Ubiquitin-specific Protease 9X Research Articles

Abrogation of USP9X Is a Potential Strategy to Decrease PEG10 Levels and Impede Tumor Progression in Cutaneous T-Cell Lymphoma

Advanced-stage cutaneous T-cell lymphomas (CTCL) are notorious for its highly aggressive behavior, resistance to conventional treatments and poor prognosis, particularly when large-cell transformation (LCT) occurs. Paternally expressed gene 10 (PEG10) has been recently proposed as a potent driver for LCT in CTCL. However, the targeting of PEG10 continues to present a formidable clinical challenge that has yet to be addressed. Here we report an important post-translational regulatory mechanism of PEG10 in CTCL. Ubiquitin-specific protease 9X (USP9X), a deubiquitinase, interacted with and deubiquitinated PEG10, thereby stabilizing PEG10. Knockdown of USP9X or pharmacological targeting of USP9X resulted in a prominent downregulation of PEG10 and its downstream pathway in CTCL. Moreover, USP9X inhibition conferred tumor cell growth disadvantage and enhanced apoptosis in vitro, an effect that occurred in part through its regulation on PEG10. Furthermore, we demonstrated that inhibition of USP9X obviously restrained CTCL tumor growth in vivo, and that high expression of USP9X is associated with poor survival in CTCL patients. Collectively, our findings uncover USP9X as a key post-translational regulator in the stabilization of PEG10 and suggest that targeting PEG10 stabilization through USP9X inhibition may represent a promising therapeutic strategy for advanced-stage CTCL.

RANBP2 and USP9x regulate nuclear import of adenovirus minor coat protein IIIa.

As intracellular parasites, viruses exploit cellular proteins at every stage of infection. Adenovirus outbreaks are associated with severe acute respiratory illnesses and conjunctivitis, with no specific antiviral therapy available. An adenoviral vaccine based on human adenovirus species D (HAdV-D) is currently in use for COVID-19. Herein, we investigate host interactions of HAdV-D type 37 (HAdV-D37) protein IIIa (pIIIa), identified by affinity purification and mass spectrometry (AP-MS) screens. We demonstrate that viral pIIIa interacts with ubiquitin-specific protease 9x (USP9x) and Ran-binding protein 2 (RANBP2). USP9x binding did not invoke its signature deubiquitination function but rather deregulated pIIIa-RANBP2 interactions. In USP9x-knockout cells, viral genome replication and viral protein expression increased compared to wild type cells, supporting a host-favored mechanism for USP9x. Conversely, RANBP2-knock down reduced pIIIa transport to the nucleus, viral genome replication, and viral protein expression. Also, RANBP2-siRNA pretreated cells appeared to contain fewer mature viral particles. Transmission electron microscopy of USP9x-siRNA pretreated, virus-infected cells revealed larger than typical paracrystalline viral arrays. RANBP2-siRNA pretreatment led to the accumulation of defective assembly products at an early maturation stage. CRM1 nuclear export blockade by leptomycin B led to the retention of pIIIa within cell nuclei and hindered pIIIa-RANBP2 interactions. In-vitro binding analyses indicated that USP9x and RANBP2 bind to C-terminus of pIIIa amino acids 386–563 and 386–510, respectively. Surface plasmon resonance testing showed direct pIIIa interaction with recombinant USP9x and RANBP2 proteins, without competition. Using an alternative and genetically disparate adenovirus type (HAdV-C5), we show that the demonstrated pIIIa interaction is also important for a severe respiratory pathogen. Together, our results suggest that pIIIa hijacks RANBP2 for nuclear import and subsequent virion assembly. USP9x counteracts this interaction and negatively regulates virion synthesis. This analysis extends the scope of known adenovirus-host interactions and has potential implications in designing new antiviral therapeutics.

USP9X Is Required to Maintain Cell Survival in Response to High-LET Radiation.

