Senile osteoporosis can cause bone fragility and increased risk for fractures and has been one of the most prevalent and severe diseases affecting the elderly population worldwidely. The underlying mechanisms are currently intensive areas of investigation. In age-related bone loss, decreased bone formation overweighs increased bone resorption. The molecular mechanisms underlying defective bone formation in age-related bone loss are not completely understood. In particular, the specific role of histone acetylation in age-related bone loss has not been examined thoroughly. We employed 6- and 18-month-old mice to investigate the mechanisms of defective bone formation in age-related bone loss. Bone marrow stromal cells (BMSCs) were induced to undergo in vitro osteogenic differentiation. Chromatin immunoprecipitation (ChIP) was used to investigate the binding of histone deacetylases (HDACs) on Runx2 promoter in BMSCs. Luciferase reporter and transient transfection assay were employed to study Runx2 gene expression modulation by HDAC and androgen receptor (AR). siRNA and HDAC6 inhibitor, Tubastatin A, were used to inhibit HDAC6 in vitro. And systemic administration of Tubastatin A was used to block HDAC6 in vivo. Age-related trabecular bone loss was observed in 18-month-old mice compared with 6-month-old mice. In vitro osteogenic differentiation potential of BMSCs from 18-month-old mice was weaker than 6-month-old mice, in which there was Runx2 expression inactivation in BMSCs of 18-month-old mice compared with 6-month-old mice, which was attributable to HDAC6-mediated histone hypoacetylation in Runx2 promoter. There was competitive binding of HDAC6 and AR on Runx2 promoter to modulate Runx2 expression in BMSCs. More importantly, through siRNA- or specific inhibitor-mediated HDAC6 inhibition, we could activate Runx2 expression, rescue in vitro osteogenesis potential of BMSCs, and alleviate in vivo age-related bone loss of mice. HDAC6 accumulation and histone hypoacetylation on Runx2 promoter contributed to the attenuation of in vitro osteogenic differentiation potential of BMSCs from aged mice. Through HDAC6 inhibition, we could activate Runx2 expression and osteogenic differentiation potential of BMSCs from aged mice and alleviate the age-related bone loss of aged mice. Our study will benefit not only for understanding the age-related bone loss, but also for finding new therapies to treat senile osteoporosis.

中文翻译：

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HDAC6 使 Runx2 启动子失活以阻断小鼠年龄相关性骨丢失中骨髓基质细胞的成骨

老年性骨质疏松症可导致骨脆性和骨折风险增加，并且已成为影响全球老年人口的最普遍和最严重的疾病之一。潜在的机制目前是深入研究的领域。在与年龄相关的骨质流失中，骨形成的减少超过了骨吸收的增加。年龄相关性骨丢失中骨形成缺陷的分子机制尚不完全清楚。特别是，尚未彻底检查组蛋白乙酰化在与年龄相关的骨质流失中的具体作用。我们使用 6 个月和 18 个月大的小鼠来研究与年龄相关的骨质流失中骨形成缺陷的机制。诱导骨髓基质细胞（BMSCs）进行体外成骨分化。染色质免疫沉淀 (ChIP) 用于研究组蛋白去乙酰化酶 (HDAC) 与 BMSCs 中 Runx2 启动子的结合。荧光素酶报告基因和瞬时转染试验用于研究 HDAC 和雄激素受体 (AR) 对 Runx2 基因表达的调节作用。siRNA 和 HDAC6 抑制剂 Tubastatin A 用于体外抑制 HDAC6。并且全身给药 Tubastatin A 用于在体内阻断 HDAC6。与 6 个月大的小鼠相比，在 18 个月大的小鼠中观察到与年龄相关的骨小梁丢失。18月龄小鼠骨髓间充质干细胞体外成骨分化潜能弱于6月龄小鼠，其中18月龄小鼠骨髓间充质干细胞与6月龄小鼠相比存在Runx2表达失活。可归因于 Runx2 启动子中 HDAC6 介导的组蛋白低乙酰化。HDAC6 和 AR 在 Runx2 启动子上存在竞争性结合，以调节 BMSCs 中 Runx2 的表达。更重要的是，通过 siRNA 或特异性抑制剂介导的 HDAC6 抑制，我们可以激活 Runx2 表达，挽救 BMSCs 的体外成骨潜力，并减轻小鼠体内与年龄相关的骨丢失。Runx2 启动子上的 HDAC6 积累和组蛋白低乙酰化有助于减弱老年小鼠骨髓间充质干细胞的体外成骨分化潜能。通过抑制 HDAC6，我们可以激活老年小鼠骨髓间充质干细胞的 Runx2 表达和成骨分化潜能，并减轻老年小鼠与年龄相关的骨丢失。我们的研究不仅有利于了解与年龄相关的骨质流失，也有利于寻找治疗老年性骨质疏松症的新疗法。