[Show abstract][Hide abstract] ABSTRACT: Systemic hypertension is one of the most prevalent cardiovascular diseases. Sleep-disordered breathing (SDB) with recurrent apnea is a major risk factor for developing essential hypertension. Chronic intermittent hypoxia (CIH) is a hallmark manifestation of recurrent apnea. Rodent models patterned after the O2 profiles seen with SDB patients showed that CIH is the major stimulus for causing systemic hypertension. This article reviews the physiological and molecular basis of CIH-induced hypertension. Physiological studies have identified that augmented carotid body chemosensory reflex and the resulting increase in sympathetic nerve activity are major contributors to CIH-induced hypertension. Analysis of molecular mechanisms revealed that CIH activates hypoxia-inducible factor (HIF)-1 and suppresses HIF-2-mediated transcription. Dysregulation of HIF-1- and HIF-2-mediated transcription leads to imbalance of pro-oxidant and anti-oxidant enzyme gene expression resulting in increased reactive oxygen species (ROS) generation in the chemosensory reflex which is central for developing hypertension.
No preview · Article · Mar 2015 · Journal of Molecular Medicine
[Show abstract][Hide abstract] ABSTRACT: Arterial blood oxygen (O2) levels are detected by specialized sensory organs called carotid bodies. Voltage-gated Ca(2+) channels (VGCC) are important for carotid body O2 sensing. Given that T-type VGCC contributes to nociceptive sensation, we hypothesized that they participate in carotid body O2 sensing. The rat carotid body expresses high levels of mRNA encoding α1H subunit and α1H protein is localized to glomus cells, the primary O2 sensing cells in the chemoreceptor tissue suggesting that CaV 3.2 is the major T-Type VGCC isoform expressed in the carotid body. Mibefradil and TTA-A2, selective blockers of T-type VGCC, markedly attenuated hypoxia-evoked [Ca(2+)]i elevation, catecholamine (CA) secretion from glomus cells and sensory excitation of the rat carotid body. Similar results were also obtained in the carotid body and glomus cells from CaV 3.2 knockout mice (Cacna1h(-/-)). Since cystathionine-γ-lyase (CSE)-derived hydrogen sulfide (H2S) is a critical mediator of carotid body response to hypoxia, the role of T-type VGCC in H2S-mediated O2 sensing was examined. Like hypoxia, NaHS, a H2S donor increased [Ca(2+)]i and augmented carotid body sensory nerve activity in wild type mice and these effects were markedly attenuated in Cacna1h(-/-) mice. In wild type mice, TTA-A2 markedly attenuated glomus cell and carotid body sensory nerve responses to hypoxia and these effects were absent in CSE knockout mice. These results demonstrate that CaV 3.2 T-type VGCC contribute to H2S mediated carotid body response to hypoxia.
Full-text · Article · Nov 2014 · AJP Cell Physiology
[Show abstract][Hide abstract] ABSTRACT: Previous studies reported that chronic intermittent hypoxia (CIH) results in an imbalanced expression of hypoxia inducible factor-α (HIF-α) isoforms and oxidative stress in rodents, which may be either due to direct effect of CIH or indirectly via hitherto uncharacterized mechanism(s). Since neural activity is a potent regulator of gene transcription, we hypothesized that carotid body (CB) neural activity contributes to CIH-induced HIF-α isoform expression and oxidative stress in the chemo reflex pathway. Experiments were performed on adult rats exposed to CIH for 10 days. Rats exposed to CIH exhibited: increased HIF-1α and decreased HIF-2α expression; increased NADPH oxidase 2 (Nox2) and decreased superoxide dismutase 2 (Sod2) expression; and oxidative stress in the nucleus tractus solitarius (nTS) and rostral ventrolateral medulla (RVLM) as well as in the adrenal medulla (AM), a major end organ of the sympathetic nervous system. Selective ablation of the CB abolished these effects. In