Enhancing Energy Expending Adipocytes in White Adipose Tissue

描述（由申请人提供）： 肥胖症已达到流行病的程度，导致2型糖尿病和心血管疾病的发病率急剧增加。由于白色脂肪（WAT）细胞内甘油三酯（TG）的过度积累，肥胖个体中脂肪组织的扩张是这些疾病的直接原因。有两种主要类型的脂肪，白色储存TG和棕色（BAT）氧化它们产生热量。直到最近，人们还认为BAT只存在于新生儿的肩胛间区域，但最近的几项研究已经确定BAT在颈部，锁骨上，腋部 和脊椎旁区域。BAT对动物静息代谢率和健康体重稳态的贡献现已得到充分证实。BAT是一种柔性组织，可以通过刺激招募，并在没有刺激的情况下萎缩。事实上，研究已经暗示了棕色脂肪细胞对WAT的募集，以解释响应于不同效应物的能量平衡的变化。我们最近发现，缺乏MRTF-A（血清反应因子的转录共激活因子，SRF），在小鼠中导致招募棕色样（米色）脂肪细胞WAT。在一系列体外研究中，我们已经证明了TGF β超家族的两个成员TGF β和BMP 7对MRTF-A活性和这些效应子将间充质干细胞转化为棕色/米色脂肪形成谱系与血管谱系的能力具有相反的作用。具体而言，BMP 7通过抑制Rho激酶（ROCK）活性诱导棕色脂肪形成，导致F-肌动蛋白解聚和细胞质G-肌动蛋白积累。这通过阻止MRTF-A易位到细胞核中来减弱SRF靶基因的表达。用小分子抑制剂CCG 1423抑制SRF活性促进MSC向不依赖于BMP 7的脂肪细胞谱系的定型。转化生长因子另一方面，通过促进MRTF-A运动进入细胞核来激活SRF活性，这导致抑制脂肪细胞基因和激活血管基因，包括平滑肌（SM）肌动蛋白、SM重链肌球蛋白（SM-MHC）和SM 22。基于这些令人兴奋的数据，我们假设WAT的布朗宁是通过涉及抑制SRF靶基因表达的机制来调节祖细胞的募集。我们提出了三个目标来检验这一假设。在目的1中，我们将确定调节BMP 7和TGF？对ROCK活性和间充质干细胞形态的相反作用的机制。在目标2中，我们将确定MRTF-A/SRF调节祖细胞向成脂与血管谱系的命运的机制。在目标3中，我们将确定MRTF-A缺乏对小鼠中白色脂肪组织的布朗宁和能量平衡的影响。了解生理效应物调节WAT“布朗宁”的分子机制并鉴定促进该过程的化合物将显著有助于开发肥胖相关疾病的治疗方法。

DESCRIPTION (provided by applicant): Obesity has reached pandemic proportions contributing to the dramatic increases in the incidence of type 2- diabetes and cardiovascular disease. The expansion of adipose tissue in obese individuals is a direct cause of these diseases due to an excessive accumulation of triglycerides (TGs) within white adipose (WAT) adipocytes. There are two major types of adipose, white that stores TGs and brown (BAT) that oxidizes them to produce heat. Until recently, it was thought that BAT only existed within the interscapular regions of newborns, but several recent investigations have identified BAT depots in the cervical, supraclavicular, axillary and paravertebral regions of adult humans. The contribution of BAT to resting metabolic rate and healthy body weight homeostasis in animals is now well established. BAT is a flexible tissue that can be recruited by stimuli and atrophies in the absence of a stimulus. In fact, studies have implicated the recruitment of brown adipocytes to WAT to explain changes in energy balance in response to different effectors. We have recently shown that lack of MRTF-A (the transcriptional coactivator of serum response factor, SRF), in mice leads to recruitment of brown-like (beige) adipocytes to WAT. In a series of in vitro studies, we have demonstrated that two members of the TGF¿ superfamily, TGF¿ and BMP7 have opposing effects on MRTF-A activity and the ability of these effectors to commit mesenchymal stem cells to an brown/beige adipogenic versus vascular lineage. Specifically, BMP7 induces brown adipogenesis by suppressing Rho kinase (ROCK) activity leading to depolymerization of F-actin and accumulation of cytoplasmic G-actin. This attenuates expression of SRF target genes by preventing translocation of MRTF- A into the nucleus. Inhibition of SRF activity with a small molecule inhibitor, CCG1423 promotes commitment of MSCs to the adipocyte lineage independent of BMP7. TGF¿ on the other hand activates SRF activity by promoting MRTF-A movement into the nucleus, which leads to suppression of adipocyte genes and activation of vascular genes including smooth muscle (SM) actin, SM heavy chain myosin (SM- MHC) and SM22. Based on these exciting data, we hypothesize that the browning of WAT is regulated by recruitment of progenitors through mechanisms involving suppression of SRF target gene expression. We propose three aims to test this hypothesis. In Aim 1, we will identify the mechanisms regulating the opposing effects of BMP7 and TGF¿ on ROCK activity and the morphology of mesenchymal stem cells. In Aim 2, we will identify the mechanisms by which MRTF-A/SRF regulates the fate of progenitors to an adipogenic versus vascular lineage. In Aim 3, we will determine the effect of MRTF-A deficiency on browning of white adipose tissue and energy balance in mice. Understanding the molecular mechanisms by which physiological effectors regulate the "browning" of WAT and identifying compounds that promote this process will significantly contribute to the development of therapeutics for obesity-associated disorders.