Journal of lipid mediators and cell signalling最新文献

Sphingomyelin (SM) biosynthesis in cultured oligodendrocytes (OC) was evaluated: (i) with [¹⁴C] tracers (choline, ethanolamine, serine) to pinpoint the major metabolic routes; (ii) with fluorescent and truncated, radiolabeled ceramide analogs to determine the relative activities of SM-synthase in intra- and extra-Golgi compartments of OC. In contrast to a general contention in the literature that SM synthase is absent from the brain, our data show that (choline → CDP-choline → phosphatidylcholine (PC) → SM) is the major anabolic route with only a minor contribution to PC via methylation of phosphatidylethanolamine (PE). SM synthase activity was found to be equally divided between intra- and extra-Golgi compartments of OC. Moreover, significant SM-synthase activity was recovered in purified myelin preparations. Our results shed new light on the possible involvement of sphingolipid-derived mediators in myelination.

Neuroblastoma and glioma cells differentially express isoforms of protein kinase C (PKC) and myristoylated PKC substrates (e.g. MARCKS). Correlation with metabolism of membrane phospholipids suggests that PKC-α and MRCKS may be required to mediate phosphatidylcholine turnover stimulated by phorbol ester (β-TPA). To evaluate relationships to neural cell differentiation, SK-N-SH human neuroblastoma cells were treated with 20 nM β-TPA. In β-TPA-treated cells, growth arrest and differentiation occurred (neurite extension; 40–60% decrease in cell number, total protein and RNA). By day 4, mRNA for PKC-α and MARCKS increased and, after an initial decrease, PKC-α protein also increased. At day 4, phosphatidylcholine synthesis was 3–5 fold greater than in control cells. In contrast, C6 glioma cells treated with β-TPA showed no growth arrest, decreased PKC-α protein (<20%) and lower phosphatidylcholine synthesis. Thus, induced differentiation of human neuroblastoma cells involved increased expression of PKC-α and MARCKS and synthesis of phosphatidylcholine, consistent with involvement of PKC-α and MARCKS in regulation of phosphatidylcholine turnover during neurite growth.

The activation of phospholipase D (PLD) in response to cell stimulation by extracellular signal molecules is a widespread phenomenon. A variety of extracellular signal molecules cause a rapid and dramatic stimulation of PLD activity. G proteins and protein kinases appear to be involved in the receptor-mediated regulation of PLD. There is indirect evidence for the existence of multiple PLD subtypes, both membrane-associated and cytosolic. Recent studies indicate that PLD activities require a lipid cofactor, phosphatidylinositol 4,5-bisphosphate (PIP₂). Addition of PIP₂ at physiological concentrations stimulates both membrane-associated and partially purified PLD activity. Other acidic phospholipids have little or no effect. Neomycin, a high affinity ligand of PIP₂, inhibits membrane PLD activity, presumably by binding to endogenous PIP₂. A monoclonal antibody to phosphatidylinositol 4-kinase inhibits PIP₂ synthesis in permeabilized U937 cells and blocks PLD activation by GTPγS and TPA. These results indicate that PIP₂ synthesis is required for G protein- and protein kinase C-mediated activation of PLD in the cells. Recent evidence has implicated PLD and phosphoinositide kinases in vesicular trafficking. The main lipid mediator produced by PLD, phosphatidic acid, could regulate membrane traffic events by direct regulation of target proteins involved in vesicle targeting, docking and fusion. In addition, under certain circumstances, the formation of phosphatidic acid may lead to changes in lipid bilayer properties that would facilitate vesicle budding and fusion events in the course of intracellular membrane traffic.

Phospholipase D (PLD) is believed to play an important role in cell signal transduction: PLD catalyzes the hydrolysis primarily of phosphatidylcholine (PC) to produce phosphatidic acid that may serve as a lipid second messenger. Although the mechanism of PLD activation has not yet been fully understood, a member of the low molecular weight GTP-binding protein (small G protein) superfamily, ADP-ribosylation factor (ARF), has been identified as a PLD-activating factor. In addition to ARF, we found that RhoA, another member of the small G proteins, activated rat brain PLD, and that ARF and RhoA synergistically stimulated the enzyme activity. When proteins of bovine brain cytosol were subjected to anion exchange column chromatography and then reconstituted with rat brain PLD partially purified from the membranes, fractions eluted at 60 mM NaCl, where ARF was not detected, activated the enzyme in a guanosine 5′-O-(3-thiotriphosphate)-dependent manner. This PLD-stimulating activity seemed to be attributed to a small G protein RhoA. Evidence provided includes the findings that: (1) the partially purified preparation of the PLD-activating factor by subsequent column chromatographies contained a 22 kDa substrate for botulinum C3 exoenzyme ADP-ribosyltransferase; (2) the 22 kDa protein strongly reacted with anti-RhoA antibody; (3) the treatment of the partially purified PLD-activating factor with C3 exoenzyme and NAD together, but not individually, significantly inhibited the PLD-stimulating activity; and (4) recombinant isoprenylated RhoA activated the PLD. On the contrary, recombinant nonisoprenylated RhoA failed to activate the PLD. Interestingly, the partially purified PLD-activating factor and ARF synergistically activated rat brain PLD, and recombinant isoprenylated RhoA could substitute for the partially purified preparation. These results conclude that rat brain PLD is regulated by RhoA in concert with ARF, and that the post-translational modification of RhoA is essential for its function as the PLD activator.

