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

The Ubiquitin-Proteasome System in Tumor Metabolism

Metabolic reprogramming, which is considered a hallmark of cancer, can maintain the homeostasis of the tumor environment and promote the proliferation, survival, and metastasis of cancer cells. For instance, increased glucose uptake and high glucose consumption, known as the "Warburg effect," play an essential part in tumor metabolic reprogramming. In addition, fatty acids are harnessed to satisfy the increased requirement for the phospholipid components of biological membranes and energy. Moreover, the anabolism/catabolism of amino acids, such as glutamine, cystine, and serine, provides nitrogen donors for biosynthesis processes, development of the tumor inflammatory environment, and signal transduction. The ubiquitin-proteasome system (UPS) has been widely reported to be involved in various cellular biological activities. A potential role of UPS in the metabolic regulation of tumor cells has also been reported, but the specific regulatory mechanism has not been elucidated. Here, we review the role of ubiquitination and deubiquitination modification on major metabolic enzymes and important signaling pathways in tumor metabolism to inspire new strategies for the clinical treatment of cancer.

Neurodegenerative Disorder Risk in Krabbe Disease Carriers

Krabbe disease (KD) is a rare autosomal recessive disorder caused by mutations in the galactocerebrosidase gene (GALC). Defective GALC causes aberrant metabolism of galactolipids present almost exclusively in myelin, with consequent demyelinization and neurodegeneration of the central and peripheral nervous system (NS). KD shares some similar features with other neuropathies and heterozygous carriers of GALC mutations are emerging with an increased risk in developing NS disorders. In this work, we set out to identify possible variations in the proteomic profile of KD-carrier brain to identify altered pathways that may imbalance its homeostasis and that may be associated with neurological disorders. The differential analysis performed on whole brains from 33-day-old twitcher (galc ^−/−), heterozygous (galc ^+/−), and wild-type mice highlighted the dysregulation of several multifunctional factors in both heterozygous and twitcher mice. Notably, the KD-carrier mouse, despite its normal phenotype, presents the deregulation of vimentin, receptor of activated protein C kinase 1 (RACK1), myelin basic protein (MBP), 2′,3′-cyclic-nucleotide 3′-phosphodiesterase (CNP), transitional endoplasmic reticulum ATPase (VCP), and N-myc downstream regulated gene 1 protein (NDRG1) as well as changes in the ubiquitinated-protein pattern. Our findings suggest the carrier may be affected by dysfunctions classically associated with neurodegeneration: (i) alteration of (mechano) signaling and intracellular trafficking, (ii) a generalized affection of proteostasis and lipid metabolism, with possible defects in myelin composition and turnover, and (iii) mitochondrion and energy supply dysfunctions.

Inhibition of proteasomal deubiquitinases USP14 and UCHL5 overcomes tyrosine kinase inhibitor resistance in chronic myeloid leukaemia

Background

Chronic myeloid leukaemia (CML) is a haematological cancer featured by the presence of BCR‐ABL fusion protein with abnormal tyrosine kinase activation. Classical tyrosine kinase inhibitor (TKI)‐based therapies are available to patients with CML. However, acquired resistance to TKI has been a challenging obstacle, especially stubborn T315I mutation is the most common cause. Therefore, it is especially urgent to find more effective targets to overcome TKI resistance induced by BCR‐ABL^T315I. Proteasomal deubiquitinases (USP14 and UCHL5) have fundamental roles in the ubiquitin‐proteasome system and possess multiple functions during cancer progression.

Methods

The human peripheral blood mononuclear cells were collected to measure the mRNA expression of USP14 and UCHL5, as well as to detect the toxicity effect of b‐AP15. We explored the effect of b‐AP15 on the activity of proteasomal deubiquitinases. We detected the effects of b‐AP15 on BCR‐ABL^WT and BCR‐ABL^T315I CML cells in vitro and in the subcutaneous tumour model. We knocked down USP14 and/or UCHL5 by shRNA to explore whether these proteasomal deubiquitinases are required for cell proliferation of CML.

