Kynurenine-3-monooxygenase inhibition prevents multiple organ failure in rodent models of acute pancreatitis

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune disorders.

Recent Advances in Current Uptake Situation, Metabolic and Nutritional Characteristics, Health, and Safety of Dietary Tryptophan

Tryptophan (Trp) is an essential amino acid which is unable to be synthesized in the body. Main sources of Trp are uptake of foods such as oats and bananas. In this review, we describe the status of current dietary consumption, metabolic pathways and nutritional characteristics of Trp, as well as its ingestion and downstream metabolites for maintaining body health and safety. This review also summarizes the recent advances in Trp metabolism, particularly the 5-HT, KYN, and AhR activation pathways, revealing that its endogenous host metabolites are not only differentially affected in the body but also are closely linked to health. More attention should be paid to targeting its specific metabolic pathways and utilizing food molecules and probiotics for manipulating Trp metabolism. However, the complexity of microbiota-host interactions requires further exploration to precisely refine targets for innovating the gut microbiota-targeted diagnostic approaches and informing subsequent studies and targeted treatments of diseases.

A novel causative role of imbalanced kynurenine pathway in ulcerative colitis: Upregulation of KMO and KYNU promotes intestinal inflammation

The kynurenine pathway (KP) is the principal metabolic route for the essential amino acid tryptophan (TRP). Recent advances have highlighted a pivotal role for several KP metabolites in inflammatory diseases, including ulcerative colitis (UC). However, the alterations of KP enzymes and their functional impact in UC remain poorly defined. Here, we focused on kynurenine 3-monooxygenase (KMO) and kynureninase (KYNU), which serve as critical branching enzymes in the KP. We observed that dextran sodium sulfate (DSS)-induced colitis mice exhibited disturbed TRP metabolism along with KMO and KYNU upregulated. In patients with active UC, both the expression of KMO and KYNU were positively correlated with inflammatory factors TNF-α and IL-1β. Pharmacological blockade of KMO or genetic silencing of KYNU suppressed IL-1β-triggered proinflammatory cytokines expression in intestinal epithelial cells. Furthermore, blockage of KMO by selective inhibitor Ro 61-8048 alleviated the symptoms of DSS-induced colitis in mice, accompanied by an expanded NAD+ pool and redox balance restoration. The protective role of Ro 61-8048 may be partly due to its effect on KP regulation, particularly in enhancing kynurenic acid production. In summary, our study provides new evidence for the proinflammatory property of KMO and KYNU in intestinal inflammation, hinting at a promising therapeutic approach in UC through targeting these enzymes.

The kynurenine pathway presents multi-faceted metabolic vulnerabilities in cancer

The kynurenine pathway (KP) and associated catabolites play key roles in promoting tumor progression and modulating the host anti-tumor immune response. To date, considerable focus has been on the role of indoleamine 2,3-dioxygenase 1 (IDO1) and its catabolite, kynurenine (Kyn). However, increasing evidence has demonstrated that downstream KP enzymes and their associated metabolite products can also elicit tumor-microenvironment immune suppression. These advancements in our understanding of the tumor promotive role of the KP have led to the conception of novel therapeutic strategies to target the KP pathway for anti-cancer effects and reversal of immune escape. This review aims to 1) highlight the known biological functions of key enzymes in the KP, and 2) provide a comprehensive overview of existing and emerging therapies aimed at targeting discrete enzymes in the KP for anti-cancer treatment.

Kynurenine pathway alteration in acute pancreatitis and its role as a biomarker of infected necrosis

INTRODUCTION

Infected pancreatic necrosis (IPN) is a major cause of mortality in acute pancreatitis (AP). Currently, no specific strategies are available to predict the development of IPN. Earlier we reported that persistent down-regulation of HLA-DR increases risk of developing IPN. Altered kynurenine pathway (KP) metabolites showed poor prognosis in sepsis. Here we evaluated the role of HLA-DR and KP in IPN.

METHODS

Patients with ANP and healthy controls were enrolled. Demographic and clinical parameters were recorded. Circulating interleukin (IL)-8, 6, 1β, 10, Tumor necrosis factor-α were quantified using flowcytometry. Plasma procalcitonin, endotoxin, and KP (tryptophan, kynurenine) concentrations were estimated using ELISA. qRT-PCR was conducted to evaluate mRNA expression of HLA-DR, IL-10, Toll like receptor-4 (TLR-4), and kynurenine-3-monooxygenase (KMO) genes on peripheral blood mononuclear cells. Plasma metabolites were quantified using gas chromatography mass spectrometry (GC-MS/MS). Standard statistical methods were used to compare study groups. Metaboanalyst was used to analyse/visualize the metabolomics data.

RESULTS

We recruited 56 patients in Cohort-1 (IPN:26,Non-IPN:30), 78 in Cohort-2 (IPN:57,Non-IPN:21), 26 healthy controls. Increased cytokines, endotoxin, and procalcitonin were observed in patients with IPN compared to Non-IPN. HLA-DR and KMO gene expressions were significantly down-regulated in IPN groups, showed positive correlation with one another but negatively correlated with IL-6 and endotoxin concentrations. Increased IDO and decreased plasma tryptophan were observed in IPN patients. Metabolome analysis showed significant reduction in several essential amino acids including tryptophan in IPN patients. Tryptophan, at a concentration of 9 mg/ml showed an AUC of 91.9 (95%CI 86.5-97.4) in discriminating IPN.

CONCLUSION

HLA-DR downregulation and KP alteration are related to IPN. The KP metabolite plasma tryptophan can act as a potential biomarker for IPN.

Kynurenine monooxygenase regulates inflammation during critical illness and recovery in experimental acute pancreatitis

Kynurenine monooxygenase (KMO) blockade protects against multiple organ failure caused by acute pancreatitis (AP), but the link between KMO and systemic inflammation has eluded discovery until now. Here, we show that the KMO product 3-hydroxykynurenine primes innate immune signaling to exacerbate systemic inflammation during experimental AP. We find a tissue-specific role for KMO, where mice lacking Kmo solely in hepatocytes have elevated plasma 3-hydroxykynurenine levels that prime inflammatory gene transcription. 3-Hydroxykynurenine synergizes with interleukin-1β to cause cellular apoptosis. Critically, mice with elevated 3-hydroxykynurenine succumb fatally earlier and more readily to experimental AP. Therapeutically, blockade with the highly selective KMO inhibitor GSK898 rescues the phenotype, reducing 3-hydroxykynurenine and protecting against critical illness and death. Together, our findings establish KMO and 3-hydroxykynurenine as regulators of inflammation and the innate immune response to sterile inflammation. During critical illness, excess morbidity and death from multiple organ failure can be rescued by systemic KMO blockade.

Discovery of Coixol Derivatives as Potent Anti-inflammatory Agents

Coixol, a derivative of 2-benzoxazolinone extracted from coix (Coix lachryma-jobi L. var. ma-yuen Stapf), has demonstrated promising anti-inflammatory activity and low cytotoxicity. In this study, 26 coixol derivatives were designed and synthesized by hybridization with cinnamic acid to identify new anti-inflammatory agents. The anti-inflammatory activities of the derivatives were screened using LPS-induced overexpression of nitric oxide (NO) in RAW264.7 macrophages. On the basis of the screening results, compounds containing furan (9c) or nitrofuran (9j) moieties displayed more pronounced activity than coixol and celecoxib. Mechanistic investigations revealed that 9c and 9j suppressed the expression of induced nitric oxide synthase (iNOS), tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β, which was associated with the inhibition of the nuclear factor (NF)-κB signaling pathway. In vivo studies confirmed the anti-inflammatory activity of 9c and 9j in a xylene-induced mice auricles edema model. The preliminary in vitro and in vivo research findings suggest that 9c and 9j have the potential to be developed as anti-inflammatory agents.

