Inducible Loss of the Aryl Hydrocarbon Receptor Activates Perigonadal White Fat Respiration and Brown Fat Thermogenesis via Fibroblast Growth Factor 21

Differential cell type-specific function of the aryl hydrocarbon receptor and its repressor in diet-induced obesity and fibrosis

OBJECTIVE

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor regulating xenobiotic responses as well as physiological metabolism. Dietary AhR ligands activate the AhR signaling axis, whereas AhR activation is negatively regulated by the AhR repressor (AhRR). While AhR-deficient mice are known to be resistant to diet-induced obesity (DIO), the influence of the AhRR on DIO has not been assessed so far.

METHODS

In this study, we analyzed AhRR-/- mice and mice with a conditional deletion of either AhRR or AhR in myeloid cells under conditions of DIO and after supplementation of dietary AhR ligands. Moreover, macrophage metabolism was assessed using Seahorse Mito Stress Test and ROS assays as well as transcriptomic analysis.

RESULTS

We demonstrate that global AhRR deficiency leads to a robust, but not as profound protection from DIO and hepatosteatosis as AhR deficiency. Under conditions of DIO, AhRR-/- mice did not accumulate TCA cycle intermediates in the circulation in contrast to wild-type (WT) mice, indicating protection from metabolic dysfunction. This effect could be mimicked by dietary supplementation of AhR ligands in WT mice. Because of the predominant expression of the AhRR in myeloid cells, AhRR-deficient macrophages were analyzed for changes in metabolism and showed major metabolic alterations regarding oxidative phosphorylation and mitochondrial activity. Unbiased transcriptomic analysis revealed increased expression of genes involved in de novo lipogenesis and mitochondrial biogenesis. Mice with a genetic deficiency of the AhRR in myeloid cells did not show alterations in weight gain after high fat diet (HFD) but demonstrated ameliorated liver damage compared to control mice. Further, deficiency of the AhR in myeloid cells also did not affect weight gain but led to enhanced liver damage and adipose tissue fibrosis compared to controls.

CONCLUSIONS

AhRR-deficient mice are resistant to diet-induced metabolic syndrome. Although conditional ablation of either the AhR or AhRR in myeloid cells did not recapitulate the phenotype of the global knockout, our findings suggest that enhanced AhR signaling in myeloid cells deficient for AhRR protects from diet-induced liver damage and fibrosis, whereas myeloid cell-specific AhR deficiency is detrimental.

Deficiency of Adipose Aryl Hydrocarbon Receptor Protects against Diet-Induced Metabolic Dysfunction through Sexually Dimorphic Mechanisms

The molecular mechanisms underlying diet-induced obesity are complex and remain unclear. The activation of the aryl hydrocarbon receptor (AhR), a xenobiotic sensor, by obesogens may contribute to diet-induced obesity through influences on lipid metabolism and insulin resistance acting at various sites, including adipose tissue. Thus, our hypothesis was that conditional AhR depletion, specifically from mature adipose tissue (CadKO), would improve high-fat diet (HFD)-induced metabolic dysfunction. CadKO protects mice from HFD-induced weight gain. CadKO females eat fewer calories, leading to increased energy expenditure (EE) and improved glucose tolerance on HFD. Our exploration of adipose tissue biology suggests that the depletion of AhR from adipocytes provides female mice with an increased capacity for adipogenesis and lipolysis, allowing for the maintenance of a healthy adipocyte phenotype. The HFD-induced leptin rise was reduced in CadKO females, but the hypothalamic leptin receptor (LepR) was increased in the energy regulatory regions of the hypothalamus, suggesting an increased sensitivity to leptin. The estrogen receptor α (ERα) was higher in CadKO female adipose tissue and the hypothalamus. CadKO males displayed a delayed progression of obesity and insulin resistance. In males, CadKO ameliorated proinflammatory adipocytokine secretion (such as TNFα, IL1β, IL6) and displayed reduced inflammatory macrophage infiltration into adipose depots. Overall, CadKO improves weight control and systemic glucose homeostasis under HFD challenge but to a more profound extent in females. CadKO facilitates a lean phenotype in females and mediates healthy adipose-hypothalamic crosstalk. In males, adipose-specific AhR depletion delays the development of obesity and insulin resistance through the maintenance of healthy crosstalk between adipocytes and immune cells.

