ABSTRACT
Two novel approaches are described, in which metabolically competent human derived cells were used for the detection of genotoxic effects of environmental carcinogens. In the first, human hepatoma (Hep G2) cells were used for micronucleus and single cells were used for micronucleus and single cell gel electrophoresis (SCGE) assays. These cells have retained the activities of phase I and phase II enzymes which are usually lost during cultivation. We demonstrated that these cells are suitable for the detection of the genotoxic effects of representatives of various classes of DNA-reactive procarcinogens such as benzo(a) pyrene (B(a)P), 2-amino-3-methylimidazo-[4,5-f]-quinoline (IQ), cyclophosphamide (CP), and N-nitrosodimethylamine (NDMA), isatidine and aflatoxin B1 (AFB1). Furthermore, we found that these tests also detect the mutagenic effects of rodent carcinogens such as safrole and hexamethylphosphoramide (HEMPA), which give negative results in conventional in vitro procedures. Additional experimental series showed that genotoxicity assays with Hep G2 cells are also useful for the detection of co- and antimutagens, in particular for compounds which act via induction of activating and detoxifying enzymes. In the second approach, a protocol for stable co-cultivation sandwich cultures with primary human hepatocytes was used. The cultivation of the cells under organotypical conditions leads to an extension of their life span and results in an improved expression of drug metabolising enzymes. Two different experimental models were developed: In the first, the induction of HPRT mutations in V-79 cells was used as an endpoint, in the seconds, single strand breaks were measured in human K562 cells in SCGE assays. Experiments which were carried out with B(a)P and 7,12-diemethylbenz(a)anthracene as model compounds indicate that in both systems positive results are obtained. In conclusion, our data show that tests with human Hep G2 cells as well as sandwich cultures with primary human liver cells are useful for the detection of environmental carcinogens and probably reflect their effects in humans better than conventional in vitro assays with metabolically incompetent cells which are currently used in most mutagenicity studies.
ABSTRACT
The human hepatoma cell line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflects the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In recent years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell electrophoresis assays). It has been demonstrated that various classes of environmental carcinogens, such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrol and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA- reactive metabolites and intracellular inhibition of activating enzymes.