IKKβ-mediated nuclear factor-κB activation attenuates smac mimetic-induced apoptosis in cancer cells

Smac mimetics (SM) have been recently reported to kill cancer cells through the extrinsic apoptosis pathway mediated by autocrine tumor necrosis factor (TNF). SM also activates nuclear factor-kappa B (NF-kappa B). However, how SM induces NF-kappa B and the role of NF-kappa B in Sm-induced cancer cell death has not been well elucidated. We found that effective blockage of NF-kappa B had no detectable effect on SM compound 3 (SMC3)-induced TNF secretion, suggesting that the induction of TNF by SMC3 is independent of NF-kappa B. Conversely, SMC3-induced NF-kappa B activation was found to be mediated by autocrine TNF because this effect of SMC3 was effectively inhibited when TNF was blocked with either a TNF neutralizing antibody or TNF small interfering RNA. In addition, although SIVIC3 dramatically reduced c-IAP1 level, it had marginal effect on c-IAP2 expression, TNF-induced RIP modification, NF-kappa B activation, and downstream antiapoptosis NF-kappa B target expression. Furthermore, blocking NF-kappa B by targeting IKK beta or ReIA substantially potentiated SMC3-induced cytotoxicity, suggesting that the NF-kappa B pathway inhibits SMC3-induced apoptosis in cancer cells. Our results show that through TNF autocrine, SM induces an IKK beta-mediated NF-kappa B activation pathway that protects cancer cells against SM-induced apoptosis, and thus, NF-kappa B blockage could be an effective approach for improving the anticancer value of SM. [Mol Cancer Ther 2009;8(6):1636-45]