the AM, sympathetic activation by CB chemo reflex mediates CIH-induced HIF-α isoform imbalance via muscarinic acetylcholine receptor-mediated Ca2+ influx, and the resultant activation of mTOR pathway and calpain proteases. Rats exposed to CIH presented with hypertension, elevated sympathetic activity and increased circulating catecholamines. Selective ablation of either the CB (afferent pathway) or sympathetic innervation to the AM (efferent pathway) abolished these effects. These observations uncover CB neural activity-dependent regulation of HIF-α isoforms and the redox state by CIH in the central and peripheral nervous systems associated with the chemo reflex.This article is protected by copyright. All rights reserved
Full-text · Article · Jun 2014 · The Journal of Physiology
[Show abstract][Hide abstract] ABSTRACT: The objective of the present study was to determine the impact of simulated apnea with intermittent hypoxia (IH) on endothelial barrier function and assess the underlying mechanism(s). Experiments were performed on human lung micro-vascular endothelial cells exposed to IH consisting alternating cycles of 1.5% O2 for 30s followed by 20% O2 for 5 min. IH decreased trans-endothelial electrical resistance (TEER) suggesting attenuated endothelial barrier function. The effect of IH on TEER was stimulus-dependent and reversible after re-oxygenation. IH exposed cells exhibited stress fiber formation and redistribution of cortactin, vascular endothelial-cadherins and zona occludens-1, junction proteins along with increased intercellular gaps at cell-cell boundaries. Extracellular signal regulated kinase (ERK) and c-jun NH2-terminal kinase (JNK) were phosphorylated in IH exposed cells. Inhibiting either ERK or JNK prevented IH-induced decrease in TEER and the reorganization of the cytoskeleton and junction proteins. IH increased reactive oxygen species (ROS) levels and manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride, a membrane permeable anti-oxidant prevented ERK and JNK phosphorylation as well as IH-induced changes in endothelial barrier function. These results demonstrate that IH via ROS-dependent activation of MAP kinases leads to re-organization of cytoskeleton and junction proteins resulting in endothelial barrier dysfunction.
Full-text · Article · Jan 2014 · AJP Cell Physiology
[Show abstract][Hide abstract] ABSTRACT: Oxygen (O2) sensing by the carotid body and its chemosensory reflex is critical for homeostatic regulation of breathing and blood pressure. Humans and animals exhibit substantial interindividual variation in this chemosensory reflex response, with profound effects on cardiorespiratory functions. However, the underlying mechanisms are not known. Here, we report that inherent variations in carotid body O2 sensing by carbon monoxide (CO)-sensitive hydrogen sulfide (H2S) signaling contribute to reflex variation in three genetically distinct rat strains. Compared with Sprague-Dawley (SD) rats, Brown-Norway (BN) rats exhibit impaired carotid body O2 sensing and develop pulmonary edema as a consequence of poor ventilatory adaptation to hypobaric hypoxia. Spontaneous Hypertensive (SH) rat carotid bodies display inherent hypersensitivity to hypoxia and develop hypertension. BN rat carotid bodies have naturally higher CO and lower H2S levels than SD rat, whereas SH carotid bodies have reduced CO and greater H2S generation. Higher CO levels in BN rats were associated with higher substrate affinity of the enzyme heme oxygenase 2, whereas SH rats present lower substrate affinity and, thus, reduced CO generation. Reducing CO levels in BN rat carotid bodies increased H2S generation, restoring O2 sensing and preventing hypoxia-induced pulmonary edema. Increasing CO levels in SH carotid bodies reduced H2S generation, preventing hypersensitivity to hypoxia and controlling hypertension in SH rats.
