AhR, ARNT, and CYP1A1 mRNA quantitation in cultured human embryonic palates exposed to TCDD and comparison with mouse palate in vivo and in culture

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is developmentally toxic in many species and induces cleft palate in the C57BL/6N mouse embryo. Palatogenes is in mouse and human embryos involves homologous processes at the morpholo gical, cellular, and molecular levels. In organ culture, mouse and human pa lates respond similarly to TCDD. The present study quantitates the expressi on of AhR, ARNT, and CYP1A1 mRNA in human embryonic palates in organ cultur e. Palatal tissues were exposed to 1 x 10(-10), 1 x 10(-9), or 1 x 10(-8) M TCDD or control medium and sampled at 0, 2, 4, and 6 hours for quantitativ e RT-PCR using a synthetic RNA internal standard. Similar measurements of C YP1A1 gene expression were collected for mouse palates cultured in this mod el. In human palates, AhR expression correlated with ARNT and CYP1A1 mRNA e xpression. TCDD induction of CYP1A1 was time- and concentration-dependent. The expression of these genes presented a uniform and continuous distributi on across the group of embryos, with no subset of either high or low expres sors/responders. The ratio of AhR to ARNT was approximately 4:1. AhR mRNA i ncreased during the culture period in both treated and control subjects; ho wever, ARNT expression was relatively constant. TCDD did not alter either A hR or ARNT expression in a consistent dose- or time-related manner. Compari son of human and mouse data showed a high correlation across species for th e induction of CYP1A1. Human embryos expressed approximately 350 times less AhR mRNA than the mouse, and in earlier studies it was shown that human pa lates required 200 times more TCDD to produce the same effects. When the mo rphological, cellular, and molecular responses to TCDD between mouse and hu man are compared, it seems highly unlikely that human embryos could be expo sed to sufficient TCDD to achieve changes in palatal differentiation that w ould lead to cleft palate.