IDO1/TDO dual inhibitor RY103 targets Kyn-AhR pathway and exhibits preclinical efficacy on pancreatic cancer

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzing the conversion of tryptophan (Trp) to kynurenine (Kyn) in kynurenine pathway (KP) is involved in the immunosuppression in pancreatic cancer (PC), but the value of IDO1 as an independent prognostic marker for PC is uncertain. Moreover, the correlation between tryptophan 2,3-dioxygenase (TDO), an isozyme of IDO1, and PC is largely unknown. Using TCGA database, the correlation between IDO1 and/or TDO expression and PC patients' survival was analyzed. The expressions of IDO1 and TDO in PC cells and PC mice were examined. The effects of IDO1, TDO or dual inhibition on IDO1 and TDO effector pathway (Aryl hydrocarbon receptor, AhR) and on migration and invasion of PC cells were investigated. The block effect of IDO1/TDO dual inhibitor RY103 on KP was evaluated. The preclinical efficacy of RY103 and its immunomodulatory effect on KPIC orthotopic PC mice and Pan02 tumor-bearing mice were explored. Results showed that IDO1/TDO co-expression is an independent prognostic marker for PC. RY103 can significantly block KP and target Kyn-AhR pathway to blunt the migration and invasion of PC cells, exhibit preclinical efficacy and ameliorate IDO1/TDO-mediated immunosuppression in PC mice.

The Role of Indoleamine 2,3-Dioxygenase in Diethylnitrosamine-Induced Liver Carcinogenesis

Indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing intracellular enzyme of the L-kynurenine pathway, causes preneoplastic cells and tumor cells to escape the immune system by inducing immune tolerance; this mechanism might be associated with the development and progression of human malignancies. In the present study, we investigated the role of IDO in diethylnitrosamine (DEN)-induced hepatocarcinogenesis by using IDO-knockout (KO) mice. To induce hepatocellular carcinoma (HCC), hepatic adenoma, and preneoplastic hepatocellular lesions termed foci of cellular alteration (FCA), male IDO-wild-type (WT) and IDO-KO mice with a C57BL/6J background received a single intraperitoneal injection of DEN at 2 weeks of age. The mice were sacrificed to evaluate the development of FCA and hepatocellular neoplasms. HCC overexpressed IDO and L-kynurenine compared to surrounding normal tissue in the DEN-treated IDO-WT mice. The number and cell proliferative activity of FCAs, and the incidence and multiplicity of HCC were significantly greater in the IDO-WT than in the IDO-KO mice. The expression levels of the IDO protein, of L-kynurenine, and of IFN-γ, COX-2, TNF-α, and Foxp3 mRNA were also significantly increased in the DEN-induced hepatic tumors that developed in the IDO-WT mice. The mRNA expression levels of CD8, perforin and granzyme B were markedly increased in hepatic tumors developed in IDO-KO mice. Moreover, Foxp3-positive inflammatory cells had infiltrated into the livers of DEN-treated IDO-WT mice, whereas fewer cells had infiltrated into the livers of IDO-KO mice. Induction of IDO and elevation of L-kynurenine might play a critical role in both the early and late phase of liver carcinogenesis. Our findings suggest that inhibition of IDO might offer a promising strategy for the prevention of liver cancer.

Danzhi Xiaoyao San ameliorates depressive-like behavior by shifting toward serotonin via the downregulation of hippocampal indoleamine 2,3-dioxygenase

ETHNOPHARMACOLOGICAL RELEVANCE

Danzhi Xiaoyao San (DXS) is a canonical Chinese medicine formula from Principles of Internal Medicine, which was written during the Ming dynasty. This formula is approved and commercialized for use in the prevention and treatment of affective disorders. This study is aimed to investigate the hypothesis that DXS treats depressive-like behavior by shifting the balance of the kynurenine (Kyn)/serotonin (5-HT) pathway toward the 5-HT pathway through the downregulation of hippocampal indoleamine 2,3-dioxygenase (IDO).

MATERIALS AND METHODS

Chemical fingerprints of gardenoside, paeoniflorin, ferulic acid, paeonol, and ligustilide in standard extraction were used as the material bases of DXS. Rats with depressive-like behavior induced by chronic unpredictable mild stress (CUMS) were randomly divided into four groups, namely the control, model, DXS, and fluoxetine groups. Cytokines, IDO, and tryptophan (Trp) catabolites were analyzed by enzyme-linked immunosorbent assay, western blot, and liquid chromatography-electrospray ionization tandem mass spectrometry, respectively.

RESULTS

DXS significantly increased crossing grid numbers, sucrose consumption, and body weight. This treatment significantly decreased the serum levels of tumor necrosis factor-α and interleukin 6 (IL-6). However, DXS elicited no significant effects on IL-1β, IL-2, and interferon γ. DXS downregulated the activity of IDO and subsequent production of Kyn in the hippocampus. This treatment upregulated the hippocampal contents of Trp and 5-HT but did not influence 5-HT turnover.

CONCLUSIONS

DXS exhibited antidepressant-like effects on rats exposed to CUMS. DXS reduced IDO activity to shift the balance of the Kyn/5-HT pathway toward the 5-HT pathway.

Involvement of the kynurenine pathway in human glioma pathophysiology

The kynurenine pathway (KP) is the principal route of L-tryptophan (TRP) catabolism leading to the production of kynurenine (KYN), the neuroprotectants, kynurenic acid (KYNA) and picolinic acid (PIC), the excitotoxin, quinolinic acid (QUIN) and the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD(+)). The enzymes indoleamine 2,3-dioxygenase-1 (IDO-1), indoleamine 2,3-dioxygenase-2 (IDO-2) and tryptophan 2,3-dioxygenase (TDO-2) initiate the first step of the KP. IDO-1 and TDO-2 induction in tumors are crucial mechanisms implicated to play pivotal roles in suppressing anti-tumor immunity. Here, we report the first comprehensive characterisation of the KP in 1) cultured human glioma cells and 2) plasma from patients with glioblastoma (GBM). Our data revealed that interferon-gamma (IFN-γ) stimulation significantly potentiated the expression of the KP enzymes, IDO-1 IDO-2, kynureninase (KYNU), kynurenine hydroxylase (KMO) and significantly down-regulated 2-amino-3-carboxymuconate semialdehyde decarboxylase (ACMSD) and kynurenine aminotransferase-I (KAT-I) expression in cultured human glioma cells. This significantly increased KP activity but significantly lowered the KYNA/KYN neuroprotective ratio in human cultured glioma cells. KP activation (KYN/TRP) was significantly higher, whereas the concentrations of the neuroreactive KP metabolites TRP, KYNA, QUIN and PIC and the KYNA/KYN ratio were significantly lower in GBM patient plasma (n = 18) compared to controls. These results provide further evidence for the involvement of the KP in glioma pathophysiology and highlight a potential role of KP products as novel and highly attractive therapeutic targets to evaluate for the treatment of brain tumors, aimed at restoring anti-tumor immunity and reducing the capacity for malignant cells to produce NAD(+), which is necessary for energy production and DNA repair.

