Multiple genetic changes are observed in lung tumors and other tumors in smokers. Among these, changes in K-ras and p53 have been the most extensively documented. How do these genetic changes arise? They occur when covalent binding products of carcinogens to DNA, called DNA adducts, evade cellular DNA repair mechanisms, and cause miscoding. The 1.1 billion smokers in the world are chronically exposed to a complex mixture of carcinogens. For at least 60 of these carcinogens, there is sufficient evidence for carcinogenicity in either laboratory animals or humans, according to evaluations by the International Agency for Research on Cancer. These carcinogens include polycyclic aromatic hydrocarbons (PAH), volatile nitrosamines, tobacco-specific nitrosamines such as NNK and NNN, aromatic amines and heterocyclic aromatic amines, aldehydes, ethylene oxide, butadiene, benzene, miscellaneous volatile organics, metals, and others. Most tobacco smoke carcinogens require a metabolic activation process, generally catalyzed by cytochrome P450 enzymes, to be converted to metabolites that are electrophilic and react with DNA. These reactions produce the DNA adducts that cause miscoding and are critical in the carcinogenic process. The paramount importance of DNA adducts in carcinogenesis has been firmly established in the past fifty years. Evidence for DNA adduct formation in the lungs of smokers is strong, based on studies using a variety of techniques. Many investigations have demonstrated higher adduct levels in lung and other respiratory tissues of smokers than in non-smokers using the relatively non-specific 32P-postlabelling and immunoassay techniques. Characterization of specific DNA adducts in the lungs of smokers has been accomplished less frequently. There is convincing evidence for the presence of adducts resulting from reaction of DNA with benzo[a]pyrene-7,8-diol-9,10-epoxide, ethylene oxide, methylating and pyridyloxobutylating agents derived from volatile and tobacco-specific nitrosamines, N-hydroxyaminobiphenyl, and oxygen radicals. This presentation will focus on the pathway leading to carcinogen DNA adduct formation in smokers, with emphasis on two important classes of tobacco smoke carcinogens: PAH and tobacco-specific nitrosamines. The uptake of these compounds in smokers has been conclusively demonstrated by measurement of specific PAH metabolites such as 1-hydroxypyrene and phenanthrene tetraol, and the NNK metabolite NNAL in urine or blood. The metabolic activation of PAH and NNK, catalyzed by P450 enzymes, has also been firmly established in studies using human tissues. Analysis of DNA from smokers’ lungs by HPLC-fluorescence or mass spectrometry convincingly demonstrates the presence of alkylation products of benzo[a]pyrene-7,8-diol-9,10-epoxide and metabolically activated tobacco-specific nitrosamines, although it should be noted that these adducts are not detected in the pulmonary DNA of all smokers. Additional data indicate that these carcinogen DNA adducts cause specific mutations in K-ras and p53. Collectively, the evidence that tobacco smoke carcinogens and their metabolically activated forms, through the production of DNA adducts, cause the multiple genetic changes observed in tumors from smokers lungs is extremely strong, although there is still considerable uncertainty regarding the relationship between particular carcinogen DNA adducts and specific mutations in human tumors.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]