Aerobic glycolysis refers to a condition in which glucose is converted to lactate in the presence of oxygen. This condition is also referred to as the Warburg Effect. This term has been utilized in the literature dating back to almost 100 years. The final stages of yield the production of extracellular lactic acid, which is counterintuitively produced even though sufficient oxygen is present to allow oxidative phosphorylation (OXPHOS) to proceed.
All tissues have a requirement for ATP to function normally. Human cells produce ATP through a careful drop in oxidation state from energy-rich molecules like glucose, through the process of cellular respiration, down to the end product CO2. It can occur aerobically or anaerobically, depending on whether oxygen is available. Glycolysis is the first step in the pathway of glucose metabolism and occurs in the cytosol of all cells. Oxygen presence is significant because of oxidation of glucose under aerobic conditions results in approximately 32 mol of ATP per mol of glucose (tissue-specific). However, under anaerobic conditions, only 2 mol of ATP can be produced. Aerobic glycolysis specifically occurs in 2 steps. The first occurs in the cytosol and involves the conversion of glucose to pyruvate with resultant production of NADH. This process alone generates two molecules of ATP. Normally, if oxygen is available, then the free energy contained in NADH is further released via re-oxidization of the mitochondrial electron chain and results in the release of 30 more mol of ATP per mol of glucose. However, when oxygen is in short supply or when cells are undergoing aerobic glycolysis, this NADH is instead re-oxidized, reducing pyruvate to lactate. This limits the amount of ATP formed per mole of glucose when compared to the complete oxidation of glucose to CO2 utilizing OXPHOS. In situations where there is an imbalance of oxygen use and oxygen delivery, for example in sepsis or heart failure, anaerobic glycolysis results, followed by lactate accumulation, inefficient glucose usage, and higher glucose intake demands.
Cancer stem cells (CSC) within a tumor are notorious for aerobic glycolysis. Thus extensive aerobic glycolysis has been indicative of aggressive cancer. Many studies of normal and cancerous cells have shown one of two dichotomous behaviors: (1) that of highly differentiated cells, which perform OXPHOS, are not exceedingly sensitive to ROS, exhibit a differentiated cell morphology, and can divide a finite number of times before dying (mortal cell line); and (2) that of stem cells, which mainly perform aerobic glycolysis, are fairly sensitive to ROS (DNA damage), exhibit an undifferentiated cell morphology, and can divide an infinite number of times (immortal cell line). It should be noted that many noncancerous cells exhibit the stem cell behavior and are required for normal human development, such as hematopoietic stem cells (HSCs) in the bone marrow of adults and the embryonic stem cells responsible for neural fold/crest development in the growing fetus. The key with CSCs is that they are no longer subject to many of the regulating pathways that govern normal stem cells. For example, the master transcription factor MYC becomes constitutively expressed in CSCs, and as a result, these cells become addicted to growth/division, a process that requires extensive anabolic precursors which can only be met through continual aerobic glycolysis. Such cells consume far more glucose than differentiated cells, and extensive modification of their metabolic pathways to further meet this demand can easily be seen as a "glucose addiction." Further, P53-dependent apoptosis becomes disrupted in CSCs.
Normally, when MYC is inappropriately expressed in cells, such cells will undergo apoptosis. In CSCs, this no longer occurs. This disrupted P53 pathway also has been implicated in the further upregulation of glycolytic enzymes by mutations deleting the normal glucose-6-phosphate dehydrogenase, inhibiting the ability of P53. Mainly due to upregulation of many membrane transporters, such as H+/Na anti-transporter (NHE-1), monocarboxylate transporters (MCT1-4, which cotransport H+ and either lactate, pyruvate, or ketone bodies), and carbonic anhydrases (CA9 or CA12), the extracellular pH and intracellular pH polarity become reversed. Intracellular pH rises markedly, becoming significantly alkaline, and extracellular pH becomes much more acidic than normal. The increased intracellular pH is thought to play a role in the disruption of P53 regulation, as PPIs such as esomeprazole have been shown to restore apoptosis in cancer cells. High intracellular pH has also been shown to decrease the intracellular uptake of many chemotherapeutic drugs, specifically targeting cancer tissues (mostly drugs which are chemically weak bases, such as doxorubicin, mitoxantrone, chloroquine, vincristine, and vinblastine) and inhibiting cytotoxic T cells.
In many studies, it has been demonstrated that pyruvate kinase (PKM2, which is isoenzyme M2) is a key metabolic enzyme in CSCs. PKM2 is found in normal proliferating cells, embryonic stem cells, and CSCs. Very few differentiated tissues express PKM2 in significant quantity, as PKM2 is the last step in glycolysis to decide the fate of glucose substituents, either for synthesis (nucleogenic) or energy (glycogenic), and its expression is required in dividing cells. In tumors, the PKM2 isoform is predominant, expressed in great enough occurrence that tumors secrete it into the bloodstream, and for mucosal tumors, into bile, feces, and urine. ELISA quantification of such fluids has been well correlated with tumor staging in studies of melanoma, breast cancer, thyroid cancer, kidney cancer, lung cancer, esophageal cancer, gastric cancer, colorectal cancer, pancreatic cancer, ovarian cancer, cervical cancer, skin cancer, and renal cancer. Note, aberrantly high plasma levels of PKM2 also can be present in cases limited to severe inflammation.
The conditions that promote CSC development in normal tissues (the "niche") are lower oxygen levels, such as with HSCs in bone marrow. Here, developing CSCs release hypoxia-inducible factors (HIF1 alpha and HIF2 alpha) that exert several physiological effects mediating survival.
The two main pillars of energy production in tumor cells are aerobic glycolysis (glucose) and glutaminolysis (glutamine/glutamate). Drug treatment aimed at metabolic disruption of tumors must take into account both pathways to be successful. For example, a study demonstrated that even hyperactivation of PKM2 could be effective in preventing tumor cell growth, as long as this is carried out with high glucose concentrations and under hypoxic conditions, which discourages glutaminolysis from simultaneously occurring. In that study, peptide aptamers were bound to PKM2 under high glucose conditions, which inhibited cell proliferation. Conversely, these same peptide aptamers promoted cell proliferation and discouraged apoptosis under low glucose conditions, apparently due to more efficient shunting of the scarcely available glucose into synthesis pathways while concurrently maintaining cellular energy through glutaminolysis.
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