Aryl Hydrocarbon Receptor Plays Protective Roles against High Fat Diet (HFD)-induced Hepatic Steatosis and the Subsequent Lipotoxicity via Direct Transcriptional Regulation of Socs3 Gene Expression

Drug-Induced Fatty Liver Disease (DIFLD): A Comprehensive Analysis of Clinical, Biochemical, and Histopathological Data for Mechanisms Identification and Consistency with Current Adverse Outcome Pathways

Drug induced fatty liver disease (DIFLD) is a form of drug-induced liver injury (DILI), which can also be included in the more general metabolic dysfunction-associated steatotic liver disease (MASLD), which specifically refers to the accumulation of fat in the liver unrelated to alcohol intake. A bi-directional relationship between DILI and MASLD is likely to exist: while certain drugs can cause MASLD by acting as pro-steatogenic factors, MASLD may make hepatocytes more vulnerable to drugs. Having a pre-existing MASLD significantly heightens the likelihood of experiencing DILI from certain medications. Thus, the prevalence of steatosis within DILI may be biased by pre-existing MASLD, and it can be concluded that the genuine true incidence of DIFLD in the general population remains unknown. In certain individuals, drug-induced steatosis is often accompanied by concomitant injury mechanisms such as oxidative stress, cell death, and inflammation, which leads to the development of drug-induced steatohepatitis (DISH). DISH is much more severe from the clinical point of view, has worse prognosis and outcome, and resembles MASH (metabolic-associated steatohepatitis), as it is associated with inflammation and sometimes with fibrosis. A literature review of clinical case reports allowed us to examine and evaluate the clinical features of DIFLD and their association with specific drugs, enabling us to propose a classification of DIFLD drugs based on clinical outcomes and pathological severity: Group 1, drugs with low intrinsic toxicity (e.g., ibuprofen, naproxen, acetaminophen, irinotecan, methotrexate, and tamoxifen), but expected to promote/aggravate steatosis in patients with pre-existing MASLD; Group 2, drugs associated with steatosis and only occasionally with steatohepatitis (e.g., amiodarone, valproic acid, and tetracycline); and Group 3, drugs with a great tendency to transit to steatohepatitis and further to fibrosis. Different mechanisms may be in play when identifying drug mode of action: (1) inhibition of mitochondrial fatty acid β-oxidation; (2) inhibition of fatty acid transport across mitochondrial membranes; (3) increased de novo lipid synthesis; (4) reduction in lipid export by the inhibition of microsomal triglyceride transfer protein; (5) induction of mitochondrial permeability transition pore opening; (6) dissipation of the mitochondrial transmembrane potential; (7) impairment of the mitochondrial respiratory chain/oxidative phosphorylation; (8) mitochondrial DNA damage, degradation and depletion; and (9) nuclear receptors (NRs)/transcriptomic alterations. Currently, the majority of, if not all, adverse outcome pathways (AOPs) for steatosis in AOP-Wiki highlight the interaction with NRs or transcription factors as the key molecular initiating event (MIE). This perspective suggests that chemical-induced steatosis typically results from the interplay between a chemical and a NR or transcription factors, implying that this interaction represents the primary and pivotal MIE. However, upon conducting this exhaustive literature review, it became evident that the current AOPs tend to overly emphasize this interaction as the sole MIE. Some studies indeed support the involvement of NRs in steatosis, but others demonstrate that such NR interactions alone do not necessarily lead to steatosis. This view, ignoring other mitochondrial-related injury mechanisms, falls short in encapsulating the intricate biological mechanisms involved in chemically induced liver steatosis, necessitating their consideration as part of the AOP's map road as well.

Blautia Coccoides is a Newly Identified Bacterium Increased by Leucine Deprivation and has a Novel Function in Improving Metabolic Disorders

Gut microbiota is linked to human metabolic diseases. The previous work showed that leucine deprivation improved metabolic dysfunction, but whether leucine deprivation alters certain specific species of bacterium that brings these benefits remains unclear. Here, this work finds that leucine deprivation alters gut microbiota composition, which is sufficient and necessary for the metabolic improvements induced by leucine deprivation. Among all the affected bacteria, B. coccoides is markedly increased in the feces of leucine-deprived mice. Moreover, gavage with B. coccoides improves insulin sensitivity and reduces body fat in high-fat diet (HFD) mice, and singly colonization of B. coccoides increases insulin sensitivity in gnotobiotic mice. The effects of B. coccoides are mediated by metabolizing tryptophan into indole-3-acetic acid (I3AA) that activates the aryl hydrocarbon receptor (AhR) in the liver. Finally, this work reveals that reduced fecal B. coccoides and I3AA levels are associated with the clinical metabolic syndrome. These findings suggest that B. coccoides is a newly identified bacterium increased by leucine deprivation, which improves metabolic disorders via metabolizing tryptophan into I3AA.

AhR, PXR and CAR: From Xenobiotic Receptors to Metabolic Sensors

Traditionally, xenobiotic receptors are known for their role in chemical sensing and detoxification, as receptor activation regulates the expression of various key enzymes and receptors. However, recent studies have highlighted that xenobiotic receptors also play a key role in the regulation of lipid metabolism and therefore function also as metabolic sensors. Since dyslipidemia is a major risk factor for various cardiometabolic diseases, like atherosclerosis and non-alcoholic fatty liver disease, it is of major importance to understand the molecular mechanisms that are regulated by xenobiotic receptors. In this review, three major xenobiotic receptors will be discussed, being the aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Specifically, this review will focus on recent insights into the metabolic functions of these receptors, especially in the field of lipid metabolism and the associated dyslipidemia.

Microbial sensing in the intestine

The gut microbiota plays a key role in host health and disease, particularly through their interactions with the immune system. Intestinal homeostasis is dependent on the symbiotic relationships between the host and the diverse gut microbiota, which is influenced by the highly co-evolved immune-microbiota interactions. The first step of the interaction between the host and the gut microbiota is the sensing of the gut microbes by the host immune system. In this review, we describe the cells of the host immune system and the proteins that sense the components and metabolites of the gut microbes. We further highlight the essential roles of pattern recognition receptors (PRRs), the G protein-coupled receptors (GPCRs), aryl hydrocarbon receptor (AHR) and the nuclear receptors expressed in the intestinal epithelial cells (IECs) and the intestine-resident immune cells. We also discuss the mechanisms by which the disruption of microbial sensing because of genetic or environmental factors causes human diseases such as the inflammatory bowel disease (IBD).

Effects of In Utero PFOS Exposure on Epigenetics and Metabolism in Mouse Fetal Livers

Prenatal exposure to perfluorooctanesulfonate (PFOS) increases fetus' metabolic risk; however, the investigation of the underlying mechanism is limited. In this study, pregnant mice in the gestational days (GD, 4.5-17.5) were exposed to PFOS (0.3 and 3 μg/g of body weight). At GD 17.5, PFOS perturbed maternal lipid metabolism and upregulated metabolism-regulating hepatokines (Angptl4, Angptl8, and Selenop). Mass-spectrometry imaging and whole-genome bisulfite sequencing revealed, respectively, selective PFOS localization and deregulation of gene methylation in fetal livers, involved in inflammation, glucose, and fatty acid metabolism. PCR and Western blot analysis of lipid-laden fetal livers showed activation of AMPK signaling, accompanied by significant increases in the expression of glucose transporters (Glut2/4), hexose-phosphate sensors (Retsat and ChREBP), and the key glycolytic enzyme, pyruvate kinase (Pk) for glucose catabolism. Additionally, PFOS modulated the expression levels of PPARα and PPARγ downstream target genes, which simultaneously stimulated fatty acid oxidation (Cyp4a14, Acot, and Acox) and lipogenesis (Srebp1c, Acaca, and Fasn). Using human normal hepatocyte (MIHA) cells, the underlying mechanism of PFOS-elicited nuclear translocation of ChREBP, associated with a fatty acid synthesizing pathway, was revealed. Our finding implies that in utero PFOS exposure altered the epigenetic landscape associated with dysregulation of fetal liver metabolism, predisposing postnatal susceptibility to metabolic challenges.

