Epigenetic and Cell Cycle Functions of Glucocorticoids in Erythropoietic Stress

描述(申请人提供)：糖皮质激素(GC)是压力的荷尔蒙调节剂。它们加快了红细胞的生成速度，这一效应是由小鼠遗传学、体外红细胞生成系统和GCs失调的人类疾病综合征很好地确立的。然而，使用GCs治疗贫血是复杂的，因为它们有严重的副作用，因此仅限于促红细胞生成素(EPO)治疗无效或禁忌症的情况，包括钻石黑粉贫血和其他骨髓衰竭综合征。GC的翻译重要性从它们在目前正在开发的用于体外产生用于输血的红细胞的系统中的使用中可见一斑。了解GCs在红系祖细胞中的分子作用有助于开发副作用比GCs少的新型红系造血刺激剂，并提高体外生成红细胞的效率。在功能上，GCs通过延迟红系祖细胞从自我更新到分化的转换而增加红系祖细胞的造血率。这种作用背后的分子机制在很大程度上是未知的。基于我们最近发表的工作和初步数据，我们提出了一个新的GC作用假说，该假说涉及细胞周期S期和DNA甲基化作为新的调控靶点。我们最近发现，胎儿和成人的红细胞生成都需要基因组范围的DNA去甲基化，这是体细胞中一种独特的全球表观遗传修饰(Searstone等人，科学，2011年)。全局性去甲基化与红系基因启动子的去甲基化密切相关，是其转录激活的限速因素。此外，整体去甲基化依赖于细胞周期的S时相的显著变化，随着从自我更新到分化的转换，细胞周期变得更短，速度快50%。细胞周期蛋白依赖性激酶抑制因子(CDKI)p57Kip2是这种转换的关键负性调节因子。P57Kip2也是GCs的直接转录靶点。我们的初步数据显示，在GC存在的情况下，红系祖细胞未能下调p57Kip2，未能加速S期，未能进行DNA去甲基化，从而延迟了红系基因的转录。在这个方案中，我们研究了一种假设，即在红系应激过程中，高水平的GCs通过诱导p57Kip2，从而抑制S期加速、DNA整体去甲基化和红系基因诱导，从而抑制从自我更新到分化的转换。我们将使用红系应激小鼠模型和Gc受体p57Kip2或DNA甲基转移酶1(DNMT1)缺失或突变的小鼠在体内验证这一假说，目的如下三：1)确定p57Kip2在GC介导的红系应激反应中的作用2)确定GC在应激过程中是否延长红系祖细胞的S期3)确定GC是否推迟应激过程中全局DNA去甲基化的开始。这项工作集中在一种独特的表观遗传修饰上，并有可能识别概念上新的调节机制，并对EPO耐药贫血的治疗具有翻译意义。

DESCRIPTION (provided by applicant): Glucocorticoids (GCs) are hormonal regulators of stress. They accelerate red blood cell production rate, an effect that is well established by mouse genetics, by in vitro erythropoiesis systems, and by human disease syndromes in which GCs are dysregulated. The use of GCs in the treatment of anemia is complicated, however, by their severe side effects, and is therefore limited to conditions where Erythropoietin (Epo) treatment is refractory or contraindicated, including Diamond Blackfan Anemia and other bone-marrow failure syndromes. The translational importance of GCs is apparent from their use in systems currently under development for the in-vitro generation of red blood cells for transfusion. Understanding the molecular action of GCs in erythroid progenitors could facilitate the development of novel erythropoiesis-stimulating agents that have fewer side-effects than GCs, and that improve the efficiency of generating red blood cells in vitro. Functionally, GCs increase erythropoietic rate by delaying the switch from self-renewal to differentiation in erythroid progenitors. The molecular mechanisms underlying this action are largely unknown. Based on our recently published work and on preliminary data, we propose a novel hypothesis of GC action that implicates the cell cycle S phase and DNA methylation as novel regulatory targets. We recently showed that both fetal and adult erythropoiesis entail genome-wide DNA demethylation, a unique global epigenetic modification in somatic cells (Shearstone et al., Science 2011). Global demethylation is tightly correlated with demethylation at erythroid gene promoters, and is a rate-limiting for their transcriptional activation. Further, global demethylatin is dependent on a marked change in S phase of the cell cycle, which becomes shorter and 50% faster with the switch from self-renewal to differentiation. The cyclin-dependent kinase inhibitor (CDKI) p57KIP2 is a key negative regulator of this switch. p57KIP2 is also a direct transcriptional target of GCs. Our preliminary data show that, in the presence of GCs, erythroid progenitors fail to downregulate p57KIP2, fail to accelerate S phase, and fail to undergo DNA demethylation, thereby delaying erythroid gene transcription. In this proposal, we investigate the hypothesis that high levels of GCs during erythropoietic stress inhibit the switch from self-renewal to differentiation by inducing p57KIP2, thereby inhibiting S phase acceleration, global DNA demethylation and erythroid gene induction. We will test this hypothesis in vivo using mouse models of erythropoietic stress and mice deleted or mutated for either p57KIP2, the GC receptor, or DNA methyl transferase 1 (Dnmt1), with the following three aims: 1) Determine the role of p57KIP2 in the GC-mediated erythropoietic stress response 2) Determine whether GCs prolong S phase in erythroid progenitors during stress 3) Determine whether GCs delay the onset of global DNA demethylation during stress. This work focuses on a unique epigenetic modification and has the potential to identify conceptually novel regulatory mechanisms, with translational implications for therapy of Epo-resistant anemia.