Targeting the UPS as therapy in multiple myeloma

Research strategies of small molecules as chemotherapeutics to overcome multiple myeloma resistance

Multiple myeloma (MM), a cancer of plasma cells, is the second most common hematological malignancy which is characterized by aberrant plasma cells infiltration in the bone marrow and complex heterogeneous cytogenetic abnormalities. Over the past two decades, novel treatment strategies such as proteasome inhibitors, immunomodulators, and monoclonal antibodies have significantly improved the relative survival rate of MM patients. However, the development of drug resistance results in the majority of MM patients suffering from relapse, limited treatment options and uncontrolled disease progression after relapse. There are urgent needs to develop and explore novel MM treatment strategies to overcome drug resistance and improve efficacy. Here, we review the recent small molecule therapeutic strategies for MM, and introduce potential new targets and corresponding modulators in detail. In addition, this paper also summarizes the progress of multi-target inhibitor therapy and protein degradation technology in the treatment of MM.

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.

Kinase Inhibition in Multiple Myeloma: Current Scenario and Clinical Perspectives

Multiple myeloma (MM) is a blood cell neoplasm characterized by excessive production of malignant monoclonal plasma cells (activated B lymphocytes) by the bone marrow, which end up synthesizing antibodies or antibody fragments, called M proteins, in excess. The accumulation of this production, both cells themselves and of the immunoglobulins, causes a series of problems for the patient, of a systemic and local nature, such as blood hyperviscosity, renal failure, anemia, bone lesions, and infections due to compromised immunity. MM is the third most common hematological neoplasm, constituting 1% of all cancer cases, and is a disease that is difficult to treat, still being considered an incurable disease. The treatments currently available cannot cure the patient, but only extend their lifespan, and the main and most effective alternative is autologous hematopoietic stem cell transplantation, but not every patient is eligible, often due to age and pre-existing comorbidities. In this context, the search for new therapies that can bring better results to patients is of utmost importance. Protein tyrosine kinases (PTKs) are involved in several biological processes, such as cell growth regulation and proliferation, thus, mutations that affect their functionality can have a great impact on crucial molecular pathways in the cells, leading to tumorigenesis. In the past couple of decades, the use of small-molecule inhibitors, which include tyrosine kinase inhibitors (TKIs), has been a hallmark in the treatment of hematological malignancies, and MM patients may also benefit from TKI-based treatment strategies. In this review, we seek to understand the applicability of TKIs used in MM clinical trials in the last 10 years.

PSMB7 Is a Key Gene Involved in the Development of Multiple Myeloma and Resistance to Bortezomib

Multiple myeloma (MM), the second most commonly diagnosed hematologic neoplasm, is the most significant clinical manifestation in a series of plasma cell (PC) dyscrasia. Monoclonal gammopathy of undetermined significance (MGUS) and smoldering MM (SMM), approximately 1% or 10% of which, respectively, can progress to MM per year, are the premalignant stages of MM. The overall survival (OS) of MM is significantly improved by the introduction of proteasome inhibitors (PIs), but almost all MM patients eventually relapse and resist anti-MM drugs. Therefore, it is crucial to explore the progression of MM and the mechanisms related to MM drug resistance. In this study, we used weighted gene co-expression network analysis (WGCNA) to analyze the gene expression of the dynamic process from normal plasma cells (NPC) to malignant profiling PC, and found that the abnormal gene expression was mainly concentrated in the proteasome. We also found that the expression of one of the proteasomal subunits PSMB7 was capable of distinguishing the different stages of PC dyscrasia and was the highest in ISS III. In the bortezomib (BTZ) treated NDMM patients, higher PSMB7 expression was associated with shorter survival time, and the expression of PSMB7 in the BTZ treatment group was significantly higher than in the thalidomide (Thai) treatment group. In summary, we found that PSMB7 is the key gene associated with MM disease progression and drug resistance.

Effects of bortezomib on mouse preimplantation embryo development in vitro

BACKGROUND

Bortezomib, a selective inhibitor of 26S proteasome, is commonly used as a chemotherapeutic agent against different types of human cancer cells. While cancer during pregnancy is a rare condition, the trends in becoming pregnant at an advanced age increase the risk of cancer associated with pregnancy. Cancers need to be treated carefully without harming the fetus during pregnancy for the patients who desire to continue their pregnancy. The aim of this study is to demonstrate the potential side effects of bortezomib as this drug has so far not been investigated during preimplantation embryo development.

MATERIALS AND METHODS

Two cell stage embryos were flushed from mated female CD1 mice and were cultured. Two-cell, eight-cell, and morula stage embryos were exposed to different concentrations of bortezomib in vitro. Morphologic alterations of the embryos were evaluated and the data were statistically analyzed.

