ONTOGENY AND SUBCELLULAR-LOCALIZATION OF 3-BETA-HYDROXYSTEROID DEHYDROGENASE (3-BETA-HSD) IN THE HUMAN AND RAT ADRENAL, OVARY AND TESTIS

Primates are unique in having adrenals that secrete large amounts of the precursor sex steroids (PSS) dehydroepiandrosterone (DHEA) and especially DHEA-sulfate. The adrenal PSS require the action of 3beta-hydroxysteroid dehydrogenase/5-ene-4 ene isomerase (3beta-HSD), 17beta-hydroxysteroid dehydrogenase (17beta-HSD), 5alpha-reductase and/or aromatase to form the androgen dihydrotestosterone (DHT) or the estrogens 17beta-estradiol and androst-5-ene-diol. Knowing the crucial role of 3beta-HSD and 17beta-HSD in sex steroid biosynthesis both in classical as well as in peripheral steroidogenic tissues, we have concentrated our efforts on the elucidation of the molecular structure of these enzyme families. We have thus characterized two types of human 3beta-HSD cDNA clones and their corresponding genes which encode deduced proteins of 371 and 372 amino acids and share 93.5% homology. Human type I 3beta-HSD is the almost exclusive mRNA species expressed in the placenta and skin, while human type II is the predominant mRNA species in the adrenals, ovaries and testes. We have also recently elucidated the structure of three types of rat 3beta-HSD cDNAs which all encode a 372 amino acid protein. The predicted rat type I and II 3beta-HSD proteins expressed adrenals, gonads and adipose tissue share 94% homology while they share 80% similarity with the liver-specific type III 3beta-HSD. Transient expression of human type I and II as well as rat type I and II 3beta-HSD cDNAs in HeLa human cervical carcinoma cells reveals that 3beta-ol dehydrogenase and 5-ene-4-ene isomerase activities reside within a single protein and that these cDNAs encode functional 3beta-HSD proteins. The expressed rat type III protein possesses a unique property catalyzing selectively the reduction of 3beta-androstane 5alpha-steroids such as DHT. Furthermore, we have also demonstrated by site-directed mutagenesis that the lower activity of expressed rat type II compared to rat type I 3beta-HSD protein is due to a change of four amino acid residues potentially involved in a membrane-spanning domain. In parallel, we have characterized the complete nucleotide sequence of human 17beta-HSD cDNA clones encoding a 327 amino acid protein as well as two in tandem 17beta-HSD genes. Two major 17beta-HSD mRNA species have been detected in several tissues due to a tissue-specific alternative site of initiation of transcription. It is quite clear that our attention should be focused on intracrinology, in order to better understand the physiological mechanisms controlling local steroid formation.