Identification of the phosphorylation sites of cytosolic phospholipase A2 in agonist-stimulated human platelets and HeLa cells

Bromocriptine protects perilesional spinal cord neurons from lipotoxicity after spinal cord injury

Recent studies have revealed that lipid droplets accumulate in neurons after brain injury and evoke lipotoxicity, damaging the neurons. However, how lipids are metabolized by spinal cord neurons after spinal cord injury remains unclear. Herein, we investigated lipid metabolism by spinal cord neurons after spinal cord injury and identified lipid-lowering compounds to treat spinal cord injury. We found that lipid droplets accumulated in perilesional spinal cord neurons after spinal cord injury in mice. Lipid droplet accumulation could be induced by myelin debris in HT22 cells. Myelin debris degradation by phospholipase led to massive free fatty acid production, which increased lipid droplet synthesis, β-oxidation, and oxidative phosphorylation. Excessive oxidative phosphorylation increased reactive oxygen species generation, which led to increased lipid peroxidation and HT22 cell apoptosis. Bromocriptine was identified as a lipid-lowering compound that inhibited phosphorylation of cytosolic phospholipase A2 by reducing the phosphorylation of extracellular signal-regulated kinases 1/2 in the mitogen-activated protein kinase pathway, thereby inhibiting myelin debris degradation by cytosolic phospholipase A2 and alleviating lipid droplet accumulation in myelin debris-treated HT22 cells. Motor function, lipid droplet accumulation in spinal cord neurons and neuronal survival were all improved in bromocriptine-treated mice after spinal cord injury. The results suggest that bromocriptine can protect neurons from lipotoxic damage after spinal cord injury via the extracellular signal-regulated kinases 1/2-cytosolic phospholipase A2 pathway.

MAPK-interacting kinase 1 regulates platelet production, activation, and thrombosis

The MAPK-interacting kinase (Mnk) family includes Mnk1 and Mnk2, which are phosphorylated and activated in response to extracellular stimuli. Mnk1 contributes to cellular responses by regulating messenger RNA (mRNA) translation, and mRNA translation influences platelet production and function. However, the role of Mnk1 in megakaryocytes and platelets has not previously been studied. The present study investigated Mnk1 in megakaryocytes and platelets using both pharmacological and genetic approaches. We demonstrate that Mnk1, but not Mnk2, is expressed and active in human and murine megakaryocytes and platelets. Stimulating human and murine megakaryocytes and platelets induced Mnk1 activation and phosphorylation of eIF4E, a downstream target of activated Mnk1 that triggers mRNA translation. Mnk1 inhibition or deletion significantly diminished protein synthesis in megakaryocytes as measured by polysome profiling and [35S]-methionine incorporation assays. Depletion of Mnk1 also reduced megakaryocyte ploidy and proplatelet forming megakaryocytes in vitro and resulted in thrombocytopenia. However, Mnk1 deletion did not affect the half-life of circulating platelets. Platelets from Mnk1 knockout mice exhibited reduced platelet aggregation, α granule secretion, and integrin αIIbβ3 activation. Ribosomal footprint sequencing indicated that Mnk1 regulates the translation of Pla2g4a mRNA (which encodes cPLA2) in megakaryocytes. Consistent with this, Mnk1 ablation reduced cPLA2 activity and thromboxane generation in platelets and megakaryocytes. In vivo, Mnk1 ablation protected against platelet-dependent thromboembolism. These results provide previously unrecognized evidence that Mnk1 regulates mRNA translation and cellular activation in platelets and megakaryocytes, endomitosis and thrombopoiesis, and thrombosis.

Phosphorylation of DGK

Diacylglycerol (DAG) and phosphatidic acid (PtdOH) play important roles in a variety of signaling cascades (Carrasco and Merida, 2007; Stace and Ktistakis, 2006). Therefore, the physiological roles and regulatory mechanisms controlling the levels of these lipids are important. One class of enzymes capable of coordinating the levels of these two lipids are the diacylglycerol kinases (DGKs). DGKs catalyze the transfer of the γ-phosphate of ATP to the hydroxyl group of DAG which generates PtdOH(Merida et al., 2008; Sakane et al., 2007). As DGKs reciprocally modulate the relative levels of these two signaling lipids, it is not surprising that there is increasing interest in understanding the mechanism underlying the catalysis and regulation of these kinases. While post-translational modifications (PTMs) are often involved in enzyme regulation, there is surprisingly little information regarding the PTMs on these enzymes and their roles in modulating their activity and function. In this review, we will summarize what is known about one PTM on DGKs, phosphorylation, and the possible functions of this modification.

Phosphorylation of cPLA2α at Ser505 Is Necessary for Its Translocation to PtdInsP2-Enriched Membranes

Group IVA cytosolic phospholipase A₂α (cPLA₂α) is a key enzyme in physiology and pathophysiology because it constitutes a rate-limiting step in the pathway for the generation of pro- and anti-inflammatory eicosanoid lipid mediators. cPLA₂α activity is tightly regulated by multiple factors, including the intracellular Ca²⁺ concentration, phosphorylation reactions, and cellular phosphatidylinositol (4,5) bisphosphate levels (PtdInsP₂). In the present work, we demonstrate that phosphorylation of the enzyme at Ser⁵⁰⁵ is an important step for the translocation of the enzyme to PtdInsP₂–enriched membranes in human cells. Constructs of eGFP-cPLA₂ mutated in Ser⁵⁰⁵ to Ala (S505A) exhibit a delayed translocation in response to elevated intracellular Ca²⁺, and also in response to increases in intracellular PtdInsP₂ levels. Conversely, translocation of a phosphorylation mimic mutant (S505E) is fully observed in response to cellular increases in PtdInsP₂ levels. Collectively, these results suggest that phosphorylation of cPLA₂α at Ser⁵⁰⁵ is necessary for the enzyme to translocate to internal membranes and mobilize arachidonic acid for eicosanoid synthesis.

Thrombin induces ACSL4-dependent ferroptosis during cerebral ischemia/reperfusion

Ischemic stroke represents a significant danger to human beings, especially the elderly. Interventions are only available to remove the clot, and the mechanism of neuronal death during ischemic stroke is still in debate. Ferroptosis is increasingly appreciated as a mechanism of cell death after ischemia in various organs. Here we report that the serine protease, thrombin, instigates ferroptotic signaling by promoting arachidonic acid mobilization and subsequent esterification by the ferroptotic gene, acyl-CoA synthetase long-chain family member 4 (ACSL4). An unbiased multi-omics approach identified thrombin and ACSL4 genes/proteins, and their pro-ferroptotic phosphatidylethanolamine lipid products, as prominently altered upon the middle cerebral artery occlusion in rodents. Genetically or pharmacologically inhibiting multiple points in this pathway attenuated outcomes of models of ischemia in vitro and in vivo. Therefore, the thrombin-ACSL4 axis may be a key therapeutic target to ameliorate ferroptotic neuronal injury during ischemic stroke.

Inflammation and JNK's Role in Niacin-GPR109A Diminished Flushed Effect in Microglial and Neuronal Cells With Relevance to Schizophrenia

Schizophrenia is a neuropsychiatric illness with no single definitive aetiology, making its treatment difficult. Antipsychotics are not fully effective because they treat psychosis rather than the cognitive or negative symptoms. Antipsychotics fail to alleviate symptoms when patients enter the chronic stage of illness. Topical application of niacin showed diminished skin flush in the majority of patients with schizophrenia compared to the general population who showed flushing. The niacin skin flush test is useful for identifying patients with schizophrenia at their ultra-high-risk stage, and understanding this pathology may introduce an effective treatment. This review aims to understand the pathology behind the diminished skin flush response, while linking it back to neurons and microglia. First, it suggests that there are altered proteins in the GPR109A-COX-prostaglandin pathway, inflammatory imbalance, and kinase signalling pathway, c-Jun N-terminal kinase (JNK), which are associated with diminished flush. Second, genes from the GPR109A-COX-prostaglandin pathway were matched against the 128-loci genome wide association study (GWAS) for schizophrenia using GeneCards, suggesting that G-coupled receptor-109A (GPR109A) may have a genetic mutation, resulting in diminished flush. This review also suggests that there may be increased pro-inflammatory mediators in the GPR109A-COX-prostaglandin pathway, which contributes to the diminished flush pathology. Increased levels of pro-inflammatory markers may induce microglial-activated neuronal death. Lastly, this review explores the role of JNK on pro-inflammatory mediators, proteins in the GPR109A-COX-prostaglandin pathway, microglial activation, and neuronal death. Inhibiting JNK may reverse the changes observed in the diminished flush response, which might make it a good therapeutic target.

