The deubiquitinase inhibitor b-AP15 induces strong proteotoxic stress and mitochondrial damage

Ubiquitination and deubiquitination in cancer: from mechanisms to novel therapeutic approaches

Ubiquitination, a pivotal posttranslational modification of proteins, plays a fundamental role in regulating protein stability. The dysregulation of ubiquitinating and deubiquitinating enzymes is a common feature in various cancers, underscoring the imperative to investigate ubiquitin ligases and deubiquitinases (DUBs) for insights into oncogenic processes and the development of therapeutic interventions. In this review, we discuss the contributions of the ubiquitin-proteasome system (UPS) in all hallmarks of cancer and progress in drug discovery. We delve into the multiple functions of the UPS in oncology, including its regulation of multiple cancer-associated pathways, its role in metabolic reprogramming, its engagement with tumor immune responses, its function in phenotypic plasticity and polymorphic microbiomes, and other essential cellular functions. Furthermore, we provide a comprehensive overview of novel anticancer strategies that leverage the UPS, including the development and application of proteolysis targeting chimeras (PROTACs) and molecular glues.

Targeting proteasomal deubiquitinases USP14 and UCHL5 with b-AP15 reduces 5-fluorouracil resistance in colorectal cancer cells

5-Fluorouracil (5-FU) is the first-line treatment for colorectal cancer (CRC) patients, but the development of acquired resistance to 5-FU remains a big challenge. Deubiquitinases play a key role in the protein degradation pathway, which is involved in cancer development and chemotherapy resistance. In this study, we investigated the effects of targeted inhibition of the proteasomal deubiquitinases USP14 and UCHL5 on the development of CRC and resistance to 5-FU. By analyzing GEO datasets, we found that the mRNA expression levels of USP14 and UCHL5 in CRC tissues were significantly increased, and negatively correlated with the survival of CRC patients. Knockdown of both USP14 and UCHL5 led to increased 5-FU sensitivity in 5-FU-resistant CRC cell lines (RKO-R and HCT-15R), whereas overexpression of USP14 and UCHL5 in 5-FU-sensitive CRC cells decreased 5-FU sensitivity. B-AP15, a specific inhibitor of USP14 and UCHL5, (1-5 μM) dose-dependently inhibited the viability of RKO, RKO-R, HCT-15, and HCT-15R cells. Furthermore, treatment with b-AP15 reduced the malignant phenotype of CRC cells including cell proliferation and migration, and induced cell death in both 5-FU-sensitive and 5-FU-resistant CRC cells by impairing proteasome function and increasing reactive oxygen species (ROS) production. In addition, b-AP15 inhibited the activation of NF-κB pathway, suppressing cell proliferation. In 5-FU-sensitive and 5-FU-resistant CRC xenografts nude mice, administration of b-AP15 (8 mg·kg-1·d-1, intraperitoneal injection) effectively suppressed the growth of both types of tumors. These results demonstrate that USP14 and UCHL5 play an important role in the development of CRC and resistance to 5-FU. Targeting USP14 and UCHL5 with b-AP15 may represent a promising therapeutic strategy for the treatment of CRC.

Proteotoxic stress-induced apoptosis in cancer cells: understanding the susceptibility and enhancing the potency

Leiomyosarcoma (LMS) is aggressive cancer with few therapeutic options. LMS cells are more sensitive to proteotoxic stress compared to normal smooth muscle cells. We used small compound 2c to induce proteotoxic stress and compare the transcriptomic adaptations of immortalized human uterine smooth muscle cells (HUtSMC) and LMS cells SK-UT-1. We found that the expression of the heat shock proteins (HSPs) gene family is upregulated with higher efficiency in normal cells. In contrast, the upregulation of BH3-only proteins is higher in LMS cells. HSF1, the master regulator of HSP transcription, is sequestered into transcriptionally incompetent nuclear foci only in LMS cells, which explains the lower HSP upregulation. We also found that several compounds can enhance the cell death response to proteotoxic stress. Specifically, when low doses were used, an inhibitor of salt-inducible kinases (SIKs) and the inhibitor of IRE1α, a key element of the unfolded protein response (UPR), support proteotoxic-induced cell death with strength in LMS cells and without effects on the survival of normal cells. Overall, our data provide an explanation for the higher susceptibility of LMS cells to proteotoxic stress and suggest a potential option for co-treatment strategies.

