Regulation of cytosolic phospholipase A(2) translocation

Quantification of intracellular and extracellular eicosapentaenoic acid-derived 3-series prostanoids by liquid chromatography/electrospray ionization tandem mass spectrometry

3-Series prostanoids are bioactive lipid mediators synthesized from eicosapentaenoic acid (EPA). Determination of intracellular and extracellular levels of prostanoids is needed to elucidate the mechanism of action, and we therefore developed a method for quantification of intracellular and extracellular levels of 3-series prostanoids (including prostaglandin E3 (PGE3), PGD3, PGF3α, thromboxane B3 (TXB3), and Δ(17)-6-keto PGF1α) by using liquid chromatography/electrospray ionization tandem mass spectrometry. The separation of prostanoids was performed with a CAPCELL PAK C18 MG II column (2.0mm×150mm, 3µm) with an isocratic flow of acetonitrile/water/acetic acid (40:60:0.1, v/v/v). This method was validated for measurement of both extracellular and intracellular samples with high levels of precision and accuracy. We applied this method to human lung epithelial A549 cells stimulated with calcium ionophore A23187 under the condition of arachidonic acid or EPA treatment and we could measure PGE3 in both intracellular and extracellular samples.

Co-localization of the alpha-subunit of BK-channels and c-PLA2 in GH3 cells

Large conductance, calcium-activated potassium channels (maxi K- or BK-channels) can be regulated by arachidonic acid produced by c-Phospholipase A2 (c-PLA2). Since in every excised patch from GH3 cells where there was BK-channel activity, treatment with either a stimulator or inhibitor of c-PLA2 resulted in a corresponding increase or decrease in BK-channel activity, we hypothesized that there must be a close association between BK-channel proteins and c-PLA2 in the cell membrane. To test this hypothesis, we first determined whether the two proteins would co-immunoprecipitate. We then used confocal imaging of fluorescently tagged proteins to determine where in the cells BK-channel proteins and c-PLA2 co-localize. The alpha-subunit of the BK-channel was strongly co-immunoprecipitated by c-PLA2 antibodies, suggesting that most of the BK channel alpha-subunits are associated with c-PLA2. This interaction was not affected by pharmacologically inhibiting c-PLA2 suggesting that the association does not require functionally active c-PLA2. Following dual immunohistochemical labeling and confocal microscopy, image analysis revealed that in the cytosol there was some co-localization, but most of the c-PLA2 was separate from BK-channel proteins. On the other hand, the c-PLA2 and BK-channel proteins at the plasma membrane were strongly co-localized. Immunoprecipitation experiments conducted with plasma membrane proteins support these findings. We conclude that c-PLA2 is likely physically associated with BK-channel proteins at the cell surface.

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.

Estrogen induces phospholipase A2 activation through ERK1/2 to mobilize intracellular calcium in MCF-7 cells

The principal secreted estrogen, 17beta-estradiol rapidly activates signaling cascades that regulate important physiological processes including ion transport across membranes, cytosolic pH and cell proliferation. These effects have been extensively studied in the MCF-7 estrogen-responsive human breast carcinoma cell line. Here, we demonstrate that a physiological concentration of 17beta-estradiol caused a rapid, synchronous and transient increase in intracellular calcium concentration in a confluent monolayer of MCF-7 cells 2-3 min after treatment. This response was abolished when cells were pre-incubated with the phospholipase A(2) (PLA(2)) inhibitor quinacrine or with the cyclooxygenase inhibitor indomethacin. The translocation of GFP-cPLA(2)alpha to perinuclear membranes occurred 1-2 min after 17beta-estradiol treatment; this translocation was concurrent with the transient phosphorylation of cPLA(2)alpha at serine residue 505. The phosphorylation and translocation of cPLA(2) were sensitive to inhibition of the extracellular signal regulated kinase (ERK) signaling cascade and occurred simultaneously with a transient activation of ERK. The phosphorylation of cPLA(2) could be stimulated by membrane impermeable 17beta-estradiol conjugated to bovine serum albumen and was blocked by an antagonist of the classical estrogen receptor. Here we show, for the first time, that PLA(2) and the eicosanoid biosynthetic pathway are involved in the 17beta-estradiol induced rapid calcium responses of breast cancer cells.

