Chemical Tools for Targeted Amplification of Reactive Oxygen Species in Neutrophils

Abstract

A number of chemical compounds are known, which amplify the availability of reactive oxygen species (ROS) in neutrophils both in vitro and in vivo. They can be roughly classified into NADPH oxidase 2 (NOX2)-dependent and NOX2-independent reagents. NOX2 activation is triggered by protein kinase C agonists (e.g., phorbol esters, transition metal ions), redox mediators (e.g., paraquat) or formyl peptide receptor (FPR) agonists (e.g., aromatic hydrazine derivatives). NOX2-independent mechanisms are realized by reagents affecting glutathione homeostasis (e.g., l-buthionine sulfoximine), modulators of the mitochondrial respiratory chain (e.g., ionophores, inositol mimics, and agonists of peroxisome proliferator-activated receptor γ) and chemical ROS amplifiers [e.g., aminoferrocene-based prodrugs (ABPs)]. Since a number of inflammatory and autoimmune diseases, as well as cancer and bacterial infections, are triggered or enhanced by aberrant ROS production in neutrophils, it is tempting to use ROS amplifiers as drugs for the treatment of these diseases. However, since the known reagents are not cell specific, their application for treatment likely causes systemic enhancement of oxidative stress, leading to severe side effects. Cell-targeted ROS enhancement can be achieved either by using conjugates of ROS amplifiers with ligands binding to receptors expressed on neutrophils (e.g., the GPI-anchored myeloid differentiation marker Ly6G or FPR) or by designing reagents activated by neutrophil function [e.g., phagocytic activity or enzymatic activity of neutrophil elastase (NE)]. Since binding of an artificial ligand to a receptor may trigger or inhibit priming of neutrophils the latter approach has a smaller potential for severe side effects and is probably better suitable for therapy. Here, we review current approaches for the use of ROS amplifiers and discuss their applicability for treatment. As an example, we suggest a possible design of neutrophil-specific ROS amplifiers, which are based on NE-activated ABPs.

Keywords: NADPH oxidase 2; aminoferrocenes; autoimmune disease; chronic granulomatous disease; inflammation; neutrophils; reactive oxygen species; therapy.

Figure 1

Influence of reactive oxygen species (ROS) on immune cell function. (A) The mechanism of deactivation of T-cells by ROS and reactive nitrogen species produced by neutrophils and monocytes/macrophages. This process is facilitated by the proximity of macrophages and T-cells at the sites of infection due to binding of the T-cells to antigens presented on the macrophage surface. (B) Oxidative burst in normal primed neutrophils leading to ROS production and neutrophil extracellular trap (NET) formation. (C) Aberrant response of NADPH oxidase 2-deficient neutrophils (e.g., in chronic granulomatous disease) leading to low ROS and insufficient NET formation. This deficiency can be fixed by applying ROS amplifiers.

Figure 2

Generation and transformation of reactive oxygen species in live cells. Reactive species are red colored.

Figure 3

Representative reactive oxygen species (ROS) modulators in neutrophils. (A–C): NADPH oxidase 2(NOX2)-dependent modulators. Quin-C1 is an formyl peptide receptor agonist, but does not induce ROS in cells. (D) NOX2-independent modulators. Structurally related fragments in DAC and phorbol myristate acetate are indicated with red color.

Figure 4

A general structure of aminoferrocene-based prodrugs (ABPs) and the mechanisms of their activation after cleavage of the triggering moiety (TM). The NE-sepcific TM is based on the peptide reported in Ref. (92).