Peroxisomal membrane monocarboxylate transporters: evidence for a redox shuttle system?

Bioenergetic myths of energy transduction in eukaryotic cells

The study of energy transduction in eukaryotic cells has been divided between Bioenergetics and Physiology, reflecting and contributing to a variety of Bioenergetic myths considered here: 1) ATP production = energy production, 2) energy transduction is confined to mitochondria (plus glycolysis and chloroplasts), 3) mitochondria only produce heat when required, 4) glycolysis is inefficient compared to mitochondria, and 5) mitochondria are the main source of reactive oxygen species (ROS) in cells. These myths constitute a 'mitocentric' view of the cell that is wrong or unbalanced. In reality, mitochondria are the main site of energy dissipation and heat production in cells, and this is an essential function of mitochondria in mammals. Energy transduction and ROS production occur throughout the cell, particularly the cytosol and plasma membrane, and all cell membranes act as two-dimensional energy conduits. Glycolysis is efficient, and produces less heat per ATP than mitochondria, which might explain its increased use in muscle and cancer cells.

Lactylation Modification in Cardiometabolic Disorders: Function and Mechanism

Cardiovascular disease (CVD) is recognized as the primary cause of mortality and morbidity on a global scale, and developing a clear treatment is an important tool for improving it. Cardiometabolic disorder (CMD) is a syndrome resulting from the combination of cardiovascular, endocrine, pro-thrombotic, and inflammatory health hazards. Due to their complex pathological mechanisms, there is a lack of effective diagnostic and treatment methods for cardiac metabolic disorders. Lactylation is a type of post-translational modification (PTM) that plays a regulatory role in various cellular physiological processes by inducing changes in the spatial conformation of proteins. Numerous studies have reported that lactylation modification plays a crucial role in post-translational modifications and is closely related to cardiac metabolic diseases. This article discusses the molecular biology of lactylation modifications and outlines the roles and mechanisms of lactylation modifications in cardiometabolic disorders, offering valuable insights for the diagnosis and treatment of such conditions.

Functional heterogeneity of MCT1 and MCT4 in metabolic reprogramming affects osteosarcoma growth and metastasis

BACKGROUND

Osteosarcoma is the most common primary malignant bone tumor in adolescents and children and prone to develop lung metastasis. Its prognosis has been virtually unimproved over the last few decades, especially in patients with metastases, who suffer from a dismal survival. Recently, increasing attention has been devoted to monocarboxylate transporters-related (MCTs) metabolic reprogramming. However, the role of MCT1 and MCT4 in osteosarcoma progression and the underlying mechanisms remain to be further elucidated.

METHODS

In this study, we established MCT1 and/or MCT4 knockout cell lines by CRISPR/Cas9 genome editing technology. Then, we assessed glycolysis and oxidative phosphorylation capacities by measuring lactate flux and oxygen consumption. We also performed flowcytometry to test circulating tumor cells and PET/CT to evaluate glucose uptake.

RESULTS

MCT1 was found to be involved in both glycolysis and oxidative respiration due to its ability to transport lactate in both directions. MCT1 inhibition significantly reduced circulating tumor cells and distant metastases partially by increasing oxidative stress. MCT4 was primarily related to glycolysis and responsible for lactate export when the concentration of extracellular lactate was high. MCT4 inhibition dramatically suppressed cell proliferation in vitro and impaired tumor growth with reduction of glucose uptake in vivo.

CONCLUSIONS

Our results demonstrate the functional heterogeneity and redundancy of MCT1 and MCT4 in glucose metabolism and tumor progression in osteosarcoma. Thus, combined inhibition of MCT1 and MCT4 may be a promising therapeutic strategy for treating tumors expressing both transporters.

Calcium in peroxisomes: An essential messenger in an essential cell organelle

Calcium is a central signal transduction element in biology. Peroxisomes are essential cellular organelles, yet calcium handling in peroxisomes has been contentious. Recent advances show that peroxisomes are part of calcium homeostasis in cardiac myocytes and therefore may contribute to or even shape their calcium-dependent functionality. However, the mechanisms of calcium movement between peroxisomes and other cellular sites and their mediators remain elusive. Here, we review calcium handling in peroxisomes in concert with other organelles and summarize the most recent knowledge on peroxisomal involvement in calcium dynamics with a focus on mammalian cells.

Conversion of Hyperpolarized [1-13C]Pyruvate in Breast Cancer Cells Depends on Their Malignancy, Metabolic Program and Nutrient Microenvironment

Hyperpolarized magnetic resonance spectroscopy (MRS) is a technology for characterizing tumors in vivo based on their metabolic activities. The conversion rates (k_pl) of hyperpolarized [1-¹³C]pyruvate to [1-¹³C]lactate depend on monocarboxylate transporters (MCT) and lactate dehydrogenase (LDH); these are also indicators of tumor malignancy. An unresolved issue is how glucose and glutamine availability in the tumor microenvironment affects metabolic characteristics of the cancer and how this relates to k_pl-values. Two breast cancer cells of different malignancy (MCF-7, MDA-MB-231) were cultured in media containing defined combinations of low glucose (1 mM; 2.5 mM) and glutamine (0.1 mM; 1 mM) and analyzed for pyruvate uptake, intracellular metabolite levels, LDH and pyruvate kinase activities, and ¹³C₆-glucose-derived metabolomics. The results show variability of k_pl with the different glucose/glutamine conditions, congruent with glycolytic activity, but not with LDH activity or the Warburg effect; this suggests metabolic compartmentation. Remarkably, k_pl-values were almost two-fold higher in MCF-7 than in the more malignant MDA-MB-231 cells, the latter showing a higher flux of ¹³C-glucose-derived pyruvate to the TCA-cycle metabolites ¹³C₂-citrate and ¹³C₃-malate, i.e., pyruvate decarboxylation and carboxylation, respectively. Thus, MRS with hyperpolarized [1-¹³C-pyruvate] is sensitive to both the metabolic program and the nutritional state of cancer cells.

Influence of the MCT1-T1470A polymorphism (rs1049434) on repeated sprint ability and blood lactate accumulation in elite football players: a pilot study

PURPOSE

The aim of this study is to investigate the influence of the MCT1 T1470A polymorphism (rs1049434) on repeated sprint ability (RSA) and lactate accumulation after RSA testing.

METHODS

Twenty-six elite Italian male football players (age: 17.7 ± 0.78 years; height: 179.2 ± 7.40 cm; weight: 72.1 ± 5.38 kg) performed RSA testing (6 × 30-m sprints with an active recovery between sprints), and lactate measurements were obtained at 1, 3, 5, 7, and 10 min post-exercise. Genotyping for the MCT1 T1470A polymorphism was performed using PCR.

RESULTS

Genotype distributions were in Hardy-Weinberg equilibrium, being 42% wildtype (A/A), 46% heterozygotes (T/A), and 12% mutated homozygotes (T/T). Significant differences between genotypic groups were found in the two final sprint times of the RSA test. Under a dominant model, carriers of the major A-allele (Glu-490) in the dominant model showed a significantly lower sprint time compared to footballers with the T/T (Asp/Asp) genotype (5th Sprint time: A/A + T/A = 4.60 s vs TT = 4.97 s, 95% CI 0.07-0.67, p = 0.022; 6th Sprint: A/A + T/A = 4.56 s vs T/T = 4.87 s, 95% CI 0.05-0.57, p = 0.033).

CONCLUSIONS

The T1470A (Glu490Asp) polymorphism of MCT1 was associated with RSA. Our findings suggest that the presence of the major A-allele (Glu-490) is favourable for RSA in football players.

Re-Evaluating the Oxidative Phenotype: Can Endurance Exercise Save the Western World?

