E-selectin permits communication between PAF receptors and TRPC channels in human neutrophils

Dysregulation of neutrophil granulocyte function has been implicated as having a key role in the initiation and progression of a number of inflammatory diseases. The control of events involved in leukocyte recruitment is critical for development of effective antimicrobial defenses and also for efficient wound healing. However, excessive inflammatory-cell recruitment or inappropriate cell activation leads to the development of chronic inflammation that favors fibrotic repair and, ultimately, loss of organ function. The selectin family of molecules (L-, P-, and E-selectin) mediate adhesive interactions between leukocytes and endothelial cells, representing one of the earliest events in the recruitment of inflammatory cells. Studies in vitro and in vivo have revealed the critical role of the selectin family of molecules in the initial capture and subsequent rolling adhesion on vascular endothelial ligands¹  before neutrophils firmly adhere and undergo diapedesis at sites of tissue injury and inflammation. In terms of structure, the selectins are type I transmembrane receptors that contain an amino terminal Ca²⁺-dependent (C-type) lectin domain that has been shown to be important in ligand recognition and is directly involved in mediating cell-to-cell contact through Ca²⁺-dependent interactions with cell-surface carbohydrates.² 

Several ligands for E-selectin, which all contain sialyl Lewis^x-type glycans, have been identified including P-selectin glycoprotein ligand 1 (PSGL-1),³  L-selectin,⁴  CD66,⁵  CD44,^6,7  and E-selectin ligand 1 (ESL-1).⁸  However, E-selectin ligands on neutrophils have not been fully characterized to date. The best characterized selectin ligand is PSGL-1, which is found on leukocytes and platelets. PSGL-1 binds to P-, E-, and L-selectins in vitro and represents an important functional ligand for all of these molecules.⁹ 

Recent studies using double or triple selectin knockout mice revealed that selectins have both overlapping and distinct functions.¹⁰  Single-selectin knockout mice showed only minor deficiencies in leukocyte recruitment in response to tumor necrosis factor α (TNF-α) or thioglycollate, suggesting distinct roles for the selectins in the inflammatory process. In contrast, double-mutant mice displayed more profound defects in neutrophil recruitment. For example, E- and P-selectin double-mutant mice showed an increased susceptibility to bacterial infection, with the majority of animals developing chronic inflammatory lesions of the oral mucosa and skin, suggesting that E- and P-selectin may function cooperatively.¹¹  The most severe deficiencies in neutrophil recruitment were found in E-, L-, and P-selectin triple-knockout mice, which had impaired neutrophil emigration, neutrophil rolling, and significant leukocytosis.¹² 

Although the role of selectins in leukocyte recruitment is well established, it is now becoming clear that pathways engaged in response to E-selectin receptor engagement may trigger cell activation even though the molecular mechanisms remain to be defined. Engagement of selectin receptors has been reported to activate the mitogen-activated protein kinase (MAPK) pathway or activate cell-surface receptor-associated protein tyrosine kinases.¹³  Recent findings from a number of studies suggest that soluble E-selectin may also engage selectin receptors. Soluble E-selectin levels are elevated in many chronic inflammatory conditions, including rheumatoid arthritis and asthma.^2,14  In addition, soluble forms of selectins are rapidly released from activated endothelial cells.¹⁴  One possibility is that receptor shedding may represent a mechanism for limiting further inflammatory-cell recruitment by decreasing the availability of endothelial ligands for inflammatory cells. However, it is becoming clear that soluble E-selectin may activate inflammatory cells¹⁵  and exert potentially proinflammatory effects.¹⁶ 

Early studies have suggested that selectin and platelet-activating factor (PAF) signaling act cooperatively to induce neutrophil adhesion to the endothelium.¹⁷  E-selectin is known to reduce the rolling velocity of neutrophils in vitro.¹⁸  In vivo work by Kanwar et al¹⁹  found that low concentrations of exogenous PAF induced an increase in neutrophil adhesion in slow-rolling cells, whereas fast-rolling cells were unresponsive to the same concentration of PAF. Similarly, P-selectin has been shown to slow the rolling cells so that they are able to firmly adhere in the presence of lower concentrations of PAF.²⁰  These findings would suggest that reduced neutrophil rolling velocity following adhesion to selectins confers a higher ability to adhere in the presence of an appropriate stimulus. However, one possibility is that selectin receptor engagement may facilitate integrin-mediated “firm” adhesion following exposure to a second stimulus and that there could be communication between receptors for inflammatory mediators and those involved in adhesion at a molecular level.

