Properties and Regulation of Cytosolic Phospholipase A2 (1997)

Membrane phospholipid is an important reservoir for the generation of bioactive mediators. Cellular phospholipases hydrolyze membrane phospholipid in a structurally specific manner releasing numerous lipid products that are responsible for transmitting diverse signals necessary for the induction of functional responses. Since this is a highly regulated process, the phospholipases are subject to complex mechanisms of activation and presumably deactivation. The phospholipase A2(PLA2) 1The abbreviations used are: PLA2, phospholipase A2; sPLA2, secretory PLA2; cPLA2, 85-kDa cytosolic PLA2; PKC, protein kinase C; COX, cyclooxygenase; TNF, tumor necrosis factor; CSF, colony-stimulating factor; IFNγ, interferon γ PMA, phorbol myristate acetate; CaLB, calcium-dependent phospholipid binding domain; MAP, mitogen-activated protein; MAPK, MAP kinase. enzymes hydrolyze fatty acid from the sn-2 position of phospholipid with the concomitant production of lysophospholipid. Mammalian cells contain structurally diverse forms of PLA2 including secretory PLA2 (sPLA2), calcium-independent PLA2, and the 85-kDa cytosolic PLA2(cPLA2) (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar, 2Dennis E.A. J. Biol. Chem. 1994; 269: 13057-13060Abstract Full Text PDF PubMed Google Scholar, 3Kudo I. Murakami M. Hara S. Inoue K. Biochim. Biophys. Acta. 1993; 177: 217-231Crossref Scopus (371) Google Scholar, 4Ackermann E.J. Dennis E.A. Biochim. Biophys. Acta. 1995; 1259: 125-136Crossref PubMed Scopus (129) Google Scholar). PLA2 enzymes function in the digestion of dietary lipid, microbial degradation, and regulation of phospholipid acyl turnover either in a housekeeping role for membrane repair or for the production of inflammatory lipid mediators. The presence of diverse PLA2 enzymes in mammalian cells provides multiple, differentially regulated pathways for the important process of fatty acid turnover. This review will focus on the biochemical properties and regulation of cPLA2, which plays an important role in mediating arachidonic acid release. cPLA2 shares no homology with other PLA2enzymes and is the only well characterized PLA2 that preferentially hydrolyzes sn-2 arachidonic acid (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar, 5Sharp J.D. White D.L. Chiou X.G. Goodson T. Gamboa G.C. McClure D. Burgett S. Hoskins J. Skatrud P.L. Sportsman J.R. Becker G.W. Kang L.H. Roberts E.F. Kramer R.M. J. Biol. Chem. 1991; 266: 14850-14853Abstract Full Text PDF PubMed Google Scholar, 6Clark J.D. Lin L.-L. Kriz R.W. Ramesha C.S. Sultzman L.A. Lin A.Y. Milona N. Knopf J.L. Cell. 1991; 65: 1043-1051Abstract Full Text PDF PubMed Scopus (1462) Google Scholar). It should be noted, however, that although sPLA2 and calcium-independent PLA2 do not exhibit acyl chain specificity they can also mediate arachidonic acid release depending on the cell type and agonist involved (3Kudo I. Murakami M. Hara S. Inoue K. Biochim. Biophys. Acta. 1993; 177: 217-231Crossref Scopus (371) Google Scholar, 4Ackermann E.J. Dennis E.A. Biochim. Biophys. Acta. 1995; 1259: 125-136Crossref PubMed Scopus (129) Google Scholar). Arachidonic acid is itself an important regulator of specific cellular processes including regulation of PKC and phospholipase Cγ and modulation of Ca2+ transients (7Shuttleworth T.J. J. Biol. Chem. 1996; 271: 21720-21725Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar, 8Damron D.S. Van Wagoner D.R. Moravec C.S. Bond M. J. Biol. Chem. 1993; 268: 27335-27344Abstract Full Text PDF PubMed Google Scholar, 9Graber M.N. Alfonso A. Gill D.L. J. Biol. Chem. 1996; 271: 883-888Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar, 10McPhail L.C. Clayton C.C. Snyderman R. Science. 1984; 224: 622-625Crossref PubMed Scopus (494) Google Scholar, 11Hwang S.C. Jhon D.