Full-text · Article · Jan 2014 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Breathing and blood pressure are under constant homeostatic regulation to maintain optimal oxygen delivery to the tissues. Chemosensory reflexes initiated by the carotid body and catecholamine secretion from the adrenal medulla are the principal mechanisms for maintaining respiratory and cardiovascular homeostasis; however, the underlying molecular mechanisms are not known. Here, we report that balanced activity of hypoxia-inducible factor-1 (HIF-1) and HIF-2 is critical for oxygen sensing by the carotid body and adrenal medulla, and for their control of cardio-respiratory function. In Hif2α(+/-) mice, partial HIF-2α deficiency increased levels of HIF-1α and NADPH oxidase 2, leading to an oxidized intracellular redox state, exaggerated hypoxic sensitivity, and cardio-respiratory abnormalities, which were reversed by treatment with a HIF-1α inhibitor or a superoxide anion scavenger. Conversely, in Hif1α(+/-) mice, partial HIF-1α deficiency increased levels of HIF-2α and superoxide dismutase 2, leading to a reduced intracellular redox state, blunted oxygen sensing, and impaired carotid body and ventilatory responses to chronic hypoxia, which were corrected by treatment with a HIF-2α inhibitor. None of the abnormalities observed in Hif1α(+/-) mice or Hif2α(+/-) mice were observed in Hif1α(+/-);Hif2α(+/-) mice. These observations demonstrate that redox balance, which is determined by mutual antagonism between HIF-α isoforms, establishes the set point for hypoxic sensing by the carotid body and adrenal medulla, and is required for maintenance of cardio-respiratory homeostasis.
Full-text · Article · Apr 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Recurrent apnea with intermittent hypoxia is a major clinical problem in preterm infants. Recent studies, although limited, showed that adults who were born preterm exhibit increased incidence of sleep-disordered breathing and hypertension, suggesting that apnea of prematurity predisposes to autonomic dysfunction in adulthood. Here, we demonstrate that adult rats that were exposed to intermittent hypoxia as neonates exhibit exaggerated responses to hypoxia by the carotid body and adrenal chromaffin cells, which regulate cardio-respiratory function, resulting in irregular breathing with apneas and hypertension. The enhanced hypoxic sensitivity was associated with elevated oxidative stress, decreased expression of genes encoding antioxidant enzymes, and increased expression of pro-oxidant enzymes. Decreased expression of the Sod2 gene, which encodes the antioxidant enzyme superoxide dismutase 2, was associated with DNA hypermethylation of a single CpG dinucleotide close to the transcription start site. Treating neonatal rats with decitabine, an inhibitor of DNA methylation, during intermittent hypoxia exposure prevented oxidative stress, enhanced hypoxic sensitivity, and autonomic dysfunction. These findings implicate a hitherto uncharacterized role for DNA methylation in mediating neonatal programming of hypoxic sensitivity and the ensuing autonomic dysfunction in adulthood.
Full-text · Article · Feb 2012 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Sleep-disordered breathing with recurrent apnea is associated with intermittent hypoxia (IH). Cardiovascular morbidities caused by IH are triggered by increased generation of reactive oxygen species (ROS) by pro-oxidant enzymes, especially NADPH oxidase-2 (Nox2). Previous studies showed that (i) IH activates hypoxia-inducible factor 1 (HIF-1) in a ROS-dependent manner and (ii) HIF-1 is required for IH-induced ROS generation, indicating the existence of a feed-forward mechanism. In the present study, using multiple pharmacological and genetic approaches, we investigated whether IH-induced expression of Nox2 is mediated by HIF-1 in the central and peripheral nervous system of mice as well as in cultured cells. IH increased Nox2 mRNA, protein, and enzyme activity in PC12 pheochromocytoma cells as well as in wild-type mouse embryonic fibroblasts (MEFs). This effect was abolished or attenuated by blocking HIF-1 activity through RNA interference or pharmacologic inhibition (digoxin or YC-1) or by genetic knockout of HIF-1α in MEFs. Increasing HIF-1α expression by treating PC 12 cells with the iron chelator deferoxamine for 20 h or by transfecting them with HIF-1alpha expression vector increased Nox2 expression and enzyme activity. Exposure of wild-type mice to IH (8 h/day for 10 days) up-regulated Nox2 mRNA expression in brain cortex, brain stem, and carotid body but not in cerebellum. IH did not induce Nox2 expression in cortex, brainstem, carotid body, or cerebellum of Hif1a(+/-) mice, which do not manifest increased ROS or cardiovascular morbidities in response to IH. These results establish a pathogenic mechanism linking HIF-1, ROS generation, and cardiovascular pathology in response to IH.
