Expression and Characterization of Recombinant Type 2 3a- Hydroxysteroid Dehydrogenase (HSD) from Human Prostate: Demonstration of Bifunctional 3a/17b-HSD Activity and Cellular Distribution

In androgen target tissues, 3a-hydroxysteroid dehydrogenase (3a-HSD) may regulate occupancy of the androgen receptor (AR) by catalyzing the interconversion of 5a-dihydrotestosterone (5a-DHT) (a potent androgen) and 3a-androstanediol (a weak androgen). In this study, a 3a-HSD cDNA (1170 bp) was isolated from a human prostate cDNA library. The human prostatic 3a-HSD cDNA encodes a 323amino acid protein with 69.9%, 84.1%, 99.4%, and 87.9% sequence identity to rat liver 3a-HSD and human type 1, type 2, and type 3 3a-HSDs, respectively, and is a member of the aldo-keto reductase superfamily. The close homology with human type 2 3a-HSD suggests that it is either identical to this enzyme or a structural allele. Surprisingly, when the recombinant protein was expressed and purified from Escherichia coli, the enzyme did not oxidize androsterone when measured spectrophotometrically, an activity previously assigned to recombinant type 2 3a-HSD using this assay. Complete kinetic characterization of the purified protein using spectrophotometric, fluorometric, and radiometric assays showed that the catalytic efficiency favored 3a-androstanediol oxidation over 5a-DHT reduction. Using [C]-5a-DHT as substrate, TLC analysis confirmed that the reaction product was [C]-3a-androstanediol. However, in the reverse reaction, [H]-3a-androstanediol was oxidized first to [H]-androsterone and then to [H]-androstanedione, revealing that the expressed protein possessed both 3aand 17b-HSD activities. The 17b-HSD activity accounted for the higher catalytic efficiency observed with 3a-androstanediol. These findings indicate that, in the prostate, type 2 3a-HSD does not interconvert 5a-DHT and 3a-androstanediol but inactivates 5a-DHT through its 3-ketosteroid reductase activity. Levels of 3a-HSD mRNA were measured in primary cultures of human prostatic cells and were higher in epithelial cells than stromal cells. In addition, elevated levels of 3a-HSD mRNA were observed in epithelial cells derived from benign prostatic hyperplasia and prostate carcinoma tissues. Expression of 3a-HSD was not prostate specific, since high levels of mRNA were also found in liver, small intestine, colon, lung, and kidney. This study is the first complete characterization of recombinant type 2 3a-HSD demonstrating dual activity and cellular distribution in the human prostate. (Molecular Endocrinology 11: 1971–1984, 1997)