Lack of the aryl hydrocarbon receptor accelerates aging in mice

The role of gut microbiota in cerebrovascular disease and related dementia

In recent years, increasing evidence suggests that commensal microbiota may play an important role not only in health but also in disease including cerebrovascular disease. Gut microbes impact physiology, at least in part, by metabolizing dietary factors and host-derived substrates and then generating active compounds including toxins. The purpose of this current review is to highlight the complex interplay between microbiota, their metabolites. and essential functions for human health, ranging from regulation of the metabolism and the immune system to modulation of brain development and function. We discuss the role of gut dysbiosis in cerebrovascular disease, specifically in acute and chronic stroke phases, and the possible implication of intestinal microbiota in post-stroke cognitive impairment and dementia, and we identify potential therapeutic opportunities of targeting microbiota in this context. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

Aryl hydrocarbon receptor: Linking environment to aging process in elderly patients with asthma

ABSTRACT

Aging is a significant risk factor for various diseases, including asthma, and it often leads to poorer clinical outcomes, particularly in elderly individuals. It is recognized that age-related diseases are due to a time-dependent accumulation of cellular damage, resulting in a progressive decline in cellular and physiological functions and an increased susceptibility to chronic diseases. The effects of aging affect not only the elderly but also those of younger ages, posing significant challenges to global healthcare. Thus, understanding the molecular mechanisms associated with aging in different diseases is essential. One intriguing factor is the aryl hydrocarbon receptor (AhR), which serves as a cytoplasmic receptor and ligand-activated transcription factor and has been linked to the aging process. Here, we review the literature on several major hallmarks of aging, including mitochondrial dysfunction, cellular senescence, autophagy, mitophagy, epigenetic alterations, and microbiome disturbances. Moreover, we provide an overview of the impact of AhR on these hallmarks by mediating responses to environmental exposures, particularly in relation to the immune system. Furthermore, we explore how aging hallmarks affect clinical characteristics, inflammatory features, exacerbations, and the treatment of asthma. It is suggested that AhR signaling may potentially play a role in regulating asthma phenotypes in elderly populations as part of the aging process.

Deletion of quinolinate phosphoribosyltransferase gene accelerates frailty phenotypes and neuromuscular decline with aging in a sex-specific pattern

Decline in neuromuscular function with aging is known to be a major determinant of disability and all-cause mortality in late life. Despite the importance of the problem, the neurobiology of age-associated muscle weakness is poorly understood. In a previous report, we performed untargeted metabolomics on frail older adults and discovered prominent alteration in the kynurenine pathway, the major route of dietary tryptophan degradation that produces neurotoxic intermediate metabolites. We also showed that neurotoxic kynurenine pathway metabolites are correlated with increased frailty score. For the present study, we sought to further examine the neurobiology of these neurotoxic intermediates by utilizing a mouse model that has a deletion of the quinolinate phosphoribosyltransferase (QPRT) gene, a rate-limiting step of the kynurenine pathway. QPRT-/- mice have elevated neurotoxic quinolinic acid level in the nervous system throughout their lifespan. We found that QPRT-/- mice have accelerated declines in neuromuscular function in an age- and sex-specific manner compared to control strains. In addition, the QPRT-/- mice show premature signs of frailty and body composition changes that are typical for metabolic syndrome. Our findings suggest that the kynurenine pathway may play an important role in frailty and age-associated muscle weakness.

An ERK5-NRF2 Axis Mediates Senescence-Associated Stemness and Atherosclerosis

BACKGROUND

ERK5 (extracellular signal-regulated kinase 5) is a dual kinase transcription factor containing an N-terminal kinase domain and a C-terminal transcriptional activation domain. Many ERK5 kinase inhibitors have been developed and tested to treat cancer and inflammatory diseases. However, recent data have raised questions about the role of the catalytic activity of ERK5 in proliferation and inflammation. We aimed to investigate how ERK5 reprograms myeloid cells to the proinflammatory senescent phenotype, subsequently leading to atherosclerosis.

METHODS

A ERK5 S496A (dephosphorylation mimic) knock in (KI) mouse model was generated using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated 9), and atherosclerosis was characterized by hypercholesterolemia induction. The plaque phenotyping in homozygous ERK5 S496A KI and wild type (WT) mice was studied using imaging mass cytometry. Bone marrow-derived macrophages were isolated from hypercholesterolemic mice and characterized using RNA sequencing and functional in vitro approaches, including senescence, mitochondria reactive oxygen species, and inflammation assays, as well as by metabolic extracellular flux analysis.

RESULTS

We show that atherosclerosis was inhibited in ERK5 S496A KI mice. Furthermore, ERK5 S496 phosphorylation mediates both senescence-associated secretory phenotype and senescence-associated stemness by upregulating AHR (aryl hydrocarbon receptor) in plaque and bone marrow-derived macrophages isolated from hypercholesterolemic mice. We also discovered that ERK5 S496 phosphorylation could induce NRF2 (NFE2-related factor 2) SUMOylation at a novel K518 site to inhibit NRF2 transcriptional activity without altering ERK5 catalytic activity and mediates oxidized LDL (low-density lipoprotein)-induced senescence-associated secretory phenotype. Specific ERK5 kinase inhibitors (AX15836 and XMD8-92) also inhibited ERK5 S496 phosphorylation, suggesting the involvement of ERK5 S496 phosphorylation in the anti-inflammatory effects of these ERK5 kinase inhibitors.

