Aryl hydrocarbon receptor (AHR) functions in infectious and sterile inflammation and NAD+-dependent metabolic adaptation.

Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of various physiologic AHR functions. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to exert cellular stress-mediated sterile inflammatory responses in exposed human tissues but may be lethal in sensitive species. Inflammation can be thought of as the extreme end of a spectrum ranging from homeostasis to stress responses (sterile inflammation) and to defense against infection (infectious inflammation). Defense against bacterial infection by generation of reactive oxygen species has to be strictly controlled and may use up a considerable amount of energy. NAD+-mediated energy metabolism adapts to various inflammatory responses. As examples, the present commentary tries to integrate responses of AHR and NAD+-consuming enzymes (PARP7/TiPARP, CD38 and sirtuins) into infectious and stress-induced inflammatory responses, the latter exemplified by nonalcoholic fatty liver disease (NAFLD). TCDD toxicity models in sensitive species provide hints to molecular AHR targets of energy metabolism including gluconeogenesis and glycolysis. AHR research remains challenging and promising.

Aryl hydrocarbon receptor (AHR), integrating energy metabolism and microbial or obesity-mediated inflammation.

Aryl hydrocarbon receptor (AHR) has been characterized as multifunctional sensor, integrator and ligand-activated transcription factor of the bHLH/PAS family. Regulation of inflammatory diseases and energy metabolism are among the putative functions of AHR. Challenges in AHR research include marked species differences, and cell, tissue and context dependence of AHR functions. The commentary is focused on AHR's role in the integration between energy expenditure and microbial and non-infectious inflammation, the latter exemplified by obesity-mediated nonalcoholic fatty liver disease. One of the mechanisms controlling energy-consuming inflammation is represented by a signalsome that is involved in retinoic acid-triggered neutrophil differentiation and regulation of the NADPH oxidase complex (NOX). Established signalsome components are AHR, CD38, multiple protein kinases and adaptors. To prevent chronic inflammatory diseases, the complex interplay between a range of inflammatory responses and energy expenditure must be precisely regulated. Surviving an infection requires both pathogen clearance and tissue protection from inflammatory damage. Defenses are energy-consuming anabolic programs. Therefore, anti-inflammatory, catabolic tolerance programs by metabolic reprogramming of macrophages have evolved. Therapeutic options of AHR agonists to reduce chronic inflammatory diseases are discussed.

Aryl hydrocarbon receptor (AHR)-mediated inflammation and resolution: Non-genomic and genomic signaling.

Inflammation, an old medical problem, is being recognized as an active, well orchestrated biological process. When dysregulated, chronic inflammation may ensue, leading to tissue-dependent diseases. Depending upon the ligand and cellular context, aryl hydrocarbon receptor (AHR) may accelerate or attenuate inflammation and subsequent resolution. Three examples are discussed in which AHR modulates inflammation by a mixture of genomic and non-genomic signaling pathways: (i) AHR-agonistic bacterial virulence factors are leading to both microbial defense and resolution of inflammatory responses. (ii) TCDD-mediated persistent AHR activation initially leads to inflammation by non-genomic signaling, and may potentially lead to chronic inflammation. (iii) AHR may modulate anti-inflammatory actions in obesity-mediated non-alcoholic fatty liver disease (NAFLD): Hepatic lipotoxicity triggers generation of danger-associated molecular patterns (DAMPs) that facilitate the development of hepatitis. AHR is mainly involved in the resolution phase by induction of lipoxin A4 and Il-22. Moderate AHR activation by phytochemicals and microbial AHR ligands may facilitate resolution. In control of inflammation, AHR appears to integrate environmental conditions with coordinated cellular functions.

Aryl hydrocarbon receptor (AHR) functions: Balancing opposing processes including inflammatory reactions.

Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of physiologic AHR functions. Control of AHR functions is challenged by the fact that AHR is often involved in balancing opposing processes. Two AHR functions are discussed. (i) Microbial defense: intestinal microbiota commensals secrete AHR ligands that are important for maintaining epithelial integrity and generation of anti-inflammatory IL-22 by multiple immune cells. On the other hand, in case of microbial defense, AHR-regulated neutrophils and Th17 cells are involved in generation of bactericidal reactive oxygen species and pro-inflammatory stimuli. However, during the process of infection resolution, 'disease tolerance' is achieved. (ii) Energy, NAD+ and lipid metabolism: In obese individuals AHR is involved in either generation or inhibition of fatty liver and associated hepatitis. Inhibition of hepatitis is mainly achieved by regulating NAD+-controlled SIRT1, 3 and 6 activity. Interestingly, these enzymes are synergistically modulated by CD38, an NAD-consuming NAD-glycohydrolase. It is proposed that inflammatory responses may be beneficially modulated by AHR agonistic and CD38 inhibiting phytochemicals. Caveats in presence of carcinogenicity have to be taken into account. AHR research is an exciting field but therapeutic options remain challenging.

