Hypoxia-Independent Mechanisms of HIF-1α Expression in Astrocytes after Ischemic Preconditioning

We recently demonstrated that ischemic tolerance was dependent on astrocytes, for which HIF-1 alpha had an essential role. The mild ischemia (preconditioning; PC) increased HIF-1 alpha in a biphasic pattern, that is, a quick and transient increase in neurons, followed by a slow and sustained increase in astrocytes. However, mechanisms underlying such temporal difference in HIF-1 alpha increase remain totally unknown. Here, we show that unlike a hypoxia-dependent mechanism in neurons, astrocytes increase HIF-1 alpha via a novel hypoxia-independent but P2X7-dependent mechanism. Using a middle cerebral artery occlusion (MCAO) model of mice, we found that the PC (a 15-min MCAO period)-evoked increase in HIF-1 alpha in neurons was quick and transient (from 1 to 3 days after PC), but that in astrocytes was slow-onset and long-lasting (from 3 days to at least 2 weeks after PC). The neuronal HIF-1 alpha increase was dependent on inhibition of PHD2, an oxygen-dependent HIF-1 alpha degrading enzyme, whereas astrocytic one was independent of PHD2. Astrocytes even do not possess this enzyme. Instead, they produced a sustained increase in P2X7 receptors, activation of which resulted in HIF-1 alpha increase. The hypoxia-independent but P2X7receptor- dependent mechanism could allow astrocytes to cause long-lasting HIF-1 alpha expression, thereby leading to induction of ischemic tolerance efficiently.