Cerebral cortex and lung indoleamine-2,3-dioxygenase activity is increased in type-D retrovirus infected macaques

Kynurenine Pathway in Respiratory Diseases

The kynurenine pathway is the primary route for tryptophan catabolism and has received increasing attention as its association with inflammation and the immune system has become more apparent. This review provides a broad overview of the kynurenine pathway in respiratory diseases, from the initial observations to the characterization of the different cell types involved in the synthesis of kynurenine metabolites and the underlying immunoregulatory mechanisms. With a focus on respiratory infections, the various attempts to characterize the kynurenine/tryptophan (K/T) ratio as an inflammatory marker are reviewed. Its implication in chronic lung inflammation and its exacerbation by respiratory pathogens is also discussed. The emergence of preclinical interventional studies targeting the kynurenine pathway opens the way for the future development of new therapies.

Neurological and Inflammatory Manifestations in Sjögren's Syndrome: The Role of the Kynurenine Metabolic Pathway

For decades, neurological, psychological, and cognitive alterations, as well as other glandular manifestations (EGM), have been described and are being considered to be part of Sjögren's syndrome (SS). Dry eye and dry mouth are major findings in SS. The lacrimal glands (LG), ocular surface (OS), and salivary glands (SG) are linked to the central nervous system (CNS) at the brainstem and hippocampus. Once compromised, these CNS sites may be responsible for autonomic and functional disturbances that are related to major and EGM in SS. Recent studies have confirmed that the kynurenine metabolic pathway (KP) can be stimulated by interferon-γ (IFN-γ) and other cytokines, activating indoleamine 2,3-dioxygenase (IDO) in SS. This pathway interferes with serotonergic and glutamatergic neurotransmission, mostly in the hippocampus and other structures of the CNS. Therefore, it is plausible that KP induces neurological manifestations and contributes to the discrepancy between symptoms and signs, including manifestations of hyperalgesia and depression in SS patients with weaker signs of sicca, for example. Observations from clinical studies in acquired immune deficiency syndrome (AIDS), graft-versus-host disease, and lupus, as well as from experimental studies, support this hypothesis. However, the obtained results for SS are controversial, as discussed in this study. Therapeutic strategies have been reexamined and new options designed and tested to regulate the KP. In the future, the confirmation and application of this concept may help to elucidate the mosaic of SS manifestations.

High-Fat Diet and Voluntary Chronic Aerobic Exercise Recover Altered Levels of Aging-Related Tryptophan Metabolites along the Kynurenine Pathway

Tryptophan metabolites regulate a variety of physiological processes, and their downstream metabolites enter the kynurenine pathway. Age-related changes of metabolites and activities of associated enzymes in this pathway are suggestable and would be potential intervention targets. Blood levels of serum tryptophan metabolites in C57BL/6 mice of different ages, ranging from 6 weeks to 10 months, were assessed using high-performance liquid chromatography, and the enzyme activities for each metabolic step were estimated using the ratio of appropriate metabolite levels. Mice were subjected to voluntary chronic aerobic exercise or high-fat diet to assess their ability to rescue age-related alterations in the kynurenine pathway. The ratio of serum kynurenic acid (KYNA) to 3-hydroxylkynurenine (3-HK) decreased with advancing age. Voluntary chronic aerobic exercise and high-fat diet rescued the decreased KYNA/3-HK ratio in the 6-month-old and 8-month-old mice groups. Tryptophan metabolites and their associated enzyme activities were significantly altered during aging, and the KYNA/3-HK ratio was a meaningful indicator of aging. Exercise and high-fat diet could potentially recover the reduction of the KYNA/3-HK ratio in the elderly.

Inflammation Models of Depression in Rodents: Relevance to Psychotropic Drug Discovery

Inflammation and depression are closely inter-related; inflammation induces symptoms of depression and, conversely, depressed mood and stress favor an inflammatory phenotype. The mechanisms that mediate the ability of inflammation to induce symptoms of depression are intensively studied at the preclinical level. This review discusses how it has been possible to build animal models of inflammation-induced depression based on clinical data and to explore critical mechanisms downstream of inflammation. Namely, we focus on the ability of inflammation to increase the activity of the tryptophan-degrading enzyme, indoleamine 2,3 dioxygenase, which leads to the production of kynurenine and downstream neuroactive metabolites. By acting on glutamatergic neurotransmission, these neuroactive metabolites play a key role in the development of depression-like behaviors. An important outcome of the preclinical research on inflammation-induced depression is the identification of potential novel targets for antidepressant treatments, which include targeting the kynurenine system and production of downstream metabolites, altering transport of kynurenine into the brain, and modulating glutamatergic transmission.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

Kynurenine 3-Monooxygenase: An Influential Mediator of Neuropathology

Mounting evidence demonstrates that kynurenine metabolism may play an important pathogenic role in the development of multiple neurological and neuropsychiatric disorders. The kynurenine pathway consists of two functionally distinct branches that generate both neuroactive and oxidatively reactive metabolites. In the brain, the rate-limiting enzyme for one of these branches, kynurenine 3-monooxygenase (KMO), is predominantly expressed in microglia and has emerged as a pivotal point of metabolic regulation. KMO substrate and expression levels are upregulated by pro-inflammatory cytokines and altered by functional genetic mutations. Increased KMO metabolism results in the formation of metabolites that activate glutamate receptors and elevate oxidative stress, while recent evidence has revealed neurodevelopmental consequences of reduced KMO activity. Together, the evidence suggests that KMO is positioned at a critical metabolic junction to influence the development or trajectory of a myriad of neurological diseases. Understanding the mechanism(s) by which alterations in KMO activity are able to impair neuronal function, and viability will enhance our knowledge of related disease pathology and provide insight into novel therapeutic opportunities. This review will discuss the influence of KMO on brain kynurenine metabolism and the current understanding of molecular mechanisms by which altered KMO activity may contribute to neurodevelopment, neurodegenerative, and neuropsychiatric diseases.

