APC/C (Cdh1) controls the proteasome-mediated degradation of E2F3 during cell cycle exit

Decoding the ubiquitin language: Orchestrating transcription initiation and gene expression through chromatin remodelers and histones

Eukaryotic transcription is a finely orchestrated process and it is controlled by transcription factors as well as epigenetic regulators. Transcription factors and epigenetic regulators undergo different types of posttranslational modifications including ubiquitination to control transcription process. Ubiquitination, traditionally associated with protein degradation, has emerged as a crucial contributor to the regulation of chromatin structure through ubiquitination of histone and chromatin remodelers. Ubiquitination introduces new layers of intricacy to the regulation of transcription initiation through controlling the equilibrium between euchromatin and heterochromatin states. Nucleosome, the fundamental units of chromatin, spacing in euchromatin and heterochromatin states are regulated by histone modification and chromatin remodeling complexes. Chromatin remodeling complexes actively sculpt the chromatin architecture and thereby influence the transcriptional states of genes. Therefore, understanding the dynamic behavior of nucleosome spacing is critical as it impacts various cellular functions through controlling gene expression profiles. In this comprehensive review, we discussed the intricate interplay between ubiquitination and transcription initiation, and illuminated the underlying molecular mechanisms that occur in a variety of biological contexts. This exploration sheds light on the complex regulatory networks that govern eukaryotic transcription, providing important insights into the fine orchestration of gene expression and chromatin dynamics.

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.

Identification of GNA12-driven gene signatures and key signaling networks in ovarian cancer

With the focus on defining the oncogenic network stimulated by lysophosphatidic acid (LPA) in ovarian cancer, the present study sought to interrogate the oncotranscriptome regulated by the LPA-mediated signaling pathway. LPA, LPA-receptor (LPAR) and LPAR-activated G protein 12 α-subunit, encoded by G protein subunit α 12 (GNA12), all serve an important role in ovarian cancer progression. While the general signaling mechanism regulated by LPA/LPAR/GNA12 has previously been characterized, the global transcriptomic network regulated by GNA12 in ovarian cancer pathophysiology remains largely unknown. To define the LPA/LPAR/GNA12-orchestrated oncogenic networks in ovarian cancer, transcriptomic and bioinformatical analyses were conducted using SKOV3 cells, in which the expression of GNA12 was silenced. Array analysis was performed in Agilent SurePrint G3 Human Comparative Genomic Hybridization 8×60 microarray platform. The array results were validated using Kuramochi cells. Gene and functional enrichment analyses were performed using Database for Annotation, Visualization and Integrated Discovery, Search Tool for Retrieval of Interacting Genes and Cytoscape algorithms. The results indicated a paradigm in which GNA12 drove ovarian cancer progression by upregulating a pro-tumorigenic network with AKT1, VEGFA, TGFB1, BCL2L1, STAT3, insulin-like growth factor 1 and growth hormone releasing hormone as critical hub and/or bottleneck nodes. Moreover, GNA12 downregulated a growth-suppressive network involving proteasome 20S subunit (PSM) β6, PSM α6, PSM ATPase 5, ubiquitin conjugating enzyme E2 E1, PSM non-ATPase 10, NDUFA4 mitochondrial complex-associated, NADH:ubiquinone oxidoreductase subunit B8 and anaphase promoting complex subunit 1 as hub or bottleneck nodes. In addition to providing novel insights into the LPA/LPAR/GNA12-regulated oncogenic networks in ovarian cancer, the present study identified several potential nodes in this network that could be assessed for targeted therapy.

Robust rank aggregation and cibersort algorithm applied to the identification of key genes in head and neck squamous cell cancer

OBJECTIVE

Although multiple hub genes have been identified in head and neck squamous cell cancer (HNSCC) in recent years, because of the limited sample size and inconsistent bioinformatics analysis methods, the results are not reliable. Therefore, it is urgent to use reliable algorithms to find new prognostic markers of HNSCC.

