Carcinogenesis; a comprehensive survey最新文献

The evolution of a fully malignant tumor is a multistep process resulting from the action of multiple factors, both environmental and endogenous, and involves alterations in the function of multiple cellular genes. Chemical carcinogens that initiate this process appear to do so by damaging cellular DNA. In addition to producing simple point mutations, this damage appears to induce the synthesis of a transacting factor that can induce asynchronous DNA replication. This response may result in gene amplification and/or gene rearrangement. This phenomenon may also play a role in synergistic interactions between chemicals and viruses in the causation of certain cancers. The primary target of the tumor promoters TPA, teleocidin, and aplysiatoxin appears to be cell membranes. All three of these agents act, at least in part by, enhancing the activity of the phospholipid-dependent enzyme PKC. We have proposed a stereochemical model to explain the interaction of these amphiphilic compounds with the PKC system. We have found that TPA and teleocidin markedly enhance the transformation of C3H10T1/2 mouse fibroblasts when these cells are transfected with the cloned H-ras human bladder cancer oncogene. Thus, tumor promoters can act synergistically with an activated oncogene to enhance cell transformation. Furthermore, carcinogen-transformed rodent cells display aberrations in the expression of various endogenous retrovirus-related sequences. Activation of some of these sequences may lead to insertion mutations and further aberrations in gene expression. These findings are discussed in terms of a multistep model that involves progressive changes in cellular oncogenes and aberrations in the function of DNA transcription enhancer sequences. It will be of interest to determine to what extent these concepts apply to the etiology of cancers of the respiratory tract.

Studies conducted in numerous laboratories have demonstrated that the transformation of C3H10T1/2 cells can proceed through discrete stages of initiation and promotion. Indeed, multiple operational aspects of initiation and promotion in this system closely mimic the essential characteristics of initiation and promotion on mouse skin. The sensitivity of this system to the effects of different tumor promoters also appears to parallel that of mouse skin, and there is evidence to suggest that the C3H10T1/2 system is most sensitive to agents acting as stage II tumor promoters on mouse skin. Sensitivity to compounds active at other tissue sites in rodents and perhaps man has also been observed. At this time it is difficult to assess the relevance of the C3H10T1/2 system for the study of agents capable of modulating respiratory carcinogenesis. The process of promotion can possess extreme tissue and species specificity and effects observed in murine fibroblasts of embryonic origin may have little practical bearing upon effects to be anticipated in the tracheal epithelium of the rat or the bronchial epithelium of man. This is not to say that the C3H10T1/2 system is irrelevant to respiratory carcinogenesis. However, due recognition must be taken of the probable natural limitations of this system for the study of promoters of respiratory carcinogenesis. As the data base for the use of this system is expanded, the relationship between promotion in C3H10T1/2 cells and the respiratory tract of man and rodents will become better defined. Until such time as this relationship is firmly established, it is perhaps best to regard the C3H10T1/2 system as an interesting model with which results obtained using respiratory tissue can be compared or contrasted.

A multistage transformation assay has been developed using normal primary rat tracheal epithelial cells as targets for measuring the transforming activity of a variety of test substances. The assay is suited for quantitation of cell transformation and allows the study of effects of promoters as well as inhibitors of transformation at various stages of this multiphasic process. Studies with TPA and retinoic acid have shown that the first stage of neoplastic transformation of RTE cells cannot be enhanced by TPA but is inhibited by retinoic acid. However, TPA can enhance a later stage of transformation, the conversion of the ag- to the ag+ phenotype. The cellular and biochemical mechanism of the inhibitory effects of RA are under investigation.

The evolution of a fully malignant tumor is a multistep process resulting from the action of multiple factors, both environmental and endogenous, and involves alterations in the function of multiple cellular genes. Chemical carcinogens that initiate this process appear to do so by damaging cellular DNA. In addition to producing simple point mutations, this damage appears to induce the synthesis of a trans acting factor that can induce asynchronous DNA replication. This response may result in gene amplification and/or gene rearrangement. This phenomenon may also play a role in synergistic interactions between chemicals and viruses in the causation of certain cancers. The primary target of the tumor promoters TPA, teleocidin, and aplysiatoxin appears to be the cell membrane. All three of these agents act, at least in part, by enhancing the activity of the phospholipid-dependent enzyme protein kinase C. We have proposed a stereochemical model to explain the interaction of these amphiphilic compounds with the PKC system. We have found that TPA and teleocidin markedly enhance the transformation of C3H 10T1/2 mouse fibroblasts when these cells are transfected with the cloned H-ras human bladder cancer oncogene. Thus, tumor promoters can act synergistically with an activated oncogene to enhance cell transformation. Furthermore, carcinogen-transformed rodent cells display aberrations in the expression of various endogenous retrovirus-related sequences. Activation of some of these sequences may lead to insertion mutations and further aberrations in gene expression. Thus, multistage carcinogenesis may involve both changes in cellular oncogenes and aberrations in the function of DNA sequences that control gene transcription.