YAP/TAZ对发育和肿瘤的调控及其功能的研究进展

Hippo通路是一种在进化中形成的保守的蛋白激酶级联通路,它与发育中器官的大小和肿瘤的形成有关。Hippo通路的中枢是从肿瘤抑制子Hippo到原癌蛋白YAP/TAZ的激酶级联反应。YAP/TAZ是Hippo通路下游的主要的效应分子,它们广泛表达于多种组织器官中。在哺乳动物细胞中,Hippo通路激酶级联反应通过对YAP/TAZ磷酸化作用,促使其从细胞核转入细胞质中,从而抑制了YAP/TAZ的功能作用。TEAD家族转录子被鉴定为YAP/TAZ发挥生物学功能的重要调节因子。YAP/TAZ的失调引起的相关的基因的表达改变,将会影响细胞的增殖,分化,以及凋亡,从而会影响器官的大小以及肿瘤的形成。本文综述Hippo通路的最新进展,重点关注的是该通路中的YAP/TAZ调控的缺失对发育缺陷和肿瘤的影响。这将为我们研究再生医学,组织工程技术,肿瘤的干预防治提供新的思路与策略。     

Hippo信号对卵巢生殖干细胞增殖及卵巢功能的影响

已有研究表明,Hippo信号通路对干细胞的自我更新和分化至关重要,且Hippo信号通路在调控卵泡生长中起重要作用,然而,目前关于Hippo通路对卵巢生殖干细胞的增殖和分化以及卵巢功能重塑的影响相关的研究较少。为了明确Hippo信号通路效应因子YAP1与卵巢生殖干细胞体外增殖分化的关系,以及Hippo信号通路对卵巢癌的主要功能。我们采用两步法酶促分离和磁性分离技术分别鉴定卵巢生殖干细胞,通过测定MVH和OCT4标记物的表达,然后选择YAP1作为Hippo信号通路的主要效应分子,作为研究的靶基因。将含有过表达的YAP1或YAP1靶向的shRNA的慢病毒转导入卵巢生殖干细胞中。通过将过表达YAP1或YAP1 shRNA的慢病毒载体微量注射到不育小鼠模型中,观察调节Hippo信号通路对卵巢的增殖、分化和内分泌功能的影响。研究结果表明,在分离的卵巢生殖干细胞中观察到YAP1和MVH的共表达。与对照组相比,过表达YAP1的卵巢生殖干细胞中MVH和OCT4表达水平显著增加。而YAP1敲低后,MVH和OCT4水平显著降低;不育小鼠模型中YAP1过表达15 d后,E2和FSH含量显著升高,而YAP1 shRNA表达后,小鼠血清E2和FSH含量显著降低。YAP1可用于调控卵巢生殖干细胞的增殖和分化以及小鼠的卵巢功能。本研究表明,Hippo信号通路可能是调控卵巢功能重建的一个新的分子靶点。     

Molecular insights into NF2/Merlin tumor suppressor function

The FERM domain protein Merlin, encoded by the NF2 tumor suppressor gene, regulates cell proliferation in response to adhesive signaling. The growth inhibitory function of Merlin is induced by intercellular adhesion and inactivated by joint integrin/receptor tyrosine kinase signaling. Merlin contributes to the formation of cell junctions in polarized tissues, activates anti-mitogenic signaling at tight-junctions, and inhibits oncogenic gene expression. Thus, inactivation of Merlin causes uncontrolled mitogenic signaling and tumorigenesis. Merlin’s predominant tumor suppressive functions are attributable to its control of oncogenic gene expression through regulation of Hippo signaling. Notably, Merlin translocates to the nucleus where it directly inhibits the CRL4^DCAF1 E3 ubiquitin ligase, thereby suppressing inhibition of the Lats kinases. A dichotomy in NF2 function has emerged whereby Merlin acts at the cell cortex to organize cell junctions and propagate anti-mitogenic signaling, whereas it inhibits oncogenic gene expression through the inhibition of CRL4^DCAF1 and activation of Hippo signaling. The biochemical events underlying Merlin’s normal function and tumor suppressive activity will be discussed in this Review, with emphasis on recent discoveries that have greatly influenced our understanding of Merlin biology.     

CD44 acts through RhoA to regulate YAP signaling

The Hippo pathway plays an important role in both physical and pathogenesis processes. As crucial downstream effectors of Hippo pathway, YAP is inhibited by Lats1/2 through phosphorylation. However, upstream signals that regulate the Hippo pathway have been still poorly understood. Here, we found that knockdown of CD44 reduced YAP expression and nuclear localization, but nearly had no effect on its upstream effectors, Mst1 and Lats1. Downregulated CD44 expression also significantly decreased the expression of YAP downstream effectors CTGF, Cyr61 and EDN1 at mRNA level. Our next study showed that knockdown of CD44 inhibited RhoA expression, which was consistent with RhoA knockdown mediated YAP downregulation. Furthermore, we demonstrated that over expression of the constitutively active RhoA (RhoA-V14) could block the YAP expression decrease mediated by CD44 knockdown. Moreover, downregulation of CD44 significantly promoted cell apoptosis and inhibited cell proliferation, cell cycle progression and migration, which were consistent with the effects of RNAi-mediated YAP knockdown in both A549 and HepG2 cells. Overall, data are presented showing that CD44 could act through RhoA signaling to regulate YAP expression and this study also provide new insights into the regulatory mechanisms of the Hippo–YAP pathway.     

