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BackgroundDespite their large numbers and widespread use, very little is known about the extent to which per- and polyfluoroalkyl substances (PFAS) can cross the placenta and expose the developing fetus.ObjectiveThe aim of our study is to develop a computational approach that can be used to evaluate the of extend to which small molecules, and in particular PFAS, can cross to cross the placenta and partition to cord blood.MethodsWe collected experimental values of the concentration ratio between cord and maternal blood (RCM) for 260 chemical compounds and calculated their physicochemical descriptors using the cheminformatics package Mordred. We used the compiled database to, train and test an artificial neural network (ANN). And then applied the best performing model to predict RCM for a large dataset of PFAS chemicals (n = 7982). We, finally, examined the calculated physicochemical descriptors of the chemicals to identify which properties correlated significantly with RCM.ResultsWe determined that 7855 compounds were within the applicability domain and 127 compounds are outside the applicability domain of our model. Our predictions of RCM for PFAS suggested that 3623 compounds had a log RCM > 0 indicating preferable partitioning to cord blood. Some examples of these compounds were bisphenol AF, 2,2-bis(4-aminophenyl)hexafluoropropane, and nonafluoro-tert-butyl 3-methylbutyrate.SignificanceThese observations have important public health implications as many PFAS have been shown to interfere with fetal development. In addition, as these compounds are highly persistent and many of them can readily cross the placenta, they are expected to remain in the population for a long time as they are being passed from parent to offspring.ImpactUnderstanding the behavior of chemicals in the human body during pregnancy is critical in preventing harmful exposures during critical periods of development. Many chemicals can cross the placenta and expose the fetus, however, the mechanism by which this transport occurs is not well understood. In our study, we developed a machine learning model that describes the transplacental transfer of chemicals as a function of their physicochemical properties. The model was then used to make predictions for a set of 7982 per- and polyfluorinated alkyl substances that are listed on EPA's CompTox Chemicals Dashboard. The model can be applied to make predictions for other chemical categories of interest, such as plasticizers and pesticides. Accurate predictions of RCM can help scientists and regulators to prioritize chemicals that have the potential to cause harm by exposing the fetus.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the biological and toxicological effects of an AhR lacking the entire PASB structurally diverse chemicals, including halogenated aromatic hydrocarbons. Ligand-dependent transformation of the AhR into its DNA binding form involves a ligand-dependent conformational change, heat shock protein 90 (hsp90), dissociation from the AhR complex and AhR dimerization with the AhR nuclear translocator (ARNT) protein. The mechanism of AhR transformation was examined using mutational approaches and stabilization of the AhR:hsp90 complex with sodium molybdate. Insertion of a single mutation (F281A) in the hsp90-binding region of the AhR resulted in its constitutive (ligand-independent) transformation/DNA binding in vitro. Mutations of AhR residues within the Arg-Cys-rich region (R212A, R217A, R219A) and Asp371 (D371A) impaired AhR transformation without a significant effect on ligand binding. Stabilization of AhR:hsp90 binding with sodium molybdate decreased transformation/DNA binding of the wild type AhR but had no effect on constitutively active AhR mutants. Interestingly, transformation of the AhR in the presence of molybdate allowed detection of an intermediate transformation ternary complex containing hsp90, AhR, and ARNT. These results are consistent with a stepwise transformation mechanism in which binding of ARNT to the liganded AhR:hsp90 complex results in a progressive displacement of hsp90 and conversion of the AhR into its high affinity DNA binding form. The available molecular insights into the signaling mechanism of other Per-ARNT-Sim (PAS) domains and structural information on hsp90 association with other client proteins are consistent with the proposed transformation mechanism of the AhR.