RESULTS

Our study showed that bortezomib exposure caused a statistically significant decrease in embryo survival and developmental competence and potential rate. We detected those early stages of embryos are more susceptible to bortezomib exposure than later stages.

CONCLUSION

We indicated that bortezomib adversely affects preimplantation embryo development in a dose, time, and developmental stage dependent manner. Moreover, bortezomib treatment inhibits preimplantation embryo development and induces cytoplasmic fragmentation and cell-cycle arrest on embryos.

Novel p97/VCP inhibitor induces endoplasmic reticulum stress and apoptosis in both bortezomib-sensitive and -resistant multiple myeloma cells

p97/VCP is an endoplasmic reticulum (ER)‐associated protein that belongs to the AAA (ATPases associated with diverse cellular activities) ATPase family. It has a variety of cellular functions including ER‐associated protein degradation, autophagy, and aggresome formation. Recent studies have shown emerging roles of p97/VCP and its potential as a therapeutic target in several cancer subtypes including multiple myeloma (MM). We conducted a cell‐based compound screen to exploit novel small compounds that have cytotoxic activity in myeloma cells. Among approximately 2000 compounds, OSSL_325096 showed relatively strong antiproliferative activity in MM cell lines (IC₅₀, 100‐500 nmol/L). OSSL_325096 induced apoptosis in myeloma cell lines, including a bortezomib‐resistant cell line and primary myeloma cells purified from patients. Accumulation of poly‐ubiquitinated proteins, PERK, CHOP, and IREα, was observed in MM cell lines treated with OSSL_325096, suggesting that it induces ER stress in MM cells. OSSL_325096 has a similar chemical structure to DBeQ, a known p97/VCP inhibitor. Knockdown of the gene encoding p97/VCP induced apoptosis in myeloma cells, accompanied by accumulation of poly‐ubiquitinated protein. IC₅₀ of OSSL_325096 to myeloma cell lines were found to be lower (0.1‐0.8 μmol/L) than those of DBeQ (2‐5 μmol/L). In silico protein–drug‐binding simulation suggested possible binding of OSSL_325096 to the ATP binding site in the D2 domain of p97/VCP. In cell‐free ATPase assays, OSSL_325096 showed dose‐dependent inhibition of p97/VCP ATPase activity. Finally, OSSL_325096 inhibited the growth of subcutaneous myeloma cell tumors in vivo. The present data suggest that OSSL_325096 exerts anti‐myeloma activity, at least in part through p97/VCP inhibition.

The power of proteasome inhibition in multiple myeloma

INTRODUCTION

Proteasome inhibitors (PIs) are therapeutic backbones of multiple myeloma treatment, with PI-based therapies being standards of care throughout the treatment algorithm. Proteasome inhibition affects multiple critical signaling pathways in myeloma cells and interacts synergistically with mechanisms of action of other conventional and novel agents, resulting in substantial anti-myeloma activity and at least additive effects. Areas covered: This review summarizes the biologic effects of proteasome inhibition in myeloma and provides an overview of the importance of proteasome inhibition to the current treatment algorithm. It reviews key clinical data on three PIs, specifically bortezomib, carfilzomib, and ixazomib; assesses ongoing phase 3 trials with these agents; and looks ahead to the increasingly broad role of both approved PIs and PIs under investigation in the frontline and relapsed settings. Expert commentary: Progress to date with PIs in multiple myeloma has been impressive, but there remain unmet needs and challenges, as well as increasing opportunities to optimize the use of these agents. Understanding discrepancies between PIs in terms of efficacy and safety profile is a key goal of ongoing research, along with proteomics-based efforts to identify potential biomarkers of sensitivity and resistance, thereby enabling increasingly personalized treatment approaches in the future.

Discovering proteasomal deubiquitinating enzyme inhibitors for cancer therapy: lessons from rational design, nature and old drug reposition

The ubiquitin proteasome system has been validated as a target of cancer therapies evident by the US FDA approval of anticancer 20S proteasome inhibitors. Deubiquitinating enzymes (DUBs), an essential component of the ubiquitin proteasome system, regulate cellular processes through the removal of ubiquitin from ubiquitinated-tagged proteins. The deubiquitination process has been linked with cancer and other pathologies. As such, the study of proteasomal DUBs and their inhibitors has garnered interest as a novel strategy to improve current cancer therapies, especially for cancers resistant to 20S proteasome inhibitors. This article reviews proteasomal DUB inhibitors in the context of: discovery through rational design approach, discovery from searching natural products and discovery from repurposing old drugs, and offers a future perspective.