No preview · Article · Nov 2011 · Journal of Cellular Physiology
[Show abstract][Hide abstract] ABSTRACT: Cardiorespiratory functions in mammals are exquisitely sensitive to changes in arterial O(2) levels. Hypoxia-inducible factors (e.g., HIF-1 and HIF-2) mediate transcriptional responses to reduced oxygen availability. We demonstrate that haploinsufficiency for the O(2)-regulated HIF-2α subunit results in augmented carotid body sensitivity to hypoxia, irregular breathing, apneas, hypertension, and elevated plasma norepinephrine levels in adult Hif-2α(+/-) mice. These dysregulated autonomic responses were associated with increased oxidative stress and decreased mitochondrial electron transport chain complex I activity in adrenal medullae as a result of decreased expression of major cytosolic and mitochondrial antioxidant enzymes. Systemic administration of a membrane-permeable antioxidant prevented oxidative stress, normalized hypoxic sensitivity of the carotid body, and restored autonomic functions in Hif-2α(+/-) mice. Thus, HIF-2α-dependent redox regulation is required for maintenance of carotid body function and cardiorespiratory homeostasis.
Full-text · Article · Feb 2011 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Nearly 90% of premature infants experience the stress of intermittent hypoxia (IH) as a consequence of recurrent apneas (periodic cessation of breathing). In neonates, catecholamine secretion from the adrenal medulla is critical for maintaining homeostasis under hypoxic stress. We recently reported that IH treatment enhanced hypoxia-evoked catecholamine secretion and [Ca2+]i responses in neonatal rat adrenal chromaffin cells and involves reactive oxygen species (ROS). The purpose of the present study was to identify the source(s) of ROS generation and examine the mechanisms underlying the enhanced catecholamine secretion by IH. Neonatal rats of either sex (postal day 0-5) were exposed to either IH or normoxia. IH treatment increased NADPH oxidase (NOX) activity, upregulated NOX2 and NOX4 transcription in adrenal medullae, and a NOX inhibitor prevented the effects of IH on hypoxia-evoked chromaffin cell secretion. IH upregulated Cav3.1 and Cav3.2 T-type Ca2+ channel mRNAs via NOX/ROS signaling and augmented T-type Ca2+ current in IH-treated chromaffin cells. Mibefradil, a blocker of T-type Ca2+ channels attenuated the effects of hypoxia on [Ca2+]i and catecholamine secretion in IH-treated cells. In Ca2+-free medium, IH-treated cells exhibited higher basal [Ca2+]i levels and more pronounced [Ca2+]i responses to hypoxia compared with controls, and blockade of ryanodine receptors (RyRs) prevented these effects. RyR2 and RyR3 mRNAs were upregulated, RyR2 was S-glutathionylated in IH-treated adrenal medullae, and NOX/ROS inhibitors prevented these effects. These results demonstrate that neonatal IH treatment leads to NOX/ROS-dependent recruitment of T-type Ca2+ channels and RyRs, resulting in augmented [Ca2+]i mobilization and catecholamine secretion.
No preview · Article · Aug 2010 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: We recently reported that adrenomedullary chromaffin cells (AMC) from neonatal rats treated with intermittent hypoxia (IH) exhibit enhanced catecholamine secretion by hypoxia (Souvannakitti D, Kumar GK, Fox A, Prabhakar NR. J Neurophysiol 101: 2837-2846, 2009). In the present study, we examined whether neonatal IH also facilitate AMC responses to nicotine, a potent stimulus to chromaffin cells. Experiments were performed on rats exposed to either IH (15-s hypoxia-5-min normoxia; 8 h/day) or to room air (normoxia; controls) from ages postnatal day 0 (P0) to P5. Quantitative RT-PCR analysis revealed expression of mRNAs alpha(3-), alpha(5-), alpha(7-), and beta(2-) and beta(4-)nicotinic acetylcholine receptor (nAChR) subunits in adrenal medullae from control P5 rats. Nicotine-elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) in AMC and nAChR antagonists prevented this response, suggesting that nAChRs are functional in neonatal AMC. In IH-treated rats, nAChR mRNAs were downregulated in AMC, which resulted in a markedly attenuated nicotine-evoked elevation in [Ca(2+)](i) and subsequent catecholamine secretion. Systemic administration of antioxidant prevented IH-evoked downregulation of nAChR expression and function. P35 rats treated with neonatal IH exhibited reduced nAChR mRNA expression in adrenal medullae, attenuated AMC responses to nicotine, and impaired neurogenic catecholamine secretion. Thus the response to neonatal IH lasts for at least 30 days. These observations demonstrate that neonatal IH downregulates nAChR expression and function in AMC via reactive oxygen species signaling, and the effects of neonatal IH persist at least into juvenile life, leading to impaired neurogenic catecholamine secretion from AMC.
