Internal ribosome entry segment-mediated initiation of c-Myc protein synthesis following genotoxic stress

Abstract

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.

BCL-2 Translation Is Mediated via Internal Ribosome Entry during Cell Stress

“…Our findings complement other recently published data describing the stress-induced IRES-mediated expression of apoptosis regulatory proteins, including cIAP-1, XIAP, cMyc, Apaf-1, and DAP-5 (42,(57)(58)(59)(60). However, it is important to note that several of these reports did not test IRES activity by transfecting RNA transcripts into cells.…”

Internal ribosome entry segment-mediated translation during apoptosis: the role of IRES-trans-acting factors

“…In this situation, c-Myc protein levels were maintained at the same levels as untreated cells and the data strongly suggest that under these conditions recruitment of the c-myc message to ribosome by the c-myc IRES allows the levels of c-Myc protein to be maintained 16,24 In contrast, even though Apaf-1 translation is solely initiated by internal ribosome entry, 25 the only situation where a small increase in Apaf-1 IRES function was observed was following genotoxic stress. 26 Given the importance of Apaf-1 during brain development, it is possible that the Apaf-1 IRES is required for the expression of this protein in the developing brain. In this regard, the FGF-2 IRES was shown to be active in adult brain, while in developing embryos both the FGF-2 and c-myc IRESes were active 27,28 It is therefore possible that the Apaf-1 IRES only functions in developing systems and artificial cell culture models are not appropriate for studying this IRES.…”

A gastrin transcript expressed in gastrointestinal cancer cells contains an internal ribosome entry site

“…In the present study, the basal activity of the c-myc and gastrin IRESs in PAN1 and HCT116 cells, which are of pancreatic and colonic origin, did not correlate. In addition, the activity of the c-myc IRES, which has previously been shown to be upregulated by MMC in HeLa cells (Subkhankulova et al, 2001), was not increased following exposure of these GI cancer cells to MMC, but it was responsive to hypoxia (data not shown). Taken together, these data suggest that expression from the two IRESs is controlled independently, involving different trans-acting factors.…”

“…The sequence appears to be acting as a true IRES, as activity is lost in the absence of a viral promoter. Under conditions of cellular stress, activity of the IRES is increased or maintained at higher levels than cap-dependent translation suggesting that it is controlled by a distinct mechanism, as shown for the c-myc IRES (Subkhankulova et al, 2001).…”