STABLE TRANSFECTION OF NONOSTEOGENIC CELL-LINES WITH TISSUE NONSPECIFIC ALKALINE-PHOSPHATASE ENHANCES MINERAL DEPOSITION BOTH IN THE PRESENCE AND ABSENCE OF BETA-GLYCEROPHOSPHATE - POSSIBLE ROLE FOR ALKALINE-PHOSPHATASE IN PATHOLOGICAL MINERALIZATION

It is documented that alkaline phosphatase (AP) plays an important rol e in bone mineralization. Considering that TN-AP is expressed in perio dontal ligament fibroblasts, renal epithelial cells, and vascular endo thelial cells, and that TN-AP is both a calcium-/phosphate-binding pro tein and a phosphohydrolytic enzyme, we hypothesize that membrane-boun d AP also plays an important role in the initiation of physiological a nd pathological mineralizations in tissues other than bone and cartila ge. To test this hypothesis, nonosteoblast cell lines, including a fib roblast line, a renal epithelial line, and a capillary endothelial lin e, were stably transfected to express high levels of rat bone AP on th eir cell surfaces. These rat bone AP-expressing cells were then cultur ed on filter membranes in the presence or absence of beta-glycerol pho sphate. von Kossa staining for calcium phosphate and transmission elec tron microscopy with electron diffraction analysis for minerals were e mployed to investigate the effect of membrane AP on extracellular calc ium phosphate mineralization. Our results indicated that AP expression on these nonosteoblast-like cell surfaces have induced extracellular hydroxyapatite (HAP) mineralization. Our findings support the concept that membrane-bound AP contributes to extracellular apatitic mineraliz ation by mechanisms that do not necessarily involve its hydrolase acti vity. They also suggest that AP might be important for the initiation of pathological mineralization in nonosteogenic tissues.