Indoleamine 2 3-dioxygenase knockout limits angiotensin II-induced aneurysm in low density lipoprotein receptor-deficient mice fed with high fat diet

Integrated metagenomic and metabolomic analysis reveals distinctive stage-specific gut-microbiome-derived metabolites in intracranial aneurysms

OBJECTIVE

Our study aimed to explore the influence of gut microbiota and their metabolites on intracranial aneurysms (IA) progression and pinpoint-related metabolic biomarkers derived from the gut microbiome.

DESIGN

We recruited 358 patients with unruptured IA (UIA) and 161 with ruptured IA (RIA) from two distinct geographical regions for conducting an integrated analysis of plasma metabolomics and faecal metagenomics. Machine learning algorithms were employed to develop a classifier model, subsequently validated in an independent cohort. Mouse models of IA were established to verify the potential role of the specific metabolite identified.

RESULTS

Distinct shifts in taxonomic and functional profiles of gut microbiota and their related metabolites were observed in different IA stages. Notably, tryptophan metabolites, particularly indoxyl sulfate (IS), were significantly higher in plasma of RIA. Meanwhile, upregulated tryptophanase expression and indole-producing microbiota were observed in gut microbiome of RIA. A model harnessing gut-microbiome-derived tryptophan metabolites demonstrated remarkable efficacy in distinguishing RIA from UIA patients in the validation cohort (AUC=0.97). Gut microbiota depletion by antibiotics decreased plasma IS concentration, reduced IA formation and rupture in mice, and downregulated matrix metalloproteinase-9 expression in aneurysmal walls with elastin degradation reduction. Supplement of IS reversed the effect of gut microbiota depletion.

CONCLUSION

Our investigation highlights the potential of gut-microbiome-derived tryptophan metabolites as biomarkers for distinguishing RIA from UIA patients. The findings suggest a novel pathogenic role for gut-microbiome-derived IS in elastin degradation in the IA wall leading to the rupture of IA.

Harnessing intestinal tryptophan catabolism to relieve atherosclerosis in mice

Tryptophan (Trp) is an essential amino acid, whose metabolism is a key gatekeeper of intestinal homeostasis. Yet, its systemic effects, particularly on atherosclerosis, remain unknown. Here we show that high-fat diet (HFD) increases the activity of intestinal indoleamine 2, 3-dioxygenase 1 (IDO), which shifts Trp metabolism from the production of microbiota-derived indole metabolites towards kynurenine production. Under HFD, the specific deletion of IDO in intestinal epithelial cells leads to intestinal inflammation, impaired intestinal barrier, augmented lesional T lymphocytes and atherosclerosis. This is associated with an increase in serotonin production and a decrease in indole metabolites, thus hijacking Trp for the serotonin pathway. Inhibition of intestinal serotonin production or supplementation with indole derivatives alleviates plaque inflammation and atherosclerosis. In summary, we uncover a pivotal role of intestinal IDO in the fine-tuning of Trp metabolism with systemic effects on atherosclerosis, paving the way for new therapeutic strategies to relieve gut-associated inflammatory diseases.

Progress and perspectives of metabolic biomarkers in human aortic dissection

BACKGROUND

Aortic dissection (AD) significantly threated human cardiovascular health, extensive clinical-scientific research programs have been executed to uncover the pathogenesis and prevention. Unfortunately, no specific biomarker was identified for the causality or development of human AD.

AIM OF REVIEW

Metabolomics, a high-throughput technique capable of quantitatively detecting metabolites, holds considerable promise in discovering specific biomarkers and unraveling the underlying pathways involved. Aiming to provide a metabolite prediction in human AD, we collected the metabolomics data from 2003 to 2023, and diligently scrutinized with the online system MetaboAnalyst 6.0.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Based on the data obtained, we have concluded the metabolic dynamics were highly correlated with human AD. Such metabolites (choline, serine and uridine) were frequently involved in the AD. Besides, the pathways, including amino acids metabolism and lipids metabolism, were also dysregulated in the disease. Due to the current limitation of metabolism analysis, the integrative omics data including genomics, transcriptomics, and proteomics were required for developing the specific biomarker for AD.

