Evidence that a deviation in the kynurenine pathway aggravates atherosclerotic disease in humans

Kynurenic Acid, a Small Foodborne Molecule with the Potential to Affect Human Health

Kynurenic acid (KYNA) is one of the metabolites of tryptophan (Trp), which can be supplied by endogenous synthesis or diet and involves various aspects of nutrition and metabolism. KYNA has been proven to be an antagonist of α7-nicotinic acetylcholine and NMDA receptors, which can regulate the neurotransmission of cholinergic and glutaminergic acid, so KYNA is mainly associated with neurodegenerative diseases. Other metabolites in the Trp and kynurenine metabolic pathways, such as 3-hydroxy-l-kynurenine (3-HK), 5-hydroxytryptophan (5-HT), and quinolinic acid (QA), are also involved in immune regulation and have pro-stimulatory or anti-excitatory toxicity. KYNA is present in both the human body and the daily diet, so it has the potential as a new dietary supplement. It may be of great significance in treating psychiatric disorders.

Assessment of Inflammatory and Oxidative Stress Biomarkers for Predicting of Patients with Asymptomatic Carotid Artery Stenosis

Background/Objectives: Asymptomatic carotid artery stenosis is usually detected by physicians in patients, coincidentally, during an ultrasound examination of the neck. Therefore, measurable biomarkers in blood are needed to define the presence and severity of atherosclerotic plaque in patients to identify and manage it. We hypothesized that biomarkers that indicate pathways related to the pathogenesis of atherosclerosis could be used to identify the presence and severity of atherosclerotic plaque. For this purpose, the levels of participants' inflammatory and oxidative stress biomarkers were determined. Kynurenine/tryptophan and neopterin levels were measured as relatively new biomarkers of inflammation in this study. Methods: Our study included 57 patients diagnosed with asymptomatic carotid artery stenosis and 28 healthy volunteers. Blood kynurenine and tryptophan levels were measured with LCMS/MS. Blood catalase, total superoxide dismutase (t-SOD), glutathione peroxidase (GPx), malondialdehyde, and neopterin levels were measured using the ELISA assay method. Result: The kynurenine/tryptophan ratio reflecting IDO activity was higher in patients than in healthy volunteers. Decreased tryptophan levels and increased kynurenine and neopterin levels were observed in patients who underwent carotid endarterectomy. In patients, catalase, t-SOD, and malondialdehyde levels were higher, while GPx activity was lower. These differences were found to be more significant in patients who underwent carotid endarterectomy. Conclusions: Increased kynurenine/tryptophan ratio and neopterin levels in patients with asymptomatic carotid artery stenosis were associated with the inflammatory status of the patients. Oxidative stress and inflammatory biomarkers can be considered effective diagnostic and severity indicators for asymptomatic carotid artery stenosis.

Epigenetic programming of obesity in early life through modulation of the kynurenine pathway

Childhood obesity is a global health concern that has its origins before birth. Although genetics plays a crucial role, increasing evidence suggests that epigenetic modifications during fetal life could also influence its incidence. In this model, during the fetal period, interactions between genetic makeup, intrauterine factors, and environmental conditions, increase the risk of childhood obesity. This is in accordance with the Developmental Origins of Health and Disease (DOHaD) hypothesis, in which specific intrauterine environments can have long-lasting effects on the immune system's essential functions during crucial stages of fetal growth, resulting in permanent changes to the immune function of the offspring. Consequently, dysfunction can consequently make the offspring more prone to inflammatory and immune-related disorders later in life. In this review, we examine how maternal inflammation could influence the risk of childhood obesity. We propose that during pregnancy, modification of the expression of critical genes in metabolic and signaling pathways, such as the kynurenine (Kyn) pathway, occurs due to increased levels of maternal inflammation. We also propose that such expression differences are mediated by epigenetic changes. Furthermore, we also hypothesize that the Kyn pathway produces metabolites that have immunoregulatory effects and may play a crucial role in regulating inflammation during pregnancy. As a result, interventions aimed at improving maternal inflammation may be able to help alleviate the risk of childhood obesity.

Exploring tryptophan metabolism in cardiometabolic diseases

Tryptophan (Trp) metabolism is linked to health and disease, with indoleamine 2,3-dioxygenase 1 (IDO) being a key enzyme in its breakdown outside the liver. This process produces metabolites that influence metabolic and inflammatory responses. A distinctive feature of the gut is its involvement in three major Trp catabolic pathways: the IDO-driven kynurenine pathway, bacteria-produced indoles, and serotonin. Dysregulation of these pathways is associated with gastrointestinal and chronic inflammatory diseases. Understanding these mechanisms could reveal how gut function affects overall systemic health and disease susceptibility. Here, we review current insights into Trp metabolism, its impact on host physiology and cardiometabolic diseases, and its role in the gut-periphery connection, highlighting its relevance for therapeutic innovation.

Characteristics of Gracilariopsis lemaneiformis hydrocolloids and their effects on intestine PPAR signaling and liver lipid metabolism in Oreochromis niloticus: A multiomics analysis

This study evaluated the effects of Gracilariopsis lemaneiformis hydrocolloids on Nile tilapia (Oreochromis niloticus) using an advanced multiomics approach (transcriptome and proteome) linked with genomic isoform structure to elucidate the biofunctions of G. lemaneiformis hydrocolloids. The results showed that G. lemaneiformis hydrocolloids did not affect growth, as indicated by the nonsignificant differences in growth and blood biochemical indicators. Regarding the response, both intestine and liver tissues were assessed. These findings indicate that 20 % G. lemaneiformis hydrocolloids enhanced cytokine expression, which may contribute to a biological function in the intestine and liver of O. niloticus. Genome and proteome profiles indicated that G. lemaneiformis hydrocolloids upregulated the intestine and liver peroxisome proliferator-activated receptor (PPAR) signaling pathway, nucleocytoplasmic transport, steroid biosynthesis, and histidine metabolism. In contrast, co-factor biosynthesis, nucleocytoplasmic transport, tryptophan metabolism, arginine and proline metabolism, arginine biosynthesis, and ribosome activity were downregulated. These findings indicate that G. lemaneiformis hydrocolloids significantly affect liver lipid and carbohydrate metabolism. Proteomics analysis revealed that G. lemaneiformis hydrocolloids upregulated the PPAR signaling pathway, playing a crucial role in lipid metabolism. In summary, 20 % G. lemaneiformis hydrocolloids are primarily involved in modulating the intestine and liver PPAR signaling pathway to regulate lipid metabolism.

The roles of the kynurenine pathway in COVID-19 neuropathogenesis

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the highly contagious respiratory disease Corona Virus Disease 2019 (COVID-19) that may lead to various neurological and psychological disorders that can be acute, lasting days to weeks or months and possibly longer. The latter is known as long-COVID or more recently post-acute sequelae of COVID (PASC). During acute COVID-19 infection, a strong inflammatory response, known as the cytokine storm, occurs in some patients. The levels of interferon-γ (IFN-γ), interferon-β (IFN-β), interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-α) are particularly increased. These cytokines are known to activate the enzyme indoleamine 2,3-dioxygenase 1 (IDO-1), catalysing the first step of tryptophan (Trp) catabolism through the kynurenine pathway (KP) leading to the production of several neurotoxic and immunosuppressive metabolites. There is already data showing elevation in KP metabolites both acutely and in PASC, especially regarding cognitive impairment. Thus, it is likely that KP involvement is significant in SARS-CoV-2 pathogenesis especially neurologically.

