UbcH10 has a rate-limiting role in G1 phase but might not act in the spindle checkpoint or as part of an autonomous oscillator

UBE2C promotes malignancy of cutaneous squamous cell carcinoma

BACKGROUND

Our study aimed to study the involvement of ubiquitin-conjugating enzyme E2C (UBE2C) in cutaneous squamous cell carcinoma (cSCC). As the second most common malignancy with a rising incidence, understanding the molecular mechanisms driving cSCC is crucial for improved diagnosis and treatment.

METHODS

We combined multiple datasets of cSCC in Gene Expression Omnibus (GEO) repository to investigate its expression and diagnostic value. We collected patient specimens and performed immunohistochemistry to examine its expression in patients and its correlation with tumor histological grade. Moreover, we compared UBE2C expression between cSCC cells and primary human epidermal keratinocytes. Subsequently, we explored the effects of UBE2C inhibition on tumor cell proliferation, migration and apoptosis through CCK8, wound healing, Transwell, and flow cytometry assay.

RESULTS

The integrated analysis revealed an upregulation of UBE2C level in cSCC. Immunohistochemistry demonstrated high UBE2C expression was associated with poorer tumor histological grade. Cell experiments further supported the crucial role of UBE2C in promoting the malignant behavior of cSCC cells.

CONCLUSION

Our findings indicate UBE2C is up-regulated in cSCC and contributes to its malignant behavior. These results suggest UBE2C has the potential to serve as both a cSCC biomarker and a therapeutic target.

The role of ubiquitin signaling pathway on liver regeneration in rats

The ubiquitin signalling pathway is a large system associated with numerous intracellular mechanisms. However, its function in the liver regeneration process remains unknown. This particular study investigates the intracellular effect mechanisms of the ubiquitin signalling pathway. It also determines the differences in the expression of 88 genes belonging to the ubiquitin pathway using the RT-PCR array method. To conduct this research, three genes-that differed in the expression analysis were selected. Moreover, their proteins were analysed by western blot analysis while using Ki67 immunohistochemical analysis that determines proliferation rates by hour. It was determined that BRCA1 and BARD1, which are effective in DNA repair, play an active role at PH24. Similarly, Ube2t expression, which belongs to the Fanconi anaemia pathway associated with DNA repair, was also found to be high at PH12-72 h. In addition, it was revealed that the expressions of Anapc2, Anapc11, Cdc20 belonging to the APC/C^Cdc20 complex, which participate in cell cycle regulation, differed at different hours after PH. Expression of Mul1, which is involved in mitochondrial biogenesis and mitophagy mechanisms, peaked at PH12 under the observation. Considering the Mul1 gene expression difference, MUL1-mediated mitophagy and mitochondrial fission mechanism may be associated with liver regeneration. It was also determined that PARKIN-mediated mitophagy mechanisms are not active in 0-72 h of liver regeneration since PARKIN expression did not show a significant change in PH groups.

The heterogeneity of mammalian utricular cells over the course of development

BACKGROUND

The inner ear organ is a delicate tissue consisting of hair cells (HCs) and supporting cells (SCs).The mammalian inner ear HCs are terminally differentiated cells that cannot spontaneously regenerate in adults. Epithelial non-hair cells (ENHCs) in the utricle include HC progenitors and SCs, and the progenitors share similar characteristics with SCs in the neonatal inner ear.

METHODS

We applied single-cell sequencing to whole mouse utricles from the neonatal period to adulthood, including samples from postnatal day (P)2, P7 and P30 mice. Furthermore, using transgenic mice and immunostaining, we traced the source of new HC generation.

RESULTS

We identified several sensory epithelial cell clusters and further found that new HCs arose mainly through differentiation from Sox9+ progenitor cells and that only a few cells were produced by mitotic proliferation in both neonatal and adult mouse utricles. In addition, we identified the proliferative cells using the marker UbcH10 and demonstrated that in adulthood the mitotically generated HCs were primarily found in the extrastriola. Moreover, we observed that not only Type II, but also Type I HCs could be regenerated by either mitotic cell proliferation or progenitor cell differentiation.

CONCLUSIONS

Overall, our findings expand our understanding of ENHC cell fate and the characteristics of the vestibular organs in mammals over the course of development.

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.

