Degradation of cyclin B is required for the onset of anaphase in Mammalian cells

UBE2D3 functions in mouse oocyte meiotic maturation

Ubiquitin-mediated proteolysis plays a critical role in meiotic cell-cycle regulation and must be tightly controlled to achieve correct chromosome segregation. While the role of E2 ubiquitin-conjugating enzymes in mitosis is well-documented, their functions in oocyte meiosis remain largely unexplored. In this study, we identified UBE2D3 as the most highly expressed E2 enzyme in mouse oocytes, which is essential for proper meiotic division. UBE2D3 depletion caused (metaphase I) MI arrest and Cyclin B1 accumulation, whereas its overexpression led to reduced Cyclin B1 levels, kinetochore-microtubule (K-MT) mis-attachments, spindle assembly checkpoint (SAC) dysfunction, and increased aneuploidy. Notably, UBE2D3 upregulation in oocytes from aged mice contributed to age-related meiotic defects, which were partially reversed by UBE2D3 knockdown or Cyclin B1 overexpression. This study underscores the importance of the UBE2D3-Cyclin B1 axis in maintaining meiotic fidelity and highlights its potential as a therapeutic target for improving oocyte quality and fertility in aged females.

CENcyclopedia: Dynamic Landscape of Kinetochore Architecture Throughout the Cell Cycle

The kinetochore, an intricate macromolecular protein complex located on chromosomes, plays a pivotal role in orchestrating chromosome segregation. It functions as a versatile platform for microtubule assembly, diligently monitors microtubule binding fidelity, and acts as a force coupler. Comprising over 100 distinct proteins, many of which exist in multiple copies, the kinetochore's composition dynamically changes throughout the cell cycle, responding to specific timing and conditions. This dynamicity is important for establishing functional kinetochores, yet the regulatory mechanisms of these dynamics have largely remained elusive. In this study, we employed advanced quantitative immunofluorescence techniques to meticulously chart the dynamics of kinetochore protein levels across the cell cycle. These findings offer a comprehensive view of the dynamic landscape of kinetochore architecture, shedding light on the detailed mechanisms of microtubule interaction and the nuanced characteristics of kinetochore proteins. This study significantly advances our understanding of the molecular coordination underlying chromosome segregation.

ADARp150 counteracts whole genome duplication

Impaired control of the G1/S checkpoint allows initiation of DNA replication under non-permissive conditions. Unscheduled S-phase entry is associated with DNA replication stress, demanding for other checkpoints or cellular pathways to maintain proliferation. Here, we uncovered a requirement for ADARp150 to sustain proliferation of G1/S-checkpoint-defective cells under growth-restricting conditions. Besides its well-established mRNA editing function in inversely oriented short interspersed nuclear elements (SINEs), we found ADARp150 to exert a critical function in mitosis. ADARp150 depletion resulted in tetraploidization, impeding cell proliferation in mitogen-deprived conditions. Mechanistically we show that ADAR1 depletion induced aberrant expression of Cyclin B3, which was causative for mitotic failure and whole-genome duplication. Finally, we find that also in vivo ADAR1-depletion-provoked tetraploidization hampers tumor outgrowth.

Human CKAP2L shows a cell cycle-dependent expression pattern and exhibits microtubule-stabilizing properties

Cytoskeleton-associated protein 2-like (CKAP2L) is a paralogue of cytoskeleton-associated protein 2 (CKAP2). We characterized the expression pattern, subcellular localization, and microtubule-stabilizing properties of human CKAP2L. The levels of both CKAP2L transcript and protein were cell cycle phase-dependent, peaking during the G2/M phase and relatively high in certain human tissues, including testis, intestine, and spleen. CKAP2L protein was detectable in all human cancer cell lines we tested. CKAP2L localized to the mitotic spindle apparatus during mitosis, as reported previously. During interphase, however, CKAP2L localized mainly to the nucleus. Ectopic overexpression of CKAP2L resulted in 'microtubule bundling', and, consequently, an elevated CKAP2L level led to prolonged mitosis. These findings support the mitotic role of CKAP2L during the human cell cycle.

Selective inhibition of HDAC6 by N-acylhydrazone derivative reduces the proliferation and induces senescence in carcinoma hepatocellular cells

Hepatocellular carcinoma (HCC) is a significant contributor to cancer-related deaths globally. Systemic therapy is the only treatment option for HCC at an advanced stage, with limited therapeutic response. In this study, we evaluated the antitumor potential of four N-acylhydrazone (NAH) derivatives, namely LASSBio-1909, 1911, 1935, and 1936, on HCC cell lines. We have previously demonstrated that the aforementioned NAH derivatives selectively inhibit histone deacetylase 6 (HDAC6) in lung cancer cells, but their effects on HCC cells have not been explored. Thus, the present study aimed to evaluate the effects of NAH derivatives on the proliferative behavior of HCC cells. LASSBio-1911 was the most cytotoxic compound against HCC cells, however its effects were minimal on normal cells. Our results showed that LASSBio-1911 inhibited HDAC6 in HCC cells leading to cell cycle arrest and decreased cell proliferation. There was also an increase in the frequency of cells in mitosis onset, which was associated with disturbing mitotic spindle formation. These events were accompanied by elevated levels of CDKN1A mRNA, accumulation of CCNB1 protein, and sustained ERK1 phosphorylation. Furthermore, LASSBio-1911 induced DNA damage, resulting in senescence and/or apoptosis. Our findings indicate that selective inhibition of HDAC6 may provide an effective therapeutic strategy for the treatment of advanced HCC, including tumor subtypes with integrated viral genome. Further, in vivo studies are required to validate the antitumor effect of LASSBio-1911 on liver cancer.

APC/C prevents a noncanonical order of cyclin/CDK activity to maintain CDK4/6 inhibitor-induced arrest

Regulated cell cycle progression ensures homeostasis and prevents cancer. In proliferating cells, premature S phase entry is avoided by the E3 ubiquitin ligase anaphasepromoting complex/cyclosome (APC/C), although the APC/C substrates whose degradation restrains G1-S progression are not fully known. The APC/C is also active in arrested cells that exited the cell cycle, but it is not clear whether APC/C maintains all types of arrest. Here, by expressing the APC/C inhibitor, EMI1, we show that APC/C activity is essential to prevent S phase entry in cells arrested by pharmacological cyclin-dependent kinases 4 and 6 (CDK4/6) inhibition (Palbociclib). Thus, active protein degradation is required for arrest alongside repressed cell cycle gene expression. The mechanism of rapid and robust arrest bypass from inhibiting APC/C involves CDKs acting in an atypical order to inactivate retinoblastoma-mediated E2F repression. Inactivating APC/C first causes mitotic cyclin B accumulation which then promotes cyclin A expression. We propose that cyclin A is the key substrate for maintaining arrest because APC/C-resistant cyclin A, but not cyclin B, is sufficient to induce S phase entry. Cells bypassing arrest from CDK4/6 inhibition initiate DNA replication with severely reduced origin licensing. The simultaneous accumulation of S phase licensing inhibitors, such as cyclin A and geminin, with G1 licensing activators disrupts the normal order of G1-S progression. As a result, DNA synthesis and cell proliferation are profoundly impaired. Our findings predict that cancers with elevated EMI1 expression will tend to escape CDK4/6 inhibition into a premature, underlicensed S phase and suffer enhanced genome instability.