Computed Tomography and Fluorescence Spectroscopy Blood Plasma Analysis Study for Kynurenic Acid as a Diagnostic Approach to Chronic Coenurosis in Sheep

Coenurosis is a parasitic disease caused by the larval stage of Taenia multiceps, Coenurus cerebralis, typically found in the central nervous system of different livestock such as sheep and goats. The blood plasma from fifteen clinically healthy sheep and six sheep with neurological symptoms was studied by fluorescence spectroscopy in order to establish the contribution of kynurenic acid (KYNA), the neuroprotective metabolite of the kynurenine pathway, to the total fluorescence of the plasma. CT scans were used to confirm the presence of cysts in the central nervous system of sheep with neurological symptoms. The fluorescence spectroscopy analysis and further spectra deconvolution process revealed some significantly lower KYNA contributions to the total plasma fluorescence in sheep with coenurosis compared to healthy controls. Our results indicate that KYNA emission parameters could serve as valuable diagnostic markers, particularly for detecting preclinical cases of coenurosis, thus allowing for improved farm management practices.

Efferocytosis drives a tryptophan metabolism pathway in macrophages to promote tissue resolution

Macrophage efferocytosis prevents apoptotic cell (AC) accumulation and triggers inflammation-resolution pathways. The mechanisms linking efferocytosis to resolution often involve changes in macrophage metabolism, but many gaps remain in our understanding of these processes. We now report that efferocytosis triggers an indoleamine 2,3-dioxygenase-1 (IDO1)-dependent tryptophan (Trp) metabolism pathway that promotes several key resolution processes, including the induction of pro-resolving proteins, such interleukin-10, and further enhancement of efferocytosis. The process begins with upregulation of Trp transport and metabolism, and it involves subsequent activation of the aryl hydrocarbon receptor (AhR) by the Trp metabolite kynurenine (Kyn). Through these mechanisms, macrophage IDO1 and AhR contribute to a proper resolution response in several different mouse models of efferocytosis-dependent tissue repair, notably during atherosclerosis regression induced by plasma low-density lipoprotein (LDL) lowering. These findings reveal an integrated metabolism programme in macrophages that links efferocytosis to resolution, with possible therapeutic implications for non-resolving chronic inflammatory diseases, notably atherosclerosis.

The Biology and Biochemistry of Kynurenic Acid, a Potential Nutraceutical with Multiple Biological Effects

Kynurenic acid (KYNA) is an antioxidant degradation product of tryptophan that has been shown to have a variety of cytoprotective, neuroprotective and neuronal signalling properties. However, mammalian transporters and receptors display micromolar binding constants; these are consistent with its typically micromolar tissue concentrations but far above its serum/plasma concentration (normally tens of nanomolar), suggesting large gaps in our knowledge of its transport and mechanisms of action, in that the main influx transporters characterized to date are equilibrative, not concentrative. In addition, it is a substrate of a known anion efflux pump (ABCC4), whose in vivo activity is largely unknown. Exogeneous addition of L-tryptophan or L-kynurenine leads to the production of KYNA but also to that of many other co-metabolites (including some such as 3-hydroxy-L-kynurenine and quinolinic acid that may be toxic). With the exception of chestnut honey, KYNA exists at relatively low levels in natural foodstuffs. However, its bioavailability is reasonable, and as the terminal element of an irreversible reaction of most tryptophan degradation pathways, it might be added exogenously without disturbing upstream metabolism significantly. Many examples, which we review, show that it has valuable bioactivity. Given the above, we review its potential utility as a nutraceutical, finding it significantly worthy of further study and development.

Identification and experimental validation of KMO as a critical immune-associated mitochondrial gene in unstable atherosclerotic plaque

BACKGROUND

The heightened risk of cardiovascular and cerebrovascular events is associated with the increased instability of atherosclerotic plaques. However, the lack of effective diagnostic biomarkers has impeded the assessment of plaque instability currently. This study was aimed to investigate and identify hub genes associated with unstable plaques through the integration of various bioinformatics tools, providing novel insights into the detection and treatment of this condition.

METHODS

Weighted Gene Co-expression Network Analysis (WGCNA) combined with two machine learning methods were used to identify hub genes strongly associated with plaque instability. The cell-type identification by estimating relative subsets of RNA transcripts (CIBERSORT) method was utilized to assess immune cell infiltration patterns in atherosclerosis patients. Additionally, Gene Set Variation Analysis (GSVA) was conducted to investigate the potential biological functions, pathways, and mechanisms of hub genes associated with unstable plaques. To further validate the diagnostic efficiency and expression of the hub genes, immunohistochemistry (IHC), quantitative real-time polymerase chain reaction (RT-qPCR), and enzyme-linked immunosorbent assay (ELISA) were performed on collected human carotid plaque and blood samples. Immunofluorescence co-staining was also utilized to confirm the association between hub genes and immune cells, as well as their colocalization with mitochondria.

RESULTS

The CIBERSORT analysis demonstrated a significant decrease in the infiltration of CD8 T cells and an obvious increase in the infiltration of M0 macrophages in patients with atherosclerosis. Subsequently, two highly relevant modules (blue and green) strongly associated with atherosclerotic plaque instability were identified. Through intersection with mitochondria-related genes, 50 crucial genes were identified. Further analysis employing least absolute shrinkage and selection operator (LASSO) logistic regression and support vector machine recursive feature elimination (SVM-RFE) algorithms revealed six hub genes significantly associated with plaque instability. Among them, NT5DC3, ACADL, SLC25A4, ALDH1B1, and MAOB exhibited positive correlations with CD8 T cells and negative correlations with M0 macrophages, while kynurenine 3-monooxygenas (KMO) demonstrated a positive correlation with M0 macrophages and a negative correlation with CD8 T cells. IHC and RT-qPCR analyses of human carotid plaque samples, as well as ELISA analyses of blood samples, revealed significant upregulation of KMO and MAOB expression, along with decreased ALDH1B1 expression, in both stable and unstable samples compared to the control samples. However, among the three key genes mentioned above, only KMO showed a significant increase in expression in unstable plaque samples compared to stable plaque samples. Furthermore, the expression patterns of KMO in human carotid unstable plaque tissues and cultured mouse macrophage cell lines were assessed using immunofluorescence co-staining techniques. Finally, lentivirus-mediated KMO silencing was successfully transduced into the aortas of high-fat-fed ApoE-/- mice, with results indicating that KMO silencing attenuated plaque formation and promoted plaque stability in ApoE-/- mice.

CONCLUSIONS

The results suggest that KMO, a mitochondria-targeted gene associated with macrophage cells, holds promise as a valuable diagnostic biomarker for assessing the instability of atherosclerotic plaques.

Dysregulated tryptophan metabolism and AhR pathway contributed to CXCL10 upregulation in stable non-segmental vitiligo

BACKGROUND

Tryptophan metabolism dysregulation has been observed in vitiligo. However, drawing a mechanistic linkage between this metabolic disturbance and vitiligo pathogenesis remains challenging.

OBJECTIVE

Aim to reveal the characterization of tryptophan metabolism in vitiligo and investigate the role of tryptophan metabolites in vitiligo pathophysiology.

METHODS

LC-MS/MS, dual-luciferase reporter assay, ELISA, qRT-PCR, small interfering RNA, western blotting, and immunohistochemistry were employed.

RESULTS

Kynurenine pathway activation and KYAT enzyme-associated deviation to kynurenic acid (KYNA) in the plasma of stable non-segmental vitiligo were determined. Using a public microarray dataset, we next validated the activation of kynurenine pathway was related with inflammatory-related genes expression in skin of vitiligo patients. Furthermore, we found that KYNA induced CXCL10 upregulation in keratinocytes via AhR activation. Moreover, the total activity of AhR agonist was increased while the AhR concentration per se was decreased in the plasma of vitiligo patients. Finally, higher KYAT, CXCL10, CYP1A1 and lower AhR expression in vitiligo lesional skin were observed by immunohistochemistry staining.

