Release of arachidonic acid

During hormonal stimulation of Ptdlns 4,5-P2 hydrolysis there appears to be a preferential degradation of molecules, such as diacylglycerol and phosphatidic acid, which contain arachidonate in the 2-position. Two separate pathways have been proposed for the release of arachidonic acid from these two products of the phosphoinositide response. The first proposes that diacylglycerol is the source of the liberated arachidonate and that diacylglycerol lipase acts on the DG released by hydrolysis of phosphoinositides. The second suggests that a phosphatidic acid-specific phospholipase A2 is responsible for cleaving the arachidonic acid from phosphati-date. 

The release of arachidonic acid from these lipids by the synchronised activation of phospholipase C/phospholipase A2 could occur in complexes of proteins that are 

Summary of stimuli that induce both Ptdlns 4,5-P, hydrolysis and the release of arachidonate, and some examples of target tissues (from Refs. 6 and 8) 

Muscarinic cholinergic Pancreas Vascular smooth muscle 

Platelet activating factor Platelets Iris smooth muscle 

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These steioids aie capable of preventing or suppressing the development of the sweUing, redness, local heat, and tenderness which characterize inflammation. They inhibit not only the acute symptoms of the inflammatory process, such as edema, fibrin deposition, and capillary dilatation, but also the chronic manifestations. There is evidence that glucocorticoids induce the synthesis of a protein that inhibits phosphoHpase A 2 (60), diminishing the release of arachidonic acid from phosphoHpids (Fig. 2), thereby reducing chemotaxis and inflammation. 

Calderwood, S.K., Bomstein, B., Famum, E.K.., Stevenson, M.A. (1989). Heat shock stimulates the release of arachidonic acid and the synthesis of prostaglandins and leukotriene B4 in mammalian cells. J. Cell. Physiol. 141, 325-333. 

A thiazole core has also been utilised by Solvay Pharmaceuticals [309] in the search for a novel bioisosterie replacement of the rimonabant (382) pyrazole core. The affinity of several compounds for the human CBi receptor was determined in transfected CHO cells using tritium-labelled CP 55940. Antagonism was determined in the same cell line by WIN 55212-2-induced release of arachidonic acid. The pK[ of (456) was found to be 6.9, while the p 2 value was measured as 8.7. A series of six 1,2,4-triazole analogues has been prepared by Jagerovic et al. [310], via their corresponding A-acylbenzamides (Table 6.39). 

With respect to vasodilation, niacin-elicited vasodilation requires the activation of GPR109A in skin Langerhans cells [34,35], which then triggers the release of arachidonic acid from membrane phospholipids and its subsequent metabolism to PGD2. The production of PGD2 then activates DPI receptors in dermal blood vessels to cause vasodilation [36]. 

Horrocks, L. A. and Farooqui, A. A. NMDA receptor-stimulated release of arachidonic acid mechanisms for the Bazan effect. In Municio, A. M. and Miras-Portugal, M. T. (eds), Cell Signal Transduction, Second Messengers, and Protein Phosphorylation in Health and Disease. New York Plenum Press, 1994, pp. 113-128. 

TNF-a is considered the primary mediator of sepsis, and concentrations are elevated early in the inflammatory response during sepsis, and there is a correlation with severity of sepsis. TNF-a release leads to activation of other cytokines associated with cellular damage and it stimulates release of arachidonic acid metabolites that contribute to endothelial cell damage. IL-6 is a more consistent predictor of sepsis as it remains elevated for longer periods of time than does TNF-a. 

Levine L (2003a) Does the release of arachidonic acid from cells play a role in cancer chemoprevention FASEB J 17(8) 800-802... 

As mentioned above, PAF and PAF-like molecules are rapidly synthesized by keratinocytes following UV exposure. We suggest that two mechanisms are involved. UV-induced free radical formation leads to membrane oxidation and the formation of oxidized phosphatidylcholine. The PAF-like molecules bind to PAF receptors in either a paracrine or autocrine fashion. This induces the release of arachidonic acid from the membrane, activates PI.A2 and promotes the synthesis of bona fide PAF.55 The newly synthesized PAF then binds to PAF receptors, which upregulates the production of more PAF and downstream biological modifiers such as eicosanoids and cytokines. Ultimately this activates the cascade of events that leads to immune suppression. 

In addition to the importance of Ca2+, PLA2 activity is also regulated by lipocortin (also termed lipomodulin), which is a 40-kDa protein. The inhibitory effect of lipocortin is regulated by its phosphorylation status, acting as an inhibitor of the enzyme when in the dephosphorylated state. Upon cell activation (e.g. by fMet-Leu-Phe), the lipocortin becomes phosphorylated, and PLA2 activity (usually detected as the release of arachidonic acid) increases. Protein kinase C can cause this phosphorylation, and so activation of this kinase may lead to the relief of PLA2 inhibition via phosphorylation of lipocortin. Thus, elevations in the levels of intracellular Ca2+ and production of DAG (required for protein kinase C activation) may co-ordinately activate PLA2. 

