Betz Bryan L, Strobeck Matthew W, Reisman David N, Knudsen Erik S, Weissman Bernard E
Department of Pathology and Laboratory Medicine and The Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, North Carolina, NC 27599, USA.
Oncogene. 2002 Aug 8;21(34):5193-203. doi: 10.1038/sj.onc.1205706.
Truncating mutations and homozygous deletions in the hSNF5/INI1/BAF47 subunit of human SWI/SNF complexes occur in most malignant rhabdoid tumors and some other malignancies. How loss of hSNF5 contributes to tumorigenesis remains unknown. Because the SWI/SNF subunit BRG1 is required for RB-mediated cell cycle arrest, we hypothesized that hSNF5 deficiency disrupts RB signaling. Here we demonstrate that unlike BRG1, hSNF5 deficient cells retain functional RB since ectopic expression of either p16ink4a or a constitutively active form of RB (PSM-RB) led to cell cycle arrest. To determine how hSNF5 loss might contribute to tumorigenesis, we used a retrovirus to introduce hSNF5 into multiple deficient cell lines. In all cases, re-expression inhibited colony formation and induced cell cycle arrest characterized by a flattened morphology. Flow cytometry revealed that these cells accumulated in G0/G1. Importantly, arrested cells exhibited strong induction of p16ink4a, hypophosphorylated RB, and down-regulation of cyclin A, suggesting that hSNF5 signals upstream of RB to induce growth arrest. Co-expression of SV40 T/t abolished hSNF5-induced G1 arrest and activation of RB. Likewise, HPV-16 E7 was sufficient to partially overcome cell cycle arrest. These results suggest that hSNF5 loss is not equivalent to BRG1/BRM loss in human tumor cell lines. Furthermore, hSNF5-induced cell cycle arrest of deficient cells is mediated in part through activation of p16ink4a expression. These findings provide insight into mechanisms of hSNF5-mediated tumor suppression.
人类SWI/SNF复合物的hSNF5/INI1/BAF47亚基中的截短突变和纯合缺失发生在大多数恶性横纹肌肉瘤和其他一些恶性肿瘤中。hSNF5的缺失如何促进肿瘤发生仍不清楚。由于RB介导的细胞周期停滞需要SWI/SNF亚基BRG1，我们推测hSNF5缺陷会破坏RB信号传导。在这里，我们证明，与BRG1不同，hSNF5缺陷细胞保留了功能性RB，因为p16ink4a或RB的组成型活性形式（PSM-RB）的异位表达都会导致细胞周期停滞。为了确定hSNF5缺失可能如何促进肿瘤发生，我们使用逆转录病毒将hSNF5引入多个缺陷细胞系。在所有情况下，重新表达均抑制集落形成并诱导以扁平形态为特征的细胞周期停滞。流式细胞术显示这些细胞在G0/G1期积累。重要的是，停滞的细胞表现出p16ink4a的强烈诱导、RB的低磷酸化以及细胞周期蛋白A的下调，这表明hSNF5在RB上游发出信号以诱导生长停滞。SV40 T/t的共表达消除了hSNF5诱导的G1期停滞和RB的激活。同样，HPV-16 E7足以部分克服细胞周期停滞。这些结果表明，在人类肿瘤细胞系中，hSNF5的缺失与BRG1/BRM的缺失不等同。此外，hSNF5诱导的缺陷细胞的细胞周期停滞部分是通过激活p16ink4a表达介导的。这些发现为hSNF5介导的肿瘤抑制机制提供了见解。