Disturbed Amino Acid Metabolism in HIV: Association with Neuropsychiatric Symptoms

Blood levels of the amino acid phenylalanine, as well as of the tryptophan breakdown product kynurenine, are found to be elevated in human immunodeficiency virus type 1 (HIV-1)-infected patients. Both essential amino acids, tryptophan and phenylalanine, are important precursor molecules for neurotransmitter biosynthesis. Thus, dysregulated amino acid metabolism may be related to disease-associated neuropsychiatric symptoms, such as development of depression, fatigue, and cognitive impairment. Increased phenylalanine/tyrosine and kynurenine/tryptophan ratios are associated with immune activation in patients with HIV-1 infection and decrease upon effective antiretroviral therapy. Recent large-scale metabolic studies have confirmed the crucial involvement of tryptophan and phenylalanine metabolism in HIV-associated disease. Herein, we summarize the current status of the role of tryptophan and phenylalanine metabolism in HIV disease and discuss how inflammatory stress-associated dysregulation of amino acid metabolism may be part of the pathophysiology of common HIV-associated neuropsychiatric conditions.

Kynurenic acid and kynurenine aminotransferases in retinal aging and neurodegeneration

The kynurenine aminotransferases (KATs) KAT I and KAT II are pivotal to the synthesis of kynurenic acid (KYNA), the only known endogenous glutamate receptor antagonist and neuroprotectant. KAT I and II have been found in avian, rodent, and human retina. Expression of KAT I in Müller cell endfeet and KAT II in retinal ganglion cells has been documented. Developmental changes in KAT expression and KYNA concentration in the avian and rodent retina have also been found. Studies of retinal neurodegeneration have shown alterations in KYNA synthesis in the retina in response to retinal ganglion cell loss. In DBA/2J mice, a model of ocular hypertension, an age-dependent decrease of retinal KYNA and KATs was found. In the corpora amylacea in the human retina intensive KAT I and II immunoreactivity was demonstrated. In summary, these findings point to the potential involvement of KYNA in the mechanisms of retinal aging and neurodegeneration.

Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

The neurodegenerative disease Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in early stages of the disease.

HCV patients, psychopathology and tryptophan metabolism: analysis of the effects of pegylated interferon plus ribavirin treatment

AIMS

Depression and other psychiatric disorders are frequent in HCV-infected patients, especially during interferon treatment. The molecular mechanism(s) underlying this finding is still unknown but it has been suggested that HCV and/or interferon administration may increase indoleamine 2,3-dioxygenase (IDO) activity, and reduce plasma tryptophan (TRP) levels and brain serotonin synthesis thus leading to psychopathological disorders.

METHODS

We studied 89 subjects: (a) 39 patients with chronic hepatitis C virus (HCV) infection and mild liver damage; (b) 39 healthy controls; and (c) 10 patients with chronic hepatitis B virus (HBV) infection. 15 of the patients with HCV infection were re-evaluated after antiviral treatment with pegylated interferon alpha-2a plus ribavirin leading to viral eradication. We measured serum TRP and kynurenine levels and IDO activity in macrophages. Furthermore, each patient had an accurate psychopathological evaluation.

RESULTS

HCV-infected patients had lower (-28%) serum TRP and kynurenine levels than healthy volunteers or HBV-infected patients with comparable liver damage. Depression and anxiety symptoms were particularly common in HCV patients. After viral clearance, macrophage IDO activity, plasma TRP and kynurenine levels returned toward normal values and psychopathology improved.

CONCLUSION

Our study shows that HCV patients have reduced serum TRP levels and confirms that they frequently suffer from anxiety and depression-related symptoms. The reduced IDO activity found in the macrophages of these patients suggests that HCV infection may hamper macrophage functions. After successful antiviral treatment, in spite of the expected increase of IDO activity in macrophages, we noticed that TRP and kynurenine plasma levels returned toward physiological levels and psychopathology decreased significantly.

Low serum tryptophan levels, reduced macrophage IDO activity and high frequency of psychopathology in HCV patients

Indoleamine 2,3-dioxygenase (IDO), a key enzyme of tryptophan (TRP) metabolism, is induced in various tissues of patients with bacterial and viral infection or with neoplastic diseases. This induction is considered the main cause of the decreased serum TRP levels, the reduced brain serotonin synthesis and the occurrence of psychopathological disorders often detected in patients with chronic infections or different forms of cancer. We studied 89 subjects including: (a) 39 patients with chronic hepatitis C virus (HCV) infection and mild liver damage (b) 40 healthy controls, and (c) 10 patients with chronic hepatitis B virus (HBV) infection. We measured serum TRP and kynurenine levels and IDO activity in macrophages. Furthermore, each patient had an accurate psychopathological evaluation. HCV-infected patients had lower (-28%) serum TRP concentrations than healthy volunteers or HBV-infected patients with comparable liver damage. Depression and anxiety symptoms were particularly common in HCV patients. Unexpectedly, serum kynurenine levels and IDO activity in cultured macrophages (under both basal or stimulated conditions) were lower in HCV patients than in controls. Our study shows that HCV patients have reduced serum TRP levels and confirms that they frequently suffer from anxiety and depression-related symptoms. The reduced IDO activity found in the macrophages of these patients suggest that HCV infection may hamper macrophage functions.

Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan

This paper reviews the preclinical and clinical evidence regarding the use of the dietary supplement 5-hydroxytryptophan (5-HTP) for the treatment of depression. In the absence of supplementation with exogenous 5-HTP, the amount of endogenous 5-HTP available for serotonin synthesis depends on the availability of tryptophan and on the activity of various enzymes, especially tryptophan hydroxylase, indoleamine 2,3-dioxygenase, and tryptophan 2,3-dioxygenase (TDO). Factors affecting each of these are reviewed. The amount of 5-HTP reaching the central nervous system (CNS) is affected by the extent to which 5-HTP is converted to serotonin in the periphery. This conversion is controlled by the enzyme amino acid decarboxylase, which, in the periphery, can be blocked by peripheral decarboxylase inhibitors (PDIs) such as carbidopa. Preclinical and clinical evidence for the efficacy of 5-HTP for depression is reviewed, with emphasis on double-blind, placebo-controlled (DB-PC) trials. Safety issues with 5-HTP are also reviewed, with emphasis on eosinophilia myalgia syndrome (EMS) and serotonin syndrome.

Optimised expression and purification of recombinant human indoleamine 2,3-dioxygenase

The hemoprotein indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting enzyme in mammalian tryptophan metabolism. It has received considerable attention in recent years, particularly due to its role in the pathogenesis of many diseases. Here, we report attempts to improve soluble expression and purification of hexahistidyl-tagged recombinant human IDO from Escherichia coli (EC538, pREP4, and pQE9-IDO). Significant formation of inclusion bodies was noted at the growth temperature of 37 degrees C, with reduced formation at 30 degrees C. The addition of the natural biosynthetic precursor of protoporphrin IX, delta-aminolevulinic acid (ALA), coupled with optimisation of IPTG induction levels during expression at 30 degrees C and purification by nickel-agarose and size exclusion chromatography, resulted in protein with 1 mol of heme/mol of protein and a specific activity of 160 micromol of kynurenine/h/mg of protein (both identical to native human IDO). The protein was homogeneous in terms of electrophoretic analysis. Yields of soluble protein (3-5 mg/L of bacterial culture) and heme content are greater than previously reported.

Asp274 and his346 are essential for heme binding and catalytic function of human indoleamine 2,3-dioxygenase

L-Tryptophan is the least abundant essential amino acid in humans. Indoleamine 2,3-dioxgyenase (IDO) is a cytosolic heme protein which, together with the hepatic enzyme tryptophan 2,3-dioxygenase, catalyzes the first and rate-limiting step in the major pathway of tryptophan metabolism, the kynurenine pathway. The physiological role of IDO is not fully understood but is of great interest, because IDO is widely distributed in human tissues, can be up-regulated via cytokines such as interferon-gamma, and can thereby modulate the levels of tryptophan, which is vital for cell growth. To identify which amino acid residues are important in substrate or heme binding in IDO, site-directed mutagenesis of conserved residues in the IDO gene was undertaken. Because it had been proposed that a histidine residue might be the proximal heme ligand in IDO, mutation to alanine of the three highly conserved histidines His16, His303, and His346 was conducted. Of these, only His346 was shown to be essential for heme binding, indicating that this histidine residue may be the proximal ligand and suggesting that neither His303 nor His16 act as the proximal ligand. Site-directed mutagenesis of Asp274 also compromised the ability of IDO to bind heme. This observation indicates that Asp274 may coordinate to heme directly as the distal ligand or is essential in maintaining the conformation of the heme pocket.

Alterations of kynurenic acid content in the retina in response to retinal ganglion cell damage

The present study is the first to examine the modulation of retinal kynurenic acid (KYNA) content in response to N-methyl-D-aspartate (NMDA)-induced cell death in adult rat retinal ganglion cells (RGC). Adult Brown Norway rats were intravitreally injected with NMDA or PBS. Surviving RGC were retrogradely labeled with fluorogold and counted in wholemounts of retinas 2, 7 and 14 days after injection. Retinal KYNA content was measured by HPLC at the same time points. RGC numbers decreased significantly 2, 7 and 14 days after NMDA injection if compared to control retinas. KYNA concentration increased significantly two days after NMDA-injection. However, 7 and 14 days after injection retinal KYNA content was found markedly decreased in NMDA-treated eyes as compared to controls. It is conceivable that KYNA deficiency is causally related to the pathology of excitotoxic retinal diseases.

Lipopolysaccharide induction of indoleamine 2,3-dioxygenase is mediated dominantly by an IFN-gamma-independent mechanism

Indoleamine 2,3-dioxygenase (IDO) is a rate-limiting enzyme in the L-tryptophan-kynurenine pathway, which converts an essential amino acid, L-tryptophan, to N-formylkynurenine. It has been speculated that IFN-gamma is a dominant IDO inducer in vivo. The present study used IFN-gamma or TNF-alpha gene-disrupted mice and IFN-gamma antibody-treated mice to demonstrate that lipopolysaccharide (LPS)-induced systemic IDO is largely dependent on TNF-alpha rather than IFN-gamma. IFN-gamma-independent IDO induction was also demonstrated in vitro with LPS-stimulated monocytic THP-1 cells. These findings clearly indicate that there is an IFN-gamma-independent mechanism of IDO induction in addition to the IFN-gamma-dependent mechanism.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Endogenous neurotoxins from tryptophan

In most tissues, including brain, a major proportion of the tryptophan which is not used for protein synthesis is metabolised along the kynurenine pathway. Long regarded as the route by which many mammals generate adequate amounts of the essential co-factor nicotinamide adenine dinucleotide, two components of the pathway are now known to have marked effects on neurones. Quinolinic acid is an agonist at the N-methyl-D-aspartate sensitive subtype of glutamate receptors in the brain, while kynurenic acid is an antagonist and, thus, a potential neuroprotectant. A third kynurenine, 3-hydroxykynurenine, is involved in the generation of free radicals which can also damage neurones. Quinolinic acid is increasingly implicated in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease, while kynurenic acid has become a standard for the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke.

