Alternative Promoters Determine Tissue-Specific Expression Profiles of the Human Microsomal Epoxide Hydrolase Gene (EPHX1)

Microsomal epoxide hydrolase (EPHX1) catalyzes hydration reactions that determine the cellular disposition of reactive epoxide derivatives. Whereas the previously defined EPHX1 exon 1 sequence (E1) is derived from a promoter proximal to exon 2 of the EPHX1 coding region, in this investigation, we identified an alternative EPHX1 exon 1 sequence, E1-b, originating from a gene promoter localized ∼18.5 kb upstream of exon 2. Northern hybridizations demonstrated that the E1-b variant is widely expressed and that the E1-b promoter functions as the primary driver of EPHX1 expression in human tissues. In contrast, the E1 promoter directs expression only in the liver. To examine the basis for liver-specific usage of the E1 promoter, we identified several potential cis -regulatory elements that included GATA (-110/-105) and hepatocyte nuclear factor 3 (HNF3) (-96/-88) motifs. GATA-4 was the principal GATA family member interacting with its respective motif, whereas both HNF3α and HNF3β were capable of interacting with the HNF3 element. GATA-4 and HNF3α/HNF3β DNA binding complexes were enriched in hepatic cells. Site-directed mutagenesis and transactivation analyses of the E1 promoter revealed that GATA-4 is probably a principal factor that regulates liver-specific expression of the E1 variant, with HNF3α and HNF3β acting to negatively regulate GATA-4 function in hepatic cells.