MULTIPLE SIGNALING PATHWAYS ACTIVATE THE CHICKEN PROGESTERONE-RECEPTOR

We have previously reported that treatment of CV1 cells, transiently transfected with DNA encoding the chicken progesterone receptor (cPR) and a reporter plasmid PREtkCAT, with either 8-Br-cAMP or okadaic acid resulted in ligand-independent transcriptional activation of the cPR. The surprising finding that cPR can be activated in the absence of hormone has been followed by numerous studies from other laboratories examining the effects of modulators of kinases and phosphatases on the activity of other steroid receptors. These studies have yielded mixed results: ligand-independent activation has been observed in some cases, but not in others. In order to determine whether the ligand-independent activation of cPR was restricted to a specific reporter and cell type and to better characterize this phenomenon, the studies in this report were undertaken. Using both the original reporter, PREtkCAT, and a simpler reporter, GRE(2)E1bCAT, we found that ligand-independent activation of the cPR can be induced in both CV1 and HeLa cells. The magnitude of the response and the response of the reporter alone differed in the two cell types. Further analysis of the activation of cPR by inhibitors of protein phosphatases showed that inhibition of phosphatase 1 rather than phosphatase 2A was necessary for activation of cPR. Finally, treatment with vanadate, an inhibitor of phosphotyrosine phosphatases, or epidermal growth factor resulted in activation of cPR. These studies suggest that signals transduced through multiple signaling pathways can activate cPR.