A substrate-trapping strategy to find E3 ubiquitin ligase substrates identifies Parkin and TRIM28 targets

The identification of true substrates of an E3 ligase is biologically important but biochemically difficult. In recent years, several techniques for identifying substrates have been developed, but these approaches cannot exclude indirect ubiquitination or have other limitations. Here we develop an E3 ligase substrate-trapping strategy by fusing a tandem ubiquitin-binding entity (TUBE) with an anti-ubiquitin remnant antibody to effectively identify ubiquitinated substrates. We apply this method to one of the RBR-type ligases, Parkin, and to one of the RING-type ligases, TRIM28, and identify previously unknown substrates for TRIM28 including cyclin A2 and TFIIB. Furthermore, we find that TRIM28 promotes cyclin A2 ubiquitination and degradation at the G1/S phase and suppresses premature entry into S phase. Taken together, the results indicate that this method is a powerful tool for comprehensively identifying substrates of E3 ligases.

Watanabe et al. combine two previously developed strategies to identify E3 ubiquitin ligase substrates into a method, TR-TUBE that is subsequently used to identify substrates of the Parkin and TRIM28 ligases. They identify known substrates, validating the utility of the approach, and find that TRIM28 targets Cyclin A and TFIIB for degradation.

UBE2C promotes the progression of head and neck squamous cell carcinoma

The development of head and neck squamous cell carcinoma (HNSCC) is a complex pathological process and many cellular and molecular events may occur. The ubiquitin conjugating enzyme E2 (UBE2C) was found to play an oncogenic role in several human cancers. However, its functional role in HNSCC tumorigenesis remains unknown. In this study, UBE2C gene expression in HNSCC was first evaluated using the data from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. The connection between UBE2C gene expression and patients' survival rates of HNSCC and other human cancers was also investigated. Liquid chromatography with tandem mass spectrometry was used to identify differentially expressed proteins, including UBE2C, between UMSCC1 oral cancer cells and normal human oral keratinocytes (NHOKs). Immunohistochemistry (IHC) was used to verify the differential expression of UBE2C protein between HNSCC and adjacent control tissues. Cell cycle analysis, MTT, colony formation, Transwell migration, and Matrigel invasion assays were used to study the effect of UBE2C downregulation on the malignant phenotypes of HNSCC cells. The bioinformatic analysis of the proteins interacting with UBE2C in HNSCC cells was also performed. Based on the data obtained from the cancer databases and our in vitro studies, we found that UBE2C was overexpressed in HNSCC and patients with high UBE2C expression suffered a remarkably worse overall survival rate than those with low UBE2C expression, and a similar observation was found in a number of other human cancers. UBE2C was also found to be overexpressed in HNSCC cells versus normal human oral keratinocytes and inhibition of UBE2C expression significantly suppressed the malignant phenotypes of HNSCC cells in vitro. The bioinformatic analysis indicated that UBE2C may be involved in head and neck tumorigenesis through the mediation of important pathways such as ubiquitin mediated proteolysis, proteasome, and cell cycle. In conclusion, our results suggest that UBE2C is consistently upregulated in many human solid tumors. It promotes HNSCC progression and may serve as a potential prognostic biomarker in HNSCC. Future studies are warranted to unveil the underlying molecular pathways of UBE2C in HNSCC.

UBE2C overexpression in melanoma and its essential role in G2/M transition

Ubiquitin‑conjugating enzyme E2C (UBE2C) is a key regulator of cell cycle progression, and its aberrant expression has been implicated in various malignancies. However, its clinical and biological roles in malignant melanoma is still unclear. In this study, we found a significant high expression level of UBE2C in melanoma by an in silico analysis of The Cancer Genome Atlas (TCGA) database, which was further validated using fresh melanoma samples. The KM plotter showed that UBE2C level was statistically related to the overall survival (OS) of melanoma patients (p<0.01). RNA interference of UBE2C inhibited the growth of melanoma cells via deactivating ERK/Akt signaling pathways, and blocked the G2/M transition through downregulation of both the level and the activity of mitosis promoting factor (MPF), triggering the apoptosis of melanoma cells. Further, silencing of UBE2C significantly inhibited the xenografted tumor growth on nude mice, indicating an important role of UBE2C in melanoma growth in vivo. Together, our results show that UBE2C may serve as a novel prognostic biomarker as well as a potential therapeutic target for melanoma.

UBE2C Induces Cisplatin Resistance via ZEB1/2-Dependent Upregulation of ABCG2 and ERCC1 in NSCLC Cells

Objectives

Cisplatin (DDP) is one of the most commonly used chemotherapeutic drugs for several cancers, including non-small-cell lung cancer (NSCLC). However, resistance to DDP eventually develops, limiting its further application. New therapy targets are urgently needed to reverse DDP resistance.