Orchiectomy sensitizes cortical bone in male mice to the harmful effects of kynurenine

Kynurenine (Kyn) is a tryptophan metabolite that increases with age and promotes musculoskeletal dysfunction. We previously found a sexually dimorphic pattern in how Kyn affects bone, with harmful effects more prevalent in females than males. This raises the possibility that male sex steroids might exert a protective effect that blunts the effects of Kyn in males. To test this, orchiectomy (ORX) or sham surgeries were performed on 6-month-old C57BL/6 mice, after which mice received Kyn (10 mg/kg) or vehicle via intraperitoneal injection, once daily, 5×/week, for four weeks. Bone histomorphometry, DXA, microCT, and serum marker analyses were performed after sacrifice. In vitro studies were performed to specifically test the effect of testosterone on activation of aryl hydrocarbon receptor (AhR)-mediated signaling by Kyn in mesenchymal-lineage cells. Kyn treatment reduced cortical bone mass in ORX- but not sham-operated mice. Trabecular bone was unaffected. Kyn's effects on cortical bone in ORX mice were attributed primarily to enhanced endosteal bone resorption activity. Bone marrow adipose tissue was increased in Kyn-treated ORX animals but was unchanged by Kyn in sham-operated mice. ORX surgery increased mRNA expression of the aryl hydrocarbon receptor (AhR) and its target gene Cyp1a1 in the bone, suggesting a priming and/or amplification of AhR signaling pathways. Mechanistic in vitro studies revealed that testosterone blunted Kyn-stimulated AhR transcriptional activity and Cyp1a1 expression in mesenchymal-linage cells. These data suggest a protective role for male sex steroids in blunting the harmful effects of Kyn in cortical bone. Therefore, testosterone may play an important role in regulating Kyn/AhR signaling in musculoskeletal tissues, suggesting crosstalk between male sex steroids and Kyn signaling may influence age-associated musculoskeletal frailty.

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.

Aryl hydrocarbon receptor maintains hepatic mitochondrial homeostasis in mice

OBJECTIVE

Mitophagy removes damaged mitochondria to maintain cellular homeostasis. Aryl hydrocarbon receptor (AhR) expression in the liver plays a crucial role in supporting normal liver functions, but its impact on mitochondrial function is unclear. Here, we identified a new role of AhR in the regulation of mitophagy to control hepatic energy homeostasis.

METHODS

In this study, we utilized primary hepatocytes from AhR knockout (KO) mice and AhR knockdown AML12 hepatocytes. An endogenous AhR ligand, kynurenine (Kyn), was used to activate AhR in AML12 hepatocytes. Mitochondrial function and mitophagy process were comprehensively assessed by MitoSOX and mt-Keima fluorescence imaging, Seahorse XF-based oxygen consumption rate measurement, and Mitoplate S-1 mitochondrial substrate utilization analysis.

RESULTS

Transcriptomic analysis indicated that mitochondria-related gene sets were dysregulated in AhR KO liver. In both primary mouse hepatocytes and AML12 hepatocyte cell lines, AhR inhibition strongly suppressed mitochondrial respiration rate and substrate utilization. AhR inhibition also blunted the fasting response of several essential autophagy genes and the mitophagy process. We further identified BCL2 interacting protein 3 (BNIP3), a mitophagy receptor that senses nutrient stress, as an AhR target gene. AhR is directly recruited to the Bnip3 genomic locus, and Bnip3 transcription was enhanced by AhR endogenous ligand treatment in wild-type liver and abolished entirely in AhR KO liver. Mechanistically, overexpression of Bnip3 in AhR knockdown cells mitigated the production of mitochondrial reactive oxygen species (ROS) and restored functional mitophagy.

CONCLUSIONS

AhR regulation of the mitophagy receptor BNIP3 coordinates hepatic mitochondrial function. Loss of AhR induces mitochondrial ROS production and impairs mitochondrial respiration. These findings provide new insight into how endogenous AhR governs hepatic mitochondrial homeostasis.

Longitudinal analysis of a dietary mouse model of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH)

Non-alcoholic fatty liver disease (NAFLD), and resultant non-alcoholic steatohepatitis (NASH), incidence and prevalence are rising globally due to increasing rates of obesity and diabetes. Currently, there are no approved pharmacological treatments for NAFLD, highlighting a need for additional mechanistic studies to develop prevention and/or therapeutic strategies. Diet-induced preclinical models of NAFLD can be used to examine the dynamic changes that occur during NAFLD development and progression throughout the lifespan. To date, most studies utilizing such models have focused exclusively on terminal time points and have likely missed critical early and late changes that are important for NAFLD progression (i.e, worsening). We performed a longitudinal analysis of histopathological, biochemical, transcriptomic, and microbiome changes that occurred in adult male mice fed either a control diet or a NASH-promoting diet (high in fat, fructose, and cholesterol) for up to 30 weeks. We observed progressive development of NAFLD in mice fed the NASH diet compared to the control diet. Differential expression of immune-related genes was observed at an early stage of diet-induced NAFLD development (10 weeks) and persisted into the later stages of the disease (20 and 30 weeks). Differential expression of xenobiotic metabolism related genes was observed at the late stage of diet-induced NAFLD development (30 weeks). Microbiome analysis revealed an increased abundance of Bacteroides at an early stage (10 weeks) that persisted into the later stages of the disease (20 and 30 weeks). These data provide insight into the progressive changes that occur during NAFLD/NASH development and progression in the context of a typical Western diet. Furthermore, these data are consistent with what has been reported in patients with NAFLD/NASH, supporting the preclinical use of this diet-induced model for development of strategies to prevent or treat the disease.

Potential therapeutic targets for age-related macular degeneration: The nuclear option

The functions and activities of nuclear receptors, the largest family of transcription factors in the human genome, have classically focused on their ability to act as steroid and hormone sensors in endocrine organs. However, they are responsible for a diverse array of physiological functions, including cellular homeostasis and metabolism, during development and aging. Though the eye is not a traditional endocrine organ, recent studies have revealed high expression levels of nuclear receptors in cells throughout the posterior pole. These findings have precipitated an interest in investigating the role of these transcription factors in the eye as a function of age and ocular disease, in particular age-related macular degeneration (AMD). As the leading cause of vision impairment in the elderly, identifying signaling pathways that may be targeted for AMD therapy is of great importance, given the lack of therapeutic options for over 85% of patients with this disease. Herein we review this relatively new field and recent findings supporting the hypothesis that the eye is a secondary endocrine organ, in which nuclear receptors serve as the bedrock for biological processes in cells vulnerable in AMD, including retinal pigment epithelial and choroidal endothelial cells, and discuss the therapeutic potential of targeting these receptors for AMD.

