Regulation of ErbB2 receptor status by the proteasomal DUB POH1

Deubiquitylating Enzymes in Cancer and Immunity

Deubiquitylating enzymes (DUBs) maintain relative homeostasis of the cellular ubiquitome by removing the post-translational modification ubiquitin moiety from substrates. Numerous DUBs have been demonstrated specificity for cleaving a certain type of ubiquitin linkage or positions within ubiquitin chains. Moreover, several DUBs perform functions through specific protein-protein interactions in a catalytically independent manner, which further expands the versatility and complexity of DUBs' functions. Dysregulation of DUBs disrupts the dynamic equilibrium of ubiquitome and causes various diseases, especially cancer and immune disorders. This review summarizes the Janus-faced roles of DUBs in cancer including proteasomal degradation, DNA repair, apoptosis, and tumor metastasis, as well as in immunity involving innate immune receptor signaling and inflammatory and autoimmune disorders. The prospects and challenges for the clinical development of DUB inhibitors are further discussed. The review provides a comprehensive understanding of the multi-faced roles of DUBs in cancer and immunity.

UCHL5 Promotes Proliferation and Migration of Bladder Cancer Cells by Activating c-Myc via AKT/mTOR Signaling

Ubiquitin C-terminal hydrolase L5 (UCHL5) is a deubiquitinating enzyme (DUB) that removes ubiquitin from its substrates. Associations between UCHL5 and cancer have been reported in various tissues, but the effect of UCHL5 on bladder cancer has not been thoroughly investigated. This study investigates the expression and function of UCHL5 in bladder cancer. UCHL5 was shown to be abnormally expressed using IHC of tissue microarray and Western blotting. Several procedures were performed to assess the effect of UCHL5 overexpression or knockdown on bladder cancer, such as cell proliferation, colony formation, wound-healing, and Transwell assays. In addition, RNA-Seq and Western blotting experiments were used to verify the status of downstream signaling pathways. Finally, bladder cancers with knockdown or overexpression of UCHL5 were treated with either SC79 or LY294002 to examine the participation of the AKT/mTOR signaling pathway and the expression of downstream targets c-Myc, SLC25A19, and ICAM5. In contrast to adjacent tissue samples, we discovered that UCHL5 was substantially expressed in bladder cancer samples. We also found that UCHL5 downregulation significantly suppressed both tumor growth in vivo and cell proliferation and migration in vitro. According to RNA-Seq analyses and Western blotting experiments, the expression of c-Myc, SLC25A19, and ICAM5 was modified as a result of UCHL5 activating AKT/mTOR signaling in bladder cancer cells. All things considered, our findings show that increased UCHL5 expression stimulates AKT/mTOR signaling, subsequently triggering the expression of c-Myc, SLC25A19, and ICAM5, which in turn promotes carcinogenesis in bladder cancer. UCHL5 is therefore a potential target for therapy in bladder cancer patients.

Structural and Functional Basis of JAMM Deubiquitinating Enzymes in Disease

Deubiquitinating enzymes (DUBs) are a group of proteases that are important for maintaining cell homeostasis by regulating the balance between ubiquitination and deubiquitination. As the only known metalloproteinase family of DUBs, JAB1/MPN/Mov34 metalloenzymes (JAMMs) are specifically associated with tumorigenesis and immunological and inflammatory diseases at multiple levels. The far smaller numbers and distinct catalytic mechanism of JAMMs render them attractive drug targets. Currently, several JAMM inhibitors have been successfully developed and have shown promising therapeutic efficacy. To gain greater insight into JAMMs, in this review, we focus on several key proteins in this family, including AMSH, AMSH-LP, BRCC36, Rpn11, and CSN5, and emphatically discuss their structural basis, diverse functions, catalytic mechanism, and current reported inhibitors targeting JAMMs. These advances set the stage for the exploitation of JAMMs as a target for the treatment of various diseases.

PSMD14 Targeting Triggers Paraptosis in Breast Cancer Cells by Inducing Proteasome Inhibition and Ca2+ Imbalance

PSMD14, a subunit of the 19S regulatory particles of the 26S proteasome, was recently identified as a potential prognostic marker and therapeutic target in diverse human cancers. Here, we show that the silencing and pharmacological blockade of PSMD14 in MDA-MB 435S breast cancer cells induce paraptosis, a non-apoptotic cell death mode characterized by extensive vacuolation derived from the endoplasmic reticulum (ER) and mitochondria. The PSMD14 inhibitor, capzimin (CZM), inhibits proteasome activity but differs from the 20S proteasome subunit-inhibiting bortezomib (Bz) in that it does not induce aggresome formation or Nrf1 upregulation, which underlie Bz resistance in cancer cells. In addition to proteasome inhibition, the release of Ca²⁺ from the ER into the cytosol critically contributes to CZM-induced paraptosis. Induction of paraptosis by targeting PSMD14 may provide an attractive therapeutic strategy against cancer cells resistant to proteasome inhibitors or pro-apoptotic drugs.

