L-tryptophan-kynurenine pathway metabolites regulate type I IFNs of acute viral myocarditis in mice

The activity of IDO that catalyzes the degradation of tryptophan (Trp) into kynurenine (Kyn) increases after diseases caused by different infectious agents. Previously, we demonstrated that IDO has an important immunomodulatory function in immune-related diseases. However, the pathophysiological role of IDO following acute viral infection is not fully understood. To investigate the role of IDO in the l-Trp-Kyn pathway during acute viral myocarditis, mice were infected with encephalomyocarditis virus, which induces acute myocarditis. We used IDO-deficient (IDO(-/-)) mice and mice treated with 1-methyl-d,l-Trp (1-MT), an inhibitor of IDO, to study the importance of Trp-Kyn pathway metabolites. Postinfection with encephalomyocarditis virus infection, the serum levels of Kyn increased, whereas those of Trp decreased, and IDO activity increased in the spleen and heart. The survival rate of IDO(-/-) or 1-MT-treated mice was significantly greater than that of IDO(+/+) mice. Indeed, the viral load was suppressed in the IDO(-/-) or 1-MT-treated mice. Furthermore, the levels of type I IFNs in IDO(-/-) mice and IDO(-/-) bone marrow-transplanted IDO(+/+) mice were significantly higher than those in IDO(+/+) mice, and treatment of IDO(-/-) mice with Kyn metabolites eliminated the effects of IDO(-/-) on the improved survival rates. These results suggest that IDO has an important role in acute viral myocarditis. Specifically, IDO increases the accumulation of Kyn pathway metabolites, which suppress type I IFNs production and enhance viral replication. We concluded that inhibition of the Trp-Kyn pathway ameliorates acute viral myocarditis.     

Mitochondrial targeting of α-tocopheryl succinate enhances its anti-mesothelioma efficacy

Abstract

Malignant mesothelioma (MM) is a fatal neoplastic disease with no therapeutic option. Therefore, the search for novel therapies is of paramount importance.

Methods

Since mitochondrial targeting of α-tocopheryl succinate (α-TOS) by its tagging with triphenylphosphonium enhances its cytotoxic effects to cancer cells, we tested its effect on MM cells and experimental mesotheliomas.

Results

Mitochondrially targeted vitamin E succinate (MitoVES) was more efficient in killing MM cells than α-TOS with IC₅₀ lower by up to two orders of magnitude. Mitochondrial association of MitoVES in MM cells was documented using its fluorescently tagged analogue. MitoVES caused apoptosis in MM cells by mitochondrial destabilization, resulting in the loss of mitochondrial membrane potential, generation of reactive oxygen species, and destabilization of respiratory supercomplexes. The role of the mitochondrial complex II in the activity of MitoVES was confirmed by the finding that MM cells with suppressed succinate quinone reductase were resistant to MitoVES. MitoVES suppressed mesothelioma growth in nude mice with high efficacy.

Discussion

MitoVES is more efficient in killing MM cells and suppressing experimental mesotheliomas compared with the non-targeted α-TOS, giving it a potential clinical benefit.     

Is tryptophan catabolism increased under indoleamine 2,3 dioxygenase activity during chronic lung inflammation in pigs?

In a preliminary study we observed that piglets suffering from chronic lung inflammation induced by an intravenous injection of complete Freund adjuvant showed a marked decrease in plasma tryptophan (Trp) concentration suggesting increased Trp utilisation. During the inflammatory process, a cytokine-induced enzyme called indoleamine 2,3-dioxygenase (IDO) has been shown to catabolise Trp into kynurenine (Kyn). Yet, during inflammation, increased Trp catabolism may decrease Trp availability for other functions such as growth. This metabolic pathway has never been studied in pigs. So, the objectives of this study were to measure IDO activity in pigs and to determine if the decrease in plasma Trp concentrations previously observed in piglets suffering from chronic lung inflammation could be explained by the induction of IDO activity. In order to do so, we compared IDO activity measured in the tracheo-bronchial lymph nodes and in the lungs of 7 piglets, injected with complete Freund adjuvant (CFA), to 7 pair-fed littermate healthy controls. Blood samples were taken at 0, 2, 5, 7 and 10 days following CFA injection in order to measure plasma Trp, Kyn and haptoglobin concentrations. Indoleamine 2,3-dioygenase activity in the tracheo-bronchial lymph nodes (P < 0.05), in the lungs (P < 0.07) and plasma haptoglobin (P < 0.01) were higher in pigs with lung inflammation than in the controls. Plasma Trp and Kyn were not significantly affected by CFA injection. Our data showed that IDO is activated under chronic lung inflammation in pigs. The impact of IDO activation on plasma Trp concentration and its availability is discussed according to the amount of Trp provided by the diet.     