Aryl hydrocarbon receptor (AhR) impairs circadian regulation: impact on the aging process

Circadian clocks control the internal sleep-wake rhythmicity of 24hours which is synchronized by the solar cycle. Circadian regulation of metabolism evolved about 2.5 billion years ago, i.e., the rhythmicity has been conserved from cyanobacteria and Archaea through to mammals although the mechanisms utilized have developed with evolution. While the aryl hydrocarbon receptor (AhR) is an evolutionarily conserved defence mechanism against environmental threats, it has gained many novel functions during evolution, such as the regulation of cell cycle, proteostasis, and many immune functions. There is robust evidence that AhR signaling impairs circadian rhythmicity, e.g., by interacting with the core BMAL1/CLOCK complex and disturbing the epigenetic regulation of clock genes. The maintenance of circadian rhythms is impaired with aging, disturbing metabolism and many important functions in aged organisms. Interestingly, it is known that AhR signaling promotes an age-related tissue degeneration, e.g., it is able to inhibit autophagy, enhance cellular senescence, and disrupt extracellular matrix. These alterations are rather similar to those induced by a long-term impairment of circadian rhythms. However, it is not known whether AhR signaling enhances the aging process by impairing circadian homeostasis. I will examine the experimental evidence indicating that AhR signaling is able to promote the age-related degeneration via a disruption of circadian rhythmicity.

Decapentaplegic retards lipolysis during metamorphosis in Bombyx mori and Drosophila melanogaster

Insect morphogen decapentaplegic (Dpp) functions as one of the key extracellular ligands of the Bone Morphogenetic Protein (BMP) signaling pathway. Previous studies in insects mainly focused on the roles of Dpp during embryonic development and the formation of adult wings. In this study, we demonstrate a new role for Dpp in retarding lipolysis during metamorphosis in both Bombyx mori and Drosophila melanogaster. CRISPR/Cas9-mediated mutation of Bombyx dpp causes pupal lethality, induces an excessive and premature breakdown of lipids in the fat body, and upregulates the expressions of several lipolytic enzyme genes, including brummer (bmm), lipase 3 (lip3), and hormone-sensitive lipase (hsl), and lipid storage droplet 1 (lsd1), a lipid droplets (LD)-associated protein gene. Further investigation in Drosophila reveals that salivary gland-specific knockdown of the dpp gene and fat body-specific knockdown of Mad involved in Dpp signaling phenocopy the effects of Bombyx dpp mutation on pupal development and lipolysis. Taken together, our data indicate that the Dpp-mediated BMP signaling in the fat body maintains lipid homeostasis by retarding lipolysis, which is necessary for pupa-adult transition during insect metamorphosis.

Aryl Hydrocarbon Receptor (AhR) Signaling in Colonic Cells and Tumors

The aryl hydrocarbon receptor (AhR) is overexpressed in many tumor types and exhibits tumor-specific tumor promoter and tumor suppressor-like activity. In colon cancer, most but not all studies suggest that the AhR exhibits tumor suppressor activity which is enhanced by AhR ligands acting as agonists. Our studies investigated the role of the AhR in colon tumorigenesis using wild-type and AhR-knockout mice, the inflammation model of colon tumorigenesis using mice treated with azoxymethane (AOM)/dextran sodium sulfate (DSS) and APCS580/+; KrasG12D/+ mice all of which form intestinal tumors. The effects of tissue-specific AhR loss in the intestine of the tumor-forming mice on colonic stem cells, organoid-initiating capacity, colon tumor formation and mechanisms of AhR-mediated effects were investigated. Loss of AhR enhanced stem cell and tumor growth and in the AOM/DSS model AhR-dependent suppression of FOXM1 and downstream genes was important for AhR-dependent anticancer activity. Furthermore, the effectiveness of interleukin-22 (IL22) in colonic epithelial cells was also dependent on AhR expression. IL22 induced phosphorylation of STAT3, inhibited colonic organoid growth, promoted colonic cell proliferation in vivo and enhanced DNA repair in AOM/DSS-induced tumors. In this mouse model, the AhR suppressed SOCS3 expression and enhanced IL22-mediated activation of STAT3, whereas the loss of the AhR increased levels of SOCS3 which in turn inhibited IL22-induced STAT3 activation. In the APCS580/+; KrasG12D/+ mouse model, the loss of the AhR enhanced Wnt signaling and colon carcinogenesis. Results in both mouse models of colon carcinogenesis were complemented by single cell transcriptomics on colonic intestinal crypts which also showed that AhR deletion promoted expression of FOXM1-regulated genes in multiple colonic cell subtypes. These results support the role of the AhR as a tumor suppressor-like gene in the colon.

Indoxyl Sulfate Promotes Metastatic Characteristics of Ovarian Cancer Cells via Aryl Hydrocarbon Receptor-Mediated Downregulation of the Mas Receptor

Although platinum-combination chemotherapy shows a high response rate at the primary site, epithelial ovarian cancer (EOC) treatment remains challenging because of tumor recurrence and metastasis. Recent studies have revealed that chemotherapy paradoxically promotes cancer cell survival, proliferation, and metastasis, although the reason for this remains unclear. The underlying molecular mechanisms that contribute to chemotherapy-induced metastasis need to be elucidated to establish effective therapeutic strategies. Acute kidney injury is a known side effect of cisplatin treatment, and kidney dysfunction results in the accumulation of uremic toxins in the serum. The present study aimed to investigate whether indoxyl sulfate (IS), a representative uremic toxin, affects the pathophysiology of EOC. In this study, IS reduced the expression of Mas receptor (MasR) in cultured human EOC cells. Both knockdown of the aryl hydrocarbon receptor (AhR), which is an intracellular IS receptor, and inhibition of AhR function suppressed IS-mediated downregulation of MasR in SK-OV-3 cells. IS induced the phosphorylation of signal transducer and activator of transcription 3 (STAT3) in an AhR-dependent manner. Inhibition of the STAT3 pathway or reactive oxygen species production suppressed the IS-mediated reduction of MasR. IS stimulated cell migration and invasion of SK-OV-3 cells in an AhR-dependent manner. Cisplatin-nephropathy model mice exhibited elevated levels of serum IS accompanied by elevated levels of blood urea nitrogen and serum creatinine. Furthermore, intraperitoneal administration of IS in mice promoted tumor growth and metastasis. Finally, we found that the MasR agonist Ang-(1-7) suppressed the IS-mediated effects on cell proliferation, migration, and invasion of SK-OV-3 cells. However, the knockdown of MasR expression by specific small interfering RNA in the absence of IS resulted in only minimal promotion of cell migration and invasion. These findings demonstrate that IS promotes malignancy in ovarian cancer via AhR-mediated downregulation of MasR function, whereas Ang-(1-7) attenuates this effect, thereby suggesting that Ang-(1-7) could provide a future treatment strategy for this cancer type.

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.

Atypical functions of xenobiotic receptors in lipid and glucose metabolism

Xenobiotic receptors are traditionally defined as xenobiotic chemical-sensing receptors, the activation of which transcriptionally regulates the expression of enzymes and transporters involved in the metabolism and disposition of xenobiotics. Emerging evidence suggests that "xenobiotic receptors" also have diverse endobiotic functions, including their effects on lipid metabolism and energy metabolism. Dyslipidemia is a major risk factor for cardiovascular disease, diabetes, obesity, metabolic syndrome, stroke, nonalcoholic fatty liver disease (NAFLD), and nonalcoholic steatohepatitis (NASH). Understanding the molecular mechanism by which transcriptional factors, including the xenobiotic receptors, regulate lipid homeostasis will help to develop preventive and therapeutic approaches. This review describes recent advances in our understanding the atypical roles of three xenobiotic receptors: aryl hydrocarbon receptor (AhR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR), in metabolic disorders, with a particular focus on their effects on lipid and glucose metabolism. Collectively, the literatures suggest the potential values of AhR, PXR and CAR as therapeutic targets for the treatment of NAFLD, NASH, obesity and diabetes, and cardiovascular diseases.

