Mechanism of action of ?-glycerophosphate on bone cell mineralization

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Abstract
Experiments were performed to determine whether β-glycerophosphate (β-GP) promoted mineralization in vitro by modulating bone cell metabolic activity and/or serving as a local source of inorganic phosphate ions (Pi). Using MC3T3-E1, ROS 17/2.8, and chick osteoblast-like cells in the presence of β-GP or Pi, we examined mineral formation, lactate generation, alkaline phosphatase (AP) activity, and protein and phospholipid synthesis. Neither β-GP nor Pi modulated any of the major biosynthetic activities of the bone cells. Thus, we found no change in the levels of phospholipids, and the total protein concentration remained constant. Measurement of lactate synthesis showed that β-GP did not effect the rate of anaerobic glycolysis. Evaluation of medium Pi levels clearly indicated that β-GP was hydrolyzed by bone cells; within 24 hours, almost 80% of 10 mM β-GP was hydrolyzed. It is likely that this local increase in medium Pi concentration promoted rapid mineral deposition. Chemical, energy dispersive X-ray, and Fourier transform infrared analysis of the mineral formed in the presence of β-GP showed that it was nonapatitic; moreover, mineral particles were also seen in the culture medium itself. Experiments performed with a cell-free system indicated that mineral particles formed spontaneously in the presence of AP and β-GP and were deposited into a collagen matrix. We conclude that medium supplementation with β-GP or Pi should not exceed 2 mM. If this value is exceeded, then there will be nonphysiological mineral deposition in the bone cell culture.