A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion

β cell acetate production and release are negligible

BACKGROUND

Studies suggest that short chain fatty acids (SCFAs), which are primarily produced from fermentation of fiber, regulate insulin secretion through free fatty acid receptors 2 and 3 (FFA2 and FFA3). As these are G-protein coupled receptors (GPCRs), they have potential therapeutic value as targets for treating type 2 diabetes (T2D). The exact mechanism by which these receptors regulate insulin secretion and other aspects of pancreatic β cell function is unclear. It has been reported that glucose-dependent release of acetate from pancreatic β cells negatively regulates glucose stimulated insulin secretion. While these data raise the possibility of acetate's potential autocrine action on these receptors, these findings have not been independently confirmed, and multiple concerns exist with this observation, particularly the lack of specificity and precision of the acetate detection methodology used.

METHODS

Using Min6 cells and mouse islets, we assessed acetate and pyruvate production and secretion in response to different glucose concentrations, via liquid chromatography mass spectrometry.

RESULTS

Using Min6 cells and mouse islets, we showed that both intracellular pyruvate and acetate increased with high glucose conditions; however, intracellular acetate level increased only slightly and exclusively in Min6 cells but not in the islets. Further, extracellular acetate levels were not affected by the concentration of glucose in the incubation medium of either Min6 cells or islets.

CONCLUSIONS

Our findings do not substantiate the glucose-dependent release of acetate from pancreatic β cells, and therefore, invalidate the possibility of an autocrine inhibitory effect on glucose stimulated insulin secretion.

Structural and systems characterization of phosphorylation on metabolic enzymes identifies sex-specific metabolic reprogramming in obesity

Coordination of adaptive metabolism through cellular signaling networks and metabolic response is essential for balanced flow of energy and homeostasis. Post-translational modifications such as phosphorylation offer a rapid, efficient, and dynamic mechanism to regulate metabolic networks. Although numerous phosphorylation sites have been identified on metabolic enzymes, much remains unknown about their contribution to enzyme function and systemic metabolism. In this study, we stratify phosphorylation sites on metabolic enzymes based on their location with respect to functional and dimerization domains. Our analysis reveals that the majority of published phosphosites are on oxidoreductases, with particular enrichment of phosphotyrosine (pY) sites in proximity to binding domains for substrates, cofactors, active sites, or dimer interfaces. We identify phosphosites altered in obesity using a high fat diet (HFD) induced obesity model coupled to multiomics, and interrogate the functional impact of pY on hepatic metabolism. HFD induced dysregulation of redox homeostasis and reductive metabolism at the phosphoproteome and metabolome level in a sex-specific manner, which was reversed by supplementing with the antioxidant butylated hydroxyanisole (BHA). Partial least squares regression (PLSR) analysis identified pY sites that predict HFD or BHA induced changes of redox metabolites. We characterize predictive pY sites on glutathione S-transferase pi 1 (GSTP1), isocitrate dehydrogenase 1 (IDH1), and uridine monophosphate synthase (UMPS) using CRISPRi-rescue and stable isotope tracing. Our analysis revealed that sites on GSTP1 and UMPS inhibit enzyme activity while the pY site on IDH1 induces activity to promote reductive carboxylation. Overall, our approach provides insight into the convergence points where cellular signaling fine-tunes metabolism.

The role of anaplerotic metabolism of glucose and glutamine in insulin secretion: A model approach

We propose a detailed computational beta cell model that emphasizes the role of anaplerotic metabolism under glucose and glucose-glutamine stimulation. This model goes beyond the traditional focus on mitochondrial oxidative phosphorylation and ATP-sensitive K+ channels, highlighting the predominant generation of ATP from phosphoenolpyruvate in the vicinity of KATP channels. It also underlines the modulatory role of H2O2 as a signaling molecule in the first phase of glucose-stimulated insulin secretion. In the second phase, the model emphasizes the critical role of anaplerotic pathways, activated by glucose stimulation via pyruvate carboxylase and by glutamine via glutamate dehydrogenase. It particularly focuses on the production of NADPH and glutamate as key enhancers of insulin secretion. The predictions of the model are consistent with empirical data, highlighting the complex interplay of metabolic pathways and emphasizing the primary role of glucose and the facilitating role of glutamine in insulin secretion. By delineating these crucial metabolic pathways, the model provides valuable insights into potential therapeutic targets for diabetes.

A metabolic redox relay supports ER proinsulin export in pancreatic islet β cells

ER stress and proinsulin misfolding are heralded as contributing factors to β cell dysfunction in type 2 diabetes, yet how ER function becomes compromised is not well understood. Recent data identify altered ER redox homeostasis as a critical mechanism that contributes to insulin granule loss in diabetes. Hyperoxidation of the ER delays proinsulin export and limits the proinsulin supply available for insulin granule formation. In this report, we identified glucose metabolism as a critical determinant in the redox homeostasis of the ER. Using multiple β cell models, we showed that loss of mitochondrial function or inhibition of cellular metabolism elicited ER hyperoxidation and delayed ER proinsulin export. Our data further demonstrated that β cell ER redox homeostasis was supported by the metabolic supply of reductive redox donors. We showed that limiting NADPH and thioredoxin flux delayed ER proinsulin export, whereas thioredoxin-interacting protein suppression restored ER redox and proinsulin trafficking. Taken together, we propose that β cell ER redox homeostasis is buffered by cellular redox donor cycles, which are maintained through active glucose metabolism.

NADH: the redox sensor in aging-related disorders

SIGNIFICANCE

NADH represents the reduced form of NAD+, and together they constitute the two forms of the Nicotinamide adenine dinucleotide whose balance is named as the NAD+/NADH ratio. NAD+/NADH ratio is mainly involved in redox reactions since both the molecules are responsible for carrying electrons to maintain redox homeostasis. NADH acts as a reducing agent and one of the most known processes exploiting NADH function is energy metabolism. The two main pathways generating energy and involving NADH are Glycolysis and Oxidative phosphorylation, occurring in cell cytosol and in the mitochondrial matrix, respectively.

RECENT ADVANCES

Although NADH is primarily produced through the reduction of NAD+ and consumed by its own oxidation, several are the biosynthetic and consumption pathways, reflecting the NADH role in multiple cellular processes.

CRITICAL ISSUES

This review gathers all the main current data referring to NADH in correlation with metabolic and cellular pathways, such as its coenzyme activity, effect in cell death and on modulating redox and calcium homeostasis. Data were selected following eligibility criteria accordingly to the reviewed topic. A set of electronic databases (Medline/PubMed, Scopus, Web of Sciences (WOS), Cochrane Library) have been used for a systematic search until January 2024 using MeSH keywords/terms (i.e., NADH, NAD+/NADH and NADH/NAD+ ratio, redox homeostasis, energy metabolism, aging, aging-related disorders, therapies).

FUTURE DIRECTION

Gene expression control, as well as to the potential impact on neurodegenerative, cardiac disorders and infections suggest NADH application in clinical settings.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

Kinetic modelling of β-cell metabolism reveals control points in the insulin-regulating pyruvate cycling pathways

Insulin, a key hormone in the regulation of glucose homoeostasis, is secreted by pancreatic β-cells in response to elevated glucose levels. Insulin is released in a biphasic manner in response to glucose metabolism in β-cells. The first phase of insulin secretion is triggered by an increase in the ATP:ADP ratio; the second phase occurs in response to both a rise in ATP:ADP and other key metabolic signals, including a rise in the NADPH:NADP+ ratio. Experimental evidence indicates that pyruvate-cycling pathways play an important role in the elevation of the NADPH:NADP+ ratio in response to glucose. The authors developed a kinetic model for the tricarboxylic acid cycle and pyruvate cycling pathways. The authors successfully validated the model against experimental observations and performed a sensitivity analysis to identify key regulatory interactions in the system. The model predicts that the dicarboxylate carrier and the pyruvate transporter are the most important regulators of pyruvate cycling and NADPH production. In contrast, the analysis showed that variation in the pyruvate carboxylase flux was compensated by a response in the activity of mitochondrial isocitrate dehydrogenase (ICDm ) resulting in minimal effect on overall pyruvate cycling flux. The model predictions suggest starting points for further experimental investigation, as well as potential drug targets for the treatment of type 2 diabetes.

