Hypoxia, cancer metabolism and the therapeutic benefit of targeting lactate/H(+) symporters

Abstract

Since Otto Warburg reported the 'addiction' of cancer cells to fermentative glycolysis, a metabolic pathway that provides energy and building blocks, thousands of studies have shed new light on the molecular mechanisms contributing to altered cancer metabolism. Hypoxia, through hypoxia-inducible factors (HIFs), in addition to oncogenes activation and loss of tumour suppressors constitute major regulators of not only the "Warburg effect" but also many other metabolic pathways such as glutaminolysis. Enhanced glucose and glutamine catabolism has become a recognised feature of cancer cells, leading to accumulation of metabolites in the tumour microenvironment, which offers growth advantages to tumours. Among these metabolites, lactic acid, besides imposing an acidic stress, is emerging as a key signalling molecule that plays a pivotal role in cancer cell migration, angiogenesis, immune escape and metastasis. Although interest in lactate for cancer development only appeared recently, pharmacological molecules blocking its metabolism are already in phase I/II clinical trials. Here, we review the metabolic pathways generating lactate, and we discuss the rationale for targeting lactic acid transporter complexes for the development of efficient and selective anticancer therapies.

Keywords: BASIGIN; Cancer; Lactate; MCT; Monocarboxylate Transporters; Therapy; Warburg effect.

Fig. 1

Schematic representation of glucose and glutamine metabolism in cancer cells. After entering the cell through specific transporters (GLUT), glucose is metabolised to pyruvate. In cancer cells, pyruvate is mainly converted to lactate by the lactate dehydrogenase A (LDHA), while its catabolism in the tricarboxylic acid (TCA) cycle is restricted through the inhibition of the mitochondrial pyruvate dehydrogenase (PDH) by the pyruvate dehydrogenase kinase 1 (PDK1) induced by HIF-1. Glycolysis (bold arrows) generates also another important intermediate, glucose-6-phosphate (glucose-6-P) that is metabolised by the pentose phosphate pathway (blue arrows), which produces NADPH and ribose-5-phosphate for glutathion and nucleic acids synthesis. Glutaminolysis (purple arrows) is an alternative energy source for cancer cells. First converted to glutamate by glutaminase (GLS) in the cytosol, glutamine replenishes tricarboxylic acid (TCA) cycle (anaplerosis) through the conversion of glutamate to α-ketoglutarate (α-KG). Glutaminolysis contributes also to synthesis of lipids, amino acids, nucleotides and generation of lactate that is transported out of the cell by the ubiquitous monocarboxylate transporter 1 (MCT1) and the hypoxia inducible MCT4. Hypoxia-inducible factor (HIF), glucose-6-phosphate dehydrogenase (G6PD), mitochondrial pyruvate carrier (MPC), oxaloacetate (OAA), l-type amino acid transporter 1 (LAT1), Asc-type amino acid transporter 2 (ASCT2)

Fig. 2

The different roles of cancer-generated lactic acid in promoting tumour growth and metastasis. Enhanced glycolysis and glutaminolysis generate large amounts of lactic acid that is exported by monocarboxylate transporters (MCT) 1 and 4. The accumulation of lactic acid in the extracellular milieu induces drop in extracellular pH (pHe), acidification of tumour microenvironment and promotes several cancer processes leading to cell survival, tumour growth and metastasis. Lactic acid stimulates angiogenesis by increasing the production of the vascular endothelial growth factor (VEGF) and its receptor VEGFR2 by tumour and endothelial cells. Lactate drives also angiogenesis through the activation of hypoxia-inducible factor 1 (HIF-1), N-Myc downstream-regulated gene 3 (NDRG3) protein and the stimulation of the production of interleukin 8 (IL8). Increased extracellular lactate levels influence cancer cell motility by promoting hyaluronan production, which acts on fibroblasts and cancer cell cytoskeleton through interaction with CD44. More importantly, lactate generated by altered cancer metabolism plays an important role in escape of immune surveillance, mostly through decreased cytotoxic activity of human T lymphocytes (T cells) [75, 76] and natural killer (NK) cells. Further, lactate reduces dendritic cell maturation, induces the accumulation of myeloid derived suppressor cells, and promotes M2-like polarisation of tumour-associated macrophages

Fig. 3

Schematic representation of BASIGIN (BSG) isoforms structure and interaction with monocarboxylate transporters (MCT). a Alternative transcriptional initiation and variation in splicing results in four isoforms of BSG (BSG1-4) that are composed of extracellular Ig-like domains containing glycosylation sites (red circles), a single-membrane spanning segment and a short intracellular cytoplasmic tail. BSG1 is specifically located at the retina, BSG2 (in bold) is the most prevalent isoform and BSG3/BSG4 are intracellular, lacking signal peptide and much less abundant proteins. b Dimer of BSG binds to two monomers of MCT, illustrated by 12 individual helices each, and forms a homo-oligodimer that translocate to the plasma membrane for proper expression and functionality

Fig. 4

Model of tumour microenvironment and lactate shuttles in cancer. Cells located far from the perfused blood vessels become rapidly hypoxic and rely on glycolysis for proliferation. They generate, therefore, large amount of lactic acid that is extruded by monocarboxylate transporter 4 (MCT4). Lactate is subsequently taken up by the endothelial cells via monocarboxylate transporter 1 (MCT1), and is converted into pyruvate by the lactate dehydrogenase B (LDHB), a phenomenon referred to as “vascular endothelial lactate shuttle.” Pyruvate, by stabilising hypoxia-inducible factor 1 α (HIF-1α), induces tumour angiogenesis. Normoxic cancer cells, that highly express MCT1, also preferentially take up lactate produced by hypoxic cancer cells to perform oxidative phosphorylation (OXPHOS). This “metabolic symbiosis” allows hypoxic regions of the tumour to acquire high levels of glucose and, subsequently, generate lactic acid. In addition, cancer-associated fibroblasts, which are highly glycolytic and express MCT4, also supply oxidative cancer cells with lactate. This tumour-stroma cooperation, termed “reverse Warburg effect” in addition to the other lactate shuttles, result in the establishment of lactate and glucose consumption gradients within tumours

Fig. 5

Efficiency of targeting lactate/H+ symporters for anticancer therapy. a Few oxidative cancer cells could use lactate to generate ATP, thus inhibition of monocarboxylate transporter 1 (MCT1) with AstraZeneca’s specific inhibitor AZD3965 results in growth arrest. Other type of cancer cells, glycolytic and expressing only MCT1, will be also sensitive to MCT1 inhibitor showing growth reduction, cell death and radiosensitivity. b Most of glycolytic cancer cells are expressing both MCT1 and MCT4. Due to functional redundancy between these two MCTs, AZD3965 will have no effect on hypoxic regions of the tumours. c Combined inhibition of MCT1 and MCT4 results in decreased glycolytic rate and severe growth arrest. However, increased intracellular lactic acid pool and subsequently increased intracellular pyruvate concentration, will fuel the tricarboxylic (TCA) cycle leading to metabolic shift from glycolysis towards OXPHOS. Therefore, tumour cells, although growing slowly, will survive by keeping physiological ATP pool and escape lactate export blockade. d Concomitant application of MCT inhibitors with metformin or phenformin, which inhibits OXPHOS, induces synthetic lethality resulting in “ATP crisis”. Consequently, rapid tumour cell death occurs due to “metabolic catastrophe.” Basigin (BSG); MCT4 inhibitor (MCT4 i)