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Malignant mesothelioma (MM) frequently exhibits Hippo signaling pathway inactivation (HPI) mainly due to NF2 and/or LATS2 mutations, which leads to the activation of YAP transcriptional co-activator. Here, we show antitumor effects of statin on MM cells with HPI, through the interplay of the mevalonate and Hippo signaling pathways. Statin attenuated proliferation and migration of MM cells harboring NF2 mutation by accelerating YAP phosphorylation/inactivation. CD44 expression was decreased by statin, in parallel with YAP phosphorylation/inactivation. Importantly, we discovered that YAP/TEAD activated CD44 transcription by binding to the CD44 promoter at TEAD-binding sites. On the other hand, CD44 regulated Merlin phosphorylation according to cell density and sequentially promoted YAP transcriptional co-activator, suggesting that CD44 plays two pivotal functional roles as an upstream suppressor of the Hippo pathway and one of downstream targets regulated by YAP/TEAD. Moreover, the YAP/CD44 axis conferred cancer stem cell (CSC)-like properties in MM cells leading to chemoresistance, which was blocked by statin. Together, our findings suggest that YAP mediates CD44 up-regulation at the transcriptional level, conferring CSC-like properties in MM cells, and statin represents a potential therapeutic option against MM by inactivating YAP.

// Shenqi Wang 1 , Li Jiang 1 , Yipeng Han 2 , Shan Hwu Chew 1 , Yuuki Ohara 1 , Shinya Akatsuka 1 , Liang Weng 2 , Koji Kawaguchi 3 , Takayuki Fukui 3 , Yoshitaka Sekido 4, 5 , Kohei Yokoi 3 , Shinya Toyokuni 1 1 Department of Pathology and Biological Responses, Nagoya University Graduate School of Medicine, Nagoya, 466–8550, Japan 2 Department of Tumor Pathology, Nagoya University Graduate School of Medicine, Nagoya, 466–8550, Japan 3 Department of Thoracic Surgery, Nagoya University Graduate School of Medicine, Nagoya, 466–8550, Japan 4 Department of Cancer Genetics, Nagoya University Graduate School of Medicine, Nagoya, 466–8550, Japan 5 Division of Molecular Oncology, Aichi Cancer Center Research Institute, Nagoya, 464–8681, Japan Correspondence to: Shinya Toyokuni, email: toyokuni@med.nagoya-u.ac.jp Keywords: malignant mesothelioma, uPAR, cisplatin, AKT signaling pathway Received: May 12, 2016 Accepted: August 25, 2016 Published: September 02, 2016 ABSTRACT Malignant mesothelioma (MM) is a rare neoplasm associated with asbestos exposure. The prognosis of MM is poor because it is aggressive and highly resistant to chemotherapy. Using a rat model of asbestos-induced MM, we found elevated urokinase-type plasminogen activator receptor ( uPAR; Plaur ) expression in rat tissues, which was associated with poor prognosis. The proliferation, migration and invasion of MM cells were suppressed by uPAR knockdown and increased by overexpression experiments, irrespective of urokinase-type plasminogen activator ( uPA; Plau ) levels. More importantly, we found that uPAR expression is associated with sensitivity to cisplatin in MM through the PI3K/AKT pathway, which was demonstrated with specific inhibitors, LY294002 and Akti-1/2. uPAR knockdown significantly increased sensitivity to cisplatin whereas its overexpression significantly decreased cisplatin sensitivity. Furthermore, sera and tissues from MM patients showed significantly high uPAR levels, which suggested the pathogenic role of uPAR in the tumor biology of human MM. In conclusion, our findings indicate that uPAR levels are associated with malignant characteristics and cisplatin sensitivity of MM. In addition to the potential use of uPAR as a prognostic marker, the combination of uPAR abrogation and cisplatin may reveal a promising therapeutic approach for MM.

Here we summarize the most recent update of mesothelioma research in basic science presented at the 14th iMig2018 international conference. The symposium of basic science track mainly focused on the drivers of mesothelioma initiation and progression, molecular pathogenesis, and perspectives on potential therapeutic approaches. This review covers several promising fields including strategies efficiently inhibiting YAP/TAZ functions or their critical downstream targets, heparanase inhibitors, RAN depletion, and MIF/CD74 inhibitors that may be developed as novel therapeutic approaches. In addition, targeting mesothelioma stem cells by depleting M2-polarized macrophages in tumor microenvironment or blocking Tnfsf18 (GITRL)-GITR signalling might be translated into therapeutic modalities in mesothelioma treatment.

