Induction of tumor cell apoptosis by a proteasome deubiquitinase inhibitor is associated with oxidative stress

miR-326 overexpression inhibits colorectal cancer cell growth and proteasome activity by targeting PNO1: unveiling a novel therapeutic intervention strategy

Proteasome inhibition emerges as a promising strategy for cancer prevention. PNO1, pivotal for colorectal cancer (CRC) progression, is involved in proteasome assembly in Saccharomyces cerevisiae. Hence, we aimed to explore the role of PNO1 in proteasome assembly and its up- and down-streams in CRC. Here, we demonstrated that PNO1 knockdown suppressed CRC cells growth, proteasome activities and assembly, as well as CDKN1B/p27Kip1 (p27) degradation. Moreover, p27 knockdown partially attenuated the inhibition of HCT116 cells growth by PNO1 knockdown. The up-stream studies of PNO1 identified miR-326 as a candidate miRNA directly targeting to CDS-region of PNO1 and its overexpression significantly down-regulated PNO1 protein expression, resulting in suppression of cell growth, decrease of proteasome activities and assembly, as well as increasing the stability of p27 in CRC cells. These findings indicated that miR-326 overexpression can suppress CRC cell growth, acting as an endogenous proteasome inhibitor by targeting PNO1.

Clinical Spectrum of USP8 Pathogenic Variants in Cushing's Disease

Cushing's disease (CD) is a life-threatening condition with a challenging diagnostic process and scarce treatment options. CD is caused by usually benign adrenocorticotrophic hormone (ACTH)-secreting pituitary neuroendocrine tumors (PitNETs), known as corticotropinomas. These tumors are predominantly of sporadic origin, and usually derive from the monoclonal expansion of a mutated cell. Somatic activating variants located within a hotspot of the USP8 gene are present in 11-62% of corticotropinomas, making USP8 the most frequent genetic driver of corticotroph neoplasia. In contrast, other somatic defects such as those affecting the glucocorticoid receptor gene (NR3C1), the BRAF oncogene, the deubiquitinase-encoding gene USP48, and TP53 are infrequent. Moreover, patients with familial tumor syndromes, such as multiple endocrine neoplasia, familial isolated pituitary adenoma, and DICER1 rarely develop corticotropinomas. One of the main molecular alterations in USP8-driven tumors is an overactivation of the epidermal growth factor receptor (EGFR) signaling pathway, which induces ACTH production. Hotspot USP8 variants lead to persistent EGFR overexpression, thereby perpetuating the hyper-synthesis of ACTH. More importantly, they condition a characteristic transcriptomic signature that might be useful for the clinical prognosis of patients with CD. Nevertheless, the clinical phenotype associated with USP8 variants is less well defined. Hereby we discuss the current knowledge on the molecular pathogenesis and clinical picture associated with USP8 hotspot variants. We focus on the potential significance of the USP8 mutational status for the design of tailored clinical strategies in CD.

Inhibition of USP14 enhances anti-tumor effect in vemurafenib-resistant melanoma by regulation of Skp2

BACKGROUND

The mutation of BRAF V600E often occurred in melanoma and results in tumorigenesis. BRAF mutation drives hyperactivation of the RAF-MAPK-ERK pathway. The acquired drug resistance upon prolonged use of BRAF inhibitors (such as vemurafenib) still remains the main obstacle. Previously, we have found that E3 ligase Skp2 over-expresses vemurafenib-resistant melanoma cells, and knockdown of Skp2 enhances the anti-tumor effect of vemurafenib. Interestingly, the literature has reported that the selective USP14/UCHL5 inhibitor b-AP15 displays great potential in melanoma therapy; however, the molecular mechanism still remains unknown.

METHODS

In vitro, the effect of the combination regimen of vemurafenib (Vem, PLX4032) and b-AP15 on vem-sensitive and vem-resistant melanoma has been investigated by wound healing, colony formation, transwell invasion assay, flow cytometry, lysosome staining, and ROS detection. In vivo, the combination effect on vem-resistant melanoma has been evaluated with a nude mice xenograft tumor model. GST-pulldown and co-immunoprecipitation (co-IP) assays have been applied to investigate the interactions between USP14, UCHL5, and Skp2. Cycloheximide (CHX) assay and ubiquitination assays have been used to explore the effect of USP14 on Skp2 protein half-life and ubiquitination status.

RESULTS

In the present study, we have revealed that repression of USP14 sensitizes vemurafenib resistance in melanoma through a previously unappreciated mechanism that USP14 but not UCHL5 stabilizes Skp2, blocking its ubiquitination. K119 on Skp2 is required for USP14-mediated deubiquitination and stabilization of Skp2. Furthermore, the mutated catalytic activity amino acid cysteine (C) 114 on USP14 abrogates stabilization of Skp2. Stabilization of Skp2 is required for USP14 to negatively regulate autophagy. The combination regimen of Skp2 inhibitor vemurafenib and USP14/UCHL5 inhibitor b-AP15 dramatically inhibits cell viability, migration, invasion, and colony formation in vemurafenib-sensitive and vemurafenib-resistant melanoma. Vemurafenib and b-AP15 hold cells in the S phase thus leading to apoptosis as well as the formation of the autophagic vacuole in vemurafenib-resistant SKMEL28 cells. The enhanced proliferation effect of USP14 and Skp2 is mainly due to a more effective reduction of cell apoptosis and autophagy. Further evaluation of various protein alterations has revealed that the increased expression of cleaved-PARP, LC3, and decreased Ki67 are more obvious in the combination of vemurafenib and b-AP15 treatment than those in single-drug treatment. Moreover, the co-treatment of vemurafenib and b-AP15 dramatically inhibits the growth of vemurafenib-resistant melanoma xenograft in vivo. Collectively, our findings have demonstrated that the combination of Skp2 inhibitor and USP14 inhibitor provides a new solution for the treatment of BRAF inhibitor resistance melanoma.

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.

