Role of monocarboxylate transporters in AMPK-mediated protection against excitotoxic injury in the rat retina

Activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway protects against N-methyl-D-aspartic acid (NMDA)-induced excitotoxic retinal injury. AMPK activation enhances fatty acid metabolism and ketone body synthesis. Ketone bodies are transported into neurons by monocarboxylate transporters (MCTs) and exert neuroprotective effects. In this study, we examined the distribution and expression levels of MCT1 and MCT2 in the retina and analyzed the effects of pharmacological inhibition of MCTs on the protective effects of metformin and 5-aminoimidazole-4-carboxamide (AICAR), activators of AMPK, against NMDA-induced retinal injury in rats. MCT1 was expressed in the blood vessels, processes of astrocytes and Müller cells, and inner segments of photoreceptors in the rat retina, whereas MCT2 was expressed in neuronal cells in the ganglion cell layer (GCL) and in astrocyte processes. The expression levels of MCT2, but not MCT1, decreased one day after intravitreal injection of NMDA (200 nmol). Intravitreal injection of NMDA decreased the number of cells in the GCL compared to the vehicle seven days after injection. Simultaneous injection of metformin (20 nmol) or AICAR (50 nmol) with NMDA attenuated NMDA-induced cell loss in the GCL, and these protective effects were attenuated by AR-C155858 (1 pmol), an inhibitor of MCTs. AR-C155858 alone had no significant effect on the retinal structure. These results suggest that AMPK-activating compounds protect against NMDA-induced excitotoxic retinal injury via mechanisms involving MCTs in rats. NMDA-induced neurotoxicity may be associated with retinal neurodegenerative changes in glaucoma and diabetic retinopathy. Therefore, AMPK-activating compounds may be effective in managing these retinal diseases.

MCT4 and CD147 co-localize with MMP14 in invadopodia and support matrix degradation and invasion by breast cancer cells

The lactate-proton cotransporter MCT4 and its chaperone CD147 are upregulated in breast cancers, correlating with decreased patient survival. Here, we test the hypothesis that MCT4 and CD147 favor breast cancer invasion through interdependent effects on extracellular matrix (ECM) degradation. MCT4 and CD147 expression and membrane localization were strongly reciprocally interdependent in MDA-MB-231 breast cancer cells. Knockdown (KD) and overexpression (OE) of MCT4 and/or CD174 in- and decreased, respectively, migration, invasion, and fluorescent gelatin degradation. OE of both proteins increased gelatin degradation and appearance of the matrix metalloprotease (MMP)-generated collagen-I cleavage product reC1M more than each protein alone, suggesting a concerted role in ECM degradation. MCT4 and CD147 co-localized with invadopodia markers at the plasma membrane and with MMP14, the lysosomal marker LAMP-1, and partially with the autophagosome marker LC3, in F-actin-decorated intracellular vesicles. We conclude that MCT4 and CD147 reciprocally regulate each other and interdependently support migration and invasiveness of MDA-MB-231 breast cancer cells. Mechanistically, this involves MCT4-CD147-dependent stimulation of ECM degradation and specifically of MMP-mediated collagen-I degradation. We suggest that the MCT4-CD147 complex is co-delivered to invadopodia with MMP14.

Effects of Elevated Intraocular Pressure on Retinal Ganglion Cell Density and Expression and Interaction of Retinal Aquaporin 9 and Monocarboxylate Transporters

INTRODUCTION

Astrocyte-to-neuron lactate shuttle (ANLS) plays an important role in the energy metabolism of neurons, including retinal ganglion cells (RGCs). Aquaporin 9 (AQP9), which is an aquaglyceroporin that can transport lactate, may be involved in ANLS together with monocarboxylate transporters (MCTs) to maintain RGC function and survival. This study aimed to investigate the impact of elevated intraocular pressure (IOP) on AQP9-MCT interaction and RGC survival.

METHODS

IOP was elevated in Aqp9 knock-out (KO) mice and wild-type (WT) littermates by anterior chamber microbead injection. RGC density was measured by TUBB3 immunostaining on retinal flat mounts. Immunolabeling, immunoblot, and immunoprecipitation were conducted to identify and quantitate expressions of AQP9, MCT1, MCT2, and MCT4 in whole retinas and ganglion cell layer (GCL).

RESULTS

Aqp9 KO and WT mice had similar RGC density at baseline. Microbead injection increased cumulative IOP by approximately 32% up to 4 weeks, resulting in RGC density loss of 42% and 34% in WT and Aqp9 KO mice, respectively, with no statistical difference. In the retina of WT mice, elevated IOP decreased the amount of AQP9, MCT1, and MCT2 protein and changed the AQP9 immunoreactivity and reduced MCT1 and MCT2 immunoreactivities in GCL. Meanwhile, it decreased MCT1 and increased MCT2 that interact with AQP9, without affecting MCT4 expression. Aqp9 gene deletion increased baseline MCT2 expression in the GCL and counteracted IOP elevation regarding MCT1 and MCT2 expressions.

CONCLUSION

The compensatory upregulation of MCT1 and MCT2 with Aqp9 gene deletion and ocular hypertension may reflect the need to maintain lactate transport in the retina for RGC survival.

Pyruvate-conjugation of PEGylated liposomes for targeted drug delivery to retinal photoreceptors

Despite several promising candidates, there is a paucity of drug treatments available for patients suffering from retinal diseases. An important reason for this is the lack of suitable delivery systems that can achieve sufficiently high drug uptake in the retina and its photoreceptors. A promising and versatile method for drug delivery to specific cell types involves transporter-targeted liposomes, i.e., liposomes surface-coated with substrates for transporter proteins highly expressed on the target cell. We identified strong lactate transporter (monocarboxylate transporter, MCT) expression on photoreceptors as a potential target for drug delivery vehicles. To evaluate MCT suitability for drug targeting, we used PEG-coated liposomes and conjugated these with different monocarboxylates, including lactate, pyruvate, and cysteine. Monocarboxylate-conjugated and dye-loaded liposomes were tested on both human-derived cell-lines and murine retinal explant cultures. We found that liposomes conjugated with pyruvate consistently displayed higher cell uptake than unconjugated liposomes or liposomes conjugated with lactate or cysteine. Pharmacological inhibition of MCT1 and MCT2 reduced internalization, suggesting an MCT-dependent uptake mechanism. Notably, pyruvate-conjugated liposomes loaded with the drug candidate CN04 reduced photoreceptor cell death in the murine rd1 retinal degeneration model while free drug solutions could not achieve the same therapeutic effect. Our study thus highlights pyruvate-conjugated liposomes as a promising system for drug delivery to retinal photoreceptors, as well as other neuronal cell types displaying high expression of MCT-type proteins.

Atorvastatin Exerts More Selective Inhibitory Effects on hMCT2 than on hMCT1 and hMCT4

BACKGROUND/AIM

Human monocarboxylate transporter 1 (hMCT1), hMCT2, and hMCT4 transport monocarboxylates, such as L-lactate and pyruvate, with pH dependency. They are often over-expressed in various cancer cells and mediate the energy balance and pH homeostasis. Therefore, hMCT inhibitors can potentially be used as anticancer drugs. However, isoform-selective inhibitors have not yet been well-characterized. In addition, several statins and 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors have been reported to inhibit hMCTs, but their selectivity has not yet been evaluated. In this study, we aimed to determine whether statins could inhibit hMCT1, hMCT2, and hMCT4.

MATERIALS AND METHODS

We expressed hMCT1, hMCT2, and hMCT4 in a heterologous expression system of Xenopus oocytes and performed inhibitory experiments with various statins (fluvastatin, atorvastatin, simvastatin, rosuvastatin, pravastatin, and pitavastatin). As the three-dimensional structure of hMCT2 has been recently reported, docking simulations of statins and their structures were also performed to estimate the inhibition site.

RESULTS

All statins inhibited the transport activities of hMCT1, hMCT2, and hMCT4. In addition, atorvastatin was found to be a potent isoform-selective inhibitor of hMCT2. Docking simulation indicated that atorvastatin could interact with a site surrounded by transmembrane (TM)-2, TM11, and intracellular helix in the TM6/7loop. Therefore, targeting this site may lead to the discovery of more potent hMCT2-selective inhibitors.

CONCLUSION

Atorvastatin exerts selective inhibitory effects on hMCT2. These findings provide insights into the inhibitory mechanism of statins against hMCT1, hMCT2, and hMCT4 and may aid in the development of novel anticancer agents.

