Mammalian Cdh1/Fzr mediates its own degradation

Notch controls APC/CFZR-1 to enable accumulation of chromatin regulators in germline stem cells from Caenorhabditis elegans

Originally known for its function in the cell cycle, the anaphase-promoting complex/cyclosome (APC/C) also plays a crucial role in regulating differentiation and maintaining cell identity. However, the mechanisms by which APC/C mediates developmental processes are not fully understood. In this study, we show that APC/C and its activator FZR-1 regulate the chromatin regulators MES-4 and MES-3. These proteins are part of histone methylation complexes essential for maintaining germline stem cell (GSC) identity in the germ line of Caenorhabditis elegans. APC/CFZR-1 facilitates the degradation of MES-4 and MES-3 when GSCs transition toward differentiating into oocytes. The activity of APC/CFZR-1 is restricted by the Notch signaling pathway provided by the distal tip cell, which is responsible for maintaining the stemness of the GSC pool. This negative regulation enables the accumulation of MES-3 and MES-4 in GSCs, offering an additional component by which niche activity modulates the C. elegans germ line.

UPS and Kinases-Gatekeepers of the G1/S Transition

The G1/S transition is a highly regulated and pivotal checkpoint in the cell cycle, where the cell decides whether to commit to DNA replication and subsequent division or enter a non-dividing state. This checkpoint serves as a critical control point for preventing uncontrolled cell proliferation and maintaining genomic stability. The major driving force underlying the G1/S transition is the sequential activation of Cyclin-dependent kinases (CDKs), which is regulated by the coordinated binding of Cyclin partners, as well as the phosphorylation and ubiquitin-mediated degradation of both Cyclin partners and Cyclin-dependent kinase inhibitors (CKIs). Various E3 ligase families govern the timely degradation of these regulatory proteins, with their activity intricately controlled by phosphorylation events. This coordination enables the cells to efficiently translate the environmental cues and molecular signaling inputs to determine their fate. We explore the evolution of three distinct models describing the G1/S transition, highlighting how the traditional linear model is being challenged by recent paradigm shifts and conflicting findings. These advances reveal emerging complexity and unresolved questions in the field, particularly regarding how the latest insights into coordinated phosphorylation and ubiquitination-dependent degradation integrate into contemporary models of the G1/S transition.

USP8 and Hsp70 regulate endoreplication by synergistically promoting Fzr deubiquitination and stabilization

Endoreplication is characterized by multiple rounds of DNA replication without cell division and determines the growth and final size of endoreplicating cells and tissues in eukaryotes. The cyclic ubiquitination and degradation of several cell cycle regulators are required for endoreplication progression. However, the deubiquitinase that deubiquitinates and stabilizes key factors to modulate endoreplication remains unknown. Here, we found in the endoreplicating Drosophila salivary gland and Bombyx silk gland that the depletion of ubiquitin-specific peptidase 8 (USP8) led to endoreplication arrest and a decrease in gland size. Mechanistically, we showed that USP8 interacted with the Fizzy-related (Fzr) protein, a conserved master regulator of endoreplication, thereby deubiquitinating and stabilizing Fzr to modulate endoreplication. Moreover, the molecular chaperone heat shock protein 70 (Hsp70) mediated proper folding of Fzr and increased the interaction between Fzr and USP8, thereby promoting the deubiquitination and stabilization of Fzr. Together, our study demonstrates that USP8 and Hsp70 regulate endoreplication by synergistically maintaining Fzr stability though deubiquitination.

MAD2L2 Dimerization Is Not Essential for Mitotic Regulation

MAD2L2 is a small HORMA domain protein that plays a crucial role in DNA repair and mitosis. In both TLS and shieldin, the dimerization of MAD2L2 via its HORMA domain is critical for the stability and function of these complexes. However, in mitosis, the dimerization state of MAD2L2 remains unknown. To assess the importance of MAD2L2's dimerization during mitosis, we utilized CRISPR/Cas9 to generate MAD2L2 knockout cells, which were subsequently complemented with MAD2L2 species carrying different dimer-disrupting point mutations. We assessed the ability of these MAD2L2 dimer-disrupting mutants to regulate mitosis by evaluating early mitotic events and mitotic fidelity. Our findings indicate that MAD2L2 can function in its monomeric form during mitosis, suggesting that MAD2L2 homodimerization is dispensable for early mitotic regulation. Furthermore, our results suggest that the binding of CDH1 to MAD2L2 is a key regulating factor in mitosis that may actively prevent the formation of MAD2L2 dimers, thereby shifting the cellular balance toward MAD2L2-CDH1 interaction. Thus, the equilibrium between the monomeric and dimeric forms of MAD2L2 is an important cellular factor regulating the MAD2L2-containing complexes.

Discovery of Ureido-Based Apcin Analogues as Cdc20-specific Inhibitors against Cancer

Cdc20 is a promising drug target that plays an important role in the mid-anaphase process of cellular mitosis, and Apcin is the only reported core structure of the Cdc20-specific inhibitor. Some potent Apcin derivatives were obtained in our previous research, and a structure-activity relationship was determined. In this study, we designed and synthesized a series of ureido-based Apcin derivatives. The proliferation-inhibition experiments on four cancer-cell lines showed that ureido skeleton could promote the anti-proliferation activity of purine-substituted compounds, whereas the ureido analogues with pyrimidine substitutes showed no significant improvement in the inhibitory effect compared with the original ones. Further tests confirmed that ureido-based compounds can enhance the binding affinity to Cdc20 by increasing the levels of Cdc20 downstream proteins. Compound 27 revealed a remarkably antitumor activity pattern against Hela (IC50 = 0.06 ± 0.02 μM) and potent binding affinity to Cdc20. Moreover, compound 20 induced caspase-dependent apoptosis and cell-cycle arrest at the G2/M phase, and compound 27 induced caspase-dependent apoptosis and promoted microtubule polymerization. Finally, a molecular-docking simulation was performed for compounds 20 and 27 to predict the potential ligand-protein interactions with the active sites of the Cdc20 proteins.

APC/C-Cdh1-targeted substrates as potential therapies for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder and the main cause of dementia in the elderly. The disease has a high impact on individuals and their families and represents a growing public health and socio-economic burden. Despite this, there is no effective treatment options to cure or modify the disease progression, highlighting the need to identify new therapeutic targets. Synapse dysfunction and loss are early pathological features of Alzheimer's disease, correlate with cognitive decline and proceed with neuronal death. In the last years, the E3 ubiquitin ligase anaphase promoting complex/cyclosome (APC/C) has emerged as a key regulator of synaptic plasticity and neuronal survival. To this end, the ligase binds Cdh1, its main activator in the brain. However, inactivation of the anaphase promoting complex/cyclosome-Cdh1 complex triggers dendrite disruption, synapse loss and neurodegeneration, leading to memory and learning impairment. Interestingly, oligomerized amyloid-β (Aβ) peptide, which is involved in Alzheimer's disease onset and progression, induces Cdh1 phosphorylation leading to anaphase promoting complex/cyclosome-Cdh1 complex disassembly and inactivation. This causes the aberrant accumulation of several anaphase promoting complex/cyclosome-Cdh1 targets in the damaged areas of Alzheimer's disease brains, including Rock2 and Cyclin B1. Here we review the function of anaphase promoting complex/cyclosome-Cdh1 dysregulation in the pathogenesis of Alzheimer's disease, paying particular attention in the neurotoxicity induced by its molecular targets. Understanding the role of anaphase promoting complex/cyclosome-Cdh1-targeted substrates in Alzheimer's disease may be useful in the development of new effective disease-modifying treatments for this neurological disorder.

