FoxO Transcription Factors Promote AKT Ser473 Phosphorylation and Renal Tumor Growth in Response to Pharmacologic Inhibition of the PI3K–AKT Pathway

The PI3K-AKT pathway is hyperactivated in many human cancers, and several drugs to inhibit this pathway, including the PI3K/mTOR dual inhibitor NVP-BEZ235, are currently being tested in various pre-clinical and clinical trials. It has been shown that pharmacological inhibition of the PI3K-AKT pathway results in feedback activation of other oncogenic signaling pathways, which likely will limit the clinical utilization of these inhibitors in cancer treatment. However, the underlying mechanisms of such feedback regulation remain incompletely understood. The PI3K-AKT pathway is a validated therapeutic target in renal cell carcinoma (RCC). Here we show that FoxO transcription factors serve to promote AKT phosphorylation at Ser473 in response to NVP-BEZ235 treatment in renal cancer cells. Inactivation of FoxO attenuated NVP-BEZ235-induced AKT Ser473 phosphorylation, and rendered renal cancer cells more susceptible to NVP-BEZ235-mediated cell growth suppression in vitro and tumor shrinkage in vivo. Mechanistically, we showed that FoxOs upregulated the expression of Rictor, an essential component of mammalian target of rapamycin complex 2 (mTORC2), in response to NVP-BEZ235 treatment, and revealed that Rictor is a key downstream target of FoxOs in NVP-BEZ235-mediated feedback regulation. Finally, we show that FoxOs similarly modulate the feedback response on AKT Ser473 phosphorylation and renal tumor growth by other PI3K or AKT inhibitor treatment. Together, our study reveals a novel mechanism of PI3K-AKT inhibition-mediated feedback regulation, and may identify FoxO as a novel biomarker to stratify RCC patients for PI3K or AKT inhibitor treatment, or a novel therapeutic target to synergize with PI3K-AKT inhibition in RCC treatment.