Deubiquitinating enzymes USP4 and USP17 finetune the trafficking of PDGFRβ and affect PDGF-BB-induced STAT3 signalling

Interaction of platelet-derived growth factor (PDGF) isoforms with their receptors results in activation and internalization of receptors, with a concomitant activation of downstream signalling pathways. Ubiquitination of PDGFRs serves as a mark to direct the internalization and sorting of the receptors. By overexpressing a panel of deubiquitinating enzymes (DUBs), we found that USP17 and USP4 efficiently deubiquitinate PDGF receptor β (PDGFRβ) and are able to remove both Lys63 and Lys48-linked polyubiquitin chains from the receptor. Deubiquitination of PDGFRβ did not affect its stability, but regulated the timing of its trafficking, whereby USP17 prolonged the presence of the receptor at the cell surface, while USP4 affected the speed of trafficking towards early endosomes. Induction of each of the DUBs in BJhTERT fibroblasts and U2OS osteosarcoma cells led to prolonged and/or shifted activation of STAT3 in response to PDGF-BB stimulation, which in turn led to increased transcriptional activity of STAT3. Induction of USP17 promoted acute upregulation of the mRNA expression of STAT3-inducible genes STAT3, CSF1, junB and c-myc, while causing long-term changes in the expression of myc and CDKN1A. Deletion of USP17 was lethal to fibroblasts, while deletion of USP4 led to a decreased proliferative response to stimulation by PDGF-BB. Thus, USP17- and USP4-mediated changes in ubiquitination of PDFGRβ lead to dysregulated signalling and transcription downstream of STAT3, resulting in defects in the control of cell proliferation.

Ubiquitin-proteasome system (UPS) as a target for anticancer treatment

The ubiquitin-proteasome system (UPS) plays an important role in the cellular processes for protein quality control and homeostasis. Dysregulation of the UPS has been implicated in numerous diseases, including cancer. Indeed, components of UPS are frequently mutated or abnormally expressed in various cancers. Since Bortezomib, a proteasome inhibitor, received FDA approval for the treatment of multiple myeloma and mantle cell lymphoma, increasing numbers of researchers have been seeking drugs targeting the UPS as a cancer therapeutic strategy. Here, we introduce the essential component of UPS, including ubiquitinating enzymes, deubiquitinating enzymes and 26S proteasome, and we summarize their targets and mechanisms that are crucial for tumorigenesis. In addition, we briefly discuss some UPS inhibitors, which are currently in clinical trials as cancer therapeutics.

Advances in Deubiquitinating Enzyme Inhibition and Applications in Cancer Therapeutics

Since the discovery of the ubiquitin proteasome system (UPS), the roles of ubiquitinating and deubiquitinating enzymes (DUBs) have been widely elucidated. The ubiquitination of proteins regulates many aspects of cellular functions such as protein degradation and localization, and also modifies protein-protein interactions. DUBs cleave the attached ubiquitin moieties from substrates and thereby reverse the process of ubiquitination. The dysregulation of these two paramount pathways has been implicated in numerous diseases, including cancer. Attempts are being made to identify inhibitors of ubiquitin E3 ligases and DUBs that potentially have clinical implications in cancer, making them an important target in the pharmaceutical industry. Therefore, studies in medicine are currently focused on the pharmacological disruption of DUB activity as a rationale to specifically target cancer-causing protein aberrations. Here, we briefly discuss the pathophysiological and physiological roles of DUBs in key cancer-related pathways. We also discuss the clinical applications of promising DUB inhibitors that may contribute to the development of DUBs as key therapeutic targets in the future.

The deubiquitinating enzyme PSMD14 facilitates tumor growth and chemoresistance through stabilizing the ALK2 receptor in the initiation of BMP6 signaling pathway

BACKGROUND

Although bone morphogenetic protein 6 (BMP6) signaling pathway has been implicated in many types of cancer, its role of tumorigenesis seems to be controversial and its ubiquitin-modifying mechanisms have not been fully addressed. Our study was designed to investigate how BMP6 signaling pathway is regulated by ubiquitin-modifying systems and to address molecular and clinical significance in colorectal cancers.

METHODS

Human deubiquitnase (DUB) siRNA library was used to screen the specific DUB, named PSMD14, involved in BMP6 signaling pathway. Immunoblot, immunoprecipitation and ubiquitination assays were used to analyze targets of the PSMD14. A role of PSMD14-mediated BMP6 signaling pathway for malignant cancer progression was investigated using in vitro and in vivo model of colorectal cancers as well as clinical samples of colorectal cancer patients.

FINDINGS

The deubiquitinase PSMD14 acts as a positive regulator for the initiation of the BMP6 signaling pathway through deubiquitinating K48-linked ALK2 type I receptor ubiquitination mediated by Smurf1 E3 ligase, resulting in increased stability of the ALK2. This role of PSMD14 is independent of its intrinsic role in the 26S proteasome system. Furthermore, either PSMD14 or ALK2 depletion significantly decreases tumorigenesis of HCT116 colorectal cancer cells in a xenograft model as well as cancer stemness/chemoresistance, and expression of the PSMD14 and ALK2 gene are correlated with malignant progression and the survival of colorectal cancer patients.

