Degradation Of Cyclin Research Articles

Abstract PD4-08: A microenvironment secretome screen reveals FGF2 as a mediator of resistance to anti-estrogens and PI3K/mTOR pathway inhibitors in ER+ breast cancer

Abstract Despite the clinical success of anti-estrogen therapies, phosphatidylinositol 3-kinase inhibitors (PI3Ki), and mechanistic target of rapamycin complex I inhibitors (mTORC1i) for the treatment of patients with ER+ breast cancer, disease recurrence and progression are common. We found that a tumor transcriptional profile reflecting high stromal fibroblast content was associated with poor outcome in 3 cohorts of patients with ER+ breast cancer. We hypothesized that individual factors in the tumor microenvironment (TME) significantly contribute to drug resistance. To test this hypothesis, we screened 297 recombinant secreted proteins for ability to confer resistance to the anti-estrogen fulvestrant in MCF-7 and T47D ER+ breast cancer cells. Screen results were validated, and expansion screening included the anti-estrogen tamoxifen, the PI3Ki pictilisib, and the mTORC1i everolimus in 4 cell lines. To identify hits are most likely to be relevant to ER+ breast cancer, a bioinformatics filter was developed utilizing gene and protein expression in human tissues relevant to the TMEs of ER+ breast cancer. After filtering, the top screening hit was fibroblast growth factor 2 (FGF2), which confers resistance to anti-estrogens, PI3Ki, and mTORC1i, and is highly expressed in tissues and cell types associated with ER+ breast cancer. FGF2 did not rescue cells from the CDK4/6i palbociclib or the DNA-damaging agent doxorubicin, demonstrating pathway selectivity in the rescue phenotype. FGF2 rescued cells from anti-estrogen-, PI3Ki-, and mTORC1i-induced apoptosis and cell cycle arrest via activation of FGFR signaling through FRS2a, MEK1/2, ERK1/2, and downstream upregulation of cyclin D1 and degradation of Bim. FGF2-mediated anti-cancer effects were abrogated by co-treatment with the FGF2-neutralizing antibody GAL-F2, the pan-FGFR inhibitor PD-173074, the MEK inhibitor trametinib, or palbociclib. Cell cycle- and apoptosis-specific effects of FGF2 were abrogated by RNAi targeting cyclin D1 and Bim, respectively. We generated a transcriptional signature of FGF2 response by RNA-seq of fulvestrant-treated MCF-7 and T47D cells treated +/- FGF2. In 3 cohorts of patients with ER+ breast cancer, a signature of FGF2 signaling was significantly associated with poor prognosis and predictive of anti-estrogen resistance, including in a multivariate analysis including age, tumor grade, tumor stage, and FGFR amplification status. Finally, the therapeutic potential of targeting FGF2 was confirmed in 3 mouse models of ER+ breast cancer: 1) FGF2 rescue MCF-7 xenografts from fulvestrant; 2) GAL-F2 synergized with fulvestrant to suppress growth of 59-2-HI murine mammary adenocarcinomas that recruit FGF2-secreting stroma; 3) GAL-F2 synergized with fulvestrant to induce regression of HCI-003 patient-derived xenografts. Therapeutic effects coincided with increased tumor cell apoptosis and decreased proliferation, but not changes in tumor vasculature. These findings warrant consideration of FGF2 as a novel therapeutic target in ER+ breast cancer. Citation Format: Shee K, Hinds JW, Yang W, Hampsch RA, Patel K, Varn FS, Cheng C, Jenkins NP, Kettenbach AN, Demidenko E, Owens P, Lanari C, Faber AC, Golub TR, Straussman R, Miller TW. A microenvironment secretome screen reveals FGF2 as a mediator of resistance to anti-estrogens and PI3K/mTOR pathway inhibitors in ER+ breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD4-08.

Degradation of cyclin B is critical for nuclear division in Trypanosoma brucei.

