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而促进肿瘤的发生。Akt和PKC的信号激活过程已被详细研究。然而，人们对这些信号在激活后如何关闭知之甚少。最近，我们已经确定了一个新的蛋白磷酸酶家族，PHLPP（PH结构域富含亮氨酸重复序列的蛋白磷酸酶），直接脱磷酸化Akt和PKC。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表达的缺失在人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作用。根据我们的研究结果，驱动这一建议的中心假设是，PHLPP作为一种肿瘤抑制因子，除了其关闭Akt和PKC激活的生长信号的能力外，还通过调节细胞极性。我们研究的长期目标是更好地了解PHLPP的生理作用及其对体内结肠癌发展和进展的贡献。目的1：明确PHLPP下调的分子机制。本研究的目的是研究导致PHLPP在癌症中下调的潜在机制。我们将检验以下假设：PHLPP的表达水平由细胞中的泛素蛋白酶体途径控制，并且防止PHLPP降解导致蛋白质的上调。目的2：探讨PHLPP在维持细胞极性中的作用。我们推测PHLPP通过调节细胞极性和细胞生长来发挥其作为肿瘤抑制因子的功能。将确定PHLPP对建立上皮细胞极性的功能作用。为了阐明潜在的机制，我们将测试的假设，PHLPP是需要通过调节PKC活性，通过结合到极性蛋白Scribble上皮连接形成。目的3：阐明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进展。我们将讨论当PHLPP被敲除时，无论是基础还是与其他致癌因素结合，肿瘤发病率是否会增加。此外，我们将使用基因敲除小鼠评估改变的细胞极性在肠上皮正常发育和肿瘤起始中的作用。公共卫生相关性：结直肠癌是美国癌症相关死亡的第二大原因，每年诊断出148，000例新病例，2007年估计约有50，000例死亡，在许多促成因素中，由蛋白激酶如Akt和PKC介导的致癌信号传导的过度活化引起的异常蛋白磷酸化是结直肠癌的关键原因。我们最近发现了一种新的蛋白磷酸酶PHLPP，直接去磷酸化Akt和PKC，并终止这些激酶激活的生长信号。我们建议确定PHLPP作为结直肠癌肿瘤抑制因子的功能重要性，本研究的结果将为使用PHLPP作为新靶点的潜在癌症治疗的发展提供重要见解。

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. PUBLIC HEALTH RELEVANCE: Colorectal cancer is the second leading cause of cancer-related deaths in the United States with 148,000 new cases diagnosed per year and approximately 50,000 deaths estimated in 2007 and, among many contributing factors, aberrant protein phosphorylation resulting from hyperactivation of oncogenic signaling mediated by protein kinases such as Akt and PKC, is a key cause of colorectal cancer. We recently identified a novel protein phosphatase PHLPP that directly dephosphorylates Akt and PKC and terminates the growth signals activated by these kinases. We propose to determine the functional importance of PHLPP as a tumor suppressor in colorectal cancer and the results from this study will provide significant insights into the development of potential cancer therapy using PHLPP as a novel target.