Mitochondrial targeting of vitamin E succinate enhances its pro-apoptotic and anti-cancer activity via mitochondrial complex II

Mitochondrial complex II (CII) has been recently identified as a novel target for anti-cancer drugs. Mitochondrially targeted vitamin E succinate (MitoVES) is modified so that it is preferentially localized to mitochondria, greatly enhancing its pro-apoptotic and anti-cancer activity. Using genetically manipulated cells, MitoVES caused apoptosis and generation of reactive oxygen species (ROS) in CII-proficient malignant cells but not their CII-dysfunctional counterparts. MitoVES inhibited the succinate dehydrogenase (SDH) activity of CII with IC(50) of 80 μM, whereas the electron transfer from CII to CIII was inhibited with IC(50) of 1.5 μM. The agent had no effect either on the enzymatic activity of CI or on electron transfer from CI to CIII. Over 24 h, MitoVES caused stabilization of the oxygen-dependent destruction domain of HIF1α fused to GFP, indicating promotion of the state of pseudohypoxia. Molecular modeling predicted the succinyl group anchored into the proximal CII ubiquinone (UbQ)-binding site and successively reduced interaction energies for serially shorter phytyl chain homologs of MitoVES correlated with their lower effects on apoptosis induction, ROS generation, and SDH activity. Mutation of the UbQ-binding Ser(68) within the proximal site of the CII SDHC subunit (S68A or S68L) suppressed both ROS generation and apoptosis induction by MitoVES. In vivo studies indicated that MitoVES also acts by causing pseudohypoxia in the context of tumor suppression. We propose that mitochondrial targeting of VES with an 11-carbon chain localizes the agent into an ideal position across the interface of the mitochondrial inner membrane and matrix, optimizing its biological effects as an anti-cancer drug.     

Molecular mechanism for the selective impairment of cancer mitochondrial function by a mitochondrially targeted vitamin E analogue

The effects of α-tocopheryl succinate (α-TOS), α-tocopheryl acetyl ether (α-TEA) and triphenylphosphonium-tagged vitamin E succinate (mitochondrially targeted vitamin E succinate; MitoVES) on energy-related mitochondrial functions were determined in mitochondria isolated from AS-30D hepatoma and rat liver, bovine heart sub-mitochondrial particles (SMPs), and in rodent and human carcinoma cell lines and rat hepatocytes. In isolated mitochondria, MitoVES stimulated basal respiration and ATP hydrolysis, but inhibited net state 3 (ADP-stimulated) respiration and Ca(2+) uptake, by collapsing the membrane potential at low doses (1-10μM). Uncoupled mitochondrial respiration and basal respiration of SMPs were inhibited by the three drugs at concentrations at least one order of magnitude higher and with different efficacy: MitoVES>α-TEA>α-TOS. At high doses (>10μM), the respiratory complex II (CII) was the most sensitive MitoVES target. Acting as an uncoupler at low doses, this agent stimulated total O(2) uptake, collapsed ?ψ(m), inhibited oxidative phosphorylation and induced ATP depletion in rodent and human cancer cells more potently than in normal rat hepatocytes. These findings revealed that in situ tumor mitochondria are preferred targets of the drug, indicating its clinical relevance.     

Psychological Stress-Induced,IDO1-Dependent Tryptophan Catabolism: Implications on Immunosuppression in Mice and Humans

It is increasingly recognized that psychological stress influences inflammatory responses and mood. Here, we investigated whether psychological stress (combined acoustic and restraint stress) activates the tryptophan (Trp) catabolizing enzyme indoleamine 2,3-dioxygenase 1(IDO1) and thereby alters the immune homeostasis and behavior in mice. We measured IDO1 mRNA expression and plasma levels of Trp catabolites after a single 2-h stress session and in repeatedly stressed (4.5-days stress, 2-h twice a day) naïve BALB/c mice. A role of cytokines in acute stress-induced IDO1 activation was studied after IFNγ and TNFα blockade and in IDO1^−/− mice. RU486 and 1-Methyl-L-tryptophan (1-MT) were used to study role of glucocorticoids and IDO1 on Trp depletion in altering the immune and behavioral response in repeatedly stressed animals. Clinical relevance was addressed by analyzing IDO1 activity in patients expecting abdominal surgery. Acute stress increased the IDO1 mRNA expression in brain, lung, spleen and Peyer''s patches (max. 14.1±4.9-fold in brain 6-h after stress) and resulted in a transient depletion of Trp (−25.2±6.6%) and serotonin (−27.3±4.6%) from the plasma measured 6-h after stress while kynurenine levels increased 6-h later (11.2±9.3%). IDO1 mRNA up-regulation was blocked by anti-TNFα and anti-IFNγ treatment. Continuous IDO1 blockade by 1-MT but not RU486 treatment normalized the anti-bacterial defense and attenuated increased IL-10 inducibility in splenocytes after repeated stress as it reduced the loss of body weight and behavioral alterations. Moreover, kynurenic acid which remained increased in 1-MT treated repeatedly stressed mice was identified to reduce the TNFα inducibility of splenocytes in vitro and in vivo. Thus, psychological stress stimulates cytokine-driven IDO1 activation and Trp depletion which seems to have a central role for developing stress-induced immunosuppression and behavioral alteration. Since patients showed Trp catabolism already prior to surgery, IDO is also a possible target enzyme for humans modulating immune homeostasis and mood.     

