The molecular regulation of IGFBP-3 in esophageal epthelial cells

描述（由申请人提供）：食管上皮含有增殖的基底细胞，这些基底细胞在向管腔表面迁移时经历分化，在管腔表面由于细胞凋亡而脱落。上皮不断更新，但正是这种持续的更新状态使上皮容易受到诸如感染、胃酸反流、放射、化疗和缺血等损伤性损伤。肽生长因子如表皮生长因子（EGF）和胰岛素样生长因子(IGF)-I调节食管上皮细胞的增殖、生长、迁移、分化和凋亡。它们在癌变等病理过程中也起着关键作用。在IGF结合蛋白中，IGFBP-3是IGF生物活性的主要负调控因子。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受缺氧诱导，受EGFR抑制。在食管中，IGFBP-3在癌前病变中经常上调，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在食道肿瘤中的生物学作用及其调控模式尚不清楚。我们的基本假设是IGFBP-3在微环境缺氧条件下的食管肿瘤生长中起关键作用。当食管上皮维持在生长因子（如EGF）存在的生理环境和正常的氧水平时，IGFBP-3处于失活状态，并允许igf信号通路的充分激活。然而，在食管肿瘤中，缺氧促进IGFBP-3的诱导。IGFBP-3基因受缺氧诱导因子（HIF）的转录激活。IGFBP-3 mRNA以帽依赖和帽独立的方式翻译，而后者在严重缺氧条件下被激活，以允许选择性的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，在适应缺氧微环境的一部分食道肿瘤细胞中，即使存在IGFBP-3，这些肿瘤细胞也能自主增殖。IGFBP-3抑制IGF信号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处于休眠状态，从而使肿瘤细胞受益。由于了解IGFBP-3在食管肿瘤生物学中的作用和调控非常重要，因此将通过以下相关的特定目的来实现这一假设：(1)特定目的1：确定IGFBP-3在食管肿瘤生物学中的作用；(2)特异性目的2：阐明HIF1a在IGFBP-3转录调控中的作用；(3)具体目的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.