A role for the aryl hydrocarbon receptor in regulation of ischemia-induced angiogenesis

The Impact of the Aryl Hydrocarbon Receptor on Antenatal Chemical Exposure-Induced Cardiovascular-Kidney-Metabolic Programming

Early life exposure lays the groundwork for the risk of developing cardiovascular-kidney-metabolic (CKM) syndrome in adulthood. Various environmental chemicals to which pregnant mothers are commonly exposed can disrupt fetal programming, leading to a wide range of CKM phenotypes. The aryl hydrocarbon receptor (AHR) has a key role as a ligand-activated transcription factor in sensing these environmental chemicals. Activating AHR through exposure to environmental chemicals has been documented for its adverse impacts on cardiovascular diseases, hypertension, diabetes, obesity, kidney disease, and non-alcoholic fatty liver disease, as evidenced by both epidemiological and animal studies. In this review, we compile current human evidence and findings from animal models that support the connection between antenatal chemical exposures and CKM programming, focusing particularly on AHR signaling. Additionally, we explore potential AHR modulators aimed at preventing CKM syndrome. As the pioneering review to present evidence advocating for the avoidance of toxic chemical exposure during pregnancy and deepening our understanding of AHR signaling, this has the potential to mitigate the global burden of CKM syndrome in the future.

Lung cancer associated with combustion particles and fine particulate matter (PM2.5) - The roles of polycyclic aromatic hydrocarbons (PAHs) and the aryl hydrocarbon receptor (AhR)

Air pollution is the leading cause of lung cancer after tobacco smoking, contributing to 20% of all lung cancer deaths. Increased risk associated with living near trafficked roads, occupational exposure to diesel exhaust, indoor coal combustion and cigarette smoking, suggest that combustion components in ambient fine particulate matter (PM2.5), such as polycyclic aromatic hydrocarbons (PAHs), may be central drivers of lung cancer. Activation of the aryl hydrocarbon receptor (AhR) induces expression of xenobiotic-metabolizing enzymes (XMEs) and increase PAH metabolism, formation of reactive metabolites, oxidative stress, DNA damage and mutagenesis. Lung cancer tissues from smokers and workers exposed to high combustion PM levels contain mutagenic signatures derived from PAHs. However, recent findings suggest that ambient air PM2.5 exposure primarily induces lung cancer development through tumor promotion of cells harboring naturally acquired oncogenic mutations, thus lacking typical PAH-induced mutations. On this background, we discuss the role of AhR and PAHs in lung cancer development caused by air pollution focusing on the tumor promoting properties including metabolism, immune system, cell proliferation and survival, tumor microenvironment, cell-to-cell communication, tumor growth and metastasis. We suggest that the dichotomy in lung cancer patterns observed between smoking and outdoor air PM2.5 represent the two ends of a dose-response continuum of combustion PM exposure, where tumor promotion in the peripheral lung appears to be the driving factor at the relatively low-dose exposures from ambient air PM2.5, whereas genotoxicity in the central airways becomes increasingly more important at the higher combustion PM levels encountered through smoking and occupational exposure.

Activation of the Aryl Hydrocarbon Receptor in Endothelial Cells Impairs Ischemic Angiogenesis in Chronic Kidney Disease

Rationale

Chronic kidney disease (CKD) is a strong risk factor for peripheral artery disease (PAD) that is associated with worsened clinical outcomes. CKD leads to accumulation of tryptophan metabolites that associate with adverse limb events in PAD and are ligands of the aryl hydrocarbon receptor (AHR) which may regulate ischemic angiogenesis.

Objectives

To test if endothelial cell-specific deletion of the AHR (AHRecKO) alters ischemic angiogenesis and limb function in mice with CKD subjected to femoral artery ligation.

Findings

Male AHRecKO mice with CKD displayed better limb perfusion recovery and enhanced ischemic angiogenesis compared to wildtype mice with CKD. However, the improved limb perfusion did not result in better muscle performance. In contrast to male mice, deletion of the AHR in female mice with CKD had no impact on perfusion recovery or angiogenesis. Using primary endothelial cells from male and female mice, treatment with indoxyl sulfate uncovered sex-dependent differences in AHR activating potential and RNA sequencing revealed wide ranging sex-differences in angiogenic signaling pathways.

Conclusion

Endothelium-specific deletion of the AHR improved ischemic angiogenesis in male, but not female, mice with CKD. There are sex-dependent differences in Ahr activating potential within endothelial cells that are independent of sex hormones.

Activation of the Aryl Hydrocarbon Receptor in Muscle Exacerbates Ischemic Pathology in Chronic Kidney Disease

BACKGROUND

Chronic kidney disease (CKD) accelerates the development of atherosclerosis, decreases muscle function, and increases the risk of amputation or death in patients with peripheral artery disease (PAD). However, the mechanisms underlying this pathobiology are ill-defined. Recent work has indicated that tryptophan-derived uremic solutes, which are ligands for AHR (aryl hydrocarbon receptor), are associated with limb amputation in PAD. Herein, we examined the role of AHR activation in the myopathy of PAD and CKD.

METHODS

AHR-related gene expression was evaluated in skeletal muscle obtained from mice and human PAD patients with and without CKD. AHRmKO (skeletal muscle-specific AHR knockout) mice with and without CKD were subjected to femoral artery ligation, and a battery of assessments were performed to evaluate vascular, muscle, and mitochondrial health. Single-nuclei RNA sequencing was performed to explore intercellular communication. Expression of the constitutively active AHR was used to isolate the role of AHR in mice without CKD.

RESULTS

PAD patients and mice with CKD displayed significantly higher mRNA expression of classical AHR-dependent genes (Cyp1a1, Cyp1b1, and Aldh3a1) when compared with either muscle from the PAD condition with normal renal function (P<0.05 for all 3 genes) or nonischemic controls. AHRmKO significantly improved limb perfusion recovery and arteriogenesis, preserved vasculogenic paracrine signaling from myofibers, increased muscle mass and strength, as well as enhanced mitochondrial function in an experimental model of PAD/CKD. Moreover, viral-mediated skeletal muscle-specific expression of a constitutively active AHR in mice with normal kidney function exacerbated the ischemic myopathy evidenced by smaller muscle masses, reduced contractile function, histopathology, altered vasculogenic signaling, and lower mitochondrial respiratory function.

CONCLUSIONS

These findings establish AHR activation in muscle as a pivotal regulator of the ischemic limb pathology in CKD. Further, the totality of the results provides support for testing of clinical interventions that diminish AHR signaling in these conditions.

AhR and HIF-1α Signaling Pathways in Benign Meningioma under Hypoxia

The role of hypoxia in benign meningiomas is less clear than that in the malignant meningiomas. Hypoxia-induced transcription factor 1 subunit alpha (HIF-1α) and its downstream signaling pathways play a central role in the mechanism of hypoxia. HIF-1α forms a complex with the aryl hydrocarbon receptor nuclear translocator (ARNT) protein and can compete for ARNT with aryl hydrocarbon receptor (AhR). In this work, the status of HIF-1α- and AhR-dependent signaling pathways was investigated in World Health Organization (WHO) grade 1 meningioma and patient-derived tumor primary cell culture under hypoxic conditions. mRNA levels of HIF-1α, AhR, and of their target genes as well as of ARNT and nuclear receptor coactivator NCOA2 were determined in tumor tissues from patients in whom the tumor was promptly removed either with or without prior endovascular embolization. Using the patient-derived nonembolized tumor primary cell culture, the effects of a hypoxia mimetic cobalt chloride (CoCl₂) and an activator of the AhR signaling pathway benzo(α)pyrene (B[a]P) on mRNA levels of HIF-1α, AhR, and their target genes were investigated. Our findings show active functioning of AhR signaling in meningioma tissue of patients with tumor embolization and crosstalk between HIF-1α and AhR signaling in meningeal cells under hypoxia.

