Nuclear Forkhead Box O1 Controls and Integrates Key Signaling Pathways in Hepatocytes

Insulin inhibits forkhead O class (FoxO) transcription factors, which down-regulate the expression of genes involved in metabolism, cell cycle arrest, and apoptosis. After being phosphorylated by protein kinase B (PKB) on S253 in its DNA-binding domain, Foxo1 is phosphorylated on T24 and additional sites, which overall triggers its nuclear exclusion. During this process, Foxo1 is thought to retain some transcriptional activity and signaling potential. To evaluate this Foxo1 action, we used a Foxo1-ADA mutant that is constitutively nuclear due to mutation of T24 and S316 to A and harbors a mutation of S253 to D. Adenoviral-mediated expression of Foxo1-ADA in hepatocytes activates PKB and MAPK pathways more than expression of wild-type or of a transactivation domain-deleted mutant (Δ256). PKB activation cannot be accounted for by a Foxo1-mediated increase in upstream signaling components such as insulin receptor substrate 1 or 2 or by Foxo1-mediated down-regulation of Tribbles homolog 3. In contrast, Foxo1-...