Effects of chemical inducers on human microsomal epoxide hydrolase in primary hepatocyte cultures

Human microsomal epoxide hydrolase (mEH; EC 3.3.2.3) is an important biotransformation enzyme and potential risk determinant for pathologies such as cancer and teratogenesis. Currently, the effects of chemical exposures on human mEH gene expression are largely unknown, but they may constitute a unique modifier of disease susceptibility. To examine this issue, we exposed cultures of primary human hepatocytes isolated from seven donors to prototypic chemical inducers [such as phenobarbital (PB), polyaromatic hydrocarbons, dexamethasone, butylated hydroxyanisole, and ciprofibrate]. Basal levels of mEH RNA and protein were detected readily in untreated cells. Chemical treatment of cultured hepatocytes resulted in variable mEH RNA and protein expression, but, in general, only modest modulatory effects were detected following these exposures. The maximum increase in mEH RNA expression observed was approximately 3.5-fold following Arochlor 1254 exposure. Immunochemical levels of mEH protein were quantified for all treatment groups in three cultures and demonstrated less overall variation and, in general, a lack of concordance with corresponding mEH RNA levels. Cytochrome P450 (CYP) 1A2 and 3A mRNA levels were measured before and following exposure to β-naphthaflavone and PB, respectively, to permit independent evaluation of hepatocyte inducer responsiveness. Substantial increases in RNA expression levels for both the CYP1A2 and CYP3A genes demonstrated that the hepatocyte cultures were robust and highly responsive to inducer treatment. These results indicate that the mEH gene in human hepatocytes is only modestly responsive to chemical exposures.