The UCHL5 Inhibitor b-AP15 Overcomes Cisplatin Resistance via Suppression of Cancer Stemness in Urothelial Carcinoma

Urothelial carcinoma (UC) comprises the majority of bladder cancers. Standard platinum-based chemotherapy has a response rate of approximately 50%, but drug resistance develops after short-term treatment. Deubiquitinating (DUB) enzyme inhibitors increase protein polyubiquitination and endoplasmic reticulum (ER) stress, which might further suppress cancer stemness and overcome cisplatin resistance. Therefore, we investigated the cytotoxic effect and potential mechanisms of b-AP15 on urothelial carcinoma. Our results revealed that b-AP15 induced ER stress and apoptosis in BFTC905, T24, T24/R (cisplatin-resistant), and RT4 urothelial carcinoma cell lines. Inhibition of the MYC signaling pathway and cancer stemness by b-AP15 was confirmed by RNA sequencing, RT-PCR, immunoblotting, and sphere-forming assays. In the mouse xenograft model, the combination of b-AP15 and cisplatin showed superior therapeutic effects compared with either monotherapy.

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.

Deubiquitinase Inhibitors Impair Leukemic Cell Migration Through Cofilin Oxidation and Alteration of Actin Reorganization

Actin networks are dynamically regulated through constant depolymerization and polymerization cycles. Although the fundamental mechanisms that govern these processes have been identified, the nature and role of post-translational modifications (PTMs) of actin and actin regulatory proteins are not completely understood. Here, we employed Actin CytoFRET, a method that we developed for real time detection of fluorescence resonance energy transfer (FRET) signals generated by actin dynamics, to screen a small library of PTM-interfering compounds on a biosensor leukemic T cell line. This strategy led to the identification of small molecule inhibitors of deubiquitinating enzymes (DUBs) as potent inducers of actin polymerization and blockers of chemotactic cell migration. The examination of the underlying mechanism further revealed that the actin depolymerizing protein cofilin represents a major effector of DUB inhibitor (DUBi)-induced actin reorganization. We found that DUB blockade results in the accumulation of polyubiquitinated proteins and ROS production, associated with cofilin oxidation and dephosphorylation on serine 3, which provokes uncontrolled actin polymerization impairing cell migration. Together, our study highlights DUBs as novel regulators of actin dynamics through ROS-dependent cofilin modulation, and shows that DUBi represent attractive novel tools to impede leukemic cell migration.

VLX1570 regulates the proliferation and apoptosis of human lung cancer cells through modulating ER stress and the AKT pathway

The ubiquitin‐proteasome system (UPS) possesses unique functions in tumorigenesis and has great potential for targeting tumours. The effectiveness of inhibitors targeting UPS in solid tumours remains to be studied. We screened a library of inhibitors targeting the ubiquitination system and found the highly potent, low‐concentration inhibitor molecule VLX1570 that specifically killed lung cancer cells. VLX1570 is an inhibitor of deubiquitinating enzyme activity and has also shown potential for preclinical cancer treatment. We found that VLX1570 significantly inhibited lung cancer cells proliferation and colony formation. VLX1570 induced a G2/M cell cycle arrest by downregulating CDK1 and CyclinB1. Moreover, VLX1570 significantly promoted the mitochondrial‐associated apoptosis. Mechanistically speaking, VLX1570 activated the PERK/IRE1/ATF6 pathway and induced ER stress in lung cancer cell lines. The inhibition of ER stress by tauroursodeoxycholic acid sodium or 4‐phenylbutyric acid enhanced VLX1570‐induced apoptosis. In addition, VLX1570 treatment led to the inactivation of Akt signalling and inhibited the proliferation of lung cancer cells by downregulating the Akt pathway. Moreover, combined treatment with VLX1570 and Afatinib or Gefitinib induced synergistic anti‐lung cancer activity. Our present study demonstrated a novel therapy using VLX1570, which inhibited the proliferation and increased apoptosis in human lung cancer.

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.

Advances in the Development Ubiquitin-Specific Peptidase (USP) Inhibitors

Ubiquitylation and deubiquitylation are reversible protein post-translational modification (PTM) processes involving the regulation of protein degradation under physiological conditions. Loss of balance in this regulatory system can lead to a wide range of diseases, such as cancer and inflammation. As the main members of the deubiquitinases (DUBs) family, ubiquitin-specific peptidases (USPs) are closely related to biological processes through a variety of molecular signaling pathways, including DNA damage repair, p53 and transforming growth factor-β (TGF-β) pathways. Over the past decade, increasing attention has been drawn to USPs as potential targets for the development of therapeutics across diverse therapeutic areas. In this review, we summarize the crucial roles of USPs in different signaling pathways and focus on advances in the development of USP inhibitors, as well as the methods of screening and identifying USP inhibitors.

Identification of proteasome inhibitors using analysis of gene expression profiles

Inhibitors of the 20S proteasome such as bortezomib (Velcade_®) and carfilzomib (Kypriolis_®) are in clinical use for the treatment of patients with multiple myeloma and mantle cell lymphoma. In an attempt to identify novel inhibitors of the ubiquitin-proteasome system (UPS) we used the connectivity map (CMap) resource, based on alterations of gene expression profiles by perturbagens, and performed COMPARE analyses of drug sensitivity patterns in the NCI₆₀ panel. Cmap analysis identified a large number of small molecules with strong connectivity to proteasome inhibition, including both well characterized inhibitors of the 20S proteasome and molecules previously not described to inhibit the UPS. A number of these compounds have been reported to be cytotoxic to tumor cells and were tested for their ability to decrease processing of proteasome substrates. The antibiotic thiostrepton and the natural products celastrol and curcumin induced strong accumulation of polyubiquitinated proteasome substrates in exposed cells. Other compounds elicited modest increases of proteasome substrates, including the protein phosphatase inhibitor BCI-Cl and the farnesyltransferase inhibitor manumycin A, suggesting that these compounds inhibit proteasome function. Induction of chaperone expression in the absence of proteasome inhibition was observed by a number of compounds, suggesting other effects on the UPS. We conclude that the combination of bioinformatic analyses and cellular assays resulted in the identification of compounds with potential to inhibit the UPS.

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.

