Lysophosphatidic acid-sensitive intracellular Ca2+ store does not regulate Ca2+ entry at plasma membrane in Jurkat human T-cells

Inhibition of calcium-independent phospholipase A impairs agonist-induced calcium entry in keratinocytes

Background

In many cells, depletion of intracellular calcium (Ca²⁺) reservoirs triggers Ca²⁺ entry through store-operated Ca²⁺ channels in the plasma membrane. However, the mechanisms of agonist-induced calcium entry (ACE) in keratinocytes are not fully understood.

Objectives

This study was designed to determine if pharmacological inhibition of calcium-independent phospholipase A (iPLA₂) impairs ACE in normal human epidermal keratinocytes.

Methods

Confocal laser scanning microscopy was used to monitor the dynamics of Ca²⁺ signalling in keratinocytes loaded with the calcium-sensitive dye Fluo-4. Cells were stimulated with extracellular nucleotides [adenosine triphosphate (ATP) or uridine triphosphate (UTP)] or with lysophosphatidic acid (LPA), a bioactive lipid that regulates keratinocyte proliferation and differentiation.

Results

Both ATP and UTP induced Ca²⁺ release in primary human keratinocytes. This was not followed by robust Ca²⁺ influx when the experiments were performed in low Ca²⁺ (70 μmol L⁻¹) medium. Upon elevation of extracellular Ca²⁺ to 1·2 mmol L⁻¹, however, a biphasic response consisting of an initial Ca²⁺ peak followed by an elevated plateau was observed. The plateau phase was inhibited when cells were treated with bromoenol lactone, a specific pharmacological inhibitor of iPLA₂. These findings indicate that iPLA₂ activity is required for ACE in keratinocytes. LPA also evoked Ca²⁺ release in keratinocytes but failed to induce sustained Ca²⁺ entry even when extracellular Ca²⁺ was elevated to 1·2 mmol L⁻¹.

Conclusion

Our results demonstrate for the first time an important role for iPLA₂ in regulating ACE in primary human keratinocytes.

Agonist-induced calcium entry correlates with STIM1 translocation

The mechanisms of agonist-induced calcium entry (ACE) following depletion of intracellular calcium stores have not been fully established. We report here that calcium-independent phospholipase A (iPLA₂) is required for robust Ca²⁺ entry in HaCaT keratinocytes following ATP or UTP stimulation. Lysophosphatidic acid (LPA), an unrelated agonist, evoked Ca²⁺ release without inducing robust Ca²⁺ entry. Both LPA and UTP induced the redistribution of STIM1 into puncta which localized to regions near or at the plasma membrane, as well as within the cytoplasm. Plasma membrane-associated STIM1 remained high for up to 10 min after UTP stimulation, whereas it had returned almost to baseline by that time point in LPA-stimulated cells. This correlated with faster reloading of the endoplasmic reticulum Ca²⁺ stores in LPA treated cells. Thus by differentially regulating store-refilling after agonist-mediated depletion, LPA and UTP may exert distinct effects on the duration of STIM1 localization at the plasma membrane, and thus, on the magnitude and duration of ACE.

Regulation of volume-sensitive osmolyte efflux from human SH-SY5Y neuroblastoma cells following activation of lysophospholipid receptors

The ability of the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) to promote the release of the organic osmolyte taurine in response to hypoosmotic stress has been examined. Incubation of SH-SY5Y neuroblastoma cells under hypoosmotic conditions (230 mOsM) resulted in a time-dependent release of taurine that was markedly enhanced (3-7-fold) by the addition of micromolar concentrations of either S1P or LPA. At optimal concentrations, the effects of S1P and LPA on taurine efflux were additive and mediated via distinct receptors. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino benzoic acid, or 4-[(2-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of both lysophospholipids to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. Both S1P and LPA elicited robust increases in intracellular calcium concentration that were attenuated by the removal of extracellular calcium, abolished by the depletion of intracellular calcium with thapsigargin, and were independent of phosphoinositide turnover. Taurine efflux mediated by S1P and LPA was unaffected by the removal of extracellular calcium but was attenuated by depletion of intracellular calcium (34-38%) and by inhibition of protein kinase C (PKC) with chelerythrine (38-72%). When intracellular calcium was depleted and PKC was inhibited, S1P- or LPA-stimulated taurine efflux was inhibited by 80%. Pretreatment of the cells with pertussis toxin, toxin B, or cytochalasin D had no effect on lysophospholipid-stimulated taurine efflux. The results indicate that both S1P and LPA receptors facilitate osmolyte release via a phospholipase C-independent mechanism that requires the availability of intracellular calcium and PKC activity.