Immunohistochemical evaluation and prognostic value of monocarboxylate transporter 1 (MCT1) and 4 (MCT4) in T-cell non-Hodgkin lymphoma

Tumor cells often exhibit the Warburg effect, wherein, they preferentially undergo glycolysis over oxidative phosphorylation for energy production. Monocarboxylate transporter 1 (MCT1) and 4 (MCT4) are critical symporters mediating lactate efflux and preventing intracellular acidification during tumor growth. Numerous studies have focused on inhibiting MCT1 or MCT4 in various cancers. However, its role in T-cell lymphoma (TCL) is not yet investigated owing to the low incidence of TCL. This study was designed to investigate the expression of MCT1/MCT4 in patients with TCL and determine their prognostic value in this cancer. We performed immunohistochemistry to evaluate the expression level of MCT1/MCT4 in 38 TCL tissue samples and then compared their expression among different TCL subgroups, which were formed based on different clinical characteristics. Survival analysis was performed to evaluate the relationship between MCT1/MCT4 expression and both overall survival (OS) and progression-free survival (PFS). Our results revealed that MCT1 and MCT4 expression was significantly increased in TCL tissues compared to the control group. In addition, increased MCT1 expression associated with the female sex, advanced disease stage, increased serum LDH, Ki-67 at ≥ 50%, and intermediate or high-risk groups as categorized by the International Prognostic Index (IPI) score. We also found that increased MCT1 expression may be associated with reduced OS and PFS. In conclusion, MCT1 and MCT4 are overexpressed in patients with TCL and may predict poor prognosis. MCT1 inhibition might be a novel treatment strategy for TCL, and further preclinical trials are required.

Role and Clinical Significance of Monocarboxylate Transporter 8 (MCT8) During Pregnancy

The review aims to summarize the available research focusing on the importance of monocarboxylate transporter (MCT8) in thyroid hormone trafficking across the placenta and fetal development. A systematic search was carried out in PubMed; studies available in English related to "monocarboxylate transporter", "adverse pregnancy", "fetal development," and "thyroid hormone" were identified and assessed. The references within the resulting articles were manually searched. MCT8 is a highly active and selective thyroid hormone transporter that facilitates the cellular uptake of triiodothyronine (T3), thyroxine (T4), reverse triiodothyronine (rT3), and diiodothyronine (T2) in different tissues. MCT8 is expressed in the placenta from the first trimester onwards, allowing the transport of thyroid hormone from mother to fetus. Mutations in MCT8 cause an X-linked disorder known as Allan-Herndon-Dudley syndrome (AHDS), characterized by severe psychomotor impairment and peripheral thyrotoxicosis. Hence, any maternal thyroid dysfunction may cause severe consequences for the fetus and newborn. Further research regarding MCT8 gene expression, polymorphic variation, and adverse pregnancy outcomes must be done to establish that MCT8 is a novel prognostic marker for the early detection of pregnancy-related complications.

Light-intensity exercise improves memory dysfunction with the restoration of hippocampal MCT2 and miRNAs in type 2 diabetic mice

Cognitive decline associated with type 2 diabetes mellitus (T2DM) is a risk factor to impair human health. Although light-intensity exercise prevents hippocampal memory dysfunction in pre-symptomatic T2DM animals by altering hippocampal lactate transport and neurotrophic factors, the effects of light-intensity exercise in an advanced stage of T2DM animals remain unclear. Here, ob/ob mice, an animal model of T2DM, were subjected to light-intensity exercise (5.0 m/min) for 30 min/day, five days/week for four weeks. The effects of light-intensity exercise on hippocampal complications, mRNA expressions of monocarboxylate transporter (MCT), and miRNA levels were assessed. The light-intensity exercise improved hippocampal memory retention in ob/ob mice. Downregulated hippocampal Mct2 mRNA levels in T2DM were improved with light-intensity exercise. Hippocampal mRNA levels of Mct1 and Mct4 were unchanged within groups. Based on miRNA sequencing, sedentary ob/ob mice exhibited that 71 miRNAs were upregulated, and 77 miRNAs were downregulated in the hippocampus. In addition, the exercise significantly increased 24 miRNAs and decreased 4 miRNAs in the T2DM hippocampus. The exercise reversed T2DM-induced alterations of hippocampal 9 miRNAs, including miR-200a-3p. Our findings imply that miR-200a-3p/Mct2 in the hippocampus would be a possible clinical target for treating T2DM-induced memory dysfunction.

Effects of Fibroblast Growth Factor 21 on Lactate Uptake and Usage in Mice with Diabetes-Associated Cognitive Decline

Fibroblast growth factor 21 (FGF21) is an endocrine hormone that exerts beneficial effects on glucose and lipid metabolic homeostasis. However, the impact of FGF21 on type 1 diabetes-associated cognitive decline (DACD) and its mechanisms of action remain unclear. In this study, we aimed to evaluate the effects of FGF21 on lactate uptake and usage in a mouse model of streptozotocin-induced DACD. Six-week-old male C57BL/6 mice were divided into the control, diabetic, and FGF21 (which received 2 mg/kg recombinant human FGF21) groups. At the end of the treatment period, learning and memory performance, nuclear magnetic resonance-based metabonomics, and expressions of various hippocampal protein were analyzed to determine the efficacy of FGF21. The results showed that compared to the control mice, the diabetic mice had reduced long-term memory performance after the hyperglycemic insult; decreased hippocampal levels of lactate dehydrogenase-B (LDH-B) activity, bioenergy metabolites, and monocarboxylate transporter 2 (MCT2); and increased lactate levels. Impaired phosphoinositide 3-kinase (PI3K) signaling was also observed in the diabetic mice. However, FGF21 treatment improved LDH-B activity, β-nicotinamide adenine dinucleotide, and ATP levels, and increased MCT2 expression and PI3K signaling pathway, which in turn improved the learning and memory defects. These findings demonstrated that the effects of FGF21 on DACD were associated with its ability to improve LDH-B-mediated lactate usage and MCT2-dependent lactate uptake. Further, these beneficial effects of FGF21 in the hippocampus were mediated by the PI3K signaling pathways.

Of axons that struggle to make ends meet: Linking axonal bioenergetic failure to programmed axon degeneration

Axons are the long, fragile, and energy-hungry projections of neurons that are challenging to sustain. Together with their associated glia, they form the bulk of the neuronal network. Pathological axon degeneration (pAxD) is a driver of irreversible neurological disability in a host of neurodegenerative conditions. Halting pAxD is therefore an attractive therapeutic strategy. Here we review recent work demonstrating that pAxD is regulated by an auto-destruction program that revolves around axonal bioenergetics. We then focus on the emerging concept that axonal and glial energy metabolism are intertwined. We anticipate that these discoveries will encourage the pursuit of new treatment strategies for neurodegeneration.

Transport function, regulation, and biology of human monocarboxylate transporter 1 (hMCT1) and 4 (hMCT4)

Human monocarboxylate transporter 1 (hMCT1) and 4 (hMCT4) are involved in the proton-dependent transport of monocarboxylates such as L-lactate, which play an essential role in cellular metabolism and pH regulation. hMCT1 and 4 are overexpressed in a number of cancers, and polymorphisms in hMCT1 have been reported to be associated with the prognosis of some cancers. Accordingly, recent advances have focused on the inhibition of these transporters as a novel therapeutic strategy in cancers. To screen for MCT inhibitors for clinical application, it is important to study MCT function and regulation, and the effect of compounds on them, using human-derived cells. In this review, we focus on the transport function, regulation, and biology of hMCT1 and hMCT4, and the effects of genetic variation in these transporters in humans.

Lactate released from human fibroblasts enhances Ni elution from Ni plate

Elution of Ni ions from medical devices induces inflammation and toxicity. We previously reported that elution of Ni ions from Ni wires induced COX-2 expression and increased lactate production, but whether lactate is involved in the further elution of Ni ions remains unclear. In this study, using KMST-6, a human fibroblast cell line, we examined the molecular mechanisms by which Ni ions increase lactate release and the role of lactate in enhancing the elution of Ni ions. When KMST-6 cells were incubated on a Ni plate or stimulated with NiCl₂ (1 mM), the expression of glucose transporter 1 (GLUT1), hexokinase 2 (HK2), and lactate dehydrogenase A (LDHA), and the release of lactate were enhanced. The NiCl₂ (1 mM)-induced expression of these genes was inhibited by a hypoxia-inducible factor-1α (HIF-1α) inhibitor, PX-478 (10-25 μM). Stimulation of cells with a prolyl hydroxylase domain (PHD) inhibitor, roxadustat, increased the expression of these genes, lactate release, and elution of Ni ions at 10 μM. A monocarboxylate transporter-4 (MCT4) inhibitor, syrosingopine, inhibited lactate release from roxadustat-treated cells and reduced the elution of Ni ions by the cells at 10 μM. Finally, syrosingopine (10 μM) reduced the elution of Ni ions by the cells from the Ni plate. These results suggest that elution of Ni ions from metals promotes the production of lactate via HIF-1α-mediated gene expression and causes further Ni elution. Thus, Ni ions show a positive feedback mechanism of Ni elution, and this step may be potentially targeted to protect against metal elution from metal devices.

