Inhibition of phospholipase A2 by cis-unsaturated fatty acids: evidence for the binding of fatty acid to enzyme

PLA2-responsive and SPIO-loaded phospholipid micelles

PLA2-responsive and superparamagnetic iron oxide (SPIO) nanoparticle-loaded phospholipid micelles were developed. The release of a phospholipid-conjugated dye from these micelles was triggered due to phospholipid degradation by phospholipase A2. The high relaxivity of the encapsulated SPIO could enable non-invasive magnetic resonance imaging.

Monitoring phospholipase A₂ activity with Gd-encapsulated phospholipid liposomes

To date, numerous analytical methods have been developed to monitor phospholipase A₂ (PLA₂) activity. However, many of these methods require the use of unnatural PLA₂ substrates that may alter enzyme kinetics, and probes that cannot be extended to applications in more complex environments. It would be desirable to develop a versatile assay that monitors PLA₂ activity based on interactions with natural phospholipids in complex biological samples. Here, we developed an activatable T1 magnetic resonance (MR) imaging contrast agent to monitor PLA₂ activity. Specifically, the clinically approved gadolinium (Gd)-based MR contrast agent, gadoteridol, was encapsulated within nanometer-sized phospholipid liposomes. The encapsulated Gd exhibited a low T1-weighted signal, due to low membrane permeability. However, when the phospholipids within the liposomal membrane were hydrolyzed by PLA₂, encapsulated Gd was released into bulk solution, resulting in a measureable change in the T1-relaxation time. These activatable MR contrast agents can potentially be used as nanosensors for monitoring of PLA₂ activity in biological samples with minimal sample preparation.

Inhibition by unsaturated fatty acids of type II secretory phospholipase A2 synthesis in guinea-pig alveolar macrophages evidence for the eicosanoid-independent pathway

The effect of arachidonic acid (C20:4) on the production of secretory type II phospholipase A2 (sPLA2-II) by guinea-pig alveolar macrophages was investigated. We show that incubation of these cells with 1-30 microM of arachidonic acid inhibits the synthesis of sPLA2-II in a concentration-dependent manner with an IC50 of approximately 7.5 microM. The inhibition by low concentrations (5 microM) of arachidonic acid was partially reduced by pretreatment of alveolar macrophages with cyclooxygenase or cytochrome P450 inhibitors (aspirin and 1-aminobenzotriazole, respectively), but not by lipoxygenase inhibitor, BW A4C. However, these inhibitors failed to interfere with the effect of high concentrations (30 microM) of arachidonic acid, suggesting that the latter may act on the expression of sPLA2-II, at least in part, independently of eicosanoid generation. Indeed, a similar inhibitory effect on sPLA2-II activity and mRNA expression was observed with other unsaturated fatty acids such as eicosapentaenoic (C20:5) and oleic (C18:1) acids, but not with the saturated fatty acid, palmitic acid (C16:0). In addition, arachidonic acid partially reduced the secretion of tumor necrosis factor alpha, an important intermediate in the induction of sPLA2-II synthesis by guinea-pig alveolar macrophages. However, addition of recombinant tumor necrosis factor alpha failed to reverse the inhibitory effect of arachidonic acid on sPLA2-II expression, suggesting that this process occurs downstream of tumor necrosis factor alpha secretion. We conclude that the expression of sPLA2-II in alveolar macrophages is down-regulated at the transcriptional level by arachidonic acid either directly or via its cyclooxygenase and cytochrome P450-derived metabolites.

PX-52, A novel inhibitor of 14 kDa secretory and 85 kDa cytosolic phospholipases A2

Previously we reported that PGBx, a prostaglandin oligomer with anti-inflammatory activity, inhibited 14 kDa phospholipase A2 (PLA2) activity and blocked arachidonic acid mobilization in prelabeled human neutrophils (Biochim. Biophys. Acta 1006:272-277, 278-286, 1989) This study describes a new inhibitor of phospholipase A2, PX-52, that also blocks agonist induced arachidonic acid mobilization in prelabeled cells. PX-52, a fatty acid polymer, inhibited hydrolysis of 14C-oleate labeled E.coli by a variety of 14 kDa PLA2s including human PMN, sperm, synovial fluid and disc, as well as porcine pancreas, N. naja, and bee venom in a dose-dependent manner with IC50s ranging from 1.0-3.7 uM. Inhibition of activity was comparable at different Ca2+ concentrations, but was relieved by increasing substrate concentration or by methylation of PX-52. Hydrolysis of [14C]-arachidonyl phosphatidylcholine by 85 kDa, cytosolic PLA2 from U937 cells was similarly inhibited by PX-52, the IC50 = 5 uM. Arachidonic acid mobilization induced by A23187 in prelabeled human PMNs was blocked by PX-52; IC50 = 10-15 uM while concentrations of up to 80 uM oleate had no effect. These results demonstrate that PX-52 inhibits the in vitro activity of secretory and cytosolic PLA2s and agonist-induced arachidonic acid release from human cells. Given its ability to block the arachidonic acid cascade, PX-52 may be useful in the control of inflammation.

