The molecular regulation of IGFBP-3 in esophageal epthelial cells

描述(申请人提供)：食道上皮含有增殖的基底细胞，当它们向管腔表面迁移时会发生分化，在那里它们会因为细胞凋亡而脱落。上皮不断更新，但正是这种持续的更新状态使上皮如此容易受到伤害，如感染、酸反流、辐射、化疗和缺血。多肽生长因子如表皮生长因子(EGF)和胰岛素样生长因子(IGF)-I调节食道上皮细胞的增殖、生长、迁移、分化和凋亡。它们在致癌等病理过程中也起着关键作用。在IGF结合蛋白中，IGFBP-3是IGF生物活性的主要负性调节因子。IGF-I和IGF-II都与食道癌有关。本提案的重点是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(IGF)并调节其生物活性。我们发现IGFBP-3在缺氧条件下被诱导，并被EGFR抑制。在食道中，IGFBP-3通常在癌前病变中上调，在浸润性腺癌和鳞癌中经常过表达，同时伴有EGFR过表达。然而，IGFBP3在食道肿瘤中的生物学作用(S)及其调控方式尚不清楚。我们的基本假设是IGFBP-3在低氧条件下的微环境中对食道肿瘤的生长起关键作用。当食道上皮在生长因子(如EGF)和正常氧水平下维持在生理环境中时，IGFBP-3是不活跃的，并允许IGF信号通路的完全激活。然而，在食道肿瘤中，低氧促进了IGFBP-3的诱导。低氧诱导因子(HIF)在转录水平激活IGFBP-3基因。IGFBP-3mRNA以帽依赖和帽非依赖性的方式翻译，而后者在严重缺氧条件下被激活，以允许选择性的IGFBP-3蛋白质合成。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，在已经适应低氧微环境的一组食道肿瘤细胞中，即使在IGFBP-3存在的情况下，这种肿瘤细胞也可以独立于IGF增殖。IGFBP-3对IGF信号的抑制可能通过抑制细胞的合成代谢活动，使其在缺乏足够的氧气供应和能源的情况下休眠，从而使肿瘤细胞受益。由于了解IGFBP-3在食道肿瘤生物学中的作用和调控非常重要，因此这一假说将通过下列相互关联的特定目标来实现：(1)特定目标1：确定IGFBP-3在食道肿瘤生物学中的作用；(2)特定目标2：阐明HIF1a在IGFBP-3转录调控中的作用；以及(3)特定目标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.