The Anaphase-Promoting Complex/Cyclosome Is a Cellular Ageing Regulator

The anaphase-promoting complex/cyclosome (APC/C) is a complicated cellular component that plays significant roles in regulating the cell cycle process of eukaryotic organisms. The spatiotemporal regulation mechanisms of APC/C in distinct cell cycle transitions are no longer mysterious, and the components of this protein complex are gradually identified and characterized. Given the close relationship between the cell cycle and lifespan, it is urgent to understand the roles of APC/C in lifespan regulation, but this field still seems to have not been systematically summarized. Furthermore, although several reviews have reported the roles of APC/C in cancer, there are still gaps in the summary of its roles in other age-related diseases. In this review, we propose that the APC/C is a novel cellular ageing regulator based on its indispensable role in the regulation of lifespan and its involvement in age-associated diseases. This work provides an extensive review of aspects related to the underlying mechanisms of APC/C in lifespan regulation and how it participates in age-associated diseases. More comprehensive recognition and understanding of the relationship between APC/C and ageing and age-related diseases will increase the development of targeted strategies for human health.

Reciprocal interaction between SIRT6 and APC/C regulates genomic stability

SIRT6 is an NAD⁺-dependent deacetylase that plays an important role in mitosis fidelity and genome stability. In the present study, we found that SIRT6 overexpression leads to mitosis defects and aneuploidy. We identified SIRT6 as a novel substrate of anaphase-promoting complex/cyclosome (APC/C), which is a master regulator of mitosis. Both CDH1 and CDC20, co-activators of APC/C, mediated SIRT6 degradation via the ubiquitination-proteasome pathway. Reciprocally, SIRT6 also deacetylated CDH1 at lysine K135 and promoted its degradation, resulting in an increase in APC/C-CDH1-targeted substrates, dysfunction in centrosome amplification, and chromosome instability. Our findings demonstrate the importance of SIRT6 for genome integrity during mitotic progression and reveal how SIRT6 and APC/C cooperate to drive mitosis.

The Negative Cross-Talk between SAG/RBX2/ROC2 and APC/C E3 Ligases in Regulation of Cell Cycle Progression and Drug Resistance

Anaphase-promoting complex/cyclosome (APC/C) is a well-characterized E3 ligase that couples with UBE2C and UBE2S E2s for substrate ubiquitylation by the K11 linkage. Our recent data show that SAG/RBX2/ROC2, a RING component of Cullin-RING E3 ligase, also complexes with these E2s for K11-linked substrate polyubiquitylation. Whether these two E3s cross-talk with each other was previously unknown. Here, we report that SAG competes with APC2 for UBE2C/UBE2S binding to act as a potential endogenous inhibitor of APC/C, thereby regulating the G2-to-M progression. As such, SAG knockdown triggers premature activation of APC/C, leading to mitotic slippage and resistance to anti-microtubule drugs. On the other hand, SAG itself is a substrate of APC/C^CDH1 for targeted degradation at the G1 phase. The degradation-resistant mutant of SAG-R98A/L101A accelerates the G1-to-S progression. Our study reveals that the negative cross-talk between SAG and APC/C is likely a mechanism to ensure the fidelity of cell cycle progression.

Zhang et al. provide a mechanistic insight of how negative cross-talk between E3 ligases SAG and APC/C ensures proper cell cycle progression. SAG knockdown prematurely activates APC/C to promote mitotic progression and trigger anti-microtubule drugs resistance, whereas SAG degradation by APC/C^CDH1 mainly occurs in G1 phase for proper G1-to-S transition.

FAK in the nucleus prevents VSMC proliferation by promoting p27 and p21 expression via Skp2 degradation

AIM

Vascular smooth muscle cells (VSMCs) normally exhibit a very low proliferative rate. Vessel injury triggers VSMC proliferation, in part, through focal adhesion kinase (FAK) activation, which increases transcription of cyclin D1, a key activator for cell cycle-dependent kinases (CDKs). At the same time, we also observe that FAK regulates the expression of the CDK inhibitors (CDKIs) p27 and p21. However, the mechanism of how FAK controls CDKIs in cell cycle progression is not fully understood.

METHODS AND RESULTS

We found that pharmacological and genetic FAK inhibition increased p27 and p21 by reducing stability of S-phase kinase-associated protein 2 (Skp2), which targets the CDKIs for degradation. FAK N-terminal domain interacts with Skp2 and an APC/C E3 ligase activator, fizzy-related 1 (Fzr1) in the nucleus, which promotes ubiquitination and degradation of both Skp2 and Fzr1. Notably, overexpression of cyclin D1 alone failed to promote proliferation of genetic FAK kinase-dead (KD) VSMCs, suggesting that the FAK-Skp2-CDKI signaling axis is distinct from the FAK-cyclin D1 pathway. However, overexpression of both cyclin D1 and Skp2 enables proliferation of FAK-KD VSMCs, implicating that FAK ought to control both activating and inhibitory switches for CDKs. In vivo, wire injury activates FAK in the cytosol and increased Skp2 and decreased p27 and p21 levels.

CONCLUSIONS

Both pharmacological FAK and genetic FAK inhibition reduced Skp2 expression in VSMCs upon injury, which significantly reduced intimal hyperplasia through elevated expression of p27 and p21. This study revealed that nuclear FAK-Skp2-CDKI signaling negatively regulates CDK activity in VSMC proliferation.

TRANSLATIONAL PERSPECTIVE

Increased VSMC proliferation contributes to pathological vessel narrowing in atherosclerosisand following vascular interventions. Blocking VSMC proliferation will reduce atherosclerosisprogression and increase patency of vascular interventions. We found that forced nuclear FAKlocalization by FAK inhibition reduced VSMC proliferation upon vessel injury. Nuclear FAKdecreased Skp2 protein expression by proteasomal degradation, thereby increasing theexpression of cell cycle inhibitors p27 and p21 and blocking cell cycle progression. This studyhas demonstrated the potential for FAK inhibitors in blocking VSMC proliferation to treat vessel narrowing diseases.

The role of Anaphase Promoting Complex activation, inhibition and substrates in cancer development and progression

The Anaphase Promoting Complex (APC), a multi-subunit ubiquitin ligase, facilitates mitotic and G1 progression, and is now recognized to play a role in maintaining genomic stability. Many APC substrates have been observed overexpressed in multiple cancer types, such as CDC20, the Aurora A and B kinases, and Forkhead box M1 (FOXM1), suggesting APC activity is important for cell health. We performed BioGRID analyses of the APC coactivators CDC20 and CDH1, which revealed that at least 69 proteins serve as APC substrates, with 60 of them identified as playing a role in tumor promotion and 9 involved in tumor suppression. While these substrates and their association with malignancies have been studied in isolation, the possibility exists that generalized APC dysfunction could result in the inappropriate stabilization of multiple APC targets, thereby changing tumor behavior and treatment responsiveness. It is also possible that the APC itself plays a crucial role in tumorigenesis through its regulation of mitotic progression. In this review the connections between APC activity and dysregulation will be discussed with regards to cell cycle dysfunction and chromosome instability in cancer, along with the individual roles that the accumulation of various APC substrates may play in cancer progression.

