The power of proteasome inhibition in multiple myeloma

The role of proteasomes in tumorigenesis

Protein homeostasis is the basis of normal life activities, and the proteasome family plays an extremely important function in this process. The proteasome 20S is a concentric circle structure with two α rings and two β rings overlapped. The proteasome 20S can perform both ATP-dependent and non-ATP-dependent ubiquitination proteasome degradation by binding to various subunits (such as 19S, 11S, and 200 PA), which is performed by its active subunit β1, β2, and β5. The proteasome can degrade misfolded, excess proteins to maintain homeostasis. At the same time, it can be utilized by tumors to degrade over-proliferate and unwanted proteins to support their growth. Proteasomes can affect the development of tumors from several aspects including tumor signaling pathways such as NF-κB and p53, cell cycle, immune regulation, and drug resistance. Proteasome-encoding genes have been found to be overexpressed in a variety of tumors, providing a potential novel target for cancer therapy. In addition, proteasome inhibitors such as bortezomib, carfilzomib, and ixazomib have been put into clinical application as the first-line treatment of multiple myeloma. More and more studies have shown that it also has different therapeutic effects in other tumors such as hepatocellular carcinoma, non-small cell lung cancer, glioblastoma, and neuroblastoma. However, proteasome inhibitors are not much effective due to their tolerance and singleness in other tumors. Therefore, further studies on their mechanisms of action and drug interactions are needed to investigate their therapeutic potential.

Selinexor in multiple myeloma

INTRODUCTION

Selinexor, an XPO1 inhibitor, has emerged as a promising therapeutic option in the challenging landscape of relapsed/refractory multiple myeloma (RRMM).

AREAS COVERED

This article provides a review of selinexor, with a focus on available clinical studies involving MM patients and its safety profile. Clinical trials, such as STORM and BOSTON, have demonstrated its efficacy, particularly in combination regimens, showcasing notable overall response rates (ORR) and prolonged median progressionfree survival (mPFS). Selinexor's versatility is evident across various combinations, including carfilzomibdexamethasone (XKd), lenalidomidedexamethasone (XRd), and pomalidomidedexamethasone (XPd), with efficacy observed even in tripleclass refractory and highrisk patient populations. However, challenges, including resistance mechanisms and adverse events, necessitate careful management. Realworld evidence also underscores selinexor's effectiveness in RRMM, though dose adjustments and supportive measures remain crucial. Ongoing trials are exploring selinexor in diverse combinations and settings, including pomalidomidenaïve patients and postautologous stem cell transplant (ASCT) maintenance.

EXPERT OPINION

The evolving landscape of selinexor's role in the sequencing of treatment for RRMM, its potential in highrisk patients, including those with extramedullary disease, as revealed in the most recent international meetings, and ongoing investigations signal a dynamic era in myeloma therapeutics. Selinexor emerges as a pivotal component in multidrug strategies and innovative combinations.

Selinexor: Targeting a novel pathway in multiple myeloma

Selinexor is an orally bioavailable selective inhibitor of nuclear export compound that inhibits exportin-1 (XPO1), a novel therapeutic target that is overexpressed in multiple myeloma (MM) and is responsible for the transport of ∼220 nuclear proteins to the cytoplasm, including tumour suppressor proteins. Inhibition of this process has demonstrated substantial antimyeloma activity in preclinical studies, both alone and in combination with established MM therapeutics. Based on a clinical trial programme encompassing multiple combination regimens, selinexor-based therapy has been approved for the treatment of relapsed/refractory MM (RRMM), with selinexor-dexamethasone approved in the later-relapse setting for penta-refractory patients and selinexor-bortezomib-dexamethasone approved for patients who have received ≥1 prior therapy. Here, we provide a comprehensive review of the clinical data on selinexor-based regimens, including recent updates from the 2022 American Society of Hematology annual meeting, and summarise ongoing studies of this novel targeted agent in newly diagnosed MM and RRMM.

Comprehensive Structure-Activity Relationship Studies of Cepafungin Enabled by Biocatalytic C-H Oxidations

The cepafungins are a class of highly potent and selective eukaryotic proteasome inhibitor natural products with potential to treat refractory multiple myeloma and other cancers. The structure-activity relationship of the cepafungins is not fully understood. This Article chronicles the development of a chemoenzymatic approach to cepafungin I. A failed initial route involving derivatization of pipecolic acid prompted us to examine the biosynthetic pathway for the production of 4-hydroxylysine, which culminated in the development of a 9-step synthesis of cepafungin I. An alkyne-tagged analogue enabled chemoproteomic studies of cepafungin and comparison of its effects on global protein expression in human multiple myeloma cells to the clinical drug bortezomib. A preliminary series of analogues elucidated critical determinants of potency in proteasome inhibition. Herein we report the chemoenzymatic syntheses of 13 additional analogues of cepafungin I guided by a proteasome-bound crystal structure, 5 of which are more potent than the natural product. The lead analogue was found to have 7-fold greater proteasome β5 subunit inhibitory activity and has been evaluated against several multiple myeloma and mantle cell lymphoma cell lines in comparison to the clinical drug bortezomib.

