Modulation by phorbol 12-myristate 13-acetate of arachidonic acid release from rat basophilic leukemia cells stimulated with A23187

Re-evaluation of thin layer chromatography as an alternative method for the quantification of prostaglandins from rat Kupffer cells

In contrast to conventionally used immunoassays, thin layer chromatography (TLC)--by prelabeling of cells with radioactive arachidonic acid (AA)--allows to differentiate between cellularly built and added prostanoids and thus to investigate feedback effects of prostanoids on their own release. PGD2, TXB2 and PGE2 released from zymosan-stimulated Kupffer cells were separated with distinct RF-values, corresponding to those of the pure substances. Quantification of PGD2 and PGE2 gave comparable results with TLC and immunoassays, but measurement in the presence of added prostanoids was only possible with TLC. Moreover TLC was superior to immunoassays in having a longer linear range while being comparably sensitive. Cellularly built TXB2 in its radioactively labeled form was not detectable by TLC. Inhibition of TXB2 release by externally added AA or technical artifacts were excluded, suggesting that the cellular AA-pools used for prostaglandin and thromboxane synthesis differ in their accessibility for added AA. Thus, TLC is a simple, sensitive and precise method for the quantification of cellularly built prostaglandins but not of thromboxane even in the presence of added prostanoids.

Transfer of arachidonyl groups within the lipids of two human neuroblastoma cell lines

The incorporation and mobilization of [3H]arachidonic acid in lipids of human neuroblastoma cell lines, SK-N-SHF and LA-N-5, was studied. Essentially similar results were obtained with these two cell lines. Except for phosphatidylinositol which displayed the highest specific activity, the incorporation patterns within phospholipid classes tended to reflect phospholipid composition initially. However at later stages, counts in the acid-stable phosphatidylcholine plateaued and/or decreased while those of plasmenylethaniolamine and acid-stable phosphatidylethanolamine increased steadily. When cells were pulse-labelled with [3H]arachidonic acid and chased with fresh medium, there was a movement of label from diacyl (acid-stable) phosphatidylcholine to plasmenylethanolamine and diacyl (acid-stable) phosphatidylethanolamine. Plasmenylcholine did not appear to be involved in the arachidonyl group transfer. Under these chase conditions there was extensive turnover in the 32P-labelled polar headgroup of phosphatidylinositol but not in that of the other phospholipids. In both incorporation and chase studies involving [3H]arachidonic acid, a movement of arachidonyl groups from triacylglycerol to phospholipid could be observed. The results indicated that the patterns of incorporation and redistribution of arachidonic acid in human neuroblastoma cells were effectively regulated to favor lipids such as phosphatidylinositol and the subclasses of phosphatidylethanolamine. Possible mechanisms involved in these enrichment processes are discussed.

The stimulation of arachidonic acid metabolism by organic lead and tin compounds in human HL-60 leukemia cells

The liberation of fatty acids, above all arachidonic acid, in human blood cells is involved in numerous health problems or physiological mechanisms. The activity of cellular phospholipases leads to lipid metabolites such as eicosanoids, platelet activating factor, diacylglycerol, and inositolphosphates that are capable of mediating such pathological symptoms. The results presented here demonstrate that organic heavy metal compounds induce arachidonic acid liberation or its rearrangement within the lipid classes of HL-60 cells before a loss in viability can be detected. Four of the compounds tested, triethyllead (Et3Pb+), diethyllead (Et2Pb2+), trimethyllead (Me3Pb+), and trimethyltin (Me3Sn+), show a threshold concentration at which the viability of the cells is drastically decreased after 60 to 180 min incubation, whereas dibutyltin (But2Sn2+) induces a constant increase of cell death during the whole incubation time. In the case of threshold concentrations, the compounds stimulate a loss of arachidonic acid within the phospholipids and an increase of free fatty acid and eicosanoids before cell death could be detected. An important fact is the rearrangement of arachidonic acid within the lipid classes of these cells induced by metal concentrations that were not able to kill the cells within the given time. Primarily affected is phosphatidylethanolamine which loses arachidonic acid and, to a minor extent, phosphatidylcholine. Portions of the liberated fatty acid were then metabolized and/or shifted into neutral lipids and other phospholipids. All compounds tested show comparable effects, although at different concentrations. The toxicities of the compounds can be ordered as follows: Et3Pb+ greater than or equal to Et2Pb2+ greater than But2Sn2+ greater than or equal to Me3Pb+ much greater than Me3Sn+ greater than or equal to Pb2+. The cellular shape change following incubation with metal compounds is a further strong indication of a change in the membrane lipids. The cells lose their characteristic microvilli and/or blebs and become round without a loss in viability.