Comprehensive Target Screening and Cellular Profiling of the Cancer-Active Compound b-AP15 Indicate Abrogation of Protein Homeostasis and Organelle Dysfunction as the Primary Mechanism of Action

Dienone compounds have been demonstrated to display tumor-selective anti-cancer activity independently of the mutational status of TP53. Previous studies have shown that cell death elicited by this class of compounds is associated with inhibition of the ubiquitin-proteasome system (UPS). Here we extend previous findings by showing that the dienone compound b-AP15 inhibits proteasomal degradation of long-lived proteins. We show that exposure to b-AP15 results in increased association of the chaperones VCP/p97/Cdc48 and BAG6 with proteasomes. Comparisons between the gene expression profile generated by b-AP15 to those elicited by siRNA showed that knock-down of the proteasome-associated deubiquitinase (DUB) USP14 is the closest related to drug response. USP14 is a validated target for b-AP15 and we show that b-AP15 binds covalently to two cysteines, Cys203 and Cys257, in the ubiquitin-binding pocket of the enzyme. Consistent with this, deletion of USP14 resulted in decreased sensitivity to b-AP15. Targeting of USP14 was, however, found to not fully account for the observed proteasome inhibition. In search for additional targets, we utilized genome-wide CRISPR/Cas9 library screening and Proteome Integral Solubility Alteration (PISA) to identify mechanistically essential genes and b-AP15 interacting proteins respectively. Deletion of genes encoding mitochondrial proteins decreased the sensitivity to b-AP15, suggesting that mitochondrial dysfunction is coupled to cell death induced by b-AP15. Enzymes known to be involved in Phase II detoxification such as aldo-ketoreductases and glutathione-S-transferases were identified as b-AP15-targets using PISA. The finding that different exploratory approaches yielded different results may be explained in terms of a “target” not necessarily connected to the “mechanism of action” thus highlighting the importance of a holistic approach in the identification of drug targets. We conclude that b-AP15, and likely also other dienone compounds of the same class, affect protein degradation and proteasome function at more than one level.

Targeting the Ubiquitin-Proteasome System for Cancer Therapeutics by Small-Molecule Inhibitors

The ubiquitin-proteasome system (UPS) is a critical regulator of cellular protein levels and activity. It is, therefore, not surprising that its dysregulation is implicated in numerous human diseases, including many types of cancer. Moreover, since cancer cells exhibit increased rates of protein turnover, their heightened dependence on the UPS makes it an attractive target for inhibition via targeted therapeutics. Indeed, the clinical application of proteasome inhibitors in treatment of multiple myeloma has been very successful, stimulating the development of small-molecule inhibitors targeting other UPS components. On the other hand, while the discovery of potent and selective chemical compounds can be both challenging and time consuming, the area of targeted protein degradation through utilization of the UPS machinery has seen promising developments in recent years. The repertoire of proteolysis-targeting chimeras (PROTACs), which employ E3 ligases for the degradation of cancer-related proteins via the proteasome, continues to grow. In this review, we will provide a thorough overview of small-molecule UPS inhibitors and highlight advancements in the development of targeted protein degradation strategies for cancer therapeutics.

VLX1570 induces apoptosis through the generation of ROS and induction of ER stress on leukemia cell lines