Brain and muscle adaptation to high-fat diets and exercise: Metabolic transporters, enzymes and substrates in the rat cortex and muscle

There is evidence suggesting that the effects of diet and physical activity on physical and mental well-being are the result of altered metabolic profiles. Though the central and peripheral systems work in tandem, the interactions between peripheral and central changes that lead to these altered states of well-being remains elusive. We measured changes in the metabolic profile of brain (cortex) and muscle (soleus and plantaris) tissue in rats following 5-weeks of treadmill exercise and/or a high-fat diet to evaluate peripheral and central interactions as well as identify any common adaptive mechanisms. To characterize changes in metabolic profiles, we measured relative changes in key metabolic enzymes (COX IV, hexokinase, LDHB, PFK), substrates (BHB, FFA, glucose, lactate, insulin, glycogen, BDNF) and transporters (MCT1, MCT2, MCT4, GLUT1, GLUT3). In the cortex, there was an increase in MCT1 and a decrease in glycogen following the high-fat diet, suggesting an increased reliance on monocarboxylates. Muscle changes were dependent muscle type. Within the plantaris, a high-fat diet increased the oxidative capacity of the muscle likely supported by increased glycolysis, whereas exercise increased the oxidative capacity of the muscle likely supported via increased glycogen synthesis. There was no effect of diet on soleus measurements, but exercise increased its oxidative capacity likely fueled by endogenous and exogenous monocarboxylates. For both the plantaris and soleus, combining exercise training and high-fat diet mediated results, resulting in a middling effect. Together, these results indicate the variable adaptions of two main metabolic pathways: glycolysis and oxidative phosphorylation. The results also suggest a dynamic relationship between the brain and body.

Role of pH Regulatory Proteins and Dysregulation of pH in Prostate Cancer

Prostate cancer is the fourth most commonly diagnosed cancer, and although it is often a slow-growing malignancy, it is the second leading cause of cancer-associated deaths in men and the first in Europe and North America. In many forms of cancer, when the disease is a solid tumor confined to one organ, it is often readily treated. However, when the cancer becomes an invasive metastatic carcinoma, it is more often fatal. It is therefore of great interest to identify mechanisms that contribute to the invasion of cells to identify possible targets for therapy. During prostate cancer progression, the epithelial cells undergo epithelial-mesenchymal transition that is characterized by morphological changes, a loss of cell-cell adhesion, and invasiveness. Dysregulation of pH has emerged as a hallmark of cancer with a reversed pH gradient and with a constitutively increased intracellular pH that is elevated above the extracellular pH. This phenomenon has been referred to as "a perfect storm" for cancer progression. Acid-extruding ion transporters include the Na⁺/H⁺ exchanger NHE1 (SLC9A1), the Na⁺HCO₃^- cotransporter NBCn1 (SLC4A7), anion exchangers, vacuolar-type adenosine triphosphatases, and the lactate-H⁺ cotransporters of the monocarboxylate family (MCT1 and MCT4 (SLC16A1 and 3)). Additionally, carbonic anhydrases contribute to acid transport. Of these, several have been shown to be upregulated in different human cancers including the NBCn1, MCTs, and NHE1. Here the role and contribution of acid-extruding transporters in prostate cancer growth and metastasis were examined. These proteins make significant contributions to prostate cancer progression.

Aqp9 Gene Deletion Enhances Retinal Ganglion Cell (RGC) Death and Dysfunction Induced by Optic Nerve Crush: Evidence that Aquaporin 9 Acts as an Astrocyte-to-Neuron Lactate Shuttle in Concert with Monocarboxylate Transporters To Support RGC Function and Survival

Aquaporin 9 (AQP9) is an aquaglyceroporin that can transport lactate. Accumulating evidence suggests that astrocyte-to-neuron lactate shuttle (ANLS) plays a critical role in energy metabolism in neurons, including retinal ganglion cells (RGCs). To test the hypothesis that AQP9, in concert with monocarboxylate transporters (MCTs), participates in ANLS to maintain function and survival of RGCs, Aqp9-null mice and wild-type (WT) littermates were subjected to optic nerve crush (ONC) with or without intravitreal injection of an MCT2 inhibitor. RGC density was similar between the Aqp9-null mice and WT mice without ONC, while ONC resulted in significantly more RGC density reduction in the Aqp9-null mice than in the WT mice at day 7. Positive scotopic threshold response (pSTR) amplitude values were similar between the two groups without ONC, but were significantly more reduced in the Aqp9-null mice than in the WT mice 7days after ONC. MCT2 inhibitor injection accelerated RGC death and pSTR amplitude reduction only in the WT mice with ONC. Immunolabeling revealed that both RGCs and astrocytes expressed AQP9, that ONC predominantly reduced astrocytic AQP9 expression, and that MCTs 1, 2, and 4 were co-localized with AQP9 at the ganglion cell layer. These retinal MCTs were also co-immunoprecipitated with AQP9 in the WT mice. ONC decreased the co-immunoprecipitation of MCTs 1 and 4, but did not impact co-immunoprecipitation of MCT2. Retinal glucose transporter 1 expression was increased in Aqp9-null mice. Aqp9 gene deletion reduced and increased the intraretinal L-lactate and D-glucose concentrations, respectively. Results suggest that AQP9 acts as the ANLS to maintain function and survival of RGCs.

Reducing monocarboxylate transporter MCT1 worsens experimental diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the most common complications in diabetic patients. Though the exact mechanism for DPN is unknown, it clearly involves metabolic dysfunction and energy failure in multiple cells within the peripheral nervous system. Lactate is an alternate source of metabolic energy that is increasingly recognized for its role in supporting neurons. The primary transporter for lactate in the nervous system, monocarboxylate transporter-1 (MCT1), has been shown to be critical for peripheral nerve regeneration and metabolic support to neurons/axons. In this study, MCT1 was reduced in both sciatic nerve and dorsal root ganglia in wild-type mice treated with streptozotocin (STZ), a common model of type-1 diabetes. Heterozygous MCT1 null mice that developed hyperglycemia following STZ treatment developed a more severe DPN compared to wild-type mice, as measured by greater axonal demyelination, decreased peripheral nerve function, and increased numbness to innocuous low-threshold mechanical stimulation. Given that MCT1 inhibitors are being developed as both immunosuppressive and chemotherapeutic medications, our results suggest that clinical development in patients with diabetes should proceed with caution. Collectively, our findings uncover an important role for MCT1 in DPN and provide a potential lead toward developing novel treatments for this currently untreatable disease.

Targeted sensitization of tumor cells for radiation through monocarboxylate transporters 1 and 4 inhibition in vitro

OBJECTIVES

Monocarboxylate transporters (MCT) 1, 2 and 4 play an important role in tumor metabolism. The amount of lactate transported by MCT's highly correlates with overall survival. Furthermore, glycolysis and hypoxia are possible causes for radiation resistance.

MATERIALS AND METHODS

An oral squamous cell carcinoma cell line (CAL27, ATCC) was analyzed in an in vitro cell assay. After incubation with two different inhibitors for MCT1 (AR-C122982/SR-13800 and AR-C155858/SR-13801, Tocris) or for MCT4 (simvastatin, Sigma-Aldrich and 2-cyano-3-(4-hydroxyphenyl)-2-propenoic acid (CHC), Tocris), cells were irradiated with six gray with a Gammacell 2000 (Nuklear Data). For analysis, cell counting assay, wound healing assay, MTT assay and clonogenic assay were applied.

RESULTS

Cell counting assay showed significant lower results for simvastatin, CHC and for the highest concentrations of AR-C122982 and AR-C155858 (p < 0.03). Additionally, cell counts decreased significantly with irradiation after 72 hours (p < 0.05) only for AR-C122982, CHC and simvastatin. The clonogenic assay confirmed these results with substantially reduced growth when incubated with CHC, simvastatin and AR-C155858 (p < 0.002). Furthermore, MCT1 and 4 inhibition led to highly reduced migration (p < 0.05). There again, comparing the wound healing assay of irradiated to non-irradiated tests showed contrary results (controls: p < 0.001; AR-C155858: p > 0.05; AR-C122982: p > 0.32; CHC: p > 0.1; simvastatin p > 0.1). The MTT assay presented significant effects with MCT1 and 4 inhibition (simvastatin/AR-C122982/CHC: p < 0.007). Irradiated cells showed significantly lower expression after only 48 h compared to non-irradiated cells (simvastatin/AR-C122982/CHC: p < 0.02).

CONCLUSIONS

Inhibition of MCT, especially MCT4 may represent a possible tool to overcome radiation resistance in tumor cell lines.

CLINICAL RELEVANCE

MCT Inhibitors may be used as a possible therapeutic approach to sensitize OSCC to radiation.

