THE STEROID ANTAGONIST RU486 EXERTS DIFFERENT EFFECTS ON THE GLUCOCORTICOID AND PROGESTERONE RECEPTORS

To determine whether the steroid antagonist RU486 mediates its antiglu cocorticoid and antiprogestin activities by the same or different rece ptor mechanisms, a direct comparison of RU486 interaction with glucoco rticoid (GR) and progesterone (PR) receptors was made. The effects of RU486 on transformation of GR and PR 8-10S complexes in the intact cel l and in vitro were analyzed by sucrose density gradient centrifugatio n, and the in vitro stability of receptor-heat shock protein-90 intera ctions was analyzed by coimmunoprecipitation. Compared to agonist, RU4 86 binding produced a reduction in the amount of GR converted from 8S to 4S and stabilized the GR-heat shock protein-90 complex. By contrast , PR-RU486 complexes were transformed both in vitro and in the intact cell to the same extent as receptor-agonist complexes. PR-RU486 comple xes sedimented at 5-6S, whereas PR-R5020, GR-RU486, and GR-agonist com plexes sedimented at 4S. The portion of GR that undergoes nuclear tran sformation when bound to RU486 was examined for binding to the glucoco rticoid-progesterone response element of the mouse mammary tumor virus by an immunoprecipitation assay. The nuclear-transformed GR-RU486 com plex bound the glucocorticoid-progesterone response element with the s ame affinity as the nuclear-transformed GR-triamcinolone acetonide com plex. The electrophoretic mobilities of GR-RU486 complexes and GR-agon ist complexes were the same, as determined by gel retardation assay. T hese results suggest that RU486 exerts its antiglucocorticoid activity at two levels of receptor action: prevention of complete GR transform ation and alteration of a step subsequent to GR-DNA binding. As an ant iprogestin, RU486 action is exerted predominantly at a post-DNA-bindin g step.