CONCLUSIONS

We discovered a novel mechanism by which the macrophage ERK5-NRF2 axis develops a unique senescence-associated secretory phenotype/stemness phenotype by upregulating AHR to engender atherogenesis. The finding of senescence-associated stemness phenotype provides a molecular explanation to resolve the paradox of senescence in proliferative plaque by permitting myeloid cells to escape the senescence-induced cell cycle arrest during atherosclerosis formation.

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.

Environmentally relevant uptake, elimination, and metabolic changes following early embryonic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in zebrafish

Dioxin and dioxin-like compounds are ubiquitous environmental contaminants that induce toxicity by binding to the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. The zebrafish model has been used to define the developmental toxicity observed following exposure to exogenous AHR ligands such as the potent agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin, TCDD). While the model has successfully identified cellular targets of TCDD and molecular mechanisms mediating TCDD-induced phenotypes, fundamental information such as the body burden produced by standard exposure models is still unknown. We performed targeted gas chromatography (GC) high-resolution mass spectrometry (HRMS) in tandem with non-targeted liquid chromatography (LC) HRMS to quantify TCDD uptake, model the elimination dynamics of TCDD, and determine how TCDD exposure affects the zebrafish metabolome. We found that 50 ppt, 10 ppb, and 1 ppb waterborne exposures to TCDD during early embryogenesis produced environmentally relevant body burdens: 38 ± 4.34, 26.6 ± 1.2, and 8.53 ± 0.341 pg/embryo, respectively, at 24 hours post fertilization. TCDD exposure was associated with the dysregulation of metabolic pathways that are associated with the AHR signaling pathway as well as pathways shown to be affected in mammals following TCDD exposure. In addition, we discovered that TCDD exposure affected several metabolic pathways that are critical for brain development and function including glutamate metabolism, chondroitin sulfate biosynthesis, and tyrosine metabolism. Together, these data demonstrate that existing exposure methods produce environmentally relevant body burdens of TCDD in zebrafish and provide insight into the biochemical pathways impacted by toxicant-induced AHR activation.

The Aryl Hydrocarbon Receptor, Epigenetics and the Aging Process

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, classically associated with the regulation of xenobiotic metabolism in response to environmental toxins. In recent years, transgenic rodent models have implicated AhR in aging and longevity. Moreover, several AhR ligands, such as resveratrol and quercetin, are compounds proven to extend the lifespan of model organisms. In this paper, we first review AhR biology with a focus on aging and highlight several AhR ligands with potential anti-aging properties. We outline how AhR-driven expression of xenobiotic metabolism genes into old age may be a key mechanism through which moderate induction of AhR elicits positive benefits on longevity and healthspan. Furthermore, via integration of publicly available datasets, we show that liver-specific AhR target genes are enriched among genes subject to epigenetic aging. Changes to epigenetic states can profoundly affect transcription factor binding and are a hallmark of the aging process. We suggest that the interplay between AhR and epigenetic aging should be the subject of future research and outline several key gaps in the current literature. Finally, we recommend that a broad range of non-toxic AhR ligands should be investigated for their potential to promote healthspan and longevity.

Proper modulation of AHR signaling is necessary for establishing neural connectivity and oligodendrocyte precursor cell development in the embryonic zebrafish brain

2,3,7,8-tetrachlorodibenzo-[p]-dioxin (TCDD) is a persistent global pollutant that exhibits a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. Epidemiological studies have associated AHR agonist exposure with multiple human neuropathologies. Consistent with the human data, research studies using laboratory models have linked pollutant-induced AHR activation to disruptions in learning and memory as well as motor impairments. Our understanding of endogenous AHR functions in brain development is limited and, correspondingly, scientists are still determining which cell types and brain regions are sensitive to AHR modulation. To identify novel phenotypes resulting from pollutant-induced AHR activation and ahr2 loss of function, we utilized the optically transparent zebrafish model. Early embryonic TCDD exposure impaired embryonic brain morphogenesis, resulted in ventriculomegaly, and disrupted neural connectivity in the optic tectum, habenula, cerebellum, and olfactory bulb. Altered neural network formation was accompanied by reduced expression of synaptic vesicle 2. Loss of ahr2 function also impaired nascent network development, but did not affect gross brain or ventricular morphology. To determine whether neural AHR activation was sufficient to disrupt connectivity, we used the Gal4/UAS system to express a constitutively active AHR specifically in differentiated neurons and observed disruptions only in the cerebellum; thus, suggesting that the phenotypes resulting from global AHR activation likely involve multiple cell types. Consistent with this hypothesis, we found that TCDD exposure reduced the number of oligodendrocyte precursor cells and their derivatives. Together, our findings indicate that proper modulation of AHR signaling is necessary for the growth and maturation of the embryonic zebrafish brain.

