c-Myc protein synthesis is initiated from the internal ribosome entry segment during apoptosis

Recent studies have shown that during apoptosis protein synthesis is inhibi ted and that this is in part due to the proteolytic cleavage of eukaryotic initiation factor IG (eIF4G), Initiation of translation can occur either by a cap-dependent mechanism or by internal ribosome entry. The latter mechan ism is dependent on a complex structural element located in the 5' untransl ated region of the mRNA which is termed an internal ribosome entry segment (IRES), In general, IRES-mediated translation does not require eIF4E or ful l-length eIF4G. In order to investigate whether cap-dependent and cap-indep endent translation are reduced during apoptosis, we examined the expression of c-Myc during this process, since we have shown previously that the 5' u ntranslated region of the c-myc proto-oncogene contains an IRES. c-Myc expr ession was determined in HeLa cells during apoptosis induced by tumor necro sis factor-related apoptosis-inducing ligand, Ne have demonstrated that the c-Myc protein is still expressed when more than 90% of the cells are apopt otic. The presence of the protein in apoptotic cells does not result from e ither an increase in protein stability or an increase in expression of c-my c mRNA. Furthermore, we show that during apoptosis initiation of c-myc tran slation occurs by internal ribosome entry. We have investigated the signali ng pathways that are involved in this response, and cotransfection with pla smids which harbor either wild-type or constitutively active MKK6, a specif ic immediate upstream activator of p38 mitogen-activated protein kinase (MA PK), increases IRES-mediated translation. In addition, the c-myc IRES is in hibited by SB203580, a specific inhibitor of p38 MAPK. Our data, therefore, strongly suggest that the initiation of translation via the c-myc IRES dur ing apoptosis is mediated by the p38 MAPK pathway.