S-Nitrosation of monocarboxylate transporter 1: inhibition of pyruvate-fueled respiration and proliferation of breast cancer cells

Targeting monocarboxylate transporters (MCTs) in cancer: How close are we to the clinics?

As a result of metabolic reprogramming, cancer cells display high rates of glycolysis, causing an excess production of lactate along with an increase in extracellular acidity. Proton-linked monocarboxylate transporters (MCTs) are crucial in the maintenance of this metabolic phenotype, by mediating the proton-coupled lactate flux across cell membranes, also contributing to cancer cell pH regulation. Among the proteins codified by the SLC16 gene family, MCT1 and MCT4 isoforms are the most explored in cancers, being overexpressed in many cancer types, from solid tumours to haematological malignancies. Similarly to what occurs in particular physiological settings, MCT1 and MCT4 are able to mediate lactate shuttles among cancer cells, and also between cancer and stromal cells in the tumour microenvironment. This form of metabolic cooperation is responsible for important cancer aggressiveness features, such as cell proliferation, survival, angiogenesis, migration, invasion, metastasis, immune tolerance and therapy resistance. The growing understanding of MCT functions and regulation is offering a new path to the design of novel inhibitors that can be foreseen in clinical practices. This review provides an overview of the role of MCT isoforms in cancer and summarizes the recent advances in their pharmacological targeting, highlighting the potential of new potent and selective MCT1 and/or MCT4 inhibitors in cancer therapeutics, and anticipating its inclusion in clinical practice.

A practical guide for the analysis, standardization and interpretation of oxygen consumption measurements

Measurement of oxygen consumption is a powerful and uniquely informative experimental technique. It can help identify mitochondrial mechanisms of action following pharmacologic and genetic interventions, and characterize energy metabolism in physiology and disease. The conceptual and practical benefits of respirometry have made it a frontline technique to understand how mitochondrial function can interface with-and in some cases control-cell physiology. Nonetheless, an appreciation of the complexity and challenges involved with such measurements is required to avoid common experimental and analytical pitfalls. Here we provide a practical guide to oxygen consumption measurements covering the selection of experimental models and instrumentation, as well as recommendations for the collection, interpretation and normalization of data. These guidelines are provided with the intention of aiding experimental design and enhancing the overall reputability, transparency and reliability of oxygen consumption measurements.

Monocarboxylate transporters in cancer

Background

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

Scope of review

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

Major conclusions

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

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

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Lactate exchanges are at the core of tumor metabolism.

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Transmembrane lactate trafficking depends on monocarboxylate transporters (MCTs).

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MCTs are implicated in tumor development and aggressiveness.

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Targeting MCTs is a therapeutic option for cancer treatment.

Nitric oxide might be an inducing factor in cognitive impairment in Alzheimer's disease via downregulating the monocarboxylate transporter 1

Alzheimer's disease (AD) is a typical neurodegenerative disease in central nervous system (CNS). Generally speaking, patients with severe AD are often accompanied with cognitive impairment. Oligodendrocytes (OLs) are myelin-forming cells in CNS, and myelin injury potentially has something to do with the cognitive impairment in AD. Based on the previous experimental studies, it has been recognized that nitric oxide (NO), as a signaling molecule, might have an influence on the axon and myelin by affecting the energy transport mechanism of OLs through monocarboxylate transporter 1 (MCT1). Interestingly, a novel model of cell signaling----axo-myelinic synapse (AMS) has been put forward. In the context of this model, chances are that a new way is established in which NO can influence the pathogenesis of AD by down-regulating the expression of MCT1. As a consequence, it may provide attractive prospective and underlying drug targeting effects for the treatment of AD.

Molecular Mechanisms of Nitric Oxide in Cancer Progression, Signal Transduction, and Metabolism

SIGNIFICANCE

Cancer is a complex disease, which not only involves the tumor but its microenvironment comprising different immune cells as well. Nitric oxide (NO) plays specific roles within tumor cells and the microenvironment and determines the rate of cancer progression, therapy efficacy, and patient prognosis. Recent Advances: Key understanding of the processes leading to dysregulated NO flux within the tumor microenvironment over the past decade has provided better understanding of the dichotomous role of NO in cancer and its importance in shaping the immune landscape. It is becoming increasingly evident that nitric oxide synthase 2 (NOS2)-mediated NO/reactive nitrogen oxide species (RNS) are heavily involved in cancer progression and metastasis in different types of tumor. More recent studies have found that NO from NOS2+ macrophages is required for cancer immunotherapy to be effective.

