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

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.

Dietary tryptophan and genetic susceptibility expand gut microbiota that promote systemic autoimmune activation

Tryptophan modulates disease activity and the composition of microbiota in the B6.Sle1.Sle2.Sle3 (TC) mouse model of lupus. To directly test the effect of tryptophan on the gut microbiome, we transplanted fecal samples from TC and B6 control mice into germ-free or antibiotic-treated non-autoimmune B6 mice that were fed with a high or low tryptophan diet. The recipient mice with TC microbiota and high tryptophan diet had higher levels of immune activation, autoantibody production and intestinal inflammation. A bloom of Ruminococcus gnavus (Rg), a bacterium associated with disease flares in lupus patients, only emerged in the recipients of TC microbiota fed with high tryptophan. Rg depletion in TC mice decreased autoantibody production and increased the frequency of regulatory T cells. Conversely, TC mice colonized with Rg showed higher autoimmune activation. Overall, these results suggest that the interplay of genetic and tryptophan can influence the pathogenesis of lupus through the gut microbiota.

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.

Simultaneous measurement of kynurenine metabolites and explorative metabolomics using liquid chromatography-mass spectrometry: A novel accurate method applied to serum and plasma samples from a large healthy cohort

Kynurenine metabolites are emerging as promising clinical biomarkers in several diseases, especially within psychiatry. Unfortunately, they are difficult to detect, particularly the challenging neurotoxic metabolite quinolinic acid (QUIN). The aim of this study was twofold: First, to develop a liquid chromatography-mass spectrometry method (LC-MS) for simultaneous targeted quantification of key kynurenine metabolites together with untargeted metabolomics, and second, to demonstrate the feasibility of the method by exploring serum/plasma and gender differences in 120 healthy young adults between 18 and 30 years of age. A range of analytical columns (C18 and biphenyl columns) and mobile phases (acidic and alkaline) were systematically evaluated. The optimized LC-MS method was based on a biphenyl column, a water-methanol gradient with 0.2% formic acid, and authentic isotope-labeled standards for each kynurenine metabolite. Precision and accuracy of targeted quantification of the key kynurenine metabolites tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), and QUIN were excellent, far exceeding the acceptance criteria specified by international guidelines. Median inter- and intra-day precision were < 6% in serum and plasma; the median accuracy was 2.4% in serum and 8% in plasma. Serum concentrations were ≤ 10% different from the corresponding concentrations in plasma for all kynurenine metabolites in healthy young adults. Men had higher levels (8-18%) of TRP, KYN, and KYNA than women (p ≤ 0.009), while no differences were observed for 3-HK and QUIN (p > 0.70). Incurred sample reanalysis of 10% of the samples yielded a median difference < 5% from the initial measurement, demonstrating the robustness of the method. Besides the targeted quantification of key kynurenine metabolites, our method was found to be suitable for simultaneous untargeted metabolomics analyses of hundreds of metabolites. A range of compound classes could be detected including amino acids, nucleic acids, dipeptides, antioxidants, and acylcarnitines, making explorative studies highly feasible. For example, we identified an additional kynurenine metabolite, 2-Quinolinecarboxylic acid, which was 47% higher in males than females (adjusted p-value = 0.001). In conclusion, in this study, we present a reliable and robust LC-MS method for simultaneous targeted and untargeted metabolomics ready for both research and clinical use. We show that both serum and plasma can be used for kynurenine studies, and the reported gender differences are in accordance with the literature. Future studies should consider using biphenyl-based LC-MS columns to successfully detect QUIN.

Activation of the kynurenine-aryl hydrocarbon receptor axis impairs the chondrogenic and chondroprotective effects of human umbilical cord-derived mesenchymal stromal cells in osteoarthritis rats

It has been proven that intra-articular injection of mesenchymal stromal cells (MSCs) can alleviate cartilage damage in osteoarthritis (OA) by differentiating into chondrocytes and protecting inherent cartilage. However, the mechanism by which the OA articular microenvironment affects MSCs' therapeutic efficiency is yet to be fully elucidated. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in various cellular processes, such as osteogenesis and immune regulation. Tryptophan (Trp) metabolites, most of which are endogenous ligand for AHR, are abnormally increased in synovial fluid (SF) of OA and rheumatoid arthritis (RA) patients. In this study, the effects of kynurenine (KYN), one of the most important metabolites of Trp, were evaluated on the chondrogenic and chondroprotective effects of human umbilical cord-derived mesenchymal stromal cells (hUC-MSCs). hUC-MSCs were cultured in conditioned medium containing different proportions of OA/RA SF, or stimulated with KYN directly, and then, AHR activation, proliferation, and chondrogenesis of hUC-MSCs were measured. Moreover, the chondroprotective efficiency of short hairpin-AHR-UC-MSC (shAHR-UC-MSC) was determined in a rat surgical OA model (right hind joint). OA SF could activate AHR signaling in hUC-MSCs in a concentration-dependent manner and inhibit the chondrogenic differentiation and proliferation ability of hUC-MSCs. Similar results were observed in hUC-MSCs stimulated with KYN in vitro. Notably, shAHR-UC-MSC exhibited superior therapeutic efficiency in OA rat upon intra-articular injection. Taken together, this study indicates that OA articular microenvironment is not conducive to the therapeutic effect of hUC-MSCs, which is related to the activation of the AHR pathway by tryptophan metabolites, and thus impairs the chondrogenic and chondroprotective effects of hUC-MSCs. AHR might be a promising modification target for further improving the therapeutic efficacy of hUC-MSCs on treatment of cartilage-related diseases such as OA.

