Journal of lipid mediators and cell signalling最新文献

Gangliosides have long been implicated in cell growth regulation and play an important role as modulators in protein phosphorylation. In order to better understand the glycosphingolipids and growth factors interact, we examined the modulation of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) effects on retinal Müller glial cells (RMG), following modification of their GG composition. Treatment of MG cells with GG (GM1, GT1b) and asialoGM1 resulted in modifications of several aspects of cellular responses to EGF- and FGF-receptor (R) activation: mitogenesis, cell migration, tyrosine phosphorylation of the EGF-R and FGF-R and even their cellular substrates were particularly influenced by GG. Indeed GG caused modifications of EGF-R and FGF-R autophosphorylation kinetics. GG long term effects (mitogenesis and migration) correlate with short term effects (tyrosine phosphorylation) and differences in receptor tyrosine kinase signalling could explain the specificity in growth factor responses.

Gangliosides have attracted particular attention in the field of brain research, since they were found not only to be abundant in neural tissue but also to have intricate structures in synaptic membranes. A murine neuroblastoma cell line, Neuro2a, expresses negligible amounts of GM3 and b-series gangliosides, but significant amounts of a-series gangliosides (GM1 and GD1a). With the transfection of cDNA encoding GD3 synthase, the de novo synthesis and expression of GD3 and b-series gangliosides occurred, and, furthermore, it induced the growth of axon-like neurites and cholinergic differentiation of Neuro2a cells. On the other hand, with the transfection of an α 1,2-fucosyltransferase, the axon-like neurite outgrowth was suppressed and dendrite-like neurites were outgrowth. These observations directly demonstrate the primary importance of the gene expression of a glycosyltransferase, and of the subsequent biosynthesis of gangliosides and their expression on the cell surface for neural cell development and differentiation.

Peripheral nerve possesses muscarinic cholinergic receptors, predominantly of the M₃ subtype, that stimulate phosphoinositide metabolism. Evidence suggests that one site of this response is the myelin sheath. Purified peripheral nerve myelin contains several heterotrimeric GTP-binding proteins. Furthermore, carbachol and guanosine-5′-(3-O-thio)triphosphate-stimulated hydrolysis of exogenous phosphatidylinositol-4,5-bis-phosphate that is blocked by atropine can be reconstituted in a purified peripheral myelin-rich fraction. Nerve phosphoinositide turnover is also stimulated by adenosine analogs and blocked by adenosine receptor antagonists in a pattern consistent with the presence of adenosine A₂ receptors in the tissue. Receptor-mediated phosphoinositide metabolism has also been studied in a human tumor-derived Schwann cell line (NF1T) derived from a neurofibromatosis-1 patient. By the same experimental criteria, NF1T cells also appear to contain adenosine A₂ receptors which upon activation stimulate phosphoinositide turnover. However, phosphoinositide metabolism in these cells is not increased by either carbachol or ATP. Our findings taken together with other reports suggest that Schwann cells may possess a variety of receptors which regulate phosphoinositide metabolism.

Arachidonic acid ethanolamide (anandamide) is a brain constituent that binds to the brain cannabinoid receptor (CB₁). It produces many of the pharmacological effects caused by Δ⁹-tetrahydrocannabinol (Δ⁹-THC) in mice. Anandamide parallels Δ⁹-THC in its specific interaction with the cannabinoid receptor and in inhibition of adenylate cyclase. Two additional fatty acid ethanolamides that bind to the cannabinoid receptor, homo-γ-linolenylethanolamide and docostetraenylethanolamide, have been identified in the brain. We believe that the anandamides are involved in the coordination of movement and short term memory. Depression of ambulation in an open field and the analgetic response to anandamide are not fully developed until adulthood, possibly due to an age-related increase in the CB₁ receptor concentration. This observation has clinical implications in pediatrics. A second cannabinoid receptor (CB₂) is present in the spleen. A monoglyceride, 2-arachidonyl-glycerol which binds to both CB₁ and CB₂ in transfected cells and inhibits andenylate cyclase in spleen cells was found in the gut. Its role is apparently associated with the immune system. These fatty acids amides and esters represent a new family of chemical modulators in the body.

PAF acetylhydrolase is a key enzyme of PAF inactivation. Intracellular PAF acetylhydrolase isoform Ib is a heterotrimeric enzyme composed of α-, β- and γ-subunits. The β- and γ-subunits act as a catalytic unit and their amino acid sequences are homologous. The α-subunit is not essential for catalytic activity but is a product of the causative gene for Miller-Dicker lissencephaly, suggesting this subunit plays an important role in brain development.

