Bim and Mcl-1 coordinate NVP-BEZ235-induced renal cell carcinoma cell apoptosis

Dysregulation of the PI3K/Akt/mTOR pathway has been reported in renal cell carcinoma (RCC) and is associated with an aggressive phenotype and a poor prognosis. To obtain insights into the action mechanisms of PI3K/mTOR dual inhibitors, the anti-tumor actions of NVP-BEZ235 were investigated in human 786-O and ACHN RCC cells. NVP-BEZ235 decreased cell proliferation and migration, and induced autophagic cell death. Inactivation of the Akt by NVP-BEZ235 was accompanied by forkhead box O1 (FOXO1) and extracellular signal-regulated kinase (ERK) activation as well as signal transducer and activator of transcription 3 (Stat3) inactivation. Despite the reduction of Mcl-1 and accumulation of Bim seen in NVP-BEZ235-treated cells, evidence of apoptosis was rare. Bcl-2 inhibitor ABT-737 and Mcl-1 inhibitor AZD5991 predisposed NVP-BEZ235-treated cells to transform into the apoptotic phenotype. PI3K inhibitor LY294002 and Stat3 inhibitor AG490 duplicated the sensitized actions towards NVP-BEZ235. FoxO1 had roles in NVP-BEZ235-induced Bim expression. Data on pharmacological approaches with ubiquitin proteasome inhibitor together with genetic silencing highlight a role of Bim in NVP-BEZ235-directed RCC cell apoptosis. However, the pro-apoptotic actions of Bim were limited by a compensatory activation of ERK, resulting in decreased Bim protein stability. Data of in vivo tumor-bearing studies further revealed a better anti-tumor potential in the combination treatment of NVP-BEZ235 and MEK/ERK inhibitors without obvious toxicity. Our findings suggest that the feedback activation of pro-survival machinery is likely to be the main cause of cancer cells being refractory to NVP-BEZ235, and a combination treatment is a feasible strategy to sensitize cancer cell responses.