Initiation of translation of the proto-oncogene c-myc can occur by either the cap-dependent scanning mechanism or by internal ribosome entry. The latter mechanism requires a complex RNA structural element that is located in the 5′ untranslated region of c-myc, termed an internal ribosome entry segment (IRES). Recent work has shown that IRESs are used to maintain protein expression under conditions when cap-dependent translation initiation is compromised; for example, during mitosis, apoptosis and under conditions of cell stress, such as hypoxia or heat shock. Induction of genotoxic stress also results in a large reduction in global protein synthesis rates and therefore we investigated whether the c-myc IRES was active following DNA damage. As expected, in cells treated with either ethylmethane sulphonate or mitomycin C there was a large reduction in protein synthesis, although this was brought about by two different mechanisms. However, in each case the c-myc IRES was active and c-Myc protein expression was maintained. Finally we showed that the proteins required for this process are downstream of the p38 mitogen-activated protein kinase (MAPK)/extracellular-signal-regulated protein kinase (ERK)/MEK(MAPK/ERK kinase) signalling pathways, since pre-treatment of cells with inhibitors of these pathways before DNA damage is initiated inhibits both c-myc IRES activity and expression of c-Myc protein.
Abbreviations used: eIF, eukaryotic initiation factor; 4E-BP, eIF4E-binding protein; EMS, ethylmethane sulphonate; MMC, mitomycin C; 5′ UTR, 5′-untranslated region; IRES, internal ribosome entry segment; VEGF, vascular endothelial growth factor; MAPK, mitogen-activated protein kinase; ERK, extracellular-signal-regulated protein kinase; MEK, MAPK/ERK kinase; FRAP/mTOR, FK506-binding protein rapamycin-associating protein/mammalian target of rapamycin; Apaf-1, apoptotic protease activating factor-1; XIAP, X-linked inhibitor of apoptosis; HRI, haem-regulated inhibitor; PKR, double-stranded-RNA-activated protein kinase; GCN, general control non-derepressible.