Mitochondria are popularly called the "powerhouses" of the cell. They promote energy metabolism through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, which in contrast to cytosolic glycolysis are oxygen-dependent and significantly more substrate efficient. That is, mitochondrial metabolism provides substantially more cellular energy currency (ATP) per macronutrient metabolised. Enhancement of mitochondrial density and metabolism are associated with endurance training, which allows for the attainment of high relative VO2 max values. However, the sedentary lifestyle and diet currently predominant in the Western world lead to mitochondrial dysfunction. Underdeveloped mitochondrial metabolism leads to nutrient-induced reducing pressure caused by energy surplus, as reduced nicotinamide adenine dinucleotide (NADH)-mediated high electron flow at rest leads to "electron leak" and a chronic generation of superoxide radicals (O2-). Chronic overload of these reactive oxygen species (ROS) damages cell components such as DNA, cell membranes, and proteins. Counterintuitively, transiently generated ROS during exercise contributes to adaptive reduction-oxidation (REDOX) signalling through the process of cellular hormesis or "oxidative eustress" defined by Helmut Sies. However, the unaccustomed, chronic oxidative stress is central to the leading causes of mortality in the 21st century-metabolic syndrome and the associated cardiovascular comorbidities. The endurance exercise training that improves mitochondrial capacity and the protective antioxidant cellular system emerges as a universal intervention for mitochondrial dysfunction and resultant comorbidities. Furthermore, exercise might also be a solution to prevent ageing-related degenerative diseases, which are caused by impaired mitochondrial recycling. This review aims to break down the metabolic components of exercise and how they translate to athletic versus metabolically diseased phenotypes. We outline a reciprocal relationship between oxidative metabolism and inflammation, as well as hypoxia. We highlight the importance of oxidative stress for metabolic and antioxidant adaptation. We discuss the relevance of lactate as an indicator of critical exercise intensity, and inferring from its relationship with hypoxia, we suggest the most appropriate mode of exercise for the case of a lost oxidative identity in metabolically inflexible patients. Finally, we propose a reciprocal signalling model that establishes a healthy balance between the glycolytic/proliferative and oxidative/prolonged-ageing phenotypes. This model is malleable to adaptation with oxidative stress in exercise but is also susceptible to maladaptation associated with chronic oxidative stress in disease. Furthermore, mutations of components involved in the transcriptional regulatory mechanisms of mitochondrial metabolism may lead to the development of a cancerous phenotype, which progressively presents as one of the main causes of death, alongside the metabolic syndrome.

Lactate in contemporary biology: a phoenix risen

After a century, it's time to turn the page on understanding of lactate metabolism and appreciate that lactate shuttling is an important component of intermediary metabolism in vivo. Cell‐cell and intracellular lactate shuttles fulfil purposes of energy substrate production and distribution, as well as cell signalling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. Moreover, the presence of lactate shuttling as part of postprandial glucose disposal and satiety signalling has been recognized. Mitochondrial respiration creates the physiological sink for lactate disposal in vivo. Repeated lactate exposure from regular exercise results in adaptive processes such as mitochondrial biogenesis and other healthful circulatory and neurological characteristics such as improved physical work capacity, metabolic flexibility, learning, and memory. The importance of lactate and lactate shuttling in healthful living is further emphasized when lactate signalling and shuttling are dysregulated as occurs in particular illnesses and injuries. Like a phoenix, lactate has risen to major importance in 21st century biology.

Peroxisomal Metabolite and Cofactor Transport in Humans

Peroxisomes are membrane-bound organelles involved in many metabolic pathways and essential for human health. They harbor a large number of enzymes involved in the different pathways, thus requiring transport of substrates, products and cofactors involved across the peroxisomal membrane. Although much progress has been made in understanding the permeability properties of peroxisomes, there are still important gaps in our knowledge about the peroxisomal transport of metabolites and cofactors. In this review, we discuss the different modes of transport of metabolites and essential cofactors, including CoA, NAD⁺, NADP⁺, FAD, FMN, ATP, heme, pyridoxal phosphate, and thiamine pyrophosphate across the peroxisomal membrane. This transport can be mediated by non-selective pore-forming proteins, selective transport proteins, membrane contact sites between organelles, and co-import of cofactors with proteins. We also discuss modes of transport mediated by shuttle systems described for NAD⁺/NADH and NADP⁺/NADPH. We mainly focus on current knowledge on human peroxisomal metabolite and cofactor transport, but also include knowledge from studies in plants, yeast, fruit fly, zebrafish, and mice, which has been exemplary in understanding peroxisomal transport mechanisms in general.

Targeting lactate production and efflux in prostate cancer

Prostate cancer (PCa) is the most commonly diagnosed cancer in men worldwide. Screening and management of PCa remain controversial and, therefore, the discovery of novel molecular biomarkers is urgently needed. Alteration in cancer cell metabolism is a recognized hallmark of cancer, whereby cancer cells exhibit high glycolytic rates with subsequent lactate production, regardless of oxygen availability. To maintain the hyperglycolytic phenotype, cancer cells efficiently export lactate through the monocarboxylate transporters MCT1 and MCT4. The impact of inhibiting lactate production/extrusion on PCa cell survival and aggressiveness was investigated in vitro and ex vivo using primary tumor and metastatic PCa cell lines and the chicken embryo chorioallantoic membrane (CAM) model. In this study, we showed the metastatic PCa cell line (DU125) displayed higher expression levels of MCT1/4 isoforms and glycolysis-related markers than the localized prostate tumor-derived cell line (22RV1), indicating these proteins are differentially expressed throughout prostate malignant transformation. Moreover, disruption of lactate export by MCT1/4 silencing resulted in a decrease in PCa cell growth and motility. To support these results, we pharmacological inhibited lactate production (via inhibition of LDH) and release (via inhibition of MCTs) and a reduction in cancer cell growth in vitro and in vivo was observed. In summary, our data provide evidence that MCT1 and MCT4 are important players in prostate neoplastic progression and that inhibition of lactate production/export can be explored as a strategy for PCa treatment.

Prostate Cancer Proliferation Is Affected by the Subcellular Localization of MCT2 and Accompanied by Significant Peroxisomal Alterations

Simple Summary

Fatty acid β-oxidation is a dominant bioenergetic pathway in prostate cancer. It has recently been suggested that the specific targeting of monocarboxylate transporter 2 (MCT2) to peroxisomes contributed to an increase in β-oxidation rates and maintenance of the redox balance in prostate cancer cells. Here we provide evidence demonstrating that prostate cancer streamlines peroxisome metabolism by upregulating distinct pathways involved in lipid metabolism. Importantly, we show that the localization of MCT2 at peroxisomes is required for prostate cancer cell proliferation. Our results emphasize the importance of peroxisomes for prostate cancer development and highlight different cellular mechanisms that may be further explored as possible targets for prostate cancer therapy.

Abstract

Reprogramming of lipid metabolism directly contributes to malignant transformation and progression. The increased uptake of circulating lipids, the transfer of fatty acids from stromal adipocytes to cancer cells, the de novo fatty acid synthesis, and the fatty acid oxidation support the central role of lipids in many cancers, including prostate cancer (PCa). Fatty acid β-oxidation is the dominant bioenergetic pathway in PCa and recent evidence suggests that PCa takes advantage of the peroxisome transport machinery to target monocarboxylate transporter 2 (MCT2) to peroxisomes in order to increase β-oxidation rates and maintain the redox balance. Here we show evidence suggesting that PCa streamlines peroxisome metabolism by upregulating distinct pathways involved in lipid metabolism. Moreover, we show that MCT2 is required for PCa cell proliferation and, importantly, that its specific localization at the peroxisomal membranes is essential for this role. Our results highlight the importance of peroxisomes in PCa development and uncover different cellular mechanisms that may be further explored as possible targets for PCa therapy.

The tortuous path of lactate shuttle discovery: From cinders and boards to the lab and ICU

Once thought to be a waste product of oxygen limited (anaerobic) metabolism, lactate is now known to form continuously under fully oxygenated (aerobic) conditions. Lactate shuttling between producer (driver) and consumer cells fulfills at least 3 purposes; lactate is: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. The Lactate Shuttle theory is applicable to diverse fields such as sports nutrition and hydration, resuscitation from acidosis and Dengue, treatment of traumatic brain injury, maintenance of glycemia, reduction of inflammation, cardiac support in heart failure and following a myocardial infarction, and to improve cognition. Yet, dysregulated lactate shuttling disrupts metabolic flexibility, and worse, supports oncogenesis. Lactate production in cancer (the Warburg effect) is involved in all main sequela for carcinogenesis: angiogenesis, immune escape, cell migration, metastasis, and self-sufficient metabolism. The history of the tortuous path of discovery in lactate metabolism and shuttling was discussed in the 2019 American College of Sports Medicine Joseph B. Wolffe Lecture in Orlando, FL.

