3-Hydroxykynurenine, 3-hydroxyanthranilic acid, and o-aminophenol inhibit leucine-stimulated insulin release from rat pancreatic islets

Antidiabetic Potential of a Trimeric Anthranilic Acid Peptide Isolated from Malbranchea flocciformis

Compound 3, a trimeric anthranilic acid peptide, and another three metabolites were isolated from an organic extract from the culture medium of Malbranchea flocciformis ATCC 34530. The chemical structure proposed previously for 3 was unequivocally assigned via synthesis and X-ray diffraction analysis. Tripeptide 3 showed insulinotropic properties by decreasing the postprandial peak in healthy and hyperglycemic mice. It also increased glucose-induced insulin secretion in INS-1E at 5 μM, specifically at higher glucose concentrations. These results revealed that 3 might act as an insulin sensitizer and a non-classical insulin secretagogue. Altogether, these findings are in harmony with the in vivo oral glucose tolerance test and acute oral hypoglycemic assay. Finally, the chemical composition of the extract was established by the Global Natural Products Social Molecular Network platform. Phylogenetic analysis using the internal transcribed spacer region revealed that M. flocciformis ATCC 34530 is related to the Malbrancheaceae.

Aromatic Amino Acids and Their Interactions with Gut Microbiota-Related Metabolites for Risk of Gestational Diabetes: A Prospective Nested Case-Control Study in a Chinese Cohort

INTRODUCTION

The aim of this study was to explore associations of aromatic amino acids (AAA) in early pregnancy with gestational diabetes mellitus (GDM), and whether high AAA and gut microbiota-related metabolites had interactive effects on GDM risk.

METHODS

We conducted a 1:1 case-control study (n = 486) nested in a prospective cohort of pregnant women from 2010 to 2012. According to the International Association of Diabetes and Pregnancy Study Group's criteria, 243 women were diagnosed with GDM. Binary conditional logistic regression was performed to examine associations of AAA with GDM risk. Interactions between AAA and gut microbiota-related metabolites for GDM were examined using additive interaction measures.

RESULTS

High phenylalanine and tryptophan were associated with increased GDM risk (OR: 1.72, 95% CI: 1.07-2.78 and 1.66, 1.02-2.71). The presence of high trimethylamine (TMA) markedly increased the OR of high phenylalanine alone up to 7.95 (2.79-22.71), while the presence of low glycoursodeoxycholic acid (GUDCA) markedly increased the OR of high tryptophan alone up to 22.88 (5.28-99.26), both with significant additive interactions. Furthermore, high lysophosphatidylcholines (LPC18:0) mediated both interactive effects.

CONCLUSIONS

High phenylalanine may have an additive interaction with high TMA, while high tryptophan may have an additive interaction with low GUDCA toward increased risk of GDM, both being mediated via LPC18:0.

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.

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).

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.

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.

Metabolite Signature of Albuminuria Involves Amino Acid Pathways in 8661 Finnish Men Without Diabetes

OBJECTIVE

To investigate the metabolite signature of albuminuria in individuals without diabetes or chronic kidney disease to identify possible mechanisms that result in increased albuminuria and elevated risk of type 2 diabetes (T2D).

RESEARCH DESIGN AND METHODS

The study cohort was a population-based Metabolic Syndrome In Men (METSIM) study including 8861 middle-aged and elderly Finnish men without diabetes or chronic kidney disease at baseline. A total of 5504 men participated in a 7.5-year follow-up study, and 5181 of them had metabolomics data measured by Metabolon's ultrahigh performance liquid chromatography-tandem mass spectroscopy.

RESULTS

We found 32 metabolites significantly (P < 5.8 × 10-5) and positively associated with the urinary albumin excretion (UAE) rate. These metabolites were especially downstream metabolites in the amino acid metabolism pathways (threonine, phenylalanine, leucine, arginine). In our 7.5-year follow-up study, UAE was significantly associated with a 19% increase (hazard ratio 1.19; 95% confidence interval, 1.13-1.25) in the risk of T2D after the adjustment for confounding factors. Conversion to diabetes was more strongly associated with a decrease in insulin secretion than a decrease in insulin sensitivity.

CONCLUSIONS

Metabolic signature of UAE included multiple metabolites, especially from the amino acid metabolism pathways known to be associated with low-grade inflammation, and accumulation of reactive oxygen species that play an important role in the pathogenesis of UAE. These metabolites were primarily associated with an increase in UAE and were secondarily associated with a decrease in insulin secretion and insulin sensitivity, resulting in an increased risk of incident T2D.

