Monitoring Mitochondrial Pyruvate Carrier Activity in Real Time Using a BRET-Based Biosensor: Investigation of the Warburg Effect

Neuron-astrocyte metabolic coupling facilitates spinal plasticity and maintenance of inflammatory pain

Long-lasting pain stimuli can trigger maladaptive changes in the spinal cord, reminiscent of plasticity associated with memory formation. Metabolic coupling between astrocytes and neurons has been implicated in neuronal plasticity and memory formation in the central nervous system, but neither its involvement in pathological pain nor in spinal plasticity has been tested. Here we report a form of neuroglia signalling involving spinal astrocytic glycogen dynamics triggered by persistent noxious stimulation via upregulation of the Protein Targeting to Glycogen (PTG) in spinal astrocytes. PTG drove glycogen build-up in astrocytes, and blunting glycogen accumulation and turnover by Ptg gene deletion reduced pain-related behaviours and promoted faster recovery by shortening pain maintenance in mice. Furthermore, mechanistic analyses revealed that glycogen dynamics is a critically required process for maintenance of pain by facilitating neuronal plasticity in spinal lamina 1 neurons. In summary, our study describes a previously unappreciated mechanism of astrocyte-neuron metabolic communication through glycogen breakdown in the spinal cord that fuels spinal neuron hyperexcitability.

Loss of mitochondrial pyruvate carrier 1 supports proline-dependent proliferation and collagen biosynthesis in ovarian cancer

The pyruvate transporter MPC1 (mitochondrial pyruvate carrier 1) acts as a tumour-suppressor, loss of which correlates with a pro-tumorigenic phenotype and poor survival in several tumour types. In high-grade serous ovarian cancers (HGSOC), patients display copy number loss of MPC1 in around 78% of cases and reduced MPC1 mRNA expression. To explore the metabolic effect of reduced expression, we demonstrate that depleting MPC1 in HGSOC cell lines drives expression of key proline biosynthetic genes; PYCR1, PYCR2 and PYCR3, and biosynthesis of proline. We show that altered proline metabolism underpins cancer cell proliferation, reactive oxygen species (ROS) production, and type I and type VI collagen formation in ovarian cancer cells. Furthermore, exploring The Cancer Genome Atlas, we discovered the PYCR3 isozyme to be highly expressed in a third of HGSOC patients, which was associated with more aggressive disease and diagnosis at a younger age. Taken together, our study highlights that targeting proline metabolism is a potential therapeutic avenue for the treatment of HGSOC.

Tubular mitochondrial pyruvate carrier disruption elicits redox adaptations that protect from acute kidney injury

OBJECTIVE

Energy-intensive kidney reabsorption processes essential for normal whole-body function are maintained by tubular epithelial cell metabolism. Although tubular metabolism changes markedly following acute kidney injury (AKI), it remains unclear which metabolic alterations are beneficial or detrimental. By analyzing large-scale, publicly available datasets, we observed that AKI consistently leads to downregulation of the mitochondrial pyruvate carrier (MPC). This investigation aimed to understand the contribution of the tubular MPC to kidney function, metabolism, and acute injury severity.

METHODS

We generated tubular epithelial cell-specific Mpc1 knockout (MPC TubKO) mice and employed renal function tests, in vivo renal 13C-glucose tracing, mechanistic enzyme activity assays, and tests of injury and survival in an established rhabdomyolysis model of AKI.

RESULTS

MPC TubKO mice retained normal kidney function, displayed unchanged markers of kidney injury, but exhibited coordinately increased enzyme activities of the pentose phosphate pathway and the glutathione and thioredoxin oxidant defense systems. Following rhabdomyolysis-induced AKI, compared to WT control mice, MPC TubKO mice showed increased glycolysis, decreased kidney injury and oxidative stress markers, and strikingly increased survival.

CONCLUSIONS

Our findings suggest that decreased renal tubular mitochondrial pyruvate uptake hormetically upregulates oxidant defense systems before AKI and is a beneficial adaptive response after rhabdomyolysis-induced AKI. This raises the possibility of therapeutically modulating the MPC to attenuate AKI severity.

Serum Metabolomic Profiling of Patients with Lipedema

Lipedema is a chronic condition characterized by disproportionate and symmetrical enlargement of adipose tissue, predominantly affecting the lower limbs of women. This study investigated the use of metabolomics in lipedema research, with the objective of identifying complex metabolic disturbances and potential biomarkers for early detection, prognosis, and treatment strategies. The study group (n = 25) comprised women diagnosed with lipedema. The controls were 25 lean women and 25 obese females, both matched for age. In the patients with lipedema, there were notable changes in the metabolite parameters. Specifically, lower levels of histidine and phenylalanine were observed, whereas pyruvic acid was elevated compared with the weight controls. The receiver operating characteristic (ROC) curves for the diagnostic accuracy of histidine, phenylalanine, and pyruvic acid concentrations in distinguishing between patients with lipedema and those with obesity but without lipedema revealed good diagnostic ability for all parameters, with pyruvic acid being the most promising (area under the curve (AUC): 0.9992). Subgroup analysis within matched body mass index (BMI) ranges (30.0 to 39.9 kg/m2) further revealed that differences in pyruvic acid, phenylalanine, and histidine levels are likely linked to lipedema pathology rather than BMI variations. Changes in low-density lipoprotein (LDL)-6 TG levels and significant reductions in various LDL-2-carried lipids of patients with lipedema, compared with the lean controls, were observed. However, these lipids were similar between the lipedema patients and the obese controls, suggesting that these alterations are related to adiposity. Metabolomics is a valuable tool for investigating lipedema, offering a comprehensive view of metabolic changes and insights into lipedema's underlying mechanisms.

Importance of glucose and its metabolism in neurodegenerative disorder, as well as the combination of multiple therapeutic strategies targeting α-synuclein and neuroprotection in the treatment of Parkinson's disease

According to recent findings, Phosphoglycerate Kinase 1 (pgk-1) enzyme is linked to Parkinson's disease (PD). Mutations in the PGK-1 gene lead to decreases in the pgk-1 enzyme which causes an imbalance in the levels of energy demand and supply. An increase in glycolytic adenosine triphosphate (ATP) production would help alleviate energy deficiency and sustain the acute energetic need of neurons. Neurodegeneration is caused by an imbalance or reduction in ATP levels. Recent data suggest that medications that increase glycolysis and neuroprotection can be used to treat PD. The current study focuses on treatment options for disorders associated with the pgk-1 enzyme, GLP-1, and A2A receptor which can be utilized to treat PD. A combination of metformin and terazosin, exenatide and meclizine, istradefylline and salbutamol treatments may benefit parkinsonism. The review also looked at potential target-specific new techniques that might assist in satisfying unfulfilled requirements in the treatment of PD.

Fifty years of the mitochondrial pyruvate carrier: New insights into its structure, function, and inhibition

The mitochondrial pyruvate carrier (MPC) resides in the mitochondrial inner membrane, where it links cytosolic and mitochondrial metabolism by transporting pyruvate produced in glycolysis into the mitochondrial matrix. Due to its central metabolic role, it has been proposed as a potential drug target for diabetes, non-alcoholic fatty liver disease, neurodegeneration, and cancers relying on mitochondrial metabolism. Little is known about the structure and mechanism of MPC, as the proteins involved were only identified a decade ago and technical difficulties concerning their purification and stability have hindered progress in functional and structural analyses. The functional unit of MPC is a hetero-dimer comprising two small homologous membrane proteins, MPC1/MPC2 in humans, with the alternative complex MPC1L/MPC2 forming in the testis, but MPC proteins are found throughout the tree of life. The predicted topology of each protomer consists of an amphipathic helix followed by three transmembrane helices. An increasing number of inhibitors are being identified, expanding MPC pharmacology and providing insights into the inhibitory mechanism. Here, we provide critical insights on the composition, structure, and function of the complex and we summarize the different classes of small molecule inhibitors and their potential in therapeutics.

