Fig. 4. Inhibition of PARP-1 stimulates UV-induced ssDNA focus formation. (A) GM-847 cells seeded on coverslips were transfected with PML-specific siRNAs or control siRNAs (GL2) as indicated. BrdU was added to the culture medium at 24 hours after transfection. At 48 hours post-transfection cells were treated with 10 J/m2 UV light and subsequently incubated in the presence or absence of 0.5 mM NU1025 for 1 or 2 hours before fixation and immunofluorescence analysis. Data represent the percentage of cells containing ssDNA foci. For each sample more than 300 cells were evaluated. The data represent the average of two independent experiments ± s.d. (B) Western blot of extracts prepared from GM-847 cells at 72 hours following transfection with siRNAs specific for PARP-1 and GL2 (control). The MCM2 protein is used as a loading control. (C) GM-847 cells were transfected with the siRNAs indicated. BrdU was added to the growth medium at 48 hours following transfection. At 24 hours following addition of BrdU, cells were irradiated with 10 J/m2 UV light or non-irradiated and subsequently incubated in normal growth medium for 1 or 2 hours. (NI) Non-irradiated.