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Phospholipases
Pre-step for eicosanoid production
These are enzymes who release fatty acids out of their phospholipid structure. There are 4 types of phospholipases; A1, A2, C and D. The enzymes A1 and A2 are needed to release 20-carbon fatty acids such as EPA, AA or DGLA, out of the cell membranes to be converted by COX1 or COX2 enzymes into eicosanoids (prostaglandins) or thromboxanes. The phospholipids are found in the cell membranes. The arrows show at which locations the different phospholipases are active. R1 is the outer fatty acid leg, R2 the inner and mostly unsaturated leg. Phospholipase C is used to release the phosphor group, important for cell signalling.
Phospholipase A2 is necessary for tumor necrosis factor alpha-induced ceramide generation in L929 cells.
Jayadev S, Hayter HL, Andrieu N, Gamard CJ, Liu B, Balu R, Hayakawa M, Ito F, Hannun YA. Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA. J Biol Chem. 1997 Jul 4;272(27):17196-203.
The role of cytosolic phospholipase A2 (cPLA2) in the regulation of ceramide formation was examined in a cell line (L929) responsive to the cytotoxic action of tumor necrosis factor alpha (TNFalpha). In L929 cells, the addition of TNFalpha resulted in the release of arachidonate, which was followed by a prolonged accumulation of ceramide occurring over 5-12 h and reaching 250% over base line. The formation of ceramide was accompanied by the hydrolysis of sphingomyelin and the activation of three distinct sphingomyelinases (neutral Mg2+-dependent, neutral Mg2+-independent, and acidic enzymes). The variant cell line C12, which lacks cPLA2, is resistant to the cytotoxic action of TNFalpha.
TNFalpha was able to activate nuclear factor kappaB in both the wild-type L929 cells and the C12 cells. However, TNFalpha was unable to cause the release of arachidonate or the accumulation of ceramide in C12 cells. C6-ceramide overcame the resistance to TNFalpha and caused cell death in C12 cells to a level similar to that in L929 cells. The introduction of the cPLA2 gene into C12 cells resulted in partial restoration of TNFalpha-induced arachidonate release, ceramide accumulation, and cytotoxicity. This study suggests that cPLA2 is a necessary component in the pathways leading to ceramide accumulation and cell death.
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