Targeting Ubc9 for cancer therapy

Repression of the SUMO-conjugating enzyme UBC9 is associated with lowered double minutes and reduced tumor progression

Double minutes (DMs), extrachromosomal gene fragments found within certain tumors, have been noted to carry onco- and drug resistance genes contributing to tumor pathogenesis and progression. After screening for SUMO-related molecule expression within various tumor sample and cell line databases, we found that SUMO-conjugating enzyme UBC9 has been associated with genome instability and tumor cell DM counts, which was confirmed both in vitro and in vivo. Karyotyping determined DM counts post-UBC9 knockdown or SUMOylation inhibitor 2-D08, while RT-qPCR and Western blot were used to measure DM-carried gene expression in vitro. In vivo, fluorescence in situ hybridization (FISH) identified micronucleus (MN) expulsion. Western blot and immunofluorescence staining were then used to determine DNA damage extent, and a reporter plasmid system was constructed to detect changes in homologous recombination (HR) and non-homologous end joining (NHEJ) pathways. Our research has shown that UBC9 inhibition is able to attenuate DM formation and lower DM-carried gene expression, in turn reducing tumor growth and malignant phenotype, via MN efflux of DMs and lowering NHEJ activity to increase DNA damage. These findings thus reveal a relationship between heightened UBC9 activity, increased DM counts, and tumor progression, providing a potential approach for targeted therapies, via UBC9 inhibition.

N-MYC-interacting protein enhances type II interferon signaling by inhibiting STAT1 sumoylation

Signaling desensitization is key to limiting signal transduction duration and intensity. Signal transducer and activator of transcription 1 (STAT1) can mediate type II interferon (IFNγ)-induced immune responses, which are enhanced and inhibited by STAT1 phosphorylation and sumoylation, respectively. Here, we identified an N-MYC interacting protein, NMI, which can enhance STAT1 phosphorylation and STAT1-mediated IFNγ immune responses by binding and sequestering the E2 SUMO conjugation enzyme, UBC9, and blocking STAT1 sumoylation. NMI facilitates UBC9 nucleus-to-cytoplasm translocation in response to IFNγ, thereby inhibiting STAT1 sumoylation. STAT1 phosphorylation at Y701 and sumoylation at K703 are mutually exclusive modifications that regulate IFNγ-dependent transcriptional responses. NMI could not alter the phosphorylation level of sumoylation-deficient STAT1 after IFNγ treatment. Thus, IFNγ signaling is modulated by NMI through sequestration of UBC9 in the cytoplasm, leading to inhibition of STAT1 sumoylation. Hence, NMI functions as a switch for STAT1 activation/inactivation cycles by modulating an IFNγ-induced desensitization mechanism.

The relationship between CD4+ T cell glycolysis and their functions

CD4⁺ T cells are effector T cells (Teffs) produced by the differentiation of initial T cells in peripheral lymphoid tissue after being attacked by antigens, and have an indispensable role in the development and activation of B cells and CD8⁺ T cells to regulate and assist immunity. In this review, we provide a new perspective on the relationship between CD4⁺ T cell glycolysis and its function. We summarize the effects of changes in the glycolysis level of CD4⁺ T cells on their activation, differentiation, proliferation, and survival. In addition, we emphasize that regulation of the glycolysis level of CD4⁺ T cells changes their inflammatory phenotypes and function. The study of immune metabolism has received more attention recently, but more work is needed to answer many open questions.

Impact of posttranslational modifications in pancreatic carcinogenesis and treatments

Pancreatic cancer (PC) is a highly aggressive cancer, with a 9% 5-year survival rate and a high risk of recurrence. In part, this is because PC is composed of heterogeneous subgroups with different biological and functional characteristics and personalized anticancer treatments are required. Posttranslational modifications (PTMs) play an important role in modifying protein functions/roles and are required for the maintenance of cell viability and biological processes; thus, their dysregulation can lead to disease. Different types of PTMs increase the functional diversity of the proteome, which subsequently influences most aspects of normal cell biology or pathogenesis. This review primarily focuses on ubiquitination, SUMOylation, and NEDDylation, as well as the current understanding of their roles and molecular mechanisms in pancreatic carcinogenesis. Additionally, we briefly summarize studies and clinical trials on PC treatments to advance our knowledge of drugs available to target the ubiquitination, SUMOylation, and NEDDylation PTM types. Further investigation of PTMs could be a critical field of study in relation to PC, as they have been implicated in the initiation and progression of many other types of cancer.

An Update on the Role of Ubiquitination in Melanoma Development and Therapies

The ubiquitination system plays a critical role in regulation of large array of biological processes and its alteration has been involved in the pathogenesis of cancers, among them cutaneous melanoma, which is responsible for the most deaths from skin cancers. Over the last decades, targeted therapies and immunotherapies became the standard therapeutic strategies for advanced melanomas. However, despite these breakthroughs, the prognosis of metastatic melanoma patients remains unoptimistic, mainly due to intrinsic or acquired resistances. Many avenues of research have been investigated to find new therapeutic targets for improving patient outcomes. Because of the pleiotropic functions of ubiquitination, and because each step of ubiquitination is amenable to pharmacological targeting, much attention has been paid to the role of this process in melanoma development and resistance to therapies. In this review, we summarize the latest data on ubiquitination and discuss the possible impacts on melanoma treatments.

Effects of targeting sumoylation processes during latent and induced Epstein-Barr virus infections using the small molecule inhibitor ML-792

As the second leading cause of death in the United States, cancer has a considerable impact on society, and one cellular process that is commonly dysregulated in many cancers is the post-translational modification of proteins by the Small Ubiquitin-like Modifier (SUMO; sumoylation). We documented that sumoylation processes are up-regulated in lymphoma tissues in the presence of Latent Membrane Protein-1 (LMP1), the principal oncoprotein of Epstein-Barr virus (EBV). LMP1-mediated dysregulation of cellular sumoylation processes contributes to oncogenesis, modulates innate immune responses, and aids the maintenance of viral latency. Manipulation of protein sumoylation has been proposed for anti-cancer and anti-viral therapies; however, known inhibitors of sumoylation do not only target sumoylation processes. Recently, a specific and selective small-molecule inhibitor of sumoylation (ML-792) was identified; however, nothing is known about the effect of ML-792 on LMP1-mediated dysregulation of cellular sumoylation or the EBV life-cycle. We hypothesized that ML-792 modulates viral replication and the oncogenic potential of EBV LMP1 by inhibiting protein sumoylation. Results showed that ML-792 inhibited sumoylation processes in multiple EBV-positive B cell lines and EBV-positive nasopharyngeal carcinoma cell lines but not in their EBV-negative counterparts. Focusing on its effect on B cells, ML-792 inhibited B-cell growth and promoted cell death at very low doses. ML-792 also modulated LMP1-induced cell migration and cell adhesion, which suggests the abrogation of the oncogenic potential of LMP1. Finally, while higher concentrations of ML-792 were sufficient to induce low levels EBV spontaneous reactivation, they decreased the production of new infectious virus following an induced reactivation and the infection of new cells, suggesting that ML-792 has anti-viral potential. Together, these findings suggest that ML-792 may be a potential therapeutic drug to treat EBV-associated lymphoid malignancies by targeting oncogenesis and the EBV life-cycle.