We have previously demonstrated that Ca²⁺ mobilization induced by PAF in neutrophils, an early key event in the control of motility, respiratory burst, and degranulation, is prolonged in the presence of soluble E-selectin.²¹  Neutrophil adhesion to β₂-integrin ligands (albumin-coated latex beads) induced by PAF but not by leukotriene B₄ (LTB₄) or formyl-Met-Leu-Phe (fMLP) was promoted by soluble E-selectin and this adhesion required PAF-induced Ca²⁺ mobilization from inositol 1,4,5-trisphosphate (IP₃)–sensitive intracellular stores. In this paper, we provide biochemical evidence for molecular cross-talk between these structurally distinct receptor pathways.

Hanks balanced salt solution (HBSS) was obtained from Life Technologies (Paisley, United Kingdom). Dextran T500 was obtained from Amersham Pharmacia Biotech (Buckinghamshire, United Kingdom). PAF, pertussis toxin, gadolinium(III) chloride, and wortmannin were obtained from Sigma-Aldrich (Poole, United Kingdom). ENA2 was purchased from Abcam (Cambridge, United Kingdom) and MRS1845 and ruthenium red from Tocris (Bristol, United Kingdom). U73122, U73343, LY294002 hydrochloride, LY303511, Fura2-am, PP2, PP3, SB203580, SB202474, and PD98059 were purchased from Calbiochem (Nottingham, United Kingdom). Enhanced chemiluminescence (ECL) Western blotting detection reagents were obtained from Amersham Pharmacia Biotech.

Anti-TRPC6 was purchased from Alamone Labs (Jerusalem, Israel). Phospho-Src family (Tyr416) was purchased from Cell Signaling (Hertfordshire, United Kingdom) and anti–phopho-Akt1/PKBα (Ser473), clone 11E6, was obtained from Upstate Biotechnology (Milton Keynes, United Kingdom). Monoclonal anti–β-actin antibody was purchased from Sigma-Aldrich. Goat anti–mouse and –rabbit horseradish peroxidase (HRP)–conjugated antibodies were obtained from Dako (Ely, United Kingdom).

Recombinant proteins of E-selectin were obtained using a baculovirus expression construct kindly provided by Dr Mike Bird (GlaxoSmithKline, Stevenage, United Kingdom). Recombinant human E-selectin, lacking the last 2 consensus repeats, was produced in a baculovirus insect-cell expression system as a C-terminal chimera with 2 protein A domains in tandem. High Five cells (BT1-TN-5B1-4 cell line, Invitrogen, Paisley, United Kingdom) were used to express recombinant E-selectin. High Five cells were cultured in Express Five serum-free media supplemented with l-glutamine and penicillin/streptomycin. High Five cells (9 × 10⁶/75-cm² flask) were seeded into cell-culture flasks and left to adhere for 20 minutes. After attachment of the cells, the medium was removed and the cells were infected with recombinant virus at 2 PFU/cell. Three hours later, the medium was replaced with fresh medium. After 72 hours of incubation at 27°C, the culture supernatant was collected and stored at 4°C for further purification.

Recombinant proteins were then purified from High 5 insect-cell culture supernatants using IgG affinity column chromatography using the protein A domain in the recombinant protein. A column containing IgG-agarose was equilibrated with 5 column volumes of 0.1 M phosphate buffer, pH 8.0. The supernatant was applied to the affinity column, the column was washed with 2 column volumes of 0.1 M phosphate buffer, and eluted with 100 mM glycine in 500-μL fractions. Fractions were analyzed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) for protein content. The eluted proteins were then dialyzed against PBS (Ca²⁺ and Mg²⁺ free) overnight. A typical yield was 1.5 to 2.0 mg protein/250 mL supernatant.