Scavenging of reactive oxygen species by tryptophan metabolites helps Pseudomonas aeruginosa escape neutrophil killing

Pseudomonas aeruginosa is responsible for persistent infections in cystic fibrosis patients, suggesting an ability to circumvent innate immune defenses. This bacterium uses the kynurenine pathway to catabolize tryptophan. Interestingly, many host cells also produce kynurenine, which is known to control immune system homeostasis. We showed that most strains of P. aeruginosa isolated from cystic fibrosis patients produce a high level of kynurenine. Moreover, a strong transcriptional activation of kynA (the first gene involved in the kynurenine pathway) was observed upon contact with immune cells and particularly with neutrophils. In addition, using coculture of human neutrophils with various strains of P. aeruginosa producing no (ΔkynA) or a high level of kynurenine (ΔkynU or ΔkynA pkynA), we demonstrated that kynurenine promotes bacterial survival. In addition, increasing the amount kynurenine inhibits reactive oxygen species production by activated neutrophils, as evaluated by chemiluminescence with luminol or isoluminol or SOD-sensitive cytochrome c reduction assay. This inhibition is due neither to a phagocytosis defect nor to direct NADPH oxidase inhibition. Indeed, kynurenine has no effect on oxygen consumption by neutrophils activated by PMA or opsonized zymosan. Using in vitro reactive oxygen species-producing systems, we showed that kynurenine scavenges hydrogen peroxide and, to a lesser extent, superoxide. Kynurenine׳s scavenging effect occurs mainly intracellularly after bacterial stimulation, probably in the phagosome. In conclusion, the kynurenine pathway allows P. aeruginosa to circumvent the innate immune response by scavenging neutrophil reactive oxygen species production.

Cyclooxygenase-2 (COX-2) inhibition constrains indoleamine 2,3-dioxygenase 1 (IDO1) activity in acute myeloid leukaemia cells

Indoleamine 2,3-dioxygenase 1 (IDO1) metabolizes L-tryptophan to kynurenines (KYN), inducing T-cell suppression either directly or by altering antigen-presenting-cell function. Cyclooxygenase (COX)-2, the rate-limiting enzyme in the synthesis of prostaglandins, is over-expressed by several tumours. We aimed at determining whether COX-2 inhibitors down-regulate the IFN-g-induced expression of IDO1 in acute myeloid leukaemia (AML) cells. IFN-γ at 100 ng/mL up-regulated COX-2 and IDO1 in HL-60 AML cells, both at mRNA and protein level. The increased COX-2 and IDO1 expression correlated with heightened production of prostaglandin (PG)E₂ and kynurenines, respectively. Nimesulide, a preferential COX-2 inhibitor, down-regulated IDO1 mRNA/protein and attenuated kynurenine synthesis, suggesting that overall IDO inhibition resulted both from reduced IDO1 gene transcription and from inhibited IDO1 catalytic activity. From a functional standpoint, IFN-g-challenged HL-60 cells promoted the in vitro conversion of allogeneic CD4⁺CD25⁻ T cells into bona fide CD4⁺CD25⁺FoxP3⁺ regulatory T cells, an effect that was significantly reduced by treatment of IFN-γ-activated HL-60 cells with nimesulide. Overall, these data point to COX-2 inhibition as a potential strategy to be pursued with the aim at circumventing leukaemia-induced, IDO-mediated immune dysfunction.

Effects of 1-methyltryptophan stereoisomers on IDO2 enzyme activity and IDO2-mediated arrest of human T cell proliferation

IDO2 is a newly discovered enzyme with 43 % similarity to classical IDO (IDO1) protein and shares the same critical catalytic residues. IDO1 catalyzes the initial and rate-limiting step in the degradation of tryptophan and is a key enzyme in mediating tumor immune tolerance via arrest of T cell proliferation. The role of IDO2 in human T cell immunity remains controversial. Here, we demonstrate that similar to IDO1, IDO2 also degrades tryptophan into kynurenine and is inhibited more efficiently by Levo-1-methyl tryptophan (L-1MT), an IDO1 competitive inhibitor, than by dextro-methyl tryptophan (D-1MT). Although IDO2 enzyme activity is weaker than IDO1, it is less sensitive to 1-MT inhibition than IDO1. Moreover, our results indicate that human CD4(+) and CD8(+) T cell proliferation was inhibited by IDO2, but both L-1MT and D-1MT could not reverse IDO2-mediated arrest of cell proliferation, even at high concentrations. These data indicate that IDO2 is an inhibitory mechanism in human T cell proliferation and support efforts to develop more effective IDO1 and IDO2 inhibitors in order to overcome IDO-mediated immune tolerance.

Biomarkers of inflammation, immunosuppression and stress with active disease are revealed by metabolomic profiling of tuberculosis patients

Although tuberculosis (TB) causes more deaths than any other pathogen, most infected individuals harbor the pathogen without signs of disease. We explored the metabolome of >400 small molecules in serum of uninfected individuals, latently infected healthy individuals and patients with active TB. We identified changes in amino acid, lipid and nucleotide metabolism pathways, providing evidence for anti-inflammatory metabolomic changes in TB. Metabolic profiles indicate increased activity of indoleamine 2,3 dioxygenase 1 (IDO1), decreased phospholipase activity, increased abundance of adenosine metabolism products, as well as indicators of fibrotic lesions in active disease as compared to latent infection. Consistent with our predictions, we experimentally demonstrate TB-induced IDO1 activity. Furthermore, we demonstrate a link between metabolic profiles and cytokine signaling. Finally, we show that 20 metabolites are sufficient for robust discrimination of TB patients from healthy individuals. Our results provide specific insights into the biology of TB and pave the way for the rational development of metabolic biomarkers for TB.

Increased levels of kynurenine and kynurenic acid in the CSF of patients with schizophrenia

BACKGROUND

The kynurenic acid (KYNA) hypothesis for schizophrenia is partly based on studies showing increased brain levels of KYNA in patients. KYNA is an endogenous metabolite of tryptophan (TRP) produced in astrocytes and antagonizes N-methyl-D-aspartate and α7* nicotinic receptors.

METHODS

The formation of KYNA is determined by the availability of substrate, and hence, we analyzed KYNA and its precursors, kynurenine (KYN) and TRP, in the cerebrospinal fluid (CSF) of patients with schizophrenia. CSF from male patients with schizophrenia on olanzapine treatment (n = 16) was compared with healthy male volunteers (n = 29).

RESULTS

KYN and KYNA concentrations were higher in patients with schizophrenia (60.7 ± 4.37 nM and 2.03 ± 0.23 nM, respectively) compared with healthy volunteers (28.6 ± 1.44 nM and 1.36 ± 0.08 nM, respectively), whereas TRP did not differ between the groups. In all subjects, KYN positively correlated to KYNA.

CONCLUSION

Our results demonstrate increased levels of CSF KYN and KYNA in patients with schizophrenia and further support the hypothesis that KYNA is involved in the pathophysiology of schizophrenia.

