Comparative studies of aryl hydrocarbon receptors in fish

Developing fish are especially sensitive to the toxicity of some members of a diverse class of polyhalogenated aromatic hydrocarbons (PCAHs) found in the wild. Understanding the molecular mechanism of this toxicity is essential to estimate the risk of exposure that these chemicals impose among species of fish. Zebrafish were used to characterize the mechanism of developmental toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) a potent PCAH toxicant. Work in mammalian systems has demonstrated that the aryl hydrocarbon receptor (AHR) is involved in TCDD toxicity. AHR is a transcription factor that is activated by binding ligands including many PCAHs and TCDD. AHR must bind ligand and dimerize with another transcription factor, the aryl hydrocarbon nuclear translocator (ARNT), to be functionally active. Fish possess at least two AHR genes (type 1 and type 2) while mammals have only one. A zebrafish AHR2 (zfAHR2) was previously characterized and was found to have similar functional properties to mammalian AHRs. Here a type one zebrafish AHR (zfAHR1) was cloned and characterized. It was found to be essentially nonfunctional in comparison to zfAHR2. The C-terminus of zfAHR1 was determined to be responsible for its lack of TCDD mediated transcriptional activity. zfAHR1 is not suspected to be involved in TCDD toxicity since it is expressed at very low levels in few tissues, does not appreciably bind TCDD, and is not able to activate transcription in a cell culture assay. Whole mount in situ hybridization studies determined that zfAHR2 and its dimer zfARNT2 colocalize in larvae prior to the development of signs of TCDD toxicity. The developing cardiovascular system was identified as an initial target of toxicity since both zfAHR2 and zfARNT2 were co-localized in these tissues along with the TCDD dependent expression of an AHR responsive gene, cytochrome P4501A. Two rainbow trout AHRs (rtAHR2alpha, rtAHR2beta were previously determined to have differential ability to respond to TCDD by activating AHR responsive luciferase reporters in cell culture. A single residue difference between the rtAHR2s was found to convey this differential activity. Additionally this residue was found to have functional significance in zfAHR2 and the human AHR implying it serves some conserved function.