Effect of phorbol ester on stimulus-secretion coupling mechanisms in gonadotropin releasing hormone-stimulated pituitary gonadotrophs.
The feedback regulatory control mechanism exerted by activated Ca2+/phospholipid-dependent protein C kinase upon gonadotropin releasing hormone (GnRH) binding, stimulation of phosphoinositide turnover and gonadotropin secretion was investigated in cultured pituitary cells. Addition of the tumor promoter phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), at concentrations which activate pituitary protein C kinase, to cultured pituitary cells resulted in up-regulation of GnRH receptors (155% at 4 h). The stimulatory effect of GnRH on [3H]inositol phosphates (Ins-P) production in myo-[2-3H]inositol prelabeled pituitary cells was not inhibited by prior treatment of the cells with TPA (10(-9)-10(-7) M). Higher concentrations of TPA (10(-6)-10(-5) M) inhibited the effect of GnRH on [3H]Ins-P production. Increasing concentrations of TPA or the permeable analog of diacylglycerol 1-oleoyl-2-acetylglycerol (OAG) stimulated luteinizing hormone (LH) release from cultured pituitary cells with ED50 values of 5 x 10(-9) M and 10 micrograms/ml, respectively. No consistent inhibition or additivity of LH release was observed when increasing doses of TPA or OAG were added with a submaximal dose of GnRH. These results suggest that protein C kinase might mediate the known homologous up-regulation of GnRH receptors during the reproductive cycle. Protein C kinase is positively involved in mediating the process of gonadotropin secretion. Unlike many other systems, activation of protein C kinase in pituitary gonadotrophs is not involved in negative feed-back regulation of stimulus-secretion-coupling mechanisms in GnRH-stimulated gonadotrophs.
Naor
Z
Z
Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel.
Schvartz
I
I
Hazum
E
E
Azrad
A
A
Hermon
J
J
D000818:Animals; D002478:Cells, Cultured; D005246:Feedback; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D007986:Luteinizing Hormone / Q000378:metabolism; D010703:Phorbol Esters / Q000494:pharmacology*; D010716:Phosphatidylinositols / Q000502:physiology*; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011493:Protein Kinase C / Q000502:physiology*; D051381:Rats; D011966:Receptors, LHRH / Q000378:metabolism*
D007295:Inositol Phosphates; D010703:Phorbol Esters; D010716:Phosphatidylinositols; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D011493:Protein Kinase C