Chronic activation of the aryl hydrocarbon receptor in muscle exacerbates ischemic pathology in chronic kidney disease

Chronic kidney disease (CKD) accelerates the development of atherosclerosis, decreases muscle function, and increases the risk of amputation or death in patients with peripheral artery disease (PAD). However, the cellular and physiological mechanisms underlying this pathobiology are ill-defined. Recent work has indicated that tryptophan-derived uremic toxins, many of which are ligands for the aryl hydrocarbon receptor (AHR), are associated with adverse limb outcomes in PAD. We hypothesized that chronic AHR activation, driven by the accumulation of tryptophan-derived uremic metabolites, may mediate the myopathic condition in the presence of CKD and PAD. Both PAD patients with CKD and mice with CKD subjected to femoral artery ligation (FAL) displayed significantly higher mRNA expression of classical AHR-dependent genes ( Cyp1a1 , Cyp1b1 , and Aldh3a1 ) when compared to either muscle from the PAD condition with normal renal function ( P <0.05 for all three genes) or non-ischemic controls. Skeletal-muscle-specific AHR deletion in mice (AHR mKO ) significantly improved limb muscle perfusion recovery and arteriogenesis, preserved vasculogenic paracrine signaling from myofibers, increased muscle mass and contractile function, as well as enhanced mitochondrial oxidative phosphorylation and respiratory capacity in an experimental model of PAD/CKD. Moreover, viral-mediated skeletal muscle-specific expression of a constitutively active AHR in mice with normal kidney function exacerbated the ischemic myopathy evidenced by smaller muscle masses, reduced contractile function, histopathology, altered vasculogenic signaling, and lower mitochondrial respiratory function. These findings establish chronic AHR activation in muscle as a pivotal regulator of the ischemic limb pathology in PAD. Further, the totality of the results provide support for testing of clinical interventions that diminish AHR signaling in these conditions.

AhR regulates VEGF expression by promoting STAT1 transcriptional activity, thereby affecting endothelial angiogenesis in acute limb ischemia

BACKGROUND

Angiogenesis has great potential in the treatment of acute limb ischemia (ALI). Here, we aimed to investigate the effect and mechanism of Aryl hydrocarbon receptor (AhR) on angiogenesis in ALI.

METHODS

The ALI mouse model was constructed by femoral artery ligation, and the cell ischemia injury was induced by Hypoxia/serum deprivation. The laser doppler perfusion imaging was executed to detect the limb blood flow velocity. The tube formation assay was performed to evaluate angiogenesis. The cell viability was measured by 3-(45)-dimethylthiahiazo(-z-y1)-35-di-phenytetrazoliumromide. The cell migration was detected by wound healing assay. Hematoxylin-eosin, immunohistochemistry, immunofluorescence, dual-luciferase reporter gene assay, and Chromatin immunoprecipitation assay were conducted.

RESULTS

In ALI models, AhR expression was increased and translocated from cytoplasm to nucleus. Besides, necrosis and inflammatory infiltration were also increased in gastrocnemius tissues of model mice. In addition, AhR loss (LV-sh-AhR) promoted cell viability, angiogenesis, and migration, and also elevated the levels of vascular endothelial growth factor (VEGF), Tie2, and Ang2 in HUVEC models with Hypoxia/serum deprivation injury. Meanwhile, the interaction between AhR and signal transducer and activator of transcription 1 (STAT1), as well as STAT1 and VEGF, has also been confirmed. Co-transfection of LV-sh-AhR and LV-STAT1 suppressed cell viability, angiogenesis, and migration of injured HUVECs. Furthermore, injection of AAV2/9-shAhR in vivo also promoted angiogenesis, which was consistent with the in vitro experimental results.

CONCLUSIONS

In ALI models, activated AhR was translocated to the nucleus and down-regulated VEGF expression by promoting the transcriptional activity of STAT1, thereby inhibiting endothelial angiogenesis.

Kynurenine promotes neonatal heart regeneration by stimulating cardiomyocyte proliferation and cardiac angiogenesis

Indoleamine 2,3 dioxygenase-1 (IDO1) catalyzes tryptophan-kynurenine metabolism in many inflammatory and cancer diseases. Of note, acute inflammation that occurs immediately after heart injury is essential for neonatal cardiomyocyte proliferation and heart regeneration. However, the IDO1-catalyzed tryptophan metabolism during heart regeneration is largely unexplored. Here, we find that apical neonatal mouse heart resection surgery led to rapid and consistent increases in cardiac IDO1 expression and kynurenine accumulation. Cardiac deletion of Ido1 gene or chemical inhibition of IDO1 impairs heart regeneration. Mechanistically, elevated kynurenine triggers cardiomyocyte proliferation by activating the cytoplasmic aryl hydrocarbon receptor-SRC-YAP/ERK pathway. In addition, cardiomyocyte-derived kynurenine transports to endothelial cells and stimulates cardiac angiogenesis by promoting aryl hydrocarbon receptor nuclear translocation and enhancing vascular endothelial growth factor A expression. Notably, Ahr deletion prevents indoleamine 2,3 dioxygenase -kynurenine-associated heart regeneration. In summary, increasing indoleamine 2,3 dioxygenase-derived kynurenine level promotes cardiac regeneration by functioning as an endogenous regulator of cardiomyocyte proliferation and cardiac angiogenesis.

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.

Chrysin promotes angiogenesis in rat hindlimb ischemia: Impact on PI3K/Akt/mTOR signaling pathway and autophagy

Limb ischemia occurs due to obstruction of blood perfusion to lower limbs, a manifestation that is associated with peripheral artery disease (PAD). Angiogenesis is important for adequate oxygen delivery. The present study investigated a potential role for chrysin, a naturally occurring flavonoid, in promoting angiogenesis in hindlimb ischemia (HLI) rat model. Rats were allocated into four groups: (1) sham-operated control, (2) HLI: subjected to unilateral femoral artery ligation, (3) HLI + chrysin: received 100 mg/kg, i.p. chrysin immediately after HLI, and (4) HLI + chrysin + rapamycin: received 6 mg/kg/day rapamycin i.p. for 5 days then subjected to HLI and dosed with 100 mg/kg chrysin, i.p. Rats were killed 18 h later and gastrocnemius muscles were collected and divided into parts for (1) immunohistochemistry detection of CD31 and CD105, (2) qRT-PCR analysis of eNOS and VEGFR2, (3) colorimetric analysis of NO, (4) ELISA estimation of TGF-β, VEGF, ATG5 and Beclin-1, and (5) Western blot analysis of p-PI3K, PI3K, p-Akt, Akt, p-mTOR, mTOR, and HIF-1α. Chrysin significantly enhanced microvessels growth in HLI muscles as indicated by increased CD31 and CD105 levels and decreased TGF-β. Chrysin's proangiogenic effect is potentially mediated by increased VEGF, VEGFR2 and activation of PI3K/AKT/mTOR pathway, which promoted eNOS and NO levels as it was reversed by the mTOR inhibitor, rapamycin. Chrysin also inhibited autophagy as it decreased ATG5 and Beclin-1. The current study shows that chrysin possesses a proangiogenic effect in HLI rats and might be useful in patients with PAD.

