Dysregulated pH in Tumor Microenvironment Checkmates Cancer Therapy
Abstract
Introduction: The dysregulation of pH by cancerous cells of solid tumors is able to create a unique milieu that is in favor of progression, invasion and metastasis as well as chemo-/immuno-resistance traits of solid tumors. Bioelements involved in pH dysregulation provide new set of oncotargets, inhibition of which may result in better clinical outcome. Methods: To study the impacts of pH dysregulation, we investigated the tumor development and progression in relation with Warburg effect, glycolysis and formation of aberrant tumor microenvironment. Results: The upregulation of glucose transporter GLUT-1 and several enzymes involve in glycolysis exacerbates this phenomenon. The accumulation of lactic acids in cancer cells provokes upregulation of several transport machineries (MCT-1, NHE-1, CA IX and H pump V-ATPase) resulting in reinforced efflux of proton into extracellular fluid. This deviant event makes pH to be settled at 7.4 and 6.6 respectively in cancer cells cytoplasm and extracellular fluid within the tumor microenvironment, which in return triggers secretion of lysosomal components (various enzymes in acidic milieu with pH 5) into cytoplasm. All these anomalous phenomena make tumor microenvironment (TME) to be exposed to cocktail of various enzymes with acidic pH, upon which extracellular matrix (ECM) can be remodeled and even deformed, resulting in emergence of a complex viscose TME with high interstitial fluid pressure. Conclusion: It seems that pH dysregulation is able to remodel various physiologic functions and make solid tumors to become much more invasive and metastatic. It also can cause undesired resistance to chemotherapy and immunotherapy. Hence, cancer therapy needs to be reinforced using specific inhibitors of bioelements involved in pH dysregulation of TME in solid tumors.
Schematic representation of tumor development process (TDP). During TDP, the cancer cells within the primary tumor achieve some sort of capabilities to strike the neighboring tissue (1: progression). The cancerous cells after destructing the ECM get into the lymphatic and blood vessels (2: intravasation), traverse through the vessels (3: dissemination), leave the traveling vessels (4: extravasation), negotiate with the new microenvironment, survive and proliferate (5: dormancy) forming micrometastatic secondary tumor (colonization). EMT: epithelial–mesenchymal transition. MET: mesenchymal–epithelial transition.
Schematic illustration of molecular events of the transition of normal cells to adenocarcinoma in colorectal cancer (CRC). Transitional stages (from normal colon epithelium to premalignant adenoma then to an invasive adenocarcinoma) are characterized through well-described sequence of mutations. Cell adhesion is compromised through loss of function of the adenomatous polyposis coli (APC) gene in up to 85% of all cases of CRC. KRAS is mutated in 50–60% of cases of CRC. Cell adhesion transmembrane glycoprotein E-cadherin is downregulated. MLH1 and MSH2 gene are mutated. SMAD4, which is involved in the transforming growth factor-β (TGF-β) signaling pathway, plays a key role in suppressing the epithelial-cell growth. While INK4A gene is involved in tumor-suppressor pathway, the p53 mutation appears to be the late phenomenon that makes cancer cells resistant to apoptosis. There exist overlaps between different stages. For detailed information, reader is referred to an excellent review by Kerr.13
Glucose metabolism path in normal cells and cancer cells. Cells transport glucose through GLUT1 and phosphorylate it to G6P through hexokinases (HK2). G6P is further metabolized through glycolysis path (1–15), glycogen metabolism (23–26) and pentose phosphate pathway (27–32). Cancer cells exploit the glycolysis path, which results in production of lactic acid that is pumped out of the dells by monocarboxylate transporter (MCT1). Pyruvate can also undergo mitochondrial pyruvate metabolism (16-20) toward synthesis of fatty acids and phosphatides (21-22). Highlighted path shows glycolysis in cancer cells. GLUT1: glucose transporter 1. G6P: glucose 6-phosphate. HK2: hexokinase 2. F6P: fructose 6-phosphate. F1,6P2: fructose 1,6-phosphate. GAP: glyceraldehyde phosphate. DHAP: dihydroxyacetone phosphate. TPI: triose phosphate isomerase, 1,3BPG: glycerate 1,3-bisphosphate. 3PG: glycerate 3-phosphate. 2PG: glycerate 2-phosphate. Pyr: pyruvate. G1P: glucose 1-phosphate. UDPGlu: uracil-diphosphate glucose. F2,6P2: fructose 2,6-phosphate. NAD: nicotinamide adenine dinucleotide. NADP: nicotinamide adenine dinucleotide phosphate. 6PG: 6-phosphate gluconate. Ru5P: ribulose 5-phosphate. R5P: ribose 5-phosphate. X5P: xylose 5-phosphate. S7P: sedoheptulose 7-phosphae. E4P: erythrose 4-phosphate. TKTL2: transketolase-like 2. TALD1: transaldolaselike 1. MCT1: monocarboxylate transporter 1. BT: bicarbonate transporter. CAIX: carbonic anhydrase 9. HE: H exchanger. NHE: Na-H exchanger. V-ATPase: Vacuolar-type H-ATPase. PDHK-1: pyruvate dehydrogenase kinase 1.
Regulation of pH in hypoxic and normoxic conditions in solid tumors. A) Glucose metabolism in hypoxic condition through glycolysis produces lactic acid, dysregulating pH in cancer cells. B) Glucose metabolism in normoxic condition through oxidative phosphorylation uptake lactic acid trough MCT1 transporter and consume it via oxidative phosphorylation. Expression of various transporters (MCT4, Glut1, CA-IX, V-ATPase, NHE1) at plasma membrane and mitochondria of hypoxic cancer cells favor alkalization of intracellular fluid (pHi=7.4) in cancer cells and acidification of extracellular fluid (pHe=6.6-6.9) in the tumor microenvironment. Dysregulated pH support cancer cells chemoresistance and immune system escape. MCT4: monocaroxylic acid transporter 4. Glut1: glucose transporter 1. CA-IX: carbonic anhydrase IX. V-ATPase: vacuolar-type H-ATPase. NHE1: Na/H exchanger. BT: bicarbonate transporter. HE: H exchanger. CA: carbonic anhydrase.
The key players of the pH dysregulation and main transport machineries involved in cellular trafficking of chemotherapeutics in solid tumors. Glucose is mainly taken up by glucose transporter 1 (GLUT1) and metabolized through glycolysis mechanism producing lactic acid (LA) and acidifying cytoplasm. LA is extruded by transporters such as the Na/H exchanger 1 (NHE1), monocarboxylate transporters (MCT1 and 4), carbonic anhydrase IX (CA IX) and H pump V-ATPase. These transport machineries can be inhibited with specific/delective inhibitors (red boxes). Cancer cells maintain intracellular pH (pHi) and extracellular pH (pHe) at 7.4 and 6.6, respectively. Transportation of some chemotherapeutics is via cell surface transporters (inset black box).