The Natural Protective Mechanism Against Hyperglycemia in Vascular Endothelial Cells

Abstract

OBJECTIVE—Vascular endothelial cells (VECs) downregulate their rate of glucose uptake in response to hyperglycemia by decreasing the expression of their typical glucose transporter GLUT-1. Hitherto, we discovered critical roles for the protein calreticulin and the arachidonic acid–metabolizing enzyme 12-lipoxygenase in this autoregulatory process. The hypothesis that 4-hydroxydodeca-(2E,6Z)-dienal (4-HDDE), the peroxidation product of 12-lipoxygenase, mediates this downregulatory mechanism by activating peroxisome proliferator–activated receptor (PPAR) was investigated. RESEARCH DESIGN AND METHODS—Effects of 4-HDDE and PPAR on the glucose transport system and calreticulin expression in primary bovine aortic endothelial cells were evaluated by pharmacological and molecular interventions. RESULTS—Using GW501516 (PPAR agonist) and GSK0660 (PPAR antagonist), we discovered that high-glucose–induced downregulation of the glucose transport system in VECs is mediated by PPAR. A PPAR-sensitive luciferase reporter assay in VECs revealed that high glucose markedly increased luciferase activity, while GSK0660 abolished it. High-performance liquid chromatography analysis showed that high-glucose incubation substantially elevated the generation of 4-HDDE in VECs. Treatment of VECs, exposed to normal glucose, with 4-HDDE mimicked high glucose and downregulated the glucose transport system and increased calreticulin expression. Like high glucose, 4-HDDE significantly activated PPAR in cells overexpressing human PPAR (hPPAR) but not hPPAR, -1, or -2. Moreover, silencing of PPAR prevented high-glucose–dependent alterations in GLUT-1 and calreticulin expression. Finally, specific binding of PPAR to a PPAR response element in the promoter region of the calreticulin gene was identified by utilizing a specific chromatin immunoprecipitation assay.