The rat 17β-hydroxysteroid dehydrogenase type III:: Molecular cloning and gonadotropin regulation

17 beta-Hydroxysteroid dehydrogenase (17 beta HSD), the enzyme that catalyzes the final step of testosterone biosynthesis in the testis, was cloned from a rat Leydig cell complementary DNA library to gain insights into the functional requirements, activation mechanisms, and molecular regulation. The 17 beta HSD complementary DNA encoded 306 amino acids (molecular mass of 33.7 kDa) and displayed 75% and 85% amino acid sequence homology to the human and mouse 17 beta HSD type III enzymes, respectively. Northern analysis revealed a single 1.4-kb messenger RNA (mRNA) species in rat Leydig cells, whereas ovarian mRNA was detected only by RT-PCR amplification. The cloned 17 beta HSD expressed in mammalian cell lines specifically catalyzed the reductive reaction in androgen formation with androstenedione as the preferred substrate. This reaction was significantly reduced in the absence of glucose. Expression of the endogenous 17 beta HSD gene in rat Leydig cells was inhibited by a single dose of hCG in vivo, with maximum reduction of steady state mRNA levels at 24 h and recovery at 9 days. Such agonist-induced down-regulation of 17 beta HSD expression, which preceded the marked reduction of LH receptors, resulted from changes at the transcriptional level and was accompanied by loss of enzymatic activity. These studies have demonstrated a glucose requirement for optimal activity of the enzyme in vitro and for a role of gonadotropin in regulating the expression of 17 beta HSD gene in vivo. Cloning of the 17 beta HSD type III enzyme from rat Leydig cells will facilitate further investigation of the molecular regulation of its activity in the testis.