Proinsulin C‐Peptide Effects in Retinal Pigment Epithelium Health and VEGF Release: Implications for Diabetic Retinopathy

Connecting peptide (C‐peptide) is derived from the proinsulin molecule and is released with insulin in equimolar amounts by beta cells of the pancreas. For many years, C‐peptide was known only as a marker of insulin secretion, but recent work has demonstrated that C‐peptide has important bioactivity, particularly in the microvasculature including the microvessels of the eye. In diabetic retinopathy, a leading cause of blindness in the developed world, the retinal pigment epithelium (RPE) becomes dysfunctional. The RPE normally plays vital metabolic and secretory roles in maintaining the retina through interactions with the choriocapillaris and the photoreceptor layer. However, exposure to high glucose, as in uncontrolled diabetes, may encourage transdifferentiation of RPE cells into fibroblast‐like cells that disrupt the normal functioning of this essential cell layer. Furthermore, high glucose can induce pathological hypersecretion of VEGF from the RPE, which contributes to retinopathy by stimulating retinal neovascularization and by increasing permeability of the RPE via an autocrine action. Since RPE cells express the putative C‐peptide receptor, GPR146, we hypothesized that C‐peptide in physiological concentrations might normalize RPE‐specific gene expression and inhibit VEGF release. To test this hypothesis, we first performed transdifferentiation assays using the spontaneously transformed human RPE cell line, ARPE‐19, in which cells were incubated in either high or normal glucose containing media, with or without C‐peptide for 30 days. We measured changes in gene expression of RPE functional genes using qPCR. C‐peptide enhanced expression of RPE‐specific genes, and the changes in gene expression appeared to be dependent on glucose conditions. In addition, we treated ARPE‐19 cells with C‐peptide in normal and high glucose conditions and measured VEGF secretion by ELISA. C‐peptide significantly reduced VEGF secretion; however, VEGF secretion was enhanced by treating cells with the Na + /K + ATPase inhibitor, ouabain, suggesting that C‐peptide inhibits VEGF release by increasing Na+/K+ ATPase activity. We conclude that C‐peptide prevents transdifferentiation of RPE cells to disruptive fibroblast‐like cells by enhancing the expression of RPE‐specific genes, and that C‐peptide diminishes release of VEGF, possibly by increasing Na + /K + ATPase activity. Future experiments exploring the mechanisms underlying these effects are essential next steps towards the development of C‐peptide‐based therapeutics for the treatment of diabetes‐associated eye disease. Support or Funding Information HL121456 National Institutes of Health