INTERPRETATION

These findings suggest that the PSMD14-ALK2 axis plays an essential role in initiation of the BMP6 signaling pathway and contributes to tumorigenesis and chemoresistance of colorectal cancers.

Targeting POH1 inhibits prostate cancer cell growth and enhances the suppressive efficacy of androgen deprivation and docetaxel

BACKGROUND

POH1, a member of the JAMM domain containing deubiquitinases, functions in malignant progression of certain types of cancer. However, the role of POH1 in prostate cancer (PCa) remains unclear.

METHODS

We performed RNA interference against the JAMM members in PC3 cells and analyzed cell proliferation. POH1 knockdown was established to evaluate the effects of POH1 on cell growth in vitro and in vivo. RNA-sequencing was utilized to explore the molecular details underlying the biological function of POH1 in PCa. The expression of POH1 in PCa tissues was detected by immunohistochemistry. The POH1 inhibitor capzimin was evaluated to explore whether pharmacologically inhibiting POH1 significantly affected PCa cell proliferation alone or enhanced the inhibitory efficacy of docetaxel and androgen deprivation.

RESULTS

Functional analyses identified POH1 as a JAMM deubiquitinase that is required for PCa proliferation. Importantly, expression of POH1 was higher in human PCa tissues (PCas) than that in normal prostate tissues, and a positive correlation was detected between elevated POH1 expression and higher pathological grades in PCas. In vivo experiments further demonstrated that depleting POH1 significantly suppressed the growth of PCa cell xenografts. POH1 deficiency profoundly inhibited the expression of a set of genes involving the cell cycle and caused G0/G1 phase arrest. Furthermore, the POH1 inhibitor capzimin phenotypically recapitulated the effects of POH1 knockdown and improved the efficacy of docetaxel and androgen deprivation in PCa cells.

CONCLUSIONS

POH1 was overexpressed in PCas and was correlated with pathological grades in human PCas. Inhibiting POH1 by gene silencing or pharmacological inhibition with capzimin suppressed PCa cell growth. Exploring the inhibition of POH1 in combination with other drugs may provide a strategy to benefit patients with PCa.

POH1 contributes to hyperactivation of TGF-β signaling and facilitates hepatocellular carcinoma metastasis through deubiquitinating TGF-β receptors and caveolin-1

Background

Hyper-activation of TGF-β signaling is critically involved in progression of hepatocellular carcinoma (HCC). However, the events that contribute to the dysregulation of TGF-β pathway in HCC, especially at the post-translational level, are not well understood.

Methods

Associations of deubiquitinase POH1 with TGF-β signaling activity and the outcomes of HCC patients were examined by data mining of online HCC datasets, immunohistochemistry analyses using human HCC specimens, spearman correlation and survival analyses. The effects of POH1 on the ubiquitination and stability of the TGF-β receptors (TGFBR1 and TGFBR2) and the activation of downstream effectors were tested by western blotting. Primary mouse liver tissues from polyinosinic:polycytidylic acid (poly I:C)- treated Mx-Cre+, poh1^f/f mice and control mice were used to detect the TGF-β receptors. The metastatic-related capabilities of HCC cells were studied in vitro and in mice.

Findings

Here we show that POH1 is a critical regulator of TGF-β signaling and promotes tumor metastasis. Integrative analyses of HCC subgroups classified with unsupervised transcriptome clustering of the TGF-β response, metastatic potential and outcomes, reveal that POH1 expression positively correlates with activities of TGF-β signaling in tumors and with malignant disease progression. Functionally, POH1 intensifies TGF-β signaling delivery and, as a consequence, promotes HCC cell metastatic properties both in vitro and in vivo. The expression of the TGF-β receptors was severely downregulated in POH1-deficient mouse hepatocytes. Mechanistically, POH1 deubiquitinates the TGF-β receptors and CAV1, therefore negatively regulates lysosome pathway-mediated turnover of TGF-β receptors.

Conclusion

Our study highlights the pathological significance of aberrantly expressed POH1 in TGF-β signaling hyperactivation and aggressive progression in HCC.

Dynasore-induced potent ubiquitylation of the exon 19 deletion mutant of epidermal growth factor receptor suppresses cell growth and migration in non-small cell lung cancer

Lung cancer is a leading cause of death worldwide, with mutations in EGFR frequently detected that render this receptor tyrosine kinase constantly active. Targeted therapy against EGFR has proved effective in lung cancer treatment, but secondary mutations in EGFR frequently cause drug resistance. In the efforts made to investigate alternative ways to inhibit mutant EGFR, we observed that the dynamin inhibitor dynasore effectively suppressed the exon 19-deleted mutant of EGFR. This agent inhibited cell proliferation, colony formation, cell migration, and cell cycle progression of HCC827 and H1650 cells driven by the exon 19-deleted EGFR mutant. From a mechanistic point of view, dynasore suppressed the activation of AKT and MEK in HCC827 and H1650 cells. However, dynasore failed to alter the subcellular distribution of EGFR, and another dynamin inhibitor, dyngo-4a, did not phenocopy the effects of dynasore, suggesting a dynamin activity-independent effect of dynasore. Finally, we show that dynasore induced the potent ubiquitylation of the exon 19-deleted mutant of EGFR. Our observations will shed light on the development of alternative therapeutic strategies that target mutant EGFR in lung cancer.