-Y. Bae Y.S. Kim J.H. Rhee S.G. J. Biol. Chem. 1996; 271: 18342-18349Abstract Full Text Full Text PDF PubMed Scopus (159) Google Scholar). Arachidonic acid can also be converted to potent inflammatory lipid mediators, the eicosanoids. This can occur enzymatically through the lipoxygenase or cyclooxygenase (COX) pathways for the production of leukotrienes, lipoxins, thromboxanes, or prostaglandins (12Smith W. DeWitt D. Adv. Immunol. 1996; 62: 166-215Google Scholar, 13Samuelsson B. Dhalen S.-E. Lindgren J.A. Rouzer C.A. Serhan C.N. Science. 1991; 237: 1171-1176Crossref Scopus (1984) Google Scholar). Arachidonic acid is also subject to non-enzymatic, free radical oxidation to bioactive isoprostanes and isoleukotrienes (14Murphy R.C. Nigam P. Kunkel G. Prescott S. Vargaftig B. Platelet-activating Factor and Related Lipid Mediators in Health and Disease. Plenum Press, New York1997: 51-58Google Scholar, 15Harrison K.A. Murphy R.C. J. Biol. Chem. 1995; 270: 17273-17278Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar). The important role of arachidonic acid in cellular activation ensures that its levels are tightly controlled. cPLA2 plays a role in maintaining arachidonate levels and is subject to complex mechanisms of regulation at both the transcriptional and post-translational levels. The involvement of cPLA2 in lipid mediator production makes it a potentially important pharmacological target for anti-inflammatory drugs. cPLA2 is a widely distributed enzyme, and the transcript is expressed at a fairly constant level in all human tissues with somewhat elevated levels in lung and hippocampus (16Miyashita A. Crystal R.G. Hay J.G. Nucleic Acids Res. 1995; 23: 293-301Crossref PubMed Scopus (59) Google Scholar, 17Sharp J.D. White D.L. J. Lipid Mediators. 1993; 8: 183-189PubMed Google Scholar). At the cellular level, cPLA2 is enriched in mononuclear phagocytes but has been found in most cells examined. One notable exception is mature T and B lymphocytes that do not contain cPLA2, whereas it is present in thymocytes and immature B cells (18Gilbert J.J. Stewart A. Courtney C.-A. Fleming M.-C. Reid P. Jackson C.G. Wise A. Wakelam M.J.O. Harnett M.M. J. Immunol. 1996; 156: 2054-2061PubMed Google Scholar). This suggests that expression of cPLA2 may be incompatible with the function of mature lymphocytes. The ubiquitous expression of cPLA2 is consistent with features of the 5′-flanking region of the human gene, which has certain characteristics of a housekeeping promoter in that it has no TATA box (16Miyashita A. Crystal R.G. Hay J.G. Nucleic Acids Res. 1995; 23: 293-301Crossref PubMed Scopus (59) Google Scholar, 19Tay A. Maxwell P. Li Z.G. Goldberg H. Skorecki K. Biochim. Biophys. Acta. 1994; 1217: 345-347Crossref PubMed Scopus (54) Google Scholar, 20Wu T. Ikezono T. Angus C.W. Shelhamer J.H. Nucleic Acids Res. 1994; 22: 5093-5098Crossref PubMed Scopus (75) Google Scholar). However, unlike many housekeeping promoters, it does not contain a GC-rich region or SP1 sites. The 5′-flanking region of the cPLA2 gene contains a 27-base pair region with a polypyrimidine sequence that is responsible for the basal expression of cPLA2 (16Miyashita A. Crystal R.G. Hay J.G. Nucleic Acids Res. 1995; 23: 293-301Crossref PubMed Scopus (59) Google Scholar). In addition, a 48-base purine/pyrimidine repeat (CA repeat) appears to confer an inhibitory effect on cPLA2 gene transcription (20Wu T. Ikezono T. Angus C.W. Shelhamer J.H. Nucleic Acids Res. 1994; 22: 5093-5098Crossref PubMed Scopus (75) Google Scholar). By genetic analysis, the human cPLA2 gene has been localized to chromosome 1q31-41 between markers F13B and D1S74 (16Miyashita A. Crystal R.G. Hay J.G. Nucleic Acids Res. 1995; 23: 293-301Crossref PubMed Scopus (59) Google Scholar). However, by structural analysis it has been mapped to chromosome 1q25 (21Tay A. Simon J.S. Squire J. Hamel K. Jacob H.J. Skorecki K. Genomics. 