FIGURE 2. Retinoblastoma cells show a p53 response to DNA damage.

a–h, Weri1, Y79, ML-1 and SJMRBL-8 cells were exposed to 5 Gy of ionizing radiation. Immunofluorescence (a), immunoblotting (b) and dissociated cell scoring (c–e) confirmed that Weri1, Y79 and ML-1 cells induce the p53 pathway 4.5 h after ionizing radiation (IR). p53-deficient SJMRBL-8 cells did not activate the p53 pathway (b). f–i, ML-1, Y79 and Weri1 cells exited the cell cycle and/or initiated apoptosis 24 h after exposure to ionizing radiation. j, Weri1 cells transfected with the MDMX siRNA showed a reduction in MDMX protein and induction of the p53 target p21. k–o, Weri1 cells transfected with the p53 siRNA showed loss of p53 protein expression and very few p53⁺ cells in response to ionizing radiation. Cells transfected with the MDMX siRNA showed a more robust p53 response to ionizing radiation. In c–h and l–o, the proportion of immunopositive cells were scored in duplicate (250 cells each) from independent experiments. Error bars represent the s.d. of three (c–h) or two (l–o) experiments. P values are shown above the relevant bars. DIC, differential interference contrast microscopy. Scale bars, 10 m.