Modulation of aryl hydrocarbon receptor (AHR) and the NAD+-consuming enzyme CD38: Searches of therapeutic options for nonalcoholic fatty liver disease (NAFLD).

The aryl hydrocarbon receptor (AHR) has been characterized as multifunctional, ligand-activated transcription factor. Recently, evidence has been obtained that AHR is involved in NAD+ and energy homeostasis in cooperation with NAD+-consuming enzymes including CD38, TiPARP and sirtuins. AHR and CD38 may adversely or beneficially modulate nonalcoholic fatty liver disease (NAFLD) which is associated with obesity, a worldwide major health problem. Although nutritional status and lifestyle are the major factors involved in the prevalence of obesity and NAFLD, modulation of AHR and CD38 has been demonstrated to provide therapeutic options. For example, inhibition of hepatic CD38 and activation of AHR, e.g., by dietary flavonoids may beneficially affect NAFLD. In addition, NAFLD-associated decrease of NAD+ may be restored by administration of the NAD+ precursor nicotinamide riboside.

Functions of aryl hydrocarbon receptor (AHR) and CD38 in NAD metabolism and nonalcoholic steatohepatitis (NASH).

Aryl hydrocarbon receptor (AHR), identified in studies of dioxin toxicity, has been characterized as ligand-activated transcription factor involved in diverse functions including microbial defense, cell proliferation, immunity and NAD metabolism. AHR targets of the latter function are PARPs/ARTs and CD38 that are regulating glucose and lipid metabolism via NAD-dependent sirtuins. Deregulation of these pathways may facilitate obesity and age-dependent pathologies. The present commentary is focused on AHR and CD38 signaling in liver. CD38 is functioning as ectoNADase and Ca2+ mobilizing enzyme in endoplasmic reticulum and endolysosomal membranes. Deregulation of TCDD-activated AHR and CD38 may facilitate hepatic steatosis and inflammation. However, these proteins are also involved in protection against inflammation and CD38-mediated age-related decreased NAD levels that may be responsible for neurodegeneration. Further knowledge about the complexity of these pathways is needed to avoid pathologies. Therapeutic modulation of AHR and CD38 remains a challenging task.

Aryl hydrocarbon receptor (AHR): From selected human target genes and crosstalk with transcription factors to multiple AHR functions.

Accumulating evidence including studies of AHR-deficient mice and TCDD toxicity suggests multiple physiologic AHR functions. Challenges to identify responsible mechanisms are due to marked species differences and dependence upon cell type and cellular context. Transient AHR modulation is often necessary for physiologic functions whereas TCDD-mediated sustained receptor activation has been demonstrated to be responsible for toxic outcomes. To stimulate studies on responsible action mechanisms the commentary is focused on human AHR target genes and crosstalk with transcription factors. Discussed AHR functions include chemical and microbial defense, organ development, modulation of immunity and inflammation, reproduction, and NAD+-dependent energy metabolism. Obviously, much more work is needed to elucidate action mechanisms. In particular, studies of pathways leading to NAD+-dependent energy metabolism may shed light on the puzzling species differences of TCDD-mediated lethality and provide options for treatment of obesity and age-related degenerative diseases.

Aryl hydrocarbon receptor (AHR) functions in NAD+ metabolism, myelopoiesis and obesity.

Diverse physiologic functions of AHR, a transcription factor discovered in studies of dioxin toxicity, are currently elucidated in many laboratories including chemical and microbial defense, immunity and myelopoiesis. Accumulating evidence suggests that AHR may also be involved in obesity and TCDD-mediated lethality in sensitive species. Underlying mechanisms include NAD+- and sirtuin-mediated deregulation of lipid, glucose and NAD+ homeostasis. Progress in NAD metabolome research suggests large consumption of NAD+ by NAD glycohydrolases (NADases) and NAD-dependent sirtuins. In focus are two NADases: (i) TiPARP (TCDD-induced poly(ADP-ribose) polymerase), one of several nuclear NADases, and (ii) plasma membrane-bound ectoNADase/CD38, a multifunctional enzyme and receptor. CD38 is involved in extra- and intracellular NAD degradation but acts also as differentiation marker. Both CD38 and AHR are components of a complex signalsome that enhances retinoic acid-induced differentiation of myeloid progenitor cells to granulocytes. Further advances of NAD metabolome research may lead to therapeutic options in the control of obesity and to improved risk assessment of TCDD toxicity.

