mTOR Signaling and Bone Formation in Aging Skeleton

描述(申请人提供)：与年龄相关的脂毒性被定义为毒性脂类中间体的积累，如脂肪酰辅酶A、神经酰胺和二酰甘油，它们可以抑制 组织功能的各个方面。这些代谢物是由于脂肪酸生物合成增加，脂肪酸氧化减少，和/或特定中间体生物合成增加或特定中间体清除减少而产生的。人们发现，老化的骨骼以矿化骨骼为代价积累了更多的脂肪。已发表的数据以及我们的初步数据表明，BMSC体外成骨分化和体内新骨形成对BMP对老年小鼠的刺激作用具有抵抗力。此外，我们还发现，与年轻小鼠相比，BMP-7诱导的老年小鼠异位骨中Fas水平升高。此外，我们观察到mTOR抑制剂雷帕霉素(RapA)抑制BMP-7诱导的幼年动物成骨细胞的成骨和脂肪生成。我们假设有毒的脂类在老化的骨骼中积累，阻断这些脂类的积累将恢复BMP-7诱导的成骨细胞分化。我们还假设，mTOR决定了骨细胞成骨和脂肪积累之间的平衡，这种平衡可能会向脂肪积累转移，而不是以老化骨的骨形成为代价。为了验证这些假设，我们将首先利用骨髓中的骨祖细胞进行体外培养。 通过对年轻、中年和老年小鼠的头盖骨和头盖骨的研究，确定脂质中间体是否在老年动物的细胞中更大程度地积累，并确定旨在直接减少脂质积累和抑制mTOR的药理学和分子遗传学干预是否将恢复基础成骨细胞分化和BMP-7诱导的成骨细胞分化。在目标2中，我们将利用BMP-7诱导的异位新骨形成的体内模型来确定直接抑制脂质堆积是否会恢复BMP-7促进最佳新骨形成的能力 衰老小鼠的骨形成。这项工作的结果将提供到目前为止尚未获得的数据，说明与年龄相关的骨骼脂肪毒性的机制，以及阻止有毒脂肪积累的动作是否可以恢复骨形成。这些结果将为未来旨在降低脂毒性的骨质疏松症治疗药物的开发提供必要的基础。 公共卫生相关性：骨质疏松症是老年人口中的一种严重疾病，其特征是由于骨形成速率不足而导致骨骼减弱，最近被认为是由于脂肪在老化的骨骼中积累所致。本项目将确定骨中毒性脂质堆积的原因和后果，雷帕霉素可抑制的mTOR途径是否具有拮抗作用，以增加骨骼中矿化骨和脂肪的形成，以及是否可以阻断该途径中的脂肪形成成分，以保护和优化其成骨作用。成功的结果将为实际治疗铺平道路，旨在最大限度地减少mTOR途径的脂肪形成作用，最大化其骨形成作用。

DESCRIPTION (provided by applicant): Age-related lipotoxicity is defined as the accrual of toxic lipid intermediates such as fatty acyl CoA, ceramide, and diacylglycerol which act to inhibit aspects of tissue function. These metabolites accrue due to increased fatty acid biosynthesis, decreased fatty acid oxidation, and/or increased biosynthesis of the specific intermediate or decreased clearance of the specific intermediate. Aging skeleton has been found to accumulate greater amounts of lipid at the expense of mineralizing bone. Published data as well as our preliminary data indicated BMSC osteoblastic differentiation in vitro and new bone formation in vivo are resistant to stimulatory effects of BMP in older mice. Additionally, we have found that BMP-7-induced ectopic bone in old mice contained elevated levels of FAS compared to young mice. Moreover, we observed that the mTOR inhibitor rapamycin (RAPA) inhibits BMP-7-induced osteogenesis and lipogenesis in cultured osteoblast cells from young animals. We hypothesize that toxic lipids accrue in aged bone and that blockade in the accrual of these lipids will restore BMP-7-induced osteoblast differentiation. We also hypothesize that mTOR determines the balance between osteogenesis and lipid accrual in bone cells, and that this balance may shift towards lipid accrual at the expense of bone formation in aging bone. To test these hypotheses, we will first utilize in vitro cultures of osteoprogenitor cells from bone marrow and calvariae of young, middle aged, and old mice to determine whether lipid intermediates accrue to a greater degree in cells from the older animals and to determine whether pharmacologic and molecular genetic interventions designed to directly decrease lipid accrual and to inhibit mTOR will restore basal and BMP-7-induced osteoblast differentiation. In Aim 2, we will utilize an in vivo model of BMP-7-induced ectopic new bone formation to determine whether direct inhibition of lipid accrual will restore the ability of BMP-7 to promote optimal new bone formation in aging mice. Outcome of this work will provide heretofore unavailable data that will illustrate mechanisms of age-related skeletal lipotoxicity, and whether maneuvers which block toxic lipid accrual can restore bone formation. These outcomes will provide the necessary basis for the development of future therapeutics for the treatment of osteoporosis aimed at reducing lipotoxicity. PUBLIC HEALTH RELEVANCE: Osteoporosis is a serious disease in the aging population characterized by bone weakening as the result of insufficient bone formation rates, which has recently been attributed to fat accumulating in aging bone. This project will determine the causes and consequences of toxic lipid accrual in bone, whether the rapamycin-inhibitable mTOR pathway exerts antagonistic effects to increase both formation of mineralized bone and fat in the skeleton, and whether the fat forming component of this pathway can be blocked in order to preserve and optimize its bone forming action. Successful outcomes will pave the way for practical therapies designed to minimize the fat forming action of the mTOR pathway and maximize its bone forming actions.