In the past 10 years evidence has been accumulating on the synthesis, distribution, regulation and function of platelet-activating factor (PAF) and related phospholipids in the brain. Clearly, much needs to be done to establish specific biological functions for PAF in the brain and in particular its role in disease processes. The cellular elements responsible for PAF synthesis and release in vivo is not yet clear nor has it been established whether PAF has a role in chronic pathological conditions. Hopefully, this ‘decade of the brain’ will lead us to better insights on these issues.

We found that lysophosphatidic acid (LPA) exerted two effects on Ca²⁺-movement in cultured bovine adrenal chromaffin cells. At concentrations of above 10⁻⁵ M, it induced slight, but significant ⁴⁵Ca²⁺ influx, resulting in release of a small portion of stored catecholamine. At high concentrations it also significantly increased the intracellular Ca²⁺ concentration in Fura-2-loaded cells. At concentrations as low as 10⁻⁷ M, it stimulated extracellular Na⁺-dependent ⁴⁵Ca²⁺ efflux, possibly by increasing $\text{Na}{}^{\mathrm{+}}\text{Ca}{}^{\mathrm{2+}}$ exchange. The maximal efflux of Ca²⁺ attained with 10⁻⁵ M LPA was inhibited by tyrosine kinase inhibitors, but augmented by a protein kinase C inhibitor. These results suggest that LPA-induced Ca²⁺ efflux is controlled positively and negatively by mechanisms involving tyrosine kinase and protein kinase C, respectively.

The effects of N-arachidonoylethanolamine (anandamide) and related compounds on the binding of [³H]CP55940 to rat brain synaptosomes were examined. Anandamide was shown to inhibit competitively the specific binding of [³H]CP55940 to synaptosomal membranes. The K_i value was 89 nM. In contrast, N-acylethanolamines containing saturated or monoenoic fatty acids did not exhibit high binding affinity. Several structural analogues of anandamide showed some binding activity. Among them, 2-arachidonoylglycerol is noteworthy because of its occurrence in mammalian tissues. A biosynthetic study indicated that anandamide can be synthesized via two separate synthetic pathways. The first is synthesis from free arachidonic acid and ethanolamine, and the second is the formation of N-arachidonoyl phosphatidylethanolamine (PE) from diarachidonoyl phospholipids and PE and the subsequent enzymatic release of N-arachidonoylethanolamine. The latter pathway appears to explain very well the fatty acid composition of N-acylethanolamines present in mammalian tissues.

Prostaglandin (PG) E2 exerts a variety of biological activities for the maintenance of local homeostasis in the body. The effects of PGE₂ are exerted by a variety of PGE receptors, which are different in their signal transduction properties and are classified into four subtypes, EP1, EP2, EP3 and EP4. We have isolated the mouse cDNAs for these PGE receptors and characterized the cloned receptors. EP1, EP2, EP3 and EP4 receptors consist of 405, 362, 365 and 513 amino acid residues with a putative seven hydrophobic domains, respectively. When expressed in mammalian cells, EP1 showed elevation of intracellular [Ca²⁺], EP2 and EP4 stimulated adenylate cyclase and EP3 inhibited the enzyme. Northern blot and in situ hybridization analyses have shown that these subtypes are differently localized to specific tissues and cells. We have identified multiple isoforms of the EP3 receptor (EP3α, EP3β, and EP3γ) which differ in their carboxy-terminal domains. These isoforms displayed identical agonist binding properties, but were functionally different in the efficiency of G protein activation, the specificity of G protein coupling, and sensitivity to agonist-induced desensitization. The diverse physiological actions of PGE₂ are elicited by the molecular diversity of the receptor subtypes and isoforms distributed differently in the body.

A phospholipase C activity highly specific for lysophosphoinositide (lysoPI-PLC) has been demonstrated to be present in synaptic plasma membranes of the rat brain. Several lines of evidence suggested that the lysoPI-PLC is an enzyme distinct from known isoforms of phosphoinositide-specific phospholipase C (PI-PLC). On the other hand, the occurrence of a Ca²⁺-independent phospholipase A₁ hydrolyzing PI in rat brain was also demonstrated. The lysoPI-PLC hydrolyzed the 2-acyl isomer as well as the 1-acyl isomer of lysoPI. These findings suggest possible pathways for PI metabolism through lysoPI to yield monoacylglycerol (mainly 2-arachidonoyl glycerol) and inositolphosphates in the brain, which are different from the well-characterized PI-PLC pathway.