CHARACTERIZATION OF CHONDROCYTE ALKALINE-PHOSPHATASE AS A POTENTIAL MEDIATOR IN THE DISSOLUTION OF CALCIUM PYROPHOSPHATE DIHYDRATE CRYSTALS

Objective. To characterize chondrocyte alkaline phosphatase (ALP) dist ribution and expression in cartilage and monolayer cultures. Methods. Sections of bovine articular cartilage or chondrocyte monolayer cultur es were stained for ALP activity. Surface ALP was released with bacter ial phosphatidylinositol specific phospholipase C (PI-PLC). The levels of ALP mRNA were determined by Northern blot analysis using a cDNA pr obe to bovine ALP. Results. Chondrocyte ALP activity dissolves calcium pyrophosphate dihydrate (CPPD) crystals. About 5% of the total chondr ocytes contained ALP activity. The ALP positive cells were present onl y in the deep layers of articular cartilage adjacent to the subchondra l bone. Thirty to forty percent of the total ALP activity was present on the chondrocyte surface and was released by PI-PLC. Both chondrocyt e ALP activity and mRNA levels decreased with time in culture. However , continuous dexamethasone treatment stimulated the expression of ALP in ALP positive chondrocytes, sufficiently to replace all of the chond rocyte surface ALP released following PI-PLC treatment. Conclusion. Si nce ALP hydrolyzes pyrophosphate and dissolves CPPD crystals, our data suggest that regulation of chondrocyte ALP activity and expression in cartilage may prove useful for the development of a specific therapy CPPD arthropathy.