Part A: Temporal and dose-dependent transcriptional responses in the liver of fathead minnows following short term exposure to the polycyclic aromatic hydrocarbon phenanthrene.

Phenanthrene is a low molecular weight polycyclic aromatic hydrocarbon (PAH) that is composed of three fused benzene rings. PAHs are formed naturally through incomplete combustion of organic materials, and are environmental contaminants due to anthropogenic activities (e.g. oil extraction and refining, industrial and municipal effluents, fossil fuel burning). Fish exposed to PAHs such as phenanthrene have been reported to exhibit altered reproductive axis endpoints, however the mechanisms that underlie these responses are not fully characterized. To better understand effects at the mechanistic level, we applied transcriptomics to identify molecular pathways altered after acute exposure to phenanthrene on both a dose and temporal scale. Female fathead minnow (Pimephales promelas) were exposed to an average measured concentration of either 0, 29.8, 389 or 943 μg phenanthrene/L for 24, 48, and 72 h in a static-renewal bioassay. Ovaries were assessed for oocyte distribution as well as in vitro 17β-estradiol production and gene expression for transcripts related to steroidogenesis and estrogen signalling. In addition, the liver transcriptome was measured as this tissue is the primary source of the egg yolk precursor protein vitellogenin. Exposure to 29.8 μg phenanthrene/L increased proportions of the cortical alveolar stage in the ovaries after 48 h while the proportion of cortical alveolar oocyte were decreased in fish exposed to 943 μg phenanthrene/L for 48 h. Phenanthrene did not affect 17β-estradiol production at any time or dose, and did not affect transcripts associated with hormone synthesis nor signalling pathways. In the liver, the transcriptome showed fewer genes in common across time when compared to those transcripts affected by concentration at a single time point. Cholesterol metabolism was the only pathway perturbed in the liver following all comparisons in both the dose and time course experiments. Our data suggest that transcriptome networks associated with hepatic lipid metabolism are rapidly affected by phenanthrene, and this may indirectly reduce resources available for reproductive efforts.