ABSTRACTKinetoplastids have a nucleus that contains the nuclear genome and a kinetoplast that contains the mitochondrial genome. These single-copy organelles must be duplicated and segregated faithfully to daughter cells at each cell division. In Trypanosoma brucei, although duplication of both organelles starts around the same time, segregation of the kinetoplast precedes that of the nucleus. Cytokinesis subsequently takes place so that daughter cells inherit a single copy of each organelle. Very little is known about the molecular mechanism that governs the timing of these events. Furthermore, it is thought that T. brucei lacks a spindle checkpoint that delays the onset of nuclear division in response to spindle defects. Here we show that a mitotic cyclin CYC6 has a dynamic localization pattern during the cell cycle, including kinetochore localization. Using CYC6 as a molecular cell cycle marker, we confirmed that T. brucei cannot delay the onset of anaphase in response to a bipolar spindle assembly defect. Interestingly, expression of a stabilized form of CYC6 caused the nucleus to arrest in a metaphase-like state without preventing cytokinesis. We propose that trypanosomes have an ability to regulate the timing of nuclear division by modulating the CYC6 protein level, without a spindle checkpoint.

-TrCP and Casein Kinase III Mediated Degradation of Cyclin F Controls Timely Mitotic Entry

Coordinated progressions of events in the cell cycle are necessary to ensure the replication of DNA and cell division. Checkpoint systems allow the accurate execution of each cell cycle phase. Cyclins are core components of the cell cycle, which master cell cycle transitions and checkpoint controls. The precise regulation of cyclin levels is fundamental to coordinate cell division with checkpoint control to avoid genome instability. Cyclin F has crucial roles in coordinating cell cycle events with checkpoint control at the G2/M transition, however, the mechanisms underlying cyclin F regulation are poorly understood. Here, we identify a novel mode of regulation of cyclin F by proteolysis through β-TrCP. βTrCP recognizes cyclin F through a non-canonical degron site (TSGXXS) after its phosphorylation by CKIIα and PLK1. The degradation of cyclin F by β-TrCP occurs at the G2/M transition and allows the initiation of the transcriptional programme required for mitosis entry.

Inactivation of USP14 Perturbs Ubiquitin Homeostasis and Delays the Cell Cycle in Mouse Embryonic Fibroblasts and in Fruit Fly Drosophila

Background/Aims: The 26S proteasome is the key proteolytic complex for recognition and degradation of polyubiquitinated target substrates in eukaryotes. Among numerous proteasome-associated proteins, a deubiquitinating enzyme (DUB) USP14 has been identified as an endogenous inhibitor of the proteasome. Here, we explored the complex regulatory functions of USP14 that involve ubiquitin (Ub) homeostasis and substrate degradation in flies and mammals. Methods: USP14-null primary and immortalized mouse embryonic fibroblasts (MEFs) and USP14 knocked-down Drosophila were analyzed in this study. We measured proteasome and DUB activities using fluorogenic reporter substrates and adduct-forming probes. To examine the levels of ubiquitin, we performed immunoblotting and immunohistochemistry. Mass spectrometry (MS) was used to examine polyUb chain linkages and USP14-interacing proteins. Cell cycle was analyzed by flow cytometry, BrdU labeling, and phospho-histone H3 staining. Results: The homeostasis of Ub in USP14^–/–MEFs was markedly perturbed because of facilitated clearance of Ub. This phenomenon was recapitulated in muscles of USP14-deficient Drosophila with old ages. Absolute quantitation using MS also revealed that USP14^–/– MEFs contained significantly increased amounts of Ub, compared with wild-type. The key phenotype of USP14^–/– MEFs was their delayed proliferation originated from prolonged interphase possibly through aberrant degradation of cyclins A and B1. We found that knocking down USP14 in Drosophila resulted in delayed eye development associated with reduced mitotic activity. Conclusion: Our study identifies novel cellular functions of USP14 not only in cellular Ub hometostasis but also in cell cycle progression. USP14 was also essential for proper Drosophila eye development. These results strongly suggest that the USP14-mediated proteasome activity regulation may be directly related to various human diseases including cancer.