Tryptophan metabolism in health and disease

Tryptophan (Trp) metabolism primarily involves the kynurenine, 5-hydroxytryptamine, and indole pathways. A variety of bioactive compounds produced via Trp metabolism can regulate various physiological functions, including inflammation, metabolism, immune responses, and neurological function. Emerging evidence supports an intimate relationship between Trp metabolism disorder and diseases. The levels or ratios of Trp metabolites are significantly associated with many clinical features. Additionally, studies have shown that disease progression can be controlled by modulating Trp metabolism. Indoleamine-2,3-dioxygenase, Trp-2,3-dioxygenase, kynurenine-3-monooxygenase, and Trp hydroxylase are the rate-limiting enzymes that are critical for Trp metabolism. These key regulatory enzymes can be targeted for treating several diseases, including tumors. These findings provide novel insights into the treatment of diseases. In this review, we have summarized the recent research progress on the role of Trp metabolites in health and disease along with their clinical applications.

Protocatechuic acid prevents isoproterenol-induced heart failure in mice by downregulating kynurenine-3-monooxygenase

Protocatechuic acid (3,4-dihydroxybenzoic acid) prevents oxidative stress, inflammation and cardiac hypertrophy. This study aimed to investigate the therapeutic effects of protocatechuic acid in an isoproterenol-induced heart failure mouse model and to identify the underlying mechanisms. To establish the heart failure model, C57BL/6NTac mice were given high-dose isoproterenol (80 mg/kg body weight) for 14 days. Echocardiography revealed that protocatechuic acid reversed the isoproterenol-induced downregulation of fractional shortening and ejection fraction. Protocatechuic acid attenuated cardiac hypertrophy as evidenced by the decreased heart-weight-to-body-weight ratio and the expression of Nppb. RNA sequencing analysis identified kynurenine-3-monooxygenase (Kmo) as a potential target of protocatechuic acid. Protocatechuic acid treatment or transfection with short-interfering RNA against Kmo ameliorated transforming growth factor β1-induced upregulation of Kmo, Col1a1, Col1a2 and Fn1 in vivo or in neonatal rat cardiac fibroblasts. Kmo knockdown attenuated the isoproterenol-induced increase in cardiomyocyte size, as well as Nppb and Col1a1 expression in H9c2 cells or primary neonatal rat cardiomyocytes. Moreover, protocatechuic acid attenuated Kmo overexpression-induced increases in Nppb mRNA levels. Protocatechuic acid or Kmo knockdown decreased isoproterenol-induced ROS generation in vivo and in vitro. Thus, protocatechuic acid prevents heart failure by downregulating Kmo. Therefore, protocatechuic acid and Kmo constitute a potential novel therapeutic agent and target, respectively, against heart failure.

Interactions of IDO and the Kynurenine Pathway with Cell Transduction Systems and Metabolism at the Inflammation-Cancer Interface

The mechanisms underlying a relationship between inflammation and cancer are unclear, but much emphasis has been placed on the role of tryptophan metabolism to kynurenine and downstream metabolites, as these make a substantial contribution to the regulation of immune tolerance and susceptibility to cancer. The proposed link is supported by the induction of tryptophan metabolism by indoleamine-2,3-dioxygenase (IDO) or tryptophan-2,3-dioxygenase (TDO), in response to injury, infection or stress. This review will summarize the kynurenine pathway and will then focus on the bi-directional interactions with other transduction pathways and cancer-related factors. The kynurenine pathway can interact with and modify activity in many other transduction systems, potentially generating an extended web of effects other than the direct effects of kynurenine and its metabolites. Conversely, the pharmacological targeting of those other systems could greatly enhance the efficacy of changes in the kynurenine pathway. Indeed, manipulating those interacting pathways could affect inflammatory status and tumor development indirectly via the kynurenine pathway, while pharmacological modulation of the kynurenine pathway could indirectly influence anti-cancer protection. While current efforts are progressing to account for the failure of selective IDO1 inhibitors to inhibit tumor growth and to devise means of circumventing the issue, it is clear that there are wider factors involving the relationship between kynurenines and cancer that merit detailed consideration as alternative drug targets.

The role of the circadian rhythms in critical illness with a focus on acute pancreatitis

Circadian rhythms are responsible for governing various physiological processes, including hormone secretion, immune responses, metabolism, and the sleep/wake cycle. In critical illnesses such as acute pancreatitis (AP), circadian rhythms can become dysregulated due to disease. Evidence suggests that time of onset of disease, coupled with peripheral inflammation brought about by AP will impact on the circadian rhythms generated in the central pacemaker and peripheral tissues. Cells of the innate and adaptive immune system are governed by circadian rhythms and the diurnal pattern of expression can be disrupted during disease. Peak circadian immune cell release and gene expression can coincide with AP onset, that may increase pancreatic injury, tissue damage and the potential for systemic inflammation and multiple organ failure to develop. Here, we provide an overview of the role of circadian rhythms in AP and the underpinning inflammatory mechanisms to contextualise ongoing research into the chronobiology and chronotherapeutics of AP.

Therapeutic potential of targeting kynurenine pathway in neurodegenerative diseases

Kynurenine pathway (KP), the primary pathway of L-tryptophan (Trp) metabolism in mammals, contains several neuroactive metabolites such as kynurenic acid (KA) and quinolinic acid (QA). Its imbalance involved in aging and neurodegenerative diseases (NDs) has attracted much interest in therapeutically targeting KP enzymes and KP metabolite-associated receptors, especially kynurenine monooxygenase (KMO). Currently, many agents have been discovered with significant improvement in animal models but only one aryl hydrocarbon receptor (AHR) agonist 30 (laquinimod) has entered clinical trials for treating Huntington's disease (HD). In this review, we describe neuroactive KP metabolites, discuss the dysregulation of KP in aging and NDs and summarize the development of KP regulators in preclinical and clinical studies, offering an outlook of targeting KP for NDs treatment in future.

Ubiquitylome study reveals the regulatory effect of α-lipoic acid on ubiquitination of key proteins in tryptophan metabolism pathway of pig liver

The decline in antioxidant defenses make it easily for human and animals to suffer from liver damage and diseases induced by oxidative stress, causing enormous losses to human health and livestock production. As one of the canonical protein post-translational modifications (PTMs), ubiquitination is widely involved in cell proliferation, apoptosis and damage/repair response, and is proven to be involved in the ability of mammals to resist oxidative stress. To explore whether α-lipoic acid (LA), a safe and efficient antioxidant, plays a role in regulating liver antioxidant status by PTMs, proteins in livers of pigs fed with LA were analyzed at the level of proteome and ubiquitylome. Based on proteome-wide enrichment of ubiquitination, a total of 7274 proteins were identified and 5326 were quantified, we also identified 1564 ubiquitination sites in 580 ubiquitinated proteins, among which there were 136 differentially ubiquitinated sites in 103 differentially ubiquitinated proteins upon LA. Further bioinformatics analysis showed that these differential proteins were mainly enriched in tryptophan metabolic pathway, and accompanied by significantly improvement of liver antioxidant capacity. We revealed the regulatory effect of LA on ubiquitination of kynurenine 3-monooxygenase (KMO) and other key proteins in tryptophan metabolism pathway of pig liver for the first time.

The Effect of PD-1 Inhibitor Combined with Irradiation on HMGB1-Associated Inflammatory Cytokines and Myocardial Injury

PURPOSE

To explore the effect of PD-1 inhibitors combined with irradiation on myocardial injury and the changes of HMGB1-associated inflammatory markers.