Ionizing radiation (IR) principally acts through induction of DNA damage that promotes cell death, although the biological effects of IR are more broad ranging. In fact, the impact of IR of higher-linear energy transfer (LET) on cell biology is generally not well understood. Critically, therefore, the cellular enzymes and mechanisms responsible for enhancing cell survival following high-LET IR are unclear. To this effect, we have recently performed siRNA screening to identify deubiquitylating enzymes that control cell survival specifically in response to high-LET α-particles and protons, in comparison to low-LET X-rays and protons. From this screening, we have now thoroughly validated that depletion of the ubiquitin-specific protease 9X (USP9X) in HeLa and oropharyngeal squamous cell carcinoma (UMSCC74A) cells using small interfering RNA (siRNA), leads to significantly decreased survival of cells after high-LET radiation. We consequently investigated the mechanism through which this occurs, and demonstrate that an absence of USP9X has no impact on DNA damage repair post-irradiation nor on apoptosis, autophagy, or senescence. We discovered that USP9X is required to stabilize key proteins (CEP55 and CEP131) involved in centrosome and cilia formation and plays an important role in controlling pericentrin-rich foci, particularly in response to high-LET protons. This was also confirmed directly by demonstrating that depletion of CEP55/CEP131 led to both enhanced radiosensitivity of cells to high-LET protons and amplification of pericentrin-rich foci. Our evidence supports the importance of USP9X in maintaining centrosome function and biogenesis and which is crucial particularly in the cellular response to high-LET radiation.

RUNX1 regulates SMAD1 by transcriptionally activating the expression of USP9X, regulating the activation of hepatic stellate cells and liver fibrosis

Liver fibrosis (LF) is a common pathological process with high morbidity and mortality. Runt-related transcription factor 1 (RUNX1) is a transcription factor that could cause nephropathy and renal fibrosis, but its role in LF is unclear. Therefore, this study aimed to investigate the role RUNX1 in LF. Briefly, hepatic fibrosis was detected by Sirius Red staining. Transcript levels were quantified by qPCR, and proteins were assessed by western blotting or immunofluorescence. Cell viability and cell migration were measured by CCK8 assays and wound healing assays, respectively. The binding of RUNX1 and ubiquitin-specific protease 9X (USP9X) promoter was validated by ChIP assays and luciferase report assays, while the binding of USP9X and SMAD1 was confirmed by co-immunoprecipitation (Co-IP). Our studies found that the expression of RUNX1 was upregulated in LF mice, and RUNX1 knockdown alleviated CCl4-induced LF. RUNX1 silencing reduced the viability and migration of HSCs. Besides, RUNX1, as a transcription factor, bound to the promoter of USP9X and regulated the expression of USP9X. USP9X is a deubiquitination enzyme and was found to be up-regulated in LF mice. USP9X silencing reduced the viability and migration of HSCs, thereby inhibiting LF. Further studies showed that USP9X could stabilize downstream Smad1 expression. Furthermore, we also found that RUNX1 regulated the expression of SMAD1 by transcriptionally activating the expression of USP9X, thereby regulating the activation of hepatic stellate cells and liver fibrosis.

USP9X deubiquitinates connexin43 to prevent high glucose-induced epithelial-to-mesenchymal transition in NRK-52E cells

Epithelial-to-mesenchymal transition (EMT) plays an important role in diabetic nephropathy (DN). Ubiquitin-specific protease 9X (USP9X/FAM) is closely linked to TGF-β and fibrosis signaling pathway. However, it remains unknown whether USP9X is involved in the process of EMT in DN. Our previous study has shown that connexin 43 (Cx43) activation attenuated the development of diabetic renal tubulointerstitial fibrosis (RIF). Here, we showed that USP9X is a novel negative regulator of EMT and the potential mechanism is related to the deubiquitination and degradation of Cx43. To explore the potential regulatory mechanism of USP9X, the expression and activity of USP9X were studied by CRISPR/Cas9-based synergistic activation mediator (SAM) system, short hairpin RNAs, and selective inhibitor. The following findings were observed: (1) Expression of USP9X was down-regulated in the kidney tissue of db/db diabetic mice; (2) overexpression of USP9X suppressed high glucose (HG)-induced expressions of EMT markers and extra cellular matrix (ECM) in NRK-52E cells; (3) depletion of USP9X further aggravated EMT process and ECM production in NRK-52E cells; (4) USP9X deubiquitinated Cx43 and suppressed its degradation to regulate EMT process; (5) USP9X deubiquitinated Cx43 by directly binding to the C-terminal Tyr286 of Cx43. The current study determined the protective role of USP9X in the process of EMT and the molecular mechanism clarified that the protective effects of USP9X on DN were associated with the deubiquitination of Cx43.