Microbiota-derived metabolite Indoles induced aryl hydrocarbon receptor activation and inhibited neuroinflammation in APP/PS1 mice

Gut microbiota alterations might affect the development of Alzheimer's disease (AD) through microbiota-derived metabolites. For example, microbiota-derived Indoles via tryptophan metabolism prevented Aβ accumulation and Tau hyperphosphorylation, restored synaptic plasticity, and then promoted the cognitive and behavioral ability of APP/PS1 mice. The imbalanced compositions of Indoles-producing bacteria with tryptophan deficiency were found in male APP/PS1 mice, but the molecular mechanisms remained unclear. Our current study revealed that Indoles (including indole, indole-3-acetic acid and indole-3-propionic acid) upregulated the production of aryl hydrocarbon receptor (AhR), inhibited the activation of the NF-κB signal pathway as well as the formation of the NLRP3 inflammasome, reduced the release of inflammatory cytokines, including TNF-α, IL-6, IL-1β and IL-18, alleviating the inflammatory response of APP/PS1 mice. These findings demonstrated the roles of Indoles-producing bacteria in activating the AhR pathway to regulate neuroinflammation of AD through gut microbiota-derived Indoles, which implied a novel way for AD treatment.

Peripheral blood CD3+ T-cell gene expression biomarkers correlate with clinical frailty in patients with haematological malignancies

Older patients with cancer often receive treatment regimens based on their age without considering other objective factors that may influence outcomes. Assessment of frailty can identify older patients who are robust and therefore more likely to benefit from intensive treatment, or conversely, frail and might instead be offered alternative approaches. However, such assessment requires specialised training and dedicated clinical resources. Alternative quantitative biomarkers associated with frailty are lacking. Here, we asked if expression signatures of 74 immune cell, ageing, and senescence-related messenger RNAs in purified peripheral blood T cells could identify associations with clinical frailty in patients with haematological malignancies. We studied 69 patients between the ages of 36 and 92 years (median 76 years) with leukaemia, lymphoma, or multiple myeloma, across two institutions. Expression of four genes (aryl hydrocarbon receptor [AHR], CD27, CD28, and interleukin-2 receptor subunit alpha [IL2RA; CD25]) in T cells was associated with frailty, independent of age. An expression-based regression model had 76% sensitivity and 90% specificity to assign a patient as robust. These data identify measurable peripheral blood correlates of clinical frailty and suggest biomarkers for future prospective assessment.

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.

Mutual antagonism between aryl hydrocarbon receptor and hypoxia-inducible factor-1α (AhR/HIF-1α) signaling: Impact on the aging process

The ambient oxygen level, many environmental toxins, and the rays of ultraviolet light (UV) provide a significant risk for the maintenance of organismal homeostasis. The aryl hydrocarbon receptors (AhR) represent a complex sensor system not only for environmental toxins and UV radiation but also for many endogenous ligands, e.g., L-tryptophan metabolites. The AhR signaling system is evolutionarily conserved and AhR homologs existed as many as 600 million years ago. The ancient atmosphere demanded the evolution of an oxygen-sensing system, i.e., hypoxia-inducible transcription factors (HIF) and their prolyl hydroxylase regulators (PHD). Given that both signaling systems have important roles in embryogenesis, it seems that they have been involved in the evolution of multicellular organisms. The evolutionary origin of the aging process is unknown although it is most likely associated with the evolution of multicellularity. Intriguingly, there is compelling evidence that while HIF-1α signaling extends the lifespan, that of AhR promotes many age-related degenerative processes, e.g., it increases oxidative stress, inhibits autophagy, promotes cellular senescence, and aggravates extracellular matrix degeneration. In contrast, HIF-1α signaling stimulates autophagy, inhibits cellular senescence, and enhances cell proliferation. Interestingly, there is a clear antagonism between the AhR and HIF-1α signaling pathways. For instance, (i) AhR and HIF-1α factors heterodimerize with the same factor, ARNT/HIF-1β, leading to their competition for DNA-binding, (ii) AhR and HIF-1α signaling exert antagonistic effects on autophagy, and (iii) co-chaperone p23 exhibits specific functions in the signaling of AhR and HIF-1α factors. One might speculate that it is the competition between the AhR and HIF-1α signaling pathways that is a driving force in the aging process.

Aryl hydrocarbon receptor: From pathogenesis to therapeutic targets in aging-related tissue fibrosis

Aging promotes chronic inflammation, which contributes to fibrosis and decreases organ function. Fibrosis, the excessive synthesis and deposition of extracellular matrix components, is the main cause of most chronic diseases including aging-related organ failure. Organ fibrosis in the heart, liver, and kidneys is the final manifestation of many chronic diseases. The aryl hydrocarbon receptor (AHR) is a cytoplasmic receptor and highly conserved transcription factor that is activated by a variety of small-molecule ligands to affect a wide array of tissue homeostasis functions. In recent years, mounting evidence has revealed that AHR plays an important role in multi-organ fibrosis initiation, progression, and therapy. In this review, we summarise the relationship between AHR and the pathogenesis of aging-related tissue fibrosis, and further discuss how AHR modulates tissue fibrosis by regulating transforming growth factor-β signalling, immune response, and mitochondrial function, which may offer novel targets for the prevention and treatment of this condition.

