Intermittent hypoxia degrades HIF-2α via calpains resulting in oxidative stress: Implications for recurrent apnea-induced morbidities

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-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 ...

Alterations in Carotid Body Morphology and Cellular Mechanism Under the Influence of Intermittent Hypoxia

“…We verified the NOX2 expression immunohistochemically in the rat CBs of different groups which was earlier histochemically observed to be present in the type I cells and macrophages within the CB [28]. HIF2α is associated with adaptation to HA exposure [25] and mediates the increased expression of Sod2 gene encoding mitochondrial SOD2 [29,9]. SOD2 acts as an antioxidant which converts superoxide to hydrogen peroxide whose presence was also demonstrated immunohistochemically.…”

“…They constitute a common HIFβ subunit with oxygen regulated HIF1α and HIF2α subunits respectively [6,7]. HIF1α is a potent activator of genes encoding pro-oxidant enzymes such as NADPH oxidase 2 (NOX2) [8] and HIF2α regulates antioxidant enzymes such as superoxide dismutase 2 (SOD2) [9]. The cellular redox state relies on the regulation of these enzymes by the respective HIFα isoforms that further triggers the signaling pathways such as the release of multiple NTs [10,11].…”

CIH: from sleep apnea to breath-hold diving

“…Molecular mechanisms responsible for CIH-induced chemoreceptor activation are emerging [12,13,16,17]. CIH has two major effects on arterial chemoreceptors; it increases sensitivity to hypoxia, and induces a long-lasting activation of chemoreceptor afferents that persists after return of normal arterial oxygenation [17].…”

“…NAD(P)H oxidase-generated reactive oxygen species lead to activation of transcription factors including hypoxiainducible factor-1 (HIF-1), nuclear factor of activated T cells, and nuclear factor jB (NFjB) [13,16,17]. Downregulation of HIF-2 and superoxide dismutase-2 further amplify the oxidative stress [12]. Genetic deficiency or pharmacological antagonism of these molecular signaling pathways, or treatment with antioxidants eliminates sLTF and CIH-induced sympathoexcitation [12,13,17].…”

“…Downregulation of HIF-2 and superoxide dismutase-2 further amplify the oxidative stress [12]. Genetic deficiency or pharmacological antagonism of these molecular signaling pathways, or treatment with antioxidants eliminates sLTF and CIH-induced sympathoexcitation [12,13,17]. A central role of oxidative stress in CIH-induced chemoreceptor activation is supported by the findings that patients with sleep apnea exhibit oxidative stress [11], and that apneaand exercise-induced oxidative stress is reduced in elite breath-hold divers compared with control subjects [9].…”