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

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

Ubiquitin carboxyl-terminal hydrolases: involvement in cancer progression and clinical implications

Protein ubiquitination and deubiquitination participate in a number of biological processes, including cell growth, differentiation, transcriptional regulation, and oncogenesis. Ubiquitin C-terminal hydrolases (UCHs), a subfamily of deubiquitinating enzymes (DUBs), includes four members: UCH-L1/PGP9.5 (protein gene product 9.5), UCH-L3, UCHL5/UCH37, and BRCA1-associated protein-1 (BAP1). Recently, more attention has been paid to the relationship between the UCH family and malignancies, which play different roles in the progression of different tumors. It remains controversial whether UCHL1 is a tumor promoter or suppressor. UCHL3 and UCH37 are considered to be tumor promoters, while BAP1 is considered to be a tumor suppressor. Studies have showed that UCH enzymes influence several signaling pathways that play crucial roles in oncogenesis, tumor invasion, and migration. In addition, UCH families are associated with tumor cell sensitivity to therapeutic modalities. Here, we reviewed the roles of UCH enzymes in the development of tumors, highlighting the potential consideration of UCH enzymes as new interesting targets for the development of anticancer drugs.

Deubiquitylating enzymes and drug discovery: emerging opportunities

More than a decade after a Nobel Prize was awarded for the discovery of the ubiquitin-proteasome system and clinical approval of proteasome and ubiquitin E3 ligase inhibitors, first-generation deubiquitylating enzyme (DUB) inhibitors are now approaching clinical trials. However, although our knowledge of the physiological and pathophysiological roles of DUBs has evolved tremendously, the clinical development of selective DUB inhibitors has been challenging. In this Review, we discuss these issues and highlight recent advances in our understanding of DUB enzymology and biology as well as technological improvements that have contributed to the current interest in DUBs as therapeutic targets in diseases ranging from oncology to neurodegeneration.

Meddling with Fate: The Proteasomal Deubiquitinating Enzymes

Three deubiquitinating enzymes-Rpn11, Usp14, and Uch37-are associated with the proteasome regulatory particle. These enzymes allow proteasomes to remove ubiquitin from substrates before they are translocated into the core particle to be degraded. Although the translocation channel is too narrow for folded proteins, the force of translocation unfolds them mechanically. As translocation proceeds, ubiquitin chains bound to substrate are drawn to the channel's entry port, where they can impede further translocation. Rpn11, situated over the port, can remove these chains without compromising degradation because substrates must be irreversibly committed to degradation before Rpn11 acts. This coupling between deubiquitination and substrate degradation is ensured by the Ins-1 loop of Rpn11, which controls ubiquitin access to its catalytic site. In contrast to Rpn11, Usp14 and Uch37 can rescue substrates from degradation by promoting substrate dissociation from the proteasome prior to the commitment step. Uch37 is unique in being a component of both the proteasome and a second multisubunit assembly, the INO80 complex. However, only recruitment into the proteasome activates Uch37. Recruitment to the proteasome likewise activates Usp14. However, the influence of Usp14 on the proteasome depends on the substrate, due to its marked preference for proteins that carry multiple ubiquitin chains. Usp14 exerts complex control over the proteasome, suppressing proteasome activity even when inactive in deubiquitination. A major challenge for the field will be to elucidate the specificities of Rpn11, Usp14, and Uch37 in greater depth, employing not only model in vitro substrates but also their endogenous targets.

RPN11 deubiquitinase promotes proliferation and migration of breast cancer cells

The deubiquitinase enzyme RPN11 is involved in oncogenesis in various types of cancer. However, in breast cancer, the expression levels, prognostic relevance and biological function of RPN11 remains unclear. In the present study, RPN11 expression levels in breast cancer tissues and adjacent non‑tumor tissues were determined by reverse transcription‑quantitative polymerase chain reaction and immunohistochemical staining, and the association of RPN11 with clinicopathological features of breast cancer was evaluated. RPN11 expression was upregulated in breast cancer tissues compared with healthy tissues. Additionally, high expression levels of RPN11 may be an indicator of poor prognosis, as validated by a breast cancer cohort from the Gene Expression Omnibus database. Knockdown of RPN11 in MDA‑MB‑231 and T47D cells significantly reduced cell proliferation and enhanced G0/G1 arrest and apoptosis. Exogenous overexpression of RPN11 in MCF7 and Hs578T cells promoted cell growth and inhibited apoptosis. In addition, knockdown of RPN11 abrogated cell migration and reduced epithelial‑mesenchymal transition. In conclusion, these findings suggested that RPN11 may function as an oncogene and its upregulation in breast cancer suggests that it may be a therapeutic target.

