Lactic acid efflux from white skeletal muscle is catalyzed by the monocarboxylate transporter isoform MCT3

Advances in the interaction of glycolytic reprogramming with lactylation

Lactylation is a novel post-translational modification (PTM) involving proteins that is induced by lactate accumulation. Histone lysine lactylation alters chromatin spatial configuration, influencing gene transcription and regulating the expression of associated genes. This modification plays a crucial role as an epigenetic regulatory factor in the progression of various diseases. Glycolytic reprogramming is one of the most extensively studied forms of metabolic reprogramming, recognized as a key hallmark of cancer cells. It is characterized by an increase in glycolysis and the inhibition of the tricarboxylic acid (TCA) cycle, accompanied by significant lactate production and accumulation. The two processes are closely linked by lactate, which interacts in various physiological and pathological processes. On the one hand, lactylation levels generally correlate positively with the extent of glycolytic reprogramming, being directly influenced by the lactate concentration produced during glycolytic reprogramming. On the other hand, lactylation can also regulate glycolytic pathways by affecting the transcription and structural functions of essential glycolytic enzymes. This review comprehensively outlines the mechanisms of lactylation and glycolytic reprogramming and their interactions in tumor progression, immunity, and inflammation, with the aim of elucidating the relationship between glycolytic reprogramming and lactylation.

10-20-30 exercise training improves fitness and health

Intense interval exercise training has been shown to improve performance and health of untrained and trained people. However, due to the exercise intensity causing high-perceived exertion, the participants often do not wish to continue the training. The 10-20-30 training concept consists of low intensity for 30 s, 20 s at a moderate pace, and then 10 s with high intensity either running or cycling. A 10-20-30 training session consist of two to four 5-min blocks. The 10-20-30 training improved fitness and performance as well as lowered blood pressure and body fat of both untrained and trained individuals even with a significant reduction in the training volume. Similarly, hypertensive, diabetic, and asthmatic patients lowered body fat, improved fitness, and performance during a 10-20-30-training intervention period. In addition, hypertensive patients reduced systolic and diastolic blood pressure markedly with the 10-20-30 training twice a week for 8 weeks. Diabetic patients lowered long-term blood sugar (HbA1c), which did not occur with moderate-intensity exercise training. Furthermore, asthmatic patients improved their control of asthma and asthma-related quality of life with the 10-20-30 training. The adherence for the patient groups was high (>80%), and no adverse events were reported. Thus, the 10-20-30 training seems to be time efficient and feasible for untrained and trained individuals as well as patients and may be used in the prevention and treatment of noncommunicable diseases.

Rare variant association analyses reveal the significant contribution of carbohydrate metabolic disturbance in severe adolescent idiopathic scoliosis

BACKGROUND

Adolescent idiopathic scoliosis (AIS), the predominant genetic-influenced scoliosis, results in spinal deformities without vertebral malformations. However, the molecular aetiology of AIS remains unclear.

METHODS

Using genome/exome sequencing, we studied 368 patients with severe AIS (Cobb angle >40°) and 3794 controls from a Han Chinese cohort. We performed gene-based and pathway-based weighted rare variant association tests to assess the mutational burden of genes and established biological pathways. Differential expression analysis of muscle tissues from 14 patients with AIS and 15 controls was served for validation.

RESULTS

SLC16A8, a lactate transporter linked to retinal glucose metabolism, was identified as a novel severe AIS-associated gene (p=3.08E-06, false discovery rate=0.009). Most AIS cases with deleterious SLC16A8 variants demonstrated early onset high myopia preceding scoliosis. Pathway-based burden test also revealed a significant enrichment in multiple carbohydrate metabolism pathways, especially galactose metabolism. Patients with deleterious variants in these genes demonstrated a significantly larger spinal curve. Genes related to catabolic processes and nutrient response showed divergent expression between AIS cases and controls, reinforcing our genomic findings.

CONCLUSION

This study uncovers the pivotal role of genetic variants in carbohydrate metabolism in the development of AIS, unveiling new insights into its aetiology and potential treatment.

MicroRNA-92b in the skeletal muscle regulates exercise capacity via modulation of glucose metabolism

BACKGROUND

Exercise mimetics is a proposed class of therapeutics that specifically mimics or enhances the therapeutic effects of exercise. Muscle glycogen and lactate extrusion are critical for physical performance. The mechanism by which glycogen and lactate metabolism are manipulated during exercise remains unclear. This study aimed to assess the effect of miR-92b on the upregulation of exercise training-induced physical performance.

METHODS

Adeno-associated virus (AAV)-mediated skeletal muscle miR-92b overexpression in C57BLKS/J mice, and global knockout of miR-92b mice were used to explore the function of miR-92b in glycogen and lactate metabolism in skeletal muscle. AAV-mediated UGP2 or MCT4 knockdown in WT or miR-92 knockout mice was used to confirm whether miR-92b regulates glycogen and lactate metabolism in skeletal muscle through UGP2 and MCT4. Body weight, muscle weight, grip strength, running time and distance to exhaustion, and muscle histology were assessed. The expression levels of muscle mass-related and function-related proteins were analysed by immunoblotting or immunostaining.

RESULTS

Global knockout of miR-92b resulted in normal body weight and insulin sensitivity, but higher glycogen content before exercise exhaustion (0.8538 ± 0.0417 vs. 1.043 ± 0.040, **P = 0.0087), lower lactate levels after exercise exhaustion (4.133 ± 0.2589 vs. 3.207 ± 0.2511, *P = 0.0279), and better exercise capacity (running distance to exhaustion, 3616 ± 86.71 vs. 4231 ± 90.29, ***P = 0.0006; running time to exhaustion, 186.8 ± 8.027 vs. 220.8 ± 3.156, **P = 0.0028), as compared with those observed in the control mice. Mice skeletal muscle overexpressing miR-92b (both miR-92b-3p and miR-92b-5p) displayed lower glycogen content before exercise exhaustion (0.6318 ± 0.0231 vs. 0.535 ± 0.0194, **P = 0.0094), and higher lactate accumulation after exercise exhaustion (4.5 ± 0.2394 vs. 5.467 ± 0.1892, *P = 0.01), and poorer exercise capacity (running distance to exhaustion, 4005 ± 81.65 vs. 3228 ± 149.8, ***P<0.0001; running time to exhaustion, 225.5 ± 7.689 vs. 163 ± 6.476, **P = 0.001). Mechanistic analysis revealed that miR-92b-3p targets UDP-glucose pyrophosphorylase 2 (UGP2) expression to inhibit glycogen synthesis, while miR-92b-5p represses lactate extrusion by directly target monocarboxylate transporter 4 (MCT4). Knockdown of UGP2 and MCT4 reversed the effects observed in the absence of miR-92b in vivo.

CONCLUSIONS

This study revealed regulatory pathways, including miR-92b-3p/UGP2/glycogen synthesis and miR-92b-5p/MCT4/lactate extrusion, which could be targeted to control exercise capacity.

Hidden chronic metabolic acidosis of diabetes type 2 (CMAD): Clues, causes and consequences

Interpretation of existing data revealed that chronic metabolic acidosis is a pathognomic feature for type 2 diabetes (T2D), which is described here as "chronic metabolic acidosis of T2D (CMAD)" for the first time. The biochemical clues for the CMAD are summarised in the following; low blood bicarbonate (high anionic gap), low pH of interstitial fluid and urine, and response to acid neutralization, while the causes of extra protons are worked out to be; mitochondrial dysfunction, systemic inflammation, gut microbiota (GM), and diabetic lung. Although, the intracellular pH is largely preserved by the buffer system and ion transporters, a persistent systemic mild acidosis leaves molecular signature in cellular metabolism in diabetics. Reciprocally, there are evidences that CMAD contributes to the initiation and progression of T2D by; reducing insulin production, triggering insulin resistance directly or via altered GM, and inclined oxidative stress. The details about the above clues, causes and consequences of CMAD are obtained by searching literature spanning between 1955 and 2022. Finally, the molecular bases of CMAD are discussed in details by interpretation of an up-to-date data and aid of well constructed diagrams, with a conclusion unravelling that CMAD is a major player in T2D pathophysiology. To this end, the CMAD disclosure offers several therapeutic potentials for prevention, delay or attenuation of T2D and its complications.

