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Colorectal cancer (CRC) is a common cancer in the world and its prevalence is increasing in developing countries. Deregulated cell cycle traverse is a hallmark of malignant transformation and is often observed in CRC as a result of imprecise activity of cell cycle regulatory components, viz. cyclins and cyclin-dependent kinases (CDKs). Apart from cell cycle regulation, some CDKs also regulate processes such as transcription and have also been shown to be involved in colorectal carcinogenesis. This article aims to review cyclin-dependent kinases as potential targets for CRC. Furthermore, therapeutic candidates to target CDKs are also discussed.

Magnesium is essential for cellular life, but how it is homeostatically controlled still remains poorly understood. Here, we report that members of CNNM family, which have been controversially implicated in both cellular Mg2+ influx and efflux, selectively bind to the TRPM7 channel to stimulate divalent cation entry into cells. Coexpression of CNNMs with the channel markedly increased uptake of divalent cations, which is prevented by an inactivating mutation to the channel's pore. Knockout (KO) of TRPM7 in cells or application of the TRPM7 channel inhibitor NS8593 also interfered with CNNM-stimulated divalent cation uptake. Conversely, KO of CNNM3 and CNNM4 in HEK-293 cells significantly reduced TRPM7-mediated divalent cation entry, without affecting TRPM7 protein expression or its cell surface levels. Furthermore, we found that cellular overexpression of phosphatases of regenerating liver (PRLs), known CNNMs binding partners, stimulated TRPM7-dependent divalent cation entry and that CNNMs were required for this activity. Whole-cell electrophysiological recordings demonstrated that deletion of CNNM3 and CNNM4 from HEK-293 cells interfered with heterologously expressed and native TRPM7 channel function. We conclude that CNNMs employ the TRPM7 channel to mediate divalent cation influx and that CNNMs also possess separate TRPM7-independent Mg2+ efflux activities that contribute to CNNMs' control of cellular Mg2+ homeostasis., Competing Interests: The authors have declared that no competing interests exist.

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis., (© The Author(s) 2019. Published by Oxford University Press on behalf of Society for the Study of Reproduction.)

Chronic liver injury triggers liver fibrosis and hepatocellular carcinoma (HCC), the third leading cause of cancer-related mortality. Cyclin E1 (CcnE1, formerly designated Cyclin E) is a regulatory subunit of the Cyclin-dependent kinase 2 (CDK2). It is overexpressed in approximately 70% of human HCCs correlating with poor prognosis, while the relevance of its orthologue Cyclin E2 (CcnE2) is unclear. Hepatocyte-specific deletion of NF-kappa-B essential modulator (NEMO Δhepa ) leads to chronic hepatitis, liver fibrosis, and HCC as well as CcnE upregulation. To this end, we generated NEMO Δhepa /CcnE1 -/- and NEMO Δhepa /CcnE2 -/- double knockout mice and investigated age-dependent liver disease progression in these animals. Deletion of CcnE1 in NEMO Δhepa mice decreased basal liver damage and reduced spontaneous liver inflammation in young mice. In contrast, loss of CcnE2 did not affect liver injury in NEMO Δhepa livers pointing to a unique, non-redundant function of CcnE1 in chronic hepatitis. Accordingly, basal compensatory hepatocyte proliferation in NEMO Δhepa mice was reduced by concomitant ablation of CcnE1, but not after loss of CcnE2. In aged NEMO Δhepa mice, loss of CcnE1 resulted in significant reduction of liver tumorigenesis, while deletion of CcnE2 had no effect on HCC formation. CcnE1, but not its orthologue CcnE2, substantially contributes to hepatic inflammatory response, liver disease progression, and hepatocarcinogenesis in NEMO Δhepa mice.

Cdk5 and Abl enzyme substrate 1 (Cables1) is an adaptor protein that links cyclin-dependent kinase (Cdks) with nonreceptor tyrosine kinases and regulates the activity of Cdks by enhancing their Y15 phosphorylation. Emerging evidence also shows that Cables1 can interact with, for example, p53 family proteins, 14-3-3, and β -catenin, suggesting that Cables1 may be a signaling hub for the regulation of cell growth. Abnormal expression of Cables1 has been observed in multiple types of cancers and other diseases. In this review, we summarize the characteristics of Cables1 and highlight the molecular mechanisms through which Cables1 regulates the development of cancer and other diseases. Finally, we discuss future challenges in demonstrating the role and potential application of Cables1 in cancer and other diseases., (Copyright © 2017 by The American Society for Pharmacology and Experimental Therapeutics.)

Chronic inflammation is a well-known precursor for cancer development and proliferation. We have recently demonstrated that high salt (NaCl) synergizes with sub-effective interleukin (IL)-17 to induce breast cancer cell proliferation. However, the exact molecular mechanisms mediating this effect are unclear. In our current study, we adopted a phosphoproteomic-based approach to identify salt modulated kinase-proteome specific molecular targets. The phosphoprotemics based binary comparison between heavy labelled MCF-7 cells treated with high salt (Δ0.05 M NaCl) and light labelled MCF-7 cells cultured under basal conditions demonstrated an enhanced phosphorylation of Serine-493 of SIK3 protein. The mRNA transcript and protein expression analysis of SIK3 in MCF-7 cells demonstrated a synergistic enhancement following co-treatment with high salt and sub-effective IL-17 (0.1 ng/mL), as compared to either treatments alone. A similar increase in SIK3 expression was observed in other breast cancer cell lines, MDA-MB-231, BT20, and AU565, while non-malignant breast epithelial cell line, MCF10A, did not induce SIK3 expression under similar conditions. Biochemical studies revealed mTORC2 acted as upstream mediator of SIK3 phosphorylation. Importantly, cell cycle analysis by flow cytometry demonstrated SIK3 induced G0/G1-phase release mediated cell proliferation, while SIK3 silencing abolished this effect. Also, SIK3 induced pro-inflammatory arginine metabolism, as evidenced by upregulation of the enzymes iNOS and ASS-1, along with downregulation of anti-inflammatory enzymes, arginase-1 and ornithine decarboxylase. Furthermore, gelatin zymography analysis has demonstrated that SIK3 induced expression of tumor metastatic CXCR4 through MMP-9 activation. Taken together, our data suggests a critical role of SIK3 in mediating three important hallmarks of cancer namely, cell proliferation, inflammation and metastasis. These studies provide a mechanistic basis for the future utilization of SIK3 as a key drug discovery target to improve breast cancer therapy.

