The Role of the Aryl Hydrocarbon Receptor (AHR) in Immune and Inflammatory Diseases

Immunotoxicity of petroleum hydrocarbons and microplastics alone or in combination to a bivalve species: Synergic impacts and potential toxication mechanisms

Both the frequent occurrence of accidental petroleum spills and the ubiquitous presence of microplastics (MPs) in the sea may pose severe threats to marine species. However, the immunotoxic impacts of these two types of pollutants and the underlying toxication mechanisms still remain largely unknown in sessile filter-feeding bivalve mollusks. Therefore, the impacts of exposure to petroleum hydrocarbons and MPs alone or in combination on the total count, cell type composition, and phagocytic activity of hemocytes were investigated in the blood clam, Tegillarca granosa. In addition, the intracellular ROS content, cell viability, degree of DNA damage, and expression levels of genes from immune-, apoptosis-, and immunotoxicity-related pathways were analyzed to reveal the potential toxication mechanisms. The results demonstrated that exposure to petroleum hydrocarbons and MPs alone or in combination at environmentally realistic concentrations could exert significant immunotoxic impacts on the blood clam, which may be caused by alterations in a series of physiological and molecular processes. In addition, the immunotoxicity of petroleum hydrocarbons could be significantly aggravated by the copresence of MPs, which suggests that coexposure to these two pollutants deserves closer attention.

Aryl Hydrocarbon Receptor Connects Inflammation to Breast Cancer

Aryl hydrocarbon receptor (AhR), an evolutionary conserved transcription factor, is a pleiotropic signal transductor. Thanks to its promiscuous ligand binding domain, during the evolution of eukaryotic cells its developmental functions were integrated with biosensor functions. Its activation by a multitude of endogenous and exogenous molecules stimulates its participation in several pathways, some of which are linked to inflammation and breast cancer (BC). Over time, the study of this malignancy has led to the identification of several therapeutic targets in cancer cells. An intense area of study is dedicated to BC phenotypes lacking adequate targets. In this context, due to its high constitutive activation in BC, AhR is currently gaining more and more attention. In this review, I have considered its interactions with: 1. the immune system, whose dysregulation is a renowned cancer hallmark; 2. interleukin 6 (IL6) which is a pivotal inflammatory marker and is closely correlated to breast cancer risk; 3. NF-kB, another evolutionary conserved transcription factor, which plays a key role in immunoregulatory functions, inflammatory response and breast carcinogenesis; 4. kynurenine, a tryptophan-derived ligand that activates and bridges AhR to chronic inflammation and breast carcinogenesis. Overall, the data here presented form an interesting framework where AhR is an interesting connector between inflammation and BC.

Modeling Approaches Reveal New Regulatory Networks in Aspergillus fumigatus Metabolism

Systems biology approaches are extensively used to model and reverse-engineer gene regulatory networks from experimental data. Indoleamine 2,3-dioxygenases (IDOs)-belonging in the heme dioxygenase family-degrade l-tryptophan to kynurenines. These enzymes are also responsible for the de novo synthesis of nicotinamide adenine dinucleotide (NAD+). As such, they are expressed by a variety of species, including fungi. Interestingly, Aspergillus may degrade l-tryptophan not only via IDO but also via alternative pathways. Deciphering the molecular interactions regulating tryptophan metabolism is particularly critical for novel drug target discovery designed to control pathogen determinants in invasive infections. Using continuous time Bayesian networks over a time-course gene expression dataset, we inferred the global regulatory network controlling l-tryptophan metabolism. The method unravels a possible novel approach to target fungal virulence factors during infection. Furthermore, this study represents the first application of continuous-time Bayesian networks as a gene network reconstruction method in Aspergillus metabolism. The experiment showed that the applied computational approach may improve the understanding of metabolic networks over traditional pathways.

Health Effects of Coffee: Mechanism Unraveled?

