Blocking Autophagy Prevents Bortezomib-Induced NF-κB Activation by Reducing I-κBα Degradation in Lymphoma Cells

Here we show that bortezomib induces effective proteasome inhibition and accumulation of poly-ubiquitinated proteins in diffuse large B-cell lymphoma (DLBCL) cells. This leads to induction of endoplasmic reticulum (ER) stress as demonstrated by accumulation of the protein CHOP, as well as autophagy, as demonstrated by accumulation of LC3-II proteins. Our data suggest that recruitment of both ubiquitinated proteins and LC3-II by p62 directs ubiquitinated proteins, including I-kappa B alpha, to the autophagosome. Degradation of I-kappa B alpha results in increased NF-kappa B nuclear translocation and transcription activity. Since bortezomib treatment promoted I-kappa B alpha phosphorylation, ubiquitination and degradation, this suggests that the route of I-kappa B alpha degradation was not via the ubiquitin-proteasome degradation system. The autophagy inhibitor chloroquine (CQ) significantly inhibited bortezomib-induced I-kappa B alpha degradation, increased complex formation with NF-kappa B and reduced NF-kappa B nuclear translocation and DNA binding activity. Importantly, the combination of proteasome and autophagy inhibitors showed synergy in killing DLBCL cells. In summary, bortezomib-induced autophagy confers relative DLBCL cell drug resistance by eliminating I-kappa B alpha. Inhibition of both autophagy and the proteasome has great potential to kill apoptosis-resistant lymphoma cells.