An in vitro Förster resonance energy transfer-based high-throughput screening assay identifies inhibitors of SUMOylation E2 Ubc9

SUMOylation is one of the posttranslational modifications that mediate cellular activities such as transcription, DNA repair, and signal transduction and is involved in the cell cycle. However, only a limited number of small molecule inhibitors have been identified to study its role in cellular processes. Here, we report a Förster resonance energy transfer (FRET) high-throughput screening assay based on the interaction between E2 Ubc9 and E3 PIAS1. Of the 3200 compounds screened, 34 (1.1%) showed higher than 50% inhibition and 4 displayed dose-response inhibitory effects. By combining this method with a label-free surface plasmon resonance (SPR) assay, false positives were excluded leading to discovering WNN0605-F008 and WNN1062-D002 that bound to Ubc9 with K_D values of 1.93 ± 0.62 and 5.24 ± 3.73 μM, respectively. We examined the effect of the two compounds on SUMO2-mediated SUMOylation of RanGAP1, only WNN0605-F008 significantly inhibited RanGAP1 SUMOylation, whereas WNN1062-D002 did not show any inhibition. These compounds, with novel chemical scaffolds, may serve as the initial material for developing new SUMOylation inhibitors.

Identification of sumoylated targets in proliferating mouse spermatogonia and human testicular seminomas

Spermatogenesis is regulated by a complex network of posttranslation modifications. Sumoylation (a modification by small ubiquitin-like modifiers, or SUMO proteins) was identified as an important cellular event in different cell types. SUMO proteins are highly expressed in the testis, and their role during spermatogenesis has begun to be elucidated. Given the important role of sumoylation in the regulation of mitosis and cancer progression in other tissues, the aim of the current study was to identify the targets of SUMO in proliferating mouse spermatogonia and human seminoma tissues and to initially examine the level of sumoylation in relation to the proliferative activity of the tissues. Using freshly purified spermatogonia and C18-4 spermatogonia cell line, mass spectrometry analysis identified several SUMO targets implicated into the proliferation of spermatogonia (such as heat shock protein 60 [HSP60] and prohibitin). Tissue array and western blot approaches showed that SUMO expression is a prominent feature of human seminomas and that the proliferative activity of the tumor tissues was positively correlated with the level of SUMO expression. Downregulation of sumoylation with si-RNA was not sufficient to significantly affect the proliferation of C18-4 spermatogonia; however, SUMO overexpression increased the proliferation rate of the cells. These data suggest that cells are more sensitive to an elevated level of SUMO, and that this situation may lead to an upregulated cellular proliferation and, possibly, cancer. Mass spectrometry analysis identified around a hundred SUMO targets in seminoma samples. Notably, many of the identified proteins (such as proliferating cell nuclear antigen [PCNA], DNA topoisomerase 2-alpha [Top2A], prohibitin, 14-3-3 protein, and others) were implicated in oncogenic transformation and cancer progression.

SUMOylation of PUM2 promotes the vasculogenic mimicry of glioma cells via regulating CEBPD

Glioma is the most common form of primary central nervous malignant tumors. Vasculogenic mimicry (VM) is a blood supply channel that is different from endothelial blood vessels in glioma. VM is related to tumor invasion and metastasis. Therefore, it plays an important role to target therapy for glioma VM. Our experimental results showed abnormal expression of UBE2I, PUM2, CEBPD, and DSG2 in glioma cells. The Co-IP and Immunofluorescence staining were used to detect that PUM2 can be modified by SUMO2/3. The interaction between PUM2 and CEBPD mRNA was detected by the RIP assays. The interaction between transcription factor CEBPD and promoter region of DSG2 was detected by the ChIP assays and luciferase assays. The capacity for migration in glioma cells was observed by the laser holographic microscope. The capacity for invasion in glioma cells was detected by Transwell method. The VM in glioma cells was detected by three-dimensional cell culture method. The experimental results found that the upregulation of UBE2I in glioma tissues and cells promotes the SUMOylation of PUM2, which decreases not only the stability of PUM2 protein but also decreases the inhibitory effect of PUM2 on CEBPD mRNA. The upregulation of CEBPD promotes the binding to the upstream promoter region of DSG2 gene, further upregulates the expression of DSG2, and finally promotes the development of glioma VM. In conclusion, this study found that the UBE2I/PUM2/CEBPD/DSG2 played crucial roles in regulating glioma VM. It also provides potential targets and alternative strategies for combined treatment of glioma.

Expression of SUMO enzymes is fiber type dependent in skeletal muscles and is dysregulated in muscle disuse

SUMOylation is a dynamic, reversible, enzymatic drug-targetable post-translational modification (PTM) reaction where the Small Ubiquitin-like Modifier (SUMO) moieties are attached to proteins. This reaction regulates various biological functions like cell growth, differentiation, and it is crucial for maintaining organ homeostasis. However, the actions of SUMO in skeletal muscle pathophysiology are still not investigated. In this study, we quantified the abundance of the SUMO enzymes and determined the distribution of SUMOylated proteins along the fibers of nine different muscles. We find that skeletal muscles contain a distinctive group of SUMO enzymes and SUMOylated proteins in relation to their different metabolism, functions, and fiber type composition. In addition, before the activation of protein degradation pathways, this unique set is quickly altered in response to muscle sedentariness. Finally, we demonstrated that PAX6 acts as an upstream regulator of the SUMO conjugation reaction, which can become a potential therapeutic marker to prevent muscle diseases generated by inactivity.

The Epstein-Barr Virus Oncoprotein, LMP1, Regulates the Function of SENP2, a SUMO-protease

Epstein-Barr virus (EBV) latent membrane protein-1 (LMP1) activates numerous signal transduction pathways using its C-terminal activating regions. We reported that LMP1 increased global levels of sumoylated proteins, which aided the oncogenic nature of LMP1. Because increased protein sumoylation is detected in numerous cancers, we wanted to elucidate additional mechanisms by which LMP1 modulates the sumoylation machinery. Results indicated that SUMO-protease activity decreased in a LMP1-dependent manner, so we hypothesized that LMP1 inhibits SUMO-protease activity, resulting in reduced de-sumoylation of cellular proteins, which contributes to the detected accumulation of sumoylated proteins in EBV-positive lymphomas. Focusing on SENP2, findings revealed that LMP1 expression corresponded with increased sumoylation of SENP2 at K48 and K447 in a CTAR-dependent manner. Interestingly, independent of LMP1-induced sumoylation of SENP2, LMP1 also decreased SENP2 activity, decreased SENP2 turnover, and altered the localization of SENP2, which led us to investigate if LMP1 regulated the biology of SENP2 by a different post-translational modification, specifically ubiquitination. Data showed that expression of LMP1 inhibited the ubiquitination of SENP2, and inhibition of ubiquitination was sufficient to mimic LMP1-induced changes in SENP2 activity and trafficking. Together, these findings suggest that LMP1 modulates different post-translational modifications of SENP2 in order to modulate its biology and identify a third member of the sumoylation machinery that is manipulated by LMP1 during latent EBV infections, which can affect oncogenesis.

