The role of activin A in mediating liver repopulation after cell transplantation

描述（由申请人提供）：目前终末期肝病唯一可用的治疗方法是肝移植。由于肝移植名单上的患者数量远远超过可用的供体器官数量，因此替代治疗方法，如肝细胞移植，正在深入研究中。然而，迄今为止，除了在高度病理环境下的动物模型系统中进行的研究外，对成人肝细胞的研究只取得了非常有限的成功。几年前，我们的实验室发现，胎儿肝干细胞/祖细胞（FLSPC）可以通过与宿主肝细胞的细胞竞争，取代正常成年大鼠肝脏中20-25%的肝块。最近，我们观察到，随着大鼠年龄的增长，FLSPC的组织替代水平急剧增加（5倍）。此外，我们还发现，激活素A的水平，细胞周期阻滞和细胞凋亡的中介，在衰老的大鼠肝脏中增加。我们假设，衰老肝脏中激活素A表达的增加创造了一个宿主组织微环境，有利于移植的FLSPC替代肝细胞。我们进一步假设，激活素A通过增加移植的FLSPC和宿主肝细胞之间的细胞竞争，介导了衰老肝脏中再生细胞的增加。该项目的目的是确定激活素A介导移植的FLSPC肝脏再生的具体机制。实验将进行1)鉴定特异性凋亡、抗凋亡、增殖、细胞周期和衰老相关基因，这些基因在老年和年轻细胞移植受者移植FLSPC肝脏再生过程中表达增加或减少；2)在体外研究激活素A诱导离体肝细胞生长停滞和凋亡的机制；3)在肝脏疾病（即肝纤维化）的实验模型中确定内源性分泌激活素A是否会增强FLSPC的肝脏再生。在终末期之前使用细胞移植治疗这些患者的潜在用途将具有实质性的临床益处，特别是在老年人中。因此，确定特定的细胞类型，进行有效的细胞移植，以替代老年慢性肝病患者的肝实质，恢复肝功能，将是一种特别有价值的治疗方法。拟议研究的结果也将提供对基本要求和机制的关键理解，这将作为所有年龄段患者有效肝脏再生的指导。

DESCRIPTION (provided by applicant): The only currently available treatment for end stage liver disease is liver transplantation. Since the number of patients on the liver transplant list far exceeds the number of donor organs available, alternative treatment methods, such as hepatic cell transplantation, are under intensive investigation. However, studies to date with adult hepatocytes have met with only very limited success, except for those performed in animal model systems under highly pathologic circumstances. Several years ago, our laboratory discovered that fetal liver stem/progenitor cells (FLSPC) can replace 20-25% of hepatic mass in the normal adult rat liver by entering into cell competition with host hepatocytes. Recently, we observed that the level of tissue replacement by FLSPC increases dramatically (5-fold) as rats age. In addition, we have discovered that the level of activin A, a mediator of cell cycle arrest, as well as apoptosis, is increased in the aging rat liver. We hypothesize that increased activin A expression in the aging liver creates a host tissue microenvironment favoring replacement of hepatocytes by transplanted FLSPC. We hypothesize further that activin A mediates increased repopulation in the aging liver through increased cell competition between transplanted FLSPC and host hepatocytes. The goal of this project is to determine the specific mechanism(s) by which activin A mediates liver repopulation by transplanted FLSPC. Experiments will be performed 1) to identify specific apoptosis, anti-apoptosis, proliferation, cell cycle and senescence related genes whose expression is increased or decreased during liver repopulation by transplanted FLSPC in older vs younger cell transplant recipients, 2) to study the mechanism in vitro for activin A induced growth arrest and apoptosis in isolated hepatic cells and 3) to determine in an experimental model of liver disease (i.e. hepatic fibrosis) whether liver repopulation by FLSPC is augmented by endogenous secretion of activin A. Since both the number and age of patients with end-stage liver diseases will continue to increase over the next twenty years, the potential use of cell transplantation to treat these patients before the end-stage would be of substantial clinical benefit, especially in the elderly. Therefore, identifying the specific cell type(s) for effective cell transplantation to replace hepatic parenchyma and restore liver function in elderly patients with chronic liver diseases would represent a particularly valuable therapy. Results from the proposed studies will also provide critical understanding of the basic requirements and mechanisms that will serve as a guide for effective liver repopulation in patients of all ages. PUBLIC HEALTH RELEVANCE: We have found that repopulation of adult rat liver with transplanted fetal liver cells increases with the age of the recipient and can be achieved in rats with advanced hepatic fibrosis. Since whole liver transplants are generally not performed in patients beyond age 65, if our findings are applicable to humans, this suggests that liver cell transplantation would represent a major new therapeutic approach for treatment of chronic liver diseases, especially in the elderly.