An overview of mammalian p38 mitogen-activated protein kinases, central regulators of cell stress and receptor signaling

The p38 family is a highly evolutionarily conserved group of mitogen-activated protein kinases (MAPKs) that is involved in and helps co-ordinate cellular responses to nearly all stressful stimuli. This review provides a succinct summary of multiple aspects of the biology, role, and substrates of the mammalian family of p38 kinases. Since p38 activity is implicated in inflammatory and other diseases, we also discuss the clinical implications and pharmaceutical approaches to inhibit p38.

The protein tyrosine phosphatase PTPN7 is a negative regulator of ERK activation and thromboxane generation in platelets

Protein tyrosine phosphatase nonreceptor type 7 (PTPN7), also called hematopoietic protein tyrosine phosphatase, controls extracellular signal-regulated protein kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase in T lymphocytes. Because ERK1/2 plays an important role in regulating thromboxane A₂ (TXA₂) generation in platelets, we investigated the function of PTPN7 in these cells. Using immunoblot analysis, we detected PTPN7 in both human and mouse platelets but not in PTPN7-null mice. PTPN7 KO mouse platelets exhibited increased platelet functional responses, including aggregation, dense granule secretion, and TXA₂ generation, compared with platelets from WT littermates, upon stimulation with both G protein-coupled receptor (GPCR) and glycoprotein VI (GPVI) agonists. Using the GPCR agonist AYPGKF in the presence of the COX inhibitor indomethacin, we found that PTPN7 KO mouse platelets aggregated and secreted to the same extent as WT platelets, suggesting that elevated TXA₂ is responsible for the potentiation of platelet functional responses in PTPN7-KO platelets. Phosphorylation of ERK1/2 was also elevated in PTPN7 KO platelets. Stimulation of platelets with the GPVI agonist collagen-related peptide along with the COX inhibitor indomethacin did not result in phosphorylation of ERK1/2, indicating that GPVI-mediated ERK phosphorylation occurs through TXA₂ Although bleeding times did not significantly differ between PTPN7-null and WT mice, time to death was significantly faster in PTPN7-null mice than in WT mice in a pulmonary thromboembolism model. We conclude that PTPN7 regulates platelet functional responses downstream of GPCR agonists, but not GPVI agonists, through inhibition of ERK activation and thromboxane generation.

TNF-α-Induced cPLA2 Expression via NADPH Oxidase/Reactive Oxygen Species-Dependent NF-κB Cascade on Human Pulmonary Alveolar Epithelial Cells

Tumor necrosis factor-α (TNF-α) triggers activation of cytosolic phospholipase A₂ (cPLA₂) and then enhancing the synthesis of prostaglandin (PG) in inflammatory diseases. However, the detailed mechanisms of TNF-α induced cPLA₂ expression were not fully defined in human pulmonary alveolar epithelial cells (HPAEpiCs). We found that TNF-α-stimulated increases in cPLA₂ mRNA (5.2 folds) and protein (3.9 folds) expression, promoter activity (4.3 folds), and PGE₂ secretion (4.7 folds) in HPAEpiCs, determined by Western blot, real-time PCR, promoter activity assay and PGE₂ ELISA kit. These TNF-α-mediated responses were abrogated by the inhibitors of NADPH oxidase [apocynin (APO) and diphenyleneiodonium chloride (DPI)], ROS [N-acetyl cysteine, (NAC)], NF-κB (Bay11-7082) and transfection with siRNA of ASK1, p47 ^phox , TRAF2, NIK, IKKα, IKKβ, or p65. TNF-α markedly stimulated NADPH oxidase activation and ROS including superoxide and hydrogen peroxide production which were inhibited by pretreatment with a TNFR1 neutralizing antibody, APO, DPI or transfection with siRNA of TRAF2, ASK1, or p47 ^phox . In addition, TNF-α also stimulated p47 ^phox phosphorylation and translocation in a time-dependent manner. On the other hand, TNF-α induced TNFR1, TRAF2, ASK1, and p47 ^phox complex formation in HPAEpiCs, which were attenuated by a TNF-α neutralizing antibody. We found that pretreatment with NAC, DPI, or APO also attenuated the TNF-α-stimulated IKKα/β and NF-κB p65 phosphorylation, NF-κB (p65) translocation, and NF-κB promoter activity in HPAEpiCs. Finally, we observed that TNF-α-stimulated NADPH oxidase activation and ROS generation activates NF-κB through the NIK/IKKα/β pathway. Taken together, our results demonstrated that in HPAEpiCs, up-regulation of cPLA₂ by TNF-α is, at least in part, mediated through the cooperation of TNFR1, TRAF2, ASK1, and NADPH oxidase leading to ROS generation and ultimately activates NF-κB pathway.

Cellular Assays for Evaluating Calcium-Dependent Translocation of cPLA2α to Membrane

The group IVA phospholipase A₂, commonly called cytosolic phospholipase A₂α (cPLA₂α), is a widely expressed enzyme that hydrolyzes membrane phospholipid to produce arachidonic acid and lysophospholipids, which are precursors for a number of bioactive lipid mediators. Arachidonic acid is metabolized through the cyclooxygenase and lipoxygenase pathways for production of prostaglandins and leukotrienes that regulate normal physiological processes and contribute to disease pathogenesis. cPLA₂α is composed of an N-terminal C2 domain and a C-terminal catalytic domain that contains the Ser-Asp catalytic dyad. The catalytic domain contains phosphorylation sites and basic residues that regulate the catalytic activity of cPLA₂α. In response to cell stimulation, cPLA₂α is rapidly activated by posttranslational mechanisms including increases in intracellular calcium and phosphorylation by mitogen-activated protein kinases. In resting cells, cPLA₂α is localized in the cytosol but translocates to membrane including the Golgi, endoplasmic reticulum, and the peri-nuclear membrane in response to increases in intracellular calcium. Calcium binds to the C2 domain, which promotes the interaction of cPLA₂α with membrane through hydrophobic interactions. In this chapter, we describe assays used to study the calcium-dependent translocation of cPLA₂α to membrane, a regulatory step necessary for access to phospholipid and release of arachidonic acid.

Antiplatelet action of indirubin-3'-monoxime through suppression of glycoprotein VI-mediated signal transduction: a possible role for ERK signaling in platelets

We investigated the antiplatelet activity of indirubin-3'-monoxime (I3O) and the underlying mechanisms. In a rat carotid artery injury model, oral administration (20 mg/kg/day) of I3O for 3 days significantly prolonged occlusion time, and ADP- and collagen-induced platelet aggregation. In washed platelets in vitro, I3O potently inhibited collagen-induced platelet aggregation by suppressing phospholipase Cγ2 (PLCγ2) phosphorylation, subsequently blocking diacylglycerol and arachidonic acid (AA) formation, P-selectin secretion and the production of thromboxane B2. Platelet aggregation induced by phorbol-12-myristate 13-acetate, a protein kinase C (PKC) activator, was inhibited by I3O. Both I3O and U0126, an extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor, markedly reduced collagen-induced phosphorylation of ERK1/2 and p47, resulting in the blockade of cyclooxygenase (COX)-mediated AA metabolite production in AA-treated platelets. I3O suppressed phosphorylation of JNK, p38, GSK-3β, and AKT. I3O inhibited glycoprotein VI (GPVI), as a collagen receptor, by suppressing the phosphorylation of tyrosine kinase Syk of GPVI and the phosphorylation of PLCγ2 and ERK1/2 stimulated by convulxin, as a specific stimulator. Our results indicate that an antiplatelet effect of I3O is due to the suppression of GPVI-mediated signaling pathways. In collagen-stimulated platelets, ERK1/2 phosphorylation is adenylyl cyclase-dependent and leads to the modulation of PKC-p47 signaling and COX-1-mediated AA-metabolic pathways.