Lysophosphatidic acid enhances interleukin-13 gene expression and promoter activity in T cells

Lysophosphatidic acid (LPA) is a membrane-derived lysophospholipid with wide-ranging effects on multiple lung cells including airway epithelial and smooth muscle cells. LPA can augment migration and cytokine synthesis in lymphocytes, but its potential effects on Th2 cytokines have not been well studied. We examined the effects of physiological concentrations of LPA on IL-13 gene expression in human T cells. The Jurkat T cell line and human peripheral blood CD4+ T cells were incubated with LPA alone or with 1) pharmacological agonists of different signaling pathways, or 2) antibodies directed against the T cell receptor complex and costimulatory molecules. Luciferase-based reporter constructs driven by different lengths of the human IL-13 promoter were transfected by electroporation in Jurkat cells treated with and without LPA. The effects of LPA on IL-13 mRNA stability were examined using actinomycin D to halt ongoing transcription. Expression of mRNA encoding LPA2 and LPP-1 increased with T cell activation. LPA augmented IL-13 secretion under conditions of submaximal T cell activation. This was observed using pharmacological agonists activating intracellular calcium-, PKC-, and cAMP-dependent signaling pathways, as well as antibodies directed against CD3 and CD28. LPA only slightly prolonged IL-13 mRNA half-life in submaximally stimulated Jurkat cells. In contrast, LPA significantly enhanced transcriptional activation of the IL-13 promoter via regulatory elements contained within proximal 312 bp. The effects of LPA on IL-13 promoter activation appeared to be distinct from those mediated by GATA-3. LPA can augment IL-13 gene expression in T cells, especially under conditions of submaximal activation.

Native and Recombinant Human Edg4 Receptor-Mediated Ca2+ Signalling

We have developed an assay system suitable for assessment of compound action on the Edg4 subtype of the widely expressed lysophosphatidic acid (LPA)-responsive Edg receptor family. Edg4 was stably overexpressed in the rat hepatoma cell line Rh 7777, and a Ca(2+)-based FLIPR assay developed for measurement of functional responses. In order to investigate the mechanisms linking Edg4 activation to cytosolic Ca(2+) elevation, we have also studied LPA signalling in a human neuroblastoma cell line that endogenously expresses Edg4. LPA responses displayed similar kinetics and potency in the two cell lines. The Ca(2+) signal generated by activation of LPA-sensitive receptors in these cells is mediated primarily by endoplasmic reticulum. However, there is a substantial inhibition of the LPA response by FCCP, indicating that mitochondria also play a key role in the LPA response. Partial inhibition of the response by cyclosporin A could indicate an active Ca(2+) release role for mitochondria in the LPA response. The inositol 1,4,5-triphosphate receptor antagonist 2-aminoethyl diphenyl borate markedly inhibits, but does not abolish, the Ca(2+) response to LPA, suggesting further complexity to the signalling pathways activated by Edg receptors. In comparing Edg signalling in recombinant and native cells, there is a striking overall similarity in receptor expression pattern, agonist potency, and the effect of modulators on the Ca(2+) response. This indicates that the Edg4-overexpressing Rh7777 cell line is a very useful model system for studying receptor pharmacology and signalling mechanisms, and for investigating the Edg4 receptor's downstream effects.