Ubiquitin C-Terminal Hydrolase L5 (UCHL5) Accelerates the Growth of Endometrial Cancer via Activating the Wnt/β-Catenin Signaling Pathway

Endometrial cancer (EC) is the most prevalent gynecological malignancy with high mortality. Chemotherapy plays a pivotal role both in an adjuvant setting and in exclusive treatment. However, current pharmacotherapies are limited and not ideal for improving the overall survival of EC patients. Thus, identification of the underlying molecular mechanisms responsible for initiation and progression of EC is imperative for developing novel therapeutic strategies. Ubiquitin C-terminal hydrolase L5 (UCHL5) has been found to aggravate tumor growth and metastasis in several different types of tumor models such as esophageal squamous cell carcinoma, hepatocellular carcinoma, and epithelial ovarian cancer. However, whether UCHL5 influences the growth of EC has not been elucidated. To expose the role of UCHL5 on EC, bioinformatics analysis was conducted, and it hinted that UCHL5 was overexpressed in EC tissues and associated with lower overall survival. Consistently, the overexpression of UCHL5 in EC tissues and cell lines was further confirmed by western blot (WB) and polymerase chain reaction (PCR) compared with non-tumor control. Lentivirus vectors carrying UCHL5 shRNA or CD sequences were used to reduce or overexpress the UCHL5 gene, respectively. Cell proliferation and cycle were facilitated, and cell apoptosis was decreased when the UCHL5 gene was overexpressed in EC cell lines. These results were opposite in UCHL5 knockdown EC cells. Additionally, the expression of β-catenin is positively related to UCHL5 levels and the tumorigenic effects of UCHL5 overexpression were reversed by the Wnt/β-catenin pathway inhibitor XAV939. Thus, Wnt/β-catenin pathway activation may be a partial mechanism responsible for the promoting effects of UCHL5 on EC growth. In conclusion, UCHL5 accelerated the growth of EC via the Wnt/β-catenin pathway and was expected to be an attractive target for EC treatment.

Deubiquitinase Inhibitor b-AP15 Attenuated LPS-Induced Inflammation via Inhibiting ERK1/2, JNK, and NF-Kappa B

b-AP15 is a deubiquitinase (DUB) inhibitor of 19S proteasomes, which in turn targets ubiquitin C-terminal hydrolase 5 (UCHL5) and ubiquitin-specific peptidase 14 (USP14). Nuclear factor kappa B (NF-κB) is closely linked to cellular response in macrophages when the organism is in the state of microbial infection, and it acts as a vital part in the mechanism of inflammatory reaction. However, the molecular mechanism by which DUB inhibitors, especially b-AP15, regulates inflammation remains poorly understood. This study aimed to investigate the relationship between b-AP15 and inflammation. The results showed that b-AP15 treatment significantly reduced the amounts of inflammatory indicators, such as tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) in lipopolysaccharide (LPS)-stimulated THP-1 and macrophages. Meanwhile, similar results were obtained from in vivo experiments. In addition, b-AP15 also significantly improved the survival rate of sepsis mouse via high-density LPS mediation. Furthermore, b-AP15 also inhibited the ERK1/2 and JNK phosphorylation, increased IκBα levels, and inhibited NF-κB p65 by removing them from the cytoplasm into the nucleus. All these findings suggested that b-AP15 has anti-inflammatory action and acts as a potential neoteric target drug for treating microbial infection.

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 deubiquitinating enzymes in cancer drug resistance

Drug resistance is a well-known phenomenon leading to a reduction in the effectiveness of pharmaceutical treatments. Resistance to chemotherapeutic agents can involve various intrinsic cellular processes including drug efflux, increased resistance to apoptosis, increased DNA damage repair capabilities in response to platinum salts or other DNA-damaging drugs, drug inactivation, drug target alteration, epithelial-mesenchymal transition (EMT), inherent cell heterogeneity, epigenetic effects, or any combination of these mechanisms. Deubiquitinating enzymes (DUBs) reverse ubiquitination of target proteins, maintaining a balance between ubiquitination and deubiquitination of proteins to maintain cell homeostasis. Increasing evidence supports an association of altered DUB activity with development of several cancers. Thus, DUBs are promising candidates for targeted drug development. In this review, we outline the involvement of DUBs, particularly ubiquitin-specific proteases, and their roles in drug resistance in different types of cancer. We also review potential small molecule DUB inhibitors that can be used as drugs for cancer treatment.

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.

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.

Deubiquitinating enzymes as cancer biomarkers: new therapeutic opportunities?

Cancer remains a life-threatening disease and accounts for the major mortality rates worldwide. The practice of using biomarkers for early detection, staging, and customized therapy may increase cancer patients’ survival. Deubiquitinating enzymes (DUBs) are a family of proteases that remove ubiquitin tags from proteins of interest undergoing proteasomal degradation. DUBs play several functional roles other than deubiquitination. One of the important roles of DUBs is regulation of tumor progression. Several reports have suggested that the DUB family members were highly-elevated in various cancer cells and tissues in different stages of cancer. These findings suggest that the DUBs could be used as drug targets in cancer therapeutics. In this review, we recapitulate the role of the DUB family members, including ubiquitin-specific protease, otubain protease, and important candidates from other family members. Our aim was to better understand the connection between DUB expression profiles and cancers to allow researchers to design inhibitors or gene therapies to improve diagnosis and prognosis of cancers.

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.

Inhibition of Ubiquitin-Specific Protease 14 Suppresses Cell Proliferation and Synergizes with Chemotherapeutic Agents in Neuroblastoma

Neuroblastoma is the most common extracranial malignant solid tumor in children, and drug resistance is a major reason for poor outcomes. Elevated proteasome activity plays an important role in neuroblastoma tumor development and resistance to conventional chemotherapy. Ubiquitin-specific protease 14 (USP14), one of three deubiquitinases associated with the regulatory subunit of the proteasome, is emerging as a potential therapeutic target in multiple tumor types. However, the role of USP14 in neuroblastoma is yet to be elucidated. We found that USP14 inhibition in neuroblastoma via knockdown or a specific inhibitor such as b-AP15 suppressed cell proliferation by inducing cell apoptosis. Furthermore, b-AP15 significantly inhibited neuroblastoma tumor growth in NGP and SH-SY5Y xenograft mouse models. For combination treatment, b-AP15 plus conventional chemotherapeutic agents such as doxorubicin or VP-16 resulted in synergistic antitumor effects on neuroblastoma. Our study demonstrates that USP14 is required for cell viability and is a novel therapeutic target in neuroblastoma. Moreover, USP14 inhibition may add value in combination therapy due to its powerful synergistic effects in treating neuroblastoma.