Genetic Alterations in Members of the Proteasome 26S Subunit, AAA-ATPase (PSMC) Gene Family in the Light of Proteasome Inhibitor Resistance in Multiple Myeloma

For the treatment of Multiple Myeloma, proteasome inhibitors are highly efficient and widely used, but resistance is a major obstacle to successful therapy. Several underlying mechanisms have been proposed but were only reported for a minority of resistant patients. The proteasome is a large and complex machinery. Here, we focus on the AAA ATPases of the 19S proteasome regulator (PSMC1-6) and their implication in PI resistance. As an example of cancer evolution and the acquisition of resistance, we conducted an in-depth analysis of an index patient by applying FISH, WES, and immunoglobulin-rearrangement sequencing in serial samples, starting from MGUS to newly diagnosed Multiple Myeloma to a PI-resistant relapse. The WES analysis uncovered an acquired PSMC2 Y429S mutation at the relapse after intensive bortezomib-containing therapy, which was functionally confirmed to mediate PI resistance. A meta-analysis comprising 1499 newly diagnosed and 447 progressed patients revealed a total of 36 SNVs over all six PSMC genes that were structurally accumulated in regulatory sites for activity such as the ADP/ATP binding pocket. Other alterations impact the interaction between different PSMC subunits or the intrinsic conformation of an individual subunit, consequently affecting the folding and function of the complex. Interestingly, several mutations were clustered in the central channel of the ATPase ring, where the unfolded substrates enter the 20S core. Our results indicate that PSMC SNVs play a role in PI resistance in MM.

Single-Nucleotide Variants and Epimutations Induce Proteasome Inhibitor Resistance in Multiple Myeloma

PURPOSE

Proteasome inhibitors (PI) are the backbone of various treatment regimens in multiple myeloma. We recently described the first in-patient point mutations affecting the 20S subunit PSMB5 underlying PI resistance. Notably, in vivo, the incidence of mutations in PSMB5 and other proteasome encoding genes is too low to explain the development of resistance in most of the affected patients. Thus, additional genetic and epigenetic alterations need to be explored.

EXPERIMENTAL DESIGN

We performed DNA methylation profiling by Deep Bisulfite Sequencing in PSMB5, PSMC2, PSMC5, PSMC6, PSMD1, and PSMD5, a subset of proteasome subunits that have hitherto been associated with PI resistance, recruited from our own previous research, the literature, or a meta-analysis on the frequency of somatic mutations. Methylation was followed up on gene expression level and by dual-luciferase reporter assay. The KMS11 cell line served as a model to functionally test the impact of demethylating agents.

RESULTS

We identified PSMD5 promoter hypermethylation and subsequent epigenetic gene silencing in 24% of PI refractory patients. Hypermethylation correlated with decreased expression and the regulatory impact of this region was functionally confirmed. In contrast, patients with newly diagnosed multiple myeloma, along with peripheral blood mononuclear cells and CD138+ plasma cells from healthy donors, generally show unmethylated profiles.

CONCLUSIONS

Under the selective pressure of PI treatment, multiple myeloma cells acquire methylation of the PSMD5 promoter silencing the PSMD5 gene expression. PSMD5 acts as a key orchestrator of proteasome assembly and its downregulation was described to increase the cell's proteolytic capacity. PSMD5 hypermethylation, therefore, represents a novel mechanism of PI tolerance in multiple myeloma.

The OTUD6B-LIN28B-MYC axis determines the proliferative state in multiple myeloma

Deubiquitylases (DUBs) are therapeutically amenable components of the ubiquitin machinery that stabilize substrate proteins. Their inhibition can destabilize oncoproteins that may otherwise be undruggable. Here, we screened for DUB vulnerabilities in multiple myeloma, an incurable malignancy with dependency on the ubiquitin proteasome system and identified OTUD6B as an oncogene that drives the G1/S‐transition. LIN28B, a suppressor of microRNA biogenesis, is specified as a bona fide cell cycle‐specific substrate of OTUD6B. Stabilization of LIN28B drives MYC expression at G1/S, which in turn allows for rapid S‐phase entry. Silencing OTUD6B or LIN28B inhibits multiple myeloma outgrowth in vivo and high OTUD6B expression evolves in patients that progress to symptomatic multiple myeloma and results in an adverse outcome of the disease. Thus, we link proteolytic ubiquitylation with post‐transcriptional regulation and nominate OTUD6B as a potential mediator of the MGUS‐multiple myeloma transition, a central regulator of MYC, and an actionable vulnerability in multiple myeloma and other tumors with an activated OTUD6B‐LIN28B axis.

Geranylgeranyl diphosphate synthase inhibitor and proteasome inhibitor combination therapy in multiple myeloma

Background

Multiple myeloma (MM) remains an incurable malignancy, despite the advent of therapies such as proteosome inhibitors (PIs) that disrupt protein homeostasis and induce ER stress. We have pursued inhibition of geranylgeranyl diphosphate synthase (GGDPS) as a novel mechanism by which to target protein homeostasis in MM cells. GGDPS inhibitors (GGSI) disrupt Rab geranylgeranylation, which in turn results in perturbation of Rab-mediated protein trafficking, leading to accumulation of intracellular monoclonal protein, induction of ER stress and apoptosis. Our lead GGSI, RAM2061, has demonstrated favorable pharmacokinetic properties and in vivo efficacy. Here we sought to evaluate if combination therapy with GGSI and PI would result in enhanced disruption of the unfolded protein response (UPR) and increase anti-MM efficacy.

Methods

MTT assays were conducted to evaluate the cytotoxic effects of combining RAM2061 with bortezomib in human MM cells. The effects of RAM2061 and/or PI (bortezomib or carfilzomib) on markers of UPR and apoptosis were evaluated by a combination of immunoblot (ATF4, IRE1, p-eIF2a, cleaved caspases and PARP), RT-PCR (ATF4, ATF6, CHOP, PERK, IRE1) and flow cytometry (Annexin-V). Induction of immunogenic cell death (ICD) was assessed by immunoblot (HMGB1 release) and flow cytometry (calreticulin translocation). Cell assays were performed using both concurrent and sequential incubation with PIs. To evaluate the in vivo activity of GGSI/PI, a flank xenograft using MM.1S cells was performed.

