Flow cytometry reveals different lag times in rapid cytoplasmic calcium elevations in human neutrophils in response to N-formyl peptide

Rosiglitazone induces the unfolded protein response, but has no significant effect on cell viability, in monocytic and vascular smooth muscle cells

Given the safety concerns expressed over negative cardiovascular outcomes resulting from the clinical use of rosiglitazone, and the view that rosiglitazone exerts PPARγ-independent effects alongside its insulin-sensitising PPARγ-dependent effects, we hypothesised that rosiglitazone may trigger Unfolded Protein Responses (UPRs) due to disruptions in [Ca(2+)](i) homeostasis within two cardiovascular cell types: monocytic (MM6) and vascular smooth muscle (A7r5) cells. In microsomal samples derived from both cell types, pre-incubation with rosiglitazone rapidly (30min) brought about concentration-dependent PPARγ-independent inhibition of Ca(2+)ATPase activity (IC(50) ∼2μM). Fluo-3 fluorimetric data demonstrated in intact cells that 1h treatment with 1 or 10μM rosiglitazone caused Ca(2+) ions to leak into the cytoplasm. Gene expression analysis showed that within 4h of rosiglitazone exposure, the UPR transcription factor XBP-1 was activated (likely due to corresponding ER Ca(2+) depletion), and the UPR target genes BiP and SERCA2b were subsequently upregulated within 24-72h. After 72h 1 or 10μM rosiglitazone treatment, microsomal Ca(2+)ATPase activity increased to >2-fold of that seen in control microsomes, while [Ca(2+)](i) returned to basal, indicating that UPR-triggered SERCA2b upregulation was responsible for enhanced enzymatic Ca(2+) sequestration within the ER. This appeared to be sufficient to replenish ER Ca(2+) stores and restore normal cell physiology, as cell viability levels were not decreased due to rosiglitazone treatment throughout a 2-week study. Thus, incubation with 1-10μM rosiglitazone triggers the UPR, but does not prove cytotoxic, in cells of the cardiovascular system. This observation provides an important contribution to the current debate over the use of rosiglitazone in the clinical treatment of Type-2 Diabetes.

Rosiglitazone transiently disturbs calcium homeostasis in monocytic cells

The PPARgamma agonist Rosiglitazone exerts anti-hyperglycaemic effects by regulating the long-term expression of genes involved in metabolism, differentiation and inflammation. In the present study, Rosiglitazone treatment rapidly inhibited (5-30 min) the ER Ca(2+) ATPase SERCA2b in monocytic cells (IC(50)=1.88 microM; p<0.05), thereby disrupting short-term Ca(2+) homeostasis (resting [Ca(2+)](cyto)=121.2+/-2.9% basal within 1h; p<0.05). However, extended Rosiglitazone treatment (72 h) induced dose-dependent SERCA2b up-regulation, and restored calcium homeostasis, in monocytic cells (SERCA2b mRNA: 138.7+/-5.7% basal (1 microM)/215.0+/-30.9% basal (10 microM); resting [Ca(2+)](cyto)=97.3+/-8.3% basal (10 microM)). As unfavourable cardiovascular outcomes, possibly related to disrupted cellular Ca(2+) homeostasis, have been linked to Rosiglitazone, this effect may be of clinical interest. In contrast, in PPRE-luciferase reporter-gene assays, Rosiglitazone induced non-dose-dependent PPARgamma-dependent effects (1 microM: 152.5+/-4.9% basal; 10 microM: 136.1+/-5.1% basal (p<0.05 for 1 microM vs. 10 microM)). Thus, we conclude that Rosiglitazone can exert PPARgamma-independent non-genomic effects, such as the SERCA2b inhibition seen here, but that long-term Rosiglitazone treatment did not perturb resting [Ca](cyto) in this study.

The PPAR-γ activator, Rosiglitazone, inhibits actin polymerisation in monocytes: Involvement of Akt and intracellular calcium

