Concentrations of various tryptophan metabolites are higher in patients with diabetes mellitus than in healthy aged male adults

The synergism of Lactobacillaceae, inulin, polyglucose, and aerobic exercise ameliorates hyperglycemia by modulating the gut microbiota community and the metabolic profiles in db/db mice

This study aimed to assess the impact of Lactobacillaceae (L or H represents a low or high dose), inulin (I), and polydextrose (P) combined with aerobic exercise (A) on the composition of the gut microbiota and metabolic profiles in db/db mice. After a 12-week intervention, LIP, LIPA, and HIPA groups exhibited significant improvements in hyperglycemia, glucose tolerance, insulin resistance, inflammatory response, and short-chain fatty acid (SCFA) and blood lipid levels compared to type 2 diabetes mice (MC). After treatment, the gut microbiota composition shifted favorably in the treatment groups which significantly increased the abundance of beneficial bacteria, such as Bacteroides, Blautia, Akkermansia, and Faecalibaculum, and significantly decreased the abundance of Proteus. Metabolomics analysis showed that compared to the MC group, the contents of 5-hydroxyindoleacetic acid, 3-hydroxysebacic acid, adenosine monophosphate (AMP), xanthine and hypoxanthine were significantly decreased, while 3-ketosphinganine, sphinganine, and sphingosine were significantly increased in the LIP and LIPA groups, respectively. Additionally, LIP and LIPA not only improved sphingolipid metabolism and purine metabolism pathways but also activated AMP-activated protein kinase to promote β-oxidation by increasing the levels of SCFAs. Faecalibaculum, Blautia, Bacteroides, and Akkermansia exhibited positive correlations with sphingosine, 3-ketosphinganine, and sphinganine, and exhibited negative correlations with hypoxanthine, xanthine and AMP. Faecalibaculum, Blautia, Bacteroides, and Akkermansia may have the potential to improve sphingolipid metabolism and purine metabolism pathways. These findings suggest that the synergism of Lactobacillaceae, inulin, polydextrose, and aerobic exercise provides a promising strategy for the prevention and management of type 2 diabetes.

Dual treatment with kynurenine pathway inhibitors and NAD+ precursors synergistically extends life span in Drosophila

Tryptophan catabolism is highly conserved and generates important bioactive metabolites, including kynurenines, and in some animals, NAD+. Aging and inflammation are associated with increased levels of kynurenine pathway (KP) metabolites and depleted NAD+, factors which are implicated as contributors to frailty and morbidity. Contrastingly, KP suppression and NAD+ supplementation are associated with increased life span in some animals. Here, we used DGRP_229 Drosophila to elucidate the effects of KP elevation, KP suppression, and NAD+ supplementation on physical performance and survivorship. Flies were chronically fed kynurenines, KP inhibitors, NAD+ precursors, or a combination of KP inhibitors with NAD+ precursors. Flies with elevated kynurenines had reduced climbing speed, endurance, and life span. Treatment with a combination of KP inhibitors and NAD+ precursors preserved physical function and synergistically increased maximum life span. We conclude that KP flux can regulate health span and life span in Drosophila and that targeting KP and NAD+ metabolism can synergistically increase life span.

Uremic toxins mediate kidney diseases: the role of aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) was originally identified as an environmental sensor that responds to pollutants. Subsequent research has revealed that AhR recognizes multiple exogenous and endogenous molecules, including uremic toxins retained in the body due to the decline in renal function. Therefore, AhR is also considered to be a uremic toxin receptor. As a ligand-activated transcriptional factor, the activation of AhR is involved in cell differentiation and senescence, lipid metabolism and fibrogenesis. The accumulation of uremic toxins in the body is hazardous to all tissues and organs. The identification of the endogenous uremic toxin receptor opens the door to investigating the precise role and molecular mechanism of tissue and organ damage induced by uremic toxins. This review focuses on summarizing recent findings on the role of AhR activation induced by uremic toxins in chronic kidney disease, diabetic nephropathy and acute kidney injury. Furthermore, potential clinical approaches to mitigate the effects of uremic toxins are explored herein, such as enhancing uremic toxin clearance through dialysis, reducing uremic toxin production through dietary interventions or microbial manipulation, and manipulating metabolic pathways induced by uremic toxins through controlling AhR signaling. This information may also shed light on the mechanism of uremic toxin-induced injury to other organs, and provide insights into clinical approaches to manipulate the accumulated uremic toxins.

The Synergism of Human Lactobacillaceae and Inulin Decrease Hyperglycemia via Regulating the Composition of Gut Microbiota and Metabolic Profiles in db/db Mice

This study aimed to evaluate the effects of Limosilactobacillus fermentum and Lactiplantibacillus plantarum isolated from human feces coordinating with inulin on the composition of gut microbiota and metabolic profiles in db/db mice. These supplements were administered to db/db mice for 12 weeks. The results showed that the Lactobacillaceae coordinating with inulin group (LI) exhibited lower fasting blood glucose levels than the model control group (MC). Additionally, LI was found to enhance colon tissue and increase the levels of short-chain fatty acids. 16S rRNA sequencing revealed that the abundance of Corynebacterium and Proteus, which were significantly increased in the MC group compared with NC group, were significantly decreased by the treatment of LI that also restored the key genera of the Lachnospiraceae_NK4A136_group, Lachnoclostridium, Ruminococcus_gnavus_group, Desulfovibrio, and Lachnospiraceae_UCG-006. Untargeted metabolomics analysis showed that lotaustralin, 5-hydroxyindoleacetic acid, and 13(S)-HpODE were increased while L-phenylalanine and L-tryptophan were decreased in the MC group compared with the NC group. However, the intervention of LI reversed the levels of these metabolites in the intestine. Correlation analysis revealed that Lachnoclostridium and Ruminococcus_gnavus_group were negatively correlated with 5-hydroxyindoleacetic acid and 13(S)-HpODE, but positively correlated with L-tryptophan. 13(S)-HpODE was involved in the "linoleic acid metabolism". L-tryptophan and 5-hydroxyindoleacetic acid were involved in "tryptophan metabolism" and "serotonergic synapse". These findings suggest that LI may alleviate type 2 diabetes symptoms by modulating the abundance of Ruminococcus_gnavus_group and Lachnoclostridium to regulate the pathways of "linoleic acid metabolism", "serotonergic synapse", and" tryptophan metabolism". Our results provide new insights into prevention and treatment of type 2 diabetes.

