Kynurenic acid in synovial fluid and serum of patients with rheumatoid arthritis, spondyloarthropathy, and osteoarthritis

The role of amino acid metabolism in inflammatory bowel disease and other inflammatory diseases

Inflammation is a characteristic symptom of the occurrence and development of many diseases, which is mainly characterized by the infiltration of inflammatory cells such as macrophages and granulocytes, and the increased release of proinflammatory factors. Subsequently, macrophage differentiates and T cells and other regulated factors exhibit anti-inflammatory function, releasing pro- and anti-inflammatory factors to maintain homeostasis. Although reports define various degrees of metabolic disorders in both the inflamed and non-inflamed parts of inflammatory diseases, little is known about the changes in amino acid metabolism in such conditions. This review aims to summarize amino acid changes and mechanisms involved in the progression of inflammatory bowel disease (IBD) and other inflammatory diseases. Since mesenchymal stem cells (MSCs) and their derived exosomes (MSC-EXO) have been found to show promising effects in the treatment of IBD and other inflammatory diseases,their potential in the modulation of amino acid metabolism in the treatment of inflammation is also discussed.

Synovial Fluid Metabolome Can Differentiate between Healthy Joints and Joints Affected by Osteoarthritis in Horses

Osteoarthritis (OA) is a common cause of lameness in sport horses with a significant economic impact. The prevention of OA is crucial since no effective treatment is available. This study aimed to apply untargeted metabolomic analysis to investigate the differences in synovial fluid (SF) composition between healthy and OA-affected joints in horses. SF collected from healthy (n.8) and OA (n.11) horses was analyzed using H-NMR analysis. Metabolomic analysis allowed 55 different metabolites to be identified and quantified in SF samples. Nineteen metabolites were found to be differently concentrated in OA compared to control horses. Synovial fluids from the OC group were found to be higher in 1,3-dihydroxyacetone but lower in tryptophan, phenylalanine, tyrosine, uridine, creatinine, creatine, glycine, choline, asparagine, glutamine, arginine, 3-hydroxybutyrate, valine, 2-hydroxyisovalerate, α-ketoisovaleric acid, 3-methyl-2-oxovalerate, 3-hydroxyisobutyrate, isoleucine, and methionine compared to the controls. A variety of SF metabolites significantly changed following joint disease, demonstrating the complex mechanism underlying osteoarthritis in horses and highlighting the value of applying the metabolomic approach in clinical research.

Kynurenic acid promotes osteogenesis via the Wnt/β-catenin signaling

The role of kynurenic acid (KynA) in neurological and mental diseases has been widely studied. Emerging studies disclosed that KynA has a protective effect on tissues including heart, kidney, and retina. However, the role of KynA in osteoporosis has not been reported so far. To elucidate the role of KynA in age-related osteoporosis, both control and osteoporosis mice were administrated KynA for three consecutive months, and micro-computed tomography (μCT) analysis was then performed. In addition, primary bone marrow mesenchymal stem cells (BMSCs) were isolated for osteogenic differentiation induction and treated with KynA in vitro. Our data suggested that KynA administration rescued age-related bone loss in vivo, and KynA treatment promotes BMSC osteogenic differentiation in vitro. Moreover, KynA activated the Wnt/β-catenin signaling during BMSC osteogenic differentiation. Wnt inhibitor MSAB inhibited KynA-induced osteogenic differentiation. Further data demonstrated that KynA exerted its effect on BMSC osteogenic differentiation and Wnt/β-catenin signaling activation via G protein-coupled receptor 35 (GPR35). In conclusion, the protective effect of KynA on age-related osteoporosis was disclosed. Additionally, the promoting effect of KynA on osteoblastic differentiation via Wnt/β-catenin signaling was verified and the effect dependent on GPR35. These data suggest that KynA administration potentially contributes to the treatment of age-related osteoporosis.