The association of habitual coffee consumption with a lower risk of diseases, like type 2 diabetes mellitus, chronic liver disease, certain cancer types, or with reduced all-cause mortality, has been confirmed in prospective cohort studies in many regions of the world. The molecular mechanism is still unresolved. The radical-scavenging and anti-inflammatory activity of coffee constituents is too weak to account for such effects. We argue here that coffee as a plant food has similar beneficial properties to many vegetables and fruits. Recent studies have identified a health promoting mechanism common to coffee, vegetables and fruits, i.e., the activation of an adaptive cellular response characterized by the upregulation of proteins involved in cell protection, notably antioxidant, detoxifying and repair enzymes. Key to this response is the activation of the Nrf2 (Nuclear factor erythroid 2-related factor-2) system by phenolic phytochemicals, which induces the expression of cell defense genes. Coffee plays a dominant role in that regard because it is the major dietary source of phenolic acids and polyphenols in the developed world. A possible supportive action may be the modulation of the gut microbiota by non-digested prebiotic constituents of coffee, but the available data are still scarce. We conclude that coffee employs similar pathways of promoting health as assumed for other vegetables and fruits. Coffee beans may be viewed as healthy vegetable food and a main supplier of dietary phenolic phytochemicals.

Functional genomics based on germline genome-wide association studies of endocrine therapy for breast cancer

Breast cancer is the most common invasive cancer in women worldwide. Functional follow-up of breast cancer genome-wide association studies has led to the discovery of genes that regulate endocrine therapy response in a SNP- and drug-dependent manner. Here, we will present four examples in which functional genomic studies from breast cancer clinical trials led to novel pharmacogenomic insights and molecular mechanisms of selective estrogen receptor modulators and aromatase inhibitors. The approach utilized for studying genetic variability described in this review offers substantial potential for meaningful discoveries that move the field toward precision medicine for patients.

Generation and application of a Tg(cyp1a:egfp) transgenic marine medaka (Oryzias melastigma) line as an in vivo assay to sensitively detect dioxin-like compounds in the environment

Large-range environmental pollution by dioxin and dioxin-like compounds (DLCs) is becoming a serious problem. To establish an in vivo method for the detection of DLCs in seawater, a Tg(cyp1a-12DRE:egfp) transgenic marine medaka (Oryzias melastigma) line was first developed with the modified cyp1a-12DRE promoter driving enhanced green fluorescent protein (EGFP) expression using Tol2 transgenesis technology. With increasing concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), the EGFP fluorescence intensity increased significantly. The Tg(cyp1a-12DRE:egfp) medaka possessed high sensitivity (limit of detection of 1 ng/L TCDD) and specificity and low background. This transgenic line is capable of detecting DLCs in environmental seawater in which the concentration of DLCs is at least 0.12207 ng/L TCDD after sample enrichment. The fluorescence-toxic equivalency (TEQ) values from EGFP intensity were closely correlated with the chemical-TEQ values obtained from chemical analyses. Furthermore, the Tg(cyp1a-12DRE:egfp) medaka can directly detect DLCs in seawater samples after a serious pollution accident and screen unknown aryl hydrocarbon receptor (AhR) agonists for risk assessment. For the first time, a convenient method has been established that sensitively and specifically responds to DLCs using the Tg(cyp1a-12DRE:egfp) marine medaka, which could be a highly efficient tool for detecting seawater DLCs in the future.

The role of the microbiome and psychosocial stress in the expression and activity of drug metabolizing enzymes in mice

The gut microbiota is involved in a number of different metabolic processes of the host organism, including the metabolism of xenobiotics. In our study, we focused on liver cytochromes P450 (CYPs), which can metabolize a wide range of exo- and endogenous molecules. We studied changes in mRNA expression and CYP enzyme activities, as well as the mRNA expression of transcription factors that have an important role in CYP expression, all in stressed germ-free (GF) and stressed specific-pathogen-free (SPF) mice. Besides the presence of the gut microbiota, we looked at the difference between acute and chronic stress. Our results show that stress has an impact on CYP mRNA expression, but it is mainly chronic stress that has a significant effect on enzyme activities along with the gut microbiome. In acutely stressed mice, we observed significant changes at the mRNA level, however, the corresponding enzyme activities were not influenced. Our study suggests an important role of the gut microbiota along with chronic psychosocial stress in the expression and activity of CYPs, which can potentially lead to less effective drug metabolism and, as a result, a harmful impact on the organism.

Does NLRP3 Inflammasome and Aryl Hydrocarbon Receptor Play an Interlinked Role in Bowel Inflammation and Colitis-Associated Colorectal Cancer?

