Macrophage colony stimulating factor activates phosphatidylcholine hydrolysis by cytoplasmic phospholipase A2

Calcium-mediated translocation of cytosolic phospholipase A2 to the nuclear envelope and endoplasmic reticulum

Cytosolic phospholipase A2 (cPLA2) is activated by a wide variety of stimuli to release arachidonic acid, the precursor of the potent inflammatory mediators prostaglandin and leukotriene. Specifically, cPLA2 releases arachidonic acid in response to agents that increase intracellular Ca2+. In vitro data have suggested that these agents induce a translocation of cPLA2 from the cytosol to the cell membrane, where its substrate is localized. Here, we use immunofluorescence to visualize the translocation of cPLA2 to distinct cellular membranes. In Chinese hamster ovary cells that stably overexpress cPLA2, this enzyme translocates to the nuclear envelope upon stimulation with the calcium ionophore A23187. The pattern of staining observed in the cytoplasm suggests that cPLA2 also translocates to the endoplasmic reticulum. We find no evidence for cPLA2 localization to the plasma membrane. Translocation of cPLA2 is dependent on the calcium-dependent phospholipid binding domain, as a calcium-dependent phospholipid binding deletion mutant of cPLA2 (delta CII) fails to translocate in response to Ca2+. In contrast, cPLA2 mutated at Ser-505, the site of mitogen-activated protein kinase phosphorylation, translocates normally. This observation, combined with the observed phosphorylation of delta CII, establishes that translocation and phosphorylation function independently to regulate cPLA2. The effect of these mutations on cPLA2 translocation was confirmed by subcellular fractionation. Each of these mutations abolished the ability of cPLA2 to release arachidonic acid, establishing that cPLA2-mediated arachidonic acid release is strongly dependent on both phosphorylation and translocation. These data help to clarify the mechanisms by which cPLA2 is regulated in intact cells and establish the nuclear envelope and endoplasmic reticulum as primary sites for the liberation of arachidonic acid in the cell.

5-Lipoxygenase products modulate the activity of the 85-kDa phospholipase A2 in human neutrophils

Addition of submicromolar concentrations of arachidonic acid (AA) to human neutrophils induced a 2-fold increase in the activity of a cytosolic phospholipase A2 (PLA2) when measured using sonicated vesicles of 1-stearoyl-2-[14C]arachidonoylphosphatidylcholine as substrate. A similar increase in cytosolic PLA2 activity was induced by stimulation of neutrophils with leukotriene B4 (LTB4), 5-oxoeicosatetraenoic acid, or 5-hydroxyeicosatetraenoic acid (5-HETE). LTB4 was the most potent of the agonists, showing maximal effect at 1 nM. Inhibition of 5-lipoxygenase with either eicosatetraynoic acid or zileuton prevented the AA-induced increase in PLA2 activity but had no effect on the response induced by LTB4. Furthermore, pretreatment of neutrophils with a LTB4-receptor antagonist, LY 255283, blocked the AA- and LTB4-induced activation of PLA2 but did not influence the action of 5-HETE. Treatment of neutrophils with pancreatic PLA2 also induced an increase in the activity of the cytosolic PLA2; this response was inhibited by both eicosatetraynoic acid or LY 255283. The increases in PLA2 activity in response to stimulation correlated with a shift in electrophoretic mobility of the 85-kDa PLA2, as determined by Western blot analysis, suggesting that phosphorylation of the 85-kDa PLA2 likely underlies its increase in catalytic activity. Although stimulation of neutrophils with individual lipoxygenase metabolites did not induce significant mobilization of endogenous AA, they greatly enhanced the N-formylmethionyl-leucyl-phenylalanine-induced mobilization of AA as determined by mass spectrometry analysis. Our findings support a positive-feedback model in which stimulus-induced release of AA or exocytosis of secretory PLA2 modulate the activity of the cytosolic 85-kDa PLA2 by initiating the formation of LTB4. The nascent LTB4 is then released to act on the LTB4 receptor and thereby promote further activation of the 85-kDa PLA2. Since 5-HETE and LTB4 are known to prime the synthesis of platelet-activating factor, the findings suggest that 85-kDa PLA2 plays a role in platelet-activating factor synthesis.

