Transgenerational propagation and quantitative maintenance of paternal centromeres depends on Cid/Cenp-A presence in Drosophila sperm

In Drosophila melanogaster, as in many animal and plant species, centromere identity is specified epigenetically. In proliferating cells, a centromere-specific histone H3 variant (CenH3), named Cid in Drosophila and Cenp-A in humans, is a crucial component of the epigenetic centromere mark. Hence, maintenance of the amount and chromosomal location of CenH3 during mitotic proliferation is important. Interestingly, CenH3 may have different roles during meiosis and the onset of embryogenesis. In gametes of Caenorhabditis elegans, and possibly in plants, centromere marking is independent of CenH3. Moreover, male gamete differentiation in animals often includes global nucleosome for protamine exchange that potentially could remove CenH3 nucleosomes. Here we demonstrate that the control of Cid loading during male meiosis is distinct from the regulation observed during the mitotic cycles of early embryogenesis. But Cid is present in mature sperm. After strong Cid depletion in sperm, paternal centromeres fail to integrate into the gonomeric spindle of the first mitosis, resulting in gynogenetic haploid embryos. Furthermore, after moderate depletion, paternal centromeres are unable to re-acquire normal Cid levels in the next generation. We conclude that Cid in sperm is an essential component of the epigenetic centromere mark on paternal chromosomes and it exerts quantitative control over centromeric Cid levels throughout development. Hence, the amount of Cid that is loaded during each cell cycle appears to be determined primarily by the preexisting centromeric Cid, with little flexibility for compensation of accidental losses.

Mitochondrial hyperfusion induced by loss of the fission protein Drp1 causes ATM-dependent G2/M arrest and aneuploidy through DNA replication stress

Mitochondrial fission and fusion cycles are integrated with cell cycle progression. In this paper, we demonstrate that the inhibition of mitochondrial fission protein Drp1 causes an unexpected delay in G2/M cell cycle progression and aneuploidy. In investigating the underlying molecular mechanism, we revealed that inhibiting Drp1 triggers replication stress, which is mediated by a hyperfused mitochondrial structure and unscheduled expression of cyclin E in the G2 phase. This persistent replication stress then induces an ATM-dependent activation of the G2 to M transition cell cycle checkpoint. Knockdown of ATR, an essential kinase in preventing replication stress, significantly enhanced DNA damage and cell death of Drp1-deficienct cells. Persistent mitochondrial hyperfusion also induces centrosomal overamplification and chromosomal instability, which are causes of aneuploidy. Analysis using cells depleted of mitochondrial DNA revealed that these events are not mediated by the defects in mitochondrial ATP production and reactive oxygen species (ROS) generation. Thus dysfunctional mitochondrial fission directly induces genome instability by replication stress, which then initiates the DNA damage response. Our findings provide a novel mechanism that contributes to the cellular dysfunction and diseases associated with altered mitochondrial dynamics.

Contribution of stochastic partitioning at human embryonic stem cell division to NANOG heterogeneity

Heterogeneity is an often unappreciated characteristic of stem cell populations yet its importance in fate determination is becoming increasingly evident. Although gene expression noise has received greater attention as a source of non-genetic heterogeneity, the effects of stochastic partitioning of cellular material during mitosis on population variability have not been researched to date. We examined self-renewing human embryonic stem cells (hESCs), which typically exhibit a dispersed distribution of the pluripotency marker NANOG. In conjunction with our experiments, a multiscale cell population balance equation (PBE) model was constructed accounting for transcriptional noise and stochastic partitioning at division as sources of population heterogeneity. Cultured hESCs maintained time-invariant profiles of size and NANOG expression and the data were utilized for parameter estimation. Contributions from both sources considered in this study were significant on the NANOG profile, although elimination of the gene expression noise resulted in greater changes in the dispersion of the NANOG distribution. Moreover, blocking of division by treating hESCs with nocodazole or colcemid led to a 39% increase in the average NANOG content and over 68% of the cells had higher NANOG level than the mean NANOG expression of untreated cells. Model predictions, which were in excellent agreement with these findings, revealed that stochastic partitioning accounted for 17% of the total noise in the NANOG profile of self-renewing hESCs. The computational framework developed in this study will aid in gaining a deeper understanding of how pluripotent stem/progenitor cells orchestrate processes such as gene expression and proliferation for maintaining their pluripotency or differentiating along particular lineages. Such models will be essential in designing and optimizing efficient differentiation strategies and bioprocesses for the production of therapeutically suitable stem cell progeny.