MTORC2 Assembly Is Regulated by USP9X-Mediated Deubiquitination of RICTOR

The mechanistic target of rapamycin complex 2 (mTORC2) controls cell metabolism and survival in response to environmental inputs. Dysregulation of mTORC2 signaling has been linked to diverse human diseases, including cancer and metabolic disorders, highlighting the importance of a tightly controlled mTORC2. While mTORC2 assembly is a critical determinant of its activity, the factors regulating this event are not well understood, and it is unclear whether this process is regulated by growth factors. Here, we present data, from human cell lines and mice, describing a mechanism by which growth factors regulate ubiquitin-specific protease 9X (USP9X) deubiquitinase to stimulate mTORC2 assembly and activity. USP9X removes Lys63-linked ubiquitin from RICTOR to promote its interaction with mTOR, thereby facilitating mTORC2 signaling. As mTORC2 is central for cellular homeostasis, understanding the mechanisms regulating mTORC2 activation toward its downstream targets is vital for our understanding of physiological processes and for developing new therapeutic strategies in pathology.

Genomic analysis of low-grade serous ovarian carcinoma to identify key drivers and therapeutic vulnerabilities.

Low-grade serous ovarian carcinoma (LGSOC) is associated with a poor response to existing chemotherapy, highlighting the need to perform comprehensive genomic analysis and identify new therapeutic vulnerabilities. The data presented here represent the largest genetic study of LGSOCs to date (n = 71), analysing 127 candidate genes derived from whole exome sequencing cohorts to generate mutation and copy-number variation data. Additionally, immunohistochemistry was performed on our LGSOC cohort assessing oestrogen receptor, progesterone receptor, TP53, and CDKN2A status. Targeted sequencing identified 47% of cases with mutations in key RAS/RAF pathway genes (KRAS, BRAF, and NRAS), as well as mutations in putative novel driver genes including USP9X (27%), MACF1 (11%), ARID1A (9%), NF2 (4%), DOT1L (6%), and ASH1L (4%). Immunohistochemistry evaluation revealed frequent oestrogen/progesterone receptor positivity (85%), along with CDKN2A protein loss (10%) and CDKN2A protein overexpression (6%), which were linked to shorter disease outcomes. Indeed, 90% of LGSOC samples harboured at least one potentially actionable alteration, which in 19/71 (27%) cases were predictive of clinical benefit from a standard treatment, either in another cancer's indication or in LGSOC specifically. In addition, we validated ubiquitin-specific protease 9X (USP9X), which is a chromosome X-linked substrate-specific deubiquitinase and tumour suppressor, as a relevant therapeutic target for LGSOC. Our comprehensive genomic study highlighted that there is an addiction to a limited number of unique 'driver' aberrations that could be translated into improved therapeutic paths. © 2020 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Upregulation of DR5 and Downregulation of Survivin by IITZ-01, Lysosomotropic Autophagy Inhibitor, Potentiates TRAIL-Mediated Apoptosis in Renal Cancer Cells via Ubiquitin-Proteasome Pathway.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selectively is able to increase apoptosis in cancer cells as agent with minimum toxicity to noncancerous cells. However, all cancer cells are not sensitive to TRAIL-induced apoptosis. In this study, we showed the sub-lethal concentrations of a lysosomotropic autophagy inhibitor, IITZ-01, sensitizes cancer cells (renal, lung, and breast carcinoma) to TRAIL-induced apoptosis through DR5 upregulation and survivin downregulation through ubiquitin-proteasome pathway. Knockdown of DR5 or overexpression of survivin inhibited combined treatment with IITZ-01 and TRAIL-induced apoptosis. IITZ-01 downregulated protein expression of Cbl, ubiquitin E3 ligase, and decreased expression level of Cbl markedly led to increase DR5 protein expression and TRAIL sensitivity. Moreover, IITZ-01 decreased expression level of survivin protein via downregulation of deubiquitinase ubiquitin-specific protease 9X (USP9X) expression. Taken together, these results provide the first evidence that IITZ-01 enhances TRAIL-mediated apoptosis through DR5 stabilization by downregulation of Cbl and USP9X-dependent survivin ubiquitination and degradation in renal carcinoma cells.