Dissimilar protection of tocopherol isomers against membrane hydrolysis by phospholipase A2

Porcine pancreatic phospholipase A2 (PLA2) hydrolyses phosphatidylcholine when in the lamellar state as well as in the micellar state. We have found that alpha-tocopherol, the most active form of vitamin E, is able to inhibit PLA2 activity only toward lamellar fluid membranes, thus protecting phospholipids toward this lytic enzyme. This compound decreases both the initial rate and the extent of hydrolysis. The inhibition is of the non-competitive type and the evidence strongly suggests that it is due to an effect of alpha-tocopherol on the substrate, i.e. the membrane, and not on the enzyme itself. Other tocopherols, such as the isomers beta-, gamma- and delta-tocopherol also display PLA2 inhibition but consecutively to a lower extent. The grade of inhibition of PLA2 activity by tocopherols correlates well with their biological activity and with their location in the bilayer as shown by fluorescence quenching. Cholesterol does not inhibit PLA2 activity at concentrations even higher than those of tocopherols, indicating that the effect of tocopherols is not due to alteration of membrane fluidity. The possible mechanisms underlying the different behaviour of tocopherol isomers as PLA2 inhibitors are discussed considering its biological significance as membrane stabilizers, suggesting biological actions of compounds with vitamin E activity other than their classical roles as antioxidants.

Dietary fatty acids modulate hormone responses in lactating cows: mechanistic role for 5'-deiodinase activity in tissue

Supplemental dietary fat provides excess fatty acids (FA), which can alter circulating concentrations of several hormones. To test the effects of fatty acid isomer type and possible sites of regulation, we abomasally infused fat mixtures high in cis-C18:1 FA (iCIS), high in trans-C18:1 FA (iTRS) or no infusion (NI) and performed intravenous arginine (ARG) and intramuscular thyrotropin-releasing hormone (TRH) challenges. The experimental design was a replicated 3 x 3 Latin square. Challenges were conducted on Days 10 (ARG) and 12 (TRH) after initiation of fat infusion on each of three 4-wk experimental periods. Plasma concentrations of IGF-I were lower (P < 0.01) when cows received iCIS or iTRS compared with NI. Plasma insulin concentrations increased with ARG but responses were not affected by FA. Plasma growth hormone (GH) was unchanged after ARG. Peak plasma GH and thyroid-stimulating hormone (TSH) responses to TRH were blunted (P < 0.05 and P < 0.1, respectively), whereas thyroxine (T4) and triiodothyronine (T3) responses were augmented post-TRH (P < 0.01) when cows received either FA isomer. Prolactin responses to TRH were not different between infusion treatments, although basal plasma concentrations before TRH were higher in cows infused with iTRS (P < 0.05). To focus on fat regulation of the thyroid axis, we tested directly in vitro the ability of fatty acids dissolved with sodium taurocholate to affect Type-I 5'-deiodinase (5'D) activity in bovine liver homogenates. Homogenate 5'D was not affected by C2:0-C10:0 fatty acids, but decreased linearly (P < 0.01) with increasing concentrations of C12:0-C16:0 and C18:1 isomers. Cis C18:1 decreased 5'D more than the trans-isomer (P < 0.01), but the difference was only apparent at concentrations greater than 0.25 mM. The data suggest that various aspects of pituitary hormone regulation are differentially affected by FA composition. Fatty acid infusion may accentuate end organ responses in the thyroid axis and decrease IGF-I in the somatotropic axis. The data also suggest that FA isomer may alter patterns of extrathyroidal generation of thyroid hormones via direct influences on 5'D.