A wake-up call to quiescent cancer cells - potential use of DYRK1B inhibitors in cancer therapy.

Nondividing cancer cells are relatively resistant to chemotherapeutic drugs and environmental stress factors. Promoting cell cycle re-entry of quiescent cancer cells is a potential strategy to enhance the cytotoxicity of agents that target cycling cells. It is therefore important to elucidate the mechanisms by which these cells are maintained in the quiescent state. The protein kinase dual specificity tyrosine phosphorylation-regulated kinase 1B (DYRK1B) is overexpressed in a subset of cancers and maintains cellular quiescence by counteracting G0 /G1 -S phase transition. Specifically, DYRK1B controls the S phase checkpoint by stabilizing the cyclin-dependent kinase (CDK) inhibitor p27Kip1 and inducing the degradation of cyclin D. DYRK1B also stabilizes the DREAM complex that represses cell cycle gene expression in G0 arrested cells. In addition, DYRK1B enhances cell survival by upregulating antioxidant gene expression and reducing intracellular levels of reactive oxygen species (ROS). Substantial evidence indicates that depletion or inhibition of DYRK1B drives cell cycle re-entry and enhances apoptosis of those quiescent cancer cells with high expression of DYRK1B. Furthermore, small molecule DYRK1B inhibitors sensitize cells to the cytotoxic effects of anticancer drugs that target proliferating cells. These encouraging findings justify continued efforts to investigate the use of DYRK1B inhibitors to disrupt the quiescent state and overturn chemoresistance of noncycling cancer cells.

Delayed APC/C activation extends the first mitosis of mouse embryos

The correct temporal regulation of mitosis underpins genomic stability because it ensures the alignment of chromosomes on the mitotic spindle that is required for their proper segregation to the two daughter cells. Crucially, sister chromatid separation must be delayed until all the chromosomes have attached to the spindle; this is achieved by the Spindle Assembly Checkpoint (SAC) that inhibits the Anaphase Promoting Complex/Cyclosome (APC/C) ubiquitin ligase. In many species the first embryonic M-phase is significantly prolonged compared to the subsequent divisions, but the reason behind this has remained unclear. Here, we show that the first M-phase in the mouse embryo is significantly extended due to a delay in APC/C activation. Unlike in somatic cells, where the APC/C first targets cyclin A2 for degradation at nuclear envelope breakdown (NEBD), we find that in zygotes cyclin A2 remains stable for a significant period of time after NEBD. Our findings that the SAC prevents cyclin A2 degradation, whereas over-expressed Plk1 stimulates it, support our conclusion that the delay in cyclin A2 degradation is caused by low APC/C activity. As a consequence of delayed APC/C activation cyclin B1 stability in the first mitosis is also prolonged, leading to the unusual length of the first M-phase.

Bistability analysis in mathematical model of M-phase control in Xenopus oocyte extracts

To gain more insight into the molecular mechanism that regulates eukaryotic cell cycle, we propose a modification to an existing Mathematical model of mitosis-promoting factor control in Xenopus oocyte extract. To achieve this, we redirect how cell exits from mitosis using ubiquitin ligase enzyme APCP:Cdc20 which is acting as a modifier in cyclin degradation. We present a modified wiring map using combination of mass action kinetics and Michaelis–Menten kinetics to represent the biochemical processes. This results in a system of non-linear ordinary differential equations. The equilibria of the system were analyzed analytically. Using MPF as a bifurcation parameter, we analytically derived conditions under which bistability can occur in the MPF activation module. We find that for bistability to occur, it is necessary that the rate of phosphorylation of Wee1 must be greater than the rate of its dephosphorylation and that the concentration of MPF must lie in certain range of parameter values. Numerical simulations conducted show that the time taken to exit the cell cycle is shorter than those published in some reports and the sensitivity analysis results suggest that the rate of synthesis of cyclin is the most sensitive parameter in the dynamics of cell cycle. Through numerical experiments, we found new parameter sets that will give rise to bistability in the MPF activation module.