MicroRNA-100 is a potential molecular marker of non-small cell lung cancer and functions as a tumor suppressor by targeting polo-like kinase 1

BACKGROUND

Polo-like kinase 1 (PLK1) is highly expressed in many human cancers and regulates critical steps in mitotic progression. Previously, we have reported that PLK1 was overexpressed in non-small cell lung cancer (NSCLC), but the underlying molecular mechanisms are not well understood. By using microRNA (miR) target prediction algorithms, we identified miR-100 that might potentially bind the 3'-untranslated region of PLK1 transcripts. The purpose of this study was to investigate the roles of miR-100 and its association with PLK1 in NSCLC development.

METHODS

Taqman real-time quantitative RT-PCR assay was performed to detect miR-100 expression 10 NSCLC tissues and corresponding nontumor tissues. Additionally, the expression of miR-100 in 110 NSCLC tissues and its correlation with clinicopathological factors or prognosis of patients was analyzed. Finally, the effects of miR-100 expression on growth, apoptosis and cell cycle of NSCLC cells by posttranscriptionally regulating PLK1 expression were determined.

RESULTS

MiR-100 was significantly downregulated in NSCLC tissues, and low miR-100 expression was found to be closely correlated with higher clinical stage, advanced tumor classification and lymph node metastasis of patients. The overall survival of NSCLC patients with low miR-100 was significantly lower than that of those patients with high miR-100, and univariate and multivariate analyses indicated that low miR-100 expression might be a poor prognostic factor. Also, miR-100 mimics could lead to growth inhibition, G2/M cell cycle arrest and apoptosis enhancement in NSCLC cells. Meanwhile, miR-100 mimics could significantly inhibit PLK1 mRNA and protein expression and reduce the luciferase activity of a PLK1 3' untranslated region-based reporter construct in A549 cells. Furthermore, small interfering RNA (siRNA)-mediated PLK1 downregulation could mimic the effects of miR-100 mimics while PLK1 overexpression could partially rescue the phenotypical changes of NSCLC cells induced by miR-100 mimics.

CONCLUSIONS

Our findings indicate that low miR-100 may be a poor prognostic factor for NSCLC patients and functions as a tumor suppressor by posttranscriptionally regulating PLK1 expression.

DNA-PKcs-dependent NHEJ pathway supports the progression of topoisomerase II poison-induced chromosome aberrant cells

The role of DNA double strand break (DSB) repair pathways, non-homologous end joining (NHEJ), and homologous recombination (HR) was evaluated to prevent the chromosome instability induced by the topoisomerase II (Top2) poisons, idarubicin, and etoposide in Chinese hamster cell lines. XR-C1 (DNA-PKcs deficient) and V-C8 (BRCA2 deficient) showed higher sensitivity to increased concentrations of Top2 poisons compared with their normal counterparts, CHO9 and V79. Both proficient and deficient cells exhibited a marked DSB induction in all phases of the cell cycle. Additionally, deficient cells showed persistent DNA damage 24 hr post-treatment. Chromosomal aberrations increased in the first mitosis following Top2 poison-treatments in G1 or G2 in proficient and deficient cells. CHO9 and V79 demonstrated chromosome and chromatid exchanges following treatments in G1 and G2 phases, respectively. Deficient cells showed high frequencies of chromatid exchanges following treatments in G1 and G2. Simultaneously, we analyzed the micronuclei (MN) induction in interphase cells after treatments in G1, S, or G2 of the previous cell cycle. Both Top2 poisons induced an important increase in MN in CHO9, V79, and V-C8 cells. XR-C1 exhibited an increased MN frequency when cells were treated in G1 phase but not in S or G2. This MN reduction was due to a cell accumulation at G2/M and death in G2-treated cells. Our data suggest that NHEJ and HR operate differentially throughout the cell cycle to protect from Top2 poison-induced chromosome instability, and that DNA-PKcs-dependent NHEJ pathway allows the survival of chromosome damaged cells during S/G2 to the next interphase.

