Gonadotropin-releasing hormone activates mitogen-activated protein kinase in human ovarian and placental cells

Considering that the action of gonadotropin-releasing hormone (GnRH) may be mediated via different signaling pathways in extrapituitary tissues, in th e present study we investigated the role of the human GnRH receptor (GnRHR) in activating mitogen-activated protein kinases (MAPKs), which regulate ce ll growth, division, and differentiation. The phosphorylation state of p44 and p42 MAPKs was examined using antibodies that distinguish phospho-p44/42 MAPK (P-MAPK, Thr(202)/Tyr(204)) from total p44/42 MAPK (T-MAPK, activated plus inactivated) in human ovarian and placental cells. Cell cultures were treated with various concentrations of a GnRH agonist, (D-Ala(6))-GnRH, fo r 5 min. (D-Ala(6))-GnRH stimulated a rapid activation of P-MAPK in human g ranulosa-luteal cells (hGLCs) and immortalized extravillous trophoblast (IE VT) cells. Interestingly, (D-Ala(6))-GnRH treatment of ovarian cancer (OVCA R-3) and placental carcinoma (JEG-3) cells induced a biphasic regulatory pa ttern in P-MAPK activity. In contrast, no change of T-MAPK levels was obser ved following addition of the GnRH agonist in the ovarian and placental cel ls examined. The physiological implication of MAPK activation by GnRH in th e ovarian and placental cells was also investigated. Human GLCs were treate d with (D-Ala(6))-GnRH for 24 h, and progesterone secretion was measured by an established RIA. (D-Ala(6))-GnRH induced a significant decrease in prog esterone secretion with maximum inhibition (a 45% decrease over basal level ) at 10(-7) M. This inhibitory effect was completely reversed by pretreatme nt, with MAPK/ERK kinase 1 (MEK1) inhibitor (PD98059), suggesting the invol vement of the MAPK pathway in hGLCs. Placental JEG-3 cells were treated wit h (D-Ala6)-GnRH for 24 h, and beta hCG mRNA level was measured using Northe rn blot analysis. (D-Ala(6))-GnRH stimulated the expression of beta hCG mRN A to 160% of control value in JEG-3 cells. In contrast to the ovarian cells , pretreatment of JEG-3 cells with PD98059 failed to block the stimulatory effect of GnRH on beta hCG mRNA level, suggesting that other signaling path way(s) may play a more dominant role in GnRH-induced beta hCG mRNA expressi on. To our knowledge, this is the first demonstration that (1) GnRH induces activation of the MAPK signaling pathway in normal and carcinoma cells of the human ovary and placenta, and (2) MAPK mediates the direct action of Gn RH on progesterone production in hGLCs. (C) 2000 Elsevier Science Ireland L td. All rights reserved.