Neurobiology of the Edg2 lysophosphatidic acid receptor

Lysophosphatidic acid (LPA, 1-acyl-sn-glycerol-3-phosphate) is a well-known lipid growth factor that is found widely in various tissues including brain and is reported to drive different intracellular signaling pathways. In the nervous system, LPA studies have drawn many neuroscientists' attention because it has some actions related to neurogenesis such as cell rounding and proliferation. Remarkable advances in this field have been obtained along with the discovery of the cDNA clone for its receptor, vzg1/edg2, a member of the seven transmembrane-type edg family. Successive studies have revealed that edg2 activation by LPA mediates several neurobiological actions related to neurogenesis, neuronal excitability and survival activity on developing and postnatal neurons. Here we focused their molecular basis of signaling through G proteins and in vivo roles of edg2 in such neurobiological events.

Physiological functions of cyclic ADP-ribose and NAADP as calcium messengers

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two Ca(2+) messengers derived from NAD and NADP, respectively. Although NAADP is a linear molecule, structurally distinct from the cyclic cADPR, it is synthesized by similar enzymes, ADP-ribosyl cyclase and its homolog, CD38. The crystal structure of the cyclase has been solved and its active site identified. These two novel nucleotides have now been shown to be involved in a wide range of cellular functions including: cell cycle regulation in Euglena, a protist; gene expression in plants; and in animal systems, from fertilization to neurotransmitter release and long-term depression in brain. A battery of pharmacological reagents have been developed, providing valuable tools for elucidating the physiological functions of these two novel Ca(2+) messengers. This article reviews these recent results and explores the implications of the existence of multiple Ca(2+) messengers and Ca(2+) stores in cells.

Intracellular sphingosine 1-phosphate production: a novel pathway for Ca2+ release

Sphingolipids such as sphingosine 1-phosphate (SPP) and sphingosylphosphorylcholine have long been recognized to possess Ca2+ mobilizing activity, yet to date little is known about their mechanism of action, or indeed their significance as Ca2+ mobilizing intracellular messengers. The recent discovery of extracellular receptors for the sphingolipids has further complicated the interpretation of many experiments in this field. This paper reviews the current literature in which molecular and pharmacological approaches have begun to uncover the signalling components associated with intracellular SPP production and Ca2+ mobilization. The functional significance of this novel Ca2+ release pathway is also discussed.

Intracellular Ca(2+) release mechanisms: multiple pathways having multiple functions within the same cell type?

The elevation of the cytosolic and nuclear Ca(2+) concentration is a fundamental signal transduction mechanism in almost all eukaryotic cells. Interestingly, three Ca(2+)-mobilising second messengers, D-myo-inositol 1,4,5-trisphosphate (InsP(3)), cyclic adenosine diphosphoribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP(+)) were identified in a phylogenetically wide range of different organisms. Moreover, in an as yet very limited number of cell types, sea urchin eggs, mouse pancreatic acinar cells, and human Jurkat T-lymphocytes, all three Ca(2+)-mobilising ligands have been shown to be involved in the generation of Ca(2+) signals. This situation raises the question why during evolution all three messengers have been conserved in the same cell type. From a theoretical point of view the following points may be considered: (i) redundant mechanisms ensuring intact Ca(2+) signalling even if one system does not work, (ii) the need for subcellularly localised Ca(2+) elevations to obtain a certain physiological response of the cell, and (iii) tight control of a physiological response of the cell by a temporal sequence of Ca(2+) signalling events. These theoretical considerations are compared to the current knowledge regarding the three messengers in sea urchin eggs, mouse pancreatic acinar cells, and human Jurkat T lymphocytes.

