A benzo[a]pyrene-induced cell cycle checkpoint resulting in p53-independent G1 arrest in 3T3 fibroblasts.
The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor of the basic helix-loop-helix family. Although physiological ligands for the AhR have not been identified, carcinogenic polycyclic aromatic hydrocarbons such as Benzo[a]pyrene (B[a]P) are high affinity AhR ligands that induce nuclear translocation and sequence-specific DNA binding of the AhR. AhR-regulated genes include members of the cytochrome P-450 family that are known to oxidize B[a]P to form genotoxic (DNA-damaging) metabolites. Murine Swiss 3T3 cells express high levels of AhR. Treatment of Swiss 3T3 cells with B[a]P during the G1 phase of the cell cycle resulted in growth arrest, as shown by inhibition of growth factor-stimulated DNA synthesis. By contrast, other murine 3T3 fibroblasts not expressing detectable levels of AhR did not undergo growth arrest in response to B[a]P. The AhR antagonist alpha-naphthoflavone prevented B[a]P-induced growth arrest, further demonstrating that cessation of cell growth was mediated by the activated AhR. A nongenotoxic AhR ligand (2,3,7,8-tetrachlorodibenzo-p-dioxin) did not elicit growth arrest, showing that ligand activation of the AhR alone was insufficient to block cell cycle progression. However, genomic DNA from B[a]P-treated Swiss 3T3 cells contained covalent adducts, whereas that from 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated cells did not, showing that G1 arrest correlated with DNA damage resulting from genotoxic B[a]P metabolites. B[a]P-induced DNA damage and growth arrest was coincident with elevated levels of nuclear p53 protein and induction of the p53-regulated mdm-2 proto-oncogene. However, Swiss 3T3 fibroblasts expressing "dominant negative" mutant p53, as well as primary fibroblasts from p53-/- "knockout" mice, also underwent growth arrest in response to B[a]P. Therefore, B[a]P-induced growth arrest occurs via p53-independent mechanisms.
Vaziri
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Cancer Research Center, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Faller
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DV
D016475:3T3 Cells; D000818:Animals; D001564:Benzo(a)pyrene / Q000494:pharmacology*; D001571:Benzoflavones / Q000494:pharmacology; D002453:Cell Cycle / Q000187:drug effects*; D002455:Cell Division / Q000187:drug effects; D002467:Cell Nucleus / Q000378:metabolism; D004249:DNA Damage; D016193:G1 Phase; D006133:Growth Substances / Q000494:pharmacology*; D018257:Helix-Loop-Helix Motifs; D007700:Kinetics; D051379:Mice; D016296:Mutagenesis; D009363:Neoplasm Proteins / Q000096:biosynthesis; D009687:Nuclear Proteins; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis; D051736:Proto-Oncogene Proteins c-mdm2; D011519:Proto-Oncogenes / Q000187:drug effects; D018336:Receptors, Aryl Hydrocarbon / Q000187:drug effects / Q000502:physiology*; D011994:Recombinant Proteins / Q000096:biosynthesis; D014162:Transfection; D016159:Tumor Suppressor Protein p53 / Q000096:biosynthesis / Q000378:metabolism*
D001571:Benzoflavones; D006133:Growth Substances; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D000072317:Polychlorinated Dibenzodioxins; D011518:Proto-Oncogene Proteins; D018336:Receptors, Aryl Hydrocarbon; D011994:Recombinant Proteins; D016159:Tumor Suppressor Protein p53; D001564:Benzo(a)pyrene; C011512:alpha-naphthoflavone; C497207:Mdm2 protein, mouse; D051736:Proto-Oncogene Proteins c-mdm2