Substrate stereo‐specificity in tryptophan dioxygenase and indoleamine 2,3‐dioxygenase

The first and rate‐limiting step of the kynurenine pathway, in which tryptophan (Trp) is converted to N‐formylkynurenine is catalyzed by two heme‐containing proteins, Indoleamine 2,3‐dioxygenase (IDO), and Tryptophan 2,3‐dioxygenase (TDO). In mammals, TDO is found exclusively in liver tissue, IDO is found ubiquitously in all tissues. IDO has become increasingly popular in pharmaceutical research as it was found to be involved in many physiological situations, including immune escape of cancer. More importantly, small‐molecule inhibitors of IDO are currently utilized in cancer therapy. One of the main concerns for the design of human IDO (hIDO) inhibitors is that they should be selective enough to avoid inhibition of TDO. In this work, we have used a combination of classical molecular dynamics (MD) and hybrid quantum‐classical (QM/MM) methodologies to establish the structural basis that determine the differences in (a) the interactions of TDO and IDO with small ligands (CO/O₂) and (b) the substrate stereo‐specificity in hIDO and TDO. Our results indicate that the differences in small ligand bound structures of IDO and TDO arise from slight differences in the structure of the bound substrate complex. The results also show that substrate stereo‐specificity of TDO is achieved by the perfect fit of L ‐Trp, but not D ‐Trp, which exhibits weaker interactions with the protein matrix. For hIDO, the presence of multiple stable binding conformations for L /D ‐Trp reveal the existence of a large and dynamic active site. Taken together, our data allow determination of key interactions useful for the future design of more potent hIDO‐selective inhibitors. Proteins 2010; © 2010 Wiley‐Liss, Inc.     

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.     

IDO1 Deficiency Does Not Affect Disease in Mouse Models of Systemic Juvenile Idiopathic Arthritis and Secondary Hemophagocytic Lymphohistiocytosis

Objectives

Indoleamine 2,3-dioxygenase-1 (IDO1) is an immune-modulatory enzyme that catalyzes the degradation of tryptophan (Trp) to kynurenine (Kyn) and is strongly induced by interferon (IFN)-γ. We previously reported highly increased levels of IFN-γ and corresponding IDO activity in patients with hemophagocytic lymphohistiocytosis (HLH), a hyper-inflammatory syndrome. On the other hand, IFN-γ and IDO were low in patients with systemic juvenile idiopathic arthritis (sJIA), an autoinflammatory syndrome. As HLH can occur as a complication of sJIA, the opposing levels of both IFN-γ and IDO are remarkable. In animal models for sJIA and HLH, the role of IFN-γ differs from being protective to pathogenic. In this study, we aimed to unravel the role of IDO1 in the pathogenesis of sJIA and HLH.

Methods

Wild-type and IDO1-knockout (IDO1-KO) mice were used in 3 models of sJIA or HLH: complete Freund’s adjuvant (CFA)-injected mice developed an sJIA-like syndrome and secondary HLH (sHLH) was evoked by either repeated injection of unmethylated CpG oligonucleotide or by primary infection with mouse cytomegalovirus (MCMV). An anti-CD3-induced cytokine release syndrome was used as a non-sJIA/HLH control model.

Results

No differences were found in clinical, laboratory and hematological features of sJIA/HLH between wild-type and IDO1-KO mice. As IDO modulates the immune response via induction of regulatory T cells and inhibition of T cell proliferation, we investigated both features in a T cell-triggered cytokine release syndrome. Again, no differences were observed in serum cytokine levels, percentages of regulatory T cells, nor of proliferating or apoptotic thymocytes and lymph node cells.

Conclusions

Our data demonstrate that IDO1 deficiency does not affect inflammation in sJIA, sHLH and a T cell-triggered cytokine release model. We hypothesize that other tryptophan-catabolizing enzymes like IDO2 and tryptophan 2,3-dioxygenase (TDO) might compensate for the lack of IDO1.