Control of IGF-1 Gene Transcription by Growth Hormone

描述（申请人提供）：生长激素在生理上起着关键作用，是正常体细胞生长、组织再生和修复以及中间代谢所必需的。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是一种保守的分泌蛋白，其表达是通过激活IGF-I基因转录而被生长激素强烈诱导的。正如生长激素和IGF-I与几种癌症发展的联系以及它们对衰老的集体负面影响所证明的那样，生长激素异常表达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调节以维持体内平衡。本应用的重点将是GH通过转录因子Stat5b控制igf - 1基因表达的机制。我们的研究将验证igf - 1与其他GH-Stat5b靶基因根本不同的假设，以及多个分散的stat5b结合转录增强子和其他潜在抑制元件是复杂调控程序中的关键因子，这些程序是控制具有积极和消极生物效应的强效生长因子表达所必需的。以下两个具体目标将测试这个想法：鉴定和表征负责GH-和stat5b调节的Igf1基因转录的染色体增强子和抑制子。要验证的主要假设是，具有不同功能特性的独立生长激素激活增强子与单个Igf1基因启动子相互作用，并共同负责

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.