METHOD

The Robust Rank Aggregation (RRA) method was used to integrate 8 microarray datasets of HNSCC downloaded from the Gene Expression Omnibus (GEO) database to screen differentially expressed genes (DEGs). Later, Gene Ontology (GO) functional annotation together with Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis was carried out to discover functions of those discovered DEGs. According to the KEGG results, those discovered DEGs showed tight association with the occurrence and development of HNSCC. Then cibersort algorithm was used to analyze the infiltration of immune cells of HNSCC and we found that the main infiltrated immune cells were B cells, dendritic cells and macrophages. A protein-protein interaction (PPI) network was established; moreover, key modules were also constructed to select 5 hub genes from the whole network using cytoHubba. 3 hub genes showed significant relationship with prognosis for TCGA-derived HNSCC patients.

RESULT

The potent DEGs along with hub genes were selected by the combined bioinformatic approach. AURKA, BIRC5 and UBE2C genes may be the potential prognostic biomarker and therapeutic targets of HNSCC.

CONCLUSIONS

The Robust Rank Aggregation method and cibersort algorithm method can accurately predict the potential prognostic biomarker and therapeutic targets of HNSCC through multiple GEO datasets.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

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.

Cryptochromes modulate E2F family transcription factors

Early 2 factor (E2F) family transcription factors participate in myriad cell biological processes including: the cell cycle, DNA repair, apoptosis, development, differentiation, and metabolism. Circadian rhythms influence many of these phenomena. Here we find that a mammalian circadian rhythm component, Cryptochrome 2 (CRY2), regulates E2F family members. Furthermore, CRY1 and CRY2 cooperate with the E3 ligase complex SKP-CULLIN-FBXL3 (SCFFBXL3) to reduce E2F steady state protein levels. These findings reveal an unrecognized molecular connection between circadian clocks and cell cycle regulation and highlight another mechanism to maintain appropriate E2F protein levels for proper cell growth.

EBF1 Gene mRNA Levels in Maternal Blood and Spontaneous Preterm Birth

Genetic variants of six genes (EBF1, EEFSEC, AGTR2, WNT4, ADCY5, and RAP2C) have been linked recently to gestational duration and/or spontaneous preterm birth (sPTB). Our goal was to examine sPTB in relation to maternal blood mRNA levels of these genes. We used a public gene expression dataset (GSE59491) derived from maternal blood in trimesters 2 and 3 that included women with sPTB (n = 51) and term births (n = 106) matched for maternal age, race/ethnicity, pre-pregnancy body mass index, smoking during pregnancy, and parity. T tests were used to examine mRNA mean differences (sPTB vs term) within and across trimesters, and logistic regression models with mRNA quartiles were applied to assess associations between candidate gene mRNA levels and sPTB. Based on these analyses, one significant candidate gene was used in a Gene Set Enrichment Analysis (GSEA) to identify related gene sets. These gene sets were then compared with the ones previously linked to sPTB in the same samples. Our results indicated that among women in the lowest quartile of EBF1 mRNA in the 2nd or 3rd trimester, the odds ratio for sPTB was 2.86 (95%CI 1.08, 7.58) (p = 0.0349, false discovery rate (FDR) = 0.18) and 4.43 (95%CI 1.57, 12.50) (p = 0.0049, FDR = 0.06), respectively. No other candidate gene mRNAs were significantly associated with sPTB. In GSEA, 24 downregulated gene sets were correlated with 2nd trimester low EBF1 mRNA and part of previous sPTB-associated gene sets. In conclusion, mRNA levels of EBF1 in maternal blood may be useful in detecting increased risk of sPTB as early as 2nd trimester. The potential underlying mechanism might involve maternal-fetal immune and cell cycle/apoptosis pathways.