Neratinib induces ErbB2 ubiquitylation and endocytic degradation via HSP90 dissociation in breast cancer cells

Receptor tyrosine kinase ErbB2/HER2 is frequently observed to be overexpressed in human cancers, leading to over activation of downstream signaling modules. HER2 positive is a major type of breast cancer for which ErbB2 targeting is already proving to be an effective therapeutic strategy. Apart from antibodies against ErbB2, the small molecule tyrosine kinase inhibitor lapatinib has had successful clinical outcomes, and other inhibitors such as neratinib are currently undergoing clinical investigations. In this study we report the effects of lapatinib and neratinib on the mRNA and protein levels of the ErbB2 receptor. We provide evidence that neratinib-induced down regulation of ErbB2 occurs through ubiquitin-mediated endocytic sorting and lysosomal degradation. At the mechanistic level, neratinib treatment leads to HSP90 release from ErbB2 and its subsequent ubiquitylation and endocytic degradation. Our findings provide novel insights into the mechanism of ErbB2 inhibition by neratinib.

Deubiquitinases and cancer: A snapshot

Ubiquitination is the vital system for controlling protein degradation and regulation of basic cellular processes. Deubiquitinases (DUBs) are emerging as an important regulator of several pathways related to cancer and other diseases. Their ability to detach ubiquitin from the target substrate and regulation of signaling makes it potential target to treat cancer and other fatal diseases. In the current review, we are trying to summarize deubiquitination, and their role in cancer and potential small molecules DUBs inhibitors which can be used as drugs for cancer treatment.

Hsp70 exerts oncogenic activity in the Apc mutant Min mouse model

Colorectal cancer (CRC) develops from colonic epithelial cells that lose expression of key tumor suppressor genes and/or gain expression of proproliferative and antiapoptotic genes like heat shock protein 70 (Hsp70). Heat shock protein 70 is overexpressed in CRC, but it is not known whether this is in response to the proteotoxic stress induced by transformation, or if it contributes to the process of transformation itself. Here, using the Apc (Min/+) mouse model of CRC, we show that Hsp70 regulates mitogenic signaling in intestinal epithelial cells through stabilization of proteins involved in the receptor tyrosine kinase (RTK) and WNT signaling pathways. Loss of Hsp70 reduced tumor size with decreased proliferation and increased tumor cell death. Hsp70 loss also led to decreased expression of ErbB2, Akt, ERK and β-catenin along with decreased β-catenin transcriptional activity as measured by c-myc and axin2 expression. Upregulation of RTK or WNT signals are frequent oncogenic events in CRC and many other cancers. Thus, in addition to the role of Hsp70 in cell-survival after transformation, Hsp70 stabilization of β-catenin, Akt, ERK and ErbB2 are predicted to contribute to transformation. This has important implications not only for understanding the pathophysiology of these cancers, but also for treatment since anti-EGFR antibodies are in clinical use for CRC and EGFR is a major ErbB2 heterodimeric partner. Targeting Hsp70, therefore, might provide an alternative or complementary strategy for achieving better outcomes for CRC and other related cancer types.

Deubiquitinases in cancer

Deubiquitinases are deubiquitinating enzymes (DUBs), which remove ubiquitin from proteins, thus regulating their proteasomal degradation, localization and activity. Here, we discuss DUBs as anti-cancer drug targets.

POH1 deubiquitylates and stabilizes E2F1 to promote tumour formation

Hyperactivation of the transcriptional factor E2F1 occurs frequently in human cancers and contributes to malignant progression. E2F1 activity is regulated by proteolysis mediated by the ubiquitin–proteasome system. However, the deubiquitylase that controls E2F1 ubiquitylation and stability remains undefined. Here we demonstrate that the deubiquitylase POH1 stabilizes E2F1 protein through binding to and deubiquitylating E2F1. Conditional knockout of Poh1 alleles results in reduced E2F1 expression in primary mouse liver cells. The POH1-mediated regulation of E2F1 expression strengthens E2F1-downstream prosurvival signals, including upregulation of Survivin and FOXM1 protein levels, and efficiently facilitates tumour growth of liver cancer cells in nude mice. Importantly, human hepatocellular carcinomas (HCCs) recapitulate POH1 regulation of E2F1 expression, as nuclear abundance of POH1 is increased in HCCs and correlates with E2F1 overexpression and tumour growth. Thus, our study suggests that the hyperactivated POH1–E2F1 regulation may contribute to the development of liver cancer.

The transcription factor E2F1 controls the expression of multiple genes and is frequently overactivated in cancer. Here, the authors show that E2F1 is deubiquitinated by POH1 and that this enhances the role of E2F1 in cell survival, and contributes to the pathogenesis of liver cancer.

