Cell cycle control of adipogenesis

修订摘要 最近的证据表明，创造新的脂肪细胞（脂肪生成）可以抵消肥胖的有害代谢效应，肥胖主要源于现有脂肪细胞的异常扩大（肥大）和功能障碍。因此，如何增加脂肪生成而不是肥大引起了极大的兴趣。先前的研究表明，体内和体外的前脂肪细胞必须完成一个或多个细胞周期，这一过程称为克隆扩增，然后才能分化为脂肪细胞。通过敲除细胞周期抑制剂p21和p27来操纵克隆扩增期，证明了增加脂肪形成的最引人注目的遗传学例子之一。在小鼠中敲除p21或p27导致脂肪生成和脂肪量增加2倍，但敲除两者导致急剧增加6倍。以前的工作表明，p21和p27是由非常不同的机制，并在脂肪形成过程中的不同时间活跃。在最近发表的工作中，我们确定了可以解释在p21基因敲除小鼠中观察到的脂肪形成增加的分子机制。然而，p27如何调节脂肪形成和组织质量，以及p21和p27如何协同作用的了解甚少。我们推测，在脂肪形成过程中，p21和p27的协同表达是控制分化前细胞分裂数量的主要机制，从而控制每个前脂肪细胞产生的脂肪细胞数量。我们将通过使用活细胞成像方法以及脂肪垫注射小鼠模型来验证这一假设。我们将首先使用诱导表达和快速降解p27的方法来确定p27在脂肪形成过程中何时以及如何调节细胞周期进程。然后，我们将使用CDK 4/6和CDK 2活性的活细胞报告基因来了解p27和p21何时以及如何协同调节CDK 4/6和CDK 2活性并控制产生的脂肪细胞的数量。这项工作的结果将是一个框架，如何克隆扩张期可以控制显着增加脂肪形成超过肥大，同时也确保祖细胞池得到维持。我们的研究结果可能不仅对脂肪组织的维持具有广泛的适用性，而且更普遍地对神经元、肌肉和其他终末分化组织的维持和再生具有广泛的适用性。

Revised Abstract Recent evidence shows that creating new fat cells (adipogenesis) can counteract the harmful metabolic effects of obesity, which have been shown to originate primarily from abnormal enlargement (hypertrophy) and resulting dysfunction of existing fat cells. Thus, how to increase adipogenesis over hypertrophy is of great interest. Previous work showed that preadipocytes in vivo and in vitro must complete one or more cell cycles, a process referred to as clonal expansion, before they can differentiate into adipocytes. One of the most striking genetic examples of increased adipogenesis was demonstrated by manipulating this clonal expansion period by knocking out the cell cycle inhibitors p21 and p27. Knockout of either p21 or p27 in mice results in a 2-fold increase in adipogenesis and fat mass, but knockout of both results in a dramatic 6-fold increase. Previous work suggests that p21 and p27 are regulated by very different mechanisms and are active at different times during adipogenesis. In recent published work, we identified the molecular mechanisms that could explain the increased adipogenesis observed in the p21- knockout mouse. However, how p27 regulates adipogenesis and tissue mass and how p21 and p27 synergize are poorly understood. We hypothesize that the synergistic expression of p21 and p27 during adipogenesis is the primary mechanism that controls the number of cell divisions before differentiation, and thus controls the number the adipocytes produced per preadipocyte. We will test this hypothesis by using live-cell imaging approaches, together with a fat-pad injection mouse model. We will first use methods to inducibly express and rapidly degrade p27 to determine when and how p27 regulates cell cycle progression during adipogenesis. We will then use live cell reporters for CDK4/6 and CDK2 activity to understand when and how p27 and p21 synergize to regulate CDK4/6 and CDK2 activity and control the number of adipocytes produced. The outcome of this work will be a framework how the clonal expansion period can be controlled to significantly increase adipogenesis over hypertrophy while also ensuring that the progenitor pool is maintained. Our results will likely have broad applicability not only to the maintenance of adipose tissue, but also more generally for the maintenance and regeneration of neuronal, muscle, and other terminally differentiated tissues.