Calpain inhibitors stimulate phagocyte functions via activation of human formyl peptide receptors

Neutrophils: Many Ways to Die

Neutrophils or polymorphonuclear leukocytes (PMN) are key participants in the innate immune response for their ability to execute different effector functions. These cells express a vast array of membrane receptors that allow them to recognize and eliminate infectious agents effectively and respond appropriately to microenvironmental stimuli that regulate neutrophil functions, such as activation, migration, generation of reactive oxygen species, formation of neutrophil extracellular traps, and mediator secretion, among others. Currently, it has been realized that activated neutrophils can accomplish their effector functions and simultaneously activate mechanisms of cell death in response to different intracellular or extracellular factors. Although several studies have revealed similarities between the mechanisms of cell death of neutrophils and other cell types, neutrophils have distinctive properties, such as a high production of reactive oxygen species (ROS) and nitrogen species (RNS), that are important for their effector function in infections and pathologies such as cancer, autoimmune diseases, and immunodeficiencies, influencing their cell death mechanisms. The present work offers a synthesis of the conditions and molecules implicated in the regulation and activation of the processes of neutrophil death: apoptosis, autophagy, pyroptosis, necroptosis, NETosis, and necrosis. This information allows to understand the duality encountered by PMNs upon activation. The effector functions are carried out to eliminate invading pathogens, but in several instances, these functions involve activation of signaling cascades that culminate in the death of the neutrophil. This process guarantees the correct elimination of pathogenic agents, damaged or senescent cells, and the timely resolution of the inflammation that is essential for the maintenance of homeostasis in the organism. In addition, they alert the organism when the immunological system is being deregulated, promoting the activation of other cells of the immune system, such as B and T lymphocytes, which produce cytokines that potentiate the microbicide functions.

Experimental Manipulation of Calpain Activity In Vitro

The calpain activity in cells can be experimentally manipulated in vitro by calpain inhibitors, and various types of calpain inhibitors such as peptide aldehydes and α-mercapto-acrylic acid derivatives are widely used as a valuable tool to elucidate the physiological and pathological roles of calpain. Here I describe the experimental procedures with calpain inhibitors, with human neutrophils being primarily used in this experiment. It should be noted that potent calpain inhibitors not only inhibit the calpain activity but also stimulate cell functions via direct activation of human formyl peptide receptors and/or other G protein-coupled receptors depending on the inhibitors used.

1α,25-dihydroxyvitamin D₃ counteracts the effects of cigarette smoke in airway epithelial cells

Cigarette smoke extracts (CSE) alter calpain-1 expression via ERK signaling pathway in bronchial epithelial cells. 1α,25-dihydroxyvitamin D3 (1,25D3) inhibits cigarette smoke-induced epithelial barrier disruption. This study was aimed to explore whether the 1,25D3 counteracted the CSE effects in a human bronchial epithelial cell line (16HBE). In particular, transepithelial electrical resistance (TER) and permeability, expression and distribution of E-cadherin and β-catenin, calpain-1 expression, and ERK phosphorylation were assessed in the CSE-stimulated 16HBE cells. The CSE induced the ERK phosphorylation, improved the calpain-1 expression, increased the distribution anomalies and the cleaving of E-cadherin and β-catenin, and resulted in the TER reduction and the permeability increase. The 1,25D3 reduced these pathological changes. The 1,25D3 mediated effects were associated with a reduced ERK phosphorylation. In conclusion, the present study provides compelling evidences that the 1,25D3 may be considered a possible valid therapeutic option in controlling the cigarette smoke-induced epithelial barrier disruption.

Development of small molecule non-peptide formyl peptide receptor (FPR) ligands and molecular modeling of their recognition

Formyl peptide receptors (FPRs) are G protein-coupled receptors (GPCRs) expressed on a variety of cell types. These receptors play an important role in the regulation of inflammatory reactions and sensing cellular damage. They have also been implicated in the pathogenesis of various diseases, including neurodegenerative diseases, cataract formation, and atherogenesis. Thus, FPR ligands, both agonists and antagonists, may represent novel therapeutics for modulating host defense and innate immunity. A variety of molecules have been identified as receptor subtype-selective and mixed FPR agonists with potential therapeutic value during last decade. This review describes our efforts along with recent advances in the identification, optimization, biological evaluation, and structure-activity relationship (SAR) analysis of small molecule non-peptide FPR agonists and antagonists, including chiral molecules. Questions regarding the interaction at the molecular level of benzimidazoles, pyrazolones, pyridazin-3(2H)-ones, N-phenylureas and other derivatives with FPR1 and FPR2 are discussed. Application of computational models for virtual screening and design of FPR ligands is also considered.

FPR2/ALXR agonists and the resolution of inflammation

The resolution of inflammation (RoI), once believed to be a passive process, has lately been shown to be an active and delicately orchestrated process. During the resolution phase of acute inflammation, novel mediators, including lipoxins and resolvins, which are members of the specialized pro-resolving mediators of inflammation, are produced. FPR2/ALXR, a receptor modulated by some of these lipids as well as by peptides (e.g., annexin A1), has been shown to be one of the receptors involved in the RoI. The aim of this perspective is to present the concept of the RoI from a medicinal chemistry point of view and to highlight the effort of the research community to discover and develop anti-inflammatory/pro-resolution small molecules to orchestrate inflammation by activation of FPR2/ALXR.

Stimulation of human formyl peptide receptors by calpain inhibitors: homology modeling of receptors and ligand docking simulation

Calpain inhibitors, including peptide aldehydes (N-acetyl-Leu-Leu-Nle-CHO and N-acetyl-Leu-Leu-Met-CHO) and α-mercapto-acrylic acid derivatives (PD150606 and PD151746), have been shown to stimulate phagocyte functions via activation of human formyl peptide receptor (hFPR) and/or hFPR-like 1 (hFPRL1). Using the homology modeling of the receptors and the ligand docking simulation, here we show that these calpain inhibitors could bind to the putative N-formyl-Met-Leu-Phe (fMLF) binding site on hFPR and/or hFPRL1. The studies with HEK-293 cells stably expressing hFPR or hFPRL1 showed that the concentrations of calpain inhibitors required to induce an increase in cytoplasmic free Ca(2+) ([Ca(2+)](i)) was much higher (>100 folds) than those of fMLF and Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm). HEK-293 cells expressing hFPR or hFPRL1 with the mutated fMLF binding site never exhibited the [Ca(2+)](i) response to calpain inhibitors. When the optimal concentrations of each stimulus were used, pretreatment of cells with fMLF or WKYMVm abolished an increase in [Ca(2+)](i) induced by calpain inhibitors as well as the same stimulus, whereas pretreatment of cells with calpain inhibitors significantly suppressed, but never abolished, the [Ca(2+)](i) response induced by fMLF or WKYMVm, suggesting that the binding affinity of the inhibitors to the putative fMLF binding site may be lower than that of fMLF or WKYMVm.