1995; 26: 138-141Crossref PubMed Scopus (82) Google Scholar). Of interest, the COX 2 gene has also been mapped to the 1q25 region (22Tay A. Squire J.A. Goldberg H. Skorecki K. Genomics. 1994; 23: 718-719Crossref PubMed Scopus (50) Google Scholar). A variety of cytokines and mitogens such as interleukin-1, tumor necrosis factor (TNF), colony-stimulating factor (CSF), epidermal growth factor, c-Kit ligand, and interferon γ (IFNγ) have been shown to induce activation and increase synthesis of cPLA2in diverse cell models (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). In WI-38 cells, the increase in cPLA2 synthesis induced by interleukin-1 correlates with PGE2 production, and both effects are suppressed by glucocorticoids (23Lin L.-L. Lin A.Y. DeWitt D.L. J. Biol. Chem. 1992; 267: 23451-23454Abstract Full Text PDF PubMed Google Scholar). In some models, there is coordinate up-regulation of both cPLA2 and COX 2 (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). The induction of cPLA2 gene expression by IFNγ in an epithelial cell line is proposed to occur at the transcriptional level (24Wu T. Levine S.J. Lawrence M.G. Logun C. Angus C.W. Shelhamer J.H. J. Clin. Invest. 1994; 93: 571-577Crossref PubMed Scopus (87) Google Scholar). Consistent with this, the promoter for human cPLA2 contains a putative IFNγ-activated sequence and IFNγ response elements (20Wu T. Ikezono T. Angus C.W. Shelhamer J.H. Nucleic Acids Res. 1994; 22: 5093-5098Crossref PubMed Scopus (75) Google Scholar). In addition, DNA sequence analysis has revealed two glucocorticoid response elements suggesting that steroids may act to suppress cPLA2 synthesis at the transcriptional level. There is also evidence for post-transcriptional regulation of cPLA2synthesis. In rat mesangial cells, phorbol myristate acetate (PMA), platelet-derived growth factor, and serum and epidermal growth factor increase the half-life of cPLA2 mRNA (25Tay A. Maxwell P. Li Z.-G. Goldberg H. Skorecki K. Biochem. J. 1994; 304: 417-422Crossref PubMed Scopus (39) Google Scholar). An adenosine-uridine-rich sequence in the 3′-untranslated region of cPLA2 is thought to be responsible for the instability of the cPLA2 transcript. Stimulation of cPLA2synthesis occurs over hours and results in the prolonged release of arachidonic acid and eicosanoid production. Although cPLA2 and sPLA2 catalyze the same reaction their catalytic mechanisms are different. cPLA2has multiple enzymatic activities exhibiting in addition to PLA2 activity high lysophospholipase activity and weak transacylase activity (26Leslie C.C. J. Biol. Chem. 1991; 266: 11366-11371Abstract Full Text PDF PubMed Google Scholar, 27Reynolds L.J. Hughes L.L. Louis A.I. Kramer R.M. Dennis E.A. Biochim. Biophys. Acta. 1993; 1167: 272-280Crossref PubMed Scopus (151) Google Scholar, 28Hanel A.M. Gelb M.H. Biochemistry. 1995; 34: 7807-7818Crossref PubMed Scopus (42) Google Scholar). cPLA2 can also hydrolyze fatty acid ester of 7-hydroxycoumarin, which has been used as a substrate for continuously monitoring the activity of pure cPLA2 (29Huang Z. Laliberte F. Tremblay N.M. Weech P.K. Street I.P. Anal. Biochem. 1994; 222: 110-115Crossref PubMed Scopus (45) Google Scholar). Although the transacylase activity may not have physiological relevance, there is evidence that cPLA2 may function as a lysophospholipase in cells (30de Carvalho M.G.S. Garritano J. Leslie C.C. J. Biol. Chem. 1995; 270: 20439-20446Abstract Full Text Full Text PDF PubMed Scopus (55) Google Scholar). The coordinate release of sn-2 arachidonic acid followed by the sn-1 fatty acid of diacylphospholipids provides an efficient mechanism for controlling the levels of potentially cytotoxic lysophospholipids. A lysophospholipase with immunological identity to cPLA2 has recently been purified from bovine brain and exhibits both high activity and selectivity for arachidonoyl-substituted lysophosphatidylcholine (31Pete M.J. Exton