Compensation of BRG-1 function by Brm: insight into the role of the core SWI-SNF subunits in retinoblastoma tumor suppressor signaling

• Published in: The Journal of biological chemistry 2002-02, Vol.277 (7), p.4782
• Main Authors: Strobeck, Matthew W, Reisman, David N, Gunawardena, Ranjaka W, Betz, Bryan L, Angus, Steven P, Knudsen, Karen E, Kowalik, Timothy F, Weissman, Bernard E, Knudsen, Erik S
• Format: Article
• Language: English
• Subjects: Adenoviridae - metabolism; Alleles; Antineoplastic Agents - pharmacology; Bromodeoxyuridine - metabolism; Cisplatin - pharmacology; Cyclin A - metabolism; DNA Damage; DNA Helicases; DNA-Binding Proteins - metabolism; Electrophoresis, Polyacrylamide Gel; Humans; Hyaluronan Receptors - biosynthesis; Immunoblotting; Mutation; Nuclear Proteins - chemistry; Nuclear Proteins - genetics; Phosphorylation; Plasmids - metabolism; Protein Structure, Tertiary; Retinoblastoma - metabolism; S Phase; Signal Transduction; Transcription Factors - chemistry; Transcription Factors - genetics; Transcription Factors - metabolism; Transcription Factors - physiology; Transfection; Tumor Cells, Cultured
• ISSN: 0021-9258
• DOI: 10.1074/jbc.M109532200

The BRG-1 subunit of the SWI-SNF complex is involved in chromatin remodeling and has been implicated in the action of the retinoblastoma tumor suppressor (RB). Given the importance of BRG-1 in RB function, germ line BRG-1 mutations in tumorigenesis may be tantamount to RB inactivation. Therefore, in this study we assessed the behavior of cells harboring discrete BRG-1 alleles for the RB-signaling pathway. Using p16ink4a, an upstream activator of endogenous RB, or a constitutively active RB construct (PSM-RB), we determined that the majority of tumor lines with germ line defects in BRG-1 were sensitive to RB-mediated cell cycle arrest. By contrast, A427 (lung carcinoma) cells were resistant to expression of p16ink4a and PSM-RB. Analysis of the SWI-SNF subunits in the different tumor lines revealed that A427 are deficient for BRG-1 and its homologue, Brm, whereas RB-sensitive cell lines retained Brm expression. Similarly, the RB-resistant SW13 and C33A cell lines were also deficient for both BRG-1/Brm. Reintroduction of either BRG-1 or Brm into A427 or C33A cells restored RB-mediated signaling to cyclin A to cause cell cycle arrest. Consistent with this compensatory role, we observed that Brm could also drive expression of CD44. We also determined that loss of these core SWI-SNF subunits renders SW13 cells resistant to activation of the RB pathway by the chemotherapeutic agent cisplatin, since reintroduction of either BRG-1 or Brm into SW13 cells restored the cisplatin DNA-damage checkpoint. Together, these data demonstrate that Brm can compensate for BRG-1 loss as pertains to RB sensitivity.