The Rad9-Rad1-Hus1 (9-1-1) cell cycle checkpoint complex plays a key role in the DNA damage response. Cells with a defective 9-1-1 complex have been shown to be sensitive to apoptosis induced by certain types of genotoxic stress. However, the mechanism linking the loss of a functional 9-1-1 complex to the cell death machinery has yet to be determined. Here, we report that etoposide treatment dramatically upregulates the BH3-only proteins, Bim and Puma, inHus1-deficient cells. Inhibition of either Bim or Puma expression inHus1-knockout cells confers significant resistance to etoposide-induced apoptosis, whereas knockdown of both proteins results in further resistance, suggesting that Bim and Puma cooperate in sensitizingHus1-deficient cells to etoposide treatment. Moreover, we found that Rad9 collaborates with Bim and Puma to sensitizeHus1-deficient cells to etoposide-induced apoptosis. In response to DNA damage, Rad9 localizes to chromatin inHus1-wild-type cells, whereas inHus1-deficient cells, it is predominantly located in the cytoplasm where it binds to Bcl-2. Taken together, these results suggest that loss ofHus1sensitizes cells to etoposide-induced apoptosis not only by inducing Bim and Puma expressions but also by releasing Rad9 into the cytosol to augment mitochondrial apoptosis.
Oncogene (2008) 27, 7248-7259; doi:10.1038/onc.2008.336; published online 15 September 2008