TNF-α induces cytosolic phospholipase A2 expression via Jak2/PDGFR-dependent Elk-1/p300 activation in human lung epithelial cells

Cytosolic phospholipase A2 (cPLA2) plays a pivotal role in mediating agonist-induced arachidonic acid release for prostaglandin (PG) synthesis during inflammation triggered by tumor necrosis factor-α (TNF-α). However, the mechanisms underlying TNF-α-induced cPLA2 expression in human lung epithelial cells (HPAEpiCs) were not completely understood. Here, we demonstrated that TNF-α induced cPLA2 mRNA and protein expression, promoter activity, and PGE2 secretion in HPAEpiCs. These responses induced by TNF-α were inhibited by pretreatment with the inhibitor of Jak2 (AG490), platelet-derived growth factor receptor (PDGFR) (AG1296), phosphoinositide 3 kinase (PI3K) (LY294002), or MEK1/2 (PD98059) and transfection with siRNA of Jak2, PDGFR, Akt, or p42. We showed that TNF-α markedly stimulated Jak2, PDGFR, Akt, and p42/p44 MAPK phosphorylation, which were attenuated by their respective inhibitors. Moreover, TNF-α stimulated Akt activation via a Jak2/PDGFR pathway in HPAEpiCs. In addition, TNF-α-induced p42/p44 MAPK phosphorylation was reduced by AG1296 or LY294002. On the other hand, TNF-α could induce Akt and p42/p44 MAPK translocation from the cytosol into the nucleus, which was inhibited by AG490, AG1296, or LY294002. Finally, we showed that TNF-α stimulated Elk-1 phosphorylation, which was reduced by LY294002 or PD98059. We also observed that TNF-α time dependently induced p300/Elk-1 and p300/Akt complex formation in HPAEpiCs, which was reduced by AG490, AG1296, or LY294002. The activity of cPLA2 protein upregulated by TNF-α was reflected on the PGE2 release, which was reduced by AG490, AG1296, LY294002 , or PD98059. Taken together, these results demonstrated that TNF-α-induced cPLA2 expression and PGE2 release were mediated through a Jak2/PDGFR/PI3K/Akt/p42/p44 MAPK/Elk-1 pathway in HPAEpiCs.

TNF-α induces cytosolic phospholipase A2 expression in human lung epithelial cells via JNK1/2- and p38 MAPK-dependent AP-1 activation

Background

Cytosolic phospholipase A₂ (cPLA₂) plays a pivotal role in mediating agonist-induced arachidonic acid (AA) release for prostaglandin (PG) synthesis during inflammation triggered by tumor necrosis factor-α (TNF-α). However, the mechanisms underlying TNF-α-induced cPLA₂ expression in human lung epithelial cells (HPAEpiCs) were not completely understood.

Principal Findings

We demonstrated that TNF-α induced cPLA₂ mRNA and protein expression, promoter activity, and PGE₂ secretion in HPAEpiCs. These responses induced by TNF-α were inhibited by pretreatment with the inhibitor of MEK1/2 (PD98059), p38 MAPK (SB202190), JNK1/2 (SP600125), or AP-1 (Tanshinone IIA) and transfection with siRNA of TNFR1, p42, p38, JNK2, c-Jun, c-Fos, or ATF2. We showed that TNF-α markedly stimulated p42/p44 MAPK, p38 MAPK, and JNK1/2 phosphorylation which were attenuated by their respective inhibitors. In addition, TNF-α also stimulated c-Jun and ATF2 phosphorylation which were inhibited by pretreatment with SP600125 and SB202190, respectively, but not PD98059. Furthermore, TNF-α-induced cPLA₂ promoter activity was abrogated by transfection with the point-mutated AP-1 cPLA₂ construct. Finally, we showed that TNF-α time-dependently induced p300/c-Fos/c-Jun/ATF2 complex formation in HPAEpiCs. On the other hand, TNF-α induced in vivo binding of c-Jun, c-Fos, ATF2, and p300 to the cPLA₂ promoter in these cells. In an in vivo study, we found that TNF-α induced leukocyte count in BAL fluid of mice and cPLA₂ mRNA levels in lung tissues via MAPKs and AP-1.

Significance

Taken together, these results demonstrated that TNF-α-induced cPLA₂ expression was mediated through p38 MAPK- and JNK1/2-dependent p300/c-Fos/c-Jun/ATF2 complex formation in HPAEpiCs.

Modulation of Rho-Rock signaling pathway protects oligodendrocytes against cytokine toxicity via PPAR-α-dependent mechanism

We earlier documented that lovastatin (LOV)-mediated inhibition of small Rho GTPases activity protects vulnerable oligodendrocytes (OLs) in mixed glial cell cultures stimulated with Th1 cytokines and in a murine model of multiple sclerosis (MS). However, the precise mechanism of OL protection remains unclear. We here employed genetic and biochemical approaches to elucidate the underlying mechanism that protects LOV treated OLs from Th1 (tumor necrosis factor-α) and Th17 (interleukin-17) cytokines toxicity in in vitro. Cytokines enhanced the reactive oxygen species (ROS) generation and mitochondrial membrane depolarization with corresponding lowering of glutathione (reduced) level in OLs and that were reverted by LOV. In addition, the expression of ROS detoxifying enzymes (catalase and superoxide-dismutase 2) and the transactivation of peroxisome proliferators-activated receptor (PPAR)-α/-β/-γ including PPAR-γ coactivator-1α were enhanced by LOV in similarly treated OLs. Interestingly, LOV-mediated inhibition of small Rho GTPases, i.e., RhoA and cdc42, and Rho-associated kinase (ROCK) activity enhanced the levels of PPAR ligands in OLs via extracellular signal regulated kinase (1/2)/p38 mitogen-activated protein kinase/cytoplasmic phospholipase 2/cyclooxygenase-2 signaling cascade activation. Small hairpin RNA transfection-based studies established that LOV mainly enhances PPAR-α and less so of PPAR-β and PPAR-γ transactivation that enhances ROS detoxifying defense in OLs. In support of this, the observed LOV-mediated protection was lacking in PPAR-α-deficient OLs exposed to cytokines. Collectively, these data provide unprecedented evidence that LOV-mediated inhibition of the Rho-ROCK signaling pathway boosts ROS detoxifying defense in OLs via PPAR-α-dependent mechanism that has implication in neurodegenerative disorders including MS.