Polymorphonuclear leukocytes were isolated from peripheral blood of healthy donors as described previously.²²  After centrifugation of citrated whole blood at 300g for 20 minutes and removal of platelet-rich plasma, leukocytes were separated from erythrocytes by dextran sedimentation using 0.6% (wt/vol) dextran T500. Polymorphonuclear leukocytes were then separated from mononuclear leukocytes using discontinuous isotonic Percoll gradients. Polymorphonuclear leukocytes were 95% to 98% neutrophils using morphologic criteria and cell viability was assessed by trypan blue exclusion.

Freshly isolated leukocytes were resuspended at 10⁷/mL in HBSS (Ca²⁺ and Mg²⁺ free) and were incubated with 2 μM Fura2-am at 37°C for 30 minutes in the dark. The cells were then washed twice to remove Fura2-am and resuspended at 4 × 10⁷/mL in HBSS (containing Ca²⁺ and Mg²⁺). Intracellular calcium was monitored by Fura-2 emission fluorescence at 510 nm using 340/380-nm dual wavelength excitation in a Perkin Elmer (Beaconsfield, CA) luminescence spectrometer at 37°C with constant stirring. Calibration was performed after each experiment using 100 μL 0.1% (vol/vol) Triton X (R_max) and 10 mM EGTA (R_min). [Ca²⁺]_i was calculated from the 340:380-nm fluorescence ratio.²³ 

Total RNA was extracted from freshly isolated neutrophils using the TRIzol reagent method per the manufacturer's instructions (Invitrogen, Life Technologies). Oligo(dT)–primed first-strand cDNA synthase was performed with Moloney murine leukemia virus reverse transcriptase (RT; SuperScript II, Promega, Madison, WI) using 500 ng mRNA as template in a total volume of 20 μL. The cDNA was then used for polymerase chain reaction (PCR) with Taq polymerase (Promega). Two pairs of specific primers each were used to detect the canonical transient receptor potential channel (TRPC) 1, 3, 4, and 6 and one primer pair was used for TRPC2 and 5; primers were synthesized by MWG Biotech (Ebersberg, Germany). The sequence of the primer pairs used along with the predicted size of their expected amplicons are as follows: TRPC1: forward 5′-ATGTATACAACCAGCTCTATCTTG-3′ and reverse 5′-AGTCTTTGGTGAGGGAATGATG-3′ (525 bp); TRPC1: forward 5′-TTCTGTGGATTATTATTGGGATGA-3′ and reverse 5′-CAGAACAAAGCAAAGCAGGTG-3′ (505 bp); TRPC2: forward 5′-TCTGGACCATGTTCGGTATG-3′ and reverse 5′-GCTACCTCGCTTTGCAGTC-3′ (565bp); TRPC3: forward 5′-CTGCAAATGAGAGCTTTGGC-3′ and reverse 5′-AACTTCCATTCTACATCACTGTC-3′ (388 bp); TRPC3: forward 5′-GCGAATTGTTAACTTTCCCAAATGC-3′ and reverse 5′-TCTTCCAAAAGTTCATAACGAAGGC-3′ (300 bp); TRPC4: forward 5′-ATTCATATACTGCCTTGTGTTG-3′ and reverse 5′-GGTCAGCAATCAGTTGGTAAG-3′ (329 bp); TRPC4: forward 5′-TTGCCTCTGAAAGACATAACATAAG-3′ and reverse 5′-CTACTAACACACATTGTTCACTGAG-3′ (300 bp); TRPC5: forward 5′-ACTTCTATTATGAAACCAGAGC-3′ and reverse 5′-GCATGATCGGCAATAAGCTG-3′ (289 bp); TRPC6: forward 5′-AAGACATCTTCAAGTTCATGGTC-3′ and reverse 5′-TCAGCGTCATCCTAATTTCCC-3′ (322 bp); TRPC6: forward 5′-ACAGATAATGCAAAACAGCTG-3′ and reverse 5′-ATGATGCTCTGGGCTTTG -3′ (244 bp); and GAPDH: forward 5′-TGCCTCCTGCACCACCAAGTG-3′ and reverse 5′-AATGCCAGCCCCAGCGTCAAAG-3′ (450 bp).

