Cytosolic phospholipase A2-alpha mediates endothelial cell proliferation and is inactivated by association with the Golgi apparatus

Activation of the cytosolic calcium-independent phospholipase A2 β isoform contributes to TRPC6 externalization via release of arachidonic acid

During vascular interventions, oxidized low-density lipoprotein and lysophosphatidylcholine (lysoPC) accumulate at the site of arterial injury, inhibiting endothelial cell (EC) migration and arterial healing. LysoPC activates canonical transient receptor potential 6 (TRPC6) channels, leading to a prolonged increase in intracellular calcium ion concentration that inhibits EC migration. However, an initial increase in intracellular calcium ion concentration is required to activate TRPC6, and this mechanism remains elusive. We hypothesized that lysoPC activates the lipid-cleaving enzyme phospholipase A₂ (PLA₂), which releases arachidonic acid (AA) from the cellular membrane to open arachidonate-regulated calcium channels, allowing calcium influx that promotes externalization and activation of TRPC6 channels. The focus of this study was to identify the roles of calcium-dependent and/or calcium-independent PLA₂ in lysoPC-induced TRPC6 externalization. We show that lysoPC induced PLA₂ enzymatic activity and caused AA release in bovine aortic ECs. To identify the specific subgroup and the isoform(s) of PLA₂ involved in lysoPC-induced TRPC6 activation, transient knockdown studies were performed in the human endothelial cell line EA.hy926 using siRNA to inhibit the expression of genes encoding cPLA₂α, cPLA₂γ, iPLA₂β, or iPLA₂γ. Downregulation of the β isoform of iPLA₂ blocked lysoPC-induced release of AA from EC membranes and TRPC6 externalization, as well as preserved EC migration in the presence of lysoPC. We propose that blocking TRPC6 activation and promoting endothelial healing could improve the outcomes for patients undergoing cardiovascular interventions.

The lipid paradox in neuroprogressive disorders: Causes and consequences

Chronic systemic inflammation is associated with an increased risk of cardiovascular disease in an environment of low low-density lipoprotein (LDL) and low total cholesterol and with the pathophysiology of neuroprogressive disorders. The causes and consequences of this lipid paradox are explored. Circulating activated neutrophils can release inflammatory molecules such as myeloperoxidase and the pro-inflammatory cytokines interleukin-1 beta, interleukin-6 and tumour necrosis factor-alpha. Since activated neutrophils are associated with atherosclerosis and cardiovascular disease and with major depressive disorder, bipolar disorder and schizophrenia, it seems reasonable to hypothesise that the inflammatory molecules released by them may act as mediators of the link between systemic inflammation and the development of atherosclerosis in neuroprogressive disorders. This hypothesis is tested by considering the association at a molecular level of systemic inflammation with increased LDL oxidation; increased small dense LDL levels; increased lipoprotein (a) concentration; secretory phospholipase A₂ activation; cytosolic phospholipase A₂ activation; increased platelet activation; decreased apolipoprotein A1 levels and function; decreased paroxonase-1 activity; hyperhomocysteinaemia; and metabolic endotoxaemia. These molecular mechanisms suggest potential therapeutic targets.

Phospholipase Signaling in Breast Cancer

Breast cancer progression results from subversion of multiple intra- or intercellular signaling pathways in normal mammary tissues and their microenvironment, which have an impact on cell differentiation, proliferation, migration, and angiogenesis. Phospholipases (PLC, PLD and PLA) are essential mediators of intra- and intercellular signaling. They hydrolyze phospholipids, which are major components of cell membrane that can generate many bioactive lipid mediators, such as diacylglycerol, phosphatidic acid, lysophosphatidic acid, and arachidonic acid. Enzymatic processing of phospholipids by phospholipases converts these molecules into lipid mediators that regulate multiple cellular processes, which in turn can promote breast cancer progression. Thus, dysregulation of phospholipases contributes to a number of human diseases, including cancer. This review describes how phospholipases regulate multiple cancer-associated cellular processes, and the interplay among different phospholipases in breast cancer. A thorough understanding of the breast cancer-associated signaling networks of phospholipases is necessary to determine whether these enzymes are potential targets for innovative therapeutic strategies.