Cyclin F protein, also known as FBXO1, is the largest among all cyclins and oscillates in the cell cycle like other cyclins. Apart from being a G 2 /M cyclin, cyclin F functions as the substrate-binding subunit of SCF cyclin F E3 ubiquitin ligase. In a gene expression analysis performed to identify novel gene modulations associated with cell cycle dysregulation during HIV-1 infection in CD4 + T cells, we observed down-regulation of the cyclin F gene ( CCNF ). Later, using gene overexpression and knockdown studies, we identified cyclin F as negatively influencing HIV-1 viral infectivity without any significant impact on virus production. Subsequently, we found that cyclin F negatively regulates the expression of viral protein Vif (viral infectivity factor) at the protein level. We also identified a novel host-pathogen interaction between cyclin F and Vif protein in T cells during HIV-1 infection. Mutational analysis of a cyclin F-specific amino acid motif in the C-terminal region of Vif indicated rescue of the protein from cyclin F-mediated down-regulation. Subsequently, we showed that Vif is a novel substrate of the SCF cyclin F E3 ligase, where cyclin F mediates the ubiquitination and proteasomal degradation of Vif through physical interaction. Finally, we showed that cyclin F augments APOBEC3G expression through degradation of Vif to regulate infectivity of progeny virions. Taken together, our results demonstrate that cyclin F is a novel F-box protein that functions as an intrinsic cellular regulator of HIV-1 Vif and has a negative regulatory effect on the maintenance of viral infectivity by restoring APOBEC3G expression., (© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)

Phosphorus is a pivotal element in all biochemical systems: it serves to store metabolic energy as ATP, it forms the backbone of genetic material such as RNA and DNA, and it separates cells from the environment as phospholipids. In addition to this "big hits", phosphorus has recently been shown to play an important role in other important processes such as cell cycle regulation. In the present review, we briefly summarize the biological processes in which phosphorus is involved in the yeast Saccharomyces cerevisiae before discussing our latest findings on the role of this element in the regulation of DNA replication in this eukaryotic model organism. We describe both the role of phosphorus in the regulation of G1 progression by means of the Cyclin Dependent Kinase (CDK) Pho85 and the stabilization of the cyclin Cln3, as well as the role of other molecule composed of phosphorus-the polyphosphate-in cell cycle progression, dNTP synthesis, and genome stability. Given the eminent role played by phosphorus in life, we outline the future of phosphorus in the context of one of the main challenges in human health: cancer treatment. [Int Microbiol 19(3):133-141 (2016)]., (Copyright© by the Spanish Society for Microbiology and Institute for Catalan Studies.)

This study is designated to investigate the roles of cyclin Y (CCNY) and Wnt signaling pathway in regulating ovarian cancer (OC) cell proliferation, migration, and invasion. Quantitative real-time PCR (qRT-PCR), Western blot, MTT assay, cell scratch, and transwell test were used in our study, and transplanted tumor model was constructed on nude mice. C-Myc, cyclin D1, PFTK1, ki67, OGT, and β-catenin protein expressions in tumor tissues were detected. CCNY was significantly upregulated in OC cell lines and tissues (both P < 0.05); significant association was observed between CCNY expression and clinicopathological stage, lymph node metastasis (LNM) (P < 0.05); and the CCNY expression in stages III to IV was higher than that in stages I to II, and patients with LNM had higher CCNY expression when compared with those in patients without LNM (P < 0.05); expressions of c-Myc, cyclin D, PFTK1, ki67, and OGT were upregulated in OC tissues compared with ovarian benign tissues, suggesting that these expressions were significantly different between the two groups (P < 0.05); CCNY significantly exacerbated proliferation, migration, and invasion of A2780 cells; c-Myc and cyclin D1 protein expressions increased as the expression of CCNY increased (P < 0.001); β-catenin expressions in A2780 cells with over-expression of CCNY were significantly increased in the nucleus, but significantly decreased in the cytoplasm (both P < 0.05); high expressions of CCNY exacerbated the proliferation of A2780 cells in nude mice and significantly increased c-Myc, cyclin D1, PFTK1, ki67, and OGT protein expressions in tumor tissues which were transplanted into nude mice (P < 0.01). CCNY might exacerbate the proliferation, migration, and invasion of OC cells via activating the Wnt signaling pathway. Thus, this study provides a theoretical foundation for the development of therapeutic drugs that are able to cure OC by targeting CCNY.

Cyclins play a central role in cell-cycle regulation; in mammals, the D family of cyclins consists of cyclin D1, D2, and D3. In Xenopus, only homologs of cyclins D1 and D2 have been reported, while a novel cyclin, cyclin Dx (ccndx), was found to be required for the maintenance of motor neuron progenitors during embryogenesis. It remains unknown whether zebrafish possess cyclin D3 or cyclin Dx. In this study, we identified a zebrafish ccndx gene encoding a protein which can form a complex with Cdk4. Through whole-mount in situ hybridization, we observed that zccndx mRNA is expressed in the motor neurons of hindbrain and spinal cord during development. Analysis of a 4-kb promoter sequence of the zccndx gene revealed the presence of HRE sites, which can be regulated by HIF2α. Morpholino knockdown of zebrafish Hif2α and cyclin Dx resulted in the abolishment of isl1 and oligo2 expression in the precursors of motor neurons, and also disrupted axon growth. Overexpression of cyclin Dx mRNA in Hif2α morphants partially rescued zccndx expression. Taken together, our data indicate that zebrafish cyclin Dx plays a role in maintaining the precursors of motor neurons.

