Tumor M2-PK

# Overview
Tumor M2-PK is a synonym for the dimeric form of the pyruvate kinase isoenzyme type M2 (M2-PK).
Tumor M2-PK is a key enzyme within tumor metabolism and can be used in stool (fecal) samples for the screening of colorectal tumors (= bowel cancer) and in EDTA plasma samples for the follow-up of various cancers.
Tumor M2-PK is not an organ-specific tumor marker, such as PSA. As a biomarker the amount of Tumor M2-PK in stool and EDTA-plasma reflects the specific metabolic status of the tumors.
Sandwich ELISAs based on two monoclonal antibodies which specifically recognize Tumor M2-PK (the dimeric form of M2-PK) are available for the quantification of Tumor M2-PK in stool and EDTA-plasma samples respectively.

# Tumor M2-PK screening for the early detection of colorectal tumors and polyps
Preliminary clinical studies in Germany, England and Ireland, which tested the detection of fecal Tumor M2-PK as a biomarker for the early detection of colorectal tumors (= bowel cancer), have revealed sensitivities between 73% (in the only published study) and 97 % (in an unpublished conference poster presentation). This means that the test was positive in between 73 and 97 out of every 100 colorectal tumor patients. For polyps with a diameter larger than 1 cm the sensitivity was 60 %; for polyps smaller than 1 cm the sensitivity was about 25 %. The overall sensitivity for all polyps was 40 %.
Most people are more willing to accept non-invasive preventive medical check-ups. Therefore, the measurement of Tumor M2-PK in stool samples, with follow-up by colonoscopy to clarify the Tumor M2-PK positive results, may prove to be an advance in the early detection of colorectal carcinomas.  More research is needed before the test can be recommended as a screening procedure.

# Tumor M2-PK measurements in cancer follow-up
Studies from various international working groups have revealed a significantly increased amount of Tumor M2-PK in EDTA-plasma samples of patients with renal, lung, breast, cervical and gastrointestinal tumors (oesophagus, stomach, pancreas, colon, rectum), as well as melanoma (= skin cancer), which correlated with the tumor stage.
The combination of Tumor M2-PK with the appropriate classical tumor marker, such as CEA for bowel cancer, CA 19-9 for pancreatic cancer and CA 72-4 for gastric cancer, significantly increases the sensitivity to detect various cancers.
An important application of the Tumor M2-PK test in EDTA-plasma is for follow-up during tumor therapy, to monitor the success or failure of the chosen treatment, as well as predicting the chances of a cure and survival.
If Tumor M2-PK levels decrease during therapy and then remain low after therapy it points towards successful treatment. An increase in the Tumor M2-PK values during or after therapy points towards relapse and/or metastasis.
Increased Tumor M2-PK values can sometimes also occur in severe inflammatory diseases, which must be excluded by differential diagnosis.

# Scientific background to the role of Tumor M2-PK in tumor metabolism
Pyruvate kinase catalyzes the last step within the glycolytic sequence, the dephosphorylation of phosphoenolpyruvate to pyruvate and is responsible for net energy production within the glycolytic pathway.
Depending upon the different metabolic functions of the tissues, different isoenzymes of pyruvate kinase are expressed.
M2-PK is the characteristic isoenzyme of all proliferating cells, such as normal proliferating cells (i.e. fibroblasts, embryonic cells and adult stem cells) and tumor cells.
During tumorigenesis a shift in the isoenzyme equipment of pyruvate kinase always occurs in such a way that the tissue specific isoenzymes, such as L-PK in the liver or M2-PK in the brain, disappear and M2-PK is expressed.

## Role of the tetrameric and dimeric forms of M2-PK within tumor metabolism
M2-PK can occur in two different forms in proliferating cells:
- a tetrameric form, which consists of four subunits
- a dimeric form, consisting of two subunits.
The tetrameric form of M2-PK has a high affinity to its substrate, phosphoenolpyruvate (PEP), and is highly active at physiological PEP concentrations. Furthermore, the tetrameric form of M2-PK is associated with several other glycolytic enzymes within the so-called glycolytic enzyme complex. Due to the close proximity of the enzymes, the association within the glycolytic enzyme complex leads to a highly effective conversion of glucose to lactate. 
When M2-PK is mainly in the highly active tetrameric form, which is the case in most normal proliferating cells, glucose is mostly converted to lactate under the production of energy.
The dimeric form of M2-PK is characterized by a low affinity to its substrate phosphoenolpyruvate and is nearly inactive at physiological PEP concentrations.
When M2-PK is mainly in the less active dimeric form, which is the case in tumor cells, all phosphometabolites above pyruvate kinase accumulate and are channelled into synthetic processes which branch off from glycolytic intermediates, such as nucleic acid-, phospholipid-and amino acid synthesis.
Nucleic acids, phospholipids and amino acids are important cell building blocks which are badly needed by highly proliferating cells, such as tumor cells.
Due to the key position of pyruvate kinase within glycolysis, the tetramer : dimer ratio of M2-PK determines whether glucose carbons are converted to pyruvate and lactate under the production of energy (tetrameric form) or channelled into synthetic processes (dimeric form).
In tumor cells M2-PK is mainly in the dimeric form. Therefore, the dimeric form of M2-PK has been termed Tumor M2-PK.
The dimerization of M2-PK in tumor cells is induced by the direct interaction of M2-PK with different oncoproteins.
However, the tetramer : dimer ratio of M2-PK is not constant.
Oxygen starvation or highly accumulated glycolytic intermediates, such as fructose 1,6-bisphosphate (fructose 1,6-P2) or the amino acid serine, induce the reassociation of the dimeric form of M2-PK to the tetrameric form. Consequently, due to the activation of M2-PK, glucose is converted to pyruvate and lactate under the production of energy until the fructose 1,6-P2 levels drop below a certain threshold value, which allows the dissociation of the tetrameric form of M2-PK to the dimeric form. Thereafter, the cycle of oscillation starts again when the fructose 1,6-P2 levels reach a certain upper threshold value which induces the tetramerization of M2-PK.
When M2-PK is mainly in the less active dimeric form, energy is produced by the degradation of the amino acid glutamine to aspartate, pyruvate and lactate, which is termed glutaminolysis.
In tumor cells the increased rate of lactate production in the presence of oxygen is termed the Warburg effect.