DNA adduct formation and repair in hamster and rat tracheas exposed to benzo[a]pyrene in organ culture.

Abstract

Syrian golden hamsters are much more susceptible than Wistar rats to the induction of tracheal tumors by benzo[a]pyrene (B[a]P). To investigate whether this difference is reflected in the pattern of DNA adduct induction and removal, tracheas from either species were isolated and exposed to B[a]P (5 micrograms/ml) in organ culture. At various time-points B[a]P-DNA adducts were quantified by 32P-postlabeling; unscheduled DNA synthesis (UDS) and cell proliferation were determined by [3H]thymidine incorporation during the 18 h before sampling. In an induction-repair experiment tracheas were exposed to B[a]P for 2 days, and cultured for another 4 days without B[a]P. After 2 days of exposure total B[a]P-DNA adduct levels were 10 times higher in hamster compared to rat tracheas. In hamster tracheas one major adduct was formed (95%), namely the adduct between (+)-anti-benzo[a]pyrene diolepoxide and deoxyguanosine (BPDE-N2dG). In rat tracheas BPDE-N2dG comprised approximately 60% of the total B[a]P-DNA adduct level. The other major adduct found in rat tracheas is probably derived from interaction of syn-BPDE and deoxyadenosine. During exposure to B[a]P in hamsters the adduct level increased to 36 +/- 19 adducts/10(6) nucleotides (add/10(6)n) on day 2. Two days after removal of B[a]P the B[a]P-DNA adduct level had decreased to 60% of that on day 2; there was no further decrease in the B[a]P-DNA adduct level, despite considerable cell proliferation at the end of the 6 day culture period. UDS increased during exposure to B[a]P and decreased after removal of B[a]P. In rats removal of B[a]P did not lead to a decrease in the B[a]P-DNA adduct level, which agreed with the observed absence of UDS. In a second experiment tracheas were exposed to B[a]P continuously for 15 days. In hamster tracheas the total B[a]P-DNA adduct level increased from 11 +/- 0.7 add/10(6)n after 1 day of exposure to 105 +/- 2 add/10(6)n after 15 days; also UDS increased with increasing exposure until day 11. Cell proliferation was low at the end of the culture period. In rat tracheas no progressive increase in the B[a]P-DNA adduct level was seen, UDS was not increased and cell proliferation had increased significantly at the end of the exposure period. The extent of adduct induction in the trachea of the two species corresponded with the different susceptibilities to B[a]P-induced tumor formation.