CONCLUSION

This study depicts the metabolic and genetic characterizations of tryptophan metabolism in vitiligo and proposes that KYNA, a tryptophan-derived AhR ligand, can enhance CXCL10 expression in keratinocytes.

Kynurenic Acid/AhR Signaling at the Junction of Inflammation and Cardiovascular Diseases

Persistent systemic chronic inflammatory conditions are linked with many pathologies, including cardiovascular diseases (CVDs), a leading cause of death across the globe. Among various risk factors, one of the new possible contributors to CVDs is the metabolism of essential amino acid tryptophan. Proinflammatory signals promote tryptophan metabolism via the kynurenine (KYN) pathway (KP), thereby resulting in the biosynthesis of several immunomodulatory metabolites whose biological effects are associated with the development of symptoms and progression of various inflammatory diseases. Some participants in the KP are agonists of aryl hydrocarbon receptor (AhR), a central player in a signaling pathway that, along with a regulatory influence on the metabolism of environmental xenobiotics, performs a key immunomodulatory function by triggering various cellular mechanisms with the participation of endogenous ligands to alleviate inflammation. An AhR ligand with moderate affinity is the central metabolite of the KP: KYN; one of the subsequent metabolites of KYN-kynurenic acid (KYNA)-is a more potent ligand of AhR. Understanding the role of AhR pathway-related metabolites of the KP that regulate inflammatory factors in cells of the cardiovascular system is interesting and important for achieving effective treatment of CVDs. The purpose of this review was to summarize the results of studies about the participation of the KP metabolite-KYNA-and of the AhR signaling pathway in the regulation of inflammation in pathological conditions of the heart and blood vessels and about the possible interaction of KYNA with AhR signaling in some CVDs.

Crosstalk between efferocytic myeloid cells and T-cells and its relevance to atherosclerosis

The interplay between myeloid cells and T-lymphocytes is critical to the regulation of host defense and inflammation resolution. Dysregulation of this interaction can contribute to the development of chronic inflammatory diseases. Important among these diseases is atherosclerosis, which refers to focal lesions in the arterial intima driven by elevated apolipoprotein B-containing lipoproteins, notably low-density lipoprotein (LDL), and characterized by the formation of a plaque composed of inflammatory immune cells, a collection of dead cells and lipids called the necrotic core, and a fibrous cap. As the disease progresses, the necrotic core expands, and the fibrous cap becomes thin, which increases the risk of plaque rupture or erosion. Plaque rupture leads to a rapid thrombotic response that can give rise to heart attack, stroke, or sudden death. With marked lowering of circulating LDL, however, plaques become more stable and cardiac risk is lowered-a process known as atherosclerosis regression. A critical aspect of both atherosclerosis progression and regression is the crosstalk between innate (myeloid cells) and adaptive (T-lymphocytes) immune cells. Myeloid cells are specialized at clearing apoptotic cells by a process called efferocytosis, which is necessary for inflammation resolution. In advanced disease, efferocytosis is impaired, leading to secondary necrosis of apoptotic cells, inflammation, and, most importantly, defective tissue resolution. In regression, efferocytosis is reawakened aiding in inflammation resolution and plaque stabilization. Here, we will explore how efferocytosing myeloid cells could affect T-cell function and vice versa through antigen presentation, secreted factors, and cell-cell contacts and how this cellular crosstalk may contribute to the progression or regression of atherosclerosis.

Dysregulated cellular metabolism in atherosclerosis: mediators and therapeutic opportunities

Accumulating evidence over the past decades has revealed an intricate relationship between dysregulation of cellular metabolism and the progression of atherosclerotic cardiovascular disease. However, an integrated understanding of dysregulated cellular metabolism in atherosclerotic cardiovascular disease and its potential value as a therapeutic target is missing. In this Review, we (1) summarize recent advances concerning the role of metabolic dysregulation during atherosclerosis progression in lesional cells, including endothelial cells, vascular smooth muscle cells, macrophages and T cells; (2) explore the complexity of metabolic cross-talk between these lesional cells; (3) highlight emerging technologies that promise to illuminate unknown aspects of metabolism in atherosclerosis; and (4) suggest strategies for targeting these underexplored metabolic alterations to mitigate atherosclerosis progression and stabilize rupture-prone atheromas with a potential new generation of cardiovascular therapeutics.

A novel causative role of imbalanced kynurenine pathway in ulcerative colitis: Upregulation of KMO and KYNU promotes intestinal inflammation

The kynurenine pathway (KP) is the principal metabolic route for the essential amino acid tryptophan (TRP). Recent advances have highlighted a pivotal role for several KP metabolites in inflammatory diseases, including ulcerative colitis (UC). However, the alterations of KP enzymes and their functional impact in UC remain poorly defined. Here, we focused on kynurenine 3-monooxygenase (KMO) and kynureninase (KYNU), which serve as critical branching enzymes in the KP. We observed that dextran sodium sulfate (DSS)-induced colitis mice exhibited disturbed TRP metabolism along with KMO and KYNU upregulated. In patients with active UC, both the expression of KMO and KYNU were positively correlated with inflammatory factors TNF-α and IL-1β. Pharmacological blockade of KMO or genetic silencing of KYNU suppressed IL-1β-triggered proinflammatory cytokines expression in intestinal epithelial cells. Furthermore, blockage of KMO by selective inhibitor Ro 61-8048 alleviated the symptoms of DSS-induced colitis in mice, accompanied by an expanded NAD+ pool and redox balance restoration. The protective role of Ro 61-8048 may be partly due to its effect on KP regulation, particularly in enhancing kynurenic acid production. In summary, our study provides new evidence for the proinflammatory property of KMO and KYNU in intestinal inflammation, hinting at a promising therapeutic approach in UC through targeting these enzymes.

Tryptophan Metabolism in Obesity: The Indoleamine 2,3-Dioxygenase-1 Activity and Therapeutic Options

Obesity activates both innate and adaptive immune responses in adipose tissue. Adipose tissue macrophages are functional antigen-presenting cells that promote the proliferation of interferon-gamma (IFN-γ)-producing cluster of differentiation (CD)4+ T cells in adipose tissue of obese subjects. The increased formation of neopterin and degradation of tryptophan may result in decreased T-cell responsiveness and lead to immunodeficiency. The activity of inducible indoleamine 2,3-dioxygenase-1 (IDO1) plays a major role in pro-inflammatory, IFN-γ-dominated settings. The expression of several kynurenine pathway enzyme genes is significantly increased in obesity. IDO1 in obesity shifts tryptophan metabolism from serotonin and melatonin synthesis to the formation of kynurenines and increases the ratio of kynurenine to tryptophan as well as with neopterin production. Reduction in serotonin (5-hydroxytryptamine; 5-HT) production provokes satiety dysregulation that leads to increased caloric uptake and obesity. According to the monoamine-deficiency hypothesis, a deficiency of cerebral serotonin is involved in neuropsychiatric symptomatology of depression, mania, and psychosis. Indeed, bipolar disorder (BD) and related cognitive deficits are accompanied by a higher prevalence of overweight and obesity. Furthermore, the accumulation of amyloid-β in Alzheimer's disease brains has several toxic effects as well as IDO induction. Hence, abdominal obesity is associated with vascular endothelial dysfunction. kynurenines and their ratios are prognostic parameters in coronary artery disease. Increased kynurenine/tryptophan ratio correlates with increased intima-media thickness and represents advanced atherosclerosis. However, after bariatric surgery, weight reduction does not lead to the normalization of IDO1 activity and atherosclerosis. IDO1 is involved in the mechanisms of immune tolerance and in the concept of tumor immuno-editing process in cancer development. Serum IDO1 activity is still used as a parameter in cancer development and growth. IDO-producing tumors show a high total IDO immunostaining score, and thus, using IDO inhibitors, such as Epacadostat, Navoximod, and L isomer of 1-methyl-tryptophan, seems an important modality for cancer treatment. There is an inverse correlation between serum folate concentration and body mass index, thus folate deficiency leads to hyperhomocysteinemia-induced oxidative stress. Immune checkpoint blockade targeting cytotoxic T-lymphocyte-associated protein-4 synergizes with imatinib, which is an inhibitor of mitochondrial folate-mediated one-carbon (1C) metabolism. Antitumor effects of imatinib are enhanced by increasing T-cell effector function in the presence of IDO inhibition. Combining IDO targeting with chemotherapy, radiotherapy and/or immunotherapy, may be an effective tool against a wide range of malignancies. However, there are some controversial results regarding the efficacy of IDO1 inhibitors in cancer treatment.