USP9X prevents AGEs-induced upregulation of FN and TGF-β1 through activating Nrf2-ARE pathway in rat glomerular mesangial cells

Oxidative stress is a key pathological factor for diabetic renal fibrosis by activating TGF-β/Smad pathway in glomerular mesangial cells (GMCs) to promote the synthesis of extracellular matrix such as fibronectin (FN). Nuclear factor-E2-related factor (Nrf2)- anti-oxidant response element (ARE) anti-oxidative pathway has crucial renoprotective effects, and inhibiting ubiquitin-mediated degradation of Nrf2 delays diabetic renal fibrosis development. Ubiquitin-specific protease 9X (USP9X) has close relationship with oxidative stress and TGF-β/Smad pathway, but whether it regulate diabetic renal fibrosis remains unclarified. Here, we found that advanced glycation-end products (AGEs) dose- and time-dependently reduced the protein expression and deubiquitinase activity of USP9X in GMCs. USP9X overexpression attenuated AGEs-induced upregulation of FN, TGF-β1, and Collagen Ⅳ, three fibrosis-related marker proteins, in a deubiquitinase activity-dependent manner. While USP9X depletion with siRNAs further promoted the expressions of those proteins in AGEs-treated GMCs. Under AGEs treatment conditions, USP9X overexpression markedly increased the total and nuclear levels, ARE-binding ability, and transcriptional activity of Nrf2, upregulated the protein expressions of Nrf2 downstream genes HO-1 and NQO1, and eventually reduced the excessive production of ROS. Overexpression of the deubiquitinase catalytically inactive USP9X–C1556S mutant failed to exert such effects. Silencing Nrf2 abolished the renoprotective effects of USP9X. Further study showed that upon AGEs stimulation, Nrf2 transferred into the nucleus and the interaction between USP9X and Nrf2 was weakened. AGEs also increased Nrf2 ubiquitination level, and overexpression of USP9X, instead of USP9X–C1556S, significantly reduced the ubiquitination level of Nrf2. Taken together, USP9X reduced Nrf2 ubiquitination level and promoted Nrf2-ARE pathway activation to prevent the accumulation of extracellular matrix, eventually alleviated the pathological process of diabetic renal fibrosis.

Effect of siRNA interference of ubiquitin-specific protease 9X on apoptosis and growth of diffuse large B cell lymphoma cell line

Ubiquitin-specific protease 9X (USP9X) is crucial in the diagnosis and treatment of many tumor types, but its role in Diffuse Large B Cell Lymphoma (DLBCL) has not been determined. The current study aimed to examine the effects of RNA interference on USP9X expression, and subsequently on the bioactivity of DLBCL Farage and Pfeiffer cells. There were two groups in the study: USP9X-siRNA and NC. USP9X siRNA was transiently transferred into DLBCL cells by Cationic liposome. The total RNA was extracted using Fe2O3 and was retrieved into the DNA using the MagBeads Total RNA Extraction Kit. The protein expression of USP9X in Farage, Pfeiffer, and normal human B cell line at the cellular level was observed by Western blot. The Farage and Pfeiffer cells were infected with USP9X-siRNA. Cell apoptosis and cell growth viability were analyzed by flow cytometry and CCK8, Mcl-1 protein, a potential target of USP9X, and apoptosis factor proteins (such as Bak, Cytochrome C, Caspase 3, Caspase 8, PARP) were detected by Western blot after siRNA interference. The results showed that the protein expression of USP9X in malignant B cells was four times higher than that of the normal B cells. Inhibition of USP9X reduced the Mcl-1 activity, and increased the caspase-3, Bak and Cytochrome C activity. In the malignant B cells, Mcl-1 and Bak were binding in vivo; Bak was a new partner of Mcl-1. Inhibition of USP9X reduced cell proliferation and increased apoptosis. The expression of USP9X is upregulated in Diffuse large B cell lymphoma cells, Farage, and Pfeiffer. Inhibition expression of USP9X may induce cell apoptosis, inhibit cell growth, and downregulate Mcl-1 protein expression in Diffuse large B cell lymphoma cells, Farage, and Pfeiffer. USP9X has the ability in regulating cell apoptosis.

