New Insights into the Role of E2s in the Pathogenesis of Diseases: Lessons Learned from UBE2O

Exploiting the potential of the ubiquitin-proteasome system in overcoming tyrosine kinase inhibitor resistance in chronic myeloid leukemia

The advent of tyrosine kinase inhibitors (TKI) targeting BCR-ABL has drastically changed the treatment approach of chronic myeloid leukemia (CML), greatly prolonged the life of CML patients, and improved their prognosis. However, TKI resistance is still a major problem with CML patients, reducing the efficacy of treatment and their quality of life. TKI resistance is mainly divided into BCR-ABL-dependent and BCR-ABL-independent resistance. Now, the main clinical strategy addressing TKI resistance is to switch to newly developed TKIs. However, data have shown that these new drugs may cause serious adverse reactions and intolerance and cannot address all resistance mutations. Therefore, finding new therapeutic targets to overcome TKI resistance is crucial and the ubiquitin-proteasome system (UPS) has emerged as a focus. The UPS mediates the degradation of most proteins in organisms and controls a wide range of physiological processes. In recent years, the study of UPS in hematological malignant tumors has resulted in effective treatments, such as bortezomib in the treatment of multiple myeloma and mantle cell lymphoma. In CML, the components of UPS cooperate or antagonize the efficacy of TKI by directly or indirectly affecting the ubiquitination of BCR-ABL, interfering with CML-related signaling pathways, and negatively or positively affecting leukemia stem cells. Some of these molecules may help overcome TKI resistance and treat CML. In this review, the mechanism of TKI resistance is briefly described, the components of UPS are introduced, existing studies on UPS participating in TKI resistance are listed, and UPS as the therapeutic target and strategies are discussed.

UBE2O reduces the effectiveness of interferon-α via degradation of IFIT3 in hepatocellular carcinoma

Interferon (IFN) exerts its effects through interferon-stimulated genes (ISGs), but its efficacy is limited by interferon resistance, which can be caused by the ubiquitination of key proteins. UBE2O was initially identified as a promising therapeutic target based on data from the TCGA and iUUCD 2.0 databases. Through the inhibition of UBE2O, interferon α/β signaling and overall interferon signaling were activated. Integrating data from proteomic, mass spectrometry, and survival analyses led to the identification of IFIT3, a mediator of interferon signaling, as a ubiquitination substrate of UBE2O. The results of in vitro and in vivo experiments demonstrated that the knockdown of UBE2O can enhance the efficacy of interferon-α by upregulating IFIT3 expression. K236 was identified as a ubiquitination site in IFIT3, and the results of rescue experiments confirmed that the effect of UBE2O on interferon-α sensitivity is dependent on IFIT3 activity. ATO treatment inhibited UBE2O and increased IFIT3 expression, thereby increasing the effectiveness of interferon-α. In conclusion, these findings suggest that UBE2O worsens the therapeutic effect of interferon-α by targeting IFIT3 for ubiquitination and degradation.

Activation of PI3K/AKT/mTOR signaling axis by UBE2S inhibits autophagy leading to cisplatin resistance in ovarian cancer

BACKGROUND

Epithelial ovarian cancer (OC) is the fourth leading cause of cancer-related deaths in women, with a 5-year survival rate of 30%-50%. Platinum resistance is the chief culprit for the high recurrence and mortality rates. Several studies confirm that the metabolic regulation of ubiquitinating enzymes plays a vital role in platinum resistance in OC.

METHODS

In this study, we selected ubiquitin-conjugating enzyme E2S (UBE2S) as the candidate gene for validation. The levels of UBE2S expression were investigated using TCGA, GTEx, UALCAN, and HPA databases. In addition, the correlation between UBE2S and platinum resistance in OC was analyzed using data from TCGA. Cisplatin-resistant OC cell lines were generated and UBE2S was knocked down; the transfection efficiency was verified. Subsequently, the effects of knockdown of UBE2S on the proliferation and migration of cisplatin-resistant OC cells were examined through the CCK8, Ki-67 immunofluorescence, clone formation, wound healing, and transwell assays. In addition, the UBE2S gene was also validated in vivo by xenograft models in nude mice. Finally, the relationship between the UBE2S gene and autophagy and the possible underlying regulatory mechanism was preliminarily investigated through MDC and GFP-LC3-B autophagy detection and western blotting experiments. Most importantly, experimental validation of mTOR agonist reversion (the rescuse experiments) was also performed.

