Effect of lipoprotein (a) on annexin A5 binding to cell membrane

Abstract

Background High blood lipoprotein (a) [Lp(a)] concentration is a risk factor for a thrombotic event. Annexin A5 is involved in anticoagulation on the endothelial surface. How Lp(a) affects the annexin A5 function is not clear. This study investigates annexin A5 binding on the cell membrane in the presence of Lp(a). Methods Lp(a) was isolated from human blood plasma by ultracentrifugation and annexin A5 protein was purchased commercially. The cell membrane was prepared from primary human umbilical vein endothelial cells (HUVEC) and cultured cell line HepG2 by sucrose density gradient centrifugation. Enzyme-linked immunosorbent assays (ELISA) were used to examine annexin A5 binding to the cell membrane in the presence of Lp(a). Flow cytometry was used to analyze the binding of fluorescence-labeled annexin A5 to phosphatidylserine (PS)-translocated intact cells in the presence of Lp(a). Results Annexin A5 binding to the cell membrane was attenuated by a high concentration of Lp(a) in both HUVEC and HepG2 membrane surfaces. The phenomenon was also observed with annexin A5 surface labeling of HepG2 cells and flow cytometry analysis. Conclusions The results imply that Lp(a) interferes with annexin A5 binding to the procoagulant PS which translocates to the membrane surface under stress condition and therefore may increase the risk for thrombosis.