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

摘要 肥胖影响着美国超过三分之一的成年人，肥胖是一个主要的公共健康威胁，使数百万人陷入困境。 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。与年龄有关的变化 脂肪组织是许多与年龄相关的疾病（包括骨质疏松症）的根本原因。虽然肥胖是一个特征 骨髓脂肪组织（MAT）以过量的白色脂肪组织（WAT）为特征，是研究最少的脂肪之一 MAT 可以表现出这两种特性，因此可能在骨骼健康和能量代谢中发挥重要作用 WAT 和代谢活跃的棕色脂肪组织 (BAT)。治疗肥胖及其相关疾病的一种有吸引力的治疗方法 合并症是所谓的 WAT 褐变，其中 WAT 被诱导表现出与 BAT 类似的行为。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 会导致 WAT 褐变， 全身性 TH 给药并不是治疗肥胖症的可行策略，因为 TH 对几乎所有疾病都有广泛的影响。 体内组织，甲状腺毒症的不利影响太危险了。为此，最近的研究 针对 TH 信号通路中的特定下游效应器，以利用 TH 的一些有益作用，同时 避免不必要的不利影响。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 可以减少骨髓肥胖并上调骨髓基质细胞 (BMSC) 中的 BAT 标记基因。然而， 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 拨款中测试以下两个具体目标，以调查 高选择性 TRβ 模拟物 MGL3196 在衰老和饮食诱导的肥胖过程中调节 MAT 的功能性褐变。 在目标 1 中，我们将测试以下假设：使用 MGL3196 激活 TRβ1 信号传导可减少 MAT，诱导功能性 MAT 褐变，并改善肥胖和老年小鼠的骨质量。成年（4 m）和老年（18 m）C57BL/6J 小鼠将 低脂肪或高脂肪饮食喂养 12 周，每天用 MGL3196 或媒介物治疗。 MGL3196的后果 将通过 DXA、显微 CT 和组织学评估对高脂肪饮食引起的骨髓肥胖和骨质量的治疗。 WAT 的表达、WAT 的褐变、BAT 和骨标记物将在 mRNA 和蛋白质水平上进行评估 实时 RT-PCR 和免疫组织化学。 MGL3196 对白色脂肪细胞褐变和谱系定向的影响 将评估 BMSC 对成骨细胞和脂肪细胞谱系的影响。在目标 2 中，我们将检验以下假设： MGL3196 对 MAT 褐变的影响是通过使用 TRβ147F 突变小鼠的非基因组 TRβ–PI3K 信号传导介导的 具有完整的基因组，但破坏了非基因组的 TRβ-PI3K 信号传导。我们将测量 MAT 和骨质量的变化 对 MGL3196 治疗的反应。为了确认 PI3K 信号传导在介导非基因组 MGL3196 反应中的作用， 我们将测量 BMSC 原代培养物中 MGL3196 处理后 PI3K/Akt 信号传导的变化。我们会 还确定用 PI3K 信号传导的药物抑制剂治疗 BMSC 是否会阻止 BAT 的诱导 MGL3196 标记。我们相信评估实用程序 MGL3196 治疗 MAT 和不足的潜在影响 鉴于美国肥胖相关医疗费用的预期增加，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.