A natural disruption of the secretory group II phospholipase A2 gene in inbred mouse strains

Agonist-specific alterations in receptor-phospholipase coupling following inactivation of Gi2alpha gene

Different forms of phospholipase A2, together with pertussis toxin-sensitive G-proteins, [Ca2+]i (intracellular Ca2+ concentration), protein kinase C, calmodulin, protein tyrosine kinases, mitogen-activated protein kinases and Ca2+/calmodulin-dependent protein kinase appear to play a role in agonist-mediated release of arachidonic acid. Here we report that fibroblasts from 14-day-old mouse embryos with inactivated Gi2alpha (alpha-subunit of the heterotrimeric G-protein Gi2) gene display a marked decrease in the ability of lysophosphatidic acid, thrombin and Ca2+ ionophores to release arachidonic acid compared with their normal counterparts. The requirement for Gi2alpha in the release of arachidonic acid following increased [Ca2+]i may be explained by the incomplete translocation of cytosolic phospholipase A2 observed in Gi2alpha-deficient cells. Paradoxically, inactivation of the Gi2alpha gene resulted in up-regulation of bradykinin receptors and their coupling to increased arachidonic acid release, phospholipase C activity and [Ca2+]i. A concomitant increase in basal phospholipase C activity was also observed in the Gi2alpha-deficient cells. These observations establish a pleiotropic and essential role for Gi2alpha in receptor-phospholipase coupling that contrasts with its less obligatory participation in agonist-mediated inhibition of adenylate cyclase.

Mice lacking secretory phospholipase A2 show altered apoptosis and differentiation with Helicobacter felis infection

BACKGROUND & AIMS

Infection with Helicobacter pylori uniformly leads to a chronic superficial gastritis that may progress to atrophic gastritis, a premalignant process. A mouse model of Helicobacter felis infection was used to study possible genetic determinants of the response to infection.

METHODS

Three inbred mouse strains with known secretory phospholipase A2 (sPLA2) genotypes [BALB/c (+/+), C3H/HeJ (+/+), and C57BL/6 (-/-)] were orally infected with H. felis and examined longitudinally using routine histology, immunocytochemistry, electron microscopy, proliferating cell nuclear antigen, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, and Northern and Western blot studies.

RESULTS

Only the C57BL/6 strain showed increased gastric fundic proliferation and apoptosis in response to infection. In addition, the C57BL/6 mouse showed a marked loss of parietal and chief cells, along with a marked expansion of an aberrant gastric mucous cell lineage that stained positive for spasmolytic polypeptide. In contrast, no significant change in these cell types was observed in BALB/c and C3H/HeJ strains. Increased expression of sPLA2 was observed in BALB/c and C3H/HeJ after H. felis infection, whereas sPLA2 expression was absent in C57BL/6 mice.

CONCLUSIONS

H. felis infection leads to increased apoptosis and altered cellular differentiation in the C57BL/6 mouse, a strain that lacks gastric sPLA2 expression. Because sPLA2 has been identified recently as the MOM1 (modifier of MIN) locus that influences polyp formation in the colon, these studies suggest that sPLA2 may also influence the gastric epithelial response to Helicobacter infection.

Cytosolic phospholipase A2 is required for cytokine-induced expression of type IIA secretory phospholipase A2 that mediates optimal cyclooxygenase-2-dependent delayed prostaglandin E2 generation in rat 3Y1 fibroblasts

Activation of rat fibroblastic 3Y1 cells with interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) induced delayed prostaglandin (PG) E2 generation over 6-48 h, which occurred in parallel with de novo induction of type IIA secretory phospholipase A2 (sPLA2) and cyclooxygenase (COX)-2, without accompanied by changes in the constitutive expression of type IV cytosolic PLA2 (cPLA2) and COX-1. Types V and IIC sPLA2s were barely detectable in these cells. Studies using an anti-type IIA sPLA2 antibody, sPLA2 inhibitors, and a type IIA sPLA2-specific antisense oligonucleotide revealed that IL-1 beta/TNF alpha-induced delayed PGE2 generation by these cells was largely dependent on inducible type IIA sPLA2, which was functionally linked to inducible COX-2. Delayed PGE2 generation was also suppressed markedly by the cPLA2 inhibitor arachidonoyl trifluoromethyl ketone (AACOCF3), which attenuated induction of type IIA sPLA2, but not COX-2, expression. AACOCF3 inhibited the initial phase of cytokine-stimulated arachidonic acid release, and supplementing AACOCF3-treated cells with exogenous arachidonic acid partially restored type IIA sPLA2 expression. These results suggest that certain metabolites produced by the cPLA2-dependent pathway are crucial for the subsequent induction of type IIA sPLA2 expression and attendant delayed PGE2 generation. Some lipoxygenase-derived products might be involved in this event, since IL-1 beta/TNF alpha-induced type IIA sPLA2 induction and PGE2 generation were reduced markedly by lipoxygenase, but not COX, inhibitors. In contrast, Ca2+ ionophore-stimulated immediate PGE2 generation was regulated predominantly by the constitutive enzymes cPLA2 and COX-1, even when type IIA sPLA2 and COX-2 were maximally induced after IL-1 beta/TNF alpha treatment, revealing functional segregation of the constitutive and inducible PG biosynthetic enzymes.