BRAF inhibitors target the BRAF-V600E/K mutated kinase, the driver mutation found in 50% of cutaneous melanoma. They give unprecedented anti-tumor responses but acquisition of resistance ultimately limits their clinical benefit. The master regulators driving the expression of resistance-genes remain poorly understood. Here, we demonstrate that the Aryl hydrocarbon Receptor (AhR) transcription factor is constitutively activated in a subset of melanoma cells, promoting the dedifferentiation of melanoma cells and the expression of BRAFi-resistance genes. Typically, under BRAFi pressure, death of BRAFi-sensitive cells leads to an enrichment of a small subpopulation of AhR-activated and BRAFi-persister cells, responsible for relapse. Also, differentiated and BRAFi-sensitive cells can be redirected towards an AhR-dependent resistant program using AhR agonists. We thus identify Resveratrol, a clinically compatible AhR-antagonist that abrogates deleterious AhR sustained-activation. Combined with BRAFi, Resveratrol reduces the number of BRAFi-resistant cells and delays tumor growth. We thus propose AhR-impairment as a strategy to overcome melanoma resistance.

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that modulates gene expression following its binding and activation by structurally diverse chemicals. Species differences in AhR functionality have been observed, with the mouse AhR (mAhR) and human AhR (hAhR) exhibiting significant differences in ligand binding, coactivator recruitment, gene expression and response. While the AhR agonist indirubin (IR) is a more potent activator of hAhR-dependent gene expression than the prototypical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), it is a significantly less potent activator of the mAhR. DNA binding analysis confirmed the greater potency/efficacy of IR in stimulating transformation/DNA binding of the hAhR in vitro and domain-swapping experiments demonstrated that the enhanced response to IR was primarily due to the hAhR ligand binding domain (LBD). Site-directed mutagenesis and functional analysis studies revealed that mutation of H326 and A349 in the mAhR LBD to the corresponding residues in the hAhR LBD significantly increased the potency of IR. Since these mutations had no significant effect on ligand binding, these residues likely contribute to an enhanced efficiency of transformation/DNA binding by IR-bound hAhR. Molecular docking to mAhR LBD homology models further elucidated the different roles of the A375V mutation in TCDD and IR binding, as revealed by [³H]TCDD competitive binding results. These results demonstrate the differential binding of structurally diverse ligands within the LBD of a given AhR and confirm that amino acid differences within the LBD of AhRs contribute to significant species differences in ligand response.

1,4-Dihydroxy-2-naphthoic acid (1,4-DHNA) is a bacterial-derived metabolite that binds the aryl hydrocarbon receptor (AhR) and exhibits anti-inflammatory activity in the gut. The structure-dependent AhR activity of hydroxyl/carboxy-substituted naphthoic acids (NAs) was determined in young adult mouse colonic (YAMC) cells and human Caco2 colon cancer cells using CYP1A1/CYP1B1 mRNAs as Ah-responsive genes. Compounds used in this study include 1,4-, 3,5-, and 3,7-DHNA, 1,4-dimethoxy-2-naphthoic acid (1,4-DMNA), 1- and 4-hydroxy-2-naphthoic acid (1-HNA, 4-HNA), 1- and 2-naphthoic acid (1-NA, 2-NA), and 1- and 2-naphthol (1-NOH, 2-NOH). 1,4-DHNA was the most potent compound among hydroxyl/carboxy naphthalene derivatives, and the fold induction response for CYP1A1 and CYP1B1 was similar to that observed for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in YAMC and Caco2 cells. 1- and 4-HNA were less potent than 1,4-DHNA but induced maximal (TCDD-like) response for CYP1B1 (both cell lines) and CYP1A1 (Caco2 cells). With the exception of 1- and 2-NA, all compounds significantly induced Cyp1b1 in YAMC cells and these responses were not observed in AhR-deficient YAMC cells generated using CRISPR/Cas9 technology. In addition, we also observed that 1- and 2-NOH (and 1,4-DHNA) were weak AhR agonists, and 1- and 2-NOH also exhibited partial AhR antagonist activity. Structure-activity relationship studies for CYP1A1 but not CYP1B1 were similar in both cell lines, and CYP1A1 induction required one or both 1,4-dihydroxy substituents and activity was significantly enhanced by the 2-carboxyl group. We also used computational analysis to show that 1,4-DHNA and TCDD share similar interactions within the AhR binding pocket and differ primarily due to the negatively charged group of 1,4-DHNA.

