Expression cloning of a novel estrogenic mouse 17β-hydroxysteroid dehydrogenase 17-ketosteroid reductase (m17HSD7), previously described as a prolactin receptor-associated protein (PRAP) in rat

17 beta-Hydroxysteroid dehydrogenases/17-ketosteroid reductases (17HSDs) modulate the biological activity of certain estrogens and androgens by catalyzing reductase or dehydrogenase reactions between 17-keto- and 17 beta-hydroxysteroids. In the present study, we demonstrate expression cloning of a novel type of 17HSD, chronologically named 17HSD type 7, from the HC11 cell line derived from mouse mammary gland. The cloned cDNA, 1.7 kb in size, encodes a protein of 334 amino acids with a calculated molecular mass of 37,317 Da. The primary structure contains segments characteristic of enzymes belonging to the short-chain dehydrogenase/reductase superfamily. Strikingly, mouse 17HSD type 7 (m17HSD7) shows 89% identity with a recently cloned rat protein called PRL receptor-associated protein (PRAP). The function of FRAP has not yet been demonstrated. The enzymatic characteristics of m17HSD7 and RT-PCR-cloned rat FRAP (rPRAP) were analyzed in cultured HEK-293 cells, where both of the enzymes efficiently catalyzed conversion of estrone (E-1) to estradiol (E-2). With other substrates tested no detectable 17HSD or 20 alpha-hydroxysteroid dehydrogenase activities were found. Kinetic parameters for m17HSD7 further indicate that E-1 is a preferred substrate for this enzyme. Relative catalytic efficiencies (V-max/K-m values) for E-1 and E-2 are 244 and 48, respectively. As it is the case with rPRAP, m17HSD7 is most abundantly expressed in the ovaries of pregnant animals. Further studies show that the rat enzyme is primarily expressed in the middle and second half of pregnancy, in parallel with E-2 secretion from the corpus luteum. The mRNA for m17HSD7 is also apparent in the placenta, and a slight signal for m17HSD7 is found in the ovaries of adult nonpregnant mice, in the mammary gland, liver, kidney, and testis. Altogether, because of their similar primary structures, enzymatic characteristics, and the tissue distribution of m17HSD7 and rPRAP, we suggest that rPRAP is rat 17HSD type 7. Furthermore, the results indicate that 17HSD7 is an enzyme of E-2 biosynthesis, which is predominantly expressed in the corpus luteum of the pregnant animal.