Decreased Blood Glucose and Lactate: Is a Useful Indicator of Recovery Ability in Athletes?

During low-intensity exercise stages of the lactate threshold test, blood lactate concentrations gradually diminish due to the predominant utilization of total fat oxidation. However, it is unclear why blood glucose is also reduced in well-trained athletes who also exhibit decreased lactate concentrations. This review focuses on decreased glucose and lactate concentrations at low-exercise intensity performed in well-trained athletes. During low-intensity exercise, the accrued resting lactate may predominantly be transported via blood from the muscle cell to the liver/kidney. Accordingly, there is increased hepatic blood flow with relatively more hepatic glucose output than skeletal muscle glucose output. Hepatic lactate uptake and lactate output of skeletal muscle during recovery time remained similar which may support a predominant Cori cycle (re-synthesis). However, this pathway may be insufficient to produce the necessary glucose level because of the low concentration of lactate and the large energy source from fat. Furthermore, fatty acid oxidation activates key enzymes and hormonal responses of gluconeogenesis while glycolysis-related enzymes such as pyruvate dehydrogenase are allosterically inhibited. Decreased blood lactate and glucose in low-intensity exercise stages may be an indicator of recovery ability in well-trained athletes. Athletes of intermittent sports may need this recovery ability to successfully perform during competition.

Peroxisome Metabolism in Cancer

Peroxisomes are metabolic organelles involved in lipid metabolism and cellular redox balance. Peroxisomal function is central to fatty acid oxidation, ether phospholipid synthesis, bile acid synthesis, and reactive oxygen species homeostasis. Human disorders caused by genetic mutations in peroxisome genes have led to extensive studies on peroxisome biology. Peroxisomal defects are linked to metabolic dysregulation in diverse human diseases, such as neurodegeneration and age-related disorders, revealing the significance of peroxisome metabolism in human health. Cancer is a disease with metabolic aberrations. Despite the critical role of peroxisomes in cell metabolism, the functional effects of peroxisomes in cancer are not as well recognized as those of other metabolic organelles, such as mitochondria. In addition, the significance of peroxisomes in cancer is less appreciated than it is in degenerative diseases. In this review, I summarize the metabolic pathways in peroxisomes and the dysregulation of peroxisome metabolism in cancer. In addition, I discuss the potential of inactivating peroxisomes to target cancer metabolism, which may pave the way for more effective cancer treatment.

Energy System Contributions and Physical Activity in Specific Age Groups during Exergames

Exergames have been recommended as alternative ways to increase the health benefits of physical exercise. However, energy system contributions (phosphagen, glycolytic, and oxidative) of exergames in specific age groups remain unclear. The purpose of this study was to investigate the contributions of three energy systems and metabolic profiles in specific age groups during exergames. Seventy-four healthy males and females participated in this study (older adults, n = 26: Age of 75.4 ± 4.4 years, body mass of 59.4 ± 8.7 kg, height of 157.2 ± 8.6 cm; adults, n = 24: Age of 27.8 ± 3.3 years, body mass of 73.4 ± 17.8 kg, height of 170.9 ± 11.9 cm; and adolescents, n = 24: Age of 14 ± 0.8 years, body mass of 71.3 ± 11.5 kg, height of 173.3 ± 5.2 cm). To evaluate the demands of different energy systems, all participants engaged in exergames named Action-Racing. Exergames protocol comprised whole-body exercises such as standing, sitting, stopping, jumping, and arm swinging. During exergames, mean heart rate (HR_mean), peak heart rate (HR_peak), mean oxygen uptake (VO_2mean), peak oxygen uptake (VO_2peak), peak lactate (Peak La^-), difference in lactate (ΔLa^-), phosphagen (W_PCr), glycolytic (W_La^-), oxidative (W_AER), and total energy demands (W_Total) were analyzed. The contribution of the oxidative energy system was higher than that of the phosphagen or glycolytic energy system (65.9 ± 12% vs. 29.5 ± 11.1% or 4.6 ± 3.3%, both p < 0.001). The contributions of the total energy demands and oxidative system in older adults were significantly lower than those in adults and adolescents (72.1 ± 28 kJ, p = 0.028; 70.3 ± 24.1 kJ, p = 0.024, respectively). The oxidative energy system was predominantly used for exergames applied in the current study. In addition, total metabolic work in older adults was lower than that in adolescents and adults. This was due to a decrease in the oxidative energy system. For future studies, quantification of intensity and volume is needed to optimize exergames. Such an approach plays a crucial role in encouraging physical activity in limited spaces.

In vivo detection of d-amino acid oxidase with hyperpolarized d-[1-13 C]alanine

d-amino acid oxidase (DAO) is a peroxisomal enzyme that catalyzes the oxidative deamination of several neutral and basic d-amino acids to their corresponding α-keto acids. In most mammalian species studied, high DAO activity is found in the kidney, liver, brain and polymorphonuclear leukocytes, and its main function is to maintain low circulating d-amino acid levels. DAO expression and activity have been associated with acute and chronic kidney diseases and with several pathologies related to N-methyl-d-aspartate (NMDA) receptor hypo/hyper-function; however, its precise role is not completely understood. In the present study we show that DAO activity can be detected in vivo in the rat kidney using hyperpolarized d-[1-¹³ C]alanine. Following a bolus of hyperpolarized d-alanine, accumulation of pyruvate, lactate and bicarbonate was observed only when DAO activity was not inhibited. The measured lactate-to-d-alanine ratio was comparable to the values measured when the l-enantiomer was injected. Metabolites downstream of DAO were not observed when scanning the liver and brain. The conversion of hyperpolarized d-[1-¹³ C]alanine to lactate and pyruvate was detected in blood ex vivo, and lactate and bicarbonate were detected on scanning the blood pool in the heart in vivo; however, the bicarbonate-to-d-alanine ratio was significantly lower compared with the kidney. These results demonstrate that the specific metabolism of the two enantiomers of hyperpolarized [1-¹³ C]alanine in the kidney and in the blood can be distinguished, underscoring the potential of d-[1-¹³ C]alanine as a probe of d-amino acid metabolism.

Lactate: the ugly duckling of energy metabolism

Lactate, perhaps the best-known metabolic waste product, was first isolated from sour milk, in which it is produced by lactobacilli. Whereas microbes also generate other fermentation products, such as ethanol or acetone, lactate dominates in mammals. Lactate production increases when the demand for ATP and oxygen exceeds supply, as occurs during intense exercise and ischaemia. The build-up of lactate in stressed muscle and ischaemic tissues has established lactate's reputation as a deleterious waste product. In this Perspective, we summarize emerging evidence that, in mammals, lactate also serves as a major circulating carbohydrate fuel. By providing mammalian cells with both a convenient source and sink for three-carbon compounds, circulating lactate enables the uncoupling of carbohydrate-driven mitochondrial energy generation from glycolysis. Lactate and pyruvate together serve as a circulating redox buffer that equilibrates the NADH/NAD ratio across cells and tissues. This reconceptualization of lactate as a fuel-analogous to how Hans Christian Andersen's ugly duckling is actually a beautiful swan-has the potential to reshape the field of energy metabolism.