Cannabinol Regulates the Expression of Cell Cycle-Associated Genes in Motor Neuron-like NSC-34: A Transcriptomic Analysis

Cannabinoids are reported to have neuroprotective properties and play a role in neurogenesis and neuroplasticity in in vitro and in vivo models. Cannabinol (CBN) is a minor cannabinoid produced by the degradation of Δ9-tetrahydrocannabinol in Cannabis sativa L. and exhibits anti-oxidant, analgesic, anti-bacterial, and anti-inflammatory effects. In this study, we explored the biological effects of 20 µM CBN (6.20 µg/mL) on differentiated NSC-34 cells by MTT assay and next-generation sequencing analysis on the transcriptome. KEGG and Gene Ontology enrichment analyses have been performed to evaluate potential CBN-associated processes. Our results highlighted the absence of any cytotoxic effect of CBN. The comparative transcriptomic analysis pointed out the downregulation of Cdkn2a, Cdkn2c and Cdkn2d genes, which are known to suppress the cell cycle. Ccne2, Cdk2, Cdk7, Anapc11, Anapc10, Cdc23, Cdc16, Anapc4, Cdc27, Stag1, Smc3, Smc1a, Nipbl, Pds5a, Pds5b, and Wapl genes, renowned for their role as cell cycle progression activators, were instead upregulated. Our work suggests that CBN regulates the expression of many genes related to the cell cycle, which are required for axonal maturation, migration, and synaptic plasticity, while not affecting the expression of genes involved in cell death or tumorigenesis.

The Emerging Role of Deubiquitinases in Radiosensitivity

Radiation therapy is a primary treatment for cancer, but radioresistance remains a significant challenge in improving efficacy and reducing toxicity. Accumulating evidence suggests that deubiquitinases (DUBs) play a crucial role in regulating cell sensitivity to ionizing radiation. Traditional small-molecule DUB inhibitors have demonstrated radiosensitization effects, and novel deubiquitinase-targeting chimeras (DUBTACs) provide a promising strategy for radiosensitizer development by harnessing the ubiquitin-proteasome system. This review highlights the mechanisms by which DUBs regulate radiosensitivity, including DNA damage repair, the cell cycle, cell death, and hypoxia. Progress on DUB inhibitors and DUBTACs is summarized, and their potential radiosensitization effects are discussed. Developing drugs targeting DUBs appears to be a promising alternative approach to overcoming radioresistance, warranting further research into their mechanisms.

The efficacy of natural bioactive compounds against prostate cancer: Molecular targets and synergistic activities

Globally, prostate cancer (PCa) is regarded as a challenging health issue, and the number of PCa patients continues to rise despite the availability of effective treatments in recent decades. The current therapy with chemotherapeutic drugs has been largely ineffective due to multidrug resistance and the conventional treatment has restricted drug accessibility to malignant tissues, necessitating a higher dosage resulting in increased cytotoxicity. Plant-derived bioactive compounds have recently attracted a great deal of attention in the field of PCa treatment due to their potent effects on several molecular targets and synergistic effects with anti-PCa drugs. This review emphasizes the molecular mechanism of phytochemicals on PCa cells, the synergistic effects of compound-drug interactions, and stem cell targeting for PCa treatment. Some potential compounds, such as curcumin, phenethyl-isothiocyanate, fisetin, baicalein, berberine, lutein, and many others, exert an anti-PCa effect via inhibiting proliferation, metastasis, cell cycle progression, and normal apoptosis pathways. In addition, multiple studies have demonstrated that the isolated natural compounds: d-limonene, paeonol, lanreotide, artesunate, and bicalutamide have potential synergistic effects. Further, a significant number of natural compounds effectively target PCa stem cells. However, further high-quality studies are needed to firmly establish the clinical efficacy of these phytochemicals against PCa.

APC/C prevents non-canonical order of cyclin/CDK activity to maintain CDK4/6 inhibitor-induced arrest

Regulated cell cycle progression ensures homeostasis and prevents cancer. In proliferating cells, premature S phase entry is avoided by the E3 ubiquitin ligase APC/C (anaphase promoting complex/cyclosome), although the APC/C substrates whose degradation restrains G1-S progression are not fully known. The APC/C is also active in arrested cells that exited the cell cycle, but it is not clear if APC/C maintains all types of arrest. Here by expressing the APC/C inhibitor, EMI1, we show that APC/C activity is essential to prevent S phase entry in cells arrested by pharmacological CDK4/6 inhibition (Palbociclib). Thus, active protein degradation is required for arrest alongside repressed cell cycle gene expression. The mechanism of rapid and robust arrest bypass from inhibiting APC/C involves cyclin-dependent kinases acting in an atypical order to inactivate RB-mediated E2F repression. Inactivating APC/C first causes mitotic cyclin B accumulation which then promotes cyclin A expression. We propose that cyclin A is the key substrate for maintaining arrest because APC/C-resistant cyclin A, but not cyclin B, is sufficient to induce S phase entry. Cells bypassing arrest from CDK4/6 inhibition initiate DNA replication with severely reduced origin licensing. The simultaneous accumulation of S phase licensing inhibitors, such as cyclin A and geminin, with G1 licensing activators disrupts the normal order of G1-S progression. As a result, DNA synthesis and cell proliferation are profoundly impaired. Our findings predict that cancers with elevated EMI1 expression will tend to escape CDK4/6 inhibition into a premature, underlicensed S phase and suffer enhanced genome instability.

The structural flexibility of MAD1 facilitates the assembly of the Mitotic Checkpoint Complex

The spindle assembly checkpoint (SAC) safeguards the genome during cell division by generating an effector molecule known as the Mitotic Checkpoint Complex (MCC). The MCC comprises two subcomplexes: BUBR1:BUB3 and CDC20:MAD2, and the formation of CDC20:MAD2 is the rate-limiting step during MCC assembly. Recent studies show that the rate of CDC20:MAD2 formation is significantly accelerated by the cooperative binding of CDC20 to the SAC proteins MAD1 and BUB1. However, the molecular basis for this acceleration is not fully understood. Here, we demonstrate that the structural flexibility of MAD1 at a conserved hinge near the C-terminus is essential for catalytic MCC assembly. This MAD1 hinge enables the MAD1:MAD2 complex to assume a folded conformation in vivo. Importantly, truncating the hinge reduces the rate of MCC assembly in vitro and SAC signaling in vivo. Conversely, mutations that preserve hinge flexibility retain SAC signaling, indicating that the structural flexibility of the hinge, rather than a specific amino acid sequence, is important for SAC signaling. We summarize these observations as the 'knitting model' that explains how the folded conformation of MAD1:MAD2 promotes CDC20:MAD2 assembly.

An affinity-directed phosphatase, AdPhosphatase, system for targeted protein dephosphorylation

Reversible protein phosphorylation, catalyzed by protein kinases and phosphatases, is a fundamental process that controls protein function and intracellular signaling. Failure of phospho-control accounts for many human diseases. While a kinase phosphorylates multiple substrates, a substrate is often phosphorylated by multiple kinases. This renders phospho-control at the substrate level challenging, as it requires inhibition of multiple kinases, which would thus affect other kinase substrates. Here, we describe the development and application of the affinity-directed phosphatase (AdPhosphatase) system for targeted dephosphorylation of specific phospho-substrates. By deploying the Protein Phosphatase 1 or 2A catalytic subunits conjugated to an antigen-stabilized anti-GFP nanobody, we can promote the dephosphorylation of two independent phospho-proteins, FAM83D or ULK1, knocked in with GFP-tags using CRISPR-Cas9, with exquisite specificity. By redirecting protein phosphatases to neo-substrates through nanobody-mediated proximity, AdPhosphatase can alter the phospho-status and function of target proteins and thus, offers a new modality for potential drug discovery approaches.

