Characterization of the human aldehyde reductase gene and promoter

Aldehyde reductase (EC 1.1.1.2; AKR1A1) is involved in the reduction of biogenic and xenobiotic aldehydes and is present in virtually every tissue. To study the regulation of its expression, the human aldehyde reductase gene and promoter were cloned and characterized. The protein coding region consists of eight exons, with two additional upstream exons, separated by a large intron of 9.4 kb, that code for the 5' untranslated region of the mRNA. Two mRNA transcripts that encode the same protein and that originate from alternative splicing were identified. The shorter transcript is the major form as shown by Northern blots and reverse transcription-PCR experiments. Northern blots of multiple tissues indicate that aldehyde reductase mRNA is present in all tissues examined and is most abundant in kidney, liver, and thyroid, which is consistent with the tissue enzyme distribution. The two mRNA transcripts do not exhibit differential tissue distribution. A construct containing a promoter region insert in a pGL3 vector drives transcription of a luciferase reporter gene and is 290-fold more active than a control vector without insert in transfected HepG2 cells. The activity of the full promoter construct is comparable to that of a pGL3 vector containing the SV40 promoter with an enhancer. The promoter does not contain a TATA box, but contains multiple GC-rich islands and exhibits bidirectional activity in transfection studies. The major active promoter element was localized by nested deletions and mutations to a DNA element (TGCAAT, -59 to -54) that presumptively binds the transcription factor CHOP [CAAT enhancer binding protein (C/EBP) homologous protein]. Comparison of the aldehyde reductase gene structure to all other characterized human genes of the aldo-keto reductase superfamily (aldose reductase, bile acid binder, and type I and type II 3alpha-hydroxysteroid dehydrogenases) indicates that it is more distantly related to these genes than they are among themselves.