Human AHR functions in vascular tissue: Pro- and anti-inflammatory responses of AHR agonists in atherosclerosis.

Despite decades of intense research physiologic aryl hydrocarbon receptor (AHR) functions have not been elucidated. Challenges include marked species differences and dependence on cell type and cellular context. A previous commentary on human AHR functions in skin and intestine has been extended to vascular tissue. Similar functions appear to be operating in vascular tissue including microbial defense, modulation of stem/progenitor cells as well as control of immunity and inflammation. However, AHR functions are Janus faced: Detrimental AHR functions in vascular tissue are well documented, e.g., upon exposure to polycyclic aromatic hydrocarbons in cigarette smoke leading to oxidative stress and generation of oxidized LDL. Modified LDL particles accumulate in macrophages and smooth muscle-derived pro-inflammatory foam cells, the hallmark of atherosclerosis. On the other hand, numerous anti-inflammatory AHR agonists have been identified including bilirubin and quercetin. Mechanisms as to how AHR produces pro- and anti-inflammatory responses in the vascular system need further investigation.

From TCDD-mediated toxicity to searches of physiologic AHR functions.

TCDD-mediated toxicity of human individuals together with animal studies led to identification of the aryl hydrocarbon receptor (AHR). It was characterized as multifunctional ligand-activated transcription factor and environmental sensor. Comparison of human toxic responses and animal models provide hints to physiologic AHR functions including chemical and microbial defense, homeostasis of stem/progenitor cells and modulation of the immune system in barrier organs such as skin and the gastrointestinal tract. Extrapolation from animals to humans is difficult due to marked species differences and dependence of AHR function on the cellular context. Nevertheless, therapeutic possibilities of AHR agonists and antagonists are in development. The AHR remains challenging and fascinating.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-mediated deregulation of myeloid and sebaceous gland stem/progenitor cell homeostasis.

Studies of TCDD toxicity stimulated identification of the responsible aryl hydrocarbon receptor (AHR), a multifunctional, ligand-activated transcription factor of the basic helix-loop-helix/Per-Arnt-Sim family. Accumulating evidence suggests a role of this receptor in homeostasis of stem/progenitor cells, in addition to its known role in xenobiotic metabolism. (1) Regulation of myelopoiesis is complex. As one example, AHR-mediated downregulation of human CD34+ progenitor differentiation to monocytes/macrophages is discussed. (2) Accumulation of TCDD in sebum leads to deregulation of sebocyte differentiation via Blimp1-mediated inhibition of c-Myc signaling and stimulation of Wnt-mediated proliferation of interfollicular epidermis. The resulting sebaceous gland atrophy and formation of dermal cysts may explain the pathogenesis of chloracne, the hallmark of TCDD toxicity. (3) TCDD treatment of confluent liver stem cell-like rat WB-F344 cells leads to release from cell-cell contact inhibition via AHR-mediated crosstalk with multiple signaling pathways. Further work is needed to delineate AHR function in crosstalk with other signaling pathways.

From dioxin toxicity to putative physiologic functions of the human Ah receptor in homeostasis of stem/progenitor cells.

Despite decades of intensive research physiologic Ah receptor (AHR) functions are not yet elucidated. Challenges include marked species differences and dependence of AHR function on the cell type and cellular context. Hints to physiologic functions may be derived (i) from feedback loops between endogenous ligands and substrates of major target enzymes such as CYP1A1 and UGT1A1, and (ii) from dioxin toxicity in human individuals. For example, dioxin-mediated chloracne is probably due to dysregulated homeostasis of sebocyte stem/progenitor cells. Dioxin-mediated inflammatory responses may be due to complex dysregulation of hematopoiesis. Comparison of AHR functions with those of PXR and its target enzyme CYP3A4 may be helpful to emphasize AHR functions in specialized cells: PXR is known to be mainly involved in regulation of systemic metabolism of endo- and xenobiotics. However, AHR may be mostly controlling local homeostasis of signals in specialized cells such as stem/progenitor cells. Accumulating evidence suggests that knowledge about physiologic AHR functions may stimulate drug development.

Human and rodent aryl hydrocarbon receptor (AHR): from mediator of dioxin toxicity to physiologic AHR functions and therapeutic options.