Ongoing Clinical Trials in Aging-Related Tissue Fibrosis and New Findings Related to AhR Pathways

Fibrosis is a pathological manifestation of wound healing that replaces dead/damaged tissue with collagen-rich scar tissue to maintain homeostasis, and complications from fibrosis contribute to nearly half of all deaths in the industrialized world. Ageing is closely associated with a progressive decline in organ function, and the prevalence of tissue fibrosis dramatically increases with age. Despite the heavy clinical and economic burden of organ fibrosis as the population ages, to date, there is a paucity of therapeutic strategies that are specifically designed to slow fibrosis. Aryl hydrocarbon receptor (AhR) is an environment-sensing transcription factor that exacerbates aging phenotypes in different tissues that has been brought back into the spotlight again with economic development since AhR could interact with persistent organic pollutants derived from incomplete waste combustion. In addition, gut microbiota dysbiosis plays a pivotal role in the pathogenesis of numerous diseases, and microbiota-associated tryptophan metabolites are dedicated contributors to fibrogenesis by acting as AhR ligands. Therefore, a better understanding of the effects of tryptophan metabolites on fibrosis modulation through AhR may facilitate the exploitation of new therapeutic avenues for patients with organ fibrosis. In this review, we primarily focus on how tryptophan-derived metabolites are involved in renal fibrosis, idiopathic pulmonary fibrosis, hepatic fibrosis and cardiac fibrosis. Moreover, a series of ongoing clinical trials are highlighted.

Aryl hydrocarbon receptor blocks aging-induced senescence in the liver and fibroblast cells

Aging impairs organismal homeostasis leading to multiple pathologies. Yet, the mechanisms and molecular intermediates involved are largely unknown. Here, we report that aged aryl hydrocarbon receptor-null mice (AhR-/-) had exacerbated cellular senescence and more liver progenitor cells. Senescence-associated markers β-galactosidase (SA-β-Gal), p16^Ink4a and p21^Cip1 and genes encoding senescence-associated secretory phenotype (SASP) factors TNF and IL1 were overexpressed in aged AhR-/- livers. Chromatin immunoprecipitation showed that AhR binding to those gene promoters repressed their expression, thus adjusting physiological levels in AhR+/+ livers. MCP-2, MMP12 and FGF secreted by senescent cells were overproduced in aged AhR-null livers. Supporting the relationship between senescence and stemness, liver progenitor cells were overrepresented in AhR-/- mice, probably contributing to increased hepatocarcinoma burden. These AhR roles are not liver-specific since adult and embryonic AhR-null fibroblasts underwent senescence in culture, overexpressing SA-β-Gal, p16^Ink4a and p21^Cip1. Notably, depletion of senescent cells with the senolytic agent navitoclax restored expression of senescent markers in AhR-/- fibroblasts, whereas senescence induction by palbociclib induced an AhR-null-like phenotype in AhR+/+ fibroblasts. AhR levels were downregulated by senescence in mouse lungs but restored upon depletion of p16^Ink4a-expressing senescent cells. Thus, AhR restricts age-induced senescence associated to a differentiated phenotype eventually inducing resistance to liver tumorigenesis.

Activation of the aryl hydrocarbon receptor by clozapine induces preadipocyte differentiation and contributes to endothelial dysfunction

BACKGROUND

The superior therapeutic benefit of clozapine is often associated with metabolic disruptions as obesity, insulin resistance, tachycardia, higher blood pressure, and even hypertension.

AIMS

These adverse vascular/ metabolic events under clozapine are similar to those caused by polycyclic aromatic hydrocarbons (PAHs), and clozapine shows structural similarity to well-known ligands of the aryl hydrocarbon receptor (AhR). Therefore, we speculated that the side effects caused by clozapine might rely on AhR signaling.

METHODS

We examined clozapine-induced AhR activation by luciferase reporter assays in hepatoma HepG2 cells and we proved upregulation of the prototypical AhR target gene Cyp1A1 by realtime-PCR (RT-PCR) analysis and enzyme activity. Next we studied the physiological role of AhR in clozapine's effects on human preadipocyte differentiation and on vasodilatation by myography in wild-type and AhR-/- mice.

RESULTS

In contrast to other antipsychotic drugs (APDs), clozapine triggered AhR activation and Cyp1A1 expression in HepG2 cells and adipocytes. Clozapine induced adipogenesis via AhR signaling. After PGF2α-induced constriction of mouse aortic rings, clozapine strongly reduced the maximal vasorelaxation under acetylcholine in rings from wild-type mice, but only slightly in rings from AhR-/- mice. The reduction was also prevented by pretreatment with the AhR antagonist CH-223191.

CONCLUSION

Identification of clozapine as a ligand for the AhR opens new perspectives to explain common clozapine therapy-associated adverse effects at the molecular level.

6-Formylindolo[3,2-b]carbazole, a Potent Ligand for the Aryl Hydrocarbon Receptor Produced Both Endogenously and by Microorganisms, can Either Promote or Restrain Inflammatory Responses

