Maximal epidermal growth-factor-induced cytosolic phospholipase A2 activation in vivo requires phosphorylation followed by an increased intracellular calcium concentration

A signalling cascade involving receptor-activated phospholipase A2, glycerophosphoinositol 4-phosphate, Shp1 and Src in the activation of cell motility

Background

Shp1, a tyrosine-phosphatase-1 containing the Src-homology 2 (SH2) domain, is involved in inflammatory and immune reactions, where it regulates diverse signalling pathways, usually by limiting cell responses through dephosphorylation of target molecules. Moreover, Shp1 regulates actin dynamics. One Shp1 target is Src, which controls many cellular functions including actin dynamics. Src has been previously shown to be activated by a signalling cascade initiated by the cytosolic-phospholipase A₂ (cPLA₂) metabolite glycerophosphoinositol 4-phosphate (GroPIns4P), which enhances actin polymerisation and motility. While the signalling cascade downstream Src has been fully defined, the mechanism by which GroPIns4P activates Src remains unknown.

Methods

Affinity chromatography, mass spectrometry and co-immunoprecipitation studies were employed to identify the GroPIns4P-interactors; among these Shp1 was selected for further analysis. The specific Shp1 residues interacting with GroPIns4P were revealed by NMR and validated by site-directed mutagenesis and biophysical methods such as circular dichroism, isothermal calorimetry, fluorescence spectroscopy, surface plasmon resonance and computational modelling. Morphological and motility assays were performed in NIH3T3 fibroblasts.

Results

We find that Shp1 is the direct cellular target of GroPIns4P. GroPIns4P directly binds to the Shp1-SH2 domain region (with the crucial residues being Ser 118, Arg 138 and Ser 140) and thereby promotes the association between Shp1 and Src, and the dephosphorylation of the Src-inhibitory phosphotyrosine in position 530, resulting in Src activation. As a consequence, fibroblast cells exposed to GroPIns4P show significantly enhanced wound healing capability, indicating that GroPIns4P has a stimulatory role to activate fibroblast migration. GroPIns4P is produced by cPLA₂ upon stimulation by diverse receptors, including the EGF receptor. Indeed, endogenously-produced GroPIns4P was shown to mediate the EGF-induced cell motility.

Conclusions

This study identifies a so-far undescribed mechanism of Shp1/Src modulation that promotes cell motility and that is dependent on the cPLA₂ metabolite GroPIns4P. We show that GroPIns4P is required for EGF-induced fibroblast migration and that it is part of a cPLA₂/GroPIns4P/Shp1/Src cascade that might have broad implications for studies of immune-inflammatory response and cancer.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0329-3) contains supplementary material, which is available to authorized users.

Regulation of cytosolic phospholipase A2 phosphorylation by proteolytic cleavage of annexin A1 in activated mast cells

Annexin A1 (ANXA1) is cleaved at the N terminal in some activated cells, such as macrophages, neutrophils, and epithelial cells. We previously observed that ANXA1 was proteolytically cleaved in lung extracts prepared from a murine OVA-induced asthma model. However, the cleavage and regulatory mechanisms of ANXA1 in the allergic response remain unclear. In this study, we found that ANXA1 was cleaved in both Ag-induced activated rat basophilic leukemia 2H3 (RBL-2H3) cells and bone marrow-derived mast cells. This cleavage event was inhibited when intracellular Ca(2+) signaling was blocked. ANXA1-knockdown RBL-2H3 cells produced a greater amount of eicosanoids with simultaneous upregulation of cytosolic phospholipase A(2) (cPLA(2)) activity. However, there were no changes in degranulation activity or cytokine production in the knockdown cells. We also found that cPLA(2) interacted with either full-length or cleaved ANXA1 in activated mast cells. cPLA(2) mainly interacted with full-length ANXA1 in the cytosol and cleaved ANXA1 in the membrane fraction. In addition, introduction of a cleavage-resistant ANXA1 mutant had inhibitory effects on both the phosphorylation of cPLA(2) and release of eicosanoids during the activation of RBL-2H3 cells and bone marrow-derived mast cells. These data suggest that cleavage of ANXA1 causes proinflammatory reactions by increasing the phosphorylation of cPLA(2) and production of eicosanoids during mast-cell activation.

Reactive oxygen species and boar sperm function

Boar spermatozoa are very susceptible to reactive oxygen species (ROS), but ROS involvement in damage and/or capacitation is unclear. The impact of exposing fresh boar spermatozoa to an ROS-generating system (xanthine/xanthine oxidase; XA/XO) on sperm ROS content, membrane lipid peroxidation, phospholipase (PL) A activity, and motility, viability, and capacitation was contrasted to ROS content and sperm function after cryopreservation. Exposing boar sperm (n = 4-5 ejaculates) to the ROS-generating system for 30 min rapidly increased hydrogen peroxide (H2O2) and lipid peroxidation in all sperm, increased PLA in dead sperm, and did not affect intracellular O2- (flow cytometry of sperm labeled with 2',7'-dichlorodihydrofluorscein diacetate, BODIPY 581/591 C11, bis-BODIPY-FL C11, hydroethidine, respectively; counterstained for viability). Sperm viability remained high, but sperm became immotile. Cryopreservation decreased sperm motility, viability, and intracellular O2- significantly, but did not affect H2O2. As expected, more sperm incubated in capacitating media than Beltsville thawing solution buffer underwent acrosome reactions and protein tyrosine phosphorylation (four proteins, 58-174 kDa); which proteins were tyrosine phosphorylated was pH dependent. Pre-exposing sperm to the ROS-generating system increased the percentage of sperm that underwent acrosome reactions after incubation in capacitating conditions (P < 0.025), and decreased capacitation-dependent increases in two tyrosine-phosphorylated proteins (P < or = 0.035). In summary, H2O2 is the major free radical mediating direct ROS effects, but not cryopreservation changes, on boar sperm. Boar sperm motility, acrosome integrity, and lipid peroxidation are more sensitive indicators of oxidative stress than viability and PLA activity. ROS may stimulate the acrosome reaction in boar sperm through membrane lipid peroxidation and PLA activation.

Role of phosphorylation and basic residues in the catalytic domain of cytosolic phospholipase A2alpha in regulating interfacial kinetics and binding and cellular function

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) is regulated by phosphorylation and calcium-induced translocation to membranes. Immortalized mouse lung fibroblasts lacking endogenous cPLA(2)alpha (IMLF(-/-)) were reconstituted with wild type and cPLA(2)alpha mutants to investigate how calcium, phosphorylation, and the putative phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding site regulate translocation and arachidonic acid (AA) release. Agonists that elicit distinct modes of calcium mobilization were used. Serum induced cPLA(2)alpha translocation to Golgi within seconds that temporally paralleled the initial calcium transient. However, the subsequent influx of extracellular calcium was essential for stable binding of cPLA(2)alpha to Golgi and AA release. In contrast, phorbol 12-myristate 13-acetate induced low amplitude calcium oscillations, slower translocation of cPLA(2)alpha to Golgi, and much less AA release, which were blocked by chelating extracellular calcium. AA release from IMLF(-/-) expressing phosphorylation site (S505A) and PIP(2) binding site (K488N/K543N/K544N) mutants was partially reduced compared with cells expressing wild type cPLA(2)alpha, but calcium-induced translocation was not impaired. Consistent with these results, Ser-505 phosphorylation did not change the calcium requirement for interfacial binding and catalysis in vitro but increased activity by 2-fold. Mutations in basic residues in the catalytic domain of cPLA(2)alpha reduced activation by PIP(2) but did not affect the concentration of calcium required for interfacial binding or phospholipid hydrolysis. The results demonstrate that Ser-505 phosphorylation and basic residues in the catalytic domain principally act to regulate cPLA(2)alpha hydrolytic activity.