Herring Milt and Herring Milt Protein Hydrolysate Are Equally Effective in Improving Insulin Sensitivity and Pancreatic Beta-Cell Function in Diet-Induced Obese- and Insulin-Resistant Mice

Although genetic predisposition influences the onset and progression of insulin resistance and diabetes, dietary nutrients are critical. In general, protein is beneficial relative to carbohydrate and fat but dependent on protein source. Our recent study demonstrated that 70% replacement of dietary casein protein with the equivalent quantity of protein derived from herring milt protein hydrolysate (HMPH; herring milt with proteins being enzymatically hydrolyzed) significantly improved insulin resistance and glucose homeostasis in high-fat diet-induced obese mice. As production of protein hydrolysate increases the cost of the product, it is important to determine whether a simply dried and ground herring milt product possesses similar benefits. Therefore, the current study was conducted to investigate the effect of herring milt dry powder (HMDP) on glucose control and the associated metabolic phenotypes and further to compare its efficacy with HMPH. Male C57BL/6J mice on a high-fat diet for 7 weeks were randomized based on body weight and blood glucose into three groups. One group continued on the high-fat diet and was used as the insulin-resistant/diabetic control and the other two groups were given the high-fat diet modified to have 70% of casein protein being replaced with the same amount of protein from HMDP or HMPH. A group of mice on a low-fat diet all the time was used as the normal control. The results demonstrated that mice on the high-fat diet increased weight gain and showed higher blood concentrations of glucose, insulin, and leptin, as well as impaired glucose tolerance and pancreatic β-cell function relative to those on the normal control diet. In comparison with the high-fat diet, the replacement of 70% dietary casein protein with the same amount of HMDP or HMPH protein decreased weight gain and significantly improved the aforementioned biomarkers, insulin sensitivity or resistance, and β-cell function. The HMDP and HMPH showed similar effects on every parameter except blood lipids where HMDP decreased total cholesterol and non-HDL-cholesterol levels while the effect of HMPH was not significant. The results demonstrate that substituting 70% of dietary casein protein with the equivalent amount of HMDP or HMPH protein protects against obesity and diabetes, and HMDP is also beneficial to cholesterol homeostasis.

Microbiota-Related Metabolites and the Risk of Type 2 Diabetes

OBJECTIVE

Recent studies have highlighted the significance of the microbiome in human health and disease. Changes in the metabolites produced by microbiota have been implicated in several diseases. Our objective was to identify microbiome metabolites that are associated with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Our study included 5,181 participants from the cross-sectional Metabolic Syndrome in Men (METSIM) study that included Finnish men (age 57 ± 7 years, BMI 26.5 ± 3.5 kg/m²) having metabolomics data available. Metabolomics analysis was performed based on fasting plasma samples. On the basis of an oral glucose tolerance test, Matsuda ISI and disposition index values were calculated as markers of insulin sensitivity and insulin secretion. A total of 4,851 participants had a 7.4-year follow-up visit, and 522 participants developed type 2 diabetes.

RESULTS

Creatine, 1-palmitoleoylglycerol (16:1), urate, 2-hydroxybutyrate/2-hydroxyisobutyrate, xanthine, xanthurenate, kynurenate, 3-(4-hydroxyphenyl)lactate, 1-oleoylglycerol (18:1), 1-myristoylglycerol (14:0), dimethylglycine, and 2-hydroxyhippurate (salicylurate) were significantly associated with an increased risk of type 2 diabetes. These metabolites were associated with decreased insulin secretion or insulin sensitivity or both. Among the metabolites that were associated with a decreased risk of type 2 diabetes, 1-linoleoylglycerophosphocholine (18:2) significantly reduced the risk of type 2 diabetes.

CONCLUSIONS

Several novel and previously reported microbial metabolites related to the gut microbiota were associated with an increased risk of incident type 2 diabetes, and they were also associated with decreased insulin secretion and insulin sensitivity. Microbial metabolites are important biomarkers for the risk of type 2 diabetes.

Vitamin B6 and Diabetes: Relationship and Molecular Mechanisms

Vitamin B6 is a cofactor for approximately 150 reactions that regulate the metabolism of glucose, lipids, amino acids, DNA, and neurotransmitters. In addition, it plays the role of antioxidant by counteracting the formation of reactive oxygen species (ROS) and advanced glycation end-products (AGEs). Epidemiological and experimental studies indicated an evident inverse association between vitamin B6 levels and diabetes, as well as a clear protective effect of vitamin B6 on diabetic complications. Interestingly, by exploring the mechanisms that govern the relationship between this vitamin and diabetes, vitamin B6 can be considered both a cause and effect of diabetes. This review aims to report the main evidence concerning the role of vitamin B6 in diabetes and to examine the underlying molecular and cellular mechanisms. In addition, the relationship between vitamin B6, genome integrity, and diabetes is examined. The protective role of this vitamin against diabetes and cancer is discussed.

