Novel Mechanism of PTH Effects on Bone Metabolism

描述（由申请人提供）：严重骨质疏松症影响超过1000万美国人。每年有超过30万例髋部骨折是由骨质疏松症引起的。重组甲状旁腺激素（PTH） 1-34（特利帕肽）是目前美国唯一获fda批准用于治疗骨质疏松症的合成代谢药物。间歇给药促甲状旁腺激素增加骨形成，而连续输注促甲状旁腺激素（1-34）导致骨吸收。然而，PTH受体（PTH1R）激活的不同信号通路在产生这种骨骼反应中的作用仍然不完全清楚。泛素-蛋白酶体途径在调节和控制骨代谢中起着重要作用。PTH1R泛素化是一个可逆过程。修饰PTH1R的去泛素化是由泛素特异性蛋白酶2 （USP2）介导的。成骨细胞急性暴露于甲状旁腺激素后，USP2的表达迅速增加，并随着治疗时间的延长而恢复到控制水平。其他蛋白，如G蛋白信号传导2的调节因子（RGS2）、矮子相关转录因子2 （Runx2）和b-catenin，它们的活性可以被PTH诱导并参与成骨，也通过泛素敏感途径下调。破骨细胞不表达PTH1R，对PTH的反应主要由NFkB配体受体激活因子（RANKL）介导。RANKL与RANK结合可诱导参与破骨细胞发生的基因表达。蛋白酶体抑制剂通过增加Runx2活性和b-catenin蛋白的稳定性来促进成骨细胞的形成，并通过减少rankl诱导的信号级联来抑制破骨细胞的形成。这些发现提示PTH通过泛素-蛋白酶体途径对骨代谢产生双峰效应，蛋白酶体抑制剂增强PTH对骨的合成代谢作用是一个中心假设。为了验证这一假设，提出了两个具体目标。目的1将描述PTH对成骨细胞形成的双峰效应是如何通过体外泛素-蛋白酶体途径介导的。在Aim 2中，我们将定义间歇性和连续PTH联合蛋白酶体抑制剂对骨量的体内作用。通过这些研究获得的信息不仅有助于了解甲状旁腺激素治疗骨质疏松症的机制，而且为设计治疗骨质疏松等代谢性骨病的药物提供了重要的见解。

DESCRIPTION (provided by applicant): Severe osteoporosis affects over 10 million Americans. Over 300,000 hip fractures are caused by osteoporosis annually. Recombinant parathyroid hormone (PTH) 1-34 (teriparatide) is currently the only FDA-approved anabolic agent for treatment of osteoporosis in the United States. Intermittent administration of PTH increases bone formation, whereas continuous infusion of PTH (1-34) results in bone resorption. However, the roles of distinct signaling pathways activated by PTH receptor (PTH1R) that generates this skeletal response remain incompletely understood. The ubiquitin-proteasome pathway plays an important role in regulating and controlling bone metabolism. PTH1R ubiquitination is a reversible process. Deubiquitination of modified PTH1R is mediated by ubiquitin specific protease 2 (USP2). The expression of USP2 is rapidly increased by acute exposure to PTH in osteoblasts and returns to control levels with prolonged treatment. Other proteins, such as, regulator of G protein signaling 2 (RGS2), runt-related transcription factor 2 (Runx2), and b-catenin, the activities of which can be induced by PTH and involved in osteogenesis, are also down regulated by a ubiquitin-sensitive pathway. Osteoclasts do not express PTH1R, and the response to PTH is primarily mediated by receptor activator of NFkB ligand (RANKL). The engagement of RANKL binding to RANK leads to induction of gene expression involved in osteoclastogenesis. Proteasomal inhibitor enhances osteoblastogenesis by increasing Runx2 activity and stabilization of b-catenin protein, and it inhibits osteoclastogenesis by decreasing RANKL-induced signaling cascades. These findings suggest that there is a central hypothesis that PTH exerts bimodal effects on bone metabolism mediated by the ubiquitin-proteasome pathway, and proteasomal inhibitor enhances the anabolic effects of PTH on bone. Two specific aims are developed to test this hypothesis. Aim 1 will characterize how bimodal effects of PTH on osteoblastogenesis are mediated by ubiquitin-proteasome pathway in vitro. In Aim 2, we will define the in vivo role of intermittent and continuous PTH administration in combination with proteasomal inhibitor on bone mass. The information gained through these studies will not only contribute to understanding the mechanisms of PTH treatment of osteoporosis, but also provide important insight into designing pharmacological agents for treatment of metabolic bone diseases such as osteoporosis.