Quantification of DNA adducts formed in liver, lungs, and isolated lung cells of rats and mice exposed to 14C-styrene by nose-only inhalation
Bronchiolo-alveolar tumors were observed in mice exposed chronically to 160 ppm styrene, whereas no tumors were seen in rats up to concentrations of 1000 ppm. Clara cells, which are predominant in the bronchiolo-alveolar region in mouse lungs but less numerous in rat and human lung, contain various cytochrome P450s, which may oxidize styrene to the rodent carcinogen styrene-7,8-oxide (SO) and other reactive metabolites. Reactive metabolites may form specific DNA adducts and induce the tumors observed in mice. To determine DNA adducts in specific tissues and cell types, rats and mice were exposed to 160 ppm [ring-U-14C]styrene by nose-only inhalation for 6 h in a recirculating exposure system. Liver and lungs were isolated 0 and 42 h after exposure. Fractions enriched in Type II cells and Clara cells were isolated from rat and mouse lung, respectively. DNA adduct profiles differed quantitatively and qualitatively in liver, total lung, and enriched lung cell fractions. At 0 and 42 h after exposure, the two isomeric N7-guanine adducts of SO (measured together, HPEG) were present in liver at 3.0 ± 0.2 and 1.9 ± 0.3 (rat) and 1.2 ± 0.2 and 3.2 ± 0.5 (mouse) per 108 bases. Several other, unidentified adducts were present at two to three times higher concentrations in mouse, but not in rat liver. In both rat and mouse lung, HPEG was the major adduct at 1 per 108 bases at 0 h, and these levels halved at 42 h. In both rat Type II and non-Type II cells, HPEG was the major adduct and was about three times higher in Type II cells than in total lung. For mice, DNA adduct levels in Clara cells and non-Clara cells were similar to total lung. The hepatic covalent binding index (CBI) at 0 and 42 h was 0.19 ± 0.06 and 0.14 ± 0.03 (rat) and 0.25 ± 0.11 and 0.44 ± 0.23 (mouse), respectively. The pulmonary CBIs, based on tissues combined for 0 and 42 h, were 0.17 ± 0.04 (rat) and 0.24 ± 0.04 (mouse). Compared with CBIs for other genotoxicants, these values indicate that styrene has only very weak adduct-forming potency. The overall results of this study indicate that DNA adduct formation does not play an important role in styrene tumorigenicity in chronically exposed mice.