Identification of novel kynurenine production-inhibiting benzenesulfonamide derivatives in cancer cells

Kynurenine (Kyn), a metabolite of tryptophan (Trp), is known to be a key regulator of human immune responses including cancer immune tolerance. Therefore, abrogation of Kyn production from cancer cells by small molecules may be a promising approach to anticancer therapy. Indeed, several small molecule inhibitors of indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in the catabolism of Trp to Kyn, exert antitumor effects in animal models. We screened our chemical libraries using a cell-based Kyn production assay to identify a new type of small molecules that regulate Kyn production, and for the first time identified a benzenesulfonamide derivative (compound 1) as a hit with the ability to inhibit Kyn production in interferon-γ (IFN-γ)-stimulated A431 and HeLa cells. Unlike the previously identified S-benzylisothiourea derivative, compound 2, compound 1 had little effect on the enzymatic activity of recombinant human IDO in vitro but suppressed the expression of IDO at the mRNA level in cells. Furthermore, compound 1 suppressed STAT1-dependent transcriptional activity and DNA binding, whereas no decrement in either the expression or phosphorylation level of STAT1 was observed. The inhibition of IDO expression by several benzenesulfonamide derivatives is associated with the suppression of STAT1. Thus, compound 1 and its analogs might be useful for analyzing the regulation of IDO activation, and STAT1-targeting could be an alternative to the IDO-directed approach for the regulation of Kyn levels by small molecules in the tumor microenvironment.

Molecular docking and spatial coarse graining simulations as tools to investigate substrate recognition, enhancer binding and conformational transitions in indoleamine-2,3-dioxygenase (IDO)

Indoleamine 2,3-dioxygenase (IDO) is an heme-containing enzyme involved in the regulation of important immunological responses and neurological processes. The enzyme catalyzes the oxidative cleavage of the pyrrole ring of the indole nucleus of tryptophan (Trp) to yield N-formylkynurenine, that is the initial and rate limiting step of the kynurenine pathway. Some indole derivatives have been reported to act as effectors of the enzyme by enhancing its catalytic activity. On the basis of the recent availability of the crystal structure of IDO, in this work we investigate substrate recognition and enhancer binding to IDO using molecular docking experiments. In addition, conformational transitions of IDO in response to substrate and enhancer binding are studied using coarse graining simulations with the program FIRST. The results enable us to identify (i) the binding site of enhancer modulators; (ii) the motion of an electrostatic gate that regulates the access of the substrate to the catalytic site of the enzyme; (iii) the movement of the anchoring region of a hairpin loop that may assist the shuttle of substrates/products to/from the catalytic site of IDO. These data, combined with available site-directed mutagenesis experiments, reveal that conformational transitions of IDO in response to substrate and enhancer binding are controlled by distinct combination of two conformational states (open and close) of the above structural motifs. On this basis, a molecular mechanism regarding substrate recognition and activity enhancement by indole derivatives is proposed.

Tryptophan metabolism in the central nervous system: medical implications

The metabolism of the amino acid L-tryptophan is a highly regulated physiological process leading to the generation of several neuroactive compounds within the central nervous system. These include the aminergic neurotransmitter serotonin (5-hydroxytryptamine, 5-HT), products of the kynurenine pathway of tryptophan metabolism (including 3-hydroxykynurenine, 3-hydroxyanthranilic acid, quinolinic acid and kynurenic acid), the neurohormone melatonin, several neuroactive kynuramine metabolites of melatonin, and the trace amine tryptamine. The integral role of central serotonergic systems in the modulation of physiology and behaviour has been well documented since the first description of serotonergic neurons in the brain some 40 years ago. However, while the significance of the peripheral kynurenine pathway of tryptophan metabolism has also been recognised for several decades, it has only recently been appreciated that the synthesis of kynurenines within the central nervous system has important consequences for physiology and behaviour. Altered kynurenine metabolism has been implicated in the pathophysiology of conditions such as acquired immunodeficiency syndrome (AIDS)-related dementia, Huntington's disease and Alzheimer's disease. In this review we discuss the molecular mechanisms involved in regulating the metabolism of tryptophan and consider the medical implications associated with dysregulation of both serotonergic and kynurenine pathways of tryptophan metabolism.

Kynurenine Pathway Enzymes in Dendritic Cells Initiate Tolerogenesis in the Absence of Functional IDO

Dendritic cell (DC) tryptophan catabolism has emerged in recent years as a major mechanism of peripheral tolerance. However, there are features of this mechanism, initiated by IDO, that are still unclear, including the role of enzymes that are downstream of IDO in the kynurenine pathway and the role of the associated production of kynurenines. In this study, we provide evidence that 1) murine DCs express all enzymes necessary for synthesis of the downstream product of tryptophan breakdown, quinolinate; 2) IFN-gamma enhances transcriptional expression of all of these enzymes, although posttranslational inactivation of IDO may prevent metabolic steps that are subsequent and consequent to IDO; 3) overcoming the IDO-dependent blockade by provision of a downstream quinolinate precursor activates the pathway and leads to the onset of suppressive properties; and 4) tolerogenic DCs can confer suppressive ability on otherwise immunogenic DCs across a Transwell in an IDO-dependent fashion. Altogether, these data indicate that kynurenine pathway enzymes downstream of IDO can initiate tolerogenesis by DCs independently of tryptophan deprivation. The paracrine production of kynurenines might be one mechanism used by IDO-competent cells to convert DCs lacking functional IDO to a tolerogenic phenotype within an IFN-gamma-rich environment.

Neuronal localization of indoleamine 2,3-dioxygenase in mice

Indoleamine 2,3-dioxygenase (IDO) catabolizes tryptophan to kynurenine. In the immune system, the reduction in tryptophan and increase in kynurenine act to suppress T-cell function. In the nervous system, kynurenine can be further metabolized to quinolinic acid, which can be neurotoxic. IDO is known to be expressed by microglia and its levels are upregulated by interferon-gamma (IFNgamma). We report here that IDO immunoreactivity is also localized in neurons, and that IDO is upregulated by IFNgamma in neurons of the hippocampus. Thus, neuronal IDO could contribute to the vulnerability of neurons to inflammatory conditions.

The kynurenine pathway of tryptophan degradation as a drug target

In mammalian cells, the essential amino acid tryptophan is degraded primarily by the kynurenine pathway, a cascade of enzymatic steps containing several biologically active compounds. Metabolites of this pathway, collectively termed 'kynurenines', have been shown to be involved in many diverse physiological and pathological processes. In particular, fluctuations in the levels of kynurenines have discrete effects on the nervous and immune systems. A considerable number of pharmacological tools have recently become available to probe the kynurenine pathway experimentally. Some of these 'kynurenergic' agents can be envisioned to be of therapeutic value, especially in the treatment of diseases that are associated with impaired kynurenine pathway metabolism.

Tryptophan as a link between psychopathology and somatic states

OBJECTIVE

Several somatic illnesses are associated with psychiatric comorbidity. Evidence is provided that availability of the essential amino acid tryptophan, which is the precursor of serotonin, may cause this phenomenon.

METHODS

We performed a database search to find relevant articles published between 1966 and 2002. For our search strategy, we combined several diseases from the categories hormonal, gastrointestinal, and inflammatory with the search terms "tryptophan" and "serotonin."

RESULTS

The catabolism of tryptophan is stimulated under the influence of stress, hormones and inflammation by the induction of the enzymes tryptophan pyrrolase (in the liver) and IDO (ubiquitous). Because of the reduction in blood levels of tryptophan under these circumstances the formation of cerebral serotonin is decreased.