Ligand-conjugated mesoporous silica nanorattles based on enzyme targeted prodrug delivery system for effective lung cancer therapy

Epidermal growth factor receptor antibody (EGFRAb) conjugated silica nanorattles (SNs) were synthesized and used to develop receptor mediated endocytosis for targeted drug delivery strategies for cancer therapy. The present study determined that the rate of internalization of silica nanorattles was found to be high in lung cancer cells when compared with the normal lung cells. EGFRAb can specifically bind to EGFR, a receptor that is highly expressed in lung cancer cells, but is expressed at low levels in other normal cells. Furthermore, in vitro studies clearly substantiated that the cPLA2α activity, arachidonic acid release and cell proliferation were considerably reduced by pyrrolidine-2 loaded EGFRAb-SN in H460 cells. The cytotoxicity, cell cycle arrest and apoptosis were significantly induced by the treatment of pyrrolidine-2 loaded EGFRAb-SN when compared with free pyrrolidine-2 and pyrrolidine-2 loaded SNs in human non-small cell lung cancer cells. An in vivo toxicity assessment showed that silica nanorattles and EGFRAb-SN-pyrrolidine-2 exhibited low systemic toxicity in healthy Balb/c mice. The EGFRAb-SN-pyrrolidine-2 showed a much better antitumor activity (38%) with enhanced tumor inhibition rate than the pyrrolidine-2 on the non-small cell lung carcinoma subcutaneous model. Thus, the present findings validated the low toxicity and high therapeutic potentials of EGFRAb-SN-pyrrolidine-2, which may provide a convincing evidence of the silica nanorattles as new potential carriers for targeted drug delivery systems.

Phospholipase signalling networks in cancer

Phospholipases (PLC, PLD and PLA) are essential mediators of intracellular and intercellular signalling. They can function as phospholipid-hydrolysing enzymes that can generate many bioactive lipid mediators, such as diacylglycerol, phosphatidic acid, lysophosphatidic acid and arachidonic acid. Lipid mediators generated by phospholipases regulate multiple cellular processes that can promote tumorigenesis, including proliferation, migration, invasion and angiogenesis. Although many individual phospholipases have been extensively studied, how phospholipases regulate diverse cancer-associated cellular processes and the interplay between different phospholipases have yet to be fully elucidated. A thorough understanding of the cancer-associated signalling networks of phospholipases is necessary to determine whether these enzymes can be targeted therapeutically.

C2 domain membrane penetration by group IVA cytosolic phospholipase A₂ induces membrane curvature changes

Group IVA cytosolic phospholipase A(2) (cPLA(2)α) is an 85 kDa enzyme that regulates the release of arachidonic acid (AA) from the sn-2 position of membrane phospholipids. It is well established that cPLA(2)α binds zwitterionic lipids such as phosphatidylcholine in a Ca(2+)-dependent manner through its N-terminal C2 domain, which regulates its translocation to cellular membranes. In addition to its role in AA synthesis, it has been shown that cPLA(2)α promotes tubulation and vesiculation of the Golgi and regulates trafficking of endosomes. Additionally, the isolated C2 domain of cPLA(2)α is able to reconstitute Fc receptor-mediated phagocytosis, suggesting that C2 domain membrane binding is sufficient for phagosome formation. These reported activities of cPLA(2)α and its C2 domain require changes in membrane structure, but the ability of the C2 domain to promote changes in membrane shape has not been reported. Here we demonstrate that the C2 domain of cPLA(2)α is able to induce membrane curvature changes to lipid vesicles, giant unilamellar vesicles, and membrane sheets. Biophysical assays combined with mutagenesis of C2 domain residues involved in membrane penetration demonstrate that membrane insertion by the C2 domain is required for membrane deformation, suggesting that C2 domain-induced membrane structural changes may be an important step in signaling pathways mediated by cPLA(2)α.

A VE-cadherin-PAR3-α-catenin complex regulates the Golgi localization and activity of cytosolic phospholipase A(2)α in endothelial cells

The rapid regulation of phospholipase A₂ activity is essential for vascular function. Evidence is found for a VE-cadherin–α-catenin–PAR3 complex regulating the reversible association of cPLA₂α with the Golgi apparatus in confluent endothelial cells. This regulation is important for controlling both cPLA₂α activity and angiogenesis.

Phospholipase A₂ enzymes hydrolyze phospholipids to liberate arachidonic acid for the biosynthesis of prostaglandins and leukotrienes. In the vascular endothelium, group IV phospholipase A₂α (cPLA₂α) enzyme activity is regulated by reversible association with the Golgi apparatus. Here we provide evidence for a plasma membrane cell adhesion complex that regulates endothelial cell confluence and simultaneously controls cPLA₂α localization and enzymatic activity. Confluent endothelial cells display pronounced accumulation of vascular endothelial cadherin (VE-cadherin) at cell–cell junctions, and mechanical wounding of the monolayer stimulates VE-cadherin complex disassembly and cPLA₂α release from the Golgi apparatus. VE-cadherin depletion inhibits both recruitment of cPLA₂α to the Golgi and formation of tubules by endothelial cells. Perturbing VE-cadherin and increasing the soluble cPLA₂α fraction also stimulated arachidonic acid and prostaglandin production. Of importance, reverse genetics shows that α-catenin and δ-catenin, but not β-catenin, regulates cPLA₂α Golgi localization linked to cell confluence. Furthermore, cPLA₂α Golgi localization also required partitioning defective protein 3 (PAR3) and annexin A1. Disruption of F-actin internalizes VE-cadherin and releases cPLA₂α from the adhesion complex and Golgi apparatus. Finally, depletion of either PAR3 or α-catenin promotes cPLA₂α-dependent endothelial tubule formation. Thus a VE-cadherin–PAR3–α-catenin adhesion complex regulates cPLA₂α recruitment to the Golgi apparatus, with functional consequences for vascular physiology.