Using glycyrrhizic acid to target sumoylation processes during Epstein-Barr virus latency

Cellular sumoylation processes are proposed targets for anti-viral and anti-cancer therapies. We reported that Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) dysregulates cellular sumoylation processes, contributing to its oncogenic potential in EBV-associated malignancies. Ginkgolic acid and anacardic acid, known inhibitors of sumoylation, inhibit LMP1-induced protein sumoylation; however, both drugs have adverse effects in hosts. Here we test the effects of glycyrrhizic acid, a medicinal botanical extract with anti-inflammatory, anti-carcinogenic, and anti-viral properties, on cellular sumoylation processes. While glycyrrhizic acid is known to inhibit EBV penetration, its affect on cellular sumoylation processes remains to be documented. We hypothesized that glycyrrhizic acid inhibits cellular sumoylation processes and may be a viable treatment for Epstein-Barr virus-associated malignancies. Results showed that glycyrrhizic acid inhibited sumoylation processes (without affecting ubiquitination processes), limited cell growth, and induced apoptosis in multiple cell lines. Similar to ginkgolic acid; glycyrrhizic acid targeted the first step of the sumoylation process and resulted in low levels of spontaneous EBV reactivation. Glycyrrhizic acid did not affect induced reactivation of the virus, but the presence of the extract did reduce the ability of the produced virus to infect additional cells. Therefore, we propose that glycyrrhizic acid may be a potential therapeutic drug to augment the treatment of EBV-associated lymphoid malignancies.

Epstein-Barr Virus Latent Membrane Protein-1 Induces the Expression of SUMO-1 and SUMO-2/3 in LMP1-positive Lymphomas and Cells

Epstein-Barr Virus latent membrane protein-1 (LMP1) interacts with the SUMO-conjugating enzyme Ubc9, which induces protein sumoylation and may contribute to LMP1-mediated oncogenesis. After analyzing human lymphoma tissues and EBV-positive cell lines, we now document a strong correlation between LMP1 and sumo-1/2/3 or SUMO-1/2/3 levels, and show that LMP1-induced sumo expression requires the activation of NF-κB signaling through CTAR1 and CTAR2. Together, these results point to a second mechanism by which LMP1 dysregulates sumoylation processes and adds EBV-associated lymphomas to the list of malignancies associated with increased SUMO expression.

Molecular mechanism of a covalent allosteric inhibitor of SUMO E1 activating enzyme

E1 enzymes activate ubiquitin (Ub) and ubiquitin-like modifiers (Ubls) in the first step of Ub/Ubl conjugation cascades and represent potential targets for therapeutic intervention in cancer and other life-threatening diseases. Here, we report the crystal structure of the E1 enzyme for the Ubl SUMO in complex with a recently discovered and highly specific covalent allosteric inhibitor (COH000). The structure reveals that COH000 targets a cryptic pocket distinct from the active site that is completely buried in all previous SUMO E1 structures and that COH000 binding to SUMO E1 is accompanied by a network of structural changes that altogether lock the enzyme in a previously unobserved inactive conformation. These structural changes include disassembly of the active site and a 180° rotation of the catalytic cysteine-containing SCCH domain, relative to conformational snapshots of SUMO E1 poised to catalyze adenylation. Altogether, our study provides a molecular basis for the inhibitory mechanism of COH000 and its SUMO E1 specificity, and also establishes a framework for potential development of molecules targeting E1 enzymes for other Ubls at a cryptic allosteric site.

Viral manipulation of the cellular sumoylation machinery

Viruses exploit various cellular processes for their own benefit, including counteracting anti-viral responses and regulating viral replication and propagation. In the past 20 years, protein sumoylation has emerged as an important post-translational modification that is manipulated by viruses to modulate anti-viral responses, viral replication, and viral pathogenesis. The process of sumoylation is a multi-step cascade where a small ubiquitin-like modifier (SUMO) is covalently attached to a conserved ΨKxD/E motif within a target protein, altering the function of the modified protein. Here we review how viruses manipulate the cellular machinery at each step of the sumoylation process to favor viral survival and pathogenesis.

Macrophilone A: Structure Elucidation, Total Synthesis, and Functional Evaluation of a Biologically Active Iminoquinone from the Marine Hydroid Macrorhynchia philippina

A previously uncharacterized pyrroloiminoquinone natural product, macrophilone A, was isolated from the stinging hydroid Macrorhynchia philippina. The structure was assigned utilizing long-range NMR couplings and DFT calculations and proved by a concise, five-step total synthesis. Macrophilone A and a synthetic analogue displayed potent biological activity, including increased intracellular reactive oxygen species levels and submicromolar cytotoxicity toward lung adenocarcinoma cells.

Autophagy regulates UBC9 levels during viral-mediated tumorigenesis

UBC9, the sole E2-conjugating enzyme required for SUMOylation, is a key regulator of essential cellular functions and, as such, is frequently altered in cancers. Along these lines, we recently reported that its expression gradually increases during early stages of human papillomavirus (HPV)-mediated cervical lesions transformation. However, a better understanding of how UBC9 is exploited by transforming viral oncoproteins is still needed. In the present study, we show that in human samples HPV drives UBC9 up-regulation also in very early steps of head and neck tumorigenesis, pointing to the important role for UBC9 in the HPV-mediated carcinogenic program. Moreover, using HPV-infected pre-cancerous tissues and primary human keratinocytes as the natural host of the virus, we investigate the pathological meaning and the cellular mechanisms responsible for UBC9 de-regulation in an oncoviral context. Our results show that UBC9 overexpression is promoted by transforming viral proteins to increase host cells' resistance to apoptosis. In addition, ultrastuctural, pharmacological and genetic approaches crucially unveil that UBC9 is physiologically targeted by autophagy in human cells. However, the presence of HPV E6/E7 oncoproteins negatively impacts the autophagic process through selective inhibition of autophagosome-lysosome fusion, finally leading to p53 dependent UBC9 accumulation during viral-induced cellular transformation. Therefore, our study elucidates how UBC9 is manipulated by HPV oncoproteins, details the physiological mechanism by which UBC9 is degraded in cells, and identifies how HPV E6/E7 impact on autophagy. These findings point to UBC9 and autophagy as novel hallmarks of HPV oncogenesis, and open innovative avenues towards the treatment of HPV-related malignancies.

SUMO conjugation - a mechanistic view

The regulation of protein fate by modification with the small ubiquitin-related modifier (SUMO) plays an essential and crucial role in most cellular pathways. Sumoylation is highly dynamic due to the opposing activities of SUMO conjugation and SUMO deconjugation. SUMO conjugation is performed by the hierarchical action of E1, E2 and E3 enzymes, while its deconjugation involves SUMO-specific proteases. In this review, we summarize and compare the mechanistic principles of how SUMO gets conjugated to its substrate. We focus on the interplay of the E1, E2 and E3 enzymes and discuss how specificity could be achieved given the limited number of conjugating enzymes and the thousands of substrates.

SUMO pathway components as possible cancer biomarkers

SUMOylation is a key post-translational modification that regulates crucial cellular functions and pathological processes. Recently, Small Ubiquitin-related MOdifier (SUMO) modification has emerged as a fundamental route that may drive different steps of human tumorigenesis. Indeed, alteration in expression or activity of one of the different SUMO pathway components may completely subvert cellular properties through fine-tuning modulation of protein(s) involved in carcinogenic pathways, leading to altered cell proliferation, apoptosis resistance and metastatic potential. Here we describe some of the most interesting findings pointing to a clear link between SUMO pathway and human malignancies. Importantly, a putative role for SUMO enzymes to predict cancer behavior can be speculated, and thus the possible application of alterations in SUMO pathway components as tumor biomarkers is discussed.