Sphingosine-1-phosphate activates phospholipase D in human airway epithelial cells via a G protein-coupled receptor

Sphingosine-1-phosphate (SPP) acts as a first messenger in immortalized human airway epithelial cells (CFNPE9o(-)), possibly interacting with an Edg family receptor. Expression of the SPP receptors Edg-1 and Edg-3, as well as a low level of Edg-5/H218, was detected in these cells, in agreement with their ability to specifically bind SPP. The related lipids, lysophosphatidic acid and sphingosylphosphorylcholine, were unable to displace SPP from its high affinity binding sites, suggesting that the biological responses to these different lysolipids are mediated by distinct receptors. SPP markedly inhibited forskolin-stimulated cAMP accumulation in a dose-dependent manner and caused a remarkable elevation of intracellular calcium, both effects being sensitive to pertussis toxin treatment. Most importantly, SPP stimulated phosphatidic acid formation, which was maximal after 2 min and decreased within 8-10 min. In the presence of butan-1-ol, suppression of SPP-induced phosphatidic acid formation and production of phosphatidylbutanol were found, clearly indicating activation of phospholipase D (PLD). This finding was also confirmed by analysis of the fatty acid composition of phosphatidic acid, showing an increase in the monounsaturated oleic acid only. The decrease of phosphatidic acid level after 8-10 min incubation with SPP was accompanied by a parallel increase of diacylglycerol production, which was abolished in the presence of butan-1-ol. This result indicates that activation of phospholipase D is followed by stimulation of phosphatidate phosphohydrolase activity. Phosphatidic acid formation was insensitive to protein kinase C inhibitors and almost completely inhibited by pertussis toxin treatment, suggesting that SPP activates phospholipase D via a G(i/o) protein-coupled receptor.

Lysophosphatidic acid-mediated Ca2+ mobilization in human SH-SY5Y neuroblastoma cells is independent of phosphoinositide signalling, but dependent on sphingosine kinase activation

Extracellular application of lysophosphatidic acid (LPA) elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) in human SH-SY5Y neuroblastoma cells. The maximal response to LPA occurred between 0. 1 and 1 microM, at which point [Ca(2+)](i) was increased by approx. 500 nM. This increase was of similar magnitude to that caused by the muscarinic acetylcholine receptor agonist methacholine (MCh), although the initial rate of release by LPA was slower. Both LPA and MCh released Ca(2+) from intracellular stores, as assessed by inhibition of their effects by thapsigargin, a blocker of endoplasmic reticular Ca(2+) uptake, and by the persistence of their action in nominally Ca(2+)-free extracellular medium. Similarly, both agonists appeared to stimulate store-refilling Ca(2+) entry. MCh produced a marked elevation in cellular Ins(1,4,5)P(3) and stimulated [(3)H]InsP accumulation in the presence of Li(+). In contrast, LPA failed to stimulate detectable phosphoinositide turnover. Chronic down-regulation of Ins(1,4,5)P(3) receptor (InsP(3)R) proteins with MCh did not affect Ca(2+) responses to LPA. In addition, heparin, a competitive antagonist of InsP(3)Rs, blocked Ca(2+)-mobilization in permeabilized SH-SY5Y cells in response to MCh or exogenously added Ins(1,4,5)P(3), but failed to inhibit Ca(2+)-release induced by LPA. Elevation of [Ca(2+)](i) elicited by LPA was blocked by guanosine 5'-[beta-thio]-diphosphate, indicating that this agonist acts via a G-protein-coupled receptor. However, pertussis toxin was without effect on LPA-evoked [Ca(2+)](i) responses, suggesting that G(i/o)-proteins were not involved. In the absence of extracellular Ca(2+), N,N-dimethylsphingosine (DMS, 30 microM), a competitive inhibitor of sphingosine kinase, blocked LPA-induced Ca(2+) responses by almost 90%. In addition, MCh-induced Ca(2+) responses were also diminished by the addition of DMS, although to a lesser extent than with LPA. We conclude that LPA mobilizes intracellular Ca(2+)-stores in SH-SY5Y cells independently of the generation and action of Ins(1,4,5)P(3). Furthermore, the Ca(2+)-response to LPA appears to be dependent on sphingosine kinase activation and the potential generation of the putative second messenger sphingosine 1-phosphate.