A Review of the Health Benefits of Food Enriched with Kynurenic Acid

Kynurenic acid (KYNA), a metabolite of tryptophan, is an endogenous substance produced intracellularly by various human cells. In addition, KYNA can be synthesized by the gut microbiome and delivered in food. However, its content in food is very low and the total alimentary supply with food accounts for only 1-3% of daily KYNA excretion. The only known exception is chestnut honey, which has a higher KYNA content than other foods by at least two orders of magnitude. KYNA is readily absorbed from the gastrointestinal tract; it is not metabolized and is excreted mainly in urine. It possesses well-defined molecular targets, which allows the study and elucidation of KYNA's role in various pathological conditions. Following a period of fascination with KYNA's importance for the central nervous system, research into its role in the peripheral system has been expanding rapidly in recent years, bringing some exciting discoveries. KYNA does not penetrate from the peripheral circulation into the brain; hence, the following review summarizes knowledge on the peripheral consequences of KYNA administration, presents data on KYNA content in food products, in the context of its daily supply in diets, and systematizes the available pharmacokinetic data. Finally, it provides an analysis of the rationale behind enriching foods with KYNA for health-promoting effects.

Indoleamine 2, 3 Dioxygenase 1 Impairs Chondrogenic Differentiation of Mesenchymal Stem Cells in the Joint of Osteoarthritis Mice Model

Osteoarthritis (OA) is a serious joint inflammation that leads to cartilage degeneration and joint dysfunction. Mesenchymal stem cells (MSCs) are used as a cell-based therapy that showed promising results in promoting cartilage repair. However, recent studies and clinical trials explored unsatisfied outcomes because of slow chondrogenic differentiation and increased calcification without clear reasons. Here, we report that the overexpression of indoleamine 2,3 dioxygenase 1 (IDO1) in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs in the joint of the OA mice model. The effect of MSCs mixed with IDO1 inhibitor on the cartilage regeneration was tested compared to MSCs mixed with IDO1 in the OA animal model. Further, the mechanism exploring the effect of IDO1 on chondrogenic differentiation was investigated. Subsequently, miRNA transcriptome sequencing was performed for MSCs cocultured with IDO1, and then TargetScan was used to verify the target of miR-122-5p in the SF-MSCs. Interestingly, we found that MSCs mixed with IDO1 inhibitor showed a significant performance to promote cartilage regeneration in the OA animal model, while MSCs mixed with IDO1 failed to stimulate cartilage regeneration. Importantly, the overexpression of IDO1 showed significant inhibition to Sox9 and Collagen type II (COL2A1) through activating the expression of β-catenin, since inhibiting of IDO1 significantly promoted chondrogenic signaling of MSCs (Sox9, COL2A1, Aggrecan). Further, miRNA transcriptome sequencing of SF-MSCs that treated with IDO1 showed significant downregulation of miR-122-5p which perfectly targets Wnt1. The expression of Wnt1 was noticed high when IDO1 was overexpressed. In summary, our results suggest that IDO1 overexpression in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs and cartilage regeneration through downregulation of miR-122-5p that activates the Wnt1/β-catenin pathway.

The Kynurenine Pathway as a Potential Target for Neuropathic Pain Therapy Design: From Basic Research to Clinical Perspectives

Accumulating evidence suggests the key role of the kynurenine pathway (KP) of the tryptophan metabolism in the pathogenesis of several diseases. Despite extensive research aimed at clarifying the mechanisms underlying the development and maintenance of neuropathic pain, the roles of KP metabolites in this process are still not fully known. Although the function of the peripheral KP has been known for several years, it has only recently been acknowledged that its metabolites within the central nervous system have remarkable consequences related to physiology and behavior. Both the products and metabolites of the KP are involved in the pathogenesis of pain conditions. Apart from the neuroactive properties of kynurenines, the KP regulates several neurotransmitter systems in direct or indirect ways. Some neuroactive metabolites are known to have neuroprotective properties (kynurenic acid, nicotinamide adenine dinucleotide cofactor), while others are toxic (3-hydroxykynurenine, quinolinic acid). Numerous animal models show that modulation of the KP may turn out to be a viable target for the treatment of diseases. Importantly, some compounds that affect KP enzymes are currently described to possess analgesic properties. Additionally, kynurenine metabolites may be useful for assessing response to therapy or as biomarkers in therapeutic monitoring. The following review describes the molecular site of action and changes in the levels of metabolites of the kynurenine pathway in the pathogenesis of various conditions, with a particular emphasis on their involvement in neuropathy. Moreover, the potential clinical implications of KP modulation in chronic pain therapy as well as the directions of new research initiatives are discussed.