RESULTS

UBE2S was highly expressed in OC at both nucleic acid and protein levels. The results of immunohistochemistry showed that the level of UBE2S expression in platinum-resistant samples was significantly higher relative to the platinum-sensitive samples. By cell transfection experiments, knocking down of the UBE2S gene was found to inhibit the proliferation and migration of cisplatin-resistant OC cells. Moreover, the UBE2S gene could inhibit autophagy by activating the PI3K/AKT/mTOR signaling pathway to induce cisplatin resistance in OC in vivo and in vitro.

CONCLUSION

In conclusion, we discovered a novel oncogene, UBE2S, which was associated with platinum response in OC, and examined its key role through bioinformatics and preliminary experiments. The findings may open up a new avenue for the evaluation and treatment of OC patients at high risk of cisplatin resistance.

Tumor necrosis factor mediates USE1-independent FAT10ylation under inflammatory conditions

The ubiquitin-like modifier FAT10 is up-regulated in many different cell types by IFNγ and TNFα (TNF) and directly targets proteins for proteasomal degradation. FAT10 gets covalently conjugated to its conjugation substrates by the E1 activating enzyme UBA6, the E2 conjugating enzyme USE1, and E3 ligases including Parkin. To date, USE1 was supposed to be the only E2 enzyme for FAT10ylation, and we show here that a knockout of USE1 strongly diminished FAT10 conjugation. Remarkably, under inflammatory conditions in the presence of TNF, FAT10 conjugation appears to be independent of USE1. We report on the identification of additional E2 conjugating enzymes, which were previously not associated with FAT10. We confirm their capacity to be charged with FAT10 onto their active site cysteine, and to rescue FAT10 conjugation in the absence of USE1. This finding strongly widens the field of FAT10 research by pointing to multiple, so far unknown pathways for the conjugation of FAT10, disclosing novel possibilities for pharmacological interventions to regulate FAT10 conjugation under inflammatory conditions and/or viral infections.

High Level of Ubiquitin Conjugate Enzyme E2O Indicates Poor Prognosis of Patients with Hepatocellular Carcinoma

OBJECTIVE

Ubiquitin conjugate enzyme E2O (UBE2O) is a ubiquitin-conjugating enzyme that has been reported to be involved in tumorigenesis. This study investigated the role of UBE2O in hepatocellular carcinoma (HCC).

METHODS

The expression of UBE2O was detected using qRT-PCR, Western blotting, and immunohistochemical staining. Cell proliferation and Transwell assays were used to detect proliferation, migration, and invasion of HCC cells, respectively. Bioinformatic analysis was performed to analyze the relationship between UBE2O and the clinical features, prognosis, and immune cell infiltration of HCC.

RESULTS

UBE2O was significantly over-expressed in HCC tissues. High expression of UBE2O was associated with poor tumor grade and poor prognosis. Functional experiments showed that down-regulation of UBE2O inhibited HCC cell proliferation, migration, and invasion. Co-expression gene analysis and gene set enrichment analysis showed that UBE2O was associated with protein hydrolysis, cell cycle, and cancer-related pathways in HCC. The results of immune analysis revealed that the expression of UBE2O was positively correlated with the immune infiltration and expression of immune-related chemokines of HCC.

CONCLUSIONS

UBE2O is significantly correlated with the prognosis of HCC and may be a valuable prognostic biomarker for HCC.

