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

Abstract

cIAP1 is an important member of the inhibitor of apoptosis family of proteins and is involved in the regulation of the NF-jBsignalling pathway downstream of the TNF receptor. We report here that UV irradiation leads to downregulation of cIAP1 expression because of enhanced cIAP1 mRNA destabilization. An AU-rich element located within the 3 0 untranslated region of cIAP1 mRNA is sufficient to mediate cIAP1 mRNA instability. Furthermore, we have identified hnRNP A1 as a cIAP1 3 0 UTR-binding protein. hnRNP A1 is a primarily nuclear protein, but accumulates in the cytoplasm after exposure of cells to UV irradiation. Indeed, we find that hnRNP A1 enhances the destabilization of cIAP1 mRNA during UV irradiation. Moreover, siRNAmediated knockdown of hnRNP A1 restores cIAP1 levels and prevents UV irradiation-induced activation of the NF-jB signal transduction pathway, suggesting that hnRNP A1 is an essential post-transcriptional modulator of cIAP1 expression, and thus cIAP1 activity. The inhibitor of apoptosis (IAP) family of proteins are key regulators of programmed cell death. 1 The IAP family comprises eight distinct members that participate in diverse cellular processes including cell cycle, signal transduction, ubiquitylation, and caspase inhibition. 2 Although it was initially believed that all IAPs are capable of inhibiting distinct caspases, the recent data suggest that only XIAP is a bona fide caspase inhibitor. 3 cIAP1 and cIAP2 were identified through an interaction with the TNF-receptor complex proteins TRAF1 and TRAF2, which regulate NF-kB signalling through the TNF receptor. 4 The C-terminal RING finger domain of cIAP1 possesses an ubiquitin E3 ligase activity and may sensitize cells to apoptosis by direct ubiquitylation and removal of TRAF2 following activation of the TNF receptor, resulting in the inhibition of NF-kB pathways. 5 Indeed, cIAP1 degradation induced by Smac mimetics leads to activation of both the canonical and non-canonical NF-kB pathways, strengthening the importance of IAPs in the regulation of NF-kB signalling. [6][7][8] Although cIAP1 and cIAP2 perform similar functions within the cell, their expression is regulated differently. The expression of cIAP2 is controlled primarily at the level of transcription in an NF-kB-dependent manner. 9 Also, the protein turnover of cIAP2 is regulated by the ubiquitin ligase activity of cIAP1. 10 In contrast, cIAP1 levels are controlled at the level of protein synthesis. The 5 0 untranslated region (UTR) of cIAP1 contains an upstream open reading frame that reduces the basal translation of cIAP1. 11 The 5 0 UTR of cIAP1 also harbours an internal ribosome entry site (IRES) element that mediates translational induction of cIAP1 in response to stress. 12,13 This complex regulatory network reflects the distinct spatial and temporal requirement for cIAP1 and cIAP2 proteins in response to various physiological conditions.Here, we describe an additional regulatory mechanism that governs the expression of cIAP1. We find that following UV irradiation the levels...

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