Short-term and long-term-effects of phorbol 12-myristate 13-acetate and different inhibitors on the ability of bone marrow cells to form colonies in vitro

The process of signal transduction responsible for the phorbol 12-myristate 13-acetate mediated increase in the colony-forming potential of murine (CBA) bone marrow cells was studied using known modulators of the mitogenic signal. Pretreatment of cells for 60 minutes with staurosporine (1 mumol/l), an inhibitor of protein kinase C, completely prevented colony formation in the control group of cells and significantly reduced the number of colonies formed in the phorbol ester-treated group. Brief exposure (60 min) of cells to the phospholipase A2 inhibitors, mepacrine (500 mumol/l) and heparin (1 g/l), reduced the number of colonies formed in the control group and completely abolished the increase in the number of colonies formed after treatment of the cells with phorbol ester. When inhibitors of protein kinase C or phospholipase A2 were present during the entire period of the colony forming assay (7 days), no colonies could be scored in either the control or phorbol ester-treated groups of bone marrow cells. Long-term treatment or temporary exposure (60 min) of cells to indomethacin (50 mumol/l), an inhibitor of cyclooxygenase, or nordihydroguaiaretic acid (50 mumol/l), an inhibitor of lipoxygenase, had no effect on colony formation in both groups. Pretreatment of cells for 45 min with calcium ionophore A23187 (10 mumol/l) failed to increase the number of colonies, compared with the control group. Moreover, simultaneous treatment of cells for 45 min with phorbol ester (500 nmol/l) and A23187 (10 mumol) did not produce any further increase in the number of colonies, compared with the phorbol ester-treated group, suggesting that elevation of intracellular calcium is unimportant in the phorbol ester-mediated response. Dibutyryl cyclic adenosine monophosphate (50 mumol/l) in the presence or absence of phorbol ester, failed to stimulate colony formation, indicating that cyclic AMP-dependent protein kinases are not involved in the signalling process. Temporary exposure (75 min) of bone marrow cells to okadaic acid (1 mumol/l), a potent inhibitor of serine/threonine phosphatases, or to tyrphostine AG-115 (20 mumol/l), a tyrosine kinase inhibitor, did not effect colony growth in the control or phorbol ester-treated group. The results indicate that phospholipase A2 activation is involved in the phorbol ester-mediated increase in colony formation, since, of the different agents applied, only staurosporine, an inhibitor of protein kinase C, and mepacrine and heparin, putative inhibitors of phospholipase A2, were capable of abolishing phorbol ester-mediated effects.

Cytosolic phospholipase A2

To summarize the regulation of cPLA2, we have proposed a model for the activation of cPLA2 based both on our previous studies (Clark et al., 1991; Lin et al., 1993) and the work of many others (Fig. 5). In this model, cPLA2 is tightly regulated by multiple pathways, including those that control Ca2+ concentration, phosphorylation states and cPLA2 protein levels, to exert both rapid and prolonged effects on cellular processes, such as inflammation. cPLA2 is rapidly activated by increased intracellular Ca2+ concentration and phosphorylation by MAP kinase. When cells are stimulated with a ligand for a receptor, such as ATP or PDGF, PLC is activated via either a G protein-dependent or -independent process, leading to the production of diacylglycerol (DAG) and inositol triphosphate (IP3). The rise in these intracellular messengers cause the activation of PKC and mobilization of intracellular Ca2+. Alternatively, the increase in intracellular Ca2+ can result from a Ca2+ influx. Increased Ca2+ acts through the CaLB domain to cause translocation of cPLA2 from the cytosol to the membrane where its substrate, phospholipid, is localized. This step is essential for the activation of cPLA2 and may account for the partial activation of cPLA2 in the absence of phosphorylation. MAP kinase activation can occur through both PKC-dependent and -independent mechanisms (Cobb et al., 1991; Posada and Cooper, 1992; Qiu and Leslie, 1994). In many cases, this pathway is also G protein-dependent. Activated MAP kinase phosphorylates cPLA2 at Ser-505, causing increased enzymatic activity of cPLA2, which is realized only upon translocation of cPLA2 to the membrane. Therefore, full activation of cPLA2 requires both increased cytosolic Ca2+ and cPLA2 phosphorylation at Ser-505. In a more delayed response, cPLA2 activity in the cells can be controlled by changes in its expression levels, such as in response to inflammatory cytokines and certain growth factors. Thus the expression level of cPLA2 is regulated by both transcriptional and post-transcriptional mechanisms.

Phospholipase A2 gene expression and activity in histologically normal ileal mucosa and in Crohn's ileitis

Increased activity of phospholipase A2 (PLA2) in the ileal mucosa may contribute to the inflammation in Crohn's disease. The results of this study showed that (a) three months after ileocolonic resection for Crohn's disease the neoterminal ileal mucosa showed endoscopically new inflammation and had higher PLA2 activity than at the time of the operation (n = 8); no such findings were seen in controls (n = 7), (b) histologically normal ileal mucosa (n = 3) contained mRNA for three isoforms of PLA2 (PLA2-I, PLA2-II, and cPLA2), but the amounts of PLA2-II mRNA clearly exceeded the amounts of mRNA for PLA2-I and cPLA2, (c) ileal mucosa from Crohn's patients (n = 2) contained higher values of PLA2-II mRNA than ileal mucosa from two controls, (d) ileal mucosa from Crohn's patients (n = 4) showed increased PLA2-II mRNA three months after ileocolonic resection. In conclusion, these results show that the predominating PLA2 mRNA in the human ileal mucosa is type II PLA2, and the increased synthesis of PLA2-II might be responsible for the increased PLA2 activity found in the ileal mucosa accompanying recurrent ileal inflammation in Crohn's disease.

