Melittin stimulates phosphoinositide hydrolysis and placental lactogen release: arachidonic acid as a link between phospholipase A2 and phospholipase C signal-transduction pathways

Myo-Inositol Moderates Glucose-Induced Effects on Human Placental 13C-Arachidonic Acid Metabolism

Maternal hyperglycemia is associated with disrupted transplacental arachidonic acid (AA) supply and eicosanoid synthesis, which contribute to adverse pregnancy outcomes. Since placental inositol is lowered with increasing glycemia, and since myo-inositol appears a promising intervention for gestational diabetes, we hypothesized that myo-inositol might rectify glucose-induced perturbations in placental AA metabolism. Term placental explants (n = 19) from women who underwent a mid-gestation oral glucose-tolerance-test were cultured with 13C-AA for 48 h in media containing glucose (5, 10 or 17 mM) and myo-inositol (0.3 or 60 µM). Newly synthesized 13C-AA-lipids were quantified by liquid-chromatography-mass-spectrometry. Increasing maternal fasting glycemia was associated with decreased proportions of 13C-AA-phosphatidyl-ethanolamines (PE, PE-P), but increased proportions of 13C-AA-triacylglycerides (TGs) relative to total placental 13C-AA lipids. This suggests altered placental AA compartmentalization towards storage and away from pools utilized for eicosanoid production and fetal AA supply. Compared to controls (5 mM glucose), 10 mM glucose treatment decreased the amount of four 13C-AA-phospholipids and eleven 13C-AA-TGs, whilst 17 mM glucose increased 13C-AA-PC-40:8 and 13C-AA-LPC. Glucose-induced alterations in all 13C-AA lipids (except PE-P-38:4) were attenuated by concurrent 60 µM myo-inositol treatment. Myo-inositol therefore rectifies some glucose-induced effects, but further studies are required to determine if maternal myo-inositol supplementation could reduce AA-associated pregnancy complications.

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Alveolar Proteinosis and Phospholipidoses of the Lungs

Three pulmonary disease conditions result from the accumulation of phospholipids in the lung. These conditions are the human lung disease known as pulmonary alveolar proteinosis, the lipoproteinosis that arises in the lungs of rats during acute silicosis, and the phospholipidoses induced by numerous cationic amphiphilic therapeutic agents. In this paper, the status of phospholipid metabolism in the lungs during the process of each of these lung conditions has been reviewed and possible mechanisms for their establishment are discussed. Pulmonary alveolar proteinosis is characterized by the accumulation of tubular myelin-like multilamellated structures in the alveoli and distal airways of patients. These structures appear to be formed by a process of spontaneous assembly involving surfactant protein A and surfactant phospholipids. Structures similar to tubular myelin-like multilamellated structures can be seen in the alveoli of rats during acute silicosis and, as with the human condition, both surfactant protein A and surfactant phospholipids accumulate in the alveoli. Excessive accumulation of surfactant protein A and surfactant phospholipids in the alveoli could arise from their overproduction and hypersecretion by a subpopulation of Type II cells that are activated by silica, and possibly other agents. Phospholipidoses caused by cationic amphiphilic therapeutic agents arise as a result of their inhibition of phospholipid catabolism. Inhibition of phospholipases results in the accumulation of phospholipids in the cytoplasm of alveolar macrophages and other cells. While inhibition of phospholipases by these agents undoubtedly occurs, there are many anomalous features, such as the accumulation of extracellular phospholipids and surfactant protein A, that cannot be accounted for by this simplistic hypothesis.