Abnormal tryptophan catabolism in diabetes mellitus and its complications: Opportunities and challenges

In recent years, the incidence rate of diabetes mellitus (DM), including type 1 diabetes mellitus(T1DM), type 2 diabetes mellitus(T2DM), and gestational diabetes mellitus (GDM), has increased year by year and has become a major global health problem. DM can lead to serious complications of macrovascular and microvascular. Tryptophan (Trp) is an essential amino acid for the human body. Trp is metabolized in the body through the indole pathway, kynurenine (Kyn) pathway and serotonin (5-HT) pathway, and is regulated by intestinal microorganisms to varying degrees. These three metabolic pathways have extensive regulatory effects on the immune, endocrine, neural, and energy metabolism systems of the body, and are related to the physiological and pathological processes of various diseases. The key enzymes and metabolites in the Trp metabolic pathway are also deeply involved in the pathogenesis of DM, playing an important role in pancreatic function, insulin resistance (IR), intestinal barrier, and angiogenesis. In DM and its complications, there is a disruption of Trp metabolic balance. Several therapy approaches for DM and complications have been proven to modify tryptophan metabolism. The metabolism of Trp is becoming a new area of focus for DM prevention and care. This paper reviews the impact of the three metabolic pathways of Trp on the pathogenesis of DM and the alterations in Trp metabolism in these diseases, expecting to provide entry points for the treatment of DM and its complications.

Integrated 16S rRNA sequencing and nontargeted metabolomics analysis to reveal the mechanisms of Yu-Ye Tang on type 2 diabetes mellitus rats

Introduction

Yu-Ye Tang (YYT) is a classical formula widely used in treatment of type 2 diabetes mellitus (T2DM). However, the specific mechanism of YYT in treating T2DM is not clear.

Methods

The aim of this study was to investigate the therapeutic effect of YYT on T2DM by establishing a rat model of T2DM. The mechanism of action of YYT was also explored through investigating gut microbiota and serum metabolites.

Results

The results indicated YYT had significant therapeutic effects on T2DM. Moreover, YYT could increase the abundance of Lactobacillus, Candidatus_Saccharimonas, UCG-005, Bacteroides and Blautia while decrease the abundance of and Allobaculum and Desulfovibrio in gut microbiota of T2DM rats. Nontargeted metabolomics analysis showed YYT treatment could regulate arachidonic acid metabolism, alanine, aspartate and glutamate metabolism, arginine and proline metabolism, glycerophospholipid metabolism, pentose and glucuronate interconversions, phenylalanine metabolism, steroid hormone biosynthesis, terpenoid backbone biosynthesis, tryptophan metabolism, and tyrosine metabolism in T2DM rats.

Discussion

In conclusion, our research showed that YYT has a wide range of therapeutic effects on T2DM rats, including antioxidative and anti-inflammatory effects. Furthermore, YYT corrected the altered gut microbiota and serum metabolites in T2DM rats. This study suggests that YYT may have a therapeutic impact on T2DM by regulating gut microbiota and modulating tryptophan and glycerophospholipid metabolism, which are potential key pathways in treating T2DM.

Oral administration of kynurenic acid delays the onset of type 2 diabetes in Goto-Kakizaki rats

Kynurenic acid (KYNA) is an endogenous catabolite of tryptophan that has been found to demonstrate neuroprotective properties in psychiatric disorders. Recently, accumulating data have suggested that KYNA may also play a significant role in various metabolic diseases by stimulating energy metabolism in adipose tissue and muscle. However, whether KYNA can serves as an anti-diabetes agent has yet to be studied. In this study, we investigated the potential anti-diabetic effects of administering KYNA orally through drinking water in pre-diabetic Goto-Kakizaki rats and examined how this treatment may influence energy metabolism regulation within the liver. We found that hyperglycemic Goto-Kakizaki rats showed lower plasmatic KYNA levels compared to normal rats. Oral administration of KYNA significantly delayed the onset of diabetes in Goto-Kakizaki rats compared to untreated animals. Moreover, we found that KYNA treatment significantly increased respiration exchange ratio and promoted the energy expenditure by stimulating the expression of uncoupling protein (UCP). We confirmed that KYNA stimulated the UCP expression in HepG2 cells and mouse hepatocytes at mRNA and protein levels. Our study reveals that KYNA could potentially act as an anti-diabetic agent and KYNA-induced UCP upregulation is closely associated with the regulation of energy metabolism. These results provide further evidence for the therapeutic potential of KYNA in diabetes.

Kynurenine Pathway in Diabetes Mellitus-Novel Pharmacological Target?

The tryptophan-kynurenine pathway (Trp-KYN) is the major route for tryptophan conversion in the brain and in the periphery. Kynurenines display a wide range of biological actions (which are often contrasting) such as cytotoxic/cytoprotective, oxidant/antioxidant or pro-/anti-inflammatory. The net effect depends on their local concentration, cellular environment, as well as a complex positive and negative feedback loops. The imbalance between beneficial and harmful kynurenines was implicated in the pathogenesis of various neurodegenerative disorders, psychiatric illnesses and metabolic disorders, including diabetes mellitus (DM). Despite available therapies, DM may lead to serious macro- and microvascular complications including cardio- and cerebrovascular disease, peripheral vascular disease, chronic renal disease, diabetic retinopathy, autonomic neuropathy or cognitive impairment. It is well established that low-grade inflammation, which often coincides with DM, can affect the function of KP and, conversely, that kynurenines may modulate the immune response. This review provides a detailed summary of findings concerning the status of the Trp-KYN pathway in DM based on available animal, human and microbiome studies. We highlight the importance of the molecular interplay between the deranged (functionally and qualitatively) conversion of Trp to kynurenines in the development of DM and insulin resistance. The Trp-KYN pathway emerges as a novel target in the search for preventive and therapeutic interventions in DM.

