Proteasomal degradation of polycomb-group protein CBX6 confers MMP-2 expression essential for mesothelioma invasion

Our plain-English summary of Proteasomal degradation of polycomb-group protein CBX6 confers MMP-2 expression essential for mesothelioma invasion, by K. Sakai, T. Nishiuchi, S. Tange, et al (Scientific Reports, 2020).

The study

• Authors: K. Sakai, T. Nishiuchi, S. Tange, Y. Suzuki, S. Yano, M. Terashima, T. Suzuki, K. Matsumoto
• Journal: Scientific Reports
• Published: 2020;10(1):16678
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The aggressive invasiveness of malignant mesothelioma limits cancer therapy, however, the molecular mechanisms underlying the invasiveness remain largely unknown. Here we found that the matrix metalloproteinase-2 (MMP-2) was required for the invasion of mesothelioma cells in the collagen matrix and the gene expression of MMP-2 was correlated with the invasive phenotype. The MMP-2 gene expression was regulated by DNA and histone methylation around the transcription start site, implicating the involvement of the polycomb repressive complex (PRC). Knockdown of PRC component chromobox 6 (CBX6) promoted MMP-2 expression and invasion of mesothelioma cells. Transcriptome analysis suggested that CBX6 regulates sets of genes involved in cancer cell migration and metastasis. In invasive but not non-invasive cells, CBX6 was constantly unstable owing to ubiquitination and protein degradation. In human tissues, CBX6 localized in the nuclei of normal mesothelium and benign mesothelioma, but the nuclear staining of CBX6 was lost in malignant mesothelioma. These results suggest involvement of proteasomal degradation of CBX6 in mesothelioma progression.

In plain English

This laboratory study looked at mesothelioma cancer cells. It did not examine asbestos or any other exposure. The authors tested seven human malignant mesothelioma cell lines. The enzyme MMP-2 was detected in the four lines that invaded a collagen gel but not in the three that did not. Blocking MMP-2 in invasive cells stopped invasion. In non-invasive lines, chemical marks (methylation) near the start of the MMP-2 gene kept it switched off. In public database data from 85 patients with malignant mesothelioma, higher methylation at four sites went with lower MMP-2 levels. Reducing a protein called CBX6 in a non-invasive line raised MMP-2 and invasion. In invasive cells CBX6 was being broken down. Blocking that breakdown raised CBX6 14.6-fold in invasive JMN-1B cells, compared with 2.0-fold and 1.3-fold in two non-invasive lines. In tissue samples, all 7 normal mesothelium samples and the single benign pleural mesothelioma showed CBX6 in cell nuclei. Nuclear staining was faint in 20 and weak in 6 of 26 malignant mesotheliomas. The authors concluded that CBX6 breakdown appears to be involved in mesothelioma progression.

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 knockdown, drug treatment, methylation and protein analyses), with tissue staining and analysis of a public tumor database (TCGA)
• Who was studied: Seven human malignant mesothelioma cell lines (four invasive, three non-invasive) and a normal mesothelial cell line (MeT-5A). Also tumor tissue from 3 patients at Kanazawa University, Japan; TCGA data from 85 malignant mesothelioma tumor samples; and commercial tissue arrays covering normal mesothelium (n = 7), benign mesothelioma (n = 1) and malignant mesothelioma (n = 26)
• Exposure: Not studied. The article does not examine asbestos or any fiber exposure; the experiments altered genes and applied drugs to cells in the laboratory.
• Main results:
  • MMP-2 was detected in invasive but not non-invasive mesothelioma cell lines.
  • Knocking down MMP-2 in invasive cells caused them to lose their ability to invade collagen.
  • Near the start of the MMP-2 gene, DNA was hypomethylated in invasive cell lines and hypermethylated in normal mesothelial cells and non-invasive lines.
  • In TCGA data from 85 malignant mesothelioma samples, MMP-2 expression showed a statistically significant inverse relationship with methylation at probes p1 to p4.
  • Knocking down CBX6 in non-invasive H2052 cells raised MMP-2 mRNA and significantly increased invasion.
  • In H2052 cells with CBX6 knocked down, 28 genes were upregulated and 58 downregulated.
  • In invasive JMN-1B cells, blocking protein breakdown with MG132 for 5 hours raised Flag-CBX6 levels 14.6-fold.
  • In non-invasive cells the same treatment raised Flag-CBX6 only 2.0-fold (H28) and 1.3-fold (Meso-4).
  • Breakdown of CBX6 depended on its amino acids 201 to 300.
  • Normal mesothelial cells from the pleura (6 cases) and greater omentum (1 case) clearly showed CBX6 in cell nuclei, as did most tumor cells in 1 case of benign pleural mesothelioma.
  • Among 26 malignant mesotheliomas of various tissue origins, nuclear CBX6 staining was faint in 20 and weak in 6.
• The authors' conclusion: The authors concluded that their results suggest proteasomal degradation (breakdown) of CBX6 is involved in mesothelioma progression.
• Limitations the authors state:
  • Other CBX family proteins may also be involved, because knockdown of CBX2, CBX7 and CBX8 was less efficient.
  • The authors did not test whether the genes changed by CBX6 knockdown actually take part in mesothelioma invasion.
  • The role of CBX6 in other tumor types and cellular contexts needs further study.

Topics

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

Medical terms in this study, explained in our Lexicon: Mesothelioma, Collagen, Metastasis, Mesothelium.

Related studies

• Chronic exposure to carbon nanotubes induces invasion of human mesothelial cells through matrix metalloproteinase-2 — W. Lohcharoenkal, L. Wang, et al. (2013)
• Molecular Regulation of the Polycomb Repressive-Deubiquitinase — C.J. Reddington, M. Fellner, et al. (2020)
• Epigenetic down-regulation of integrin α7 increases migratory potential and confers poor prognosis in malignant pleural mesothelioma — V. Laszlo, M.A. Hoda, et al. (2015)
• CD26-mediated regulation of periostin expression contributes to migration and invasion of malignant pleural mesothelioma cells — E. Komiya, K. Ohnuma, et al. (2014)
• Hepatocyte growth factor/scatter factor enhances the invasion of mesothelioma cell lines and the expression of matrix metalloproteinases — P. Harvey, I.M. Clark, et al. (2000)
• ERK2 is essential for the growth of human epithelioid malignant mesotheliomas — A. Shukla, J.M. Hillegass, et al. (2011)

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