Parallel decrease in neurotoxin quinolinic acid and soluble tumor necrosis factor receptor p75 in serum during highly active antiretroviral therapy of HIV type 1 disease

The chronic immune activation state in HIV disease leads to increased activity of the rate-limiting tryptophan-kynurenine pathway enzyme indoleamine 2,3-dioxygenase (2,3-IDO), thereby increasing the formation of neurotoxic tryptophan metabolites such as kynurenine and quinolinic acid. We investigated whether highly active antiretroviral therapy (HAART) (median duration, 100 days; range, 50-188 days) lowers serum levels of these metabolites in HIV-infected individuals and if so, whether this was paralleled by changes in a surrogate marker for immune activation, i.e., soluble tumor necrosis factor receptor p75 (sTNFR p75) concentrations. Baseline quinolinic acid (848 nM, 95% CI 567-1130 vs. 303 nM, 95% CI 267.1-339.5) and kynurenine (4.1 microM, 95% CI 3.3-4.9 vs. 2.7 microM, 95% CI 2.4-2.9) concentrations as well as the mean kynurenine-to-tryptophan ratio (108.2, 95% CI 76.1-140.4 vs. 51.4, 95% CI 47.6-55.3) in 17 HIV-1-infected outpatients (7 with AIDS) were significantly higher than those in 55 healthy age-matched controls (p < 0.01), respectively. Serum quinolinic acid concentrations in 14 of 17 patients decreased (mean, -44.4%) during HAART in comparison with baseline (471.2 nM, 95% CI 288-654.3; p = 0. 022). Thirteen of these 14 patients also had decreases in sTNFR p75 concentrations. Overall, the mean sTNFR p75 concentration decreased by 36.3% (13.5 ng/ml, 95% CI 9.3-17.8 vs. 8.6 ng/ml, 95% CI 5.9-11. 4; p = 0.01, n = 17). Reduction in viral load through HAART and subsequent mitigation of the pathological immune activation state in HIV disease may have reduced 2,3-IDO over activation. This eventually led to a decrease in quinolinic acid formation. The parallel reduction of the immune activation marker sTNFR p75 supports this hypothesis.

Expression and purification of recombinant human indoleamine 2, 3-dioxygenase

Indoleamine 2,3-dioxygenase, the first and rate-limiting enzyme in human tryptophan metabolism, has been implicated in the pathogenesis of many diseases. The human enzyme was expressed in Escherichia coli EC538 (pREP4) as a fusion protein to a hexahistidyl tag and purified to homogeneity in terms of electrophoretic and mass spectroscopic analysis, by a combination of phosphocellulose and nickel-agarose affinity chromatography. The yield of the fusion protein was 1.4 mg per liter of bacterial culture with an overall recovery of 56% from the crude extract. When the culture medium was supplemented with 7 microM hemin, the purified protein contained 0.8 mol of heme per mole of enzyme and exhibited an absorption spectrum consistent with the ferric form of hemoprotein. The pI value of the recombinant enzyme was 7.09 compared with 6.9 for the native enzyme. This was as expected from the addition of the hexahistidyl tag. Similar to the native enzyme, the recombinant enzyme required methylene blue and ascorbic acid for enzyme activity and oxidized not only l-tryptophan but also d-tryptophan and 5-hydroxy-l-tryptophan. The molecular activities for these substrates and their K(m) values were similar to those of the native enzyme, indicating that the addition of the hexahistidyl tag did not significantly affect catalytic activity. The recombinant protein can therefore be used to investigate properties of the native enzyme. This will aid the development of specific inhibitors of indoleamine 2,3-dioxygenase, which may be effective in halting disease progression.

Redox reactions related to indoleamine 2,3-dioxygenase and tryptophan metabolism along the kynurenine pathway

The heme enzyme indoleamine 2,3-dioxygenase (IDO) oxidizes the pyrrole moiety of L-tryptophan (Trp) and other indoleamines and represents the initial and rate-limiting enzyme of the kynurenine (Kyn) pathway. IDO is a unique enzyme in that it can utilize superoxide anion radical (O2*- ) as both a substrate and a co-factor. The latter role is due to the ability of O2*- to reduce inactive ferric-IDO to the active ferrous form. Nitrogen monoxide (*NO) and H2O2 inhibit the dioxygenase and various inter-relationships between the nitric oxide synthase- and IDO-initiated amino acid degradative pathways exist. Induction of IDO and metabolism of Trp along the Kyn pathway is implicated in a variety of physiological and pathophysiological processes, including anti-microbial and anti-tumor defense, neuropathology, immunoregulation and antioxidant activity. Antioxidant activity may arise from O2*- scavenging by IDO and formation of the potent radical scavengers and Kyn pathway metabolites, 3-hydroxyanthranilic acid and 3-hydroxykynurenine. Under certain conditions, these aminophenols and other Kyn pathway metabolites may exhibit pro-oxidant activities. This article reviews findings indicating that redox reactions are involved in the regulation of IDO and Trp metabolism along the Kyn pathway and also participate in the biological activities exhibited by Kyn pathway metabolites.

