Bereziat G
URA CNRS 1283, CHU Saint-Antoine, Paris.
C R Seances Soc Biol Fil. 1996;190(4):409-16.
Membrane phospholipids not only constitute structural membrane components, they also contain a wealth of biochemical information. They are the source of numerous lipid mediators (prostaglandins, leukotrienes, thromboxane, paf, lysophosphatidic acid and free fatty acids). These lipids act as second messengers inside the cell to modulate enzyme (e.g. PKC and GAP), ion channels (e.g. Ca2+ and K+) or the activity of factors regulating gene expression either at the transcriptional level (e.g. on the TNF alpha gene) or at the post-transcriptional level (e.g. on the GLUT4 transporter). The synthesis of lipid mediators results from the stimulation of phospholipase A2 (PLA2) activities. PLA2 cleaves membrane phospholipids to give rise to lysophospholipids and to free fatty acids from which second messengers are generated. More specifically, PLA2 provides the precursor for the eicosanoids, when the cleaved fatty acid is arachidonic acid, or for PAF, when the sn-1 position of the phospholipid is an alkyl ether linkage. Therefore, PLA2 is a key enzyme in the regulation of lipid mediators of inflammatory process. The purification and cloning of several PLA2s have demonstrated clear differences between secreted and intracellular PLA2. The secreted PLA2s are closely related proteins of low molecular weight (14 kDa) with calcium requirement in the mM range. They contain numerous bonds and retain the same amino-acids at the active site. In mammals, two types of secreted PLA2 have been identified: type I pancreatic PLA2 and type II inflammatory PLA2 which show 70% sequence homology. Recently, two others 14 kDa sPLA2 have been cloned which share also high homologies with type I and type II but contain respectively 6 and 8 disulpide bonds. In contrast, cellular PLA2s have higher molecular weights (40-110 kDa) and are either calcium independent or require microM amounts for activity. Cellular PLA2s preferentially act on sn-2-arachidonoyl phospholipids in vitro whereas sPLA2 do not display such selectivity in vitro. Both cellular and secreted PLA2s are involved in lipid mediator production. Cellular PLA2 can be activated by membrane receptors coupled to G proteins or by tyrosine kinase receptor, through the ras-raf1-MAP kinases network. Cellular PLA2s are thought to be involved in the initial production of lipid mediators after cell activation. Several lines of evidence suggest that secreted PLA2 is involved in the sustained production of lipid mediators in several cell types. These lines of evidence include the decrease in eicosanoid production by antibodies RNA of sPLA2. Furthermore, secreted PLA2s might trigger autocrine loops and proliferation responses through interaction with a specific receptor.
膜磷脂不仅构成细胞膜的结构成分，还包含丰富的生化信息。它们是众多脂质介质（前列腺素、白三烯、血栓素、血小板活化因子、溶血磷脂酸和游离脂肪酸）的来源。这些脂质在细胞内充当第二信使，以调节酶（如蛋白激酶C和GTP酶激活蛋白）、离子通道（如钙离子和钾离子通道）或在转录水平（如肿瘤坏死因子α基因）或转录后水平（如葡萄糖转运蛋白4）调节基因表达的因子的活性。脂质介质的合成源于磷脂酶A2（PLA2）活性的刺激。PLA2切割膜磷脂产生溶血磷脂和游离脂肪酸，第二信使由此产生。更具体地说，当切割的脂肪酸为花生四烯酸时，PLA2为类花生酸提供前体；当磷脂的sn-1位为烷基醚键时，PLA2为血小板活化因子提供前体。因此，PLA2是炎症过程中脂质介质调节的关键酶。几种PLA2的纯化和克隆表明，分泌型和细胞内PLA2之间存在明显差异。分泌型PLA2是低分子量（14 kDa）的密切相关蛋白质，对钙的需求在毫摩尔范围内。它们含有许多二硫键，在活性位点保留相同的氨基酸。在哺乳动物中，已鉴定出两种分泌型PLA2：I型胰腺PLA2和II型炎症PLA2，它们的序列同源性为70%。最近，又克隆了另外两种14 kDa的分泌型PLA2，它们与I型和II型也有高度同源性，但分别含有6个和8个二硫键。相比之下，细胞内PLA2分子量更高（40 - 110 kDa），要么不依赖钙，要么活性需要微摩尔量的钙。细胞内PLA2在体外优先作用于sn-2-花生四烯酰磷脂，而分泌型PLA2在体外不表现出这种选择性。细胞内和分泌型PLA2都参与脂质介质的产生。细胞内PLA2可通过与G蛋白偶联的膜受体或酪氨酸激酶受体，经ras-raf1-丝裂原活化蛋白激酶网络激活。细胞内PLA2被认为参与细胞激活后脂质介质的初始产生。几条证据表明，分泌型PLA2参与多种细胞类型中脂质介质的持续产生。这些证据包括用分泌型PLA2的抗体RNA降低类花生酸的产生。此外，分泌型PLA2可能通过与特定受体相互作用触发自分泌环和增殖反应。