METHODS

Four groups of five mice were used, each groupformed by randomly dividing 20 mice (group A control; group B PD-1 inhibitors; group C Irradiation; group D PD-1 inhibitors+irradiation; n = 5 for each). The mice were treated with either PD-1 inhibitors or a 15 Gy dose of single heart irradiation, or both. Hematoxylin-eosin staining assessed the morphology and pathology of heart tissue; Masson staining assessed heart fibrosis; Tunel staining evaluated heart apoptosis; flow cytometry detected CD3+, CD4+, and CD8+ T lymphocytes in heart tissues; enzyme linked immunosorbent assay evaluated IL-1β, IL-6, and TNF-ɑ of heart tissue; Western blot and quantitative real-time PCR (qPCR) detected the expression of protein and mRNA of HMGB1, TLR-4, and NF-κB p65 respectively.

RESULTS

The degree of heart injury, collagen volume fraction (CVF) and apoptotic index (AI) in groups B, C, and D were higher than group A, but the differences between the CVF and AI of group A and group B were not statistical significance (P>0.05). Similarly, the absolute counts and relative percentage of CD3+ and CD8+ T lymphocytes and the concentrations of IL-1β, IL-6, and TNF-α in heart tissue with group D were significantly higher than the other groups (P<0.05). In addition, compared with group A, the expression of protein and mRNA of HMGB1 and NF-κB p65 in other groups were higher, and the differences between each group were statistically significant while TLR4 was not. In addition, interaction by PD-1 inhibitors and irradiation was found in inflammatory indicators, especially in the expression of the HMGB1 and CD8+ T lymphocytes.

CONCLUSION

PD-1 inhibitors can increase the expression of HMGB1-associated inflammatory cytokines and aggravate radiation-induced myocardial injury.

Kynurenine Pathway of Tryptophan Metabolism Is Associated with Hospital Mortality in Patients with Acute Respiratory Distress Syndrome: A Prospective Cohort Study

Acute respiratory distress syndrome (ARDS) involves dysregulated immune-inflammatory responses, characterized by severe oxidative stress and high mortality. Metabolites modulating the inflammatory and immune responses may play a central role in the pathogenesis of ARDS. Most biogenic amines may induce the production of reactive oxygen species, oxidative stress, mitochondrial dysfunction, and programmed cell death. We conducted a prospective study on metabolic profiling specific to the amino acids and biogenic amines of 69 patients with ARDS. Overall, hospital mortality was 52.2%. Between day 1 and day 7 after ARDS onset, plasma kynurenine levels and the kynurenine/tryptophan ratio were significantly higher among non-survivors than in survivors (all p < 0.05). Urine metabolic profiling revealed a significantly higher prevalence of tryptophan degradation and higher concentrations of metabolites downstream of the kynurenine pathway among non-survivors than among survivors upon ARDS onset. Cox regression models revealed that plasma kynurenine levels and the plasma kynurenine/tryptophan ratio on day 1 were independently associated with hospital mortality. The activation of the kynurenine pathway was associated with mortality in patients with ARDS. Metabolic phenotypes and modulating metabolic perturbations of the kynurenine pathway could perhaps serve as prognostic markers or as a target for therapeutic interventions aimed at reducing oxidative stress and mortality in ARDS.

Kynurenine-3-monooxygenase expression is activated in the pancreatic endocrine cells by diabetes and its blockade improves glucose-stimulated insulin secretion

Type 2 diabetes is associated with an inflammatory phenotype in the pancreatic islets. We previously demonstrated that proinflammatory cytokines potently activate the tryptophan/kynurenine pathway (TKP) in INS-1 cells and in normal rat islets. Here we examined: (1) the TKP enzymes expression in the diabetic GK islets; (2) the TKP enzymes expression profiles in the GK islets before and after the onset of diabetes; (3) The glucose-stimulated insulin secretion (GSIS) in vitro in GK islets after KMO knockdown using specific morpholino-oligonucleotides against KMO or KMO blockade using the specific inhibitor Ro618048; (4) The glucose tolerance and GSIS after acute in vivo exposure to Ro618048 in GK rats. We report a remarkable induction of the kmo gene in GK islets and in human islets exposed to proinflammatory conditions. It occurred prominently in beta cells. The increased expression and activity of KMO reflected an acquired adaptation. Both KMO knockdown and specific inhibitor Ro618048 enhanced GSIS in vitro in GK islets. Moreover, acute administration of Ro618048 in vivo improved glucose tolerance, GSIS and basal blood glucose levels in GK rats. These results demonstrate that targeting islet TKP is able to correct defective GSIS. KMO inhibition could represent a potential therapeutic strategy for type 2 diabetes.

Hypertriglyceridemia Acute Pancreatitis: Animal Experiment Research

In recent years, the number of acute pancreatitis cases caused by hypertriglyceridemia has increased gradually, which has caught the attention of the medical community. However, because the exact mechanism of hypertriglyceridemic acute pancreatitis (HTG-AP) is not clear, treatment and prevention in clinical practice face enormous challenges. Animal models are useful for elucidating the pathogenesis of diseases and developing and testing novel interventions. Therefore, animal experiments have become the key research means for us to understand and treat this disease. We searched almost all HTG-AP animal models by collecting many studies and finally collated common animals such as rats, mice and included some rare animals that are not commonly used, summarizing the methods to model spontaneous pancreatitis and induce pancreatitis. We sorted them on the basis of three aspects, including the selection of different animals, analyzed the characteristics of different animals, different approaches to establish hypertriglyceridemic pancreatitis and their relative advantages and disadvantages, and introduced the applications of these models in studies of pathogenesis and drug therapy. We hope this review can provide relevant comparisons and analyses for researchers who intend to carry out animal experiments and will help researchers to select and establish more suitable animal experimental models according to their own experimental design.

Intestinal Dysbiosis, the Tryptophan Pathway and Nonalcoholic Steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) progresses from simple steatosis to steatohepatitis (NASH), which may then progress to the development of cirrhosis and hepatocarcinoma. NASH is characterized by both steatosis and inflammation. Control of inflammation in NASH is a key step for the prevention of disease progression to severe sequalae. Intestinal dysbiosis has been recognized to be an important causal factor in the pathogenesis of NASH, involving both the accumulation of lipids and aggravation of inflammation. The effects of gut dysbiosis are mediated by adverse shifts of various intestinal commensal bacterial genera and their associated metabolites such as butyrate, tryptophan, and bile acids. In this review, we focus on the roles of tryptophan and its metabolites in NASH in association with intestinal dysbiosis and discuss possible therapeutic implications.

Molecular Patterns in Acute Pancreatitis Reflect Generalizable Endotypes of the Host Response to Systemic Injury in Humans

OBJECTIVE

Acute Pancreatitis (AP) is sudden onset pancreas inflammation that causes systemic injury with a wide and markedly heterogeneous range of clinical consequences. Here, we hypothesized that this observed clinical diversity corresponds to diversity in molecular subtypes that can be identified in clinical and multiomics data.

SUMMARY BACKGROUND DATA

Observational cohort study. n = 57 for the discovery cohort (clinical, transcriptomics, proteomics, and metabolomics data) and n = 312 for the validation cohort (clinical and metabolomics data).

METHODS

We integrated coincident transcriptomics, proteomics, and metabolomics data at serial time points between admission to hospital and up to 48 hours after recruitment from a cohort of patients presenting with acute pancreatitis. We systematically evaluated 4 different metrics for patient similarity using unbiased mathematical, biological, and clinical measures of internal and external validity.We next compared the AP molecular endotypes with previous descriptions of endotypes in a critically ill population with acute respiratory distress syndrome (ARDS).

RESULTS

Our results identify 4 distinct and stable AP molecular endotypes. We validated our findings in a second independent cohort of patients with AP.We observed that 2 endotypes in AP recapitulate disease endotypes previously reported in ARDS.

CONCLUSIONS

Our results show that molecular endotypes exist in AP and reflect biological patterns that are also present in ARDS, suggesting that generalizable patterns exist in diverse presentations of critical illness.

