Hepatocellular carcinoma (HCC) is known to progress through a step often called tumor promotion. Phenobarbital (PB) is the prototype of nongenotoxic cacinogens that promote HCC in rodents. The molecular target of PB to elicit the promotion has been the subject of intense investigations over the last 30 years since it was discovered. The nuclear receptor constitutive active/androstane receptor (CAR) is activated by PB as well as by various other xenobiotics such as therapeutic drugs and environmental pollutants. CAR activation results in the transcriptional induction of numerous hepatic genes including those that encode xenobiotic-metabolizing enzymes such as a set of cytochrome P450s. In addition to PB, many CAR activators are nongenotoxic carcinogens, but the role of CAR in liver tumor promotion remains unexplored. Using Car ؊/؊ mice, we have here examined tumor promotion by chronic treatment with PB in drinking water after tumor initiation with a single dose of the genotoxic carcinogen diethylnitrosamine. None of the Car ؊/؊ mice developed either eosinophilic foci or advanced liver tumors, whereas all Car ؉/؉ mice developed HCC and/or adenoma by 39 weeks. The results indicate that CAR is the molecular target of promotion by PB and that activation of this receptor is an essential requirement for liver tumor development.

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