ABSTRACT
There is an urgent need to develop and establish new toxicological approaches to assess the potential cytotoxic and genotoxic effects of heavy metals found in the environment. In the past several decades numerous in vitro and in vivo assays have been utilized to assess the effects of environmental pollutants on their cellular targets. Increasing public interest in these issues has created a demand for alternatives to using animals in such testing. Bacterial assays are used both for fundamental studies of mutagenesis and for screening of environmental samples as potential genotoxins. Mammalian cell culture systems have also been used in risk evaluation, both for investigating mechanisms of chemical carcinogenesis and as bioassay systems for monitoring environmental genotoxins. Isolated cells have been extensively used in toxicological studies in vitro. One organ of particular importance to toxicological research is the liver. The use of in vitro hepatic systems for heavy metal toxicity studies has received increasing attention in recent years. These have been used advantageously in hepatocyte-based cytotoxic-ity and genotoxicity assays in vitro. DNA damage in hepatocytes is often measured as covalent DNA adducts or as strand breaks that occur as a result of the DNA repair process. Assessment of DNA damage induced by heavy metals can employ either primary hepatocyte cultures or established hepatic cell lines such as HepG2. The latter cell model provides a convenient and sensitive tool for rapid screening of environmental samples for potential genotoxic and cytotoxic effects. Other recently developed methods for assessing genotoxic effects include use of microarrays that express multiple genes and from which large amounts of screening data can be obtained. More recently, human cells have been used to investigate the mechanisms by which certain heavy metals such as cadmium interact with intracellular regulatory systems that control expression of genes and intracellular stability of newly synthesized proteins. An interesting new finding is the linkage between heavy metal-induced toxicity and the function of the ubiquitinproteasome system in the cell. The ubiquitin-proteasome system is involved in regulating protein stability for a wide array of important proteins involved in control of cell cycle, cell division, gene transcription, protein secretion, and many other vital cell functions. It was recently shown that expression of this ubiquitin-dependent proteolysis pathway in
yeast is activated in response to cadmium exposure and that mutants deficient in specific ubiquitin-conjugating enzymes are hypersensitive to cadmium. This indicates that a major reason for cadmium toxicity may be cadmium-induced formation of abnormal proteins. This may be a common mechanism by which heavy metals induce cytotoxicity. Furthermore, inhibition of proteasome activity may either directly or indirectly trigger apoptosis and cell death as shown for synthetic inhibitors of this multicatalytic protease system.
