Novel Mechanism of PTH Effects on Bone Metabolism

描述（由申请人提供）：严重的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（特立帕肽）是目前唯一的FDA批准的合成代谢药物用于治疗骨质疏松症在美国。PTH的间歇给药增加骨形成，而PTH（1-34）的连续输注导致骨吸收。然而，PTH受体（PTH 1 R）激活的不同信号通路产生这种骨骼反应的作用仍然不完全清楚。泛素-蛋白酶体途径在骨代谢的调控中起着重要的作用。PTH 1 R泛素化是一个可逆的过程。修饰的PTH 1 R的去泛素化由泛素特异性蛋白酶2（USP 2）介导。成骨细胞中USP 2的表达通过急性暴露于PTH而迅速增加，并且随着延长的治疗而恢复到对照水平。其他蛋白质，如G蛋白信号传导调节因子2（RGS 2）、runt-related转录因子2（Runx 2）和β-连环蛋白，其活性可由PTH诱导并参与骨生成，也通过泛素敏感途径下调。破骨细胞不表达PTH 1 R，对PTH的反应主要由NF κ B配体受体激活剂（RANKL）介导。RANKL与RANK结合导致诱导破骨细胞生成相关基因表达。蛋白酶体抑制剂通过增加Runx 2活性和b-连环蛋白的稳定性增强成骨细胞生成，并通过减少RANKL诱导的信号级联抑制破骨细胞生成。这些发现表明，有一个中心假设，即PTH发挥双峰效应的骨代谢介导的泛素-蛋白酶体途径，蛋白酶体抑制剂增强PTH对骨的合成代谢作用。两个具体的目标来测试这一假设。目的1研究PTH对成骨细胞的双峰效应是如何通过泛素-蛋白酶体途径介导的。在目标2中，我们将确定间歇性和连续性PTH给药与蛋白酶体抑制剂联合对骨量的体内作用。通过这些研究获得的信息不仅有助于理解PTH治疗骨质疏松症的机制，而且还为设计用于治疗代谢性骨病（如骨质疏松症）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.