Activation of epidermal growth factor receptor inhibits KCNQ2/3 current through two distinct pathways: membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation

KCNQ2/3 currents are the molecular basis of the neuronal M currents that play a critical role in neuron excitability. Many neurotransmitters modulate M/KCNQ currents through their G-protein-coupled receptors. Membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation are two mechanisms that have been proposed for modulation of KCNQ2/3 currents. In this study, we studied regulation of KCNQ2/3 currents by the epidermal growth factor (EGF) receptor, a member of another family of membrane receptors, receptor tyrosine kinases. We demonstrate here that EGF induces biphasic inhibition of KCNQ2/3 currents in human embryonic kidney 293 cells and in rat superior cervical ganglia neurons, an initial fast inhibition and a later slow inhibition. Additional studies indicate that the early and late inhibitions resulted from PtdIns(4,5)P2 hydrolysis and tyrosine phosphorylation, respectively. We further demonstrate that these two processes are mutually dependent. This study indicates that EGF is a potent modulator of M/KCNQ currents and provides a new dimension to the understanding of the modulation of these channels.

Role of phosphatidylinositol 4,5-bisphosphate in the activation of cytosolic phospholipase A2-α

Cytosolic phospholipase A(2)-alpha (cPLA(2)) plays an important role in the release of arachidonic acid and in cell injury. Activation of cPLA(2) is dependent on a rise in cytosolic Ca(2+) concentration, membrane association via the Ca(2+)-dependent lipid binding (CaLB) domain, and phosphorylation. This study addresses the activation of cPLA(2) via potential association with membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)), including the role of a "pleckstrin homology (PH)-like" region of cPLA(2) (amino acids 263-354). In cells incubated with complement, phorbol myristate acetate+the Ca(2+) ionophore, A23187, or epidermal growth factor+A23187, expression of the PH domain of phospholipase C-delta1 (which sequesters membrane PIP(2)) attenuated cPLA(2) activity. Stimulated cPLA(2) activity was also attenuated by the expression of cPLA(2) 135-366, or cPLA(2) 2-366, and expression of a PIP(2)-specific 5'-phosphatase. However, in a yeast-based assay that tests the ability of proteins to bind to membrane lipids, including PIP(2), with high affinity, only cPLA(2) 1-200 (CaLB domain) was able to interact with membrane lipids, whereas cPLA(2)s 135-366, 2-366, 201-648, and 1-648 were unable to do so. Therefore, cPLA(2) activity can be modulated by sequestration or depletion of cellular PIP(2), although the interaction of cPLA(2) with membrane PIP(2) appears to be indirect, or of weak affinity.

Mechanotransduction of stretch-induced prostanoid release by fetal lung epithelial cells

Mechanical ventilation is the primary supportive treatment for infants and adults suffering from severe respiratory failure. Adverse mechanical ventilation (overdistension of the lung) triggers a proinflammatory response. Along with cytokines, inflammatory mediators such as bioactive lipids are involved in the regulation of the inflammatory response. The arachidonic acid pathway is a key source of bioactive lipid mediators, including prostanoids. Although ventilation has been shown to influence the production of prostanoids in the lung, the mechanotransduction pathways are unknown. Herein, we established that cyclic stretch of fetal lung epithelial cells, but not fibroblasts, can evoke an extremely sensitive, rapid alteration in eicosanoid metabolism through a cyclooxygenase (COX)-2 dependent mechanism. Cyclic stretch significantly increased PGI(2), PGF(2alpha), PGD(2), PGE(2), and thromboxane B(2) levels in the media of epithelial cells, but did not alter leukotriene B(4) or 12-hydroxyeicosatetraenoic acid levels. Inhibition of COX-2, but not COX-1, attenuated the cyclic stretch-induced PG increase in the media, suggesting that cyclic stretch primarily affected PG synthesis. Substrate (free arachidonic acid) availability for PG generation was increased because of a cyclic stretch-induced activation of cytosolic phospholipase A(2) (cPLA(2)) via an influx of extracellular calcium and phosphorylation by mitogen-activated protein kinase, p44/42MAPK. The data are compatible with cPLA(2) and COX-2 being intimately involved in regulating the injury response to adverse mechanical ventilation.

Targeting non-human coronaviruses to human cancer cells using a bispecific single-chain antibody

To explore the potential of using non-human coronaviruses for cancer therapy, we first established their ability to kill human tumor cells. We found that the feline infectious peritonitis virus (FIPV) and a felinized murine hepatitis virus (fMHV), both normally incapable of infecting human cells, could rapidly and effectively kill human cancer cells artificially expressing the feline coronavirus receptor aminopeptidase N. Also 3-D multilayer tumor spheroids established from such cells were effectively eradicated. Next, we investigated whether FIPV and fMHV could be targeted to human cancer cells by constructing a bispecific single-chain antibody directed on the one hand against the feline coronavirus spike protein--responsible for receptor binding and subsequent cell entry through virus-cell membrane fusion--and on the other hand against the human epidermal growth factor receptor (EGFR). The targeting antibody mediated specific infection of EGFR-expressing human cancer cells by both coronaviruses. Furthermore, in the presence of the targeting antibody, infected cancer cells formed syncytia typical of productive coronavirus infection. By their potent cytotoxicity, the selective targeting of non-human coronaviruses to human cancer cells provides a rationale for further investigations into the use of these viruses as anticancer agents.

The role of omega-3 long-chain polyunsaturated fatty acids in health and disease of the retina

In this work we advance the hypothesis that omega-3 (omega-3) long-chain polyunsaturated fatty acids (LCPUFAs) exhibit cytoprotective and cytotherapeutic actions contributing to a number of anti-angiogenic and neuroprotective mechanisms within the retina. omega-3 LCPUFAs may modulate metabolic processes and attenuate effects of environmental exposures that activate molecules implicated in pathogenesis of vasoproliferative and neurodegenerative retinal diseases. These processes and exposures include ischemia, chronic light exposure, oxidative stress, inflammation, cellular signaling mechanisms, and aging. A number of bioactive molecules within the retina affect, and are effected by such conditions. These molecules operate within complex systems and include compounds classified as eicosanoids, angiogenic factors, matrix metalloproteinases, reactive oxygen species, cyclic nucleotides, neurotransmitters and neuromodulators, pro-inflammatory and immunoregulatory cytokines, and inflammatory phospholipids. We discuss the relationship of LCPUFAs with these bioactivators and bioactive compounds in the context of three blinding retinal diseases of public health significance that exhibit both vascular and neural pathology. How is omega-3 LCPUFA status related to retinal structure and function? Docosahexaenoic acid (DHA), a major dietary omega-3 LCPUFA, is also a major structural lipid of retinal photoreceptor outer segment membranes. Biophysical and biochemical properties of DHA may affect photoreceptor membrane function by altering permeability, fluidity, thickness, and lipid phase properties. Tissue DHA status affects retinal cell signaling mechanisms involved in phototransduction. DHA may operate in signaling cascades to enhance activation of membrane-bound retinal proteins and may also be involved in rhodopsin regeneration. Tissue DHA insufficiency is associated with alterations in retinal function. Visual processing deficits have been ameliorated with DHA supplementation in some cases. What evidence exists to suggest that LCPUFAs modulate factors and processes implicated in diseases of the vascular and neural retina? Tissue status of LCPUFAs is modifiable by and dependent upon dietary intake. Certain LCPUFAs are selectively accreted and efficiently conserved within the neural retina. On the most basic level, omega-3 LCPUFAs influence retinal cell gene expression, cellular differentiation, and cellular survival. DHA activates a number of nuclear hormone receptors that operate as transcription factors for molecules that modulate reduction-oxidation-sensitive and proinflammatory genes; these include the peroxisome proliferator-activated receptor-alpha (PPAR-alpha) and the retinoid X receptor. In the case of PPAR-alpha, this action is thought to prevent endothelial cell dysfunction and vascular remodeling through inhibition of: vascular smooth muscle cell proliferation, inducible nitric oxide synthase production, interleukin-1 induced cyclooxygenase (COX)-2 production, and thrombin-induced endothelin 1 production. Research on model systems demonstrates that omega-3 LCPUFAs also have the capacity to affect production and activation of angiogenic growth factors, arachidonic acid (AA)-based vasoregulatory eicosanoids, and MMPs. Eicosapentaenoic acid (EPA), a substrate for DHA, is the parent fatty acid for a family of eicosanoids that have the potential to affect AA-derived eicosanoids implicated in abnormal retinal neovascularization, vascular permeability, and inflammation. EPA depresses vascular endothelial growth factor (VEGF)-specific tyrosine kinase receptor activation and expression. VEGF plays an essential role in induction of: endothelial cell migration and proliferation, microvascular permeability, endothelial cell release of metalloproteinases and interstitial collagenases, and endothelial cell tube formation. The mechanism of VEGF receptor down-regulation is believed to occur at the tyrosine kinase nuclear factor-kappa B (NFkappaB). NFkappaB is a nuclear transcription factor that up-regulates COX-2 expression, intracellular adhesion molecule, thrombin, and nitric oxide synthase. All four factors are associated with vascular instability. COX-2 drives conversion of AA to a number angiogenic and proinflammatory eicosanoids. Our general conclusion is that there is consistent evidence to suggest that omega-3 LCPUFAs may act in a protective role against ischemia-, light-, oxygen-, inflammatory-, and age-associated pathology of the vascular and neural retina.