The Kynurenine Pathway and Kynurenine 3-Monooxygenase Inhibitors

Under normal physiological conditions, the kynurenine pathway (KP) plays a critical role in generating cellular energy and catabolizing tryptophan. Under inflammatory conditions, however, there is an upregulation of the KP enzymes, particularly kynurenine 3-monooxygenase (KMO). KMO has garnered much attention due to its production of toxic metabolites that have been implicated in many diseases and disorders. With many of these illnesses having an inadequate or modest treatment, there exists a need to develop KMO inhibitors that reduce the production of these toxic metabolites. Though prior efforts to find an appropriate KMO inhibitor were unpromising, the development of a KMO crystal structure has provided the opportunity for a rational structure-based design in the development of inhibitors. Therefore, the purpose of this review is to describe the kynurenine pathway, the kynurenine 3-monooxygenase enzyme, and KMO inhibitors and their potential candidacy for clinical use.

Small Molecule Regulators Targeting NAD+ Biosynthetic Enzymes

Nicotinamide adenine dinucleotide (NAD+) is a key player in many metabolic pathways as an activated carrier of electrons. In addition to being the cofactor for redox reactions, NAD+ also serves as the substrate for various enzymatic transformations such as adenylation and ADP-ribosylation. Maintaining cellular NAD+ homeostasis has been suggested as an effective anti-aging strategy. Given the importance of NAD+ in regulating a broad spectrum of cellular events, small molecules targeting NAD+ metabolism have been pursued as therapeutic interventions for the treatment of mitochondrial disorders and agerelated diseases. In this article, small molecule regulators of NAD+ biosynthetic enzymes will be reviewed. The focus will be given to the discovery and development of these molecules, the mechanism of action as well as their therapeutic potentials.

The Kynurenine Pathway in Acute Kidney Injury and Chronic Kidney Disease

BACKGROUND

The kynurenine pathway (KP) is the major catabolic pathway for tryptophan degradation. The KP plays an important role as the sole de novo nicotinamide adenine dinucleotide (NAD+) biosynthetic pathway in normal human physiology and functions as a counter-regulatory mechanism to mitigate immune responses during inflammation. Although the KP has been implicated in a variety of disorders including Huntington's disease, seizures, cardiovascular disease, and osteoporosis, its role in renal diseases is seldom discussed.

SUMMARY

This review summarizes the roles of the KP and its metabolites in acute kidney injury (AKI) and chronic kidney disease (CKD) based on current literature evidence. Metabolomics studies demonstrated that the KP metabolites were significantly altered in patients and animal models with AKI or CKD. The diagnostic and prognostic values of the KP metabolites in AKI and CKD were highlighted in cross-sectional and longitudinal human observational studies. The biological impact of the KP on the pathophysiology of AKI and CKD has been studied in experimental models of different etiologies. In particular, the activation of the KP was found to confer protection in animal models of glomerulonephritis, and its immunomodulatory mechanism may involve the regulation of T cell subsets such as Th17 and regulatory T cells. Manipulation of the KP to increase NAD+ production or diversion toward specific KP metabolites was also found to be beneficial in animal models of AKI. Key Messages: KP metabolites are reported to be dysregulated in human observational and animal experimental studies of AKI and CKD. In AKI, the magnitude and direction of changes in the KP depend on the etiology of the damage. In CKD, KP metabolites are altered with the onset and progression of CKD all the way to advanced stages of the disease, including uremia and its related vascular complications. The activation of the KP and diversion to specific sub-branches are currently being explored as therapeutic strategies in these diseases, especially with regards to the immunomodulatory effects of certain KP metabolites. Further elucidation of the KP may hold promise for the development of biomarkers and targeted therapies for these kidney diseases.

Kynurenic Acid and Its Analogue SZR-72 Ameliorate the Severity of Experimental Acute Necrotizing Pancreatitis

The pathophysiology of acute pancreatitis (AP) is not well understood, and the disease does not have specific therapy. Tryptophan metabolite L-kynurenic acid (KYNA) and its synthetic analogue SZR-72 are antagonists of the N-methyl-D-aspartate receptor (NMDAR) and have immune modulatory roles in several inflammatory diseases. Our aims were to investigate the effects of KYNA and SZR-72 on experimental AP and to reveal their possible mode of action. AP was induced by intraperitoneal (i.p.) injection of L-ornithine-HCl (LO) in SPRD rats. Animals were pretreated with 75-300 mg/kg KYNA or SZR-72. Control animals were injected with physiological saline instead of LO, KYNA and/or SZR-72. Laboratory and histological parameters, as well as pancreatic and systemic circulation were measured to evaluate AP severity. Pancreatic heat shock protein-72 and IL-1β were measured by western blot and ELISA, respectively. Pancreatic expression of NMDAR1 was investigated by RT-PCR and immunohistochemistry. Viability of isolated pancreatic acinar cells in response to LO, KYNA, SZR-72 and/or NMDA administration was assessed by propidium-iodide assay. The effects of LO and/or SZR-72 on neutrophil granulocyte function was also studied. Almost all investigated laboratory and histological parameters of AP were significantly reduced by administration of 300 mg/kg KYNA or SZR-72, whereas the 150 mg/kg or 75 mg/kg doses were less or not effective, respectively. The decreased pancreatic microcirculation was also improved in the AP groups treated with 300 mg/kg KYNA or SZR-72. Interestingly, pancreatic heat shock protein-72 expression was significantly increased by administration of SZR-72, KYNA and/or LO. mRNA and protein expression of NMDAR1 was detected in pancreatic tissue. LO treatment caused acinar cell toxicity which was reversed by 250 µM KYNA or SZR-72. Treatment of acini with NMDA (25, 250, 2000 µM) did not influence the effects of KYNA or SZR-72. Moreover, SZR-72 reduced LO-induced H₂O₂ production of neutrophil granulocytes. KYNA and SZR-72 have dose-dependent protective effects on LO-induced AP or acinar toxicity which seem to be independent of pancreatic NMDA receptors. Furthermore, SZR-72 treatment suppressed AP-induced activation of neutrophil granulocytes. This study suggests that administration of KYNA and its derivative could be beneficial in AP.

Tryptophan Catabolism and Inflammation: A Novel Therapeutic Target For Aortic Diseases

Aortic diseases are the primary public health concern. As asymptomatic diseases, abdominal aortic aneurysm (AAA) and atherosclerosis are associated with high morbidity and mortality. The inflammatory process constitutes an essential part of a pathogenic cascade of aortic diseases, including atherosclerosis and aortic aneurysms. Inflammation on various vascular beds, including endothelium, smooth muscle cell proliferation and migration, and inflammatory cell infiltration (monocytes, macrophages, neutrophils, etc.), play critical roles in the initiation and progression of aortic diseases. The tryptophan (Trp) metabolism or kynurenine pathway (KP) is the primary way of degrading Trp in most mammalian cells, disturbed by cytokines under various stress. KP generates several bioactive catabolites, such as kynurenine (Kyn), kynurenic acid (KA), 3-hydroxykynurenine (3-HK), etc. Depends on the cell types, these metabolites can elicit both hyper- and anti-inflammatory effects. Accumulating evidence obtained from various animal disease models indicates that KP contributes to the inflammatory process during the development of vascular disease, notably atherosclerosis and aneurysm development. This review outlines current insights into how perturbed Trp metabolism instigates aortic inflammation and aortic disease phenotypes. We also briefly highlight how targeting Trp metabolic pathways should be considered for treating aortic diseases.