Nine Amino Acids Are Associated With Decreased Insulin Secretion and Elevated Glucose Levels in a 7.4-Year Follow-up Study of 5,181 Finnish Men

Several amino acids (AAs) have been shown to be associated with insulin resistance and increased risk of type 2 diabetes, but no previous studies have investigated the association of AAs with insulin secretion in a longitudinal setting. Our study included 5,181 participants of the cross-sectional METabolic Syndrome In Men (METSIM) study having metabolomics data on 20 AAs. A total of 4,851 had a 7.4-year follow-up visit. Nine AAs (phenylalanine, tryptophan, tyrosine, alanine, isoleucine, leucine, valine, aspartate, and glutamate) were significantly (P < 5.8 × 10^-5) associated with decreases in insulin secretion (disposition index) and the elevation of fasting or 2-h glucose levels. Five of these AAs (tyrosine, alanine, isoleucine, aspartate, and glutamate) were also found to be significantly associated with an increased risk of incident type 2 diabetes after adjustment for confounding factors. Our study is the first population-based large cohort to report that AAs are associated not only with insulin resistance but also with decreased insulin secretion.

Mitochondria, Oxidative Stress and the Kynurenine System, with a Focus on Ageing and Neuroprotection

In this review, the potential causes of ageing are discussed. We seek to gain insight into the main physiological functions of mitochondria and discuss alterations in their function and the genome, which are supposed to be the central mechanisms in senescence. We conclude by presenting the potential modulating role of the kynurenine pathway in the ageing processes. Mitochondrial dynamics are supposed to have important physiological roles in maintaining cell homeostasis. During ageing, a decrease in mitochondrial dynamics was reported, potentially compromising the function of mitochondria. Mitochondrial biogenesis not only encompasses mitochondrial dynamics, but also the regulation of transcription and translation of genes, and mitochondria are supposed to play a prominent role in cell death during senescence. Defects in the mtDNA replication machinery and failure in the repair of mtDNA might result in the accumulation of mutations, leading to mitochondrial dysfunction and bioenergetic failure of the cell. The role of reactive oxygen species (ROS) in the ageing processes is widely acknowledged. Exaggerated oxidative damage to mDNA is supposed to take place during senescence, including single-nucleotide base alterations, nucleotide base pair alterations, chain breaks and cross linkage. A broad repertoire for the repair of DNA faults has evolved, but they do not function efficiently during senescence. Poly (ADP-ribose) polymerase (PARP) is an enzyme that assists in DNA repair, i.e., it participates in the repair of single-stranded DNA nicks, initiating base excision repair (BER). In the case of extensive DNA damage, PARP-1 becomes overactivated and rapidly depletes the intracellular NAD⁺ and ATP pools. This results in a profound energy loss of the cell and leads to cell dysfunction, or even cell death. Alterations in the kynurenine system have been linked with ageing processes and several age-related disorders. The kynurenine pathway degrades tryptophan (TRP) to several metabolites, among others kynurenine (KYN), kynurenic acid (KYNA) and quinolinic acid (QUIN). The end product of the route is NAD⁺. The first metabolic reaction is mediated by TRP-2,3-dioxygenase (TDO) or indolamine-2,3-dioxygenases (IDO), the latter being induced by inflammation, and it is thought to have a significant role in several disorders and in ageing. Research is currently focusing on the KYN pathway, since several intermediates possess neuro- and immunoactive properties, and hence are capable of modulating the activity of certain brain cells and inflammatory responses. During ageing, and in many age-associated disorders like obesity, dyslipidaemia, hypertension, insulin resistance and neurodegenerative diseases, low-grade, sustained inflammation and upregulation of IDO have been reported. However, TRP downstream catabolites create a negative feedback loop by weakening the activated immune system through several actions, including a decline in the Th1 response and an enhancement of Th2-type processes. The broad actions of the KYN-intermediates in brain excitation/inhibition and their role in regulating immune responses may provide the possibility of modifying the pathological processes in an array of age-associated diseases in the future.

