Reexpression of hSNF5 in malignant rhabdoid tumor cell lines causes cell cycle arrest through a p21^CIP1/WAF1-dependent mechanism

Loss of hSNF5 function is usually observed in malignant rhabdoid tumor (MRT), a highly aggressive pediatric neoplasm. Previous studies have shown that reexpression of hSNF5 inMRT cell lines causes G₁ cell cycle arrest with p16^INK4A, p21^CIP1/WAF1, and cyclin D₁ playing key roles in MRT cell growth control. However, we have shown that reexpression of hSNF5 induced cell cycle arrest in the absence of p16 ^INK4A expression. These results indicate that the mechanism of hSNF5-induced cell cycle arrest is context dependent. Here, we investigated the relationship between p21^CIP1/WAF1 and hSNF5 in the regulation of growth using several MRT cell lines. We found that G₁ cell cycle arrest occurred concomitant with an increase in p21^CIP1/WAF1 mRNAand protein levels and preceded p16^INK4A mRNA and protein upregulation. Chromatin immunoprecipitation data confirmed that hSNF5 appeared at both p21^CIP1/WAF1 and p16^INK4A promoters after reexpression. We further showed that p21^CIP1/WAF1 induction showed both p53-dependent and p53-independent mechanisms.Wealso showed that reduction of p21 ^CIP1/WAF1 expression by RNAi significantly inhibited hSNF5-induced G₁ arrest. Our results show that both p21^CIP1/WAF1 and p16^INK4A are targets for hSNF5 and that p21^CIP1/WAF1 upregulation during hSNF5-induced G₁ arrest precedes p16 ^INK4A upregulation. These findings indicate that SNF5 mediates a temporally controlled program of cyclin-dependent kinase inhibition to restrict aberrant proliferation in MRT cells.