G2/M checkpoint is p53-dependent and independent after irradiation in fivehuman 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 stat us in their response to Xrays. 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) ra nged 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 s imilar behavior in their response to irradiation with G2/M arrest and apopt osis. However, the maximal rate of apoptosis with a range from 70 to 18.0% was rather small The decrease of G2/M cells was coupled with an increased p ercentage of apoptotic cells. However, a different delay in G2/M did not re sult in a change of radiation sensitivity. Western analyses showed an incre ased 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 p 53 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 a s important as p53-independent mechanisms.