Polycyclic aromatic hydrocarbons (PAHs) may explain the paradoxical effects of cigarette use on preeclampsia (PE)

Tobacco smoking and use of snus (smokeless tobacco) are associated with adverse effects on pregnancy and neonatal outcomes. Nicotine is considered a key toxicant involved in effects caused by both smoking and snus, while pyrolysis products including polycyclic aromatic hydrocarbons (PAHs) in cigarette smoke represents the constituents most unequally divided between these two groups of tobacco products. The aim of this review was: i) to compare the impact, in terms of relative effect estimates, of cigarette smoking and use of Swedish snus on pregnancy outcomes using similar non-tobacco user controls, and ii) to examine whether exposure to PAHs from smoking could explain possible differences in impact on pregnancy outcomes. We systematically searched MEDLINE, Embase, PsycInfo, Web of Science and the Cochrane Database of Systematic Reviews up to October 2021 and identified studies reporting risks for adverse pregnancy and neonatal outcomes associated with snus use and with smoking relative to pregnant women with no use of tobacco. Both snus use and smoking were associated with increased risk of stillbirth, preterm birth, and oral cleft malformation, with comparable point estimates. These effects were likely due to comparable nicotine exposure. We also found striking differences. While both smoking and snus increased the risk of having small for gestational age (SGA) infants, risk from maternal smoking was markedly higher as was the reduction in birthweight. In contrast, the risk of preeclampsia (PE) was markedly lower in smokers than in controls, while snus use was associated with a slightly increased risk. We suggest that PAHs acting via AhR may explain the stronger effects of tobacco smoking on SGA and also to the apparent protective effect of cigarette smoking on PE. Possible mechanisms involved include: i) disrupted endocrine control of fetal development as well as placental development and function, and ii) stress adaption and immune suppression in placenta and mother.

Transcriptional and Epigenetic Factors Associated with Early Thrombosis of Femoral Artery Involved in Arteriovenous Fistula

Arteriovenous fistulas (AVFs), created for hemodialysis in end-stage renal disease patients, mature through the outward remodeling of the outflow vein. However, early thrombosis and chronic inflammation are detrimental to the process of AVF maturation and precipitate AVF maturation failure. For the successful remodeling of the outflow vein, blood flow through the fistula is essential, but early arterial thrombosis attenuates this blood flow, and the vessels become thrombosed and stenosed, leading to AVF failure. The altered expression of various proteins involved in maintaining vessel patency or thrombosis is regulated by genes of which the expression is regulated by transcription factors and microRNAs. In this study, using thrombosed and stenosed arteries following AVF creation, we delineated transcription factors and microRNAs associated with differentially expressed genes in bulk RNA sequencing data using upstream and causal network analysis. We observed changes in many transcription factors and microRNAs that are involved in angiogenesis; vascular smooth muscle cell proliferation, migration, and phenotypic changes; endothelial cell function; hypoxia; oxidative stress; vessel remodeling; immune responses; and inflammation. These factors and microRNAs play a critical role in the underlying molecular mechanisms in AVF maturation. We also observed epigenetic factors involved in gene regulation associated with these molecular mechanisms. The results of this study indicate the importance of investigating the transcriptional and epigenetic regulation of AVF maturation and maturation failure and targeting factors precipitating early thrombosis and stenosis.

Aryl-hydrocarbon receptor (AhR) activation contributes to high-fat diet-induced vascular dysfunction

BACKGROUND AND PURPOSE

Metabolic and vascular dysfunction are common features of obesity. Aryl hydrocarbons receptors (AhR) regulate lipid metabolism and vascular homeostasis, but whether vascular AhR are activated in obesity or if AhR have protective or harmful effects on vascular function in obesity are not known. Thus, our study addressed whether AhR activation contributes to obesity-associated vascular dysfunction and the mechanisms involved in the AhR effects.

EXPERIMENTAL APPROACH

Male AhRKO (AhR knockout) and WT (wild type) mice were fed either a control or a HF (high-fat) diet for ten weeks. Metabolic and inflammatory parameters were measured in serum and adipose tissue. Vascular reactivity (isometric force) was evaluated using a myography. eNOS and AhR protein expression was determined by Western blot; Cyp1A1 and eNOS gene expression by RT-PCR. Nitric oxide (NO) production was quantified by DAF fluorescence.

KEY RESULTS

HF diet increased serum total, HDL, and LDL cholesterol, as well as vascular AhR protein expression and proinflammatory cytokines in the adipose tissue. HF diet decreased endothelium-dependent vasodilation. AhR deletion protected mice from HF diet-induced dyslipidemia, weight gain, and inflammatory processes. HF diet-induced endothelial dysfunction was attenuated in AhRKO mice. Vessels from AhRKO mice exhibited a greater NO reserve. In cultured endothelial cells, lysophosphatidylcholine (LPC, a major component of LDL and oxLDL) reduced eNOS gene expression and NO production. Antagonism of AhR abrogated LPC effects on endothelial cells and LPC-induced decreased endothelium-dependent vasodilation.

CONCLUSION AND IMPLICATIONS

AhR deletion attenuates HF diet-induced dyslipidemia and vascular dysfunction by improving eNOS/NO signalling. Targeting AhR may prevent obesity-associated vascular dysfunction.

Uraemic solutes as therapeutic targets in CKD-associated cardiovascular disease

Chronic kidney disease (CKD) is characterized by the retention of a myriad of solutes termed uraemic (or uremic) toxins, which inflict damage to several organs, including the cardiovascular system. Uraemic toxins can induce hallmarks of cardiovascular disease (CVD), such as atherothrombosis, heart failure, dysrhythmias, vessel calcification and dysregulated angiogenesis. CVD is an important driver of mortality in patients with CKD; however, reliance on conventional approaches to managing CVD risk is insufficient in these patients, underscoring a need to target risk factors that are specific to CKD. Mounting evidence suggests that targeting uraemic toxins and/or pathways induced by uraemic toxins, including tryptophan metabolites and trimethylamine N-oxide (TMAO), can lower the risk of CVD in patients with CKD. Although tangible therapies resulting from our growing knowledge of uraemic toxicity are yet to materialize, a number of pharmacological and non-pharmacological approaches have the potential to abrogate the effects of uraemic toxins, for example, by decreasing the production of uraemic toxins, by modifying metabolic pathways induced by uraemic toxins such as those controlled by aryl hydrocarbon receptor signalling and by augmenting the clearance of uraemic toxins.

