Pin1 inhibits PP2A-mediated Rb dephosphorylation in regulation of cell cycle and S-phase DNA damage

Inactivation of the retinoblastoma protein (Rb) has a key role in tumorigenesis. It is well established that Rb function is largely regulated by a dynamic balance of phosphorylation and dephosphorylation. Although much research has been done to understand the mechanisms and function of RB phosphorylation, the regulation of Rb dephosphorylation is still not well understood. In this study, we demonstrate that Pin1 has an important role in the regulation of Rb function in cell cycle progression and S-phase checkpoint upon DNA damage. We show that the Rb C-pocket directly binds to the Pin1 WW domain in vitro and in vivo, and that the phosphorylation of Rb C-pocket by G1/S Cyclin/Cyclin-dependent kinase complexes is critical for mediating this interaction. We further show that Rb-mediated cell cycle arrest and Rb-induced premature cellular senescence are effectively inhibited by Pin1 expression. In addition, DNA damage induces Rb dephosphorylation in a PP2A-dependent manner, and this process is inhibited by Pin1. Furthermore, the overexpression of Pin1 promotes Rb hyperphosphorylation upon S-phase DNA damage. Importantly, both the Pin1 WW domain and isomerase activity are required for its effect on S-phase checkpoint. Moreover, the overexpression of Pin1 is correlated with Rb hyperphosphorylation in breast cancer biopsies. These results indicate that Pin1 has a critical role in the modulation of Rb function by the regulation of Rb dephosphorylation, which may have an important pathological role in cancer development.