Activation of K-ras in aflatoxin B1-induced lung tumors from AC3F1 (A/J x C3H/HeJ) mice

Abstract

In addition to being a potent hepatocarcinogen, aflatoxin B1 (AFB1) is a pulmonary carcinogen in experimental animals and epidemiological studies have shown an association between AFB1 exposure and lung cancer in humans. Since point mutations at codons 12, 13 and 61 of the K-ras protooncogene are often implicated in chemically induced mouse lung tumors and in human lung adenocarcinomas, we undertook an investigation of the role of K-ras activation in AFB1-induced pulmonary carcinogenesis. Female AC3F1 (A/J x C3H/HeJ) mice were treated with AFB1 (150 mg/kg i.p., divided into 24 doses over 8 weeks), and 6-14 months after the completion of dosing mice were killed and pulmonary adenomas and carcinomas removed. Of the 76 AFB1-induced lung tumors analyzed by single strand conformation polymorphism (SSCP) and direct sequencing, 75 possessed K-ras codon 12 mutations (46 GTT, 14 GAT, 13 TGT and 2 TTT; normal, GGT) and one had a GGC-->CGC mutation in codon 13. The observation that K-ras mutations occurred only at G:C base pairs is in agreement with N7-guanine being the primary site of AFB1-DNA adduct formation and with guanine residues being targets for AFB1-induced oxidative DNA damage via formation of 8-hydroxydeoxyguanosine (8-OHdG). The AFB1-specific nature of the observed K-ras mutation spectrum and the fact that 100% of the tumor samples examined contained K-ras mutations is consistent with K-ras activation being an early, critical event in AFB1-induced pulmonary carcinogenesis in AC3F1 mice. The parental origin of the observed K-ras mutations was determined by allele-specific PCR amplification of AFB1-induced lung tumor DNA followed by SSCP analysis. In the vast majority of tumors (73/76), the mutated K-ras allele was derived from the lung tumor susceptible A/J parent. This finding supports the existence of a link between K-ras and differences in mouse lung tumor susceptibility.