Depurinating naphthalene-DNA adducts in mouse skin related to cancer initiation

Abstract

Naphthalene has been shown to be a weak carcinogen in rats. To investigate its mechanism of metabolic activation and cancer initiation, mice were topically treated with naphthalene or one of its metabolites, 1-naphthol, 1,2-dihydrodiolnaphthalene (1,2-DDN), 1,2-dihydroxynaphthalene (1,2-DHN), and 1,2-naphthoquinone (1,2-NQ). After 4 h, the mice were sacrificed, the treated skin was excised, and the depurinating and stable DNA adducts were analyzed. The depurinating adducts were identified and quantified by ultraperformance liquid chromatography/tandem mass spectrometry, whereas the stable adducts were quantified by (32)P-postlabeling. For comparison, the stable adducts formed when a mixture of the four deoxyribonucleoside monophosphates was treated with 1,2-NQ or enzyme-activated naphthalene were also analyzed. The depurinating adducts 1,2-DHN-1-N3Ade and 1,2-DHN-1-N7Gua arise from reaction of 1,2-NQ with DNA. Similarly, the major stable adducts appear to derive from the 1,2-NQ. The depurinating DNA adducts are, in general, the most abundant. Therefore, naphthalene undergoes metabolic activation to the electrophilic ortho-quinone, 1,2-NQ, which reacts with DNA to form depurinating adducts. This is the same mechanism as other weak carcinogens, such as the natural and synthetic estrogens, and benzene.

Fig. 1

Pathways of naphthalene metabolism.

Fig. 2

Formation of depurinating adducts in mouse skin treated with naphthalene or one of its metabolites. The values are the average of two determinations that differed by <20%.

Fig. 3

PAGE analysis of ³²P-postlabeled stable DNA adducts obtained from naphthalene and its metabolites in vitro and in vivo. For the in vivo study, naphthalene (1200 or 500 nmol) or its metabolite (500 nmol) was delivered in 50 μl acetone to mouse skin.

Fig. 4

Quantitative comparison of depurinating and stable adducts formed in mouse skin treated with naphthalene (500 or 1200 nmol) or one of its metabolites (500 nmol). The values are an average of two determinations that differed by <20%.

Fig. 5

Comparable mechanisms of cancer initiation by natural estrogens and naphthalene.