The molecular regulation of IGFBP-3 in esophageal epthelial cells

描述（由申请方提供）：食管上皮含有增殖的基底细胞，这些细胞在向管腔表面迁移时发生分化，在管腔表面由于细胞凋亡而脱落斯劳。上皮不断更新，但正是这种持续的更新状态使上皮对有害的损伤如此敏感，如感染、胃酸反流、放射、化疗和局部缺血。肽类生长因子如表皮生长因子（EGF）和胰岛素样生长因子（IGF）-I调节食管上皮细胞增殖、生长、迁移、分化和凋亡。它们还在病理过程中发挥关键作用，如致癌作用。在IGF结合蛋白中，IGFBP-3是IGF生物活性的主要负调节剂。IGF-I和IGF-II都与食管癌有关。目前的建议集中在胰岛素样生长因子结合蛋白-3（IGFBP-3）。IGFBP-3结合胰岛素样生长因子（IGFs）并调节其生物活性。IGFBP-3由缺氧诱导并被EGFR抑制。在食管中，IGFBP-3通常在癌前病变中上调，在浸润性腺癌和鳞状细胞癌中经常过表达，同时伴有EGFR过表达。然而，IGFBP-3的生物学作用及其调节模式在食管肿瘤中尚不清楚。我们的基本假设是IGFBP-3在微环境中缺氧条件下的食管肿瘤生长中起关键作用。当食管上皮在生长因子（例如EGF）和正常氧水平存在下维持在生理环境中时，IGFBP-3是无活性的并且允许IGF信号传导途径的完全激活。然而，在食管肿瘤中，缺氧促进IGFBP-3诱导。IGFBP-3基因是由缺氧诱导因子（HIF）转录激活的。IGFBP-3 mRNA以帽依赖性和帽非依赖性方式翻译，而后者在严重缺氧条件下被激活以允许选择性IGFBP-3蛋白合成。一旦表达和分泌，IGFBP-3中和IGF并在肿瘤细胞中发挥抗增殖或促凋亡作用。然而，在已经适应缺氧微环境的食管肿瘤细胞的子集中，即使在IGFBP-3存在下，这样的肿瘤细胞也可以自主增殖IGF。IGFBP-3对IGF信号传导的抑制可能通过抑制细胞合成代谢活动使肿瘤细胞在缺乏足够的氧气供应和能量来源的情况下休眠而有益于肿瘤细胞。由于了解IGFBP-3在食管肿瘤生物学中的作用和调节是重要的，因此将通过以下相互关联的特定目的来追求该假设：（1）特定目的1：确定IGFBP-3在食管肿瘤生物学中的作用;（2）特定目的2：阐明HIF 1a在IGFBP-3转录调节中的作用;以及（3）特定目的3：确定缺氧如何调节IGFBP-3的翻译。

DESCRIPTION (provided by applicant): The esophageal epithelium contains proliferating basal cells that undergo differentiation as they migrate towards the luminal surface, where they slough off due to apoptosis. The epithelium is renewed continuously, but it is this constant state of renewal that makes the epithelium so susceptible to injurious insults, such as infection, acid reflux, radiation, chemotherapy, and ischemia. Peptide growth factors such as epidermal growth factor (EGF) and insulin-like growth factor (IGF)-I regulate esophageal epithelial cell proliferation, growth, migration, differentiation, and apoptosis. They also play a critical role in pathological processes such as carcinogenesis. Amongst the IGF binding proteins, IGFBP-3 is a major negative regulator of the bioactivities of IGF. Both IGF-I and IGF-II are implicated in esophageal cancer. The present proposal focuses upon insulin like growth factor binding protein-3 (IGFBP-3). IGFBP-3 binds insulin-like growth factors (IGFs) and regulates their bioactivities. IGFBP-3 is induced by hypoxia and suppressed by EGFR as shown by us. In the esophagus, IGFBP-3 is often upregulated in preneoplastic lesions and frequently overexpressed in invasive adenocarcinomas and squamous cell carcinomas with concurrent EGFR overexpression. However, the biological role(s) of IGFBP-3 and the mode of its regulation are not known in esophageal tumors. Our fundamental hypothesis is that IGFBP-3 plays a critical role in esophageal tumor growth under hypoxic conditions in the microenvironment. When the esophageal epithelium is maintained in a physiological milieu in the presence of growth factors (e.g. EGF) and a normal level of oxygen, IGFBP-3 is inactive and permissive for the full activation of the IGF-signaling pathway. However, in esophageal tumors, hypoxia fosters IGFBP-3 induction. IGFBP-3 gene is transcriptionally activated by hypoxia inducible factor (HIF). IGFBP-3 mRNA is translated in cap-dependent as well as cap-independent manners, while the latter is activated under severe hypoxic conditions to permit selective IGFBP-3 protein synthesis. Once expressed and secreted, IGFBP-3 neutralizes IGF and exerts antiproliferative or proapoptotic effects in tumor cells. However, in a subset of esophageal tumor cells, which have adapted to a hypoxic microenvironment, such tumor cells can proliferate autonomous of IGF even in the presence of IGFBP-3. Inhibition of IGF signaling by IGFBP-3 may benefit tumor cells by suppressing cellular anabolic activities to make them dormant in the absence of sufficient oxygen supply and energy sources. This hypothesis will be pursued through the following interrelated Specific Aims since it is important to understand the role and regulation of IGFBP-3 in esophageal tumor biology: (1) Specific Aim 1: To determine the role of IGFBP-3 in esophageal tumor biology; (2) Specific Aim 2: To elucidate the role of HIF1a in the transcriptional regulation of IGFBP-3; and (3) Specific Aim 3: To determine how hypoxia regulates the translation of IGFBP-3.