Effects of Crocodile Oil (Crocodylus siamensis) on Liver Enzymes: Cytochrome P450 and Glutathione S-Transferase Activities in High-fat DietFed Rats

Crocodile oil is a highly effective treatment for ailments ranging from skin conditions to cancer. However, the effects of the oil on liver detoxification pathways are not well studied. This study aimed to investigate the effects of crocodile oil on the detoxification enzyme activities and the mRNA expressions of cytochrome P450 1A2 (CYP1A2), cytochrome P450 2E1 (CYP2E1), and glutathione S-transferase (GST) in rats. The rats were divided into four groups (n = 7/group): rats received a standard diet (C), a high-fat diet or HFD (H), and HFD with 1 ml (HCO1) and 3 ml (HCO3) of the oil per kg body weight. Interestingly, the oil yields from this study presented alpha-linolenic acid (0.96%) at similar levels compared with fish oil. The results revealed that HFD significantly increased the activity and relative gene expression of CYP1A2 in the H group (P < 0.05), whereas 3% crocodile oil normalized the enzyme activities compared to the C group. This suggested inhibiting the HFD-induced expression of CYP1A2 mediated by the omega-3 fatty acids found in the oil. Also, crocodile oil supplementation did not reduce the activities of GST. However, the relative gene expression of GSTA1 was significantly decreased (P < 0.05) in the HCO1 and HCO3 groups compared to the H group, which might be attributed to the lower lipid peroxidation that occurred in the liver tissues. Therefore, it could be suggested that using crocodile oil could help in liver detoxification through the CYP1A2 even when offered with a HFD.

Cinnabarinic Acid Provides Hepatoprotection Against Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a chronic condition in which excess lipids accumulate in the liver and can lead to a range of progressive liver disorders including non-alcoholic steatohepatitis, liver cirrhosis, and hepatocellular carcinoma. While lifestyle and diet modifications have proven to be effective as NAFLD treatments, they are not sustainable in the long-term, and currently no pharmacological therapies are approved to treat NAFLD. Our previous studies demonstrated that cinnabarinic acid (CA), a novel endogenous Aryl hydrocarbon Receptor (AhR) agonist, activates the AhR target gene, Stanniocalcin 2, and confers cytoprotection against a plethora of ER/oxidative stressors. In this study, the hepatoprotective and anti-steatotic properties of CA were examined against free fatty-acid-induced in vitro and high-fat-diet fed in vivo NAFLD models. The results demonstrated that CA treatment significantly lowered weight gain and attenuated hepatic lipotoxicity both before and after the established fatty liver, thereby protecting against steatosis, inflammation, and liver injury. CA mitigated intracellular free fatty acid uptake concomitant with the downregulation of CD36/fatty acid translocase. Genes involved in fatty acid and triglyceride synthesis were also downregulated in response to CA treatment. Additionally, suppressing AhR and Stc2 expression using RNA interference in vitro verified that the hepatoprotective effects of CA were absolutely dependent on both AhR and its target, Stc2. Collectively, our results demonstrate that the endogenous AhR agonist, CA, confers hepatoprotection against NAFLD by regulating hepatic fatty acid uptake and lipogenesis. SIGNIFICANCE STATEMENT: In this study using in vitro and in vivo models, we demonstrate that cinnabarinic acid (CA), an endogenous AhR agonist, provides protection against non-alcoholic fatty liver disease. CA bestows cytoprotection against steatosis and liver injury by controlling expression of several key genes associated with lipid metabolism pathways, limiting the hepatic lipid uptake, and controlling liver inflammation. Moreover, CA-induced hepatoprotection is absolutely dependent on AhR and Stc2 expression.

Aryl hydrocarbon receptor (AhR) reveals evidence of antagonistic pleiotropy in the regulation of the aging process

The antagonistic pleiotropy hypothesis is a well-known evolutionary theory to explain the aging process. It proposes that while a particular gene may possess beneficial effects during development, it can exert deleterious properties in the aging process. The aryl hydrocarbon receptor (AhR) has a significant role during embryogenesis, but later in life, it promotes several age-related degenerative processes. For instance, AhR factor (i) controls the pluripotency of stem cells and the stemness of cancer stem cells, (ii) it enhances the differentiation of embryonal stem cells, especially AhR signaling modulates the differentiation of hematopoietic stem cells and progenitor cells, (iii) it also stimulates the differentiation of immunosuppressive Tregs, Bregs, and M2 macrophages, and finally, (iv) AhR signaling participates in the differentiation of many peripheral tissues. On the other hand, AhR signaling is involved in many processes promoting cellular senescence and pathological processes, e.g., osteoporosis, vascular dysfunction, and the age-related remodeling of the immune system. Moreover, it inhibits autophagy and aggravates extracellular matrix degeneration. AhR signaling also stimulates oxidative stress, promotes excessive sphingolipid synthesis, and disturbs energy metabolism by catabolizing NAD⁺ degradation. The antagonistic pleiotropy of AhR signaling is based on the complex and diverse connections with major signaling pathways in a context-dependent manner. The major regulatory steps include, (i) a specific ligand-dependent activation, (ii) modulation of both genetic and non-genetic responses, (iii) a competition and crosstalk with several transcription factors, such as ARNT, HIF-1α, E2F1, and NF-κB, and (iv) the epigenetic regulation of target genes with binding partners. Thus, not only mTOR signaling but also the AhR factor demonstrates antagonistic pleiotropy in the regulation of the aging process.

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.

Role of indoleamine 2,3-dioxygenase 1 (IDO1) and kynurenine pathway in the regulation of the aging process

Indoleamine 2,3-dioxygenase 1 (IDO1) is activated in chronic inflammatory states, e.g., in the aging process and age-related diseases. IDO1 enzyme catabolizes L-tryptophan (L-Trp) into kynurenine (KYN) thus stimulating the KYN pathway. The depletion of L-Trp inhibits the proliferation of immune cells in inflamed tissues and it also reduces serotonin synthesis predisposing to psychiatric disorders. Interestingly, IDO1 protein contains two immunoreceptor tyrosine-based inhibitory motifs (ITIM) which trigger suppressive signaling through the binding of PI3K p110 and SHP-1 proteins. This immunosuppressive activity is not dependent on the catalytic activity of IDO1. KYN and its metabolite, kynurenic acid (KYNA), are potent activators of the aryl hydrocarbon receptor (AhR) which can enhance immunosuppression. IDO1-KYN-AhR signaling counteracts excessive pro-inflammatory responses in acute inflammation but in chronic inflammatory states it has many harmful effects. A chronic low-grade inflammation is associated with the aging process, a state called inflammaging. There is substantial evidence that the activation of the IDO1-KYN-AhR pathway robustly increases with the aging process. The activation of IDO1-KYN-AhR signaling does not only suppress the functions of effector immune cells, probably promoting immunosenescence, but it also impairs autophagy, induces cellular senescence, and remodels the extracellular matrix as well as enhancing the development of osteoporosis and vascular diseases. I will review the function of IDO1-KYN-AhR signaling and discuss its activation with aging as an enhancer of the aging process.

