Different mutants of PSMB5 confer varying bortezomib resistance in T lymphoblastic lymphoma/leukemia cells derived from the Jurkat cell line

Dihydrolipoamide dehydrogenase (DLD) is a novel molecular target of bortezomib

Proteasome inhibitors (PIs), such as bortezomib and calfizomib, were backbone agents in the treatment of multiple myeloma (MM). In this study, we investigated bortezomib interactors in MM cells and identified dihydrolipoamide dehydrogenase (DLD) as a molecular target of bortezomib. DLD catalyzes the oxidation of dihydrolipoamide to form lipoamide, a reaction that also generates NADH. Our data showed that bortezomib bound to DLD and inhibited DLD's enzymatic function in MM cells. DLD knocked down MM cells (DLD-KD) had decreased levels of NADH. Reduced NADH suppressed assembly of proteasome complex in cells. As a result, DLD-KD MM cells had decreased basal-level proteasome activity and were more sensitive to bortezomib. Since PIs were used in many anti-MM regimens in clinics, we found that high expression of DLD correlated with inferior prognosis of MM. Considering the regulatory role of DLD in proteasome assembly, we evaluated DLD targeting therapy in MM cells. DLD inhibitor CPI-613 showed a synergistic anti-MM effect with bortezomib in vitro and in vivo. Overall, our findings elucidated DLD as an alternative molecular target of bortezomib in MM. DLD-targeting might increase MM sensitivity to PIs.

Interplay between proteasome inhibitors and NF-κB pathway in leukemia and lymphoma: a comprehensive review on challenges ahead of proteasome inhibitors

The current scientific literature has extensively explored the potential role of proteasome inhibitors (PIs) in the NF-κB pathway of leukemia and lymphoma. The ubiquitin-proteasome system (UPS) is a critical component in regulating protein degradation in eukaryotic cells. PIs, such as BTZ, are used to target the 26S proteasome in hematologic malignancies, resulting in the prevention of the degradation of tumor suppressor proteins, the activation of intrinsic mitochondrial-dependent cell death, and the inhibition of the NF-κB signaling pathway. NF-κB is a transcription factor that plays a critical role in the regulation of apoptosis, cell proliferation, differentiation, inflammation, angiogenesis, and tumor migration. Despite the successful use of PIs in various hematologic malignancies, there are limitations such as resistant to these inhibitors. Some reports suggest that PIs can induce NF-κB activation, which increases the survival of malignant cells. This article discusses the various aspects of PIs' effects on the NF-κB pathway and their limitations. Video Abstract.

Decoding the secrets: how conformational and structural regulators inhibit the human 20S proteasome

Acquired resistance to drugs that modulate specific protein functions, such as the human proteasome, presents a significant challenge in targeted therapies. This underscores the importance of devising new methodologies to predict drug binding and potential resistance due to specific protein mutations. In this work, we conducted an extensive computational analysis to ascertain the effects of selected mutations (Ala49Thr, Ala50Val, and Cys52Phe) within the active site of the human proteasome. Specifically, we sought to understand how these mutations might disrupt protein function either by altering protein stability or by impeding interactions with a clinical administered drug. Leveraging molecular dynamics simulations and molecular docking calculations, we assessed the effect of these mutations on protein stability and ligand affinity. Notably, our results indicate that the Cys52Phe mutation critically impacts protein-ligand binding, providing valuable insights into potential proteasome inhibitor resistance.

High-Throughput/High Content Imaging Screen Identifies Novel Small Molecule Inhibitors and Immunoproteasomes as Therapeutic Targets for Chordoma

Chordomas account for approximately 1-4% of all malignant bone tumors and 20% of primary tumors of the spinal column. It is a rare disease, with an incidence estimated to be approximately 1 per 1,000,000 people. The underlying causative mechanism of chordoma is unknown, which makes it challenging to treat. Chordomas have been linked to the T-box transcription factor T (TBXT) gene located on chromosome 6. The TBXT gene encodes a protein transcription factor TBXT, or brachyury homolog. Currently, there is no approved targeted therapy for chordoma. Here, we performed a small molecule screening to identify small chemical molecules and therapeutic targets for treating chordoma. We screened 3730 unique compounds and selected 50 potential hits. The top three hits were Ribociclib, Ingenol-3-angelate, and Duvelisib. Among the top 10 hits, we found a novel class of small molecules, including proteasomal inhibitors, as promising molecules that reduce the proliferation of human chordoma cells. Furthermore, we discovered that proteasomal subunits PSMB5 and PSMB8 are increased in human chordoma cell lines U-CH1 and U-CH2, confirming that the proteasome may serve as a molecular target whose specific inhibition may lead to better therapeutic strategies for chordoma.

Inhibitors of the ATPase p97/VCP: From basic research to clinical applications

Protein homeostasis deficiencies underlie various cancers and neurodegenerative diseases. The ubiquitin-proteasome system (UPS) and autophagy are responsible for most of the protein degradation in mammalian cells and, therefore, represent attractive targets for cancer therapy and that of neurodegenerative diseases. The ATPase p97, also known as VCP, is a central component of the UPS that extracts and disassembles its substrates from various cellular locations and also regulates different steps in autophagy. Several UPS- and autophagy-targeting drugs are in clinical trials. In this review, we focus on the development of various p97 inhibitors, including the ATPase inhibitors CB-5083 and CB-5339, which reached clinical trials by demonstrating effective anti-tumor activity across various tumor models, providing an effective alternative to targeting protein degradation for cancer therapy. Here, we provide an overview of how different p97 inhibitors have evolved over time both as basic research tools and effective UPS-targeting cancer therapies in the clinic.

PSMB5 overexpression is correlated with tumor proliferation and poor prognosis in hepatocellular carcinoma

Aberrant expression of members of the proteasome subunit beta (PSMB) family (including PSMB2, PSMB4, PSMB7 and PSMB8) has been reported in hepatocellular carcinoma (HCC). However the role of PSMB5 in HCC is unclear. To address this issue, we examined the expression of PSMB5 in HCC tissues using the The Cancer Genome Atlas, International Cancer Genome Consortium and Gene Expression Omnibus databases. A quantitative real-time PCR and immunohistochemistry were performed to validate the expression of PSMB5 in HCC. The survival mutation status and immune cell infiltration of PSMB5 were also evaluated in HCC. We then examined the effect of knocking down PSMB5 expression through RNA interference in the HCC cell line Huh7. High expression of PSMB5 was observed in HCC tissues and was associated with poor prognosis. PSMB5 expression and clinical characteristics were then incorporated to build a prognostic nomogram. We observed that PSMB5 expression was closely related to the abundance of B cells, CD4⁺ T cells, CD8⁺ T cells, dendritic cell macrophages and neutrophils. Moreover silencing of PSMB5 in Huh7 significantly suppressed cell proliferation and migration at the same time as increasing apoptosis. Inhibition of the phosphatidylinositol-3-kinase/Akt/mechanistic target of rapamycin pathway was observed after PSMB5 downregulation in Huh7 cells. Our findings suggest that PSMB5 may promote the proliferation of HCC cells by inactivating the phosphatidylinositol-3-kinase/Akt/mechanistic target of rapamycin signaling pathway and thus PSMB5 may have potential as a biomarker for diagnosis and prognosis of HCC.

