Depletion of Jab1 inhibits proliferation of pancreatic cancer cell lines

Jab1-siRNA Induces Cell Growth Inhibition and Cell Cycle Arrest in Gall Bladder Cancer Cells via Targeting Jab1 Signalosome

BACKGROUND

The aberrant alteration in Jab1 signalosome (COP9 Signalosome Complex Subunit 5) has been proven to be associated with the progression of several carcinomas. However the specific role and mechanism of action of Jab1 signalosome in carcinogenesis of gall bladder cancer (GBC) are poorly understood.

OBJECTIVE

The main objective of our study was to elucidate the role and mechanism of Jab1 signalosome in gall bladder cancer by employing siRNA.

METHODS

Jab1 overexpression was identified in gall bladder cancer tissue sample. The role of Jab1-siRNA approach in cell growth inhibition and apoptotic induction was then examined by RT-PCR, Western Blotting, MTT, ROS, Hoechst and FITC/Annexin-V staining.

RESULTS

In the current study, we have shown that overexpression of Jab1 stimulated the proliferation of GBC cells; whereas downregulation of Jab1 by using Jab1-siRNA approach resulted incell growth inhibition and apoptotic induction. Furthermore, we found that downregulation of Jab1 induces cell cycle arrest at G1 phase and upregulated the expression of p27, p53 and Bax gene. Moreover, Jab1-siRNA induces apoptosis by enhancing ROS generation and caspase-3 activation. In addition, combined treatment with Jab1-siRNA and gemicitabine demonstrated an enhanced decline in cell proliferation which further suggested increased efficacy of gemcitabine at a very lower dose (5μM) in combination with Jab1-siRNA.

CONCLUSION

In conclusion, our study strongly suggests that targeting Jab1 signalosome could be a promising therapeutic target for the treatment of gall bladder cancer.

Jab1 Silencing Inhibits Proliferation and Sensitizes to Cisplatin in Biliary Tract Cancer

Purpose

Jab1 is a coactivator of c-Jun that enhances the transcriptional function of c-Jun. Jab1 is frequently overexpressed in various cancers and is associatedwith poor prognosis of cancer patients. Thus, Jab1 could be a potential therapeutic target in cancer. However, the role of Jab1 in biliary tract cancer (BTC) has not been studied.

Materials and Methods

We performed in vitro and in vivo experiments to evaluate the therapeutic potential ofJab1 inhibition in BTC.

Results

Among 8 BTC cell lines, many showed higher Jab1 expression levels. In addition, Jab1 silencing by siRNA increased p27 expression levels. SNU478 and HuCCT-1 cells exhibited profound Jab1 knockdown and increased p27 expression by Jab1-specific siRNA transfection. Jab1 silencing induced anti-proliferative and anti-migratory effects and resulted in G1 cell cycle arrest in SNU478 and HuCCT-1 cells. In addition, Jab1 silencing potentiated the anti-proliferative and anti-migratory effects of cisplatin by increasing DNA damage. Interestingly,Jab1 knockdown increased PTEN protein half-life, resulting in increased PTEN expression. In the HuCCT-1 mouse xenograft model, stable knockdown of Jab1 by shRNA also showed anti-proliferative effects in vivo, with decreased Ki-67 expression and AKT phosphorylation and increased Terminal deoxynucleotidyl transferase–mediated dUTP nick end labeling and p27 expression.

Conclusion

Jab1 knockdown demonstrated anti-proliferative and anti-migratory effects in BTC cells by increasing DNA damage and stabilizing PTEN, resulting in G1 cell cycle arrest. In addition, Jab1 silencing potentiated the anti-proliferative effects of cisplatin. Our data suggest that Jab1 may be a potential therapeutic target in BTC that is worthy of further investigations.