Activation of cytosolic phospholipase A2 in Her14 fibroblasts by hydrogen peroxide: a p42/44MAPK-dependent and phosphorylation-independent mechanism

Reactive oxygen species (ROS) have been implicated in the pathogenesis of diseases as well as various normal cellular processes. It has been suggested that ROS function as mediators of signal transduction, given that they can mimic growth factor-induced signaling. The ROS H2O2 has been reported to activate phospholipase A2 (PLA2) and, therefore, we investigated if and through which pathway ROS activate cytosolic PLA2 (cPLA2) in Her14 fibroblasts. cPLA2 was activated concentration-dependently by H2O2 in a transient manner. In addition, the lipophilic cumene hydroperoxide was shown to induce cPLA2 activity in the same manner. H2O2-induced cPLA2 activity in Her14 cells was partially phosphorylation-dependent, which was mediated through the Raf-MEK-p42/44(MAPK) pathway and occurred partially through a phosphorylation-independent mechanism. ROS can lead to changes in the (micro) viscosity of membranes due to the presence oxidized lipids, thereby increasing the substrate availability for cPLA2. In support of this, treatment of Her14 cells with H2O2 induced lipid peroxidation time-dependently as determined from degradation of lipid arachidonate and linoleate and the formation of aldehydic degradation products. Furthermore, H2O2 induced translocation of cPLA2 to the membrane fraction in a calcium-independent fashion, with a concomitant increase in cPLA2 activity. Collectively, the results suggest that oxidative stress-induced cPLA2 activity is partially phosphorylation-dependent and is further increased due to increased substrate availability by the action of ROS on membranes.

Henk van den Bosch: chemist and biochemist

Henk van den Bosch is a native of The Netherlands and recently retired from his position as Professor at Utrecht University. This article summarizes the many scientific achievements of Dr. van den Bosch. He enjoys an international reputation for his research on phospholipases A, cardiolipin biosynthesis in eukaryotes, lysophospholipases, phosphatidylcholine biosynthesis for lung surfactant, plasmalogen biosynthesis in peroxisomes, diagnosis of peroxisomal disorders and most recently his work on alkyl-dihydroxyacetone phosphate synthase. During his research career Henk van den Bosch published approximately 280 articles and presented 110 invited lectures.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

Inhibition of the MEK/ERK pathway has no effect on agonist-induced aggregation of human platelets

The activation of human platelets by a variety of agonists is accompanied by the phosphorylation of the extracellular signal-regulated kinase (ERK) isoforms of mitogen-activated protein (MAP) kinases. However, the role(s) of, and the substrate(s) for, these enzymes in platelet function remain unclear. Studies on ERKs in platelets have relied on pharmacological tools, including an inhibitor of ERK activation, U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene]. In the present study, the effects of U0126 and its "inactive" analogue, U0125 [1,4-diamino-2,3-dicyano-1,4-bis(phenylthio)butadiene], on human platelet aggregation and MAP kinase activity were examined. Several agonists with a variety of signaling pathways were studied including thrombin, a thromboxane analogue, arachidonic acid, collagen, calcium ionophores, and the phorbol ester phorbol myristate acetate (PMA). U0126, at concentrations consistent with inhibition of the isolated enzyme, inhibited ERK phosphorylation, and therefore MEK activation, in response to each agonist. Under such conditions, U0126 did not affect the phosphorylation of a second MAP kinase, p38(MAPK); however, platelet aggregation was also unaffected. Higher concentrations of U0126, and of U0125, inhibited platelet aggregation in response to collagen and PMA with no effect on that induced by the other agonists. These results dissociate ERK activation from platelet aggregation, suggesting an alternative role for ERKs in platelet function. In addition, the effects of higher concentrations of U0126 are likely due to an action on protein kinase C, likely unrelated to ERK inhibition, suggesting that the inhibitor concentration is crucial to the interpretation of such studies.

Hyperosmotic stress induces phosphorylation of cytosolic phospholipase A(2) in HaCaT cells by an epidermal growth factor receptor-mediated process

Cytosolic phospholipase A(2) (cPLA(2)) is an enzyme involved in the formation of proinflammatory mediators by catalyzing the release of arachidonic acid, thereby mediating eicosanoid biosynthesis. Using HaCaT keratinocytes as a model system, we present experimental evidence that in these cells, cPLA(2) is constitutively phosphorylated and that the degree of phosphorylation dramatically increases in cells under hyperosmotic stress induced by sorbitol. In parallel, a rapid release of arachidonic acid followed by prostaglandin E(2) formation was detected. Elucidating the mechanism of cPLA(2) upregulation, we observed that it is mediated via epidermal growth factor receptor (EGFR) activation, since tyrphostin AG1478, a selective inhibitor of EGFR tyrosine kinase, completely inhibited cPLA(2) phosphorylation. Furthermore, addition of PD98059, which is an inhibitor of MEK1 activation, but not of SB203580, which is an inhibitor of p38 stress kinase, inhibited cPLA(2) phosphorylation, indicating that the ras-raf-MEK cascade is the major signalling pathway involved in cPLA(2) phosphorylation. In addition, depletion of the cells from intracellular calcium does not prevent sorbitol-elicited cPLA(2) phosphorylation, suggesting that this process is independent of the presence of calcium. Together, our results demonstrate that hyperosmotic stress phosphorylates cPLA(2) in human keratinocytes by an EGFR-mediated process.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Regulation of phospholipase A(2) activity in cockroach (Periplaneta americana) fat body by hypertrehalosemic hormone: evidence for the participation of protein kinase C

Phospholipase A(2) (PLA(2)) associated with the membrane fraction of trophocytes from Periplaneta americana fat body increases by as much as 100% when the cells are incubated with hypertrehalosemic hormone (HTH-II). Activation with HTH-II is approximately halved by inclusion of the PKC inhibitor sphingosine in the incubation medium. Because activation of PLA(2) by HTH-II is blocked by the GDP analogue GDP-beta-S, and the unactivated enzyme is activated by the GTP analogue GTP-gamma-S it is likely that a G protein is involved in activation of the enzyme. Activation of PLA(2) was also achieved by treating the trophocytes with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol in the presence of thapsigargin. This supports the view that protein kinase C is also involved in the activation process.

Cytosolic phospholipase A(2) activity during the ongoing cell cycle

Cytosolic phospholipase A(2) (cPLA(2)) is of special interest because it selectively releases arachidonic acid from membrane phospholipids. Arachidonic acid has been implicated to play an important role in various cellular responses. Recently arachidonic acid release and prostaglandin synthesis have been shown to be cell cycle dependent and therefore the activity of cPLA(2) during the ongoing cell cycle was investigated, using the mitotic shake off method for cell synchronisation. cPLA(2) activity was high in mitotic cells and decreased rapidly in the early G1 phase. A strong increase in activity was measured following the G1/S transition in both neuroblastoma and Chinese hamster ovary cells. The changes in activity were not due to a difference in cPLA(2) expression but due to phosphorylation of cPLA(2). Phosphorylation of cPLA(2) occurs through MAPK since the use of a specific MAPK kinase inhibitor and serum depletion of synchronised cells inhibited cPLA(2) activity.

