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

We have examined the effects of aluminum (Al) on osteoprogenitor proliferat ion and differentiation, cell survival, and bone formation in long-term rat calvaria (RC) cell cultures. RC cells were grown in alpha minimal essentia l medium containing 10% fetal bovine serum, 50 mu g/ml ascorbic acid, and 1 0 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 no dule numbers at later times (day 17). Time course experiments showed that A l increased nodule number beginning from day 7. Alkaline phosphatase activi ty, 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 d ecreased saturation density of the treated cultures. Al concentrations of 3 0 mu M and above resulted in degeneration of the cell layer and an increasi ng fibrillar appearance of the matrix present in between or adjacent to nod ules when cultures were maintained for more than 15 days. The presence of A l significantly decreased the viability of cells obtained from 13-17 days c ultures, 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 w hile concomitantly inhibiting nodule mineralization. However, concentration s that accelerate differentiation appear to be cytotoxic in long-term cultu res.