C/EBP alpha in Aging Liver

描述（由申请方提供）：部分肝切除术后，老化的肝脏丧失增殖能力，导致老年人死亡率较高。我们的实验室研究控制肝脏增殖的分子机制。肝脏特异性蛋白CCAAT/增强子结合蛋白α（C/EBPalpha）是肝脏增殖的强抑制剂。C/EBPalpha通过与细胞周期蛋白依赖性激酶2和4的直接相互作用抑制年轻肝脏的增殖。在脂肪组织中，C/EBPalpha通过抑制E2 F转录引起生长停滞。我们最近发现，衰老将肝脏中的C/EBPalpha从cdks的抑制转换为E2 F的抑制，E2 F是通常在脂肪组织中起作用的途径。在老年肝脏中，在高MW复合物C/EBPalpha-Rb-E2 F4-Brm中观察到C/EBPalpha。该复合物占据E2 F依赖性启动子并阻断肝增殖所需表达的基因的激活。部分肝切除术后未能减少这种复合物似乎是老年肝脏增殖反应降低的主要原因。复合物的三个组分Rb、C/EBPalpha和E2 F4在老年肝脏中差异磷酸化。C/EBPalpha在Serl 93处的磷酸化是年龄特异性复合物形成中的关键事件。我们最近鉴定了cdk 4和PP 2A作为调节C/EBPalpha的Serl 93的磷酸化-去磷酸化的酶。在本申请中，我们建议检查老化对这些信号转导途径的影响，并确定它们的作用-在C/EBPalpha-Rb-E2 F4-Brm复合物的出现和增殖反应的减少。我们的工作假设是：1）C/EBPalpha、E2 F4和Rb的磷酸化促进年龄相关复合物的形成，2）肝脏中C/EBPalpha的Serl 93的磷酸化由cdk 4介导，并通过PI 3 K-Akt-PP 2A途径的作用逆转，3）衰老的肝脏通过减弱PI 3 K-Akt-PP 2A途径和通过cdk 4的活化来增加年龄特异性复合物。在具体目标1中，我们将研究衰老激活cdk 4并下调PP 2A活性的机制。特异性目的2检查Rb和E2 F4的磷酸化是否影响它们形成年龄特异性C/EBPalpha复合物的能力。在特定目标3中，将鉴定复合物的其他蛋白质，并检查它们在肝脏增殖中的作用。根据本项目获得的数据，我们计划开始制定一项纠正老年人肝脏增殖的策略。

DESCRIPTION (provided by applicant): An aging liver loses the ability to proliferate after partial hepatic resections leading to a higher mortality in the elderly. Our laboratory' investigates molecular mechanisms that control liver proliferation. Liver specific protein, CCAAT/Enhancer Binding Protein alpha (C/EBPalpha), is a strong inhibitor of liver proliferation. C/EBPalpha inhibits proliferation of young livers through direct interactions with cyclin dependent kinases 2 and 4. In adipose tissues, C/EBPalpha causes growth arrest via repression of E2F transcription. We have recently found that aging switches C/EBPalpha in liver from inhibition of cdks to repression of E2F, the pathway that normally operates in adipose tissues. In old livers, C/EBPalpha is observed in a high MW complex C/EBPalpha-Rb-E2F4-Brm. This complex occupies E2F-dependent promoters and blocks activation of genes whose expression is required for liver proliferation. A failure to diminish this complex after partial hepatectomy seems to be a major cause for the reduced proliferative response in old livers. Three components of the complex, Rb, C/EBPalpha and E2F4, are differentially phosphorylated in old livers. Phosphorylation of C/EBPalpha at Serl93 is a key event in the formation of the age-specific complex. We have recently identified cdk4 and PP2A as enzymes that regulate phosphorylation-dephosphorylation of Serl93 of C/EBPalpha. In this application, we propose to examine the effects of aging on these signal transduction pathways and determine their roles -in the appearance of the C/EBPalpha-Rb-E2F4-Brm complex and in the reduction of the proliferative response. Our working hypotheses are that: 1) Phosphorylation of C/EBPalpha, E2F4, and Rb promotes the formation of the age related complex, 2) Phosphorylation of Serl93 of C/EBPalpha in the liver is mediated by cdk4 and is reversed by the action of the PI3K-Akt-PP2A pathway, 3) Aging liver increases the age-specific complex by diminishing the PI3K-Akt-PP2A pathway and by activation of cdk4. In Specific Aim 1, we will examine mechanisms by which aging activates cdk4 and down-regulates activity of PP2A. Specific Aim 2 examines whether phosphorylation of Rb and E2F4 affects their ability to form the age-specific C/EBPalpha complex. In Specific Aim 3, additional proteins of the complex will be identified and their role in liver proliferation will be examined. Based on data obtained in this project, we are planning to start developing a strategy to correct liver proliferation in elderly.