Augmenter of liver regeneration: A key protein in liver regeneration and pathophysiology

Liver is constantly exposed to pathogens, viruses, chemicals, and toxins, and several of them cause injury, leading to the loss of liver mass and sometimes resulting in cirrhosis and cancer. Under physiological conditions, liver can regenerate if the loss of cells is less than the proliferation of hepatocytes. If the loss is more than the proliferation, the radical treatment available is liver transplantation. Due to this reason, the search for an alternative therapeutic agent has been the focus of liver research. Liver regeneration is regulated by several growth factors; one of the key factors is augmenter of liver regeneration (ALR). Involvement of ALR has been reported in crucial processes such as oxidative phosphorylation, maintenance of mitochondria and mitochondrial biogenesis, and regulation of autophagy and cell proliferation. Augmenter of liver regeneration has been observed to be involved in liver regeneration by not only overcoming cell cycle inhibition but by maintaining the stem cell pool as well. These observations have created curiosity regarding the possible role of ALR in maintenance of liver health. Thus, this review brings a concise presentation of the work done in areas exploring the role of ALR in normal liver physiology and in liver health maintenance by fighting liver diseases, such as liver failure, non-alcoholic fatty liver disease/non-alcoholic steatohepatitis, viral infections, cirrhosis, and hepatocellular carcinoma.

Porcine JAB1 significantly enhances apoptosis induced by staurosporine

c-Jun activation domain-binding protein-1 (JAB1), also known as the subunit 5 of the COP9 signalosome, is a multifunctional protein that regulates cell proliferation, apoptosis and oncogenesis by interacting with and subsequently degrading a large number of proteins. Although human JAB1 (hJAB1) has been studied for a long time, studies on porcine JAB1 (pJAB1) have never been reported. In the present study, we cloned and characterized the pJAB1 gene. The genomic structure of the pJAB1 gene was determined. The open-reading frame of pJAB1 encoded 334 amino acids. The deduced amino acid sequence was highly similar to homologs in other species. Furthermore, the tertiary structure analysis and phylogenetic analysis indicated that JAB1 was highly conservative among species. pJAB1 may interact with several proteins according to protein–protein interactions analysis. In addition, pJAB1 was found to be universally expressed in porcine tissues. Subcellular localization analysis showed that GFP–pJAB1 fusion protein distributed specifically in the cytoplasm. Flow cytometric analysis proved that pJAB1 significantly enhanced apoptosis induced by staurosporine, which at least partially depended on the activation of caspase-9 and caspase-3. This study is useful for understanding the function of pJAB1 and offers a potential molecular model for the investigation of diseases related to hJAB1.

CSN5/JAB1 interacts with the centromeric components CENP-T and CENP-W and regulates their proteasome-mediated degradation

The CENP-T·CENP-W complex is a recently identified inner centromere component that plays crucial roles in the formation of a functional kinetochore involved in cell division during mitosis. Using yeast two-hybrid screening, we identified an interaction between CENP-T and CSN5, the fifth component of the COP9 signalosome and a key modulator of the cell cycle and cancer. Co-immunoprecipitation revealed that CSN5 directly interacts with both CENP-T and CENP-W. Ectopically expressed CSN5 promoted the ubiquitin- and proteasome-dependent degradation of CENP-T·CENP-W. The formation of a CENP-T·CENP-W complex greatly enhanced the stabilities of the respective proteins, possibly by blocking CSN5-mediated degradation. Furthermore, dysregulation of CSN5 induced severe defects in the recruitment of CENP-T·CENP-W to the kinetochore during the prophase stage of mitosis. Thus, our results indicate that CSN5 regulates the stability of the inner kinetochore components CENP-T and CENP-W, providing the first direct link between CSN5 and the mitotic apparatus, highlighting the role of CSN5 as a multifunctional cell cycle regulator.

COP9 signalosome subunit Csn8 is involved in maintaining proper duration of the G1 phase

The COP9 signalosome (CSN) is a conserved protein complex known to be involved in developmental processes of eukaryotic organisms. Genetic disruption of a CSN gene causes arrest during early embryonic development in mice. The Csn8 subunit is the smallest and the least conserved subunit, being absent from the CSN complex of several fungal species. Nevertheless, Csn8 is an integral component of the CSN complex in higher eukaryotes, where it is essential for life. By characterizing the mouse embryonic fibroblasts (MEFs) that express Csn8 at a low level, we found that Csn8 plays an important role in maintaining the proper duration of the G1 phase of the cell cycle. A decreased level of Csn8, either in Csn8 hypomorphic MEFs or following siRNA-mediated knockdown in HeLa cells, accelerated cell growth rate. Csn8 hypomorphic MEFs exhibited a shortened G1 duration and affected expression of G1 regulators. In contrast to Csn8, down-regulation of Csn5 impaired cell proliferation. Csn5 proteins were found both as a component of the CSN complex and outside of CSN (Csn5-f), and the amount of Csn5-f relative to CSN was increased in the Csn8 hypomorphic cells. We conclude that CSN harbors both positive and negative regulators of the cell cycle and therefore is poised to influence the fate of a cell at the crossroad of cell division, differentiation, and senescence.