The toxic effects of dioxins and related compounds (DRCs) are mediated by the aryl hydrocarbon receptor (AHR). Our previous study identified AHR1 and AHR2 genes from the red seabream (Pagrus major). Moreover, we found that AHR2 mRNA levels were notably elevated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in the early life stage of red seabream embryos, while AHR1 mRNA level was not altered. In this study, to investigate the regulatory mechanism of these AHR transcripts, we cloned and characterized 5'-flanking regions of AHR1 and AHR2 genes. Both of the 5'-flanking regions in these AHR genes contained three potential xenobiotic-responsive elements (XREs). To assess whether the 5'-flanking region is transactivated by rsAHR1 and rsAHR2 proteins, we measured the transactivation potency of the luciferase reporter plasmids containing the 5'-flanking regions by AHR1 and AHR2 proteins that were transiently co-expressed in COS-7. Only reporter plasmid (pGL4-rsAHR2-3XREs) that contained three putative XRE sites in the 5'-flanking region of AHR2 gene showed a clear TCDD dose-dependent transactivation by AHR1 and AHR2 proteins. TCDD-EC50 values for the rsAHR2-derived XRE transactivation were 1.3 and 1.4 nM for AHR1 and AHR2, respectively. These results suggest that the putative XREs of AHR2 gene have a function for AHR1- and AHR2-mediated transactivation, supporting our in ovo observation of an induction of AHR2 mRNA levels by TCDD exposure. Mutations in XREs of AHR2 gene led to a decrease in luciferase induction. Electrophoretic mobility shift assay showed that XRE1, the closest XRE from the start codon in AHR2 gene, is mainly responsible for the binding with TCDD-activated AHR. This suggests that TCDD-activated AHR1 and AHR2 up-regulate the AHR2 mRNA levels and this auto-induced AHR2 may amplify the signal transduction of its downstream targets including CYP1A in the red seabream.

The Aryl hydrocarbon Receptor (AhR) is a transcription factor that mediates the biochemical response to xenobiotics and the toxic effects of a number of environmental contaminants, including dioxins. Recently, endogenous regulatory roles for the AhR in normal physiology and development have also been reported, thus extending the interest in understanding its molecular mechanisms of activation. Since dimerization with the AhR Nuclear Translocator (ARNT) protein, occurring through the Helix-Loop-Helix (HLH) and PER-ARNT-SIM (PAS) domains, is needed to convert the AhR into its transcriptionally active form, deciphering the AhR:ARNT dimerization mode would provide insights into the mechanisms of AhR transformation. Here we present homology models of the murine AhR:ARNT PAS domain dimer developed using recently available X-ray structures of other bHLH-PAS protein dimers. Due to the different reciprocal orientation and interaction surfaces in the different template dimers, two alternative models were developed for both the PAS-A and PAS-B dimers and they were characterized by combining a number of computational evaluations. Both well-established hot spot prediction methods and new approaches to analyze individual residue and residue-pairwise contributions to the MM-GBSA binding free energies were adopted to predict residues critical for dimer stabilization. On this basis, a mutagenesis strategy for both the murine AhR and ARNT proteins was designed and ligand-dependent DNA binding ability of the AhR:ARNT heterodimer mutants was evaluated. While functional analysis disfavored the HIF2α:ARNT heterodimer-based PAS-B model, most mutants derived from the CLOCK:BMAL1-based AhR:ARNT dimer models of both the PAS-A and the PAS-B dramatically decreased the levels of DNA binding, suggesting this latter model as the most suitable for describing AhR:ARNT dimerization. These novel results open new research directions focused at elucidating basic molecular mechanisms underlying the functional activity of the AhR.