Suppression of endoplasmic reticulum stress improves endothelium-dependent contractile responses in aorta of the spontaneously hypertensive rat

A contributing factor to increased peripheral resistance seen during hypertension is an increased production of endothelium-derived contractile factors (EDCFs). The main EDCFs are vasoconstrictor prostanoids, metabolites of arachidonic acid (AA) produced by Ca(2+)-dependent cytosolic phospholipase A2 (cPLA2) following phosphorylation (at Ser(505)) mediated by extracellular signal-regulated kinase (ERK1/2) and cyclooxygenase (COX) activations. Although endoplasmic reticulum (ER) stress has been shown to contribute to pathophysiological alterations in cardiovascular diseases, the relationship between ER stress and EDCF-mediated responses remains unclear. We tested the hypothesis that ER stress plays a role in EDCF-mediated responses via activation of the cPLA2/COX pathway in the aorta of the spontaneously hypertensive rat (SHR). Male SHR and Wistar-Kyoto rats (WKY) were treated with ER stress inhibitor, tauroursodeoxycholic acid or 4-phenlybutyric acid (TUDCA or PBA, respectively, 100 mg·kg(-1)·day(-1) ip) or PBS (control, 300 μl/day ip) for 1 wk. There was a decrease in systolic blood pressure in SHR treated with TUDCA or PBA compared with control SHR (176 ± 3 or 181 ± 5, respectively vs. 200 ± 2 mmHg). In the SHR, treatment with TUDCA or PBA normalized aortic (vs. control SHR) 1) contractions to acetylcholine (ACh), AA, and tert-butyl hydroperoxide, 2) ACh-stimulated releases of prostanoids (thromboxane A2, PGF2α, and prostacyclin), 3) expression of COX-1, 4) phosphorylation of cPLA2 and ERK1/2, and 5) production of H2O2. Our findings demonstrate a novel interplay between ER stress and EDCF-mediated responses in the aorta of the SHR. Moreover, ER stress inhibition normalizes such responses by suppressing the cPLA2/COX pathway.

Arachidonic acid depletion extends survival of cold-stored platelets by interfering with the [glycoprotein Ibα--14-3-3ζ] association

BACKGROUND

Cold storage of platelets reduces bacterial growth and preserves their hemostatic properties better than current procedures do. However, storage at 0°C induces [14-3-3ζ-glycoprotein Ibα] association, 14-3-3ζ release from phospho-Bad, Bad activation and apoptosis.

DESIGN AND METHODS

We investigated whether arachidonic acid, which also binds 14-3-3ζ, contributes to coldinduced apoptosis.

RESULTS

Cold storage activated P38-mitogen-activated protein kinase and released arachidonic acid, which accumulated due to cold inactivation of cyclooxygenase-1/thromboxane synthase. Accumulated arachidonic acid released 14-3-3ζ from phospho-Bad and decreased the mitochondrial membrane potential, which are steps in the induction of apoptosis. Addition of arachidonic acid did the same and its depletion made platelets resistant to cold-induced apoptosis. Incubation with biotin-arachidonic acid revealed formation of an [arachidonic acid-14-3-3ζ-glycoprotein Ibα] complex. Indomethacin promoted complex formation by accumulating arachidonic acid and released 14-3-3ζ from cyclo-oxygenase-1. Arachidonic acid depletion prevented the cold-induced reduction of platelet survival in mice.

CONCLUSIONS

We conclude that cold storage induced apoptosis through an [arachidonic acid-14-3-3ζ-glycoprotein Ibα] complex, which released 14-3-3ζ from Bad in an arachidonic acid-dependent manner. Although arachidonic acid depletion reduced agonist-induced thromboxane A(2) formation and aggregation, arachidonic acid repletion restored these functions, opening ways to reduce apoptosis during storage without compromising hemostatic functions post-transfusion.

Thrombin and vascular inflammation

Vascular endothelium is a key regulator of homeostasis. In physiological conditions it mediates vascular dilatation, prevents platelet adhesion, and inhibits thrombin generation. However, endothelial dysfunction caused by physical injury of the vascular wall, for example during balloon angioplasty, acute or chronic inflammation, such as in atherothrombosis, creates a proinflammatory environment which supports leukocyte transmigration toward inflammatory sites. At the same time, the dysfunction promotes thrombin generation, fibrin deposition, and coagulation. The serine protease thrombin plays a pivotal role in the coagulation cascade. However, thrombin is not only the key effector of coagulation cascade; it also plays a significant role in inflammatory diseases. It shows an array of effects on endothelial cells, vascular smooth muscle cells, monocytes, and platelets, all of which participate in the vascular pathophysiology such as atherothrombosis. Therefore, thrombin can be considered as an important modulatory molecule of vascular homeostasis. This review summarizes the existing evidence on the role of thrombin in vascular inflammation.

Activation and induction of cytosolic phospholipase A2 by TNF-α mediated through Nox2, MAPKs, NF-κB, and p300 in human tracheal smooth muscle cells

Cytosolic phospholipase A(2) (cPLA(2)) plays a pivotal role in mediating agonist-induced arachidonic acid (AA) release for prostaglandin (PG) synthesis during inflammation triggered by tumor necrosis factor-α (TNF-α). However, the mechanisms underlying TNF-α-induced cPLA(2) expression and PGE(2) synthesis in human tracheal smooth muscle cells (HTSMCs) remain unknown. Here, we report that TNF-α-induced cPLA(2) protein and mRNA expression, PGE(2) production, and phosphorylation of p42/p44 MAPK, p38 MAPK, and JNK1/2, which were attenuated by pretreatment with a ROS scavenger [N-acetyl-L-cysteine, (NAC)] and the inhibitors of NADPH oxidase [apocynin (APO) and diphenyleneiodonium chloride (DPI)], MEK1/2 (U0126), p38 MAPK (SB202190), and JNK1/2 (SP600125) or transfection with siRNA of Nox2, p47(phox) , MEK1, p42, p38, or JNK2. TNF-α-induced cPLA(2) expression was also inhibited by pretreatment with a selective NF-κB inhibitor [helenalin (HLN)] or transfection with dominant negative mutants of NF-κB inducing kinase (NIK) or IκB kinase (IKK)α/β. TNF-α-induced NF-κB translocation was blocked by pretreatment with NAC, DPI, APO, or HLN, but not by U0126, SB202190, or SP600125. In addition, pretreatment with curcumin (a p300 inhibitor) or transfection with p300 siRNA blocked cPLA(2) expression and PGE(2) synthesis induced by TNF-α. We further confirmed that p300 was associated with the cPLA(2) promoter which was dynamically linked to histone H4 acetylation stimulated by TNF-α, determined by chromatin immunoprecipitation assay. Association of p300 and histone H4 to cPLA(2) promoter was inhibited by U0126, SB202190, and SP600125. These results suggested that in HTSMCs, activation of p47(phox) , MAPKs, NF-κB, and p300 is essential for TNF-α-induced cPLA(2) expression and PGE(2) release.

Identification of a 42-kDa Group IV cPLA2-activating protein, cPLAPγ, as a GTP-binding protein in the bovine brain

Brain tissue contains multiple forms of Phospholipase A(2) (PLA(2)) whose activities are involved in intracellular and intercellular signalling related to normal functions such as long-term potentiation, neurotransmitter release, cell growth and differentiation. Among them, we focused on regulatory mechanism of cPLA(2)α (Group IVA cytosolic PLA(2)) in brain tissue. In the present study, we report the identification of a cPLA(2)-activating protein (cPLAP) in the bovine brain. cPLAP activity appeared as two major peaks with molecular masses of 200 and 42 kDa in a Superose 12 gel filtration FPLC column. The 42-kDa form of cPLAP, designated cPLAPγ, was further purified using a Mono S FPLC column to near homogeneity and characterized to as a GTP-binding protein (G protein). Metabolic labelling and immunoprecipitation studies revealed that cPLAPγ associates with cPLA(2) in vitro and co-immunoprecipitates with [(35)S]-cPLA(2). Notably, cPLAPγ rendered cPLA(2) fully activated at submicromolar concentrations of Ca(2+). These results suggest that cPLAPγ may act as a G protein, activating cPLA(2)α prior to reaching full intracellular Ca(2+) concentrations.

Over-expression of caveolin-1 aggravate LPS-induced inflammatory response in AT-1 cells via up-regulation of cPLA2/p38 MAPK

OBJECTIVE AND DESIGN

The aim of this study was to study the effect of caveolin-1 on the cytosolic phospholipase A2 (cPLA2), p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor kappaB (NF-kappaB) in mouse lung alveolar type-1 cells' (AT-1 cells) inflammatory response induced by LPS.

