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Index: WOUNDS. 2012;24(7):178–184.

  Abstract:This study observed the quantity and functions of wound neutrophils after topical insulin treatment and investigated the effect of insulin on the wound inflammatory response, as well as the mechanism of insulin-induced wound healing. Methods. Full-thickness excisional wounds were made on the dorsal symmetrical site of C57BL/6J mice. The wounds were treated with either 0.03U insulin/20 µL saline or 20 µL saline. The healing times and healing rates of the wounds were recorded. The wounds and adjacent tissues were collected during the first 3 consecutive days after the injury. Quantification of myeloid differentiation antigen Gr-1, myeloperoxidase (MPO), malondialdehyde (MDA), and macrophage inflammatory protein-2 (MIP-2) were measured by Western blotting, biochemical analysis, and real-time polymerase chain reaction (PCR), respectively.

Results. The healing time of insulin-treated wounds was significantly decreased compared with the saline-treated wounds. The wound closure rates at days 5 and 7 after injury were significantly higher with insulin treatment than with saline treatment. Additionally, wound neutrophil infiltration was suppressed by insulin treatment within the first 2 days after injury. Real-time PCR revealed insulin treatment significantly inhibited MIP-2 expression within 2 days after injury (P <0.05), whereas MIP-2 expression in both groups decreased to a similar level at 3 days after injury. There was no statistical difference in MPO or MDA content between the insulin treatment and control group. Conclusion. Topical insulin application decreased neutrophil infiltration by inhibiting MIP-2 expression and advanced neutrophil resolution. It has been reported that insulin promotes neutrophil functions; therefore, the present results, along with previous findings, suggest there is a delicate regulation of insulin on wound inflammatory response during the healing process.

Introduction

  Wound healing is divided into 3 sequential, yet overlapping, phases: inflammatory, proliferation, and remodeling. In the early 20th century, insulin was first used to treat diseases other than diabetes.^1,2 Various animal models showed that systemic insulin treatment accelerated healing from fractures, skin cuts, and skin ulcers.3–6 Low-dose topical insulin application also promoted healing of thermal traumas in rats and incision wounds in rabbits.^7,8 In the proliferation phase of wound healing, the authors’ previous research showed that low-dose topical insulin stimulated migration of keratinocytes and vascular endothelial cells through the insulin receptor-mediated PI3K-Akt-Rac1 signal pathway. These molecular events could trigger re-epithelialization and angiogenesis, and hence, promote wound healing.^9,10

  In the inflammatory phase, the initiation of wound healing, which consists of a series of biological events including coagulation and release of vasoactive substances, chemokines, and cytokines, is important for regulating the consequent phases of proliferation and remodeling.¹¹ Systemic insulin treatment was reported to alleviate the systemic inflammatory response via inhibiting the expression of monocyte chemoattractant protein-1(MCP-1), cytokine-induced neutrophil chemoattractant 1(CINC-1), and CINC-2.¹² However, it remains unknown whether topical insulin application could help regulate the traumatic inflammatory response, and if so, whether this insulin-regulated inflammatory response was one of the underlying mechanisms of insulin-accelerated wound healing.

  Neutrophils are the main type of cells that are involved in the inflammatory response. They clean exogenous pathogens through phagocytosis and release enzymes and reactive oxygen species (ROS) to kill bacteria and other intruders.