Chromosomal radiosensitivity in Ukrainian breast cancer patients and healthy individuals

AIM

Recent studies showed that increased chromosomal damage induced by ionizing radiation is observed among patients with different tumor types. The aim of the study was evaluation of chromosomal radiosensitivity in breast cancer (BC) patients (n = 37) and healthy women (n = 44).

METHODS

Chromosomal radiosensitivity was assessed with G0 and G2 assay. For G0 assay lymphocytes were exposed in vitro to 1,5 Gy of X-rays before culture setting. For G2 assay lymphocytes were irradiated with 0,5 Gy of X-rays after 47 h of incubation.

RESULTS

Significant differences in mean scores both of G0 and G2 assay between breast cancer patients and controls were observed indicating the increased chromosomal radiosensitivity of lymphocytes of cancer patients. 11% of healthy women and 38% of BC patients were determined to be radiosensitive with G2 assay.

CONCLUSION

Obtained results support the concept of association between elevated individual G2 chromosomal radiosensitivity and predisposition to BC.

Menthol induces cell-cycle arrest in PC-3 cells by down-regulating G2/M genes, including polo-like kinase 1

Menthol, a naturally occurring monoterpene, is used in foods, cosmetic products, and topical therapeutic preparations. It also exerts cytotoxic activity against several cancer cell types, including prostate cancer cells. However, little is known about the mechanism of menthol action against prostate cancer cells. In this study, we investigated the effect of menthol on the gene expression profile of PC-3 prostate cancer cells using DNA microarray analyses. Gene set enrichment analysis revealed that menthol primarily affects the expression of cell cycle-related genes. Subsequent experimental analyses validated that menthol induces G2/M arrest. Particularly, menthol markedly down-regulated polo-like kinase 1 (PLK1), a key regulator of G2/M phase progression and inhibited its downstream signaling. Our computational analyses and experimental validation provide a basis for future investigations that are aimed at elucidating the action of menthol on cell cycle control in prostate cancer cells.

Interacting factors and cellular localization of SR protein-specific kinase Dsk1

Schizosaccharomyces pombe Dsk1 is an SR protein-specific kinase (SRPK), whose homologs have been identified in every eukaryotic organism examined. Although discovered as a mitotic regulator with protein kinase activity toward SR splicing factors, it remains largely unknown about what and how Dsk1 contributes to cell cycle and pre-mRNA splicing. In this study, we investigated the Dsk1 function by determining interacting factors and cellular localization of the kinase. Consistent with its reported functions, we found that pre-mRNA processing and cell cycle factors are prominent among the proteins co-purified with Dsk1. The identification of these factors led us to find Rsd1 as a novel Dsk1 substrate, as well as the involvement of Dsk1 in cellular distribution of poly(A)(+) RNA. In agreement with its role in nuclear events, we also found that Dsk1 is mainly localized in the nucleus during G(2) phase and at mitosis. Furthermore, we revealed the oscillation of Dsk1 protein in a cell cycle-dependent manner. This paper marks the first comprehensive analysis of in vivo Dsk1-associated proteins in fission yeast. Our results reflect the conserved role of SRPK family in eukaryotic organisms, and provide information about how Dsk1 functions in pre-mRNA processing and cell-division cycle.