Selective phospholipids of synaptic membranes are reservoirs for lipid second messengers. 1-Alkyl-2-arachidonoyl glycero-3-phosphocholine is hydrolyzed by phospholipase A₂ (PLA₂) into two products: lyso-PAF, which is transacetylated to yield platelet-activating factor (PAF), and free arachidonic acid (20:4), which can undergo oxidative metabolism to eicosanoids. Alternative pathways of PAF synthesis, such as CoA-independent transacylase and the de novo route of synthesis, remain to be explored and compared to the PLA₂-dependent route. At low concentrations, PAF is a retrograde messenger of LTP in CA₁ hippocampal neurons, and is also a memory enhancer in inhibitory avoidance tasks. PAF enhances excitatory amino acid release in synaptic pairs from primary hippocampal cultures by a presynaptic mechanism. Ischemia and convulsions activate synaptic PLA₂. Thus, increased concentrations of PAF promote massive glutamate exocytosis, glutamate receptor activation, and elevated intracellular calcium levels in target cells. As a result, calcium-sensitive cascades are affected. PAF thus has dual roles as a lipid mediator: under physiological conditions it modulates neurotransmitter release, but at high concentrations it becomes neurotoxic. Through an intracellular high affinity binding site, PAF activates the expression of immediate-early genes. Some of these genes encode transcription factors (e.g. zif-268, c-fos), and others encode enzymes (COX-2 or inducible prostaglandin synthase). PAF also activates the expression of metalloproteinases which participate in the remodeling of the extracellular matrix. These effects have been studied in cells in culture as well as in the brain. A PAF antagonist specific for the intracellular binding site inhibits COX-2 expression elicited by a single electroconvulsive shock or vasogenic edema. COX-1, the constitutive prostaglandin synthase, is not induced and is unaffected by the antagonist. Most of the cerebral induction occurs in the hippocampus and results from transcriptional activation. PAF mediated gene expression may be involved in neural plasticity as well as in pathophysiological conditions in which the neural tissue activates repair-injury pathways.

Long-term potentiation (LTP) is a neurophysiological process that has been implicated in memory formation. The elevation of intracellular Ca²⁺ levels in postsynaptic neurons, an essential step in the induction of LTP in the hippocampus, can lead to activation of the enzyme acetyl-CoA:lyso-PAF acetyltransferase that is required for PAF synthesis in neurons. Thus, during the induction of LTP, stimulation of Ca²⁺ influx by glutamate receptors would lead to a postsynaptic increase in PAF biosynthesis. A main target for PAF action in neurons is the stimulation of neurotransmitter release via Ca²⁺-dependent vesicular exocytosis, a process that occurs presynaptically. In this article we describe the evidence obtained to-date for the pre- and postsynaptic events outlined above, and demonstrate for the first time that during the induction of LTP by high-frequency stimulation (HFS) a 9-fold increase in PAF release to the extracellular environment occurs within 60 min following HFS. This finding provides the evidence that PAF can diffuse from postsynaptic sites of synthesis to presynaptic sites of action, and thus function as a retrograde messenger in the induction of LTP. Based on these data, we present a scheme in which postsynaptic glutamate receptors cooperate with presynaptic PAF receptors in a reverberating cycle that can amplify the transmission in a Hebbian synapse.

In both immortalized cat iris sphincter smooth muscle cells (SV-CISM-2 cells) and cat iris sphincter, endothelin-1 (ET-1) markedly increased the activities of phospholipase A₂ (PLA₂), as measured by the release of arachidonic acid (AA), phospholipase C (PLC), as measured by the production of inositol triphosphate (IP₃), and phospholipase D (PLD), as measured by the formation of phosphatidylethanol (PEt). In SV-CISM-2 cells, ET-1 induced AA release, IP₃ production and PEt formation in a dose- and time-dependent manner. The dose-response studies showed that the peptide is more potent in activating PLD (EC₅₀ = 1.2 nM) than in activating PLC (EC₅₀ = 1.5 nM) or PLA₂ (EC₅₀ = 1.7 nM). The time course studies revealed that ET-1 activated the phospholipases in a temporal sequence in which PLA₂ was stimulated first ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 12}\text{s}$), followed by PLC ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 48}\text{s}$) and lastly PLD ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 106}\text{s}$). In SV-CISM-2 cells, in contrast to the intact iris sphincter, sarafotoxin-c, an ET_B receptor agonist, had no effect on the phospholipases, and indomethacin, a cyclooxygenase inhibitor, had no effect on the stimulatory effect of ET-1 on the phospholipases. These results suggest that in this smooth muscle cell line, ET-1 interacts with the ET_A receptor subtype to activate, via G proteins, phospholipases A₂, C and D in a temporal sequence.