GnRH is the first key hormone of reproduction. The role of protein kinase C (PKC) isoforms in GnRH-stimulated MAPK [ERK and Jun N-terminal kinase (JNK)] was examined in the αT3-1 and LβT2 gonadotrope cells. Incubation of the cells with GnRH resulted in a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2. Gonadotropes express conventional PKCα and conventional PKCβII, novel PKCδ, novel PKCε, and novel PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein-PKC constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. Interestingly, PKCα, PKCβII, and PKCε translocation to the plasma membrane was more pronounced and more prolonged in phorbol-12-myristate-13-acetate (PMA) than in GnRH-treated cells. The use of selective inhibitors and dominant-negative plasmids for the various PKCs has revealed that PKCβII, PKCδ, and PKCε mediate ERK2 activation by GnRH, whereas PKCα, PKCβII, PKCδ, and PKCε mediate ERK2 activation by PMA. Also, PKCα, PKCβII, PKCδ, and PKCε are involved in GnRH and PMA stimulation of JNK1 in a cell-context-dependent manner. We present preliminary evidence that persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane may dictate its selective role in ERK or JNK activation. Thus, we have described the contribution of selective PKCs to ERK and JNK activation by GnRH.

The GnRH receptor (GnRHR) mediates the pituitary functions of GnRH, as well as its anti-proliferative effects in sex hormone-dependent cancer cells. Here we compare the signaling of GnRHR in pituitary gonadotrope cell lines vs. prostate cancer cell lines. We first noticed that the expression level of PKCα, PKCβII and PKCε is much higher in αT3-1 and LβT2 gonadotrope cell lines vs. LNCaP and DU-145 cell lines, while the opposite is seen for PKCδ. Activation of PKCα, PKCβII and PKCε by GnRH is relatively transient in αT3-1 and LβT2 gonadotrope cell lines and more prolonged in LNCaP and DU-145 cell lines. On the otherhand, the activation and re-distribution of the above PKCs by PMA was similar for both gonadotrope cell lines and prostate cancer cell lines. Activation of ERK1/2 by GnRH and PMA was robust in the gonadotrope cell lines, with a smaller effect observed in the prostate cancer cell lines. The Ca2+ ionophore A23187 stimulated ERK1/2 in gonadotrope cell lines but not in prostate cancer cell lines. GnRH, PMA and A23187 stimulated JNK activity in gonadotrope cell lines, with a more sustained effect in prostate cancer cell lines. Sustained activation of p38 was observed for PMA and A23187 in Du-145 cells, while p38 activation by GnRH, PMA and A23187 in LβT2 cells was transient. Thus, differential expression and re-distribution of PKCs by GnRH and the transient vs. the more sustained nature of the activation of the PKC-MAPK cascade by GnRH in gonadotrope cell lines vs. prostate cancer cell lines respectively, may provide the mechanistic basis for the cell context-dependent differential biological responses observed in GnRH interaction with pituitary gonadotropes vs. prostate cancer cells.

We recently described a novel GnRH receptor signaling pathway mediated by the prostaglandins (PGs) F(2alpha) and PGI(2), which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and inhibit LH but not FSH release. Here we further explore the cross talk between GnRH and the PG receptors. GnRH stimulates arachidonic acid (AA) release from LbetaT2 gonadotrope cells via the Ca(2+)-independent phospholipase A(2) (iPLA(2)) and not via the more common Ca(2+)-dependent cytosolic phospholipase A(2)alpha (cPLA(2)alpha). AA release was followed by a marked induction of cyclooxygenase (COX)-1 and COX-2 by GnRH via the protein kinase C/c-Src/phosphatidylinositol 3-kinase/MAPK pathway. COX-2 transcription by GnRH is mediated by the two nuclear factor-kappaB sites and the CCAAT/enhancer-binding protein site within its promoter. Indeed, GnRH stimulates p65/RelA phosphorylation (22-fold) in LbetaT2 cells and the two nuclear factor-kappaB sites apparently act as a composite response element. Although GnRH stimulates cAMP formation in LbetaT2 cells, we found no role for cAMP acting via the cAMP response element site in the COX-2 promoter. PGF(2alpha), PGI(2), or PGE(2) had no effect on GnRH-stimulated ERK, c-Jun N-terminal kinase, and p38MAPK activation or on GnRH- and high K(+)-stimulated intracellular Ca(2+) elevation in LbetaT2 and gonadotropes in primary culture. Although, PGF(2alpha), PGI(2), and PGE(2) reduced GnRH-stimulated cAMP formation, we could not correlate it to the inhibition of GnRH receptor expression, which is exerted only by PGF(2alpha) and PGI(2.) Hence, the inhibition by PGF(2alpha) and PGI(2) of the autoregulation of GnRH receptor expression is most likely mediated via inhibition of GnRH-stimulated phosphoinositide turnover and not by inhibition of Ca(2+) elevation and MAPK activation.

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