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),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.<Phosphoglucose isomerase expression
Phosphoglucose isomerase (PGI) is a housekeeping cytosolic enzyme with roles in both glycolysis and gluconeogenesis pathways. It is responsible for catalyzing the interconversion of glucose 6-phosphate and fructose 6-phosphate. Extracellularly, PGI is known as an autocrine motility factor (AMF) eliciting mitogenic, motogenic, differentiation functions as well as tumor progression and metastasis. Activation of PGI through proposed HIF-1 induced mechanisms results in increased conversion of glucose 6-phosphate to fructose 6-phosphate and also contributes to cell motility and invasion during cancer metastasis.6-Phosphofructo-2-kinase/fructose 2,6-bisphosphatases expression
6-Phosphofructo-2-kinases/fructose 2,6-bisphosphatases (PFKFBs) belong to a family of bifunctional ATP-dependent enzymes responsible for controlling the level of glycolysis intermediate fructose-1,6-bisphosphate. HIF-1-induced expression of these enzymes (PFK-2/FBPase-2) subsequently alters the balance of fructose-2,6-bisphosphate which plays an important role as an allosteric activator of phospho-fructokinase 1 (PFK-1). PFK-1 is an enzyme that controls one of the most critical steps of glycolysis. Regulation of PFK-1 is also mediated by the cellular energy status in result of ATP's inhibitory effect. Greater quantities of fructose-2,6-bisphosphate in cancer cells, in result of HIF-1 expression of PFK-2/FBPase-2, thus activates PFK-1allowing for an increased glycolytic flux converting fructose-6-phosphate to fructose-1,6-bisphosphate. Allosteric regulation of glycolysis by fructose-2,6-bisphosphate allows cancer cells to maintain a glycolytic balance to match their bioenergetic and biosynthetic demands.Fructose-1,6-bisphosphate aldolase expression
Fructose-1,6-bisphosphate aldolase (ALDO) belongs to a family include aldolase A, B and C. Unique in glycolysis, aldolase enzymes cleave fructose-1,6-bisphosphate into two 3-C molecules including glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). With the HIF-1 mediated expression of aldolase A under hypoxic conditions, the catalysis of fructose-2,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate is increased thus leading to increased glycolytic flux.Glyceraldehyde-3-phosphate dehydrogenase expression
The glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is responsible for the oxidative conversion of glyceraldehyde-3-phosphate (GADP) to 1,3-bisphosphoglycerate (1,3BPG). Up-regulation of glyceraldehyde-3-phosphate dehydrogenase expression is maximal (4-5 fold) following hypoxic conditions of ~24 hours in vascular endothelial cells. Various models for the exact glyceraldehyde-3-phosphate dehydrogenase activation mechanisms have been proposed.Phosphoglycerate kinase 1 expression
Hypoxia has been shown to induce a 10-fold accumulation of phosphoglycerate kinase 1 (PGK-1) mRNA in mouse hepatoma (Hepa 1c1c7) cells. Phosphoglycerate kinase 1 is an enzyme involved in the conversion of 1,3-bisphosphoglycerate (1,3-BPG) to 3-phosphoglycerate (3-P-G) leading the production of ATP from ADP. Induction of gene expression by HIF-1 is thought to be dependent on the presence of aromatic hydrocarbon receptor nuclear translocator (ARNT1). Arnt's N-terminal region and HIF-1 are thought to work together to induce transcription of phosphoglycerate kinase 1.Phosphoglycerate mutase expression
Phosphoglycerate mutase B (PGM-B) is one of the latter glycolytic enzymes responsible for the conversion of 3-phosphoglycerate (3PG) to 2-phosphoglycerate (2PG). Both protein and mRNA levels were shown to increase 2-3-fold in research exposing fetal rat lung fibroblasts to hypoxic conditions. Increased levels appeared to be regulated at the transcriptional level as per many of the other glycolytic enzymes. Maximum up regulation was shown following 16 hours thus supporting its role in contributing to an increased glycolytic flux for adaption of cells to hypoxia.Enolase 1 expression
Enolase 1, also known as α-enolase, is encoded by the ENOA gene and is responsible for converting 2-phosphoglycerate to phosphoenolpyruvate in the glycolytic pathway. Both enolase 1 overexpression and its post-translational modifications could be of value for diagnostic and prognostic work in terms of cancer. Although the exact roles of post-translational modifications have not been completely elucidated, patterns are shown between certain cancer cell types suggesting they may have important influence on function, localization and immunogenicity. Aside from its role in promoting glycolytic flux and anaerobic energy production, it has been shown to induce a specific humoral and cellular immune response. On all levels, hypoxia-induced over-expression of enolase 1 may possess important roles in hypoxic tumors including the most straightforward increase in anaerobic respiration.Pyruvate kinase expression
