Gaseous messengers, nitric oxide and carbon monoxide, have been implicated in O 2 sensing by the carotid body, a sensory organ that monitors arterial blood O 2 levels and stimulates breathing in response to hypoxia. We now show that hydrogen sulfide (H 2 S) is a physiologic gasotransmitter of the carotid body, enhancing its sensory response to hypoxia. Glomus cells, the site of O 2 sensing in the carotid body, express cystathionine γ-lyase (CSE), an H 2 Sgenerating enzyme, with hypoxia increasing H 2 S generation in a stimulus-dependent manner. Mice with genetic deletion of CSE display severely impaired carotid body response and ventilatory stimulation to hypoxia, as well as a loss of hypoxia-evoked H 2 S generation. Pharmacologic inhibition of CSE elicits a similar phenotype in mice and rats. Hypoxia-evoked H 2 S generation in the carotid body seems to require interaction of CSE with hemeoxygenase-2, which generates carbon monoxide. CSE is also expressed in neonatal adrenal medullary chromaffin cells of rats and mice whose hypoxia-evoked catecholamine secretion is greatly attenuated by CSE inhibitors and in CSE knockout mice.I n adult mammals, carotid bodies are the sensory organs responsible for monitoring arterial blood O 2 concentrations and relay sensory information to the brainstem neurons associated with regulation of breathing and the cardiovascular system (1). Carotid bodies are comprised mainly of two cell types: glomus (also called type I) and sustanticular (or type II) cells. Glomus cells, of neuronal nature, are considered the main hypoxiasensing cells. The gaseous messengers, carbon monoxide (CO) and nitric oxide (NO), generated by hemeoxygenase-2 (HO-2) and neuronal nitric oxide synthase (nNOS), respectively, physiologically inhibit carotid body activity (2-4). Because HO-2 and nNOS require molecular O 2 for their activity, stimulation of carotid body activity by hypoxia may reflect in part reduced formation of CO and NO (5).Like NO and CO, hydrogen sulfide (H 2 S) is a gasotransmitter physiologically regulating neuronal transmission (6) and vascular tone (7). Cystathionine γ-lyase (CSE) (EC 4.4.1.1) and cystathionine β-synthase (CBS) (4.2.1.22) are the major enzymes associated with generation of endogenous H 2 S (8, 9). CBS is the predominant H 2 S-synthesizing enzyme in the brain, CSE preponderates in the peripheral tissues whose H 2 S levels are reduced 90% in CSE −/− mice (7-10). Given that carotid bodies are peripheral organs and that H 2 S is redox active, we hypothesized that CSE-derived H 2 S plays a role in hypoxic sensing by the carotid body. We examined carotid body response to hypoxia in wild-type (CSE +/+ ) and CSE −/− mice as well as in rats treated with CSE inhibitor. Genetic deletion or pharmacologic inhibition of CSE dramatically impairs hypoxic sensing by the carotid body as well as in neonatal adrenal medullary chromaffin cells (AMC). ResultsLoss of Carotid Body Response to Hypoxia in CSE −/− Mice. CSE immunoreactivity was seen in glomus cells of carotid bodies from CSE +/+ mice...
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 1α (HIF-1α) do not develop cardio-respiratory responses to chronic IH. HIF-1α 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-1α 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-1α accumulation is due to increased generation of ROS by NADPH oxidase. We further demonstrate that ROS-dependent Ca 2+ signaling pathways involving phospholipase Cγ and protein kinase C activation are required for IH-evoked HIF-1α accumulation. IH leads to activation of mTOR and S6 kinase and rapamycin partially inhibited IHinduced HIF-1α accumulation. IH also decreased hydroxylation of HIF-1α protein and antioxidants as well as inhibitors of Ca +2 signaling prevented this response. Thus, both increased mTOR-dependent HIF-1α synthesis and decreased hydroxylase-dependent HIF-1α degradation contribute to IH-evoked HIF-1α accumulation. Following IH, HIF-1α and phosphorylated mTOR levels remained elevated during 90 min of re-oxygenation despite re-activation of prolyl hydroxylase. Rapamycin or cycloheximide, blocked increased HIF-1α levels during reoxygenation indicating that mTOR-dependent protein synthesis is required for the persistent elevation of HIF-1α levels during re-oxygenation.
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-2␣ is an orthologue of HIF-1␣, we examined the effects of IH on HIF-2␣, the O2-regulated subunit expression, in pheochromocytoma 12 cell cultures. In contrast to the up-regulation of HIF-1␣, HIF-2␣ was down-regulated by IH. Similar down-regulation of HIF-2␣ 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-2␣ whereas inhibitors of prolyl hydroxylases or proteosome were ineffective. IH activated calpain proteases and down-regulated the endogenous calpain inhibitor calpastatin. IH-evoked HIF-2␣ degradation led to inhibition of SOD2 transcription, resulting in oxidative stress. Over-expression of transcriptionally active HIF-2␣ prevented IH-evoked oxidative stress and restored SOD2 activity. Systemic treatment of IH-exposed rats with ALLM rescued HIF-2␣ 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-2␣ via a calpain-dependent signaling pathway and results in oxidative stress as well as autonomic morbidities.calcium signaling ͉ hypoxia inducible factors S leep-disordered breathing with recurrent apneas is a leading cause of morbidity and mortality affecting an estimated 18 million people in the United States alone (1-4). Recurrent apneas are characterized by transient, repetitive cessations of breathing (Ϸ10 sec in adults) resulting in periodic decreases in arterial blood O 2 or intermittent hypoxia (IH). Patients with recurrent apneas are at risk for developing several comorbidities including hypertension, sympathetic activation, breathing abnormalities, atherosclerosis, and stroke (4-8). Exposure of rodents to IH alone induces several co-morbidities reported in patients with recurrent apnea (9-11). However, little information is available on the molecular mechanisms underlying the morbidities associated with IH.Hypoxia-inducible factors (HIFs) mediate transcriptional responses to low O 2 (12). HIF-1 is the prototypical member of the HIF family and comprises an O 2 -regulated ␣ subunit and a constitutive  subunit (13). HIF-1 transcriptional activity is induced under continuous hypoxia (CH) as a result of HIF-1␣ protein accumulation resulting from decreased O 2 -dependent proline hydroxylation, ubiquitination, and proteasomal degradation (14). Recent studies showed that IH leads to HIF-1␣ accumulation and utilizes signaling pathways distinct from those identified with CH (15). The importance of HIF-1 to IHassociated physiological and pathophysiological responses was studied in mice with heterozygous deficiency of HIF-1␣. IHevoked cardio-respiratory and metabolic morbidities were absent ...
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