Functional Activation of Mutant p53 by Platinum Analogues in Cisplatin-Resistant Cells Is Dependent on Phosphorylation

Dysfunctionality of the p53 tumor suppressor is a major cause of therapeutic drug resistance in cancer. Recently, we reported that mutant, but otherwise functional, p53v172F was inactivated in cisplatin-resistant 2780CP/Cl-16 and 2780CP/Cl-24 human ovarian tumor cells by increased recruitment of the inhibitor MDM4. The current study demonstrates that, unlike cisplatin, platinum analogues oxaliplatin and DACH-diacetato-dichloro-Pt(IV) (DAP) strongly stabilize and activate p53v172F in resistant cells, as indicated by prolonged p53 half-life and transactivation of targets p21 (CDKN1A) and MDM2. This increase in MDM2 reduced MDM4 levels in cell lysates as well as the p53 immunocomplex and prevented reversion of p53 to the inactive p53-MDM2-MDM4–bound state. Phosphorylation of p53 at Ser15 was demonstrated by all three drugs in sensitive A2780 and corresponding resistant 2780CP/Cl-16 and 2780CP/Cl-24 cell lines. However, cisplatin induced Ser20 phosphorylation in A2780 cells only, but not in resistant cells; in contrast, both DAP and oxaliplatin induced this phosphorylation in all three cell lines. The inference that Ser20 phosphorylation is more important for p53 activation was confirmed by ectopic expression of a phosphomimetic (S20D) mutant p53 that displayed reduced binding, relative to wild-type p53, to both MDM2 and MDM4 in p53-knockout A2780 cells. In consonance, temporal studies demonstrated drug-induced Ser15 phosphorylation coincided with p53 stabilization, whereas Ser20 phosphorylation coincided with p53 transactivation. Implications: Cisplatin fails to activate the pathway involved in phosphorylating mutant p53v172F at Ser20 in resistant cells, but this phosphorylation is restored by oxaliplatin and DAP that reactivates p53 function and circumvents cisplatin resistance. Mol Cancer Res; 15(3); 328–39. ©2016 AACR.