The Ah Receptor from Toxicity to Therapeutics: Report from the 5th AHR Meeting at Penn State University, USA, June 2022

The aryl hydrocarbon receptor (AHR) is a sensor of low-molecular-weight molecule signals that originate from environmental exposures, the microbiome, and host metabolism. Building upon initial studies examining anthropogenic chemical exposures, the list of AHR ligands of microbial, diet, and host metabolism origin continues to grow and has provided important clues as to the function of this enigmatic receptor. The AHR has now been shown to be directly involved in numerous biochemical pathways that influence host homeostasis, chronic disease development, and responses to toxic insults. As this field of study has continued to grow, it has become apparent that the AHR is an important novel target for cancer, metabolic diseases, skin conditions, and autoimmune disease. This meeting attempted to cover the scope of basic and applied research being performed to address possible applications of our basic knowledge of this receptor on therapeutic outcomes.

Role of Hepatic Aryl Hydrocarbon Receptor in Non-Alcoholic Fatty Liver Disease

Numerous nuclear receptors including farnesoid X receptor, liver X receptor, peroxisome proliferator-activated receptors, pregnane X receptor, hepatic nuclear factors have been extensively studied within the context of non-alcoholic fatty liver disease (NAFLD). Following the first description of the Aryl hydrocarbon Receptor (AhR) in the 1970s and decades of research which unveiled its role in toxicity and pathophysiological processes, the functional significance of AhR in NAFLD has not been completely decoded. Recently, multiple research groups have utilized a plethora of in vitro and in vivo models that mimic NAFLD pathology to investigate the functional significance of AhR in fatty liver disease. This review provides a comprehensive account of studies describing both the beneficial and possible detrimental role of AhR in NAFLD. A plausible reconciliation for the paradox indicating AhR as a 'double-edged sword' in NAFLD is discussed. Finally, understanding AhR ligands and their signaling in NAFLD will facilitate us to probe AhR as a potential drug target to design innovative therapeutics against NAFLD in the near future.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Obesity I: Overview and molecular and biochemical mechanisms

Obesity is a chronic, relapsing condition characterized by excess body fat. Its prevalence has increased globally since the 1970s, and the number of obese and overweight people is now greater than those underweight. Obesity is a multifactorial condition, and as such, many components contribute to its development and pathogenesis. This is the first of three companion reviews that consider obesity. This review focuses on the genetics, viruses, insulin resistance, inflammation, gut microbiome, and circadian rhythms that promote obesity, along with hormones, growth factors, and organs and tissues that control its development. It shows that the regulation of energy balance (intake vs. expenditure) relies on the interplay of a variety of hormones from adipose tissue, gastrointestinal tract, pancreas, liver, and brain. It details how integrating central neurotransmitters and peripheral metabolic signals (e.g., leptin, insulin, ghrelin, peptide YY_3-36) is essential for controlling energy homeostasis and feeding behavior. It describes the distinct types of adipocytes and how fat cell development is controlled by hormones and growth factors acting via a variety of receptors, including peroxisome proliferator-activated receptor-gamma, retinoid X, insulin, estrogen, androgen, glucocorticoid, thyroid hormone, liver X, constitutive androstane, pregnane X, farnesoid, and aryl hydrocarbon receptors. Finally, it demonstrates that obesity likely has origins in utero. Understanding these biochemical drivers of adiposity and metabolic dysfunction throughout the life cycle lends plausibility and credence to the "obesogen hypothesis" (i.e., the importance of environmental chemicals that disrupt these receptors to promote adiposity or alter metabolism), elucidated more fully in the two companion reviews.

The metabolic regulation of Fuzhuan brick tea in high-fat diet-induced obese mice and the potential contribution of gut microbiota

This study investigated the metabolic effects of Fuzhuan brick tea (FBT) in high-fat diet (HFD)-induced obese mice and the potential contribution of gut microbiota. The results showed that FBT ameliorated the HFD-induced glycerophospholipid metabolic aberrance, specifically increased the serum levels of phosphatidylcholines (PCs), lysophosphatidylcholines (LysoPCs), and the ratio of PC to phosphatidylethanolamines (PE). Besides, FBT increased the serum level of gut microbiota-derived aryl hydrocarbon receptor (AhR) ligand, 3-indole propionic acid, as well as the relative abundance of intestinal AhR-ligand producing bacteria such as Clostridiaceae, Bacteroidales_S24-7_group, and Lactobacillaceae. However, the metabolic benefits of FBT were weakened when the gut microbiota were depleted by antibiotic treatment, thereby suggesting that gut microbiota was required for FBT to regulate glycerophospholipid metabolism. Indeed, the metabolites regulated by FBT were significantly correlated with the AhR-ligand producing bacteria. The KEGG pathway enrichment analysis and expressions of AhR target genes indicated that FBT would improve the glycerophospholipid metabolism via the AhR-Pemt signal axis, in which the gut microbiota and their metabolites played pivotal mediators. Overall, FBT could be a functional beverage to improve HFD-induced metabolic disorders in a gut microbiota dependent manner.

