r~UTTEI~WQRTH I~IE I N E M A N N The two faces of a steroid antagonist: When an antagonist isn't Steven K. Nordeen,* Betty J. Bona,* Candace A. Beck,* Dean P. Edwards,* Kristina C. Borror,? and Donald B. DeFranco? *Department of Pathology, University of Colorado Health Sciences Center, Denver, Colorado, USA and ?Dept. of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania USA Activation of protein kinase A potentiates the transcriptional response mediated by the glucocorticoid receptor in responsive fibroblasts and in mammary carcinoma cells. This potentiation is ligand-dependent and occurs without detectable change in the phosphorylation of receptor. The transcriptional response to glucocorticoid or progestin agonists can be blocked by potent antagonists like RU 486. However, upon activation of protein kinase A, the antagonist action of RU 486 on both receptors is blunted. Indeed, RU 486 can itself activate transcription of a hormone-responsive promoter. The conditional agonist activity is observed with type H antagonists, those which recapitulate many of the early steps of ligand-dependent receptor activation, but not type I antagonists, which do not. These studies have now been extended to antimineralocorticoids. In COS-1 cells transfected with a mineralocorticoid receptor expression vector, treatment with 8-BromocAMP potentiates the response to the agonist aldosterone and elicits additional agonist activity in mineralocorticoid antagonists. A model is proposed wherein type H antagonist-receptor complexes occupy receptor binding sites on the genome. The antagonist, however, fails to promote a receptor conformation that can interact productively with a coactivator mediating the communication between receptor and the basal transcription apparatus. Activation of protein kinase A results in the recruitment or activation of a coactivator that permits recovery of receptor-mediated activation function. The recent documentation of conditional agonist activity in antagonists of several different classes of steroids couM have significant implications for the use of steroid antagonists in the clinical setting, representing a previously unrecognized mechanism for the development of steroid resistance. (Steroids 60:97-11M, 1995) Keywords: Glucocorticoid antagonists; mineralocorticoid antagonists; progesterone antagonists;proteinkinase A, RU 486 Introduction Steroid antagonists play an important role in the physician's pharmacopeia, especially in the management of hyperten- sion and endocrine-dependent neoplasia. Nonetheless, the mechanism of action of steroid antagonists is not com- pletely understood. Indeed, the potency of certain antago- nists such as the antiglucocorticoid, antiprogestin RU 486 cannot be accounted for simply by competition with agonist for binding to the receptor. Another conundrum is offered by the observation that antagonists like RU 486 recapitulate many of the steps elicited by agonists, including dissocia- tion of receptor from heat-shock proteins and acquisition of high affinity for target DNA binding, yet fail to induce a transcriptional response. 1-3 A third significant question re- garding steroid antagonists concerns the development of Address reprint requests to Steven K. Nordeen, Ph.D., Departmentof Pathology B-216, Universityof Colorado Health Sciences Center, 4200 East 9th Avenue, Denver, CO 80262 USA. Steroids 60:97-104, 1995 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010 resistance to the growth-suppressing effects of hormone, a phenomenon that remains a significant problem in cancer therapy. In the present work, we summarize results which grew out of studies on the coupling of steroid response pathways with other cellular signal transduction pathways. It is shown that activation of protein kinase A results in the elicitation of antagonist activity in certain steroid antago- nists, including antimineralocorticoids. The implication of these results for the mechanism of antagonist action and the development of resistance to that action is discussed. Experimental Steroids The antiglucorticoid/antiprogestin ZK 98 299 [ 1113-(4-dimethyl- aminophenyl)- 17et-hydroxy- 1713-(3-hydroxypropyl)- 13ot-methyl- 4,9-gonadiene-3-one]4 was obtained as a generous gift from Dr. D. Henderson (Schering AG, Berlin, Germany). The antiminer- alocorticoids ZK 30 595 [dihydrospirorenone: 613,713; 1513,1613- dimethylene- 3-oxo- 17ot-pregn-4-ene-21,17-carbolactone]~-s and ZK 91 587 [7ot-methoxycarbonyl-1513,161~-methylene-3-oxo-17ot- 0039-128X/95/$10.00 SSDI 0039-128X(94)0000 I-S