Glycolytic enzyme inhibitors effectively kill cancer cells – Part 2

September 11, 2012, Featured in Cancer and Natural Medicines, 0 Comments

As discussed in our previous article, PFK1 (phosphofructokinase type 1) is a major regulatory enzyme in glycolysis.

Anti-tumor activity of Herbalzym Vinegar by disrupting cancer cell metabolism

PFK1 is an allosteric enzyme made of 4 subunits and controlled by many activators and inhibitors. PFK-1 catalyzes the important “committed” step of glycolysis, the conversion of fructose 6-phosphate and ATP to fructose 1,6-bisphosphate and ADP.

PFK1 is allosterically inhibited by high levels of ATP but AMP reverses the inhibitory action of ATP. Therefore, the activity of the enzyme increases when the cellular ATP/AMP ratio is lowered. Glycolysis is thus stimulated when energy charge falls. PFK1 has two sites with different affinities for ATP which is both a substrate and an inhibitor.

PFK1 is inhibited by glucagon through repression of synthesis. Glucagon is one of the major hormonal regulators of glucose metabolism, counteracting the hepatic effects of insulin when the concentration of glucose in the bloodstream falls below a certain level.

PFK1 is also inhibited by low pH levels which augment the inhibitory effect of ATP. The pH falls when muscle is functioning anaerobically and producing excessive quantities of lactic acid. This inhibitory effect serves to protect the muscle from damage that would result from the accumulation of too much acid.

Finally, PFK1 is allosterically inhibited by both PEP (Phosphoenolpyruvic acid) and citrate. PEP is a product further downstream the glycolytic pathway. Citrate is the salt or ester of citric acid. It is a well-known physiological inhibitor of PFK1. Because the presence of citrate is proportional to the rate of ATP production, and the citrate slows the pathway down. The level of citrate inside the cells is both an essential intermediary of the metabolism and a key regulator of the energy production. Understanding its central position should be important to propose new strategies for counteracting cancer cells proliferation and overcoming chemoresistance.

Posttranslational modification of 6-phosphofructo-1-kinase as an important feature of cancer metabolism.

Evolution of the allosteric ligand sites of mammalian phosphofructo-1-kinase.

Glucose fatty acid interactions and the regulation of glucose disposal.

Citrate is an early intermediate in the citric acid cycle. The citric acid cycle is a series of chemical reactions to generate energy. A large number of compounds—for example, fatty acids and amino acids—can be metabolized to citric acid cycle intermediates. Citrate buildup is a sign of the citric acid cycle reaching saturation and thus glycolysis slows down as there is no longer any need to commit more glucose to degradation. Inhibition of PFK1 is total when citrate is abundant.

Because citrate promotes the acetylation of histones, it could play a role in adjusting the level of various key regulator enzymes. Many genes in cancer cells are silent due to excessive DNA methylation and poor acetylation of gene associated histones. Some of these silent genes are powerful tumor suppressors. Increasing the acetylation of lysine (K) residues on histones does contribute to the activation of previously silent genes.

Citrate also inhibits tumor angiogenesis. It is well established that tumors cannot grow larger than a few millimeters without a blood supply. Angiogenesis process, the growth of new blood vessels into tumors, is permissive for both cancer growth in general and metastasis.

Citrate demonstrated in vitro anti-cancer properties when administered in excess, and sensitized cancer cells to chemotherapy.  Citrate induced an early diminution of the expression of the anti-apoptotic protein Mcl-1, which is a protein member of the Bcl-2 family playing a key role, with Bcl-xL, in the chemoresistance of malignant cancers. Indeed, concomitant inhibition of these two anti-apoptotic proteins by specific siRNA (directed against Mcl-1 or Bcl-xL) caused complete cancer cell death, whereas inhibition of only one of these two anti-apoptotic molecules, even combined with cisplatin at a low dose (5 g per ml), was not sufficient to eradicate cultured cells. Since citrate sensitizes cancer cells to chemotherapy, administration of this molecule at high dosage should be considered as a new “targeting metabolism strategy” in cancer therapy.   

Understanding the central role of citrate in the metabolism of cancer cells.

D-Amino acid oxidase-induced oxidative stress, 3-bromopyruvate and citrate inhibit angiogenesis, exhibiting potent anticancer effects.

Histone acetylation modulation by small molecules: a chemical approach.

ATP-citrate lyase links cellular metabolism to histone acetylation.

Biochemistry. A glucose-to-gene link.

The engine driving the ship: metabolic steering of cell proliferation and death.

Hypothesis proved…citric acid (citrate) does improve cancer: a case of a patient suffering from medullary thyroid cancer.

Emerging metabolic targets in cancer therapy.

Targeting cancer metabolism: a therapeutic window opens.

Usually produced in powder form, citric acid is naturally found in citrus fruits. However, the available data confirm the low acute and (sub)chronic toxicity profile of citric acid (in various organs such as liver, heart, lung, kidney, etc.). Citrate also causes anticoagulation by chelation of calcium. Chelation is a process by which an ion is attached to a neighbouring atom by at least 2 bonds. This process creates a strong bond that markedly decreases the ionised calcium available to the coagulation process. Citrate is likely to lead to magnesium chelation as well.

Citrate is metabolised in the liver and it accumulates in blood during liver failure. It should not be used in patients with known liver disease, because too much citrate would have to be given to anticoagulate the greater amount of blood and the liver would be unable to metabolise it. If citrate accumulates, it can cause any combination of these 3 complications:

1. Metabolic acidosis

2. Hypocalcemia

3. Systemic hypocoagulability


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