The Role of PHLPP in Colon Cancer

描述(申请人提供)：结直肠癌是美国癌症相关死亡的第二大原因，2007年估计有11.2万例新诊断病例和约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而参与肿瘤的发生。对Akt和PKC的信号激活过程进行了较为详细的研究。然而，人们对这些信号一旦被激活后如何关闭知之甚少。最近，我们发现了一个新的蛋白磷酸酶家族，PHLPP(PH域富含亮氨酸重复蛋白磷酸酶)，它直接使Akt和PKC去磷酸化。然而，PHLPP在癌症中的作用尚未确定。在前期的研究中，我们发现PHLPP的表达缺失在人类结直肠癌标本中发生的频率很高。此外，我们的研究表明，PHLPP在调节细胞极性方面发挥了作用。根据我们的发现，支持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来发挥肿瘤抑制作用。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发展中的作用。具体目的是：目的1：明确PHLPP下调的分子机制。这一目标的目的是研究导致PHLPP在癌症中下调的潜在机制。我们将检验这一假设，即PHLPP的表达水平受细胞中泛素蛋白酶体途径的控制，阻止PHLPP的降解导致蛋白质上调。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而发挥其肿瘤抑制作用。PHLPP在建立上皮细胞极性方面的功能作用将被确定。为了阐明其潜在的机制，我们将测试PHLPP是上皮连接形成所必需的假设，通过与极性蛋白Scribble结合来调节PKC的活性。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。我们将解决的问题是，当PHLPP基因被敲除时，肿瘤发病率是否会增加，无论是从基础上还是与其他致癌因素相结合。此外，我们将利用基因敲除小鼠评估细胞极性改变在肠道上皮细胞正常发育和肿瘤启动中的作用。

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.