Small molecular weight protein kinase inhibitors are frequently used tools to unravel the complex network of cellular signal transduction under certain physiological and pathophysiological conditions. 4-amino-5-(4-chlorophenyl)-7-(dimethylethyl)pyrazolo[3,4-d]pyrimidine (PP2) is a widely used compound to block the activity of Src family kinases, the major group of non-receptor tyrosine kinases, which trigger multiple cellular signaling pathways. Here, we show that PP2 induces cytochrome P450 1A1 mRNA expression and enzyme activity in a dose-dependent manner in human HepG2 hepatoma cells and NCTC 2544 keratinocytes. By means of reporter gene assays, RNA interference, electrophoretic mobility shift assay, and competitive ligand-binding assay, we further demonstrate that PP2 is a ligand for the aryl hydrocarbon receptor (AHR), an intracellular chemosensor that regulates xenobiotic metabolism, environmental stress responses, and immune functions. Upon ligand-dependent activation, the AHR translocates into the nucleus and dimerizes with the AHR nuclear translocator (ARNT) to modulate the expression of its target genes. In addition, AHR activation is frequently accompanied by an activation of the tyrosine kinase c-Src, resulting in stimulation of cell-surface receptors and downstream signal transduction. As PP2 activates the AHR/ARNT pathway by simultaneously blocking c-Src-mediated alternative signaling routes, this compound may be a suitable tool to study the contribution of the different AHR-dependent signaling pathways to biological processes and adverse outcomes. On the other hand, the unexpected property of PP2 to stimulate AHR/ARNT signaling should be carefully taken into account in future investigations in order to avoid a false interpretation of experimental results and molecular interrelations.

Malassezia furfur yeast strains isolated from diseased human skin preferentially biosynthesize indole alkaloids which can be detected in the human skin and are highly potent activators of the aryl hydrocarbon receptor (AhR) and AhR-dependent gene expression. Chemical analysis of an EtOAc extract of a M. furfur strain obtained from diseased human skin and grown on l-tryptophan agar revealed several known AhR active tryptophan metabolites along with a previously unidentified compound, pityriazepin. While its structure resembled that of the known alkaloid pityriacitrin, the comprised pyridine ring had been transformed into an azepinone. The indoloazepinone scaffold of pityriazepin is extremely rare in nature and has only been reported once previously. Pityriazepin, like the other isolated compounds, was found to be a potent activator of the AhR-dependent reporter gene assay in recombinant cell lines derived from four different species, although significant species differences in relative potency were observed. The ability of pityriazepin to competitively bind to the AhR and directly stimulate AhR DNA binding classified it as a new naturally-occurring potent AhR agonist. M. furfur produces an expanded collection of extremely potent naturally occurring AhR agonists, which produce their biological effects in a species-specific manner.

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that can be activated by structurally diverse chemicals. To examine the mechanisms responsible for the promiscuity in AhR ligand binding, we determined the effects of mutations within the AhR ligand-binding domain (LBD) on the activity of diverse AhR ligands. Site-directed mutagenesis identified Ile319 of the mouse AhR and, to a lesser extent, Phe318 as residues involved in ligand-selective modulation of AhR transformation using a panel of 12 AhR ligands. These ligands could be categorized into four distinct structurally related groups based on their ability to activate AhR mutants at position 319 in vitro. The mutation I319K was selectively activated by FICZ and not by other examined ligands in vitro and in cell culture. F318L and F318A mutations resulted in the conversion of AhR agonists β-naphthoflavone and 3-methylcholanthrene, respectively, into partial agonists/antagonists. Hsp90 binding to the AhR was decreased with several mutations and was inversely correlated with AhR ligand-binding promiscuity. Together, these data define overlapping amino acid residues within the AhR LBD involved in the selectivity of ligand binding, the agonist or antagonist mode of ligand binding, and hsp90 binding and provide insights into the ligand diversity of AhR activators.