Revisiting Proteasome Inhibitors: Molecular Underpinnings of Their Development, Mechanisms of Resistance and Strategies to Overcome Anti-Cancer Drug Resistance

Proteasome inhibitors have shown relevant clinical activity in several hematological malignancies, namely in multiple myeloma and mantle cell lymphoma, improving patient outcomes such as survival and quality of life, when compared with other therapies. However, initial response to the therapy is a challenge as most patients show an innate resistance to proteasome inhibitors, and those that respond to the therapy usually develop late relapses suggesting the development of acquired resistance. The mechanisms of resistance to proteasome inhibition are still controversial and scarce in the literature. In this review, we discuss the development of proteasome inhibitors and the mechanisms of innate and acquired resistance to their activity—a major challenge in preclinical and clinical therapeutics. An improved understanding of these mechanisms is crucial to guiding the design of new and more effective drugs to tackle these devastating diseases. In addition, we provide a comprehensive overview of proteasome inhibitors used in combination with other chemotherapeutic agents, as this is a key strategy to combat resistance.

Syrbactin-class dual constitutive- and immuno-proteasome inhibitor TIR-199 impedes myeloma-mediated bone degeneration in vivo

Proteasome-addicted neoplastic malignancies present a considerable refractory and relapsed phenotype with patients exhibiting drug resistance and high mortality rates. To counter this global problem, novel proteasome-based therapies are being developed. In the current study, we extensively characterize TIR-199, a syrbactin-class proteasome inhibitor derived from a plant virulence factor of bacterium Pseudomonas syringae pv syringae. We report that TIR-199 is a potent constitutive and immunoproteasome inhibitor, capable of inducing cell death in multiple myeloma, triple-negative breast cancer, (TNBC) and non-small cell lung cancer lines. TIR-199 also effectively inhibits the proteasome in primary myeloma cells of patients, and bypasses the PSMB5 A49T+A50V bortezomib-resistant mutant. TIR-199 treatment leads to accumulation of canonical proteasome substrates in cells, it is specific, and does not inhibit 50 other enzymes tested in vitro. The drug exhibits synergistic cytotoxicity in combination with proteasome-activating kinase DYRK2 inhibitor LDN192960. Furthermore, low-doses of TIR-199 exhibits in vivo activity by delaying myeloma-mediated bone degeneration in a mouse xenograft model. Together, our data indicates that proteasome inhibitor TIR-199 could indeed be a promising next-generation drug within the repertoire of proteasome-based therapeutics.

The Role of Proteasome Inhibitors in Treating Acute Lymphoblastic Leukaemia

Acute lymphoblastic leukaemia (ALL) is an aggressive haematolymphoid malignancy. The prognosis of ALL is excellent in paediatric population, however the outcome of relapse/refractory disease is dismal. Adult ALL has less favourable prognosis and relapse/refractory disease is not uncommonly encountered. Bortezomib is the first generation proteasome inhibitor licensed to treat plasma cell myeloma and mantle cell lymphoma with favourable side effect profile. Efficacy of bortezomib had been proven in other solid tumors. Clinical studies showed promising response for proteasome inhibitors in treating relapse/refractory ALL. Thus, proteasome inhibitors are attractive alternative agents for research in treating ALL. In the review article, we will introduce different proteasome inhibitors and their difference in pharmacological properties. Moreover, the mechanism of action of proteasome inhibitors on ALL will be highlighted. Finally, results of various clinical studies on proteasome inhibitors in both paediatric and adult ALL will be discussed. This review article provides the insights on the use of proteasome inhibitors in treating ALL with a summary of mechanism of action in ALL which facilitates future research on its use to improve the outcome of ALL.

The Ubiquitin-Proteasome Pathway and Resistance Mechanisms Developed Against the Proteasomal Inhibitors in Cancer Cells

BACKGROUND

The ubiquitin-proteasome pathway is crucial for all cellular processes and is, therefore, a critical target for the investigation and development of novel strategies for cancer treatment. In addition, approximately 30% of newly synthesized proteins never attain their final conformations due to translational errors or defects in post-translational modifications; therefore, they are also rapidly eliminated by the ubiquitin-proteasome pathway.

OBJECTIVE

Here, an effort was made to outline the recent findings deciphering the new molecular mechanisms involved in the regulation of ubiquitin-proteasome pathway as well as the resistance mechanisms developed against proteasome inhibitors in cell culture experiments and in the clinical trials.

RESULTS

Since cancer cells have higher proliferation rates and are more prone to translational errors, they require the ubiquitin-proteasome pathway for selective advantage and sustained proliferation. Therefore, drugs targeting the ubiquitin-proteasome pathway are promising agents for the treatment of both hematological and solid cancers.

CONCLUSION

A number of proteasome inhibitors are approved and used for the treatment of advanced and relapsed multiple myeloma. Unfortunately, drug resistance mechanisms may develop very fast within days of the start of the proteasome inhibitor-treatment either due to the inherent or acquired resistance mechanisms under selective drug pressure. However, a comprehensive understanding of the mechanisms leading to the proteasome inhibitor-resistance will eventually help the design and development of novel strategies involving new drugs and/or drug combinations for the treatment of a number of cancers.

Proteasomal adaptations to FDA-approved proteasome inhibitors: a potential mechanism for drug resistance?

With proteasome inhibitors (PIs) becoming clinically available since 2003, outcomes for patients with multiple myeloma (MM) have dramatically changed, improving quality of life and survival. Despite the impressive treatment success, however, almost all MM patients who initially respond to these PIs eventually develop resistance. Furthermore, a portion of MM patients is inherently unresponsive to the PIs. Extensive mechanistic investigations identified several non-proteasomal signaling pathways suspected to be linked to the PI resistance, for which several excellent reviews are currently available. On the other hand, it is still unclear how cancer cells under high PI environments adapt to spare proteasome activity essential for survival and proliferation regardless of cancer evolution stages. This review outlines current progress towards understanding the proteasomal adaptations of cells in response to PI treatment to maintain necessary proteasome activity. A better understanding of cellular proteasomal changes in response to the PIs could provide a rationale to develop new therapeutics that could be used to overcome resistance to existing PI drugs.