Inflammation is a hallmark in many forms of cancer; with colitis-associated colorectal cancer (CAC) being a progressive intestinal inflammation due to inflammatory bowel disease (IBD). While this is an exemplification of the negatives of inflammation, it is just as crucial to have some degree of the inflammatory process to maintain a healthy immune system. A pivotal component in the maintenance of such intestinal homeostasis is the innate immunity component, inflammasomes. Inflammasomes are large, cytosolic protein complexes formed following stimulation of microbial and stress signals that lead to the expression of pro-inflammatory cytokines. The NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome has been extensively studied in part due to its strong association with colitis and CAC. The aryl hydrocarbon receptor (AhR) has recently been acknowledged for its connection to the immune system aside from its role as an environmental sensor. AhR has been described to play a role in the inhibition of the NLRP3 inflammasome activation pathway. This review will summarise the signalling pathways of both the NLRP3 inflammasome and AhR; as well as new-found links between these two signalling pathways in intestinal immunity and some potential therapeutic agents that have been found to take advantage of this link in the treatment of colitis and CAC.

Particulate Matter Toxicity Is Nrf2 and Mitochondria Dependent: The Roles of Metals and Polycyclic Aromatic Hydrocarbons

Particulate matter (PM), an important component of air pollution, induces significant adverse health effects. Many of the observed health effects caused by inhaled PM are associated with oxidative stress and inflammation. This association has been linked in particular to the particles’ chemical components, especially the inorganic/metal and the organic/polycyclic aromatic hydrocarbon (PAH) fractions, and their ability to generate reactive oxygen species in biological systems. The transcription factor NF-E2 nuclear factor erythroid-related factor 2 (Nrf2) is activated by redox imbalance and regulates the expression of phase II detoxifying enzymes. Nrf2 plays a key role in preventing PM-induced toxicity by protecting against oxidative damage and inflammation. This review focuses on specific PM fractions, particularly the dissolved metals and PAH fractions, and their roles in inducing oxidative stress and inflammation in cell and animal models with respect to Nrf2 and mitochondria.

Ozone-Induced Aryl Hydrocarbon Receptor Activation Controls Lung Inflammation via Interleukin-22 Modulation

Airborne ozone exposure causes severe lung injury and inflammation. The aryl hydrocarbon Receptor (AhR) (1), activated in pollutant-induced inflammation, is critical for cytokine production, especially IL-22 and IL-17A. The role of AhR in ozone-induced lung inflammation is unknown. We report here that chronic ozone exposure activates AhR with increased tryptophan and lipoxin A4 production in mice. AhR-/- mice show increased lung inflammation, airway hyperresponsiveness, and tissue remodeling with an increased recruitment of IL-17A and IL-22-expressing cells in comparison to control mice. IL-17A- and IL-22-neutralizing antibodies attenuate lung inflammation in AhR-/- and control mice. Enhanced lung inflammation and recruitment of ILC3, ILC2, and T cells were observed after T cell-specific AhR depletion using the AhRCD4cre-deficient mice. Together, the data demonstrate that ozone exposure activates AhR, which controls lung inflammation, airway hyperresponsiveness, and tissue remodeling via the reduction of IL-22 expression.

Strategies to Dissect Host-Microbial Immune Interactions That Determine Mucosal Homeostasis vs. Intestinal Inflammation in Gnotobiotic Mice

When identifying the key immunologic-microbial interactions leading to either mucosal homeostasis in normal hosts or intestinal inflammatory responses in genetically susceptible individuals, it is important to not only identify microbial community correlations but to also define the functional pathways involved. Gnotobiotic rodents are a very effective tool for this purpose as they provide a highly controlled environment in which to identify the function of complex intestinal microbiota, their individual components, and metabolic products. Herein we review specific strategies using gnotobiotic mice to functionally evaluate the role of various intestinal microbiota in host responses. These studies include basic comparisons between host responses in germ-free (GF), specific-pathogen-free or conventionally raised wild-type mice or those with underlying genetic susceptibilities to intestinal inflammation. We also discuss what can be learned from studies in which GF mice are colonized with single wild-type or genetically-modified microbial isolates to examine the functions of individual bacteria and their targeted bacterial genes, or colonized by multiple defined isolates to determine interactions between members of defined consortia. Additionally, we discuss studies to identify functions of complex microbial communities from healthy or diseased human or murine hosts via fecal transplant into GF mice. Finally, we conclude by suggesting ways to improve studies of immune-microbial interactions using gnotobiotic mice.