Emerging concepts and opportunities for endocrine disruptor screening of the non-EATS modalities

Endocrine disrupting chemicals (EDCs) are ubiquitous in the environment and involve diverse chemical-receptor interactions that can perturb hormone signaling. The Organization for Economic Co-operation and Development has validated several EDC-receptor bioassays to detect endocrine active chemicals and has established guidelines for regulatory testing of EDCs. Focus on testing over the past decade has been initially directed to EATS modalities (estrogen, androgen, thyroid, and steroidogenesis) and validated tests for chemicals that exert effects through non-EATS modalities are less established. Due to recognition that EDCs are vast in their mechanisms of action, novel bioassays are needed to capture the full scope of activity. Here, we highlight the need for validated assays that detect non-EATS modalities and discuss major international efforts underway to develop such tools for regulatory purposes, focusing on non-EATS modalities of high concern (i.e., retinoic acid, aryl hydrocarbon receptor, peroxisome proliferator-activated receptor, and glucocorticoid signaling). Two case studies are presented with strong evidence amongst animals and human studies for non-EATS disruption and associations with wildlife and human disease. This includes metabolic syndrome and insulin signaling (case study 1) and chemicals that impact the cardiovascular system (case study 2). This is relevant as obesity and cardiovascular disease represent two of the most significant health-related crises of our time. Lastly, emerging topics related to EDCs are discussed, including recognition of crosstalk between the EATS and non-EATS axis, complex mixtures containing a variety of EDCs, adverse outcome pathways for chemicals acting through non-EATS mechanisms, and novel models for testing chemicals. Recommendations and considerations for evaluating non-EATS modalities are proposed. Moving forward, improved understanding of the non-EATS modalities will lead to integrated testing strategies that can be used in regulatory bodies to protect environmental, animal, and human health from harmful environmental chemicals.

Loss of aryl hydrocarbon receptor suppresses the response of colonic epithelial cells to IL22 signaling by upregulating SOCS3

IL22 signaling plays an important role in maintaining gastrointestinal epithelial barrier function, cell proliferation, and protection of intestinal stem cells from genotoxicants. Emerging studies indicate that the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, promotes production of IL22 in gut immune cells. However, it remains to be determined if AhR signaling can also affect the responsiveness of colonic epithelial cells to IL22. Here, we show that IL22 treatment induces the phosphorylation of STAT3, inhibits colonic organoid growth, and promotes colonic cell proliferation in vivo. Notably, intestinal cell-specific AhR knockout (KO) reduces responsiveness to IL22 and compromises DNA damage response after exposure to carcinogen, in part due to the enhancement of suppressor of cytokine signaling 3 (SOCS3) expression. Deletion of SOCS3 increases levels of pSTAT3 in AhR KO organoids, and phenocopies the effects of IL22 treatment on wild-type (WT) organoid growth. In addition, pSTAT3 levels are inversely associated with increased azoxymethane/dextran sulfate sodium (AOM/DSS)-induced colon tumorigenesis in AhR KO mice. These findings indicate that AhR function is required for optimal IL22 signaling in colonic epithelial cells and provide rationale for targeting AhR as a means of reducing colon cancer risk.NEW & NOTEWORTHY AhR is a key transcription factor controlling expression of IL22 in gut immune cells. In this study, we show for the first time that AhR signaling also regulates IL22 response in colonic epithelial cells by modulating SOCS3 expression.

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.

Impaired beta-oxidation increases vulnerability to influenza A infection

Influenza A virus (IAV) infection casts a significant burden on society. It has particularly high morbidity and mortality rates in patients suffering from metabolic disorders. The aim of this study was to relate metabolic changes with IAV susceptibility using well-characterized inbred mouse models. We compared the highly susceptible DBA/2J (D2) mouse strain for which IAV infection is lethal with the C57BL/6J (B6) strain, which exhibits a moderate course of disease and survives IAV infection. Previous studies showed that D2 has higher insulin and glucose levels and is predisposed to develop diet-induced type 2 diabetes. Using high-resolution liquid chromatography–coupled MS, the plasma metabolomes of individual animals were repeatedly measured up to 30 days postinfection. The biggest metabolic difference between these strains in healthy and infected states was in the levels of malonylcarnitine, which was consistently increased 5-fold in D2. Other interstrain and intrastrain differences in healthy and infected animals were observed for acylcarnitines, glucose, branched-chain amino acids, and oxidized fatty acids. By mapping metabolic changes to canonical pathways, we found that mitochondrial beta-oxidation is likely disturbed in D2 animals. In noninfected D2 mice, this leads to increased glycerolipid production and reduced acylcarnitine production, whereas in infected D2 animals, peroxisomal beta-oxidation becomes strongly increased. From these studies, we conclude that metabolic changes caused by a distortion of mitochondrial and peroxisomal metabolism might impact the innate immune response in D2, leading to high viral titers and mortality.

In Utero Exposure to Persistent Organic Pollutants and Childhood Lipid Levels

Animal studies have shown that developmental exposures to polybrominated diphenyl ethers (PBDE) permanently affect blood/liver balance of lipids. No human study has evaluated associations between in utero exposures to persistent organic pollutants (POPs) and later life lipid metabolism. In this pilot, maternal plasma levels of PBDEs (BDE-47, BDE-99, BDE-100, and BDE-153) and polychlorinated biphenyls (PCB-138, PCB-153, and PCB-180) were determined at delivery in participants of GESTation and Environment (GESTE) cohort. Total cholesterol (TCh), triglycerides (TG), low- and high-density lipoproteins (LDL-C and HDL-C), total lipids (TL), and PBDEs were determined in serum of 147 children at ages 6-7. General linear regression was used to estimate the relationship between maternal POPs and child lipid levels with adjustment for potential confounders, and adjustment for childhood POPs. In utero BDE-99 was associated with lower childhood levels of TG (p = 0.003), and non-significantly with HDL-C (p = 0.06) and TL (p = 0.07). Maternal PCB-138 was associated with lower childhood levels of TG (p = 0.04), LDL-C (p = 0.04), and TL (p = 0.02). Our data indicate that in utero exposures to POPs may be associated with long lasting decrease in circulating lipids in children, suggesting increased lipid accumulation in the liver, a mechanism involved in NAFLD development, consistent with previously reported animal data.

Ingestion of Faecalibaculum rodentium causes depression-like phenotypes in resilient Ephx2 knock-out mice: A role of brain-gut-microbiota axis via the subdiaphragmatic vagus nerve

BACKGROUND

The brain-gut-microbiota axis plays a crucial role in the bidirectional interactions between the brain and the gut. Soluble epoxide hydrolase (coded by the Ephx2 gene) plays an important role in inflammation, which has been implicated in stress-related depression. Ephx2 knock-out (KO) mice exposed to chronic social defeat stress (CSDS) did not show depression-like behaviors, indicating stress resilience. Here we examined whether the brain-gut-microbiota axis influences the resilience in Ephx2 KO mice.

METHODS

Effects of fecal microbiota transplantation (FMT) from CSDS-susceptible (or control) mice in wild-type (WT) mice and Ephx2 KO mice treated with an antibiotic cocktail (ABX) were investigated. Behavioral, biochemical tests and 16S ribosome RNA analysis were performed.

