Expression of cyclins E1 and E2 during mouse development and in neoplasia

Transcriptional repression and enhancer decommissioning silence cell cycle genes in postmitotic tissues

The mechanisms that maintain a non-cycling status in postmitotic tissues are not well understood. Many cell cycle genes have promoters and enhancers that remain accessible even when cells are terminally differentiated and in a non-cycling state, suggesting their repression must be maintained long term. In contrast, enhancer decommissioning has been observed for rate-limiting cell cycle genes in the Drosophila wing, a tissue where the cells die soon after eclosion, but it has been unclear if this also occurs in other contexts of terminal differentiation. In this study, we show that enhancer decommissioning also occurs at specific, rate-limiting cell cycle genes in the long-lived tissues of the Drosophila eye and brain, and we propose this loss of chromatin accessibility may help maintain a robust postmitotic state. We examined the decommissioned enhancers at specific rate-limiting cell cycle genes and show that they encode dynamic temporal and spatial expression patterns that include shared, as well as tissue-specific elements, resulting in broad gene expression with developmentally controlled temporal regulation. We extend our analysis to cell cycle gene expression and chromatin accessibility in the mammalian retina using a published dataset, and find that the principles of cell cycle gene regulation identified in terminally differentiating Drosophila tissues are conserved in the differentiating mammalian retina. We propose a robust, non-cycling status is maintained in long-lived postmitotic tissues through a combination of stable repression at most cell cycle gens, alongside enhancer decommissioning at specific rate-limiting cell cycle genes.

c-Myc uses Cul4b to preserve genome integrity and promote antiviral CD8+ T cell immunity

During infection, virus-specific CD8+ T cells undergo rapid bursts of proliferation and differentiate into effector cells that kill virus-infected cells and reduce viral load. This rapid clonal expansion can put T cells at significant risk for replication-induced DNA damage. Here, we find that c-Myc links CD8+ T cell expansion to DNA damage response pathways though the E3 ubiquitin ligase, Cullin 4b (Cul4b). Following activation, c-Myc increases the levels of Cul4b and other members of the Cullin RING Ligase 4 (CRL4) complex. Despite expressing c-Myc at high levels, Cul4b-deficient CD8+ T cells do not expand and clear the Armstrong strain of lymphocytic choriomeningitis virus (LCMV) in vivo. Cul4b-deficient CD8+ T cells accrue DNA damage and succumb to proliferative catastrophe early after antigen encounter. Mechanistically, Cul4b knockout induces an accumulation of p21 and Cyclin E2, resulting in replication stress. Our data show that c-Myc supports cell proliferation by maintaining genome stability via Cul4b, thereby directly coupling these two interdependent pathways. These data clarify how CD8+ T cells use c-Myc and Cul4b to sustain their potential for extraordinary population expansion, longevity and antiviral responses.

Giant Cells of Various Lesions Are Characterised by Different Expression Patterns of HLA-Molecules and Molecules Involved in the Cell Cycle, Bone Metabolism, and Lineage Affiliation: An Immunohistochemical Study with a Review of the Literature

Giant cells (GCs) are thought to originate from the fusion of monocytic lineage cells and arise amid multiple backgrounds. To compare GCs of different origins, we immunohistochemically characterised the GCs of reactive and neoplastic lesions (n = 47). We studied the expression of 15 molecules including HLA class II molecules those relevant to the cell cycle, bone metabolism and lineage affiliation. HLA-DR was detectable in the GCs of sarcoidosis, sarcoid-like lesions, tuberculosis, and foreign body granuloma. Cyclin D1 was expressed by the GCs of neoplastic lesions as well as the GCs of bony callus, fibroid epulis, and brown tumours. While cyclin E was detected in the GCs of all lesions, p16 and p21 showed a heterogeneous expression pattern. RANK was expressed by the GCs of all lesions except sarcoid-like lesions and xanthogranuloma. All GCs were RANK-L-negative, and the GCs of all lesions were osteoprotegerin-positive. Osteonectin was limited to the GCs of chondroblastoma. Osteopontin and TRAP were detected in the GCs of all lesions except xanthogranuloma. RUNX2 was heterogeneously expressed in the reactive and neoplastic cohort. The GCs of all lesions except foreign body granuloma expressed CD68, and all GCs were CD163- and langerin-negative. This profiling points to a functional diversity of GCs despite their similar morphology.

Nannocystin ax, an eEF1A inhibitor, induces G1 cell cycle arrest and caspase-independent apoptosis through cyclin D1 downregulation in colon cancer in vivo

Colorectal cancer (CRC) is one of the most common causes of cancer-related death worldwide. Nannocystin ax (NAN), a 21-membered cyclodepsipeptide initially isolated from myxobacteria of the Nannocystis genus, was found to target the eukaryotic elongation factor 1A (eEF1A). The current study was designed to evaluate the anticancer effect and underlying mechanisms of NAN with in vitro and in vivo models. Results showed that NAN induced G1 phase cell cycle arrest and caspase-independent apoptosis in HCT116 and HT29 human CRC cells. NAN significantly downregulated cyclin D1 level in a short time, but NAN did not affect the transcription level and ubiquitin-dependent degradation of cyclin D1. Furthermore, NAN treatment directly targeted eEF1A and partially decreased the synthesis of new proteins, contributing to the downregulation of cyclin D1. Besides, NAN significantly suppressed tumor growth in the zebrafish xenograft model. In conclusion, NAN triggered G1 phase cell cycle arrest through cyclin D1 downregulation and eEF1A-targeted translation inhibition and promoted caspase-independent apoptosis in CRC cells.

Cyclin E in normal physiology and disease states

E-type cyclins, collectively called cyclin E, represent key components of the core cell cycle machinery. In mammalian cells, two E-type cyclins, E1 and E2, activate cyclin-dependent kinase 2 (CDK2) and drive cell cycle progression by phosphorylating several cellular proteins. Abnormally elevated activity of cyclin E-CDK2 has been documented in many human tumor types. Moreover, cyclin E overexpression mediates resistance of tumor cells to various therapeutic agents. Recent work has revealed that the role of cyclin E extends well beyond cell proliferation and tumorigenesis, and it may regulate a diverse array of physiological and pathological processes. In this review, we discuss these various cyclin E functions and the potential for therapeutic targeting of cyclin E and cyclin E-CDK2 kinase.

The requirement for cyclin E in c-Myc overexpressing breast cancers

Basal-like triple-negative breast cancers frequently express high levels of c-Myc. This oncoprotein signals to the core cell cycle machinery by impinging on cyclin E. High levels of E-type cyclins (E1 and E2) are often seen in human triple-negative breast tumors. In the current study, we examined the requirement for E-type cyclins in the c-Myc-driven mouse model of breast cancer (MMTV-c-Myc mice). To do so, we crossed cyclin E1- (E1^-/-) and E2- (E2^-/-) deficient mice with MMTV-c-Myc animals, and observed the resulting cyclin E1^-/-/MMTV-c-Myc and cyclin E2^-/-/MMTV-c-Myc females for breast cancer incidence. We found that mice lacking cyclins E1 or E2 developed breast cancers like their cyclin Ewild-type counterparts. In contrast, further reduction of the dosage of E-cyclins in cyclin E1^-/-E2^+/-/MMTV-c-Myc and cyclin E1^+/-E2^-/-/MMTV-c-Myc animals significantly decreased the incidence of mammary carcinomas, revealing arole for E-cyclins in tumor initiation. We also observed that depletion of E-cyclins in human triple-negative breast cancer cell lines halted cell cycle progression, indicating that E-cyclins are essential for tumor cell proliferation. In contrast, we found that the catalytic partner of E-cyclins, the cyclin-dependent kinase 2 (CDK2), is dispensable for the proliferation of these cells. These results indicate that E-cyclins, but not CDK2, play essential and rate-limiting roles in driving the proliferation of c-Myc overexpressing breast cancer cells.