Inhibition of the ʟ-glutamine transporter ASCT2 sensitizes plasma cell myeloma cells to proteasome inhibitors

Proteasome inhibitors (PIs), used in the treatment of plasma cell myeloma (PCM), interfere with the degradation of misfolded proteins leading to activation of unfolded protein response (UPR) and cell death. However, despite initial strong antimyeloma effects, PCM cells eventually develop acquired resistance to PIs. The pleiotropic role of ʟ-glutamine (Gln) in cellular functions makes inhibition of Gln metabolism a potentially good candidate for combination therapy. Here, we show that PCM cells, both sensitive and resistant to PIs, express membrane Gln transporter (ASCT2), require extracellular Gln for survival, and are sensitive to ASCT2 inhibitors (ASCT2i). ASCT2i synergistically potentiate the cytotoxic activity of PIs by inducing apoptosis and modulating autophagy. Combination of ASCT2 inhibitor V9302 and proteasome inhibitor carfilzomib upregulates the intracellular levels of ROS and oxidative stress markers and triggers catastrophic UPR as shown by upregulated spliced Xbp1 mRNA, ATF3 and CHOP levels. Moreover, analysis of RNA sequencing revealed that the PI in combination with ASCT2i reduced the levels of Gln metabolism regulators such as MYC and NRAS. Analysis of PCM patients' data revealed that upregulated ASCT2 and other Gln metabolism regulators are associated with advanced disease stage and with PIs resistance. Altogether, we identified a potent therapeutic approach that may prevent acquired resistance to PIs and may contribute to the improvement of treatment of patients suffering from PCM.

The Prognostic Value of Whole-Blood PSMB5, CXCR4, POMP, and RPL5 mRNA Expression in Patients with Multiple Myeloma Treated with Bortezomib

Proteasome inhibitors, like bortezomib, play a key role in the treatment of multiple myeloma (MM); however, most patients eventually relapse and eventually show multiple drug resistance, and the molecular mechanisms of this resistance remain unclear. The aim of our study is to assess the expression of previously described genes that may influence the resistance to bortezomib treatment at the mRNA level (ABCB1, CXCR4, MAF, MARCKS, POMP, PSMB5, RPL5, TXN, and XBP1) and prognosis of MM patients. mRNA expression was determined in 73 MM patients treated with bortezomib-based regimens (30 bortzomib-sensitive and 43 bortezomib-refractory patients) and 11 healthy controls. RPL5 was significantly down-regulated in multiple myeloma patients as compared with healthy controls. Moreover, POMP was significantly up-regulated in MM patients refractory to bortezomib-based treatment. In multivariate analysis, high expression of PSMB5 and CXCR and autologous stem cell transplantation were independent predictors of progression-free survival, and high expression of POMP and RPL5 was associated with shorter overall survival.

Updates to the drug-resistant mechanism of proteasome inhibitors in multiple myeloma

Proteasome inhibitors (PIs) have been a kind of backbone therapies for newly diagnosed as well as relapsed or refractory myeloma patients in the last two decades. Bortezomib, the first-in-class PI, was approved by the United States Food and Drug Administration in 2003. The key roles of this class of agents are targeting at the overstressed 26S proteasome, which are involved in the pathogenesis of the disease. Despite recent advancements in clinical antimyeloma treatment, the acquisition of resistance is a major limitation in PI therapy. This review aims at a better understanding of the pathways and biomarkers involved in MM drug resistance.

ZHX2 mediates proteasome inhibitor resistance via regulating nuclear translocation of NF-κB in multiple myeloma

Background

Multiple myeloma (MM) is an incurable hematological malignancy. Although proteasome inhibitors and immunomodulators have significantly improved patient outcomes, some patients respond poorly to treatment and almost all patients will relapse. Mechanisms of proteasome inhibitor resistance in multiple myeloma have not been fully elucidated. ZHX2 is a transcription regulator degraded via proteasome and presents both oncogenic or tumor suppressive effect in different cancers, however, it is still unknown that the role of ZHX2 in myeloma. In this study, we aim to demonstrate the effect and mechanism of ZHX2 on proteasome inhibitor resistance in MM.

Methods

GSE24080 gene expression profile datasets from Gene Expression Omnibus (GEO) were analyzed to evaluate the relationship between ZHX2 expression level and survival in MM. Expression of ZHX2 in human MM cell lines at baseline and after bortezomib (BTZ) treatment was determined by Western blotting (WB). The proliferation and apoptosis rate of MM cells treated with BTZ after the knockdown of ZHX2 were analyzed by flow cytometry. Nuclear translocation of NF‐κB after the knockdown of ZHX2 was evaluated by WB and immunofluorescence, and the expression of NF‐κB target genes was measured by real‐time quantitative PCR. Co‐immunoprecipitation (Co‐IP) and WB were used to detect the interaction of ZHX2 with NF‐κB.

Results

We found that higher ZHX2 expression was correlated with poorer clinical outcomes of patients. In addition, ZHX2 expression was relatively higher in RPMI‐8226 and MM.1S cell lines and the level of ZHX2 protein was upregulated after BTZ treatment. Knockdown of ZHX2 significantly enhanced the sensitivity of MM cells to BTZ, inhibited nuclear translocation of NF‐κB, and reduced mRNA expression of NF‐κB target genes. It was also revealed that ZHX2 directly binds to NF‐κB.

Conclusion

Our study showed that ZHX2 can promote proteasome inhibitor resistance in MM cells by regulating the nuclear translocation of NF‐κB.

The Immunoproteasome: An Emerging Target in Cancer and Autoimmune and Neurological Disorders

The immunoproteasome (iCP) is an isoform of the 20S proteasome that is expressed when cells are stressed or receive an inflammatory signal. The primary role of the iCP is to hydrolyze proteins into peptides that are compatible with being loaded into a MHC-I complex. When the activity of the iCP is dysregulated or highly expressed, it can lead to unwanted cell death. Some cancer types express the iCP rather than the standard proteasome, and selective inhibitors have been developed to exploit this difference. Here, we describe diseases known to be influenced by iCP activity and the current status for targeting the iCP to elicit a therapeutic response. We also describe a variety of chemical tools that have been developed to monitor the activity of the iCP in cells. Finally, we present the future outlook for targeting the iCP in a variety of disease types and with mechanisms besides inhibition.

Delanzomib, a novel proteasome inhibitor, sensitizes breast cancer cells to doxorubicin-induced apoptosis

Background

Delanzomib, a novel proteasome inhibitor, has demonstrated promising efficacy and antitumor ability in human multiple myeloma cell lines and patient‐derived cells. However, the potential therapeutic effects of delanzomib on breast cancer remain unknown. In this study, we show that delanzomib has antitumor effects and synergizes with doxorubicin (Dox) in human breast cancer cell lines.

Methods

Cell proliferation assay and flow cytometry were used to evaluate cell viability and apoptosis in eight human breast cancer cell lines after treatment with delanzomib or Dox. Essential molecules of the p53, MAPK, and apoptosis signaling pathways were analyzed by Western blotting.

Results

Delanzomib induced cell death and demonstrated synergism with Dox in all tested breast cancer cell lines. In addition, delanzomib enhanced the Dox‐induced phosphorylation of p38/JNK and the expression of transcriptional target proteins of p53, such as p21, p27, NOXA, and PUMA.

Conclusion

The combined regimen of the proteasome inhibitor delanzomib with Dox chemotherapy may become an effective strategy for breast cancer therapy.

