Recent studies with the synthetic progestin 6amethylprogesterone (6MP) suggested that tissue specificity, steroid metabolism, and multiple steroid-receptors play a role in how this steroid modifies the action of androgens. In the present study we compared the in vivo nuclear uptake of [3H]testosterone ([3H]T) and [3H]6MP in kidney and prostate-seminal vesicle from mice pretreated with radioinert steroids. The results indicate that the uptake and metabolism of these steroids are regulated differently. The nuclear uptake of [3H]T in kidney and prostate-seminal vesicle was inhibited by both T and 6MP commensurate with interaction with androgen receptors. In prostate-seminal vesicle nuclei, [3H]6MP uptake was inhibited by T but not 6MP. Paradoxically, in renal nuclei the uptake of [3H]6MP was increased 2- to 3-fold (potentiation) by both T and 6MP. A similar potentiation of [3H]6MP uptake by kidney nuclei was produced by medroxyprogeterone acetate (2- to 3-fold), cyproterone acetate (2- to 3-fold), and dexamethasone (3- to 6-fold) but not estradiol treatment. None of these steroids increased the uptake of [3H]6MP into nuclei of prostate-seminal vesicle. The ability of kidney nuclei from Tfm/Y mice to concentrate [3H]6MP indicated that this occurred in the absence of a functional androgen receptor. This is in contrast to the negligible uptakes of [3H]T and [3H]medroxyprogesterone acetate that are known to require androgen receptors. The uptake of [3H]6MP into kidney nuclei of Tfm/Y mice was increased by nonradioactive 6MP but not by T, indicating that the androgen receptor is required only when T is the potentiating steroid. A metabolite, 20α-hydroxy-6α-methylprogesterone (20α-OH-6MP) was isolated from kidney nuclei of mice receiving [3H]6MP. The nuclear concentration of this metabolite exceeded that of the parent compound in control animals receiving only [3H]6MP, whereas unmetabolized [3H]6MP was the predominant nuclear steroid from mice pretreated with radioinert steroids. Nuclei from prostate-seminal vesicle contained predominantly 6MP and little 20α-OH-6MP regardless of treatment. These results suggest that1) 6MP may interact with the androgen receptor, as well as with other high capacity, low affinity macromolecule(s) involved in nuclear steroid uptake; 2) the nuclear uptake of 6MP in kidney but not prostate-seminal vesicle nuclei is increased by androgens, progestins, and glucocorticoids but not by estrogens; 3) the androgen receptor is required for T but not for other steroids to potentiate the uptake of [3H]6MP; and 4) the renal nuclear distribution of 6MP and its metabolite 20α-OH-6MP is influenced by steroid treatment.
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