A PLA1-2 punch regulates the Golgi complex

The mammalian Golgi complex, trans Golgi network (TGN) and ER-Golgi intermediate compartment (ERGIC) are comprised of membrane cisternae, coated vesicles and membrane tubules, all of which contribute to membrane trafficking and maintenance of their unique architectures. Recently, a new cast of players was discovered to regulate the Golgi and ERGIC: four unrelated cytoplasmic phospholipase A (PLA) enzymes, cPLA(2)α (GIVA cPLA(2)), PAFAH Ib (GVIII PLA(2)), iPLA(2)-β (GVIA-2 iPLA(2)) and iPLA(1)γ. These ubiquitously expressed enzymes regulate membrane trafficking from specific Golgi subcompartments, although there is evidence for some functional redundancy between PAFAH Ib and cPLA(2)α. Three of these enzymes, PAFAH Ib, cPLA(2)α and iPLA(2)-β, exert effects on Golgi structure and function by inducing the formation of membrane tubules. We review our current understanding of how PLA enzymes regulate Golgi and ERGIC morphology and function.

Attachment of HeLa cells during early G1 phase

Both growth factor directed and integrin dependent signal transduction were shown to take place directly after completion of mitosis. The local activation of these signal transduction cascades was investigated in early G1 cells. Interestingly, various key signal transduction proteins were found in blebs at the cell membrane within 30 min after mitosis. These membrane blebs appeared in round, mitotic-like cells and disappeared rapidly during spreading of the cells in G1 phase. In addition to tyrosine-phosphorylated proteins, the blebs contained also phosphorylated FAK and phosphorylated MAP kinase. The formation of membrane blebs in round, mitotic cells before cell spreading is not specific for mitotic cells, because similar features were observed in trypsinized cells. Just before cell spreading also these cells exhibited membrane blebs containing active signal transduction proteins. Inhibition of signal transduction did not affect membrane bleb formation, suggesting that the membrane blebs were formed independent of signal transduction.

GVI phospholipase A2 role in the stimulatory effect of sphingosine-1-phosphate on TRPC5 cationic channels

The Transient Receptor Potential Canonical 5 (TRPC5) protein forms calcium-permeable cationic channels that are stimulated by G protein-coupled receptor agonists. The signaling pathways of such agonist effects are poorly understood. Here we investigated the potential for involvement of lysophosphatidylcholine (LPC) and arachidonic acid generated by group 6 (GVI) phospholipase A2 (PLA2) enzymes, focusing on stimulation of TRPC5 by sphingosine-1-phosphate (S1P) which acts via a pertussis toxin-sensitive (Gi/o protein) pathway without Ca²⁺-release. Experiments were on HEK 293 cells containing conditional expression of human TRPC5. Channel activity was recorded using an intracellular calcium indicator or whole-cell patch-clamp and PLA2 activity was detected using ³H-arachidonic acid. S1P stimulated PLA2 and TRPC5 activities. Both effects were suppressed by the GVI PLA2 inhibitor bromoenol lactone. Knock-down of GVI PLA2 by RNA interference suppressed channel activity evoked by S1P whereas activity evoked by the direct channel stimulator LPC was unaffected. Arachidonic acid did not stimulate the channels. Prior exposure of channels to LPC but not arachidonic acid suppressed channel activity evoked by S1P but not gadolinium, a putative direct stimulator of the channels. The data suggest roles of LPC and GVI PLA2 in S1P-evoked TRPC5 activity.

Ligand-stimulated VEGFR2 signaling is regulated by co-ordinated trafficking and proteolysis

Vascular endothelial growth factor A (VEGF-A)-induced signaling through VEGF receptor 2 (VEGFR2) regulates both physiological and pathological angiogenesis in mammals. However, the temporal and spatial mechanism underlying VEGFR2-mediated intracellular signaling is not clear. Here, we define a pathway for VEGFR2 trafficking and proteolysis that regulates VEGF-A-stimulated signaling and endothelial cell migration. Ligand-stimulated VEGFR2 activation and ubiquitination preceded proteolysis and cytoplasmic domain removal associated with endosomes. A soluble VEGFR2 cytoplasmic domain fragment displayed tyrosine phosphorylation and activation of downstream intracellular signaling. Perturbation of endocytosis by the depletion of either clathrin heavy chain or an ESCRT-0 subunit caused differential effects on ligand-stimulated VEGFR2 proteolysis and signaling. This novel VEGFR2 proteolysis was blocked by the inhibitors of 26S proteasome activity. Inhibition of proteasome activity prolonged VEGF-A-induced intracellular signaling to c-Akt and endothelial nitric oxide synthase (eNOS). VEGF-A-stimulated endothelial cell migration was dependent on VEGFR2 and VEGFR tyrosine kinase activity. Inhibition of proteasome activity in this assay stimulated VEGF-A-mediated endothelial cell migration. VEGFR2 endocytosis, ubiquitination and proteolysis could also be stimulated by a protein kinase C-dependent pathway. Thus, removal of the VEGFR2 carboxyl terminus linked to phosphorylation, ubiquitination and trafficking is necessary for VEGF-stimulated endothelial signaling and cell migration.