Plasma Tryptophan-Kynurenine Pathway Metabolites and Risk for Progression to End-Stage Kidney Disease in Patients With Type 2 Diabetes

OBJECTIVE

We sought to study the associations between plasma metabolites in the tryptophan-kynurenine pathway and the risk of progression to end-stage kidney disease (ESKD) in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Plasma tryptophan, kynurenine, 3-hydroxykynurenine, kynurenic acid, and xanthurenic acid concentrations were measured in discovery (n = 1,915) and replication (n = 346) cohorts. External validation was performed in Chronic Renal Insufficiency Cohort (CRIC) participants with diabetes (n = 1,312). The primary outcome was a composite of incident ESKD (progression to estimated glomerular filtration rate [eGFR] <15 mL/min/1.73 m2, sustained dialysis, or renal death). The secondary outcome was annual eGFR decline.

RESULTS

In the discovery cohort, tryptophan was inversely associated with risk for ESKD, and kynurenine-to-tryptophan ratio (KTR) was positively associated with risk for ESKD after adjustment for clinical risk factors, including baseline eGFR and albuminuria (adjusted hazard ratios [HRs] 0.62 [95% CI 0.51, 0.75] and 1.48 [1.20, 1.84] per 1 SD). High levels of kynurenic acid and xanthurenic acid were associated with low risks of ESKD (0.74 [0.60, 0.91] and 0.74 [0.60, 0.91]). Consistently, high levels of tryptophan, kynurenic acid, and xanthurenic acid were independently associated with a slower eGFR decline, while a high KTR was predictive of a faster eGFR decline. Similar outcomes were obtained in the replication cohort. Furthermore, the inverse association between kynurenic acid and risk of ESKD was externally validated in CRIC participants with diabetes (adjusted HR 0.78 [0.65, 0.93]).

CONCLUSIONS

Accelerated catabolism of tryptophan in the kynurenine pathway may be involved in progressive loss of kidney function. However, shunting the kynurenine pathway toward the kynurenic acid branch may potentially slow renal progression.

Tryptophan metabolites and incident cardiovascular disease: The EPIC-Norfolk prospective population study

BACKGROUND AND AIMS

Cardiovascular disease (CVD) remains the largest cause of death globally due to various risk factors. One novel potential contributor to CVD might be the metabolism of the essential amino acid tryptophan (Trp), which through many pathways can produce immunomodulatory metabolites such as kynurenine, indole-3-propionate and serotonin. We aim to identify the metabolites with the strongest association with cardiovascular disease, utilizing a substantial and diverse cohort of individuals. In our pursuit of this aim, our primary focus is to validate and reinforce the findings from previous cross-sectional studies.

METHODS

We used the community-based EPIC-Norfolk cohort (46.3 % men, age 59.8 ± 9.0) with a median follow-up of 22.1 (17.6-23.3) years to study associations between the relative levels of Trp metabolites measured with untargeted metabolomics and incident development of CVD. Serum from n = 11,972 apparently healthy subjects was analysed, of which 6982 individuals had developed CVD at the end of follow-up. Cox proportional hazard models were used to study associations, adjusted for sex, age, conventional cardiovascular risk factors and CRP. All metabolites were Ln-normalised prior to analysis.

RESULTS

Higher levels of Trp were inversely associated with mortality (HR 0.73; CI 0.64-0.83) and fatal CVD (HR 0.76; CI 0.59-0.99). Higher levels of kynurenine (HR 1.33; CI 1.19-1.49) and the [Kynurenine]/[Tryptophan]-ratio (HR 1.24; CI 1.14-1.35) were associated with a higher incident development of CVD. Serotonin was not associated with overall CVD, but we did find associations for myocardial infarction and stroke. Adjustment for CRP did not yield any discernible differences in effect size.

CONCLUSIONS

Tryptophan levels were inversely correlated with CVD, while several of its major metabolites (especially kynurenine and serotonin) were positively correlated. These findings indicate that mechanistic studies are required to understand the role of Trp metabolism in CVD with the goal to identify new therapeutic targets.

Host-microbe tryptophan partitioning in cardiovascular diseases

The functional interdependencies between the molecular components of a biological process demand for a network medicine platform that integrates systems biology and network science, to explore the interactions among biological components in health and disease. Access to large-scale omics datasets (genomics, transcriptomics, proteomics, metabolomics, metagenomics, phenomics, etc.) has significantly advanced our opportunity along this direction. Studies utilizing these techniques have begun to provide us with a deeper understanding of how the interaction between the intestinal microbes and their host affects the cardiovascular system in health and disease. Within the framework of a multiomics network approach, we highlight here how tryptophan metabolism may orchestrate the host-microbes interaction in cardiovascular diseases and the implications for precision medicine and therapeutics, including nutritional interventions.

Elevated indoleamine 2,3-dioxygenase activity is associated with endothelial dysfunction in people living with HIV and ROS production in human aortic endothelial cells in vitro

The precise role of indoleamine 2,3-dioxygenase (IDO) in cardiovascular diseases (CVD) among people living with HIV (PLWH) is still under debate, despite recognized links. This study aimed to investigate the impact of elevated IDO activity on endothelial dysfunction in PLWH. A total of 38 PLWH, who had not previously received anti-retroviral therapy (ART), were enrolled in the study. These participants were monitored for 36 months following the initiation of ART. Measurements including plasma levels of IDO activity, markers of endothelial dysfunction, inflammatory factors, and lipids. In vitro, human aortic endothelial cells (HAEC) were exposed to interferon-γ, an IDO inhibitor, a kynurenine 3-hydroxylase (KMO) inhibitor, as well as different concentrations of kynurenine. Pre-ART, PLWH demonstrated notably elevated plasma concentrations of soluble intercellular adhesion molecule 1 (sICAM-1), soluble vascular cell adhesion molecule 1(sVCAM-1), and IDO activity in comparison to healthy controls. Post-ART, both IDO activity and sICAM-1 levels experienced a significant decrease, with IDO activity reaching levels comparable to those observed in healthy controls. Furthermore, a positive correlation was observed between IDO activity and sICAM-1 (p = 0.0002), as well as sVCAM-1 (p < 0.0001) before ART. In vitro, the augmentation of kynurenine concentration in the medium and the induction of IDO expression in HAEC resulted in increased production of reactive oxygen species (ROS), with minimal impact on endothelial dysfunction. From these findings, it can be concluded that long-term ART has the potential to restore the heightened IDO activity observed in PLWH. The overexpression of IDO primarily influences the expression of ROS in HAEC.

The pathophysiology and management of depression in cardiac surgery patients

Background

Depression is common in the cardiac surgery population. This contemporary narrative review aims to explore the main pathophysiological disturbances underpinning depression specifically within the cardiac surgery population. The common non-pharmacological and pharmacological management strategies used to manage depression within the cardiac surgery patient population are also explored.

Methods

A total of 1291 articles were identified through Ovid Medline and Embase. The findings from 39 studies were included for qualitative analysis in this narrative review.