Nuclear-cytoplasmic compartmentalization of cyclin B1-Cdk1 promotes robust timing of mitotic events

The cyclin-dependent kinase (Cdk1) oscillator is widely characterized in homogenized cytosolic extracts, leaving unclear the impact of nucleocytoplasmic compartmentalization. Here, by developing a Förster resonance energy transfer (FRET) biosensor, we track Cdk1 spatiotemporal dynamics in reconstituted cells with or without side by side and find compartmentalization significantly modulates clock properties previously found in bulk studies. Although nucleus-absent cells display highly tunable frequency, the nucleus-present cells maintain constant frequency against cyclin B1 variations. Despite high expression variability, cyclin degraded within the same duration, enabling a robust mitotic phase. Moreover, Cdk1 and cyclin B1 cycle rigorously out-of-phase, ensuring wide phase-plane orbits, essential for oscillation robustness. Although Cdk1 in homogeneous extracts is well known for delayed switch-like activation, we find active cyclin B1-Cdk1 accumulates in nuclei, without delay, until the nuclear envelope breakdown (NEB) when another abrupt activation triggers anaphase. Cdk1 biphasic activation and spatial compartmentalization may together coordinate the accurate ordering of different downstream events.

Zinc supplementation prevents mitotic accumulation in human keratinocyte cell lines upon environmentally relevant arsenic exposure

Disrupted cell cycle progression underlies the molecular pathogenesis of multiple diseases. Chronic exposure to inorganic arsenic (iAs) is a global health issue leading to multi-organ cancerous and non-cancerous diseases. Exposure to supratherapeutic concentrations of iAs causes cellular accumulation in G2 or M phase of the cell cycle in multiple cell lines by inducing cyclin B1 expression. It is not clear if iAs exposure at doses corresponding to serum levels of chronically exposed populations (∼100 nM) has any effect on cell cycle distribution. In the present study we investigated if environmentally relevant iAs exposure induced cell cycle disruption and mechanisms thereof employing two human keratinocyte cell lines (HaCaT and Ker-CT), flow cytometry, immunoblots and quantitative real-time PCR (qRT-PCR). iAs exposure (100 nM; 24 h) led to mitotic accumulation of cells in both cell lines, along with the stabilization of ANAPC11 ubiquitination targets cyclin B1 and securin, without affecting their steady state mRNA levels. This result suggested that induction of cyclin B1 and securin is modulated at the level of protein degradation. Moreover, zinc supplementation successfully prevented iAs-induced mitotic accumulation and stabilization of cyclin B1 and securin without affecting their mRNA levels. Together, these data suggest that environmentally relevant iAs exposure leads to mitotic accumulation possibly by displacing zinc from the RING finger subunit of anaphase promoting complex/cyclosome (ANAPC11), the cell cycle regulating E3 ubiquitin ligase. This early cell cycle disruptive effect of environmentally relevant iAs concentration could underpin the molecular pathogenesis of multiple diseases associated with chronic iAs exposure.

Farrerol Induces Cancer Cell Death via ERK Activation in SKOV3 Cells and Attenuates TNF-α-Mediated Lipolysis

Farrerol (FA) is a flavanone isolated from the Chinese herbal medicine “Man-shan-hong” (Rhododendron dauricum L.). In the present study, FA decreased the viability of SKOV3 cells in a dose- and time-dependent manner, and it induced G2/M cell cycle arrest and cell apoptosis. Cell cycle distribution analysis via flow cytometry showed that FA decreased G1 populations and increased G2/M populations in SKOV3 cells. Additionally, Western blotting confirmed an increase in the expression level of proteins involved in the cell cycle, e.g., CDK and cyclins. FA-induced apoptosis in SKOV3 cells was also investigated using a TUNEL assay, and increased expression levels of proapoptotic factors, including Caspase-3 and poly ADP ribose polymerase (PARP), through the Extracellular signal-regulated kinase (ERK)/MAPK pathway were investigated. Proinflammatory cytokines (e.g., IL-6, TNF-α, and IL-1) have been identified as a driver of the pathological mechanisms underlying involuntary weight loss and impaired physical function, i.e., cachexia, during cancer; in the present study, we showed that farrerol attenuates TNF-α-induced lipolysis and increases adipogenic differentiation in 3T3-L1 cells. Thus, farrerol could potentially be used as an anticancer agent or anticachetic drug.

Cfdp1 controls the cell cycle and neural differentiation in the zebrafish cerebellum and retina

BACKGROUND

Although the cell cycle and cell differentiation should be coordinately regulated to generate a variety of neurons in the brain, the molecules that are involved in this coordination still remain largely unknown. In this study, we analyzed the roles of a nuclear protein Cfdp1, which is thought to be involved in chromatin remodeling, in zebrafish neurogenesis.

RESULTS

Zebrafish cfdp1 mutants maintained the progenitors of granule cells (GCs) in the cerebellum, but showed defects in their differentiation to GCs. cfdp1 mutants showed an increase in phospho-histone 3 (pH 3)-positive cells and apoptotic cells, as well as a delayed cell cycle transition from the G2 to the M phase in the cerebellum. The inhibition of tp53 prevented apoptosis but not GC differentiation in the cfdp1 mutant cerebellum. A similar increase in apoptotic cells and pH 3-positive cells, and defective cell differentiation, were observed in the cfdp1 mutant retina. Although mitotic spindles formed, mitosis was blocked before anaphase in both the cerebellum and retina of cfdp1 mutant larvae. Furthermore, expression of the G2/mitotic-specific cyclin B1 gene increased in the cfdp1 mutant cerebellum.

CONCLUSIONS

Our findings suggest that Cfdp1 regulates the cell cycle of neural progenitors, thereby promoting neural differentiation in the brain.

Spindle Architectural Features Must Be Considered Along With Cell Size to Explain the Timing of Mitotic Checkpoint Silencing

Mitosis proceeds through a defined series of events that is largely conserved, but the amount of time needed for their completion can vary in different cells and organisms. In many systems, mitotic duration depends on the time required to satisfy and silence the spindle assembly checkpoint (SAC), also known as the mitotic checkpoint. Because SAC silencing involves trafficking SAC molecules among kinetochores, spindle, and cytoplasm, the size and geometry of the spindle relative to cell volume are expected to affect mitotic duration by influencing the timing of SAC silencing. However, the relationship between SAC silencing, cell size, and spindle dimensions is unclear. To investigate this issue, we used four DLD-1 tetraploid (4N) clones characterized by small or large nuclear and cell size. We found that the small 4N clones had longer mitotic durations than the parental DLD-1 cells and that this delay was due to differences in their metaphase duration. Leveraging a previous mathematical model for spatiotemporal regulation of SAC silencing, we show that the difference in metaphase duration, i.e., SAC silencing time, can be explained by the distinct spindle microtubule densities and sizes of the cell, spindle, and spindle poles in the 4N clones. Lastly, we demonstrate that manipulating spindle geometry can alter mitotic and metaphase duration, consistent with a model prediction. Our results suggest that spindle size does not always scale with cell size in mammalian cells and cell size is not sufficient to explain the differences in metaphase duration. Only when a number of spindle architectural features are considered along with cell size can the kinetics of SAC silencing, and hence mitotic duration, in the different clones be explained.