Tryptophan intake, not always the more the better

Objectives

To investigate the effects of excessive tryptophan intake on the body and the effects of tryptophan metabolism-related aryl hydrocarbon receptor (AhR) pathway in healthy rats and chronic kidney disease rats, to study the adverse effects of excess tryptophan.

Design

In Part I Experiment, the healthy rats were fed with diet containing 0.6, 1.2 and 1.8% tryptophan for 12 weeks. After the intervention, the blood and kidney tissues were collected. Serum creatinine and blood urea nitrogen were detected. Hematoxylin-eosin (H&E) staining was used to observe renal pathological changes. Enzyme-linked immunosorbent assay was used to detect serum kynurenic acid and AhR levels. The kidney levels of AhR, CyP1A1 and CyP1B1 were detected by western-blot. In Part II Experiment, the chronic kidney disease (CKD) model was induced by intra-gastric gavage with adenine for 4 weeks. Then the CKD rats were given tryptophan at a dose of 100 mg/kg or 500 mg/kg for eight weeks. Rat survival curve, renal function, renal tissue pathology and serum AhR were detected. Tryptophan-targeted ultra-high-performance liquid chromatography coupled with multiple reaction monitoring mass spectrometry (UHPLC-MRM-MS) was employed to quantitatively access the tryptophan-targeted metabolites in two parts experiments.

Results

In part I experiment, high tryptophan diet can increase the level of blood urea nitrogen (BUN) in healthy rats and induce focal renal tubulointerstitial injury. Tryptophan-targeted analyzes showed that high tryptophan diet feeding can significantly increase the concentration of kynurenine and indole metabolites. The serum AhR level and kidney AhR, CyP1A1 and CyP1B1 were also significantly increased in high tryptophan diet rats. In part II experiment, high tryptophan intervention induced a significant increase in mortality, serum creatinine, urea nitrogen levels, and renal pathological damage in CKD rats. The levels of tryptophan-targeted metabolites, kynurenine, xanthurenate, picolinic acid, 5-hydroxyindole-3-acetic acid, indole-3-lactic acid, indoleacetate and indoxyl sulfate, showed an upward trend in the high-dose tryptophan group (Ade + Trp-H) compared with the adenine group. The serum AhR of Ade + Trp-H rats was significantly higher than those of adenine rats.

Conclusion

Moderate tryptophan intake may be beneficial, but excessive tryptophan can lead to accumulation of kynurenine and indole metabolites, activate AhR pathway and induce kidney injury.

Untargeted approach to investigating the metabolomics profile of type 2 diabetes emiratis

Type 2 Diabetes (T2D) is expected to be the seventh most significant cause of death worldwide by 2030. Although research into its mechanism has received the attention it deserves, our understanding of T2D is still limited. This case-control study employs untargeted metabolomics to explore novel T2D plasma biomarkers in the Emirati population. Ninety-two UAE nationals were included in the cohort, with fifty T2D and forty-two non-T2D profiles. Participants were then stratified into three groups based on metabolic profiles, clinically verified diabetic status, and current HbA1c values: namely controlled diabetics, uncontrolled diabetics and prediabetics, and non-diabetics. The study identified fifteen significant differentially abundant metabolites between the uncontrolled diabetics group and the prediabetics or controlled diabetics group. Interestingly, some metabolites essential for the corticosteroid and thyroid signaling pathways were found to be significantly elevated in poorly controlled T2D, including cortisol, glycocholic acid, bile acids, thyroxine, and the tryptophan metabolite, 5-hydroxyindoleacetic acid. These findings align with those from prior western cohorts and suggest an intriguing linkage between T2D glycemic control and thyroid and adrenal signaling that may provide new diagnostic and prognostic indicators. RESEARCH SIGNIFICANCE: This study investigates the underlooked metabolomic role and correlation with T2D in the UAE population. The report indicates fifteen significant differentially abundant metabolites between on diabetics, uncontrolled diabetics and or controlled diabetics or prediabetics. This panel of metabolites such as thyroxine and corticosteroids should be considered further as potential diagnostic or prognostic biomarkers for T2D in the region.

Association of plasma kynurenine pathway metabolite concentrations with metabolic health risk in prepubertal Asian children

BACKGROUND

The tryptophan-kynurenine (KYN) pathway is linked to obesity-related systemic inflammation and metabolic health. The pathway generates multiple metabolites, with little available data on their relationships to early markers of increased metabolic disease risk in children. The aim of this study was to examine the association of multiple KYN pathway metabolites with metabolic risk markers in prepubertal Asian children.

METHODS

Fasting plasma concentrations of KYN pathway metabolites were measured using liquid chromatography-tandem mass spectrometry in 8-year-old children (n = 552) from the Growing Up in Singapore Towards healthy Outcomes (GUSTO) prospective mother-offspring cohort study. The child's weight and height were used to ascertain overweight and obesity using local body mass index (BMI)-for-age percentile charts. Body fat percentage was measured by quantitative magnetic resonance. Abdominal circumference, systolic and diastolic blood pressure, homeostatic model assessment for insulin resistance (HOMA-IR), triglyceride, and HDL-cholesterol were used for the calculation of Metabolic syndrome scores (MetS). Serum triglyceride, BMI, gamma-glutamyl transferase (GGT), and abdominal circumference were used in the calculation of the Fatty liver index (FLI). Associations were examined using multivariable regression analyses.