Analysis of transcription factors regulating induction of indoleamine 2,3-dioxygenase by IFN-gamma

IFN-gamma treatment of the human carcinoma cell line ME180 causes cell death due to induction of indoleamine 2,3-dioxygenase (IDO) and resulting starvation for tryptophan. A mutant cell line 3B6A derived from ME180 was resistant to IFN-gamma because of loss of IDO activity. Cotransfecting an IDO promoter-chloramphenicol acetyl transferase (CAT) construct with IFN regulatory factor-1 (IRF-1) resulted in induction of CAT activity in both ME180 and 3B6A cells even in the absence of IFN-gamma. This induction was reduced by cotransfection with IRF-2. However, IRF-1 was not able to restore IDO activity, suggesting a possible repressor site outside the IDO promoter region. Stat1alpha (p91) restored both CAT and IDO activities in 3B6A cells following IFN-gamma treatment. 3B6A cells doubly treated with IFN-gamma and IFN-alpha or IFN-beta restored IDO activity, although neither cytokine on its own could induce IDO. Western blot analysis showed that both constitutive expression and induction of Stat1alpha by IFN-gamma were reduced in 3B6A cells, and double treatment of IFN-gamma with IFN-alpha or IFN-beta restored the expression level of Statla. Electrophoretic mobility shift assays indicated that Stat1 binds to the IFN-gamma-activated sequence (GAS) region in the IDO promoter in ME180 cells following IFN-gamma treatment. Our results indicated that the defect in 3B6A cells was reduced expression of Stat1alpha and that IRF-1, NF-kappaB, and PKR were all involved to some extent in the induction of IDO following IFN-gamma treatment.

The anti-inflammatory effects of quinolinic acid in the rat

Quinolinic acid (QUIN) levels are elevated in patients and animals suffering from chronic infectious diseases. In the present study, male Sprague-Dawley rats were used to test the anti-inflammatory effects of QUIN using the carrageenan (CGN)-induced paw edema assay and the CGN sponge assay. Results of these studies indicate that QUIN (30, 100 or 300 mg/kg i.p.) caused a reduction of carrageenan-induced inflammation by as much as 80% at the highest dose. Moreover, QUIN reduced exudate volume and inhibited leukocyte migration in the sponge granuloma assay. In another experiment, the anti-inflammatory activity of QUIN was eliminated in adrenalectomized rats. QUIN did not reduce edema caused by arachidonic acid, bradykinin or compound 48/80. Neither morphine nor naloxone altered the anti-inflammatory activity of QUIN. These results may suggest that QUIN exerts its anti-inflammatory activity through a direct action on neutrophils or vascular permeability.

Serum kynurenine-to-tryptophan ratio increases with progressive disease in HIV-infected patients

An alternative pathway of Trp metabolism involves the conversion of Trp to kynurenine by indoleamine-2,3-dioxygenase, which leads to synthesis of the neurotoxin, quinolinic acid. This study explores the relationship of indoleamine-2,3-dioxygenase activity with stages of HIV infection. Sera from 206 HIV-positive and 72 seronegative subjects were analyzed for Trp and kynurenine. The kynurenine-to-Trp (KT) ratio was calculated. The mean KT ratio of seronegative controls was 36.6 ± 10.9, and the median ratio was 34.9. The upper limit of the seronegative KT ratio, defined as mean + 2 SD, was 58.4. Patients with HIV infection showed a reciprocal relationship between the KT ratio, the CD4 count, and the stage of the disease. The median KT ratios for asymptomatic and AIDS patients were 50.5 and 117.0, respectively. This study shows that the serum Trp concentration is markedly decreased and that the kynurenine concentration is increased with immune stimulation in HIV infection. This may lead to changes in quinolinic acid and explain some of the pathogenesis of AIDS dementia.

Frontal cortex indoleamine-2,3-dioxygenase activity is increased in HIV-1-associated dementia

HIV-associated dementia is a frequent consequence of HIV infection and relates to neuronal damage, possibly as a result of increased neurotoxic kynurenine metabolites. Indoleamine-2,3-dioxygenase (IDO) activity, which regulates kynurenine metabolism, may thus be increased in HIV infection. We measured IDO activity in post-mortem brain tissue from AIDS patients, including a subgroup that exhibited dementia, and age-matched control subjects. IDO activity was increased, but not significantly, in the AIDS group as well as the non-dementia group, compared to controls. Enzyme activity was significantly increased in the dementia group when compared to control values. IDO activity is increased in HIV-associated dementia and is thus likely to increase kynurenine pathway metabolites, such as 3-hydroxykynurenine and quinolinic acid, and elevated levels of these neurotoxins may contribute to the neuronal deficits underlying HIV-associated dementia.