J.H. J. Biol. Chem. 1996; 271: 18114-18121Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar). It is suggested that the combined action of brain phospholipase A1 and this lysophospholipase may provide significant amounts of arachidonic acid in the brain. The transacylase activity provides evidence that the cPLA2reaction proceeds by formation of an acyl-enzyme intermediate, either as an acyl-serine or acyl-cysteine. cPLA2, which contains 9 cysteine residues, is most stable in the presence of reducing agents (32Li B. Copp L. Castelhano A.L. Feng R. Stahl M. Yuan Z. Krantz A. Biochemistry. 1994; 33: 8594-8603Crossref PubMed Scopus (19) Google Scholar). This is in contrast to sPLA2 enzymes, which are stabilized by disulfide linkages and inactivated by reducing agents. Sulfhydryl-modifying reagents such as iodoacetamide or dithiobisnitrobenzoic acid inactivate cPLA2; however, the cysteine residues are not essential for catalytic activity based on mutagenesis studies (32Li B. Copp L. Castelhano A.L. Feng R. Stahl M. Yuan Z. Krantz A. Biochemistry. 1994; 33: 8594-8603Crossref PubMed Scopus (19) Google Scholar, 33Sharp J.C. Pickard R.T. Chiou X.G. Manetta J.V. Kovacevic S. Miller J.R. Varshavsky A.D. Roberts E.F. Strifler B.A. Brems D.N. Kramer R.M. J. Biol. Chem. 1994; 269: 23250-23254Abstract Full Text PDF PubMed Google Scholar). Cys331 is the residue that is targeted by sulfhydryl-modifying reagents leading to a loss of cPLA2 activity, which suggests that it is located in a sensitive region of cPLA2, possibly near the active site (34Li B. Xia L. Krantz A. Yuan Z. Biochemistry. 1996; 35: 3156-3161Crossref PubMed Scopus (10) Google Scholar, 35Pickard R.T. Chiou X.G. Strifler B.A. DeFelippis M.R. Hyslop P.A. Tebbe A.L. Yee Y.K. Reynolds L.J. Dennis E.A. Kramer R.M. Sharp J.D. J. Biol. Chem. 1996; 271: 19225-19231Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar) (Fig. 1). Several lines of evidence have demonstrated that cPLA2contains a serine nucleophile at the active site. NMR analysis of cPLA2 complexed with an inhibitory trifluoromethylketone analogue of arachidonic acid has revealed a similarity with α-chymotrypsin complexed with a peptidyltrifluoromethyl ketone inhibitor (36Trimble L.A. Street I.P. Perrier H. Tremblay N.M. Weech P.K. Bernstein M.A. Biochemistry. 1993; 32: 12560-12565Crossref PubMed Scopus Google Scholar). This suggests that the cPLA2 complex as a and an active site some sequence homology with phospholipase B which exhibits both lysophospholipase of a serine in cPLA2 that with a sequence of phospholipase B both PLA2 and lysophospholipase activities suggesting a role for in J.C. Pickard R.T. Chiou X.G. Manetta J.V. Kovacevic S. Miller J.R. Varshavsky A.D. Roberts E.F. Strifler B.A. Brems D.N. Kramer R.M. J. Biol. Chem. 1994; 269: 23250-23254Abstract Full Text PDF PubMed Google Scholar) (Fig. 1). The role of as the active site nucleophile has recently been Z. P. K. Biochemistry. 1996; 35: PubMed Scopus Google Scholar). cells are with COX and forms of cPLA2 in which is with or they provide arachidonic acid for production in response to unlike cells with type Although the cPLA2 is on phospholipid it exhibits activity unlike the results an acyl-enzyme and that can be by cysteine as the active site nucleophile with catalytic located in the sequence that is proposed to be an active site sequence for a of serine Sharp A.M. Biochem. 