Interaction of plant lipids with 14 kDa phospholipase A2 enzymes

Several structurally related plant lipids were isolated and their effect was assessed on the enzyme activity of group I (pancreatic and Naja mocambique venom) and group II (Crotalus atrox venom) phospholipase A2 (PLA2) enzymes, with labelled Escherichia coli as an enzyme substrate. The neutral monogalactosyldiacylglycerol (MGDG) and negatively charged diacylglyceryl alpha-D-glucuronide (DGGA) did not influence the enzyme activity of either group. Digalactosyldiacylglycerol (DGDG), another uncharged glycolipid, inhibited PLA2 activity in a dose-dependent manner to 60-70% of the control. Sulphoquinovosyldiacylglycerol (SQDG), which is also anionic, activated both groups of PLA2 enzyme. A similar activation was observed with the zwitterionic diacylglyceryl-O-(N,N,N-trimethylhomoserine) (DGTS) and diacylglyceryl-O-(hydroxymethyl)(N,N, N-trimethyl)-beta-alanine (DGTA). DGDG, SQDG and DGTS are dispersed homogeneously with low critical micelle concentrations (CMCs). The hydrodynamic radius of neutral DGDG is an order of magnitude larger than the charged lipids SQDG and DGTS. The inhibition of pig pancreatic PLA2 by DGDG was dependent on substrate concentration. The intrinsic fluorescence spectra of the enzyme was not changed in the presence of native or hydrogenated DGDG. Thus the inhibition is most probably due to a non-specific interaction of plant lipids with the substrate. Different lengths and saturations of the fatty acyl chains of DGDG did not alter the inhibition of PLA2, whereas deacylation abrogated the inhibitory effect. Both SQDG and DGTS activated pig pancreatic PLA2 in a dose-dependent manner. Saturation of the double bonds of these lipids decreased the activating effect. The fluorescence of pig pancreatic PLA2 incubated with SQDG and DGTS was enhanced by 2-fold and 3-fold respectively, suggesting the formation of a complex between enzyme and lipids. In conclusion, the effect of different plant lipids on PLA2 activity depends on different structural elements of the polar head group and their charge as well as the degree of unsaturation of the fatty acyl chains.

Polyunsaturated fatty acids and signalling via phospholipase C-beta and A2 in myocardium

Dietary n-6 and n-3 polyunsaturated fatty acids (PUFAs) have potent biological effects on the blood(cells), the vasculature and they myocardium. In the epidemiological studies in which the benefit from the regular ingestion of n-3 PUFAs was reported, the responsible mechanisms remain obscure. A great deal of the PUFA-effect can be explained by the known interference with the eicosanoid metabolism. Many processes, believed to be involved in atherogenesis such as adhesion and infiltration of bloodcells (in)to the vasculature, platelet aggregation, secretion of endothelium-derived factors and mitogenic responses of vascular smooth muscle cells are partially mediated by receptor-activated phospholipases C-beta and A2. As PUFAs take part at many steps of the signalling pathways, the latter could represent important action sites to beneficially interfere with atherogenesis. In this brief review, we have discussed the results of studies on the influence of alteration of PUFA composition of the membrane phospholipids or of exogenously administered non-esterified PURAs on phospholipid signalling. For convenience, we have mainly focused our discussion on those studies available on the myocardium. By changing the PUFA composition of the phospholipids, the endogenous substrates for the membrane-associated phospholipase C-beta and A2 are changed. This is accompanied by changes in their hydrolytic action on these substrates resulting in altered products (the molecular species of 1,2-diacylglycerols and the non-esterified PUFAs) which on their turn evoke changes in events downstream of the signalling cascades: activation of distinct protein kinase C isoenzymes, formation of distinct eicosanoids and non-esterified PUFA effects on Ca2+ channels. It has also become more clear that the membrane physicochemical properties, in terms of fluidity and cholesterol content of the bilayer, might undergo changes due to altered PUFA incorporation into the membrane phospholipids. The latter effects could have consequences for the receptor functioning, receptor-GTP-binding protein coupling, GTP-binding protein-phospholipase C-beta or A2 coupling as well. It should be noted that most of these studies have been carried out with cardiomyocytes isolated from hearts of animals on PUFA diet or incubation of cultured cardiomyocytes with non-esterified PUFAs in the presence of albumin. Studies need to be performed to prove that the PUFA-diet induced modulations of the phospholipid signalling reactions do occur in vivo and that these effects are involved in the mechanism of beneficial effects of dietary PUFAs on the process of atherosclerosis.

