The Role of Matrix Metalloproteinases (MMPs) in Platelet Endothelial Cell Adhesion Molecule‐1 (PECAM‐1) Loss in Diabetic Retinopathy: In Vivo and in Vitro Studies

Abstract

OBJECTIVEThe aim of this study is to understand the mechanisms involved in the diabetes‐induced loss of PECAM‐1 in the retina, and whether it can be attributed to PECAM‐1 cleavage by matrix metalloproteinases (MMPs).INTRODUCTIONPlatelet endothelial cell adhesion molecule‐1 (PECAM‐1) is expressed on the surface of endothelial cells of the vasculature, and has major roles in inflammation, permeability, and angiogenesis. In addition, PECAM‐1 functions in cell signaling, cell‐cell adhesion, and cell survival. However, its physiological role in the pathophysiology of diabetic retinopathy (DR), the leading cause of blindness among working age adults worldwide, has not yet been investigated. We have previously found a significant decrease (50–90%) in retinal PECAM‐1 levels in a diabetic model of type I diabetes in rats.DESIGN AND METHODSFor our animal studies, we injected age‐matched male Wistar rats with either vehicle (control) or streptozotocin (STZ, diabetic), with the latter at 30 mg/kg/day for three consecutive days to elicit a model of type I diabetes. For our in vitro studies, primary rat retinal microvascular endothelial cells (RRMECs) were grown in either normal glucose (NG, 5mM) or high glucose (HG, 25 mM) media, and treated for 6 days with 0 or 10% plasma collected from control or diabetic rats 8 weeks post‐injection. PECAM‐1, and MMP levels or activity, were assessed using immunofluorescence staining, western blotting, or zymography.RESULTSFor RRMECs grown in NG conditions, treatment with 10% diabetic plasma resulted in significantly lower levels of PECAM‐1 than treatment with 10% control plasma (27%, p<0.05). PECAM‐1 loss was further exaggerated when cells were grown under HG conditions and treated with 10% diabetic plasma (74%, p<0.01). In contrast, control plasma added to RRMECs in NG conditions increased the expression of PECAM‐1 in a dose‐dependent manner. These findings indicate that normal plasma contributes to the expression of PECAM‐1, but some factor(s) in diabetic plasma has the reverse effect. MMPs, which are calcium‐dependent zinc‐containing endopeptidases, are known to cleave PECAM‐1. Plasma samples from diabetic rats revealed a significant increase in MMP‐2 levels (2‐fold, p<0.05), and a significant increase in MMP‐7 activity (p<0.01). Moreover, RRMECs showed a significant increase in MMP2 levels (12‐fold, p<0.01) when grown in HG media.CONCLUSIONThese results indicate a possible role for MMPs in the diabetes‐induced loss of PECAM‐1. Further studies will be conducted to investigate the potential benefits of protecting the retinal expression of PECAM‐1 by targeting the actions of MMPs.Support or Funding InformationNIH RO1 EY017599