Cytosolic phospholipase A2-p11 interaction controls arachidonic acid release as a function of epithelial cell confluence

Recent Advances in Molecular and Cellular Functions of S100A10

S100A10 (p11, annexin II light chain, calpactin light chain) is a multifunctional protein with a wide range of physiological activity. S100A10 is unique among the S100 family members of proteins since it does not bind to Ca2+, despite its sequence and structural similarity. This review focuses on studies highlighting the structure, regulation, and binding partners of S100A10. The binding partners of S100A10 were collated and summarized.

Abnormal n-6 fatty acid metabolism in cystic fibrosis contributes to pulmonary symptoms

Cystic fibrosis (CF) is a recessively inherited fatal disease that is the subject of extensive research and ongoing development of therapeutics targeting the defective protein, cystic fibrosis transmembrane conductance regulator (CFTR). Despite progress, the link between CFTR and clinical symptoms is incomplete. The severe CF phenotypes are associated with a deficiency of linoleic acid, which is the precursor of arachidonic acid. The release of arachidonic acid from membranes via phospholipase A₂ is the rate-limiting step for eicosanoid synthesis and is increased in CF, which contributes to the observed inflammation. A potential deficiency of docosahexaenoic acid may lead to decreased levels of specialized pro-resolving mediators. This pathophysiology may contribute to an early and sterile inflammation, mucus production, and to bacterial colonization, which further increases inflammation and potentiates the clinical symptoms. Advances in lipid technology will assist in elucidating the role of lipid metabolism in CF, and stimulate therapeutic modulations of inflammation.

Potentiation of endothelin-1-induced prostaglandin E2 formation by Ni(2+) and Sr(2+) in murine osteoblastic MC3T3-E1 cells

Cation recognition mechanisms beyond calcium-sensing receptors are still largely unexplored and consequently there is surprisingly little information on linking of this primary event to key metabolic features of different cell systems, such as arachidonic acid metabolism. However, information on the modulatory role of extracellular cations in cellular function is scarce. In this study we have demonstrated, that Ni(2+) and Sr(2+) potentiate endothelin-1 induced prostaglandin E2 formation in the osteoblastic cell line, MC3T3-E1, even in the absence of extracellular calcium. The effect is strictly dependent of receptor-mediated signal transduction processes evoked by endothelin-1 and arachidonate release involves cytosolic phospholipase A2 activity. The ligation sites, at least for Ni(2+) are extracellular. The data suggest a novel activation mechanism for arachidonate release and subsequent prostaglandin formation that does not require calcium.

Cytosolic phospholipase A2 and eicosanoids modulate life, death and function of human osteoclasts in vitro

INTRODUCTION

Eicosanoids are important in bone physiology but the specific function of phopholipase enzymes has not been determined in osteoclasts. The objective of this is study was to determine the presence of cPLA2 in human in vitro-differentiated osteoclasts as well as osteoclasts in situ from bone biopsies.

MATERIALS AND METHODS

Osteoclastogenesis, apoptosis, bone resorption and the modulation of actin cytoskeleton assays were performed on osteoclasts differentiated in vitro. Immunohistochemistry was done in differentiated osteoclasts as well as on bone biopsies.

RESULTS

Human osteoclasts from normal, fetal, osteoarthritic, osteoporotic and Pagetic bone biopsies express cPLA2 and stimulation with RANKL increases cPLA2 phosphorylation in vitro. Inhibition of cPLA2 increased osteoclastogenesis and decreased apoptosis but decreased the capacity of osteoclasts to generate actin rings and to resorb bone.

DISCUSSION AND CONCLUSIONS

These results suggest that cPLA2 modulates osteoclast functions and could be a useful target in bone diseases with hyperactivated osteoclasts.

