Suppression of
protein synthesis in the brain following an ischemic insult has been thought to occur because of inhibition of translation initiation. All eukaryotic mRNAs, with the exception of heat-shock transcripts, require the activity of eukaryotic
initiation factor (
eIF) 4E for formation of the translation initiation complex, and
eIF-4E availability is rate-limiting. The response of brain
eIF-4E concentration and phosphorylation following
decapitation ischemia was studied in rat brain homogenates after electrophoresis and western blotting with
antibodies against
eIF-4E and
phosphoserine, respectively. There was no change in level of
eIF-4E after 5 min of
ischemia (p = 0.82 vs. time 0), but it had decreased 32 (p = 0.01) and 57% (p = 0.006) after 10 and 20 min of
ischemia, respectively. There was no loss of
serine phosphorylation on
eIF-4E beyond signal loss observed due to degradation of the
protein itself (p = 0.31). In vitro exposure of
eIF-4E to activated
mu-calpain resulted in a 50% loss in 10 min of
eIF-4E on western blots. If active
eIF-4E is required for translation of its own mRNA, degradation of this
protein during
ischemia, possibly by activated
mu-calpain, could be a direct mechanism of irreversible neuronal injury, and the rate of proteolysis of
eIF-4E could place an upper time limit on the maximal duration of global
brain ischemia compatible with neurologic recovery.