CSN5 specifically interacts with CDK2 and controls senescence in a cytoplasmic cyclin E-mediated manner

The fifth component (CSN5) of the mammalian COP9 signalosome complex plays an essential role in cell proliferation and senescence, but its molecular mediator remains to be determined. Here, we searched for interactors among various cell cycle regulators, and found that CSN5, but not the CSN holo-complex, bound to CDK2 in vivo and in vitro. Depletion of CSN5 enhanced phosphorylation of CDK2 by Akt, resulting in cytoplasmic accumulation of CDK2 together with cyclin E in a leptomycin B-resistant manner, and impaired phosphorylation of the retinoblastoma protein. Additional knockdown of CDK2, which reduced the expression of cyclin E to the normal level, did not restore cell proliferation, but significantly suppressed senescence in CSN5-depleted cells. Enforced expression of cytoplasmic cyclin E induced premature senescence in immortalized cell lines. These results show that CSN5 functions through CDK2 to control premature senescence in a novel way, depending on cyclin E in the cytoplasm.

Targeting Jab1/CSN5 in nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is an Epstein-Barr virus-associated head and neck cancer that is most common in eastern Asia. Epstein-Barr virus infection, environmental factors, and genetic susceptibility play important roles in NPC pathogenesis. Jab1/CSN5 is a multifunctional protein that participates in affecting integrin signaling, controlling cell proliferation and apoptosis, and regulating genomic instability and DNA repair. Correlation of Jab1/CSN5 overexpression with poor prognosis for NPC provides evidence that it is involved in the tumorigenic process. In this review, we highlight recent advances in studies of the oncogenic role of Jab1/CSN5 in NPC and its potential as a therapeutic target for this cancer.

JAB1 is essential for B cell development and germinal center formation and inversely regulates Fas ligand and Bcl6 expression

Jun activation domain-binding protein 1 (JAB1) regulates ubiquitin-dependent protein degradation by deneddylation of cullin-based ubiquitin ligases and, therefore, plays a central role in regulating proliferation and apoptosis. Because these processes are decisive for B cell development, we investigated JAB1 functions in B cells by establishing a mouse strain with a B cell-specific JAB1 deletion. We show that JAB1 is essential for early B cell development, because the ablation of JAB1 expression blocks B cell development between the pro-B and pre-B cell stages. Furthermore, JAB1 deletion leads to aberrant expression of the apoptosis-triggering protein Fas ligand in pro-B cells. Concomitant B cell-specific overexpression of the antiapoptotic protein Bcl2 partially reverses the block in B cell development; rescued JAB1-deficient B cells reach the periphery and produce protective class-switched Abs after Borrelia burgdorferi infection. Interestingly, B cell-rescued mice exhibit no germinal centers but a striking extrafollicular plasma cell accumulation. In addition, JAB1 is essential for Bcl6 expression, a transcriptional repressor required for germinal center formation. These findings identify JAB1 as an important factor in checkpoint control during early B cell development, as well as in fate decisions in mature Ag-primed B cells.

Jab1 expression is associated with TGF-β1 signaling in chronic rhinosinusitis and nasal polyposis

Jab1, which is a fifth component of COP9 signalosome, plays an essential role in cell growth and proliferation. Jab1 is also shown to regulate transforming growth factor-beta (TGF-β) signaling in carcinoma cells. The aim of the present study was to investigate the expression and the correlation of Jab1 and TGF-β1 in chronic rhinosinusitis and nasal polyposis. Here, we show the elevated expression of Jab1 and TGF-β1 in diseased mucosa without nasal polyps and a correlation between Jab1 and TGF-β1 expression. Forty-six samples (26 patients with nasal polyps, 10 patients with chronic rhinosinusitis and 10 control subjects) were included to this study. Immunohistochemistry and Western blotting were performed for the assessment of Jab1 and TGF-β1 localization and the expression of proteins. Double staining of both proteins showed that Jab1 and TGF-β1 were colocalized in the epithelium, inflammatory cells and the vascular endothelium of nasal mucosa. There was a significant increase in the expression of TGF-β1 and Jab1 in patients without nasal polyps and a significant decrease in patients with nasal polyps compared to controls. Moreover, correlation was detected between the expression of Jab1 and TGF-β1 in chronic rhinosinusitis and nasal polyposis. Our results demonstrate that chronic rhinosinusitis is characterized by elevated expression of Jab1 and TGF-β1 compared to nasal polyposis and Jab1 may play a vital role in the pathogenesis of both chronic rhinosinusitis and nasal polyposis.

