Tear Levels of IGFBP-3: A Potential Biomarker for Diabetic Nerve Changes in the Cornea

INTRODUCTION: Type 2 diabetes mellitus has reached epidemic levels in the United States and worldwide. Ocular complications from this disease include diabetic retinopathy and keratopathy, both of which can lead to significant vision loss. While frequently underappreciated, diabetic keratopathy is associated with painful ocular surface disorders, including corneal erosions and delayed wound healing. Recent work in our laboratory has focused on the role of the insulin-like growth factor (IGF) system in diabetic corneal disease. METHODS: Here, we review recent findings on the presence of IGF-1, insulin, and the insulin-like binding protein (IGFBP-3) in human tear fluid and evaluate their potential use as biomarkers in diabetes. We further examine clinical evidence using in vivo confocal microscopy as an important imaging biomarker in diabetes and discuss associations between tear film changes in diabetes and corneal nerve loss. RESULTS: IGFBP-3 was the only tear film marker significantly associated with nerve loss in type 2 diabetes, whereas tear levels of IGF-1 were associated with aging. Interestingly, tear levels of IGFBP-3 were not directly related to serum levels of HbA1c, suggesting that hyperglycemia alone is not driving increased secretion of this protein. CONCLUSIONS: Overwhelming evidence supports the use of in vivo confocal microscopy as a tool to evaluate corneal nerve and epithelial changes induced by diabetes in research settings. The newly identified relationship between morphological changes in the corneal subbasal nerve plexus in diabetes and the increase in tear levels of IGFBP-3 suggest that this protein may represent an innovative new biomarker to assess risk of ocular and nonocular complications in type 2 diabetes mellitus.