Ubiquitin signaling in cell cycle control and tumorigenesis

Cell cycle progression is a tightly regulated process by which DNA replicates and cell reproduces. The major driving force underlying cell cycle progression is the sequential activation of cyclin-dependent kinases (CDKs), which is achieved in part by the ubiquitin-mediated proteolysis of their cyclin partners and kinase inhibitors (CKIs). In eukaryotic cells, two families of E3 ubiquitin ligases, anaphase-promoting complex/cyclosome and Skp1-Cul1-F-box protein complex, are responsible for ubiquitination and proteasomal degradation of many of these CDK regulators, ensuring cell cycle progresses in a timely and precisely regulated manner. In the past couple of decades, accumulating evidence have demonstrated that the dysregulated cell cycle transition caused by inefficient proteolytic control leads to uncontrolled cell proliferation and finally results in tumorigenesis. Based upon this notion, targeting the E3 ubiquitin ligases involved in cell cycle regulation is expected to provide novel therapeutic strategies for cancer treatment. Thus, a better understanding of the diversity and complexity of ubiquitin signaling in cell cycle regulation will shed new light on the precise control of the cell cycle progression and guide anticancer drug development.

ULK1 inhibitor induces spindle microtubule disorganization and inhibits phosphorylation of Ser10 of histone H3

Certain tumors are dependent on autophagy for survival; thus, the use of unc‐51‐like autophagy activating kinase (ULK) 1 inhibitors to block autophagy has the potential for tumor treatment. However, ULK1 inhibitors affect processes other than autophagy. Herein, we report that the ULK1 inhibitors SBI‐0206965/MRT68921 not only inhibit phosphorylation of histone H3 (Ser10) and delay chromatin condensation but also induce spindle microtubule disorganization to form short and fragmented microtubule polymers. Although the delay in chromatin condensation also delayed mitotic entry, the disorganized microtubule polymers resulted in unsegregated chromosomes and polyploidy. Although the effect on mitotic entry was moderate, polyploidy formation was decreased in ULK1 knockout cells with or without ULK2 knockdown. In conclusion, it will be helpful to consider the roles of ULK1 inhibitors in mitotic dysregulation, as well as autophagy, when evaluating their antitumor efficacy.

A novel orally active microtubule destabilizing agent S-40 targets the colchicine-binding site and shows potent antitumor activity

The tubulin colchicine binding site has been recognized as an attractive drug target to combat cancer, but none of the candidate drugs have been approved for medical treatment. We recently identified a structurally distinct small molecule S-40 as an oral potent tubulin destabilizing agent. Crystal structure analysis of S-40 in a complex with tubulin at a resolution of 2.4 Å indicated that S-40 occupies all 3 zones in the colchicine pocket with interactions different from known microtubule inhibitors, presenting unique effects on assembly and curvature of tubulin dimers. S-40 overcomes paclitaxel resistance and lacks neurotoxicity, which are the main obstacles limiting clinical applications of paclitaxel. Moreover, S-40 harbors the ability to inhibit growth of cancer cell lines as well as patient-derived organoids, induce mitotic arrest and cell apoptosis. Xenograft mouse models of human prostate cancer DU145, non-small cell lung cancer NCI-H1299 and paclitaxel-resistant A549 were strongly restrained without apparent side effects by S-40 oral administration once daily. These findings provide evidence for the development of S-40 as the next generation of orally effective microtubule inhibitors for cancer therapy.

Maintenance of genome integrity and active homologous recombination in embryonic stem cells

Embryonic stem cells (ESCs) possess specific gene expression patterns that confer the ability to proliferate indefinitely and enable pluripotency, which allows ESCs to differentiate into diverse cell types in response to developmental signals. Compared to differentiated cells, ESCs harbor an elevated level of homologous recombination (HR)-related proteins and exhibit exceptional cell cycle control, characterized by a high proliferation rate and a prolonged S phase. HR is involved in several aspects of chromosome maintenance. For instance, HR repairs impaired chromosomes and prevents the collapse of DNA replication forks during cell proliferation. Thus, HR is essential for the maintenance of genomic integrity and prevents cellular dysregulation and lethal events. In addition, abundant HR proteins in the prolonged S phase can efficiently protect ESCs from external damages and protect against genomic instability caused by DNA breaks, facilitating rapid and accurate DNA break repair following chromosome duplication. The maintenance of genome integrity is key to preserving the functions of ESCs and reducing the risks of cancer development, cell cycle arrest, and abnormal replication. Here, we review the fundamental links between the stem cell-specific HR process and DNA damage response as well as the different strategies employed by ESCs to maintain genomic integrity.

Embryonic stem cells (ESCs), which give rise to the many specialized cells of the body, have highly effective molecular processes of DNA maintenance and repair which protect their genetic information from damage. Keun Pil Kim and colleagues at Chung-Ang University, Seoul, South Korea, review the strategies found in ESCs to maintain the integrity of their DNA as they develop and multiply. A key feature is the process of homologous recombination (HR) in which one copy of a section of DNA acts as the template allowing a damaged version of the DNA to be repaired. HR also facilitates swapping of sections of DNA when sperm and egg cells form, promoting genetic diversity. HR appears to be especially significant in maintaining ESC DNA as ESCs possess higher levels of key proteins involved in its maintenance and regulation.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

An mTORC1-to-CDK1 Switch Maintains Autophagy Suppression during Mitosis

Since nuclear envelope breakdown occurs during mitosis in metazoan cells, it has been proposed that macroautophagy must be inhibited to maintain genome integrity. However, repression of macroautophagy during mitosis remains controversial and mechanistic detail limited to the suggestion that CDK1 phosphorylates VPS34. Here, we show that initiation of macroautophagy, measured by the translocation of the ULK complex to autophagic puncta, is repressed during mitosis, even when mTORC1 is inhibited. Indeed, mTORC1 is inactive during mitosis, reflecting its failure to localize to lysosomes due to CDK1-dependent RAPTOR phosphorylation. While mTORC1 normally represses autophagy via phosphorylation of ULK1, ATG13, ATG14, and TFEB, we show that the mitotic phosphorylation of these autophagy regulators, including at known repressive sites, is dependent on CDK1 but independent of mTOR. Thus, CDK1 substitutes for inhibited mTORC1 as the master regulator of macroautophagy during mitosis, uncoupling autophagy regulation from nutrient status to ensure repression of macroautophagy during mitosis.

Prognostic value of mitotic checkpoint protein BUB3, cyclin B1, and pituitary tumor-transforming 1 expression in prostate cancer

The mitotic checkpoint protein BUB3, cyclin B1 (CCNB1) and pituitary tumor-transforming 1 (PTTG1) regulates cell division, and are sparsely studied in prostate cancer. Deregulation of these genes can lead to genomic instability, a characteristic of more aggressive tumors. We aimed to determine the expression levels of BUB3, CCNB1, and PTTG1 as potential prognostic markers of recurrence after radical prostatectomy. Protein levels were determined by immunohistochemistry on three formalin-fixed paraffin-embedded tissue sections from each of the 253 patients treated with radical prostatectomy. Immunohistochemistry scores were obtained by automated image analysis for CCNB1 and PTTG1. Recurrence, defined as locoregional recurrence, distant metastasis or death from prostate cancer, was used as endpoint for survival analysis. Tumors having both positive and negative tumor areas for cytoplasmic BUB3 (30%), CCNB1 (28%), or PTTG1 (35%) were considered heterogeneous. Patients with ≥1 positive tumor area had significantly increased risk of disease recurrence in univariable analysis compared with patients where all tumor areas were negative for cytoplasmic BUB3 (hazard ratio [HR] = 2.18, 95% confidence interval [CI] 1.41-3.36), CCNB1 (HR = 2.98, 95% CI 1.93-4.61) and PTTG1 (HR = 1.91, 95% CI 1.23-2.97). Combining the scores of cytoplasmic BUB3 and CCNB1 improved risk stratification when integrated with the Cancer of the Prostate Risk Assessment post-Surgical (CAPRA-S) score (difference in concordance index = 0.024, 95% CI 0.001-0.05). In analysis of multiple tumor areas, prognostic value was observed for cytoplasmic BUB3, CCNB1, and PTTG1.

