Fatty acid oxidation alleviates the energy deficiency caused by the loss of MPC1 in MPC1+/- mice

Study on the Neuroprotective Effects of Eight Iridoid Components Using Cell Metabolomics

Iridoid components have been reported to have significant neuroprotective effects. However, it is not yet clear whether the efficacy and mechanisms of iridoid components with similar structures are also similar. This study aimed to compare the neuroprotective effects and mechanisms of eight iridoid components (catalpol (CAT), genipin (GE), geniposide (GEN), geniposidic acid (GPA), aucubin (AU), ajugol (AJU), rehmannioside C (RC), and rehmannioside D (RD)) based on corticosterone (CORT)-induced injury in PC12 cells. PC12 cells were randomly divided into a normal control group (NC), model group (M), positive drug group (FLX), and eight iridoid administration groups. Firstly, PC12 cells were induced with CORT to simulate neuronal injury. Then, the MTT method and flow cytometry were applied to evaluate the protective effects of eight iridoid components on PC12 cell damage. Thirdly, a cell metabolomics study based on ultra-performance liquid chromatography-quadrupole-time-of-flight mass spectrometry (UPLC-Q/TOF-MS) was performed to explore changes in relevant biomarkers and metabolic pathways following the intervention of administration. The MTT assay and flow cytometry analysis showed that the eight iridoid components can improve cell viability, inhibit cell apoptosis, reduce intracellular ROS levels, and elevate MMP levels. In the PCA score plots, the sample points of the treatment groups showed a trend towards approaching the NC group. Among them, AU, AJU, and RC had a weaker effect. There were 38 metabolites (19 metabolites each in positive and negative ion modes, respectively) identified as potential biomarkers during the experiment, among which 23 metabolites were common biomarkers of the eight iridoid groups. Pathway enrichment analysis revealed that the eight iridoid components regulated the metabolism mainly in relation to D-glutamine and D-glutamate metabolism, arginine biosynthesis, the TCA cycle, purine metabolism, and glutathione metabolism. In conclusion, the eight iridoid components could reverse an imbalanced metabolic state by regulating amino acid neurotransmitters, interfering with amino acid metabolism and energy metabolism, and harmonizing the level of oxidized substances to exhibit neuroprotective effects.

Mitochondrial pyruvate carrier 1 regulates fatty acid synthase lactylation and mediates treatment of nonalcoholic fatty liver disease

BACKGROUND AND AIMS

NAFLD has become a major metabolic disease worldwide. A few studies have reported the potential relationship between mitochondrial pyruvate carrier 1 (MPC1) and inflammation, fibrosis, and insulin sensitivity in obese or NASH mouse models. However, the impact of MPC1 on NAFLD-related liver lipid metabolism and its role in the NAFLD progression require further investigation.

APPROACH AND RESULTS

MPC1 expression was measured in liver tissues from normal controls and patients with NAFLD. We characterized the metabolic phenotypes and expression of genes involved in hepatic lipid accumulation in MPC1 systemic heterozygous knockout (MPC1 +/- ) mice. Hepatic protein lactylation was detected using Tandem Mass Tags proteomics and verified by the overexpression of lactylation mutants in cells. Finally, the effect of MPC1 inhibition on liver inflammation was examined in mice and AML-12 cells. Here, we found that MPC1 expression was positively correlated to liver lipid deposition in patients with NAFLD. MPC1 +/- mice fed with high-fat diet had reduced hepatic lipid accumulation but no change in the expression of lipid synthesis-related genes. MPC1 knockout affected the lactylation of several proteins, especially fatty acid synthase, through the regulation of lactate levels in hepatocytes. Lactylation at the K673 site of fatty acid synthase inhibited fatty acid synthase activity, which mediated the downregulation of liver lipid accumulation by MPC1. Moreover, although MPC1 knockout caused lactate accumulation, inflammation level was controlled because of mitochondrial protection and macrophage polarization.

CONCLUSIONS

In NAFLD, MPC1 levels are positively correlated with hepatic lipid deposition; the enhanced lactylation at fatty acid synthase K673 site may be a downstream mechanism.

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.

Mitochondrial pyruvate carrier 1 alleviates hypoxic-ischemic brain injury in rats

AIMS

Mitochondrial dysfunction is a critical pathological change in cerebral ischemia. Mitochondrial pyruvate carrier 1 (MPC1) is a mitochondrial inner membrane protein carrier participating in pyruvate transport. The work is aiming to figure out the effect of MPC1 on cerebral ischemia.

