Tumor hypoxia is the situation where tumor cells have been deprived of oxygen. As a tumor grows, it rapidly outgrows its blood supply, leaving portions of the tumor with regions where the oxygen concentration is significantly lower than in healthy tissues. Hypoxic microenvironements in solid tumors are a result of available oxygen being consumed within 70 to 150 μm of tumour vasculature by rapidly proliferating tumor cells thus limiting the amount of oxygen available to diffuse further into the tumor tissue. In order to support continuous growth and proliferation in challenging hypoxic environments, cancer cells are found to alter their metabolism. Furthermore, hypoxia is known to change cell behavior and is associated with extracellular matrix remodeling and increased migratory and metastatic behavior.
Changes in the glycolytic pathway
A particular change in metabolism, historically known as the Warburg effect results in high rates of glycolysis in both normoxic and hypoxic cancer cells. Expression of genes responsible for glycolytic enzymes and glucose transporters are enhanced by numerous oncogenes including RAS, SRC, and MYC.
HIF-1 induced changes in gene expression
During cancer progression, tumor cells acquire comprehensive metabolic reprogramming, and tissue hypoxia is a prominent feature of solid tumors leading to cell metabolism adaptive changes. Hypoxia inducible factor-1α (HIF-1α) is a key oxygen-regulated transcriptional activator, playing a fundamental role in the adaptation of tumor cells to hypoxia by upregulating the transcription of target genes related to multiple biological processes, including cell survival, proliferation, angiogenesis and anti-apoptosis. Significant HIF1A expression has been noted in most solid tumors studied, which include cancers of the gastric and colon.
These genes include: solute carrier family 2 (GLUT1), hexokinase (HK), phosphoglucose isomerase (PGI), phosphofructokinase (PFKL), fructose-bisphosphate aldolase (ALDO), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), phosphoglycerate mutase (PGM), enolase 1 (ENOA), pyruvate kinase (PK), pyruvate dehydrogenase kinase, isozyme 1 (PDK1) and lactate dehydrogenase A (LDH-A).
In addition to alterations in oxygen concentration associated with hypoxic microenvironments, glucose concentration gradients found in tumors also influence the rate of aerobic and anaerobic glycolysis. A carbohydrate-response element (ChoRE) is responsible for regulating glycolytic enzyme gene expression in response to changing glucose concentrations through a binding interaction at the same consensus sequence as HIF-1. Interactions of HIF-1 and ChoRE with the DNA sequence 5’-RCGTG-3’ leads to increased expression of genes listed above.
GLUT1 transporter expression
GLUT1 is a member of the GLUT transporter family of 14 hexose transporters responsible for facilitating the transport of hexose sugars along the concentration gradient. GLUT1 is the most abundantly expressed of the family thought to maintain basal glucose transport in almost all cell types. GLUT1 levels, in response to hypoxic conditions, have been shown to increase with changes at both the mRNA and protein levels. Moreover, transport of GLUT1 has been shown to increase under these hypoxic conditions. With the role of transporting sugars from the extracellular to the intracellular environment, GLUT1, along with other members of the GLUT family, can be rate-controlling for cellular glycolytic metabolism. Having an increased level of GLUT1, in the case of hypoxic tumors, increases the flux of glucose into the cells allowing for a higher rate of glycolysis and thus greater risks of metastasis (as elaborated upon below).
Hexokinase 2 expression
Hexokinase (HK) is the first enzyme in the glycolytic pathway converting glucose to glucose-6-phosphate through an ATP-dependent phosphorylation event. Important for glycolysis to proceed, the hexokinase reaction activates glucose for subsequent steps. In hypoxic tumors, hexokinase mRNA abundance is significantly increased as well as protein levels. Increased expression of hexokinase 2, in some cases nearly 10-fold, allows for an increased flux of glucose through the glycolytic pathway subsequent to the increased uptake by GLUT1.<