Extracellular citrate and metabolic adaptations of cancer cells

It is well established that cancer cells acquire energy via the Warburg effect and oxidative phosphorylation. Citrate is considered to play a crucial role in cancer metabolism by virtue of its production in the reverse Krebs cycle from glutamine. Here, we review the evidence that extracellular citrate is one of the key metabolites of the metabolic pathways present in cancer cells. We review the different mechanisms by which pathways involved in keeping redox balance respond to the need of intracellular citrate synthesis under different extracellular metabolic conditions. In this context, we further discuss the hypothesis that extracellular citrate plays a role in switching between oxidative phosphorylation and the Warburg effect while citrate uptake enhances metastatic activities and therapy resistance. We also present the possibility that organs rich in citrate such as the liver, brain and bones might form a perfect niche for the secondary tumour growth and improve survival of colonising cancer cells. Consistently, metabolic support provided by cancer-associated and senescent cells is also discussed. Finally, we highlight evidence on the role of citrate on immune cells and its potential to modulate the biological functions of pro- and anti-tumour immune cells in the tumour microenvironment. Collectively, we review intriguing evidence supporting the potential role of extracellular citrate in the regulation of the overall cancer metabolism and metastatic activity.

Insulin-Induced Recurrent Hypoglycemia Up-Regulates Glucose Metabolism in the Brain Cortex of Chemically Induced Diabetic Rats

Diabetes is a chronic metabolic disease that seriously compromises human well-being. Various studies highlight the importance of maintaining a sufficient glucose supply to the brain and subsequently safeguarding cerebral glucose metabolism. The goal of the present work is to clarify and disclose the metabolic alterations induced by recurrent hypoglycemia in the context of long-term hyperglycemia to further comprehend the effects beyond brain harm. To this end, chemically induced diabetic rats underwent a protocol of repeatedly insulin-induced hypoglycemic episodes. The activity of key enzymes of glycolysis, the pentose phosphate pathway and the Krebs cycle was measured by spectrophotometry in extracts or isolated mitochondria from brain cortical tissue. Western blot analysis was used to determine the protein content of glucose and monocarboxylate transporters, players in the insulin signaling pathway and mitochondrial biogenesis and dynamics. We observed that recurrent hypoglycemia up-regulates the activity of mitochondrial hexokinase and Krebs cycle enzymes (namely, pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and succinate dehydrogenase) and the protein levels of mitochondrial transcription factor A (TFAM). Both insults increased the nuclear factor erythroid 2–related factor 2 (NRF2) protein content and induced divergent effects in mitochondrial dynamics. Insulin-signaling downstream pathways were found to be down-regulated, and glycogen synthase kinase 3 beta (GSK3β) was found to be activated through both decreased phosphorylation at Ser9 and increased phosphorylation at Y216. Interestingly, no changes in the levels of cAMP response element-binding protein (CREB), which plays a key role in neuronal plasticity and memory, were caused by hypoglycemia and/or hyperglycemia. These findings provide experimental evidence that recurrent hypoglycemia, in the context of chronic hyperglycemia, has the capacity to evoke coordinated adaptive responses in the brain cortex that will ultimately contribute to sustaining brain cell health.

The anti-angiogenic compound dimethyl fumarate inhibits the serine synthesis pathway and increases glycolysis in endothelial cells

A pathological and persistent angiogenesis is observed in several diseases like retinopathies, diabetes, psoriasis and cancer. Dimethyl fumarate, an ester from the Krebs cycle intermediate fumarate, is approved as a drug for the treatment of psoriasis and multiple sclerosis, and its anti-angiogenic activity has been reported in vitro and in vivo. However, it is not known whether dimethyl fumarate is able to modulate endothelial cell metabolism, considered an essential feature for the angiogenic switch. By means of different experimental approximations, including proteomics, isotope tracing and metabolomics experimental approaches, in this work we studied the possible role of dimethyl fumarate in endothelial cell energetic metabolism. We demonstrate for the first time that dimethyl fumarate promotes glycolysis and diminishes cell respiration, which could be a consequence of a down-regulation of serine and glycine synthesis through inhibition of PHGDH activity in endothelial cells. This new target can open a new field of study regarding the mechanism of action of dimethyl fumarate.