The balance between mitotic death and mitotic slippage in acute leukemia: a new therapeutic window?

Mitosis is the process whereby an eukaryotic cell divides into two identical copies. Different multiprotein complexes are involved in the fine regulation of cell division, including the mitotic promoting factor and the anaphase promoting complex. Prolonged mitosis can result in cellular division, cell death, or mitotic slippage, the latter leading to a new interphase without cellular division. Mitotic slippage is one of the causes of genomic instability and has an important therapeutic and clinical impact. It has been widely studied in solid tumors but not in hematological malignancies, in particular, in acute leukemia. We review the literature data available on mitotic regulation, alterations in mitotic proteins occurring in acute leukemia, induction of prolonged mitosis and its consequences, focusing in particular on the balance between cell death and mitotic slippage and on its therapeutic potentials. We also present the most recent preclinical and clinical data on the efficacy of second-generation mitotic drugs (CDK1-Cyclin B1, APC/CCDC20, PLK, Aurora kinase inhibitors). Despite the poor clinical activity showed by these drugs as single agents, they offer a potential therapeutic window for synthetic lethal combinations aimed to selectively target leukemic cells at the right time, thus decreasing the risk of mitotic slippage events.

TIP60 up-regulates ΔNp63α to promote cellular proliferation

An estimated 5.4 million cases of nonmelanoma skin cancer are reported in the United States at an associated cost of $4.8 billion. ΔNp63α, a proto-oncogene in the p53 family of transcription factors, is overexpressed in squamous cell carcinoma (SCC) and associated with poor prognosis and survival. ΔNp63α elicits its tumorigenic effects in part by promoting cellular proliferation and cell survival. Despite its importance in SCC, the upstream regulation of ΔNp63α is poorly understood. In this study, we identify TIP60 as a novel upstream regulator of ΔNp63α. Using a combination of overexpression, silencing, stable expression, and pharmacological approaches in multiple cell lines, we showed that TIP60 up-regulates ΔNp63α expression. Utilizing cycloheximide treatment, we showed that TIP60 catalytic activity is required for stabilization of ΔNp63α protein levels. We further showed that TIP60 coexpression inhibits ΔNp63α ubiquitination and proteasomal degradation. Stabilization of ΔNp63α protein was further associated with TIP60-mediated acetylation. Finally, we demonstrated that TIP60-mediated regulation of ΔNp63α increases cellular proliferation by promoting G₂/M progression through MTS assays and flow cytometry. Taken together, our findings provide evidence that TIP60 may contribute to SCC progression by increasing ΔNp63α protein levels, thereby promoting cellular proliferation.

A kinetochore-based ATM/ATR-independent DNA damage checkpoint maintains genomic integrity in trypanosomes

DNA damage-induced cell cycle checkpoints serve as surveillance mechanisms to maintain genomic stability, and are regulated by ATM/ATR-mediated signaling pathways that are conserved from yeast to humans. Trypanosoma brucei, an early divergent microbial eukaryote, lacks key components of the conventional DNA damage-induced G2/M cell cycle checkpoint and the spindle assembly checkpoint, and nothing is known about how T. brucei controls its cell cycle checkpoints. Here we discover a kinetochore-based, DNA damage-induced metaphase checkpoint in T. brucei. MMS-induced DNA damage triggers a metaphase arrest by modulating the abundance of the outer kinetochore protein KKIP5 in an Aurora B kinase- and kinetochore-dependent, but ATM/ATR-independent manner. Overexpression of KKIP5 arrests cells at metaphase through stabilizing the mitotic cyclin CYC6 and the cohesin subunit SCC1, mimicking DNA damage-induced metaphase arrest, whereas depletion of KKIP5 alleviates the DNA damage-induced metaphase arrest and causes chromosome mis-segregation and aneuploidy. These findings suggest that trypanosomes employ a novel DNA damage-induced metaphase checkpoint to maintain genomic integrity.

The effects of neuromedin S on the hypothalamic-pituitary-testicular axis in male pigs in vitro

Evidence has shown that neuromedin S (NMS) and its receptor (NMU2R) are expressed in the hypothalamus, pituitary, and testis of pigs. To determine the potential mechanisms of NMS, we systematically investigated the direct effects of NMS on the hypothalamic-pituitary-testicular (HPT) axis of male pigs in vitro. We initially confirmed that NMU2R distributed in isolated hypothalamic cells, anterior pituitary cells and Leydig cells using immunocytochemistry. Subsequently we investigated the direct effects of NMS on hormone secretion from cells (anterior pituitary cells and Leydig cells) treated with different doses of NMS. The results showed that NMS increase the release of LH and FSH from anterior pituitary cells and testosterone from Leydig cells. NMS up-regulated the expression of NMU2R and GnRH mRNAs in hypothalamic cells, NMU2R, LH and FSH mRNAs in anterior pituitary cells, and NMU2R, STAR, P450 and 3β-HSD mRNAs and the expression of PCNA and Cyclin B1 protein in Leydig cells; moreover, it down-regulated the expression of GnIH mRNA in hypothalamic cells. Using immunofluorescence staining and confocal microscopy, we also demonstrated the colocalization of NMU2R and AR or GnIH in Leydig cells. These data in vitro indicated that NMS may regulate the release and/or synthesis of LH, FSH and testosterone at different levels of the reproductive axis through NMU2R, which provided novel evidence of the potential roles of NMS in regulation of pig reproduction.

FAM83D directs protein kinase CK1α to the mitotic spindle for proper spindle positioning

The concerted action of many protein kinases helps orchestrate the error-free progression through mitosis of mammalian cells. The roles and regulation of some prominent mitotic kinases, such as cyclin-dependent kinases, are well established. However, these and other known mitotic kinases alone cannot account for the extent of protein phosphorylation that has been reported during mammalian mitosis. Here we demonstrate that CK1α, of the casein kinase 1 family of protein kinases, localises to the spindle and is required for proper spindle positioning and timely cell division. CK1α is recruited to the spindle by FAM83D, and cells devoid of FAM83D, or those harbouring CK1α-binding-deficient FAM83DF283A/F283A knockin mutations, display pronounced spindle positioning defects, and a prolonged mitosis. Restoring FAM83D at the endogenous locus in FAM83D-/- cells, or artificially delivering CK1α to the spindle in FAM83DF283A/F283A cells, rescues these defects. These findings implicate CK1α as new mitotic kinase that orchestrates the kinetics and orientation of cell division.

A direct interaction between survivin and myosin II is required for cytokinesis

An acto-myosin contractile ring, which forms after anaphase onset and is highly regulated in time and space, mediates cytokinesis, the final step of mitosis. The chromosomal passenger complex (CPC), composed of Aurora-B kinase, INCENP, borealin and survivin (also known as BIRC5), regulates various processes during mitosis, including cytokinesis. It is not understood, however, how CPC regulates cytokinesis. We show that survivin binds to non-muscle myosin II (NMII), regulating its filament assembly. Survivin and NMII interact mainly in telophase, and Cdk1 regulates their interaction in a mitotic-phase-specific manner, revealing the mechanism for the specific timing of survivin-NMII interaction during mitosis. The survivin-NMII interaction is indispensable for cytokinesis, and its disruption leads to multiple mitotic defects. We further show that only the survivin homodimer binds to NMII, attesting to the biological importance for survivin homodimerization. We suggest a novel function for survivin in regulating the spatio-temporal formation of the acto-NMII contractile ring during cytokinesis and we elucidate the role of Cdk1 in regulating this process.This article has an associated First Person interview with the first author of the paper.