Polychlorinated biphenyls induce arachidonic acid release in human platelets in a tamoxifen sensitive manner via activation of group IVA cytosolic phospholipase A2-alpha

Polychlorinated biphenyls (PCBs) are stable compounds commonly found in nature as environmental pollutants. PCBs can affect the endocrine function of hormones such as steroid-hormones. Also, PCBs are known to be inducers of arachidonic acid release in various cells. We report, here, the effects of PCBs on eicosanoid formation, arachidonic acid release and cytosolic phospholipase A2-alpha (cPLA2-alpha) activation in human platelets. Ortho-substituted PCBs induced a time and dose-dependent release of arachidonic acid and the concomitant formation of 12(S)-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid (12-HETE) and 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid (12-HHT) in human platelets. The release of arachidonic acid and the formation of 12-HETE was completely blocked by the cPLA2-alpha inhibitors AACOCF3 or pyrrolidine-1. PCB-treatment of platelets demonstrated that the cPLA2-alpha protein as well as PLA2 activity translocated to the membrane fraction, independent of a rise in intracellular Ca2+. Furthermore, electrophoretic gel mobility shift analysis of cPLA2-alpha on SDS-PAGE demonstrated a PCB-dependent phosphorylation of cPLA2-alpha. The effects of 17beta-estradiol and two structurally unrelated anti-estrogens, nafoxidin and tamoxifen on PCB-induced arachidonic acid release in platelets were also investigated. Both nafoxidin and tamoxifen inhibited PCB-induced arachidonic acid release as well as 12-HETE and 12-HHT formation. Interestingly, platelets incubated with PCBs did not aggregate despite the fact that robust release of arachidonic acid was observed. In summary, these results demonstrate that certain PCBs induce activation of cPLA2-alpha independent of a rise in intracellular calcium and a robust release of arachidonic acid release with resulting eicosanoid formation in human platelets.

Cytosolic phospholipase A2 Group IValpha but not secreted phospholipase A2 Group IIA, V, or X induces interleukin-8 and cyclooxygenase-2 gene and protein expression through peroxisome proliferator-activated receptors gamma 1 and 2 in human lung cells

It has been reported that interleukin-8 (IL-8) and cyclooxygenase-2 (COX-2) expression is regulated by peroxisome proliferator-activated receptor (PPAR)-gamma synthetic ligands. We have shown previously that cytosolic phospholipase A2 (cPLA2) is able to activate gene expression through PPAR-gamma response elements (Pawliczak, R., Han, C., Huang, X. L., Demetris, A. J., Shelhamer, J. H., and Wu, T. (2002) J. Biol. Chem. 277, 33153-33163). In this study we investigated the influence of cPLA2 and secreted phospholipase A2 (sPLA2) Group IIA, Group V, and Group X on IL-8 and COX-2 expression in human lung epithelial cells (A549 cells). We also studied the results of cPLA2 activation by epidermal growth factor (EGF) and calcium ionophore (A23187) on IL-8 and COX-2 reporter gene activity, mRNA level, and protein synthesis. cPLA2 overexpression and activation increased both IL-8 and COX-2 reporter gene activity. Overexpression and activation of Group IIA, Group V, or Group X sPLA2s did not increase IL-8 and COX-2 reporter gene activity. Methyl arachidonyl fluorophosphate, a cPLA2 inhibitor, inhibited the effect of A23187 and of EGF on both IL-8 and COX-2 reporter gene activity, steady state levels of IL-8 and COX-2 mRNA, and IL-8 and COX-2 protein expression. Small inhibitory RNAs directed against PPAR-gamma1 and -gamma2 blunted the effect of A23187 and of EGF on IL-8 and COX-2 protein expression. Moreover small inhibitory RNAs directed against cPLA2 decreased the effect of A23187 and EGF on IL-8 and COX-2 protein expression. These results demonstrate that cPLA2 has an influence on IL-8 and COX 2 gene and protein expression at least in part through PPAR-gamma.

The calcium binding loops of the cytosolic phospholipase A2 C2 domain specify targeting to Golgi and ER in live cells

Translocation of cytosolic phospholipase A2 (cPLA2) to Golgi and ER in response to intracellular calcium mobilization is regulated by its calcium-dependent lipid-binding, or C2, domain. Although well studied in vitro, the biochemical characteristics of the cPLA2C2 domain offer no predictive value in determining its intracellular targeting. To understand the molecular basis for cPLA2C2 targeting in vivo, the intracellular targets of the synaptotagmin 1 C2A (Syt1C2A) and protein kinase Calpha C2 (PKCalphaC2) domains were identified in Madin-Darby canine kidney cells and compared with that of hybrid C2 domains containing the calcium binding loops from cPLA2C2 on Syt1C2A and PKCalphaC2 domain backbones. In response to an intracellular calcium increase, PKCalphaC2 targeted plasma membrane regions rich in phosphatidylinositol-4,5-bisphosphate, and Syt1C2A displayed a biphasic targeting pattern, first targeting phosphatidylinositol-4,5-bisphosphate-rich regions in the plasma membrane and then the trans-Golgi network. In contrast, the Syt1C2A/cPLA2C2 and PKCalphaC2/cPLA2C2 hybrids targeted Golgi/ER and colocalized with cPLA2C2. The electrostatic properties of these hybrids suggested that the membrane binding mechanism was similar to cPLA2C2, but not PKCalphaC2 or Syt1C2A. These results suggest that primarily calcium binding loops 1 and 3 encode structural information specifying Golgi/ER targeting of cPLA2C2 and the hybrid domains.