NONGENOTOXIC CARCINOGENS - DEVELOPMENT OF DETECTION METHODS BASED ON MECHANISMS - A EUROPEAN PROJECT

While the accumulation of genetic changes in a somatic cell is conside red essential for the genesis of a cancer, it has become clear that no t all carcinogens are genotoxic, suggesting that some carcinogens indi rectly participate in the generation of genetic changes during carcino genesis, A European project funded by the European Community was thus conceived to study mechanisms of nongenotoxic aspects of carcinogenesi s, Two main strategical approaches were adapted: (i) to study whether and how Syrian hamster embryo (SHE), Syrian hamster dermal (SHD) and B ALB/c 3T3 cell transformation systems simulate in vivo carcinogenesis, and to examine whether they can detect nongenotoxic carcinogens; (ii) to study, refine and validate mechanisms-based end-points for detecti on of nongenotoxic carcinogens. For mechanisms-based research, the pro posed end-points included gap junctional intercellular communication ( GJIC) inhibition, altered expression of critical genes, immortalizatio n and aberrant cell proliferation. We also selected model compounds co mmonly usable for various endpoints. Our major results can be summariz ed as follows: (1) SHE and BALB/c 3T3 transformation systems reflect b oth genotoxic and nongenotoxic carcinogenic events; they detect not on ly genotoxic but also many although not all, nongenotoxic carcinogens. This is further supported by the fact that both genotoxic and nongeno toxic carcinogens were able to immortalize SHD cells. (2) Many nongeno toxic carcinogens, although not all, inhibit GJIC in vitro as well as in vivo. Mechanistic studies suggest an important role of blocked GJIC in carcinogenesis and that different mechanisms are involved in inhib ition of the communication by different agents used. However, inhibiti on of GJIC is not a prerequisite for the enhancement (or induction) of transformation of SHE or BALB/c 3T3 cells. (3) Among compounds examin ed, there was a good correlation between induction of micronuclei and cell transformation in SHE cells while no such correlation was found b etween the induction of cell transformation and ornithine decarboxylas e activity. (4) Two transgenic mouse mutation assays (lacI and lacZ) w ere established and validated. The genotoxin dimethylnitrosamine was s hown to be mutagenic to the liver in both assays. Ortho-anisidine, a b ladder-specific carcinogen that was inactive in standard rodent geneti c toxicity assays was uniquely mutagenic to the bladder of the transge nic mice. The peroxisome proliferator methyl clofenipate was establish ed as nonmutagenic to the liver of both transgenic mice. That eliminat ed DNA damage as a cause of the liver tumours produced by this chemica l and weakened the idea that induced cell division leads to mutation i nduction. (5) With an in vitro DNA replication model, it was found tha t DNA damage induced by genotoxic agents can be responsible for inhibi tion of DNA replication, while certain nongenotoxic agents such as pho rbol esters increase DNA replication. (6) An attempt to use structure- activity relationship for subfamilies of nongenotoxic carcinogens, e.g ., receptor-mediated carcinogens, has been initiated with some promisi ng results. Our results support the idea that there are multiple nonge notoxic mechanisms in carcinogenesis, and that working hypothesis-orie nted approaches are encouraged rather than simple screening of chemica ls in developing test systems for the detection of nongenotoxic carcin ogens.