c-Myc Protein Synthesis Is Initiated from the Internal Ribosome Entry Segment during Apoptosis

Abstract

Recent studies have shown that during apoptosis protein synthesis is inhibited and that this is in part due to the proteolytic cleavage of eukaryotic initiation factor 4G (eIF4G). Initiation of translation can occur either by a cap-dependent mechanism or by internal ribosome entry. The latter mechanism is dependent on a complex structural element located in the 5 untranslated region of the mRNA which is termed an internal ribosome entry segment (IRES). In general, IRES-mediated translation does not require eIF4E or full-length eIF4G. In order to investigate whether cap-dependent and cap-independent translation are reduced during apoptosis, we examined the expression of c-Myc during this process, since we have shown previously that the 5 untranslated region of the c-myc proto-oncogene contains an IRES. c-Myc expression was determined in HeLa cells during apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand. We have demonstrated that the c-Myc protein is still expressed when more than 90% of the cells are apoptotic. The presence of the protein in apoptotic cells does not result from either an increase in protein stability or an increase in expression of c-myc mRNA. Furthermore, we show that during apoptosis initiation of c-myc translation occurs by internal ribosome entry. We have investigated the signaling pathways that are involved in this response, and cotransfection with plasmids which harbor either wild-type or constitutively active MKK6, a specific immediate upstream activator of p38 mitogen-activated protein kinase (MAPK), increases IRES-mediated translation. In addition, the c-myc IRES is inhibited by SB203580, a specific inhibitor of p38 MAPK. Our data, therefore, strongly suggest that the initiation of translation via the c-myc IRES during apoptosis is mediated by the p38 MAPK pathway.

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

“…23 In a study to determine the mechanisms used to maintain c-Myc expression during apoptosis, HeLa cells were treated with TRAIL and this resulted in apoptosis of 90% of the cells. 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 each of these diverse conditions, it has been found that approximately 3% of the messages remain associated with the polysomes including c-myc mRNA. 33 However, while there is some overlap between the genes found associated, many of the mRNAs identified are distinct, suggesting that up to 10% of all cellular mRNAs can be translated by an alternative mechanism from global regulation. In addition, as not all mRNAs that are selected for translation during conditions of pathophysiological stress contain IRESs, it is likely that other hitherto unidentified mechanisms exist to recruit the ribosome.…”

“…70,71 It has been shown for the c-Myc IRES that proteins that mediate IRES-initiated expression are downstream of p38MAPK, and the p38 inhibitor SB203580 blocks both the function of the cmyc IRES and the expression of c-Myc during apoptosis. 33 However, it is likely that multiple signalling events are required for ITAF activation and it has been shown recently that the cmyc-IRES-ITAF, hnRNPK, interacts with c-Src kinase, leading to c-Src activation and tyrosine phosphorylation of hnRNPK in vivo and in vitro. 72 This raises the possibility that regulation of hnRNPK through changes in phosphorylation could affect c-myc IRES function.…”

DAP5 and IRES-mediated translation during programmed cell death

“…Some of these cellular IRESes are utilized during different stress conditions such as hypoxia or heat shock. [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] In light of the above, a major question that became of interest to the cell death community, and will be addressed in detail in this issue of CDD, is whether translational switches are part of the PCD process, and more specifically, whether a shift from cap-dependent to IRES-mediated translation occurs under certain settings of cell death. In fact, it turned out that induction of type I PCD, for example, is associated with a rapid and substantial shut-off of overall protein synthesis in the cell.…”

“…Extensive work over the past several years revealed that this group comprise mRNAs encoding both proapoptotic proteins such as DAP5, c-Myc and Apaf-1, and antiapoptotic proteins such as XIAP and HIAP2, all of which harbor functional IRES elements within their 5 0 UTR, known as 'death IRESes'. 38,42,48,69,70 In theory, de novo synthesis of proapoptotic proteins provides positive feedback, reinforcing the death cascade. This might be especially important when dealing with short-lived protein products (as in the case of c-myc).…”