The high metabolic demand of cancer cells leads to an accumulation of H ions in the tumour microenvironment. The disorganized tumour vasculature prevents an efficient wash-out of H ions released into the extracellular medium but also favours the development of tumour hypoxic regions associated with a shift towards glycolytic metabolism. Under hypoxia, the final balance of glycolysis, including breakdown of generated ATP, is the production of lactate and a stoichiometric amount of H ions. Another major source of H ions results from hydration of CO produced in the more oxidative tumour areas. All of these events occur at high rates in tumours to fulfil bioenergetic and biosynthetic needs. This Review summarizes the current understanding of how H-generating metabolic processes segregate within tumours according to the distance from blood vessels and inversely how ambient acidosis influences tumour metabolism, reducing glycolysis while promoting mitochondrial activity. The Review also presents novel insights supporting the participation of acidosis in cancer progression via stimulation of autophagy and immunosuppression. Finally, recent advances in the different therapeutic modalities aiming to either block pH-regulatory systems or exploit acidosis will be discussed.

Bioenergetic preferences of cancer cells foster tumor acidosis that in turn leads to dramatic reduction in glycolysis and glucose-derived acetyl-coenzyme A (acetyl-CoA). Here, we show that the main source of this critical two-carbon intermediate becomes fatty acid (FA) oxidation in acidic pH-adapted cancer cells. FA-derived acetyl-CoA not only fuels the tricarboxylic acid (TCA) cycle and supports tumor cell respiration under acidosis, but also contributes to non-enzymatic mitochondrial protein hyperacetylation, thereby restraining complex I activity and ROS production. Also, while oxidative metabolism of glutamine supports the canonical TCA cycle in acidic conditions, reductive carboxylation of glutamine-derived α-ketoglutarate sustains FA synthesis. Concomitance of FA oxidation and synthesis is enabled upon sirtuin-mediated histone deacetylation and consecutive downregulation of acetyl-CoA carboxylase ACC2 making mitochondrial fatty acyl-CoA degradation compatible with cytosolic lipogenesis. Perturbations of these regulatory processes lead to tumor growth inhibitory effects further identifying FA metabolism as a critical determinant of tumor cell proliferation under acidosis.

Acidosis, a common characteristic of the tumor microenvironment, is associated with alterations in metabolic preferences of cancer cells and progression of the disease. Here we identify the TGF-β2 isoform at the interface between these observations. We document that acidic pH promotes autocrine TGF-β2 signaling, which in turn favors the formation of lipid droplets (LD) that represent energy stores readily available to support anoikis resistance and cancer cell invasiveness. We find that, in cancer cells of various origins, acidosis-induced TGF-β2 activation promotes both partial epithelial-to-mesenchymal transition (EMT) and fatty acid metabolism, the latter supporting Smad2 acetylation. We show that upon TGF-β2 stimulation, PKC-zeta-mediated translocation of CD36 facilitates the uptake of fatty acids that are either stored as triglycerides in LD through DGAT1 or oxidized to generate ATP to fulfill immediate cellular needs. We also address how, by preventing fatty acid mobilization from LD, distant metastatic spreading may be inhibited.

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Lactate exchange between glycolytic and oxidative cancer cells is proposed to optimize tumor growth. Blocking lactate uptake through monocarboxylate transporter 1 (MCT1) represents an attractive therapeutic strategy but may stimulate glucose consumption by oxidative cancer cells. We report here that inhibition of mitochondrial pyruvate carrier (MPC) activity fulfils the tasks of blocking lactate use while preventing glucose oxidative metabolism. Using in vitro 13C-glucose and in vivo hyperpolarized 13C-pyruvate, we identify 7ACC2 as a potent inhibitor of mitochondrial pyruvate transport which consecutively blocks extracellular lactate uptake by promoting intracellular pyruvate accumulation. Also, while in spheroids MCT1 inhibition leads to cytostatic effects, MPC activity inhibition induces cytotoxic effects together with glycolysis stimulation and uncompensated inhibition of mitochondrial respiration. Hypoxia reduction obtained with 7ACC2 is further shown to sensitize tumor xenografts to radiotherapy. This study positions MPC as a control point for lactate metabolism and expands on the anticancer potential of MPC inhibition.

Extracellular tumor acidosis largely results from an exacerbated glycolytic flux in cancer and cancerassociated cells. Conversely, little is known about how tumor cells adapt their metabolism to acidosis. Here, we demonstrate that long-term exposure of cancer cells to acidic pH leads to a metabolic reprogramming toward glutamine metabolism. This switch is triggered by the need to reduce the production of protons from glycolysis and further maintained by the NAD þ -dependent increase in SIRT1 deacetylase activity to ensure intracellular pH homeostasis. A consecutive increase in HIF2a activity promotes the expression of various transporters and enzymes supporting the reductive and oxidative glutamine metabolism, whereas a reduction in functional HIF1a expression consolidates the inhibition of glycolysis. Finally, in vitro and in vivo experiments document that acidosis accounts for a net increase in tumor sensitivity to inhibitors of SIRT1 and glutaminase GLS1. These findings highlight the influence that tumor acidosis and metabolism exert on each other.

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The precursor of nerve growth factor (proNGF) has been described as a biologically active polypeptide able to induce apoptosis in neuronal cells, via the neurotrophin receptor p75 NTR and the sortilin receptor. Herein, it is shown that proNGF is produced and secreted by breast cancer cells, stimulating their invasion. Using Western blotting and mass spectrometry, proNGF was detected in a panel of breast cancer cells as well as in their conditioned media. Immunohistochemical analysis indicated an overproduction of proNGF in breast tumors, when compared with benign and normal breast biopsies, and a relationship to lymph node invasion in ductal carcinomas. Interestingly, siRNA against proNGF induced a decrease of breast cancer cell invasion that was restored by the addition of noncleavable proNGF. The activation of TrkA, Akt, and Src, but not the MAP kinases, was observed. In addition, the proNGF invasive effect was inhibited by the Trk pharmacological inhibitor K252a, a kinase-dead TrkA, and siRNA against TrkA sortilin, neurotensin, whereas siRNA against p75 NTR and the MAP kinase inhibitor PD98059 had no impact. These data reveal the existence of an autocrine loop stimulated by proNGF and mediated by TrkA and sortilin, with the activation of Akt and Src, for the stimulation of breast cancer cell invasion. Nerve growth factor (NGF),5 the prototypical member of the neurotrophin family of polypeptides, is essential for the survival and differentiation of central and peripheral neurons, and its role in the development and regeneration of the sympathetic and sensory nervous systems has been extensively described (1). NGF binds to the tropomyosin-related kinase A (TrkA) receptor, a tyrosine kinase receptor, and to the p75 neurotrophin receptor (p75 NTR ), a member of the tumor necrosis factor receptor family, to induce its neurotrophic effects. NGF is synthesized as a 25-kDa precursor protein, named proNGF, that yields the mature NGF polypeptide of 13.5 kDa and an inactive prosegment of 11.5 kDa, released from the N terminus intracellularly by furin, or extracellularly by plasmin as well as by several matrix metalloproteases (2). Importantly, proNGF can be secreted without being processed to mature NGF and can have its own biological effects (3). As more than just a source for NGF, proNGF was shown to induce neuronal death by apoptosis where mature NGF induced survival and differentiation (4, 5). For inducing its proapoptotic effect on neurons, proNGF forms a trimeric complex with two plasma membrane receptors: p75 NTR and sortilin (4). Sortilin, a 95-kDa type I receptor,