Fetuin-A uptake in bovine vascular smooth muscle cells is calcium dependent and mediated by annexins

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Abstract
Fetuin-A is a known inhibitor of vascular calcification in vitro. In arteries with calcification, there is increased immunostaining for fetuin-A. However, vascular smooth muscle cells (VSMC) do not synthesize fetuin-A, suggesting fetuin-A may be endocytosed to exert its inhibitory effects. To examine the mechanism by which fetuin-A is taken up in bovine VSMC (BVSMC), we examined living cells by confocal microscopy and determined the uptake of Cy5-labeled fetuin-A. The results demonstrated that fetuin-A was taken up in BVSMC only in the presence of extracellular calcium, whereas phosphorus had no effect. Additional studies demonstrated the calcium-dependent uptake was specific for fetuin-A and only observed in BVSMC and osteoblasts, but not epithelial, endothelial, or adipose cells. The uptake was dose dependent, but could not be inhibited by excess unlabeled fetuin-A, suggesting a fluid phase rather than a receptor-mediated process. Fetuin-A also induced a sustained increase in intracellular calcium in BVSMC in the presence of extracellular calcium, whereas there was no increase in the absence of extracellular calcium. To further characterize the uptake, we utilized an inhibitor of annexin calcium channel activity, demonstrating inhibition of both fetuin-A uptake and intracellular calcium increase. Finally, we demonstrate that fetuin-A binds to annexin II at the cell membrane of BVSMC. In summary, our study demonstrates calcium- and annexin-dependent uptake of fetuin-A that leads to a sustained rise in intracellular calcium. This regulated uptake may be a mechanism by which fetuin-A inhibits VSMC calcification in the presence of excess calcium.