CONCLUSIONS

It is argued that the coupling of peripheral tryptophan levels and cerebral serotonin levels has physiological significance. The clinical implications and therapeutic consequences of changes in tryptophan and consequently serotonin metabolism are discussed.

CTLA-4-Ig regulates tryptophan catabolism in vivo

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) plays a critical role in peripheral tolerance. However, regulatory pathways initiated by the interactions of CTLA-4 with B7 counterligands expressed on antigen-presenting cells are not completely understood. We show here that long-term survival of pancreatic islet allografts induced by the soluble fusion protein CTLA-4-immunoglobulin (CTLA-4-Ig) is contingent upon effective tryptophan catabolism in the host. In vitro, we show that CTLA-4-Ig regulates cytokine-dependent tryptophan catabolism in B7-expressing dendritic cells. These data suggest that modulation of tryptophan catabolism is a means by which CTLA-4 functions in vivo and that CTLA-4 acts as a ligand for B7 receptor molecules that transduce intracellular signals.

Increased levels of 3-hydroxykynurenine in different brain regions of rats with chronic renal insufficiency

Tryptophan (TRP) metabolism via the kynurenine pathway leads to formations of neuroactive substances like kynurenine (KYN) and 3-hydroxykynurenine (3-HK), which may be involved in the pathogenesis of several human brain diseases. 3-Hydroxykynurenine especially is known to have strong neurotoxic properties. The generation of reactive oxygen species (ROS) leads to neuronal cell death with apoptotic features. Because the chronic renal insufficiency (CRI) results in disturbances in the functioning of the central nervous system (CNS), it is conceivable that the metabolism of some kynurenines may be altered and could play an important role in uremic encephalopathy. The levels of TRP, KYN and 3-HK were measured in the plasma and in different brain regions of uremic rats. The total plasma concentration of TRP as well as in all the studied brain samples was significantly diminished during uremia. Surprisingly, the level of KYN and 3-HK were elevated both in the plasma and different brain regions of CRI animals. KYN concentrations were approximately two times higher in the cerebellum, midbrain and cortex compared to the control group. The changes of 3-HK levels were more pronounced in the striatum and medulla than in other structures. This data suggests that CRI results in deep disturbances on the kynurenine pathway in CNS, which could be responsible for neurological abnormalities seen in uremia.

Enzyme activities along the tryptophan-nicotinic acid pathway in alloxan diabetic rabbits

Recent data from our laboratory have indicated that the rabbit is a suitable animal model for the study of enzyme activities of the tryptophan-nicotinic acid pathway. We report here the pattern of tryptophan metabolism in rabbits made diabetic with alloxan treatment, and hypercholesterolemic with a high-cholesterol diet. A group of rabbits with only hypercholesterolemia was also considered. The enzymes assayed were: liver tryptophan 2,3-dioxygenase (TDO), intestine indoleamine 2,3-dioxygenase (IDO), liver and kidney kynurenine 3-monooxygenase, kynurenine-oxoglutarate transaminase, kynureninase, 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase.TDO showed a reduction of specific activity in liver of diabetic-hyperlipidemic and hyperlipidemic rabbits compared to controls. Intestine IDO activities and liver and kidney kynurenine monooxygenase were unchanged with respect to controls.Kynurenine-oxoglutarate transaminase and kynureninase activities were reduced in the kidneys, but not in the liver, of diabetic-hyperlipidemic rabbits. The main finding was the reduction of 3-hydroxyanthranilate 3,4-dioxygenase activity (expressed as activity per g of fresh tissue) in the liver and kidneys of diabetic-hypercholesterolemic and hyperlipidemic rabbits compared to controls. Conversely, aminocarboxymuconate-semialdehyde decarboxylase activity was significantly higher in diabetic hypercholesterolemic rabbits in comparison with control and hypercholesterolemic rabbits. These data demonstrate that also in diabetic rabbits there is an alteration of tryptophan metabolism at the level of 3-hydroxyanthranilic acid-->nicotinic acid step. Also dyslipidemia seems to be involved in enzyme activity variations of the tryptophan metabolism along the kynurenine pathway.

Novel protein modification by kynurenine in human lenses

It is known that human lenses increase in color and fluorescence with age, but the molecular basis for this is not well understood. We demonstrate here that proteins isolated from human lenses contain significant levels of the UV filter kynurenine covalently bound to histidine and lysine residues. Identification was confirmed by synthesis of the kynurenine amino acid adducts and comparison of the chromatographic retention times and mass spectra of these authentic standards with those of corresponding adducts isolated from human lenses following acid hydrolysis. Using calf lens proteins as a model, covalent binding of kynurenine to lens proteins has been shown to proceed via side chain deamination in a manner analogous to that observed for the related UV filter, 3-hydroxykynurenine O-beta-D-glucoside. Levels of histidylkynurenine and lysylkynurenine were low in human lenses in subjects younger than 30, but thereafter increased in concentration with the age of the individual. Post-translational modification of lens proteins by tryptophan metabolites therefore appears to be responsible, at least in part, for the age-dependent increase in coloration and fluorescence of the human lens, and this process may also be important in other tissues in which up-regulation of tryptophan catabolism occurs.

Expression of indoleamine 2,3-dioxygenase, tryptophan degradation, and kynurenine formation during in vivo infection with Toxoplasma gondii: induction by endogenous gamma interferon and requirement of interferon regulatory factor 1

The induction of indoleamine 2,3-dioxygenase (INDO) expression and the tryptophan (Trp)-kynurenine (Kyn) metabolic pathway during in vivo infection with Toxoplasma gondii was investigated. Decreased levels of Trp and increased formation of Kyn were observed in the lungs, brain, and serum from mice infected with T. gondii. Maximal INDO mRNA expression and enzyme activity were detected in the lungs at 10 to 20 days postinfection. Further, the induction of INDO mRNA expression, Trp degradation and Kyn formation were completely absent in tissues from mice deficient in IFN-gamma (IFN-gamma(-/-)) or IFN regulatory factor -1 (IRF-1(-/-)). These findings indicate the important role of endogenous IFN-gamma and IRF-1 in the in vivo induction of the Trp-Kyn metabolic pathway during acute infection with T. gondii. In contrast, expression of INDO mRNA and its activity was preserved in the tissues of TNF-receptor p55- or inducible nitric oxide synthase-deficient mice infected with T. gondii. Together with the results showing the extreme susceptibility of the IFN-gamma(-/-) and the IRF-1(-/-) mice to infection with T. gondii, our results indicate a possible involvement of INDO and Trp degradation in host resistance to early infection with this parasite.