Proteomics and metabolic phenotyping define principal roles for the aryl hydrocarbon receptor in mouse liver

Dioxin-like molecules have been associated with endocrine disruption and liver disease. To better understand aryl hydrocarbon receptor (AHR) biology, metabolic phenotyping and liver proteomics were performed in mice following ligand-activation or whole-body genetic ablation of this receptor. Male wild type (WT) and Ahr^–/– mice (Taconic) were fed a control diet and exposed to 3,3′,4,4′,5-pentachlorobiphenyl (PCB126) (61 nmol/kg by gavage) or vehicle for two weeks. PCB126 increased expression of canonical AHR targets (Cyp1a1 and Cyp1a2) in WT but not Ahr^–/–. Knockouts had increased adiposity with decreased glucose tolerance; smaller livers with increased steatosis and perilipin-2; and paradoxically decreased blood lipids. PCB126 was associated with increased hepatic triglycerides in Ahr^–/–. The liver proteome was impacted more so by Ahr^–/– genotype than ligand-activation, but top gene ontology (GO) processes were similar. The PCB126-associated liver proteome was Ahr-dependent. Ahr principally regulated liver metabolism (e.g., lipids, xenobiotics, organic acids) and bioenergetics, but it also impacted liver endocrine response (e.g., the insulin receptor) and function, including the production of steroids, hepatokines, and pheromone binding proteins. These effects could have been indirectly mediated by interacting transcription factors or microRNAs. The biologic roles of the AHR and its ligands warrant more research in liver metabolic health and disease.

Association of aryl hydrocarbon receptor (AhR) serum level and gene rs10247158 polymorphism with anthropometric, biochemical parameters and food consumption in overweight/obese patients

AIM

Aryl hydrocarbon receptor (AhR) plays a role in xenobiotic metabolism, which can be also activated by dietary patterns and components. AhR ligands in circulation are reported to induce weight gain, glucose intolerance and suggested to contribute to the development of obesity. In this study, we aimed to examine the relationship of the AhR gene and its polymorphisms with obesity and food consumption.

METHODS

The study was conducted with 117 individuals of whom 52 had a body mass index (BMI) of <25 (normal weight) and 65 had a BMI of ≥25 (overweight/obese). The distribution of the serum level and polymorphism (rs10247158) of the participants were determined in venous blood samples using the ELISA and PCR method. Body composition and skinfold thickness of the individuals were measured and their food consumption records were analysed in the BeBiS program.

RESULTS

The serum AhR, HOMA-IR, fasting blood glucose and basal insulin levels were found to be significantly higher (P < .001); however, no relationship was found between AhR polymorphisms in the overweight/obese individuals. In the overweight/obese group, the serum AhR level had a negative correlation with potassium, coffee and alcohol consumption and a positive correlation with suprailiac skinfold thickness. Dietary patterns expected to be related with increased serum AhR levels, such as fat and derivatives, were not observed in overweight/obese group; on the other hand, there was a negative correlation in normal group.

CONCLUSION

In our study, the serum AhR levels of the overweight/obese individuals were found to be significantly higher. Some dietary patterns were determined to be correlated with serum AhR levels in overweight/obese group. However, the results need to be confirmed for ethnic differences and larger samples.

Liver-Specific Nonviral Gene Delivery of Fibroblast Growth Factor 21 Protein Expression in Mice Regulates Body Mass and White/Brown Fat Respiration