Bilirubin as a signaling molecule

For long time bilirubin was only considered as a potentially dangerous sign of liver diseases, but it now appears clear that it is also a powerful signaling molecule. Together with potent antioxidant activities that were only reported in the last few decades, many other biological effects have now been clearly described. These include especially profound inhibitory effects on almost all effectors of the immune system, with their clinical consequences in the bilirubin-mediated protection against autoimmune and inflammatory diseases. Separate from these, bilirubin activates various nuclear and cytoplasmic receptors, resembling the endocrine activities of actual hormonal substances. This is true for the "classical" hepatic nuclear receptors, including the aryl hydrocarbon receptor, or the constitutive androstane receptor; and also for some lesser-explored receptors such as peroxisome proliferator-activated receptors α and γ; Mas-related G protein-coupled receptor; or other signaling molecules including fatty acid binding protein 1, apolipoprotein D, or reactive oxygen species. All of these targets have broad metabolic effects, which in turn may offer protection against obesity, diabetes mellitus, and other metabolic diseases. The (mostly experimental) data are also supported by clinical evidence. In fact, data from the last three decades have convincingly demonstrated the protective effects of mildly elevated serum bilirubin concentrations against various "diseases of civilization." Additionally, even tiny, micromolar changes of serum bilirubin concentrations have been associated with substantial alteration in the risks of these diseases. It is highly likely that all of the biological activities of bilirubin have yet to be exhaustively explored, and thus we can expect further clinical discoveries about this evolutionarily old molecule into the future.

Aberrant DNA methylation profile exacerbates inflammation and neurodegeneration in multiple sclerosis patients

Multiple sclerosis (MS) is an autoimmune and demyelinating disease of the central nervous system characterised by incoordination, sensory loss, weakness, changes in bladder capacity and bowel function, fatigue and cognitive impairment, creating a significant socioeconomic burden. The pathogenesis of MS involves both genetic susceptibility and exposure to distinct environmental risk factors. The gene x environment interaction is regulated by epigenetic mechanisms. Epigenetics refers to a complex system that modifies gene expression without altering the DNA sequence. The most studied epigenetic mechanism is DNA methylation. This epigenetic mark participates in distinct MS pathophysiological processes, including blood-brain barrier breakdown, inflammatory response, demyelination, remyelination failure and neurodegeneration. In this study, we also accurately summarised a list of environmental factors involved in the MS pathogenesis and its clinical course. A literature search was conducted using MEDLINE through PubMED and Scopus. In conclusion, an exhaustive study of DNA methylation might contribute towards new pharmacological interventions in MS by use of epigenetic drugs.

AhR and IDO1 in pathogenesis of Covid-19 and the "Systemic AhR Activation Syndrome:" a translational review and therapeutic perspectives

Covid-19 is the acute illness caused by SARS-CoV-2 with initial clinical symptoms such as cough, fever, malaise, headache, and anosmia. After entry into cells, corona viruses (CoV) activate aryl hydrocarbon receptors (AhRs) by an indoleamine 2,3-dioxygenase (IDO1)-independent mechanism, bypassing the IDO1-kynurenine-AhR pathway. The IDO1-kynurenine-AhR signaling pathway is used by multiple viral, microbial and parasitic pathogens to activate AhRs and to establish infections. AhRs enhance their own activity through an IDO1-AhR-IDO1 positive feedback loop prolonging activation induced by pathogens. Direct activation of AhRs by CoV induces immediate and simultaneous up-regulation of diverse AhR-dependent downstream effectors, and this, in turn, results in a "Systemic AhR Activation Syndrome" (SAAS) consisting of inflammation, thromboembolism, and fibrosis, culminating in multiple organ injuries, and death. Activation of AhRs by CoV may lead to diverse sets of phenotypic disease pictures depending on time after infection, overall state of health, hormonal balance, age, gender, comorbidities, but also diet and environmental factors modulating AhRs. We hypothesize that elimination of factors known to up-regulate AhRs, or implementation of measures known to down-regulate AhRs, should decrease severity of infection. Although therapies selectively down-regulating both AhR and IDO1 are currently lacking, medications in clinical use such as dexamethasone may down-regulate both AhR and IDO1 genes, as calcitriol/vitamin D3 may down-regulate the AhR gene, and tocopherol/vitamin E may down-regulate the IDO1 gene. Supplementation of calcitriol should therefore be subjected to epidemiological studies and tested in prospective trials for prevention of CoV infections, as should tocopherol, whereas dexamethasone could be tried in interventional trials. Because lack of physical exercise activates AhRs via the IDO1-kynurenine-AhR signaling pathway increasing risk of infection, physical exercise should be encouraged during quarantines and stay-at-home orders during pandemic outbreaks. Understanding which factors affect gene expression of both AhR and IDO1 may help in designing therapies to prevent and treat humans suffering from Covid-19.