Kynurenic acid metabolism in the brain of HIV-1 infected patients

Patients who are infected with human immunodeficiency virus type 1 (HIV-1) frequently present with neurological and psychiatric symptoms. Kynurenic acid (KYNA), an intermediate metabolite of L-kynurenine (L-KYN), is a neuroprotectant and a broad-spectrum antagonist at excitatory amino acid (EAA) receptors. The present study examines the biosynthetic machinery of KYNA in the frontal cortex and cerebellum of 25 HIV-1 and 16 control (CO) patients. We measured the contents of L-KYN and KYNA and the activity of enzymes synthesizing KYNA, kynurenine aminotransferases I and II (KAT I and KAT II). The KYNA level was significantly increased in the frontal cortex (209 +/- 38% of CO; p < 0.05) and moderately increased in the cerebellum (164 +/- 31% of CO) of HIV-1 brains as compared with controls. The bioprecursor of KYNA, L-KYN, was increased in frontal cortex (188 +/- 45% of CO) and cerebellum (151 +/- 16% of CO; p < 0.05). The elevated KYNA in frontal cortex correlated with significant increases of KAT I (341 +/- 95% of CO; p < 0.05) and KAT II (141 +/- 8% of CO; p < 0.05). In the cerebellum, a high KYNA content was in the line with increased KAT I (262 +/- 52% of CO; p < 0.05) activity, while KAT II was in a control range (85 +/- 12% of CO). This study demonstrates that HIV-1 infection associates with elevated KYNA synthesis in the brain. In contrast to KAT II, KAT I was prominently increased in both brain regions investigated. Differences in neurochemical parameters of KYNA metabolism between frontal cortex and cerebellum suggests selective tissue damage. Drugs which influence the synthesis of the endogenous neuroprotectant KYNA may become useful in the therapy of neuropsychiatric manifestations of HIV-1 infected patients.

Indoleamine 2,3-dioxygenase in the human lens, the first enzyme in the synthesis of UV filters

Tryptophan-derived UV filters have recently been shown to bind to human lens proteins. These UV filter adducts increase in amount with age and appear to be mainly responsible for the yellowing of the lens in man. On the basis of research performed in other tissues, it has been assumed that indoleamine 2,3-dioxygenase (IDO) may be the first and probably rate-limiting enzyme in UV filter biosynthesis. In this study, 25 human lenses were examined by a reliable and sensitive assay method with a monoclonal antibody specific for IDO. IDO activity was detected in all lenses ranging from 26 to 80 years, and there was no clear relationship of IDO activity with age. The mean activity was 0.85 +/- 0.49 nmol of kynurenine formed hr(-1)per lens. IDO expression was found to be localized in the anterior cortex of the lens with little or no activity in the posterior cortex or nucleus. The level in the iris/ciliary body was negligible (<0.05 nmol of kynurenine formed hr(-1)). The lens IDO activity is consistent with UV filter turnover values obtained previously. These findings indicate that IDO is the first enzyme in the UV filter pathway and that UV filter biosynthesis is active even in aged lenses. Yellowing of the aged lens may therefore be preventable by drug-induced suppression of lens IDO activity.

Combined inhibition of indoleamine 2,3-dioxygenase and nitric oxide synthase modulates neurotoxin release by interferon-gamma-activated macrophages

We evaluated the synthesis of nitric oxide (NO) and of the neurotoxic kynurenine metabolites 3OH-kynurenine and quinolinic acid (QUIN) in interferon-gamma (IFN-gamma)-activated macrophages of the murine BACl.2F5 cell line with the aim of investigating the roles of mononuclear phagocytes in inflammatory neurological disorders. IFN-gamma induced indoleamine 2,3-dioxygenase (IDO) and NO synthase (NOS) and increased the synthesis of 3OH-kynurenine, QUIN, and NO that accumulated in the incubation medium where they reached neurotoxic levels. Macrophage exposure to norharmane, an IDO inhibitor, resulted in a decreased formation of not only the kynurenine metabolites but also NO. The inhibition of NO synthesis could not be ascribed to reduced NADPH availability or decreased NOS induction. Norharmane inhibited NOS activity also in coronary vascular endothelial cells and in isolated aortic rings. Our findings suggest that activated macrophages release large amounts of neurotoxic molecules and that norharmane may represent a prototype compound to study macrophage involvement in inflammatory brain damage.

Early kynurenergic impairment in Huntington's disease and in a transgenic animal model

Several neuroactive metabolites of the kynurenine pathway of tryptophan degradation have been speculatively linked to the pathophysiology of Huntington's Disease (HD). Here we demonstrate that the levels of two of these metabolites, the free radical generator 3-hydroxykynurenine (3HK) and the neuroprotectant kynurenate (KYNA), are increased in the neostriatum of stage 1 HD patients and in the brain of mice transgenic for full-length mutant huntingtin. In both cases, the elevation in 3HK was far more pronounced, resulting in significant increases in the 3HK/KYNA ratios. These data suggest that abnormal kynurenine pathway metabolism may play a role during the early phases of the neurodegenerative process in HD.

Human placental indoleamine 2,3-dioxygenase: cellular localization and characterization of an enzyme preventing fetal rejection

In order to test the hypothesis (Munn, Zhou, Attwood, Bondarev, Conway, Marshall, Brown, Mellor, Science 281 (1998) 1191-1193) that localized placental tryptophan catabolism prevents immune rejection of the mammalian fetus, the cellular localization and characteristics of human placental indoleamine 2,3-dioxygenase (EC 1.13.11.42) were studied. The localization of indoleamine 2, 3-dioxygenase activity was determined quantitatively using cell fractionation by differential and discontinuous sucrose gradient centrifugation. Enzyme activity was looked for in isolated brush border microvillous plasma membranes of placental syncytiotrophoblast. We found that this membrane preparation (which showed a 32.4-fold purification from the starting homogenate with reference to the activity of a membrane marker enzyme, alkaline phosphatase (EC 3.1.3.1)) was strongly negatively enriched with indoleamine 2,3-dioxygenase (which showed a one twenty-fifth decrease in its specific activity). Placental indoleamine 2, 3-dioxygenase is thus not expressed in the maternal facing brush border membrane of syncytiotrophoblast. 1-Methyl-DL-tryptophan which was used by Munn et al. as a key experimental tool for inhibiting indoleamine 2,3-dioxygenase in the murine model showed a competitive inhibition of human placental indoleamine 2,3-dioxygenase with L-tryptophan. The hypothesis, based on experiments performed in mouse, may therefore be applicable to avoidance of immune rejection of the fetus in human pregnancy.

Tryptophan metabolism in pregnant sheep: increased fetal kynurenine production in response to maternal tryptophan loading

OBJECTIVE

The effects of a tryptophan load on the plasma concentration of kynurenine, the precursor for the production in the brain of the neuroactive products kynurenic acid and quinolinic acid, were determined in pregnant sheep at midgestation and late gestation and in nonpregnant sheep.

STUDY DESIGN

Pregnant ewes were given an intravenous infusion of 100 mg/kg L-tryptophan during 2 hours at 95 to 98 days' gestation (n = 4) or 135 to 138 days' gestation (n = 10). Nonpregnant ewes (n = 6) were studied in late estrus. Arterial blood samples taken from 2 hours before to 48 hours after the start of the infusion were used for analysis of plasma tryptophan, kynurenine, and cortisol concentrations.