Cyclins, Cyclin-Dependent Kinases, and Cyclin-Dependent Kinase Inhibitors in the Mouse Nervous System

Development and normal physiology of the nervous system require proliferation and differentiation of stem and progenitor cells in a strictly controlled manner. The number of cells generated depends on the type of cell division, the cell cycle length, and the fraction of cells that exit the cell cycle to become quiescent or differentiate. The underlying processes are tightly controlled and modulated by cyclin-dependent kinases (Cdks) and their interactions with cyclins and Cdk inhibitors (CKIs). Studies performed in the nervous system with mouse models lacking individual Cdks, cyclins, and CKIs, or combinations thereof, have shown that many of these molecules control proliferation rates in a cell-type specific and time-dependent manner. In this review, we will provide an update on the in vivo studies on cyclins, Cdks, and CKIs in neuronal and glial tissue. The goal is to highlight their impact on proliferation processes during the development of the peripheral and central nervous system, including and comparing normal and pathological conditions in the adult.

Tissue nonspecific alkaline phosphatase promotes calvarial progenitor cell cycle progression and cytokinesis via Erk1,2

Bone growth is dependent upon the presence of self-renewing progenitor cell populations. While the contribution of Tissue Nonspecific Alkaline Phosphatase (TNAP) enzyme activity in promoting bone mineralization when expressed in differentiated bone forming cells is well understood, little is known regarding the role of TNAP in bone progenitor cells. We previously found diminished proliferation in the calvarial MC3T3E1 cell line upon suppression of TNAP by shRNA, and in calvarial cells and tissues of TNAP^-/- mice. These findings indicate that TNAP promotes cell proliferation. Here we investigate how TNAP mediates this effect. Results show that TNAP is essential for calvarial progenitor cell cycle progression and cytokinesis, and that these effects are mediated by inorganic phosphate and Erk1/2. Levels of active Erk1/2 are significantly diminished in TNAP deficient cranial cells and tissues even in the presence of inorganic phosphate. Moreover, in the absence of TNAP, FGFR2 expression levels are high and FGF2 rescues phospho-Erk1/2 levels and cell cycle abnormalities to a significantly greater extent than inorganic phosphate. Based upon the data we propose a model in which TNAP stimulates Erk1/2 activity via both phosphate dependent and independent mechanisms to promote cell cycle progression and cytokinesis in calvarial bone progenitor cells. Concomitantly, TNAP feeds back to inhibit FGFR2 expression. These results identify a novel mechanism by which TNAP promotes calvarial progenitor cell renewal and indicate that converging pathways exist downstream of FGF signaling and TNAP activity to control craniofacial skeletal development.

Comparative transcriptomics analysis of the river pufferfish (Takifugu obscurus) by tributyltin exposure: Clues for revealing its toxic injury mechanism

TBT residual in water had become a noticeable ecological problem for aquatic ecosystems. The river pufferfish (Takifugu obscurus) is a kind of an anadromous fish species and widely distributed in the East China Sea and the Yellow Sea. Because of the water contamination, the pufferfish wild resource had a sudden decline in recent years. Therefore, the study on the response of pufferfish to the TBT exposure may contribute to reveal toxic injury mechanism of T. obscurus under TBT exposure. In this study, the transcriptional library of T. obscurus liver and gill was constructed and sequenced by an improved Illumina HiseqX10 high-throughput sequencing platform under different concentrations of TBT acute stress. The blood cell numbers distinctly decreased after TBT exposure, showing the adverse effects of TBT invasion and self-adjusting ability of the pufferfish. The production of reactive oxygen species increased, demonstrating the oxidation resistance of T. obscurus when exposed to TBT. The obtained data were compared with the genome data of Takifugu rubripes and transcriptional resource database. On this basis, gene function annotation, analysis and classification were carried out by bioinformatics method, and differential genes related to toxic injury function were screened out. Meanwhile, new toxic related genes and related signal pathways were sought to provide new theoretical guidance for the pathogenesis of T. obscurus exposed to TBT. This study not only enriched the transcriptome data of T. obscurus under environmental stress, but also provided a new research method for the response mechanism of T. obscurus under the stimulation of environmental factors.

Analysis of the putative tumor suppressor gene cdkn2ab in pigment cells and melanoma of Xiphophorus and medaka

In humans, the CDKN2A locus encodes two transcripts, INK4A and ARF. Inactivation of either one by mutations or epigenetic changes is a frequent signature of malignant melanoma and one of the most relevant entry points for melanomagenesis. To analyze whether cdkn2ab, the fish ortholog of CDKN2A, has a similar function as its human counterpart, we studied its action in fish models for human melanoma. Overexpression of cdkn2ab in a Xiphophorus melanoma cell line led to decreased proliferation and induction of a senescence-like phenotype, indicating a melanoma-suppressive function analogous to mammals. Coexpression of Xiphophorus cdkn2ab in medaka transgenic for the mitfa:xmrk melanoma-inducing gene resulted in full suppression of melanoma development, whereas CRISPR/Cas9 knockout of cdkn2ab resulted in strongly enhanced tumor growth. In summary, this provides the first functional evidence that cdkn2ab acts as a potent tumor suppressor gene in fish melanoma models.

Prognostic Values of CCNE1 Amplification and Overexpression in Cancer Patients: A Systematic Review and Meta-analysis

A number of studies revealed that CCNE1 copy number amplification and overexpression (on mRNA or protein expression level) were associated with prognosis of diverse cancers, however, the results were inconsistent among studies. So we conducted this systematic review and meta-analysis to investigate the prognostic values of CCNE1 amplification and overexpression in cancer patients. PubMed, Cochrane library, Embase, CNKI and WanFang database (last update by February 15, 2018) were searched for literatures. A total of 20 studies were included and 5 survival assessment parameters were measured in this study, which included overall survival (OS), progression free survival (PFS), recurrence free survival (RFS), cancer specific survival (CSS) and distant metastasis free survival (DMFS). Pooled analyses showed that CCNE1 amplification might predict poor OS (HR=1.59, 95% CI: 1.05-2.40, p=0.027) rather than PFS (HR=1.49, 95% CI: 0.83-2.67, p=0.177) and RFS (HR=0.982, 95% CI: 0.2376-4.059, p=0.9801) in various cancers; CCNE1 overexpression significantly correlated with poor OS (HR=1.52, 95% CI: 1.05-2.20, p=0.027), PFS (HR=1.20, 95% CI: 1.07-1.34, p=0.001) and DMFS (HR=1.62, 95% CI: 1.09-2.40, p=0.017) rather than RFS (HR=1.68, 95% CI: 0.81-3.50, p=0.164) and CSS (HR=1.54, 95% CI: 0.74-3.18, p=0.246). On the whole, these results indicated CCNE1 amplification and overexpression were associated with poor survival of patients with cancer, suggesting that CCNE1 might be an effective prognostic signature for cancer patients.

Loss of Cyclin E1 attenuates hepatitis and hepatocarcinogenesis in a mouse model of chronic liver injury

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.

Regulation of cellular quiescence by YAP/TAZ and Cyclin E1 in colon cancer cells: Implication in chemoresistance and cancer relapse

Our aim was to decipher the role and clinical relevance of the YAP/TAZ transcriptional coactivators in the regulation of the proliferation/quiescence balance in human colon cancer cells (CCC) and survival after 5FU-based chemotherapy. The prognostic value of YAP/TAZ on tumor relapse and overall survival was assessed in a five-year follow-up study using specimens of liver metastases (n = 70) from colon cancer patients. In 5FU-chemoresistant HT29-5F31 and -chemosensitive HCT116 and RKO CCC, a reversible G0 quiescent state mediated by Cyclin E1 down-regulation was induced by 5FU in 5F31 cells and recapitulated in CCC by either YAP/TAZ or Cyclin E1 siRNAs or the YAP inhibitor Verteporfin. Conversely, the constitutive active YAPdc-S127A mutant restricted cellular quiescence in 5FU-treated 5F31 cells and sustained high Cyclin E1 levels through CREB Ser-133 phosphorylation and activation. In colon cancer patients, high YAP/TAZ level in residual liver metastases correlated with the proliferation marker Ki-67 (p < 0.0001), high level of the YAP target CTGF (p = 0.01), shorter disease-free and overall survival (p = 0.008 and 0.04, respectively). By multivariate analysis and Cox regression model, the YAP/TAZ level was an independent factor of overall (Hazard ratio [CI 95%] 2.06 (1.02–4.16) p = 0.045) and disease-free survival (Hazard ratio [CI 95%] 1.98 (1.01–3.86) p = 0.045). Thus, YAP/ TAZ pathways contribute to the proliferation/quiescence switch during 5FU treatment according to the concerted regulation of Cyclin E1 and CREB. These findings provide a rationale for therapeutic interventions targeting these transcriptional regulators in patients with residual chemoresistant liver metastases expressing high YAP/TAZ levels.