LMP1-Induced Sumoylation Influences the Maintenance of Epstein-Barr Virus Latency through KAP1

As a herpesvirus, Epstein-Barr virus (EBV) establishes a latent infection that can periodically undergo reactivation, resulting in lytic replication and the production of new infectious virus. Latent membrane protein-1 (LMP1), the principal viral oncoprotein, is a latency-associated protein implicated in regulating viral reactivation and the maintenance of latency. We recently found that LMP1 hijacks the SUMO-conjugating enzyme Ubc9 via its C-terminal activating region-3 (CTAR3) and induces the sumoylation of cellular proteins. Because protein sumoylation can promote transcriptional repression, we hypothesized that LMP1-induced protein sumoylation induces the repression of EBV lytic promoters and helps maintain the viral genome in its latent state. We now show that with inhibition of LMP1-induced protein sumoylation, the latent state becomes less stable or leakier in EBV-transformed lymphoblastoid cell lines. The cells are also more sensitive to viral reactivation induced by irradiation, which results in the increased production and release of infectious virus, as well as increased susceptibility to ganciclovir treatment. We have identified a target of LMP1-mediated sumoylation that contributes to the maintenance of latency in this context: KRAB-associated protein-1 (KAP1). LMP1 CTAR3-mediated sumoylation regulates the function of KAP1. KAP1 also binds to EBV OriLyt and immediate early promoters in a CTAR3-dependent manner, and inhibition of sumoylation processes abrogates the binding of KAP1 to these promoters. These data provide an additional line of evidence that supports our findings that CTAR3 is a distinct functioning regulatory region of LMP1 and confirm that LMP1-induced sumoylation may help stabilize the maintenance of EBV latency.

IMPORTANCE

Epstein-Barr virus (EBV) latent membrane protein-1 (LMP1) plays an important role in the maintenance of viral latency. Previously, we documented that LMP1 targets cellular proteins to be modified by a ubiquitin-like protein (SUMO). We have now identified a function for this LMP1-induced modification of cellular proteins in the maintenance of EBV latency. Because latently infected cells have to undergo viral reactivation in order to be vulnerable to antiviral drugs, these findings identify a new way to increase the rate of EBV reactivation, which increases cell susceptibility to antiviral therapies.

Characterization of amino acid residues within the N-terminal region of Ubc9 that play a role in Ubc9 nuclear localization

As the sole E2 enzyme for SUMOylation, Ubc9 is predominantly nuclear. However, the underlying mechanisms of Ubc9 nuclear localization are still not well understood. Here we show that RNAi-depletion of Imp13, an importin known to mediate Ubc9 nuclear import, reduces both Ubc9 nuclear accumulation and global SUMOylation. Furthermore, Ubc9-R13A or Ubc9-H20D mutation previously shown to interrupt the interaction of Ubc9 with nucleus-enriched SUMOs reduces the nuclear enrichment of Ubc9, suggesting that the interaction of Ubc9 with the nuclear SUMOs may enhance Ubc9 nuclear retention. Moreover, Ubc9-R17E mutation, which is known to disrupt the interaction of Ubc9 with both SUMOs and Imp13, causes a greater decrease in Ubc9 nuclear accumulation than Ubc9-R13A or Ubc9-H20D mutation. Lastly, Ubc9-K74A/S89D mutations that perturb the interaction of Ubc9 with nucleus-enriched SUMOylation-consensus motifs has no effect on Ubc9 nuclear localization. Altogether, our results have elucidated that the amino acid residues within the N-terminal region of Ubc9 play a pivotal role in regulation of Ubc9 nuclear localization.

Sumo E2 enzyme UBC9 is required for efficient protein quality control in cardiomyocytes

RATIONALE

Impairment of proteasomal function is pathogenic in several cardiac proteinopathies and can eventually lead to heart failure. Loss of proteasomal activity often results in the accumulation of large protein aggregates. The ubiquitin proteasome system (UPS) is primarily responsible for cellular protein degradation, and although the role of ubiquitination in this process is well studied, the function of an ancillary post-translational modification, SUMOylation, in protein quality control is not fully understood.

OBJECTIVE

To determine the role of ubiquitin-conjugating enzyme 9 (UBC9), a small ubiquitin-like modifier-conjugating enzyme, in cardiomyocyte protein quality control.

METHODS AND RESULTS

Gain- and loss-of-function approaches were used to determine the importance of UBC9. Overexpression of UBC9 enhanced UPS function in cardiomyocytes, whereas knockdown of UBC9 by small interfering RNA caused significant accumulations of aggregated protein. UPS function and relative activity was analyzed using a UPS reporter protein consisting of a short degron, CL1, fused to the COOH-terminus of green fluorescent protein (GFPu). Subsequently, the effects of UBC9 on UPS function were tested in a proteotoxic model of desmin-related cardiomyopathy, caused by cardiomyocyte-specific expression of a mutated αB crystallin, CryAB(R120G). CryAB(R120G) expression leads to aggregate formation and decreased proteasomal function. Coinfection of UBC9-adenovirus with CryAB(R120G) virus reduced the proteotoxic sequelae, decreasing overall aggregate concentrations. Conversely, knockdown of UBC9 significantly decreased UPS function in the model and resulted in increased aggregate levels.

CONCLUSIONS

UBC9 plays a significant role in cardiomyocyte protein quality control, and its activity can be exploited to reduce toxic levels of misfolded or aggregated proteins in cardiomyopathy.

Sumoylation of human argonaute 2 at lysine-402 regulates its stability

Gene silencing by small RNAs has emerged as a powerful post-transcriptional regulator of gene expression, however processes underlying regulation of the small RNA pathway in vivo are still largely elusive. Here, we identified sumoylation as a novel post-translational modification acting on Ago2, the main effector of small RNA-mediated gene silencing. We demonstrate that Ago2 can be modified by SUMO1 and SUMO2/3 and identified Lys402 as the major Ago2 sumoylation site in vivo. Ago2 physically interacts with the SUMO E2 conjugating enzyme Ubc9 and the E3 ligase RanBP2 facilitates Ago2 sumoylation in vitro. Mutation of Lys402 enhances the stability of Ago2 protein and impairment of cellular sumoylation by siRNA- or shRNA-mediated extinction of Ubc9 or in Ubc9 knockout mouse tissues results in increased steady-state levels and enhanced stability of Ago2. Similarly, knockdown of RanBP2 or of the SAE2 E1 enzyme enhances Ago2 protein levels. Lys402 is located in the L2g1 loop linking the PAZ and PIWI domains of Ago2, in the immediate vicinity of Tyr393 which can be phosphorylated, implying that the L2g1 linker represents an easily accessible hot spot for post-translational modifications. Altogether, our results show that sumoylation of Ago2 at Lys402 negatively regulates its stability, thereby establishing a first link between SUMO and the small RNA machinery.

SUMO Ubc9 enzyme as a viral target

Viruses alter specific host cell targets to counteract possible defense mechanisms aimed at eliminating infectivity and viral propagation. The SUMO conjugating enzyme Ubc9 functions as a hub for protein sumoylation, whilst also providing an interactive surface for sumoylated proteins through noncovalent interactions. The targeting of Ubc9 by viruses and viral proteins is thus highly beneficial for the disruption of both protein modification and protein-protein interaction mechanisms with which proteins increase their functional repertoire in cells. This review explores some of the clever mechanisms adopted by viruses to deregulate Ubc9, influence effector pathways and positively impact viral persistence consequently.

