Biochemical Properties of Carcinogen-Metabolizing Enzymes in Cultured Hepatoma Cells

Abstract

We have previously demonstrated the inducibility of both cytochrome P-448- and P-450-dependent mono-oxygenases in the differentiated rat hepatoma cell line MH₁C₁. Further experiments with these cells on the expression of different forms of cytochrome P-450, inducible not only by phenobarbital (PB) and 3-methylcholanthrene (MC), but also by metyrapone (MP), ethanol (E), and beta-naphthoflavone (BNF) are reported here. The effects of the in vitro addition of the inhibitors alpha-naphthoflavone and beta-naphthoflavone on the aryl hydroxylase activity (AHH) and the influence of protein synthesis on the induction of cytochrome P-450 were also assessed. Cultures were exposed to the inducers PB, MC, BNF, and MP during the last 6 days of culture and to E for 10 days. The inhibition of protein synthesis was obtained by adding cycloheximide (CY) to the cultured cells during the last 24 hr. The exposure of MH₁C₁ cells to various concentrations of MP resulted in a dose-dependent increase in AHH activity. The treatment of MH₁C₁ cells with different concentrations of ethanol produced a significant dose-dependent increase of monooxygenases. AHH activity, induced by the various treatments, was inhibited in a dose-dependent way by alpha-naphthoflavone and beta-naphthoflavone. CY reduced the concentration of cytochrome P-450 and the AHH activity induced by the various treatments, thus indicating an implication of the protein synthesis in the mechanism(s) of induction.

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