Complex Cartography: Regulation of E2F Transcription Factors by Cyclin F and Ubiquitin

The E2F family of transcriptional regulators sits at the center of cell cycle gene expression and plays vital roles in normal and cancer cell cycles. Whereas control of E2Fs by the retinoblastoma family of proteins is well established, much less is known about their regulation by ubiquitin pathways. Recent studies placed the Skp1-Cul1-F-box-protein (SCF) family of E3 ubiquitin ligases with the F-box protein Cyclin F at the center of E2F regulation, demonstrating temporal proteolysis of both activator and atypical repressor E2Fs. Importantly, these E2F members, in particular activator E2F1 and repressors E2F7 and E2F8, form a feedback circuit at the crossroads of cell cycle and cell death. Moreover, Cyclin F functions in a reciprocal circuit with the cell cycle E3 ligase anaphase-promoting complex/cyclosome (APC/C), which also controls E2F7 and E2F8. This review focuses on the complex contours of feedback within this circuit, highlighting the deep crosstalk between E2F, SCF-Cyclin F, and APC/C in regulating the oscillator underlying human cell cycles.

Chk1-mediated phosphorylation of Cdh1 promotes the SCFβTRCP-dependent degradation of Cdh1 during S-phase and efficient cell-cycle progression

APC/CCdh1 is a ubiquitin ligase with roles in numerous diverse processes, including control of cellular proliferation and multiple aspects of the DNA damage response. Precise regulation of APC/CCdh1 activity is central to efficient cell-cycle progression and cellular homeostasis. Here, we have identified Cdh1 as a direct substrate of the replication stress checkpoint effector kinase Chk1 and demonstrate that Chk1-mediated phosphorylation of Cdh1 contributes to its recognition by the SCFβTRCP ubiquitin ligase, promotes efficient S-phase entry, and is important for cellular proliferation during otherwise unperturbed cell cycles. We also find that prolonged Chk1 activity in late S/G2 inhibits Cdh1 accumulation. In addition to promoting control of APC/CCdh1 activity by facilitating Cdh1 destruction, we find that Chk1 also antagonizes activity of the ligase by perturbing the interaction between Cdh1 and the APC/C. Overall, these data suggest that the rise and fall of Chk1 activity contributes to the regulation of APC/CCdh1 activity that enhances the replication process.

APC/C ubiquitin ligase: Functions and mechanisms in tumorigenesis

The anaphase promoting complex/ cyclosome (APC/C), is an evolutionarily conserved protein complex essential for cellular division due to its role in regulating the mitotic transition from metaphase to anaphase. In this review, we highlight recent work that has shed light on our understanding of the role of APC/C coactivators, Cdh1 and Cdc20, in cancer initiation and development. We summarize the current state of knowledge regarding APC/C structure and function, as well as the distinct ways Cdh1 and Cdc20 are dysregulated in human cancer. We also discuss APC/C inhibitors, novel approaches for targeting the APC/C as a cancer therapy, and areas for future work.

Ubiquitin-Regulated Cell Proliferation and Cancer

Ubiquitin ligases (E3) play a crucial role in the regulation of different cellular processes such as proliferation and differentiation via recognition, interaction, and ubiquitination of key cellular proteins in a spatial and temporal regulated manner. The type of ubiquitin chain formed determines the fate of the substrates. The ubiquitinated substrates can be degraded by the proteasome, display altered subcellular localization, or can suffer modifications on their interaction with functional protein complexes. Deregulation of E3 activities is frequently found in various human pathologies, including cancer. The illegitimated or accelerated degradation of oncosuppressive proteins or, inversely, the abnormally high accumulation of oncoproteins, contributes to cell proliferation and transformation. Anomalies in protein abundance may be related to mutations that alter the direct or indirect recognition of proteins by the E3 enzymes or alterations in the level of expression or activity of ubiquitin ligases. Through a few examples, we illustrate here the complexity and diversity of the molecular mechanisms related to protein ubiquitination involved in cell cycle regulation. We will discuss the role of ubiquitin-dependent degradation mediated by the proteasome, the role of non-proteolytic ubiquitination during cell cycle progression, and the consequences of this deregulation on cellular transformation. Finally, we will highlight the novel opportunities that arise from these studies for therapeutic intervention.

A novel human Cdh1 mutation impairs anaphase promoting complex/cyclosome activity resulting in microcephaly, psychomotor retardation, and epilepsy

The Fizzy-related protein 1 (Fzr1) gene encodes Cdh1 protein, a coactivator of the E3 ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C). Previously, we found that genetic ablation of Fzr1 promotes the death of neural progenitor cells leading to neurogenesis impairment and microcephaly in mouse. To ascertain the possible translation of these findings in humans, we searched for mutations in the Fzr1 gene in 390 whole exomes sequenced in trio in individuals showing neurodevelopmental disorders compatible with a genetic origin. We found a novel missense (p.Asp187Gly) Fzr1 gene mutation (c.560A>G) in a heterozygous state in a 4-year-old boy, born from non-consanguineous Spanish parents, who presents with severe antenatal microcephaly, psychomotor retardation, and refractory epilepsy. Cdh1 protein levels in leucocytes isolated from the patient were significantly lower than those found in his parents. Expression of the Asp187Gly mutant form of Cdh1 in human embryonic kidney 293T cells produced less Cdh1 protein and APC/C activity, resulting in altered cell cycle distribution when compared with cells expressing wild-type Cdh1. Furthermore, ectopic expression of the Asp187Gly mutant form of Cdh1 in cortical progenitor cells in primary culture failed to abolish the enlargement of the replicative phase caused by knockout of endogenous Cdh1. These results indicate that the loss of function of APC/C-Cdh1 caused by Cdh1 Asp187Gly mutation is a new cause of prenatal microcephaly, psychomotor retardation, and severe epilepsy. Read the Editorial Highlight for this article on page 8. Cover Image for this issue: doi: 10.1111/jnc.14524.

Regulation of the anaphase promoting complex/cyclosome by the degradation of its unassembled catalytic subunit, Apc11

One of the challenges encountered by the protein quality control machinery is the need to ensure that members of multiprotein complexes are available in the correct proportions. In this study, we demonstrate that the ubiquitin proteasome system (UPS) mediates the degradation of Apc11, the catalytic core subunit of the anaphase promoting complex/cyclosome (APC/C). In vitro studies have shown that Apc11, together with its E2 enzyme, is sufficient to ubiquitinate substrates independently of the APC/C. Here, we establish that this can occur in living yeast cells. We show that the tight controls regulating the function of the fully assembled APC/C can be circumvented when its substrates are ubiquitinated by the excess levels of Apc11 independently of the assembled complex. We thus suggest that the UPS-mediated degradation of Apc11 is an overlooked mechanism ensuring that proper function of the APC/C is limited to suitably delimited holoenzymes and that an imbalance in protein expression may result in detrimental gain-of-function activity, rather than merely the disruption of protein complex stoichiometry.-Volpe, M., Levinton, N., Rosenstein, N., Prag, G., Ben-Aroya, S. Regulation of the anaphase promoting complex/cyclosome by the degradation of its unassembled catalytic subunit, Apc11.

