Latent agonist activity of the steroid antagonist, RU486, is unmasked in cells treated with activators of protein kinase A.

RU486 is a glucocorticoid and progesterone antagonist. In glucocorticoid-responsive fibroblasts, it mediates little or no induction of a truncated, hormone-responsive mouse mammary tumor virus promoter; moreover, it abrogates the induction mediated by the glucocorticoid agonist, dexamethasone. However, when the fibroblasts are treated with activators of protein kinase A, 8-Br-cAMP or forskolin, along with RU486, the steroid now acts as a partial agonist, capable of mediating an induction of hormone-responsive reporter genes. In addition, the ability of RU486 to block the action of the glucocorticoid agonist, dexamethasone, is compromised by concomitant treatment with 8-Br-cAMP. Activators of protein kinase C fail to elicit these phenomena. Induction of gene expression in the presence of 8-Br-cAMP is dependent on the dose of RU486 over a range consistent with a glucocorticoid receptor-mediated mechanism. An antagonist, ZK98 299, which unlike RU486 is not thought to permit receptor binding to DNA, is not activated by 8-Br-cAMP. The elicitation of RU486 agonist activity cannot be attributed solely to idiosyncrasies of the cell line or the promoter. Similar phenomena are observed in another glucocorticoid-responsive fibroblast line. Furthermore, RU486 can induce a minimal promoter bearing two copies of a synthetic receptor target site. However, we have identified at least one promoter toward which RU486 still behaves as an antagonist despite 8-Br-cAMP treatment. These observations suggest that the unmasking of latent agonist activity in a type II antagonist is not an isolated phenomenon and may, therefore, be seen with other receptors and antagonists. The finding that modulation of cellular signal transduction pathways can unmask agonist activity in an otherwise effective steroid antagonist has significant implications for the use of steroid antagonists in the clinical setting and could represent a heretofore unrecognized mechanism for the development of steroid resistance.