Combined Actions and Interactions of Chemicals in Mixtures

act in synergy when they elicit their effect by different mechanisms. As in the case
of initiation, no formal guidelines exist for testing of promotion. The identification
of tumour promoters is complicated by the fact that they may be organ specific
and, possibly, initiator specific. However, good models seem to exist for mouse
skin, rat liver and rat bladder (Autrup and Dragsted, 1987).
7.3.4 Combination effect at later stages
7.3.4.1 Conversion
It has been shown by Hennings et al. (Hennings, 1991) that the tumour response
after initiation and promotion may be enhanced further in the mouse skin by
subsequent treatments with a direct-acting genotoxic compound. Hennings termed
this effect, conversion. Similar results were observed in rat liver carcinogenesis by
Scherer and co-workers (Scherer et al., 1984; Scherer, 1984), and the effect is also
known from in vitro transformation of "normal" cells to malignant cells (see
section on mutagenesis). Thus, two terms, conversion and transformation may be
used for this effect. No formal test systems exist for conversion testing although the
methods of Hennings and of Scherer cited above may be generally applicable. The
genotoxins active in conversion or transformation may increase the risk of cells
loosing the second allele of the gene targeted by the initiation treatment. This risk
is much larger after promotion since tumour promoters increase dramatically the
pool of initiated (daughter) cells with identical genetic lesions. It is therefore likely
that many genotoxic compounds are not only initiators but also converters leading
to the formation of "the first" malignant cell in the tumourigenic process.
No studies have been concerned with interactions at this stage of carcinogenesis,
but most of the observations related to initiation may be relevant here as well, i.e.
co-initiation would be similar to co-conversion. In humans the enhancing effects of
beta-carotene on smoking-induced cancers of the lung at a very late stage (Omenn
et al., 1996; ATBC cancer prevention study group, 1994) may be an example of
interaction at the conversion level, but the mechanism behind the effect is still
obscure. No formal test guidelines for converters exist, but the mouse skin and rat
liver models might be used to develop such guidelines.
7.3.5 Anticarcinogenesis
Anticarcinogens may be defined as compounds, which decrease the response of
carcinogens. Their effects can be elicited at any stage during carcinogenesis by
counteracting the effects of the carcinogenic treatments. Anti-initiators are often
termed 'blocking agents' since they may prevent carcinogenesis altogether
(Wattenberg, 1980). A large number of different assays have been applied to
identify anticarcinogens, but few, if any, have been formally validated, so no
formal test system exists. They can act as enzyme inducers as described above in
section 3.3.2.1, or they may specifically induce detoxifying (Phase II) enzymes
only. An in vitro assay has been proposed for the identification of such
anticarcinogens (Talalay et al., 1988). They may also decrease the absorption of
carcinogens by affinity binding in the gut. An example is the sequestering of
aflatoxin in the trout gut by chlorophyllin (Breinholt et al., 1999). Finally they may
protect DNA by as yet undefined mechanisms. An example here is the preventive
actions of many polyphenols on carcinogen-DNA binding (Webster et al., 1996;
Giri and Lu, 1995; Huber et al., 1997; Malaveille et al., 1998). One of the most
studied effect of anticarcinogens is the scavenging of activated, DNA-binding
carcinogens, e.g. the binding by ellagic acid of benzo[a]pyrenediolepoxides (Barch
et al., 1996; Dixit et al., 1985) and the scavenging of radicals by antioxidants and
spin-trapping agents (Poulsen et al., 1998).
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