FOXO3a induces differentiation of Bcr-Abl-transformed cells through transcriptional down-regulation of Id1.
Leukemic transformation often requires activation of protein kinase B (PKB/c-Akt) and is characterized by increased proliferation, decreased apoptosis, and a differentiation block. PKB phosphorylates and inactivates members of the FOXO subfamily of Forkhead transcription factors. It has been suggested that hyperactivation of PKB maintains the leukemic phenotype through actively repressing FOXO-mediated regulation of specific genes. We have found expression of the transcriptional repressor Id1 (inhibitor of DNA binding 1) to be abrogated by FOXO3a activation. Inhibition of PKB activation or growth factor deprivation also resulted in strong down-regulation of Id1 promoter activity, Id1 mRNA, and protein expression. Id1 is highly expressed in Bcr-Abl-transformed K562 cells, correlating with high PKB activation and FOXO3a phosphorylation. Inhibition of Bcr-Abl by the chemical inhibitor STI571 resulted in activation of FOXO3a and down-regulation of Id1 expression. By performing chromatin immunoprecipitation assays and promoter-mutation analysis, we demonstrate that FOXO3a acts as a transcriptional repressor by directly binding to the Id1 promoter. STI571 treatment, or expression of constitutively active FOXO3a, resulted in erythroid differentiation of K562 cells, which was inhibited by ectopic expression of Id1. Taken together our data strongly suggest that high expression of Id1, through PKB-mediated inhibition of FOXO3a, is critical for maintenance of the leukemic phenotype.
Birkenkamp
Kim U
KU
Molecular Immunology Laboratory, Department of Immunology, University Medical Center, KC.02.085.2, Lundiaan 6, 3584-CX Utrecht, The Netherlands.
Essafi
Abdelkader
A
van der Vos
Kristan E
KE
da Costa
Marco
M
Hui
Rosaline C-Y
RC
Holstege
Frank
F
Koenderman
Leo
L
Lam
Eric W-F
EW
Coffer
Paul J
PJ
D000818:Animals; D001483:Base Sequence; D001854:Bone Marrow Cells / Q000378:metabolism / Q000473:pathology; D002454:Cell Differentiation / Q000235:genetics; D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic; D015536:Down-Regulation; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000235:genetics* / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D016313:Genes, abl; D006801:Humans; D051798:Inhibitor of Differentiation Protein 1 / Q000235:genetics* / Q000378:metabolism; D020014:K562 Cells; D007938:Leukemia / Q000235:genetics / Q000473:pathology; D051379:Mice; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D014158:Transcription, Genetic
D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; C474033:FoxO3 protein, mouse; C497455:Idb1 protein, mouse; D051798:Inhibitor of Differentiation Protein 1; D051057:Proto-Oncogene Proteins c-akt