Cyclin F Controls Cell-Cycle Transcriptional Outputs by Directing the Degradation of the Three Activator E2Fs

E2F1, E2F2, and E2F3A, the three activators of the E2F family of transcription factors, are key regulators of the G1/S transition, promoting transcription of hundreds of genes critical for cell-cycle progression. We found that during late S and in G2, the degradation of all three activator E2Fs is controlled by cyclin F, the substrate receptor of 1 of 69 human SCF ubiquitin ligase complexes. E2F1, E2F2, and E2F3A interact with the cyclin box of cyclin F via their conserved N-terminal cyclin binding motifs. In the short term, E2F mutants unable to bind cyclin F remain stable throughout the cell cycle, induce unscheduled transcription in G2 and mitosis, and promote faster entry into the next S phase. However, in the long term, they impair cell fitness. We propose that by restricting E2F activity to the S phase, cyclin F controls one of the main and most critical transcriptional engines of the cell cycle.

Two Distinct E2F Transcriptional Modules Drive Cell Cycles and Differentiation

Orchestrating cell-cycle-dependent mRNA oscillations is critical to cell proliferation in multicellular organisms. Even though our understanding of cell-cycle-regulated transcription has improved significantly over the last three decades, the mechanisms remain untested in vivo. Unbiased transcriptomic profiling of G0, G1-S, and S-G2-M sorted cells from FUCCI mouse embryos suggested a central role for E2Fs in the control of cell-cycle-dependent gene expression. The analysis of gene expression and E2F-tagged knockin mice with tissue imaging and deep-learning tools suggested that post-transcriptional mechanisms universally coordinate the nuclear accumulation of E2F activators (E2F3A) and canonical (E2F4) and atypical (E2F8) repressors during the cell cycle in vivo. In summary, we mapped the spatiotemporal expression of sentinel E2F activators and canonical and atypical repressors at the single-cell level in vivo and propose that two distinct E2F modules relay the control of gene expression in cells actively cycling (E2F3A-8-4) and exiting the cycle (E2F3A-4) during mammalian development.

The Anaphase Promoting Complex/Cyclosome (APC/C): A Versatile E3 Ubiquitin Ligase

In the present chapter we discuss the essential roles of the human E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) in mitosis as well as the emerging evidence of important APC/C roles in cellular processes beyond cell division control such as regulation of genomic integrity and cell differentiation of the nervous system. We consider the potential incipient role of APC/C dysregulation in the pathophysiology of the neurological disorder Alzheimer's disease (AD). We also discuss how certain Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA) viruses take control of the host's cell division regulatory system through harnessing APC/C ubiquitin ligase activity and hypothesise the plausible molecular mechanisms underpinning virus manipulation of the APC/C. We also examine how defects in the function of this multisubunit protein assembly drive abnormal cell proliferation and lastly argue the potential of APC/C as a promising therapeutic target for the development of innovative therapies for the treatment of chronic malignancies such as cancer.

Transcriptome analysis of pancreatic cancer cell response to treatment with grape seed proanthocyanidins

Grape seed proanthocyanidins (GSPs) have been demonstrated to exhibit potential chemotherapeutic efficacy against various cancer types. To determine the underlying molecular mechanisms involved in GSP-induced apoptosis, the present study prepared pancreatic cancer (PC) cells samples, S3, S12 and S24, which were treated with 20 µg/ml GSPs for 3, 12 and 24 h, respectively. Control cell samples, C3, C12 and C24, were also prepared. Using RNA-sequencing, transcriptome comparisons were performed, which identified 966, 3,543 and 4,944 differentially-expressed genes (DEGs) in S3 vs. C3, S12 vs. C12 and S24 vs. C24, respectively. Gene Ontology analysis of the DEGs, revealed that treatment with GSPs is associated with disruption of the cell cycle (CC) in PC cells. Additionally, disruption of transcription, DNA replication and DNA repair were associated with GSP-treatment in PC cells. Network analysis demonstrated that the common DEGs involved in the CC, transcription, DNA replication and DNA repair were integrated, and served essential roles in the control of CC progression in cancer cells. In summary, GSPs may exhibit a potential chemotherapeutic effect on PC cell proliferation.