1993; Full Text PDF PubMed Scopus Google Scholar). and are also essential for cPLA2 activity R.T. Chiou X.G. Strifler B.A. DeFelippis M.R. Hyslop P.A. Tebbe A.L. Yee Y.K. Reynolds L.J. Dennis E.A. Kramer R.M. Sharp J.D. J. Biol. Chem. 1996; 271: 19225-19231Abstract Full Text Full Text PDF PubMed Scopus (96) Google Scholar) (Fig. 1). The is with the acid residue in the catalytic site of the a residue does not to a role in cPLA2 The mechanism for the involvement of is not cPLA2contains a catalytic that is responsible for its multiple catalytic cPLA2 for however, unlike sPLA2, is necessary for binding membrane or phospholipid for cPLA2 contains an calcium-dependent phospholipid binding that shares homology with the in the of PKC, phospholipase and to a (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar) (Fig. 1). An of are and many a role in and membrane E.A. J.J. 1996; PubMed Scopus Google Scholar). The of has been the most characterized and contains residues that coordinate two B.A. A.M. Cell. 1995; Full Text PDF PubMed Scopus Google Scholar, B.A. J. Science. 1996; PubMed Scopus Google Scholar). The of cPLA2 from acid residue to E.A. J.J. 1996; PubMed Scopus Google Scholar). of the that are thought to Ca2+ are present in the of cPLA2 and The residue in cPLA2 is a E.A. J.J. 1996; PubMed Scopus Google Scholar). at binding of cPLA2 to membrane and correlates with the of necessary to catalytic phospholipid as substrate J.D. Lin L.-L. Kriz R.W. Ramesha C.S. Sultzman L.A. Lin A.Y. Milona N. Knopf J.L. Cell. 1991; 65: 1043-1051Abstract Full Text PDF PubMed Scopus (1462) Google Scholar, J. Leslie C.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). A of cPLA2 the membrane at the same of for the enzyme, whereas a the to membrane but catalytic activity phospholipid E.A. Sultzman L.A. Kriz R.W. Knopf J.L. J.D. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). This two structural and functional of The properties of cPLA2 are consistent with the in cells at and to membrane levels It is well that arachidonic acid release is in cells by many (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). In a rat cell cPLA2 is cytosolic in cells but to membrane in response to or S. Carvalho T. M. Leslie C.C. Gelb M.H. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). The of membrane binding correlates with the of arachidonic acid which is with with In cells cPLA2, of the to and which is on the A.R. Lin L.-L. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus (425) Google Scholar). In of the site of MAP kinase has no effect on cPLA2 although it arachidonic acid release. The of cPLA2 can be on cell M.R. J.R. J.S. Invest. 1996; Google Scholar). In cells, cPLA2 is but in cells most of the cPLA2 is of and cells to a variety of of to a region and the In the cells to of the This is the cell in which cPLA2 has been shown to be with the In rat type cells, of cPLA2 to membrane results in loss of arachidonic acid from this site functional activation of cPLA2 at the membrane M. K. T. T. Biochem. J. 1996; PubMed Scopus Google Scholar). and COX 2 are also with membrane suggesting that enzymes a functional complex for the production of C.N. Leslie C.C. J. 1996; PubMed Scopus Google Scholar). It also that there may be a specific role for eicosanoid production at the of cPLA2 to the membrane suggests that there may be specific or binding that mediate this However, it should be in that cPLA2 is also important in mediating eicosanoid production in which do not contain a R.M. Roberts E.F. Manetta J.V. Hyslop P.A. J.A. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). In many cell a stable increase in cPLA2 activity occurs to diverse and can be by (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). of cPLA2 on serine residues has been and the cPLA2 activity in cells is to on by L.-L. Lin A.Y. Knopf J.L. S. A. 1992; PubMed Scopus Google Scholar, Carvalho Leslie C.C. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar, Carvalho M.G.S. A.L. F. Gelb M.H. J.R. Leslie C.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). cPLA2 has site for and of this site in by either or and a in its or S. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar, L.-L. M. Lin A.Y. Knopf J.L. A. Cell. 1993; Full Text PDF PubMed Scopus Google Scholar) (Fig. 1). of is important in the activation of cPLA2 in of cPLA2 in cells to arachidonic acid release as type is expressed L.