Toxicogenomics: Applications of new functional genomics technologies in toxicology

Toxicogenomics studies toxic effects of substances on organisms in relation to the composition of the genome. It applies the functional genomics technologies transcriptomics, proteomics and metabolomics that determine expression of the genes, proteins and metabolites in a sample. These methods could facilitate toxicological research and eventually, toxicogenomics could improve human health risk assessment. This thesis evaluated applications of toxicogenomics, especially to investigate mechanisms of toxicity, to obtain new toxicity markers and to assess toxicity of mixtures.

In the first studies, protein and gene expression were characterised in livers of rats treated with bromobenzene, well-known to cause liver damage. The metabolite contents of plasma and urine were also measured. Many changes were found related to biotransformation, glutathione metabolism, oxidative stress and to unexpectedly involved processes like cholesterol, fatty acid and protein metabolism. The studies enabled to identify crucial events in the mechanisms of hepatotoxic by bromobenzene.

The specificity of bromobenzene-induced liver gene expression changes was delineated by comparison with published effects of high doses of acetaminophen (paracetamol), that also caused liver necrosis. Subsequently, effects of abundant chemicals benzene and trichloroethylene, not typical liver toxicants, were analysed. Benzene primarily causes hematotoxicity and trichloroethylene nephrotoxicity. However, both compounds induced liver weight. They were also found to modulate liver gene expression. Effects were related to biotransformation, fatty acid metabolism and other processes. Benzene, trichloroethylene, bromobenzene and acetaminohen induced several common and many specific changes in liver gene expression.

Transcriptomics, proteomics and metabolomics were applied to obtain new markers for identification of liver toxicity at early stages and low exposure levels. Potential markers were correlated to the liver damage induced by bromobenzene. Furthermore, characteristic profiles of metabolites in urine enabled specific and robust assessment of exposure to bromobenzene, benzene and trichloroethylene.

A study with mixtures of benzene and trichloroethylene showed for the first time how transcriptomics may facilitate assessment of mixture toxicity. The gene expression changes by trichloroethylene were found to be enhanced by benzene. More importantly, transcriptomics provided insights in molecular mechanisms of joint action.

Transcriptomics and metabolomics proved more sensitive than conventional toxicity parameters. However, it remains to be clarified whether observed effects are adverse. At present, transcriptomics is the most informative of the toxicogenomics methods, while proteomics and metabolomics wil! rapidly become more relevant. Linking the technologies could provide detailed insights in (dynamic) processes within the eell in relation to environmental stimuli.

In conclusion, toxicogenomics methods provided detailed insights in molecular mechanisms of toxicity. They enabled more sensitive and early detection of effects and facilitated assessment of effects of mixtures of compounds. Thus, toxicogenomics methods may be used to improve human health risk assessment.