Secretion of osteopontin from MG-63 cells under a physiological level of mechanical strain in vitro--a [35S] incorporation approach.

To gain insight into the early response of osteoblastic cells to a physiological level of mechanical strain in vitro, the secretion of osteopontin by MG-63 osteosarcoma cells was assessed by [35S] incorporation and autoradiography. First, osteopontin secreted from MG-63 cells was immunolocalized at 60-64 kDa (Mr) by polyacrylamide gel electrophoresis. A uniform physiological level of strain was generated by a vacuum added to the convex side of a half-ball shaped silicon rubber membrane on which the cells were cultured on the concave side. After labelling proteins with [35S]-methionine/cysteine (147 microCi/ml), the membranes were exposed to a strain of 0.5 per cent (5000 microepsilon), 3 cycles/minute (sine wave) with 10 minutes on and off. At 1, 2 and 4 hours after strain, the supernatants were collected and analysed by 10 per cent sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography. The results showed that osteopontin was secreted by the strained cells at significantly higher amounts than the non-strained cells at all three time points (P < 0.05), with the first hour being the most prominent. A physiological level of mechanical strain increased the secretion of osteopontin from MG-63 cells in an early phase. This finding implies an accelerated process of bone remodelling, which suggests that the application of light and intermittent forces would result in the cellular reaction identified in relation to orthodontic tooth movement. The results indirectly indicate that the level of force presently used might be too high.