Efficacy of Perioperative Infusion of N(2)-L-alanyl-L-glutamine in Glycemic Control for Patients With Uncontrolled Diabetes Mellitus Presented for Urgent Coronary Artery Bypass Surgery: A Randomized Controlled Trial

OBJECTIVES

To evaluate the efficacy of preoperative glutamine infusion in reducing insulin requirements in patients with uncontrolled type 2 diabetes, defined as glycated hemoglobin (HbA1c) >7%, undergoing urgent coronary artery bypass graft (CABG) surgery.

DESIGN

A randomized controlled trial.

SETTING

At Ain Shams University Hospital, Cardiothoracic Academy.

PARTICIPANTS

Ninety-three patients (of both sexes) with uncontrolled diabetes presenting for urgent CABG were categorized into 2 groups.

INTERVENTIONS

The dipeptiven group (n = 46) was given an infusion of dipeptiven 1.5 mL/kg body weight dissolved in normal saline (200 mL) over 3 hours before surgery. The control group (n = 47) received a normal saline infusion (200 mL).

MEASUREMENTS AND MAIN RESULTS

The dipeptiven group demonstrated statistically significant lower intraoperative (173.74 ± 19.97 mg/dL v 198.22 ±14.64 mg/dL) and postoperative (162.36 ±13.11 mg/dL v 176.13 ±14.86 mg/dL) mean blood glucose levels. In addition, dipeptiven infusion was found to reduce mean total insulin requirements intraoperatively by 3.64 ± 0.56 units/h and postoperatively by 37.109 ± 4.30 units/24 h in comparison to placebo (50.98 ± 16.55 units/24 h and 5.10 ± 2.28 units/h, respectively).

CONCLUSION

A preoperative infusion of dipeptiven can contribute to ameliorating stress hyperglycemia in uncontrolled diabetic patients undergoing urgent CABG.

Isotopomer analyses with the tricarboxylic acid cycle intermediates and exchanging metabolites from the rat kidney

Renal metabolism is essential for kidney functions and energy homeostasis in the body. The TCA cycle is the hub of metabolism, but the metabolic activities of the cycle in the kidney have rarely been investigated. This study is to assess metabolic processes at the level of the TCA cycle in the kidney based on isotopomer distributions in multiple metabolites. Isolated rat kidneys were perfused with media containing common substrates including lactate and alanine for an hour. One group of kidneys received [U-13 C3 ]lactate instead of natural abundance lactate while the other group received [U-13 C3 ]alanine instead of natural abundance alanine. Perfused kidneys and effluent were prepared for analysis using NMR spectroscopy. 13 C-labeling patterns in glutamate, fumarate, aspartate and succinate from the kidney extracts showed that pyruvate carboxylase and oxidative metabolism through the TCA cycle were comparably very active, but pyruvate cycling and pyruvate dehydrogenase were relatively less active. Isotopomer analyses with fumarate and malate from effluent, however, indicated that pyruvate carboxylase was much more active than the TCA cycle and other metabolic processes. The reverse equilibrium of oxaloacetate with four-carbon intermediates of the cycle was nearly complete (92%), based on the ratio of [2,3,4-13 C3 ]/[1,2,3-13 C3 ] in aspartate or malate. 13 C enrichment in glucose with 13 C-lactate supply was higher than that with 13 C-alanine. Isotopomer analyses with multiple metabolites (i.e., glutamate, fumarate, aspartate, succinate and malate) allowed us to assess relative metabolic processes in the TCA cycle in the kidney supplied with [U-13 C3 ]lactate. Data from the analytes were generally consistent, indicating highly active pyruvate carboxylase and oxidative metabolism through the TCA cycle. Different 13 C-labeling patterns in analytes from the kidney extracts versus effluent suggested metabolic compartmentalization.

The regulatory mechanisms and inhibitors of isocitrate dehydrogenase 1 in cancer

Reprogramming of energy metabolism is one of the basic characteristics of cancer and has been proved to be an important cancer treatment strategy. Isocitrate dehydrogenases (IDHs) are a class of key proteins in energy metabolism, including IDH1, IDH2, and IDH3, which are involved in the oxidative decarboxylation of isocitrate to yield α-ketoglutarate (α-KG). Mutants of IDH1 or IDH2 can produce d-2-hydroxyglutarate (D-2HG) with α-KG as the substrate, and then mediate the occurrence and development of cancer. At present, no IDH3 mutation has been reported. The results of pan-cancer research showed that IDH1 has a higher mutation frequency and involves more cancer types than IDH2, implying IDH1 as a promising anti-cancer target. Therefore, in this review, we summarized the regulatory mechanisms of IDH1 on cancer from four aspects: metabolic reprogramming, epigenetics, immune microenvironment, and phenotypic changes, which will provide guidance for the understanding of IDH1 and exploring leading-edge targeted treatment strategies. In addition, we also reviewed available IDH1 inhibitors so far. The detailed clinical trial results and diverse structures of preclinical candidates illustrated here will provide a deep insight into the research for the treatment of IDH1-related cancers.

Targeting Apollo-NADP+ to Image NADPH Generation in Pancreatic Beta-Cell Organelles

NADPH/NADP⁺ redox state supports numerous reactions related to cell growth and survival; yet the full impact is difficult to appreciate due to organelle compartmentalization of NADPH and NADP⁺. To study glucose-stimulated NADPH production in pancreatic beta-cell organelles, we targeted the Apollo-NADP⁺ sensor by first selecting the most pH-stable version of the single-color sensor. We subsequently targeted mTurquoise2-Apollo-NADP⁺ to various organelles and confirmed activity in the cytoplasm, mitochondrial matrix, nucleus, and peroxisome. Finally, we measured the glucose- and glutamine-stimulated NADPH responses by single- and dual-color imaging of the targeted sensors. Overall, we developed multiple organelle-targeted Apollo-NADP⁺ sensors to reveal the prominent role of beta-cell mitochondria in determining NADPH production in the cytoplasm, nucleus, and peroxisome.

The roles of IDH1 in tumor metabolism and immunity

IDH1 is a key metabolic enzyme for cellular respiration in the tricarboxylic acid (TCA) cycle that can convert isocitrate into α-ketoglutarate (α-KG) and generate NADPH. The reduction of IDH1 may affect dioxygenase activity and damage the body's detoxification mechanism. Many studies have shown that IDH1 is closely related to the occurrence and development of tumors, and the changes in IDH1 expression levels or gene mutations have appeared in many tumor tissues and produced a series of metabolic and immunity changes at the same time. To better understand the relationship between IDH1 and tumor development, this article reviews the latest advances in IDH1 and tumor metabolism, tumor immunity, IDH1 regulatory mechanisms and IDH1 target inhibitors.

