Author of

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  P3.02 - Poster Session 3 - Novel Cancer Genes and Pathways (ID 149)

  • Event: WCLC 2013
  • Type: Poster Session
  • Track: Biology
  • Presentations: 1
  • Moderators:
  • Coordinates: 10/30/2013, 09:30 - 16:30, Exhibit Hall, Ground Level

  • 12390

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    P3.02-004 - Dysregulation of Hippo tumor-suppressive pathway in malignant mesothelioma (ID 1039)

    09:30 - 09:30  |  Author(s): M. Fujii
    • Abstract

    Background
    Malignant pleural mesothelioma (MPM) is a highly aggressive tumor caused by asbestos exposure after a long latency. The neurofibromatosis type 2 (NF2) tumor suppressor gene is mutated in around 50% of MPM cases, which encodes Merlin that regulates the Hippo tumor-suppressive signaling pathway. We previously reported occasional genetic alteration in the LATS2 and SAV1 genes in MMs, which encode components of the Hippo pathway. The LATS2 inactivation was shown to lead to constitutive activation of YAP, a prooncogenic protein and transcriptional coactivator, which enhances multiple cell cycle regulation genes including cyclin D1 (CCDN1). To further delineate the exact inactivation mechanism of this pathway in MMs, we analyzed MM cell lines for other multiple components that regulate activation or inactivation of this pathway.

    Methods
    We studied several new components that have been identified to be involved in the Hippo signaling pathway including a LIM protein Ajuba. Expression analyses such as conventional western blot and real time RT-PCR were performed. Using MPM cell lines and their transplanted mouse models, biological assays were conducted to study the effects of cell proliferation, motility and invasion after the induction of overexpression or knockdown of candidate genes. Immunohistochemical analysis with primary MPM tissues and xenografts was also carried out.

    Results
    We found that the expression of Ajuba was significantly down-regulated in 6 of 20 MM cell lines compared to an immortalized normal mesothelial cell line, MeT-5A. Interestingly, the 6 cell lines with low Ajuba expression showed decreased phosphorylation (activation) levels of YAP. We infected the MM cells with Ajuba-expressing lentivirus and found that exogenous Ajuba increased phosphorylation (inactivation) of YAP and inhibited cell proliferation. Dual-luciferase reporter assays demonstrated that Ajuba suppressed the promoter activities of YAP-transcriptional targets including CCND1. Knockdown of LATS2 effectively increased these promoter activities, suggesting the mediation of LATS2 for Ajuba to inhibit this cascade. Immunohistochemical analysis also showed frequent downregulation of Ajuba in primary MMs.

    Conclusion
    Inactivation of Hippo signaling is a key mechanism for MPM cell proliferation and progression. Our findings suggest that Ajuba is also one of the target components for Hippo signaling inactivation; Ajuba negatively regulates YAP in the presence of LATS2, and thus the downregulation of Ajuba serves to enhance constitutive activation of YAP in MM cells. Our study also indicates that a strategy to normalize this signaling cascade may be the rationale for developing a new target therapy against MPMs.