SUMOylation regulates p27Kip1 stability and localization in response to TGFβ

Exposure of normal and tumor-derived cells to TGF beta results in different outcomes, depending on the regulation of key targets. The CDK inhibitor p27(Kip1) is one of these TGF beta targets and is essential for the TGF beta-induced cell cycle arrest. TGF beta treatment inhibits p27(Kip1) degradation and induces its nuclear translocation, through mechanisms that are still unknown. Recent evidences suggest that SUMOylation, a post-translational modification able to modulate the stability and subcellular localization of target proteins, critically modifies members of the TGF beta signaling pathway. Here, we demonstrate that p27(Kip1) is SUMOylated in response to TGF beta treatment. Using different p27(Kip1) point mutants, we identified lysine 134 (K134) as the residue modified by small ubiquitin-like modifier 1 (SUMO1) in response to TGF beta treatment. TGF beta-induced K134 SUMOylation increased protein stability and nuclear localization of both endogenous and exogenously expressed p27(Kip1). We observed that SUMOylation regulated p27(Kip1) binding to CDK2, thereby governing its nuclear proteasomal degradation through the phosphorylation of threonine 187. Importantly, p27(Kip1) SUMOylation was necessary for proper cell cycle exit following TGF beta treatment. These data indicate that SUMOylation is a novel regulatory mechanism that modulates p27(Kip1) function in response to TGF beta stimulation. Given the involvement of TGF beta signaling in cancer cell proliferation and invasion, our data may shed light on an important aspect of this pathway during tumor progression.