Low expression of isocitrate dehydrogenase 1 (IDH1) R132H is associated with advanced pathological features in laryngeal squamous cell carcinoma

INTRODUCTION

Recent developments in genomic sequencing have led to the identification of somatic mutations in isocitrate dehydrogenase 1 (IDH1) in various malignancies. IDH1 R132H is the most common mutation of IDH1, which affects codon 132 and results in the conversion of amino acid residue arginine (R) to histidine (H). This study is designed to evaluate the association between the expression of IDH1 R132H and clinicopathological characteristics in laryngeal squamous cell carcinoma (LSCC).

METHODS

The expression pattern and clinical significance of IDH1 R132H were investigated in tissue microarrays (TMAs) of 50 LSCC tumors as well as adjacent normal tissues using immunohistochemistry. Then the exons of the 12 tumor samples with negative/weak positive staining were sequenced by applying polymerase chain reaction (PCR).

RESULTS

The results demonstrated that the cytoplasmic expression of IDH1 R132H was downregulated in tumor cells compared to adjacent normal tissues. A statistically significant association was found between a low level of cytoplasmic expression of IDH1 R132H protein and an increase in histological grade (p < 0.001), perineural invasion (p = 0.019), and lymph node involvement (p < 0.001). The exon4 sequencing results showed that only one sample was positive for IDH1 R132H mutation. IDH1 R132H expression was observed in 39 (78.0%) LSCC samples.

CONCLUSION

These findings indicate that low cytoplasmic expression of IDH1 R132H may have clinical significance in LSCC patients and is associated with more aggressive tumor behavior and progression of the disease, which can help improve potential treatment in patients with LSCC. Further investigations are needed to understand the biological function of IDH1 R132H and larger sample size to confirm our findings.

Role of Reactive Oxygen Species in Glucose Metabolism Disorder in Diabetic Pancreatic β-Cells

The dysfunction of pancreatic β-cells plays a central role in the onset and progression of type 2 diabetes mellitus (T2DM). Insulin secretory defects in β-cells are characterized by a selective impairment of glucose stimulation, and a reduction in glucose-induced ATP production, which is essential for insulin secretion. High glucose metabolism for insulin secretion generates reactive oxygen species (ROS) in mitochondria. In addition, the expression of antioxidant enzymes is very low in β-cells. Therefore, β-cells are easily exposed to oxidative stress. In islet studies using a nonobese T2DM animal model that exhibits selective impairment of glucose-induced insulin secretion (GSIS), quenching ROS generated by glucose stimulation and accumulated under glucose toxicity can improve impaired GSIS. Acute ROS generation and toxicity cause glucose metabolism disorders through different molecular mechanisms. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcription factor, is a master regulator of antioxidant defense and a potential therapeutic target in oxidative stress-related diseases, suggesting the possible involvement of Nrf2 in β-cell dysfunction caused by ROS. In this review, we describe the mechanisms of insulin secretory defects induced by oxidative stress in diabetic β-cells.

Reduced nicotinamide adenine dinucleotide phosphate in redox balance and diseases: a friend or foe?

The nicotinamide adenine dinucleotide (NAD⁺/NADH) and nicotinamide adenine dinucleotide phosphate (NADP⁺/NADPH) redox couples function as cofactors or/and substrates for numerous enzymes to retain cellular redox balance and energy metabolism. Thus, maintaining cellular NADH and NADPH balance is critical for sustaining cellular homeostasis. The sources of NADPH generation might determine its biological effects. Newly-recognized biosynthetic enzymes and genetically encoded biosensors help us better understand how cells maintain biosynthesis and distribution of compartmentalized NAD(H) and NADP(H) pools. It is essential but challenging to distinguish how cells sustain redox couple pools to perform their integral functions and escape redox stress. However, it is still obscure whether NADPH is detrimental or beneficial as either deficiency or excess in cellular NADPH levels disturbs cellular redox state and metabolic homeostasis leading to redox stress, energy stress, and eventually, to the disease state. Additional study of the pathways and regulatory mechanisms of NADPH generation in different compartments, and the means by which NADPH plays a role in various diseases, will provide innovative insights into its roles in human health and may find a value of NADPH for the treatment of certain diseases including aging, Alzheimer's disease, Parkinson's disease, cardiovascular diseases, ischemic stroke, diabetes, obesity, cancer, etc.

Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology

The citrate carrier (CIC) is an integral protein of the inner mitochondrial membrane which catalyzes the efflux of mitochondrial citrate (or other tricarboxylates) in exchange with a cytosolic anion represented by a tricarboxylate or a dicarboxylate or phosphoenolpyruvate. In this way, the CIC provides the cytosol with citrate which is involved in many metabolic reactions. Several studies have been carried out over the years on the structure, function and regulation of this metabolite carrier protein both in mammals and in many other organisms. A lot of data on the characteristics of this protein have therefore accumulated over time thereby leading to a complex framework of metabolic and physiological implications connected to the CIC function. In this review, we critically analyze these data starting from the multiple roles played by the mitochondrial CIC in many cellular processes and then examining the regulation of its activity in different nutritional and hormonal states. Finally, the metabolic significance of the citrate flux, mediated by the CIC, across distinct subcellular compartments is also discussed.

Metabolic cycles and signals for insulin secretion

In this review, we focus on recent developments in our understanding of nutrient-induced insulin secretion that challenge a key aspect of the "canonical" model, in which an oxidative phosphorylation-driven rise in ATP production closes KATP channels. We discuss the importance of intrinsic β cell metabolic oscillations; the phasic alignment of relevant metabolic cycles, shuttles, and shunts; and how their temporal and compartmental relationships align with the triggering phase or the secretory phase of pulsatile insulin secretion. Metabolic signaling components are assigned regulatory, effectory, and/or homeostatic roles vis-à-vis their contribution to glucose sensing, signal transmission, and resetting the system. Taken together, these functions provide a framework for understanding how allostery, anaplerosis, and oxidative metabolism are integrated into the oscillatory behavior of the secretory pathway. By incorporating these temporal as well as newly discovered spatial aspects of β cell metabolism, we propose a much-refined MitoCat-MitoOx model of the signaling process for the field to evaluate.

Metabolism-secretion coupling in glucose-stimulated insulin secretion

Pancreatic β-cells in the islets of Langerhans secrete insulin in response to blood glucose levels. Precise control of the amount of insulin secreted is of critical importance for maintaining systemic carbohydrate homeostasis. It is now well established that glucose induced production of ATP from ADP and the K_ATP channel closure elevate cytosolic Ca²⁺, triggering insulin exocytosis in β-cells. However, for full activation of insulin secretion by glucose, other mechanisms besides Ca²⁺ elevation are needed. These mechanisms are the targets of current research and include intracellular metabolic pathways branching from glycolysis. They are metabolic pathways originating from the TCA cycle intermediates, the glycerolipid/free fatty acid cycle and the pentose phosphate pathway. Signaling effects of these pathways including degradation (removal) of protein SUMOylation, modulation of insulin vesicular energetics, and lipid modulation of exocytotic machinery may converge to fulfill insulin secretion, though the precise mechanisms have yet to be elucidated. This mini-review summarize recent advances in research on metabolic coupling mechanisms functioning in insulin secretion.

The Molecular Mechanisms of Resistance to IDH Inhibitors in Acute Myeloid Leukemia

Gain-of-function mutations of isocitrate dehydrogenases 1/2 (IDH1/2) play crucial roles in the development and progression of acute myeloid leukemia (AML), which provide promising therapeutic targets. Two small molecular inhibitors, ivosidenib and enasidenib have been approved for the treatment of IDH1- and IDH2-mutant AML, respectively. Although these inhibitors benefit patients with AML clinically, drug resistance still occurs and have become a major problem for targeted therapies of IDH-mutant AML. A number of up-to-date studies have demonstrated molecular mechanisms of resistance, providing rationales of novel therapeutic strategies targeting mutant IDH1/2. In this review, we discuss mechanisms of resistance to ivosidenib and enasidenib in patients with AML.