Do annexins participate in lipid messenger mediated intracellular signaling? A question revisited

Annexins are physiologically important proteins that play a role in calcium buffering but also influence membrane structure, participate in Ca²⁺-dependent membrane repair events and in remodelling of the cytoskeleton. Thirty years ago several peptides isolated from lung perfusates, peritoneal leukocytes, neutrophiles and renal cells were proven inhibitory to the activity of phospholipase A₂. Those peptides were found to derive from structurally related proteins: annexins AnxA1 and AnxA2. These findings raised the question whether annexins may participate in regulation of the production of lipid second messengers and, therefore, modulate numerous lipid mediated signaling pathways in the cell. Recent advances in the field of annexins made also with the use of knock-out animal models revealed that these proteins are indeed important constituents of specific signaling pathways. In this review we provide evidence supporting the hypothesis that annexins, as membrane-binding proteins and organizers of the membrane lateral heterogeneity, may participate in lipid mediated signaling pathways by affecting the distribution and activity of lipid metabolizing enzymes (most of the reports point to phospholipase A₂) and of protein kinases regulating activity of these enzymes. Moreover, some experimental data suggest that annexins may directly interact with lipid metabolizing enzymes and, in a calcium-dependent or independent manner, with some of their substrates and products. On the basis of these observations, many investigators suggest that annexins are capable of linking Ca²⁺, redox and lipid signaling to coordinate vital cellular responses to the environmental stimuli.

S100A10-mediated translocation of annexin-A2 to SNARE proteins in adrenergic chromaffin cells undergoing exocytosis

In neuroendocrine cells, annexin-A2 is implicated as a promoter of monosialotetrahexosylganglioside (GM1)-containing lipid microdomains that are required for calcium-regulated exocytosis. As soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) require a specific lipid environment to mediate granule docking and fusion, we investigated whether annexin-A2-induced lipid microdomains might be linked to the SNAREs present at the plasma membrane. Stimulation of adrenergic chromaffin cells induces the translocation of cytosolic annexin-A2 to the plasma membrane, where it colocalizes with SNAP-25 and S100A10. Cross-linking experiments performed in stimulated chromaffin cells indicate that annexin-A2 directly interacts with S100A10 to form a tetramer at the plasma membrane. Here, we demonstrate that S100A10 can interact with vesicle-associated membrane protein 2 (VAMP2) and show that VAMP2 is present at the plasma membrane in resting adrenergic chromaffin cells. Tetanus toxin that cleaves VAMP2 solubilizes S100A10 from the plasma membrane and inhibits the translocation of annexin-A2 to the plasma membrane. Immunogold labelling of plasma membrane sheets combined with spatial point pattern analysis confirmed that S100A10 is present in VAMP2 microdomains at the plasma membrane and that annexin-A2 is observed close to S100A10 and to syntaxin in stimulated chromaffin cells. In addition, these results showed that the formation of phosphatidylinositol (4,5)-bisphosphate (PIP(2)) microdomains colocalized with S100A10 in the vicinity of docked granules, suggesting a functional interplay between annexin-A2-mediated lipid microdomains and SNAREs during exocytosis.

Eicosanoid release is increased by membrane destabilization and CFTR inhibition in Calu-3 cells