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.

Cytosolic phospholipase A2 activation correlates with HER2 overexpression and mediates estrogen-dependent breast cancer cell growth

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) catalyzes the hydrolysis of membrane glycerol-phospholipids to release arachidonic acid as the first step of the eicosanoid signaling pathway. This pathway contributes to proliferation in breast cancer, and numerous studies have demonstrated a crucial role of cyclooxygenase 2 and prostaglandin E(2) release in breast cancer progression. The role of cPLA(2)alpha activation is less clear, and we recently showed that 17beta-estradiol (E2) can rapidly activate cPLA(2)alpha in MCF-7 breast cancer cells. Overexpression or gene amplification of HER2 is found in approximately 30% of breast cancer patients and correlates with a poor clinical outcome and resistance to endocrine therapy. This study reports the first evidence for a correlation between cPLA(2)alpha enzymatic activity and overexpression of the HER2 receptor. The activation of cPLA(2)alpha in response to E2 treatment was biphasic with the first phase dependent on trans-activation through the matrix metalloproteinase-dependent release of heparin-bound epidermal growth factor. EGFR/HER2 heterodimerization resulted in downstream signaling through the ERK1/2 cascade to promote cPLA(2)alpha phosphorylation at Ser505. There was a correlation between HER2 and cPLA(2)alpha expression in six breast cancer cell lines examined, and inhibition of HER2 activation or expression in the SKBR3 cell line using herceptin or HER2-specific small interfering RNA, respectively, resulted in decreased activation and expression of cPLA(2)alpha. Pharmacological blockade of cPLA(2)alpha using a specific antagonist suppressed the growth of both MCF-7 and SKBR3 cells by reducing E2-induced proliferation and by stimulating cellular apoptosis and necrosis. This study highlights cPLAalpha(2) as a potential target for therapeutic intervention in endocrine-dependent and endocrine-independent breast cancer.

Novel role of cPLA(2)alpha in membrane and actin dynamics

Actin-directed processes such as membrane ruffling and cell migration are regulated by specific signal transduction pathways that become activated by growth factor receptors. The same signaling pathways that lead to modifications in actin dynamics also activate cPLA(2)alpha. Moreover, arachidonic acid, the product of cPLA(2)alpha activity, is involved in regulation of actin dynamics. Therefore, it was investigated whether cPLA(2)alpha plays a role in actin dynamics, more specifically during growth factor-induced membrane ruffling and cell migration. Upon stimulation of ruffling and cell migration by growth factors, endogenous cPLA(2)alpha and its active phosphorylated form were shown to relocate at protrusions of the cell membrane involved in actin and membrane dynamics. Inhibition of cPLA(2)alpha activity with specific inhibitors blocked growth factor-induced membrane and actin dynamics, suggesting an important role for cPLA(2)alpha in these processes.

Golgi-associated cPLA2alpha regulates endothelial cell-cell junction integrity by controlling the trafficking of transmembrane junction proteins

In endothelial cells specifically, cPLA2alpha translocates from the cytoplasm to the Golgi complex in response to cell confluence. Considering the link between confluence and cell-cell junction formation, and the emerging role of cPLA2alpha in intracellular trafficking, we tested whether Golgi-associated cPLA2alpha is involved in the trafficking of junction proteins. Here, we show that the redistribution of cPLA2alpha from the cytoplasm to the Golgi correlates with adherens junction maturation and occurs before tight junction formation. Disruption of adherens junctions using a blocking anti-VE-cadherin antibody reverses the association of cPLA2alpha with the Golgi. Silencing of cPLA2alpha and inhibition of cPLA2alpha enzymatic activity using various inhibitors result in the diminished presence of the transmembrane junction proteins VE-cadherin, occludin, and claudin-5 at cell-cell contacts, and in their accumulation at the Golgi. Altogether, our data support the idea that VE-cadherin triggers the relocation of cPLA2alpha to the Golgi and that in turn, Golgi-associated cPLA2alpha regulates the transport of transmembrane junction proteins through or from the Golgi, thereby controlling the integrity of endothelial cell-cell junctions.