MCT2 overexpression rescues metabolic vulnerability and protects retinal ganglion cells in two models of glaucoma

Improving cellular access to energy substrates is one strategy to overcome observed declines in energy production and utilization in the aged and pathologic central nervous system. Monocarboxylate transporters (MCTs), the movers of lactate, pyruvate, and ketone bodies into or out of a cell, are significantly decreased in the DBA/2 J mouse model of glaucoma. In order to confirm MCT decreases are disease-associated, we decreased MCT2 in the retinas of MCT2^fl/+ mice using an injection of AAV2-cre, observing significant decline in ATP production and visual evoked potential. Restoring MCT2 levels in retinal ganglion cells (RGCs) via intraocular injection of AAV2-GFP-MCT2 in two models of glaucoma, the DBA/2 J (D2), and a magnetic bead model of ocular hypertension (OHT), preserved RGCs and their function. Viral-mediated overexpression of MCT2 increased RGC density and axon number, reduced energy imbalance, and increased mitochondrial function as measured by cytochrome c oxidase and succinate dehydrogenase activity in both models of glaucoma. Ocular hypertensive mice injected with AAV2:MCT2 had significantly greater P1 amplitude as measured by pattern electroretinogram than mice with OHT alone. These findings indicate overexpression of MCT2 improves energy homeostasis in the glaucomatous visual system, suggesting that expanding energy input options for cells is a viable option to combat neurodegeneration.

Lactate: More Than Merely a Metabolic Waste Product in the Inner Retina

The retina is an extension of the central nervous system and has been considered to be a simplified, more tractable and accessible version of the brain for a variety of neuroscience investigations. The optic nerve displays changes in response to underlying neurodegenerative diseases, such as stroke, multiple sclerosis, and Alzheimer's disease, as well as inner retinal neurodegenerative disease, e.g., glaucoma. Neurodegeneration has increasingly been linked to dysfunctional energy metabolism or conditions in which the energy supply does not meet the demand. Likewise, increasing lactate levels have been correlated with conditions consisting of unbalanced energy supply and demand, such as ischemia-associated diseases or excessive exercise. Lactate has thus been acknowledged as a metabolic waste product in organs with high energy metabolism. However, in the past decade, numerous beneficial roles of lactate have been revealed in the central nervous system. In this context, lactate has been identified as a valuable energy substrate, protecting against glutamate excitotoxicity and ischemia, as well as having signaling properties which regulate cellular functions. The present review aims to summarize and discuss protective roles of lactate in various model systems (in vitro, ex vivo, and in vivo) reflecting the inner retina focusing on lactate metabolism and signaling in inner retinal homeostasis and disease.

Effects of sildenafil citrate on peripheral fatigue and exercise performance after exhaustive swimming exercise in rats

Sildenafil citrate is a potent and selective inhibitor of phosphodiesterase type-5 used to treat erectile dysfunction. We investigated the effects of sildenafil citrate treatment on peripheral fatigue and exercise performance after exhaustive swimming exercise in rats. The rats in the sildenafil citrate-treated groups received sildenafil citrate orally once a day for 14 consecutive days at respective dosage. On the 14 days after starting experiment, each animal was submitted to swimming test with intensity equivalent to overload. The exhaustion was defined as a state in which coordinated movements did not return to the water surface for breathing within 10 sec. Western blot for monocarboxylate transporter (MCT)1, MCT4, and neuronal nitric oxide synthase (nNOS) were performed. Exhaustive swimming exercise decreased time of exhaustion and increased lactate concentration, however, sildenafil citrate enhanced time of exhaustion and decreased lactate concentration. Exhaustive swimming exercise increased MCT1 and MCT4 expressions in the gastrocnemius muscles and sildenafil citrate further enhanced MCT1 and MCT4 expressions in the exhaustive swimming exercise rats. Exhaustive swimming exercise decreased nNOS expression in the gastrocnemius muscles and sildenafil citrate enhanced nNOS expression in the exhaustive swimming exercise rats. The most potent effect appeared in the 20-mg/kg sildenafil citrate. Sildenafil citrate might be proposed as a potential ergogenic aid through antiperipheral fatigue.

Differential expression and immunoreactivity of thyroid hormone transporters MCT8 and OATP1C1 in rat ovary

Thyroid hormones (THs) regulate several physiological processes in female mammals, many of which are related to reproduction such as steroidogenesis in the ovary, oocyte and granulosa cells maturation, follicular development and differentiation, and ovulation. THs actions require the presence of THs transporters to facilitate their cellular uptake and efflux. MCT8 and OATP1C1 are the principal THs transporters. The aim of the present study was to determine the gene expression and cellular localization of MCT8 and OATP1C1 in the rat ovary during the diestrus-II cycle phase. Ovaries of virgin adult rats were histologically processed. Reverse Transcription-PCR and immunohistochemistry analyses for MCT8 and OATP1C1 were done. MCT8 gene expression level was significantly higher (P ≤ 0.01) than that of OATP1C1 in the rat ovary. MCT8 and OATP1C1 were found in all types of ovarian cells but with different immunoreactivity. MCT8 showed stronger immunoreactivity in tertiary and Graafian follicles, corpus luteum and blood vessels, whereas OATP1C1's immunoreactivity was stronger in stroma cells, tunica albuginea, and blood vessels. Our results provide evidence that THs and their transporters are both necessary for ovarian function and that any alteration in these transporters could interfere with reproductive processes such as ovulation and steroidogenesis, compromising fertility.

The impact of lactic acid and medium chain triglyceride on blood glucose, lactate and diurnal motor activity: A re-examination of a treatment of major depression using lactic acid

While investigating the effect of alternative energy substrates on extracellular brain glucose or lactate, Béland-Millar (2017) noted a reduction of physical activity after intraperitoneal administration of lactate and ketone bodies. These observations were similar to an older study that examined the impact of drinking a sodium lactate/lactic acid solution before sleep in hospitalized patients with major depression. Patients and control participants self-reported drowsiness, early sleep onset and better overall sleep after consumption. Some patients showed improved mood after several days of treatment. We re-evaluated the effects of the solution used (0.59 g/kg) as well as several smaller doses (0.47, 0.35, 0.24 and 0.12 g/kg) on blood lactate and glucose in CD-1 mice and on sleep onset associated activity reduction. Because of adverse effects with the lactate/lactic acid solution, we also examined the effects of a medium chain triglyceride (MCT) solution (10, 5, 2.5, and 1 ml/kg) on blood lactate and glucose. Oral gavage administration of lactic acid/lactate produced adverse effects particularly for the largest doses. However consumption of 10 and 5 ml/kg volumes of MCT oils significantly increased blood lactate concentration to levels comparable to Lowenbach's solution without piloerection indicative of adverse effects. To evaluate pre-sleep activity reduction produced by lactate, mice were intraperitoneally administered diluted sodium lactate (2.0 g/kg, 1.0 g/kg, 0.5 g/kg, 0.25 g/kg, or saline) for 6 days, 120 min before their sleep period and their running activity was measured. Larger lactate doses reduced pre-sleep running each day up to 60 min post injection. Smaller doses reduced running after a single treatment only. These results suggest that the modulation of blood lactate levels may be useful in treating sleep onset problems associated with depression.

Monocarboxylate transporter 1 and monocarboxylate transporter 4 in cancer-endothelial co-culturing microenvironments promote proliferation, migration, and invasion of renal cancer cells

Background

The Warburg effect demonstrates the importance of glycolysis in the development of primary and metastatic cancers. We aimed to explore the role of monocarboxylate transporter 1 (MCT1) and MCT4, two essential transporters of lactate, in renal cancer progression during cancer-endothelial cell co-culturing.

Methods

Renal cancer cells (786-O) and human vascular endothelial cells (HUVECs) were single-cultured or co-cultured in transwell membranes in the presence or absence of a MCT-1/MCT-4 specific blocker, 7ACC1. Cell proliferation was evaluated with the CCK-8 kit, while cell migration, after a scratch and invasion in transwell chambers, was evaluated under a microscope. Real-time qPCR and western blot were employed to determine the mRNA and protein levels of MCT1 and MCT4, respectively. The concentration of lactic acid in the culture medium was quantified with an l-Lactic Acid Assay Kit.

Results

786-O cells and HUVECs in the co-culturing mode exhibited significantly enhanced proliferation and migration ability, compared with the cells in the single-culturing mode. The expression of MCT1 and MCT4 was increased in both 786-O cells and HUVECs in the co-culturing mode. Co-culturing promoted the invasive ability of 786-O cells, and markedly increased extracellular lactate. Treatments with 7ACC1 attenuated cell proliferation, migration, and invasion, and down-regulated the levels of MCT1/MCT4 expression and extracellular lactate.

Conclusions

The Warburg effect accompanied with high MCT1/MCT4 expression in the cancer-endothelial microenvironments contributed significantly to renal cancer progression, which sheds new light on targeting MCT1/MCT4 and glycolytic metabolism in order to effectively treat patients with renal cancers.

