Abstract 4313: A LIM protein ajuba suppresses malignant mesothelioma cell proliferation via Hippo signaling cascade.

Abstract

Abstract Malignant mesothelioma (MM) is a highly aggressive neoplasm which is associated with asbestos exposure and around 50% of MM tumors have a mutation of the NF2 tumor suppressor gene. NF2 encodes merlin, a member of the ezrin/radixin/moesin protein family, and merlin is an upstream regulator of the Hippo signaling cascade. Activation of the Hippo pathway induces the activation of serine/threonine kinases LATS1/2, which phosphorylate and inactivate a transcriptional coactivator, YAP. The Hippo pathway activation negatively regulates tissue growth and its inactivation is involved in carcinogenesis. We previously found that, besides NF2 mutation, a subset of MM tumors harbor LATS2 mutation or YAP oncogene amplification. However, there are yet other MM cases that don't have any of these gene mutations, leading us to hypothesize that other components of this pathway might be altered. Recently, the LIM protein Jub in Droshophila was shown to activate Yoki (ortholog of human YAP) through suppression of the Hippo pathway. To determine whether the human ortholog of Jub, Ajuba, is involved in the Hippo signaling downregulation in MM, we analyzed 20 MM cell lines for the alteration of Ajuba. We found that the expression of Ajuba was significantly downregulated in 6 of 20 MM cell lines compared to an immortalized normal mesothelial cell line, Met-5A. Interestingly, three of the 6 cell lines with low Ajuba expression showed YAP activation, whereas Met-5A cells showed high Ajuba expression and YAP inactivation, both of which suggested that Ajuba might act as a tumor suppressor instead of oncogenic protein in mesothelial cells. We infected Ajuba-expressing lentivirus into these MM cell lines and found that exogenous Ajuba induced YAP phosphorylation and inhibited MM cell proliferation. Since the CCND1 and connective tissue growth factor (CTGF) genes are well-known transcriptional targets of YAP, we conducted a dual-luciferase reporter assay using these promoter regions to see if Ajuba affects their transcriptional activities. The dual-luciferase assay demonstrated that Ajuba suppressed the promoter activity of these genes and that the suppression of them was attenuated by LATS2 knockdown. These results indicated that Ajuba negatively regulates YAP through activation of LATS2. Our results suggest that the inactivation of Ajuba is one of the mechanisms for constitutive activation of YAP, which results in deregulation of MM cell proliferation. Citation Format: Ichidai Tanaka, Hirotaka Osada, Makiko Fujii, Yoshitaka Sekido. A LIM protein ajuba suppresses malignant mesothelioma cell proliferation via Hippo signaling cascade. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4313. doi:10.1158/1538-7445.AM2013-4313