The proteasome and proteasome inhibitors in multiple myeloma

Proteasome inhibitors are one of the most important classes of agents to have emerged for the treatment of multiple myeloma in the past two decades, and now form one of the backbones of treatment. Three agents in this class have been approved by the United States Food and Drug Administration-the first-in-class compound bortezomib, the second-generation agent carfilzomib, and the first oral proteasome inhibitor, ixazomib. The success of this class of agents is due to the exquisite sensitivity of myeloma cells to the inhibition of the 26S proteasome, which plays a critical role in the pathogenesis and proliferation of the disease. Proteasome inhibition results in multiple downstream effects, including the inhibition of NF-κB signaling, the accumulation of misfolded and unfolded proteins, resulting in endoplasmic reticulum stress and leading to the unfolded protein response, the downregulation of growth factor receptors, suppression of adhesion molecule expression, and inhibition of angiogenesis; resistance to proteasome inhibition may arise through cellular responses mediating these downstream effects. These multiple biologic consequences of proteasome inhibition result in synergistic or additive activity with other chemotherapeutic and targeted agents for myeloma, and proteasome inhibitor-based combination regimens have become established as a cornerstone of therapy throughout the myeloma treatment algorithm, incorporating agents from the other key classes of antimyeloma agents, including the immunomodulatory drugs, monoclonal antibodies, and histone deacetylase inhibitors. This review gives an overview of the critical role of the proteasome in myeloma and the characteristics of the different proteasome inhibitors and provides a comprehensive summary of key clinical efficacy and safety data with the currently approved proteasome inhibitors.

UCHL1 is a biomarker of aggressive multiple myeloma required for disease progression

The success of proteasome inhibition in multiple myeloma highlights the critical role for the ubiquitin-proteasome system (UPS) in this disease. However, there has been little progress in finding more specific targets within the UPS involved in myeloma pathogenesis. We previously found the ubiquitin hydrolase UCH-L1 to be frequently over-expressed in B-cell malignancies, including myeloma, and showed it to be a potent oncogene in mice. Here we show that UCH-L1 is a poor prognostic factor that is essential for the progression of myeloma. We found high levels of UCHL1 to predict early progression in newly diagnosed patients; a finding reversed by the inclusion of bortezomib. We also found high UCHL1 levels to be a critical factor in the superiority of bortezomib over high-dose dexamethasone in relapsed patients. High UCHL1 partially overlaps with, but is distinct from, known genetic risks including 4p16 rearrangement and 1q21 amplification. Using an orthotopic mouse model, we found UCH-L1 depletion delays myeloma dissemination and causes regression of established disease. We conclude that UCH-L1 is a biomarker of aggressive myeloma that may be an important marker of bortezomib response, and may itself be an effective target in disseminated disease.

Bortezomib inhibits Burkitt's lymphoma cell proliferation by downregulating sumoylated hnRNP K and c-Myc expression

Bortezomib (Velcal) was the first proteasome inhibitor to be approved by the US Food and Drug Administration to treat patients with relapsed/refractory multiple myelomas. Previous studies have demonstrated that bortezomib inhibits tumor cell proliferation and induces apoptosis by blocking the nuclear factor (NF)-κB pathway. However, the exact mechanism by which bortezomib induces cancer cell apoptosis is still not well understood. In this study, we found that bortezomib significantly inhibited cell proliferation in both human Burkitt's lymphoma CA46 and Daudi cells. Through proteomic analysis, we found that bortezomib treatment changed the expression of various proteins in distinct functional categories including unfolding protein response (UPS), RNA processing, protein targeting and biosynthesis, apoptosis, and signal transduction. Among the proteins with altered expression, hnRNP K, hnRNP H, Hsp90α, Grp78, and Hsp7C were common to both Daudi and CA46 cells. Interestingly, bortezomib treatment downregulated the expression of high-molecular-weight (HMw) hnRNP K and c-Myc but upregulated the expression of low-molecular-weight (LMw) hnRNP K. Moreover, cell proliferation was significantly correlated with high expression of HMw hnRNP K and c-Myc. HMw and LMw hnRNP K were identified as sumoylated and desumoylated hnRNP K, respectively. Using transient transfection, we found that sumoylated hnRNP K increased c-Myc expression at the translational level and contributed to cell proliferation, and that Lys422 of hnRNP K is the candidate sumoylated residue. Our results suggest that besides inhibiting the ubiquitin-proteasome pathway, bortezomib may inhibit cell proliferation by downregulating sumoylated hnRNP K and c-Myc expression in Burkitt's lymphoma cells.