Phosphorylation of p42/44(MAPK) by various signal transduction pathways activates cytosolic phospholipase A(2) to variable degrees

Arachidonic acid has been implicated to play a role in physiological and pathophysiological processes and is selectively released by the 85-kDa cytosolic phospholipase A(2) (cPLA(2)). The activity of cPLA(2) is regulated by calcium, translocating the enzyme to its substrate, and by phosphorylation by a mitogen-activated protein kinase (MAPK) family member and a MAPK-activated protein kinase. In this study, the signal transduction pathways in growth factor-induced phosphorylation of p42/44(MAPK) and cPLA(2) activation were investigated in Her14 fibroblasts. p42/44(MAPK) in response to epidermal growth factor was not only phosphorylated via the Raf-MEK pathway but mainly through protein kinase C (PKC) or a related or unrelated kinase in which the phosphorylated p42/44(MAPK) corresponded with cPLA(2) activity. Serum-induced phosphorylation of p42/44(MAPK) also corresponded with cPLA(2) activity but is predominantly mediated via Raf-MEK and partly through PKC or a related or unrelated kinase. In contrast, activation of PKC by phorbol ester did not result in increased cPLA(2) activity, while p42/44(MAPK) is phosphorylated, mainly via Raf-MEK and through MEK. Moreover, p42/44(MAPK) phosphorylation is present in quiescent and proliferating cells, and p42/44(MAPK) is entirely phosphorylated via Raf-MEK, but it only corresponds to cPLA(2) activity in the former cells. Collectively, these data show that p42/44(MAPK) in proliferating, quiescent, and stimulated cells is phosphorylated by various signal transduction pathways, suggesting the activation of different populations of p42/44(MAPK) and cPLA(2).

Incorporation of map kinases into the platelet cytoskeleton

Erk1 (p44) and erk2 (p42) mitogen-activated protein (MAP) kinases are activated in agonist-stimulated platelets, although their role(s) in the activation process is unknown. In the present study, erk1, erk2 and the phosphorylated forms of both enzymes became associated with the contractile cytoskeleton in thrombin-stimulated platelets. Enzyme incorporation was accompanied by an increase in MAP kinase activity in the cytoskeleton, which was inhibited by PD98059. Pretreatment of the platelets with the arginine-glycine-aspartic acid-serine (RGDS) polypeptide enhanced both the cytoskeletal association and the enzyme activity, but cytochalasin D had no significant effect. Platelets from a patient with Glanzmann's thrombasthenia lack the alpha(IIb)beta(3) integrin and form only a rudimentary cytoskeleton, however, this cytoskeleton is enriched with both erk1 and erk2. These data suggest either that MAP kinases play a role in cytoskeletal rearrangement or that the cytoskeleton act as a frame to align MAP kinases with substrates in a highly integrated signal transduction pathway.

Cytosolic phospholipase A2-alpha associates with plasma membrane, endoplasmic reticulum and nuclear membrane in glomerular epithelial cells

Eicosanoids mediate complement-dependent glomerular epithelial injury in experimental membranous nephropathy. The release of arachidonic acid from phospholipids by cytosolic phospholipase A(2) (cPLA(2)) is the rate-limiting step in eicosanoid synthesis. The present study examines the association of cPLA(2) with membranes of organelles. Glomerular epithelial cells were disrupted by homogenization in Ca(2+)-free buffer; organelles were separated by gradient centrifugation. The distribution of cPLA(2) and organelles was analysed by immunoblotting with antibodies against cPLA(2) and organelle markers, or by enzyme assay. In cells incubated with or without the Ca(2+) ionophore ionomycin plus PMA, cPLA(2) co-localized with plasma membrane, endoplasmic reticulum and nuclei, but not with mitochondria or Golgi. A greater amount of cPLA(2) was associated with membranes in stimulated cells, but membrane-associated cPLA(2) was readily detectable under resting conditions. The pattern of association of cPLA(2) with membrane in cells treated with antibody and complement was similar to that in cells stimulated with ionomycin plus PMA; however, complement did not enhance the membrane association of cPLA(2) protein. To determine the functional role of membrane association of cPLA(2), phospholipids were labelled with [(3)H]arachidonic acid. Cells were then incubated with or without antibody and complement and were fractionated. Complement induced a loss of radioactivity from the plasma membrane, endoplasmic reticulum and nuclei, but not from the mitochondrial fraction. Thus the release of arachidonic acid by cPLA(2) is due to the hydrolysis of phospholipids at multiple subcellular membrane sites, including the endoplasmic reticulum, plasma membrane and nucleus.

Localization and regulation of cytosolic phospholipase A(2)

Liberation of arachidonic acid by cytosolic phospholipase A(2) (cPLA(2)) upon cell activation is often the initial and rate-limiting step in leukotriene and prostaglandin biosynthesis. This review discusses the essential features of cPLA(2) isoforms and addresses intriguing insights into the catalytic and regulatory mechanisms. Gene expression, posttranslational modification and subcellular localization can regulate these isoforms. Translocation of cPLA(2)alpha from the cytosol to the perinuclear region in response to calcium transients is critical for the immediate arachidonic acid release. Therefore, particular emphasis is placed on the mechanism of the translocation and the role of the proteins and lipids implicated in this process. The regional distribution and cellular localization of cPLA(2) may help to better understand its function as an arachidonic acid supplier to downstream enzymes and as a regulator of specific cellular processes.

Regulation of cytosolic phospholipase A(2) in a new perspective: recruitment of active monomers from an inactive clustered pool

cPLA(2) plays a key role in many signal transduction cascades by hydrolyzing arachidonic acid from membrane phospholipids. Tight control of cPLA(2) activity by a number of regulatory mechanisms is essential to its cellular function. We recently described the localization of cPLA(2) in clusters in fibroblasts and now propose that these clusters reflect a localized inactive pool from which active monomers can be recruited to keep cPLA(2) activity under control on the subcellular level. Using an electron microscopic in vitro approach, we show that cPLA(2) monomers, but not the clusters, bind to membranes in a Ca(2+)-dependent manner. This binding is accompanied by hydrolytic activity. The present data combined with our previous observation of a relative abundance of clusters over monomers in fixed fibroblasts [Bunt, G., de Wit, J., van den Bosch, H., Verkleij, A., and Boonstra, J. (1997) J. Cell Sci. 110, 2449-2459] gives rise to a concept of cPLA(2) regulation in which small amounts of active monomers are recruited to fulfill their function upon stimulation. This is in contrast to processes described for inflammatory cells, where a substantial part of the cytoplasmically localized cPLA(2) translocates to the perinuclear region upon stimulation to become active. Small-scale regulation of cPLA(2) by the proposed cluster-monomer cycle allows local and strictly confined control of cPLA(2) activity, apparently necessary for its cellular role in fibroblasts.

The MAP kinase inhibitors, PD098059, UO126 and SB203580, inhibit IL-1beta-dependent PGE(2) release via mechanistically distinct processes

1. In common with human bronchial epithelial cells, pulmonary A549 cells release prostaglandin (PG) E(2) in response to pro-inflammatory cytokines. We have therefore used these cells to examine the effect of the selective mitogen activated protein (MAP) kinase inhibitors; PD098059, a mitogen activated and extracellular regulated kinase kinase (MEK) 1 inhibitor, UO126, a dual MEK1 & MEK2 inhibitor, and SB203580, a p38 MAP kinase inhibitor in the IL-1beta-dependent release of PGE(2). 2. Following IL-1beta treatment the extracellular regulated kinases (ERKs) and the p38 MAP kinases were rapidly phosphorylated. 3. PD09059, UO126 and SB203580 prevented IL-1beta-induced PGE(2) release at doses that correlated closely with published IC(50) values. Small or partial effects at the relevant doses were observed on induction of cyclo-oxygenase (COX) activity or COX-2 protein suggesting that the primary effects were at the level of arachidonate availability. 4. Neither PD098059 nor SB203580 showed any effect on IL-1beta-induced arachidonate release. We therefore speculate that the MEK1/ERK and p38 kinase cascades play a role in the functional coupling of arachidonate release to COX-2. 5. In contrast, UO126 was highly effective at inhibiting IL-1beta-dependent arachidonate release, implicating MEK2 in the activation of the PLA(2) that is involved in IL-1beta-dependent PGE(2) release. 6. We conclude that the MEK1, MEK2 and p38 MAP kinase inhibitors, PD098059, UO126 and SB203580, are highly potent in respect of inflammatory PG release. Finally, we conclude that these inhibitors act via mechanistically distinct processes, which may have anti-inflammatory benefits.

