The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological and toxic effects of its xenobiotic ligands. Previous cell culture studies have shown that, in addition to controlling the xenobiotic detoxification response, AHR activation leads to G 0 -G 1 arrest, diminished capacity for DNA replication, and inhibition of cell proliferation. In fact, recent work from our own and from other laboratories suggests that AHR may function as a tumor suppressor gene that becomes silenced during the process of tumor formation. To test this hypothesis and determine whether the mouse Ahr gene acts as a tumor suppressor gene in vivo, we have examined the role of Ahr ablation in liver tumorigenesis induced by the genotoxic chemical diethylnitrosamine (DEN), a hepatic carcinogen that is not an AHR ligand. In mice given a single i.p. injection of DEN, AHR antagonized liver tumor formation and growth by regulating cell proliferation, inflammatory cytokine expression, and DNA damage, parameters which were significantly elevated in the livers of control and, more so, of DEN-exposed Ahr −/− mice. Ahr −/− hepatocytes also showed significantly higher numbers of 4N cells, increased expression of proliferative markers, and repression of tumor suppressor genes. These data support the concept that in its basal state in the absence of a xenobiotic ligand, the Ahr gene functions as a tumor suppressor gene, and that its silencing may be associated with cancer progression.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the toxic effects of its xenobiotic ligands and acts as an environmental checkpoint during the cell cycle. We expressed stably integrated, Tet-Off-regulated AHR variants in fibroblasts from AHR-null mice to further investigate the AHR role in cell cycle regulation. Ahr ؉/؉ fibroblasts proliferated significantly faster than Ahr ؊/؊ fibroblasts did, and exposure to a prototypical AHR ligand or deletion of the ligand-binding domain did not change their proliferation rates, indicating that the AHR function in cell cycle was ligand independent. Growth-promoting genes, such as cyclin and cyclin-dependent kinase genes, were significantly down-regulated in Ahr ؊/؊ cells, whereas growth-arresting genes, such as the transforming growth factor ␤1 (TGF-␤1) gene, extracellular matrix (ECM)-related genes, and cyclin-dependent kinase inhibitor genes, were up-regulated. Ahr ؊/؊ fibroblasts secreted significantly more TGF-␤1 into the culture medium than Ahr ؉/؉ fibroblasts did, and Ahr The Ah receptor (AHR) is a ligand-activated transcription factor that belongs to the basic region helix-loop-helix/PerArnt-Sim (PAS) family of proteins. Prototypical AHR ligands include many polycyclic and halogenated aromatic compounds, such as benzo[a]pyrene (B[a]P) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The unliganded AHR is considered to be a cytosolic protein that translocates to the nucleus upon ligand binding and dimerizes with a second basic region helixloop-helix/PAS protein, the AHR nuclear translocator (ARNT), to form a heterodimeric transcription factor (27). The AHR/ ARNT complex binds to canonical DNA recognition sequences (termed AhREs, XREs, or DREs) found in the genes coding for many phase I and phase II detoxification enzymes, such as the cytochromes P450 CYP1A1, CYP1A2, and CYP1B1, and initiates gene transcription (68).Besides its role in the regulation of drug metabolism, the AHR also has an important physiologic role in liver and vascular development, immune system function, and cell growth and differentiation (18,56). Evidence dating back more than 20 years has shown that TCDD, the prototypical AHR ligand, can inhibit cell proliferation and that this effect is AHR dependent. Confluent mouse epithelial cell cultures exhibited a diminished capacity for DNA replication in the presence of as little as 10 pM TCDD (22). TCDD also inhibited DNA synthesis in rat primary hepatocytes (30) and in rat and mouse liver following partial hepatectomy (3, 46). Inhibition of G 1 phase entry by TCDD was first reported for rat 5L hepatoma cells and was found to involve increases in the levels of the CDK2 inhibitor p27KIP1 (37). The activated Ah receptor has also been shown to form protein-protein complexes with the retinoblastoma protein RB (16,21,51) to slow down entry into S phase (51) and to silence the expression of S-phase-specific E2F target genes (42).These observations indicate that the activated AHR slows down cell proliferation, suppor...