Muscle allele-specific expression QTLs may affect meat quality traits in Bos indicus

Single nucleotide polymorphisms (SNPs) located in transcript sequences showing allele-specific expression (ASE SNPs) were previously identified in the Longissimus thoracis muscle of a Nelore (Bos indicus) population consisting of 190 steers. Given that the allele-specific expression pattern may result from cis-regulatory SNPs, called allele-specific expression quantitative trait loci (aseQTLs), in this study, we searched for aseQTLs in a window of 1 Mb upstream and downstream from each ASE SNP. After this initial analysis, aiming to investigate variants with a potential regulatory role, we further screened our aseQTL data for sequence similarity with transcription factor binding sites and microRNA (miRNA) binding sites. These aseQTLs were overlapped with methylation data from reduced representation bisulfite sequencing (RRBS) obtained from 12 animals of the same population. We identified 1134 aseQTLs associated with 126 different ASE SNPs. For 215 aseQTLs, one allele potentially affected the affinity of a muscle-expressed transcription factor to its binding site. 162 aseQTLs were predicted to affect 149 miRNA binding sites, from which 114 miRNAs were expressed in muscle. Also, 16 aseQTLs were methylated in our population. Integration of aseQTL with GWAS data revealed enrichment for traits such as meat tenderness, ribeye area, and intramuscular fat . To our knowledge, this is the first report of aseQTLs identification in bovine muscle. Our findings indicate that various cis-regulatory and epigenetic mechanisms can affect multiple variants to modulate the allelic expression. Some of the potential regulatory variants described here were associated with the expression pattern of genes related to interesting phenotypes for livestock. Thus, these variants might be useful for the comprehension of the genetic control of these phenotypes.

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.

Indoxyl sulfate impairs angiogenesis via chronic aryl hydrocarbon receptor activation

Chronic kidney disease (CKD) is associated with a substantial increased risk of cardiovascular disease. There is growing evidence that uremic metabolites, which accumulate in the blood with CKD, have detrimental impacts on endothelial cell health and function. However, the molecular mechanisms by which uremic metabolites negatively impact endothelial cell biology are not fully understood. In this study, activation of the aryl hydrocarbon receptor (AHR) via indoxyl sulfate, a known uremic metabolite, was found to impair endothelial cell tube formation and proliferation but not migratory function. Moreover, aortic ring cultures treated with indoxyl sulfate also exhibited decreased sprouting and high AHR activation. Next, genetic knockdown of the AHR using shRNA was found to rescue endothelial cell tube formation, proliferation, and aortic ring sprouting. Similarly, pharmacological AHR antagonism using resveratrol and CH223191 were also found to rescue angiogenesis in cell and aortic ring cultures. Finally, a constitutively active AHR (CAAHR) vector was generated and used to confirm AHR-specific effects. Expression of the CAAHR recapitulated the impaired tube formation and proliferation in cultured endothelial cells and decreased sprouting in aortic ring cultures. Taken together, these data define the impact of AHR activation on angiogenesis and highlight the potential for therapeutic AHR antagonists, which may improve angiogenesis in the context of CKD and cardiovascular disease.

AHR canonical pathway: in vivo findings to support novel antihypertensive strategies

Essential hypertension (HTN) is a disease where genetic and environmental factors interact to produce a high prevalent set of almost indistinguishable phenotypes. The weak definition of what is under the umbrella of HTN is a consequence of the lack of knowledge on the players involved in environment-gene interaction and their impact on blood pressure (BP) and mechanisms. The disclosure of these mechanisms that sense and (mal)adapt to toxic-environmental stimuli might at least determine some phenotypes of essential HTN and will have important therapeutic implications. In the present manuscript, we looked closer to the environmental sensor aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor involved in cardiovascular physiology, but better known by its involvement in biotransformation of xenobiotics through its canonical pathway. This review aims to disclose the contribution of the AHR-canonical pathway to HTN. For better mirror the complexity of the mechanisms involved in BP regulation, we privileged evidence from in vivo studies. Here we ascertained the level of available evidence and a comprehensive characterization of the AHR-related phenotype of HTN. We reviewed clinical and rodent studies on AHR-HTN genetic association and on AHR ligands and their impact on BP. We concluded that AHR is a druggable mechanistic linker of environmental exposure to HTN. We conclude that is worth to investigate the canonical pathway of AHR and the expression/polymorphisms of its related genes and/or other biomarkers (e.g. tryptophan-related ligands), in order to identify patients that may benefit from an AHR-centered antihypertensive treatment.

Aryl Hydrocarbon Receptor: Its Regulation and Roles in Transformation and Tumorigenesis

AhR is an environmental response gene that mediates cellular responses to a variety of xenobiotic compounds that frequently function as AhR ligands. Many AhR ligands are classified as carcinogens or pro-carcinogens. Thus, AhR itself acts as a major mediator of the carcinogenic effect of many xenobiotics in vivo. In this concise review, mechanisms by which AhR trans-activates downstream target gene expression, modulates immune responses, and mediates malignant transformation and tumor development are discussed. Moreover, activation of AhR by post-translational modifications and crosstalk with other transcription factors or signaling pathways are also summarized.

The Role of Aryl Hydrocarbon Receptor (AhR) in Brain Tumors

Primary brain tumors, both malignant and benign, are diagnosed in adults at an incidence rate of approximately 23 people per 100 thousand. The role of AhR in carcinogenesis has been a subject of debate, given that this protein may act as either an oncogenic protein or a tumor suppressor in different cell types and contexts. Lately, there is growing evidence that aryl hydrocarbon receptor (AhR) plays an important part in the development of brain tumors. The role of AhR in brain tumors is complicated, depending on the type of tumor, on ligands that activate AhR, and other features of the pathological process. In this review, we summarize current knowledge about AhR in relation to brain tumors and provide an overview of AhR’s potential as a therapeutic target.

Ablation of aryl hydrocarbon receptor promotes angiotensin II-induced cardiac fibrosis through enhanced c-Jun/HIF-1α signaling

Aryl hydrocarbon receptor (AHR) is a transcription factor that binds to DNA as a heterodimer with the AHR nuclear translocator (ARNT) after interaction with ligands, such as polycyclic and halogenated aromatic hydrocarbons and other xenobiotics. The endogenous ligands and functions of AHR have been the subject of many investigations. In the present study, the potential role of AHR signaling in the development of left ventricular hypertrophy and cardiac fibrosis by angiotensin II (Ang II) infusion was investigated in mice lacking the AHR gene (Ahr^-/-). We also assessed the hypothesis that fenofibrate, a peroxisome proliferator-activated receptor-α (PPARα) activator, reduces cardiac fibrosis through the c-Jun signaling. Male Ahr^-/- and age-matched wild-type mice (n = 8 per group) were infused with Ang II at 100 ng/kg/min daily for 2 weeks. Treatment with Ang II increased systolic blood pressure to comparable levels in Ahr^-/- and wild-type mice. However, Ahr^-/- mice developed severe cardiac fibrosis after Ang II infusion compared with wild-type mice. Ang II infusion also significantly increased the expression of endothelin in the left ventricles of Ahr^-/- mice, but not in wild-type mice, and significantly increased the c-Jun signaling in Ahr^-/- mice. Ang II infusion also significantly enhanced the expression of hypoxia-inducible factor-1α (HIF-1α) and the downstream target vascular endothelial growth factor (VEGF) in the left ventricles of Ahr^-/- mice. These results suggested pathogenic roles for the AHR signaling pathway in the development of cardiac fibrosis. Treatment with fenofibrate reduced cardiac fibrosis and abrogated the effects of Ang II on the expression of endothelin, HIF-1α, and VEGF. The inhibitory effect of fenofibrate on cardiac fibrosis was mediated by suppression of VEGF expression through modulation of c-Jun/HIF-1α signaling.