Metabolism of aryl hydrocarbons and toxicity of dioxins led to the discovery of the aryl hydrocarbon receptor (AHR). Tremendous advances have been made on multiplicity of AHR signaling and identification of endogenous ligands including the tryptophan metabolites FICZ and kynurenine. However, human AHR functions are still poorly understood due to marked species differences as well as cell-type- and cell context-dependent AHR functions. Observations in dioxin-poisoned individuals may provide hints to physiologic AHR functions in humans. Based on these observations three human AHR functions are discussed: (1) Chemical defence and homeostasis of endobiotics. The AHR variant Val381 in modern humans leads to reduced AHR affinity to aryl hydrocarbons in comparison with Neanderthals and primates expressing the Ala381 variant while affinity to indoles remains unimpaired. (2) Homeostasis of stem/progenitor cells. Dioxins dysregulate homeostasis in sebocyte stem cells. (3) Modulation of immunity. In addition to microbial defence, AHR may be involved in a 'disease tolerance defence pathway'. Further characterization of physiologic AHR functions may lead to therapeutic options.

Toward elucidation of dioxin-mediated chloracne and Ah receptor functions.

Target cells and molecular targets responsible for dioxin-mediated chloracne, the hallmark of dioxin toxicity, are reviewed. The dioxin TCDD accumulates in sebum, and thereby persistently activates the Ah receptor (AhR), expressed in bipotential stem/progenitor cells of the sebaceous gland. AhR operates in cooperation with other transcription factors including c-Myc, Blimp1 and ß-Catenin/TCF: c-Myc stimulates exit of stem cells from quiescence to proliferating sebocyte progenitors; Blimp1 is a major c-Myc repressor, and ß-Catenin/TCF represses sebaceous gland differentiation and stimulates differentiation to interfollicular epidermis. TCDD has been demonstrated to induce Blimp1 expression in the sebocyte stem/progenitor cell line SZ95, leading to sebocyte apoptosis and proliferation of interfollicular epidermis cells. These findings explain observations in TCDD-poisoned individuals, and identify target cells and molecular targets of dioxin-mediated chloracne. They clearly demonstrate that the AhR operates in a cell context-dependent manner, and provide hints to homeostatic functions of AhR in stem/progenitor cells.

The UDP-glycosyltransferase (UGT) superfamily expressed in humans, insects and plants: Animal-plant arms-race and co-evolution.

UDP-glycosyltransferases (UGTs) are major phase II enzymes of a detoxification system evolved in all kingdoms of life. Lipophilic endobiotics such as hormones and xenobiotics including phytoalexins and drugs are conjugated by vertebrates mainly with glucuronic acid, by invertebrates and plants mainly with glucose. Plant-herbivore arms-race has been the major driving force for evolution of large UGT and other enzyme superfamilies. The UGT superfamily is defined by a common protein structure and signature sequence of 44 amino acids responsible for binding the UDP moiety of the sugar donor. Plants developed toxic phytoalexins stored as glucosides. Upon herbivore attack these conjugates are converted to highly reactive compounds. In turn, animals developed large families of UGTs in their intestine and liver to detoxify these phytoalexins. Interestingly, phytoalexins, exemplified by quercetin glucuronides and glucosinolate-derived isocyanates, are known insect attractant pigments in plants, and antioxidants, anti-inflammatory and chemopreventive compounds of humans. It is to be anticipated that phytochemicals may provide a rich source in beneficial drugs.

Roles of human UDP-glucuronosyltransferases in clearance and homeostasis of endogenous substrates, and functional implications.

Human UDP-glucuronosyltransferases (UGTs) are major phase II enzymes in the drug metabolism system. Despite major advances in characterization of UGT gene family members, their role in clearance and homeostasis of endogenous substrates is insufficiently understood. Endobiotic substrates including bilirubin, serotonin, eicosanoids, steroid hormones, bile acids, thyroxine and fat-soluble vitamins A and D are discussed. Species- and tissue/cell-dependent regulation of UGT expression by ligand-activated transcription factors is often involved in endobiotic homeostasis. However, roles of particular UGTs are often difficult to delineate since they function together with other enzymes and transporters. Better knowledge of endobiotic UGT substrates and consequences of their conjugation may help to understand evolutionary conserved UGT functions.

Homeostatic control of xeno- and endobiotics in the drug-metabolizing enzyme system.