Proteasome inhibitor b-AP15 induces enhanced proteotoxicity by inhibiting cytoprotective aggresome formation

Proteasome inhibitors have been shown to induce cell death in cancer cells by triggering an acute proteotoxic stress response characterized by accumulation of poly-ubiquitinated proteins, ER stress and the production of reactive oxygen species. The aggresome pathway has been described as an escape mechanism from proteotoxicity by sequestering toxic cellular aggregates. Here we show that b-AP15, a small-molecule inhibitor of proteasomal deubiquitinase activity, induces poly-ubiquitin accumulation in absence of aggresome formation. b-AP15 was found to affect organelle transport in treated cells, raising the possibility that microtubule-transport of toxic protein aggregates is inhibited, leading to enhanced cytotoxicity. In contrast to the antiproliferative effects of the clinically used proteasome inhibitor bortezomib, the effects of b-AP15 are not further enhanced by the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA). Our results suggest an inhibitory effect of b-AP15 on the transport of misfolded proteins, resulting in a lack of aggresome formation, and a strong proteotoxic stress response.

Clogging the Ubiquitin-Proteasome Machinery with Marine Natural Products: Last Decade Update

The ubiquitin-proteasome pathway (UPP) is the central protein degradation system in eukaryotic cells, playing a key role in homeostasis maintenance, through proteolysis of regulatory and misfolded (potentially harmful) proteins. As cancer cells produce proteins inducing cell proliferation and inhibiting cell death pathways, UPP inhibition has been exploited as an anticancer strategy to shift the balance between protein synthesis and degradation towards cell death. Over the last few years, marine invertebrates and microorganisms have shown to be an unexhaustive factory of secondary metabolites targeting the UPP. These chemically intriguing compounds can inspire clinical development of novel antitumor drugs to cope with the incessant outbreak of side effects and resistance mechanisms induced by currently approved proteasome inhibitors (e.g., bortezomib). In this review, we report about (a) the role of the UPP in anticancer therapy, (b) chemical and biological properties of UPP inhibitors from marine sources discovered in the last decade, (c) high-throughput screening techniques for mining natural UPP inhibitors in organic extracts. Moreover, we will tell about the fascinating story of salinosporamide A, the first marine natural product to access clinical trials as a proteasome inhibitor for cancer treatment.

(Immuno)proteasomes as therapeutic target in acute leukemia

The clinical efficacy of proteasome inhibitors in the treatment of multiple myeloma has encouraged application of proteasome inhibitor containing therapeutic interventions in (pediatric) acute leukemia. Here, we summarize the positioning of bortezomib, as first-generation proteasome inhibitor, and second-generation proteasome inhibitors in leukemia treatment from a preclinical and clinical perspective. Potential markers for proteasome inhibitor sensitivity and/or resistance emerging from leukemia cell line models and clinical sample studies will be discussed focusing on the role of immunoproteasome and constitutive proteasome (subunit) expression, PSMB5 mutations, and alternative mechanisms of overcoming proteolytic stress.

Proteasome-associated deubiquitinases and cancer

Maintenance of protein homeostasis is a crucial process for the normal functioning of the cell. The regulated degradation of proteins is primarily facilitated by the ubiquitin proteasome system (UPS), a system of selective tagging of proteins with ubiquitin followed by proteasome-mediated proteolysis. The UPS is highly dynamic consisting of both ubiquitination and deubiquitination steps that modulate protein stabilization and degradation. Deregulation of protein stability is a common feature in the development and progression of numerous cancer types. Simultaneously, the elevated protein synthesis rate of cancer cells and consequential accumulation of misfolded proteins drives UPS addiction, thus sensitizing them to UPS inhibitors. This sensitivity along with the potential of stabilizing pro-apoptotic signaling pathways makes the proteasome an attractive clinical target for the development of novel therapies. Targeting of the catalytic 20S subunit of the proteasome is already a clinically validated strategy in multiple myeloma and other cancers. Spurred on by this success, promising novel inhibitors of the UPS have entered development, targeting the 20S as well as regulatory 19S subunit and inhibitors of deubiquitinating and ubiquitin ligase enzymes. In this review, we outline the manner in which deregulation of the UPS can cause cancer to develop, current clinical application of proteasome inhibitors, and the (pre-)clinical development of novel inhibitors of the UPS.

Exocytosis of polyubiquitinated proteins in bortezomib-resistant leukemia cells: a role for MARCKS in acquired resistance to proteasome inhibitors

PSMB5 mutations and upregulation of the β5 subunit of the proteasome represent key determinants of acquired resistance to the proteasome inhibitor bortezomib (BTZ) in leukemic cells in vitro. We here undertook a multi-modality (DNA, mRNA, miRNA) array-based analysis of human CCRF-CEM leukemia cells and BTZ-resistant subclones to determine whether or not complementary mechanisms contribute to BTZ resistance. These studies revealed signatures of markedly reduced expression of proteolytic stress related genes in drug resistant cells over a broad range of BTZ concentrations along with a high upregulation of myristoylated alanine-rich C-kinase substrate (MARCKS) gene expression. MARCKS upregulation was confirmed on protein level and also observed in other BTZ-resistant tumor cell lines as well as in leukemia cells with acquired resistance to other proteasome inhibitors. Moreover, when MARCKS protein expression was demonstrated in specimens derived from therapy-refractory pediatric leukemia patients (n = 44), higher MARCKS protein expression trended (p = 0.073) towards a dismal response to BTZ-containing chemotherapy. Mechanistically, we show a BTZ concentration-dependent association of MARCKS protein levels with the emergence of ubiquitin-containing vesicles in BTZ-resistant CEM cells. These vesicles were found to be extruded and taken up in co-cultures with proteasome-proficient acceptor cells. Consistent with these observations, MARCKS protein associated with ubiquitin-containing vesicles was also more prominent in clinical leukemic specimen with ex vivo BTZ resistance compared to BTZ-sensitive leukemia cells. Collectively, we propose a role for MARCKS in a novel mechanism of BTZ resistance via exocytosis of ubiquitinated proteins in BTZ-resistant cells leading to quenching of proteolytic stress.

Unbiased compound-protein interface mapping and prediction of chemoresistance loci through forward genetics in haploid stem cells

Forward genetic screens in haploid mammalian cells have recently emerged as powerful tools for the discovery and investigation of recessive traits. Use of the haploid system provides unique genetic tractability and resolution. Upon positive selection, these screens typically employ analysis of loss-of-function (LOF) alleles and are thus limited to non-essential genes. Many relevant compounds, including anti-cancer therapeutics, however, target essential genes, precluding positive selection of LOF alleles. Here, we asked whether the use of random and saturating chemical mutagenesis might enable screens that identify essential biological targets of toxic compounds. We compare and contrast chemical mutagenesis with insertional mutagenesis.

Selecting mutagenized cells with thapsigargin, an inhibitor of the essential Ca²⁺ pump SERCA2, insertional mutagenesis retrieved cell clones overexpressing SERCA2. With chemical mutagenesis, we identify six single amino acid substitutions in the known SERCA2-thapsigargin binding interface that confer drug resistance. In a second screen, we used the anti-cancer drug MG132/bortezomib (Velcade), which inhibits proteasome activity. Using chemical mutagenesis, we found 7 point mutations in the essential subunit Psmb5 that map to the bortezomib binding surface. Importantly, 4 of these had previously been identified in human tumors with acquired bortezomib resistance. Insertional mutagenesis did not identify Psmb5 in this screen, demonstrating the unique ability of chemical mutagenesis to identify relevant point mutations in essential genes.