UBE2C promotes rectal carcinoma via miR-381

We aimed to characterize the expression pattern of UBE2C in rectal carcinoma and elucidate its fundamental involvement in rectal carcinoma biology. The relative expression of UBE2C in rectal carcinoma was determined by immunoblotting and QPCR. The cell viability was measured using CCK-8 assay. The anchorage-independent growth was evaluated with soft agar assay. Cell apoptosis was detected by Annexin V-PI staining. Invasion capacity was determined by transwell chamber. Tumor growth was monitored in xenograft mice model. We demonstrated that UBE2C was aberrantly up-regulated in rectal carcinoma. SiRNA-mediated knockdown of UBE2C significantly inhibited cell viability, proliferation, colony formation, invasion and induced apoptosis in vitro. Moreover, tumor growth in xenograft mice was markedly suppressed upon UBE2C silencing. Furthermore, we have identified that miR-381 was involved in regulation of UBE2C in rectal carcinoma. Here we demonstrated that UBE2C was over-expressed in rectal carcinoma, which was subjected to miR-381 modulation and in turn promoted cell proliferation, invasion and inhibited cell apoptosis.

New Functions of APC/C Ubiquitin Ligase in the Nervous System and Its Role in Alzheimer's Disease

The E3 ubiquitin ligase Anaphase Promoting Complex/Cyclosome (APC/C) regulates important processes in cells, such as the cell cycle, by targeting a set of substrates for degradation. In the last decade, APC/C has been related to several major functions in the nervous system, including axon guidance, synaptic plasticity, neurogenesis, and neuronal survival. Interestingly, some of the identified APC/C substrates have been related to neurodegenerative diseases. There is an accumulation of some degradation targets of APC/C in Alzheimer’s disease (AD) brains, which suggests a dysregulation of the protein complex in the disorder. Moreover, recently evidence has been provided for an inactivation of APC/C in AD. It has been shown that oligomers of the AD-related peptide, Aβ, induce degradation of the APC/C activator subunit cdh1, in vitro in neurons in culture and in vivo in the mouse hippocampus. Furthermore, in the AD mouse model APP/PS1, lower cdh1 levels were observed in pyramidal neurons in CA1 when compared to age-matched wildtype mice. In this review, we provide a complete list of APC/C substrates that are involved in the nervous system and we discuss their functions. We also summarize recent studies that show neurobiological effects in cdh1 knockout mouse models. Finally, we discuss the role of APC/C in the pathophysiology of AD.

Dosage-dependent copy number gains in E2f1 and E2f3 drive hepatocellular carcinoma

Disruption of the retinoblastoma (RB) tumor suppressor pathway, either through genetic mutation of upstream regulatory components or mutation of RB1 itself, is believed to be a required event in cancer. However, genetic alterations in the RB-regulated E2F family of transcription factors are infrequent, casting doubt on a direct role for E2Fs in driving cancer. In this work, a mutation analysis of human cancer revealed subtle but impactful copy number gains in E2F1 and E2F3 in hepatocellular carcinoma (HCC). Using a series of loss- and gain-of-function alleles to dial E2F transcriptional output, we have shown that copy number gains in E2f1 or E2f3b resulted in dosage-dependent spontaneous HCC in mice without the involvement of additional organs. Conversely, germ-line loss of E2f1 or E2f3b, but not E2f3a, protected mice against HCC. Combinatorial mapping of chromatin occupancy and transcriptome profiling identified an E2F1- and E2F3B-driven transcriptional program that was associated with development and progression of HCC. These findings demonstrate a direct and cell-autonomous role for E2F activators in human cancer.