Color recovery after photoconversion of H2B::mEosFP allows detection of increased nuclear DNA content in developing plant cells

Many higher plants are polysomatic whereby different cells possess variable amounts of nuclear DNA. The conditional triggering of endocycles results in higher nuclear DNA content (C value) that in some cases has been correlated to increased cell size. While numerous multicolored fluorescent protein (FP) probes have revealed the general behavior of the nucleus and intranuclear components, direct visualization and estimation of changes in nuclear-DNA content in live cells during their development has not been possible. Recently, monomeric Eos fluorescent protein (mEosFP) has emerged as a useful photoconvertible protein whose color changes irreversibly from a green to a red fluorescent form upon exposure to violet-blue light. The stability and irreversibility of red fluorescent mEosFP suggests that detection of green color recovery would be possible as fresh mEosFP is produced after photoconversion. Thus a ratiometric evaluation of the red and green forms of mEosFP following photoconversion could be used to estimate production of a core histone such as H2B during its concomitant synthesis with DNA in the synthesis phase of the cell cycle. Here we present proof of concept observations on transgenic tobacco (Nicotiana tabacum) Bright Yellow 2 cells and Arabidopsis (Arabidopsis thaliana) plants stably expressing H2B::mEosFP. In Arabidopsis seedlings an increase in green fluorescence is observed specifically in cells known to undergo endoreduplication. The detection of changes in nuclear DNA content by correlating color recovery of H2B::mEosFP after photoconversion is a novel approach involving a single FP. The method has potential for facilitating detailed investigations on conditions that favor increased cell size and the development of polysomaty in plants.

[Nucleus DNA content measurement methods features]

We suggest a new theoretical method of the flow cytometry DNA histograms and apply it for Drosophila melanogaster imaginal discs cells. The model gives a possibility to determine the proportions of cells in G1, G2 (M) and S cell cycle phases. We show that the precision of G1 and G2 (M) DNA content measurements is limited by the precision of device zero signal arrangement. The usage of calculated device zero and dividing cells as the DNA content standards may improve the precision of DNA content measurements. We also compared the precisions of different DNA content methods and draw the conclusion that the current precisions of different methods are similar and lie within 2-6% interval.

Sin3a is essential for the genome integrity and viability of pluripotent cells

The Sin3a/HDAC co-repressor complex is a critical regulator of transcription networks that govern cell cycle control and apoptosis throughout development. Previous studies have identified Sin3a as essential for embryonic development around the time of implantation, during which the epiblast cell cycle is uniquely structured to achieve very rapid divisions with little tolerance of DNA damage. This study investigates the specific requirement for Sin3a in the early mouse embryo and shows that embryos lacking Sin3a suffer unresolved DNA damage and acute p53-independent apoptosis specifically in the E3.5–4.5 epiblast. Surprisingly, Myc and E2F targets in Sin3a-null ICMs are downregulated, suggesting a central but non-canonical role for Sin3a in regulating the pluripotent embryonic cell cycle. ES cells deleted for Sin3a mount a DNA damage response indicative of unresolved double-strand breaks, profoundly arrest at G2, and undergo apoptosis. These results indicate that Sin3a protects the genomic integrity of pluripotent embryonic cells and governs their unusual cell cycle.