Targeting metabolism in aortic aneurysm and dissection: from basic research to clinical applications

Aortic aneurysm and dissection (AAD) are a group of insidious and lethal cardiovascular diseases that characterized by seriously threatening the life and health of people, but lack effective nonsurgical interventions. Alterations in metabolites are increasingly recognized as universal features of AAD because metabolic abnormalities have been identified not only in arterial tissue but also in blood and vascular cells from both patients and animal models with this disease. Over the past few decades, studies have further supported this notion by linking AAD to various types of metabolites such as those derived from gut microbiota or involved in TCA cycle or lipid metabolism. Many of these altered metabolites may contribute to the pathogenesis of AAD. This review aims to illustrate the close association between body metabolism and the occurrence and development of AAD, as well as summarize the significance of metabolites correlated with the pathological process of AAD. This provides valuable insight for developing new therapeutic agents for AAD. Therefore, we present a brief overview of metabolism in AAD biology, including signaling pathways involved in these processes and current clinical studies targeting AAD metabolisms. It is necessary to understand the metabolic mechanisms underlying AAD to provides significant knowledge for AAD diagnosis and new therapeutics for treatment.

Comparison of moderate-intensity continuous training and high-intensity interval training effects on the Ido1-KYN-Ahr axis in the heart tissue of rats with occlusion of the left anterior descending artery

Myocardial infarction (MI) affects many molecular pathways in heart cells, including the Ido1-KYN-Ahr axis. This pathway has recently been introduced as a valuable therapeutic target in infarction. We examined the effects of moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) on the axis in the heart tissue of male Wistar rats with occluded left anterior descending (OLAD). Thirty rats (age 10-12 weeks, mean weight 275 ± 25 g) were divided into five groups with 6 animals: Control (Ct) group, MICT group, rats with OLAD as MI group, rats with OLAD treated with MICT (MIMCT group) and rats with OLAD treated with HIIT (MIHIIT group). Rats performed the training protocols for 8 weeks, 5 days a week. HIIT included 7 sets of 4 min running with an intensity of 85-90% VO₂max and 3 min of recovery activation between sets. MICT included continuous running at the same distance as HIIT with an intensity of 50-60% VO₂max for 50 min. The expressions of Ahr, Cyp1a1, and Ido1 were assayed by real-time PCR. Malondialdehyde (MDA) and Kynurenine levels, and AHR, CYP1A1, and IDO1 proteins were detected using ELISA. Data were analyzed using the ANOVA and MANOVA tests. Compared to the CT group, MI caused an increase in all studied factors, but only statistically significant (P < 0.05) for MDA and IDO1. With a greater effect of HIIT, both protocols significantly lowered the proteins expressions in the MIHIIT and MIMCT groups, compared with the MI group (P < 0.001). In healthy rats, only AHR protein significantly decreased in the MICT group compared to the Ct group (P < 0.05). HIIT and MICT protocols significantly reduced the gene and protein expression of Cyp1a1 (P < 0.05) and Ido1 (P < 0.01), and HIIT had a greater effect. In conclusion, both protocols were effective at reducing the levels of Ido1-Kyn-Ahr axis components and oxidative stress in the infarcted heart tissue and HIIT had a higher significant effect.

Gut microbiome sheds light on the development and treatment of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is an inflammatory vascular disease with high disability and mortality. Its susceptible risk factors include old age, being male, smoking, hypertension, and aortic atherosclerosis. With the improvement of screening techniques, AAA incidence and number of deaths caused by aneurysm rupture increase annually, attracting much clinical attention. Due to the lack of non-invasive treatment, early detection and development of novel treatment of AAA is an urgent clinical concern. The pathophysiology and progression of AAA are characterized by inflammatory destruction. The gut microbiota is an "invisible organ" that directly or indirectly affects the vascular wall inflammatory cell infiltration manifested with enhanced arterial wall gut microbiota and metabolites, which plays an important role in the formation and progression of AAA. As such, the gut microbiome may become an important risk factor for AAA. This review summarizes the direct and indirect effects of the gut microbiome on the pathogenesis of AAA and highlights the gut microbiome-mediated inflammatory responses and discoveries of relevant therapeutic targets that may help manage the development and rupture of AAA.