RESULTS

Tryptophan loading at both gestational ages resulted in significantly greater increases in kynurenine concentrations in fetal plasma (at 95-98 days' gestation, from 5.7 +/- 1.2 micromol/L [baseline] to 247.9 +/- 86.7 micromol/L (peak); at 135-138 days' gestation, from 9.0 +/- 2.3 micromol/L [baseline] to 289.0 +/- 194.0 micromol/L [peak]) than in maternal plasma [at 95-98 days' gestation, from 4.6 +/- 0.8 micromol/L [baseline] to 118.0 +/- 79.7 micromol/L [peak]; at 135-138 days' gestation, from 4.8 +/- 2.9 micromol/L [baseline] to 98.3 +/- 67.8 micromol/L [peak]). It took longer for kynurenine concentrations to return to basal values in the fetus (24-30 hours) than in the ewe (8-12 hours). The kynurenine responses in pregnant and nonpregnant ewes were not different from each other.

CONCLUSION

The production of kynurenine from tryptophan is significantly greater in the fetal lamb than in the pregnant or nonpregnant adult ewe.

Regulation of dopa decarboxylase expression during colour pattern formation in wild-type and melanic tiger swallowtail butterflies

The eastern tiger swallowtail butterfly Papilio glaucus shows a striking example of Batesian mimicry. In this species, females are either wild type (yellow and black) or melanic (where most of the yellow colour is replaced by black). In order to understand how these different colour patterns are regulated, we examined the temporal order of wing pigment synthesis via precursor incorporation studies, enzyme assays, and in situ hybridisation to mRNA encoding a key enzyme, dopa decarboxylase. We show that dopa decarboxylase provides dopamine to both of the two major colour pigments, papiliochrome (yellow) and melanin (black). Interestingly, however, dopa decarboxylase activity is spatially and temporally regulated, being utilised early in presumptive yellow tissues and later in black. Further, in melanic females, both dopa decarboxylase activity and early papiliochrome synthesis are suppressed in the central forewing and this normally yellow area is later melanised. These results show that the regulation of enzyme synthesis observed in the yellow/black pattern of a single wing, is similar to that involved in melanism. We infer that dopa decarboxylase activity must be regulated in concert with downstream enzymes of either the melanin and/or the papiliochrome specific pathways, forming part of a developmental switch between yellow or black. This modification of multiple enzyme activities in concert is consistent with a model of melanisation involving coordinate regulation of the underlying synthetic pathways by a single Y-linked (female) factor.

Formation of N-formylkynurenine suggests the involvement of apolipoprotein B-100 centered tryptophan radicals in the initiation of LDL lipid peroxidation

Tryptophan oxidation products were determined in pronase E digests of apo B of delipidated, native and Cu2+ oxidized LDL using a sensitive and specific HPLC method. Oxidized LDL contained N-formylkynurenine, kynurenine and tryptamine but no oxindolylalanine and 5-hydroxytryptophan. N-Formylkynurenine increased from an initial value of 0.21 to 1.67 mol/mol apo B within 5 h. Apo B of native LDL also contained kynurenine (0.80 mol/mol) and tryptamine (0.13 mol/ mol). The results support the assumption that oxidation of Trp residues in apo B is an early event and possibly an elementary reaction involved in initiating LDL oxidation.

Ultraviolet filter compounds in human lenses: 3-hydroxykynurenine glucoside formation

Experiments have been conducted on various aspects of the biosynthetic pathway leading to the formation of 3-hydroxykynurenine glucoside (3OHKG), the major tryptophan-derived human lens UV filter compound. Measurable levels of the reactive metabolite, 3-hydroxykynurenine (3OHKyn) were found in normal human lenses, including lens nuclei. 3OHKyn was able to enter organ-cultured lenses and to serve as a substrate for production of 3OHKG. Thus lenticular 3OKHyn can potentially be derived either by metabolism of Trp in the lens or from transport into the lens. Lens incubation followed by dissection showed that the equatorial region was probably the major site of synthesis of 3OHKG. The concentration of 3OHKG was relatively constant along the visual axis but was significantly lower in the equatorial region: a pattern also found for Kyn and 3OHKyn. No evidence, however could be obtained for hydrolysis of 3OHKG by lens homogenates.

Quantification of L-tryptophan and L-kynurenine by liquid chromatography/electron capture negative ion chemical ionization mass spectrometry

In a number of infectious and inflammatory diseases, stimulation of the immune system can lead to increased accumulation of tryptophan metabolites via induction of kynurenine pathway enzymes in extrahepatic tissues. We developed a liquid chromatographic/mass spectrometric (LC/MS) method suitable for tracing the disposition of 13C isotopomers of L-tryptophan and L-kynurenine in various cultured cell, tissue slice, and whole animal model systems used to investigate tryptophan flux through the kynurenine pathway. The method employs extractive derivatization of the analytes and their 2H internal standards with pentafluorobenzyl bromide in order to enhance the negative ion chemical ionization (NICI) mass spectrometric response. Normal-phase liquid chromatographic separation of derivatized analytes was optimized using a silica column with organic solvents, followed by particle beam transfer and NICI-MS. Standard curves were linear over the range 1-250 ng per sample. Particle beam and mass spectrometric operating parameters were optimized with direct flow injections of 1-(methylamino) anthraquinone, which is an ideal test compound for the evaluation of LC/NICI-MS. The developed method was used to quantify the conversion of (13C6)L-tryptophan to (13C6)L-kynurenine by human monocytes (THP-1) stimulated with interferon-gamma, lung and brain tissue slices obtained from gerbils immune-stimulated with pokeweed mitogen. The effect of whole body immune stimulation on the plasma levels of endogenous L-kynurenine in mice stimulated with interferon-gamma was also quantified.

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.

Neuropharmacology of quinolinic and kynurenic acids

In a little more than 10 years, the kynurenine metabolites of tryptophan have emerged from their former position as biochemical curiosities, to occupy a prominent position in research on the causes and treatment of several major CNS disorders. The pathway includes two compounds, quinolinic acid and kynurenic acid, which are remarkably specific in their pharmacological profiles: one is a selective agonist at receptors sensitive to NMDA, whereas the other is a selective antagonist at low concentrations at the strychnine-resistant glycine modulatory site associated with the NMDA receptor. It has been argued that these agents cannot be of physiological or pathological relevance because their normal extracellular concentrations, in the nanomolar range, are at least 3 orders of magnitude lower than those required to act at NMDA receptors. This is a facile argument, however, that ignores at least two possibilities. One is that both quinolinate and kynurenate may be present in very high concentrations locally at some sites in the brain that cannot be reflected in mean extracellular levels. Similar considerations apply to many neuroactive agents in the CNS. The fact that both compounds appear to be synthesised in, and thus emerge from, glial cells that are well recognised as enjoying a close physical and chemical relationship with some neurones in which the intercellular space may be severely restricted may support such a view. Certainly the realisation that NMDA receptors may not be fully saturated functionally with glycine would be consistent with the possibility that even quite low concentrations of kynurenate could maintain a partial antagonism at the glycine receptor. A second possibility is that there may be a subpopulation of NMDA receptors (or, indeed, for a quite different amino acid) that possesses a glycine modulatory site with a much lower sensitivity to glycine or higher sensitivity to kynurenate, making it more susceptible to fluctuations of endogenous kynurenine levels. Whatever the specific nature of their physiological roles, the presence of an endogenous selective agonist and antagonist acting at NMDA receptors must continue to present exciting possibilities for understanding the pathological basis of several CNS disorders as well as developing new therapeutic approaches. An imbalance in the production or removal of either of these substances would be expected to have profound implications for brain function, especially if that imbalance were present chronically.(ABSTRACT TRUNCATED AT 400 WORDS)