Resistance to endotoxic shock in phospholipase A2 receptor-deficient mice

Mammals possess various types of secretory phospholipase A2, which differ in the primary structure and tissue distribution. The phosholipase A2 receptor (PLA2R) recognizes group IB phospholipase A2 (PLA2-IB) and mediates the PLA2-IB-induced biological responses in non-digestive organs, including eicosanoid production and contraction of airway smooth muscles. In this study, we generated PLA2R-deficient mice to define its biological roles further. These mice are viable, fertile, and without evident histopathological abnormalities. There was no difference in the clearance of circulating PLA2-IB between wild-type and mutant mice. After challenge with bacterial lipopolysaccharide (LPS), PLA2R-deficient mice exhibited longer survival than wild-type mice. The mutant mice were also resistant to lethal effects of exogenous PLA2-IB after sensitization with sublethal dose of LPS. The plasma levels of tumor necrosis factor-alpha and interleukin-1beta elevated after LPS treatment were significantly reduced in mutant mice compared with wild-type mice. These findings suggest a potential role of PLA2R in the progression of endotoxic shock.

Role of cytosolic phospholipase A2 in allergic response and parturition

Phospholipase A2 (PLA2) comprises a superfamily of enzymes that hydrolyse the ester bond of phospholipids at the sn-2 position. Among the members of this superfamily, cytosolic PLA2 has attracted attention because it preferentially hydrolyses arachidonoyl phospholipids and is activated by submicromolar concentrations of Ca2+ ions and by phosphorylation by mitogen-activated protein kinases (MAP kinases). Here we investigate the function of cytosolic PLA2 in vivo by using homologous recombination to generate mice deficient in this enzyme. These mice showed a marked decrease in their production of eicosanoids and platelet-activating factor in peritoneal macrophages. Their ovalbumin-induced anaphylactic responses were significantly reduced, as was their bronchial reactivity to methacholine. Female mutant mice failed to deliver offspring, but these could be rescued by administration of a progesterone-receptor antagonist to the mother at term. Considered together with previous findings, our results indicate that cytosolic PLA2 plays a non-redundant role in allergic responses and reproductive physiology.

Detection of secretory phospholipase A2s related but not identical to type IIA isozyme in cultured mast cells

We previously reported that BALB/cJ mouse-derived bone marrow-derived mast cells (BMMC) exhibited two sequential phases of prostaglandin D2 (PGD2) generation in response to Fc(epsilon) receptor I (Fc(epsilon)RI) crosslinking and cytokine stimulation, the late phase of which was suppressed by an antibody raised against type IIA secretory phospholipase A2 (sPLA2). Here we report that BMMC derived from C57BL/6J mice, which are genetically deficient in type IIA sPLA2, display both immediate and delayed PGD2 generation normally. Lysates of C57BL/6J-derived BMMC contained a Ca2+-dependent PLA2 that was absorbed to a column conjugated with anti-type IIA sPLA2 antibody and had a similar molecular mass of 14 kDa, as assessed by immunoblotting. Therefore we speculate that a sPLA2 similar to, but distinct from, type IIA sPLA2 would compensate for type IIA sPLA2 deficiency in C57BL/6J-derived BMMC. We found that the two type IIA-related sPLA2 family members, type V and type IIC sPLA2s, were expressed in BMMC as well as in rat mastocytoma RBL-2H3 cells.