Targeting MAGE-C1/CT7 expression increases cell sensitivity to the proteasome inhibitor bortezomib in multiple myeloma cell lines

The MAGE-C1/CT7 encodes a cancer/testis antigen (CTA), is located on the chromosomal region Xq26–27 and is highly polymorphic in humans. MAGE-C1/CT7 is frequently expressed in multiple myeloma (MM) that may be a potential target for immunotherapy in this still incurable disease. MAGEC1/CT7 expression is restricted to malignant plasma cells and it has been suggested that MAGE-C1/CT7 might play a pathogenic role in MM; however, the exact function this protein in the pathophysiology of MM is not yet understood. Our objectives were (1) to clarify the role of MAGE-C1/CT7 in the control of cellular proliferation and cell cycle in myeloma and (2) to evaluate the impact of silencing MAGE-C1/CT7 on myeloma cells treated with bortezomib. Myeloma cell line SKO-007 was transduced for stable expression of shRNA-MAGE-C1/CT7. Downregulation of MAGE-C1/CT7 was confirmed by real time quantitative PCR and western blot. Functional assays included cell proliferation, cell invasion, cell cycle analysis and apoptosis. Western blot showed a 70–80% decrease in MAGE-C1/CT7 protein expression in inhibited cells (shRNA-MAGE-C1/CT7) when compared with controls. Functional assays did not indicate a difference in cell proliferation and DNA synthesis when inhibited cells were compared with controls. However, we found a decreased percentage of cells in the G2/M phase of the cell cycle among inhibited cells, but not in the controls (p<0.05). When myeloma cells were treated with bortezomib, we observed a 48% reduction of cells in the G2/M phase among inhibited cells while controls showed 13% (empty vector) and 9% (ineffective shRNA) reduction, respectively (p<0.01). Furthermore, inhibited cells treated with bortezomib showed an increased percentage of apoptotic cells (Annexin V+/PI-) in comparison with bortezomib-treated controls (p<0.001). We found that MAGE-C1/CT7 protects SKO-007 cells against bortezomib-induced apoptosis. Therefore, we could speculate that MAGE-C1/CT7 gene therapy could be a strategy for future therapies in MM, in particular in combination with proteasome inhibitors.

Apical migration of nuclei during G2 is a prerequisite for all nuclear motion in zebrafish neuroepithelia

Nuclei in the proliferative pseudostratified epithelia of vastly different organisms exhibit a characteristic dynamics - the so-called interkinetic nuclear migration (IKNM). Although these movements are thought to be intimately tied to the cell cycle, little is known about the relationship between IKNM and distinct phases of the cell cycle and the role that this association plays in ensuring balanced proliferation and subsequent differentiation. Here, we perform a quantitative analysis of modes of nuclear migration during the cell cycle using a marker that enables the first unequivocal differentiation of all four phases in proliferating neuroepithelial cells in vivo. In zebrafish neuroepithelia, nuclei spend the majority of the cell cycle in S phase, less time in G1, with G2 and M being noticeably shorter still in comparison. Correlating cell cycle phases with nuclear movements shows that IKNM comprises rapid apical nuclear migration during G2 phase and stochastic nuclear motion during G1 and S phases. The rapid apical migration coincides with the onset of G2, during which we find basal actomyosin accumulation. Inhibiting the transition from G2 to M phase induces a complete stalling of nuclei, indicating that IKNM and cell cycle continuation cannot be uncoupled and that progression from G2 to M is a prerequisite for rapid apical migration. Taken together, these results suggest that IKNM involves an actomyosin-driven contraction of cytoplasm basal to the nucleus during G2, and that the stochastic nuclear movements observed in other phases arise passively due to apical migration in neighboring cells.