The roles of cyclins and their catalytic partners, the cyclin-dependent kinases (CDKs), as core components of the machinery that drives cell cycle progression are well established. Increasing evidence indicates that mammalian cyclins and CDKs also carry out important functions in other cellular processes, such as transcription, DNA damage repair, control of cell death, differentiation, the immune response and metabolism. Some of these non-canonical functions are performed by cyclins or CDKs, independently of their respective cell cycle partners, suggesting that there was a substantial divergence in the functions of these proteins during evolution.

Cyclin Y-like 1 (Ccnyl1) is a newly-identified member of the cyclin family and is highly similar in protein sequences to Cyclin Y (Ccny). However, the function of Ccnyl1 is poorly characterized in any organism. Here we found that Ccnyl1 was most abundantly expressed in the testis of mice and was about seven times higher than the level of Ccny. Male Ccnyl1-/- mice were infertile, whereas both male and female Ccny-/- mice displayed normal fertility. These results suggest that Ccnyl1, but not Ccny, is indispensable for male fertility. Spermatozoa obtained from Ccnyl1-/- mice displayed significantly impaired motility, and represented a thinned annulus region and/or a bent head. We found that the protein, but not the mRNA, level of cyclin-dependent kinase 16 (CDK16) was decreased in the testis of Ccnyl1-/- mice. Further study demonstrated that CCNYL1 interacted with CDK16 and this interaction mutually increased the stability of these two proteins. Moreover, the interaction increased the kinase activity of CDK16. In addition, we observed an alteration of phosphorylation levels of CDK16 in the presence of CCNYL1. We identified the phosphorylation sites of CDK16 by mass spectrometry and revealed that several phosphorylation modifications on the N-terminal region of CDK16 were indispensable for the CCNYL1 binding and the modulation of CDK16 kinase activity. Our results therefore reveal a previously unrecognized role of CCNYL1 in regulating spermatogenesis through the interaction and modulation of CDK16.

Unlabelled: Walleye dermal sarcoma virus (WDSV) infection is associated with the seasonal development and regression of walleye dermal sarcoma. Previous work showed that the retroviral cyclin (RV-cyclin), encoded by WDSV, has separable cyclin box and transcription activation domains. It binds to cyclin-dependent kinase 8 (CDK8) and enhances its kinase activity. CDK8 is evolutionarily conserved and is frequently overexpressed in human cancers. It is normally activated by cyclin C and is required for transcription elongation of the serum response genes (immediate early genes [IEGs]) FOS, EGR1, and cJUN. The IEGs drive cell proliferation, and their expression is brief and highly regulated. Here we show that constitutive expression of RV-cyclin in the HCT116 colon cancer cell line significantly increases the level of IEG expression in response to serum stimulation. Quantitative reverse transcription-PCR (RT-PCR) and nuclear run-on assays provide evidence that RV-cyclin does not alter the initiation of IEG transcription but does enhance the overall rate of transcription elongation and maintains transcription reinitiation. RV-cyclin does not increase activating phosphorylation events in the mitogen-activated protein kinase pathway and does not inhibit decay of IEG mRNAs. At the EGR1 gene locus, RV-cyclin increases and maintains RNA polymerase II (Pol II) occupancy after serum stimulation, in conjunction with increased and extended EGR1 gene expression. The RV-cyclin increases CDK8 occupancy at the EGR1 gene locus before and after serum stimulation. Both of RV-cyclin's functional domains, i.e., the cyclin box and the activation domain, are necessary for the overall enhancement of IEG expression. RV-cyclin presents a novel and ancient mechanism of retrovirus-induced oncogenesis., Importance: The data reported here are important to both virology and cancer biology. The novel mechanism pinpoints CDK8 in the development of walleye dermal sarcoma and sheds light on CDK8's role in many human cancers. CDK8 controls expression from highly regulated genes, including the interferon-stimulated genes. Its function is likely the target of many viral interferon-resistance mechanisms. CDK8 also controls cellular responses to metabolic stimuli, stress, and hypoxia, in addition to the serum response. The retroviral cyclin (RV-cyclin) represents a highly selected probe of CDK8 function. RV-cyclin does not control CDK8 specificity but instead enhances CDK8's effects on regulated genes, an important distinction for its use to delineate natural CDK8 targets. The outcomes of this research are applicable to investigations of normal and abnormal CDK8 functions. The mechanisms defined here will contribute directly to the dermal sarcoma model in fish and clarify an important path for oncogenesis and innate resistance to viruses., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

Mucociliary clearance and fluid transport along epithelial surfaces are carried out by multiciliated cells (MCCs). Recently, human mutations in Cyclin O (CCNO) were linked to severe airway disease. Here, we show that Ccno expression is restricted to MCCs and the genetic deletion of Ccno in mouse leads to reduced numbers of multiple motile cilia and characteristic phenotypes of MCC dysfunction including severe hydrocephalus and mucociliary clearance deficits. Reduced cilia numbers are caused by compromised generation of centrioles at deuterosomes, which serve as major amplification platform for centrioles in MCCs. Ccno-deficient MCCs fail to sufficiently generate deuterosomes, and only reduced numbers of fully functional centrioles that undergo maturation to ciliary basal bodies are formed. Collectively, this study implicates CCNO as first known regulator of deuterosome formation and function for the amplification of centrioles in MCCs., (© 2015 The Authors.)