Kinase-independent function of E-type cyclins in liver cancer

E-type cyclins (cyclins E1 and E2) are components of the core cell cycle machinery and are overexpressed in many human tumor types. E cyclins are thought to drive tumor cell proliferation by activating the cyclin-dependent kinase 2 (CDK2). The cyclin E1 gene represents the site of recurrent integration of the hepatitis B virus in the pathogenesis of hepatocellular carcinoma, and this event is associated with strong up-regulation of cyclin E1 expression. Regardless of the underlying mechanism of tumorigenesis, the majority of liver cancers overexpress E-type cyclins. Here we used conditional cyclin E knockout mice and a liver cancer model to test the requirement for the function of E cyclins in liver tumorigenesis. We show that a ubiquitous, global shutdown of E cyclins did not visibly affect postnatal development or physiology of adult mice. However, an acute ablation of E cyclins halted liver cancer progression. We demonstrated that also human liver cancer cells critically depend on E cyclins for proliferation. In contrast, we found that the function of the cyclin E catalytic partner, CDK2, is dispensable in liver cancer cells. We observed that E cyclins drive proliferation of tumor cells in a CDK2- and kinase-independent mechanism. Our study suggests that compounds which degrade or inhibit cyclin E might represent a highly selective therapeutic strategy for patients with liver cancer, as these compounds would selectively cripple proliferation of tumor cells, while sparing normal tissues.

Coatomer subunit beta 2 (COPB2), identified by label-free quantitative proteomics, regulates cell proliferation and apoptosis in human prostate carcinoma cells

Label-free quantitative proteomics has broad applications in the identification of differentially expressed proteins. Here, we applied this method to identify differentially expressed proteins (such as coatomer subunit beta 2 [COPB2]) and evaluated the functions and molecular mechanisms of these proteins in prostate cancer (PCA) cell proliferation. Proteins extracted from surgically resected PCA tissues and adjacent tissues of 3 patients were analyzed by label-free quantitative proteomics. The target protein was confirmed by bioinformatics and GEO dataset analyses. To investigate the role of the target protein in PCA, we used lentivirus-mediated small-interfering RNA (siRNA) to knockdown protein expression in the prostate carcinoma cell line, CWR22RV1 cells and assessed gene and protein expression by reverse transcription quantitative polymerase chain reaction and western blotting. CCK8 and colony formation assays were conducted to evaluate cell proliferation. Cell cycle distributions and apoptosis were assayed by flow cytometry. We selected the differentiation-related protein COPB2 as our target protein based on the results of label-free quantitative proteomics. High expression of COPB2 was found in PCA tissue and was related to poor overall survival based on a public dataset. Cell proliferation was significantly inhibited in COPB2-knockdown CWR22RV1 cells, as demonstrated by CCK8 and colony formation assays. Additionally, the apoptosis rate and percentage of cells in the G₁ phase were increased in COPB2-knockdown cells compared with those in control cells. CDK2, CDK4, and cyclin D1 were downregulated, whereas p21 Waf1/Cip1 and p27 Kip1 were upregulated, affecting the cell cycle signaling pathway. COPB2 significantly promoted CWR22RV1 cell proliferation through the cell cycle signaling pathway. Thus, silencing of COPB2 may have therapeutic applications in PCA.

Mechanism study of isoflavones as an anti-retinoblastoma progression agent

Isoflavones, bioactive soy compounds, are known to exhibit anticancer activities. The present study investigated the anticancer activities of isoflavones on human retinoblastoma Y79 cells in vitro and in vivo. An MTT cell viability assay showed that the half maximal inhibitory concentration value of isoflavones against human retinoblastoma Y79 cells is 1.23 ± 0.42 μmol/l. Flow cytometry analysis indicated that isoflavones blocked G1/S progression. Western blot analysis demonstrated that the mammalian target of rapamycin (mTOR) pathway in Y79 cells was inhibited by isoflavones, with a concomitant decrease in cyclin E1, which accounted for the isoflavone-mediated G1 phase arrest. Isoflavones also inhibited human retinoblastoma growth in vivo; western blot analysis showed inhibition of mTOR and downregulation of cyclin E1 in an isoflavone-treated xenograft mouse model. Together, these results illustrate that isoflavones inhibit retinoblastoma tumour growth in vitro and vivo and that inactivation of the mTOR pathway and downregulation of cyclin E1 is involved in this action. The results of this study suggest that isoflavones could be tested as promising anti-retinoblastoma agent.

Carfilzomib induces G2/M cell cycle arrest in human endometrial cancer cells via upregulation of p21Waf1/Cip1 and p27Kip1

OBJECTIVE

Carfilzomib is a second-generation tetrapeptide epoxyketone proteasome inhibitor used in current clinical therapy of hematologic malignancies. The mechanism of proteasome inhibition in endometrial cancer is not very clear. Carfilzomib inhibition of type I endometrial carcinoma cell proliferation by inducing cell cycle arrest at the G2/M phase was investigated in our study.

MATERIALS AND METHODS

HEC-1-A and Ishikawa endometrial carcinoma cell lines and three tumor cell lines were treated by different concentrations of carfilzomib. Methyl thiazolyl tetrazolium (MTT) assay was used to detect cell viability. Flow cytometry was used to analyze the cell cycle. Western blot was used to detect proteins involved in cell cycle progression.

RESULTS

Carfilzomib impaired viability of myelogenous leukemia cell line K562, cervical cancer cell line HeLa, hepatocellular carcinoma cell line SMCC-7721, and endometrial carcinoma cell lines HEC-1-A and Ishikawa. The cell cycle was arrested at the G2/M phase in carfilzomib-treated HEC-1-A endometrial carcinoma cells, while it was arrested at both S and G2/M phases in carfilzomib-treated Ishikawa cells. Carfilzomib treatment significantly induced p21^{Waf1/ Cip1} and p27, while substantially reduced cyclin D3 and cyclin-dependent kinase 1.

CONCLUSION

This study showed that carfilzomib inhibited endometrial cancer proliferation by upregulating cyclin-dependent kinase inhibitors p21^Waf1/Cip1 and p27^Kip1, and reducing cyclin-dependent kinase 1 to arrest the cell cycle at the G2/M phase.

Analysis of Cyclin E1 Functions in Porcine Preimplantation Embryonic Development by Fluorescence Microscopy

Cyclin E1 (CCNE1) is a core component of cell cycle regulation that drives the transition into the S phase. CCNE1 plays critical roles in cell cycle, cell proliferation, and cellular functions. However, the function of CCNE1 in early embryonic development is limited. In the present study, the function and expression of Ccne1 in porcine early parthenotes were examined. Immunostaining experiments showed that CCNE1 localized in the nucleus, starting at the four-cell stage. Knockdown of Ccne1 by double-stranded RNA resulted in the failure of blastocyst formation and induced blastocyst apoptosis. Ccne1 depletion increased expression of the pro-apoptotic gene Bax, and decreased the expression of Oct4 and the rate of inner cell mass (ICM)/trophectoderm formation. The results indicated that CCNE1 affects blastocyst formation by inducing cell apoptosis and ICM formation during porcine embryonic development.