Metabolic Oscillations and Glycolytic Phenotypes of Cancer Cells

Cancer cells show several metabolic phenotypes depending on the cancer types and the microenvironments in tumor tissues. The glycolytic phenotype is one of the hallmarks of cancer cells and is considered to be one of the crucial features of malignant cancers. Here, we show glycolytic oscillations in the concentrations of metabolites in the glycolytic pathway in two types of cancer cells, HeLa cervical cancer cells and DU145 prostate cancer cells, and in two types of cellular morphologies, spheroids and monolayers. Autofluorescence from nicotinamide adenine dinucleotide (NADH) in cells was used for monitoring the glycolytic oscillations at the single-cell level. The frequencies of NADH oscillations were different among the cellular types and morphologies, indicating that more glycolytic cancer cells tended to exhibit oscillations with higher frequencies than less glycolytic cells. A mathematical model for glycolytic oscillations in cancer cells reproduced the experimental results quantitatively, confirming that the higher frequencies of oscillations were due to the higher activities of glycolytic enzymes. Thus, glycolytic oscillations are expected as a medical indicator to evaluate the malignancy of cancer cells with glycolytic phenotypes.

NMR and Patch-Clamp Characterization of Yeast Mitochondrial Pyruvate Carrier Complexes

The mitochondrial pyruvate carrier (Mpc) plays an indispensable role in the transport of pyruvates across the mitochondrial inner membrane. Despite the two distinct homologous proteins, Mpc1 and Mpc2, were identified in 2012, there are still controversies on the basic functional units and oligomeric state of Mpc complexes. In this study, yeast Mpc1 and Mpc2 proteins were expressed in a prokaryotic heterologous system. Both homo- and hetero-dimers were successfully reconstituted in mixed detergents. Interactions among Mpc monomers were recorded utilizing paramagnetic relaxation enhancement (PRE) nuclear magnetic resonance (NMR) methods. By single-channel patch-clamp assays, we discovered that both the Mpc1-Mpc2 hetero-dimer and Mpc1 homo-dimer are able to transport K⁺ ions. Furthermore, the Mpc1-Mpc2 hetero-dimer demonstrated the ability to transport pyruvates, at a rate significantly higher than that of the Mpc1 homo-dimer, indicating that it could be the basic functional unit of Mpc complexes. Our findings provide valuable insights for further structural determination and the study of the transport mechanism of Mpc complexes.

The Hepatic Mitochondrial Pyruvate Carrier as a Regulator of Systemic Metabolism and a Therapeutic Target for Treating Metabolic Disease

Pyruvate sits at an important metabolic crossroads of intermediary metabolism. As a product of glycolysis in the cytosol, it must be transported into the mitochondrial matrix for the energy stored in this nutrient to be fully harnessed to generate ATP or to become the building block of new biomolecules. Given the requirement for mitochondrial import, it is not surprising that the mitochondrial pyruvate carrier (MPC) has emerged as a target for therapeutic intervention in a variety of diseases characterized by altered mitochondrial and intermediary metabolism. In this review, we focus on the role of the MPC and related metabolic pathways in the liver in regulating hepatic and systemic energy metabolism and summarize the current state of targeting this pathway to treat diseases of the liver. Available evidence suggests that inhibiting the MPC in hepatocytes and other cells of the liver produces a variety of beneficial effects for treating type 2 diabetes and nonalcoholic steatohepatitis. We also highlight areas where our understanding is incomplete regarding the pleiotropic effects of MPC inhibition.

Tubular Mitochondrial Pyruvate Carrier Disruption Elicits Redox Adaptations that Protect from Acute Kidney Injury

Energy-intensive kidney reabsorption processes essential for normal whole-body function are maintained by tubular epithelial cell metabolism. Tubular metabolism changes markedly following acute kidney injury (AKI), but which changes are adaptive versus maladaptive remain poorly understood. In publicly available data sets, we noticed a consistent downregulation of the mitochondrial pyruvate carrier (MPC) after AKI, which we experimentally confirmed. To test the functional consequences of MPC downregulation, we generated novel tubular epithelial cell-specific Mpc1 knockout (MPC TubKO) mice. 13C-glucose tracing, steady-state metabolomic profiling, and enzymatic activity assays revealed that MPC TubKO coordinately increased activities of the pentose phosphate pathway and the glutathione and thioredoxin oxidant defense systems. Following rhabdomyolysis-induced AKI, MPC TubKO decreased markers of kidney injury and oxidative damage and strikingly increased survival. Our findings suggest that decreased mitochondrial pyruvate uptake is a central adaptive response following AKI and raise the possibility of therapeutically modulating the MPC to attenuate AKI severity.

The mitochondrial pyruvate carrier at the crossroads of intermediary metabolism

Pyruvate metabolism, a central nexus of carbon homeostasis, is an evolutionarily conserved process and aberrant pyruvate metabolism is associated with and contributes to numerous human metabolic disorders including diabetes, cancer, and heart disease. As a product of glycolysis, pyruvate is primarily generated in the cytosol before being transported into the mitochondrion for further metabolism. Pyruvate entry into the mitochondrial matrix is a critical step for efficient generation of reducing equivalents and ATP and for the biosynthesis of glucose, fatty acids, and amino acids from pyruvate. However, for many years, the identity of the carrier protein(s) that transported pyruvate into the mitochondrial matrix remained a mystery. In 2012, the molecular-genetic identification of the mitochondrial pyruvate carrier (MPC), a heterodimeric complex composed of protein subunits MPC1 and MPC2, enabled studies that shed light on the many metabolic and physiological processes regulated by pyruvate metabolism. A better understanding of the mechanisms regulating pyruvate transport and the processes affected by pyruvate metabolism may enable novel therapeutics to modulate mitochondrial pyruvate flux to treat a variety of disorders. Herein, we review our current knowledge of the MPC, discuss recent advances in the understanding of mitochondrial pyruvate metabolism in various tissue and cell types, and address some of the outstanding questions relevant to this field.

Dynamic Reconfiguration of Subcompartment Architectures in Artificial Cells

Artificial cells are minimal structures constructed from biomolecular building blocks designed to mimic cellular processes, behaviors, and architectures. One near-ubiquitous feature of cellular life is the spatial organization of internal content. We know from biology that organization of content (including in membrane-bound organelles) is linked to cellular functions and that this feature is dynamic: the presence, location, and degree of compartmentalization changes over time. Vesicle-based artificial cells, however, are not currently able to mimic this fundamental cellular property. Here, we describe an artificial cell design strategy that addresses this technological bottleneck. We create a series of artificial cell architectures which possess multicompartment assemblies localized either on the inner or on the outer surface of the artificial cell membrane. Exploiting liquid-liquid phase separation, we can also engineer spatially segregated regions of condensed subcompartments attached to the cell surface, aligning with coexisting membrane domains. These structures can sense changes in environmental conditions and respond by reversibly transitioning from condensed multicompartment layers on the membrane surface to a dispersed state in the cell lumen, mimicking the dynamic compartmentalization found in biological cells. Likewise, we engineer exosome-like subcompartments that can be released to the environment. We can achieve this by using two types of triggers: chemical (addition of salts) and mechanical (by pulling membrane tethers using optical traps). These approaches allow us to control the compartmentalization state of artificial cells on population and single-cell levels.

