Interferon (IFN)-γ-mediated inflammation and the kynurenine pathway in relation to bone mineral density: the Hordaland Health Study

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

A Review of the Evidence for Tryptophan and the Kynurenine Pathway as a Regulator of Stem Cell Niches in Health and Disease

Stem cells are ubiquitously found in various tissues and organs in the body, and underpin the body's ability to repair itself following injury or disease initiation, though repair can sometimes be compromised. Understanding how stem cells are produced, and functional signaling systems between different niches is critical to understanding the potential use of stem cells in regenerative medicine. In this context, this review considers kynurenine pathway (KP) metabolism in multipotent adult progenitor cells, embryonic, haematopoietic, neural, cancer, cardiac and induced pluripotent stem cells, endothelial progenitor cells, and mesenchymal stromal cells. The KP is the major enzymatic pathway for sequentially catabolising the essential amino acid tryptophan (TRP), resulting in key metabolites including kynurenine, kynurenic acid, and quinolinic acid (QUIN). QUIN metabolism transitions into the adjoining de novo pathway for nicotinamide adenine dinucleotide (NAD) production, a critical cofactor in many fundamental cellular biochemical pathways. How stem cells uptake and utilise TRP varies between different species and stem cell types, because of their expression of transporters and responses to inflammatory cytokines. Several KP metabolites are physiologically active, with either beneficial or detrimental outcomes, and evidence of this is presented relating to several stem cell types, which is important as they may exert a significant impact on surrounding differentiated cells, particularly if they metabolise or secrete metabolites differently. Interferon-gamma (IFN-γ) in mesenchymal stromal cells, for instance, highly upregulates rate-limiting enzyme indoleamine-2,3-dioxygenase (IDO-1), initiating TRP depletion and production of metabolites including kynurenine/kynurenic acid, known agonists of the Aryl hydrocarbon receptor (AhR) transcription factor. AhR transcriptionally regulates an immunosuppressive phenotype, making them attractive for regenerative therapy. We also draw attention to important gaps in knowledge for future studies, which will underpin future application for stem cell-based cellular therapies or optimising drugs which can modulate the KP in innate stem cell populations, for disease treatment.

Altered kynurenine pathway metabolism and association with disease activity in patients with systemic lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease accompanied by increased release of proinflammatory cytokines that are known to activate the indoleamine 2,3-dioxygenase (IDO-1) enzyme, which catalyzes the rate-limiting step of the kynurenine pathway (KP). This study aimed to measure KP metabolite levels in patients with SLE and investigate the relationship between disease activity, clinical findings, and KP. The study included 100 patients with SLE and 100 healthy controls. Serum tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxyanthranilic acid (3HAA), 3-hydroxykynurenine (3HK), quinolinic acid (QA) concentrations were measured with tandem mass spectrometry. Serum KYN, KYNA, 3HAA, 3HK, and QA levels of the patients with SLE were significantly higher than the control group. Serum QA levels were elevated in patients with neurological involvement (four patients with peripheral neuropathy and two patients with mononeuropathy), serum KYN levels and KYN/TRP ratio increased in patients with joint involvement, and serum KYN, 3HK, and 3HAA levels and the KYN/TRP ratio were increased in patients with renal involvement. Moreover, KYN and KYN/TRP ratios were positively correlated with the disease activity score. These findings indicated that imbalances in KP metabolites may be associated with the pathogenesis, activation, and clinical manifestations of SLE.

Application and prospect of metabolomics in the early diagnosis of osteoporosis: a narrative review

This paper reviews the application of metabolomics in the early diagnosis of osteoporosis in recent years. The authors searched electronic databases for the keywords "metabolomics", "osteoporosis" and "biomarkers", then analyzed the relationship between functional markers and osteoporosis using categorical summarization. Lipid metabolism, amino acid metabolism and energy metabolism are closely related to osteoporosis development and can become early diagnostic markers of the condition. However, the existing studies in metabolomics suffer from varying application methods, difficulty in identifying isomers, small study cohorts and insufficient research on metabolic mechanisms. Consequently, it is important for future research to focus on broadening and standardizing the scope of the application of metabolomics. High-quality studies on a large scale should also be conducted while promoting the early diagnosis of osteoporosis in a more precise, comprehensive and sensitive manner.

The Association of Tryptophan and Its Metabolites With Incident Hip Fractures, Mortality, and Prevalent Frailty in Older Adults: The Cardiovascular Health Study

Amino acids are the building blocks of proteins, and sufficient protein intake is important for skeletal health. We utilized stored serum from the Cardiovascular Health Study in 1992-1993 to examine the relationship between levels of the essential amino acid tryptophan (trp) and its oxidized and nonoxidized metabolites to risk for incident hip fractures and mortality over 12 years of follow-up. We included 131 persons who sustained a hip fracture during this time period and 131 without a hip fracture over these same 12 years of follow-up; 58% female and 95% White. Weighted multivariable Cox hazards models were used to estimate the hazard ratios (HR) and 95% confidence intervals (CI) of incident hip fracture associated with a one standard deviation (SD) higher trp or its metabolites exposure. Relative risk regression was used to evaluate the cross-sectional association of trp and its metabolites with frailty. Higher serum levels of trp were significantly associated with lower risk of incident hip fractures (HR = 0.75 per SD of trp (95% CI 0.57-0.99) but were not significantly associated with mortality or frailty status by Freid's frailty index. There were no statistically significant associations between any of the oxidized or nonoxidized products of trp with incident hip fractures (p ≥ 0.64), mortality (p ≥ 0.20), or cross-sectional frailty status (p ≥ 0.13) after multiple testing adjustment. Randomized clinical trials examining whether increasing trp intake is beneficial for osteoporosis are needed. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

The Role of the Kynurenine Pathway in the Pathophysiology of Frailty, Sarcopenia, and Osteoporosis

Tryptophan is an essential nutrient required to generate vitamin B3 (niacin), which is mainly involved in energy metabolism and DNA production. Alterations in tryptophan metabolism could have significant effects on aging and musculoskeletal health. The kynurenine pathway, essential in tryptophan catabolism, is modulated by inflammatory factors that are increased in older persons, a process known as inflammaging. Osteoporosis, sarcopenia, osteosarcopenia, and frailty have also been linked with chronically increased levels of inflammatory factors. Due to the disruption of the kynurenine pathway by chronic inflammation and/or changes in the gut microbiota, serum levels of toxic metabolites are increased and are associated with the pathophysiology of those conditions. In contrast, anabolic products of this pathway, such as picolinic acid, have demonstrated a positive effect on skeletal muscle and bone. In addition, physical activity can modulate this pathway by promoting the secretion of anabolic kynurenines. According to the evidence collected, kynurenines could have a promising role as biomarkers for osteoporosis sarcopenia, osteosarcopenia, and frailty in older persons. In addition, some of these metabolites could become important targets for developing new pharmacological treatments for these conditions.

