Actions of calcium influx blockers in human neutrophils support a role for receptor-operated calcium entry

Calcium Pathways in Human Neutrophils-The Extended Effects of Thapsigargin and ML-9

In neutrophils, intracellular Ca²⁺ levels are regulated by several transporters and pathways, namely SERCA [sarco(endo)plasmic reticulum Ca²⁺-ATPase], SOCE (store-operated calcium entry), and ROCE (receptor-operated calcium entry). However, the exact mechanisms involved in the communication among these transporters are still unclear. In the present study, thapsigargin, an irreversible inhibitor of SERCA, and ML-9, a broadly used SOCE inhibitor, were applied in human neutrophils to better understand their effects on Ca²⁺ pathways in these important cells of the immune system. The thapsigargin and ML-9 effects in the intracellular free Ca²⁺ flux were evaluated in freshly isolated human neutrophils, using a microplate reader for monitoring fluorimetric kinetic readings. The obtained results corroborate the general thapsigargin-induced intracellular pattern of Ca²⁺ fluctuation, but it was also observed a much more extended effect in time and a clear sustained increase of Ca²⁺ levels due to its influx by SOCE. Moreover, it was obvious that ML-9 enhanced the thapsigargin-induced emptying of the internal stores. Indeed, ML-9 does not have this effect by itself, which indicates that, in neutrophils, thapsigargin does not act only on the influx by SOCE, but also by other Ca²⁺ pathways, that, in the future, should be further explored.

STIM1 and STIM2 differently regulate endogenous Ca2+ entry and promote TGF-β-induced EMT in breast cancer cells

The Ca²⁺ sensor proteins STIM1 and STIM2 are crucial elements of store-operated calcium entry (SOCE) in breast cancer cells. Increased SOCE activity may contribute to epithelial-mesenchymal transitions (EMT) and increase cell migration and invasion. However, the roles of STIM1 and STIM2 in TGF-β-induced EMT are still unclear. In this study, we demonstrate roles of STIMs in TGF-β-induced EMT in breast cancer cells. In particular, STIM1 and STIM2 expression affected TGF-β-induced EMT by mediating SOCE in MDA-MB-231 and MCF-7 breast cancer cells. The specific SOCE inhibitor YM58483 blocked TGF-β-induced EMT, and differing effects of STIM1 and STIM2 on TGF-β-induced EMT correlated with differing roles in SOCE. Finally, we showed that STIM2 is associated with non-store-operated calcium entry (non-SOCE) during TGF-β-induced EMT, whereas STIM1 is not. What's more, non-SOCE have a large possibility to be ROCE. In conclusion, STIM1 and STIM2 proteins play important roles in TGF-β-induced EMT and these effects are related to both SOCE and non-SOCE.

Inhibition of neutrophil superoxide generation by shikonin is associated with suppression of cellular Ca(2+) fluxes

Shikonin, an anti-inflammatory compound of “Shikon”, inhibits the neutrophil superoxide (O₂^•−) generation by NADPH oxidase 2 (Nox2); however, the mechanisms of how shikonin affects Nox2 activity remained unclear. We aimed to elucidate the relationship between the inhibition of Nox2 activity and influences on intracellular Ca²⁺ concentration ([Ca²⁺]_i) by shikonin. For this purpose, we used a simultaneous monitoring system for detecting changes in [Ca²⁺]_i (by fluorescence) and O₂^•− generation (by chemiluminescence) and evaluated the effects of shikonin on neutrophil-like HL-60 cells stimulated with N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP). Since fMLP activates Nox2 by elevation in [Ca²⁺]_i via fluxes such as inositol 1,4,5-trisphosphate-induced Ca²⁺ release (IICR) and store-operated Ca²⁺ entry (SOCE), we also evaluated the effects of shikonin on IICR and SOCE. Shikonin dose-dependently inhibited the fMLP-induced elevation in [Ca²⁺]_i and O₂^•− generation (IC₅₀ values of 1.45 and 1.12 µM, respectively) in a synchronized manner. Analyses of specific Ca²⁺ fluxes showed that shikonin inhibits IICR and IICR-linked O₂^•− generation (IC₅₀ values: 0.28 and 0.31 µM for [Ca²⁺]_i and O₂^•−, respectively), as well as SOCE and SOCE-linked O₂^•− generation (IC₅₀ values: 0.39 and 0.25 µM for [Ca²⁺]_i and O₂^•−, respectively). These results suggested that shikonin inhibits the O₂^•− generation by Nox2 in fMLP-stimulated neutrophils by targeting Ca²⁺ fluxes such as IICR and SOCE.

