Carcinogenicity, DNA adduct formation and K-ras activation by 7H-dibenzo[c, g]carbazole in strain A/J mouse lung

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Abstract
N-Heterocyclic polynuclear aromatic hydrocarbons (NHA) are environmental pollutants formed during the combustion of organic materials. 7H-Dibenzo[c,g]carbazole (DBC) is a potent carcinogen in lung, liver and skin. We undertook these studies to determine whether tissue specificity for DBC lung carcinogenicity inthe strain A/J mouse is mirrored by formation of DBC-DNA adducts in lung tissue and whether these adducts are consistent with mutation patterns in the K-ras gene. Strain A/J mice were given a single i.p. injectionof DBC at doses of 0, 5, 10, 20 or 40 mg/kg and levels of DNA adducts in the lung were monitored by 32P-postlabeling on days 1,3,5,7,14 and 21. The remaining animals were sacrificed 8 months after DBC treatment and lung tumor multiplicity and K-ras mutation patterns in the tumors were determined. The lung tumor response to DBC was dose related, with an average of 4.7 ± 1.2 tumors/mouse at 5 mg/kg and 48.1 ± 5.5 tumors/mouse at 40 mg/kg. As many as seven DBC-DNA adducts were observed in the lung. DNA binding levels in the lung were highest at 40 mg/kg, with maximum binding at 5-7 days. At lower dose levels the maximum binding to DNA decreased and shifted to earlier time points. The DBC-DNA adduct in the lung with the highest level of binding at all dose levels was DBC-DNA adduct 3. The majority of DBC-induced mutations in the K-ras gene in the lung were A↑T(80%) transversions in the third base of codon 61, a mutation that has not been previously observed in chemically induced lung tumors in strain A/J mice.

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