A novel proteasome deubiquitinase inhibitor, VLX1570, has been highlighted as a promising therapeutic agent mainly for lymphoid neoplasms and solid tumors. We examined in vitro effects of VLX1570 on eight myeloid and three lymphoid leukemia cell lines. From cell culture studies, 10 out of 11 cell lines except K562 were found to be susceptible to VLX1570 treatment and it inhibited cell growth mainly by apoptosis. Next, to identify the signaling pathways associated with apoptosis, we performed gene expression profiling using HL‐60 with or without 50 nmol/L of VLX1570 for 3 hours and demonstrated that VLX1570 induced the genetic pathway involved in “heat shock transcription factor 1 (HSF1) activation”, “HSF1 dependent transactivation”, and “Regulation of HSF1 mediated heat shock response”. VLX1570 increased the amount of high molecular weight polyubiquitinated proteins and the expression of HSP70 as the result of the suppression of ubiquitin proteasome system, the expression of heme oxygenase‐1, and the amount of phosphorylation in JNK and p38 associated with the generation of reactive oxygen species (ROS) induced apoptosis and the amount of phosphorylation in eIF2α, inducing the expression of ATF4 and endoplasmic reticulum (ER) stress dependent apoptosis protein, CHOP, and the amount of phosphorylation slightly in IRE1α, leading to increased expression of XBP‐1s in leukemia cell lines. In the present study, we demonstrate that VLX1570 induces apoptosis and exerts a potential anti‐leukemic effect through the generation of ROS and induction of ER stress in leukemia cell lines.

Deubiquitinating enzymes as possible drug targets for schistosomiasis

Deubiquitinating enzymes (DUBs) are conserved in Schistosoma mansoni and may be linked to the 26S proteasome. Previous results from our group showed that b-AP15, an inhibitor of the 26S proteasome DUBs UCHL5 and USP14 induced structural and gene expression changes in mature S. mansoni pairs. This work suggests the use of the nonselective DUB inhibitor PR-619 to verify whether these enzymes are potential target proteins for new drug development. Our approach is based on previous studies with DUB inhibitors in mammalian cells that have shown that these enzymes are associated with apoptosis, autophagy and the transforming growth factor beta (TGF-β) signaling pathway. PR-619 inhibited oviposition in parasite pairs in vitro, leading to mitochondrial changes, autophagic body formation, and changes in expression of SmSmad2 and SmUSP9x, which are genes linked to the TGF-β pathway that are responsible for parasite oviposition and SmUCHL5 and SmRpn11 DUB maintenance. Taken together, these results indicate that DUBs may be used as targets for the development of new drugs against schistosomiasis.

Enhancing Proteotoxic Stress in Leiomyosarcoma Cells Triggers Mitochondrial Dysfunctions, Cell Death, and Antitumor Activity in vivo

Leiomyosarcomas are rare and aggressive tumors characterized by a complex karyotype. Surgical resection with or without radiotherapy and chemotherapy is the standard curative treatment. Unfortunately, a high percentage of leiomyosarcomas recurs and metastasizes. In these cases, doxorubicin and ifosfamide represent the standard treatment but with low response rates. Here, we evaluated the induction of proteotoxic stress as a possible strategy to kill leiomyosarcoma cells in a therapeutic perspective. We show that aggressive leiomyosarcomas coexist with high levels of proteotoxic stress. As a consequence, we hypothesized that leiomyosarcoma cells are vulnerable to further increases of proteotoxic stress. The small compound 2c is a strong inducer of proteotoxic stress. In leiomyosarcoma cells, it triggers cell death coupled to a profound reorganization of the mitochondrial network. By using stimulated emission depletion microscopy, we have unveiled the existence of DIABLO/SMAC clusters that are modulated by 2c. Finally, we have engineered a new version of 2c linked to polyethylene glycol though a short peptide, named 2cPP. This new prodrug is specifically activated by proteases present in the tumor microenvironment. 2cPP shows a strong antitumor activity in vivo against leiomyosarcomas and no toxicity against normal cells.

Promising Anti-Mitochondrial Agents for Overcoming Acquired Drug Resistance in Multiple Myeloma