Thus, chemical mutagenesis in haploid embryonic stem cells can define the interaction of toxic small molecules with essential proteins at amino acid resolution, fully mapping small molecule-protein binding interfaces.

PSMB5 is associated with proliferation and drug resistance in triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, characterized by advanced disease stage and poor prognosis. Moreover, due to the lack of therapeutic markers, TNBC patients can't benefit fully from currently available targeted therapies.

METHODS

To fully understand the molecular basis of TNBC, we used gene set enrichment analysis (GSEA) to screen out the most altered functional module in TNBC, from publicly available microarray data and studied the association of the candidate gene with TNBC development.

RESULTS

We found that the proteasome was significantly activated in TNBC. As compared with other breast cancer subtypes and normal tissue, proteasome subunit beta 5 (PSMB5), the key regulator of proteasome function, was overexpressed in TNBC tissue and predictive of poor prognosis. Moreover, we also found that PSMB5 knockdown induced TNBC apoptosis and significantly enhanced cancer cell sensitivity to the chemotherapeutic agents bortezomib and paclitaxel.

CONCLUSIONS

Our results suggest a potential role for PSMB5 as a biomarker and therapeutic target for TNBC.

Proteasome inhibitors in cancer therapy

The ubiquitin proteasome pathway was discovered in the 1980s to be a central component of the cellular protein-degradation machinery with essential functions in homeostasis, which include preventing the accumulation of misfolded or deleterious proteins. Cancer cells produce proteins that promote both cell survival and proliferation, and/or inhibit mechanisms of cell death. This notion set the stage for preclinical testing of proteasome inhibitors as a means to shift this fine equilibrium towards cell death. Since the late 1990s, clinical trials have been conducted for a variety of malignancies, leading to regulatory approvals of proteasome inhibitors to treat multiple myeloma and mantle-cell lymphoma. First-generation and second-generation proteasome inhibitors can elicit deep initial responses in patients with myeloma, for whom these drugs have dramatically improved outcomes, but relapses are frequent and acquired resistance to treatment eventually emerges. In addition, promising preclinical data obtained with proteasome inhibitors in models of solid tumours have not been confirmed in the clinic, indicating the importance of primary resistance. Investigation of the mechanisms of resistance is, therefore, essential to further maximize the utility of this class of drugs in the era of personalized medicine. Herein, we discuss the advances and challenges resulting from the introduction of proteasome inhibitors into the clinic.

Glutaminase inhibitor CB-839 synergizes with carfilzomib in resistant multiple myeloma cells

Curative responses in the treatment of multiple myeloma (MM) are limited by the emergence of therapeutic resistance. To address this problem, we set out to identify druggable mechanisms that convey resistance to proteasome inhibitors (PIs; e.g., bortezomib), which are cornerstone agents in the treatment of MM. In isogenic pairs of PI sensitive and resistant cells, we observed stark differences in cellular bioenergetics between the divergent phenotypes. PI resistant cells exhibited increased mitochondrial respiration driven by glutamine as the principle fuel source. To target glutamine-induced respiration in PI resistant cells, we utilized the glutaminase-1 inhibitor, CB-839. CB-839 inhibited mitochondrial respiration and was more cytotoxic in PI resistant cells as a single agent. Furthermore, we found that CB-839 synergistically enhanced the activity of multiple PIs with the most dramatic synergy being observed with carfilzomib (Crflz), which was confirmed in a panel of genetically diverse PI sensitive and resistant MM cells. Mechanistically, CB-839 enhanced Crflz-induced ER stress and apoptosis, characterized by a robust induction of ATF4 and CHOP and the activation of caspases. Our findings suggest that the acquisition of PI resistance involves adaptations in cellular bioenergetics, supporting the combination of CB-839 with Crflz for the treatment of refractory MM.

Directed evolution using dCas9-targeted somatic hypermutation in mammalian cells

Engineering and study of protein function by directed evolution has been limited by the technical requirement to use global mutagenesis or introduce DNA libraries. Here, we develop CRISPR-X, a strategy to repurpose the somatic hypermutation machinery for protein engineering in situ. Using catalytically inactive dCas9 to recruit variants of cytidine deaminase (AID) with MS2-modified sgRNAs, we can specifically mutagenize endogenous targets with limited off-target damage. This generates diverse libraries of localized point mutations and can target multiple genomic locations simultaneously. We mutagenize GFP and select for spectrum-shifted variants, including EGFP. Additionally, we mutate the target of the cancer therapeutic bortezomib, PSMB5, and identify known and novel mutations that confer bortezomib resistance. Finally, using a hyperactive AID variant, we mutagenize loci both upstream and downstream of transcriptional start sites. These experiments illustrate a powerful approach to create complex libraries of genetic variants in native context, which is broadly applicable to investigate and improve protein function.

CNS uptake of bortezomib is enhanced by P-glycoprotein inhibition: implications for spinal muscular atrophy

The development of therapeutics for neurological disorders is constrained by limited access to the central nervous system (CNS). ATP-binding cassette (ABC) transporters, particularly P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), are expressed on the luminal surface of capillaries in the CNS and transport drugs out of the endothelium back into the blood against the concentration gradient. Survival motor neuron (SMN) protein, which is deficient in spinal muscular atrophy (SMA), is a target of the ubiquitin proteasome system. Inhibiting the proteasome in a rodent model of SMA with bortezomib increases SMN protein levels in peripheral tissues but not the CNS, because bortezomib has poor CNS penetrance. We sought to determine if we could inhibit SMN degradation in the CNS of SMA mice with a combination of bortezomib and the ABC transporter inhibitor tariquidar. In cultured cells we show that bortezomib is a substrate of P-gp. Mass spectrometry analysis demonstrated that intraperitoneal co-administration of tariquidar increased the CNS penetrance of bortezomib, and reduced proteasome activity in the brain and spinal cord. This correlated with increased SMN protein levels and improved survival and motor function of SMA mice. These findings show that CNS penetrance of treatment for this neurological disorder can be improved by inhibiting drug efflux at the blood-brain barrier.

The small heat shock protein B8 (HSPB8) confers resistance to bortezomib by promoting autophagic removal of misfolded proteins in multiple myeloma cells

Velcade is one of the inescapable drug to treat patient suffering from multiple myeloma (MM) and resistance to this drug represents a major drawback for patients. However, the mechanisms underlying velcade resistance remain incompletely understood. We derived several U266 MM cell clones that resist to velcade. U266-resistant cells were resistant to velcade-induced cell death but exhibited a similar sensitivity to various proapoptotic stimuli. Careful analysis of proteosomal subunits and proteasome enzymatic activities showed that neither the composition nor the activity of the proteasome was affected in velcade-resistant cells. Elimination of velcade-induced poly-ubiquitinated proteins and protein aggregates was drastically stimulated in the resistant cells and correlated with increased cell survival. Inhibition of the lysosomal activity in velcade-resistant cells resulted in an increase of cell aggregates and decrease survival, indicating that aggregates are eliminated through lysosomal degradation. In addition, pangenomic profiling of velcade-sensitive and resistant cells showed that the small heat shock protein HSPB8 was overexpressed in resistant cells. Finally, gain and loss of function experiment demonstrated that HSPB8 is a key factor for velcade resistance. In conclusion, HSPB8 plays an important role for the elimination of aggregates in velcade-resistant cells that contributes to their enhanced survival.