Cytosolic phospholipase A2 is required for macrophage arachidonic acid release by agonists that Do and Do not mobilize calcium. Novel role of mitogen-activated protein kinase pathways in cytosolic phospholipase A2 regulation

The 85-kDa cytosolic phospholipase A(2) (cPLA(2)) mediates agonist-induced arachidonic acid release and eicosanoid production. Calcium and phosphorylation on Ser-505 by mitogen-activated protein kinases (MAPKs) regulate cPLA(2). Arachidonic acid release and eicosanoid production induced by stimuli that do (A23187, zymosan) or do not (phorbol myristate acetate (PMA), okadaic acid) mobilize calcium were quantitatively suppressed in cPLA(2)-deficient mouse peritoneal macrophages. The contribution of MAPKs to cPLA(2)-mediated arachidonic acid release was investigated. Both extracellular signal-regulated kinases (ERKs) and p38 contributed to cPLA(2) phosphorylation on Ser-505. However, although ERK inhibition did not affect A23187-induced arachidonic acid release, it suppressed zymosan-, PMA-, and okadaic acid-induced arachidonic acid release under conditions where phosphorylation of cPLA(2) on Ser-505 was unaffected. This indicates an additional regulatory mechanism for the ERK pathway. A role for transcriptional regulation is suggested by data showing that cycloheximide and actinomycin D inhibited arachidonic acid release induced by zymosan, PMA and, okadaic acid but not by A23187. Our results show that MAPK pathways contribute to arachidonic acid release in macrophages through alternative mechanisms in addition to their ability to phosphorylate cPLA(2) on Ser-505 and suggest a role for new protein synthesis.

A Ca(2+)-independent activation of a type IV cytosolic phospholipase A(2) underlies the receptor stimulation of arachidonic acid-dependent noncapacitative calcium entry

The oscillatory [Ca(2+)](i) signals typically seen following physiologically relevant stimulation of phospholipase C-linked receptors are associated with a receptor-activated entry of Ca(2+), which plays a critical role in driving the oscillations and influencing their frequency. We have recently shown that this receptor-activated entry of Ca(2+) does not conform to the widely accepted "capacitative" model and, instead, reflects the activity of a distinct, novel Ca(2+) entry pathway regulated by arachidonic acid (Shuttleworth, T. J., and Thompson, J. L. (1998) J. Biol. Chem. 273, 32636-32643). We now show that the generation of arachidonic acid under these conditions results from the activity of a type IV cytosolic phospholipase A(2) (cPLA(2)). Although cPLA(2) activation commonly involves a Ca(2+)-dependent translocation to the membrane, at these low agonist concentrations cPLA(2) activation was independent of increases in [Ca(2+)](i), and no detectable translocation to the membrane occurs. Nevertheless, stimulation of cPLA(2) activity was confined to the membrane fraction, where an increase in phosphorylation of the enzyme was observed. We suggest that, at the low agonist concentrations associated with oscillatory [Ca(2+)](i) signals, cPLA(2) activation involves an increased phosphorylation of a discrete pool of the total cellular cPLA(2) that is already localized within the membrane fraction at resting [Ca(2+)](i).

N-terminal and C-terminal plasma membrane anchoring modulate differently agonist-induced activation of cytosolic phospholipase A2

The 85 kDa cytosolic phospholipase A2 (cPLA2) plays a key role in liberating arachidonic acid from the sn-2 position of membrane phospholipids. When activated by extracellular stimuli, cPLA2 undergoes calcium-dependent translocation from cytosol to membrane sites which are still a matter of debate. In order to evaluate the effect of plasma membrane association on cPLA2 activation, we constructed chimeras of cPLA2 constitutively targeted to the plasma membrane by the N-terminal targeting sequence of the protein tyrosine kinase Lck (Lck-cPLA2) or the C-terminal targeting signal of K-Ras4B (cPLA2-Ras). Constitutive expression of these chimeras in Chinese hamster ovary cells overproducing the alpha2B adrenergic receptor (CHO-2B cells) did not affect the basal release of [3H]arachidonic acid, indicating that constitutive association of cPLA2 with cellular membranes did not ensure the hydrolysis of membrane phospholipids. However, Lck-cPLA2 increased [3H]arachidonic acid release in response to receptor stimulation and to increased intracellular calcium, whereas cPLA2-Ras inhibited it, compared with parental CHO-2B cells and CHO-2B cells producing comparable amounts of recombinant wild-type cPLA2. The lack of stimulation of cPLA2-Ras was not due to a decreased enzymatic activity as measured using an exogenous substrate, or to a decreased phosphorylation of the protein. These results show that the plasma membrane is a suitable site for cPLA2 activation when orientated correctly.

Signaling mechanisms involved in the activation of arachidonic acid metabolism in human astrocytoma cells by tumor necrosis factor-alpha: phosphorylation of cytosolic phospholipase A2 and transactivation of cyclooxygenase-2

Tumor necrosis factor-alpha (TNF-alpha) is a cytokine that elicits cell responses by activating the mitogen-activated protein kinase (MAP kinase) cascade and transcription factors such as nuclear factor-kappaB (NF-kappaB). As these elements play a central role in the mechanisms of signaling involved in the activation of cytosolic phospholipase A2 (cPLA2) and cyclooxygenase-2 (COX-2), the effect of TNF-alpha on arachidonate (AA) metabolism in 1321N1 astrocytoma cells was assayed. TNF-alpha produced a phosphorylation of cPLA2, which was preceded by an activation of both c-Jun N-terminal kinase (JNK) and p38-MAP kinase, and this was associated with the release of [3H]AA. In contrast, TNF-alpha did not activate the extracellular signal-regulated kinase (MAP kinase) p42, nor did it elicit a mitogenic response. Analysis of [3H]AA metabolites by reverse-phase HPLC showed that all of the [3H]AA released during the first hour after TNF-alpha addition eluted as authentic AA, whereas in samples obtained at 24 h after addition of TNF-alpha, 25% of the [3H]AA had been converted into COX products as compared with only 9% in control cells. In keeping with these findings, TNF-alpha produced an increase of COX-2 expression, as judged from both RT-PCR studies and immunoblot of COX-2 protein, and a long-lasting activation of NF-kappaB. These data show that TNF-alpha produces in astrocytoma cells an early activation of both p38-MAP kinase and JNK, which is followed by the phosphorylation of cPLA2 and the release of AA. On the other hand, the activation of NF-kappaB may explain the induction of the expression of COX-2 and the delayed generation of prostanoids.

Phosphorylation-dependent stimulation of prostanoid synthesis by nigericin in cerebral endothelial cells

Nigericin decreases intracellular pH (pH(i)) and stimulates prostanoid (PG) synthesis in endothelial cells from cerebral microvessels of newborn pigs. Nigericin-induced PG production was abolished by protein tyrosine kinase (PTK) inhibitors and amplified by phorbol 12-myristate 13-acetate (PMA) or protein tyrosine phosphatase (PTP) inhibitors. Nigericin-induced PG production in PMA-primed cells was potentiated by PTP inhibitors and abrogated by PTK inhibitors. Phospholipase A(2) (PLA(2)) activity was stimulated by nigericin in a phosphorylation-dependent manner. Nigericin's effects on PG production and PLA(2) activity were reproduced by ionomycin, which activates cytosolic PLA(2) (cPLA(2)). cPLA(2) was immunodetected in endothelial cell lysates. We found no evidence that nigericin's effects are mediated via mitogen-activated protein (MAP) kinase [extracellularly regulated kinase 1 (ERK1) and ERK2] activation: although nigericin stimulated detergent-soluble MAP kinase, its effects were not amplified by PMA or PTP inhibitors. Phosphorylation-dependent stimulation of PG synthesis was also observed when pH(i) was decreased by sodium propionate or a high level of CO(2). Altogether, our data indicate that nigericin and decreased pH(i) stimulate PG synthesis by a protein phosphorylation-dependent mechanism involving cross talk between pathways mediated by PTK and PTP and by protein kinase C; cPLA(2) appears to be a key enzyme affected by nigericin and decreased pH(i).