Viability of diffuse large B-cell lymphoma cells is regulated by kynurenine 3-monooxygenase activity

Diffuse large B-cell lymphoma (DLBCL) is a clinically heterogeneous lymphoid malignancy that is the most common type of lymphoma in Japan. Previous studies have demonstrated that patients with DLBCL have a poor prognosis due to increased levels of indoleamine 2,3-dioxygnase and kynurenine (KYN). However, the roles of metabolites acting downstream of KYN and associated enzymes are not fully understood. The present study investigated the role of kynurenine 3-monooxygenase (KMO), which catalyzes the conversion of KYN to 3-hydroxykynurenine (3-HK), using serum samples from patients with DLBCL and human DLBCL cell lines with different KMO expression [STR-428 cells with high levels of KMO expression (KMO^high) and KML-1 cells with low levels of KMO expression (KMO^low)]. Serum samples from 28 patients with DLBCL and 34 healthy volunteers were used to investigate the association between prognosis and KMO activity or 3-HK levels. Furthermore, to investigate the roles of KMO and its related metabolites, STR-428 and KML-1 cell lines, and the lymph nodes of patients with DLBCL were analyzed by reverse transcription-quantitative PCR for KMO, KYNU, 3-hydroxyanthranilate-3,4-dioxygenase and quinolinate phosphoribosyltransferase, by western blotting, and immunohistochemical or immunofluorescence staining for KMO, and by cell viability and NAD⁺/NADH assays. KYN pathway metabolites in serum samples were measured by HPLC. Serum 3-HK levels were regulated independently of serum KYN levels, and increased serum 3-HK levels and KMO activity were found to be associated with worse disease progression. Notably, the addition of KMO inhibitors and 3-HK negatively and positively regulated the viability of DLBCL cells, respectively. Furthermore, NAD⁺ levels in KMO^high STR-428 cells were significantly higher than those in KMO^low KML-1 cells. These results suggested that 3-HK generated by KMO activity may be involved in the regulation of DLBCL cell viability via NAD⁺ synthesis.

Interleukin-22 Attenuates Acute Pancreatitis-Associated Intestinal Mucosa Injury in Mice via STAT3 Activation

Background/Aims

Interleukin-22 (IL-22) is an important cytokine maintaining homeostasis at barrier surfaces. In this study, the role of IL-22 in acute pancreatitis-associated intestinal injury was further explored.

Methods

Severe acute pancreatitis (SAP) was induced by administration of L-arginine in Balb/c mice at different time gradients. Histopathological examinations were made in both the pancreas and small intestine. Furthermore, recombinant murine IL-22 (rIL-22) was administrated to L-arginine-induced SAP mice by intraperitoneal injection. The mRNA levels of IL-22R1, Reg-IIIβ, Reg-IIIγ, Bcl-2, and Bcl-xL were detected in the small intestine by real-time polymerase chain reaction, and protein levels of total and phosphorylated STAT3 were assessed via Western blot.

Results

Compared with normal control group, 72 hours of L-arginine exposure induced the most characteristic histopathological changes of SAP, evidenced by pathological changes and serum amylase levels. Meanwhile, significant pancreatitis-associated intestinal mucosa injury was also observed. The gene expression levels of antimicrobial proteins Reg-IIIβ, Reg-IIIγ and anti-apoptosis proteins Bcl-2, Bcl-xL were downregulated in small intestine. Furthermore, Larginine- induced SAP was attenuated by rIL-22 treatment. Importantly, pancreatitis-associated intestinal mucosa injury was also ameliorated, reflected by improved pathological changes and significant increase in gene expression levels of Reg-IIIβ, Reg-IIIγ, Bcl-2 and Bcl-xL. Consistently, serum amylase levels and mortality were decreased in mice treated with rIL-22. Mechanistically, the upregulated expressions of these protective genes were achieved by activating STAT3.

Conclusions

Exogenous rIL-22 attenuates L-arginine-induced acute pancreatitis and intestinal mucosa injury in mice, via activating STAT3 signaling pathway and enhancing the expression of antimicrobial peptides and antiapoptotic genes.

Macrophage targeting in cancer

Tumorigenesis is not only determined by the intrinsic properties of cancer cells but also by their interactions with components of the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) are among the most abundant immune cells in the TME. During initial stages of tumor development, macrophages can either directly promote antitumor responses by killing tumor cells or indirectly recruit and activate other immune cells. As genetic changes occur within the tumor or T helper 2 (TH 2) cells begin to dominate the TME, TAMs begin to exhibit an immunosuppressive protumor phenotype that promotes tumor progression, metastasis, and resistance to therapy. Thus, targeting TAMs has emerged as a strategy for cancer therapy. To date, TAM targeting strategies have focused on macrophage depletion and inhibition of their recruitment into the TME. However, these strategies have shown limited therapeutic efficacy, although trials are still underway with combination therapies. The fact that macrophages have the potential for antitumor activity has moved the TAM targeting field toward the development of TAM-reprogramming strategies to support this antitumor immune response. Here, we discuss the various roles of TAMs in cancer therapy and their immunosuppressive properties, as well as implications for emerging checkpoint inhibitor-based immunotherapies. We review state-of-the-art TAM-targeting strategies, focusing on current ones at the preclinical and clinical trial stages that aim to reprogram TAMs as an oncological therapy.

Hypoxic preconditioning protects against ischemic kidney injury through the IDO1/kynurenine pathway

Prolonged cellular hypoxia leads to energetic failure and death. However, sublethal hypoxia can trigger an adaptive response called hypoxic preconditioning. While prolyl-hydroxylase (PHD) enzymes and hypoxia-inducible factors (HIFs) have been identified as key elements of oxygen-sensing machinery, the mechanisms by which hypoxic preconditioning protects against insults remain unclear. Here, we perform serum metabolomic profiling to assess alterations induced by two potent cytoprotective approaches, hypoxic preconditioning and pharmacologic PHD inhibition. We discover that both approaches increase serum kynurenine levels and enhance kynurenine biotransformation, leading to preservation of NAD+ in the post-ischemic kidney. Furthermore, we show that indoleamine 2,3-dioxygenase 1 (Ido1) deficiency abolishes the systemic increase of kynurenine and the subsequent renoprotection generated by hypoxic preconditioning and PHD inhibition. Importantly, exogenous administration of kynurenine restores the hypoxic preconditioning in the context of Ido1 deficiency. Collectively, our findings demonstrate a critical role of the IDO1-kynurenine axis in mediating hypoxic preconditioning.

Stromal Neutrophil Extracellular Trap Density Is an Independent Prognostic Factor for Cervical Cancer Recurrence

BACKGROUND

Emerging evidence indicates that the tumor microenvironment influences tumor progression and patient prognosis through various inflammatory cells. Polymorphonuclear neutrophils (PMNs) and their functional structures termed neutrophil extracellular traps (NETs) are prominent constituents of several malignant tumors and affect the tumor microenvironment and cancer evolution. Here, we investigate the prognostic value of PMNs and NETs for recurrence in patients with cervical cancer.

METHODS

The study comprised 126 cervical cancer patients who were retrospectively enrolled. CD66b+ neutrophils and myeloperoxidase/citrullinated histone H3 (MPO/H3Cit)-labeled NETs were assessed by immunofluorescence, and the relationships with clinical and histopathologic features and patient outcome were evaluated.

RESULTS

The highest density of CD66b+ neutrophils were observed in the stromal compartment (median 55 cells/mm2). Above median densities of stromal CD66b+ neutrophils and NETs were significantly associated with short recurrence-free survival (RFS) (P = 0.041 and P = 0.006, respectively). Multivariate analysis identified high clinical stage (hazard ratio [HR] 6.40; 95% confidence interval [CI] 3.51-11.64; P < 0.001), lymph node metastases (HR 4.69; 95% CI 3.09-9.66; P = 0.006) and high density of NETs (HR 2.66; 95% CI 1.21-5.82; P = 0.015) as independent prognostic factors for short RFS, whereas a high density of CD66b+ neutrophils was not significant. Patients with a high NET density showed worse recurrence status in every stage, but the difference was only significant for stage I (P = 0.042), not stages II, III, or IV (all P > 0.05). Combining stromal NET density and the tumor, nodes, metastasis (TNM) staging system had better prognostic accuracy for cervical cancer than the TNM staging system alone at five and six years respectively (P = 0.010 and P = 0.023).