Proteomic Landscape of Tissue-Specific Cyclin E Functions in Vivo

E-type cyclins (cyclins E1 and E2) are components of the cell cycle machinery that has been conserved from yeast to humans. The major function of E-type cyclins is to drive cell division. It is unknown whether in addition to their 'core' cell cycle functions, E-type cyclins also perform unique tissue-specific roles. Here, we applied high-throughput mass spectrometric analyses of mouse organs to define the repertoire of cyclin E protein partners in vivo. We found that cyclin E interacts with distinct sets of proteins in different compartments. These cyclin E interactors are highly enriched for phosphorylation targets of cyclin E and its catalytic partner, the cyclin-dependent kinase 2 (Cdk2). Among cyclin E interactors we identified several novel tissue-specific substrates of cyclin E-Cdk2 kinase. In proliferating compartments, cyclin E-Cdk2 phosphorylates Lin proteins within the DREAM complex. In the testes, cyclin E-Cdk2 phosphorylates Mybl1 and Dmrtc2, two meiotic transcription factors that represent key regulators of spermatogenesis. In embryonic and adult brains cyclin E interacts with proteins involved in neurogenesis, while in adult brains also with proteins regulating microtubule-based processes and microtubule cytoskeleton. We also used quantitative proteomics to demonstrate re-wiring of the cyclin E interactome upon ablation of Cdk2. This approach can be used to study how protein interactome changes during development or in any pathological state such as aging or cancer.

Cyclin A2 regulates erythrocyte morphology and numbers

Cyclin A2 is an essential gene for development and in haematopoietic stem cells and therefore its functions in definitive erythropoiesis have not been investigated. We have ablated cyclin A2 in committed erythroid progenitors in vivo using erythropoietin receptor promoter-driven Cre, which revealed its critical role in regulating erythrocyte morphology and numbers. Erythroid-specific cyclin A2 knockout mice are viable but displayed increased mean erythrocyte volume and reduced erythrocyte counts, as well as increased frequency of erythrocytes containing Howell-Jolly bodies. Erythroblasts lacking cyclin A2 displayed defective enucleation, resulting in reduced production of enucleated erythrocytes and increased frequencies of erythrocytes containing nuclear remnants. Deletion of the Cdk inhibitor p27^Kip1 but not Cdk2, ameliorated the erythroid defects resulting from deficiency of cyclin A2, confirming the critical role of cyclin A2/Cdk activity in erythroid development. Loss of cyclin A2 in bone marrow cells in semisolid culture prevented the formation of BFU-E but not CFU-E colonies, uncovering its essential role in BFU-E function. Our data unveils the critical functions of cyclin A2 in regulating mammalian erythropoiesis.

Mammalian E-type cyclins control chromosome pairing, telomere stability and CDK2 localization in male meiosis

Loss of function of cyclin E1 or E2, important regulators of the mitotic cell cycle, yields viable mice, but E2-deficient males display reduced fertility. To elucidate the role of E-type cyclins during spermatogenesis, we characterized their expression patterns and produced additional deletions of Ccne1 and Ccne2 alleles in the germline, revealing unexpected meiotic functions. While Ccne2 mRNA and protein are abundantly expressed in spermatocytes, Ccne1 mRNA is present but its protein is detected only at low levels. However, abundant levels of cyclin E1 protein are detected in spermatocytes deficient in cyclin E2 protein. Additional depletion of E-type cyclins in the germline resulted in increasingly enhanced spermatogenic abnormalities and corresponding decreased fertility and loss of germ cells by apoptosis. Profound meiotic defects were observed in spermatocytes, including abnormal pairing and synapsis of homologous chromosomes, heterologous chromosome associations, unrepaired double-strand DNA breaks, disruptions in telomeric structure and defects in cyclin-dependent-kinase 2 localization. These results highlight a new role for E-type cyclins as important regulators of male meiosis.

Understanding the control of meiosis is fundamental to deciphering the origin of male infertility. Although the mechanisms controlling meiosis are poorly understood, key regulators of mitosis, such as cyclins, appear to be critical. In this regard, male mice deficient for cyclin E2 exhibit subfertility and defects in spermatogenesis; however, neither the stages of germ cell differentiation affected nor the responsible mechanisms are known. We investigated how E-type cyclins control male meiosis by examining their expression in spermatogenesis and the consequences that multiple deletions of Ccne1 and Ccne2 alleles produce. Loss of Ccne2 expression increases cyclin E1 levels as a compensatory effect, but there are still meiotic defects and subfertility. Further, loss of one Ccne1 allele in the absence of cyclin E2 results in infertility as does loss of the remaining Ccne1 allele, but with even more severe meiotic abnormalities. We further found that cyclin E1 is involved in sex chromosome synapsis while E2 is involved with homologous pairing and chromosome and telomere integrity. These processes and structures were severely disrupted in absence of both cyclin E1 and E2, uncovering new roles for the E-type cyclins in regulating male meiosis.

A non-redundant function of cyclin E1 in hematopoietic stem cells

A precise balance between quiescence and proliferation is crucial for the lifelong function of hematopoietic stem cells (HSCs). Cyclins E1 and E2 regulate exit from quiescence in fibroblasts, but their role in HSCs remains unknown. Here, we report a non-redundant role for cyclin E1 in mouse HSCs. A long-term culture-initiating cell (LTC-IC) assay indicated that the loss of cyclin E1, but not E2, compromised the colony-forming activity of primitive hematopoietic progenitors. Ccne1^−/− mice showed normal hematopoiesis in vivo under homeostatic conditions but a severe impairment following myeloablative stress induced by 5-fluorouracil (5-FU). Under these conditions, Ccne1^−/− HSCs were less efficient in entering the cell cycle, resulting in decreased hematopoiesis and reduced survival of mutant mice upon weekly 5-FU treatment. The role of cyclin E1 in homeostatic conditions became apparent in aged mice, where HSC quiescence was increased in Ccne1^−/− animals. On the other hand, loss of cyclin E1 provided HSCs with a competitive advantage in bone marrow serial transplantation assays, suggesting that a partial impairment of cell cycle entry may exert a protective role by preventing premature depletion of the HSC compartment. Our data support a role for cyclin E1 in controlling the exit from quiescence in HSCs. This activity, depending on the physiological context, can either jeopardize or protect the maintenance of hematopoiesis.

Cyclin E2 induces genomic instability by mechanisms distinct from cyclin E1

Cyclins E1 drives the initiation of DNA replication, and deregulation of its periodic expression leads to mitotic delay associated with genomic instability. Since it is not known whether the closely related protein cyclin E2 shares these properties, we overexpressed cyclin E2 in breast cancer cells. This did not affect the duration of mitosis, nor did it cause an increase in p107 association with CDK2. In contrast, cyclin E1 overexpression led to inhibition of the APC complex, prolonged metaphase and increased p107 association with CDK2. Despite these different effects on the cell cycle, elevated levels of either cyclin E1 or E2 led to hallmarks of genomic instability, i.e., an increased proportion of abnormal mitoses, micronuclei and chromosomal aberrations. Cyclin E2 induction of genomic instability by a mechanism distinct from cyclin E1 indicates that these two proteins have unique functions in a cancer setting.

Cyclin E1 controls proliferation of hepatic stellate cells and is essential for liver fibrogenesis in mice

Liver fibrogenesis is associated with the transition of quiescent hepatocytes and hepatic stellate cells (HSCs) into the cell cycle. Exit from quiescence is controlled by E-type cyclins (cyclin E1 [CcnE1] and cyclin E2 [CcnE2]). Thus, the aim of the current study was to investigate the contribution of E-type cyclins for liver fibrosis in man and mice. Expression of CcnE1, but not of its homolog, CcnE2, was induced in fibrotic and cirrhotic livers from human patients with different etiologies and in murine wild-type (WT) livers after periodical administration of the profibrotic toxin, CCl(4). To further evaluate the potential function of E-type cyclins for liver fibrogenesis, we repetitively treated constitutive CcnE1(-/-) and CcnE2(-/-) knock-out mice with CCl(4) to induce liver fibrosis. Interestingly, CcnE1(-/-) mice were protected against CCl(4)-mediated liver fibrogenesis, as evidenced by reduced collagen type I α1 expression and the lack of septum formation. In contrast, CcnE2(-/-) mice showed accelerated fibrogenesis after CCl(4) treatment. We isolated primary HSCs from WT, CcnE1(-/-), and CcnE2(-/-) mice and analyzed their activation, proliferation, and survival in vitro. CcnE1 expression in WT HSCs was maximal when they started to proliferate, but decreased after the cells transdifferentiated into myofibroblasts. CcnE1(-/-) HSCs showed dramatically impaired survival, cell-cycle arrest, and strongly reduced expression of alpha smooth muscle actin, indicating deficient HSC activation. In contrast, CcnE2-deficient HSCs expressed an elevated level of CcnE1 and showed enhanced cell-cycle activity and proliferation, compared to WT cells.

