Journal of lipid mediators最新文献

Studies carried out in many laboratories have demonstrated the activation of phospholipase D (PLD) by a variety of receptor agonists and in many cell types. The signal-dependent formation of phosphatidic acid (PA), by PLD-catalyzed hydrolysis of phosphatidylcholine (PC), may represent a novel and ubiquitous signal transduction pathway in mammalian cells. The mode(s) of coupling between agonist receptors and PLD activation are not well understood. Studies utilizing NIH-3T3 fibroblasts indicated that PLD activation by different mitogens involves distinct mechanisms. Protein kinase C (PKC) seems to play a role both as a mediator and as a modulator of PLD activation. The role of PKC was further examined in Swiss/3T3-derived fibroblasts which stably overexpress PKC-alpha. In these cells, both basal and agonist-stimulated PLD activity are higher than in control cells. In vitro analysis of PLD activity in detergent-solubilized cell membranes, utilizing exogenous C6-NBD-PC as fluorescent substrate, showed nearly 2-fold higher activity in membranes from cells that overexpress PKC-alpha. These results suggest that PKC-alpha may play a role in regulating PLD expression. The PLD product PA was identified as a precursor of 'late phase' diacylglycerol which, at least in some cases, was temporally correlated and causally related to the sustained activation of PKC. However, PA may itself act as an intracellular messenger in its own right, although immediate targets for its action have not yet been identified. Activation of phosphoinositide-phospholipase C, PLD and phospholipase A2 seems to comprise a signaling cascade which is typically utilized by most (if not all) Ca(2+)-mobilizing agonists.

A factor responsible for releasing PAF produced in stimulated human polymorphonuclear leukocytes, which had been previously reported in human serum (Miwa et al. (1992) J. Immunol. 148, 872-880), was also confirmed to be present in inflammatory exudate. PAF-releasing factor, partially purified from human serum, was shown to possess higher affinity for PAF than for triacylglycerol, cholesterol ester, fatty acid or phosphatidylcholine. PAF bound to this factor aggregated washed rabbit platelets to the same extent as that bound to BSA, but was difficult to be hydrolyzed using PAF acetylhydrolase. These observations strongly suggest that PAF-releasing factor functions as a PAF carrier in blood and in the inflammatory response.

In the present study, we have compared the responses to platelet-activating factor (PAF) of A/J and BALB/c inbred mouse strains. Two PAF-induced events were analyzed: increased vasopermeability, measured by extravasation of Evans blue dye (EB), and mortality. PAF injected into the peritoneal cavity induced a bell-shaped dose-response curve of EB extravasation in both strains of mouse. In A/J mice, maximal EB extravasation was reached with 0.1 microgram of PAF/mouse, whereas in BALB/c mice maximal extravasation was attained at a 10-fold greater PAF concentration. PAF-induced mortality also differed among these mouse strains; the LD50 was 12.1 micrograms/kg in A/J and 21.2 micrograms/kg in BALB/c mice. Thus, these strains differ significantly regarding both events mediated by PAF. Surprisingly, the F1 hybrid (A/J x BALB/c) mice were as sensitive as the A/J strain to PAF-induced extravasation but were as resistant as the BALB/c mice to PAF-induced mortality. The effects of the PAF antagonists BN 52021 and WEB 2086 were compared in the F1 hybrids. It was found that 1.0 mg/kg of WEB 2086 affected PAF-induced extravasation at almost all PAF doses tested (0.03-3.0 micrograms) while 15 mg/kg of BN 52021 was only effective at doses of PAF below 0.3 microgram. Both antagonists prevented PAF-induced mortality. Our results indicate that the two events induced by PAF may be controlled by different genes.

Low levels of 5-lipoxygenase (5-LO), the first committed enzyme in the synthesis of leukotrienes (LTs), have been reported in the porcine pancreas. We have quantitated 5-LO activity in subcellular fractions of pancreas samples from three human donors. 5-LO activity was detectable in all samples, although enzyme activity was lower than in human leukocytes. 5-LO in human pancreas samples displayed highest specific activity in membrane fractions, and did not require arachidonic acid (AA) addition for activity. These unusual characteristics of pancreatic 5-LO appear to be due, at least in part, to the presence of unesterified AA in the pancreas samples. Western blot analysis demonstrated that the human pancreas contains low levels of 5-lipoxygenase-activating protein (FLAP) in addition to 5-LO.

Previous studies from our laboratory have shown that sphingosine (Zhang et al. (1990) J. Biol. Chem. 265, 76-81) and sphingosine 1-phosphate (Zhang et al. (1991) J. Cell. Biol. 114, 155-167), metabolites of membrane sphingolipids, stimulate release of calcium from internal sources and increase proliferation of quiescent Swiss 3T3 fibroblasts acting in a fundamentally different, protein kinase C-independent pathway. The mitogenic effect of sphingosine was accompanied by an increase in the levels of phosphatidic acid (PA), a potent mitogen for a variety of cell types, that may function as an intracellular second messenger (Zhang et al. (1990) J. Biol. Chem. 265, 21309-21316). Sphingosine also induced early increases in sphingosine 1-phosphate (SPP) levels that preceded the increase in PA (Desai et al. (1992) J. Biol. Chem. 267, 23122-23128). SPP itself produced a more rapid increase in PA, thus suggesting that it may mediate the effects of sphingosine on PA accumulation. The concentration dependence for the formation of PA induced by SPP correlated with its effect on DNA synthesis. Similar to sphingosine, SPP also stimulated the activity of phospholipase D, although a significant effect was observed at a much lower concentration. However, in contrast to previous reports with sphingosine, SPP did not inhibit the PA phosphohydrolase activity in cell homogenates. Thus, in addition to its effect on mobilization of calcium, SPP can increase the level of PA, most likely via activation of phospholipase D. We suggest that SPP mediates the effect of sphingosine on PA accumulation in Swiss 3T3 fibroblasts and may regulate cellular proliferation by affecting multiple transmembrane signaling pathways.

Thrombin-stimulated formation of malondialdehyde in isolated rat platelets was time- and dose-dependent and occurred in parallel with thromboxane B2 production and platelet aggregation. beta-Adrenoceptor blocking drugs inhibited thrombin-stimulated malondialdehyde formation according to their liposolubility in the rank order of potency: atenolol < practolol < oxprenolol < metipranolol approximately alprenolol approximately propranolol. In platelets pretreated with beta-blockers and stimulated with thrombin, a positive correlation was found between malondialdehyde formation and inhibition of aggregation, arachidonic acid liberation from membrane phospholipids as well as thromboxane production. Measurement of malondialdehyde was shown to be an indicator for the arachidonic acid pathway in stimulated and inhibited isolated platelets.

The local release of sensory neuropeptides from capsaicin-sensitive primary afferents elicits prominent motor and inflammatory actions in mammalian airways. This neurogenic inflammation can contribute to the pathophysiology of asthma and airway hyperreactivity. In this review evidence will be presented regarding the involvement of this peptidergic neural pathway in the mediation of some pulmonary actions of lipid mediators such as eicosanoids and PAF.

Lipocortin 1 is a member of a group of calcium and phospholipid binding proteins named lipocortins (Di Rosa et al. (1984) Prostglandins 28, 441-442) and also known as annexins (Crumpton (1990) Nature 345, 212). In this study we have shown that human recombinant lipocortin-1 can increase the activated partial thromboplastin time but not the prothrombin time of human plasma in vitro. This effect is dose and time dependent and is reversed by polyclonal anti-lipocortin-1 antibodies. To our knowledge, this is the first demonstration that the human recombinant protein has the same activity as the native protein on human plasma.