The reaction solution (50 μL) contained 0.4 μM of each primer (forward and reverse), 2 mM MgCl₂, 0.2 mM dNTPs, 1.25 U Taq polymerase, and 1 μL cDNA. The PCR conditions were: 95°C for 10 minutes, followed by 35 cycles, each consisting of denaturation at 95°C for 1.5 minutes, annealing at 63°C for 2 minutes and extension at 72°C for 2 minutes, and a final extension at 72°C for 10 minutes. After PCR amplification, the reaction mixtures were applied to 1% (wt/vol) agarose gel for electrophoresis and DNA fragments were detected by ethidium bromide staining.

Neutrophils (5 × 10¹⁰ cells/L/sample) were lysed, following stimulation as detailed in the figure legends, in lysis buffer containing Tris HCl (100 mM, pH 8.0), NaCl (100 mM), EDTA (2 mM), Nonidet NP-40 (1% vol/vol), Na₃VO₄ (5 mM), NaF (50 mM), and protease inhibitor cocktail (Sigma-Aldrich) for 30 minutes at 4°C. Samples were centrifuged for 20 minutes at 15 000g at 4°C and supernatants were reduced with electrophoresis sample buffer containing Tris HCl (0.25 M, pH 6.8), SDS (8% wt/vol), β-mercaptoethanol (10% wt/vol), glycerol (30% vol/vol), and bromophenol blue (0.02% wt/vol). Each sample was loaded onto a 10% SDS-polyacrylamide gel and proteins were separated and electrophoretically transferred to nitrocellulose. The membranes were then blocked for 1 hour in 5% (wt/vol) dried milk and probed with primary antibody overnight. After washing with Tris-buffered saline containing 0.1% (vol/vol) Tween 20 (TBST), blots were incubated with goat anti–mouse HRP (1:2500) or with goat anti–rabbit HRP (1:2500) antibodies for 1 hour at room temperature. The membranes were incubated with ECL reagent (Amersham Biosciences, Buckinghamshire, United Kingdom), placed under BioMax MS-1 x-ray–sensitive film, and processed through an x-ray developer (X-Ograph Imaging Systems, Wilts, United Kingdom).

Statistical analysis was carried out using the one-way ANOVA test with a Newman-Keuls multiple comparison posttest analysis, with statistical significance being achieved when P was below .05.

Incubation of human neutrophils in the presence of soluble E-selectin did not induce Ca²⁺ mobilization (Figure 1A). However, in agreement with our previous studies,²¹  preincubation of neutrophils with soluble E-selectin caused a subsequent sustained increase in [Ca²⁺]_i in response to PAF without affecting the initial increase of [Ca²⁺]_i (Figure 1A). To facilitate comparison of experimental data, we have calculated the area under the Ca²⁺ curve to provide a measure of the Ca²⁺ mobilization observed. Inhibition of soluble E-selectin interaction with neutrophils by ENA2, an anti–E-selectin monoclonal antibody that binds to the lectin domain of E-selectin, inhibited the sustained Ca²⁺ levels without affecting the initial rapid rise in [Ca²⁺]_i observed in response to PAF treatment (Figure 1B). These data demonstrate that binding of soluble E-selectin via the lectin domain to a counterreceptor present on neutrophils is required for the prolongation of Ca²⁺ mobilization in neutrophils in response to PAF.

