THE TIME-DEPENDENT INCREASE IN THE BINDING OF BENZO[A]PYRENE TO DNA THROUGH (+)-ANTI-BENZO[A]PYRENE-7,8-DIOL-9,10-EPOXIDE IN PRIMARY RAT HEPATOCYTE CULTURES RESULTS FROM INDUCTION OF CYTOCHROME P450IA1 BY BENZO[A]PYRENE TREATMENT

The proportion and amount of benzo[a]pyrene (B[a]P) that binds to DNA through the carcinogenic (+)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide [(+)-anti-BPDE] increases with time of exposure to B[a]P in cell cultures derived from a number of species. Pretreatment of primary rat hepatocyte cultures for 12 h with 1-mu-g B[a]P/ml medium increased the subsequent metabolism of [H-3]B[a]P by 47% and [H-3]B[a]P-DNA binding by 53% compared with acetone-pretreated hepatocytes. The amount of (+)-anti-BPDE bound to DNA in the B[a]P-pretreated hepatocytes increased 175%. B[a]P pretreatment also increased DNA-binding 2-fold in hepatocytes treated with [H-3]7,8-dihydroxy-7,8-dihydro-B[a]P but had no effect on DNA binding in cells treated with anti-B[a]P-7,8-diol-9,10-epoxide. Western blotting showed that cytochrome P450IA1, which was not detectable prior to B[a]P treatment, was selectively increased by B[a]P treatment. A monoclonal antibody that specifically inhibits cytochrome P450IA1 reduced the binding of B[a]P to DNA by > 90% in microsomal preparations from B[a]P-pretreated hepatocytes. These results indicate that the time-dependent increase in the formation of (+)-anti-BPDE-DNA adducts results from an increase in the amount and proportion of B[a]P metabolized to this ultimate carcinogen by P450IA1 that is induced by the B[a]P treatment. The importance of P450IA1 induction by the B[a]P for its activation to this ultimate carcinogenic metabolite suggests that long-term exposure of cells to B[a]P could result in activation of a higher proportion of the B[a]P to the carcinogenic (+)-anti-BPDE.