Glucocorticoid-regulated crosstalk between arachidonic acid and endocannabinoid biochemical pathways coordinates cognitive-, neuroimmune-, and energy homeostasis-related adaptations to stress

Arachidonic acid and its derivatives constitute the major group of signaling molecules involved in the innate immune response and its communication with all cellular and systemic aspects involved on homeostasis maintenance. Glucocorticoids spread throughout the organism their influences over key enzymatic steps of the arachidonic acid biochemical pathways, leading, in the central nervous system, to a shift favoring the synthesis of anti-inflammatory endocannabinoids over proinflammatory metabolites, such as prostaglandins. This shift modifies local immune-inflammatory response and neuronal activity to ultimately coordinate cognitive, behavioral, neuroendocrine, neuroimmune, physiological, and metabolic adjustments to basal and stress conditions. In the hypothalamus, a reciprocal feedback between glucocorticoids and arachidonate-containing molecules provides a mechanism for homeostatic control. This neurochemical switch is susceptible to fine-tuning by neuropeptides, cytokines, and hormones, such as leptin and interleukin-1beta, assuring functional integration between energy homeostasis control and the immune/stress response.

Glucocorticoids shift arachidonic acid metabolism toward endocannabinoid synthesis: a non-genomic anti-inflammatory switch

Glucocorticoids are capable of exerting both genomic and non-genomic actions in target cells of multiple tissues, including the brain, which trigger an array of electrophysiological, metabolic, secretory and inflammatory regulatory responses. Here, we have attempted to show how glucocorticoids may generate a rapid anti-inflammatory response by promoting arachidonic acid-containing endocannabinoids biosynthesis. According to our hypothesized model, non-genomic action of glucocorticoids results in the global shift of membrane lipid metabolism, subverting metabolic pathways toward the synthesis of the anti-inflammatory endocannabinoids, anandamide (AEA) and 2-arachidonoyl-glycerol (2-AG), and away from arachidonic acid production. Post-transcriptional inhibition of cyclooxygenase-2 (COX(2)) synthesis by glucocorticoids assists this mechanism by suppressing the synthesis of pro-inflammatory prostaglandins as well as endocannabinoid-derived prostanoids. In the central nervous system (CNS) this may represent a major neuroprotective system, which may cross-talk with leptin signaling in the hypothalamus allowing for the coordination between energy homeostasis and the inflammatory response.

Endocrine cell lines from the placenta

Cell-lines derived from human placenta and chorion have been used extensively to model the endocrine functions of human trophoblast. In general terms, the endocrine functions of the primary cells and tissues are at least partially replicated within the cell-lines, suggesting that they may be used as appropriate models. There are, however, two major provisos that compromise this generalisation. Firstly, the endocrine function of placenta represents a complex interaction between cytotrophoblast, syncytiotrophoblast and multiple regulators, so a single cell population digested from the normal environment is unlikely to represent this. Secondly, the characterisation of primary trophoblast populations and of cell-lines is incomplete, complicating the assignment of functions to trophoblast populations. Despite these difficulties, useful information has been obtained from the available cell-lines, regardless of whether they have arisen spontaneously, been transformed in vitro, or derived from cancers in vivo.

AHNAK, a protein that binds and activates phospholipase C-gamma1 in the presence of arachidonic acid

We have recently shown that phospholipase C-gamma (PLC-gamma) is activated by tau, a neuronal cell-specific microtubule-associated protein, in the presence of arachidonic acid. We now report that non-neuronal tissues also contain a protein that can activate PLC-gamma in the presence of arachidonic acid. Purification of this activator from bovine lung cytosol yielded several proteins with apparent molecular sizes of 70-130 kDa. They were identified as fragments derived from an unusually large protein (approximately 700 kDa) named AHNAK, which comprises about 30 repeated motifs each 128 amino acids in length. Two AHNAK fragments containing one and four of the repeated motifs, respectively, were expressed as glutathione S-transferase fusion proteins. Both recombinant proteins activated PLC-gamma1 at nanomolar concentrations in the presence of arachidonic acid, suggesting that an intact AHNAK molecule contains multiple sites for PLC-gamma activation. The role of arachidonic acid was to promote a physical interaction between AHNAK and PLC-gamma1, and the activation by AHNAK and arachidonic acid was mainly attributable to reduction in the enzyme's apparent Km toward the substrate phosphatidylinositol 4,5-bisphosphate. Our results suggest that arachidonic acid liberated by phospholipase A2 can act as an additional trigger for PLC-gamma activation, constituting an alternative mechanism that is independent of tyrosine phosphorylation.