Molecular targeting of CSN5 in human hepatocellular carcinoma: a mechanism of therapeutic response

Development of targeted therapy for hepatocellular carcinoma (HCC) remains a major challenge. We have recently identified an elevated expression of the fifth subunit of COP9 signalosome (CSN5) in early HCC as compared to dysplastic stage. In the present study, we explored the possibility of CSN5 being a potential therapeutic target for HCC. Our results show that CSN5 knockdown by small interfering (si) RNA caused a strong induction of apoptosis and inhibition of cell cycle progression in HCC cells in vitro. The downregulation of CSN5 was sufficient to interfere with CSN function as evidenced by the accumulation of neddylated Cullin1 and changes in the protein levels of CSN controlled substrates SKP2, p53, p27 and NF-kB, albeit to a different degree depending on the HCC cell line, which could account for the CSN5 knockdown phenotype. The transcriptomic analysis of CSN5 knockdown signature showed that the anti-proliferative effect was driven by a common subset of molecular alterations including downregulation of CDK6 and ITGB1, which were functionally interconnected with key oncogenic regulators MYC and TGFβ1 involved in the control of proliferation, apoptotic cell death and HCC progression. Consistent with microarray analysis, western blotting revealed that CSN5 depletion increased phosphorylation of Smad 2/3, key mediators of TGFβ1 signaling, decreased the protein levels of ITGB1, CDK6, and cyclin D1 and caused reduced expression of anti-apoptotic Bcl-2 while elevating the levels of pro-apoptotic Bak. A chemically modified variant of CSN5 siRNA was then selected for in vivo application based on the growth inhibitory effect and minimal induction of unwanted immune response. Systemic delivery of the CSN5 3/8 variant by stable-nucleic-acid-lipid-particles (SNALP) significantly suppressed the tumor growth in Huh7-luc⁺ orthotopic xenograft model. Taken together, these results indicate that CSN5 plays a pivotal role in HCC pathogenesis and maybe an attractive molecular target for systemic HCC therapy.

Physical interaction of Jab1 with human serotonin 6 G-protein-coupled receptor and their possible roles in cell survival

The 5-HT(6) receptor (5-HT(6)R) is one of the most recently cloned serotonin receptors, and it plays important roles in Alzheimer disease, depression, and learning and memory disorders. However, unlike the other serotonin receptors, the cellular mechanisms of 5-HT(6)R are poorly elucidated relative to its significance in human brain diseases. Here, using a yeast two-hybrid assay, we found that the human 5-HT(6)R interacts with Jun activation domain-binding protein-1 (Jab1). We also confirmed a physical interaction between 5-HT(6)R and Jab1 using glutathione S-transferase pulldown, fluorescence resonance energy transfer, co-immunoprecipitation, and immunocyto(histo)chemistry assays. The manipulation of Jab1 expression using Jab1 small interference RNA decreased 5-HT(6)R-mediated activity and cell membrane expression of 5-HT(6)R, whereas overexpression of Jab1 produced no significant effect. In addition, we demonstrated that the activation of 5-HT(6)R induced the translocation of Jab1 into the nucleus and increased c-Jun phosphorylation and the interaction between Jab1 and c-Jun. Furthermore, we found that 5-HT(6)R and Jab1 were up-regulated in middle cerebral artery occlusion-induced focal cerebral ischemic rats and in cultured cells exposed to hypoxic insults, suggesting possible protective roles for 5-HT(6)R and Jab1. These findings suggest that Jab1 provides a novel signal transduction pathway for 5-HT(6)R and may play an important role in 5-HT(6)R-mediated behavior changes in the brain.

