Molecular Mechanisms Underlying PML Nuclear Body Plasticity

The promyelocytic leukaemia(PML) protein is a major component to govern the PML nuclear body(NB) assembly and function.Although it is well defined that PML NB is a site recruiting SUMOylated proteins,the mechanism by which PML protein regulates the process remains unclear.Here we show that PML3,a specific PML isoform,interacts with and recruits TIP60 to PML NBs.Our biochemical characterization demonstrates that PML3 physically interacts with TIP60 via its N-terminal 364 amino acids.Importantly, this portion of TIP60 is sufficient to target to the PML NBs,suggesting that PML3-TIP60 interaction is sufficient for targeting TIP60 to the NBs.The PML3-TIP60 interaction is specific as the region of TIP60-binding is not conserved in other PML isoforms.The physical interaction between PML3 and TIP60 protects TIP60 from Mdm2-mediated degradation,suggesting that PML3 competes with Mdm2 for binding to TIP60. Fluorescence recovery after photobleaching analysis indicates that the PML3-TIP60 interaction modulates the nuclear body distribution and mobility of TIP60.Conversely, the distribution and mobility of TIP60 are perturbed in PML3-deficient cells, accompanied by aberrations in DNA damage repairing response.Thus,PML3 orchestrates the distribution,dynamics and function of TIP60.Our findings suggest a novel regulatory mechanism by which the PML3 and TIP60 tumor suppressors cooperate to ensure genomic stability.PML-NBs are dynamic structures that undergo significant changes in number,size and position both under normal circumstances and particularly in response to cellular stress,For instance,heat shock and heavy metals cause the redistribution of PML-NBs into numerous small nuclear dots.Similarly,DNA-damaging agents such as cisplatin and alkylating agents cause the dispersal of PML-NBs into smaller bodies or even into a diffuse pattern.Although previous studies try to research those phenotypes,the function of PML microstructures under stress is still unknown.In our recent study,we show that CdCl₂ and TNF-αcould induce two different phenomenons and mediated two different cell fates.When Hela cells were treated with CdCl₂,PML-NBs was disrupted in the nuclear,and most of the PML proteins transferred into cytoplasm.Interestingly, cytoplasmic localization of PML was colocalized with Ubquitin and degraded.The ultimate effect of CdCl₂ on Hela cells was to induce apoptosis.However,TNF-αcould incease the number and size of PML-NBs through promoting the sumoylation of PML, and there was no apoptosis cells detected under stress of TNF-α.Our observations offer some evidences to further understand the function of PML and PML-NBs,and will be helpful for screenning drugs for APL chemotherapy.