Calcium influx in Trypanosoma brucei can be induced by amphiphilic peptides and amines

The following study was undertaken to determine whether an inducible calcium influx pathway is present in intact bloodstream forms of Trypanosoma brucei. Fura-2 fluorescence was used to demonstrate that amphiphilic peptides and amines, including melittin, mastoparan and compound 48/80, each produced a dose dependent calcium influx across the plasma membrane. Calcium influx did not result from general disruption of membrane integrity, since a corresponding influx of ethidium bromide or other divalent cations was not observed. Instead, the calcium influx was selectively blocked by the calcium channel antagonists, La3+, Cd2+ or Ni2+, and was not affected by the Na+ channel antagonists, tetrodotoxin or amiloride. Activation of the trypanosome calcium influx pathway was dependent upon an intact membrane potential, and the rise in intracellular free calcium concentration ([Ca2+]i) was reversed upon membrane depolarization with gramicidin D. Changes in Ins(1,4,5)P3 did not accompany the calcium influx. Overall, these data provide the first evidence of an inducible calcium influx pathway in T. brucei, and describe methods to selectively manipulate this pathway.

Activation of phospholipase C-gamma by the concerted action of tau proteins and arachidonic acid

Phospholipase C-gamma (PLC-gamma) isozymes are thought to be activated by receptor-induced tyrosine phosphorylation. Proteins that activate PLC-gamma1 have now been purified from bovine brain and identified as members of the tau family of microtubule-associated proteins. Activation of PLC-gamma by tau was enhanced in the presence of unsaturated fatty acids such as arachidonic acid, saturated fatty acids being ineffective. Maximal (15-20-fold) activation was apparent in the presence of 0.15 microM tau and 25 microM arachidonic acid (AA). The effect of tau and AA was specific to PLC-gamma isozymes in the presence of submicromolar concentrations of Ca2+ and was markedly inhibited by phosphatidylcholine. These results suggest that in cells that express tau, receptors coupled to cytosolic phospholipase A2 may activate PLC-gamma isozymes indirectly in the absence of tyrosine phosphorylation through the hydrolysis of phosphatidylcholine to generate AA.

Induction by melittin of protein phosphorylation in bovine mammary gland and suppression of the phosphorylation by phosphatidylserine

The protein kinase C (PKC) inhibitor sphingosine induces phosphorylation of an 18-kDa protein in Jurkat T cells and of a 19-kDa protein in bovine mammary gland, and suppresses phosphorylation of substrate proteins for PKC. Melittin, a toxic amphiphilic peptide from bee venom known to inhibit PKC activity, was examined to determine whether it, like sphingosine, induced phosphorylation of the 19-kDa protein. Melittin inhibited PKC activity in both cytosolic and total particulate fractions of bovine mammary gland, with IC50 values (concentrations causing 50% inhibition) of 5-7 microM. Melittin suppressed phosphorylation of such PKC substrate proteins as a 91-kDa protein in cytosol and a 36-kDa protein in the particulate fraction. Besides the suppression, melittin induced phosphorylation of cytosolic 105-kDa, 94-kDa, 27-kDa, 24-kDa and 19-kDa and particulate 110-kDa, 53-kDa and 43-kDa proteins, which were not clearly observed in the absence of melittin. The induction could be detected at a 10 microM concentration. Phosphorylation of these proteins was reversed by excess addition of the PKC cofactor phosphatidylserine, but not by other cofactors such as 1-oleoyl-2-acetyl-sn-glycerol or Ca2+. PKC inhibitors other than melittin [1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, gossypol, palmitoylcarnitine and adriamycin] had little effect on the induction, except at a high concentration (500 microM) of adriamycin. These results suggest that melittin, like sphingosine, has both suppressive and stimulatory effects on protein phosphorylation in bovine mammary gland.