Molecular pathways: translational potential of deubiquitinases as drug targets

The ubiquitin proteasome system (UPS) is the main system for controlled protein degradation and a key regulator of fundamental cellular processes. The dependency of cancer cells on a functioning UPS coupled with the clinical success of bortezomib for the treatment of multiple myeloma have made the UPS an obvious target for drug development. Deubiquitinases (DUB) are components of the UPS that encompass a diverse family of ubiquitin isopeptidases that catalyze the removal of ubiquitin moieties from target proteins or from polyubiquitin chains, resulting in altered signaling or changes in protein stability. Increasing evidence has implicated deregulation of DUB activity in the initiation and progression of cancer. The altered pattern of DUB expression observed in many tumors can potentially serve as a clinical marker for predicting disease outcome and therapy response. The finding of DUB overexpression in tumor cells suggests that they may serve as novel targets for the development of anticancer therapies. Several specific and broad-spectrum DUB inhibitors are shown to have antitumor activity in preclinical in vivo models with low levels of systemic toxicity. Future studies will hopefully establish the clinical potential for DUB inhibitors as a strategy to treat cancer.

Deubiquitinating Enzymes as Novel Targets for Cancer Therapies

Most ubiquitinated proteins can be recognized and degraded by the 26S proteasome. In the meantime, protein deubiquitination by various deubiquitinating enzymes (DUBs) regulates protein stability within cells, and it can counterbalance intracellular homeostasis mediated by ubiquitination. Numerous reports have demonstrated that an aberrant process of the ubiquitin-proteasome pathway (UPP) regulated by the ubiquitination and deubiquitination systems results in failure of balancing between protein stability and degradation, and this failure can lead to tumorigenesis in various organs and tissues of mammals. The identification of molecular properties for various DUBs is very critical to understand cancer development and tumorigenesis. Therefore, knowledge of DUBs and their association with cancer and diseases is indispensible for developing effective inhibitors for DUBs. This chapter describes various features and functions of cancer-related DUBs. In addition, we summarize several inhibitors that specifically target certain DUBs in cancer and suggest that DUBs may be one of the most ideal and attractive therapeutic targets.

DNA damage emergency: cellular garbage disposal to the rescue?

The proteasome is a cellular machine found in the cytosol, nucleus and on chromatin that performs much of the proteolysis in eukaryotic cells. Recent reports show it is enriched at sites of double-stranded DNA breaks (DSBs) in mammalian cells. What is it doing there? This review will address three possibilities suggested by recent reports: in degrading proteins after their ubiquitination at and eviction from chromatin; as a deubiquitinase, specific to the antagonism of ubiquitin conjugates generated as part of the signalling of a DSB; and as a functional component of DNA repair mechanism itself. These findings add complexity to the proteasome as a potential therapeutic target in cancer treatment.

Epidermal growth factor cytoplasmic domain affects ErbB protein degradation by the lysosomal and ubiquitin-proteasome pathway in human cancer cells

The cytoplasmic domains of EGF-like ligands, including EGF cytoplasmic domain (EGFcyt), have important biological functions. Using specific constructs and peptides of human EGF cytoplasmic domain, we demonstrate that EGFcyt facilitates lysosomal and proteasomal protein degradation, and this coincided with growth inhibition of human thyroid and glioma carcinoma cells. EGFcyt and exon 22-23-encoded peptide (EGF22.23) enhanced procathepsin B (procathB) expression and procathB-mediated lysosomal degradation of EGFR/ErbB1 as determined by inhibitors for procathB and the lysosomal ATPase inhibitor BafA1. Presence of mbEGFctF, EGFcyt, EGF22.23, and exon 23-encoded peptides suppressed the expression of the deubiqitinating enzyme ubiquitin C-terminal hydrolase-L1 (UCH-L1). This coincided with hyperubiquitination of total cellular proteins and ErbB1/2 and reduced proteasome activity. Upon small interfering RNA-mediated silencing of endogenously expressed UCH-L1, a similar hyperubiquitinylation phenotype, reduced ErbB1/2 content, and attenuated growth was observed. The exon 23-encoded peptide region of EGFcyt was important for these biologic actions. Structural homology modeling of human EGFcyt showed that this molecular region formed an exposed surface loop. Peptides derived from this EGFcyt loop structure may aid in the design of novel peptide therapeutics aimed at inhibiting growth of cancer cells.

Expression and clinical significance of UCH37 in human esophageal squamous cell carcinoma

BACKGROUND

Ubiquitin carboxyl-terminal hydrolase 37 (UCH37), a member of the DUBs, was found to play an important role in oncogenesis through promoting some Proto-oncogenes' expression and stem cell-like characteristics in the cell in previous research. The aim of this study was to assess the value of UCH37 in predicting tumor recurrence after curative resection in esophageal squamous cell carcinoma (ESCC) patients.