Kynurenine pathway enzymes in brain: responses to ischemic brain injury versus systemic immune activation

Accumulation of L-kynurenine and quinolinic acid (QUIN) in the brain occurs after either ischemic brain injury or after systemic administration of pokeweed mitogen. Although conversion of L-[13C6]tryptophan to [13C6]-QUIN has not been demonstrated in brain either from normal gerbils or from gerbils given pokeweed mitogen, direct conversion in brain tissue does occur 4 days after transient cerebral ischemia. Increased activities of enzymes distal to indoleamine-2,3-dioxygenase may determine whether L-kynurenine is converted to QUIN. One day after 10 min of cerebral ischemia, the activities of kynureninase and 3-hydroxy-3,4-dioxygenase were increased in the hippocampus, but local QUIN levels and the activities of the indoleamine-2,3-dioxygenase and kynurenine-3-hydroxylase were unchanged. By days 2 and 4 after ischemia, however, the activities of all these enzymes in the hippocampus as well as QUIN levels were significantly increased. Kynurenine aminotransferase activity in the hippocampus was unchanged on days 1 and 2 after ischemia but was decreased on day 4, at a time when local kynurenic acid levels were unchanged. A putative precursor of QUIN, [13C6]anthranilic acid, was not converted to [13C6]QUIN in the hippocampus of either normal or 4-day post-ischemic gerbils. Gerbil macrophages stimulated by endotoxin in vitro converted L-[13C6]tryptophan to [13C6]QUIN. Kinetic analysis of kynurenine-3-hydroxylase activity in the cerebral cortex of postischemic gerbils showed that Vmax increased, without changes in Km. Systemic administration of pokeweed mitogen increased indoleamine-2,3-dioxygenase and kynureninase activities in the brain without significant changes in kynurenine-3-hydroxylase or 3-hydroxyanthranilate-3,4-dioxygenase activities. Increases in kynurenine-3-hydroxylase activity, in conjunction with induction of indoleamine-2,3-dioxygenase, kynureninase, and 3-hydroxyanthranilate-3,4-dioxygenase in macrophage infiltrates at the site of brain injury, may explain the ability of postischemic hippocampus to convert L-[13C6]tryptophan to [13C6]QUIN.

Induction of pterin synthesis is not required for cytokine-stimulated tryptophan metabolism

Activation of the immune system which occurs in inflammatory disease leads to parallel increases in pterin synthesis and increased production of neuroactive L-tryptophan metabolites. Several model systems were studied to determine whether pterins, which are cofactors for hydroxylation reactions, could be required in the oxidative kynurenine pathway of L-tryptophan degradation. Treatment of mice with interferon-gamma increased L-tryptophan metabolism without any corresponding change in tissue biopterin concentrations. Cytokine-treated human fibroblasts, macrophages and glioblastoma cells all showed increases in kynurenine production, which were completely independent of pterin synthesis. When pterin synthesis de novo was blocked, either by an inhibitor of GTP cyclohydrolase or because of a genetic deficiency of one of the enzymes of the pathway of pterin biosynthesis, cytokine-stimulated increases in tryptophan metabolism were unaffected. Furthermore, increasing intracellular tetrahydrobiopterin concentrations by treating cells with sepia-pterin also had no effect on markers of tryptophan metabolism. Therefore, both normal and cytokine-stimulated L-tryptophan metabolism appears to be completely independent of pterin biosynthesis.

Skin L-tryptophan-2,3-dioxygenase and rat hair growth

We have identified a new enzyme, skin L-tryptophan-2,3-dioxygenase (skin TDO), that catalyzes the degradation of L-tryptophan into formylkynurenine in rats. The rate of this degradation peaks in all rats at 5 to 6 weeks after birth, and also, among rats depilated at 8 weeks old, at 10 to 11 weeks after birth. We have also observed that the properties of this enzyme are closer to those of hepatic L-tryptophan-2,3-dioxygenase (hepatic TDO) than to indoleamine 2,3-dioxygenase. Although an intraperitoneal injection of L-tryptophan increased the activity of skin TDO to approximately 2.2 times greater than control values, an intraperitoneal injection of hydrocortisone and alpha-methyl-DL-tryptophan, both compounds known to affect hepatic TDO activity, had no effect on skin TDO activity. The molecular weight of skin TDO was estimated to be 16.0 kDa, which is close to the molecular weight of hepatic TDO, yet a much larger molecule than indoleamine-2,3-dioxygenase. Increased hair growth rates paralleled increased levels of skin TDO activity in 5- to 6-week-old rats, and marked increases in the activity of skin TDO occurred 2 or 3 weeks after depilation. Enzyme activity was also greatest 2 days before the time of maximum hair root length. Therefore, skin TDO may play an important role in the initiation or suppression of rat hair growth.

4-Chloro-3-hydroxyanthranilate, 6-chlorotryptophan and norharmane attenuate quinolinic acid formation by interferon-gamma-stimulated monocytes (THP-1 cells)

Accumulation of quinolinic acid and L-kynurenine occurs in the brain and/or blood following immune activation, and may derive from L-tryptophan following induction of indoleamine 2,3-dioxygenase and other kynurenine-pathway enzymes. In the present study a survey of various cell lines derived from either brain or systemic tissues showed that, while all cells examined responded to interferon-gamma by increased conversion of L-[13C6]tryptophan into L-kynurenine (human: B-lymphocytes, neuroblastoma, glioblastoma, lung, liver, kidney; rat brain: microglia, astrocytes and oligodendrocytes), only macrophage-derived cells (peripheral-blood mononuclear cells; THP-1, U-937) and certain liver cells (SKHep1) synthesized [13C6]quinolinic acid. Tumour necrosis factor-alpha enhanced the effects of interferon-gamma in THP-1 cells. Norharmane, 6-chloro-DL-tryptophan and 4-chloro-3-hydroxyanthranilate attenuated quinolinic acid formation by THP-1 cells with IC50 values of 51 microM, 58 microM and 0.11 microM respectively. Norharmane and 6-chloro-DL-tryptophan attenuated L-kynurenine formation with IC50 values of 43 microM and 51 microM respectively, whereas 4-chloro-3-hydroxyanthranilate had no effect on L-kynurenine accumulation. The reductions in L-kynurenine and quinolinic acid formation are consistent with the reports that norharmane is an inhibitor of indoleamine 2,3-dioxygenase, 6-chloro-DL-tryptophan is metabolized through the kynurenine pathway, and 4-chloro-3-hydroxyanthranilate is an inhibitor of 3-hydroxyanthranilate 3,4-dioxygenase. These results suggest that many tissues may contribute to the production of L-kynurenine following indoleamine 2,3-dioxygenase induction and immune activation. Quinolinic acid may be directly synthesized from L-tryptophan in both macrophages and certain types of liver cells, although uptake of quinolinic acid precursors from blood may contribute to quinolinic acid synthesis in cells that cannot convert L-kynurenine into quinolinic acid.