Synergistic anti-myeloma activity of the proteasome inhibitor marizomib and the IMiD immunomodulatory drug pomalidomide

The proteasome inhibitor bortezomib is an effective therapy for the treatment of relapsed and refractory multiple myeloma (RRMM); however, prolonged treatment can be associated with toxicity, peripheral neuropathy and drug resistance. Our earlier studies showed that the novel proteasome inhibitor marizomib is distinct from bortezomib in its chemical structure, mechanisms of action and effects on proteasomal activities, and that it can overcome bortezomib resistance. Pomalidomide, like lenalidomide, has potent immunomodulatory activity and has been approved by the US Food and Drug Administration for the treatment of RRMM. Here, we demonstrate that combining low concentrations of marizomib with pomalidomide induces synergistic anti-MM activity. Marizomib plus pomalidomide-induced apoptosis is associated with: (i) activation of caspase-8, caspase-9, caspase-3 and PARP cleavage, (ii) downregulation of cereblon (CRBN), IRF4, MYC and MCL1, and (iii) suppression of chymotrypsin-like, caspase-like, and trypsin-like proteasome activities. CRBN-siRNA attenuates marizomib plus pomalidomide-induced MM cells death. Furthermore, marizomib plus pomalidomide inhibits the migration of MM cells and tumour-associated angiogenesis, as well as overcomes cytoprotective effects of bone marrow microenvironment. In human MM xenograft model studies, the combination of marizomib and pomalidomide is well tolerated, inhibits tumour growth and prolongs survival. These preclinical studies provide the rationale for on-going clinical trials of combined marizomib and pomalidomide to improve outcome in patients with RRMM.

Bortezomib treatment causes long-term testicular dysfunction in young male mice

Background

With increased long-term survivors of childhood cancer patients, therapy-associated infertility has become one of the most common late side-effects and significantly affects their life-quality. Therefore, evaluation of anti-cancer agents on male reproduction and infertility prevention are urgently demanding. The proteasome inhibitor bortezomib has been launched in clinical trials for childhood cancers, however, its potential side effects on reproduction have so far been neither investigated experimentally nor reported in treated children. Thus the present study is designed to explore the impact of bortezomib on male reproductive function and to gain insights into how bortezomib exerts its adverse effects on man gonad, thereby providing pediatric oncologists relevant information.

Methods

35 day-old male mice were treated with one 11-day cycle of bortezomib and then sacrificed 2 days, 45 days, or 6 months later. A mating study was performed in the group followed for 6 months, and their pups were analyzed on postnatal day 50. Serum follicle-stimulating hormone (FSH) and testicular testosterone levels were measured. Testicular morphology was evaluated by light- and electron microscopy, and the underlying mechanisms and pathways of testis damage were investigated.

Results

Testicular damage was visible already 2 days after stopping bortezomib and increased in severity by day 45. Then 80% of seminiferous tubules exhibited hypospermatogenesis with arrest at the levels of spermatogonia, spermatocytes and round spermatids. Germ cells were specifically targeted by bortezomib as evidenced by increased apoptosis mediated through activation of p53 and caspases. Even six months after the bortezomib treatment, testis weight, sperm concentration and seminiferous tubule length remained at a decreased level, indicating that spermatogenesis and tubular outgrowth could not fully recover. Combined with persistently increased serum levels of FSH in these mice, our results demonstrate that bortezomib can have long-term effects on testicular function, although fertility of bortezomib-exposed males remained and their offspring looked healthy.

Conclusion

Bortezomib treatment causes long-term gonadal dysfunction in male mice. Careful monitoring of gonadal function in male childhood cancer patients treated with bortezomib is thus strongly recommended.

Bortezomib inhibits C2C12 growth by inducing cell cycle arrest and apoptosis

Proteosome inhibitors such as bortezomib (BTZ) have been used to treat muscle wasting in animal models. However, direct effect of BTZ on skeletal muscle cells has not been reported. In the present study, our data showed that C2C12 cells exhibited a dose-dependent decrease in cell viability in response to increasing concentrations of BTZ. Consistent with the results of cell viability, Annexin V/PI analysis showed a significant increase in apoptosis after exposing the cells to BTZ for 24h. The detection of cleaved caspase-3 further confirmed apoptosis. The apoptosis induced by BTZ was associated with reduced expression of p-ERK. Cell cycle analysis revealed that C2C12 cells underwent G2/M cell cycle arrest when incubated with BTZ for 24h. Furthermore, BTZ inhibited formation of multinucleated myotubes. The inhibition of myotube formation was accompanied by decreased expression of Myogenin. Our data suggest that BTZ induces cell death and inhibits differentiation of C2C12 cells at clinically relevant doses.

