Insulin Can Delay Neutrophil Extracellular Trap Formation In Vitro—Implication for Diabetic Wound Care?

SIMPLE SUMMARY: Diabetes is a globally developing disease. Diabetic patients suffer from several side diseases, including impaired healing abilities. In general, every injury is characterized by an immediate immune response at the wound site. Neutrophils are the first immune cells arriving at the fracture site, to defend the body against pathogens. However, neutrophils may also induce tissue damage. By releasing their DNA together with proteins, so-called neutrophil extracellular traps (NETs), they can induce an overshooting inflammation and directly harm tissue, especially during wound healing. Some anti-diabetic drugs were already found to influence this process but for one of the major treatments of diabetes, insulin, nothing is known so far. We found that insulin can modulate the process of NET formation and instead induce the anti-bacterial capacity of neutrophils. Thus, insulin may be a tool to improve diabetic wound healing by regulation of NET formation. ABSTRACT: Diabetes is a worldwide evolving disease with many associated complications, one of which is delayed or impaired wound healing. Appropriate wound healing strongly relies on the inflammatory reaction directly after injury, which is often altered in diabetic wound healing. After an injury, neutrophils are the first cells to enter the wound site. They have a special defense mechanism, neutrophil extracellular traps (NETs), consisting of released DNA coated with antimicrobial proteins and histones. Despite being a powerful weapon against pathogens, NETs were shown to contribute to impaired wound healing in diabetic mice and are associated with amputations in diabetic foot ulcer patients. The anti-diabetic drugs metformin and liraglutide have already been shown to regulate NET formation. In this study, the effect of insulin was investigated. NET formation after stimulation with PMA (phorbol myristate acetate), LPS (lipopolysaccharide), or calcium ionophore (CI) in the presence/absence of insulin was analyzed. Insulin led to a robust delay of LPS- and PMA-induced NET formation but had no effect on CI-induced NET formation. Mechanistically, insulin induced reactive oxygen species, phosphorylated p38, and ERK, but reduced citrullination of histone H3. Instead, bacterial killing was induced. Insulin might therefore be a new tool for the regulation of NET formation during diabetic wound healing, either in a systemic or topical application.