Results

Depression is associated with several pathophysiological and behavioral factors which increase the likelihood of developing coronary heart disease which may ultimately require surgical intervention. The main pathophysiological factors contributing to depression are well characterized and include autonomic nervous system dysregulation, excessive inflammation and disruption of the hypothalamic-pituitary-adrenal axis. There are also several behavioral factors in depressed patients associated with the development of coronary heart disease including poor diet, insufficient exercise, poor compliance with medications and reduced adherence to cardiac rehabilitation. The common preventative and management modalities used for depression following cardiac surgery include preoperative and peri-operative education, cardiac rehabilitation, cognitive behavioral therapy, religion/prayer/spirituality, biobehavioral feedback, anti-depressant medications, and statins.

Conclusion

This contemporary review explores the pathophysiological mechanisms leading to depression following cardiac surgery and the current management modalities. Further studies on the preventative and management strategies for postoperative depression in the cardiac surgery patient population are warranted.

Aryl hydrocarbon receptor: A bridge linking immuno-inflammation and metabolism in atherosclerosis

Cardiovascular disease is the leading cause of death worldwide, and atherosclerosis is a major contributor to this etiology. The ligand-activated transcription factor, known as the aryl hydrocarbon receptor (AhR), plays an essential role in the interactions between genes and the environment. In a number of human diseases, including atherosclerosis, the AhR signaling pathway has recently been shown to be aberrantly expressed and activated. It's reported that AhR can regulate the immuno-inflammatory response and metabolism pathways in atherosclerosis, potentially serving as a bridge that links these processes. In this review, we highlight the involvement of AhR in atherosclerosis. From the literature, we conclude that AhR is a potential target for controlling atherosclerosis through precise interventions.

Kynurenine pathway dynamics in patients with schizophrenia spectrum disorders across the disease trajectory

The aim of this study was to evaluate how schizophrenia spectrum disorders (SSD) and applied long-term (5.1 years) antipsychotic (AP) treatment affect the serum levels of tryptophan (Trp) metabolites. A total of 112 adults (54 first-episode psychosis [FEP] patients and 58 control subjects [CSs]) participated in the study. The investigated changes in the metabolite levels appeared against a background of persistent increase in BMI and waist circumference among the patients. Regarding the kynurenine (KYN) pathway, the strongest changes were seen in AP-naïve FEP patients. Trp, KYN, kynurenic acid (KYNA), and anthranilic acid (ANT) levels were significantly reduced in blood samples from patients in the early stage of the disease. Furthermore, 3-OH-kynurenine (3-HK) and quinolinic acid (QUIN) levels were somewhat lower in these patients. Most of these changes in the KYN pathway became weaker with AP treatment. The levels of serotonin and its metabolite 5-HIAA tended to be higher at 5.1 years in patients showing the relation of elevated serotonin turnover to increased BMI and waist circumference. The similar trend was evident for the ratio between xanthurenic acid (XA) and KYNA with strong link to the elevated BMI. Altogether, the present study supports the role of Trp-metabolites in the development of obesity and metabolic syndrome in SSD patients.

Role of kinurenic acid in the systemic sclerosis renal involvement

Systemic sclerosis (SSc) subclinical renal vasculopathy is characterized by progressive increase of intrarenal stiffness and reduction of parenchymal thickness due to post ischemic fibrosis secondary to the renal Raynaud phenomenon. Aims of this study were to evaluate kinurenic acid (KYNA) serum level in SSc patients and healthy controls (HC) and to assess the role of KYNA in SSc subclinical nephropathy. Serum level of KYNA was evaluated in 52 SSc patients and 20 HC, matched for sex and age. Renal function was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation to estimate glomerular filtration rate (eGFR) and renal doppler ultrasound was performed to evaluate kidneys' morphology and indices of intrarenal stiffness. The parameters registered were renal longitudinal length, atrophy index (AI), renal sinus, parenchymal thickness, renal resistive index (RRI), pulsatile index (PI) and systolic/diastolic ratio (S/D). SSc patients had lower median value of KYNA than HC [54.43 ng/ml (IQR 44.44-63.64) vs 61.94 ng/ml (IQR 55.23-88.75), p < 0.001]. SSc patients with AI ≥ 0.70 had lower KYNA than SSc patients with AI < 0.70 [47.85 ng/ml (IQR 41.16-59.91) vs 55.5 ng/ml (IQR 49.99-67.33), p < 0.05] and a slightly significant negative linear correlation was found between KYNA and AI (r =  - 0.249, p < 0.05). SSc patient with RRI ≥ 0.70 had higher KYNA than SSc patients with RRI < 0.70 [58.25 ng/ml (IQR 50.49-69.68) vs 50.07 ng/ml (IQR 42.70-56.31), p < 0.05] and a significant positive correlation was found between KYNA and RRI (r = 0.318, p < 0.05). KYNA may be used as a marker to evaluate the renal involvement in SSc patients.

Serum kynurenic acid is lower in systemic sclerosis patients with microvascular damage of hands

BACKGROUND

Endothelial dysfunction occurs early in systemic sclerosis (SSc), leading to tissue hypoxia, vasoconstriction and fibrosis. It has been demonstrated that endothelial cells (ECs) are able to produce kynurenic acid (KYNA) in response to vascular inflammation, due to its anti-inflammatory and anti-oxidants activity. In SSc patients, blood perfusion of hands, assessed by laser speckle contrast analysis (LASCA), correlated negatively with the extent of the nailfold microvascular damage, scored according to nailfold videocapillaroscopy (NVC) classification. Aim of this study was to evaluate the difference of serum KYNA in SSc patients with different stages of microvascular damage.

METHODS

Serum KYNA was assessed in 40 SSc patients at the enrolment. NVC was performed to evaluate capillaroscopic patterns (early, active and late). LASCA was performed to evaluate mean peripheral blood perfusion (PBP) of both hands and to evaluate the proximal-distal gradient (PDG).

RESULTS

Median PDG was significantly lower in SSc patients with late NVC pattern compared to SSc patients with early and active NVC pattern [3.79 pU (IQR -8.55-18.16) vs 23.55 pU (IQR 14.92-43.80), p < 0.01]. Serum KYNA was significantly lower in SSc patients with late NVC pattern compared to SSc patient with early and active NVC pattern [45.19 ng/mL (IQR 42.70-54.74) vs 52.65 ng/mL (IQR 49.99-60.29), p < 0.05]. Moreover, SSc patients without PDG had significantly lower serum KYNA than in SSc patients with PDG [48.03 ng/mL (IQR 43.87-53.68) vs 59.27 ng/mL (IQR 49.15-71.00), p < 0.05].

CONCLUSION

KYNA is lower in SSc patients with late NCV pattern and without PDG. KYNA may be associated with early endothelial dysfunction.

A tryptophan metabolite prevents depletion of circulating endothelial progenitor cells in systemic low-grade inflammation

Background

Chronic systemic inflammation reduces the bioavailability of circulating endothelial progenitor cells (EPCs). Indoleamine 2,3-dioxygenase 1 (IDO1), a key enzyme of immune tolerance catalyzing the initial step of tryptophan degradation along the so-called l-kynurenine (l-kyn) pathway, that is induced by inflammatory stimuli and exerts anti-inflammatory effects. A specific relationship between IDO1 activity and circulating EPC numbers has not yet been investigated.

Methods

In this study, circulating EPCs were examined in mice treated with low doses of lipopolysaccharide (LPS) to mimic low-grade inflammation. Moreover, the association between IDO1 activity and circulating EPCs was studied in a cohort of 277 patients with variable systemic low-grade inflammation.

