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

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% O₂) or 60 cycles of IH (30 sec at 1.5% O₂ followed by 5 min at 20% O₂). 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 Ca²⁺ signaling pathways involving phospholipase Cγ (PLCγ) and protein kinase C activation are required for IH-evoked HIF-1α accumulation. IH leads to activation of mTOR and S6 kinase (S6K) and rapamycin partially inhibited IH-induced HIF-1α accumulation. IH also decreased hydroxylation of HIF-1α protein and anti-oxidants 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 re-oxygenation indicating that mTOR-dependent protein synthesis is required for the persistent elevation of HIF-1α levels during re-oxygenation.