No preview · Article · Aug 2010 · AJP Cell Physiology
[Show abstract][Hide abstract] ABSTRACT: We recently reported that adrenomedullary chromaffin cells (AMC) from neonatal rats treated with intermittent hypoxia (IH) exhibit enhanced catecholamine secretion by hypoxia. In the present study we examined whether neonatal IH also facilitate AMC responses to nicotine, a potent stimulus to chromaffin cells. Experiments were performed on rats exposed to either IH (15s hypoxia-5min normoxia; 8h/day) or to room air (normoxia; controls) from ages P0-P5. Quantitative RT-PCR analysis revealed expression of mRNAs encoding alpha3, alpha5, alpha7 and beta2 and beta4 nAChR subunits in adrenal medullae from control P5 rats. Nicotine elevated [Ca(2+)](i) in AMC and nAChR antagonists prevented this response suggesting that nAChRs are functional in neonatal AMC. In IH treated rats, nAChR mRNAs were down-regulated in AMC which resulted in a markedly attenuated nicotine-evoked elevation in [Ca(2+)](i) and subsequent catecholamine secretion. Systemic administration of anti-oxidant prevented IH-evoked down-regulation of nAChR expression and function. P35 rats treated with neonatal IH exhibited reduced nAChR mRNA expression in adrenal medullae, attenuated AMC responses to nicotine and impaired neurogenic catecholamine secretion. Thus the response to neonatal IH lasts for at least 30 days. These observations demonstrate that neonatal IH down-regulates nAChR expression and function in AMC via reactive oxygen species signaling and the effects of neonatal IH persist at least into juvenile life leading to impaired neurogenic catecholamine secretion from AMC.
No preview · Article · May 2010 · AJP Cell Physiology
[Show abstract][Hide abstract] ABSTRACT: Previous studies identified NADPH oxidases (Nox) and mitochondrial electron transport chain at complex I as major cellular sources of reactive oxygen species (ROS) mediating systemic and cellular responses to intermittent hypoxia (IH). In the present study, we investigated potential interactions between Nox and the mitochondrial complex I and assessed the contribution of mitochondrial ROS in IH-evoked elevation in blood pressure. IH treatment led to stimulus-dependent activation of Nox and inhibition of complex I activity in rat pheochromocytoma (PC)12 cells. After re-oxygenation, Nox activity returned to baseline values within 3 h, whereas the complex I activity remained downregulated even after 24 h. IH-induced complex I inhibition was prevented by Nox inhibitors, Nox2 but not Nox 4 siRNA, in cell cultures and was absent in gp91(phox-/Y) (Nox2 knock-out; KO) mice. Using pharmacological inhibitors, we show that ROS generated by Nox activation mobilizes Ca(2+) flux from the cytosol to mitochondria, leading to S-glutathionylation of 75- and 50-kDa proteins of the complex I and inhibition of complex I activity, which results in elevated mitochondrial ROS. Systemic administration of mito-tempol prevented the sustained but not the acute elevations of blood pressure in IH-treated rats, suggesting that mitochondrial-derived ROS contribute to sustained elevation of blood pressure.