The Footprint of Kynurenine Pathway in Cardiovascular Diseases

Kynurenine pathway is the main route of tryptophan metabolism and produces several metabolites with various biologic properties. It has been uncovered that several cardiovascular diseases are associated with the overactivation of kynurenine pathway and kynurenine and its metabolites have diagnostic and prognostic value in cardiovascular diseases. Furthermore, it was found that several kynurenine metabolites can differently affect cardiovascular health. For instance, preclinical studies have shown that kynurenine, xanthurenic acid and cis-WOOH decrease blood pressure; kynurenine and 3-hydroxyanthranilic acid prevent atherosclerosis; kynurenic acid supplementation and kynurenine 3-monooxygenase (KMO) inhibition improve the outcome of stroke. Indoleamine 2,3-dioxygenase (IDO) overactivity and increased kynurenine levels improve cardiac and vascular transplantation outcomes, whereas exacerbating the outcome of myocardial ischemia, post-ischemic myocardial remodeling, and abdominal aorta aneurysm. IDO inhibition and KMO inhibition are also protective against viral myocarditis. In addition, dysregulation of kynurenine pathway is observed in several conditions such as senescence, depression, diabetes, chronic kidney disease (CKD), cirrhosis, and cancer closely connected to cardiovascular dysfunction. It is worth defining the exact effect of each metabolite of kynurenine pathway on cardiovascular health. This narrative review is the first review that separately discusses the involvement of kynurenine pathway in different cardiovascular diseases and dissects the underlying molecular mechanisms.

Angiotensin-converting enzyme 2, coronavirus disease 2019, and abdominal aortic aneurysms

OBJECTIVE

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the etiologic agent of the current, world-wide coronavirus disease 2019 (COVID-19) pandemic. Angiotensin-converting enzyme 2 (ACE2) is the SARS-CoV-2 host entry receptor for cellular inoculation and target organ injury. We reviewed ACE2 expression and the role of ACE2-angiotensin 1-7-Mas receptor axis activity in abdominal aortic aneurysm (AAA) pathogenesis to identify potential COVID-19 influences on AAA disease pathogenesis.

METHODS

A comprehensive literature search was performed on PubMed, National Library of Medicine. Key words included COVID-19, SARS-CoV-2, AAA, ACE2, ACE or angiotensin II type 1 (AT1) receptor inhibitor, angiotensin 1-7, Mas receptor, age, gender, respiratory diseases, diabetes, and autoimmune diseases. Key publications on the epidemiology and pathogenesis of COVID-19 and AAAs were identified and reviewed.

RESULTS

All vascular structural cells, including endothelial and smooth muscle cells, fibroblasts, and pericytes express ACE2. Cigarette smoking, diabetes, chronic obstructive pulmonary disease, lupus, certain types of malignancies, and viral infection promote ACE2 expression and activity, with the magnitude of response varying by sex and age. Genetic deficiency of AT1 receptor, or pharmacologic ACE or AT1 inhibition also increases ACE2 and its catalytic product angiotensin 1-7. Genetic ablation or pharmacologic inhibition of ACE2 or Mas receptor augments, whereas ACE2 activation or angiotensin 1-7 treatment attenuates, progression of experimental AAAs. The potential influences of SARS-CoV-2 on AAA pathogenesis include augmented ACE-angiotensin II-AT1 receptor activity resulting from decreased reciprocal ACE2-angiotensin 1-7-Mas activation; increased production of proaneurysmal mediators stimulated by viral spike proteins in ACE2-negative myeloid cells or by ACE2-expressing vascular structural cells; augmented local or systemic cross-talk between viral targeted nonvascular, nonleukocytic ACE2-expressing cells via ligand recognition of their cognate leukocyte receptors; and hypoxemia and increased systemic inflammatory tone experienced during severe COVID-19 illness.

CONCLUSIONS

COVID-19 may theoretically influence AAA disease through multiple SARS-CoV-2-induced mechanisms. Further investigation and clinical follow-up will be necessary to determine whether and to what extent the COVID-19 pandemic will influence the prevalence, progression, and lethality of AAA disease in the coming decade.

