The Role of PHLPP in Colon Cancer

描述（由申请人提供）：结直肠癌是美国癌症相关死亡的第二大原因，每年诊断出112,000例新病例，2007年估计约有52,000例死亡。Akt和蛋白激酶C （PKC）的失调通过促进细胞增殖和抑制细胞凋亡参与肿瘤发生。Akt和PKC的信号激活过程已被详细研究。然而，人们对这些信号一旦被激活是如何被关闭的知之甚少。最近，我们发现了一个新的蛋白磷酸酶家族，PHLPP （PH结构域富含亮氨酸重复蛋白磷酸酶），可直接使Akt和PKC去磷酸化。然而，PHLPP在癌症中的作用尚未明确。在初步研究中，我们发现PHLPP表达缺失在人类结直肠癌标本中发生频率很高。此外，我们的研究表明，PHLPP在调节细胞极性方面发挥作用。根据我们的研究结果，推动这一提议的中心假设是，PHLPP除了能够关闭Akt和PKC激活的生长信号外，还通过调节细胞极性作为肿瘤抑制因子。我们研究的长期目标是更好地了解PHLPP的生理作用及其在体内结肠癌发生和进展中的作用。具体目的：目的1：明确PHLPP下调的分子机制。目的是研究导致PHLPP在癌症中下调的潜在机制。我们将验证细胞中PHLPP的表达水平受泛素蛋白酶体途径控制，阻止PHLPP降解导致该蛋白上调的假设。目的2：确定PHLPP在维持细胞极性中的作用。我们假设PHLPP通过调节细胞极性和细胞生长来发挥其肿瘤抑制作用。PHLPP对上皮细胞极性建立的功能作用将被确定。为了阐明潜在的机制，我们将通过与极性蛋白Scribble结合来调节PKC的活性，从而验证PHLPP是上皮连接形成所必需的假设。目的3：探讨PHLPP在体内肿瘤发生中的作用。推动这一目标的假设是PHLPP表达的缺失有助于结直肠肿瘤的发生和发展。我们将探讨当PHLPP被敲除后，无论是单纯敲除还是与其他致癌因素联合敲除，肿瘤发病率是否会增加。此外，我们将利用敲除小鼠评估改变的细胞极性对肠道上皮正常发育和肿瘤起始的贡献。

DESCRIPTION (provided by applicant): Colorectal cancer is the second leading cause of cancer-related deaths in the United States with 112,000 new cases diagnosed per year and approximately 52,000 deaths estimated in 2007. Dysregulation of Akt and protein kinase C (PKC) contributes to tumorigenesis by promoting cell proliferation and inhibiting apoptosis. The signaling activation process of Akt and PKC has been studied in great detail. However, little is known about how the signals are turned off once activated. Recently, we have identified a family of novel protein phosphatases, PHLPP (PH domain Leucine-rich-repeats Protein Phosphatase) that directly dephosphorylates Akt and PKC. However, the role of PHLPP in cancer has not been defined. In the preliminary studies, we found that loss of PHLPP expression occurs with high frequency in human colorectal cancer specimens. Furthermore, our studies have suggested that PHLPP plays a role in regulating cell polarity. In light of our findings, the central hypothesis driving this proposal is that PHLPP serves as a tumor suppressor by regulating cell polarity in addition to its ability of turning off the growth signaling activated by Akt and PKC. The long-term goal of our studies is to better understand the physiological role of PHLPP and its contribution to colon cancer development and progression in vivo. The Specific Aims are: Aim 1: To define the molecular mechanism of PHLPP downregulation. The goal of this aim is to investigate the potential mechanism leading to PHLPP downregulation in cancer. We will test the hypothesis that the expression level of PHLPP is controlled by the ubiquitin proteasome pathway in cells, and preventing PHLPP degradation leads to upregulation of the protein. Aim 2: To determine the role PHLPP in maintaining cell polarity. We hypothesize that PHLPP exerts its function as a tumor suppressor by regulating cell polarity and cell growth. The functional effect of PHLPP on establishing epithelial cell polarity will be determined. To elucidate the underlying mechanism, we will test the hypothesis that PHLPP is required for epithelial junction formation by modulating PKC activity via binding to the polarity protein Scribble. Aim 3: To delineate the role of PHLPP in tumorigenesis in vivo. The hypothesis driving this aim is that loss of PHLPP expression contributes to the initiation and progression of colorectal tumors. We will address the question whether there is an increase of tumor incidence when PHLPP is knocked out, both basally and in combination with other carcinogenic factors. Furthermore, we will assess the contribution of altered cell polarity in normal development of gut epithelium and tumor initiation using the knockout mice.