In vivo pH in metabolic-defective Ras-transformed fibroblast tumors. Key role of the monocarboxylate transporter, MCT4, for inducing an alkaline intracellular pH.
Résumé
We present an investigation of tumor pH regulation, designed to support a new anticancer therapy concept that we had previously proposed. This study employs 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 intra and extracellular tumor pH that can be detected and quantified non-invasively 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, NHE1, and/or in the monocarboxylate transporter, MCT4; further variants were deficient in glycolysis or respiration. MCT4 expression dramatically increased the gradient between intra 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 paralled by increased tumor growth performance and diminished necrotic regions, and 50 % of the CCL39 variant expressing neither MCT4 nor NHE1, 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-1α expression was detected. Our in vivo results support a multi-pronged 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.