The aryl hydrocarbon receptor (AHR) binds major physiological modifiers of the immune system. The endogenous 6-formylindolo[3,2-b]carbazole (FICZ), which binds with higher affinity than any other compound yet tested, including TCDD, plays a well-documented role in maintaining the homeostasis of the intestines and skin. The effects of transient activation of AHR by FICZ differ from those associated with continuous stimulation and, depending on the dose, include either differentiation into T helper 17 cells that express proinflammatory cytokines or into regulatory T cells or macrophages with anti-inflammatory properties. Moreover, in experimental models of human diseases high doses stimulate the production of immunosuppressive cytokines and suppress pathogenic autoimmunity. In our earlier studies we characterized the formation of FICZ from tryptophan via the precursor molecules indole-3-pyruvate and indole-3-acetaldehyde. In the gut formation of these precursor molecules is catalyzed by microbial aromatic-amino-acid transaminase ArAT. Interestingly, tryptophan can also be converted into indole-3-pyruvate by the amino-acid catabolizing enzyme interleukin-4 induced gene 1 (IL4I1), which is secreted by host immune cells. By thus generating derivatives of tryptophan that activate AHR, IL4I1 may have a role to play in anti-inflammatory responses, as well as in a tumor escape mechanism that reduces survival in cancer patients. The realization that FICZ can be produced from tryptophan by sunlight, by enzymes expressed in our cells (IL4I1), and by microorganisms as well makes it highly likely that this compound is ubiquitous in humans. A diurnal oscillation in the level of FICZ that depends on the production by the fluctuating number of microbes might influence not only intestinal and dermal immunity locally, but also systemic immunity.

The Role of AhR in the Hallmarks of Brain Aging: Friend and Foe

In recent years, aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, has been considered to be involved in aging phenotypes across several species. This receptor is a highly conserved biosensor that is activated by numerous exogenous and endogenous molecules, including microbiota metabolites, to mediate several physiological and toxicological functions. Brain aging hallmarks, which include glial cell activation and inflammation, increased oxidative stress, mitochondrial dysfunction, and cellular senescence, increase the vulnerability of humans to various neurodegenerative diseases. Interestingly, many studies have implicated AhR signaling pathways in the aging process and longevity across several species. This review provides an overview of the impact of AhR pathways on various aging hallmarks in the brain and the implications for AhR signaling as a mechanism in regulating aging-related diseases of the brain. We also explore how the nature of AhR ligands determines the outcomes of several signaling pathways in brain aging processes.

Aryl Hydrocarbon Receptor Defect Attenuates Mitogen-Activated Signaling through Leucine-Rich Repeats and Immunoglobulin-like Domains 1 (LRIG1)-Dependent EGFR Degradation

Aryl hydrocarbon receptor (AHR) genomic pathway has been well-characterized in a number of respiratory diseases. In addition, the cytoplasmic AHR protein may act as an adaptor of E3 ubiquitin ligase. In this study, the physiological functions of AHR that regulate cell proliferation were explored using the CRISPR/Cas9 system. The doubling-time of the AHR-KO clones of A549 and BEAS-2B was observed to be prolonged. The attenuation of proliferation potential was strongly associated with either the induction of p27Kip1 or the impairment in mitogenic signal transduction driven by the epidermal growth factor (EGF) and EGF receptor (EGFR). We found that the leucine-rich repeats and immunoglobulin-like domains 1 (LRIG1), a repressor of EGFR, was induced in the absence of AHR in vitro and in vivo. The LRIG1 tends to degrade via a proteasome dependent manner by interacting with AHR in wild-type cells. Either LRIG1 or a disintegrin and metalloprotease 17 (ADAM17) were accumulated in AHR-defective cells, consequently accelerating the degradation of EGFR, and attenuating the response to mitogenic stimulation. We also affirmed low AHR but high LRIG1 levels in lung tissues of chronic obstructive pulmonary disease (COPD) patients. This might partially elucidate the sluggish tissue repairment and developing inflammation in COPD patients.

Aryl Hydrocarbon Receptor and Its Diverse Ligands and Functions: An Exposome Receptor

The aryl hydrocarbon receptor (AhR) is a transcriptional factor that regulates multiple functions following its activation by a variety of ligands, including xenobiotics, natural products, microbiome metabolites, and endogenous molecules. Because of this diversity, the AhR constitutes an exposome receptor. One of its main functions is to regulate several lines of defense against chemical insults and bacterial infections. Indeed, in addition to its well-established detoxication function, it has several functions at physiological barriers, and it plays a critical role in immunomodulation. The AhR is also involved in the development of several organs and their homeostatic maintenance. Its activity depends on the type of ligand and on the time frame of the receptor activation, which can be either sustained or transient, leading in some cases to opposite modes of regulations as illustrated in the regulation of different cancer pathways. The development of selective modulators and their pharmacological characterization are important areas of research. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Examination of CA1 Hippocampal DNA Methylation as a Mechanism for Closing of Estrogen's Critical Window

There is a critical window for estrogen replacement therapy, beyond which estradiol (E2) fails to enhance cognition and N-methyl-D-aspartate (NMDA) receptor function, and E2-responsive transcription decreases. Much less attention has been given to the mechanism for closing of the critical window, which is thought to involve the decline in estrogen signaling cascades, possibly involving epigenetic mechanisms, including DNA methylation. This study investigated changes in DNA methylation in region CA1 of the hippocampus of ovariectomized female rats over the course of brain aging and in response to E2-treatment, using whole genome bisulfite sequencing. Differential methylation of CpG and non-CpG (CHG and CHH) sites and associated genes were characterized in aged controls (AC), middle-age controls (MC), and young controls (YC) and differential methylation in response to E2-treatment (T) was examined in each age group (AT-AC, MT-MC, and YT-YC). Possible candidate genes for the closing of the critical window were defined as those that were hypomethylated by E2-treatment in younger animals, but were unresponsive in aged animals. Gene ontology categories for possible critical window genes were linked to response to hormones (Adcyap1, Agtr2, Apob, Ahr, Andpro, Calm2, Cyp4a2, Htr1b, Nr3c2, Pitx2, Pth, Pdk4, Slc2a2, Tnc, and Wnt5a), including G-protein receptor signaling (Gpr22 and Rgs4). Other possible critical window genes were linked to glutamate synapses (Nedd4, Grm1, Grm7, and Grin3a). These results suggest that decreased E2 signaling with advanced age, and/or prolonged E2 deprivation, results in methylation of E2-responsive genes, including those involved in rapid E2 signaling, which may limit subsequent transcription.

