Smad4 controls bone homeostasis through regulation of osteoblast/osteocyte viability

Regulation of osteoblast and osteocyte viability is essential for bone homeostasis. Smad4, a major transducer of bone morphogenetic protein and transforming growth factor-β signaling pathways, regulates apoptosis in various cell types through a mitochondrial pathway. However, it remains poorly understood whether Smad4 is necessary for the regulation of osteoblast and osteocyte viability. In this study, we analyzed Smad4ΔOs mice, in which Smad4 was subjected to tissue-specific disruption under the control of the 2.3-kb Col1a1 promoter, to understand the functional significance of Smad4 in regulating osteoblast/osteocyte viability during bone formation and remodeling. Smad4ΔOs mice showed a significant increase in osteoblast number and osteocyte density in the trabecular and cortical regions of the femur, whereas osteoclast activity was significantly decreased. The proliferation of osteoblasts/osteocytes did not alter, as shown by measuring 5′-bromo-2′deoxyuridine incorporation. By contrast, the percentage of TUNEL-positive cells decreased, together with a decrease in the Bax/Bcl-2 ratio and in the proteolytic cleavage of caspase 3, in Smad4ΔOs mice. Apoptosis in isolated calvaria cells from Smad4ΔOs mice decreased after differentiation, which was consistent with the results of the TUNEL assay and western blotting in Smad4ΔOs mice. Conversely, osteoblast cells overexpressing Smad4 showed increased apoptosis. In an apoptosis induction model of Smad4ΔOs mice, osteoblasts/osteocytes were more resistant to apoptosis than were control cells, and, consequently, bone remodeling was attenuated. These findings indicate that Smad4 has a significant role in regulating osteoblast/osteocyte viability and therefore controls bone homeostasis. A protein called Smad4 plays a significant role in regulating viability of and balance between cells that build and maintain healthy bones. Making bones requires regulated cooperation among different cells. Osteoblast cells in the bone marrow produce proteins that form the intercellular ‘matrix’ that holds bone cells together. Osteoclast cells break down bone tissue when necessary for maintenance, remodeling and repair. Smad4 was known to control the activity of several proteins and signaling pathways involved in bone formation. Eui-Sic Cho and co-workers at Chonbuk National University in South Korea have now further clarified the role of Smad4 in osteoblasts and osteoclasts. Investigating the effects of controlled disruption of Smad4 activity in mice confirmed that the protein participates in maintaining viable and suitably balanced populations of these specific types of cell.