Thyroid hormone receptor β1 agonist therapy for the treatment of bone marrow adiposity in aging and obesity

摘要 在美国，肥胖症影响着超过三分之一的成年人，是一个主要的公共健康威胁，使数百万人处于... 骨质疏松症、2型糖尿病、心血管疾病和全因死亡率的风险增加。与联合国有关的变化 脂肪组织是已知的许多与年龄有关的疾病包括骨质疏松症的根本原因。虽然肥胖是一种慢性病， 以过量的白色脂肪组织（WAT）为特征，骨髓脂肪组织（MAT）是研究最少的脂肪组织之一。 MAT是一种储存库，可能在骨骼健康和能量代谢中发挥重要作用，因为MAT可以表现出两者的特性。 WAT和代谢活性棕色脂肪组织（BAT）。一种有吸引力的治疗肥胖症的方法及其 共病是所谓的WAT的布朗宁，其中WAT被诱导表现得与BAT相似。甲状腺激素 (TH)是脂肪组织和能量代谢的重要调节剂。虽然已知TH诱导WAT的布朗宁， 全身性TH给药不是治疗肥胖的可行策略，因为TH对几乎每一个 甲状腺毒症的副作用是非常危险的。因此，最近的研究表明， 靶向TH信号通路中的特定下游效应物，以利用TH的一些有益作用， 避免不希望的不利影响。在我们的初步研究中，我们发现用TRβ1特异性激动剂治疗， GC-1可降低骨髓肥胖，并上调骨髓基质细胞（BMSCs）中BAT标记基因。但是，在这方面， 现在已知GC-1会产生对其他组织有害的脱靶效应。根据我们新的初步数据，我们 建议在R21赠款中测试以下两个具体目标，以调查 高选择性TRβ模拟物MGL 3196在衰老和饮食诱导的肥胖过程中调节MAT的功能性布朗宁。 在目的1中，我们将检验使用MGL 3196激活TRβ1信号转导减少MAT，诱导功能性MAT， MAT的布朗宁，并改善肥胖和老年小鼠的骨质量。成年（4个月）和老年（18个月）C57 BL/6 J小鼠将 用低脂肪或高脂肪饮食喂养12周，每天用MGL 3196或媒介物处理。MGL 3196的结果 将通过DXA、微CT和组织学评价对高脂肪饮食诱导的骨髓肥胖和骨质量的治疗。 WAT的表达、WAT的布朗宁、BAT和骨标志物将通过以下方法在mRNA和蛋白质水平上进行评价： 实时RT-PCR和免疫组织化学。MGL 3196对白色脂肪细胞的布朗宁和谱系转化的影响。 将评估BMSC向成骨细胞和脂肪细胞谱系的分化。在目标2中，我们将检验以下假设： 使用TRβ 147 F突变小鼠，通过非基因组TRβ-PI 3 K信号转导介导MGL 3196对MAT布朗宁的影响 基因组完整但非基因组TRβ-PI 3 K信号转导被破坏。我们将测量MAT和骨质量的变化， 对MGL 3196治疗的反应。为了证实PI 3 K信号传导在介导非基因组MGL 3196应答中的作用， 我们将测量BMSC的原代培养物中响应于MGL 3196处理的PI 3 K/Akt信号传导的变化。我们将 还确定用PI 3 K信号传导的药理学抑制剂处理BMSC是否阻断BAT的诱导 MGL 3196标记。我们认为，评估MGL 3196治疗MAT和低血糖的效用的潜在影响， 基于美国与肥胖相关的医疗费用的预期增加，它的作用机制是巨大的。

Abstract Affecting more than 1 in 3 adults in the United States, obesity is a major public health threat, putting millions at in- creased risk of osteoporosis, type 2 diabetes, cardiovascular disease, and all-cause mortality. Age-related changes in the adipose tissue are known underlying causes for many age-related diseases including osteoporosis. Although obesity is char- acterized by an excess of white adipose tissue (WAT), bone marrow adipose tissue (MAT) is among the least studied adipose depots and may play an important role in skeletal health and energy metabolism, as MAT can exhibit properties of both WAT and metabolically active brown adipose tissue (BAT). An attractive therapeutic approach for treating obesity and its comorbidities is the so-called browning of WAT in which WAT is induced to behave similarly to BAT. Thyroid hormone (TH) is an important regulator of adipose tissues and energy metabolism. While TH is known to induce browning of WAT, systemic TH administration is not a viable strategy for treating obesity as TH exerts a wide range of effects on nearly every tissue in the body, and the adverse effects of thyrotoxicosis are much too dangerous. For this reason, recent studies have targeted specific downstream effectors in the TH signaling pathway to leverage some of TH’s beneficial effects while avoiding unwanted adverse effects. In our preliminary studies, we have found that treatment with a TRβ1 specific agonist, GC-1, can decrease marrow adiposity and upregulate BAT marker genes in bone marrow stromal cells (BMSCs). However, GC-1 is now known to exert off-target effects that are detrimental to other tissues. Based on our new preliminary data, we propose to test the following two specific aims in this R21 grant to investigate the role and mechanism of action of the highly selective TRβ mimetic, MGL3196, in regulating functional browning of MAT during aging and diet-induced obesity. In Aim 1, we will test the hypothesis that activation of TRβ1 signaling using MGL3196 reduces MAT, induces functional browning of MAT, and improves bone quality in obese and aged mice. Adult (4 m) and aged (18 m) C57BL/6J mice will be fed with low- or high-fat diet for 12 weeks and treated daily with MGL3196 or vehicle. The consequence of MGL3196 treatment on high-fat diet-induced marrow adiposity and bone quality will be evaluated by DXA, micro-CT, and histology. The expression of WAT, browning of WAT, BAT, and bone markers will be evaluated at the mRNA and protein levels by real-time RT-PCR and immunohistochemistry. The MGL3196 effect on browning of white adipocytes and lineage commit- ment of BMSCs towards osteoblastic and adipocytic lineages will be evaluated. In Aim 2, we will test the hypothesis that MGL3196 effects on browning of MAT are mediated via nongenomic TRβ–PI3K signaling by using TRβ147F mutant mice with intact genomic but disrupted nongenomic TRβ–PI3K signaling. We will measure changes in MAT and bone quality in response to MGL3196 treatment. To confirm the role of PI3K signaling in mediating the nongenomic MGL3196 response, we will measure changes in PI3K/Akt signaling in response to MGL3196 treatment in primary cultures of BMSCs. We will also determine if treatment of BMSCs with pharmacological inhibitors of PI3K signaling blocks the induction of BAT markers by MGL3196. We believe that the potential impact of evaluating the utility MGL3196 to treat MAT and under- standing its mechanism of action is huge based on the anticipated increase in obesity-related healthcare expenses in the U.S.