Response of AMP-activated protein kinase and lactate metabolism of largemouth bass (Micropterus salmoides) under acute hypoxic stress

To study adaptation of largemouth bass (Micropterus salmoides) to hypoxic stress, we investigated physiological responses and lactate metabolism of the fish under acute hypoxia. The objectives of this study were to (a) observe changes in glucose, glycogen, and lactate content; (b) detect the activity of lactate dehydrogenase (LDH) in serum, brain, heart, and liver tissues; and (c) quantify the dynamic gene expression of AMP activated protein kinase alpha (AMPKα), hypoxia-inducible factor-1 alpha (HIF-1α), monocarboxylate transporter 1 (MCT1), monocarboxylate transporter 4 (MCT4), and lactate dehydrogenase-a (LDHa) following exposure to hypoxia. The fish were subjected to two hypoxia stresses (dissolved oxygen [DO] 1.20 ± 0.2 mg/L and 3.50 ± 0.3 mg/L, respectively) for 24 h. Our results showed that hypoxic stress significantly increased the decomposition of liver glycogen and significantly increased the concentration of blood glucose; however, the muscle glycogen content was not significantly decreased, which indicates that liver glycogen was the main energy source under acute hypoxia. Moreover, hypoxia led to accumulation of a large amount of lactic acid in tissues, possibly due to the activity of lactic acid dehydrogenase, but this process was delayed in the heart and brain relative to the liver. Additionally, hypoxia induced the expression of AMPKα, HIF-1α, MCT1, MCT4, and LDHa, suggesting that glycometabolism had switched from aerobic to anaerobic. Our results contribute to a better understanding of the molecular mechanisms of the response to hypoxia in largemouth bass.

Identification of the multivalent PDZ protein PDZK1 as a binding partner of sodium-coupled monocarboxylate transporter SMCT1 (SLC5A8) and SMCT2 (SLC5A12)

Sodium-coupled monocarboxylate transporters SMCT1 (SLC5A8) and SMCT2 (SLC5A12) mediate the high- and low-affinity transport of lactate in the kidney, but their regulatory mechanism is still unknown. Since these two transporters have the PDZ-motif at their C-terminus, the function of SMCTs may be modulated by a protein-protein interaction. To investigate the binding partner(s) of SMCTs in the kidney, we performed yeast two-hybrid (Y2H) screenings of a human kidney cDNA library with the C-terminus of SMCT1 (SMCT1-CT) and SMCT2 (SMCT2-CT) as bait. PDZK1 was identified as a partner for SMCTs. PDZK1 coexpression in SMCT1-expressing HEK293 cells enhanced their nicotinate transport activity. PDZK1, SMCT1, and URAT1 in vitro assembled into a single tri-molecular complex and their colocalization was confirmed in the renal proximal tubule in vivo by immunohistochemistry. These results indicate that the SMCT1-PDZK1 interaction thus plays an important role in both lactate handling as well as urate reabsorption in the human kidney.

Norepinephrine control of ventromedial hypothalamic nucleus glucoregulatory neurotransmitter expression in the female rat: Role of monocarboxylate transporter function

The ventromedial hypothalamic nucleus (VMN) is a critical component of the neural circuitry that regulates glucostasis. Astrocyte glycogen is a vital reserve of glucose and its oxidizable metabolite L-lactate. In hypoglycemic female rats, estradiol-dependent augmentation of VMN glycogen phosphorylase (GP) protein requires hindbrain catecholamine input. Research here investigated the premise that norepinephrine (NE) regulation of VMN astrocyte metabolism shapes local glucoregulatory neurotransmitter signaling in this sex. Estradiol-implanted ovariectomized rats were pretreated by intra-VMN administration of the monocarboxylate transporter inhibitor alpha-cyano-4-hydroxy-cinnamic acid (4CIN) or vehicle before NE delivery to that site. NE caused 4CIN-reversible reduction or augmentation of VMN glycogen synthase and phosphorylase expression. 4CIN prevented NE stimulation of gluco-inhibitory (glutamate decarboxylase65/67) and suppression of gluco-stimulatory (neuronal nitric oxide synthase) neuron marker proteins. These outcomes imply that effects of noradrenergic stimulation of VMN astrocyte glycogen depletion on glucoregulatory transmitter signaling may be mediated, in part, by glycogen-derived substrate fuel provision. NE control of astrocyte glycogen metabolism may involve down-regulated adrenoreceptor (AR), e.g. alpha1 and alpha2, alongside amplified beta1 AR and estrogen receptor-beta signaling. Noradrenergic hypoglycemia was refractory to 4CIN, implying that additional NE-sensitive VMN glucoregulatory neurochemicals may be insensitive to monocarboxylate uptake. Augmentation of circulating free fatty acids by combinatory NE and 4CIN, but not NE alone implies that acute hypoglycemia induced here is an insufficient stimulus for mobilization of these fuels, but is adequate when paired with diminished brain monocarboxylate fuel availability.

An overview of MCT1 and MCT4 in GBM: small molecule transporters with large implications

Monocarboxylate transporters (MCTs) represent a diverse group of transmembrane proteins encoded by the SLC16 gene family found ubiquitously across mammalian species. Two members of this family, MCT1 and MCT4, have been linked to key roles in the metabolic activity of tissues through the proton-coupled transport of monocarboxylates, most notably L-lactate, ketone bodies, and pyruvate. This review aims to provide an overview of MCT1 and MCT4, followed by the implications of their expression in a multitude of cancers and in glioblastoma (GBM) specifically. Further, the possible mechanisms underlying these effects will be discussed. Given the relationships between MCT1 and MCT4 and cancer, they offer a unique opportunity for novel treatment strategies. We aim to explore current therapies focused on MCT1 and MCT4 and propose future studies to better understand their role in GBM to optimize future treatment regimens.

Valproate sensitizes human glioblastoma cells to 3-bromopyruvate-induced cytotoxicity

Glioblastoma (GBM) is the most common brain tumor; however, no effective treatment for it is available yet. Monocarboxylate transporters, which are highly expressed in GBM, play a role in transporting antitumor agents, such as 3-bromopyruvate (3-BrPA). Valproate, primarily used to treat epilepsy, has been considered a possible treatment option for malignant GBM. In this study, we aimed to investigate the combined effects of 3-BrPA and valproate on GBM cell growth and elucidate the underlying mechanisms. Valproate enhanced 3-BrPA-induced cell death in T98G cells, used as a GBM model. Multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) mRNA levels significantly increased after valproate treatment. 3-BrPA-induced cell death, which was enhanced by valproate, was inhibited in the presence of MK571, a MRP inhibitor, or Ko143, a BCRP inhibitor. In addition, treatment with 3-BrPA and valproate for 48 h reduced cellular ATP levels compared to those in the 3-BrPA alone treatment group. However, cellular ATP levels were recovered in the presence of MK571 or Ko143, compared to those in the 3-BrPA and valproate treatment groups. In conclusion, we suggested that valproate enhanced 3-BrPA-induced cell death. This might be attributable to the increase in cellular ATP consumption owing to valproate-induced MRP2 or BCRP expression.

Monocarboxylate transporters MCT1 and MCT4 are independent prognostic biomarkers for the survival of patients with clear cell renal cell carcinoma and those receiving therapy targeting angiogenesis

BACKGROUND

Prognostic biomarkers for patients with clear cell renal cell carcinoma (ccRCC), particularly those receiving therapy targeting angiogenesis, are not well established. In this study, we examined the correlations of monocarboxylate transporter 1 (MCT1) and MCT4, 2 critical transporters for glycolytic metabolism, with various clinicopathological parameters as well as survival of patients with ccRCC and those treated with vascular endothelial growth factor receptor (VEGFR) inhibitors.

METHODS

A cohort of 150 ccRCC patients were recruited into this study. All patients underwent radical or partial nephrectomy as the first-line treatment, and 38 received targeted therapy (sorafenib or sunitinib) after the surgery. Expression levels of MCT1, MCT4, and CD34 were examined by immunohistochemistry. Correlations between MCT1 or MCT4 expression and different clinicopathological parameters or patient survival were analyzed among all as well as patients receiving targeted therapy.

RESULTS

MCT1 or MCT4 expression did not significantly correlate with sex, age, tumor diameter, microvascular density, tumor staging, pathological Furmann grade, or MSKCC (P>0.05). High expression of either MCT1 or MCT4 significantly correlated with reduced overall survival (OS) and progression-free survival (PFS) among the total cohort of ccRCC patients. For patients receiving targeted therapy, high expression of either MCT1 or MCT4 significantly correlated with reduced PFS, but not OS. Both conditions were independent prognostic biomarkers for reduced PFS among all patients or those receiving targeted therapy.

CONCLUSION

MCT1 and MCT4 are prognostic biomarkers for patients with ccRCC or those receiving targeted therapy. High expression of these 2 proteins predicts reduced PFS in these patients.