Proteomic analysis reveals that proteasome subunit beta 6 is involved in hypoxia-induced pulmonary vascular remodeling in rats

Background

Chronic hypoxia (CH) is known to be one of the major causes of pulmonary hypertension (PH), which is characterized by sustained elevation of pulmonary vascular resistance resulting from vascular remodeling. In this study, we investigated whether the ubiquitin proteasome system (UPS) was involved in the mechanism of hypoxia-induced pulmonary vascular remodeling. We isolated the distal pulmonary artery (PA) from a previously defined chronic hypoxic pulmonary hypertension (CHPH) rat model, performed proteomic analyses in search of differentially expressed proteins belonging to the UPS, and subsequently identified their roles in arterial remodeling.

Results

Twenty-two proteins were differently expressed between the CH and normoxic group. Among them, the expression of proteasome subunit beta (PSMB) 1 and PSMB6 increased after CH exposure. Given that PSMB1 is a well-known structural subunit and PSMB6 is a functional subunit, we sought to assess whether PSMB6 could be related to the multiple functional changes during the CHPH process. We confirmed the proteomic results by real-time PCR and Western blot. With the increase in quantity of the active subunit, proteasome activity in both cultured pulmonary artery smooth muscle cells (PASMCs) and isolated PA from the hypoxic group increased. An MTT assay revealed that the proteasome inhibitor MG132 was able to attenuate the hypoxia-induced proliferation of PASMC in a dose-dependent manner. Knockdown of PSMB6 using siRNA also prevented hypoxia-induced proliferation.

Conclusion

The present study revealed the association between increased PSMB6 and CHPH. CH up-regulated proteasome activity and the proliferation of PASMCs, which may have been related to increased PSMB6 expression and the subsequently enhanced functional catalytic sites of the proteasome. These results suggested an essential role of the proteasome during CHPH development, a novel finding requiring further study.

Endoplasmic reticulum stress and the unfolded protein response: targeting the Achilles heel of multiple myeloma

Multiple myeloma is characterized by the malignant proliferating antibody-producing plasma cells in the bone marrow. Despite recent advances in therapy that improve the survival of patients, multiple myeloma remains incurable and therapy resistance is the major factor causing lethality. Clearly, more effective treatments are necessary. In recent years it has become apparent that, as highly secretory antibody-producing cells, multiple myeloma cells require an increased capacity to cope with unfolded proteins and are particularly sensitive to compounds targeting proteostasis such as proteasome inhibitors, which represent one of the most prominent new therapeutic strategies. Because of the increased requirement for dealing with secretory proteins within the endoplasmic reticulum, multiple myeloma cells are heavily reliant for survival on a set of signaling pathways, known as the unfolded protein response (UPR). Thus, directly targeting the UPR emerges as a new promising therapeutic strategy. Here, we provide an overview of the current understanding of the UPR signaling in cancer, and outline its important role in myeloma pathogenesis and treatment. We discuss new therapeutic approaches based on targeting the protein quality control machinery and particularly the IRE1α/XBP1 axis of the UPR.

Ubiquitin- and ubiquitin-like proteins-conjugating enzymes (E2s) in breast cancer

Breast cancer is the most common malignancy in women and a significant cause of morbidity and mortality. Sub-types of breast cancer defined by the expression of steroid hormones and Her2/Neu oncogene have distinct prognosis and undergo different therapies. Besides differing in their phenotype, sub-types of breast cancer display various molecular lesions that participate in their pathogenesis. BRCA1 is one of the common hereditary cancer predisposition genes and encodes for an ubiquitin ligase. Ubiquitin ligases or E3 enzymes participate together with ubiquitin activating enzyme and ubiquitin conjugating enzymes in the attachment of ubiquitin (ubiquitination) in target proteins. Ubiquitination is a post-translational modification regulating multiple cell functions. It also plays important roles in carcinogenesis in general and in breast carcinogenesis in particular. Ubiquitin conjugating enzymes are a central component of the ubiquitination machinery and are often perturbed in breast cancer. This paper will discuss ubiquitin and ubiquitin-like proteins conjugating enzymes participating in breast cancer pathogenesis, their relationships with other proteins of the ubiquitination machinery and their role in phenotype of breast cancer sub-types.

