Testosterone and 5α-dihydrotestosterone interact differently with the androgen receptor to enhance transcription of the MMTV-CAT reporter gene

Testosterone and its 5α-reduced derivative 5α-dihydrotestosterone exert different actions in the male during embryogenesis and in postnatal life. Nevertheless the two hormones bind to the same intracellular androgen receptor, and genetic and endocrinological studies in the Tfm mouse suggest that the actions of both hormones are mediated by this single receptor. Previous studies indicate that dihydrotestosterone binds more tightly to the androgen receptor but that the B_max of binding of the two hormones is the same. To determine whether these differences in binding parameters could explain the mechanism by which the two hormones exert different physiological actions via the same receptor, we introduced a plasmid encoding the androgen receptor cDNA and a reporter plasmid encoding MMTV-CAT into Chinese hamster ovary cells. These cells do not express endogenous androgen receptor and do not convert testosterone to dihydrotestosterone. Therefore, it was possible to examine the relation between the concentration of each of the steroids and reporter gene expression. Both hormones enhanced CAT activity, but dihydrotestosterone was approximately 10 times as potent (half maximal of 0.018 nM) as testosterone (half maximal of 0.2 nM); the maximal activity achieved was the same for the two androgens. These findings are nearly identical to the apparent K_d values for the interaction of the two hormones with the androgen receptor. Although testosterone and dihydrotestosterone may influence the expression of other genes differently, these findings are compatible with a model system in which the differential effects can be explained as a consequence of different binding affinities to the receptor. In peripheral target tissues dihydrotestosterone formation would be necessary for androgen action whereas in tissues in which testosterone concentration is high (testes, wolffian ducts) dihydrotestosterone formation may not be essential for virilization.