Regulation of osteogenic differentiation of human bone marrow stromal cells: Interaction between transforming growth factor-beta and 1,25(OH)(2) vitamin D-3 in vitro

Bone marrow stromal cells are believed to play a major role in bone formati on as a major source of osteoprogenitor cells, however, very little is know n about how the osteogenic differentiation of these cells is regulated by s ystemic hormones and local growth factors. We examined the effects of TGF-b eta and its interaction with 1,25(OH)(2) Vitamin D-3 [1,25(OH)(2)D-3] on th e differentiation and proliferation of human bone marrow stromal cells (hBM SC) in secondary cultures. Alkaline phosphatase (ALP) activity was inhibite d by TGF-beta (0.1-10 ng/ml) and increased by 1,25(OH)(2)D-3 (50 nM), howev er, co-treatment of TGF-beta and 1,25(OH)(2)D-3 synergistically enhanced AL P activity with maximal stimulation occurring at about 8 days after treatme nt. This synergistic effect was independent of proliferation because, in co ntrast to TGF-beta alone, combined treatment with TGF-beta and 1,25(OH)(2)D -3 had no effect on hBMSC proliferation. As no synergistic effect was seen with combinations of 1,25(OH)(2)D-3 and other osteotrophic growth factors, including BMP-2, IGF-I, and basic fibroblast growth factor (bFGF), it would seem likely that the synergistic interaction is specific for TGF-beta. The increased ALP activity was due to an enhancement of 1,25(OH)(2)D-3-induced ALP activity by TGF-beta, rather than vice versa. In contrast, TGF-beta in hibited 1,25(OH)(2)D-3-induced osteocalcin production. Taken together, thes e results indicate that TGF-beta and 1,25(OH)(2)D-3 act synergistically to stimulate the recruitment of BMSC to the osteoblast lineage. This interacti on may play an important role in bone remodeling.