Role of FoxOs in Skeletal Homeostasis

描述(申请人提供)：WNT/2-连环蛋白信号对成骨细胞的形成和骨的形成是必不可少的。在导致这一应用的研究中，阐明了Wnt/2-catenin拮抗的一种新的机制，即激活FoxO转录因子，将有限的2-catenin池从Wnt诱导的TCF/Lef转录转移到FoxO介导的转录。这种骨骼中的拮抗级联反应是由活性氧物种(ROS)启动的，代表着氧化应激的中介和防御机制。FOXOS通过调节细胞周期、DNA修复和寿命相关基因的表达来介导氧化应激反应。组蛋白脱乙酰酶Sirt1参与了ROS对Wnt/2-catenin的抑制作用；2-catenin共同激活了促进成骨细胞在应激条件下存活的FoxO靶基因的表达。此外，有证据表明，糖皮质激素或TNF1对骨骼的作用至少部分是通过ROS诱导的FoxO激活来介导的。上述观察结果形成了假设的基础，即氧化应激激活FoxO转录因子代表了Wnt/2-catenin拮抗的一种先前未被认识的细胞自主机制，该机制通过将2-catenin从TCF-介导的转录转移到FoxO-介导的转录，从而导致衰老、糖皮质激素过剩和炎性细胞因子对骨骼的不利影响。为了验证这一假设，我们将研究ROS诱导的FoxO翻译后修饰对成骨细胞中FoxOS与2-连环蛋白结合的作用，以及ROS是否导致2-连环蛋白重新聚集到FoxO靶基因的启动子上(目标1)。此外，还将进行体外研究，以确定FOXO激活对成骨细胞寿命和分化的影响(目标2)。最后，在成骨细胞前体及其后代(成骨细胞、骨细胞)中过表达FOXO3a的小鼠，以及有条件地缺失三个主要FoxO亚型FoxO1、3a和4的小鼠，将被用于研究FoxO在骨骼动态平衡中的作用(目标3)。这项工作将促进人们对衰老、糖皮质激素过剩或炎性细胞因子如何降低骨量的了解。此外，它应该更好地了解如何优化这种情况的治疗。 与公共健康相关：拟议的研究试图确定衰老、糖皮质激素过量或炎性细胞因子导致骨丢失的方式。这将通过研究控制骨形成细胞的蛋白质功能的变化来实现。进一步了解控制骨形成的机制将为开发保持或增加骨量和强度的治疗方法提供重要信息，从而降低骨质疏松性骨折的风险。

DESCRIPTION (provided by applicant): Wnt/2-catenin signaling is indispensable for osteoblastogenesis and bone formation. In studies leading to this application, a novel mechanism of Wnt/2-catenin antagonism, whereby activation of FoxO transcription factors diverts the limited pool of 2-catenin from Wnt induced TCF/Lef- transcription to FoxO-mediated transcription was elucidated. This antagonistic cascade in bone is initiated by reactive oxygen species (ROS) and represents both a mediator of, and a defense mechanism against, oxidative stress. FoxOs mediate oxidative stress responses by regulating the expression of genes involved in cell cycle, DNA repair, and lifespan. The involvement of the histone deacetylase Sirt1 in the inhibition of Wnt/2-catenin by ROS; and co- activation by 2-catenin of the expression of FoxO-target genes that promote osteoblast survival under stress conditions was also established. Moreover, evidence was obtained that the actions of glucocorticoids or TNF1 on bone are mediated, at least in part, by ROS-induced FoxO activation. The above observations form the foundation of the hypothesis that activation of FoxO transcription factors by oxidative stress represents a previously unappreciated cell-autonomous mechanism of Wnt/2-catenin antagonism which contributes to the adverse effects of aging, glucocorticoid excess and inflammatory cytokines on bone, by diverting 2-catenin from TCF- to FoxO-mediated transcription. To test this hypothesis, the role of ROS-induced FoxO post- translational modifications on the binding of FoxOs to 2-catenin, in osteoblastic cells, and whether ROS lead to recruitment of 2-catenin to the promoter of FoxO-target genes will be investigated (Aim 1). In addition, in vitro studies will be done to establish the consequences of FoxO activation in osteoblast lifespan and differentiation (Aim 2). Finally, mice overexpressing FoxO3a in osteoblast precursors and their progeny (osteoblasts, osteocytes); and mice in which the three main FoxO isoforms FoxO1, 3a, and 4 are conditionally deleted will be used to examine the role of FoxOs in skeletal homeostasis (Aim 3). This work should advance knowledge of how aging, glucocorticoid excess or inflammatory cytokines decrease bone mass. Furthermore, it should provide a better understanding of how to optimize the treatment of this condition. PUBLIC HEALTH RELEVANCE: The proposed studies seek to identify the means by which aging, glucocorticoid excess or inflammatory cytokines cause bone loss. This will be accomplished by studying changes in the function of proteins that control bone-forming cells. Increased understanding of the mechanisms that control bone formation will provide important information for the development of therapies to maintain or increase bone mass and strength, thereby reducing the risk of osteoporotic fractures