LIM-domain protein AJUBA suppresses malignant mesothelioma cell proliferation via Hippo signaling cascade

Our plain-English summary of LIM-domain protein AJUBA suppresses malignant mesothelioma cell proliferation via Hippo signaling cascade, by I. Tanaka, H. Osada, M. Fujii, et al (Oncogene, 2015).

The study

• Authors: I. Tanaka, H. Osada, M. Fujii, A. Fukatsu, T. Hida, Y. Horio, Y. Kondo, A. Sato, Y. Hasegawa, T. Tsujimura, Y. Sekido
• Journal: Oncogene
• Published: 2015;34(1):73-83
• Cited by: 67 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Malignant mesothelioma (MM) is one of the most aggressive neoplasms usually associated with asbestos exposure and is highly refractory to current therapeutic modalities. MMs show frequent activation of a transcriptional coactivator Yes-associated protein (YAP), which is attributed to the neurofibromatosis type 2 (NF2)-Hippo pathway dysfunction, leading to deregulated cell proliferation and acquisition of a malignant phenotype. However, the whole mechanism of disordered YAP activation in MMs has not yet been well clarified. In the present study, we investigated various components of the NF2-Hippo pathway, and eventually found that MM cells frequently showed downregulation of LIM-domain protein AJUBA, a binding partner of large tumor suppressor type 2 (LATS2), which is one of the last-step kinases of the NF2-Hippo pathway. Although loss of AJUBA expression was independent of the alteration status of other Hippo pathway components, MM cell lines with AJUBA inactivation showed a more dephosphorylated (activated) level of YAP. Immunohistochemical analysis showed frequent downregulation of AJUBA in primary MMs, which was associated with YAP constitutive activation. We found that AJUBA transduction into MM cells significantly suppressed promoter activities of YAP-target genes, and the suppression of YAP activity by AJUBA was remarkably canceled by knockdown of LATS2. In connection with these results, transduction of AJUBA-expressing lentivirus significantly inhibited the proliferation and anchorage-independent growth of the MM cells that harbored ordinary LATS family expression. Taken together, our findings indicate that AJUBA negatively regulates YAP activity through the LATS family, and inactivation of AJUBA is a novel key mechanism in MM cell proliferation.

In plain English

This was a laboratory study of a protein called AJUBA in malignant mesothelioma (MM) cells. Asbestos exposure itself was not studied. The authors examined 24 MM cell lines against an immortalized normal mesothelial cell line and stained 20 primary MM tumor specimens. AJUBA was significantly reduced in 18 of 24 (75%) cell lines. In 21 of 24 lines, YAP was activated; YAP is a protein linked to uncontrolled cell growth. Adding AJUBA to three low-AJUBA MM cell lines lowered YAP activity and significantly reduced growth in all three. Colony growth in soft agar also fell in the two such lines tested. These effects were weak or absent in two lines lacking LATS2, and blocking LATS2 cancelled AJUBA's effect on a YAP target gene. Of 20 tumor specimens, 16 (80%) showed negative or weak AJUBA staining. All 5 AJUBA-negative cases showed nuclear YAP, which the authors read as YAP activation. The authors concluded that AJUBA negatively regulates YAP activity through the LATS family and that loss of AJUBA is a novel key mechanism in MM cell proliferation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: laboratory study of human mesothelioma cell lines (gene transfer, gene silencing, growth assays) plus immunohistochemical staining of primary tumor specimens
• Who was studied: 24 malignant mesothelioma cell lines (19 previously described and 5 newly established; 18 were Japanese lines established in the authors' laboratory) compared with the immortalized normal mesothelial cell line MeT-5A; plus 20 primary MM tumor specimens (source and tumor site not stated in the text provided)
• Exposure: No asbestos exposure was studied. The experimental exposure was introduction of the AJUBA gene into MM cells with a lentivirus, and RNA-based silencing of AJUBA, LATS1 and LATS2
• Main results:
  • AJUBA expression was significantly reduced in 18 of 24 (75%) MM cell lines (undetectable in 15, significantly low in 3) compared with the normal mesothelial line MeT-5A
  • 21 of 24 MM cell lines showed YAP activation (underphosphorylation). The article prints this as 85%, but 21 of 24 is 87.5%
  • All six cell lines with intact NF2, SAV1 and LATS1/2 showed AJUBA downregulation; four of these six showed YAP activation
  • Low AJUBA expression was also seen in 11 of 15 MM cell lines with NF2 inactivation
  • Among NF2-inactivated lines, YAP phosphorylation tended to be lower in lines with low AJUBA than in lines that kept AJUBA, but the difference was not statistically significant (P = 0.057)
  • Adding AJUBA to three low-AJUBA MM cell lines (NCI-H290, Y-MESO-8D, NCI-H28) markedly increased YAP phosphorylation and decreased CCND1 expression
  • Added AJUBA markedly suppressed the activity of two YAP-target gene promoters, CCND1 and CTGF
  • Knocking down LATS2 cancelled AJUBA's suppression of CCND1 promoter activity in NCI-H290 cells
  • AJUBA significantly inhibited proliferation of all three MM cell lines that retained both LATS1 and LATS2 (NCI-H28, NCI-H290, Y-MESO-8D)
  • Significant suppression of proliferation was not seen in two MM cell lines with LATS2 deletion (Y-MESO-14 and Y-MESO-27)
  • AJUBA markedly reduced the number and size of soft agar colonies of NCI-H290 and Y-MESO-8D cells
  • Of 20 primary MM specimens, AJUBA staining was negative in 5 (25%), weak in 11 (55%) and strong in 4
  • YAP was stained more strongly in the nucleus than in the cytoplasm (constitutive activation) in 13 (65%) of 20 primary MMs; all 5 AJUBA-negative cases showed this nuclear YAP pattern
• The authors' conclusion: The authors concluded that AJUBA negatively regulates YAP activity through the LATS family, and that inactivation of AJUBA is a novel key mechanism in MM cell proliferation
• Limitations the authors state:
  • The effect of LATS1 knockdown may have been underestimated, because silencing LATS1 increased LATS2 protein
  • The antibody used for WTIP may have had low specificity, so mRNA analysis was also done
  • YAP phosphorylation status differed in several cell lines from the authors' previous study, possibly because of different culture conditions
  • The authors state that further studies are needed to clarify how the AJUBA family proteins interact in the Hippo pathway in MM cells

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Malignant Mesothelioma, Neoplasm, Asbestos Exposure.

Related studies

• Convergent signaling in the regulation of connective tissue growth factor in malignant mesothelioma: TGFβ signaling and defects in the Hippo signaling cascade — M. Fujii, H. Nakanishi, et al. (2012)
• Hippo pathway gene mutations in malignant mesothelioma: revealed by RNA and targeted exon sequencing — A. Miyanaga, M. Masuda, et al. (2015)
• Inactivation of Merlin in malignant mesothelioma cells and the Hippo signaling cascade dysregulation — Y. Sekido (2011)
• Immunohistochemistry for NF2, LATS1/2, and YAP/TAZ Fails to Separate Benign From Malignant Mesothelial Proliferations — B.S. Sheffield, J. Lorette, et al. (2016)
• TAZ activation by Hippo pathway dysregulation induces cytokine gene expression and promotes mesothelial cell transformation — A. Matsushita, T. Sato, et al. (2019)
• The Emerging Role of the Hippo Pathway in Lung Cancers: Clinical Implications — S.L. Teoh, S. Das (2017)

Browse more research in Oncogene, or return to medical research.