Promotion of Corneal Epithelial Wound Healing by a Tetrapeptide (SSSR) Derived from IGF-1

PURPOSE. A prior study showed that a tetrapeptide (FGLMamide) derived from the carboxyl terminus of substance P (SP) and a 12-residue peptide corresponding to the C domain of insulin-like growth factor (IGF)-1 mimic the synergistic effect of the full-length molecules on corneal epithelial wound healing. To develop an effective treatment for persistent corneal epithelial defects, the current study was conducted to investigate the minimal sequence within the C domain of IGF-1 that is required for such synergism with SP or FGLM-amide. METHODS. The effects of IGF-1-derived peptides on corneal epithelial migration were evaluated with a rabbit corneal organ-culture system. RESULTS. A tetrapeptide (SSSR; Ser 33 -Ser-Ser-Arg) derived from the C domain of IGF-1 was sufficient for the synergistic promotion with FGLM-amide both of corneal epithelial migration in vitro and of wound closure in vivo. The activity of the SSSR peptide was sequence specific and its potency was similar to that of IGF-1. The SSSR peptide by itself also promoted corneal epithelial migration in vitro at higher concentrations. It was devoid, however, of both the mitogenic action of IGF-1 and the ability of the full-length molecule to induce neovascularization. CONCLUSIONS. The SSSR sequence mediates the synergistic effect of IGF-1 with SP on corneal epithelial wound healing. Clinical application of the SSSR peptide would be expected to be free of potentially deleterious side effects associated with treatment with full-length IGF-1. Local administration of the SSSR tetrapeptide, alone or in combination with FGLM-amide, is thus a potential new strategy for the treatment of nonhealing epithelial wounds. (Invest Ophthalmol Vis Sci. 2006;47: 3286 -3292) DOI:10.1167/iovs.05-1205 E pithelia provide an essential barrier that protects organs and tissues from the external environment and contributes to maintenance of the internal environment of the body. Although epithelial defects in general heal relatively quickly, the persistence of such defects under certain conditions, such as those that exist in individuals with diabetes mellitus or peripheral neuropathy, can have serious pathologic consequences. The persistence of epithelial defects of the cornea often results in the development of corneal ulcer, which can lead to corneal perforation and loss of vision. There is currently no consistently effective mode of treatment for persistent corneal epithelial defects that is free of potentially adverse consequences. Damage to the corneal epithelium is followed by three phases of epithelial wound healing. Immediately after injury, the remaining epithelial cells begin to migrate to cover the area of the defect. After the defect is covered, contact inhibition results in the termination of epithelial cell migration and the epithelial cells enter a proliferative phase. Finally, the newly generated cells differentiate to form the stratified structure of the corneal epithelium. 1 Epithelial migration is thus an important first step in corneal epithelial wound healing. To develop an effective treatment for persistent corneal epithelial defects, we have established an organ-culture system for the rabbit cornea that allows quantitative evaluation of the effects of test agents on epithelial migration. 2 With this system, we have previously shown that several biological factors, including epidermal growth factor, 3 fibronectin, 2 interleukin-6, 4 and the combination of insulin-like growth factor (IGF)-1 and substance P (SP) 5 promote epithelial migration in vitro. Verification of the relevance of this in vitro system was provided by the observation that the combination of IGF-1 and SP also facilitates closure of rabbit corneal epithelial wounds in vivo. 11 The carboxyl-terminal four amino acids of SP (Phe-Gly-Leu-Metamide, or FGLM-amide) are sufficient for this effect. 12 The synergistic effect of IGF-1 with SP on epithelial migration is mimicked by IGF-2 but not by insulin. 13 IGFs and insulin share many structural similarities in their A and B domains. 14 However, the C domain of IGFs does not exhibit sequence homology to the C peptide of proinsulin, which is not retained in the mature insulin molecule. We have recently shown that the C domain of IGF-1 or -2 is responsible for the synergistic facilitation with SP of corneal epithelial migration