Results

Isobologram analysis of cytotoxicity data revealed that sequential treatment of bortezomib with RAM2061 has a synergistic effect in MM cells, while concurrent treatment was primarily additive or mildly antagonistic. The effect of PIs on augmenting RAM2061-induced upregulation of UPR and apoptotic markers was dependent on timing of the PI exposure. Combination treatment with RAM2061 and bortezomib enhanced activation of ICD pathway markers. Lastly, combination treatment slowed MM tumor growth and lengthened survival in a MM xenograft model without evidence of off-target toxicity.

Conclusion

We demonstrate that GGSI/PI treatment can potentiate activation of the UPR and apoptotic pathway, as well as induce upregulation of markers associated with the ICD pathway. Collectively, these findings lay the groundwork for future clinical studies evaluating combination GGSI and PI therapy in patients with MM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40164-022-00261-6.

DKK1 suppresses WWP2 to enhance bortezomib resistance in multiple myeloma via regulating GLI2 ubiquitination

Bortezomib-based chemotherapy represents the most prevalent regimens for multiple myeloma (MM), whereas acquired drug resistance remains a major obstacle. Myeloma cells often produce excessive amount of dickkopf-1 (DKK1), giving rise to myeloma bone disease. However, it remains obscure about the effects and mechanisms of DKK1 in the progression and bortezomib responsiveness of MM cells. In the current study, we found WWP2, an E3 ubiquitin-protein ligase, was downregulated in the bortezomib-resistant cells along with high expression of DKK1. Further investigation revealed that WWP2 was a direct target of Wnt/β-catenin signaling pathway, and DKK1 suppressed the expression of WWP2 via canonical Wnt signaling. We further identified that WWP2 mediated the ubiquitination and degradation of GLI2, a main transcriptional factor of the Hedgehog (Hh) pathway. Therefore, DKK1-induced WWP2 downregulation improved GLI2 stability and activation of Hh signaling pathway, contributing to the resistance to bortezomib of MM cells. Clinical data also validated that WWP2 expression was associated with the treatment response and clinic outcomes of MM patients. WWP2 overexpression restricted MM progression and enhanced cell sensitivity to bortezomib treatment in vitro and in vivo. Taken together, our findings demonstrate that DKK1 facilitates the generation of bortezomib resistance in MM via downregulating WWP2 and activating Hh pathway. Thus, the manipulation of DKK1-WWP2-GLI2 axis might sensitize myeloma cells to proteasome inhibitors.

Carfilzomib Promotes the Unfolded Protein Response and Apoptosis in Cetuximab-Resistant Colorectal Cancer

Cetuximab is a common treatment option for patients with wild-type K-Ras colorectal carcinoma. However, patients often display intrinsic resistance or acquire resistance to cetuximab following treatment. Here we generate two human CRC cells with acquired resistance to cetuximab that are derived from cetuximab-sensitive parental cell lines. These cetuximab-resistant cells display greater in vitro proliferation, colony formation and migration, and in vivo tumour growth compared with their parental counterparts. To evaluate potential alternative therapeutics to cetuximab-acquired-resistant cells, we tested the efficacy of 38 current FDA-approved agents against our cetuximab-acquired-resistant clones. We identified carfilzomib, a selective proteosome inhibitor to be most effective against our cell lines. Carfilzomib displayed potent antiproliferative effects, induced the unfolded protein response as determined by enhanced CHOP expression and ATF6 activity, and enhanced apoptosis as determined by enhanced caspase-3/7 activity. Overall, our results indicate a potentially novel indication for carfilzomib: that of a potential alternative agent to treat cetuximab-resistant colorectal cancer.

Real-world evidence for carfilzomib dosing intensity on overall survival and treatment progression in multiple myeloma patients

INTRODUCTION

Carfilzomib dosing as a single agent or in combination with dexamethasone (Kd) has evolved from the initial 27 mg/m² twice-weekly (legacy dose), to more recently approved doses of 56 mg/m² twice-weekly and 70 mg/m² once-weekly (optimized doses). The objective of this study was to evaluate the overall survival (OS), and time to next treatment (TTNT) among multiple myeloma patients treated with Kd optimized vs legacy doses.

METHODS

A retrospective analysis of patients receiving Kd between 01/01/2013-07/31/2017 was conducted using IQVIA's oncology electronic medical records database. Kd dose was estimated based on body surface area. OS was measured from the Kd-initiation date until death. TTNT was defined as the time from Kd-initiation until the start of subsequent treatment. Kaplan-Meier analysis and Cox models were used to evaluate OS and TTNT.

RESULTS

Of the 1,469 patients evaluated, 129 (8.8%) received optimized dose and 1,340 (91.2%) received legacy dose. Risk of mortality was 64% lower for patients receiving the optimized doses (HR: 0.36, 95% CI: 0.178-0.745). Patients receiving the optimized doses had significantly longer TTNT compared to patients receiving the legacy dose (median TTNT: 17.5 months [95% CI: 14.8-NE] and 13.2 months, [95% CI: 12.4-14.4], respectively; p = 0.023), and 33% lower risk of progressing to the subsequent treatment (HR: 0.67, 95% CI: 0.48-0.93).

CONCLUSIONS

Patient outcomes could be improved if eligible MM patients are treated with the optimized, recently approved Kd doses (56 mg/m² twice-weekly and 70 mg/m² once-weekly).