Methods

The mRNA expression of UBE2C, ZEB1/2, ABCG2, and ERCC1 was analyzed by reverse transcription-polymerase chain reaction. The protein levels of these molecules were analyzed by Western blotting and immunofluorescent staining. Cell proliferation was detected by CCK8 and MTT assays. Cell migration and invasion were analyzed by wound healing assay and Transwell assays. Promoter activities and gene transcription were analyzed by luciferase reporter assay.

Results

In this study, we examined the effect of UBE2C and ZEB1/2 expression levels in DDP-resistant cells of NSCLC. We confirmed that aberrant expression of UBE2C and ZEB1/2 plays a critical role in repressing the DDP sensitivity to NSCLC cells. Additionally, knockdown of UBE2C significantly sensitized resistant cells to DDP by repressing the expression of ZEB1/2. Mechanistic investigations indicated that UBE2C transcriptionally regulated ZEB1/2 by accelerating promoter activity. This study revealed that ZEB1/2 promotes the epithelial mesenchymal transition and expression of ABCG2 and ERCC1 to participate in UBE2C-mediated NSCLC DDP-resistant cell progression, metastasis, and invasion.

Conclusion

UBE2C may be a novel therapy target for NSCLC for sensitizing cells to the chemotherapeutic agent DDP.

Who guards the guardian? Mechanisms that restrain APC/C during the cell cycle

The cell cycle is principally controlled by Cyclin Dependent Kinases (CDKs), whose oscillating activities are determined by binding to Cyclin coactivators. Cyclins exhibit dynamic changes in abundance as cells pass through the cell cycle. The sequential, timed accumulation and degradation of Cyclins, as well as many other proteins, imposes order on the cell cycle and contributes to genome maintenance. The destruction of many cell cycle regulated proteins, including Cyclins A and B, is controlled by a large, multi-subunit E3 ubiquitin ligase termed the Anaphase Promoting Complex/Cyclosome (APC/C). APC/C activity is tightly regulated during the cell cycle. Its activation state increases dramatically in mid-mitosis and it remains active until the end of G1 phase. Following its mandatory inactivation at the G1/S boundary, APC/C activity remains low until the subsequent mitosis. Due to its role in guarding against the inappropriate or untimely accumulation of Cyclins, the APC/C is a core component of the cell cycle oscillator. In addition to the regulation of Cyclins, APC/C controls the degradation of many other substrates. Therefore, it is vital that the activity of APC/C itself be tightly guarded. The APC/C is most well studied for its role and regulation during mitosis. However, the APC/C also plays a similarly important and conserved role in the maintenance of G1 phase. Here we review the diverse mechanisms counteracting APC/C activity throughout the cell cycle and the importance of their coordinated actions on cell growth, proliferation, and disease.

Irreversible APC(Cdh1) Inactivation Underlies the Point of No Return for Cell-Cycle Entry

Proliferating cells must cross a point of no return before they replicate their DNA and divide. This commitment decision plays a fundamental role in cancer and degenerative diseases and has been proposed to be mediated by phosphorylation of retinoblastoma (Rb) protein. Here, we show that inactivation of the anaphase-promoting complex/cyclosome (APC(Cdh1)) has the necessary characteristics to be the point of no return for cell-cycle entry. Our study shows that APC(Cdh1) inactivation is a rapid, bistable switch initiated shortly before the start of DNA replication by cyclin E/Cdk2 and made irreversible by Emi1. Exposure to stress between Rb phosphorylation and APC(Cdh1) inactivation, but not after APC(Cdh1) inactivation, reverted cells to a mitogen-sensitive quiescent state, from which they can later re-enter the cell cycle. Thus, APC(Cdh1) inactivation is the commitment point when cells lose the ability to return to quiescence and decide to progress through the cell cycle.

APC/C and SCF(cyclin F) Constitute a Reciprocal Feedback Circuit Controlling S-Phase Entry

The anaphase promoting complex/cyclosome (APC/C) is an ubiquitin ligase and core component of the cell-cycle oscillator. During G1 phase, APC/C binds to its substrate receptor Cdh1 and APC/C(Cdh1) plays an important role in restricting S-phase entry and maintaining genome integrity. We describe a reciprocal feedback circuit between APC/C and a second ubiquitin ligase, the SCF (Skp1-Cul1-F box). We show that cyclin F, a cell-cycle-regulated substrate receptor (F-box protein) for the SCF, is targeted for degradation by APC/C. Furthermore, we establish that Cdh1 is itself a substrate of SCF(cyclin F). Cyclin F loss impairs Cdh1 degradation and delays S-phase entry, and this delay is reversed by simultaneous removal of Cdh1. These data indicate that the coordinated, temporal ordering of cyclin F and Cdh1 degradation, organized in a double-negative feedback loop, represents a fundamental aspect of cell-cycle control. This mutual antagonism could be a feature of other oscillating systems.