Rab GTPase regulation of VEGFR2 trafficking and signaling in endothelial cells

OBJECTIVE

Vascular endothelial growth factor receptor 2 (VEGFR2) is a receptor tyrosine kinase that regulates vascular physiology. However, mechanism(s) by which VEGFR2 signaling and trafficking is coordinated are not clear. Here, we have tested endocytic Rab GTPases for regulation of VEGFR2 trafficking and signaling linked to endothelial cell migration.

METHODS AND RESULTS

Quiescent VEGFR2 displays endosomal localization and colocalization with the Rab5a GTPase, an early endosome fusion regulator. Expression of GTP or GDP-bound Rab5a mutants block activated VEGFR2 trafficking and degradation. Manipulation of Rab7a GTPase activity associated with late endosomes using overexpression of wild-type or mutant proteins blocks activated VEGFR2 trafficking and degradation. Depletion of Rab7a decreased VEGFR2 Y1175 phosphorylation but increased p42/44 (pERK1/2) MAPK phosphorylation. Endothelial cell migration was increased by Rab5a depletion but decreased by Rab7a depletion.

CONCLUSIONS

Rab5a and Rab7a regulate VEGFR2 trafficking toward early and late endosomes. Our data suggest that VEGFR2-mediated regulation of endothelial function is dependent on different but specific Rab-mediated GTP hydrolysis activity required for endosomal trafficking.

Potential role of protease-activated receptor-2-stimulated activation of cytosolic phospholipase A(2) in intestinal myofibroblast proliferation: Implications for stricture formation in Crohn's disease

BACKGROUND AND AIMS

Myofibroblast hyperplasia contributes to muscularis mucosae thickening and stricture formation in Crohn's disease (CD). Protease-activated receptor-2 (PAR-2) and cytosolic phospholipase A(2) (cPLA(2)) are known regulators of cell growth, but their significance in intestinal myofibroblast proliferation remain to be elucidated. The principle aims of the present study were to investigate if PAR-2 is expressed in the expanded muscularis mucosa in ileal CD specimens, if inflammatory cytokines may stimulate PAR-2 expression in intestinal myofibroblasts, and if PAR-2 and cPLA(2) may regulate intestinal myofibroblast growth.

METHODS

Immunohistochemistry was used for detection of PAR-2 in ileal CD specimens. Studies on PAR-2 expression, PLA(2) activation and cell growth were performed in a human intestinal myofibroblast cell line, CCD-18Co. PAR-2 expression was investigated by RT-PCR and immunocytochemistry. PLA(2) activity was analyzed by quantification of released (14)C-arachidonic acid ((14)C-AA). Cell growth was examined by (3)H-thymidine incorporation and cell counting.

RESULTS

The thickened muscularis mucosae of the CD specimens showed strong PAR-2 expression. In cultured myofibroblasts, tumor necrosis factor-α (TNF-α) up-regulated PAR-2 mRNA and protein, and potentiated PAR-2-stimulated (14)C-AA release by two known PAR-2 activators, trypsin and SLIGRL-NH(2). The release of (14)C-AA was dependent on cPLA(2). Trypsin stimulated the proliferation of serum-starved cells, and inhibition of cPLA(2) reduced normal cell growth and abolished the growth-promoting effect of trypsin.

CONCLUSIONS

The results suggest that PAR-2-mediated cPLA(2) activation might be of importance in intestinal myofibroblast proliferation. The results also point to the possibility that PAR-2 up-regulation by inflammatory cytokines, like TNF-α, may modulate this effect.

Regulation of Wnt/beta-catenin pathway by cPLA2alpha and PPARdelta

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) is a rate-limiting key enzyme that releases arachidonic acid (AA) from membrane phospholipid for the production of biologically active lipid mediators including prostaglandins, leukotrienes and platelet-activating factor. cPLA(2)alpha is translocated to nuclear envelope in response to intracellular calcium increase and the enzyme is also present inside the cell nucleus; however, the biological function of cPLA(2)alpha in the nucleus remains unknown. Here we show a novel role of cPLA(2)alpha for activation of peroxisome proliferator-activated receptor-delta (PPARdelta) and beta-catenin in the nuclei. Overexpression of cPLA(2)alpha in human cholangiocarcinoma cells induced the binding of PPARdelta to beta-catenin and increased their association with the TCF/LEF response element. These effects are inhibited by the cPLA(2)alpha siRNA and inhibitors as well as by siRNA knockdown of PPARdelta. Overexpression of PPARdelta or treatment with the selective PPARdelta ligand, GW501516, also increased beta-catenin binding to TCF/LEF response element and increased its reporter activity. Addition of AA and GW501516 to nuclear extracts induced a comparable degree of beta-catenin binding to TCF/LEF response element. Furthermore, cPLA(2)alpha protein is present in the PPARdelta and beta-catenin binding complex. Thus the close proximity between cPLA(2)alpha and PPARdelta provides a unique advantage for their efficient functional coupling in the nucleus, where AA produced by cPLA(2)alpha becomes immediately available for PPARdelta binding and subsequent beta-catenin activation. These results depict a novel interaction linking cPLA(2)alpha, PPARdelta and Wnt/beta-catenin signaling pathways and provide insight for further understanding the roles of these key molecules in human cells and diseases.