Sequential incorporation of 3H- and 14C-arachidonic acid during 24 h in rabbit colonic explants cultured in vitro

Arachidonic acid (AA) might be released from a number of different intracellular pools. This aspect was studied by sequential incorporation of 3H- and 14C-AA during 24 h in rabbit colonic biopsies in culture. Mucosal explants were subjected to 3H-AA (0-6 h) and 14C-AA (6-24 h) followed by a 2-h washout. Lipid extraction of biopsies revealed a distinct pattern where 3H, in relative terms, dominated in phosphatidylinositol (PI) and phosphatidylethanolamine (PE) and 14C was more abundant in neutral lipids, whereas the remaining lipids had equal proportions of labellings. When stimulating double labelled biopsies with 1 microM phorbol 12-myristate 13-acetate (PMA), 10 micrograms/ml A23187 and a combination thereof, the 14C-labelling was in general released in larger amounts, i.e. a maximum of 7.5% with the combined drugs vs 5% for 3H. However, the relative release of 3H increased upon stimulation especially for PMA + A23187. The release of labelling was directly coupled to a stimulation dependent production of PGE2 in the order PMA less than A23187 less than PMA + A23187, maximum release = 8.2 +/- 2.4 ng/h and 3 biopsies. This study shows the existence of different AA-pools, where PI and PE (3H-labelling) appears to have a slow turnover and representing a terminus in a redistribution chain and where neutral lipids (14C-labelling) have a more rapid turnover. Upon stimulation more AA appears to be recruited from pools with slow turnover.

Differential actions of diacyl- and alkylacylglycerols in priming phospholipase A2, 5-lipoxygenase and acetyltransferase activation in human neutrophils

One aspect of human neutrophil (PMN) function during inflammation is formation of platelet-activating factor (PAF), leukotriene B4 (LTB4), and 5-hydroxyeicosatetraenoic acid (5-HETE), but production of these lipid mediators is limited if PMN are directly stimulated with soluble, physiologic agonists. In vitro, PMN activities can be enhanced by the process of primed-stimulation where cells are sequentially treated with non-stimulatory concentrations of different agonists. Many agents that prime PMN also induce production of 1,2-diacyl- and 1-O-alkyl-2-acylglycerols. Therefore, we investigated whether diglycerides were involved in priming PMN for production of lipid mediators. We previously described the ability of the diacylglycerol, 1-oleoyl-2-acetylglycerol (OAG), and its alkylacylglycerol analog, 1-O-octadecenyl-2-acetylglycerol (EAG), to prime phospholipase A2 (PLA2) for subsequent activation by a second stimulus. However, while OAG also primed 5-lipoxygenase activity (LTB4 and 5-HETE production), EAG priming inhibited LTB4 and 5-HETE formation. We now report the effects of diglyceride priming on acetyltransferase activation (PAF formation). PMN, prelabeled with 1-O-[9',10'-3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine, were primed with OAG or EAG before stimulation. Neither OAG nor EAG induced formation of labeled PAF. Treatment of PMN with the chemotactic peptide, N-formyl-met-leu-phe (FMLP), induced low but significant production of PAF; PAF formation doubled in PMN primed with 20 microM OAG before FMLP stimulation while priming with 20 microM EAG more than tripled the level of PAF. Calcium ionophore strongly induced PAF formation; OAG priming before ionophore challenge had no effect but EAG priming further enhanced PAF formation. These results suggests a role for alkylacylglycerols in modulating the production of lipid mediators of inflammation.

