In Vivo antagonism of AhR-mediated gene induction by 3 '-methoxy4 '-nitroflavone in TCDD-responsive lacZ mice

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription fac tor that is a member of the bHLH-PAS family of proteins. The highest-affini ty ligand of this receptor is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), w hich is a potent immunological, reproductive, and developmental toxicant. T he mechanism of TCDD-induced toxicity and the gene modulations that result in toxicity have not been fully defined. The majority of work to date explo ring AhR function has focused on agonist-activated AhR signaling. However, it is expected that a better understanding of AhR antagonism will lead to a n improved understanding of TCDD toxicity and other AhR-mediated events. Th is study contributes to such investigations by utilizing the AhR antagonist 3 ' -methoxy-4 ' -nitroflavone (3 ' M4 ' NF) and a dioxin-responsive lacZ transgenic mouse model to characterize antagonism of the receptor system in vivo. The dose-response and time course of TCDD-induced transgene activati on were evaluated in transgenic mice to provide information necessary to de sign 3 ' M4 ' NF in vivo studies. TCDD induction of the transgene was noted as early as 8 h after exposure in the lung. 3-mug/kg body weight TCDD was the lowest dose found to induce the reporter transgene. Finally, experiment s were performed to evaluate the in vivo efficacy of 3 ' M4 ' NF. We found that 3 ' M4 ' NF inhibits TCDD-mediated reporter gene activation and CYP1A1 induction in vivo. Based on these findings, it is clear that DRF-lacZ anim als and the antagonist 3 ' M4 ' NF represent important tools which will hel p in the identification of tissues where AhR is active, and to further char acterize AhR-mediated signaling.