Diabetes causes endothelial dysfunction, which is the initial trigger for vascular complications in diabetic patients. Hyperglycemia initiates a cascade of events that alters protein expression and secretion by endothelial cells. Tissue transglutaminase-2 (tTG2) is an enzyme that under physiologic conditions is sequestered inside the endothelial cell, but under pathologic conditions causing decreased bioavailability of nitric oxide, tTG2 is secreted, activated, and catalyzes irreversible crosslinking of proteins in the extracellular matrix (ECM). Exendin-4 (Ex-4) is a glucagon-like peptide-1 receptor (GLP-1R) agonist, used in the treatment of type 2 diabetes, which has vasculo-protective effects. We hypothesized that hyperglycemic stress would induce secretion of tTG2, and that this effect would be attenuated by Ex-4. Mouse cardiac microvascular endothelial cells (MCECs) were serum-starved and exposed to control (5.5 mM glucose) or hyperglycemic (25 mM glucose) conditions, with or without Ex-4 (10 nM) x 72 hrs. Proteins from conditioned media were isolated, trypsinized, and analyzed using LC-MS/MS (LTQ Orbitrap Velos). Immunoblots from cell homogenate were probed for tTG protein expression. Conditioned media from MCECs exposed to high-glucose but not Ex-4 contained tTG2, which was absent in media from cells exposed to high-glucose and Ex-4, as well as in media from control cells, suggesting that Ex-4 prevented the secretion of tTG2 induced by hyperglycemic stress. Protein expression in cell lysate was not different. These findings may have important implications for the etiology of diabetic vascular complications, and for the role of Ex-4 to prevent the pathologic ECM remodeling associated with diabetic vasculopathy. Further studies are ongoing to determine the mechanisms of glucose-induced secretion of tTG2, as well as the mechanisms by which Ex-4 prevents this effect.
Acute stimulation of glucose transport by histamine in cardiac microvascular endothelial cells