CRITICAL ISSUES

NO/RNS, unlike other molecules, are unique in their ability to target a plethora of oncogenic pathways during cancer progression. In this review, we subcategorize the different levels of NO produced by cells and shed light on the context-dependent temporal effects on cancer signaling and metabolic shift in the tumor microenvironment.

FUTURE DIRECTIONS

Understanding the source of NO and its spaciotemporal profile within the tumor microenvironment could help improve efficacy of cancer immunotherapies by improving tumor infiltration of immune cells for better tumor clearance.

Insights in pathogenesis of multiple sclerosis: nitric oxide may induce mitochondrial dysfunction of oligodendrocytes

Demyelinating diseases, such as multiple sclerosis (MS), are kinds of common diseases in the central nervous system (CNS), and originated from myelin loss and axonal damage. Oligodendrocyte dysfunction is the direct reason of demyelinating lesions in the CNS. Nitric oxide (NO) plays an important role in the pathological process of demyelinating diseases. Although the neurotoxicity of NO is more likely mediated by peroxynitrite rather than NO itself, NO can impair oligodendrocyte energy metabolism through mediating the damaging of mitochondrial DNA, mitochondrial membrane and mitochondrial respiratory chain complexes. In the progression of MS, NO can mainly mediate demyelination, axonal degeneration and cell death. Hence, in this review, we extensively discuss endangerments of NO in oligodendrocytes (OLs), which is suggested to be the main mediator in demyelinating diseases, e.g. MS. We hypothesize that NO takes part in MS through impairing the function of monocarboxylate transporter 1, especially causing axonal degeneration. Then, it further provides a new insight that NO for OLs may be a reliable therapeutic target to ameliorate the course of demyelinating diseases.

In situ measurements of mitochondrial matrix enzyme activities using plasma and mitochondrial membrane permeabilization agents

Activities of enzymes localized to the mitochondrial matrix of mammalian cells are often critical regulatory steps in cellular metabolism. As such, measurement of matrix enzyme activities in response to genetic modifications or drug interventions is often desired. However, measurements in intact cells are often hampered by the presence of other isozymes in the cytoplasm as well as the inability to deliver enzyme substrates across cellular membranes. Classic approaches to liberate matrix enzymes utilize harsh treatments that disrupt intracellular architecture or require significant starting material to allow mitochondrial isolation prior to sample extraction. We describe a method using permeabilization reagents for both the plasma and mitochondrial membranes to allow in situ measurement of matrix enzyme activities. It is applied to adherent cell monolayers in 96-well plates treated with perfringolysin O to permeabilize the plasma membrane and alamethicin to permeabilize the mitochondrial inner membrane. We present three examples validated with inhibitor sensitivity: (i) Complex I-mediated oxygen consumption driven by NADH, (ii) ATP hydrolysis by the F₁F_O complex measuring pH changes in an Agilent Seahorse XF Analyzer, and (iii) Mitochondrial glutaminase (GLS1) activity in a coupled reaction monitoring NADH fluorescence in a plate reader.

Loops and layers of post-translational modifications of drug transporters

Drug transporters encoded by solute carrier (SLC) family are distributed in multiple organs including kidney, liver, placenta, brain, and intestine, where they mediate the absorption, distribution, and excretion of a diverse array of environmental toxins and clinically important drugs. Alterations in the expression and function of these transporters play important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. While it is clear that the regulation of these transporters tightly depends on genetic mechanisms, many studies have demonstrated that these transporters are the target of various post-translational modifications. This review article summarizes the recent advances in identifying the posttranslational modifications underlying the regulation of the drug transporters of SLC family. Such mechanisms are pivotal not only in physiological conditions, but also in diseases.

Effect of nitric oxide to axonal degeneration in multiple sclerosis via downregulating monocarboxylate transporter 1 in oligodendrocytes

Multiple sclerosis (MS) is a neurodegenerative disease of the central nervous system (CNS). Axonal degeneration, one of the main pathological characteristics of MS, is affected by nitric oxide (NO). In turn, NO induces mitochondrial dysfunction of neurons and glial cells. Inadequate glucose causes monocarboxylate transporter 1 (MCT1) to transfer lactate from oligodendrocytes (OLs) to neurons, which decreases MCT1 and results in energy substrate deficit (mainly lactate) in axons. The condition gradually leads to axonal degeneration. This study proposes that NO-induced MCT1 down-regulation in OLs may be involved in the pathological process of axonal degeneration, which eventually leads to MS.