Peroxisomal oxidation of erucic acid suppresses mitochondrial fatty acid oxidation by stimulating malonyl-CoA formation in the rat liver

Feeding of rapeseed (canola) oil with a high erucic acid concentration is known to cause hepatic steatosis in animals. Mitochondrial fatty acid oxidation plays a central role in liver lipid homeostasis, so it is possible that hepatic metabolism of erucic acid might decrease mitochondrial fatty acid oxidation. However, the precise mechanistic relationship between erucic acid levels and mitochondrial fatty acid oxidation is unclear. Using male Sprague-Dawley rats, along with biochemical and molecular biology approaches, we report here that peroxisomal β-oxidation of erucic acid stimulates malonyl-CoA formation in the liver and thereby suppresses mitochondrial fatty acid oxidation. Excessive hepatic uptake and peroxisomal β-oxidation of erucic acid resulted in appreciable peroxisomal release of free acetate, which was then used in the synthesis of cytosolic acetyl-CoA. Peroxisomal metabolism of erucic acid also remarkably increased the cytosolic NADH/NAD⁺ ratio, suppressed sirtuin 1 (SIRT1) activity, and thereby activated acetyl-CoA carboxylase, which stimulated malonyl-CoA biosynthesis from acetyl-CoA. Chronic feeding of a diet including high-erucic-acid rapeseed oil diminished mitochondrial fatty acid oxidation and caused hepatic steatosis and insulin resistance in the rats. Of note, administration of a specific peroxisomal β-oxidation inhibitor attenuated these effects. Our findings establish a cross-talk between peroxisomal and mitochondrial fatty acid oxidation. They suggest that peroxisomal oxidation of long-chain fatty acids suppresses mitochondrial fatty acid oxidation by stimulating malonyl-CoA formation, which might play a role in fatty acid-induced hepatic steatosis and related metabolic disorders.

Slc25a17 Gene Trapped Mice: PMP34 Plays a Role in the Peroxisomal Degradation of Phytanic and Pristanic Acid

Mice lacking PMP34, a peroxisomal membrane transporter encoded by Slc25a17, did not manifest any obvious phenotype on a Swiss Webster genetic background, even with various treatments designed to unmask impaired peroxisomal functioning. Peroxisomal α- and β-oxidation rates in PMP34 deficient fibroblasts or liver slices were not or only modestly affected and in bile, no abnormal bile acid intermediates were detected. Peroxisomal content of cofactors like CoA, ATP, NAD+, thiamine-pyrophosphate and pyridoxal-phosphate, based on direct or indirect data, appeared normal as were tissue plasmalogen and very long chain fatty acid levels. However, upon dietary phytol administration, the knockout mice displayed hepatomegaly, liver inflammation, and an induction of peroxisomal enzymes. This phenotype was partially mediated by PPARα. Hepatic triacylglycerols and cholesterylesters were elevated and both phytanic acid and pristanic acid accumulated in the liver lipids, in females to higher extent than in males. In addition, pristanic acid degradation products were detected, as wells as the CoA-esters of all these branched fatty acids. Hence, PMP34 is important for the degradation of phytanic/pristanic acid and/or export of their metabolites. Whether this is caused by a shortage of peroxisomal CoA affecting the intraperoxisomal formation of pristanoyl-CoA (and perhaps of phytanoyl-CoA), or the SCPx-catalyzed thiolytic cleavage during pristanic acid β-oxidation, could not be proven in this model, but the phytol-derived acyl-CoA profile is compatible with the latter possibility. On the other hand, the normal functioning of other peroxisomal pathways, and especially bile acid formation, seems to exclude severe transport problems or a shortage of CoA, and other cofactors like FAD, NAD(P)+, TPP. Based on our findings, PMP34 deficiency in humans is unlikely to be a life threatening condition but could cause elevated phytanic/pristanic acid levels in older adults.

Lactate as a fulcrum of metabolism

Mistakenly thought to be the consequence of oxygen lack in contracting skeletal muscle we now know that the L-enantiomer of the lactate anion is formed under fully aerobic conditions and is utilized continuously in diverse cells, tissues, organs and at the whole-body level. By shuttling between producer (driver) and consumer (recipient) cells lactate fulfills at least three purposes: 1] a major energy source for mitochondrial respiration; 2] the major gluconeogenic precursor; and 3] a signaling molecule. Working by mass action, cell redox regulation, allosteric binding, and reprogramming of chromatin by lactylation of lysine residues on histones, lactate has major influences in energy substrate partitioning. The physiological range of tissue [lactate] is 0.5–20 mM and the cellular Lactate/Pyruvate ratio (L/P) can range from 10 to >500; these changes during exercise and other stress-strain responses dwarf other metabolic signals in magnitude and span. Hence, lactate dynamics have rapid and major short- and long-term effects on cell redox and other control systems. By inhibiting lipolysis in adipose via HCAR-1, and muscle mitochondrial fatty acid uptake via malonyl-CoA and CPT1, lactate controls energy substrate partitioning. Repeated lactate exposure from regular exercise results in major effects on the expression of regulatory enzymes of glycolysis and mitochondrial respiration. Lactate is the fulcrum of metabolic regulation in vivo.

The Role of Sodium Hydrogen Exchanger 1 in Dysregulation of Proton Dynamics and Reprogramming of Cancer Metabolism as a Sequela

Cancer cells have an unusual regulation of hydrogen ion dynamics that are driven by poor vascularity perfusion, regional hypoxia, and increased glycolysis. All these forces synergize/orchestrate together to create extracellular acidity and intracellular alkalinity. Precisely, they lead to extracellular pH (pH_e) values as low as 6.2 and intracellular pH values as high as 8. This unique pH gradient (∆pH_i to ∆pH_e) across the cell membrane increases as the tumor progresses, and is markedly displaced from the electrochemical equilibrium of protons. These unusual pH dynamics influence cancer cell biology, including proliferation, metastasis, and metabolic adaptation. Warburg metabolism with increased glycolysis, even in the presence of Oxygen with the subsequent reduction in Krebs’ cycle, is a common feature of most cancers. This metabolic reprogramming confers evolutionary advantages to cancer cells by enhancing their resistance to hypoxia, to chemotherapy or radiotherapy, allowing rapid production of biological building blocks that support cellular proliferation, and shielding against damaging mitochondrial free radicals. In this article, we highlight the interconnected roles of dysregulated pH dynamics in cancer initiation, progression, adaptation, and in determining the programming and re-programming of tumor cell metabolism.

Monocarboxylate transporters in cancer

Background

Tumors are highly plastic metabolic entities composed of cancer and host cells that can adopt different metabolic phenotypes. For energy production, cancer cells may use 4 main fuels that are shuttled in 5 different metabolic pathways. Glucose fuels glycolysis that can be coupled to the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS) in oxidative cancer cells or to lactic fermentation in proliferating and in hypoxic cancer cells. Lipids fuel lipolysis, glutamine fuels glutaminolysis, and lactate fuels the oxidative pathway of lactate, all of which are coupled to the TCA cycle and OXPHOS for energy production. This review focuses on the latter metabolic pathway.

Scope of review

Lactate, which is prominently produced by glycolytic cells in tumors, was only recently recognized as a major fuel for oxidative cancer cells and as a signaling agent. Its exchanges across membranes are gated by monocarboxylate transporters MCT1-4. This review summarizes the current knowledge about MCT structure, regulation and functions in cancer, with a specific focus on lactate metabolism, lactate-induced angiogenesis and MCT-dependent cancer metastasis. It also describes lactate signaling via cell surface lactate receptor GPR81.

Major conclusions

Lactate and MCTs, especially MCT1 and MCT4, are important contributors to tumor aggressiveness. Analyses of MCT-deficient (MCT^+/- and MCT^−/-) animals and (MCT-mutated) humans indicate that they are druggable, with MCT1 inhibitors being in advanced development phase and MCT4 inhibitors still in the discovery phase. Imaging lactate fluxes non-invasively using a lactate tracer for positron emission tomography would further help to identify responders to the treatments.

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In cancer, hypoxia and cell proliferation are associated to lactic acid production.

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Lactate exchanges are at the core of tumor metabolism.

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Transmembrane lactate trafficking depends on monocarboxylate transporters (MCTs).

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MCTs are implicated in tumor development and aggressiveness.

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Targeting MCTs is a therapeutic option for cancer treatment.