RESULTS

In overweight or obese children (n = 93; 16.9% of the cohort), all KYN pathway metabolites were significantly increased, relative to normal weight children. KYN, kynurenic acid (KA), xanthurenic acid (XA), hydroxyanthranilic acid (HAA) and quinolinic acid (QA) all showed significant positive associations with body fat percentage (B(95% CI) = 0.32 (0.22,0.42) for QA), HOMA-IR (B(95% CI) = 0.25 (0.16,0.34) for QA), and systolic blood pressure (B(95% CI) = 0.14(0.06,0.22) for QA). All KYN metabolites except 3-hydroxykynurenine (HK) significantly correlated with MetS (B (95% CI) = 0.29 (0.21,0.37) for QA), and FLI (B (95% CI) = 0.30 (0.21,0.39) for QA).

CONCLUSIONS

Higher plasma concentrations of KYN pathway metabolites are associated with obesity and with increased risk for metabolic syndrome and fatty liver in prepubertal Asian children.

The Kynurenine Pathway in Obese Middle-Aged Women with Normoglycemia and Type 2 Diabetes

We examined the relationships of tryptophan (Trp) and the metabolites of the kynurenine pathway (KP) to the occurrence of type 2 diabetes (T2D) and metabolic risk factors in obese middle-aged women. The study included 128 obese women divided into two subgroups: a normoglycemic group (NG, n = 65) and a T2D group (n = 63). The concentrations of serum tryptophan (Trp), kynurenine (Kyn), 3-hydroxykynurenine (3HKyn), quinolinic acid (QA), and kynurenic acid (Kyna) were analyzed using ultra-high-performance liquid chromatography coupled with electrospray ionization/triple quadrupole mass spectrometry. Blood biochemical parameters and anthropometric parameters were measured. The women with T2D had significantly higher Trp, Kyna, Kyna/QA ratio, and Kyna/3HKyn ratio values than the NG women. Logistic regression analysis showed that the concentrations of Trp and Kyna and the values of the Kyna/3HKyn ratio were most strongly associated with T2D occurrence, even after controlling for confounding factors. The model with Trp level and Kyna/3HKyn ratio accounted for 20% of the variation in the presence of T2D. We also showed a different pattern of correlations between kynurenines and metabolic factors in the NG and T2D women, which was mostly reflected in the stronger relationship between BMI and KP metabolites in the NG obese women. An increase in Trp and Kyna levels with an accompanying increase in Kyna/3HKyn ratio value is associated with the occurrence of T2D in obese middle-aged women.

Analysis of plasma free amino acids in diabetic rat and the intervention of Ginkgo biloba leaves extract using hydrophilic interaction liquid chromatography coupled with tandem mass-spectrometry

Amino acids (AAs) are important metabolites that are related with diabetes. However, their roles in the initiation and development of diabetes mellitus (DM), especially in the treatment of Ginkgo biloba leaves extract (GBE) have not been fully explored. Thus, we investigated the roles that AAs played in the progression and GBE supplementation of DM rat induced by streptozotocin. The rats were randomly divided into a normal control group treated with drug-free solution, a normal control group treated with GBE, a DM group treated with drug-free solution, and DM group treated with GBE; and maintained on this protocol for 9 weeks. Rat plasma was collected from the sixth week to the ninth week and then analyzed with the optimized hydrophilic interaction liquid chromatography coupled with tandem mass spectrometry method. A total of 17 AAs with differential levels were monitored to indicate dysfunction of AAs metabolism to confirm the occurrence and development of DM. Treatment with GBE partially reversed the changes seen in seven AAs including leucine, isoleucine, tyrosine, glutamic acid, asparagines, lysine and alanine in DM rats, indicating that GBE could prevent the occurrence and development of DM by acting on AAs metabolism. The improvement of those AAs metabolism disorders may play a considerable role in the treatment of GBE on the occurrence and development of DM. Those findings potentially promote the understanding of the pathogenic progression of DM and reveal the therapeutic mechanism of GBE against DM.

Comparative metabolomic study of high-flux hemodialysis and high volume online hemodiafiltration in the removal of uremic toxins using 1H NMR spectroscopy

Uremic toxins (UTs) accumulate in the circulation of patients with chronic kidney disease (CKD). High volume hemodiafiltration (HDF) improves clearance of low and medium molecular weight UTs compared to HD. The present study is a post-hoc analysis comparing the metabolomic profile in serum from patients under high flux HD (hf-HD) and HDF in HDFIT, a multicentric randomized controlled trial (RCTs). Per protocol, serum samples were collected pre- and post- dialysis treatments at randomization (baseline) and at the end of the follow up (6 months) and stored in a biorepository. Random (pre- and post-dialysis) samples from nine patients in study arm were selected at baseline and at the end of the follow up. To compare the samples, 26 possibly matching metabolites were identified by a t-test among the four groups using ¹H nuclear magnetic resonance (NMR). To evaluate the comparison between the modalities is a single treatment session, the clearance rates (CRs) of each metabolite were calculated based on pre-dialysis and post-dialysis samples. In addition, to evaluate to effect of UT removal during the trial follow up period, the pre-dialysis metabolite concentrations at the baseline and at 6 months were compared among the two arms of the study. There was no significant difference between in the single session CRs of metabolites when hf-HD and HDF were compared. On the other hand, the comparison between baseline and 6-month (long-term evolution) led to the identification of 16 metabolites that differentiated the hf-HD and the HDF evolutions. Most of these 16 metabolites are involved in several important metabolic pathways, such as metabolism of phenylalanine and biosynthesis of phenylalanine, tyrosine, and tryptophan, which are related to UTs and cardiovascular disease development. Although no difference was observed between hf-HD and HDF samples before and after a single session, concentrations of CKD-relevant metabolites and associated pathologies were stable in the HDF samples, but not in the hf-HD samples, over the six-month period, suggesting that HDF enhances long-term stability.