Nimbolide exerts protective effects in complete Freund's adjuvant induced inflammatory arthritis via abrogation of STAT-3/NF-κB/Notch-1 signaling

AIM

Activation of transmembrane Notch-1 receptors through inflammatory cytokines is highly regulated by STAT-3 and NF-κB phosphorylation. Nimbolide (NMB) exhibits potent anti-inflammatory, anti-fibrotic, anticancer activities by targeting various pathways. Here, we have investigated the effect of NMB in regulation of STAT-3/NF-κB/Notch-1 axis in complete Freund's adjuvant (CFA) induced inflammatory arthritis (IA) model.

MAIN METHODS

The anti-inflammatory and anti-arthritic activity of NMB was evaluated both in vitro (IL-1β stimulated HIG-82 synovial fibroblasts) and in vivo (CFA induced rat model of IA) models. In vitro anti-arthritic activity was assessed by anti-migratory effect, while in vivo effects were evaluated through radiological and histological analysis. The effect of NMB on STAT-3, NF-κB, Notch-1 signaling pathways and proinflammatory cytokines were studied using western blot, immunohistochemistry and ELISA methods. Key findings NMB attenuated the migration of synovial fibroblasts in vitro. It reduced the progression of arthritis as evidenced from the improved radiological and histological abnormalities in arthritic rats. NMB significantly suppressed the nitrosooxidative stress and levels of pro-inflammatory cytokines. NMB also exhibited remarkable protective activity against upregulation of MAPK, STAT-3 and NF-κB phosphorylation mediated Notch-1 signaling pathway in synovial tissue of arthritic rats.

SIGNIFICANCE

NMB may have clinical therapeutic value in rheumatoid arthritis by inhibiting STAT-3/NF-κB/Notch-1 axis and also by reducing the levels of proinflammatory cytokines.

Chronic dietary supplementation with kynurenic acid, a neuroactive metabolite of tryptophan, decreased body weight without negative influence on densitometry and mandibular bone biomechanical endurance in young rats

Kynurenic acid (KYNA) is a neuroactive metabolite of tryptophan. KYNA naturally occurs in breast milk and its content increases with lactation, indicating the role of neonatal nutrition in general growth with long-term health effects. KYNA is also an antagonist of ionotropic glutamate receptors expressed in bone cells. The aim of this study was to establish the effects of chronic KYNA supplementation on bone homeostasis in young rats, using mandible as a model bone. Female and male newborn Wistar rats were divided into control and KYNA-administered groups until 60 days of age (25x10¹ mg/L or 25x10² mg/L in drinking water). Hemimandibles were subjected to densitometry, computed tomography analysis and mechanical testing. Rats supplemented with KYNA at both doses showed a decrease in body weight. There were no effects of KYNA administration and mandible histomorphometry. In males, a significant quadratic effect (P < 0.001) was observed in the densitometry of the hemimandible, where BMD increased in the group supplemented with 2.5x10¹ mg/L of KYNA. Analysis of mechanical tests data showed that when fracture forces were corrected for bone geometry and rats body weight the improvement of bone material properties was observed in male and female rats supplemented with lower dose of KYNA. This study showed that chronic supplementation with KYNA may limit weight gain in the young, without adversely affecting the development of the skeleton.

Quinaldic acid induces changes in the expression of p53 tumor suppressor both on protein and gene level in colon cancer LS180 cells

BACKGROUND

Origin, synthesis and activity of quinaldic acid (QA), proposed derivative of kynurenic acid, have been poorly studied to date. Previously, we have demonstrated the antiproliferative effect of QA in a colon cancer model in vitro. The goal of present study was to verify QA activity to modify the expression of p53 tumor suppressor in colon cancer cells, and to relate it to its cancer cell growth inhibiting activity in vitro.

METHODS

LS180 colon cancer cells possessing the wild type form of p53 were used in the study. Real-time PCR and immunobloting techniques were used to test the expression of p53 at gene and protein level, respectively. Next, immunocytochemistry was used to visualize the localization of p53 protein within the cells. Furthermore, the antiproliferative activity of QA was retested in cells with siRNA silenced P53 gene.

RESULTS

The activity of QA to modify both the expression and phosphorylation of p53 protein as well as the level of P53 gene is shown. Concomitantly, the nuclear and cytoplasmic localization of phospho-p53 protein upon QA treatment is also presented. Moreover, reduced activity of QA in colon cancer cells with silenced p53 expression is observed.

CONCLUSION

QA affects the expression of p53 tumor suppressor, both at gene and protein level. The prominent contribution of p53 to the antiproliferative effect of QA in LS180 colon cancer cells can be suggested.