Induction of HIF‐1α expression by intermittent hypoxia: Involvement of NADPH oxidase, Ca²⁺ signaling, prolyl hydroxylases, and mTOR

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

“…1A). Consistent with a previous report (15), HIF-1␣ expression increased Ϸ4-fold in cells exposed to IH 60 . In striking contrast, HIF-2␣ levels markedly decreased in cells exposed to IH 60 ( Fig.…”

“…Similar results were also obtained with MG-132 (5 M), a proteasome inhibitor ( Fig. S1 C and D (15). To assess whether Ca 2ϩ signaling contributes to HIF-2␣ degradation by IH, we determined the effect of altered Ca 2ϩ levels on HIF-2␣ levels.…”

“…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␣.…”

Accelerated tumor growth under intermittent hypoxia is associated with hypoxia-inducible factor-1-dependent adaptive responses to hypoxia

“…ROS upregulates HIF-1α expression by stimulating synthesis or increasing its stability through Ca 2+ -dependent signaling pathways, including those involving phospholipase Cγ or protein kinase C and mTOR [30,31]. In humans, OSA severity positively correlates with oxidative stress parameters [32,33].…”

The role of epigenetics in age-related macular degeneration

“…This results in the elicited transcriptional activity. 58,59 These molecular responses ultimately lead to the increased expression of vascular endothelial growth factor (VEGF) that is the predominant growth factor-stimulating choroidal neovascularization (CNV) in exudative AMD. 60 Although different aspects of HIF regulation are well known, it is still unclear by which precise mechanism HIFs activate transcription of their target genes.…”