Antiproliferative Effects of the Natural Oxadiazine Nocuolin A Are Associated With Impairment of Mitochondrial Oxidative Phosphorylation

Natural products are interesting sources for drug discovery. The natural product oxadiazine Nocuolin A (NocA) was previously isolated from the cyanobacterial strain Nodularia sp. LEGE 06071 and here we examined its cytotoxic effects against different strains of the colon cancer cell line HCT116 and the immortalized epithelial cell line hTERT RPE-1. NocA was cytotoxic against colon cancer cells and immortalized cells under conditions of exponential growth but was only weakly active against non-proliferating immortalized cells. NocA induced apoptosis by mechanism(s) resistant to overexpression of BCL family members. Interestingly, NocA affected viability and induced apoptosis of HCT116 cells grown as multicellular spheroids. Analysis of transcriptome profiles did not match signatures to any known compounds in CMap but indicated stress responses and induction of cell starvation. Evidence for autophagy was observed, and a decrease in various mitochondrial respiration parameter within 1 h of treatment. These results are consistent with previous findings showing that nutritionally compromised cells in spheroids are sensitive to impairment of mitochondrial energy production due to limited metabolic plasticity. We conclude that the antiproliferative effects of NocA are associated with effects on mitochondrial oxidative phosphorylation.

Repurposing of auranofin: Thioredoxin reductase remains a primary target of the drug

Auranofin is a gold (I)-containing compound used for the treatment of rheumatic arthritis. Auranofin has anticancer activity in animal models and is approved for clinical trials for lung and ovarian carcinomas. Both the cytosolic and mitochondrial forms of the selenoprotein thioredoxin reductase (TrxR) are well documented targets of auranofin. Auranofin was recently reported to also inhibit proteasome activity at the level of the proteasome-associated deubiquitinases (DUBs) UCHL5 and USP14. We here set out to re-examine the molecular mechanism underlying auranofin cytotoxicity towards cultured cancer cells. The effects of auranofin on the proteasome were examined in cells and in vitro, effects on DUB activity were assessed using different substrates. The cellular response to auranofin was compared to that of the 20S proteasome inhibitor bortezomib and the 19S DUB inhibitor b-AP15 using proteomics. Auranofin was found to inhibit mitochondrial activity and to an induce oxidative stress response at IC₅₀ doses. At 2-3-fold higher doses, auranofin inhibits proteasome processing in cells. At such supra-pharmacological concentrations USP14 activity was inhibited. Analysis of protein expression profiles in drug-exposed tumor cells showed that auranofin induces a response distinct from that of the 20S proteasome inhibitor bortezomib and the DUB inhibitor b-AP15, both of which induced similar responses. Our results support the notion that the primary mechanism of action of auranofin is TrxR inhibition and suggest that proteasome DUB inhibition is an off-target effect. Whether proteasome inhibition will contribute to the antineoplastic effect of auranofin in treated patients is unclear but remains a possibility.

A new pyridazinone exhibits potent cytotoxicity on human cancer cells via apoptosis and poly-ubiquitinated protein accumulation

In the last 15 years, pyridazinone derivatives have acquired extensive attention due to their widespread biological activities and pharmacological applications. Pyridazinones are well known for their anti-microbial, anti-viral, anti-inflammatory, anti-cancer, and cardiovascular activities, among others. In this study, we evaluated the anti-cancer activity of a new pyridazinone derivative and propose it as a potential anti-neoplastic agent in acute promyelocytic leukemia cells. Pyr-1 cytotoxicity was assessed on several human cancer and two non-cancerous cell lines by the DNS assay. Pyr-1 demonstrated potent cytotoxicity against 22 human cancer cell lines, exhibiting the most favorable selective cytotoxicity on leukemia (CEM and HL-60), breast (MDA-MB-231 and MDA-MB-468), and lung (A-549) cancer cell lines, when compared with non-cancerous breast epithelial MCF-10A cells. Analyses of apoptosis/necrosis pathways, reactive oxygen species (ROS) production, mitochondria health, caspase-3 activation, and cell cycle profile were performed via flow cytometry. Both hmox-1 RNA and protein expression levels were evaluated by quantitative real-time PCR and Western blotting assays, respectively. Pyr-1 induced apoptosis in acute promyelocytic leukemia cells as confirmed by phosphatidylserine externalization, mitochondrial depolarization, caspase-3 activation, DNA fragmentation, and disrupted cell cycle progression. Additionally, it was determined that Pyr-1 generates oxidative and proteotoxic stress by provoking the accumulation of ROS, resulting in the overexpression of the stress-related hmox-1 mRNA transcripts and protein and a marked increase in poly-ubiquitinated proteins. Our data demonstrate that Pyr-1 induces cell death via the intrinsic apoptosis pathway by accumulating ROS and by impairing proteasome activity.

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.

Reactive Oxygen and Nitrogen Species-Induced Protein Modifications: Implication in Carcinogenesis and Anticancer Therapy

Cancer is a complex disorder extremely dependent on its microenvironment and highly regulated by multiple intracellular and extracellular stimuli. Studies show that reactive oxygen and nitrogen species (RONS) play key roles in cancer initiation and progression. Accumulation of RONS caused by imbalance between RONS generation and activity of antioxidant system (AOS) has been observed in many cancer types. This leads to alterations in gene expression levels, signal transduction pathways, and protein quality control machinery, that is, processes that regulate cancer cell proliferation, migration, invasion, and apoptosis. This review focuses on the latest advancements evidencing that RONS-induced modifications of key redox-sensitive residues in regulatory proteins, that is, cysteine oxidation/S-sulfenylation/S-glutathionylation/S-nitrosylation and tyrosine nitration, represent important molecular mechanisms underlying carcinogenesis. The oxidative/nitrosative modifications cause alterations in activities of intracellular effectors of MAPK- and PI3K/Akt-mediated signaling pathways, transcription factors (Nrf2, AP-1, NFκB, STAT3, and p53), components of ubiquitin/proteasomal and autophagy/lysosomal protein degradation systems, molecular chaperones, and cytoskeletal proteins. Redox-sensitive proteins, RONS-generating enzymes, and AOS components can serve as targets for relevant anticancer drugs. Chemotherapeutic agents exert their action via RONS generation and induction of cancer cell apoptosis, while drug resistance associates with RONS-induced cancer cell survival; this is exploited in selective anticancer therapy strategies. Cancer Res; 78(21); 6040-7. ©2018 AACR.

