Eukaryotic Initiation Factor 2α Kinase and Phosphatase Activity during Postischemic Brain Reperfusion

When ischemic brain is reperfused, there is in vulnerable neurons immediate inhibition of protein synthesis associated with a large increase in phosphorylation of the α-subunit of eukaryotic initiation factor 2 [eIF2α, phosphorylated form eIF2α(P)]. We examined eIF2α kinase and eIF2α(P) phosphatase activity in brain homogenate postmitochondrial supernatants obtained from rats after 3 to 30 min of global brain ischemia (cardiac arrest), after 5 min of ischemia and 5 min of reperfusion (5R), and after 10 min of ischemia and 90 min reperfusion (90R). Because it has been suggested that PKR might be specifically responsible for producing eIF2α(P) during reperfusion, we also examined in brain homogenates from wild-type and PKR0/0C57BL/6J × 129/SV mice the effect of 5 min of ischemia and 5 min of reperfusion on eIF2α(P). Cytosolic brain eIF2α(P) in the 5R and 90R rats was 18- and 23-fold that of nonischemic controls without any change in the rate of eIF2α(P) dephosphorylation. There was no change in eIF2α kinase activity between 3 and 30 min of ischemia but an 85% decrease in the 5R group; the 90R group was similar to controls. In wild-type and PKR0/0mice total eIF2α was identical, and there was an identical 16-fold increase in eIF2α(P) at 5 min of reperfusion. Our observations contradict hypotheses that PKR activation, loss of eIF2α(P) phosphatase activity, or any general increase in eIF2α kinase activity are responsible for reperfusion-induced phosphorylation of eIF2α, and we suggest that the mechanism may involve regulation of the availability of eIF2α to a kinase.