Monocyte hyperactivation as seen in diabetes results in increased cytoskeletal rigidity and reduced cell deformability leading to microchannel occlusions and microvascular complications. The thiazolidinediones (TZDs) are PPAR-gamma agonists that have been reported to exert beneficial non-metabolic effects on the vasculature. This study demonstrates that the TZD, Rosiglitazone, significantly reduces f-MLP-induced actin polymerisation in human monocytic cells (p < 0.05). Two of the key signalling processes known to be involved in the regulation of cytoskeletal remodelling were investigated: PI(3)K-dependent Akt phosphorylation and intracellular calcium concentration [Ca(2+)](i). The PI(3)K inhibitor, Wortmannin, ameliorated f-MLP-induced actin polymerisation (p < 0.05), while the Ca(2+) sequestration inhibitor, thapsigargin, induced actin depolymerisation (p < 0.05), confirming the involvement of both processes in cytoskeletal remodelling. Rosiglitazone significantly reduced f-MLP activation of Akt (p < 0.05), and significantly increased [Ca(2+)](i) in both resting and f-MLP-stimulated cells (p < 0.05). Therefore, Rosiglitazone interacts with signalling events downstream of occupancy of the f-MLP receptor, to modulate cytoskeletal remodelling in a PPAR-gamma-independent manner. To our knowledge, these results are the first to present evidence that a PPAR-gamma agonist can modulate actin remodelling in monocytes, and may therefore be protective against microvascular damage in diabetes.

Human eotaxin represents a potent activator of the respiratory burst of human eosinophils

Increased numbers of eosinophils are found in parasitic infections, autoimmune diseases and allergic diseases such as allergic asthma. They are activated by distinct cytokines and chemokines leading to the immigration in the inflamed tissue and mediate tissue damage by releasing reactive oxygen species. Here, the effect of the recently cloned CC chemokine human eotaxin was investigated for its ability to affect different eosinophil effector functions and compared to the CC chemokines MCP-3 and RANTES. Human eotaxin induced chemotaxis of human eosinophils in a dose-dependent manner. The range of efficacy of the CC chemokines compared to the well-known chemotaxin C5a was eotaxin = RANTES > MCP-3 = C5a. In addition, eotaxin induced rapid and transient actin polymerization, a prerequisite for cell migration, in eosinophils in the same range of efficacy as observed for chemotaxis. To investigate whether eotaxin was able to activate the respiratory burst of eosinophils, release of reactive oxygen species was measured by lucigenin-dependent chemiluminescence. Eotaxin induced production of significantly high amounts of reactive oxygen species at a concentration between 10 ng/ml and 500 ng/ml. Surprisingly, the effect of eotaxin was comparable to the well-known eosinophil activator C5a. The range of efficacy of the CC chemokines compared to C5a in the activation of the respiratory burst was eotaxin = C5a > MCP-3 > RANTES. Production of reactive oxygen species was inhibited by pertussis toxin, staurosporin, genestein and wortmannin. Furthermore, eotaxin induced transient increases in intracellular calcium concentration ([Ca2+]i) in human eosinophils. Therefore, pertussis toxin-sensitive Gi-proteins, protein kinase C, tyrosine kinase, phosphatidylinositol-3-kinase and transient increases in [Ca2+]i are involved in the signal transduction of eosinophils following stimulation with eotaxin. In summary, this study reveals the importance of the CC chemokine eotaxin as a potent activator of the respiratory burst, actin polymerization and chemotaxis. Eotaxin, therefore, plays an important role not only by attracting eosinophils to the site of inflammation but also by damaging tissue by its capacity to induce the release of reactive oxygen species.

Detection of C5a receptors on human eosinophils and inhibition of eosinophil effector functions by anti-C5a receptor (CD88) antibodies

Eosinophils and complement activation are reported to play a crucial role in the pathogenesis of connective tissue diseases. Depositions of antigens and antigen-antibody complexes lead to complement activation with the generation of anaphylatoxins, particularly C5a, which is thought to be responsible for the infiltration and activation of eosinophils in the tissue. Previous studies suggested that the eosinophil C5a receptor differs structurally from the receptor expressed on neutrophils. In this study, we investigated the expression and functional properties of C5a receptors on human eosinophils using the C5a receptor monoclonal antibody S5/1 (anti-CD88 mAb). Flow cytometric analysis demonstrated that the anti-CD88 mAb bound homogeneously on the surface of human eosinophils from nonatopic healthy donors. In addition, no subpopulations with respect to C5a receptor expression were identified in normodense or hypodense eosinophils of patients with hypereosinophilia. Pre-incubation of eosinophils with anti-CD88 specifically inhibited C5a-induced intracellular calcium concentration transients. C5a-induced chemotactic activity of eosinophils was significantly inhibited after pre-incubation of cells with anti-CD88 mAb in a dose-dependent manner. Furthermore, anti-CD88 mAb inhibited dose-dependently the release of reactive oxygen species by eosinophils following stimulation with C5a. Thus, the human eosinophil C5a receptor is homogeneously expressed on normal eosinophils from healthy donors as well as on hypodense and normodense eosinophil subpopulations from patients with hypereosinophilia. Based on the inhibitory effect of the S5/1 mAb on C5a-stimulated eosinophil effector functions, we conclude that a single C5a receptor type exists on human eosinophils. In addition, the inhibitory effect of the S5/1 mAb on C5a functions may enable a new experimental approach to the treatment of diseases that have been associated with C5a-mediated activation.

