Use of human-derived liver cell lines for the detection of environmental and dietary genotoxicants; current state of knowledge.
This article gives an overview of the results of genotoxicity tests, which have been conducted within the last 5 years with the human liver cell line HepG2. It is an update of an earlier review from 1998 (by Knasmüller et al.). In addition, a number of publications are discussed which are relevant for the use of human derived liver cell lines in genetic toxicology. They concern the establishment of new endpoints, the development of new cell lines and possible pitfalls and problems. HepG2 cells have been used to test a wide variety of compounds over the last years. The most interesting observations are that the cells are highly sensitive toward polycyclic aromatic hydrocarbons and that genotoxic effects are seen with a number of carcinogenic mycotoxins, that give negative results in other in vitro assays. Carcinogenic metals such as As and Cd caused positive results as well, whereas only marginal or negative results were seen with nitrosamines. The low sensitivity toward these latter carcinogens is probably due to a lack of cytochrome P4502E1 which catalyses their activation. Also, a number of structurally different synthetic pesticides as well as bioactive plant constituents ("natural pesticides") have been tested and with some of them genotoxic effects were found. In most experiments, the formation of micronuclei was used as an endpoint; however also the single cell gel electrophoresis assay is increasingly used. Several transfectant lines of HepG2 have been constructed which express increased levels of phase I enzymes (such as CYP1A1, CYP1A2, CYP2E1 etc.); furthermore, cell lines became available which express human glutathione-S-transferases. These new clones might be particularly useful for the investigation of specific classes of genotoxicants and also for mechanistic studies. Apart from HepG2 cells, a number of other human derived liver cell lines have been isolated, but so far no data from genotoxicity experiments are available, except for Hep3B cells, which were compared with HepG2 and found to be less sensitive in general. Studies with HepG2 clones of a different origin indicate that the cells differ in regard to their sensitivity toward genotoxicants; also medium effects and the cultivation time might affect the outcome of genotoxicity studies. Overall, the results support the assumption that HepG2 cells are a suitable tool for genotoxicity testing.