Targeted Disruption of the Microsomal Epoxide Hydrolase Gene

Abstract

Microsomal epoxide hydrolase (mEH) is a conserved enzyme that is known to hydrolyze many drugs and carcinogens, and a few endogenous steroids and bile acids. mEH-null mice were produced and found to be fertile and have no phenotypic abnormalities thus indicating that mEH is not critical for reproduction and physiological homeostasis. mEH has also been implicated in participating in the metabolic activation of polycyclic aromatic hydrocarbon carcinogens. Embryonic fibroblast derived from the mEH-null mice were unable to produce the proximate carcinogenic metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens. They were also resistant to DMBA-mediated toxicity. Using the two-stage initiation-promotion skin cancer bioassay, the mEH-null mice were found to be highly resistant to DMBA-induced carcinogenesis. In a complete carcinogenesis bioassay, the mEH mice were totally resistant to tumorigenesis. These data establish in an intact animal model that mEH is a key genetic determinant in DMBA carcinogenesis through its role in production of the ultimate carcinogenic metabolite of DMBA, the 3,4-diol-1,2-epoxide.Microsomal epoxide hydrolase (mEH) 1 is a critical phase I biotransformation enzyme that catalyzes hydrolysis of a large number of epoxide intermediates (1, 2). mEH is highly conserved in different mammalian species, is expressed in the embryo (3-5) and multiple organs, and is active toward some endogenous epoxy-steroids (6) and bile acids (7) thus suggesting that it plays a critical physiological role. mEH is usually thought to play a pivotal role in protection against the toxicity of reactive epoxide intermediates, because metabolism of epoxides by this enzyme results in the production of less reactive and less toxic dihydrodiol intermediates of drugs such as phenytoin and carbamazepine (8, 9) and epoxides of environmental toxins (10, 11). In contrast to this protective effect, mEH is thought to be required for the metabolic activation of the potent carcinogen 7,12-dimethylbenz[a]anthracene (DMBA), a widely studied experimental prototype for the polycylic aromatic hydrocarbon class of chemical carcinogens (12). P450s and mEH metabolize DMBA to both inert metabolites and metabolites that are electrophilic and capable of producing DNA adducts (Fig. 1). Cytochrome P450 CYP1B1 oxidizes DMBA to the 3,4-epoxide (13). This is followed by hydrolysis of the epoxide by mEH to the proximate carcinogenic metabolite, DMBA-3,4-diol. This metabolite can be further oxidized by either CYP1A1 or CYP1B1 to the principal ultimate carcinogenic metabolite, DMBA-3,4-diol-1,2-epoxide, that is capable of producing DNA adducts (14 -17). Other ring hydroxylations and methyl hydroxylations of DMBA result in inactive metabolites that do not bind DNA. Based on this scheme, mEH should be a critical enzyme in the pathway leading to the carcinogenic activity of DMBA. However a role for mEH in DMBA carcinogenesis has not been established i...

Genetic polymorphism and gene expression of microsomal epoxide hydrolase in non-small cell lung cancer

“…When 295-and 62-bp fragments appeared, the patient had homozygous allele of His139. When 174-, 121-and 62-bp bands appeared, the patient was identified as homozygous Arg139 allele, and when all four DNA fragments appeared, the patient was heterozygous with His139/Arg139 (1,(8)(9)(10)(11)(12)(13)(14)(15).…”

“…Epoxide hydrolase (EH) is a phase I biotransformation enzyme that catabolizes epoxides, which are converted from hydrophorbic polycyclic aromatic hydrocarbons (PAH), into dihydrodiols (1)(2)(3). In contrast to highly reactive epoxides, dihydrodiols are mostly inert and can be excreted after conjugating to glutathione (4).…”

“…However, EH may not always be protective. Hydrolysis of certain epoxides by EH may result in carcinogenic metabolites that can inauspiciously form DNA adducts (1).…”

“…Two polymorphic sites have been reported within the coding region of EPHX1. One is located in exon 3 at amino acid residue 113, in which tyrosine (Tyr113, TAC) or histidine (His113, CAC) is expressed, and the other is located in exon 4 at amino acid residue 139, in which histidine (His139) or arginine (Arg139) is expressed (1,8,9). The enzyme with Tyr113 (fast type) has 50% higher activity than that with His113 (slow type), whereas mEH with His139 (slow type) has only 25% activity of that with Arg139 (fast type) (8).…”

The EPHX1 rs1051740 Polymorphism Is Associated with Childhood Acute Lymphoblastic Leukemia in a Korean Population

“…Recent studies have suggested that the interaction between these genes and environmental factors may also affect the susceptibility to leukemia [10]- [12]. Microsomal epoxide hydrolase (EPHX1), along with CYP, another family of xenobiotic-metabolizing enzymes, plays an important role in both the detoxification and activation of procarcinogens [13] [14] and is considered to be a protective enzyme [15]. Two polymorphisms in EPHX1, Tyr113 His in exon 3 (rs1051740) and His139Arg in exon 4 (rs2234922), have been identified to affect enzyme activity.…”