Kynurenines: Tryptophan's metabolites in exercise, inflammation, and mental health

Kynurenine metabolites are generated by tryptophan catabolism and regulate biological processes that include host-microbiome signaling, immune cell response, and neuronal excitability. Enzymes of the kynurenine pathway are expressed in different tissues and cell types throughout the body and are regulated by cues, including nutritional and inflammatory signals. As a consequence of this systemic metabolic integration, peripheral inflammation can contribute to accumulation of kynurenine in the brain, which has been associated with depression and schizophrenia. Conversely, kynurenine accumulation can be suppressed by activating kynurenine clearance in exercised skeletal muscle. The effect of exercise training on depression through modulation of the kynurenine pathway highlights an important mechanism of interorgan cross-talk mediated by these metabolites. Here, we discuss peripheral mechanisms of tryptophan-kynurenine metabolism and their effects on inflammatory, metabolic, oncologic, and psychiatric disorders.

IDO chronic immune activation and tryptophan metabolic pathway: A potential pathophysiological link between depression and obesity

Obesity and depression are among the most pressing health problems in the contemporary world. Obesity and depression share a bidirectional relationship, whereby each condition increases the risk of the other. By inference, shared pathways may underpin the comorbidity between obesity and depression. Activation of cell-mediated immunity (CMI) is a key factor in the pathophysiology of depression. CMI cytokines, including IFN-γ, TNFα and IL-1β, induce the catabolism of tryptophan (TRY) by stimulating indoleamine 2,3-dioxygenase (IDO) resulting in the synthesis of kynurenine (KYN) and other tryptophan catabolites (TRYCATs). In the CNS, TRYCATs have been related to oxidative damage, inflammation, mitochondrial dysfunction, cytotoxicity, excitotoxicity, neurotoxicity and lowered neuroplasticity. The pathophysiology of obesity is also associated with a state of aberrant inflammation that activates aryl hydrocarbon receptor (AHR), a pathway involved in the detection of intracellular or environmental changes as well as with increases in the production of TRYCATs, being KYN an agonists of AHR. Both AHR and TRYCATS are involved in obesity and related metabolic disorders. These changes in the TRYCAT pathway may contribute to the onset of neuropsychiatric symptoms in obesity. This paper reviews the role of immune activation, IDO stimulation and increased TRYCAT production in the pathophysiology of depression and obesity. Here we suggest that increased synthesis of detrimental TRYCATs is implicated in comorbid obesity and depression and is a new drug target to treat both diseases.

The Role of the Microbial Metabolites Including Tryptophan Catabolites and Short Chain Fatty Acids in the Pathophysiology of Immune-Inflammatory and Neuroimmune Disease

There is a growing awareness that gut commensal metabolites play a major role in host physiology and indeed the pathophysiology of several illnesses. The composition of the microbiota largely determines the levels of tryptophan in the systemic circulation and hence, indirectly, the levels of serotonin in the brain. Some microbiota synthesize neurotransmitters directly, e.g., gamma-amino butyric acid, while modulating the synthesis of neurotransmitters, such as dopamine and norepinephrine, and brain-derived neurotropic factor (BDNF). The composition of the microbiota determines the levels and nature of tryptophan catabolites (TRYCATs) which in turn has profound effects on aryl hydrocarbon receptors, thereby influencing epithelial barrier integrity and the presence of an inflammatory or tolerogenic environment in the intestine and beyond. The composition of the microbiota also determines the levels and ratios of short chain fatty acids (SCFAs) such as butyrate and propionate. Butyrate is a key energy source for colonocytes. Dysbiosis leading to reduced levels of SCFAs, notably butyrate, therefore may have adverse effects on epithelial barrier integrity, energy homeostasis, and the T helper 17/regulatory/T cell balance. Moreover, dysbiosis leading to reduced butyrate levels may increase bacterial translocation into the systemic circulation. As examples, we describe the role of microbial metabolites in the pathophysiology of diabetes type 2 and autism.

Loss of arylformamidase with reduced thymidine kinase expression leads to impaired glucose tolerance

Tryptophan metabolites have been linked in observational studies with type 2 diabetes, cognitive disorders, inflammation and immune system regulation. A rate-limiting enzyme in tryptophan conversion is arylformamidase (Afmid), and a double knockout of this gene and thymidine kinase (Tk) has been reported to cause renal failure and abnormal immune system regulation. In order to further investigate possible links between abnormal tryptophan catabolism and diabetes and to examine the effect of single Afmid knockout, we have carried out metabolic phenotyping of an exon 2 Afmid gene knockout. These mice exhibit impaired glucose tolerance, although their insulin sensitivity is unchanged in comparison to wild-type animals. This phenotype results from a defect in glucose stimulated insulin secretion and these mice show reduced islet mass with age. No evidence of a renal phenotype was found, suggesting that this published phenotype resulted from loss of Tk expression in the double knockout. However, despite specifically removing only exon 2 of Afmid in our experiments we also observed some reduction of Tk expression, possibly due to a regulatory element in this region. In summary, our findings support a link between abnormal tryptophan metabolism and diabetes and highlight beta cell function for further mechanistic analysis.

