Disruption of cell cycle kinetics by benzo[a]pyrene: Inverse expression patterns of BRCA-1 and p53 in MCF-7 cells arrested in S and G(2)

The effects of a ligand of the aromatic hydrocarbon receptor (AhR),benzo[a] pyrene (B[a]P),and its metabolite, BPDE (7r,8t-dihydroxy-9t,10t-epoxy-7,8, 9,10-tetrahydro-benzo [a]pyrene), on BRCA-1 levels and cell cycle kinetics were determined in MCF-7 breast cancer cells. Exposure of asynchronous MCF- 7 cells for 72 hours to a non-cytotoxic dose of 0.5 muM B[a]P triggered a t hree-fold reduction in BRCA-1 protein. In MCF-7 cells resistant (20% to 30% ) to genotoxic concentrations of B[a]P (1 to 5 muM), the loss of BRCA-1 pro tein was coupled with pausing in S-phase and G(2)/M, and accumulation of p5 3, mdm2 and p21, Treatment of MCF-7 cells synchronized in S-phase (72%) wit h B[a]P prolonged the arrest in S-phase, although this checkpoint was trans ient since cells resumed to G(2)/M after 12 hours with reduced levels of BR CA-1, In these cells, levels of p53 were increased, whereas the cellular co ntent of p21 remained unaltered. In contrast, the co-treatment with the AhR antagonist, cu-naphthoflavone (ANF), abrogated the deleterious effects of B[a]P on BRCA-1 expression, while preventing the accumulation of p53 and di sruption of cell cycle profile. These findings suggest that the AhR mediate d the inverse expression patterns of BRCA-1 and p53 upon exposure to B[a]P. The treatment with BPDE induced S-phase arrest and reduced BRCA-1 mRNA lev els. The negative effects of BPDE on BRCA-1 expression were not transient s ince removal of BPDE did not allow complete reversal of the repression. The se cumulative data suggest that the B[a]P metabolite, BPDE, may play a key role in disruption of BRCA-1 expression and cell cycle kinetics in breast e pithelial cells.