Feedback regulation between atypical E2Fs and APC/CCdh1 coordinates cell cycle progression

E2F transcription factors control the oscillating expression pattern of multiple target genes during the cell cycle. Activator E2Fs, E2F1-3, induce an upswing of E2F targets, which is essential for the G1-to-S phase transition, whereas atypical E2Fs, E2F7 and E2F8, mediate a downswing of the same targets during late S, G2, and M phases. Expression of atypical E2Fs is induced by E2F1-3, but it is unknown how atypical E2Fs are inactivated in a timely manner. Here, we demonstrate that E2F7 and E2F8 are substrates of the anaphase-promoting complex/cyclosome (APC/C). Removal of CDH1, or mutating the CDH1-interacting KEN boxes, stabilized E2F7/8 from anaphase onwards and during G1. Expressing KEN mutant E2F7 during G1 impairs S phase entry and eventually results in cell death. Furthermore, we show that E2F8, but not E2F7, interacts also with APC/C(C) (dc20). Importantly, atypical E2Fs can activate APC/C(C) (dh1) by repressing its inhibitors cyclin A, cyclin E, and Emi1. In conclusion, we discovered a feedback loop between atypical E2Fs and APC/C(C) (dh1), which ensures balanced expression of cell cycle genes and normal cell cycle progression.

E2F Transcription Factors Control the Roller Coaster Ride of Cell Cycle Gene Expression

Initially, the E2F transcription factor was discovered as a factor able to bind the adenovirus E2 promoter and activate viral genes. Afterwards it was shown that E2F also binds to promoters of nonviral genes such as C-MYC and DHFR, which were already known at that time to be important for cell growth and DNA metabolism, respectively. These findings provided the first clues that the E2F transcription factor might be an important regulator of the cell cycle. Since this initial discovery in 1987, several additional E2F family members have been identified, and more than 100 targets genes have been shown to be directly regulated by E2Fs, the majority of these are important for controlling the cell cycle. The progression of a cell through the cell cycle is accompanied with the increased expression of a specific set of genes during one phase of the cell cycle and the decrease of the same set of genes during a later phase of the cell cycle. This roller coaster ride, or oscillation, of gene expression is essential for the proper progression through the cell cycle to allow accurate DNA replication and cell division. The E2F transcription factors have been shown to be critical for the temporal expression of the oscillating cell cycle genes. This review will focus on how the oscillation of E2Fs and their targets is regulated by transcriptional, post-transcriptional and post-translational mechanism in mammals, yeast, flies, and worms. Furthermore, we will discuss the functional impact of E2Fs on the cell cycle progression and outline the consequences when E2F expression is disturbed.

USP17- and SCFβTrCP--regulated degradation of DEC1 controls the DNA damage response

In response to genotoxic stress, DNA damage checkpoints maintain the integrity of the genome by delaying cell cycle progression to allow for DNA repair. Here we show that the degradation of the basic helix-loop-helix (bHLH) transcription factor DEC1, a critical regulator of cell fate and circadian rhythms, controls the DNA damage response. During unperturbed cell cycles, DEC1 is a highly unstable protein that is targeted for proteasome-dependent degradation by the SCF(βTrCP) ubiquitin ligase in cooperation with CK1. Upon DNA damage, DEC1 is rapidly induced in an ATM/ATR-dependent manner. DEC1 induction results from protein stabilization via a mechanism that requires the USP17 ubiquitin protease. USP17 binds and deubiquitylates DEC1, markedly extending its half-life. Subsequently, during checkpoint recovery, DEC1 proteolysis is reestablished through βTrCP-dependent ubiquitylation. Expression of a degradation-resistant DEC1 mutant prevents checkpoint recovery by inhibiting the downregulation of p53. These results indicate that the regulated degradation of DEC1 is a key factor controlling the DNA damage response.