Azole antifungal ketoconazole (KET) was demonstrated to activate aryl hydrocarbon receptor (AhR). Since clinically used KET is a racemic mixture of two cis-enantiomers (2R,4S)-(+)-KET and (2S,4R)-(-)-KET, we examined the effects of KET enantiomers on AhR signaling pathway. (+)-KET dose-dependently activated AhR in human gene reporter cell line AZ-AHR, and displayed 5-20× higher agonist activity (efficacy), as compared to (-)-KET; both enantiomers were AhR antagonists with equal potency (IC50). Consistently, (+)-KET strongly induced CYP1A1 mRNA and protein in human HepG2 cells, while (-)-KET exerted less than 10% of (+)-KET activity. In primary human hepatocytes, both enantiomers preferentially induced CYP1A2 over CYP1A1 mRNA and protein, and the potency of (+)-KET was slightly higher as compared to (-)-KET. Ligand binding assay with guinea pig liver cytosols revealed that both (+)-KET and (-)-KET are weak ligands of AhR that displaced [3H]-TCDD with comparable potency. Similarly, both enantiomers weakly transformed AhR to DNA-binding form with similar potency, as showed by EMSA, in guinea pig liver cytosolic extracts and nuclear extracts from mouse Hepa-1 cells. We also examined effects of KET on glucocorticoid receptor (GR), a regulator of AhR activity. Both KET enantiomers antagonized GR with similar potency, as revealed by gene reporter assay in AZ-GR cell line and down-regulation of tyrosine aminotransferase mRNA in human hepatocytes. Finally, we demonstrate enantiospecific antifungal activities of KET enantiomers in six Candida spp. strains. In conclusion, the significance of current study is providing the first evidence of enatiospecific effects of cis-enantiomers of ketoconazole on AhR-CYP1A pathway.

Abstract Background The plant genus Fallopia is well-known in Chinese traditional medicine and includes many species that contain bioactive compounds, namely phytoestrogens. Consumption of phytoestrogens may be linked to decreased incidence of breast and prostate cancers therefore discovery of novel phytoestrogens and novel sources of phytoestrogens is of interest. Although phytoestrogen content has been analyzed in the rhizomes of various Fallopia sp., seeds of a Fallopia sp. have never been examined for phytoestrogen presence. Methods Analytical chemistry techniques were used with guidance from an in vitro estrogen receptor bioassay (a stably transfected human ovarian carcinoma cell line) to isolate and identify estrogenic components from seeds of Fallopia convolvulus. A transiently transfected human breast carcinoma cell line was used to characterize the biological activity of the isolated compounds on estrogen receptors (ER) α and β. Results Two compounds, emodin and the novel flavan-3-ol, (−)-epiafzelechin-3-O-p-coumarate (rhodoeosein), were identified to be responsible for estrogenic activity of F. convolvulus seed extract. Absolute stereochemistry of rhodoeosein was determined by 1 and 2D NMR, optical rotation and circular dichroism. Emodin was identified by HPLC/DAD, LC/MS/MS, and FT/ICR-MS. When characterizing the ER specificity in biological activity of rhodoeosein and emodin, rhodoeosein was able to exhibit a four-fold greater relative estrogenic potency (REP) in breast cells transiently-transfected with ERβ as compared to those transfected with ERα, and emodin exhibited a six-fold greater REP in ERβ-transfected breast cells. Cell type-specific differences were observed with rhodoeosein but not emodin; rhodoeosein produced superinduction of reporter gene activity in the human ovarian cell line (> 400% of maximum estradiol [E2] induction) but not in the breast cell line. Conclusion This study is the first to characterize the novel flavan-3-ol compound, rhodoeosein, and its ability to induce estrogenic activity in human cell lines. Rhodoeosein and emodin may have potential therapeutic applications as natural products activating ERβ, and further characterization of rhodoeosein is necessary to evaluate its selectivity as a cell type-specific ER agonist.

