Effect of arsenite on induction of CYP1A, CYP2B, and CYP3A in primary cultures of rat hepatocytes

In earlier studies, sodium arsenite treatment was shown to decrease inducti on of enzymatic activities associated with hepatic CYPs in rats. Here we in vestigated the effect of sodium arsenite on induction of CYP2B, CYP1A, and CYP3A in primary cultures of rat hepatocytes. Arsenite decreased the induct ion of all three families of CYP, as measured enzymatically and immunochemi cally. These decreases in CYPs occurred at concentrations of arsenite (2.5- 10 mu M) at which no toxicity was observed; however, toxicity was observed at 25 mu M arsenite. With 3-methylcholanthrene as inducer, 5 mu M arsenite caused a 55% decrease in CYP1A1 immunoreactive protein and enzyme activity, but only a 25% decrease in CYP1A1 mRNA. With phenobarbital (PB) as the ind ucer, 2.5 mu M arsenite decreased CYP2B enzyme activity and immunoreactive protein 50%, with only a 25% decrease in CYP2B1 mRNA. 5 mu M Arsenite decre ased CYP2B enzyme activity and immunoreactive protein 80%, but decreased CY P2B1 mRNA only 50%, while CYP3A protein was decreased greater than 75% with no decrease in CYP3A23 mRNA. With dexamethasone (DEX) as inducer, 5 mu M s odium arsenite caused a 50% decrease in immunoreactive CYP3A and a 30% decr ease in CYP3A23 mRNA. Although arsenite-mediated increases in heme oxygenase (HO) inversely corre lated with decreases in CYP2B or CYP1A activity, inclusion of heme in cultu res treated with inducers of CYP1A or CYP2B did not prevent the arsenite-me diated decreases in these CYPs. Even though added heme induced HO to simila r levels with and without arsenite, decreases in CYPs were only observed in the presence of arsenite. These results suggest that, in rat hepatocytes, elevated levels of HO alone are not responsible for arsenite-mediated decre ases in CYP. (C) 1999 Academic Press.