Mixed lineage kinase 3 modulates β-catenin signaling in cancer cells

Expression of β-catenin is strictly regulated in normal cells via the glycogen synthase kinase 3β (GSK3β)- adenomatous polyposis coli-axin-mediated degradation pathway. Mechanisms leading to inactivation of this pathway (example: activation of Wnt/β-catenin signaling or mutations of members of the degradation complex) can result in β-catenin stabilization and activation of β-catenin/T-cell factor (TCF) signaling. β-Catenin-mediated cellular events are diverse and complex. A better understanding of the cellular signaling networks that control β-catenin pathway is important for designing effective therapeutic strategies targeting this axis. To gain more insight, we focused on determining any possible cross-talk between β-catenin and mixed lineage kinase 3 (MLK3), a MAPK kinase kinase member. Our studies indicated that MLK3 can induce β-catenin expression via post-translational stabilization in various cancer cells, including prostate cancer. This function of MLK3 was dependent on its kinase activity. MLK3 can interact with β-catenin and phosphorylate it in vitro. Overexpression of GSK3β-WT or the S9A mutant was unable to antagonize MLK3-induced stabilization, suggesting this to be independent of GSK3β pathway. Surprisingly, despite stabilizing β-catenin, MLK3 inhibited TCF transcriptional activity in the presence of both WT and S37A β-catenin. These resulted in reduced expression of β-catenin/TCF downstream targets Survivin and myc. Immunoprecipitation studies indicated that MLK3 did not decrease β-catenin/TCF interaction but promoted interaction between β-catenin and KLF4, a known repressor of β-catenin/TCF transcriptional activity. In addition, co-expression of MLK3 and β-catenin resulted in significant G(2)/M arrest. These studies provide a novel insight toward the regulation of β-catenin pathway, which can be targeted to control cancer cell proliferation, particularly those with aberrant activation of β-catenin signaling.

Mutations in MYB3R1 and MYB3R4 cause pleiotropic developmental defects and preferential down-regulation of multiple G2/M-specific genes in Arabidopsis

R1R2R3-Myb proteins represent an evolutionarily conserved class of Myb family proteins important for cell cycle regulation and differentiation in eukaryotic cells. In plants, this class of Myb proteins are believed to regulate the transcription of G2/M phase-specific genes by binding to common cis-elements, called mitosis-specific activator (MSA) elements. In Arabidopsis (Arabidopsis thaliana), MYB3R1 and MYB3R4 act as transcriptional activators and positively regulate cytokinesis by activating the transcription of KNOLLE, which encodes a cytokinesis-specific syntaxin. Here, we show that the double mutation myb3r1 myb3r4 causes pleiotropic developmental defects, some of which are due to deficiency of KNOLLE whereas other are not, suggesting that multiple target genes are involved. Consistently, microarray analysis of the double mutant revealed altered expression of many genes, among which G2/M-specific genes showed significant overrepresentation of the MSA motif and a strong tendency to be down-regulated by the double mutation. Our results demonstrate, on a genome-wide level, the importance of the MYB3R-MSA pathway for regulating G2/M-specific transcription. In addition, MYB3R1 and MYB3R4 may have diverse roles during plant development by regulating G2/M-specific genes with various functions as well as genes possibly unrelated to the cell cycle.

DNA damage during G2 phase does not affect cell cycle progression of the green alga Scenedesmus quadricauda

DNA damage is a threat to genomic integrity in all living organisms. Plants and green algae are particularly susceptible to DNA damage especially that caused by UV light, due to their light dependency for photosynthesis. For survival of a plant, and other eukaryotic cells, it is essential for an organism to continuously check the integrity of its genetic material and, when damaged, to repair it immediately. Cells therefore utilize a DNA damage response pathway that is responsible for sensing, reacting to and repairing damaged DNA. We have studied the effect of 5-fluorodeoxyuridine, zeocin, caffeine and combinations of these on the cell cycle of the green alga Scenedesmus quadricauda. The cells delayed S phase and underwent a permanent G2 phase block if DNA metabolism was affected prior to S phase; the G2 phase block imposed by zeocin was partially abolished by caffeine. No cell cycle block was observed if the treatment with zeocin occurred in G2 phase and the cells divided normally. CDKA and CDKB kinases regulate mitosis in S. quadricauda; their kinase activities were inhibited by Wee1. CDKA, CDKB protein levels were stabilized in the presence of zeocin. In contrast, the protein level of Wee1 was unaffected by DNA perturbing treatments. Wee1 therefore does not appear to be involved in the DNA damage response in S. quadricauda. Our results imply a specific reaction to DNA damage in S. quadricauda, with no cell cycle arrest, after experiencing DNA damage during G2 phase.