Gut-Brain Connection: Microbiome, Gut Barrier, and Environmental Sensors

The gut is an important organ with digestive and immune regulatory function which consistently harbors microbiome ecosystem. The gut microbiome cooperates with the host to regulate the development and function of the immune, metabolic, and nervous systems. It can influence disease processes in the gut as well as extra-intestinal organs, including the brain. The gut closely connects with the central nervous system through dynamic bidirectional communication along the gut-brain axis. The connection between gut environment and brain may affect host mood and behaviors. Disruptions in microbial communities have been implicated in several neurological disorders. A link between the gut microbiota and the brain has long been described, but recent studies have started to reveal the underlying mechanism of the impact of the gut microbiota and gut barrier integrity on the brain and behavior. Here, we summarized the gut barrier environment and the 4 main gut-brain axis pathways. We focused on the important function of gut barrier on neurological diseases such as stress responses and ischemic stroke. Finally, we described the impact of representative environmental sensors generated by gut bacteria on acute neurological disease via the gut-brain axis.

Neurons expressing the aryl hydrocarbon receptor in the locus coeruleus and island of Calleja major are novel targets of dioxin in the mouse brain

The aryl hydrocarbon receptor (AhR) acts as a receptor that responds to ligands, including dioxin. The AhR-ligand complex translocates from the cytoplasm into the nucleus to induce gene expression. Because dioxin exposure impairs cognitive and neurobehavioral functions, AhR-expressing neurons need to be identified for elucidation of the dioxin neurotoxicity mechanism. Immunohistochemistry was performed to detect AhR-expressing neurons in the mouse brain and confirm the specificity of the anti-AhR antibody using Ahr^-/- mice. Intracellular distribution of AhR and expression level of AhR-target genes, Cyp1a1, Cyp1b1, and Ahr repressor (Ahrr), were analyzed by immunohistochemistry and quantitative RT-PCR, respectively, using mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The mouse brains were shown to harbor AhR in neurons of the locus coeruleus (LC) and island of Calleja major (ICjM) during developmental period in Ahr^+/+ mice but not in Ahr^-/- mice. A significant increase in nuclear AhR of ICjM neurons but not LC neurons was found in 14-day-old mice compared to 5- and 7-day-old mice. AhR was significantly translocated into the nucleus in LC and ICjM neurons of TCDD-exposed adult mice. Additionally, the expression levels of Cyp1a1, Cyp1b1, and Ahrr genes in the brain, a surrogate of TCDD in the tissue, were significantly increased by dioxin exposure, suggesting that dioxin-activated AhR induces gene expression in LC and ICjM neurons. This histochemical study shows the ligand-induced nuclear translocation of AhR at the single-neuron level in vivo. Thus, the neurotoxicological significance of the dioxin-activated AhR in the LC and ICjM warrants further studies.

"GrimAge," an epigenetic predictor of mortality, is accelerated in major depressive disorder

Major depressive disorder (MDD) is associated with premature mortality and is an independent risk factor for a broad range of diseases, especially those associated with aging, such as cardiovascular disease, diabetes, and Alzheimer's disease. However, the pathophysiology underlying increased rates of somatic disease in MDD remains unknown. It has been proposed that MDD represents a state of accelerated cellular aging, and several measures of cellular aging have been developed in recent years. Among such metrics, estimators of biological age based on predictable age-related patterns of DNA methylation (DNAm), so-called 'epigenetic clocks', have shown particular promise for their ability to capture accelerated aging in psychiatric disease. The recently developed DNAm metric known as 'GrimAge' is unique in that it was trained on time-to-death data and has outperformed its predecessors in predicting both morbidity and mortality. Yet, GrimAge has not been investigated in MDD. Here we measured GrimAge in 49 somatically healthy unmedicated individuals with MDD and 60 age-matched healthy controls. We found that individuals with MDD exhibited significantly greater GrimAge relative to their chronological age ('AgeAccelGrim') compared to healthy controls (p = 0.001), with a median of 2 years of excess cellular aging. This difference remained significant after controlling for sex, current smoking status, and body-mass index (p = 0.015). These findings are consistent with prior suggestions of accelerated cellular aging in MDD, but are the first to demonstrate this with an epigenetic metric predictive of premature mortality.