A recent study showed THC to be toxic to hippocampal neurons, while sparing cortical neurons in concentrations likely to be reached in normal human doses (0.5 pM) (Chan et al. 1998). Apoptotic changes were noted in the hippocampus such as shrinkage of neuronal cell bodies and nuclei as well as DNA strand breaks. THC stimulates release of arachidonic acid, and it was hypothesized that this neurotoxic effect is mediated by cyclooxygenase pathways and free radical generation. In support of this, the toxicity is inhibited by nonsteroidal anti-inflammatory drugs, such as aspirin, and antioxidants such as vitamin E. 

Manoalide (164), a marine natural product which inhibits the release of arachidonic acid from phospholipids by phospholipase A2 [397,398], showed topical anti-inflammatory activity in mouse ear models [399]. Activity in ISN and cRBL (< 1 M) have also been reported [400]. A series of analogues consisting of the furanone ring of manoalide bearing simple unsaturated 16-20 carbon chains showed similar activity in rabbit neutrophils and isolated guinea-pig neutrophil 5-LO [401] interestingly, however, topical anti-inflammatory activity was seen in phorbol ester ear oedema but not in AAE [399]. The importance of 5-LO inhibition to the anti-inflammatory activity of manoalide is unknown effects on phospholipase C and calcium channels have also been shown [402, 403]. 

Another important aspect of the inflammatory cascade is arachidonic acid metabolism, leading to the synthesis of the proinflammatory prostaglandins and leukotrienes. Through the formation of Upocortin, an inhibitor of phospholipase A2, glucocorticoids depress the release of arachidonic acid from phospholipids and hence the production of arachidonic acid metabolites. 

Metabolites of arachidonic acid, including prostaglandins (PG), thromboxanes, and leukotrienes, are considered strong candidates as mediators of the inflammatory process. Steroids may exert a primary effect at the inflammatory site by inducing the synthesis of a group of proteins called lipocortins. These proteins suppress the activation of phospholipase A2, thereby decreasing the release of arachidonic acid and the production of proinflammatory eicosanoids (Fig. 60.6). 

The substrates of the MAP kinase pathway are very diverse and include both cytosolic and nuclear localized proteins. Phospholipase A2 and transcription factors of the Ets family are well characterized substrates of the ERK pathway. Phosphorylation of a Ser residue of phospholipase A2 by ERK proteins leads to activation of the lipase activity. Consequently, there is an increase in release of arachidonic acid and of lyso-phospholipids, which can act immediately as diffusible signal molecules or may represent first stages in the formation of second messenger molecules. 

Pharmacological approaches include the inhibition of release of arachidonic acid by inhibition of phospholipase A2 and the inhibition of acylcarnitine transferase I by and oxfenicine, the latter of which has been shown to prevent or at least delay ischemia-induced uncoupling. There are at present no data available on the possible effects of inhibitors of arachidonic acid release on ischemia-induced uncoupling. 

Arachidonic acid is not present in significant amounts in tissues as the free acid but is stored as a fatty acid at the sn-2 position of phospholipids. Prostaglandin biosynthesis is initiated by the interaction of a stimulus with the cell surface. Depending on the cell type, the stimulus can take the form of a hormone, such as angiotensin II or antidiuretic hormone, or a protease such as thrombin (involved in blood clotting), or both hormone and protease. These agents bind to a specific receptor that activates a phospholipase A2 that specifically releases the arachidonic acid from a phospholipid such as phosphatidylcholine. The release of arachidonic acid by phospholipase A2 is believed to be the rate-limiting step for the biosynthesis of eicosanoids. 

Fig. 4.4 Hypothetical model showing the modulation of glutamate transporter by arachidonic acid. Interactions of glutamate with its receptor result in depolarization and Ca2+ entry into the cell. Ca2+-mediated stimulation of PLA2 results in breakdown of neural membrane phospholipids and the release of arachidonic acid. Arachidonic acid not only modulates proton conductance associated with neuronal excitability, but also provides eicosanoids, which may control the glutamate transporter (modified from Fairman and Amara, 1999)...

Sanfeliu C., Hunt A., and Patel A. J. (1990). Exposure to N-methyl-D-aspartate increases release of arachidonic acid in primary cultures of rat hippocampal neurons and not in astrocytes. Brain Res. 526 241-248. 

Stella N., Pellerin L., and Magistretti P. J. (1995). Modulation of the glutamate-evoked release of arachidonic acid from mouse cortical neurons Involvement of a pH-sensitive membrane phospholipase A2. J. Neurosci. 15 3307-3317. 

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