Hyperphosphorylation of CDH1 in Glioblastoma Cancer Stem Cells Attenuates APC/CCDH1 Activity and Pharmacologic Inhibition of APC/CCDH1/CDC20 Compromises Viability

Glioblastoma (GBM) is the most common and lethal primary brain tumor and remains incurable. This is in part due to the cellular heterogeneity within these tumors, which includes a subpopulation of treatment-resistant cells called cancer stem-like cells (CSC). We previously identified that the anaphase-promoting complex/cylosome (APC/C), a key cell-cycle regulator and tumor suppressor, had attenuated ligase activity in CSCs. Here, we assessed the mechanism of reduced activity, as well as the efficacy of pharmacologically targeting the APC/C in CSCs. We identified hyperphosphorylation of CDH1, but not pseudosubstrate inhibition by early mitotic inhibitor 1 (EMI1), as a major mechanism driving attenuated APC/C^CDH1 activity in the G₁-phase of the cell cycle in CSCs. Small-molecule inhibition of the APC/C reduced viability of both CSCs and nonstem tumor cells (NSTCs), with the combination of proTAME and apcin having the biggest impact. Combinatorial drug treatment also led to the greatest mitotic arrest and chromosomal abnormalities. IMPLICATIONS: Our findings demonstrate how the activity of the APC/C^CDH1 tumor suppressor is reduced in CSCs and also validates small-molecule inhibition of the APC/C as a promising therapeutic target for the treatment of GBM.

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.

Ubiquitin in Cell-Cycle Regulation and Dysregulation in Cancer

Uncontrolled cell proliferation and genomic instability are common features of cancer and can arise from, respectively, the loss of cell-cycle control and defective checkpoints. Ubiquitin-mediated proteolysis, ultimately executed by ubiquitin-ligating enzymes (E3s), plays a key part in cell-cycle regulation and is dominated by two multisubunit E3s, the anaphase-promoting complex (or cyclosome) (APC/C) and SKP1–cullin-1–F-box (SCF) complex. We highlight the role of APC/C and the SCF bound to F-box proteins, FBXW7, SKP2, and β-TrCP, in regulating the abundance of select fundamental proteins, primarily during the cell cycle, that are associated with human cancer. The clinical success of the first proteasome inhibitor, bortezomib, in treating multiple myeloma and mantle-cell lymphoma set the precedent for viewing the ubiquitin–proteasome system as a druggable target for cancer. Given that there are more E3s than kinases, selective, small-molecule E3 inhibitors have the potential of opening up another dimension in the therapeutic armamentarium for the treatment of cancer.

TGF-β activates APC through Cdh1 binding for Cks1 and Skp2 proteasomal destruction stabilizing p27kip1 for normal endometrial growth

We previously reported that aberrant TGF-β/Smad2/3 signaling in endometrial cancer (ECA) leads to continuous ubiquitylation of p27(kip1)(p27) by the E3 ligase SCF-Skp2/Cks1 causing its degradation, as a putative mechanism involved in the pathogenesis of this cancer. In contrast, normal intact TGF-β signaling prevents degradation of nuclear p27 by SCF-Skp2/Cks1 thereby accumulating p27 to block Cdk2 for growth arrest. Here we show that in ECA cell lines and normal primary endometrial epithelial cells, TGF-β increases Cdh1 and its binding to APC/C to form the E3 ligase complex that ubiquitylates Cks1 and Skp2 prompting their proteasomal degradation and thus, leaving p27 intact. Knocking-down Cdh1 in ECA cell lines increased Skp2/Cks1 E3 ligase activity, completely diminished nuclear and cytoplasmic p27, and obviated TGF-β-mediated inhibition of proliferation. Protein synthesis was not required for TGF-β-induced increase in nuclear p27 and decrease in Cks1 and Skp2. Moreover, half-lives of Cks1 and Skp2 were extended in the Cdh1-depleted cells. These results suggest that the levels of p27, Skp2 and Cks1 are strongly or solely regulated by proteasomal degradation. Finally, an inverse relationship of low p27 and high Cks1 in the nucleus was shown in patients in normal proliferative endometrium and grade I-III ECAs whereas differentiated secretory endometrium showed the reverse. These studies implicate Cdh1 as the master regulator of TGF-β-induced preservation of p27 tumor suppressor activity. Thus, Cdh1 is a potential therapeutic target for ECA and other human cancers showing an inverse relationship between Cks1/Skp2 and p27 and/or dysregulated TGF-β signaling.

Aneuploidy in Cancer and Aging

Chromosomal instability (CIN), the persistent inability of a cell to faithfully segregate its genome, is a feature of many cancer cells. It stands to reason that CIN enables the acquisition of multiple cancer hallmarks; however, there is a growing body of evidence suggesting that CIN impairs cellular fitness and prevents neoplastic transformation. Here, we suggest a new perspective to reconcile this apparent paradox and share an unexpected link between aneuploidy and aging that was discovered through attempts to investigate the CIN-cancer relationship. Additionally, we provide a comprehensive overview of the function and regulation of the anaphase-promoting complex, an E3 ubiquitin ligase that mediates high-fidelity chromosome segregation, and describe the mechanisms that lead to whole-chromosome gain or loss. With this review, we aim to expand our understanding of the role of CIN in cancer and aging with the long-term objective of harnessing this information for the advancement of patient care.

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.

APC/C and retinoblastoma interaction: cross-talk of retinoblastoma protein with the ubiquitin proteasome pathway

The ubiquitin (Ub) ligase anaphase promoting complex/cyclosome (APC/C) and the tumour suppressor retinoblastoma protein (pRB) play key roles in cell cycle regulation. APC/C is a critical regulator of mitosis and G₁-phase of the cell cycle whereas pRB keeps a check on proliferation by inhibiting transition to the S-phase. APC/C and pRB interact with each other via the co-activator of APC/C, FZR1, providing an alternative pathway of regulation of G₁ to S transition by pRB using a post-translational mechanism. Both pRB and FZR1 have complex roles and are implicated not only in regulation of cell proliferation but also in differentiation, quiescence, apoptosis, maintenance of chromosomal integrity and metabolism. Both are also targeted by transforming viruses. We discuss recent advances in our understanding of the involvement of APC/C and pRB in cell cycle based decisions and how these insights will be useful for development of anti-cancer and anti-viral drugs.

Imaging Cell Cycle Phases and Transitions of Living Cells from Yeast to Woman

The eukaryotic cell cycle is comprised of different phases that take place sequentially once, and normally only once, every division cycle. Such a dynamic process is best viewed in real time in living dividing cells. The insights that can be gained from such methods are considerably larger than any alternative technique that only generates snapshots. A great number of studies can gain from live cell imaging; however this method often feels somewhat intimidating to the novice. The purpose of this chapter is to demonstrate that imaging cell cycle phases in living cells from yeast to human is relatively easy and can be performed with equipment that is available in most research institutes. We present the different approaches, review different types of reporters, and discuss in depth all the aspects to be considered to obtain optimal results. We also describe our latest cell cycle markers, which afford unprecedented "sub"-phase temporal resolution.

O-GlcNAcylation Antagonizes Phosphorylation of CDH1 (CDC20 Homologue 1)

The anaphase promoting complex/cyclosome (APC/C) orchestrates various aspects of the eukaryotic cell cycle. One of its co-activators, Cdh1, is subject to myriad post-translational modifications, such as phosphorylation and ubiquitination. Herein we identify the O-linked N-acetylglucosamine (O-GlcNAc) modification that occurs on Cdh1. Cdh1 is O-GlcNAcylated in cultured cells and mouse brain extracts. Mass spectrometry identifies an O-GlcNAcylated peptide that neighbors a known phosphorylation site. Cell synchronization and mutation studies reveal that O-GlcNAcylation of Cdh1 may antagonize its phosphorylation. Our results thus reveal a pivotal role of O-GlcNAcylation in regulating APC/C activity.