[Show abstract][Hide abstract] ABSTRACT: Respiratory motoneuron response to hypoxia is reflex in nature and carotid body sensory receptor constitutes the afferent limb of this reflex. Recent studies showed that repetitive exposures to hypoxia evokes long term facilitation of sensory nerve discharge (sLTF) of the carotid body in rodents exposed to chronic intermittent hypoxia (CIH). Although studies with anti-oxidants suggested the involvement of reactive oxygen species (ROS)-mediated signaling in eliciting sLTF, the source of and the mechanisms associated with ROS generation have not yet been investigated. We tested the hypothesis that ROS generated by NADPH oxidase (NOX) mediate CIH-evoked sLTF. Experiments were performed on ex vivo carotid bodies from rats and mice exposed either to 10 d of CIH or normoxia. Acute repetitive hypoxia evoked a approximately 12-fold increase in NOX activity in CIH but not in control carotid bodies, and this effect was associated with upregulation of NOX2 mRNA and protein, which was primarily localized to glomus cells of the carotid body. sLTF was prevented by NOX inhibitors and was absent in mice deficient in NOX2. NOX activation by CIH required 5-HT release and activation of 5-HT(2) receptors coupled to PKC signaling. Studies with ROS scavengers revealed that H(2)O(2) generated from O(2).(-) contributes to sLTF. Priming with H(2)O(2) elicited sLTF of carotid bodies from normoxic control rats and mice, similar to that seen in CIH-treated animals. These observations reveal a novel role for NOX-induced ROS signaling in mediating sensory plasticity of the carotid body.
Full-text · Article · May 2009 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: We previously reported that intermittent hypoxia (IH) augments hypoxic sensory response (HSR) and increases the number of glomus cells in neonatal carotid bodies. In the present study, we tested the hypothesis that recruitment of endothelin-1 (ET-1) signaling by reactive oxygen species (ROS) plays a critical role in IH-evoked changes in neonatal carotid bodies. Experiments were performed on neonatal rats exposed either to 10 days of IH (P0-P10; 8 h/day) or to normoxia. IH augmented HSR of the carotid bodies ex vivo and resulted in hyperplasia of glomus cells. The effects of IH were associated with enhanced basal release of ET-1 under normoxia, sensitization of carotid body response to exogenous ET-1, and upregulation of ET(A) but not an ET(B) receptor mRNA without altering the ET-1 content. An ET(A) but not ET(B) receptor antagonist prevented augmented HSR by IH. ROS levels were elevated in carotid bodies from IH-treated rat pups as evidenced by increased levels of malondialdehyde. Systemic administration of manganese (III) tetrakis(1-methyl-4-pyridyl)porphyrin pentachloride (MnTMPyP; 5 mg/kg ip), a scavenger of O(2)(*-), prevented IH-induced elevation of ROS, basal release of ET-1, upregulation of ET(A) mRNA, and augmented HSR. In striking contrast, MnTMPyP treatment had no significant effect on IH-induced hyperplasia of glomus cells. These results demonstrate that IH-evoked increase in HSR involve a ROS-mediated increase in basal ET-1 release and upregulation of ET(A) receptor mRNA.
No preview · Article · Mar 2009 · AJP Regulatory Integrative and Comparative Physiology
[Show abstract][Hide abstract] ABSTRACT: Intermittent hypoxia (IH) occurs in many pathological conditions including recurrent apneas. Hypoxia-inducible factors (HIFs) 1 and 2 mediate transcriptional responses to low O(2). A previous study showed that HIF-1 mediates some of the IH-evoked physiological responses. Because HIF-2alpha is an orthologue of HIF-1alpha, we examined the effects of IH on HIF-2alpha, the O(2)-regulated subunit expression, in pheochromocytoma 12 cell cultures. In contrast to the up-regulation of HIF-1alpha, HIF-2alpha was down-regulated by IH. Similar down-regulation of HIF-2alpha was also seen in carotid bodies and adrenal medullae from IH-exposed rats. Inhibitors of calpain proteases (ALLM, ALLN) prevented IH-evoked degradation of HIF-2alpha whereas inhibitors of prolyl hydroxylases or proteosome were ineffective. IH activated calpain proteases and down-regulated the endogenous calpain inhibitor calpastatin. IH-evoked HIF-2alpha degradation led to inhibition of SOD2 transcription, resulting in oxidative stress. Over-expression of transcriptionally active HIF-2alpha prevented IH-evoked oxidative stress and restored SOD2 activity. Systemic treatment of IH-exposed rats with ALLM rescued HIF-2alpha degradation and restored SOD2 activity, thereby preventing oxidative stress and hypertension. These observations demonstrate that, unlike continuous hypoxia, IH leads to down-regulation of HIF-2alpha via a calpain-dependent signaling pathway and results in oxidative stress as well as autonomic morbidities.