Results

Repeated low doses of LPS caused a decrease in circulating EPCs and l-kyn supplementation, mimicking IDO1 activation, significantly increased EPC numbers under homeostatic conditions preventing EPC decline in low-grade endotoxemia. Accordingly, in patients with variable systemic low-grade inflammation, there was a significant interaction between IDO1 activity and high-sensitivity C-reactive protein (hs-CRP) in predicting circulating EPCs, with high hs-CRP associated with significantly lower EPCs at low IDO1 activity but not at high IDO1 activity.

Interpretation

Overall, these findings demonstrate that systemic low-grade inflammation reduces circulating EPCs. However, high IDO1 activity and l-kyn supplementation limit circulating EPC loss in low-grade inflammation.

Dendritic cell-expressed IDO alleviates atherosclerosis by expanding CD4+CD25+Foxp3+Tregs through IDO-Kyn-AHR axis

Atherosclerosis is a chronic inflammatory disease, in which immune disorders constitute an essential part of vascular pathogenesis. Accumulating evidence indicates that dendritic cells (DCs) and their tryptophan metabolisms regulate host immune responses. However, the mechanistic involvement of metabolic products from DCs in dysregulating vascular immunity during the development of atherosclerosis is far from clear. Flow cytometry examination showed immune cells were accumulated and gradually increased in the atherosclerotic lesions during the atherosclerosis progression, in which IDO⁺DCs were enriched. To study the role of DC-expressed IDO in the development of atherosclerosis, we made a stable IDO-overexpressing DC line (IDO^oeDCs) by lentiviral infection for adoptive transfer into pro-atherosclerotic mice. Compared with DCs containing empty vector (Vector^CtrlDC)-treated group, treatment of IDO^oeDCs led to a significant reduction of atherosclerotic lesions in the aorta, with decreased aortic infiltration of Th1 immune cells and reduced vascular inflammation. Importantly, IDO^oeDCs increased aortic kynurenine (Kyn) concentration and aryl hydrocarbon receptor (AHR) expression, concomitant with CD4⁺CD25⁺Foxp3⁺Treg expansion in the aortic tissues, which were abrogated by AHR antagonist treatment. These results indicate that DC-expressed IDO reduces atherosclerotic lesions by inducing aortic CD4⁺CD25⁺Foxp3⁺Treg expansion through IDO-Kyn-AHR axis, which may represent a novel possibility for treatment or prevention of atherosclerosis.

Immunomodulatory properties of mesenchymal stromal/stem cells: The link with metabolism

BACKGROUND

Mesenchymal stromal/stem cells (MSCs) are the most promising stem cells for the treatment of multiple inflammatory and immune diseases due to their easy acquisition and potent immuno-regulatory capacities. These immune functions mainly depend on the MSC secretion of soluble factors. Recent studies have shown that the metabolism of MSCs plays critical roles in immunomodulation, which not only provides energy and building blocks for macromolecule synthesis but is also involved in the signaling pathway regulation.

AIM OF REVIEW

A thorough understanding of metabolic regulation in MSC immunomodulatory properties can provide new sights to the enhancement of MSC-based therapy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

MSC immune regulation can be affected by cellular metabolism (glucose, adenosine triphosphate, lipid and amino acid metabolism), which further mediates MSC therapy efficiency in inflammatory and immune diseases. The enhancement of glycolysis of MSCs, such as signaling molecule activation, inflammatory cytokines priming, or environmental control can promote MSC immune functions and therapeutic potential. Besides glucose metabolism, inflammatory stimuli also alter the lipid molecular profile of MSCs, but the direct link with immunomodulatory properties remains to be further explored. Arginine metabolism, glutamine-glutamate metabolism and tryptophan-kynurenine via indoleamine 2,3-dioxygenase (IDO) metabolism all contribute to the immune regulation of MSCs. In addition to the metabolism dictating the MSC immune functions, MSCs also influence the metabolism of immune cells and thus determine their behaviors. However, more direct evidence of the metabolism in MSC immune abilities as well as the underlying mechanism requires to be uncovered.

A novel interleukin-2-based fusion molecule, HCW9302, differentially promotes regulatory T cell expansion to treat atherosclerosis in mice

Atherosclerosis is a chronic inflammatory disease caused by deposition of oxidative low-density lipoprotein (LDL) in the arterial intima which triggers the innate immune response through myeloid cells such as macrophages. Regulatory T cells (Tregs) play an important role in controlling the progression or regression of atherosclerosis by resolving macrophage-mediated inflammatory functions. Interleukin-2 (IL-2) signaling is essential for homeostasis of Tregs. Since recombinant IL-2 has an unfavorable pharmacokinetic profile limiting its therapeutic use, we constructed a fusion protein, designated HCW9302, containing two IL-2 domains linked by an extracellular tissue factor domain. We found that HCW9302 exhibited a longer serum half-life with an approximately 1000-fold higher affinity for the IL-2Rα than IL-2. HCW9302 could be administered to mice at a dosing range that expanded and activated Tregs but not CD4⁺ effector T cells. In an ApoE^-/- mouse model, HCW9302 treatment curtailed the progression of atherosclerosis through Treg activation and expansion, M2 macrophage polarization and myeloid-derived suppressor cell induction. HCW9302 treatment also lessened inflammatory responses in the aorta. Thus, HCW9302 is a potential therapeutic agent to expand and activate Tregs for treatment of inflammatory and autoimmune diseases.

Tryptophan catabolites and depression in the general population: results from the Gutenberg Health Study

Previous studies reported significantly altered tryptophan catabolite concentrations in major depression. Thus, tryptophan catabolites were considered as potential biomarkers of depression and their modulators as potential targets for psychopharmacotherapy. However, the results were based mainly on studies with small sample sizes limiting their generalizability. Against this background, we investigated the relationship of peripheral tryptophan catabolites with depression in a population-based sample with n = 3,389 participants (with fasting status ≥ 8 h and C-reactive protein < 10 mg/L). N = 248 had clinically significant depression according to a PHQ-9 score of ≥ 10, n = 1,101 subjects had mild depressive symptoms with PHQ-9 scores between 5 and 9, and n = 2,040 had no depression. After multivariable adjustment, clinically significant depression was associated with lower kynurenine and kynurenic acid. Spearman correlation coefficients of the tryptophan catabolites with the severity of depression were very small (rho ≤ 0.080, p ≤ 0.015). None of the tryptophan catabolites could diagnostically separate depressed from not depressed persons. Concerning linear associations, kynurenine and kynurenic acid were associated only with the severity and the cognitive dimension of depression but not its somatic dimension. Tryptophan catabolites were not associated with persistence or recurrence of depression at the 5 year follow-up. The results replicated the association between kynurenine and kynurenic acid with depression. However, the associations were small raising doubts about their clinical utility. Findings underline the complexity of the relationships between depression and tryptophan catabolites. The search for subgroups of depression with a potentially higher impact of depression might be warranted.

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.

Genetic Deficiency of Indoleamine 2,3-dioxygenase Aggravates Vascular but Not Liver Disease in a Nonalcoholic Steatohepatitis and Atherosclerosis Comorbidity Model

Nonalcoholic steatohepatitis (NASH) is a chronic liver disease that increases cardiovascular disease risk. Indoleamine 2,3-dioxygenase-1 (IDO1)-mediated tryptophan (Trp) metabolism has been proposed to play an immunomodulatory role in several diseases. The potential of IDO1 to be a link between NASH and cardiovascular disease has never been investigated. Using Apoe^−/− and Apoe^−/−Ido1^−/− mice that were fed a high-fat, high-cholesterol diet (HFCD) to simultaneously induce NASH and atherosclerosis, we found that Ido1 deficiency significantly accelerated atherosclerosis after 7 weeks. Surprisingly, Apoe^−/−Ido1^−/− mice did not present a more aggressive NASH phenotype, including hepatic lipid deposition, release of liver enzymes, and histopathological parameters. As expected, a lower L-kynurenine/Trp (Kyn/Trp) ratio was found in the plasma and arteries of Apoe^−/−Ido1^−/− mice compared to controls. However, no difference in the hepatic Kyn/Trp ratio was found between the groups. Hepatic transcript analyses revealed that HFCD induced a temporal increase in tryptophan 2,3-dioxygenase (Tdo2) mRNA, indicating an alternative manner to maintain Trp degradation during NASH development in both Apoe^−/− and Apoe^−/−Ido1^−/mice⁻. Using HepG2 hepatoma cell and THP1 macrophage cultures, we found that iron, TDO2, and Trp degradation may act as important mediators of cross-communication between hepatocytes and macrophages regulating liver inflammation. In conclusion, we show that Ido1 deficiency aggravates atherosclerosis, but not liver disease, in a newly established NASH and atherosclerosis comorbidity model. Our data indicate that the overexpression of TDO2 is an important mechanism that helps in balancing the kynurenine pathway and inflammation in the liver, but not in the artery wall, which likely determined disease outcome in these two target tissues.

