Caveolin-1 in extracellular matrix vesicles secreted from osteoblasts

Caveolin-1 is an essential and signature protein of caveolae, which are small invaginations of the plasma membrane enriched in cholesterol and sphingolipids. Although high levels of expression of caveolin-1 have been demonstrated in osteoblasts as well as endothelial cells, fibroblasts, and muscular cells, the role of caveolin-1 in osteoblasts has not been clarified. Here, we show that caveolin-1 is secreted from osteoblasts in the form of matrix vesicles; extracellular vesicles released from the plasma membrane of osteoblasts. In this study, caveolae and matrix vesicles were similarly enriched in cholesterol and sphingomyelin in fractions isolated from mineralizing MC3T3-E1 cells. Interestingly, in the MC3T3-E1 cells caveolin-1 was enriched in the matrix vesicle fraction as well as the caveolar membrane fraction, and the amount of caveolin-1 in the matrix vesicle fraction increased as differentiation progressed. Localization of caveolin-1 in matrix vesicles was also confirmed in murine tibia. Furthermore, overexpression of caveolin-1 enhanced matrix calcification in MC3T3-E1 cells, whereas knockdown of caveolin-1 diminished it. These results suggest that secreted caveolin-1 as a component of matrix vesicles may play an important role in osteoblast calcification.