MATERIALS AND METHODS

Gene clone technique was used to over-express caveolin-1 in AT-1 cells by lentivirus vector. The level of tumor necrosis factor alpha (TNF-alpha), interleukin 6 (IL-6), cPLA2, p38 MAPK and NF-kappaB was measured by ELISA, western blotting and EMSA.

TREATMENT

AT-1 cells were treated with LPS.

RESULTS

Over-expression of caveolin-1 not only increased the production of pro-inflammatory cytokine TNF-alpha and IL-6, but also enhanced the expression of the cPLA2, p38 MAPK, and NF-kappaB.

CONCLUSIONS

Our data demonstrated that over-expression of caveolin-1 aggravates the AT-1 injury induced by LPS, involving in modulation of the cPLA2 mediated by the cPLA2/p38 MAPK pathway.

Localization and function of cytosolic phospholipase A2alpha at the Golgi

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha, Group IVA phospholipase A(2)) is a central mediator of arachidonate release from cellular phospholipids for the biosynthesis of eicosanoids. cPLA(2)alpha translocates to intracellular membranes including the Golgi in response to a rise in intracellular calcium level. The enzyme's calcium-dependent phospholipid-binding C2 domain provides the targeting specificity for cPLA(2)alpha translocation to the Golgi. However, other features of cPLA(2)alpha regulation are incompletely understood such as the role of phosphorylation of serine residues in the catalytic domain and the function of basic residues in the cPLA(2)alpha C2 and catalytic domains that are proposed to interact with anionic phospholipids in the membrane to which cPLA(2)alpha is targeted. Increasing evidence strongly suggests that cPLA(2)alpha plays a role in regulating Golgi structure, tubule formation and intra-Golgi transport. For example, recent data suggests that cPLA(2)alpha regulates the transport of tight junction and adherens junction proteins through the Golgi to cell-cell contacts in confluent endothelial cells. However, there are now examples where data based on knockdown using siRNA or pharmacological inhibition of enzymatic activity of cPLA(2)alpha affects fundamental cellular processes yet these phenotypes are not observed in cells from cPLA(2)alpha deficient mice. These results suggest that in some cases there may be compensation for the lack of cPLA(2)alpha. Thus, there is continued need for studies employing highly specific cPLA(2)alpha antagonists in addition to genetic deletion of cPLA(2)alpha in mice.

Chronic imipramine but not bupropion increases arachidonic acid signaling in rat brain: is this related to 'switching' in bipolar disorder?

Agents effective against mania in bipolar disorder are reported to decrease turnover of arachidonic acid (AA) in phospholipids and expression of calcium-dependent AA-selective cytosolic phospholipase A(2) (cPLA(2)) in rat brain. In contrast, fluoxetine, an antidepressant that is reported to switch bipolar depressed patients to mania, increases cPLA(2) expression and AA turnover in rat brain. We therefore hypothesized that antidepressants that increase switching to mania generally increase cPLA(2) and AA turnover in brain. To test this hypothesis, adult male CDF-344 rats were administered imipramine and bupropion, with reported high and low switching rates, respectively, at daily doses of 10 and 30 mg kg(-1) i.p., respectively, or i.p. saline (control) for 21 days. Frontal cortex expression of different PLA(2) enzymes and AA turnover rates in brain when the rats were unanesthetized were measured. Compared with chronic saline, chronic imipramine but not bupropion significantly increased cortex cPLA(2) mRNA activity, protein and phosphorylation, expression of the cPLA(2) transcription factor, activator protein-2alpha (AP-2alpha) and AA turnover in phospholipids. Protein levels of secretory phospholipase A(2), calcium-independent phospholipase A(2), cyclooxygenase (COX)-1 and COX-2 were unchanged, and prostaglandin E(2) was unaffected. These results, taken with prior data on chronic fluoxetine in rats, suggest that antidepressants that increase the switching tendency of bipolar depressed patients to mania do so by increasing AA recycling and metabolism in brain. Mania in bipolar disorder thus may involve upregulated brain AA metabolism.

Analysis of the mechanism for the development of allergic skin inflammation and the application for its treatment: aspirin modulation of IgE-dependent mast cell activation: role of aspirin-induced exacerbation of immediate allergy

Aspirin (acetylsalicylic acid) is a well-known nonsteroidal anti-inflammatory drug that can potentiate some acute allergies and causes adverse immunological reactions collectively referred to as aspirin intolerance, a disorder that induces urticaria, asthma, and anaphylaxis in response to oral administration of the drug. Aspirin also potentiates some acute allergies such as food-dependent exercise-induced anaphylaxis (FDEIA), a food allergy induced by physical exercise. The anti-inflammatory actions as well as the adverse immunological effects have been thought to be primarily due to inhibition of cyclooxygenase activity. However, a growing body of evidence suggests that mechanisms unrelated to inhibition of prostaglandin synthesis are involved. One key feature of aspirin intolerance is the overproductions of cysteinyl leukotrienes (LTs), in which mast cells have been implicated to play a role. In this review, we provide an overview of our current knowledge about the regulatory mechanisms of LTC(4) secretion in mast cells and its modulation by aspirin, with a special emphasis on the role of Ca(2+) signals. We also introduced our recent findings that mast cells express dihydropyridine-sensitive L-type Ca(2+) channels (LTCCs) and that Ca(2+) channels of this kind mediate aspirin modulation of LTC(4) secretion in mast cells.

In retinal vein occlusion platelet response to thrombin is increased

INTRODUCTION

Retinal vein occlusion is a major cause of ocular morbidity. The precise mechanism leading to thrombosis in retinal vein occlusion has not yet been clearly elucidated. Several risk factors have been identified, including hypertension diabetes, history of cardiovascular disease, hypercholesterolemia, hyperhomocysteinaemia, increased ocular pressure and glaucoma. Although thrombus formation in the vein plays a significant role in the onset of retinal vein occlusion, the relationship between platelet aggregation and retinal vein occlusion remains to be clarified.

MATERIALS AND METHODS

In the present study the platelet response to thrombin in a selected group of retinal vein occlusion patients was investigated. Retinal vein occlusion patients were compared to a group of healthy subjects matched for age, sex, clinical and metabolic characteristics. In resting and activated platelets of both groups of subjects total protein tyrosine phosphorylation, p38MAPK and cytosolic phospholipase A(2) phosphorylation, arachidonic acid release, intracellular calcium levels, thromboxane B(2) and superoxide anion formation were measured.

RESULTS

Results show that platelets of patients were more responsive to thrombin than healthy subjects. In resting or in thrombin stimulated platelets of patients total protein tyrosine phosphorylation, p38MAPK and cytosolic phospholipase A(2) phosphorylation were increased. Also arachidonic acid release, thromboxane B(2) and superoxide anion formation were higher in patients than in healthy subjects. In addition intracellular calcium rise induced by thrombin was increased in patients.

CONCLUSIONS

Altogether data suggest that platelet hyperaggregability inducing thrombus formation might be an important factor in the onset and/or development of retinal vein occlusion.

CalDAG-GEFI is at the nexus of calcium-dependent platelet activation

The importance of the second messengers calcium (Ca(2+)) and diacylglycerol (DAG) in platelet signal transduction was established more than 30 years ago. Whereas protein kinase C (PKC) family members were discovered as the targets of DAG, little is known about the molecular identity of the main Ca(2+) sensor(s). We here identify Ca(2+) and DAG-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) as a critical molecule in Ca(2+)-dependent platelet activation. CalDAG-GEFI, through activation of the small GTPase Rap1, directly triggers integrin activation and extracellular signal-regulated kinase-dependent thromboxane A(2) (TxA(2)) release. CalDAG-GEFI-dependent TxA(2) generation provides crucial feedback for PKC activation and granule release, particularly at threshold agonist concentrations. PKC/P2Y12 signaling in turn mediates a second wave of Rap1 activation, necessary for sustained platelet activation and thrombus stabilization. Our results lead to a revised model for platelet activation that establishes one molecule, CalDAG-GEFI, at the nexus of Ca(2+)-induced integrin activation, TxA(2) generation, and granule release. The preferential activation of CalDAG-GEFI over PKC downstream of phospholipase C activation, and the different kinetics of CalDAG-GEFI- and PKC/P2Y12-mediated Rap1 activation demonstrate an unexpected complexity to the platelet activation process, and they challenge the current model that DAG/PKC-dependent signaling events are crucial for the initiation of platelet adhesion.