UbiSite approach for comprehensive mapping of lysine and N-terminal ubiquitination sites

Ubiquitination is a post-translational modification (PTM) that is essential for balancing numerous physiological processes. To enable delineation of protein ubiquitination at a site-specific level, we generated an antibody, denoted UbiSite, recognizing the C-terminal 13 amino acids of ubiquitin, which remain attached to modified peptides after proteolytic digestion with the endoproteinase LysC. Notably, UbiSite is specific to ubiquitin. Furthermore, besides ubiquitination on lysine residues, protein N-terminal ubiquitination is readily detected as well. By combining UbiSite enrichment with sequential LysC and trypsin digestion and high-accuracy MS, we identified over 63,000 unique ubiquitination sites on 9,200 proteins in two human cell lines. In addition to uncovering widespread involvement of this PTM in all cellular aspects, the analyses reveal an inverse association between protein N-terminal ubiquitination and acetylation, as well as a complete lack of correlation between changes in protein abundance and alterations in ubiquitination sites upon proteasome inhibition.

Proteasome inhibition promotes autophagy and protects from endoplasmic reticulum stress in rat alveolar macrophages exposed to hypoxia-reoxygenation injury

Alveolar macrophages play vital roles in acute lung injury, and macrophage response to hypoxia play relevant roles to disease mechanisms. There is growing evidence that cell death pathways play crucial roles in physiological and pathological settings and that the ubiquitin-proteasome system is involved in the regulation of these processes. However, the functional role of proteasome in alveolar macrophages exposed to hypoxia-reoxygenation (H/R) injury is unknown. We aimed to investigate the function of proteasome on alveolar macrophages exposed to H/R and the underlying mechanisms. NR8383 cells were pretreated with proteasome activator sulforaphane (SFN) or inhibitor MG-132 for 1 hr, and then submitted to 2/6 hr, 4/6 hr, and 6/6 hr H/R treatment. Cell viability was assessed with MTT assay. Autophagy was monitored using electron transmission microscope and flow cytometry and western blotting. The endoplasmic reticulum (ER) stress and unfolded protein response (UPR) pathways were equally analyzed by western blotting. Cell apoptosis was detected by immunohistochemistry, caspase3/7 activity, and western blotting. The viability of NR8383 cells exposed to H/R was affected by proteasome activity and proteasome inhibition significantly inhibited cell death. Treatment with MG-132 led to autophagy activation and induced the survival of NR8383 cells exposed to H/R. Pretreatment with SFN significantly decreased cell autophagy and induced cell death. ER stress was activated in H/R-treated NR8383 cells, and SFN further promoted ER stress whereas proteasome inhibition led to contrary results. Proteasome inhibtion hindered cell apoptosis as demonstrated by decreased caspase-3/7 activity, immunolabelling, and western blot results. Proteasome inhibition might be a promising approach for treating H/R injury-related lung diseases.

Metal-based proteasomal deubiquitinase inhibitors as potential anticancer agents

Deubiquitinases (DUBs) play an important role in protein quality control in eukaryotic cells due to their ability to specifically remove ubiquitin from substrate proteins. Therefore, recent findings have focused on the relevance of DUBs to cancer development, and pharmacological intervention on these enzymes has become a promising strategy for cancer therapy. In particular, several DUBs are physically and/or functionally associated with the proteasome and are attractive targets for the development of novel anticancer drugs. The successful clinical application of cisplatin in cancer treatment has prompted researchers to develop various metal-based anticancer agents with new properties. Recently, we have reported that several metal-based drugs, such as the antirheumatic gold agent auranofin (AF), the antifouling paint biocides copper pyrithione (CuPT) and zinc pyrithione (ZnPT), and also our two synthesized complexes platinum pyrithione (PtPT) and nickel pyrithione (NiPT), can target the proteasomal DUBs UCHL5 and USP14. In this review, we summarize the recently reported small molecule inhibitors of proteasomal DUBs, with a focus on discussion of the unique nature of metal-based proteasomal DUB inhibitors and their anticancer activity.

Proteasome-associated deubiquitinases and cancer

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

Deubiquitinase inhibitor b-AP15 activates endoplasmic reticulum (ER) stress and inhibits Wnt/Notch1 signaling pathway leading to the reduction of cell survival in hepatocellular carcinoma cells

b-AP15, a potent and selective inhibitor of the ubiquitin-specific peptidase 14 (USP14), displays in vitro and in vivo antitumor abilities on some types of cancer cells. However, the mechanism underlying its action is not well elucidated. The purposes of the present study are to observe the potential impacts of b-AP15 on cell survival of hepatocellular carcinoma cells and to investigate whether and how this compound inhibits some survival-promoting signaling pathways. We found that b-AP15 significantly decreased cell viability and increased cell apoptosis in a dose-dependent manner in hepatocellular carcinoma cells, along with the perturbation of cell cycle and the decreased expressions of cell cycle-related proteins. We also demonstrated that the endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) were enhanced by b-AP15 supplementation. The inhibition of ER stress/UPR only partly attenuated the cytotoxicity of b-AP15 on hepatocellular carcinoma cells. In addition, b-AP15 treatment inhibited Wnt/β-catenin and Notch1 signaling pathways, and suppressed phosphorylation of STAT3, Akt, and Erk1/2, which were not restored by the inhibition of ER stress/UPR. Furthermore, the expression levels of signaling molecules in Notch1 were reduced by specific inhibitor of Wnt/β-catenin pathway. Notably, either Wnt or Notch1 signaling inhibitor mitigated phosphorylation of STAT3, Akt, and Erk1/2, and mimicked the cytotoxicity of b-AP15 on hepatocellular carcinoma cells. These results clearly indicate that b-AP15 induced cytotoxic response to hepatocellular carcinoma cells by augmenting ER stress/UPR and inhibiting Wnt/Notch1 signaling pathways. This new finding provides a novel mechanism by which b-AP15 produces its antitumor therapeutic effects.

