Ligand-independent activation of pituitary ER: dependence on PKA-stimulated pathways.
In pituitary and other target tissues, estrogen acts through ERs, which are ligand-activated nuclear transcription factors. ERs can also be activated by intracellular signaling pathways in a ligand-independent manner in some cells. Because the pituitary is the target of several cAMP-activating factors, we examined the ability of cAMP to activate ERs in the alphaT3 gonadotrope cell line. Forskolin, 8-bromo-cAMP, and pituitary adenylate cyclase-activating polypeptide all enhanced ER-dependent promoter activity, which was inhibited by antiestrogen or a pituitary-specific inhibitory ER variant. Activation was PKA dependent and was blocked by the PKA inhibitor H89 or cotransfection of the inhibitor PKI. Although cAMP activated MAPK in alphaT3 cells, inhibition of MAPK with the MEK inhibitor PD98059 did not prevent forskolin-induced ER activation. Similarly, epidermal growth factor did not stimulate ER activity, although it increased MAPK activation. Forskolin-induced activation of ER was enhanced by cotransfection of steroid receptor coactivator-1 and was inhibited by the repressor of ER action, suggesting that cAMP does not alter the normal interactions between ER and cofactors. In contrast to results with estrogen, cAMP treatment did not decrease ER protein levels. These results demonstrate that in the pituitary, cAMP activates ER in a ligand-independent manner exclusively through PKA.
Schreihofer
D A
DA
Department of Internal Medicine, Division of Endocrinology, University of Virginia, Charlottesville, Virginia 22908, USA.
Resnick
E M
EM
Lin
V Y
VY
Shupnik
M A
MA
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D047628:Estrogen Receptor alpha; D004967:Estrogens / Q000494:pharmacology; D051548:Histone Acetyltransferases; D008024:Ligands; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000502:physiology; D056920:Nuclear Receptor Coactivator 1; D010902:Pituitary Gland / Q000378:metabolism*; D000091402:Prohibitins; D011960:Receptors, Estrogen / Q000187:drug effects / Q000378:metabolism*; D012097:Repressor Proteins / Q000494:pharmacology; D014157:Transcription Factors / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology
D047628:Estrogen Receptor alpha; D004967:Estrogens; D008024:Ligands; D000091402:Prohibitins; D011960:Receptors, Estrogen; D012097:Repressor Proteins; D014157:Transcription Factors; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D051548:Histone Acetyltransferases; C538861:Ncoa1 protein, mouse; D056920:Nuclear Receptor Coactivator 1; D017868:Cyclic AMP-Dependent Protein Kinases; D020928:Mitogen-Activated Protein Kinases