The role of 14-3-3 sigma in the c -Myc -Pin1 complex: Impacting upon the c-Myc degradation process and c-Myc-induced DNA damage

Tumorigenesis is a process that accumulates mutations in tumor suppressors and oncogenes, resulting in genomic instability. Epigenetic silencing of tumor suppressor 143-3ahas been found in 90% of breast cancers. Of note, these 14-3-3σ silenced cells exhibit genomic abnormalities, such as DNA breaks. Similarly, somatic-cell knockout of 14-3-3σ in colorectal cancer cell line HCT116 also showed chromosomal aberrations, suggesting that 14-3-3σ is critical in protecting genomic integrity. However, how 14-3-3σ maintains genomic stability is still unknown. Our data show that reactive oxygen species (ROS) and γ-H2AX (phosphorylation of histone M2AX) foci are frequently observed in 14-3-3σ null cells. In addition, ARF (Alternative Reading Frame) is also upregulated in 14-3-3σ null cells. These observations prompt us to hypothesize that 14-3-3σ functions to suppress some oncogenes that can cause DNA damage. Because there is a link between over-expression of proto-oncogene c-Myc, ROS induction, and DNA double-strand breaks, we examined whether c-Myc is deregulated, thereby inducing DNA double-strand breaks in 14-3-3σ -/- cells. We showed that c-Myc is degraded following the induction of 14-3-3σ in HCT116 cells after DNA damage, while the level of c-Myc is not changed in HCT116 14-3-3σ -/- cells. In addition, robustly ubiquitinated c-Myc was detected in 14-3-3σ over-expression conditions. These results indicate that 14-3-3σ is essential for c-Myc degradation. Treating 14-3-3σ knocked-down cells with c-Myc inhibitor diminishes ROS signal, suggesting that stabilized c-Myc in 14-3-3σ null cells is responsible for DNA damage in cells. The peptidyl-prolyl isomerase Pin1 has been shown to be an important negative regulator of c-Myc; the association of Pin1 with c-Myc will result in degradation of c-Myc via the 26S proteasome pathway. In this study, higher levels of Pin1 were found to bind to c-Myc after DNA damage, which concurred with the induction of 14-3-3σ. Furthermore, the binding affinity between Pin1 and c-Myc is weaker in the absence of 14-3-3σ, suggesting that 14-3-3σ enhances the interaction of Pin1 and c-Myc. Collectively, our studies show that loss of 14-3-3σ results in c-Myc stabilization, which initiates DNA damage thereby leading to genomic instability