Abstract 310: Enhanced stability of D-type cyclins correlates with glioblastoma resistance to ionizing radiation

Abstract Transition from G1 to S phase of the cell cycle is promoted by D-type cyclins (D1, D2 and D3), regulatory subunits critical for the activation of cyclin dependent kinases 4 and 6 (CDK4/6), which phosphorylate and inactivate the retinoblastoma protein (pRb) and activate the transcription factor E2F1 to promote the expression of genes essential for DNA synthesis. Deregulated expression of D-type cyclins is frequently observed in human malignancies and correlates with increased proliferation. Cyclin Ds overexpression is also associated with radioresistance, leading to radiotherapy failure and disease recurrence. Cyclin D1 undergoes ubiquitin-dependent degradation following the exposure of cancer cells to ionizing radiation (IR), and this has been shown to be critical for IR-induced G1 growth arrest and sensitivity of cancer cells to IR. While the role of cyclin D1 overexpression in drug resistance is already well documented, less is known about the role of cyclins D2 and D3 in radio-resistance in human cancer cells, particularly in brain neoplasms. In the present study, we investigated the effect of IR on different D-type cyclins (D1, D2, D3) in human glioblastoma cell lines as well as in tumor-derived mouse oligodendrocytes progenitor cells (OPCs). Here, we show that exposure of human astrocytes to increasing levels of IR led to a rapid proteasomal degradation of all three D-type cyclins. On the other hand, IR failed to induce significant cyclin Ds degradation in a number of human glioblastoma cells (U87, A172, U251, Snb19). Analysis of cell cycle profile of all glioblastoma cells tested and exposed to IR showed prominent cell cycle arrest in G1 starting at 24 hours following exposure. Furthermore, we show that the steady-state levels of D-type cyclin is higher in OPCs compared to mouse NIH3T3 cells and fail to undergo proteasomal degradation following IR exposure. Collectively, these results suggest that the degradation of D-type cyclins is not critical for IR-induced G1 cell cycle arrest in GMB and may underlie their resistance to IR. Understanding the role of cyclin Ds in promoting radioresistance may lead to a useful pharmacological strategy to enhance radiotherapy outcome and help implement a more effective treatment modality. Citation Format: Fadila Guessous, Mouadh Benamar, Hui Zhong, Tarek Abbas. Enhanced stability of D-type cyclins correlates with glioblastoma resistance to ionizing radiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 310. doi:10.1158/1538-7445.AM2017-310

Predicting network modules of cell cycle regulators using relative protein abundance statistics

BackgroundParameter estimation in systems biology is typically done by enforcing experimental observations through an objective function as the parameter space of a model is explored by numerical simulations. Past studies have shown that one usually finds a set of “feasible” parameter vectors that fit the available experimental data equally well, and that these alternative vectors can make different predictions under novel experimental conditions. In this study, we characterize the feasible region of a complex model of the budding yeast cell cycle under a large set of discrete experimental constraints in order to test whether the statistical features of relative protein abundance predictions are influenced by the topology of the cell cycle regulatory network.ResultsUsing differential evolution, we generate an ensemble of feasible parameter vectors that reproduce the phenotypes (viable or inviable) of wild-type yeast cells and 110 mutant strains. We use this ensemble to predict the phenotypes of 129 mutant strains for which experimental data is not available. We identify 86 novel mutants that are predicted to be viable and then rank the cell cycle proteins in terms of their contributions to cumulative variability of relative protein abundance predictions. Proteins involved in “regulation of cell size” and “regulation of G1/S transition” contribute most to predictive variability, whereas proteins involved in “positive regulation of transcription involved in exit from mitosis,” “mitotic spindle assembly checkpoint” and “negative regulation of cyclin-dependent protein kinase by cyclin degradation” contribute the least. These results suggest that the statistics of these predictions may be generating patterns specific to individual network modules (START, S/G2/M, and EXIT). To test this hypothesis, we develop random forest models for predicting the network modules of cell cycle regulators using relative abundance statistics as model inputs. Predictive performance is assessed by the areas under receiver operating characteristics curves (AUC). Our models generate an AUC range of 0.83-0.87 as opposed to randomized models with AUC values around 0.50.ConclusionsBy using differential evolution and random forest modeling, we show that the model prediction statistics generate distinct network module-specific patterns within the cell cycle network.

