Transforming Growth Factor-β Controls Human Osteoclastogenesis through the p38 MAPK and Regulation of RANK Expression*

Abstract

Although RANK-L is essential for osteoclast formation, factors such as transforming growth factor-β (TGF-β) are potent modulators of osteoclastogenic stimuli. To systematically investigate the role of TGF-β in human osteoclastogenesis, monocytes were isolated from peripheral blood by three distinct approaches, resulting in either a lymphocyte-rich, a lymphocyte-poor, or a pure osteoclast precursor (CD14-positive) cell population. In each of these osteoclast precursor populations, the effect of TGF-β on proliferation, TRAP activity, and bone resorption was investigated with respect to time and length of exposure. When using the highly pure CD14 osteoclast precursor cell population, the effect of TGF-β was strongly dependent on the stage of osteoclast maturation. When monocytes were exposed to TGF-β during the initial culture period (days 1–7), TRAP activity and bone resorption were increased by 40%, whereas the cell number was reduced by 25%. A similar decrease in cell number was observed when TGF-β was present during the entire culture period (days 1–21), but in direct contrast, TRAP activity, cell fusion, cathepsin K, and matrix metalloproteinase (MMP)-9 expression as well as bone resorption were almost completely abrogated. Moreover, we found that latent TGF-β was strongly activated by incubation with MMP-9 and suggest this to be a highly relevant mechanism for regulating osteoclast activity. To further investigate the molecular mechanism responsible for the divergent effects of continuous versus discontinuous exposure to TGF-β, we examined RANK expression and p38 MAPK activation. We found the TGF-β strongly induced p38 MAPK in monocytes, but not in mature osteoclasts, and that continuous exposure of TGF-β to monocytes down-regulated RANK expression. The current results suggest that TGF-β promotes human osteoclastogenesis in monocytes through stimulation of the p38 MAPK, whereas continuous exposure to TGF-β abrogates osteoclastogenesis through down-regulation of RANK expression and therefore attenuation of RANK-RANK-L signaling.