Cdk5-mediated inhibition of APC/C-Cdh1 switches on the cyclin D1-Cdk4-pRb pathway causing aberrant S-phase entry of postmitotic neurons

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that regulates cell cycle progression in proliferating cells. To enter the S-phase, APC/C must be inactivated by phosphorylation of its cofactor, Cdh1. In post-mitotic cells such as neurons APC/C-Cdh1 complex is highly active and responsible for the continuous degradation of mitotic cyclins. However, the specific molecular pathway that determines neuronal cell cycle blockade in post-mitotic neurons is unknown. Here, we show that activation of glutamatergic receptors in rat cortical primary neurons endogenously triggers cyclin-dependent kinase-5 (Cdk5)-mediated phosphorylation of Cdh1 leading to its cytoplasmic accumulation and disassembly from the APC3 core protein, causing APC/C inactivation. Conversely, pharmacological or genetic inhibition of Cdk5 promotes Cdh1 ubiquitination and proteasomal degradation. Furthermore, we show that Cdk5-mediated phosphorylation and inactivation of Cdh1 leads to p27 depletion, which switches on the cyclin D1-cyclin-dependent kinase-4 (Cdk4)-retinoblastoma protein (pRb) pathway to allow the S-phase entry of neurons. However, neurons do not proceed through the cell cycle and die by apoptosis. These results indicate that APC/C-Cdh1 actively suppresses an aberrant cell cycle entry and death of neurons, highlighting its critical function in neuroprotection.

Cyclin A/Cdk2 regulates Cdh1 and claspin during late S/G2 phase of the cell cycle

Whereas many components regulating the progression from S phase through G2 phase into mitosis have been identified, the mechanism by which these components control this critical cell cycle progression is still not fully elucidated. Cyclin A/Cdk2 has been shown to regulate the timing of Cyclin B/Cdk1 activation and progression into mitosis although the mechanism by which this occurs is only poorly understood. Here we show that depletion of Cyclin A or inhibition of Cdk2 during late S/early G2 phase maintains the G2 phase arrest by reducing Cdh1 transcript and protein levels, thereby stabilizing Claspin and maintaining elevated levels of activated Chk1 which contributes to the G2 phase observed. Interestingly, the Cyclin A/Cdk2 regulated APC/C(Cdh1) activity is selective for only a subset of Cdh1 targets including Claspin. Thus, a normal role for Cyclin A/Cdk2 during early G2 phase is to increase the level of Cdh1 which destabilises Claspin which in turn down regulates Chk1 activation to allow progression into mitosis. This mechanism links S phase exit with G2 phase transit into mitosis, provides a novel insight into the roles of Cyclin A/Cdk2 in G2 phase progression, and identifies a novel role for APC/C(Cdh1) in late S/G2 phase cell cycle progression.

Degradation of Ndd1 by APC/C(Cdh1) generates a feed forward loop that times mitotic protein accumulation

Ndd1 activates the Mcm1-Fkh2 transcription factor to transcribe mitotic regulators. The anaphase-promoting complex/cyclosome activated by Cdh1 (APC/C(Cdh1)) mediates the degradation of proteins throughout G1. Here we show that the APC/C(Cdh1) ubiquitinates Ndd1 and mediates its degradation, and that APC/C(Cdh1) activity suppresses accumulation of Ndd1 targets. We confirm putative Ndd1 targets and identify novel ones, many of them APC/C(Cdh1) substrates. The APC/C(Cdh1) thus regulates these proteins in a dual manner—both pretranscriptionally and post-translationally, forming a multi-layered feedforward loop (FFL). We predict by mathematical modelling and verify experimentally that this FFL introduces a lag between APC/C(Cdh1) inactivation at the end of G1 and accumulation of genes transcribed by Ndd1 in G2. This regulation generates two classes of APC/C(Cdh1) substrates, early ones that accumulate in S and late ones that accumulate in G2. Our results show how the dual state APC/C(Cdh1) activity is converted into multiple outputs by interactions between its substrates.

Casein kinase 1δ is an APC/C(Cdh1) substrate that regulates cerebellar granule cell neurogenesis

Although casein kinase 1δ (CK1δ) is at the center of multiple signaling pathways, its role in the expansion of CNS progenitor cells is unknown. Using mouse cerebellar granule cell progenitors (GCPs) as a model for brain neurogenesis, we demonstrate that the loss of CK1δ or treatment of GCPs with a highly selective small molecule inhibits GCP expansion. In contrast, CK1δ overexpression increases GCP proliferation. Thus, CK1δ appears to regulate GCP neurogenesis. CK1δ is targeted for proteolysis via the anaphase-promoting complex/cyclosome (APC/C(Cdh1)) ubiquitin ligase, and conditional deletion of the APC/C(Cdh1) activator Cdh1 in cerebellar GCPs results in higher levels of CK1δ. APC/C(Cdh1) also downregulates CK1δ during cell-cycle exit. Therefore, we conclude that APC/C(Cdh1) controls CK1δ levels to balance proliferation and cell-cycle exit in the developing CNS. Similar studies in medulloblastoma cells showed that CK1δ holds promise as a therapeutic target.

Spatiotemporal regulation of the anaphase-promoting complex in mitosis

The appropriate timing of events that lead to chromosome segregation during mitosis and cytokinesis is essential to prevent aneuploidy, and defects in these processes can contribute to tumorigenesis. Key mitotic regulators are controlled through ubiquitylation and proteasome-mediated degradation. The APC/C (anaphase-promoting complex; also known as the cyclosome) is an E3 ubiquitin ligase that has a crucial function in the regulation of the mitotic cell cycle, particularly at the onset of anaphase and during mitotic exit. Co-activator proteins, inhibitor proteins, protein kinases and phosphatases interact with the APC/C to temporally and spatially control its activity and thus ensure accurate timing of mitotic events.

Bioinformatics Approaches for Predicting Disordered Protein Motifs

Short, linear motifs (SLiMs) in proteins are functional microdomains consisting of contiguous residue segments along the protein sequence, typically not more than 10 consecutive amino acids in length with less than 5 defined positions. Many positions are 'degenerate' thus offering flexibility in terms of the amino acid types allowed at those positions. Their short length and degenerate nature confers evolutionary plasticity meaning that SLiMs often evolve convergently. Further, SLiMs have a propensity to occur within intrinsically unstructured protein segments and this confers versatile functionality to unstructured regions of the proteome. SLiMs mediate multiple types of protein interactions based on domain-peptide recognition and guide functions including posttranslational modifications, subcellular localization of proteins, and ligand binding. SLiMs thus behave as modular interaction units that confer versatility to protein function and SLiM-mediated interactions are increasingly being recognized as therapeutic targets. In this chapter we start with a brief description about the properties of SLiMs and their interactions and then move on to discuss algorithms and tools including several web-based methods that enable the discovery of novel SLiMs (de novo motif discovery) as well as the prediction of novel occurrences of known SLiMs. Both individual amino acid sequences as well as sets of protein sequences can be scanned using these methods to obtain statistically overrepresented sequence patterns. Lists of putatively functional SLiMs are then assembled based on parameters such as evolutionary sequence conservation, disorder scores, structural data, gene ontology terms and other contextual information that helps to assess the functional credibility or significance of these motifs. These bioinformatics methods should certainly guide experiments aimed at motif discovery.