The yeast cyclin C-Cdk8 kinase forms a complex with Med13p to repress the transcription of genes involved in the stress response and meiosis. In response to oxidative stress, cyclin C displays nuclear to cytoplasmic relocalization that triggers mitochondrial fission and promotes programmed cell death. In this report, we demonstrate that Med13p mediates cyclin C nuclear retention in unstressed cells. Deleting MED13 allows aberrant cytoplasmic cyclin C localization and extensive mitochondrial fragmentation. Loss of Med13p function resulted in mitochondrial dysfunction and hypersensitivity to oxidative stress-induced programmed cell death that were dependent on cyclin C. The regulatory system controlling cyclin C-Med13p interaction is complex. First, a previous study found that cyclin C phosphorylation by the stress-activated MAP kinase Slt2p is required for nuclear to cytoplasmic translocation. This study found that cyclin C-Med13p association is impaired when the Slt2p target residue is substituted with a phosphomimetic amino acid. The second step involves Med13p destruction mediated by the 26S proteasome and cyclin C-Cdk8p kinase activity. In conclusion, Med13p maintains mitochondrial structure, function, and normal oxidative stress sensitivity through cyclin C nuclear retention. Releasing cyclin C from the nucleus involves both its phosphorylation by Slt2p coupled with Med13p destruction., (© 2014 Khakhina et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

Meiotic crossovers (COs) are crucial for ensuring accurate homologous chromosome segregation during meiosis I. Because the double-strand breaks (DSBs) that initiate meiotic recombination greatly outnumber eventual COs, this process requires exquisite regulation to narrow down the pool of DSB intermediates that may form COs. In this paper, we identify a cyclin-related protein, CNTD1, as a critical mediator of this process. Disruption of Cntd1 results in failure to localize CO-specific factors MutLγ and HEI10 at designated CO sites and also leads to prolonged high levels of pre-CO intermediates marked by MutSγ and RNF212. These data show that maturation of COs is intimately coupled to deselection of excess pre-CO sites to yield a limited number of COs and that CNTD1 coordinates these processes by regulating the association between the RING finger proteins HEI10 and RNF212 and components of the CO machinery., (© 2014 Holloway et al.)

Unlabelled: The liver has a strong regenerative capacity. After injury, quiescent hepatocytes can reenter the mitotic cell cycle to restore tissue homeostasis. This G(0) /G(1) -S cell-cycle transition of primed hepatocytes is regulated by complexes of cyclin-dependent kinase 2 (Cdk2) with E-type cyclins (CcnE1 or CcnE2). However, single genetic ablation of either E-cyclin or Cdk2 does not affect overall liver regeneration. Here, we systematically investigated the contribution of CcnE1, CcnE2, and Cdk2 for liver regeneration after partial hepatectomy (PH) by generating corresponding double- and triple-knockout (KO) mouse mutants. We demonstrate that conditional deletion of Cdk2 alone in hepatocytes resulted in accelerated induction of CcnE1, but otherwise normal initiation of S phase in vivo and in vitro. Excessive CcnE1 did not contribute to a noncanonical kinase activity, but was located at chromatin together with components of the pre-replication complex (pre-RC), such as the minichromosome maintenance (MCM) helicase. Concomitant ablation of Cdk2 and CcnE1 in hepatocytes caused a defect in pre-RC formation and further led to dramatically impaired S-phase progression by down-regulation of cyclin A2 and cell death in vitro and substantially reduced hepatocyte proliferation and liver regeneration after PH in vivo. Similarly, combined loss of CcnE1 and CcnE2, but also the Cdk2/CcnE1/CcnE2 triple KO in liver, significantly inhibited S-phase initiation and liver mass reconstitution after PH, whereas concomitant ablation of CcnE2 and Cdk2 had no effect., Conclusion: In the absence of Cdk2, CcnE1 performs crucial kinase-independent functions in hepatocytes, which are capable of driving MCM loading on chromatin, cyclin A2 expression, and S-phase progression. Thus, combined inactivation of Cdk2 and CcnE1 is the minimal requirement for blocking S-phase machinery in vivo., (© 2013 by the American Association for the Study of Liver Diseases.)

Background: A single nucleotide polymorphism (rs7914558) within the cyclin M2 (CNNM2) gene was recently identified as a common risk variant for schizophrenia. The mechanism by which CNNM2 confers risk is unknown., Aims: To determine the impact of the rs7914558 risk 'G' allele [corrected] on measures of neurocognition, social cognition and brain structure., Method: Patients with schizophrenia (n = 400) and healthy controls (n = 160) completed measures of neuropsychological function and social cognition. Structural magnetic resonance imaging data were also acquired from an overlapping sample of Irish healthy controls (n = 159) and an independent sample of Italian patients (n = 82) and healthy controls (n = 39)., Results: No effects of genotype on neuropsychological test performance were observed. However, a dosage effect of the risk allele was found for an index of social cognition (i.e. attributional style), such that risk status was associated with reduced self-serving bias across groups (GG>AG>AA, P<0.05). Using voxel-based morphometry to investigate neuroanatomical regions putatively supporting social cognition, risk carriers had relatively increased grey matter volume in the right temporal pole and right anterior cingulate cortex (Pcorrected<0.05) in the Irish healthy controls sample; neuroanatomical associations between CNNM2 and grey matter volume in anterior cingulate cortex were also observed in the Italian schizophrenia and healthy controls samples., Conclusions: Although the biological role of CNNM2 in schizophrenia remains unknown, these data suggest that this CNNM2 risk variant rs7914558 may have an impact on neural systems relevant to social cognition. How such effects may mediate the relationship between genotype and disease risk remains to be established.

The order and timing of cell-cycle events is controlled by changing substrate specificity and different activity thresholds of cyclin-dependent kinases (CDKs). However, it is not understood how a single protein kinase can trigger hundreds of switches in a sufficiently time-resolved fashion. We show that cyclin-Cdk1-Cks1-dependent phosphorylation of multisite targets in Saccharomyces cerevisiae is controlled by key substrate parameters including distances between phosphorylation sites, distribution of serines and threonines as phosphoacceptors and positioning of cyclin-docking motifs. The component mediating the key interactions in this process is Cks1, the phosphoadaptor subunit of the cyclin-Cdk1-Cks1 complex. We propose that variation of these parameters within networks of phosphorylation sites in different targets provides a wide range of possibilities for differential amplification of Cdk1 signals, thus providing a mechanism to generate a wide range of thresholds in the cell cycle.