Proteasome inhibitor treatment in alcoholic liver disease

Oxidative stress, generated by chronic ethanol consumption, is a major cause of hepatotoxicity and liver injury. Increased production of oxygen-derived free radicals due to ethanol metabolism by CYP2E1 is principally located in the cytoplasm and in the mitochondria, which does not only injure liver cells, but also other vital organs, such as the heart and the brain. Therefore, there is a need for better treatment to enhance the antioxidant response elements. To date, there is no established treatment to attenuate high levels of oxidative stress in the liver of alcoholic patients. To block this oxidative stress, proteasome inhibitor treatment has been found to significantly enhance the antioxidant response elements of hepatocytes exposed to ethanol. Recent studies have shown in an experimental model of alcoholic liver disease that proteasome inhibitor treatment at low dose has cytoprotective effects against ethanol-induced oxidative stress and liver steatosis. The beneficial effects of proteasome inhibitor treatment against oxidative stress occurred because antioxidant response elements (glutathione peroxidase 2, superoxide dismutase 2, glutathione synthetase, glutathione reductase, and GCLC) were up-regulated when rats fed alcohol were treated with a low dose of PS-341 (Bortezomib, Velcade^®). This is an important finding because proteasome inhibitor treatment up-regulated reactive oxygen species removal and glutathione recycling enzymes, while ethanol feeding alone down-regulated these antioxidant elements. For the first time, it was shown that proteasome inhibition by a highly specific and reversible inhibitor is different from the chronic ethanol feeding-induced proteasome inhibition. As previously shown by our group, chronic ethanol feeding causes a complex dysfunction in the ubiquitin proteasome pathway, which affects the proteasome system, as well as the ubiquitination system. The beneficial effects of proteasome inhibitor treatment in alcoholic liver disease are related to proteasome inhibitor reversibility and the rebound of proteasome activity 72 h post PS-341 administration.

Regulation of intracellular decorin via proteasome degradation in rat mesangial cells

Decorin (DCN) is a member of small leucine-rich proteoglycan family that neutralizes the bioactivity of transforming growth factor-beta1 (TGF-β1). It has been proven to be a promising anti-fibrotic agent to treat glomerulonephritis. But the underlining mechanism for regulating and degrading intracellular DCN is still not fully understood. In this study, we investigated the roles of ubiquitination in the regulation of cytoplasmic DCN metabolism in rat mesangial cells (MC) by immunoprecipitation and Western blot. The results showed that a proportion of cytoplasmic DCN was ubiquitinated in normal MC and was enhanced in N-glycosylation inhibitor (tunicamycin)-treated MC. After being treated with the proteasome inhibitor MG132, ubiquitinated DCN accumulated and displayed a prolonged half-life, accompanied by decreased TGF-β1 expression and reduced collagen IV mRNA level in MC. This study demonstrated that the stability and function of cytoplasmic DCN can be regulated by ubiquitin-proteasome system (UPS) in MC, which implies that regulating the ubiquitination and degradation of DCN might be a novel approach for modulating MC bioactivity.

Novel drugs in myeloma: harnessing tumour biology to treat myeloma

Steroids and alkylating agents have formed the backbone of myeloma therapy for decades with the result that patient outcomes improved very little over this period. The situation has changed recently with the advent of immunomodulatory agents and bortezomib, and patient outcomes are now improving. The introduction of bortezomib can be viewed as particularly successful as it was designed in the laboratory to fit a target that had been identified through biological research. As such, it has formed the template for new drug discovery in myeloma, with an increased understanding of the biology of the myeloma cell leading to the definition of upregulated pathways which are then targeted with a specific agent. This chapter will examine novel agents currently in development in the context of the abnormal biology of the myeloma cell and its microenvironment.

Emerging therapies for the treatment of relapsed or refractory multiple myeloma

Encouraging progress has been made in the treatment of patients with relapsed/refractory multiple myeloma (MM). The rapidly evolving understanding of key pathways responsible for tumor growth and survival has led to the development of novel agents (including immunomodulatory drugs, proteasome inhibitors, histone deacetylase inhibitors, and other targeted agents) with the potential to provide significant improvements in response and survival, and influence treatment guidelines. This review summarizes recent advances in understanding of the biology of relapsed/refractory MM and clinical trials with novel targeted agents that are currently under investigation for patients with this disease.

Enhancement of proteasome activity by a small-molecule inhibitor of USP14

Proteasomes, the primary mediators of ubiquitin-protein conjugate degradation, are regulated through complex and poorly understood mechanisms. Here we show that USP14, a proteasome-associated deubiquitinating enzyme, can inhibit the degradation of ubiquitin-protein conjugates both in vitro and in cells. A catalytically inactive variant of USP14 has reduced inhibitory activity, indicating that inhibition is mediated by trimming of the ubiquitin chain on the substrate. A high-throughput screen identified a selective small-molecule inhibitor of the deubiquitinating activity of human USP14. Treatment of cultured cells with this compound enhanced degradation of several proteasome substrates that have been implicated in neurodegenerative disease. USP14 inhibition accelerated the degradation of oxidized proteins and enhanced resistance to oxidative stress. Enhancement of proteasome activity through inhibition of USP14 may offer a strategy to reduce the levels of aberrant proteins in cells under proteotoxic stress.