Mechanism of delayed increases in kynurenine pathway metabolism in damaged brain regions following transient cerebral ischemia

Delayed increases in the levels of an endogenous N-methyl-D-aspartate receptor agonist, quinolinic acid (QUIN), have been demonstrated following transient ischemia in the gerbil and were postulated to be secondary to induction of indoleamine-2,3-dioxygenase (IDO) and other enzymes of the L-tryptophan-kynurenine pathway. In the present study, proportional increases in IDO activity and QUIN concentrations were found 4 days after 10 min of cerebral ischemia, with both responses in hippocampus > striatum > cerebral cortex > thalamus. These increases paralleled the severity of local brain injury and inflammation. IDO activity and QUIN concentrations were unchanged in the cerebellum of postischemic gerbils, which is consistent with the preservation of blood flow and resultant absence of pathology in this region. Blood QUIN and L-kynurenine concentrations were not affected by ischemia. Brain tissue QUIN levels at 4 days postischemia exceeded blood concentrations, minimizing a role for breakdown of the blood-brain barrier. Marked increases in the activity of kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase were also detected in hippocampus but not in cerebellum on day 4 of recirculation. In vivo synthesis of [13C6]QUIN was demonstrated, using mass spectrometry, in hippocampus but not in cerebellum of 4-day postischemic animals 1 h after intracisternal administration of L-[13C6]tryptophan. However, accumulation of QUIN was demonstrated in both cerebellum and hippocampus of control gerbils following an intracisternal injection of 3-hydroxyanthranilic acid, which verifies the availability of precursor to both regions when administered intracisternally. Notably, although IDO activity and QUIN concentrations were unchanged in the cerebellum of ischemic gerbils, both IDO activity and QUIN content were increased in cerebellum to approximately the same degree as in hippocampus, striatum, cerebral cortex, and thalamus 24 h after immune stimulation by systemic pokeweed mitogen administration, demonstrating that the cerebellum can increase IDO activity and QUIN content in response to immune activation. No changes in kynurenic acid concentrations in either hippocampus, cerebellum, or cerebrospinal fluid were observed in the postischemic gerbils compared with controls, in accordance with the unaffected activity of kynurenine aminotransferase activity. Collectively, these results support roles for IDO, kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase in accelerating the conversion of L-tryptophan and other substrates to QUIN in damaged brain regions following transient cerebral ischemia. Immunocytochemical results demonstrated the presence of macrophage infiltrates in hippocampus and other brain regions that parallel the extent of these biochemical changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of immune activation on quinolinic acid and neuroactive kynurenines in the mouse

Accumulation of quinolinic acid and neuroactive kynurenines derived from tryptophan are of potential significance in human neuropathologic diseases because of their neurotoxic and convulsant properties. Clinical studies have established that sustained elevations of quinolinic acid, L-kynurenine and kynurenic acid within the cerebrospinal fluid occur in patients with a broad spectrum of inflammatory diseases and correlate with markers of immune activation and interferon-gamma activity. The present study describes an animal model that replicates these clinical observations and investigates the role of interferon-gamma as a mediator between immune activation and increased kynurenine pathway metabolism. Marked elevations in quinolinic acid, L-kynurenine and 3-hydroxykynurenine as well as an increased ratio of quinolinic acid: kynurenic acid in brain occurred 24 h after systemic pokeweed mitogen administration to C57BL6 mice. In plasma, L-tryptophan and kynurenic acid levels were reduced by pokeweed mitogen, while the concentrations of L-kynurenine, 3-hydroxykynurenine and quinolinic acid were increased. Interferon-gamma, pokeweed mitogen and lipopolysaccharide induced indoleamine-2,3-dioxygenase, the first enzyme of the kynurenine pathway, and increased both L-kynurenine and quinolinic acid concentrations of brain and systemic tissues, particularly in the lung, gastrointestinal tract and spleen. In contrast, hepatic tryptophan-2,3-dioxygenase activity was either reduced or unaffected. Increases in kynurenine pathway metabolism were sustained in mice given daily injections of interferon-gamma for seven days and subsequent responses to interferon-gamma were further enhanced. In contrast, daily administration of lipopolysaccharide was associated with subsequent attenuated responsiveness (tolerance) to lipopolysaccharide, pokeweed mitogen and interferon-gamma. Systemic administration of a monoclonal antibody to mouse interferon-gamma either attenuated or abolished the responses of kynurenine pathway metabolism to pokeweed mitogen and interferon-gamma. We conclude that acute and chronic increases in quinolinic acid and neuroactive kynurenines follow immune stimulation in mice, and result from indoleamine-2,3-dioxygenase induction. The results demonstrate that interferon-gamma is an important mediator between immune stimulation and indoleamine-2,3-dioxygenase induction. These increases in kynurenine pathway metabolism closely parallel the responses documented in patients with a broad spectrum of inflammatory diseases. Mice treated with immune stimuli are a useful model to investigate the relationships between immune activation and kynurenine pathway metabolism.

