In vivo pH in metabolic‐defective Ras‐transformed fibroblast tumors: Key role of the monocarboxylate transporter, MCT4, for inducing an alkaline intracellular pH

Abstract

We present an investigation of tumor pH regulation, designed to support a new anticancer therapy concept that we had previously proposed. Our study uses a tumor model of ras-transformed hamster fibroblasts, CCL39, xenografted in the thighs of nude mice. We demonstrate, for the first time, that genetic modifications of specific mechanisms of proton production and/or proton transport result in distinct, reproducible changes in intracellular and extracellular tumor pH that can be detected and quantified noninvasively in vivo, simultaneously with determinations of tumor energetic status and necrosis in the same experiment. The CCL39 variants used were deficient in the sodium/proton exchanger, NHE-1, and/or in the monocarboxylate transporter, MCT4; further, variants were deficient in glycolysis or respiration. MCT4 expression markedly increased the gradient between intracellular and extracellular pH from 0.14 to 0.43 when compared to CCL39 wild-type tumors not expressing MCT4. The other genetic modifications studied produced smaller but significant increases in intracellular and decreases in extracellular pH. In general, increased pH gradients were paralleled by increased tumor growth performance and diminished necrotic regions, and 50% of the CCL39 variant expressing neither MCT4 nor NHE-1, but possessing full genetic capacity for glycolysis and oxidative phosphorylation, underwent regression before reaching a 1-cm diameter. Except for CCL39 wild-type tumors, no significant HIF-1a expression was detected. Our in vivo results support a multipronged approach to tumor treatment based on minimizing intracellular pH by targeting several proton production and proton transport processes, among which the very efficient MCT4 proton/lactate co-transport deserves particular attention.Over the past decade, the study of tumor metabolism has gained new momentum, as the importance of biological processes that are closely linked to the phenotype is widely recognized in the current postgenomic era.1-3 Moreover, the crucial role of the local extracellular environment in tumorigenesis, cancer diagnosis and cancer treatment is increasingly being investigated, in addition to the behavior of cancer cells. 4 The communication between cancer cells and their microenvironment is complex, and involves metabolic events, blood perfusion, reactions of the immune system, cooperation with stromal cells and close interaction with the extracellular matrix.4 Among the multitude of factors influencing the formation, survival and growth of cancer cells, pH regulation occupies a central place.5 Indeed, the metabolism of rapidly growing cells is generating enormous amounts of acid. Mammalian cells can only survive at neutral or slightly alkaline intracellular pH values (pH i ). Therefore, cells need to tightly regulate their pH i to allow optimal functioning of the metabolic pathways. Furthermore, normal cells usually exist in an environment that is somewhat more alkaline (physiological value 7.35-7.45 6 ) than the intracellular space and are easily...

Genetic Disruption of Lactate/H+ Symporters (MCTs) and Their Subunit CD147/BASIGIN Sensitizes Glycolytic Tumor Cells to Phenformin

“…However, the high glycolytic rate associated with a production of lactic acid rapidly creates a hostile acidic and hypoxic microenvironment that induces dissemination of metastatic variants (11). Hypoxia via HIF1 controls intracellular pH (pHi) by inducing the membrane-bound carbonic anhydrases IX and XII (12)(13)(14) and a major H þ /lactate À symporter, MCT4 (15)(16)(17). Considering the key role of glycolysis in "glucoseaddicted" tumors, targeting this bioenergetic and anabolic pathway represents an attractive therapeutic approach (18)(19)(20)(21)(22).…”

Cancer cell behaviors mediated by dysregulated pH dynamics at a glance

“…Supporting this idea, lowering pH i through genetic knockdown or inhibition of ion transporters reduces cell proliferation and migration Amith et al, 2016a;Le Floch et al, 2011), cell invasion (Yang et al, 2010), and suppresses tumorigenesis in xenograft models (Amith et al, 2015;Lagarde et al, 1988;Sonveaux et al, 2008;Colen et al, 2011;Chiche et al, 2012). Additionally, we have recently shown in Drosophila animal models and human clonal cells that, by lowering the pH i , we can induce synthetic lethality with expression of activated oncogenes -notably Raf and Raspossibly by limiting the efflux of metabolically generated acids (Grillo-Hill et al, 2015).…”

The real face of HIF1α in the tumor process

“…24,25 However, hypoxia always decreases the pH i value of the cell because of an alteration in the metabolic response (e.g., lactate and ammonia generation). [26][27][28][29] Recently, Ward and Thompson ascertained that altered metabolism by itself can be oncogenic and might be a hallmark of cancer. 30 Most toxic metabolic waste products generated by an overstimulated/ HIF1.…”