Overexpressed MDM2 inactivates wild-type (wt) p53 in various human
tumors. However, whether and how the wild-type p53 can be activated by anticancer
drug treatment in the presence of excess MDM2 is still unclear. In the present study, we showed that the
topoisomerase II inhibitor of widely used anticancer drugs
etoposide and
doxorubicin activated wt p53 in BL2, a
Burkitt's lymphoma cell line which overexpressed MDM2. Activation of p53 was followed by apoptosis in BL2 cells, while the same
drug treatment did not induce apoptosis in Raji cells, another
Burkitt's lymphoma cell line which carried mutant p53. Activation of p53 was accompanied by phosphorylation of p53 at Ser-15 and elevated p21 and MDM2, both of which were at least partly blocked by
wortmannin, a
kinase inhibitor against
proteins with a
PI3 kinase domain. Although
MDM2 protein was rapidly cleaved and degraded after anticancer
drug treatment, cotreatment with
caspase inhibitor Z-VAD blocked degradation, while wt p53 remained activated, suggesting MDM2 degradation not to be essential for the activation of p53. Treatment with
proteasome inhibitor stabilized p53 without being further phosphorylated. This p53 was co-immunoprecipitated with MDM2, but p53 activated by
etoposide or
doxorubicin barely complexed with MDM2. These results suggest that the wild-type p53 in MDM2-overexpressing cells can be activated by anticancer drugs through phosphorylation of p53, alleviating inhibitory action by MDM2, and activating
caspases which in turn downregulates MDM2. The activation of p53 in MDM2-overexpressing
tumor cells, which does not require the downregulation of MDM2, may have important implications in
cancer therapy.