REGULATION OF EXPRESSION OF MALE-SPECIFIC RAT-LIVER MICROSOMAL 3-BETA-HYDROXYSTEROID DEHYDROGENASE

In the steroidogenic pathways present in the gonads and adrenal cortex, 3-beta-hydroxysteroid dehydrogenase isomerase (3-beta-HSD) is a key enzyme which controls the formation of DELTA-4-3-ketosteroids from DELTA-5-3-beta-hydroxysteroids. Herein, we used an antibody against human placental 3-beta-HSD and a rat testicular 3-beta-HSD cDNA probe to study the expression of rat liver 3-beta-HSD mRNA and protein. Rat liver microsomal 3-beta-HSD activity has been previously reported to exhibit a significant sex difference, with much higher activity in the male. We have shown an age-dependent increase in levels of immunoreactive 3-beta-HSD through the time of maturation of the male rat. The immunoreactive protein, of similar molecular size to the human placental and rat testicular 3-beta-HSD, was localized to the microsomal fraction of liver and was concentrated in pericentral locations. Immunoreactive protein was not detected in liver of immature (before 25 days of age) rats of either sex or in adult female liver. Northern blot analysis of liver and testicular RNA with a rat testicular 3-beta-HSD cDNA probe revealed the presence of a 1.6-kilobase mRNA species in addition to the major 2.1-kilobase mRNA species in adult male liver, neither of which was detected in immature or adult female liver RNA. Hypophysectomy of female rats or treatment with testosterone implants caused induction of liver 3-beta-HSD protein, while continuous infusion of GH to male rats decreased the level of 3-beta-HSD protein. Similarly, the levels of the mRNA species were decreased after GH treatment. Using [3-alpha-H-3]dehydroepiandrosterone as substrate for 3-beta-HSD activity, we determined the apparent K(m) for liver microsomal NAD+-dependent 3-beta-HSD activity to be 20-mu-M in both adult male and female liver and was much greater than the K(m) of rat Leydig tumor 3-beta-HSD activity (0.2-mu-M). Liver 3-beta-HSD activity was inhibited by trilostane, a proven inhibitor of gonadal and adrenal 3-beta-HSD activity. A rat liver 3-beta-HSD cDNA was isolated from a male liver cDNA library that was closely related to the type II 3-beta-HSD form of rat ovary but different from type III liver 3-beta-HSD. The enzyme obtained upon expression of this cDNA had properties characteristic of male-specific NAD+-dependent liver microsomal 3-beta-HSD (i.e. high apparent K(m) for dehydroepiandrosterone) and distinct from those of the high affinity gonadal type I 3-beta-HSD. We conclude that liver 3-beta-HSDs are distinct members of the 3-beta-HSD family of enzymes, and their male-specific expression is modulated via the sex-specific pattern of GH secretion.