Detection of in vivo DNA repair synthesis in mouse liver and lung induced by treatment with benzo(a)pyrene or 4-nitroquinoline 1-oxide.

We examined in vivo DNA repair synthesis in liver and lung of A/HeJ mice treated with benzo(a)pyrene (BP) or 4-nitroquinoline 1-oxide. To differentiate between the removal of carcinogen metabolite:DNA adducts due to cell turnover and DNA repair, we measured unscheduled DNA synthesis (UDS) in the nonreplicating DNA fraction. Mice were exposed to bromodeoxyuridine pellets 1 hr prior to carcinogen treatment. Immediately following carcinogen exposure, mice received 4 hourly i.v. doses of [3H]thymidine. Mice were sacrificed 5 hr post-carcinogen treatment, and DNA was isolated. Purified DNA was then separated into newly replicated and nonreplicated DNA by ultracentrifugation in alkaline CsCl gradients. BP induced UDS in the liver at p.o. doses of 0.3 and 3.0 mg/mouse, whereas we failed to detect UDS in the lung. However, 4-nitroquinoline 1-oxide, another lung carcinogen, induced a definite repair response in the lung but not in the liver. It is not clear why mouse lung cells have the capacity to repair 4-nitroquinoline 1-oxide-induced damage to DNA and not the damage induced by BP, since both of these lung carcinogens form bulky adducts with DNA. These results demonstrate that (a) the in vivo disappearance of BP metabolite:DNA adducts from the lung of the A/HeJ mouse is due to cell turnover, whereas the disappearance of adducts from the liver is due, in part, to DNA repair and (b) induction of in vivo UDS after treatment with two different lung carcinogens is both tissue and carcinogen dependent in this mouse strain.