Unraveling the differential impact of PAHs and dioxin-like compounds on AKR1C3 reveals the EGFR extracellular domain as a critical determinant of the AHR response.

Polycyclic aromatic hydrocarbons (PAHs), dioxin-like compounds (DLCs) and structurally-related environmental pollutants may contribute to the pathogenesis of various diseases and disorders, primarily by activating the aryl hydrocarbon receptor (AHR) and modulating downstream cellular responses. Accordingly, AHR is considered an attractive molecular target for preventive and therapeutic measures. However, toxicological risk assessment of AHR-modulating compounds as well as drug development is complicated by the fact that different ligands elicit remarkably different AHR responses. By elucidating the differential effects of PAHs and DLCs on aldo-keto reductase 1C3 expression and associated prostaglandin D2 metabolism, we here provide evidence that the epidermal growth factor receptor (EGFR) substantially shapes AHR ligand-induced responses in human epithelial cells, i.e. primary and immortalized keratinocytes and breast cancer cells. Exposure to benzo[a]pyrene (B[a]P) and dioxin-like polychlorinated biphenyl (PCB) 126 resulted in a rapid c-Src-mediated phosphorylation of EGFR. Moreover, both AHR agonists stimulated protein kinase C activity and enhanced the ectodomain shedding of cell surface-bound EGFR ligands. However, only upon B[a]P treatment, this process resulted in an auto-/paracrine activation of EGFR and a subsequent induction of aldo-keto reductase 1C3 and 11-ketoreduction of prostaglandin D2. Receptor binding and internalization assays, docking analyses and mutational amino acid exchange confirmed that DLCs, but not B[a]P, bind to the EGFR extracellular domain, thereby blocking EGFR activation by growth factors. Finally, nanopore long-read RNA-seq revealed hundreds of genes, whose expression is regulated by B[a]P, but not by PCB126, and sensitive towards pharmacological EGFR inhibition. Our data provide novel mechanistic insights into the ligand response of AHR signaling and identify EGFR as an effector of environmental chemicals

Neurodevelopmental toxicity assessment of flame retardants using a human DNT in vitro testing battery

Due to their neurodevelopmental toxicity, flame retardants (FRs) like polybrominated diphenyl ethers are banned from the market and replaced by alternative FRs, like organophosphorus FRs, that have mostly unknown toxicological profiles. To study their neurodevelopmental toxicity, we evaluated the hazard of several FRs including phased-out polybrominated FRs and organophosphorus FRs: 2,2′,4,4′-tetrabromodiphenylether (BDE-47), 2,2′,4,4′,5-pentabromodiphenylether (BDE-99), tetrabromobisphenol A, triphenyl phosphate, tris(2-butoxyethyl) phosphate and its metabolite bis-(2-butoxyethyl) phosphate, isodecyl diphenyl phosphate, triphenyl isopropylated phosphate, tricresyl phosphate, tris(1,3-dichloro-2-propyl) phosphate, tert-butylphenyl diphenyl phosphate, 2-ethylhexyl diphenyl phosphate, tris(1-chloroisopropyl) phosphate, and tris(2-chloroethyl) phosphate. Therefore, we used a human cell–based developmental neurotoxicity (DNT) in vitro battery covering a large variety of neurodevelopmental endpoints. Potency according to the respective most sensitive benchmark concentration (BMC) across the battery ranked from 10 μM range (3 FRs). Evaluation of the data with the ToxPi tool revealed a distinct ranking (a) than with the BMC and (b) compared to the ToxCast data, suggesting that DNT hazard of these FRs is not well predicted by ToxCast assays. Extrapolating the DNT in vitro battery BMCs to human FR exposure via breast milk suggests low risk for individual compounds. However, it raises a potential concern for real-life mixture exposure, especially when different compounds converge through diverse modes-of-action on common endpoints, like oligodendrocyte differentiation in this study. This case study using FRs suggests that human cell–based DNT in vitro battery is a promising approach for neurodevelopmental hazard assessment and compound prioritization in risk assessment. GRAPHICAL ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10565-021-09603-2