Search

The release of [3H]arachidonic acid was studied in the 1321N1 astrocytoma cell line upon stimulation with thrombin. The effect of thrombin was antagonized by hirudin only when both compounds were added simultaneously, which suggests activation of thrombin receptor. Evidence that the cytosolic phospholipase A2 (cPLA2) takes part in thrombin-induced arachidonate release was provided by the finding that thrombin induced retardation of the mobility of cPLA2 in SDS/polyacrylamide gels, which is a feature of the activation of cPLA2 by mitogen-activated protein (MAP) kinases. Thrombin induced activation of two members of the MAP kinase family whose consensus primary sequence appears in cPLA2, namely p42-MAP kinase and c-Jun kinase. However, the activation of c-Jun kinase preceded the phosphorylation of cPLA2 more clearly than the activation of p42-MAK kinase did. Both cPLA2 and c-Jun kinase activation were not affected by PD-98059, a specific inhibitor of MAP kinase kinases, which indeed completely blocked p42-MAP kinase shift. Heat shock, a well-known activator of c-Jun kinase, also phosphorylated cPLA2 but not p42-MAP kinase. These data indicate the existence in astrocytoma cells of a signalling pathway triggered by thrombin receptor stimulation that activates a kinase cascade acting on the Pro-Leu-Ser-Pro consensus primary sequence, activates cPLA2, and associates the release of arachidonate with nuclear signalling pathways.

Phospholipase A2 (PLA2) activity was purified 12,544-fold with a 13% yield from the plasma of patients diagnosed of septic shock by the sequential use of heparin-agarose affinity chromatography, gel filtration, and reverse-phase f.p.l.c. Gel-filtration chromatography of plasma omitting high-ionic-strength buffer revealed a molecular mass different from that of purified PLA2 and co-elution with apolipoprotein A-I peaks, which suggests its association with high-density lipoproteins (HDL). N-terminal analysis of the enzyme activity protein band, electroblotted from a SDS-acrylamide gel and with an assessed molecular mass of 19 kDa, showed an identical sequence to that of alpha-chain of human C3 complement component, suggesting the presence in this band of a complex formed by a complement C3-derived anaphylatoxin (C3a)-related fragment and the PLA2 linked side-by-side. Because the preparation of plasma enzyme showed lower activity than the enzyme obtained from fibroblasts transfected with the coding sequence of human group-II PLA2, and because the addition of C3-derived anaphylatoxins from human serum inhibited the activity of this recombinant PLA2, it was considered that C3a-related peptides behave as inhibitors of group-II PLA2. The enzyme showed optimal activity on [14C]oleate-labelled autoclaved E. coli, on synthetic phosphatidylethanolamine, and on [3H]arachidonate-labelled membranes of the monoblast cell line U937, but it did not show any activity on the release of [3H]arachidonate from pre-labelled human polymorphonuclear leukocytes (PMNs). In short, PLA2 from plasma of sepsis patients shows unique associations with other plasma proteins which may influence its functional properties. The association with C3-related peptides shows an inhibitory effect on the enzyme activity, whereas the association with HDL might influence its environment and/or its interaction with cells. The study of the catalytic properties shows a prominent effect on bacterial phospholipids, synthetic phosphatidylethanolamine, and membranes from U937 monoblasts, but not on synthetic phosphatidylcholine or on PMNs, even when these cells were maintained in culture to allow spontaneous apoptosis and became a good substrate for pancreatic type PLA2.

The role of Ca2+ in the activation of the enzyme lyso-(platelet-activating factor): acetyl-CoA acetyltransferase was studied in rat peritoneal macrophages in response to complement-coated zymosan particles and ionophore A23187. By using Ca2+-containing buffers, a threshold concentration of extracellular Ca2+ above 1 microM was found to be necessary to observe the activation of the enzyme in response to zymosan. By contrast, a significant role of intracellular Ca2+ in this process could be ruled out, since the putative intracellular calcium-transport antagonist TMB-8 [8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate] did not inhibit the activation of the acetyltransferase induced by zymosan in the presence of extracellular Ca+. The link between acetyltransferase activation and extracellular Ca2+ transport was studied by measuring Ca2+ uptake in response to the stimuli. Zymosan particles induced a rapid increment in cell-associated Ca2+ which correlated well with the extent of acetyltransferase activation (r = 0.91) and with the release of platelet-activating factor (r = 0.95) in response to different doses of zymosan. Cellular Ca2+ efflux in response to zymosan particles was also measured and found to be increased, as compared with controls, when the activation of the acetyltransferase declined. In short, the data suggest that the entry of extracellular Ca2+ into the cell is a crucial event in the activation of acetyltransferase and, thereby, in the formation of platelet-activating factor in rat peritoneal macrophages.

We have previously observed that the phagocytosis of zymosan particles coated with complement by human polymorphonuclear leucocytes is accompanied by a time- and dose-dependent inhibition of phosphatidylcholine synthesis by transmethylation [García Gil, Alonso, Sánchez Crespo & Mato (1981) Biochem. Biophys. Res. Commun.101, 740-748]. The present studies show that phosphatidylcholine synthesis by a cholinephosphotransferase reaction is enhanced, up to 3-fold, during phagocytosis by polymorphonuclear cells. This effect was tested by both measuring the incorporation of radioactivity into phosphatidylcholine in cells labelled with [Me-(14)C]choline, and by assaying the activity of CDP-choline:diacylglycerol cholinephosphotransferase. The time course of CDP-choline:diacylglycerol cholinephosphotransferase activation by zymosan mirrors the inhibition of phospholipid methyltransferase activity previously reported. The extent of incorporation of radioactivity into phosphatidylcholine induced by various doses of zymosan correlates with the physiological response of the cells to this stimulus. This effect was specific for phosphatidylcholine, and phosphatidyl-ethanolamine turnover was not affected by zymosan. The purpose of this enhanced phosphatidylcholine synthesis is not to provide phospholipid molecules rich in arachidonic acid. The present studies show that about 80% of the arachidonic acid generated in response to zymosan derives from phosphatidylinositol. A transient accumulation of arachidonoyldiacylglycerol has also been observed, which indicates that a phospholipase C is responsible, at least in part, for the generation of arachidonic acid. Finally, isobutylmethylxanthine and quinacrine, inhibitors of phosphatidylinositol turnover, inhibit both arachidonic acid generation and phagocytosis, indicating a function for this pathway during this process.