Results

In this study, we found that increased expression of the proteasomal deubiquitinase USP14 and UCHL5 in primary cancer cells from CML patients compared to healthy donors. b‐AP15, an inhibitor of USP14 and UCHL5, exhibited potent tumour‐killing activity in BCR‐ABL^WT and BCR‐ABL^T315I CML cell lines, as well as in CML xenografts and primary CML cells. Mechanically, pharmacological or genetic inhibition of USP14 and UCHL5 induced cell apoptosis and decreased the protein level of BCR‐ABL in CML cells expressing BCR‐ABL^WT and BCR‐ABL^T315I. Moreover, b‐AP15 synergistically enhanced the cytotoxic effect caused by TKI imatinib in BCR‐ABL^WT and BCR‐ABL^T315I CML cells.

Conclusion

Collectively, our results demonstrate targeting USP14 and UCHL5 as a potential strategy for combating TKI resistance in CML.

Autophagic sequestration of SQSTM1 disrupts the aggresome formation of ubiquitinated proteins during proteasome inhibition

Aggresome formation is a protective cellular response to counteract proteasome dysfunction by sequestering misfolded proteins and reducing proteotoxic stress. Autophagic degradation of the protein aggregates is considered to be a key compensating mechanism for balancing proteostasis. However, the precise role of autophagy in proteasome inhibition-induced aggresome biogenesis remains unclear. Herein, we demonstrate that in the early stage of proteasome inhibition, the maturation of the autophagosome is suppressed, which facilitates aggresome formation of misfolded proteins. Proteasome inhibition-induced phosphorylation of SQSTM1 T269/S272 inhibits its autophagic receptor activity and promotes aggresome formation of misfolded proteins. Inhibiting SQSTM1 T269/S272 phosphorylation using Doramapimod aggravates proteasome inhibitor-mediated cell damage and tumor suppression. Taken together, our data reveal a negative effect of autophagy on aggresome biogenesis and cell damage upon proteasome inhibition. Our study suggests a novel therapeutic intervention for proteasome inhibitor-mediated tumor treatment.

Siah2-GRP78 interaction regulates ROS and provides a proliferative advantage to Helicobacter pylori-infected gastric epithelial cancer cells

Helicobacter pylori-mediated gastric carcinogenesis involves upregulation of the E3 ubiquitin ligase Siah2 and its phosphorylation-mediated stabilization. This study elucidates a novel mechanism of oxidative stress regulation by phosphorylated Siah2 in H. pylori-infected gastric epithelial cancer cells (GECs). We identify that H. pylori-mediated Siah2 phosphorylation at the 6th serine residue (P-S6-Siah2) enhances proteasomal degradation of the 78-kDa glucose-regulated protein (GRP78) possessing antioxidant functions. S6 phosphorylation stabilizes Siah2 and P-S6-Siah2 potentiates H. pylori-mediated reactive oxygen species (ROS) generation. However, infected S6A phospho-null Siah2-expressing cells have decreased cellular GRP78 level as surprisingly these cells release GRP78 to a higher extent and accumulate significantly higher ROS than the wild type (WT) Siah2 construct-expressing cells. Ectopic expression of GRP78 prevents the loss of mitochondrial membrane potential and cellular ROS accumulation caused by H. pylori. H. pylori-induced mitochondrial damage and mitochondrial membrane potential loss are potentiated in Siah2-overexpressing cells but these effects are further enhanced in S6A-expressing cells. This study also confirms that while phosphorylation-mediated Siah2 stabilization optimally upregulates aggresome accumulation, it suppresses autophagosome formation, thus decreasing the dependency on the latter mechanism in regulating cellular protein abundance. Disruption of the phospho-Siah2-mediated aggresome formation impairs proliferation of infected GECs. Thus, Siah2 phosphorylation has diagnostic and therapeutic significance in H. pylori-mediated gastric cancer (GC).