Role of aryl hydrocarbon receptor (AHR) in overall retinoid metabolism: Response comparisons to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure between wild-type and AHR knockout mice

Young adult wild-type and aryl hydrocarbon receptor knockout (AHRKO) mice of both sexes and the C57BL/6J background were exposed to 10 weekly oral doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; total dose of 200 μg/kg bw) to further characterize the observed impacts of AHR as well as TCDD on the retinoid system. Unexposed AHRKO mice harboured heavier kidneys, lighter livers and lower serum all-trans retinoic acid (ATRA) and retinol (REOH) concentrations than wild-type mice. Results from the present study also point to a role for the murine AHR in the control of circulating REOH and ATRA concentrations. In wild-type mice, TCDD elevated liver weight and reduced thymus weight, and drastically reduced the hepatic concentrations of 9-cis-4-oxo-13,14-dihydro-retinoic acid (CORA) and retinyl palmitate (REPA). In female wild-type mice, TCDD increased the hepatic concentration of ATRA as well as the renal and circulating REOH concentrations. Renal CORA concentrations were substantially diminished in wild-type male mice exclusively following TCDD-exposure, with a similar tendency in serum. In contrast, TCDD did not affect any of these toxicity or retinoid system parameters in AHRKO mice. Finally, a distinct sex difference occurred in kidney concentrations of all the analysed retinoid forms. Together, these results strengthen the evidence of a mandatory role of AHR in TCDD-induced retinoid disruption, and suggest that the previously reported accumulation of several retinoid forms in the liver of AHRKO mice is a line-specific phenomenon. Our data further support participation of AHR in the control of liver and kidney development in mice.

Anti-Aging Physiological Roles of Aryl Hydrocarbon Receptor and Its Dietary Regulators

The vast majority of the literature on the aryl hydrocarbon receptor is concerned with its functions in xenobiotic detoxification. However, in the course of evolution, this receptor had to have physiological (rather than toxicological) functions. Our aim was to review the aryl hydrocarbon receptor’s role in the physiological functions involved in aging. This study was performed by searching the MEDLINE and Google Academic databases. A total of 34 articles were selected that focused specifically on the aryl hydrocarbon receptor and aging, the aryl hydrocarbon receptor and physiological functions, and the combination of both. This receptor’s main physiological functions (mediated by the modulation of gene expression) were cell regeneration, the immune reaction, intestinal homeostasis, and cell proliferation. Furthermore, it was shown that the loss of this receptor led to premature aging. This process may be caused by the dysregulation of hematopoietic stem cells, loss of glucose and lipid homeostasis, increase in inflammation, and deterioration of the brain. We conclude that the aryl hydrocarbon receptor, apart from its well-established role in xenobiotic detoxication, plays an important role in physiological functions and in the aging process. Modulation of the signaling pathway of this receptor could be a therapeutic target of interest in aging.

Indole-3-carbinol regulates microglia homeostasis and protects the retina from degeneration

BACKGROUND

Retinal degenerative diseases significantly contribute to visual impairment and blindness. Microglia reactivity is a hallmark of neurodegenerative diseases including retinal cell death and immunomodulation emerges as a therapeutic option. Indole-3-carbinol (I3C) is a natural ligand of aryl hydrocarbon receptor (AhR), with potent immunomodulatory properties. Here, we hypothesized that I3C may inhibit microglia reactivity and exert neuroprotective effects in the light-damaged murine retina mimicking important immunological aspects of retinal degeneration.

METHODS

BV-2 microglia were treated in vitro with I3C followed by lipopolysaccharide (LPS) stimulation to analyze pro-inflammatory and anti-oxidant responses by quantitative real-time PCR (qRT-PCR) and Western blots. Nitric oxide (NO) secretion, caspase 3/7 levels, phagocytosis rates, migration, and morphology were analyzed in control and AhR knockdown cells. I3C or vehicle was systemically applied to light-treated BALB/cJ mice as an experimental model of retinal degeneration. Pro-inflammatory and anti-oxidant responses in the retina were examined by qRT-PCR, ELISA, and Western blots. Immunohistochemical staining of retinal flat mounts and cryosections were performed. The retinal thickness and structure were evaluated by in vivo imaging using spectral domain-optical coherence tomography (SD-OCT).

RESULTS

The in vitro data showed that I3C potently diminished LPS-induced pro-inflammatory gene expression of I-NOS, IL-1ß, NLRP3, IL-6, and CCL2 and induced anti-oxidants gene levels of NQO1, HMOX1, and CAT1 in BV-2 cells. I3C also reduced LPS-induced NO secretion, phagocytosis, and migration as important functional microglia parameters. siRNA-mediated knockdown of AhR partially prevented the previously observed gene regulatory events. The in vivo experiments revealed that I3C treatment diminished light-damage induced I-NOS, IL-1ß, NLRP3, IL-6, and CCL2 transcripts and also reduced CCL2, I-NOS, IL-1ß, p-NFkBp65 protein levels in mice. Moreover, I3C increased anti-oxidant NQO1 and HMOX1 protein levels in light-exposed retinas. Finally, I3C therapy prevented the accumulation of amoeboid microglia in the subretinal space and protected from retinal degeneration.

CONCLUSIONS

The AhR ligand I3C potently counter-acts microgliosis and light-induced retinal damage, highlighting a potential treatment concept for retinal degeneration.

