Superfund Research Program
Mechanisms and Impacts of PCB Resistant Fish
Project Leader: Mark E. Hahn (Woods Hole Oceanographic Institution)
Co-Investigators: Sibel I. Karchner (Woods Hole Oceanographic Institution), Neelakanteswar Aluru (Woods Hole Oceanographic Institution)
Grant Number: P42ES007381
Funding Period: 1995-2020
Dr. Hahn and his team are studying a population of estuarine fish, Atlantic killifish (Fundulus heteroclitus), inhabiting New Bedford Harbor, Massachusetts (NBH), a PCB-contaminated Superfund site. NBH killifish are much less sensitive to PCBs than killifish from a reference site, Scorton Creek (SC). PCBs cause toxicity by activating aryl hydrocarbon receptor (AHR) proteins. Killifish have two AHRs (AHR1 and AHR2). The specific objectives of the researchers are: 1) to understand the mechanisms by which NBH killifish are less sensitive to the developmental toxicity of PCBs and 2) to determine the impact of evolved PCB resistance on the sensitivity to low oxygen (hypoxia).
Previously, the researchers identified numerous polymorphic variants of AHR1 and AHR2 in killifish. This year, the team performed population-genetic analyses of these variants. The analyses revealed substantial genetic structure among several killifish populations, including statistically significant differences between SC and NBH at these loci, suggesting selection acting on one or both populations.
To determine whether the resistance of NBH fish to PCBs involves a limited number of AHR target genes or is more extensive, the researchers performed an experiment in which SC and NBH embryos were exposed to PCB-126 and sampled at 5, 10, and 15 days post fertilization (dpf). Analysis of gene expression with a ~4,000-feature killifish microarray showed that the resistance extended to a large set of AHR target genes; changes in gene expression were more numerous and more dramatic in the SC fish.
The researchers also continued to investigate the impact of the PCB-resistant phenotype on the response to hypoxia. Killifish F1 larvae (50 dpf) from each site were exposed to PCB-126 and then held under varied oxygen tensions (ambient, 20% O2, 10% O2, or 5% O2) for 48 hr. Larvae from NBH exhibited enhanced lethality at 5% O2. The hypoxia-responsive gene IGFBP1 was induced by hypoxia in SC fish but only when co-exposed to PCB-126. In contrast, IGFBP1 was not induced in NBH fish under any conditions. The results suggest that the inability of NBH fish to mount an adaptive response to hypoxia in the presence of PCBs may make them more sensitive to hypoxia-induced lethality.
In collaboration with Dr. Schlezinger’s research, the team identified a novel mechanism of cross-talk between AHR and hypoxia-signaling pathways. These studies showed that AHR repressor (AHRR), in addition to being induced in an AHR-dependent manner and inhibiting AHR signaling, is also capable of repressing hypoxia signaling. This result suggests that AHR agonists can interfere with HIF signaling through AHR-mediated induction of AHRR.
The existence of dioxin-sensitive and dioxin-resistant populations of killifish provides a unique opportunity to understand the molecular mechanisms of differential dioxin sensitivity and the impact of evolved resistance on the sensitivity of fish to other environmental stressors. The results obtained this year, demonstrating evidence for selection on the AHR loci and showing that all AHR target genes are affected, further implicate the AHRs in the mechanism of resistance. The researchers also obtained evidence that the PCB resistance affects the sensitivity to other stressors such as hypoxia.