Biosynthesis of steroid hormones requires a battery of oxidative enzymes located in both mitochondria and endoplasmic reticulum. The rate-limiting step in this process is the transport of free cholesterol from the cytoplasm into mitochondria. Within mitochondria, cholesterol is converted to pregnenolone by an enzyme in the inner membrane called CYP11A1. Pregnenolone itself is not a hormone, but is the immediate precursor for the synthesis of all of the steroid hormones. The following table delineates the enzymes required to synthesize the major classes of steroid hormones.
Steroidogenesis in the testis is regulated by a negative feedback mechanism through the hypothalamus-pituitary-testis axis. Recent studies suggest that besides this long-loop regulation, testicular steroidogenesis is also locally regulated by androgen. However, the molecular mechanism behind this additional regulatory pathway has been poorly addressed. In the present study, we demonstrate that liganded androgen receptor (AR) suppresses the transcriptional activity of Nur77 on steroidogenic enzyme gene promoters, affecting testicular steroidogenesis. AR physically interacts and colocalizes with Nur77 in the nucleus in the presence of androgen. AR inhibits Nur77 transactivation by competing mainly with coactivators such as SRC-1 for Nur77 binding. These results suggest that androgen, through binding to AR, directly acts as a signal inhibiting the expression of steroidogenic enzyme genes in Leydig cells, eventually resulting in decreased testicular steroidogenesis. These findings strongly support the hypothesis that androgen acts locally to regulate testicular steroidogenesis, and may provide its action mechanism.