Aryl hydrocarbon receptor in mesenchymal stromal cells: new frontiers in AhR biology

Mesenchymal stromal cells (MSCs) are nonhematopoietic cells that have been clinically explored as investigational cellular therapeutics for tissue injury regeneration and immune-mediated diseases. Their pharmaceutical properties arise from activation of endogenous receptors and transcription factors leading to a paracrine effect which mirror the biology of progenitors from which they arise. The aryl hydrocarbon receptor (AhR) is a transcription factor that has been extensively studied as an environmental sensor for xenobiotics, but recent findings suggest it can modulate immunological functions. Both genetic and pharmacological investigations revealed that MSCs express AhR and that it plays roles in inflammation, immunomodulation, and mesodermal plasticity of endogenous MSCs. Further, AhR has been shown to interact with key signaling cascades associated with these conditions. Therefore, AhR has potential to be an attractive target in both endogenous and culture-adapted MSCs for novel therapeutics to treat inflammation and other age-related disorders.

Identification of endogenous 1-aminopyrene as a novel mediator of progressive chronic kidney disease via aryl hydrocarbon receptor activation

BACKGROUND AND PURPOSE

Increasing evidence has indicated that the high risk of cardiovascular disease in chronic kidney disease (CKD) patients cannot be sufficiently explained by classic risk factors.

EXPERIMENTAL APPROACH

Based on the least absolute shrinkage and selection operator method, we identified significantly altered renal tissue metabolites during progressive CKD in a 5/6 nephrectomized rat model and in CKD patients.

KEY RESULTS

Six aryl-containing metabolites (ACMs) were significantly increased from Week 1 to Week 20. They were associated with the activation of aryl hydrocarbon receptor (AhR) and its target genes including CYP1A1, CYP1A2 and CYP1B1, which were further validated by molecular docking. Our study further demonstrated that AhR signalling could be activated by ACM in patients with idiopathic membranous nephropathy, diabetic nephropathy and IgA nephropathy. Most importantly, 1-aminopyrene (AP) showed strong positive and negative correlation with serum creatinine and creatinine clearance, respectively. AP significantly up-regulated the mRNA expressions of AhR and its three target genes in both mice and NRK-52E cells, while this effect was partially weakened in AhR small hairpin RNA-treated mice and NRK-52E cells. Furthermore, dietary flavonoid supplementation ameliorated CKD and renal fibrosis through partially inhibiting the AhR activity via lowering the ACM levels. The antagonistic effect of flavonoids on AhR was deeply influenced by the number and location of hydroxyl and glycosyl groups.

CONCLUSION AND IMPLICATIONS

We uncovered that endogenous AP is a novel mediator of CKD progression via AhR activation; thus, AhR might serve as a promising target for CKD treatment.

Aryl Hydrocarbon Receptor in Post-Mortem Hippocampus and in Serum from Young, Elder, and Alzheimer's Patients

One of the characteristics of the cerebral aging process is the presence of chronic inflammation through glial cells, which is particularly significant in neurodegeneration. On the other hand, it has been demonstrated that the aryl hydrocarbon receptor (AHR) participates in the inflammatory response. Currently, evidence in animal models shows that the hallmarks of aging are associated with changes in the AHR levels. However, there is no information concerning the behavior and participation of AHR in the human aging brain or in Alzheimer’s disease (AD). We evaluated the expression of AHR in human hippocampal post-mortem tissue and its association with reactive astrocytes by immunohistochemistry. Besides this, we analyzed through ELISA the AHR levels in blood serum from young and elder participants, and from AD patients. The levels of AHR and glial fibrillar acid protein were higher in elder than in young post-mortem brain samples. AHR was localized mainly in the cytosol of astrocytes and displayed a pattern that resembles extracellular vesicles; this latter feature was more conspicuous in AD subjects. We found higher serum levels of AHR in AD patients than in the other participants. These results suggest that AHR participates in the aging process, and probably in the development of neurodegenerative diseases like AD.

The Aryl Hydrocarbon Receptor (AhR) in the Aging Process: Another Puzzling Role for This Highly Conserved Transcription Factor

Aging is the most important risk factor for the development of major life-threatening diseases such as cardiovascular disorders, cancer, and neurodegenerative disorders. The aging process is characterized by the accumulation of damage to intracellular macromolecules and it is concurrently shaped by genetic, environmental and nutritional factors. These factors influence the functionality of mitochondria, which play a central role in the aging process. Mitochondrial dysfunction is one of the hallmarks of aging and is associated with increased fluxes of ROS leading to damage of mitochondrial components, impaired metabolism of fatty acids, dysregulated glucose metabolism, and damage of adjacent organelles. Interestingly, many of the environmental (e.g., pollutants and other toxicants) and nutritional (e.g., flavonoids, carotenoids) factors influencing aging and mitochondrial function also directly or indirectly affect the activity of a highly conserved transcription factor, the Aryl hydrocarbon Receptor (AhR). Therefore, it is not surprising that many studies have already indicated a role of this versatile transcription factor in the aging process. We also recently found that the AhR promotes aging phenotypes across species. In this manuscript, we systematically review the existing literature on the contradictory studies indicating either pro- or anti-aging effects of the AhR and try to reconcile the seemingly conflicting data considering a possible dependency on the animal model, tissue, as well as level of AhR expression and activation. Moreover, given the crucial role of mitochondria in the aging process, we summarize the growing body of evidence pointing toward the influence of AhR on mitochondria, which can be of potential relevance for aging.

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.