Abstract
The glutathione S-transferases (GSTs) are a multifunctional family of phase II enzymes that detoxify a variety of environmental chemicals, reactive intermediates, and secondary products of oxidative damage. GST mRNA expression and catalytic activity have been used as biomarkers of exposure to environmental chemicals. However, factors such as species differences in induction, partial analyses of multiple GST isoforms, and lack of understanding of fish GST gene regulation, have confounded the use of GSTs as markers of pollutant exposure. In the present study, we examined the effect of exposure to cadmium (Cd), a prototypical environmental contaminant and inducer of mammalian GST, on GST mRNA expression in coho salmon (Oncorhynchus kisutch) liver, gill, and olfactory tissues. GST expression data were compared to those for metallothionein (MT), a prototypical biomarker of metal exposure. Data mining of genomic databases led to the development of quantitative real-time PCR (qPCR) assays for salmon GST isoforms encompassing 9 subfamilies, including alpha, mu, pi, theta, omega, kappa, rho, zeta and microsomal GST. In vivo acute (8-48 h) exposures to low (3.7 ppb) and high (347 ppb) levels of Cd relevant to environmental scenarios elicited a variety of transient, albeit minor changes (<2.5-fold) in tissue GST profiles, including some reductions in GST mRNA expression. In general, olfactory GSTs were the earliest to respond to cadmium, whereas, more pronounced effects in olfactory and gill GST expression were observed at 48 h relative to earlier time points. Although evaluation of GSTs reflected a cadmium-associated oxidative stress response, there was no clear GST isoform in any tissue that could serve as a reliable biomarker of acute cadmium exposure. By contrast, metallothionein (MT) mRNA was consistently and markedly induced in all three tissues by cadmium, and among the tissues examined, olfactory MT was the most sensitive marker of cadmium exposures. In summary, coho salmon exhibit a complex GST tissue profile consisting of at least 9 isoforms, all of which are present in the peripheral olfactory system. Short-term exposure to environmental levels of Cd causes transient changes in salmon GST consistent with oxidative stress, and in some cases, includes a loss of GST. In a biomarker context, however, monitoring of tissue MT mRNA expression, especially in the peripheral olfactory system, may be of greater utility for assessing short-term environmental exposures to cadmium.