Deubiquitylating enzymes as cancer stem cell therapeutics

The focus of basic and applied research on core stem cell transcription factors has paved the way to initial delineation of their characteristics, their regulatory mechanisms, and the applicability of their regulatory proteins for protein-induced pluripotent stem cells (protein-IPSC) generation and in further clinical settings. Striking parallels have been observed between cancer stem cells (CSCs) and stem cells. For the maintenance of stem cells and CSC pluripotency and differentiation, post translational modifications (i.e., ubiquitylation and deubiquitylation) are tightly regulated, as these modifications result in a variety of stem cell fates. The identification of deubiquitylating enzymes (DUBs) involved in the regulation of core stem cell transcription factors and CSC-related proteins might contribute to providing novel insights into the implications of DUB regulatory mechanisms for governing cellular reprogramming and carcinogenesis. Moreover, we propose the novel possibility of applying DUBs coupled with core transcription factors to improve protein-iPSC generation efficiency. Additionally, this review article further illustrates the potential of applying DUB inhibitors as a novel therapeutic intervention for targeting CSCs. Thus, defining DUBs as core pharmacological targets implies that future endeavors to develop their inhibitors may revolutionize our ability to regulate stem cell maintenance and differentiation, somatic cell reprogramming, and cancer stem cells.

Acute lymphoblastic leukemia cells are sensitive to disturbances in protein homeostasis induced by proteasome deubiquitinase inhibition

The non-genotoxic nature of proteasome inhibition makes it an attractive therapeutic option for the treatment of pediatric malignancies. We recently described the small molecule VLX1570 as an inhibitor of proteasome deubiquitinase (DUB) activity that induces proteotoxic stress and apoptosis in cancer cells. Here we show that acute lymphoblastic leukemia (ALL) cells are highly sensitive to treatment with VLX1570, resulting in the accumulation of polyubiquitinated proteasome substrates and loss of cell viability. VLX1570 treatment increased the levels of a number of proteins, including the chaperone HSP70B', the oxidative stress marker heme oxygenase-1 (HO-1) and the cell cycle regulator p21Cip1. Unexpectedly, polybiquitin accumulation was found to be uncoupled from ER stress in ALL cells. Thus, increased phosphorylation of eIF2α occurred only at supra-pharmacological VLX1570 concentrations and did not correlate with polybiquitin accumulation. Total cellular protein synthesis was found to decrease in the absence of eIF2α phosphorylation. Furthermore, ISRIB (Integrated Stress Response inhibitor) did not overcome the inhibition of protein synthesis. We finally show that VLX1570 can be combined with L-asparaginase for additive or synergistic antiproliferative effects on ALL cells. We conclude that ALL cells are highly sensitive to the proteasome DUB inhibitor VLX1570 suggesting a novel therapeutic option for this disease.

Hinokitiol copper complex inhibits proteasomal deubiquitination and induces paraptosis-like cell death in human cancer cells

The ubiquitin-proteasome system (UPS) plays a central role in the regulation of proteins that control cell growth and apoptosis and has therefore become an important target for anticancer therapy. Several constitutive subunits of the 19S proteasome display deubiquitinase (DUB) activity, suggesting that ubiquitin modification of proteins is dynamically regulated. Our study and others have shown that metal complexes, such as copper complexes, can induce cancer cell apoptosis through inhibiting 19S proteasome-associated DUBs and/or 20S proteasome activity. In this study, we found that (1) Hinokitiol copper complex (HK-Cu) induces striking accumulation of ubiquitinated proteins in A549 and K562 cells (2) HK-Cu potently inhibits the activity of the 19S proteasomal DUBs much more effectively than it does to the chymotrypsin-like activity of the 20S proteasome (3) HK-Cu effectively induces caspase-independent and paraptosis-like cell death in A549 and K562 cells, and (4) HK-Cu-induced cell death depends on ATF4-assosiated ER stress but is apparently not related to ROS generation. Altogether, these data indicate that HK-Cu can inhibit the activity of the 19S proteasomal DUBs and induce paraptosis-like cell death, representing a new drug candidate for cancer treatment.

The proteasome deubiquitinase inhibitor b-AP15 enhances DR5 activation-induced apoptosis through stabilizing DR5

b-AP15 and its derivatives block proteasome deubiquitinase (DUB) activity and have been developed and tested in the clinic as potential cancer therapeutic agents. b-AP15 induces apoptosis in cancer cells, but the underlying mechanisms are largely undefined. The current study focuses on studying the modulatory effects of b-AP15 on death receptor 5 (DR5) levels and DR5 activation-induced apoptosis as well as on understanding the underlying mechanisms. Treatment with b-AP15 potently increased DR5 levels including cell surface DR5 in different cancer cell lines with limited or no effects on the levels of other related proteins including DR4, c-FLIP, FADD, and caspase-8. b-AP15 substantially slowed the degradation of DR5, suggesting that it stabilizes DR5. Moreover, b-AP15 effectively augmented apoptosis when combined with TRAIL or the DR5 agonistic antibody AMG655; these effects are DR5-dependent because DR5 deficiency abolished the ability of b-AP15 to enhance TRAIL- or AMG655-induced apoptosis. Therefore, it is clear that b-AP15, and possibly its derivatives, can stabilize DR5 and increase functional cell surface DR5 levels, resulting in enhancement of DR5 activation-induced apoptosis. Our findings suggest that b-AP15 and its derivatives may have potential in sensitizing cancer cells to DR5 activation-based cancer therapy.

