Dynamic Methylation of Numb by Set8 Regulates Its Binding to p53 and Apoptosis

• Numb binds to p53 in the nucleus and promotes apoptosis
• Numb is methylated by Set8 in the PTB domain
• Lys methylation of Numb abolishes its binding to and protection of p53
• Doxorubicin induces apoptosis by promoting Numb demethylation

SummaryAlthough Numb exhibits its tumor-suppressive function in breast cancer in part by binding to and stabilizing p53, it is unknown how the Numb-p53 interaction is regulated in cells. We found that Numb is methylated in its phosphotyrosine-binding (PTB) domain by the lysine methyltransferase Set8. Moreover, methylation uncouples Numb from p53, resulting in increased p53 ubiquitination and degradation. While Numb promotes apoptosis in a p53-dependent manner, the apoptotic function is abolished when Numb is methylated by Set8 or the Lys methylation sites in Numb are mutated. Conversely, the Numb-p53 interaction and Numb-mediated apoptosis are significantly enhanced by depletion of Set8 from cancer cells or by treating the cells with doxorubicin, a chemotherapeutic drug that causes a reduction in the mRNA and protein levels of Set8. Our work identifies the Set8-Numb-p53 signaling axis as an important regulatory pathway for apoptosis and suggests a therapeutic strategy by targeting Numb methylation.