Prognostic Role of the Ubiquitin Proteasome System in Clear Cell Renal Cell Carcinoma: A Bioinformatic Perspective

Background: Clear cell renal cell carcinoma (ccRCC) is a common malignant tumor of the urinary system. The ubiquitin proteasome system (UPS) plays an important role in the generation, metabolism and survival of tumor. We are aimed to make a comprehensive exploration of the UPS's role in ccRCC with bioinformatic tools, which may contribute to the understanding of UPS in ccRCC, and give insight for further research. Methods: The UPS-related genes (UPSs) were collected by an integrative approach. The expression and clinical data were downloaded from TCGA database. R soft was used to perform the differentially expressed UPSs analysis, functional enrichment analysis. We also estimated prognostic value of each UPS with the help of GEPIA database. Two predicting models were constructed with the differentially expressed UPSs and prognosis-related genes, respectively. The correlations of risk score with clinical characteristics were also evaluated. Data of GSE29609 cohort were obtained from GEO database to validate the prognostic models. Results: We finally identified 91 differentially expressed UPSs, 48 prognosis related genes among them, and constructed a prognostic model with 18 UPSs successfully, the AUC was 0.760. With the help of GEPIA, we found 391 prognosis-related UPSs, accounting for 57.84% of all UPSs. Another prognostic model was constructed with 28 prognosis-related genes of them, and with a better AUC of 0.825. Additionally, our models can also stratify patients into high and low risk groups accurately in GSE29609 cohort. Similar prognostic values of our models were observed in the validated GSE29609 cohort. Conclusions: UPS is dysregulated in ccRCC. UPS related genes have significant prognostic value in ccRCC. Models constructed with UPSs are effective and applicable. An abnormal ubiquitin proteasome system should play an important role in ccRCC and be worthy of further study.

Reinvigorating the Chiral Pool: Chemoenzymatic Approaches to Complex Peptides and Terpenoids

Biocatalytic transformations that leverage the selectivity and efficiency of enzymes represent powerful tools for the construction of complex natural products. Enabled by innovations in genome mining, bioinformatics, and enzyme engineering, synthetic chemists are now more than ever able to develop and employ enzymes to solve outstanding chemical problems, one of which is the reliable and facile generation of stereochemistry within natural product scaffolds. In recognition of this unmet need, our group has sought to advance novel chemoenzymatic strategies to both expand and reinvigorate the chiral pool. Broadly defined, the chiral pool comprises cheap, enantiopure feedstock chemicals that serve as popular foundations for asymmetric total synthesis. Among these building blocks, amino acids and enantiopure terpenes, whose core structures can be mapped onto several classes of structurally and pharmaceutically intriguing natural products, are of particular interest to the synthetic community.In this Account, we summarize recent efforts from our group in leveraging biocatalytic transformations to expand the chiral pool, as well as efforts toward the efficient application of these transformations in natural products total synthesis, the ultimate testing ground for any novel methodology. First, we describe several examples of enzymatic generation of noncanonical amino acids as means to simplify the synthesis of peptide natural products. By extracting amino acid hydroxylases from native biosynthetic pathways, we obtain efficient access to hydroxylated variants of proline, lysine, arginine, and their derivatives. The newly installed hydroxyl moiety then becomes a chemical handle that can facilitate additional complexity generation, thereby expanding the pool of amino acid-derived building blocks available for peptide synthesis. Next, we present our efforts in enzymatic C-H oxidations of diverse terpene scaffolds, in which traditional chemistry can be combined with strategic applications of biocatalysis to selectively and efficiently derivatize several commercial terpenoid skeletons. The synergistic logic of this approach enables a small handful of synthetic intermediates to provide access to a plethora of terpenoid natural product families. Taken together, these findings demonstrate the advantages of applying enzymes in total synthesis in conjunction with established methodologies, as well as toward the expansion of the chiral pool to enable facile incorporation of stereochemistry during synthetic campaigns.

Resistance of osteosarcoma cells to the proapoptotic effects of carfilzomib involves activation of mitogen activated protein kinase pathways

NEW FINDINGS

What is the central question of this study? Carfilzomib, a second-generation proteasome inhibitor approved for the treatment of multiple myeloma, shows efficacy against osteosarcoma. However, drug resistance remains a major challenge. What is the role of carfilzomib-induced changes in mitogen-activated protein kinase (MAPK) pathways in the sensitivity of osteosarcoma cells to the proapoptotic effects of the drug? What is the main finding and its importance? The dose-dependent antiapoptotic effects in osteosarcoma are associated with activation of MAPK signalling. Combinational targeting of MAPK signalling pathways can synergistically enhance carfilzomib-induced cell apoptosis, suggesting that MAPK inhibitors in combination with proteasome inhibitors can serve as a novel therapeutic tool for osteosarcoma.

ABSTRACT

Osteosarcoma is the most common primary bone malignancy. Despite efforts to improve outcomes, the overall survival rates for osteosarcoma have remained unchanged over the past three decades. In this study, we assessed the proapoptotic effects of the second-generation proteasome inhibitor carfilzomib on osteosarcoma and investigated the potential mechanisms underlying the synergistic proapoptotic action when combined with mitogen-activated protein kinase (MAPK) inhibitors. We found that carfilzomib alone significantly inhibited cell proliferation and induced apoptosis in a dose-dependent manner, characterized by the induction of cleaved caspase 3 and poly (ADP-ribose) polymerase. More importantly, focusing on the changes of antiapoptotic B-cell lymphoma 2 (Bcl-2) family members and signalling pathways, we found a striking induction of myeloid cell leukaemia 1 (Mcl-1) and the activation of MAPK pathways. Furthermore, we observed that combinational targeting of the MAPK pathways using the specific inhibitors U0126, SP600125 or SB203580 synergistically enhanced carfilzomib-induced cell apoptosis. Notably, we found that the combinational inhibition of extracellular signal-regulated kinase or c-Jun N-terminal kinase MAPK pathways significantly decreased the expression of the three antiapoptotic Bcl-2 family proteins, and in particular this reversed induction of Mcl-1 by carfilzomib. Collectively, our findings show that activation of the MAPK pathways contributes to the mechanisms of drug resistance to carfilzomib. In addition, the synergistic proapoptotic action of MAPK and proteasome inhibitors in osteosarcoma cells suggests that combinational therapy with both drug types may serve as a novel strategy for the clinical management of osteosarcoma.