The impact of bariatric surgery on serum tryptophan-kynurenine pathway metabolites

This study aims to explore the immediate effects of bariatric surgery on serum tryptophan–kynurenine pathway metabolites in individuals with type 2 diabetes and BMI > 30. With the goal of providing insight into the link between tryptophan pathway metabolites, type 2 diabetes, and chronic obesity-induced inflammation. This longitudinal study included 20 participants. Half were diagnosed with type 2 diabetes. 11 and 9 underwent RYGB and SG respectively. Blood samples were obtained at pre-operative and 3 months post-operative timepoints. Tryptophan and downstream metabolites of the kynurenine pathway were quantified with an ultrahigh-performance liquid chromatography tandem mass spectrometry with electrospray ionisation method. At 3 months post-operation, RYGB led to significant reductions in tryptophan, kynurenic acid and xanthurenic acid levels when compared to baseline. Significant reductions of the same metabolites after surgery were also observed in individuals with T2D irrespective of surgical procedure. These metabolites were significantly correlated with serum HbA1c levels and BMI. Bariatric surgery, in particular RYGB reduces serum levels of tryptophan and its downstream kynurenine metabolites. These metabolites are associated with T2D and thought to be potentially mechanistic in the systemic processes of obesity induced inflammation leading to insulin resistance. Its reduction after surgery is associated with an improvement in glycaemic control (HbA1c).

Revealing the Hypoglycemic Effects and Mechanism of GABA-Rich Germinated Adzuki Beans on T2DM Mice by Untargeted Serum Metabolomics

Type 2 diabetes mellitus (T2DM) is one of the most common metabolic diseases, and exploring strategies to prevent and treat diabetes has become extremely important. In recent decades the search for new therapeutic strategies for T2DM involving dietary interventions has attracted public attention. We established a diabetic mouse model by feeding mice a high-fat diet combined with injection of low-dose streptozotocin, intending to elucidate the effects and possible mechanisms of different dosages of γ-aminobutyric acid (GABA)-rich germinated adzuki beans on the treatment of diabetes in mice. The mice were treated for 6 weeks either with increasing doses of GABA-enriched germinated adzuki beans, with non-germinated adzuki beans, with GABA, or with the positive control drug metformin. Then, the blood glucose levels and blood lipid biochemical indicators of all the mice were measured. At the same time, serum differential metabolite interactions were explored by UPLC-Q/TOF-MS-based serum metabolomic analysis. The results showed that body weight and fasting blood glucose levels were significantly reduced (P < 0.05). We also report improved levels of total cholesterol, triglycerides, aspartate aminotransferase, alanine aminotransferase, urea, and serum creatinine. We observed a significant improvement in the homeostasis model assessment of the beta cell function and insulin resistance (HOMA-β and HOMA-IR) scores (P < 0.05) in the group of mice treated with the highest dose of GABA-enriched germinated adzuki beans. In addition, the metabolic profiles of the serum were analyzed, and 31 differential metabolites including amino acids and lipids were obtained. According to the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis, this was found to be correlated with nine significantly enriched metabolic pathways involving the up-regulation of levels of L-serine, SM (d18:1/22:1(13Z)), L-histidine, creatine, and 3-indoleacetic acid. Our data suggest that the hypoglycemic effect of GABA-enriched germinated adzuki beans on diabetic mice may be related to improving tryptophan metabolism, glycerol phospholipid metabolism, sphingosline metabolism, and the glycine, serine, and threonine metabolic pathways. This study provides a reference for the application of GABA-enriched germinated foods in type 2 diabetes and could provide a cue for searching biomarkers to be adopted for T2DM diagnosis.

The Effect of Mild Renal Dysfunction on the Assessment of Plasma Amino Acid Concentration and Insulin Resistance in Patients with Type 2 Diabetes Mellitus

BACKGROUND

An increase in the levels of branched-chain amino acids (BCAAs) and certain aromatic amino acids, such as alanine, in plasma is correlated with insulin resistance (IR) in type 2 diabetes mellitus (T2DM). T2DM is a leading risk factor for chronic kidney disease. Meanwhile, renal dysfunction causes changes in plasma amino acid levels. To date, no study has examined how mild renal dysfunction and IR interact with plasma amino acid levels. This study examines the effects of IR and renal dysfunction on plasma amino acid concentrations in T2DM.

METHODS

Data were collected from healthy male participants (controls) and male patients with T2DM between May 2018 and February 2022. Blood samples were collected after overnight fasting. IR and renal function were evaluated using the homeostasis model assessment of IR (HOMA-IR) and serum cystatin C (CysC), respectively.

RESULTS

A total of 49 and 93 participants were included in the control and T2DM groups, respectively. In the T2DM group, eight amino acids (alanine, glutamic acid, glutamine, glycine, isoleucine, leucine, tyrosine, and valine) and total BCAA showed a significant correlation with HOMA-IR (p < 0.01), whereas six amino acids (γ-aminobutyric acid, citrulline, cysteine, glycine, methionine, and valine) and total BCAA showed a significant correlation with 1/CysC (p < 0.02). However, only alanine, glutamic acid, and each BCAA showed significant differences between the control group and the IR T2DM subgroup. Increases in the BCAA levels with T2DM were canceled by renal dysfunction (CysC ≥ 0.93) in patients with intermediate IR.

CONCLUSION

To use plasma BCAA concentration as a marker of IR, renal function must be considered, even in mild renal dysfunction. Increased alanine and glutamic acid levels indicate IR, regardless of mild renal dysfunction.

Tryptophan Metabolism in Atherosclerosis and Diabetes

The essential amino acid tryptophan (Trp) undergoes catabolism through several pathways, producing biologically active metabolites that significantly impact physiological processes. The metabolic pathway responsible for the majority of Trp catabolism is the kynurenine synthesis pathway (KP). Serotonin and melatonin are among the most essential Trp pathways degradation products. It has emerged that a strong relationship exists between alterations in Trp metabolism and the onset and progression of atherosclerosis and diabetes. Atherosclerosis is a chronic inflammatory disease of the small and medium arteries wall caused by maladaptive local immune responses, which underpins several cardiovascular diseases (CVD). Systemic low-grade immune-mediated inflammation is implicated in atherosclerosis where pro-inflammatory cytokines, such as interferon-γ (IFN-γ), play a significant role. IFN-γ upregulates the enzyme indoleamine 2,3-dioxygenase (IDO), decreasing serum levels of the Trp and increasing metabolite levels of kynurenine. Increased IDO expression and activity could accelerate the atherosclerosis process. Therefore, activated IDO inhibition could offer possible treatment options regarding atherosclerosis management. Diabetes is a chronic metabolic disease characterized by hyperglycemia that, over time, leads to severe damage to the heart, blood vessels, eyes, kidneys, and peripheral nerves. Trp serum levels and lower activity of IDO were higher in future type 2 diabetes (T2DM) patients. This article reviews recent findings on the link between mammalian Trp metabolism and its role in atherosclerosis and diabetes and outlines the intervention strategies.