Kynurenic Acid Analog Attenuates the Production of Tumor Necrosis Factor-α, Calgranulins (S100A 8/9 and S100A 12), and the Secretion of HNP1-3 and Stimulates the Production of Tumor Necrosis Factor-Stimulated Gene-6 in Whole Blood Cultures of Patients With Rheumatoid Arthritis

Objectives: Rheumatoid arthritis (RA) is a chronic, inflammatory joint disease with complex pathogenesis involving a variety of immunological events. Recently, it has been suggested that kynurenic acid (KYNA) might be a potential regulator of inflammatory processes in arthritis. KYNA has a definitive anti-inflammatory and immunosuppressive function. The aim of the present study is to investigate the complex effects of a newly synthesized KYNA analog—SZR72 on the in vitro production of tumor necrosis factor-α (TNF-α), tumor necrosis factor-stimulated gene-6 (TSG-6), calprotectin (SA1008/9), SA100 12 (EN-RAGE), and HNP1–3 (defensin-α) in the peripheral blood of patients with RA and the various effects of the disease.

Methods: Patients with RA (n = 93) were selected based on the DAS28 score, medication, and their rheumatoid factor (RF) status, respectively. Peripheral blood samples from 93 patients with RA and 50 controls were obtained, and activated by heat-inactivated S. aureus. Parallel samples were pretreated before the activation with the KYNA analog N-(2-N, N-dimethylaminoethyl)-4-oxo-1H-quinoline-2-carboxamide hydrochloride. Following the incubation period (18 h), the supernatants were tested for TNF-α, TSG-6, calprotectin, S100A12, and HNP1–3 content by ELISA.

Results: SZR72 inhibited the production of the following inflammatory mediators: TNF-α, calprotectin, S100A12, and HNP1–3 in whole blood cultures. This effect was observed in each group of patients in various phases of the disease. The basic (control) levels of these mediators were higher in the blood of patients than in healthy donors. In contrast, lower TSG-6 levels were detected in patients with RA compared to healthy controls. In addition, the KYNA analog exerted a stimulatory effect on the TSG-6 production ex vivo in human whole blood cultures of patients with RA in various phases of the disease.

Conclusion: These data further support the immunomodulatory role of KYNA in RA resulting in anti-inflammatory effects and draw the attention to the importance of the synthesis of the KYNA analog, which might have a future therapeutic potential.

Kynurenic Acid: The Janus-Faced Role of an Immunomodulatory Tryptophan Metabolite and Its Link to Pathological Conditions

Tryptophan metabolites are known to participate in the regulation of many cells of the immune system and are involved in various immune-mediated diseases and disorders. Kynurenic acid (KYNA) is a product of one branch of the kynurenine pathway of tryptophan metabolism. The influence of KYNA on important neurophysiological and neuropathological processes has been comprehensively documented. In recent years, the link of KYNA to the immune system, inflammation, and cancer has become more apparent. Given this connection, the anti-inflammatory and immunosuppressive functions of KYNA are of particular interest. These characteristics might allow KYNA to act as a "double-edged sword." The metabolite contributes to both the resolution of inflammation and the establishment of an immunosuppressive environment, which, for instance, allows for tumor immune escape. Our review provides a comprehensive update of the significant biological functions of KYNA and focuses on its immunomodulatory properties by signaling via G-protein-coupled receptor 35 (GPR35)- and aryl hydrocarbon receptor-mediated pathways. Furthermore, we discuss the role of KYNA-GPR35 interaction and microbiota associated KYNA metabolism for gut homeostasis.