Ubiquitination and Ubiquitin-Like Modifications in Multiple Myeloma: Biology and Therapy

Simple Summary

Multiple myeloma is a cancer of plasma cells causing bone fractures, anemia, renal insufficiency and hypercalcemia. Despite the introduction of new drugs in the past years, it still remains incurable and most patients die from the disease. Multiple myeloma cells are characterized by the production of high amounts of monoclonal antibodies. Therefore, maintaining protein homeostasis from synthesis through folding to degradation is crucial for multiple myeloma cells. While protein ubiquitination and organized degradation are typically considered critical for cellular health, an emerging strategy is to block these processes to induce cell death in disease-state cells characterized by protein over-production. Recent development of compounds that alter the ubiquitin proteasome pathway and drugs that affect ubiquitin-like modifications appear promising in both preclinically and in clinical trials. This review summarizes the impact of protein modifications such as ubiquitination and ubiquitin-like modifications in the biology of multiple myeloma and how it can be exploited to develop new effective therapies for multiple myeloma.

Abstract

Multiple myeloma is a genetically heterogeneous plasma cell malignancy characterized by organ damage and a massive production of (in-)complete monoclonal antibodies. Coping with protein homeostasis and post-translational regulation is therefore essential for multiple myeloma cells to survive. Furthermore, post-translational modifications such as ubiquitination and SUMOylation play key roles in essential pathways in multiple myeloma, including NFκB signaling, epigenetic regulation, as well as DNA damage repair. Drugs modulating the ubiquitin–proteasome system, such as proteasome inhibitors and thalidomide analogs, are approved and highly effective drugs in multiple myeloma. In this review, we focus on ubiquitin and ubiquitin-like modifications in the biology and current developments of new treatments for multiple myeloma.

Molecular and cellular dynamics of the 26S proteasome

In eukaryotic cells, the ubiquitin-proteasome system serves to remove proteins that are either dysfunctional or no longer needed. The 26S proteasome is a 2.5 MDa multisubunit complex comprising the 20S core particle, where degradation is executed, and one or two regulatory particles which prepare substrates for degradation. Whereas the 20S core particles of several species had been studied extensively by X-ray crystallography, the 26S holocomplex structure had remained elusive for a long time. Recent advances in single-particle cryo-electron microscopy have changed the situation and provided atomic resolution models of this intriguing molecular machine and its dynamics. Besides, cryo-electron tomography enables structural studies in situ, providing molecular resolution images of macromolecules inside pristinely preserved cellular environments. This has greatly contributed to our understanding of proteasome dynamics in the context of cells.

Curcumin inhibits proteasome activity in triple-negative breast cancer cells through regulating p300/miR-142-3p/PSMB5 axis

BACKGROUND

Curcumin functions as a proteasome inhibitor. However, the molecular mechanisms behind this action need more detailed explanations.

PURPOSE

This study aimed to investigate the inhibitory effect of curcumin on 20S proteasome activity and to elucidate its exact mechanism in triple-negative breast cancer (TNBC) MDA-MB-231 cells.

METHODS

Proteasomal peptidase activities were assayed using synthetic fluorogenic peptide substrates. Knockdown or overexpression of microRNA (miRNA or miR) or protein was used to investigate its functional effect on downstream cellular processes. BrdU (5‑bromo‑2'-deoxyuridine) assay was performed to identify cell proliferation. Western blot and quantitative real-time PCR(qRT-PCR) were carried out to determine protein abundance and miRNA expression, respectively. Correlations between protein expressions, miRNA levels, and proteasome activities were analyzed in TNBC tissues. Xenograft tumor model was performed to observe the in vivo effect of curcumin on 20S proteasome activity.

RESULTS

Curcumin significantly reduced PSMB5 protein levels, accompanied with a reduction in the chymotrypsin-like (CT-l) activity of proteasome 20S core. Loss of PSMB5 markedly inhibited the CT-l activity of 20S proteasome. Furthermore, curcumin treatment significantly elevated miR-142-3p expression. PSMB5 was a direct target of miR-142-3p and its protein levels were negatively regulated by miR-142-3p. Moreover, histone acetyltransferase p300 suppressed miR-142-3p expression. Overexpression of p300 mitigated the promotive effect of curcumin on miR-142-3p expression. The correlations among p300 abundances, miR-142-3p levels, PSMB5 expressions, and the CT-l activities of 20S proteasome were evidenced in TNBC tissues. In addition, loss of p300 and PSMB5 reduced cell proliferation. Inhibition of miR-142-3p significantly attenuated the inhibitory impact of curcumin on cell proliferation. These curcumin-induced changes on p300, miR-142-3p, PSMB5, and 20S proteasome activity were further confirmed in in vivo solid tumor model.

CONCLUSION

These findings demonstrated that curcumin suppressed p300/miR-142-3p/PSMB5 axis leading to the inhibition of the CT-l activity of 20S proteasome. These results provide a novel and alternative explanation for the inhibitory effect of curcumin on proteasome activity and also raised potential therapeutic targets for TNBC treatment.

