Hyperglycemia‐induced Ubiquitination and Degradation of β‐Catenin with the Loss of Platelet Endothelial Cell Adhesion Molecule‐1 (PECAM‐1) in Retinal Endothelial Cells
Authors: Eshaq Randa , Harris Norman 
Published: November 2023

OBJECTIVEThe aim of this study is to understand the molecular mechanisms leading to the breakdown of the blood‐retinal barrier (BRB) due to diabetic retinopathy (DR), and the role of PECAM‐1 diabetes‐induced loss in this process via the β‐catenin pathway.BACKGROUNDEarly events occurring in DR include altered vascular permeability leading to complications such as macular edema. Platelet endothelial cell adhesion molecule‐1 (PECAM‐1) is expressed heavily on the surface of endothelial cells of the vasculature, and has major roles in permeability, endothelial cell‐endothelial cell adhesion, and cell signaling. PECAM‐1 acts as a catenin reservoir by sequestering it to the plasma membrane. Moreover, the Src homology2 (SH2) domain‐containing protein‐tyrosine phosphatase, SHP‐2, associates with the cytoplasmic domain of PECAM‐1 as it becomes tyrosine‐phosphorylated. Activated SHP‐2 can then dephosphorylate β‐catenin, leading to β‐catenin stabilization and its binding to VE‐cadherin which maintains the adherens junction. Thus, the loss of PECAM‐1 may play a role in modulating β‐catenin phosphorylation levels, allowing it to be targeted for ubiquitination and subsequent degradation, which may lead to the disruption of the adherens junction via decreased VE‐cadherin/β‐catenin binding. We have previously found a significant decrease (50–90%, P<0.05) of PECAM‐1 levels in the diabetic retina, and in cultured rat retinal microvascular endothelial cells grown under high glucose conditions (30–40%, P<0.05), which might contribute to the increased vascular permeability in DR.DESIGN AND METHODSFor our in vitro studies, primary rat retinal microvascular endothelial cells (RRMECs) were grown in normal glucose (NG, 5 mM glucose) or high glucose (HG, 25 mM) media for six days. For our animal studies, age‐matched male Wistar rats were injected with either vehicle (control), or Streptozotocin (STZ, 30 mg/kg/day) for three consecutive days to establish a model of type I diabetes, with retinas collected eight weeks post‐STZ injections for analysis. PECAM‐1 and β‐catenin levels and interactions were assessed using western blotting, immunofluorescence, and immunoprecipitation assays.RESULTSβ‐catenin levels were significantly decreased in the diabetic retina by 55% (P<0.05), and in RRMECs grown under HG conditions by 32% (P<0.05), which was accompanied by a threefold increase in β‐catenin ubiquitination in RRMECs grown under HG condition (P<0.05). Moreover, PECAM‐1/β‐catenin binding was significantly decreased (50%, P<0.05) in RRMECs grown under HG conditions.CONCLUSIONThese results show a potential role of the diabetes‐induced loss of PECAM‐1 in the associated degradation of β‐catenin, possibly leading to the loss of the adherens junction. Further studies will be conducted to identify the specific mechanisms, and the possible benefits of protecting the diabetic retina from PECAM‐1 and β‐catenin loss.Support or Funding InformationNIH EY025632AHA 16PRE30080003