Proteasome inhibition enhances the efficacy of volasertib-induced mitotic arrest in AML in vitro and prolongs survival in vivo.

Elderly and frail patients, diagnosed with acute myeloid leukemia (AML) and ineligible to undergo intensive treatment, have a dismal prognosis. The small molecule inhibitor volasertib induces a mitotic block via inhibition of polo-like kinase 1 and has shown remarkable anti-leukemic activity when combined with low-dose cytarabine. We have demonstrated that AML cells are highly vulnerable to cell death in mitosis yet manage to escape a mitotic block through mitotic slippage by sustained proteasome-dependent slow degradation of cyclin B. Therefore, we tested whether interfering with mitotic slippage through proteasome inhibition arrests and kills AML cells more efficiently during mitosis. We show that therapeutic doses of bortezomib block the slow degradation of cyclin B during a volasertib-induced mitotic arrest in AML cell lines and patient-derived primary AML cells. In a xenotransplant mouse model of human AML, mice receiving volasertib in combination with bortezomib showed superior disease control compared to mice receiving volasertib alone, highlighting the potential therapeutic impact of this drug combination.

Gcn5 determines the fate of Drosophila germline stem cells through degradation of Cyclin A.

The fluctuating CDK-CYCLIN complex plays a general role in cell-cycle control. Many types of stem cells use unique features of the cell cycle to facilitate asymmetric division. However, the manner in which these features are established remains poorly understood. The cell cycle of Drosophila female germline stem cells (GSCs) is characterized by short G1 and very long G2 phases, making it an excellent model for the study of cell cycle control in stem cell fate determination. Using a Drosophila female GSC model, we found Gcn5, the first discovered histone acetyltransferase, to maintain germline stem cells in Drosophila ovaries. Results showed that Gcn5 is dispensable for the transcriptional silencing of bam, but interacts with Cyclin A to facilitate proper turnover in GSCs. Results also showed that Gcn5 promotes Cyclin A ubiquitination, which is dependent on its acetylating activity. Finally, results showed that knockdown of Cyclin A rescued the GSC-loss phenotype caused by lack of Gcn5. Collectively, these findings support the conclusion that Gcn5 acts through acetylation to facilitate Cyclin A ubiquitination and proper turnover, thereby determining the fate of GSCs.-Liu, T., Wang, Q., Li, W., Mao, F., Yue, S., Liu, S., Liu, X., Xiao, S., Xia, L. Gcn5 determines the fate of Drosophila germline stem cells through degradation of Cyclin A.

A comparative study of the degradation of yeast cyclins Cln1 and Cln2.

The yeast cyclins Cln1 and Cln2 are very similar in both sequence and function, but some differences in their functionality and localization have been recently described. The control of Cln1 and Cln2 cellular levels is crucial for proper cell cycle initiation. In this work, we analyzed the degradation patterns of Cln1 and Cln2 in order to further investigate the possible differences between them. Both cyclins show the same half‐life but, while Cln2 degradation depends on ubiquitin ligases SCFG rr1 and SCFC dc4, Cln1 is affected only by SCFG rr1. Degradation analysis of chimeric cyclins, constructed by combining fragments from Cln1 and Cln2, identifies the N‐terminal sequence of the proteins as responsible of the cyclin degradation pattern. In particular, the N‐terminal region of Cln2 is required to mediate degradation by SCFC dc4. This region is involved in nuclear import of Cln1 and Cln2, which suggests that differences in degradation may be due to differences in localization. Moreover, a comparison of the cyclins that differ only in the presence of the Cln2 nuclear export signal indicates a greater instability of exported cyclins, thus reinforcing the idea that cyclin stability is influenced by their localization.