Metabolome-Wide Mendelian Randomization Assessing the Causal Relationship Between Blood Metabolites and Bone Mineral Density

Mounting evidence has supported osteoporosis (OP) as a metabolic disorder. Recent metabolomics studies have discovered numerous metabolites related to bone mineral density (BMD). However, the causal effects of metabolites on BMD at distinct sites remained underexplored. Leveraging genome-wide association datasets, we conducted two-sample Mendelian randomization (MR) analyses to investigate the causal relationship between 486 blood metabolites and bone mineral density at five skeletal sites including heel (H), total body (TB), lumbar spine (LS), femoral neck (FN), and ultra-distal forearm (FA). Sensitivity analyses were performed to test the presence of the heterogeneity and the pleiotropy. To exclude the influences of reverse causation, genetic correlation, and linkage disequilibrium (LD), we further performed reverse MR, linkage disequilibrium regression score (LDSC), and colocalization analyses. In the primary MR analyses, 22, 10, 3, 7, and 2 metabolite associations were established respectively for H-BMD, TB-BMD, LS-BMD, FN-BMD, and FA-BMD at the nominal significance level (IVW, P < 0.05) and passing sensitivity analyses. Among these, one metabolite, androsterone sulfate showed a strong effect on four out of five BMD phenotypes (Odds ratio [OR] for H-BMD = 1.045 [1.020, 1.071]; Odds ratio [OR] for TB-BMD = 1.061 [1.017, 1.107]; Odds ratio [OR] for LS-BMD = 1.088 [1.023, 1.159]; Odds ratio [OR] for FN-BMD = 1.114 [1.054, 1.177]). Reverse MR analysis provided no evidence for the causal effects of BMD measurements on these metabolites. Colocalization analysis have found that several metabolite associations might be driven by shared genetic variants such as mannose for TB-BMD. This study identified some metabolites causally related to BMD at distinct sites and several key metabolic pathways, which shed light on predictive biomarkers and drug targets for OP.

The Imbalance of Cytokines and Lower Levels of Tregs in Elderly Male Primary Osteoporosis

INTRODUCTION

Osteoporosis (OP) is a debilitating disease that brings a heavy burden to individuals and society with reduced quality of life and lifespan. However, it's frequently overlooked and poorly studied in elderly male patients. Worse still, few anti-osteoporosis drugs are effective at the prevention and treatment of osteoporosis in men. It has been reported that the cells of bone and the immune system share common progenitors, cytokines and growth factors, and that reciprocal interactions occur during health and disease. Nevertheless, the role of immune system in OP is not fully understood, especially in male patients. Therefore, this study aimed to investigate molecular alterations in immune cells in men with OP and to identify immunomodulatory strategies with potential therapeutic value.

MATERIALS AND METHODS

A population of 121 men aged between 51 and 80 years old was recruited. Bone mineral density (BMD) was measured at the lumbar spine L1-4 and femoral neck using dual-energy X-ray absorptiometry (DXA). Twenty people were healthy, 66 people had osteopenia and 35 people had OP. Bone metabolic markers, Th1, Th2, Tregs and immune molecules were evaluated at the time of enrollment.

RESULTS

Smoking was a risk factor for OP. C-terminal crosslinking of type I collagen (β-CTX) and the ratio of receptor activator of nuclear factor-κB ligand (RANKL) to osteoprotegerin (OPG) were higher in OP group, which had lower 25-hydroxyvitamin D [25(OH)D] levels. OP had the higher levels of IL-6 and TNF-α and lower levels of IFN-γ and IL-10. CD4⁺CD25⁺CD127^-/low Tregs were significantly lower in the OP group. The imbalance of Th1/Th2 cells may play an important role in the development of OP. 25(OH)D may play essential roles in maintaining bone health. The low level of Tregs is also one of the underlying immune mechanism that leads to male primary OP.

CONCLUSION

The active function of osteoclasts and the decline in osteoblasts were characteristics of OP, and the imbalance in cytokines and lower levels of Tregs were observed in elderly male patients with primary OP.

Evaluating the toxicity of escalating dose of oral picolinic acid in Sprague-Dawley rats

Picolinic acid (PIC) is a byproduct of tryptophan catabolism through the kynurenine pathway, with anabolic effects on bone in vivo and in vitro. Hence, PIC has been nominated as a possible candidate to treat and/or prevent osteoporosis. However, the effective dose and toxicity of PIC are not known yet. To test the effect of escalating and very high doses of oral PIC, male Sprague-Dawley rats were gavaged PIC: Group 1 (n = 3) received incremental doses of 125, 250 and 500 mg/kg/day PIC on days 1, 3 and 5. Group 2 (n = 3) received 500 mg/kg BID (8 h apart; i.e. 1000 mg/kg/day) PIC on Day 1. Group 3 (n = 3) received 125 mg/kg/day PIC for seven consecutive days. Group 4 (n = 3) received 250 mg/kg/day PIC for seven consecutive days. Groups 1, 3 and 4 rats were euthanized on Day 8. Group 5 (n = 6) received 500 mg/kg/day PIC for two consecutive days and then once a week dose (Days 9, 16 and 23) of 500 mg/kg/dose PIC, until euthanasia (Day 30). Blood and cerebrospinal fluid (CSF) were sampled at euthanasia, and tissues showing abnormalities at necropsy underwent histopathology evaluation. All rats displayed some degree of mild hypercalcemia and hyperkalemia. Rats receiving high doses (500 or 1000 mg/kg/day) of PIC died or were euthanized on humane grounds within the first week after showing clinical neurological signs, with animals later revealed to have brain necrosis and hemorrhage at histopathology. Rats receiving lower doses (125 or 250 mg/kg/day) of PIC completed treatment course without apparent clinical adverse events. In summary, very high doses of PIC (≥500 mg/kg/day) were vascular-neurotoxic. Possible future experiments must consider significantly lower doses.

Role of Kynurenine Pathway in Oxidative Stress during Neurodegenerative Disorders

Neurodegenerative disorders are chronic and life-threatening conditions negatively affecting the quality of patients’ lives. They often have a genetic background, but oxidative stress and mitochondrial damage seem to be at least partly responsible for their development. Recent reports indicate that the activation of the kynurenine pathway (KP), caused by an activation of proinflammatory factors accompanying neurodegenerative processes, leads to the accumulation of its neuroactive and pro-oxidative metabolites. This leads to an increase in the oxidative stress level, which increases mitochondrial damage, and disrupts the cellular energy metabolism. This significantly reduces viability and impairs the proper functioning of central nervous system cells and may aggravate symptoms of many psychiatric and neurodegenerative disorders. This suggests that the modulation of KP activity could be effective in alleviating these symptoms. Numerous reports indicate that tryptophan supplementation, inhibition of KP enzymes, and administration or analogs of KP metabolites show promising results in the management of neurodegenerative disorders in animal models. This review gathers and systematizes the knowledge concerning the role of metabolites and enzymes of the KP in the development of oxidative damage within brain cells during neurodegenerative disorders and potential strategies that could reduce the severity of this process.

Kynurenines as a Novel Target for the Treatment of Malignancies

Malignancies are unquestionably a significant public health problem. Their effective treatment is still a big challenge for modern medicine. Tumors have developed a wide range of mechanisms to evade an immune and therapeutic response. As a result, there is an unmet clinical need for research on solutions aimed at overcoming this problem. An accumulation of tryptophan metabolites belonging to the kynurenine pathway can enhance neoplastic progression because it causes the suppression of immune system response against cancer cells. They are also involved in the development of the mechanisms responsible for the resistance to antitumor therapy. Kynurenine belongs to the most potent immunosuppressive metabolites of this pathway and has a significant impact on the development of malignancies. This fact prompted researchers to assess whether targeting the enzymes responsible for its synthesis could be an effective therapeutic strategy for various cancers. To date, numerous studies, both preclinical and clinical, have been conducted on this topic, especially regarding the inhibition of indoleamine 2,3-dioxygenase activity and their results can be considered noteworthy. This review gathers and systematizes the knowledge about the role of the kynurenine pathway in neoplastic progression and the findings regarding the usefulness of modulating its activity in anticancer therapy.