Hypoxia-induced exosomal circPDK1 promotes pancreatic cancer glycolysis via c-myc activation by modulating miR-628-3p/BPTF axis and degrading BIN1

Background

circRNA has been established to play a pivotal role in tumorigenesis development in a variety of cancers; nevertheless, the biological functions and molecular mechanisms of hypoxia-induced exosomal circRNAs in pancreatic cancer remain largely unknown.

Methods

Differentially expressed circRNAs in exosomes between hypoxic exosomes and normoxic exosomes in PC cells were verified by RNA sequencing. The expression of circPDK1 in PC tumors and PC patients was evaluated by qRT-PCR and ISH, and the biological functions of circPDK1 in PC were verified through a series of in vitro and in vivo experiments. Using Western blotting, Co-IP, RNA pull-down, ChIP, RIP, dual-luciferase assays, and rescue experiments, the underlying mechanism of circPDK1 was verified.

Results

CircPDK1 was highly abundant in PC tumor tissues and serum exosomes and was associated with poor survival. Exosomal circPDK1 significantly promoted PC cell proliferation, migration, and glycolysis both in vitro and in vivo. Mechanistically, circPDK1 could be activated by HIF1A at the transcriptional level and sponges miR-628-3p to activate the BPTF/c-myc axis. In addition, circPDK1 serves as a scaffold that enhances the interaction between UBE2O and BIN1, inducing the UBE2O-mediated degradation of BIN1.

Conclusions

We found that circPDK1 was activated by HIF1A at the transcriptional level by modulating the miR-628-3p/BPTF axis and degrading BIN1. Exosomal circPDK1 is a promising biomarker for PC diagnosis and prognosis and represents a potential therapeutic target for PC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13045-022-01348-7.

UBE2S promotes the development of ovarian cancer by promoting PI3K/AKT/mTOR signaling pathway to regulate cell cycle and apoptosis

Background

Ovarian cancer is one of the important factors that seriously threaten women's health and its morbidity and mortality ranks eighth among female cancers in the world. It is critical to identify potential and promising biomarkers for prognostic evaluation and molecular therapy of OV. Ubiquitin-conjugating enzyme E2S (UBE2S), a potential oncogene, regulates the malignant progression of various tumors; however, its role in OV is still unclear.

Methods

The expression and prognostic significance of UBE2S at the pan-cancer level were investigated through high-throughput gene expression analysis and clinical prognostic data from TCGA, GEPIA, and GEO databases. 181 patients with OV were included in this study. Cell culture and cell transfection were performed on OV cell lines (SKOV3 and A2780) and a normal ovarian cell line (IOSE80). The expression level and prognostic significance of UBE2S in OV were verified by western blot, immunohistochemistry, and Kaplan–Meier survival analysis. Through cell transfection, CCK-8, Ki-67 immunofluorescence, wound healing, Transwell, clonogenic, and flow cytometry assays, the effect and detailed mechanism of UBE2S knockdown on the malignant biological behavior of OV cells were explored.

Results

UBE2S exhibited abnormally high expression at the pan-cancer level. The results of RT-qPCR and Western blotting indicated that UBE2S was significantly overexpressed in ovarian cancer cell lines compared with normal cell lines (P < 0.05). Kaplan–Meier survival analysis and Immunohistochemistry indicated that overexpression of UBE2S was related to poor prognosis of OV (HR > 1, P < 0.05). Results of in vitro experiments indicated that UBE2S gene knockdown might inhibit the proliferation, invasion, and prognosis of OV cells by inhibiting the PI3K/AKT/mTOR signaling pathway, thereby blocking the cell cycle and promoting apoptosis (P < 0.05).

Conclusion

UBE2S is a potential oncogene strongly associated with a poor prognosis of OV patients. Knockdown of UBE2S could block the cell cycle and promote apoptosis by inhibiting the PI3K/AKT/mTOR pathway and ultimately inhibit the proliferation, migration and prognosis of ovarian cancer, which suggested that UBE2S might be used for molecular therapy and prognostic evaluation of ovarian cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00489-2.

