SMG1 and NIK regulate apoptosis induced by Smac mimetic compounds

Smac mimetic compounds (SMCs) are experimental small molecules that induce tumour necrosis factor alpha (TNF alpha)-dependent cancer cell death by targeting the inhibitor of apoptosis proteins. However, many cancer cell lines are resistant to SMC-mediated apoptosis despite the presence of TNF alpha. To add insight into the mechanism of SMC-resistance, we used functional siRNA-based kinomic and focused chemical screens and identified suppressor of morphogenesis in genitalia-1 (SMG1) and NF-kappa B-inducing kinase (NIK) as novel protective factors. Both SMG1 and NIK prevent SMC-mediated apoptosis likely by maintaining FLICE inhibitory protein (c-FLIP) levels to suppress caspase-8 activation. In SMC-resistant cells, the accumulation of NIK upon SMC treatment enhanced the activity of both the classical and alternative nuclear factor-kappa B pathways, and increased c-FLIP mRNA levels. In parallel, persistent SMG1 expression in SMC-resistant cells repressed SMC-mediated TNF alpha-induced JNK activation and c-FLIP levels were sustained. Importantly, SMC-resistance is overcome by depleting NIK and SMG1, which appear to facilitate the downregulation of c-FLIP in response to SMC and TNF alpha treatment, leading to caspase-8-dependent apoptosis. Collectively, these data show that SMG1 and NIK function as critical repressors of SMC-mediated apoptosis by potentially converging on the regulation of c-FLIP metabolism. Cell Death and Disease (2011) 2, e146; doi: 10.1038/cddis.2011.25; published online 14 April 2011