CONCLUSION

Stromal NET density is an independent prognostic factor for RFS in cervical cancer. Combining NETs with the TNM staging system may further improve prognostic stratification.

The kynurenine pathway in chronic diseases: a compensatory mechanism or a driving force?

The kynurenine (KYN) pathway (KP) of tryptophan (TRP) metabolism is dysregulated in inflammation-driven pathologies including oncological and brain diseases [e.g., multiple sclerosis (MS), depression] and thus is a promising therapeutic target. Both pathological and compensatory mechanisms underlie disease-associated KP activation. There is growing evidence for bioenergetic roles of certain KP metabolites such as kynurenic acid (KA), or quinolinic acid (QA) as an NAD⁺ precursor, which may explain its frequently observed 'pathological' overactivation. Disease- and tissue-specific aspects, negative feedback on inflammatory signals, and the balance of downstream metabolites are likely to be decisive factors in the interpretation of an imbalanced KP. Therapeutic strategies should consider the compensatory actions and bioenergetic roles of KP metabolites to successfully design future theragnostic approaches aimed at attenuating disease progression.

Assessment of the safety, pharmacokinetics and pharmacodynamics of GSK3335065, an inhibitor of kynurenine monooxygenase, in a randomised placebo-controlled first-in-human study in healthy volunteers

GSK3335065 is an inhibitor of kynurenine monooxygenase (KMO) being developed for the treatment of acute pancreatitis. Healthy male volunteers were administered ascending doses of GSK3335065 or matched placebo as a single intravenous bolus injection to assess safety, tolerability, pharmacokinetics and pharmacodynamics. GSK3335065 displayed an apparent volume of distribution between 20.6 L and 44.6 L, a clearance between 0.462 L/h and 0.805 L/hr and a terminal half-life between 31.3 and 34.5 hr. In the single subject who received 1.3 mg GSK3335065, changes in tryptophan pathway metabolites were observed consistent with the changes seen in preclinical species suggesting that KMO enzyme activity was partially inhibited. However, a broad complex ventricular tachycardia was observed in this subject, which was judged to be a Serious Adverse Event (SAE) and resulted in early termination of the study. While development of GSK3335065 was subsequently discontinued, significant confounding factors hinder a clear interpretation that the tachycardia was directly related to administration of the compound.

Protective Role of Kynurenine 3-Monooxygenase in Allograft Rejection and Tubular Injury in Kidney Transplantation

Renal tubular epithelial cells (TECs) are the primary targets of ischemia-reperfusion injury (IRI) and rejection by the recipient's immune response in kidney transplantation (KTx). However, the molecular mechanism of rejection and IRI remains to be identified. Our previous study demonstrated that kynurenine 3-monooxygenase (KMO) and kynureninase were reduced in ischemia-reperfusion procedure and further decreased in rejection allografts among mismatched pig KTx. Herein, we reveal that TEC injury in acutely rejection allografts is associated with alterations of Bcl2 family proteins, reduction of tight junction protein 1 (TJP1), and TEC-specific KMO. Three cytokines, IFN γ , TNFα, and IL1β, reported in our previous investigation were identified as triggers of TEC injury by altering the expression of Bcl2, BID, and TJP1. Allograft rejection and TEC injury were always associated with a dramatic reduction of KMO. 3HK and 3HAA, as direct and downstream products of KMO, effectively protected TEC from injury via increasing expression of Bcl-xL and TJP1. Both 3HK and 3HAA further prevented allograft rejection by inhibiting T cell proliferation and up-regulating aryl hydrocarbon receptor expression. Pig KTx with the administration of DNA nanoparticles (DNP) that induce expression of indoleamine 2,3-dioxygenase (IDO) and KMO to increase 3HK/3HAA showed an improvement of allograft rejection as well as murine skin transplant in IDO knockout mice with the injection of 3HK indicated a dramatic reduction of allograft rejection. Taken together, our data provide strong evidence that reduction of KMO in the graft is a key mediator of allograft rejection and loss. KMO can effectively improve allograft outcome by attenuating allograft rejection and maintaining graft barrier function.

Inflammation control and improvement of cognitive function in COVID-19 infections: is there a role for kynurenine 3-monooxygenase inhibition?

The novel respiratory virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), emerged during late 2019 and spread rapidly across the world. It is now recognised that the nervous system can be affected in COVID-19, with several studies reporting long-term cognitive problems in patients. The metabolic pathway of tryptophan degradation, known as the kynurenine pathway (KP), is significantly activated in patients with COVID-19. KP metabolites have roles in regulating both inflammatory/immune responses and neurological functions. In this review, we speculate on the effects of KP activation in patients with COVID-19, and how modulation of this pathway might impact inflammation and reduce neurological symptoms.

Pharmacophore-Based Virtual Screening of Novel Competitive Inhibitors of the Neurodegenerative Disease Target Kynurenine-3-Monooxygenase

The pathogenesis of several neurodegenerative diseases such as Alzheimer’s or Huntington’s disease has been associated with metabolic dysfunctions caused by imbalances in the brain and cerebral spinal fluid levels of neuroactive metabolites. Kynurenine monooxygenase (KMO) is considered an ideal therapeutic target for the regulation of neuroactive tryptophan metabolites. Despite significant efforts, the known KMO inhibitors lack blood–brain barrier (BBB) permeability and upon the mimicking of the substrate binding mode, are subject to produce reactive oxygen species as a side reaction. The computational drug design is further complicated by the absence of complete crystal structure information for human KMO (hKMO). In the current work, we performed virtual screening of readily available compounds using several protein–ligand complex pharmacophores. Each of the pharmacophores accounts for one of three distinct reported KMO protein-inhibitor binding conformations. As a result, six novel KMO inhibitors were discovered based on an in vitro fluorescence assay. Compounds VS1 and VS6 were predicted to be BBB permeable and avoid the hydrogen peroxide production dilemma, making them valuable, novel hit compounds for further drug property optimization and advancement in the drug design pipeline.

NAD+ Metabolism, Metabolic Stress, and Infection

Nicotinamide adenine dinucleotide (NAD⁺) is an essential metabolite with wide-ranging and significant roles in the cell. Defects in NAD⁺ metabolism have been associated with many human disorders; it is therefore an emerging therapeutic target. Moreover, NAD⁺ metabolism is perturbed during colonization by a variety of pathogens, either due to the molecular mechanisms employed by these infectious agents or by the host immune response they trigger. Three main biosynthetic pathways, including the de novo and salvage pathways, contribute to the production of NAD⁺ with a high degree of conservation from bacteria to humans. De novo biosynthesis, which begins with l-tryptophan in eukaryotes, is also known as the kynurenine pathway. Intermediates of this pathway have various beneficial and deleterious effects on cellular health in different contexts. For example, dysregulation of this pathway is linked to neurotoxicity and oxidative stress. Activation of the de novo pathway is also implicated in various infections and inflammatory signaling. Given the dynamic flexibility and multiple roles of NAD⁺ intermediates, it is important to understand the interconnections and cross-regulations of NAD⁺ precursors and associated signaling pathways to understand how cells regulate NAD⁺ homeostasis in response to various growth conditions. Although regulation of NAD⁺ homeostasis remains incompletely understood, studies in the genetically tractable budding yeast Saccharomyces cerevisiae may help provide some molecular basis for how NAD⁺ homeostasis factors contribute to the maintenance and regulation of cellular function and how they are regulated by various nutritional and stress signals. Here we present a brief overview of recent insights and discoveries made with respect to the relationship between NAD⁺ metabolism and selected human disorders and infections, with a particular focus on the de novo pathway. We also discuss how studies in budding yeast may help elucidate the regulation of NAD⁺ homeostasis.

Protective Effects of Necrostatin-1 in Acute Pancreatitis: Partial Involvement of Receptor Interacting Protein Kinase 1

Acute pancreatitis (AP) is a severe and potentially fatal disease caused predominantly by alcohol excess and gallstones, which lacks a specific therapy. The role of Receptor-Interacting Protein Kinase 1 (RIPK1), a key component of programmed necrosis (Necroptosis), is unclear in AP. We assessed the effects of RIPK1 inhibitor Necrostatin-1 (Nec-1) and RIPK1 modification (RIPK1K45A: kinase dead) in bile acid (TLCS-AP), alcoholic (FAEE-AP) and caerulein hyperstimulation (CER-AP) mouse models. Involvement of collateral Nec-1 target indoleamine 2,3-dioxygenase (IDO) was probed with the inhibitor Epacadostat (EPA). Effects of Nec-1 and RIPK1K45A were also compared on pancreatic acinar cell (PAC) fate in vitro and underlying mechanisms explored. Nec-1 markedly ameliorated histological and biochemical changes in all models. However, these were only partially reduced or unchanged in RIPK1K45A mice. Inhibition of IDO with EPA was protective in TLCS-AP. Both Nec-1 and RIPK1K45A modification inhibited TLCS- and FAEE-induced PAC necrosis in vitro. Nec-1 did not affect TLCS-induced Ca2+ entry in PACs, however, it inhibited an associated ROS elevation. The results demonstrate protective actions of Nec-1 in multiple models. However, RIPK1-dependent necroptosis only partially contributed to beneficial effects, and actions on targets such as IDO are likely to be important.

Significance of Kynurenine 3-Monooxygenase Expression in Colorectal Cancer

Colorectal cancer (CRC) is a leading cause of cancer-related deaths. Because of the lack of reliable prognostic and predictive biomarkers for CRC, most patients are often diagnosed at a late stage. The tryptophan–kynurenine pathway plays a crucial role in promoting cancer progression. Kynurenine is considered an oncometabolite in colon cancer, and its downstream metabolites are also associated with CRC. Kynurenine 3-monooxygenase (KMO), a pivotal enzyme that catalyzes kynurenine metabolism, is essential for several cellular processes. In the current study, we explored the role of KMO in CRC. Immunohistochemical results showed that KMO was upregulated in CRC tissues relative to paired healthy tissue and polyps. Moreover, CRC patients with higher KMO expression were associated with higher metastasis and poorer survival rates. Knockdown of KMO decreased the expression of cancer stem cell markers, as well as the sphere-forming, migration, and invasion abilities of CRC cells. Additionally, blockade of the enzymatic activity of KMO using an inhibitor suppressed sphere formation and cell motility in CRC cells. These findings suggest the clinical relevance of KMO in CRC tumorigenesis and aggressiveness.

Metabolomic-based clinical studies and murine models for acute pancreatitis disease: A review

Acute pancreatitis (AP) is one of the most common gastroenterological disorders requiring hospitalization and is associated with substantial morbidity and mortality. Metabolomics nowadays not only help us to understand cellular metabolism to a degree that was not previously obtainable, but also to reveal the importance of the metabolites in physiological control, disease onset and development. An in-depth understanding of metabolic phenotyping would be therefore crucial for accurate diagnosis, prognosis and precise treatment of AP. In this review, we summarized and addressed the metabolomics design and workflow in AP studies, as well as the results and analysis of the in-depth of research. Based on the metabolic profiling work in both clinical populations and experimental AP models, we described the metabolites with potential utility as biomarkers and the correlation between the altered metabolites and AP status. Moreover, the disturbed metabolic pathways correlated with biological function were discussed in the end. A practical understanding of current and emerging metabolomic approaches applicable to AP and use of the metabolite information presented will aid in designing robust metabolomics and biological experiments that result in identification of unique biomarkers and mechanisms, and ultimately enhanced clinical decision-making.

Tryptophan Metabolism via Kynurenine Pathway: Role in Solid Organ Transplantation

Solid organ transplantation is a gold standard treatment for patients suffering from an end-stage organ disease. Patient and graft survival have vastly improved during the last couple of decades; however, the field of transplantation still encounters several unique challenges, such as a shortage of transplantable organs and increasing pool of extended criteria donor (ECD) organs, which are extremely prone to ischemia-reperfusion injury (IRI), risk of graft rejection and challenges in immune regulation. Moreover, accurate and specific biomarkers, which can timely predict allograft dysfunction and/or rejection, are lacking. The essential amino acid tryptophan and, especially, its metabolites via the kynurenine pathway has been widely studied as a contributor and a therapeutic target in various diseases, such as neuropsychiatric, autoimmune disorders, allergies, infections and malignancies. The tryptophan-kynurenine pathway has also gained interest in solid organ transplantation and a variety of experimental studies investigating its role both in IRI and immune regulation after allograft implantation was first published. In this review, the current evidence regarding the role of tryptophan and its metabolites in solid organ transplantation is presented, giving insights into molecular mechanisms and into therapeutic and diagnostic/prognostic possibilities.

Early diagnosis of serum sICAM-1 and sRAGE in severe acute pancreatitis, and efficacy and prognosis prediction of glutamine combined with ulinastatin

Acute pancreatitis (AP) is a common gastrointestinal disease that can become severe, so that intensive care may be required. This study was to examine serum soluble intercellular adhesion molecule-1 (sICAM-1), and soluble receptor for advanced glycation end products (sRAGE) for efficacy and prognosis prediction of glutamine (Glu) combined with ulinastatin (UTI) on severe acute pancreatitis (SAP). Fifty-four mild acute pancreatitis (MAP) patients admitted to Yidu Central Hospital of Weifang were selected as the MAP group (MAPG), 80 with SAP were divided as the SAP group (SAPG), and 60 healthy individuals who came to Yidu Central Hospital of Weifang for physical examination during the same period were included to the normal group (NG). Serum sICAM-1 and sRAGE were measured and their predictive value of efficacy and prognosis were analyzed. In view of the treatment effectiveness and prognosis, the patients were divided into effective group (EG) and ineffective group (IG), good prognosis group (GPG) and poor prognosis group (PPG). The levels of D-lactate, diamine oxidase (DAO), endotoxin and T-lymphocyte subsets (CD3⁺, CD4⁺, CD8⁺ and CD4⁺/CD8⁺) were measured and the changes before and after treatment were analyzed. The AUC values of NG and MAPG, NG and SAPG, MAPG and SAPG were 0.857, 0.939 and 0.856, respectively, those of predicting efficacy were 0.920 and 0.874, respectively, and those of poor prognosis in the SAPG were 0.914 and 0.879, respectively. In the SAPG, D-lactate, DAO, endotoxin and CD8⁺ decreased markedly after treatment, but CD3⁺, CD4⁺, and CD4⁺/CD8⁺ were opposite. SICAM-1 and sRAGE were also independent risk factors for poor prognosis in the SAPG. Serum sICAM-1 and sRAGE have high predictive value for early diagnosis, efficacy and prognosis of Glu combined with UTI.

Full-length in meso structure and mechanism of rat kynurenine 3-monooxygenase inhibition

The structural mechanisms of single-pass transmembrane enzymes remain elusive. Kynurenine 3-monooxygenase (KMO) is a mitochondrial protein involved in the eukaryotic tryptophan catabolic pathway and is linked to various diseases. Here, we report the mammalian full-length structure of KMO in its membrane-embedded form, complexed with compound 3 (identified internally) and compound 4 (identified via DNA-encoded chemical library screening) at 3.0 Å resolution. Despite predictions suggesting that KMO has two transmembrane domains, we show that KMO is actually a single-pass transmembrane protein, with the other transmembrane domain lying laterally along the membrane, where it forms part of the ligand-binding pocket. Further exploration of compound 3 led to identification of the brain-penetrant compound, 5. We show that KMO is dimeric, and that mutations at the dimeric interface abolish its activity. These results will provide insight for the drug discovery of additional blood-brain-barrier molecules, and help illuminate the complex biology behind single-pass transmembrane enzymes.