Prostaglandin E2 amplifies cytosolic phospholipase A2- and cyclooxygenase-2-dependent delayed prostaglandin E2 generation in mouse osteoblastic cells. Enhancement by secretory phospholipase A2

We used the MC3T3-E1 cell line, which originates from C57BL/6J mouse that is genetically type IIA secretory phospholipase A2 (sPLA2)-deficient, to reveal the type IIA sPLA2-independent route of the prostanglandin (PG) biosynthetic pathway. Kinetic and pharmacological studies showed that delayed PGE2 generation by this cell line in response to interleukin (IL)-1beta and tumor necrosis factor alpha (TNFalpha) was dependent upon cytosolic phospholipase A2 (cPLA2) and cyclooxygenase (COX)-2. Expression of these two enzymes was reduced by cPLA2 or COX-2 inhibitors and restored by adding exogenous arachidonic acid or PGE2, indicating that PGE2 produced by these cells acted as an autocrine amplifier of delayed PGE2 generation through enhanced cPLA2 and COX-2 expression. Exogenous addition or enforced expression of type IIA sPLA2 significantly increased IL-1beta/TNFalpha-initiated PGE2 generation, which was accompanied by increased expression of both cPLA2 and COX-2 and suppressed by inhibitors of these enzymes. Thus, our results revealed a particular cross-talk between the two PLA2 enzymes and COX-2 for delayed PGE2 biosynthesis by a type IIA sPLA2-deficient cell line. cPLA2 is responsible for initiating COX-2-dependent delayed PGE2 generation, and sPLA2, if introduced, enhances PGE2 generation by increasing cPLA2 and COX-2 expression via endogenous PGE2.

Expression of human group II phospholipase A2 in transgenic mice

Group II phospholipase A2 (PLA2) has been proposed to play an important role in inflammation and defense against bacterial infection. We investigated tissues of transgenic mice expressing the human group II PLA2 gene by immunohistochemistry using rabbit anti-human group II PLA2 antibodies, and by in situ hybridization by probing with human group II PLA2 mRNA anti-sense (test) and sense (control) riboprobes. By immunohistochemistry, human group II PLA2 was found in various mouse tissues and cell types including hepatocytes, proximal tubule cells of the kidney, epithelial cells of the renal pelvis, urinary bladder and ureter, granulosa cells of Graafian follicles, aortic intima and media, cartilage, epiphyseal bone, bronchial epithelial cells, and connective tissue cells in the dermis. By in situ hybridization, group II PLA2 mRNA was localized in hepatocytes, epidermal cells, dermal cells, connective tissue fibroblasts, epithelial and smooth muscle cells of the urinary bladder, and cells of Bowman's capsule. These results show that human group II PLA2 is expressed in large amounts in hepatocytes and many extrahepatic tissues of the transgenic mice. These animals provide a useful new tool for studies on the metabolism, in vivo effects, and physiological and pathological roles of phospholipase A2.

A novel cytosolic calcium-independent phospholipase A2 contains eight ankyrin motifs

We report the purification, molecular cloning, and expression of a novel cytosolic calcium-independent phospholipase A2 (iPLA2) from Chinese hamster ovary cells, which lacks extended homology to other phospholipases. iPLA2 is an 85-kDa protein that exists as a multimeric complex of 270-350 kDa with a specific activity of 1 micromol/min/mg. The full-length cDNA clone encodes a 752-amino acid cytoplasmic protein with one lipase motif (GXS465XG) and eight ankyrin repeats. Expression of the cDNA in mammalian cells generates an active 85-kDa protein. Mutagenesis studies show that Ser465 and the ankyrin repeats are required for activity. We demonstrate that iPLA2 selectively hydrolyzes the sn-2 over sn-1 fatty acid by 5-fold for 1,2-dipalmitoyl phosphatidylcholine in a mixed micelle. Moreover, we found the fatty acid preference at the sn-2 position to be highly dependent upon substrate presentation. However, iPLA2 does have a marked preference for 1,2-dipalmitoyl phosphatidic acid presented in a vesicle, generating the lipid second messenger lysophosphatidic acid. Finally the enzyme is able to hydrolyze the acetyl moiety at the sn-2 position of platelet-activating factor.

Localization of group IIc low molecular weight phospholipase A2 mRNA to meiotic cells in the mouse

We use in situ hybridization to demonstrate that the testicular expression of a novel, mouse, low molecular weight phospholipase A2 (PLA2 Group IIc) mRNA is specific to cells undergoing meiosis. A complete cDNA (1421 bp) encoding the mouse Pla2g2c gene was generated with reverse transcription-PCR (RT-PCR) and 5' and 3' RACE (rapid amplification of cDNA ends) RT-PCR, and its nucleotide sequence was determined. Northern blots of RNA from different tissues revealed a single 1.6 kb transcript only in testis. In situ hybridization indicated that this mouse gene is transcribed mainly in pachytene spermatocytes, secondary spermatocytes, and round spermatids. Expression of the gene is seen in all stages of the seminiferous epithelium, especially in stages VI-VII.

