DIFFERENTIAL ESTROGEN SUBSTRATE SPECIFICITIES FOR TRANSIENTLY EXPRESSED HUMAN PLACENTAL 17-BETA-HYDROXYSTEROID DEHYDROGENASE AND AN ENDOGENOUS ENZYME EXPRESSED IN CULTURED COS-M6 CELLS

The metabolism of estrogens catalyzed by human placental 17beta-hydroxysteroid dehydrogenase (17HSD) transiently expressed in COS-m6 cells was studied, and the properties of the enzyme were compared with those of an endogenous hydroxysteroid dehydrogenase (HSD) expressed in the cells. In cultured cells, the endogenous HSD had almost exclusively oxidative activity, converting estradiol to estrone (oxidative and reductive activity, 0.84 +/- 0.164 and 0.034 +/- 0.01 nmol/mg protein . h, respectively). This was, nevertheless, opposed to the activity of the transiently expressed human placental 17HSD, as a high reductive activity (0.86 +/- 0.30 nmol/mg protein . h) appeared in the cells after transfection, whereas oxidative activity was not significantly induced. In the different transfections, the reductive activity was induced 13- to 34-fold, and the oxidative activity in the 17HSD-transfected cells was 65-162% of that in the mock-transfected cells. Thus, in cultured cells, these two enzymes preferentially catalyze opposite reactions. When the metabolism of the estrogens was followed up to 20 h, the two enzymes were found to regulate the proportion of estrone to estradiol in the culture medium. The different properties found for the enzymes show that the endogenous HSD expressed in the COS-m6 cells is an additional member of the family of 17HSD enzymes. It is suggested that different 17HSD enzymes exist, with differential estrogen substrate specificities in cultured cells. Thus, in addition to cofactor and substrate availability, the biological activity of estrogens in different cell types may be regulated by the expression of different forms of 17HSD enzymes, resulting in the dominance of either estradiol or estrone production.