A reversible and highly selective inhibitor of the proteasomal ubiquitin receptor rpn13 is toxic to multiple myeloma cells

The proteasome is a multisubunit complex responsible for most nonlysosomal turnover of proteins in eukaryotic cells. Proteasome inhibitors are of great interest clinically, particularly for the treatment of multiple myeloma (MM). Unfortunately, resistance arises almost inevitably to these active site-targeted drugs. One strategy to overcome this resistance is to inhibit other steps in the protein turnover cascade mediated by the proteasome. Previously, Anchoori et al. identified Rpn13 as the target of an electrophilic compound (RA-190) that was selectively toxic to MM cells (Cancer Cell 2013, 24, 791-805), suggesting that this subunit of the proteasome is also a viable cancer drug target. Here we describe the discovery of the first highly selective, reversible Rpn13 ligands and show that they are also selectively toxic to MM cells. These data strongly support the hypothesis that Rpn13 is a viable target for the development of drugs to treat MM and other cancers.

Trial Watch: Proteasomal inhibitors for anticancer therapy

The so-called "ubiquitin-proteasome system" (UPS) is a multicomponent molecular apparatus that catalyzes the covalent attachment of several copies of the small protein ubiquitin to other proteins that are generally (but not always) destined to proteasomal degradation. This enzymatic cascade is crucial for the maintenance of intracellular protein homeostasis (both in physiological conditions and in the course of adaptive stress responses), and regulates a wide array of signaling pathways. In line with this notion, defects in the UPS have been associated with aging as well as with several pathological conditions including cardiac, neurodegenerative, and neoplastic disorders. As transformed cells often experience a constant state of stress (as a result of the hyperactivation of oncogenic signaling pathways and/or adverse microenvironmental conditions), their survival and proliferation are highly dependent on the integrity of the UPS. This rationale has driven an intense wave of preclinical and clinical investigation culminating in 2003 with the approval of the proteasomal inhibitor bortezomib by the US Food and Drug Administration for use in multiple myeloma patients. Another proteasomal inhibitor, carfilzomib, is now licensed by international regulatory agencies for use in multiple myeloma patients, and the approved indications for bortezomib have been extended to mantle cell lymphoma. This said, the clinical activity of bortezomib and carfilzomib is often limited by off-target effects, innate/acquired resistance, and the absence of validated predictive biomarkers. Moreover, the antineoplastic activity of proteasome inhibitors against solid tumors is poor. In this Trial Watch we discuss the contribution of the UPS to oncogenesis and tumor progression and summarize the design and/or results of recent clinical studies evaluating the therapeutic profile of proteasome inhibitors in cancer patients.

Emerging Therapeutic Strategies for Overcoming Proteasome Inhibitor Resistance

The debut of the proteasome inhibitor bortezomib (Btz; Velcade®) radically and immediately improved the treatment of multiple myeloma (MM), an incurable malignancy of the plasma cell. Therapeutic resistance is unavoidable, however, and represents a major obstacle to maximizing the clinical potential of the drug. To address this challenge, studies have been conducted to uncover the molecular mechanisms driving Btz resistance and to discover new targeted therapeutic strategies and combinations that restore Btz activity. This review discusses the literature describing molecular adaptations that confer Btz resistance with a primary disease focus on MM. Also discussed are the most recent advances in therapeutic strategies that overcome resistance, approaches that include redox-modulating agents, murine double minute 2 inhibitors, therapeutic monoclonal antibodies, and new epigenetic-targeted drugs like bromodomain and extra terminal domain inhibitors.

Regulation of PSMB5 protein and β subunits of mammalian proteasome by constitutively activated signal transducer and activator of transcription 3 (STAT3): potential role in bortezomib-mediated anticancer therapy

The ubiquitin-proteasome system facilitates the degradation of ubiquitin-tagged proteins and performs a regulatory role in cells. Elevated proteasome activity and subunit expression are found in several cancers. However, the inherent molecular mechanisms responsible for increased proteasome function in cancers remain unclear despite the well investigated and defined role of the mammalian proteasome. This study was initiated to elucidate the mechanisms involved in the regulation of β subunits of the mammalian proteasome. Suppression of STAT3 tyrosine phosphorylation coordinately decreased the mRNA and protein levels of the β subunits of the 20 S core complex in DU145 cells. Notably, PSMB5, a molecular target of bortezomib, was shown to be a target of STAT3. Knockdown of STAT3 decreased PSMB5 protein. Inhibition of phospho-STAT3 substantially reduced PSMB5 protein levels in cells expressing constitutively active-STAT3. Accumulation of activated STAT3 resulted in the induction of PSMB5 promoter and protein levels. In addition, a direct correlation was observed between the endogenous levels of PSMB5 and constitutively active STAT3. PSMB5 and STAT3 protein levels remained unaltered following the inhibition of proteasome activity. The EGF-induced concerted increase of β subunits was blocked by inhibition of the EGF receptor or STAT3 but not by the PI3K/AKT or MEK/ERK pathways. Decreased proteasome activities were due to reduced protein levels of catalytic subunits of the proteasome in STAT3-inhibited cells. Combined treatments with bortezomib and inhibitor of STAT3 abrogated proteasome activity and enhanced cellular apoptosis. Overall, we demonstrate that aberrant activation of STAT3 regulates the expression of β subunits, in particular PSMB5, and the catalytic activity of the proteasome.

BAK and NOXA are critical determinants of mitochondrial apoptosis induced by bortezomib in mesothelioma

Based on promising preclinical efficacy associated with the 20S proteasome inhibitor bortezomib in malignant pleural mesothelioma (MPM), two phase II clinical trials have been initiated (EORTC 08052 and ICORG 05-10). However, the potential mechanisms underlying resistance to this targeted drug in MPM are still unknown. Functional genetic analyses were conducted to determine the key mitochondrial apoptotic regulators required for bortezomib sensitivity and to establish how their dysregulation may confer resistance. The multidomain proapoptotic protein BAK, but not its orthologue BAX, was found to be essential for bortezomib-induced apoptosis in MPM cell lines. Immunohistochemistry was performed on tissues from the ICORG-05 phase II trial and a TMA of archived mesotheliomas. Loss of BAK was found in 39% of specimens and loss of both BAX/BAK in 37% of samples. However, MPM tissues from patients who failed to respond to bortezomib and MPM cell lines selected for resistance to bortezomib conserved BAK expression. In contrast, c-Myc dependent transactivation of NOXA was abrogated in the resistant cell lines. In summary, the block of mitochondrial apoptosis is a limiting factor for achieving efficacy of bortezomib in MPM, and the observed loss of BAK expression or NOXA transactivation may be relevant mechanisms of resistance in the clinic.