Activation of cytosolic phospholipase A2 by platelet-derived growth factor is essential for cyclooxygenase-2-dependent prostaglandin E2 synthesis in mouse osteoblasts cultured with interleukin-1

The synthesis of prostaglandins (PGs) is regulated by the arachidonic acid release by phospholipase A2 (PLA2) and its conversion to PGs by cyclooxygenase (COX). In the present study, we examined the regulation of PG synthesis by interleukin (IL)-1alpha in primary mouse osteoblastic cells isolated from mouse calvaria. Although IL-1alpha greatly enhanced cox-2 mRNA expression and its protein levels, PGE2 was not produced until 24 h. When arachidonic acid was added to osteoblastic cells precultured with IL-1alpha for 24 h, PGE2 was produced within 10 min. Of several growth factors tested, platelet-derived growth factor (PDGF) specifically initiated the rapid synthesis of PGE2, which was markedly suppressed by a selective inhibitor of cox-2 (NS-398). In mouse osteoblastic cells, cytosolic PLA2 (cPLA2) mRNA and its protein were constitutively expressed and increased approximately 2-fold by IL-1alpha, but secretory PLA2 mRNA was not detected. PDGF rapidly stimulated PLA2 activity, which was blocked completely by a cPLA2 inhibitor (arachidonyltrifluoromethyl ketone). The PDGF-induced cPLA2 activation was accompanied by phosphorylation of its protein. These results indicate that cox-2 induction by IL-1alpha is not sufficient, but cPLA2 activation by PDGF is crucial for IL-1alpha-induced PGE2 synthesis in mouse osteoblasts.

Macrophage arachidonate release via both the cytosolic Ca(2+)-dependent and -independent phospholipases is necessary for cell spreading

We have observed that phospholipase A2 (PLA2) activation and arachidonate (AA) release are essential for monocyte/macrophage adherence and spreading. In this study, we addressed the relationship between AA release and cell adherence/spreading in murine resident peritoneal macrophages, and the roles of specific PLA2S in these processes. The PLA2-specific inhibitors, (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (BEL, specific for the Ca(2+)-independent PLA2 (iPLA2)) and methyl arachidonoyl fluorophosphonate (MAFP, specific for the Ca(2+)-dependent phospholipase (cPLA2)) inhibited AA release and cell spreading in a correlated fashion but only modestly decreased cell adherence. Cell spreading was normalized by the addition of AA to PLA2-inhibited cells. AA release during spreading was also inhibited by Ca2+ depletion or protein kinase C (PKC) inhibition, and was accompanied by increased (but transient) phosphorylation of cPLA2-Inhibition of macrophage spreading, however, only partially inhibited AA release. Moreover, constitutive AA release was seen in fully spread macrophages which was inhibited by BEL, but not MAFP or Ca2+ depletion. BEL also reversed the phenotype of fully spread cells. These data suggest that macrophage spreading requires the release of AA by the iPLA2 (which appears to be constitutively active) and cPLA2 (which appears to be stimulated by adherence/spreading). Maintenance of macrophage spreading, in contrast, appears to be principally dependent on the iPLA2.

Type II secretory phospholipase A2 associated with cell surfaces via C-terminal heparin-binding lysine residues augments stimulus-initiated delayed prostaglandin generation

Type II secretory phospholipase A2 (sPLA2) has been shown to be induced by a variety of proinflammatory stimuli and, therefore, has been implicated in the inflammatory process. In order to determine whether association of sPLA2 with cell surfaces via heparan sulfate proteoglycan is important for its effects on cellular functions, we have identified the critical domain in sPLA2 for heparin and cell surface binding and examined its role in cellular prostaglandin (PG) biosynthesis. Replacement of several conserved Lys residues in the C-terminal region of mouse and rat sPLA2s by Glu resulted in a marked reduction of their capacities to bind to heparin and mammalian cell surfaces without affecting their enzymatic activities toward dispersed phospholipid as a substrate. CHO cells stably transfected with wild-type sPLA2 released about twice as much arachidonic acid (AA) during culture for 10 h with fetal calf serum and interleukin-1beta than cells transfected with vector alone, whereas the ability to enhance AA release was impaired in sPLA2 mutants incapable of binding to cell surfaces. AA released by wild-type sPLA2-transfected CHO cells was metabolized to prostaglandin E2 via prostaglandin endoperoxide H synthase (PGHS)-2 after IL-1beta stimulation, revealing a particular functional linkage of sPLA2 to PGHS-2. In contrast, A23187-initiated immediate AA release over 30 min was not affected by sPLA2 overexpression. Taken together, these results suggest that sPLA2 expressed endogenously and anchored on cell surfaces via its C-terminal heparin-binding domain is involved in the PGHS-2-dependent delayed PG biosynthesis initiated by growth factors and cytokines during long term culture.