Activation of cytosolic phospholipase A2-{alpha} as a novel mechanism regulating endothelial cell cycle progression and angiogenesis

Release of endothelial cells from contact-inhibition and cell cycle re-entry is required for the induction of new blood vessel formation by angiogenesis. Using a combination of chemical inhibition, loss of function, and gain of function approaches, we demonstrate that endothelial cell cycle re-entry, S phase progression, and subsequent angiogenic tubule formation are dependent upon the activity of cytosolic phospholipase A(2)-alpha (cPLA(2)alpha). Inhibition of cPLA(2)alpha activity and small interfering RNA (siRNA)-mediated knockdown of endogenous cPLA(2)alpha reduced endothelial cell proliferation. In the absence of cPLA(2)alpha activity, endothelial cells exhibited retarded progression from G(1) through S phase, displayed reduced cyclin A/cdk2 expression, and generated less arachidonic acid. In quiescent endothelial cells, cPLA(2)alpha is inactivated upon its sequestration at the Golgi apparatus. Upon the stimulation of endothelial cell proliferation, activation of cPLA(2)alpha by release from the Golgi apparatus was critical to the induction of cyclin A expression and efficient cell cycle progression. Consequently, inhibition of cPLA(2)alpha was sufficient to block angiogenic tubule formation in vitro. Furthermore, the siRNA-mediated retardation of endothelial cell cycle re-entry and proliferation was reversed upon overexpression of an siRNA-resistant form of cPLA(2)alpha. Thus, activation of cPLA(2)alpha acts as a novel mechanism for the regulation of endothelial cell cycle re-entry, cell cycle progression, and angiogenesis.

Arachidonic acid-induced Ca2+ entry is involved in early steps of tumor angiogenesis

Growth factor-induced intracellular calcium signals in endothelial cells regulate cytosolic and nuclear events involved in the angiogenic process. Among the intracellular messengers released after proangiogenic stimulation, arachidonic acid (AA) plays a key role and its effects are strictly related to calcium homeostasis and cell proliferation. Here, we studied AA-induced intracellular calcium signals in endothelial cells derived from human breast carcinomas (B-TEC). AA promotes B-TEC proliferation and organization of vessel-like structures in vitro. The effect is directly mediated by the fatty acid without a significant contribution of its metabolites. AA induces Ca(2+)(i) signals in the entire capillary-like structure during the early phases of tubulogenesis in vitro. No such responses are detectable in B-TECs organized in more structured tubules. In B-TECs growing in monolayer, AA induces two different signals: a Ca(2+)(i) increase due to Ca(2+) entry and an inhibition of store-dependent Ca(2+) entry induced by thapsigargin or ATP. An inhibitor of Ca(2+) entry and angiogenesis, carboxyamidotriazole, significantly and specifically decreases AA-induced B-TEC tubulogenesis, as well as AA-induced Ca(2+) signals in B-TECs. We conclude that (a) AA-activated Ca(2+) entry is associated with the progression through the early phases of angiogenesis, mainly involving proliferation and tubulogenesis, and it is down-regulated during the reorganization of tumor-derived endothelial cells in capillary-like structures; and (b) inhibition of AA-induced Ca(2+) entry may contribute to the antiangiogenic action of carboxyamidotriazole.

Cytosolic phospholipase A2 regulates viability of irradiated vascular endothelium

Radiosensitivity of various normal tissues is largely dependent on radiation-triggered signal transduction pathways. Radiation simultaneously initiates distinct signaling from both DNA damage and cell membrane. Specifically, DNA strand breaks initiate cell-cycle delay, strand-break repair or programmed cell death, whereas membrane-derived signaling through phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) enhances cell viability. Here, activation of cytosolic phospholipase A(2) (cPLA(2)) and production of the lipid second-messenger lysophosphatidylcholine were identified as initial events (within 2 min) required for radiation-induced activation of Akt and ERK1/2 in vascular endothelial cells. Inhibition of cPLA(2) significantly enhanced radiation-induced cytotoxicity due to an increased number of multinucleated giant cells and cell cycle-independent accumulation of cyclin B1 within 24-48 h of irradiation. Delayed programmed cell death was detected at 72-96 h after treatment. Endothelial functions were also affected by inhibition of cPLA(2) during irradiation resulting in attenuated cell migration and tubule formation. The role of cPLA(2) in the regulation of radiation-induced activation of Akt and ERK1/2 and cell viability was confirmed using human umbilical vein endothelial cells transfected with shRNA for cPLA(2)alpha and cultured embryonic fibroblasts from cPLA(2)alpha(-/-) mice. In summary, an immediate radiation-induced cPLA(2)-dependent signaling was identified that regulates cell viability and, therefore, represents one of the key regulators of radioresistance of vascular endothelial cells.