Targeting of the E3 ubiquitin-protein ligase HUWE1 impairs DNA repair capacity and tumor growth in preclinical multiple myeloma models

Experimental evidence suggests that ubiquitin-protein ligases regulate a number of cellular processes involved in tumorigenesis. We analysed the role of the E3 ubiquitin-protein ligase HUWE1 for pathobiology of multiple myeloma (MM), a still incurable blood cancer. mRNA expression analysis indicates an increase in HUWE1 expression levels correlated with advanced stages of myeloma. Pharmacologic as well as RNAi-mediated HUWE1 inhibition caused anti-proliferative effects in MM cell lines in vitro and in an MM1.S xenotransplantation mouse model. Cell cycle analysis upon HUWE1 inhibition revealed decreased S phase cell fractions. Analyses of potential HUWE1-dependent molecular functions did not show involvement in MYC-dependent gene regulation. However, HUWE1 depleted MM cells displayed increased DNA tail length by comet assay, as well as changes in the levels of DNA damage response mediators such as pBRCA1, DNA-polymerase β, γH2AX and Mcl-1. Our finding that HUWE1 might thus be involved in endogenous DNA repair is further supported by strongly enhanced apoptotic effects of the DNA-damaging agent melphalan in HUWE1 depleted cells in vitro and in vivo. These data suggest that HUWE1 might contribute to tumour growth by endogenous repair of DNA, and could therefore potentially be exploitable in future treatment developments.

Concise Chemoenzymatic Total Synthesis and Identification of Cellular Targets of Cepafungin I

The natural product cepafungin I was recently reported to be one of the most potent covalent inhibitors of the 20S proteasome core particle through a series of in vitro activity assays. Here, we report a short chemoenzymatic total synthesis of cepafungin I featuring the use of a regioselective enzymatic oxidation to prepare a key hydroxylated amino acid building block in a scalable fashion. The strategy developed herein enabled access to a chemoproteomic probe, which in turn revealed the exceptional selectivity and potency of cepafungin I toward the β2 and β5 subunits of the proteasome. Further structure-activity relationship studies suggest the key role of the hydroxyl group in the macrocycle and the identity of the lipid tail in modulating the potency of this natural product family. This study lays the groundwork for further medicinal chemistry exploration to fully realize the anticancer potential of cepafungin I.

Cell cycle exit during bortezomib-induced osteogenic differentiation of mesenchymal stem cells was mediated by Xbp1s-upregulated p21Cip1 and p27Kip1

Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating into a variety of cell types. Bortezomib, the first approved proteasome inhibitor used for the treatment of multiple myeloma (MM), has been shown to induce osteoblast differentiation, making it beneficial for myeloma bone disease. In the present study, we aimed to investigate the effects and underlying mechanisms of bortezomib on the cell cycle during osteogenic differentiation. We confirmed that low doses of bortezomib can induce MSCs towards osteogenic differentiation, but high doses are toxic. In the course of bortezomib-induced osteogenic differentiation, we observed cell cycle exit characterized by G₀ /G₁ phase cell cycle arrest with a significant reduction in cell proliferation. Additionally, we found that the cell cycle exit was tightly related to the induction of the cyclin-dependent kinase inhibitors p21^Cip1 and p27^Kip1 . Notably, we further demonstrated that the up-regulation of p21^Cip1 and p27^Kip1 is transcriptionally dependent on the bortezomib-activated ER stress signalling branch Ire1α/Xbp1s. Taken together, these findings reveal an intracellular pathway that integrates proteasome inhibition, osteogenic differentiation and the cell cycle through activation of the ER stress signalling branch Ire1α/Xbp1s.

Proteasome Subunits Differentially Control Myeloma Cell Viability and Proteasome Inhibitor Sensitivity

We generated eight multiple myeloma cell lines resistant to bortezomib; five acquired PSMB5 mutations. In 1,500 patients such mutations were rare clinically. To better understand disruption of proteasomes on multiple myeloma viability and drug sensitivity, we systematically deleted the major proteasome catalytic subunits. Multiple myeloma cells without PSMB5 were viable. Drug-resistant, PSMB5-mutated cell lines were resensitized to bortezomib by PSMB5 deletion, implying PSMB5 mutation is activating in its drug resistance function. In contrast, PSMB6 knockout was lethal to multiple myeloma cell lines. Depleting PSMB6 prevented splicing of the major catalytic subunits PSMB5, PSMB7, PSMB8, and PSMB10; however, PSMB6 engineered without splicing function or catalytic activity, also restored viability, inferring the contribution of PSMB6 to proteasome structure to be more important than functional activity. Supporting this, bortezomib sensitivity was restored in drug-resistant multiple myeloma cell lines by low level expression of mutated PSMB6 lacking splicing function. Loss of PSMB8 and PSMB9 was neither lethal nor restored bortezomib sensitivity. Significant codependency of PSMB5, PSMB6, and PSMB7 expression was observed. We demonstrated elevated levels of PSMB6 and 7, but not 8 and 9, in some, but not all, serial patient samples exposed to proteasome inhibitors. In summary, we show PSMB6 and PSMB7, but not PSMB5, to be essential for multiple myeloma cell survival, this dependency is structural and that upregulation or activating mutation of PSMB5, 6, and 7 confers proteasome inhibitor resistance, while depletion confers sensitivity. IMPLICATIONS: These findings support modulation of PSMB5, PSMB6, or PSMB7 expression as a new therapeutic strategy.

