Acetylated low-density lipoprotein stimulates human vascular smooth muscle cell calcification by promoting osteoblastic differentiation and inhibiting phagocytosis

Abstract

Background: Vascular smooth muscle cells (VSMCs) in atherosclerotic lesions display an osteogenic phenotype, and calcification commonly occurs in association with lipid. We therefore tested the hypothesis that lipid components in atherosclerotic lesions influenced VSMC phenotype and calcification using an in vitro model of calcification.

Methods and results: In situ hybridization of human atherosclerotic plaques (n=10) collected from patients undergoing carotid endarterectomy demonstrated that subsets of lipid-filled VSMCs adjacent to sites of calcification expressed alkaline phosphatase, bone Gla protein, and bone sialoprotein, suggesting an osteogenic phenotype. Treatment of VSMCs in culture with acetylated low-density lipoprotein (acLDL) or lipoprotein-deficient serum altered the time course of bone-associated protein gene expression and calcification. AcLDL increased nodule calcification 3-fold, whereas lipoprotein-deficient serum significantly inhibited it. Reverse transcriptase-polymerase chain reaction and Western analysis demonstrated the presence of the acLDL receptor, SRA1, exclusively in calcifying nodular VSMCs, and blockade of SRA with polyinosinic acid inhibited acLDL-induced calcification. Because apoptotic bodies can serve as nucleation sites for calcification, we investigated whether acLDL could stimulate apoptosis in nodules. Apoptosis of nodular VSMCs was unaltered, but the number of apoptotic bodies per nodule increased approximately 3-fold, implying a defect in phagocytosis. Consistent with these observations, binding of apoptotic bodies to VSMCs was decreased in the presence of acLDL.

Conclusions: These studies suggest that modified lipoproteins stimulate calcification by enhancing osteogenic differentiation of VSMCs and by a novel mechanism whereby acLDL interacts with SRA on VSMCs and blocks phagocytic removal of apoptotic bodies.