Comparative analyses of aryl hydrocarbon receptor structure and function in marine mammals

NOAA National Sea Grant College Program, Grant No. NA16RG2273, Grant No. NA86RG0075, NOAA Right Whale Grants Program, Grant No. NA03NMF4720475, American Association of University Women, American Dissertation Fellowship

Marine mammals possess high body burdens of persistent organic pollutants,including PCBs and dioxin-like compounds (DLC). Chronic environmental ordietary exposure to these chemicals can disrupt the function of reproductive andimmune systems, as well as cause developmental defects in laboratory animals.The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor,mediating the expression of a suite of genes in response to exposure to DLC andstructurally related chemicals. Species-specific differences in AHR structure canaffect an organism’s susceptibility to the effects of DLC. The structures andfunctions of several cetacean AHRs were investigated using in vitro molecularcloning and biochemical techniques. Using a novel combination of remotebiopsy and molecular cloning methods, RNA was extracted from smallintegument samples from living North Atlantic right whales to identify the cDNAsequence for AHR and other genes of physiological importance. Biopsy-derivedRNA was found to be of higher quality than RNA extracted from strandedcetaceans, and proved a good source for identifying cDNA sequences forexpressed genes. The molecular sequences, binding constants, andtranscriptional activities for North Atlantic right whale and humpback whale AHRscDNAs were determined using in vitro and cell culture methods. Whale AHRsare capable of specifically binding dioxin and initiating transcription of reportergenes. The properties of these AHRs were compared with those from othermammalian species, including human, mouse, hamster, and guinea pig, andother novel marine mammal AHRs, using biochemical, phylogenetic, andhomology modeling analyses. The relative binding affinities for some marinemammal AHRs fall between those for the high-affinity mouse AHRb-1 and thelower affinity human AHR. Species-specific variability in two regions of the AHRligand binding domain were identified as having the greatest potential impact onAHR tertiary structure, yet does not sufficiently explain differences observed inligand binding assays. Additional studies are necessary to link exposure toenvironmental contaminants with potential reproductive effects in marinemammals, especially via interactions with steroid hormone receptor pathways.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2007