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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2006;26:1985.)
© 2006 American Heart Association, Inc.

Vascular Biology

Endocytosis of Extracellular Superoxide Dismutase Into Endothelial Cells

Role of the Heparin-Binding Domain

Yi Chu; Robert Piper; Simon Richardson; Yoshimasa Watanabe; Pragnesh Patel; Donald D. Heistad

From the Cardiovascular Center and Departments of Internal Medicine (Y.C., Y.W., P.P., D.D.H.), Physiology and Biophysics (R.P., S.R.), and Pharmacology (D.D.H.), University of Iowa, Roy J. and Lucille A. Carver College of Medicine, and the VA Medical Center (D.D.H.), Iowa City, Iowa.

Correspondence to Donald D. Heistad, Department of Internal Medicine, University of Iowa Carver College of Medicine, Iowa City, IA 52242. E-mail donald-heistad{at}uiowa.edu

Objective— Extracellular superoxide dismutase (EC-SOD) is a secreted antioxidant enzyme that binds to the outer plasma membrane and extracellular matrix through its heparin-binding domain (HBD). Carriers of a common genetic variant of EC-SOD (EC-SOD_R213G, within the HBD) have higher plasma concentration of EC-SOD and increased risk for vascular disease. In the present study, we used confocal fluorescence microscopy to examine mechanisms of endocytosis of EC-SOD to determine whether EC-SOD translocates to the nucleus of endothelial cells, and to test the hypothesis that EC-SOD, but not EC-SOD_R213G, is endocytosed into endothelial cells.

Methods and Results— Mouse endothelial cells (MS-1) were incubated with EC-SOD, EC-SOD_R213G, or HBD-deleted EC-SOD (EC-SODHBD). Binding to MS-1 was observed only with EC-SOD, but not EC-SOD_R213G or EC-SODHBD. Endocytosis of EC-SODs was monitored after coincubation of MS-1 cells with EC-SODs and BSA-Texas Red (BSA-TR), which marks endosomes and lysosomes. Only EC-SOD was endocytosed, colocalizing with BSA-TR. EC-SOD also colocalized with early endosome antigen 1 (EEA-1), a specific marker for endocytosis. Endocytosis of EC-SOD was inhibited by chlorpromazine, but not by methyl-ß-cyclodextrin or nystatin, which suggests that endocytosis of EC-SOD is mediated by clathrin but not by caveolae. Minimal or no localization of EC-SOD in the nucleus of MS-1 cells was detected.

Conclusions— Our findings indicate that EC-SOD, but not EC-SOD_R213G, is endocytosed into endothelial cells through clathrin-mediated pathway, but does not translocate to the nucleus. We speculate that impairment of endocytosis may contribute to high plasma levels of EC-SOD_R213G in R213G carriers.

Mechanisms of endocytosis of EC-SOD were examined. The findings indicate that EC-SOD, but not the gene variant EC-SOD_R213G, is endocytosed into endothelial cells through clathrin-mediated pathway, but does not translocate to the nucleus. Impairment of endocytosis may contribute to high plasma levels of EC-SOD_R213G.

Key Words: antioxidant enzyme • superoxide dismutase • endothelium • endocytosis