Valproic acid transport in the choriocarcinoma placenta cell line JEG-3 proceeds independently of the proton-dependent transporters MCT1 and MCT4

Medication therapy is the first line of treatment in the management of epilepsy. Fetal exposure to valproic acid (VPA), an antiepileptic drug, poses an elevated risk of teratogenicity in early pregnancy. Some studies have reported that monocarboxylate transporters (MCTs) may be involved in the placental transport of VPA. However, it has not been determined which MCTs contribute to VPA transport into the placenta. Therefore, the aim of this study was to determine how MCTs contribute to VPA transport into the placenta using the human placenta choriocarcinoma cell line JEG-3. VPA uptake was investigated using JEG-3 cells and radiolabeled VPA. MCT expression in JEG-3 cells was detected using RT-PCR and western blotting. Knockdown of MCTs was carried out using siRNAs. VPA uptake into JEG-3 cells was pH- and concentration-dependent, and described by using the Michaelis-Menten equation (K_m = 0.95 ± 0.17 mM; V_max = 19.3 ± 1.21 nmol/mg protein/15 s). MCT1 and MCT4 expression was found in JEG-3 cells, and typical MCT inhibitors significantly inhibited VPA uptake into JEG-3 cells. However, knockdown of MCT1 and MCT4 did not alter VPA uptake. In conclusion, VPA transport is mediated by a proton-dependent transporter in JEG-3 cells, but not by MCT1 and MCT4.

Differential effects of type 2 diabetes on brain glycometabolism in rats: focus on glycogen and monocarboxylate transporter 2

Astrocyte-neuron lactate shuttle (ANLS) is a pathway that supplies glycogen-derived lactate to active neurons via monocarboxylate transporter 2 (MCT2), and is important for maintaining brain functions. Our study revealed alterations of ANLS with hippocampal hyper-glycogen levels and downregulated MCT2 protein levels underlying hippocampal dysfunctions as a complication in type 2 diabetic (T2DM) animals. Since T2DM rats exhibit brain dysfunctions involving several brain regions, we examined whether there might also be T2DM effects on ANLS's disturbances in other brain loci. OLETF rats exhibited significantly higher glycogen levels in the hippocampus, hypothalamus, and cerebral cortex than did LETO rats. MCT2 protein levels in OLETF rats decreased significantly in the hippocampus and hypothalamus compared to their controls, but a significant correlation with glycogen levels was only observed in the hippocampus. This suggests that the hippocampus may be more vulnerable to T2DM compared to other brain regions in the context of ANLS disruption.

Glucose-dependent growth arrest of leukemia cells by MCT1 inhibition: Feeding Warburg's sweet tooth and blocking acid export as an anticancer strategy

This study aims to investigate the utilization of The Warburg Effect, cancer's "sweet tooth" and natural greed for glucose to enhance the effect of monocarboxylate transporter inhibition on cellular acidification. By simulating hyperglycemia with high glucose we may increase the effectiveness of inhibition of lactate and proton export on the dysregulation of cell pH homeostasis causing cell death or disruption of growth in cancer cells. MCT1 and MCT4 expression was determined in MCF7 and K562 cell lines using RT-PCR. Cell viability, growth, intracellular pH and cell cycle analysis was measured in the cell lines grown in 5 mM and 25 mM glucose containing media in the presence and absence of the MCT1 inhibitor AR-C155858 (1 μM) and the NHE1 inhibitor cariporide (10 μM). The MCT1 inhibitor, AR-C155858 had minimal effect on the viability, growth and intracellular pH of MCT4 expressing MCF7 cells. AR-C155858 had no effect on the viability of the MCT1 expressing K562 cells, but decreased intracellular pH and cell proliferation, by a glucose-dependent mechanism. Inhibition of NHE1 on its own had a no effect on cell growth, but together with AR-C155858 showed an additive effect on inhibition of cell growth. In cancer cells that only express MCT1, increased glucose concentrations in the presence of an MCT1 inhibitor decreased intracellular pH and reduced cell growth by G1 phase cell-cycle arrest. Thus we propose a transient hyperglycemic-clamp in combination with proton export inhibitors be evaluated as an adjunct to cancer treatment in clinical studies.

Identification of a selective inhibitor of human monocarboxylate transporter 4

The human monocarboxylate transporters (hMCTs/SLC16As) mediate the uptake of various monocarboxylates. Several isoforms of hMCTs are expressed in cancerous tissue as well as in normal tissue. In cancerous tissue, hypoxia induces the expression of hMCT4, which transports the energetic metabolite l-lactate across the plasma membrane. Since hMCT4 is involved in pH regulation and the transport of l-lactate in cancer cells, an hMCT4 inhibitor could function as an anticancer agent. Although several non specific hMCT inhibitors have been developed, a selective hMCT4 inhibitor has not yet been identified. The aim of this study was therefore to identify a selective hMCT4 inhibitor for use as a pharmacological tool for studying hMCT4. The heterologous expression system of the Xenopus oocyte was used to assess the effects of test compounds on hMCT4, whereupon isobutyrate derivatives, fibrates, and bindarit (2-[(1-benzyl-1H-indazol-3-yl)methoxy]-2-methylpropanoic acid) were demonstrated to exhibit selective inhibitory effects against this transporter. It is suggested that the structure formed from the joining of an isobutyrate moiety and two aromatic rings by appropriate linkers is important for acquiring the selective hMCT4-inhibiting activity. These findings provide novel insights into the ligand recognition of hMCT4, and contribute to the development of novel anticancer agents.

A perillyl alcohol-conjugated analog of 3-bromopyruvate without cellular uptake dependency on monocarboxylate transporter 1 and with activity in 3-BP-resistant tumor cells

The anticancer agent 3-bromopyruvate (3-BP) is viewed as a glycolytic inhibitor that preferentially kills glycolytic cancer cells through energy depletion. However, its cytotoxic activity is dependent on cellular drug import through transmembrane monocarboxylate transporter 1 (MCT-1), which restricts its anticancer potential to MCT-1-positive tumor cells. We created and characterized an MCT-1-independent analog of 3-BP, called NEO218. NEO218 was synthesized by covalently conjugating 3-BP to perillyl alcohol (POH), a natural monoterpene. The responses of various tumor cell lines to treatment with either compound were characterized in the presence or absence of supplemental pyruvate or antioxidants N-acetyl-cysteine (NAC) and glutathione (GSH). Drug effects on glyceraldehyde 3-phosphate dehydrogenase (GAPDH) enzyme activity were investigated by mass spectrometric analysis. The development of 3-BP resistance was investigated in MCT-1-positive HCT116 colon carcinoma cells in vitro. Our results show that NEO218: (i) pyruvylated GAPDH on all 4 of its cysteine residues and shut down enzymatic activity; (ii) severely lowered cellular ATP content below life-sustaining levels, and (iii) triggered rapid necrosis. Intriguingly, supplemental antioxidants effectively prevented cytotoxic activity of NEO218 as well as 3-BP, but supplemental pyruvate powerfully protected cells only from 3-BP, not from NEO218. Unlike 3-BP, NEO218 exerted its potent cytotoxic activity irrespective of cellular MCT-1 status. Treatment of HCT116 cells with 3-BP resulted in prompt development of resistance, based on the emergence of MCT-1-negative cells. This was not the case with NEO218, and highly 3-BP-resistant cells remained exquisitely sensitive to NEO218. Thus, our study identifies a mechanism by which tumor cells develop rapid resistance to 3-BP, and presents NEO218 as a superior agent not subject to this cellular defense. Furthermore, our results offer alternative interpretations of previously published models on the role of supplemental antioxidants: Rather than quenching reactive oxygen species (ROS), supplemental NAC or GSH directly interact with 3-BP, thereby neutralizing the drug's cytotoxic potential before it can trigger ROS production. Altogether, our study introduces new aspects of the cytotoxic mechanism of 3-BP, and characterizes NEO218 as an analog able to overcome a key cellular defense mechanism towards this drug.

1α,25(OH) 2D3 Sensitive Cytosolic pH Regulation and Glycolytic Flux in Human Endometrial Ishikawa Cells

BACKGROUND/AIMS

Tumor cell proliferation is modified by 1,25-Dihydroxy-Vitamin D3 (1,25(OH)2D3), a steroid hormone predominantly known for its role in calcium and phosphorus metabolism. Key properties of tumor cells include enhanced glycolytic flux with excessive consumption of glucose and formation of lactate. As glycolysis is highly sensitive to cytosolic pH, maintenance of glycolysis requires export of H+ ions and lactate, which is in part accomplished by Na+/H+ exchangers, such as NHE1 and monocarboxylate transporters, such as MCT4. An effect of 1,25(OH)2D3 on those transport processes has, however, never been reported. As cytosolic pH impacts on apoptosis, the study further explored the effect of 1,25(OH)2D3 on apoptosis and on the apoptosis regulating kinase AKT, transcription factor Forkhead box O-3 (FOXO3A) and B-cell lymphoma protein BCL-2.