RESULTS

FMT from CSDS-susceptible mice produced anhedonia-like behavior in ABX-treated WT and Ephx2 KO mice. The 16S ribosome RNA analysis showed that Faecalibaculum rodentium (F. rodentium) may be responsible for the observed anhedonia-like behavior following FMT from CSDS-susceptible mice. Ingestion of F. rodentium for 14 days produced depression- and anhedonia-like behaviors, higher blood levels of interleukin-6, and reduced expression of synaptic proteins in the prefrontal cortex of ABX-treated Ephx2 KO mice. Furthermore, subdiaphragmatic vagotomy blocked the development of these behavioral abnormalities after ingestion of F. rodentium.

LIMITATIONS

Detailed mechanisms are unclear.

CONCLUSIONS

These findings suggest that F. rodentium might contribute to the conversion of resilient Ephx2 KO mice into KO mice with depression-like phenotypes. The brain-gut-microbiota axis via the subdiaphragmatic vagus nerve plays a crucial role in susceptibility and resilience to stress.

Role of the Aryl Hydrocarbon Receptor (AhR) in Mediating the Effects of Coffee in the Colon

SCOPE

This study investigates the mechanism of action and functional effects of coffee extracts in colonic cells, on intestinal stem cell growth, and inhibition of dextran sodium sulfate (DSS)-induced intestinal barrier damage in mice.

METHODS AND RESULTS

Aqueous coffee extracts induced Ah receptor (AhR) -responsive CYP1A1, CYP1B1, and UGT1A1 gene expression in colon-derived Caco2 and YAMC cells. Tissue-specific AhR knockout (AhR^f/f x Lgr5-GFP-CreERT2 x Villin-Cre), wild-type (Lgr5-CreERT2 x Villin-Cre) mice are sources of stem cell enriched organoids and both coffee extracts and norharman, an AhR-active component of these extracts inhibited stem cell growth. Coffee extracts also inhibit DSS-induced damage to intestinal barrier function and DSS-induced mucosal inflammatory genes such as IL-6 and TGF-β1 in wild-type (AhR^+/+ ) but not AhR^-/- mice. In contrast, coffee does not exhibit protective effects in intestinal-specific AhR knockout mice. Coffee extracts also enhanced overall formation of AhR-active microbial metabolites.

CONCLUSIONS

In colon-derived cells and in the mouse intestine, coffee induced several AhR-dependent responses including gene expression, inhibition of intestinal stem cell-enriched organoid growth, and inhibition of DSS-induced intestinal barrier damage. We conclude that the anti-inflammatory effects of coffee in the intestine are due, in part, to activation of AhR signaling.

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.

Hepatoprotective Effects of Indole, a Gut Microbial Metabolite, in Leptin-Deficient Obese Mice

BACKGROUND

The gut microbiota plays a role in the occurrence of nonalcoholic fatty liver disease (NAFLD), notably through the production of bioactive metabolites. Indole, a bacterial metabolite of tryptophan, has been proposed as a pivotal metabolite modulating inflammation, metabolism, and behavior.

OBJECTIVES

The aim of our study was to mimic an upregulation of intestinal bacterial indole production and to evaluate its potential effect in vivo in 2 models of NAFLD.

METHODS

Eight-week-old leptin-deficient male ob/ob compared with control ob/+ mice (experiment 1), and 4-5-wk-old C57BL/6JRj male mice fed a low-fat (LF, 10 kJ%) compared with a high-fat (HF, 60 kJ%) diet (experiment 2), were given plain water or water supplemented with a physiological dose of indole (0.5 mM, n ≥6/group) for 3 wk and 3 d, respectively. The effect of the treatments on the liver, intestine, adipose tissue, brain, and behavior was assessed.

RESULTS

Indole reduced hepatic expression of genes involved in inflammation [C-C motif chemokine ligand 2 (Ccl2), C-X-C motif chemokine ligand 2 (Cxcl2); 3.3- compared with 5.0-fold, and 2.4- compared with 3.3-fold of control ob/+ mice, respectively, P < 0.05], and in macrophage activation [Cd68, integrin subunit α X (Itgax); 2.1- compared with 2.5-fold, and 5.0- compared with 6.4-fold of control ob/+ mice, respectively, P < 0.01] as well as markers of hepatic damage (alaninine aminotransferase; -32%, P < 0.001) regardless of genotype in experiment 1. Indole had no effect on hepatic inflammation in mice fed the LF or HF diet in experiment 2. Indole did not change hepatic lipid content, anxiety-like behavior, or inflammation in the ileum, adipose tissue, and brain in experiment 1.

CONCLUSIONS

Our results support the efficacy of indole to reduce hepatic damage and associated inflammatory response and macrophage activation in ob/ob mice. These modifications appear to be attributable to direct effects of indole on the liver, rather than through effects on the adipose tissue or intestinal barrier.

The aryl hydrocarbon receptor in liver inflammation

The aryl hydrocarbon receptor (AHR) is a ubiquitously expressed ligand-activated transcription factor with multifaceted physiological functions. In the immune system, AHR has been unequivocally identified as a key regulatory factor that can integrate environmental, dietary, or microbial signals into innate and adaptive immune responses. Correspondingly, AHR activity seems to be most important at barrier organs, such as the gut, skin, and lung. The liver is likewise prominently exposed to gut-derived dietary or microbial AHR ligands and, moreover, generates plenty of AHR ligands itself. Yet, surprisingly little is known about the role of AHR in the regulation of hepatic immune responses, which are normally biased towards tolerance, preventing harmful inflammation in response to innocuous stimuli. In this review, we summarize the current knowledge about the role of AHR in hepatic immune responses in the healthy liver as well as in inflammatory liver disease. Moreover, we discuss AHR as a potential therapeutic target in hepatic disorders, including autoimmune liver disease, liver fibrosis, and liver cancer.

Aryl hydrocarbon receptor promotes lipid droplet biogenesis and metabolic shift in respiratory Club cells

Club cells are critical in maintaining airway integrity via, in part, secretion of immunomodulatory Club cell 10 kd protein (CC10) and xenobiotic detoxification. Aryl hydrocarbon receptor (AhR) is important in xenobiotic metabolism, but its role in Club cell function is unclear. To this end, an AhR ligand, 6-formylindolo[3,2-b]carbazole (FICZ, 10 nM) was found to induce, in a ligand and AhR-dependent manner, endoplasmic reticulum stress, phospholipid remodeling, free fatty acid and triglyceride synthesis, leading to perilipin 2-dependent lipid droplet (LD) biogenesis in a Club cell-like cell line, NL20. The increase in LDs was due, in part, to the blockade of adipose triglyceride lipase to LDs, while perilipin 5 facilitated LDs-mitochondria connection, leading to the breakdown of LDs via mitochondrial β-oxidation and acetyl-coA generation. In FICZ-treated cells, increased CC10 secretion and its intracellular association with LDs were noted. Administration of low (0.28 ng), medium (1.42 ng), and high (7.10 ng) doses of FICZ in C57BL/6 mice significantly enhanced lipopolysaccharide (LPS, 0.1 μg)-induced airway inflammation, mucin secretion, pro-inflammatory cytokines and CC10 in the bronchoalveolar lavage fluids, as compared to those seen in mice receiving LPS alone, suggesting the importance of AhR signaling in controlling the metabolic homeostasis and functions of Club cells.