The neurofibromatosis type 2 (NF2) gene encodes merlin, a tumor suppressor protein frequently inactivated in schwannoma, meningioma, and malignant mesothelioma (MM). The sequence of merlin is similar to that of ezrin/radixin/moesin (ERM) proteins which crosslink actin with the plasma membrane, suggesting that merlin plays a role in transducing extracellular signals to the actin cytoskeleton. Merlin adopts a distinct closed conformation defined by specific intramolecular interactions and regulates diverse cellular events such as transcription, translation, ubiquitination, and miRNA biosynthesis, many of which are mediated through Hippo and mTOR signaling, which are known to be closely involved in cancer development. MM is a very aggressive tumor associated with asbestos exposure, and genetic alterations in NF2 that abrogate merlin’s functional activity are found in about 40% of MMs, indicating the importance of NF2 inactivation in MM development and progression. In this review, we summarize the current knowledge of molecular events triggered by NF2/merlin inactivation, which lead to the development of mesothelioma and other cancers, and discuss potential therapeutic targets in merlin-deficient mesotheliomas.

Malignant pleural mesothelioma (MPM) is a rare and highly aggressive neoplasm that arises from the pleural, pericardial, or peritoneal lining. Although surgery, chemotherapy, radiotherapy, and combinations of these therapies are used to treat MPM, the median survival of such patients is dismal. Therefore, there is a compelling need to develop novel therapeutics with different modes of action. Ganglioside GM2 is a glycolipid that has been shown to be overexpressed in various types of cancer. However, there are no published reports regarding the use of GM2 as a potential therapeutic target in cases of MPM. In this study, we evaluated the efficacy of the anti-GM2 antibody BIW-8962 as an anti-MPM therapeutic using in vitro and in vivo assays. Consequently, the GM2 expression in the MPM cell lines was confirmed using flow cytometry. In addition, eight of 11 cell lines were GM2-positive (73%), although the GM2 expression was variable. BIW-8962 showed a significant antibody-dependent cellular cytotoxicity activity against the GM2-expressing MPM cell line MSTO-211H, the effect of which depended on the antibody concentration and effector/target ratio. In an in vivo orthotropic mouse model using MSTO-211H cells, BIW-8962 significantly decreased the incidence and size of tumors. Additionally, the GM2 expression was confirmed in the MPM clinical specimens. Fifty-eight percent of the MPM tumors were positive for GM2, with individual variation in the intensity and frequency of staining. These data suggest that anti-GM2 antibodies may become a therapeutic option for MPM patients. In this study, the anti-GM2 antibody BIW-8962 first showed a significant ADCC activity against malignant pleural mesothelioma (MPM) cell line and therapeutic activity in an in vivo orthotropic mouse model using the cell line. Additionally, the GM2 expression was confirmed in the MPM clinical specimens. These data suggest that anti-GM2 antibodies may become a therapeutic option for MPM patients. © 2014 The Authors. Cancer Science published by Wiley Publishing Asia Pty Ltd on behalf of Japanese Cancer Association.

Malignant pleural mesothelioma (MPM) is a highly aggressive tumor with a poor prognosis. Thus, novel therapeutic agents need to be developed for treating it. We recently reported the isolation of the novel anti-MPM compound designated as JBIR-23 from Streptomyces sp. AK-AB27. In this study, JBIR-23 exerted its cytotoxic effect on MPM cells by promotion of tubulin polymerization and G₂/M arrest, which was followed by apoptosis induction via the caspase pathway through phosphorylation of p38 mitogen-activated protein kinase and c-jun N-terminal kinase. Furthermore, in vivo analysis demonstrated that JBIR-23 prevented tumor growth in tumor-bearing nude mice without evident side effects.

Malignant pleural mesothelioma (MPM) is resistant to chemotherapy and thus shows a dismal prognosis. Osteopontin (OPN), a secreted noncollagenous and phosphoprotein, is suggested to be involved in the pathogenesis of MPM. However, the precise role of OPN, especially in the multidrug resistance of MPM, remains to be elucidated. We therefore established stable transfectants (ACC-MESO-1/OPN), which constitutively express OPN, to determine its role in the chemoresistance observed in MPM. The introduction of the OPN gene provides MPM cells with upregulated multidrug resistance through the mechanism of enhanced hyaluronate (HA) binding. The expression of CD44 variant isoforms, which inhibit HA binding, significantly decreased in ACC-MESO-1/OPN cells in comparison to control transfectants. Interestingly, the inhibition of the HA-CD44 interaction abrogated multidrug resistance in the ACC-MESO-1/OPN, thus suggesting the involvement of the surviving signal emanating from the HA-CD44 interaction. An enhanced level of the p-Akt in ACC-MESO-1/OPN cells was observed, and was diminished by CD44 siRNA. Inhibition of the Akt phosphorylation increased in number of the cells underwent apoptosis induced by NVB, VP-16 and GEM. Collectively, these results indicate that OPN is strongly involved in multidrug resistance by enhancing the CD44 binding to HA.

The screening for active compounds against malignant pleural mesothelioma (MPM) cells produced by Streptomyces sp. AK-AB27 resulted in the isolation of two compounds with a dodecahydrodibenzo[b,d]furan skeleton named JBIR-23 (1) and -24 (2). Their structures were established on the basis of extensive NMR and MS analyses. Compounds 1 and 2 exhibited cytotoxic effects against several MPM cell lines.