The Strategic Location of Glycogen and Lactate: From Body Energy Reserve to Brain Plasticity

Brain energy metabolism has been the object of intense research in recent years. Pioneering work has identified the different cell types involved in energy production and use. Recent evidence has demonstrated a key role of L-Lactate in brain energy metabolism, producing a paradigm-shift in our understanding of the neuronal energy metabolism. At the center of this shift, is the identification of a central role of astrocytes in neuroenergetics. Thanks to their morphological characteristics, they are poised to take up glucose from the circulation and deliver energy substrates to neurons. Astrocyte neuron lactate shuttle (ANLS) model, has shown that the main energy substrate that astrocytes deliver to neurons is L-Lactate, to sustain neuronal oxidative metabolism. L-Lactate can also be produced from glycogen, the storage form of glucose, which is exclusively localized in astrocytes. Inhibition of glycogen metabolism and the ensuing inhibition of L-Lactate production leads to cognitive dysfunction. Experimental evidence indicates that the role of lactate in cognitive function relates not only to its role as a metabolic substrate for neurons but also as a signaling molecule for synaptic plasticity. Interestingly, a similar metabolic uncoupling appears to exist in peripheral tissues plasma, whereby glucose provides L-Lactate as the substrate for cellular oxidative metabolism. In this perspective article, we review the known information on the distribution of glycogen and lactate within brain cells, and how this distribution relates to the energy regime of glial vs. neuronal cells.

Melatonin Therapy Modulates Cerebral Metabolism and Enhances Remyelination by Increasing PDK4 in a Mouse Model of Multiple Sclerosis

Metabolic disturbances have been implicated in demyelinating diseases including multiple sclerosis (MS). Melatonin, a naturally occurring hormone, has emerged as a potent neuroprotective candidate to reduce myelin loss and improve MS outcomes. In this study, we evaluated the effect of melatonin, at both physiological and pharmacological doses, on oligodendrocytes metabolism in an experimental autoimmune encephalomyelitis (EAE) mouse model of MS. Results showed that melatonin decreased neurological disability scores and enhanced remyelination, significantly increasing myelin protein levels including MBP, MOG, and MOBP. In addition, melatonin attenuated inflammation by reducing pro-inflammatory cytokines (IL-1β and TNF-α) and increasing anti-inflammatory cytokines (IL-4 and IL-10). Moreover, melatonin significantly increased brain concentrations of lactate, N-acetylaspartate (NAA), and 3-hydroxy-3-methylglutaryl-coenzyme-A reductase (HMGCR). Pyruvate dehydrogenase kinase-4 (PDK-4) mRNA and protein expression levels were also increased in melatonin-treated, compared to untreated EAE mice. However, melatonin significantly inhibited active and total pyruvate dehydrogenase complex (PDC), an enzyme under the control of PDK4. In summary, although PDC activity was reduced by melatonin, it caused a reduction in inflammatory mediators while stimulating oligodendrogenesis, suggesting that oligodendrocytes are forced to use an alternative pathway to synthesize fatty acids for remyelination. We propose that combining melatonin and PDK inhibitors may provide greater benefits for MS patients than the use of melatonin therapy alone.

Intracellular processing of silica-coated superparamagnetic iron nanoparticles in human mesenchymal stem cells

Silica-coated superparamagnetic iron nanoparticles (SiMAGs) are an exciting biomedical technology capable of targeted delivery of cell-based therapeutics and disease diagnosis. However, in order to realise their full clinical potential, their intracellular fate must be determined. The analytical techniques of super-resolution fluorescence microscopy, particle counting flow cytometry and pH-sensitive nanosensors were applied to elucidate mechanisms of intracellular SiMAG processing in human mesenchymal stem cell (hMSCs). Super-resolution microscopy showed SiMAG fluorescently-tagged nanoparticles are endocytosed and co-localised within lysosomes. When exposed to simulated lysosomal conditions SiMAGs were solubilised and exhibited diminishing fluorescence emission over 7 days. The in vitro intracellular metabolism of SiMAGs was monitored in hMSCs using flow cytometry and co-localised pH-sensitive nanosensors. A decrease in SiMAG fluorescence emission, which corresponded to a decrease in lysosomal pH was observed, mirroring ex vivo observations, suggesting SiMAG lysosomal exposure degrades fluorescent silica-coatings and iron cores. These findings indicate although there is a significant decrease in intracellular SiMAG loading, sufficient particles remain internalised (>50%) to render SiMAG treated cells amenable to long-term magnetic cell manipulation. Our analytical approach provides important insights into the understanding of the intracellular fate of SiMAG processing, which could be readily applied to other particle therapeutics, to advance their clinical translation.

Redox Signaling from and to Peroxisomes: Progress, Challenges, and Prospects

SIGNIFICANCE

Peroxisomes are organelles that are best known for their role in cellular lipid and hydrogen peroxide (H₂O₂) metabolism. Emerging evidence suggests that these organelles serve as guardians and modulators of cellular redox balance, and that alterations in their redox metabolism may contribute to aging and the development of chronic diseases such as neurodegeneration, diabetes, and cancer. Recent Advances: H₂O₂ is an important signaling messenger that controls many cellular processes by modulating protein activity through cysteine oxidation. Somewhat surprisingly, the potential involvement of peroxisomes in H₂O₂-mediated signaling processes has been overlooked for a long time. However, recent advances in the development of live-cell approaches to monitor and modulate spatiotemporal fluxes in redox species at the subcellular level have opened up new avenues for research in redox biology and boosted interest in the concept of peroxisomes as redox signaling platforms.

CRITICAL ISSUES

This review first introduces the reader to what is known about the role of peroxisomes in cellular H₂O₂ production and clearance, with a focus on mammalian cells. Next, it briefly describes the benefits and drawbacks of current strategies used to investigate the complex interplay between peroxisome metabolism and cellular redox state. Furthermore, it integrates and critically evaluates literature dealing with the interrelationship between peroxisomal redox metabolism, cell signaling, and human disease.

FUTURE DIRECTIONS

As the precise molecular mechanisms underlying many of these associations are still poorly understood, a key focus for future research should be the identification of primary targets for peroxisome-derived H₂O₂.

The peroxisome: an update on mysteries 2.0

Peroxisomes are key metabolic organelles, which contribute to cellular lipid metabolism, e.g. the β-oxidation of fatty acids and the synthesis of myelin sheath lipids, as well as cellular redox balance. Peroxisomal dysfunction has been linked to severe metabolic disorders in man, but peroxisomes are now also recognized as protective organelles with a wider significance in human health and potential impact on a large number of globally important human diseases such as neurodegeneration, obesity, cancer, and age-related disorders. Therefore, the interest in peroxisomes and their physiological functions has significantly increased in recent years. In this review, we intend to highlight recent discoveries, advancements and trends in peroxisome research, and present an update as well as a continuation of two former review articles addressing the unsolved mysteries of this astonishing organelle. We summarize novel findings on the biological functions of peroxisomes, their biogenesis, formation, membrane dynamics and division, as well as on peroxisome-organelle contacts and cooperation. Furthermore, novel peroxisomal proteins and machineries at the peroxisomal membrane are discussed. Finally, we address recent findings on the role of peroxisomes in the brain, in neurological disorders, and in the development of cancer.

The Science and Translation of Lactate Shuttle Theory

Once thought to be a waste product of anaerobic metabolism, lactate is now known to form continuously under aerobic conditions. Shuttling between producer and consumer cells fulfills at least three purposes for lactate: (1) a major energy source, (2) the major gluconeogenic precursor, and (3) a signaling molecule. "Lactate shuttle" (LS) concepts describe the roles of lactate in delivery of oxidative and gluconeogenic substrates as well as in cell signaling. In medicine, it has long been recognized that the elevation of blood lactate correlates with illness or injury severity. However, with lactate shuttle theory in mind, some clinicians are now appreciating lactatemia as a "strain" and not a "stress" biomarker. In fact, clinical studies are utilizing lactate to treat pro-inflammatory conditions and to deliver optimal fuel for working muscles in sports medicine. The above, as well as historic and recent studies of lactate metabolism and shuttling, are discussed in the following review.

Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H₂O₂) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a “redox triangle” formed by mitochondria, peroxisomes and the ER, assembling “redoxosomes” that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca²⁺ ions, since many Ca²⁺ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP₃Rs), SERCA pumps or regulators of the mitochondrial Ca²⁺ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

Two NAD-linked redox shuttles maintain the peroxisomal redox balance in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, peroxisomes are the sole site of fatty acid β-oxidation. During this process, NAD⁺ is reduced to NADH. When cells are grown on oleate medium, peroxisomal NADH is reoxidised to NAD⁺ by malate dehydrogenase (Mdh3p) and reduction equivalents are transferred to the cytosol by the malate/oxaloacetate shuttle. The ultimate step in lysine biosynthesis, the NAD⁺-dependent dehydrogenation of saccharopine to lysine, is another NAD⁺-dependent reaction performed inside peroxisomes. We have found that in glucose grown cells, both the malate/oxaloacetate shuttle and a glycerol-3-phosphate dehydrogenase 1(Gpd1p)-dependent shuttle are able to maintain the intraperoxisomal redox balance. Single mutants in MDH3 or GPD1 grow on lysine-deficient medium, but an mdh3/gpd1Δ double mutant accumulates saccharopine and displays lysine bradytrophy. Lysine biosynthesis is restored when saccharopine dehydrogenase is mislocalised to the cytosol in mdh3/gpd1Δ cells. We conclude that the availability of intraperoxisomal NAD⁺ required for saccharopine dehydrogenase activity can be sustained by both shuttles. The extent to which each of these shuttles contributes to the intraperoxisomal redox balance may depend on the growth medium. We propose that the presence of multiple peroxisomal redox shuttles allows eukaryotic cells to maintain the peroxisomal redox status under different metabolic conditions.

The D-lactate dehydrogenase MoDLD1 is essential for growth and infection-related development in Magnaporthe oryzae

Rice blast disease caused by Magnaporthe oryzae is initiated by the attachment of conidia to plant surfaces. Germ tubes emerging from conidia develop melanized appressoria to physically penetrate the host surface. Previous studies revealed that appressorium development requires the breakdown of storage lipids and glycogen that occur in peroxisomes and the cytosol respectively, culminating in production of pyruvate. However, the downstream product(s) entering the mitochondria for further oxidation is unclear. In this study, we aimed to investigate the molecular basis underlying the metabolic flux towards the mitochondria associated with the infectious-related development in M. oryzae. We showed that D-lactate is a key intermediate metabolite of the mobilization of lipids and glycogen, and its oxidative conversion to pyruvate is catalysed by a mitochondrial D-lactate dehydrogenase MoDLD1. Deletion of MoDLD1 caused defects in conidiogenesis and appressorium formation, and subsequently the loss of fungal pathogenicity. Further analyses demonstrated that MoDLD1 activity is involved in the maintenance of redox homeostasis during conidial germination. Thus, MoDLD1 is a critical modulator that channels metabolite flow to the mitochondrion coupling cellular redox state, and contributes to development and virulence of M. oryzae.

Patients with Fever of Unknown Origin and Splenomegaly: Diagnostic Value of Splenectomy and Preoperative Risk Factors Suggestive of Underlying Lymphomas

BACKGROUND

We reviewed patients with fever of unknown origin (FUO) and splenomegaly and assessed the diagnostic value of splenectomy and measured risk factors suggestive of an underlying lymphoma.

METHODS

FUO patients (n = 83) who had splenomegaly and underwent splenectomy were enrolled into this retrospective single-center study. Clinical presentations were documented and risk factors suggestive of an underlying lymphoma were tested.

RESULTS

Seventy-four patients (89.2%) had a diagnosis of lymphoma or not after splenectomy and follow-up. Of those (55.4%) diagnosed with lymphoma, 29 had B-cell non-Hodgkin lymphoma and 12 had T-cell non-Hodgkin lymphoma. The remaining 33 (44.6%) had diseases other than lymphoma. Using multivariate logistic analysis, the following 3 independent risk factors were found to be related to a final diagnosis of lymphoma: age (continuous) (HR 1.086; 95% CI 1.033-1.141; p = 0.001), massively enlarged spleen (HR 7.797; 95% CI 1.267-47.959; p = 0.027), and enlarged intra-abdominal lymph nodes (HR 63.925; 95% CI 7.962-513.219; p < 0.001). The calibration of the model was satisfactory (p = 0.248 using the Hosmer-Lemeshow test), and the discrimination power was good (area under the receiver operating characteristic curve 0.925; 95% CI 0.863-0.987).

CONCLUSIONS

Splenectomy is an effective diagnostic procedure for patients with FUO and splenomegaly and lymphoma is a common cause. Older age, a massively enlarged spleen, and enlarged intra-abdominal lymph nodes are risk factors suggesting an underlying lymphoma, and surgery for high-risk patients should be considered.

The Peroxisome-Mitochondria Connection: How and Why?

Over the past decades, peroxisomes have emerged as key regulators in overall cellular lipid and reactive oxygen species metabolism. In mammals, these organelles have also been recognized as important hubs in redox-, lipid-, inflammatory-, and innate immune-signaling networks. To exert these activities, peroxisomes must interact both functionally and physically with other cell organelles. This review provides a comprehensive look of what is currently known about the interconnectivity between peroxisomes and mitochondria within mammalian cells. We first outline how peroxisomal and mitochondrial abundance are controlled by common sets of cis- and trans-acting factors. Next, we discuss how peroxisomes and mitochondria may communicate with each other at the molecular level. In addition, we reflect on how these organelles cooperate in various metabolic and signaling pathways. Finally, we address why peroxisomes and mitochondria have to maintain a healthy relationship and why defects in one organelle may cause dysfunction in the other. Gaining a better insight into these issues is pivotal to understanding how these organelles function in their environment, both in health and disease.

Peroxisomal Dysfunction in Age-Related Diseases

Peroxisomes carry out many key functions related to lipid and reactive oxygen species (ROS) metabolism. The fundamental importance of peroxisomes for health in humans is underscored by the existence of devastating genetic disorders caused by impaired peroxisomal function or lack of peroxisomes. Emerging studies suggest that peroxisomal function may also be altered with aging and contribute to the pathogenesis of a variety of diseases, including diabetes and its related complications, neurodegenerative disorders, and cancer. With increasing evidence connecting peroxisomal dysfunction to the pathogenesis of these acquired diseases, the possibility of targeting peroxisomal function in disease prevention or treatment becomes intriguing. Here, we review recent developments in understanding the pathophysiological implications of peroxisomal dysfunctions outside the context of inherited peroxisomal disorders.

Functional Translational Readthrough: A Systems Biology Perspective

Translational readthrough (TR) has come into renewed focus because systems biology approaches have identified the first human genes undergoing functional translational readthrough (FTR). FTR creates functional extensions to proteins by continuing translation of the mRNA downstream of the stop codon. Here we review recent developments in TR research with a focus on the identification of FTR in humans and the systems biology methods that have spurred these discoveries.

Tumor metabolism, cancer cell transporters, and microenvironmental resistance

Cancer cells reprogram their metabolic machineries to enter into permanent glycolytic pathways. The full reason for such reprogramming takes place is unclear. However, this metabolic switch is not made in vain for the lactate that is generated and exported outside cells is reused by other cells. This results in the generation of a pH gradient between the low extracellular pH that is acidic (pHe) and the higher cytosolic alkaline or near neutral pH (pHi) environments that are tightly regulated by the overexpression of several pumps and ion channels (e.g. NHE-1, MCT-1, V-ATPase, CA9, and CA12). The generation of this unique pH gradient serves as a determining factor in defining "tumor fitness". Tumor fitness is the capacity of the tumor to invade and metastasize due to its ability to reduce the efficiency of the immune system and confer resistance to chemotherapy. In this article, we highlight the importance of tumor microenvironment in mediating the failure of chemotherapeutic agents.

