Recombinant human transforming growth factor β1 modulates bone remodeling in a mineralizing bone organ culture

TGF-beta 1 increases cell proliferation and collagen synthesis in osteoblast-like cells and bone organ cultures. However, the effects of TGF-beta 1 on bone resorption remain contradictory. Therefore, the exact role that this growth factor plays in the process of bone remodeling is still not clear. We studied the effects of recombinant human TGF-beta 1 (rhTGF-beta 1) on bone formation and resorption in a mineralizing bone organ culture system. Parietal bones from 20-day-old fetal rat calvariae were cultured up to 7 days in serum-free BGJb medium. They responded to a 1 day pulse or continuous treatment of rhTGF-beta 1 with dose-dependent increases in dry weight, [3H]thymidine ([3H]TdR) incorporation, and collagen synthesis. In contrast, rhTGF-beta 1 reduced the calcium content of the bones. This is not due to increased bone resorption but rather to failure of calcium deposition. The following responses occurred at 1 nM rhTGF-beta 1. Dry weight was increased 25-50% after 6 days in culture. DNA synthesis was increased to a maximum at day 1, reaching twofold of the control level. Adding hydroxyurea at day 0 reduced [3H]TdR incorporation in rhTGF-beta 1 treated bones to 20% of the control and indomethacin abrogated the increase in [3H]TdR stimulated by rhTGF-beta 1 to the control level. Both treatments completely blocked the increase in dry weight induced by rhTGF-beta 1 at day 6. rhTGF-beta 1 stimulated collagen synthesis to reach its maximum at day 2, with a twofold increase in [3H]proline incorporation. Basal alkaline phosphatase activity fell continuously in culture, reaching 35% of day 0 level at day 6. Enzyme activity was not altered by rhTGF-beta 1. Morphologic observations by light and electron microscopy confirmed these findings. In summary, rhTGF-beta 1 altered bone remodeling by increasing organic components and decreasing calcification in a mineralizing bone organ culture system.