UV filters in human lenses: tryptophan catabolism

Primate lenses are unique in that they convert tryptophan (trp) into 3-hydroxykynurenine glucoside (30HKG). This is the major short-wave absorbing pigment present in human lenses and it may play a role in protecting the eye from UV-induced photodamage. A study has been performed on aspects of this metabolic pathway in human lenses. A significant rate of synthesis could be observed in a 24-hr period using intact lenses to which radiolabelled tryptophan had been added. Label was found in kynurenine (Kyn), 3-hydroxykynurenine (30HKyn) and 30HKG, although always to the greatest extent in the latter metabolite. Considerable variation in the proportion of label incorporated into 30HKG was observed. Older lenses tended to accumulate a greater percentage into the glucoside; the data indicating a generally greater flux through the trp catabolic pathway in lenses above 60 years of age. Pulse-chase experiments on lens pairs suggested that there may be a significant loss or metabolism of 30HKG. Biosynthesis of 30HKG was found to take place in the lens epithelial cells. A linear rate of 30HKG efflux from organ cultured lenses was observed indicating that one pathway for removal of this compound involves diffusion through the lens capsule. That this pathway also occurs in vivo was confirmed by analysing samples of human vitreous humour. Based on efflux rates from cultured lenses (1.07 x 10(-3) +/- 0.293 x 10(-3) mumol hr-1, n = 5), half-life values for 30HKG in the lens ranging between 7 and 40 hr were calculated.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of [14C]-labeled 3-hydroxy-L-kynurenine in a butterfly, Heliconius charitonia L. (Heliconidae), and precursor studies in butterfly wing ommatins

A method was developed to produce radiolabeled 3-hydroxy-L-kynurenine by injection of [14C]-L-tryptophan into pupae of the heliconid butterfly, Heliconius charitonia, which was converted into [14C]-3-hydroxy-L-kynurenine and deposited as a wing pigment. Extractions of 3-hydroxykynurenine (3-OHK) with 60% methanol from wings yielded in 14.4 micrograms per mg dry weight. In extracts from yellow wing areas, 3-OHK represented 100% of detectable amino acids. Resulting specific radioactivity of [14C]-3-OHK was between 0.05 and 0.07 mCi/mmol when 0.5 microCi [14C]-tryptophan was injected into pupae 1 or 2 days before emergence of the butterfly. Incorporation of [14C]-3-OHK into wing ommochromes was studied in nymphalid butterflies, Araschnia levana and Precis coenia. After injection into pupae [14C]-3-OHK as well as [14C]-tryptophan were specifically incorporated into red and red-brown wing scales as shown by autoradiography. The same incorporation occurred in isolated wings after incubation in Grace's medium containing [14C]-3-OHK. In Araschnia levana, [14C]-3-OHK offered to left wing pairs was incorporated into dihydroxanthommatin six times more effectively than [14C]-tryptophan offered to right wing pairs from the same specimen. Therefore, 3-OHK seems to be the ultimate precursor of wing ommatins.

Alteration of mice L-tryptophan metabolism by the organophosphorous acid triester diazinon

Diazinon [O,O-diethyl O-(2-isopropyl-6-methyl-4- pyrimidinyl)phosphorothioate] altered the formation of several L-tryptophan metabolites associated with the L-kynurenine pathway in mice. Liver kynurenine formamidase was inhibited almost completely by diazinon (10 mg/kg). The enzyme inhibition resulted in reduced L-kynurenine biosynthesis in livers with a concomitant accumulation of N-formyl-L-kynurenine. In contrast to the liver, plasma L-kynurenine increased up to 5-fold in diazinon-treated mice. Consequently, the urinary excretion of xanthurenic acid and kynurenic acid was raised 5- to 15-fold. The revelation of this novel mechanism of diazinon action is an important piece of information needed for a better understanding of the noncholinergic toxicity of organophosphorous acid triesters and methylcarbamates.

Serotonin-GABA treatment is hypothesized for self-injury in Lesch-Nyhan syndrome

The self-injurious movements of Lesch-Nyhan patients are considered extremely severe and currently intractable. Lesch-Nyhan syndrome is a genetic disorder of purine metabolism resulting in several abnormalities, one of which is elevated levels of xanthine. The author reasons that elevated plasma xanthine sets off a chain of events that produce excessive endogenous convulsants and diminished endogenous anticonvulsants. Treatment is proposed for self-injury in Lesch-Nyhan that entails reducing production of two endogenous convulsants (kynurenine and quinolinic acid) and enhancing two endogenous anticonvulsants (serotonin and gamma aminobutyric acid).

Oxidative tryptophan metabolism in renal allograft recipients: increased kynurenine synthesis is associated with inflammation and OKT3 therapy

Serum concentrations of tryptophan (TRP) and kynurenine (KYN) were determined in renal allograft recipients (RAR) as an index of interferon-gamma-induced, indoleamine-dioxygenase-catalysed TRP degradation. Serum TRP and KYN in RAR during periods of stable graft function were typically within the normal range, however, the median values for serum KYN demonstrated significant increases 5-7 days prior to biopsy-confirmed acute rejection (1.6-fold, P less than 0.01) and on the day of biopsy (1.7-fold, P less than 0.001). Serum KYN was also markedly elevated in patients who contracted viral or Gram-negative bacterial infections in the absence of graft rejection. Serum KYN was not correlated with serum creatinine in RAR nor were serum TRP or KYN affected by antirejection therapy with high dose steroids. Retrospective analysis of intra-patient changes in serum KYN demonstrated that KYN monitoring was a useful adjunct to serum creatinine in the early detection of first acute rejection episodes. The first course of OKT3 therapy was associated with low serum TRP and significant increases in serum KYN (two- to three-fold) following the first three doses. The time course of these abnormalities corresponded to that over which many of the side effects of the OKT3 'first dose reaction' have been reported to occur. Significant changes in serum KYN were not observed in patients receiving repeat courses of OKT3 therapy. Significant decreases in serum TRP and significant increases in serum KYN were both prevalent and frequent in RAR during the first two postoperative months.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between interferon-gamma, indoleamine 2,3-dioxygenase, and tryptophan catabolism

Interferons have been shown to be potential anti-cancer agents and to inhibit tumor cell growth in culture. The in vivo mechanism of the anti-proliferative effect may be direct or indirect through the immune system; however, in vitro a primary mechanism of cytotoxicity is through the depletion of tryptophan. In particular, interferon-gamma (IFN-gamma) induces an enzyme of tryptophan catabolism, indoleamine 2,3-dioxygenase (IDO), which is responsible for conversion of tryptophan and other indole derivatives to kynurenine. The inhibitory effect of interferon on many intracellular parasites such as Toxoplasma gondii and Chlamydia trachomatis is by the same mechanism. Elevated kynurenine levels have been found in humans in a number of diseases and after interferon treatment, and the enzyme is induced in rodents after administration of interferon inducers, or influenza virus. IDO induction also occurs in vivo during rejection of allogeneic tumors, indicating a possible role for this enzyme in the tumor rejection process. The gene for IDO has been cloned and shown to be differentially regulated by IFN-alpha and IFN-gamma. IDO induction has been correlated with induction of GTP-cyclohydrolase, the key enzyme in pteridine biosynthesis. A direct role for IDO in pteridine synthesis has not been shown, and this parallel induction may reflect coordinate regulation of genes induced by IFN-gamma. A possible role for IDO in O2-radical scavenging and in inflammation is discussed.

