Department of Orthopaedic Surgery, University of Toledo, College of Medicine and Life Sciences, 3000 Arlington Avenue, Toledo, OH 43614, USA; Center for Diabetes and Endocrine Research, University of Toledo, College of Medicine and Life Sciences, 3000 Arlington Avenue, Toledo, OH 43614, USA.
The Wertheim UF Scripps Institute, University of Florida, Jupiter, FL 33458, USA.
Mol Metab. 2024 Oct;88:102000. doi: 10.1016/j.molmet.2024.102000. Epub 2024 Jul 27.
The skeleton is one of the largest organs in the body, wherein metabolism is integrated with systemic energy metabolism. However, the bioenergetic programming of osteocytes, the most abundant bone cells coordinating bone metabolism, is not well defined. Here, using a mouse model with partial penetration of an osteocyte-specific PPARG deletion, we demonstrate that PPARG controls osteocyte bioenergetics and their contribution to systemic energy metabolism independently of circulating sclerostin levels, which were previously correlated with metabolic status of extramedullary fat depots.
In vivo and in vitro models of osteocyte-specific PPARG deletion, i.e. Dmp1Pparγ male and female mice (γOT) and MLO-Y4 osteocyte-like cells with either siRNA-silenced or CRISPR/Cas9-edited Pparγ. As applicable, the models were analyzed for levels of energy metabolism, glucose metabolism, and metabolic profile of extramedullary adipose tissue, as well as the osteocyte transcriptome, mitochondrial function, bioenergetics, insulin signaling, and oxidative stress.
Circulating sclerostin levels of γOT male and female mice were not different from control mice. Male γOT mice exhibited a high energy phenotype characterized by increased respiration, heat production, locomotion and food intake. This high energy phenotype in males did not correlate with "beiging" of peripheral adipose depots. However, both sexes showed a trend for reduced fat mass and apparent insulin resistance without changes in glucose tolerance, which correlated with decreased osteocytic responsiveness to insulin measured by AKT activation. The transcriptome of osteocytes isolated from γOT males suggested profound changes in cellular metabolism, fuel transport, mitochondria dysfunction, insulin signaling and increased oxidative stress. In MLO-Y4 osteocytes, PPARG deficiency correlated with highly active mitochondria, increased ATP production, and accumulation of reactive oxygen species (ROS).
PPARG in male osteocytes acts as a molecular break on mitochondrial function, and protection against oxidative stress and ROS accumulation. It also regulates osteocyte insulin signaling and fuel usage to produce energy. These data provide insight into the connection between osteocyte bioenergetics and their sex-specific contribution to the balance of systemic energy metabolism. These findings support the concept that the skeleton controls systemic energy expenditure via osteocyte metabolism.
骨骼是人体最大的器官之一，其中代谢与全身能量代谢相整合。然而，作为协调骨代谢的最丰富的骨细胞，成骨细胞的生物能量编程尚未得到很好的定义。在这里，我们使用一种成骨细胞特异性部分穿透性 PPARG 缺失的小鼠模型，证明 PPARG 独立于循环骨硬化蛋白水平控制成骨细胞的生物能量及其对全身能量代谢的贡献，而骨硬化蛋白水平先前与骨髓外脂肪沉积的代谢状态相关。
利用成骨细胞特异性 PPARG 缺失的体内和体外模型，即 Dmp1Pparγ 雄性和雌性小鼠（γOT）和具有 siRNA 沉默或 CRISPR/Cas9 编辑 Pparγ 的 MLO-Y4 成骨细胞样细胞。在适用的情况下，分析能量代谢、葡萄糖代谢和骨髓外脂肪组织代谢谱，以及成骨细胞转录组、线粒体功能、生物能量、胰岛素信号和氧化应激。
γOT 雄性和雌性小鼠的循环骨硬化蛋白水平与对照小鼠无差异。雄性 γOT 小鼠表现出高能量表型，其特征为呼吸、产热、运动和食物摄入增加。这种高能量表型与外周脂肪沉积的“褐变”无关。然而，两性均表现出脂肪量减少和明显的胰岛素抵抗，而葡萄糖耐量无变化，这与 AKT 激活测量的成骨细胞对胰岛素的反应性降低相关。从雄性 γOT 中分离出的成骨细胞的转录组表明细胞代谢、燃料转运、线粒体功能障碍、胰岛素信号和氧化应激增加发生了深刻变化。在 MLO-Y4 成骨细胞中，PPARG 缺乏与高度活跃的线粒体、增加的 ATP 产生和活性氧物质（ROS）的积累相关。
雄性成骨细胞中的 PPARG 作为线粒体功能的分子制动器，并防止氧化应激和 ROS 积累。它还调节成骨细胞的胰岛素信号和燃料利用以产生能量。这些数据提供了成骨细胞生物能量与其对全身能量代谢平衡的性别特异性贡献之间联系的深入了解。这些发现支持骨骼通过成骨细胞代谢控制全身能量消耗的概念。
