Proteasome inhibitor-adapted myeloma cells are largely independent from proteasome activity and show complex proteomic changes, in particular in redox and energy metabolism

Abstract

Adaptive resistance of myeloma to proteasome inhibition represents a clinical challenge, whose biology is poorly understood. Proteasome mutations were implicated as underlying mechanism, while an alternative hypothesis based on low activation status of the unfolded protein response was recently suggested (IRE1/XBP1-low model). We generated bortezomib- and carfilzomib-adapted, highly resistant multiple myeloma cell clones (AMO-BTZ, AMO-CFZ), which we analyzed in a combined quantitative and functional proteomic approach. We demonstrate that proteasome inhibitor-adapted myeloma cells tolerate subtotal proteasome inhibition, irrespective of a proteasome mutation, and uniformly show an 'IRE1/XBP1-low' signature. Adaptation of myeloma cells to proteasome inhibitors involved quantitative changes in >600 protein species with similar patterns in AMO-BTZ and AMO-CFZ cells: proteins involved in metabolic regulation, redox homeostasis, and protein folding and destruction were upregulated, while apoptosis and transcription/translation were downregulated. The quantitatively most upregulated protein in AMO-CFZ cells was the multidrug resistance protein (MDR1) protein ABCB1, and carfilzomib resistance could be overcome by MDR1 inhibition. We propose a model where proteasome inhibitor-adapted myeloma cells tolerate subtotal proteasome inhibition owing to metabolic adaptations that favor the generation of reducing equivalents, such as NADPH, which is supported by oxidative glycolysis. Proteasome inhibitor resistance may thus be targeted by manipulating the energy and redox metabolism.

Analysis of the genomic landscape of multiple myeloma highlights novel prognostic markers and disease subgroups

“…However, in individual cases, rare gene mutations may impact treatment decisions. For example, CRBN , IKZF1 or IKZF3 mutations (totaling a combined 1.6% of patients in our cohort) would predict resistance to IMiDs 26 , whereas XBP1 deletions could predict bortezomib-resistance 38 , suggesting future clinical-grade targeted platforms must include rare driver events that have predictive value for treatment.…”

The start of a new wave: Developments in proteasome inhibition in multiple myeloma

“…62 A similar pattern of selectivity is observed for carfilzomib binding to the immunoproteasome. 63 More pertinently, lower levels of off-target protease inhibition are observed with carfilzomib than with bortezomib, including activity at the HtrA2/Omi protease; this may explain the difference in incidence of peripheral neuropathy associated with the 2 proteasome inhibitors. 58…”

“…Mutations in the PSMB5 gene, which codes for the proteasome β5-subunit protein, by diminishing proteasome inhibitor binding, may lead to the development of resistance 93 ; however, this has not been corroborated. 63 It is now believed that several pathways are involved in the development of proteasome inhibitor resistance. An in vitro study showed that the adaptation of MM cells exposed to bortezomib Krüppel-like transcription factor 4 (which leads to prosurvival cell destruction).…”

“…An in vitro study showed that the adaptation of MM cells exposed to bortezomib Krüppel-like transcription factor 4 (which leads to prosurvival cell destruction). 63,94,95 Early-phase clinical data are available that support the link between UPR downregulation and bortezomib resistance. The addition of the protease inhibitor nelfinavir, which has been shown to activate the UPR response, enabled bortezomib resistance to be overcome in patients with advanced hematological malignancies.…”

“…A study investigating protein expression in proteasome inhibitor-resistant MM cells showed that the protein most upregulated in carfilzomib-resistant cells was ABCB1, an efflux pump (also known as P-glycoprotein), but upregulation of ABCB1 was not observed in bortezomib-resistant cells. 63 Furthermore, a recent in vitro study found that the protease inhibitors nelfinavir and lopinavir-potent functional modulators of ABCB1-mediated drug export-can restore carfilzomib activity at clinically tolerable levels. 99 An improved understanding of the -mechanisms underlying proteasome inhibitor resistance is required to inform the development of effective strategies against this emerging challenge in MM.…”