Tryptophan Catabolism and Inflammation: A Novel Therapeutic Target For Aortic Diseases

Aortic diseases are the primary public health concern. As asymptomatic diseases, abdominal aortic aneurysm (AAA) and atherosclerosis are associated with high morbidity and mortality. The inflammatory process constitutes an essential part of a pathogenic cascade of aortic diseases, including atherosclerosis and aortic aneurysms. Inflammation on various vascular beds, including endothelium, smooth muscle cell proliferation and migration, and inflammatory cell infiltration (monocytes, macrophages, neutrophils, etc.), play critical roles in the initiation and progression of aortic diseases. The tryptophan (Trp) metabolism or kynurenine pathway (KP) is the primary way of degrading Trp in most mammalian cells, disturbed by cytokines under various stress. KP generates several bioactive catabolites, such as kynurenine (Kyn), kynurenic acid (KA), 3-hydroxykynurenine (3-HK), etc. Depends on the cell types, these metabolites can elicit both hyper- and anti-inflammatory effects. Accumulating evidence obtained from various animal disease models indicates that KP contributes to the inflammatory process during the development of vascular disease, notably atherosclerosis and aneurysm development. This review outlines current insights into how perturbed Trp metabolism instigates aortic inflammation and aortic disease phenotypes. We also briefly highlight how targeting Trp metabolic pathways should be considered for treating aortic diseases.

Tryptophan: From Diet to Cardiovascular Diseases

Cardiovascular disease (CVD) is one of the major causes of mortality worldwide. Inflammation is the underlying common mechanism involved in CVD. It has been recently related to amino acid metabolism, which acts as a critical regulator of innate and adaptive immune responses. Among different metabolites that have emerged as important regulators of immune and inflammatory responses, tryptophan (Trp) metabolites have been shown to play a pivotal role in CVD. Here, we provide an overview of the fundamental aspects of Trp metabolism and the interplay between the dysregulation of the main actors involved in Trp metabolism such as indoleamine 2, 3-dioxygenase 1 (IDO) and CVD, including atherosclerosis and myocardial infarction. IDO has a prominent and complex role. Its activity, impacting on several biological pathways, complicates our understanding of its function, particularly in CVD, where it is still under debate. The discrepancy of the observed IDO effects could be potentially explained by its specific cell and tissue contribution, encouraging further investigations regarding the role of this enzyme. Thus, improving our understanding of the function of Trp as well as its derived metabolites will help to move one step closer towards tailored therapies aiming to treat CVD.

Effects of indoleamine 2, 3-dioxygenase (IDO) silencing on immunomodulatory function and cancer-promoting characteristic of adipose-derived mesenchymal stem cells (ASCs)

Indoleamine 2, 3-dioxygenase (IDO) catabolizes tryptophan, mediates immunomodulatory functions, and is released by stromal cells such as mesenchymal stem cells. The aims of this study were to investigate the effects of IDO silencing on immunosuppressive function of adipose-derived mesenchymal stem cells (ASCs), T cells phenotype, and the proliferation/migration of tumor cells. ASCs isolated from adipose tissues of healthy women were transfected with IDO-siRNA. Galectin-3, transforming growth factor-β1, hepatocyte growth factor, and interleukin-10 as immunomodulators were measured in ASCs using qRT-PCR. T cells phenotype, interferon-γ, and interleukin-17 expression were evaluated in peripheral blood lymphocytes (PBLs) cocultured with IDO silenced-ASCs by flow cytometry and qRT-PCR, respectively. Scratch assay was applied to assess the proliferation/migration of MDA-MB-231 cell line. Galectin-3 was upregulated (p ˂ 0.05) while hepatocyte growth factor was downregulated (p ˂ 0.05) in IDO-silenced ASCs compared to control groups. Regulatory T cells were inhibited in PBLs cocultured with IDO-silenced ASCs; also T helper2 was decreased in PBLs cocultured with IDO-silenced ASCs relative to the scramble group. IDO-silenced ASCs caused interferon-γ overexpression but interleukin-17 downregulation in PBLs. The proliferation/migration of MDA-MB-231 was suppressed after exposing to condition media of IDO-silenced ASCs compared with condition media of untransfected (p < 0.01) and scramble-transfected ASCs (p < 0.05). The results exhibited the weakened capacity of IDO-silenced ASCs for suppressing the immune cells and promoting the tumor cells' proliferation/migration. IDO suppression may be utilized as a strategy for cancer treatment. Simultaneous blocking of immunomodulators along with IDO inhibitors may show more effects on boosting the efficiency of immune-based cancer therapies.

Kallistatin limits abdominal aortic aneurysm by attenuating generation of reactive oxygen species and apoptosis

Inflammation, vascular smooth muscle cell apoptosis and oxidative stress are believed to play important roles in abdominal aortic aneurysm (AAA) pathogenesis. Human kallistatin (KAL; gene SERPINA4) is a serine proteinase inhibitor previously shown to inhibit inflammation, apoptosis and oxidative stress. The aim of this study was to investigate the role of KAL in AAA through studies in experimental mouse models and patients. Serum KAL concentration was negatively associated with the diagnosis and growth of human AAA. Transgenic overexpression of the human KAL gene (KS-Tg) or administration of recombinant human KAL (rhKAL) inhibited AAA in the calcium phosphate (CaPO₄) and subcutaneous angiotensin II (AngII) infusion mouse models. Upregulation of KAL in both models resulted in reduction in the severity of aortic elastin degradation, reduced markers of oxidative stress and less vascular smooth muscle apoptosis within the aorta. Administration of rhKAL to vascular smooth muscle cells incubated in the presence of AngII or in human AAA thrombus-conditioned media reduced apoptosis and downregulated markers of oxidative stress. These effects of KAL were associated with upregulation of Sirtuin 1 activity within the aortas of both KS-Tg mice and rodents receiving rhKAL. These results suggest KAL-Sirtuin 1 signalling limits aortic wall remodelling and aneurysm development through reductions in oxidative stress and vascular smooth muscle cell apoptosis. Upregulating KAL may be a novel therapeutic strategy for AAA.

Indoleamine 2, 3-dioxygenase 1 aggravates acetaminophen-induced acute liver failure by triggering excess nitroxidative stress and iron accumulation

Acetaminophen (APAP) is the leading cause of acute liver failure (ALF), which is characterized by GSH depletion, oxidative stress and mitochondrial dysfunction. However, the specific mechanism of APAP-induced ALF remains to be clarified. In this study, we demonstrated that indoleamine 2,3-dioxygenase 1 (IDO1) aggravated APAP-induced ALF associated with excess lipid peroxidation, which was reversed by lipid peroxidation inhibitor (ferrostatin-1). Meanwhile, IDO1 deficiency effectively decreased the accumulation of reactive nitrogen species. Additionally, IDO1 deficiency prevented against APAP-induced liver injury through suppressing the activation of macrophages, thereby reduced their iron uptake and export, eventually reduced iron accumulation in hepatocytes through transferrin and transferrin receptor axis. In summary, our study confirmed that APAP-induced IDO1 aggravated ALF by triggering excess oxidative and nitrative stress and iron accumulation in liver. These results offer new insights for the clinical treatment of ALF or iron-dysregulated liver diseases in the future.

Advanced Research of Abdominal Aortic Aneurysms on Metabolism

Abdominal aortic aneurysm (AAA) is a cardiovascular disease with a high risk of death, seriously threatening the life and health of people. The specific pathogenesis of AAA is still not fully understood. In recent years, researchers have found that amino acid, lipid, and carbohydrate metabolism disorders play important roles in the occurrence and development of AAA. This review is aimed to summarize the latest research progress of the relationship between AAA progression and body metabolism. The body metabolism is closely related to the occurrence and development of AAA. It is necessary to further investigate the pathogenesis of AAA from the perspective of metabolism to provide theoretical basis for AAA diagnosis and drug development.

Upregulated Kynurenine Pathway Enzymes in Aortic Atherosclerotic Aneurysm: Macrophage Kynureninase Downregulates Inflammation

AIMS

Inflammation and hypertension contribute to the progression of atherosclerotic aneurysm in the aorta. Vascular cell metabolism is regarded to modulate atherogenesis, but the metabolic alterations that occur in atherosclerotic aneurysm remain unknown. The present study aimed to identify metabolic pathways and metabolites in aneurysmal walls and examine their roles in atherogenesis.

METHODS

Gene expression using microarray and metabolite levels in the early atherosclerotic lesions and aneurysmal walls obtained from 42 patients undergoing aortic surgery were investigated (early lesion n=11, aneurysm n=35) and capillary electrophoresis-time-of-flight mass spectrometry (early lesion n=14, aneurysm n=38). Using immunohistochemistry, the protein expression and localization of the identified factors were examined (early lesion n=11, non-aneurysmal advanced lesion n=8, aneurysm n=11). The roles of the factors in atherogenesis were analyzed in macrophages derived from human peripheral blood mononuclear cells.

