Novel Mechanism of PTH Effects on Bone Metabolism

描述(申请人提供)：严重的骨质疏松症影响着1000多万美国人。每年有超过30万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(Teriparatide)是目前美国FDA批准的唯一治疗骨质疏松症的合成代谢药物。间歇给予甲状旁腺素可促进骨形成，而持续注射甲状旁腺素(1-34)则会导致骨吸收。然而，由甲状旁腺素受体(PTH1R)激活的不同信号通路产生这种骨骼反应的作用仍不完全清楚。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泛素化是一个可逆的过程。修饰的PTH1R的去泛素化是由泛素特异性蛋白水解酶2(USP2)介导的。在急性甲状旁腺激素作用下，成骨细胞中USP2的表达迅速增加，随着治疗时间的延长，USP2的表达恢复到对照水平。其他蛋白，如G蛋白信号转导调节因子2(RGS2)、矮小相关转录因子2(Runx2)和b-catenin，其活性可由PTH诱导并参与成骨，也受到泛素敏感途径的下调。破骨细胞不表达PTH1R，对PTH的反应主要由NFkB受体激活剂配体(RANKL)介导。RANKL与RANK结合可诱导破骨细胞发生相关基因的表达。蛋白酶体抑制剂通过增加Runx2的活性和稳定b-catenin蛋白来促进成骨细胞的形成，并通过减少RANKL诱导的信号转导来抑制破骨细胞的形成。这些结果表明，有一个中心假说，即PTH通过泛素-蛋白酶体途径对骨代谢起双峰作用，蛋白酶体抑制剂增强PTH对骨的合成代谢作用。为了检验这一假说，我们制定了两个具体目标。目的1研究甲状旁腺激素(PTH)在体外通过泛素-蛋白酶体途径对成骨细胞的双峰作用。在目标2中，我们将确定间歇性和持续性甲状旁腺素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PTH治疗骨质疏松症的机制，而且为设计治疗骨质疏松症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.