A heparin-sensitive phospholipase A2 and prostaglandin endoperoxide synthase-2 are functionally linked in the delayed phase of prostaglandin D2 generation in mouse bone marrow-derived mast cells

BALB/cJ mouse bone marrow-derived mast cells (BMMC) developed with interleukin (IL)-3 can be stimulated by c-kit ligand (KL) in the presence of IL-10 and IL-1beta for sequential immediate and delayed generation of prostaglandin (PG) D2 through utilization of constitutive prostaglandin endoperoxide synthase (PGHS) -1 and induced PGHS-2, respectively (Murakami, M., Matsumoto, R., Austen, K. F., and Arm, J. P. (1994) J. Biol. Chem. 269, 22269-22275). We now report that BALB/cJ BMMC stimulated with KL + IL-10 + IL-1beta also exhibit the biphasic release of [3H]arachidonic acid with an immediate phase over the first 10 min followed by a delayed phase from 2 to 7 h. The delayed phase of arachidonic acid release and of PGD2 generation was inhibited by heparin, which concomitantly released a phospholipase (PL) A2 from the cells into the supernatant. Both dexamethasone and a type II PLA2 inhibitor, 12-epi-scalaradial, suppressed delayed-phase PGD2 generation at concentrations that did not affect immediate eicosanoid generation. Transcripts for type IIA PLA2, as assessed by reverse transcription-polymerase chain reaction, were progressively induced in BALB/cJ BMMC treated for 2 to 7 h with KL + IL-10 + IL-1beta; the induction of these transcripts was down-regulated by 10(-6) M dexamethasone. The expression of steady-state transcripts and protein for cytosolic PLA2 (cPLA2) did not change. PGHS-2-dependent delayed-phase PGD2 generation elicited by IgE-dependent activation of BALB/cJ BMMC primed with KL + IL-10 was also accompanied by the induction of type IIA PLA2 transcripts and was suppressed by heparin, with concomitant release of PLA2 into the supernatant. However, both the direct, cytokine-stimulated and the cytokine-primed, IgE-dependent, delayed-phase PGD2 generation occurred in BMMC from C57BL/6J mice, which have a natural disruption of the type IIA PLA2 gene. Thus, kinetic, pharmacologic, and genetic analyses suggest that an inducible, heparin-sensitive PLA2, rather than cPLA2, provides arachidonic acid to concomitantly induced PGHS-2 for delayed-phase PGD2 biosynthesis in activated BMMC. Furthermore, this heparin-sensitive PLA2 likely represents a novel PLA2 or a new function for a known low molecular weight PLA2.

Release of secretory phospholipase A2 from rat neuronal cells and its possible function in the regulation of catecholamine secretion

Here we show that secretory phospholipase A2 (sPLA2) that is immunochemically indistinguishable from type II sPLA2 is (i) stored in neuroendocrine cells, (ii) released in response to neurotransmitters or depolarization, and (iii) involved in the regulation of catecholamine secretion by these cells. Rat brain synaptic vesicle fractions contained PLA2 activity, which was neutralized completely by an antibody raised against rat type II sPLA2. sPLA2 immunoreactive with anti-(type II sPLA2) antibody was released from synaptosomes in response to depolarization evoked by a high concentration of potassium in the presence of Ca2+. Rat pheochromocytoma PC12 cells, which differentiated into adherent cells similar to sympathetic neurons in response to nerve growth factor, were used for the detailed analysis of the dynamics and function of sPLA2 in neuronal cells. Antibody against rat type II sPLA2 precipitated approximately 80% of the PLA2 activity in PC12 cell lysates. Transcript for type II sPLA2 was detected in PC12 cells by reverse transcriptase-PCR. When neuronally differentiated PC12 cells were stimulated with carbamylcholine or potassium, sPLA2 was released into the medium and reached a maximal approximately 40% release by 15 min. Inhibitors specific to type II sPLA2 suppressed catecholamine secretion by PC12 cells which had been activated by carbamylcholine. Furthermore, treatment of PC12 cells with exogenous type II sPLA2 alone elicited catecholamine secretion. These observations indicate that sPLA2 released from neuronal cells may regulate the degranulation process leading to release of neurotransmitters and are compatible with our earlier finding that this enzyme is involved in the degranulation of rat mast cells.