Drug-metabolizing Phase I and II enzyme families, drug transporters (Phase III) and their ligand-activated transcription factors probably evolved as a system involved in homeostatic control of lipophilic endobiotics and detoxification of xenobiotics. The review is focused on CYP, UGT enzymes and the Ah receptor as transcription factor. The hypothesis of a system is supported by (i) coordinate regulation of subsets of these enzyme families and transporters by transcription factors including the AhR, and (ii) feedback loops between endobiotic AhR agonists and substrates of major catabolic target genes/proteins; for example, 6-formylindolo[3,2-b]carbazole as substrate of CYP1A1, and bilirubin, as substrate of UGT1A1. In the latter case the AhR is one of multiple transcription factors contributing to bilirubin homeostasis. Interestingly, xenobiotics including dietary phytochemicals, products of microbiota, ubiquitous environmental pollutants such as benzo[a]pyrene may also have shaped this system in intestinal epithelia during millions of years of evolution. Most lipophilic drugs are metabolized by the same system since drug-metabolizing enzymes are broad substrate spectrum enzymes. Better understanding of this system may lead to generation of drugs with desirable therapeutic properties.

The human Ah receptor: hints from dioxin toxicities to deregulated target genes and physiological functions.

Marked species differences of dioxin toxicity prompted the review of three well-studied human dioxin toxicities (chloracne, inflammation and cancer) and deregulated Ah receptor (AhR) target genes to obtain hints as to the physiological functions of this receptor. Dioxin here stands for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Microarray analysis of dermal cysts from a dioxin-poisoned patient revealed, in addition to induced CYP1A1, increased expression of gremlin, an antagonist of bone morphogenetic proteins. Dioxin-mediated skin and intestinal inflammation is associated with deregulated T cell differentiation. In the supernatant of CD4+ T cells obtained from the dioxin-poisoned patient, increased interleukin-22 was detected, a cytokine that may be controlled in part by AhR-regulated Notch. Cancer is one of the long-term consequences of chronic inflammation. In line with dioxin-sensitive lymphoid tissue, enhanced death of lymphoid cancer was observed in the dioxin-exposed Seveso population 25 years after poisoning. Accumulating evidence suggests that endogenous AhR ligands, notably the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole, in contrast to TCDD, is rapidly metabolized by AhR-induced CYP1A1. The feedback loop between 6-formylindolo[3,2-b]carbazole, AhR and CYP1A1 guarantees transient activation that, in contrast to sustained activation by TCDD, may be essential for a putative role of the AhR in stem/progenitor cell homeostasis.

Human UDP-glucuronosyltransferases: feedback loops between substrates and ligands of their transcription factors.

Expression profiles of human adult and fetal hepatic and intestinal UDP-glucuronosyltransferases (UGTs), information about their endo- and xenobiotic substrates, and their transcriptional regulation suggests regulatory circuits between some UGT substrates and ligands of their transcription factors. For examples: (i) bilirubin is solely conjugated by UGT1A1 and activates its transcription factors Ah receptor, PXR and CAR. (ii) Hepatotoxic lithocholic acid (LCA) is oxidized to hyodeoxycholic acid, the latter conjugated by UGT2B4 and UGT2B7. LCA is also an agonist of FXR and PPARα, which are controlling these UGTs. (iii) Similar feedback loops possibly exist between some eicosanoids, PPARα and UGTs. (iv) Regulatory circuits may also have evolved between dietary polyphenols, which are efficient substrates of UGTs and activators of the Ah receptor. Although many newly developed drugs are conjugated by promiscuous UGTs, the discussed regulatory circuits may provide hints to evolutionary important UGT substrates.

Ah receptor- and Nrf2-gene battery members: modulators of quinone-mediated oxidative and endoplasmic reticulum stress.

Quinones are ubiquitously present in mammals and their environment. They are involved in physiologic functions such as electron transport but are also toxic compounds. In particular, quinone-quinol redox cycles may lead to oxidative stress, and arylating quinones have been demonstrated to activate endoplasmic reticulum (ER) stress. To detoxify quinones coordinately regulated Ah receptor and Nrf2 gene batteries evolved. Two pathways are emphasized: (i) glutathione S-transferases, and (ii) NAD(P)H:quinone oxidoreductases NQO1 and NQO2 acting together with UDP-glucuronosyltransferases and sulfotransferases. Coupling between these enzymes may prevent oxidative and ER stress in a tissue-dependent manner, as discussed using benzo[a]pyrene detoxification in enterocytes, catecholestrogen metabolism in breast tissue and endometrium, and aminochromes in neurones and astrocytes. Possible consequences of chronic ER stress such as apoptosis and inflammation as well as therapeutic possibilities of modulating Ah receptor and Nrf2 are discussed. In conclusion, tight coupling of Ah receptor- and Nrf2-regulated enzymes may prevent quinone-mediated oxidative and ER stress.