Illumination of cell cycle progression by multi-fluorescent sensing system

Multi-fluorescent imaging of cell cycle progression is essential for the study of cell proliferation in vitro and in vivo. However, there remain challenges, particularly to image cell cycle progression in living cell with available imaging techniques due to lacking the suitable probe. Here, we design a triple fluorescent sensors system making the cell cycle progression visible. Multi-fluorescent sensor shows the proliferating or proliferated cells with different colors. We thus generate the construct and adenovirus to probe cell cycle progression in living cell lines and primary cardiomyocytes. Furthermore, we create the knock-in transgenic mouse to monitor cell cycle progression in vivo. Together, the system can be applied to investigate cell proliferation or cell cycle progression in living cells and animals.

Cytotoxicity and cell cycle analysis of Asparagus laricinus Burch. and Senecio asperulus DC. on breast and prostate cancer cell lines

Aims

Medicinal plants play an important role in our African communities for treatment and prevention of various diseases including cancer. This study was aimed on evaluating the cytotoxicity activities of Asparagus laricinus Burch. and Senecio asperulus DC.

Main methods

In vitro cytotoxicity screening was carried out using fluorescent cellular stains on human prostate cancer (PC3), human breast cancer (MCF-7) and the non-cancerous African green monkey kidney (Vero) cell lines. The cells were imaged with the ImageXpress Micro XLS Widefield fluorescent Microscope, and the acquired images were analysed using the MetaXpress software and the Multi-Wavelength cell scoring application module. Melphalan was used as a positive control in all experiments.

Key findings

Asparagus laricinus methanol and Senecio asperulus DC. dichloromethane extracts exhibited cytotoxicity activity against breast cancer cells with IC₅₀ values of 97.6 μg/mL and 69.15 μg/mL, respectively. Cell cycle analysis suggested that Asparagus laricinus methanol extract induced cell death selectively through apoptosis observed from Annexin V-FITC and PI stain. Cell cycle analysis also showed that Senecio asperulus DC. dichloromethane extracts induced breast cancer cells death through cell arrest at the synthesis phase and G2 phase. Senecio asperulus DC. dichloromethane extracts further showed cytotoxicity activity against prostate cancer cells with IC₅₀ values of 69.25 μg/mL due to cell arrest at the G2 and early mitotic (G2/M) phase.

Significance

We, therefore, propose that the methanol extract of Asparagus laricinus is a suitable aspirant for future breast cancer chemotherapeutic drug, due to its selective cytotoxicity on cancer cells and not on non-cancerous cells.

Renal medullary carcinomas depend upon SMARCB1 loss and are sensitive to proteasome inhibition

Renal medullary carcinoma (RMC) is a rare and deadly kidney cancer in patients of African descent with sickle cell trait. We have developed faithful patient-derived RMC models and using whole-genome sequencing, we identified loss-of-function intronic fusion events in one SMARCB1 allele with concurrent loss of the other allele. Biochemical and functional characterization of these models revealed that RMC requires the loss of SMARCB1 for survival. Through integration of RNAi and CRISPR-Cas9 loss-of-function genetic screens and a small-molecule screen, we found that the ubiquitin-proteasome system (UPS) was essential in RMC. Inhibition of the UPS caused a G2/M arrest due to constitutive accumulation of cyclin B1. These observations extend across cancers that harbor SMARCB1 loss, which also require expression of the E2 ubiquitin-conjugating enzyme, UBE2C. Our studies identify a synthetic lethal relationship between SMARCB1-deficient cancers and reliance on the UPS which provides the foundation for a mechanism-informed clinical trial with proteasome inhibitors.

Renal medullary carcinoma (RMC for short) is a rare type of kidney cancer that affects teenagers and young adults. These patients are usually of African descent and carry one of the two genetic changes that cause sickle cell anemia. RMC is an aggressive disease without effective treatments and patients survive, on average, for only six to eight months after their diagnosis.

Recent genetic studies found that most RMC cells have mutations that prevent them from producing a protein called SMARCB1. SMARCB1 normally acts as a so-called tumor suppressor, preventing cells from becoming cancerous. However, it was not clear whether RMCs always have to lose SMARCB1 if they are to survive and grow.

Often, diseases are studied using laboratory-grown cells and tissues that have certain features of the disease. No such models had been created for RMC, which has slowed efforts to understand how the disease develops and find new treatments for it. Hong et al. therefore worked with patients to develop new lines of cells that can be used to study RMC in the laboratory. These RMC cells started dying when they were given copies of the SMARCB1 gene, which supports the theory that RMCs have to lose SMARCB1 in order to grow.

Hong et al. then used a set of genetic reagents that can suppress or delete genes that are targeted by drugs, and followed this by testing a range of drugs on the RMC cells. Drugs and genetic reagents that reduced the activity of the proteasome – the structure inside cells that gets rid of old or unwanted proteins – caused the RMC cells to die. These proteasome inhibitor drugs also killed other kinds of cancer cells with SMARCB1 mutations.

Proteasome inhibitors are already used to treat different types of cancer. Potentially, a clinical trial could be run to see if they will treat patients whose cancers lack SMARCB1. Further work is also needed to determine the exact link between SMARCB1 and the proteasome.

Kinetochore protein depletion underlies cytokinesis failure and somatic polyploidization in the moss Physcomitrella patens

Lagging chromosome is a hallmark of aneuploidy arising from errors in the kinetochore–spindle attachment in animal cells. However, kinetochore components and cellular phenotypes associated with kinetochore dysfunction are much less explored in plants. Here, we carried out a comprehensive characterization of conserved kinetochore components in the moss Physcomitrella patens and uncovered a distinct scenario in plant cells regarding both the localization and cellular impact of the kinetochore proteins. Most surprisingly, knock-down of several kinetochore proteins led to polyploidy, not aneuploidy, through cytokinesis failure in >90% of the cells that exhibited lagging chromosomes for several minutes or longer. The resultant cells, containing two or more nuclei, proceeded to the next cell cycle and eventually developed into polyploid plants. As lagging chromosomes have been observed in various plant species in the wild, our observation raised a possibility that they could be one of the natural pathways to polyploidy in plants.

Plants and animals, like all living things, are made of self-contained units called cells that are able to grow and multiply as required. Each cell contains structures called chromosomes that provide the genetic instructions needed to perform every task in the cell. When a cell is preparing to divide to make two identical daughter cells – a process called mitosis – it first needs to duplicate its chromosomes and separate them into two equal-sized sets. This process is carried out by complex cell machinery known as the spindle.

Structures called kinetochores assemble on the chromosomes to attach them to the spindle. Previous studies in animal cells have shown that, if the kinetochores do not work properly, one or more chromosomes may be left behind when the spindle operates. These ‘lagging’ chromosomes may ultimately land up in the wrong daughter cell, resulting in one of the cells having more chromosomes than the other. This can lead to cancer or other serious diseases in animals. However, it was not known what happens in plant cells when kinetochores fail to work properly.

To address this question, Kozgunova et al. used a technique called RNA interference (or RNAi for short) to temporarily interrupt the production of kinetochores in the cells of a moss called Physcomitrella patens. Unexpectedly, the experiments found that most of the moss cells with lagging chromosomes were unable to divide. Instead, they remained as single cells that had twice the number of chromosomes as normal, a condition known as polyploidy. After the effects of the RNAi wore off, these polyploid moss cells were able to divide normally and were successfully grown into moss plants with a polyploid number of chromosomes.

