Fibroblast growth factor activation of the rat PRL promoter is mediated byPKC delta

Fibroblast growth factors play a critical role in cell growth, development, and differentiation and are also implicated in the formation and progressi on of tumors in a variety of tissues including pituitary. We have previousl y shown that fibroblast growth factor activation of the rat PRL promoter in GH4T2 pituitary tumor cells is mediated via MAP kinase in a Ras/Raf-1-inde pendent manner. Herein we show using biochemical, molecular, and pharmacolo gical approaches that PKC delta is a critical component of the fibroblast g rowth factor signaling pathway. PKC inhibitors, or down-regulation of PKC, rendered the rat PRL promoter refractory to subsequent stimulation by fibro blast growth factors, implying a role for PKC in fibroblast growth factor s ignal transduction. FGFs caused specific translocation of PKC delta from cy tosolic to membrane fractions, consistent with enzyme activation. In contra st, other PKCs expressed in GH4T2 cells (alpha, betaI, beta II, and epsilon ) did not translocate in response to fibroblast growth factors. The PKC del ta subtype-selective inhibitor, rottlerin, or expression of a dominant nega tive PKC delta adenoviral construct also blocked fibroblast growth factor i nduction of rat PRL promoter activity, confirming a role for the novel PKC delta isoform. PKC inhibitors selective for the conventional alpha and beta isoforms or dominant negative PKC alpha adenoviral expression constructs h ad no effect. Induction of the endogenous PRL gene was also blocked by aden oviral dominant negative PKC delta expression but not by an analogous domin ant negative PKC alphaa construct. Finally, rottlerin significantly attenua ted FGF-induced MAP kinase phosphorylation. Together, these results indicat e that MAP kinase-dependent fibroblast growth factor stimulation of the rat PRL promoter in pituitary cells is mediated by PKC delta.