The antituberculosis drug ethionamide is activated by a flavoprotein monooxygenase

Ethionamide (ETA), a prodrug that must undergo metabolic activation to exert its cytotoxic effects, is a second line drug against tuberculosis, a disease that infects more than a third of the world's population. It has been proposed, on the basis of genetic experiments, that ETA is activated in Mycobacterium tuberculosis by the protein encoded by the gene Rv3854c (DeBarber, A. E., Mdluli, K., Bosman, M., Bekker, L.-G., and Barry, C. E., III (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 9677-9682; Baulard, A. R., Betts, J. C., Engohang-Ndong, J., Quan, S., McAdam, R. A., Brennan, P. J., Locht, C., and Besra, G. S. (2000) J. Biol. Chem. 275, 28326-28331). We report here the expression, purification, and characterization of the protein encoded by this gene. Our results establish that the enzyme (EtaA) is an FAD-containing enzyme that oxidizes ETA to the corresponding S-oxide. The S-oxide, which has a similar biological activity as ETA, is further oxidized by EtaA to 2-ethyl-4-amidopyridine, presumably via the unstable doubly oxidized sulfinic acid intermediate. This flavoenzyme also oxidizes thiacetazone, thiobenzamide, and isothionicotinamide and thus is probably responsible, as suggested by the observation of crossover resistance, for the oxidative activation of other thioamide antitubercular drugs.     

Characterisation of kynurenine pathway metabolism in human astrocytes and implications in neuropathogenesis

Abstract

The role of astrocytes in the production of the neurotoxin quinolinic acid (QUIN) and other products of the kynurenine pathway (KP) is controversial. Using cytokine-stimulated human astrocytes, we assayed key enzymes and products of the KP. We found that astrocytes lack kynurenine-hydroxylase so that large amounts of kynurenine (KYN) and kynurenic acid (KYNA) were produced, while minor amounts of QUIN were synthesised that were completely degraded. We then showed that kynurenine added to macrophages led to significant production of QUIN. These results suggest that astrocytes alone are neuroprotective by minimising QUIN production and maximising synthesis of KYNA. However, it is likely that, in the presence of macrophages and/or microglia, astrocytes are neurotoxic by producing large concentrations of KYN that can be metabolised by neighbouring monocytic cells to QUIN.     

Characterisation of kynurenine pathway metabolism in human astrocytes and implications in neuropathogenesis

The role of astrocytes in the production of the neurotoxin quinolinic acid (QUIN) and other products of the kynurenine pathway (KP) is controversial. Using cytokine-stimulated human astrocytes, we assayed key enzymes and products of the KP. We found that astrocytes lack kynurenine-hydroxylase so that large amounts of kynurenine (KYN) and kynurenic acid (KYNA) were produced, while minor amounts of QUIN were synthesised that were completely degraded. We then showed that kynurenine added to macrophages led to significant production of QUIN. These results suggest that astrocytes alone are neuroprotective by minimising QUIN production and maximising synthesis of KYNA. However, it is likely that, in the presence of macrophages and/or microglia, astrocytes are neurotoxic by producing large concentrations of KYN that can be metabolised by neighbouring monocytic cells to QUIN.     

The kynurenine pathway and quinolinic acid: pivotal roles in HIV associated neurocognitive disorders

This brief review will first consider HIV associated neurocognitive disorder followed by the current understanding of its neuropathogenesis. Against this background the role of the kynurenine pathway will be detailed. Evidence both direct and indirect will be discussed for involvement of the kynurenine pathway at each step in the neuropathogenesis of HIV associated neurocognitive disorder.     

