Molecular Pharmacology, Vol 11, 94-104, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Aryl Hydrocarbon Hydroxylase Induction in Mammalian Liver-Derived Cell Cultures

Stimulation of "Cytochrome P₁450-Associated" Enzyme Activity by Many Inducing Compounds

¹ Section on Developmental Pharmacology, Neonatal and Pediatric Medicine Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20014

Increases in aryl hydrocarbon (benzo[a]pyrene) hydroxylase activity are found in fetal rat liver primary cultures and in cell lines derived from rat Reuber Hepatoma H-35 or mouse Hepatoma BW 7756, when the cells are treated with any one of a large number of hydrophobic compounds in the growth medium. Compounds which are effective inducers include 3-methylcholanthrene, sodium phenobarbital, isoproterenol, -naphthoflavone, -naphthoflavone, 2,5-diphenyloxazole, 2-(4'-chlorophenyl)benzothiazole, 2-(4'-formylphenyl)benzothiazole, metyrapone, 1-(2-isopropylphenyl)imidazole, 2-diethylaminoethyl-2,2-diphenyl valerate HCl (SKF 525-A), piperonyl butoxide, diethylstilbestrol, sodium laurate, allylisopropylacetamide, aniline, and aminopyrine. Regardless of which inducer is used, the hydroxylase activity found in liver- or hepatoma-derived cell cultures seems to be (a) particularly sensitive to such inhibitors in vitro as -naphthoflavone or 2,5-diphenyloxazole, (b) less inhibited in vitro by such drugs as metyrapone or SKF 525-A, and (c) associated with a blue spectral shift in the Soret peak of the reduced hemoprotein-CO complex. The hydroxylase activity in control cultures and the hydroxylase activity induced by phenobarbital in liver- or hepatoma-derived cell cultures thus differ from the hepatic enzyme activities in the control and phenobarbital-treated intact animal; our findings presumably indicate that cytochrome P₁450 is induced by some factor in the control medium or by phenobarbital in culture, whereas other P450 species predominate in the control or phenobarbital-treated adult intact animal. Certain compounds are known to be metabolized differently by P₁450 than by P450. For studies with liver- or hepatoma-derived cultures and concerning evaluation of cytotoxicity, drug metabolism, or chemical carcinogenesis by certain compounds known to be metabolized via the monooxygenase metabolic pathways, we therefore suggest caution in the extrapolation of certain results found in cultured cells to the actual situation existing in the intact animal.

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ACKNOWLEDGMENT We thank Dr. Jacques E. Gielen for important contributions to this work.

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