Activation of the aryl hydrocarbon receptor by dioxin during embryonic stem cell differentiation disrupts the expression of homeobox transcription factors that control cardiomyogenesis. Mus musculus strain:C57BL/6J

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that regulates the expression of xenobiotic detoxification genes and is a critical mediator of gene-environment interactions. In addition, many AHR target genes that have been identified by genome-wide profiling have morphogenetic functions, suggesting that AHR activation may play a role in embryonic development. To address this hypothesis, we studied the consequences of AHR activation by TCDD, its prototypical ligand, during spontaneous mouse ES cell differentiation into contractile cardiomyocytes. Treatment with TCDD or shRNA-mediated AHR knockdown significantly decreased the ability of cardiomyocytes to contract and the expression of cardiac markers in these cells. An AHR-positive embryonic stem cell lineage was generated that expressed puromycin resistance and eGFP under the control of the AHR-responsive Cyp1a1 promoter. Cells of this lineage were over 90% pure and expressed AHR as well as cardiomyocyte markers. Analysis of temporal trajectories of global gene expression in these cells shows that activation of the AHR/TCDD axis disrupts the concerted expression of genes that regulate multiple signaling pathways involved in cardiac and neural morphogenesis and differentiation, including dozens of genes encoding homeobox transcription factors and Polycomb and Trithorax Group genes. More than 50% of the homeobox factors so regulated do not have AhRE sites in their promoters, indicating that AHR activation may establish a complex regulatory network that reaches beyond direct AHR signaling and is capable of disrupting various aspects of embryonic development, including cardiomyocyte differentiation.

芳香烃受体（aryl hydrocarbon receptor, AHR）是一类配体激活的转录因子，可调控异生物质解毒基因的表达，同时是基因-环境互作的关键介导因子。目前已通过全基因组分析（genome-wide profiling）鉴定出诸多AHR靶基因，这类基因多具备形态发生功能，提示AHR激活可能在胚胎发育过程中发挥作用。为验证这一假说，我们针对其经典配体TCDD激活AHR的效应展开研究，观察其在小鼠胚胎干细胞自发分化为收缩型心肌细胞过程中的影响。经TCDD处理或短发夹RNA（short hairpin RNA, shRNA）介导的AHR敲低后，心肌细胞的收缩能力及该类细胞中心肌标志物的表达水平均显著降低。我们构建了一株AHR阳性的胚胎干细胞谱系，该谱系在AHR应答性Cyp1a1启动子的调控下，表达嘌呤霉素抗性基因与增强型绿色荧光蛋白（enhanced green fluorescent protein, eGFP）。该谱系的细胞纯度超过90%，同时表达AHR与心肌细胞标志物。对上述细胞的全局基因表达时序轨迹进行分析后发现，AHR/TCDD信号轴的激活会干扰一系列协同表达的基因，这些基因参与调控心脏与神经形态发生及分化的多条信号通路，其中包含数十种编码同源框转录因子的基因，以及多梳蛋白家族（Polycomb Group, PcG）与三胸蛋白家族（Trithorax Group, TrxG）基因。在这类受调控的同源框因子中，超过50%的基因启动子区域不含有芳香烃受体应答元件（aryl hydrocarbon receptor response element, AhRE），这表明AHR激活可能构建出一套复杂的调控网络，其作用范围远超直接的AHR信号传导，能够干扰胚胎发育的多个环节，包括心肌细胞分化。