Disruption of Glycolysis, TCA Cycle, Respiratory Chain, Calcium and Iron Homeostasis in Doxorubicin Induced Cardiomyopathy-An In-silico Approach

Doxorubicin is a well-known anthracycline antibiotic that is frequently used to treat a variety of malignancies. However, its clinical use is limited due to its adverse consequences, most notably cardiomyopathy. In the present work, we evaluated the molecular mechanisms behind the impairment of cardiac energetics in doxorubicin-induced cardiomyopathy. According to molecular docking, the interaction of doxorubicin with phosphofructokinase (PKF) and α-enolase is likely to negatively affect glycolysis. The interaction between doxorubicin with HMOX1 results in the accumulation of free iron. The free iron contributes to the heme-driven toxicity and the oxidizing environment that results in reactive oxygen species (ROS) production resulting from cell death. Additionally, the interaction of doxorubicin with HMOX1 impairs the availability of iron required for the Krebs cycle and ETC function. The interaction between doxorubicin and PINK1 results in a reduced membrane potential, which results in calcium accumulation. On the other hand, a lack of iron and calcium in the mitochondrial matrix results in ATP depletion, impairing the Krebs cycle activity. At the same time, the primary cause of doxorubicin-induced cardiomyopathy is cardiac energy metabolism. Thus, our work shows that doxorubicin impairs the activity of PFK, α-enolase, HMOX1, and PINK1, resulting in ATP production failure. As a result of changes in the heart energy metabolism, this ultimately leads to dilated cardiomyopathy caused by doxorubicin. Understanding the critical function of cardiac energy metabolism in doxorubicin-induced cardiomyopathy is critical for overcoming the obstacles that effectively limit the clinical effectiveness of this life-saving anti-cancer treatment.

Physiological Doses of Hydroxytyrosol Modulate Gene Expression in Skeletal Muscle of Exercised Rats

We tested whether physiological doses of hydroxytyrosol (HT) may alter the mRNA transcription of key metabolic genes in exercised skeletal muscle. Two groups of exercise-trained Wistar rats, HTlow and HTmid, were supplemented with 0.31 and 4.61 mg/kg/d of HT, respectively, for 10 weeks. Another two groups of rats were not supplemented with HT; one remained sedentary and the other one was exercised. After the experimental period, the soleus muscle was removed for qRT-PCR and western blot analysis. The consumption of 4.61 mg/kg/d of HT during exercise increased the mRNA expression of important metabolic proteins. Specifically, 4.61 mg/kg/d of HT may upregulate long-chain fatty acid oxidation, lactate, and glucose oxidation as well as mitochondrial Krebs cycle in trained skeletal muscle. However, a 4.61 mg/kg/d of HT may alter protein translation, as in spite of the increment showed by CD36 and GLUT4 at the mRNA level this was not translated to higher protein content.

The Echocardiographic Parameters of Systolic Function Are Associated with Specific Metabolomic Fingerprints in Obstructive and Non-Obstructive Hypertrophic Cardiomyopathy

The purpose of this study was to assess whether metabolomics, associated with echocardiography, was able to highlight pathophysiological differences between obstructive (OHCM) or non-obstructive (NOHCM) hypertrophic cardiomyopathy. Thirty-one HCM patients underwent standard and advanced echocardiography; a plasma sample was collected for metabolomic analysis. Results. Patients with OHCM compared with subjects with NOHCM had higher values of 2DLVEF (66.5 ± 3.3% vs. 60.6 ± 1.8%, p < 0.01), S wave (7.6 ± 1.1 vs. 6.3 ± 0.7 cm/s, p < 0.01) and 3D global longitudinal strain (17.2 ± 4.2%, vs. 13.4 ± 1.3%, p < 0.05). A 2-group PLS-Discriminant Analysis was performed to verify whether the two HCM groups differed also based on the metabolic fingerprint. A clear clustering was shown (ANOVA p = 0.014). The most discriminating metabolites resulted as follows: in the NOHCM Group, there were higher levels of threitol, aminomalonic acid, and sucrose, while the OHCM Group presented higher levels of amino acids, in particular those branched chains, of intermediates of glycolysis (lactate) and the Krebs cycle (fumarate, succinate, citrate), of fatty acids (arachidonic acid, palmitoleic acid), of ketone bodies (2-OH-butyrate). Our data point out a different systolic function related to a specific metabolic activity in the two HCM phenotypic forms, with specific metabolites associated with better contractility in OHCM.

