Inhibition of phospholipase C-independent exocytotic responses in rat peritoneal mast cells by U73122
ABSTRACT The aminosteroid U73122 has been established as potent, selective, and cell-permeable inhibitor C-type phosphatidylinositol-specific phospholipases (PI-PLCs), and has been used to define a contribution of PI-PLCs as part of exocytotic signalling pathways in rat peritoneal mast cells (RPMCs). However, doubts have been raised regarding its PI-PLC selectivity of action. Therefore, in the present study, U73122 was tested in RPMCs under experimental conditions allowing to elicit exocytosis PI-PLC independently (streptolysin O [SLO]-permeabilised cells; stimulated by GTPgammaS; in the presence of low concentrations of free Ca2+). The release of [3H]5-hydroxytryptamine ([3H]5-HT) from [3H]5-HT-loaded RPMCs served as measure of secretion. U73122 potently inhibited the exocytotic response induced by 10 microM GTPgammaS (Ca2+: 10(-6) M) in permeabilised cells (IC50: 0.6 microM, n=5) in an insurmountable manner. In intact RPMCs, with a nearly equal potency (IC50: 4 microM, n=4), U73122 also inhibited the PI-PLC-dependent exocytotic response induced by concomitant application of nerve growth factor and lyso-phosphatidylserine (NGF/lyso-PS). CONCLUSION: U73122 exerts potent PI-PLC-independent secretostatic effects, limiting its use to define PI-PLC function within exocytotic processes.
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ABSTRACT: A variety of agents, including imidazoles, such as miconazole, elicit elevations of calcium in human leukemic HL-60 cells through a stimulation of influx of calcium and/or a release of intracellular calcium. Release of intracellular calcium by such agents may involve stimulation of IP3 formation. The steroidal maleimide U73122, a potent phospholipase C (PLC) inhibitor, at 3–10 µM blocked elevations of calcium elicited in HL-60 cells by ATP. At a 10&!hyphen; µM concentration, U73122 either blocked or altered elevations of calcium elicited by imidazoles, such as miconazole, SKF 96365, and clotrimazole that are often used to block capacitative calcium entry, and by calmidazolium, a quaternary imidazole that can trigger massive influx of calcium in HL-60 cells. Elevations of calcium elicited by thapsigargin were not affected by 3 µM U73122, but were reduced by higher concentrations. Remarkably, U73122 at 10 µM did not block, but instead slowed and potentiated the elevation of calcium elicited by higher concentrations of imidazoles, an effect that may involve inhibition of Ca2+ -ATPases of the endoplasmic reticulum. The steroidal succinimide U73343 had minimal effects on calcium elevations in HL-60 cells. The results indicate that U73122 requires cautious use in assessing the role of PLC in controlling calcium levels in HL-60 cells, in particular when used with imidazoles, such as SKF 96365, which are used to block capacitative calcium entry. The effects of U73122 on cell calcium are reviewed. In toto, the results suggest that many lipophilic agents may affect intracellular calcium by activating PLC through unknown mechanisms. Drug Dev. Res. 67:519–534, 2006. Published 2006 Wiley-Liss, Inc.Drug Development Research 06/2006; 67(6):519 - 534. DOI:10.1002/ddr.20111 · 0.77 Impact Factor
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ABSTRACT: The inositol lipid phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] is involved in a myriad of cellular processes, including the regulation of exocytosis and endocytosis. In this paper, we address the role of PtdIns(4,5)P2 in compound exocytosis from rat peritoneal mast cells. This process involves granule-plasma membrane fusion as well as homotypic granule membrane fusion and occurs without any immediate compensatory endocytosis. Using a novel quantitative immunofluorescence technique, we report that plasma membrane PtdIns(4,5)P2 becomes transiently depleted upon activation of exocytosis, and is not detected on the membranes of fusing granules. Depletion is caused by phospholipase C activity, and is mandatory for exocytosis. Although phospholipase C is required for Ca2+ release from internal stores, the majority of the requirement for PtdIns(4,5)P2 hydrolysis occurs downstream of Ca2+ signalling - as shown in permeabilised cells, where the inositol (1,4,5)-trisphosphate-Ca2+ pathway is bypassed. Neither generation of the PtdIns(4,5)P2 metabolite, diacylglycerol (DAG) or simple removal and/or sequestration of PtdIns(4,5)P2 are sufficient for exocytosis to occur. However, treatment of permeabilised cells with DAG induces a small potentiation of exocytosis, indicating that it may be required. We propose that a cycle of PtdIns(4,5)P2 synthesis and breakdown is crucial for exocytosis to occur in mast cells, and may have a more general role in all professional secretory cells.Journal of Cell Science 06/2006; 119(Pt 10):2084-94. DOI:10.1242/jcs.02912 · 5.43 Impact Factor
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ABSTRACT: Malignant breast cancer cells that have entered the blood circulation from primary mammary fat pad tumors or are grown in end-over-end suspension culture assemble a characteristic, multi-globular polymeric fibronectin (polyFn) coat on their surfaces. Surface polyFn is critical for pulmonary metastasis, presumably by facilitating lung vascular arrest via endothelial dipeptidylpeptidase IV (CD26). Here, we show that cell-surface polyFn assembly is initiated by the state of suspension, is dependent upon the synthesis and secretion of cellular Fn, and is augmented in a dose- and time-dependent manner by plasma Fn. PolyFn assembly is regulated by protein kinase Cepsilon (PKCepsilon), which translocates rapidly and in increasing amounts from the cytosol to the plasma membrane and is phosphorylated. PolyFn assembly is impeded by select inhibitors of this kinase, i.e. bisindolylmaleimide I, Ro-32-0432, Gö6983, and Rottlerin, by the phorbol 12-myristate 13-acetate-mediated and time-dependent loss of PKCepsilon protein and decreased plasma membrane translocation, and more specifically, by stable transfection of lung-metastatic MTF7L breast cancer cells with small interfering RNA-PKCepsilon and dominant-negative PKCepsilon constructs (e.g. RD-PKCepsilon). The inability to assemble a cell surface-associated polyFn coat by knockdown of endogenous Fn or PKCepsilon impedes cancer cells from metastasis to the lungs. The present studies identify a novel regulatory mechanism for polyFn assembly on blood-borne breast cancer cells and depict its effect on pulmonary metastasis.Journal of Biological Chemistry 04/2008; 283(12):7616-27. DOI:10.1074/jbc.M705839200 · 4.57 Impact Factor