Control of IGF-1 Gene Transcription by Growth Hormone

描述(申请人提供)：生长激素在生理学中起着关键作用，对正常的躯体生长、组织再生和修复以及中间代谢是必不可少的。生长激素的许多生物学效应是由胰岛素样生长因子I(IGF-I)介导的，IGF-I是一种保守的分泌性蛋白，其表达是由GH通过激活IGF-I基因转录而有效地诱导的。生长激素(GH)和胰岛素样生长因子-I(IGF-I)与多种癌症的发生发展相关联，以及它们对衰老的共同负面影响，表明GH异常表达IGF-I可能具有有害的致病后果，这意味着它的产生必须受到严格的调控，以维持体内平衡。这一应用的重点将是GH通过转录因子Stat5b控制IGF-I基因表达的机制。我们的研究将检验这一具有挑衅性的假设，即IGF-I与其他GH-Stat5b靶基因根本不同，并且多个分散的Stat5b结合转录增强子和其他潜在的抑制元件是控制强大生长因子表达所必需的复杂调控程序的关键因子，具有积极和消极的生物学效应。以下两个特定的目的将检验这一想法：1.鉴定和鉴定负责GH和Stat5b调节的Igf1基因转录的染色体增强子和抑制子。有待检验的主要假设是，具有不同功能特性的离散的生长激素激活的增强子与单个Igf1基因启动子相互作用，并共同负责 介导对生长激素的急性转录反应。一个推论是，Igf1染色质中的一些GH调节元件不是转录增强子，而是假定的负调节因子，在缺乏GH或隔离GH刺激的阳性部位的Stat5b的情况下干扰Igf1启动子的功能。2.明确GH和Stat5b在调节Igf1基因染色质可塑性中的作用。需要检验的主要假设是，持续的生长激素介导的信号转导需要在Igf1启动子处建立开放的染色质环境，但Stat5b是必不可少的。一个必然的假设是，Stat5b负责 GH快速诱导Igf1基因转录所必需的急性染色质修饰。所提出的研究对建立生长激素作用于调控IGF-I基因表达的新机制具有潜在的影响，并导致对生长激素介导的信号如何控制表观遗传途径、染色质可塑性和基因调控的新的生理学意义的见解。 与公共健康相关：GH的作用对于人类和其他物种的正常躯体生长、组织维持和修复以及正常的中间代谢是必不可少的，主要由IGF-I介导，IGF-I是一种保守的分泌蛋白，其表达由GH诱导。GH-IGF-I途径也与癌症的发展和衰老的负面方面有关，这意味着它的异常调节具有有害的致病后果，必须至少在成人中限制其活性以维持体内平衡。这一应用的重点是GH通过保守的转录因子Stat5b调节IGF-I基因表达的机制，这是一个长期目标的一部分，目的是确定GH如何控制表观遗传途径，以及染色质修饰如何以具有生理意义的方式有效地调节Stat5b的功能。

DESCRIPTION (provided by applicant): GH plays a pivotal role in physiology, and is essential for normal somatic growth, tissue regeneration and repair, and intermediary metabolism. Many of the biological effects of GH are mediated by insulin- like growth factor I (IGF-I), a conserved secreted protein whose expression is potently induced by GH by activation of IGF-I gene transcription. As evidenced by linkage of GH and IGF-I with development of several cancers, and by their collective negative impact on aging, aberrant expression of IGF-I by GH may have deleterious pathogenic consequences, implying that its production must be tightly regulated to maintain homeostasis. The focus of this application will be on mechanisms by which GH controls IGF-I gene expression via the transcription factor Stat5b. Our studies will test the provocative hypothesis that IGF-I is fundamentally different from other GH-Stat5b target genes, and that multiple dispersed Stat5b-binding transcriptional enhancers, and other potentially inhibitory elements, are key agents in a complex regulatory program necessary to control expression of a potent growth factor with both positive and negative biological effects. The following two Specific Aims will test this idea: 1. To identify and characterize the chromosomal enhancers and repressors responsible for GH- and Stat5b-regulated Igf1 gene transcription. The major hypothesis to be tested is that discrete GH- activated enhancers with distinct functional properties interact with individual Igf1 gene promoters and are responsible collectively for mediating the acute transcriptional response to GH. A corollary hypothesis is that some GH-regulated elements in Igf1 chromatin are not transcriptional enhancers, but rather are putative negative regulators, and interfere with Igf1 promoter function in the absence of GH or sequester GH-stimulated Stat5b from positive sites. 2. To define the roles of GH and Stat5b in regulating chromatin plasticity of the Igf1 gene. The major hypothesis to be tested is that sustained GH-mediated signaling is required to establish an open chromatin environment at the Igf1 promoters, but that Stat5b is dispensable. A corollary hypothesis is that Stat5b is responsible for the acute chromatin modifications necessary for rapid induction of Igf1 gene transcription by GH. Proposed research has the potential impact to establish a new paradigm about mechanisms of GH action to regulate IGF-I gene expression, and to lead to new physiologically significant insights about how GH-mediated signaling controls epigenetic pathways, chromatin plasticity, and gene regulation. PUBLIC HEALTH RELEVANCE: GH actions are essential for normal somatic growth, for tissue maintenance and repair, and for normal intermediary metabolism in humans and other species, and are primarily mediated by IGF-I, a conserved secreted protein whose expression is induced by GH. The GH - IGF-I pathway also has been implicated in cancer development and in the negative aspects of aging, implying that its aberrant regulation has deleterious pathogenic consequences, and that its activity must be constrained at least in the adult to maintain homeostasis. The focus of this application is on the mechanisms by which GH regulates IGF-I gene expression through the conserved transcription factor, Stat5b, and is part of a longer-term goal to define how GH actions control epigenetic pathways and how chromatin modifications can potently regulate Stat5b functions in physiologically meaningful ways.