Beta-glycerophosphate accelerates calcification in cultured bovine vascular smooth muscle cells

Calcification is a common feature of advanced atherosclerotic lesions and is being reemphasized as a clinically significant element of vascular disease. However, the scarcity of in vitro models of vascular calcification preclude studying its molecular and cellular mechanism. In the present study, we describe an in vitro calcification in which diffuse calcification can be induced by culturing bovine vascular smooth muscle cells (BVSMC) in the presence of beta-glycerophosphate, ascorbic acid, and insulin in a manner analogous to in vitro mineralization by osteoblasts. Calcification was confirmed by von Kossa staining and 45Ca accumulation. Factor analysis revealed that beta-glycerophosphate is the most important factor for this calcification process, suggesting that alkaline phosphatase (ALP) may be involved. As predicted, high levels of ALP expression were detected by ALP assay and Northern blot analysis. Functional significance of ALP was confirmed by demonstrating that levamisole, a specific inhibitor of ALP, inhibited BVSMC calcification in a dose-dependent manner. Bisphosphonates such as etidronate and pamidronate potently inhibited BVSMC calcification, suggesting that hydroxyapatite formation may be involved. Importantly, expression of osteopontin mRNA was dramatically increased in calcified BVSMC compared with uncalcified control cells. These data suggest that beta-glycerophosphate can induce diffuse calcification by an ALP-dependent mechanism and that this in vitro calcification system is useful for analyzing the molecular and cellular mechanisms of vascular calcification.