BRG-1 is required for RB-mediated cell cycle arrest

The antiproliferative action of the retinoblastoma tumor suppressor protein , RB, is disrupted in the majority of human cancers. Disruption of RB activ ity occurs through several disparate mechanisms, including viral oncoprotei n binding, deregulated RB phosphorylation, and mutation of the RB gene. Her e we report disruption of RB-signaling in tumor cells through loss of a cri tical cooperating factor. We have previously reported that C33A cells fail to undergo cell cycle inhibition in the presence of constitutively active R B (PSM-RB). To determine how C33A cells evade RB-mediated arrest, cell fusi on experiments were performed with RB-sensitive cells. The resulting fusion s were arrested by PSM-RB, indicating that C33A cells lack a factor require d for RB-mediated cell cycle inhibition. C33A cells are deficient in BRG-1, a SWI/SNF family member known to stimulate RB activity. Consistent with BR G-1 deficiency underlying resistance to RB-mediated arrest, we identified t wo other ERG-l-deficient cell lines (SW13 and PANC-1) and demonstrate that these tumor lines are also resistant to cell cycle inhibition by PSM-RB and pl6ink4a, which activates endogenous RB. In cell lines lacking BRG-1, we n oted a profound defect in RB-mediated repression of the cyclin A promoter. This deficiency in RB-mediated transcriptional repression and cell cycle in hibition was rescued through ectopic coexpression of BRG-1. We also demonst rate that 3T3-derived cells, which inducibly express a dominant-negative BR G-1, arrest by PSM-RB and p16ink4a in the absence of dominant-negative BRG- 1 expression; however, cell cycle arrest was abrogated on induction of domi nant-negative BRG-1. These findings demonstrate that BRG-1 loss renders cel ls resistant to RB-mediated cell cycle progression, and that disruption of RB signaling through loss of cooperating factors occurs in cancer cells.