| Microsomal epoxide hydrolase is an important biotransformation enzyme that detoxifies a large number of activated xenobiotics. It is readily inducible in animals, but the effects of chemical inducing agents on expression of human microsomal epoxide hydrolase are largely unknown. This project investigates the effects of several prototypic inducers of drug- and xenobiotic-metabolizing enzymes on human microsomal epoxide hydrolase mRNA, protein and enzyme activity levels.; One hour treatment of HepG2 cell lysates with several xenobiotics, known to be prototypic inducers of drug- and xenobiotic-metabolizing enzymes, did not affect the microsomal epoxide hydrolase enzyme activity, indicating that xenobiotics do not have a direct effect on human microsomal epoxide hydrolase.; In contrast, continuous treatment of cells in culture for 12 to 72 hours resulted in an early and sustained 2- to 3-fold increase in microsomal epoxide hydrolase mRNA levels. Expression of cytochrome P4501A1 monooxygenase mRNA was simultaneously determined as an internal control of cellular responsiveness to aryl hydrocarbon receptor- and XRE-regulated gene expression. Increases in microsomal epoxide hydrolase protein levels and microsomal epoxide hydrolase enzyme activity in response to stimulation were preceded by specific mRNA elevation. These results indicate that the expression of human microsomal epoxide hydrolase is regulated, at least in part, at the transcriptional level.; Microsomal epoxide hydrolase mRNA expression was stimulated following 12 hours of exposure to β-naphthoflavone (BNF), a compound known to upregulate several xenobiotic-metabolizing enzymes via both the antioxidant responsive element/electrophile responsive element (ARE/EpRE) and the xenobiotic responsive element (XRE). Microsomal epoxide hydrolase mRNA expression was also stimulated by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (selective for XRE) and tert-butylhydroquinone (selective for ARE/EpRE) alone, suggesting that both elements may playa role in regulation of microsomal epoxide hydrolase expression inhuman liver. Computer-aided analysis of the 5′-flanking region of human microsomal epoxide hydrolase gene revealed the presence of the multiple sequences with identity or a high resemblance to those of consensus XRE and ARE/EpRE sites.; Finally, concomitant exposure of cells in culture to the xenobiotic inducing agent BNF, and to the cytokine tumor necrosis factor-α (TNFα), resulted in a decrease of microsomal epoxide hydrolase enzyme activity, suggesting a negative regulatory role for TNFα and, possibly, other cytokines in regulation of expression of human microsomal epoxide hydrolase. |
