Blocking NF-kappa B and Akt by Hsp90 inhibition sensitizes Smac mimetic compound 3-induced extrinsic apoptosis pathway and results in synergistic cancer cell death

NF-kappa B and Akt are two main cell survival pathways that attenuate the anticancer efficacy of therapeutics. Our previous studies demonstrated that the Smac mimetic compound 3 (SMC3) specifically suppresses c-IAP1 and induces TNF-alpha autocrine to kill cancer cells. However, SMC3 also induces a cell survival signal through NF-kappa B activation. In this report, we further found that SMC3 potently activates Akt, which inhibits SMC3-induced cancer cell death. Strikingly, concurrent blocking NF-kappa B and Akt resulted in a significantly potentiated cytotoxicity. Because heat shock protein 90 (Hsp90) plays an important role in maintaining the integrity of both the NF-kappa B and Akt pathways in cancer cells, we examined if suppression of Hsp90 is able to potentiate SMC3-induced cancer cell death. The results show that targeting Hsp90 does not interfere with SMC3-induced c-IAP1 degradation and TNF-alpha autocrine, the key processes for SMC3-induced cancer cell apoptosis. However, Hsp90 inhibitors effectively blocked SMC3-induced NF-kappa B activation through degradation of RIP1 and IKK beta, two key components of the NF-kappa B activation pathway, and reduced both the constitutive and SMC3-induced Akt activity through degradation of the Akt protein. Consistently, with the co-treatment of SMC3 and Hsp90 inhibitors, apoptosis was markedly sensitized and a synergistic cytotoxicity was observed. The results suggest that concurrent targeting c-IAP1 and Hsp90 by combination of SMC3 and Hsp90 inhibitors is an effective approach for improving the anticancer value of SMC3.