HIF-1 activated pyruvate kinase M comes in multiple isoforms known as PKM1 and PKM2. Pyruvate kinase is shown to convert phosphoenolpyruvate to pyruvate forming ATP from ADP. Along with phospho-fructokinase 1, pyruvate kinase is also allosterically activated by fructose-2,6-bisphosphate. In cancer cells pyruvate kinase M2 has been shown to interact directly with HIF-1α enhancing HIF-1 binding and p300 recruitment to hypoxia response elements. This positive feedback loop leads to HIF-1 transactivation and an amplified effect on glucose metabolism. Pyruvate kinase M2 is often considered the main regulator of cancer metabolism with roles in various parallel, feed-forward, positive and negative feedback mechanisms. The genetic difference between pyruvate kinase M1 and pyruvate kinase M2 is only 22 out of 531 amino acids which makes an immense difference. Pyruvate kinase M2 has metabolic activity regulated by post-translational modifications including acetylation, oxidation, phosphorylation, hydroxylation and sumoylation. These different modifications can cause the shift from the metabolically active tetrameric form to the in-active monomeric form. The well-known EGFR-activated extracellular signal-regulated kinase 2 (ERK2) and death-associated protein kinase are both shown to bind and directly phosphorylate pyruvate kinase M2 leading to increased activity in the glycolysis pathway. In hypoxic conditions found in a solid tumor, pyruvate kinase M2 plays a large role in promoting anaerobic energy production.Pyruvate dehydrogenase kinase expression
Pyruvate dehydrogenase directly follows the glycolytic pathway and is responsible for the conversion of pyruvate to acetyl-CoA which enters into the TCA cycle. The TCA cycle, although not directly requiring oxygen, requires the cycling of NADH to NAD+ as performed by the electron transport chain under aerobic conditions. Under anaerobic conditions, such as those found in hypoxic tumors, the TCA cycle provides little ATP yield due to the lack of electron transport chain function. In order to direct the glycolytically produced pyruvate away from the TCA cycle, pyruvate dehydrogenase kinase is over-expressed in response to hypoxic conditions. Pyruvate dehydrogenase kinase is not a glycolytic enzymes but more of a glycolytic regulator. Pyruvate dehydrogenase kinases, transcriptionally activated by HIF-1 in hypoxic conditions, are responsible for phosphorylating the E1 subunit of pyruvate dehydrogenase ultimately suppressing its function. By inhibiting this specific pathway, the glycolytic products are directed away from the mitochondrial TCA cycle and towards lactate dehydrogenase.Lactate dehydrogenase expression
Activated expression of lactate dehydrogenase A (LDH-A), parallels with deactivation of pyruvate dehydrogenase mediated by pyruvate dehydrogenase kinase. Subsequent inactivation of pyruvate dehydrogenase following phosphorylation and increased expression of lactate dehydrogenase A shunts pyruvate away from the mitochondrial TCA cycle. In many different tumor types lactate dehydrogenase A is found at elevated levels and has even been linked to poor prognosis and a greater metastatic potential The high levels of lactate production surface the question of whether lactate has some influence on the aggressive behaviour shown in hypoxic tumors.Overview of glycolytic changes and consequences
Increased expression of almost every glycolytic enzyme is seen in hypoxic tumor conditions. The over-expression of these proteins is mediated by HIF-1 and completely alters normal cellular metabolism. With decreases in the rate of mitochondrial oxidation, lactate and protons begin to accumulate. High levels of glycolysis and the production of lactate, as shown in hypoxic tumor cells, is hallmark of cancer cells even in the presence of oxygen. To relieve tumors cells ofLactate and cancer
It is commonly accepted that cancer cells (both hypoxic and normoxic) produce large amounts of lactate in result of a large metabolic shift fromAcidic microenvironment and metastasis