Viral-mediated in vivo gene delivery methods currently dominate among therapeutic strategies within the clinical and experimental settings, albeit with well documented limitations arising from immunologic constraints. In this study, we demonstrate the utility of nonviral hepatotropic in vivo gene delivery of unpackaged expression constructs, including one encoding fibroblast growth factor 21 (FGF21). FGF21 is an important hepatokine whose expression positively correlates with therapeutic outcomes across various animal models of obesity. Our data demonstrate that FGF21 expression can be restored into the livers of immunocompetent FGF21 knockout mice for at least 2 weeks after a single injection with an FGF21 expression plasmid. In wild-type C57BL6/J mice, in vivo transfection with an FGF21-expressing plasmid induced weight loss, decreased adiposity, and activated thermogenesis in white fat within 2 weeks. Furthermore, in vivo FGF21 gene delivery protected C57BL6/J mice against diet-induced obesity by decreasing adiposity and increasing uncoupling protein 1-dependent thermogenesis in brown fat and by boosting respiratory capacity in subcutaneous and perigonadal white fat. Together, the data illustrate a facile and effective methodology for delivering prolonged protein expression specifically to the liver. We contend that this method will find utility in basic science research as a practical means to enhance in vivo studies characterizing liver protein function. We further believe our data provide a rationale for further exploring the potential clinical utility of nonviral gene therapy in mouse models of disease. SIGNIFICANCE STATEMENT: This study presents a valuable method for nonviral gene delivery in mice that improves upon existing techniques. The data provide a rationale for further exploring the potential clinical utility of nonviral gene therapy in mouse models of disease and will likely enhance in vivo studies characterizing liver protein function.

Effects of a high-fat diet and global aryl hydrocarbon receptor deficiency on energy balance and liver retinoid status in male Sprague-Dawley rats

The physiological functions of the aryl hydrocarbon receptor (AHR) are only beginning to unfold. Studies in wildtype and AHR knockout (AHRKO) mice have recently disclosed that AHR activity is required for obesity and steatohepatitis to develop when mice are fed with a high-fat diet (HFD). In addition, a line of AHRKO mouse has been reported to accumulate retinoids in the liver. Whether these are universal manifestations across species related to AHR activity level is not known yet. Therefore, we here subjected wildtype and AHRKO male rats (on Sprague-Dawley background) to HFD feeding coupled with free access to 10% sucrose solution and water; controls received a standard diet and water. Although the HFD-fed rats consumed more energy throughout the 24-week feeding regimen, they did not get overweight. However, relative weights of the brown and epididymal adipose tissues were elevated in HFD-fed rats, while that of the liver was lower in AHRKO than wildtype rats. Moreover, the four groups exhibited diet- or genotype-dependent differences in biochemical variables, some of which suggested marked dissimilarities from AHRKO mice. Expression of pro- and anti-inflammatory genes was induced in livers of HFD-fed AHRKO rats, but histologically they did not differ from others. HFD reduced the hepatic concentrations of retinyl palmitate, 9-cis-4-oxo-13,14-dihydroretinoic acid and (suggestively) retinol, whereas AHR status had no effect. Hence, the background strain/line of AHRKO rat is resistant to diet-induced obesity, and AHR does not modulate this or liver retinoid concentrations. Yet, subtle AHR-dependent differences in energy balance-related factors exist despite similar weight development.

Induction of aryl hydrocarbon receptor in granulosa cells by endoplasmic reticulum stress contributes to pathology of polycystic ovary syndrome

Recent studies have uncovered the critical role of aryl hydrocarbon receptor (AHR) in various diseases, including obesity and cancer progression, independent of its previously identified role as a receptor for endocrine-disrupting chemicals (EDCs). We previously showed that endoplasmic reticulum (ER) stress, a newly recognized local factor in the follicular microenvironment, is activated in granulosa cells from patients with polycystic ovary syndrome (PCOS) and a mouse model of the disease. By affecting diverse functions of granulosa cells, ER stress contributes to PCOS pathology. We hypothesized that expression of AHR and activation of its downstream signaling were upregulated by ER stress in granulosa cells, irrespective of the presence of EDCs, thereby promoting PCOS pathogenesis. In this study, we found that AHR, AHR nuclear translocator (ARNT), and AHR target gene cytochrome P450 1B1 (CYP1B1) were upregulated in the granulosa cells of PCOS patients and model mice. We examined CYP1B1 as a representative AHR target gene. AHR and ARNT were upregulated by ER stress in human granulosa-lutein cells (GLCs), resulting in an increase in the expression and activity of CYP1B1. Administration of the AHR antagonist CH223191 to PCOS mice restored estrous cycling and decreased the number of atretic antral follicles, concomitant with downregulation of AHR and CYP1B1 in granulosa cells. Taken together, our findings indicate that AHR activated by ER stress in the follicular microenvironment contributes to PCOS pathology, and that AHR represents a novel therapeutic target for PCOS.