Aryl hydrocarbon receptor: Its roles in physiology

Aryl hydrocarbon receptor (AHR) was initially discovered as a cellular protein involved in mediating the detoxification of xenobiotic compounds. Extensive research in the past two decades has identified several families of physiological ligands and uncovered important functions of AHR in normal development and homeostasis. Deficiency in AHR expression disrupts major signaling systems and transcriptional programs, which appear to be responsible for the development of numerous developmental abnormalities including cardiac hypertrophy and epidermal hyperplasia. This mini review primarily summarizes recent advances in our understanding of AHR functions in normal physiology with an emphasis on the cardiovascular, gastrointestinal, integumentary, nervous, and immunomodulatory systems.

Integrated omics analysis unraveled the microbiome-mediated effects of Yijin-Tang on hepatosteatosis and insulin resistance in obese mouse

BACKGROUND

Gut microbiota play important roles in insulin homeostasis and the pathogenesis of non-alcoholic fatty liver diseases (NAFLD). Yijin-Tang (YJT), a traditional Korean and Chinese medicine, is used in the treatment of gastrointestinal diseases and obesity-related disorders such as insulin resistance (IR) and NAFLD.

PURPOSE

Our aim was to identify the microbiome-mediated effects of YJT on IR and associated NAFLD by integrating metagenomics and hepatic lipid profile.

METHODS

C57BL/6J mice were fed a normal chow diet (NC) or high-fat/high-cholesterol (HFHC) diet with or without YJT treatment. Hepatic lipid profiles were analyzed using liquid chromatography/mass spectrometry, and the composition of gut microbiota was investigated using 16S rRNA sequencing. Then, hepatic lipid profiles, gut microbiome, and inflammatory marker data were integrated using multivariate analysis and bioinformatics tools.

RESULTS

YJT improved NAFLD, and 39 hepatic lipid metabolites were altered by YJT in a dose-dependent manner. YJT also altered the gut microbiome composition in HFHC-fed mice. In particular, Faecalibaculum rodentium and Bacteroides acidifaciens were altered by YJT in a dose-dependent manner. Also, we found significant correlation among hepatic phosphatidylglycerol metabolites, F. rodentium, and γδ-T cells. Moreover, interleukin (IL)-17, which is secreted by the γδ-T cell when it recognizes lipid antigens, were elevated in HFHC mice and decreased by YJT treatment. In addition, YJT increased the relative abundance of B. acidifaciens in NC or HFHC-fed mice, which is a gut microbiota that mediates anti-obesity and anti-diabetic effects by modulating the gut environment. We also confirmed that YJT ameliorated the gut tight junctions and increased short chain fatty acid (SCFA) levels in the intestine, which resulted in improved IR.

CONCLUSION

These data demonstrated that gut microbiome and hepatic lipid profiles are regulated by YJT, which improved the IR and NAFLD in mice with diet-induced obesity.

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.

Pdgfrα-Cre mediated knockout of the aryl hydrocarbon receptor protects mice from high-fat diet induced obesity and hepatic steatosis

Aryl hydrocarbon receptor (AHR) agonists such as dioxin have been associated with obesity and the development of diabetes. Whole-body Ahr knockout mice on high-fat diet (HFD) have been shown to resist obesity and hepatic steatosis. Tissue-specific knockout of Ahr in mature adipocytes via adiponectin-Cre exacerbates obesity while knockout in liver increases steatosis without having significant effects on obesity. Our previous studies demonstrated that treatment of subcutaneous preadipocytes with exogenous or endogenous AHR agonists disrupts maturation into functional adipocytes in vitro. Here, we used platelet-derived growth factor receptor alpha (Pdgfrα)-Cre mice, a Cre model previously established to knock out genes in preadipocyte lineages and other cell types, but not liver cells, to further define AHR's role in obesity. We demonstrate that Pdgfrα-Cre Ahr-floxed (Ahrfl/fl) knockout mice are protected from HFD-induced obesity compared to non-knockout Ahrfl/fl mice (control mice). The Pdgfrα-Cre Ahrfl/fl knockout mice were also protected from increased adiposity, enlargement of adipocyte size, and liver steatosis while on the HFD compared to control mice. On a regular control diet, knockout and non-knockout mice showed no differences in weight gain, indicating the protective phenotype arises only when animals are challenged by a HFD. At the cellular level, cultured cells from brown adipose tissue (BAT) of Pdgfrα-Cre Ahrfl/fl mice were more responsive than cells from controls to transcriptional activation of the thermogenic uncoupling protein 1 (Ucp1) gene by norepinephrine, suggesting an ability to burn more energy under certain conditions. Collectively, our results show that knockout of Ahr mediated by Pdgfrα-Cre is protective against diet-induced obesity and suggest a mechanism by which enhanced UCP1 activity within BAT might confer these effects.

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.

AhR activation defends gut barrier integrity against damage occurring in obesity

Objective

Obesity is characterized by systemic and low-grade tissue inflammation. In the intestine, alteration of the intestinal barrier and accumulation of inflammatory cells in the epithelium are important contributors of gut inflammation. Recent studies demonstrated the role of the aryl hydrocarbon receptor (AhR) in the maintenance of immune cells at mucosal barrier sites. A wide range of ligands of external and local origin can activate this receptor. We studied the causal relationship between AhR activation and gut inflammation in obesity.

Methods

Jejunum samples from subjects with normal weight and severe obesity were phenotyped according to T lymphocyte infiltration in the epithelium from lamina propria and assayed for the mRNA level of AhR target genes. The effect of an AhR agonist was studied in mice and Caco-2/TC7 cells. AhR target gene expression, permeability to small molecules and ions, and location of cell-cell junction proteins were recorded under conditions of altered intestinal permeability.

Results

We showed that a low AhR tone correlated with a high inflammatory score in the intestinal epithelium in severe human obesity. Moreover, AhR activation protected junctional complexes in the intestinal epithelium in mice challenged by an oral lipid load. AhR ligands prevented chemically induced damage to barrier integrity and cytokine expression in Caco-2/TC7 cells. The PKC and p38MAPK signaling pathways were involved in this AhR action.

Conclusions

The results of these series of human, mouse, and cell culture experiments demonstrate the protective effect of AhR activation in the intestine targeting particularly tight junctions and cytokine expression. We propose that AhR constitutes a valuable target to protect intestinal functions in metabolic diseases, which can be achieved in the future via food or drug ligands.

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Intestinal inflammation in obese subjects correlates with low AhR activity.

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AhR activation protects intestinal epithelium from damages.

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AhR activation prevents cytokine induction in response to damages.

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Protective role of AhR activation targets junctional complexes.

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Protein kinases-dependent signaling pathways are involved in AhR action.

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.

PCB126 blocks the thermogenic beiging response of adipocytes

Subcutaneous white adipose tissue is capable of becoming thermogenic in a process that is referred to as "beiging." Beiging is associated with activation of the uncoupling protein, UCP1, and is known to be important for preventing adipose hypertrophy and development of insulin resistance. Polychlorinated biphenyls (PCBs) accumulate in fat, and it is hypothesized that disruption of adipogenesis and adipocyte function by PCBs may be causative in the development of obesity and diabetes. We developed immortal human subcutaneous preadipocytes that, when differentiated, are capable of beiging. Preadipocytes that were treated with polychlorinated biphenyl congener 126 (PCB126), followed by differentiation, were suppressed for their ability to activate UCP1 upon β-adrenergic stimulation with norepinephrine (NE), demonstrating a block in the beiging response. Treatment of preadipocytes with another known endogenous AhR agonist, indoxyl sulfate (IS), followed by differentiation also blocked the NE-stimulated upregulation of UCP1. Knockdown of the aryl hydrocarbon receptor (AhR) caused the preadipocytes to be refractory to PCB126 and IS effects. The chemical AhR antagonist, CH223191, was effective at preventing the effects of PCB126 but not IS, indicating AhR ligand specificity of CH223191. Repression of NE-induced UCP1 upregulation was also observed when already-differentiated mature adipocytes were treated with PCB126 but not IS. These results indicate that exposure of preadipocytes to endogenous (IS) or exogenous (PCB126) AhR agonists is effective at blocking them from becoming functional adipocytes that are capable of the beiging response. Mature adipocytes may have differential responses. This finding suggests a mechanism by which dioxin-like PCBs such as PCB126 could lead to disruption in energy homeostasis, potentially leading to obesity and diabetes.