Actin polymerization in human eosinophils, unlike human neutrophils, depends on intracellular calcium mobilization

Eosinophils represent major effector cells in the allergic inflammation. In contrast to neutrophils, the mechanism of eosinophil activation during the inflammatory response is poorly understood. In this study, the relation between calcium fluxes, chemotaxis, and actin polymerization in eosinophils from healthy non-atopic donors was investigated. Pre-incubation of eosinophils with the intracellular calcium chelator BAPTA dose-dependently prevented an increase in the intracellular calcium concentration ([Ca2+]i), whereas the depletion of extracellular calcium in the test medium had no effect. The chemotactic response of eosinophils, which was measured by the modified boyden chamber technique upon stimulation with RANTES, C5a and PAF, was dose-dependently inhibited by the chelation of intracellular calcium as well as inactivation of the cells in Ca2+ -depleted medium. To evaluate whether other cell functions which are involved in the migratory response of eosinophils might be dependent on intracellular and extracellular calcium, actin polymerization was investigated. Flow-cytometric measurement of F-actin with NBD-phallacidin revealed that actin polymerization in human eosinophils in response to RANTES, C5a, and PAF was dose-dependently inhibited by the intracellular calcium chelator BAPTA. Since it is well known that actin polymerization in neutrophils is not affected by chelation of intracellular calcium, actin polymerization in these cells was investigated under the same conditions as for eosinophils. In contrast to eosinophils, BAPTA did not inhibit actin polymerization in neutrophils. In summary, these data demonstrate that intracellular calcium fluxes represent a prerequisite for eosinophil chemotaxis and actin polymerization in human eosinophils. Furthermore, regulation of actin polymerization in eosinophils differed from that of neutrophils on the level of intracellular calcium fluxes.

Activation of the respiratory burst in human eosinophils by chemotaxins requires intracellular calcium fluxes

Eosinophils represent major effector cells in the allergic inflammatory response. Following activation, these cells are capable of mediating tissue damage, particularly by the release of reactive oxygen species. In this study, the role of extracellular and intracellular calcium in the induction of the respiratory burst of human eosinophils was investigated in healthy non-atopic individuals. Pre-incubation of Fura-2-loaded eosinophils with the intracellular calcium chelator 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid prevented the increase of the [Ca++]i following stimulation by RANTES, C5a and PAF, in concentration-dependent fashion, whereas depletion of extracellular calcium in the test medium by ethyl=eneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid was ineffective. To investigate the potential role of extracellular and intracellular calcium on the production of reactive oxygen species, flow-cytometric measurement of H2O2 production by dihydrorhodamine 123 and lucigenin-dependent chemiluminescence were carried out. Chelation of both intracellular and extracellular calcium prevented production of reactive oxygen species after stimulation with C5a, PAF, or RANTES. However, production of reactive oxygen species after stimulation by phorbol myristate acetate, which bypasses post-receptor events by direct activation of protein kinase C, was prevented only after chelation of intracellular but not extracellular calcium. This suggested a Ca(++)-sensitive form of protein kinase C in the activation process of the respiratory burst. These data demonstrate that intracellular and extracellular calcium represent a prerequisite of chemotaxin-induced activation of the respiratory burst in human eosinophils. Thus, intracellular calcium seems to play a central role in the modulation of the respiratory burst in eosinophils and might therefore be an interesting target for drugs that interfere with calcium homeostasis and reduce the tissue destructive power of eosinophils.

[Ca2+]i-transients and actin polymerization in human neutrophils under stimulation with GRO alpha and complement fragment C5a

The neutrophil chemotaxins, complement fragment C5a (C5a) and GRO alpha, induced the mobilization of Ca2+ from intracellular stores and the polymerization of actin in human neutrophils as assayed by flow cytometric measurements. [Ca2+]i-transients developed as an "all-or-none" response. Individual neutrophils required different threshold concentrations of added ligand to induce [Ca2+]i-transients which were then always maximal. In contrast, chemotaxin-induced formation of actin filaments in single neutrophils occurred in a dose-dependent manner. Pertussis toxin blocked chemotaxin-induced actin polymerization and [Ca2+]i-transients indicating that both cell responses shared initial activation steps such as ligand binding and activation of guanine nucleotide-binding proteins (G-proteins).