Perspectives in the treatment of multiple myeloma

INTRODUCTION

The development of proteasome inhibitor (PI) and immunomodulatory drugs (IMiDs) and advances in supportive care have considerably changed the treatment paradigm of multiple myeloma (MM) and significantly improved survival. Nevertheless, almost all patients show disease relapse and develop drug resistance.

AREAS COVERED

We review the prognostic stratification and therapeutic strategy for newly diagnosed MM patients. Furthermore, mechanisms of drug resistance are discussed. Data regarding newer drugs, currently undergoing examination, such as PI (carfilzomib, ONX0912, MLN9708, and marizomib), IMiDs (pomalidomide), histone deacetylase inhibitors (vorinostat and panobinostat), kinase inhibitors (temsirolimus, everolimus, and tanespimycin), and immune-based therapies (elotuzumab, siltuximab, MOR03087, and MMBT062) are reported.

EXPERT OPINION

The use of three to four drug combination therapies including PI and IMiDs has significantly impacted on MM patient outcome. Moreover, new insights into MM biology from high-throughput technologies and availability of newer and more efficacious drugs will continue to influence our approach to MM treatment. In the immediate future molecular subgroup-specific trials using targeted agents may represent a very important step toward evaluating impact of interfering with relevant signaling pathways in MM. With the continued rapid evolution of progress in this field, MM will become a chronic illness having sustained complete response in a significant number of patients.

Homopiperazine derivatives as a novel class of proteasome inhibitors with a unique mode of proteasome binding

The proteasome is a proteolytic machinery that executes the degradation of polyubiquitinated proteins to maintain cellular homeostasis. Proteasome inhibition is a unique and effective way to kill cancer cells because they are sensitive to proteotoxic stress. Indeed, the proteasome inhibitor bortezomib is now indispensable for the treatment of multiple myeloma and other intractable malignancies, but is associated with patient inconvenience due to intravenous injection and emerging drug resistance. To resolve these problems, we attempted to develop orally bioavailable proteasome inhibitors with distinct mechanisms of action and identified homopiperazine derivatives (HPDs) as promising candidates. Biochemical and crystallographic studies revealed that some HPDs inhibit all three catalytic subunits (ß 1, ß 2 and ß 5) of the proteasome by direct binding, whereas bortezomib and other proteasome inhibitors mainly act on the ß5 subunit. Proteasome-inhibitory HPDs exhibited cytotoxic effects on cell lines from various hematological malignancies including myeloma. Furthermore, K-7174, one of the HPDs, was able to inhibit the growth of bortezomib-resistant myeloma cells carrying a ß5-subunit mutation. Finally, K-7174 had additive effects with bortezomib on proteasome inhibition and apoptosis induction in myeloma cells. Taken together, HPDs could be a new class of proteasome inhibitors, which compensate for the weak points of conventional ones and overcome the resistance to bortezomib.

The resistance mechanisms of proteasome inhibitor bortezomib

The proteasome inhibitor, bortezomib, a boronic dipeptide which reversibly inhibit the chymotrypsin-like activity at the β5-subunit of proteasome (PSMB5), has marked efficacy against multiple myeloma and several non-Hodgkin's lymphoma subtypes, and has a potential therapeutic role against other malignancy diseases. However, intrinsic and acquired resistance to bortezomib may limit its efficacy. In this article, we discuss recent advances in the molecular understanding of bortezomib resistance. Resistance mechanisms discussed include mutations of PSMB5 and the up-regulation of proteasome subunits, alterations of gene and protein expression in stress response, cell survival and antiapoptotic pathways, and multidrug resistance.

Bortezomib and TRAIL: a perfect match for apoptotic elimination of tumour cells?

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine that selectively eradicates tumour cells via specific cell surface receptors and is intensively explored for use as a novel anticancer approach. To enhance the efficacy of TRAIL receptor agonists the proteasome inhibitor bortezomib is one of the most potent sensitizers. Here we review the main mechanisms underlying bortezomib-dependent TRAIL sensitization, including stimulation of apoptosis by increasing expression of TRAIL receptors, reduction of cFLIP and enhancement of caspase 8 activation, and modulation of Bcl-2 family proteins and inhibitor of apoptosis proteins (IAPs). Concomitantly, pro-survival signals are suppressed such as elicited by NF-κB and Akt. The different preclinical tumour models explored with this combination, including primary tumour (stem) cells, stroma co-culture and mice models, are discussed, as well as possible hurdles for clinical activity. Collectively, anticipating a solid rationale for bortezomib-TRAIL combination and very promising preclinical results, its clinical activity remains to be demonstrated.

Overcoming bortezomib resistance in human B cells by anti-CD20/rituximab-mediated complement-dependent cytotoxicity and epoxyketone-based irreversible proteasome inhibitors

Background

In clinical and experimental settings, antibody-based anti-CD20/rituximab and small molecule proteasome inhibitor (PI) bortezomib (BTZ) treatment proved effective modalities for B cell depletion in lymphoproliferative disorders as well as autoimmune diseases. However, the chronic nature of these diseases requires either prolonged or re-treatment, often with acquired resistance as a consequence.

Methods

Here we studied the molecular basis of acquired resistance to BTZ in JY human B lymphoblastic cells following prolonged exposure to this drug and examined possibilities to overcome resistance by next generation PIs and anti-CD20/rituximab-mediated complement-dependent cytotoxicity (CDC).

Results

Characterization of BTZ-resistant JY/BTZ cells compared to parental JY/WT cells revealed the following features: (a) 10–12 fold resistance to BTZ associated with the acquisition of a mutation in the PSMB5 gene (encoding the constitutive β5 proteasome subunit) introducing an amino acid substitution (Met45Ile) in the BTZ-binding pocket, (b) a significant 2–4 fold increase in the mRNA and protein levels of the constitutive β5 proteasome subunit along with unaltered immunoproteasome expression, (c) full sensitivity to the irreversible epoxyketone-based PIs carfilzomib and (to a lesser extent) the immunoproteasome inhibitor ONX 0914. Finally, in association with impaired ubiquitination and attenuated breakdown of CD20, JY/BTZ cells harbored a net 3-fold increase in CD20 cell surface expression, which was functionally implicated in conferring a significantly increased anti-CD20/rituximab-mediated CDC.

Conclusions

These results demonstrate that acquired resistance to BTZ in B cells can be overcome by next generation PIs and by anti-CD20/rituximab-induced CDC, thereby paving the way for salvage therapy in BTZ-resistant disease.

Relapsed/Refractory multiple myeloma: defining refractory disease and identifying strategies to overcome resistance

Despite the development of more effective therapies for multiple myeloma (MM) over the past decade, nearly all patients will eventually experience disease relapse and require further therapy. Designing the next generation of therapies for relapsed and refractory disease will depend on understanding the complex molecular pathogenesis of MM and mechanisms of resistance. Oncogenomic studies have identified many potential therapeutic targets and have led to emerging models of the multistep molecular pathogenesis of MM. The key to overcoming resistance may depend on interrupting the complex interactions between MM cells and the bone microenvironment. Direct interaction between MM cells and bone marrow cells activates pleiotropic signaling pathways that mediate growth, survival, and migration of MM cells as well as resistance to chemotherapy (known as cell adhesion-mediated drug resistance). The bone marrow also secretes growth factors and cytokines that maintain MM cells and inhibit apoptosis. Therefore, successful therapeutic strategies must target not only the MM plasma cell but also the bone microenvironment. The benefit of immunomodulatory drugs such as thalidomide and lenalidomide and the proteasome inhibitor bortezomib in relapsed/refractory MM is related to their ability to target both. Novel agents and combination strategies are building on the success of these agents and targeting synergistic pathways.