Shared metabolic and neuroimmune mechanisms underlying Type 2 Diabetes Mellitus and Major Depressive Disorder

Type 2 Diabetes Mellitus (T2DM) is a chronic metabolic disease with symptoms that go beyond the domain of glucose metabolism. In fact, research has shown that T2DM is accompanied by neurodegeneration and neuroinflammation. Interestingly, Major Depressive Disorder (MDD), a mood disorder characterized mainly by depressed mood and anhedonia is a key feature of T2DM. A body of evidence demonstrates that there are many shared neuroimmune mechanisms underlying the pathophysiology of T2DM and MDD. Therefore, here we review the state-of-art regarding the underlying factors common to both T2DM and MDD. Furthermore, we briefly discuss how depressive symptoms in diabetic patients could be tackled by using novel therapeutic approaches uncovered by these shared mechanisms. Understanding the comorbidity of depression in diabetic patients is essential to fully address T2DM pathophysiology and treatment.

Crosstalk between Tryptophan Metabolism via Kynurenine Pathway and Carbohydrate Metabolism in the Context of Cardio-Metabolic Risk-Review

Scientific interest in tryptophan metabolism via the kynurenine pathway (KP) has increased in the last decades. Describing its metabolites helped to increase their roles in many diseases and disturbances, many of a pro-inflammatory nature. It has become increasingly evident that KP can be considered an important part of emerging mediators of diabetes mellitus and metabolic syndrome (MS), mostly stemming from chronic systemic low-grade inflammation resulting in the aggravation of cardiovascular complications. An electronic literature search of PubMed and Embase up to March 2021 was performed for papers reporting the effects of tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), xanthurenic acid (XA), anthranilic acid (AA), and quinolinic acid (QA), focusing on their roles in carbohydrate metabolism and the cardiovascular system. In this review, we discussed the progress in tryptophan metabolism via KP research, focusing particular attention on the roles in carbohydrate metabolism and its complications in the cardiovascular system. We examined the association between KP and diabetes mellitus type 2 (T2D), diabetes mellitus type 1 (T1D), and cardiovascular diseases (CVD). We concluded that tryptophan metabolism via KP serves as a potential diagnostic tool in assessing cardiometabolic risk for patients with T2D.

Decreased serum human leukocyte antigen-G levels are associated with gestational diabetes mellitus

AIM

This study was designed to determine serum human leukocyte antigen-G (HLA-G) levels and establish whether serum HLA-G level is related with gestational diabetes mellitus (GDM).

METHODS

Twenty-five GDM patients aged between 24 and 34 years and 24 healthy pregnant women aged between 22 and 33 years were included in this study. Health status of subjects was determined by medical history, physical, and obstetric examinations. Absence of family history for Type 2 diabetes mellitus, absence of diagnosis or clinical evidence of any major disease, and absence of medication use altering glucose metabolism constituted the inclusion criteria. GDM cases who are at their 24-28 weeks of pregnancy and who have not been using any oral hypoglycemic agents were included in GDM group. Control group consisted of healthy pregnant women at their 24-28 weeks of pregnancy.

RESULTS

Women with GDM had significantly lower levels of serum HLA-G than controls. HLA-G levels were negatively correlated with all parameters of glucose metabolism including insulin (r = - 0.14; p = 0.338), fasting blood glucose (r = - 0.220; p = 0.129), 1-h oral glucose tolerance test (OGTT) blood glucose (r = - 0.18; p = 0.271), 2-h OGTT blood glucose (r = - 0.314; p = 0.172), homeostasis model assessment (r = - 0.226; p = 0.119), HbA1C (r = -0.342; p = 0.108), and WBC (r = -0.149; p = 0.307). There was a negative correlation between HLA-G and BMI (r = -0.341; p = 0.016).

CONCLUSION

Decreased circulating HLA-G level is found to be associated with insulin resistance and GDM, in this study.

Allysine and α-Aminoadipic Acid as Markers of the Glyco-Oxidative Damage to Human Serum Albumin under Pathological Glucose Concentrations

Understanding the molecular basis of the disease is of the utmost scientific interest as it contributes to the development of targeted strategies of prevention, diagnosis, and therapy. Protein carbonylation is a typical feature of glyco-oxidative stress and takes place in health disorders such as diabetes. Allysine as well as its oxidation product, the α-amino adipic acid (α-AA) have been found to be markers of diabetes risk whereas little is known about the chemistry involved in its formation under hyperglycemic conditions. To provide insight into this issue, human serum albumin was incubated in the presence of FeCl3 (25 μM) and increasing glucose concentrations for 32 h at 37 °C. These concentrations were selected to simulate (i) physiological fasting plasma concentration (4 mM), (ii) pathological pre-diabetes fasting plasma concentration (8 mM), and pathological diabetes fasting plasma concentration (12 mM) of glucose. While both allysine and α-AA were found to increase with increasing glucose concentrations, the carboxylic acid was only detected at pathological glucose concentrations and appeared to be a more reliable indicator of glyco-oxidative stress. The underlying chemical mechanisms of lysine glycation as well as of the depletion of tryptophan and formation of fluorescent and colored advanced glycation products are discussed.

Plasma Targeted Metabolomics Analysis for Amino Acids and Acylcarnitines in Patients with Prediabetes, Type 2 Diabetes Mellitus, and Diabetic Vascular Complications

BACKGROUND

We hypothesized that specific amino acids or acylcarnitines would have benefits for the differential diagnosis of diabetes. Thus, a targeted metabolomics for amino acids and acylcarnitines in patients with diabetes and its complications was carried out.