Phytohormones: Multifunctional nutraceuticals against metabolic syndrome and comorbid diseases

Metabolic syndrome is characterized by the co-occurrence of diverse symptoms initiating the development of type 2 diabetes, cardiovascular diseases, and a variety of comorbid diseases. The complex constellation of numerous comorbidities makes it difficult to develop common therapeutic approaches that ameliorate these pathological features simultaneously. The plant hormones abscisic acid, salicylic acid, auxin, and cytokinins, have shown promising anti-inflammatory and pro-metabolic effects that could mitigate several disorders relevant to metabolic syndrome. Intriguingly, besides plants, human cells and gut microbes also endogenously produce these molecules, indicating a role in the complex interplay between inflammatory responses associated with metabolic syndrome, the gut microbiome, and nutrition. Here, we introduce how bioactive phytohormones can be generated endogenously and through the gut microbiome. These molecules subsequently influence immune responses and metabolism. We also elaborate on how phytohormones can beneficially modulate metabolic syndrome comorbidities, and propose them as nutraceuticals.

The Other Obesity Epidemic-Of Drugs and Bugs

Chronic psychiatric patients with schizophrenia and related disorders are frequently treatment-resistant and may require higher doses of psychotropic drugs to remain stable. Prolonged exposure to these agents increases the risk of weight gain and cardiometabolic disorders, leading to poorer outcomes and higher medical cost. It is well-established that obesity has reached epidemic proportions throughout the world, however it is less known that its rates are two to three times higher in mentally ill patients compared to the general population. Psychotropic drugs have emerged as a major cause of weight gain, pointing to an urgent need for novel interventions to attenuate this unintended consequence. Recently, the gut microbial community has been linked to psychotropic drugs-induced obesity as these agents were found to possess antimicrobial properties and trigger intestinal dysbiosis, depleting Bacteroidetes phylum. Since germ-free animals exposed to psychotropics have not demonstrated weight gain, altered commensal flora composition is believed to be necessary and sufficient to induce dysmetabolism. Conversely, not only do psychotropics disrupt the composition of gut microbiota but the later alter the metabolism of the former. Here we review the role of gut bacterial community in psychotropic drugs metabolism and dysbiosis. We discuss potential biomarkers reflecting the status of Bacteroidetes phylum and take a closer look at nutritional interventions, fecal microbiota transplantation, and transcranial magnetic stimulation, strategies that may lower obesity rates in chronic psychiatric patients.

Aryl hydrocarbon receptor (AhR) regulates adipocyte differentiation by assembling CRL4B ubiquitin ligase to target PPARγ for proteasomal degradation

Peroxisome proliferator-activated receptor γ (PPARγ) is the central regulator of adipogenesis, and its dysregulation is linked to obesity and metabolic diseases. Identification of the factors that regulate PPARγ expression and activity is therefore crucial for combating obesity. Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a known role in xenobiotic detoxification. Recent studies have suggested that AhR also plays essential roles in energy metabolism. However, the detailed mechanisms remain unclear. We previously reported that experiments with adipocyte-specific Cullin 4b (Cul4b)-knockout mice showed that CUL4B suppresses adipogenesis by targeting PPARγ. Here, using immunoprecipitation, ubiquitination, real-time PCR, and GST-pulldown assays, we report that AhR functions as the substrate receptor in CUL4B-RING E3 ubiquitin ligase (CRL4B) complex and is required for recruiting PPARγ. AhR overexpression reduced PPARγ stability and suppressed adipocyte differentiation, and AhR knockdown stimulated adipocyte differentiation in 3T3-L1 cells. Furthermore, we found that two lysine sites on residues 268 and 293 in PPARγ are targeted for CRL4B-mediated ubiquitination, indicating cross-talk between acetylation and ubiquitination. Our findings establish a critical role of AhR in regulating PPARγ stability and suggest that the AhR-PPARγ interaction may represent a potential therapeutic target for managing metabolic diseases arising from PPARγ dysfunction.

Gut Microbiota-Derived Tryptophan Metabolites Modulate Inflammatory Response in Hepatocytes and Macrophages

The gut microbiota plays a significant role in the progression of fatty liver disease; however, the mediators and their mechanisms remain to be elucidated. Comparing metabolite profile differences between germ-free and conventionally raised mice against differences between mice fed a low- and high-fat diet (HFD), we identified tryptamine and indole-3-acetate (I3A) as metabolites that depend on the microbiota and are depleted under a HFD. Both metabolites reduced fatty-acid- and LPS-stimulated production of pro-inflammatory cytokines in macrophages and inhibited the migration of cells toward a chemokine, with I3A exhibiting greater potency. In hepatocytes, I3A attenuated inflammatory responses under lipid loading and reduced the expression of fatty acid synthase and sterol regulatory element-binding protein-1c. These effects were abrogated in the presence of an aryl-hydrocarbon receptor (AhR) antagonist, indicating that the effects are AhR dependent. Our results suggest that gut microbiota could influence inflammatory responses in the liver through metabolites engaging host receptors.

Maternal Exposure to Bisphenol A Combined with High-Fat Diet-Induced Programmed Hypertension in Adult Male Rat Offspring: Effects of Resveratrol

Maternal exposure to endocrine disrupting chemicals (EDCs) and a high-fat intake may induce the developmental programming of hypertension in adult offspring. Bisphenol A (BPA) is one of the most commonly environmental EDCs. As the nitric oxide (NO) and aryl hydrocarbon receptor (AHR) signaling pathways both contribute to the pathogenesis of hypertension, we evaluated whether resveratrol, an antioxidant and an AHR antagonist, can prevent hypertension programmed by a maternal BPA and HF diet. Sixteen-week-old male rat offspring were assigned to six groups (n = 8 per group): Control, HF (D12331, Research Diets), BPA (50 μg/kg/day), HF + BPA, BPA + R (resveratrol 50mg/L in drinking water throughout pregnancy and lactation), and HF + BPA + R. Maternal BPA exposure exacerbated hypertension programmed by HF consumption in adult male offspring, which was protected by maternal resveratrol therapy. The BPA and HF diet synergistically induced oxidative stress in offspring kidneys, which resveratrol treatment prevented. We observed that HF + BPA-induced programmed hypertension was associated with a decreased NO bioavailability, increased oxidative stress, and an activated AHR signaling pathway. The beneficial effects of resveratrol are relevant to restoring NO bioavailability, reducing oxidative stress, and antagonizing the AHR signaling pathway. Our results cast a new light on resveratrol as a reprogramming strategy to protect against hypertension programmed by combined BPA and HF exposure, but this strategy has yet to be translated into clinical applications.

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.