PGC-1β regulates HER2-overexpressing breast cancer cells proliferation by metabolic and redox pathways

Breast cancer is a prevalent neoplastic disease among women worldwide which treatments still present several side effects and resistance. Considering that cancer cells present derangements in their energetic homeostasis, and that peroxisome proliferator-activated receptor- gamma coactivator 1 (PGC-1) is crucial for cellular metabolism and redox signaling, the main objective of this study was to investigate whether there is a relationship between PGC-1 expression, the proliferation of breast cancer cells and the mechanisms involved. We initially assessed PGC-1β expression in complementary DNA (cDNA) from breast tumor of patients bearing luminal A, luminal B, and HER2-overexpressed and triple negative tumors. Our data showed that PGC-1β expression is increased in patients bearing HER2-overexpressing tumors as compared to others subtypes. Using quantitative PCR and immunoblotting, we showed that breast cancer cells with HER2-amplification (SKBR-3) have greater expression of PGC-1β as compared to a non-tumorous breast cell (MCF-10A) and higher proliferation rate. PGC-1β expression was knocked down with short interfering RNA in HER2-overexpressing cells, and cells decreased proliferation. In these PGC-1β-inhibited cells, we found increased citrate synthase activity and no marked changes in mitochondrial respiration. Glycolytic pathway was decreased, characterized by lower intracellular lactate levels. In addition, after PGC-1β knockdown, SKBR-3 cells showed increased reactive oxygen species production, no changes in antioxidant activity, and decreased expression of ERRα, a modulator of metabolism. In conclusion, we show an association of HER2-overexpression and PGC-1β. PGC-1β knockdown impairs HER2-overexpressing cells proliferation acting on ERRα signaling, metabolism, and redox balance.

Cellular targets of the myeloperoxidase-derived oxidant hypothiocyanous acid (HOSCN) and its role in the inhibition of glycolysis in macrophages

Myeloperoxidase (MPO) released at sites of inflammation catalyzes the formation of the oxidants hypochlorous acid (HOCl) and hypothiocyanous acid (HOSCN) from H2O2 and halide and pseudo-halide ions. HOCl, a major oxidant produced under physiological conditions reacts rapidly with many biological molecules, and is strongly linked with tissue damage during inflammatory disease. The role of HOSCN in disease is less clear, though it can initiate cellular damage by pathways involving the selective oxidation of thiol-containing proteins. Utilizing a thiol-specific proteomic approach, we explored the cellular targets of HOSCN in macrophages (J774A.1). We report that multiple thiol-containing proteins involved in metabolism and glycolysis; fructose bisphosphate aldolase, triosephosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and creatine kinase, together with a number of chaperone, antioxidant and structural proteins, were modified in a reversible manner in macrophages treated with HOSCN. The modification of the metabolic enzymes was associated with a decrease in basal glycolysis, glycolytic reserve, glycolytic capacity and lactate release, which was only partly reversible on further incubation in the absence of HOSCN. Inhibition of glycolysis preceded cell death and was seen in cells exposed to low concentrations (≤25µM) of HOSCN. The ability of HOSCN to inhibit glycolysis and perturb energy production is likely to contribute to the cell death seen in macrophages on further incubation after the initial treatment period, which may be relevant for the propagation of inflammatory disease in smokers, who have elevated plasma levels of the HOSCN precursor, thiocyanate.

Analysis and interpretation of microplate-based oxygen consumption and pH data

Breakthrough technologies to measure cellular oxygen consumption and proton efflux are reigniting the study of cellular energetics by increasing the scope and pace with which discoveries are made. As we learn the variation in metabolism between cell types is large, it is helpful to continually provide additional perspectives and update our roadmap for data interpretation. In that spirit, this chapter provides the following for those conducting microplate-based oxygen consumption experiments: (i) a description of the standard parameters for measuring respiration in intact cells, (ii) a framework for data analysis and normalization, and (iii) examples of measuring respiration in permeabilized cells to follow up results observed with intact cells. Additionally, rate-based measurements of extracellular pH are increasingly used as a qualitative indicator of glycolytic flux. As a resource to help interpret these measurements, this chapter also provides a detailed accounting of proton production during glucose oxidation in the context of plate-based assays.