Analysis of the role of the mitogen-activated protein kinase in mediating cyclic-adenosine 3′,5′-Monophosphate effects on prolactin promoter activity

The mechanisms mediating cAMP effects to stimulate transcription of the PRL gene have been examined. Treatments that elevate intracellular cAMP concentrations were found to stimulate the mitogen-activated protein kinase (MAPK) in GH₃ cells. Elevated cAMP was also found to stimulate activation of the GTP-binding protein, Rap1. Rap1GAP1 reduced cAMP-induced phosphorylation of MAPK, offering evidence that Rap1 may play a role in mediating activation of MAPK. Treatment of GH₃ cells with PD98059, an inhibitor of the MAPK pathway, reduced the ability of forskolin to activate a PRL reporter gene, providing evidence that MAPK contributes to cAMP-mediated effects on the PRL promoter. As previous studies have implicated Ets factor binding sites within the PRL promoter in mediating responses to MAPK, we expected that the Ets sites would also play a role in cAMP responsiveness. Surprisingly, mutation of all of the consensus Ets factor binding sites in the proximal PRL promoter greatly reduced responsiveness to epidermal growth factor (EGF) and TRH but did not reduce cAMP responsiveness. Experiments using an expression vector for adenovirus 12S E1a provided evidence that the coactivators, CREB binding protein and/or p300, probably play a role in cAMP responsiveness of the PRL promoter. Interestingly, the ability of a GAL4-p300 fusion protein to enhance reporter gene activity was stimulated by cAMP in a MAPK-dependent manner. These findings provide evidence for a model for cAMP-induced PRL transcription involving Rap1-induced MAPK activity leading to stimulation of the transcriptional coactivators, CBP and p300.