Summary: Mice homozygous for a tm1b deleted allele have impaired glucose tolerance due to reduced insulin secretion associated with reduced islet mass, but no evidence of renal disease, suggesting a link between abnormal tryptophan metabolism and diabetes.

Expression of the kynurenine pathway enzymes in the pancreatic islet cells. Activation by cytokines and glucolipotoxicity

The tryptophan/kynurenine pathway (TKP) is the main route of tryptophan degradation and generates several neuroactive and immunomodulatory metabolites. Experimental and clinical data have clearly established that besides fat, muscle and liver, pancreatic islet tissue itself is a site of inflammation during obesity and type 2 diabetes. Therefore it is conceivable that pancreatic islet exposure to increased levels of cytokines may induce upregulation of islet kynurenine metabolism in a way resembling that seen in the brain in many neurodegenerative disorders. Using normal rat islets and the INS-1 β-cell line, we have demonstrated for the first time that: 1/only some TKP genes are constitutively expressed, both in β-cells as well as non β-cells; 2/ the regulatory enzyme indoleamine 2,3-dioxygenase (IDO1) is not constitutively expressed; 3/ IDO1 and kynurenine 3-monoxygenase (KMO) expression are potently activated by proinflammatory cytokines (IFN-γ, IL-1β) and glucolipotoxicity respectively, rather in β-cells than in non β-cells; 4/ Islet kynurenine/kynurenic acid production ratio is enhanced following IFN-γ and glucolipotoxicity; 5/ acute exposure to KYN potentiates glucose-induced insulin secretion by normal islets; and 6/ oxidative stress or glucocorticoid modulates TKP genes only marginally. Pancreatic islets may represent a new target tissue for inflammation and glucolipotoxicity to activate the TKP. Since inflammation is now recognized as a crucial mechanism in the development of the metabolic syndrome and more specifically at the islet level, it is needed to evaluate the potential induction of the TKP in the endocrine pancreas during obesity and/or diabetes and its relationship to the islet cell functional alterations.

Serotonin-kynurenine hypothesis of depression: historical overview and recent developments

This mini-review focuses on the studies of late Prof. IP Lapin (1903 - 2012) and his research team on the role of methoxyindole and kynurenine (KYN) pathways of tryptophan (TRP) metabolism in the pathogenesis of depression and action mechanisms of antidepressant effect. In the late 60s of the last century Prof. IP Lapin suggested that "intensification of central serotoninergic processes is a determinant of the thymoleptic (mood elevating) component" while "activation of noradrenergic processes is responsible for psychoenergetic and motor-stimulating component of the clinical antidepressant effect". The cause of serotonin deficiency in depression was attributed to the shunt of TRP "metabolism away from serotonin production towards KYN production" due to cortisol-induced activation of liver enzyme, tryptophan 2,3- dioxygenase, the rate-limiting enzyme of TRP - KYN pathway. Prof. Lapin suggested and discovered that KYN and its metabolites affect brain functions, and proposed the role of neurokynurenines in pathogenesis of depression and action mechanisms of antidepressant effect (kynurenine hypothesis). Further research suggested the antidepressant and cognition- enhancing effects of post-serotonin metabolite, N-acetylserotonin (NAS), an agonist to tyrosine kinase B (TrkB) receptor; and link between depression and chronic inflammation-associated disorders (e.g., insulin resistance, hepatitis C virus) via inflammation-induced activation of indoleamine 2,3- dioxygenase, brain located rate-limiting enzyme of TRY - KYN metabolism. NAS and kynurenines might be the targets for prevention and treatment of depression and associated conditions.

