ACTL6A suppresses p21Cip1 tumor suppressor expression to maintain an aggressive mesothelioma cancer cell phenotype

Our plain-English summary of ACTL6A suppresses p21Cip1 tumor suppressor expression to maintain an aggressive mesothelioma cancer cell phenotype, by S. Shrestha, G. Adhikary, W. Naselsky, et al (Oncogenesis, 2021).

The study

• Authors: S. Shrestha, G. Adhikary, W. Naselsky, W. Xu, J.S. Friedberg, R.L. Eckert
• Journal: Oncogenesis
• Published: 2021;10(10):70
• Cited by: 4 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Mesothelioma is a poor prognosis cancer of the mesothelial lining that develops in response to exposure to various agents including asbestos. Actin-Like Protein 6A (ACTL6A, BAF53a) is a SWI/SNF regulatory complex protein that is elevated in cancer cells and has been implicated as a driver of cancer cell survival and tumor formation. In the present study, we show that ACTL6A drives mesothelioma cancer cell proliferation, spheroid formation, invasion, and migration, and that these activities are markedly attenuated by ACTL6A knockdown. ACTL6A expression reduces the levels of the p21Cip1 cyclin-dependent kinase inhibitor and tumor suppressor protein. DNA binding studies show that ACTL6A interacts with Sp1 and p53 binding DNA response elements in the p21Cip1 gene promoter and that this is associated with reduced p21Cip1 promoter activity and p21Cip1 mRNA and protein levels. Moreover, ACTL6A suppression of p21Cip1 expression is required for maintenance of the aggressive mesothelioma cancer cell phenotype suggesting that p21Cip1 is a mediator of ACTL6A action. p53, a known inducer of p21Cip1 expression, is involved ACTL6A in regulation of p21Cip1 in some but not all mesothelioma cells. In addition, ACTL6A knockout markedly reduces tumor formation and this is associated with elevated tumor levels of p21Cip1. These findings suggest that ACTL6A suppresses p21Cip1 promoter activity to reduce p21Cip1 protein as a mechanism to maintain the aggressive mesothelioma cell phenotype.

© 2021. The Author(s).

In plain English

This laboratory study examined the protein ACTL6A (BAF53a) in two mesothelioma cell lines, Meso-1 (peritoneal) and NCI-Meso-17 (pleural), in mouse tumour experiments, and in tumour tissue from three pleural mesothelioma patients. Mesothelioma is described in the introduction as an asbestos-triggered cancer; asbestos exposure itself was not measured. Lowering ACTL6A with siRNA, or deleting it with CRISPR, raised the p21Cip1 tumour suppressor protein and reduced cell proliferation, spheroid formation, invasion and migration; restoring ACTL6A lowered p21Cip1 again. Knocking p21Cip1 down in ACTL6A-deficient cells partially restored the aggressive behaviour. p21Cip1 promoter activity rose when ACTL6A was lost, and chromatin immunoprecipitation found ACTL6A enriched at the promoter's Sp1 and p53 response elements. The authors suggested that ACTL6A acts at these sites to suppress transcription, and said the mechanism needs further study. p53 protein was absent in Meso-1 cells but present in NCI-Meso-17 cells. In mice (five NSG mice per group, 10 tumours per group, monitored 0 to 6 weeks), ACTL6A knockout cells formed smaller tumours with higher p21Cip1. The authors concluded that ACTL6A suppresses p21Cip1 transcription to maintain the aggressive mesothelioma phenotype.

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 experimental study: mesothelioma cell line experiments (siRNA knockdown, CRISPR knockout, promoter-reporter and chromatin immunoprecipitation assays), mouse tumour xenograft experiments, and immunoblot of patient tumour samples
• Who was studied: Human mesothelioma cell lines Meso-1 (peritoneal) and NCI-Meso-17 (pleural); NSG mice (five per treatment group, 10 tumours per group); tumour samples from three pleural mesothelioma patients
• Exposure: Not an exposure study. The factors the authors changed were ACTL6A level (siRNA knockdown, CRISPR knockout, forced expression), p21Cip1 siRNA and p53 siRNA. Asbestos is mentioned only in the introduction, as a cause of mesothelioma
• Main results:
  • In Meso-1 cells, ACTL6A knockdown with siRNA raised p21Cip1 protein and decreased spheroid number and size, invasion and migration
  • The decreases after ACTL6A knockdown were reported as significant, based on triplicate cell culture experiments
  • The ACTL6A knockout clones (ACTL6A-KOc1-1 and ACTL6A-KOc1-5) showed increased p21Cip1 and reduced proliferation, spheroid number and diameter, and invasion
  • Knocking down p21Cip1 in ACTL6A knockdown or knockout cells partially restored invasion, spheroid formation and migration
  • ACTL6A knockout increased the activity of the full-length (p21-2326) and proximal (p21-124) p21Cip1 promoter constructs. Mutating the six Sp1 sites abolished activity. Chromatin immunoprecipitation found ACTL6A enriched at the Sp1, p53-1 and p53-2 sites, which the authors read as suggesting suppression at these sites
  • p53 protein was not detected in Meso-1 cells, whether or not they expressed ACTL6A, and p53 siRNA did not alter Meso-1 spheroid formation or invasion. NCI-Meso-17 cells did express p53, and their p53 increased after ACTL6A knockdown
  • In mice, ACTL6A knockout cells (Meso1-ACTL6A-KOc1-5-1) showed reduced tumour growth, and their tumours had increased p21Cip1 (n = 10 tumours, p less than or equal to 0.001)
  • NCI-Meso-17 cells had the highest ACTL6A level and were negative for p21Cip1. The three pleural mesothelioma patient tumour samples had lower ACTL6A and basal p21Cip1
• The authors' conclusion: The authors concluded that ACTL6A suppresses p21Cip1 promoter activity, lowering p21Cip1 protein, as a mechanism to maintain the aggressive mesothelioma cell phenotype
• Limitations the authors state:
  • The authors stated that further work is needed to describe how ACTL6A binding at the Sp1 sites inhibits transcription
  • The authors noted that they examined only major regulatory sites, and that ACTL6A may also act through other promoter regions and transcription factors
  • The authors stated that the relationship between ACTL6A, p53 and p21Cip1 in NCI-Meso-17 cells is not fully understood

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Prognosis.

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