Aryl Hydrocarbon Receptor: A New Player of Pathogenesis and Therapy in Cardiovascular Diseases

The aryl hydrocarbon receptor (AhR) is a DNA binding protein that acts as a nuclear receptor mediating xenobiotic metabolism and environmental responses. Owing to the evolutionary conservation of this gene and its widespread expression in the immune and circulatory systems, AhR has for many years been almost exclusively studied by the pharmacological/toxicological field for its role in contaminant toxicity. More recently, the functions of AhR in environmental adaption have been examined in the context of the occurrence, development, and therapy of cardiovascular diseases. Increasing evidence suggests that AhR is involved in maintaining homeostasis or in triggering pathogenesis by modulating the biological responses of critical cell types in the cardiovascular system. Here, we describe the structure, distribution, and ligands of AhR and the AhR signaling pathway and review the impact of AhR on cardiovascular physiology. We also discuss the potential contribution of AhR as a new potential factor in the targeted treatment of cardiovascular diseases.

Aryl hydrocarbon receptor (AHR): "pioneer member" of the basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family of "sensors" of foreign and endogenous signals

The basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family comprises many transcription factors, found throughout all three kingdoms of life; bHLH/PAS members "sense" innumerable intracellular and extracellular "signals" - including endogenous compounds, foreign chemicals, gas molecules, redox potential, photons (light), gravity, heat, and osmotic pressure. These signals then initiate downstream signaling pathways involved in responding to that signal. The term "PAS", abbreviation for "per-Arnt-sim" was first coined in 1991. Although the mouse Arnt gene was not identified until 1991, evidence of its co-transcriptional binding partner, aryl hydrocarbon receptor (AHR), was first reported in 1974 as a "sensor" of foreign chemicals, up-regulating cytochrome P450 family 1 (CYP1) and other enzyme activities that usually metabolize the signaling chemical. Within a few years, AHR was proposed also to participate in inflammation. The mouse [Ah] locus was shown (1973-1989) to be relevant to chemical carcinogenesis, mutagenesis, toxicity and teratogenesis, the mouse Ahr gene was cloned in 1992, and the first Ahr(-/-) knockout mouse line was reported in 1995. After thousands of studies from the early 1970s to present day, we now realize that AHR participates in dozens of signaling pathways involved in critical-life processes, affecting virtually every organ and cell-type in the animal, including many invertebrates.

VEGF expression is regulated by HIF-1α and ARNT in 3D KYSE-70, esophageal cancer cell spheroids

In 3D cultured cell systems, the cells form 3D spheroids that mimic cancer cell spheroids in vivo. Cancer cells form cell spheroids as they grow. The in vivo spheroids do not contain a vascular network; therefore, oxygen and nutrition supplies are insufficient. Specifically, the cells in the core region of the cluster are exposed to higher stress levels than the cells in the outer spheroid layer. As a result, the cells in the spheroid are exposed to low nutrition and hypoxia conditions. To overcome these shortages, angiogenesis is induced in cancer spheroids in vivo. Vascular endothelial growth factor (VEGF) is an important molecule involved in angiogenesis. VEGF is secreted by cancer cells in vivo in response to stress conditions such as hypoxia. VEGF expression in cancer cells is mediated by hypoxia-inducible factor 1α (HIF1α), which accumulates in cancer cells during hypoxia. In this report, we show that VEGF expression is regulated by HIF1α and that VEGF is secreted to the outside of the spheroid in vitro. Several investigators have reported that HIF1α forms a protein-protein complex with aryl hydrocarbon receptor translocator (ARNT). We report here that not only HIF1α but also ARNT regulates VEGF expression in 3D cancer spheroids. Our results suggest the utility of the in vitro 3D cancer spheroid model for investigating angiogenesis in cancerous tissues.

An Aryl Hydrocarbon Receptor-Mediated Amplification Loop That Enforces Cell Migration in ER-/PR-/Her2- Human Breast Cancer Cells

The endogenous ligand-activated aryl hydrocarbon receptor (AHR) plays an important role in numerous biologic processes. As the known number of AHR-mediated processes grows, so too does the importance of determining what endogenous AHR ligands are produced, how their production is regulated, and what biologic consequences ensue. Consequently, our studies were designed primarily to determine whether ER⁻/PR⁻/Her2⁻ breast cancer cells have the potential to produce endogenous AHR ligands and, if so, how production of these ligands is controlled. We postulated that: 1) malignant cells produce tryptophan-derived AHR ligand(s) through the kynurenine pathway; 2) these metabolites have the potential to drive AHR-dependent breast cancer migration; 3) the AHR controls expression of a rate-limiting kynurenine pathway enzyme(s) in a closed amplification loop; and 4) environmental AHR ligands mimic the effects of endogenous ligands. Data presented in this work indicate that primary human breast cancers, and their metastases, express high levels of AHR and tryptophan-2,3-dioxygenase (TDO); representative ER⁻/PR⁻/Her2⁻ cell lines express TDO and produce sufficient intracellular kynurenine and xanthurenic acid concentrations to chronically activate the AHR. TDO overexpression, or excess kynurenine or xanthurenic acid, accelerates migration in an AHR-dependent fashion. Environmental AHR ligands 2,3,7,8-tetrachlorodibenzo[p]dioxin and benzo[a]pyrene mimic this effect. AHR knockdown or inhibition significantly reduces TDO2 expression. These studies identify, for the first time, a positive amplification loop in which AHR-dependent TDO2 expression contributes to endogenous AHR ligand production. The net biologic effect of AHR activation by endogenous ligands, which can be mimicked by environmental ligands, is an increase in tumor cell migration, a measure of tumor aggressiveness.

The aryl hydrocarbon receptor promotes aging phenotypes across species

The ubiquitously expressed aryl hydrocarbon receptor (AhR) induces drug metabolizing enzymes as well as regulators of cell growth, differentiation and apoptosis. Certain AhR ligands promote atherosclerosis, an age-associated vascular disease. Therefore, we investigated the role of AhR in vascular functionality and aging. We report a lower pulse wave velocity in young and old AhR-deficient mice, indicative of enhanced vessel elasticity. Moreover, endothelial nitric oxide synthase (eNOS) showed increased activity in the aortas of these animals, which was reflected in increased NO production. Ex vivo, AhR activation reduced the migratory capacity of primary human endothelial cells. AhR overexpression as well as treatment with a receptor ligand, impaired eNOS activation and reduced S-NO content. All three are signs of endothelial dysfunction. Furthermore, AhR expression in blood cells of healthy human volunteers positively correlated with vessel stiffness. In the aging model Caenorhabditis elegans, AhR-deficiency resulted in increased mean life span, motility, pharynx pumping and heat shock resistance, suggesting healthier aging. Thus, AhR seems to have a negative impact on vascular and organismal aging. Finally, our data from human subjects suggest that AhR expression levels could serve as an additional, new predictor of vessel aging.

Aryl hydrocarbon receptor agonists upregulate VEGF secretion from bronchial epithelial cells

Chronic airway diseases, such as asthma and chronic obstructive pulmonary disease, are characterized by airway remodeling. Vascular endothelial growth factor (VEGF) is a critical regulator of angiogenesis and vascular remodeling, important components of airway remodeling. The aryl hydrocarbon receptor (AhR) is the principle receptor for many environmental toxicants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which may contribute to the pathogenesis of asthma and chronic obstructive pulmonary disease. However, the regulatory role of AhR on the expression of VEGF in bronchial epithelial cells (BECs) remains elusive. This study was conducted to determine the role of AhR in regulating bronchial epithelial VEGF expression, which might contribute to angiogenesis of airway remodeling. The plasma VEGF levels of asthmatic patients and healthy subjects were compared. By treating HBE-135, Beas-2B, and primary human BECs with AhR agonists, the mechanisms through which AhR modulated VEGF expression in human BECs were investigated. The plasma VEGF level was significantly higher in asthmatic patients than in healthy subjects. AhR agonists significantly upregulated VEGF secretion from human BECs, which promoted the migratory and tube-forming ability of human umbilical vein endothelial cells. The secretion of VEGF was increased via a canonical AhR pathway, followed by the 15-LOX/15-HETE/STAT3 pathway. C57BL/6JNarl mice treated with TCDD intratracheally also showed increased VEGF expression in BECs. This hitherto unrecognized pathway may provide a potential target for the treatment of airway remodeling in many pulmonary diseases, especially those related to environmental toxicants.

