Apoptosis induced by adenovirus-mediated p53 gene transfer in human glioma correlates with site-specific phosphorylation

Therapeutic replacement of the p53 gene using an adenovirus vector (Ad-p53) may be an effective alternative to conventional therapies for the treatment of glioma. We have previously demonstrated that the introduction of Ad-p53 into glioma cells containing mutant p53 induces apoptosis, whereas glioma cells containing wild-type p53 are resistant. However, Ad-p53 will enhance the radiosensitivity of wild-type p53 glioma cells by increasing their tendency for apoptosis. The mechanism underlying these different responses to Ad-p53 has not been elucidated to date. Because phosphorylation of p53 at serines 15, 20, and 392 may play a role in regulating p53-mediated apoptotic activity, we determined the phosphorylation status of exogenous p53 in mutant and wild-type gliomas after Ad-p53 transfer. Monolayer cultures of glioma cell lines expressing mutant p53 (U251 and U373) or wild-type p53 (U87 and D54) were infected with Ad-p53 and analyzed by Western blotting. High levels of exogenous p53 were detected in both cell lines after Ad-p53 transfer. However, only apoptotic mutant p53 cells expressed high levels of phospho-Ser15-p53 and phospho-Ser20-p53. The levels of phospho-Ser15-p53 and phospho-Ser20-p53 were very low in wild-type p53 cells after Ad-p53 infection alone. When wild-type p53 glioma cells were exposed to radiation after Ad-p53 infection, phospho-Ser15-p53 and phospho-Ser20-p53 were detected at high levels, and the cells subsequently underwent apoptosis; no change in serine 392 was detected. The induction of apoptosis and the expression of phospho-Ser15 and phospho-Ser20 in these cells were also enhanced by the combination of Ad-p53 and other DNA-damaging agents such as cisplatin and bichloroethyl nitrosourea. Furthermore, the expression of phospho-Ser15-p53 and phospho-Ser20-p53 correlated with the amount of apoptosis; the apoptotic activity of p53 in glioma cells was partially inhibited by a mutation of p53 at serine 15. These results suggest that phosphorylation of p53 at serine 15 and serine 20 is critical for apoptosis induction in p53 gene therapy for gliomas.