Glucose and glutamine metabolism control by APC and SCF during the G1-to-S phase transition of the cell cycle

Recent studies have given us a clue as to how modulations of both metabolic pathways and cyclins by the ubiquitin system influence cell cycle progression. Among these metabolic modulations, an aerobic glycolysis and glutaminolysis represent an initial step for metabolic machinery adaptation. The enzymes 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) and glutaminase-1 (GLS1) maintain a high abundance in glycolytic intermediates (for synthesis of non-essential amino acids, the use of ribose for the synthesis of nucleotides and hexosamine biosynthesis), as well as tricarboxylic acid cycle intermediates (replenishing the loss of mitochondrial citrate), respectively. On the one hand, regulation of these key metabolic enzymes by ubiquitin ligases anaphase-promoting complex/cyclosome (APC/C) and Skp1/cullin/F-box (SCF) has revealed the importance of anaplerosis by both glycolysis and glutaminolysis to overcome the restriction point of the G1 phase by maintaining high levels of glycolytic and glutaminolytic intermediates. On the other hand, only glutaminolytic intermediates are necessary to drive cell growth through the S and G2 phases of the cell cycle. It is interesting to appreciate how this reorganization of the metabolic machinery, which has been observed beyond cellular proliferation, is a crucial determinant of a cell's decision to proliferate. Here, we explore a unifying view of interactions between the ubiquitin system, metabolic activity, and cyclin-dependent kinase complexes activity during the cell cycle.

ATR-Chk1-APC/CCdh1-dependent stabilization of Cdc7-ASK (Dbf4) kinase is required for DNA lesion bypass under replication stress

Cdc7 kinase regulates DNA replication. However, its role in DNA repair and recombination is poorly understood. Here we describe a pathway that stabilizes the human Cdc7-ASK (activator of S-phase kinase; also called Dbf4), its regulation, and its function in cellular responses to compromised DNA replication. Stalled DNA replication evoked stabilization of the Cdc7-ASK (Dbf4) complex in a manner dependent on ATR-Chk1-mediated checkpoint signaling and its interplay with the anaphase-promoting complex/cyclosome(Cdh1) (APC/C(Cdh1)) ubiquitin ligase. Mechanistically, Chk1 kinase inactivates APC/C(Cdh1) through degradation of Cdh1 upon replication block, thereby stabilizing APC/C(Cdh1) substrates, including Cdc7-ASK (Dbf4). Furthermore, motif C of ASK (Dbf4) interacts with the N-terminal region of RAD18 ubiquitin ligase, and this interaction is required for chromatin binding of RAD18. Impaired interaction of ASK (Dbf4) with RAD18 disables foci formation by RAD18 and hinders chromatin loading of translesion DNA polymerase η. These findings define a novel mechanism that orchestrates replication checkpoint signaling and ubiquitin-proteasome machinery with the DNA damage bypass pathway to guard against replication collapse under conditions of replication stress.

Aurora-A controls pre-replicative complex assembly and DNA replication by stabilizing geminin in mitosis

Geminin, an essential factor for DNA replication, directly binds to the licensing factor Cdt1 and inhibits pre-replicative complex formation to prevent re-replication. In G1, geminin levels are controlled by the anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase complex, which targets geminin for proteasomal degradation to allow pre-replicative complex formation. Conversely, from S to G2, geminin is stabilized due to APC/C ubiquitin ligase complex inhibition, ensuring the inhibition of pre-replicative complex formation. However, mitotic regulation of geminin has hitherto not been described. Here we show that Aurora-A phosphorylates geminin on Thr25 during M phase, and this event induces geminin stabilization by preventing its APC/C ubiquitin ligase complex-mediated degradation during mitosis. In turn, stabilized geminin inhibits SCF^Skp2-mediated degradation of Cdt1 to ensure pre-replicative complex formation in the ensuing S phase. The Aurora-A–geminin–Cdt1 axis therefore represents a critical regulator of proper DNA replication.

Geminin blocks the inappropriate assembly of pre-replication complexes on DNA, and this activity is inhibited in G1 by its proteasomal degradation. Tsunematsu et al. demonstrate that geminin is stabilized during mitosis due to its phosphorylation by the mitotic kinase Aurora-A.

The eukaryotic linear motif resource ELM: 10 years and counting

The eukaryotic linear motif (ELM http://elm.eu.org) resource is a hub for collecting, classifying and curating information about short linear motifs (SLiMs). For >10 years, this resource has provided the scientific community with a freely accessible guide to the biology and function of linear motifs. The current version of ELM contains ∼200 different motif classes with over 2400 experimentally validated instances manually curated from >2000 scientific publications. Furthermore, detailed information about motif-mediated interactions has been annotated and made available in standard exchange formats. Where appropriate, links are provided to resources such as switches.elm.eu.org and KEGG pathways.

Sequestration of CDH1 by MAD2L2 prevents premature APC/C activation prior to anaphase onset

MAD2L2 is rapidly degraded by APC/C^CDC20 at the onset of anaphase, allowing release of sequestered CDH1 to activate the dephosphorylated APC/C.

The switch from activation of the anaphase-promoting complex/cyclosome (APC/C) by CDC20 to CDH1 during anaphase is crucial for accurate mitosis. APC/C^CDC20 ubiquitinates a limited set of substrates for subsequent degradation, including Cyclin B1 and Securin, whereas APC/C^CDH1 has a broader specificity. This switch depends on dephosphorylation of CDH1 and the APC/C, and on the degradation of CDC20. Here we show, in human cells, that the APC/C inhibitor MAD2L2 also contributes to ensuring the sequential activation of the APC/C by CDC20 and CDH1. In prometaphase, MAD2L2 sequestered free CDH1 away from the APC/C. At the onset of anaphase, MAD2L2 was rapidly degraded by APC/C^CDC20, releasing CDH1 to activate the dephosphorylated APC/C. Loss of MAD2L2 led to premature association of CDH1 with the APC/C, early destruction of APC/C^CDH1 substrates, and accelerated mitosis with frequent mitotic aberrations. Thus, MAD2L2 helps to ensure a robustly bistable switch between APC/C^CDC20 and APC/C^CDH1 during the metaphase-to-anaphase transition, thereby contributing to mitotic fidelity.

SCF-mediated Cdh1 degradation defines a negative feedback system that coordinates cell-cycle progression

Proper cell-cycle transitions are driven by waves of ubiquitin-dependent degradation of key regulators by the anaphase-promoting complex (APC) and Skp1-Cullin1-F-box (SCF) E3 ubiquitin ligase complexes. But precisely how APC and SCF activities are coordinated to regulate cell-cycle progression remains largely unclear. We previously showed that APC/Cdh1 earmarks the SCF component Skp2 for degradation. Here, we continue to report that SCF(β-TRCP) reciprocally controls APC/Cdh1 activity by governing Cdh1 ubiquitination and subsequent degradation. Furthermore, we define both cyclin A and Plk1, two well-known Cdh1 substrates, as upstream modifying enzymes that promote Cdh1 phosphorylation to trigger Cdh1 ubiquitination and subsequent degradation by SCF(β-TRCP). Thus, our work reveals a negative repression mechanism for SCF to control APC, thereby illustrating an elegant dual repression system between these two E3 ligase complexes to create the ordered cascade of APC and SCF activities governing timely cell-cycle transitions.

