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G2/M checkpoint is p53-dependent and independent after irradiation in five human sarcoma cell lines.

To determine the role of p53 in G2/M arrest, G2/M transition and apoptosis, we investigated five human sarcoma cell lines with different p53 gene status in their response to X-rays. The p53 status of the cell lines was mutant (US 8-93, LMS 6-93 and RD), null (SAOS-2) and wildtype (A-204). Clonogenic survival of the cell lines varied as the survival fraction at 2 Gy (SF2) ranged from 0.28 to 0.79. Compared with the mutated p53 cell lines (SF2 with a range from 0.46 to 0.79) the clonogenic survival of the wildtype p53 (wt-p53) cell line A-204 (SF2 = 0.34) was lower. The p53 null cell line (SAOS-2) was also sensitive to X-rays (SF2 = 0.28). We detected, in all cell lines a similar behavior in their response to irradiation with G2/M arrest and apoptosis. However, the maximal rate of apoptosis with a range from 7.0 to 18.0% was rather small. The decrease of G2/M cells was coupled with an increased percentage of apoptotic cells. However, a different delay in G2/M did not result in a change of radiation sensitivity. Western analyses showed an increased P53 level only for the cell line A-204 (wt-p53) after irradiation. Our results point out that there is not always a simple relationship between p53 gene status and radiation sensitivity. We suggest, that wt-p53 plays an active role in G2/M arrest and in decreasing the number of G2/M cells as a response to apoptosis. Therefore, p53-dependent regulation in G2/M may be as important as p53-independent mechanisms.