Arachidonic acid release by basophilic leukemia cells and macrophages stimulated by Ca2+ ionophores, antigen and diacylglycerol: essential role for protein kinase C and prevention by glucocorticosteroids

The role of protein kinase C in phospholipase A2 (PLA2) activation in rat basophilic leukemia cells (RBL-2H3) and macrophages was investigated. 12-O-Tetradecanoyl phorbol 13-acetate (TPA) doubled ionomycin-induced PLA2 activity, assessed by [3H]arachidonate release. Protein kinase C inhibitors, staurosporine and K252a (100 nM) or H-7 (15 micrograms/ml) inhibited ionomycin-stimulation of PLA2 activity by 62, 75 and 80%, respectively. Down-regulation of protein kinase C by prolonged treatment with TPA inhibited Ca2(+)-ionophore A23187 or antigen-stimulation of [3H]arachidonate release by 80%. We examined whether the inhibitory effect of dexamethasone (DEX) on PLA2 activity is related to modulation of protein kinase C activity. The 50% inhibition by DEX of ionomycin elevation of [3H]arachidonate release was almost overcome by addition of TPA. The Ca2+ ionophore and antigen-induced increase in [3H]TPA binding to intact RBL cells was not impaired by DEX. However, DEX markedly reduced phosphorylation of several proteins. 1-Oleoyl-2-acetyl-glycerol (OAG) had a sustained stimulatory effect on PLA2 activity in isolated plasma membranes derived from treated bone-marrow intact mouse macrophages, while both DEX and staurosporine reduced elevated PLA2 activity by 68 and 84%, respectively. The results support an essential role for protein kinase C in regulation of PLA2 activity.

Oxysterol activation of arachidonic acid release and prostaglandin E2 biosynthesis in NRK 49F cells is partially dependent on protein kinase C activity [corrected]

We previously demonstrated that the oxysterol potentiation of arachidonic acid release and prostaglandin biosynthesis induced by foetal calf serum activation of normal rat kidney (NRK) cells (fibroblastic clone 49F) was not related to a direct effect of oxysterols on cell free Ca2+ level. Since both Ca2+ variations and protein kinase C are involved in arachidonic acid release in some models, we looked for a possible modulation by protein kinase C in the oxysterol effect on arachidonic acid release. We show that when the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a protein kinase C activator, was added to the culture medium, the oxysterol effect on arachidonic acid release and prostaglandin synthesis clearly increased. Moreover, the effect of TPA was dose-dependent and TPA EC50 (4 x 10(-9) M) was unchanged in the presence of the oxysterol. Preincubation of cells with TPA for 24 h prevented the arachidonic acid release induced by TPA alone, whereas the oxysterol effect was decreased but not abolished. In the absence of serum, TPA and ionomycin added together induced the same noticeable (arachidonic acid) release and PGE2 synthesis as serum alone. Nevertheless, the potentiating effect of cholest-5-ene-3 beta, 25-diol was much higher when serum itself was used to activate NRK cells than it was in the present serum-mimicking experimental conditions. Thus, the presence of growth factors is probably required to obtain a full oxysterol effect. We conclude that the oxysterol effect was synergistic with, but not fully dependent on, protein kinase C and Ca2+ ion fluxes, therefore oxysterols could affect earlier events triggered by serum growth factor binding to their cell membrane receptors.

Relationship between arachidonic acid release and Ca2(+)-dependent exocytosis in digitonin-permeabilized bovine adrenal chromaffin cells

The relationship between Ca2(+)-dependent arachidonic acid release and exocytosis from digitonin-permeabilized bovine adrenal chromaffin cells was investigated. The phospholipase A2 inhibitors mepacrine, nordihydroguaiaretic acid and indomethacin had no effect on either arachidonic acid release or secretion. The phospholipase A2 activator melittin had no effect on secretion. The specific diacylglycerol lipase inhibitor RG80267 had no effect on secretion, but decreased basal arachidonic acid release to such an extent that the level of arachidonic acid in treated cells in response to 10 microM-Ca2+ was equivalent to that of control cells in the absence of Ca2+. Staurosporine, a protein kinase C inhibitor, was found to abolish Ca2(+)-dependent arachidonic acid release completely, but had only a slight inhibitory effect on Ca2(+)-dependent secretion. It is concluded that arachidonic acid is not essential for Ca2(+)-dependent exocytosis in adrenal chromaffin cells.