CONCLUSIONS

CcnE1 and CcnE2 have antagonistic roles in liver fibrosis. CcnE1 is indispensable for the activation, proliferation, and survival of HSCs and thus promotes the synthesis of extracellular matrix and liver fibrogenesis.

TIGAR induces p53-mediated cell-cycle arrest by regulation of RB-E2F1 complex

BACKGROUND

p53 induces cell-cycle arrest and apoptosis in cancer cells and negatively regulates glycolysis via TIGAR. Glycolysis is crucial for cancer progression although TIGAR provides protection from reactive oxygen species and apoptosis. The relation between TIGAR-mediated inhibition of glycolysis and p53 tumour-suppressor activity is unknown.

METHODS

RT-PCR, western blot, luciferase and chromatin immunoprecipitation assays were used to study TIGAR gene regulation. Co-IPP was used to determine the role of TIGAR protein in regulating the protein-protein interaction between retinoblastoma (RB) and E2F1. MCF-7 tumour xenografts were utilised to study the role of TIGAR in tumour regression.

RESULTS

Our study shows that TIGAR promotes p21-independent, p53-mediated G1-phase arrest in cancer cells. p53 activates the TIGAR promoter only in cells exposed to repairable doses of stress. TIGAR regulates the expression of genes involved in cell-cycle progression; suppresses synthesis of CDK-2, CDK-4, CDK-6, Cyclin D, Cyclin E and promotes de-phosphorylation of RB protein. RB de-phosphorylation stabilises the complex between RB and E2F1 thus inhibiting the entry of cell cycle from G1 phase to S phase.

CONCLUSION

TIGAR mediates de-phosphorylation of RB and stabilisation of RB-E2F1 complex thus delaying the entry of cells in S phase of the cell cycle. Thus, TIGAR inhibits proliferation of cancer cells and increases drug-mediated tumour regression by promoting p53-mediated cell-cycle arrest.

Late intervention with anti-BRAF(V600E) therapy induces tumor regression in an orthotopic mouse model of human anaplastic thyroid cancer

Human anaplastic thyroid cancer (ATC) is a lethal disease with an advanced clinical presentation and median survival of 3 months. The BRAF(V600E) oncoprotein is a potent transforming factor that causes human thyroid cancer cell progression in vitro and in vivo; therefore, we sought to target this oncoprotein in a late intervention model of ATC in vivo. We used the human ATC cell line 8505c, which harbors the BRAF(V600E) and TP53(R248G) mutations. Immunocompromised mice were randomized to receive the selective anti-BRAF(V600E) inhibitor, PLX4720, or vehicle by oral gavage 28 d after tumor implantation, 1 wk before all animals typically die due to widespread metastatic lung disease and neck compressive symptoms in this model. Mice were euthanized weekly to evaluate tumor volume and metastases. Control mice showed progressive tumor growth and lung metastases by 35 d after tumor implantation. At that time, all control mice had large tumors, were cachectic, and were euthanized due to their tumor-related weight loss. PLX4720-treated mice, however, showed a significant decrease in tumor volume and lung metastases in addition to a reversal of tumor-related weight loss. Mouse survival was extended to 49 d in PLX4720-treated animals. PLX4720 treatment inhibited cell cycle progression from 28 d to 49 d in vivo. PLX4720 induces striking tumor regression and reversal of cachexia in an in vivo model of advanced thyroid cancer that harbors the BRAF(V600E) mutation.

Cyclin E constrains Cdk5 activity to regulate synaptic plasticity and memory formation

Cyclin E is a component of the core cell cycle machinery, and it drives cell proliferation by regulating entry and progression of cells through the DNA synthesis phase. Cyclin E expression is normally restricted to proliferating cells. However, high levels of cyclin E are expressed in the adult brain. The function of cyclin E in quiescent, postmitotic nervous system remains unknown. Here we use a combination of in vivo quantitative proteomics and analyses of cyclin E knockout mice to demonstrate that in terminally differentiated neurons cyclin E forms complexes with Cdk5 and controls synapse function by restraining Cdk5 activity. Ablation of cyclin E led to a decreased number of synapses, reduced number and volume of dendritic spines, and resulted in impaired synaptic plasticity and memory formation in cyclin E-deficient animals. These results reveal a cell cycle-independent role for a core cell cycle protein, cyclin E, in synapse function and memory.

Impact of cyclin E overexpression on Smad3 activity in breast cancer cell lines

Smad3, a component of the TGFβ signaling pathway, contributes to G1 arrest in breast cancer cells. Overexpression of the cell cycle mitogen, cyclin E, is associated with poor prognosis in breast cancer, and cyclin E/CDK2 mediated phosphorylation of Smad3 has been linked with inhibition of Smad3 activity. We hypothesized that the biological aggressiveness of cyclin E overexpressing breast cancer cells would be associated with CDK2 phosphorylation and inhibition of the tumor suppressant action of Smad3. Expression constructs containing empty vector, wild type (WT) Smad3, or Smad3 with CDK phosphorylation site mutations were co-transfected with a Smad3-responsive reporter construct into parental, vector control (A1), or cyclin E overexpressing (EL1) MCF7 cells. Smad3 function was evaluated by luciferase reporter assay and mRNA analysis. The impact of a Cdk2 inhibitor and cdk2 siRNA on Smad3 activity was also assessed. Cells expressing Smad3 containing mutations of the CDK phosphorylation sites had higher p15 and p21 and lower c-myc mRNA levels, as well as higher Smad3-responsive reporter activity, compared with controls or cells expressing WT Smad3. Transfection of cdk2 siRNA resulted in a significant increase in Smad3-responsive reporter activity compared with control siRNA; reporter activity was also increased after the treatment with a Cdk2 inhibitor. Thus, cyclin E-mediated inhibition of Smad3 is regulated by CDK2 phosphorylation of the Smad3 protein in MCF7 cells. Inhibition of CDK2 may lead to restoration of Smad3 tumor suppressor activity in breast cancer cells, and may represent a potential treatment approach for cyclin E overexpressing breast cancers.

Expression of cyclin E in postmitotic neurons during development and in the adult mouse brain

Cyclin E, a member of the G1 cyclins, is essential for the G1/S transition of the cell cycle in cultured cells, but its roles in vivo are not fully defined. The present study characterized the spatiotemporal expression profile of cyclin E in two representative brain regions in the mouse, the cerebral and cerebellar cortices. Western blotting showed that the levels of cyclin E increased towards adulthood. In situ hybridization and immunohistochemistry showed the distributions of cyclin E mRNA and protein were comparable in the cerebral cortex and the cerebellum. Immunohistochemistry for the proliferating cell marker, proliferating cell nuclear antigen (PCNA) revealed that cyclin E was expressed by both proliferating and non-proliferating cells in the cerebral cortex at embryonic day 12.5 (E12.5) and in the cerebellum at postnatal day 1 (P1). Subcellular localization in neurons was examined using immunofluorescence and western blotting. Cyclin E expression was nuclear in proliferating neuronal precursor cells but cytoplasmic in postmitotic neurons during embryonic development. Nuclear cyclin E expression in neurons remained faint in newborns, increased during postnatal development and was markedly decreased in adults. In various adult brain regions, cyclin E staining was more intense in the cytoplasm than in the nucleus in most neurons. These data suggest a role for cyclin E in the development and function of the mammalian central nervous system and that its subcellular localization in neurons is important. Our report presents the first detailed analysis of cyclin E expression in postmitotic neurons during development and in the adult mouse brain.

Distinct and redundant functions of cyclin E1 and cyclin E2 in development and cancer

The highly conserved E-type cyclins are core components of the cell cycle machinery, facilitating the transition into S phase through activation of the cyclin dependent kinases, and assembly of pre-replication complexes on DNA. Cyclin E1 and cyclin E2 are assumed to be functionally redundant, as cyclin E1-/- E2-/- mice are embryonic lethal while cyclin E1-/- and E2-/- single knockout mice have primarily normal phenotypes. However more detailed studies of the functions and regulation of the E-cyclins have unveiled potential additional roles for these proteins, such as in endoreplication and meiosis, which are more closely associated with either cyclin E1 or cyclin E2. Moreover, expression of each E-cyclin can be independently regulated by distinct transcription factors and microRNAs, allowing for context-specific expression. Furthermore, cyclins E1 and E2 are frequently expressed independently of one another in human cancer, with unique associations to signatures of poor prognosis. These data imply an absence of co-regulation of cyclins E1 and E2 during tumorigenesis and possibly different contributions to cancer progression. This is supported by in vitro data identifying divergent regulation of the two genes, as well as potentially different roles in vivo.