METHODS

In human endometrial adenocarcinoma (Ishikawa) cells, cytosolic pH (pHi) was determined utilizing (2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein [BCECF] fluorescence, Na+/H+ exchanger activity from Na+ dependent realkalinization after an ammonium pulse, NHE1 and MCT4 transcript levels using qRT-PCR, NHE1, MCT4, total & phospho AKT, total & phospho-FOXO3A and BCL-2 protein abundance by Western blotting, lactate concentration in the supernatant utilizing a colorimetric enzyme assay and cell death quantification using CytoTox 96®, Annexin V and Propidium Iodide staining.

RESULTS

A 24 hours treatment with 1,25(OH)2D3 (100 nM) significantly increased cytosolic pH (pHi), significantly decreased Na+/H+ exchanger activity, NHE1 and MCT4 transcript levels as well as protein abundance and significantly increased lactate concentration in the supernatant. Treatment of Ishikawa cells with 1,25(OH)2D3 (100 nM) further triggered apoptosis, an effect paralleled by decreased phosphorylation of AKT and FOXO3A as well as decreased abundance of BCL-2.

CONCLUSIONS

In Ishikawa cells 1,25(OH)2D3 is a powerful stimulator of glycolysis, an effect presumably due to cytosolic alkalinization. Despite stimulation of glycolysis, 1,25(OH)2D3 stimulates slightly but significantly suicidal cell death, an effect presumably in part due to decreased activation of AKT with decreased inhibition of pro-apoptotic transcription factor FOXO3A and downregulation of the anti-apoptotic protein BCL-2.

Inhibition of monocarboxylate transporter 1 suppresses the proliferation of glioblastoma stem cells

Recent evidence suggests that a minor subset of cancer cells, termed cancer stem cells (CSCs), have self-renewal and tumorigenic potential. Therefore, the characterization of CSCs is important for developing therapeutic strategies against cancer. Cancer cells rely on anaerobic glycolysis to produce ATP even under normoxic conditions, resulting in the generation of excess acidic substances. Cancer cells maintain a weakly alkaline intracellular pH to support functions. Glioblastoma is an aggressive malignancy with a poor 5-year survival rate. Based on the hypothesis that ion transport-related molecules regulate the viability and function of CSCs, we investigated the expression of ion transport-related molecules in glioblastoma CSCs (GSCs). Quantitative RT-PCR analysis showed that monocarboxylate transporter1 (MCT1) were upregulated in GSCs, and inhibition of MCT1 decreased the viability of GSCs compared with that of non-GSCs. Our findings indicate that MCT1 is involved in the maintenance of GSCs and is a promising therapeutic target for glioblastoma.

Mechanism of antineoplastic activity of lonidamine

Lonidamine (LND) was initially introduced as an antispermatogenic agent. It was later found to have anticancer activity sensitizing tumors to chemo-, radio-, and photodynamic-therapy and hyperthermia. Although the mechanism of action remained unclear, LND treatment has been known to target metabolic pathways in cancer cells. It has been reported to alter the bioenergetics of tumor cells by inhibiting glycolysis and mitochondrial respiration, while indirect evidence suggested that it also inhibited l-lactic acid efflux from cells mediated by members of the proton-linked monocarboxylate transporter (MCT) family and also pyruvate uptake into the mitochondria by the mitochondrial pyruvate carrier (MPC). Recent studies have demonstrated that LND potently inhibits MPC activity in isolated rat liver mitochondria (K_i 2.5μM) and cooperatively inhibits l-lactate transport by MCT1, MCT2 and MCT4 expressed in Xenopus laevis oocytes with K_0.5 and Hill coefficient values of 36-40μM and 1.65-1.85, respectively. In rat heart mitochondria LND inhibited the MPC with similar potency and uncoupled oxidation of pyruvate was inhibited more effectively (IC₅₀~7μM) than other substrates including glutamate (IC₅₀~20μM). LND inhibits the succinate-ubiquinone reductase activity of respiratory Complex II without fully blocking succinate dehydrogenase activity. LND also induces cellular reactive oxygen species through Complex II and has been reported to promote cell death by suppression of the pentose phosphate pathway, which resulted in inhibition of NADPH and glutathione generation. We conclude that MPC inhibition is the most sensitive anti-tumour target for LND, with additional inhibitory effects on MCT-mediated l-lactic acid efflux, Complex II and glutamine/glutamate oxidation.

The anticancer agent 3-bromopyruvate: a simple but powerful molecule taken from the lab to the bedside

At the beginning of the twenty-first century, 3-bromopyruvate (3BP), a simple alkylating chemical compound was presented to the scientific community as a potent anticancer agent, able to cause rapid toxicity to cancer cells without bystander effects on normal tissues. The altered metabolism of cancers, an essential hallmark for their progression, also became their Achilles heel by facilitating 3BP's selective entry and specific targeting. Treatment with 3BP has been administered in several cancer type models both in vitro and in vivo, either alone or in combination with other anticancer therapeutic approaches. These studies clearly demonstrate 3BP's broad action against multiple cancer types. Clinical trials using 3BP are needed to further support its anticancer efficacy against multiple cancer types thus making it available to more than 30 million patients living with cancer worldwide. This review discusses current knowledge about 3BP related to cancer and discusses also the possibility of its use in future clinical applications as it relates to safety and treatment issues.

Hyperlactatemia in type 2 diabetes: Can physical training help?

Type 2 diabetic patients often exhibit hyperlactatemia in association with a reduced aerobic-oxidative capacity and a restricted lactate transport. Studies suggest a link between increased lactate levels and the manifestation and progression of insulin resistance. However, the specificities of molecular mechanisms remain unclear, and it is not entirely clear whether elevated lactate levels are a cause or consequence of type 2 diabetes. This review focuses on lactate as a key molecule in diabetes and provides an overview of how regular physical activity can be helpful in normalizing elevated lactate levels in type 2 diabetic patients. Physical training may reduce lactate production and reinforce lactate transport and clearance among this particular patient group. We emphasize the crucial role physical training plays in the therapy of type 2 diabetes due to evidence that pharmacological treatment with metformin, which is commonly used in the first-line therapy of type 2 diabetes, does not help reducing lactate levels.

Organized metabolic crime in prostate cancer: The coexpression of MCT1 in tumor and MCT4 in stroma is an independent prognosticator for biochemical failure

BACKGROUND

Lactate import or export over cell membranes is facilitated by monocarboxylate transporters (MCTs) 1 and 4. Expression profiles can be markers of an oxidative or glycolytic phenotype. Descriptive studies and functional studies in neoplastic cells and fibroblasts in prostate cancer (PC) have suggested a distinct phenotype. We aimed to explore expression of MCT1 and MCT4 in PC cells and surrounding stroma in a large cohort. Additionally, we wanted to find out if distinct expression profiles were associated with biochemical failure-free survival (BFFS).

METHODS

Tissue microarrays were constructed from 535 patients with radical prostatectomies between January 1, 1995, and December 31, 2005. Immunohistochemistry was used to detect expression, and degrees of expression were evaluated semiquantitatively by 2 pathologists using light microscopy.

RESULTS

For MCT1, there was only epithelial expression, whereas there was a low level of expression of MCT4 in tumor and stroma. A total of 172 patients had a low expression of MCT1 in tumor and MCT4 in stroma. There were 232 patients who had a high expression of MCT1 and a low expression of MCT4 in stroma. Only 11 patients had a low tumoral MCT1 expression and a high stromal MCT4 expression, and 26 patients (5%) had a high expression of both. Patients with a high-high combination had a significantly reduced BFFS (P = 0.011), and when adjusting for other factors, its effect was significant and independent (HR = 1.99, CI 95%: 1.09-3.62; P = 0.024).

CONCLUSIONS

This study adds to the current understanding of the reversed Warburg effect to be a significant phenotype in PC. High coexpression of MCT1 in tumor and MCT4 in stroma is independently associated to a worse BFFS, and the strength of this association is as strong as having a Gleason score of ≥9.

Deficiency in monocarboxylate transporter 1 (MCT1) in mice delays regeneration of peripheral nerves following sciatic nerve crush

Peripheral nerve regeneration following injury occurs spontaneously, but many of the processes require metabolic energy. The mechanism of energy supply to axons has not previously been determined. In the central nervous system, monocarboxylate transporter 1 (MCT1), expressed in oligodendroglia, is critical for supplying lactate or other energy metabolites to axons. In the current study, MCT1 is shown to localize within the peripheral nervous system to perineurial cells, dorsal root ganglion neurons, and Schwann cells by MCT1 immunofluorescence in wild-type mice and tdTomato fluorescence in MCT1 BAC reporter mice. To investigate whether MCT1 is necessary for peripheral nerve regeneration, sciatic nerves of MCT1 heterozygous null mice are crushed and peripheral nerve regeneration was quantified electrophysiologically and anatomically. Compound muscle action potential (CMAP) recovery is delayed from a median of 21 days in wild-type mice to greater than 38 days in MCT1 heterozygote null mice. In fact, half of the MCT1 heterozygote null mice have no recovery of CMAP at 42 days, while all of the wild-type mice recovered. In addition, muscle fibers remain 40% more atrophic and neuromuscular junctions 40% more denervated at 42 days post-crush in the MCT1 heterozygote null mice than wild-type mice. The delay in nerve regeneration is not only in motor axons, as the number of regenerated axons in the sural sensory nerve of MCT1 heterozygote null mice at 4 weeks and tibial mixed sensory and motor nerve at 3 weeks is also significantly reduced compared to wild-type mice. This delay in regeneration may be partly due to failed Schwann cell function, as there is reduced early phagocytosis of myelin debris and remyelination of axon segments. These data for the first time demonstrate that MCT1 is critical for regeneration of both sensory and motor axons in mice following sciatic nerve crush.