Kynurenine induces an age-related phenotype in bone marrow stromal cells

Advanced age is one of the important contributing factors for musculoskeletal deterioration. Although the exact mechanism behind this degeneration is unknown, it has been previously established that nutritional signaling plays a vital role in musculoskeletal pathophysiology. Our group established the vital role of the essential amino acid, tryptophan, in aging musculoskeletal health. With advanced age, inflammatory factors activate indoleamine 2,3-dioxygenase (IDO1) and accumulate excessive intermediate tryptophan metabolites such as Kynurenine (KYN). With age, Kynurenine accumulates and suppresses osteogenic differentiation, impairs autophagy, promotes early senescence, and alters cellular bioenergetics of bone marrow stem cells. Recent studies have shown that Kynurenine negatively impacts bone marrow stromal cells (BMSCs) and, consequently, promotes bone loss. Overall, understanding the mechanism behind BMSCs losing their ability for osteogenic differentiation can provide insight into the prevention of osteoporosis and the development of targeted therapies. Therefore, in this article, we review Kynurenine and how it plays a vital role in BMSC dysfunction and bone loss with age.

Tryptophan and Kynurenine Enhances the Stemness and Osteogenic Differentiation of Bone Marrow-Derived Mesenchymal Stromal Cells In Vitro and In Vivo

Aging tissues present a progressive decline in homeostasis and regenerative capacities, which has been associated with degenerative changes in tissue-specific stem cells and stem cell niches. We hypothesized that amino acids could regulate the stem cell phenotype and differentiation ability of human bone marrow-derived mesenchymal stromal cells (hBMSCs). Thus, we performed a screening of 22 standard amino acids and found that D-tryptophan (10 μM) increased the number of cells positive for the early stem cell marker SSEA-4, and the gene expression levels of OCT-4, NANOG, and SOX-2 in hBMSCs. Comparison between D- and L-tryptophan isomers showed that the latter presents a stronger effect in inducing the mRNA levels of Oct-4 and Nanog, and in increasing the osteogenic differentiation of hBMSCs. On the other hand, L-tryptophan suppressed adipogenesis. The migration and colony-forming ability of hBMSCs were also enhanced by L-tryptophan treatment. In vivo experiments delivering L-tryptophan (50 mg/kg/day) by intraperitoneal injections for three weeks confirmed that L-tryptophan significantly increased the percentage of cells positive for SSEA-4, mRNA levels of Nanog and Oct-4, and the migration and colony-forming ability of mouse BMSCs. L-kynurenine, a major metabolite of L-tryptophan, also induced similar effects of L-tryptophan in enhancing stemness and osteogenic differentiation of BMSCs in vitro and in vivo, possibly indicating the involvement of the kynurenine pathway as the downstream signaling of L-tryptophan. Finally, since BMSCs migrate to the wound healing site to promote bone healing, surgical defects of 1 mm in diameter were created in mouse femur to evaluate bone formation after two weeks of L-tryptophan or L-kynurenine injection. Both L-tryptophan and L-kynurenine accelerated bone healing compared to the PBS-injected control group. In summary, L-tryptophan enhanced the stemness and osteoblastic differentiation of BMSCs and may be used as an essential factor to maintain the stem cell properties and accelerate bone healing and/or prevent bone loss.

Picolinic Acid, a Catabolite of Tryptophan, Has an Anabolic Effect on Bone In Vivo

Fractures attributable to osteoporosis have a severe impact on our older population. Reports of side effects with commonly prescribed osteoporosis drugs have led to the investigation of new and safer treatments with novel mechanisms of action. Picolinic acid (PIC), a catabolite of tryptophan, induces in vitro osteogenic differentiation of mesenchymal stem cells. Here we demonstrate that PIC has an anabolic effect on bone in vivo by increasing bone formation, bone mass, and bone strength in normal and ovariectomized C57BL/6 mice. Activation of the osteogenic pathways triggered this osteoanabolic response without any cross-related effects on mineral absorption or calciotropic hormones. Because PIC was also well tolerated and absorbed with no side effects, it is an ideal potential candidate for the treatment of osteoporosis. © 2020 American Society for Bone and Mineral Research.

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.

The Role of Tryptophan Metabolites in Musculoskeletal Stem Cell Aging

Although aging is considered a normal process, there are cellular and molecular changes that occur with aging that may be detrimental to health. Osteoporosis is one of the most common age-related degenerative diseases, and its progression correlates with aging and decreased capacity for stem cell differentiation and proliferation in both men and women. Tryptophan metabolism through the kynurenine pathway appears to be a key factor in promoting bone-aging phenotypes, promoting bone breakdown and interfering with stem cell function and osteogenesis; however, little data is available on the impact of tryptophan metabolites downstream of kynurenine. Here we review available data on the impact of these tryptophan breakdown products on the body in general and, when available, the existing evidence of their impact on bone. A number of tryptophan metabolites (e.g., 3-hydroxykynurenine (3HKYN), kynurenic acid (KYNA) and anthranilic acid (AA)) have a detrimental effect on bone, decreasing bone mineral density (BMD) and increasing fracture risk. Other metabolites (e.g., 3-hydroxyAA, xanthurenic acid (XA), picolinic acid (PIA), quinolinic acid (QA), and NAD+) promote an increase in bone mineral density and are associated with lower fracture risk. Furthermore, the effects of other tryptophan breakdown products (e.g., serotonin) are complex, with either anabolic or catabolic actions on bone depending on their source. The mechanisms involved in the cellular actions of these tryptophan metabolites on bone are not yet fully known and will require further research as they are potential therapeutic targets. The current review is meant as a brief overview of existing English language literature on tryptophan and its metabolites and their effects on stem cells and musculoskeletal systems. The search terms used for a Medline database search were: kynurenine, mesenchymal stem cells, bone loss, tryptophan metabolism, aging, and oxidative stress.

Modulation of the Paracrine Kynurenic System in Bone as a New Regulator of Osteoblastogenesis and Bone Mineral Status in an Animal Model of Chronic Kidney Disease Treated with LP533401

An increase in the peripheral synthesis of serotonin and kynurenine, observed during the chronic kidney disease (CKD) course, is negatively associated with bone health. Serotonin and kynurenine are connected by the common precursor, tryptophan. LP533401 is an inhibitor of peripheral serotonin synthesis. This study aimed to establish if the inhibition of serotonin synthesis by LP533401 may affect the kynurenine pathway activity in bone tissue and its potential consequence with regard to osteogenesis and bone mineral status. Nephrectomized rats were treated with LP533401 at a dose of 30 and 100 mg/kg daily for eight weeks. Tryptophan and kynurenine concentrations were determined, and tryptophan 2,3-dioxygenase (TDO) expression was assessed. We discovered the presence of a TDO-dependent, paracrine kynurenic system in the bone of rats with CKD. Its modulation during LP533401 treatment was associated with impaired bone mineral status. Changes in TDO expression affecting the kynurenine pathway activity were related to the imbalance between peripheral serotonin and 25-hydroxyvitamin D. There were also close associations between the expression of genes participating in osteoblastogenesis and activation of the kynurenine pathway in the bones of LP53301-treated rats. Our results represent the next step in studying the role of tryptophan metabolites in renal osteodystrophy.

