Department of Pathology and Molecular Medicine, Queen's University, Kingston, Ontario, Canada.
Oncogene. 2012 Mar 1;31(9):1189-95. doi: 10.1038/onc.2011.318. Epub 2011 Jul 25.
The Hippo signaling network is proving to be an essential regulator within the cell, participating in multiple cellular phenotypes including cell proliferation, apoptosis, cell migration and organ size control. Much of this pathway is conserved from flies to mammals; however, how the upstream components, namely Expanded, affect downstream processes in mammalian systems has remained elusive. Only recently has human Expanded (hEx), also known as FRMD6 or Willin, been identified. However, its functional significance with respect to its putative tumor suppressor function and activation of the Hippo pathway has not been studied. In this study, we show for the first time that hEx possesses several tumor suppressor properties. First, hEx dramatically inhibits cell proliferation in two human cancer cell lines, MDA-MB-231 and MDA-MB-436 cells, and sensitizes these cells to the chemotherapeutic drug Taxol. Furthermore, downregulation of hEx in the immortalized MCF10A breast cell line leads to enhanced proliferation and resistance to Taxol treatment. As evidence for its tumor suppressor function, overexpression of hEx inhibits colony formation, soft agar colony growth in vitro and in vivo tumor growth in nude mice. Although Drosophila expanded (ex) can activate the Hippo pathway, surprisingly no significant alterations were discovered in the phosphorylation status of any of the Hippo pathway components, including downstream tumor suppressor LATS1, upon overexpression of hEx. In addition, knockdown of both LATS1 and LATS2 in hEx-overexpressing cells was unable to rescue the hEx phenotype, suggesting that hEx functions independently of the Hippo pathway in this cell line. Alternatively, we propose a mechanism through which hEx inhibits progression through the S phase of the cell cycle by upregulating p21(Cip1) and downregulating Cyclin A. This is the first study to functionally characterize hEx and show that hEx acts in a distinct manner compared with Drosophila expanded.
Hippo 信号通路被证明是细胞内的一个重要调节因子，参与多种细胞表型，包括细胞增殖、凋亡、细胞迁移和器官大小控制。这条通路在从苍蝇到哺乳动物的过程中有很大的保守性；然而，上游组件（即 Expanded）如何影响哺乳动物系统中的下游过程仍然难以捉摸。直到最近，人类 Expanded（hEx），也称为 FRMD6 或 Willin，才被发现。然而，它在假定的肿瘤抑制功能和 Hippo 通路激活方面的功能意义尚未得到研究。在这项研究中，我们首次表明 hEx 具有几种肿瘤抑制特性。首先，hEx 显著抑制了两种人类癌细胞系 MDA-MB-231 和 MDA-MB-436 中的细胞增殖，并使这些细胞对化疗药物 Taxol 敏感。此外，在永生化 MCF10A 乳腺细胞系中下调 hEx 会导致增殖增强和对 Taxol 治疗的耐药性。作为其肿瘤抑制功能的证据，hEx 的过表达抑制集落形成、体外软琼脂集落生长和裸鼠体内肿瘤生长。尽管果蝇扩展（ex）可以激活 Hippo 通路，但令人惊讶的是，在 hEx 过表达时，没有发现 Hippo 通路任何成分（包括下游肿瘤抑制因子 LATS1）的磷酸化状态发生显著改变。此外，在 hEx 过表达细胞中敲低 LATS1 和 LATS2 都不能挽救 hEx 表型，这表明 hEx 在该细胞系中独立于 Hippo 通路发挥作用。或者，我们提出了一种机制，通过该机制，hEx 通过上调 p21(Cip1)和下调 Cyclin A 来抑制细胞周期 S 期的进展。这是首次对 hEx 进行功能表征的研究，并表明 hEx 的作用方式与果蝇扩展不同。
