Enhanced expression of rat microsomal epoxide hydrolase gene by organosulfur compounds

The effects of organosulfur compounds including allylsulfide (AS), allylmercaptan (AM) and allylmethylsulfide (AMS) on the expression of microsomal epoxide hydrolase (mEH) protein and its mRNA were examined in rats. The levels of mEH induction were examined with or without concomitant treatment of animals with pyrazine, a strong inducer of mEH, in order to establish whether a common molecular basis exists for mEH induction between these structurally different xenobiotics. Immunoblot analyses using anti-rat mEH antibody showed that treatment with AS caused an approximately 4-fold increase in hepatic mEH protein levels relative to controls whereas treatment with both AS and pyrazine resulted in only minimal additive increases in the elevation of mEH. Administration of AM to rats resulted in a comparable increase in mEH levels to that caused by AS, whereas an approximately 2-fold increase was noted after AMS treatment, as compared to control. mEH levels in the hepatic microsomes isolated from animals treated with both AMS and pyrazine were, however, approximately 50% less than those from pyrazine-treated rats. Thus, AS and AM appeared to be more effective than AMS in elevating mEH, as evidenced by immunoblot analyses. The levels of mEH mRNA were increased 10-16-fold following treatment with either AS or AM, while AMS caused a 3-7-fold increase relative to control, as assessed by slot blot analysis probed with a 1.3 kb mEH cDNA. Time-dependent increases in mRNA levels by each of these organosulfur compounds were consistent with those in mEH protein levels at 3 days. A marginal additive increase in mEH mRNA levels was noted following co-administration of either AS or AM with pyrazine, whereas treatment with both AMS and pyrazine decreased mEH mRNA levels by 55%. Significant mEH mRNA increases in poly(A)+ RNA fractions were confirmed by northern blot analysis. The results demonstrate that these organosulfur compounds are inducers of mEH and that the induction involves increases in its mRNA.