RESULTS

Enrichment analysis using 35 significantly upregulated genes (log2 ratio, ＞3) revealed the alteration of the kynurenine pathway. Metabolite levels of tryptophan, kynurenine, and quinolinic acid and the kynurenine-to-tryptophan ratio were increased in the aneurysmal walls. Gene and protein expression of kynureninase and kynurenine 3-monooxygenase were upregulated and localized in macrophages in the aneurysmal walls. The silencing of kynureninase in the cultured macrophages enhanced the expression of interleukin-6 and indoleamine 2,3-dioxygenase 1.

CONCLUSION

Our study suggests the upregulation of the kynurenine pathway in macrophages in aortic atherosclerotic aneurysm. Kynureninase may negatively regulate inflammation via the kynurenine pathway itself in macrophages.

Endothelial Cell Indoleamine 2, 3-Dioxygenase 1 Alters Cardiac Function After Myocardial Infarction Through Kynurenine

BACKGROUND

Ischemic cardiovascular diseases, particularly acute myocardial infarction (MI), is one of the leading causes of mortality worldwide. Indoleamine 2, 3-dioxygenase 1 (IDO) catalyzes 1 rate-limiting step of L-tryptophan metabolism, and emerges as an important regulator of many pathological conditions. We hypothesized that IDO could play a key role to locally regulate cardiac homeostasis after MI.

METHODS

Cardiac repair was analyzed in mice harboring specific endothelial or smooth muscle cells or cardiomyocyte or myeloid cell deficiency of IDO and challenged with acute myocardial infarction.

RESULTS

We show that kynurenine generation through IDO is markedly induced after MI in mice. Total genetic deletion or pharmacological inhibition of IDO limits cardiac injury and cardiac dysfunction after MI. Distinct loss of function of IDO in smooth muscle cells, inflammatory cells, or cardiomyocytes does not affect cardiac function and remodeling in infarcted mice. In sharp contrast, mice harboring endothelial cell-specific deletion of IDO show an improvement of cardiac function as well as cardiomyocyte contractility and reduction in adverse ventricular remodeling. In vivo kynurenine supplementation in IDO-deficient mice abrogates the protective effects of IDO deletion. Kynurenine precipitates cardiomyocyte apoptosis through reactive oxygen species production in an aryl hydrocarbon receptor-dependent mechanism.

CONCLUSIONS

These data suggest that IDO could constitute a new therapeutic target during acute MI.

The immunometabolic role of indoleamine 2,3-dioxygenase in atherosclerotic cardiovascular disease: immune homeostatic mechanisms in the artery wall

Coronary heart disease and stroke, the two most common cardiovascular diseases worldwide, are triggered by complications of atherosclerosis. Atherosclerotic plaques are initiated by a maladaptive immune response triggered by accumulation of lipids in the artery wall. Hence, disease is influenced by several non-modifiable and modifiable risk factors, including dyslipidaemia, hypertension, smoking, and diabetes. Indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme in the kynurenine pathway of tryptophan (Trp) degradation, is modulated by inflammation and regarded as a key molecule driving immunotolerance and immunosuppressive mechanisms. A large body of evidence indicates that IDO-mediated Trp metabolism is involved directly or indirectly in atherogenesis. This review summarizes evidence from basic and clinical research showing that IDO is a major regulatory enzyme involved in the maintenance of immunohomeostasis in the vascular wall, as well as current knowledge about promising targets for the development of new anti-atherosclerotic drugs.

Indoleamine 2, 3-dioxygenase 1enhanceshepatocytes ferroptosis in acute immune hepatitis associated with excess nitrative stress

