17β-Estradiol (E2)-stimulated estrogem receptor (ERα) transcription is accompanied by protein degradation via the 26S-proteasome pathway. Inhibition of proteasome activity stabilizes ERα protein and abolishes E2-activated transcription, suggesting functional linkages between transcription and degradation. It is not known whether ligand-independent ERalpha; activation is coupled to proteolysis. In pituitary cells, forskolin (FSK) stimulates ERα transcription through the protein kinase A (PKA) pathway. This study examined interactions between E2-dependent and PKA-stimulated pathways in GH3 cells by measuring transcription of a transfected reporter gene and endogenous ERα levels. E2 stimulated estrogen response element-mediated transcription 2- to 3-fold and decreased ERα protein levels to 40%. In contrast, FSK stimulated ERα transcription without decreasing ERα protein. Treatment with FSK plus E2 resulted in synergistic ERα transactivation, and FSK specifically prevented E2-induced ERα degradation. PKA is required for protection and was prevented by H89 (a PKA inhibitor), but not PD98059 (a MAPK kinase inhibitor). Propyl-pyrazole-triol and R,R-diethyl-tetrahydrochrysene, selective ERα agonists, reduced ERα protein by 50% while stimulating ERα transcriptional activity 4-to 8-fold. The antagonist ICI 182,780 similarly decreased ERα levels, but prevented ER activation. FSK prevented all ligand-induced ERα degradation. Lactacystin, a proteasome inhibitor, abolished E2-stimulated, but not FSK-stimulated, ERα transcription. Thus, stimulation of ERα transcription by the PKA-dependent pathway is dissociated from receptor degradation and proteasome activity. These data suggest a mechanism of ERα transcriptional activation by PKA that is distinct from E2 activation and that may contribute to the synergistic transcriptional activation of ERα by ligand-dependent and PKA-dependent pathways.
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