FOXO1 Inhibition and FADD Knockdown Have Opposing Effects on Anticancer Drug-Induced Cytotoxicity and p21 Expression in Osteosarcoma Cells.

Forkhead box class O1 (FOXO1) and fas-associated death domain (FADD) regulate cell death pathways and homeostatic processes such as cell cycle progression and apoptosis. FADD phosphorylation promotes nuclear localization of FOXO1, and FOXO1 regulates FADD expression. Therefore, it is plausible that FOXO1 and FADD have synergistic or antagonistic effects on cell cycle regulation and the response to anticancer drug treatment in cancer cells. In the present study, we report that AS1842856-mediated inhibition of FOXO1 reverses anticancer drug-induced cytotoxicity, while FADD knockdown increases anticancer drug-induced cytotoxicity in osteosarcoma (OS). Reversed anticancer drug-induced cytotoxicity was accompanied by G2/M cell cycle arrest and increased expression of p21. The anticancer function of FOXO1 was further supported by the observation that OS cells that express higher basal levels of FOXO1 had increased sensitivity to camptothecin-induced cytotoxicity. FADD knockdown reversed the FOXO1 inhibition-induced increase in p21 expression. The results presented in this study indicate that FOXO1 has a tumor suppressor function, while FADD has a tumor-promoting function in OS following anticancer drug treatment. The experimental approach used in this investigation also indicates that FADD antagonizes the effect of FOXO1 on p21 expression in OS.