Epidermal growth factor (EGF) induces serine phosphorylation-dependent activation and calcium-dependent translocation of the cytosolic phospholipase A2

Phospholipase A2 (PLA2) is a key enzyme in the release of arachidonic acid and subsequent production of eicosanoids, which play an important role in a variety of biological processes, including mitogenic signalling by epidermal growth factor (EGF). In a previous study [Spaargaren, M. et al. (1992) Biochem J. 287, 37-43] we identified the EGF-activated PLA2 as being similar to the recently cloned high-molecular-mass cytosolic phospholipase A2 (cPLA2). In the present study we demonstrate a rapid transient EGF-induced activation of this cPLA2 and an EGF-induced increase in phosphorylation of the cPLA2. The EGF-induced activation of cPLA2 is reversed upon phosphatase treatment showing phosphorylation-dependent activation of the cPLA2. No direct association of the cPLA2 to the EGF receptor was detected under conditions where such an association with phospholipase C-gamma was demonstrated. Phosphoamino acid analysis of this cPLA2 showed that EGF induced an increase in serine phosphorylation exclusively, no tyrosine phosphorylation being observed. EGF treatment of the cells resulted in a Ca(2+)-dependent translocation of the cPLA2 from the cytosol to the membrane fraction. This is due to an EGF-induced [Ca2+]i rise which is dependent on the influx of extracellular Ca2+ via voltage-independent Ca2+ channels. It is shown that the Ca(2+)-dependent association of cPLA2 to membranes does not require accessory membrane molecules.

Discordance between macrophage arachidonate metabolic phenotype and the expression of cytosolic phospholipase A2 and cyclooxygenase

Macrophages (M phi s) exhibit variations in their ability to release and metabolize arachidonate (AA) depending on their state of activation, differentiation, and tissue origin. In order to understand these variations on a molecular level, we determined whether differences in AA release and metabolism by murine peritoneal M phi s could be explained in terms of cytosolic phospholipase A2 (cPLA2) and cyclooxygenase (COX) expression. Resident M phi s exhibited greater COX capacity (conversion of exogenous AA to PGE2) but lower phospholipase (PLase) activity (release of endogenous AA) than elicited M phi s. Activation of resident M phi s in vivo with endotoxin increased both their PLase activity and COX capacity. Despite the observed differences in PLase activity, peritoneal M phi s under all conditions expressed similar amounts of cPLA2 mRNA and protein. All M phi s exhibited COX-1 mRNA and protein (i.e., the constitutive isoform of COX), although elicited M phi s exhibited increased mRNA for COX-1 but decreased levels of protein, relative to resident M phi s. Elicited (but not resident) cells also exhibited COX-2 mRNA but not COX-2 protein (i.e., the inducible form of COX). Despite the increased COX capacity of resident cells with in vivo activation, their expression of COX-2 mRNA and protein was equivalent to that of unactivated cells, becoming apparent only after cell adherence in vitro. In sum, there is no simple relationship between the ability of M phi s to release and metabolize AA, and the expression of cPLA2 or COX isoforms. Moreover, adherence appears to be important for the expression of COX-2 by M phi s.

Interleukin-1 beta induces the synthesis and activity of cytosolic phospholipase A2 and the release of prostaglandin E2 in human amnion-derived WISH cells

The objective of this study was to examine the expression and activity of cytosolic phospholipase A2 (cPLA2) in relation to prostaglandin E2 (PGE2) synthesis in human amnion-derived WISH cells in response to stimulation by interleukin-1 beta (IL-1 beta). cPLA2 activity was characterized by sensitivity to heat and acid treatment, stability to dithiothreitol, and inhibition by the specific inhibitor, arachidonyl trifluoromethyl ketone (AACOCF3). Treatment of WISH cells with IL-1 beta (0.01-1 ng/mL) for up to 24 h resulted in a significant increase in PGE2 release in a concentration- and time-dependent manner accompanied by increases both in total cellular cPLA2 activity and in cPLA2 protein levels detected by Western blot analysis. The parallel increase in total cellular cPLA2 activity and cPLA2 protein level indicates that IL-1 beta may induce the synthesis of cPLA2. Incubation of the cells with 10 microM AACOCF3 for 24 h significantly inhibited IL-1 beta-induced PGE2 production strongly suggesting that cPLA2 mediates IL-1 beta-induced PGE2 formation. In unstimulated cells, there is appreciable total cellular cPLA2 activity and protein, but these cells produce low amounts of PGE2 until stimulated by IL-1 beta, suggesting that cPLA2 translocation from cytosol to the membrane is necessary for its bioactivity. In contrast to IL-1 beta, treatment with phorbol ester (12-O-tetradecanoyl phorbol-13-acetate, TPA, 10(-10)-10(-6)M) for 24 h significantly inhibited total cellular cPLA2 activity in a concentration-dependent manner. The amount of total cellular cPLA2 protein seen on Western blot remained unchanged following TPA treatment. These data suggest that in WISH cells, IL-1 beta induces both translocation to the membrane and de novo synthesis of cPLA2 protein to sustain prostaglandin (PG) synthesis. In contrast, TPA may only cause cPLA2 translocation but no increase in cPLA2 protein synthesis, resulting in limited PG synthesis. Our results provide a mechanism for the effect of IL-1 beta on prostaglandin synthesis in human amnion cells and provide support for a role of cPLA2 in the mechanism initiating human parturition.

