Non-hypoxic stabilization of hypoxia-inducible factor alpha (HIF-alpha): relevance in neural progenitor/stem cells.

Hypoxia-inducible factor-1 (HIF-1) plays an important role in neural progenitor cell (NPC) propagation and dopaminergic differentiation. In the presence of oxygen and iron, hypoxia-inducible factor 1 alpha (HIF-1alpha) is rapidly degraded via the prolyl hydroxylase (PHD)/VHL pathway. In addition to hypoxia, various non-hypoxic stimuli can stabilize HIF-1alpha in NPCs and influence the transcription of HIF-regulated genes. Here, we investigate various hypoxia mimetics: deferoxamine (DFO), ciclopirox olamine (CPX), dimethyloxallyl glycine (DMOG), a novel HIF-PHD inhibitor (FG-4497) and cobalt chloride (CoCl(2)) with respect to their ability to enhance in vitro proliferation, neurogenesis and dopaminergic differentiation of human fetal mesencephalic NPCs (hmNPCs) in ambient oxygen (21%). Although able to stabilize HIF-1alpha, iron chelators (DFO and CPX) and DMOG were toxic to hmNPCs. CoCl(2) was beneficial only towards neuronal and dopaminergic differentiation, while FG-4497 enhanced proliferation, neurogenesis and dopaminergic differentiation of hmNPCs. Both CoCl(2) and FG-4497 were protective to human dopaminergic neurons. Finally, exposure to hyperbaric oxygen (HBO) also stabilized HIF-1alpha in hmNPCs and induced neurogenesis in vitro. These findings suggest that several HIF stabilizing agents or conditions can rescue impaired neurons and promote neurogenesis in vitro.