Aberrant or a sustained activation of

Aberrant or a sustained activation of AhR signaling pathway, triggered by exogenous toxic ligands such as dioxins, PAHs or polychlorinated biphenyls (PCBs), is a well-described biological process engaged during tumor initiation and/or promotion in various tissues (e.g. liver, lung, skin)(reviewed in (Kolluri et al., 2017; Mimura and Fujii-Kuriyama, 2003; Murray et al., 2014)). Studies reporting effects of chronic TCDD exposure in Harlan Sprague-Dawley rats clearly identified lung tissue as a target organ for dioxin and dioxin-like compounds, which induce various pathologies, including cancer development (Sells et al., 2007). Some toxic AhR ligands are mutagenic compounds acting as genotoxins, via formation of DNA adducts and oxidative stress (e.g. benzo[a]pyrene – BaP), while others seem to act primarily as tumor promoters, which do not cause a direct DNA damage. This second group of AhR ligands may deregulate cellular homeostasis via direct interference with processes and signaling pathways orchestrating the balance among cell survival, apoptosis and cell fate determination (as documented e.g. for TCDD). Therefore, among the most important aspects of their toxic effects could be those forcing the exposed cells to turn on cellular program(s) necessary for cellular adaptation to acute stress, which results from severe and acute disruption of endogenous AhR functions. The majority of endogenous AhR ligands described so far, are transient products of e.g. tryptophan and indole metabolism, originating from endogenous metabolic sources, diet or microbiota (for review see e.g. (Bock, 2013; Tian et al., 2015)). A typical feature of these compounds is their limited stability, since they are rapidly metabolized, which limits investigation of their effects to first several hours post exposure. In lung tissue, 2-(1H-indole-3-carbonyl)-thiazole-4-carboxylic price scd australia methyl ester (ITE) has been identified as an endogenous AhR ligand that efficiently competes with TCDD for binding to AhR of human, mouse and fish origin (Song et al., 2002). When acute effects (up to 4 h) of ITE and TCDD on mouse lung fibroblast transcriptome were analyzed, ITE elicited similar changes in global gene expression profile as TCDD (Henry et al., 2010). A number of synthetic AhR antagonists have also been reported. Among them, CH223191 is a potent AhR antagonist that prevents binding of TCDD to AhR and unlike other compounds previously reported to be AhR antagonists (e.g. ɑ-naphthoflavone or 3′-methoxy-4′-nitroflavone), CH223191 does not display, even at higher concentrations, any AhR agonist-like activity (Kim et al., 2006). Its antagonistic character has been shown to be both ligand- and species-selective, being particularly active against the TCDD-induced AhR activation (Zhao et al., 2010). Nevertheless, recent studies have reported the ability of CH223191 to antagonize also 6-formylindolo(3,2-b)carbazole (FICZ)- induced AhR activation (Lowe et al., 2014; Stanford et al., 2016; Yin et al., 2016). Given the potential role of AhR in the lung carcinogenesis and considering its overexpression observed in human lung adenocarcinomas, we aimed to analyze global gene expression profiles in a well-annotated in vitro model of human A549 lung adenocarcinoma cells, following their acute exposure (6 h) to toxic AhR agonist, TCDD, and to synthetic AhR antagonist, CH223191, either alone or in combination. Established gene expression profiles were then compared together with previously reported list of peaks bound by AhR antibody and identified in MCF7 cells after their exposure to TCDD (Yang et al., 2017). Similarly, global gene expression profile detected in CH223191-treated A549 cells was compared to that reported in MCF7 cells, where AhR levels were downregulated by specific siRNA molecules, in order to identify genes regulated by “basal” AhR activity (Salisbury et al., 2014). All these steps enabled us to focus on the central theme of this study: identification of specific cellular candidates (genes, pathways and transcription factors), which expression and/or activation status is directly changed alongside with changes in AhR signaling in model of lung cancer cells.