Abstract
Smac mimetics target cancer cells in a TNFalpha-dependent manner, partly via proteasome degradation of cellular inhibitor of apoptosis 1 (cIAP1) and cIAP2. Degradation of cIAPs triggers the release of receptor interacting protein kinase (RIPK1) from TNF receptor I (TNFR1) to form a caspase-8 activating complex together with the adaptor protein Fas-associated death domain (FADD). We report here a means through which cancer cells mediate resistance to Smac mimetic/TNFalpha-induced apoptosis and corresponding strategies to overcome such resistance. These human cancer cell lines evades Smac mimetic-induced apoptosis by up-regulation of cIAP2, which although initially degraded, rebounds and is refractory to subsequent degradation. cIAP2 is induced by TNFalpha via NF-kappaB and modulation of the NF-kappaB signal renders otherwise resistant cells sensitive to Smac mimetics. In addition, other signaling pathways, including phosphatidyl inositol-3 kinase (PI3K), have the potential to concurrently regulate cIAP2. Using the PI3K inhibitor, LY294002, cIAP2 up-regulation was suppressed and resistance to Smac mimetics-induced apoptosis was also overcome.