Double-Stranded RNA Structural Elements Holding the Key to Translational Regulation in Cancer: The Case of Editing in RNA-Binding Motif Protein 8A

Our plain-English summary of Double-Stranded RNA Structural Elements Holding the Key to Translational Regulation in Cancer: The Case of Editing in RNA-Binding Motif Protein 8A, by A. Abukar, M. Wipplinger, A. Hariharan, et al (Cells, 2021).

The study

• Authors: A. Abukar, M. Wipplinger, A. Hariharan, S. Sun, M. Ronner, M. Sculco, A. Okonska, J. Kresoja-Rakic, H. Rehrauer, W. Qi, V.W.V. Beusechem, E. Felley-Bosco
• Journal: Cells
• Published: 2021;10(12)
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Mesothelioma is an aggressive cancer associated with asbestos exposure. RNA-binding motif protein 8a (RBM8A) mRNA editing increases in mouse tissues upon asbestos exposure. The aim of this study was to further characterize the role of RBM8A in mesothelioma and the consequences of its mRNA editing. RBM8A protein expression was higher in mesothelioma compared to mesothelial cells. Silencing RBM8A changed splicing patterns in mesothelial and mesothelioma cells but drastically reduced viability only in mesothelioma cells. In the tissues of asbestos-exposed mice, editing of Rbm8a mRNA was associated with increased protein immunoreactivity, with no change in mRNA levels. Increased adenosine deaminase acting on dsRNA (ADAR)-dependent editing of Alu elements in the RBM8A 3'UTR was observed in mesothelioma cells compared to mesothelial cells. Editing stabilized protein expression. The unedited RBM8A 3'UTR had a stronger interaction with Musashi (MSI) compared to the edited form. The silencing of MSI2 in mesothelioma or overexpression of Adar2 in mesothelial cells resulted in increased RBM8A protein levels. Therefore, ADAR-dependent editing contributes to maintaining elevated RBM8A protein levels in mesothelioma by counteracting MSI2-driven downregulation. A wider implication of this mechanism for the translational control of protein expression is suggested by the editing of similarly structured Alu elements in several other transcripts.

In plain English

This laboratory study examined RBM8A, a protein that helps cells process genetic messages (mRNA), in mesothelioma. It compared human mesothelioma cell lines with normal mesothelial (body-cavity lining) cells. Mouse tissue from an earlier crocidolite (blue asbestos) injection experiment was also reanalyzed. Average RBM8A protein levels were 4-fold higher in 14 mesothelioma cell lines than in 5 mesothelial cultures. When silencing reduced RBM8A protein, mesothelial cells kept 92% of their viability, while mesothelioma cell survival fell to 46%. In asbestos-exposed mice, RNA editing (a chemical change) of the Rbm8a message increased, and more protein was seen without a significant rise in the message. In human cells, editing at selected sites was higher in mesothelioma cells. In test-tube experiments, an unedited RBM8A message fragment associated with more MSI2, a protein that holds back protein production, than an edited one did, directly or indirectly. The authors concluded that editing helps keep RBM8A levels high in mesothelioma by counteracting MSI2. They noted too few cell lines to study the role of the gene BAP1, and pooled mouse samples because few were available.

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 using human mesothelioma and normal mesothelial cell lines, plus reanalysis of RNA-sequencing data and tissue from an earlier asbestos-exposure mouse experiment
• Who was studied: Human malignant pleural mesothelioma (MPM) cell lines and normal mesothelial cell cultures, studied at University Hospital Zürich. The protein comparison used 14 mesothelioma lines and 5 mesothelial cultures. The silencing experiments used 6 MPM and 2 mesothelial lines. The study also used tissue from mice in an earlier experiment, in three groups: sham-exposed, crocidolite-exposed with preneoplastic lesions, and crocidolite-exposed with tumors. Mouse numbers are not given in this article.
• Exposure: In the mouse data, mice received crocidolite (blue asbestos) by repeated injection into the abdomen (intraperitoneally) over 21 weeks, or sham exposure. They were killed at 33 weeks, 12 weeks after the last exposure. The cell experiments involved no asbestos exposure. In those experiments, RBM8A, ADAR1, ADAR2 and MSI2 levels were reduced by silencing, or Adar2 was added experimentally.
• Main results:
  • Average RBM8A protein levels were 4-fold higher in mesothelioma cell lines (n = 14) than in normal mesothelial cells (n = 5), p less than 0.05.
  • After RBM8A silencing, which decreased its protein levels, mesothelial cells kept 92% of cell viability, while MPM cell survival decreased to 46% (p less than 0.0001).
  • In crocidolite-exposed mice, Rbm8a was among the genes significantly more edited than in sham mice.
  • In asbestos-exposed mice, nuclear Rbm8a immunoreactivity increased in mesothelial cells and mesothelioma tumors. No significant increase in Rbm8a mRNA was detected.
  • Three MPM cell lines were compared with three mesothelial cell lines. Editing levels were significantly higher in the MPM cells at selected sites in the AluJo and AluSz6 elements of the RBM8A message.
  • In four MPM cell lines, silencing ADAR1 significantly reduced editing at all AluSz6 sites. Silencing ADAR2 reduced editing at only four sites, and only in cell lines with higher ADAR2 levels.
  • In reporter tests, the AluY element alone reduced reporter protein expression by 62% in mesothelial cells and 37% in MPM cells.
  • The authors made an in-vitro-transcribed RBM8A 3′UTR fragment (AluY-AluSz6) and incubated it with nuclear extracts of mesothelioma cells. The unedited fragment associated with more MSI2 than the edited version. The authors described this association as either direct or indirect.
  • In NCI-H226 and Mero95 mesothelioma cells, silencing MSI2 increased RBM8A protein levels. RBM8A mRNA levels did not change significantly.
  • In mesothelial cells (LP9/TERT-1), adding Adar2 significantly increased RBM8A editing in the AluY and AluSz6 elements. RBM8A protein levels rose, with no change in mRNA levels.
• The authors' conclusion: The authors concluded that ADAR-dependent editing contributes to maintaining elevated RBM8A protein levels in mesothelioma by counteracting MSI2-driven downregulation.
• Limitations the authors state:
  • The small number of MPM cell lines studied did not allow the authors to examine whether BAP1 status affects sensitivity to RBM8A silencing.
  • Because few samples were available, all crocidolite-exposed mouse samples (tumor and inflamed mesothelium) were pooled and compared with sham-treated mice.
  • The authors could not exclude that other controls, such as silencing by a microRNA, also contribute to regulating RBM8A.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Asbestos Exposure, Mesothelial Cells.

Related studies

• RNA editing in mesothelioma: a look forward — A. Hariharan, S. Sun, et al. (2020)
• Endogenous retrovirus expression activates type-I interferon signaling in an experimental mouse model of mesothelioma development — S. Sun, F. Frontini, et al. (2021)
• ADAR2 Regulates Malignant Behaviour of Mesothelioma Cells Independent of RNA-editing Activity — K.I. Sakata, K. Maeda, et al. (2020)
• How asbestos drives the tissue towards tumors: YAP activation, macrophage and mesothelial precursor recruitment, RNA editing, and somatic mutations — H. Rehrauer, L. Wu, et al. (2018)
• Practical utility of insulin-like growth factor II mRNA-binding protein 3, glucose transporter 1, and epithelial membrane antigen for distinguishing malignant mesotheliomas from benign mesothelial proliferations — S. Chang, M.H. Oh, et al. (2014)
• Regulation of calretinin in malignant mesothelioma is mediated by septin 7 binding to the CALB2 promoter — W. Blum, L. Pecze, et al. (2018)

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