Figure 5. Spontaneous downregulation of E2 in HPV-18 E2-transduced cells induces replicative senescence. (A) Flow cytometric analysis of HFK-E2 cells at passages 8, 15 and 20 exhibited normal cell cycles with similar proportion of cells at G0/G1, S and G2/M phases. At p26, cells were arrested at G2/M, similar to HFK treated with 100 nM colchicine as positive control. (B) As shown before, HFK and HFK-ZsG at passage 8 were positive to SA-β-galactosidase activity, whereas HFK-E2 cells at p8 and p16 were negative to this senescence marker (DAPI stain is shown for both fields). At p26, HFK-E2 cells became positive for SA-β-galactosidase staining and a higher magnification shows many binucleated cells (arrowheads). Bars = 50 μm.