Androgen-Controlled Subcellular Distribution of Its Receptor in the Rat Epididymis: 5α-Dihydrotestosterone-Induced Translocation Is Blocked by Antiandrogens*

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Abstract
The objectives of these studies were to investigate the influence of androgen on the intracellular distribution of its receptor in the rat epididymis and to examine the mechanism of action of antiandrogens in this organ. A ligand exchange assay for cytosol and nuclear androgen receptors was developed. Our results demonstrated a complete and reversible exchange between [3H]R1881 ([3H]17β-hydroxy-17α-methyl-estra-4,9,lltrien- 3-one) and the endogenous hormone [presumably 5α-dihydrotestosterone (5α-DHT)] as well as a marked specificity of the epididymal androgen receptor for binding biologically active androgens and antiandrogens. Control rats had the following number of epididymal androgen-binding sites: 12 ± 2 fmol/mg protein in cytosol and 1540 ± 140 fmol/mg DNA in nuclei. Two days after castration, binding sites in the cytosol increased to 22.5 ± 1.5 fmol/mg protein; those in nuclei decreased to 575 ± 35 fmol/mg DNA. The injection of 5α-DHT into castrated rats (for 2 days) induced a dose-dependent translocation of cytoplasmic binding sites into nuclei. Approximately 30% of the binding sites were transferred by a dose of 0.15 tig 5α-DHT, and a maximal (70%) translocation was obtained with 15 μg. A highly significant correlation (r = 0.98; P < 0.01) was obtained between the decrease in cytoplasmic sites and the increase in nuclear sites. Higher doses of 5α-DHT up to 1 mg were unable to induce translocation of the residual 30% of the sites remaining in cytoplasm. While the Kd values for nuclear sites were identical in all experimental groups (2.5 ± 0.4 nM), a difference (P < 0.01; n = 5) was found in the Kd of cytoplasmic receptor in castrated rats (1.33 ± 0.67 nM) vs. that in 5α-DHT-injected castrated rats (0.67 ± 0.15 nM). The total number of binding sites per cell was decreased in castrated vs control rats, but there was no difference between castrated groups regardless of 5α-DHT injection. The simultaneous administration of the antiandrogens cyproterone acetate (6α-chloro-17α-hydroxy-lα,2α-methylene-4,6- pregnadiene-3,20-dione-17-acetate) or RU 23908 [5,5-dimethyl- 3,4- nitro - 3 - 3 - (trifluoromethyl) phenyl -2,4- imidazolildinedione] blocked the 5±-DHT-induced translocation of cytoplasmic receptor into nuclei, while these compounds by themselves were unable to modify the intracellular distribution of binding sites. We postulate that antiandrogens act by forming complexes with cytoplasmic receptor that are unable to undergo translocation into nuclei. Our results also suggest the existence of two types of cytoplasmic epididymal androgen-binding sites, and that the occupancy of a limited number of sites by antiandrogen can block most of the biological response of the target cell to androgen.

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