Activation of p53 in MDM2-overexpressing cells through phosphorylation

Overexpressed MDM2 inactivates wild-type (wt) p53 in various human tumors. However, whether and how the wild-type p53 can be activated by anticancer d rug treatment in the presence of excess MDM2 is still unclear. In the prese nt study, we showed that the topoisomerase II inhibitor of widely used anti cancer drugs etoposide and doxorubicin activated wt p53 in BL2, a Burkitt's lymphoma cell Line which overexpressed MDM2. Activation of p53 was followe d by apoptosis in BL2 cells, while the same chug treatment did not induce a poptosis 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 blocke d by wortmannin, a kinase inhibitor against proteins with a PI3 kinase doma in. Although MDM2 protein was rapidly cleaved and degraded after anticancer drug treatment, cotreatment with caspase inhibitor Z-VAD blocked degradati on, while wt p53 remained activated, suggesting MDM2 degradation not to be essential for the activation of p53. Treatment with proteasome inhibitor st abilized p53 without being further phosphorylated. This p53 was co-immunopr ecipitated with MDM2, but p53 activated by etoposide or doxorubicin barely complexed with MDM2. These results suggest that the mild-type p53 in MDM2-o verexpressing cells can be activated by anticancer drugs through phosphoryl ation of p53, alleviating inhibitory action by MDM2, and activating caspase s which in turn downregulates MDM2. The activation of p53 in MDM2-overexpre ssing tumor cells, which does not require the downregulation of MDM2, mau h ave important implications in cancer therapy. (C) 1999 Academic Press.