Growth hormone signaling leading to CYP2C12 gene expression in rat hepatocytes involves phospholipase A2

The expression of CYP2C12 is liver-specific and regulated at the transcriptional level by growth hormone (GH). In attempts to elucidate the nature of signaling molecules mediating the GH regulation of this gene in rat hepatocytes, a role for phospholipase A2 (PLA2) as a transducer of GH-induced levels of P4502C12 mRNA was investigated. GH was shown to induce tyrosyl-phosphorylation of p42 and p44 microtubule-associated protein (MAP) kinases and to reduce the electrophoretic mobility of a 100-kDa protein, immunologically related to cPLA2. These events were observed in parallel with GH-stimulated release of [3H]arachidonic acid ([3H]AA) from cellular phospholipids of rat hepatocytes labeled with [3H]AA. These rapid effects of GH action, as well as the GH-induced expression of CYP2C12, were inhibited in cells treated with the tyrosine kinase inhibitor herbimycin A. Similarly, when the GH-induced liberation of [3H]AA was blocked by the PLA2 inhibitor mepacrine or the Ca2+ channel blocker verapamil, GH-induced accumulation of P4502C12 mRNA was absent. These results suggest a correlation between PLA2 activity and GH regulation of the CYP2C12 gene. The inhibitory effect of mepacrine on GH induction of P4502C12 mRNA was reversed by AA addition, further supporting a role for eicosanoids in the regulation of CYP2C12. Finally, inhibitors of P450-mediated AA metabolism, SKF-525A and ketoconazole as well as eicosatetraynoic acid, blocked the GH-mediated induction of P4502C12 mRNA, whereas more specific inhibitors of cyclooxygenase or lipoxygenase metabolism did not. Based on these results, we suggest that GH signaling in rat hepatocytes, leading to increased expression of CYP2C12, involves PLA2 activation and subsequent P450-catalyzed formation of an active AA metabolite.

Differential regulation of elicited-peritoneal macrophage 14 kDa and 85 kDa phospholipase A2(s) by transforming growth factor-beta

Elicited guinea pig macrophages collected from inflammatory peritoneal exudate release soluble 14 kDa phospholipase A2 (PLA2) and prostaglandin E2 (PGE2) into surrounding media during culture (Marshall et al. (1994) J. Lipid Med. 10, 295-313). The effect of transformation growth factor beta 1 (TGF beta), an immunoregulatory growth factor, was examined in this system. Exposure of cultured macrophages to TGF beta reduced both the activity and protein levels of 14 kDa PLA2 measured in conditioned media. This inhibition occurred within the first 6-8 h, was prevalent through 72 h of exposure and was dependent on TGF beta concentration. The reduction, however, never reached more than 40-60%. Evaluation of the cellular PLA2 activity confirmed the existence of an immunologically-related 14 kDa PLA2 (ELISA) in the particulate fraction and an 85 kDa PLA2 (Western analysis) in the cytosol. Exposure to TGF beta halved the particulate activity and protein levels of 14 kDa PLA2 which was consistent with the reduction in the secreted form. Alternatively, TGF beta induced an increase in cytosolic 85 kDa PLA2 (activity and protein) which was not apparent until 12 h and significant at 20-24 h of exposure. This demonstrates that TGF beta differentially regulates the production of these two enzymes. Despite this, neither PGE2 synthesis nor the up-regulated cyclooxygenase -II were altered by TGF beta treatment suggesting that maximal prostanoid synthesis had been reached.

