REGULATIONS OF BETA CATENIN BY PROTEIN PHOSPHATASE 2A

对结直肠癌的分析表明，Wnt通路的失调是其恶性转化的重要步骤。因此，了解这一途径的调控可能有助于改善人类癌症的预防和治疗。Wnt通路在结肠中的一个关键功能是通过磷酸化调节的信号转导通路调节β -连环蛋白依赖性转录。虽然已经有几种激酶被证明可以调节Wnt通路，但之前没有磷酸酶被明确地确定为该通路的一个组成部分。我们的数据表明，蛋白磷酸酶2A （PP2A）的B56调控亚基与大肠腺瘤性息肉病（APC）蛋白相互作用，B56过表达导致β -连环蛋白丰度降低，导致哺乳动物组织培养细胞和爪蟾胚外植体中β -连环蛋白依赖基因表达降低。B56的过度表达似乎激活了磷酸化依赖的蛋白酶体介导的β -连环蛋白降解。此外，如果PP2A抑制Wnt信号传导，磷酸酶抑制剂冈田酸会导致β -连环蛋白水平升高。因此，我们的研究结果表明，含有b56的PP2A全酶是Wnt信号转导途径的关键调节因子。本提案的目的是明确B56和PP2A在Wnt信号转导通路和肠癌发展中的确切作用。我们建议(1)确定作为PP2A:B56底物的关键Wnt通路组分，(2)确定与B56通路的其他组分相互作用，(3)测试Wnt是否部分通过调节PP2A:B56活性来传播其信号，以及(4)使用转基因小鼠来验证B56过表达抑制结肠癌发生的假设，而显性阴性B56等位基因促进结肠癌发生。

The analysis of colorectal carcinomas suggests that deregulation of the Wnt pathway is an essential step in their malignant transformation. Understanding the regulation of this pathway may therefore lead to improved prevention and treatment of human carcinomas. One crucial function of the Wnt pathway in the colon is to regulate beta-catenin-dependent transcription via a phosphorylation-regulated signal transduction pathway. Although several kinases have been shown to regulate the Wnt pathway, no phosphatase has previously been clearly identified as a component of this pathway. Our data show that the B56 regulatory subunits of protein phosphatase 2A (PP2A) interact with the adenomatous polyposis coli (APC) protein, and that B56 overexpression causes decreased beta-catenin abundance, leading to decreased beta-catenin-dependent gene expression in mammalian tissue culture cells and Xenopus embryo explants. The overexpression of B56 appears to activate the phosphorylation-dependent, proteasome-mediated, degradation of beta-catenin. In addition, the phosphatase inhibitor okadaic acid causes increased levels of beta-catenin, as would be expected if PP2A inhibited Wnt signaling. Our results suggest, therefore, that the B56-containing PP2A holoenzyme is a key regulator of a Wnt signal transduction pathway. The objective of this proposal is to define the precise role of B56 and PP2A in the Wnt signal transduction pathway and the development of intestinal cancers. We propose to (1) identify critical Wnt pathway components that are PP2A:B56 substrates, (2) determine with which additional components of the pathway B56 interacts, (3) test if Wnt propagates its signal in part by regulating PP2A:B56 activity, and (4) use transgenic mice to test the hypothesis that overexpression of B56 suppresses colon carcinogenesis, while dominant-negative B56 alleles promote it.