MAIN METHODS

Bilateral internal carotid artery embolization (BICAO) rats model and cells model from oxygen glucose deprivation/reoxygenation (OGD/R) were used to simulate cerebral ischemia in vivo and in vitro. The effect of MPC1 on cerebral ischemia was detected by imaging, behavioral test, immunofluorescence, flow cytometry, transmission electron microscopy, Western blot and RT-Q-PCR. RNA-sequence (RNA-seq) was applied to explore the potential molecular mechanisms underlying the role of MPC1 in cerebral ischemia.

KEY FINDING

After BICAO or OGD/R treatment, MPC1 expression in ischemic cortical neurons was significantly decreased, and MPC1 deficiency significantly reduced cerebral blood flow, decreased locomotion activities, and exacerbated neuronal injury. Moreover, MPC1 deficiency obviously aggravated oxidative stress, structural disruption and dysfunction of mitochondria, autophagy and calcium overload of ischemic cortical neurons. Interestingly, MPC1 overexpression remarkably reversed neuronal loss and persisting neuronal deficits induced by OGD. Using RNA-seq, 38 MPC1-associated differentially expressed genes were involved in oxidative stress, autophagy and calcium overload. Our results further confirmed that MPC1 could alleviate autophagy via the PI3K/Akt/mTOR pathway in the ischemic cortical neurons.

SIGNIFICANCE

MPC1 may exert neuroprotective effects by attenuating oxidative stress, mitochondrial dysfunction, calcium overload and autophagy during cerebral ischemia. MPC1-related genes identified by RNA-seq may be a novel therapeutic target for cerebral ischemia.

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.

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.

The role of SAMM50 in non-alcoholic fatty liver disease: from genetics to mechanisms

Non‐alcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid accumulation. SAMM50 encodes Sam50, a mitochondrial outer membrane protein involved in the removal of reactive oxygen species, mitochondrial morphology and regulation of mitophagy. Certain single nucleotide polymorphisms of SAMM50 have been reported to be correlated with NAFLD. However, the contribution of SAMM50 polymorphisms to the occurrence and severity of fatty liver in the Chinese Han cohort has rarely been reported. Here, we investigated the association between SAMM50 polymorphisms (rs738491 and rs2073082) and NAFLD in a Chinese Han cohort, as well as the mechanistic basis of this association. Clinical information and blood samples were collected from 380 NAFLD cases and 380 normal subjects for the detection of genotypes and biochemical parameters. Carriers of the rs738491 T allele or rs2073082 G allele of SAMM50 exhibit increased susceptibility to NAFLD [odds ratio (OR) = 1.39; 95% confidence interval (CI) = 1.14–1.71, P = 0.001; OR = 1.31; 95% CI = 1.05–1.62, P = 0.016, respectively] and are correlated with elevated serum triglyceride, alanine aminotransferase and aspartate aminotransferase levels. The presence of the T allele (TT + CT) of rs738491 (P < 0.01) or G allele (AG + GG) of rs2073082 (P = 0.03) is correlated with the severity of fatty liver in the NAFLD cohort. In vitro studies indicated that SAMM50 gene polymorphisms decrease its expression and SAMM50 deficiency results in increased lipid accumulation as a result of a decrease in fatty acid oxidation. Overexpression of SAMM50 enhances fatty acid oxidation and mitigates intracellular lipid accumulation. Our results confirm the association between the SAMM50 rs738491 and rs2073082 polymorphisms and the risk of fatty liver in a Chinese cohort. The underlying mechanism may be related to decreased fatty acid oxidation caused by SAMM50 deficiency.

Mitochondrial pyruvate carrier as a key regulator of fever and neuroinflammation

The mitochondrial pyruvate carrier (MPC) is an inner-membrane transporter that facilitates pyruvate uptake from the cytoplasm into mitochondria. We previously reported that MPC1 protein levels increase in the hypothalamus of animals during fever induced by lipopolysaccharide (LPS), but how this increase contributes to the LPS responses remains to be studied. Therefore, we investigated the effect of UK 5099, a classical MPC inhibitor, in a rat model of fever, on hypothalamic mitochondrial function and neuroinflammation in LPS-stimulated preoptic area (POA) primary microcultures. Intracerebroventricular administration of UK 5099 reduced the LPS-induced fever. High-resolution respirometry revealed an increase in oxygen consumption and oxygen flux related to ATP synthesis in the hypothalamic homogenate from LPS-treated animals linked to mitochondrial complex I plus II. Preincubation with UK 5099 prevented the LPS-induced increase in oxygen consumption, ATP synthesis and spare capacity only in complex I-linked respiration and reduced mitochondrial H₂O₂ production. In addition, treatment of rat POA microcultures with UK 5099 reduced the secretion of the proinflammatory and pyrogenic cytokines TNFα and IL-6 as well as the immunoreactivity of inflammatory transcription factors NF-κB and NF-IL6 four hours after LPS stimulation. These results suggest that the regulation of mitochondrial pyruvate metabolism through MPC inhibition may be effective in reducing neuroinflammation and fever.