Modulation of phospholipases A2 and C activities against dilauroylphosphorylcholine in mixed monolayers with semisynthetic derivatives of ganglioside and sphingosine

Most of the semisynthetic ganglioside and sphingosine derivatives studied here decreased the rate as well as the extent of hydrolysis of monomolecular layers of dilauroylphosphorylcholine (dlPC) by both phospholipase A2 (PLA2) and C (PLC). For PLA2, the rate of enzymatic activity was inversely correlated (p < 0.001) with the duration of the latency period of the enzymatic reaction. The correlation between the rate of activity and the latency period was not statistically significant for PLC. The potency to inhibit PLA2 and PLC was not significantly correlated with the presence of specific chemical groups. Also, the inhibitory effect is not correlated to changes of the substrate intermolecular spacing or surface potential caused by the sphingolipids (SLs). Conversely, for PLA2 the variation of the kinetic parameters of the reaction with the molecular polarization vector of the SLs are statistically significant (p < 0.01). The rate of phospholipid degradation by PLA2 decreased, and the lag times tended to become longer, with increasing values of the SLs' polarization vector. The kinetic parameters of the reaction with PLC did not show statistically significant correlation with the polarization vector of the SLs. Our results suggest that the configuration of the electrostatic field across the interface is more important than are formal charges or specific chemical moieties in modulating the activity of PLA2. Inhibition of phospholipase activities of these types by specific SLs or their metabolites may be important as supramolecular regulatory steps at the membrane level of the amplification of lipid-mediated signaling pathways.

Diacylglycerol: formation and function in phospholipid-mediated signal transduction

1. Properties, distribution and multiplicity of phosphoinositidases (phospholipase C, PLC) are investigated. 2. Generation of diacylglycerol (DAG) by a variety of enzymes such as phosphoinositide and phosphatidylcholine specific PLC, by a combination of phospholipase D and phosphatidic hydrolase, and by triglyceride lipase is examined. 3. Ca2+ and phospholipid-dependent protein kinase C act as the target of DAG messenger action. 4. There are differences in the formation of DAG in normal and transformal cell.

Possible mechanisms of action of cobra snake venom cardiotoxins and bee venom melittin

Cobra snake venom cardiotoxins and bee venom melittin share a number of pharmacological properties in intact tissues including hemolysis, cytolysis, contractures of muscle, membrane depolarization and activation of tissue phospholipase C and, to a far lesser extent, an arachidonic acid-associated phospholipase A2. The toxins have also been demonstrated to open the Ca2+ release channel (ryanodine receptor) and alter the activity of the Ca(2+)+Mg(2+)-ATPase in isolated sarcoplasmic reticulum preparations derived from cardiac or skeletal muscle. However, a relationship of these actions in isolated organelles to contracture induction has not yet been established. The toxins also bind to and, in some cases, alter the function of a number of other proteins in disrupted tissues. The most difficult tasks in understanding the mechanism of action of these toxins have been dissociating the primary from secondary effects and distinguishing between effects that only occur in disrupted tissues and those that occur in intact tissue. The use of cardiotoxin and melittin fractions contaminated with trace ('undetectable') amounts of venom-derived phospholipases A2 has continued to be common practice, despite the problems associated with the synergism between the toxins and enzymes and the availability of methods to overcome this problem. With adequate precautions taken with regard to methodology and interpretation of results, the cobra venom cardiotoxins and bee venom melittin may prove to be useful probes of a number of cell processes, including lipid metabolism and Ca2+ regulation in skeletal and cardiac muscle.

Arachidonic acid and cell signalling in the ovary and placenta

Arachidonic acid (AA) and its metabolites make up a diverse group of signalling molecules important to mediation of metabolic and endocrine function of ovarian and placental cell membranes. This paper reviews recent literature examining AA and eicosanoid involvement in the functional dynamics of follicular development, ovulation and corpus luteum function. The putative roles of AA metabolites in establishment and maintenance of pregnancy are reviewed with reference to decidualization, trophoblast invasion and implantation, maintenance of perfusion of the feto-placental unit and lipid transfer. Finally, recent evidence implicating AA metabolism in mediation of enzyme activity following hormone-receptor coupling within various cells types comprising the placental membranes is reviewed.

