Apoptosis in small intestinal epithelia from p53-null mice: Evidence for a delayed, p53-indepdendent G2/M-associated cell death after gamma-irradiation

The death of small intestinal epithelial cells has been characterized and quantitated after irradiation of mice rendered homozygously null for the p53 gene, In wild-type animals homozygous for p53 a rapid (4.5 h) elevation of p53 protein was observed in the proliferative compartment of the crypts after 8 Gy of irradiation, Cells underwent cell death by apoptosis in this region, We had reported previously a total repression of apoptosis in small intestinal crypt epithelia 4.5 h after the gamma-irradiation (8 Gy) of p53 homozygously null animals, Thus, while 400 apoptotic cells mere observed in 200 half crypts taken from mild-type animals at 4.5 h, this fell to background levels (10-30) in the p53 null animals (Merritt et al., 1994) and did not increase by 12 h, However, we have nom found a delayed initiation of a p53-independent apoptosis after 8 Gy of gamma-radiation: at 24 h, approximately 100 apoptotic cells were observed in 200 half crypts, This late wave of apoptosis was not observed after 1 Gy of gamma-radiation. The morphological appearance of this p53-independent apoptosis suggested that death may have arisen as the result of aberrant mitosis, Analysis of the regeneration of crypts 3 days after irradiation of mice with between 11 and 17 Gy showed that there was no significant increase (P=0.135) in the potential of clonogenic cells from the p53 nub animals to repopulate the crypts, The data support the idea that a p53-independent apoptotic mechanism permits the engagement of apoptosis, probably by a mitotic catastrophe, after 8 Gy of gamma-irradiation in,live and that a loss of p53 does not make these epithelial cells radioresistant lit vivo to doses of 8 Gy and above, In contrast, irradiation with 1 GS failed to induce a p53-independent apoptosis in vivo, suggesting that the p53 'sensor' of damage was more sensitive than that engaging the p53-independent mechanism of cell death.