Polycomb group (PcG) proteins are transcriptional repressors of genes involved in development and differentiation, and also maintain repression of key genes involved in the cell cycle, indirectly regulating cell proliferation. The human SCML2 gene, a mammalian homologue of the Drosophila PcG protein SCM, encodes two protein isoforms: SCML2A that is bound to chromatin and SCML2B that is predominantly nucleoplasmic. Here, we purified SCML2B and found that it forms a stable complex with CDK/CYCLIN/p21 and p27, enhancing the inhibitory effect of p21/p27. SCML2B participates in the G1/S checkpoint by stabilizing p21 and favoring its interaction with CDK2/CYCE, resulting in decreased kinase activity and inhibited progression through G1. In turn, CDK/CYCLIN complexes phosphorylate SCML2, and the interaction of SCML2B with CDK2 is regulated through the cell cycle. These findings highlight a direct crosstalk between the Polycomb system of cellular memory and the cell-cycle machinery in mammals., Competing Interests: The authors have declared that no competing interests exist.

Cyclin-dependent kinases (Cdks) are serine/threonine kinases and their catalytic activities are modulated by interactions with cyclins and Cdk inhibitors (CKIs). Close cooperation between this trio is necessary for ensuring orderly progression through the cell cycle. In addition to their well-established function in cell cycle control, it is becoming increasingly apparent that mammalian Cdks, cyclins and CKIs play indispensable roles in processes such as transcription, epigenetic regulation, metabolism, stem cell self-renewal, neuronal functions and spermatogenesis. Even more remarkably, they can accomplish some of these tasks individually, without the need for Cdk/cyclin complex formation or kinase activity. In this Review, we discuss the latest revelations about Cdks, cyclins and CKIs with the goal of showcasing their functional diversity beyond cell cycle regulation and their impact on development and disease in mammals.

Skin wound healing is an interactive process involving soluble mediators, ECM, resident cells, and infiltrating cells. Little is known about wound healing in the presence of lymphedema. In this study, we investigated wound healing using kCYC⁺/⁻ mice, which demonstrate severe lymphatic dysfunction. Wound healing was delayed significantly in kCYC⁺/⁻ mice when compared with WT mice. In wounded skin of kCYC⁺/⁻ mice, mast cell numbers were increased compared with WT mice, whereas macrophage numbers were decreased. Moreover, IL-10 expression by mast cells was increased, and expression of bFGF, mainly produced by macrophages, was decreased in wounded skin of kCYC⁺/⁻ mice compared with WT mice. We next crossed kCYC⁺/⁻ mice with IL-10⁻/⁻ mice, which were reported to show accelerated wound closure. In kCYC⁺/⁻ IL-10⁺/⁻ mice, time course of wound healing, numbers of macrophages, and IL-10 mRNA expression levels in wounded skin were comparable with WT IL-10⁺/⁻ mice. Similar results were obtained using a different lymphedema model, in which circumferential skin excision was performed on the tails of mice to remove the superficial lymphatics. In summary, these findings suggest that IL-10 plays an important role in delayed wound healing in the setting of lymphatic dysfunction.

In eukaryotes, an oscillating network of protein kinase activities drives the order and timing of the cell cycle progression. Complexes formed by cyclins associated to cyclin-dependent kinases (CDKs) are the central components of this network. Cyclins act as the activating subunits and their abundance is regulated by different mechanisms in order to promote or prevent kinase activity. Protein synthesis, proteasomal degradation and/or differential subcellular compartmentalization modulate cyclin expression levels along the cell cycle. We describe in this work the characterization of Trypanosoma cruzi Cyclin 2 (TcCYC2), which contributes to a better understanding of the cell cycle regulation in this protozoan parasite. We found TcCYC2 exhibited cyclin function in a yeast complementation assay and over-expression of hemagglutinin tagged TcCYC2-HA rendered shorter duplication times and smaller cell sizes in the epimastigote form of the parasite. Analysis of synchronized cultures showed that over-expression of TcCYC2-HA altered the timing epimastigotes pass through G2/M boundary or cytokinesis. Taken together, our results showed that TcCYC2 is a functional cyclin whose over-expression modifies the dynamics of the cell cycle as well as the morphology of epimastigote forms of T. cruzi, suggesting it plays an important role in the cell cycle regulation machinery., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Precise timing coordinates cell proliferation with embryonic morphogenesis. As Drosophila melanogaster embryos approach cell cycle 14 and the midblastula transition, rapid embryonic cell cycles slow because S phase lengthens, which delays mitosis via the S-phase checkpoint. We probed the contributions of each of the three mitotic cyclins to this timing of interphase duration. Each pairwise RNA interference knockdown of two cyclins lengthened interphase 13 by introducing a G2 phase of a distinct duration. In contrast, pairwise cyclin knockdowns failed to introduce a G2 in embryos that lacked an S-phase checkpoint. Thus, the single remaining cyclin is sufficient to induce early mitotic entry, but reversal of the S-phase checkpoint is compromised by pairwise cyclin knockdown. Manipulating cyclin levels revealed that the diversity of cyclin types rather than cyclin level influenced checkpoint reversal. We conclude that different cyclin types have distinct abilities to reverse the checkpoint but that they collaborate to do so rapidly.

The adult brain most probably reaches its highest degree of plasticity with the lifelong generation and integration of new neurons in the hippocampus and olfactory system. Neural precursor cells (NPCs) residing both in the subgranular zone of the dentate gyrus and in the subventricular zone of the lateral ventricles continuously generate neurons that populate the dentate gyrus and the olfactory bulb, respectively. The regulation of NPC proliferation in the adult brain has been widely investigated in the past few years. Yet, the intrinsic cell cycle machinery underlying NPC proliferation remains largely unexplored. In this review, we discuss the cell cycle components that are involved in the regulation of NPC proliferation in both neurogenic areas of the adult brain.