Aryl Hydrocarbon Receptor in Cutaneous Vascular Endothelial Cells Restricts Psoriasis Development by Negatively Regulating Neutrophil Recruitment

Vascular endothelial cells (VECs) that line the interiors of blood vessels participate in physiological and inflammatory processes. All skin cell types express the aryl hydrocarbon receptor (AhR), which is involved in the pathogenesis of psoriasis. However, the role of the cutaneous VEC AhR in the pathogenesis of psoriasis remains elusive. In the present study, we found that AhR protein expression and activation were downregulated in psoriatic VECs. Furthermore, cutaneous VEC-specific AhR-knockout (AhR^cVECs-KO) mice were established. Using imiquimod and IL-23-induced psoriasis models, we found that skin inflammation was exacerbated with excessive neutrophil recruitment in AhR^cVECs-KO mice. Furthermore, neutrophil neutralization alleviates exacerbated inflammation in imiquimod-treated AhR^cVECs-KO mice. In addition, cutaneous VECs in AhR^cVECs-KO mice exhibited increased dilation and activation compared with those in control mice. Finally, AhR-deficient microvascular endothelial cells stimulated by proinflammatory cytokines showed increased ICAM-1 expression in vivo and in vitro, which may have facilitated neutrophil recruitment. In summary, our study demonstrates that AhR in dermal VECs restricts psoriasis development by negatively regulating neutrophil recruitment, thereby providing insight into the pathogenesis of psoriasis.

Changes in Deoxyribonucleic Acid Methylation Contribute to the Pathophysiology of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by loss of coordination, weakness, dysfunctions in bladder capacity, bowel movement, and cognitive impairment. Thus, the disease leads to a significant socioeconomic burden. In the pathophysiology of the disease, both genetic and environmental risk factors are involved. Gene x environment interaction is modulated by epigenetic mechanisms. Epigenetics refers to a sophisticated system that regulates gene expression with no changes in the DNA sequence. The most studied epigenetic mechanism is the DNA methylation. In this review, we summarize the data available from the current literature by grouping sets of differentially methylated genes in distinct biological categories: the immune system including innate and adaptive response, the DNA damage, and the central nervous system.

Multiple Sclerosis: Melatonin, Orexin, and Ceramide Interact with Platelet Activation Coagulation Factors and Gut-Microbiome-Derived Butyrate in the Circadian Dysregulation of Mitochondria in Glia and Immune Cells

Recent data highlight the important roles of the gut microbiome, gut permeability, and alterations in mitochondria functioning in the pathophysiology of multiple sclerosis (MS). This article reviews such data, indicating two important aspects of alterations in the gut in the modulation of mitochondria: (1) Gut permeability increases toll-like receptor (TLR) activators, viz circulating lipopolysaccharide (LPS), and exosomal high-mobility group box (HMGB)1. LPS and HMGB1 increase inducible nitric oxide synthase and superoxide, leading to peroxynitrite-driven acidic sphingomyelinase and ceramide. Ceramide is a major driver of MS pathophysiology via its impacts on glia mitochondria functioning; (2) Gut dysbiosis lowers production of the short-chain fatty acid, butyrate. Butyrate is a significant positive regulator of mitochondrial function, as well as suppressing the levels and effects of ceramide. Ceramide acts to suppress the circadian optimizers of mitochondria functioning, viz daytime orexin and night-time melatonin. Orexin, melatonin, and butyrate increase mitochondria oxidative phosphorylation partly via the disinhibition of the pyruvate dehydrogenase complex, leading to an increase in acetyl-coenzyme A (CoA). Acetyl-CoA is a necessary co-substrate for activation of the mitochondria melatonergic pathway, allowing melatonin to optimize mitochondrial function. Data would indicate that gut-driven alterations in ceramide and mitochondrial function, particularly in glia and immune cells, underpin MS pathophysiology. Aryl hydrocarbon receptor (AhR) activators, such as stress-induced kynurenine and air pollutants, may interact with the mitochondrial melatonergic pathway via AhR-induced cytochrome P450 (CYP)1b1, which backward converts melatonin to N-acetylserotonin (NAS). The loss of mitochnodria melatonin coupled with increased NAS has implications for altered mitochondrial function in many cell types that are relevant to MS pathophysiology. NAS is increased in secondary progressive MS, indicating a role for changes in the mitochondria melatonergic pathway in the progression of MS symptomatology. This provides a framework for the integration of diverse bodies of data on MS pathophysiology, with a number of readily applicable treatment interventions, including the utilization of sodium butyrate.