E3 Ubiquitin Ligases in Breast Cancer Metastasis: A Systematic Review of Pathogenic Functions and Clinical Implications

Female breast cancer has become the most commonly occurring cancer worldwide. Although it has a good prognosis under early diagnosis and appropriate treatment, breast cancer metastasis drastically causes mortality. The process of metastasis, which includes cell epithelial–mesenchymal transition, invasion, migration, and colonization, is a multistep cascade of molecular events directed by gene mutations and altered protein expressions. Ubiquitin modification of proteins plays a common role in most of the biological processes. E3 ubiquitin ligase, the key regulator of protein ubiquitination, determines the fate of ubiquitinated proteins. E3 ubiquitin ligases target a broad spectrum of substrates. The aberrant functions of many E3 ubiquitin ligases can affect the biological behavior of cancer cells, including breast cancer metastasis. In this review, we provide an overview of these ligases, summarize the metastatic processes in which E3s are involved, and comprehensively describe the roles of E3 ubiquitin ligases. Furthermore, we classified E3 ubiquitin ligases based on their structure and analyzed them with the survival of breast cancer patients. Finally, we consider how our knowledge can be used for E3s’ potency in the therapeutic intervention or prognostic assessment of metastatic breast cancer.

Ubiquitin-Conjugating Enzymes in Cancer

The ubiquitin-mediated degradation system is responsible for controlling various tumor-promoting processes, including DNA repair, cell cycle arrest, cell proliferation, apoptosis, angiogenesis, migration and invasion, metastasis, and drug resistance. The conjugation of ubiquitin to a target protein is mediated sequentially by the E1 (activating)‒E2 (conjugating)‒E3 (ligating) enzyme cascade. Thus, E2 enzymes act as the central players in the ubiquitination system, modulating various pathophysiological processes in the tumor microenvironment. In this review, we summarize the types and functions of E2s in various types of cancer and discuss the possibility of E2s as targets of anticancer therapeutic strategies.

The Molecular Basis of Ubiquitin-Conjugating Enzymes (E2s) as a Potential Target for Cancer Therapy

Ubiquitin-conjugating enzymes (E2s) are one of the three enzymes required by the ubiquitin-proteasome pathway to connect activated ubiquitin to target proteins via ubiquitin ligases. E2s determine the connection type of the ubiquitin chains, and different types of ubiquitin chains regulate the stability and activity of substrate proteins. Thus, E2s participate in the regulation of a variety of biological processes. In recent years, the importance of E2s in human health and diseases has been particularly emphasized. Studies have shown that E2s are dysregulated in variety of cancers, thus it might be a potential therapeutic target. However, the molecular basis of E2s as a therapeutic target has not been described systematically. We reviewed this issue from the perspective of the special position and role of E2s in the ubiquitin-proteasome pathway, the structure of E2s and biological processes they are involved in. In addition, the inhibitors and microRNAs targeting E2s are also summarized. This article not only provides a direction for the development of effective drugs but also lays a foundation for further study on this enzyme in the future.

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.

UBE2T-regulated H2AX monoubiquitination induces hepatocellular carcinoma radioresistance by facilitating CHK1 activation

Background

Radioresistance is the major obstacle in radiation therapy (RT) for hepatocellular carcinoma (HCC). Dysregulation of DNA damage response (DDR), which includes DNA repair and cell cycle checkpoints activation, leads to radioresistance and limits radiotherapy efficacy in HCC patients. However, the underlying mechanism have not been clearly understood.

Methods

We obtained 7 pairs of HCC tissues and corresponding non-tumor tissues, and UBE2T was identified as one of the most upregulated genes. The radioresistant role of UBE2T was examined by colony formation assays in vitro and xenograft tumor models in vivo. Comet assay, cell cycle flow cytometry and γH2AX foci measurement were used to investigate the mechanism by which UBE2T mediating DDR. Chromatin fractionation and immunofluorescence staining were used to assess cell cycle checkpoint kinase 1(CHK1) activation. Finally, we analyzed clinical data from HCC patients to verify the function of UBE2T.

