Induction of Mxi1-SR alpha by FOXO3a contributes to repression of Myc-dependent gene expression.
Forkhead transcription factors of the O class (FOXOs) are important targets of the phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway. FOXOs have been implicated in the regulation of cell cycle progression, oxidative stress resistance, and apoptosis. Using DNA microarrays, we analyzed the transcriptional response to FOXO3a activation by gene expression analysis in DLD-1 colon cancer cells stably expressing a FOXO3a.A3-ER fusion protein. We found that activation of FOXO3a resulted in repression of a number of previously identified Myc target genes. Furthermore, FOXO3a activation induced expression of several members of the Mad/Mxd family of transcriptional repressors, most notably Mxi1. The induction of Mxi1 by FOXO3a was specific to the Mxi1-SR alpha isoform and was mediated by three highly conserved FOXO binding sites within the first intron of the gene. Activation of FOXO3a in response to inhibition of Akt also resulted in activation of Mxi1-SR alpha expression. Silencing of Mxi1 by small interfering RNA (siRNA) reduced FOXO3a-mediated repression of a number of Myc target genes. We also observed that FOXO3a activation induced a switch in promoter occupancy from Myc to Mxi1 on the E-box containing promoter regions of two Myc target genes, APEX and FOXM1. siRNA-mediated transient silencing of Mxi1 or all Mad/Mxd proteins reduced exit from S phase in response to FOXO3a activation, and stable silencing of Mxi1 or Mad1 reduced the growth inhibitory effect of FOXO3a. We conclude that induction of Mad/Mxd proteins contributes to the inhibition of proliferation in response to FOXO3a activation. Our results provide evidence of direct regulation of Mxi1 by FOXO3a and imply an additional mechanism through which the PI3-kinase/Akt/FOXO pathway can modulate Myc function.
Delpuech
Oona
O
Gene Expression Analysis Laboratory, Cancer Research UK London Research Institute, 44 Lincoln's Inn Fields, London WC2A 3PX, United Kingdom.
Griffiths
Beatrice
B
East
Philip
P
Essafi
Abdelkader
A
Lam
Eric W-F
EW
Burgering
Boudewijn
B
Downward
Julian
J
Schulze
Almut
A
D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics*; D002453:Cell Cycle; D045744:Cell Line, Tumor; D017124:Conserved Sequence; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic; D020868:Gene Silencing; D006801:Humans; D007438:Introns / Q000235:genetics; D011485:Protein Binding; D020033:Protein Isoforms / Q000235:genetics; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics; D012097:Repressor Proteins / Q000378:metabolism*; D014158:Transcription, Genetic; D025521:Tumor Suppressor Proteins / Q000235:genetics*
D051792:Basic Helix-Loop-Helix Proteins; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D020033:Protein Isoforms; D051057:Proto-Oncogene Proteins c-akt; D016271:Proto-Oncogene Proteins c-myc; D012097:Repressor Proteins; D025521:Tumor Suppressor Proteins; C499517:FOXO3 protein, human; C079440:MXI1 protein, human