Phosphorylation by Cdk2 is required for Myc to repress Ras-induced senescence in cotransformation

The MYC and RAS oncogenes are frequently activated in cancer and, together, are sufficient to transform rodent cells. The basis for this cooperativity remains unclear. We found that although Ras interfered with Myc-induced apoptosis, Myc repressed Ras-induced senescence, together abrogating two main barriers of tumorigenesis. Inhibition of cellular senescence required phosphorylation of Myc at Ser-62 by cyclin E/cyclin-dependent kinase (Cdk) 2. Cdk2 interacted with Myc at promoters, where it affected Myc-dependent regulation of genes, including Bmi-1, p16, p21, and hTERT, which encode proteins known to control senescence. Repression of senescence by Myc was abrogated by the Cdk inhibitor p27Kip1, which is induced by antiproliferative signals like IFN-gamma or by pharmacological inhibitors of Cdk2 but not by inhibitors of other Cdks. In contrast, a phospho-mimicking Myc-S62D mutant was resistant to these manipulations. Inhibition of cyclin E/Cdk2 reversed the senescence-associated gene expression pattern imposed by Myc/cyclin E/Cdk2. This indicates a role of Cdk2 as a transcriptional cofactor and activator of the antisenescence function of Myc and provides mechanistic insight into the Myc-p27Kip1 antagonism. Finally, our findings highlight that pharmacological inhibition of Cdk2 activity is a potential therapeutical principle for cancer therapy, in particular for tumors with activated Myc or Ras.

Estrogen regulation of cyclin E2 requires cyclin D1 but not c-Myc

During estrogen-induced proliferation, c-Myc and cyclin D1 initiate independent pathways that activate cyclin E1-Cdk2 by sequestration and/or downregulation of the CDK inhibitor p21(Waf1/Cip1), without significant increases in cyclin E1 protein levels. In contrast, cyclin E2 undergoes a marked increase in expression, which occurs within 9 to 12 h of estrogen treatment of antiestrogen-pretreated MCF-7 breast cancer cells. Both E cyclins are important to estrogen action, as small interfering RNA (siRNA)-mediated knockdown of either cyclin E1 or cyclin E2 attenuated estrogen-mediated proliferation. Inducible expression of cyclin D1 upregulated cyclin E2, while siRNA-mediated knockdown of cyclin D1 attenuated estrogen effects on cyclin E2. However, manipulation of c-Myc levels did not profoundly affect cyclin E2. Cyclin E2 induction by estrogen was accompanied by recruitment of E2F1 to the cyclin E1 and E2 promoters, and cyclin D1 induction was sufficient for E2F1 recruitment. siRNA-mediated knockdown of the chromatin remodelling factor CHD8 prevented cyclin E2 upregulation. Together, these data indicate that cyclin E2-Cdk2 activation by estrogen occurs via E2F- and CHD8-mediated transcription of cyclin E2 downstream of cyclin D1. This contrasts with the predominant regulation of cyclin E1-Cdk2 activity via CDK inhibitor association downstream of both c-Myc and cyclin D1 and indicates that cyclins E1 and E2 are not always coordinately regulated.

Cdc6 and cyclin E2 are PTEN-regulated genes associated with human prostate cancer metastasis

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is frequently inactivated in metastatic prostate cancer, yet the molecular consequences of this and their association with the metastatic phenotype are incompletely understood. We performed transcriptomic analysis and identified genes altered by conditional PTEN reexpression in C4-2, a human metastatic prostate cancer cell line with inactive PTEN. PTEN-regulated genes were disproportionately represented among genes altered in human prostate cancer progression and metastasis but not among those associated with tumorigenesis. From the former set, we identified two novel putative PTEN targets, cdc6 and cyclin E2, which were overexpressed in metastatic human prostate cancer and up-regulated as a function of PTEN depletion in poorly metastatic DU145 human prostate cancer cells harboring a wild type PTEN. Inhibition of cdc6 and cyclin E2 levels as a consequence of PTEN expression was associated with cell cycle G(1) arrest, whereas use of PTEN activity mutants revealed that regulation of these genes was dependent on PTEN lipid phosphatase activity. Computational and promoter-reporter evaluations implicated the E2F transcription factor in PTEN regulation of cdc6 and cyclin E2 expression. Our results suggest a hypothetical model whereby PTEN loss upregulates cell cycle genes such as cdc6 and cyclin E2 that in turn promote metastatic colonization at distant sites.

Involvement of cyclins in mammalian spermatogenesis

Mammalian spermatogenesis is a complicated developmental process by which undifferentiated germ cells continuously produce mature sperm throughout a lifetime. Stringent control of the cell cycle during spermatogenesis is required to ensure self-renewal of male germ line cells and differentiation of appropriate numbers of cells for the various lineages. Cyclins are key factors of cell cycle regulation and play crucial roles in governing both the mitotic and meiotic divisions that characterize spermatogenesis. Abnormal expression of some types of cyclins in the testes can induce apoptosis, infertility, testicular tumors, and other problems related to spermatogenesis in mammals. In this review, available data regarding cellular and molecular regulation of several different types of cyclins during mammalian spermatogenesis are collected and further discussed.

Genome-wide and locus-specific DNA hypomethylation in G9a deficient mouse embryonic stem cells

In the mammalian genome, numerous CpG-rich loci define tissue-dependent and differentially methylated regions (T-DMRs). Euchromatin from different cell types differs in terms of its tissue-specific DNA methylation profile as defined by these T-DMRs. G9a is a euchromatin-localized histone methyltransferase (HMT) and catalyzes methylation of histone H3 at lysines 9 and 27 (H3-K9 and -K27). To test whether HMT activity influences euchromatic cytosine methylation, we analyzed the DNA methylation status of approximately 2000 CpG-rich loci, which are predicted in silico, in G9a(-/-) embryonic stem cells by restriction landmark genomic scanning (RLGS). While the RLGS profile of wild-type cells contained about 1300 spots, 32 new spots indicating DNA demethylation were seen in the profile of G9a(-/-) cells. Virtual-image RLGS (Vi-RLGS) allowed us to identify the genomic source of ten of these spots. These were confirmed to be cytosine demethylated, not just at the Not I site detected by the RLGS but extending over several kilobase pairs in cis. Chromatin immunoprecipitation (ChIP) confirmed these loci to be targets of G9a, with decreased H3-K9 and/or -K27 dimethylation in the G9a(-/-) cells. These data indicate that G9a site-selectively contributes to DNA methylation.

RB and cell cycle progression

The Rb protein is a tumor suppressor, which plays a pivotal role in the negative control of the cell cycle and in tumor progression. It has been shown that Rb protein (pRb) is responsible for a major G1 checkpoint, blocking S-phase entry and cell growth. The retinoblastoma family includes three members, Rb/p105, p107 and Rb2/p130, collectively referred to as 'pocket proteins'. The pRb protein represses gene transcription, required for transition from G1 to S phase, by directly binding to the transactivation domain of E2F and by binding to the promoter of these genes as a complex with E2F. pRb represses transcription also by remodeling chromatin structure through interaction with proteins such as hBRM, BRG1, HDAC1 and SUV39H1, which are involved in nucleosome remodeling, histone acetylation/deacetylation and methylation, respectively. Loss of pRb functions may induce cell cycle deregulation and so lead to a malignant phenotype. Gene inactivation of pRB through chromosomal mutations is one of the principal reasons for retinoblastoma tumor development. Functional inactivation of pRb by viral oncoprotein binding is also shown in many neoplasias such as cervical cancer, mesothelioma and AIDS-related Burkitt's lymphoma.

