Figure 4.

(Above and opposite) Effects of inhibition of PARP activity by GPI 6150 on the development of tetraploidy in immortalized wild-type and PARP^–/– fibroblasts. FACS analysis was performed using unsynchronized immortalized wild-type (left) and PARP^–/– (right) fibroblasts (A). Cells were harvested and nuclei were prepared and stained with propidium iodide for FACS analysis. In addition to the two major peaks of nuclei at G₀/G₁ and G₂/M apparent in the DNA histograms of unsynchronized wild-type cells, the DNA histograms of PARP^–/– cells exhibit a third peak corresponding to the G₂/M peak of an unstable tetraploid cell population (arrows). FACS analysis was performed after continuous exposure of wild-type (B) or PARP^–/– (C) fibroblasts to GPI 6150 (20 µM) or to vehicle (DMSO) alone for 1, 2 or 3 weeks to detect the presence of an unstable tetraploid population in the GPI 6150-treated wild-type cells. The G₂/M peak of the unstable tetraploid cell population in the DNA histograms of PARP^–/– cells is indicated by arrows. Essentially identical results were obtained in three independent experiments.