Silencing of argininosuccinate lyase inhibits colorectal cancer formation.

Arginine and nitric oxide (NO) are important mediators of tumorigenesis in various types of cancer. Dysregulation of NO content by argininosuccinate lyase (ASL) has been previously demonstrated to inhibit the proliferation of liver and breast cancer cells. However, the function of ASL in colon cancer is not well defined. The present study aimed to determine the effect of ASL on colon cancer. Western blot analysis indicated that ASL expression was induced by endoplasmic reticulum stress in HCT116 and SW480 colon cancer cells. Additionally, the expression of ASL in colon cancer tissues was enhanced compared with that in the adjacent normal tissues, and the patients with colon cancer with higher ASL expression exhibited poorer survival rates. Transfection of ASL-targeting short hairpin RNA (shRNA) into HCT116 cells inhibited cell proliferation and decreased anchorage-independent growth in a soft agar assay. In addition, when injected subcutaneously into NOD/SCID mice, stable transfectant ASL-downregulated HCT116 cells exhibited decreased invivo tumorigenic ability. Flow cytometric analysis of cell cycle progression indicated that ASL-targeting shRNA induced G2/Marrest, and western blot analysis showed that the inhibition of ASL was accompanied by cyclinA2 degradation. Furthermore, ASL-targeting shRNA resulted in increased autophagosomes and decreased NO levels. Inhibition of NO by the NO synthase inhibitor L-NMMA significantly reduced cell proliferation and colony formation. In summary, the results of the present study indicated that ASL-targeting shRNA-induced growth inhibition is associated with decreased cyclinA2 expression and NO content in colon cancer.

Abstract A03: SCFCyclin F connects AKT signaling to the core cell cycle oscillator

Abstract Cell proliferation is governed by the presence of growth factors, and in their absence normal cells cease to progress through the cell cycle. The waves of Cyclin Dependent Kinase (CDK) activity constituting the core of the cell cycle oscillator are driven by cyclin synthesis and its subsequent destruction by the Anaphase Promoting Complex/Cyclosome (APC), a multi-subunit E3 ubiquitin ligase that controls degradation of Cyclins A and B. How growth factor signaling is integrated into the core cell cycle machinery remains largely elusive. Modular Cullin RING ubiquitin ligases (CRLs) represent the largest E3 ligase family in humans, and control the destruction of hundreds of proteins involved in virtually all aspects of cellular physiology. Here we establish a bi-directional ubiquitylation circuit between the APC substrate receptor Cdh1, and Cyclin F, a substrate receptor for the SCF (Skp1-Cul1-F box) family of CRL E3 ubiquitin ligases. We demonstrate that Cdh1 is regulated by SCF(Cyclin F), and conversely, Cyclin F is regulated by APC(Cdh1). AKT modulates the circuit by phosphorylating Cyclin F, enhancing SCF(Cyclin F) ligase assembly, and thereby catalyzing Cdh1 degradation. Later in the cell cycle, CDK phosphorylates Cdh1, preventing its degradation by altering its localization, and allowing both proteins to accumulate. The mutually antagonistic relationship between Cdh1 and Cyclin F represents the first described direct, reciprocal ubiquitylation circuit. Further, phospho-dependent ligase assembly signifies a novel regulatory mechanism for SCF ligases. We propose that AKT is part of a positive feedback loop involved in cell cycle control. This discovery connects growth factor signaling to the core cell cycle machinery, and has potential implications in malignancies where AKT is activated. Citation Format: Michael J. Emanuele, Rajarshi Choudhury, Jennifer Kernan, Christine Allie Mills, Debojyoti Lahiri, Daniel J. Burke. SCFCyclin F connects AKT signaling to the core cell cycle oscillator. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr A03.

BUB1 and SURVIVIN proteins are not degraded after a prolonged mitosis and accumulate in the nuclei of HCT116 cells.