KEY MESSAGE

AhR agonists increase VEGF secretion from bronchial epithelial cells. The mechanism involves the canonical AhR pathway and 15-LOX/15-HETE/STAT3 pathway. Asthmatic patients have higher plasma VEGF level. Mice treated with intratracheal TCDD show increased VEGF expression in BECs. This novel regulatory pathway is a potential target for treating asthma and COPD.

Aryl hydrocarbon receptor knock-out exacerbates choroidal neovascularization via multiple pathogenic pathways

The aryl hydrocarbon receptor (AhR) is a heterodimeric transcriptional regulator with pleiotropic functions in xenobiotic metabolism and detoxification, vascular development and cancer. Herein, we report a previously undescribed role for the AhR signalling pathway in the pathogenesis of the wet, neovascular subtype of age-related macular degeneration (AMD), the leading cause of vision loss in the elderly in the Western world. Comparative analysis of gene expression profiles of aged AhR^−/− and wild-type (wt) mice, using high-throughput RNA sequencing, revealed differential modulation of genes belonging to several AMD-related pathogenic pathways, including inflammation, angiogenesis and extracellular matrix regulation. To investigate AhR regulation of these pathways in wet AMD, we experimentally induced choroidal neovascular lesions in AhR^−/− mice and found that they measured significantly larger in area and volume compared to age-matched wt mice. Furthermore, these lesions displayed a higher number of ionized calcium-binding adaptor molecule 1-positive (Iba1⁺) microglial cells and a greater amount of collagen type IV deposition, events also seen in human wet AMD pathology specimens. Consistent with our in vivo observations, AhR knock-down was sufficient to increase choroidal endothelial cell migration and tube formation in vitro. Moreover, AhR knock-down caused an increase in collagen type IV production and secretion in both retinal pigment epithelial (RPE) and choroidal endothelial cell cultures, increased expression of angiogenic and inflammatory molecules, including vascular endothelial growth factor A (VEGFA) and chemokine (C–C motif) ligand 2 (CCL2) in RPE cells, and increased expression of secreted phosphoprotein 1 (SPP1) and transforming growth factor-β1 (TGFβ1) in choroidal endothelial cells. Collectively, our findings identify AhR as a regulator of multiple pathogenic pathways in experimentally induced choroidal neovascularization, findings that are consistent with a possible role of AhR in wet AMD. The data discussed in this paper have been deposited in NCBI's Gene Expression Omnibus; GEO Submission No. GSE56983, NCBI Tracking System No. 17021116.

Regulatory crosstalk and interference between the xenobiotic and hypoxia sensing pathways at the AhR-ARNT-HIF1α signaling node

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the responses to toxic environmental chemicals such as TCDD or dioxin-like PCBs. To regulate gene expression, the AhR requires its binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT). ARNT is also required by the hypoxia-inducible factor-1α (HIF-1α), a crucial regulator of responses to conditions of reduced oxygen. The important role of ARNT in both the AhR and HIF-1α signaling pathways establishes a meaningful foundation for a possible crosstalk between these two vitally important signaling pathways. This crosstalk might lead to interference between the two signaling pathways and thus might play a role in the variety of cellular responses after exposure to AhR ligands and reduced oxygen availability. This review focuses on studies that have analyzed the effect of low oxygen environments and hypoxia-mimetic agents on AhR signaling and conversely, the effect of AhR ligands, with a special emphasis on PCBs, on HIF-1α signaling. We highlight studies that assess the role of ARNT, elucidate the mechanism of the crosstalk, and discuss the physiological implications for exposure to AhR-inducing compounds in the context of hypoxia.

Persistent organic pollutants meet adipose tissue hypoxia: does cross-talk contribute to inflammation during obesity?

Lipophilic persistent organic pollutants (POPs) accumulate in lipid-rich tissues such as human adipose tissue. This is particularly problematic in individuals with excess adiposity, a physiological state that may be additionally characterized by local adipose tissue hypoxia. Hypoxic patches occur when oxygen diffusion is insufficient to reach all hypertrophic adipocytes. POPs and hypoxia independently contribute to the development of adipose tissue-specific and systemic inflammation often associated with obesity. Inflammation is induced by increased proinflammatory mediators such as tumour necrosis factor-alpha, interleukin-6, and monocyte chemotactic protein-1, as well as reduced adiponectin release, an anti-inflammatory and insulin-sensitizing adipokine. The aryl hydrocarbon receptor (AhR) mediates the cellular response to some pollutants, while hypoxia responses occur through the oxygen-sensitive transcription factor hypoxia-inducible factor (HIF)-1. There is some overlap between the two signalling pathways since both require a common subunit called the AhR nuclear translocator. As such, it is unclear how adipocytes respond to simultaneous POP and hypoxia exposure. This brief review explores the independent contribution of POPs and adipose tissue hypoxia as factors underlying the inflammatory response from adipocytes during obesity. It also highlights that the combined effect of POPs and hypoxia through the AhR and HIF-1 signalling pathways remains to be tested.

The pathological roles of environmental and redox stresses in cardiovascular diseases

Oxidative stress and inflammation are implicated in cardiovascular diseases such as atherosclerosis, reperfusion injury, hypertension, and heart failure. High levels of oxidative stress resulting from increased cardiac generation of reactive oxygen species (ROS) is thought to contribute to contractile and endothelial dysfunction, apoptosis and necrosis of myocytes, and extracellular matrix remodeling in the heart. ROS activate several transcription factors known as redox-regulated transcription factors, and these transcription factors play important roles in the pathophysiology of cardiovascular diseases. This review focuses on the pathological roles of environmental and redox stresses in cardiovascular diseases, especially severe cardiac dysfunction and the transition from compensated hypertrophy to heart failure. The aryl hydrocarbon receptor (AHR) and NF-E2 p45-related factor (Nrf2) are transcription factors involved in the regulation of drug-metabolizing enzymes. AHR has been studied as a receptor for environmental contaminants and as a mediator of chemical toxicity. However, other roles for AHR in cardiac and vascular development have recently been described. Moreover, Nrf2 protects against oxidative stress by increasing the transcription of genes, including those for several antioxidant enzymes. The roles of these transcription factors, AHR and Nrf2 in angiogenesis are also discussed in this review.

Antiangiogenic activity of indole-3-carbinol in endothelial cells stimulated with activated macrophages

The effect of indole-3-carbinol (I3C), a major indolic metabolite in cruciferous vegetables, on lipopolysaccharide (LPS)-activated macrophage-induced tube formation and its associated factors in endothelial EA hy926 cells was investigated. LPS significantly enhanced the capillary-like structure of endothelial cells (ECs) co-cultured with macrophages, but no such effect was observed in single-cultured ECs. I3C, on the other hand, suppressed such enhancement in concert with decreased secretions of vascular endothelial growth factor (VEGF), nitric oxide (NO), interleukin-6 (IL-6), and matrix metalloproteinases (MMPs). The results obtained from cultivating ECs with conditioned medium (CM) collected from macrophages suggested that both ECs and macrophages were inactivated by I3C. These results indicate that I3C from cruciferous vegetables may possess potential roles in preventing inflammation-associated angiogenic diseases.