Oxidised LDL internalisation by the LOX-1 scavenger receptor is dependent on a novel cytoplasmic motif and is regulated by dynamin-2

The LOX-1 scavenger receptor recognises pro-atherogenic oxidised low-density lipoprotein (OxLDL) particles and is implicated in atherosclerotic plaque formation, but this mechanism is not well understood. Here we show evidence for a novel clathrin-independent and cytosolic-signal-dependent pathway that regulates LOX-1-mediated OxLDL internalisation. Cell surface labelling in the absence or presence of OxLDL ligand showed that LOX-1 is constitutively internalised from the plasma membrane and its half-life is not altered upon ligand binding and trafficking. We show that LOX-1-mediated OxLDL uptake is disrupted by overexpression of dominant-negative dynamin-2 but unaffected by CHC17 or mu2 (AP2) depletion. Site-directed mutagenesis revealed a conserved and novel cytoplasmic tripeptide motif (DDL) that regulates LOX-1-mediated endocytosis of OxLDL. Taken together, these findings indicate that LOX-1 is internalised by a clathrin-independent and dynamin-2-dependent pathway and is thus likely to mediate OxLDL trafficking in vascular tissues.

iPLA2beta: front and center in human monocyte chemotaxis to MCP-1

Monocyte chemoattractant protein-1 (MCP-1) directs migration of blood monocytes to inflamed tissues. Despite the central role of chemotaxis in immune responses, the regulation of chemotaxis by signal transduction pathways and their in vivo significance remain to be thoroughly deciphered. In this study, we examined the intracellular location and functions of two recently identified regulators of chemotaxis, Ca²⁺-independent phospholipase (iPLA₂β) and cytosolic phospholipase (cPLA₂α), and substantiate their in vivo importance. These enzymes are cytoplasmic in unstimulated monocytes. Upon MCP-1 stimulation, iPLA₂β is recruited to the membrane-enriched pseudopod. In contrast, cPLA₂α is recruited to the endoplasmic reticulum. Although iPLA₂β or cPLA₂α antisense oligodeoxyribonucleotide (ODN)–treated monocytes display reduced speed, iPLA₂β also regulates directionality and actin polymerization. iPLA₂β or cPLA₂α antisense ODN–treated adoptively transferred mouse monocytes display a profound defect in migration to the peritoneum in vivo. These converging observations reveal that iPLA₂β and cPLA₂α regulate monocyte migration from different intracellular locations, with iPLA₂β acting as a critical regulator of the cellular compass, and identify them as potential targets for antiinflammatory strategies.

The Confluence-dependent Interaction of Cytosolic Phospholipase A2-α with Annexin A1 Regulates Endothelial Cell Prostaglandin E2 Generation

The regulated generation of prostaglandins from endothelial cells is critical to vascular function. Here we identify a novel mechanism for the regulation of endothelial cell prostaglandin generation. Cytosolic phospholipase A(2)-alpha (cPLA(2)alpha) cleaves phospholipids in a Ca(2+)-dependent manner to yield free arachidonic acid and lysophospholipid. Arachidonic acid is then converted into prostaglandins by the action of cyclooxygenase enzymes and downstream synthases. By previously undefined mechanisms, nonconfluent endothelial cells generate greater levels of prostaglandins than confluent cells. Here we demonstrate that Ca(2+)-independent association of cPLA(2)alpha with the Golgi apparatus of confluent endothelial cells correlates with decreased prostaglandin synthesis. Golgi association blocks arachidonic acid release and prevents functional coupling between cPLA(2)alpha and COX-mediated prostaglandin synthesis. When inactivated at the Golgi apparatus of confluent endothelial cells, cPLA(2)alpha is associated with the phospholipid-binding protein annexin A1. Furthermore, the siRNA-mediated knockdown of endogenous annexin A1 significantly reverses the inhibitory effect of confluence on endothelial cell prostaglandin generation. Thus the confluence-dependent interaction of cPLA(2)alpha and annexin A1 at the Golgi acts as a novel molecular switch controlling cPLA(2)alpha activity and endothelial cell prostaglandin generation.