Key features of inhibitor binding to the human mitochondrial pyruvate carrier hetero-dimer

OBJECTIVE

The mitochondrial pyruvate carrier (MPC) has emerged as a promising drug target for metabolic disorders, including non-alcoholic steatohepatitis and diabetes, metabolically dependent cancers and neurodegenerative diseases. A range of structurally diverse small molecule inhibitors have been proposed, but the nature of their interaction with MPC is not understood, and the composition of the functional human MPC is still debated. The goal of this study was to characterise the human MPC protein in vitro, to understand the chemical features that determine binding of structurally diverse inhibitors and to develop novel higher affinity ones.

METHODS

We recombinantly expressed and purified human MPC hetero-complexes and studied their composition, transport and inhibitor binding properties by establishing in vitro transport assays, high throughput thermostability shift assays and pharmacophore modeling.

RESULTS

We determined that the functional unit of human MPC is a hetero-dimer. We compared all different classes of MPC inhibitors to find that three closely arranged hydrogen bond acceptors followed by an aromatic ring are shared characteristics of all inhibitors and represent the minimal requirement for high potency. We also demonstrated that high affinity binding is not attributed to covalent bond formation with MPC cysteines, as previously proposed. Following the basic pharmacophore properties, we identified 14 new inhibitors of MPC, one outperforming compound UK5099 by tenfold. Two are the commonly prescribed drugs entacapone and nitrofurantoin, suggesting an off-target mechanism associated with their adverse effects.

CONCLUSIONS

This work defines the composition of human MPC and the essential MPC inhibitor characteristics. In combination with the functional assays we describe, this new understanding will accelerate the development of clinically relevant MPC modulators.

Molecular sensors for detection of tumor-stroma crosstalk

In most solid tumors, malignant cells coexist with non-cancerous host tissue comprised of a variety of extracellular matrix components and cell types, notably fibroblasts, immune cells, and endothelial cells. It is becoming increasingly evident that the non-cancerous host tissue, often referred to as the tumor stroma or the tumor microenvironment, wields tremendous influence in the proliferation, survival, and metastatic ability of cancer cells. The tumor stroma has an active biological role in the transmission of signals, such as growth factors and chemokines that activate oncogenic signaling pathways by autocrine and paracrine mechanisms. Moreover, the constituents of the stroma define the mechanical properties and the physical features of solid tumors, which influence cancer progression and response to therapy. Inspired by the emerging importance of tumor-stroma crosstalk and oncogenic physical forces, numerous biosensors, or advanced imaging and analysis techniques have been developed and applied to investigate complex and challenging questions in cancer research. These techniques facilitate measurements and biological readouts at scales ranging from subcellular to tissue-level with unprecedented level of spatial and temporal precision. Here we examine the application of biosensor technology for studying the complex and dynamic multiscale interactions of the tumor-host system.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.

Oscillations and Dynamic Symbiosis in Cellular Metabolism in Cancer

The grade of malignancy differs among cancer cell types, yet it remains the burden of genetic studies to understand the reasons behind this observation. Metabolic studies of cancer, based on the Warburg effect or aerobic glycolysis, have also not provided any clarity. Instead, the significance of oxidative phosphorylation (OXPHOS) has been found to play critical roles in aggressive cancer cells. In this perspective, metabolic symbiosis is addressed as one of the ultimate causes of the grade of cancer malignancy. Metabolic symbiosis gives rise to metabolic heterogeneities which enable cancer cells to acquire greater opportunities for proliferation and metastasis in tumor microenvironments. This study introduces a real-time new imaging technique to visualize metabolic symbiosis between cancer-associated fibroblasts (CAFs) and cancer cells based on the metabolic oscillations in these cells. The causality of cellular oscillations in cancer cells and CAFs, connected through lactate transport, is a key point for the development of this novel technique.

Identification of Novel Mitochondrial Pyruvate Carrier Inhibitors by Homology Modeling and Pharmacophore-Based Virtual Screening

The mitochondrial pyruvate carrier (MPC) is an inner-mitochondrial membrane protein complex that has emerged as a drug target for treating a variety of human conditions. A heterodimer of two proteins, MPC1 and MPC2, comprises the functional MPC complex in higher organisms; however, the structure of this complex, including the critical residues that mediate binding of pyruvate and inhibitors, remain to be determined. Using homology modeling, we identified a putative substrate-binding cavity in the MPC dimer. Three amino acid residues (Phe66 (MPC1) and Asn100 and Lys49 (MPC2)) were validated by mutagenesis experiments to be important for substrate and inhibitor binding. Using this information, we developed a pharmacophore model and then performed a virtual screen of a chemical library. We identified five new non-indole MPC inhibitors, four with IC₅₀ values in the nanomolar range that were up to 7-fold more potent than the canonical inhibitor UK-5099. These novel compounds possess drug-like properties and complied with Lipinski's Rule of Five. They are predicted to have good aqueous solubility, oral bioavailability, and metabolic stability. Collectively, these studies provide important information about the structure-function relationships of the MPC complex and for future drug discovery efforts targeting the MPC.

Imaging Approaches for the Study of Metabolism in Real Time Using Genetically Encoded Reporters

Metabolism comprises of two axes in order to serve homeostasis: anabolism and catabolism. Both axes are interbranched with the so-called bioenergetics aspect of metabolism. There is a plethora of analytical biochemical methods to monitor metabolites and reactions in lysates, yet there is a rising need to monitor, quantify and elucidate in real time the spatiotemporal orchestration of complex biochemical reactions in living systems and furthermore to analyze the metabolic effect of chemical compounds that are destined for the clinic. The ongoing technological burst in the field of imaging creates opportunities to establish new tools that will allow investigators to monitor dynamics of biochemical reactions and kinetics of metabolites at a resolution that ranges from subcellular organelle to whole system for some key metabolites. This article provides a mini review of available toolkits to achieve this goal but also presents a perspective on the open space that can be exploited to develop novel methodologies that will merge classic biochemistry of metabolism with advanced imaging. In other words, a perspective of "watching metabolism in real time."

Andrographolide inhibits non-small cell lung cancer cell proliferation through the activation of the mitochondrial apoptosis pathway and by reprogramming host glucose metabolism

Background

The main aim of this research was to explore the role and mechanism of Andrographolide (Andro) in controlling non-small cell lung cancer (NSCLC) cell proliferation.

Methods

Human NSCLC H1975 cells were treated with Andro (0–20 µM) for 4–72 h. B-cell leukemia/lymphoma 2 (Bcl-2)-antagonist/killer (Bak)-small interfering RNA (siRNA) (Bak-siRNA) and fructose-1,6-bisphosphatase (FBP1)-siRNA were transfected into H1975 cells to inhibit the endogenic Bak and FBP1 expression, respectively, and their expressions were detected by real-time quantitative reverse transcription–polymerase chain reaction (qRT-PCR) and western blotting (WB). Cellular proliferation ability was determined through various assessments, including 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), colony formation, and cell counting kit-8 (CCK-8) assays. Cell apoptosis ability was measured using flow cytometry. Pro-apoptotic-related proteins (cleaved caspase 9, cleaved caspase 8, and cleaved caspase 3) and mitochondrial apoptosis pathway proteins [Bcl2-associated X (Bax), Bak, Bcl-2, and cytochrome C (cyto C)] were assessed by WB. Aerobic glycolysis-associated genes [pyruvate kinase M2 (PKM2), lactate dehydrogenase A (LDHA), and glucose transporter 1 (GLUT1)] and gluconeogenesis genes [phosphoenolpyruvate carboxykinase 1 (PEPCK1), fructose-1,6-bisphosphatase 1 (FBP1), and phosphofructokinase (PFK)] were measured by qRT-PCR. The mitochondrial membrane depolarization sensor, 5, 50, 6, 60-tetrachloro-1, 10, 3, 30 tetraethyl benzimidazolo carbocyanine iodide (JC-1) assay was used for the measurement of mitochondrial membrane potential (ΔΨm). Additionally, glycolytic metabolism, lactate production, and adenosine triphosphate (ATP) synthesis were also analyzed.

Results

Andro inhibited human NSCLC cellular proliferation and induced apoptosis in a dose-time or dose-dependent manner via activation of the mitochondrial apoptosis pathway. Andro inhibited glycolysis, promoted the gluconeogenesis pathway, and increased the levels of cleaved caspase 9, cleaved caspase 8, cleaved caspase 3, Bax, Bak, PEPCK1, FBP1, and PFK, and decreased the levels of Bcl-2, PKM2, LDHA, and GLUT1. Moreover, it also decreased the ΔΨm and facilitated the release of cyto C from mitochondria into the cytoplasm. Furthermore, Andro enhanced the mitochondrial translocation of Bak, glucose uptake, lactate release, and intracellular ATP synthesis. Suppression of endogenic Bak and FBP1 expression significantly reduced the effects of Andro in H1975 cells.

Conclusions

Andro represses NSCLC cell proliferation through the activation of the mitochondrial apoptosis pathway and by reprogramming glucose metabolism.

Mitochondrial pyruvate carrier inhibitors improve metabolic parameters in diet-induced obese mice

The mitochondrial pyruvate carrier (MPC) is an inner mitochondrial membrane complex that plays a critical role in intermediary metabolism. Inhibition of the MPC, especially in liver, may have efficacy for treating type 2 diabetes mellitus. Herein, we examined the antidiabetic effects of zaprinast and 7ACC2, small molecules which have been reported to act as MPC inhibitors. Both compounds activated a bioluminescence resonance energy transfer–based MPC reporter assay (reporter sensitive to pyruvate) and potently inhibited pyruvate-mediated respiration in isolated mitochondria. Furthermore, zaprinast and 7ACC2 acutely improved glucose tolerance in diet-induced obese mice in vivo. Although some findings were suggestive of improved insulin sensitivity, hyperinsulinemic–euglycemic clamp studies did not detect enhanced insulin action in response to 7ACC2 treatment. Rather, our data suggest acute glucose-lowering effects of MPC inhibition may be due to suppressed hepatic gluconeogenesis. Finally, we used reporter sensitive to pyruvate to screen a chemical library of drugs and identified 35 potentially novel MPC modulators. Using available evidence, we generated a pharmacophore model to prioritize which hits to pursue. Our analysis revealed carsalam and six quinolone antibiotics, as well as 7ACC1, share a common pharmacophore with 7ACC2. We validated that these compounds are novel inhibitors of the MPC and suppress hepatocyte glucose production and demonstrated that one quinolone (nalidixic acid) improved glucose tolerance in obese mice. In conclusion, these data demonstrate the feasibility of therapeutic targeting of the MPC for treating diabetes and provide scaffolds that can be used to develop potent and novel classes of MPC inhibitors.

Genetically Encoded Fluorescent Biosensors for Biomedical Applications

One of the challenges of modern biology and medicine is to visualize biomolecules in their natural environment, in real-time and in a non-invasive fashion, so as to gain insight into their physiological behavior and highlight alterations in pathological settings, which will enable to devise appropriate therapeutic strategies. Genetically encoded fluorescent biosensors constitute a class of imaging agents that enable visualization of biological processes and events directly in situ, preserving the native biological context and providing detailed insight into their localization and dynamics in cells. Real-time monitoring of drug action in a specific cellular compartment, organ, or tissue type; the ability to screen at the single-cell resolution; and the elimination of false-positive results caused by low drug bioavailability that is not detected by in vitro testing methods are a few of the obvious benefits of using genetically encoded fluorescent biosensors in drug screening. This review summarizes results of the studies that have been conducted in the last years toward the fabrication of genetically encoded fluorescent biosensors for biomedical applications with a comprehensive discussion on the challenges, future trends, and potential inputs needed for improving them.

Mitochondrial pyruvate carrier 1: a novel prognostic biomarker that predicts favourable patient survival in cancer

Mitochondrial pyruvate carrier 1 (MPC1) is a key metabolic protein that regulates the transport of pyruvate into the mitochondrial inner membrane. MPC1 deficiency may cause metabolic reprogramming. However, whether and how MPC1 controls mitochondrial oxidative capacity in cancer are still relatively unknown. MPC1 deficiency was recently found to be strongly associated with various diseases and cancer hallmarks. We utilized online databases and uncovered that MPC1 expression is lower in many cancer tissues than in adjacent normal tissues. In addition, MPC1 expression was found to be substantially altered in five cancer types: breast-invasive carcinoma (BRCA), kidney renal clear cell carcinoma (KIRC), lung adenocarcinoma (LUAD), pancreatic adenocarcinoma (PAAD), and prostate adenocarcinoma (PRAD). However, in KIRC, LUAD, PAAD, and PRAD, high MPC1 expression is closely associated with favourable prognosis. Low MPC1 expression in BRCA is significantly associated with shorter overall survival time. MPC1 expression shows strong positive and negative correlations with immune cell infiltration in thymoma (THYM) and thyroid carcinoma (THCA). Furthermore, we have comprehensively summarized the current literature regarding the metabolic reprogramming effects of MPC1 in various cancers. As shown in the literature, MPC1 expression is significantly decreased in cancer tissue and associated with poor prognosis. We discuss the potential metabolism-altering effects of MPC1 in cancer, including decreased pyruvate transport ability; impaired pyruvate-driven oxidative phosphorylation (OXPHOS); and increased lactate production, glucose consumption, and glycolytic capacity, and the underlying mechanisms. These activities facilitate tumour progression, migration, and invasion. MPC1 is a novel cancer biomarker and potentially powerful therapeutic target for cancer diagnosis and treatment. Further studies aimed at slowing cancer progression are in progress.

Heart failure-emerging roles for the mitochondrial pyruvate carrier

The mitochondrial pyruvate carrier (MPC) is the entry point for the glycolytic end-product pyruvate to the mitochondria. MPC activity, which is controlled by its abundance and post-translational regulation, determines whether pyruvate is oxidised in the mitochondria or metabolised in the cytosol. MPC serves as a crucial metabolic branch point that determines the fate of pyruvate in the cell, enabling metabolic adaptations during health, such as exercise, or as a result of disease. Decreased MPC expression in several cancers limits the mitochondrial oxidation of pyruvate and contributes to lactate accumulation in the cytosol, highlighting its role as a contributing, causal mediator of the Warburg effect. Pyruvate is handled similarly in the failing heart where a large proportion of it is reduced to lactate in the cytosol instead of being fully oxidised in the mitochondria. Several recent studies have found that the MPC abundance was also reduced in failing human and mouse hearts that were characterised by maladaptive hypertrophic growth, emulating the anabolic scenario observed in some cancer cells. In this review we discuss the evidence implicating the MPC as an important, perhaps causal, mediator of heart failure progression.

The Metabolic Fates of Pyruvate in Normal and Neoplastic Cells

Pyruvate occupies a central metabolic node by virtue of its position at the crossroads of glycolysis and the tricarboxylic acid (TCA) cycle and its production and fate being governed by numerous cell-intrinsic and extrinsic factors. The former includes the cell’s type, redox state, ATP content, metabolic requirements and the activities of other metabolic pathways. The latter include the extracellular oxygen concentration, pH and nutrient levels, which are in turn governed by the vascular supply. Within this context, we discuss the six pathways that influence pyruvate content and utilization: 1. The lactate dehydrogenase pathway that either converts excess pyruvate to lactate or that regenerates pyruvate from lactate for use as a fuel or biosynthetic substrate; 2. The alanine pathway that generates alanine and other amino acids; 3. The pyruvate dehydrogenase complex pathway that provides acetyl-CoA, the TCA cycle’s initial substrate; 4. The pyruvate carboxylase reaction that anaplerotically supplies oxaloacetate; 5. The malic enzyme pathway that also links glycolysis and the TCA cycle and generates NADPH to support lipid bio-synthesis; and 6. The acetate bio-synthetic pathway that converts pyruvate directly to acetate. The review discusses the mechanisms controlling these pathways, how they cross-talk and how they cooperate and are regulated to maximize growth and achieve metabolic and energetic harmony.

The Importance of Mitochondrial Pyruvate Carrier in Cancer Cell Metabolism and Tumorigenesis

Simple Summary

The characteristic metabolic hallmark of cancer cells is the massive catabolism of glucose by glycolysis, even under aerobic conditions—the so-called Warburg effect. Although energetically unfavorable, glycolysis provides “building blocks” to sustain the unlimited growth of malignant cells. Aberrant glycolysis is also responsible for lactate accumulation and acidosis in the tumor milieu, which fosters hypoxia and immunosuppression. One of the mechanisms used by cancer cells to increase glycolytic flow is the negative regulation of the proteins that conform the mitochondrial pyruvate carrier (MPC) complex, which transports pyruvate into the mitochondrial matrix to be metabolized in the tricarboxylic acid (TCA) cycle. Evidence suggests that MPC downregulation in tumor cells impacts many aspects of tumorigenesis, including cancer cell-intrinsic (proliferation, invasiveness, stemness, resistance to therapy) and -extrinsic (angiogenesis, anti-tumor immune activity) properties. In many cancers, but not in all, MPC downregulation is associated with poor survival. MPC regulation is therefore central to tackling glycolysis in tumors.

Abstract

Pyruvate is a key molecule in the metabolic fate of mammalian cells; it is the crossroads from where metabolism proceeds either oxidatively or ends with the production of lactic acid. Pyruvate metabolism is regulated by many enzymes that together control carbon flux. Mitochondrial pyruvate carrier (MPC) is responsible for importing pyruvate from the cytosol to the mitochondrial matrix, where it is oxidatively phosphorylated to produce adenosine triphosphate (ATP) and to generate intermediates used in multiple biosynthetic pathways. MPC activity has an important role in glucose homeostasis, and its alteration is associated with diabetes, heart failure, and neurodegeneration. In cancer, however, controversy surrounds MPC function. In some cancers, MPC upregulation appears to be associated with a poor prognosis. However, most transformed cells undergo a switch from oxidative to glycolytic metabolism, the so-called Warburg effect, which, amongst other possibilities, is induced by MPC malfunction or downregulation. Consequently, impaired MPC function might induce tumors with strong proliferative, migratory, and invasive capabilities. Moreover, glycolytic cancer cells secrete lactate, acidifying the microenvironment, which in turn induces angiogenesis, immunosuppression, and the expansion of stromal cell populations supporting tumor growth. This review examines the latest findings regarding the tumorigenic processes affected by MPC.

Defective Mitochondrial Pyruvate Flux Affects Cell Bioenergetics in Alzheimer's Disease-Related Models

Mitochondria are key organelles for brain health. Mitochondrial alterations have been reported in several neurodegenerative disorders, including Alzheimer's disease (AD), and the comprehension of the underlying mechanisms appears crucial to understand their relationship with the pathology. Using multiple genetic, pharmacological, imaging, and biochemical approaches, we demonstrate that, in different familial AD cell models, mitochondrial ATP synthesis is affected. The defect depends on reduced mitochondrial pyruvate oxidation, due to both lower Ca²⁺-mediated stimulation of the Krebs cycle and dampened mitochondrial pyruvate uptake. Importantly, this latter event is linked to glycogen-synthase-kinase-3β (GSK-3β) hyper-activation, leading, in turn, to impaired recruitment of hexokinase 1 (HK1) to mitochondria, destabilization of mitochondrial-pyruvate-carrier (MPC) complexes, and decreased MPC2 protein levels. Remarkably, pharmacological GSK-3β inhibition in AD cells rescues MPC2 expression and improves mitochondrial ATP synthesis and respiration. The defective mitochondrial bioenergetics influences glutamate-induced neuronal excitotoxicity, thus representing a possible target for future therapeutic interventions.

TRIB3 Promotes Lung Adenocarcinoma Progression via an Enhanced Warburg Effect

Background

The pseudokinase Tribbles 3 (TRIB3) is involved in many cellular processes and various cancers. In recent years, the importance of metabolic transformation in the maintenance of malignant tumors has become increasingly prominent. Abnormal metabolism of cancer cells is considered a hallmark of cancer. However, the exact role and molecular mechanism of TRIB3 in lung adenocarcinoma (LUAD) cell reprogramming is largely unknown.

Methods

The oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) of cells were examined with a Seahorse XF Extracellular Flux Analyzer. In vitro and in vivo RT-qPCR, Western blotting, and functional assays were performed to explore the functional roles of TRIB3 in LUAD.

Results

In the present study, we demonstrated that TRIB3 is remarkably upregulated in LUAD cell lines as well as tissues. TRIB3 knockdown significantly inhibited LUAD cell growth and suppressed LUAD cell invasion, while TRIB3 overexpression conferred the opposite effects. Moreover, silencing TRIB3 suppressed the tumorigenesis and metastatic ability of LUAD cells. Mechanistically, we demonstrated that silencing TRIB3 significantly impaired aerobic glycolysis ability in LUAD cells. Furthermore, our data indicated that TRIB3 knockdown decreased hypoxia-inducible factor (HIF)1α levels and targeted the glycolytic genes regulated by HIF1α.

Conclusion

Together, our findings revealed a previously unappreciated function of TRIB3 in cancer cell metabolism and tumor progression, illustrating that TRIB3 could be considered a valuable therapeutic target for LUAD patients.

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.

Hyperpolarized [1-13C]pyruvate-to-[1-13C]lactate conversion is rate-limited by monocarboxylate transporter-1 in the plasma membrane

Hyperpolarized [1-¹³C]pyruvate magnetic resonance spectroscopic imaging (MRSI) is a noninvasive metabolic-imaging modality that probes carbon flux in tissues and infers the state of metabolic reprograming in tumors. Prevailing models attribute elevated hyperpolarized [1-¹³C]pyruvate-to-[1-¹³C]lactate conversion rates in aggressive tumors to enhanced glycolytic flux and lactate dehydrogenase A (LDHA) activity (Warburg effect). By contrast, we find by cross-sectional analysis using genetic and pharmacological tools in mechanistic studies applied to well-defined genetically engineered cell lines and tumors that initial hyperpolarized [1-¹³C]pyruvate-to-[1-¹³C]lactate conversion rates as well as global conversion were highly dependent on and critically rate-limited by the transmembrane influx of [1-¹³C]pyruvate mediated predominately by monocarboxylate transporter-1 (MCT1). Specifically, in a cell-encapsulated alginate bead model, induced short hairpin (shRNA) knockdown or overexpression of MCT1 quantitatively inhibited or enhanced, respectively, unidirectional pyruvate influxes and [1-¹³C]pyruvate-to-[1-¹³C]lactate conversion rates, independent of glycolysis or LDHA activity. Similarly, in tumor models in vivo, hyperpolarized [1-¹³C]pyruvate-to-[1-¹³C]lactate conversion was highly dependent on and critically rate-limited by the induced transmembrane influx of [1-¹³C]pyruvate mediated by MCT1. Thus, hyperpolarized [1-¹³C]pyruvate MRSI measures primarily MCT1-mediated [1-¹³C]pyruvate transmembrane influx in vivo, not glycolytic flux or LDHA activity, driving a reinterpretation of this maturing new technology during clinical translation. Indeed, Kaplan-Meier survival analysis for patients with pancreatic, renal, lung, and cervical cancers showed that high-level expression of MCT1 correlated with poor overall survival, and only in selected tumors, coincident with LDHA expression. Thus, hyperpolarized [1-¹³C]pyruvate MRSI provides a noninvasive functional assessment primarily of MCT1 as a clinical biomarker in relevant patient populations.

Polydatin alleviates parkinsonism in MPTP-model mice by enhancing glycolysis in dopaminergic neurons

Parkinson's disease (PD) is a common neurodegenerative disease. Damage to energy metabolism and reduced adenosine triphosphate (ATP) levels in dopaminergic neurons are common features of PD. Previous studies suggested that the occurrence of PD often affects glucose metabolism and ATP production in the brain, and increased glycolysis or ATP production protects dopaminergic neuronal degeneration in the brain of PD patients. These systems may provide new potential therapeutic targets for the prevention of PD. The present study investigated the inhibitory action of polydatin (PLD) on early dopaminergic neuronal degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The results showed that PLD protected against MPTP-induced early dopaminergic neuronal degeneration. PLD reduced the MPTP-induced loss of dopaminergic neurons in substantia nigra and striatum, inhibited the occurrence of neural apoptosis, and restored motor function in mice. PLD also increased the continuous activity duration and rhythm amplitude in mice during the circadian activity test. PLD improved glucose metabolism in the brain and restored ATP production levels. These observations suggest that PLD attenuates MPTP-induced early PD-like symptoms, and its mechanism of action may be associated with the promotion of glucose metabolism in neurons.

On the Detection and Functional Significance of the Protein-Protein Interactions of Mitochondrial Transport Proteins

Protein-protein assemblies are highly prevalent in all living cells. Considerable evidence has recently accumulated suggesting that particularly transient association/dissociation of proteins represent an important means of regulation of metabolism. This is true not only in the cytosol and organelle matrices, but also at membrane surfaces where, for example, receptor complexes, as well as those of key metabolic pathways, are common. Transporters also frequently come up in lists of interacting proteins, for example, binding proteins that catalyze the production of their substrates or that act as relays within signal transduction cascades. In this review, we provide an update of technologies that are used in the study of such interactions with mitochondrial transport proteins, highlighting the difficulties that arise in their use for membrane proteins and discussing our current understanding of the biological function of such interactions.

The Multifaceted Pyruvate Metabolism: Role of the Mitochondrial Pyruvate Carrier

Pyruvate, the end product of glycolysis, plays a major role in cell metabolism. Produced in the cytosol, it is oxidized in the mitochondria where it fuels the citric acid cycle and boosts oxidative phosphorylation. Its sole entry point into mitochondria is through the recently identified mitochondrial pyruvate carrier (MPC). In this review, we report the latest findings on the physiology of the MPC and we discuss how a dysfunctional MPC can lead to diverse pathologies, including neurodegenerative diseases, metabolic disorders, and cancer.

Lethal Poisoning of Cancer Cells by Respiratory Chain Inhibition plus Dimethyl α-Ketoglutarate

Inhibition of oxidative phosphorylation (OXPHOS) by 1-cyclopropyl-4-(4-[(5-methyl-3-(3-[4-(trifluoromethoxy)phenyl]-1,2,4-oxadiazol-5-yl)-1H-pyrazol-1-yl)methyl]pyridin-2-yl)piperazine (BAY87-2243, abbreviated as B87), a complex I inhibitor, fails to kill human cancer cells in vitro. Driven by this consideration, we attempted to identify agents that engage in synthetically lethal interactions with B87. Here, we report that dimethyl α-ketoglutarate (DMKG), a cell-permeable precursor of α-ketoglutarate that lacks toxicity on its own, kills cancer cells when combined with B87 or other inhibitors of OXPHOS. DMKG improved the antineoplastic effect of B87, both in vitro and in vivo. This combination caused MDM2-dependent, tumor suppressor protein p53 (TP53)-independent transcriptional reprogramming and alternative exon usage affecting multiple glycolytic enzymes, completely blocking glycolysis. Simultaneous inhibition of OXPHOS and glycolysis provoked a bioenergetic catastrophe culminating in the activation of a cell death program that involved disruption of the mitochondrial network and activation of PARP1, AIFM1, and APEX1. These results unveil a metabolic liability of human cancer cells that may be harnessed for the development of therapeutic regimens.

Enhancing glycolysis attenuates Parkinson's disease progression in models and clinical databases

Parkinson's disease (PD) is a common neurodegenerative disease that lacks therapies to prevent progressive neurodegeneration. Impaired energy metabolism and reduced ATP levels are common features of PD. Previous studies revealed that terazosin (TZ) enhances the activity of phosphoglycerate kinase 1 (PGK1), thereby stimulating glycolysis and increasing cellular ATP levels. Therefore, we asked whether enhancement of PGK1 activity would change the course of PD. In toxin-induced and genetic PD models in mice, rats, flies, and induced pluripotent stem cells, TZ increased brain ATP levels and slowed or prevented neuron loss. The drug increased dopamine levels and partially restored motor function. Because TZ is prescribed clinically, we also interrogated 2 distinct human databases. We found slower disease progression, decreased PD-related complications, and a reduced frequency of PD diagnoses in individuals taking TZ and related drugs. These findings suggest that enhancing PGK1 activity and increasing glycolysis may slow neurodegeneration in PD.

A highly responsive pyruvate sensor reveals pathway-regulatory role of the mitochondrial pyruvate carrier MPC

Mitochondria generate ATP and building blocks for cell growth and regeneration, using pyruvate as the main substrate. Here we introduce PyronicSF, a user-friendly GFP-based sensor of improved dynamic range that enables real-time subcellular quantitation of mitochondrial pyruvate transport, concentration and flux. We report that cultured mouse astrocytes maintain mitochondrial pyruvate in the low micromolar range, below cytosolic pyruvate, which means that the mitochondrial pyruvate carrier MPC is poised to exert ultrasensitive control on the balance between respiration and anaplerosis/gluconeogenesis. The functionality of the sensor in living tissue is demonstrated in the brain of Drosophila melanogaster larvae. Mitochondrial subpopulations are known to coexist within a given cell, which differ in their morphology, mobility, membrane potential, and vicinity to other organelles. The present tool can be used to investigate how mitochondrial diversity relates to metabolism, to study the role of MPC in disease, and to screen for small-molecule MPC modulators.

A Chemical Proteomic Probe for the Mitochondrial Pyruvate Carrier Complex

Target engagement assays are crucial for establishing the mechanism-of-action of small molecules in living systems. Integral membrane transporters can present a challenging protein class for assessing cellular engagement by small molecules. The chemical proteomic discovery of alpha-chloroacetamide (αCA) compounds that covalently modify cysteine-54 (C54) of the MPC2 subunit of the mitochondrial pyruvate carrier (MPC) is presented. This finding is used to create an alkyne-modified αCA, YY4-yne, that serves as a cellular engagement probe for MPC2 in click chemistry-enabled western blotting or global mass spectrometry-based proteomic experiments. Studies with YY4-yne revealed that UK-5099, an alpha-cyanocinnamate inhibitor of the MPC complex, engages MPC2 with remarkable selectivity in human cells. These findings support a model where UK-5099 inhibits the MPC complex by binding to C54 of MPC2 in a covalent reversible manner that can be quantified in cells using the YY4-yne probe.

Long Noncoding RNA MALAT1 Regulates Cancer Glucose Metabolism by Enhancing mTOR-Mediated Translation of TCF7L2

Reprogrammed glucose metabolism of enhanced aerobic glycolysis (or the Warburg effect) is known as a hallmark of cancer. The roles of long noncoding RNAs (lncRNA) in regulating cancer metabolism at the level of both glycolysis and gluconeogenesis are mostly unknown. We previously showed that lncRNA metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) acts as a proto-oncogene in hepatocellular carcinoma (HCC). Here, we investigated the role of MALAT1 in regulating cancer glucose metabolism. MALAT1 upregulated the expression of glycolytic genes and downregulated gluconeogenic enzymes by enhancing the translation of the metabolic transcription factor TCF7L2. MALAT1-enhanced TCF7L2 translation was mediated by upregulation of SRSF1 and activation of the mTORC1-4EBP1 axis. Pharmacological or genetic inhibition of mTOR and Raptor or expression of a hypophosphorylated mutant version of eIF4E-binding protein (4EBP1) resulted in decreased expression of TCF7L2. MALAT1 expression regulated TCF7L2 mRNA association with heavy polysomes, probably through the TCF7L2 5'-untranslated region (UTR), as determined by polysome fractionation and 5'UTR-reporter assays. Knockdown of TCF7L2 in MALAT1-overexpressing cells and HCC cell lines affected their metabolism and abolished their tumorigenic potential, suggesting that the effects of MALAT1 on glucose metabolism are essential for its oncogenic activity. Taken together, our findings suggest that MALAT1 contributes to HCC development and tumor progression by reprogramming tumor glucose metabolism. SIGNIFICANCE: These findings show that lncRNA MALAT1 contributes to HCC development by regulating cancer glucose metabolism, enhancing glycolysis, and inhibiting gluconeogenesis via elevated translation of the transcription factor TCF7L2.

The yeast mitochondrial pyruvate carrier is a hetero-dimer in its functional state

The mitochondrial pyruvate carrier (MPC) is critical for cellular homeostasis, as it is required in central metabolism for transporting pyruvate from the cytosol into the mitochondrial matrix. MPC has been implicated in many diseases and is being investigated as a drug target. A few years ago, small membrane proteins, called MPC1 and MPC2 in mammals and Mpc1, Mpc2 and Mpc3 in yeast, were proposed to form large protein complexes responsible for this function. However, the MPC complexes have never been isolated and their composition, oligomeric state and functional properties have not been defined. Here, we identify the functional unit of MPC from Saccharomyces cerevisiae In contrast to earlier hypotheses, we demonstrate that MPC is a hetero-dimer, not a multimeric complex. When not engaged in hetero-dimers, the yeast Mpc proteins can also form homo-dimers that are, however, inactive. We show that the earlier described substrate transport properties and inhibitor profiles are embodied by the hetero-dimer. This work provides a foundation for elucidating the structure of the functional complex and the mechanism of substrate transport and inhibition.

Mitochondrial pyruvate carrier 1 functions as a tumor suppressor and predicts the prognosis of human renal cell carcinoma

Invasion and subsequent metastasis are major characteristics of malignant human renal cell carcinoma (RCC), though the mechanisms remain elusive. Mitochondrial pyruvate carrier (MPC), a key factor that controls pyruvate transportation in mitochondria, is frequently dysregulated in tumor cells and loss of MPC predicts poor prognosis in various types of cancer. However, the clinical relevance and functional significance of MPC in RCC remain to be elucidated. In this study, we investigated the expression of MPC1 and MPC2 in specimens from RCC patients and observed downregulation of MPC1, but not MPC2, in RCC tissues compared with adjacent non-cancerous tissue. Moreover, RCC patients with higher MPC1 expression exhibited longer overall survival rate than those with lower MPC1. Functionally, MPC1 suppressed the invasion of RCC cells in vitro and reduced the growth of RCC cells in vivo, possibly through inhibition of MMP7 and MMP9. Further studies revealed that loss of MPC1 was induced by hypoxia in RCC cells, and notably, MPC1 expression, was negatively correlated with HIF1α expression in RCC cells and patient samples. Taken together, our results identify anti-tumor function of MPC1 in RCC and revealed MPC1 as a novel prognostic biomarker to predict better patient survival.

Neurons rely on glucose rather than astrocytic lactate during stimulation

Brain metabolism increases during stimulation, but this increase does not affect all energy metabolism equally. Briefly after stimulation, there is a local increase in cerebral blood flow and in glucose uptake, but a smaller increase in oxygen uptake. This indicates that temporarily the rate of glycolysis is faster than the rate of oxidative metabolism, with a corresponding temporary increase in lactate production. This minireview discusses the long-standing controversy about which cell type, neurons or astrocytes, are involved in this increased aerobic glycolysis. Recent biosensor studies measuring metabolic changes in neurons, in acute brain slices or in vivo, are placed in the context of other data bearing on this question. The most direct measurements indicate that, although both neurons and astrocytes may increase glycolysis after stimulation, neurons do not rely on import of astrocytic-produced lactate, and instead they increase their own glycolytic rate and become net exporters of lactate. This temporary increase in neuronal glycolysis may provide rapid energy to meet the acute energy demands of neurons.

Adipose tissues of MPC1± mice display altered lipid metabolism-related enzyme expression levels

Mitochondrial pyruvate carrier 1 (MPC1) is a component of the MPC1/MPC2 heterodimer that facilitates the transport of pyruvate into mitochondria. Pyruvate plays a central role in carbohydrate, fatty, and amino acid catabolism. The present study examined epididymal white adipose tissue (eWAT) and intrascapular brown adipose tissue (iBAT) from MPC1^± mice following 24 weeks of feeding, which indicated low energy accumulation as evidenced by low body and eWAT weight and adipocyte volume. To characterize molecular changes in energy metabolism, we analyzed the transcriptomes of the adipose tissues using RNA-Sequencing (RNA-Seq). The results showed that the fatty acid oxidation pathway was activated and several genes involved in this pathway were upregulated. Furthermore, qPCR and western blotting indicated that numerous genes and proteins that participate in lipolysis were also upregulated. Based on these findings, we propose that the energy deficiency caused by reduced MPC1 activity can be alleviated by activating the lipolytic pathway.

Including the mitochondrial metabolism of L-lactate in cancer metabolic reprogramming

Glucose avidity, high glycolysis and L-lactate production, regardless of oxygen availability, are the main traits of cancer metabolic reprogramming. The idea that mitochondria are dysfunctional in cancer, thus causing a glycolysis increase for ATP production and L-lactate accumulation as a dead-end product of glucose catabolism, has oriented cancer research for many years. However, it was shown that mitochondrial metabolism is essential for cancer cell proliferation and tumorigenesis and that L-lactate is a fundamental energy substrate with tumor growth-promoting and signaling capabilities. Nevertheless, the known ability of mitochondria to take up and oxidize L-lactate has remained ignored by cancer research. Beginning with a brief overview of the metabolic changes occurring in cancer, we review the present knowledge of L-lactate formation, transport, and intracellular oxidation and underline the possible role of L-lactate metabolism as energetic, signaling and anabolic support for cancer cell proliferation. These unexplored aspects of cancer biochemistry might be exploited for therapeutic benefit.

Targeting energy metabolism via the mitochondrial pyruvate carrier as a novel approach to attenuate neurodegeneration

Several molecular pathways are currently being targeted in attempts to develop disease-modifying therapies to slow down neurodegeneration in Parkinson’s disease. Failure of cellular energy metabolism has long been implicated in sporadic Parkinson’s disease and recent research on rare inherited forms of Parkinson’s disease have added further weight to the importance of energy metabolism in the disease pathogenesis. There exists a new class of anti-diabetic insulin sensitizers in development that inhibit the mitochondrial pyruvate carrier (MPC), a protein which mediates the import of pyruvate across the inner membrane of mitochondria. Pharmacological inhibition of the MPC was recently found to be strongly neuroprotective in multiple neurotoxin-based and genetic models of neurodegeneration which are relevant to Parkinson’s disease. In this review, we summarize the neuroprotective effects of MPC inhibition and discuss the potential putative underlying mechanisms. These mechanisms involve augmentation of autophagy via attenuation of the activity of the mammalian target of rapamycin (mTOR) in neurons, as well as the inhibition of neuroinflammation, which is at least partly mediated by direct inhibition of MPC in glia cells. We conclude that MPC is a novel and potentially powerful therapeutic target that warrants further study in attempts to slow Parkinson’s disease progression.

Fueling thought: Management of glycolysis and oxidative phosphorylation in neuronal metabolism

Yellen reviews how cellular metabolism responds acutely to the intense energy requirements of neurons when they are stimulated.

The brain’s energy demands are remarkable both in their intensity and in their moment-to-moment dynamic range. This perspective considers the evidence for Warburg-like aerobic glycolysis during the transient metabolic response of the brain to acute activation, and it particularly addresses the cellular mechanisms that underlie this metabolic response. The temporary uncoupling between glycolysis and oxidative phosphorylation led to the proposal of an astrocyte-to-neuron lactate shuttle whereby during stimulation, lactate produced by increased glycolysis in astrocytes is taken up by neurons as their primary energy source. However, direct evidence for this idea is lacking, and evidence rather supports that neurons have the capacity to increase their own glycolysis in response to stimulation; furthermore, neurons may export rather than import lactate in response to stimulation. The possible cellular mechanisms for invoking metabolic resupply of energy in neurons are also discussed, in particular the roles of feedback signaling via adenosine diphosphate and feedforward signaling by calcium ions.

Function of mitochondrial pyruvate carriers in hepatocellular carcinoma patients

Mitochondrial pyruvate carriers (MPC) have been identified as a critical component of energy metabolism in the cancer cells of multiple malignant tumor types. The aim of the present study was to investigate the association between the expression of MPC1 and MPC2 and the prognosis of patients with hepatocellular carcinoma (HCC). A total of 85 formalin-fixed paraffin-embedded HCC tissues were assessed using immunohistochemistry. A further 20 fresh pathological specimens, including cancer and adjacent normal liver tissues from patients who had undergone a hepatectomy, were analyzed using western blotting and reverse transcription-quantitative polymerase chain reaction. The relative expression of MPC1 and MPC2 was quantified using Image-Pro Plus software, and the association between MPC expression and clinical outcomes was analyzed by Student's t-test. MPC1 and MPC2 protein expression was significantly downregulated in HCC, but no association was identified between the expression of MPC1 or MPC2 and the clinicopathological characteristics of the patients. MPC1 mRNA levels were decreased in each cancer sample, while a mixture of increased and decreased MPC2 mRNA levels observed in the HCC samples. Multivariate regression analysis indicated that the protein level and the microvascular invasion of MPC1 were positively associated with the recurrence of HCC (P=0.000 and P=0.017, respectively). MPC1 may therefore serve as an attractive biomarker for the identification of patients with HCC at a high risk of recurrence following curative resection.

Mitochondrial pyruvate carrier function is negatively linked to Warburg phenotype in vitro and malignant features in esophageal squamous cell carcinomas

Aerobic glycolysis is one of the emerging hallmarks of cancer cells. In this study, we investigated the relationship between blocking mitochondrial pyruvate carrier (MPC) with MPC blocker UK5099 and the metabolic alteration as well as aggressive features of esophageal squamous carcinoma. It was found that blocking pyruvate transportation into mitochondria attenuated mitochondrial oxidative phosphorylation (OXPHOS) and triggered aerobic glycolysis, a feature of Warburg effect. In addition, the HIF-1α expression and ROS production were also activated upon UK5099 application. It was further revealed that the UK5099-treated cells became significantly more resistant to chemotherapy and radiotherapy, and the UK5099-treated tumor cells also exhibited stronger invasive capacity compared to the parental cells. In contrast to esophageal squamous epithelium cells, decreased MPC protein expression was observed in a series of 157 human squamous cell carcinomas, and low/negative MPC1 expression predicted an unfavorable clinical outcome. All these results together revealed the potential connection of altered MPC expression/activity with the Warburg metabolic reprogramming and tumor aggressiveness in cell lines and clinical samples. Collectively, our findings highlighted a therapeutic strategy targeting Warburg reprogramming of human esophageal squamous cell carcinomas.

Human mitochondrial pyruvate carrier 2 as an autonomous membrane transporter

The active transport of glycolytic pyruvate across the inner mitochondrial membrane is thought to involve two mitochondrial pyruvate carrier subunits, MPC1 and MPC2, assembled as a 150 kDa heterotypic oligomer. Here, the recombinant production of human MPC through a co-expression strategy is first described; however, substantial complex formation was not observed, and predominantly individual subunits were purified. In contrast to MPC1, which co-purifies with a host chaperone, we demonstrated that MPC2 homo-oligomers promote efficient pyruvate transport into proteoliposomes. The derived functional requirements and kinetic features of MPC2 resemble those previously demonstrated for MPC in the literature. Distinctly, chemical inhibition of transport is observed only for a thiazolidinedione derivative. The autonomous transport role for MPC2 is validated in cells when the ectopic expression of human MPC2 in yeast lacking endogenous MPC stimulated growth and increased oxygen consumption. Multiple oligomeric species of MPC2 across mitochondrial isolates, purified protein and artificial lipid bilayers suggest functional high-order complexes. Significant changes in the secondary structure content of MPC2, as probed by synchrotron radiation circular dichroism, further supports the interaction between the protein and ligands. Our results provide the initial framework for the independent role of MPC2 in homeostasis and diseases related to dysregulated pyruvate metabolism.

Mitochondrial pyruvate carrier 1 mediates abscisic acid-regulated stomatal closure and the drought response by affecting cellular pyruvate content in Arabidopsis thaliana

BACKGROUND

Stomata are micropores surrounded by pairs of guard cells, and their opening is finely controlled to balance water vapor loss as transpiration and CO₂ absorption for photosynthesis. The regulatory signaling network for stomatal movement is complicated, and increasing numbers of new genes have been shown to be involved in this process. Our previous study indicated that a member of the plant putative mitochondrial pyruvate carrier (MPC) family, NRGA1, is a negative regulator of guard cell abscisic acid (ABA) signaling. In this study, we identified novel physiological roles of pyruvate and MPC1, another member of the MPC family, in the regulation of stomatal closure in Arabidopsis.

RESULTS

Loss-of-function mutants of MPC1 (mpc1) were hypersensitive to ABA-induced stomatal closure and ABA-activated guard cell slow-type anion currents, and showed a reduced rate of water loss upon drought treatment compared with wild-type plants. In contrast, plants overexpressing MPC1 showed a hyposensitive ABA response and increased sensitivity to drought stress. In addition, mpc1 mutants accumulated more pyruvate after drought or ABA treatment. The increased pyruvate content also induced stomatal closure and activated the slow-type anion channels of guard cells, and this process was dependent on the function of RbohD/F NADPH oxidases and reactive oxygen species concentrations in guard cells.

CONCLUSIONS

Our findings revealed the essential roles of MPC1 and pyruvate in stomatal movement and plant drought resistance.