Development of a Screening Assay for Surrogate Markers of Chk1 Inhibitor-Induced Cell Cycle Release

Chk1 is a key regulator of the S and G2/M checkpoints and is activated following DNA damage by agents such as the topoisomerase I inhibitor camptothecin (CPT). It has been proposed that Chk1 inhibitors used in combination with such a DNA damaging agent to treat tumors would potentiate cytotoxicity and increase the therapeutic index, particularly in tumors lacking functional p53. The aim of this study was to determine whether gene expression analysis could be used to inform lead optimization of a novel series of Chk1 inhibitors. The candidate small-molecule Chk1 inhibitors were used in combination with CPT to identify potential markers of functional Chk1 inhibition, as well as resulting cell cycle progression, using cDNA-based microarrays. Differential expression of several of these putative marker genes was further validated by RT-PCR for use as a medium-throughput assay. In the presence of DNA damage, Chk1 inhibitors altered CPT-dependent effects on the expression of cell cycle and DNA repair genes in a manner consistent with a Chk1-specific mechanism of action. Furthermore, differential expression of selected marker genes, cyclin E2, EGR1, and DDIT3, was dose dependent for Chk1 inhibition. RT-PCR results for these genes following treatment with a panel of Chk1 inhibitors showed a strong correlation between marker gene response and the ability of each compound to abrogate cell cycle arrest in situ following CPT-induced DNA damage. These results demonstrate the utility of global expression analysis to identify surrogate markers, providing an alternative method for rapid compound characterization to support advancement decisions in early drug discovery.

Mouse models of cell cycle regulators: new paradigms

In yeast, a single cyclin-dependent kinase (Cdk) is able to regulate diverse cell cycle transitions (S and M phases) by associating with multiple stage-specific cyclins. The evolution of multicellular organisms brought additional layers of cell cycle regulation in the form of numerous Cdks, cyclins and Cdk inhibitors to reflect the higher levels of organismal complexity. Our current knowledge about the mammalian cell cycle emerged from early experiments using human and rodent cell lines, from which we built the current textbook model of cell cycle regulation. In this model, the functions of different cyclin/Cdk complexes were thought to be specific for each cell cycle phase. In the last decade, studies using genetically engineered mice in which cell cycle regulators were targeted revealed many surprises. We discovered the in vivo functions of cell cycle proteins within the context of a living animal and whether they are essential for animal development. In this review, we discuss first the textbook model of cell cycle regulation, followed by a global overview of data obtained from different mouse models. We describe the similarities and differences between the phenotypes of different mouse models including embryonic lethality, sterility, hematopoietic, pancreatic, and placental defects. We also describe the role of key cell cycle regulators in the development of tumors in mice, and the implications of these data for human cancer. Furthermore, animal models in which two or more genes are ablated revealed which cell cycle regulators interact genetically and functionally complement each other. We discuss for example the interaction of cyclin D1 and p27 and the compensation of Cdk2 by Cdc2. We also focus on new functions discovered for certain cell cycle regulators such as the regulation of S phase by Cdc2 and the role of p27 in regulating cell migration. Finally, we conclude the chapter by discussing the limitations of animal models and to what extent can the recent findings be reconciled with the past work to come up with a new model for cell cycle regulation with high levels of redundancy among the molecular players.

The p110 isoform of the CDP/Cux transcription factor accelerates entry into S phase

The CDP/Cux transcription factor was previously found to acquire distinct DNA binding and transcriptional properties following a proteolytic processing event that takes place at the G1/S transition of the cell cycle. In the present study, we have investigated the role of the CDP/Cux processed isoform, p110, in cell cycle progression. Populations of cells stably expressing p110 CDP/Cux displayed a faster division rate and reached higher saturation density than control cells carrying the empty vector. p110 CDP/Cux cells reached the next S phase faster than control cells under various experimental conditions: following cell synchronization in G0 by growth factor deprivation, synchronization in S phase by double thymidine block treatment, or enrichment in G2 by centrifugal elutriation. In each case, duration of the G1 phase was shortened by 2 to 4 h. Gene inactivation confirmed the role of CDP/Cux as an accelerator of cell cycle progression, since mouse embryo fibroblasts obtained from Cutl1z/z mutant mice displayed a longer G1 phase and proliferated more slowly than their wild-type counterparts. The delay to enter S phase persisted following immortalization by the 3T3 protocol and transformation with H-RasV12. Moreover, CDP/Cux inactivation hindered both the formation of foci on a monolayer and tumor growth in mice. At the molecular level, expression of both cyclin E2 and A2 was increased in the presence of p110 CDP/Cux and decreased in its absence. Overall, these results establish that p110 CDP/Cux functions as a cell cycle regulator that accelerates entry into S phase.

The importance of being big

The ultimate stem cell, the oocyte, is frequently very large. For example, Drosophila and Xenopus oocytes are approximately 10(5) times larger than normal somatic cells. Importantly, once the large oocytes are fertilized, the resulting embryonic cells proliferate rapidly. Moreover, these divisions occur in the absence of cell growth and are not governed by normal cell cycle controls. Observations suggest that mitogens and cell growth signals modulate proliferation by upregulating G1-phase cyclins, which in turn promote cell division. Like embryonic cells, the proliferation of cancer cells is largely independent of mitogens and growth factors. This occurs, in part, because many proteins that are known to modulate G1-phase cyclin activity are frequently mutated in cancer cells. Interestingly, we have found that both the expression and the activity of G1-phase cyclins is modulated by growth rate and cell size in yeast. These and other data suggest that proliferative capacity correlates with cell size. Thus, a major goal of our laboratory is to use yeast to investigate the relationship between proliferation rate, G1-phase cyclins, growth rate, and cell size. The elucidation of this relationship will help clarify the role of cell size in promoting proliferation in both normal and cancer cells.

Aberrant cell cycle regulation in cervical carcinoma

Carcinoma of the uterine cervix is one of the most common malignancies among women worldwide. Human papillomaviruses (HPV) have been identified as the major etiological factor in cervical carcinogenesis. However, the time lag between HPV infection and the diagnosis of cancer indicates that multiple steps, as well as multiple factors, may be necessary for the development of cervical cancer. The development and progression of cervical carcinoma have been shown to be dependent on various genetic and epigenetic events, especially alterations in the cell cycle checkpoint machinery. In mammalian cells, control of the cell cycle is regulated by the activity of cyclin-dependent kinases (CDKs) and their essential activating coenzymes, the cyclins. Generally, CDKs, cyclins, and CDK inhibitors function within several pathways, including the p16(INK4A)-cyclin D1-CDK4/6-pRb-E2F, p21(WAF1)- p27(KIP1)-cyclinE-CDK2, and p14(ARF)-MDM2-p53 pathways. The results from several studies showed aberrant regulation of several cell cycle proteins, such as cyclin D, cyclin E, p16(INK4A), p21(WAF1), and p27(KIP1), as characteristic features of HPV- infected and HPV E6/E7 oncogene-expressing cervical carcinomas and their precursors. These data suggested further that interactions of viral proteins with host cellular proteins, particularly cell cycle proteins, are involved in the activation or repression of cell cycle progression in cervical carcinogenesis.

Identification of early-responsive genes correlated to valproic acid-induced neural tube defects in mice

BACKGROUND

Valproic acid (VPA) causes the failure of neural tube closure in newborn mice. However, the molecular mechanism of its teratogenesis is unknown. This study was conducted to investigate the genomewide effects of VPA disruption of normal neural tube development in mice.

METHODS

Microarray analysis was performed on the head part of NMRI mouse embryos treated for 1 hr with VPA on gestational day (GD) 8. Subsequently, we attempted to isolate genes that changed in correlation with the teratogenic action of VPA by employing reduced teratogenic VPA analogs, valpromide (VPD) and valnoctamide (VCD), in a real-time PCR study.

RESULTS

Microarray results demonstrated that during neurulation, many genes, some of whose functions are known and some unknown, were either increased or decreased after VPA injection. Some genes were affected by VPD or VCD in the same way as VPA, but others were not changed by the analogs. In this way, our system identified 11 increased and 20 decreased genes. Annotation analysis revealed that the increased genes included gadd45b, ier5, per1, phfl3, pou3f1, and sox4, and the decreased genes included ccne2, ccnl, gas5, egr2, sirt1, and zfp105.

