Phorbol esters reduce gonadotrope responsiveness to protein kinase C activators but not to Ca2+-mobilizing secretagogues. Does protein kinase C mediate gonadotropin-releasing hormone action?
The demonstration that activators of the Ca2+-activated, phospholipid-dependent protein kinase (protein kinase C), such as phorbol esters and diacylglycerols, can provoke luteinizing hormone (LH) release from pituitary gonadotropes, suggests a possible role for protein kinase C in stimulus-release coupling. We now report that administration of phorbol myristate acetate (PMA) to pituitary cell cultures causes a sustained reduction in Triton X-100-extracted protein kinase C activity. Further, phorbol ester- and diacylglycerol-stimulated LH release, as well as inhibition by PMA of gonadotropin-releasing hormone (GnRH)-stimulated inositol phosphate production, were reduced by pretreatment with PMA. The effects of phorbol ester pretreatment on PMA-stimulated LH release and protein kinase C activity were dose-dependent, sustained (greater than or equal to 24 h) and specific (no measurable effect with 4 alpha-phorbol didecanoate). The effect on PMA-stimulated LH release was apparently Ca2+-independent. In pituitary cell cultures with reduced protein kinase C activity, the gonadotropes have reduced responsiveness to PMA but release a similar proportion of cellular LH in response to Ca2+-mobilizing secretagogues (GnRH and A23187) as do control cells. The normal responsiveness to GnRH of cells with reduced responsiveness to protein kinase C activators calls into question the requirement for this enzyme for GnRH-stimulated LH release.
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004075:Diglycerides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D007295:Inositol Phosphates / Q000096:biosynthesis; D007986:Luteinizing Hormone / Q000378:metabolism; D017830:Octoxynol; D010703:Phorbol Esters / Q000494:pharmacology*; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism; D010906:Pituitary Hormone-Releasing Hormones / Q000494:pharmacology*; D011092:Polyethylene Glycols; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D004075:Diglycerides; D007295:Inositol Phosphates; D010703:Phorbol Esters; D010906:Pituitary Hormone-Releasing Hormones; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; D011092:Polyethylene Glycols; D007986:Luteinizing Hormone; D017830:Octoxynol; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium