SUCNR1-mediated chemotaxis of macrophages aggravates obesity-induced inflammation and diabetes

G-Protein-Coupled Receptors in Chronic Kidney Disease Induced by Hypertension and Diabetes

Hypertension and diabetes are two common causes of chronic kidney disease. Hypertension can induce renal vascular injury, glomerular damage, podocyte loss, and tubular injury, leading to tubulointerstitial fibrosis. A number of factors influence the regulation of hypertension, among which G-protein-coupled receptors (GPCRs) have been studied extensively because they are desirable targets for drug development. Compared to hypertension, the regulatory effects of GPCRs on hypertensive kidney disease (HKD) are less generalized. In this review, we discussed the GPCRs involved in hypertensive kidney disease, such as angiotensin II receptors (AT1R and AT2R), Mas receptor (MasR), Mas-related G-protein-coupled receptor member D (MrgD), relaxin family receptor 1 (RXFP1), adenosine receptors (A1, A2A, A2B, and A3), purinergic P2Y receptors, and endothelin receptors (ETA and ETB). The progression of HKD is rarely reversed but can be retarded by ameliorating the hypertensive microenvironment in the kidneys. However, simply reducing blood pressure cannot stop the progression of HKD. Diabetic nephropathy (DN) is the most common cause of end-stage renal disease (ESRD), which is a major cause of morbidity and mortality in diabetes. Many GPCRs are involved in DN. Here, we select some well-studied GPCRs that are directly associated with the pathogenesis of DN to illustrate their mechanisms. The main purpose of this review is to provide an overview of the GPCRs involved in the occurrence and progression of HKD and DN and their probable pathophysiological mechanisms, which we hope will help in developing new therapeutic strategies.

Trans-right ventricle metabolite gradients in obesity highlight multiple metabolic pathways

Obesity and metabolic dysfunction are associated with pulmonary vascular remodeling, yet molecular mechanisms remain poorly understood. We sought to study trans-right ventricular (RV) metabolite gradients to elucidate potential molecular pathways operant among individuals with obesity and pulmonary hypertension. In this study, 38 individuals with obesity (mean age 58 years, 68% women, average BMI 36.6 kg/m2) underwent invasive right heart catheterization. Multi-site blood sampling from the superior vena cava and pulmonary artery was performed to assess trans-RV gradients, with targeted metabolite profiling using liquid chromatography-mass spectrometry. We found 56 metabolites with significant trans-RV gradients (FDR q < 0.05), including intermediates of fatty acid oxidation, the tricarboxylic acid cycle, and nucleotide metabolism. Further, trans-RV gradients in lipid and purine metabolism were associated with BMI and related cardiometabolic traits, such as waist circumference, insulin resistance, and serum lipids. Finally, differential levels of bile acids, intermediates of lipid peroxidation, and nucleotide metabolism across the RV were associated with pulmonary hypertension. In conclusion, trans-RV metabolite gradients among individuals with obesity reveal alterations in metabolites representative of molecular pathways such as fatty acid oxidation, and others correlated with cardiometabolic traits and/or pulmonary hypertension, including orotic acid, bile acids, and acylcarnitines.

The Evolving Role of Macrophage Metabolic Reprogramming in Obesity

Recent research has extensively explored the critical role of energy metabolism in shaping the inflammatory response and polarization of macrophages in obesity. This rapidly growing field emphasizes the need to understand the connection between metabolic processes that support macrophage polarization in obesity. Although most published research in this area has focused on glucose and fatty acids, how the flux through other metabolic pathways (such as ketone and amino acid oxidation) in macrophages is altered in obesity is not well defined. This review summarizes the main alterations in uptake, storage, and oxidation of oxidative substrates (glucose, fatty acids, ketone bodies, and amino acids) in macrophages and how these alterations are linked to macrophage polarization and contribution to augmented inflammatory markers in obesity. The review also discusses how oxidative substrates could modulate macrophage energy metabolism and inflammatory responses via feeding into other nonoxidative pathways (such as the pentose phosphate pathway, triacylglycerol synthesis/accumulation), via acting as signalling molecules, or via mediating post-translational modifications (such as O-GlcNAcylation or β-hydroxybutyrylation). The review also identifies several critical unanswered questions regarding the characteristics (functional and metabolic) of macrophages from different origins (adipose tissue, skeletal muscle, bone marrow) in obesity and how these characteristics contribute to early vs late phases of obesity. We also identified a number of new therapeutic targets that could be evaluated in future investigations. Targeting macrophage metabolism in obesity is an exciting and active area of research with significant potential to help identify new treatments to limit the detrimental effects of inflammation in obesity.

Dietary succinate supplementation alleviates DSS-induced colitis via the IL-4Rα/Hif-1α Axis

Inflammatory bowel disease (IBD) remains a pressing global health challenge, necessitating novel therapeutic strategies. Succinate, a metabolite known for its role in type 2 immunity and tuft cell activation in the small intestine, presents its potential in IBD management. However, its impact on colonic inflammation has not been explored. Here, we demonstrate that succinate administration induces a type 2 immune response, significantly alleviating dextran sulfate sodium (DSS)-induced colonic inflammation. Succinate enhances antibacterial capacity, reduces intestinal permeability, and reshapes the colonic cytokine milieu. Mechanistically, succinate promotes myeloid cell expansion in peripheral blood, mesenteric lymph nodes, and the colonic lamina propria. The protective effects of succinate were abolished in Ccr2-/- mice, confirming the role of monocyte recruitment, but persisted in Rag1-/- mice, indicating independence from adaptive immunity. Adoptive transfer of monocytes from succinate-treated donors mitigated intestinal inflammation in recipient mice. Transcriptomic analysis revealed heightened expression of Il1b and Il6, and higher lactate production in monocytes upon lipopolysaccharide (LPS) stimulation, highlighting a reprogrammed pro-inflammatory trained immunity phenotype. Finally, we identify the IL-4Rα/Hif-1α axis is critical for succinate-mediated protection. These findings reveal the ability of succinate to reprogram monocytes into protective intestinal macrophages via induction of type 2 response, restoring homeostasis through enhanced barrier function and immune modulation. Our study positions thus uncover succinate as a promising therapeutic candidate for IBD.

Succinate receptor 1 signaling mutually depends on subcellular localization and cellular metabolism

Succinate is a pivotal tricarboxylic acid cycle metabolite but also specifically activates the Gi- and Gq-coupled succinate receptor 1 (SUCNR1). Contradictory roles of succinate and succinate-SUCNR1 signaling include reports about its anti- or pro-inflammatory effects. The link between cellular metabolism and localization-dependent SUCNR1 signaling qualifies as a potential cause for the reported conflicts. To systematically address this connection, we used a diverse set of methods, including several bioluminescence resonance energy transfer-based biosensors, dynamic mass redistribution measurements, second messenger and kinase phosphorylation assays, calcium imaging, and metabolic analyses. Different cellular metabolic states were mimicked using glucose (Glc) or glutamine (Gln) as available energy substrates to provoke differential endogenous succinate (SUC) production. We show that SUCNR1 signaling, localization, and metabolism are mutually dependent, with SUCNR1 showing distinct spatial and energy substrate-dependent Gi and Gq protein activation. We found that Gln-consumption associated with a higher rate of oxidative phosphorylation causes increased extracellular SUC concentrations, accompanied by a higher rate of SUCNR1 internalization, reduced miniGq protein recruitment to the plasma membrane, and lower Ca2+ signals. In Glc, under basal conditions, SUCNR1 causes stronger Gq than Gi protein activation, while the opposite is true upon stimulation with an agonist. In addition, SUCNR1 specifically interacts with miniG proteins in endosomal compartments. In THP-1 cells, polarized to M2-like macrophages, endogenous SUCNR1-mediated Gi signaling stimulates glycolysis, while Gq signaling inhibits the glycolytic rate. Our results suggest that the metabolic context determines spatially dependent SUCNR1 signaling, which in turn modulates cellular energy homeostasis and mediates adaptations to changes in SUC concentrations.

SUCNR1 Deficiency Alleviates Liver Ischemia-Reperfusion Injury by Regulating Kupffer Cell Activation and Polarization Through the ERK/NF-κB Pathway in Mice

Succinate regulates inflammation through its receptor, succinate receptor 1 (SUCNR1). However, the effects of this interaction on Kupffer cell (KC)-driven inflammation during liver ischemia-reperfusion injury (IRI) remain unclear. Herein, we investigated the succinate/SUCNR1 axis in the progression of liver IRI. In this study, succinate levels and SUCNR1 expression were analyzed in mice underwent segmental liver IRI. Sucnr1 deficiency (Sucnr1-/-) and Wild-type mice were treated with or without clodronate before liver IRI modeling, and a co-culture system was established to assess the impact of Sucnr1 deficiency in KCs on hepatocyte viability and apoptosis. KC activation status and polarization were determined, in vivo and in vitro. Furthermore, the downstream pathways in regulating KC polarization were investigated. We observed a significant increase in succinate levels in the serum and liver, and SUCNR1 expression in KCs after IRI. Sucnr1 deletion alleviated liver IRI and hepatocyte apoptosis either in vivo or in vitro. However, the aforementioned hepatoprotective effects were abolished by the depletion of KCs with clodronate. Sucnr1 deletion inhibited KC activation and M1 polarization, and dampened proinflammatory cytokine release after liver IRI. In addition, Sucnr1 knockout reversed the increasing phosphorylation of ERK and NF-κB p65 in KCs following liver IRI. The phosphorylation of ERK/NF-κB p65 and M1 polarization in KCs were also inhibited by the SUCNR1 antagonist Compound 4C or ERK inhibitor SCH772984. Together, these findings suggest that SUCNR1 deficiency protects against liver IRI by modulating KC activation and polarization probably through the ERK/NF-κB pathway.

Gut commensals-derived succinate impels colonic inflammation in ulcerative colitis

Gut microbiota-derived metabolites play a crucial role in modulating the inflammatory response in inflammatory bowel disease (IBD). In this study, we identify gut microbiota-derived succinate as a driver of inflammation in ulcerative colitis (UC) by activating succinate-responsive, colitogenic helper T (Th) cells that secrete interleukin (IL)-9. We demonstrate that colitis is associated with an increase in succinate-producing gut bacteria and decrease in succinate-metabolizing gut bacteria. Similarly, UC patients exhibit elevated levels of succinate-producing gut bacteria and luminal succinate. Intestinal colonization by succinate-producing gut bacteria or increased succinate availability, exacerbates colonic inflammation by activating colitogenic Th9 cells. In contrast, intestinal colonization by succinate-metabolizing gut bacteria, blocking succinate receptor signaling with an antagonist, or neutralizing IL-9 with an anti-IL-9 antibody alleviates inflammation by reducing colitogenic Th9 cells. Our findings underscore the role of gut microbiota-derived succinate in driving colitogenic Th9 cells and suggesting its potential as a therapeutic target for treating IBD.

Intestinal inflammation exacerbates endometritis through succinate production by gut microbiota and SUCNR1-mediated proinflammatory response

Endometritis poses higher health risks to women. Clinical practice has found that gastrointestinal dysfunction is more likely to lead to the occurrence of endometritis. However, the mechanism is unclear. This study explored the influence and mechanism of DSS-induced intestinal inflammation on endometritis. Our findings demonstrate that DSS-induced intestinal inflammation can worsen LPS-induced endometritis in mice, and this effect is dependent on the gut microbiota, as depleting the gut microbiota eliminates this protective effect. Similarly, FMT from DSS-treated mice to recipient mice exacerbates LPS-induced endometritis. In addition, treatment of DSS disrupted an imbalance of succinate-producing and succinate-consuming bacteria and increased the levels of succinate in the gut and uterine tissues. Furthermore, treatment with succinate aggravates LPS-induced endometritis by activating the succinate receptor 1 (SUCNR1), evidenced by inhibition of the activation of SUCNR1 reversed the inflammatory response in uterine tissues induced by succinate during endometritis induced by LPS. Collectively, the results suggested that dysbiosis of the gut microbiota exacerbates LPS-induced endometritis by production and migration of succinate from gut to uterine tissues via the gut-uterus axis, then activates the SUCNR1. This identifies gut-derived succinate as a novel target for treating endometritis, and it indicates that targeting the gut microbiota and its metabolism could be a potential strategy for intervention in endometritis.

miR-369-3p ameliorates diabetes-associated atherosclerosis by regulating macrophage succinate-GPR91 signalling

AIMS

Diabetes leads to dysregulated macrophage immunometabolism, contributing to accelerated atherosclerosis progression. Identifying critical factors to restore metabolic alterations and promote resolution of inflammation remains an unmet goal. MicroRNAs orchestrate multiple signalling events in macrophages, yet their therapeutic potential in diabetes-associated atherosclerosis remains unclear.

METHODS AND RESULTS

miRNA profiling revealed significantly lower miR-369-3p expression in aortic intimal lesions from Ldlr-/- mice on a high-fat sucrose-containing (HFSC) diet for 12 weeks. miR-369-3p was also reduced in peripheral blood mononuclear cells from diabetic patients with coronary artery disease (CAD). Cell-type expression profiling showed miR-369-3p enrichment in aortic macrophages. In vitro, oxLDL treatment reduced miR-369-3p expression in mouse bone marrow-derived macrophages (BMDMs). Metabolic profiling in BMDMs revealed that miR-369-3p overexpression blocked the oxidized low density lipoprotein (oxLDL)-mediated increase in the cellular metabolite succinate and reduced mitochondrial respiration (OXPHOS) and inflammation [Interleukin (lL)-1β, TNF-α, and IL-6]. Mechanistically, miR-369-3p targeted the succinate receptor (GPR91) and alleviated the oxLDL-induced activation of inflammasome signalling pathways. Therapeutic administration of miR-369-3p mimics in HFSC-fed Ldlr-/- mice reduced GPR91 expression in lesional macrophages and diabetes-accelerated atherosclerosis, evident by a decrease in plaque size and pro-inflammatory Ly6Chi monocytes. RNA-Seq analyses showed more pro-resolving pathways in plaque macrophages from miR-369-3p-treated mice, consistent with an increase in macrophage efferocytosis in lesions. Finally, a GPR91 antagonist attenuated oxLDL-induced inflammation in primary monocytes from human subjects with diabetes.

CONCLUSION

These findings establish a therapeutic role for miR-369-3p in halting diabetes-associated atherosclerosis by regulating GPR91 and macrophage succinate metabolism.

Intermedin Alleviates Diabetic Cardiomyopathy by Up-Regulating CPT-1β through Activation of the Phosphatidyl Inositol 3 Kinase/Protein Kinase B Signaling Pathway

Diabetic cardiomyopathy (DCM), one of the most serious long-term consequences of diabetes, is closely associated with myocardial fatty acid metabolism. Carnitine palmitoyltransferase-1β (CPT-1β) is the rate-limiting enzyme responsible for β-oxidation of long-chain fatty acids. Intermedin (IMD) is a pivotal bioactive small molecule peptide, participating in the protection of various cardiovascular diseases. However, the role and underlying mechanisms of IMD in DCM are still unclear. In this study, we investigated whether IMD alleviates DCM via regulating CPT-1β. A rat DCM model was established by having rats to drink fructose water for 12 weeks. A mouse DCM model was induced by feeding mice a high-fat diet for 16 weeks. We showed that IMD and its receptor complexes levels were significantly down-regulated in the cardiac tissues of DCM rats and mice. Reduced expression of IMD was also observed in neonatal rat cardiomyocytes treated with palmitic acid (PA, 300 μM) in vitro. Exogenous and endogenous IMD mitigated cardiac hypertrophy, fibrosis, dysfunction, and lipid accumulation in DCM rats and IMD-transgenic DCM mice, whereas knockout of IMD worsened these pathological processes in IMD-knockout DCM mice. In vitro, IMD alleviated PA-induced cardiomyocyte hypertrophy and cardiac fibroblast activation. We found that CPT-1β enzyme activity, mRNA and protein levels, and acetyl-CoA content were increased in T2DM patients, rats and mice. IMD up-regulated the CPT-1β levels and acetyl-CoA content in T2DM rats and mice. Knockdown of CPT-1β blocked the effects of IMD on increasing acetyl-CoA content and on inhibiting cardiomyocyte hypertrophy and cardiac fibroblast activation. IMD receptor antagonist IMD17-47 and the phosphatidyl inositol 3 kinase (PI3K)/protein kinase B (Akt) inhibitor LY294002 reversed the effects of IMD on up-regulating CPT-1β and acetyl-CoA expression and on inhibiting cardiomyocyte hypertrophy and cardiac fibroblast activation. We revealed that IMD alleviates DCM by up-regulating CPT-1β via calcitonin receptor-like receptor/receptor activity-modifying protein (CRLR/RAMP) receptor complexes and PI3K/Akt signaling. IMD may serve as a potent therapeutic target for the treatment of DCM.

Dry blood spots as a sampling strategy to identify insulin resistance markers during a dietary challenge

This study aimed to identify markers of postprandial dysglycemia in the blood of self-described healthy individuals using dry blood spots (DBS) as a sampling strategy. A total of 54 volunteers, including 31 women, participated in a dietary challenge. They consumed a high-fat, high-sugar mixed meal and underwent multiple blood sampling over the course of 150 min to track their postprandial responses. Blood glucose levels were monitored with a portable glucometer and individuals were classified into two groups based on the glucose area under the curve (AUC): High-AUC (H-AUC) and Low-AUC (L-AUC). DBS sampling was performed at the same time points as the assessment of glycemia using Whatman 903 Protein Saver filter paper. A gas chromatography-mass spectrometry-based metabolite profiling was conducted in the DBS samples to assess postprandial changes in blood metabolome. Higher concentrations of metabolites associated with insulin resistance were observed in individuals from the H-AUC group, including sugars and sugar-derived products such as fructose and threonic acid, as well as organic acids and fatty acids such as succinate and stearic acid. Several metabolites detected in the GC-MS analysis remained unidentified, indicating that other markers of hyperglycemia remain to be discovered in DBS. Based on these observations, we demonstrated that the use of DBS as a non-invasive and inexpensive sampling tool allows the identification of metabolites markers of dysglycemia in the postprandial period.

What do stimulated beta cells have in common with cancer cells?

This study investigates the metabolic parallels between stimulated pancreatic beta cells and cancer cells, focusing on glucose and glutamine metabolism. Addressing the significant public health challenges of Type 2 Diabetes (T2D) and cancer, we aim to deepen our understanding of the mechanisms driving insulin secretion and cellular proliferation. Our analysis of anaplerotic cycles and the role of NADPH in biosynthesis elucidates their vital functions in both processes. Additionally, we point out that both cell types share an antioxidative response mediated by the Nrf2 signaling pathway, glutathione synthesis, and UCP2 upregulation. Notably, UCP2 facilitates the transfer of C4 metabolites, enhancing reductive TCA cycle metabolism. Furthermore, we observe that hypoxic responses are transient in beta cells post-stimulation but persistent in cancer cells. By synthesizing these insights, the research may suggest novel therapeutic targets for T2D, highlighting the shared metabolic strategies of stimulated beta cells and cancer cells. This comparative analysis not only illuminates the metabolic complexity of these conditions but also emphasizes the crucial role of metabolic pathways in cell function and survival, offering fresh perspectives for tackling T2D and cancer challenges.

Cellular succinate metabolism and signaling in inflammation: implications for therapeutic intervention

Succinate, traditionally viewed as a mere intermediate of the tricarboxylic acid (TCA) cycle, has emerged as a critical mediator in inflammation. Disruptions within the TCA cycle lead to an accumulation of succinate in the mitochondrial matrix. This excess succinate subsequently diffuses into the cytosol and is released into the extracellular space. Elevated cytosolic succinate levels stabilize hypoxia-inducible factor-1α by inhibiting prolyl hydroxylases, which enhances inflammatory responses. Notably, succinate also acts extracellularly as a signaling molecule by engaging succinate receptor 1 on immune cells, thus modulating their pro-inflammatory or anti-inflammatory activities. Alterations in succinate levels have been associated with various inflammatory disorders, including rheumatoid arthritis, inflammatory bowel disease, obesity, and atherosclerosis. These associations are primarily due to exaggerated immune cell responses. Given its central role in inflammation, targeting succinate pathways offers promising therapeutic avenues for these diseases. This paper provides an extensive review of succinate's involvement in inflammatory processes and highlights potential targets for future research and therapeutic possibilities development.

Obesity-dependent molecular alterations in fatal COVID-19: A retrospective postmortem study of metabolomic profile of adipose tissue

We investigated the effects of obesity on metabolic, inflammatory, and oxidative stress parameters in the adipose tissue of patients with fatal COVID-19. Postmortem biopsies of subcutaneous adipose tissue were obtained from 25 unvaccinated inpatients who passed from COVID-19, stratified as nonobese (N-OB; body mass index [BMI], 26.5 ± 2.3 kg m-2) or obese (OB BMI 34.2 ± 5.1 kg m-2). Univariate and multivariate analyses revealed that body composition was responsible for most of the variations detected in the metabolome, with greater dispersion observed in the OB group. Fifteen metabolites were major segregation factors. Results from the OB group showed higher levels of creatinine, myo-inositol, O-acetylcholine, and succinate, and lower levels of sarcosine. The N-OB group showed lower levels of glutathione peroxidase activity, as well as higher content of IL-6 and adiponectin. We revealed significant changes in the metabolomic profile of the adipose tissue in fatal COVID-19 cases, with high adiposity playing a key role in these observed variations. These findings highlight the potential involvement of metabolic and inflammatory pathways, possibly dependent on hypoxia, shedding light on the impact of obesity on disease pathogenesis and suggesting avenues for further research and possible therapeutic targets.

Pulmonary succinate receptor 1 elevation in high-fat diet mice exacerbates lipopolysaccharides-induced acute lung injury via sensing succinate

BACKGROUND

Individuals with obesity have higher level of circulating succinate, which acts as a signaling factor that initiates inflammation. It is obscure whether succinate and succinate receptor 1 (SUCNR1) are involved in the process of obesity aggravating acute lung injury (ALI).

METHODS

The lung tissue and blood samples from patients with obesity who underwent lung wedgectomy or segmental resection were collected. Six-week-old male C57BL/6J mice were fed a high-fat diet for 12 weeks to induce obesity and lipopolysaccharides (LPS) were injected intratracheally (100 μg, 1 mg/ml) for 24 h to establish an ALI model. The pulmonary SUCNR1 expression and succinate level were measured. Exogenous succinate was supplemented to assess whether succinate exacerbated the LPS-induced lung injury. We next examined the cellular localization of pulmonary SUCNR1. Furthermore, the role of the succinate-SUCNR1 pathway in LPS-induced inflammatory responses in MH-s macrophages and obese mice was investigated.

RESULT

The pulmonary SUCNR1 expression and serum succinate level were significantly increased in patients with obesity and in HFD mice. Exogenous succinate supplementation significantly increased the severity of ALI and inflammatory response. SUCNR1 was mainly expressed on lung macrophages. In LPS-stimulated MH-s cells, knockdown of SUCNR1 expression significantly inhibited pro-inflammatory cytokines' expression, the increase of hypoxia-inducible factor-1α (HIF-1α) expression, inhibitory κB-α (IκB-α) phosphorylation, p65 phosphorylation and p65 translocation to nucleus. In obese mice, SUCNR1 inhibition significantly alleviated LPS-induced lung injury and decreased the HIF-1α expression and IκB-α phosphorylation.

CONCLUSION

The high expression of pulmonary SUCNR1 and serum succinate accumulation at least partly participate in the process of obesity aggravating LPS-induced lung injury.

SUCNR1 regulates insulin secretion and glucose elevates the succinate response in people with prediabetes

Pancreatic β cell dysfunction is a key feature of type 2 diabetes, and novel regulators of insulin secretion are desirable. Here, we report that succinate receptor 1 (SUCNR1) is expressed in β cells and is upregulated in hyperglycemic states in mice and humans. We found that succinate acted as a hormone-like metabolite and stimulated insulin secretion via a SUCNR1-Gq-PKC-dependent mechanism in human β cells. Mice with β cell-specific Sucnr1 deficiency exhibited impaired glucose tolerance and insulin secretion on a high-fat diet, indicating that SUCNR1 is essential for preserving insulin secretion in diet-induced insulin resistance. Patients with impaired glucose tolerance showed an enhanced nutrition-related succinate response, which correlates with the potentiation of insulin secretion during intravenous glucose administration. These data demonstrate that the succinate/SUCNR1 axis is activated by high glucose and identify a GPCR-mediated amplifying pathway for insulin secretion relevant to the hyperinsulinemia of prediabetic states.

Monocytes and macrophages: Origin, homing, differentiation, and functionality during inflammation

Monocytes and macrophages are essential components of innate immune system and have versatile roles in homeostasis and immunity. These phenotypically distinguishable mononuclear phagocytes play distinct roles in different stages, contributing to the pathophysiology in various forms making them a potentially attractive therapeutic target in inflammatory conditions. Several pieces of evidence have supported the role of different cell surface receptors expressed on these cells and their downstream signaling molecules in initiating and perpetuating the inflammatory response. In this review, we discuss the current understanding of the monocyte and macrophage biology in inflammation, highlighting the role of chemoattractants, inflammasomes, and integrins in the function of monocytes and macrophages during events of inflammation. This review also covers the recent therapeutic interventions targeting these mononuclear phagocytes at the cellular and molecular levels.

Tricarboxylic acid cycle metabolites: new players in macrophage

Metabolic remodeling is a key feature of macrophage activation and polarization. Recent studies have demonstrated the role of tricarboxylic acid (TCA) cycle metabolites in the innate immune system. In the current review, we summarize recent advances in the metabolic reprogramming of the TCA cycle during macrophage activation and polarization and address the effects of these metabolites in modulating macrophage function. Deciphering the crosstalk between the TCA cycle and the immune response might provide novel potential targets for the intervention of immune reactions and favor the development of new strategies for the treatment of infection, inflammation, and cancer.

Oral streptococci S. anginosus and S. mitis induce distinct morphological, inflammatory, and metabolic signatures in macrophages

Oral streptococci, key players in oral biofilm formation, are implicated in oral dysbiosis and various clinical conditions, including dental caries, gingivitis, periodontal disease, and oral cancer. Specifically, Streptococcus anginosus is associated with esophageal, gastric, and pharyngeal cancers, while Streptococcus mitis is linked to oral cancer. However, no study has investigated the mechanistic links between these Streptococcus species and cancer-related inflammatory responses. As an initial step, we probed the innate immune response triggered by S. anginosus and S. mitis in RAW264.7 macrophages. These bacteria exerted time- and dose-dependent effects on macrophage morphology without affecting cell viability. Compared with untreated macrophages, macrophages infected with S. anginosus exhibited a robust proinflammatory response characterized by significantly increased levels of inflammatory cytokines and mediators, including TNF, IL-6, IL-1β, NOS2, and COX2, accompanied by enhanced NF-κB activation. In contrast, S. mitis-infected macrophages failed to elicit a robust inflammatory response. Seahorse Xfe96 analysis revealed an increased extracellular acidification rate in macrophages infected with S. anginosus compared with S. mitis. At the 24-h time point, the presence of S. anginosus led to reduced extracellular itaconate, while S. mitis triggered increased itaconate levels, highlighting distinct metabolic profiles in macrophages during infection in contrast to aconitate decarboxylase expression observed at the 6-h time point. This initial investigation highlights how S. anginosus and S. mitis, two Gram-positive bacteria from the same genus, can prompt distinct immune responses and metabolic shifts in macrophages during infection.IMPORTANCEThe surge in head and neck cancer cases among individuals devoid of typical risk factors such as Human Papilloma Virus (HPV) infection and tobacco and alcohol use sparks an argumentative discussion around the emerging role of oral microbiota as a novel risk factor in oral squamous cell carcinoma (OSCC). While substantial research has dissected the gut microbiome's influence on physiology, the oral microbiome, notably oral streptococci, has been underappreciated during mucosal immunopathogenesis. Streptococcus anginosus, a viridans streptococci group, has been linked to abscess formation and an elevated presence in esophageal cancer and OSCC. The current study aims to probe the innate immune response to S. anginosus compared with the early colonizer Streptococcus mitis as an important first step toward understanding the impact of distinct oral Streptococcus species on the host immune response, which is an understudied determinant of OSCC development and progression.

Monocarboxylate transporters facilitate succinate uptake into brown adipocytes

Uptake of circulating succinate by brown adipose tissue (BAT) and beige fat elevates whole-body energy expenditure, counteracts obesity and antagonizes systemic tissue inflammation in mice. The plasma membrane transporters that facilitate succinate uptake in these adipocytes remain undefined. Here we elucidate a mechanism underlying succinate import into BAT via monocarboxylate transporters (MCTs). We show that succinate transport is strongly dependent on the proportion that is present in the monocarboxylate form. MCTs facilitate monocarboxylate succinate uptake, which is promoted by alkalinization of the cytosol driven by adrenoreceptor stimulation. In brown adipocytes, we show that MCT1 primarily facilitates succinate import. In male mice, we show that both acute pharmacological inhibition of MCT1 and congenital depletion of MCT1 decrease succinate uptake into BAT and consequent catabolism. In sum, we define a mechanism of succinate uptake in BAT that underlies its protective activity in mouse models of metabolic disease.

Real-time OXPHOS capacity analysis in wounded skin from diabetic mice: A pilot study

INTRODUCTION

Diabetes mellitus (DM) impairs wound healing. The aim was to determine whether DM influences mitochondrial respiration in wounded skin (WS) and non-wounded skin (NWS), in a pre-clinical wound healing model of streptozotocin (STZ)-induced diabetes.

METHODS

Six weeks after diabetes induction, two wounds were created in the back of C57BL/J6 mice. Using high-resolution respirometry (HRR), oxygen flux was measured, in WS and NWS, using two substrate-uncoupler-inhibitor titration protocols, at baseline (day 0), day 3 and 10 post-wounding, in STZ-DM and non-diabetic (NDM) mice. Flux control ratios for the oxidative phosphorylation (OXPHOS) capacity were calculated.

RESULTS

A significant increase in mitochondrial respiration was observed in STZ-DM skin compared to control skin at baseline. The OXPHOS capacity was decreased in WS under diabetes at day 3 post-wounding (inflammation phase). However, at day 10 post-wounding (remodeling phase), the OXPHOS capacity was higher in WS from STZ-DM compared to NDM mice, and compared to NWS from STZ-DM mice. A significant relative contribution of pyruvate, malate and glutamate (PMG) oxidation to the OXPHOS capacity was observed in WS compared to NWS from STZ-DM mice, at day 10, while the relative contribution of fatty acid oxidation to the OXPHOS capacity was higher in NWS. The OXPHOS capacity is altered in WS from STZ-DM compared to NDM mice across the healing process, and so is the substrate contribution in WS and NWS from STZ-DM mice, at each time point.

CONCLUSION

HRR may be a sensitive tool to evaluate the underlying mechanisms of tissue repair during wound healing.

Type 2 diabetes and succinate: unmasking an age-old molecule

Beyond their conventional roles in intracellular energy production, some traditional metabolites also function as extracellular messengers that activate cell-surface G-protein-coupled receptors (GPCRs) akin to hormones and neurotransmitters. These signalling metabolites, often derived from nutrients, the gut microbiota or the host's intermediary metabolism, are now acknowledged as key regulators of various metabolic and immune responses. This review delves into the multi-dimensional aspects of succinate, a dual metabolite with roots in both the mitochondria and microbiome. It also connects the dots between succinate's role in the Krebs cycle, mitochondrial respiration, and its double-edge function as a signalling transmitter within and outside the cell. We aim to provide an overview of the role of the succinate-succinate receptor 1 (SUCNR1) axis in diabetes, discussing the potential use of succinate as a biomarker and the novel prospect of targeting SUCNR1 to manage complications associated with diabetes. We further propose strategies to manipulate the succinate-SUCNR1 axis for better diabetes management; this includes pharmacological modulation of SUCNR1 and innovative approaches to manage succinate concentrations, such as succinate administration and indirect strategies, like microbiota modulation. The dual nature of succinate, both in terms of origins and roles, offers a rich landscape for understanding the intricate connections within metabolic diseases, like diabetes, and indicates promising pathways for developing new therapeutic strategies.

The pathogenic role of succinate-SUCNR1: a critical function that induces renal fibrosis via M2 macrophage

BACKGROUND

Renal fibrosis significantly contributes to the progressive loss of kidney function in chronic kidney disease (CKD), with alternatively activated M2 macrophages playing a crucial role in this progression. The serum succinate level is consistently elevated in individuals with diabetes and obesity, both of which are critical factors contributing to CKD. However, it remains unclear whether elevated succinate levels can mediate M2 polarization of macrophages and contribute to renal interstitial fibrosis.

METHODS

Male C57/BL6 mice were administered water supplemented with 4% succinate for 12 weeks to assess its impact on renal interstitial fibrosis. Additionally, the significance of macrophages was confirmed in vivo by using clodronate liposomes to deplete them. Furthermore, we employed RAW 264.7 and NRK-49F cells to investigate the underlying molecular mechanisms.

RESULTS

Succinate caused renal interstitial macrophage infiltration, activation of profibrotic M2 phenotype, upregulation of profibrotic factors, and interstitial fibrosis. Treatment of clodronate liposomes markedly depleted macrophages and prevented the succinate-induced increase in profibrotic factors and fibrosis. Mechanically, succinate promoted CTGF transcription via triggering SUCNR1-p-Akt/p-GSK3β/β-catenin signaling, which was inhibited by SUCNR1 siRNA. The knockdown of succinate receptor (SUCNR1) or pretreatment of anti-CTGF(connective tissue growth factor) antibody suppressed the stimulating effects of succinate on RAW 264.7 and NRK-49F cells.

CONCLUSIONS

The causative effects of succinate on renal interstitial fibrosis were mediated by the activation of profibrotic M2 macrophages. Succinate-SUCNR1 played a role in activating p-Akt/p-GSK3β/β-catenin, CTGF expression, and facilitating crosstalk between macrophages and fibroblasts. Our findings suggest a promising strategy to prevent the progression of metabolic CKD by promoting the excretion of succinate in urine and/or using selective antagonists for SUCNR1.

Phloretin Ameliorates Succinate-Induced Liver Fibrosis by Regulating Hepatic Stellate Cells

BACKGRUOUND

Hepatic stellate cells (HSCs) are the major cells which play a pivotal role in liver fibrosis. During injury, extracellular stimulators can induce HSCs transdifferentiated into active form. Phloretin showed its ability to protect the liver from injury, so in this research we would like to investigate the effect of phloretin on succinate-induced HSCs activation in vitro and liver fibrosis in vivo study.

METHODS

In in vitro, succinate was used to induce HSCs activation, and then the effect of phloretin on activated HSCs was examined. In in vivo, succinate was used to generated liver fibrosis in mouse and phloretin co-treated to check its protection on the liver.

RESULTS

Phloretin can reduce the increase of fibrogenic markers and inhibits the proliferation, migration, and contraction caused by succinate in in vitro experiments. Moreover, an upregulation of proteins associated with aerobic glycolysis occurred during the activation of HSCs, which was attenuated by phloretin treatment. In in vivo experiments, intraperitoneal injection of phloretin decreased expression of fibrotic and glycolytic markers in the livers of mice with sodium succinate diet-induced liver fibrosis. These results suggest that aerobic glycolysis plays critical role in activation of HSCs and succinate can induce liver fibrosis in mice, whereas phloretin has therapeutic potential for treating hepatic fibrosis.

CONCLUSION

Intraperitoneal injection of phloretin attenuated succinate-induced hepatic fibrosis and alleviates the succinate-induced HSCs activation.

Succinate signaling attenuates high-fat diet-induced metabolic disturbance and intestinal barrier dysfunction

Succinate is a vital signaling metabolite produced by the host and gut microbiota. Succinate has been shown to regulate host metabolic homeostasis and inhibit obesity-associated inflammation in macrophages by engaging its cognate receptor, SUCNR1. However, the contribution of the succinate-SUCNR1 axis to intestinal barrier dysfunction in obesity remains unclear. In the present study, we explored the effects of succinate-SUCNR1 signaling on high-fat diet (HFD)-induced intestinal barrier dysfunction. Using a SUCNR1-deficient mouse model under HFD feeding conditions, we identified the effects of succinate-SUCNR1 axis on obesity-associated intestinal barrier impairment. Our results showed that HFD administration decreased goblet cell numbers and mucus production, promoted intestinal pro-inflammatory responses, induced gut microbiota composition imbalance, increased intestinal permeability, and caused mucosal barrier dysfunction. Dietary succinate supplementation was sufficient to activate a type 2 immune response, trigger the differentiation of barrier-promoting goblet cells, suppress intestinal inflammation, restore HFD-induced mucosal barrier impairment and intestinal dysbiosis, and eventually exert anti-obesity effects. However, SUNNR1-deficient mice failed to improve the intestinal barrier function and metabolic phenotype in HFD mice. Our data indicate the protective role of the succinate-SUCNR1 axis in HFD-induced intestinal barrier dysfunction.

Metabolite-sensing GPCRs controlling interactions between adipose tissue and inflammation

Metabolic disorders including obesity, diabetes and non-alcoholic steatohepatitis are a group of conditions characterised by chronic low-grade inflammation of metabolic tissues. There is now a growing appreciation that various metabolites released from adipose tissue serve as key signalling mediators, influencing this interaction with inflammation. G protein-coupled receptors (GPCRs) are the largest family of signal transduction proteins and most historically successful drug targets. The signalling pathways for several key adipose metabolites are mediated through GPCRs expressed both on the adipocytes themselves and on infiltrating macrophages. These include three main groups of GPCRs: the FFA4 receptor, which is activated by long chain free fatty acids; the HCA₂ and HCA₃ receptors, activated by hydroxy carboxylic acids; and the succinate receptor. Understanding the roles these metabolites and their receptors play in metabolic-immune interactions is critical to establishing how these GPCRs may be exploited for the treatment of metabolic disorders.

Extracellular Succinate: A Physiological Messenger and a Pathological Trigger

When tissues are under physiological stresses, such as vigorous exercise and cold exposure, skeletal muscle cells secrete succinate into the extracellular space for adaptation and survival. By contrast, environmental toxins and injurious agents induce cellular secretion of succinate to damage tissues, trigger inflammation, and induce tissue fibrosis. Extracellular succinate induces cellular changes and tissue adaptation or damage by ligating cell surface succinate receptor-1 (SUCNR-1) and activating downstream signaling pathways and transcriptional programs. Since SUCNR-1 mediates not only pathological processes but also physiological functions, targeting it for drug development is hampered by incomplete knowledge about the characteristics of its physiological vs. pathological actions. This review summarizes the current status of extracellular succinate in health and disease and discusses the underlying mechanisms and therapeutic implications.

Metabolomic Profiles, Ideal Cardiovascular Health, and Risk of Heart Failure and Atrial Fibrillation: Insights From the Framingham Heart Study

Background The American Heart Association's framework "ideal cardiovascular health" (CVH) focuses on modifiable risk factors to reduce cardiovascular disease (CVD). Metabolomics provides important pathobiological insights into risk factors and CVD development. We hypothesized that metabolomic signatures associate with CVH status, and that metabolites, at least partially, mediate the association of CVH score with atrial fibrillation (AF) and heart failure (HF). Methods and Results We studied 3056 adults in the FHS (Framingham Heart Study) cohort to evaluate CVH score and incident outcomes of AF and HF. Metabolomics data were available in 2059 participants; mediation analysis was performed to evaluate the mediation of metabolites in the association of CVH score and incident AF and HF. In the smaller cohort (mean age, 54 years; 53% women), CVH score was associated with 144 metabolites, with 64 metabolites shared across key cardiometabolic components (body mass index, blood pressure, and fasting blood glucose) of the CVH score. In mediation analyses, 3 metabolites (glycerol, cholesterol ester 16:1, and phosphatidylcholine 32:1) mediated the association of CVH score with incident AF. Seven metabolites (glycerol, isocitrate, asparagine, glutamine, indole-3-proprionate, phosphatidylcholine C36:4, and lysophosphatidylcholine 18:2), partly mediated the association between CVH score and incident HF in multivariable-adjusted models. Conclusions Most metabolites that associated with CVH score were shared the most among 3 cardiometabolic components. Three main pathways: (1) alanine, glutamine, and glutamate metabolism; (2) citric acid cycle metabolism; and (3) glycerolipid metabolism mediated CVH score with HF. Metabolomics provides insights into how ideal CVH status contributes to the development of AF and HF.

Metaflammation in glucolipid metabolic disorders: Pathogenesis and treatment

The public health issue of glucolipid metabolic disorders (GLMD) has grown significantly, posing a grave threat to human wellness. Its prevalence is rising yearly and tends to affect younger people. Metaflammation is an important mechanism regulating body metabolism. Through a complicated multi-organ crosstalk network involving numerous signaling pathways such as NLRP3/caspase-1/IL-1, NF-B, p38 MAPK, IL-6/STAT3, and PI3K/AKT, it influences systemic metabolic regulation. Numerous inflammatory mediators are essential for preserving metabolic balance, but more research is needed to determine how they contribute to the co-morbidities of numerous metabolic diseases. Whether controlling the inflammatory response can influence the progression of GLMD determines the therapeutic strategy for such diseases. This review thoroughly examines the role of metaflammation in GLMD and combs the research progress of related therapeutic approaches, including inflammatory factor-targeting drugs, traditional Chinese medicine (TCM), and exercise therapy. Multiple metabolic diseases, including diabetes, non-alcoholic fatty liver disease (NAFLD), cardiovascular disease, and others, respond therapeutically to anti-inflammatory therapy on the whole. Moreover, we emphasize the value and open question of anti-inflammatory-based means for treating GLMD.

The Role of Next-Generation Probiotics in Obesity and Obesity-Associated Disorders: Current Knowledge and Future Perspectives

Obesity and obesity-associated disorders pose a major public health issue worldwide. Apart from conventional weight loss drugs, next-generation probiotics (NGPs) seem to be very promising as potential preventive and therapeutic agents against obesity. Candidate NGPs such as Akkermansia muciniphila, Faecalibacterium prausnitzii, Anaerobutyricum hallii, Bacteroides uniformis, Bacteroides coprocola, Parabacteroides distasonis, Parabacteroides goldsteinii, Hafnia alvei, Odoribacter laneus and Christensenella minuta have shown promise in preclinical models of obesity and obesity-associated disorders. Proposed mechanisms include the modulation of gut flora and amelioration of intestinal dysbiosis, improvement of intestinal barrier function, reduction in chronic low-grade inflammation and modulation of gut peptide secretion. Akkermansia muciniphila and Hafnia alvei have already been administered in overweight/obese patients with encouraging results. However, safety issues and strict regulations should be constantly implemented and updated. In this review, we aim to explore (1) current knowledge regarding NGPs; (2) their utility in obesity and obesity-associated disorders; (3) their safety profile; and (4) their therapeutic potential in individuals with overweight/obesity. More large-scale, multicentric and longitudinal studies are mandatory to explore their preventive and therapeutic potential against obesity and its related disorders.

Partially hydrolyzed guar gum increased colonic mucus layer in mice via succinate-mediated MUC2 production

Colonic mucus layers protect intestinal tissues against intestinal bacteria. We investigated the effects of dietary fiber and its metabolites on mucus production in the colonic mucosa. Mice were fed a partially hydrolyzed guar gum (PHGG)-containing diet and a fiber-free diet (FFD). The colon mucus layer, fecal short-chain fatty acid (SCFA) levels, and gut microbiota were evaluated. Mucin 2 (MUC2) expression was assessed in SCFA-treated LS174T cells. The role of AKT in MUC2 production was investigated. The mucus layer in the colonic epithelium was significantly increased in the PHGG group compared with that in the FFD group. In the PHGG group, an increase in Bacteroidetes in the stool was observed, and fecal acetate, butyrate, propionate, and succinate levels were significantly increased. However, MUC2 production was significantly increased only in succinate-stimulated LS174T cells. The succinate-induced MUC2 production was associated with AKT phosphorylation. Succinate mediated the PHGG-induced increase in the colon mucus layer.

Mechanisms of obesity- and diabetes mellitus-related pancreatic carcinogenesis: a comprehensive and systematic review

Research on obesity- and diabetes mellitus (DM)-related carcinogenesis has expanded exponentially since these two diseases were recognized as important risk factors for cancers. The growing interest in this area is prominently actuated by the increasing obesity and DM prevalence, which is partially responsible for the slight but constant increase in pancreatic cancer (PC) occurrence. PC is a highly lethal malignancy characterized by its insidious symptoms, delayed diagnosis, and devastating prognosis. The intricate process of obesity and DM promoting pancreatic carcinogenesis involves their local impact on the pancreas and concurrent whole-body systemic changes that are suitable for cancer initiation. The main mechanisms involved in this process include the excessive accumulation of various nutrients and metabolites promoting carcinogenesis directly while also aggravating mutagenic and carcinogenic metabolic disorders by affecting multiple pathways. Detrimental alterations in gastrointestinal and sex hormone levels and microbiome dysfunction further compromise immunometabolic regulation and contribute to the establishment of an immunosuppressive tumor microenvironment (TME) for carcinogenesis, which can be exacerbated by several crucial pathophysiological processes and TME components, such as autophagy, endoplasmic reticulum stress, oxidative stress, epithelial-mesenchymal transition, and exosome secretion. This review provides a comprehensive and critical analysis of the immunometabolic mechanisms of obesity- and DM-related pancreatic carcinogenesis and dissects how metabolic disorders impair anticancer immunity and influence pathophysiological processes to favor cancer initiation.

SUCNR1 signaling in adipocytes controls energy metabolism by modulating circadian clock and leptin expression

Adipose tissue modulates energy homeostasis by secreting leptin, but little is known about the factors governing leptin production. We show that succinate, long perceived as a mediator of immune response and lipolysis, controls leptin expression via its receptor SUCNR1. Adipocyte-specific deletion of Sucnr1 influences metabolic health according to nutritional status. Adipocyte Sucnr1 deficiency impairs leptin response to feeding, whereas oral succinate mimics nutrient-related leptin dynamics via SUCNR1. SUCNR1 activation controls leptin expression via the circadian clock in an AMPK/JNK-C/EBPα-dependent manner. Although the anti-lipolytic role of SUCNR1 prevails in obesity, its function as a regulator of leptin signaling contributes to the metabolically favorable phenotype in adipocyte-specific Sucnr1 knockout mice under standard dietary conditions. Obesity-associated hyperleptinemia in humans is linked to SUCNR1 overexpression in adipocytes, which emerges as the major predictor of adipose tissue leptin expression. Our study establishes the succinate/SUCNR1 axis as a metabolite-sensing pathway mediating nutrient-related leptin dynamics to control whole-body homeostasis.

Monocarboxylate transporters facilitate succinate uptake into brown adipocytes

Uptake of circulating succinate by brown adipose tissue (BAT) and beige fat elevates whole body energy expenditure, counteracts obesity, and antagonizes systemic tissue inflammation in mice. The plasma membrane transporters that facilitate succinate uptake in these adipocytes remain undefined. Here we elucidate a mechanism underlying succinate import into BAT via monocarboxylate transporters (MCTs). We show that succinate transport is strongly dependent on the proportion of it present in the monocarboxylate form. MCTs facilitate monocarboxylate succinate uptake, which is promoted by alkalinization of the cytosol driven by adrenoreceptor stimulation. In brown adipocytes, we show that MCT1 primarily facilitates succinate import, however other members of the MCT family can partially compensate and fulfill this role in the absence of MCT1. In mice, we show that acute pharmacological inhibition of MCT1 and 2 decreases succinate uptake into BAT. Conversely, congenital genetic depletion of MCT1 alone has little effect on BAT succinate uptake, indicative of additional transport mechanisms with high capacity in vivo . In sum, we define a mechanism of succinate uptake in BAT that underlies its protective activity in mouse models of metabolic disease.

Bioinformatics and Next-Generation Data Analysis for Identification of Genes and Molecular Pathways Involved in Subjects with Diabetes and Obesity

Background and Objectives: A subject with diabetes and obesity is a class of the metabolic disorder. The current investigation aimed to elucidate the potential biomarker and prognostic targets in subjects with diabetes and obesity. Materials and Methods: The next-generation sequencing (NGS) data of GSE132831 was downloaded from Gene Expression Omnibus (GEO) database. Functional enrichment analysis of DEGs was conducted with ToppGene. The protein-protein interactions network, module analysis, target gene-miRNA regulatory network and target gene-TF regulatory network were constructed and analyzed. Furthermore, hub genes were validated by receiver operating characteristic (ROC) analysis. A total of 872 DEGs, including 439 up-regulated genes and 433 down-regulated genes were observed. Results: Second, functional enrichment analysis showed that these DEGs are mainly involved in the axon guidance, neutrophil degranulation, plasma membrane bounded cell projection organization and cell activation. The top ten hub genes (MYH9, FLNA, DCTN1, CLTC, ERBB2, TCF4, VIM, LRRK2, IFI16 and CAV1) could be utilized as potential diagnostic indicators for subjects with diabetes and obesity. The hub genes were validated in subjects with diabetes and obesity. Conclusion: This investigation found effective and reliable molecular biomarkers for diagnosis and prognosis by integrated bioinformatics analysis, suggesting new and key therapeutic targets for subjects with diabetes and obesity.

Gut microbiota-derived succinate aggravates acute lung injury after intestinal ischaemia/reperfusion in mice

INTRODUCTION

Acute lung injury (ALI) is a major cause of morbidity and mortality after intestinal ischaemia/reperfusion (I/R). The gut microbiota and its metabolic byproducts act as important modulators of the gut-lung axis. This study aimed to define the role of succinate, a key microbiota metabolite, in intestinal I/R-induced ALI progression.

METHODS

Gut and lung microbiota of mice subjected to intestinal I/R were analysed using 16S rRNA gene sequencing. Succinate level alterations were measured in germ-free mice or conventional mice treated with antibiotics. Succinate-induced alveolar macrophage polarisation and its effects on alveolar epithelial apoptosis were evaluated in succinate receptor 1 (Sucnr1)-deficient mice and in murine alveolar macrophages transfected with Sucnr1-short interfering RNA. Succinate levels were measured in patients undergoing cardiopulmonary bypass, including intestinal I/R.

RESULTS

Succinate accumulated in lungs after intestinal I/R, and this was associated with an imbalance of succinate-producing and succinate-consuming bacteria in the gut, but not the lungs. Succinate accumulation was absent in germ-free mice and was reversed by gut microbiota depletion with antibiotics, indicating that the gut microbiota is a source of lung succinate. Moreover, succinate promoted alveolar macrophage polarisation, alveolar epithelial apoptosis and lung injury during intestinal I/R. Conversely, knockdown of Sucnr1 or blockage of SUCNR1 in vitro and in vivo reversed the effects of succinate by modulating the phosphoinositide 3-kinase-AKT/hypoxia-inducible factor-1α pathway. Plasma succinate levels significantly correlated with intestinal I/R-related lung injury after cardiopulmonary bypass.

CONCLUSION

Gut microbiota-derived succinate exacerbates intestinal I/R-induced ALI through SUCNR1-dependent alveolar macrophage polarisation, identifying succinate as a novel target for gut-derived ALI in critically ill patients.

Succinate-SUCNR1 induces renal tubular cell apoptosis

Succinate has long been known to be only an intermediate product of the tricarboxylic acid cycle until identified as a natural ligand for SUCNR1 in 2004. SUCNR1 is widely expressed throughout the body, especially in the kidney. Abnormally elevated succinate is associated with many diseases, including obesity, type 2 diabetes, nonalcoholic fatty liver disease, and ischemia injury, but it is not known whether succinate can cause kidney damage. This study showed that succinate induced apparent renal injury after treatment for 12 wk, characterized by a reduction in 24 h urine and the significant detachment of the brush border of proximal tubular epithelial cells, tubular dilation, cast formation, and vacuolar degeneration of tubular cells in succinate-treated mice. Besides, succinate caused tubular epithelial cell apoptosis in kidneys and HK-2 cells. Mechanistically, succinate triggered cell apoptosis via SUCNR1 activation. In addition, succinate upregulated ERK by binding to SUCNR1, and inhibition of ERK using PD98059 abolished the proapoptotic effects of succinate in HK-2 cells. In summary, our study provides the first evidence that succinate acts as a risk factor and contributes to renal injury, and further research is required to discern the pathological effects of succinate on renal functions.

Inflammation driven metabolic regulation and adaptation in macrophages

Macrophages are a diverse population of phagocytic immune cells involved in the host defense mechanisms and regulation of homeostasis. Usually, macrophages maintain healthy functioning at the cellular level, but external perturbation in their balanced functions can lead to acute and chronic disease conditions. By sensing the cues from the tissue microenvironment, these phagocytes adopt a plethora of phenotypes, such as inflammatory or M1 to anti-inflammatory (immunosuppressive) or M2 subtypes, to fulfill their spectral range of functions. The existing evidence in the literature supports that in macrophages, regulation of metabolic switches and metabolic adaptations are associated with their functional behaviors under various physiological and pathological conditions. Since these macrophages play a crucial role in many disorders, therefore it is necessary to understand their heterogeneity and metabolic reprogramming. Consequently, these macrophages have also emerged as a promising target for diseases in which their role is crucial in driving the disease pathology and outcome (e.g., Cancers). In this review, we discuss the recent findings that link many metabolites with macrophage functions and highlight how this metabolic reprogramming can improve our understanding of cellular malfunction in the macrophages during inflammatory disorders. A systematic analysis of the interconnecting crosstalk between metabolic pathways with macrophages should inform the selection of immunomodulatory therapies for inflammatory diseases.

Succinylation and redox status in cancer cells

Succinylation is a post-translational modification (PTM) event that associates metabolic reprogramming with various pathological disorders including cancers via transferring a succinyl group to a residue of the target protein in an enzymic or non-enzymic manner. With our incremental knowledge on the roles of PTM played in tumor initiation and progression, relatively little has been focused on succinylation and its clinical implications. By delineating the associations of succinylation with cancer hallmarks, we identify the, in general, promotive roles of succinylation in manifesting cancer hallmarks, and conceptualize two working modes of succinylation in driving oncogenic signaling, i.e., via altering the structure and charge of target proteins towards enhanced stability and activity. We also characterize succinylation as a reflection of cellular redox homeostatic status and metabolic state, and bring forth the possible use of hyper-succinylated genome for early cancer diagnosis or disease progression indication. In addition, we propose redox modulation tools such as cold atmospheric plasma as a promising intervention approach against tumor cells and cancer stemness via targeting the redox homeostatic environment cells established under a pathological condition such as hypoxia. Taken together, we emphasize the central role of succinylation in bridging the gap between cellular metabolism and redox status, and its clinical relevance as a mark for cancer diagnosis as well as a target in onco-therapeutics.

Gemigliptin Alleviates Succinate-Induced Hepatic Stellate Cell Activation by Ameliorating Mitochondrial Dysfunction

BACKGRUOUND

Dipeptidyl peptidase-4 inhibitors (DPP-4Is) are used clinically as oral antidiabetic agents. Although DPP-4Is are known to ameliorate liver fibrosis, the protective mechanism of DPP-4Is in liver fibrosis remains obscure. In this study, gemigliptin was used to investigate the potential of DPP-4Is to alleviate the progression of liver fibrosis.

METHODS

To clarify the effects and mechanisms of gemigliptin, we conducted various experiments in LX-2 cells (immortalized human hepatic stellate cells [HSCs], the principal effectors of hepatic fibrogenesis), which were activated by succinate and exhibited elevated expression of α-smooth muscle actin, collagen type 1, and pro-inflammatory cytokines and increased cell proliferation. In vivo, we examined the effects and mechanisms of gemigliptin on a high-fat, high-cholesterol-induced mouse model of nonalcoholic steatohepatitis (NASH).

RESULTS

Gemigliptin decreased the expression of fibrogenesis markers and reduced the abnormal proliferation of HSCs. In addition, gemigliptin reduced the succinate-induced production of mitochondrial reactive oxygen species (ROS), intracellular ROS, and mitochondrial fission in HSCs. Furthermore, in the mouse model of NASH-induced liver fibrosis, gemigliptin alleviated both liver fibrosis and mitochondrial dysfunction.

CONCLUSION

Gemigliptin protected against HSC activation and liver fibrosis by alleviating mitochondrial dysfunction and ROS production, indicating its potential as a strategy for preventing the development of liver disease.

Cancer-derived extracellular succinate: a driver of cancer metastasis

Succinate is a tricarboxylic acid (TCA) cycle intermediate normally confined to the mitochondrial matrix. It is a substrate of succinate dehydrogenase (SDH). Mutation of SDH subunits (SDHD and SDHB) in hereditary tumors such as paraganglioma or reduction of SDHB expression in cancer results in matrix succinate accumulation which is transported to cytoplasma and secreted into the extracellular milieu. Excessive cytosolic succinate is known to stabilize hypoxia inducible factor-1α (HIF-1α) by inhibiting prolyl hydroxylase. Recent reports indicate that cancer-secreted succinate enhances cancer cell migration and promotes cancer metastasis by activating succinate receptor-1 (SUCNR-1)-mediated signaling and transcription pathways. Cancer-derived extracellular succinate enhances cancer cell and macrophage migration through SUCNR-1 → PI-3 K → HIF-1α pathway. Extracellular succinate induces tumor angiogenesis through SUCNR-1-mediated ERK1/2 and STAT3 activation resulting in upregulation of vascular endothelial growth factor (VEGF) expression. Succinate increases SUCNR-1 expression in cancer cells which is considered as a target for developing new anti-metastasis drugs. Furthermore, serum succinate which is elevated in cancer patients may be a theranostic biomarker for selecting patients for SUCNR-1 antagonist therapy.

Uncovering the characteristics of the gut microbiota in patients with acute ischemic stroke and phlegm-heat syndrome

Growing evidence has indicated that the characteristics of gut microbiota are associated with acute ischemic stroke (AIS). Phlegm-heat syndrome (PHS), a specific pathological state of the AIS, is one of the common traditional Chinese syndromes of stroke. The long duration of PHS in patients with AIS could lead to poor clinical outcomes. Gut microbiota characteristics in patients with both AIS and PHS, and their relationship remains unknown. This study was designed to investigate the alterations in gut microbiota in patients with AIS and PHS through a cross-sectional study. Fecal samples were collected from 10 patients with AIS and non-PHS (ntAIS), 7 patients with AIS and PHS (tAIS), and 10 healthy controls (HC). Samples were profiled via Illumina sequencing of the 16S rRNA V3-V4. Stroke severity was assessed at admission by the National Institutes of Health Stroke Scale (NIHSS) and modified Rankin scale (mRS); their correlation with gut microbiota was investigated. The alpha-diversity of the bacterial communities was significantly higher in the fecal samples of patients with tAIS than in patients with ntAIS (Shannon index, P = 0.037). In addition, the combined tAIS and ntAIS group (tntAIS) exhibited higher microbiotic diversity when compared with HC (chao1, P = 0.019). The structure of intestinal microbiota was effectively distinguished between the tAIS and ntAIS group (ANOSIM, r = 0.337, P = 0.007). Additionally, the gut microbiota structure was significantly different between the tntAIS and HC groups (ANOSIM, r = 0.217, P = 0.005). The genera, Ruminococcaceae_ UCG_002 and Christensenellaceae_R-7_group, were implicated in the discrimination of PHS from non-PHS. The order Lactobacillales and family Lachnospiraceae were significantly negatively correlated with NIHSS and mRS at admission (P < 0.05). By contrast, the order Desulfovibrionales, families Christensenellaceae and Desulfovibrionaceae, and genera Ruminococcaceae UCG-014 and Ruminococcaceae UCG-002 were significantly positively correlated with NIHSS and mRS at admission (P < 0.05). This study is the first to profile the characteristics of gut microbiota in patients with AIS and PHS, compared with those with non-PHS. The genera, Ruminococcaceae_ UCG_002 and Christensenellaceae_R-7_group, may be objective indicators of this traditional Chinese medicine (TCM) syndrome in AIS. Furthermore, it provides a microbe-inspired biological basis for TCM syndrome differentiation.

High mobility group box 1 and Dickkopf-related protein 1 as biomarkers of glucose toxicity, atherogenicity, and lower β cell function in patients with type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is associated with increased atherogenicity and inflammatory responses, which may be related to high mobility group box 1 (HMGB1) and Dickkopf-related protein 1 (DKK1). The role of HMGB1 and DKK1 in T2DM is examined in association with lipid and insulin profiles. Serum HMGB1 and DKK1 were measured in T2DM with and without hypertension and compared with controls. The results showed that HMGB1 and DKK1 are higher in T2DM irrespective of hypertension. A large part of the variance in the β-cell index and glucose toxicity was explained by the combined effects of HMGB1 and DKK1. In conclusion, both HMGB1 and DKK1 may contribute to increased atherogenicity in T2DM. Moreover, both biomarkers may cause more deficits in β-cell function and increase glucose toxicity leading to the development of more inflammation and diabetic complications. HMGB1 and the Wnt pathways are other drug targets in treating T2DM.

Tryptamine, a Microbial Metabolite in Fermented Rice Bran Suppressed Lipopolysaccharide-Induced Inflammation in a Murine Macrophage Model

Fermentation is thought to alter the composition and bioavailability of bioactive compounds in rice bran. However, how this process affects the anti-inflammatory effects of rice bran and the bioactive compounds that might participate in this function is yet to be elucidated. This study aimed to isolate bioactive compounds in fermented rice bran that play a key role in its anti-inflammatory function. The fermented rice bran was fractionated using a succession of solvent and solid-phase extractions. The fermented rice bran fractions were then applied to lipopolysaccharide (LPS)-activated murine macrophages to evaluate their anti-inflammatory activity. The hot water fractions (FRBA), 50% ethanol fractions (FRBB), and n-hexane fractions (FRBC) were all shown to be able to suppress the pro-inflammatory cytokine expression from LPS-stimulated RAW 264.7 cells. Subsequent fractions from the hot water fraction (FRBF and FRBE) were also able to reduce the inflammatory response of these cells to LPS. Further investigation revealed that tryptamine, a bacterial metabolite of tryptophan, was abundantly present in these extracts. These results indicate that tryptamine may play an important role in the anti-inflammatory effects of fermented rice bran. Furthermore, the anti-inflammatory effects of FRBE and tryptamine may depend on the activity of the aryl hydrocarbon receptor.

Orally administered Odoribacter laneus improves glucose control and inflammatory profile in obese mice by depleting circulating succinate

BACKGROUND

Succinate is produced by both human cells and by gut bacteria and couples metabolism to inflammation as an extracellular signaling transducer. Circulating succinate is elevated in patients with obesity and type 2 diabetes and is linked to numerous complications, yet no studies have specifically addressed the contribution of gut microbiota to systemic succinate or explored the consequences of reducing intestinal succinate levels in this setting.

RESULTS

Using germ-free and microbiota-depleted mouse models, we show that the gut microbiota is a significant source of circulating succinate, which is elevated in obesity. We also show in vivo that therapeutic treatments with selected bacteria diminish the levels of circulating succinate in obese mice. Specifically, we demonstrate that Odoribacter laneus is a promising probiotic based on its ability to deplete succinate and improve glucose tolerance and the inflammatory profile in two independent models of obesity (db/db mice and diet-induced obese mice). Mechanistically, this is partly mediated by the succinate receptor 1. Supporting these preclinical findings, we demonstrate an inverse correlation between plasma and fecal levels of succinate in a cohort of patients with severe obesity. We also show that plasma succinate, which is associated with several components of metabolic syndrome including waist circumference, triglycerides, and uric acid, among others, is a primary determinant of insulin sensitivity evaluated by the euglycemic-hyperinsulinemic clamp.

CONCLUSIONS

Overall, our work uncovers O. laneus as a promising next-generation probiotic to deplete succinate and improve glucose tolerance and obesity-related inflammation. Video Abstract.

Recent Progress in the Diagnosis and Management of Type 2 Diabetes Mellitus in the Era of COVID-19 and Single Cell Multi-Omics Technologies

Type 2 diabetes mellitus (T2DM) is one of the world's leading causes of death and life-threatening conditions. Therefore, we review the complex vicious circle of causes responsible for T2DM and risk factors such as the western diet, obesity, genetic predisposition, environmental factors, and SARS-CoV-2 infection. The prevalence and economic burden of T2DM on societal and healthcare systems are dissected. Recent progress on the diagnosis and clinical management of T2DM, including both non-pharmacological and latest pharmacological treatment regimens, are summarized. The treatment of T2DM is becoming more complex as new medications are approved. This review is focused on the non-insulin treatments of T2DM to reach optimal therapy beyond glycemic management. We review experimental and clinical findings of SARS-CoV-2 risks that are attributable to T2DM patients. Finally, we shed light on the recent single-cell-based technologies and multi-omics approaches that have reached breakthroughs in the understanding of the pathomechanism of T2DM.

Macrophages, Low-Grade Inflammation, Insulin Resistance and Hyperinsulinemia: A Mutual Ambiguous Relationship in the Development of Metabolic Diseases

Metabolic derangement with poor glycemic control accompanying overweight and obesity is associated with chronic low-grade inflammation and hyperinsulinemia. Macrophages, which present a very heterogeneous population of cells, play a key role in the maintenance of normal tissue homeostasis, but functional alterations in the resident macrophage pool as well as newly recruited monocyte-derived macrophages are important drivers in the development of low-grade inflammation. While metabolic dysfunction, insulin resistance and tissue damage may trigger or advance pro-inflammatory responses in macrophages, the inflammation itself contributes to the development of insulin resistance and the resulting hyperinsulinemia. Macrophages express insulin receptors whose downstream signaling networks share a number of knots with the signaling pathways of pattern recognition and cytokine receptors, which shape macrophage polarity. The shared knots allow insulin to enhance or attenuate both pro-inflammatory and anti-inflammatory macrophage responses. This supposedly physiological function may be impaired by hyperinsulinemia or insulin resistance in macrophages. This review discusses the mutual ambiguous relationship of low-grade inflammation, insulin resistance, hyperinsulinemia and the insulin-dependent modulation of macrophage activity with a focus on adipose tissue and liver.

The Succinate Receptor SUCNR1 Resides at the Endoplasmic Reticulum and Relocates to the Plasma Membrane in Hypoxic Conditions

The GPCR SUCNR1/GPR91 exerts proangiogenesis upon stimulation with the Krebs cycle metabolite succinate. GPCR signaling depends on the surrounding environment and intracellular localization through location bias. Here, we show by microscopy and by cell fractionation that in neurons, SUCNR1 resides at the endoplasmic reticulum (ER), while being fully functional, as shown by calcium release and the induction of the expression of the proangiogenic gene for VEGFA. ER localization was found to depend upon N-glycosylation, particularly at position N8; the nonglycosylated mutant receptor localizes at the plasma membrane shuttled by RAB11. This SUCNR1 glycosylation is physiologically regulated, so that during hypoxic conditions, SUCNR1 is deglycosylated and relocates to the plasma membrane. Downstream signal transduction of SUCNR1 was found to activate the prostaglandin synthesis pathway through direct interaction with COX-2 at the ER; pharmacologic antagonism of the PGE2 EP4 receptor (localized at the nucleus) was found to prevent VEGFA expression. Concordantly, restoring the expression of SUCNR1 in the retina of SUCNR1-null mice renormalized vascularization; this effect is markedly diminished after transfection of the plasma membrane-localized SUCNR1 N8A mutant, emphasizing that ER localization of the succinate receptor is necessary for proper vascularization. These findings uncover an unprecedented physiologic process where GPCR resides at the ER for signaling function.

Nutritional and metabolic signalling through GPCRs

Deregulated metabolism is a well-known feature of several challenging diseases, including diabetes, obesity and cancer. Besides their important role as intracellular bioenergetic molecules, dietary nutrients and metabolic intermediates are released in the extracellular environment. As such, they may achieve unconventional roles as hormone-like molecules by activating cell-surface G-protein-coupled receptors (GPCRs) that regulate several pathophysiological processes. In this review, we provide an insight into the role of lactate, succinate, fatty acids, amino acids, ketogenesis-derived and β-oxidation-derived intermediates as extracellular signalling molecules. Moreover, the mechanisms by which their cognate metabolite-sensing GPCRs integrate nutritional and metabolic signals with specific intracellular pathways will be described. A better comprehension of these aspects is of fundamental importance to identify GPCRs as novel druggable targets.

Inhibiting Succinate Release Worsens Cardiac Reperfusion Injury by Enhancing Mitochondrial Reactive Oxygen Species Generation

Background

The metabolite succinate accumulates during cardiac ischemia. Within 5 minutes of reperfusion, succinate returns to baseline levels via both its release from cells and oxidation by mitochondrial complex II. The latter drives reactive oxygen species (ROS) generation and subsequent opening of the mitochondrial permeability transition (PT) pore, leading to cell death. Targeting succinate dynamics (accumulation/oxidation/release) may be therapeutically beneficial in cardiac ischemia–reperfusion (IR) injury. It has been proposed that blocking MCT1 (monocarboxylate transporter 1) may be beneficial in IR injury, by preventing succinate release and subsequent engagement of downstream inflammatory signaling pathways. In contrast, herein we hypothesized that blocking MCT1 would retain succinate in cells, exacerbating ROS generation and IR injury.

Methods and Results

Using the mitochondrial ROS probe mitoSOX and a custom‐built murine heart perfusion rig built into a spectrofluorometer, we measured ROS generation in situ during the first moments of reperfusion. We found that acute MCT1 inhibition enhanced mitochondrial ROS generation at reperfusion and worsened IR injury (recovery of function and infarct size). Both of these effects were abrogated by tandem inhibition of mitochondrial complex II, suggesting that succinate retention worsens IR because it drives more mitochondrial ROS generation. Furthermore, using the PT pore inhibitor cyclosporin A, along with monitoring of PT pore opening via the mitochondrial membrane potential indicator tetramethylrhodamine ethyl ester, we herein provide evidence that ROS generation during early reperfusion is upstream of the PT pore, not downstream as proposed by others. In addition, pore opening was exacerbated by MCT1 inhibition.

Conclusions

Together, these findings highlight the importance of succinate dynamics and mitochondrial ROS generation as key determinants of PT pore opening and IR injury outcomes.