Spectomycin B1 as a novel SUMOylation inhibitor that directly binds to SUMO E2

Conjugation of small ubiquitin-like modifier (SUMO) to protein (SUMOylation) regulates multiple biological systems by changing the functions and fates of a large number of proteins. Consequently, abnormalities in SUMOylation have been linked to multiple diseases, including breast cancer. Using an in situ cell-based screening system, we have identified spectomycin B1 and related natural products as novel SUMOylation inhibitors. Unlike known SUMOylation inhibitors such as ginkgolic acid, spectomycin B1 directly binds to E2 (Ubc9) and selectively blocks the formation of the E2-SUMO intermediate; that is, Ubc9 is the direct target of spectomycin B1. Importantly, either spectomycin B1 treatment or Ubc9 knockdown inhibited estrogen-dependent proliferation of MCF7 human breast-cancer cells. Our findings suggest that Ubc9 inhibitors such as spectomycin B1 have potential as therapeutic agents against hormone-dependent breast cancers.

An electrophoretic mobility shift assay identifies a mechanistically unique inhibitor of protein sumoylation

The dynamic, posttranslational modification of proteins with a small ubiquitin-like modifier (SUMO) tag has been recognized as an important cellular regulatory mechanism relevant to a number of cancers as well as normal embryonic development. As part of a program aimed toward the identification of inhibitors of SUMO-conjugating enzymes, we developed a microfluidic electrophoretic mobility shift assay to monitor sumoylation events in real time. We disclose herein the use of this assay to identify a cell-permeable compound capable of blocking the transfer of SUMO-1 from the E2 enzyme Ubc9 to the substrate. We screened a small collection of compounds and identified an oxygenated flavonoid derivative that inhibits sumoylation in vitro. Next, we carried out an in-depth mechanistic analysis that ruled out many common false-positive mechanisms such as aggregation or alkylation. Furthermore, we report that this flavonoid inhibits a single step in the sumoylation cascade: the transfer of SUMO from the E2 enzyme (Ubc9) thioester conjugate to the substrate. In addition to having a unique mechanism of action, this inhibitor has a discrete structure-activity relationship uncharacteristic of a promiscuous inhibitor. Cell-based studies showed that the flavonoid inhibits the sumoylation of topoisomerase-I in response to camptothecin treatment in two different breast cancer cell lines, while isomeric analogs are inactive. Importantly, this compound blocks sumoylation while not affecting ubiquitylation in cells. This work identifies a point of entry for pharmacologic inhibition of the sumoylation cascade and may serve as the basis for continued study of additional pharmacophores that modulate SUMO-conjugating enzymes such as Ubc9.

SUMO rules: regulatory concepts and their implication in neurologic functions

Posttranslational modification of proteins by the small ubiquitin-like modifier (SUMO) is a potent regulator of various cellular events. Hundreds of substrates have been identified, many of them involved in vital processes like transcriptional regulation, signal transduction, protein degradation, cell cycle regulation, DNA repair, chromatin organization, and nuclear transport. In recent years, protein sumoylation increasingly attracted attention, as it could be linked to heart failure, cancer, and neurodegeneration. However, underlying mechanisms involving how modification by SUMO contributes to disease development are still scarce thus necessitating further research. This review aims to critically discuss currently available concepts of the SUMO pathway, thereby highlighting regulation in the healthy versus diseased organism, focusing on neurologic aspects. Better understanding of differential regulation in health and disease may finally allow to uncover pathogenic mechanisms and contribute to the development of disease-specific therapies.

Identification of interconnected markers for T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a complex disease, resulting from proliferation of differentially arrested immature T cells. The molecular mechanisms and the genes involved in the generation of T-ALL remain largely undefined. In this study, we propose a set of genes to differentiate individuals with T-ALL from the nonleukemia/healthy ones and genes that are not differential themselves but interconnected with highly differentially expressed ones. We provide new suggestions for pathways involved in the cause of T-ALL and show that network-based classification techniques produce fewer genes with more meaningful and successful results than expression-based approaches. We have identified 19 significant subnetworks, containing 102 genes. The classification/prediction accuracies of subnetworks are considerably high, as high as 98%. Subnetworks contain 6 nondifferentially expressed genes, which could potentially participate in pathogenesis of T-ALL. Although these genes are not differential, they may serve as biomarkers if their loss/gain of function contributes to generation of T-ALL via SNPs. We conclude that transcription factors, zinc-ion-binding proteins, and tyrosine kinases are the important protein families to trigger T-ALL. These potential disease-causing genes in our subnetworks may serve as biomarkers, alternative to the traditional ones used for the diagnosis of T-ALL, and help understand the pathogenesis of the disease.

Starting and stopping SUMOylation. What regulates the regulator?

A large number of proteins are modified post-translationally by the ubiquitin-like protein (Ubl) SUMO. This process, known as sumoylation, regulates the function, localisation and activity of target proteins as part of normal cellular metabolism, e.g., during development, and through the cell cycle, as well as in response to a range of stresses. In order to be effective, the sumoylation pathway itself must also be regulated. This review describes how the SUMOylation process is regulated. In particular, regulation of the SUMO conjugation and deconjugation machinery at the level of transcription and by post-translational modifications is discussed.

It takes two to tango: Ubiquitin and SUMO in the DNA damage response

The complexity of living cells is primarily determined by the genetic information encoded in DNA and gets fully disclosed upon translation. A major determinant of complexity is the reversible post-translational modification (PTM) of proteins, which generates variants displaying distinct biological properties such as subcellular localization, enzymatic activity and the ability to assemble in complexes. Decades of work on phosphorylation have unambiguously proven this concept. In recent years, the covalent attachment of Ubiquitin or Small Ubiquitin-like Modifiers (SUMO) to amino acid residues of target proteins has been recognized as another crucial PTM, re-directing protein fate and protein-protein interactions. This review focuses on the role of ubiquitylation and sumoylation in the control of DNA damage response proteins. To lay the ground, we begin with a description of ubiquitylation and sumoylation, providing established examples of DNA damage response elements that are controlled through these PTMs. We then examine in detail the role of PTMs in the cellular response to DNA double-strand breaks illustrating hierarchy, cross-talk, synergism or antagonism between phosphorylation, ubiquitylation and sumoylation. We conclude offering a perspective on Ubiquitin and SUMO pathways as targets in cancer therapy.

Ubiquitin-conjugating enzyme 9 promotes epithelial ovarian cancer cell proliferation in vitro

Epithelial ovarian cancer (EOC) is one of the leading causes of cancer deaths in women worldwide. Ubiquitin-conjugating enzyme 9 (Ubc9), the sole conjugating enzyme for sumoylation, regulates protein function and plays an important role in sumoylation-mediated cellular pathways. Although sumoylation plays a key role in DNA repair and tumorgenesis, whether Ubc9 is involved in EOC progression remains unknown. In the present study, we constructed Ubc-9 expressed recombined plasmid pEGFP-N1-Ubc9. The mRNA levels of Ubc9 were confirmed in human ovarian cell lines before and after transfection with pEGFP-N1-Ubc9 or small interfering RNA (siRNA) targeted Ubc9 by real-time polymerase chain reaction (PCR). The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was used to observe the effect of Ubc9 on cell proliferation. The protein levels of Ubc9, and proliferation-related signals Akt and physphorylated Akt were determined by Western blot. Our results showed that proliferation of EOC cells increased significantly in Ubc9 overexpressing cells, but decreased in Ubc9 knockdown cells. The physphorylation of Akt showed similar trends. In addition, the inhibitor of PI3K/Akt signaling pathway, LY294002, dramatically inhibited the growth of Ubc9 overexpressing cells. Therefore, Ubc9 gene plays an important role in cell proliferation in EOC through PI3K/Akt signaling pathway.