Induction of CYP1 family members under low-glucose conditions requires AhR expression and occurs through the nuclear translocation of AhR

Cross-talk between the aryl hydrocarbon receptor (AhR) pathway and the typical stress response is thought to be an important signal transduction in response to nutrient-stress conditions, such as glucose deprivation in liver cells. In the present study, we demonstrate that reduction of glucose concentration in the medium of HepG2 cells, a human hepatocellular carcinoma cell line, induces the CYP1 family and Nrf2. RNAi for AhR abolishes the induction of expression of CYP1 and Nrf2. These inductions are accompanied by the translocation of AhR into the nucleus in response to low-glucose conditions. Endogenous compounds are recruited as AhR ligands to induce various gene expressions, and our present results suggest that an endogenous AhR ligand is produced under low-glucose conditions and that the role of AhR as a transcription factor is related to the low-glucose response. The recommended glucose concentration (4.5 g/L) in the medium for culture of HepG2 was used as the high-glucose concentration in this study. We adopted 1.0 g/L as the low-glucose condition for elucidation of mechanisms of the stress response. These results will be useful to understand the relationship between drug-metabolizing enzymes and mechanisms of the anti-stress response of tumor cells, and will also be useful for investigating preventive remedies against tumor angiogenesis.

An activated renin-angiotensin system maintains normal blood pressure in aryl hydrocarbon receptor heterozygous mice but not in null mice

It has been postulated that fetal vascular abnormalities in aryl hydrocarbon receptor null (ahr(-/-)) mice may alter cardiovascular homeostasis in adulthood. We tested the hypothesis that blood pressure regulation in adult heterozygous mice (ahr(+/-)) would be normal, compared to ahr(-/-) mice, since no vascular abnormalities have been reported in the heterozygote animals. Mean arterial blood pressure (MAP) was measured using radiotelemetry prior to and during treatment with inhibitors of the autonomic nervous system, nitric oxide synthase (NOS), angiotensin converting enzyme (ACE), or endothelin-1 A receptor (ET(A)). Also, indices of renin-angiotensin system (RAS) activation were measured. ahr(+/-) and ahr(-/-) mice were normotensive and hypotensive, respectively, compared to wild-type (ahr(+/+)) littermates. Responses of all genotypes to autonomic nervous system inhibition were normal. ahr(+/-) mice responded normally to NOS inhibition, while the responses of ahr(-/-) mice were significantly blunted. In contrast, ahr(+/-) mice were significantly more responsive to inhibition of ACE, an ET(A) antagonist, or both, while ahr(-/-) mice were significantly less responsive to ACE inhibition and more responsive to an ET(A) antagonist. ahr(+/-) mice also exhibited significant increases in plasma renin and ACE activity, plasma sodium, and urine osmolality, indicative of RAS activation. Thus, normotension in ahr(+/-) mice appears to be maintained by increased RAS and ET-1 signaling, while hypotension in ahr(-/-) mice may result from decreased RAS signaling. In conclusion, despite the lack of overt fetal vascular abnormalities in ahr(+/-) mice, the loss of a single ahr allele has a significant effect on blood pressure regulation.

Inducible nitric oxide synthase modulates angiogenesis in ischemic hindlimb of rat

BACKGROUND

Angiogenesis plays an important role in maintaining adequate oxygen delivery, and nitric oxide (NO) is a potential regulator of angiogenesis. NO is synthesized through three isoforms of NO synthase (NOS). It is hypothesized that the NO derived from inducible NOS (iNOS) may promote survival of ischemic tissue through angiogenesis. To test this hypothesis, we investigated the effect of iNOS deficiency (by L-NIL) on angiogenesis in a hindlimb ischemia model.

METHODS

Thirty-two male wistar rats randomly divided into four groups. In groups 1 & 2, hindlimb ischemia was induced by ligation of femoral artery and they received L-NIL and saline respectively. The animals in groups 3 and 4 also received L-NIL and saline respectively without surgical procedure. After 21 days, the serum concentration of nitrite, capillary density and expression of HIF1alpha were determined.

RESULTS

Serum nitrite levels were significantly lower in L-NIL groups (p<0.05). The capillary density in group 1 (ischemia+L-NIL) was significantly different from group 2 (ischemia+saline); group 1: 360.33+/-77.02, group 2: 549+/-81.85 /mm2, p<0.05) .In addition, expression of HIF1alpha was significantly increased in ischemic groups (p<0.05).

CONCLUSION

Selective inhibition of iNOS by L-NIL inhibits angiogenesis in a hindlimb ischemic rat model. In addition, ischemia induces expression of HIF1alpha in hypoxic tissue.

Dioxin receptor deficiency impairs angiogenesis by a mechanism involving VEGF-A depletion in the endothelium and transforming growth factor-beta overexpression in the stroma

Angiogenesis has key roles in development and in the progression of human diseases such as cancer. Consequently, identifying the novel markers and regulators of angiogenesis is a critical task. The dioxin receptor (AhR) contributes to vascular homeostasis and to the endothelial response to toxins, although the mechanisms involved are largely uncharacterized. Here, we show that AhR-null mice (AhR(-/-)) have impaired angiogenesis in vivo that compromises tumor xenograft growth. Aortic rings emigration experiments and RNA interference indicated that AhR(-/-) endothelial cells failed to branch and to form tube-like structures. Such a phenotype was found to be vascular endothelial growth factor (VEGF)-dependent, as AhR(-/-) aortic endothelial cells (MAECs) secreted lower amounts of active VEGF-A and their treatment with VEGF-A rescued angiogenesis in culture and in vivo. Further, the addition of anti-VEGF antibody to AhR(+/+) MAECs reduced angiogenesis. Treatment under hypoxic conditions with 2-methoxyestradiol suggested that HIF-1alpha modulates endothelial VEGF expression in an AhR-dependent manner. Importantly, AhR-null stromal myofibroblasts produced increased transforming growth factor-beta (TGFbeta) activity, which inhibited angiogenesis in human endothelial cells (HMECs) and AhR(-/-) mice, whereas the co-culture of HMECs with AhR(-/-) myofibroblasts or with their conditioned medium inhibited branching, which was restored by an anti-TGFbeta antibody. Moreover, VEGF and TGFbeta activities cooperated in modulating angiogenesis, as the addition of TGFbeta to AhR(-/-) MAECs further reduced their low basal VEGF-A activity. Thus, AhR modulates angiogenesis through a mechanism requiring VEGF activation in the endothelium and TGFbeta inactivation in the stroma. These data highlight the role of AhR in cardiovascular homeostasis and suggest that this receptor can be a novel regulator of angiogenesis during tumor development.

