Compensation of BRG-1 Function by Brm: INSIGHT INTO THE ROLE OF THE CORE SWI·SNF SUBUNITS IN RETINOBLASTOMA TUMOR SUPPRESSOR SIGNALING

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

Unbiased Analysis of RB-mediated Transcriptional Repression Identifies Novel Targets and Distinctions from E2F Action

The retinoblastoma tumor suppressor, RB, is thought to inhibit cell cycle progression through transcriptional repression. E2F-regulated genes have been viewed as presumptive targets of RB-mediated repression. However, we found that specific E2F targets were not regulated in a consistent manner by the action of a RB allele that is refractory to cyclin-dependent kinase/cyclin-mediated phosphorylation (PSM-RB) when compared with E2F2 overproduction. Therefore, we used Affymetrix GeneChips as an unbiased approach to identify RB targets. We found that expression of PSM-RB significantly attenuates \u3e200 targets, the majority of which are involved in cell cycle control (DNA replication or G2-M), DNA repair, or transcription/chromatin structure. The observed repression was due to the action of RB and not merely a manifestation of altered cell cycle distribution. Additionally, the majority of RB repression targets were confirmed through the blockade of endogenous RB phosphorylation via p16ink4a overexpression. Thus, these results have utility in assigning RB pathway activation in more complex systems of cell cycle inhibition (e.g., mitogen withdrawal, senescence, or DNA damage checkpoint). As expected, a significant fraction of RB-repressed genes have promoters that are bound/regulated by E2F family members. However, targets were identified that are distinct from genes known to be stimulated by overexpression of specific E2F proteins. Moreover, the relative action of RB versus E2F2 overexpression on specific genes demonstrates that a simple opposition model does not explain the relative contribution of RB to gene regulation. Thus, this study provides the first unbiased description of RB-repressed genes, thereby delineating new aspects of RB-mediated transcriptional control and novel targets involved in diverse cellular processes

Unbiased Analysis of RB-mediated Transcriptional Repression Identifies Novel Targets and Distinctions from E2F Action

The retinoblastoma tumor suppressor, RB, is thought to inhibit cell cycle progression through transcriptional repression. E2F-regulated genes have been viewed as presumptive targets of RB-mediated repression. However, we found that specific E2F targets were not regulated in a consistent manner by the action of a RB allele that is refractory to cyclin-dependent kinase/cyclin-mediated phosphorylation (PSM-RB) when compared with E2F2 overproduction. Therefore, we used Affymetrix GeneChips as an unbiased approach to identify RB targets. We found that expression of PSM-RB significantly attenuates \u3e200 targets, the majority of which are involved in cell cycle control (DNA replication or G2-M), DNA repair, or transcription/chromatin structure. The observed repression was due to the action of RB and not merely a manifestation of altered cell cycle distribution. Additionally, the majority of RB repression targets were confirmed through the blockade of endogenous RB phosphorylation via p16ink4a overexpression. Thus, these results have utility in assigning RB pathway activation in more complex systems of cell cycle inhibition (e.g., mitogen withdrawal, senescence, or DNA damage checkpoint). As expected, a significant fraction of RB-repressed genes have promoters that are bound/regulated by E2F family members. However, targets were identified that are distinct from genes known to be stimulated by overexpression of specific E2F proteins. Moreover, the relative action of RB versus E2F2 overexpression on specific genes demonstrates that a simple opposition model does not explain the relative contribution of RB to gene regulation. Thus, this study provides the first unbiased description of RB-repressed genes, thereby delineating new aspects of RB-mediated transcriptional control and novel targets involved in diverse cellular processes