Characterization of R 5020 and RU 486 binding to progesterone receptor from calf uterus

Abstract

We have examined and compared the binding characteristics of the progesterone agonist R5020 [promegestone, 17,21-dimethylpregna-4,9(10)-diene-3,20-dione] and the progesterone antagonist RU486 [mifepristone, 17.beta.-hydroxy-11.beta.-[4-(dimethylamino)phenyl]-17.alpha.-(prop-1-ynyl)-estra-4,9-dien-3-one] in calf uterine cytosol. Both steroids bound cytosol macromolecule(s) with high affinity, exhibiting Kd values of 5.6 and 3.6 nM for R5020 and RU486 binding, respectively. The binding of the steroids to the macromolecule(s) was rapid at 4.degree. C, showing saturation of binding sites at 1-2 h for [3H]progesterone and 2-4 h for both [3H]R5020 and [3H]RU486. Addition of molybdate and glycerol to cytosol increased the extent of [3H]R5020 binding. The extent of [3H]RU486 binding remained unchanged in the presence of molybdate, whereas glycerol had an inhibitory effect. Molybdate alone or in combination with glycerol stabilized the [3H]R5020- and [3H]RU486-receptor complexes at 37.degree. C. Although the rate of association of [3H]RU486 with the cytosolic macromolecule was slower than that of [3H]R5020, its dissociation from the ligand-macromolecule complex was significantly slower than [3H]R5020. Competitive steroid binding analysis revealed that [3H]progesterone, [3H]R5020, and [3H]RU486 complete for the same site(s) in the uterine cytosol, suggesting that all three bind to the progesterone receptor (PR). Sedimentation rate analysis showed that both steroids were bound to a molecule that sediments in the 8S region. The 8S [3H]R5020 and [3H]RU486 peaks were abolished by excess radioinert progesterone, RU486, or R5020. The results of this study suggest that, although there are some differences in the nature of their interaction with the PR, both R5020 and RU486 bind to the same 8S receptor in calf uterine cytosol.