Naphthalene DNA Adduct Formation and Tolerance in the Lung

Abstract

Naphthalene (NA) is a respiratory toxicant and possible human carcinogen. NA is a ubiquitous combustion product and significant component of jet fuel. The National Toxicology Program found that NA forms tumors in two species, in rats (nose) and mice (lung). However, it has been argued that NA does not pose a cancer risk to humans because NA is bioactivated by cytochrome P450 monooxygenase enzymes that have very high efficiency in the lung tissue of rodents but low efficiency in the lung tissue of humans. It is thought that NA carcinogenesis in rodents is related to repeated cycles of lung epithelial injury and repair, an indirect mechanism. Repeated in vivo exposure to NA leads to development of tolerance, with the emergence of cells more resistant to NA insult. We tested the hypothesis that tolerance involves reduced susceptibility to the formation of NA-DNA adducts. NA-DNA adduct formation in tolerant mice was examined in individual, metabolically-active mouse airways exposed ex vivo to 250 μΜ ¹⁴C-NA. Ex vivo dosing was used since it had been done previously and the act of creating a radioactive aerosol of a potential carcinogen posed too many safety and regulatory obstacles. Following extensive rinsing to remove unbound ¹⁴C-NA, DNA was extracted and ¹⁴C-NA-DNA adducts were quantified by AMS. The tolerant mice appeared to have slightly lower NA-DNA adduct levels than non-tolerant controls, but intra-group variations were large and the difference was statistically insignificant. It appears the tolerance may be more related to other mechanisms, such as NA-protein interactions in the airway, than DNA-adduct formation.

Keywords: DNA adducts; carcinogen; naphthalene; tolerance test.

Fig. 1.

Microdissected mouse airway.

Fig. 2.

Adduct levels of individual samples and means for NA tolerance (A) and control tolerance (B) groups. The averages are compared to each other and the undosed controls in (C). The undosed control average was −0.7±2.1 adducts/pg DNA. Each mouse cell contains 5.6 pg of nuclear DNA.