Constitutive Activation of p62/Sequestosome-1-Mediated Proteaphagy Regulates Proteolysis and Impairs Cell Death in Bortezomib-Resistant Mantle Cell Lymphoma

Abstract

Protein ubiquitylation coordinates crucial cellular events in physiological and pathological conditions. A comparative analysis of the ubiquitin proteome from bortezomib (BTZ)-sensitive and BTZ-resistant mantle cell lymphoma (MCL) revealed an enrichment of the autophagy–lysosome system (ALS) in BTZ-resistant cells. Pharmacological inhibition of autophagy at the level of lysosome-fusion revealed a constitutive activation of proteaphagy and accumulation of proteasome subunits within autophagosomes in different MCL cell lines with acquired or natural resistance to BTZ. Inhibition of the autophagy receptor p62/SQSTM1 upon verteporfin (VTP) treatment disrupted proteaphagosome assembly, reduced co-localization of proteasome subunits with autophagy markers and negatively impacted proteasome activity. Finally, the silencing or pharmacological inhibition of p62 restored the apoptosis threshold at physiological levels in BTZ-resistant cells both in vitro and in vivo. In total, these results demonstrate for the first time a proteolytic switch from the ubiquitin–proteasome system (UPS) to ALS in B-cell lymphoma refractory to proteasome inhibition, pointing out a crucial role for proteaphagy in this phenomenon and paving the way for the design of alternative therapeutic venues in treatment-resistant tumors.

Autophagy and oxidative stress modulation mediate Bortezomib resistance in prostate cancer

“…Preliminary data from other researchers have indicated an induction of autophagy in Bortezomib-resistant cells as a balancing mechanism to obtain nutrients from damaged molecules, banish accumulated non-functional proteins, and suppress the activation of pro-apoptotic pathways [ 25 , 27 , 31 ]. Additionally, the joint assembly of proteasomes and autophagosomes and the emerging process called proteaphagy have been documented to increase after the administration of proteasome inhibitors, potentially in an effort to eliminate blocked proteasomes [ 55 ]. To assess these phenomena, the lysosomal activity was studied using the alkaline dye Lysotracker RED, which can visualize ( Fig 5B ) and quantify ( Fig 5C ) acidic protein content.…”

The ERK1/2-Elk1, JNK-cJun, and JAK-STAT Transcriptional Axes as Potential Bortezomib Resistance Mediators in Prostate Cancer

“…In the first case, other autophagic markers would be absent due to significant downregulation, while in the second case, increased p62 would be necessary to carry an increased load toward degradation. The role of p62 in Bortezomib resistance has only recently been uncovered, linking its expression with proteasome inhibitor-resistant cells (56). In our experiments, p62 was found to be significantly increased in the naïve cells after Bortezomib administration in a dose-dependent manner.…”

“…In contrast to Yerlikaya and Okur (2019) where a Bortezomib-resistant cell line had also been created, we observed the mature PSMB5 overexpressed and no accumulation of the precursor form, indicting a different pathway of resistance (93). Recently, the role of p62 in proteasome inhibitor resistance has gained interest, given the protein’s role as a connection molecule between ubiquitin-proteasome degradation and ubiquitin-dependent microautophagy (56,57,61). The resistant cells were found to overexpress p62, confirming data from previous studies that all the autophagy markers assessed indicated a strong indication of the pathway during resistance (27,56,57,101).…”

“…Recently, the role of p62 in proteasome inhibitor resistance has gained interest, given the protein’s role as a connection molecule between ubiquitin-proteasome degradation and ubiquitin-dependent microautophagy (56,57,61). The resistant cells were found to overexpress p62, confirming data from previous studies that all the autophagy markers assessed indicated a strong indication of the pathway during resistance (27,56,57,101). Autophagy modulation as an alternative proteolytic mechanism could explain the cells’ ability to control the turnover of various kinases and cell cycle inhibitors that otherwise accumulate and lead the cell to apoptotic death.…”

“…The idea that autophagy can substitute for dysregulated UPS has been documented in several studies of different cancer types, including prostate cancer (27,35,56,57). To monitor the autophagic activity, the lysosomal marker Lysotracker RED was used.…”