Fibrinolytic proteins in apoptotic human umbilical vein endothelial cells

Endothelial cells express fibrinolytic proteins including: urokinase (u-PA) and tissue type (t-PA) plasminogen activators, type-1 (PAI-1) and 2 (PAI-2) plasminogen activator inhibitors, and u-PA receptor (u-PAR). Apoptotic endothelial cells detach, potentially forming both local and circulating microthrombi in vivo. In this article, apoptotic human umbilical vein endothelium was obtained by serum starvation and compared with nonapoptotic cells rescued from death with fresh medium containing serum. Antigen levels for t-PA, PAI-1, PAI-2, and u-PAR were reduced greatly in apoptosis (p< 0.05). In contrast, u-PA levels were similar in apoptotic as compared with rescued cells (p<0.05). Radioactive amino acids were used to determine absolute levels of protein synthesis and degradation in these cells. Reduced antigen levels likely were due to proteolysis as there was 98% total protein degradation and very little protein synthesis in apoptotic endothelial cells. Also, u-PA levels in apoptotic endothelial cells were not affected by the protein synthesis inhibitor cycloheximide. Endothelial cells in inflammatory sites are exposed to cytokines, which increase both apoptosis and u-PA levels. Data from this article support the idea that maintained u-PA levels in apoptotic endothelium may protect from micro-thrombosis in inflammatory sites as well as in the circulation.