Effects of Endurance Training on Metabolic Enzyme Activity and Transporter Proteins in Skeletal Muscle of Ovariectomized Mice

PURPOSE

Estrogen deficiency or insufficiency can occur under several conditions, leading to negative health outcomes. To establish an effective countermeasure against estrogen loss, we investigated the effects of endurance training on ovariectomy (OVX)-induced metabolic disturbances.

METHODS

Female Institute of Cancer Research mice underwent OVX or sham operations. On day 7 of recovery, the mice were randomized to remain either sedentary or undergo 5 wk of treadmill running (15-20 m·min -1 , 60 min, 5 d·wk -1 ). During week 5 of the training, all animals performed a treadmill running test (15 m·min -1 , 60 min).

RESULTS

After the experimental period, OVX resulted in greater gains in body mass, fat mass, and triglyceride content in the gastrocnemius muscle. OVX enhanced phosphofructokinase activity in the plantaris muscle and decreased lactate dehydrogenase activity in the plantaris and soleus muscles. OVX decreased the protein content of NDUFB8, a mitochondrial respiratory chain subunit, but did not decrease other mitochondrial proteins or enzyme activities. Endurance training significantly enhanced mitochondrial enzyme activity and protein content in the skeletal muscles. Although OVX increased the respiratory exchange ratio during the treadmill running test, and postexercise blood lactate levels, endurance training normalized these parameters.

CONCLUSIONS

The present findings suggest that endurance training is a viable strategy to counteract the negative metabolic consequences in hypoestrogenism.

Maternal and Fetal Expression of ATP-Binding Cassette and Solute Carrier Transporters Involved in the Brain Disposition of Drugs

Evidence from preclinical and clinical studies demonstrate that pregnancy is a physiological state capable of modifying drug disposition. Factors including increased hepatic metabolism and renal excretion are responsible for impacting disposition, and the role of membrane transporters expressed in biological barriers, including the placental- and blood-brain barriers, has received considerable attention. In this regard, the brain disposition of drugs in the mother and fetus has been the subject of studies attempting to characterize the mechanisms by which pregnancy could alter the expression of ATP-binding cassette (ABC) and solute carrier (SLC) transporters. This chapter will summarize findings of the influence of pregnancy on the maternal and fetal expression of ABC and SLC transporters in the brain and the consequences of such changes on the disposition of therapeutic drugs.

Tracing the lactate shuttle to the mitochondrial reticulum

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial-venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ-Organ, Cell-Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Effect of increasing zinc supplementation on post-transit performance, behavior, blood and muscle metabolites, and gene expression in growing beef feedlot steers

Fifty-four Angus-cross steers (297 kg ± 12) were stratified by body weight (BW) to pens (six steers per pen) to determine the effects of supplemental Zn on posttransit growth performance and blood and muscle metabolites. Dietary treatments started 25 d before trucking: control (CON; analyzed 54 mg Zn/kg DM), industry (IND; CON + 70 mg supplemental Zn/kg DM), and supranutritional Zn (SUPZN; CON + 120 mg supplemental Zn/kg DM). Supplemental Zn was bis-glycinate bound Zn (Plexomin Zn; Phytobiotics North America, Cary, NC). On day 0, steers were loaded onto a commercial trailer and transported in 18 h (1,822 km). Individual BW was recorded on days –26, –25, –1, and 0 (pre-transit), 1 (posttransit), 6, 27, and 28. Blood was collected on days –1, 1, 6, and 27. Longissimus thoracis biopsies were collected on days –1, 1, and 28. Daily individual feed disappearance was recorded via GrowSafe bunks. Data were analyzed using Proc Mixed of SAS with fixed effect of diet and steer as the experimental unit (growth performance, blood: n = 18 steers per treatment; muscle: n = 12 steers per treatment). Individual initial BW was used as a covariate in BW analysis. Contrast statements to test linear, quadratic, and Zn effects were used to analyze performance and blood parameters. Repeated measures analysis was used for posttransit DMI recovery and weekly posttransit DMI and Zn intake with the repeated effect of time. MetaboAnalyst 5.0 was utilized for statistical analysis of day 1 (off truck) muscle metabolites. Plasma Zn linearly increased due to Zn on days 1, 6, and 27 (P = 0.01), and off-truck (day 1) serum lactate increased over day –1 by 20%, 0%, and 20% in CON, IND, and SUPZN, respectively (Quadratic: P = 0.01). Muscle lactate tended to increase posttransit in CON and IND (P ≤ 0.07) but not SUPZN. Muscle metabolites relating to amino acid and nitrogen metabolism were increased in all treatments posttransit (P ≤ 0.02), and alanine-glucose cycle metabolites tended to increase in CON and IND (P ≤ 0.07). Steers supplemented with Zn recovered pretransit DMI quicker than CON (by d 2: P = 0.01), while IND had greater overall posttransit DMI than CON with SUPZN intermediate (P = 0.04), and Zn-fed steers had greater ADG posttransit (P = 0.04). Zinc supplementation mitigated muscle or serum lactate increases due to transit and increased posttransit ADG.

Effects of endurance training on metabolic enzyme activity and transporter protein levels in the skeletal muscles of orchiectomized mice

This study investigated whether endurance training attenuates orchiectomy (ORX)-induced metabolic alterations. At 7 days of recovery after sham operation or ORX surgery, the mice were randomized to remain sedentary or undergo 5 weeks of treadmill running training (15-20 m/min, 60 min, 5 days/week). ORX decreased glycogen concentration in the gastrocnemius muscle, enhanced phosphofructokinase activity in the plantaris muscle, and decreased lactate dehydrogenase activity in the plantaris and soleus muscles. Mitochondrial enzyme activities and protein content in the plantaris and soleus muscles were also decreased after ORX, but preserved, in part, by endurance training. In the treadmill running test (15 m/min, 60 min) after 4 weeks of training, orchiectomized sedentary mice showed impaired exercise performance, which was restored by endurance training. Thus, endurance training could be a potential therapeutic strategy to prevent the hypoandrogenism-induced decline in muscle mitochondrial content and physical performance.

Uncovering Novel Plasma Membrane Carboxylate Transporters in the Yeast Cyberlindnera jadinii

The yeast Cyberlindnera jadinii has great potential in the biotechnology industry due to its ability to produce a variety of compounds of interest, including carboxylic acids. In this work, we identified genes encoding carboxylate transporters from this yeast species. The functional characterization of sixteen plasma membrane carboxylate transporters belonging to the AceTr, SHS, TDT, MCT, SSS, and DASS families was performed by heterologous expression in Saccharomyces cerevisiae. The newly identified C. jadinii transporters present specificity for mono-, di-, and tricarboxylates. The transporters CjAto5, CjJen6, CjSlc5, and CjSlc13-1 display the broadest substrate specificity; CjAto2 accepts mono- and dicarboxylates; and CjAto1,3,4, CjJen1-5, CjSlc16, and CjSlc13-2 are specific for monocarboxylic acids. A detailed characterization of these transporters, including phylogenetic reconstruction, 3D structure prediction, and molecular docking analysis is presented here. The properties presented by these transporters make them interesting targets to be explored as organic acid exporters in microbial cell factories.

Lactate administration does not affect denervation-induced loss of mitochondrial content and muscle mass in mice

Lactate is considered to be a signaling molecule that induces mitochondrial adaptation and muscle hypertrophy. The purpose of this study was to examine whether lactate administration attenuates denervation-induced loss of mitochondrial content and muscle mass. Eight-week-old male Institute of Cancer Research mice underwent unilateral sciatic nerve transection surgery. The contralateral hindlimb served as a sham-operated control. From the day of surgery, mice were injected intraperitoneally with PBS or sodium lactate (equivalent to 1 g·kg-1 body weight) once daily for 9 days. After 10 days of denervation, gastrocnemius muscle weight decreased to a similar extent in both the PBS- and lactate-injected groups. Denervation significantly decreased mitochondrial enzyme activity, protein content, and MCT4 protein content in the gastrocnemius muscle. However, lactate administration did not have any significant effects. The current observations suggest that daily lactate administration for 9 days does not affect denervation-induced loss of mitochondrial content and muscle mass.

