Antiapoptotic Effect of Serum and Glucocorticoid-Inducible Protein Kinase Is Mediated by Novel Mechanism Activating IκB Kinase

Serum and glucocorticoid inducible protein kinase (SGK) plays a crucial role in promoting cell survival, but the mechanisms for this response are not clear. We show that SGK is involved in the regulation of apoptosis in breast cancer cells by modulating the transcriptional activity of nuclear transcription factor kappaB (NF-kappaB). High levels of SGK expression were observed in human breast cancer samples. When SGK was reduced the apoptotic rate increased, and increased SGK activity prevents serum withdrawal-induced apoptosis. SGK-induced cell survival was abolished by a dominant-negative form of IkappaB kinase beta (IKKbeta, K44A) or a null mutation of IKKbeta in mouse embryonic fibroblast cells indicating involvement of the NF-kappaB pathway. Serum-induced SGK or increased expression of SGK activated NF-kappaB transcriptional activity, whereas small interference RNA to SGK blocked NF-kappaB activity. Coexpression of SGK and IKKbeta significantly increased the activation of NF-kappaB (versus expression of IKKbeta alone). Expression of dominant-negative IKKbeta K44A, IkappaBalpha AA, and kinase-dead SGK (127KM) blocked the ability of SGK to stimulate NF-kappaB activity, suggesting that IKKbeta is a target of SGK. We also show that SGK enhances the ability of IKKbeta to phosphorylate endogenous IkappaBalpha in cells or recombinant glutathione S-transferase-IkappaBalpha in vitro and increases IkappaBalpha degradation; SGK physically associates with and activates IKKbeta in MDA231 cells via phosphorylation of Ser(181) in IKKbeta. Taken together, we conclude that SGK acts as an oncogene in breast cancer cells through activation of the IKK-NF-kappaB pathway, thereby preventing apoptosis. Blocking SGK expression/activity represents a potential therapeutic approach for breast cancer treatment.