A Benzo[a]pyrene-induced Cell Cycle Checkpoint Resulting in p53-independent G1 Arrest in 3T3 Fibroblasts

Abstract

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.