To determine whether in vitro cell transformation systems can be used to detect nongenotoxic carcinogens.
The research effort is to develop detection methods of nongenotoxic carcinogens based on mechanisms of nongenotoxic aspects of carcinogenesis. This includes the validation of in vitro cell transformation systems and other mechanisms derived endpoints.
The following methods, assays and techniques have been developed and/or established in the participating laboratories: cell transformation assay of Syrian hamster embryo cells and BALB/c 3T3 cells;
in vitro as well as in vivo methods to measure gap junctional intercellular communications;
in vitro deoxyribonucleic acid (DNA) replication system;
transgenic mice which carry model genes for mutation studies;
cell immortalization assays;
a novel method to assay genetic loss;
a nuclear matrix alteration assay. Employing the available tools described above, it has been shown that inhibition of intercellular communication, induction of micronuclei, induction of ornithine decarboxylase activity and nuclear matrix changes are associated with cell transformation and/or in vivo carcinogenesis. While the causal relationship between these biological effects and carcinogenesis is not yet clear, these results support the idea of developing these endpoints for carcinogen screening. Both BALB/c 3T3 and Syrian hamster embryo cell transformation assays can detect genotoxic and nongenotoxic carcinogens. Using oncogene activation and intercellular communication inhibition as genotoxic and nongenotoxic markers, respectively, we have shown that both of these events are required for chemical induced cell transformation. Use of various cells also suggested that nongenotoxic carcinogens exert tissue specific effects.
There is growing concern about our ability to detect carcinogens that do not fit into the genotoxic model of chemical carcinogenecity. A decade ago these exceptions were few and were considered to be negligible, but the increasing number of carcinogenecity bioassays conducted in recent years, have led to the realization that about one third of all carcinogens determined by such bioassays present problems in this respect. For convenience, these agents are grouped under the heading of nongenotoxic carcinogens, but it is unlikely that they form a coherent group inducing cancer by a single mechanism of action. Risk estimation for humans exposed to nongenotoxic carcinogens is complicated by the absence of the usual markers of exposure to genotoxins.
In attempting to approach the phenomenon of nongenotoxic carcinogenicity, it is recognized that primary emphasis should be on understanding the mechanisms of the phenomenon. This, if successful, will lead to the development of predictive assays but a primary search for empirically based assays has been avoided. Since many nongenotoxic carcinogens demonstrate organospecificity and the liver is often such a target in experimental animals, and since rodent liver represents a good model of multistage carcinogenesis, the rodent liver model is a focus of this study. The following broad endpoints that seem likely to be key events in the action of nongenotoxic carcinogens are studied:
induction of cell immortalization;
induction of unscheduled cell proliferation;
and blocked gap junctional intercellular communication.
Proposed biochemical and molecular mechanisms involved in these processes include chromatin conformation changes, deoxyribonucleic acid (DNA) hypomethylation, oxidative DNA damage, genome loss and phosphorylation of gap junction proteins. This proposal is based on the idea that genotoxic and nongenotoxic processes are not mutually exclusive mechanisms in carcinogenesis, but rather are complementary and interdependent in the completion of carcinogenesis. Part of the research studies the relationship between these 2 mechanisms (and possibly agents), so as to improve the basis for risk assessment of these 2 classes of chemicals. This coordinated project will include, in its final phase, the validation of proposed endpoints by testing selected nongenotoxic carcinogens.
Funding SchemeCSC - Cost-sharing contracts
6700 AA Wageningen
SK10 4TJ Macclesfield