Our group investigates toxic effects of environmental chemical contaminants (dioxins and other persistent organic pollutants and trace elements) in a variety of wildlife including invertebrates, fish, amphibians, reptiles, birds and mammals. Also we attempt to understand the molecular mechanisms underlying species-specific responses and susceptibility to chemical exposure, focusing on the functions of hydrophobic ligand receptors and xenobiotic metabolizing enzymes. Our goal is to assess the risk of the chemical contaminants in the wild population of species of concern.

The responses and susceptibility to chemicals are greatly different among species. This is mostly because the difference in genes coding hydrophobic ligand receptors and metallothioneins is a critical factor to account for the interspecies differences. For example, it is well known that the aryl hydrocarbon receptor (AHR), a dioxin-activated intracellular protein, plays a central role in mediating various toxic effects of dioxins, and interspecies difference in the sensitivity to dioxins, at least partly, is accounted for by the functional difference in AHR. Comparative studies on the function of such gene products in key species, representing phylogenetically and ecotoxicologically relevant groups, may lead to more fundamental understanding of diversity of toxicities and sensitivity to chemicals. Based on such a perspective, we focus on hydrophobic ligand receptors including AHR, constitutive androstane receptor (CAR), pregnane X receptor (PXR) and peroxisome proliferator-activated receptor (PPAR) and metallothioneins of wildlife. Our group has cloned the cDNAs and constructed in vitro assay systems using the clones in order to evaluate species-specific responses of signal transduction triggered by environmental chemicals. The in vitro systems can potentially be a valuable tool for exploring unknown toxic chemicals and for assessing the risk in wild species of concern.

One of target genes of hydrophobic ligand receptors, cytochrome P450 (CYP) comprises a large superfamily of heme-thiolate enzymes that are critical for the metabolism of physiologically relevant endogenous substrates and a vast range of xenobiotics. It has been well documented that expression levels of some CYP members are altered by numerous chemical contaminants. Such CYP members can thus be involved in the onset of biological/toxic effects by chemicals through the disruption of hormone levels, the metabolic activation of chemicals and the production of reactive oxygen species. Hence, chronic induction of CYPs by chemicals can be a potential biomarker of chemical exposure and effects. We have found significant positive correlations between concentrations of environmental chemicals and expression levels of CYPs in some wild populations of birds and aquatic mammals. This indicates that some CYPs are induced in the specimens that have been exposed to elevated levels of the chemicals. Moreover, to evaluate the metabolic potencies of environmental and endogenous compounds in wild species, we have characterized the catalytic functions of individual CYP isozymes, of which the cDNA clones were isolated from the species of concern and were heterologously expressed.

Since organisms react to exposure of environmental chemicals by altering the expression level of genes, analysis of transcriptome and proteome in relation to chemical exposure may thus be a valid approach to predict the potential effects and the mode of action. In order to screen contaminant-responsive genes, we have constructed custom oligo arrays of some species. For the interpretation of gene expression profiles in terms of chemical exposure, we have carried out comprehensive correlation analyses between concentrations of environmental contaminants and gene expression patterns in the wild population. Proteome analysis is also within the scope of our framework.