Using a range of preincubation times with soluble E-selectin, it was demonstrated that the effects of soluble E-selectin on prolongation of [Ca²⁺]_i in neutrophils were maximal by 15 minutes (Figure 2A), suggesting that downstream signaling pathways may be involved to induce prolonged Ca²⁺ mobilization in response to PAF rather than a rapid and direct physical interaction causing conformational changes. Furthermore, the effects of soluble E-selectin on prolongation of [Ca²⁺]_i levels were maintained whether soluble E-selectin was present or removed by washing; no significant difference was evident between samples (Figure 2B). Thus, the effects of soluble E-selectin are unlikely to be attributed to nonspecific effects such as sequestration/buffering of extracellular Ca²⁺ or changes in associated molecules that affect Ca²⁺ ion movements. These data suggest that soluble E-selectin triggers intracellular signaling pathways to modulate Ca²⁺ entry. We have previously shown that soluble E-selectin promotes Ca²⁺ mobilization and adhesion selectively to PAF but not to stimulation by fMLP or LTB₄.²¹  Pretreatment of neutrophils with pertussis toxin, which ADP ribosylates G_i and G₀, was found to inhibit fMLP- and LTB₄-induced Ca²⁺ mobilization but had no effect on Ca²⁺ responses (Figure 3A), thereby suggesting a role for a pertussis toxin-insensitive G protein in mediating the effects of PAF.

PAF-induced Ca²⁺ mobilization is dependent on activation of phospholipase C and release of IP₃ because this response is sensitive to complete inhibition by U73122, a specific phospholipase C inhibitor (Figure 3B), whereas the inactive analog U73343 had no effect. Our previous studies also demonstrated that the initial PAF-induced [Ca²⁺]_i spike was abolished by TMB-8, which blocks Ca²⁺ release from intracellular stores, and that a relatively small second phase of Ca²⁺ mobilization was sensitive to inhibition by the receptor-operated channel inhibitor SKF96365.²¹  Importantly, soluble E-selectin–induced promotion of Ca²⁺ mobilization in this latter phase was insensitive to SKF96365,²¹  suggesting it occurs through distinct ion channels. We therefore sought to further define the molecular mechanism by which soluble E-selectin induced prolonged elevation of [Ca²⁺]_i. Neutrophils treated with ruthenium red, an inhibitor of Ca²⁺-induced Ca²⁺ release from ryanodine-sensitive stores, had no effect on mobilization of Ca²⁺ in response to PAF in the presence of E-selectin (Figure 3C). Chelation of extracellular Ca²⁺ by EGTA showed that only the E-selectin–mediated prolongation of [Ca²⁺]_i was sensitive to blockade and that PAF-induced Ca²⁺ release from stores was unaffected, indicating that soluble E-selectin affected Ca²⁺ influx rather than release from an intracellular store. The initial elevation of [Ca²⁺]_i in response to PAF, known to be due to release of Ca²⁺ from IP₃-sensitive stores, was unaffected by MRS1845, a store-operated channel inhibitor; however, soluble E-selectin–induced sustained [Ca²⁺]_i following stimulation with PAF was sensitive to inhibition by MRS1845 (Figure 3D). These results indicate that soluble E-selectin was allowing activation of a store-operated channel by PAF-induced Ca²⁺ store emptying, an effect we have termed “permissive” store-operated Ca²⁺ entry (SOCE). We next tested whether Gd³⁺, a specific transient receptor potential family of cation channels (TRPC) inhibitor, would affect the E-selectin–induced [Ca²⁺]_i response to PAF. As shown in Figure 3C, the prolonged rise in [Ca²⁺]_i observed in the presence of soluble E-selectin was sensitive to Gd³⁺, which is consistent with a role for TRPC in this response.

To determine the profile of TRPC expression in human neutrophils, multiple specific primer pairs were used to screen for the presence of TRPC1-TRPC6 mRNA species in highly purified human neutrophils. The expression profile for members of the TRPC family is illustrated in a representative gel of RT-PCR products shown in Figure 4A. PCR products for TRPC6 were found in all neutrophil samples (n = 10), whereas TRPC3 was only found in 20% of samples. We did not observe signals for TRPC1, 2, 4, and 5 in any of the preparations, despite positive RT-PCR controls demonstrating that these PCR conditions were optimal. To confirm TRPC6 protein expression, we assayed crude membrane preparations from freshly isolated human neutrophils using Western blotting techniques. A specific antibody for TRPC6 revealed a strong band in the appropriate 90- to 100-kDa range, which could be blocked by a TRPC6-blocking peptide (Figure 4B), confirming the presence of protein and the RT-PCR data.

