Regulation of PTH-induced Osteoanabolism by Inflammatory Lipids

描述（申请人提供）：甲状旁腺激素（PTH）是骨代谢的关键调节因子，被用作治疗骨质疏松症的合成代谢药物疗法。最近的证据表明，脂质可能影响骨代谢，间歇性甲状旁腺激素治疗可能在高脂血症的存在失败。这在临床上很重要，因为高脂血症在骨质疏松症患者中普遍存在，即使考虑到年龄。在高脂血症条件下，低密度脂蛋白（LDL）的生物活性衍生物在组织内皮下空间产生，引发慢性炎症反应，包括氧化应激和细胞因子和趋化因子的表达。我们发现这些炎性脂蛋白/脂质也存在于骨中，它们抑制成骨细胞分化。在其他研究中，我们和其他研究人员证明，与WT小鼠相比，高脂血症小鼠的骨密度降低。我们在体外和体内的初步研究表明，脂质通过减少环AMP的产生，抑制pth诱导的直接早期基因，包括Nurr1（成骨基因的转录调节因子），高脂血症减弱pth诱导的体内骨合成代谢，主要是在皮质骨中。这些发现强烈提示高脂血症诱导甲状旁腺激素抵抗。甲状旁腺素耐药性是否在分子和/或组织水平上仍有待确定。由于骨质疏松症和高脂血症在治疗后仍然普遍存在，了解脂质对基础（内源性）和间歇（外源性）甲状旁腺激素的影响可能为骨质疏松症提供新的途径。我们假设炎症脂蛋白，在高脂血症中增加，减少甲状旁腺激素的合成代谢作用。在我们前期研究的基础上，在Specific Aim 1中，我们将在体外测试脂质对PTH诱导的环AMP产生的抑制机制是在PTH受体表达、受体运输水平，还是在g蛋白激活水平的下游，还是在腺苷酸环化酶激活水平的下游。在特异性目标2中，我们将确定脂质诱导的甲状旁腺激素抵抗在体内发生的水平：骨髓祖细胞向成骨和成脂谱系分化的水平；成熟成骨细胞/骨细胞的合成代谢反应；和/或成骨细胞瞬间表达破骨细胞因子。我们将产生表达针对成骨细胞和骨细胞的绿色荧光蛋白的Ldlr-/-小鼠。在Specific Aim 3中，我们将通过测量高脂血症（Ldlr-/-）小鼠的骨密度、组织形态学参数和骨转换标志物来测试降低高脂血症或抑制脂质氧化是否会在体内逆转甲状旁腺激素抵抗，这些小鼠接受肝脏X受体激动剂治疗或过度表达抗氧化酶对氧酶-1。这些拟议的研究将提供关于炎性脂质如何抑制PTH诱导的骨合成代谢的见解，查明PTH信号通路中抑制作用的位置，并展示逆转脂质诱导的PTH失调的方法。公共卫生相关性：与公共卫生的相关性高胆固醇在低骨密度疾病、骨质疏松症患者中很常见。甲状旁腺激素治疗是一种很有前途的治疗骨质疏松症的方法，但有证据表明，它对高胆固醇患者的效果可能要差得多。拟议的研究将确定不受控制的胆固醇水平如何降低治疗效果，以便制定纠正策略。

DESCRIPTION (provided by applicant): Parathyroid hormone (PTH) is a key regulator of bone metabolism and is used as anabolic pharmacotherapy for treatment of osteoporosis. Recent evidence suggests that lipids may affect bone metabolism and that intermittent PTH therapy may fail in the presence of hyperlipidemia. This is clinically important because hyperlipidemia is prevalent in patients with osteoporosis even accounting for age. In the hyperlipidemic condition, bioactive derivatives of low-density lipoproteins (LDL) are generated in the subendothelial space of tissues, triggering chronic inflammatory responses including oxidant stress and expression of cytokines and chemokines. We have found that these inflammatory lipoproteins/lipids are also present in bone and that they inhibit osteoblastic differentiation. In additional studies, we and other investigators demonstrated that hyperlipidemic mice have reduced bone density compared to WT mice. Our preliminary studies, both in vitro and in vivo, now show that lipids inhibit PTH-induced immediate early genes, including Nurr1, a transcriptional regulator of osteoblastic genes, by attenuating cyclic AMP production and that hyperlipidemia blunts PTH-induced osteoanabolism in vivo, primarily in cortical bone. These findings strongly suggest that hyperlipidemia induces PTH resistance. Whether PTH resistance is at the molecular and/or tissue level remains to be determined. Since osteoporosis and hyperlipidemia remain widespread despite treatment, understanding effects of lipids on basal (endogenous) and intermittent (exogenous) PTH may provide new approaches to osteoporosis. We hypothesize that inflammatory lipoproteins, which are increased in hyperlipidemia, reduce PTH anabolic effects. Based on our preliminary studies, in Specific Aim 1, we will test in vitro whether the inhibitory mechanism of lipids on PTH-induced cyclic AMP production is at the level of PTH receptor expression, receptor trafficking, downstream at the level of G-protein activation, or further downstream at the level of adenylate cyclase activation. In Specific Aim 2, we will identify the level at which lipid-induced PTH resistance occurs in vivo: at the level of differentiation of marrow progenitors toward osteogenic vs. adipogenic lineages; anabolic responses of mature osteoblasts/osteocytes; and/or transient expression of osteoclastogenic factors by osteoblasts. We will generate Ldlr-/- mice that express green fluorescent protein targeted to osteoblasts and osteocytes. In Specific Aim 3, we will test whether reducing hyperlipidemia or inhibiting lipid oxidation will reverse PTH resistance in vivo by measuring bone density, histomorphometric parameters and bone turnover markers in the hyperlipidemic (Ldlr-/-) mice that are treated with liver X receptor agonists or that overexpress the anti-oxidant enzyme, paraoxonase-1. These proposed studies will provide insights into how inflammatory lipids inhibit PTH-induced osteoanabolism, pinpoint the site of inhibitory action within the PTH signaling pathway, and demonstrate approaches to reverse lipid-induced PTH dysregulation. PUBLIC HEALTH RELEVANCE: Relevance to Public Health High cholesterol is common in patients with the low bone density disease, osteoporosis. Parathyroid hormone treatment is a promising therapy for osteoporosis, but evidence suggests that it may be much less effective in patients with high cholesterol. The proposed studies will determine how unmanaged cholesterol level reduces efficacy of the treatment so that corrective strategies can be developed.