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

摘要 在美国，超过三分之一的成年人受到肥胖的影响，肥胖是一个主要的公共健康威胁，导致数百万人患有肥胖症。 骨质疏松症、2型糖尿病、心血管疾病和全因死亡的风险增加。与年龄相关的变化 脂肪组织是已知的许多年龄相关疾病的潜在原因，包括骨质疏松症。尽管肥胖是一种-- 骨髓脂肪组织(MAT)的特点是白色脂肪组织(WAT)过多，是研究最少的脂肪之一 在骨骼健康和能量代谢方面可能发挥重要作用，因为垫子可以表现出两者的特性 WAT和代谢活跃的棕色脂肪组织(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褐化，其中Wat被诱导表现得与蝙蝠相似。甲状腺激素 (TH)是脂肪组织和能量代谢的重要调节因子。虽然已知这会导致Wat的褐变， 全身应用TH并不是治疗肥胖症的可行策略，因为TH对几乎所有的 体内的组织和甲状腺机能亢进症的副作用太危险了。出于这个原因，最近的研究已经 靶向TH信号通路中的特定下游效应器，以利用TH的一些有益效果 避免不必要的不良影响。在我们的初步研究中，我们发现使用TRβ1特异性激动剂治疗， GC-1可降低骨髓肥胖度，上调骨髓基质细胞(BMSCs)的BAT标志基因。然而， 现在已知GC-1会产生对其他组织有害的非靶点效应。根据我们新的初步数据，我们 建议在这项R21拨款中测试以下两个具体目标，以调查 高选择性的TRβ模拟物，MGL3196，在调节垫在老化和饮食诱导的肥胖过程中的功能性褐变。 在目标1中，我们将测试这样一个假设，即使用MGL3196激活TRβ1信号减少MAT，诱导功能性 改善肥胖和衰老小鼠的骨骼质量。成年(4米)和老年(18米)C57BL/6J小鼠将 用低脂或高脂饲料喂养12周，每日服用MGL3196或赋形剂。MGL3196的后果 对高脂饮食诱导的骨髓肥胖症和骨质量的治疗将通过DXA、微型CT和组织学进行评估。 Wat的表达、Wat的褐变、BAT和骨标记物将通过以下方法在mRNA和蛋白质水平上进行评估 实时荧光定量RT-PCR和免疫组织化学检测。MGL3196对白色脂肪细胞褐变的影响及传代承诺 将评估骨髓间充质干细胞向成骨细胞和脂肪细胞方向的分化。在目标2中，我们将检验假设 MGL3196通过非基因组β-PI3K信号转导trβ147F突变小鼠的MAT褐变 具有完整的基因组但中断的非基因组β-PI3K信号。我们将测量垫和骨质量的变化 对MGL3196治疗的反应。为了证实PI3K信号在非基因组MGL3196反应中的作用， 我们将在原代培养的骨髓间充质干细胞中检测PI3K/Akt信号通路在MGL3196处理后的变化。我们会 同时确定用PI3K信号通路的药物抑制剂处理骨髓间充质干细胞是否会阻断BAT的诱导 MGL3196标记。我们相信，评估公用事业MGL3196处理垫子和以下- 根据美国肥胖相关医疗费用的预期增长，其作用机制是巨大的。

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.