The E3 ligase HUWE1 inhibition as a therapeutic strategy to target MYC in multiple myeloma

Proteasome inhibitors have provided a significant advance in the treatment of multiple myeloma (MM). Consequently, there is increasing interest in developing strategies to target E3 ligases, de-ubiquitinases, and/or ubiquitin receptors within the ubiquitin proteasome pathway, with an aim to achieve more specificity and reduced side-effects. Previous studies have shown a role for the E3 ligase HUWE1 in modulating c-MYC, an oncogene frequently dysregulated in MM. Here we investigated HUWE1 in MM. We identified elevated expression of HUWE1 in MM compared with normal cells. Small molecule-mediated inhibition of HUWE1 resulted in growth arrest of MM cell lines without significantly effecting the growth of normal bone marrow cells, suggesting a favorable therapeutic index. Studies using a HUWE1 knockdown model showed similar growth inhibition. HUWE1 expression positively correlated with MYC expression in MM bone marrow cells and correspondingly, genetic knockdown and biochemical inhibition of HUWE1 reduced MYC expression in MM cell lines. Proteomic identification of HUWE1 substrates revealed a strong association of HUWE1 with metabolic processes in MM cells. Intracellular glutamine levels are decreased in the absence of HUWE1 and may contribute to MYC degradation. Finally, HUWE1 depletion in combination with lenalidomide resulted in synergistic anti-MM activity in both in vitro and in vivo models. Taken together, our data demonstrate an important role of HUWE1 in MM cell growth and provides preclinical rationale for therapeutic strategies targeting HUWE1 in MM.

Mitochondrial Functions, Energy Metabolism and Protein Glycosylation are Interconnected Processes Mediating Resistance to Bortezomib in Multiple Myeloma Cells

The proteasome inhibitor bortezomib (BTZ) has emerged as an effective drug for the treatment of multiple myeloma even though many patients relapse from BTZ therapy. The present study investigated the metabolic pathways underlying the acquisition of bortezomib resistance in multiple myeloma. We used two different clones of multiple myeloma cell lines exhibiting different sensitivities to BTZ (U266 and U266-R) and compared them in terms of metabolic profile, mitochondrial fitness and redox balance homeostasis capacity. Our results showed that the BTZ-resistant clone (U266-R) presented increased glycosylated UDP-derivatives when compared to BTZ-sensitive cells (U266), thus also suggesting higher activities of the hexosamine biosynthetic pathway (HBP), regulating not only protein O- and N-glycosylation but also mitochondrial functions. Notably, U266-R displayed increased mitochondrial biogenesis and mitochondrial dynamics associated with stronger antioxidant defenses. Furthermore, U266-R maintained a significantly higher concentration of substrates for protein glycosylation when compared to U266, particularly for UDP-GlcNac, thus further suggesting the importance of glycosylation in the BTZ pharmacological response. Moreover, BTZ-treated U266-R showed significantly higher ATP/ADP ratios and levels of ECP and also exhibited increased mitochondrial fitness and antioxidant response. In conclusions, our findings suggest that the HBP may play a major role in mitochondrial fitness, driving BTZ resistance in multiple myeloma and thus representing a possible target for new drug development for BTZ-resistant patients.

Proteasome mapping reveals sexual dimorphism in tissue-specific sensitivity to protein aggregations

Defects in the proteasome can result in pathological proteinopathies. However, the pathogenic role of sex‐ and tissue‐specific sensitivity to proteotoxic stress remains elusive. Here, we map the proteasome activity across nine tissues, in male and female mice, and demonstrate strong sexual dimorphism in proteasome activity, where females have significantly higher activity in several tissues. Further, we report drastic differences in proteasome activity among tissues, independently of proteasome concentration, which are exacerbated under stress conditions. Sexual dimorphism in proteasome activity is confirmed in a SOD1 ALS mouse model, in which the spinal cord, a tissue with comparatively low proteasome activity, is severely affected. Our results offer mechanistic insight into tissue‐specific sensitivities to proteostasis stress and into sex differences in the progression of neurodegenerative proteinopathies.

Syrbactin proteasome inhibitor TIR-199 overcomes bortezomib chemoresistance and inhibits multiple myeloma tumor growth in vivo

Multiple myeloma (MM) and mantle cell lymphoma (MCL) are blood cancers that respond to proteasome inhibitors. Three FDA-approved drugs that block the proteasome are currently on the market, bortezomib, carfilzomib, and ixazomib. While these proteasome inhibitors have demonstrated clinical efficacy against refractory and relapsed MM and MCL, they are also associated with considerable adverse effects including peripheral neuropathy and cardiotoxicity, and tumor cells often acquire drug resistance. TIR-199 belongs to the syrbactin class, which constitutes a novel family of irreversible proteasome inhibitors. In this study, we compare TIR-199 head-to-head with three FDA-approved proteasome inhibitors. We demonstrate that TIR-199 selectively inhibits to varying degrees the sub-catalytic proteasomal activities (C-L/β1, T-L/β2, and CT-L/β5) in three actively dividing MM cell lines, with Ki₅₀ (CT-L/β5) values of 14.61 ± 2.68 nM (ARD), 54.59 ± 10.4 nM (U266), and 26.8 ± 5.2 nM (MM.1R). In most instances, this range was comparable with the activity of ixazomib. However, TIR-199 was more effective than bortezomib, carfilzomib, and ixazomib in killing bortezomib-resistant MM and MCL cell lines, as judged by a low resistance index (RI) between 1.7 and 2.2, which implies that TIR-199 indiscriminately inhibits both bortezomib-sensitive and bortezomib-resistant MM and MCL cells at similar concentrations. Importantly, TIR-199 reduced the tumor burden in a MM mouse model (p < 0.01) confirming its potency in vivo. Given the fact that there is still no cure for MM, the further development of TIR-199 or similar molecules that belong to the syrbactin class of proteasome inhibitors is warranted.

