Regulation of matrix vesicle phospholipid metabolism is cell maturation-dependent

We have developed a chondrocyte culture model for assessing the regulation of matrix vesicles at two different stages of chondrogenic maturation. These chondrocytes, resting zone (RC) and growth zone (GC), retain their phenotypic markers in culture, including production of matrix vesicles with distinctive lipid compositions and enzyme activities. Isolated matrix vesicles incubated in vitro with 1,25-(OH)2D3 (1,25) or 24,25-(OH)2D3 (24,25) respond differentially. 1,25 stimulates phospholipase A2 (PA2) in GC vesicles, but not on those from RC. 24,25 inhibits PA2 in RC vesicles, but has no effect on GC. PA2 activity is required for fatty acid turnover and is the rate-limiting step in prostaglandin production. Plasma membrane phospholipids are more susceptible to the release of arachidonic acid by PA2 than are matrix vesicle phospholipids. Matrix vesicles are distinct from the plasma membrane in terms of lipid composition and arachidonic acid incorporation. 1,25 and 24,25 stimulate arachidonic acid turnover in their target cells, but by different mechanisms. 1,25 has no effect on arachidonic acid turnover in RC; however, 24,25 inhibits turnover in RC and GC. 1,25 and 24,25 also affect isolated matrix vesicle membrane fluidity. These results suggest that vitamin D metabolites modulate PA2 activity, change the composition of membrane phospholipids by altering fatty acid composition, and affect calcium transport. The effects are mediated by altering membrane fluidity and is dependent on the stage of cell differentiation.