The Aryl Hydrocarbon Receptor in Energy Balance: The Road from Dioxin-Induced Wasting Syndrome to Combating Obesity with Ahr Ligands

The aryl hydrocarbon receptor (AHR) has been studied for over 40 years, yet our understanding of this ligand-activated transcription factor remains incomplete. Each year, novel findings continually force us to rethink the role of the AHR in mammalian biology. The AHR has historically been studied within the context of potent activation via AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with a focus on how the AHR mediates TCDD toxicity. Research has subsequently revealed that the AHR is actively involved in distinct physiological processes ranging from the development of the liver and reproductive organs, to immune system function and wound healing. More recently, the AHR was implicated in the regulation of energy metabolism and is currently being investigated as a potential therapeutic target for obesity. In this review, we re-trace the steps through which the early toxicological studies of TCDD led to the conceptual framework for the AHR as a potential therapeutic target in metabolic disease. We additionally discuss the key discoveries that have been made concerning the role of the AHR in energy metabolism, as well as the current and future directions of the field.

The sulfate metabolite of 3,3'-dichlorobiphenyl (PCB-11) impairs Cyp1a activity and increases hepatic neutral lipids in zebrafish larvae (Danio rerio)

The environmental contaminant 3,3'-dichlorobiphenyl (PCB-11) is widely detected in environmental samples, and this parent compound along with its metabolites 4-OH-PCB-11 and 4-PCB-11-Sulfate are detected in human serum. Our previous research in zebrafish (Danio rerio) embryos shows exposure to 20 μM PCB-11 inhibits Cyp1a enzyme activity and perturbs lipid metabolism pathways. In this study, wildtype AB embryos underwent acute exposures from 1 to 4 days post fertilization (dpf) to 0.002-20 μM 4-OH-PCB-11 or 0.2-20 μM 4-PCB-11-Sulfate, with and without co-exposures to 100 μg/L benzo[a]pyrene (B[a]P) or 5 nM 3,3',4,4',5-pentachlorobiphenyl (PCB-126), and were assessed for in vivo EROD activity and morphometrics. Chronic exposures from 1 to 15 dpf to assess lipid accumulation using Oil-Red-O staining were also conducted with 0.2 μM parent or metabolite compounds, alongside a co-exposure experiment of 0.002-0.2 μM 4-PCB-11-Sulfate and 10 μg/L B[a]P. For acute experiments, 2 and 20 μM 4-OH-PCB-11 was lethal but no Cyp1a or morphological effects were observed at lower concentrations; 20 μM 4-PCB-11-Sulfate significantly lowered the Cyp1a activity of B[a]P and PCB-126 but did not alter morphological development. For chronic experiments, 0.2 μM 4-PCB-11-Sulfate significantly increased lipid accumulation 30% in single exposures and 44% in co-exposures with B[a]P. Further long-term studies would better elucidate the effects of this contaminant, particularly in the context of environmentally-relevant mixtures.

Relationship between FGF21 and drug or nondrug therapy of type 2 diabetes mellitus

Sedentary and high-calorie diets are associated with increased risk of obesity and type 2 diabetes mellitus, while exercise and diet control are also important nondrug treatments for diabetes. Fibroblast growth factor 21 (FGF21) is an important cytokine, which is mainly expressed in liver, fat and muscle tissue responding to nutrition and exercise, and plays an important role in the improvement of glucose and lipid metabolism. Due to the increasing serum FGF21 level in obesity and diabetes, FGF21 can be used as a predictor or biomarker of diabetes. A variety of clinical antidiabetic drugs can reduce the content of FGF21, possibly for the improvement of FGF21 sensitivity. In this paper, we reviewed the interactions between FGF21 and nondrug therapy (diet and exercise) for diabetes and explored the potential value of the combined application of clinical antidiabetic drugs and FGF21.