Effects of the semisynthetic bis-indole derivative KAR-2 on store-operated calcium entry in human neutrophils

We studied the effect of KAR-2 on cytosolic Ca(2+) level in human neutrophils by using a fluorescent dye (Fura-2) trapped in the cells. KAR-2 is a semisynthetic bis-indole derivative that shares vinblastine anti-microtubular properties, but does not share the vinblastine antagonistic effect on calmodulin. Therefore KAR-2 offers a convenient mean of studying the effect of microtubule destabilization, without concomitant calmodulin alterations. We found that KAR-2 induces Ca(2+) release from intracellular stores, whereby the stores are depleted. In addition KAR-2 reduces store-operated entry of extracellular Ca(2+) induced by agonists such as thapsigargin or ATP. On the other hand, in Ca(2+) refilled cells, KAR-2 promotes limited entry of extracellular Ca(2+) in the absence of agonist, but still interferes prominently with Ca(2+) entry triggered by ATP and with Ca(2+) uptake by intracellular stores. We suggest that Ca(2+) traffic through the plasma membrane is operated by two diverse pathways: the prominent pathway is interfered with by microtubule destabilization, while an alternate and minor pathway is actually favored (or uncovered) following microtubule destabilization.

Simultaneous monitoring of superoxides and intracellular calcium ions in neutrophils by chemiluminescence and fluorescence: evaluation of action mechanisms of bioactive compounds in foods

We have developed a measuring system for simultaneous monitoring of chemiluminescence and fluorescence, which indicate respectively, (i) generation of superoxide anion radicals (O2(-•)) and (ii) change in the intracellular calcium ion concentration ([Ca(2+)]i) of neutrophils triggered by the mechanism of innate immune response. We applied this measuring system for establishing a method to distinguish between anti-inflammatory actions and antioxidant actions caused by bioactive compounds. We evaluated anti-inflammatory agents (zinc ion [Zn(2+)] and ibuprofen) and antioxidants (superoxide dismutase [SOD] and ascorbic acid). It was shown that ibuprofen and Zn(2+) were anti-inflammatory while SOD and ascorbic acid were anti-oxidative. We conclude that it is possible to determine the mechanism of action of bioactive compounds using this method.

Requirement for both receptor-operated and store-operated calcium entry in N-formyl-methionine-leucine-phenylalanine-induced neutrophil polarization

Tissue penetration of neutrophils is a key process in many inflammatory diseases. In response to inflammatory stimuli such as N-formyl-methionine-leucine-phenylalanine (fMLP), neutrophils polarize and migrate towards the chemotactic gradient of the stimulus. Elevated intracellular Ca(2+) concentration is known to play a critical role in neutrophil polarization and migration; however, the exact mechanism remains elusive. Here, we demonstrated that fMLP stimulation caused not only store-operated calcium entry (SOCE), but also receptor-operated calcium entry (ROCE) in neutrophils by using both pharmacological and neutralizing monoclonal antibody approaches. We also investigated neither Rac2 nor Cdc42 activation could take place if either SOCE or ROCE was inhibited. This study thus provides the first evidence for coordination of Ca(2+) influx by SOCE and ROCE to regulate neutrophil polarization.

Pharmacology of receptor operated calcium entry in human neutrophils

In neutrophils, increases in intracellular calcium [Ca(2+)](i) provide a crucial link between inflammatory mediators and inflammatory responses. The modulation of [Ca(2+)](i) fluxes in non-excitable cells such as neutrophils has been studied for more than 25 years yet remains to be resolved. In these cells, the Ca(2+) influx can occur through at least two mechanisms, as follows: one dependent on the state of filling of the endoplasmic reticulum Ca(2+) stores, termed store operated calcium entry (SOCE), and the other less studied mechanism in neutrophils which is not dependent on the state of the Ca(2+) stores but is regulated by receptor occupation, termed receptor operated calcium entry (ROCE). Over the past ten years, the molecular components of SOCE have been extensively characterized, but in neutrophils, the molecular components of ROCE have only recently been explored. In this review, we discuss recent research findings that have demonstrated an important role for ROCE in human neutrophils. In addition, an overview of pharmacological approaches used to discriminate between ROCE and SOCE will be discussed. The elucidation of the molecular components of ROCE may well provide important pharmacological targets for the development of novel anti-inflammatory drugs.

Discrimination between receptor- and store-operated Ca(2+) influx in human neutrophils

Ca(2+) and Sr(2+) entry pathways activated by pro-inflammatory agonists FMLP, LTB(4) and PAF have been compared to thapsigargin in human neutrophils. 2-APB (10microM) increased Ca(2+) influx and to a greater extent in agonist than in thapsigargin stimulated neutrophils. This action of 2-APB was specific to Ca(2+) because 2-APB did not augment Sr(2+) entry in agonist and thapsigargin stimulated neutrophils. This suggests that Ca(2+) and Sr(2+) entry can be used to discriminate between receptor and non-receptor (store)-operated Ca(2+) influx. Our data show for the first time that Pyr3 whilst partially inhibiting agonist induced Ca(2+) influx almost completely abolished Ca(2+) influx after thapsigargin stimulation.