Regulation of calcium signalling in T lymphocytes by the second messenger cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a natural compound that mobilizes calcium ions in several eukaryotic cells. Although it can lead to the release of calcium ions in T lymphocytes, it has not been firmly established as a second messenger in these cells. Here, using high-performance liquid chromatography analysis, we show that stimulation of the T-cell receptor/CD3 (TCR/CD3) complex results in activation of a soluble ADP-ribosyl cyclase and a sustained increase in intracellular levels of cADPR. There is a causal relation between increased cADPR concentrations, sustained calcium signalling and activation of T cells, as shown by inhibition of TCR/CD3-stimulated calcium signalling, cell proliferation and expression of the early- and late-activation markers CD25 and HLA-DR by using cADPR antagonists. The molecular target for cADPR, the type-3 ryanodine receptor/calcium channel, is expressed in T cells. Increased cADPR significantly and specifically stimulates the apparent association of [3H]ryanodine with the type-3 ryanodine receptor, indicating a direct modulatory effect of cADPR on channel opening. Thus we show the presence, causal relation and biological significance of the major constituents of the cADPR/calcium-signalling pathway in human T cells.

Intracellular calcium mobilization and phospholipid degradation in sphingosylphosphorylcholine-stimulated human airway epithelial cells

Extracellular sphingosylphosphorylcholine (SPC) caused a remarkable elevation in the intracellular Ca2+ concentration ([Ca2+]i) in immortalized human airway epithelial cells (CFNP9o-). An increase in total inositol phosphates formation was determined; however, the dose responses for [Ca2+]i elevation and inositol phosphates production were slightly different and, furthermore, PMA and pertussis toxin almost completely inhibited [Ca2+]i mobilization by SPC, whereas inositol phosphates production was only partially reduced. The possible direct interaction of SPC with Ca2+ channels of intracellular stores was determined by experiments with permeabilized cells, where SPC failed to evoke Ca2+ release, whereas lysophosphatidic acid was shown to be effective. The level of phosphatidic acid was increased by SPC only in the presence of AACOCF3, a specific inhibitor of phospholipase A2 (PLA2) and blocked by both pertussis toxin and R59022, an inhibitor of diacylglycerol kinase. R59022 enhanced diacylglycerol production by SPC and also significantly reduced [Ca2+]i mobilization. Only polyunsaturated diacylglycerol and phosphatidic acid were generated by SPC. Lastly, SPC caused stimulation of arachidonic acid release, indicating the involvement of PLA2. Taken together, these data suggest that, after SPC stimulation, phospholipase C-derived diacylglycerol is phosphorylated by a diacylglycerol kinase to phosphatidic acid, which is further hydrolysed by PLA2 activity to arachidonic and lysophosphatidic acids. We propose that lysophosphatidic acid might be the intracellular messenger able to release Ca2+ from internal stores.

Sphingosylphosphorylcholine and sphingosine-1-phosphate mobilize cytosolic calcium through different mechanisms in human airway epithelial cells

The sphingosine derivatives sphingosylphosphorylcholine (SPC) and sphingosine-1-phosphate (S1P) caused a similar elevation of the intracellular Ca2+ concentration ([Ca2+]i) in an immortalized airway epithelial cell line (CFNP9o-) incubated in Ca(2+)-free medium. The maximal effect was obtained with 2 microM SPC and 0.1 microM S1P and was sensitive to pre-incubation with pertussis toxin, indicating the involvement of a Gi/G(o) type of G protein. In Ca2+ containing medium, [Ca2+]i elevation by SPC was significantly higher than that by S1P, due to the fact that SPC was able to stimulate Mn2+ entry, whereas S1P was ineffective. SPC, but not S1P, caused a dose-dependent production of total inositol phosphates. Conversely, S1P, but not SPC, increased the level of phosphatidic acid. These findings suggest the presence of two distinct receptors, specific for SPC and S1P, respectively. Depletion of intracellular Ca2+ stores by SPC makes cells unable to respond to a subsequent addition of S1P. Conversely, cells do respond to SPC after a challenge with S1P, suggesting that the two receptors likely share one or more intracellular signalling component(s).