It is now clear that critical regulatory elements that control TRPC activity include phosphorylation and Src family tyrosine kinases in particular.²⁴  We therefore used specific protein kinase inhibitors to test their involvement in soluble E-selectin–mediated SOCE. Inhibition of p38 MAPK by SB203580 (10 μM) or the negative control SB202474 (10 μM) had no effect on soluble E-selectin–induced Ca²⁺ influx following stimulation of neutrophils with PAF (Figure 5B). Similarly, a lack of effect by PD98059 (10 μM) on soluble E-selectin–induced modulation of Ca²⁺ influx identified that MEK1 was not involved in mediating these responses (Figure 5B). However, the specific Src family tyrosine kinase inhibitor PP2 (5 μM) selectively inhibited the soluble E-selectin–induced SOCE to levels observed in PAF-only stimulated neutrophils (Figure 5A-B), whereas PP3, the inactive analog, had no effect on soluble E-selectin–induced Ca²⁺ influx. These data suggest either a direct role for Src in modulating TRPC6 channel activity or potentially a role for Src in the downstream signaling events following soluble E-selectin binding to its putative receptor on neutrophils. Because PI 3-kinase is known to be a key regulator of ion channels in a variety of other cell types, we pretreated neutrophils with the specific PI 3-kinase inhibitors wortmannin (100 nM) or LY294002 (10 μM) and LY303511, an inactive structural analog. PI 3-kinase inhibition also inhibited the soluble E-selectin–induced SOCE to controls levels (Figure 5C-D), the inactive analog having no effect, thus identifying PI 3-kinase as a key regulator in the signaling pathway, which mediates the effects of soluble E-selectin on Ca²⁺ influx.

Western blot analysis of neutrophil protein lysates using a phosphorylation state-specific antibody (Tyr(P)⁴¹⁶), which correlates with Src activation, showed significant phosphorylation above control levels with soluble E-selectin treatment (Figure 6A). Interestingly, stimulation of neutrophils with PAF alone had no effect on the levels of phospho-Src. Pretreatment of neutrophils with PP2, prior to stimulation with soluble E-selectin, inhibited active phospho-Src to below control levels. In addition, LY294002 (10 μM) had no significant effect on soluble E-selectin–induced phosphorylation and activation of Src (Figure 6A), indicating that PI 3-kinase may be involved in a parallel pathway or acts downstream of Src in regulating Ca²⁺ influx. Soluble E-selectin caused activation of PI 3-kinase as assessed by a significant increase in phosphorylated Akt, a downstream target of PI 3-kinase, compared with control untreated cells, whereas PAF did not induce any Akt phosphorylation or activation (Figure 6B). Soluble E-selectin–induced increases in phospho-Akt levels could be inhibited completely by LY294002, confirming that it is a target of PI 3-kinase, and interestingly the Src tyrosine kinase inhibitor PP2 also showed complete inhibition of phospho-Akt accumulation following treatment with soluble E-selectin (Figure 6B). These data would support the hypothesis that the pathway that regulates permissive SOCE induced by soluble E-selectin is mediated primarily by Src with PI 3-kinase acting downstream.

The selectin family of receptors is critical for the appropriate recruitment of neutrophils to sites of infection or tissue injury and the initiation and progression of the inflammatory response. We have previously shown that soluble E-selectin acts to promote neutrophil adhesion, inhibit migration, and amplify destructive responses,²¹  raising the possibility that elevated levels of soluble E-selectin in patients with inflammatory diseases, such as rheumatoid arthritis, and associated with tumor growth, have a proinflammatory effect.²⁵ 

In leukocytes, PAF acts through a specific G protein-coupled receptor to induce chemotaxis. Neutrophil activation by PAF has been shown to be insensitive to pertussis toxin, implicating a G_q/11 family member. Neutrophil responses to stimulation by LTB₄ and fMLP are sensitive to pertussis toxin, suggesting G_o or G_i involvement.^26-28  β₂-Integrin-mediated neutrophil adhesion to albumin-coated latex beads induced by PAF but not fMLP and LTB₄ was promoted by soluble E-selectin.²¹  Furthermore, we have also shown that soluble E-selectin specifically prolongs elevation of [Ca²⁺]_i in response to PAF but not fMLP or LTB₄.²¹  We have demonstrated that pertussis toxin does not affect PAF-induced Ca²⁺ mobilization but abolishes fMLP- and LTB₄-induced Ca²⁺ mobilization. Taken together, this would indicate that only signals from a pertussis toxin-insensitive G_q/11-coupled receptor such as the PAF receptor are able to communicate through a G protein-derived signal to allow prolonged Ca²⁺ signaling to occur in the presence of soluble E-selectin.

