Threshold level of p53 required for the induction of apoptosis in X-irradiated MOLT-4 cells.

Purpose: To determine the threshold level for the initiation of apoptosis by studying the quantitative aspect of p53 response to DNA damage in individual cells, to better understand the process in X-ray-induced p53-dependent apoptosis.

Methods And Materials: Time-sequential changes in p53 protein level were obtained for X-irradiated MOLT-4 cells using flow cytometry and analyzed.

Results: The changes in the cellular frequency distribution pattern of p53 content could be divided into two parts at a certain p53 level. The p53 vs. side-scatter in flow cytometry showed the sequential changes of p53 increase followed by an increase in cell death. On the basis of these results we determined a threshold level of p53 for the initiation of apoptosis. The level was estimated to be (1.08 +/- 0.05) x 10(5) molecules per cell, which was approximately threefold higher than the mean content of control cells. The minimum times for p53 level to reach this threshold level were independent of X-ray dose and 1.4-1.6 h. The times for the signal transduction from the p53 accumulation to disruption of the mitochondrial membrane potential, caspase-3 activation, and cell death were 1.6, 2.1, and 2.8 h, respectively.

Conclusions: The threshold level of p53 for the initiation of apoptosis and the time sequence in the course of apoptotic events were determined in X-irradiated MOLT-4 cells.