Phospholipase A2 activity in platelets of patients with primary thrombocythemia

Patients with primary thrombocythemia (PT) have both, bleeding and thrombotic events. Although platelet aggregation tests are usually abnormal, synthesis of thromboxane B2 (TxB2) by platelets is increased. This feature could be the consequence of an increased phospholipase activity or a facilitated metabolism of arachidonate by prostaglandin synthetase pathway. We studied the activity of phospholipase A2 as well the arachidonate metabolism in platelets of patients suffering from PT. Eleven patients and 11 controls were included. Platelets were labelled with [14C]arachidonic acid ([14C]AA). Lost of radioactivity from phospholipids and new radioactive prostanoids were evaluated in calcium ionophore A23187 activated platelets, to explore phospholipase A2 activity. This assay was also carried out in aspirin-incubated platelets. We also studied the formation of prostanoids in platelets activated by radioactive free arachidonic acid. Platelet aggregation studies of patients were abnormal. [14C]AA incorporation in platelet phospholipids was normal. Ionophore activated platelets from patients and controls lost 26.1 +/- 8.3% and 24.1 +/- 10.5% of radioactivity, respectively, mainly from phosphatidylcholine. The main arachidonate metabolite was 12-L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE), which comprised 14.1 +/- 5.1% of the radioactivity released from phospholipids in patients, and a similar amount in the controls (14.4 +/- 7.5%). Formation of TxB2 was also similar in patients (5.5 +/- 1.2%) and controls (4.9 +/- 2.9%). Formation of 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) was also normal. Ionophore A23187 activation of aspirinized platelets of patients released 19.5 +/- 7.4% of radioactivity from phospholipids, which was completely metabolized to HETE. Formation of prostanoids HETE, HHT and TxB2 by arachidonic acid activated platelets of patients was normal. Phospholipase A2 activity as well both cyclooxygenase and lipoxygenase activities in platelets of patients with PT were found to be normal.

Phospholipase A2 biochemistry

The phospholipase A(2) (PLA(2)) superfamily consists of many different groups of enzymes that catalyze the hydrolysis of the sn-2 ester bond in a variety of different phospholipids. The products of this reaction, a free fatty acid, and lysophospholipid have many different important physiological roles. There are five main types of PLA(2): the secreted sPLA(2)'s, the cytosolic cPLA(2)'s, the Ca(2+)independent iPLA(2)'s, the PAF acetylhydrolases, and the lysosomal PLA(2)'s. This review focuses on the superfamily of PLA(2) enzymes, and then uses three specific examples of these enzymes to examine the differing biochemistry of the three main types of these enzymes. These three examples are the GIA cobra venom PLA(2), the GIVA cytosolic cPLA(2), and the GVIA Ca(2+)-independent iPLA(2).

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.

Regulation of arachidonate remodeling enzymes impacts eosinophil survival during allergic asthma

Although the role of arachidonic acid (AA) metabolism to eicosanoids has been well established in allergy and asthma, recent studies in neoplastic cells have revealed that AA remodeling through phospholipids impacts cell survival. This study tests the hypothesis that regulation of AA/phospholipid-remodeling enzymes, cytosolic phospholipase A(2) alpha(cPLA(2)-alpha, gIValphaPLA(2)) and CoA-independent transacylase (CoA-IT), provides a mechanism for altered eosinophil survival during allergic asthma. In vitro incubation of human eosinophils (from donors without asthma) with IL-5 markedly increased cell survival, induced gIValphaPLA(2) phosphorylation, and increased both gIValphaPLA(2) and CoA-IT activity. Furthermore, treatment of eosinophils with nonselective (ET18-O-CH(3)) and selective (SK&F 98625) inhibitors of CoA-IT triggered apoptosis, measured by changes in morphology, membrane phosphatidylserine exposure, and caspase activation, completely reversing IL-5-induced eosinophil survival. To determine if similar activation occurs in vivo, human blood eosinophils were isolated from either normal individuals at baseline or from subjects with mild asthma, at both baseline and 24 hours after inhaled allergen challenge. Allergen challenge of subjects with allergic asthma induced a marked increase in cPLA(2) phosphorylation, augmented gIValphaPLA(2) activity, and increased CoA-IT activity. These findings indicate that both in vitro and in vivo challenge of eosinophils activated gIValphaPLA(2) and CoA-IT, which may play a key role in enhanced eosinophil survival.

Aspirin and salicylates modulate IgE-mediated leukotriene secretion in mast cells through a dihydropyridine receptor-mediated Ca(2+) influx

Aspirin is a well-known nonsteroidal anti-inflammatory drug (NSAID) that may potentiate some acute allergies and causes adverse immunological reactions collectively referred to as aspirin intolerance. Aspirin intolerance is accompanied by increased leukotriene (LT) synthesis, and high levels of serum IgE are a risk factor for NSAID sensitivity. Here we demonstrate that aspirin modulates LTC(4) secretion in mast cells. Therapeutic levels of aspirin and salicylates (<or=0.3 mM, i.e., the concentrations observed in vivo in the use of antipyretic analgesic) increased IgE-mediated LTC(4) secretion. Aspirin-induced stimulation was accompanied by increased Ser-505 phosphorylation of cytosolic phospholipase A(2), which occurred independently of extracellular signal-regulated protein kinase-1/2 and p38 mitogen-activated protein kinase pathways. Aspirin also increased IgE-mediated Ca(2+) influx, whereas aspirin at concentrations of >or=0.3 mM dose-dependently reduced Ca(2+) store emptying and Ca(2+) release-activated Ca(2+) channel activation. Instead, aspirin facilitated a dihydropyridine receptor-mediated Ca(2+) influx, resulting in increased LTC(4) secretion. This novel action of aspirin may play roles in exacerbation of immediate allergy and aspirin intolerance.

Mitogen-activated protein kinases in hemostasis and thrombosis

The mitogen-activated protein (MAP) kinases ERK2, p38 and JNK1 are present in platelets and are activated by various stimuli, such as thrombin, collagen, von Willebrand factor (VWF) and ADP. Until recently, MAP kinases were only studied in the conventional model of agonist-induced platelet aggregation mediated by fibrinogen and integrin alphaIIbbeta3. However, this approach is likely to be too limited for a physiological understanding of platelet MAP kinases and their signaling pathways. Recent studies with varying blood-flow conditions and animal models of thrombosis have provided deeper insight into the role of MAP kinases in thrombus formation and the dependence of these kinases on shear conditions. This review summarizes and discusses the physiological functions of these kinases in hemostasis and thrombosis as revealed by various technical approaches.

