Metformin sensitizes human bladder cancer cells to TRAIL-induced apoptosis through mTOR/S6K1-mediated downregulation of c-FLIP.
Metformin, an oral antidiabetic agent, has been reported to potentiate chemotherapeutic-induced cytotoxicity. In this study, we investigated the effects and molecular mechanisms of metformin in sensitizing tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in human bladder cancer cells. Metformin alone did not induce apoptosis, but markedly potentiated TRAIL-induced apoptosis in 253J and RT4 bladder cancer cells. To elucidate the underlying mechanism, we examined the modulatory effects of metformin on the key components of the TRAIL signaling pathway and found that metformin did not alter the expression levels of death receptor 4 (DR4) and death receptor 5 (DR5), but significantly reduced the cellular Fas-associated death domain (FADD)-like interleukin-1β-converting enzyme (FLICE) inhibitory protein (c-FLIP) levels, contributing toward the sensitization to TRAIL. Further experiments showed that metformin did not affect the mRNA level, proteasomal degradation, and protein stability of c-FLIPL. However, metformin inhibited the mTOR/S6K1 pathway in 253J and RT4 cells, which usually regulates protein translation; moreover, knockdown of S6K1 effectively reduced the levels of c-FLIPL, indicating that metformin downregulates c-FLIP through inhibition of the mTOR/S6K1 pathway. In addition, AMP-activated protein kinase (AMPK) inhibitor compound C did not prevent the inhibitory effects of metformin on the mTOR/S6K1 pathway and metformin-mediated sensitization to TRAIL. Taken together, our results indicate that metformin sensitizes human bladder cancer cells to TRAIL-induced apoptosis through downregulation of c-FLIP, which is mediated by the mTOR/S6K1 pathway, but independent of AMPK; furthermore, these findings provide a rationale for the combined application of metformin with TRAIL in the treatment of bladder cancer.
aMinimally Invasive Urology Center, Provincial Hospital Affiliated to Shandong University, Jinan bDepartment of Urology, The First Hospital of Xi'an Jiaotong University cOncology Research Laboratory, Key Laboratory of Environment and Genes Related to Diseases, Ministry of Education, Xi'an, People's Republic of China.
Tao
T
Zhang
Xinyang
X
Wang
Dalin
D
He
Xunbo
X
Jin
Peng
P
Guo
D055372:AMP-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D015536:Down-Regulation; D004357:Drug Synergism; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D008687:Metformin / Q000494:pharmacology*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism*; D015398:Signal Transduction; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000378:metabolism* / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D001749:Urinary Bladder Neoplasms / Q000378:metabolism / Q000473:pathology*
D051017:Apoptosis Regulatory Proteins; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; C506386:CFLAR protein, human; D007004:Hypoglycemic Agents; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; D008687:Metformin; C546842:MTOR protein, human; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; C448901:ribosomal protein S6 kinase, 70kD, polypeptide 1; D055372:AMP-Activated Protein Kinases