Multiple myeloma (MM) remains an incurable tumor due to the high rate of relapse that still occurs. Acquired drug resistance represents the most challenging obstacle to the extension of survival and several studies have been conducted to understand the mechanisms of this phenomenon. Mitochondrial pathways have been extensively investigated, demonstrating that cancer cells become resistant to drugs by reprogramming their metabolic assessment. MM cells acquire resistance to proteasome inhibitors (PIs), activating protection programs, such as a reduction in oxidative stress, down-regulating pro-apoptotic, and up-regulating anti-apoptotic signals. Knowledge of the mechanisms through which tumor cells escape control of the immune system and acquire resistance to drugs has led to the creation of new compounds that can restore the response by leading to cell death. In this scenario, based on all literature data available, our review represents the first collection of anti-mitochondrial compounds able to overcome drug resistance in MM. Caspase-independent mechanisms, mainly based on increased oxidative stress, result from 2-methoxyestradiol, Artesunate, ascorbic acid, Dihydroartemisinin, Evodiamine, b-AP15, VLX1570, Erw-ASNase, and TAK-242. Other agents restore PIs' efficacy through caspase-dependent tools, such as CDDO-Im, NOXA-inhibitors, FTY720, GCS-100, LBH589, a derivative of ellipticine, AT-101, KD5170, SMAC-mimetics, glutaminase-1 (GLS1)-inhibitors, and thenoyltrifluoroacetone. Each of these substances improved the efficacy rates when employed in combination with the most frequently used antimyeloma drugs.

Dienone Compounds: Targets and Pharmacological Responses

The biological responses to dienone compounds with a 1,5-diaryl-3-oxo-1,4-pentadienyl pharmacophore have been studied extensively. Despite their expected general thiol reactivity, these compounds display considerable degrees of tumor cell selectivity. Here we review in vitro and preclinical studies of dienone compounds including b-AP15, VLX1570, RA-9, RA-190, EF24, HO-3867, and MCB-613. A common property of these compounds is their targeting of the ubiquitin–proteasome system (UPS), known to be essential for the viability of tumor cells. Gene expression profiling experiments have shown induction of responses characteristic of UPS inhibition, and experiments using cellular reporter proteins have shown that proteasome inhibition is associated with cell death. Other mechanisms of action such as reactivation of mutant p53, stimulation of steroid receptor coactivators, and induction of protein cross-linking have also been described. Although unsuitable as biological probes due to widespread reactivity, dienone compounds are cytotoxic to apoptosis-resistant tumor cells and show activity in animal tumor models.

Proteotoxic Stress and Cell Death in Cancer Cells

To maintain proteostasis, cells must integrate information and activities that supervise protein synthesis, protein folding, conformational stability, and also protein degradation. Extrinsic and intrinsic conditions can both impact normal proteostasis, causing the appearance of proteotoxic stress. Initially, proteotoxic stress elicits adaptive responses aimed at restoring proteostasis, allowing cells to survive the stress condition. However, if the proteostasis restoration fails, a permanent and sustained proteotoxic stress can be deleterious, and cell death ensues. Many cancer cells convive with high levels of proteotoxic stress, and this condition could be exploited from a therapeutic perspective. Understanding the cell death pathways engaged by proteotoxic stress is instrumental to better hijack the proliferative fate of cancer cells.

Induction of ER Stress in Acute Lymphoblastic Leukemia Cells by the Deubiquitinase Inhibitor VLX1570

The proteasome is a validated target of cancer therapeutics. Inhibition of proteasome activity results in the activation of the unfolded protein response (UPR) characterized by phosphorylation of eukaryotic initiation factor 2α (eIF2α), global translational arrest, and increased expression of the proapoptotic CHOP (C/EBP homologous protein) protein. Defects in the UPR response has been reported to result in altered sensitivity of tumor cells to proteasome inhibitors. Here, we characterized the effects of the deubiquitinase (DUB) inhibitor VLX1570 on protein homeostasis, both at the level of the UPR and on protein translation, in acute lymphoblastic leukemia (ALL). Similar to the 20S inhibitor bortezomib, VLX1570 induced accumulation of polyubiquitinated proteins and increased expression of the chaperone Grp78/Bip in ALL cells. Both compounds induced cleavage of PARP (Poly (ADP-ribose) polymerase) in ALL cells, consistent with induction of apoptosis. However, and in contrast to bortezomib, VLX1570 treatment resulted in limited induction of the proapoptotic CHOP protein. Translational inhibition was observed by both bortezomib and VLX1570. We report that in distinction to bortezomib, suppression of translation by VXL1570 occurred at the level of elongation. Increased levels of Hsc70/Hsp70 proteins were observed on polysomes following exposure to VLX1570, possibly suggesting defects in nascent protein folding. Our findings demonstrate apoptosis induction in ALL cells that appears to be uncoupled from CHOP induction, and show that VLX1570 suppresses protein translation by a mechanism distinct from that of bortezomib.