Regulation of Hippo signaling pathway in cancer: A MicroRNA perspective

Recent studies have suggested that Hippo signaling is not only involved in controlling organ size in Drosophila but can also regulate cell proliferation, tissue homeostasis, differentiation, apoptosis and regeneration. Any dysregulation of Hippo signaling, especially the hyper activation of its downstream effectors YAP/TAZ, can lead to uncontrolled cell proliferation and malignant transformation. In majority of cancers, expression of YAP/TAZ is extremely high and this increased expression of YAP/TAZ has been shown to be an independent predictor of prognosis and indicator of increased cell proliferation, metastasis and poor survival. In this review, we have summarized the most recent findings about the cross talk of Hippo signaling pathway with other signaling pathways and its regulation by different miRNAs in various cancer types. Recent evidence has suggested that Hippo pathway is also involved in mediating the resistance of different cancer cells to chemotherapeutic drugs and in a few cancer types, this is brought about by regulating miRNAs. Therefore, the delineation of the underlying mechanisms regulating the chemotherapeutic resistance might help in developing better treatment options. This review has attempted to provide an overview of different drugs/options which can be utilized to target oncogenic YAP/TAZ proteins for therapeutic interventions.     

Hippo信号通路在乳腺癌中的调控机制及作用

Hippo信号通路是调控器官大小和肿瘤发生发展的关键通路,近年来受到广泛的关注。TAZ/YAP作为哺乳动物中Hippo信号通路两个核心下游效应分子,通过Hippo信号通路依赖性和非依赖性的机制受到细胞内外信号的严密调控。除了参与正常乳腺组织发育,Hippo信号通路还在人乳腺癌细胞的增殖、分化、凋亡、迁移、侵袭、上皮-间质转化和干性维持等多个过程中起着关键性作用。本文总结了Hippo信号通路的调控机制和调节信号,阐述了Hippo信号通路异常在乳腺癌发生发展中的作用,并讨论了其在乳腺癌中作为治疗靶点的临床策略。     

YAP1 increases organ size and expands undifferentiated progenitor cells

The mechanisms that regulate mammalian organ size are poorly understood. It is unclear whether the pathways that control organ size also impinge on stem/progenitor cells. A highly expressed gene in stem cells is YAP1, the ortholog of Drosophila Yorkie, a downstream component of the Hippo pathway. Mutations in components of this pathway produce tissue overgrowth phenotypes in the fly whereas mammalian orthologs, like salvador, merlin, LATS, and YAP1, have been implicated in tumorigenesis. We report here that YAP1 increases organ size and causes aberrant tissue expansion in mice. YAP1 activation reversibly increases liver size more than 4-fold. In the intestine, expression of endogenous YAP1 is restricted to the progenitor/stem cell compartment, and activation of YAP1 expands multipotent undifferentiated progenitor cells, which differentiate upon cessation of YAP1 expression. YAP1 stimulates Notch signaling, and administration of gamma-secretase inhibitors suppressed the intestinal dysplasia caused by YAP1. Human colorectal cancers expressing higher levels of YAP1 share molecular aspects with YAP1-induced dysplastic growth in the mouse. Our data show that the Hippo signaling pathway regulates organ size in mammals and can act on stem cell compartments, indicating a potential link between stem/progenitor cells, organ size, and cancer.     

Merlin is a negative regulator of human melanoma growth

Merlin is encoded by the neurofibromatosis type 2 (NF2) gene and is a member of the Band 4.1 protein family. This protein acts as a linker that connects cell surface proteins to the actin cytoskeleton. Defects caused by mutations of the NF2 gene give rise to NF2 disease, which is generally characterized by the formation of bilateral vestibular schwannomas and, to a lesser extent, meningiomas and ependymomas. In addition to these tumor types, NF2 is mutated and/or merlin expression is reduced or lost in numerous non-NF2 associated tumors, including melanoma. However, the role of merlin in human melanoma growth and the mechanism underlying its effect are currently unknown. In the present study, we show that merlin knockdown enhances melanoma cell proliferation, migration, and invasion in vitro and that decreased merlin expression promotes subcutaneous melanoma growth in immunocompromised mice. Concordantly, we find that increased expression of merlin in a metastatic melanoma cell line reduced their in vitro migration and proliferation, and diminished their ability to grow in an anchorage independent manner. Increased merlin expression also inhibits in vivo growth of these melanoma cells. Lastly, we demonstrate that higher merlin levels in human melanoma cells promote the H(2)O(2)-induced activation of MST1/2 Ser/Thr kinases, which are known tumor suppressors in the Hippo signaling pathway. Taken together, these results provide for the first time evidence that merlin negatively regulates human melanoma growth, and that loss of merlin, or impaired merlin function, results in an opposite effect. In addition, we show that increased merlin expression leads to enhanced activation of the MTS1/2 kinases, implying the potential roles of MST1/2 in mediating the anti-melanoma effects of merlin.