Astrocyte responses to injury: VEGF simultaneously modulates cell death and proliferation

Abstract

Hypoxia is linked to changes in blood-brain barrier (BBB) permeability, and loss of BBB integrity is characteristic of many pathological brain diseases including stroke. In particular, astrocytes play a central role in brain homeostasis and BBB function. We investigated how hypoxia affects astrocyte survival and assessed whether VEGF release through hypoxia-inducible factor-1α (HIF-1α) induction plays a role in tolerance of these cells to insult. Thus primary astrocytes were subjected to normoxic (21% O2), hypoxic (1% O2), or near-anoxic (<0.1% O2) conditions in the presence or absence of glucose. Cell death was significantly initiated after combined oxygen glucose deprivation, and, surprisingly, astrocyte proliferation increased concomitantly. Near anoxic, but not hypoxic, conditions stabilized HIF-1α protein and provoked DNA binding activity, whereas oxygen and glucose deprivation accelerated HIF-1α accumulation. Unexpectedly, Hif-1α knockdown studies showed that elevated VEGF levels following increased insult was only partially due to HIF-1α induction, suggesting alternative mechanisms of VEGF regulation. Notably, endogenous VEGF signaling during insult was essential for cell fate since VEGF inhibition appreciably augmented cell death and reduced proliferation. These data suggest Hif-1 only partially contributes to VEGF-mediated astrocyte responses during chronic injury (as occurs in clinical hypoxic/ischemic insults) that may ultimately be responsible for disrupting BBB integrity.