METHODS

We analyzed UCH37 protein expression in 111 clinicopathologically characterized ESCC cases, from those who underwent curative resection between 2007 and 2008, by immunohistochemistry. The prognostic significance was assessed using Kaplan-Meier survival estimates and log-rank tests.

RESULTS

We found that UCH37 expression was higher in the cancer tissue than in non-tumorous control tissue at protein level and was overexpressed in tumor tissues of recurrent patients. There was a significant difference of UCH37 expression in patients categorized according to TNM stage (p = 0.038) and lymph nodes metastasis condition (p = 0.009). Univariate analyses revealed that UCH37 was a significant predictor for overall survival and disease-free survival, and multivariate analyses showed that UCH37 was an independent prognostic marker for ESCC recurrence. A prognostic significance of UCH37 was also found in the subgroup of lymph nodes metastasis condition classification. About 90 % of the recurrent patients recurred within 2 years, of which 84.4 % were predicted by UCH37.

CONCLUSION

UCH37 is associated with outcome and recurrence of ESCC and can be a novel predictor for poor prognosis of ESCC patients after curative resection.

Proteasome deubiquitinases as novel targets for cancer therapy

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

Recycling of EGFR and ErbB2 is associated with impaired Hrs tyrosine phosphorylation and decreased deubiquitination by AMSH

ErbB receptors play an important role in normal cellular growth, differentiation and development, but overexpression or poor downregulation can result in enhanced signaling and cancerous growth. ErbB signaling is terminated by clathrin-dependent receptor-mediated endocytosis, followed by incorporation in multi-vesicular bodies and subsequent degradation in lysosomes. In contrast to EGFR, ErbB2 displays poor ligand-induced downregulation and enhanced recycling, but the molecular mechanisms underlying this difference are poorly understood. Given our previous observation that both EGFR and an EGFR-ErbB2 chimera undergo Cbl-mediated K63-polyubiquitination, we investigated in the present study whether activation of the EGFR and the EGFR-ErbB2 chimera is associated with tyrosine phosphorylation of the ESCRT-0 complex subunit Hrs and AMSH-mediated deubiquitination. EGF stimulation of the EGFR resulted in efficient Hrs tyrosine phosphorylation and deubiquitination by the K63-polyubiquitin chain-specific deubiquitinating enzyme AMSH. In contrast, EGF activation of EGFR-ErbB2 showed significantly decreased Hrs tyrosine phosphorylation and deubiquitination by AMSH. To test whether this phenotype is the result of endosomal recycling, we induced recycling of the EGFR by stimulation with TGFα. Indeed, even though TGFα-stimulation of EGFR is associated with efficient ligand-stimulated K63-polyubiquitination, we observed that Hrs tyrosine phosphorylation as well as AMSH-mediated deubiquitination is significantly reduced under these conditions. Using various EGFR-ErbB2 chimeras, we demonstrate that enhanced recycling, decreased Hrs tyrosine phosphorylation and decreased AMSH mediated deubiquitination of EGFR-ErbB2 chimeras is primarily due to the presence of ErbB2 sequences or the absence of EGFR sequences C-terminal to the Cbl binding site. We conclude that endosomal recycling of the EGFR and ErbB2 receptors is associated with significantly impaired tyrosine phosphorylation of the ESCRT-0 subunit Hrs as well as decreased deubiquitination by AMSH, which is consistent with the finding that recycling receptors are not efficiently incorporated in the MVB pathway.

Deubiquitinases in cancer: new functions and therapeutic options

Deubiquitinases (DUBs) have fundamental roles in the ubiquitin system through their ability to specifically deconjugate ubiquitin from targeted proteins. The human genome encodes at least 98 DUBs, which can be grouped into 6 families, reflecting the need for specificity in their function. The activity of these enzymes affects the turnover rate, activation, recycling and localization of multiple proteins, which in turn is essential for cell homeostasis, protein stability and a wide range of signaling pathways. Consistent with this, altered DUB function has been related to several diseases, including cancer. Thus, multiple DUBs have been classified as oncogenes or tumor suppressors because of their regulatory functions on the activity of other proteins involved in tumor development. Therefore, recent studies have focused on pharmacological intervention on DUB activity as a rationale to search for novel anticancer drugs. This strategy may benefit from our current knowledge of the physiological regulatory mechanisms of these enzymes and the fact that growth of several tumors depends on the normal activity of certain DUBs. Further understanding of these processes may provide answers to multiple remaining questions on DUB functions and lead to the development of DUB-targeting strategies to expand the repertoire of molecular therapies against cancer.

Protease addiction and synthetic lethality in cancer

The “oncogene addiction” concept refers to the dependence of cancer cells on the function of the oncogenes responsible for their transformed phenotype, while the term “non-oncogene addiction” has been introduced to define the exacerbated necessity of the normal function of non-mutated genes. In this Perspective, we focus on the importance of proteolytic enzymes to maintain the viability of cancer cells and hypothesize that most, if not all, tumors present “addiction” to a number of proteolytic activities, which in turn may represent valuable targets of anti-cancer therapies, even without being mutated or over-expressed by the malignant cells.

