Inhibition of Ubiquitin-Proteasome Pathway Causes Growth Suppression and Induces Apoptosis in Diffuse Large B Cell Lymphoma.

SKP2, an F-box protein targets cell cycle regulators including cycle-dependent kinase inhibitor p27KIP1 via ubiquitin-mediated degradation. SKP2 is frequently overexpressed in variety of cancer cells and has been implicated in oncogenesis, however its role in diffuse large B-cell lymphoma (DLBCL) has not been elucidated. Therefore, we investigated the role of SKP2 and its ubiquitin-proteasome pathway in DLBL using a panel of cell lines and clinical samples. MG132, a reversible and specific inhibitor of ubiquitin-proteasome pathway that acts through blocking ubiquitin-mediated proteolysis via binding to 20S and 26S proteasomes. Our study showed that treatment of DLBCL cell lines (SUDHL4, SUDHL5, SUDHL8 and OCILY19) with MG132 suppressed growth and induced apoptosis. Treatment of DLBCL cells with MG132 caused downregulation of SKP2, accumulation and stabilization of p27KIP1 leading to the apoptosis. Further downstream, MG132 induced activation of caspase-8 and cleavage of Bid subsequently leading to loss of mitochondrial membrane potential and release of cytochrome c from mitochondria into cytosol, resulting in activation of caspase-3 and cleavage of PARP. zVAD-fmk, a universal inhibitor of caspases prevented caspase-3 activation and abrogated apoptosis induced by MG132 treatment. Finally, using immunohistochemistry, SKP2 was detected in 94 (32.9%) of 286 DLBCL tumors. Furthermore, patients with high SKP2 expression were associated with high expression of Ki67, a proliferative marker. Altogether, these results suggest that SKP2 and ubiquitin-proteasome pathway may be a potential target for therapeutic intervention in diffuse large B-cell lymphoma.