A novel deubiquitinase inhibitor b-AP15 triggers apoptosis in both androgen receptor-dependent and -independent prostate cancers

Prostate cancer (PCa) remains a leading cause of cancer-related death in men. Especially, a subset of patients will eventually progress to the metastatic castrate-resistant prostate cancer (CRPC) which is currently incurable. Deubiquitinases (DUBs) associated with the 19S proteasome regulatory particle are increasingly emerging as significant therapeutic targets in numerous cancers. Recently, a novel small molecule b-AP15 is identified as an inhibitor of the USP14/UCHL5 (DUBs) of the 19S proteasome, resulting in cell growth inhibition and apoptosis in several human cancer cell lines. Here, we studied the therapeutic effect of b-AP15 in PCa, and our results indicate that (i) b-AP15 decreases viability, proliferation and triggers cytotoxicity to both androgen receptor-dependent and -independent PCa cells in vitro and in vivo, associated with caspase activation, inhibition of mitochondria function, increased reactive oxygen species (ROS) generation and endoplasmic reticulum (ER) stress; (ii) pan-caspase inhibitor z-VAD-FMK and ROS scavenger N-acetyl-L-cysteine (NAC) efficiently block apoptosis but not proteasome inhibition induced by exposure of b-AP15; (iii) treatment with b-AP15 in androgen-dependent prostate cancer (ADPC) cells down-regulates the expression of androgen receptor (AR), which is degraded via the ubiquitin proteasome system. Hence, the potent anti-tumor effect of b-AP15 on both androgen receptor-dependent and -independent PCa cells identifies a new promising therapeutic strategy for prostate cancer.

Coinhibition of the deubiquitinating enzymes, USP14 and UCHL5, with VLX1570 is lethal to ibrutinib- or bortezomib-resistant Waldenstrom macroglobulinemia tumor cells

The survival of Waldenstrom macroglobulinemia (WM) tumor cells hinges on aberrant B-cell receptor (BCR) and MYD88 signaling. WM cells upregulate the proteasome function to sustain the BCR-driven growth while maintaining homeostasis. Clinically, two treatment strategies are used to disrupt these complementary yet mutually exclusive WM survival pathways via ibrutinib (targets BTK/MYD88 node) and bortezomib (targets 20 S proteasome). Despite the success of both agents, WM patients eventually become refractory to treatment, highlighting the adaptive plasticity of WM cells and underscoring the need for development of new therapeutics. Here we provide a comprehensive preclinical report on the anti-WM activity of VLX1570, a novel small-molecule inhibitor of the deubiquitinating enzymes (DUBs), ubiquitin-specific protease 14 (USP14) and ubiquitin carboxyl-terminal hydrolase isozyme L5 (UCHL5). Both DUBs reside in the 19 S proteasome cap and their inhibition by VLX1570 results in rapid and tumor-specific apoptosis in bortezomib- or ibrutinib-resistant WM cells. Notably, treatment of WM cells with VLX1570 downregulated BCR-associated elements BTK, MYD88, NFATC, NF-κB and CXCR4, the latter whose dysregulated function is linked to ibrutinib resistance. VLX1570 administered to WM-xenografted mice resulted in decreased tumor burden and prolonged survival (P=0.0008) compared with vehicle-treated mice. Overall, our report demonstrates significant value in targeting USP14/UCHL5 with VLX1570 in drug-resistant WM and carries a high potential for clinical translation.

Anti-infective Activity of 2-Cyano-3-Acrylamide Inhibitors with Improved Drug-Like Properties against Two Intracellular Pathogens

Due to the rise of antibiotic resistance and the small number of effective antiviral drugs, new approaches for treating infectious diseases are urgently needed. Identifying targets for host-based therapies represents an emerging strategy for drug discovery. The ubiquitin-proteasome system is a central mode of signaling in the eukaryotic cell and may be a promising target for therapies that bolster the host's ability to control infection. Deubiquitinase (DUB) enzymes are key regulators of the host inflammatory response, and we previously demonstrated that a selective DUB inhibitor and its derivative promote anti-infective activities in host cells. To find compounds with anti-infective efficacy but improved toxicity profiles, we tested a library of predominantly 2-cyano-3-acrylamide small-molecule DUB inhibitors for anti-infective activity in macrophages against two intracellular pathogens: murine norovirus (MNV) and Listeria monocytogenes We identified compound C6, which inhibited DUB activity in human and murine cells and reduced intracellular replication of both pathogens with minimal toxicity in cell culture. Treatment with C6 did not significantly affect the ability of macrophages to internalize virus, suggesting that the anti-infective activity interferes with postentry stages of the MNV life cycle. Metabolic stability and pharmacokinetic assays showed that C6 has a half-life in mouse liver microsomes of ∼20 min and has a half-life of approximately 4 h in mice when administered intravenously. Our results provide a framework for targeting the host ubiquitin system in the development of host-based therapies for infectious disease. Compound C6 represents a promising tool with which to elucidate the role of DUBs in the macrophage response to infection.

The proteasome deubiquitinase inhibitor VLX1570 shows selectivity for ubiquitin-specific protease-14 and induces apoptosis of multiple myeloma cells

Inhibition of deubiquitinase (DUB) activity is a promising strategy for cancer therapy. VLX1570 is an inhibitor of proteasome DUB activity currently in clinical trials for relapsed multiple myeloma. Here we show that VLX1570 binds to and inhibits the activity of ubiquitin-specific protease-14 (USP14) in vitro, with comparatively weaker inhibitory activity towards UCHL5 (ubiquitin-C-terminal hydrolase-5). Exposure of multiple myeloma cells to VLX1570 resulted in thermostabilization of USP14 at therapeutically relevant concentrations. Transient knockdown of USP14 or UCHL5 expression by electroporation of siRNA reduced the viability of multiple myeloma cells. Treatment of multiple myeloma cells with VLX1570 induced the accumulation of proteasome-bound high molecular weight polyubiquitin conjugates and an apoptotic response. Sensitivity to VLX1570 was moderately affected by altered drug uptake, but was unaffected by overexpression of BCL2-family proteins or inhibitors of caspase activity. Finally, treatment with VLX1570 was found to lead to extended survival in xenograft models of multiple myeloma. Our findings demonstrate promising antiproliferative activity of VLX1570 in multiple myeloma, primarily associated with inhibition of USP14 activity.