Jab1/CSN5 induces the cytoplasmic localization and degradation of RUNX3

Runt-related (RUNX) transcription factors play pivotal roles in neoplastic development and have tissue-specific developmental roles in hematopoiesis (RUNX1), osteogenesis (RUNX2), as well as neurogenesis and thymopoiesis (RUNX3). RUNX3 is a tumor suppressor in gastric carcinoma, and its expression is frequently inactivated by DNA methylation or its protein mislocalized in many cancer types, including gastric and breast cancer. Jun-activation domain-binding protein 1 (Jab1/CSN5), a component of the COP9 signalosome (CSN), is critical for nuclear export and the degradation of several tumor suppressor proteins, including p53, p27(Kip1), and Smad4. Here, we find that Jab1 facilitates nuclear export of RUNX3 that is controlled by CSN-associated kinases. RUNX3 sequestered in the cytoplasm is rapidly degraded through a proteasome-mediated pathway. Our results identify a novel mechanism of regulating nuclear export and protein stability of RUNX3 by the CSN complex.

Analysis of the role of COP9 Signalosome (CSN) subunits in K562; the first link between CSN and autophagy

Background

The COP9/signalosome (CSN) is a highly conserved eight subunit complex that, by deneddylating cullins in cullin-based E3 ubiquitin ligases, regulates protein degradation. Although studied in model human cell lines such as HeLa, very little is known about the role of the CSN in haemopoietic cells.

Results

Greater than 95% knockdown of the non-catalytic subunit CSN2 and the deneddylating subunit CSN5 of the CSN was achieved in the human myeloid progenitor cell line K562. CSN2 knockdown led to a reduction of both CSN5 protein and mRNA whilst CSN5 knockdown had little effect on CSN2. Both knockdowns inhibited CSN deneddylase function as demonstrated by accumulation of neddylated Cul1. Furthermore, both knockdowns resulted in the sequential loss of Skp2, Cdc4 and β-TrCP F-box proteins. These proteins were rescued by the proteasome inhibitor MG132, indicating the autocatalytic degradation of F-box proteins upon loss of CSN2 or CSN5. Interestingly, altered F-box protein gene expression was also observed in CSN2 and CSN5 knockdowns, suggesting a potential role of the CSN in regulating F-box protein transcription.

Loss of either CSN subunit dramatically reduced cell growth but resulted in distinct patterns of cell death. CSN5 knockdown caused mitotic defects, G2/M arrest and apoptotic cell death. CSN2 knockdown resulted in non-apoptotic cell death associated with accumulation of both the autophagy marker LC3-II and autophagic vacuoles. Treatment of vector control K562 cells with the autophagy inhibitors 3-methyladenine and bafilomycin A1 recapitulated the growth kinetics, vacuolar morphology and LC3-II accumulation of CSN2 knockdown cells indicating that the cellular phenotype of CSN2 cells arises from autophagy inhibition. Finally, loss of CSN2 was associated with the formation of a CSN5 containing subcomplex.

Conclusion

We conclude that CSN2 is required for CSN integrity and the stability of individual CSN subunits, and postulate that CSN2 loss results in a phenotype distinct from that of cells lacking CSN5 possibly as a consequence of altered CSN5 activity within a resultant CSN subcomplex. Our data present the first evidence for the sequential loss of F-box proteins upon CSN manipulation and are the first to identify a potential link between CSN function and autophagy.

Overexpression of Jab1 in hepatocellular carcinoma and its inhibition by peroxisome proliferator-activated receptor{gamma} ligands in vitro and in vivo

PURPOSE

Jun activation domain-binding protein 1 (Jab1) is the fifth subunit of the COP9 signalosome and exhibits oncogenic activity. We investigated Jab1 expression in hepatocellular carcinoma (HCC) tissues and cell lines and tested the effect of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on Jab1 expression.

EXPERIMENTAL DESIGN

Jab1 expression in HCC tissues and cell lines was studied by real-time reverse transcription-PCR, immunohistochemical staining, and Western blotting. Promoter activity and chromatin immunoprecipitation assays were done to address the inhibition of Jab1 promoter by PPARgamma ligands. RNA interference was used to clarify PPARgamma ligand-induced inhibition of Jab1. Anticancer and Jab1-suppressing activity of PPARgamma ligands was tested in nude mice.

