pH, Lactate, and Hypoxia: Reciprocity in Regulating High-Affinity Monocarboxylate Transporter Expression in Glioblastoma

Abstract

Highly malignant brain tumors harbor the aberrant propensity for aerobic glycolysis, the excessive conversion of glucose to lactic acid even in the presence of ample tissue oxygen. Lactic acid is rapidly effluxed to the tumor microenvironment via a group of plasma-membrane transporters denoted monocarboxylate transporters (MCTs) to prevent “self-poisoning.” One isoform, MCT2, has the highest affinity for lactate and thus should have the ability to respond to microenvironment conditions such as hypoxia, lactate, and pH to help maintain high glycolytic flux in the tumor. Yet, MCT2 is considered to not respond to hypoxia, which is counterintuitive. Its response to tumor lactate has not been reported. In this report, we experimentally identify the transcription initiation site/s for MCT2 in astrocytes (normal) and glioma (tumor). We then use a BACmid library to isolate a 4.2-kbp MCT2 promoter-exon I region and examine promoter response to glycolysis-mediated stimuli in glioma cells. Reporter analysis of nested-promoter constructs indicated response of MCT2 to hypoxia, pH, lactate, and glucose, the major physiological “players” that facilitate a tumor's growth and proliferation. Immunoblot analysis of native MCT2 expression under altered pH and hypoxia reflected the reporter data. The pH-mediated gene-regulation studies we describe are the first to record H+-based reporter studies for any mammalian system and demonstrate the exquisite response of the MCT2 gene to minute changes in tumor pH. Identical promoter usage also provides the first evidence of astrocytes harnessing the same gene regulatory regions to facilitate astrocyte-neuron lactate shuttling, a metabolic feature of normal brain.

1H-NMR metabolomics reveals the Glabrescione B exacerbation of glycolytic metabolism beside the cell growth inhibitory effect in glioma

“…These effects could be a consequence of the GlaB-induced reduction of intracellular levels of ATP [49], thus leading to the hypothesis that GlaB could favor energy stress in glioma. We also found that the increased lactate extrusion induced by GlaB treatment is important to prevent lactate “self-poisoning”, as already described for astrocytes and glioma cells [26]. Indeed, the blockade of lactate efflux by ACCA induced GL261 cell death by apoptosis, as already reported [25, 26].…”

“…At this aim, cells were treated with ACCA (250 μ m ), a small-molecule competitive inhibitor of lactate transporters, and analyzed for cell growth, apoptosis and necrosis in the presence or absence of GlaB (5 μ m ). Previous studies demonstrated that ACCA induced tumor cell death, both in vivo and in vitro [25, 26].…”

Acidosis Maintains the Function of Brain Mitochondria in Hypoxia-Tolerant Triplefin Fish: A Strategy to Survive Acute Hypoxic Exposure?

“…Here, we were able to follow mt respiration, ΔΨm and pH or JO 2 , Vol mt and pH simultaneously. However, as lactate may modulate the mt function and is a possible substrate of neurons (Philp et al, 2005; Chen et al, 2016; Caruso et al, 2017), we divided tissues from single brains to provide a control and reference for comparisons between buffered and unbuffered samples. We show that while intertidal HTS suppress JO 2 they preserve ΔΨm and ATP production as pH declines.…”

Stable Isotope Tracing Metabolomics to Investigate the Metabolic Activity of Bioactive Compounds for Cancer Prevention and Treatment

“…Human melanoma cells, when treated with αCHC, had reduced intracellular pH at low extracellular pH, which resulted in reduced survival of these cells [77]. Treatment of malignant glioma cells with αCHC resulted in altered glycolytic metabolism, making them more radiosensitive [78] and adversely impacting the invasiveness and proliferative ability [79]. In human pancreatic cancer cells, αCHC inhibited migration and proliferation and induced cell death, performing better when applied in combination with metformin [80].…”