USP9X mRNA expression predicts clinical outcome for esophageal squamous cell carcinoma treated with cisplatin-based therapy

Combination therapy with cisplatin is the conventionalfirst-line treatment in patients with locally advanced or metastatic esophageal squamous cell carcinoma (ESCC). Ubiquitin-specific protease 9X (USP9X) has been shown to be associated with resistance to chemotherapy drugs in several cancers. The purpose of this study was to explore the predictive effects of USP9X on advanced ESCC patients treated with cisplatin-based regimens. The subjects were 69 advanced ESCC patients who received first-line cisplatin-based chemotherapy or chemoradiotherapy. The quantitative real-time PCR was performed to measure USP9X mRNA expression. The correlation of USP9X expression with clinical parameters and tumor response was analyzed. The Kaplan-Meier method and Cox analysis were employed to analyze differences in overall survival (OS). USP9X mRNA expression was positively associated with the TMN stage at initial diagnosis. Patients with low USP9X mRNA expression had a significantly higher objective response rate (57.1% vs. 17.6%, P=0.001) and longer median OS (25.0 vs. 14.0 months, P<0.001) than those with high expression in all patients or in different treatment subgroups (all P<0.05). Multivariate analysis showed that low mRNA expression of USP9X emerged as an independent prognostic factor indicating prolonged OS (hazard ratio 0.50, 95% CI 0.34-0.73; P<0.001). These findings suggest that high USP9X mRNA expression predicts poor clinical efficacyand survival to cisplatin-based therapy in patients with advanced ESCC.

Non-canonical regulation of homologous recombination DNA repair by the USP9X deubiquitylase.

In order to prevent the deleterious effects of genotoxic agents, cells have developed complex surveillance mechanisms and DNA repair pathways that allow them to maintain genome integrity. The ubiquitin-specific protease 9X (USP9X) contributes to genome stability during DNA replication and chromosome segregation. Depletion of USP9X leads to DNA double-strand breaks, some of which are triggered by replication fork collapse. Here, we identify USP9X as a novel regulator of homologous recombination (HR) DNA repair in human cells. By performing cellular HR reporter, irradiation-induced focus formation and colony formation assays, we show that USP9X is required for efficient HR. Mechanistically, we show USP9X is important to sustain the expression levels of key HR factors, namely BRCA1 and RAD51 through a non-canonical regulation of their mRNA abundance. Intriguingly, we find that the contribution of USP9X to BRCA1 and RAD51 expression is independent of its known catalytic activity. Thus, this work identifies USP9X as a regulator of HR, demonstrates a novel mechanism by which USP9X can regulate protein levels, and provides insights in to the regulation of BRCA1 and RAD51 mRNA.This article has an associated First Person interview with the first author of the paper.