RESULTS

Jab1 was detected in the nucleus and cytoplasm of HCC tissues and 37% (37 of 99) of tissues exhibited Jab1 overexpression. Jab1 expression correlated with sex and hepatitis C virus infection, whereas it was negatively associated with hepatitis B virus infection. Additionally, Jab1 was overexpressed in HCC cell lines. PPARgamma ligands troglitazone and rosiglitazone down-regulated Jab1 expression in HCC cells, and troglitazone directly suppressed Jab1 promoter activity by inhibiting Sp1- and Tcf4-mediated transcription. This suppression was mediated via both PPARgamma-dependent and PPARgamma-independent mechanisms. Ectopic expression of Jab1 counteracted troglitazone-induced growth inhibition. Animal studies verified that intratumor or i.p. injection of troglitazone attenuated HCC growth and reduced Jab1 expression in tumor tissues.

CONCLUSIONS

Our results indicate that Jab1 is overexpressed in HCC and PPARgamma ligands may suppress Jab1 to inhibit the proliferation of HCC cells.

Stable form of JAB1 enhances proliferation and maintenance of hematopoietic progenitors

Overexpression of JAB1 is observed in a variety of human cancers, but how JAB1 is involved in tumor development remained to be investigated. Here we analyzed mice with modified Jab1 expression. Mice ectopically expressing a more stable form of JAB1 protein under the control of a constitutive promoter were rescued from the embryonic lethality caused by the Jab1(-/-) allele and developed a myeloproliferative disorder in a gene dosage-dependent manner. Hematopoietic cells from the bone marrow of Jab1 transgenic mice had a significantly larger stem cell population and exhibited higher and transplantable proliferative potential. In contrast, Jab1(+/-) mice, which express approximately 70% as much JAB1 protein as their wild-type littermates, showed inefficient hematopoiesis. Expression of the tumor suppressor p16(INK4a) was inversely correlated with that of JAB1, and the oncoprotein SMYD3, a newly identified JAB1 interactor, suppressed transcription of p16 in cooperation with JAB1. Thus, the expression and function of JAB1 are critical for the proliferation and maintenance of hematopoietic progenitors.

Jab1 is overexpressed in human breast cancer and is a downstream target for HER-2/neu

Jab1 is a coactivator of AP-1 transcription factor and the fifth subunit of the COP9 signalosome. This protein is a potential oncogene and is involved in the mediation of nuclear exportation and degradation of the tumor suppressor p27(Kip1). However, control of Jab1 gene expression and its de-regulation in cancer cells are largely unknown. In this study, we demonstrated that Jab1 is overexpressed in 53 (80.3%) of a series of 66 human breast tumor tissues. In addition, its expression is significantly correlated with HER-2/neu overexpression (P=0.0318). HER-2/neu-overexpressing MDA-MB-453 human breast cancer cells exhibited higher expression of Jab1 than that of MCF-7 breast cancer cells. Promoter activity assay suggested that HER-2/neu oncogene upregulated Jab1 via transcriptional activation. Inhibition of HER-2/neu activity by Herceptin or AG825 significantly attenuated Jab1 expression in HER-2/neu-overexpressing MDA-MB-453 cells. On the contrary, ectopic expression of HER-2/neu stimulated Jab1 expression in MCF-7 cells. Knockdown of Jab1 expression by siRNA resulted in p27(Kip1) upregulation and G1 growth arrest in Jab1-overexpressing MDA-MB-453 cells. Taken together, our results suggest that Jab1 is a downstream target for HER-2/neu and its overexpression is linked with HER-2/neu expression in breast cancer.

CSN5 isopeptidase activity links COP9 signalosome activation to breast cancer progression

CSN5 has been implicated as a candidate oncogene in human breast cancers by genetic linkage with activation of the poor-prognosis, wound response gene expression signature. CSN5 is a subunit of the eight-protein COP9 signalosome, a signaling complex with multiple biochemical activities; the mechanism of CSN5 action in cancer development remains poorly understood. Here, we show that CSN5 isopeptidase activity is essential for breast epithelial transformation and progression. Amplification of CSN5 is required for transformation of primary human breast epithelial cells by defined oncogenes. The transforming effects of CSN5 require CSN subunits for assembly of the full COP9 signalosome and the isopeptidase activity of CSN5, which potentiates the transcriptional activity of MYC. Transgenic inhibition of CSN5 isopeptidase activity blocks breast cancer progression evoked by MYC and RAS in vivo. These results highlight CSN5 isopeptidase activity in breast cancer progression, suggesting it as a therapeutic target in aggressive human breast cancers.