Drosophila-Cdh1 (Rap/Fzr) a regulatory subunit of APC/C is required for synaptic morphology, synaptic transmission and locomotion

The assembly of functional synapses requires the orchestration of the synthesis and degradation of a multitude of proteins. Protein degradation and modification by the conserved ubiquitination pathway has emerged as a key cellular regulatory mechanism during nervous system development and function (Kwabe and Brose, 2011). The anaphase promoting complex/cyclosome (APC/C) is a multi-subunit ubiquitin ligase complex primarily characterized for its role in the regulation of mitosis (Peters, 2002). In recent years, a role for APC/C in nervous system development and function has been rapidly emerging (Stegmuller and Bonni, 2005; Li et al., 2008). In the mammalian central nervous system the activator subunit, APC/C-Cdh1, has been shown to be a regulator of axon growth and dendrite morphogenesis (Konishi et al., 2004). In the Drosophila peripheral nervous system (PNS), APC2, a ligase subunit of the APC/C complex has been shown to regulate synaptic bouton size and activity (van Roessel et al., 2004). To investigate the role of APC/C-Cdh1 at the synapse we examined loss-of-function mutants of Rap/Fzr (Retina aberrant in pattern/Fizzy related), a Drosophila homolog of the mammalian Cdh1 during the development of the larval neuromuscular junction in Drosophila. Our cell biological, ultrastructural, electrophysiological, and behavioral data showed that rap/fzr loss-of-function mutations lead to changes in synaptic structure and function as well as locomotion defects. Data presented here show changes in size and morphology of synaptic boutons, and, muscle tissue organization. Electrophysiological experiments show that loss-of-function mutants exhibit increased frequency of spontaneous miniature synaptic potentials, indicating a higher rate of spontaneous synaptic vesicle fusion events. In addition, larval locomotion and peristaltic movement were also impaired. These findings suggest a role for Drosophila APC/C-Cdh1 mediated ubiquitination in regulating synaptic morphology, function and integrity of muscle structure in the peripheral nervous system.

A novel function of CRL4(Cdt2): regulation of the subunit structure of DNA polymerase δ in response to DNA damage and during the S phase

DNA polymerase δ (Pol δ4) is a heterotetrameric enzyme, whose p12 subunit is degraded in response to DNA damage, leaving behind a trimer (Pol δ3) with altered enzymatic characteristics that participate in gap filling during DNA repair. We demonstrate that CRL4(Cdt2), a key regulator of cell cycle progression that targets replication licensing factors, also targets the p12 subunit of Pol δ4 in response to DNA damage and on entry into S phase. Evidence for the involvement of CRL4(Cdt2) included demonstration that p12 possesses a proliferating cell nuclear antigen-interacting protein-degron (PIP-degron) and that knockdown of the components of the CRL4(Cdt2) complex inhibited the degradation of p12 in response to DNA damage. Analysis of p12 levels in synchronized cell populations showed that p12 is partially degraded in S phase and that this is affected by knockdowns of CUL4A or CUL4B. Laser scanning cytometry of overexpressed wild type p12 and a mutant resistant to degradation showed that the reduction in p12 levels during S phase was prevented by mutation of p12. Thus, CRL4(Cdt2) also regulates the subunit composition of Pol δ during the cell cycle. These studies reveal a novel function of CRL4(Cdt2), i.e. the direct regulation of DNA polymerase δ, adding to its known functions in the regulation of the licensing of replication origins and expanding the scope of its overall control of DNA replication. The formation of Pol δ3 in S phase as a normal aspect of cell cycle progression leads to the novel implications that it is involved in DNA replication as well as DNA repair.

Control the host cell cycle: viral regulation of the anaphase-promoting complex

Viruses commonly manipulate cell cycle progression to create cellular conditions that are most beneficial to their replication. To accomplish this feat, viruses often target critical cell cycle regulators in order to have maximal effect with minimal input. One such master regulator is the large, multisubunit E3 ubiquitin ligase anaphase-promoting complex (APC) that targets effector proteins for ubiquitination and proteasome degradation. The APC is essential for cells to progress through anaphase, exit from mitosis, and prevent a premature entry into S phase. These far-reaching effects of the APC on the cell cycle are through its ability to target a number of substrates, including securin, cyclin A, cyclin B, thymidine kinase, geminin, and many others. Recent studies have identified several proteins from a number of viruses that can modulate APC activity by different mechanisms, highlighting the potential of the APC in driving viral replication or pathogenesis. Most notably, human cytomegalovirus (HCMV) protein pUL21a was recently identified to disable the APC via a novel mechanism by targeting APC subunits for degradation, both during virus infection and in isolation. Importantly, HCMV lacking both viral APC regulators is significantly attenuated, demonstrating the impact of the APC on a virus infection. Work in this field will likely lead to novel insights into viral replication and pathogenesis and APC function and identify novel antiviral and anticancer targets. Here we review viral mechanisms to regulate the APC, speculate on their roles during infection, and identify questions to be addressed in future studies.

Selective enhancing effect of early mitotic inhibitor 1 (Emi1) depletion on the sensitivity of doxorubicin or X-ray treatment in human cancer cells

Chemotherapy and radiation in addition to surgery has proven useful in a number of different cancer types, but the effectiveness in normal tissue cannot be avoided in these therapies. To improve the effectiveness of these therapies selectively in cancer tissue is important for avoiding side effects. Early mitotic inhibitor 1 (Emi1) is known to have the function to inhibit anaphase-promoting complex/cyclosome ubiquitin ligase complex, which ubiquitylates the cell cycle-related proteins. It recently has been shown that Emi1 knockdown prevents transition from S to G2 phase by down-regulating geminin via anaphase-promoting complex/cyclosome activation. At present, anticancer drugs for targeting DNA synthesis to interfere with rapidly dividing cells commonly are used. As Emi1 depletion interferes with completion of DNA synthesis in cancer cells, we thought that Emi1 knockdown might enhance the sensitivity for anticancer agents. Here, we confirmed that Emi1 siRNA induced polyploidy for preventing transition from S to G2 phase in several cancer cell lines. Then, we treated Emi1 depleted cells with doxorubicin. Interestingly, increased apoptotic cells were observed after doxorubicin treatment in Emi1 siRNA-treated cancer cells. In addition, Emi1 depletion enhanced the sensitivity of x-ray irradiation in cancer cells. Importantly, synergistic effect of Emi1 knockdown in these combination therapies was not observed in normal cells. These results suggest that Emi1 siRNA can be a useful tool for enhancing of sensitivity of cancer cells to anticancer reagents and radiation.

Emerging regulatory mechanisms in ubiquitin-dependent cell cycle control

The covalent modification of proteins with ubiquitin is required for accurate cell division in all eukaryotes. Ubiquitylation depends on an enzymatic cascade, in which E3 enzymes recruit specific substrates for modification. Among ~600 human E3s, the SCF (Skp1-cullin1-F-box) and the APC/C (anaphase-promoting complex/cyclosome) are known for driving the degradation of cell cycle regulators to accomplish irreversible cell cycle transitions. The cell cycle machinery reciprocally regulates the SCF and APC/C through various mechanisms, including the modification of these E3s or the binding of specific inhibitors. Recent studies have provided new insight into the intricate relationship between ubiquitylation and the cell division apparatus as they revealed roles for atypical ubiquitin chains, new mechanisms of substrate and E3 regulation, as well as extensive crosstalk between ubiquitylation enzymes. Here, we review these emerging regulatory mechanisms of ubiquitin-dependent cell cycle control and discuss how their manipulation might provide therapeutic benefits in the future.