It is thought that the low pH of hypoxic tumors in result of high levels of lactic acid can promote tumor cell invasion by destruction of adjacent non-cancerous tissue.> Carbonic anhydrase 9 involved in maintaining a slightly alkaline intracellular pH does so by removing carbonate from the extracellular space consequently acidifying the cells surroundings. Moreover, proton pumping from the hypoxic tumor cells further decreases the surrounding pH. On a completely different note, as briefly discussed above, the autocrine function of phosphoglucose isomerase also promotes cell motility and metastasis.Metabolic symbiosis
With the hypoxic tumor cells consuming large amounts of glucose to maintain energyThe pentose phosphate pathway
In order to meet the demands of rapid tumor growth, the tumor must find ways to support the synthesis of a complete daughter cell while facing depleting nutrient supplies. They must coordinate production of precursors for macromolecular synthesis as well as maintain cellular bioenergetics without impairing cell growth, proliferation and viability. One way of doing this is by shuffling glycolytic intermediates such as glucose-6-phosphate into the pentose phosphate pathway to give ribose-5-phosphate and NADPH. Ribose-5-phosphate acts as an intermediate for the production of nucleotides thus providing a connection between glycolysis and nucleotide synthesis in hypoxic tumor cells. In cases where glycolysis remains highly active in normoxic conditions, NADPH acts as a mediator of antioxidative reactions to protect cells from oxidative damage.Cancer treatments and tumor hypoxia
Radiotherapy
The presence or absence of oxygen has a strong influence on ionizing radiation to cause cell death of tumour and normal cells. This is called the oxygen effect. Under hypoxic conditions it has been shown that cells obtain radioresistance through HIF-1 mediated mechanisms. To overcome this problem, radiation oncologists have developed powerful tools and approaches such as simultaneous integrated boost intensity-modulated radiation therapy (SIB-IMRT), which enables a booster dose of radiation to be delivered to small target fractions in a malignant tumor, hypoxia-selective cytotoxins/drugs, and HIF-1 inhibitors. Moreover, it is possible to treat a hypoxic tumor by means of ion beam therapy, in particular with 12C. As ions' damage is direct, OER (Other treatment options
Bioreductive prodrugs play a significant part in dealing with these kinds of cells: they can kill the oxygen-deficient tumor cells selectively as hypoxia-activated prodrugs. Example drugs include Tirapazamine and Evofosfamide. The study of tumors in such conditions was pioneered by Dr L. H. Gray.Targeting tumor hypoxia to overcome metastasis
An association between tumor hypoxia and metastatic progression has been shown through numerous publications.Drug development
Several approaches have been taken to address tumor hypoxia. Some companies tried to develop drugs that are activated in hypoxic environments (Novacea, Inc. Proacta, Inc, and Threshold Pharmaceuticals, Inc), while others are currently seeking to reduce tumor hypoxia (Diffusion Pharmaceuticals, Inc. and NuvOx Pharma, LLC). Several companies have tried to develop drugs that are activated in hypoxic environments. These drug candidates target levels of hypoxia that are common in tumors but are rare in normal tissues. The hypoxic zones of tumors generally evade traditional chemotherapeutic agents and ultimately contribute to relapse. In the literature, hypoxia has been demonstrated to be associated with a worse prognosis, making it a determinant of cancer progression and therapeutic response Several review articles summarize the current status of hypoxic cytotoxins ( hypoxia activated prodrugs). Companies that have tried drugs that are activated in hypoxic environments included Novacea, Inc. Proacta, and Threshold Pharmaceuticals. Novacea Inc discontinued development of its hypoxia activated drug. Proacta's drug PR610 failed a Phase I clinical trial due to toxicity. Threshold Pharmaceuticals discontinued the hypoxia activated prodrug, TH-302, after Phase III trials failed to show statistically significant overall survival. Niacinamide, the active form of vitamin B3, acts as a chemo- and radio-sensitizing agent by enhancing tumor blood flow, thereby reducing tumor hypoxia. Niacinamide also inhibits poly(ADP-ribose) polymerases (PARP-1), enzymes involved in the rejoining of DNA strand breaks induced by radiation or chemotherapy. As of August 2016, no clinical trials appear to be in progress for this indication. Another approach to the treatment of tumor hypoxia is the use of anSee also
* HypoxiaReferences
Further reading
* {{DEFAULTSORT:Tumor Hypoxia Oncology Oxygen