What sustains the multidrug resistance phenotype beyond ABC efflux transporters? Looking beyond the tip of the iceberg

Identification of multidrug (MDR) efflux transporters that belong to the ATP-Binding Cassette (ABC) superfamily, represented an important breakthrough for understanding cancer multidrug resistance (MDR) and its possible overcoming. However, recent data indicate that drug resistant cells have a complex intracellular physiology that involves constant changes in energetic and oxidative-reductive metabolic pathways, as well as in the molecular circuitries connecting mitochondria, endoplasmic reticulum (ER) and lysosomes. The aim of this review is to discuss the key molecular mechanisms of cellular reprogramming that induce and maintain MDR, beyond the presence of MDR efflux transporters. We specifically highlight how cancer cells characterized by high metabolic plasticity - i.e. cells able to shift the energy metabolism between glycolysis and oxidative phosphorylation, to survive both the normoxic and hypoxic conditions, to modify the cytosolic and mitochondrial oxidative-reductive metabolism, are more prone to adapt to exogenous stressors such as anti-cancer drugs and acquire a MDR phenotype. Similarly, we discuss how changes in mitochondria dynamics and mitophagy rates, changes in proteome stability ensuring non-oncogenic proteostatic mechanisms, changes in ubiquitin/proteasome- and autophagy/lysosome-related pathways, promote the cellular survival under stress conditions, along with the acquisition or maintenance of MDR. After dissecting the complex intracellular crosstalk that takes place during the development of MDR, we suggest that mapping the specific adaptation pathways underlying cell survival in response to stress and targeting these pathways with potent pharmacologic agents may be a new approach to enhance therapeutic efficacy against MDR tumors.

Missense variant in TPI1 (Arg189Gln) causes neurologic deficits through structural changes in the triosephosphate isomerase catalytic site and reduced enzyme levels in vivo

Mutations in the gene triosephosphate isomerase (TPI) lead to a severe multisystem condition that is characterized by hemolytic anemia, a weakened immune system, and significant neurologic symptoms such as seizures, distal neuropathy, and intellectual disability. No effective therapy is available. Here we report a compound heterozygous patient with a novel TPI pathogenic variant (NM_000365.5:c.569G>A:p.(Arg189Gln)) in combination with the common (NM_000365.5:c.315G>C:p.(Glu104Asp)) allele. We characterized the novel variant by mutating the homologous Arg in Drosophila using a genomic engineering system, demonstrating that missense mutations at this position cause a strong loss of function. Compound heterozygote animals were generated and exhibit motor behavioural deficits and markedly reduced protein levels. Furthermore, examinations of the TPI^Arg189Gln/TPI^Glu104Asp patient fibroblasts confirmed the reduction of TPI levels, suggesting that Arg189Gln may also affect the stability of the protein. The Arg189 residue participates in two salt bridges on the backside of the TPI enzyme dimer, and we reveal that a mutation at this position alters the coordination of the substrate-binding site and important catalytic residues. Collectively, these data reveal a new human pathogenic variant associated with TPI deficiency, identify the Arg189 salt bridge as critical for organizing the catalytic site of the TPI enzyme, and demonstrates that reduced TPI levels are associated with human TPI deficiency. These findings advance our understanding of the molecular pathogenesis of the disease, and suggest new therapeutic avenues for pre-clinical trials.

Carfilzomib for relapsed and refractory multiple myeloma

Although the prognosis of multiple myeloma (MM) patients has dramatically improved during recent years, virtually all patients eventually develop relapsed refractory disease. Several new therapeutics have been developed in the last few years, including carfilzomib, a second-generation proteasome inhibitor (PI) that has been approved by the US Food and Drug Administration (FDA) in the setting of relapsed and/or refractory MM, as a single agent with or without dexamethasone, and in combination with lenalidomide in 2012 and 2015, respectively. Other promising combinations with carfilzomib are being investigated. Carfilzomib has shown superiority over the first-generation PI bortezomib on both efficacy and toxicity. In particular, profoundly lower incidence in polyneuropathy compared to bortezomib has been described. However, carfilzomib has a different toxicity profile, with more cardiovascular adverse events. Therefore, caution should be taken with the use of carfilzomib for elderly and cardiovascularly compromised patients. The once-weekly administration of carfilzomib, recently approved by the FDA in combination with dexamethasone, will lead to a lower burden for the patient and caregivers compared to the twice-weekly schemes that were routinely used until recently. This review has a focus on clinical trial data that has led to drug approval, as well as new promising combination studies, and provides advice for treating physicians who are now prescribing this drug to patients.

TRAF6 Silencing Attenuates Multiple Myeloma Cell Adhesion to Bone Marrow Stromal Cells

The bone marrow (BM) microenvironment plays an important role in supporting proliferation, survival and drug resistance of Multiple Myeloma (MM) cells. MM cells adhere to bone marrow stromal cells leading to the activation of tumour-promoting signaling pathways. Activation of the NFκB pathway, in particular, is central to the pathogenesis of MM. Tumour necrosis factor receptor-associated factor 6 (TRAF6) is a key mediator of NFκB activation and has previously been highlighted as a potential therapeutic target in MM. Here, we demonstrate that adherence of MM cell lines to stromal cells results in a reciprocal increase in TRAF6 expression. Knockdown of TRAF6 expression attenuates the ability of MM cells to bind to stromal cells and this is associated with a decrease in NFκB-induced expression of the adhesion molecules ICAM1 and VCAM1. Finally, we show that knockdown of TRAF6 sensitizes MM cells to treatment with bortezomib when co-cultured with stromal cells. Inhibiting TRAF6 represents a promising strategy to target MM cells in the BM microenvironment.