METHODS

A cohort of 54 normal individuals and 156 patients with type 2 diabetes mellitus and/or diabetic complications enrolled from the First Affiliated Hospital of Jinzhou Medical University was studied. The subjects were divided into five main groups: normal individuals, impaired fasting glucose, overt diabetes, diabetic microvascular complications, and diabetic peripheral vascular disease. The technique of tandem mass spectrometry was applied to obtain the plasma metabolite profiles. Metabolomics multivariate statistics were applied for the metabolic data analysis and the differential metabolites determination.

RESULTS

A total of 10 cross-comparisons within diabetes and its complications were designed to explore the differential metabolites. The results demonstrated that eight comparisons existed and yielded significant metabolic differences. A total number of 24 differential metabolites were determined from six selected comparisons, including up-regulated amino acids, down-regulated medium-chain and long-chain acylcarnitines. Altered differential metabolites provided six panels of biomarkers, which were helpful in distinguishing diabetic patients.

CONCLUSION

Our results demonstrated that the biomarker panels consisted of specific amino acids and acylcarnitines which could reflect the metabolic variations among the different stages of diabetes and might be useful for the differential diagnosis of prediabetes, overt diabetes and diabetic complications.

Obesity Connected Metabolic Changes in Type 2 Diabetic Patients Treated With Metformin

Metformin is widely used in the treatment of Type 2 Diabetes Mellitus (T2DM). However, it is known to have beneficial effects in many other conditions, including obesity and cancer. In this study, we aimed to investigate the metabolic effect of metformin in T2DM and its impact on obesity. A mass spectrometry (MS)-based metabolomics approach was used to analyze samples from two cohorts, including healthy lean and obese control, and lean as well as obese T2DM patients on metformin regimen in the last 6 months. The results show a clear group separation and sample clustering between the study groups due to both T2DM and metformin administration. Seventy-one metabolites were dysregulated in diabetic obese patients (30 up-regulated and 41 down-regulated), and their levels were unchanged with metformin administration. However, 30 metabolites were dysregulated (21 were up-regulated and 9 were down-regulated) and then restored to obese control levels by metformin administration in obese diabetic patients. Furthermore, in obese diabetic patients, the level of 10 metabolites was dysregulated only after metformin administration. Most of these dysregulated metabolites were dipeptides, aliphatic amino acids, nucleic acid derivatives, and urea cycle components. The metabolic pattern of 62 metabolites was persistent, and their levels were affected by neither T2DM nor metformin in obesity. Interestingly, 9 metabolites were significantly dysregulated between lean and obese cohorts due to T2DM and metformin regardless of the obesity status. These include arginine, citrulline, guanidoacetic acid, proline, alanine, taurine, 5-hydroxyindoleacetic acid, and 5-hydroxymethyluracil. Understanding the metabolic alterations taking place upon metformin treatment would shed light on possible molecular targets of metformin, especially in conditions like T2DM and obesity.

Gut microbiota-derived tryptophan metabolism mediates renal fibrosis by aryl hydrocarbon receptor signaling activation

The gut microbiota has a crucial effect on regulating the intestinal mucosal immunity and maintaining intestinal homeostasis both in health and in disease state. Many effects are mediated by gut microbiota-derived metabolites and tryptophan, an essential aromatic amino acid, is considered important among many metabolites in the crosstalk between gut microbiota and the host. Kynurenine, serotonin, and indole derivatives are derived from the three major tryptophan metabolism pathways modulated by gut microbiota directly or indirectly. Aryl hydrocarbon receptor (AHR) is a cytoplasmic ligand-activated transcription factor involved in multiple cellular processes. Tryptophan metabolites as ligands can activate AHR signaling in various diseases such as inflammation, oxidative stress injury, cancer, aging-related diseases, cardiovascular diseases (CVD), and chronic kidney diseases (CKD). Accumulated uremic toxins in the body fluids of CKD patients activate AHR and affect disease progression. In this review, we will elucidate the relationship between gut microbiota-derived uremic toxins by tryptophan metabolism and AHR activation in CKD and its complications. This review will provide therapeutic avenues for targeting CKD and concurrently present challenges and opportunities for designing new therapeutic strategies against renal fibrosis.

Liquid chromatography-mass spectrometry untargeted metabolomics reveals increased levels of tryptophan indole metabolites in urine of metabolic syndrome patients

Metabolic syndrome (MetS) is a multifactor condition predisposing for diabetes, cardiovascular diseases and other degenerative disorders. Although several diagnostic criteria have been established, none of them is specific and there is a call for better pathophysiological explanation of MetS and for the discovery of molecular biomarkers. Phenotype characterization at metabolome level might be useful for both purposes. To this end, our aim was to perform comparative untargeted metabolomics of urines from MetS patients and from the control group. The study participants included 52 diagnosticated and 50 healthy individuals from Leon city in central Mexico; 23 anthropometric and clinical parameters were measured and submitted to Principal Component Analysis (PCA). The obtained PCA model allowed us for selection of 11 MetS patients and 13 control subjects, correspondingly representative for each of the two groups (clearly separated in PCA). The first morning urines from these subjects were ambulatory collected and, after methanol extraction and acidification, were submitted to capillary liquid chromatography-high resolution mass spectrometry (LC-HRMS). The obtained data were analyzed on MetaboScape® platform (Bruker Daltonics). Specifically, t-test applied to LC-HRMS data revealed several ions presenting at least 3-fold higher intensities in MetS with respect to the control samples (p < 0.05). Data analysis and complementary experiments yielded the identification of the following metabolites: indole-3-acetic acid, indole-3-acetic acid-O-glucuronide, N-(indol-3-ylacetyl) glutamine, indole-3-carbaldehyde and hydroxyhexanoycarnitine. Additionally, indole-3-carboxylic acid was annotated with 2.13-fold higher abundance in MetS patients. To assess the contribution of individual metabolites in the difference between two groups of subjects, partial least square discriminant analysis was performed for LC-HRMS data and the obtained values of variable importance in projection (VIP), confirmed the association of six above mentioned compounds with MetS. Overall, this study provides direct evidence on the disturbed catabolism of tryptophan in metabolic syndrome.

