Abstract 313: Regulation of p53 Tetramerization and Nuclear Export by ARC
ARC (Apoptosis Repressor with Caspase recruitment domain) is an endogenous inhibitor of apoptosis that, under normal conditions, is abundantly expressed in both the cytoplasm and nucleus of terminally differentiated cells such as cardiac myocytes. Although cytoplasmic ARC is known to inhibit both extrinsic and intrinsic pathways of apoptosis by direct interactions with Fas, FADD and Bax, the function of nuclear ARC is unknown. ARC overexpression decreases infarct size following ischaemia-reperfusion injury while ARC deficient mice develop larger infarcts after ischaemia-reperfusion and transition more rapidly and severely to dilated cardiomyopathy following aortic constriction. We have recently found that ARC is upregulated in many different cancers. Inactivation of the pro-apoptotic transcription factor p53 is central to carcinogenesis. We therefore tested the hypothesis that ARC regulates p53 function and here we show that nuclear ARC inactivates p53 through dual mechanisms. In the nucleus, ARC binds directly to the p53 tetramerization domain and inhibits p53 tetramerization. By doing so, ARC exposes a nuclear export signal on p53 and triggers Crm1-dependent p53 hyperexport from the nucleus. The transcriptional activity of p53 is highly dependent on the p53 tetrameric state and therefore by disrupting p53 tetramerization, ARC also disrupts p53 function independently of its effect on p53 nuclear export. Knockdown of endogenous ARC results in spontaneous tetramerization of endogenous p53, its nuclear accumulation, and activation of p53 target genes. To further demonstrate the biological significance of ARC-mediated inactivation of p53, we used a tissue microarray of human breast cancers. We show that in primary human breast cancer, nuclear ARC is associated with wild type p53. In contrast, breast cancers with mutant p53 lack nuclear ARC, in keeping with the observation in cancer that important genetic changes that contribute to carcinogenesis are largely non-redundant. Although other proteins can bind to the tetramerization domain of p53, we have now identified ARC to be the first endogenous protein that couples inhibition of p53 tetramerization to p53 nuclear export. We conclude that nuclear ARC is an important negative regulator of p53.