Characterization of insulin inhibition of transactivation by a C-terminal fragment of the forkhead transcription factor Foxo1 in rat hepatoma cells.

The transcription factor Foxo1 controls the expression of genes involved in fundamental cellular processes. In keeping with its important physiological roles, Foxo1 activity is negatively regulated in response to growth factors and cytokines that activate a phosphatidylinositol 3-kinase (PI 3-kinase) protein kinase B (PKB)/Akt pathway. PKB/Akt-mediated phosphorylation of Foxo1 has been shown to result in the inhibition of target gene transcription and to trigger the export of Foxo1 from the nucleus, which is generally believed to explain the subsequent decrease of transcription. In the present study, using a chimeric protein in which a C-terminal fragment of Foxo1 (amino acids 208-652) containing the transactivation domain is fused to the yeast Gal4 DNA binding domain, we present evidence showing that insulin can directly regulate transactivation by Foxo1 in H4IIE rat hepatoma cells. Insulin inhibition of Foxo1-(208-652)-stimulated transactivation is mediated by PI 3-kinase but in contrast to full-length Foxo1, does not require either of the two PKB/Akt phosphorylation sites (Ser253 and Ser316) present in the protein fragment. Using mutational and deletion studies, we identify two potential phosphorylation sites, Ser319 and Ser499, as well as a 15-amino acid region located between residues 350 and 364 that are critical for insulin inhibition of transactivation by Foxo1-(208-652). We conclude that the transcriptional activity of Foxo1 is regulated at different levels by insulin: transactivation, as well as DNA binding and nuclear exclusion. These different regulatory mechanisms allow the precise control of transcription of Foxo1 target genes by insulin.