Molecular mechanisms in chemically induced cancer.

bout one third of the agents established as carcinogenic to man have affinities for lung. These include defined compounds and the complex mixtures found in tars, oils, soot, car exhausts, and tobacco smoke.’ To understand mechanisms of initiation of these cancers, detailed studies of the biologic actions of specific classes of chemicals have been made. These have implicated DNA as a critical target molecule and highlighted the role of metabolism in the generation of the chemically reactive form of the carcinogen which can bind to macromolecules. Benzo(a)pyrene, for example, which occurs in engine exhausts, cigarette smoke, and generally as a product of incomplete combustion, has been used as a classic polycyc!ic aromatic hydrocarbon to study metabolism and binding to DNA in cells from individuals from defined family groups. These data indicate that such parameters are genetically controlled, thereby implicating an inherited predisposition as a potential factor in the development of lung cancer.3 Benzo(a)pyrene is, however, one of over 100 chemicals identified in cigarette smoke. More than 40 are carcinogenic in animals, and several are N-nitroso compounds, a highly potent group of carcinogens producing tumors at diverse sites in animals.’ Exposure to these agents can also arise from foodstuffs, beverages, industrial atmospheres, and from endogenous sources due to the N-nitrosation of ingested precursors.4 Many of the N-nitroso compounds require metabolism to form chemically reactive intermediates and those which transfer small alkyl (eg, methyl and ethyl) groups to DNA show affinities for respiratory tissues. The methylating agent 4-(methylnitrosamino)1-(3-pyridyl)-1butanone, for example, is one of the tobacco-specific nitrosaniines and the most potent lung carcinogen known in laboratory animals. Mechanisms of carcinogenesis induced by alkylating agents have been reviewed extensively.” Factors that determine the organ specificity of tumor induction include dose, route of administration, metabolism, DNA alkylation, and the capacity for the repair of specific DNA lesions. Of the 12 lesions produced in DNA by these agents, only 2 are highly promutagenic (ie, O’-alkylthymine and O”-alkylguanine). They are implicated in mechanisms of cancer initiation, since during DNA synthesis, due either to normal cell replacement or arising from carcinogen induced tissue toxicity, replication over these modified bases gives rise to CC-’AT or AT-’CC transitions, thereby conferring a heritable change on the cell. The presence of DNA adducts during cell division may also lead to clastogenic events, vvhich could place a gene under the influence of different controlling elements, thereby leading to aberrant expression. These random, mutational changes may affect any gene, but of particular relevance for carcinogenesis are the so-called oncogenes, which are capable of diverting cells toward the malignant phenotype if they are affected by changes of the kind described above.” Protection against the biologic affects of environmental alkylating agents is afforded by specific DNA repair mechanisms capable of restoring the integrity of the genetic message and is best illustrated by reference to the well characterized Escherichia coil repair protein encoded by the ada gene. This 37 kDa methyltransferase (MT) removes the methyl group from the O”-position of guanine or the O position of thymine in DNA and binds it covalently to a cysteine residue within the protein itself. A second cysteine residue within the same protein removes the alkyl group from the phosphate moiety of the sugar-phosphate DNA backbone. This autoinactivating process is expensive to the cell’s economy as synthesis of new repair protein is required.” Its value to the cell has been demonstrated by transfection of the cloned ada gene into repair-deficient Chinese hamster ovary cells in culture. Such studies have shown that this prokaryotic gene can function in a mammalian environment and that it can effectively protect against the toxic, mutagenic and SCE-induced effects of treatment with MNU . These observations confirm the hazardous nature of specific DNA alkylation products, particularly of O”-methylguanine (0”MeG), and similar studies with transgenic animals should eventually demonstrate their role in carcinogenesis. The nuclei and mitochondria of mammalian cells possess a smaller (24 kDa) MT protein capable only of repairing O6 MeG.” Repair of O -methylthymine is via a separate protein, and methylphosphotriesters are repaired slowly, if at all, in mammalian cells. The repair of 06-MeG from individual nuclei can be followed in situ by immunohistochemical procedures.b0 In rats treated with the classic methylating agent N-nitrosodimethylamine (NDMA; 40 mg/kg), 0”MeG-positive nuclei are distributed sporadically throughout the lung tissue. In some bronchioles, for example, the nuclei of the lining cells are intensely positive, but 12 days later the O”-MeC has been removed from most of the cells. Some, however, still show highly positive nuclei and are clearly repair-deficient. Lung tissue evidently contains cells which are both metabolically proficient and repair deficient, characteristics which are genetically determined, represent a double risk factor and could be affected by genetic predisposition. We are now attempting to relate these high-dose animal studies to the human situation by using low-doses of NDMA (1#{128}, 1-100 g/kg). Although the 06-MeC introduced into hepatic DNA by such doses is rapidly repaired, the efficiency of repair is markedly increased in rats previously exposed to the inducing agent phenobarbital.” Thus, at environmentally relevant doses O”-MeC does not escape the repair process, it is not sequestered in DNA, and prospects for increasing repair efficiency using membrane active agents are now indicated. More sensitive, immunohistochemical assays of O”-MeG for use with human tissue (eg, smoker’s lung) are also being developed. Sensitive radioimmunoassay techniques already permit direct measurements of the exposure of human tissues to environmental alkylating agents.’2’3 The tissue DNA of patients undergoing surgery for cancers of the esophagus in northern China’2 and Southeast Asia’3 or CI problems in Manchester’s contain O”-MeG in the range 0.01->0.2