Activation of the aryl hydrocarbon receptor by dioxin during embryonic stem cell differentiation disrupts the expression of homeobox transcription factors that control cardiomyogenesis
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. Overall design: mRNA profiles of WT and selected AHR positive cells at different differentiation days treated with and without TCDD in duplicates