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

Abstract

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-κBα, to the autophagosome. Degradation of I-κBα results in increased NF-κB nuclear translocation and transcription activity. Since bortezomib treatment promoted I-κBα phosphorylation, ubiquitination and degradation, this suggests that the route of I-κBα degradation was not via the ubiquitin-proteasome degradation system. The autophagy inhibitor chloroquine (CQ) significantly inhibited bortezomib-induced I-κBα degradation, increased complex formation with NF-κB and reduced NF-κB 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-κBα. Inhibition of both autophagy and the proteasome has great potential to kill apoptosis-resistant lymphoma cells.

Combined autophagy and proteasome inhibition

“…16,19,[24][25][26][27][28][29] Our results demonstrate the feasibility of this approach, as well as intracellular accumulation of AVs during combined stimulation of the autophagy pathway and inhibition of its final step.…”

“…Autophagy inhibition augments the efficacy of many anticancer therapies in cell lines and animal models. [23][24][25][26][27] Specifically, preclinical evidence suggests that autophagy inhibition synergistically increases bortezomib cytotoxicity in laboratory models of myeloma, [28][29][30] colon carcinoma, 31 and hepatocellular carcinoma.…”

MAP17 (PDZKIP1) Expression Determines Sensitivity to the Proteasomal Inhibitor Bortezomib by Preventing Cytoprotective Autophagy and NFκB Activation in Breast Cancer

“…Furthermore, the relapse of multiple myeloma tumors observed in patients is correlated with higher levels of NFkB (17). It has also been shown that bortezomib induces I-kBa degradation, which is removed by the autophagic process, and activates NFkB transcriptional activity (18). These and other results suggest that NFkB is a key target of bortezomib.…”

Autophagy and SQSTM1 on the RHOA(d) again

“…2H). [30][31][32][33][34][35] However, this often occurs under stressful conditions such as nutrient starvation, extracellular matrix (ECM) detachment, HSP90 inhibition, proteasome inhibition, infections, and treatment with chemotherapy drugs. 20,[23][24][25][26][27]29,[31][32][33]35 Therefore, the ability of autophagy to degrade signaling proteins under basal growth conditions and thereby to ensure tumor-suppressive functions remains to be established.…”

“…2H). [27][28][29][30]32,33,35 Importantly, upon inhibition of autophagy by chloroquine treatment, we identified that RHOA is ubiquitinated and recognized by the autophagy receptor SQSTM1. 17 We therefore propose that RHOA, once activated, undergoes ubiquitination, recruiting the scaffold protein SQSTM1 that bridges RHOA to the autophagosome marker LC3 for selective autophagic targeting.…”