Spatial control of cAMP signaling by Epacs

描述（由申请人提供）：该提案检查了一类新型cAMP调节蛋白的生理功能，即由cAMP激活的交换蛋白（Epac），并测试了每种亚型（Epac 1和ERpac 2）在细胞中执行不同生理功能的新假设。该蛋白质家族直接结合cAMP，并代表cAMP的主要细胞内靶标而不是PKA本身。我们对小G蛋白Rap 1的Epac激活的研究表明，交换剂的亚细胞定位可以决定它们激活的G蛋白所利用的效应物的选择（Wang et al. 2006; Liu et al. 2008）。我们已经在Epac 1和Epac 2中鉴定了不同的靶向结构域，称为Ras缔合（RA）结构域（Liu et al.2008）。我们建议，这些结构域允许特定的小G蛋白连接到Epac 1和Epac 2，分别激活不同的Rap 1亚细胞池。我们提出Epac 1和Epac 2的RA结构域分别将cAMP信号传导连接到小G蛋白Ran和Ras。这可以解释Ran如何耦合到Rap 1（通过Epac 1交换器）以及Ras如何耦合到Rap 1（通过Epac 2交换器）。该模型及其对Epac 1和Epac 2的生理后果在本提案中进行了研究。我们已经发现Epac 1在核孔处与小G蛋白Ran结合。我们将检验Epac 1的RA结构域将Ran-GTP偶联到Rap 1以调节蛋白质进出细胞核的转运的假设。这代表了Ran与细胞内信号级联的第一个联系，并确定了Epac 1和Rap的新功能。这一发现的意义在于，通过Ran循环的蛋白质核转位控制细胞生长和分化，其失调是许多癌症的标志。我们发现Epac 1调节癌蛋白2-catenin的核功能。我们认为Epac 1的这一功能和相关功能可以解释cAMP的一些抗增殖作用。与Epac 1相反，我们认为Epac 2与质膜上激活的Ras结合，将cAMP和Ras与Rap 1激活联系起来。Epac 2作为通过Ras和cAMP的信号的符合检测器的功能的这种模型对糖尿病具有重要意义，因为现在已知Epac是磺酰脲类药物的靶点。我们在这里首次提出了一个模型，Ras和cAMP信号收敛于Epac 2，以加强胰岛素的释放。如果这是真的，这将建立一个创新的方法来治疗糖尿病，包括将Ras激活剂与拟胰岛素激素和磺酰脲类药物相结合。 公共卫生相关性：药物治疗靶向环腺苷酸（cAMP）信号通路比任何其他途径。称为Epac的小G蛋白Rap的鸟嘌呤核苷酸交换器是cAMP信号传导途径的新成员，并且直接被cAMP结合激活。该提案测试了Epac促进健康和糖尿病患者胰岛素释放的假设，并通过两种Epac亚型Epac 1和Epac 2的非重叠作用调节蛋白质进入正常和癌细胞的细胞核。

DESCRIPTION (provided by applicant): This proposal examines the physiological function of a novel class of cAMP-regulated proteins, the exchange proteins activated by cAMP (Epacs) and tests the novel hypothesis that each isoform (Epac1 and ERpac2) carries out distinct physiological functions in the cell. This family of proteins binds cAMP directly and represents the major intracellular target of cAMP other than PKA itself. Our studies of Epac activation of small G protein Rap1 have demonstrated that subcellular localization of exchangers can dictate the choice of effectors utilized by the G proteins they activate (Wang et al. 2006; Liu et al. 2008). We have identified distinct targeting domains in both Epac 1 and Epac2 called Ras association (RA) domains (Liu et al. 2008). We propose that these domains allow specific small G proteins to link to Epac1 and Epac2, respectively, to the activation of distinct subcellular pools of Rap1. We propose that the RA domains of Epac1 and Epac2 link cAMP signaling to the small G proteins, Ran and Ras, respectively. This can explain how Ran couples to Rap1 (via the Epac1 exchanger) and how Ras can couple to Rap1 (via the Epac2 exchanger). This model and its physiological consequences for both Epac1 and Epac2 are examined in this proposal. We have discovered that Epac1 binds to the small G protein Ran at the nuclear pore. We will test the hypothesis that the RA domain of Epac1 couples Ran-GTP to Rap1 to regulate the transport of proteins in and out of the nucleus. This represents the first link of Ran to intracellular signaling cascades and identifies a novel function for both Epac1 and Rap. The significance of this finding is that the nuclear translocation of proteins via the Ran cycle governs cell growth and differentiation, and its dysregulation is a hallmark of many cancers. We show that Epac1 regulates the nuclear functions of the oncoprotein 2-catenin. We propose that this and related functions of Epac1 may explain some of the anti-proliferative effects of cAMP. In contrast to Epac1, we propose that Epac2 binds to activated Ras at the plasma membrane to link cAMP and Ras to Rap1 activation. This model of Epac2 function as a coincidence detector for signals through Ras and cAMP has significant implications for diabetes, as Epacs are now known to be targets of the sulfonylureas. We propose here for the first time a model that Ras and cAMP signals converge on Epac2 to potentiate insulin release. If true, this would establish an innovative approach to diabetes involving combining Ras activators with both insulinomimetic hormones and the sulfonylurea drugs. PUBLIC HEALTH RELEVANCE: Pharmacological therapeutics target the cyclic AMP (cAMP) signaling pathway more than any other pathway. The guanine nucleotide exchanger for the small G protein Rap called Epac is a new member of the cAMP signaling pathways and is directly activated by cAMP binding. This proposal tests the hypotheses that Epac promotes insulin release in both healthy and diabetic patients, and regulates the entry of proteins into the nucleus of normal and cancer cells through the non-overlapping actions of the two Epac isoforms, Epac1 and Epac2.