Monocyte colony-stimulating factor stimulates binding of phosphatidylinositol 3-kinase to Grb2.Sos complexes in human monocytes

Monocyte colony-stimulating factor (M-CSF) is required for the proliferation of mononuclear phagocytes. The activated M-CSF receptor associates with phosphatidylinositol 3-kinase (PI 3-kinase). In the present studies, we demonstrate that M-CSF also induces direct interaction of PI 3-kinase (p85 alpha subunit) with the SH2/SH3 adaptor protein Grb2. Tyrosine-phosphorylated PI 3-kinase interacts with the SH2 domain of Grb2. A pYRNE (pY408) site in PI 3-kinase is potentially involved in this interaction. The results also demonstrate that the PI 3-kinase.Grb2 complex associates with the guanine nucleotide exchange protein Sos. Since Sos binds to the SH3 domains of Grb2 and thereby associates with Ras at the cell membrane, formation of the PI 3-kinase.Grb2.Sos complex provides a potential mechanism for growth factor-induced interactions of PI 3-kinase and Ras.

c-kit ligand mediates increased expression of cytosolic phospholipase A2, prostaglandin endoperoxide synthase-1, and hematopoietic prostaglandin D2 synthase and increased IgE-dependent prostaglandin D2 generation in immature mouse mast cells

We have examined the cytokine regulation of IgE-dependent prostaglandin (PG) D2 generation in mouse mast cells by assessing the changes in the levels of the transcript, translated protein, and activity of the enzymes involved in the synthesis of PGD2 from endogenous arachidonic acid. When mouse mast cells, derived by culture of bone marrow cells with WEHI-3 cell-conditioned medium as a source of interleukin (IL)-3 (BMMC), were cultured in recombinant ckit ligand (KL), sensitized with IgE, and stimulated with antigen, PGD2 generation increased 3-fold; when KL was combined with IL-3, IL-9, or IL-10, PGD2 generation increased 6-8-fold above that produced by the cells cultured in IL-3 alone. The increased IgE-dependent PGD2 generation by BMMC was apparent after 1 day of culture, reached a maximum after 2-4 days of culture, and was dose-dependent for KL and for each of the accessory cytokines. IgE-dependent generation of leukotriene C4 increased 2-fold after the cells were cultured with KL and was not increased by the addition of IL-3, IL-9, or IL-10. Assays for steady-state transcripts by RNA blotting, for protein by SDS-PAGE/immunoblotting, and for function by enzymatic activities revealed that KL alone stimulated the increased expression of cytosolic phospholipase A2 (cPLA2), prostaglandin endoperoxide synthase (PGHS)-1, and the terminal enzyme, hematopoietic PGD2 synthase, without a change in expression of 5-lipoxygenase. IL-3, IL-9, and IL-10 each enhanced the KL-induced expression of PGHS-1. In contrast, transcripts for PGHS-2, which were detected transiently after the cells had been cultured for 5 h in KL+IL-3, were not expressed during the period of subsequent increase in IgE-dependent PGD2 generation. These findings demonstrate that KL up-regulates expression of cPLA2, PGHS-1, and hematopoietic PGD2 synthase, leading to a relatively selective increase in IgE-dependent production of PGD2 from endogenously released arachidonic acid in BMMC, and they provide the first example of cytokine regulation of hematopoietic PGD2 synthase.

Activation of cytosolic phospholipase A2 by basic fibroblast growth factor via a p42 mitogen-activated protein kinase-dependent phosphorylation pathway in endothelial cells

Basic fibroblast growth factor (FGF) stimulates the proliferation, differentiation, and motility of multiple cell types. Signal transduction by FGF is mediated by high affinity FGF receptors that have autophosphorylating tyrosine kinase activity and also elicit the release of low molecular weight signaling molecules, including inositol 1,4,5-trisphosphate, diacylglycerol, and arachidonate. We have shown previously that basic FGF-stimulated, phospholipase A2 (PLA2)-mediated arachidonate release regulates endothelial cell (EC) motility (Sa, G., and Fox, P.L. (1994) J. Biol. Chem. 269, 3219-3225). Here we identify the phospholipase responsible for basic FGF-mediated arachidonate release as cytosolic PLA2 (cPLA2) by demonstrating in EC lysates a requirement for micromolar Ca2+, dithiothreitol insensitivity, and inactivation by anti-cPLA2 antiserum. The role of cPLA2 is also indicated by the observed mechanisms of activation which show a requirement for p42 mitogen-activated protein kinase activity, cPLA2 phosphorylation, and cPLA2 translocation from cytosol to membranes. Phosphorylation of cPLA2, arachidonate release from prelabeled intact cells, and cell motility all have similar concentration dependencies on basic FGF. Since arachidonate release is required for basic FGF-stimulated motility of EC, our results show that p42 mitogen-activated protein kinase activation of cPLA2 may be a regulatory event in stimulation of cellular release of this important eicosanoid precursor during cellular responses to basic FGF.

