A constitutively active dioxin/aryl hydrocarbon receptor promotes hepatocarcinogenesis in mice

The dioxin/aryl hydrocarbon receptor (AhR) functions as a ligand-activated transcription factor regulating transcription of a battery of genes encoding enzymes involved in drug metabolism. Known ligands include polycyclic aromatic hydrocarbons, certain polychlorinated biphenyls, and the polyhalogenated dioxins including 2,3,7,8-tetrachlorodibenzo-p-dioxin. Both polyhalogenated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin are potent promoters of rodent hepatocarcinogenesis in two-stage initiation-promotion experiments. Although several lines of evidence indicate the involvement of the AhR in toxic effects mediated by polyhalogenated biphenyls and dioxins, its involvement in tumor promotion has not been unequivocally proven. In the present study, a transgenic mouse line expressing a constitutively active AhR (CA-AhR) has been used to investigate the role of the AhR in hepatocarcinogenesis. Male AhR wild-type and CA-AhR-transgenic B6C3F1-mice were treated with a single injection of the hepatocarcinogen N-nitrosodiethylamine at 6 weeks of age and were subsequently kept untreated on control diet. Thirty five weeks after carcinogen treatment, mice were sacrificed, and the prevalence and multiplicity of liver tumors were determined. Whereas only 1 small liver tumor was observed in 15 AhR-wild-type mice, 19 tumors (two >1 cm in diameter) were present in 18 CA-AhR-transgenic mice. This result demonstrates the oncogenic potential of the activated AhR and implicates an important role of the receptor in promotion of hepatocarcinogenesis. A microarray-based gene expression-profiling analysis revealed down-regulation in the liver of CA-AhR-transgenic mice of a cluster of genes encoding heat shock proteins, including GRP78/BiP, Herp1, Hsp90, DnaJ (Hsp40) homologue B1, and Hsp105, which are important for protein folding and quality control.