Active FKHRL1 overcomes imatinib resistance in chronic myelogenous leukemia‐derived cell lines via the production of tumor necrosis factor‐related apoptosis‐inducing ligand

FKHRL1 (also called FOXO3a) is a member of the Forkhead Box, class O (FOXO) subfamily of forkhead transcription factors and functions downstream of Bcr–Abl tyrosine kinase as a phosphorylated inactive form in chronic myelogenous leukemia (CML). The Bcr–Abl tyrosine kinase inhibitor imatinib induces cell cycle arrest and subsequent apoptosis via the conversion of FKHRL1 from the phosphorylated inactive form to the dephosphorylated active form in CML-derived cell lines. In the present study, we examined whether active FKHRL1 can overcome resistance to imatinib. To this end, we generated a 4-hydroxytamoxifen-inducible active FKHRL1 (FKHRL1-TM; a triple mutant of FKHRL1 in which all three Akt phosphorylation sites have been mutated)–estrogen receptor fusion protein expression system in CML-derived imatinib-resistant cell lines. 4-Hydroxytamoxifen inhibited cell growth and cell cycle progression, and subsequently induced apoptosis, accompanied by upregulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Thus, active FKHRL1 antagonized deregulated proliferation and induced apoptosis in these cell lines. In addition, imatinib-resistant cells underwent apoptosis after transfection with full-length TRAIL cDNA. Collectively, our results suggest that active FKHRL1 can overcome imatinib resistance in CML cells, in part via TRAIL production. (Cancer Sci 2007; 98: 1949–1958)