Three types of 17β-hydroxysteroid dehydrogenases involved in Japanese eel ovarian steroidogenesis

The synthesis of estradiol-17β (E2), an important steroid hormone in piscine gametogenesis, is catalyzed by various steroidogenic enzymes under the control of pituitary gonadotropins (Gths), follicle-stimulating hormone (Fsh), and luteinizing hormone (Lh). Among these enzymes, 17β-hydroxysteroid dehydrogenases (Hsd17bs) with 17β-hydroxysteroid dehydrogenase/17-ketosteroid reductase activities are crucial enzymes in the synthesis of sex steroid hormones in gonadal and extra-gonadal tissues. In teleost species, although Hsd17b1 is recognized as essential for E2 synthesis, other Hsd17b types have recently been shown to have the potential to produce E2; however, information is limited. In this study, we isolated cDNA encoding Hsd17b7 from Japanese eels. We then characterized both hsd17b7 and hsd17b12a and compared their properties with those of hsd17b1 to identify the enzymes involved in E2 synthesis during ovarian development. The isolated hsd17b7 cDNAs putatively encoded 345 amino acid residues with conserved signatures of the short-chain dehydrogenase family. Recombinant Hsd17b7 and Hsd17b12a expressed in Human kidney 293T cells transformed androstenedione (A4) into testosterone (T) and estrone (E1) into E2. Tissue distribution analysis using quantitative real-time PCR revealed that hsd17b1, hsd17b7, and hsd17b12a mRNA were present in the ovaries of immature female Japanese eels. Furthermore, among the three hsd17bs, only hsd17b1 expression was upregulated during ovarian development. These results suggest that Hsd17b1, Hsd17b7, and Hsd17b12a are involved in E2 synthesis in the ovaries of the Japanese eel, and that Hsd17b1 plays a particularly important role in E2 production during oogenesis.