Maturation of beta cells: lessons from in vivo and in vitro models

The ability to maintain normoglycaemia, through glucose-sensitive insulin release, is a key aspect of postnatal beta cell function. However, terminally differentiated beta cell identity does not necessarily imply functional maturity. Beta cell maturation is therefore a continuation of beta cell development, albeit a process that occurs postnatally in mammals. Although many important features have been identified in the study of beta cell maturation, as of yet no unified mechanistic model of beta cell functional maturity exists. Here, we review recent findings about the underlying mechanisms of beta cell functional maturation. These findings include systemic hormonal and nutritional triggers that operate through energy-sensing machinery shifts within beta cells, resulting in primed metabolic states that allow for appropriate glucose trafficking and, ultimately, insulin release. We also draw attention to the expansive synergistic nature of these pathways and emphasise that beta cell maturation is dependent on overlapping regulatory and metabolic networks.

More than meets the islet: aligning nutrient and paracrine inputs with hormone secretion in health and disease

The pancreatic islet is responsive to an array of endocrine, paracrine, and nutritional inputs that adjust hormone secretion to ensure accurate control of glucose homeostasis. Although the mechanisms governing glucose-coupled insulin secretion have received the most attention, there is emerging evidence for a multitude of physiological signaling pathways and paracrine networks that collectively regulate insulin, glucagon, and somatostatin release. Moreover, the modulation of these pathways in conditions of glucotoxicity or lipotoxicity are areas of both growing interest and controversy. In this review, the contributions of external, intrinsic, and paracrine factors in pancreatic β-, α-, and δ-cell secretion across the full spectrum of physiological (i.e., fasting and fed) and pathophysiological (gluco- and lipotoxicity; diabetes) environments will be critically discussed.

Contribution of Mitochondria to Insulin Secretion by Various Secretagogues

Significance: Mitochondria determine glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells by elevating ATP synthesis. As the metabolic and redox hub, mitochondria provide numerous links to the plasma membrane channels, insulin granule vesicles (IGVs), cell redox, NADH, NADPH, and Ca²⁺ homeostasis, all affecting insulin secretion. Recent Advances: Mitochondrial redox signaling was implicated in several modes of insulin secretion (branched-chain ketoacid [BCKA]-, fatty acid [FA]-stimulated). Mitochondrial Ca²⁺ influx was found to enhance GSIS, reflecting cytosolic Ca²⁺ oscillations induced by action potential spikes (intermittent opening of voltage-dependent Ca²⁺ and K⁺ channels) or the superimposed Ca²⁺ release from the endoplasmic reticulum (ER). The ATPase inhibitory factor 1 (IF1) was reported to tune the glucose sensitivity range for GSIS. Mitochondrial protein kinase A was implicated in preventing the IF1-mediated inhibition of the ATP synthase. Critical Issues: It is unknown how the redox signal spreads up to the plasma membrane and what its targets are, what the differences in metabolic, redox, NADH/NADPH, and Ca²⁺ signaling, and homeostasis are between the first and second GSIS phase, and whether mitochondria can replace ER in the amplification of IGV exocytosis. Future Directions: Metabolomics studies performed to distinguish between the mitochondrial matrix and cytosolic metabolites will elucidate further details. Identifying the targets of cell signaling into mitochondria and of mitochondrial retrograde metabolic and redox signals to the cell will uncover further molecular mechanisms for insulin secretion stimulated by glucose, BCKAs, and FAs, and the amplification of secretion by glucagon-like peptide (GLP-1) and metabotropic receptors. They will identify the distinction between the hub β-cells and their followers in intact and diabetic states. Antioxid. Redox Signal. 36, 920-952.

Nutrient Regulation of Pancreatic Islet β-Cell Secretory Capacity and Insulin Production

Pancreatic islet β-cells exhibit tremendous plasticity for secretory adaptations that coordinate insulin production and release with nutritional demands. This essential feature of the β-cell can allow for compensatory changes that increase secretory output to overcome insulin resistance early in Type 2 diabetes (T2D). Nutrient-stimulated increases in proinsulin biosynthesis may initiate this β-cell adaptive compensation; however, the molecular regulators of secretory expansion that accommodate the increased biosynthetic burden of packaging and producing additional insulin granules, such as enhanced ER and Golgi functions, remain poorly defined. As these adaptive mechanisms fail and T2D progresses, the β-cell succumbs to metabolic defects resulting in alterations to glucose metabolism and a decline in nutrient-regulated secretory functions, including impaired proinsulin processing and a deficit in mature insulin-containing secretory granules. In this review, we will discuss how the adaptative plasticity of the pancreatic islet β-cell's secretory program allows insulin production to be carefully matched with nutrient availability and peripheral cues for insulin signaling. Furthermore, we will highlight potential defects in the secretory pathway that limit or delay insulin granule biosynthesis, which may contribute to the decline in β-cell function during the pathogenesis of T2D.

Glucose-6-phosphatase catalytic subunit 2 negatively regulates glucose oxidation and insulin secretion in pancreatic β-cells

Elevated fasting blood glucose (FBG) is associated with increased risks of developing type 2 diabetes (T2D) and cardiovascular-associated mortality. G6PC2 is predominantly expressed in islets, encodes a glucose-6-phosphatase catalytic subunit that converts glucose-6-phosphate (G6P) to glucose, and has been linked with variations in FBG in genome-wide association studies. Deletion of G6pc2 in mice has been shown to lower FBG without affecting fasting plasma insulin levels in vivo. At 5 mM glucose, pancreatic islets from G6pc2 knockout (KO) mice exhibit no glucose cycling, increased glycolytic flux, and enhanced glucose-stimulated insulin secretion (GSIS). However, the broader effects of G6pc2 KO on β-cell metabolism and redox regulation are unknown. Here we used CRISPR/Cas9 gene editing and metabolic flux analysis in βTC3 cells, a murine pancreatic β-cell line, to examine the role of G6pc2 in regulating glycolytic and mitochondrial fluxes. We found that deletion of G6pc2 led to ∼60% increases in glycolytic and citric acid cycle (CAC) fluxes at both 5 and 11 mM glucose concentrations. Furthermore, intracellular insulin content and GSIS were enhanced by approximately two-fold, along with increased cytosolic redox potential and reductive carboxylation flux. Normalization of fluxes relative to net glucose uptake revealed upregulation in two NADPH-producing pathways in the CAC. These results demonstrate that G6pc2 regulates GSIS by modulating not only glycolysis but also, independently, citric acid cycle activity in β-cells. Overall, our findings implicate G6PC2 as a potential therapeutic target for enhancing insulin secretion and lowering FBG, which could benefit individuals with prediabetes, T2D, and obesity.

Isoform-specific Roles of Prolyl Hydroxylases in the Regulation of Pancreatic β-Cell Function

Pancreatic β-cells can secrete insulin via 2 pathways characterized as KATP channel -dependent and -independent. The KATP channel-independent pathway is characterized by a rise in several potential metabolic signaling molecules, including the NADPH/NADP+ ratio and α-ketoglutarate (αKG). Prolyl hydroxylases (PHDs), which belong to the αKG-dependent dioxygenase superfamily, are known to regulate the stability of hypoxia-inducible factor α. In the current study, we assess the role of PHDs in vivo using the pharmacological inhibitor dimethyloxalylglycine (DMOG) and generated β-cell-specific knockout (KO) mice for all 3 isoforms of PHD (β-PHD1 KO, β-PHD2 KO, and β-PHD3 KO mice). DMOG inhibited in vivo insulin secretion in response to glucose challenge and inhibited the first phase of insulin secretion but enhanced the second phase of insulin secretion in isolated islets. None of the β-PHD KO mice showed any significant in vivo defects associated with glucose tolerance and insulin resistance except for β-PHD2 KO mice which had significantly increased plasma insulin during a glucose challenge. Islets from both β-PHD1 KO and β-PHD3 KO had elevated β-cell apoptosis and reduced β-cell mass. Isolated islets from β-PHD1 KO and β-PHD3 KO had impaired glucose-stimulated insulin secretion and glucose-stimulated increases in the ATP/ADP and NADPH/NADP+ ratio. All 3 PHD isoforms are expressed in β-cells, with PHD3 showing the most distinct expression pattern. The lack of each PHD protein did not significantly impair in vivo glucose homeostasis. However, β-PHD1 KO and β-PHD3 KO mice had defective β-cell mass and islet insulin secretion, suggesting that these mice may be predisposed to developing diabetes.

The Human Islet: Mini-Organ With Mega-Impact

This review focuses on the human pancreatic islet-including its structure, cell composition, development, function, and dysfunction. After providing a historical timeline of key discoveries about human islets over the past century, we describe new research approaches and technologies that are being used to study human islets and how these are providing insight into human islet physiology and pathophysiology. We also describe changes or adaptations in human islets in response to physiologic challenges such as pregnancy, aging, and insulin resistance and discuss islet changes in human diabetes of many forms. We outline current and future interventions being developed to protect, restore, or replace human islets. The review also highlights unresolved questions about human islets and proposes areas where additional research on human islets is needed.

Early Cytokine-Induced Transient NOX2 Activity Is ER Stress-Dependent and Impacts β-Cell Function and Survival

In type 1 diabetes (T1D) development, proinflammatory cytokines (PIC) released by immune cells lead to increased reactive oxygen species (ROS) production in β-cells. Nonetheless, the temporality of the events triggered and the role of different ROS sources remain unclear. Isolated islets from C57BL/6J wild-type (WT), NOX1 KO and NOX2 KO mice were exposed to a PIC combination. We show that cytokines increase O₂^•− production after 2 h in WT and NOX1 KO but not in NOX2 KO islets. Using transgenic mice constitutively expressing a genetically encoded compartment specific H₂O₂ sensor, we show, for the first time, a transient increase of cytosolic/nuclear H₂O₂ in islet cells between 4 and 5 h during cytokine exposure. The H₂O₂ increase coincides with the intracellular NAD(P)H decrease and is absent in NOX2 KO islets. NOX2 KO confers better glucose tolerance and protects against cytokine-induced islet secretory dysfunction and death. However, NOX2 absence does not counteract the cytokine effects in ER Ca²⁺ depletion, Store-Operated Calcium Entry (SOCE) increase and ER stress. Instead, the activation of ER stress precedes H₂O₂ production. As early NOX2-driven ROS production impacts β-cells’ function and survival during insulitis, NOX2 might be a potential target for designing therapies against early β-cell dysfunction in the context of T1D onset.

Mitochondrial Efflux of Citrate and Isocitrate Is Fully Dispensable for Glucose-Stimulated Insulin Secretion and Pancreatic Islet β-Cell Function

The defining feature of pancreatic islet β-cell function is the precise coordination of changes in blood glucose levels with insulin secretion to regulate systemic glucose homeostasis. While ATP has long been heralded as a critical metabolic coupling factor to trigger insulin release, glucose-derived metabolites have been suggested to further amplify fuel-stimulated insulin secretion. The mitochondrial export of citrate and isocitrate through the citrate-isocitrate carrier (CIC) has been suggested to initiate a key pathway that amplifies glucose-stimulated insulin secretion, though the physiological significance of β-cell CIC-to-glucose homeostasis has not been established. Here, we generated constitutive and adult CIC β-cell knockout (KO) mice and demonstrate that these animals have normal glucose tolerance, similar responses to diet-induced obesity, and identical insulin secretion responses to various fuel secretagogues. Glucose-stimulated NADPH production was impaired in β-cell CIC KO islets, whereas glutathione reduction was retained. Furthermore, suppression of the downstream enzyme cytosolic isocitrate dehydrogenase (Idh1) inhibited insulin secretion in wild-type islets but failed to impact β-cell function in β-cell CIC KO islets. Our data demonstrate that the mitochondrial CIC is not required for glucose-stimulated insulin secretion and that additional complexities exist for the role of Idh1 and NADPH in the regulation of β-cell function.

Pyruvate carboxylase and cancer progression

Pyruvate carboxylase (PC) is a mitochondrial enzyme that catalyzes the ATP-dependent carboxylation of pyruvate to oxaloacetate (OAA), serving to replenish the tricarboxylic acid (TCA) cycle. In nonmalignant tissue, PC plays an essential role in controlling whole-body energetics through regulation of gluconeogenesis in the liver, synthesis of fatty acids in adipocytes, and insulin secretion in pancreatic β cells. In breast cancer, PC activity is linked to pulmonary metastasis, potentially by providing the ability to utilize glucose, fatty acids, and glutamine metabolism as needed under varying conditions as cells metastasize. PC enzymatic activity appears to be of particular importance in cancer cells that are unable to utilize glutamine for anaplerosis. Moreover, PC activity also plays a role in lipid metabolism and protection from oxidative stress in cancer cells. Thus, PC activity may be essential to link energy substrate utilization with cancer progression and to enable the metabolic flexibility necessary for cell resilience to changing and adverse conditions during the metastatic process.

Reductive TCA cycle metabolism fuels glutamine- and glucose-stimulated insulin secretion

Metabolic fuels regulate insulin secretion by generating second messengers that drive insulin granule exocytosis, but the biochemical pathways involved are incompletely understood. Here we demonstrate that stimulation of rat insulinoma cells or primary rat islets with glucose or glutamine + 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (Gln + BCH) induces reductive, "counter-clockwise" tricarboxylic acid (TCA) cycle flux of glutamine to citrate. Molecular or pharmacologic suppression of isocitrate dehydrogenase-2 (IDH2), which catalyzes reductive carboxylation of 2-ketoglutarate to isocitrate, results in impairment of glucose- and Gln + BCH-stimulated reductive TCA cycle flux, lowering of NADPH levels, and inhibition of insulin secretion. Pharmacologic suppression of IDH2 also inhibits insulin secretion in living mice. Reductive TCA cycle flux has been proposed as a mechanism for generation of biomass in cancer cells. Here we demonstrate that reductive TCA cycle flux also produces stimulus-secretion coupling factors that regulate insulin secretion, including in non-dividing cells.

Pancreatic β cells put the glutamine engine in reverse

The metabolism of nutrients other than glucose influences insulin secretion by pancreatic β cells, but the mechanisms involved are incompletely understood. In this issue of Cell Metabolism, Zhang et al. (2020) report that reductive glutamine metabolism generates cytosolic NADPH to promote insulin secretion by β cells.

The Pancreatic β-Cell: The Perfect Redox System

Pancreatic β-cell insulin secretion, which responds to various secretagogues and hormonal regulations, is reviewed here, emphasizing the fundamental redox signaling by NADPH oxidase 4- (NOX4-) mediated H₂O₂ production for glucose-stimulated insulin secretion (GSIS). There is a logical summation that integrates both metabolic plus redox homeostasis because the ATP-sensitive K⁺ channel (K_ATP) can only be closed when both ATP and H₂O₂ are elevated. Otherwise ATP would block K_ATP, while H₂O₂ would activate any of the redox-sensitive nonspecific calcium channels (NSCCs), such as TRPM2. Notably, a 100%-closed K_ATP ensemble is insufficient to reach the -50 mV threshold plasma membrane depolarization required for the activation of voltage-dependent Ca²⁺ channels. Open synergic NSCCs or Cl^- channels have to act simultaneously to reach this threshold. The resulting intermittent cytosolic Ca²⁺-increases lead to the pulsatile exocytosis of insulin granule vesicles (IGVs). The incretin (e.g., GLP-1) amplification of GSIS stems from receptor signaling leading to activating the phosphorylation of TRPM channels and effects on other channels to intensify integral Ca²⁺-influx (fortified by endoplasmic reticulum Ca²⁺). ATP plus H₂O₂ are also required for branched-chain ketoacids (BCKAs); and partly for fatty acids (FAs) to secrete insulin, while BCKA or FA β-oxidation provide redox signaling from mitochondria, which proceeds by H₂O₂ diffusion or hypothetical SH relay via peroxiredoxin "redox kiss" to target proteins.

Mechanisms controlling pancreatic islet cell function in insulin secretion

Metabolic homeostasis in mammals is tightly regulated by the complementary actions of insulin and glucagon. The secretion of these hormones from pancreatic β-cells and α-cells, respectively, is controlled by metabolic, endocrine, and paracrine regulatory mechanisms and is essential for the control of blood levels of glucose. The deregulation of these mechanisms leads to various pathologies, most notably type 2 diabetes, which is driven by the combined lesions of impaired insulin action and a loss of the normal insulin secretion response to glucose. Glucose stimulates insulin secretion from β-cells in a bi-modal fashion, and new insights about the underlying mechanisms, particularly relating to the second or amplifying phase of this secretory response, have been recently gained. Other recent work highlights the importance of α-cell-produced proglucagon-derived peptides, incretin hormones from the gastrointestinal tract and other dietary components, including certain amino acids and fatty acids, in priming and potentiation of the β-cell glucose response. These advances provide a new perspective for the understanding of the β-cell failure that triggers type 2 diabetes.

Nrf2: The Master and Captain of Beta Cell Fate

Prolonged hyperglycemia is toxic to pancreatic β cells, generating excessive reactive oxygen species, defective glucose-stimulated insulin secretion, decreased insulin production, and eventually β cell death and diabetes. Nrf2 is a master regulator of cellular responses to counteract dangerous levels of oxidative stress. Maintenance of β cell mass depends on Nrf2 to promote the survival, function, and proliferation of β cells. Indeed, Nrf2 activation decreases inflammation, increases insulin sensitivity, reduces body weight, and preserves β cell mass. Therefore, numerous pharmacological activators of Nrf2 are being tested in clinical trials for the treatment of diabetes and diabetic complications. Modulating Nrf2 activity in β cells is a promising therapeutic approach for the treatment of diabetes.

Augmented mitochondrial energy metabolism is an early response to chronic glucose stress in human pancreatic beta cells

AIMS/HYPOTHESIS

In islets from individuals with type 2 diabetes and in islets exposed to chronic elevated glucose, mitochondrial energy metabolism is impaired. Here, we studied early metabolic changes and mitochondrial adaptations in human beta cells during chronic glucose stress.

METHODS

Respiration and cytosolic ATP changes were measured in human islet cell clusters after culture for 4 days in 11.1 mmol/l glucose. Metabolomics was applied to analyse intracellular metabolite changes as a result of glucose stress conditions. Alterations in beta cell function were followed using insulin secretion assays or cytosolic calcium signalling after expression of the calcium probe YC3.6 specifically in beta cells of islet clusters.

RESULTS

At early stages of glucose stress, mitochondrial energy metabolism was augmented in contrast to the previously described mitochondrial dysfunction in beta cells from islets of diabetic donors. Following chronic glucose stress, mitochondrial respiration increased (by 52.4%, p < 0.001) and, as a consequence, the cytosolic ATP/ADP ratio in resting human pancreatic islet cells was elevated (by 27.8%, p < 0.05). Because of mitochondrial overactivation in the resting state, nutrient-induced beta cell activation was reduced. In addition, chronic glucose stress caused metabolic adaptations that resulted in the accumulation of intermediates of the glycolytic pathway, the pentose phosphate pathway and the TCA cycle; the most strongly augmented metabolite was glycerol 3-phosphate. The changes in metabolites observed are likely to be due to the inability of mitochondria to cope with continuous nutrient oversupply. To protect beta cells from chronic glucose stress, we inhibited mitochondrial pyruvate transport. Metabolite concentrations were partially normalised and the mitochondrial respiratory response to nutrients was markedly improved. Furthermore, stimulus-secretion coupling as assessed by cytosolic calcium signalling, was restored.

CONCLUSION/INTERPRETATION

We propose that metabolic changes and associated mitochondrial overactivation are early adaptations to glucose stress, and may reflect what happens as a result of poor blood glucose control. Inhibition of mitochondrial pyruvate transport reduces mitochondrial nutrient overload and allows beta cells to recover from chronic glucose stress. Graphical abstract.

Muscle-Liver Trafficking of BCAA-Derived Nitrogen Underlies Obesity-Related Glycine Depletion

Glycine levels are inversely associated with branched-chain amino acids (BCAAs) and cardiometabolic disease phenotypes, but biochemical mechanisms that explain these relationships remain uncharted. Metabolites and genes related to BCAA metabolism and nitrogen handling were strongly associated with glycine in correlation analyses. Stable isotope labeling in Zucker fatty rats (ZFRs) shows that glycine acts as a carbon donor for the pyruvate-alanine cycle in a BCAA-regulated manner. Inhibition of the BCAA transaminase (BCAT) enzymes depletes plasma pools of alanine and raises glycine levels. In high-fat-fed ZFRs, dietary glycine supplementation raises urinary acyl-glycine content and lowers circulating triglycerides but also results in accumulation of long-chain acyl-coenzyme As (acyl-CoAs), lower 5' adenosine monophosphate-activated protein kinase (AMPK) phosphorylation in muscle, and no improvement in glucose tolerance. Collectively, these studies frame a mechanism for explaining obesity-related glycine depletion and also provide insight into the impact of glycine supplementation on systemic glucose, lipid, and amino acid metabolism.

Pancreatic β-cells respond to fuel pressure with an early metabolic switch

Pancreatic β-cells become irreversibly damaged by long-term exposure to excessive glucose concentrations and lose their ability to carry out glucose stimulated insulin secretion (GSIS) upon damage. The β-cells are not able to control glucose uptake and they are therefore left vulnerable for endogenous toxicity from metabolites produced in excess amounts upon increased glucose availability. In order to handle excess fuel, the β-cells possess specific metabolic pathways, but little is known about these pathways. We present a study of β-cell metabolism under increased fuel pressure using a stable isotope resolved NMR approach to investigate early metabolic events leading up to β-cell dysfunction. The approach is based on a recently described combination of ¹³C metabolomics combined with signal enhanced NMR via dissolution dynamic nuclear polarization (dDNP). Glucose-responsive INS-1 β-cells were incubated with increasing concentrations of [U-¹³C] glucose under conditions where GSIS was not affected (2–8 h). We find that pyruvate and DHAP were the metabolites that responded most strongly to increasing fuel pressure. The two major divergence pathways for fuel excess, the glycerolipid/fatty acid metabolism and the polyol pathway, were found not only to operate at unchanged rate but also with similar quantity.

The role of α-ketoglutarate and the hypoxia sensing pathway in the regulation of pancreatic β-cell function

Anaplerosis and the associated mitochondrial metabolite transporters generate unique cytosolic metabolic signaling molecules that can regulate insulin release from pancreatic β-cells. It has been shown that mitochondrial metabolites, transported by the citrate carrier (CIC), dicarboxylate carrier (DIC), oxoglutarate carrier (OGC), and mitochondrial pyruvate carrier (MPC) play a vital role in the regulation of glucose-stimulated insulin secretion (GSIS). Metabolomic studies on static and biphasic insulin secretion, suggests that several anaplerotic derived metabolites, including α-ketoglutarate (αKG), are strongly associated with nutrient regulated insulin secretion. Support for a role of αKG in the regulation of insulin secretion comes from studies looking at αKG dependent enzymes, including hypoxia-inducible factor-prolyl hydroxylases (PHDs) in clonal β-cells, and rodent and human islets. This review will focus on the possible link between defective anaplerotic-derived αKG, PHDs, and the development of type 2 diabetes (T2D).

Gene of the month: IDH1

Isocitrate dehydrogenase 1 (IDH1) encodes a protein which catalyses the oxidative decarboxylation of isocitrate to α-ketoglutarate. Mutant IDH1 favours the production of 2-hydroxyglutarate, an oncometabolite with multiple downstream effects which promote tumourigenesis. IDH1 mutations have been described in a number of neoplasms most notably low-grade diffuse gliomas, conventional central and periosteal cartilaginous tumours and cytogenetically normal acute myeloid leukaemia. Post zygotic somatic mutations of IDH1 characterise the majority of cases of Ollier disease and Maffucci syndrome. IDH1 mutations are uncommon in epithelial neoplasia but have been described in cholangiocarcinoma.

Glucose-Stimulated Insulin Secretion Fundamentally Requires H2O2 Signaling by NADPH Oxidase 4

NADPH facilitates glucose-stimulated insulin secretion (GSIS) in pancreatic islets (PIs) of β-cells through an as yet unknown mechanism. We found NADPH oxidase isoform 4 (NOX4) to be the main producer of cytosolic H₂O₂, which is essential for GSIS; an increase in ATP alone was insufficient for GSIS. The fast GSIS phase was absent from PIs from NOX4-null, β-cell-specific knockout mice (NOX4βKO) (though not from NOX2 knockout mice) and from NOX4-silenced or catalase-overexpressing INS-1E cells. Lentiviral NOX4 overexpression or H₂O₂ rescued GSIS in PIs from NOX4βKO mice. NOX4 silencing suppressed Ca²⁺ oscillations, and the patch-clamped K_ATP channel opened more frequently when glucose was high. Mitochondrial H₂O₂, decreasing upon GSIS, provided alternative redox signaling when 2-oxo-isocaproate or fatty acid oxidation formed superoxides through electron-transfer flavoprotein:Q-oxidoreductase. Unlike GSIS, such insulin secretion was blocked with mitochondrial antioxidant SkQ1. Both NOX4 knockout and NOX4βKO mice exhibited impaired glucose tolerance and peripheral insulin resistance. Thus, the redox signaling previously suggested to cause β-cells to self-check hypothetically induces insulin resistance when it is absent. In conclusion, increases in ATP and H₂O₂ constitute an essential signal that switches on insulin exocytosis for glucose and branched-chain oxoacids as secretagogues (it does so partially for fatty acids). Redox signaling could be impaired by cytosolic antioxidants; hence, those targeting mitochondria should be preferred for clinical applications to treat (pre)diabetes at any stage.

Improved glucose tolerance with DPPIV inhibition requires β-cell SENP1 amplification of glucose-stimulated insulin secretion

Pancreatic islet insulin secretion is amplified by both metabolic and receptor-mediated signaling pathways. The incretin-mimetic and DPPIV inhibitor anti-diabetic drugs increase insulin secretion, but in humans this can be variable both in vitro and in vivo. We examined the correlation of GLP-1 induced insulin secretion from human islets with key donor characteristics, glucose-responsiveness, and the ability of glucose to augment exocytosis in β-cells. No clear correlation was observed between several donor or organ processing parameters and the ability of Exendin 4 to enhance insulin secretion. The ability of glucose to facilitate β-cell exocytosis was, however, significantly correlated with responses to Exendin 4. We therefore studied the effect of impaired glucose-dependent amplification of insulin exocytosis on responses to DPPIV inhibition (MK-0626) in vivo using pancreas and β-cell specific sentrin-specific protease-1 (SENP1) mice which exhibit impaired metabolic amplification of insulin exocytosis. Glucose tolerance was improved, and plasma insulin was increased, following either acute or 4 week treatment of wild-type (βSENP1+/+ ) mice with MK-0626. This DPPIV inhibitor was ineffective in βSENP1+/- or βSENP1- / - mice. Finally, we confirm impaired exocytotic responses of β-cells and reduced insulin secretion from islets of βSENP1- / - mice and show that the ability of Exendin 4 to enhance exocytosis is lost in these cells. Thus, an impaired ability of glucose to amplify insulin exocytosis results in a deficient effect of DPPIV inhibition to improve in vivo insulin responses and glucose tolerance.

Tumor mutational burden predicts survival in patients with low-grade gliomas expressing mutated IDH1

Background

Gliomas are the most common primary brain tumors. High-Grade Gliomas have a median survival (MS) of 18 months, while Low-Grade Gliomas (LGGs) have an MS of approximately 7.3 years. Seventy-six percent of patients with LGG express mutated isocitrate dehydrogenase (mIDH) enzyme. Survival of these patients ranges from 1 to 15 years, and tumor mutational burden ranges from 0.28 to 3.85 somatic mutations/megabase per tumor. We tested the hypothesis that the tumor mutational burden would predict the survival of patients with tumors bearing mIDH.

Methods

We analyzed the effect of tumor mutational burden on patients' survival using clinical and genomic data of 1199 glioma patients from The Cancer Genome Atlas and validated our results using the Glioma Longitudinal AnalySiS consortium.

Results

High tumor mutational burden negatively correlates with the survival of patients with LGG harboring mIDH (P = .005). This effect was significant for both Oligodendroglioma (LGG-mIDH-O; MS = 2379 vs 4459 days in high vs low, respectively; P = .005) and Astrocytoma (LGG-mIDH-A; MS = 2286 vs 4412 days in high vs low respectively; P = .005). There was no differential representation of frequently mutated genes (eg, TP53, ATRX, CIC, and FUBP) in either group. Gene set enrichment analysis revealed an enrichment in Gene Ontologies related to cell cycle, DNA-damage response in high versus low tumor mutational burden. Finally, we identified 6 gene sets that predict survival for LGG-mIDH-A and LGG-mIDH-O.

Conclusions

we demonstrate that tumor mutational burden is a powerful, robust, and clinically relevant prognostic factor of MS in mIDH patients.

The Landscape of the Anti-Kinase Activity of the IDH1 Inhibitors

Isocitrate dehydrogenases constitute a class of enzymes that are crucial for cellular metabolism. The overexpression or mutation of isocitrate dehydrogenases are often found in leukemias, glioblastomas, lung cancers, and ductal pancreatic cancer among others. Mutation R132H, which changes the functionality of an enzyme to produce mutagenic 2-hydroxyglutarate instead of a normal product, is particularly important in this field. A series of inhibitors were described for these enzymes of which ivosidenib was the first to be approved for treating leukemia and bile duct cancers in 2018. Here, we investigated the polypharmacological landscape of the activity for known sulfamoyl derivatives that are inhibitors, which are selective towards IDH1 R132H. These compounds appeared to be effective inhibitors of several non-receptor kinases at a similar level as imatinib and axitinib. The antiproliferative activity of these compounds against a panel of cancer cells was tested and is explained based on the relative expression levels of the investigated proteins. The multitargeted activity of these compounds makes them valuable agents against a wide range of cancers, regardless of the status of IDH1.

The Loss of ARNT/HIF1β in Male Pancreatic β-Cells Is Protective Against High-Fat Diet-Induced Diabetes

The transcription factor aryl hydrocarbon receptor nuclear translocator (ARNT)/hypoxia-inducible factor (HIF)-1β (ARNT/HIF1β) plays a key role in maintaining β-cell function and has been shown to be one of the most downregulated transcription factors in islets from patients with type 2 diabetes. We have shown a role for ARNT/HIF1β in glucose sensing and insulin secretion in vitro and no defects in in vivo glucose homeostasis. To gain a better understanding of the role of ARNT/HIF1β in the development of diabetes, we placed control (+/+/Cre) and β-cell-specific ARNT/HIF1β knockout (fl/fl/Cre) mice on a high-fat diet (HFD). Unlike the control (+/+/Cre) mice, HFD-fed fl/fl/Cre mice had no impairment in in vivo glucose tolerance. The lack of impairment in HFD-fed fl/fl/Cre mice was partly due to an improved islet glucose-stimulated NADPH/NADP+ ratio and glucose-stimulated insulin secretion. The effects of the HFD-rescued insulin secretion in fl/fl/Cre islets could be reproduced by treating low-fat diet (LFD)-fed fl/fl/Cre islets with the lipid signaling molecule 1-monoacylglcyerol. This suggests that the defects seen in LFD-fed fl/fl/Cre islet insulin secretion involve lipid signaling molecules. Overall, mice lacking ARNT/HIF1β in β-cells have altered lipid signaling in vivo and are resistant to an HFD's ability to induce diabetes.

Integrated In Vivo Quantitative Proteomics and Nutrient Tracing Reveals Age-Related Metabolic Rewiring of Pancreatic β Cell Function

Pancreatic β cell physiology changes substantially throughout life, yet the mechanisms that drive these changes are poorly understood. Here, we performed comprehensive in vivo quantitative proteomic profiling of pancreatic islets from juvenile and 1-year-old mice. The analysis revealed striking differences in abundance of enzymes controlling glucose metabolism. We show that these changes in protein abundance are associated with higher activities of glucose metabolic enzymes involved in coupling factor generation as well as increased activity of the coupling factor-dependent amplifying pathway of insulin secretion. Nutrient tracing and targeted metabolomics demonstrated accelerated accumulation of glucose-derived metabolites and coupling factors in islets from 1-year-old mice, indicating that age-related changes in glucose metabolism contribute to improved glucose-stimulated insulin secretion with age. Together, our study provides an in-depth characterization of age-related changes in the islet proteome and establishes metabolic rewiring as an important mechanism for age-associated changes in β cell function.

Organismal age impacts fundamental aspects of β cell physiology. Wortham et al. apply proteomics and targeted metabolomics to islets from juvenile and adult mice, revealing age-related changes in metabolic enzyme abundance and production of coupling factors that enhance insulin secretion. This work provides insight into age-associated changes to the β cell.

Real-time hyperpolarized 13C magnetic resonance detects increased pyruvate oxidation in pyruvate dehydrogenase kinase 2/4-double knockout mouse livers

The pyruvate dehydrogenase complex (PDH) critically regulates carbohydrate metabolism. Phosphorylation of PDH by one of the pyruvate dehydrogenase kinases 1-4 (PDK1-4) decreases the flux of carbohydrates into the TCA cycle. Inhibition of PDKs increases oxidative metabolism of carbohydrates, so targeting PDKs has emerged as an important therapeutic approach to manage various metabolic diseases. Therefore, it is highly desirable to begin to establish imaging tools for noninvasive measurements of PDH flux in rodent models. In this study, we used hyperpolarized (HP) ¹³C-magnetic resonance spectroscopy to study the impact of a PDK2/PDK4 double knockout (DKO) on pyruvate metabolism in perfused livers from lean and diet-induced obese (DIO) mice and validated the HP observations with high-resolution ¹³C-nuclear magnetic resonance (NMR) spectroscopy of tissue extracts and steady-state isotopomer analyses. We observed that PDK-deficient livers produce more HP-bicarbonate from HP-[1-¹³C]pyruvate than age-matched control livers. A steady-state ¹³C-NMR isotopomer analysis of tissue extracts confirmed that flux rates through PDH, as well as pyruvate carboxylase and pyruvate cycling activities, are significantly higher in PDK-deficient livers. Immunoblotting experiments confirmed that HP-bicarbonate production from HP-[1-¹³C]pyruvate parallels decreased phosphorylation of the PDH E1α subunit (pE1α) in liver tissue. Our findings indicate that combining real-time hyperpolarized ¹³C NMR spectroscopy and ¹³C isotopomer analysis provides quantitative insights into intermediary metabolism in PDK-knockout mice. We propose that this method will be useful in assessing metabolic disease states and developing therapies to improve PDH flux.

Mitochondrial fragmentation, elevated mitochondrial superoxide and respiratory supercomplexes disassembly is connected with the tamoxifen-resistant phenotype of breast cancer cells

Tamoxifen resistance remains a clinical obstacle in the treatment of hormone sensitive breast cancer. It has been reported that tamoxifen is able to target respiratory complex I within mitochondria. Therefore, we established two tamoxifen-resistant cell lines, MCF7 Tam5R and T47D Tam5R resistant to 5 μM tamoxifen and investigated whether tamoxifen-resistant cells exhibit mitochondrial changes which could help them survive the treatment. The function of mitochondria in this experimental model was evaluated in detail by studying i) the composition and activity of mitochondrial respiratory complexes; ii) respiration and glycolytic status; iii) mitochondrial distribution, dynamics and reactive oxygen species production. We show that Tam5R cells exhibit a significant decrease in mitochondrial respiration, low abundance of assembled mitochondrial respiratory supercomplexes, a more fragmented mitochondrial network connected with DRP1 Ser637 phosphorylation, higher glycolysis and sensitivity to 2-deoxyglucose. Tam5R cells also produce significantly higher levels of mitochondrial superoxide but at the same time increase their antioxidant defense (CAT, SOD2) through upregulation of SIRT3 and show phosphorylation of AMPK at Ser 485/491. Importantly, MCF7 ρ0 cells lacking functional mitochondria exhibit a markedly higher resistance to tamoxifen, supporting the role of mitochondria in tamoxifen resistance. We propose that reduced mitochondrial function and higher level of reactive oxygen species within mitochondria in concert with metabolic adaptations contribute to the phenotype of tamoxifen resistance.

Metabolomics for Investigating Physiological and Pathophysiological Processes

Metabolomics uses advanced analytical chemistry techniques to enable the high-throughput characterization of metabolites from cells, organs, tissues, or biofluids. The rapid growth in metabolomics is leading to a renewed interest in metabolism and the role that small molecule metabolites play in many biological processes. As a result, traditional views of metabolites as being simply the "bricks and mortar" of cells or just the fuel for cellular energetics are being upended. Indeed, metabolites appear to have much more varied and far more important roles as signaling molecules, immune modulators, endogenous toxins, and environmental sensors. This review explores how metabolomics is yielding important new insights into a number of important biological and physiological processes. In particular, a major focus is on illustrating how metabolomics and discoveries made through metabolomics are improving our understanding of both normal physiology and the pathophysiology of many diseases. These discoveries are yielding new insights into how metabolites influence organ function, immune function, nutrient sensing, and gut physiology. Collectively, this work is leading to a much more unified and system-wide perspective of biology wherein metabolites, proteins, and genes are understood to interact synergistically to modify the actions and functions of organelles, organs, and organisms.