Concentration–response relationships and temporal patterns in hepatic gene expression of Chinook salmon (Oncorhynchus tshawytscha) exposed to sewage

Changes in liver gene expression were examined in juvenile Chinook salmon (Oncorhynchus tshawytscha) exposed in vivo for 8 d to seawater (control) or one of 5 concentrations of sewage (environmentally-relevant dilutions of 0.05%, 0.1%, and 0.7%; 2%, 5% or 10%) and subsequently transferred to clean seawater for an 8-d recovery period. Livers were sampled on days 1, 4, 8 (sewage-exposed) and 16 (8 d of sewage exposure plus 8 d of recovery). A custom cDNA microarray using a universal DNA reference design was used to examine trends of altered gene expression across sewage concentrations, across timepoints, and at the end of the recovery period. Alterations in gene expression followed four distinct concentration-dependent patterns: (1) concentration response (e.g. estrogen receptor alpha), (2) inverse-concentration response (e.g. insulin receptor beta), U-shaped (e.g. mineralocorticoid receptor), (3) inverse U-shaped (e.g. benzodiazepine receptor), and (4) concentration-independent responses (e.g. ubiquitin). Temporal trends included: (1) peak gene expression at one of the sewage exposure timepoints with recovery to baseline levels after the depuration phase (e.g. vitelline envelope protein beta), (2) gene expression alterations that did not recover (e.g. glucose transporter 3), and (3) delayed gene expression alterations initiated only at the recovery timepoint (e.g. insulin-like growth factor 2). In summary, patterns in gene expression changes were found across sewage concentrations and exposure timepoints. This study is the first to show gene expression trends of this nature.