A role for Akt in mediating the estrogenic functions of epidermal growth factor and insulin-like growth factor I

This study examines whether the serine/threonine protein kinase, Akt, is in volved in the cross-talk between epidermal growth factor (EGF) and insulin- related growth factor I (IGF-I) receptors and ER-alpha. Treatment of MCF-7 cells with either EGF or IGF-I resulted in a rapid phosphorylation of Akt a nd a 14- to 16-fold increase in Akt activity, respectively. Akt activation was blocked by inhibitors of phosphatidylinositol 3-kinase, but not by an i nhibitor of the ribosomal protein kinase p70(S6K). Stable transfection of c ells with a dominant negative Akt mutant blocked the effects of EGF and IGF -I on ER-alpha expression and activity, whereas stable transfection of cell s with a constitutively active Akt mutant mimicked the effects of EGF and I GF-I. In the latter cells, there was a decrease in the amount of ER-alpha p rotein and messenger RNA (70-80%) and an increase in the amount of progeste rone receptor protein, messenger RNA (4- to 9- and by 3.5- to 7-fold, respe ctively) and pS2 (3- to 5-fold). Coexpression of wild-type ER-cu and the do minant negative Akt mutant in COS-1 cells also blocked the growth factor-st imulated activation of ER-alpha, but coexpression of the wild-type receptor with the constitutively active Akt mutant increased ER-alpha activity. Rec eptor activation was blocked by an antiestrogen. Studies using mutants of E R-alpha demonstrated that Akt increased estrogen receptor activity through the amino-terminal activation function-1 (AF-1). Serines S104 S106, S118, a nd S167 appear to play a role in the activation of ER-alpha by Akt.