USP9X stabilizes BRCA1 and confers resistance to DNA‐damaging agents in human cancer cells

BRCA1, a multifunctional protein with an important role in DNA double‐strand break repair by homologous recombination (HR), is subjected to ubiquitin‐dependent degradation. To date, several E3 ubiquitin ligases have been identified to govern BRCA1 stability, but the deubiquitinase that counteracts its turnover remains undefined. In this study, we report that the ubiquitin‐specific protease 9X (USP9X) is a bona fide deubiquitinase for BRCA1 in human cancer cells. Reciprocal immunoprecipitation assays demonstrated that USP9X interacted with BRCA1. Depletion of USP9X by short interfering RNAs or inhibition of USP9X by the small‐molecular inhibitor WP1130 significantly reduced BRCA1 protein abundance, without affecting its mRNA levels. In contrast, overexpression of wild‐type USP9X, but not its deubiquitinase activity‐defective mutant (C1566S), resulted in an upregulation of BRCA1 protein levels. Moreover, USP9X depletion reduced the half‐life of BRCA1, accompanied by an increase in its ubiquitination. HR assays showed that knockdown of USP9X significantly reduced HR efficiency, which was partially rescued by reintroduction of BRCA1 into USP9X‐depleted cells. In support of these findings, USP9X knockdown significantly enhanced sensitivity to PARP inhibitor Olaparib and methyl methanesulfonate (MMS). Collectively, these results establish USP9X as a deubiquitinase for BRCA1 and reveal a previously unrecognized role of USP9X in the regulation of HR repair and the sensitivity of cancer cells to DNA‐damaging agents.

Deubiquitinase USP9X regulates the invasion of prostate cancer cells by regulating the ERK pathway and mitochondrial dynamics.

The ubiquitin-specific protease 9X (USP9X) is a conserved deubiquitinase that has been investigated in several types of human cancer. However, the clinical significance and the biological roles of USP9X in prostate cancer remain unexplored. In the present study, an investigation into the expression and clinical significance of USP9X in prostate cancer revealed that USP9X expression was downregulated in prostate cancer tissues compared with that in healthy tissues. In addition, decreased USP9X expression was associated with a higher Gleason score and local invasion. Depletion of USP9X in prostate cancer LNCaP and PC-3 cells by small interfering RNA promoted cell invasion and migration. Furthermore, USP9X depletion upregulated matrix metalloproteinase 9 (MMP9) and the phosphorylation of dynamin-related protein 1 (DRP1). Notably, a significant increase in phosphorylated extracellular signal-regulated kinase (ERK), an upstream activator of MMP9 and DRP1, was observed. To investigate whether ERK activation was able to increase MMP9 protein levels and induce DRP1 phosphorylation, an ERK inhibitor was used, demonstrating that ERK-mediated MMP9 production and change in mitochondrial function was critical for the biological function of USP9X in prostate cancer cells. In conclusion, the present study demonstrated that USP9X is downregulated in prostate cancer and functions as an inhibitor of tumor cell invasion, possibly through the regulation of the ERK signaling pathway.

USP9X deubiquitinates ALDH1A3 and maintains mesenchymal identity in glioblastoma stem cells.

The mesenchymal (MES) subtype of glioblastoma (GBM) stem cells (GSCs) represents a subpopulation of cancer cells that are notorious for their highly aggressive nature and resistance to conventional therapy. Aldehyde dehydrogenase 1A3 (ALDH1A3) has been recently suggested as a key determinant for the maintenance of MES features of GSCs. However, the mechanisms underpinning aberrant ALDH1A3 expression remain elusive. Here, we identified ubiquitin-specific protease 9X (USP9X) as a bona fide deubiquitinase of ALDH1A3 in MES GSCs. USP9X interacted with, depolyubiquitylated, and stabilized ALDH1A3. Moreover, we showed that FACS-sorted USP9Xhi cells were enriched for MES GSCs with high ALDH1A3 activity and potent tumorigenic capacity. Depletion of USP9X markedly downregulated ALDH1A3, resulting in a loss of self-renewal and tumorigenic capacity of MES GSCs, which could be largely rescued by ectopic expression of ALDH1A3. Furthermore, we demonstrated that the USP9X inhibitor WP1130 induced ALDH1A3 degradation and showed marked therapeutic efficacy in MES GSC-derived orthotopic xenograft models. Additionally, USP9X strongly correlated with ALDH1A3 expression in primary human GBM samples and had a prognostic value for patients with the MES subgroup. Collectively, our findings unveil USP9X as a key deubiquitinase for ALDH1A3 protein stabilization and a potential target for GSC-directed therapy.

