Exposure of murine hepatoma (Hepa lclc7) cells to a variety of chemical agents known to protect animals against the neoplastic, mutagenic, and other toxic effects of chemical carcinogens results in dose-and time-dependent inductions of NAD(P)H:quinone reductase (EC 1.6.99.2). This enzyme protects against quinone toxicity by promoting obligatory two-electron reductions that divert quinones from oxidative cycling or direct interactions with critical nucleophiles. Quinone reductase levels are stable in culture, are easily measured, and are useful markers for the inductive effects of chemoprotective agents. The Hepa lclc7 system responds to chemoprotective compounds such as phenolic antioxidants {e.g., BHA [3(2)-tert-butyl-4-hydroxyanisole], BHT (3,5-ditert-butyl-4-hydroxytoluene), and tert-butylhydroquinone}, lipophilic azo dyes belonging to the 1,1'-azonaphthalene, Sudan I (1-phenylazo-2-naphthol), and Sudan m [1-(4-phenylazophenylazo)-2-naphtholl families, polycyclic aromatic hydrocarbons, coumarin and various other lactones, flavonoids, and certain sulfur compounds (e.g., benzylisothiocyanate, dithiolthiones, and dithiocarbamates), all of which are recognized enzyme inducers and chemoprotectors in vivo. Quinone reductase induction in Hepa lclc7 cells therefore provides a simple, versatile, and reliable system for the evaluation of the potency, kinetics, and mechanism of action of anticarcinogens.An astonishing variety of structurally unrelated compounds, including phenolic antioxidants, coumarins, azo dyes, flavonoids, polycycic aromatics, and certain sulfur compounds, protect laboratory animals against the neoplastic, mutagenic, and other toxic effects of chemical carcinogens (1, 2). Clarification of the molecular mechanisms underlying these protective actions is of importance in devising strategies for protecting man against cancer. Much evidence indicates that protection by these agents depends on altering the metabolism of carcinogens (3, 4). Some protectors (e.g., polycyclic hydrocarbons, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and azo dyes) are potent inducers of both phase I and phase II drug-metabolizing enzymest, whereas other protective agents (e.g., phenolic antioxidants) induce phase II enzymes only (for review, see ref.3). Although the balance between these two types of enzymes required for protective action remains unclear, current evidence indicates that induction of such phase II enzymes as glutathione Stransferases, quinone reductase, and epoxide hydrolase is important for protection (1)(2)(3)(4)(6)(7)(8).We have developed a murine hepatic cell culture system (Hepa lclc7) that mimics many animal tissues in responding to a wide variety of chemoprotective agents by induction of cytosolic quinone reductase [NAD(P)H:(quinone-acceptor) oxidoreductase, EC 1.6.99.2; also known as menadione reductase, DT diaphorase, or vitamin K reductase]. These cells are a suitable model for studying chemoprotection because (i) they retain many characteristics ofnormal tissues, particularly the capacity for carci...