A role of protein kinase C in the regulation of cytosolic phospholipase A(2) in bradykinin-induced PGI(2) synthesis by human vascular endothelial cells

The purpose of this study was to elucidate the mechanism by which bradykinin (BK) enhances prostacyclin (PGI(2)) production in human umbilical vein endothelial cells (HUVEC). BK-induced enhancement of PGI(2) synthesis was observed in a dose- and time-dependent manner, and it also increased [Ca(2+)](i) followed by enhancement of cytosolic phospholipase A(2) (cPLA(2)) activity. The PKC inhibitors GF109203X and H7 attenuated the BK-induced increase in [Ca(2+)](i) and inhibited the BK-induced PGI(2) synthesis. Phorbol 12-myristate 13-acetate increased cPLA(2) activity and PGI(2) synthesis but failed to alter [Ca(2+)](i). BK increased cPLA(2) mRNA eightfold by 15 min, and this increase was inhibited by pretreatment with the PKC inhibitors. In response to cycloheximide pretreatment, cPLA(2) mRNA was superinduced. These results suggest that BK stimulates PGI(2) synthesis in HUVEC by activation of cPLA(2) by dual mechanisms: an elevation of [Ca(2+)](i) and a PKC-dependent pathway. Moreover, changes in calcium kinetics and expression of cPLA(2) mRNA may underlie the BK-induced PGI(2) enhancement in these cells.

Potentiating effects of hyper-osmolality and epidermal growth factor on the release of arachidonic acid in human glomerular mesangial cells

Studies were performed to determine the interactive effects of high concentrations of glucose (HG) and epidermal growth factor (EGF) on the release of arachidonic acid (3H-AA) in human glomerular mesangial cells (MC) in culture. Since high glucose has been reported to increase the mass of diacylglycerol (DAG) in MC, the HG-induced release of 3H-AA was compared to that initiated by the phorbol ester, PMA. It was found that when media contained high levels (25 mM) of glucose, the release of 3H-AA was increased significantly by 8.4% (change from control) after 1 h of exposure and was maintained at values not significantly different from this level for the next 2 h. After 3-h exposure, there was no significant difference between 25 and 50 mM glucose, suggesting that the effects of glucose are saturating at 25 mM. After 1-h exposure, 3H-AA release was also increased by PMA; however, the increase was larger and the peak increase was delayed until after 1 h. 3H-AA release was significantly increased by epidermal growth factor (EGF) by 8.5% after 1 h and was maintained at this level after 2 and 3 h of exposure. In the presence of HG, EGF increased 3H-AA release by 24.6% after the 1st hour and by 20.4 and 19.4%, after the 2nd and 3rd hours, respectively. Mannitol (20 mM), added as an osmotic control, increased 3H-AA release by 6.2% and also significantly enhanced the effects of EGF after 3 h. The experimental values (19.0%) for the release of 3H-AA after 3-h exposure to EGF in combination with either high glucose or mannitol were significantly greater than the expected (added) values (12.1%). These results demonstrate that as a result of an elevated solution osmolality, high glucose acts synergistically with EGF to increase the release of 3H-AA in human mesangial cells.

Extracellular calcium regulates HeLa cell morphology during adhesion to gelatin: role of translocation and phosphorylation of cytosolic phospholipase A2

Attachment of HeLa cells to gelatin induces the release of arachidonic acid (AA), which is essential for cell spreading. HeLa cells spreading in the presence of extracellular Ca2+ released more AA and formed more distinctive lamellipodia and filopodia than cells spreading in the absence of Ca2+. Addition of exogenous AA to cells spreading in the absence of extracellular Ca2+ restored the formation of lamellipodia and filopodia. To investigate the role of cytosolic phospholipase A2 (cPLA2) in regulating the differential release of AA and subsequent formation of lamellipodia and filopodia during HeLa cell adhesion, cPLA2 phosphorylation and translocation from the cytosol to the membrane were evaluated. During HeLa cell attachment and spreading in the presence of Ca2+, all cPLA2 became phosphorylated within 2 min, which is the earliest time cell attachment could be measured. In the absence of extracellular Ca2+, the time for complete cPLA2 phosphorylation was lengthened to <4 min. Maximal translocation of cPLA2 from cytosol to membrane during adhesion of cells to gelatin was similar in the presence or absence of extracellular Ca2+ and remained membrane associated throughout the duration of cell spreading. The amount of total cellular cPLA2 translocated to the membrane in the presence of extracellular Ca2+ went from <20% for unspread cells to >95% for spread cells. In the absence of Ca2+ only 55-65% of the total cPLA2 was translocated to the membrane during cell spreading. The decrease in the amount translocated could account for the comparable decrease in the amount of AA released by cells during spreading without extracellular Ca2+. Although translocation of cPLA2 from cytosol to membrane was Ca2+ dependent, phosphorylation of cPLA2 was attachment dependent and could occur both on the membrane and in the cytosol. To elucidate potential activators of cPLA2, the extracellular signal-related protein kinase 2 (ERK2) and protein kinase C (PKC) were investigated. ERK2 underwent a rapid phosphorylation upon early attachment followed by a dephosphorylation. Both rates were enhanced during cell spreading in the presence of extracellular Ca2+. Treatment of cells with the ERK kinase inhibitor PD98059 completely inhibited the attachment-dependent ERK2 phosphorylation but did not inhibit cell spreading, cPLA2 phosphorylation, translocation, or AA release. Activation of PKC by phorbol ester (12-O-tetradecanoylphorbol-13-acetate) induced and attachment-dependent phosphorylation of both cPLA2 and ERK2 in suspension cells. However, in cells treated with the PKC inhibitor Calphostin C before attachment, ERK2 phosphorylation was inhibited, whereas cPLA2 translocation and phosphorylation remained unaffected. In conclusion, although cPLA2-mediated release of AA during HeLa cell attachment to a gelatin substrate was essential for cell spreading, neither ERK2 nor PKC appeared to be responsible for the attachment-induced cPLA2 phosphorylation and the release of AA.

The linear C-terminal regions of epidermal growth factor (EGF) and transforming growth factor-alpha bind to different epitopes on the human EGF receptor

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGFalpha) bind with similar affinities in a competitive fashion to the human EGF receptor, and basically induce similar mitogenic responses. In spite of the fact that EGF and TGFalpha are structurally alike, it is still not clear if the two growth factors bind the receptor in an identical manner. The observation that the 13A9 antibody blocks binding of TGFalpha, but not that of EGF, to the human EGF receptor [Winkler, O'Connor, Winget and Fendly (1989) Biochemistry 28, 6373-6378] suggests that their binding characteristics are not identical. In the present study we have made use of a set of EGF/TGFalpha chimaeric molecules to show that the 13A9 antibody blocks receptor binding of ligands with TGFalpha sequences, but not of ligands with EGF sequences, in their C-terminal linear regions. Using HaCaT human keratinocyte cells in culture, it was determined that ligands that are able to bind the EGF receptor in the presence of 13A9 are also able to induce calcium release from intracellular stores in these cells, indicating that these ligands have the ability to activate the EGF receptor in the presence of the antibody. From these data it is concluded that the flexible C-terminal linear domains of EGF and TGFalpha bind to separate sequences on the EGF receptor, such that the binding domain of TGFalpha, but not that of EGF, overlaps with the binding epitope of the 13A9 antibody.

Sustained Phosphorylation of Cytosolic Phospholipase A2 Accompanies Cycloheximide- and Adenovirus-Induced Susceptibility to TNF

In this report we examine the phosphorylation state of cytosolic phospholipase A2 (cPLA2) in C3HA fibroblasts that have been treated with TNF, cycloheximide (CHI), or a combination of both compounds. Our experiments show that TNF and CHI, when used independently, caused the rapid phosphorylation of cPLA2 (within 10 min). In both cases, cPLA2 was subsequently dephosphorylated to pretreatment levels by 40 min. In addition, under these conditions [3H]arachidonic acid was not released, and we could not detect a change in the activity of cPLA2 in vitro. In contrast, in cells treated with a combination of TNF and CHI, we found that the dephosphorylation of cPLA2 was inhibited, and cPLA2 remained phosphorylated for up to 2 h. In vitro we found that sustained phosphorylation of cPLA2 was accompanied by a 60 to 80% increase in the activity of cPLA2. The sustained phosphorylation of cPLA2 also occurred in cells infected with the adenovirus mutant dl309, suggesting that sustained phosphorylation may be a general requirement for the activation of cPLA2 in apoptotic cells. We also found that sustained phosphorylation of phosphoproteins is not a general consequence of apoptotic death, since the phosphorylation of p42 mitogen-activated protein kinase was not sustained. Finally, we show that the phosphatase inhibitor orthovanadate acts as does CHI to render cells susceptible to TNF, suggesting that resistance to TNF may depend on TNF’s ability to induce the expression of tyrosine or dual specificity phosphatase(s).

