PGF2α triggers the demise of the corpus luteum whereby progesterone synthesis is inhibited, the luteal structure regresses, and the estrus cycle resumes. Upon binding to its heterotrimeric G-protein-coupled receptors, PGF2α initiates the phospholipase C/diacylglycerol and inositol-l,4,5-trisphosphate/Ca2+-protein kinase C (PKC) signaling pathway. More recently, we have demonstrated that PGF2α activates extracellular signal-regulated kinase (ERK) mitogen-activated protein (MAP) kinase signaling through a Raf-dependent mechanism in bovine luteal cells. However, the relationship between PKC and ERK activation in PGF2α signaling has not been clearly defined. Moreover, the signaling pathway that PGF2α uses to regulate gene expression is unknown. In this report, primary cultures of bovine luteal cells were used to address the role of PKC in ERK activation and the signaling pathway for induction of c-fos and c-jun messenger RNA (mRNA) expression in response to PGF2α. By using a PKC inhibitor and a PKC-deficient luteal cell model, we observed that phorbol ester-responsive isoforms of PKC were required for ERK phosphorylation and activation by PGF2α (1 μM) or phorbol 12myristate 13-acetate (PMA) (20 nM). In PGF2α- and PMA-treated cells, active ERK MAP kinase was localized in the nucleus. PGF2α-induced ERK phosphorylation was dose-dependently inhibited by the MEK1 inhibitor PD098059 (1-50 μM). The expression of c-fos and c-jun mRNA in luteal cells was markedly increased by treatment with PGF2α (1 μM) or PMA (20 nM) for 30 min. We also observed that activation of ERK MAP kinase was required for the expression of c-fos and c-jun mRNA in response to PGF2α and PMA because it was abrogated by blocking the ERK pathway with PD098059. In addition, PGF2α and PMA-induced c-fos and c-jun mRNA expression was abolished in the PKC-deficient cells. Taken together, our data demonstrate that a PKC-dependent ERK MAP kinase pathway mediates the expression of c-fos and c-jun mRNA in PGF2α-treated bovine luteal cells.
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