Drug Development Targeting the Ubiquitin-Proteasome System (UPS) for the Treatment of Human Cancers

Cancer cells are characterized by a higher rate of protein turnover and greater demand for protein homeostasis compared to normal cells. In this scenario, the ubiquitin-proteasome system (UPS), which is responsible for the degradation of over 80% of cellular proteins within mammalian cells, becomes vital to cancer cells, making the UPS a critical target for the discovery of novel cancer therapeutics. This review systematically categorizes all current reported small molecule inhibitors of the various essential components of the UPS, including ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin ligases (E3s), the 20S proteasome catalytic core particle (20S CP) and the 19S proteasome regulatory particles (19S RP), as well as their mechanism/s of action and limitations. We also discuss the immunoproteasome which is considered as a prospective therapeutic target of the next generation of proteasome inhibitors in cancer therapies.

The role of ubiquitination in tumorigenesis and targeted drug discovery

Ubiquitination, an important type of protein posttranslational modification (PTM), plays a crucial role in controlling substrate degradation and subsequently mediates the "quantity" and "quality" of various proteins, serving to ensure cell homeostasis and guarantee life activities. The regulation of ubiquitination is multifaceted and works not only at the transcriptional and posttranslational levels (phosphorylation, acetylation, methylation, etc.) but also at the protein level (activators or repressors). When regulatory mechanisms are aberrant, the altered biological processes may subsequently induce serious human diseases, especially various types of cancer. In tumorigenesis, the altered biological processes involve tumor metabolism, the immunological tumor microenvironment (TME), cancer stem cell (CSC) stemness and so on. With regard to tumor metabolism, the ubiquitination of some key proteins such as RagA, mTOR, PTEN, AKT, c-Myc and P53 significantly regulates the activity of the mTORC1, AMPK and PTEN-AKT signaling pathways. In addition, ubiquitination in the TLR, RLR and STING-dependent signaling pathways also modulates the TME. Moreover, the ubiquitination of core stem cell regulator triplets (Nanog, Oct4 and Sox2) and members of the Wnt and Hippo-YAP signaling pathways participates in the maintenance of CSC stemness. Based on the altered components, including the proteasome, E3 ligases, E1, E2 and deubiquitinases (DUBs), many molecular targeted drugs have been developed to combat cancer. Among them, small molecule inhibitors targeting the proteasome, such as bortezomib, carfilzomib, oprozomib and ixazomib, have achieved tangible success. In addition, MLN7243 and MLN4924 (targeting the E1 enzyme), Leucettamol A and CC0651 (targeting the E2 enzyme), nutlin and MI-219 (targeting the E3 enzyme), and compounds G5 and F6 (targeting DUB activity) have also shown potential in preclinical cancer treatment. In this review, we summarize the latest progress in understanding the substrates for ubiquitination and their special functions in tumor metabolism regulation, TME modulation and CSC stemness maintenance. Moreover, potential therapeutic targets for cancer are reviewed, as are the therapeutic effects of targeted drugs.

Repurposing old drugs as new inhibitors of the ubiquitin-proteasome pathway for cancer treatment

The ubiquitin-proteasome system (UPS) plays a central role in the degradation of cellular proteins. Targeting protein degradation has been validated as an effective strategy for cancer therapy since 2003. Several components of the UPS have been validated as potential anticancer targets, including 20S proteasomes, 19S proteasome-associated deubiquitinases (DUBs) and ubiquitin ligases (E3s). 20S proteasome inhibitors (such as bortezomib/BTZ and carfilzomib/CFZ) have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of multiple myeloma (MM) and some other liquid tumors. Although survival of MM patients has been improved by the introduction of BTZ-based therapies, these clinical 20S proteasome inhibitors have several limitations, including emergence of resistance in MM patients, neuro-toxicities, and little efficacy in solid tumors. One of strategies to improve the current status of cancer treatment is to repurpose old drugs with UPS-inhibitory properties as new anticancer agents. Old drug reposition represents an attractive drug discovery approach compared to the traditional de novo drug discovery process which is time-consuming and costly. In this review, we summarize status of repurposed inhibitors of various UPS components, including 20S proteasomes, 19S-associated DUBs, and ubiquitin ligase E3s. The original and new mechanisms of action, molecular targets, and potential anticancer activities of these repurposed UPS inhibitors are reviewed, and their new uses including combinational therapies for cancer treatment are discussed.

