production rates at nonpulmonary sites. If CO has antiproliferative properties, the lack of HO enzyme activity in the lung may be permissive for pulmonary vascular proliferation in hypoxia.