Inter-relationships between quinolinic acid, neuroactive kynurenines, neopterin and beta 2-microglobulin in cerebrospinal fluid and serum of HIV-1-infected patients

Quinolinic acid (QUIN) is an neurotoxic N-methyl-D-aspartate receptor agonist and an L-tryptophan metabolite of the kynurenine pathway. Increased concentrations of QUIN occur in both cerebrospinal fluid (CSF) and blood of patients infected with human immunodeficiency virus (HIV)-1, particularly those with neurologic disturbances. In the present study of HIV-1 infected patients in Walter Reed stages 4, 5 and 6, reductions in L-tryptophan accompanied proportional increases in L-kynurenine and QUIN in both serum and CSF. Further, close inter-correlations exist between QUIN kynurenic acid and L-kynurenine with both beta 2-microglobulin and neopterin in CSF and serum. These correlations support the hypotheses that the kynurenine pathway is activated in association with inflammation and induction of indoleamine-2,3-dioxygenase. There were no relationships between CSF QUIN, L-kynurenine or kynurenic acid with the ratio of serum:CSF albumin concentrations, which indicates that the increases in CSF QUIN, L-kynurenine or kynurenic acid were not dependent on a breakdown of the blood-brain barrier. Kynurenic acid is also a kynurenine pathway metabolite that can attenuate the excitotoxic effects of QUIN when present in higher molar concentrations. While CSF kynurenic acid levels were increased in HIV-1-infected patients, the magnitude of the increases were smaller than those of QUIN and the molar concentrations of kynurenic acid were consistently lower than QUIN by at least one order of magnitude. We conclude that immune activation increases the levels of neuroactive kynurenines within the central nervous system of HIV-1-infected patients secondary to activation of indoleamine-2,3-dioxygenase.

Neuroscience findings in AIDS: a review of research sponsored by the National Institute of Mental Health

1. The human immunodeficiency virus (HIV-1) infects cells in both the immune system and the brain, but these effects are not independent. 2. Research funded by the National Institute of Mental Health (NIMH) has been directed at identifying some of the mechanisms by which HIV-1 infects the brain, produces pathology, causes behavioral changes, and alters immune responses. 3. HIV-1-associated peptides have been shown to produce immunological changes without active virus present and there is also evidence that neurological damage may result not from direct viral action, by via excitotoxin production. 4. Rhesus macaque monkeys infected with simian immunodeficiency virus (SIV) are proving to be a useful model of the neurological and behavioral changes identified in human AIDS patients; behavioral changes observed in monkeys are similar to those seen in humans infected with HIV-1. 5. Studies examining the relationship between the brain and immune system are identifying the role that the macrophage cytokine interleukin-1 may play in suppressing T-lymphocyte activity by two pathways, both mediated by corticotropin releasing factor (CRF). 6. One pathway involves the pituitary-adrenal axis and release of glucocorticoids while the other involves direct interaction with the sympathetic noradrenergic nervous system, which has been shown to innervate T-lymphocytes in the spleen and thymus. 7. These observations are relevant because macrophages infected with HIV-1 infiltrate the brain and may release substances that damage the brain. 8. Stress may affect these pathways via the CRF-mediated release of glucocorticoids; a model of stress has also demonstrated that stress can suppress the cellular immune response.

Relationship of neurologic status in macaques infected with the simian immunodeficiency virus to cerebrospinal fluid quinolinic acid and kynurenic acid

Increased concentrations of the excitotoxin quinolinic acid (QUIN) have been implicated in the neurologic deficits and brain atrophy that may accompany infection with the human immunodeficiency virus type-1. Key neuropathologic features of the AIDS encephalitis are replicated in some macaques following infection with the simian immunodeficiency virus (SIV). In the present studies, cerebrospinal fluid (CSF) QUIN concentrations increased within 2 weeks following infection of 11 rhesus macaques (Macaca mulatta) with a neurotropic sooty mangabey isolate of the simian immunodeficiency virus (SIVsm) and were sustained to greater than 2 standard deviations above uninfected control macaques. Highest CSF QUIN concentrations (up to 400-fold above pre-inoculation levels) were observed in 6 SIVsm-infected macaques with motor and behavioral abnormalities during life, brain atrophy on MRI scan and inflammatory lesions within the brain and meninges. Four of the 6 neurologic macaques deteriorated rapidly within 12 weeks after inoculation and had substantially larger increases in CSF QUIN levels than 2 other neurologic macaques and 5 macaques without neurologic signs which survived for longer than 37 weeks. Increases in serum QUIN and CSF kynurenic acid also occurred but generally to a lesser degree than the increases in CSF QUIN. In some animals, increases in serum L-kynurenine concentrations and reductions in CSF and serum L-tryptophan occurred and were consistent with activation of indoleamine-2, 3-dioxygenase, the first enzyme of the kynurenine pathway in extrahepatic tissues. CSF QUIN exceeded serum QUIN in 8.8% of samples from macaques with neurologic signs, supporting increased QUIN synthesis within the central nervous system. Production of [13C6]QUIN was demonstrated in one SIVsm-infected macaque and one uninfected control macaque following an intracisternal injection of [13C6]L-tryptophan and suggests that L-tryptophan is a substrate for QUIN synthesis within the nervous system or meninges, although the cellular localization of QUIN synthesis remain to be determined. We conclude that increases in kynurenine pathway metabolism occur in SIV-infected macaques and are most prominent in macaques with neurologic signs. Macaques infected with SIV offer a model to investigate the relationship between the metabolism of neuroactive kynurenines and neurologic disturbances associated with retroviral infection.