Prostaglandin E2-induced arachidonic acid release and catecholamine secretion from cultured bovine adrenal chromaffin cells

We recently reported that prostaglandin E2 (PGE2) and arachidonic acid (AA) each induced a gradual secretion of catecholamines from cultured bovine adrenal chromaffin cells in the presence of ouabain by stimulation of phosphoinositide metabolism. In the present study, we examined the relationship between phospholipase A2 and C activation and catecholamine secretion by PGE2 in chromaffin cells. The phospholipase A2 inhibitors p-bromophenacyl bromide and mepacrine did not affect the basal and ouabain-induced release, but dose-dependently blocked PGE2-evoked phosphoinositide metabolism and the consequent catecholamine release at an IC50 value of 3 microM. PGE2 induced rapid hydrolysis of [3H]AA from prelabeled phospholipid pools: the release of [3H]AA could be detected at as early as 15 sec and reached a plateau after 1 min. While the phospholipase C inhibitor neomycin did not inhibit PGE2-induced AA release, phospholipase A2 inhibitors dose-dependently inhibited it at IC50 values comparable to those for catecholamine release. Pretreatment of intact cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, but not with pertussis toxin, prevented AA release by PGE2. These results demonstrate that PGE2 activates phospholipase A2 as well as phospholipase C in a pertussis toxin-insensitive manner and suggest that the released arachidonic acid may be involved in PGE2-induced catecholamine release from chromaffin cells.

Thimerosal blocks stimulated but not basal release of endothelium-derived relaxing factor (EDRF) in dog isolated coronary artery

1. The effect of an acetly-coA lysolecithin acyltransferase inhibitor, thimerosal, on the release of endothelium-derived relaxing factor (EDRF) was examined in the greyhound isolated coronary artery. 2. Thimerosal (1-10 microM) relaxed fully, ring segments of coronary artery which were contracted with the thromboxane A2-mimetic, U46619 (30 nM). The response was endothelium-dependent, slow in both onset and time to reach maximum. The maximum relaxation to the highest concentration of thimerosal (10 microM) was maintained for 10-20 min before the tissue slowly regained active force (1-2 h) to the same or higher level as that prior to the addition of thimerosal. At this time the endothelium-dependent relaxation responses to acetylcholine (ACh), substance P (SP), bradykinin (BK) and the calcium ionophores, ionomycin and A23187 were abolished. The endothelium-dependent contractions to the nitric oxide synthase inhibitors, NG-nitro-L-arginine (L-NNA; 10-100 microM) and NG-monomethyl-L-arginine (L-NMMA: 10-100 microM), however, were unaffected. 3. Thimerosal (10 microM) did not affect the relaxation curve to sodium nitroprusside (SNP) nor the contraction curve to the thromboxane A2-mimetic, U46619. 4. Both the relaxation response to thimerosal and the selective block of the relaxation responses to stimulated EDRF release were unaffected by either indomethacin (10 microM) or superoxide dismutase (150 u ml-1). 5. L-NNA (100 microM) significantly blocked the relaxation curves to thimerosal and A23187 but not that to SNP.6. Abolition of stimulated EDRF-mediated responses with thimerosal was unlikely to result from maximal and maintained stimulation of EDRF release even when active U46619-induced force had returned to pre-thimerosal levels, since the relaxation curves to glyceryl trinitrate (GTN) and SNP were markedly attenuated in the presence of SNP and GTN respectively when active force was restored with endothelin-1 (ET-1).7. Melittin (1 microM), ionomycin (1 microM) and A23187 (1 microM) each had selective effects on stimulated but not basal EDRF responses, similar to those of thimerosal.8. We propose that stimulated but not 'basal' release of EDRF is dependent on the release of arachidonic acid or one of its non-cyclo-oxygenase metabolites, possibly by Ca2'-dependent activation of phospholipase A2.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.