NOVEL REGULATORY MECHANISMS CONTROLLING BONE REPAIR AND OSTEOPOROSIS

描述（由申请人提供）：骨质疏松症是一种以低骨量为特征的疾病，导致骨骼的隐性恶化，使其脆弱易骨折。骨质疏松症是由于多种遗传、激素、营养和物理因素单独或共同作用而导致骨骼完整性下降。雌激素替代通过阻断破骨细胞介导的骨脱矿来减缓骨质流失。在这种情况下，甲状旁腺激素（PTH）能够介导骨生长。我们最近做了一系列的观察，表明雌激素缺乏可能导致骨质疏松症的另一个潜在的重要机制。具体来说，我们发现被认为是无活性的氨基末端截断的甲状旁腺激素片段以细胞特异性的方式促进骨中甲状旁腺激素受体（PTH1R）的快速内化。这些不具有生物活性的PTH片段在不首先激活受体的情况下降低了细胞表面PTH1R的丰度。这种PTH1R激活和失活的解耦是由于细胞质接头蛋白EBP50 （ezrin结合蛋白，50 kDa）的存在。EBP50含有一个雌激素反应元件。缺乏EBP50的实验动物出现骨质减少和骨质疏松。我们假设雌激素增加EBP50的表达，从而保护成骨细胞PTH1R免受PTH片段的失活作用。本课题的具体目的是：(1)研究性激素对EBP50的调节；(2)确定雌激素诱导EBP50的表达是否有助于恢复PTH对骨的合成代谢作用。性激素对EBP50表达的影响将在人和大鼠成骨细胞系中测定。第二组实验由两部分组成。首先，从ebp50缺失小鼠骨髓中制备的异位小骨将被用作模型系统，分析PTH和雌激素对骨的影响，独立于全身效应或供体动物。第二部分将研究雌二醇单独或联合PTH对去卵巢ebp50缺失小鼠骨恢复的影响。拟议的研究将导致更好地理解影响骨骼健康的激素事件，并重要地影响对骨骼负荷的反应。新的脆弱性预测措施和治疗干预的潜在途径可能被开发出来。这些新研究的目的是描述女性激素对骨骼的影响是如何被某种适配蛋白调节的。长期目标是确定可用于治疗绝经后骨质疏松症的新化合物。

DESCRIPTION (provided by applicant): Osteoporosis is a disease that is characterized by low bone mass that results in insidious deterioration of the skeleton leaving it fragile and vulnerable to fracture. Osteoporosis is due to multiple genetic, hormonal, nutritional, and physical factors that act alone or together to diminish skeletal integrity. Estrogen replacement blunts the loss of bone by blocking osteoclast-mediated bone demineralization. In this setting, parathyroid hormone (PTH) is able to mediate bone growth. We recently made a series of observations that suggest another potentially important mechanism by which estrogen deficiency may contribute to osteoporosis. Specifically, we found that amino-terminally truncated PTH fragments that are thought to be inactive promoted rapid internalization of the PTH receptor (PTH1R) in a cell-specific manner in bone. These otherwise biologically inactive PTH fragments reduced the abundance of PTH1R on the cell surface without first activating the receptor. This uncoupling of PTH1R activation and inactivation is due to the presence of a cytoplasmic adaptor protein, termed EBP50 (ezrin-binding protein, 50 kDa). EBP50 contains an estrogen response element. Experimental animals lacking EBP50 have osteopenia and develop osteoporosis. We hypothesize that estrogens increase EBP50 expression, thereby protecting the osteoblast PTH1R against the inactivating effects of PTH fragments. The Specific Aims of this project are: (1) To examine gonadal hormone regulation of EBP50, and (2) to determine if estrogen induction of EBP50 expression contributes to restoring the anabolic action of PTH on bone. The effects of gonadal hormones on EBP50 expression will be determined in osteoblastic human and rat cell lines. The second group of experiments consists of two parts. In the first, ectopic ossicles prepared from bone marrow of EBP50-null mice will be used as a model system to analyze the effects of PTH and estrogens on bone, independent of systemic effects or the donor animal. The second part will examine the effects of estradiol administration alone or in combination with PTH on restoring bone in ovariectomized EBP50-null mice. The proposed studies will lead to a better understanding of the hormonal events that impact bone health and importantly influence the response to bone loading. Novel predictive measures of fragility and potential avenues for therapeutic intervention may be developed. The goal of these new studies is to characterize how the effect of female hormones on bone may be regulated by a certain adapter protein. The long-term objective is to identify novel compounds that can be used to treat post-menopausal osteoporosis.