STRUCTURE AND TISSUE-SPECIFIC EXPRESSION OF A NOVEL MEMBER OF THE RAT 3-BETA-HYDROXYSTEROID DEHYDROGENASE/DELTA(5)-DELTA(4) ISOMERASE (3-BETA-HSD) FAMILY - THE EXCLUSIVE 3-BETA-HSD GENE EXPRESSED IN THE SKIN

Structures of cDNA clones encoding three members of the rat 3beta-hydroxysteroid dehydrogenase/DELTA5-DELTA4 isomerase (3beta-HSD) family were characterized. To search for potential new types of 3beta-HSD, rat types I and II 3beta-HSD cDNAs were used as probes to screen a rat genomic DNA library. Among the clones isolated, one encodes a novel predicted rat 3beta-HSD isoenzyme, chronologically designated type IV. The corresponding full-length cDNA was thereafter isolated by selective polymerase chain reaction amplification from rat ovary and day- 15 placenta cDNA libraries. The rat type IV 3beta-HSD cDNA encodes a predicted 372-amino acid protein of 41,854 daltons, which shares 90.9, 87.9, and 78.8% sequence identity with rat types 1, II, and III proteins, respectively. Ribonuclease protection assay reveals that type IV 3beta-HSD is the sole 3beta-HSD mRNA species detectable in the skin and represents the predominant species in the placenta while being also detectable in the ovary and, to a lower degree, in the adrenal gland. Transient expression of type IV cDNA in SW- 13 cells indicates 3beta-HSD activity similar to that of rat type I 3beta-HSD. The presence of multiple 3beta-HSD genes should permit differential and tissue-specific regulation of this rate-limiting enzymatic activity essential in the biosynthesis of all classes of steroid hormones in both classical steroidogenic and intracrine peripheral tissues.