Here we presented that the expression of RUNX3 was significantly decreased in 75 cases of clear cell renal cell carcinoma (CCRCC) tissues (p<0.05). Enforced RUNX3 expression mediated 786-O cells to exhibit inhibition of growth, G1 cell-cycle arrest and metastasis in vitro, and to lost tumorigenicity in nude mouse model in vivo. RUNX3-induced growth suppression was found partially to regulate various proteins, including inhibition of cyclinD1, cyclinE, cdk2, cdk4 and p-Rb, but increase of p27(Kip1), Rb and TIMP-1. Therefore, RUNX3 had the function of inhibiting the proliferative and metastatic abilities of CCRCC cells by regulating cyclins and TIMP1.

Cell cycle progression corresponds to a series of events, which succeed one another to end in the division of a mother cell to give two daughter cells. The processes that allow the cell to divide are very precisely controlled by a multitude of enzymatic reactions among which protein phosphorylation, carried out by protein kinases, plays a key role. Serine/threonine kinases are enzymes that catalyse the transfer of a phosphate from ATP to a protein substrate, more precisely on a serine or threonine amino acid residue. Three important families of serine/threonine kinases are involved in the regulation of cell cycle progression, the cyclin dependent kinase (CDK) the polo-like kinase (PLK) and those of the Aurora family. The cancer is described as an uncontrolled cell division process. Cancer cells proliferate indeed in an anarchic way, and carry out cycles of cellular division by being unaware of the signals of alarm. A simple idea thus appeared soon: to stop or to slow down cell cycle progression would result in inhibiting cell proliferation and thus fighting against cancer. Cell cycle progression being controlled in particular by protein kinases of the CDK, PLK and Aurora families, it was rapidly decided to look for inhibitors of those protein kinases. We will first make a general recall on cell cycle progression and the mechanisms that control it. The functions of protein kinases of the CDK, PLK and Aurora families will then be described by concentrating on the sensitive phase of the cell cycle progression, i.e. mitosis. Finally, we will approach the consequences of the inhibition of these protein kinases within the framework of the fight against cancer.

The wood frog (Rana sylvatica) exhibits a well-developed natural anoxia and dehydration tolerance. The degree of stress tolerance depends on numerous biochemical adaptations, including stress-induced hypometabolism that helps to preserve long-term viability by reducing ATP demand. We hypothesized that the mechanisms involved in cell cycle control could act to aid in the establishment of the hypometabolic state required for stress survival. Selected proteins involved in the proliferation of cells were evaluated using immunoblotting in liver and skeletal muscle of wood frogs comparing controls with animals subjected to either 24-hr anoxia exposure under a nitrogen gas atmosphere or dehydration to 40% of total body water lost (all at 5°C). Levels of cyclins (type A, B, D, and E) decreased significantly under both stresses in liver and skeletal muscle. Similar reductions were seen for Cyclin-dependant kinases (Cdk) types 2, 4, and 6 in both liver and skeletal muscle; however, an increase in the relative amount of phosphorylated inactive p-Cdk (Thr14/Tyr15) was observed in liver under both stresses. Levels of positive regulators of Cdk activity (Cdc25 type A and C) were significantly reduced in both tissues under both stresses, whereas negative regulators of Cdk activity (p16(INK4a) and p27(KIP1) ) increased significantly in liver under both anoxia and dehydration stress (but not in muscle). This study provides the first report of differential regulation of cell cycle components in an anoxia and dehydration tolerant vertebrate, the wood frog, suggesting that cell cycle suppression is an active part of stress resistance and life extension in hypometabolic states., (Copyright © 2011 Wiley-Liss, Inc., A Wiley Company.)

The process of cell division in mammalian cells is orchestrated by cell-cycle-dependent oscillations of cyclin protein levels. Cyclin levels are controlled by redundant transcriptional, post-translational and degradation feedback loops. How each of these separate loops contributes to the regulation of the key cell cycle events and to the connection between the G1-S transition and the subsequent mitotic events is under investigation. Here, we present an integrated computational model of the mammalian cell cycle based on the sequential activation of cyclins. We validate the model against experimental data on liver cells (hepatocytes), which undergo one or two rounds of synchronous circadian-clock gated cell divisions during liver regeneration, after partial hepatectomy (PH). The model exhibits bandpass filter properties that allow the system to ignore strong but transient, or sustained but weak damages after PH. Bifurcation analysis of the model suggests two different threshold mechanisms for the progression of the cell through mitosis. These results are coherent with the notion that the mitotic exit in mammalian cells is bistable, and suggests that Cdc20 homologue 1 (Cdh1) is an important regulator of mitosis. Regulation by Cdh1 also explains the observed G2/M phase prolongation after hepatocyte growth factor (HGF) stimulation during S phase., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

The assembly and maturation of neural circuits require a delicate balance between synapse formation and elimination. The cellular and molecular mechanisms that coordinate synaptogenesis and synapse elimination are poorly understood. In C. elegans, DD motoneurons respecify their synaptic connectivity during development by completely eliminating existing synapses and forming new synapses without changing cell morphology. Using loss- and gain-of-function genetic approaches, we demonstrate that CYY-1, a cyclin box-containing protein, drives synapse removal in this process. In addition, cyclin-dependent kinase-5 (CDK-5) facilitates new synapse formation by regulating the transport of synaptic vesicles to the sites of synaptogenesis. Furthermore, we show that coordinated activation of UNC-104/Kinesin3 and Dynein is required for patterning newly formed synapses. During the remodeling process, presynaptic components from eliminated synapses are recycled to new synapses, suggesting that signaling mechanisms and molecular motors link the deconstruction of existing synapses and the assembly of new synapses during structural synaptic plasticity., (Copyright © 2011 Elsevier Inc. All rights reserved.)

