Pharmacological control of neutrophil-mediated inflammation: strategies targeting calcium handling by activated polymorphonuclear leukocytes

Abstract

Unlike most other effector cells of the innate, as well as the adaptive immune systems, the neutrophil is a relatively undiscerning aggressor with scant regard for damage limitation. Although this highly combative, professional phagocyte has become increasingly implicated in the immunopathogenesis of many acute and chronic inflammatory disorders, of both infective and noninfective origin, effective pharmacological strategies to counter neutrophil aggression have remained elusive. Activation of neutrophils results in rapid mobilization of both stored and extracellular Ca(2+), resulting in abrupt, usually transient increases in cytosolic Ca(2+), which precede, and are a prerequisite for activation of the Ca(2+)-dependent pro-inflammatory activities of these cells. Mobilization of Ca(2+) by, and restoration of Ca(2+) homeostasis to activated neutrophils are multistep processes which present a number of potential targets, some well recognized and others novel and unconventional, for the pharmacological control of neutrophil-mediated inflammation. Uncovering these targets represents the primary focus of this review.

Keywords: NADPH oxidase; calcium; cyclic AMP; neutrophils; sodium-calcium exchanger.

Figure 1

Activation of neutrophils by chemoattractants such as FMLP, PAF, C5a, and LTB₄ increases cytosolic Ca²⁺ concentrations with resultant generation of toxic reactive oxygen species (ROS) and release of proteases, matrix metalloproteinases (MMPs) and lipid mediators. The tissue injury that may be associated with release of these harmful molecules into the vicinity of innocent bystander host tissues contributes to the pathogenesis of numerous diseases, including chronic obstructive pulmonary disease (COPD) and the acute respiratory distress syndrome (ARDS). Abbreviations: FMLP, N-formylated peptides/polypeptides; LTB₄, leukotriene B₄; PAF, platelet-activating factor.

Figure 2

Calcium-mobilizing stimuli interact with membrane G-protein coupled receptors (GPCR) to activate phospholipase C (PLC) generating inositol triphosphate (IP₃) which interacts with IP₃ receptors (IP₃R) releasing Ca²⁺ from storage vesicles. Cytosolic Ca²⁺ phospholipase A₂ (cPLA₂) Which mobilizes arachidonic acid (AA) for the 5-lipoxygenase (5-LO) pathway. The AA metabolite leukotriene B₄ (LTB₄) is actively transported to the cell exterior where it binds to its receptor to activate PLC, completing a positive feedback autocrine loop. Ca²⁺ released into the cytosol is rapidly extruded from the cell by the plasma membrane Ca²⁺ ATPase and resequested into storage vesicles by the protein kinase A (PKA)-sensitive endomembrane Ca²⁺ ATPase. Protein kinase C (PKC) activated by Ca²⁺ and diacylglycerol (DAG) facilitates assembly and activation of NADPH oxidase on the outer membrane which generates reactive oxygen species (ROS) with concomitant membrane depolarization. The depolarized membrane potential delays Ca²⁺ entry through store operated channels (SOCCs) until the Ca²⁺-activatible Na⁺/Ca²⁺ exchanger, operating in reverse mode, mediates recovery of the membrane potential promoting Ca²⁺ reuptake via SOCCs. PKC down-regulates PLC as part of a negative feedback loop to terminate IP₃ production