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Am J Physiol Renal Physiol (September 12, 2006). doi:10.1152/ajprenal.00303.2006
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Submitted on August 2, 2006
Accepted on September 8, 2006

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

Neal X Chen1*, Kalisha D O'Neill1, Xianming Chen1, Danxia Duan1, Exing Wang1, and Sharon M Moe2

1 Medicine/Nephrology, Indiana University, Indianapolis, Indiana, United States
2 Medicine/Nephrology, Indiana University, Indianapolis, Indiana, United States; Richard L. Roudebush VAMC, Indianapolis, Indiana, United States

* To whom correspondence should be addressed. E-mail: xuechen{at}iupui.edu.

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.




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M. Mizobuchi, D. Towler, and E. Slatopolsky
Vascular Calcification: The Killer of Patients with Chronic Kidney Disease
J. Am. Soc. Nephrol., July 1, 2009; 20(7): 1453 - 1464.
[Abstract] [Full Text] [PDF]




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