Skeletal Toxicity of Coplanar Polychlorinated Biphenyl Congener 126 in the Rat Is Aryl Hydrocarbon Receptor Dependent

Epidemiological evidence links polychlorinated biphenyls (PCBs) to skeletal toxicity, however mechanisms whereby PCBs affect bone are poorly studied. In this study, coplanar PCB 126 (5 μmol/kg) or corn oil vehicle was administered to N = 5 and 6 male and female, wild type (WT) or AhR -/- rats via intraperitoneal injection. Animals were sacrificed after 4 weeks. Bone length was measured; bone morphology was assessed by microcomputed tomography and dynamic histomorphometry. Reduced bone length was the only genotype-specific effect and only observed in males (p < .05). WT rats exposed to PCB 126 had reduced serum calcium, and smaller bones with reduced tibial length, cortical area, and medullary area relative to vehicle controls (p < .05). Reduced bone formation rate observed in dynamic histomorphometry was consistent with inhibition of endosteal and periosteal bone growth. The effects of PCB 126 were abolished in AhR -/- rats. Gene expression in bone marrow and shaft were assessed by RNA sequencing. Approximately 75% of the PCB-regulated genes appeared AhR dependent with 89 genes significantly (p < .05) regulated by both PCB 126 and knockout of the AhR gene. Novel targets significantly induced by PCB 126 included Indian hedgehog (Ihh) and connective tissue growth factor (Ctgf/Ccn2), which regulate chondrocyte proliferation and differentiation in the bone growth plate and cell-matrix interactions. These data suggest the toxic effects of PCB 126 on bone are mediated by AhR, which has direct effects on the growth plate and indirect actions related to endocrine disruption. These studies clarify important mechanisms underlying skeletal toxicity of dioxin-like PCBs and highlight potential therapeutic targets.

Reversal of obesity and liver steatosis in mice via inhibition of aryl hydrocarbon receptor and altered gene expression of CYP1B1, PPARα, SCD1, and osteopontin

BACKGROUND/OBJECTIVES

Obesity is a global epidemic and the underlying basis for numerous comorbidities. We report that the aryl hydrocarbon receptor (AHR) plays a key role in the metabolism of obesity. The AHR is a promiscuous, ligand-activated nuclear receptor primarily known for regulating genes involved in xenobiotic metabolism and T cell polarization. The aims of the work reported here were to understand the underlying mechanism of AHR-based obesity and to determine whether inhibition of AHR activity would reverse obesity.

METHODS

Mice were fed control (low fat) and Western (high fat) diets with and without the AHR antagonist alpha-naphthoflavone (aNF). Gene expression of identified AHR-regulated genes from liver and adipose tissue was characterized. To determine the role of the AHR in obesity reversal, selected mice in control and Western diet regimens were switched at midpoint to the respective control and Western diets containing aNF, and the identified AHR-regulated genes characterized.

RESULTS

AHR inhibition prevented obesity in mice on a 40-week diet regimen. The likely AHR-regulated and cross-regulated downstream effectors of AHR-based obesity were shown to be CYP1B1, PPARα-target genes, SCD1, and SPP1 (osteopontin). Western diet caused an increase of mRNA and protein expression of the Cyp1b1, Scd1, and Spp1, and PPARα-target genes in the liver, and inhibition of the AHR maintained expression of these genes near control levels. The body weight of obese mice on Western diet switched to Western diet containing aNF decreased to that of mice on control diet concurrently with a reduction in the expression of liver CYP1B1, PPARα-target genes, SCD1, and SPP1. AHR inhibition prevented hypertrophy and hyperplasia in visceral adipose tissue and limited expression levels of CYP1B1 and SPP1 to that of mice on control diet.

CONCLUSIONS

AHR inhibition prevents and reverses obesity by likely reducing liver expression of the Cyp1b1, Scd1, Spp1, and PPARα-target genes; and the AHR is a potentially potent therapeutic target for the treatment and prevention of obesity and linked diseases.

Kynurenine signaling through the aryl hydrocarbon receptor: Implications for aging and healthspan

The tryptophan metabolite kynurenine increases with aging and inflammation, and appears to contribute directly to the development and progression of several age-related conditions. Kynurenine is now known to signal through the aryl hydrocarbon receptor (Ahr) to modulate levels of reactive oxygen species (ROS). The Ahr promoter region contains several sites for NF-kB binding, indicating that inflammation is a key factor modulating Ahr expression. Furthermore, kynurenine activation of Ahr is observed to stimulate expression of the enzyme IDO1, which generates kynurenine by degrading tryptophan, representing a positive feedback loop that may link inflammation with ROS production. On the other hand, the antioxidant system-inducing transcription factor Nrf2 can be stimulated by Ahr, and Nrf2 can itself activate Ahr expression. The balance between pro- and antioxidant functions of Ahr mediated by kynurenine may therefore regulate healthy versus unhealthy aging in different tissues and organ systems. Potential therapeutic approaches to target this pathway include exercise to alter kynurenine production or molecules such as metformin or resveratrol that may suppress Ahr activity.