We provide important new information relating to the mechanism by which soluble E-selectin prolongs PAF-induced Ca²⁺ signaling in neutrophils. Stimulation of neutrophils with PAF causes primarily a rapid release of Ca²⁺ from IP₃-sensitive stores and a relatively minor influx of Ca²⁺ through SKF96365-sensitive channels, most likely receptor-operated channels.²¹  Soluble E-selectin alone does not cause release of Ca²⁺ from intracellular stores or via Ca²⁺ influx but acts to prolong Ca²⁺ signals induced by PAF receptor ligation in a novel manner by allowing “permissive” SOCE. We have demonstrated that prolongation of PAF-induced Ca²⁺ signaling by E-selectin is due to Ca²⁺ influx due to its sensitivity to blockade by EGTA rather than further release from ryanodine-sensitive intracellular stores. Furthermore, an obligate requirement for IP₃-mediated release of Ca²⁺ from intracellular stores to act as a trigger for the Ca²⁺ influx permitted by soluble E-selectin was demonstrated by inhibition of phospholipase C causing a complete loss of any Ca²⁺ signaling. In addition, the susceptibility of E-selectin–permitted Ca²⁺ influx to blockade by MRS1845, a store-operated channel (SOC) inhibitor, identified this as SOCE. A role for TRPCs as candidates for mediating this novel SOCE was proposed based on the ability of Gd³⁺ to cause selective inhibition of prolonged Ca²⁺ entry following PAF stimulation in the presence of soluble E-selectin. Proteins homologous to the Drosophila transient receptor potential gene (trp) Ca²⁺ channels that assemble into tetrameric ion channels are known to be involved in the generation of store-operated Ca²⁺ entry (SOCE). Our RT-PCR studies found that only TRPC3 and TRPC6 mRNA were expressed in polymorphonuclear leukocytes (Figure 4A), in agreement with Heiner et al.²⁹  TRPC6 appears to represent the principal TRPC family member present, being detected at both the level of mRNA and protein.

Thus, soluble E-selectin acts to promote a novel form of molecular cross-talk involving TRPCs that allow a putative E-selectin receptor to influence PAF-induced signaling pathways. Our data also suggest that both G_q/11- and soluble E-selectin–mediated signals are required to communicate with TRPC6 before release of Ca²⁺ from intracellular stores can trigger SOCE. We are currently investigating potential mechanisms for this effect, for example, whether a regulatory protein becomes recruited to TRPCs to permit SOCE, or promotion of a physical interaction between IP₃ channels in the endoplasmic reticulum (ER) with TRPC in the plasma membrane, or alternatively, TRPCs may become sensitized to the signals that mediate SOCE, such as calcium influx factor (CIF).³⁰  It has recently been shown that TRPC6 is externalized to the plasma membrane by the stimulation of a G_q protein-coupled receptor,³¹  and it has also been shown that expression of TRPC6 in COS cells increases Ca²⁺ entry in response to stimulation of a G_q protein-coupled receptor.³²  We are therefore currently investigating whether soluble E-selectin preincubation leads to the up-regulation of TRPC6 on the cell surface and if this is sensitive to activation via G_q-coupled receptor-induced signals specifically.

