The Role of Beta-catenin/Tcf Pathway Defects in Cancer

描述(申请人提供)：WNTS是一个高度保守的分泌蛋白家族，在发育和成人组织中具有关键作用，包括调节细胞命运决定、增殖、细胞存活和运动的功能。WNTs与细胞表面的同源卷曲受体和低密度脂蛋白受体相关蛋白(LRP)共受体复合体结合，通过不同的途径介导细胞内信号事件。可以说，“典型的”Wnt途径是已发现的Wnt依赖途径中最具特征的，而在典型途径中的关键因素的突变已被最明显地与癌症有关。在典型的途径中，β-连环蛋白是一个重要的效应器，某些Wnt配体(例如，Wnt-1)起稳定β-连环蛋白的作用。糖原合成酶激酶3β(GSK3β)蛋白与Axin和APC(结肠腺瘤性息肉病)肿瘤抑制蛋白和其他激酶协同作用，在其氨基(N)末端的多个丝氨酸和苏氨酸残基上磷酸化(-catenin)。磷酸化的β-连环蛋白被迅速泛素化，然后被26S蛋白酶体降解。WNT配体与FrizzledLRP5/6共受体复合体结合导致GSK3β抑制，从而稳定β-连环蛋白。在人类癌症中起主要作用的突变机制包括APC或Axin1肿瘤抑制基因失活或激活β-连环素N端磷酸化和降解基序的(致癌)突变。通过Wnts或与癌症相关的突变稳定β-catenin，导致β-catenin核聚集，并增强其与T细胞因子(TCF)家族转录因子的结合。反过来，β-连环蛋白/TCF复合体调节调控细胞命运、增殖和其他过程的一整套基因产物的表达。已经提供了关于β-连环蛋白/TCF参与特定靶基因调控的数据，包括关键的促生长基因，如c-myc和细胞周期蛋白d1。然而，对β-连环蛋白/TCF通路缺陷在结肠癌和其他癌症发病机制中的作用的了解还远未完全。鉴于这一背景，我们提出了以下目标：1)继续努力寻找在结肠癌和其他癌细胞中受β-连环蛋白/TCF调控表达的下游基因。2)利用强大的体外系统来评估选定的β-连环蛋白/TCF靶基因及其蛋白产物在结肠癌和其他癌症表型改变中的作用。3)利用小鼠转基因和基因敲除模型，继续成功地评估β-连环蛋白/TCF和选定的下游靶基因在肿瘤发生中的作用。我们的研究将突出导致结肠和其他癌细胞表型特征改变的关键分子。对癌症的新诊断标记物和新的治疗目标和方法的洞察力也是可望的。

DESCRIPTION (provided by applicant): The Wnts are a highly conserved family of secreted proteins with critical roles during development and in adult tissues, including functions in regulating cell fate determination, proliferation, cell survival, and motility. Wnts bind to cognate frizzled receptor and LDL-Receptor-Related Protein (LRP) co-receptor complexes on the cell surface and mediate intracellular signaling events through distinct pathways. The "canonical" Wnt pathway is arguably the best characterized of the Wnt-dependent pathways uncovered, and mutations in key factors in the canonical pathway have been most clearly implicated in cancer. In the canonical pathway, the beta-catenin protein is a vital effector, with certain Wnt ligands (e.g., Wnt-1) acting to stabilize beta-catenin. The glycogen synthase kinase 3beta (GSK3beta) protein functions in concert with the Axin and APC (adenomatous polyposis coli) tumor suppressor proteins and other kinases to phosphorylate (-catenin at multiple serine and threonine residues in its amino (N)-terminus. The phosphorylated beta-catenin is rapidly ubiquitinated and then degraded by the 26S proteasome. Wnt ligand binding to the Frizzled-LRP5/6 co- receptor complex leads to GSK3beta inhibition, with resultant beta-catenin stabilization. Mutational mechanisms with major roles in dysregulating beta-catenin in human cancer, include inactivation of the APC or Axin1 tumor suppressors or activating (oncogenic) mutations in beta-catenin's N-terminal phophorylation and degradation motif. Stabilization of beta-catenin via Wnts or cancer-related mutations leads to beta-catenin nuclear accumulation and its enhanced binding to T cell factor (TCF) family of transcription factors. In turn, beta-catenin/TCF complexes regulate expression of the panoply of gene products that regulate cell fate, proliferation, and other processes. Data implicating beta-catenin/TCF in the regulation of specific target genes, including key growth promoting genes, such as c-MYC and cyclin D1, have been offered. However, understanding of the role of beta- catenin/TCF pathway defects in the pathogenesis of colon and other cancers remains far from complete. Given this background, we propose the following aims: 1) To continue productive efforts to identify downstream genes whose expression is regulated by beta-catenin/TCF in colon and other cancer cells. 2) To utilize robust in vitro systems to assess the role of selected beta-catenin/TCF target genes and their protein products in the altered phenotype of colon and other cancers. 3) To continue successful efforts to assess the role of beta-catenin/TCF and selected downstream target genes in tumorigenesis, using mouse transgenic and knockout models. Our studies will highlight critical molecules contributing to the altered phenotypic traits of colon and other cancer cells. Insights into new diagnostic markers and novel therapeutic targets and approaches for cancer are also expected.