Recent insights into the structure, function and biology of cPLA2

The 85-kDa cytosolic PLA2 (cPLA2) is present in most cells and tissues and its structural and functional properties have been described. Different agonists, growth factors and cytokines activate cPLA2 to hydrolyze cellular phospholipids thereby providing the precursor substrates for the biosynthesis of eicosanoids and platelet-activating factor (PAF), the well-known mediators of inflammatory and allergic reactions. Recent studies discussed here suggest that cPLA2 is a receptor-regulated enzyme involved in the inflammatory response. Therefore, inhibitors of cPLA2 may be useful as therapeutic agents in the treatment of inflammatory diseases.

Molecular cloning of the 31 kDa cytosolic phospholipase A2, as an antigen recognized by the lung cancer-specific human monoclonal antibody, AE6F4

The human monoclonal antibody AE6F4 specifically reacts with human lung cancer tissues but does not with normal tissues. This monoclonal antibody recognizes a cytosolic 31 kDa antigen in the cancer cells. In a previous study, we elucidated that the 31 kDa antigen belonged to a family of proteins collectively designated as 14-3-3 proteins, which were known as protein kinase-dependent activators of tyrosine/trytophan hydroxylases, or protein kinase C inhibitor proteins. Here we report molecular cloning of the 31 kDa antigen from the human lung adenocarcinoma cell line, A549. Sequencing analysis indicates that the cloned cDNA is identical to that of previously reported human placental cytosolic phospholipase A2 (cPLA2), which is also a member of the 14-3-3 protein family. Western analysis demonstrated that a 31 kDa recombinant cPLA2 expressed in monkey COS cells was recognized by the AE6F4 monoclonal antibody. Binding of the monoclonal antibody to the recombinant cPLA2 was abolished when treated with sodium periodate, suggesting that not only are carbohydrate chains associated with the cPLA2, but they also play a crucial role in antigen recognition by the monoclonal antibody.

Thromboxane A2 synthase activity in platelet free human monocytes

Soon after platelets, the highest amounts of thromboxane A2 (TXA2) can be detected in human monocytes activated by serum. Using platelet-free human monocytes, we have shown that foetal calf serum (FCS) induces prostaglandin H synthase (PGH synthase) after 16 h of incubation, as shown by the use of transcriptional inhibitors and Western blotting. The effect of serum can be in part mimicked by recombinant colony stimulating factor-1 (hr CSF-1). It is not known whether the limiting step leading from arachidonate to TXA2 is represented solely by the level of PGH synthase or also by the level of TXA2 synthase. We approached this problem by using a Western blot specific for the enzyme, as well as by using PGH2 as substrate. The results show that TXA2 synthase is constitutively expressed in monocytes, i.e., its levels were high soon after their isolation, and similar to those observed after 24 h of incubation with serum. However TXA2 failed to be synthesized until at least 3 h of incubation, and the pattern of synthesis was dependent on the kinetics of PGH synthase induction. In any condition in which TXA2 synthase was immunodetectable, using PGH2 as substrate a high rate of conversion to TXB2 could be detected. Experiments with actinomycin D and cycloheximide indicate that the half-life of TXA2 synthase was longer than 16 h, therefore much longer than that of PGH synthase, that the gene coding for it is fully active in resting monocytes, and that the conversion of arachidonate to TXA2 induced by serum or CSF-1 is dependent solely on the de novo synthesis of PGH synthase.

Endothelin-1 stimulates cytosolic phospholipase A2 activity and gene expression in rat glomerular mesangial cells

Endothelin-1 (ET-1) stimulates vascular smooth muscle and mesangial cells to release prostaglandin E2 (PGE2), which can attenuate the vasoconstrictor and mitogenic effects of this peptide. Phospholipase A2 (PLA2)-mediated release of arachidonic acid from the sn-2 position of membrane phospholipids is thought to be one of the rate-limiting steps in prostaglandin (PG) synthesis. We evaluated the role of ET-1 to regulate gene expression, protein synthesis and enzymatic activity of cytosolic PLA2 (cPLA2), an intracellular form of the PLA2 enzyme family, in cultured rat mesangial cells using both acute and chronic incubation protocols. Acute ET-1-induced stimulation of cPLA2 activity was maximal after 10 minutes (181.1 +/- 6.84% of control), persisted for 40 minutes and did not require new protein synthesis. Heparin, a potent inhibitor of intracellular Ca2+ increase as well as mitogen-activated protein (MAP) kinase activation and cell proliferation, did not affect the rapid cPLA2 stimulation by ET-1. Chronic incubation of glomerular mesangial cells with ET-1 (1 to 24 hr) led to time- and dose-dependent increases in cPLA2 mRNA expression which was maximal after six hours, persisted up to 24 hours and which was accompanied by both cPLA2 protein formation, as assessed by Western analysis, as well as by stimulation of enzymatic activity. Inhibition of protein synthesis by cycloheximide increased basal cPLA2 mRNA accumulation in quiescent mesangial cells, and the combination of ET-1 and cycloheximide resulted in a greater induction of cPLA2 gene expression when compared to ET-1 alone. Actinomycin D treatment blocked the effect of ET-1 on cPLA2 mRNA accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipases A2 in ras-transformed and immortalized human mammary epithelial cells