Expression of Monocarboxylate Transporter Isoforms in Rat Skeletal Muscle Under Hypoxic Preconditioning and Endurance Training

Previously, we have reported the regulation of monocarboxylate transporters (MCT)1 and MCT4 by physiological stimuli such as hypoxia and exercise. In the present study, we have evaluated the effect of hypoxic preconditioning and training on expression of different MCT isoforms in muscles. We found the increased mRNA expression of MCT1, MCT11, and MCT12 after hypoxic preconditioning with cobalt chloride and training. However, the expression of other MCT isoforms increased marginally or even reduced after hypoxic preconditioning. Only the protein expression of MCT1 increased after hypoxia preconditioning. MCT2 protein expression increased after training only and MCT4 protein expression decreased both in preconditioning and hypoxic training. Furthermore, we found decreased plasma lactate level during hypoxia preconditioning (0.74-fold), exercise (0.78-fold), and hypoxia preconditioning along with exercise (0.67-fold), which indicates increased lactate uptake by skeletal muscle. The protein-protein interactions with hypoxia inducible factor-1 and MCT isoforms were also evaluated, but no interaction was found. In conclusion, we say that almost all MCTs are expressed in red gastrocnemius muscle at the mRNA level and their expression is regulated differently under hypoxia preconditioning and exercise condition.

2-Hydroxy Acids in Plant Metabolism

Glycolate, malate, lactate, and 2-hydroxyglutarate are important 2-hydroxy acids (2HA) in plant metabolism. Most of them can be found as D- and L-stereoisomers. These 2HA play an integral role in plant primary metabolism, where they are involved in fundamental pathways such as photorespiration, tricarboxylic acid cycle, glyoxylate cycle, methylglyoxal pathway, and lysine catabolism. Recent molecular studies in Arabidopsis thaliana have helped elucidate the participation of these 2HA in in plant metabolism and physiology. In this chapter, we summarize the current knowledge about the metabolic pathways and cellular processes in which they are involved, focusing on the proteins that participate in their metabolism and cellular/intracellular transport in Arabidopsis.

Epigenetic and oncogenic regulation of SLC16A7 (MCT2) results in protein over-expression, impacting on signalling and cellular phenotypes in prostate cancer

Monocarboxylate Transporter 2 (MCT2) is a major pyruvate transporter encoded by the SLC16A7 gene. Recent studies pointed to a consistent overexpression of MCT2 in prostate cancer (PCa) suggesting MCT2 as a putative biomarker and molecular target. Despite the importance of this observation the mechanisms involved in MCT2 regulation are unknown. Through an integrative analysis we have discovered that selective demethylation of an internal SLC16A7/MCT2 promoter is a recurrent event in independent PCa cohorts. This demethylation is associated with expression of isoforms differing only in 5'-UTR translational control motifs, providing one contributing mechanism for MCT2 protein overexpression in PCa. Genes co-expressed with SLC16A7/MCT2 also clustered in oncogenic-related pathways and effectors of these signalling pathways were found to bind at the SLC16A7/MCT2 gene locus. Finally, MCT2 knock-down attenuated the growth of PCa cells. The present study unveils an unexpected epigenetic regulation of SLC16A7/MCT2 isoforms and identifies a link between SLC16A7/MCT2, Androgen Receptor (AR), ETS-related gene (ERG) and other oncogenic pathways in PCa. These results underscore the importance of combining data from epigenetic, transcriptomic and protein level changes to allow more comprehensive insights into the mechanisms underlying protein expression, that in our case provide additional weight to MCT2 as a candidate biomarker and molecular target in PCa.

Endogenous Nutritive Support after Traumatic Brain Injury: Peripheral Lactate Production for Glucose Supply via Gluconeogenesis

We evaluated the hypothesis that nutritive needs of injured brains are supported by large and coordinated increases in lactate shuttling throughout the body. To that end, we used dual isotope tracer ([6,6-(2)H2]glucose, i.e., D2-glucose, and [3-(13)C]lactate) techniques involving central venous tracer infusion along with cerebral (arterial [art] and jugular bulb [JB]) blood sampling. Patients with traumatic brain injury (TBI) who had nonpenetrating head injuries (n=12, all male) were entered into the study after consent of patients' legal representatives. Written and informed consent was obtained from healthy controls (n=6, including one female). As in previous investigations, the cerebral metabolic rate (CMR) for glucose was suppressed after TBI. Near normal arterial glucose and lactate levels in patients studied 5.7±2.2 days (range of days 2-10) post-injury, however, belied a 71% increase in systemic lactate production, compared with control, that was largely cleared by greater (hepatic+renal) glucose production. After TBI, gluconeogenesis from lactate clearance accounted for 67.1% of glucose rate of appearance (Ra), which was compared with 15.2% in healthy controls. We conclude that elevations in blood glucose concentration after TBI result from a massive mobilization of lactate from corporeal glycogen reserves. This previously unrecognized mobilization of lactate subserves hepatic and renal gluconeogenesis. As such, a lactate shuttle mechanism indirectly makes substrate available for the body and its essential organs, including the brain, after trauma. In addition, when elevations in arterial lactate concentration occur after TBI, lactate shuttling may provide substrate directly to vital organs of the body, including the injured brain.

Localization of MCT2 at peroxisomes is associated with malignant transformation in prostate cancer

Previous studies on monocarboxylate transporters expression in prostate cancer (PCa) have shown that monocarboxylate transporter 2 (MCT2) was clearly overexpressed in prostate malignant glands, pointing it out as a putative biomarker for PCa. However, its localization and possible role in PCa cells remained unclear. In this study, we demonstrate that MCT2 localizes mainly at peroxisomes in PCa cells and is able to take advantage of the peroxisomal transport machinery by interacting with Pex19. We have also shown an increase in MCT2 expression from non-malignant to malignant cells that was directly correlated with its peroxisomal localization. Upon analysis of the expression of several peroxisomal β-oxidation proteins in PIN lesions and PCa cells from a large variety of human prostate samples, we suggest that MCT2 presence at peroxisomes is related to an increase in β -oxidation levels which may be crucial for malignant transformation. Our results present novel evidence that may not only contribute to the study of PCa development mechanisms but also pinpoint novel targets for cancer therapy.

Inhibition of lactate dehydrogenase activity as an approach to cancer therapy

In the attempt of developing innovative anticancer treatments, growing interest has recently focused on the peculiar metabolic properties of cancer cells. In this context, LDH, which converts pyruvate to lactate at the end of glycolysis, is emerging as one of the most interesting molecular targets for the development of new inhibitors. In fact, because LDH activity is not needed for pyruvate metabolism through the TCA cycle, inhibitors of this enzyme should spare glucose metabolism of normal non-proliferating cells, which usually completely degrade the glucose molecule to CO2. This review is aimed at summarizing the available data on LDH biology in normal and neoplastic cells, which support the anticancer therapeutic approach based on LDH inhibition. These data encouraged pharmaceutical industries and academic institutions in the search of small-molecule inhibitors and promising candidates have recently been identified. The availability of inhibitors with drug-like properties will allow the evaluation in the near future of the real potential of LDH inhibition in anticancer treatment, also making the identification of the most responsive neoplastic conditions possible.

Peroxisomal lactate dehydrogenase is generated by translational readthrough in mammals

Translational readthrough gives rise to low abundance proteins with C-terminal extensions beyond the stop codon. To identify functional translational readthrough, we estimated the readthrough propensity (RTP) of all stop codon contexts of the human genome by a new regression model in silico, identified a nucleotide consensus motif for high RTP by using this model, and analyzed all readthrough extensions in silico with a new predictor for peroxisomal targeting signal type 1 (PTS1). Lactate dehydrogenase B (LDHB) showed the highest combined RTP and PTS1 probability. Experimentally we show that at least 1.6% of the total cellular LDHB is targeted to the peroxisome by a conserved hidden PTS1. The readthrough-extended lactate dehydrogenase subunit LDHBx can also co-import LDHA, the other LDH subunit, into peroxisomes. Peroxisomal LDH is conserved in mammals and likely contributes to redox equivalent regeneration in peroxisomes.