Cryptogein-induced cell cycle arrest at G2 phase is associated with inhibition of cyclin-dependent kinases, suppression of expression of cell cycle-related genes and protein degradation in synchronized tobacco BY-2 cells

Induction of defense responses by pathogens or elicitors is often accompanied by growth inhibition in planta, but its molecular mechanisms are poorly understood. In this report, we characterized the molecular events that occur during cryptogein-induced cell cycle arrest at G(2) phase in synchronously cultured tobacco Bright Yellow-2 (BY-2) cells. Concomitant with the proteinaceous elicitor-induced G(2) arrest, we observed inhibition of the histone H1 kinase activity of cyclin-dependent kinases (CDKs), which correlated with a decrease in mRNA and protein levels of CDKB1. In contrast, the amount of CDKA was almost unaffected by cryptogein even at M phase. Cryptogein rapidly inhibited the expression not only of positive, e.g. A- and B-type cyclins and NtCAK, but also of negative cell cycle regulators such as WEE1, suggesting that cryptogein affects multiple targets to inactivate CDKA to induce G(2) arrest by mechanisms distinct from known checkpoint regulation. Moreover, we show that CDKB1 and cyclin proteins are also rapidly degraded by cryptogein and that the proteasome-dependent protein degradation has a crucial role in the control of cryptogein-induced hypersensitive cell death.

Correct mRNA processing at a mutant TT splice donor in FANCC ameliorates the clinical phenotype in patients and is enhanced by delivery of suppressor U1 snRNAs

The U1 small nuclear RNA (U1 snRNA) as a component of the major U2-dependent spliceosome recognizes 5' splice sites (5'ss) containing GT as the canonical dinucleotide in the intronic positions +1 and +2. The c.165+1G>T germline mutation in the 5'ss of exon 2 of the Fanconi anemia C (FANCC) gene commonly predicted to prevent correct splicing was identified in nine FA patients from three pedigrees. RT-PCR analysis of the endogenous FANCC mRNA splicing pattern of patient-derived fibroblasts revealed aberrant mRNA processing, but surprisingly also correct splicing at the TT dinucleotide, albeit with lower efficiency. This consequently resulted in low levels of correctly spliced transcript and minute levels of normal posttranslationally processed FANCD2 protein, indicating that this naturally occurring TT splicing might contribute to the milder clinical manifestations of the disease in these patients. Functional analysis of this FANCC 5'ss within splicing reporters revealed that both the noncanonical TT dinucleotide and the genomic context of FANCC were required for the residual correct splicing at this mutant 5'ss. Finally, use of lentiviral vectors as a delivery system to introduce expression cassettes for TT-adapted U1 snRNAs into primary FANCC patient fibroblasts allowed the correction of the DNA-damage-induced G2 cell-cycle arrest in these cells, thus representing an alternative transcript-targeting approach for genetic therapy of inherited splice-site mutations.

Polo-like kinase 1 activated by the hepatitis B virus X protein attenuates both the DNA damage checkpoint and DNA repair resulting in partial polyploidy