Lactate and Myocadiac Energy Metabolism

The myocardium is capable of utilizing different energy substrates, which is referred to as “metabolic flexibility.” This process assures ATP production from fatty acids, glucose, lactate, amino acids, and ketones, in the face of varying metabolic contexts. In the normal physiological state, the oxidation of fatty acids contributes to approximately 60% of energy required, and the oxidation of other substrates provides the rest. The accumulation of lactate in ischemic and hypoxic tissues has traditionally be considered as a by-product, and of little utility. However, recent evidence suggests that lactate may represent an important fuel for the myocardium during exercise or myocadiac stress. This new paradigm drives increasing interest in understanding its role in cardiac metabolism under both physiological and pathological conditions. In recent years, blood lactate has been regarded as a signal of stress in cardiac disease, linking to prognosis in patients with myocardial ischemia or heart failure. In this review, we discuss the importance of lactate as an energy source and its relevance to the progression and management of heart diseases.

The pyruvate-lactate axis modulates cardiac hypertrophy and heart failure

The metabolic rewiring of cardiomyocytes is a widely accepted hallmark of heart failure (HF). These metabolic changes include a decrease in mitochondrial pyruvate oxidation and an increased export of lactate. We identify the mitochondrial pyruvate carrier (MPC) and the cellular lactate exporter monocarboxylate transporter 4 (MCT4) as pivotal nodes in this metabolic axis. We observed that cardiac assist device-induced myocardial recovery in chronic HF patients was coincident with increased myocardial expression of the MPC. Moreover, the genetic ablation of the MPC in cultured cardiomyocytes and in adult murine hearts was sufficient to induce hypertrophy and HF. Conversely, MPC overexpression attenuated drug-induced hypertrophy in a cell-autonomous manner. We also introduced a novel, highly potent MCT4 inhibitor that mitigated hypertrophy in cultured cardiomyocytes and in mice. Together, we find that alteration of the pyruvate-lactate axis is a fundamental and early feature of cardiac hypertrophy and failure.

Skeletal muscle proteins important for work capacity are altered with type 2 diabetes - Effect of 10-20-30 training

The study examined whether men with type 2 diabetes exhibit lower expression of muscle proteins important for exercise capacity, and whether exercise training promotes adaptations in these proteins. In a cross-sectional and longitudinal study, conducted at the University of Copenhagen. Twelve men with type 2 diabetes (T2D) were compared to eleven nondiabetes counterparts (ND) matched for age and body composition (body fat percentage). T2D underwent 10 weeks of high-intensity interval exercise training (10-20-30 training). T2D had lower expression of SOD1 (-62%; p < 0.001) and ETC complex V (-34%; p = 0.003), along with higher expression of ETC complex IV (+66%; p = 0.007), MFN2 (+62%; p = 0.001), and DRP1 (+30%; p = 0.028) compared to ND. T2D had higher (p < 0.001) expression of Na⁺ /K⁺ α1 (+98%), α2 (+114%), and NHE1 (+144%) than ND. In T2D, training increased exercise capacity (+9%; p < 0.001) as well as expression of SOD2 (+44%; p = 0.029), ETC complex II (+25%; p = 0.035), III (+52%; p = 0.041), IV (+23%; p = 0.005), and V (+21%; p = 0.035), CS activity (+32%; p = 0.006) as well as Na⁺ /K⁺ α1 (+24%; p = 0.034), Kir6.2 (+36%; p = 0.029), and MCT1 (+20%; p = 0.007). Men with type 2 diabetes exhibited altered expression of a multitude of skeletal muscle proteins important for exercise capacity. Ten weeks of 10-20-30 training upregulated expression of muscle proteins regulating antioxidant defense, mitochondrial function, and ion handling while enhancing exercise capacity in men with type 2 diabetes.

Intracellular pH Regulation of Skeletal Muscle in the Milieu of Insulin Signaling

Type 2 diabetes (T2D), along with obesity, is one of the leading health problems in the world which causes other systemic diseases, such as cardiovascular diseases and kidney failure. Impairments in glycemic control and insulin resistance plays a pivotal role in the development of diabetes and its complications. Since skeletal muscle constitutes a significant tissue mass of the body, insulin resistance within the muscle is considered to initiate the onset of diet-induced metabolic syndrome. Insulin resistance is associated with impaired glucose uptake, resulting from defective post-receptor insulin responses, decreased glucose transport, impaired glucose phosphorylation, oxidation and glycogen synthesis in the muscle. Although defects in the insulin signaling pathway have been widely studied, the effects of cellular mechanisms activated during metabolic syndrome that cross-talk with insulin responses are not fully elucidated. Numerous reports suggest that pathways such as inflammation, lipid peroxidation products, acidosis and autophagy could cross-talk with insulin-signaling pathway and contribute to diminished insulin responses. Here, we review and discuss the literature about the defects in glycolytic pathway, shift in glucose utilization toward anaerobic glycolysis and change in intracellular pH [pH]_i within the skeletal muscle and their contribution towards insulin resistance. We will discuss whether the derangements in pathways, which maintain [pH]_i within the skeletal muscle, such as transporters (monocarboxylate transporters 1 and 4) and depletion of intracellular buffers, such as histidyl dipeptides, could lead to decrease in [pH]_i and the onset of insulin resistance. Further we will discuss, whether the changes in [pH]_i within the skeletal muscle of patients with T2D, could enhance the formation of protein aggregates and activate autophagy. Understanding the mechanisms by which changes in the glycolytic pathway and [pH]_i within the muscle, contribute to insulin resistance might help explain the onset of obesity-linked metabolic syndrome. Finally, we will conclude whether correcting the pathways which maintain [pH]_i within the skeletal muscle could, in turn, be effective to maintain or restore insulin responses during metabolic syndrome.

SLC16 Family: From Atomic Structure to Human Disease

The solute carrier 16 (SLC16) family represents a diverse group of membrane proteins mediating the transport of monocarboxylates across biological membranes. Family members show a variety of functional roles ranging from nutrient transport and intracellular pH regulation to thyroid hormone homeostasis. Changes in the expression levels and transport function of certain SLC16 transporters are manifested in severe health disorders including cancer, diabetes, and neurological disorders. L-Lactate-transporting SLC16 family members play essential roles in the metabolism of certain tumors and became validated drug targets. This review illuminates the SLC16 family under a new light using structural information obtained from a SLC16 homolog. Furthermore, the role of these transporters in cancer metabolism and how their inhibition can contribute to anticancer therapy are discussed.

Effect of post-exercise lactate administration on glycogen repletion and signaling activation in different types of mouse skeletal muscle

Lactate is not merely a metabolic intermediate that serves as an oxidizable and glyconeogenic substrate, but it is also a potential signaling molecule. The objectives of this study were to investigate whether lactate administration enhances post-exercise glycogen repletion in association with cellular signaling activation in different types of skeletal muscle. Eight-week-old male ICR mice performed treadmill running (20 m/min for 60 min) following overnight fasting (16 h). Immediately after the exercise, animals received an intraperitoneal injection of phosphate-buffered saline or sodium lactate (equivalent to 1 g/kg body weight), followed by oral ingestion of water or glucose (2 g/kg body weight). At 60 min of recovery, glucose ingestion enhanced glycogen content in the soleus, plantaris, and gastrocnemius muscles. In addition, lactate injection additively increased glycogen content in the plantaris and gastrocnemius muscles, but not in the soleus muscle. Nevertheless, lactate administration did not significantly alter protein levels related to glucose uptake and oxidation in the plantaris muscle, but enhanced phosphorylation of TBC1D1, a distal protein regulating GLUT4 translocation, was observed in the soleus muscle. Muscle FBP2 protein content was significantly higher in the plantaris and gastrocnemius muscles than in the soleus muscle, whereas MCT1 protein content was significantly higher in the soleus muscle than in the plantaris and gastrocnemius muscles. The current findings suggest that an elevated blood lactate concentration and post-exercise glucose ingestion additively enhance glycogen recovery in glycolytic phenotype muscles. This appears to be associated with glyconeogenic protein content, but not with enhanced glucose uptake, attenuated glucose oxidation, or lactate transport protein.

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.

