Transactivation activity of human, zebrafish, and rainbow trout aryl hydrocarbon receptors expressed in COS-7 cells: Greater insight into species differences in toxic potency of polychlorinated dibenzo-p-dioxin, dibenzofuran, and biphenyl congeners

Transactivation assays were used to compare the potency and efficacy of pol ychlorinated dibenzo-p-dioxin (PCDD), dibenzofuran (PCDF), and biphenyl (PC B) congeners in activating aryl hydrocarbon receptors (AhRs) from rainbow t rout (rtAhR2 alpha and rtAhR2 beta), zebrafish (zfAhR2), and human (huAhR), respectively. All AhRs were expressed with their species-specific AhR nucl ear translocator (ARNT) in COS-7 cells. Transactivation activity was determ ined for two PCDD, two PCDF, and seven PCB congeners with each of the four AhR/ARNT pairs using prt1Aluc, a luciferase reporter driven by two dioxin-r esponsive enhancer elements (DREs) from the rainbow trout cyp1A gene. Maxim al-fold induction, EC50, and relative potency values were calculated for co ngeners that exhibited dose-related activity in the assay. Of the four AhR/ ARNT pairs tested with PCDD, PCDF, and non-ortho PCB congeners, three exhib ited high activity (rainbow trout AhR2 alpha, zebrafish AhR2, and human AhR ), while rainbow trout AhR2 beta had very weak or no activity. Comparisons between these AhRs showed that while mono-ortho PCBs were able to activate the human AhR, they were generally ineffective in activating rainbow trout and zebrafish AhR2s. This supports the hypothesis that structural differenc es between mammalian and fish AhRs may account for differences in relative potencies of the mono-ortho PCBs between mammals and fish. Another importan t finding was a significant difference in transactivation activity between the two rainbow trout AhR2 isoforms despite the fact that they are 95% iden tical at the amino acid level. For all PCDD, PCDF, and PCB agonists tested, rainbow trout AhR2 alpha was significantly more active than AhR2 beta. How ever, rainbow trout AhR2 beta is active as a 2,3,7,8-tetrachlorodibenzo-p-d ioxin (TCDD)-activated transcription factor, with enhancer elements from th e mouse cyp1A gene. This suggests that AkR2 beta may have evolved to serve a different physiological function than AhR2 alpha in salmonid fish species . (C) 1999 Academic Press.