Internal Ribosome Entry Site-mediated Translation of Apaf-1, but Not XIAP, Is Regulated during UV-induced Cell Death

Abstract

Components of the cellular translation machinery are targets of caspase-mediated cleavage during apoptosis that correlates with the inhibition of protein synthesis, which accompanies apoptosis. Paradoxically, protein synthesis is required for apoptosis to occur in many experimental settings. Previous studies showed that two proteins that regulate apoptosis by controlling caspase activity, XIAP and Apaf-1, are translated by a unique, cap-independent mechanism mediated by an internal ribosome entry site (IRES) that is used preferentially under conditions in which normal cap-dependent translation is repressed. We investigated the regulation of XIAP and Apaf-1 following UVC irradiation. We show that UVC irradiation leads to the inhibition of translation and cell death. Furthermore, IRES-mediated translation of Apaf-1, but not XIAP, is enhanced by UVC irradiation, and this increase in Apaf-1 translation correlated with cell death. The enhanced Apaf-1 IRES-mediated translation is caspase-independent but is negatively modulated by the eIF2␣ kinase protein kinase RNA-like endoplasmic reticulum kinase. These data suggest that progression of UVinduced apoptosis requires IRES-mediated translation of Apaf-1 to ensure continuous levels of Apaf-1 despite an overall suppression of protein synthesis.

Cytoplasmic Relocalization of Heterogeneous Nuclear Ribonucleoprotein A1 Controls Translation Initiation of Specific mRNAs

“…We next investigated the involvement of hnRNP A1 in apaf-1 mRNA translation, because apaf-1 IRES-mediated translation is activated after UVC irradiation in human embryonic kidney (293T) cells (Ungureanu et al, 2006) and hnRNP A1 is redistributed to the cytoplasm after UVC irradiation ( Figure 5A; as expected from van der Houven van Oordt et al, 2000). To first confirm the effect of UVC irradiation on endogenous apaf-1 mRNA translation, we examined the polysomal association of this mRNA in 293T cells.…”

“…To first confirm the effect of UVC irradiation on endogenous apaf-1 mRNA translation, we examined the polysomal association of this mRNA in 293T cells. Polysome gradients were prepared from untreated cells or cells irradiated with a UVC dose of 300J/m 2 , a dose previously shown to induce 50% cell death (Ungureanu et al, 2006). UVC irradiation caused a large decrease in polysomal mRNAs and a corresponding increase in free ribosomes and ribosomal subunits, reflecting a global reduction in translation initiation efficiency ( Figure 5B).…”

“…Furthermore, activation of the mitogen-activated protein kinase(3/6)-p38 signaling pathway in mammalian cells stressed by osmotic shock or UVC irradiation results in phosphorylation-dependent cytoplasmic accumulation of hnRNP A1 (van der Houven van Oordt et al, 2000;Allemand et al, 2005). Interestingly, UVC-dependent IRES-mediated translational control has recently been demonstrated for the apoptotic peptidase activating factor 1 (apaf-1) mRNA (Ungureanu et al, 2006), which encodes a proapoptotic protein involved in the formation of the apoptosome (Schafer and Kornbluth, 2006).…”

hnRNP A1 regulates UV-induced NF-κB signalling through destabilization of cIAP1 mRNA

“…We have earlier reported that UVR results in an inhibition of global protein synthesis, whereas simultaneously enhancing translation of the proapoptotic protein Apaf-1 through an IRES-dependent mode of translation. 14 In the course of these experiments we observed that the levels of the anti-apoptotic gene cIAP1 were significantly reduced following UVR (Figure 1a). We wished to determine if this reduction in cIAP1 levels is because of changes in protein or mRNA stability.…”

“…37 Previous work showed that exposure of cells to UVR results in an inhibition of global protein synthesis. 14,38,39 This observation leads to the question of why an additional regulatory mechanism, such as degradation of cIAP1 mRNA, would be employed by cells to silence the expression of target genes following UVR. It is interesting to note that several genes involved in the regulation of cell proliferation and survival are translated by an IRES-dependent mode of translation that escapes some of the control steps that govern global, cap-dependent translation.…”