p16^INK4a Is Required for hSNF5 Chromatin Remodeler-induced Cellular Senescence in Malignant Rhabdoid Tumor Cells

The hSNF5 chromatin-remodeling factor is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). A number of studies have shown that hSNF5 re-expression blocks MRT cell proliferation. However, the pathway through which hSNF5 acts remains unknown. To address this question, we generated MRT-derived cell lines in which restoration of hSNF5 expression leads to an accumulation in G₀/G₁, induces cellular senescence and increased apoptosis. Following hSNF5 expression, we observed transcriptional activation of the tumor suppressor p16^INK4a but not of p14^ARF, repression of several cyclins and CD44, a cell surface glycoprotein implicated in metastasis. Chromatin immunoprecipitations indicated that hSNF5 activates p16^INK4a transcription and CD44 down-regulation by mediating recruitment of the SWI/SNF complex. Thus, hSNF5 acts as a dualistic co-regulator that, depending on the promoter context, can either mediate activation or repression. Three lines of evidence established that p16^INK4a is an essential effector of hSNF5-induced cell cycle arrest. 1) Overexpression of p16^INK4a mimics the effect of hSNF5 induction and leads to cellular senescence. 2) Expression of a p16 ^INK4a-insensitive form of CDK4 obstructs hSNF5-induced cell cycle arrest. 3) Inhibition of p16^INK4a activation by siRNA blocks hSNF5-mediated cellular senescence. Collectively, these results indicate that in human MRT cells, the p16^INK4a/pRb, rather than the p14 ^ARF/p53 pathway, mediates hSNF5-induced cellular senescence.