Dietary lipid and iron modify normal colonic mucosa without affecting phospholipase A2 activity

Phospholipase A2 (PLA2) functions as the rate-limiting step in arachidonic acid metabolism and in the removal of damaged or peroxidized membrane lipids. It is elevated in some human tumors and may be involved with mechanisms of tumor promotion. In vitro systems have shown PLA2 activity to be altered by variations in fatty acid and antioxidant components. This study encompassed two objectives. First, PLA2, activity in colon tumors produced using azoxymethane (AOM) in Fischer-344 rats was examined. Secondly, this study tested the effect of iron supplementation as a potential pro-oxidant in diets of varied fatty acid composition on PLA2 activity. Diets included 35 or 140 mg/kg or iron in AIN-76A based diets high in corn oil, menhaden oil, or beef tallow. Results for the first objective showed PLA2 activity to be significantly higher in colon tumors than in normal mucosa with the increase due primarily to an increase in activity within a particulate subcellular fraction. In the second objective, fatty acid composition of colon mucosa was altered by both dietary fat and iron. Animals fed beef tallow had the highest level of oleic acid and corn oil-fed animals had the highest level of linoleic acid. Animals fed menhaden oil had the lowest level of arachidonic acid and highest level of alpha-linolenic, eicosapentaenoic, docosapentaenoic, and docosahexaenoic acids. Iron supplementation in diets high in corn oil resulted in decreased membrane composition of palmitoleic and delta-linolenic acid. In spite of these changes in membrane composition, there were no changes in PLA2 activity. These results show that PLA2 activity is increased in AOM-induced tumors but that diet alone does not influence PLA2 activity in this model.

Hyperlipidemic effects of trans fatty acids are accentuated by dietary cholesterol in gerbils

Trans isomers of dietary fatty acids, generated during the commercial hydrogenation of unsaturated fats, may contribute to coronary heart disease (CHD) in humans by interfering with lipid metabolism. To examine this possibility in a fat-sensitive model, the Mongolian gerbil (Meriones unguiculatus) was used to compare the cholesterolemic and triglyceridemic potential of modest increments of trans fatty acids from partially hydrogenated soybean oil with other saturated fatty acids in the presence and absence of dietary cholesterol. Age-, dose-, and time-dependent effects were examined in weanling, 6-month-old, and 1-year-old gerbils. Although lipoprotein metabolism in weanling gerbils was initially refractory to trans fat, even as perturbations by saturated fatty acids were demonstrable, these gerbils eventually (after 16 weeks) developed a trans-induced hypercholesterolemia that was intermediate between the response to 16:0 and 12:0 + 14:0. The hepatic and plasma 18:1/18:2 cholesteryl ester (CE) ratio was depressed by trans in a manner similar to saturated fatty acids. The 6-month-old gerbils readily developed hypertriglyceridemia but not hypercholesterolemia, again revealing a decrease in the plasma 18:1/18:2 CE ratio. The 1-year-old gerbils revealed a dose-related (0, 5, 10%en as trans) elevation in total cholesterol (TC), and especially triglycerides (TG), that was accentuated by 0.04% dietary cholesterol. Increases in plasma lipids were again accompanied by a significant decrease in the mass of hepatic esterified cholesterol, particularly 18:1-cholesteryl esters. Thus, dietary trans-fatty acids induce age-, time-, and dose-dependent modulations in gerbil plasma lipids associated with decreased 18:1 cholesteryl esters. Further investigation with gerbils may reveal mechanisms by which trans fat consumption disturbs lipoprotein metabolism.

Specific competitive inhibitor of secreted phospholipase A2 from berries of Schinus terebinthifolius

Two structurally related triterpenoids 1 and 2 from pink peppercorn (berries of Schinus terebinthifolius) are identified and characterized as active site-directed specific competitive inhibitors of the three classes of secreted 14 kDa phospholipase A2. The inhibitors not only protect the active site histidine from alkylation but also inhibit the action of secreted phospholipase A2 from pig pancreas, human synovial fluid, and bee venom. Detailed X-ray crystallographic results on the structures of the inhibitors are provided. By physical methods and X-ray crystallography the triterpenoids were identified as masticadienoic acid and masticadienolic acid (schinol). Several other triterpenoids were ineffective as inhibitors of phospholipase A2; however certain ganoderic acid derivatives showed noticeable inhibition. Results show that the side chain of these acidic tetracyclic terpenoids can access the catalytic-site region of phospholipase A2, whereas the acyclic nucleus is at the interfacial recognition region. The selectivity of the assay protocol used here is demonstrated by the fact that the original screen of ethyl acetate extracts of 60 commercially available spices and herbs was carried out with phospholipase A2 from pig pancreas, and only one extract showed inhibitory action on the hydrolytic activity in the scooting mode. Under such assay conditions the enzyme remains tightly bound to the surface of the substrate vesicles. In this way, nonspecific effects of additives that promote desorption of the enzyme from the substrate vesicle surface, under conditions in which the binding of the enzyme to the vesicle is weak, are avoided. The assay protocol is useful for the kinetic characterization of the inhibitors of phospholipase A2, and it does not give false positive results with amphiphilic and hydrophobic compounds, as is the case with virtually all assay systems in use.