Activation of microglia by endotoxin suppresses the secretion of glial cell line-derived neurotrophic factor (GDNF) through the action of protein kinase C alpha (PKCalpha) and mitogen-activated protein kinases (MAPKS)

The ability of microglia to produce/secrete glial cell line-derived neurotrophic factor (GDNF) in vitro was examined. Immunoblotting analysis revealed that nonstimulated microglia release limited amounts of GDNF with molecular sizes of 14 and 17 kDa. However, the secreted amounts significantly decreased when the microglia were activated with the endotoxin lipopolysaccharide (LPS). Comparison of the amounts of GDNF in the cells and the conditioned medium between the nonstimulated microglia and LPS-stimulated microglia clarified that the secretion of GDNF, but not its production, is strongly suppressed when the microglia are activated with LPS. The inhibitor experiments suggested that the GDNF secretion is depressed by a signaling cascade associated with protein kinase C alpha (PKCalpha) and/or mitogen-activated protein kinases (MAPKs). As expected from the above results, a PKC activator suppressed the secretion of GDNF in nonstimulated microglia. Taken together, these results demonstrated that microglia have the ability to produce and secrete GDNF in vitro, and that the secretion is suppressed by stimulation with endotoxin, probably due to a signaling mechanism involving PKCalpha and/or MAPKs.

A role for plasmin in platelet aggregation: differential regulation of IGF release from IGF-IGFBP complexes?

OBJECTIVES

To determine if plasmin differentially augments platelet aggregation through variable efficiencies of IGF-IGFBP complex cleavage.

METHODS

We utilized ADP-triggered platelet aggregation assays to test the effects of IGF-I versus IGF-II in complex with IGFBP-2 or IGFBP-3 upon the efficiency of plasmin (a known IGFBP protease) as a pro-aggregatory stimulus. In vitro proteolysis assays were performed as controls.

RESULTS

We found that IGF-I complexes augmented platelet-mediated aggregation whereas IGF-II either had no effect (IGFBP-2) or inhibited platelet-mediated aggregation (IGFBP-3). In vitro proteolysis assays of IGFBP-2 and IGFBP-3 using plasmin revealed that three of the four aggregation findings were explained by the disparate efficiencies of IGFBP proteolysis associated with each IGF. Only IGF-II-IGFBP-2 complex resulted in a finding that could not be explained by the concept of differential regulation of plasmin's proteolysis efficiency by the two IGF ligands.

CONCLUSIONS

Our findings demonstrate that the plasmin can differentially modulate platelet aggregation in response to intrinsic heterogeneities within the IGF axis.

Regulation of cytosolic phospholipase A2alpha by hsp90 and a p54 kinase in okadaic acid-stimulated macrophages

In resident mouse peritoneal macrophages, group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) mediates arachidonic acid (AA) release and eicosanoid production in response to diverse agonists such as A23187, phorbol myristate acetate, zymosan, and the enterotoxin, okadaic acid (OA). cPLA(2)alpha is regulated by phosphorylation and by calcium that binds to the C2 domain and induces translocation from the cytosol to membranes. In contrast, OA activates cPLA(2)alpha-induced AA release and translocation to the Golgi in macrophages without an apparent increase in calcium. Inhibitors of heat shock protein 90 (hsp90), geldanamycin, and herbimycin blocked AA release in response to OA but not to A23187, PMA, or zymosan. OA, but not the other agonists, induced activation of a cytosolic serine/threonine 54-kDa kinase (p54), which phosphorylated cPLA(2)alpha in in-gel kinase assays and was associated with cPLA(2)alpha in immunoprecipitates. Activation of the p54 kinase was inhibited by geldanamycin. The kinase coimmunoprecipitated with hsp90 in unstimulated macrophages, and OA induced its loss from hsp90, concomitant with its association with cPLA(2)alpha. The results demonstrate a role for hsp90 in regulating cPLA(2)alpha-mediated AA release that involves association of a p54 kinase with cPLA(2)alpha upon OA stimulation.

cPLA2 phosphorylation at serine-515 and serine-505 is required for arachidonic acid release in vascular smooth muscle cells

Cytosolic phospholipase A(2) (cPLA(2)) is activated by phosphorylation at serine-505 (S505) by extracellular regulated kinase 1/2 (ERK1/2). However, rat brain calcium/calmodulin-dependent kinase II (CaMKII) phosphorylates recombinant cPLA(2) at serine-515 (S515) and increases its activity in vitro. We have studied the sites of cPLA(2) phosphorylation and their significance in arachidonic acid (AA) release in response to norepinephrine (NE) in vivo in rabbit vascular smooth muscle cells (VSMCs) using specific anti-phospho-S515- and -S505 cPLA(2) antibodies and by mutagenesis of S515 and S505 to alanine. NE increased the phosphorylation of cPLA(2) at S515, followed by phosphorylation of ERK1/2 and consequently phosphorylation of cPLA(2) at S505. The CaMKII inhibitor 2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzene-sulfonyl)]amino-N-(4-chlorocinnamyl)-methylbenzylamine attenuated cPLA(2) at S515 and S505, whereas the ERK1/2 inhibitor U0126 reduced phosphorylation at S505 but not at S515. NE in cells transduced with adenovirus carrying enhanced cyan fluorescent protein cPLA(2) wild type caused phosphorylation at S515 and S505 and increased AA release. Expression of the S515A mutant in VSMCs reduced the phosphorylation of S505, ERK1/2, and AA release in response to NE. Transduction with a double mutant (S515A/S505A) blocked the phosphorylation of cPLA(2) and AA release. These data suggest that the NE-stimulated phosphorylation of cPLA(2) at S515 is required for the phosphorylation of S505 by ERK1/2 and that both sites of phosphorylation are important for AA release in VSMCs.

Mechanism of regulation of group IVA phospholipase A2 activity by Ser727 phosphorylation

Although group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) has been reported to be phosphorylated at multiple Ser residues, the mechanisms by which phosphorylation at different sites regulates cPLA(2)alpha activities are not fully understood. To explore the possibility that phosphorylation of Ser(727) modulates cellular protein-protein interactions, we measured the effect of Ser(727) mutations on the interaction of cPLA(2)alpha with a reported cPLA(2)alpha-binding protein, p11. In vitro activity assays and membrane binding measurements by surface plasmon resonance analysis showed that a heterotetramer (A2t) of p11 and annexin A2, but not p11 or annexin A2 alone, directly binds cPLA(2)alpha via Ser(727), which keeps the enzyme from binding the membrane and catalyzing the phospholipid hydrolysis. Phosphorylation of Ser(727) disrupts this inhibitory cPLA(2)alpha-A2t interaction, thereby activating cPLA(2)alpha. Subcellular translocation and activity measurements in HEK293 cells cotransfected with cPLA(2)alpha and p11 also showed that p11, in the form of A2t, inhibits cPLA(2)alpha by the same mechanism and that phosphorylation of Ser(727) activates cPLA(2)alpha by interfering with the inhibitory cPLA(2)alpha-A2t interaction. Collectively, these studies provide new insight into the regulatory mechanism of cPLA(2)alpha through Ser(727) phosphorylation.

Chronic NMDA administration to rats up-regulates frontal cortex cytosolic phospholipase A2 and its transcription factor, activator protein-2

Excessive N-methyl-D-aspartate (NMDA) signaling is thought to contribute to bipolar disorder symptoms. Lithium and carbamazepine, effective against bipolar mania, are reported in rats to reduce brain transcription of an arachidonic acid selective calcium-dependent cytosolic phospholipase A(2) (cPLA(2)), as well as expression of one of its transcription factors, activator protein (AP)-2. In this study, we determined if chronic administration of NMDA (25 mg/kg i.p.) to rats would increase brain cPLA(2) and AP-2 expression, as these antimanic drugs are known to down-regulate excessive NMDA signaling. Administration of a daily subconvulsive dose of NMDA to rats for 21 days decreased frontal cortex NMDA receptor (NR)-1 and NR-3A subunits and increased cPLA(2) activity, phosphorylation, protein, and mRNA levels. The activity and protein levels of secretory phospholipase A(2) or calcium-independent phospholipase A(2) were not changed significantly. Chronic NMDA also increased the DNA-binding activity of AP-2 and the protein levels of its alpha and beta subunits. These changes were absent following acute (3 h earlier) NMDA administration. The changes, opposite to those found following chronic lithium or carbamazepine, are consistent with up-regulated arachidonic acid release due to excessive NR signaling and may be a contributing factor to bipolar mania.

