Induction of epoxide hydrolase in cultured rat hepatocytes and hepatoma cell lines

Abstract

The expression of epoxide hydrolases was studied in cultured rat hepatocytes and hepatoma cell lines. Styrene 7,8-oxide and benzo[a]pyrene 4,5-oxide were used as substrates for microsomal epoxide hydrolase and trans-stilbene oxide for the cytosolic form of this enzyme. In freshly isolated hepatocytes from control rats, microsomal epoxide hydrolase activity was 7.7 and 10.8 nmoles/mg cellular protein/min with benzo[a]pyrene 4,5-oxide and styrene 7,8-oxide as substrates respectively. This enzyme activity increased by more than 2-fold in hepatocytes after 24 hr in culture and remained elevated throughout 96 hr using both substrates. In cultured hepatocytes from rats pretreated in vivo with phenobarbital, trans-stilbene oxide, 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene, both benzo[a]pyrene 4,5-oxide and styrene 7,8-oxide hydrolase activities were increased greater than 1.8 relative to controls. Hepatocytes from 2-acetylaminofluorene-pretreated animals at 24 hr in culture had approximately 9-fold higher activities than control hepatocytes. In marked contrast to microsomal epoxide hydrolase activity, the cytosolic enzyme showed an initial activity of 191 pmoles/mg cellular protein/min in freshly isolated hepatocytes, decreased by 75% after 24 hr in culture, and was barely detectable at 96 hr. A similar trend was apparent in hepatocytes from the pretreated animals. In vitro treatment of hepatocytes with trans-stilbene oxide and phenobarbital increased microsomal epoxide hydrolase, while this activity was refractory to 2-acetylaminofluorene treatment. Styrene 7,8-oxide hydrolase activity was increased in the McA-RH-7777 rat hepatoma cell line by phenobarbital, trans-stilbene oxide and 2-acetylaminofluorene treatment. Similarly, benzo[a]pyrene 4,5-oxide hydrolase activity was also increased in this cell line by treatment with phenobarbital and trans-stilbene oxide but not by 2-acetylaminofluorene. Microsomal epoxide hydrolase activity in rat H4-II-E hepatoma cells was refractory to induction, except by trans-stilbene oxide treatment, which caused a 70% increase in benzo[a]pyrene 4,5-oxide hydrolase activity.