SGK3 Mediates INPP4B-Dependent PI3K Signaling in Breast Cancer

Oncogenic mutations in PIK3CA, the gene encoding the catalytic subunit of phosphoinositide 3-kinase (PI3K), occur with high frequency in breast cancer. The protein kinase Akt is considered to be the primary effector of PIK3CA, although mechanisms by which PI3K mediates Akt-independent tumorigenic signals remain obscure. We show that serum and glucocorticoid-regulated kinase 3 (SGK3) is amplified in breast cancer and activated downstream of PIK3CA in a manner dependent on the phosphoinositide phosphatase INPP4B. Expression of INPP4B leads to enhanced SGK3 activation and suppression of Akt phosphorylation. Activation of SGK3 downstream of PIK3CA and INPP4B is required for 3D proliferation, invasive migration, and tumorigenesis in vivo. We further show that SGK3 targets the metastasis suppressor NDRG1 for degradation by Fbw7. We propose a model in which breast cancers harboring oncogenic PIK3CA activate SGK3 signaling while suppressing Akt, indicative of oncogenic functions for both INPP4B and SGK3 in these tumors.