Aberrant self-renewal and quiescence contribute to the aggressiveness of glioblastoma

Cancer cells with enhanced self-renewal capacity influence tumour growth in glioblastoma. So far, a variety of surrogate markers have been proposed to enrich these cells, emphasizing the need to devise new characterization methods. Here, we screen a large panel of glioblastoma cultures (n = 21) cultivated under stem cell-permissive conditions and identify several cell lines with enhanced self-renewal capacity. These cell lines are capable of matrix-independent growth and form fast-growing, orthotopic tumours in mice. Employing isolation, re-plating, and label-retention techniques, we show that self-renewal potential of individual cells is partitioned asymmetrically between daughter cells in a robust and cell line-specific fashion. This yields populations of fast- and slow-cycling cells, which differ in the expression of cell cycle-associated transcripts. Intriguingly, fast-growing cells keep their slow-cycling counterparts in a reversible state of quiescence associated with high chemoresistance. Our results suggest that two different subpopulations of tumour cells contribute to aberrant growth and tumour recurrence after therapy in glioblastoma.

Proteasome-dependent degradation of transcription factor activating enhancer-binding protein 4 (TFAP4) controls mitotic division

TFAP4, a basic helix-loop-helix transcription factor that regulates the expression of a multitude of genes involved in the regulation of cellular proliferation, stemness, and epithelial-mesenchymal transition, is up-regulated in colorectal cancer and a number of other human malignancies. We have found that, during the G2 phase of the cell division cycle, TFAP4 is targeted for proteasome-dependent degradation by the SCF(βTrCP) ubiquitin ligase. This event requires phosphorylation of TFAP4 on a conserved degron. Expression of a stable TFAP4 mutant unable to interact with βTrCP results in a number of mitotic defects, including chromosome missegregation and multipolar spindles, which eventually lead to the activation of the DNA damage response. Our findings reveal that βTrCP-dependent degradation of TFAP4 is required for the fidelity of mitotic division.

Functional characterization of Anaphase Promoting Complex/Cyclosome (APC/C) E3 ubiquitin ligases in tumorigenesis

The Anaphase Promoting Complex/Cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that primarily governs cell cycle progression. APC/C is composed of at least 14 core subunits and recruits its substrates for ubiquitination via one of the two adaptor proteins, Cdc20 or Cdh1, in M or M/early G1 phase, respectively. Furthermore, recent studies have shed light on crucial functions for APC/C in maintaining genomic integrity, neuronal differentiation, cellular metabolism and tumorigenesis. To gain better insight into the in vivo physiological functions of APC/C in regulating various cellular processes, particularly development and tumorigenesis, a number of mouse models of APC/C core subunits, coactivators or inhibitors have been established and characterized. However, due to their essential role in cell cycle regulation, most of the germline knockout mice targeting the APC/C pathway are embryonic lethal, indicating the need for generating conditional knockout mouse models to assess the role in tumorigenesis for each APC/C signaling component in specific tissues. In this review, we will first provide a brief introduction of the ubiquitin-proteasome system (UPS) and the biochemical activities and cellular functions of the APC/C E3 ligase. We will then focus primarily on characterizing genetic mouse models used to understand the physiological roles of each APC/C signaling component in embryogenesis, cell proliferation, development and carcinogenesis. Finally, we discuss future research directions to further elucidate the physiological contributions of APC/C components during tumorigenesis and validate their potentials as a novel class of anti-cancer targets.

Flipping the switch from g1 to s phase with e3 ubiquitin ligases

The cell cycle ensures genome maintenance by coordinating the processes of DNA replication and chromosome segregation. Of particular importance is the irreversible transition from the G1 phase of the cell cycle to S phase. This transition marks the switch from preparing chromosomes for replication ("origin licensing") to active DNA synthesis ("origin firing"). Ubiquitin-mediated proteolysis is essential for restricting DNA replication to only once per cell cycle and is the major mechanism regulating the G1 to S phase transition. Although some changes in protein levels are attributable to regulated mRNA abundance, protein degradation elicits very rapid changes in protein abundance and is critical for the sharp and irreversible transition from one cell cycle stage to the next. Not surprisingly, regulation of the G1-to-S phase transition is perturbed in most cancer cells, and deregulation of key molecular events in G1 and S phase drives not only cell proliferation but also genome instability. In this review we focus on the mechanisms by which E3 ubiquitin ligases control the irreversible transition from G1 to S phase in mammalian cells.