Transforming growth factor beta enhances epithelial cell survival via Akt-dependent regulation of FKHRL1

The Forkhead family of transcription factors participates in the induction of death-related genes. In NMuMG and 4T1 mammary epithelial cells, transfor ming growth factor beta (TGF beta) induced phosphorylation and cytoplasmic retention of the Forkhead factor FKHRL1, while reducing FHKRL1-dependent tr anscriptional activity. TGF beta -induced FKHRL1 phosphorylation and nuclea r exclusion were inhibited by LY294002, an inhibitor of phosphatidylinosito l-3 kinase. A triple mutant of FKHRL1, in which all three Akt phosphorylati on sites have been mutated (TMFKHRL1), did not translocate to the cytoplasm in response to TGF beta. In HaCaT keratinocytes, expression of dominant-ne gative Akt prevented TGF beta -induced 1) reduction of Forkhead-dependent t ranscription, 2) FKHRL1 phosphorylation, and 3) nuclear exclusion of FKRHL1 . Forced expression of either wild-type (WT) or TM-FKHRL1, but not a FKHRL1 mutant with deletion of the transactivation domain, resulted in NMuMG mamm ary cell apoptosis., Evidence of nuclear fragmentation colocalized to cells with expression of WT- or TM-FKHRL1. The apoptotic effect of WT-FKHRL1 but not TM-FKHRL1 was prevented by exogenous TGF beta. Serum starvation-induce d apoptosis was also inhibited by TGF beta in NMuMG and HaCaT cells. Finall y, dominant-negative Akt abrogated the antiapoptotic effect of TGF beta. Ta ken together, these data suggest that TGF beta may play a role in epithelia l cell survival via Akt-dependent regulation of FKHRL1.