The Spatial Organization of Proton and Lactate Transport in a Rat Brain Tumor

Abstract

Tumors create a heterogeneous acidic microenvironment which assists their growth and which must be taken into account in the design of drugs and their delivery. In addition, the acidic extracellular pH (pHe) is itself exploited in several experimental techniques for drug delivery. The way the acidity is created is not clear. We report here the spatial organization of key proton-handling proteins in C6 gliomas in rat brain. The mean profiles across the tumor rim of the Na+/H+ exchanger NHE1, and the lactate-H+ cotransporter MCT1, both showed peaks. NHE1, which is important for extension and migration of cells in vitro, showed a peak 1.55 times higher than in extratumoural tissue at 0.33 mm from the edge. MCT1 had a broader peak, further into the tumor (maximum 1.76 fold at 1.0 mm from the edge). In contrast, MCT4 and the carbonic anhydrase CAIX, which are associated with hypoxia, were not significantly upregulated in the rim. The spatial distribution of MCT4 was highly correlated with that of CAIX, suggesting that their expression is regulated by the same factors. Since protons extruded by NHE1 diffuse away through extracellular clefts, NHE1 requires a continuous source of intracellular protons. From the stoichiometries of metabolic pathways that produce or consume H+, and the greater availability of glucose compared to oxygen in most parts of a tumor, we support the classic view that most of the net proton efflux from C6 gliomas originates in glycolytic formation of lactate and H+ inside the tumor, but add that some lactate is taken up into cells in the rim on MCT1, and some lactate diffuses away, leaving its associated protons available to re-enter cells for extrusion on NHE1. Therapeutic inhibition of NHE1, MCT1 or CAIX is predicted to affect different parts of a tumor.

Regulation of Monocarboxylate Transporter MCT1 Expression by p53 Mediates Inward and Outward Lactate Fluxes in Tumors

“…Similar lactate exchange and uptake through MCT1 is reported in the heart (8), red blood cells (9), and even in the liver, to support gluconeogenesis (10). Within tumors also, we and others recently documented that cancer cells could import lactate through MCT1 from the most glycolytic tumor cells (11)(12)(13)(14) or tumor-associated fibroblasts (15,16) to fuel mitochondrial respiration and thereby spare glucose for hypoxic tumor cells. In many tumor cell lines, characterized by an elevated glycolytic flux even in the presence of oxygen (the Warburg effect; ref.…”

Family of pH (low) insertion peptides for tumor targeting

“…An acidic extracellular milieu favors tumor growth, invasion, and development (15)(16)(17)(18)(19)(20), and the pHLIP peptides can exploit tumor acidity as a useful biomarker. Based on the results of our previous and current investigations, design principles can be formulated to set directions for different clinical uses:…”

Cancer cell behaviors mediated by dysregulated pH dynamics at a glance

“…Hypoxia has been shown to increase pH i , suggesting that anoxic cells at a tumor center can have a higher pH i than peripheral cells. However, a recent study of rat brain gliomas found that NHE1 and MCT1 are more abundant at the tumor edge (Grillon et al, 2011), although pH i was not determined. Furthermore, increased pH i has recently been proposed to be necessary for epithelial-to-mesenchymal transition (Amith et al, 2016b), and epithelial-to-mesenchymal transition is implicated in initiating metastasis.…”