Results

Here, we found that ubiquitin-conjugating enzyme E2T (UBE2T) was upregulated in HCC tissues, and the HCC patients with higher UBE2T levels exhibited poorer outcomes. Functional studies indicated that UBE2T increased HCC radioresistance in vitro and in vivo. Mechanistically, UBE2T-RNF8, was identified as the E2-E3 pair, physically bonded with and monoubiquitinated histone variant H2AX/γH2AX upon radiation exposure. UBE2T-regulated H2AX/γH2AX monoubiquitination facilitated phosphorylation of CHK1 for activation and CHK1 release from the chromatin to cytosol for degradation. The interruption of UBE2T-mediated monoubiquitination on H2AX/γH2AX, including E2-enzyme-deficient mutation (C86A) of UBE2T and monoubiquitination-site-deficient mutation (K119/120R) of H2AX, cannot effectively activate CHK1. Moreover, genetical and pharmacological inhibition of CHK1 impaired the radioresistant role of UBE2T in HCC. Furthermore, clinical data suggested that the HCC patients with higher UBE2T levels exhibited worse response to radiotherapy.

Conclusion

Our results revealed a novel role of UBE2T-mediated H2AX/γH2AX monoubiquitination on facilitating cell cycle arrest activation to provide sufficient time for radiation-induced DNA repair, thus conferring HCC radioresistance. This study indicated that disrupting UBE2T-H2AX-CHK1 pathway maybe a promising potential strategy to overcome HCC radioresistance.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s13046-020-01734-4.

UBE2O targets Mxi1 for ubiquitination and degradation to promote lung cancer progression and radioresistance

UBE2O, an E2/E3 hybrid ubiquitin-protein ligase, has been implicated in the regulation of adipogenesis, erythroid differentiation, and tumor proliferation. However, its role in cancer radioresistance remains completely unknown. Here, we uncover that UBE2O interacts and targets Mxi1 for ubiquitination and degradation at the K46 residue. Furthermore, we show that genetical or pharmacological blockade of UBE2O impairs tumor progression and radioresistance in lung cancer in vitro and in vivo, and these effects can be restored by Mxi1 inhibition. Moreover, we demonstrate that UBE2O is overexpressed and negatively correlated with Mxi1 protein levels in lung cancer tissues. Collectively, our work reveals that UBE2O facilitates tumorigenesis and radioresistance by promoting Mxi1 ubiquitination and degradation, suggesting that UBE2O is an attractive radiosensitization target for the treatment of lung cancer.

Association Study of Coronary Artery Disease-Associated Genome-Wide Significant SNPs with Coronary Stenosis in Pakistani Population

Genome-wide association studies (GWAS) of coronary artery disease (CAD) have revealed multiple genetic risk loci. We assessed the association of 47 genome-wide significant single-nucleotide polymorphisms (SNPs) at 43 CAD loci with coronary stenosis in a Pakistani sample comprising 663 clinically ascertained and angiographically confirmed cases. Genotypes were determined using the iPLEX Gold technology. All statistical analyses were performed using R software. Linkage disequilibrium (LD) between significant SNPs was determined using SNAP web portal, and functional annotation of SNPs was performed using the RegulomeDB and Genotype-Tissue Expression (GTEx) databases. Genotyping comparison was made between cases with severe stenosis (≥70%) and mild/minimal stenosis (<30%). Five SNPs demonstrated significant associations: three with additive genetic models PLG/rs4252120 (p = 0.0078), KIAA1462/rs2505083 (p = 0.005), and SLC22A3/rs2048327 (p = 0.045) and two with recessive models SORT1/rs602633 (p = 0.005) and UBE2Z/rs46522 (p = 0.03). PLG/rs4252120 was in LD with two functional PLG variants (rs4252126 and rs4252135), each with a RegulomeDB score of 1f. Likewise, KIAA1462/rs2505083 was in LD with a functional SNP, KIAA1462/rs3739998, having a RegulomeDB score of 2b. In the GTEx database, KIAA1462/rs2505083, SLC22A3/rs2048327, SORT1/rs602633, and UBE2Z/rs46522 SNPs were found to be expression quantitative trait loci (eQTLs) in CAD-associated tissues. In conclusion, five genome-wide significant SNPs previously reported in European GWAS were replicated in the Pakistani sample. Further association studies on larger non-European populations are needed to understand the worldwide genetic architecture of CAD.

