EFFECT OF ARSENITE ON INDUCTION OF CYP1A AND CYP2H IN PRIMARY CULTURES OF CHICK HEPATOCYTES

In earlier studies, treatment with sodium arsenite was shown to decrea se total hepatic CYP in rats. A concomitant increase in heme oxygenase , the rate-limiting step in heme degradation to biliverdin, was consid ered responsible for the decrease in CYP. Here we investigated the eff ect of sodium arsenite on induction of CYP2H, CYP1A, and heme oxygenas e in primary cultures of chicken embryo hepatocytes. When added simult aneously with inducer, arsenite inhibited phenobarbital-mediated incre ases in CYP2H and 3-methylcholanthrene-mediated increases in CYP1A, as measured enzymatically and immunochemically. Near maximal decreases w ere observed in these forms of CYP at a concentration of 2.5 mu M sodi um arsenite. The concentration-dependent decreases in CYP2H and CYP1A by sodium arsenite were concomitant with increases in heme oxygenase. Sodium arsenite was not toxic at concentrations as high as 10 mu M, as indicated by protein synthesis and the reduction of MTT by intact cel ls. Sodium arsenite had no effect on induction of CYP2H1 mRNA, suggest ing that the decreases in this form of CYP occurred post-transcription ally. Treatment of cells with tin mesoporphyrin (SnMeso), an inhibitor of heme oxygenase, resulted in inhibition of arsenite-induced heme ox ygenase. However, SnMeso did not alter the effect of arsenite to preve nt phenobarbital-mediated increases in CYP2H protein. SnMeso alone inh ibited phenobarbital-mediated increases in CYP2H, Inclusion of 2 or 5 mu M exogenous heme with arsenite did not prevent the arsenite-mediate d decrease in CYP2H. Combined treatment with heme and phenobarbital in duced heme oxygenase to the same extent as treatment with heme, arseni te, and phenobarbital. However, CYP2H activity was decreased only when the treatment included arsenite. These results suggest that elevated levels of heme oxygenase alone are not responsible for arsenite-mediat ed decreases in CYP2H. (C) 1998 Academic Press.