Insulin resistance and dysregulation of tryptophan-kynurenine and kynurenine-nicotinamide adenine dinucleotide metabolic pathways

Insulin resistance (IR) underlines aging and aging-associated medical (diabetes, obesity, dyslipidemia, hypertension) and psychiatric (depression, cognitive decline) disorders. Molecular mechanisms of IR in genetically or metabolically predisposed individuals remain uncertain. Current review of the literature and our data presents the evidences that dysregulation of tryptophan (TRP)-kynurenine (KYN) and KYN-nicotinamide adenine dinucleotide (NAD) metabolic pathways is one of the mechanisms of IR. The first and rate-limiting step of TRP-KYN pathway is regulated by enzymes inducible by pro-inflammatory factors and/or stress hormones. The key enzymes of KYN-NAD pathway require pyridoxal-5-phosphate (P5P), an active form of vitamin B6, as a cofactor. Deficiency of P5P diverts KYN-NAD metabolism from production of NAD to the excessive formation of xanthurenic acid (XA). Human and experimental studies suggested that XA and some other KYN metabolites might impair production, release, and biological activity of insulin. We propose that one of the mechanisms of IR is inflammation- and/or stress-induced upregulation of TRP-KYN metabolism in combination with P5P deficiency-induced diversion of KYN-NAD metabolism towards formation of XA and other KYN derivatives affecting insulin activity. Monitoring of KYN/P5P status and formation of XA might help to identify subjects at risk for IR. Pharmacological regulation of the TRP-KYN and KYN-NAD pathways and maintaining of adequate vitamin B6 status might contribute to prevention and treatment of IR in conditions associated with inflammation/stress-induced excessive production of KYN and deficiency of vitamin B6, e.g., type 2 diabetes, obesity, cardiovascular diseases, aging, menopause, pregnancy, and hepatitis C virus infection.

Tryptophan-kynurenine metabolism and insulin resistance in hepatitis C patients

Chronic hepatitis C virus (HCV) infection is associated with 50% incidence of insulin resistance (IR) that is fourfold higher than that in non-HCV population. IR impairs the outcome of antiviral treatment. The molecular mechanisms of IR in HCV are not entirely clear. Experimental and clinical data suggested that hepatitis C virus per se is diabetogenic. However, presence of HCV alone does not affect IR. It was proposed that IR is mediated by proinflammatory cytokines, mainly by TNF-alpha. TNF-alpha potentiates interferon-gamma-induced transcriptional activation of indoleamine 2,3-dioxygenase, the rate-limiting enzyme of tryptophan- (TRP-) kynurenine (KYN) metabolism. Upregulation of TRP-KYN metabolism was reported in HCV patients. KYN and some of its derivatives affect insulin signaling pathways. We hypothesized that upregulation of TRP-KYN metabolism might contribute to the development of IR in HCV. To check this suggestion, we evaluated serum concentrations of TRP and KYN and HOMA-IR and HOMA-beta in 60 chronic HCV patients considered for the treatment with IFN-alpha. KYN and TRP concentrations correlated with HOMA-IR and HOMA-beta scores. Our data suggest the involvement of KYN and its metabolites in the development of IR in HCV patients. TRP-KYN metabolism might be a new target for prevention and treatment of IR in HCV patients.

Vitamin B6: Beyond Adequacy

There is good agreement concerning average requirements and reference intakes for vitamin B6 but less agreement over safe upper levels from supplements. High-dose supplements cause sensory nerve damage. Supplements of vitamin B6 have been advocated for treatment of the premenstrual syndrome, with little evidence of efficacy. There are plausible mechanisms for an antidepressant action and protection against steroid hormone—dependent cancers but no evidence from clinical trials. Pyridoxamine reduces the glycation of proteins and so could be beneficial in preventing the adverse effects of poor glycemic control in diabetes. There are plausible mechanisms for an antihypertensive action but only suggestive evidence from small intervention trials. There is no evidence that supplements of vitamin B6 have any beneficial effect in hyperhomocysteinemia. There is neither a plausible mechanism nor any evidence from controlled trials for any effect of supplements of vitamin B6 in preventing a decline in cognitive function with aging, amelioration of dementia or autism, or improvement of the carpal tunnel syndrome.

Non-nutritional uses of vitamin B6

Vitamin B6is a water-soluble vitamin, and is readily metabolized and excreted, so it has generally been assumed to have negligible toxicity, although at very high levels of intake it can cause peripheral nerve damage. Nutritional deficiency disease is extremely rare, although a significant proportion of the population shows biochemical evidence of inadequate status, despite apparently adequate levels of intake. The vitamin has been used to treat a wide variety of conditions, which may or may not be related to inadequate intake. In some conditions use of vitamin B6supplements has been purely empirical; in other conditions there is a reasonable physiological or metabolic mechanism to explain why supplements of the vitamin many times greater than average requirements may have therapeutic uses. However, even in such conditions there is little evidence of efficacy from properly conducted controlled trials.