Polyploidy is actually widespread in the plant kingdom, and it has major impacts on plant evolution. It is also known to increase the amount of food that crops produce. However, it is still unclear why polyploidy is so common in plants. By showing that errors in mitosis may also be able to double the number of chromosomes in plant cells, the findings of Kozgunova et al. provide new insights into plant evolution and, potentially, a method to increase polyploidy in crop plants in the future.

The dynein adaptor Hook2 plays essential roles in mitotic progression and cytokinesis

Hook proteins are evolutionarily conserved dynein adaptors that promote assembly of highly processive dynein-dynactin motor complexes. Mammals express three Hook paralogs, namely Hook1, Hook2, and Hook3, that have distinct subcellular localizations and expectedly, distinct cellular functions. Here we demonstrate that Hook2 binds to and promotes dynein-dynactin assembly specifically during mitosis. During the late G2 phase, Hook2 mediates dynein-dynactin localization at the nuclear envelope (NE), which is required for centrosome anchoring to the NE. Independent of its binding to dynein, Hook2 regulates microtubule nucleation at the centrosome; accordingly, Hook2-depleted cells have reduced astral microtubules and spindle positioning defects. Besides the centrosome, Hook2 localizes to and recruits dynactin and dynein to the central spindle. Dynactin-dependent targeting of centralspindlin complex to the midzone is abrogated upon Hook2 depletion; accordingly, Hook2 depletion results in cytokinesis failure. We find that the zebrafish Hook2 homologue promotes dynein-dynactin association and was essential for zebrafish early development. Together, these results suggest that Hook2 mediates assembly of the dynein-dynactin complex and regulates mitotic progression and cytokinesis.

Quantitative analyses of the metaphase-to-anaphase transition reveal differential kinetic regulation for securin and cyclin B1

Separation of sister chromatids is a drastic and irreversible step in the cell cycle. The key biochemistry behind this event is the proteolysis mediated by the ubiquitin ligase called the anaphase promoting complex, or APC/C. Securin and cyclin B1 are the two established substrates for APC/C whose degradation releases separase and inactivates cyclin B1-dependent kinase 1 (cdk1), respectively, at the metaphase-to-anaphase transition. In this study, we have combined biochemical quantifications with mathematical simulations to characterize the kinetic regulation of securin and cyclin B1, in the cytoplasmic and chromosomal compartments, and found that they are differentially distributed and degraded with different rates. Modeling their interaction with separase predicted that activation timing of separase well coincides with the decline of securin-separase concentration in the cytoplasm. Notably, it also coincides with the peak of cyclin B1-separase level on chromosomes, which appeared crucial to coordinate the timing for separase activation and cdk1 inhibition. We have also conducted phosphoproteomic analysis and identified Ki67 as a chromosomal cdk1 substrate whose dephosphorylation is facilitated by cyclin B1-separase interaction in anaphase.

ARF6 protects sister chromatid cohesion to ensure the formation of stable kinetochore-microtubule attachments

Sister chromatid cohesion, facilitated by the cohesin protein complex, is crucial for the establishment of stable bipolar attachments of chromosomes to the spindle microtubules and their faithful segregation. Here, we demonstrate that the GTPase ARF6 prevents the premature loss of sister chromatid cohesion. During mitosis, ARF6-depleted cells normally completed chromosome congression. However, at the metaphase plate, chromosomes failed to establish stable kinetochore-microtubule attachments because of the impaired cohesion at centromeres. As a result, the spindle assembly checkpoint (SAC) was active and cyclin B ubiquitylation and degradation were blocked. Chromosomes and/or chromatids in these cells scattered gradually from the metaphase plate to the two poles of the cell or remained blocked at the metaphase plate for hours. Our study demonstrates that the small GTP-binding protein ARF6 is essential for maintaining centromeric cohesion between sister chromatids, which is necessary for the establishment of stable k-fibres, SAC satisfaction and the onset of anaphase.

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

Kinetoplastids 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.

Importance of tuberin in carcinogenesis

The cell cycle is a dynamic process with multiple phases regulating cell growth. The proper regulation is essential for avoiding errors and activation of cell death. Tumour suppressor proteins, including tuberin, are crucial in coordinating adequate cell growth and properly timed cell division. So, the present review article is focused on the latest aspects of the tuberin in the process of carcinogenesis. The PubMed was the main database used for the collection of latest data relating to multiple aspects of tuberin especially in context of cancer. Most of the recent studies revealed that mutation, truncation, and deregulation of the tuberin protein could definitely lead to cancer. Recent studies are also devoted to explore implications towards better understanding the progression of disease involving mis-regulated tuberin.

The ubiquitin ligase CRL2ZYG11 targets cyclin B1 for degradation in a conserved pathway that facilitates mitotic slippage

Cells arrested in mitosis by inactivation of the APC/C complex sometimes manage to exit mitosis in a process called mitotic slippage, which helps cancer cells circumvent chemotherapy drugs. Balachandran et al. show that mitotic slippage occurs as a result of targeting of cyclin B1 for degradation by the ligase CRL2^ZYG11.

The anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase is known to target the degradation of cyclin B1, which is crucial for mitotic progression in animal cells. In this study, we show that the ubiquitin ligase CRL2^ZYG-11 redundantly targets the degradation of cyclin B1 in Caenorhabditis elegans and human cells. In C. elegans, both CRL2^ZYG-11 and APC/C are required for proper progression through meiotic divisions. In human cells, inactivation of CRL2^ZYG11A/B has minimal effects on mitotic progression when APC/C is active. However, when APC/C is inactivated or cyclin B1 is overexpressed, CRL2^ZYG11A/B-mediated degradation of cyclin B1 is required for normal progression through metaphase. Mitotic cells arrested by the spindle assembly checkpoint, which inactivates APC/C, often exit mitosis in a process termed “mitotic slippage,” which generates tetraploid cells and limits the effectiveness of antimitotic chemotherapy drugs. We show that ZYG11A/B subunit knockdown, or broad cullin–RING ubiquitin ligase inactivation with the small molecule MLN4924, inhibits mitotic slippage in human cells, suggesting the potential for antimitotic combination therapy.

Loss of the Greatwall Kinase Weakens the Spindle Assembly Checkpoint

The Greatwall kinase/Mastl is an essential gene that indirectly inhibits the phosphatase activity toward mitotic Cdk1 substrates. Here we show that although Mastl knockout (Mastl^NULL) MEFs enter mitosis, they progress through mitosis without completing cytokinesis despite the presence of misaligned chromosomes, which causes chromosome segregation defects. Furthermore, we uncover the requirement of Mastl for robust spindle assembly checkpoint (SAC) maintenance since the duration of mitotic arrest caused by microtubule poisons in Mastl^NULL MEFs is shortened, which correlates with premature disappearance of the essential SAC protein Mad1 at the kinetochores. Notably, Mastl^NULL MEFs display reduced phosphorylation of a number of proteins in mitosis, which include the essential SAC kinase MPS1. We further demonstrate that Mastl is required for multi-site phosphorylation of MPS1 as well as robust MPS1 kinase activity in mitosis. In contrast, treatment of Mastl^NULL cells with the phosphatase inhibitor okadaic acid (OKA) rescues the defects in MPS1 kinase activity, mislocalization of phospho-MPS1 as well as Mad1 at the kinetochore, and premature SAC silencing. Moreover, using in vitro dephosphorylation assays, we demonstrate that Mastl promotes persistent MPS1 phosphorylation by inhibiting PP2A/B55-mediated MPS1 dephosphorylation rather than affecting Cdk1 kinase activity. Our findings establish a key regulatory function of the Greatwall kinase/Mastl->PP2A/B55 pathway in preventing premature SAC silencing.

