Lactate/GPR81 signaling and proton motive force in cancer: Role in angiogenesis, immune escape, nutrition, and Warburg phenomenon

Abstract

Reprogramming of biochemical pathways is a hallmark of cancer cells, and generation of lactic acid from glucose/glutamine represents one of the consequences of such metabolic alterations. Cancer cells export lactic acid out to prevent intracellular acidification, not only increasing lactate levels but also creating an acidic pH in extracellular milieu. Lactate and protons in tumor microenvironment are not innocuous bystander metabolites but have special roles in promoting tumor-cell proliferation and growth. Lactate functions as a signaling molecule by serving as an agonist for the G-protein-coupled receptor GPR81, involving both autocrine and paracrine mechanisms. In the autocrine pathway, cancer cell-generated lactate activates GPR81 on cancer cells; in the paracrine pathway, cancer cell-generated lactate activates GPR81 on immune cells, endothelial cells, and adipocytes present in tumor stroma. The end result of GPR81 activation is promotion of angiogenesis, immune evasion, and chemoresistance. The acidic pH creates an inwardly directed proton gradient across the cancer-cell plasma membrane, which provides driving force for proton-coupled transporters in cancer cells to enhance supply of selective nutrients. There are several molecular targets in the pathways involved in the generation of lactic acid by cancer cells and its role in tumor promotion for potential development of novel anticancer therapeutics.

Introduction

Otto Warburg's findings from the 1920s highlighted the preferential production of lactic acid in glycolysis even in the presence of oxygen (Warburg, 1956). These were unexpected findings because in normal cells glycolysis produces pyruvic acid which enters mitochondria and undergoes complete oxidation into CO₂ in the presence of oxygen with consequent production of ATP via oxidative phosphorylation (OXPHOS). ATP is a potent negative feedback regulator of glycolysis via inhibition of the rate-limiting enzyme in the glycolytic pathway, namely phosphofructokinase-1; therefore, oxygen suppresses the rate of glycolysis in normal cells. In general, glycolysis yields lactic acid in normal cells only under conditions of hypoxia. As lactic acid is produced in normal cells only in the absence of oxygen, the process is referred to as “anaerobic glycolysis”, and the altered pathway seen in cancer cells where lactic acid is produced even in the presence of oxygen is now referred to as “aerobic glycolysis”. Warburg proposed that this altered metabolism constitutes the origin of cancer (Warburg, 1956). But this novel theory of cancer metabolism did not receive much attention in Warburg's days nor the decades following; Warburg's findings in cancer cells were generally thought to represent a symptom of cancer rather than the primary cause. This notion has changed dramatically in the past two decades, and we are seeing a rekindling of interest in recent years in cancer-cell metabolism. Notwithstanding this increased interest in cancer-cell metabolism because of the original Warburg's findings, it has to be borne in mind that “aerobic glycolysis” is a phenomenon observed in cancer cells in culture. The extent to which this phenomenon occursin tumors in vivo is not clear. Nonetheless, the fact remains that there has certainly been an increased focus not only on glycolysis as it pertains directly to Warburg's original findings but also on the citric acid cycle, electron transport chain, oxidative phosphorylation, hexose monophosphate shunt, fatty acid metabolism, and amino acid metabolism in relation to tumor growth and metastasis.

The present review focuses on biologic and metabolic relevance of lactic acid generated by cancer cells, highlighting the notion that this glycolytic metabolite is not a waste product of proliferating cancer cells but actually elicits a broad spectrum of effects that are critical for tumor progression and metastasis. To avoid intracellular acidification, cancer cells rapidly export lactate via monocarboxylate transporters (MCTs) (Ganapathy, Thangaraju, & Prasad, 2009; Parks & Pouysségur, 2017). This maintains low intracellular concentrations of lactic acid and allows for aerobic glycolysis and lactate production to continue, and simultaneously increases lactic acid concentrations in the extracellular tumor microenvironment (TME). The pKa of lactic acid is ~3.9; therefore, lactic acid exists mostly in the ionized state at physiological pH. Thus, lactic acid has two components, lactate and H⁺; tumor-promoting effects of lactic acid involve both components. In addition to the role of lactate as an energy-rich metabolite, lactate has now emerged as an important signaling molecule acting through the cell-surface G_i/o-protein-coupled receptor GPR81 (also known as hydroxycarboxylic acid receptor 1 or HCAR1). High lactate concentrations in the extracellular environment of tumors is associated with a poor prognosis (Sun, Li, Chen, & Qian, 2017). Accumulation of lactic acid in the TME acidifies the extracellular milieu due to H⁺, thus creating a transmembrane electrochemical H⁺ gradient (also known as proton motive force) across the plasma membrane of cancer cells. As the proton motive force provides energy for active entry of several essential nutrients (amino acids, small peptides, folic acid, and Fe²⁺) in normal cells, a similar phenomenon might occur also in cancer cells. In the current review, we will detail the biological roles of both lactate and H⁺ in cancer cells and in the TME as they pertain to cancer progression and metastasis.