Quinaldic acid in synovial fluid of patients with rheumatoid arthritis and osteoarthritis and its effect on synoviocytes in vitro

BACKGROUND

Previously, we have demonstrated that kynurenic acid (KYNA), an endogenous metabolite of tryptophan formed along kynurenine pathway, is present in synovial fluid of rheumatoid arthritis (RA) and osteoarthritis (OA) patients. In this study, the goal was to investigate the presence of quinaldic acid (QUDA), a putative metabolite of KYNA, in synovial fluid of RA and OA patients.

METHODS

The effect of QUDA on proliferation and motility of synovial fibroblasts and its interaction with KYNA were determined in vitro. The study was conducted on synovial fluid obtained from 38 patients with RA and 15 patients with OA. QUDA was identified and quantified using the gas chromatography-mass spectrometry (GC-MS) method. In vitro experiments were conducted on rabbit synoviocyte cell line HIG-82.

RESULTS

Presence of QUDA was detected in all 53 samples of synovial fluid. The concentration of QUDA in synovial fluid obtained from patients with RA was 28.6 ± 14.9 pmol/ml, which was lower in comparison with OA 42.3 ± 10.0 pmol/ml. QUDA content positively correlated with the number of tender joints and negatively with the total cell counts determined in synovial fluid of RA patients. It did not correlate with KYNA content. QUDA reduced both proliferation and motility of synoviocytes in a dose-dependent manner. The enhancement of antiproliferative action of QUDA by KYNA was evidenced.

CONCLUSIONS

Data show a local deficit of QUDA in RA patients and suggest its potential role as an endogenous substance controlling synoviocyte viability.

Phytomedicine in Joint Disorders

Chronic joint inflammatory disorders such as osteoarthritis and rheumatoid arthritis have in common an upsurge of inflammation, and oxidative stress, resulting in progressive histological alterations and disabling symptoms. Currently used conventional medication (ranging from pain-killers to biological agents) is potent, but frequently associated with serious, even life-threatening side effects. Used for millennia in traditional herbalism, medicinal plants are a promising alternative, with lower rate of adverse events and efficiency frequently comparable with that of conventional drugs. Nevertheless, their mechanism of action is in many cases elusive and/or uncertain. Even though many of them have been proven effective in studies done in vitro or on animal models, there is a scarcity of human clinical evidence. The purpose of this review is to summarize the available scientific information on the following joint-friendly medicinal plants, which have been tested in human studies: Arnica montana, Boswellia spp., Curcuma spp., Equisetum arvense, Harpagophytum procumbens, Salix spp., Sesamum indicum, Symphytum officinalis, Zingiber officinalis, Panax notoginseng, and Whitania somnifera.

Carotid surgery affects plasma kynurenic acid concentration: a pilot study

BACKGROUND

An increase in plasma kynurenic acid (KYNA) concentration has been observed following surgery, inflammation, and cerebral pathologies. The aim of the present study was to analyze the changes in plasma KYNA concentration in patients undergoing carotid surgery (CS).

MATERIAL AND METHODS

Adult patients undergoing elective carotid endarterectomy (CEA) or carotid angioplasty with stent placement (CAS) were studied. Plasma KYNA concentrations were analyzed before surgery and at 4 time points after CS. The amount of inflammation was measured as neutrophil-lymphocyte ratio (NLR).

RESULTS

Forty patients (10 female and 30 male) aged 55-86 years of age were evaluated in this study. In patients with unstable carotid plaque, the plasma KYNA concentration was higher than in patients with stable carotid plaque. Moreover, the NLR was significantly higher in patients with unstable carotid plaque undergoing CEA than in patients undergoing CAS. Plasma KYNA concentration increased after surgery in patients undergoing CEA and CAS. There was a strong correlation between plasma KYNA concentration and NLR in patients with postoperative neurological disorders.

CONCLUSIONS

CS increases plasma KYNA concentration, and changes in plasma KYNA concentration can indicate neurologic outcomes in patients undergoing CS.