The recent finding that diacylglycerol directly activates TRPC3 and TRPC6 may represent an alternative mechanism for activation of these channels via phospholipase C-linked receptors,³³  allowing regulation to occur through a lipid mediator. Recent studies have shown that tyrosine phosphorylation by Src family protein tyrosine kinases represents a potential regulatory mechanism of TRPC6 activity.²⁴  It has been suggested that 2 simultaneous events, opening of the channel by DAG and modulation by Src-induced tyrosine phosphorylation, contribute to the efficient influx of calcium through TRPC6. We found that PP2 specifically inhibited soluble E-selectin–mediated SOCE without affecting PAF-induced responses. In parallel, soluble E-selectin caused phosphorylation and activation of Src, which was sensitive to inhibition by PP2 but was unaffected by PI 3-kinase inhibition. These findings strongly suggest that Src activity is involved in modulating TRPC6 activity to regulate Ca²⁺ influx in human neutrophils.

Inhibition of PI 3-kinase selectively blocked the soluble E-selectin–induced SOCE in neutrophils. Several potential intracellular regulatory motifs have been identified on TRPC6 including PI3K-SH2 recognition domains, suggesting a mechanism by which these channels might interact with the PI 3-kinase signaling pathway.³⁴  Several groups^35,36  have discovered that the PI 3-kinase lipid product phosphatidylinositol 3,4,5-trisphosphate (PIP₃) mediates calcium influx through a mechanism independent of phospholipase C (PLC) activity or store depletion in several cell lines. The activation of receptor tyrosine kinase cascades leads to the membrane colocalization of PLCγ and PI 3-kinase, both of which use phosphatidylinositol 4,5-bisphosphate (PIP₂) as a substrate to generate IP₃ and PIP₃, respectively. These 2 signaling intermediates trigger the activation of calcium channels at different cellular compartments, giving rise to elevated levels of [Ca²⁺]_i. Soluble E-selectin was demonstrated to cause an increase in phospho-Akt, a downstream target of PI 3-kinase. We would speculate that PI 3-kinase acts to modulate TRPC6 activity and that PI 3-kinase lies downstream of Src in the regulation of soluble E-selectin–mediated permissive SOCE.

Several putative glycoprotein selectin ligands have been isolated from hematopoietic cells using in vitro affinity purification techniques, but the exact identity and contribution of physiologic E-selectin ligands on neutrophils is unknown.³⁷  In this paper, we have demonstrated that E-selectin binds via the lectin domain to cause permissive SOCE in neutrophils in response to PAF, presumably through a putative E-selectin receptor present on neutrophils. A cell-adhesion molecule suggested to play a role in E-selectin adhesion is CD66 or carcinoembryonic antigen (CEA). Neutrophils are known to express several CEA family members, which are all highly glycosylated molecules with multiple sialyl and fucosyl residues. In preliminary experiments, CD66 ligation with antibodies caused prolonged PAF-induced Ca²⁺ mobilization in a similar manner to that caused by soluble E-selectin (data not shown). We are currently investigating the possibility that CD66 and other adhesion receptors need to coengage via soluble E-selectin to regulate Ca²⁺ signaling in response to PAF.

Receptor-mediated activation of leukocytes by inflammatory stimuli requires Ca²⁺ mobilization and influx as a critical common activation mechanism. Selective modulation of distinct components of these Ca²⁺ signals may represent potentially attractive strategies for developing anti-inflammatory drugs to attenuate leukocyte activation. Our report is the first demonstration of soluble E-selectin causing permissive SOCE to occur following activation of neutrophils by PAF and that this SOCE most likely occurs through TRPC6. We have identified a novel form of permissive SOCE induced by soluble E-selectin in human neutrophils, which occurs through a Src/PI 3-kinase–dependent pathway and also requires a G_q/11-derived signal to sensitize or prime TRPCs to open on increased intracellular Ca²⁺ and depletion of Ca²⁺ stores, but the precise order of these molecular events is yet to be fully explored (Figure 7). This novel mechanism of molecular cross-talk integrates signals from pertussis toxin-insensitive G_q/11-coupled receptors and TRPCs and could be critical for fine tuning adhesion and migratory responses during neutrophil recruitment during inflammation.

We are very grateful to Dr M. Bird and Dr Girish Shah (GlaxoSmithKline, Stevenage, United Kingdom) for their generous gift of the recombinant-selectin baculovirus expression constructs and to Dr Gavin Nicoll for his kind help with the baculovirus expression technique.