The identification of new members in the Cdk and cyclin families, functions for many of which are still emerging, has added new facets to the cell cycle regulatory network. With roles extending beyond the classical regulation of cell cycle progression, these new players are involved in diverse processes such as transcription, neuronal function, and ion transport. Members closely related to Cdks and cyclins such as the Speedy/RINGO proteins offer fresh insights and hope for filling in the missing gaps in our understanding of cell division. This chapter will present a broad outlook on the cell cycle and its key regulators with special emphasis on the less-studied members and their emerging roles.

Cyclin-dependent kinase 9 (CDK9), with its cyclin T regulatory subunit, is a component of the positive transcription elongation factor b (P-TEFb) complex, which stimulates transcription elongation and also functions in co-transcriptional histone modification, mRNA processing, and mRNA export. CDK9 also binds to cyclin K but the function of this CDK9-cyclin K complex is less clear. We and others have recently shown that CDK9 functions directly in maintaining genome integrity. This activity is restricted to CDK9-cyclin K. Depletion of CDK9 or its cyclin K but not cyclin T regulatory subunit impairs cell cycle recovery in response to replication stress and induces spontaneous DNA damage in replicating cells. CDK9-cyclin K also interacts with ATR and other DNA damage response and DNA repair proteins. CDK9 accumulates on chromatin and limits the amount of single-stranded DNA in response to replication stress. Collectively, these data are consistent with a model in which CDK9 responds to replication stress by localizing to chromatin to reduce the breakdown of stalled replication forks and promote recovery from replication arrest. The direct role of CDK9-cyclin K in pathways that maintain genome integrity in response to replication stress appear to be evolutionarily conserved.

Recent reports have better elucidated the components of the Polycomb Repressive Complex 2 (PRC2) and its functional role in embryonic stem cells (ESCs) and their differentiated derivatives. The depletion of a newly described mammalian PRC2 associated protein, PCL2, leads to an increase in ESC self-renewal and a delay in differentiation, a phenotype similar to knockouts of the core PRC2 members. Genomic and cell biology data suggest that PCL2 is important in cell fate decisions and may play a role in recruitment of PRC2 to target genes and histone methylation. Importantly, depletion of PCL2 in ESCs leads to a decrease in 3meH3K27 at the proximal promoter regions of pluripotency transcription factors Tbx3, Klf4, Foxd3 and a concomitant increase in gene expression. These proteins subsequently activate expression of Oct4, Nanog and Sox2 through a feed-forward gene regulatory circuit, altering the core pluripotency network and driving cell fate decisions towards self-renewal. We propose a model whereby alteration of the epigenetic state of Tbx3, Klf4, and Foxd3 results in the enforced expression of the pluripotency network, preventing differentiation.

Cyclin Y is one of the most highly conserved members of the cyclin superfamily of proteins, which are famous for their crucial roles in regulating the cell cycle and transcription. Despite this high degree of conservation, very little was known about Cyclin Y function prior to a handful of studies published in this past year. Cyclins typically function by activating cyclin-dependent kinases (Cdks) and one insight has come from the identification of a Cdk that is activated by Cyclin Y. Yeast two-hybrid data first linked Cyclin Y with Cdk14, known as Eip63E in Drosophila or PFTAIRE1 in vertebrates. In Drosophila, both Cyclin Y and Eip63E are essential at many stages of development, from embryogenesis to metamorphosis and null mutants show a similar spectrum of developmental defects. In cultured cells, Cyclin Y and Eip63E were shown to phosphorylate the Wg/Wnt co-receptor Arrow/LRP6 in a ligand-independent manner. Eip63E is recruited to LRP6 at the plasma membrane by interacting with Cyclin Y, which is tethered to the membrane through an N-terminal myristoylation. Cyclin Y-dependent LRP6 phosphorylation appears to prime the receptor for subsequent ligand-dependent phosphorylation and activation of the canonical Wnt signaling pathway. Interestingly, Wnt receptor phosphorylation and signaling is maximal in G₂/M when Cyclin Y is at its highest levels, suggesting that Cyclin Y may serve to entrain Wnt signaling to the cell cycle. Given the wide range of roles for Wnt signaling during development, these studies may help explain why Cyclin Y is required at several developmental stages and in turn why these proteins are so well conserved in metazoans.

The pathogenesis of gastric cancer is complex and related to multiple factors. Dysregulation of intracellular signaling pathways represents a common pathogenic mechanism and may be amenable to drug targeting. Multiple well-established oncogenic pathways, such as those mediated by cell cycle regulators, nuclear factor-kappaB, cyclooxygenase-2 and epidermal growth factor receptor are implicated in gastric carcinogenesis. Emerging evidence also underscores the importance of signaling pathways involved in the developmental process, including transforming growth factor-beta/bone morphogenetic protein signaling, Wnt/beta-catenin signaling, Hedgehog signaling and Notch signaling. Understanding their biological significance will provide a rational basis for drug development. Their relative importance and cross-talk in gastric carcinogenesis, however, are still not completely understood and warrant further investigation., (2010 Elsevier Ireland Ltd. All rights reserved.)

Pituitary tumours are benign neoplasms that may cause major endocrine dysfunction. Transgenic disruption of the cell cycle machinery frequently leads to pituitary adenoma formation in animal models. The molecular analysis of human pituitary tumours has found various alterations in the expression of cell cycle regulators: cyclins, cyclin-dependent kinases and their inhibitors. There are also different mechanisms (e.g. hypermethylation, frameshift mutations, increased proteasome degradation) responsible for changed expression in cyclin mRNA and protein. It is probable that the primary initiating events lie beyond the cell cycle and may be related to co-activation of Akt, MAP-kinase and beta-catenin pathways. Nevertheless, molecular CDK inhibitors may play a role in pituitary tumour treatment in the future., (2010 Elsevier Ireland Ltd. All rights reserved.)