The antiinflammatory protein annexin-1 (ANXA1) and the adaptor S100A10 (p11), inhibit cytosolic phospholipase A2 (cPLA2α) by direct interaction. Since the latter is responsible for the cleavage of arachidonic acid at membrane phospholipids, all three proteins modulate eicosanoid production. We have previously shown the association of ANXA1 expression with that of CFTR, the multifactorial protein mutated in cystic fibrosis. This could in part account for the abnormal inflammatory status characteristic of this disease. We postulated that CFTR participates in the regulation of eicosanoid release by direct interaction with a complex containing ANXA1, p11 and cPLA2α. We first analyzed by plasmon surface resonance the in vitro binding of CFTR to the three proteins. A significant interaction between p11 and the NBD1 domain of CFTR was found. We observed in Calu-3 cells a rapid and partial redistribution of all four proteins in detergent resistant membranes (DRM) induced by TNF-α. This was concomitant with increased IL-8 synthesis and cPLA2α activation, ultimately resulting in eicosanoid (PGE2 and LTB4) overproduction. DRM destabilizing agent methyl-β-cyclodextrin induced further cPLA2α activation and eicosanoid release, but inhibited IL-8 synthesis. We tested in parallel the effect of short exposure of cells to CFTR inhibitors Inh172 and Gly-101. Both inhibitors induced a rapid increase in eicosanoid production. Longer exposure to Inh172 did not increase further eicosanoid release, but inhibited TNF-α-induced relocalization to DRM. These results show that (i) CFTR may form a complex with cPLA2α and ANXA1 via interaction with p11, (ii) CFTR inhibition and DRM disruption induce eicosanoid synthesis, and (iii) suggest that the putative cPLA2/ANXA1/p11/CFTR complex may participate in the modulation of the TNF-α-induced production of eicosanoids, pointing to the importance of membrane composition and CFTR function in the regulation of inflammation mediator synthesis.

Cell culture models demonstrate that CFTR dysfunction leads to defective fatty acid composition and metabolism

Cystic fibrosis (CF) is associated with fatty acid alterations characterized by low linoleic and docosahexaenoic acid. It is not clear whether these fatty acid alterations are directly linked to cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction or result from nutrient malabsorption. We hypothesized that if fatty acid alterations are a result of CFTR dysfunction, those alterations should be demonstrable in CF cell culture models. Two CF airway epithelial cell lines were used: 16HBE, sense and antisense CFTR cells, and C38/IB3-1 cells. Wild-type (WT) and CF cells were cultured in 10% fetal bovine serum (FBS) or 10% horse serum. Fatty acid levels were analyzed by GC-MS. Culture of both WT and CF cells in FBS resulted in very low linoleic acid levels. When cells were cultured in horse serum containing concentrations of linoleic acid matching those found in human plasma, physiological levels of linoleic acid were obtained and fatty acid alterations characteristic of CF tissues were then evident in CF compared with WT cells. Kinetic studies with radiolabeled linoleic acid demonstrated in CF cells increased conversion to longer and more-desaturated fatty acids such as arachidonic acid. In conclusion, these data demonstrate that CFTR dysfunction is associated with altered fatty acid metabolism in cultured airway epithelial cells.

Cytosolic phospholipase A2 regulation in the hibernating thirteen-lined ground squirrel

Cytosolic calcium-dependent phospholipase A₂ (cPLA₂) has multiple roles including production of arachidonic acid (a key player in cellular signaling pathways) and membrane remodeling. Additionally, since catabolism of arachidonic acid generates free radicals, the enzyme is also implicated in ischemic injury to mammalian organs. Regulation of cPLA₂ could be important in the suppression and prioritization of cellular pathways in animals that undergo reversible transitions into hypometabolic states. The present study examines the responses and regulation of cPLA₂ in skeletal muscle and liver of hibernating thirteen-lined ground squirrels, Spermophilus tridecemlineatus. cPLA₂ activity decreased significantly by 43% in liver during hibernation, compared with euthermic controls, and K_m values for arachidonoyl thio-PC substrate fell in both organs during hibernation to 61% in liver and 28% in muscle of the corresponding euthermic value. To determine whether these responses were due to a change in the phosphorylation state of the enzyme, Western blotting was employed using antibodies recognizing phospho-Ser⁵⁰⁵ on α-cPLA₂. The amount of phosphorylated α-cPLA₂ in hibernator liver was just 38% of the value in euthermic liver. Furthermore, incubation of liver extracts under conditions that enhanced protein phosphatase action caused a greater reduction in the detectable amount of phospho-Ser⁵⁰⁵ enzyme content in euthermic, versus hibernator, extracts. The data are consistent with a suppression of cPLA₂ function during torpor via enzyme dephosphorylation, an action that may contribute to the well-developed ischemia tolerance and lack of oxidative damage found in hibernating species over cycles of torpor and arousal.

