Differential requirements for caspase-8 activity in the mechanism of phosphorylation of elF2 alpha, cleavage of eIF4GI and signaling events associated with the inhibition of protein synthesis in apoptotic Jurkat T cells

Previously we have reported that induction of apoptosis in Jurkat cells res ults in an inhibition of overall protein synthesis with the selective and r apid cleavage of eukaryotic initiation factor (eIF) 4GI, For the cleavage o f eIF4GI, caspase-3 activity is both necessary and sufficient in vivo, in a process which does not require signaling through the p38 MAP kinase pathwa y. We now show that activation of the Fas/CD95 receptor promotes an early, transient increase in the level of eIF2 alpha phosphorylation, which is tem porally correlated with the onset of the inhibition of translation. This is associated with a modest increase in the autophosphorylation of the protei n kinase activated by double-stranded RNA. Using a Jurkat cell line that is deficient in caspase-8 and resistant to anti-Fas-induced apoptosis, we sho w that whilst the cleavage of eIF4GI is caspase-8-dependent, the enhancemen t of eIF2 alpha phosphorylation does not require caspase-8 activity and occ urs prior to the cleavage of eIF4GI, In addition, activation of the Fas/CD9 5 receptor results in the caspase-8-dependent dephosphorylation and degrada tion of p70(S6K), the enhanced binding of 4E-BP1 to eIF4E, and, at later ti mes, the cleavage of eIF2a, These data suggest that apoptosis impinges upon the activity of several polypeptides which are central to the regulation o f protein synthesis and that multiple signaling pathways are involved in vi vo. (C) 2000 Federation of European Biochemical Societies. Published by Els evier Science B.V. All rights reserved.