Deubiquitinase ubiquitin-specific protease 9X regulates the stability and function of E3 ubiquitin ligase ring finger protein 115 in breast cancer cells.

The E3 ubiquitin ligase ring finger protein 115 (RNF115) is overexpressed in more than half of human breast tumors and is implicated in the pathogenesis and progression of breast cancer. However, the mechanism behind RNF115 overexpression in breast tumors remains largely unknown. Here we report that ubiquitin‐specific protease 9X (USP9X), a substrate‐specific deubiquitinating enzyme, stabilizes RNF115 and thereby regulates its biological functions in breast cancer cells. Immunoprecipitation and GST pull‐down assays showed that USP9X interacted with RNF115. Depletion of RNF115 by siRNAs or overexpression of RNF115 did not significantly affect USP9X expression. In contrast, knockdown of USP9X in breast cancer cells by siRNAs reduced RNF115 protein abundance, which was partially restored following treatment with proteasome inhibitor MG‐132. Moreover, depletion of USP9X reduced the half‐life of RNF115 and increased its ubiquitination. Conversely, overexpression of USP9X resulted in an accumulation of RNF115 protein, accompanied by a decrease in its ubiquitination. RNF115 mRNA levels were unaffected by overexpression or knockdown of USP9X. Furthermore, USP9X protein expression levels correlated positively with RNF115 in breast cancer cell lines and breast tumor samples. Importantly, reintroduction of RNF115 in USP9X‐depleted cells partially rescued the reduced proliferation, migration, and invasion of breast cancer cells by USP9X knockdown. Collectively, these findings indicate that USP9X is a stabilizer of RNF115 protein and that the USP9X‐RNF115 signaling axis is implicated in the breast cancer malignant phenotype.

USP9X destabilizes pVHL and promotes cell proliferation.

Numerous mutations of the Von Hippel-Lindau (VHL) gene have been reported to cause dysfunction of VHL protein (pVHL) and lead to processes related to tumor progression. pVHL acts as an E3 ligase and degrades downstream targets, such as hypoxia-inducible transcription factor (HIF) which is essential for tumor growth. Previous studies reported reduction of VHL protein, rather than mRNA in VHL-related tumor patients, suggesting that instability of the pVHL protein itself is a primary cause of dysfunction. Regulation of pVHL stability has therefore been a major focus of research. We report that ubiquitin-specific protease 9X (USP9X), which is a deubiquitinase binds and promotes degradation of both wild-type and mutants of pVHL that retain E3 ligase function, thus activating the HIF pathway. USP9X degrades pVHL through protection of its substrate, the newly identified pVHL E3 ligase Smurf1. In addition, USP9X activates glycolysis and promotes cell proliferation through pVHL. Treatment with a USP9X inhibitor shows an effect similar to USP9X knockdown in pVHL induction, and suppresses HIF activity. Our findings demonstrate that USP9X is a novel regulator of pVHL stability, and USP9X may be a therapeutic target for treatment of VHL-related tumors.

Deubiquitylating enzyme USP9x regulates radiosensitivity in glioblastoma cells by Mcl-1-dependent and -independent mechanisms.

