PLA2-activating protein

The OP group of receptois share common effector mechanisms. All receptois couple via pertussis toxin-sensitive Go and Gi proteins leading to (i) inhibition of adenylate cyclase (ii) reduction of Ca2+ currents via diverse Ca2+ channels (hi) activation of inward rectifying K+ channels. In addition, the majority of these receptors cause the activation of phospholipase A2 (PLA2), phospholipase C 3 (PLC 3), phospholipase D2 and of MAP (mitogen-activated protein) kinase (Table 3). 

H, receptors also can stimulate the activity of phospholipase A2 (PLA2), with the subsequent release of arachido-nate and its metabolites. In platelets, this response does not require activation of the phosphoinositide cycle and is inhibited by pertussis toxin, suggesting a second, distinct Gj/o-protein-mediated transduction mechanism. In cells transfected with the H, receptor, PLA2 activation is partially inhibited by pertussis toxin, also suggesting at least two transduction systems [30,34],... 

Reports that AA is released primarily by G-protein-mediated PLA2 activation remain to be confirmed [84, 85]. In addition, modulation of PLA2 by Ca2+ and protein kinase needs to be better defined. It is clear that NMDA receptor activation promotes the release of AA [86], and that a variety of eicosanoids are then generated (Fig 33-2,33-3). The modulatory events that channel AA towards specific eicosanoids are not understood. The endocannabinoid family of lipid messengers will remain an active focus of interest because of the growing evidence of their actions in synaptic function, learning, memory, and other forms of behavior [56,87]. 

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. 

The role of protein kinase C in many neutrophil functions is undisputed and has been recognised for some time. For many years it was believed that the source of DAG, the activator of protein kinase C, was derived from the activity of PLC on membrane phosphatidylinositol lipids. Whilst this enzyme undoubtedly does generate some DAG (which may then activate protein kinase C), there are many reasons to indicate that this enzyme activity is insufficient to account for all the DAG generated by activated neutrophils. More recently, experimental evidence has been provided to show that a third phospholipase (PLD) is involved in neutrophil activation, and that this enzyme is probably responsible for the majority of DAG that is formed during cell stimulation. The most important substrate for PLD is phosphatidylcholine, the major phospholipid found in neutrophil plasma membranes, which accounts for over 40% of the phospholipid pool. The sn-1 position of phosphatidylcholine is either acyl linked or alkyl linked, whereas the sn-2 position is invariably acyl linked. In neutrophils, alkyl-phosphatidylcholine (1-0-alky 1-PC) represents about 40% of the phosphatidylcholine pool (and is also the substrate utilised for PAF formation), whereas the remainder is diacyl-phosphatidylcholine. Both of these types of phosphatidylcholine are substrates for PLD and PLA2. 

Numerous studies have suggested that PLA2 activity changes in neural disorders [12-17], cancer [18-21], inflammatory disorders [8,22-24], and parturition [25,26], In addition, lysophosphatidylcholine (LPC) causes demyelination [27], Recent studies suggested that LPC, converted from oxidized low-density lipoprotein (ox-LDL), induces endothelium-dependent vasoconstriction [28] and abolishes the formation of the receptor G-protein complex [29],... 

Studies of membrane-associated effects of a series of drugs on PLA2 have been undertaken to investigate the possibility that some proteins and drugs interact with the bilayer at phase boundaries or with defect structures necessary for PLA2 activity. The direct or indirect effect of drugs on such boundaries or defects could then affect membrane-protein interactions. 

Stretch-activated proteins in animal cell membranes that are candidates for osmosensing activity include mechanosensitive ion channels and the membrane-localized enzyme phospholipase A2 (PLA2). The former proteins remain to be conclusively linked to osmosensing. Activity of PLA2 is sensitive to packing of the lipid bilayer of the cell and is responsive to osmotic changes, two attributes that mark it as a prime candidate for a stretch-activated sensor (Lehtonen and Kinnunen, 1995). 

Some evidence exists for phospholipase A2 (PLA2) activity that could be regulated by cannabinoid receptors. Cannabinoid-induced arachidonic acid release has been observed in several cell culture systems, and this is believed to be mediated both by phospholipase activity and G proteins (Burstein 1991 Burstein et al. 1994 Shivachar et al. 1996). 

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