Blocking mitochondrial pyruvate import in brown adipocytes induces energy wasting via lipid cycling

Combined fatty acid esterification and lipolysis, termed lipid cycling, is an ATP‐consuming process that contributes to energy expenditure. Therefore, interventions that stimulate energy expenditure through lipid cycling are of great interest. Here we find that pharmacological and genetic inhibition of the mitochondrial pyruvate carrier (MPC) in brown adipocytes activates lipid cycling and energy expenditure, even in the absence of adrenergic stimulation. We show that the resulting increase in ATP demand elevates mitochondrial respiration coupled to ATP synthesis and fueled by lipid oxidation. We identify that glutamine consumption and the Malate‐Aspartate Shuttle are required for the increase in Energy Expenditure induced by MPC inhibition in Brown Adipocytes (MAShEEBA). We thus demonstrate that energy expenditure through enhanced lipid cycling can be activated in brown adipocytes by decreasing mitochondrial pyruvate availability. We present a new mechanism to increase energy expenditure and fat oxidation in brown adipocytes, which does not require adrenergic stimulation of mitochondrial uncoupling.

Chronic cold exposure enhances glucose oxidation in brown adipose tissue

The cultured brown adipocytes can oxidize glucose in vitro, but it is still not fully clear whether brown adipose tissue (BAT) could completely oxidize glucose in vivo. Although positron emission tomography (PET) with ¹⁸F‐fluorodeoxyglucose (¹⁸F‐FDG) showed a high level of glucose uptake in the activated BAT, the non‐metabolizable ¹⁸F‐FDG cannot fully demonstrate intracellular glucose metabolism. Through in vivo [U‐¹³C]glucose tracing, here we show that chronic cold exposure dramatically activates glucose oxidation in BAT and the browning/beiging subcutaneous white adipose tissue (sWAT). Specifically, chronic cold exposure enhances glucose flux into the mitochondrial TCA cycle. Metabolic flux analysis models that β3‐adrenergic receptor (β3‐AR) agonist significantly enhances the flux of mitochondrial pyruvate uptake through mitochondrial pyruvate carrier (MPC) in the differentiated primary brown adipocytes. Furthermore, in vivo MPC inhibition blocks cold‐induced glucose oxidation and impairs body temperature maintenance in mice. Together, mitochondrial pyruvate uptake and oxidation serve an important energy source in the chronic cold exposure activated BAT and beige adipose tissue, which supports a role for glucose oxidation in brown fat thermogenesis.

The Role of Pi, Glutamine and the Essential Amino Acids in Modulating the Metabolism in Diabetes and Cancer

Purpose

Re-examine the current metabolic models.

Methods

Review of literature and gene networks.

Results

Insulin activates Pi uptake, glutamine metabolism to stabilise lipid membranes. Tissue turnover maintains the metabolic health. Current model of intermediary metabolism (IM) suggests glucose is the source of energy, and anaplerotic entry of fatty acids and amino acids into mitochondria increases the oxidative capacity of the TCA cycle to produce the energy (ATP). The reduced cofactors, NADH and FADH2, have different roles in regulating the oxidation of nutrients, membrane potentials and biosynthesis. Trans-hydrogenation of NADH to NADPH activates the biosynthesis. FADH2 sustains the membrane potential during the cell transformations. Glycolytic enzymes assume the non-canonical moonlighting functions, enter the nucleus to remodel the genetic programmes to affect the tissue turnover for efficient use of nutrients. Glycosylation of the CD98 (4F2HC) stabilises the nutrient transporters and regulates the entry of cysteine, glutamine and BCAA into the cells. A reciprocal relationship between the leucine and glutamine entry into cells regulates the cholesterol and fatty acid synthesis and homeostasis in cells. Insulin promotes the Pi transport from the blood to tissues, activates the mitochondrial respiratory activity, and glutamine metabolism, which activates the synthesis of cholesterol and the de novo fatty acids for reorganising and stabilising the lipid membranes for nutrient transport and signal transduction in response to fluctuations in the microenvironmental cues. Fatty acids provide the lipid metabolites, activate the second messengers and protein kinases. Insulin resistance suppresses the lipid raft formation and the mitotic slippage activates the fibrosis and slow death pathways.

