Selective effects of 8-Br-cAMP on agonists and antagonists of the glucocorticoid receptor.

RU486 has been reported to be a glucocorticoid receptor (GR) and a progesterone receptor (PR) antagonist. We have analysed RU486 activity on the GR in WCL-2 (CHO) cells and in COS-7 cells transiently transfected with the mouse GR and with the reporter MMTVCAT (MCAT). These cell lines do not contain any active progesterone or androgen receptors. In both cell lines RU486 is a partial agonist of the GR with 10-15% of the activity of dexamethasone. As expected, RU486 is also a partial antagonist of the GR. Treatment of COS-7 cells with 8-Br-cAMP increases the agonist activity of both dexamethasone and RU486. This cAMP induced superactivation is seen with all steroids that have full or partial agonist activity. In contrast, the activities of ZK98.299 and R5020, which are complete antagonists of the GR without any agonist activity, are not affected by 8-Br-cAMP treatment. This effect of 8-Br-cAMP is not seen in WCL2 cells. 8-Br-cAMP, therefore, is not a switch which changes antagonists to agonists but is, rather, a cell specific activator of all agonists whether they have full or only partial agonist activity.