Immunosuppressive IDO in Cancer: Mechanisms of Action, Animal Models, and Targeting Strategies

Indoleamine 2, 3-dioxygenase 1 (IDO; IDO1; INDO) is a rate-limiting enzyme that metabolizes the essential amino acid, tryptophan, into downstream kynurenines. Canonically, the metabolic depletion of tryptophan and/or the accumulation of kynurenine is the mechanism that defines how immunosuppressive IDO inhibits immune cell effector functions and/or facilitates T cell death. Non-canonically, IDO also suppresses immunity through non-enzymic effects. Since IDO targeting compounds predominantly aim to inhibit metabolic activity as evidenced across the numerous clinical trials currently evaluating safety/efficacy in patients with cancer, in addition to the recent disappointment of IDO enzyme inhibitor therapy during the phase III ECHO-301 trial, the issue of IDO non-enzyme effects have come to the forefront of mechanistic and therapeutic consideration(s). Here, we review enzyme-dependent and -independent IDO-mediated immunosuppression as it primarily relates to glioblastoma (GBM); the most common and aggressive primary brain tumor in adults. Our group's recent discovery that IDO levels increase in the brain parenchyma during advanced age and regardless of whether GBM is present, highlights an immunosuppressive synergy between aging-increased IDO activity in cells of the central nervous system that reside outside of the brain tumor but collaborate with GBM cell IDO activity inside of the tumor. Because of their potential value for the in vivo study of IDO, we also review current transgenic animal modeling systems while highlighting three new constructs recently created by our group. This work converges on the central premise that maximal immunotherapeutic efficacy in subjects with advanced cancer requires both IDO enzyme- and non-enzyme-neutralization, which is not adequately addressed by available IDO-targeting pharmacologic approaches at this time.

Age-related increase of kynurenine enhances miR29b-1-5p to decrease both CXCL12 signaling and the epigenetic enzyme Hdac3 in bone marrow stromal cells

Mechanisms leading to age-related reductions in bone formation and subsequent osteoporosis are still incompletely understood. We recently demonstrated that kynurenine (KYN), a tryptophan metabolite, accumulates in serum of aged mice and induces bone loss. Here, we report on novel mechanisms underlying KYN's detrimental effect on bone aging. We show that KYN is increased with aging in murine bone marrow mesenchymal stem cells (BMSCs). KYN reduces bone formation via modulating levels of CXCL12 and its receptors as well as histone deacetylase 3 (Hdac3). BMSCs responded to KYN by significantly decreasing mRNA expression levels of CXCL12 and its cognate receptors, CXCR4 and ACKR3, as well as downregulating osteogenic gene RUNX2 expression, resulting in a significant inhibition in BMSCs osteogenic differentiation. KYN's effects on these targets occur by increasing regulatory miRNAs that target osteogenesis, specifically miR29b-1-5p. Thus, KYN significantly upregulated the anti-osteogenic miRNA miR29b-1-5p in BMSCs, mimicking the up-regulation of miR-29b-1-5p in human and murine BMSCs with age. Direct inhibition of miR29b-1-5p by antagomirs rescued CXCL12 protein levels downregulated by KYN, while a miR29b-1-5p mimic further decreased CXCL12 levels. KYN also significantly downregulated mRNA levels of Hdac3, a target of miR-29b-1-5p, as well as its cofactor NCoR1. KYN is a ligand for the aryl hydrocarbon receptor (AhR). We hypothesized that AhR mediates KYN's effects in BMSCs. Indeed, AhR inhibitors (CH-223191 and 3',4'-dimethoxyflavone [DMF]) partially rescued secreted CXCL12 protein levels in BMSCs treated with KYN. Importantly, we found that treatment with CXCL12, or transfection with an miR29b-1-5p antagomir, downregulated the AhR mRNA level, while transfection with miR29b-1-5p mimic significantly upregulated its level. Further, CXCL12 treatment downregulated IDO, an enzyme responsible for generating KYN. Our findings reveal novel molecular pathways involved in KYN's age-associated effects in the bone microenvironment that may be useful translational targets for treating osteoporosis.

Kynurenine suppresses osteoblastic cell energetics in vitro and osteoblast numbers in vivo

Aging is a progressive process associated with declining tissue function over time. Kynurenine, an oxidized metabolite of the essential amino acid tryptophan that increases in abundance with age, drives cellular processes of aging and dysfunction in many tissues, and recent work has focused on understanding the pathways involved in the harmful effects of kynurenine on bone. In this study, we sought to investigate the effects of controlled kynurenine administration on osteoblast bioenergetics, in vivo osteoblast abundance, and marrow fat accumulation. Additionally, as an extension of earlier studies with dietary administration of kynurenine, we investigated the effects of kynurenine on Hdac3 and NCoR1 expression and enzymatic deacetylase activity as potential mechanistic contributors to the effects of kynurenine on osteoblasts. Kynurenine administration suppressed cellular metabolism in osteoblasts at least in part through impaired mitochondrial respiration, and suppressed osteoblastic numbers in vivo with no concurrent effects on marrow adiposity. Deleterious effects of kynurenine treatment on osteoblasts were more pronounced in female models as compared to males. However, kynurenine treatment did not inhibit Hdac3's enzymatic deacetylase activity nor its repression of downstream glucocorticoid signaling. As such, future work will be necessary to determine the mechanisms by which increased kynurenine contributes to aging bone bioenergetics. The current study provides novel further support for the idea that kynurenine contributes to impaired osteoblastic function, and suggests that impaired matrix production by kynurenine-affected osteoblasts is attributed in part to impaired osteoblastic bioenergetics. As circulating kynurenine levels in increase with age, and human bone density inversely correlates with the serum kynurenine to tryptophan ratio, these mechanisms may have important relevance in the etiology and pathogenesis of osteoporosis in humans.

IDO activation, inflammation and musculoskeletal disease

The IDO/kynurenine pathway is now established as a major regulator of immune system function. The initial enzyme, indoleamine 2,3-dioxygenase (IDO1) is induced by IFNγ, while tryptophan-2,3-dioxygenase (TDO) is induced by corticosteroids. The pathway is therefore positioned to mediate the effects of systemic inflammation or stress-induced steroids on tissue function and its expression increases with age. Disorders of the musculoskeletal system are a common feature of ageing and many of these conditions are characterized by an inflammatory state. In inflammatory arthritis and related disorders, kynurenine protects against the development of disease, while inhibition or deletion of IDO1 increases its severity. The long-term regulation of autoimmune disorders may be influenced by the epigenetic modulation of kynurenine pathway genes, with recent data suggesting that methylation of IDO may be involved. Osteoporosis is also associated with abnormalities of the kynurenine pathway, reflected in an inversion of the ratio between blood levels of the metabolites anthranilic acid and 3-hydroxy-anthranilic acid. This review discusses evidence to date on the role of the IDO/kynurenine pathway and the highly prevalent age-related disorders of osteoporosis and rheumatoid arthritis and identifies key areas that require further research.

Circulating amino acids are associated with bone mineral density decline and ten-year major osteoporotic fracture risk in older community-dwelling adults

With aging, poor bone mineral density (BMD) and accelerated decrease in BMD are strong risk factors for fracture. Reports of the associations of dietary protein intake with bone strength are inconsistent, possibly owing to differences in protein sources and amino acid (AA) composition. We examined the associations of serum AA with 4-year hip BMD loss and subsequent fracture risk within 10 years in older community-dwelling adults, and further addressed whether lifestyle, dietary protein intake and its source, and body composition would affect the associations. In 1424 men and 1573 women (mean age 72 years), using binary logistic regression, higher serum valine, leucine, isoleucine and tryptophan concentrations were associated (or approaching a borderline significance in case of the last three ones) with less hip BMD decline (defined as BMD loss ≥ 2.8 times the precision error of the BMD measurement at femoral neck) in 4 years later, with the OR (95%CI) /SD of AA increase, ranging from 0.83 (0.75, 0.91) to 0.92 (0.87, 0.98) after multiple adjustments for baseline age, gender, BMI, BMD, estimated glomerular filtration rate (eGFR), dietary protein intake (animal- and plant-derived protein intakes), calcium intake, established lifestyles (physical activity level, smoking and alcohol drinking status), osteoporosis medications, and changes of body fat and lean muscle mass. Higher serum total homocysteine (tHcy) concentration was independently associated with BMD decline 4 years later (OR (95%CI) /SD of 1.16 (1.05, 1.27)). Using multivariate Cox regression, higher serum tryptophan concentration potentially predicted low risk of incident major osteoporotic fractures (MOFs) (HR/SD (95%CI)=0.86 (0.75, 0.98)) after multiple adjustments. Higher serum tHcy was associated with MOFs (HR/SD (95%CI)=1.29 (1.12, 1.50)) risk after multiple adjustments in men. These findings suggest that a specific AA profile correlates with greater BMD and lower subsequent fracture risk, independent of diet and lifestyle factors.