Targeting Aryl hydrocarbon receptor for next-generation immunotherapies: Selective modulators (SAhRMs) versus rapidly metabolized ligands (RMAhRLs)

Aryl Hydrocarbon Receptor (AhR) constitutes a major network hub of genomic and non-genomic signaling pathways, connecting host's immune cells to environmental factors. It shapes innate and adaptive immune processes to environmental stimuli with species-, cell- and tissue-type dependent specificity. Although an ever increasing number of studies has thrust AhR into the limelight as attractive target for the development of next-generation immunotherapies, concerns exist on potential safety issues associated with small molecule modulation of the receptor. Selective AhR modulators (SAhRMs) and rapidly metabolized AhR ligands (RMAhRLs) are two classes of receptor agonists that are emerging as interesting lead compounds to bypass AhR-related toxicity in favor of therapeutic effects. In this article, we discuss SAhRMs and RMAhRLs reported in literature, covering concepts underlying their definitions, specific binding modes, structure-activity relationships and AhR-mediated functions.

bHLH-PAS Proteins: Their Structure and Intrinsic Disorder

The basic helix–loop–helix/Per-ARNT-SIM (bHLH–PAS) proteins are a class of transcriptional regulators, commonly occurring in living organisms and highly conserved among vertebrates and invertebrates. These proteins exhibit a relatively well-conserved domain structure: the bHLH domain located at the N-terminus, followed by PAS-A and PAS-B domains. In contrast, their C-terminal fragments present significant variability in their primary structure and are unique for individual proteins. C-termini were shown to be responsible for the specific modulation of protein action. In this review, we present the current state of knowledge, based on NMR and X-ray analysis, concerning the structural properties of bHLH–PAS proteins. It is worth noting that all determined structures comprise only selected domains (bHLH and/or PAS). At the same time, substantial parts of proteins, comprising their long C-termini, have not been structurally characterized to date. Interestingly, these regions appear to be intrinsically disordered (IDRs) and are still a challenge to research. We aim to emphasize the significance of IDRs for the flexibility and function of bHLH–PAS proteins. Finally, we propose modern NMR methods for the structural characterization of the IDRs of bHLH–PAS proteins.

Single Nucleotide Polymorphisms at a Distance from Aryl Hydrocarbon Receptor (AHR) Binding Sites Influence AHR Ligand-Dependent Gene Expression

Greater than 90% of significant genome-wide association study (GWAS) single-nucleotide polymorphisms (SNPs) are in noncoding regions of the genome, but only 25.6% are known expression quantitative trait loci (eQTLs). Therefore, the function of many significant GWAS SNPs remains unclear. We have identified a novel type of eQTL for which SNPs distant from ligand-activated transcription factor (TF) binding sites can alter target gene expression in a SNP genotype-by-ligand–dependent fashion that we refer to as pharmacogenomic eQTLs (PGx-eQTLs)—loci that may have important pharmacotherapeutic implications. In the present study, we integrated chromatin immunoprecipitation-seq with RNA-seq and SNP genotype data for a panel of lymphoblastoid cell lines to identify 10 novel cis PGx-eQTLs dependent on the ligand-activated TF aryl hydrocarbon receptor (AHR)—a critical environmental sensor for xenobiotic (drug) and immune response. Those 10 cis PGx-eQTLs were eQTLs only after AHR ligand treatment, even though the SNPs did not create/destroy an AHR response element—the DNA sequence motif recognized and bound by AHR. Additional functional studies in multiple cell lines demonstrated that some cis PGx-eQTLs are functional in multiple cell types, whereas others displayed SNP-by-ligand–dependent effects in just one cell type. Furthermore, four of those cis PGx-eQTLs had previously been associated with clinical phenotypes, indicating that those loci might have the potential to inform clinical decisions. Therefore, SNPs across the genome that are distant from TF binding sites for ligand-activated TFs might function as PGx-eQTLs and, as a result, might have important clinical implications for interindividual variation in drug response.