Arachidonic acid and free fatty acids as second messengers and the role of protein kinase C

In addition to serving as the precursor to a plethora of eicosanoids and other bioactive molecules, arachidonate may function as a bona fide second messenger. A number of studies have documented the ability of arachidonate to regulate the function of multiple targets in vitro systems. This has been particularly well established and studied with the activation of protein kinase C by arachidonate in a mechanism distinct from activation by diacylglycerol. In cells, arachidonate induces a number of activities, many of which may be independent of further metabolism to eicosanoids; suggesting possible direct action of arachidonate. This review summarizes the current state of knowledge on the possible second messenger function of arachidonate with specific emphasis on the regulation of protein kinase C.

Competitive inhibition of lipolytic enzymes. X. Further delineation of the active site of pancreatic phospholipases A2 from pig, ox and horse by comparing the inhibitory power of a number of (R)-2-acylamino phospholipid analogues

Two series of (R)-phospholipid analogues, each containing a n-propyl group at the C-1 position and various acylamino functions at the C-2 position have been synthesized and their inhibitory properties towards three mammalian pancreatic phospholipases A2 have been determined. The members of the first series of analogues all contained the zwitter-ionic phosphocholine headgroup which in the second series was replaced by the anionic phosphoglycol function. In the saturated 2-acylamino phospholipids the length of the acyl chain ranged from 8 to 18 carbon atoms. The unsaturated 2-acylamino analogues possessed a chain length of 11 or 18 carbon atoms and contained one, two, three or four double bonds. For inhibitors with a saturated acylamino group, the phospholipases A2 from pig, ox and horse show a sharp optimum in inhibitory power Z for an acyl chain length of 10 carbon atoms. The inhibitory behaviour of the unsaturated acylamino analogues is more complex: both the zwitter-ionic and the anionic inhibitors demonstrate an increase in Z with an increasing number of cis-double bonds but the degree of improvement is dependent on the position of the double bonds. Subsequently the influence of polar groups at carbon position 12 of the dodecanoylamino phospholipids on Z was analyzed. Substitution of the terminal methyl group by an OH-function lowers the inhibitory potency of the three enzymes by a factor of 4 to 5 both in the phosphocholine and phosphoglycol series. Replacement of the methyl group by potentially charged functions (-NH2, -COOH) resulted in a complete loss of inhibitory properties. Blocking of the amino group and carboxyl function by t-butyloxycarbonylation and esterification, respectively, fully restored the inhibitory power. Finally we investigated how changes in the polar headgroup and the presence of aromatic rings at the C-1 or C-2 position influenced the inhibitory potency of the analogues.

Structure-activity relationships leading to WAY-121,520, a tris aryl-type, indomethacin-based, phospholipase A2 (PLA2)/leukotriene biosynthesis inhibitor

We were intrigued by reports of the inhibition of phospholipase A2 (PLA2) by indomethacin. In order to increase the potency of the indomethacin system as an inhibitor of PLA2, it was decided to make more lipophilic analogs. Indeed, covalent attachment of a quinoline ring to the methoxy substituent of indomethacin affords WAY-122,220 which is almost an order of magnitude more potent than indomethacin in inhibiting human synovial fluid PLA2 (IC50 = 15 and 145 microM, respectively). The N-p-chloro-benzyl analog of this compound, WAY-121,520, was an even more potent inhibitor of PLA2 (IC50 = 4 microM). Structural analyses and molecular modeling suggest that these compounds may inhibit PLA2 by mimicking arachidonic acid. WAY-121,520 is also a potent leukotriene biosynthesis inhibitor both in the rat PMN and mouse macrophage assays (IC50 = 10 and 4 nM, respectively), possibly acting via a 5-LO (5-lipoxygenase) translocation inhibition mechanism. The multiple actions of WAY-121,520 may contribute to its favorable anti-inflammatory profile.