Phospholipase A2 (PLA2) activities of non-tumorigenic and tumorigenic human mammary epithelial cells, 184B5 cells (immortalized cell line from a reduction mammoplasty) and B5KTu cells (cells from a tumor induced by ras-transformed 184B5 cells), are characterized, with emphasis on lipid biomediator-related phospholipases A2. Phospholipases A2 associated with regulation of arachidonic acid metabolism include the high molecular mass cytosolic PLA2 (cPLA2) and group II PLA2. The major PLA2 activity in the mammary epithelial cells has the characteristics of cPLA2; this activity is higher in the B5KTu cells. In contrast, the 184B5 and B5KTu cells have similar levels of a Ca(2+)-independent, cytosolic PLA2 activity and low levels of a particulate fraction PLA2 activity, which does not have the properties of group II PLA2. Thus, cPLA2 activity is selectively elevated in the tumorigenic human mammary epithelial cells and this may result in increased generation of lipid biomediators such as arachidonic acid metabolites.

Role of colony-stimulating factor-1 in bone metabolism

Colony-stimulating factor-1 (CSF-1) is a cytokine required for proliferation, differentiation, activity, and survival of cells of the mononuclear phagocytic system. The growth factor is synthesized as a soluble, matrix, or membrane associated molecule. The specific functions of these forms are not clear. However, some data suggest a dependence of the development of various populations of tissue macrophages on the locally expressed and presented cytokine. Deficiency in CSF-1, as is the case in the murine mutant strain op/op, results in low numbers of macrophages and monocytes and, most striking, leads to osteopetrosis due to a virtual absence of osteoclasts. Using the op/op mutation as a model, CSF-1 was established as one of the growth factors for osteoclasts. The expression of CSF-1 receptors, encoded by the proto-oncogene c-fms, by osteoclast precursors and osteoclasts, suggested an effect of this cytokine not only during osteoclast formation but also on the mature cells. In fact, CSF-1 was shown to inhibit the resorbing activity, to stimulate migration, and to support survival of isolated osteoclasts in vitro. By these actions on cells of the osteoclast lineage, CSF-1 induces recruitment of new osteoclasts, leading to a net increase of bone resorption, and might govern the spatial distribution of resorption sites within the bone. During these processes, locally expressed and presented forms of the growth factor may play a crucial role, as will be discussed in this article.

Two novel colony-stimulating factor 1 (CSF-1) properties: it post-transcriptionally inhibits interferon-specific induction of class II antigens and reduces the risk of fetal abortion

Local production of 250 U/ml of Colony Stimulating Factor 1 (CSF-1) by decidual cells on the 11th day of murine pregnancy prompted us to investigate the role of this mononuclear phagocyte factor during gestation. We show that the presence of CSF-1 accounts for down-regulating induction of class II antigens on the placenta, antigens whose presence leads to fetal death. It is known from previous studies that gamma interferon (IFN-gamma) induces the expression of MHC-Ia antigens on the placenta causing thus fetal rejection due to an immune reaction the mother develops against the fetus. This Ia induction occurs both in vitro and in vivo, however, an extra dose of CSF-1, in addition to the physiological levels already present, overwhelms the ability of IFN-gamma to such induction and the embryos are rescued. Thus, CSF-1 appears to be an inhibitor of IFN-gamma induced class II expression, a property that may permit CSF-1 to be employed during gestation to safeguard the outcome of pregnancy and alleviate, by extrapolation to the human system, the recurrent abortion issue. This action, however, is not unique during gestation only. Other inducible IFN-gamma cells (WEHI-3, HL-60 and HeLa) are also blocked by CSF-1 not to express surface class II antigens, as it has been recently reported on glia cells. At the mRNA level, however, CSF-1 is unable to block class II induction, a finding that suggests a post-transcriptional regulation pattern.

Isolation of promoter for cytosolic phospholipase A2 (cPLA2)

Cytosolic phospholipase A2 (cPLA2) releases arachidonic acid from membrane phospholipids and is believed to be the rate-limiting enzyme in the arachidonic acid pathway. We report herein the isolation of a 3 kb fragment of rodent genomic DNA containing part of the first intron, the first exon and 5'-flanking sequence. The start site of transcription was mapped by 5'-rapid amplification of cDNA ends and corroborated by ribonuclease protection assay. The gene has a TATAless promoter with no classical Sp1 binding sites or initiator element. A microsatellite series of CA repeats was noted in the 5'-flanking region of both the rodent and human promoters. Deletion constructs have been analysed for luciferase activity and confirmed promoter activity.