CONCLUSIONS

These findings demonstrate that expression changes in genes having roles in the cell cycle and apoptosis pathways of neural tube cells were strongly expected to relate to the teratogenic, but not antiepileptic, activity of VPA. Our approach has allowed the expansion of the catalog of molecules immediately affected by VPA in the developing neural tube.

Cyclins and cdks in development and cancer: a perspective

A fundamental aspect of cancer is dysregulated cell cycle control. Unlike normal cells that only proliferate when compelled to do so by developmental or other mitogenic signals in response to tissue growth needs, the proliferation of cancer cells proceeds essentially unchecked. This does not mean that cancer cell cycles are necessarily different from those found in normal cycling cells, but rather implies that cancer cells proliferate because they are no longer subject to proliferation-inhibitory influences arising from the stroma or from gene expression pattern changes consequent to 'terminal' differentiation, nor do they necessarily require extrinsic growth factors to recruit them into or maintain their proliferative state. Finally, cancer cells have also often avoided normal controls linked to cell cycle progression that halt proliferation in the presence of damaged DNA or other physiological insults. The result of these alterations is the inappropriate proliferation commonly associated with cancerous tumor formation. This review will summarize the current understanding of dysregulation of the G0/G1-to-S-phase transition in cancer cells, with particular emphasis on recent in vivo studies that suggest a need to rethink existing models of cell cycle control in development and tumorigenesis.

Differential roles of E-type cyclins during transformation of murine E2F-1-deficient cells

Deregulation of retinoblastoma gene product (pRB) is a hallmark of cancer, which acts as a transcriptional repressor by targeting E2F transcription factors. A transcription factor E2F-1 is not only important for S phase entry of cell cycle, but also stimulates gene expression of pro-apoptotic molecules. To investigate roles of E2F-1 and its target genes in cellular transformation, we studied murine E2F-1-deficient embryonic fibroblasts. Compared with control wild-type cells, E2F-1-deficient cells at early passages were less sensitive to exposure to gamma-radiation and showed an increase of colony formation, while their growth was slow. After sequential passages, the growth of E2F-1-deficient cells reached closely to that of wild-type cells. Immunoblot study of E2F target genes showed that multiple passages of E2F-1-deficient cells resulted in preferential increase of cyclin E2 expression. Furthermore, carcinogenicity study using N-nitrosomethylbenzylamine demonstrated that, compared to wild-type mice, fore-stomach tumors in E2F-1-deficient mice expressed an increased amount of cyclin E2, but not cyclin E1. Taken together, the present study shows that differential roles of E-type cyclins are involved at least partially in the process of cellular transformation, supporting the concept of important roles of the E2F regulatory pathway in carcinogenesis.

The tumor suppressor DAL-1/4.1B modulates protein arginine N-methyltransferase 5 activity in a substrate-specific manner

We previously identified DAL-1/4.1B as a growth suppression protein involved in the pathogenesis of lung, breast, and meningioma tumors. Using yeast two-hybrid interaction cloning, protein arginine N-methyltransferase 3 (PRMT3) was originally identified as a DAL-1/4.1B-interacting protein. PRMTs catalyze the sequential transfer of methyl groups from S-adeonsyl-l-methionine to the guanidino nitrogens of arginine residues in proteins, the effect of which can include regulation of signal transduction, transcription regulation, and RNA transport, suggesting that modulating this event may have far-reaching impact. In this study, we assessed the impact of DAL-1/4.1B binding on the activity of another family member, PRMT5, both in vitro and in cells. In contrast to PRMT3, DAL-1/4.1B was found to mediate PRMT5 by either inhibiting (Sm proteins) or enhancing (myelin basic protein) protein methylation. We propose that this interaction between a tumor suppressor and a post-translational methylation enzyme is of biological importance in controlling tumorigenesis.

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

The NF-kappaB/IkappaB signaling system: a molecular target in breast cancer therapy

The nuclear factor kappaB (NFkappaB) superfamily of eukaryotic transcription factors plays an important role in carcinogenesis. NF-kappaB and its regulators are linked to various signal transduction pathways as well as transcriptional activation events that mediate critical stages of cell proliferation. These intracellular signaling processes are thought to regulate chromatin structure to accommodate transcription, apoptosis, cell-cycle control, and cell transformation. In this capacity, uncontrolled or aberrant NF-kappaB activity may, in part, be responsible for breast cancer progression. Constitutive NF-kappaB expression may predict the metastatic potential of breast tumors, indicating early use of adjuvant therapy and suggesting NF-kappaB inhibition as a novel treatment. In this review, we discuss the regulatory mechanisms and physiological significance of NF-kappaB activation, and highlight recent advances in the development of NF-kappaB as an integral mediator of mammary carcinogenesis.

The A-type cyclins and the meiotic cell cycle in mammalian male germ cells

There are two mammalian A-type cyclins, cyclin Al and A2. While cyclin A1 is limited to male germ cells, cyclin A2 is widely expressed. Cyclin A2 promotes both Gl/S and G2/M transitions in somatic cells and cyclin A2-deficient mice are early embryonic lethal. We have shown that cyclin Al is essential for passage of spermatocytes into meiosis I (MI) by generating mice null for the cyclin A1 gene Ccna1. Both Ccna1(-/-) males and females were healthy but the males were sterile because of a cell cycle arrest before MI. This arrest was associated with desynapsis abnormalities, low M-phase promoting factor activity, and apoptosis. We have now determined that human cyclin A1 is expressed in similar stages of spermatogenesis and are exploring its role in human male infertility and whether it may be a novel target for new approaches for male contraception.

Sonic hedgehog signaling regulates Gli3 processing, mesenchymal proliferation, and differentiation during mouse lung organogenesis

Lack of Sonic hedgehog (Shh) signaling, mediated by the Gli proteins, leads to severe pulmonary hypoplasia. However, the precise role of Gli genes in lung development is not well established. We show Shh signaling prevents Gli3 proteolysis to generate its repressor forms (Gli3R) in the developing murine lung. In Shh(-/-) or cyclopamine-treated wild-type (WT) lung, we found that Gli3R level is elevated, and this upregulation appears to contribute to defects in proliferation and differentiation observed in the Shh(-/-) mesenchyme, where Gli3 is normally expressed. In agreement, we found Shh(-/-);Gli3(-/-) lungs exhibit enhanced growth potential. Vasculogenesis is also enhanced; in contrast, bronchial myogenesis remains absent in Shh(-/-);Gli3(-/-) compared with Shh(-/-) lungs. Genes upregulated in Shh(-/-);Gli3(-/-) relative to Shh(-/-) lung include Wnt2 and, surprisingly, Foxf1 whose expression has been reported to be Shh-dependent. Cyclins D1, D2, and D3 antibody labelings also reveal distinct expression patterns in the normal and mutant lungs. We found significant repression of Tbx2 and Tbx3, both linked to inhibition of cellular senescence, in Shh(-/-) and partial derepression in Shh(-/-); Gli3(-/-) lungs, while Tbx4 and Tbx5 expressions are less affected in the mutants. Our findings shed light on the role of Shh signaling on Gli3 processing in lung growth and differentiation by regulating several critical genes.

The cell cycle and how it is steered by Kaposi's sarcoma-associated herpesvirus cyclin

A timely coordination of cellular DNA synthesis and division cycles is governed by the temporal and spatial activation of cyclin-dependent kinases (Cdks). The primary regulation of Cdk activation is through binding to partner cyclin proteins. Several gammaherpesviruses encode a viral homologue of cellular cyclin D, which may function to deregulate host cell cycle progression. One of these is encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) and is called K cyclin or viral cyclin (v-cyclin). v-Cyclin is expressed in most of the malignant cells that are associated with KSHV infection in humans, labelling v-cyclin as a putative viral oncogene. Here are described some of the major structural and functional properties of mammalian cyclin/Cdk complexes, some of which are phenocopied by v-cyclin. In addition, the molecular events leading to orderly progression through the G1/S and G/M cell cycle phases are reviewed. This molecular picture serves as a platform on which to explain v-cyclin-specific functional properties. Interesting but largely speculative issues concern the interplay between v-cyclin-mediated cell cycle deregulation and molecular progression of KSHV-associated neoplasms.