Regulation of osteoblast and osteocyte viability is essential for bone homeostasis. Smad4, a major transducer of bone morphogenetic protein and transforming growth factor-¥â 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 Smad4¥ÄOs 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. Smad4¥ÄOs 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¡Ç-bromo-2¡Çdeoxyuridine 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 Smad4¥ÄOs mice. Apoptosis in isolated calvaria cells from Smad4¥ÄOs mice decreased after differentiation, which was consistent with the results of the TUNEL assay and western blotting in Smad4¥ÄOs mice. Conversely, osteoblast cells overexpressing Smad4 showed increased apoptosis. In an apoptosis induction model of Smad4¥ÄOs 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.
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