Appropriate expression of Ube2C and Ube2S controls the progression of the first meiotic division

Timely degradation of protein regulators of the cell cycle is essential for the completion of cell division. This degradation is promoted by the E3 anaphase-promoting complex/cyclosome (APC/C) and mediated by the E2 ubiquitin-conjugating enzymes (Ube2s). Unlike the ample information gathered regarding the meiotic E3 APC/C, the E2s participating in this cell division have never been studied. We identified Ube2C, -S, and -D3 as the E2 enzymes that regulate APC/C activity during meiosis of mouse oocytes. Their depletion reduces the levels of the first meiotic cytokinesis by 50%, and their overexpression doubles and accelerates its completion (50% as compared with 4% at 11 h). We also demonstrated that these E2s take part in ensuring appropriate spindle formation. It is noteworthy that high levels of Ube2C bring about the resumption of the first meiotic division, regardless of the formation of the spindle, overriding the spindle assembly checkpoint. Thus, alongside their canonical function in protein degradation, Ube2C and -S also control the extrusion of the first polar body. Overall, our study characterizes new regulators and unveils the novel roles they play during the meiotic division. These findings shed light on faithful chromosome segregation in oocytes and may contribute to better understanding of aneuploidy and its consequent genetic malformations.

Co-activator independent differences in how the metaphase and anaphase APC/C recognise the same substrate

The Anaphase Promoting Complex or Cyclosome (APC/C) is critical to the control of mitosis. The APC/C is an ubiquitin ligase that targets specific mitotic regulators for proteolysis at distinct times in mitosis, but how this is achieved is not well understood. We have addressed this question by determining whether the same substrate, cyclin B1, is recognised in the same way by the APC/C at different times in mitosis. Unexpectedly, we find that distinct but overlapping motifs in cyclin B1 are recognised by the APC/C in metaphase compared with anaphase, and this does not depend on the exchange of Cdc20 for Cdh1. Thus, changes in APC/C substrate specificity in mitosis can potentially be conferred by altering interaction sites in addition to exchanging Cdc20 for Cdh1.

Using in vivo biotinylated ubiquitin to describe a mitotic exit ubiquitome from human cells

Mitotic division requires highly regulated morphological and biochemical changes to the cell. Upon commitment to exit mitosis, cells begin to remove mitotic regulators in a temporally and spatially controlled manner to bring about the changes that reestablish interphase. Ubiquitin-dependent pathways target these regulators to generate polyubiquitin-tagged substrates for degradation by the 26S proteasome. However, the lack of cell-based assays to investigate in vivo ubiquitination limits our knowledge of the identity of substrates of ubiquitin-mediated regulation in mitosis. Here we report an in vivo ubiquitin tagging system used in human cells that allows efficient purification of ubiquitin conjugates from synchronized cell populations. Coupling purification with mass spectrometry, we have identified a series of mitotic regulators targeted for polyubiquitination in mitotic exit. We show that some are new substrates of the anaphase-promoting complex/cyclosome and validate KIFC1 and RacGAP1/Cyk4 as two such targets involved respectively in timely mitotic spindle disassembly and cell spreading. We conclude that in vivo biotin tagging of ubiquitin can provide valuable information about the role of ubiquitin-mediated regulation in processes required for rebuilding interphase cells.

High-resolution live-cell imaging reveals novel cyclin A2 degradation foci involving autophagy

Cyclin A2 is a key player in the regulation of the cell cycle. Its degradation in mid-mitosis relies on the ubiquitin-proteasome system (UPS). Using high-resolution microscopic imaging, we find that cyclin A2 persists beyond metaphase. Indeed, we identify a novel cyclin-A2-containing compartment that forms dynamic foci. Förster (or fluorescence) resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) analyses show that cyclin A2 ubiquitylation takes place predominantly in these foci before spreading throughout the cell. Moreover, inhibition of autophagy in proliferating cells induces the stabilisation of a subset of cyclin A2, whereas induction of autophagy accelerates the degradation of cyclin A2, thus showing that autophagy is a novel regulator of cyclin A2 degradation.