Characteristics of lysophospholipase activity expressed by cytosolic phospholipase A2

Evidence has accumulated to suggest that a wide variety of mammalian cells and tissues express a cytosolic phospholipase A2 with arachidonoyl preference (cPLA2). Purified rabbit platelet-derived cPLA2, as well as the human recombinant enzyme originally identified in the monocytic leukemic cell line U937, exhibit significant lysophospholipase activity. Several series of experiments indicated that a single protein mediated both activities. Treatment of the purified enzyme with p-bromophenacylbromide or an anti-(rabbit platelet cPLA2) monoclonal antibody, RHY-5, suppressed the activity of phospholipase A2 without any appreciable effect on lysophospholipase activity, suggesting that the domain(s) required for phospholipase A2 activity may be located separately from that for lysophospholipase activity. Lysophospholipase activity was appreciably detected above the critical micellar concentration of the substrate. Lysophosphatidylcholine was also hydrolyzed efficiently when it was incorporated into liposomes made of dialkylphosphatidylcholine. The hydrolysis of lysophospholipid was dependent on the fatty acid bound at the sn1 position; the relative rates of hydrolysis of 1-oleoyllysophosphatidylcholine, 1-palmitoyllysophosphatidylcholine, and 1-stearoyllysophosphatidylcholine were 23, 8, and 1, respectively. A similar order of reactivity was observed with lysophospholipid incorporated into dialkylphosphatidylcholine liposomes. cPLA2 may function not only as an arachidonate liberation enzyme but also as an enzyme responsible for degradation of certain molecular species of lysophospholipids formed in membranes.

Interleukin-1 beta-induced cytosolic phospholipase A2 activity and protein synthesis is blocked by dexamethasone in rat mesangial cells

Interleukin-1 beta induces gene expression and secretion of the secretory phospholipase A2 (sPLA2) and prostaglandin E2 (PGE2) release from rat mesangial cells. We have previously shown that prolonged treatment of rat mesangial cells with interleukin-1 beta (IL-1 beta) also enhances the cytosolic phospholipase A2 (cPLA2) activity. This effect of IL-1 beta on the cPLA2 activity is inhibited by actinomycin D and cycloheximide, indicating that both transcription and translation are involved. Here, we describe that IL-1 beta increases mRNA levels and protein synthesis of cPLA2 itself. In parallel with the effect of dexamethasone on the sPLA2, this glucocorticoid inhibits the IL-1 beta-enhanced cPLA2 activity as a result of the suppression of IL-1 beta-induced cPLA2 gene expression. This report suggests that the pro-inflammatory action of interleukin-1 beta may, in part, be mediated by its effects on cPLA2 activity.

Gravidin: an endogenous inhibitor of phospholipase A2

Gravidin is a recently discovered protein inhibitor of phospholipase A2 and therefore prostaglandin synthesis. Transformed or rapidly growing cells are unaffected by gravidin, but in slow-growing cells arachidonate release is inhibited. Gravidin may play a physiological role in pregnancy maintenance. This review summarizes our current knowledge of the properties and activity of gravidin.

Secretory phospholipase A2 is not required for arachidonic acid liberation during platelet activation

The subcellular localization of secretory phospholipase A2 (sPLA2) and cytosolic phospholipase A2 (cPLA2) in resting and activated platelets, and their involvement in arachidonic acid liberation during platelet activation, were studied. The amounts of sPLA2 and cPLA2 recovered were not modified during platelet activation. sPLA2 was mainly associated with the organelles of resting platelets (71% of total activity) and was released into the extracellular medium during cell activation (60% of total activity), whereas the majority of cPLA2 was localized in the cytosol of resting and activated platelets. The secretion of sPLA2 correlated with the release of ATP. sPLA2-depleted platelets aggregated as much as control platelets and produced similar amounts of thromboxane B2 upon thrombin activation. These results indicate that sPLA2 is not involved in the liberation of arachidonic acid during platelet activation.

cPLA2 is phosphorylated and activated by MAP kinase

Treatment of cells with agents that stimulate the release of arachidonic acid causes increased serine phosphorylation and activation of cytosolic phospholipase A2 (cPLA2). Here we report that cPLA2 is a substrate for mitogen-activated protein (MAP) kinase. Moreover, phosphorylation by MAP kinase increases the enzymatic activity of cPLA2. The site of cPLA2 phosphorylation by MAP kinase, Ser-505, is identical to the major site of cPLA2 phosphorylation observed in phorbol ester-treated cells. Replacement of Ser-505 with Ala resulted in a mutant cPLA2 that is not a substrate for MAP kinase and causes little or no enhanced agonist-stimulated arachidonate release from intact cells. Taken together, these data indicate that MAP kinase mediates, at least in part, the agonist-induced activation of cPLA2.