Mitochondrial Pyruvate Carrier Function in Health and Disease across the Lifespan

As a nodal mediator of pyruvate metabolism, the mitochondrial pyruvate carrier (MPC) plays a pivotal role in many physiological and pathological processes across the human lifespan, from embryonic development to aging-associated neurodegeneration. Emerging research highlights the importance of the MPC in diverse conditions, such as immune cell activation, cancer cell stemness, and dopamine production in Parkinson’s disease models. Whether MPC function ameliorates or contributes to disease is highly specific to tissue and cell type. Cell- and tissue-specific differences in MPC content and activity suggest that MPC function is tightly regulated as a mechanism of metabolic, cellular, and organismal control. Accordingly, recent studies on cancer and diabetes have identified protein–protein interactions, post-translational processes, and transcriptional factors that modulate MPC function. This growing body of literature demonstrates that the MPC and other mitochondrial carriers comprise a versatile and dynamic network undergirding the metabolism of health and disease.

Characteristic Analysis of Homo- and Heterodimeric Complexes of Human Mitochondrial Pyruvate Carrier Related to Metabolic Diseases

Human mitochondrial pyruvate carriers (hMPCs), which are required for the uptake of pyruvate into mitochondria, are associated with several metabolic diseases, including type 2 diabetes and various cancers. Yeast MPC was recently demonstrated to form a functional unit of heterodimers. However, human MPC-1 (hMPC-1) and MPC-2 (hMPC-2) have not yet been individually isolated for their detailed characterization, in particular in terms of their structural and functional properties, namely, whether they exist as homo- or heterodimers. In this study, hMPC-1 and hMPC-2 were successfully isolated in micelles and they formed stable homodimers. However, the heterodimer state was found to be dominant when both hMPC-1 and hMPC-2 were present. In addition, as heterodimers, the molecules exhibited a higher binding capacity to both substrates and inhibitors, together with a larger structural stability than when they existed as homodimers. Taken together, our results demonstrated that the hetero-dimerization of hMPCs is the main functional unit of the pyruvate metabolism, providing a structural insight into the transport mechanisms of hMPCs.

Mitochondrial Pyruvate Carriers are not Required for Adipogenesis but are Regulated by High-Fat Feeding in Brown Adipose Tissue

OBJECTIVE

The objectives of this study were to assess the role of mitochondrial pyruvate carriers (MPCs) in adipocyte development in vitro and determine whether MPCs are regulated in vivo by high-fat feeding in male and female C57BL/6J mice.

METHODS

This study utilized small interfering RNA-mediated knockdown to assess the requirement of MPC1 for adipogenesis in the 3T3-L1 model system. Treatment with UK-5099, a potent pharmacological MPC inhibitor, was also used to assess the loss of MPC activity. Western blot analysis was performed on adipose tissue samples from mice on a low-fat diet or a high-fat diet (HFD) for 12 weeks.

RESULTS

The loss of MPC expression via small interfering RNA-mediated knockdown or pharmacological inhibition did not affect adipogenesis of 3T3-L1 cells. In vivo studies indicated that expression of MPCs was significantly decreased in brown adipose tissue of male mice, but not female, on an HFD.

CONCLUSIONS

Although MPCs are essential for pyruvate transport, MPCs are not required for adipogenesis in vitro, suggesting that other substrates can be used for energy production when the MPC complex is not functional. Also, a significant decrease in MPC1 and 2 expression occurred in brown fat, but not white fat, of male mice fed an HFD.

Maternal Lipid Metabolism Directs Fetal Liver Programming following Nutrient Stress

The extreme metabolic demands of pregnancy require coordinated metabolic adaptations between mother and fetus to balance fetal growth and maternal health with nutrient availability. To determine maternal and fetal contributions to metabolic flexibility during gestation, pregnant mice with genetic impairments in mitochondrial carbohydrate and/or lipid metabolism were subjected to nutrient deprivation. The maternal fasting response initiates a fetal liver transcriptional program marked by upregulation of lipid- and peroxisome proliferator-activated receptor alpha (Pparα)-regulated genes. Impaired maternal lipid metabolism alters circulating lipid metabolite concentrations and enhances the fetal response to fasting, which is largely dependent on fetal Pparα. Maternal fasting also improves metabolic deficits in fetal carbohydrate metabolism by increasing the availability of alternative substrates. Impairment of both carbohydrate and lipid metabolism in pregnant dams further exacerbates the fetal liver transcriptional response to nutrient deprivation. Together, these data demonstrate a regulatory role for mitochondrial macronutrient metabolism in mediating maternal-fetal metabolic communication, particularly when nutrients are limited.

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.