Oral Lactate Administration Additively Enhances Endurance Training-Induced Increase in Cytochrome C Oxidase Activity in Mouse Soleus Muscle

We tested the hypothesis that oral lactate supplementation increases mitochondrial enzyme activity given the potential role of lactate for inducing mitochondrial biogenesis. In this study, mice were assigned to a saline-ingested sedentary group (S+S; n = 8), a lactate-ingested sedentary group (L+S; n = 9), a saline-ingested training group (S+T; n = 8), and a lactate-ingested training group (L+T; n = 8). Mice in the S+S and S+T groups received saline, whereas mice in the L+S and L+T groups received sodium lactate (equivalent to 5 g/kg of body weight) via oral gavage 5 days a week for 4 weeks. At 30 min after the ingestion, mice in the S+T and L+T groups performed endurance training (treadmill running, 20 m/min, 30 min, 5 days/week). At 30 min after lactate ingestion, the blood lactate level reached peak value (5.8 ± 0.4 mmol/L) in the L+S group. Immediately after the exercise, blood lactate level was significantly higher in the L+T group (9.3 ± 0.9 mmol/L) than in the S+T group (2.7 ± 0.3 mmol/L) (p < 0.01). Following a 4-week training period, a main effect of endurance training was observed in maximal citrate synthase (CS) (p < 0.01; S+T: 117 ± 3% relative to S+S, L+T: 110 ± 3%) and cytochrome c oxidase (COX) activities (p < 0.01; S+T: 126 ± 4%, L+T: 121 ± 4%) in the plantaris muscle. Similarly, there was a main effect of endurance training in maximal CS (p < 0.01; S+T: 105 ± 3%, L+T: 115 ± 2%) and COX activities (p < 0.01; S+T: 113 ± 3%, L+T: 122 ± 3%) in the soleus muscle. In addition, a main effect of oral lactate ingestion was found in maximal COX activity in the soleus (p < 0.05; L+S: 109 ± 3%, L+T: 122 ± 3%) and heart muscles (p < 0.05; L+S: 107 ± 3%, L+T: 107 ± 2.0%), but not in the plantaris muscle. Our results suggest that lactate supplementation may be beneficial for increasing mitochondrial enzyme activity in oxidative phenotype muscle.

Altered expression of lactate dehydrogenase and monocarboxylate transporter involved in lactate metabolism in broiler wooden breast

Wooden breast (WB) results in significant losses to the broiler industry due to reductions in meat quality. While the etiology of WB is unknown, it is believed to be associated with localized hypoxia and decreased lactate levels in skeletal muscles, indicating the presence of altered lactate metabolism in WB. We hypothesized that the expression levels of the major signaling molecules that control lactate metabolism, including lactate dehydrogenases (LDHA and LDHB) and monocarboxylate transporters (MCT1 and MCT4), were altered in WB. Therefore, the objectives of this study were to evaluate whether there were changes in mRNA and protein levels of LDHA, LDHB, MCT1, and MCT4 in WB compared to normal breast (NB) muscles. Biochemical analysis for LDH enzyme activity in NB and WB muscles was studied. MicroRNA375 (miR-375) expression, known to be inversely associated with LDHB protein expression in human cells, was also investigated. The level of LDHA mRNA was 1.7-fold lower in WB tissues than in NB tissues (P < 0.0001). However, the LDHA protein levels were similar in WB and NB tissues. In contrast, the levels of LDHB mRNA and protein were 8.4-fold higher (P < 0.002) and 13.6-fold higher (P < 0.02) in WB than in NB tissues, respectively. The level of miR-375 was not different between WB and NB muscles. The specific LDH isoenzyme activity that converted lactate to pyruvate was 1.8-fold lower in WB compared to NB tissues (P < 0.01). The level of MCT1 mRNA was 2.3-fold higher in WB than those in NB muscles (P < 0.02). However, this upregulation was not observed with MCT1 protein expression levels. The expression levels of MCT4 mRNA and protein were elevated 2.8-fold (P < 0.02) and 3.5-fold (P < 0.004) in WB compared to NB tissues, respectively. Our current findings suggest the potential roles of LDHB and MCT4 on lactate metabolism and provide a unique molecular elucidation for altered lactate homeostasis in WB muscles of broilers.

New Insights into the Lactate Shuttle: Role of MCT4 in the Modulation of the Exercise Capacity

Lactate produced by muscle during high-intensity activity is an important end product of glycolysis that supports whole body metabolism. The lactate shuttle model suggested that lactate produced by glycolytic muscle fibers is utilized by oxidative fibers. MCT4 is a proton coupled monocarboxylate transporter preferentially expressed in glycolytic muscle fibers and facilitates the lactate efflux. Here we investigated the exercise capacity of mice with disrupted lactate shuttle due to global deletion of MCT4 (MCT4^-/-) or muscle-specific deletion of the accessory protein Basigin (iMSBsg^-/-). Although MCT4^-/- and iMSBsg^-/- mice have normal muscle morphology and contractility, only MCT4^-/- mice exhibit an exercise intolerant phenotype. In vivo measurements of compound muscle action potentials showed a decrement in the evoked response in the MCT4^-/- mice. This was accompanied by a significant structural degeneration of the neuromuscular junctions (NMJs). We propose that disruption of the lactate shuttle impacts motor function and destabilizes the motor unit.

Effects of lactate administration on mitochondrial enzyme activity and monocarboxylate transporters in mouse skeletal muscle

Growing evidence shows that lactate is not merely an intermediate metabolite, but also a potential signaling molecule. However, whether daily lactate administration induces physiological adaptations in skeletal muscle remains to be elucidated. In this study, we first investigated the effects of daily lactate administration (equivalent to 1 g/kg of body weight) for 3 weeks on mitochondrial adaptations in skeletal muscle. We demonstrated that 3-week lactate administration increased mitochondrial enzyme activity (citrate synthase, 3-hydroxyacyl CoA dehydrogenase, and cytochrome c oxidase) in the plantaris muscle, but not in the soleus muscle. MCT1 and MCT4 protein contents were not different after 3-week lactate administration. Next, we examined whether lactate administration enhances training-induced adaptations in skeletal muscle. Lactate administration prior to endurance exercise training (treadmill running, 20 m/min, 60 min/day), which increased blood lactate concentration during exercise, enhanced training-induced mitochondrial enzyme activity in the skeletal muscle after 3 weeks. MCT protein content and blood lactate removal were not different after 3-week lactate administration with exercise training compared to exercise training alone. In a single bout experiment, lactate administration did not change the phosphorylation state of AMPK, ACC, p38 MAPK, and CaMKII in skeletal muscle. Our results suggest that lactate can be a key factor for exercise-induced mitochondrial adaptations, and that the efficacy of high-intensity training is, at least partly, attributed to elevated blood lactate concentration.

Prostate Cancer Energetics and Biosynthesis

Cancers must alter their metabolism to satisfy the increased demand for energy and to produce building blocks that are required to create a rapidly growing tumor. Further, for cancer cells to thrive, they must also adapt to an often changing tumor microenvironment, which can present new metabolic challenges (ex. hypoxia) that are unfavorable for most other cells. As such, altered metabolism is now considered an emerging hallmark of cancer. Like many other malignancies, the metabolism of prostate cancer is considerably different compared to matched benign tissue. However, prostate cancers exhibit distinct metabolic characteristics that set them apart from many other tumor types. In this chapter, we will describe the known alterations in prostate cancer metabolism that occur during initial tumorigenesis and throughout disease progression. In addition, we will highlight upstream regulators that control these metabolic changes. Finally, we will discuss how this new knowledge is being leveraged to improve patient care through the development of novel biomarkers and metabolically targeted therapies.

Lactic Acid: No Longer an Inert and End-Product of Glycolysis

For decades, lactic acid has been considered a dead-end product of glycolysis. Research in the last 20+ years has shown otherwise. Through its transporters (MCTs) and receptor (GPR81), lactic acid plays a key role in multiple cellular processes, including energy regulation, immune tolerance, memory formation, wound healing, ischemic tissue injury, and cancer growth and metastasis. We summarize key findings of lactic acid signaling, functions, and many remaining questions.

Clinical and Functional Relevance of the Monocarboxylate Transporter Family in Disease Pathophysiology and Drug Therapy

The solute carrier (SLC) SLC16 gene family comprises 14 members and encodes for monocarboxylate transporters (MCTs), which mediate the absorption and distribution of monocarboxylic compounds across plasma membranes. As the knowledge about their physiological function, activity, and regulation increases, their involvement and contribution to cancer and other diseases become increasingly evident. Moreover, promising opportunities for therapeutic interventions by directly targeting their endogenous functions or by exploiting their ability to deliver drugs to specific organ sites emerge.