Exploring a new frontier in cancer treatment: targeting the ubiquitin and ubiquitin-like activating enzymes

The labeling of proteins with small ubiquitin (Ub) and ubiquitin-like (Ubl) modifiers regulates a plethora of activities within the cell, such as protein recycling, cell cycle modifications, and protein translocation. These processes are often overactive in diseased cells, leading to unregulated cell growth and disease progression. Therefore, in systems where Ub/Ubl protein labeling is dysregulated, the development of drugs to selectively and potently disrupt Ub/Ubl protein labeling offers a targeted molecular approach for sensitizing these diseased cells. This Perspective outlines the progress that has been made in the context of inhibitor development for targeting Ub/Ubl pathways.

Ubc9 expression predicts chemoresistance in breast cancer

Ubiquitin-conjugating enzyme 9 (Ubc9), the sole conjugating enzyme for sumoylation, regulates protein function and plays an important role in tumorigenesis. Whether Ubc9 is involved in the chemoresistance of breast cancer remains unknown. In this study, we aimed to evaluate the contribution of Ubc9 in the chemoresistance of breast cancer. Immunohistochemistry (IHC) was used to examine the expression level of Ubc9. Chi-square test, Wilcoxon test, and one-way ANOVA were applied to analyze the relationship between Ubc9 expression, clinicopathologic features, and clinical response to neoadjuvant chemotherapy. The significance of variables for survival was analyzed by the Cox proportional hazards model in a multivariate analysis. Kaplan-Meier survival curves were plotted and log-rank test was performed. The proportion of Ubc9-positive cells was higher in invasive ductal carcinoma than in normal breast tissues [(48.48 ± 17.94)% vs. (5.82 ± 2.80)%, P < 0.001]. High Ubc9 expression was associated with poor differentiation (Χ² = 6.538, P = 0.038), larger tumor size (Χ² = 4.701, P = 0.030), advanced clinical stage (Χ² = 4.651, P = 0.031), lymph node metastasis (Χ² = 9.913, P = 0.010), basal-like phenotype (Χ² = 8.660, P = 0.034), and poor clinical response to neoadjuvant chemotherapy (Χ² = 11.09, P = 0.001). The expected 6-year cumulative disease-free survival rate was 87.32% in patients with low Ubc9 expression compared to 68.78% in those with high Ubc9 expression (Χ² = 4.289, P = 0.038). These data indicate that high Ubc9 expression correlates with poor response to chemotherapy and poor clinical prognosis.

Modification by ubiquitin-like proteins: significance in apoptosis and autophagy pathways

Ubiquitin-like proteins (Ubls) confer diverse functions on their target proteins. The modified proteins are involved in various biological processes, including DNA replication, signal transduction, cell cycle control, embryogenesis, cytoskeletal regulation, metabolism, stress response, homeostasis and mRNA processing. Modifiers such as SUMO, ATG12, ISG15, FAT10, URM1, and UFM have been shown to modify proteins thus conferring functions related to programmed cell death, autophagy and regulation of the immune system. Putative modifiers such as Domain With No Name (DWNN) have been identified in recent times but not fully characterized. In this review, we focus on cellular processes involving human Ubls and their targets. We review current progress in targeting these modifiers for drug design strategies.

Gold nanoparticles as a platform for creating a multivalent poly-SUMO chain inhibitor that also augments ionizing radiation

Protein-protein interactions mediated by ubiquitin-like (Ubl) modifications occur as mono-Ubl or poly-Ubl chains. Proteins that regulate poly-SUMO (small ubiquitin-like modifier) chain conjugates play important roles in cellular response to DNA damage, such as those caused by cancer radiation therapy. Additionally, high atomic number metals, such as gold, preferentially absorb much more X-ray energy than soft tissues, and thus augment the effect of ionizing radiation when delivered to cells. In this study, we demonstrate that conjugation of a weak SUMO-2/3 ligand to gold nanoparticles facilitated selective multivalent interactions with poly-SUMO-2/3 chains leading to efficient inhibition of poly-SUMO-chain-mediated protein-protein interactions. The ligand-gold particle conjugate significantly sensitized cancer cells to radiation but was not toxic to normal cells. This study demonstrates a viable approach for selective targeting of poly-Ubl chains through multivalent interactions created by nanoparticles that can be chosen based on their properties, such as abilities to augment radiation effects.

Reversible SUMOylation of TBL1-TBLR1 regulates β-catenin-mediated Wnt signaling

Dysregulation of Wnt signaling has been implicated in tumorigenesis. The role of Transducin β-like proteins TBL1-TBLR1 in the promotion of Wnt/β-catenin-mediated oncogenesis has recently been emphasized; however, the molecular basis of activation of Wnt signaling by the corepressor TBL1-TBLR1 is incompletely understood. Here, we show that both TBL1 and TBLR1 are SUMOylated in a Wnt signaling-dependent manner, and that this modification is selectively reversed by SUMO-specific protease I (SENP1). SUMOylation dismissed TBL1-TBLR1 from the nuclear hormone receptor corepressor (NCoR) complex, increased recruitment of the TBL1-TBLR1-β-catenin complex to the promoter of Wnt target genes, and consequently led to activation of Wnt signaling. Conversely, SENP1 decreased formation of the TBL1-TBLR1-β-catenin complex, leading to inhibition of β-catenin-mediated transcription. Importantly, inhibition of SUMOylation significantly decreased the tumorigenicity of SW480 colon cancer cells. Thus, our data reveal a mechanism for activation of Wnt signaling via the SUMOylation-dependent disassembly of the corepressor complex.

Human pathogens and the host cell SUMOylation system

Since posttranslational modification (PTM) by the small ubiquitin-related modifiers (SUMOs) was discovered over a decade ago, a huge number of cellular proteins have been found to be reversibly modified, resulting in alteration of differential cellular pathways. Although the molecular consequences of SUMO attachment are difficult to predict, the underlying principle of SUMOylation is altering inter- and/or intramolecular interactions of the modified substrate, changing localization, stability, and/or activity. Unsurprisingly, many different pathogens have evolved to exploit the cellular SUMO modification system due to its functional flexibility and far-reaching functional downstream consequences. Although the extensive knowledge gained so far is impressive, a definitive conclusion about the role of SUMO modification during virus infection in general remains elusive and is still restricted to a few, yet promising concepts. Based on the available data, this review aims, first, to provide a detailed overview of the current state of knowledge and, second, to evaluate the currently known common principles/molecular mechanisms of how human pathogenic microbes, especially viruses and their regulatory proteins, exploit the host cell SUMO modification system.

SUMOylation in carcinogenesis

SUMOylation is a post-translational modification characterized by covalent and reversible binding of small ubiquitin-like modifier (SUMO) to a target protein. In mammals, four different isoforms, termed SUMO-1, -2, -3 and -4 have been identified so far. SUMO proteins are critically involved in the modulation of nuclear organization and cell viability. Their expression is significantly increased in processes associated with carcinogenesis such as cell growth, differentiation, senescence, oxidative stress and apoptosis. Little is known about the role of SUMOylation in cancer development. Therefore the present review focuses on possible implications of SUMOylation in carcinogenesis highlighting its impact as an important regulatory cell cycle protein. Moreover, novel opportunities for therapeutic approaches are discussed. The differential expression levels, the target protein preferences and the function of the SUMO pathway in different cancer subtypes raises unexpected issues questioning our understanding of the implication of SUMO in carcinogenesis.