Kynurenine 3-monooxygenase upregulates pluripotent genes through β-catenin and promotes triple-negative breast cancer progression

Background

Triple-negative breast cancer (TNBC) is aggressive and has a poor prognosis. Kynurenine 3-monooxygenase (KMO), a crucial kynurenine metabolic enzyme, is involved in inflammation, immune response and tumorigenesis. We aimed to study the role of KMO in TNBC.

Methods

KMO alteration and expression data from public databases were analyzed. KMO expression levels in TNBC samples were analyzed using immunohistochemistry. Knockdown of KMO in TNBC cells was achieved by RNAi and CRISPR/Cas9. KMO functions were examined by MTT, colony-forming, transwell migration/invasion, and mammosphere assays. The molecular events were analyzed by cDNA microarrays, Western blot, quantitative real-time PCR and luciferase reporter assays. Tumor growth and metastasis were detected by orthotopic xenograft and tail vein metastasis mouse models, respectively.

Findings

KMO was amplified and associated with worse survival in breast cancer patients. KMO expression levels were higher in TNBC tumors compared to adjacent normal mammary tissues. In vitro ectopic KMO expression increased cell growth, colony and mammosphere formation, migration, invasion as well as mesenchymal marker expression levels in TNBC cells. In addition, KMO increased pluripotent gene expression levels and promoter activities in vitro. Mechanistically, KMO was associated with β-catenin and prevented β-catenin degradation, thereby enhancing the transcription of pluripotent genes. KMO knockdown suppressed tumor growth and the expression levels of β-catenin, CD44 and Nanog. Furthermore, mutant KMO (known with suppressed enzymatic activity) could still promote TNBC cell migration/invasion. Importantly, mice bearing CRISPR KMO-knockdown TNBC tumors showed decreased lung metastasis and prolonged survival.

Interpretation

KMO regulates pluripotent genes via β-catenin and plays an oncogenic role in TNBC progression.

New frontiers in flavin-dependent monooxygenases

Flavin-dependent monooxygenases catalyze a wide variety of redox reactions in important biological processes and are responsible for the synthesis of highly complex natural products. Although much has been learned about FMO chemistry in the last ~80 years of research, several aspects of the reactions catalyzed by these enzymes remain unknown. In this review, we summarize recent advancements in the flavin-dependent monooxygenase field including aspects of flavin dynamics, formation and stabilization of reactive species, and the hydroxylation mechanism. Novel catalysis of flavin-dependent N-oxidases involving consecutive oxidations of amines to generate oximes or nitrones is presented and the biological relevance of the products is discussed. In addition, the activity of some FMOs have been shown to be essential for the virulence of several human pathogens. We also discuss the biomedical relevance of FMOs in antibiotic resistance and the efforts to identify inhibitors against some members of this important and growing family enzymes.

Critical thresholds: key to unlocking the door to the prevention and specific treatments for acute pancreatitis

Acute pancreatitis (AP), an acute inflammatory disorder of the exocrine pancreas, is one of the most common gastrointestinal diseases encountered in emergency departments with no specific treatments. Laboratory-based research has formed the cornerstone of endeavours to decipher the pathophysiology of AP, because of the limitations of such study in human beings. While this has provided us with substantial understanding, we cannot answer several pressing questions. These are: (a) Why is it that only a minority of individuals with gallstones, or who drink alcohol excessively, or are exposed to other causative factors develop AP? (b) Why do only some develop more severe manifestations of AP with necrosis and/or organ failure? (c) Why have we been unable to find an effective therapeutic for AP? This manuscript provides a state-of-the-art review of our current understanding of the pathophysiology of AP providing insights into the unanswered clinical questions. We describe multiple protective factors operating in most people, and multiple stressors that in a minority induce AP, independently or together, via amplification loops. We present testable hypotheses aimed at halting progression of severity for the development of effective treatments for this common unpredictable disease.

Experimental Acute Pancreatitis Models: History, Current Status, and Role in Translational Research

Acute pancreatitis is a potentially severe inflammatory disease that may be associated with a substantial morbidity and mortality. Currently there is no specific treatment for the disease, which indicates an ongoing demand for research into its pathogenesis and development of new therapeutic strategies. Due to the unpredictable course of acute pancreatitis and relatively concealed anatomical site in the retro-peritoneum, research on the human pancreas remains challenging. As a result, for over the last 100 years studies on the pathogenesis of this disease have heavily relied on animal models. This review aims to summarize different animal models of acute pancreatitis from the past to present and discuss their main characteristics and applications. It identifies key studies that have enhanced our current understanding of the pathogenesis of acute pancreatitis and highlights the instrumental role of animal models in translational research for developing novel therapies.

Advantages of brain penetrating inhibitors of kynurenine-3-monooxygenase for treatment of neurodegenerative diseases

Kynurenine-3-monooxygenase (KMO) is an important therapeutic target for several brain disorders that has been extensively studied in recent years. Potent inhibitors towards KMO have been developed and tested within different disease models, showing great therapeutic potential, especially in models of neurodegenerative disease. The inhibition of KMO reduces the production of downstream toxic kynurenine pathway metabolites and shifts the flux to the formation of the neuroprotectant kynurenic acid. However, the efficacy of KMO inhibitors in neurodegenerative disease has been limited by their poor brain permeability. Combined with virtual screening and prodrug strategies, a novel brain penetrating KMO inhibitor has been developed which dramatically decreases neurotoxic metabolites. This review highlights the importance of KMO as a drug target in neurological disease and the benefits of brain permeable inhibitors in modulating kynurenine pathway metabolites in the central nervous system.

Tryptophan Metabolism as a Pharmacological Target

L-Tryptophan is an essential amino acid required for protein synthesis. It undergoes an extensive and complex metabolism along several pathways, resulting in many bioactive molecules acting in various organs through different action mechanisms. Enzymes involved in its metabolism, metabolites themselves, or their receptors, represent potential therapeutic targets, which are the subject of dynamic research. Disruptions in L-tryptophan metabolism are reported in several neurological, metabolic, psychiatric, and intestinal disorders, paving the way to develop drugs to target it. This review will briefly describe L-tryptophan metabolism and present and discuss the most recent pharmacological developments targeting it.

A novel role for kynurenine 3-monooxygenase in mitochondrial dynamics

The enzyme kynurenine 3-monooxygenase (KMO) operates at a critical branch-point in the kynurenine pathway (KP), the major route of tryptophan metabolism. As the KP has been implicated in the pathogenesis of several human diseases, KMO and other enzymes that control metabolic flux through the pathway are potential therapeutic targets for these disorders. While KMO is localized to the outer mitochondrial membrane in eukaryotic organisms, no mitochondrial role for KMO has been described. In this study, KMO deficient Drosophila melanogaster were investigated for mitochondrial phenotypes in vitro and in vivo. We find that a loss of function allele or RNAi knockdown of the Drosophila KMO ortholog (cinnabar) causes a range of morphological and functional alterations to mitochondria, which are independent of changes to levels of KP metabolites. Notably, cinnabar genetically interacts with the Parkinson's disease associated genes Pink1 and parkin, as well as the mitochondrial fission gene Drp1, implicating KMO in mitochondrial dynamics and mitophagy, mechanisms which govern the maintenance of a healthy mitochondrial network. Overexpression of human KMO in mammalian cells finds that KMO plays a role in the post-translational regulation of DRP1. These findings reveal a novel mitochondrial role for KMO, independent from its enzymatic role in the kynurenine pathway.