Comparison of annexin II, p63 and alpha-methylacyl-CoA racemase immunoreactivity in prostatic tissue: a tissue microarray study

BACKGROUND

Current ancillary markers for diagnosis in prostate biopsies include p63 and alpha-methylacyl-CoA racemase (AMACR). Annexin II (ANXII), a calcium and phospholipid binding protein, is lost in prostate cancer.

AIMS

To investigate ANXII expression in order to assess its utility as a novel diagnostic marker in comparison to p63 and AMACR.

METHODS

Using immunohistochemistry on six tissue microarrays, ANXII, p63, and AMACR expression was analysed from 210 radical prostatectomy cases. Staining was evaluated in benign and atrophic glands, high-grade prostatic intraepithelial neoplasia (HGPIN), and prostatic adenocarcinoma. Separate scores were given for ANXII, AMACR and p63 expression.

RESULTS

Diffuse cytoplasmic expression of ANXII correlated with p63 reactivity in basal cells. Benign glands were positive for ANXII in 286/292 cores (98%) and negative for AMACR in all 292 cores. HGPIN showed heterogeneous expression of AMACR and ANXII. A significantly larger proportion of HGPIN glands were correctly identified as ANXII negative than as positive for AMACR. ANXII loss in prostate cancer was found in 282/320 cores (88%) and correlated with positive AMACR expression (272/320 cores, 85%), which was not statistically significant. There was no statistically significant correlation between ANXII scores and the clinical parameters examined.

CONCLUSIONS

Immunohistochemical staining for ANXII is a consistent and reliable marker of prostatic neoplasia. The findings of this study suggest the potential utility of ANXII as a diagnostic aid in prostate cancer histopathology.

Arachidonic acid cascade and epithelial barrier function during Caco-2 cell differentiation

The small intestinal epithelium is a highly dynamic system continuously renewed by a process involving cell proliferation and differentiation. The intestinal epithelium constitutes a permeability barrier regulating the vectorial transport of ions, water, and solutes. Morphological changes during cell differentiation, as well as changes in the activity of brush-border enzymes and the expression of transport proteins, are well established. However, little is known about the arachidonic acid (AA) cascade underlying epithelial cell differentiation or its role in the development of epithelial barrier function. The main purpose of this study was to examine the activity of the high-molecular-weight phospholipases A(2) (PLA(2)) and cyclooxygenase (COX) pathway during differentiation, with particular emphasis on paracellular permeability. PLA(2) activity, AA release, COX-2 expression, prostaglandin E(2) (PGE(2)) production, and paracellular permeability were studied in preconfluent, confluent, and differentiated Caco-2 cell cultures. Our results show that Caco-2 differentiation induces a decrease in both calcium-independent PLA(2) activity and COX-2 expression and, consequently, a decrease in AA release and PGE(2) synthesis in parallel with a reduction in paracellular permeability. Moreover, the addition of PGE(2) to differentiated cells, at concentrations similar to those detected in nondifferentiated cultures, induces the disruption of epithelial barrier function. These results suggest that AA release by calcium-independent PLA(2), COX-2 expression, and subsequent PGE(2) release are important for the maintenance of paracellular permeability in differentiated Caco-2 cells.

Calcium-dependent and -independent interactions of the S100 protein family

The S100 proteins comprise at least 25 members, forming the largest group of EF-hand signalling proteins in humans. Although the proteins are expressed in many tissues, each S100 protein has generally been shown to have a preference for expression in one particular tissue or cell type. Three-dimensional structures of several S100 family members have shown that the proteins assume a dimeric structure consisting of two EF-hand motifs per monomer. Calcium binding to these S100 proteins, with the exception of S100A10, results in an approx. 40 degrees alteration in the position of helix III, exposing a broad hydrophobic surface that enables the S100 proteins to interact with a variety of target proteins. More than 90 potential target proteins have been documented for the S100 proteins, including the cytoskeletal proteins tubulin, glial fibrillary acidic protein and F-actin, which have been identified mostly from in vitro experiments. In the last 5 years, efforts have concentrated on quantifying the protein interactions of the S100 proteins, identifying in vivo protein partners and understanding the molecular specificity for target protein interactions. Furthermore, the S100 proteins are the only EF-hand proteins that are known to form both homo- and hetero-dimers, and efforts are underway to determine the stabilities of these complexes and structural rationales for their formation and potential differences in their biological roles. This review highlights both the calcium-dependent and -independent interactions of the S100 proteins, with a focus on the structures of the complexes, differences and similarities in the strengths of the interactions, and preferences for homo- compared with hetero-dimeric S100 protein assembly.

