Aluminum accelerates osteoblastic differentiation but is cytotoxic in long-term rat calvaria cell cultures

We have examined the effects of aluminum (Al) on osteoprogenitor proliferation and differentiation, cell survival, and bone formation in long-term rat calvaria (RC) cell cultures. RC cells were grown in alpha minimal essential medium containing 10% fetal bovine serum, 50 mu g/ml ascorbic acid, and 10 mM beta-glycerophosphate with or without Al added to final concentrations of 1 mu M-1 mM. Al caused a dose-dependent increase in the number of bone nodules present at early times (day II) but had no significant effect on nodule numbers at later times (day 17). Time course experiments showed that Al increased nodule number beginning from day 7. Alkaline phosphatase activity, assessed at four stages during the differentiation sequence of RC cell cultures (from 4 to 13 days) was stimulated by Al at all times. However, Al decreased colony formation, inhibited cell growth in late log phase, and decreased saturation density of the treated cultures. Al concentrations of 30 mu M and above resulted in degeneration of the cell layer and an increasing fibrillar appearance of the matrix present in between or adjacent to nodules when cultures were maintained for more than 15 days. The presence of Al significantly decreased the viability of cells obtained from 13-17 days cultures, as determined by plating efficiency and trypan blue exclusion. We frequently observed cellular toxicity (in 8 of 10 experiments) in cultures containing 300 mu M Al, and by days 17-19, cells, nodules, and matrix were disintegrating in these cultures. We conclude that Al accelerates the rate of osteoprogenitor cell differentiation and the formation of bone nodules while concomitantly inhibiting nodule mineralization. However, concentrations that accelerate differentiation appear to be cytotoxic in long-term cultures.