Differential regulation of gene expression by insulin and IGF-1 receptors correlates with phosphorylation of a single amino acid residue in the forkhead transcription factor FKHR

The transcription factor FKHR is inhibited by phosphorylation in response to insulin and IGF-1 through Akt kinase. Here we show that FKHR phosphorylation in hepatocytes conforms to a hierarchical pattern in which phosphorylation of the Akt site at S-253, in the forkhead DNA binding domain, is a prerequisite for the phosphorylation of two additional potential Akt sites, T-24 and S-316. Using insulin receptor-deficient hepatocytes, we show that T-24 fails to be phosphorylated by IGF-1 receptors, suggesting that this residue is targeted by a kinase specifically activated by insulin receptors, Lack of T-24 phosphorylation is associated with the failure of IGF-1 to induce nuclear export of FKHR, and to inhibit expression of a reporter gene under the transcriptional control of the IGF binding protein-1 insulin response element. We propose that site-specific phosphorylation of FKHR is one of the mechanisms by which insulin and IGF-I receptors exert different effects on gene expression.