The Drug of Abuse Gamma-Hydroxybutyric Acid Exhibits Tissue-Specific Nonlinear Distribution

The drug of abuse γ-hydroxybutyric acid (GHB) demonstrates complex toxicokinetics with dose-dependent metabolic and renal clearance. GHB is a substrate of monocarboxylate transporters (MCTs) which are responsible for the saturable renal reabsorption of GHB. MCT expression is observed in many tissues and therefore may impact the tissue distribution of GHB. The objective of the present study was to evaluate the tissue distribution kinetics of GHB at supratherapeutic doses. GHB (400, 600, and 800 mg/kg iv) or GHB 600 mg/kg plus L-lactate (330 mg/kg iv bolus followed by 121 mg/kg/h infusion) was administered to rats and blood and tissues were collected for up to 330 min post-dose. K _p values for GHB varied in both a tissue- and dose-dependent manner and were less than 0.5 (except in the kidney). Nonlinear partitioning was observed in the liver (0.06 at 400 mg/kg to 0.30 at 800 mg/kg), kidney (0.62 at 400 mg/kg to 0.98 at 800 mg/kg), and heart (0.15 at 400 mg/kg to 0.29 at 800 mg/kg), with K _p values increasing with dose consistent with saturation of transporter-mediated efflux. In contrast, lung partitioning decreased in a dose-dependent manner (0.43 at 400 mg/kg to 0.25 at 800 mg/kg) suggesting saturation of active uptake. L-lactate administration decreased K _p values in liver, striatum, and hippocampus and increased K _p values in lung and spleen. GHB demonstrates tissue-specific nonlinear distribution consistent with the involvement of monocarboxylate transporters. These observed complexities are likely due to the involvement of MCT1 and 4 with different affinities and directionality for GHB transport.

Monocarboxylate transporter 1 contributes to growth factor-induced tumor cell migration independent of transporter activity

Tumor progression to metastatic disease contributes to the vast majority of incurable cancer. Understanding the processes leading to advanced stage cancer is important for the development of future therapeutic strategies. Here, we establish a connection between tumor cell migration, a prerequisite to metastasis, and monocarboxylate transporter 1 (MCT1). MCT1 transporter activity is known to regulate aspects of tumor progression and, as such, is a clinically relevant target for treating cancer. Knockdown of MCT1 expression caused decreased hepatocyte growth factor (HGF)-induced as well as epidermal growth factor (EGF)-induced tumor cell scattering and wound healing. Western blot analysis suggested that MCT1 knockdown (KD) hinders signaling through the HGF receptor (c-Met) but not the EGF receptor. Exogenous, membrane-permeable MCT1 substrates were not able to rescue motility in MCT1 KD cells, nor was pharmacologic inhibition of MCT1 able to recapitulate decreased cell motility as seen with MCT1 KD cells, indicating transporter activity of MCT1 was dispensable for EGF- and HGF-induced motility. These results indicate MCT1 expression, independent of transporter activity, is required for growth factor-induced tumor cell motility. The findings presented herein suggest a novel function for MCT1 in tumor progression independent of its role as a monocarboxylate transporter.

Reexamining cancer metabolism: lactate production for carcinogenesis could be the purpose and explanation of the Warburg Effect

Herein, we use lessons learned in exercise physiology and metabolism to propose that augmented lactate production (‘lactagenesis’), initiated by gene mutations, is the reason and purpose of the Warburg Effect and that dysregulated lactate metabolism and signaling are the key elements in carcinogenesis. Lactate-producing (‘lactagenic’) cancer cells are characterized by increased aerobic glycolysis and excessive lactate formation, a phenomenon described by Otto Warburg 93 years ago, which still remains unexplained. After a hiatus of several decades, interest in lactate as a player in cancer has been renewed. In normal physiology, lactate, the obligatory product of glycolysis, is an important metabolic fuel energy source, the most important gluconeogenic precursor, and a signaling molecule (i.e. a ‘lactormone’) with major regulatory properties. In lactagenic cancers, oncogenes and tumor suppressor mutations behave in a highly orchestrated manner, apparently with the purpose of increasing glucose utilization for lactagenesis purposes and lactate exchange between, within and among cells. Five main steps are identified (i) increased glucose uptake, (ii) increased glycolytic enzyme expression and activity, (iii) decreased mitochondrial function, (iv) increased lactate production, accumulation and release and (v) upregulation of monocarboxylate transporters MTC1 and MCT4 for lactate exchange. Lactate is probably the only metabolic compound involved and necessary in all main sequela for carcinogenesis, specifically: angiogenesis, immune escape, cell migration, metastasis and self-sufficient metabolism. We hypothesize that lactagenesis for carcinogenesis is the explanation and purpose of the Warburg Effect. Accordingly, therapies to limit lactate exchange and signaling within and among cancer cells should be priorities for discovery.

Transcriptome Analysis Reveals Long Intergenic Noncoding RNAs Contributed to Growth and Meat Quality Differences between Yorkshire and Wannanhua Pig

There are major differences between Yorkshire (lean-type) and Wannanhua pig (fat-type) in terms of growth performance and meat quality. Long intergenic noncoding RNAs (lincRNAs) are a class of regulators that are involved in numerous biological processes and widely identified in many species. However, the role of lincRNAs in pig is largely unknown, and the mechanisms by which they affect growth and meat quality are elusive. In this study, we used published data to identify 759 lincRNAs in porcine longissimus dorsi muscle. These putative lincRNAs shared many features with mammalian lincRNAs, such as shorter length and fewer exons. Gene ontology and pathway analysis indicated that many potential target genes (PTGs) of lincRNAs were involved in muscle growth-related and meat quality-related biological processes. Moreover, we constructed a co-expression network between differentially expressed lincRNAs (DELs) and their PTGs, and found a potential mechanism that most DELs can use to upregulate their PTGs, which may finally contribute to the growth and meat quality differences between the two breeds through an unknown manner. This work details some lincRNAs and their PTGs related to muscle growth or meat quality, and facilitates future research on the roles of lincRNAs in these two types of pig, as well as molecular-assisted breeding for pig.

Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice

The purpose of this study was to investigate the effects of a partial suppression of monocarboxylate transporter (MCT)-1 on skeletal muscle pH, energetics, and function (MCT1^+/- mice). Twenty-four MCT1^+/- and 13 wild-type (WT) mice were subjected to a rest-exercise-recovery protocol, allowing assessment of muscle energetics (by magnetic resonance spectroscopy) and function. The study included analysis of enzyme activities and content of protein involved in pH regulation. Skeletal muscle of MCT1^+/- mice had lower MCT1 (-61%; P < 0.05) and carbonic anhydrase (CA)-II (-54%; P < 0.05) contents. Although intramuscular pH was higher in MCT1^+/- mice at rest (P < 0.001), the mice showed higher acidosis during the first minute of exercise (P < 0.01). Then, the pH time course was similar among groups until exercise completion. MCT1^+/- mice had higher specific peak (P < 0.05) and maximum tetanic (P < 0.01) forces and lower fatigability (P < 0.001) when compared to WT mice. We conclude that both MCT1 and CAII are involved in the homeostatic control of pH in skeletal muscle, both at rest and at the onset of exercise. The improved muscle function and resistance to fatigue in MCT1^+/- mice remain unexplained.-Chatel, B., Bendahan, D., Hourdé, C., Pellerin, L., Lengacher, S., Magistretti, P., Fur, Y. L., Vilmen, C., Bernard, M., Messonnier, L. A. Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice.

Molecular Characteristics, Regulation, and Function of Monocarboxylate Transporters

Lactate transporters play an important role in the glutamate recycling. Here their kinetics and tissue distribution with emphasis on the brain are addressed. Recent evidence shows their participation in important brain functions that involve intercellular communication, such as hypothalamic glucose sensing. Furthermore, we describe the regulation of their expression and some animal models that have allowed clarification of their functions.