Transgenic model for the discovery of novel human secretory non-pancreatic phospholipase A2 inhibitors

Transgenic mice were created which overexpress human secretory non-pancreatic phospholipase A2 (sPLA2) pansomatically as a potential disease and drug-testing model. The mice were produced using a DNA construct in which the inducible mouse metallothionein gene promoter drives expression of a human sPLA2 minigene. High levels of sPLA2 were detected in several tissues by immunofluorescence localization. Expression in the testes caused hypospermia and male infertility. Circulating catalytically active sPLA2 could be induced to levels observed in patients undergoing a systemic inflammatory response but had no detectable effect on the mice. Therefore, these results suggest that sPLA2 hyperphospholipasemia alone may have only limited pathophysiological consequences. We further show that 3-[3-acetamide-1-benzyl-2-ethylindolyl-5-oxy]propane phosphonic acid LY311727), a potent new inhibitor of phospholipase A2 catalysis developed by our group, dramatically suppresses the circulating enzyme activity in these animals whereas 3-[3-acetamide-1-benzyl-2-propylindolyl-5-oxy]propane phosphonic acid (LY314024), a substantially less potent LY311727 analog, is without effect. These later results thus motivate the further development of this compound as a potential new therapeutic agent and valuable research tool.

Expression of human group II PLA2 in transgenic mice results in epidermal hyperplasia in the absence of inflammatory infiltrate

Group II PLA2 has been implicated in inflammatory processes in both man and other animals and has been shown to be involved in inflammatory conditions, such as arthritis and sepsis. Transgenic mice expressing the human group II PLA2 gene have been generated using a 6.2-kb genomic fragment. These mice express the group II PLA2 gene abundantly in liver, lung, kidney, and skin, and have serum PLA2 activity levels approximately eightfold higher than nontransgenic littermates. The group II PLA2 transgenic mice reported here exhibit epidermal and adnexal hyperplasia, hyperkeratosis, and almost total alopecia. The chronic epidermal hyperplasia and hyperkeratosis seen in these mice is similar to that seen in a variety of dermatopathies, including psoriasis. However, unlike what is seen with these dermatopathies, no significant inflammatory-cell influx was observed in the skin of these animals, or in any other tissue examined. These mice provide an important tool for examining group II PLA2 expression, and for determining the role of group II PLA2 in normal and disease physiology. They serve as an in vivo model for identifying inhibitors of group II PLA2 activity and gene expression.

Inhibition of the activity of pro-inflammatory secretory phospholipase A(2) by acute phase proteins

Pro-Inflammatory non-pancreatic phospholipase A₂ (sPLA₂) is markedly over-expressed in acute systemic and chronic local inflammatory processes. Since in acute phase reaction sPLA₂ is often over-expressed simultaneously with acute phase proteins (APP), it is important to determine whether APP interacts with sPLA₂. We tested ten APPs for interaction with sPLA₂ using as a substrate multilamellar Hposomes composed either of PC:Lyso PC or PE:Lyso PE. Using PC:Lyso PC substrate, CRP, lactoferrin and SAP were found to inhibit sPLA₂ activity with an IC₅₀ of 25 μg/ml, 7.5 μg/ml and 50 μg/ml, respectively, corresponding to 0.21 μM, 0.1 μM and 0.21 μM respectively. Using PE:Lyso PE substrate only SAP was inhibitory, with an IC₅₀ of 10 μg/ml (0.04 μM). Phosphorylcholine abolished the inhibitory activity of CRP but not of SAP or lactoferrin. Addition of phosphorylethanolamine or of excess calcium had no effect on the inhibitory activity of APP. Limulin, lysozyme, transferrin, β₂-microglobulin, α₂-macroglobulin, human and bovine albumins had no effect on sPLA2 activity. Therefore neither the structure of pentraxins, or ironbinding, bacteriostatic property or amyloidogenic property preclude whether APP modulates sPLA₂ activity. Inhibition of pro-inflammatory sPLA₂ by APP may be one of the protective mechanisms of the acute phase reaction.