Clinical insights into the kynurenine pathway in age-related diseases

Accumulating evidence from diverse experiments, including heterochronic parabiosis-the surgical joining of two animals of different ages-has highlighted the importance of systemic factors in the progressive functional decline of various organs and tissues during aging. The major metabolic pathway of tryptophan, an essential amino acid in humans, is the kynurenine pathway (KP) in which indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) catalyze the conversion of tryptophan into kynurenine. Importantly, circulating kynurenine produced by this enzymatic breakdown, as a primary driver of the aging process, has been linked to higher mortality in humans. This review discusses the potential roles of tryptophan derivatives as biomarkers for the risk of frailty in the elderly, based on human observational studies as well as the KP as a therapeutic target for age-related diseases.

Elevated Systemic L-Kynurenine/L-Tryptophan Ratio and Increased IL-1 Beta and Chemokine (CX3CL1, MCP-1) Proinflammatory Mediators in Patients with Long-Term Titanium Dental Implants

Titanium is the mean biocompatible metal found in dental titanium alloys (Ti-6Al-4V). The safety of certain dental biomaterial amalgams has been questioned in patients. The levels of several systemic cytokines (interleukin (IL)-1 beta, IL-4: pg/mL) and chemokines (monocyte chemoattractant protein-1 (MCP-1), soluble fractalkine (CX3CL1: pg/mL) were determined using ELISA and compared between these study groups. The study included 30 controls without dental materials (cont), 57 patients with long-term titanium dental implants plus amalgams (A + I group) as well as 55 patients with long-term dental amalgam alone (A group). All patients (except controls) have had dental titanium implants (Ti-6Al-4V) and/or amalgams for at least 10 years (average: 15 years). We evaluated whether systemic levels of cytokines/chemokines, kyn/L-trp ratio and aromatic amino acid levels (HPLC: mM/L, Phe, L-Trp, His, Treo) could be altered in patients with long-term dental titanium and/or amalgams. These systemic markers were evaluated in 142 patients. The A + I group had higher L-Kynurenine/L-Tryptophan ratios than patients with long-term dental amalgam fillings alone (A). In addition, levels of IL-1 Beta cytokine, CX3CL1 and MCP-1 chemokines were higher in the A + I group than in the A group (A). The increased L-kyn/L-trp ratio and MCP-1 and fractalkine receptor (CX3CR1) elevations could suggest enhanced chemotactic responses by these chemokines in the A + I group.

The Association of Aromatic Amino Acids with Incident Hip Fracture, aBMD, and Body Composition from the Cardiovascular Health Study

In 5187 persons from the Cardiovascular Health Study, there was no significant association of dietary intakes of aromatic amino acids (AAA) with areal BMD of the hip or body composition. However, those who had the lowest dietary intakes of AAA were at increased risk for incident hip fractures. Prior studies of the association of protein intake with osteoporosis are conflicting and have not directly examined the relationship of aromatic amino acids (AAA) with fractures, areal bone mineral density (aBMD), and body composition. We sought to determine the relationship of dietary intakes of AAA with osteoporosis parameters in elderly men and women. 5187 men and women aged ≥ 65 years from the Cardiovascular Health Study (CHS) with dietary intakes of AAA (tryptophan, phenylalanine, tyrosine) estimated by food frequency questionnaire (FFQ) were included. We examined the relationship between a one-time estimate of daily dietary AAA intake with risk of incident hip fractures over a median of 13.2 years of fracture follow-up. A subset (n = 1336) who had dual energy X-ray absorptiometry (DXA) performed were included in a cross-sectional analysis of the association of dietary AAA intake with aBMD of the total hip and measurements of body composition. In multivariable models adjusted for demographic and clinical variables, medication use, and diet, higher dietary AAA intake was not significantly associated with incident hip fractures. All hazard ratios (HR) were less than one (tryptophan, HR 0.14, 95% CI 0.01 to 1.89; phenylalanine, HR 0.60, 95% CI 0.23 to 1.55; tyrosine, HR 0.59, 95% CI 0.27 to 1.32), but confidence intervals were wide and included no difference. However, in post hoc analyses, the lowest quartile of intake for each AAA was associated with an increased risk for hip fracture compared to higher quartiles (p ≤ 0.047 for all). Dietary AAA intakes were not significantly associated with total hip aBMD or any measurements of body composition. Overall, there was no significant association of dietary AAA intake with hip fractures, aBMD of the hip, or body composition. However, there may be a subset of elderly individuals with low dietary intakes of AAA who are at increased for hip fractures.

The Detrimental Effects of Kynurenine, a Tryptophan Metabolite, on Human Bone Metabolism

CONTEXT

Studies in aged mice support a role for kynurenine, a tryptophan metabolite, in age-induced bone loss; however, the role of kynurenine in human bone metabolism is not well understood.

OBJECTIVE

To assess whether the kynurenine level in bone marrow (BM) aspirates, directly reflecting the bone microenvironment, is associated with osteoporosis-related phenotypes and bone biochemical markers.

DESIGN AND SETTING

A case-control study conducted in a clinical unit.

PARTICIPANTS AND MAIN OUTCOME MEASURES

BM samples were collected from 72 patients at the time of hip surgery for either fragility hip fracture (HF) (n = 27) or for other causes (n = 45). In these samples, kynurenine was measured by liquid chromatography-tandem mass spectrometry, and the levels of tartrate-resistant acid phosphatase 5b (TRAP5b), bone-specific alkaline phosphatase (BSALP), receptor activator of nuclear factor-κB ligand (RANKL), and osteoprotegerin (OPG) were measured by immunoassay.

RESULTS

Age was positively correlated with BM kynurenine level. After adjustment for confounders, subjects with fragility HF had a 39.7% higher BM kynurenine level than those without, and the OR per SD increment in BM kynurenine level for fragility HF was 3.80. The BM kynurenine level was inversely associated with bone mass at the total femur. Higher kynurenine concentrations were significantly associated with higher TRAP-5b and RANKL levels, but not with BSALP and OPG levels, in BM plasma.

CONCLUSION

These results suggest that increased kynurenine levels during aging may contribute to the bone fragility seen in the elderly through increased bone resorption, with a resultant imbalance in bone remodeling.

Association between Mycobacterium avium complex lung disease and serum vitamin D status, antimicrobial peptide levels, and bone mineral density

Vitamin D maintains calcium balance and has immunomodulatory effects. Only few studies have revealed the relationship between vitamin D and its associated factors in Mycobacterium avium complex (MAC) infection. This study aimed to investigate the effects of MAC infection on serum vitamin D, human cationic antimicrobial protein 18, its C-terminal 37 amino acid fragment (hCAP18/LL-37) levels, and bone mineral density (BMD).We enrolled 58 patients with MAC lung disease and 15 control participants. Serum 25-hydroxyvitamin D and hCAP18/LL-37 levels were measured via enzyme-linked immunosorbent assay. Lastly, computed tomography scan density readings of the BMD of the thoracic and lumbar vertebral bones (Th4, Th7, Th10, and L1) were assessed.No significant differences in patient characteristics and serum vitamin D levels were observed. Patients with MAC lung disease had significantly low serum hCAP18/LL-37 levels (P = .049). Moreover, low BMD of the mean thoracic and lumbar vertebrae was observed (mean Th, P = .012; L1, P = .48, respectively). A higher prevalence of scoliosis (P = .031) was observed in the participants with low BMD compared with the control participants. Based on a multivariate analysis, patients with MAC lung disease had significantly lower body mass index [odds ratio (OR), 19.1; 95% confidence interval (CI), 2.0-419.0; P < .01] and vertebral BMD (OR, 12.4; 95% CI, 1.7-160.6; P = .012) than control participants.Serum hCAP18/LL-37 level and BMD were significantly decreased in patients with MAC lung disease without relation to serum vitamin D level. The vitamin D-independent pathway might affect the waning of antimicrobial peptides and decrease in BMD.