Ubiquitin Subproteome of Brain Mitochondria and Its Changes Induced by Experimental Parkinsonism and Action of Neuroprotectors

The review summarizes the data of our research and published studies on the ubiquitination of brain mitochondrial proteins and its changes during the development of experimental parkinsonism and administration of the neuroprotector isatin (indole-2,3-dione) with special attention to the mitochondrial ubiquitin-conjugating system and location of ubiquitinated proteins in these organelles. Incubation of brain mitochondrial fraction with biotinylated ubiquitin in vitro resulted in the incorporation of biotinylated ubiquitin in both mitochondrial and mitochondria-associated proteins. According to the interactome analysis, the identified non-ubiquitinated proteins are able to form tight complexes with ubiquitinated proteins or their partners and components of mitochondrial membranes, in which interactions of ubiquitin chains with the ubiquitin-binding protein domains play an important role. The studies of endogenous ubiquitination in the total brain mitochondrial fraction of C57Bl mice performed in different laboratories have shown that mitochondrial proteins represent about 30% of all ubiquitinated proteins. However, comparison of brain subproteomes of mitochondrial ubiquitinated proteins reported in the literature revealed significant differences both in their composition and involvement of identified ubiquitinated proteins in biological processes listed in the Gene Ontology database. The development of experimental parkinsonism in C57Bl mice induced by a single-dose administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) resulted in a decrease in the total number of mitochondrial ubiquitinated proteins and increase in the number of oxidized mitochondrial proteins containing the ubiquitin signature (K-ε-GG). Comparison of ubiquitinated proteins associated with the mouse brain mitochondrial fraction and mouse brain mitochondrial proteins bound to the proteasome ubiquitin receptor (Rpn10 subunit) did not reveal any common proteins. This suggests that ubiquitination of brain mitochondrial proteins is not directly related to their degradation in the proteasomes. Proteomic profiling of brain isatin-binding proteins identified enzymes involved in the ubiquitin-conjugating system functioning. Mapping of the identified isatin-binding proteins to known metabolic pathways indicates their participation in the parkin (E3 ubiquitin ligase)-associated pathway (CH000000947). The functional links involving brain mitochondrial ubiquitinated proteins were found only in the group of animals with the MPTP-induced parkinsonism, but not in animals treated with MPTP/isatin or isatin only. This suggests that the neuroprotective effect of isatin may be associated with the impaired functional relationships of proteins targeted to subsequent degradation.

The ubiquitin-conjugating enzyme UBE2QL1 coordinates lysophagy in response to endolysosomal damage

The autophagic clearance of damaged lysosomes by lysophagy involves extensive modification of the organelle with ubiquitin, but the underlying ubiquitination machinery is still poorly characterized. Here, we use an siRNA screening approach and identify human UBE2QL1 as a major regulator of lysosomal ubiquitination, lysophagy, and cell survival after lysosomal damage. UBE2QL1 translocates to permeabilized lysosomes where it associates with damage sensors, ubiquitination targets, and lysophagy effectors. UBE2QL1 knockdown reduces ubiquitination and accumulation of the critical autophagy receptor p62 and abrogates recruitment of the AAA-ATPase VCP/p97, which is essential for efficient lysophagy. Crucially, it affects association of LC3B with damaged lysosomes indicating that autophagosome formation was impaired. Already in unchallenged cells, depletion of UBE2QL1 leads to increased lysosomal damage, mTOR dissociation from lysosomes, and TFEB activation pointing to a role in lysosomal homeostasis. In line with this, mutation of the homologue ubc-25 in Caenorhabditis elegans exacerbates lysosome permeability in worms lacking the lysosome stabilizing protein SCAV-3/LIMP2. Thus, UBE2QL1 coordinates critical steps in the acute endolysosomal damage response and is essential for maintenance of lysosomal integrity.

