Fibroblast Growth Factor Activation of the Rat PRL Promoter is Mediated by PKCδ

Fibroblast growth factors play a critical role in cell growth, development, and differentiation and are also implicated in the formation and progression of tumors in a variety of tissues including pituitary. We have previously 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-independent manner. Herein we show using biochemical, molecular, and pharmacological approaches that PKCδ is a critical component of the fibroblast growth factor signaling pathway. PKC inhibitors, or down-regulation of PKC, rendered the rat PRL promoter refractory to subsequent stimulation by fibroblast growth factors, implying a role for PKC in fibroblast growth factor signal transduction. FGFs caused specific translocation of PKCδ from cytosolic to membrane fractions, consistent with enzyme activation. In contrast, other PKCs expressed in GH4T2 cells (α, βI, βII, andε ) did not translocate in response to fibroblast growth factors. The PKCδ subtype-selective inhibitor, rottlerin, or expression of a dominant negative PKCδ adenoviral construct also blocked fibroblast growth factor induction of rat PRL promoter activity, confirming a role for the novel PKCδ isoform. PKC inhibitors selective for the conventional α and β isoforms or dominant negative PKCα adenoviral expression constructs had no effect. Induction of the endogenous PRL gene was also blocked by adenoviral dominant negative PKCδ expression but not by an analogous dominant negative PKCα construct. Finally, rottlerin significantly attenuated FGF-induced MAP kinase phosphorylation. Together, these results indicate that MAP kinase-dependent fibroblast growth factor stimulation of the rat PRL promoter in pituitary cells is mediated by PKCδ.