Virilization of the urogenital sinus of the tammar wallaby is not unique to 5α-androstane-3α,17β-diol

Abstract

The androgen 5α-androstane-3α,17β-diol (5α-adiol) is synthesized in testes and secreted into plasma of male tammar wallaby pouch young and appears to virilize the urogenital sinus. To provide insight into its mechanism of action, a dose response study showed that administration of 1 μg 5α-adiol monoenanthate per g body wt. per week for 3 weeks to 24-day-old female pouch young induced prostate bud formation equivalent to that of males of the same age. Administration of this same dose of the enanthates of testosterone, dihydrotestosterone, and 5α-adiol to female pouch young caused equivalent virilization of the urogenital sinus. The fact that 5α-adiol does not exert a unique effect, together with our earlier findings in this species that 5α-adiol and testosterone are converted to dihydrotestosterone in the urogenital sinus and that virilization of the urogenital sinus is prevented by the androgen receptor antagonist flutamide, suggest that 5α-adiol is a circulating precursor for dihydrotestosterone formation in this tissue.

Introduction

Development of the male urogenital tract in all mammalian species is controlled by testicular androgens (George and Wilson, 1994, Wilson et al., 1995). In the eutherian mammal, formation of the male urogenital tract occurs over a short time early in intrauterine life, and it has not been possible to identity the circulating androgen responsible. However, in the marsupial the male phenotype develops after birth and over a longer period of time (Renfree et al., 1995). For example, in the tammar wallaby Macropus eugenii prostate buds begin to form between days 20 and 30 of pouch life (Shaw et al., 1988), and development of the male phallus starts between days 80 and 100 (Butler et al., 1999). The fact that these processes take place post-natally during extrauterine development and in distinct phases makes it possible to investigate the formation of the male phenotype in detail.

It was generally assumed that virilization during male development occurs by the same mechanism as that in the adult male, namely that testosterone is synthesized in the testes, secreted into plasma, and transported to androgen target tissues, where it acts directly via the androgen receptor (Wolffian ducts) or is locally 5α-reduced to dihydrotestosterone, which also acts via the androgen receptor (urogenital sinus and urogenital tubercle) (George and Wilson, 1994). In the early tammar pouch young testosterone concentrations are higher in testes than ovaries (Renfree et al., 1992), virilization of the urogenital tract of female pouch young can be induced by grafting developing testes under the skin (Tyndale-Biscoe and Hinds, 1989), and virilization of the male pouch young can be inhibited by inhibition of dihydrotestosterone formation (Ryorchuk et al., 1997) or of dihydrotestosterone binding to the androgen receptor (Lucas et al., 1997). However, in the tammar wallaby (Wilson et al., 1999) and in the gray short-tailed opossum (Fadem and Harder, 1992, Xie et al., 1998) there is no sexual dimorphism in the levels of dihydrotestosterone or testosterone at the time of prostate development. The circulating androgen that virilizes the prostate in the tammar wallaby appears to be another 5α-reduced androgen, 5α-androstane-3α,17β-diol (5α-adiol), which is synthesized in the testes, is secreted into male plasma, and induces prostate formation when administered to female pouch young (Shaw et al., 2000). 5α-Adiol does not appear to activate the androgen receptor (Penning, 1997), and the mechanism by which it promotes virilization of the urogenital sinus is unclear. 5α-Adiol could serve as a unique mediator by combining with an unidentified receptor or by participating in a specific transport process into cells (Ding et al., 1998), or it could be converted to dihydrotestosterone in target cells (Shaw et al., 2000) and act via the androgen receptor (Bruchovsky, 1971).

To provide insight into its mechanism of action, we investigated the effects of different dosages of 5α-adiol on prostate formation in female pouch young and then compared the effects of 5α-adiol, dihydrotestosterone, and testosterone on this process.