Progestins possess androgenic, antiandrogenic, and synandrogenic activities on androgen-responsive tissues. Several recent studies suggest that some or all of these actions may be initiated in mouse kidney via progestin interaction with the androgen receptor. All of the progestins studied, including medroxyprogesterone acetate [6a-methyl-17a-hydroxy-pregn-4-ene-3, 20-dione acetate (MPA)], cyproterone acetate [6-chloro-17a-hydroxy-la, 2a-methylene-pregna-4, 6-diene-3, 20-dione acetate (CPA)], 6a-methylprogesterone (6MP), and progesterone(P), effectively competed with [3H]testosterone ([3H]T) for binding sites on androgen receptors in mouse kidney cytosol. Their relative binding affinities were T >> 6MP > CPA > MPA = P, indicating that MPA, the most biologically potent androgenic progestin, bound to the cytoplasmic androgen receptor with less affinity than 6MP or the antiandrogenic progestin CPA. To further investigate the relationship between binding affinity and biological activity of progestins, an assay using kidney minces was established to study the effect of progestins on the nuclear uptake of the androgen receptor-steroid complex by intact cells. Kidney minces were incubated with [3H]T in the presence or absence of unlabeled steroids. All of the specifically bound nuclear steroid was [3H]T under optimal assay conditions (at 22 C for 45 min; 5 × 10-1 M [yH]T). Nuclear binding was diminished at temperatures above or below 22 C, indicating temperature dependence of nuclear translocation and lability of the androgen receptor. Protease inhibitors, such as leupeptin and antipain, did not significantly stabilize the androgen receptor at 22 C. In contrast to their relative binding affinities in cytosol, the competitive nuclear uptake of progestins (10-5 × 10 (iM) relative to T in kidney minces was T >> MPA = 6 MP > P > CPA. A similar order for the androgenic progestins was obtained with in vivo studies of the relative inhibitory potencies of progestins on nuclear [3H]T uptake by mouse kidney. These observations indicate that the competitive nuclear uptake of progestins relative to T correlates with their androgenic but not with their antiandrogenic and synandrogenic actions. In conclusion, the multiple actions of progestins in mouse kidney cannot be accurately predicted from their binding affinities to androgen receptors. Furthermore, the discrepancy between cytoplasmic binding and nuclear uptake suggested that some property of the receptor in addition to its affinity for a particular steroid is recognized by the cell as a determinant of biological activity.
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