Beyond Warburg effect--dual metabolic nature of cancer cells

Acidosis-Induced Changes in Proteome Patterns of the Prostate Cancer-Derived Tumor Cell Line AT-1

Under various pathological conditions, such as inflammation, ischemia and in solid tumors, physiological parameters (local oxygen tension or extracellular pH) show distinct tissue abnormalities (hypoxia and acidosis). For tumors, the prevailing microenvironment exerts a strong influence on the phenotype with respect to proliferation, invasion, and metastasis formation and therefore influences prognosis. In this study, we investigate the impact of extracellular metabolic acidosis (pH 7.4 versus 6.6) on the proteome patterns of a prostate cancer-derived tumor cell type (AT-1) using isobaric labeling and LC-MS/MS analysis. In total, 2710 proteins were identified and quantified across four biological replicates, of which seven were significantly affected with changes >50% and used for validation. Glucose transporter 1 and farnesyl pyrophosphatase were found to be down-regulated after 48 h of acidic treatment, and metallothionein 2A was reduced after 24 h and returned to control values after 48 h. After 24 and 48 h at pH 6.6, glutathione S transferase A3 and NAD(P)H dehydrogenase 1, cellular retinoic acid-binding protein 2, and Na-bicarbonate transporter 3 levels were found to be increased. The changes in protein levels were confirmed by transcriptome and functional analyses. In addition to the experimental in-depth investigation of proteins with changes >50%, functional profiling (statistical enrichment analysis) including proteins with changes >20% revealed that acidosis upregulates GSH metabolic processes, citric acid cycle, and respiratory electron transport. Metabolism of lipids and cholesterol biosynthesis were downregulated. Our data comprise the first comprehensive report on acidosis-induced changes in proteome patterns of a tumor cell line.

High-capacity glycolytic and mitochondrial oxidative metabolisms mediate the growth ability of glioblastoma

Among the primary brain tumors, glioblastoma multiforme (GBM) has a radical proliferation ability that complicates the therapeutic modulation of cancer progression. The majority of GBM patients have a low survival rate (<1 year) due to radical tumor growth and late cancer diagnosis. Previous reports have shown that astrocytes have a specific metabolic organization that includes the production of lactate, the storage of glycogen, and use of lactate to support neurons which possess higher capacity of metabolism compared to neurons. We hypothesized that these characteristics of astrocytes could contribute to enhanced proliferation of GBM compared to neuroblastoma (NB). Here, we show that U87MG cells (a model of GBM) proliferate more rapidly than SH-SY5Y cells (a model of NB). A higher extracellular acidification rate and maximal mitochondrial oxygen consumption rate were observed in U87MG cells compared to SH-SY5Y cells. The expression levels of lactate dehydrogenase (LDH)-A and LDH-B were higher in U87MG cells and primary cultured astrocytes than in SH-SY5Y cells and neurons. Furthermore, the mRNA levels of succinate dehydrogenase and peroxisome proliferator-activated receptor-γ were high in U87MG cells, suggesting that these cells have high capacity for mitochondrial metabolism and uptake of fatty acids related to synthesis of the cell membrane, respectively. Taken together, we demonstrate that GBM cells are characterized by activation of the LDH-expression-related glycolytic pathway and mitochondrial metabolic capacity, suggesting two innate properties of astrocytes that could provide a driving force for the growth ability of GBM. Based on these findings, we propose that therapeutic approaches aimed at treating GBM could target LDH for modulating the metabolic properties of GBM cells.

Type B lactic acidosis: a rare but life threatening hematologic emergency. A case illustration and brief review

Major strides have been made in improving the treatment of medical emergencies associated with malignancies. Nonetheless, metabolic emergencies in cancer patients can often times be life-threatening. Type B lactic acidosis is a rare but potentially fatal paraneoplastic phenomenon that has been described in association with hematologic and solid malignancies and portends a poor prognosis if not rapidly recognized and treated. It is believed that this occurs as a result of cancer cells switching their glucose metabolism from an oxidative oxygen- dependent pathway towards a glycolytic phenotype, also known as the "Warburg effect". Though rare, it is important to consider this entity in the differential diagnosis of type B lactic acidosis since prompt identification and treatment may help improve outcomes in this otherwise fatal process. We present a case of type B lactic acidosis in a patient with chronic lymphocytic leukemia along with a brief review of the literature.