Histone deacetylase inhibitor, panobinostat, exerts anti-proliferative effect with partial normalization from aberrant epigenetic states on granulosa cell tumor cell lines

The prognosis of the patients with inoperable or advanced granulosa cell tumors (GCTs) is still poor, and therefore it is important to establish a novel treatment strategy. Here we investigated the in vitro effects of a histone deacetylase inhibitor, panobinostat (PS) on two GCT cell lines (KGN and COV434). GCT cell lines were found to be susceptible to PS treatment and it inhibited cell growth mainly by apoptosis. In cell cycle analysis, PS reduced only the ratio of S phase in GCT cell lines. Combined treatment of PS with a deubiquitinase inhibitor, VLX1570 enhanced the expression of p21, cleaved PARP, cleaved caspase-9, heme oxygenase-1, and the acetylation of histone H4 and α-tubulin, leading to an additive anti-proliferative effect on KGN and COV434. The gene set enrichment analysis revealed that PS treatment suppressed DNA replication- or cell cycle-related gene expression which led to chemotherapeutic cell death and in addition, this treatment induced activation of the gene set of adherens junction towards a normalized direction as well as activation of neuron-related gene sets that might imply unexpected differentiation potential due to epigenetic modification by a HDAC inhibitor in KGN cells. Exposure of KGN and COV434 cells to PS increased the expression of E-cadherin, one of the principal regulators associated with adherens junction in quantitative RT-PCR and immunoblotting analysis. In the present study, we indicate a basis of a novel therapeutic availability of a HDAC inhibitor for the treatment of GCTs and further investigations will be warranted.

Apoptotic mechanism in human brain microvascular endothelial cells triggered by 4'-iodo-α-pyrrolidinononanophenone: Contribution of decrease in antioxidant properties

In this study, we newly synthesized four α-pyrrolidinononanophenone (α-PNP) derivatives [4'-halogenated derivatives and α-pyrrolidinodecanophenone (α-PDP)], and then performed the structure-cytotoxicity relationship analyses. The results showed the rank order for the cytotoxic effects, α-PNP < α-PDP < 4'-fluoro-α-PNP < 4'-chrolo-α-PNP < 4'-bromo-α-PNP < 4'-iodo-α-PNP (I-α-PNP), and suggest that cytotoxicities of 4'-halogenated derivatives were more intensive than that of elongation of the hydrocarbon chain (α-PDP). We also surveyed the apoptotic mechanism of I-α-PNP in brain microvascular endothelial (HBME) cells that are utilized as the in vitro model of the blood-brain barrier. HBME cell treatment with I-α-PNP facilitated the apoptotic events (caspase-3 activation, externalization of phosphatidylserine, and DNA fragmentation), which were almost completely abolished by pretreating with antioxidants. In addition, the immunofluorescent staining revealed the enhanced production of hydroxyl radical in mitochondria by the I-α-PNP treatment, inferring that the I-α-PNP treatment triggers the apoptotic mechanism dependent on the enhanced ROS production in mitochondria. The treatment with I-α-PNP increased the production of cytotoxic aldehyde 4-hydroxy-2-nonenal and decreased the amount of reduced glutathione. Additionally, the treatment decreased the 26S proteasome-based proteolytic activities and aggresome formation. These results suggest that decrease in the antioxidant properties is also ascribable to HBME cell apoptosis elicited by I-α-PNP.

Treatment with b-AP15 to Inhibit UCHL5 and USP14 Deubiquitinating Activity and Enhance p27 and Cyclin E1 for Tumors with p53 Deficiency

Background: TP53 protein is lost or mutated in about half of all types of human cancers and small molecules to regulate mutant p53 repair, or interrupt ubiquitination degradation of p53 induced by E3-ubiquitin ligase Mdm2 have a potential application in clinical application. Methods: To inhibit the deubiquitinase activity of 19S proteasome and restore the p53 protein level, in this study, we utilized p53 knockout mice to test the anti-cancer effect of a specific USP14 and UCH37 inhibitor b-AP15. Results: Our results show that UCHL5, USP14 and COPS5 are upregulated in p53-related tumors, and higher expression of these genes results in a shorter overall survival in patients with p53 deficiency. Treatment with b-AP15, a UCHL5 and USP14 deubiquitinating activity inhibitor in 19S regulatory subunit, induces tumor regression and prolong the survival period of tumor-loaded mice through down-regulation of COPS5 and its downstream AP-1 and E2F1, and up-regulation of the cell cycle-related proteins p27 and Cyclin E1. Conclusions: Thus, our results suggested that inhibition of UCHL5 and USP14 deubiquitinating activity in 19S proteasome may contribute an extensive approach to preventing tumor progress due to p53 deficiency.

