PMA-sensitive protein kinase C is not necessary in TRH-stimulated prolactin release from female rat primary pituitary cells.
In GH3 cells and other clonal rat pituitary tumor cells, TRH has been shown to mediate its effects on prolactin release via a rise of cytosolic Ca2+ and activation of protein kinase C. In this study, we examined the role of protein kinase C in TRH-stimulated prolactin release from female rat primary pituitary cell culture. Both TRH and PMA stimulated prolactin release in a dose-dependent manner. When present together at maximal concentrations, TRH and PMA produced an effect which was slightly less than additive. Pretreatment of rat pituitary cells with 10(-6) M PMA for 24 hrs completely down-regulated protein kinase C, since such PMA-pretreated cells did not release prolactin in response to a second dose of PMA. Interestingly, protein kinase C down-regulation had no effect on TRH-induced prolactin release from rat pituitary cells. In contrast, PMA-pretreated GH3 cells did not respond to a subsequent stimulation by either PMA or TRH. Pretreatment of rat pituitary cells with TRH (10(-7) M, 24 hrs) inhibited the subsequent response to TRH, but not PMA. Forskolin, an adenylate cyclase activator, stimulated prolactin release by itself and in a synergistic manner when incubated together with TRH or PMA. The synergistic effects of forskolin on prolactin release was greater in the presence of PMA than TRH. Down-regulation of protein kinase C by PMA pretreatment abolished the synergistic effect produced by PMA and forskolin but had no effect on those generated by TRH and forskolin. sn-1,2-Dioctanylglycerol (DOG) pretreatment attenuated the subsequent response to DOG and PMA but not TRH. The effect of TRH, but not PMA, on prolactin release required the presence of extracellular Ca2+. In conclusion, the mechanism by which TRH causes prolactin release from rat primary pituitary cells is different from that of GH3 cells; the former is a protein kinase C-independent process whereas the latter is at least partially dependent upon the activation of protein kinase C.
Cheng
K
K
Department of Growth Biochemistry and Physiology, Merck Research Laboratories, Rahway, NJ 07065.
Chan
W W
WW
Arias
R
R
Barreto
A
A
Butler
B
B
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D004075:Diglycerides / Q000494:pharmacology; D015536:Down-Regulation / Q000187:drug effects; D004789:Enzyme Activation; D005260:Female; D010903:Pituitary Gland, Anterior / Q000187:drug effects* / Q000378:metabolism; D011388:Prolactin / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism*; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D014407:Tumor Cells, Cultured
D004075:Diglycerides; C044295:1,2-dioctanoylglycerol; D005576:Colforsin; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium