The long and winding road to the mitochondrial pyruvate carrier

Exploring the mystery of tumor metabolism: Warburg effect and mitochondrial metabolism fighting side by side

The metabolic reconfiguration of tumor cells constitutes a pivotal aspect of tumor proliferation and advancement. This study delves into two primary facets of tumor metabolism: the Warburg effect and mitochondrial metabolism, elucidating their contributions to tumor dominance. The Warburg effect facilitates efficient energy acquisition by tumor cells through aerobic glycolysis and lactic acid fermentation, offering metabolic advantages conducive to growth and proliferation. Simultaneously, mitochondrial metabolism, serving as the linchpin of sustained tumor vitality, orchestrates the tricarboxylic acid cycle and electron transport chain, furnishing a steadfast and dependable wellspring of biosynthesis for tumor cells. Regarding targeted therapy, this discourse examines extant strategies targeting tumor glycolysis and mitochondrial metabolism, underscoring their potential efficacy in modulating tumor metabolism while envisaging future research trajectories and treatment paradigms in the realm of tumor metabolism. By means of a thorough exploration of tumor metabolism, this study aspires to furnish crucial insights into the regulation of tumor metabolic processes, thereby furnishing valuable guidance for the development of novel therapeutic modalities. This comprehensive deliberation is poised to catalyze advancements in tumor metabolism research and offer novel perspectives and pathways for the formulation of cancer treatment strategies in the times ahead.

A pyruvate transporter in the apicoplast of apicomplexan parasites

Pyruvate lies at a pivotal node of carbon metabolism in eukaryotes. It is involved in diverse metabolic pathways in multiple organelles, and its interorganelle shuttling is crucial for cell fitness. Many apicomplexan parasites harbor a unique organelle called the apicoplast that houses metabolic pathways like fatty acid and isoprenoid precursor biosyntheses, requiring pyruvate as a substrate. However, how pyruvate is supplied in the apicoplast remains enigmatic. Here, deploying the zoonotic parasite Toxoplasma gondii as a model apicomplexan, we identified two proteins residing in the apicoplast membranes that together constitute a functional apicoplast pyruvate carrier (APC) to mediate the import of cytosolic pyruvate. Depletion of APC results in reduced activities of metabolic pathways in the apicoplast and impaired integrity of this organelle, leading to parasite growth arrest. APC is a pyruvate transporter in diverse apicomplexan parasites, suggesting a common strategy for pyruvate acquisition by the apicoplast in these clinically relevant intracellular pathogens.

Mitochondria at the crossroads of health and disease

Mitochondria reside at the crossroads of catabolic and anabolic metabolism-the essence of life. How their structure and function are dynamically tuned in response to tissue-specific needs for energy, growth repair, and renewal is being increasingly understood. Mitochondria respond to intrinsic and extrinsic stresses and can alter cell and organismal function by inducing metabolic signaling within cells and to distal cells and tissues. Here, we review how the centrality of mitochondrial functions manifests in health and a broad spectrum of diseases and aging.

Emerging experimental drugs in clinical trials for migraine: observations and key talking points

INTRODUCTION

There have been significant advances in the treatment of migraine. In response to the clinical success of monoclonal antibodies targeting calcitonin gene-related peptide, there is interest in the clinical trial outcomes of alternative emerging drugs that act on novel targets associated with migraine pathophysiology. As approximately 50% of patients do not respond to CGRP therapies, there is significant value in future drug innovation. Emerging drugs in clinical trials for the treatment of migraine aim to fill this need.

AREAS COVERED

The emerging drugs that will be discussed in this review include zavegepant, lasmiditan, delta opioid receptor agonists, neuronal nitric oxide synthase inhibitors, monoclonal antibodies targeting pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, dual orexin receptor antagonists, metabotropic glutamate receptor 5 antagonists, and inducers of ketosis.

EXPERT OPINION

When considering the preclinical and clinical research related to the emerging drug classes discussed in this review, most therapies are derived from highly supported targets of migraine pathogenesis. Although the individual drugs discussed in this review may be of dubious clinical value, the importance of the therapeutic targets on which they act cannot be understated. Future research is necessary to appropriately target the pathways elucidated by preclinical studies.

Mitochondrial pyruvate carrier-mediated metabolism is dispensable for the classical activation of macrophages

Glycolysis is essential for the classical activation of macrophages (M1), but how glycolytic pathway metabolites engage in this process remains to be elucidated. Glycolysis leads to production of pyruvate, which can be transported into the mitochondria by the mitochondrial pyruvate carrier (MPC) followed by utilization in the tricarboxylic acid cycle. Based on studies that used the MPC inhibitor UK5099, the mitochondrial route has been considered to be of significance for M1 activation. Using genetic approaches, here we show that the MPC is dispensable for metabolic reprogramming and activation of M1 macrophages. In addition, MPC depletion in myeloid cells has no impact on inflammatory responses and macrophage polarization toward the M1 phenotype in a mouse model of endotoxemia. While UK5099 reaches maximal MPC inhibitory capacity at approximately 2-5 μM, higher concentrations are required to inhibit inflammatory cytokine production in M1 and this is independent of MPC expression. Taken together, MPC-mediated metabolism is dispensable for the classical activation of macrophages and UK5099 inhibits inflammatory responses in M1 macrophages due to effects other than MPC inhibition.

miRNA-1 promotes acute myeloid leukemia cell pathogenesis through metabolic regulation

Acute myeloid leukemia (AML) is a heterogeneous and deadly disease characterized by uncontrolled expansion of malignant blasts. Altered metabolism and dysregulated microRNA (miRNA) expression profiles are both characteristic of AML. However, there is a paucity of studies exploring how changes in the metabolic state of the leukemic cells regulate miRNA expression leading to altered cellular behavior. Here, we blocked pyruvate entry into mitochondria by deleting the Mitochondria Pyruvate Carrier (MPC1) gene in human AML cell lines, which decreased Oxidative Phosphorylation (OXPHOS). This metabolic shift also led to increased expression of miR-1 in the human AML cell lines tested. AML patient sample datasets showed that higher miR-1 expression correlates with reduced survival. Transcriptional and metabolic profiling of miR-1 overexpressing AML cells revealed that miR-1 increased OXPHOS, along with key metabolites that fuel the TCA cycle such as glutamine and fumaric acid. Inhibition of glutaminolysis decreased OXPHOS in miR-1 overexpressing MV4-11 cells, highlighting that miR-1 promotes OXPHOS through glutaminolysis. Finally, overexpression of miR-1 in AML cells exacerbated disease in a mouse xenograft model. Together, our work expands current knowledge within the field by uncovering novel connections between AML cell metabolism and miRNA expression that facilitates disease progression. Further, our work points to miR-1 as a potential new therapeutic target that may be used to disrupt AML cell metabolism and thus pathogenesis in the clinic.

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.

PM2.5 promotes NSCLC carcinogenesis through translationally and transcriptionally activating DLAT-mediated glycolysis reprograming

Background

Airborne fine particulate matter (PM2.5) has been associated with lung cancer development and progression in never smokers. However, the molecular mechanisms underlying PM2.5-induced lung cancer remain largely unknown. The aim of this study was to explore the mechanisms by which PM2.5 regulated the carcinogenesis of non-small cell lung cancer (NSCLC).

Methods

Paralleled ribosome sequencing (Ribo-seq) and RNA sequencing (RNA-seq) were performed to identify PM2.5-associated genes for further study. Quantitative real time-PCR (qRT-PCR), Western blot, and immunohistochemistry (IHC) were used to determine mRNA and protein expression levels in tissues and cells. The biological roles of PM2.5 and PM2.5-dysregulated gene were assessed by gain- and loss-of-function experiments, biochemical analyses, and Seahorse XF glycolysis stress assays. Human tissue microarray analysis and ¹⁸F-FDG PET/CT scans in patients with NSCLC were used to verify the experimental findings. Polysome fractionation experiments, chromatin immunoprecipitation (ChIP), and dual-luciferase reporter assay were implemented to explore the molecular mechanisms.

Results

We found that PM2.5 induced a translation shift towards glycolysis pathway genes and increased glycolysis metabolism, as evidenced by increased L-lactate and pyruvate concentrations or higher extracellular acidification rate (ECAR) in vitro and in vivo. Particularly, PM2.5 enhanced the expression of glycolytic gene DLAT, which promoted glycolysis but suppressed acetyl-CoA production and enhanced the malignancy of NSCLC cells. Clinically, high expression of DLAT was positively associated with tumor size, poorer prognosis, and SUVmax values of ¹⁸F-FDG-PET/CT scans in patients with NSCLC. Mechanistically, PM2.5 activated eIF4E, consequently up-regulating the expression level of DLAT in polysomes. PM2.5 also stimulated transcription factor Sp1, which further augmented transcription activity of DLAT promoter.

Conclusions

This study demonstrated that PM2.5-activated overexpression of DLAT and enhancement in glycolysis metabolism contributed to the tumorigenesis of NSCLC, suggesting that DLAT-associated pathway may be a therapeutic target for NSCLC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02437-8.

Mitochondrial Pyruvate Carrier Subunits Are Essential for Pyruvate-Driven Respiration, Infectivity, and Intracellular Replication of Trypanosoma cruzi

Trypanosoma cruzi is the causative agent of Chagas disease. Pyruvate is the end product of glycolysis, and its transport into the mitochondrion is mediated by the mitochondrial pyruvate carrier (MPC) subunits.

Pyruvate is the final metabolite of glycolysis and can be converted into acetyl coenzyme A (acetyl-CoA) in mitochondria, where it is used as the substrate for the tricarboxylic acid cycle. Pyruvate availability in mitochondria depends on its active transport through the heterocomplex formed by the mitochondrial pyruvate carriers 1 and 2 (MPC1/MPC2). We report here studies on MPC1/MPC2 of Trypanosoma cruzi, the etiologic agent of Chagas disease. Endogenous tagging of T. cruziMPC1 (TcMPC1) and TcMPC2 with 3×c-Myc showed that both encoded proteins colocalize with MitoTracker to the mitochondria of epimastigotes. Individual knockout (KO) of TcMPC1 and TcMPC2 genes using CRISPR/Cas9 was confirmed by PCR and Southern blot analyses. Digitonin-permeabilized TcMPC1-KO and TcMPC2-KO epimastigotes showed reduced O₂ consumption rates when pyruvate, but not succinate, was used as the mitochondrial substrate, while α-ketoglutarate increased their O₂ consumption rates due to an increase in α-ketoglutarate dehydrogenase activity. Defective mitochondrial pyruvate import resulted in decreased Ca²⁺ uptake. The inhibitors UK5099 and malonate impaired pyruvate-driven oxygen consumption in permeabilized control cells. Inhibition of succinate dehydrogenase by malonate indicated that pyruvate needs to be converted into succinate to increase respiration. TcMPC1-KO and TcMPC2-KO epimastigotes showed little growth differences in standard or low-glucose culture medium. However, the ability of trypomastigotes to infect tissue culture cells and replicate as intracellular amastigotes was decreased in TcMPC-KOs. Overall, T. cruzi MPC1 and MPC2 are essential for cellular respiration in the presence of pyruvate, invasion of host cells, and replication of amastigotes.

Sodium Pyruvate Ameliorates Influenza a Virus Infection In Vivo

Influenza A virus (IAV) causes seasonal epidemics annually and pandemics every few decades. Most antiviral treatments used for IAV are only effective if administered during the first 48 h of infection and antiviral resistance is possible. Therapies that can be initiated later during IAV infection and that are less likely to elicit resistance will significantly improve treatment options. Pyruvate, a key metabolite, and an end product of glycolysis, has been studied for many uses, including its anti-inflammatory capabilities. Sodium pyruvate was recently shown by us to decrease inflammasome activation during IAV infection. Here, we investigated sodium pyruvate’s effects on IAV in vivo. We found that nebulizing mice with sodium pyruvate decreased morbidity and weight loss during infection. Additionally, treated mice consumed more chow during infection, indicating improved symptoms. There were notable improvements in pro-inflammatory cytokine production (IL-1β) and lower virus titers on day 7 post-infection in mice treated with sodium pyruvate compared to control animals. As pyruvate acts on the host immune response and metabolic pathways and not directly on the virus, our data demonstrate that sodium pyruvate is a promising treatment option that is safe, effective, and unlikely to elicit antiviral resistance.

Reprogramming of aerobic glycolysis in non-transformed mouse liver with pyruvate dehydrogenase complex deficiency

The Pyruvate Dehydrogenase Complex (PDC), a key enzyme in glucose metabolism, catalyzes an irreversible oxidative decarboxylation reaction of pyruvate to acetyl‐CoA, linking the cytosolic glycolytic pathway to mitochondrial tricarboxylic acid cycle and oxidative phosphorylation. Earlier we reported a down‐regulation of several key hepatic lipogenic enzymes and their upstream regulators in liver‐specific PDC‐deficient mouse (L‐PDCKO model by deleting the Pdha1 gene). In this study we investigated gene expression profiles of key glycolytic enzymes and other proteins that respond to various metabolic stresses in liver from L‐PDCKO mice. Transcripts of several, such as hexokinase 2, phosphoglycerate kinase 1, pyruvate kinase muscle‐type 2, and lactate dehydrogenase B as well as those for the nonglycolysis‐related proteins, CD‐36, C/EBP homologous protein, and peroxisome proliferator‐activated receptor γ, were up‐regulated in L‐PDCKO liver whereas hypoxia‐induced factor‐1α, pyruvate dehydrogenase kinase 1 and Sirtuin 1 transcripts were down‐regulated. The protein levels of pyruvate kinase muscle‐type 2 and lactate dehydrogenase B were increased whereas that of lactate dehydrogenase A was decreased in PDC‐deficient mouse liver. Analysis of endoplasmic reticulum and oxidative stress indicators suggests that the L‐PDCKO liver showed evidence of the former but not the latter. These findings indicate that (i) liver‐specific PDC deficiency is sufficient to induce “aerobic glycolysis characteristic” in mouse liver, and (ii) the mechanism(s) responsible for these changes appears distinct from that which induces the Warburg effect in some cancer cells.

Insights on the Quest for the Structure-Function Relationship of the Mitochondrial Pyruvate Carrier

Simple Summary

The atomic structure of a biological macromolecule determines its function. Discovering how one or more amino acid chains fold and interact to form a protein complex is critical, from understanding the most fundamental cellular processes to developing new therapies. However, this is far from a straightforward task, especially when studying a membrane protein. The functional link between the oligomeric state and complex composition of the proteins involved in the active mitochondrial transport of cytosolic pyruvate is a decades-old question but remains urgent. We present a brief historical review beginning with the identification of the so-called mitochondrial pyruvate carrier (MPC) proteins, followed by a rigorous conceptual analysis of technical approaches in more recent biochemical studies that seek to isolate and reconstitute the functional MPC complex(es) in vitro. We correlate these studies with early kinetic observations and current experimental and computational knowledge to assess their main contributions, identify gaps, resolve ambiguities, and better define the research goal.

Abstract

The molecular identity of the mitochondrial pyruvate carrier (MPC) was presented in 2012, forty years after the active transport of cytosolic pyruvate into the mitochondrial matrix was first demonstrated. An impressive amount of in vivo and in vitro studies has since revealed an unexpected interplay between one, two, or even three protein subunits defining different functional MPC assemblies in a metabolic-specific context. These have clear implications in cell homeostasis and disease, and on the development of future therapies. Despite intensive efforts by different research groups using state-of-the-art computational tools and experimental techniques, MPCs’ structure-based mechanism remains elusive. Here, we review the current state of knowledge concerning MPCs’ molecular structures by examining both earlier and recent studies and presenting novel data to identify the regulatory, structural, and core transport activities to each of the known MPC subunits. We also discuss the potential application of cryogenic electron microscopy (cryo-EM) studies of MPC reconstituted into nanodiscs of synthetic copolymers for solving human MPC2.

Complement's favourite organelle-Mitochondria?

The complement system, well known for its central role in innate immunity, is currently emerging as an unexpected, cell‐autonomous, orchestrator of normal cell physiology. Specifically, an intracellularly active complement system—the complosome—controls key pathways of normal cell metabolism during immune cell homeostasis and effector function. So far, we know little about the exact structure and localization of intracellular complement components within and among cells. A common scheme, however, is that they operate in crosstalk with other intracellular immune sensors, such as inflammasomes, and that they impact on the activity of key subcellular compartments. Among cell compartments, mitochondria appear to have built a particularly early and strong relationship with the complosome and extracellularly active complement—not surprising in view of the strong impact of the complosome on metabolism. In this review, we will hence summarize the current knowledge about the close complosome–mitochondria relationship and also discuss key questions surrounding this novel research area.

Pyruvate affects inflammatory responses of macrophages during influenza A virus infection

Pyruvate is the end product of glycolysis and transported into the mitochondria for use in the tricarboxylic acid (TCA) cycle. It is also a common additive in cell culture media. We discovered that inclusion of sodium pyruvate in culture media during infection of mouse bone marrow derived macrophages with influenza A virus impaired cytokine production (IL-6, IL-1β, and TNF-α). Sodium pyruvate did not inhibit viral RNA replication. Instead, the addition of sodium pyruvate alters cellular metabolism and diminished mitochondrial reactive oxygen species (ROS) production and lowered immune signaling. Overall, sodium pyruvate affects the immune response produced by macrophages but does not inhibit virus replication.

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.

Regulation of Tumor Initiation by the Mitochondrial Pyruvate Carrier

Although metabolic adaptations have been demonstrated to be essential for tumor cell proliferation, the metabolic underpinnings of tumor initiation are poorly understood. We found that the earliest stages of colorectal cancer (CRC) initiation are marked by a glycolytic metabolic signature, including downregulation of the mitochondrial pyruvate carrier (MPC), which couples glycolysis and glucose oxidation through mitochondrial pyruvate import. Genetic studies in Drosophila suggest that this downregulation is required because hyperplasia caused by loss of the Apc or Notch tumor suppressors in intestinal stem cells can be completely blocked by MPC overexpression. Moreover, in two distinct CRC mouse models, loss of Mpc1 prior to a tumorigenic stimulus doubled the frequency of adenoma formation and produced higher grade tumors. MPC loss was associated with a glycolytic metabolic phenotype and increased expression of stem cell markers. These data suggest that changes in cellular pyruvate metabolism are necessary and sufficient to promote cancer initiation.

Lactate and Lactate Transporters as Key Players in the Maintenance of the Warburg Effect

Reprogramming of energy metabolism is a key hallmark of cancer. Most cancer cells display a glycolytic phenotype, with increased glucose consumption and glycolysis rates, and production of lactate as the end product, independently of oxygen concentrations. This phenomenon, known as "Warburg Effect", provides several survival advantages to cancer cells and modulates the metabolism and function of neighbour cells in the tumour microenvironment. However, due to the presence of metabolic heterogeneity within a tumour, cancer cells can also display an oxidative phenotype, and corruptible cells from the microenvironment become glycolytic, cooperating with oxidative cancer cells to boost tumour growth. This phenomenon is known as "Reverse Warburg Effect". In either way, lactate is a key mediator in the metabolic crosstalk between cancer cells and the microenvironment, and lactate transporters are expressed differentially by existing cell populations, to support this crosstalk.In this review, we will focus on lactate and on lactate transporters in distinct cells of the tumour microenvironment, aiming at a better understanding of their role in the acquisition and maintenance of the direct/reverse "Warburg effect" phenotype, which modulate cancer progression.

Androgen-induced expression of DRP1 regulates mitochondrial metabolic reprogramming in prostate cancer

Androgen receptor (AR) signaling plays a central role in metabolic reprogramming for prostate cancer (PCa) growth and progression. Mitochondria are metabolic powerhouses of the cell and support several hallmarks of cancer. However, the molecular links between AR signaling and the mitochondria that support the metabolic demands of PCa cells are poorly understood. Here, we demonstrate increased levels of dynamin-related protein 1 (DRP1), a mitochondrial fission mediator, in androgen-sensitive and castration-resistant AR-driven PCa. AR signaling upregulates DRP1 to form the VDAC-MPC2 complex, increases pyruvate transport into mitochondria, and supports mitochondrial metabolism, including oxidative phosphorylation and lipogenesis. DRP1 inhibition activates the cellular metabolic stress response, which involves AMPK phosphorylation, induction of autophagy, and the ER unfolded protein response, and attenuates androgen-induced proliferation. Additionally, DRP1 expression facilitates PCa cell survival under diverse metabolic stress conditions, including hypoxia and oxidative stress. Moreover, we found that increased DRP1 expression was indicative of poor prognosis in patients with castration-resistant PCa. Collectively, our findings link androgen signaling-mediated mitochondrial dynamics to metabolic reprogramming; moreover, they have important implications for understanding PCa progression.

Targeting the Mitochondrial Pyruvate Carrier for Neuroprotection

The mitochondrial pyruvate carriers mediate pyruvate import into the mitochondria, which is key to the sustenance of the tricarboxylic cycle and oxidative phosphorylation. However, inhibition of mitochondria pyruvate carrier-mediated pyruvate transport was recently shown to be beneficial in experimental models of neurotoxicity pertaining to the context of Parkinson’s disease, and is also protective against excitotoxic neuronal death. These findings attested to the metabolic adaptability of neurons resulting from MPC inhibition, a phenomenon that has also been shown in other tissue types. In this short review, I discuss the mechanism and potential feasibility of mitochondrial pyruvate carrier inhibition as a neuroprotective strategy in neuronal injury and neurodegenerative diseases.

Effective breast cancer combination therapy targeting BACH1 and mitochondrial metabolism

Mitochondrial metabolism is an attractive target for cancer therapy^1,2. Reprogramming metabolic pathways could improve the ability of metabolic inhibitors to suppress cancers with limited treatment options, such as triple-negative breast cancer (TNBC)^1,3. Here we show that BTB and CNC homology1 (BACH1)⁴, a haem-binding transcription factor that is increased in expression in tumours from patients with TNBC, targets mitochondrial metabolism. BACH1 decreases glucose utilization in the tricarboxylic acid cycle and negatively regulates transcription of electron transport chain (ETC) genes. BACH1 depletion by shRNA or degradation by hemin sensitizes cells to ETC inhibitors such as metformin^5,6, suppressing growth of both cell line and patient-derived tumour xenografts. Expression of a haem-resistant BACH1 mutant in cells that express a short hairpin RNA for BACH1 rescues the BACH1 phenotype and restores metformin resistance in hemin-treated cells and tumours⁷. Finally, BACH1 gene expression inversely correlates with ETC gene expression in tumours from patients with breast cancer and in other tumour types, which highlights the clinical relevance of our findings. This study demonstrates that mitochondrial metabolism can be exploited by targeting BACH1 to sensitize breast cancer and potentially other tumour tissues to mitochondrial inhibitors.

MPC1 deficiency accelerates lung adenocarcinoma progression through the STAT3 pathway

Mitochondrial pyruvate carrier 1 (MPC1), a key factor that controls pyruvate transportation in the mitochondria, is known to be frequently dysregulated in tumor initiation and progression. However, the clinical relevance and potential molecular mechanisms of MPC1 in lung adenocarcinoma (LAC) progression remain to be illustrated. Herein, MPC1 was lowly expressed in LAC tissues and significantly associated with favorable survival of patients with LAC. Functionally, MPC1 markedly suppressed stemness, invasion, and migration in vitro and spreading growth of LAC cells in vivo. Further study revealed that MPC1 could interact with mitochondrial signal transducer and activator of transcription 3 (mito-STAT3), disrupting the distribution of STAT3 and reducing cytoplasmic signal transducer and activator of transcription 3 (cyto-STAT3) as well as its phosphorylation, while the activation of cyto-STAT3 by IL-6 reversed the attenuated malignant progression in MPC1-overexpression LAC cells. Collectively, we reveal that MPC1/STAT3 axis plays an important role in the progression of LAC, and our work may promote the development of new therapeutic strategies for LAC.

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.

Mitochondrial pyruvate carrier function determines cell stemness and metabolic reprogramming in cancer cells

One of the remarkable features of cancer cells is aerobic glycolysis, a phenomenon known as the "Warburg Effect", in which cells rely preferentially on glycolysis instead of oxidative phosphorylation (OXPHOS) as the main energy source even in the presence of high oxygen tension. Cells with dysfunctional mitochondria are unable to generate sufficient ATP from mitochondrial OXPHOS, and then are forced to rely on glycolysis for ATP generation. Here we report our results in a prostate cancer cell line in which the mitochondrial pyruvate carrier 1 (MPC1) gene was knockout. It was discovered that the MPC1 gene knockout cells revealed a metabolism reprogramming to aerobic glycolysis with reduced ATP production, and the cells became more migratory and resistant to both chemotherapy and radiotherapy. In addition, the MPC1 knockout cells expressed significantly higher levels of the stemness markers Nanog, Hif1α, Notch1, CD44 and ALDH. To further verify the correlation of MPC gene function and cell stemness/metabolic reprogramming, MPC inhibitor UK5099 was applied in two ovarian cancer cell lines and similar results were obtained. Taken together, our results reveal that functional MPC may determine the fate of metabolic program and the stemness status of cancer cells in vitro.

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.

Proteomic Analysis of Differentially-Expressed Proteins in the Liver of Streptozotocin-Induced Diabetic Rats Treated with Parkia biglobosa Protein Isolate

Protein isolate from Parkia biglobosa seeds is believed to possess excellent anti-diabetic properties. The purpose of this study was to identify differentially expressed proteins in liver of streptozotocin-induced diabetic rats treated with Parkia biglobosa seeds protein isolate (PBPi). In this study, total proteins extracted from rat liver were separated on one-dimensional SDS polyacrylamide gel (1D SDS-PAGE) and stained with Coomassie brilliant blue (CBB) to visualize protein bands. We observed that protein bands in the region of 10-15 kDa were altered by the different treatments; these bands were selected and excised for in-gel digestion and peptide extraction followed by nLC-MS, MALDI-TOF MS, and LIFT MS/MS. A database search with the Mascot algorithm positively identified four differentially expressed proteins. These proteins are known to be responsible for diverse biological functions within various organs and tissues. The present result gives insight and understanding into possible molecular mechanisms by which streptozotocin causes various alterations in proteins found in the liver of diabetic rats and the possible modulatory role of PBPi in the management of streptozotocin-induced diabetes.

Rethinking the evolution of eukaryotic metabolism: novel cellular partitioning of enzymes in stramenopiles links serine biosynthesis to glycolysis in mitochondria

Background

An important feature of eukaryotic evolution is metabolic compartmentalization, in which certain pathways are restricted to the cytosol or specific organelles. Glycolysis in eukaryotes is described as a cytosolic process. The universality of this canon has been challenged by recent genome data that suggest that some glycolytic enzymes made by stramenopiles bear mitochondrial targeting peptides.

Results

Mining of oomycete, diatom, and brown algal genomes indicates that stramenopiles encode two forms of enzymes for the second half of glycolysis, one with and the other without mitochondrial targeting peptides. The predicted mitochondrial targeting was confirmed by using fluorescent tags to localize phosphoglycerate kinase, phosphoglycerate mutase, and pyruvate kinase in Phytophthora infestans, the oomycete that causes potato blight. A genome-wide search for other enzymes with atypical mitochondrial locations identified phosphoglycerate dehydrogenase, phosphoserine aminotransferase, and phosphoserine phosphatase, which form a pathway for generating serine from the glycolytic intermediate 3-phosphoglycerate. Fluorescent tags confirmed the delivery of these serine biosynthetic enzymes to P. infestans mitochondria. A cytosolic form of this serine biosynthetic pathway, which occurs in most eukaryotes, is missing from oomycetes and most other stramenopiles. The glycolysis and serine metabolism pathways of oomycetes appear to be mosaics of enzymes with different ancestries. While some of the noncanonical oomycete mitochondrial enzymes have the closest affinity in phylogenetic analyses with proteins from other stramenopiles, others cluster with bacterial, plant, or animal proteins. The genes encoding the mitochondrial phosphoglycerate kinase and serine-forming enzymes are physically linked on oomycete chromosomes, which suggests a shared origin.

Conclusions

Stramenopile metabolism appears to have been shaped through the acquisition of genes by descent and lateral or endosymbiotic gene transfer, along with the targeting of the proteins to locations that are novel compared to other eukaryotes. Colocalization of the glycolytic and serine biosynthesis enzymes in mitochondria is apparently necessary since they share a common intermediate. The results indicate that descriptions of metabolism in textbooks do not cover the full diversity of eukaryotic biology.

Electronic supplementary material

The online version of this article (10.1186/s12862-017-1087-8) contains supplementary material, which is available to authorized users.

Overexpression of MPC1 inhibits the proliferation, migration, invasion, and stem cell-like properties of gastric cancer cells

Invasion and metastasis are major malignant characteristics of human gastric cancer (GC), but the molecular mechanisms underlying the invasion and metastasis of GC cells remain elusive. MPC1, a key factor that controls pyruvate transportation through the inner mitochondrial membrane, was reported to be downregulated and correlated with poor prognosis in several cancers. However, the effects of MPC1 on human GC have not been illustrated. In this study, we investigated the potential role of MPC1 in the proliferation, migration, invasion, and stem cell-like properties of human GC cells and evaluated its prognostic significance for patients with GC. We found that MPC1 protein and mRNA levels were significantly decreased in GC tissues and cell lines. Low MPC1 expression was associated with tumor T stage, N stage, and advanced tumor node metastasis stage. Decreased MPC1 expression was an independent prognostic marker and correlated with poor overall survival of patients with GC. Furthermore, overexpression of MPC1 inhibited the proliferation, migration, invasion, and stem cell-like properties of GC cells. These findings suggest that MPC1 may be a novel prognostic marker and a potential therapeutic target in human GC.

Combining metabolic and process engineering strategies to improve recombinant glycoprotein production and quality

Increasing recombinant protein production while ensuring a high and consistent protein quality remains a challenge in mammalian cell culture process development. In this work, we combined a nutrient substitution approach with a metabolic engineering strategy that improves glucose utilization efficiency. This combination allowed us to tackle both lactate and ammonia accumulation and investigate on potential synergistic effects on protein production and quality. To this end, HEK293 cells overexpressing the pyruvate yeast carboxylase (PYC2) and their parental cells, both stably producing the therapeutic glycoprotein interferon α2b (IFNα2b), were cultured in media deprived of glutamine but containing chosen substitutes. Among the tested substitutes, pyruvate led to the best improvement in growth (integral of viable cell density) for both cell lines in batch cultures, whereas the culture of PYC2 cells without neither glutamine nor any substitute displayed surprisingly enhanced IFNα2b production. The drastic reduction in both lactate and ammonia in the cultures translated into extended high viability conditions and an increase in recombinant protein titer by up to 47% for the parental cells and the PYC2 cells. Product characterization performed by surface plasmon resonance biosensing using Sambucus nigra (SNA) lectin revealed that the increase in yield was however accompanied by a reduction in the degree of sialylation of the product. Supplementing cultures with glycosylation precursors and a cofactor were effective at counterbalancing the lack of glutamine and allowed improvement in IFNα2b quality as evaluated by lectin affinity. Our study provides a strategy to reconcile protein productivity and quality and highlights the advantages of PYC2-overexpressing cells in glutamine-free conditions.

Metabolic Diversity and Novelties in the Oomycetes

The eukaryotic microbes called oomycetes include many important saprophytes and pathogens, with the latter exhibiting necrotrophy, biotrophy, or obligate biotrophy. Understanding oomycete metabolism is fundamental to understanding these lifestyles. Genome mining and biochemical studies have shown that oomycetes, which belong to the kingdom Stramenopila, secrete suites of carbohydrate- and protein-degrading enzymes adapted to their environmental niches and produce unusual lipids and energy storage compounds. Despite having limited secondary metabolism, many oomycetes make chemicals for communicating within their species or with their hosts. Horizontal and endosymbiotic gene transfer events have diversified oomycete metabolism, resulting in biochemical pathways that often depart from standard textbook descriptions by amalgamating enzymes from multiple sources. Gene fusions and duplications have further shaped the composition and expression of the enzymes. Current research is helping us learn how oomycetes interact with host and environment, understand eukaryotic diversity and evolution, and identify targets for drugs and crop protection chemicals.

Functional Properties of the Mitochondrial Carrier System

The mitochondrial carrier system (MCS) transports small molecules between mitochondria and the cytoplasm. It is integral to the core mitochondrial function to regulate cellular chemistry by metabolism. The mammalian MCS comprises the transporters of the 53-member canonical SLC25A family and a lesser number of identified noncanonical transporters. The recent discovery and investigations of the mitochondrial pyruvate carrier (MPC) illustrate the diverse effects a single mitochondrial carrier may exert on cellular function. However, the transport selectivities of many carriers remain unknown, and most have not been functionally investigated in mammalian cells. The mechanisms coordinating their function as a unified system remain undefined. Increased accessibility to molecular genetic and metabolomic technologies now greatly enables investigation of the MCS. Continued investigation of the MCS may reveal how mitochondria encode complex regulatory information within chemical thermodynamic gradients. This understanding may enable precision modulation of cellular chemistry to counteract the dysmetabolism inherent in disease.

A metabolic switch controls intestinal differentiation downstream of Adenomatous polyposis coli (APC)

Elucidating signaling pathways that regulate cellular metabolism is essential for a better understanding of normal development and tumorigenesis. Recent studies have shown that mitochondrial pyruvate carrier 1 (MPC1), a crucial player in pyruvate metabolism, is downregulated in colon adenocarcinomas. Utilizing zebrafish to examine the genetic relationship between MPC1 and Adenomatous polyposis coli (APC), a key tumor suppressor in colorectal cancer, we found that apc controls the levels of mpc1 and that knock down of mpc1 recapitulates phenotypes of impaired apc function including failed intestinal differentiation. Exogenous human MPC1 RNA rescued failed intestinal differentiation in zebrafish models of apc deficiency. Our data demonstrate a novel role for apc in pyruvate metabolism and that pyruvate metabolism dictates intestinal cell fate and differentiation decisions downstream of apc.

DOI:http://dx.doi.org/10.7554/eLife.22706.001

Colon cancer remains an important problem in healthcare. Cancer researchers are looking for new ways to detect the disease earlier and treat it more effectively. This is challenging because many of the genetic and molecular causes of colon cancer are still poorly understood. Mutations in the gene that encodes a protein called APC are one of the major causes of the disease. The APC protein normally keeps cells from growing and dividing too fast or in an uncontrolled way and is hence referred to as a tumor suppressor. For example, APC induces stem cells in the intestine to develop into specialized cells that keep the gut working normally. Mutations in tumor suppressor genes are common in many cancers.

Other research has shown that cancer cells must reprogram their own metabolism – in other words, all the chemical processes that keep the cell alive – to meet the demands of proliferating rapidly. In particular, recent studies reveal that colon cancer cells produce less of a protein called mpc1, which is involved in metabolism. These discoveries raised the following questions: does APC have an additional role in maintaining normal metabolism in cells by controlling how much mpc1 is produced? Do mutations in the gene for APC lead to colon cancer because they alter the cell’s metabolism?

Sandoval et al. have now discovered a connection between APC and changes in cancer cells that help them to adapt to a new metabolic program. Experiments with zebrafish – a model animal that is now commonly used in the field of cancer biology – showed that APC acts via mpc1 to regulate how the cell uses energy. This regulation goes awry in colon cells that have abnormal APC activity; however, restoring the cell’s metabolism back to normal was enough to induce cells in the intestine to develop properly.

Together, these findings suggest that restoring the normal balance of energy production in colon cancer cells may be an effective way to make the cells behave normally. This hypothesis remains to be tested and, if confirmed, further studies will be needed to determine whether it will lead to new treatments for colon cancer in humans.

DOI:http://dx.doi.org/10.7554/eLife.22706.002

Expression and putative role of mitochondrial transport proteins in cancer

Cancer cells undergo major changes in energy and biosynthetic metabolism. One of them is the Warburg effect, in which pyruvate is used for fermentation rather for oxidative phosphorylation. Another major one is their increased reliance on glutamine, which helps to replenish the pool of Krebs cycle metabolites used for other purposes, such as amino acid or lipid biosynthesis. Mitochondria are central to these alterations, as the biochemical pathways linking these processes run through these organelles. Two membranes, an outer and inner membrane, surround mitochondria, the latter being impermeable to most organic compounds. Therefore, a large number of transport proteins are needed to link the biochemical pathways of the cytosol and mitochondrial matrix. Since the transport steps are relatively slow, it is expected that many of these transport steps are altered when cells become cancerous. In this review, changes in expression and regulation of these transport proteins are discussed as well as the role of the transported substrates. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

In situ lactate dehydrogenase activity: a novel renal cortical imaging biomarker of tubular injury?

Renal ischemia-reperfusion injury is the state of which a tissue experiences injury after a phase of restrictive blood supply and recirculation. Ischemia-reperfusion injury (I/R-I) is a leading cause of acute kidney injury (AKI) in several disease states, including kidney transplantation, sepsis, and hypovolemic shock. The most common methods to evaluate AKI are creatinine clearance, plasma creatinine, blood urea nitrogen, or renal histology. However, currently, there are no precise methods to directly assess renal injury state noninvasively. Hyperpolarized 13C-pyruvate MRI enables noninvasive accurate quantification of the in vivo conversion of pyruvate to lactate, alanine, and bicarbonate. In the present study, we investigated the in situ alterations of metabolic conversion of pyruvate to lactate, alanine, and bicarbonate in a unilateral I/R-I rat model with 30 min and 60 min of ischemia followed by 24 h of reperfusion. The pyruvate conversion was unaltered compared with sham in the 30 min I/R-I group, while a significant reduced metabolic conversion was found in the postischemic kidney after 60 min of ischemia. This indicates that after 30 min of ischemia, the kidney maintains normal metabolic function in spite of decreased kidney function, whereas the postischemic kidney after 60 min of ischemia show a generally reduced metabolic enzyme activity concomitant with a reduced kidney function. We have confidence that these findings can have a high prognostic value in prediction of kidney injury and the outcome of renal injury.

Decreased Mitochondrial Pyruvate Transport Activity in the Diabetic Heart: ROLE OF MITOCHONDRIAL PYRUVATE CARRIER 2 (MPC2) ACETYLATION

Alterations in mitochondrial function contribute to diabetic cardiomyopathy. We have previously shown that heart mitochondrial proteins are hyperacetylated in OVE26 mice, a transgenic model of type 1 diabetes. However, the universality of this modification and its functional consequences are not well established. In this study, we demonstrate that Akita type 1 diabetic mice exhibit hyperacetylation. Functionally, isolated Akita heart mitochondria have significantly impaired maximal (state 3) respiration with physiological pyruvate (0.1 mm) but not with 1.0 mm pyruvate. In contrast, pyruvate dehydrogenase activity is significantly decreased regardless of the pyruvate concentration. We found that there is a 70% decrease in the rate of pyruvate transport in Akita heart mitochondria but no decrease in the mitochondrial pyruvate carriers 1 and 2 (MPC1 and MPC2). The potential role of hyperacetylation in mediating this impaired pyruvate uptake was examined. The treatment of control mitochondria with the acetylating agent acetic anhydride inhibits pyruvate uptake and pyruvate-supported respiration in a similar manner to the pyruvate transport inhibitor α-cyano-4-hydroxycinnamate. A mass spectrometry selective reactive monitoring assay was developed and used to determine that acetylation of lysines 19 and 26 of MPC2 is enhanced in Akita heart mitochondria. Expression of a double acetylation mimic of MPC2 (K19Q/K26Q) in H9c2 cells was sufficient to decrease the maximal cellular oxygen consumption rate. This study supports the conclusion that deficient pyruvate transport activity, mediated in part by acetylation of MPC2, is a contributor to metabolic inflexibility in the diabetic heart.

Value of pH regulators in the diagnosis, prognosis and treatment of cancer

Altered metabolism, associated with acidification of the extracellular milieu, is one of the major features of cancer. As pH regulation is crucial for the maintenance of all biological functions, cancer cells rely on the activity of lactate exporters and proton transporters to regulate their intracellular pH. The major players in cancer pH regulation are proton pump ATPases, sodium-proton exchangers (NHEs), monocarboxylate transporters (MCTs), carbonic anhydrases (CAs) and anion exchangers (AEs), which have been shown to be upregulated in several human malignancies. Thanks to the activity of the proton pumps and transporters, tumours acidify their microenvironment, becoming more aggressive and resistant to therapy. Thus, targeting tumour pH may contribute to more effective anticancer strategies for controlling tumour progression and therapeutic resistance. In the present study, we review the role of the main pH regulators expressed in human cancer cells, including their diagnostic and prognostic value, as well as their usefulness as therapeutic targets.

Malnutrition-associated liver steatosis and ATP depletion is caused by peroxisomal and mitochondrial dysfunction

BACKGROUND & AIMS

Severe malnutrition in young children is associated with signs of hepatic dysfunction such as steatosis and hypoalbuminemia, but its etiology is unknown. Peroxisomes and mitochondria play key roles in various hepatic metabolic functions including lipid metabolism and energy production. To investigate the involvement of these organelles in the mechanisms underlying malnutrition-induced hepatic dysfunction we developed a rat model of malnutrition.

METHODS

Weanling rats were placed on a low protein or control diet (5% or 20% of calories from protein, respectively) for four weeks. Peroxisomal and mitochondrial structural features were characterized using immunofluorescence and electron microscopy. Mitochondrial function was assessed using high-resolution respirometry. A novel targeted quantitative proteomics method was applied to analyze 47 mitochondrial proteins involved in oxidative phosphorylation, tricarboxylic acid cycle and fatty acid β-oxidation pathways.

RESULTS

Low protein diet-fed rats developed hypoalbuminemia and hepatic steatosis, consistent with the human phenotype. Hepatic peroxisome content was decreased and metabolomic analysis indicated peroxisomal dysfunction. This was followed by changes in mitochondrial ultrastructure and increased mitochondrial content. Mitochondrial function was impaired due to multiple defects affecting respiratory chain complex I and IV, pyruvate uptake and several β-oxidation enzymes, leading to strongly reduced hepatic ATP levels. Fenofibrate supplementation restored hepatic peroxisome abundance and increased mitochondrial β-oxidation capacity, resulting in reduced steatosis and normalization of ATP and plasma albumin levels.

CONCLUSIONS

Malnutrition leads to severe impairments in hepatic peroxisomal and mitochondrial function, and hepatic metabolic dysfunction. We discuss the potential future implications of our findings for the clinical management of malnourished children.

LAY SUMMARY

Severe malnutrition in children is associated with metabolic disturbances that are poorly understood. In order to study this further, we developed a malnutrition animal model and found that severe malnutrition leads to an impaired function of liver mitochondria which are essential for energy production and a loss of peroxisomes, which are important for normal liver metabolic function.

MPC1-like Is a Placental Mammal-specific Mitochondrial Pyruvate Carrier Subunit Expressed in Postmeiotic Male Germ Cells

Selective transport of pyruvate across the inner mitochondrial membrane by the mitochondrial pyruvate carrier (MPC) is a fundamental step that couples cytosolic and mitochondrial metabolism. The recent molecular identification of the MPC complex has revealed two interacting subunits, MPC1 and MPC2. Although in yeast, an additional subunit, MPC3, can functionally replace MPC2, no alternative MPC subunits have been described in higher eukaryotes. Here, we report for the first time the existence of a novel MPC subunit termed MPC1-like (MPC1L), which is present uniquely in placental mammals. MPC1L shares high sequence, structural, and topological homology with MPC1. In addition, we provide several lines of evidence to show that MPC1L is functionally equivalent to MPC1: 1) when co-expressed with MPC2, it rescues pyruvate import in a MPC-deleted yeast strain; 2) in mammalian cells, it can associate with MPC2 to form a functional carrier as assessed by bioluminescence resonance energy transfer; 3) in MPC1 depleted mouse embryonic fibroblasts, MPC1L rescues the loss of pyruvate-driven respiration and stabilizes MPC2 expression; and 4) MPC1- and MPC1L-mediated pyruvate imports show similar efficiency. However, we show that MPC1L has a highly specific expression pattern and is localized almost exclusively in testis and more specifically in postmeiotic spermatids and sperm cells. This is in marked contrast to MPC1/MPC2, which are ubiquitously expressed throughout the organism. To date, the biological importance of this alternative MPC complex during spermatogenesis in placental mammals remains unknown. Nevertheless, these findings open up new avenues for investigating the structure-function relationship within the MPC complex.

Mitochondrial pyruvate carrier function and cancer metabolism

Metabolic reprogramming in cancer supports the increased biosynthesis required for unchecked proliferation. Increased glucose utilization is a defining feature of many cancers that is accompanied by altered pyruvate partitioning and mitochondrial metabolism. Cancer cells also require mitochondrial tricarboxylic acid cycle activity and electron transport chain function for biosynthetic competency and proliferation. Recent evidence demonstrates that mitochondrial pyruvate carrier (MPC) function is abnormal in some cancers and that increasing MPC activity may decrease cancer proliferation. Here we examine recent findings on MPC function and cancer metabolism. Special emphasis is placed on the compartmentalization of pyruvate metabolism and the alternative routes of metabolism that maintain the cellular biosynthetic pools required for unrestrained proliferation in cancer.

The regulation of neuronal mitochondrial metabolism by calcium

Calcium signalling is fundamental to the function of the nervous system, in association with changes in ionic gradients across the membrane. Although restoring ionic gradients is energetically costly, a rise in intracellular Ca(2+) acts through multiple pathways to increase ATP synthesis, matching energy supply to demand. Increasing cytosolic Ca(2+) stimulates metabolite transfer across the inner mitochondrial membrane through activation of Ca(2+) -regulated mitochondrial carriers, whereas an increase in matrix Ca(2+) stimulates the citric acid cycle and ATP synthase. The aspartate-glutamate exchanger Aralar/AGC1 (Slc25a12), a component of the malate-aspartate shuttle (MAS), is stimulated by modest increases in cytosolic Ca(2+) and upregulates respiration in cortical neurons by enhancing pyruvate supply into mitochondria. Failure to increase respiration in response to small (carbachol) and moderate (K(+) -depolarization) workloads and blunted stimulation of respiration in response to high workloads (veratridine) in Aralar/AGC1 knockout neurons reflect impaired MAS activity and limited mitochondrial pyruvate supply. In response to large workloads (veratridine), acute stimulation of respiration occurs in the absence of MAS through Ca(2+) influx through the mitochondrial calcium uniporter (MCU) and a rise in matrix [Ca(2+) ]. Although the physiological importance of the MCU complex in work-induced stimulation of respiration of CNS neurons is not yet clarified, abnormal mitochondrial Ca(2+) signalling causes pathology. Indeed, loss of function mutations in MICU1, a regulator of MCU complex, are associated with neuromuscular disease. In patient-derived MICU1 deficient fibroblasts, resting matrix Ca(2+) is increased and mitochondria fragmented. Thus, the fine tuning of Ca(2+) signals plays a key role in shaping mitochondrial bioenergetics.

The mitochondrial pyruvate carrier in health and disease: To carry or not to carry?

Mitochondria play a key role in energy metabolism, hosting the machinery for oxidative phosphorylation, the most efficient cellular pathway for generating ATP. A major checkpoint in this process is the transport of pyruvate produced by cytosolic glycolysis into the mitochondrial matrix, which is accomplished by the recently identified mitochondrial pyruvate carrier (MPC). As the gatekeeper for pyruvate entry into mitochondria, the MPC is thought to be of fundamental importance in establishing the metabolic programming of a cell. This is especially relevant in the context of the aerobic glycolysis, also known as the Warburg effect, which is a hallmark in many types of cancer, and MPC loss of function promotes cancer growth. Moreover, mitochondrial pyruvate uptake is needed for efficient hepatic gluconeogenesis and the regulation of blood glucose levels. In this review we discuss recent advances in our knowledge of the MPC, and we argue that it may offer a promising target in diseases like cancer and type 2 diabetes. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Identification of active elementary flux modes in mitochondria using selectively permeabilized CHO cells

Metabolic compartmentation is a key feature of mammalian cells. Mitochondria are the powerhouse of eukaryotic cells, responsible for respiration and the TCA cycle. We accessed the mitochondrial metabolism of the economically important Chinese hamster ovary (CHO) cells using selective permeabilization. We tested key substrates without and with addition of ADP. Based on quantified uptake and production rates, we could determine the contribution of different elementary flux modes to the metabolism of a substrate or substrate combination. ADP stimulated the uptake of most metabolites, directly by serving as substrate for the respiratory chain, thus removing the inhibitory effect of NADH, or as allosteric effector. Addition of ADP favored substrate metabolization to CO2 and did not enhance the production of other metabolites. The controlling effect of ADP was more pronounced when we supplied metabolites to the first part of the TCA cycle: pyruvate, citrate, α-ketoglutarate and glutamine. In the second part of the TCA cycle, the rates were primarily controlled by the concentrations of C4-dicarboxylates. Without ADP addition, the activity of the pyruvate carboxylase-malate dehydrogenase-malic enzyme cycle consumed the ATP produced by oxidative phosphorylation, preventing its accumulation and maintaining metabolic steady state conditions. Aspartate was taken up only in combination with pyruvate, whose uptake also increased, a fact explained by complex regulatory effects. Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase were identified as the key regulators of the TCA cycle, confirming existent knowledge from other cells. We have shown that selectively permeabilized cells combined with elementary mode analysis allow in-depth studying of the mitochondrial metabolism and regulation.

Role of host cell-derived amino acids in nutrition of intracellular Salmonella enterica

The facultative intracellular pathogen Salmonella enterica resides in a specific membrane-bound compartment termed the Salmonella-containing vacuole (SCV). Despite being segregated from access to metabolites in the host cell cytosol, Salmonella is able to efficiently proliferate within the SCV. We set out to unravel the nutritional supply of Salmonella in the SCV with focus on amino acids. We studied the availability of amino acids by the generation of auxotrophic strains for alanine, asparagine, aspartate, glutamine, and proline in a macrophage cell line (RAW264.7) and an epithelial cell line (HeLa) and examined access to extracellular nutrients for nutrition. Auxotrophies for alanine, asparagine, or proline attenuated intracellular replication in HeLa cells, while aspartate, asparagine, or proline auxotrophies attenuated intracellular replication in RAW264.7 macrophages. The different patterns of intracellular attenuation of alanine- or aspartate-auxotrophic strains support distinct nutritional conditions in HeLa cells and RAW264.7 macrophages. Supplementation of medium with individual amino acids restored the intracellular replication of mutant strains auxotrophic for asparagine, proline, or glutamine. Similarly, a mutant strain deficient in succinate dehydrogenase was complemented by the extracellular addition of succinate. Complementation of the intracellular replication of auxotrophic Salmonella by external amino acids was possible if bacteria were proficient in the induction of Salmonella-induced filaments (SIFs) but failed in a SIF-deficient background. We propose that the ability of intracellular Salmonella to redirect host cell vesicular transport provides access of amino acids to auxotrophic strains and, more generally, is essential to continuously supply bacteria within the SCV with nutrients.

DLAT subunit of the pyruvate dehydrogenase complex is upregulated in gastric cancer-implications in cancer therapy

An iTRAQ-based tandem mass spectrometry approach was employed to relatively quantify proteins in the membrane proteome of eleven gastric cancer cell lines relative to a denominator non-cancer gastric epithelial cell line HFE145. Of the 882 proteins detected, 57 proteins were found to be upregulated with > 1.3-fold change in at least 6 of the 11 cell lines. Bioinformatics analysis revealed that these proteins are significantly associated with cancer, cell growth and proliferation, death, survival and cell movement. The catalogue of membrane proteins presented that are potential regulators/effectors of gastric cancer progression has implications in cancer therapy. DLAT, a subunit of the pyruvate dehydrogenase complex, was selected as a candidate protein for further studies as its function in gastric cancer has yet to be established. SiRNA studies supported a role of DLAT in gastric cancer cell proliferation and carbohydrate metabolism, reprogramming of which is a hallmark of cancer. Our study contributes to recent interest and discussion in cancer energetics and related phenomena such as the Warburg and Reverse Warburg effects. Future mechanistic studies should lead to the elucidation of the mode of action of DLAT in human gastric cancer and establish DLAT as a viable drug target.

Deregulation of energetic metabolism in the clear cell renal cell carcinoma: A multiple pathway analysis based on microarray profiling

Clear cell renal cell carcinoma (ccRCC) is the most frequent type of kidney cancer. In order to better understand the biology of ccRCC, we accomplished the gene profiling of fresh tissue specimens from 11 patients with the renal tumors (9 ccRCCs, 1 oncocytoma and 1 renal B-lymphoma), in which the tumor-related data were compared to the paired healthy kidney tissues from the same patients. All ccRCCs exhibited a considerably elevated transcription of the gene coding for carbonic anhydrase IX (CAIX). Moreover, the ccRCC tumors consistently displayed increased expression of genes encoding the glycolytic pathway enzymes, e.g. hexokinase II (HK2) and lactate dehydrogenase A (LDHA) and a decreased expression of genes for the mitochondrial electron transport chain components, indicating an overall reprogramming of the energetic metabolism in this tumor type. This appears to be accompanied by altered expression of the genes of the pH regulating machinery, including ion and lactate transporters. Immunohistochemical staining of tumor tissue sections confirmed the increased expression of CAIX, HK2 and LDHA in ccRCC, validating the microarray data and supporting their potential as the energetic metabolism-related biomarkers of the ccRCC.

Mitochondrial pyruvate transport: a historical perspective and future research directions

Pyruvate is the end-product of glycolysis, a major substrate for oxidative metabolism, and a branching point for glucose, lactate, fatty acid and amino acid synthesis. The mitochondrial enzymes that metabolize pyruvate are physically separated from cytosolic pyruvate pools and rely on a membrane transport system to shuttle pyruvate across the impermeable inner mitochondrial membrane (IMM). Despite long-standing acceptance that transport of pyruvate into the mitochondrial matrix by a carrier-mediated process is required for the bulk of its metabolism, it has taken almost 40 years to determine the molecular identity of an IMM pyruvate carrier. Our current understanding is that two proteins, mitochondrial pyruvate carriers MPC1 and MPC2, form a hetero-oligomeric complex in the IMM to facilitate pyruvate transport. This step is required for mitochondrial pyruvate oxidation and carboxylation-critical reactions in intermediary metabolism that are dysregulated in several common diseases. The identification of these transporter constituents opens the door to the identification of novel compounds that modulate MPC activity, with potential utility for treating diabetes, cardiovascular disease, cancer, neurodegenerative diseases, and other common causes of morbidity and mortality. The purpose of the present review is to detail the historical, current and future research investigations concerning mitochondrial pyruvate transport, and discuss the possible consequences of altered pyruvate transport in various metabolic tissues.

Alterations in cancer cell metabolism: the Warburg effect and metabolic adaptation

The Warburg effect means higher glucose uptake of cancer cells compared to normal tissues, whereas a smaller fraction of this glucose is employed for oxidative phosphorylation. With the advent of high throughput technologies and computational systems biology, cancer cell metabolism has been reinvestigated over the last decades toward identifying various events underlying "how" and "why" a cancer cell employs aerobic glycolysis. Significant progress has been shaped to revise the Warburg effect. In this study, we have integrated the gene expression of 13 different cancer cells with the genome-scale metabolic network of human (Recon1) based on the E-Flux method, and analyzed them based on constraint-based modeling. Results show that regardless of significant up- and down-regulated metabolic genes, the distribution of metabolic changes is similar in different cancer types. These findings support the theory that the Warburg effect is a consequence of metabolic adaptation in cancer cells.

Phenylalanine sensitive K562-D cells for the analysis of the biochemical impact of excess amino acid

Although it is recognized that the abnormal accumulation of amino acid is a cause of the symptoms in metabolic disease such as phenylketonuria (PKU), the relationship between disease severity and serum amino acid levels is not well understood due to the lack of experimental model. Here, we present a novel in vitro cellular model using K562-D cells that proliferate slowly in the presence of excessive amount of phenylalanine within the clinically observed range, but not phenylpyruvate. The increased expression of the L-type amino acid transporter (LAT2) and its adapter protein 4F2 heavy chain appeared to be responsible for the higher sensitivity to phenylalanine in K562-D cells. Supplementation with valine over phenylalanine effectively restored cell proliferation, although other amino acids did not improve K562-D cell proliferation over phenylalanine. Biochemical analysis revealed mammalian target of rapamycin complex (mTORC) as a terminal target of phenylalanine in K562-D cell proliferation, and supplementation of valine restored mTORC1 activity. Our results show that K562-D cell can be a potent tool for the investigation of PKU at the molecular level and to explore new therapeutic approaches to the disease.