Spindle poisons activate the spindle assembly checkpoint and prevent mitotic exit until cells die or override the arrest. Several studies have focused on spindle poison-mediated cell death, but less is known about consequences in cells that survive a mitotic arrest. During mitosis, proteins such as CYCLIN B, SECURIN, BUB1 and SURVIVIN are degraded in order to allow mitotic exit, and these proteins are maintained at low levels in the next interphase. In contrast, exit from a prolonged mitosis depends only on degradation of CYCLIN B; it is not known whether the levels of other proteins decrease or remain high. Here, we analyzed the levels and localization of the BUB1 and SURVIVIN proteins in cells that escaped from a paclitaxel-mediated prolonged mitosis. We compared cells with a short arrest (HCT116 cells) with cells that spent more time in mitosis (HT29 cells) after paclitaxel treatment. BUB1 and SURVIVIN were not degraded and remained localized to the nuclei of HCT116 cells after a mitotic arrest. Moreover, BUB1 nuclear foci were observed; BUB1 did not colocalize with centromere proteins. In HT29 cells, the levels of BUB1 and SURVIVIN decreased during the arrest, and these proteins were not present in cells that reached the next interphase. Using time-lapse imaging, we observed morphological heterogeneity in HCT116 cells that escaped from the arrest; this heterogeneity was due to the cytokinesis-like mechanism by which the cells exited mitosis. Thus, our results show that high levels of BUB1 and SURVIVIN can be maintained after a mitotic arrest, which may promote resistance to cell death.

Melanoma antigen-A11 regulates substrate-specificity of Skp2-mediated protein degradation

Melanoma antigen-A11 (MAGE-A11) is a proto-oncogene involved in androgen receptor signaling and androgen-dependent cell growth. In this report we provide evidence that MAGE-A11 interacts with Skp2 (S phase kinase-associated protein), the substrate recognition protein of the Skp1-Cullin1-F-box E3 ubiquitin ligase, and with Skp2 binding protein, cyclin A. A similar cyclin A binding motif in MAGE-A11 and Skp2 was consistent with a competitive relationship between MAGE-A11 and Skp2 in binding cyclin A. Skp2 inhibited MAGE-A11 interaction with cyclin A. Differential effects of MAGE-A11 on Skp2-mediated protein degradation were also revealed. MAGE-A11 increased Skp2-mediated degradation of cyclin A and retinoblastoma-related protein p130. In contrast, MAGE-A11 decreased Skp2-mediated degradation of E2F1 and Skp2 self-ubiquitination. Stabilization of E2F1 by MAGE-A11 was associated with sequestration and inactivation of Skp2 through the formation of an E2F1-MAGE-A11-Skp2 complex. We conclude that direct interactions of MAGE-A11 with Skp2 and cyclin A regulate the substrate-specificity of Skp2-mediated protein degradation.

Investigating ego modules involved in TGFβ3-induced chondrogenesis in mesenchymal stem cells based on ego network

ObjectiveThis paper aimed to investigate ego modules for TGFβ3-induced chondrogenesis in mesenchymal stem cells (MSCs) using ego network algorithm. MethodsThe ego network algorithm comprised three parts, extracting differential expression network (DEN) based on gene expression data and protein-protein interaction (PPI) data; exploring ego genes by reweighting DEN; and searching ego modules by ego gene expansions. Subsequently, permutation test was carried out to evaluate the statistical significance of the ego modules. Finally, pathway enrichment analysis was conducted to investigate ego pathways enriched by the ego modules. ResultsA total of 15 ego genes were obtained from the DEN, such as PSMA4, HNRNPM and WDR77. Starting with each ego genes, 15 candidate modules were gained. When setting the thresholds of the area under the receiver operating characteristics curve (AUC) ≥0.9 and gene size ≥4, three ego modules (Module 3, Module 8 and Module 14) were identified, and all of them had statistical significances between normal and TGFβ3-induced chondrogenesis in MSCs. By mapping module genes to confirmed pathway database, their ego pathways were detected, Cdc20:Phospho-APC/C mediated degradation of Cyclin A for Module 3, Mitotic G1-G1/S phases for Module 8, and mRNA Splicing for Module 14. ConclusionsWe have successfully identified three ego modules, evaluated their statistical significances and investigated their functional enriched ego pathways. The findings might provide potential biomarkers and give great insights to reveal molecular mechanism underlying this process.

