Contaminated sediment in the Detroit River provokes acclimated responses in wild brown bullhead (Ameiurus nebulosus) populations

In a previous study, adaptive responses to a single polycyclic aromatic hydrocarbon (PAH), benzo[a]pyrene (BaP), were identified in brown bullhead (Ameiurus nebulosus) captured from contaminated sites across the Great Lakes. The tumor suppressor p53 and phase I toxin metabolizing CYP1A genes showed a protective and refractory response, respectively, up to the F1 generation (Williams and Hubberstey, 2014). As an extension to the first study, bullhead were exposed to sediment collected from sites along the Detroit River to see if these adaptive responses are attainable when fish from a contaminated site are exposed to a mixture of contaminants, instead of a single compound. p53 and CYP1A proteins were measured in both studies with the addition of phase II glutathione-s-transferase (GST) activity in the second. Three treatment groups were measured: acute (treated immediately), cleared (depurated for three months and subsequent treatment), and farm raised F1 offspring. All three treatment groups were exposed to clean and contaminated sediment for 24 and 96 hours. Acute fish from contaminated sites exposed to contaminated sediment revealed an initial elevated p53 response that was not reached in cleared fish exposed to contaminated sediment. Instead, cleared and F1 bullhead from clean and contaminated sites had overlapping p53 expression patterns in response to contaminated sediment by 96 hours. Acute fish from contaminated sites exposed to contaminated sediment revealed refractory CYP1A expression, which disappeared in cleared fish and whose F1 refractory response overlapped with clean site F1 offspring. Decreasing GST activity was evident in both clean and contaminated fish over time, with clean site fish responding to contaminated sediment more deliberately. By 96 hours, the response patterns of F1 offspring from clean and contaminated sites to clean and contaminated sediment exposures were similar.  

Because p53, CYP1A and GST activity responses to contaminated sediment dosing overlapped in clean and contaminated farm-raised F1 offspring, these results suggest that contaminated fish have acclimated to the contaminants present in their environments by reaching a tolerance threshold and no evidence of adaptation was detected in these biomarkers.

Methods
Brown bullhead (Ameiurus nebulosus) were collected from two reference 'clean' (Peche Island, Belle River) and contaminated sites (Trenton Channel, Belle Isle) in the Great Lakes and exposed to clean (Peche Island) and contaminated (Trenton Channel) sediment for 24 or 96 hours to confirm previously identified adaptive properties between different bullhead  populations in the Detroit River. Instead of exposing fish to a single polycyclic aromatic hydrocarbon (Benzo[a]pyrene) as in the previous study, we exposed fish to either clean or contaminated sediment from sites where fish were collected using a petite ponar.

Liver tissue samples were preserved in RNA later or flash frozen in liquid nitrogen at time of collection. 

Differences in CYP1A and p53 protein expression (phase I metabolism) and glutathion-s-transferase (phase II) enzyme activity was measured and quantified in F1 farm-raised, Acute and Cleared (for 3 months) fish. Protein expression in liver tissue was measured using western blotting technique and ANOVA was used to identify significant differences in test groups. Microsomal preparation and cytosolic fractionation of liver tissue was used to measure glutathion-s-transferase (GST) activity differences between fish populations and significance was identified by unpaired t-tests.