Inhibition of SKP2 Activity Impaired ATM-Mediated DNA Repair and Enhanced Sensitivity of Cisplatin-Resistant Mantle Cell Lymphoma Cells.

Mantle cell lymphoma (MCL) is associated with poor patient prognosis mainly due to incomplete response to chemotherapy. S-phase kinase-related protein 2 (SKP2) is an oncoprotein that promotes cell cycle progression and proliferation. A recent study revealed that SKP2 is also involved in DNA damage response mechanisms. SKP2 induces activation of the Ataxia-telangiectasia-mutated (ATM) protein kinase by regulating NBS1 ubiquitination. The authors thus hypothesized that SKP2-mediated ATM activation is associated with MCL resistance to cisplatin (DDP). DDP-resistant MCL cell lines JeKo-1/DDP and Mino/DDP were established by culturing JeKo-1 and Mino cells, respectively, with increasing concentrations of DDP. Protein expression levels of SKP2, ATM, and phosphorylated ATM (p-ATM) in the cell lines were assessed using western blotting. The extent of NBS1 ubiquitination was determined with immunoprecipitation assays. Cell viability, apoptosis, and DNA damage were analyzed using specific detection kits. JeKo-1/DDP and Mino/DDP cells showed higher levels of SKP2 and p-ATM proteins than JeKo-1 and Mino cells, respectively. SKP2 knockdown resulted in a reduced NBS1 ubiquitination and p-ATM protein level in JeKo-1/DDP cells. Both SKP2 knockdown and treatment with an ATM inhibitor enhanced DDP-induced DNA damage in JeKo-1/DDP cells by decreasing amounts of RAD51 and FANCD2, which are factors responsible for DNA repair. Consequently, both SKP2 knockdown and ATM inhibition increased the sensitivity of JeKo-1/DDP cells to DDP treatment, with a more pronounced effect observed by SKP2 depletion. These results suggest that SKP2 is likely to be a more promising target than ATM in the treatment of DDP-resistant MCL.