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that mediates many of the biological and toxicological actions of structurally diverse chemicals. In this study, we examined the ability of a series of ginsenosides extracted from ginseng, a traditional Chinese medicine, to bind to and activate/inhibit the AHR and AHR signal transduction. Utilizing a combination of ligand and DNA binding assays, molecular docking and reporter gene analysis, we demonstrated the ability of selected ginsenosides to directly bind to and activate the guinea pig cytosolic AHR, and to stimulate/inhibit AHR-dependent luciferase gene expression in a recombinant guinea pig cell line. Comparative studies revealed significant species differences in the ability of ginsenosides to stimulate AHR-dependent gene expression in guinea pig, rat, mouse and human cell lines. Not only did selected ginsenosides preferentially activate the AHR from one species and not others, mouse cell line was also significantly less responsive to these chemicals than rat and guinea pig cell lines, but the endogenous gene CYP1A1 could still be inducted in mouse cell line. Overall, the ability of these compounds to stimulate AHR signal transduction demonstrated that these ginsenosides are a new class of naturally occurring AHR agonists.

Comparative structural modeling studies of the ligand binding domains of six additional high-affinity mammalian aryl hydrocarbon receptor (AhR) were performed to detect the 'TCDD binding-fingerprint' of conserved residues within the ligand binding cavity necessary for high-affinity 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binding and TCDD-dependent AhR transformation DNA binding. The results obtained provide better insight into the molecular determinants of TCDD binding, species differences in AhR sensitivity to TCDD and the mechanistic basis for the dramatic diversity in AhR ligand structure.

The structural and functional characterization of the ligand binding domain (LBD) of the aryl hydrocarbon receptor (mAhR) through rational mutational analysis is presented. The results have validated the structural features of the improved homology model of the mAhR Per-Arnt-Sim (PAS) B LBD and have also provided a framework for developing and testing the key events involved in the mechanisms of ligand binding and ligand-dependent activation of the AhR.

1,4-Dihydroxy-2-naphthoic acid (1,4-DHNA) is a bacterial-derivedmetabolite that binds the aryl hydrocarbon receptor (AhR) and exhibits anti-inflammatory activity in the gut. The structure-dependent AhR activity of hydroxyl/carboxy-substituted naphthoic acids (NAs)was determined in young adultmouse colonic (YAMC) cells and human Caco2 colon cancer cells using CYP1A1/CYP1B1mRNAs asAh-responsive genes. Compounds used in this study include 1,4-, 3,5-, and 3,7-DHNA, 1,4-dimethoxy-2-naphthoic acid (1,4-DMNA), 1- and 4-hydroxy-2-naphthoic acid (1-HNA, 4-HNA), 1- and 2-naphthoic acid (1-NA, 2-NA), and 1- and 2-naphthol (1-NOH, 2-NOH). 1,4-DHNAwas themost potent compoundamong hydroxyl/carboxy naphthalene derivatives, and the fold induction response for CYP1A1 and CYP1B1was similar to that observed for 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) in YAMC and Caco2 cells. 1- and 4-HNA were less potent than 1,4-DHNA but inducedmaximal (TCDD-like) response for CYP1B1 (both cell lines) and CYP1A1 (Caco2 cells).With the exception of 1- and 2-NA, all compounds significantly induced Cyp1b1 inYAMCcells and these responseswere not observed in AhR-deficient YAMC cells generated using CRISPR/ Cas9 technology. In addition, we also observed that 1- and 2-NOH(and 1,4-DHNA)wereweakAhR agonists, and 1- and 2-NOH also exhibited partialAhR antagonist activity. Structure-activity relationship studies for CYP1A1 but not CYP1B1were similar in both cell lines, and CYP1A1 induction required one or both 1,4-dihydroxy substituents and activitywas significantly enhanced by the 2-carboxyl group. We also used computational analysis to showthat 1,4-DHNAand TCDDshare similar interactions within the AhR binding pocket and differ primarily due to the negatively charged group of 1,4-DHNA. [ABSTRACT FROM AUTHOR]