RNA interference in the treatment of colon cancer

Colorectal cancer is the third most common cancer in both men and women and has shown a progressive increase over the past 20 years. Current chemotherapy has major limitations, and a novel therapeutic approach is required. Given that neoplastic transformation of colon epithelial cells is a consequence of genetic and epigenetic alterations, RNA interference (RNAi) has been proposed as a new therapeutic strategy that offers important advantages over conventional treatments, with high specificity and potency and low toxicity. RNAi has been employed as an effective tool to study the function of genes, preventing their expression and leading to the development of new approaches to cancer treatment. In malignancies, including colon cancer, RNAi is being used for "silencing" genes that are deregulated by different processes such as gene amplification, mutation, or overexpression and may be the cause of oncogenesis. This strategy not only provides information on the involvement of certain genes in colon cancer, but also opens up a new perspective for its treatment. However, most studies have used adenovirus or lentivirus vectors to transport RNAi into tumor cells or tumors in animal models, because several technical obstacles must be overcome before RNAi can be used in the clinical setting. The aim of this study was to review current knowledge on the use of RNAi techniques in the treatment of colon cancer.

Ubiquitin-conjugating enzyme E2C: a potential cancer biomarker

The ubiquitin-conjugating enzymes 2C (UBE2C) is an integral component of the ubiquitin proteasome system. UBE2C consists of a conserved core domain containing the catalytic Cys residue and an N-terminal extension. The core domain is required for ubiquitin adduct formation by interacting with the ubiquitin-fold domain in the E1 enzyme, and contributes to the E3 enzyme binding. UBE2C N-terminal extension regulates E3 enzyme activity as a part of an intrinsic inhibitory mechanism. UBE2C is required for the destruction of mitotic cyclins and securin, which are essential for spindle assembly checkpoint and mitotic exit. The UBE2C mRNA and/or protein levels are aberrantly increased in many cancer types with poor clinical outcomes. Accumulation of UBE2C stimulates cell proliferation and anchorage-independent growth. UBE2C transgenic mice are prone to develop spontaneous tumors and carcinogen-induced tumor with evidence of chromosome aneuploidy.

Panta rhei: the APC/C at steady state

The anaphase-promoting complex or cyclosome (APC/C) is a conserved, multisubunit E3 ubiquitin (Ub) ligase that is active both in dividing and in postmitotic cells. Its contributions to life are especially well studied in the domain of cell division, in which the APC/C lies at the epicenter of a regulatory network that controls the directionality and timing of cell cycle events. Biochemical and structural work is shedding light on the overall organization of APC/C subunits and on the mechanism of substrate recognition and Ub chain initiation and extension as well as on the molecular mechanisms of a checkpoint that seizes control of APC/C activity during mitosis. Here, we review how these recent advancements are modifying our understanding of the APC/C.

K11-linked ubiquitin chains as novel regulators of cell division

Modification of proteins with ubiquitin chains is an essential regulatory event in cell cycle control. Differences in the connectivity of ubiquitin chains are believed to result in distinct functional consequences for the modified proteins. Among eight possible homogenous chain types, canonical Lys48-linked ubiquitin chains have long been recognized to drive the proteasomal degradation of cell cycle regulators, and Lys48 is the only essential lysine residue of ubiquitin in yeast. It thus came as a surprise that in higher eukaryotes atypical K11-linked ubiquitin chains regulate the substrates of the anaphase-promoting complex and control progression through mitosis. We discuss recent findings that shed light on the assembly and function of K11-linked chains during cell division.

APC15 drives the turnover of MCC-CDC20 to make the spindle assembly checkpoint responsive to kinetochore attachment

Faithful chromosome segregation during mitosis depends on the spindle assembly checkpoint (SAC), which monitors kinetochore attachment to the mitotic spindle. Unattached kinetochores generate mitotic checkpoint proteins complexes (MCCs) that bind and inhibit the anaphase-promoting complex, or cyclosome (APC/C). How the SAC proficiently inhibits the APC/C but still allows its rapid activation when the last kinetochore attaches to the spindle is important for the understanding of how cells maintain genomic stability. We show that the APC/C subunit APC15 is required for the turnover of the APC/C co-activator CDC20 and release of MCCs during SAC signalling but not for APC/C activity per se. In the absence of APC15, MCCs and ubiquitylated CDC20 remain 'locked' onto the APC/C, which prevents the ubiquitylation and degradation of cyclin B1 when the SAC is satisfied. We conclude that APC15 mediates the constant turnover of CDC20 and MCCs on the APC/C to allow the SAC to respond to the attachment state of kinetochores.