Induction of indoleamine 2,3-dioxygenase by interferon-gamma in human islets

Indoleamine 2,3-dioxygenase (IDO) catalyzes the initial, rate-limiting step of tryptophan (Trp) catabolism along the kynurenine (KYN) pathway, and its induction in cells of the immune system in response to cytokines has been implicated in the regulation of antigen presentation and responses to cell-mediated immune attack. Microarray and quantitative PCR analyses of isolated human islets incubated with interferon (IFN)-gamma for 24 h revealed increased expression of IDO mRNA (>139-fold) and Trp-tRNA synthase (WARS) (>17-fold) along with 975 other transcripts more than threefold, notably the downstream effectors janus kinase (JAK)2, signal transducer and activator of transcription (STAT)1, IFN-gamma regulatory factor-1, and several chemokines (CXCL9/MIG, CXCL10/IP10, CXCL11/1-TAC, CCL2, and CCL5/RANTES) and their receptors. IDO protein expression was upregulated in IFN-gamma-treated islets and accompanied by increased intracellular IDO enzyme activity and the release of KYN into the media. The response to IFN-gamma was countered by interleukin-4 and 1alpha-methyl Trp. Immunohistochemical localization showed IDO to be induced in cells of both endocrine, including pancreatic duodenal homeobox 1-positive beta-cells, and nonendocrine origin. We postulate that in the short term, IDO activation may protect islets from cytotoxic damage, although chronic exposure to various Trp metabolites could equally lead to beta-cell attrition.

The probable involvement of soluble and deposited melanins, their intermediates and the reactive oxygen side-products in human diseases and aging

The plasma soluble melanins (PSM) form spontaneously in vitro and in vivo and their formation involves oxidative polymerization and copolymerization of dopa, catecholamines, homogentisic acid, 3-hydroxyanthranilic acid, p-aminophenol, p-phenylenediamine, and other end(ex)ogenous ortho and para polyhydroxy-, (poly)hydroxy(poly)amino- and polyamino-phenyl compounds. The build up of PSM is visible within 2-3 h after the start of incubation at 37 degrees C with 1 mg/ml of plasma. PSM also form similarly in blood and these processes cause hemolysis. The mean quantity of PSM in normal human plasma is 1.61+/-0.1 (S.D.) mg/ml (n = 20) and in normal human urine is 1.1+/-1.2 g/24 h collection (n = 8). They contribute to the yellow color of plasma and urine. Antioxidants delay the formation of PSM. The deposited melanins also form from these precursors. Reactive oxygen side products (ROSP) are generated during and after melanogenesis. Melanins in vivo are generally associated with proteins or with proteins and lipids. The PSM-protein-lipid complexes are called plasma soluble lipofuscins (PSL), because they have histochemical and fluorescence properties similar to those of solid lipofuscins. The soluble and deposited melanins (SDM) and their intermediates have similar toxic chemical reactivities. The oxidizing quinoid (they can produce partially and completely substituted conjugates) and the semiquinoid free radical intermediates are also moieties in most human melanin structures. Soluble melanins formed from dopa, or dopamine, or norepinephrine in weak alkaline solution have been shown to be toxic to human CD4+ lymphoblastic cells (MT-2) at higher than 10 microg/ml concentrations. Alkaptonuria with high levels of homogentisic acid in the plasma is a potentially fatal disease, exhibiting the toxic effects of the homogentisic acid melanin (soluble and deposited), its intermediates and the ROSP. Patients with alkaptonuria develop arthritis and often suffer from other diseases too, including cardiovascular disease (frequent cause of death) and kidney disease. Pheochromocytoma, with high levels of catecholamines in the plasma is another potentially fatal disease. The catecholamine PSM of pheochromocytoma have very light yellow or practically no colors, due to the concentrations and chemical structures. Pheochromocytomas can cause hypertension, cardiovascular disease (frequent cause of death), kidney disease, stroke, cancer, amyloid formation and can mimic many other diseases, including acute pancreatitis, carcinoid, neuroblastoma, psychiatric illness, hypercalcemia, retinal vascular lesions, and diabetes mellitus. Pheochromocytoma is potentially fatal even in patients without hypertension. Following trauma and surgery, heavily pigmented eyes are apt to experience greater inflammation than lightly pigmented eyes. In Parkinson's disease those neurons are lost first in the substantia nigra and locus ceruleus which contain the greatest amounts of neuromelanins. The antihypertensive alphamethyldopa causes Parkinson's syndrome. It forms PSM in a short time in vitro. The side effects of L-dopa (immobility episodes alternate with normal or involuntary movements; psychotic abnormalities) suggest that the SDM, their intermediates and the ROSP present naturally in vivo are involved in the cause of Parkinson's disease and Alzheimer's disease. There is a large overlap between these two diseases. (ABSTRACT TRUNCATED)

