Signaling by Hippo

Human Hippo signaling is a network of reactions that regulates cell proliferation and apoptosis, centered on a three-step kinase cascade. The cascade was discovered by analysis of Drosophila mutations that lead to tissue overgrowth, and human homologues of its components have since been identified and characterized at a molecular level. Data from studies of mice carrying knockout mutant alleles of the genes as well as from studies of somatic mutations in these genes in human tumors are consistent with the conclusion that in mammals, as in flies, the Hippo cascade is required for normal regulation of cell proliferation and defects in the pathway are associated with cell overgrowth and tumorigenesis (Oh and Irvine 2010; Pan 2010; Zhao et al. 2010). This group of reactions is also notable for its abundance of protein:protein interactions mediated by WW domains and PPxY sequence motifs (Sudol and Harvey 2010).<p>There are two human homologues of each of the three Drosophila kinases, whose functions are well conserved: expression of human proteins rescues fly mutants. The two members of each pair of human homologues have biochemically indistinguishable functions. Autophosphorylated STK3 (MST2) and STK4 (MST1) (homologues of Drosophila Hippo) catalyze the phosphorylation and activation of LATS1 and LATS2 (homologues of Drosophila Warts) and of the accessory proteins MOB1A and MOB1B (homologues of Drosophila Mats). LATS1 and LATS2 in turn catalyze the phosphorylation of the transcriptional co-activators YAP1 and WWTR1 (TAZ) (homologues of Drosophila Yorkie).<p>In their unphosphorylated states, YAP1 and WWTR1 freely enter the nucleus and function as transcriptional co-activators. In their phosphorylated states, however, YAP1 and WWTR1 are instead bound by 14-3-3 proteins, YWHAB and YWHAE respectively, and sequestered in the cytosol.<p>Several accessory proteins are required for the three-step kinase cascade to function. STK3 (MST2) and STK4 (MST1) each form a complex with SAV1 (homologue of Drosophila Salvador), and LATS1 and LATS2 form complexes with MOB1A and MOB1B (homologues of Drosophila Mats).<p>In Drosophila a complex of three proteins, Kibra, Expanded, and Merlin, can trigger the Hippo cascade. A human homologue of Kibra, WWC1, has been identified and indirect evidence suggests that it can regulate the human Hippo pathway (Xiao et al. 2011). A molecular mechanism for this interaction has not yet been worked out and the molecular steps that trigger the Hippo kinase cascade in humans are unknown.<p>Four additional processes related to human Hippo signaling, although incompletely characterized, have been described in sufficient detail to allow their annotation. All are of physiological interest as they are likely to be parts of mechanisms by which Hippo signaling is modulated or functionally linked to other signaling processes. First, the caspase 3 protease cleaves STK3 (MST2) and STK4 (MST1), releasing inhibitory carboxyterminal domains in each case, leading to increased kinase activity and YAP1 / TAZ phosphorylation (Lee et al. 2001). Second, cytosolic AMOT (angiomotin) proteins can bind YAP1 and WWTR1 (TAZ) in their unphosphorylated states, a process that may provide a Hippo-independent mechanism to down-regulate the activities of these proteins (Chan et al. 2011). Third, WWTR1 (TAZ) and YAP1 bind ZO-1 and 2 proteins (Remue et al. 2010; Oka et al. 2010). Fourth, phosphorylated WWTR1 (TAZ) binds and sequesters DVL2, providing a molecular link between Hippo and Wnt signaling (Varelas et al. 2010).

人类 Hippo 信号传导是一组调控细胞增殖和凋亡的反应网络，其核心为三步激酶级联反应。该级联反应的发现源于对果蝇突变体的分析，这些突变体导致组织过度生长，而其组分在人类中的同源基因随后在分子水平上被鉴定并进行了表征。关于携带敲除突变等位基因的基因小鼠以及人类肿瘤中这些基因的体细胞突变的研究数据均与以下结论一致：在哺乳动物中，与果蝇类似，Hippo 级联反应对于正常调节细胞增殖至关重要，该通路中的缺陷与细胞过度生长和肿瘤发生相关（Oh 和 Irvine 2010；Pan 2010；Zhao 等人 2010）。这一反应群还因其富含由 WW 结构域和 PPxY 序列基序介导的蛋白质-蛋白质相互作用而著称（Sudol 和 Harvey 2010）。<p>对于每一个果蝇激酶，均存在两个人类同源基因，其功能得到高度保守：人类蛋白的表达能够拯救果蝇突变体。每一对人类同源基因的两个成员在生化功能上无法区分。自磷酸化的 STK3（MST2）和 STK4（MST1）（果蝇 Hippo 的同源物）催化 LATS1 和 LATS2（果蝇 Warts 的同源物）以及辅助蛋白 MOB1A 和 MOB1B（果蝇 Mats 的同源物）的磷酸化和激活。LATS1 和 LATS2 随后催化转录共激活因子 YAP1 和 WWTR1（TAZ）（果蝇 Yorkie 的同源物）的磷酸化。<p>在非磷酸化状态下，YAP1 和 WWTR1 自由进入细胞核并作为转录共激活因子发挥作用。然而，在磷酸化状态下，YAP1 和 WWTR1 分别与 14-3-3 蛋白 YWHAB 和 YWHAE 结合，并在细胞质中被隔离。<p>三个步骤的激酶级联反应功能需要多种辅助蛋白。STK3（MST2）和 STK4（MST1）各自与 SAV1（果蝇 Salvador 的同源物）形成复合物，而 LATS1 和 LATS2 与 MOB1A 和 MOB1B（果蝇 Mats 的同源物）形成复合物。<p>在果蝇中，由三种蛋白 Kibra、Expanded 和 Merlin 组成的复合物可以触发 Hippo 级联反应。Kibra 的人类同源物 WWC1 已被鉴定，间接证据表明它可以调节人类 Hippo 通路（Xiao 等人 2011）。这一相互作用的分子机制尚未阐明，人类触发 Hippo 激酶级联反应的分子步骤尚不清楚。<p>与人类 Hippo 信号传导相关的四个额外过程，尽管表征尚不完整，但已足够详细，可以对其进行注释。所有这些过程都具有生理学意义，因为它们很可能是调节 Hippo 信号传导或与其它信号传导过程功能关联的机制的一部分。首先，caspase 3 蛋白酶切割 STK3（MST2）和 STK4（MST1），分别释放抑制性的羧基末端结构域，导致激酶活性和 YAP1/TAZ 磷酸化增加（Lee 等人 2001）。其次，细胞质中的 AMOT（angiomotin）蛋白可以结合未磷酸化的 YAP1 和 WWTR1（TAZ），这一过程可能为这些蛋白提供一种 Hippo 独立的下调活性机制（Chan 等人 2011）。第三，WWTR1（TAZ）和 YAP1 结合 ZO-1 和 2 蛋白（Remue 等人 2010；Oka 等人 2010）。第四，磷酸化的 WWTR1（TAZ）结合并隔离 DVL2，为 Hippo 和 Wnt 信号传导之间的分子联系提供桥梁（Varelas 等人 2010）。