Sensitivity of Acute Myelocytic Leukemia Cells to the Dienone Compound VLX1570 Is Associated with Inhibition of the Ubiquitin-Proteasome System

Dienone compounds with a 1,5-diaryl-3-oxo-1,4-pentadienyl pharmacophore have been widely reported to show tumor cell selectivity. These compounds target the ubiquitin-proteasome system (UPS), known to be essential for the viability of tumor cells. The induction of oxidative stress, depletion of glutathione, and induction of high-molecular-weight (HMW) complexes have also been reported. We here examined the response of acute myeloid leukemia (AML) cells to the dienone compound VLX1570. AML cells have relatively high protein turnover rates and have also been reported to be sensitive to depletion of reduced glutathione. We found AML cells of diverse cytogenetic backgrounds to be sensitive to VLX1570, with drug exposure resulting in an accumulation of ubiquitin complexes, induction of ER stress, and the loss of cell viability in a dose-dependent manner. Caspase activation was observed but was not required for the loss of cell viability. Glutathione depletion was also observed but did not correlate to VLX1570 sensitivity. Formation of HMW complexes occurred at higher concentrations of VLX1570 than those required for the loss of cell viability and was not enhanced by glutathione depletion. To study the effect of VLX1570 we developed a zebrafish PDX model of AML and confirmed antigrowth activity in vivo. Our results show that VLX1570 induces UPS inhibition in AML cells and encourage further work in developing compounds useful for cancer therapeutics.

Gatekeepers of the Gut: The Roles of Proteasomes at the Gastrointestinal Barrier

The gut epithelial barrier provides the first line of defense protecting the internal milieu from the environment. To circumvent the exposure to constant challenges such as pathogenic infections and commensal bacteria, epithelial and immune cells at the gut barrier require rapid and efficient means to dynamically sense and respond to stimuli. Numerous studies have highlighted the importance of proteolysis in maintaining homeostasis and adapting to the dynamic changes of the conditions in the gut environment. Primarily, proteolytic activities that are involved in immune regulation and inflammation have been examined in the context of the lysosome and inflammasome activation. Yet, the key to cellular and tissue proteostasis is the ubiquitin–proteasome system, which tightly regulates fundamental aspects of inflammatory signaling and protein quality control to provide rapid responses and protect from the accumulation of proteotoxic damage. In this review, we discuss proteasome-dependent regulation of the gut and highlight the pathophysiological consequences of the disarray of proteasomal control in the gut, in the context of aberrant inflammatory disorders and tumorigenesis.

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.

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.

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.

Inhibition of USP14 Deubiquitinating Activity as a Potential Therapy for Tumors with p53 Deficiency

Functional elimination of p53 is a common feature of a large percentage of human malignancies. Here, we report the development of a pharmacological strategy aimed at restoring p53 function and its use for targeted therapy in p53-deficient mice. Specific inhibition of deubiquitinases ubiquitin-specific peptidase 14 (USP14) resulted in durable tumor regressions of autochthonous lymphomas and sarcomas in p53-deficient mice without affecting normal tissues, and therapeutic response was correlated with an increase in the ubiquitination of constitutive photomorphogenesis 9 (COP9) signalosome subunit 5 (COPS5), a key negative regulatory effector for p53. Inhibition of USP14 resulted in durable tumor regression through COPS5 deubiquitilation and a p53-dependent and -independent regulation mechanism by USP14. This series highlights the utility of proteasome deubiquitinating activity inhibition as a novel treatment paradigm for p53-deficient cancers. In addition, it provides preliminary evidence that inhibition of USP14 resulted in durable tumor regression through COPS5 deubiquitilation and p53-dependent and -independent regulation mechanism by USP14. These findings suggest that the deubiquitinating activity of the 19S regulatory particle is a new anticancer drug target for patients with p53 deficiency.

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.

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.