Endometrial cancer cell survival and apoptosis is regulated by protein kinase C alpha and delta

Endometrial cancer is the most common invasive gynecologic malignancy but the molecular mechanisms underlying its onset and progression are poorly understood. Paradoxically, endometrial tumors exhibit increased apoptosis, correlating with disease progression and poor patient prognosis. Endometrial tumors also show altered activity and expression of protein kinase C (PKC) isoforms, implicated in the regulation of programmed cell death; however, PKC modulation of apoptosis in endometrial cancer cells has not been investigated. We detected nine out of ten PKC isoforms in Ishikawa endometrial cancer cell lines, and demonstrated expression of both PKCalpha and delta in human endometrial tumors. To determine the functional roles of PKCalpha and delta in apoptosis in endometrial cancer, Ishikawa cells were treated with selective PKC inhibitors or adenoviral constructs encoding wild-type or isoform-specific, dominant-negative mutants. Apoptosis was assessed by DNA fragmentation and caspase-mediated poly-(ADP-ribose)-polymerase cleavage. The inhibition of PKCdelta suppressed etoposide-induced apoptosis, while overexpression of PKCdelta enhanced it. In contrast, inhibition of PKCalpha elevated basal levels of apoptosis and potentiated etoposide-induced cell death. Etoposide treatment also selectively activated PKCdelta, but resulted in both cytosolic translocation and decreased activity of PKCalpha. A fraction of PKCdelta also underwent caspase-dependent cleavage, in response to etoposide. Our results suggest that changes in apoptosis and PKC expression in endometrial cancer are mechanistically linked, such that PKCdelta is required for DNA damage-induced apoptosis, while PKCalpha mediates a survival response. Thus, PKCalpha and delta expression and signaling may be important in endometrial tumorigenesis and could serve as potential prognostic indicators and/or novel targets for therapeutic intervention.

Differential regulation of cell growth and gene expression by FGF-2 and FGF-4 in pituitary lactotroph GH4 cells

Fibroblast growth factors, FGF-2 and FGF-4, are reported to play divergent roles in pituitary differentiation and tumor formation, stimulating cell differentiation or proliferation, respectively. However, mitogenic responses to FGFs have not been extensively characterized and little is known about the molecular mechanisms by which specific FGF isoforms may mediate distinct biological responses. Here we show that FGF-4 but not FGF-2 stimulated DNA synthesis and cell proliferation in GH4 cells. Microarray analyses revealed that FGF-4 induced expression of several oncogenes, growth factor receptors and cell cycle control proteins (e.g. cyclin D3/cdk4, N-myc, c-Raf, insulin and thyroid hormone receptors) while FGF-2 had no effect or down regulated these same genes. These transcriptional responses are consistent with a proliferative and/or tumorigenic role for FGF-4 versus a growth inhibitory effect of FGF-2. FGF-2 and FGF-4 also differentially regulated MAP kinase phosphorylation, which may underlie their isoform-specific effects on cell growth and gene expression.

Fibroblast growth factors regulate prolactin transcription via an atypical Rac-dependent signaling pathway

Fibroblast growth factors (FGFs) play a critical role in pituitary development and in pituitary tumor formation and progression. We have previously characterized FGF signal transduction and regulation of the tissue-specific rat prolactin (rPRL) promoter in GH4 pituitary cells. FGF induction of rPRL transcription is independent of Ras, but mediated by a protein kinase C-delta (PKCdelta)-dependent activation of MAPK (ERK). Here we demonstrate a functional role for the Rho family monomeric G protein, Rac1, in FGF regulation of PRL gene expression via an atypical signaling pathway. Expression of dominant negative Rac, but not RhoA or Cdc42, selectively inhibited FGF-induced rPRL promoter activity. Moreover, expression of dominant negative Rac also attenuated FGF-2 and FGF-4 stimulation of MAPK (ERK). However, in contrast to other Rac-dependent signaling pathways, FGF activation of rPRL promoter activity was independent of the c-Jun N-terminal kinase (JNK) and phosphoinositide 3-kinase/Akt cascades. FGFs failed to activate JNK1 or JNK2, and expression of dominant negative JNK or Akt constructs did not block FGF-induced PRL transcription. Consistent with the role of PKCdelta in FGF regulation of PRL gene expression, activation of the rPRL promoter was blocked by an inhibitor of phospholipase Cgamma (PLCgamma) activity. FGF treatment also induced rapid tyrosine phosphorylation of PLCgamma in a Rac-dependent manner. These results suggest that FGF-2 and FGF-4 activate PRL gene expression via a novel Rac1, PLCgamma, PKCdelta, and ERK cascade, independent of phosphoinositol-3-kinase and JNK.

Fibroblast growth factor activation of the rat PRL promoter is mediated by PKCdelta

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 PKCdelta 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 PKCdelta from cytosolic to membrane fractions, consistent with enzyme activation. In contrast, other PKCs expressed in GH4T2 cells (alpha, betaI, betaII, and epsilon) did not translocate in response to fibroblast growth factors. The PKCdelta subtype-selective inhibitor, rottlerin, or expression of a dominant negative PKCdelta adenoviral construct also blocked fibroblast growth factor induction of rat PRL promoter activity, confirming a role for the novel PKCdelta isoform. PKC inhibitors selective for the conventional alpha and beta isoforms or dominant negative PKCalpha adenoviral expression constructs had no effect. Induction of the endogenous PRL gene was also blocked by adenoviral dominant negative PKCdelta expression but not by an analogous dominant negative PKCalpha 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 PKCdelta.