Secondary elements of blood pH variation can influence the effort effectiveness based on adaptive changes within a group of elite athletes

Aim

pH is the direct indicator of the body reaction following the activities performed. Establishing precise correlations between pH and blood biochemical parameters might support the balancing of values during periods of marked physical activity.

Method

We conducted a case study in a group of elite rowers. Twelve athletes were included in the study. Monitoring was carried out by collecting biological samples several times a day: in the morning, 80 minutes pre-workout, 12 hours after the last physical effort performed, at two different times, 10 days apart. Determinations were aimed at adapting the reported biochemical parameters depending on the effort performed. The following parameters were monitored: pH, HCO3, pCO2, pO2, BE, SBE, SBC, Ca++, Mg++, LDH, GPT, T-Pro, and Alb.

Results

The mean value of pH found in athletes was 7.41±0.024. The value obtained was significantly correlated to biochemical parameters such as BE (2.32±1.79), SBC (1.67±1.45), SBE (2.70±1.75). However, bicarbonate (HCO3) was statistically significantly related with SBE, SBC, SBE, and pO2, but did not present a strong association with the pH value (p=0.094). However, values such as Alb, Ca++, LDH, BE, SBC are related to pH value as a result of variations in the data submitted.

Conclusions

The processed data evidence the fact that blood pH, in this case, is significantly influenced by a number of indices that correlate energy system activity, individual adaptation to effort, and the recovery process. The parameters under investigation (SBE, SBC, SBE, CPK, LDH) are associated with pH changes that could confirm the recovery efficiency of the athlete, along with a possible metabolic acidosis/alkalosis.

Localisation of Lactate Transporters in Rat and Rabbit Placentae

The distribution of monocarboxylate transporter (MCT) isoforms 1 and 4, which mediate the plasmalemmal transport of l-lactic and pyruvic acids, has been identified in the placentae of rats and rabbits at different ages of gestation. Groups of three pregnant Sprague-Dawley rats and New Zealand White rabbits were sacrificed on gestation days (GD) 11, 14, 18, or 20 and on GD 13, 18, or 28, respectively. Placentae were removed and processed for immunohistochemical detection of MCT1 and MCT4. In the rat, staining for MCT1 was associated with lakes and blood vessels containing enucleated red blood cells (maternal vessels) while staining for MCT4 was associated with vessels containing nucleated red blood cells (embryofoetal vessels). In the rabbit, staining for MCT1 was associated with blood vessels containing nucleated red blood cells while staining for MCT4 was associated with vessels containing enucleated red blood cells. Strength of staining for MCT1 decreased during gestation in both species, but that for MCT4 was stronger than that for MCT1 and was consistent between gestation days. The results imply an opposite polarity of MCT1 and MCT4 across the trophoblast between rat and rabbit.

Induction of triglyceride accumulation and mitochondrial maintenance in muscle cells by lactate

Muscle exercise induces intramuscular triglyceride (TG) accumulation and promotes mitochondrial maintenance in myotubes. However, the mechanism underlying exercise effects remains unknown. In this study, lactic acid was tested as a signaling molecule in C2C12 myotubes to understand the mechanism. Intracellular TG storage was induced in the cells by sodium lactate. The lactate activity was observed with an inhibition of the cAMP-PKA pathway as indicated by a reduction in the phosphorylation status of CREB (pCREB). Induction of pCREB signal by forskolin was blocked by pretreatment of cells with lactate. The impact of lactate on mitochondrial function was examined with a focus on the activities of two enzymes, MCAT (malonylCoA:ACP transferase) and PDH (pyruvate dehydrogenase). The enzyme activities were induced in the cells by lactate. Expression of the lactate receptor (GPR81) and lactate transporters (MCT1/4) were induced as well by lactate. The lactate activities were observed at concentrations between 4-64 mM, and were not dependent on the increase in intracellular pyruvate. Pyruvate treatment did not generate the same effects in the cells. Those results suggest that lactate may induce intramuscular TG storage and mitochondrial maintenance in myotubes through inhibition of the cAMP pathway by activation of GPR81 in a positive feedback manner.

Changes in expression of monocarboxylate transporters, heat shock proteins and meat quality of Large White Yorkshire and Ghungroo pigs during hot summer period

OBJECTIVE

Present study explores the effect of hot summer period on the glycolytic rate of early post-mortem meat quality of Ghungroo and Large White Yorkshire (LWY) pig and comparative adaptability to high temperature between above breeds by shifting the expression of stress related genes like mono-carboxylate transporters (MCTs) and heat shock proteins (HSPs).

METHODS

Healthy pigs of two different breeds, viz., LYW and Ghungroo (20 from each) were maintained during hot summer period (May to June) with a mean temperature of about 38°C. The pigs were slaughtered and meat samples from the longissimus dorsi (LD) muscles were analyzed for pH, glycogen and lactate content and mRNA expression. Following 24 h of chilling, LD muscle was also taken from the carcasses to evaluate protein solubility and different meat quality measurements.

RESULTS

LWY exhibited significantly (p<0.01) higher plasma cortisol and lactate dehydrogenase concentration than Ghungroo indicating their higher sensitivity to high temperature. LD muscle from LWY pigs revealed lower initial and ultimate pH values and higher drip loss compared to Ghungroo, indicating a faster rate of pH fall. LD muscle of Ghungroo had significantly lower lactate content at 45 min postmortem indicating normal postmortem glycolysis and much slower glycolytic rate at early postmortem. LD muscle of LWY showed rapid postmortem glycolysis, higher drip loss and higher degrees of protein denaturation. Ghungroo exhibited slightly better water holding capacity, lower cooking loss and higher protein solubility. All HSPs (HSP27, HSP70, and HSP90) and MCTs (MCT1, MCT2, and MCT4) in the LD muscle of pigs inclined to increase more in Ghungroo than LWY when exposed to high temperature.

CONCLUSION

Effect of high temperature on the variation of HSPs and MCTs may play a crucial role in thermal tolerance and adaptation to different climatic conditions, pH regulation, muscle acidification, drip loss, protein denaturation and also in postmortem meat quality development.

Continuous Aerobic Training in Individualized Intensity Avoids Spontaneous Physical Activity Decline and Improves MCT1 Expression in Oxidative Muscle of Swimming Rats

Although aerobic training has been shown to affect the lactate transport of skeletal muscle, there is no information concerning the effect of continuous aerobic training on spontaneous physical activity (SPA). Because every movement in daily life (i.e., SPA) is generated by skeletal muscle, we think that it is possible that an improvement of SPA could affect the physiological properties of muscle with regard to lactate transport. The aim of this study was to evaluate the effect of 12 weeks of continuous aerobic training in individualized intensity on SPA of rats and their gene expressions of monocarboxylate transporters (MCT) 1 and 4 in soleus (oxidative) and white gastrocnemius (glycolytic) muscles. We also analyzed the effect of continuous aerobic training on aerobic and anaerobic parameters using the lactate minimum test (LMT). Sixty-day-old rats were randomly divided into three groups: a baseline group in which rats were evaluated prior to initiation of the study; a control group (Co) in which rats were kept without any treatment during 12 weeks; and a chronic exercise group (Tr) in which rats swam for 40 min/day, 5 days/week at 80% of anaerobic threshold during 12 weeks. After the experimental period, SPA of rats was measured using a gravimetric method. Rats had their expression of MCTs determined by RT-PCR analysis. In essence, aerobic training is effective in maintaining SPA, but did not prevent the decline of aerobic capacity and anaerobic performance, leading us to propose that the decline of SPA is not fully attributed to a deterioration of physical properties. Changes in SPA were concomitant with changes in MCT1 expression in the soleus muscle of trained rats, suggestive of an additional adaptive response toward increased lactate clearance. This result is in line with our observation showing a better equilibrium on lactate production-remotion during the continuous exercise (LMT). We propose an approach to combat the decline of SPA of rats in their home cages. This new finding is worth for scientists who work with animal models to study the protective effects of exercise.