Glioblastoma is a very aggressive form of brain tumor with limited therapeutic options. Usually, glioblastoma is treated with ionizing radiation (IR) and chemotherapy after surgical removal. However, radiotherapy is frequently unsuccessful, among others owing to resistance mechanisms the tumor cells have developed. Antiapoptotic B-cell leukemia (Bcl)-2 family members can contribute to radioresistance by interfering with apoptosis induction in response to IR. Bcl-2 and the closely related Bcl-xL and Mcl-1 are often overexpressed in glioblastoma cells. In contrast to Bcl-2 and Bcl-xL, Mcl-1 is a short-lived protein whose stability is closely regulated by ubiquitylation-dependent proteasomal degradation. Although ubiquitin ligases facilitate degradation, the deubiquitylating enzyme ubiquitin-specific protease 9x (USP9x) interferes with degradation by removing polyubiquitin chains from Mcl-1, thereby stabilizing this protein. Thus, an inability to downregulate Mcl-1 by enhanced USP9x activity might contribute to radioresistance. Here we analyzed the impact of USP9x on Mcl-1 levels and radiosensitivity in glioblastoma cells. Correlating Mcl-1 and USP9x expressions were significantly higher in human glioblastoma than in astrocytoma. Downregulation of Mcl-1 correlated with apoptosis induction in established glioblastoma cell lines. Although Mcl-1 knockdown by siRNA increased apoptosis induction after irradiation in all glioblastoma cell lines, USP9x knockdown significantly improved radiation-induced apoptosis in one of four cell lines and slightly increased apoptosis in another cell line. In the latter two cell lines, USP9x knockdown also increased radiation-induced clonogenic death. The massive downregulation of Mcl-1 and apoptosis induction in A172 cells transfected with USP9x siRNA shows that the deubiquitinase regulates cell survival by regulating Mcl-1 levels. In contrast, USP9x regulated radiosensitivity in Ln229 cells without affecting Mcl-1 levels. We conclude that USP9x can control survival and radiosensitivity in glioblastoma cells by Mcl-1-dependent and Mcl-1-independent mechanisms.

Ubiquitin-specific protease 9X in host cells interacts with herpes simplex virus 1 ICP0.

Herpes simplex virus 1 (HSV-1) expresses infected cell protein 0 (ICP0), a multi-functional protein with E3 ubiquitin ligase activity and a critical regulator of the viral life cycle. To obtain novel insights into the molecular mechanism by which ICP0 regulates HSV-1 replication, we analyzed HEp-2 cells infected with HSV-1 by tandem affinity purification and mass spectrometry-based proteomics. This screen identified 50 host-cell proteins that potentially interact with ICP0, including ubiquitin-specific protease 9X (USP9X). The interaction between ICP0 and USP9X was confirmed by co-immunoprecipitation. Notably, USP9X depletion increased the ICP0 abundance and promoted viral replication. These results suggest that USP9X-dependent regulation of ICP0 expression is part of a complex feedback mechanism that facilitates optimal HSV-1 replication.

Reduced ubiquitin-specific protease 9X expression induced by RNA interference inhibits the bioactivity of hepatocellular carcinoma cells.

Ubiquitin-specific protease 9X (USP9X) is crucial in many tumor types, but not in hepatocellular carcinoma (HCC). The current study aimed to examine the effects of RNA interference on USP9X expression, and subsequently on the bioactivity of HCC SMMC7721 and HepG2 cells. The protein expression of USP9X in SMMC7721, HepG2 and normal human liver cell line L02 at the cellular level was determined by western blot analysis; USP9X was knocked down by small interfering RNA (siRNA) in HCC SMMC7721 and HepG2 cells. In vitro cell viability was assessed by MTT assay, apoptosis was determined by flow cytometry (FCM) and cell migration was evaluated by Transwell assays. The protein expression of USP9X in SMMC7721 and HepG2 were both significantly higher than that in L02 (P<0.01). The results of western blot demonstrated that the USP9X-siRNA could efficiently inhibit USP9X expression when compared with that of the negative control (NC) group (P<0.01) and MTT assay demonstrated that cell proliferation in USP9X-blocked cells was significantly reduced when compared with that of the NC group (P<0.01). The results of FCM revealed that apoptosis was significantly increased in USP9X-blocked cells when compared with that of the NC group (P<0.01). The results of transwell assay showed that cell migration was significantly inhibited in USP9X-blocked cells when compared with that of the NC group (P<0.01). These results show that expression of USP9X is upregulated in hepatoma cells SMMC7721 and HepG2, and that downregulating USP9X by siRNA may induce cell apoptosis, inhibit cell growth and cell migration in the HCC SMMC7721 and HepG2 cell lines. USP9X may therefore be a potential target for HCC treatment and early detection.