Molecular Control of Brown Adipose Cell Fate and Energy Metabolism

描述(由申请者提供)：当长期摄入的能量超过总的能量消耗时，就会发生肥胖。目前，大多数抗肥胖药物通过抑制食欲或抑制肠道脂肪吸收来抑制能量摄入。然而，由于副作用，包括抑郁，油性大便和脂肪泻，迫切需要替代方法。由于棕色脂肪组织(BAT)在抵御寒冷和肥胖时会消耗能量产生热量，因此改变分子途径以增加BAT的数量或生热活性可能会导致一种替代的有效治疗干预措施，以对抗人类肥胖和代谢障碍。我们的长期目标是了解调节棕色脂肪细胞命运的分子回路，并研究它们在能量动态平衡中的生理作用。我们之前已经证明，棕色脂肪细胞通过转录因子PRDM16的作用来自皮肤肌肉前体的子集；然而，目前尚不清楚PRDM16在成肌细胞向棕色脂肪转换中的作用是如何调节的。我们发现赖氨酸甲基转移酶EHMT1是PRDM16转录复合体的关键成分。EHMT1在BAT中的表达最高，且受PRDM16的高度诱导。值得注意的是，EHMT1似乎是棕色脂肪细胞与肌肉细胞之间的发育开关。重要的是，EHMT1基因的缺失与小鼠和人类的肥胖有关；然而，其潜在的机制仍然完全不清楚。因此，我们目前的目标是研究EHMT1在体内控制棕色脂肪细胞命运的生理功能和机制。基于我们的初步数据，我们将检验EHMT1作为一个发育开关在能量平衡中发挥关键作用的假设，该开关通过调节PRDM16复合体的功能来控制棕色脂肪细胞的命运。为了验证这一假设，我们将追求以下具体目标：在Aim1中，我们将确定EHMT1在棕色脂肪细胞体外和体内命运指定和维持中的遗传要求。在AIM2中，我们将分析脂肪特异的EHMT1基因敲除小鼠和EHMT1杂合缺失小鼠的代谢表型，并关键地表征EHMT1在体内控制能量消耗和葡萄糖动态平衡中的生理作用。在Aim3中，我们将进行生化分析，并使用培养的细胞来阐明EHMT1作为棕色脂肪家族的发育开关的机制。这些研究的预期结果是描述一种全新的棕色脂肪细胞命运指定的上游调控途径。我们的发现将产生重大影响，因为据我们所知，这项研究将表征第一种控制细胞命运在棕色脂肪和骨骼肌之间切换的酶。已知的机制将使我们能够通过药理学方法操纵这一发育途径，这可能提供一个可能的治疗靶点。 公共卫生相关性：肥胖及其代谢后果仍然是当今美国最重要的生物医学挑战之一。由于棕色脂肪通过释放能量产生热量来抵御寒冷和肥胖，了解棕色脂肪发育和功能的分子控制将为人类肥胖提供新的、有希望的治疗策略。因此，这项拟议的研究与美国国立卫生研究院的任务密切相关。

DESCRIPTION (provided by applicant): Obesity develops when energy intake chronically exceeds total energy expenditure. Currently, most anti- obesity medications act to repress energy intake, either by suppressing appetite or by inhibiting intestinal fat absorption. However, due to side effects including depression, oily bowel movements and steatorrhea, there is an urgent need for alternative approaches. Because brown adipose tissue (BAT) dissipates energy to produce heat as a defense against cold and obesity, altering the molecular pathway to increase the amount or thermogenic activity of BAT may lead to an alternative and effective therapeutic intervention to counteract human obesity and metabolic disorders. Our long-term goals are to understand the molecular circuits that regulate the fate specification of brown adipose cells and to investigate their physiological roles in energy homeostasis. We have previously shown that brown adipocytes arise from a subset of dermomytomal precursors through the action of a transcription factor, PRDM16; however, it remains unclear how the PRDM16 action in the myoblast-to-brown fat switch is regulated. We identified a lysine methyltransferase, EHMT1 as a critical component of the PRDM16 transcriptional complex. EHMT1 is expressed at its highest in BAT and is highly induced by PRDM16. Notably, EHMT1 appears to act as a developmental switch of brown adipocytes versus myocytes. Importantly, loss of the EHMT1 gene is associated with obesity in mice and in humans; however, its underlying mechanism remains completely unknown. Our current objective is thus to investigate the physiological function and mechanism of EHMT1 that controls brown adipose cell fate in vivo. Based on our preliminary data, we will test the hypothesis that EHMT1 plays a pivotal role in energy homeostasis as a developmental switch that controls brown adipose cell fate through modulating the function of the PRDM16 complex. To test this hypothesis, we will pursue the following specific aims: In Aim1, we will determine the genetic requirement of EHMT1 in the fate specification and maintenance of brown adipose cells in vitro and in vivo. In Aim2, we will analyze the metabolic phenotypes of adipose-specific EHMT1 knockout mice and EHMT1 heterozygous null mice and critically characterize EHMT1's physiological role in controlling energy expenditure and glucose homeostasis in vivo. In Aim3, we will conduct biochemical analyses and use cultured cells to elucidate the mechanism by which EHMT1 acts as a developmental switch of brown fat lineage. The expected outcome of these studies is to characterize a completely novel upstream regulatory pathway of brown adipose cell fate specification. Our findings will have a significant impact, because, to our knowledge, this study will characterize the first enzyme that controls the cell fate switch between brown adipose versus skeletal muscle. The identified mechanism will allow us to manipulate this developmental pathway by pharmacological approaches, which may provide a possible therapeutic target. PUBLIC HEALTH RELEVANCE: Obesity and its metabolic consequences continue to be among the most important biomedical challenges in the USA today. Because brown fat dissipates energy to produce heat as a defense against cold and obesity, understanding the molecular control of brown fat development and function will provide new and promising therapeutic strategies for human obesity. Hence, the proposed research is closely aligned with the part of NIH's mission.