Molecular mechanisms of acquired proteasome inhibitor resistance

The development of proteasome inhibitors (PIs) has transformed the treatment of multiple myeloma and mantle cell lymphoma. To date, two PIs have been FDA approved, the boronate peptide bortezomib and, most recently, the epoxyketone peptide carfilzomib. However, intrinsic and acquired resistance to PIs, for which the underlying mechanisms are poorly understood, may limit their efficacy. In this Perspective, we discuss recent advances in the molecular understanding of PI resistance through acquired bortezomib resistance in human cell lines and evolved salinosporamide A (marizomib) resistance in bacteria. Resistance mechanisms discussed include the up-regulation of proteasome subunits and mutations of the catalytic β-subunits. Additionally, we explore potential strategies to overcome PI resistance.

Carfilzomib: a novel treatment in relapsed and refractory multiple myeloma

Carfilzomib is a second-generation proteasome inhibitor with well-documented clinical activity as a single agent in patients with relapsed/refractory multiple myeloma. Carfilzomib can partially overcome resistance in bortezomib-refractory patients and has significant efficacy in bortezomib-naïve patients. Responses generally occur rapidly and are durable in the majority of cases. Carfilzomib can be safely administered in patients with renal failure and adverse cytogenetics do not seem to interfere with its activity. Moreover, carfilzomib has the advantage of a favorable safety profile, especially a low incidence of peripheral neuropathy, which is often the dose-limiting factor in thalidomide and bortezomib-based regimens. The most frequently observed high-grade adverse event is cytopenia. However, long-term tolerability is good with no cumulative toxicity. The place of carfilzomib in the treatment of the advanced and the newly diagnosed myeloma patient is currently under examination in several ongoing phase 3 clinical trials.

Sequence analysis of β-subunit genes of the 20S proteasome in patients with relapsed multiple myeloma treated with bortezomib or dexamethasone

Variations within proteasome β (PSMB) genes, which encode the β subunits of the 20S proteasome, may affect proteasome function, assembly, and/or binding of proteasome inhibitors. To investigate the potential association between PSMB gene variants and treatment-emergent resistance to bortezomib and/or long-term outcomes, in the present study, PSMB gene sequence variation was characterized in tumor DNA samples from patients who participated in the phase 3 Assessment of Proteasome Inhibition for Extending Remissions (APEX) study of bortezomib versus high-dose dexamethasone for treatment of relapsed multiple myeloma. Twelve new PSMB variants were identified. No associations were found between PSMB single nucleotide polymorphism genotype frequency and clinical response to bortezomib or dexamethasone treatment or between PSMB single nucleotide polymorphism allelic frequency and pooled overall survival or time to progression. Although specific PSMB5 variants have been identified previously in preclinical models of bortezomib resistance, these variants were not detected in patient tumor samples collected after clinical relapse from bortezomib, which suggests that alternative mechanisms underlie bortezomib insensitivity.

Targeting the insulin-like growth factor-1 receptor to overcome bortezomib resistance in preclinical models of multiple myeloma

Proteasome inhibition with bortezomib is a validated approach to the treatment of multiple myeloma, but drug resistance often emerges and limits its utility in the retreatment setting. To begin to identify some of the mechanisms involved, we developed bortezomib-resistant myeloma cell lines that, unlike previously reported models, showed no β5 subunit mutations. Instead, up-regulation of the insulin-like growth factor (IGF)-1 axis was identified, with increased autocrine and paracrine secretion of IGF-1, leading to increased activation of the IGF-1 receptor (IGF-1R). Exogenous IGF-1 reduced cellular sensitivity to bortezomib, whereas pharmacologic or small hairpin RNA-mediated IGF-1R suppression enhanced bortezomib sensitivity in cell lines and patient samples. In vitro studies with OSI-906, a clinically relevant dual IGF-1R and insulin receptor inhibitor, showed it acted synergistically with bortezomib, and potently resensitized bortezomib-resistant cell lines and patient samples to bortezomib. Importantly, OSI-906 in combination with bortezomib also overcame bortezomib resistance in an in vivo model of myeloma. Taken together, these data support the hypothesis that signaling through the IGF-1/IGF-1R axis contributes to acquired bortezomib resistance, and provide a rationale for combining bortezomib with IGF-1R inhibitors like OSI-906 to overcome or possibly prevent the emergence of bortezomib-refractory disease in the clinic.

Proteasome deubiquitinases as novel targets for cancer therapy

The ubiquitin-proteasome system (UPS) is a conserved pathway regulating numerous biological processes including protein turnover, DNA repair, and intracellular trafficking. Tumor cells are dependent on a functioning UPS, making it an ideal target for the development of novel anti-cancer therapies. The development of bortezomib (Velcade(®)) as a treatment for multiple myeloma and mantle cell lymphoma has verified this and suggests that targeting other components of the UPS may be a viable strategy for the treatment for cancer. We recently described a novel class of proteasome inhibitors that function by an alternative mechanism of action (D'Arcy et al., 2011). The small molecule b-AP15 blocks the deubiquitinase (DUB) activity of the 19S regulatory particle (19S RP) without inhibiting the proteolytic activities of the 20S core particle (20S CP). b-AP15 inhibits two proteasome-associated DUBs, USP14 and UCHL5, resulting in a rapid accumulation of high molecular weight ubiquitin conjugates and a functional proteasome shutdown. Interestingly, b-AP15 displays several differences to bortezomib including insensitivity to over-expression of the anti-apoptotic mediator Bcl-2 and anti-tumor activity in solid tumor models. In this review we will discuss the potential of proteasome deubiquitinase inhibitors as additions to the therapeutic arsenal against cancer.

Inhibitors for the immuno- and constitutive proteasome: current and future trends in drug development

Proteolytic degradation is an essential cellular process which is primarily carried out by the 20S proteasome core particle (CP), a protease of 720 kDa and 28 individual subunits. As a result of its central functional role, the proteasome represents an attractive drug target that has been extensively investigated during the last decade and validated by the approval of bortezomib by the US Food and Drug Administration (FDA). Currently, several optimized second-generation proteasome inhibitors are being explored as anticancer drugs in clinical trials, and most of them target both constitutive proteasomes (cCPs) and immunoproteasomes (iCPs). However, selective inhibition of the iCPs, a distinct class of proteasomes predominantly expressed in immune cells, appears to be a promising therapeutic rationale for the treatment of autoimmune disorders. Although a few selective agents have already been identified, the recently determined crystal structure of the iCP will further promote the development and optimization of iCP-selective compounds.