Ferroptosis is a recently recognized form of regulated cell death that is characterized by lipid peroxidation. However, the molecular mechanisms of ferroptosis in acute immune hepatitis (AIH) are largely unknown. In this study, we investigated the classical ferroptotic events in the livers of mice with concanavalin A (ConA) to induce AIH. The dramatically upregulated gene indoleamine 2, 3-dioxygenase 1 (IDO1) was identified with AIH, and its role in generation of ferroptosis and reactive nitrogen species (RNS) was assessed both in vitro and in vivo by genetic deletion or pharmacologic inhibition of IDO1. We observed that ferroptosis contributed to the ConA-induced hepatic damage, which was confirmed by the therapeutical effects of ferroptosis inhibitor (ferrostatin-1). Noteworthy, upregulation of hepatic IDO1 and nitrative stress in ConA-induced hepatic damage were also remarkably inhibited by the ferroptosis abolishment. Additionally, IDO1 deficiency contributed to ferroptosis resistance by activating solute carrier family 7 member 11 (SLC7A11; also known as xCT) expression, accompanied with the reductions of murine liver lesions and RNS. Meanwhile, IDO inhibitor 1-methyl tryptophan alleviated murine liver damage with the reduction of inducible nitric oxide synthase and 3-nitrotyrosine expression. Consistent with the results in vivo, hepatocytes-specific knockdown of IDO1 led to ferroptosis resistance upon exposure to ferroptosis-inducing compound (Erastin) in vitro, whereas IDO1 overexpression aggravated the classical ferroptotic events, and the RNS stress. Overall, these results revealed a novel molecular mechanism of ferroptosis with the key feature of nitrative stress in ConA-induced liver injury, and also identified IDO1-dependent ferroptosis as a potential target for the treatment of AIH.

Amino Acids in Circulatory Function and Health

Cardiovascular disease is the major cause of global mortality and disability. Abundant evidence indicates that amino acids play a fundamental role in cardiovascular physiology and pathology. Decades of research established the importance of L-arginine in promoting vascular health through the generation of the gas nitric oxide. More recently, L-glutamine, L-tryptophan, and L-cysteine have also been shown to modulate vascular function via the formation of a myriad of metabolites, including a number of gases (ammonia, carbon monoxide, hydrogen sulfide, and sulfur dioxide). These amino acids and their metabolites preserve vascular homeostasis by regulating critical cellular processes including proliferation, migration, differentiation, apoptosis, contractility, and senescence. Furthermore, they exert potent anti-inflammatory and antioxidant effects in the circulation, and block the accumulation of lipids within the arterial wall. They also mitigate known risk factors for cardiovascular disease, including hypertension, hyperlipidemia, obesity, and diabetes. However, in some instances, the metabolism of these amino acids through discrete pathways yields compounds that fosters vascular disease. While supplementation with amino acid monotherapy targeting the deficiency has ameliorated arterial disease in many animal models, this approach has been less successful in the clinic. A more robust approach combining amino acid supplementation with antioxidants, anti-inflammatory agents, and/or specific amino acid enzymatic pathway inhibitors may prove more successful. Alternatively, supplementation with amino acid-derived metabolites rather than the parent molecule may elicit beneficial effects while bypassing potentially harmful pathways of metabolism. Finally, there is an emerging recognition that circulating levels of multiple amino acids are perturbed in vascular disease and that a more holistic approach that targets all these amino acid derangements is required to restore circulatory function in diseased blood vessels.

Immunomodulatory Effects of Genetic Alterations Affecting the Kynurenine Pathway

Several enzymes and metabolites of the kynurenine pathway (KP) have immunomodulatory effects. Modulation of the activities and levels of these molecules might be of particular importance under disease conditions when the amelioration of overreacting immune responses is desired. Results obtained by the use of animal and tissue culture models indicate that by eliminating or decreasing activities of key enzymes of the KP, a beneficial shift in disease outcome can be attained. This review summarizes experimental data of models in which IDO, TDO, or KMO activity modulation was achieved by interventions affecting enzyme production at a genomic level. Elimination of IDO activity was found to improve the outcome of sepsis, certain viral infections, chronic inflammation linked to diabetes, obesity, aorta aneurysm formation, and in anti-tumoral processes. Similarly, lack of TDO activity was advantageous in the case of anti-tumoral immunity, while KMO inhibition was found to be beneficial against microorganisms and in the combat against tumors, as well. On the other hand, the complex interplay among KP metabolites and immune function in some cases requires an increase in a particular enzyme activity for the desired immune response modulation, as was shown by the exacerbation of liver fibrosis due to the elimination of IDO activity and the detrimental effects of TDO inhibition in a mouse model of autoimmune gastritis. The relevance of these studies concerning possible human applications are discussed and highlighted. Finally, a brief overview is presented on naturally occurring genetic variants affecting immune functions via modulation of KP enzyme activity.

