Monocarboxylate transporter

The monocarboxylate transporters,[1] or MCTs, are a family of proton-linked plasma membrane transporters that carry molecules having one carboxylate group (monocarboxylates), such as lactate and pyruvate, across biological membranes.

Lactate and the Cori cycle

Lactate has long been considered a byproduct resulting from glucose breakdown through glycolysis during anaerobic metabolism. Glycolysis requires the coenzyme NAD+, and reduces it to NADH. As a means of regenerating NAD+ to allow glycolysis to continue, lactate dehydrogenase catalyzes the conversion of pyruvate to lactate in the cytosol, oxidizing NADH to NAD+. Lactate is then transported from the peripheral tissues to the liver. There it is reformed into pyruvate and ultimately to glucose, which can travel back to the peripheral tissues, completing the Cori cycle.

Thus, lactate has traditionally considered a toxic metabolic byproduct that could give rise to fatigue and muscle pain during anaerobic respiration. Lactate can be thought of essentially as payment for "oxygen debt", defined by Hill and Lupton as the "total amount of oxygen used, after cessation of exercise in recovery there from".[2]

Clinical significance

Highly malignant tumors rely heavily on anaerobic glycolysis (metabolism of glucose to lactic acid even under ample tissue oxygen; Warburg Effect) and thus need to efflux lactic acid via MCTs to the tumor micro-environment to maintain a robust glycolytic flux and to prevent the tumor from being "pickled to death".[3] The MCTs have been successfully targeted in pre-clinical studies using RNAi[4] and a small-molecule inhibitor alpha-cyano-4-hydroxycinnamic acid (ACCA; CHC) to show that inhibiting lactic acid efflux is a very effective therapeutic strategy against highly glycolytic malignant tumors.[5][6][7]

See also

Monocarboxylate transporters:

References

  1. ↑ Halestrap AP, Meredith D (2004). "The SLC16 gene family-from monocarboxylate transporters (MCTs) to aromatic amino acid transporters and beyond". Pflugers Arch. 447 (5): 619–28. doi:10.1007/s00424-003-1067-2. PMID 12739169.
  2. ↑ Lupton, H. (1923). "An analysis of the effects of speed on the mechanical efficiency of human muscular movement". J Physiol.
  3. ↑ Mathupala SP, Colen CB, Parajuli P, Sloan AE (2007). "Lactate and malignant tumors: a therapeutic target at the end stage of glycolysis (Review)". J Bioenerg Biomembr. 39 (1): 73–77. doi:10.1007/s10863-006-9062-x. PMID 17354062.
  4. ↑ Mathupala SP, Parajuli P, Sloan AE (2004). "Silencing of monocarboxylate transporters via small interfering ribonucleic acid inhibits glycolysis and induces cell death in malignant glioma: an in vitro study". Neurosurgery. 55 (6): 1410–1419. doi:10.1227/01.neu.0000143034.62913.59. PMID 15574223.
  5. ↑ Colen, CB, PhD Thesis (2005) http://elibrary.wayne.edu/record=b3043899~S47
  6. ↑ Colen CB, Seraji-Bozorgzad N, Marples B, Galloway MP, Sloan AE, Mathupala SP (2006). "Metabolic remodeling of malignant gliomas for enhanced sensitization during radiotherapy: an in vitro study". Neurosurgery. 59 (6): 1313–1323. doi:10.1227/01.NEU.0000249218.65332.BF. PMID 17277695.
  7. ↑ Colen CB, Shen Y, Ghoddoussi F, Yu P, Francis TB, Koch BJ, Monterey MD, Galloway MP, Sloan AE, Mathupala SP (2011). "Metabolic targeting of lactate efflux by malignant glioma inhibits invasiveness and induces necrosis: an in vivo study". Neoplasia. 13 (7): 620–632. doi:10.1593/neo.11134. PMC 3132848Freely accessible. PMID 21750656.
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