Involvement of Monocarboxylate Transporter 4 Expression in Statin-Induced Cytotoxicity

Statins, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, are the most widely used cholesterol-lowering agents for prevention of obstructive cardiovascular events. However, statins can cause a variety of skeletal muscle problems, and exercise leads to an increase in statin-induced muscle injury. Exercise induces the protein content of monocarboxylate transporter 4 (MCT4), which is expressed strongly in skeletal muscle and is thought to play a major role in the transport of metabolically important monocarboxylates such as l-lactate. We previously reported that α-cyano-4-hydroxycinnamate, an MCT4 inhibitor, increased the inhibition of growth of RD cells, a prototypic embryonal rhabdomyosarcoma cell line (an RD cell line), as a model of in vitro skeletal muscle, induced by a statin. However, it is unclear whether statin-induced RD cell cytotoxicity is associated with MCT4 expression. We, therefore, examined the relationship between statin-induced cytotoxicity and MCT4 expression in RD cells. Atorvastatin reduced the number of viable cells and upregulated MCT4, but not MCT1, mRNA level in a concentration-dependent manner. MCT4 knockdown suppressed atorvastatin-, simvastatin-, and fluvastatin-induced reduction of cell viability and apoptosis compared with negative control-treated cells. In this study, we demonstrated that MCT4 expression is associated with statin-induced cytotoxicity.

Understanding Muscle Dysfunction in Chronic Fatigue Syndrome

Introduction. Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a debilitating disorder of unknown aetiology, characterised by severe disabling fatigue in the absence of alternative diagnosis. Historically, there has been a tendency to draw psychological explanations for the origin of fatigue; however, this model is at odds with findings that fatigue and accompanying symptoms may be explained by central and peripheral pathophysiological mechanisms, including effects of the immune, oxidative, mitochondrial, and neuronal pathways. For example, patient descriptions of their fatigue regularly cite difficulty in maintaining muscle activity due to perceived lack of energy. This narrative review examined the literature for evidence of biochemical dysfunction in CFS/ME at the skeletal muscle level. Methods. Literature was examined following searches of PUB MED, MEDLINE, and Google Scholar, using key words such as CFS/ME, immune, autoimmune, mitochondria, muscle, and acidosis. Results. Studies show evidence for skeletal muscle biochemical abnormality in CFS/ME patients, particularly in relation to bioenergetic dysfunction. Discussion. Bioenergetic muscle dysfunction is evident in CFS/ME, with a tendency towards an overutilisation of the lactate dehydrogenase pathway following low-level exercise, in addition to slowed acid clearance after exercise. Potentially, these abnormalities may lead to the perception of severe fatigue in CFS/ME.

Prolonged hypoxia induces monocarboxylate transporter-4 expression in mesenchymal stem cells resulting in a secretome that is deleterious to cardiovascular repair

MSCs encounter extended hypoxia in the wound microenvironment yet little is known about their adaptability to this prolonged hypoxic milieu. In this study, we evaluated the cellular and molecular response of MSCs in extended hypoxia (1% O2 ) versus normoxia (20% O2 ) culture. Prolonged hypoxia induced a switch toward anaerobic glycolysis transcriptome and a dramatic increase in the transcript and protein levels of monocarboxylate transporter-4 (MCT4) in MSCs. To clarify the impact of MCT4 upregulation on MSC biology, we generated MSCs which stably overexpressed MCT4 (MCT4-MSCs) at levels similar to wild-type MSCs following prolonged hypoxic culture. Consistent with its role to efflux lactate to maintain intracellular pH, MCT4-MSCs demonstrated reduced intracellular lactate. To explore the in vivo significance of MCT4 upregulation in MSC therapy, mice were injected intramuscularly following MI with control (GFP)-MSCs, MCT4-MSCs, or MSCs in which MCT4 expression was stably silenced (KDMCT4-MSCs). Overexpression of MCT4 worsened cardiac remodeling and cardiac function whereas silencing of MCT4 significantly improved cardiac function. MCT4-overexpressing MSC secretome induced reactive oxygen species-mediated cardiomyocyte but not fibroblast apoptosis in vitro and in vivo; lactate alone recapitulated the effects of the MCT4-MSC secretome. Our findings suggest that lactate extruded by MCT4-overexpressing MSCs preferentially induced cell death in cardiomyocytes but not in fibroblasts, leading ultimately to a decline in cardiac function and increased scar size. A better understanding of stem cells response to prolonged hypoxic stress and the resultant stem cell-myocyte/fibroblast cross-talk is necessary to optimize MSC-based therapy for cardiac regeneration.

Direct dynamic measurement of intracellular and extracellular lactate in small-volume cell suspensions with (13)C hyperpolarised NMR

Hyperpolarised (HP) (13)C NMR allows enzymatic activity to be probed in real time in live biological systems. The use of in vitro models gives excellent control of the cellular environment, crucial in the understanding of enzyme kinetics. The increased conversion of pyruvate to lactate in cancer cells has been well studied with HP (13)C NMR. Unfortunately, the equally important metabolic step of lactate transport out of the cell remains undetected, because intracellular and extracellular lactate are measured as a single resonance. Furthermore, typical experiments must be performed using tens of millions of cells, a large amount which can lead to a costly and sometimes highly challenging growing procedure. We present a relatively simple set-up that requires as little as two million cells with the spectral resolution to separate the intracellular and extracellular lactate resonances. The set-up is tested with suspensions of prostate cancer carcinoma cells (PC3) in combination with HP [1-(13)C]pyruvate. We obtained reproducible pyruvate to lactate label fluxes of 1.2 and 1.7 nmol/s per million cells at 2.5 and 5.0 mM pyruvate concentrations. The existence of a 3-Hz chemical shift difference between intracellular and extracellular lactate enabled us to determine the lactate transport rates in PC3. We deduced a lactate export rate of 0.3 s(-1) and observed a decrease in lactate transport on addition of the lactate transport inhibitor α-cyano-4-hydroxycinnamic acid.

Identification of key binding site residues of MCT1 for AR-C155858 reveals the molecular basis of its isoform selectivity

The proton-linked monocarboxylate transporters (MCTs) are required for lactic acid transport into and out of all mammalian cells. Thus, they play an essential role in tumour cells that are usually highly glycolytic and are promising targets for anti-cancer drugs. AR-C155858 is a potent MCT1 inhibitor (K_i ~2 nM) that also inhibits MCT2 when associated with basigin but not MCT4. Previous work [Ovens, M.J. et al. (2010) Biochem. J. 425, 523–530] revealed that AR-C155858 binding to MCT1 occurs from the intracellular side and involves transmembrane helices (TMs) 7–10. In the present paper, we generate a molecular model of MCT4 based on our previous models of MCT1 and identify residues in the intracellular substrate-binding cavity that differ significantly between MCT4 and MCT1/MCT2 and so might account for differences in inhibitor binding. We tested their involvement using site-directed mutagenesis (SDM) of MCT1 to change residues individually or in combination with their MCT4 equivalent and determined inhibitor sensitivity following expression in Xenopus oocytes. Phe³⁶⁰ and Ser³⁶⁴ were identified as important for AR-C155858 binding with the F³⁶⁰Y/S³⁶⁴G mutant exhibiting >100-fold reduction in inhibitor sensitivity. To refine the binding site further, we used molecular dynamics (MD) simulations and additional SDM. This approach implicated six more residues whose involvement was confirmed by both transport studies and [³H]-AR-C155858 binding to oocyte membranes. Taken together, our data imply that Asn¹⁴⁷, Arg³⁰⁶ and Ser³⁶⁴ are important for directing AR-C155858 to its final binding site which involves interaction of the inhibitor with Lys³⁸, Asp³⁰² and Phe³⁶⁰ (residues that also play key roles in the translocation cycle) and also Leu²⁷⁴ and Ser²⁷⁸.

A combination of molecular modelling, site-directed mutagenesis and molecular dynamics simulations define the binding site of MCT1 for AR-C155858, a potent and specific inhibitor. Key amino acids within the binding site differ between MCT1 and MCT4 accounting for isoform specificity.

