17 beta-hydroxysteroid dehydrogenases in normal human mammary epithelial cells and breast tissue

17 beta-hydroxysteroid dehydrogenase activity represents a group of several isoenzymes (17HSDs) that catalyze the interconversion between highly activ e 17 beta-hydroxy- and low activity 17-ketosteroids and thereby regulate th e biological activity of sex steroids. The present study was carried out to characterize the expression of 17HSD isoenzymes in human mammary epithelia l cells and breast tissue. In normal breast tissues 17HSD types 1 and 2 mRN As were both evenly expressed in glandular epithelium. In two human mammary epithelial cell lines, mRNAs for 17HSD types 1, 2 and 4 were detected. In enzyme activity measurements only oxidative 17HSD activity, corresponding t o either type 2 or type 4 enzyme, was present. The role of 17HSD type 4 in estrogen metabolism was further investigated, using several cell lines orig inating from various tissues. No correlation between the presence of 17HSD type 4 mRNA and 17HSD activity in different cultured cell lines was detecte d. Instead, oxidative 17HSD activity appeared in cell lines where 17HSD typ e 2 was expressed and reductive 17HSD activity was present in cells express ing 17HSD type 1. These data strongly suggest that in mammary epithelial ce ll lines the oxidative activity is due to type 2 17HSD and that oxidation o f 17 beta-hydroxysteroids is not the primary activity of the 17HSD type 4 e nzyme.