Abstract
In addition to increased differentiation of vascular smooth muscle cells into osteoblast-like phenotypes, the limited accumulation of osteoclasts in atherosclerotic plaques or their dysfunction may participate in potential mechanisms for vascular calcification. N-acetylglucosamine-1-phosphate transferase containing alpha and beta subunits (GNPTAB) is a transmembrane enzyme complex that mediates the vesicular transport of lysosomal hydrolases. GNPTAB may also regulate the biogenesis of lysosomal hydrolases from bone-marrow derived osteoclasts. In this study, the areas surrounding calcification in human atherosclerotic plaques contained high levels of GNPTAB and low levels of lysosomal hydrolases such as cathepsin K (CTSK) and tartrate-resistant acid phosphatase (TRAP), as demonstrated by immunohistochemistry and laser-capture microdissection-assisted mRNA expression analysis. We therefore hypothesized that GNPTAB secretion may suppress the release of CTSK and TRAP by vascular osteoclast-like cells, thus causing their dysfunction and reducing the resorption of calcification. We used human primary macrophages derived from peripheral blood mononuclear cells, an established osteoclast differentiation model. GNPTAB siRNA silencing accelerated the formation of functional osteoclasts as detected by increased secretion of CTSK and TRAP and increased their bone resorption activity as gauged by resorption pits assay. We concluded that high levels of GNPTAB inhibit secretion of lysosomal hydrolases in dysfunctional osteoclasts, thereby affecting their resorption potential in cardiovascular calcification.
Highlights
Previous studies have proposed several mechanisms for vascular calcification, including osteoblastic differentiation of smooth muscle cells (SMCs) [1,2,3], limited accumulation of osteoclasts and their dysfunction [4,5,6,7], loss of endogenous inhibitors of mineralization [8,9], cell death [10], and thermodynamic mechanisms via elevated calcium and phosphate levels [11]
The evidence has suggested the discrepancy between limited numbers of osteoclasts and abundance of macrophages, precursors of osteoclasts, in calcifying atherosclerotic plaques
Our histopathological observations showed that two major osteoclast lysosomal hydrolases, tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CTSK), had negligible expression within GNPTAB-immunoreactive cells, surrounding calcifying regions, indicating that GNPTAB may affect TRAP and CTSK
Summary
Previous studies have proposed several mechanisms for vascular calcification, including osteoblastic differentiation of smooth muscle cells (SMCs) [1,2,3], limited accumulation of osteoclasts and their dysfunction [4,5,6,7], loss of endogenous inhibitors of mineralization [8,9], cell death [10], and thermodynamic mechanisms via elevated calcium and phosphate levels [11]. To solubilize the mineral component of bone, osteoclasts secrete various enzymes such as tartrate-resistant acid phosphatase (TRAP) [15], cathepsin K (CTSK) [16], and carbonic anhydrase II [17]. TRAP, an osteoclast differentiation marker, regulates bone mineral matrix resorption [15]; CTSK cleaves collagen [16]; and carbonic anhydrase II facilitates proton production and maintains acidification of the resorption lacuna [17,18]
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