Characteristics of a highly labile human type 5 17β-hydroxysteroid dehydrogenase

17 beta-Hydroxysteroid dehydrogenases (17 beta HSDs) play an essential role in the formation of active intracellular sex steroids. Six types of 17 beta HSD have been described to date, which only share approximately 20% homology. Human type 5 17 beta HSD complementary DNA is unique among the 17 beta HSDs because it belongs to the aldo-keto reductase family, whereas the others are members of the short chain alcohol dehydrogenases. The characteristics of human type 5 17 beta HSD were investigated in human embryonic (293) cells stably transfected with human and mouse type 5 17 beta HSD, as well as human type 3 3 alpha HSD. Using intact transfected cells, type 5 17 beta HSD shows a substrate specificity pattern comparable to those of human type 3 17 beta HSD and mouse type 5 17 beta HSD. These enzymes catalyze more efficiently the transformation of androstenedione (4-dione) to testosterone, whereas the transformation of dihydrotestosterone to 5 alpha-androstane-3 alpha,17 beta-diol is much lower. In contrast, type 3 3 alpha HSD catalyzes more efficiently the transformation of dihydrotestosterone to 5 alpha-androstane-3 alpha,17 beta-diol, whereas the transformation of 4-dione to testosterone represents only 7% of the 3 alpha HSD activity. However. upon homogenization, human type 5 17 beta HSD activity decreases to approximately 10% of the activity in intact cells and remains stable at this level together with the 3 alpha HSD activity. Under the same conditions, however, the mouse Enzyme is not altered by homogenization. Indeed, using purified human 17 beta HSD overexpressed in Escherichia coli, we could confirm that a much greater amount of protein is required to produce activity similar to the enzymatic activity measured in intact transfected cells, The present data provide the answer to the question of why previous researchers could hardly detect type 5 17 beta HSD activity. Indeed, all previous publications used cell or tissue homogenates or purified enzymes. Under such conditions, only the low level, but stable, 3 alpha BSD and 17 beta HSD activities could be measured, whereas the high level, but highly unstable, 17 beta HSD activity could not be measured. As type 5 17 beta HSD shares 84%, 86%, and 88% amino acid identity with types 1 and 3 3 alpha HSD and 20 alpha HSDs, respectively, Northern blot analysis used in previous studies could not provide unequivocal information. In this report, we used a more specific ribonuclease protection assay and could thus show that human type 5 17 beta HSD is expressed in the liver, adrenal, and prostate; in prostatic cancer cell lines DU-145 and LNCaP: as well as in bone carcinoma (MG-63) cells. By analogy with type 3 17 beta HSD, which is responsible for the formation of androgens in the testis, the expression of type 5 17 beta HSD in the prostate and bone cells suggests that this enzyme is involved in the formation of active intracellular androgens in these tissues.