Effects of the mitogen-activated protein (MAP) kinase kinase inhibitor 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059) on human platelet activation

The role of mitogen-activated protein (MAP) kinase cascades in platelet function remains to be determined. Several studies have suggested a role in the activation of phospholipase A2; however, other functions seem likely. The object of the present study was to determine the role of the MAP kinase cascade in platelet function. An inhibitor of the mitogen-activated protein kinase kinase MEK1, 2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059), was used, at concentrations consistent with those reported to inhibit MEK1, to examine the role that this enzyme plays in platelet function. PD98059 inhibited aggregation in response to low-dose collagen and arachidonic acid, but not that in response to high-dose collagen, thrombin, thrombin receptor-activating peptide (TRAP), 9,11-dideoxy-11alpha, 9alpha-epoxymethano-prostaglandin F2alpha (U46619), or phorbol ester. Thrombin, thrombin receptor-activating peptide, U46619, collagen, and arachidonic acid each caused the release of [3H]serotonin from dense granules, but only that elicited by low-dose collagen and arachidonic acid was inhibited by PD98059. The release of [3H]arachidonic acid in response to thrombin or collagen was unaffected by PD98059 pretreatment. In contrast, collagen- and arachidonic acid-induced thromboxane formation was inhibited by PD98059. These data suggest that MEK1 is not involved in the platelet response to thrombin or U46619. Furthermore, the inhibitory effects of PD98059 on collagen- and arachidonic acid-induced responses suggest that PD98059 may inhibit the conversion of arachidonic acid to thromboxane, in addition to its reported effects on MEK1.

Overexpression of EGFR and c-erbB2 causes enhanced cell migration in human breast cancer cells and NIH3T3 fibroblasts

Overexpression of EGFR and c-erbB2 frequently occurs in human breast cancers, correlating with poor prognosis. Here we show that overexpression of EGFR and c-erbB2 in cell lines increases cell migration, an important step in metastasis formation. The effect of EGFR on migration is dependent on the addition of EGF to the cells. In contrast, c-erbB2 seems to act independently of its ligand in these assays. Overexpression of this receptor is sufficient to induce cell migration. In addition, we investigated the involvement of a number of signal transduction pathways known to be activated by the EGFR. We found that inactivation of MAPKK results in a decreased migration, while inactivation of PI3K increases migration.

The regulation of vascular function by P2 receptors: multiple sites and multiple receptors

Although the effects of nucleotides in the cardiovascular system have been known for almost 70 years, it is only in the past few years that some of the P2 receptors at which they act have been cloned and characterized. It is now clear that the control of cardiovascular function by nucleotides is complex, involving multiple receptors and multiple effects in the different cell types of importance. In this review Mike Boarder and Susanna Hourani summarize the P2 receptors that are present in endothelial cells, platelets, smooth muscle and nerves, the signalling pathways that they activate and the responses that are produced. They also discuss the important role of nucleotides in the interactions between the different cell types, and the implications of this in vascular disease.

Generation of reactive oxygen species in a human keratinocyte cell line: role of calcium

In the human keratinocyte cell line HaCaT, reactive oxygen species (ROS) were generated in a dose- and time-dependent manner in response to epidermal growth factor (EGF), bradykinin, thapsigargin, and the Ca(2+)-ionophore A23187, agonists that interact with different primary cell targets. ROS formation was assessed by both chemiluminescence- and fluorescence-based methods. The ROS evoked by EGF and bradykinin decayed within 8 and 4 min, respectively, this transient effect resulting probably from down-regulation of the specific agonist receptors or dissipation of the secondary signals. In contrast, the response to thapsigargin and A23187 was sustained for at least 15 min. Extracellular Ca2+ and a rise in intracellular Ca2+ concentration ([Ca2+]i) proved essential for ROS production. Chelation by BAPTA suppressed ROS formation. Direct measurement of [Ca2+]i using fura fluorescence revealed that EGF and bradykinin evoked a modest, transient [Ca2+]i elevation of less than twofold, whereas with thapsigargin and A23187 there was a sustained two- to fourfold elevation. For each agonist, the kinetics of the rise and decay of [Ca2+]i were similar to those of ROS. The enzyme(s) involved in ROS formation were inhibited by diphenyleneiodonium, indicating dependence on FAD. Our results suggest a close link between ROS and changes in [Ca2+]i generated by growth factors and hormones. This is a particularly interesting connection because elevation of ROS and/ or [Ca2+]i has been linked to cell proliferation, differentiation, and apoptosis.

ANG II-induced translocation of cytosolic PLA2 to the nucleus in vascular smooth muscle cells

The accumulation of radiolabeled arachidonic acid (AA), immunoblot analysis of subcellular fractions, and immunofluorescence tagging of proteins in intact cells were used to examine the coupling of ANG II receptors with the activity and location of a cytosolic phospholipase A2 (cPLA2) in vascular smooth muscle cells (VSMC). ANG II induced the accumulation of AA, which peaked by 10 min and was downregulated by 20 min. A large proportion of the AA released in response to ANG II was due to the activation of a Ca(2+)-dependent lipasc coupled to an AT1 receptor. However, regulation of Ca2+ availability failed to completely block AA release, and a small but significant reduction in ANG II-mediated AA release was observed in the presence of an AT2 antagonist. These findings, coupled with a 25% reduction in the ANG II-induced AA release by an inhibitor specific for a Ca(2+)-independent PLA2, are consistent with the presence and activation of a Ca(2+)-independent PLA2. In contrast, immunoblot analysis and immunofluorescence detection showed that the ANG II-mediated translocation of cPLA2 to a membrane fraction was exclusively AT1 dependent and regulated by Ca2+ availability. Furthermore, the nucleus was the membrane target. We conclude that ANG II regulates the Ca(2+)-dependent activation and translocation of cPLA2 through an AT1 receptor and that this event is targeted at the nucleus in VSMC.

Differential routes of Ca2+ influx in Swiss 3T3 fibroblasts in response to receptor stimulation

Ca2+ influx into cells in response to stimulation of various receptors was studied with Swiss 3T3 fibroblasts. The mechanisms involved were found to be so diverse that they were classified into four groups, Type I to IV. Type-I influx occurred, via pertussis toxin-susceptible G-proteins, immediately after internal Ca2+ mobilization by bradykinin, thrombin, endothelin, vasopressin or angiotensin II. Type-II influx induced by bombesin differed from Type I in its insusceptibility to pertussis toxin treatment. Ca2+ influx induced by prostaglandin E1, referred to as Type-III influx, was unique in that phospholipase C was apparently not activated without extracellular Ca2+, strongly suggesting that the Ca2+ influx preceded and was responsible for InsP3 generation and internal Ca2+ mobilization. More Ca2+ entered the cells more slowly via the Type-IV route opened by platelet-derived and other growth factors. These types of Ca2+ influx could be differentiated by their different susceptibilities to protein kinase C maximally activated by 1 h of exposure of cells to PMA, which inhibited phospholipase Cbeta coupled to receptors involved in Type-I and -II influx but did not inhibit growth-factor-receptor-coupled phospholipase Cgamma. Type-I and -II Ca2+ influxes, together with store-operated influx induced by thapsigargin, were not directly inhibited by exposure of cells to PMA, but Type-III and -IV influxes were completely inhibited. In addition, stimulation of receptors involved in Type-I and -IV Ca2+ influx, but not Type-II and -III influx, led to phospholipase A2 activation in the presence of extracellular Ca2+. Inhibition of Type-I and -IV Ca2+ influxes by their respective inhibitors, diltiazem and nifedipine, resulted in abolition of phospholipase A2 activation induced by the respective receptor agonists, in agreement with the notion that Ca2+ influx via these routes is responsible for receptor-mediated phospholipase A2 activation.

