Search for compounds that inhibit the genotoxic and carcinogenic effects of heterocyclic aromatic amines.

Over the last 30 years approximately 160 reports have been published on dietary compounds that protect from the mutagenic and carcinogenic effects of heterocyclic aromatic amines (HAAs). In the first section of this review, the current state of knowledge is briefly summarized. Based on the evaluation of the available data, various protective mechanisms are described, and the use of different methodologies for the detection of protective effects is critically discussed. In most antimutagenicity studies (>70%) bacterial indicators (predominantly Salmonella strain TA98) were used, and about 600 individual compounds and complex mixtures have been identified that attenuate the effects of HAAs. The most frequently used in vivo method to detect protective effects are adduct measurements; anticarcinogenic dietary factors were identified by aberrant crypt foci assays and liver foci tests with rats. The mechanisms of protection include inactivation of HAAs and their metabolites by direct binding, inhibition of enzymes involved in the metabolic activation of the amines, induction of detoxifying enzymes, and interaction with DNA repair processes. The detection spectrum of conventional in vitro mutagenicity assays with metabolically incompetent indicator cells is limited. These procedures reflect only simple mechanisms such as direct binding of the HAAs to pyrroles and fibers. It has been shown that these compounds are also effective in rodents. More complex mechanisms, namely, interactions with metabolic activation reactions are not adequately represented in in vitro assays with exogenous enzyme homogenates, and false-negative as well as false-positive results may be obtained. More appropriate approaches for the detection of protective effects are recently developed test systems with metabolically competent cells such as the human Hep G2 line or primary hepatocytes. SCGE tests and DNA adduct measurements with laboratory rodents enable the detection of antigenotoxic effects in different organs, including those that are targets for tumor induction by the amines. Medium term assays based on aberrant crypt foci in colon and liver foci tests have been used to prove that certain compounds that prevented DNA damage by HAAs also reduced their carcinogenic effects. These experiments are costly and time consuming and, due to the weak induction capacity of the amines, only pronounced anticarcinogenic effects can be detected. Over the years, a large bulk of data on HAA protective compounds has accumulated, but only for a few (e.g., fibers, pyrroles, constituents of teas, and lactic acid bacteria) is there sufficient evidence to support the assumption that they are protective in humans as well.

Genotoxic effects of heterocyclic aromatic amines in human derived hepatoma (HepG2) cells.

In order to study the mutagenic effects of heterocyclic aromatic amines (HAAs) in cells of human origin, five compounds, namely 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), 2-amino-3, 4-dimethyl-imidazo[4,5-f]quinoline (MeIQ), 2-amino-3, 8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx), the pyridoimidazo derivative 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), were tested in micronucleus (MN) assays with a human derived hepatoma (HepG2) cell line. All HAAs caused significant, dose-dependent effects. The activities of IQ, MeIQ, MeIQx and PhIP were similar (lowest effective concentrations 25-50 microM), whereas Trp-P-1 was effective at a dose of >/=2.1 microM. In addition, the HAAs were tested in MN assays with Chinese hamster ovary (CHO) cells and in Salmonella strain YG1024 using HepG2 cell homogenates as an activation mix. In the CHO experiments, positive results were obtained with Trp-P-1 and PhIP, whereas the other compounds were devoid of activity under all experimental conditions. The discrepancy in the responsivity of the two cell lines is probably due to differences in their acetylation capacity: enzyme measurements with 2-aminofluorene as a substrate revealed that the cytosolic acetyltransferase activity in the HepG2 cells is approximately 40-fold higher than that of the CHO cells. In the bacterial assays all five HAAs gave positive results but the ranking order was completely different from that seen in the HepG2/MN experiments (IQ > MeIQ > Trp-P-1 >/= MeIQx >> PhIP) and the mutagenic potencies of the various compounds varied over several orders of magnitude. The order obtained in bacterial tests with rat liver S9 mix was more or less identical to that seen in the tests with HepG2 cell homogenates but the concentrations of the amines required to give positive results were in general substantially lower (10(-5)-10(-1) microM). Overall, the results of the present study indicate that MN/HepG2 tests might reflect the mutagenic effects of HAAs more adequately than other in vitro mammalian cell systems due to the presence of enzymes involved in the metabolic conversion of the amines.