Structure and activity of the murine type 5 17β-hydroxysteroid dehydrogenase gene

Abstract

17β-Hydroxysteroid dehydrogenases (17β-HSDs) play a crucial role in the control of active sex steroid intracellular levels. Seven types of 17β-HSD have been described. In this study, we report the cloning and characterization of the mouse type 5 17β-HSD belonging to the aldo-keto reductase superfamily, in contrast with types 1, 2, 3, 4, 6, and 7 17β-HSD which belong to the short-chain alcohol dehydrogenase family. The gene spans 16 kb and contains 9 exons separated by 8 introns. Primer extension analysis identified a major transcription start site beginning 50 nucleotides upstream from the ATG initiation codon. Northern blot analysis showed a high mRNA expression level in the liver and a weaker signal in the kidney. To determine more precisely the substrate specificity of the enzyme, we established a stable cell line expressing mouse type 5 17β-HSD in transformed human embryonic kidney (293) cells. The transfected cell line preferentially catalyzes the transformation of 4-androstenedione (4-dione) and androstanedione (A-dione) into testosterone (T) and dihydrotestosterone (DHT), respectively. This data is somewhat in contradiction with a previous study that described the enzyme as estradiol 17β-dehydrogenase. Our results indicate that the rate of transformation of estradiol (E 2) to estrone (E 1) represents only 1% of the rate of transformation of 4-dione to T. Mouse type 5 17β-HSD shares 76% amino acid sequence identity with human type 5 17β-HSD; 71%, 76%, 76% with rat 3α-HSD and human types 1 and 3 3α-HSDs, respectively; and 71%, 69% and 77% with mouse, rat and human 20α-HSD, respectively.