Kynurenine, a Tryptophan Metabolite That Accumulates With Age, Induces Bone Loss

Age-dependent bone loss occurs in humans and in several animal species, including rodents. The underlying causal mechanisms are probably multifactorial, although an age-associated increase in the generation of reactive oxygen species has been frequently implicated. We previously reported that aromatic amino acids function as antioxidants, are anabolic for bone, and that they may potentially play a protective role in an aging environment. We hypothesized that upon oxidation the aromatic amino acids would not only lose their anabolic effects but also potentially become a catabolic byproduct. When measured in vivo in C57BL/6 mice, the tryptophan oxidation product and kynurenine precursor, N-formylkynurenine (NFK), was found to increase with age. We tested the direct effects of feeding kynurenine (kyn) on bone mass and also tested the short-term effects of intraperitoneal kyn injection on bone turnover in CD-1 mice. μCT analyses showed kyn-induced bone loss. Levels of serum markers of osteoclastic activity (pyridinoline [PYD] and RANKL) increased significantly with kyn treatment. In addition, histological and histomorphometric studies showed an increase in osteoclastic activity in the kyn-treated groups in both dietary and injection-based studies. Further, kyn treatment significantly increased bone marrow adiposity, and BMSCs isolated from the kyn-injected mice exhibited decreased mRNA expression of Hdac3 and its cofactor NCoR1 and increased expression of lipid storage genes Cidec and Plin1. A similar pattern of gene expression is observed with aging. In summary, our data show that increasing kyn levels results in accelerated skeletal aging by impairing osteoblastic differentiation and increasing osteoclastic resorption. These data would suggest that kyn could play a role in age-induced bone loss. © 2017 American Society for Bone and Mineral Research.

Elevated Levels of Peripheral Kynurenine Decrease Bone Strength in Rats with Chronic Kidney Disease

The diagnosis and treatment of bone disorders in patients with chronic kidney disease (CKD) represent a clinical challenge. CKD leads to mineral and bone complications starting early in the course of renal failure. Recently, we have observed the positive relationship between intensified central kynurenine turnover and bone strength in rats with subtotal 5/6 nephrectomy (5/6 Nx)-induced CKD. The aim of the present study was to determine the association between peripheral kynurenine pathway metabolites and bone strength in rats with 5/6 Nx-induced CKD. The animals were sacrificed 1 and 3 months after 5/6 Nx or sham operation. Nephrectomized rats presented higher concentrations of serum creatinine, urea nitrogen, and parathyroid hormone both 1 and 3 months after nephrectomy. These animals revealed higher concentrations of kynurenine and 3-hydroxykynurenine in the serum and higher gene expression of aryl hydrocarbon receptor (AhR) as a physiological receptor for kynurenine and AhR-dependent cytochrome in the bone tissue. Furthermore, nephrectomy significantly increased the number of osteoclasts in the bone without affecting their resorptive activity measured in serum. These changes were particularly evident in rats 1 month after 5/6 Nx. The main bone biomechanical parameters of the tibia were unchanged between nephrectomized and sham-operated rats but were significantly increased in older compared to younger animals. A similar trend was observed for geometrical parameters measured with calipers, bone mineral density based on Archimedes' method and image of bone microarchitecture obtained from micro-computed tomography analyses of tibial cortical bone. In nephrectomized animals, peripheral kynurenine levels correlated negatively with the main parameters of bone biomechanics, bone geometry, and bone mineral density values. In conclusion, our data suggest that CKD-induced elevated levels of peripheral kynurenine cause pathological changes in bone structure via AhR pathway. This finding opens new opportunities for the treatment/prevention of osteoporosis in CKD.

Metabolites related to renal function, immune activation, and carbamylation are associated with muscle composition in older adults

Reduced skeletal muscle density in older adults is associated with insulin resistance, decreased physical function, and an increased all-cause mortality risk. To elucidate mechanisms that may underlie the maintenance of skeletal muscle density, we conducted a secondary analysis of previously published muscle composition and serum metabolomic data in 73 older adults (average age, 78y). Multivariable-adjusted linear regression was used to examine associations between 321 metabolites with muscle composition, defined as the ratio between normal density (NDM) with low density (LDM) thigh muscle cross sectional area (NDM/LDM). Sixty metabolites were significantly (p≤0.05 and q<0.30) associated with NDM/LDM. Decreased renal function and the immune response have been previously linked with reduced muscle density, but the mechanisms underlying these connections are less clear. Metabolites that were significantly associated with muscle composition were then tested for their association with circulating markers of renal function (blood urea nitrogen, creatinine, uric acid), and with the immune response (neutrophils/lymphocytes) and activation (kynurenine/tryptophan). 43 significant NDM/LDM metabolites (including urea) were co-associated with at least 1 marker of renal function; 23 of these metabolites have been previously identified as uremic solutes. The neutrophil/lymphocyte ratio was significantly associated with NDM/LDM (β±SE: -0.3±0.1, p=0.01, q=0.04). 35 significant NDM/LDM metabolites were co-associated with immune activation. Carbamylation (defined as homocitrulline/lysine) was identified as a pathway that may link renal function and immune activation with muscle composition, as 29 significant NDM/LDM metabolites were co-associated with homocitrulline/lysine, with at least 2 markers of renal function, and with kynurenine/tryptophan. When considering that elevated urea and uremic metabolites have been linked with an increased systemic microbial burden, that antimicrobial defense can be reduced in the presence of carbamylation, and that adipocytes can promote host defense, we propose the novel hypothesis that the age-related increase in adipogenesis within muscle may be a compensatory antimicrobial response to protect against an elevated microbial burden.

Metabolites Associated With Circulating Interleukin-6 in Older Adults

Background

Circulating levels of the pro-inflammatory cytokine interleukin-6 (IL-6) levels are elevated in older adults, but mechanisms are unclear. In the current study, we used an untargeted metabolomic approach to develop an improved understanding about mechanisms related to circulating IL-6 in older adults.

Methods

Serum IL-6 values were log-transformed to normalize its distribution. Multivariable-adjusted linear regression was used to examine the association between 324 serum metabolites with log IL-6. Backward elimination linear regression was used to develop a metabolite predictor set representative of log IL-6.

