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発表内容

Title:
Wnt5a: Its Signaling, Cellular Functions, and Diseases

Akira Kikuchi, M.D. Ph.D
Professor, Department of Molecular Biology and Biochemistry,
Graduate School of Medicine , Osaka University

Abstract:
The genetics of cancer and development have converged in the identification of intra- and extra-cellular signaling pathways that are aberrantly regulated in cancer and are also central to embryonic pattering. The Wnt signaling pathway has provided an outstanding example of this. Wnt proteins are a large family of cysteine-rich secreted molecules and Wnt family members activate multiple intracellular signaling systems. The most well-established pathway, the b -catenin-dependent pathway, is known to regulate cellular proliferation and differentiation. Genetic alterations of proteins that constitute this pathway are often observed in human cancer. In these cancer cells, b -catenin is accumulated in the cytoplasm and nucleus, which leads to abnormal expression of various genes. Therefore, to clarify the molecular mechanism by which the Wnt/ b -catenin pathway is activated is important for the understanding of tumorigenesis and useful for the diagnosis and therapy of cancer. Some of Wnt proteins activate a b -catenin-independent pathway that primary modulates cell movement and polarity, as first observed during embryogenesis. However, how the b -catenin-independent pathway regulates these cellular functions or whether the abnormal activation of this pathway is involved in tumorigenesis is not clear.
  Since Wnt5a is a representative ligand that activates the b -catenin-independent pathway, we analyzed the molecular mechanism by which Wnt5a activates signaling pathway, regulates cell adhesion and migration, and affects aggressiveness of cancers. Wnt5a bound to Fz2 and Ror1/2 on cell surface and induced the internalization of the receptors in a clathrin-dependent manner. The receptor-mediated endocytosis is necessary for the activation of small G protein Rac. Knockdown of Wnt5a reduced the dynamics of focal adhesion turnover and suppressed cell migration activities. Dvl (dishevelled) and APC (adenomatous polyposis coli) formed a complex with each other, and Wnt5a enhanced the complex formation. Dvl and APC bound to FAK (focal adhesion kinase) and paxillin, respectively, both of which are important components of focal adhesions, and Dvl and APC were necessary for cell-to-substrate adhesion-dependent activation of FAK and phosphorylation of paxillin. Furthermore, Fz2 and integrin were present closely on substrate-facing surface of cells and formed a complex in the present of a cross-linker. These results suggest that Wnt5a and integrin signaling cooperate to regulate cell adhesion and migration.
  Immunohistochmical analyses with anti-Wnt5a antibody revealed that Wnt5a is overexpressed in about 30% of gastric cancer samples. The positivity of Wnt5a expression was correlated with advanced stages and poor prognosis of gastric cancer. Knockdown of Wnt5a in gastric cancer cells suppressed their liver metastatic ability in nude mice. Furthermore, anti-Wnt5a antibody suppressed gastric cancer cell migration. These results suggest that Wnt5a stimulates cell migration by regulating focal adhesion complexes and that Wnt5a is not only a prognostic factor but also a good therapeutic target for gastric cancer. In this seminar , we will discuss about our recent observations in both the functions and mechanisms of the Wnt5a/ b -catenin-independent pathway with an emphasis on its functional contribution to tumor progression.

References:
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3 . Sakane, H., Yamamoto, H., and Kikuchi, A. L RP6 is internalized by Dkk1 to suppress its phosphorylation in the lipid raft and is recycled for reuse. J. Cell Sci. 123, 360-368, 2010

4. Yamamoto, H., Oue, N., Sato, A., Hasegawa, Y., Matsubara, A., Yamamoto, H., Yasui, W., and Kikuch i , A. Wnt5a signaling is involved in the aggressiveness of prostate cancer and expression of metalloproteinase. Oncogene , 29, 2036-2046, 2010

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1 1 . Yamamoto, H., Komekado, H., and Kikuchi, A. Caveolin is necessary for Wnt-3a-induced internalization of LRP6 and accumulation of b -catenin. Dev. Cell , 11, 213-223, 2006