UBE2E1 Is Preferentially Expressed in the Cytoplasm of Slow-Twitch Fibers and Protects Skeletal Muscles from Exacerbated Atrophy upon Dexamethasone Treatment

Skeletal muscle mass is reduced during many diseases or physiological situations (disuse, aging), which results in decreased strength and increased mortality. Muscle mass is mainly controlled by the ubiquitin-proteasome system (UPS), involving hundreds of ubiquitinating enzymes (E2s and E3s) that target their dedicated substrates for subsequent degradation. We recently demonstrated that MuRF1, an E3 ubiquitin ligase known to bind to sarcomeric proteins (telethonin, α-actin, myosins) during catabolic situations, interacts with 5 different E2 enzymes and that these E2-MuRF1 couples are able to target telethonin, a small sarcomeric protein, for degradation. Amongst the E2s interacting with MuRF1, E2E1 was peculiar as the presence of the substrate was necessary for optimal MuRF1-E2E1 interaction. In this work, we focused on the putative role of E2E1 during skeletal muscle atrophy. We found that E2E1 expression was restricted to type I and type IIA muscle fibers and was not detectable in type IIB fibers. This strongly suggests that E2E1 targets are fiber-specific and may be strongly linked to the contractile and metabolic properties of the skeletal muscle. However, E2E1 knockdown was not sufficient for preserving the protein content in C2C12 myotubes subjected to a catabolic state (dexamethasone treatment), suggesting that E2E1 is not involved in the development of muscle atrophy. By contrast, E2E1 knockdown aggravated the atrophying process in both catabolic C2C12 myotubes and the Tibialis anterior muscle of mice, suggesting that E2E1 has a protective effect on muscle mass.

Ubiquitin-conjugating enzyme UBE2O regulates cellular clock function by promoting the degradation of the transcription factor BMAL1

Dysregulation of the circadian rhythm is associated with many diseases, including diabetes, obesity, and cancer. Aryl hydrocarbon receptor nuclear translocator-like protein 1 (Arntl or Bmal1) is the only clock gene whose loss disrupts circadian locomotor behavior in constant darkness. BMAL1 levels are affected by proteasomal inhibition and by several enzymes in the ubiquitin-proteasome system, but the exact molecular mechanism remains unclear. Here, using immunoprecipitation and MS analyses, we discovered an interaction between BMAL1 and ubiquitin-conjugating enzyme E2 O (UBE2O), an E3-independent E2 ubiquitin-conjugating enzyme (i.e. hybrid E2/E3 enzyme). Biochemical experiments with cell lines and animal tissues validated this specific interaction and uncovered that UBE2O expression reduces BMAL1 levels by promoting its ubiquitination and degradation. Moreover, UBE2O expression/knockdown diminished/increased, respectively, BMAL1-mediated transcriptional activity but did not affect BMAL1 gene expression. Bioluminescence experiments disclosed that UBE2O knockdown elevates the amplitude of the circadian clock in human osteosarcoma U2OS cells. Furthermore, mapping of the BMAL1-interacting domain in UBE2O and analyses of BMAL1 stability and ubiquitination revealed that the conserved region 2 (CR2) in UBE2O significantly enhances BMAL1 ubiquitination and decreases BMAL1 protein levels. A Cys-to-Ser substitution experiment identified the critical Cys residue in the CR2 domain responsible for BMAL1 ubiquitination. This work identifies UBE2O as a critical regulator in the ubiquitin-proteasome system, which modulates BMAL1 transcriptional activity and circadian function by promoting BMAL1 ubiquitination and degradation under normal physiological conditions.