Group VIA Calcium-independent Phospholipase A2 Mediates Endothelial Cell S Phase Progression

Arachidonic acid and its metabolites have been previously implicated in the regulation of endothelial cell proliferation. Arachidonic acid may be liberated from cellular phospholipids by the action of group VIA calcium-independent phospholipase A2 (iPLA2-VIA). Consequently, we tested the hypothesis that iPLA2-VIA activity is linked to the regulation of endothelial cell proliferation. Inhibition of iPLA2 activity by bromoenol lactone (BEL) was sufficient to entirely block endothelial cell growth. BEL dose-dependently inhibited endothelial cell DNA synthesis in a manner that was reversed upon the exogenous addition of arachidonic acid. DNA synthesis was inhibited by the S-isomer and not by the R-isomer of BEL, demonstrating that endothelial cell proliferation is mediated specifically by iPLA2-VIA. iPLA2-VIA activity was critical to the progression of endothelial cells through S phase and is required for the expression of the cyclin A/cdk2 complex. Thus, inhibition of iPLA2-VIA blocks S phase progression and results in exit from the cell cycle. Inhibition of iPLA2-VIA-mediated endothelial cell proliferation is sufficient to block angiogenic tubule formation in co-culture assays. Consequently, iPLA2-VIA is a novel regulator of endothelial cell S phase progression, cell cycle residence, and angiogenesis.

Roles of cPLA2alpha and arachidonic acid in cancer

Phospholipase A(2)s (PLA(2)s) are key enzymes that catalyze the hydrolysis of membrane phospholipids to release bioactive lipids that play an important role in normal cellular homeostasis. Under certain circumstances, disrupted production of key lipid mediators may adversely impact physiological processes, leading to pathological conditions such as inflammation and cancer. In particular, cytosolic PLA(2)alpha (cPLA(2)alpha) has a high selectivity for liberating arachidonic acid (AA) that is subsequently metabolized by a panel of downstream enzymes for eicosanoid production. Although concentrations of free AA are maintained at low levels in resting cells, alterations in AA production, often resulting from dysregulation of cPLA(2)alpha activity, are observed in transformed cells. In this review, we summarize recent evidence that cPLA(2)alpha plays a role in the pathogenesis of many human cancers. Much of this evidence has been accumulated from functional studies using cPLA(2)alpha-deficient mice, as well as mechanistic studies in cell culture. We also discuss the potential contribution of cPLA(2)alpha and AA to apoptosis, and the regulatory mechanisms leading to aberrant expression of cPLA(2)alpha.

A novel pathway of cell growth regulation mediated by a PLA2α‐derived phosphoinositide metabolite

The phosphoinositides have well-defined roles in the control of cellular functions, including cytoskeleton dynamics, membrane trafficking, and cell signaling. However, the interplay among the phosphoinositides and their diffusible derivatives that originate through phospholipase A2 action (the lysophosphoinositides and glycerophosphoinositols) remains to be fully elucidated. Here we demonstrate that in PCCl3 rat thyroid cells, the intracellular levels of glycerophosphoinositol are finely modulated by ATP and norepinephrine through the P2Y metabotropic and alpha-adrenergic receptors, respectively. The enzyme involved here is phospholipase A2 IValpha (PLA2 IValpha), which in these cells specifically hydrolyzes phosphatidylinositol, forming lysophosphatidylinositol, glycerophosphoinositol, and arachidonic acid. This receptor-mediated activation of PLA2 IValpha leads to stimulation of PCCl3 cell growth. The involvement of a PLA2 IValpha-mediated pathway is demonstrated by inhibition of the increase in intracellular glycerophosphoinositol levels and cell proliferation by specific inhibitors, RNA interference, and overexpression of the dominant-negative PLA2 IValpha(1-522). Modulation of PCCl3 cell growth is not seen with inhibitors of arachidonic acid metabolism. In conclusion, these data characterize glycerophosphoinositol as a mediator of the purinergic and adrenergic regulation of PCCl3 cell proliferation, defining a novel regulatory cascade specifically involving this soluble phosphoinositide derivative and widening the involvement of the phosphoinositides in the regulation of cell function.

Distribution and functional characterization of equilibrative nucleoside transporter-4, a novel cardiac adenosine transporter activated at acidic pH

Adenosine plays multiple roles in the efficient functioning of the heart by regulating coronary blood flow, cardiac pacemaking, and contractility. Previous studies have implicated the equilibrative nucleoside transporter family member equilibrative nucleoside transporter-1 (ENT1) in the regulation of cardiac adenosine levels. We report here that a second member of this family, ENT4, is also abundant in the heart, in particular in the plasma membranes of ventricular myocytes and vascular endothelial cells but, unlike ENT1, is virtually absent from the sinoatrial and atrioventricular nodes. Originally described as a monoamine/organic cation transporter, we found that both human and mouse ENT4 exhibited a novel, pH-dependent adenosine transport activity optimal at acidic pH (apparent K(m) values 0.78 and 0.13 mmol/L, respectively, at pH 5.5) and absent at pH 7.4. In contrast, serotonin transport by ENT4 was relatively insensitive to pH. ENT4-mediated nucleoside transport was adenosine selective, sodium independent and only weakly inhibited by the classical inhibitors of equilibrative nucleoside transport, dipyridamole, dilazep, and nitrobenzylthioinosine. We hypothesize that ENT4, in addition to playing roles in cardiac serotonin transport, contributes to the regulation of extracellular adenosine concentrations, in particular under the acidotic conditions associated with ischemia.