How APC/C-Cdc20 changes its substrate specificity in mitosis

Progress through mitosis requires that the right protein be degraded at the right time. One ubiquitin ligase, the anaphase-promoting complex or cyclosome (APC/C) targets most of the crucial mitotic regulators by changing its substrate specificity throughout mitosis. The spindle assembly checkpoint (SAC) acts on the APC/C co-activator, Cdc20 (cell division cycle 20), to block the degradation of metaphase substrates (for example, cyclin B1 and securin), but not others (for example, cyclin A). How this is achieved is unclear. Here we show that Cdc20 binds to different sites on the APC/C depending on the SAC. Cdc20 requires APC3 and APC8 to bind and activate the APC/C when the SAC is satisfied, but requires only APC8 to bind the APC/C when the SAC is active. Moreover, APC10 is crucial for the destruction of cyclin B1 and securin, but not cyclin A. We conclude that the SAC causes Cdc20 to bind to different sites on the APC/C and this alters APC/C substrate specificity.

State of the APC/C: organization, function, and structure

The ubiquitin-proteasome protein degradation system is involved in many essential cellular processes including cell cycle regulation, cell differentiation, and the unfolded protein response. The anaphase-promoting complex/cyclosome (APC/C), an evolutionarily conserved E3 ubiquitin ligase, was discovered 15 years ago because of its pivotal role in cyclin degradation and mitotic progression. Since then, we have learned that the APC/C is a very large, complex E3 ligase composed of 13 subunits, yielding a molecular machine of approximately 1 MDa. The intricate regulation of the APC/C is mediated by the Cdc20 family of activators, pseudosubstrate inhibitors, protein kinases and phosphatases and the spindle assembly checkpoint. The large size, complexity, and dynamic nature of the APC/C represent significant obstacles toward high-resolution structural techniques; however, over the last decade, there have been a number of lower resolution APC/C structures determined using single particle electron microscopy. These structures, when combined with data generated from numerous genetic and biochemical studies, have begun to shed light on how APC/C activity is regulated. Here, we discuss the most recent developments in the APC/C field concerning structure, substrate recognition, and catalysis.

Substrate-specific regulation of ubiquitination by the anaphase-promoting complex

By orchestrating the sequential degradation of a large number of cell cycle regulators, the ubiquitin ligase anaphase-promoting complex (APC/C) is essential for proliferation in all eukaryotes. The correct timing of APC/C-dependent substrate degradation, a critical feature of progression through mitosis, was long known to be controlled by mechanisms targeting the core APC/C-machinery. Recent experiments, however, have revealed an important contribution of substrate-specific regulation of the APC/C to achieve accurate cell division. In this perspective, we describe different mechanisms of substrate-specific APC/C-regulation and discuss their importance for cell division.

Structure, function and mechanism of the anaphase promoting complex (APC/C)

The complex molecular events responsible for coordinating chromosome replication and segregation with cell division and growth are collectively known as the cell cycle. Progression through the cell cycle is orchestrated by the interplay between controlled protein synthesis and degradation and protein phosphorylation. Protein degradation is primarily regulated through the ubiquitin proteasome system, mediated by two related E3 protein ubiquitin ligases, the Skp1 cullin F-box (SCF) and the anaphase promoting complex (also known as the cyclosome) (APC/C). The APC/C is a multi-subunit cullin-RING E3 ubiquitin ligase that regulates progression through the mitotic phase of the cell cycle and controls entry into S phase by catalysing the ubiquitylation of cyclins and other cell cycle regulatory proteins. Selection of APC/C targets is controlled through recognition of short destruction motifs, predominantly the D-box and KEN-box. APC/C-mediated coordination of cell cycle progression is achieved through the temporal regulation of APC/C activity and substrate specificity, exerted through a combination of co-activator subunits, reversible phosphorylation and inhibitory proteins and complexes. The aim of this article is to discuss the APC/C from a structural and mechanistic perspective. Although an atomic structure of the APC/C is still lacking, a combination of genetic, biochemical, electron microscopy studies of intact APC/C and crystallographic analysis of individual subunits, together with analogies to evolutionarily related E3 ligases of the RING family, has provided deep insights into the molecular mechanisms of catalysis and substrate recognition, and structural organisation of the APC/C.

Hematopoietic cells from Ube1L-deficient mice exhibit an impaired proliferation defect under the stress of bone marrow transplantation

Following bone marrow transplantation, donor stem cells are recruited from their quiescent status to promote the rapid reconstitution in recipients. This dynamic process is tightly regulated by a complex of internal and external signals. Protein modification by the ubiquitin like modifier ISG15 (ISGylation) is strongly induced by type I interferons (IFNs). There are higher levels of type I IFNs and protein ISGylation in the bone marrow of recipients shortly after transplantation. In order to clarify the physiological function of protein ISGylation, we generated a mouse model that lacks protein ISGylation due to deficiency of ISG15 conjugating enzyme Ube1L (Ube1L(-/-)). In this report, we focused on the analysis of the hematopoietic system in Ube1L(-)(/)(-) mice in steady-state hematopoiesis and its potential protective role during bone marrow reconstitution. Here we demonstrated that In Ube1L(-/-) mice, steady-state hematopoiesis was unperturbed. However, transplantation experiment revealed a 50% reduction in repopulation potential of Ube1L-deficient cells at 3weeks posttransplantation, but no differences at 6 and 12weeks. A competitive transplantation experiment magnified and extended this phenotype. Cell cycle analysis revealed that under the condition with high levels of IFNs and protein ISGylation, the Ube1L deficiency can cause G2/M phase block of cell cycle in hematopoietic multipotential progenitors. These observations indicate that although protein ISGylation is dispensable for steady-state hematopoiesis, it plays a significant role during interferon related stress response, such as bone marrow transplantation.