Aerobic tryptophan degradation pathway in bacteria: novel kynurenine formamidase

While a variety of chemical transformations related to the aerobic degradation of L-tryptophan (kynurenine pathway), and most of the genes and corresponding enzymes involved therein have been predominantly characterized in eukaryotes, relatively little was known about this pathway in bacteria. Using genome comparative analysis techniques we have predicted the existence of the three-step pathway of aerobic L-tryptophan degradation to anthranilate (anthranilate pathway) in several bacteria. Based on the chromosomal gene clustering analysis, we have identified a previously unknown gene encoding for kynurenine formamidase (EC 3.5.1.19) involved with the second step of the anthranilate pathway. This functional prediction was experimentally verified by cloning, expression and enzymatic characterization of recombinant kynurenine formamidase orthologs from Bacillus cereus, Pseudomonas aeruginosa and Ralstonia metallidurans. Experimental verification of the inferred anthranilate pathway was achieved by functional expression in Escherichia coli of the R. metallidurans putative kynBAU operon encoding three required enzymes: tryptophan 2,3-dioxygenase (gene kynA), kynurenine formamidase (gene kynB), and kynureninase (gene kynU). Our data provide the first experimental evidence of the connection between these genes (only one of which, kynU, was previously characterized) and L-tryptophan aerobic degradation pathway in bacteria.     

Peripheral distribution of kynurenine metabolites and activity of kynurenine pathway enzymes in renal failure.

We investigated L-kynurenine distribution and metabolism in rats with experimental chronic renal failure of various severity, induced by unilateral nephrectomy and partial removal of contralateral kidney cortex. In animals with renal insufficiency the plasma concentration and the content of L-tryptophan in homogenates of kidney, liver, lung, intestine and spleen were significantly decreased. These changes were accompanied by increase activity of liver tryptophan 2,3-dioxygenase, the rate-limiting enzyme of kynurenine pathway in rats, while indoleamine 2,3-dioxygenase activity was unchanged. Conversely, the plasma concentration and tissue content of L-kynurenine, 3-hydroxykynurenine, and anthranilic, kynurenic, xanthurenic and quinolinic acids in the kidney, liver, lung, intestine, spleen and muscles were increased. The accumulation of L-kynurenine and the products of its degradation was proportional to the severity of renal failure and correlated with the concentration of renal insufficiency marker, creatinine. Kynurenine aminotransferase, kynureninase and 3-hydroxyanthranilate-3,4-dioxygenase activity was diminished or unchanged, while the activity of kynurenine 3-hydroxylase was significantly increased. We conclude that chronic renal failure is associated with the accumulation of L-kynurenine metabolites, which may be involved in the pathogenesis of certain uremic syndromes.     

Soluble methane monooxygenase: activation of dioxygen and methane

The mechanisms by which soluble methane monooxygenase uses dioxygen to convert methane selectively to methanol have come into sharp focus. Diverse techniques have clarified subtle details about each step in the reaction, from binding and activating dioxygen, to hydroxylation of alkanes and other substrates, to the electron transfer events required to complete the catalytic cycle.     

53BP1 functions in an ATM-dependent checkpoint pathway that is constitutively activated in human cancer

53BP1 is a conserved nuclear protein that is implicated in the DNA damage response. After irradiation, 53BP1 localizes rapidly to nuclear foci, which represent sites of DNA double strand breaks, but its precise function is unclear. Using small interference RNA (siRNA), we demonstrate that 53BP1 functions as a DNA damage checkpoint protein. 53BP1 is required for at least a subset of ataxia telangiectasia-mutated (ATM)-dependent phosphorylation events at sites of DNA breaks and for cell cycle arrest at the G2-M interphase after exposure to irradiation. Interestingly, in cancer cell lines expressing mutant p53, 53BP1 was localized to distinct nuclear foci and ATM-dependent phosphorylation of Chk2 at Thr 68 was detected, even in the absence of irradiation. In addition, Chk2 was phosphorylated at Thr 68 in more than 50% of surgically resected lung and breast tumour specimens from otherwise untreated patients [corrected]. We conclude that the constitutive activation of the DNA damage checkpoint pathway may be linked to the high frequency of p53 mutations in human cancer, as p53 is a downstream target of Chk2 and ATM.     

The phosphoinositide 3-kinase/Akt pathway is activated by daunorubicin in human acute myeloid leukemia cell lines.