Mitochondrial Management of Reactive Oxygen Species

Mitochondria in aerobic eukaryotic cells are both the site of energy production and the formation of harmful species, such as radicals and other reactive oxygen species, known as ROS. They contain an efficient antioxidant system, including low-molecular-mass molecules and enzymes that specialize in removing various types of ROS or repairing the oxidative damage of biological molecules. Under normal conditions, ROS production is low, and mitochondria, which are their primary target, are slightly damaged in a similar way to other cellular compartments, since the ROS released by the mitochondria into the cytosol are negligible. As the mitochondrial generation of ROS increases, they can deactivate components of the respiratory chain and enzymes of the Krebs cycle, and mitochondria release a high amount of ROS that damage cellular structures. More recently, the feature of the mitochondrial antioxidant system, which does not specifically deal with intramitochondrial ROS, was discovered. Indeed, the mitochondrial antioxidant system detoxifies exogenous ROS species at the expense of reducing the equivalents generated in mitochondria. Thus, mitochondria are also a sink of ROS. These observations highlight the importance of the mitochondrial antioxidant system, which should be considered in our understanding of ROS-regulated processes. These processes include cell signaling and the progression of metabolic and neurodegenerative disease.

Tritium Incorporation From Tritium Water as an Index of Metabolism

<p>Part A The metabolism of mustard (Sinapis alba) seeds wet at sub-germination temperatures has been studied using tritium incorporation as an index of metabolism. The theory and scope of the method are discussed. The enzymic reactions known in 1964 are surveyed one by one, suggesting which will, or will not, incorporate tritium from THO into specified metabolites, or cannot be confidently predicted either way. Improvements have been made in the chromatography procedure. At 0°, many of the normal germination chemical reactions proceed, but about one tenth as fast as at 24°. Amino-acids are being metabolised within 2 h of wetting the seeds, and malic and citric acids within 4 h. Within 24 h lipids and fructose are undergoing reactions. An unidentified compound “M”, not reported in normal germination, is being metabolised within 48 h. Another aberration from normal is the absence of detectable succinate metabolism. Labelling of the solid residue (insoluble in ethanol and in water) always occurs, shown to be largely non-metabolic. To explain the non-germination of seeds at temperatures near 0°, it is hypothesized that the Krebs cycle is qualitatively altered, perhaps by “wasting away” of glutamate to 4-aminobutyrate instead of its routing into the Krebs cycle as alpha-oxoglutarate. Part B A method has been developed for studying the metabolism of dry seeds, spores and pollen by exposure to THO vapour. Dry Pinus radiata pollen labels many compounds. A few have been identified and are common metabolites. It may be that the metabolism of dry pollen is not qualitatively different from its germination reactions. Dry mustard seeds and spores of the fungus Pithomyces chartarum give, in contrast to pollen, patterns of incorporation very different from those in early germination.</p>

Tritium Incorporation From Tritium Water as an Index of Metabolism

<p>Part A The metabolism of mustard (Sinapis alba) seeds wet at sub-germination temperatures has been studied using tritium incorporation as an index of metabolism. The theory and scope of the method are discussed. The enzymic reactions known in 1964 are surveyed one by one, suggesting which will, or will not, incorporate tritium from THO into specified metabolites, or cannot be confidently predicted either way. Improvements have been made in the chromatography procedure. At 0°, many of the normal germination chemical reactions proceed, but about one tenth as fast as at 24°. Amino-acids are being metabolised within 2 h of wetting the seeds, and malic and citric acids within 4 h. Within 24 h lipids and fructose are undergoing reactions. An unidentified compound “M”, not reported in normal germination, is being metabolised within 48 h. Another aberration from normal is the absence of detectable succinate metabolism. Labelling of the solid residue (insoluble in ethanol and in water) always occurs, shown to be largely non-metabolic. To explain the non-germination of seeds at temperatures near 0°, it is hypothesized that the Krebs cycle is qualitatively altered, perhaps by “wasting away” of glutamate to 4-aminobutyrate instead of its routing into the Krebs cycle as alpha-oxoglutarate. Part B A method has been developed for studying the metabolism of dry seeds, spores and pollen by exposure to THO vapour. Dry Pinus radiata pollen labels many compounds. A few have been identified and are common metabolites. It may be that the metabolism of dry pollen is not qualitatively different from its germination reactions. Dry mustard seeds and spores of the fungus Pithomyces chartarum give, in contrast to pollen, patterns of incorporation very different from those in early germination.</p>