Epstein-Barr virus latent membrane protein 1 (LMP1) C-terminal-activating region 3 contributes to LMP1-mediated cellular migration via its interaction with Ubc9

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1), the principal viral oncoprotein and a member of the tumor necrosis factor receptor superfamily, is a constitutively active membrane signaling protein that regulates multiple signal transduction pathways via its C-terminal-activating region 1 (CTAR1) and CTAR2, and also the less-studied CTAR3. Because protein sumoylation among other posttranslational modifications may regulate many signaling pathways induced by LMP1, we investigated whether during EBV latency LMP1 regulates sumoylation processes that control cellular activation and cellular responses. By immunoprecipitation experiments, we show that LMP1 interacts with Ubc9, the single reported SUMO-conjugating enzyme. Requirements for LMP1-Ubc9 interactions include enzymatically active Ubc9: expression of inactive Ubc9 (Ubc9 C93S) inhibited the LMP1-Ubc9 interaction. LMP1 CTAR3, but not CTAR1 and CTAR2, participated in the LMP1-Ubc9 interaction, and amino acid sequences found in CTAR3, including the JAK-interacting motif, contributed to this interaction. Furthermore, LMP1 expression coincided with increased sumoylation of cellular proteins, and disruption of the Ubc9-LMP1 CTAR3 interaction almost completely abrogated LMP1-induced protein sumoylation, suggesting that this interaction promotes the sumoylation of downstream targets. Additional consequences of the disruption of the LMP1 CTAR3-Ubc9 interaction revealed effects on cellular migration, a hallmark of oncogenesis. Together, these data demonstrate that LMP1 CTAR3 does in fact function in intracellular signaling and leads to biological effects. We propose that LMP1, by interaction with Ubc9, modulates sumoylation processes, which regulate signal transduction pathways that affect phenotypic changes associated with oncogenesis.

Ubiquitin-like protein conjugation and the ubiquitin-proteasome system as drug targets

Ubiquitin is a highly conserved 76 amino-acid protein that covalently attaches to protein substrates targeted for degradation by the 26S proteasome. The coordinated effort of a series of enzymes, including an activating enzyme (E1), a conjugating enzyme (E2) and a ligase (E3), uses ATP to ultimately form an isopeptide bond between ubiquitin and a substrate.

Another class of enzymes called deubiquitylating enzymes (DUBs) deconstruct these linkages and also have an essential role in ubiquitin function. In addition, ubiquitin-like proteins (UBLs), including NEDD8, SUMO and ISG15, share a characteristic three-dimensional fold with ubiquitin but have their own dedicated enzyme cascades and distinct (although sometimes overlapping) biological functions.

The ubiquitin–proteasome system (UPS) and UBL conjugation pathways have important roles in various human diseases, including numerous types of cancer, cardiovascular disease, viral diseases and neurodegenerative disorders. The proteasome inhibitor bortezomib (Velcade; Millennium Pharmaceuticals) is the first clinically validated drug to target the UPS and is approved for the treatment of multiple myeloma. This suggests that other diseases may conceivably be targeted by modulating components of the UPS and UBL conjugation pathways using small-molecule inhibitors.

A significant hurdle to identifying drug-like inhibitors of enzyme targets within the UPS and UBL conjugation pathways is the limited understanding of the molecular mechanisms and biological consequences of UBL conjugation.

Here, we provide an overview of the enzyme classes in the UPS and UBL pathways that are potential therapeutic targets, and highlight considerations that are important for drug discovery. We also discuss the progress in the development of small-molecule inhibitors, and review developments in understanding of the role of the components of the UPS and the UBL pathways in disease and their potential for therapeutic intervention.

The ubiquitin–proteasome system (UPS) and ubiquitin-like protein (UBL) conjugation pathways are integral to cellular protein homeostasis, and their functional importance in various diseases, including cancer, cardiovascular disease and neurodegenerative disorders, is now beginning to emerge. Brownell and colleagues review developments in understanding of the role of the components of the UPS and the UBL pathways in disease and their potential for therapeutic intervention.

The ubiquitin–proteasome system (UPS) and ubiquitin-like protein (UBL) conjugation pathways are integral to cellular protein homeostasis. The growing recognition of the fundamental importance of these pathways to normal cell function and in disease has prompted an in-depth search for small-molecule inhibitors that selectively block the function of these pathways. However, our limited understanding of the molecular mechanisms and biological consequences of UBL conjugation is a significant hurdle to identifying drug-like inhibitors of enzyme targets within these pathways. Here, we highlight recent advances in understanding the role of some of these enzymes and how these new insights may be the key to developing novel therapeutics for diseases including immuno-inflammatory disorders, cancer, infectious diseases, cardiovascular disease and neurodegenerative disorders.

Therapeutically targeting the SUMOylation, Ubiquitination and Proteasome pathways as a novel anticancer strategy

The ubiquitin (Ub)+proteasome proteolytic pathway is responsible for the selective degradation of the majority of nuclear and cytosolic proteins. The proteasome is a high molecular weight multicatalytic protease that serves as the catalytic core of the complex Ub-dependent protein degradation pathway and is an exciting new target for the development of novel anticancer therapies. Inhibition of the proteasome, and consequently Ub-dependent proteolysis, with the small molecule pharmacologic agent bortezomib led to approval by the US Food and Drug Administration (FDA) for the treatment of multiple myeloma (MM) that has subsequently been extended to other hematologic malignancies. Inhibition of the proteasome results in the intracellular accumulation of many ubiquitinated proteins that control essential cellular functions such as cellular growth and apoptosis. The accumulation of high molecular weight Ub~protein conjugates eventually triggers apoptosis, with tumor cells more susceptible to proteasome inhibition than non-malignant cells. The defined mechanism of action for proteasome inhibitors has not been completely characterized, not all patients respond to proteasome inhibitor-based therapy, and inevitably patients develop resistance to proteasome inhibitors. Further investigation of the Ub+proteasome system (UPS) is needed to develop more effective inhibitors, to develop agents that overcome bortezomib resistance and to avoid adverse effects such as neuropathy. Furthermore, there are newly uncovered pathways, e.g., the SUMOylation and NEDDylation pathways, which similarly attach Ub-like proteins (ULPs) to protein substrates. The functional consequence of these modifications is only beginning to emerge, but these pathways have been linked to tumorigenesis and may similarly provide therapeutic targets. The immunoproteasome is a specialized form of the proteasome that produces peptides that are presented at the cell surface as major histocompatibility complex (MHC) class I antigens. Proteasome inhibitors decrease the presentation of antigenic peptides to reduce tumor cell recognition by cytotoxic T cells (CTLs) but unexpectedly increase tumor cell recognition by natural killer (NK) cells. Inhibitors of the UPS are validated, cytotoxic agents that may be further exploited in immunotherapy since they modulate tumor cell recognition by effectors of the immune system. Targeting the UPS, SUMOylation and NEDDylation pathways offers great promise in the treatment of hematologic and solid malignancies.

SUMOylation modulates the function of Aurora-B kinase

Aurora kinases are central regulators of mitotic-spindle assembly, chromosome segregation and cytokinesis. Aurora B is a member of the chromosomal passenger complex (CPC) with crucial functions in regulation of the attachment of kinetochores to microtubules and in cytokinesis. We report here that Aurora B contains a conserved SUMO modification motif within its kinase domain. Aurora B can bind SUMO peptides in vitro when bound to the IN-box domain of its CPC partner INCENP. Mutation of Lys207 to arginine (Aurora B(K207R)) impairs the formation of conjugates of Aurora B and SUMO in vivo. Expression of the SUMO-null form of Aurora B results in abnormal chromosome segregation and cytokinesis failure and it is not able to rescue mitotic defects in Aurora-B-knockout cells. These defects are accompanied by increased levels of the CPC on chromosome arms and defective centromeric function, as detected by decreased phosphorylation of the Aurora-B substrate CENP-A. The Aurora-B(K207R) mutant does not display reduced kinase activity, suggesting that functional defects are probably a consequence of the altered localization, rather than decreased intrinsic kinase activity. These data suggest that SUMOylation of Aurora B modulates its function, possibly by mediating the extraction of CPC complexes from chromosome arms during prometaphase.

