A role for the mitogen-activated protein kinase in mediating the ability of thyrotropin-releasing hormone to stimulate the prolactin promoter.
The hypothalamic hormone, TRH, stimulates PRL secretion and gene transcription. We have examined the possibility that the mitogen-activated protein kinase (MAPK) may play a role in mediating TRH effects on the PRL gene. TRH was found to stimulate sustained activation of MAPK in PRL-producing, GH3 cells, consistent with a possible role in transcriptional regulation. A kinase-defective, interfering MAPK kinase (MAPKK) mutant reduced TRH induction of the PRL promoter. Treatment with the MAPKK inhibitor, PD98059, blocked TRH-induced activation of MAPK and also reduced TRH induction of a PRL-luciferase reporter gene, confirming that MAPK activation is necessary for TRH effects on PRL gene expression. Previous studies have demonstrated that the PRL promoter contains binding sites for members of the Ets family of transcription factors, which are important for mediating MAPK responsiveness of the PRL promoter. Mutation of specific Ets sites within the PRL promoter reduced responsiveness to both TRH and MAPK. The finding that DNA elements required for MAPK responsiveness of the PRL gene colocalize with DNA elements required for TRH responsiveness further supports a role for MAPK in mediating TRH effects on the PRL gene. We also explored the signaling mechanisms that link the TRH receptor to MAPK induction. Occupancy of the TRH receptor results in activation of protein kinase C (PKC) as well as increases in the concentration of Ca2+ due to release from intracellular stores and entry of Ca2+ through Ca2+ channels. A PKC inhibitor, GF109203X, and an L-type Ca2+ channel blocker, nimodipine, both partially reduced TRH-induced MAPK activation and PRL promoter activity. The effects of the two inhibitors were additive. These studies are consistent with a signaling pathway involving PKC- and Ca2+-dependent activation of MAPK, which leads to phosphorylation of an Ets transcription factor and activation of the PRL promoter.
Wang
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YH
Department of Cell and Developmental Biology, Oregon Health Sciences University, Portland 97201, USA.
Maurer
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RA
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000276:immunology / Q000378:metabolism*; D004268:DNA-Binding Proteins; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D008301:Maleimides / Q000494:pharmacology; D019950:Mitogen-Activated Protein Kinase 1; D009154:Mutation; D010911:Pituitary Neoplasms / Q000378:metabolism; D011233:Precipitin Tests; D011388:Prolactin / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D050783:Proto-Oncogene Proteins c-ets; D051381:Rats; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D013973:Thyrotropin-Releasing Hormone / Q000378:metabolism* / Q000494:pharmacology; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014407:Tumor Cells, Cultured; D050807:ets-Domain Protein Elk-1
D004268:DNA-Binding Proteins; C494114:Elk1 protein, rat; D004791:Enzyme Inhibitors; D005419:Flavonoids; D007211:Indoles; D008301:Maleimides; D011518:Proto-Oncogene Proteins; D050783:Proto-Oncogene Proteins c-ets; D011994:Recombinant Proteins; D014157:Transcription Factors; D050807:ets-Domain Protein Elk-1; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D017346:Protein Serine-Threonine Kinases; D011493:Protein Kinase C; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; C070515:bisindolylmaleimide I; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one; D002118:Calcium