Smac mimetics and TNFalpha: a dangerous liaison?

Inhibitor of apoptosis proteins (IAPs) such as XIAP, cIAP1, and cIAP2 are upregulated in many cancer cells. It has been thought that small-molecule mimetics of Smac, an endogenous IAP antagonist, might potentiate apoptosis in cancer cells by promoting caspase activation. However, three recent papers, two in Cell (Vince et al., 2007; Varfolomeev et al., 2007) and one in Cancer Cell (Petersen et al., 2007), now report that Smac mimetics primarily kill cancer cells via a different mechanism, the induction of autoubiquitination and degradation of cIAPs, which culminates in TNFalpha-mediated cell death.