Figure 3. Senescence induction and tumor promotion by ionizing radiation are p21-dependent.

A, p21+/+ or p21−/− mice were mock-irradiated or irradiated with 10 Gy of X-rays and then allowed to recover for 30 days (6 mice per cohort). Senescence was measured at 30 days post-IR by staining brain sections for SA-β-Gal activity. Representative Images of SA-β-Gal and DAPI (blue) staining of the cortex of brains of mock-irradiated and irradiated mice are shown. Scale bar=100 μm. B, Plot shows numbers of SA-β-Gal cells per 40X microscopic field in brains of mock-irradiated and irradiated mice (n=6 for each cohort, P=0.3371, error bars S.D). C, Images show immunofluorescence staining of the cortex of mock-irradiated and irradiated p21−/− brains for GFAP (green), Lamin B1 (red) and DAPI (blue). Scale bar=50 μm. D, Plot shows the percentage of GFAP-positive cells that stain positive for Lamin B1 in mock-irradiated and irradiated brains (n=3 per cohort, P= 0.6117, error bars S.D). Note that irradiated p21−/− brains do not stain for senescence markers. E, p21+/+ or p21−/− mice were mock irradiated or cranially irradiated with 10 Gy of X-rays (6 mice per cohort). After 30 days, mice were intra-cranially implanted with 2,500 GL261 cells expressing firefly luciferase. Tumor growth was monitored by BLI imaging. Plot represents average signal intensity (photons per second) for each cohort versus time post-injection. Note promotion of tumor growth in irradiated p21+/+ mice but not in p21−/− mice (p21−/− IR vs p21+/+ IR P= 0.0082, p21−/− Mock vs p21−/− IR P=0.6448, error bars S.E.M). F, H&E-stained sections of mock-irradiated or irradiated p21+/+ and p21−/− mouse brains bearing GL261 tumors.