Multiple Environmental Stressors: Biological Interactions Between Parameters of Climate Change and Perfluorinated Alkyl Substances in Fish

Abstract

Aquatic organisms continuously acclimatize to fluctuations in environment parameters and contaminant insult. The aim of this thesis was to investigate effects of combined exposure to quantifiable measures of climate change (hypercapnia and hypoxia) and perfluorinated alkyl substances (PFASs) on oxidative stress, lipid homeostasis and endocrine disruption in fish. The thesis consists of four papers (Paper I-IV). In Paper I and II, Atlantic cod (Gadus morhua) juveniles was exposed to perfluorooctane sulfonic acid (PFOS) (0, 100 or 200 μg/L) 1 H/day for 5 days, followed by exposure to different concentrations of carbon dioxide (CO2) (848, 2735 or 7963 ppm). Sampling was performed 3, 6 and 9 days after initiated CO2 exposure. We observed interactions between PFOS and CO2 exposure on transcription of gill glutathione peroxidase 1 (GPx1), gill manganese superoxide dismutase (MnSOD) and liver phosphatidylethanolamine N-methyltransferase (PEMT) (Paper I). Peroxisome proliferator-activated receptor β (PPAR-β) was increased in cod liver by CO2, and the response was higher in the presence of PFOS (Paper I). In Paper II, PCA-biplots showed clustering of samples based on CO2 concentration, and the distribution of response parameters indicated that hypercapnia was the main driver of hormone responses. Changes in estradiol-17β (E2), testosterone (T) and 11-ketotestosterone (11-KT) levels and E2-responsive genes were increased by hypercapnia alone at day 3 and 9, and by combined exposure scenarios (Paper II). Hypercapnia increased cytochrome P450 1A (CYP1A) mRNA alone and in combination with PFOS (Paper II). In Paper III and IV, primary Atlantic salmon (Salmo salar) hepatocytes were exposed to perfluorooctane sulfonamide (PFOSA) (0, 25 and 50 μM) singly, and in combination with hypoxia-inducible compounds (cobalt chloride: CoCl2 and deferoxamine: DFO) for 24 and 48 h. We observed that combined exposure generally altered the transcription of antioxidant responses and lipid regulation, showing higher effect of hypoxia, compared to PFOSA (PCA-plots Paper III-IV). Hypoxic condition alone, and in combination with PFOSA, increased transcription of E2-responsive genes, CYP1A and CYP3A responses (Paper III). Transcription of PPAR-α, -β and –γ, and the ω6:ω3 ratio was increased by DFO and these responses were modulated in the presence of PFOSA (Paper IV). In addition, we observed a parallel increase of hypoxia-inducible factor 1α (HIF-1α), acyl coenzyme A oxidase (ACOX) and PPAR-γ, indicating a link between lipid metabolism and hypoxic responses (Paper IV). Changes in PPAR system and FA β-oxidation (ACOD and ACOX) suggests complex changes in the regulation of lipid homeostasis and FA metabolism, where PFASs modulated the responses produced by hypercapnia or hypoxia alone (Paper I and IV). Changes in the composition of ω6 and ω3 FAs in salmon hepatocytes and reduced PEMT transcription in cod liver were observed (Paper I and IV), indicating that climate change variables alone and in combination with PFASs may potentially alter membrane lipid composition and possibly produce overt physiological consequences. These studies indicate that parameters of climate change and PFASs affect sex steroids, E2-responsive genes and the CYP system, and that combined exposure generally produced higher response than single exposure (Paper II and III). These findings show that processes involved in endocrine signaling and biotransformation are very complex and hard to predict due to biphasic responses and possibly interaction between stressors. This thesis provides new and valuable insight on the combined effects of quantifiable parameters of climate change and environmental pollutants. Potential interaction between environmental stressors and the relevance of considering exposure duration were revealed, emphasizing the importance of investigating combined and chronic exposure scenarios