The Role Of The MDM2/MDMX-p53 Axis In Tumor And Tumor Microenvironment

As one of the most important tumor suppressors,p53 is universally inactivated in human cancers.So far,many p53 activators have been developed for cancer therapy.While p53 activators have shown clinical efficacy,the resulting toxicities to normal tissues severely limit their use.MDM2 and MDMX are two important negative regulators of p53.How MDM2/MDMX heterodimer regulates p53 in adults under physiological condition still remains fairly understudied.The tumor cells with their surrounding stromal tissues and infiltrated immune cells constitute the complex tumor microenvironment and this tumor microenvironment protects tumor from therapeutic effects on clinic.p53 dysfunction facilitates the formation of an immunosuppressive microenvironment.Here,we will investigate the role of MDM2/MDMX-p53 axis in regulating tumor cells and the tumor microenvironment and its potential therapeutic role on clinic application.Our experiments are based on Mdmx^C462A and Mdmx^S314A transgenic mouse models and thus we would be able to investigate the role of MDM2/MDMX heterodimer in regulating p53 under contexts of physiological,tumor and the tumor microenvironment.Main results of this study are as follow:?1?Under physiological condition,we injected adult conditional CreER Mdmx^C462A mice with tamoxifen to avoid the embryonic lethal phenotype of Mdmx^C462A/C462A.Disassociation of MDM2/MDMX complex caused by MDMX RING mutant failed to control p53activity and mice exhibited no discernable phenotype.These data demonstrate that regulation of p53 through MDM2/MDMX heterodimer is dispensable or redundant under homeostatic conditions.?2?Next,to investigate the role of MDM2/MDMX heterodimer in regulating p53 during tumor development,we crossed ACTB-Cre Mdmx^C462A mouse with E?-Myc transgenic mouse and we also transfected cells with different p53 functional mutated plasmids.Overall,our results indicate that partially detached MDM2/MDMX complex preserves p53 and preserved p53 can effectively inhibit MEF cells proliferation in vitro and suppress lymphomas development in vivo without overt transcriptional activation.?3?Thirdly,we created a knock-in mouse model expressing Mdmx^S314A along with isolated bone marrow-derived macrophages to investigate function of MDM2/MDMX-p53 axis in the tumor microenvironment by using a combination of the syngeneic mouse tumor model,flow cytometry assay,quantitative real-time PCR and immunohistochemistry.We showed that impeding p53decline in the surrounding stromal tissues via blocking MDMX phosphorylation can neutralize the immunosuppressive tumor microenvironment,as demonstrated by the significant increased immune cells infiltration and macrophage M1 polarization in Mdmx^S314A mice,which correlated with tumor growth delay.These results reveal a novel function of p53 in regulating anti-tumor immune response in microenvironment.Our data collectively demonstrate that targeting MDM2/MDMX heterodimer through MDMX RING mutant or ablation of MDMX phosphorylation is dispensable for regulating p53 in the adult under homeostatic conditions,and uncover the novel roles of preserved p53 in inhibiting tumor growth and regulating tumor immune microenvironment.This study provides new view and reference to develop a safer and more effective strategy for cancer therapy based on the MDM2/MDMX-p53 axis.