Kynurenine Pathway in Chronic Kidney Disease: What’s Old, What’s New, and What’s Next?

Impaired kidney function and increased inflammatory process occurring in the course of Chronic Kidney Disease (CKD) contribute to the development of complex amino-acid alterations. The essential amino-acid tryptophan (TRP) undergoes extensive metabolism along several pathways, resulting in the production of many biologically active compounds. The results of many studies have shown that its metabolism via the kynurenine pathway is potently increased in the course of CKD. Metabolites of this pathway exhibit differential, sometimes opposite, roles in several biological processes. Their accumulation in the course of CKD may induce oxidative cell damage which stimulates inflammatory processes. They can also modulate the activity of numerous cellular signaling pathways through activation of the aryl hydrocarbon receptor, leading to the disruption of homeostasis of various organs. As a result, they can contribute to the development of the systemic disorders accompanying the course of chronic renal failure. This review gathers and systematizes reports concerning the knowledge connecting the kynurenine pathway metabolites to systemic disorders accompanying the development of CKD.

Plasma Free Amino Acids and Risk of Cardiovascular Disease in Chinese Patients With Type 2 Diabetes

OBJECTIVES

This study aimed to explore associations between plasma free amino acids (PFAA) and risk of cardiovascular disease (CVD) in Chinese with Type 2 diabetes (T2D).

METHODS

We retrieved 741 inpatients with T2D consecutively from tertiary hospital. Twenty-three PFAA were measured. CVD was defined as having coronary heart disease (CHD) or stroke. Principal component analysis was used to extract factors of PFAA. Factors and their components were introduced into binary logistic regressions as continuous and tertiles to obtain OR (odds ratio) and 95% confidence interval (CI) for CVD (or its components) risk.

RESULTS

Of 741 inpatients, 282 (38.1%) had CVD (CHD alone: 122, stroke alone: 109, both: 51). Five factors were extracted, accounting for 65% of the total variance. Factor 3 composed of glutamate and tryptophan was associated with increased CVD risk (ORs, 95%CI of top vs. bottom tertiles: 1.60, 1.02-2.50 for CVD; 2.19, 1.17-4.07 for stroke, 1.51, 0.83-2.73 for CHD); the ORs (top vs. bottom tertiles) of glutamate were 2.62 (95%CI, 1.18-5.84) for stroke and 1.44 (0.80-2.61) for CHD; the ORs (top vs. bottom tertiles) of tryptophan were 1.50 (0.81-2.75) for stroke and 1.07 (0.58-1.97) for CHD. Comparable results were observed according to important confounders (all P for interaction >0.05).

CONCLUSIONS

Elevated factor 3 composed of glutamate and tryptophan was associated with increased CVD, especially stroke in T2D in China.

Emerging role for kynurenines in metabolic pathologies

PURPOSE OF REVIEW

So far, the tryptophan catabolites generated in the kynurenine pathway have been mainly studied in relation to oncologic and mental health disorders. The current review provides an update on the emerging biomedical interest for kynurenine pathway activity in the field of energy homeostasis and metabolic diseases.

RECENT FINDINGS

Kynurenine pathway enzymes are expressed in tissues relevant for energy homeostasis such as fat, skeletal muscle, liver and endocrine pancreas, blood vessel and heart, and are regulated by nutritional and inflammatory signals. Kynurenine pathway metabolites have been proposed as biomarkers for initiation and progression of atherosclerosis and diabetes. Exercise training activation of kynurenine pathway in skeletal muscles increases lipid metabolism and thermogenesis, and it limits weight gain, inflammation, insulin resistance, and glucose intolerance in rodents fed a high-fat diet. Manipulation of kynurenine pathway metabolism through administration of enzyme inhibitors or kynurenine pathway metabolites can serve as novel therapeutic strategy for atherosclerosis, obesity, glucose intolerance, or impaired insulin secretion.

SUMMARY

Although we are far from a complete understanding of the role of kynurenine pathway in the modulation of energy homeostasis, targeting kynurenine pathway harbors high potential to expand the range of therapies to prevent and treat metabolic diseases.

Association of Tryptophan Metabolites with Incident Type 2 Diabetes in the PREDIMED Trial: A Case-Cohort Study

BACKGROUND

Metabolites of the tryptophan-kynurenine pathway (i.e., tryptophan, kynurenine, kynurenic acid, quinolinic acid, 3-hydroxyanthranilic) may be associated with diabetes development. Using a case-cohort design nested in the Prevención con Dieta Mediterránea (PREDIMED) study, we studied the associations of baseline and 1-year changes of these metabolites with incident type 2 diabetes (T2D).

METHODS

Plasma metabolite concentrations were quantified via LC-MS for n = 641 in a randomly selected subcohort and 251 incident cases diagnosed during 3.8 years of median follow-up. Weighted Cox models adjusted for age, sex, body mass index, and other T2D risk factors were used.

RESULTS

Baseline tryptophan was associated with higher risk of incident T2D (hazard ratio = 1.29; 95% CI, 1.04-1.61 per SD). Positive changes in quinolinic acid from baseline to 1 year were associated with a higher risk of T2D (hazard ratio = 1.39; 95% CI, 1.09-1.77 per SD). Baseline tryptophan and kynurenic acid were directly associated with changes in homeostatic model assessment for insulin resistance (HOMA-IR) from baseline to 1 year. Concurrent changes in kynurenine, quinolinic acid, 3-hydroxyanthranilic acid, and kynurenine/tryptophan ratio were associated with baseline-to-1-year changes in HOMA-IR.

CONCLUSIONS

Baseline tryptophan and 1-year increases in quinolinic acid were positively associated with incident T2D. Baseline and 1-year changes in tryptophan metabolites predicted changes in HOMA-IR. Tryptophan levels may initially increase and then deplete as diabetes progresses in severity.