Since the discovery of cyclins, the role of cell cycle regulators in the control of cell proliferation has been extensively studied. It is clear that proliferation requires an adapted metabolic response of the cells; hence the regulation of cell cycle must be linked to metabolic control. While at a much slower pace, the impact that the activities of cell cycle regulators such as cyclins, cyclin dependent kinases or E2F factor, transcription factor have on cell metabolism are also being uncovered. Here we will focus on recent data implicating cell cycle regulators in metabolic control, with particular attention to studies performed using mouse models. Furthermore, we will discuss the possible relevance of these findings in the context of metabolic disorders such as obesity or diabetes.

Key components of the cell cycle machinery are the regulatory subunits, the cyclins, and their catalytic partners the cyclin-dependent kinases. Regulating the cell cycle in the male germ line cells represents unique challenges for this machinery given the constant renewal of gametes throughout the reproductive lifespan and the induction of the unique process of meiosis, a highly specialized kind of cell division. With challenges come opportunities to the critical eye, recognizing that understanding these specialized modes of regulation will provide considerable insight into both normal differentiation as well as disease conditions, including infertility and oncogenesis.

Background: The evolutionarily conserved Ccr4-Not and Bur1/2 kinase complexes are functionally related in Saccharomyces cerevisiae. In this study, we further explore the relationship between the subunits Not4p and Bur2p., Methodology/principal Findings: First, we investigated the presence of post-translational modifications on the Ccr4-Not complex. Using mass spectrometry analyses we identified several SP/TP phosphorylation sites on its Not4p, Not1p and Caf1p subunits. Secondly, the influence of Not4p phosphorylation on global H3K4 tri-methylation status was examined by immunoblotting. This histone mark is severely diminished in the absence of Not4p or of Bur2p, but did not require the five identified Not4p phosphorylation sites. Thirdly, we found that Not4p phosphorylation is not affected by the kinase-defective bur1-23 mutant. Finally, phenotypic analyses of the Not4p phosphomutant (not4S/T5A) and bur2Delta strains showed overlapping sensitivities to drugs that abolish cellular stress responses. The double-mutant not4S/T5A and bur2Delta strain even revealed enhanced phenotypes, indicating that phosphorylation of Not4p and BUR2 are active in parallel pathways for drug tolerance., Conclusions: Not4p is a phospho-protein with five identified phosphorylation sites that are likely targets of a cyclin-dependent kinase(s) other than the Bur1/2p complex. Not4p phosphorylation on the five Not4 S/T sites is not required for global H3K4 tri-methylation. In contrast, Not4p phosphorylation is involved in tolerance to cellular stresses and acts in pathways parallel to BUR2 to affect stress responses in Saccharomyces cerevisiae.

Cyclin Y (CCNY) is a novel cyclin and almost nothing is known about its expression level and role in human cancers. In the present study, we found that lentivirus-mediated RNA interference (RNAi) of CCNY significantly downregulated its expression level. More importantly, knockdown of CCNY inhibited cell proliferation, colony formation, and cell cycle progression in glioma cells. These data suggest that CCNY might play an important role in glioma tumorigenisis.

Cellular rhythms represent a field of choice for studies in system biology. The examples of circadian rhythms and of the cell cycle show how the experimental and modeling approaches contribute to clarify the conditions in which periodic behavior spontaneously arises in regulatory networks at the cellular level. Circadian rhythms originate from intertwined positive and negative feedback loops controlling the expression of several clock genes. Models can be used to address the dynamical bases of physiological disorders related to dysfunctions of the mammalian circadian clock. The cell cycle is driven by a network of cyclin-dependent kinases (Cdks). Modeled in the form of four modules coupled through multiple regulatory interactions, the Cdk network operates in an oscillatory manner in the presence of sufficient amounts of growth factor. For circadian rhythms and the cell cycle, as for other recently observed cellular rhythms, periodic behavior represents an emergent property of biological systems related to their regulatory structure.

The cell cycle is one of the best studied cellular mechanisms at the experimental and theoretical levels. Although most of the important biochemical components and reactions of the cell cycle are probably known, the precise way the cell cycle dynamics are driven is still under debate. This phenomenon is not atypical to many other biological systems where the knowledge of the molecular building blocks and the interactions between them does not lead to a coherent picture of the appropriate dynamics. We here propose a methodology to develop plausible models for the driving mechanisms of embryonic and cancerous cell cycles. We first define a key property of the system (a cyclic behaviour in the case of the embryonic cell cycle) and set mathematical constraints on the types of two variable simplified systems robustly reproducing such a cyclic behaviour. We then expand these robust systems to three variables and reiterate the procedure. At each step, we further limit the type of expanded systems to fit the known microbiology until a detailed description of the system is obtained. This methodology produces mathematical descriptions of the required biological systems that are more robust to changes in the precise function and rate constants. This methodology can be extended to practically any type of subcellular mechanism.

As a partner of cyclin-dependent kinase (CDK) 3, Cyclin C controls cellular proliferation and, together with CDK8, represses gene transcription. In this study, we showed that the highly expressed Cyclin C gene is a direct target of the nuclear hormone all-trans retinoic acid (RA) in HEK293 human embryonal kidney cells. The RA receptor (RAR) gamma associates with a Cyclin C promoter region containing two RAR binding sites. The Cyclin C gene also directly responds to the cAMP activator Forskolin via the transcription factor CREB1 (cAMP response element-binding protein 1), for which we identified four binding sites within the first 2250 bp of its promoter. RARgamma and CREB1 show functional convergence via the corepressor NCoR1, which controls in particular the Forskolin response of Cyclin C. The histone deacetylases 1, 5, 6, 7 and 11 are involved in the basal expression of Cyclin C, but in HEK293 and MCF-7 human breast carcinoma cells the antiproliferative effects of the histone deacetylase inhibitor SAHA (suberoylanilide hydroxamic acid) are not mediated by Cyclin C. However, cell cycle progressing effects of all-trans RA and Forskolin are dependent on Cyclin C expression levels. This suggests that the primary regulation of Cyclin C by all-trans RA and Forskolin mediates some of the cell cycle control actions of these compounds.