Tissue distribution and thyroid hormone effects on mRNA abundance for membrane transporters Mct8, Mct10, and organic anion-transporting polypeptides (Oatps) in a teleost fish

Many of the actions of thyroid hormones (THs) occur via TH binding to intracellular receptors. Although it was long thought that THs diffused passively across plasma membranes, it is now recognized that cellular entry is mediated by a variety of membrane transporter proteins. In this study, we identified cDNAs encoding the TH transporters monocarboxylate transferases 8 (mct8) and 10 (mct10) as well as eight distinct organic anion-transporting polypeptide (oatp) proteins from fathead minnow (Pimephales promelas). Analysis of the tissue distribution of transporter mRNAs revealed that mct8 and mct10 transcripts were both abundant in liver, but also present at lower levels in brain, gonad and other tissues. Transcripts encoding oatp1c1 were highly abundant in brain, liver and gonad, and exhibited significant sex differences in the liver and gonad. Treatment of adult male minnows with 3,5,3'-triiodothyronine (T3) or the goitrogen methimazole altered gene transcript abundance for several transporters. Fish given exogenous T3 had reduced mct8 and oapt1c1 mRNA levels in the liver compared to methimazole-treated fish. In the brain, transcripts for mct8, mct10, oatp2b1, and oatp3a1 were each reduced in abundance in fish with elevated T3. As a whole, these results provide evidence that TH status influences the transcriptional dynamics of mct8, mct10 and several Oatp genes including oatp1c1 in teleost fish.

Synthesis and pharmacological evaluation of carboxycoumarins as a new antitumor treatment targeting lactate transport in cancer cells

Under hypoxia, cancer cells consume glucose and release lactate at a high rate. Lactate was recently documented to be recaptured by oxygenated cancer cells to fuel the TCA cycle and thereby to support tumor growth. Monocarboxylate transporters (MCT) are the main lactate carriers and therefore represent potential therapeutic targets to limit cancer progression. In this study, we have developed and implemented a stepwise in vitro screening procedure on human cancer cells to identify new potent MCT inhibitors. Various 7-substituted carboxycoumarins and quinolinone derivatives were synthesized and pharmacologically evaluated. Most active compounds were obtained using a palladium-catalyzed Buchwald-Hartwig type coupling reaction, which proved to be a quick and efficient method to obtain aminocarboxycoumarin derivatives. Inhibition of lactate flux revealed that the most active compound 19 (IC50 11 nM) was three log orders more active than the CHC reference compound. Comparison with warfarin, a conventional anticoagulant coumarin, further showed that compound 19 did not influence the prothrombin time which, together with a good in vitro ADME profile, supports the potential of this new family of compounds to act as anticancer drugs through inhibition of lactate flux.

Deletion of the Basigin gene results in reduced mitochondria in the neural retina

Basigin-null mice are characterized as blind from the time of eye opening, with degeneration of the retina beginning at 8weeks of age, and progressing until the entire photoreceptor cell layer is destroyed. It is likely that a metabolic deficiency underlies the blindness and degeneration phenotypes, as it has been determined that Basigin-null mice do not express the transporter protein monocarboxylate transporter one on the membrane of photoreceptor cells and inner segments, nor Müller cells of the neural retina, as is observed in normal mice. The purpose of the present study was to assess the health of mitochondria in normal and Basigin-null mice, specifically to determine if mitochondria within the Basigin-null mouse neural retina are metabolically active. This was achieved via a measurement of cytochrome C concentration and the expression of autophagy-specific proteins via ELISA analyses. Additionally, Mitotracker dyes were used to assess the number and relative activity of mitochondria. It was determined that cytochrome C concentrations and expression of autophagy-specific proteins were not increased in Basigin-null animals, as compared to control animals. Also, while Basigin-null mice do have metabolically active mitochondria, the amount of mitochondria was greatly reduced, when compared to control animals. The results suggest that a reduction in mitochondria is a result, rather than the cause, of the metabolic deficiency observed in Basigin-null mice, and likely occurs because of reduced metabolic activity in the absence of MCT1 expression.

Pyruvate minimizes rtPA toxicity from in vitro oxygen-glucose deprivation and reoxygenation

Clinical application of recombinant tissue plasminogen activator (rtPA) for stroke is limited by hemorrhagic transformation, which narrows rtPA's therapeutic window. In addition, mounting evidence indicates that rtPA is potentially neurotoxic if it traverses a compromised blood brain barrier. Here, we demonstrated that pyruvate protects cultured HT22 neuronal and primary microvascular endothelial cells co-cultured with primary astrocytes from oxygen glucose deprivation (OGD)/reoxygenation stress and rtPA cytotoxicity. After 3 or 6h OGD, cells were reoxygenated with 11mmol/L glucose±pyruvate (8mmol/L) and/or rtPA (10µg/ml). Measured variables included cellular viability (calcein AM and annexin-V/propidium iodide), reactive oxygen species (ROS; mitosox red and 2',7'-dichlorofluorescein diacetate), NADPH, NADP(+) and ATP contents (spectrophotometry), matrix metalloproteinase-2 (MMP2) activities (gelatin zymography), and cellular contents of MMP2, tissue inhibitor of metalloproteinase-2 (TIMP2), and phosphor-activation of anti-apoptotic p70s6 kinase, Akt and Erk (immunoblot). Pyruvate prevented the loss of HT22 cells after 3h OGD±rtPA. After 6h OGD, rtPA sharply lowered cell viability; pyruvate dampened this effect. Three hours OGD and 4h reoxygenation with rtPA increased ROS formation by about 50%. Pyruvate prevented this ROS formation and doubled cellular NADPH/NADP(+) ratio and ATP content. In endothelial cell monolayers, 3h OGD and 24h reoxygenation increased FITC-dextran leakage, indicating disruption of intercellular junctions. Although rtPA exacerbated this effect, pyruvate prevented it while sharply lowering MMP2/TIMP2 ratio and increasing phosphorylation of p70s6 kinase, Akt and Erk. Pyruvate protects neuronal cells and microvascular endothelium from hypoxia-reoxygenation and cytotoxic action of rtPA while reducing ROS and activating anti-apoptotic signaling. These results support the proposed use of pyruvate as an adjuvant to dampen the side effects of rtPA treatment, thereby extending rtPA's therapeutic window.

Monocarboxylate Transporter Inhibition with Osmotic Diuresis Increases γ-Hydroxybutyrate Renal Elimination in Humans: A Proof-of-Concept Study

BACKGROUND AND OBJECTIVE

The purpose of the current study was to demonstrate proof-of-concept that monocarboxylate transporter (MCT) inhibition with L-lactate combined with osmotic diuresis increases renal clearance of γ-hydroxybutyrate (GHB) in human subjects. GHB is a substrate for human and rodent MCTs, which are responsible for GHB renal reabsorption, and this therapy increases GHB renal clearance in rats.

METHODS

Ten healthy volunteers were administered GHB orally as sodium oxybate 50 mg/kg (4.5 gm maximum dose) on two different study days. On study day 1, GHB was administered alone. On study day 2, treatment of L-lactate 0.125 mmol/kg and mannitol 200 mg/kg followed by L-lactate 0.75 mmol/kg/hr was administered intravenously 30 minutes after GHB ingestion. Blood and urine were collected for 6 hours, analyzed for GHB, and pharmacokinetic and statistical analyses performed.

RESULTS

L-lactate/mannitol administration significantly increased GHB renal clearance compared to GHB alone, 439 vs. 615 mL/hr (P=0.001), and increased the percentage of GHB dose excreted in the urine, 2.2 vs. 3.3% (P=0.021). Total clearance was unchanged.

CONCLUSIONS

MCT inhibition with L-lactate combined with osmotic diuresis increases GHB renal elimination in humans. No effect on total clearance was observed in this study due to the negligible contribution of renal clearance to total clearance at this low GHB dose. Considering the nonlinear renal elimination of GHB, further research in overdose cases is warranted to assess the efficacy of this treatment strategy for increasing renal and total clearance at high GHB doses.