NAD+ Levels Are Augmented in Aortic Tissue of ApoE-/- Mice by Dietary Omega-3 Fatty Acids

BACKGROUND

Maintaining bioenergetic homeostasis provides a means to reduce the risk of cardiovascular events during chronological aging. Nicotinamide adenine dinucleotide (NAD⁺) acts as a signaling molecule, and its levels were used to govern several biological pathways, for example, promoting angiogenesis by SIRT1 (sirtuin 1)-mediated inhibition of Notch signaling to rejuvenate capillary density of old-aged mice. NAD⁺ modulation shows promise in the vascular remodeling of endothelial cells. However, NAD⁺ distribution in atherosclerotic regions remains uncharacterized. Omega-3 polyunsaturated fatty acids consumption, such as docosahexaenoic acid and eicosapentaenoic acid, might increase the abundance of cofactors in blood vessels due to omega-3 polyunsaturated fatty acids metabolism.

METHODS

Apolipoprotein E-deficient (ApoE^-/-) mice were fed a Western diet, and the omega-3 polyunsaturated fatty acids-treated groups were supplemented with docosahexaenoic acid (1%, w/w) or eicosapentaenoic acid (1%, w/w) for 3 weeks. Desorption electrospray ionization mass spectrometry imaging was exploited to detect exogenous and endogenous NAD⁺ imaging.

RESULTS

NAD⁺, NADH, NADP⁺, NADPH, FAD⁺, FADH, and nicotinic acid adenine dinucleotide of the aortic arches were detected higher in the omega-3 polyunsaturated fatty acids-treated mice than the nontreated control. Comparing the distribution in the outer and inner layers of the arterial walls, only NADPH was detected slightly higher in the outer part in eicosapentaenoic acid-treated mice.

CONCLUSIONS

Supplementation of adding docosahexaenoic acid or eicosapentaenoic acid to the Western diet led to a higher NAD⁺, FAD⁺, and their metabolites in the aortic arch. Considering the pleiotropic roles of NAD⁺ in biology, this result serves as a beneficial therapeutic strategy in the animal model counter to pathological conditions.

Metabolism in atherosclerotic plaques: immunoregulatory mechanisms in the arterial wall

Over the last decade, there has been a growing interest to understand the link between metabolism and the immune response in the context of metabolic diseases but also beyond, giving then birth to a new field of research. Termed 'immunometabolism', this interdisciplinary field explores paradigms of both immunology and metabolism to provided unique insights into different disease pathogenic processes, and the identification of new potential therapeutic targets. Similar to other inflammatory conditions, the atherosclerotic inflammatory process in the artery has been associated with a local dysregulated metabolic response. Thus, recent studies show that metabolites are more than just fuels in their metabolic pathways, and they can act as modulators of vascular inflammation and atherosclerosis. In this review article, we describe the most common immunometabolic pathways characterised in innate and adaptive immune cells, and discuss how macrophages' and T cells' metabolism may influence phenotypic changes in the plaque. Moreover, we discuss the potential of targeting immunometabolism to prevent and treat cardiovascular diseases (CVDs).

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

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

Kynurenine Pathway Metabolites as Potential Clinical Biomarkers in Coronary Artery Disease

Coronary artery disease (CAD) is one of the leading cause of mortality worldwide. Several risk factors including unhealthy lifestyle, genetic background, obesity, diabetes, hypercholesterolemia, hypertension, smoking, age, etc. contribute to the development of coronary atherosclerosis and subsequent coronary artery disease. Inflammation plays an important role in coronary artery disease development and progression. Pro-inflammatory signals promote the degradation of tryptophan via the kynurenine pathway resulting in the formation of several immunomodulatory metabolites. An unbalanced kynurenic pathway has been implicated in the pathomechanisms of various diseases including CAD. Significant improvements in detection methods in the last decades may allow simultaneous measurement of multiple metabolites of the kynurenine pathway and such a thorough analysis of the kynurenine pathway may be a valuable tool for risk stratification and determination of CAD prognosis. Nevertheless, imbalance in the activities of different branches of the kynurenine pathway may require careful interpretation. In this review, we aim to summarize clinical evidence supporting a possible use of kynurenine pathway metabolites as clinical biomarkers in various manifestations of CAD.

Intestinal Dysbiosis, the Tryptophan Pathway and Nonalcoholic Steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) progresses from simple steatosis to steatohepatitis (NASH), which may then progress to the development of cirrhosis and hepatocarcinoma. NASH is characterized by both steatosis and inflammation. Control of inflammation in NASH is a key step for the prevention of disease progression to severe sequalae. Intestinal dysbiosis has been recognized to be an important causal factor in the pathogenesis of NASH, involving both the accumulation of lipids and aggravation of inflammation. The effects of gut dysbiosis are mediated by adverse shifts of various intestinal commensal bacterial genera and their associated metabolites such as butyrate, tryptophan, and bile acids. In this review, we focus on the roles of tryptophan and its metabolites in NASH in association with intestinal dysbiosis and discuss possible therapeutic implications.

Abnormal Activation of Tryptophan-Kynurenine Pathway in Women With Polycystic Ovary Syndrome

BACKGROUND

Women with polycystic ovary syndrome (PCOS) suffer from dysfunctional metabolism and studies have reported increased levels of tryptophan in patients with PCOS. However, the changes of downstream metabolites in tryptophan catabolism pathways remain unclear.

METHODS

This is a cross-sectional study that included 200 PCOS patients and 200 control women who were recruited from the Reproductive Medicine Center of Peking University Third Hospital from October 2017 to June 2019. The PCOS patients and the control group were further divided into subtypes of normal weight and overweight/obesity. Fasting blood samples from all subjects were collected on days 2~3 of a natural menstrual cycle or when amenorrhea for over 40 days with follicle diameter not exceeding 10 mm. The plasma levels of tryptophan metabolites were quantitatively determined by the liquid chromatograph mass spectrometer, including tryptophan, serotonin, kynurenine, kynurenic acid, 3-hydroxykynurenine, and quinolinic acid.

RESULTS

The tryptophan-kynurenine pathway was dysregulated in women with PCOS, along with significantly elevated levels of tryptophan, serotonin, kynurenine, kynurenic acid, and quinolinic acid. Moreover, levels of tryptophan, kynurenine, and kynurenic acid were positively correlated with luteinizing hormone, anti-Müllerian hormone, fasting insulin, HOMA-IR. tryptophan, and kynurenine and quinolinic acid had an obvious association with C-reactive protein levels. Furthermore, logistic regression showed that tryptophan, serotonin, kynurenine, kynurenic acid and quinolinic acid were all associated significantly with the increased risk of PCOS with the adjustment for potential confounding factors. Additionally, tryptophan, kynurenine, and kynurenic acid had good diagnostic performances for PCOS, and their combination exhibited higher sensitivity and specificity to diagnostic efficiency, with the area under the ROC curve of 0.824 (95% CI 0.777-0.871), which was comparable to the endocrine indicators.