Inhibition of ischemia-induced angiogenesis by benzo[a]pyrene in a manner dependent on the aryl hydrocarbon receptor

We have investigated the effect of benzo[a]pyrene (B[a]P), a carcinogen of tobacco smoke and an agonist for the aryl hydrocarbon receptor (AHR), on hypoxia-induced angiogenesis. Ischemia was induced by femoral artery ligation in wild-type and AHR-null mice, and the animals were subjected to oral administration of B[a]P (125 mg/kg) once a week. Exposure to B[a]P up-regulated the expression of metallothionein in the ischemic hindlimb and markedly inhibited ischemia-induced angiogenesis in wild-type mice. The amounts of interleukin-6 and of vascular endothelial growth factor (VEGF) mRNA in the ischemic hindlimb of wild-type mice were reduced by exposure to B[a]P. These various effects of B[a]P were markedly attenuated in AHR-null mice. Our observations suggest that the loss of the inhibitory effect of B[a]P on ischemia-induced angiogenesis apparent in AHR-null mice may be attributable to maintenance of interleukin-6 expression and consequent promotion of angiogenesis through up-regulation of VEGF expression.

Dioxin-induced toxicity on vascular remodeling of the placenta

Arylhydrocarbon receptor (AhR) activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) triggers its downstream signaling pathway to exert adverse effects on vasculature development, which can be initiated by vasculogenesis, followed by angiogenesis, or vascular remodeling, in a variety of animals including avians, piscines and mammals. The placenta, a mammalian organ rich in vasculature, consists of endothelial and trophoblast cells of fetal origin, which proliferate and differentiate under hypoxic condition in the uterine horn. Our studies demonstrated that vascular remodeling occurs prominently in the placenta of the control Holtzman rat strain during the late period of gestation, and induces changes in cell shape and elimination by apoptosis of trophoblasts. As a result, the net volumes of both maternal and fetal blood in the placenta increase to cope with the essential requirements of oxygen and nutrients in the late period of gestation. On the other hand, in utero exposure to TCDD markedly suppressed the development of sinusoids and trophoblast cells and the apoptosis of trophoblast cells with a concomitant increase in the incidence of fetal death under hypoxic condition. A crosstalk between the hypoxia-inducible factor (HIF)-mediated pathway and AhR-mediated pathway is considered to play an important role in this physiological process. No such changes were observed in the Sprague-Dawley rat strain that turned out to have an AhR conformation identical to that of the Holtzman rat strain. In this commentary, we will discuss a possible link of the TCDD toxicities with the AhR signaling pathway and gestation-related diseases.

Genetic polymorphism and variability of chemical carcinogenesis

Risk assessment in chemical carcinogenesis involves ratios of several factors. Individual responses of an organism to carcinogenic agents depend on polymorphism of enzymes responsible for metabolic activation/detoxification of carcinogens, DNA repair, and apoptosis, as well as promotion and progression in malignantly transformed cells. The effects of a particular polymorphic variant are manifested only in the case of its high penetrance. An integral effect is formed by the ratio of procarcinogenic and anticarcinogenic effects. The complexity of risk assessment depends on the gene polymorphism mosaic involved, directly or indirectly, in tumorigenesis and upstream/downstream interactions of gene products.

Aryl hydrocarbon receptor is activated by glucose and regulates the thrombospondin-1 gene promoter in endothelial cells

Hyperglycemia is an independent risk factor for development of diabetic vascular complications. The molecular mechanisms that are activated by glucose in vascular cells and could explain the development of vascular complications are still poorly understood. A putative binding site for the transcription factor aryl hydrocarbon receptor (AhR) was identified in the glucose-responsive fragment of the promoter of thrombospondin-1, a potent antiangiogenic and proatherogenic protein involved in development of diabetic vascular complications. AhR was expressed in aortic endothelial cells (ECs), activated, and bound to the promoter in response to high glucose stimulation of ECs. The constitutively active form of AhR induced activation of the thrombospondin-1 gene promoter. In response to high glucose stimulation, AhR was found in complex with Egr-1 and activator protein-2, which are 2 other nuclear transcription factors activated by glucose in ECs that have not been previously detected in complex with AhR. The activity of the DNA-binding complex was regulated by glucose through the activation of hexosamine pathway and intracellular glycosylation. This is the first report of activation of AhR (a receptor for xenobiotic compounds) by a physiological stimulus. This report links the activation of AhR to the pathological effects of hyperglycemia in the vasculature.

Loss of the aryl hydrocarbon receptor induces hypoxemia, endothelin-1, and systemic hypertension at modest altitude

The aryl hydrocarbon receptor (AHR) is a basic helix-loop-helix Per-Arnt-Sim transcription factor that mediates induction of metabolic enzymes and toxicity of certain environmental pollutants. Although AHR knockout (KO) mice develop cardiac hypertrophy, conflicting reports associate this pathology with hypotension or endothelin (ET)-1-dependent hypertension. Because hypertension occurred at modest altitude, we tested the hypothesis that loss of AHR increases the sensitivity to hypoxia-induced ET-1, contributing to systemic hypertension. We found that AHR KO mice were hypertensive at modest altitude (1632 m) but hypotensive at low altitude (225 m). When AHR KO mice residing at 1632 m were exposed to the partial pressure of inspired oxygen (PIO(2)) at sea level for 11 days, blood pressure declined to levels measured at 225 m. Although plasma ET-1 in AHR KO mice was significantly elevated at 1632 m and decreased at 225 m and sea level PIO(2), pulmonary prepro-ET-1 mRNA was significantly reduced at 1632 m and decreased further at 225 m and sea level PIO(2). Blood gas analysis revealed that AHR KO mice were hypoxemic, hypercapnic, and acidotic at 1632 m, values that were attenuated and normalized after 24 hours and 11 days under sea level PIO(2), respectively. Lastly, AHR inactivation in endothelial cells by small interfering RNA significantly reduced basal prepro-ET-1 mRNA but did not alter hypoxia-induced expression. Our studies establish the AHR KO mouse as a model in which modest decreases in PIO(2) lead to hypoxemia, increased plasma ET-1, and systemic hypertension without increased pulmonary prepro-ET-1 mRNA expression.

Reversibility of the Adverse Effects of 1-Bromopropane Exposure in Rats

Previous experiments indicated that 1-bromopropane (1-BP), an alternative to chloroflurocarbons, is neurotoxic and inhibits spermiation in the testis. Here we investigated the reversibility of the toxic effects of 1-BP in rats. Male Wistar rats were divided into three equal groups of 24 each and exposed by inhalation to 0, 400 or 1000 ppm of 1-BP for 6 weeks (8 hrs/day, 7 days/week). Eight rats from each group were sacrificed at the end of 6 weeks exposure, and at 4 and 14 weeks after the end of exposure, to assess the recovery processes. We studied sperm count, motility, morphology and testicular histopathology, as well as blood pressure, skin temperature and hindlimb muscle strength. At the end of 6 weeks of exposure to 1000 ppm (0 week recovery), testicular weight, epididymal weight, sperm count and motility were low, morphologically abnormal sperm were increased and spermatogenic cells showed diffuse degeneration. These changes did not show full recovery at 14 weeks recovery, with the exception of the prostate and seminal vesicular weights, which recovered back to control values. At 400 ppm, increased retained spermatids at 0 week recovery returned to normal at 4 weeks recovery. Exposure to 1000 ppm produced sustained reduction of hindlimb muscle strength at 14 weeks recovery, whereas normalization of the skin temperature and blood pressure was noted after transient changes. Our study showed that the effect of 1-BP on spermatogenesis is dose-dependent; low exposure inhibited spermiation and hormone-dependent organ weight reduction and these changes were transient, while a higher dose of 1000 ppm 1-BP caused persistent depletion of spermatogenic cells.