-L. M. Lin A.Y. Knopf J.L. A. Cell. 1993; Full Text PDF PubMed Scopus Google Scholar). such as PKC and protein kinase A can cPLA2 in but this does not in a significant increase in cPLA2 activity or a However, there is evidence for a role for PKC in the activation of PLA2 and regulation of arachidonic acid release. PKC activation can a role by a kinase leading to activation Leslie C.C. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). There is no evidence at this that PKC cPLA2 in and activation of cPLA2 correlates with the activation of in many cell models (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). However, it has been shown that of cPLA2 in or and occurs of or activation Biochem. J. 1995; PubMed Scopus Google Scholar, Biochem. Biophys. Res. 1995; PubMed Scopus Google Scholar, R.M. Roberts E.F. Hyslop P.A. J.A. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). Since cPLA2 exhibits a and activity in models, it suggests that the is by a kinase on and kinase has recently been R.M. Roberts E.F. Strifler B.A. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google J.L. Biochem. J. 1996; PubMed Scopus Google Scholar). The and can cPLA2 in and induce a cPLA2 on M. Z. J. Leslie C. J. 1996; Scholar). However, it has recently been shown in an inhibitor of kinase that of cPLA2 is not for of arachidonic acid in response to R.M. Roberts E.F. M.J. J.A. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). This suggests that mechanisms are involved in functional activation of cPLA2 in or that an increase in is Although on is important for cPLA2 activation in certain cells it is not for activation leading to arachidonic acid release (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). In is a of and and an increase in activity of However, does not induce arachidonic acid but it with C. Z. Leslie C. S. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar). This suggests that an increase in necessary for cPLA2 and cPLA2 which enzymatic activity, can act to of the increase in Ca2+ to the for arachidonic acid release in cells, and this suggests that cPLA2 may not be for it is to membrane C.G. G. R. H. J. Biochem. J. 1996; PubMed Scopus (55) Google Scholar). However, there is evidence suggesting pathways for regulation of cPLA2 activation and arachidonic acid release. In and can induce an increase in and catalytic activity of cPLA2 as well as arachidonic acid and this occurs an increase in Carvalho Leslie C.C. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar, J. J.D. P. J. Clin. Invest. 1994; 93: PubMed Scopus Google Scholar, Z. Leslie C. J. 1996; Scholar). However, in other cell models arachidonic acid release of cPLA2 and be combined with a agonist L.-L. Lin A.Y. Knopf J.L. S. A. 1992; PubMed Scopus Google Scholar). This suggests that there are pathways for regulation of cPLA2 activation and arachidonic acid release that are acid also arachidonic acid release in and this correlates with of a site on cPLA2 Z. Leslie C. J. 1996; Scholar). cPLA2 expressed in cells with on cPLA2 have been Carvalho M.G.S. A.L. F. Gelb M.H. J.R. Leslie C.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). cPLA2 is expressed as an in cells it is on Carvalho M.G.S. A.L. F. Gelb M.H. J.R. Leslie C.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, K. P. F. Z. Street I. Biochim. Biophys. Acta. 1995; PubMed Scopus Google Scholar). However, cells cPLA2 exhibit release of arachidonic but release can be by with acid or Carvalho M.G.S. A.L. F. Gelb M.H. J.R. Leslie C.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). The residues on the expressed cPLA2 have been as and Carvalho M.G.S. A.L. F. Gelb M.H. J.R. Leslie C.C. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar) (Fig. 1). and are the on the from In response to an increase in preferentially occurs on only a increase in of all the sites. There is also evidence that is on cPLA2 in human and Since acid does not induce an increase in the results mechanisms for cPLA2 activation of and are in cPLA2 from consistent with important functional (1Clark J.D. Schievella A.R. Nalefski E.A. Lin L.-L. J. Lipid Mediat. Cell Signal. 1995; 12: 83-117Crossref PubMed Scopus (425) Google Scholar). The kinase that has not been but it is not a MAP kinase a sequence for a kinase There is evidence that cPLA2 is also on In with growth factor cPLA2 is in R. G. F. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). In cells, cPLA2 is in cells, and this with S.J. J. 1996; PubMed Scopus Google Scholar). the kinase with cPLA2 in cells, and this is not on arachidonic acid release in cells, and has been shown to be for this In cells, cPLA2 is for of gene factor S.J. J. 1996; PubMed Scopus Google Scholar). The mechanism cPLA2 this response is not but is not to an arachidonic acid The results of cPLA2 are and by acid In models in which serine of cPLA2 has been by acid analysis, no of cPLA2 has been This suggests that the is a cell and cPLA2 plays a role in mediating forms of cell In some models the mechanism involved has not been whereas in other models, arachidonic acid are has been shown to induce activation and synthesis of cPLA2 in and human A. J. S. J. 1996; 270: PubMed Google Scholar). is a potent of and this is thought to be important in its to act as a tumor The production of plays an important role in mediating and cPLA2 is the responsible for production in human A. J. S. J. 1996; 270: PubMed Google Scholar). In human correlates with synthesis of cPLA2, which is by formation of free is to induce a potent cPLA2 synthesis is by and agents can increase cPLA2synthesis A. A. Biochim. Biophys. Acta. 1996; PubMed Scopus Google Scholar). results that cPLA2 is regulated by through generation of free and plays an important role in In other models, cellular have also been shown to In cells, serine and activation of cPLA2 R.W. Biochim. Biophys. Acta. 1995; PubMed Scopus Google Scholar). In this and arachidonic acid production are in the induction of and and the response induced by In of epithelial cells with been shown to cPLA2, which to A. R. J.V. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). In cells either cPLA2 or sPLA2, only cells cPLA2 are to cell Of interest, and is consistent with the role of in cPLA2 However, with to not induce suggesting that other cellular induced by membrane lipid with cPLA2 activation to the The that arachidonic acid does not and that the cells that may not to cell cPLA2 is also involved in cell is an important that the response to J. Cell. Biochem. 1995; PubMed Scopus Google Scholar). cPLA2 and the arachidonic acid is as a regulator of activation leading to production. it has been shown that a cell line is to but this response is cPLA2 is expressed M. N. K. S. T. N. F. M. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar). However, is of cPLA2 this cell well in or cells M. H. M. F. S. J. Biochem. 1996; PubMed Scopus Google Scholar). the and type act through pathways for cell are not by TNF, but some can be to in with of transcription or and this also correlates with the level of cPLA2 present C. J. Immunol. 1996; 156: Google Scholar). The role that cPLA2 plays in mediating cell has not been In cPLA2 is subject to diverse mechanisms of regulation that are only to cPLA2 can be by post-translational mechanisms to arachidonic acid for eicosanoid production. It is also subject to synthesis and activation leading to prolonged production of lipid mediators. mechanisms of regulation are important in the diverse functional of on the or cell cPLA2 has been to function in cellular such as and cPLA2 can a functional role through its to arachidonic acid release and eicosanoid production. However, in many the mechanisms have not been and it is that cPLA2 may act by in membrane or by important for