FUNCTIONAL COMPONENTS OF FIBROBLAST GROWTH-FACTOR (FGF) SIGNAL-TRANSDUCTION IN PITUITARY-CELLS - IDENTIFICATION OF FGF RESPONSE ELEMENTS INTHE PROLACTIN GENE

Fibroblast growth factors (FGFs) have been implicated in pituitary lac totroph tumorigenesis; however, little is known about the molecular me chanisms of FGF signal transduction. We used a transient transfection approach, in GH4 cells, to identify components of the FGF signaling pa thway leading to activation of the rat prolactin (rPRL) promoter. Usin g dominant-negative constructs of p21(Ras), Raf-1 kinase, and mitogen activated protein (MAP) kinase, we show that FGF activation of the rPR L promoter is independent of Pas and Raf-1 but requires MAP kinase. Fu rthermore, MAP kinase but not Raf-1 kinase catalytic activity is stimu lated by FGFs. The rPRL promoter FGF response maps to two Ets binding sites, centered at -212 (FRE1) and -96 (FRE2), and co transfection of dominant-negative Ets inhibits FGF activation, FRE1 co-localizes with a composite, Ets/GHF-1, has response element. However, overexpression of Ets-1 and GHF-1, which potentiate the Ras response, inhibits FGF st imulation of the rPRL promoter, implying that Ras and FGF signaling pa thways target distinct factors to elicit their effects. These data sug gest that Ets factors serve to sort and integrate MAP kinase-dependent growth factor signals, allowing highly specific transcriptional respo nses to be mediated via the interaction of distinct Ets proteins and c ofactors at common response elements.