Mechanism of P73-Dependent Tumor Suppression

描述（由申请人提供）：p73基因是p53家族的一员，由两个交替启动子控制。当上游P1启动子用于p73表达时，产生了几种TAp73亚型。当使用内含子3中的P2启动子表达p73时，会产生几种DNp73异构体。由于DN亚型缺乏p53中保守的n端激活结构域，因此它们被认为在转录中不活跃。有趣的是，我们发现DNp73中的13个独特残基与n端PXXP基序一起构成了一个新的激活域。此外，我们发现TAp73和DNp73既调节常见的靶基因群，也调节不同的靶基因群。由于与p53序列相似，p73具有许多类似p53的活性。这些包括诱导细胞周期阻滞、凋亡、分化和衰老的能力。因此，p73是一种肿瘤抑制因子。与此一致的是，p73的状态与生存率的增加有关，p73表达的缺失与许多类型的人类癌症转移潜力的增加有关。同样，p73杂合的小鼠容易增加肿瘤负荷和转移率，这在p53和/或p63杂合等位基因的小鼠中更为复杂。然而，p73并不是一个典型的肿瘤抑制因子，因为突变小鼠也容易出现发育缺陷、慢性感染和炎症。之前，我们和其他人发现p73的表达被应激信号激活，比如DNA损伤。已知这些应激信号主要通过翻译后修饰来稳定和激活p53。相反，p73基因的转录调控被发现对DNA损伤时p73的激活至关重要。为了表征p73的功能域和转录活性，我们发现组蛋白去乙酰化酶（hdac）是p73的共同抑制因子。我们还发现了几个新的p73靶点，包括Dec1（一种基本的螺旋-环-螺旋转录因子）和RNPC1（一种rna结合蛋白）。有趣的是，我们发现hdac、Dec1和RNPC1能够调控p73的表达。因此，我们假设在响应应激信号时，hdac、RNPC1和Dec1作为p73通路的效应器和调节剂。为了验证这一点，我们提出了以下四个具体目标：(1)确定hdac如何差异调节各种p73亚型；(2)确定p73的基本螺旋-环-螺旋转录因子Dec1对不同p73亚型的差异调控；(3)确定不同p73亚型是如何被RNPC1（一种rna结合蛋白和p73的靶标）差异调控的；(4)确定hdac、Dec1和RNPC1调控p73的生物学后果。

DESCRIPTION (provided by applicant): The p73 gene, a member of the p53 family, is controlled by two alternate promoters. When the upstream P1 promoter is used for p73 expression, several TAp73 isoforms are produced. When the P2 promoter in intron 3 is used for p73 expression, several DNp73 isoforms are produced. Since the DN isoforms lack the N-terminal activation domain conserved in p53, they are assumed to be inactive in transcription. Interestingly, we found that the thirteen unique residues together with the N-terminal PXXP motif in DNp73 constitute a novel activation domain. Furthermore, we found that TAp73 and DNp73 regulate both common and distinct groups of target genes. Due to its sequence similarity with p53, p73 has many p53-like activities. These include the ability to induce cell cycle arrest, apoptosis, differentiation, and senescence. Thus, p73 is a tumor suppressor. Consistent with this, p73 status is linked to increased survival rates and loss of p73 expression is linked to increased metastasis potentials in many types of human cancer. Likewise, mice heterozygous of p73 are prone to increased tumor burdens and metastasis rates, which are compounded in mice harboring heterozygous alleles of p53 and/or p63. However, p73 is not a classic tumor suppressor since mutant mice are also prone to developmental defects, chronic infection and inflammation. Previously, we and others showed that p73 expression is activated by stress signals, such as DNA damage. These stress signals are known to stabilize and activate p53 primarily via post-translational modifications. In contrast, transcriptional regulation of the p73 gene is found to be critical for activation of p73 upon DNA damage. In an effort to characterize p73 functional domains and transcriptional activity, we identified histone deacetylases (HDACs) as a co-repressor of p73. We also identified several novel p73 targets, including Dec1, a basic helix-loop-helix transcription factor, and RNPC1, a RNA-binding protein. Interestingly, we found that HDACs, Dec1 and RNPC1 are capable of regulating p73 expression. Thus, we hypothesize that in response to stress signals, HDACs, RNPC1 and Dec1 serve as an effector and a modulator of the p73 pathway. To test this, the following four specific aims are proposed to: (1) determine how various p73 isoforms are differentially regulated by HDACs; (2) determine how various p73 isoforms are differentially regulated by Dec1, a basic helix-loop-helix transcription factor and a target of p73; (3) determine how various p73 isoforms are differentially regulated by RNPC1, a RNA-binding protein and a target of p73; and (4) to determine the biological consequences of p73 regulation by HDACs, Dec1, and RNPC1.