Full-text · Article · Feb 2009 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Sleep-disordered breathing with recurrent apnea (periodic cessation of breathing) results in chronic intermittent hypoxia (IH), which leads to cardiovascular and respiratory pathology. Molecular mechanisms underlying IH-evoked cardio-respiratory co-morbidities have not been delineated. Mice with heterozygous deficiency of hypoxia-inducible factor 1alpha (HIF-1alpha) do not develop cardio-respiratory responses to chronic IH. HIF-1alpha protein expression and HIF-1 transcriptional activity are induced by IH in PC12 cells. In the present study, we investigated the signaling pathways associated with IH-evoked HIF-1alpha accumulation. PC12 cells were exposed to aerobic conditions (20% O(2)) or 60 cycles of IH (30 sec at 1.5% O(2) followed by 5 min at 20% O(2)). Our results show that IH-induced HIF-1alpha accumulation is due to increased generation of ROS by NADPH oxidase. We further demonstrate that ROS-dependent Ca(2+) signaling pathways involving phospholipase Cgamma (PLCgamma) and protein kinase C activation are required for IH-evoked HIF-1alpha accumulation. IH leads to activation of mTOR and S6 kinase (S6K) and rapamycin partially inhibited IH-induced HIF-1alpha accumulation. IH also decreased hydroxylation of HIF-1alpha protein and anti-oxidants as well as inhibitors of Ca(+2) signaling prevented this response. Thus, both increased mTOR-dependent HIF-1alpha synthesis and decreased hydroxylase-dependent HIF-1alpha degradation contribute to IH-evoked HIF-1alpha accumulation. Following IH, HIF-1alpha, and phosphorylated mTOR levels remained elevated during 90 min of re-oxygenation despite re-activation of prolyl hydroxylase. Rapamycin or cycloheximide, blocked increased HIF-1alpha levels during re-oxygenation indicating that mTOR-dependent protein synthesis is required for the persistent elevation of HIF-1alpha levels during re-oxygenation.
Preview · Article · Dec 2008 · Journal of Cellular Physiology
[Show abstract][Hide abstract] ABSTRACT: Previous studies suggest that reactive oxygen species (ROS) play an important role in physiological responses to hypoxia. In the present study, we examined the effects of hypoxia on human ether-a-go-go related gene (hERG) channel protein expression and assessed the role of ROS. Hypoxia, in a stimulus- and time-dependent manner, decreased hERG protein with marked reduction in hERG K+ conductance in human embryonic kidney cells stably expressing the hERG alpha subunit. Down-regulation of hERG by hypoxia was not due to increased proteasomal degradation or decreased transcription but due to decreased synthesis of the protein. Hypoxia increased ROS in a time-dependent manner. Antioxidants prevented hypoxia-evoked down-regulation of hERG protein and exogenous oxidants mimicked the effects of hypoxia. Hypoxia-evoked down-regulation of hERG protein and elevation in ROS were absent in p(O) cells, which are devoid of mitochondrial DNA. Inhibitors of NADPH oxidase failed to prevent the effects of hypoxia. These results demonstrate that hypoxia enhances the production of ROS in the mitochondria, resulting in down-regulation of hERG translation and decreased hERG-mediated K+ conductance.
No preview · Article · Sep 2008 · Biochemical and Biophysical Research Communications