Tamoxifen inhibits the release of arachidonic acid stimulated by thapsigargin in estrogen receptor-negative A549 cells

In pre-labelled A549 cells the tumour promoter thapsigargin (50 nM) stimulates the release of [5,6,8,9,11,12,14,15-3H(N)]-arachidonic acid (3H-AA) by ca. 300% above basal levels. A549 cells are estrogen receptor negative (ER-), yet this stimulation by thapsigargin is inhibited in a dose-dependent manner by a 3 h pre-treatment with the anti-estrogen tamoxifen (1-20 microM). Moreover, the presence of excess (100 microM) estradiol does not reverse this effect of tamoxifen. Thapsigargin stimulated 3H-AA release is not inhibited over the same concentration range by 4 hydroxy-tamoxifen nor by the steroidal anti-estrogen ICI 164384. However, the steroidal anti-estrogen ICI 182780 inhibits thapsigargin stimulated 3H-AA release in a similar manner to tamoxifen and this effect is also not reversed by the presence of excess estradiol. Stimulation of 3H-AA release by EGF (10 nM), IL-1beta (1 ng ml-1) and bradykinin (100 nM) was unaffected by these concentrations of tamoxifen. Ionomycin (10 microM) stimulates 3H-AA release by ca. 700% and A23187 (10 microM) by ca. 300% above basal levels. Pre-treatment with tamoxifen (1-20 microM) inhibits 3H-AA release stimulated by both these agents and again the presence of excess estradiol does not reverse this effect. Unlike the effects of glucocorticoids on 3H-AA release in A549 cells the effects of tamoxifen are not reversed by neutralizing anti-bodies to lipocortin 1. Arachidonic acid release is central to cell proliferation in A549 cells and we propose that this action of tamoxifen could explain the anti-proliferative effect seen in these cells and could have important implications for control of cell proliferation of ER- cells in general.

Phosphorylation and calcium influx are not sufficient for the activation of cytosolic phospholipase A2 in U937 cells: requirement for a Gi alpha-type G-protein

Differentiation with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line toward a monocyte/granulocyte-like cell results in the cell acquiring an ability to release arachidonate upon stimulation. In contrast, the calcium ionophore ionomycin was able to stimulate phospholipase C, as measured by inositol 1,4,5-trisphosphate formation, to equal extents in both undifferentiated and dBcAMP-differentiated U937 cells. The role and regulation of cytosolic phospholipase A2 (cPLA2) in the production of arachidonate in these cells when either the chemotactic peptide fMLP or ionomycin are used as stimulus were investigated. The ionomycin- and fMLP-stimulated release of arachidonate were sensitive to the cPLA2 inhibitor arachidonyl trifluoromethylketone (IC50 values of 32 and 18 microM, respectively), but were not inhibited by E-6-(bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2 H-pyran-2-one, a bromoenol lactone inhibitor of the calcium-independent phospholipase A2. These results, coupled with the inhibition of ionomycin-induced arachidonate production by electroporation of differentiated cells to introduce an anti-cPLA2, demonstrate that the cPLA2 is the enzyme responsible for arachidonate release in differentiated cells. SDS-PAGE and immunoblot analysis of differentiated cells showed the cells to contain both phosphorylated and unphosphorylated forms of cPLA2 (ratio of about 2: 3). Surprisingly, undifferentiated cells contain 30% more enzyme than differentiated cells and contain a higher percentage (approximately 75%) of the phosphorylated in the absence of stimulation. The inability of undifferentiated cells to produce arachidonate is not due to insufficient intracellular calcium concentrations since ionomycin induces large (820-940 nM) influxes of intracellular calcium in both differentiated and undifferentiated cells. This demonstrates that phosphorylation of cPLA2 andan influx of intracellular calcium are not sufficient to activate the enzyme to produce arachidonate. Instead, activation of a pertussis toxin-sensitive Gi alpha-type G-protein is required as evidenced by the production of arachidonate in undifferentiated cells stimulated with mastoparan, an activator of Gi alpha subunits, in combination with ionomycin. This activation of a Gi alpha-type G-protein is independent of modulations of adenylyl cyclase activity since cellular cAMP levels were not modulated upon treatment with mastoparan and ionomycin.

Presence of glycerol masks the effects of phosphorylation on the catalytic efficiency of cytosolic phospholipase A2

Cytosolic phospholipase A2 catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. The enzymatic activity of cPLA2 is affected by several mechanisms, including substrate presentation and the phosphorylation state of the enzyme. Using covesicles of 1-palmitoy1-2-arachidonoyl-[arachidonoyl-1-14C]-8n-glycero-3 -phosphocholine and 1,2-dimyristoyl-phosphatidylmethanol as substrate, the effects of phosphorylation on the interfacial binding and catalytic constants were investigated. Phosphorylated and dephosphorylated enzyme forms were shown to have identical values of 2.6 microM for KMapp, an equilibrium dissociation constant which consists of the intrinsic dissociation constant from the lipid/water interface (Ks) and the dissociation constant for phospholipid from the active site (KM*). Moreover, the values of KM* for phosphorylated and dephosphorylated enzyme did not differ significantly (0.4 +/- 0.1 and 0.2 +/- 0.1, respectively). However, dephosphorylation of the enzyme reduced the value of kcat by 39%. The phosphorylation state of the enzyme had no effect on either the cooperativity shown by this enzyme or the thermal stability of the enzyme. Surprisingly, the presence of glycerol (4 M) masks the effect of phosphorylation on kcat. Instead, glycerol increased the value of kcat by 440% for the phosphorylated enzyme and by 760% for the dephosphorylated form. Moreover, addition of glycerol had only small effects on KMapp. the increase in the kcat upon addition of glycerol results from a substantial decrease in the activation energy from 29.4 to 14.8 kcal. mol-1. To determine whether the effects of phosphorylation of the enzyme or addition of glycerol are unique to this artificial substrate, membranes from U937 cells were isolated and used as substrate. With these membranes, the dephosphorylated enzyme was only 21% less active than the phosphorylated enzyme. In the presence of glycerol, there was no detectable difference the two enzyme forms, and the rate of hydrolysis was increased by 300-390% over that measured in the absence of glycerol. These results suggest that the catalytic efficiency of the phosphorylated enzyme is not particularly relevant to its activation in vivo. Moreover, it may be that glycerol is mimicking the effect of some unidentified factor which greatly enhances the catalytic efficiency of the enzyme.

Phosphorylation and activation of p42 and p44 mitogen-activated protein kinase are required for the P2 purinoceptor stimulation of endothelial prostacyclin production

Extracellular ATP and ADP, released from platelets and other sites stimulate the endothelial production of prostacyclin (PGI2) by acting on G-protein-coupled P2Y2 and P2Y2 purinoceptors, contributing to the maintenance of a non-thrombogenic surface. The mechanism, widely described as being dependent on elevated cytosolic [Ca2+], also requires protein tyrosine phosphorylation. Here we show that activation of both these P2 receptor types leads to the tyrosine phosphorylation and activation of both the p42 and p44 forms of mitogen-activated protein kinase (MAPK). 2-Methylthio-ATP and UTP, selectively activating P2Y1 and P2Y2 purinoceptors respectively, and ATP, a non-selective agonist at these two receptors, stimulate the tyrosine phosphorylation of both p42mapk and p44mapk, as revealed by Western blots with an antiserum specific for the tyrosine-phosphorylated forms of the enzymes. By using separation on Resource Q columns, peptide kinase activity associated with the phosphorylated MAPK enzymes distributes into two peaks, one mainly p42mapk and one mainly p44mapk, both of which are stimulated by ATP with respect to kinase activity and phospho-MAPK immunoreactivity. Stimulation of P2Y1 or P2Y2 purinoceptors leads to a severalfold increase in PGI2 efflux; this was blocked in a dose-dependent manner by the selective MAPK kinase inhibitor PD98059. This drug also blocked the agonist-stimulated increase in phospho-MAPK immunoreactivity for both p42mapk and p44mapk but left the phospholipase C response to P2 agonists essentially unchanged. Olomoucine has been reported to inhibit p44mapk activity. Here we show that in the same concentration range olomoucine inhibits activity in both peaks from the Resource Q column and also the agonist stimulation of 6-keto-PGF1, but has no effect on agonist-stimulated phospho-MAPK immunoreactivity. These results provide direct evidence for the involvement of p42 and p44 MAPK in the PGI2 response of intact endothelial cells: we have shown that both the endothelial P2Y purinoceptors are linked to activation of MAPK, and that activation of this pathway is a requirement for the stimulation by ATP/ADP of endothelial PGI2 production.