Para-aminophenol and structurally related compounds as intermediates in lipofuscin formation and in renal and other tissue toxicities

P-aminophenol is considered a minor nephrotoxic metabolite of phenacetin and acetaminophen (paracetamol) in man. Our experiments show that p-aminophenol readily undergoes oxidative polymerization during incubation in human blood or plasma, to form melanin, as a component of soluble lipofuscin. Haemolysis accompanies this process in whole blood. Unmetabolized phenacetin and acetaminophen do not form soluble lipofuscins. Long-term excessive use of phenacetin or acetaminophen has been associated with chronic renal disease, haemolytic anaemia, and increased solid lipofuscin deposition in tissues. Excessive use of phenacetin has also been associated with cancer of renal pelvis and bladder. It appears to us that p-aminophenol and other o- and p-aminophenol metabolites of these drugs are intermediates not only in the etiology of chronic renal disease, but in the other developments as well. P-aminophenol and other ex(end)ogenous aminohydroxyphenyl, aminopolyhydroxyphenyl, polyhydroxyphenyl and polyaminophenyl compounds with these groups in ortho and para positions (such as 3-hydroxyanthranilic acid, 6-aminodopamine, dopamine, p-phenylenediamine, etc.) can undergo autoxidations and metal-catalyzed and enzymatic oxidations in man to produce toxic (semi)quinones(imines), (semi)quinonediimines and reactive oxygen species. After depletion of antioxidants these very reactive (semi)quinones(imines) and (semi)quinonediimine intermediates, many of which are precursors of plasma soluble lipofuscins and melanoproteins, react with essential proteins, DNA, other macromolecules and can cause or contribute to renal and other tissue toxicity, haemolytic anaemia, neoplasia, and granular lipofuscin formation. The reactive oxygen species can also deplete antioxidants, damage essential proteins, DNA, and other macromolecules, and thereby injure cells and extracellular matrix.

Superoxide dismutase enhances the formation of hydroxyl radicals in the reaction of 3-hydroxyanthranilic acid with molecular oxygen

Superoxide dismutase (SOD) enhanced the formation of hydroxyl radicals, which were detected by using the e.s.r. spin-trapping technique, in a reaction mixture containing 3-hydroxyanthranilic acid (or p-aminophenol), Fe3+ ions, EDTA and potassium phosphate buffer, pH 7.4. The hydroxyl-radical formation enhanced by SOD was inhibited by catalase and desferrioxamine, and stimulated by EDTA and diethylenetriaminepenta-acetic acid, suggesting that both hydrogen peroxide and iron ions participate in the reaction. The hydroxyl-radical formation enhanced by SOD may be considered to proceed via the following steps. First, 3-hydroxyanthranilic acid is spontaneously auto-oxidized in a process that requires molecular oxygen and yields superoxide anions and anthranilyl radicals. This reaction seems to be reversible. Secondly, the superoxide anions formed in the first step are dismuted by SOD to generate hydrogen peroxide and molecular oxygen, and hence the equilibrium in the first step is displaced in favour of the formation of superoxide anions. Thirdly, hydroxyl radicals are generated from hydrogen peroxide through the Fenton reaction. In this Fenton reaction Fe2+ ions are available since Fe3+ ions are readily reduced by 3-hydroxyanthranilic acid. The superoxide anions do not seem to participate in the reduction of Fe3+ ions, since superoxide anions are rapidly dismuted by SOD present in the reaction mixture.

Lack of influence of vitamin D deficiency on insulin release from the isolated pancreatic islets of rats

Pancreatic islets were isolated from young (100 g) and adult (390 g), normal and vitamin D deficient male Sprague-Dawley rats. The release of insulin from leucine-stimulated or glucose-stimulated islet was not altered by vitamin D deficiency. The in vitro addition of either 25-hydroxy- or 1,25-dihydroxyvitamin-D had no effect on insulin release from either normal or vitamin D deficient islets. We conclude that the earlier report (Normal et al., Science 209 (1980) 823-825) on vitamin D deficiency depressing insulin secretion from the perfused pancreas must be related to the vitamin's effect on insulin synthesis and not the islet's release of insulin.