APC16 is a conserved subunit of the anaphase-promoting complex/cyclosome

Error-free chromosome segregation depends on timely activation of the multi-subunit E3 ubiquitin ligase APC/C. Activation of the APC/C initiates chromosome segregation and mitotic exit by targeting critical cell-cycle regulators for destruction. The APC/C is the principle target of the mitotic checkpoint, which prevents segregation while chromosomes are unattached to spindle microtubules. We now report the identification and characterization of APC16, a conserved subunit of the APC/C. APC16 was found in association with tandem-affinity-purified mitotic checkpoint complex protein complexes. APC16 is a bona fide subunit of human APC/C: it is present in APC/C complexes throughout the cell cycle, the phenotype of APC16-depleted cells copies depletion of other APC/C subunits, and APC16 is important for APC/C activity towards mitotic substrates. APC16 sequence homologues can be identified in metazoans, but not fungi, by four conserved primary sequence stretches. We provide evidence that the C. elegans gene K10D2.4 and the D. rerio gene zgc:110659 are functional equivalents of human APC16. Our findings show that APC/C is composed of previously undescribed subunits, and raise the question of why metazoan APC/C is molecularly different from unicellular APC/C.

Genome-wide microarray evidence that 8-cell human blastomeres over-express cell cycle drivers and under-express checkpoints

PURPOSE

To understand cell cycle controls in the 8-Cell human blastomere.

METHODS

Data from whole human genome (43,377 elements) microarray analyses of RNAs from normal 8-Cell human embryos were compiled with published microarrays of RNAs from human fibroblasts, before and after induced pluripotency, and embryonic stem cells. A sub database of 3,803 genes identified by high throughput RNA knock-down studies, plus genes that oscillate in human cells, was analyzed.

RESULTS

Thirty-five genes over-detected at least 7-fold specifically on the 8-Cell arrays were enriched for cell cycle drivers and for proteins that stabilize chromosome cohesion and spindle attachment and limit DNA and centrosome replication to once per cycle.

CONCLUSIONS

These results indicate that 8-cell human blastomere cleavage is guided by cyclic over-expression of key proteins, rather than canonical checkpoints, leading to rapidly increasing gene copy number and a susceptibility to chromosome and cytokinesis mishaps, well-noted characteristics of preimplantation human embryos.

Overexpression of the E2 ubiquitin-conjugating enzyme UbcH10 causes chromosome missegregation and tumor formation

An overabundance of UbcH10 disrupts mitotic checkpoint signaling as a result of a degradation of cyclin B, increasing spontaneous and carcinogen-induced tumor formation in transgenic mice.

The anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase functions with the E2 ubiquitin–conjugating enzyme UbcH10 in the orderly progression through mitosis by marking key mitotic regulators for destruction by the 26-S proteasome. UbcH10 is overexpressed in many human cancer types and is associated with tumor progression. However, whether UbcH10 overexpression causes tumor formation is unknown. To address this central question and to define the molecular and cellular consequences of UbcH10 overexpression, we generated a series of transgenic mice in which UbcH10 was overexpressed in graded fashion. In this study, we show that UbcH10 overexpression leads to precocious degradation of cyclin B by the APC/C, supernumerary centrioles, lagging chromosomes, and aneuploidy. Importantly, we find that UbcH10 transgenic mice are prone to carcinogen-induced lung tumors and a broad spectrum of spontaneous tumors. Our results identify UbcH10 as a prominent protooncogene that causes whole chromosome instability and tumor formation over a wide gradient of overexpression levels.

