Genomic structure of rat 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase (3 alpha-HSD/DD, AKR1C9)

Rat liver 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase (3 alpha-HSD/DD) is a member of the aldo-keto reductase (AKR) superfamily. It is involved i n the inactivation of steroid hormones and the metabolic activation of poly cyclic aromatic hydrocarbons (PAH) by converting trans-dihydrodiols into re active and redox-active o-quinones. The structure of the 5'-flanking region of the gene and factors involved in the constitutive and regulated express ion of this gene have been reported [H.-K. Lin, T.M. Penning, Cloning, sequ encing, and functional analysis of the 5'-flanking region of the rat 3 alph a-hydroxysteroid/dihydrodiol dehydrogenase gene, Cancer Res. 55 (1995) 4105 -4113]. We now describe the complete genomic structure of the rat type 1 3 alpha-HSD/DD gene. Charon 4A and P1 genomic clones contained at least three rat genes (type 1, type 2 and type 3 3 alpha-HSD/DD) each of which encoded for the same open reading frame (ORF) but differed in their exon-intron or ganization. 5'-RACE confirmed that the type 1 3 alpha-HSD/DD gene encodes f or the dominant transcript in rat liver and it was the regulation of this g ene that was previously studied. The rat type 1 3 alpha-HSD/DD gene is 30 k b in length and consists of nine exons and eight introns. Exon 9 encodes +9 31 to 966 bp of the ORF and the 1292 bp 3'-UTR implicated in mRNA stability . This genomic structure is nearly identical to the homologous human genes, type 1 3 alpha-HSD (chlordecone reductase/DD4, AKR1C4), type 2 3 alpha-HSD (AKR1C3) and type 3 3 alpha-HSD (bile-acid binding protein, AKR1C2) genes. Three different cDNA's containing identical ORFs for 3 alpha-HSD have been reported suggesting that all three genes may be expressed in rat liver. Us ing 5' primers corresponding to the 5'-UTR's of the three different cDNA's only one PCR fragment was obtained and corresponded to the type 1 3 alpha-H SD/DD gent. These data suggested that the type 2 and type 3 3 alpha-HSD/DD genes are not abundantly expressed in rat liver. It is unknown whether the type 2 and type 3 3 alpha-HSD/DD genes represent pseudo-genes or whether th ey represent genes that are differentially expressed in other rat tissues. (C) 1999 Elsevier Science Ltd. All rights reserved.