Cdk1 phosphorylates many substrates in mitosis and simultaneoulsy reduces the activity of the corresponding phosphatase PP2A through the Greatwall kinase/Mastl. When Mastl is deleted, cells progress through mitosis with missegregated chromosomes, which become unraveled. However, the molecular mechansims by which Mastl promotes proper chromosome segregation and mitotic progression remain elusive. In this study, we show that the Cdk1->Greatwall kinase/Mastl->PP2A pathway plays a central role in regulating the spindle assembly checkpoint (SAC) by preventing premature SAC silencing. We further demonstrate that Mastl is required for multi-site phosphorylation of the essntial SAC protein MPS1 as well as robust MPS1 kinase activity in mitosis by inhibiting PP2A/B55-mediated MPS1 dephosphorylation. Our findings establish the requirement of Mastl for robust SAC maintenance.

Inhibition of autophagy enhances dynamin inhibitor-induced apoptosis via promoting Bak activation and mitochondrial damage in human Jurkat T cells

Treatment of Jurkat T cells with the dynamin inhibitor, myristyl trimethyl ammonium bromides (MiTMAB) caused cytokinesis impairment and apoptotic DNA fragmentation along with down-regulation of anti-apoptotic BAG3 and Mcl-1 levels, Bak activation, mitochondrial membrane potential (Δψm) loss, activation of caspase-9 and -3, and PARP cleavage, without accompanying necrosis. Bcl-xL overexpression completely abrogated these MiTMAB-induced mitochondrial damage and resultant caspase cascade activation, except for impaired cytokinesis and down-regulated BAG3 and Mcl-1 levels. Additionally, autophagic responses including Akt-mTOR pathway inhibition, formation of acridine orange-stainable acidic vesicular organelles, LC3-I/II conversion, and p62/SQSTM1 down-regulation were detected regardless of Bcl-xL overexpression. The autophagy inhibitors 3-methyladenine and LY294002 enhanced MiTMAB-induced apoptotic sub-G1 peak, BAG3 and Mcl-1 down-regulation, Bak activation, Δψm loss, and caspase activation. These results indicate that MiTMAB-caused cytokinesis failure leads to concomitant induction of apoptosis and cytoprotective autophagy, and suggest that inhibition of autophagy is a promising strategy to augment antitumor activity of MiTMAB.

Mps1 kinase regulates tumor cell viability via its novel role in mitochondria

Targeting mitotic kinase monopolar spindle 1 (Mps1) for tumor therapy has been investigated for many years. Although it was suggested that Mps1 regulates cell viability through its role in spindle assembly checkpoint (SAC), the underlying mechanism remains less defined. In an endeavor to reveal the role of high levels of mitotic kinase Mps1 in the development of colon cancer, we unexpectedly found the amount of Mps1 required for cell survival far exceeds that of maintaining SAC in aneuploid cell lines. This suggests that other functions of Mps1 besides SAC are also employed to maintain cell viability. Mps1 regulates cell viability independent of its role in cytokinesis as the genetic depletion of Mps1 spanning from metaphase to cytokinesis affects neither cytokinesis nor cell viability. Furthermore, we developed a single-cycle inhibition strategy that allows disruption of Mps1 function only in mitosis. Using this strategy, we found the functions of Mps1 in mitosis are vital for cell viability as short-term treatment of mitotic colon cancer cell lines with Mps1 inhibitors is sufficient to cause cell death. Interestingly, Mps1 inhibitors synergize with microtubule depolymerizing drug in promoting polyploidization but not in tumor cell growth inhibition. Finally, we found that Mps1 can be recruited to mitochondria by binding to voltage-dependent anion channel 1 (VDAC1) via its C-terminal fragment. This interaction is essential for cell viability as Mps1 mutant defective for interaction fails to main cell viability, causing the release of cytochrome c. Meanwhile, deprivation of VDAC1 can make tumor cells refractory to loss of Mps1-induced cell death. Collectively, we conclude that inhibition of the novel mitochondrial function Mps1 is sufficient to kill tumor cells.

The benefits of local depletion: The centrosome as a scaffold for ubiquitin-proteasome-mediated degradation

The centrosome is the major microtubule-organizing center in animal cells but is dispensable for proper microtubule spindle formation in many biological contexts and is thus thought to fulfill additional functions. Recent observations suggest that the centrosome acts as a scaffold for proteasomal degradation in the cell to regulate a variety of biological processes including cell fate acquisition, cell cycle control, stress response, and cell morphogenesis. Here, we review the body of studies indicating a role for the centrosome in promoting proteasomal degradation of ubiquitin-proteasome substrates and explore the functional relevance of this system in different biological contexts. We discuss a potential role for the centrosome in coordinating local degradation of proteasomal substrates, allowing cells to achieve stringent spatiotemporal control over various signaling processes.

IPP51, a chalcone acting as a microtubule inhibitor with in vivo antitumor activity against bladder carcinoma

We previously identified 1-(2,4-dimethoxyphenyl)-3-(1-methylindolyl) propenone (IPP51), a new chalcone derivative that is capable of inducing prometaphase arrest and subsequent apoptosis of bladder cancer cells. Here, we demonstrate that IPP51 selectively inhibits proliferation of tumor-derived cells versus normal non-tumor cells. IPP51 interfered with spindle formation and mitotic chromosome alignment. Accumulation of cyclin B1 and mitotic checkpoint proteins Bub1 and BubR1 on chromosomes in IPP51 treated cells indicated the activation of spindle-assembly checkpoint, which is consistent with the mitotic arrest. The antimitotic actions of other chalcones are often associated with microtubule disruption. Indeed, IPP51 inhibited tubulin polymerization in an in vitro assay with purified tubulin. In cells, IPP51 induced an increase in soluble tubulin. Furthermore, IPP51 inhibited in vitro capillary-like tube formation by endothelial cells, indicating that it has anti-angiogenic activity. Molecular docking showed that the indol group of IPP51 can be accommodated in the colchicine binding site of tubulin. This characteristic was confirmed by an in vitro competition assay demonstrating that IPP51 can compete for colchicine binding to soluble tubulin. Finally, in a human bladder xenograft mouse model, IPP51 inhibited tumor growth without signs of toxicity. Altogether, these findings suggest that IPP51 is an attractive new microtubule-targeting agent with potential chemotherapeutic value.

Peloruside A, a microtubule-stabilizing agent, induces aneuploidy in ovarian cancer cells

To ensure proper chromosome segregation, mitosis is tightly regulated by the spindle assembly checkpoint (SAC). Low concentrations of microtubule-stabilizing agents can induce aneuploid populations of cells in the absence of G2/M block, suggesting pertubation of the spindle checkpoint. We investigated the effects of peloruside A, a microtubule-stabilizing agent, on expression levels of several key cell cycle proteins, MAD2, BUBR1, p55CDC and cyclin B1. Synchronized 1A9 ovarian carcinoma cells were allowed to progress through the cell cycle in the presence or absence of peloruside A. Co-immunoprecipitation and Western blotting were used to probe the cell cycle kinetics of MAD2 and BUBR1 dissociation from p55CDC. Using confocal microscopy, we investigated whether premature dissociation of MAD2 and BUBR1 at low (40 nM) but not high (100 nM) concentrations of peloruside A was caused by defects in the attachment of chromosomes to the mitotic spindle. An increased frequency of polar chromosomes was observed at low concentrations of peloruside A, suggesting that an increased frequency of pseudo-metaphase cells, which are not detected by the spindle assembly checkpoint, may be underlying the induction of aneuploidy.