DAF-16 recruits the CREB-binding protein coactivator complex to the insulin-like growth factor binding protein 1 promoter in HepG2 cells

Insulin negatively regulates expression of the insulin-like growth factor b inding protein 1 (IGFBP-1) gene by means of an insulin-responsive element ( IRE) that also contributes to glucocorticoid stimulation of this gene. We f ind that the Caenorhabditis elegans protein DAF-16 binds the IGFBP-1 IRE wi th specificity similar to that of the forkhead (FKH) factor(s) that act bot h to enhance glucocorticoid responsiveness and to mediate the negative effe ct of insulin at this site. In HepG2 cells, DAF-16 and its mammalian homolo gs, FKHR, FKHRL1. and AFX, activate transcription through the IGFBP-1.IRE; this effect is inhibited by the viral oncoprotein E1A, but not by mutants o f E1A that fail to interact with the coactivator p300/CREB-binding protein (CBP), We show that DAF-16 and FKHR can interact with both the KIX and E1A/ SRC interaction domains of p300/CBP, as well as the steroid receptor coacti vator (SRC). A C-terminal deletion mutant of DAF-16 that is nonfunctional i n C. elegans fails to bind the KIX domain of CBP, fails to activate transcr iption through the IGFBP-1-IRE, and inhibits activation of the IGFBP-1 prom oter by glucocorticoids. Thus, the interaction of DAF-16 homologs with the KIX domain of CBP is essential to basal and glucocorticoid-stimulated trans activation, Although AFX interacts with the KIX domain of CBP, it does not interact with SRC and does not respond to glucocorticoids or insulin. Thus, we conclude that DAF-16 and FKHR act as accessory factors to the glucocort icoid response, by recruiting the p300/CBP/SRC coactivator complex to an FK H factor site in the IGFBP-1 promoter, which allows the cell to integrate t he effects of glucocorticoids and insulin on genes that carry this site.