Sensitivity of avian species to the aryl hydrocarbon receptor ligand 6-formylindolo [3,2-b] carbazole (FICZ)

Avian species differ in sensitivity to the toxic effects of dioxin-like compounds (DLCs) and recent reports have provided insight into the molecular mechanisms underlying this variability. The sensitivity of avian species to DLCs is associated with the identity of amino acids at positions 324 and 380 within the ligandbinding domain (LBD) of the aryl hydrocarbon receptor I (AHRI). 6-formylindolo [3,2-b] carbazole (FICZ), a naturally produced photo-oxidation product of tryptophan, is a highly potent AHR ligand. Few studies have attempted to determine if there are species differences in AHR activation by FICZ in a systematic manner. Here we describe results from an in vitro assay that measures AHRI -mediated luciferase reporter gene activity to determine concentration-dependent effects of FICZ and 2,3,7,8-tetrachlorodibenzo-pdioxin (TCDD) in COS-7 cells transfected with AHR1 constructs from chicken (Gallus gallus domesticus), ring-necked pheasant (Phasianus colchicus), Japanese quail (Coturrzix japonica) and common tern (Sterna hi rondo), and three mutant AHR1 constructs. Data were used to (a) compare the potency of FICZ and TCDD for each AHR1 construct (relative potency; Rep) and (b) the sensitivity of each construct to AHR1 activation by FICZ and TCDD (relative sensitivity; ReS). The results show that (1) FICZ was considerably more potent than TCDD in cells transfected with chicken AHR1 (RePavg = 41), ring-necked pheasant AHRI (RePavg = 93), Japanese quail AHRI (RePavg = 1392) and common tern AHRI (RePavg = 1534), (2) there were no significant differences in sensitivity to FICZ in cells expressing chicken, pheasant, quail and tern AHRI, but there were significant differences in sensitivity to TCDD, (3) alteration of amino acids at positions 324 and 380 had no effect on avian AHR1 activity in response to FICZ, (4) there was no timedependent change in the relative potency of FICZ in COS-7 cells, and (5) neither FICZ nor TCDD induced ethoxyresorufin O-deethylase (EROD activity) in COS-7 cells. Our results suggest that FICZ and TCDD activate avian AHR1 by different modes of interaction with AHR1. Crown Copyright (C) 2014 Published by Elsevier Ireland Ltd. All rights reserved.