H. Yamasaki et al., NONGENOTOXIC CARCINOGENS - DEVELOPMENT OF DETECTION METHODS BASED ON MECHANISMS - A EUROPEAN PROJECT, Mutation research, 353(1-2), 1996, pp. 47-63
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.