Daunorubicin induces apoptosis in myeloid leukemia cells by activation of neutral sphingomyelinase and ceramide generation occurring 4-10 min after daunorubicin addition. We show here that daunorubicin is able to increase the phosphoinositide 3-kinase activity and enhance intracellular phosphoinositide 3-kinase lipid products prior to ceramide generation. Daunorubicin activates Akt, a downstream phosphoinositide 3-kinase effector. Interestingly, the phosphoinositide 3-kinase inhibitors wortmannin and LY294002 accelerate daunorubicin-induced apoptosis in U937 cells. The phosphoinositide 3-kinase/Akt pathway has been involved in cell survival following serum deprivation, tumor necrosis factor alpha, anti-Fas and UV radiations. Our results suggest that anti-tumor agents such as daunorubicin may also activate anti-apoptotic signals that could contribute to drug resistance.     

The FANC pathway is activated by adenovirus infection and promotes viral replication-dependent recombination

Deciphering the crosstalk between a host cell and a virus during infection is important not only to better define viral biology but also to improve our understanding of cellular processes. We identified the FANC pathway as a helper of viral replication and recombination by searching for cellular targets that are modified by adenovirus (Ad) infection and are involved in its outcome. This pathway, which is involved in the DNA damage response and checkpoint control, is altered in Fanconi anaemia, a rare cancer predisposition syndrome. We show here that Ad5 infection activates the FANC pathway independent of the classical DNA damage response. Infection with a non-replicating Ad shows that the presence of viral DNA is not sufficient to induce the monoubiquitination of FANCD2 but still activates the DNA damage response coordinated by phospho-NBS1 and phospho-CHK1. E1A expression alone fails to induce FANCD2 monoubiquitination, indicating that a productive viral infection and/or replication is required for FANC pathway activation. Our data indicate that Ad5 infection induces FANCD2 activation to promote its own replication. Specifically, we show that FANCD2 is involved in the recombination process that accompanies viral DNA replication. This study provides evidence of a DNA damage-independent function of the FANC pathway and identifies a cellular system involved in Ad5 recombination.     

The kynurenine pathway modulates neurodegeneration in a Drosophila model of Huntington's disease

Neuroactive metabolites of the kynurenine pathway (KP) of tryptophan degradation have been implicated in the pathophysiology of neurodegenerative disorders, including Huntington's disease (HD) [1]. A central hallmark of HD is neurodegeneration caused by a polyglutamine expansion in the huntingtin (htt) protein [2]. Here we exploit a transgenic Drosophila melanogaster model of HD to interrogate the therapeutic potential of KP manipulation. We observe that genetic and pharmacological inhibition of kynurenine 3-monooxygenase (KMO) increases levels of the neuroprotective metabolite kynurenic acid (KYNA) relative to the neurotoxic metabolite 3-hydroxykynurenine (3-HK) and ameliorates neurodegeneration. We also find that genetic inhibition of tryptophan 2,3-dioxygenase (TDO), the first and rate-limiting step in the pathway, leads to a similar neuroprotective shift toward KYNA synthesis. Importantly, we demonstrate that the feeding of KYNA and 3-HK to HD model flies directly modulates neurodegeneration, underscoring the causative nature of these metabolites. This study provides the first genetic evidence that inhibition of KMO and TDO activity protects against neurodegenerative disease in an animal model, indicating that strategies targeted?at?two key points within the KP may have therapeutic relevance in HD, and possibly other neurodegenerative disorders.     

Rat supernatant protein factor-like protein stimulates squalene monooxygenase and is activated by protein kinase A

Rat supernatant protein factor-like protein (SPF2) shares 90% sequence identity with rat SPF and 77% identity with human SPF, both of which have been shown to stimulate squalene monooxygenase in the cholesterol biosynthetic pathway. SPF2 appears to be predominantly expressed in respiratory and epithelial tissues, whereas SPF is expressed in liver. To determine if SPF2 was also able to stimulate squalene monooxygenase activity, we have cloned, expressed, and purified the protein following heterologous expression in Escherichia coli. SPF2 was only half as effective as SPF in stimulating squalene epoxidation and was more strongly inhibited by GTP and GDP. The inhibition by guanine nucleotides was fully prevented by alpha-tocopherol, a reported ligand for these proteins. Incubation of SPF2 with protein kinase A and ATP increased its activity by about twofold, has been found for SPF. These results indicate that SPF2 activity is modulated by guanine nucleotides and alpha-tocopherol, as well as by phosphorylation.     

