Expression of Class A Scavenger Receptor Is Enhanced by High Glucose in Vitro and under Diabetic Conditions in Vivo

In the early stage of atherosclerosis, macrophages take up chemically modified low density lipoproteins (LDL) through the scavenger receptors, leading to foam cell formation in atherosclerotic lesions. To get insight into a role of the scavenger receptors in diabetes-enhanced atherosclerotic complications, the effects on class A scavenger receptor (SR-A) of high glucose exposure in vitro as well as the diabetic conditions in vivo were determined in the present study. The in vitro experiments demonstrated that high glucose exposure to human monocyte-derived macrophages led to an increased SR-A expression with a concomitant increase in the endocytic uptake of acetylated LDL and oxidized LDL. The endocytic process was significantly suppressed by an anti-SR-A neutralizing antibody. Stability analyses revealed a significant increased stability of SR-A at a mRNA level but not a protein level, indicating that high glucose-induced up-regulation of SR-A is due largely to increased stability of SR-A mRNA. High glucose-enhanced SR-A expression was prevented by protein kinase C and NAD(P)H oxidase inhibitors as well as antioxidants. High glucose-enhanced production of intracellular peroxides was visualized in these cells, which was attenuated by an antioxidant. The in vivo experiments demonstrated that peritoneal macrophages from streptozotocin-induced diabetic mice increased SR-A expression when compared with those from nondiabetic mice. Endocytic degradation of acetylated LDL and oxidized LDL were also increased with these macrophages but not with the corresponding macrophages from diabetic SR-A knock-out mice. These in vitro and in vivo results probably suggest that reactive oxygen species generated from a protein kinase C-dependent NAD(P)H oxidase pathway plays a role in the high glucose-induced up-regulation of SR-A, leading to the increased endocytic degradation of modified LDL for foam cell formation. This could be one mechanism for an increased rate of atherosclerosis in patients with diabetes.