A member of Forkhead transcription factor FKHRL1 is a downstream effector of STI571-induced cell cycle arrest in BCR-ABL-expressing cells.
A member of the Forkhead transcription factor family, FKHRL1, lies downstream of the phosphatidylinositol 3-kinase-Akt activation pathway in cytokine signaling. Because the phosphatidylinositol 3-kinase-Akt activation pathway is required for BCR-ABL-mediated transformation and survival signaling in chronic myelogenous leukemia (CML), in this study we examined the involvement of FKHRL1 in the BCR-ABL-mediated signaling pathway. FKHRL1 was constitutively phosphorylated in BCR-ABL-expressing cell lines KCL22 and KU812, and its phosphorylation was inhibited by treatment with STI571, a specific inhibitor of BCR-ABL tyrosine kinase. Concomitantly, STI571 induced cell cycle arrest at the G(0)/G(1) phase, accompanied by up-regulation of a cyclin-dependent kinase inhibitor p27/Kip1 in KCL22 cells. In addition, FKHRL1 was constitutively phosphorylated in the TF-1/bcr-abl cell line ectopically expressing BCR-ABL but not in the parent TF-1 cell line. Considering several lines of evidence that phosphorylated FKHRL1 has lost transcriptional activity and that p27/Kip1 expression is positively regulated by dephosphorylated "active" FKHRL1, BCR-ABL may down-regulate p27/Kip1 expression via the loss of FKHRL1 function as a transcription factor. To demonstrate this hypothesis, we generated a tamoxifen-inducible "active FKHRL1" FKHRL1-TM (a triple mutant of FKHRL1, in which all three Akt phosphorylation sites have been mutated), estrogen receptor system in the KCL22 cell line. The addition of tamoxifen inhibited the cell growth indicating that overexpression of FKHRL1 in the nucleus antagonized deregulated proliferation of CML cells. Collectively, FKHRL1 regulates the expression of p27/Kip1 as a downstream molecule of BCR-ABL signaling in CML cells. BCR-ABL-induced loss of FKHRL1 function may be involved in oncogenic transformation of CML partially via the down-regulation of p27/Kip1 proteins.
Komatsu
Norio
N
Division of Hematology, Department of Medicine, Jichi Medical School, Tochigi 329-0498, Japan. nkomatsu@jichi.ac.jp
Watanabe
Tomoko
T
Uchida
Mie
M
Mori
Masaki
M
Kirito
Keita
K
Kikuchi
Satoru
S
Liu
Qifa
Q
Tauchi
Tetsuzo
T
Miyazawa
Keisuke
K
Endo
Hitoshi
H
Nagai
Tadashi
T
Ozawa
Keiya
K
D000818:Animals; D001549:Benzamides; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology*; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000378:metabolism*; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D016044:Fusion Proteins, bcr-abl / Q000235:genetics* / Q000378:metabolism; D016193:G1 Phase / Q000187:drug effects; D016195:G2 Phase / Q000187:drug effects; D006801:Humans; D000068877:Imatinib Mesylate; D015464:Leukemia, Myelogenous, Chronic, BCR-ABL Positive; D010766:Phosphorylation; D010879:Piperazines; D011743:Pyrimidines / Q000494:pharmacology*; D014157:Transcription Factors / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured
D001549:Benzamides; D004268:DNA-Binding Proteins; C085126:FOXO1 protein, human; C499517:FOXO3 protein, human; D000071161:Forkhead Box Protein O1; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D010879:Piperazines; D011743:Pyrimidines; D014157:Transcription Factors; D000068877:Imatinib Mesylate; D016044:Fusion Proteins, bcr-abl