Down-regulation of nuclear factor kappaB is required for p53-dependent apoptosis in X-ray-irradiated mouse lymphoma cells and thymocytes.

Transcription factors p53 and nuclear factor kappaB (NF-kappaB) have been implicated in apoptosis induced by DNA-damaging agents, but the relationship between these two factors at the molecular level is largely unknown. We have isolated apoptosis-resistant mutant sublines from a radiosensitive mouse lymphoma 3SB cell line that undergoes p53-depen-dent apoptosis after X-ray irradiation, and we have analyzed the NF-kappaB activity. Two of these apoptosis-resistant sublines expressed mutant p53 protein and exhibited a defect in the induction of cyclin-dependent kinase inhibitor p21 after X-ray irradiation. A decrease in the DNA binding activity of NF-kappaB was observed in the parental 3SB cells after exposure to X-rays, whereas the same activity was unaffected by radiation in the two mutant sublines. A similar down-regulation of NF-kappaB activity by X-rays was observed in thymocytes derived from p53 wild-type and heterozygous mice, but not in thymocytes from p53 homozygous knock-out mice. These results suggest that NF-kappaB inactivation is p53 dependent and is required for X-ray-induced apoptosis in thymic lymphoma cells and normal thymocytes.