Small-molecule inhibitors of USP7 induce apoptosis through oxidative and endoplasmic reticulum stress in cancer cells

USP7 is a deubiquitinating enzyme that involves the ubiquitin proteasome system (UPS) to maintain regulation of protein synthesis and degradation. The well-known substrate of USP7 is the Mdm2-p53 complex. In fact, several studies have reported that functional inhibition of USP7 induces cancer cell apoptosis through activation of p53. However, the contribution of oxidative or endoplasmic reticulum (ER) stress, which is commonly induced by inhibition of the UPS for USP7 inhibitor-mediated apoptosis in cancer cells, has not been investigated. In contrast to previous reports, we show that p53 is not critical during USP7 inhibitor-induced apoptosis in several cancer cells. Inhibition of deubiquitinating enzyme activities by USP7 inhibitors causes ER stress by accumulating polyubiquitinated proteins in cancer cells. Furthermore, we demonstrate that USP7 inhibitors increase intracellular reactive oxygen species and mainly cause cancer cell apoptosis. Taken together, our results reveal that oxidative and ER stress, rather than the Mdm2-p53 axis, mainly contributes to USP7 inhibitor-mediated apoptosis in cancer cells.

Downregulation of ubiquitin-specific protease 14 (USP14) inhibits breast cancer cell proliferation and metastasis, but promotes apoptosis

Breast cancer is the second leading cause of cancer-related death in women. Previously, evidence suggested that ubiquitin-specific protease 14 (USP14) was associated with various signal transduction pathways and tumourigenesis. In this study, we demonstrate that USP14 is a novel therapeutic target in breast cancer. A Western blot analysis of USP14 was performed using seven breast cancer tissues and paired adjacent normal tissues and showed that the expression of USP14 was increased in the breast cancer tissues. Immunohistochemistry was conducted on formalin-fixed paraffin-embedded sections of breast cancer samples from 100 cases. Using Pearson's χ(2) test, it was demonstrated that USP14 expression was associated with the histological grade, lymph node status and Ki-67 expression in the tumour. The Kaplan-Meier analysis revealed that increased USP14 expression in patients with breast cancer was associated with a poorer prognosis. In in vitro experiments, the highly migratory MDA-MB-231 cells that were treated with USP14-shRNA (shUSP14) exhibited decreased motility using Transwell migration assays. Next, we employed a starvation and re-feeding assay, and the CCK-8 assay demonstrated that USP14 regulated breast cancer cell proliferation. Furthermore, we used flow cytometry to analyse cellular apoptosis following USP14 knockdown. Taken together, our results suggested that USP14 was involved in the progression of breast cancer.

Novel role of TRPV2 in promoting the cytotoxicity of H2O2-mediated oxidative stress in human hepatoma cells

Oxidative stress is important for the initiation and progression of cancers, which confers the cells with a survival advantage by inducing oxidative adaption and drug resistance. Therefore, developing strategies to promote oxidative stress-induced cytotoxicity could be important for cancer therapy. Herein, we found that H2O2-mediated oxidative stress increases TRPV2 expression in human hepatoma (HepG2 and Huh-7) cells. This occurred at the mRNA and protein levels in a dose-dependent manner. The significance of TRPV2 in promoting H2O2-induced cell death was demonstrated in gain and loss of function studies with overexpression and knockdown of TRPV2, respectively. Mechanistically, H2O2-induced cell death involves inhibition of pro-survival signaling proteins (Akt, Nrf2) and activation of pro-death signaling proteins (p38, JNK1). Overexpression of TRPV2 in H2O2-treated hepatoma cells aggravates the inhibition of Akt and Nrf2, while it enhances the activation of p38 and JNK1 at the early stage of cell death. Interestingly, increased expression of TRPV2 in HepG2 cells improved the efficacy of stress-associated chemicals to induce cell death. Our findings suggest that TRPV2 acts as an important enhancer for H2O2-induced cytotoxicity. This process occurred by the inhibition of Akt and Nrf2 as well as the early activation of p38 and JNK1. These findings have important implications for inhibition of oxidative adaption and drug resistance.

USP11, Deubiquitinating Enzyme, Associated with Neuronal Apoptosis Following Intracerebral Hemorrhage

Protein ubiquitination is a dynamic two-way process that can be reversed or regulated by deubiquitinating enzymes (DUB). USP11, located on the X chromosome, 6 is a member of USP subclass of the DUB family. Here, we demonstrate that USP11 may be involved in neuronal apoptosis in the processes of intracerebral hemorrhage (ICH). From the results of Western blot, immunohistochemistry, and immunofluorescence, we obtained a significant up-regulation of USP11 in neurons adjacent to the hematoma following ICH. Increasing USP11 level was found to be accompanied by the up-regulation of active caspase-3, Fas receptor (Fas), Fas ligand (FasL), and active caspase-8. Besides, USP11 co-localized well with active caspase-3 in neurons, indicating its potential role in neuronal apoptosis. What is more, knocking down USP11 by RNA-interference in PC12 cells reduced active caspase-3 expression. Thus, USP11 may play a role in promoting the brain secondary damage following ICH.

Deubiquitinases (DUBs) and DUB inhibitors: a patent review

INTRODUCTION

Deubiquitinating-enzymes (DUBs) are key components of the ubiquitin-proteasome system (UPS). The fundamental role of DUBs is specific removal of ubiquitin from substrates. DUBs contribute to activation/deactivation, recycling and localization of numerous regulatory proteins, and thus play major roles in diverse cellular processes. Altered DUB activity is associated with a multitudes of pathologies including cancer. Therefore, DUBs represent novel candidates for target-directed drug development.

AREAS COVERED

The article is a thorough review/accounting of patented compounds targeting DUBs and stratifying/classifying the patented compounds based on: chemical-structures, nucleic-acid compositions, modes-of-action, and targeting sites. The review provides a brief background on the UPS and the involvement of DUBs. Furthermore, methods for assessing efficacy and potential pharmacological utility of DUB inhibitor (DUBi) are discussed.

EXPERT OPINION

The FDA's approval of the 20S proteasome inhibitors (PIs): bortezomib and carfilzomib for treatment of hematological malignancies established the UPS as an anti-cancer target. Unfortunately, many patients are inherently resistant or develop resistance to PIs. One potential strategy to combat PI resistance is targeting upstream components of the UPS such as DUBs. DUBs represent a promising potential therapeutic target due to their critical roles in various cellular processes including protein turnover, localization and cellular homeostasis. While considerable efforts have been undertaken to develop DUB modulators, significant advancements are necessary to move DUBis into the clinic.