Ubiquitin ligases and cell cycle control

The ubiquitin-proteasome system plays a pivotal role in the sequence of events leading to cell division known as the cell cycle. Not only does ubiquitin-mediated proteolysis constitute a critical component of the core oscillator that drives the cell cycle in all eukaryotes, it is also central to the mechanisms that ensure that the integrity of the genome is maintained. These functions are primarily carried out by two families of E3 ubiquitin ligases, the Skp/cullin/F-box-containing and anaphase-promoting complex/cyclosome complexes. However, beyond those functions associated with regulation of central cell cycle events, many peripheral cell cycle-related processes rely on ubiquitylation for signaling, homeostasis, and dynamicity, involving additional types of ubiquitin ligases and regulators. We are only beginning to understand the diversity and complexity of this regulation.

The ubiquitin proteasome system - implications for cell cycle control and the targeted treatment of cancer

Two families of E3 ubiquitin ligases are prominent in cell cycle regulation and mediate the timely and precise ubiquitin-proteasome-dependent degradation of key cell cycle proteins: the SCF (Skp1/Cul1/F-box protein) complex and the APC/C (anaphase promoting complex or cyclosome). While certain SCF ligases drive cell cycle progression throughout the cell cycle, APC/C (in complex with either of two substrate recruiting proteins: Cdc20 and Cdh1) orchestrates exit from mitosis (APC/C(Cdc20)) and establishes a stable G1 phase (APC/C(Cdh1)). Upon DNA damage or perturbation of the normal cell cycle, both ligases are involved in checkpoint activation. Mechanistic insight into these processes has significantly improved over the last ten years, largely due to a better understanding of APC/C and the functional characterization of multiple F-box proteins, the variable substrate recruiting components of SCF ligases. Here, we review the role of SCF- and APC/C-mediated ubiquitylation in the normal and perturbed cell cycle and discuss potential clinical implications of SCF and APC/C functions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

The E3 ligase APC/C-Cdh1 is required for associative fear memory and long-term potentiation in the amygdala of adult mice

The anaphase promoting complex/cyclosome (APC/C) is an E3 ligase regulated by Cdh1. Beyond its role in controlling cell cycle progression, APC/C-Cdh1 has been detected in neurons and plays a role in long-lasting synaptic plasticity and long-term memory. Herein, we further examined the role of Cdh1 in synaptic plasticity and memory by generating knockout mice where Cdh1 was conditionally eliminated from the forebrain post-developmentally. Although spatial learning and memory in the Morris water maze (MWM) was normal, the Cdh1 conditional knockout (cKO) mice displayed enhanced reversal learning in the MWM and in a water-based Y maze. In addition, we found that the Cdh1 cKO mice had impaired associative fear memory and exhibited impaired long-term potentiation (LTP) in amygdala slices. Finally, we observed increased expression of Shank1 and NR2A expression in amygdalar slices from the Cdh1 cKO mice following the induction of LTP, suggesting a possible molecular mechanism underlying the behavioral and synaptic plasticity impairments displayed in these mice. Our findings are consistent with a role for the APC/C-Cdh1 in fear memory and synaptic plasticity in the amygdala.

Mitochondrial hyperfusion induced by loss of the fission protein Drp1 causes ATM-dependent G2/M arrest and aneuploidy through DNA replication stress

Mitochondrial fission and fusion cycles are integrated with cell cycle progression. In this paper, we demonstrate that the inhibition of mitochondrial fission protein Drp1 causes an unexpected delay in G2/M cell cycle progression and aneuploidy. In investigating the underlying molecular mechanism, we revealed that inhibiting Drp1 triggers replication stress, which is mediated by a hyperfused mitochondrial structure and unscheduled expression of cyclin E in the G2 phase. This persistent replication stress then induces an ATM-dependent activation of the G2 to M transition cell cycle checkpoint. Knockdown of ATR, an essential kinase in preventing replication stress, significantly enhanced DNA damage and cell death of Drp1-deficienct cells. Persistent mitochondrial hyperfusion also induces centrosomal overamplification and chromosomal instability, which are causes of aneuploidy. Analysis using cells depleted of mitochondrial DNA revealed that these events are not mediated by the defects in mitochondrial ATP production and reactive oxygen species (ROS) generation. Thus dysfunctional mitochondrial fission directly induces genome instability by replication stress, which then initiates the DNA damage response. Our findings provide a novel mechanism that contributes to the cellular dysfunction and diseases associated with altered mitochondrial dynamics.

Estrogen and progesterone regulate p27kip1 levels via the ubiquitin-proteasome system: pathogenic and therapeutic implications for endometrial cancer

The levels of proteins that control the cell cycle are regulated by ubiquitin-mediated degradation via the ubiquitin-proteasome system (UPS) by substrate-specific E3 ubiquitin ligases. The cyclin-dependent kinase inhibitor, p27kip1 (p27), that blocks the cell cycle in G1, is ubiquitylated by the E3 ligase SCF-Skp2/Cks1 for degradation by the UPS. In turn, Skp2 and Cks1 are ubiquitylated by the E3 ligase complex APC/Cdh1 for destruction thereby maintaining abundant levels of nuclear p27. We previously showed that perpetual proteasomal degradation of p27 is an early event in Type I endometrial carcinogenesis (ECA), an estrogen (E2)-induced cancer. The present studies demonstrate that E2 stimulates growth of ECA cell lines and normal primary endometrial epithelial cells (EECs) and induces MAPK-ERK1/2-dependent phosphorylation of p27 on Thr187, a prerequisite for p27 ubiquitylation by nuclear SCF-Skp2/Cks1 and subsequent degradation. In addition, E2 decreases the E3 ligase [APC]Cdh1 leaving Skp2 and Cks1 intact to cause p27 degradation. Furthermore, knocking-down Skp2 prevents E2-induced p27 degradation and growth stimulation suggesting that the pathogenesis of E2-induced ECA is dependent on Skp2-mediated degradation of p27. Conversely, progesterone (Pg) as an inhibitor of endometrial proliferation increases nuclear p27 and Cdh1 in primary EECs and ECA cells. Pg, also increases Cdh1 binding to APC to form the active E3ligase. Knocking-down Cdh1 obviates Pg-induced stabilization of p27 and growth inhibition. Notably, neither E2 nor Pg affected transcription of Cdh1, Skp2, Cks1 nor p27. These studies provide new insights into hormone regulation of cell proliferation through the UPS. The data implicates that preventing nuclear p27 degradation by blocking Skp2/Cks1-mediated degradation of p27 or increasing Cdh1 to mediate degradation of Skp2-Cks1 are potential strategies for the prevention and treatment of ECA.

Regulation of APC/C-Cdh1 and its function in neuronal survival

Neurons are post-mitotic cells that undergo an active downregulation of cell cycle-related proteins to survive. The activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that regulates cell cycle progression in proliferating cells, plays a relevant role in post-mitotic neurons. Recent advances in the study of the regulation of APC/C have documented that the APC/C-activating cofactor, Cdh1, is essential for the function(s) of APC/C in neuronal survival. Here, we review the normal regulation of APC/C activity in proliferating cells and neurons. We conclude that in neurons the APC/C-Cdh1 complex actively downregulates the stability of the cell cycle protein cyclin B1 and the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3. Keeping these proteins destabilized is critical both for preventing the aberrant reentry of post-mitotic neurons into the cell cycle and for maintaining their reduced antioxidant status. Further understanding of the pathophysiological regulation of these proteins by APC/C-Cdh1 in neurons will be important for the search for novel therapeutic targets against neurodegeneration.