Chronic fluoxetine increases cytosolic phospholipase A(2) activity and arachidonic acid turnover in brain phospholipids of the unanesthetized rat

RATIONALE

Fluoxetine is used to treat unipolar depression and is thought to act by increasing the concentration of serotonin (5-HT) in the synaptic cleft, leading to increased serotonin signaling. The 5-HT(2A/2C) receptor subtypes are coupled to a phospholipase A(2) (PLA(2)). We hypothesized that chronic fluoxetine would increase the brain activity of PLA(2) and the turnover rate of arachidonic acid (AA) in phospholipids of the unanesthetized rat.

MATERIALS AND METHODS

To test this hypothesis, rats were administered fluoxetine (10 mg/kg) or vehicle intraperitoneally daily for 21 days. In the unanesthetized rat, [1-(14)C]AA was infused intravenously and arterial blood plasma was sampled until the animal was killed at 5 min and its brain was subjected to chemical, radiotracer, or enzyme analysis.

RESULTS

Using equations from our fatty acid model, we found that chronic fluoxetine compared with vehicle increased the turnover rate of AA within several brain phospholipids by 75-86%. The activity and protein levels of brain cytosolic PLA(2) (cPLA(2)) but not of secretory or calcium-independent PLA(2) were increased in rats administered fluoxetine. In a separate group of animals that received chronic fluoxetine followed by a 3-day saline washout, the turnover of AA and activity and protein levels of cPLA(2) were not significantly different from controls. The protein levels of cyclooxygenases 1 and 2 as well as the concentration of prostaglandin E(2) in rats chronically administered fluoxetine did not differ significantly from controls.

CONCLUSION

The results support the hypothesis that fluoxetine increases the cPLA(2)-mediated turnover of AA within brain phospholipids.

Ca2+ influx through CRAC channels activates cytosolic phospholipase A2, leukotriene C4 secretion, and expression of c-fos through ERK-dependent and -independent pathways in mast cells

Cytosolic phospholipase A2 (cPLA2) is a Ca2+-dependent enzyme that mediates agonist-dependent arachidonic acid release in most cell types. Arachidonic acid can then be metabolized by the 5-lipoxygenase enzyme to generate the proinflammatory signal leukotriene C4 (LTC4). Here we report that Ca2+ entry through store-operated CRAC (Ca2+ release-activated Ca2+) channels activates the extracellular signal-regulated kinases (ERKs), members of the mitogen-activated protein kinase family, within minutes and this is necessary for stimulation of cPLA2. Ca2+ entry activates ERK indirectly, via recruitment of Ca2+-dependent protein kinase C alpha and betaI. Ca2+ influx also promotes translocation of cytosolic 5-lipoxygenase to the nuclear membrane, a key step in the activation of this enzyme. Translocation is dependent on ERK activation. A role for gene activation is shown by the finding that CRAC channel opening results in increased transcription and translation of c-fos. Inhibition of ERK activation failed to prevent c-fos expression. Our results show that CRAC channel activation elicits short-term effects through the co-coordinated regulation of two metabolic pathways (cPLA2 and 5-lipoxygenase), which results in the generation of both intra- and intercellular messengers within minutes, as well as longer term changes involving gene activation. These short-term effects are mediated via ERK, whereas, paradoxically, c-fos expression is not. Ca2+ influx through CRAC channels can therefore activate different signaling pathways at the same time, culminating in a range of temporally diverse responses.

Phospholipase A2-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets

In the present study, phospholipase A(2) (PLA(2))-catalyzed hydrolysis of platelet membrane phospholipids was investigated by measuring PLA(2) activity, phospholipid hydrolysis, arachidonic acid release and choline lysophospholipid production in thrombin-stimulated human platelets. Thrombin-stimulated platelets demonstrated selective hydrolysis of arachidonylated plasmenylcholine and plasmenylethanolamine, with little change in diacyl phospholipids. Accelerated plasmalogen hydrolysis was accompanied by increased arachidonic acid and thromboxane B(2) release and increased lysoplasmenylcholine production. Thrombin stimulation caused an increase in PLA(2) activity measured in the cytosolic fraction with plasmenylcholine only; no increase in activity was measured with phosphatidylcholine. No change in membrane-associated PLA(2) activity was observed with either substrate tested. Pretreatment with the Ca(2+)-independent PLA(2)-selective inhibitor, bromoenol lactone, inhibited completely any thrombin-stimulated phospholipid hydrolysis. Thus, thrombin stimulation of human platelets activates a cytosolic PLA(2) that selectively hydrolyzes arachidonylated plasmalogen phospholipids.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Oxidative signaling in renal epithelium: Critical role of cytosolic phospholipase A2 and p38(SAPK)

Previous studies from this laboratory have demonstrated a critical role of cytosolic phospholipase A2 (cPLA2) and arachidonic acid in angiotensin II (Ang II) AT2 receptor-mediated signal transduction in renal epithelium. In primary proximal tubular epithelial cells exposed to hydrogen peroxide (H2O2), both the selective cPLA2 inhibitors and the cPLA2 antisense oligonucleotides significantly attenuated H2O2-induced arachidonic acid liberation and activation of p38(SAPK), ERK1/2, and Akt1. This H2O2-induced kinase activation was significantly attenuated by a Src kinase inhibitor PP2, or by transient transfection of carboxyl-terminal Src kinase (CSK) that maintained Src in the dormant form. Under basal conditions, Src coimmunoprecipitated with epidermal growth factor receptor (EGFR), while H2O2 increased EGFR phosphorylation in the complex. We observed that inhibition of EGFR kinase activity with AG1478 significantly attenuated H2O2-induced p38(SAPK) and ERK1/2 activation, but did not inhibit Akt1 activation. Furthermore, it seems that p38(SAPK) is upstream of ERK1/2 and Akt1, since a p38(SAPK) inhibitor SB203580 significantly blocked H2O2-induced activation of ERK1/2 and Akt1. Interestingly, overexpression of the dominant-negative p38(SAPK) isoform alpha inhibited ERK1/2 but not Akt1 activation. Our observations demonstrate that in these nontransformed cells, activation of cPLA2 is a converging point for oxidative stress and Ang II, which share common downstream signaling mechanisms including Src and EGFR. In addition, p38(SAPK) provides a positive input to both growth and antiapoptotic signaling pathways induced by acute oxidative stress.

Chronic fluoxetine upregulates activity, protein and mRNA levels of cytosolic phospholipase A2 in rat frontal cortex

Chronic lithium and carbamazepine, which are effective against mania in bipolar disorder, decrease the activity of cytosolic phospholipase A(2) (cPLA(2)) and the turnover rate of arachidonic acid in phospholipids in rat brain. Assuming that stages of bipolar disorder are related to brain arachidonic acid metabolism, we hypothesized that drugs effective in depression would increase cPLA(2) activity. To test this hypothesis, adult male CDF-344 rats were administered fluoxetine (10 mg/kg intraperitoneally (i.p.) or saline (control) (i.p.) chronically for 21 days. Frontal cortex cPLA(2) protein, phosphorylated cPLA(2), activity and mRNA levels were increased after chronic fluoxetine. Transcription factors (activator protein-1, activator protein-2, glucocorticoid response element, polyoma enhancer element-3 and nuclear factor-kappa B) that are known to regulate cPLA(2) gene expression were not significantly changed by chronic fluoxetine, but nuclear AU-rich element/poly(U)-binding/degradation factor-1 RNA-stabilizing protein was increased significantly. The results suggest that chronic fluoxetine increases brain cPLA(2) gene expression post-transcriptionally by increasing cPLA(2) mRNA stabilization. Chronic fluoxetine's effect on cPLA(2) expression was opposite to the effect reported with chronic lithium or carbamazepine administration, and may be part of fluoxetine's mode of action.