Isopeptidases in anticancer therapy: looking for inhibitors

Addition of polypeptides belonging to the ubiquitin family to selected lysines residues is a widespread post-translation modification (PTM) that controls many fundamental aspects of cell's life. Specific alterations in the normal turnover of this PTM are frequently observed in tumors. The conjugation/deconjugation cycle of ubiquitin (Ub) or ubiquitin-like (Ubl) proteins influences the activities of oncogenes and tumor suppressor genes. Two families of enzymes work in antagonizing manner to add or remove Ub and Ubl-proteins on target proteins: the E3 ligases and the isopeptidases. These enzymes are the subjects of fervent research with the ambition to comprehend their regulation, their mechanisms of action, their involvement in human diseases, and to develop specific inhibitors for therapeutic intervention. Here we will discuss of isopeptidases, the deconjugating enzymes, with particular emphasis on the proapoptotic activities of the relative inhibitors identified so far.

Bortezomib decreases Rb phosphorylation and induces caspase-dependent apoptosis in Imatinib-sensitive and -resistant Bcr-Abl1-expressing cells

The use of c-abl-specific inhibitors such as Imatinib (IM) or Dasatinib has revolutionized the treatment of chronic myeloid leukemia (CML). However, a significant percentage of patients become resistant to IM. In this report, we have analyzed the possibility of using the proteasome as a molecular target in CML. Our results show that cells that express Bcr-Abl1 are more sensitive to the inhibition of the proteasome with Bortezomib (Btz) than control cells. This treatment reduces the proliferation of Bcr-Abl1-expressing cells, by inactivating NF-kappaB2 and decreasing the phosphorylation of Rb, eventually leading to an increase in caspase-dependent apoptosis. Furthermore, we show that Btz also induces cell-cycle arrest and apoptosis in cells expressing Bcr-Abl1 mutants that are resistant to IM. These results unravel a new molecular target of Btz, that is the Rb pathway, and open new possibilities in the treatment of CML especially for patients that become resistant to IM because of the presence of the T315I mutation.

Drug discovery and assay development in the ubiquitin-proteasome system

The observation that tumour cells are more sensitive to pharmacological inhibition of the proteasome than normal cells has led to the development of the proteasome inhibitor bortezomib. To date, this is the only proteasome inhibitor that has been approved for clinical use. The clinical success of bortezomib, combined with the occurrence of adverse effects and the development of clinical resistance against this compound, has initiated the development of a broad range of second-generation proteasome inhibitors as well as of assays that can be used to establish a relationship between the extent and type of proteasome inhibition and the effectiveness of a particular drug. In the present paper, we discuss new strategies that may be used in the future to overcome drug resistance and to broaden the use of proteasome inhibitors for the treatment of both cancer and infectious and autoimmune disease.

Fem1b, a proapoptotic protein, mediates proteasome inhibitor-induced apoptosis of human colon cancer cells

In the treatment of colon cancer, the development of resistance to apoptosis is a major factor in resistance to therapy. New molecular approaches to overcome apoptosis resistance, such as selectively upregulating proapoptotic proteins, are needed in colon cancer therapy. In a mouse model with inactivation of the adenomatous polyposis coli (Apc) tumor suppressor gene, reflecting the pathogenesis of most human colon cancers, the gene encoding feminization-1 homolog b (Fem1b) is upregulated in intestinal epithelium following Apc inactivation. Fem1b is a proapoptotic protein that interacts with apoptosis-inducing proteins Fas, tumor necrosis factor receptor-1 (TNFR1), and apoptotic protease activating factor-1 (Apaf-1). Increasing Fem1b expression induces apoptosis of cancer cells, but effects on colon cancer cells have not been reported. Fem1b is a homolog of feminization-1 (FEM-1), a protein in Caenorhabditis elegans that is regulated by proteasomal degradation, but whether Fem1b is likewise regulated by proteasomal degradation is unknown. Herein, we found that Fem1b protein is expressed in primary human colon cancer specimens, and in malignant SW620, HCT-116, and DLD-1 colon cancer cells. Increasing Fem1b expression, by transfection of a Fem1b expression construct, induced apoptosis of these cells. We found that proteasome inhibitor treatment of SW620, HCT-116, and DLD-1 cells caused upregulation of Fem1b protein levels, associated with induction of apoptosis. Blockade of Fem1b upregulation with morpholino antisense oligonucleotide suppressed the proteasome inhibitor-induced apoptosis of these cells. In conclusion, the proapoptotic protein Fem1b is downregulated by the proteasome in malignant colon cancer cells and mediates proteasome inhibitor-induced apoptosis of these cells. Therefore, Fem1b could represent a novel molecular target to overcome apoptosis resistance in therapy of colon cancer.

