Regulation of Matrix Vesicle Metabolism by Vitamin D Metabolites

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Abstract
Matrix vesicles are membrane organelles found in the extracellular matrix of calcifying cells. Vitamin D-responsive alkaline phosphatase specific activity has been localized to matrix vesicles in chondrocyte and osteoblast cultures. The effect of hormone is both metabolite and cell specific. Alkaline phosphatase in matrix vesicles produced by resting zone chondrocytes is stimulated by 24,25(OH)2D3 whereas alkaline phosphatase in matrix vesicles produced by growth zone chondrocytes is responsive to 1,25(OH)2D3. However, mesenchymal cell cultures, which exhibit a chondrogenic phenotype when exposed to bone inductive proteins in vitro, produce vesicles with alkaline phosphatase activity that is unaffected by either 1,25(OH)2D3 or 24,25(OH)2D3. Incorporation and release of arachidonic acid into phosphatidylethanolamine is also differentially regulated by 1,25(OH)2D3 and 24,25(OH)2D3 in chondrocytes. These data suggest that vitamin D metabolites may regulate endochondral ossification by altering matrix vesicle enzyme activities, perhaps through changes in membrane phospholipid metabolism.