Tryptophan Dietary Impacts Gut Barrier and Metabolic Diseases

The intestine has a major role in the digestion and absorption of nutrients, and gut barrier is the first defense line against harmful pathogens. Alteration of the intestinal barrier is associated with enhanced intestinal permeability and development of numerous pathological diseases including gastrointestinal and cardiometabolic diseases. Among the metabolites that play an important role within intestinal health, L Tryptophan (Trp) is one of the nine essential amino acids supplied by diet, whose metabolism appears as a key modulator of gut microbiota, with major impacts on physiological, and pathological pathways. Recently, emerging evidence showed that the Trp catabolism through one major enzyme indoleamine 2,3-dioxygenase 1 (IDO1) expressed by the host affects Trp metabolism by gut microbiota to generate indole metabolites, thereby altering gut function and health in mice and humans. In this mini review, I summarize the most recent advances concerning the role of Trp metabolism in host–microbiota cross-talk in health, and metabolic diseases. This novel aspect of IDO1 function in intestine will better explain its complex roles in a broad range of disease states where the gut function affects local as well as systemic health, and will open new therapeutic strategies.

Quantification of IDO1 enzyme activity in normal and malignant tissues

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the first and rate-limiting reaction of l-tryptophan (Trp) conversion into l-kynurenine (Kyn). The depletion of Trp, and the accumulation of Kyn have been proposed as mechanisms that contribute to the suppression of the immune response-primarily evidenced by in vitro study. IDO1 is therefore considered to be an immunosuppressive modulator and quantification of IDO1 metabolism may be critical to understanding its role in select immunopathologies, including autoimmune- and oncological-conditions, as well as for determining the potency of IDO1 enzyme inhibitors. Because tryptophan 2,3-dioxygenase (TDO), and to a significantly lesser extent, IDO2, also catabolize Trp into Kyn, it's important to differentiate the contribution of each enzyme to Trp catabolism and Kyn generation. Moreover, a great variety of detection methods have been developed for the quantification of Trp metabolites, but choosing the suitable protocol remains challenging. Here, we review the differential expression of IDO1/TDO/IDO2 in normal and malignant tissues, followed by a comprehensive analysis of methodologies for quantifying Trp and Kyn in vitro and in vivo, with an emphasis on the advantages/disadvantages for each application.

Butyrate Produced by Commensal Bacteria Down-Regulates Indolamine 2,3-Dioxygenase 1 (IDO-1) Expression via a Dual Mechanism in Human Intestinal Epithelial Cells

Commensal bacteria are crucial for the development and maintenance of a healthy immune system therefore contributing to the global well-being of their host. A wide variety of metabolites produced by commensal bacteria are influencing host health but the characterization of the multiple molecular mechanisms involved in host-microbiota interactions is still only partially unraveled. The intestinal epithelial cells (IECs) take a central part in the host-microbiota dialogue by inducing the first microbial-derived immune signals. Amongst the numerous effector molecules modulating the immune responses produced by IECs, indoleamine 2,3-dioxygenase-1 (IDO-1) is essential for gut homeostasis. IDO-1 expression is dependent on the microbiota and despites its central role, how the commensal bacteria impacts its expression is still unclear. Therefore, we investigated the impact of individual cultivable commensal bacteria on IDO-1 transcriptional expression and found that the short chain fatty acid (SCFA) butyrate was the main metabolite controlling IDO-1 expression in human primary IECs and IEC cell-lines. This butyrate-driven effect was independent of the G-protein coupled receptors GPR41, GPR43, and GPR109a and of the transcription factors SP1, AP1, and PPARγ for which binding sites were reported in the IDO-1 promoter. We demonstrated for the first time that butyrate represses IDO-1 expression by two distinct mechanisms. Firstly, butyrate decreases STAT1 expression leading to the inhibition of the IFNγ-dependent and phosphoSTAT1-driven transcription of IDO-1. In addition, we described a second mechanism by which butyrate impairs IDO-1 transcription in a STAT1-independent manner that could be attributed to its histone deacetylase (HDAC) inhibitor property. In conclusion, our results showed that IDO-1 expression is down-regulated by butyrate via a dual mechanism: the reduction of STAT1 level and the HDAC inhibitor property of SCFAs.