Loss of the mammalian APC/C activator FZR1 shortens G1 and lengthens S phase but has little effect on exit from mitosis

The anaphase-promoting complex/cyclosome (APC/C) is essential for progression through mitosis. At anaphase onset, the APC/C requires the activator protein CDC20 to target securin and cyclin B1 for proteasome-dependent degradation, but then depends on the CDC20-related protein FZR1 (also known as CDH1) to remain active until the onset of the next S phase. To investigate the role of FZR1 in mammalian cells, we used RNAi in human cell lines and conditional gene targeting in mouse embryonic fibroblasts. In neither case was FZR1 required for exit from mitosis, but in cells lacking FZR1, the G1 phase was shortened and the S phase was prolonged. In several normal and transformed human cell lines, loss of FZR1 function induced DNA-damage responses and impaired proliferation independently of the p53 status. Constitutive knockdown of p53 in U2OS cells with inducible FZR1 siRNA also failed to restore their proliferative capacity. Thus, the proliferation defects are a direct consequence of the genetic damage inflicted by loss of FZR1 function and are largely independent of p53. In summary, mammalian FZR1 is not required for the completion of mitosis, but is an important regulator of G1 phase and is required for efficient DNA replication in human and mouse somatic cells.

UBE2S elongates ubiquitin chains on APC/C substrates to promote mitotic exit

The anaphase-promoting complex (APC/C), a ubiquitin ligase, is the target of the spindle-assembly checkpoint (SAC), and it ubiquitylates protein substrates whose degradation regulates progress through mitosis. The identity of the ubiquitin-conjugating (E2) enzymes that work with the APC/C is unclear. In an RNA interference (RNAi) screen for factors that modify release from drug-induced SAC activation, we identified the E2 enzyme UBE2S as an APC/C auxiliary factor that promotes mitotic exit. UBE2S is dispensable in a normal mitosis, but its depletion prolongs drug-induced mitotic arrest and suppresses mitotic slippage. In vitro, UBE2S elongates ubiquitin chains initiated by the E2 enzymes UBCH10 and UBCH5, enhancing the degradation of APC/C substrates by the proteasome. Indeed, following release from SAC-induced mitotic arrest, UBE2S-depleted cells neither degrade crucial APC/C substrates, nor silence this checkpoint, whereas bypassing the SAC through BUBR1 depletion or Aurora-B inhibition negates the requirement for UBE2S. Thus, UBE2S functions with the APC/C in a two-step mechanism to control substrate ubiquitylation that is essential for mitotic exit after prolonged SAC activation, providing a new model for APC/C function in human cells.

Identification of a physiological E2 module for the human anaphase-promoting complex

Ubiquitination by the anaphase-promoting complex (APC/C) is essential for proliferation in all eukaryotes. The human APC/C promotes the degradation of mitotic regulators by assembling K11-linked ubiquitin chains, the formation of which is initiated by its E2 UbcH10. Here, we identify the conserved Ube2S as a K11-specific chain elongating E2 for human and Drosophila APC/C. Ube2S depends on the cell cycle-dependent association with the APC/C activators Cdc20 and Cdh1 for its activity. While depletion of Ube2S already inhibits APC/C in cells, the loss of the complete UbcH10/Ube2S-module leads to dramatic stabilization of APC/C substrates, severe spindle defects, and a strong mitotic delay. Ube2S and UbcH10 are tightly co-regulated in the cell cycle by APC/C-dependent degradation. We conclude that UbcH10 and Ube2S constitute a physiological E2-module for APC/C, the activity of which is required for spindle assembly and cell division.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

Overexpression of UbcH10 alternates the cell cycle profile and accelerate the tumor proliferation in colon cancer

Background

UbcH10 participates in proper metaphase to anaphase transition, and abrogation of UbcH10 results in the premature separation of sister chromatids. To assess the potential role of UbcH10 in colon cancer progression, we analyzed the clinicopathological relevance of UbcH10 in colon cancer.

Methods

We firstly screened the expression profile of UbcH10 in various types of cancer tissues as well as cell lines. Thereafter, using the colon cancer cells line, we manipulated the expression of UbcH10 and evaluated the cell cycle profile and cellular proliferations. Furthermore, the clinicopathological significance of UbcH10 was immunohistologically evaluated in patients with colon cancer. Statistical analysis was performed using the student's t-test and Chi-square test.

Results

Using the colon cancer cells, depletion of UbcH10 resulted in suppression of cellular growth whereas overexpression of UbcH10 promoted the cellular growth and oncogenic cellular growth. Mitotic population was markedly alternated by the manipulation of UbcH10 expression. Immunohistochemical analysis indicated that UbcH10 was significantly higher in colon cancer tissue compared with normal colon epithelia. Furthermore, the clinicopathological evaluation revealed that UbcH10 was associated with high-grade histological tumors.

Conclusion

The results show the clinicopathological significance of UbcH10 in the progression of colon cancer. Thus UbcH10 may act as a novel biomarker in patients with colon cancer.