Proteasome inhibitors in the treatment of multiple myeloma

Targeting intracellular protein turnover by inhibiting the ubiquitin-proteasome pathway as a strategy for cancer therapy is a new addition to our chemotherapeutic armamentarium, and has seen its greatest successes against multiple myeloma. The first-in-class proteasome inhibitor, bortezomib, was initially approved for treatment of patients in the relapsed/refractory setting as a single agent, and was recently shown to induce even greater benefits as part of rationally designed combinations that overcome chemoresistance. Modulation of proteasome function is also a rational approach to achieve chemosensitization to other antimyeloma agents, and bortezomib has now been incorporated into the front-line setting. Bortezomib-based induction regimens are able to achieve higher overall response rates and response qualities than was the case with prior standards of care, and unlike these older approaches, maintain efficacy in patients with clinically and molecularly defined high-risk disease. Second-generation proteasome inhibitors with novel properties, such as NPI-0052 and carfilzomib, are entering the clinical arena, and showing evidence of antimyeloma activity. In this spotlight review, we provide an overview of the current state of the art use of bortezomib and other proteasome inhibitors against multiple myeloma, and highlight areas for future study that will further optimize our ability to benefit patients with this disease.

The double-edged sword of autophagy modulation in cancer

Macroautophagy (autophagy) is a lysosomal degradation pathway for the breakdown of intracellular proteins and organelles. Although constitutive autophagy is a homeostatic mechanism for intracellular recycling and metabolic regulation, autophagy is also stress responsive, in which it is important for the removal of damaged proteins and organelles. Autophagy thereby confers stress tolerance, limits damage, and sustains viability under adverse conditions. Autophagy is a tumor-suppression mechanism, yet it enables tumor cell survival in stress. Reconciling how loss of a prosurvival function can promote tumorigenesis, emerging evidence suggests that preservation of cellular fitness by autophagy may be key to tumor suppression. As autophagy is such a fundamental process, establishing how the functional status of autophagy influences tumorigenesis and treatment response is important. This is especially critical as many current cancer therapeutics activate autophagy. Therefore, efforts to understand and modulate the autophagy pathway will provide new approaches to cancer therapy and prevention.

De-ubiquitylation is the most critical step in the ubiquitin-mediated homeostatic control of the NF-kappaB/IKK basal activity

The role of ubiquitylation in signal-induced activation of nuclear factor -kappaB (NF-kappaB) has been well established, while its involvement in maintaining NF-kappaB basal activity is less certain. Recent evidences demonstrate that in unstimulated cells, NF-kappaB homeostasis is actually the result of opposing forces: pro-activating activity of the IkappaB Kinase (IKK) and inhibitory activity of the Inhibitor of -kappaB (IkappaB) proteins. It is well known that endogenous de-ubiquitylating mechanisms are less effective on Ub motifs containing UbG76A. Here, we show that overexpression of a ubiquitin (Ub) G76A mutant leads to persistent activation of the IKK/NF-kappaB pathway in the absence of extra-cellular stimuli. In contrast, no effects on NF-kappaB activation were observed upon expression of UbK48R and UbK63R mutants, which are known to impair elongation of Lys(48)- and Lys(63)-linked poly-ubiquitin chains, respectively. Overall, these findings indicate that under basal conditions, the rate of de-ubiquitylation, rather than that of substrate ubiquitylation, is critical for the maintenance of appropriate levels of IKK/NF-kappaB activity.

The ubiquitin system, disease, and drug discovery

The ubiquitin system of protein modification has emerged as a crucial mechanism involved in the regulation of a wide array of cellular processes. As our knowledge of the pathways in this system has grown, so have the ties between the protein ubiquitin and human disease. The power of the ubiquitin system for therapeutic benefit blossomed with the approval of the proteasome inhibitor Velcade in 2003 by the FDA. Current drug discovery activities in the ubiquitin system seek to (i) expand the development of new proteasome inhibitors with distinct mechanisms of action and improved bioavailability, and (ii) validate new targets. This review summarizes our current understanding of the role of the ubiquitin system in various human diseases ranging from cancer, viral infection and neurodegenerative disorders to muscle wasting, diabetes and inflammation. I provide an introduction to the ubiquitin system, highlight some emerging relationships between the ubiquitin system and disease, and discuss current and future efforts to harness aspects of this potentially powerful system for improving human health.

Republished from Current BioData's Targeted Proteins database (TPdb; ).