Metformin pharmacogenomics: a genome-wide association study to identify genetic and epigenetic biomarkers involved in metformin anticancer response using human lymphoblastoid cell lines.

Metformin is currently considered as a promising anticancer agent in addition to its anti-diabetic effect. To better individualize metformin therapy and explore novel molecular mechanisms in cancer treatment, we conducted a pharmacogenomic study using 266 lymphoblastoid cell lines (LCLs). Metformin cytotoxicity assay was performed using the MTS assay. Genome-wide association (GWA) analyses were performed in LCLs using 1.3 million SNPs, 485k DNA methylation probes, 54k mRNA expression probe sets, and metformin cytotoxicity (IC50s). Top candidate genes were functionally validated using siRNA screening, followed by MTS assay in breast cancer cell lines. Further study of one top candidate, STUB1, was performed to elucidate the mechanisms by which STUB1 might contribute to metformin action. GWA analyses in LCLs identified 198 mRNA expression probe sets, 12 SNP loci, and 5 DNA methylation loci associated with metformin IC50 with P-values <10−4 or <10−5. Integrated SNP/methylation loci-expression-IC50 analyses found 3 SNP loci or 5 DNA methylation loci associated with metformin IC50 through trans-regulation of expression of 11 or 26 genes with P-value <10−4. Functional validation of top 61 candidate genes in 4 IPA networks indicated down regulation of 14 genes significantly altered metformin sensitivity in two breast cancer cell lines. Mechanistic studies revealed that the E3 ubiquitin ligase, STUB1, could influence metformin response by facilitating proteasome-mediated degradation of cyclin A. GWAS using a genomic data-enriched LCL model system, together with functional and mechanistic studies using cancer cell lines, help us to identify novel genetic and epigenetic biomarkers involved in metformin anticancer response.

An APC/C-Cdh1 Biosensor Reveals the Dynamics of Cdh1 Inactivation at the G1/S Transition.

B-type cyclin-dependent kinase activity must be turned off for mitotic exit and G1 stabilization. B-type cyclin degradation is mediated by the anaphase-promoting complex/cyclosome (APC/C); during and after mitotic exit, APC/C is dependent on Cdh1. Cdh1 is in turn phosphorylated and inactivated by cyclin-CDK at the Start transition of the new cell cycle. We developed a biosensor to assess the cell cycle dynamics of APC/C-Cdh1. Nuclear exit of the G1 transcriptional repressor Whi5 is a known marker of Start; APC/C-Cdh1 is inactivated 12 min after Whi5 nuclear exit with little measurable cell-to-cell timing variability. Multiple phosphorylation sites on Cdh1 act in a redundant manner to repress its activity. Reducing the number of phosphorylation sites on Cdh1 can to some extent be tolerated for cell viability, but it increases variability in timing of APC/C-Cdh1 inactivation. Mutants with minimal subsets of phosphorylation sites required for viability exhibit striking stochasticity in multiple responses including budding, nuclear division, and APC/C-Cdh1 activity itself. Multiple cyclin-CDK complexes, as well as the stoichiometric inhibitor Acm1, contribute to APC/C-Cdh1 inactivation; this redundant control is likely to promote rapid and reliable APC/C-Cdh1 inactivation immediately following the Start transition.

Editorial: Maintenance of Genome Integrity: DNA Damage Sensing, Signaling, Repair, and Replication in Plants.

Editorial: Maintenance of Genome Integrity: DNA Damage Sensing, Signaling, Repair, and Replication in Plants.