Tissue-, Region-, and Gene-Specific Induction of Microsomal Epoxide Hydrolase Expression and Activity in the Mouse Intestine by Arsenic in Drinking Water

The Abstract

This comprehensive investigation sought to thoroughly characterize the specific effects of arsenic exposure on the transcriptional and translational expression profiles of two key epoxide hydrolase enzymes: microsomal epoxide hydrolase (mEH), encoded by the EPHX1 gene, and soluble epoxide hydrolase (sEH), encoded by the EPHX2 gene, within both the liver and the small intestine. To achieve this, C57BL/6 mice were systemically exposed to sodium arsenite, dissolved in their drinking water, at various precisely controlled doses over a period extending up to 28 days.

Our findings revealed distinct and tissue-specific patterns of mEH induction. Notably, mEH messenger RNA (mRNA) and protein expression were significantly induced by arsenic exposure at a concentration of 25 parts per million (ppm) specifically within the small intestine, and this induction was observed consistently in both male and female mice. In contrast, hepatic (liver) mEH expression required higher arsenic concentrations, becoming induced only at doses of 50 ppm or 100 ppm. This induction of mEH was found to be highly gene-specific, as the identical arsenic exposure conditions did not lead to any measurable induction of sEH expression in either the liver or the small intestinal tissue. Furthermore, within the small intestine, the induction of mEH expression was spatially restricted, occurring exclusively in the proximal segments of the small intestine, but not in the more distal regions.

The observed induction of intestinal mEH was not merely a change in gene expression; it was functionally significant. This upregulation was accompanied by measurable increases in microsomal enzymatic activities. These enhanced activities were demonstrated *in vitro* against a classic model mEH substrate, cis-stilbene oxide, and critically, against an epoxide-containing pharmaceutical compound, oprozomib. To confirm the *in vivo* relevance of these findings, we observed corresponding increases in the levels of PR-176, which is the primary hydrolysis metabolite of oprozomib, specifically in the proximal small intestine of mice treated with oprozomib following arsenic exposure.

These compelling findings collectively suggest that intestinal mEH, an enzyme primarily involved in converting exogenous xenobiotic epoxides into less reactive diols, plays a relevant role in the adverse effects associated with arsenic exposure. Conversely, sEH, which typically prefers endogenous epoxides as substrates, does not appear to be similarly implicated. The study highlights that further, more in-depth investigations into the complex interactions between chronic drinking water arsenic exposure and its potential impact on the disposition, or pharmacokinetic profile, of epoxide-containing drugs and other xenobiotic compounds within the intestinal environment are critically warranted.

Significance Statement

The global consumption of water contaminated with arsenic has been unequivocally linked to an elevated risk of a diverse array of adverse health effects in human populations, including chronic conditions such as diabetes. While the epithelial cells lining the small intestine represent the primary anatomical site for the absorption of ingested arsenic into the body, the precise characterization of arsenic exposure-related changes within these intestinal cells has, until now, remained largely underdeveloped. This study makes a significant contribution by identifying specific gene expression alterations within the small intestine. These identified changes may be mechanistically connected to the insidious adverse health effects observed following chronic arsenic exposure. Furthermore, the findings illuminate possible interactions between the ingestion of arsenic and the *in vivo* pharmacokinetics of epoxide-containing pharmaceutical drugs, as well as other xenobiotic compounds, specifically within the intestinal lumen. These insights open crucial avenues for understanding arsenic’s systemic effects and its potential to modulate drug efficacy and toxicity in humans.