The role of activin A in the human osteoblast cell cycle: a preliminary experimental in vitro study.

Animal studies have previously shown that activin A enhances osteoblast proliferation IN VITRO and increases bone formation and bone mechanical strength IN VIVO. For the further understanding of its action in human osteoblast, we studied the pattern of a cell cycle response to the treatment with activin A. We hypothesize that activin A alters the cell cycle pattern of human osteoblast. Primary cultures of human osteoblast-like cells were treated by activin A in a biologically effective concentration (100 ng/mL). The cells in cultured samples were counted, assayed for cellular alkaline phosphatase activity and calcitonin expression, LDH activity in the medium, cellular BrdU incorporation, cell cycle cytometry and compared to untreated controls. The treated by activin A cells responded by a significant shift toward the G1 phase of the cell cycle with parallel decrease in cell death rate (lower LDH activity and less necrotic cells in cytometric analysis). The treated cells also showed a lower alkaline phosphatase activity and calcitonin expression, indicating their undifferentiated state, and didn't change their proliferation rate. The number of cells in culture increased following treatment with activin A. We show that activin A increases the net osteoblast number in culture by reducing the cell death rate without affecting the cell proliferation. These findings should be part of cellular pathways that are involved in the initial stages of bone tissue generation.