Exploring the quantitative relationship between metabolism and enzymatic phenotype by physiological modeling of glucose metabolism and lactate oxidation in solid tumors

Molecular imaging using PET or hyperpolarized MRI can characterize tumor phenotypes by assessing the related metabolism of certain substrates. However, the interpretation of the substrate turnover in terms of a pathophysiological understanding is not straightforward and only semiquantitative. The metabolism of imaging probes is influenced by a number of factors, such as the microvascular structure or the expression of key enzymes. This study aims to use computational simulation to investigate the relationship between the metabolism behind molecular imaging and the underlying tumor phenotype. The study focused on the pathways of glucose metabolism and lactate oxidation in order to establish the quantitative relationship between the expression of several transporters (GLUT, MCT1 and MCT4), expression of the enzyme hexokinase (HK), microvasculature and the metabolism of glucose or lactate and the extracellular pH distribution. A computational model for a 2D tumor tissue phantom was constructed and the spatio-temporal evolution of related species (e.g. oxygen, glucose, lactate, protons, bicarbonate ions) was estimated by solving reaction-diffusion equations. The proposed model was tested by the verification of the simulation results using in vivo and in vitro literature data. The influences of different expression levels of GLUT, MCT1, MCT4, HK and microvessel distribution on substrate concentrations were analyzed. The major results are consistent with experimental data (e.g. GLUT is more influential to glycolytic flux than HK; extracellular pH is not correlated with MCT expressions) and provide theoretical interpretation of the co-influence of multiple factors of the tumor microenvironment. This computational simulation may assist the generation of hypotheses to bridge the discrepancy between tumor metabolism and the functions of transporters and enzymes. It has the potential to accelerate the development of multi-modal imaging strategies for assessment of tumor phenotypes.

Novel therapeutic targets in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic immune-mediated liver disease characterized by progressive cholestasis, biliary fibrosis and eventually cirrhosis. It results in characteristic symptoms with marked effects on life quality. The advent of large patient cohorts has challenged the view of PBC as a benign condition treated effectively by the single licensed therapy-ursodeoxycholic acid (UDCA). UDCA nonresponse or under-response has a major bearing on outcome, substantially increasing the likelihood that liver transplantation will be required or that patients will die of the disease. In patients with high-risk, treatment-unresponsive or highly symptomatic disease the need for new treatment approaches is clear. Evolution in our understanding of disease mechanisms is rapidly leading to the advent of new and re-purposed therapeutic agents targeting key processes. Notable opportunities are offered by targeting what could be considered as the 'upstream' immune response, 'midstream' biliary injury and 'downstream' fibrotic processes. Combination therapy targeting several pathways or the development of novel agents addressing multiple components of the disease pathway might be required. Ultimately, PBC therapeutics will require a stratified approach to be adopted in practice. This Review provides a current perspective on potential approaches to PBC treatment, and highlights the challenges faced in evaluating and implementing those treatments.

MCT1 and MCT4 kinetic of mRNA expression in different tissues after aerobic exercise at maximal lactate steady state workload

We evaluate the mRNA expression of monocarboxylate transporters 1 and 4 (MCT1 and MCT4) in skeletal muscle (soleus, red and white gastrocnemius), heart and liver tissues in mice submitted to a single bout of swimming exercise at the maximal lactate steady state workload (MLSSw). After 72 h of MLSS test, the animals were submitted to a swimming exercise session for 25 min at individual MLSSw. Tissues and muscle samples were obtained at rest (control, n=5), immediately (n=5), 5 h (n=5) and 10 h (n=5) after exercise for determination of the MCT1 and MCT4 mRNA expression (RT-PCR). The MCT1 mRNA expression in liver increased after 10 h in relation to the control, immediate and 5 h groups, but the MCT4 remained unchanged. The MCT1 mRNA expression in heart increased by 31 % after 10 h when compared to immediate, but no differences were observed in relation to the control group. No significant differences were observed for red gastrocnemius in MCT1 and MCT4 mRNA expression. However, white gastrocnemius increased MCT1 mRNA expression immediately when compared to rest, 5 and 10 h test groups. In soleus muscle, the MCT1 mRNA expression increased immediately, 5 and 10 h after exercise when compared to the control. In relation to MCT4 mRNA expression, the soleus increased immediately and 10 h after acute exercise when compared to the control group. The soleus, liver and heart were the main tissues that showed improved the MCT1 mRNA expression, indicating its important role in controlling MLSS concentration in mice.

Sublethal toxicity of commercial formulations of deltamethrin and permethrin on selected biochemical constituents and enzyme activities in liver and muscle tissues of Anabas testudineus

The freshwater fish Anabas testudineus was exposed for 21 days to two commercial formulations of synthetic pyrethroids deltamethrin and permethrin at sublethal concentrations of 0.007 and 0.0007 mg L(-1), and 0.093 and 0.0093 mg L(-1), that represented 10% and 1%, respectively, of the 96 h LC50 of these two pesticides for this fish. The glycogen, protein and lactic acid contents, along with succinate dehydrogenase (SDH), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) enzyme activities in liver and muscle tissues of control and pesticide-exposed fish were estimated. When compared with those of control fish, significant depletion of glycogen content was observed in liver, and that of protein in muscle tissue of fish treated with both the pesticides at their higher as well as lower concentrations. Lactic acid reduction was significant only in fish muscle treated with deltamethrin. SDH level was reduced significantly in both liver and muscle tissues except in fish exposed to 0.0093 mg L(-1) permethrin. AST level was reduced significantly in liver and muscle tissues and ALT in muscle tissue of deltamethrin treated fish only. It is concluded that deltamethrin, a type-II pyrethroid, is more toxic to fish than the type-I pyrethroid permethrin and is capable of rendering toxicity at a dose as low as 1% of its LC50 value.

Translational insight into statin-induced muscle toxicity: from cell culture to clinical studies

Statins are lipid-lowering drugs used widely to prevent and treat cardiovascular and coronary heart diseases. These drugs are among the most commonly prescribed medicines intended for long-term use. In general, statins are well tolerated. However, muscular adverse effects appear to be the most common obstacle that limits their use, resulting in poor patient compliance or even drug discontinuation. In addition, rare but potentially fatal cases of rhabdomyolysis have been reported with the use of these drugs, especially in the presence of certain risk factors. Previous reports have investigated statin-induced myotoxicity in vivo and in vitro using a number of cell lines, muscle tissues, and laboratory animals, in addition to randomized clinical trials, observational studies, and case reports. None of them have compared directly results from laboratory investigations with clinical observations of statin-related muscular adverse effects. To the best of our knowledge this is the first review article that combines laboratory investigation with clinical aspects of statin-induced myotoxicity. By reviewing published literature of in vivo, in vitro, and clinically relevant studies of statin myotoxicity, we aim to translate this important drug-related problem to establish a clear picture of proposed mechanisms that explain the risk factors and describe the diagnostic approaches currently used for evaluating the degree of muscle damage induced by these agents. This review provides baseline novel translational insight that can be used to enhance the safety profile, to minimize the chance of progression of these adverse effects to more severe and potentially fatal rhabdomyolysis, and to improve the overall patient compliance and adherence to long-term statin therapy.

Effects of sciatic nerve transection on glucose uptake in the presence and absence of lactate in the frog dorsal root ganglia and spinal cord

Frogs have been used as an alternative model to study pain mechanisms because the simplicity of their nervous tissue and the phylogenetic aspect of this question. One of these models is the sciatic nerve transection (SNT), which mimics the clinical symptoms of “phantom limb”, a condition that arises in humans after amputation or transverse spinal lesions. In mammals, the SNT increases glucose metabolism in the central nervous system, and the lactate generated appears to serve as an energy source for nerve cells. An answerable question is whether there is elevated glucose uptake in the dorsal root ganglia (DRG) after peripheral axotomy. As glucose is the major energy substrate for frog nervous tissue, and these animals accumulate lactic acid under some conditions, bullfrogs Lithobates catesbeianus were used to demonstrate the effect of SNT on DRG and spinal cord 1-[14C] 2-deoxy-D-glucose (14C-2-DG) uptake in the presence and absence of lactate. We also investigated the effect of this condition on the formation of 14CO2 from 14C-glucose and 14C-L-lactate, and plasmatic glucose and lactate levels. The 3-O-[14C] methyl-D-glucose (14C-3-OMG) uptake was used to demonstrate the steady-state tissue/medium glucose distribution ratio under these conditions. Three days after SNT, 14C-2-DG uptake increased, but 14C-3-OMG uptake remained steady. The increase in 14C-2-DG uptake was lower when lactate was added to the incubation medium. No change was found in glucose and lactate oxidation after SNT, but lactate and glucose levels in the blood were reduced. Thus, our results showed that SNT increased the glucose metabolism in the frog DRG and spinal cord. The effect of lactate on this uptake suggests that glucose is used in glycolytic pathways after SNT.