Bankers‐Fulbright, Jennifer L.

Abstract Eosinophils play a key role in the pathogenesis of asthma and other allergic inflammatory diseases. We have previously shown that treatment of eosinophils with lidocaine preferentially inhibits IL-5-induced survival. This inhibition cannot be overcome by increasing concentrations of IL-5 and is not due to the blocking of Na+ channels by lidocaine. Here we report that one class of K+ channel blockers, the sulfonylureas, inhibits eosinophil survival in a manner similar to lidocaine. The sulfonylurea glyburide inhibits eosinophil survival even at high concentrations of IL-5. In contrast, increasing concentrations of IL-3 or granulocyte-macrophage CSF overcome glyburide inhibition. Glyburide also blocks cytokine-induced eosinophil superoxide production. Similar results were seen with the sulfonylureas tolbutamide and glipizide. Interestingly, the effects of glyburide are not antagonized by the ATP-sensitive K+ channel openers cromakalim, pinacidil, or diazoxide. Although Scatchard analysis of [3H]glyburide binding to eosinophil membranes indicated that the high affinity sulfonylurea receptor (SUR1) is not present on eosinophils, human eosinophils do express mRNA homologous to the sulfonylurea receptor family, in keeping with the presence of a sulfonylurea receptor. Finally, coculture of eosinophils with combinations of glyburide, lidocaine, and dexamethasone resulted in synergistic inhibition of cytokine-mediated eosinophil survival and superoxide production. These results have intriguing clinical implications for the treatment of eosinophil-associated diseases.

Abstract Eosinophils and cytokines active on eosinophils, especially IL-5, are believed to be critically involved in chronic allergic diseases. IL-5 activates eosinophils and enhances their survival in vitro by delaying apoptosis. In this study, we found that lidocaine and six analogues blunt responses of eosinophils to IL-5. Lidocaine and its derivatives inhibit IL-5-mediated eosinophil survival in a concentration-dependent manner (IC50 = 110 μM for 30 pg/ml IL-5). At suboptimal lidocaine concentrations, the eosinophil survival response to IL-5 shifts and more IL-5 is required to maintain survival. The inhibitory effect requires at least 24-h exposure of eosinophils to lidocaine, and the protein kinase C activator, PMA, completely reverses the inhibition. A multiparameter flow-cytometric analysis shows that lidocaine hastens the apoptosis of eosinophils normally delayed by IL-5. Lidocaine does not affect IL-5R expression or IL-5-induced protein tyrosine phosphorylation. Lidocaine also inhibits eosinophil survival mediated by IL-3 or granulocyte-macrophage CSF, although less potently than that mediated by IL-5. Furthermore, lidocaine inhibits eosinophil superoxide production stimulated by IL-5, granulocyte-macrophage CSF, or IL-3, but not that stimulated by platelet-activating factor, immobilized IgG, or PMA. Lidocaine and its derivatives show novel immunomodulatory properties and are able to blunt eosinophil responses to cytokines in addition to their local anesthetic or antiarrhythmic properties. Thus, lidocaine and its derivatives may represent a new class of therapeutic agents to treat patients with allergic diseases.

The AML14 cell line and its AML14.3D10 subclone was established as a model cell system that exhibits phenotypic characteristics similar to that of human eosinophils. We investigated the effects of purinergic receptor stimulation on the physiological response of the AML 14.3D10 cell line to better evaluate its role as a suitable model for eosinophils using both molecular and functional approaches. Ca 2+ imaging experiments using Fura‐2 demonstrated the existence of purinergic receptors responsive to UTP (EC 50 of 8.9 x 10 −8 ), ATP γ‐s (EC 50 of 1.87 x 10 −7 ), and UDP (EC 50 of 7.44x 10 −6 ). Molecular identification of the purinergic receptor subtypes was confirmed by QRT‐PCR which demonstrated the presence of P2Y 1 , P2Y 2 , P2Y 4 , and P2Y 6 receptors. Ratio imaging experiments using the membrane potential sensitive indicator DiBAC‐3 showed that stimulation with UTP produced a membrane hyperpolarization of approximately 10 mV that returned to the original resting potential within 10 minutes after stimulation. Whole cell perforated patch clamp experiments using amphotericin B revealed the presence of voltage‐gated potassium channel currents that were stimulated following treatment with UTP. Analysis of mRNA by QRT‐PCR indicated the existence of several voltage‐gated potassium channel α subunits including, Kv1.2, Kv1.3, Kv1.4, and Kv1.6. Previous studies of Kv1.3 channel function in T‐lymphocytes showed that the channel was involved in activation and proliferation, suggesting the possibility of a similar role related to tumorogenesis in the AML 14.3D10 cell line.

The effects of platelet-activating factor (PAF) and IL-5 on intracellular pH were investigated in human eosinophils. Purified peripheral blood eosinophils were loaded with the ratiometric fluorescent pH indicator BCECF-AM ester. Stimulation of eosinophils with PAF produced time-dependent alkalinization of the cytoplasm from an initial pH of 7.1±0.04 to 7.5±0.05. A similar alkalinization response was produced by the calcium ionophore, ionomycin and by the calcium ATPase inhibitor, thapsigargin. These compounds as well as PAF produce significant increases in cytoplasmic calcium ([Ca2+]i). In contrast, IL-5 and the protein kinase C (PKC) activator phorbol myristate acetate (PMA) did not produce cytoplasmic alkalinization and had no effect on [Ca2+]i in eosinophils. PAF-stimulated alkalinization was not inhibited under conditions that blocked plasma membrane Na+-H+ exchange, proton channel or plasma membrane H+-ATPase activities. Measurements of intragranule pH with a cell permeant pH indicator (LysoSensor Yellow/Blue DND-160), which partitions into intracellular acidic compartments, revealed that PAF-stimulated cytosolic alkalinization correlated with intragranule acidification. These results suggest that the increase in [Ca2+]i after PAF stimulation activates a H+-ATPase present in the granule membranes, leading to enhanced granule acidification and cytoplasmic alkalinization. We propose that granule acidification is an important step in solubilization of major basic protein crystals, which are stored within the granule core, in preparation for degranulation and release of these proteins.

Protein kinase C (PKC) activation in human eosinophils increases NADPH oxidase activity, which is associated with plasma membrane depolarization. In this study, membrane potential measurements of eosinophils stimulated with phorbol ester (phorbol 12-myristate 13-acetate; PMA) were made using a cell-permeable oxonol membrane potential indicator, diBAC4(3). Within 10 minutes after PMA stimulation, eosinophils depolarized from–32.9±5.7 mV to +17.3±1.8 mV. The time courses of depolarization and proton channel activation were virtually identical. Blocking the proton conductance with 250 μM ZnCl2(+43.0±4.2 mV) or increasing the proton channel activation threshold by reducing the extracellular pH to 6.5 (+44.4±1.4 mV) increased depolarization compared with PMA alone. Additionally, the protein kinase C(PKC) δ-selective blocker, rottlerin, inhibited PMA-stimulated depolarization, indicating that PKCδ was involved in regulating depolarization associated with eosinophil NADPH oxidase activity. Thus, the membrane depolarization that is associated with NADPH oxidase activation in eosinophils is sufficient to produce marked proton channel activation under physiological conditions.