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

Arachidonic acid and its metabolites are implicated in regulating endothelial cell proliferation. Cytosolic phospholipase A2-alpha (cPLA2alpha) is responsible for receptor-mediated arachidonic acid evolution. We tested the hypothesis that cPLA2alpha activity is linked to endothelial cell proliferation. The specific cPLA2alpha inhibitor, pyrrolidine-1, inhibited umbilical vein endothelial cell (HUVEC) proliferation in a dose-dependent manner. Exogenous arachidonic acid addition reversed this inhibitory effect. Inhibition of sPLA2 did not affect HUVEC proliferation. The levels of cPLA2alpha did not differ between subconfluent and confluent cultures of cells. However, using fluorescence microscopy we observed a novel, confluence-dependent redistribution of cPLA2alpha to the distal Golgi apparatus in HUVECs. Association of cPLA2alpha with the Golgi was linked to the proliferative status of HUVECs. When associated with the Golgi apparatus, cPLA2alpha activity was seen to be 87% inhibited. Relocation of cPLA2alpha to the cytoplasm and nucleus, and cPLA2alpha enzyme activity were required for cell cycle entry upon mechanical wounding of confluent monolayers. Thus, cPLA2alpha activity and function in controlling endothelial cell proliferation is regulated by reversible association with the Golgi apparatus.

Role of group VIA calcium-independent phospholipase A2 in arachidonic acid release, phospholipid fatty acid incorporation, and apoptosis in U937 cells responding to hydrogen peroxide

Group VIA calcium-independent phospholipase A2 (iPLA2) has been shown to play a major role in regulating basal phospholipid deacylation reactions in certain cell types. More recently, roles for this enzyme have also been suggested in the destruction of membrane phospholipid during apoptosis and after oxidant injury. Proposed iPLA2 roles have rested heavily on the use of bromoenol lactone as an iPLA2-specific inhibitor, but this compound actually inhibits other enzymes and lipid pathways unrelated to PLA2, which makes it difficult to define the contribution of iPLA2 to specific functions. In previous work, we pioneered the use of antisense technology to decrease cellular iPLA2 activity as an alternative approach to study iPLA2 functions. In the present study, we followed the opposite strategy and prepared U937 cells that exhibited enhanced iPLA activity by stably expressing a plasmid containing iPLA2 cDNA. Compared with control cells, the iPLA2 -overexpressing U937 cells showed elevated responses to hydrogen peroxide with regard to both arachidonic acid mobilization and incorporation of the fatty acid into phospholipids, thus providing additional evidence for the key role that iPLA2 plays in these events. Long-term exposure of the cells to hydrogen peroxide resulted in cell death by apoptosis, and this process was accelerated in the iPLA2-overexpressing cells. Increased phospholipid hydrolysis and fatty acid release also occurred in these cells. Unexpectedly, however, abrogation of U937 cell iPLA2 activity by either methyl arachidonyl fluorophosphonate or an antisense oligonucleotide did not delay or decrease the extent of apoptosis induced by hydrogen peroxide. These results indicate that, although iPLA2-mediated phospholipid hydrolysis occurs during apoptosis, iPLA2 may actually be dispensable for the apoptotic process to occur. Thus, beyond a mere destructive role, iPLA2 may play other roles during apoptosis.