Results

Thirty-six metabolites were significantly associated (p < 0.05 and q < 0.30) with log IL-6 in 73 older adults (average age, 78 years). Metabolites related to tryptophan metabolism (kynurenine, 3-indoxyl sulfate, indoleacetate, indolepropionate, C-glycosyltryptophan), infectious burden (C-glycosyltryptophan, N6-carbamoylthreonyladenosine, 1-methylurate, N-formylmethionine, N1-methyladenosine, 3-indoxyl sulfate, bilirubin (E,E), indoleacetate, γ-CEHC, N-acetylneuraminate), aryl hydrocarbon receptor activation and cytochrome P450 (CYP) 1A expression (kynurenine, 3-indoxyl sulfate, indoleacetate, N6-carbamoylthreonyladenosine, bilirubin, 1-methylurate) were positively associated, whereas metabolites related to CYP-mediated ω-oxidation (adipate, 8-hydroxyoctanoate, azelate, sebacate, undecanedioate, γ-CEHC), and peroxisome proliferator activated receptor-alpha (PPAR-α) activation (13 + 9-HODE, bilirubin, 5-oxoproline, cholesterol, glycerate, uridine) were negatively associated with log IL-6. The use of backward elimination regression identified tyrosine, cysteine, uridine, bilirubin, N-formylmethionine, indoleacetate, and 3-indoxyl sulfate to collectively explain 51% of the variance inherent in log IL-6.

Conclusions

These data suggest roles for tryptophan metabolism, infectious burden, activation of host defense, and detoxification through CYP1A-mediated pathways in mechanisms related to elevated inflammation, whereas CYP-mediated ω-oxidation and PPAR-α activation may be related to decreased inflammation in older adults.

Comprehensive metabolic characterization of serum osteocalcin action in a large non-diabetic sample

Recent research suggested a metabolic implication of osteocalcin (OCN) in e.g. insulin sensitivity or steroid production. We used an untargeted metabolomics approach by analyzing plasma and urine samples of 931 participants using mass spectrometry to reveal further metabolic actions of OCN. Several detected relations between OCN and metabolites were strongly linked to renal function, however, a number of associations remained significant after adjustment for renal function. Intermediates of proline catabolism were associated with OCN reflecting the implication in bone metabolism. The association to kynurenine points towards a pro-inflammatory state with increasing OCN. Inverse relations with intermediates of branch-chained amino acid metabolism suggest a link to energy metabolism. Finally, urinary surrogate markers of smoking highlight its adverse effect on OCN metabolism. In conclusion, the present study provides a read-out of metabolic actions of OCN. However, most of the associations were weak arguing for a limited role of OCN in whole-body metabolism.

Potential Benefits of Ameliorating Metabolic and Nutritional Abnormalities in People With Profound Developmental Disabilities

Background:

People with profound developmental disabilities have some of the most severe neurological impairments seen in society, have accelerated mortality due to huge medical challenges, and yet are often excluded from scientific studies. They actually have at least 2 layers of conditions: (1) the original disability and (2) multiple under-recognized and underexplored metabolic and nutritional imbalances involving minerals (calcium, zinc, and selenium), amino acids (taurine, tryptophan), fatty acids (linoleic acid, docosahexaenoic acid, arachidonic acid, adrenic acid, Mead acid, plasmalogens), carnitine, hormones (insulinlike growth factor 1), measures of oxidative stress, and likely other substances and systems.

Summary:

This review provides the first list of metabolic and nutritional abnormalities commonly found in people with profound developmental disabilities and, based on the quality of life effects of similar abnormalities in neurotypical people, indicates the potential effects of these abnormalities in this population which often cannot communicate symptoms.

Key messages:

We propose that improved understanding and management of these disturbed mechanisms would enhance the quality of life of people with profound developmental disabilities. Such insights may also apply to people with other conditions associated with disability, including some diseases requiring stem cell implantation and living in microgravity.

A link between central kynurenine metabolism and bone strength in rats with chronic kidney disease

BACKGROUND

Disturbances in mineral and bone metabolism represent one of the most complex complications of chronic kidney disease (CKD). Serotonin, a monoamine synthesized from tryptophan, may play a potential role in bone metabolism. Brain-derived serotonin exerts a positive effect on the bone structure by limiting bone resorption and enhancing bone formation. Tryptophan is the precursor not only to the serotonin but also and primarily to kynurenine metabolites. The ultimate aim of the present study was to determine the association between central kynurenine metabolism and biomechanical as well as geometrical properties of bone in the experimental model of the early stage of CKD.

METHODS

Thirty-three Wistar rats were randomly divided into two groups (sham-operated and subtotal nephrectomized animals). Three months after surgery, serum samples were obtained for the determination of biochemical parameters, bone turnover biomarkers, and kynurenine pathway metabolites; tibias were collected for bone biomechanical, bone geometrical, and bone mass density analysis; brains were removed and divided into five regions for the determination of kynurenine pathway metabolites.

RESULTS

Subtotal nephrectomized rats presented higher serum concentrations of creatinine, urea nitrogen, and parathyroid hormone, and developed hypocalcemia. Several biomechanical and geometrical parameters were significantly elevated in rats with experimentally induced CKD. Subtotal nephrectomized rats presented significantly higher kynurenine concentrations and kynurenine/tryptophan ratio and significantly lower tryptophan levels in all studied parts of the brain. Kynurenine in the frontal cortex and tryptophan in the hypothalamus and striatum correlated positively with the main parameters of bone biomechanics and bone geometry.

DISCUSSION

In addition to the complex mineral, hormone, and metabolite changes, intensified central kynurenine turnover may play an important role in the development of bone changes in the course of CKD.

Inflammation, vitamin B6 and related pathways

The active form of vitamin B6, pyridoxal 5'-phosphate (PLP), serves as a co-factor in more than 150 enzymatic reactions. Plasma PLP has consistently been shown to be low in inflammatory conditions; there is a parallel reduction in liver PLP, but minor changes in erythrocyte and muscle PLP and in functional vitamin B6 biomarkers. Plasma PLP also predicts the risk of chronic diseases like cardiovascular disease and some cancers, and is inversely associated with numerous inflammatory markers in clinical and population-based studies. Vitamin B6 intake and supplementation improve some immune functions in vitamin B6-deficient humans and experimental animals. A possible mechanism involved is mobilization of vitamin B6 to the sites of inflammation where it may serve as a co-factor in pathways producing metabolites with immunomodulating effects. Relevant vitamin B6-dependent inflammatory pathways include vitamin B6 catabolism, the kynurenine pathway, sphingosine 1-phosphate metabolism, the transsulfuration pathway, and serine and glycine metabolism.

The kynurenine pathway of tryptophan degradation is activated during osteoblastogenesis

The mechanisms involved in the anabolic effect of interferon gamma (IFNγ) on bone have not been carefully examined. Using microarray expression analysis, we found that IFNγ upregulates a set of genes associated with a tryptophan degradation pathway, known as the kynurenine pathway, in osteogenic differentiating human mesenchymal stem cells (hMSC). We, therefore, hypothesized that activation of the kynurenine pathway plays a role in osteoblastogenesis even in the absence of IFNγ. Initially, we observed a strong increase in tryptophan degradation during osteoblastogenesis with and without IFNγ in the media. We next blocked indoleamine 2,3-dioxygenase-1 (IDO1), the most important enzyme in the kynurenine pathway, using a siRNA and pharmacological approach and observed a strong inhibition of osteoblastogenesis with a concomitant decrease in osteogenic factors. We next examined the bone phenotype of Ido1 knockout (Ido1(-/-)) mice. Compared to their wild-type littermates, Ido1(-/-) mice exhibited osteopenia associated with low osteoblast and high osteoclast numbers. Finally, we tested whether the end products of the kynurenine pathway have an osteogenic effect on hMSC. We identified that picolinic acid had a strong and dose-dependent osteogenic effect in vitro. In summary, we demonstrate that the activation of the kynurenine pathway plays an important role during the commitment of hMSC into the osteoblast lineage in vitro, and that this process can be accelerated by exogenous addition of IFNγ. In addition, we found that mice lacking IDO1 activity are osteopenic. These data therefore support a new role for the kynurenine pathway and picolinic acid as essential regulators of osteoblastogenesis and as potential new targets of bone-forming cells in vivo.

