Determination of the developmental expression and activation of the aryl hydrocarbon receptor in TCDD-responsive LacZ transgenic mice

The A&barbelow;ryl-h&barbelow;ydrocarbon R&barbelow;eceptor (AhR) is a ligand-dependent transcription factor that mediates the toxicity of halogenated aromatic hydrocarbons including 2,3,7,8-tetra-chlorodibenzo p-dioxin (TCDD). These compounds are potent developmental, reproductive, and immunologic toxicants. This is believed to be dependent on their ability to alter gene expression as well as cellular proliferation and differentiation through AhR signaling. The exact mechanism underlying this toxicity is not known and the most sensitive tissue targets and times during development remain to be identified. These gaps in our understanding have also prevented the identification of specific gene targets in toxicity. It is hypothesized that the AhR serves some normal function in both adult and developing organisms and that disruption of this normal function, by compounds like TCDD, leads to toxicity. The transgenic model presented in this work was developed to delineate the temporal and spatial context of AhR activation through the use of a dioxin-responsive element (DRE)-mediated LacZ reporter. The reporter consists of a synthetic promoter containing two DREs, a minimal TATA box, and the LacZ gene. The transgene was found to be strongly induced by TCDD. Transgenic animals expressing the LacZ construct showed TCDD-dependent β-galactosidase activity in the palate, genital tubercle, toothbud, paws, and other tissues during gestation after TCDD treatment in utero. Activation of the reporter transgene was found to be dose-dependent. The ability of other ligands, such as benzo[a]pyrene and cigarette smoke, to activate the reporter was confirmed. Studies utilizing the AhR antagonist 3′M4 ′NF showed its ability to antagonize TCDD-dependent activation of the reporter. This finding confirmed the AhR-dependence of the transgene and provided a model for the evaluation of endogenous AhR activity. In addition, the model identified the paw as a target of TCDD, and morphological evaluation of in utero TCDD treated animals at birth revealed subtle changes in epithelia and digit organization of this previously unidentified target. These finding emphasize the ability of the model to delineate new target tissues. The identification of such tissues will likely improve our understanding of AhR's involvement in both toxicity and perhaps normal physiology. Such information will improve risk analysis and risk management involving the AhR ligand exposure.