Implications of interferon-induced tryptophan catabolism in cancer, auto-immune diseases and AIDS

Tryptophan (Trp) is an indispensable amino acid required for biosynthesis of proteins, serotonin and niacin. Indoleamine 2,3-dioxygenase (IDO) is induced by infections, viruses, lipopolysaccharides, or interferons (IFNs) and this results in significant catabolism of Trp along the kynurenine (Kyn) pathway. Intracellular growth of Toxoplasma gondii and Chlamydia psittaci in human fibroblasts in vitro is inhibited by IFN-gamma and this inhibition is negated by extra Trp in the medium. Similarly, growth of a number of human cell lines in vitro is inhibited by IFN-gamma and addition of extra Trp restores growth. Thus, in some in vitro systems, antiproliferative effects of IFN-gamma are mediated by induced depletion of Trp. We find that cancer patients given Type I or Type II IFNs can induce IDO which results in decreased serum Trp levels (20-50% of pretreatment) and increased urinary metabolites of the Kyn pathway (5 to 500 fold of pretreatment). We speculate that in vivo antineoplastic effects of IFNs and clinical side effects are mediated, at least in part, by a general or localized depletion of Trp. In view of reported increases of IFNs in autoimmune diseases and our earlier findings of elevated urinary Trp metabolites in autoimmune diseases, it seems likely that systemic or local depletion of Trp occurs in autoimmune diseases and may relate to degeneration, wasting and other symptoms in such diseases. We find high levels of IDO in cells isolated from synovia of arthritic joints. IFNs are also elevated in human immunodeficiency virus (HIV) patients and increasing IFN levels are associated with a worsening prognosis. We propose that IDO is induced chronically by HIV infection, is further increased by opportunistic infections, and that this chronic loss of Trp initiates mechanisms responsible for the cachexia, dementia, diarrhea and possibly immunosuppression of AIDS patients. In these symptoms, AIDS resembles classical pellagra due to dietary deficiency of Trp and niacin. In preliminary studies, others report low levels of Trp and serotonin, and elevated levels of Kyn and quinolinic acid in AIDS patients. The implications of these data in cancer, autoimmune diseases and AIDS are discussed.

Induction of indoleamine 2,3-dioxygenase in tumor cells transplanted into allogeneic mouse: interferon-gamma is the inducer

Tryptophan depletion observed during induction of indoleamine 2,3-dioxygenase (IDO) in cultured cells has been suggested to involve a mechanism identical to that employed in self-defense against inhaled microorganisms and tumor growth. We recently reported that a dramatic induction of IDO occurred in i.p. transplanted tumor (Meth-A) cells undergoing rejection from allogeneic mice (C57BL/6), and that soluble factor(s) released from infiltrated host cells was responsible for the IDO induction. Here we report on the characterization of the soluble factor. To assay the factor, we used a 35 mm special culture dish (Transwell), which consisted of two wells divided vertically with a membrane (0.4 micron pore). Host cells (mainly lymphocytes) that infiltrated into the transplantation loci were cultured in the upper well, and untreated Meth-A cells in the lower well. With this in vitro system, the membrane-permeable factor, released by the host cells (upper well), induced IDO in the tumor cells (lower well). The culture superna tants, obtained by centrifuging the culture media from the upper and lower wells, contained the IDO inducer. The inducer activity was completely neutralized by the addition of antibody against interferon-gamma (IFN-gamma) but not by antibody against IFN-alpha/beta. The concentration of IFN-gamma in the medium after 1-day culture with a Transwell culture dish was found to be 2-3 U/ml based on the neutralization curve with the antibody. At this concentration, recombinant IFN-gamma induced IDO in Meth-A cells to the same extent as the inducer in the culture medium. These observations indicate that the in vivo factor for IDO induction in the allografted tumor cells is IFN-gamma.

Inhibition of tumor cell growth by interferon-gamma is mediated by two distinct mechanisms dependent upon oxygen tension: induction of tryptophan degradation and depletion of intracellular nicotinamide adenine dinucleotide

Growth of a variety of human tumor cell lines is inhibited by interferon-gamma (IFN-gamma) in vitro. This mechanism is not well understood. The present experiments identify two separate mechanisms which account for the growth inhibitory activity of IFN-gamma. Cell lines most sensitive to IFN-gamma (inhibited by 10-30 U/ml IFN-gamma in 3 d) were stimulated by IFN-gamma to oxidize tryptophan in media to kynurenine and completely eliminated tryptophan from the culture media after 48-72 h. Addition of L-tryptophan, but not other aromatic amino acids, other essential amino acids, or D-tryptophan, prevented inhibition of cell growth by IFN-gamma. The amount of IFN-gamma required to yield 50% inhibition of cell growth was directly related to the concentration of L-tryptophan in culture media and increased from approximately 3 to 600 U/ml as the concentration of tryptophan in the media was increased from 25 to 1,000 microM. By contrast, inhibition of growth of the cell lines, BT20 and HT29, was not prevented by addition of tryptophan. Inhibition by IFN-gamma (100-300 U/ml after 5-6 d) was, however, completely prevented by addition of two inhibitors of adenosine diphosphate-ribosyl transferase (ADP-RT), 3-aminobenzamide or nicotinamide. Activity of ADP-RT was increased in these cell lines after addition of IFN-gamma. ADP-RT catalyzes the incorporation of the ADP moiety of nicotinamide adenine dinucleotide (NAD) into proteins and causes depletion of intracellular NAD. All tumor cell lines tested had reduced levels of intracellular NAD after treatment with IFN-gamma and loss of NAD preceded inhibition of cell growth by 12-24 h. Inhibitors of IFN-gamma-mediated inhibition of cell growth prevented loss of levels of intracellular NAD. Generation of reactive oxygen species lead to DNA strand breaks which result in activation of ADP-RT. Increased DNA strand breaks were induced in BT20 and HT29 cells but not ME180 and A549 cells after culture with IFN-gamma. The two enzymes known to catalyze the decyclization of tryptophan to kynurenine require superoxide anion for activity. Increased amounts of superoxide anion were released from ME180 and A549 cells after culture with IFN-gamma. Reduced oxygen concentration decreased the ability of IFN-gamma to inhibit tumor cell growth in vitro. Intracellular glutathione has been shown to protect cells against oxidative damage by various agents. Elevation or reduction of intracellular glutathione concentrations lowered or raised sensitivity of cell lines to IFN-gamma, respectively. These data indicate that at least two distinct mechanisms can account for IFN-gamma-madiated inhibition of tumor cell growth. Both mechanisms appear to be sensitive to oxygen tension and to changes in intracellular glutathione concentrations, and both mechanisms lead to loss of intracellular NAD.