2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental contaminant that produces adverse biological effects including carcinogenesis, reproductive toxicity, immune dysfunction, hyperkeratosis, hepatotoxicity, thymic involution, and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. Glucocorticoids are endogenous steroid compounds that have an important role in development, but are teratogenic at pharmacological doses. The synthetic glucocorticoid, hydrocortisone (HC), induces cleft palate and a potent, synergistic interaction has been observed between TCDD and HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor, the Ah receptor (AhR) and the glucocorticoid receptor (GR), respectively. The morphology and etiology of TCDD and HC-induced clefts are distinctly different, as HC clefting is due to formation of small palatal shelves, while TCDD-treated shelves fail to fuse due to effects on epithelial cell proliferation and differentiation. The present study examines the expression of AhR and GR in the embryonic palate following exposure to TCDD, HC, and HC + TCDD. C57BL/6N pregnant mice were treated with HC (25 or 100 mg/kg/day GD10-13, sc), TCDD (3 μg/kg/day GD10-13, or 24 μg/kg GD10, orally), or HC + TCDD (25 mg/kg/day sc and 3 μg/kg/day orally, GD10-13). Craniofacial tissues were collected from the embryos on GD14 and examined for AhR and GR expression using in situ hybridization, Northern blots, and immunohistochemistry. We found that in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC + TCDD produced increased expression of both receptors. Effects on AhR appeared to be regulated at the transcriptional level, as both protein and mRNA were altered in similar directions. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.
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