ProTargetMiner as a proteome signature library of anticancer molecules for functional discovery

Deconvolution of targets and action mechanisms of anticancer compounds is fundamental in drug development. Here, we report on ProTargetMiner as a publicly available expandable proteome signature library of anticancer molecules in cancer cell lines. Based on 287 A549 adenocarcinoma proteomes affected by 56 compounds, the main dataset contains 7,328 proteins and 1,307,859 refined protein-drug pairs. These proteomic signatures cluster by compound targets and action mechanisms. The targets and mechanistic proteins are deconvoluted by partial least square modeling, provided through the website http://protargetminer.genexplain.com. For 9 molecules representing the most diverse mechanisms and the common cancer cell lines MCF-7, RKO and A549, deep proteome datasets are obtained. Combining data from the three cell lines highlights common drug targets and cell-specific differences. The database can be easily extended and merged with new compound signatures. ProTargetMiner serves as a chemical proteomics resource for the cancer research community, and can become a valuable tool in drug discovery.

Anticancer drugs often have widespread effects on the cellular proteome. Here, the authors generate a proteome signature library of drug-treated cancer cell lines and develop a software tool to deconvolute drug targets and gain insights into their mechanisms of action.

Emerging role of ubiquitin-specific protease 14 in oncogenesis and development of tumor: Therapeutic implication

Ubiquitin (Ub) is a small protein that can be attached to substrate proteins to direct their degradation via the proteasome. Deubiquitinating enzymes (DUBs) reverse this process by removing ubiquitin from its substrate protein. Over the past few decades, ubiquitin-specific protease 14 (USP14), a member of the DUBs, has emerged as an important player in various types of cancers. In this article, we review and summarize biological function of USP14 in tumorigenesis and multiple signaling pathways. To determine its role in cancer, we analyzed USP14 gene expression across a panel of tumors, and discussed that it could serve as a novel bio-marker in several types of cancer. And recent contributions indicated that USP14 has been shown to act as a tumor-promoting gene via the AKT, NF-κB, MAPK pathways etc. Besides, drugs targeting USP14 have shown potential anti-tumor effect and clinical significance. We focus on recent studies that explore the link between USP14 and cancer, and further discuss USP14 as a novel target for cancer therapy.

Oxidative Stress Induced by the Deubiquitinase Inhibitor b-AP15 Is Associated with Mitochondrial Impairment

Inhibitors of the 20S proteasome such as bortezomib are cytotoxic to tumor cells and have been proven to be valuable for the clinical management of multiple myeloma. The therapeutic efficacy of bortezomib is, however, hampered by the emergence of acquired resistance. Available data suggest that blocking proteasome activity at the level of proteasome-associated deubiquitinases (DUBs) provides a mechanism to overcome resistance to bortezomib and also to other cancer therapies. The small molecule b-AP15 is an inhibitor of proteasome-associated DUB activity that induces both proteotoxic stress and increases in the levels of reactive oxygen species (ROS) in tumor cells. Antioxidants have been shown to decrease apoptosis induction by b-AP15 and we here addressed the question of the mechanism of redox perturbation by this compound. We show that oxidative stress induction by b-AP15 is abrogated in cells deprived of mitochondrial DNA (ρ⁰ cells). We also show associations between the level of proteotoxic stress, the degree of mitochondrial dysfunction, and the extent of induction of hemeoxygenase-1 (HO-1), a target of the redox-regulated Nrf-2 transcription factor. Decreased expression of COX5b (cytochrome c oxidase subunit 5b) and TOMM34 (translocase of outer mitochondrial membrane 34) was observed in b-AP15-treated cells. These findings suggest a mitochondrial origin of the increased levels of ROS observed in cells exposed to the DUB inhibitor b-AP15.

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.