DOI:http://dx.doi.org/10.7554/eLife.03640.001

Amino acids are the building blocks of proteins, and the order of the amino acids in a protein is determined by the order in which ‘codons’ appear in a messenger RNA molecule. Most codons represent a specific amino acid, but there are also three stop codons that are used to mark the end of a protein.

When the cellular machinery that ‘translates’ the messenger RNA molecule into a protein encounters a stop codon, it stops and releases the completed protein. Sometimes, however, the stop codon is not interpreted as a stop signal, and the translation of the messenger RNA molecule continues until another stop codon is encountered. This process is known as readthrough.

Some organisms, in particular viruses and fungi, use readthrough to produce a wider range of proteins than their genomes would otherwise allow. While readthrough also occurs in higher organisms such as mammals, it is not known if the resulting proteins perform extra functions that the original protein does not perform.

A number of factors affect whether readthrough occurs when an mRNA template is being translated. For example, each of the three stop codons has a different likelihood of having its stop signal misinterpreted, and the mRNA sequence that surrounds the stop codon can also affect the likelihood of readthrough.

Schueren et al. have developed a computational model that estimates how common this form of translational readthrough is in the human genome. The model was based on the identity of the stop codons themselves and the surrounding mRNA sequence. This model was then combined with another model that identifies proteins that are targeted to a structure inside a cell called the peroxisome, which is where a number of essential energy-releasing reactions take place. The combined model enabled Schueren et al. to identify proteins that both perform functions in the peroxisome and are likely to be formed by readthrough.

The combined model suggested a protein that is a part of lactate dehydrogenase: an enzyme that speeds up chemical reactions that are important for the cell to produce energy. Low levels of lactate dehydrogenase had previously been found in the peroxisome, despite it apparently lacking a specific sequence of amino acids that proteins need to have to enter the peroxisome. However, Schueren et al. confirmed experimentally that readthrough does occur for the lactate dehydrogenase component identified by the model, revealing that it contains a ‘hidden’ peroxisome-targeting region. Furthermore, when more translational readthrough occurred, more lactate dehydrogenase was found in the peroxisomes.

This unusual way that lactate dehydrogenase enters the peroxisome is an example of how the cell optimizes the used of the genetic information encoded in the genome and in messenger RNA. Translational readthrough always ensures that a certain proportion of lactate dehydrogenase will be brought to the peroxisome. The computational model developed here will be a valuable tool to identify other such proteins produced from genomes, including the human genome and those of other species.

DOI:http://dx.doi.org/10.7554/eLife.03640.002

Prognostic significance of serum lactate dehydrogenase level in osteosarcoma: a meta-analysis

BACKGROUND

A number of studies have investigated the role of serum lactate dehydrogenase (LDH) level in patients with osteosarcoma but have yielded inconsistent and inconclusive results. Thus, we conducted a meta-analysis to assess its prognostic value more precisely.

METHODS

Systematic computerized searches of PubMed, Embase and Web of Science databases were performed. The pooled hazard ratio (HR) with 95 % confidence intervals (95 % CI) of overall survival was used to assess the prognostic role of serum LDH level.

RESULTS

Ten studies published between 1997 and 2013 with a total of 943 osteosarcoma patients were included. Overall, the pooled HR for all ten eligible studies evaluating high LDH level on overall survival was 1.92(95 % CI 1.53-2.40). Sensitivity analysis suggested that the pooled HR was stable and omitting a single study did not change the significance of the pooled HR. Funnel plots and Egger's tests revealed there was some possibility of publication bias risk in the meta-analysis.

CONCLUSION

This meta-analysis shows that high serum LDH level is obviously associated with lower overall survival rate in patients with osteosarcoma, and it is an effective biomarker of prognosis.

White matter lactate--does it matter?

About half of the human brain is white matter, characterized by axons covered in myelin, which facilitates the high speed of nerve signals from one brain area to another. At the time of myelination, the oligodendrocytes that synthesize myelin require a large amount of energy for this task. Conditions that deprive the tissue of energy can kill the oligodendrocytes. During brain development, the oligodendrocytes may use lactate as an alternative source of energy and material for myelin formation. Mature oligodendrocytes, however, can release lactate through the myelin sheath as nutrient for axons. In addition, lactate carries signals as a volume transmitter. Myelin thus seems to serve as a provider of substrates and signals for axons, and not as a mere insulator. We review the fluxes of lactate in white matter and their significance in brain function.

The proteome of human liver peroxisomes: identification of five new peroxisomal constituents by a label-free quantitative proteomics survey

The peroxisome is a key organelle of low abundance that fulfils various functions essential for human cell metabolism. Severe genetic diseases in humans are caused by defects in peroxisome biogenesis or deficiencies in the function of single peroxisomal proteins. To improve our knowledge of this important cellular structure, we studied for the first time human liver peroxisomes by quantitative proteomics. Peroxisomes were isolated by differential and Nycodenz density gradient centrifugation. A label-free quantitative study of 314 proteins across the density gradient was accomplished using high resolution mass spectrometry. By pairing statistical data evaluation, cDNA cloning and in vivo colocalization studies, we report the association of five new proteins with human liver peroxisomes. Among these, isochorismatase domain containing 1 protein points to the existence of a new metabolic pathway and hydroxysteroid dehydrogenase like 2 protein is likely involved in the transport or β-oxidation of fatty acids in human peroxisomes. The detection of alcohol dehydrogenase 1A suggests the presence of an alternative alcohol-oxidizing system in hepatic peroxisomes. In addition, lactate dehydrogenase A and malate dehydrogenase 1 partially associate with human liver peroxisomes and enzyme activity profiles support the idea that NAD⁺ becomes regenerated during fatty acid β-oxidation by alternative shuttling processes in human peroxisomes involving lactate dehydrogenase and/or malate dehydrogenase. Taken together, our data represent a valuable resource for future studies of peroxisome biochemistry that will advance research of human peroxisomes in health and disease.

Alanine: Glyoxylate aminotransferase 1 is required for mobilization and utilization of triglycerides during infection process of the rice blast pathogen, Magnaporthe oryzae

The rice blast pathogen, Magnaporthe oryzae has been widely used as a model pathogen to study plant infection-related fungal morphogenesis, such as penetration via appressorium and plant-microbe interactions at the molecular level. Previously, we identified a gene encoding peroxisomal alanine: glyoxylate aminotransferase 1 (AGT1) in M. oryzae and demonstrated that the AGT1 was indispensable for pathogenicity. The AGT1 knockout mutants were unable to penetrate the host plants, such as rice and barley, and therefore were non-pathogenic. The inability of ∆Moagt1 mutants to penetrate the susceptible plants was likely due to the disruption in coordination of the β-oxidation and the glyoxylate cycle resulted from a blockage in lipid droplet mobilization and eventually utilization during conidial germination and appressorium morphogenesis, respectively. Here, we further demonstrate the role of AGT1 in lipid mobilization by in vitro germination assays and confocal microscopy.

Detecting and investigating substrate cycles in a genome-scale human metabolic network

Substrate cycles, also known as futile cycles, are cyclic metabolic routes that dissipate energy by hydrolysing cofactors such as ATP. They were first described to occur in the muscles of bumblebees and brown adipose tissue in the 1970s. A popular example is the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate and back. In the present study, we analyze a large number of substrate cycles in human metabolism that consume ATP and discuss their statistics. For this purpose, we use two recently published methods (i.e. EFMEvolver and the K-shortest EFM method) to calculate samples of 100,000 and 15,000 substrate cycles, respectively. We find an unexpectedly high number of substrate cycles in human metabolism, with up to 100 reactions per cycle, utilizing reactions from up to six different compartments. An analysis of tissue-specific models of liver and brain metabolism shows that there is selective pressure that acts against the uncontrolled dissipation of energy by avoiding the coexpression of enzymes belonging to the same substrate cycle. This selective force is particularly strong against futile cycles that have a high flux as a result of thermodynamic principles.