KIT (c-kit oncogene product) pathway is constitutively activated in human testicular germ cell tumors

We investigated the expression of KIT (product of c-kit oncogene), gain-of-function mutations, and activation of its downstream signal transduction in human testicular cancers. KIT was expressed in 88% (22/25) of seminomas and in 44.4% (4/9) of non-seminomas compared to adjacent normal testicular tissue. Nine of the KIT-expressing seminomas had mutations (40.9%; 9/22) in the c-kit gene; two cases in exon 11 and 7 cases in exon 17. Two of these mutations in exon 17 were novel, and the other seven mutations were identical to the already known gain-of-function mutations which cause activation of KIT without ligand stem cell factor. All of the mutant KIT and 53.8% (7/13) of wild-type KIT were phosphorylated (activated) and associated with phosphorylated phosphatidylinositol 3-kinase (PI3K). Akt was also phosphorylated in these seminomas, suggesting that the KIT-PI3K-Akt pathway is activated in seminoma. These findings suggest that the KIT-PI3K-Akt pathway is constitutively activated in testicular germ cell tumors, due to overexpression of KIT protein and/or gain-of-function mutations in the c-kit gene.     

Metabolome analysis reveals the association between the kynurenine pathway and human herpesvirus 6 encephalopathy in immunocompetent children

Introduction

Human herpesvirus 6 (HHV-6) is the second most common causative pathogen of acute encephalopathy in immunocompetent children in Japan. Identification of biomarkers associated the pathophysiology is desirable to monitor disease severity, progression, and prognosis.

Objectives

To investigate potential biomarkers for HHV-6 encephalopathy, serum metabolome profiling was analyzed and candidate metabolites were investigated the function in the diseases.

Methods

Pediatric patients with HHV-6 encephalopathy (n?=?8), febrile seizure (n?=?20), and febrile infection without febrile seizure (n?=?7) were enrolled in this study, and serum metabolites were identified and quantified. For further analysis, serum samples of HHV-6 infected patients were analyzed by absolute quantification assay for kynurenine (KYN) and quinolinic acid (QUIN) in a total of 38 patients with or without encephalopathy. An in vitro blood–brain barrier (BBB) model was used to evaluate the effect of KYN and QUIN on BBB integrity because BBB damage induces brain edema.

Results

Metabolome profiling identified 159 metabolites in serum samples. The levels of KYN and QUIN, which belong to the tryptophan-KYN pathway, were significantly higher in the HHV-6 encephalopathy group than the other two groups. When quantified in the larger patient group, remarkably high levels of KYN and QUIN were observed exclusively in the encephalopathy group. Trans-endothelial electrical resistance of the BBB model was significantly decreased after QUIN treatment in culture.

Conclusion

Metabolome analysis revealed that KYN and QUIN may be associated with the pathophysiology of HHV-6 encephalopathy. In particular, QUIN may damage BBB integrity.

     

The classical activation pathway of the human complement system is specifically inhibited by calreticulin from Trypanosoma cruzi

The high resistance of Trypanosoma cruzi trypomastigotes, the causal agent of Chagas' disease, to complement involves several parasite strategies. In these in vitro studies, we show that T. cruzi calreticulin (TcCRT) and two subfragments thereof (TcCRT S and TcCRT R domains) bind specifically to recognition subcomponents of the classical and lectin activation pathways (i.e., to collagenous tails of C1q and to mannan-binding lectin) of the human complement system. As a consequence of this binding, specific functional inhibition of the classical pathway and impaired mannan-binding lectin to mannose were observed. By flow cytometry, TcCRT was detected on the surface of viable trypomastigotes and, by confocal microscopy, colocalization of human C1q with surface TcCRT of infective trypomastigotes was visualized. Taken together, these findings imply that TcCRT may be a critical factor contributing to the ability of trypomastigotes to interfere at the earliest stages of complement activation.