Molecular Connections between Cancer Cell Metabolism and the Tumor Microenvironment

Cancer cells preferentially utilize glycolysis, instead of oxidative phosphorylation, for metabolism even in the presence of oxygen. This phenomenon of aerobic glycolysis, referred to as the “Warburg effect”, commonly exists in a variety of tumors. Recent studies further demonstrate that both genetic factors such as oncogenes and tumor suppressors and microenvironmental factors such as spatial hypoxia and acidosis can regulate the glycolytic metabolism of cancer cells. Reciprocally, altered cancer cell metabolism can modulate the tumor microenvironment which plays important roles in cancer cell somatic evolution, metastasis, and therapeutic response. In this article, we review the progression of current understandings on the molecular interaction between cancer cell metabolism and the tumor microenvironment. In addition, we discuss the implications of these interactions in cancer therapy and chemoprevention.

Heterogeneity of glycolysis in cancers and therapeutic opportunities

Upregulated glycolysis, both in normoxic and hypoxic environments, is a nearly universal trait of cancer cells. The enormous difference in glucose metabolism offers a target for therapeutic intervention with a potentially low toxicity profile. The past decade has seen a steep rise in the development and clinical assessment of small molecules that target glycolysis. The enzymes in glycolysis have a highly heterogeneous nature that allows for the different bioenergetic, biosynthetic, and signaling demands needed for various tissue functions. In cancers, these properties enable them to respond to the variable requirements of cell survival, proliferation and adaptation to nutrient availability. Heterogeneity in glycolysis occurs through the expression of different isoforms, posttranslational modifications that affect the kinetic and regulatory properties of the enzyme. In this review, we will explore this vast heterogeneity of glycolysis and discuss how this information might be exploited to better target glucose metabolism and offer possibilities for biomarker development.

The contributions of respiration and glycolysis to extracellular acid production

BACKGROUND

The rate at which cells acidify the extracellular medium is frequently used to report glycolytic rate, with the implicit assumption that conversion of uncharged glucose or glycogen to lactate(-)+H(+) is the only significant source of acidification. However, another potential source of extracellular protons is the production of CO2 during substrate oxidation: CO2 is hydrated to H2CO3, which then dissociates to HCO3(-)+H(+).

METHODS

O2 consumption and pH were monitored in a popular platform for measuring extracellular acidification (the Seahorse XF Analyzer).

RESULTS

We found that CO2 produced during respiration caused almost stoichiometric release of H(+) into the medium. With C2C12 myoblasts given glucose, respiration-derived CO2 contributed 34% of the total extracellular acidification. When glucose was omitted or replaced by palmitate or pyruvate, this value was 67-100%. Analysis of primary cells, cancer cell lines, stem cell lines, and isolated synaptosomes revealed contributions of CO2-produced acidification that were usually substantial, ranging from 3% to 100% of the total acidification rate.

CONCLUSION

Measurement of glycolytic rate using extracellular acidification requires differentiation between respiratory and glycolytic acid production.

GENERAL SIGNIFICANCE

The data presented here demonstrate the importance of this correction when extracellular acidification is used for quantitative measurement of glycolytic flux to lactate. We describe a simple way to correct the measured extracellular acidification rate for respiratory acid production, using simultaneous measurement of oxygen consumption rate.

SUMMARY STATEMENT

Extracellular acidification is often assumed to result solely from glycolytic lactate production, but respiratory CO2 also contributes. We demonstrate that extracellular acidification by myoblasts given glucose is 66% glycolytic and 34% respiratory and describe a method to differentiate these sources.