cIAP2 represses IKKα/β-mediated activation of MDM2 to prevent p53 degradation

Cellular inhibitor of apoptosis proteins (cIAP1 and cIAP2) function to prevent apoptosis and are often overexpressed in various cancers. However, mutations in cIAP1/2 can activate the alternative NF kappa B pathway through IB alpha-kinase-alpha (IKK alpha) and are associated with hematopoetic malignancies. In the current study, we found that knockdown of cIAP2 in human mammary epithelial cells resulted in activation of MDM2 through increased SUMOylation and profound reduction of the pool of MDM2 not phosphorylated at Ser166. cIAP2 siRNA markedly decreased p53 levels, which were rescued by addition of the MDM2 inhibitor, Nutlin3a. An IAP antagonist, which induces cIAP degradation, transiently increased MDM2 mRNA. Simultaneous transfection of siRNA for cIAP2 and IKKa reduced MDM2 protein, while expression of a kinase-dead IKK beta strongly increased non-Ser166 P-MDM2. Inhibition of either IKK alpha or -beta partially rescued p53 levels, while concomitant IKK alpha/beta inhibition fully rescued p53 after cIAP2 knockdown. Surprisingly, IKK alpha knockdown alone increased SUMO-MDM2, suggesting that in the absence of activation, IKK alpha can prevent MDM2 SUMOylation. cIAP2 knockdown disrupted the interaction between the MDM2 SUMO ligase, PIAS1 and IKK alpha. Partial knockdown of cIAP2 cooperated with H-V12-ras-transfected mammary epithelial cells to enhance colony formation. In summary, our data identify a novel role for cIAP2 in maintaining wild-type p53 levels by preventing both an NF kappa B-mediated increase and IKK alpha/-beta-dependent transcriptional and post-translational modifications of MDM2. Thus, mutations or reductions in cIAP2 could contribute to cancer promotion, in part, through downregulation of p53.