Engineered bivalent ligands to bias ErbB receptor-mediated signaling and phenotypes

The ErbB receptor family is dysregulated in many cancers, and its therapeutic manipulation by targeted antibodies and kinase inhibitors has resulted in effective chemotherapies. However, many malignancies remain refractory to current interventions. We describe a new approach that directs ErbB receptor interactions, resulting in biased signaling and phenotypes. Due to known receptor-ligand affinities and the necessity of ErbB receptors to dimerize to signal, bivalent ligands, formed by the synthetic linkage of two neuregulin-1β (NRG) moieties, two epidermal growth factor (EGF) moieties, or an EGF and a NRG moiety, can potentially drive homotypic receptor interactions and diminish formation of HER2-containing heterodimers, which are implicated in many malignancies and are a prevalent outcome of stimulation by native, monovalent EGF, or NRG. We demonstrate the therapeutic potential of this approach by showing that bivalent NRG (NN) can bias signaling in HER3-expressing cancer cells, resulting in some cases in decreased migration, inhibited proliferation, and increased apoptosis, whereas native NRG stimulation increased the malignant potential of the same cells. Hence, this new approach may have therapeutic relevance in ovarian, breast, lung, and other cancers in which HER3 has been implicated.

An atlas of altered expression of deubiquitinating enzymes in human cancer

Background

Deubiquitinating enzymes (DUBs) are proteases that process ubiquitin (Ub) or ubiquitin-like gene products, remodel polyubiquitin(-like) chains on target proteins, and counteract protein ubiquitination exerted by E3 ubiquitin-ligases. A wealth of studies has established the relevance of DUBs to the control of physiological processes whose subversion is known to cause cellular transformation, including cell cycle progression, DNA repair, endocytosis and signal transduction. Altered expression of DUBs might, therefore, subvert both the proteolytic and signaling functions of the Ub system.

Methodology/Principal Findings

In this study, we report the first comprehensive screening of DUB dysregulation in human cancers by in situ hybridization on tissue microarrays (ISH-TMA). ISH-TMA has proven to be a reliable methodology to conduct this kind of study, particularly because it allows the precise identification of the cellular origin of the signals. Thus, signals associated with the tumor component can be distinguished from those associated with the tumor microenvironment. Specimens derived from various normal and malignant tumor tissues were analyzed, and the “normal” samples were derived, whenever possible, from the same patients from whom tumors were obtained. Of the ∼90 DUBs encoded by the human genome, 33 were found to be expressed in at least one of the analyzed tissues, of which 22 were altered in cancers. Selected DUBs were subjected to further validation, by analyzing their expression in large cohorts of tumor samples. This analysis unveiled significant correlations between DUB expression and relevant clinical and pathological parameters, which were in some cases indicative of aggressive disease.

Conclusions/Significance

The results presented here demonstrate that DUB dysregulation is a frequent event in cancer, and have implications for therapeutic approaches based on DUB inhibition.

The potential role of ubiquitin c-terminal hydrolases in oncogenesis

Deubiquitinating enzymes (DUBs), capable of removing ubiquitin (Ub) from protein substrates, are involved in numerous biological processes. The ubiquitin C-terminal hydrolases (UCHs) subfamily of DUBs consists of four members: UCH-L1, UCH-L3, UCH37 and BRCA1-associated protein-1 (BAP1). UCH-L1 possesses deubiquitinating activity and dimerization-dependent ubiquitin ligase activity, and functions as a mono-ubiquitin stabilizer; UCH-L3 does both deubiquitinating and deneddylating activity, except dimerization or ligase activity, and unlike UCH-L1, can interact with Lys48-linked Ub dimers to protect it from degradation and in the meanwhile to inhibit its hydrolase activity; UCH37 is responsible for the deubiquitinating activity in the 19S proteasome regulatory complex, and as indicated by the recent study, UCH37 is also associated with the human Ino80 chromatin-remodeling complex (hINO80) in the nucleus and can be activated via transient association of 19S regulatory particle- or proteasome-bound hRpn13 with hINO80; BAP1, binding to the wild-type BRCA1 RING finger domain, is regarded as a tumor suppressor, but for such suppressing activity, as demonstrated otherwise, both deubiquitinating activity and nucleus localization are required. There is growing evidence that UCH enzymes and human malignancies are closely correlated. Previous studies have shown that UCH enzymes play a crucial role in some signalings and cell-cycle regulation. In this review, we provided an insight into the relation between UCH enzymes and oncogenesis.

Emerging roles of deubiquitinases in cancer-associated pathways

Deubiquitinases (DUBs) are emerging as important regulators of many pathways germane to cancer. They may regulate the stability of key oncogenes, exemplified by USP28 stabilisation of c-Myc. Alternatively they can negatively regulate ubiquitin-dependent signalling cascades such as the NF-kappaB activation pathway. We review the current literature that associates DUBs with cancer and discuss their suitability as drug targets of the future.