CONCLUSION S

The tryptophan-kynurenine pathway was abnormally activated in PCOS patients.

Neopterin and kynurenic acid as predictors of stroke recurrence and mortality: a multicentre prospective cohort study on biomarkers of inflammation measured three months after ischemic stroke

BACKGROUND

Chronic low-grade inflammation is associated with both ischemic stroke and sedentary behaviour. The aim of this study was to investigate the predictive abilities of biomarkers of inflammation and immune modulation associated with sedentary behaviour for ischemic stroke recurrence and mortality in a stroke population.

METHODS

Patients admitted to hospital for acute stroke were recruited to the prospective multicentre cohort study, the Norwegian Cognitive Impairment After Stroke (Nor-COAST) study, from May 2015 until March 2017. Patients with ischemic stroke, blood samples available from the three-month follow-up, and no stroke recurrence before the three-month follow-up were included. Serum was analysed for C-reactive protein (CRP) with high-sensitive technique, and plasma for interleukin-6 (IL-6), neopterin, pyridoxic acid ratio index (PAr-index: 4-pyridoxic acid: [pyrioxal+pyridoxal-5'-phosphate]) and kynurenic acid (KA). Ischemic stroke recurrence and death were identified by the Norwegian Stroke Registry and the Cause of Death Registry until 31 December 2018.

RESULTS

The study included 354 patients, 57% male, mean age 73 (SD 11) years, mean observation time 2.5 (SD 0.6) years, and median National Institute of Health Stroke Scale of 0 (IQR 1) at three months. CRP was associated with mortality (HR 1.40, CI 1.01, 1.96, p = 0.046), and neopterin was associated with the combined endpoint (recurrent ischemic stroke or death) (HR 1.52, CI 1.06, 2.20, p = 0.023), adjusted for age, sex, prior cerebrovascular disease, modified Rankin Scale, and creatinine. When adding neopterin and KA to the same model, KA was negatively associated (HR 0.57, CI 0.33, 0.97, p = 0.038), and neopterin was positively associated (HR 1.61, CI 1.02, 2.54, p = 0.040) with mortality. Patients with cardioembolic stroke at baseline had higher levels of inflammation at three months.

CONCLUSION

Neopterin might be a valuable prognostic biomarker in stroke patients. The use of KA as a measure of anti-inflammatory capacity should be investigated further.

TRIAL REGISTRATION

The study was registered at Clinicaltrials.gov ( NCT02650531 ). First posted on 08/01/2016.

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.

Investigating novel biomarkers of immune activation and modulation in the context of sedentary behaviour: a multicentre prospective ischemic stroke cohort study

Background

Sedentary behaviour is associated with disease, but the molecular mechanisms are not understood. Valid biomarkers with predictive and explanatory properties are required. Therefore, we have investigated traditional and novel biomarkers of inflammation and immune modulation and their association to objectively measured sedentary behaviour in an ischemic stroke population.

Methods

Patients admitted to hospital with acute ischemic stroke were included in the multicentre Norwegian Cognitive Impairment After Stroke (Nor-COAST) study (n = 815). For this sub-study (n = 257), sedentary behaviour was registered 3 months after stroke using position transition data from the body-worn sensor, ActivPal®. Blood samples were analysed for high sensitive C-reactive protein (hsCRP), the cytokines interleukin-6 (IL-6) and 10 (IL-10), neopterin, tryptophan (Trp), kynurenine (kyn), kynurenic acid (KA), and three B6 vitamers, pyridoxal 5′-phosphate (PLP), pyridoxal (PL), and pyridoxic acid (PA). The kynurenine/tryptophan ratio (KTR) and the pyridoxic acid ratio index (PAr = PA: PL + PLP) were calculated.

Results

Of the 815 patients included in the main study, 700 attended the three-month follow-up, and 257 fulfilled the inclusion criteria for this study. Sedentary time was significantly associated with levels of hsCRP, IL-6, neopterin, PAr-index, and KA adjusted for age, sex, waist circumference, and creatinine. In a fully adjusted model including all the significant biomarkers except hsCRP (because of missing values), sedentary time was independently positively associated with the PAr-index and negatively with KA. We did not find an association between sedentary behaviour, IL-10, and KTR.

Conclusions

The PAr-index is known to capture several modes of inflammation and has previously shown predictive abilities for future stroke. This novel result indicates that the PAr-index could be a useful biomarker in future studies on sedentary behaviour and disease progression. KA is an important modulator of inflammation, and this finding opens new and exciting pathways to understand the hazards of sedentary behaviour.

Trial registration

The study was registered at Clinicaltrials.gov (NCT02650531). First posted 08/01/2016.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02343-0.

Disruption of GPR35 Signaling in Bone Marrow-Derived Cells Does Not Influence Vascular Inflammation and Atherosclerosis in Hyperlipidemic Mice

G-protein-coupled receptor-35 (GPR35) has been identified as a receptor for the tryptophan metabolite kynurenic acid (KynA) and suggested to modulate macrophage polarization in metabolic tissues. Whether GPR35 can influence vascular inflammation and atherosclerosis has however never been tested. Lethally irradiated LdlrKO mice were randomized to receive GPR35KO or wild type (WT) bone marrow transplants and fed a high cholesterol diet for eight weeks to develop atherosclerosis. GPR35KO and WT chimeric mice presented no difference in the size of atherosclerotic lesions in the aortic arch (2.37 ± 0.58% vs. 1.95 ± 0.46%, respectively) or in the aortic roots (14.77 ± 3.33% vs. 11.57 ± 2.49%, respectively). In line with these data, no changes in the percentage of VCAM-1+, IA^b + cells, and CD3+ T cells, as well as alpha smooth muscle cell actin expression, was observed between groups. Interestingly, the GPR35KO group presented a small but significant increase in CD68+ macrophage infiltration in the plaque. However, in vitro culture experiments using bone marrow-derived macrophages from both groups indicated that GPR35 plays no role in modulating the secretion of major inflammatory cytokines. Our study indicates that GPR35 expression does not play a direct role in macrophage activation, vascular inflammation, and the development of atherosclerosis.

The Kynurenine Pathway Is Upregulated by Methyl-deficient Diet and Changes Are Averted by Probiotics

SCOPE

Probiotics exert immunomodulatory effects and may influence tryptophan metabolism in the host. Deficiency of nutrients related to C1 metabolism might stimulate inflammation by enhancing the kynurenine pathway. This study used Sprague Dawley rats to investigate whether a methyl-deficient diet (MDD) may influence tryptophan/kynurenine pathways and cytokines and whether probiotics can mitigate these effects.

METHODS AND RESULTS

Rats are fed a control or MDD diet. Animals on the MDD diet received vehicle, probiotics (L. helveticus R0052 and B. longum R0175), choline, or probiotics + choline for 10 weeks (n = 10 per group). Concentrations of plasma kynurenine metabolites and the methylation and inflammatory markers in plasma and liver are measured.

RESULTS

MDD animals (vs controls) show upregulation of plasma kynurenine, kynurenic acid, xanthurenic acid, 3-hydroxyxanthranilic acid, quinolinic acid, nicotinic acid, and nicotinamide (all p < 0.05). In the MDD rats, the probiotics (vs vehicle) cause lower anthranilic acid and a trend towards lower kynurenic acid and picolinic acid. Compared to probiotics alone, probiotics + choline is associated with a reduced enrichment of the bacterial strains in cecum. The interventions have no effect on inflammatory markers.

CONCLUSIONS

Probiotics counterbalance the effect of MDD diet and downregulate downstream metabolites of the kynurenine pathway.