Analysis of serum metabolic profile by ultra-performance liquid chromatography-mass spectrometry for biomarkers discovery: application in a pilot study to discriminate patients with tuberculosis

BACKGROUND

Tuberculosis (TB) is a chronic wasting inflammatory disease characterized by multisystem involvement, which can cause metabolic derangements in afflicted patients. Metabolic signatures have been exploited in the study of several diseases. However, the serum that is successfully used in TB diagnosis on the basis of metabolic profiling is not by much.

METHODS

Orthogonal partial least-squares discriminant analysis was capable of distinguishing TB patients from both healthy subjects and patients with conditions other than TB. Therefore, TB-specific metabolic profiling was established. Clusters of potential biomarkers for differentiating TB active from non-TB diseases were identified using Mann-Whitney U-test. Multiple logistic regression analysis of metabolites was calculated to determine the suitable biomarker group that allows the efficient differentiation of patients with TB active from the control subjects.

RESULTS

From among 271 participants, 12 metabolites were found to contribute to the distinction between the TB active group and the control groups. These metabolites were mainly involved in the metabolic pathways of the following three biomolecules: Fatty acids, amino acids, and lipids. The receiver operating characteristic curves of 3D, 7D, and 11D-phytanic acid, behenic acid, and threoninyl-γ-glutamate exhibited excellent efficiency with area under the curve (AUC) values of 0.904 (95% confidence interval [CI]: 0863-0.944), 0.93 (95% CI: 0.893-0.966), and 0.964 (95% CI: 00.941-0.988), respectively. The largest and smallest resulting AUCs were 0.964 and 0.720, indicating that these biomarkers may be involved in the disease mechanisms. The combination of lysophosphatidylcholine (18:0), behenic acid, threoninyl-γ-glutamate, and presqualene diphosphate was used to represent the most suitable biomarker group for the differentiation of patients with TB active from the control subjects, with an AUC value of 0.991.

CONCLUSION

The metabolic analysis results identified new serum biomarkers that can distinguish TB from non-TB diseases. The metabolomics-based analysis provides specific insights into the biology of TB and may offer new avenues for TB diagnosis.

Increased inflammatory potential of diet is associated with bone mineral density among postmenopausal women in Iran

PURPOSE

Diet has been shown to be associated with bone mineral density (BMD); however, the inflammatory potential of diet in modulating BMD has not yet been studied.

METHODS

We examined the association between a newly developed dietary inflammatory index (DII) and BMD in a sample of postmenopausal Iranian women. In this cross-sectional study, 160 postmenopausal women aged 50-85 years were studied and their femoral neck and lumbar spine BMDs were measured by dual-energy X-ray absorptiometry. The DII was computed based on dietary intake assessed using a previously validated, 168-item semiquantitative food frequency questionnaire. Logistic and linear regression models were fit to derive beta estimates and odds ratios (ORs), with DII fit as continuous and as a dichotomous variable.

RESULTS

After adjusting for potential confounders, women with higher DII scores were more likely to have BMD below the median in the lumbar spine with the DII being used as both a continuous variable [ORcontinuous 1.64, 95 % confidence interval (CI) 1.11-2.43, p value = 0.01; one-unit increase corresponding to ≈17 % of its range in the current study] and a categorical variable (ORDII>-0.06/≤ 2.30, 95 % CI 1.05-5.07, p value = 0.04). Similar associations were observed when lumbar spine BMD was used as a continuous outcome. No significant association was observed with BMD in femoral neck, although the direction was along expected lines.

CONCLUSION

These data suggest a pro-inflammatory diet, as indicated by increasing DII score, may be a risk factor for lower BMD in lumbar spine in postmenopausal Iranian women.

Antidepressants may lead to a decrease in niacin and NAD in patients with poor dietary intake

The term niacin is the generic name for the two compounds nicotinic acid and nicotinamide, the major dietary precursors for two important coenzymes, nicotinamide adenine dinucleotide (NAD) and its phosphorylated form, NADP. Niacin is important for the maintenance of cellular integrity and energy production and is involved in more than 500 intracellular reactions. Deficiencies of niacin may contribute to neuropsychiatric and neurodegenerative disorders. Patients who develop nutritional deficiencies as a result of poor dietary intake, especially inadequate intake of proteins and vitamins, could potentially suffer from niacin deficiency and NAD depletion. However, de novo synthesis of niacin and NAD in the kynurenine pathway of tryptophan metabolism may compensate for impaired dietary intake. The rate of synthesis of NAD and niacin from tryptophan oxidation depends on the induction of the enzyme indoleamine 2,3-dioxygenase (IDO) by pro-inflammatory cytokines such as interferon-gamma. Niacin synthesis is not limited by a decrease in tryptophan and excessive IDO activity may therefore lead to a decline in tryptophan levels. Antidepressants have an anti-inflammatory effect, including reduction of interferon-gamma and therefore inhibition of IDO, the rate-limiting enzyme of the kynurenine pathway. In theory, this could account for increased serotonin as more tryptophan becomes available for serotonin synthesis. However, the downside may be that less NAD and niacin are synthesised downstream, which could exacerbate common psychiatric problems. It is our hypothesis that patients with poor dietary intake, who are treated with antidepressants, are at risk of developing niacin/NAD deficiency with possible development of associated neuropsychiatric symptoms. We therefore propose that niacin supplementation be considered in patients with inadequate diets who are treated with antidepressants. We believe that if this does not happen, a subclinical niacin deficiency may result, which would be difficult to detect as it would cause the same symptoms of the original illness (e.g. depression). Niacin deficiency should be considered and ruled out in all patients with treatment-resistant depression, who have a poor response to antidepressants. This is potentially a cost-effective and easy intervention, which could be examined in a randomized controlled trial.

Interferon gamma (IFN-γ)-mediated inflammation and the kynurenine pathway in relation to risk of hip fractures: the Hordaland Health Study

The cytokine interferon gamma (IFN-γ) stimulates neopterin release and tryptophan degradation into kynurenines through the kynurenine pathway. High levels of neopterin were associated with increased hip fracture risk, as were some of the kynurenines, suggesting a role of IFN-γ-mediated inflammation in the processes leading to hip fracture.

INTRODUCTION

Low-grade systemic inflammation has been associated with bone loss and risk of fractures. Interferon gamma (IFN-γ) initiates macrophage release of neopterin and also stimulates degradation of tryptophan along the kynurenine pathway as part of cell-mediated immune activation. Plasma neopterin and the kynurenine/tryptophan ratio (KTR) are thus markers of IFN-γ-mediated inflammation. Risk of hip fracture was investigated in relation to markers of inflammation and metabolites in the kynurenine pathway (kynurenines).

METHODS

Participants (71 to 74 years, N = 3,311) in the community-based Hordaland Health Study (HUSK) were followed for hip fractures from enrolment (1998-2000) until 31 December 2009. Plasma C-reactive protein (CRP), neopterin, KTR, and six kynurenines were investigated as predictors of hip fracture, using Cox proportional hazards regression analyses.

RESULTS

A hazard ratio (HR) of 1.9 (95% confidence interval (CI) 1.3-2.7) for hip fracture was found in the highest compared to the lowest quartile of neopterin (p trend across quartiles <0.001). CRP and KTR were not related to hip fracture risk. Among the kynurenines, a higher risk of fracture was found in the highest compared to the lowest quartiles of anthranilic acid and 3-hydroxykynurenine. For subjects in the highest quartiles of neopterin, CRP, and KTR compared to those in no top quartiles, HR was 2.5 (95% CI 1.6-4.0).

CONCLUSIONS

This may indicate a role for low-grade immune activation in the pathogenic processes leading to hip fracture.