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In earlier studies, sodium arsenite treatment was shown to decrease induction of enzymatic activities associated with hepatic CYPs in rats. Here we investigated the effect of sodium arsenite on induction of CYP2B, CYP1A, and CYP3A in primary cultures of rat hepatocytes. Arsenite decreased the induction of all three families of CYP, as measured enzymatically and immunochemically. These decreases in CYPs occurred at concentrations of arsenite (2.5-10 microM) at which no toxicity was observed; however, toxicity was observed at 25 microM arsenite. With 3-methylcholanthrene as inducer, 5 microM 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 inducer, 2.5 microM arsenite decreased CYP2B enzyme activity and immunoreactive protein 50%, with only a 25% decrease in CYP2B1 mRNA. 5 microM Arsenite decreased CYP2B enzyme activity and immunoreactive protein 80%, but decreased CYP2B1 mRNA only 50%, while CYP3A protein was decreased greater than 75% with no decrease in CYP3A23 mRNA. With dexamethasone (DEX) as inducer, 5 microM sodium arsenite caused a 50% decrease in immunoreactive CYP3A and a 30% decrease in CYP3A23 mRNA. Although arsenite-mediated increases in heme oxygenase (HO) inversely correlated with decreases in CYP2B or CYP1A activity, inclusion of heme in cultures treated with inducers of CYP1A or CYP2B did not prevent the arsenite-mediated decreases in these CYPs. Even though added heme induced HO to similar 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 decreases in CYP.

We had reported previously that 2.5–5 μM sodium arsenite decreased the phenobarbital-mediated induction of CYP2H activity and protein but not CYP2H1 mRNA in chick-embryo hepatocyte cultures. Induction of a CYP1A activity and protein by 3-methylcholanthrene was also decreased by low arsenite concentrations; however, CYP1A mRNAs were not measured in those studies. We report here that low concentrations of arsenite decreased induction of activities and mRNAs of two chicken cytochromes P450, CYP1A (1A4 and 1A5), by 3-methylcholanthrene in chick-embryo hepatocyte cultures. Arsenite treatment did not affect the turnover of either mRNA, nor did it decrease the superinduction of each mRNA caused by treatment with cycloheximide in addition to 3-methylcholanthrene. Glutathione depletion enhanced the effect of arsenite to decrease induction of CYP1A4. These results indicate the induction of CYP1A4 and 1A5 is inhibited by sodium arsenite at the level of transcription, suggesting that the Ah receptor complex may be involved.