Identification of a molecular recognition feature in the E1A oncoprotein that binds the SUMO conjugase UBC9 and likely interferes with polySUMOylation

Hub proteins have central roles in regulating cellular processes. By targeting a single cellular hub, a viral oncogene may gain control over an entire module in the cellular interaction network that is potentially comprised of hundreds of proteins. The adenovirus E1A oncoprotein is a viral hub that interacts with many cellular hub proteins by short linear motifs/molecular recognition features (MoRFs). These interactions transform the architecture of the cellular protein interaction network and virtually reprogram the cell. To identify additional MoRFs within E1A, we screened portions of E1A for their ability to activate yeast pseudohyphal growth or differentiation. This identified a novel functional region within E1A conserved region 2 comprised of the sequence EVIDLT. This MoRF is necessary and sufficient to bind the N-terminal region of the SUMO conjugase UBC9, which also interacts with SUMO noncovalently and is involved in polySUMOylation. Our results suggest that E1A interferes with polySUMOylation, but not with monoSUMOylation. These data provide the first insight into the consequences of the interaction of E1A with UBC9, which was initially described in 1996. We further demonstrate that polySUMOylation regulates pseudohyphal growth and promyelocytic leukemia body reorganization by E1A. In conclusion, the interaction of the E1A oncogene with UBC9 mimics the normal binding between SUMO and UBC9 and represents a novel mechanism to modulate polySUMOylation.

Ubc9 promotes breast cell invasion and metastasis in a sumoylation-independent manner

Ubc9 is an E2-conjugating enzyme that transfers the activated small ubiquitin-like modifier (SUMO) to protein substrates, and thus it has an important function in sumoylation-mediated cellular pathways. We have earlier reported that Ubc9 promotes tumor growth in the xenograft mouse model using breast cancer cell line MCF-7 in part through regulation of Bcl-2 expression. In this study, we show that ectopic expression of wild-type Ubc9 (Ubc9-WT) promotes cell invasion and metastasis. Surprisingly, the dominant negative mutant Ubc9 (Ubc9-DN) also causes the same phenotype, indicating that the ability of Ubc9 to promote invasion and metastasis is distinct from its ability to conjugate SUMO to protein substrates. Of considerable interest, several microRNAs such as miR-224 are regulated by Ubc9. Although ectopic expression of Ubc9 causes downregulation of miR-224, suppression of Ubc9 by Ubc9-siRNAs leads to its upregulation. We further show that miR-224 can inhibit cell invasion and directly targets CDC42 and CXCR4, and that suppression of CDC42 and CXCR4 by RNAi causes inhibition of Ubc9-mediated invasion. Together, these results show a molecular link between Ubc9 and the metastasis genes such as CDC42 and CXCR4, and thus provide new insight into the mechanism by which Ubc9 promotes tumor invasion and metastasis.

Identification and characterization of proteins that selectively interact with the LHR mRNA binding protein (LRBP) in rat ovaries

Luteinizing hormone receptor (LHR) mRNA binding protein (LRBP), identified as mevalonate kinase, has been shown to be a trans factor mediating the post-transcriptional regulation of LHR mRNA expression in ovaries. LRBP binds to the coding region of LHR mRNA and accelerates its degradation. Our previous studies in an in vitro system showed that LRBP represses the translation of LHR mRNA by forming an untranslatable ribonucleoprotein (mRNP) complex, further suggesting that the untranslatable mRNP complex is directed to the mRNA repression/decay machinery for subsequent mRNA turnover. In the present studies, we used yeast two-hybrid system to screen a cDNA library which was constructed from LHR down-regulated ovaries. Two proteins were identified interacting with LRBP: ribosomal protein S20 (RP S20) and ubiquitin conjugating enzyme 2i (UBCE2i). Their interactions with LRBP were confirmed by the mating assay, co-immunoprecipitation analyses and in vitro sumoylation assays. Furthermore, we show that LRBP is a target for modification by SUMO2/3 but not by SUMO1, at K256 and/or K345. Mutation of both lysine residues is sufficient to abrogate the sumoylation of LRBP. These findings suggest that the direct interaction of LRBP with the translation machinery, through RP S20, may be responsible for the transition of LHR mRNA to an untranslatable complex, and that sumoylation of LRBP may play a role in targeting the untranslatable mRNP complex to the mRNA decay machinery in specific cytoplasmic foci.

Sumo control

Sumoylation, the covalent attachment of SUMO peptide to cellular proteins, is an essential regulator of protein function involved in a wide range of cellular events. Deregulation of the SUMO pathway is implicated in the pathogenesis of several diseases, so it is important to understand how this system is controlled. Sumoylation is a highly dynamic regulatory mechanism, involving an energy dependent enzyme cascade for conjugation and another set of enzymes for deconjugation. In this chapter we will highlight the different mechanisms controlling the SUMO system.

SUMOylated IRF-1 shows oncogenic potential by mimicking IRF-2

Interferon regulatory factor-1 (IRF-1) is an interferon-induced transcriptional activator that suppresses tumors by impeding cell proliferation. Recently, we demonstrated that the level of SUMOylated IRF-1 is elevated in tumor cells, and that SUMOylation of IRF-1 attenuates its tumor-suppressive function. Here we report that SUMOylated IRF-1 mimics IRF-2, an antagonistic repressor, and shows oncogenic potential. To demonstrate the role of SUMOylated IRF-1 in tumorigenesis, we used SUMO-IRF-1 recombinant protein. Stable expression of SUMO-IRF-1 in NIH3T3 cells resulted in focus formation and anchorage-independent growth in soft agar. Inoculation of SUMO-IRF-1-transfected cells into athymic nude mice resulted in tumor formation and infiltration of adipose tissues. Finally, we demonstrated that SUMO-IRF-1 transforms NIH3T3 cells in a dose-dependent manner suggesting that SUMOylated IRF-1 may act as an oncogenic protein in tumor cells.

Expression of a novel marker, Ubc9, in squamous cell carcinoma of the head and neck

BACKGROUND

Ubiquitin-conjugating enzyme (Ubc9) is a novel enzyme involved in posttranslational modification of cellular proteins. The objective of this study was to determine the expression of Ubc9 in squamous cell carcinoma of the head and neck (SCCHN).

METHODS

SCCHN specimens were stained with anti-Ubc9 antibodies, scored using a semiquantitative method, and statistically analyzed.

RESULTS

Forty-six tumors were stained, 26 of which included adjacent mucosa. Ubc9 was significantly upregulated in the malignant and peritumoral tissues compared with mucosa from normal individuals. In peritumoral tissues, Ubc9 expression was detected in the basal and suprabasal epithelial layers. No Ubc9 was detected in epithelial cells in normal mucosa. These differences in Ubc9 expression were statistically significant (p < .0001). Tumor Ubc9 expression significantly correlated with clinical and pathologic stage.

CONCLUSIONS

Ubc9 is significantly overexpressed in the primary SCCHN tumors and peritumoral mucosa compared with normal epithelial cells. These findings suggest that Ubc9 may play an important role in tumorigenesis and tumor progression of SCCHN.