Journal of lipid mediators and cell signalling最新文献

Multiple cellular responses are regulated through the generation of lipid second messengers upon activation of phospholipases. One such response concerns the activity of a class of kinase constituting the protein kinase C family. The production of specific molecular species of lipid second messengers may be therefore of prime importance in the activation of a member of the PKC isoforms. Prompted by this possibility we investigated the production of 1,2 diacyl-sn-glycerol (DAG) and phosphatidic acid (PtdOH) in LA-N-1 neuroblastoma cells under various physiological states. 12-0-Tetradecanoylphorbol 13-acetate (TPA) stimulation activated a phospholipase D (PLD) specific for phosphatidylcholine (PtdCho) in proliferating cells and a phospholipase C (PLC) specific for phosphatidylethanolamine (PtdEtn) in retinoic acid (RA) differentiated cells. These separate activations produced different molecular species of DAG or PtdOH. PtdOH was able to stimulate the Ca²⁺ dependent protein kinase C (PKC) by a mechanism which differed from the action of DAG. PtdOH did not induce the translocation of the PKC to the membrane. Moreover PtdOH, in contrast to DAG, prevented PKC degradation by inhibiting the enzymatic hydrolysis by m-calpain. These observations suggest that the stimulation of cells by agonists elicited the production of specific molecular species of lipid second messengers depending on the physiological status of the cells, and probably on the nature of the stimulus. It seems therefore likely that the generation of specific lipid second messengers may activate specific PKC isoforms resulting in a specific cellular response.

We have examined the Ca²⁺ release activity of sphingolipid-derivatives from rat brain microsomes using a Fura-2 cytofluorometric assay. Sphingosylphosphorylcholine, lysosphingomyelin, elicited a rapid Ca²⁺ release from both cerebral and cerebellar microsomes. Other compounds including sphingosine and sphingosine-1-phosphate were incapable of causing the Ca²⁺ release. The pharmacological properties suggest that the sphingosylphosphorylcholine-elicited Ca²⁺ mobilization is not mediated by inositol 1,4,5-triphosphate receptors. Immunocytochemical study showed the occurrence of sphingomyelin, a putative precursor for sphingosylphosphorylcholine, in the somatodendritic membrane domains of cerebellar neurons. These observations imply that sphingosylphosphorylcholine is a potent Ca²⁺ releaser in brain neurons.

Stimulation of PC12 cells with carbachol (Cch) or phorbol 12-myristate 13-acetate (PMA) induced [³H]phosphatidylbutanol (PBut) production. Depletion of extracellular Ca²⁺ abolished the Cch-mediated phospholipase D (PLD) activation, indicating the requirement of Ca²⁺ influx. Two different types of PLD activity, oleate-dependent and GTPγS-dependent, were examined by using exogenous [³H]phosphatidylcholine substrate. PLD activity of the membrane fraction of PC12 cells was highly dependent on oleate and independent of GTPγS. This profile is in sharp contrast to that observed in HL60 cells showing the profound GTPγS-induced activation of PLD. The oleate-dependent PLD activity of PC12 membrane was inhibited by high concentrations of Ca²⁺ and Mg²⁺. These results indicate that Ca²⁺ may not directly activate PLD but through some Ca²⁺-dependent mechanism(s) in Cch-stimulated cells.

We studied the localization of N-acyl phosphatidylethanolamine (NAPE), a putative cannabinoid precursor, in primary cultures of striatal and cortical neurons from the rat brain. We probed intact neurons with various exogenous phospholipases, including S. chromofuscus phospholipase D (PLD). S. chromofuscus PLD does not penetrate into neurons (as demonstrated by a lack of internalization of ¹²⁵I-labeled PLD), and does not cause gross damage to the neuronal membrane (as demonstrated by a lack of effect of PLD on [³H]γ-aminobutyric acid release). When neurons, labeled to isotopic equilibrium with [³H]ethanolamine, were incubated for 10 min with S. chromofuscus PLD, approximately 50% of neuronal NAPE was hydrolysed. This hydrolysis was accompanied by the release of a family of N-acyl ethanolamines (NAE) (assessed by high performance liquid chromatography), which included the cannabinoid receptor agonist, anandamide. Exogenous phospholipase A₂ (PLA₂) (Apis mellifera) and PLC (B. cereus) mobilized [³H]arachidonate and [³H]diacylglycerol, respectively, but had no effect on NAE formation under these conditions. These experiments indicate that ∼ 50% of neuronal NAPE is localized in a compartment that is easily accessible to extracellular PLD, possibly the plasmalemma, where it would also be easily hydrolyzed upon stimulation to produce NAE.

Long term potentiation (LTP) is a widely studied form of synaptic plasticity. Brief tetanic stimulation of synaptic afferents in several areas of the brain, most notably the hippocampus, produces long lasting changes in the synaptic strength. The induction of LTP requires in the limit a significant activation of the NMDA receptor and the subsequent entry of calcium into the post synaptic cell. The maintenance of LTP requires at least in part a change in presynaptic function. This review addresses the current thinking in the literature on how a post synaptic induction event may be communicated to the presynaptic terminal and subsequently lead to a series of poorly defined biochemical process that ultimately lead to an enhancement in the efficiency of the potentiated terminal.

Three different cDNA clones for diacylglycerol (DG) kinase were isolated from a rat brain cDNA library and designated DGK-I, DGK-II and DGK-III. These three encode distinct polypeptides with 58% identity to each other and contain EF-hand motifs, cysteine-rich zinc finger-like sequences and putative ATP-binding sites. A high kinase activity is shown in COS cells transfected with either one of the three cDNAs without substrate specificity among DG species, and the kinase activity is Ca-dependent. The activity for DGK-I is recovered dominantly in the soluble fraction of the cell, that for DGK-II in the particulate fraction; and that for DGK-III equally in both of the fractions. The difference in their expression localization is most noticeable: DGK-I is expressed in oligodendrocytes of the brain as well as T-lymphocytes in the thymus and spleen; DGK-II is expressed in neurons of the caudate-putamem, accumbens nucleus and olfactory tubercle; and DGK-III in the cerebellar Purkinje cells and granule cells. The functional significance of the discovery of three DG kinase isozymes is briefly discussed.

Agonist-stimulated inositol phospholipid hydrolysis by phospholipase C was once thought to be the sole mechanism to produce diacylglycerol that transduce extracellular signals into intracellular events through activation of protein kinase C. It is becoming clear that agonist-induced hydrolysis, of the other membrane phospholipids, particularly phosphatidylcholine, by phospholipase A₂ and phospholipase D also takes part in cellular responses such as cell proliferation and differentiation. Possibly, the members of the protein kinase C family may be activated differently by various combinations of phospholipid degradation products, and play each distinct role in signal transduction for the control of various cellular functions.