Figure 4. IFN treatment induces cellular senescence in a p53 SUMOylation-dependent manner. (A) Increased sensitivity of p53-WT cells to IFN-α. The clonogenic capacity of H1299 cells treated with IFN-α was assessed. The colony number relative to untreated cells (average +/− SD of triplicates) is shown. *P < 0.05, Student t test when comparing the reduction in the clonogenicity of pcDNA or p53-K386R- and p53-WT-expressing cells. (B) SUMOylation of p53 contributes to the senescence program induced by IFN. Representative pictures of SA-β-Gal staining (upper-left panel) and quantification of data, showing the percentage of SA-β-Gal-positive cells (upper-right panel). Data shown average +/−SD from 2 independent experiments. Fold change in the fraction of p53-WT- or p53-K386R-expressing cells that were positive for SA-β-Gal staining after IFN-α treatment relative to the change observed in the control cells (lower-right panel) (average +/−SD from 4 independent experiments) *P < 0.05, ***P < 0.0005, Student t test. Western blot analysis of protein extracts from cells treated or not with IFN-α (lower-left panel). Numbers under the bands, densitometric analysis showing fold change in p21 levels compared with untreated cells.