Hepatitis B virus X protein (pX), implicated in hepatocarcinogenesis, induces DNA damage because of re-replication and allows propagation of damaged DNA, resulting in partial polyploidy and oncogenic transformation. The mechanism by which pX allows cells with DNA damage to continue proliferating is unknown. Herein, we show pX activates Polo-like kinase 1 (Plk1) in the G(2) phase, thereby attenuating the DNA damage checkpoint. Specifically, in the G(2) phase of pX-expressing cells, the checkpoint kinase Chk1 was inactive despite DNA damage, and protein levels of claspin, an adaptor of ataxia telangiectasia-mutated and Rad3-related protein-mediated Chk1 phosphorylation, were reduced. Pharmacologic inhibition or knockdown of Plk1 restored claspin protein levels, Chk1 activation, and p53 stabilization. Also, protein levels of DNA repair protein Mre11 were decreased in the G(2) phase of pX-expressing cells but not with Plk1 knockdown. Interestingly, in pX-expressing cells, Mre11 co-immunoprecipitated with transfected Plk1 Polo-box domain, and inhibition of Plk1 increased Mre11 stability in cycloheximide-treated cells. These results suggest that pX-activated Plk1 by down-regulating Mre11 attenuates DNA repair. Importantly, concurrent inhibition of Plk1, p53, and Mre11 increased the number of pX-expressing cells with DNA damage entering mitosis, relative to Plk1 inhibition alone. By contrast, inhibition or knockdown of Plk1 reduced pX-induced polyploidy while increasing apoptosis. We conclude Plk1, activated by pX, allows propagation of DNA damage by concurrently attenuating the DNA damage checkpoint and DNA repair, resulting in polyploidy. We propose this novel Plk1 mechanism initiates pX-mediated hepatocyte transformation.

Centrobin/NIP2 is a microtubule stabilizer whose activity is enhanced by PLK1 phosphorylation during mitosis

Centrobin/NIP2 is a centrosomal protein that is required for centrosome duplication. It is also critical for microtubule organization in both interphase and mitotic cells. In the present study, we observed that centrobin is phosphorylated in a cell cycle stage-specific manner, reaching its maximum at M phase. PLK1 is a kinase that is responsible for M phase-specific phosphorylation of centrobin. The microtubule forming activity of centrobin was enhanced by PLK1 phosphorylation. Furthermore, mitotic spindles were not assembled properly with the phospho-resistant mutant of centrobin. Based on these results, we propose that centrobin functions as a microtubule stabilizing factor and PLK1 enhances centrobin activity for proper spindle formation during mitosis.

Cell proliferation and cell cycle alterations in oesophageal p53-mutated cancer cells treated with cisplatin in combination with photodynamic therapy

OBJECTIVES

The major goal of anti-cancer therapies is selective destruction of tumour cells with minimum side effects on normal cells. Towards this aim, combination of different therapeutic modalities has been evaluated for improving control of neoplastic diseases and quality of life for the patient. Photodynamic therapy (PDT) is a procedure for treatment of various types of cancer, but its combination with other established treatments has not been evaluated in detail. We have used KYSE-510 cells from a human oesophageal carcinoma as an in vitro model to investigate whether cisplatin (CDDP) could be combined with PDT to increase cell death with respect to single treatments.

MATERIALS AND METHODS

p53-mutated KYSE-510 cells were treated with CDDP alone or in combination with PDT. Analyses of cell viability, cell cycle progression and apoptosis induction were carried out at specific times after treatments.

RESULTS

Decrease in cell viability, cell cycle arrest at the G(2)/M- and S-phases boundary, and apoptosis induction were observed after single and combined treatments.

CONCLUSIONS

Our results show that low CDDP doses (0.25-1 microm) induce cell mortality and cell cycle perturbation, which were more evident when given in combination with PDT, but in contrast to work of other authors no synergistic activity was found. Apoptosis occurred via intrinsic pathways in treated cells, although it did not represent the predominant mode of cell death.

SP600125 suppresses Cdk1 and induces endoreplication directly from G2 phase, independent of JNK inhibition

Cell cycle controls ensure that DNA replication (S phase) follows mitosis resulting in two precise copies of the genome. A failure of the control mechanisms can result in multiple rounds of DNA replication without cell division. In endoreplication, cells with replicated genomes bypass mitosis, then replicate their DNA again, resulting in polyploidy. Endoreplication from G2 phase lacks all hallmarks of mitosis. Using synchronized cells, we show that the c-Jun N-terminal kinase (JNK) inhibitor, SP600125, prevents the entry of cells into mitosis and leads to endoreplication of DNA from G2 phase. We show that cells proceed from G2 phase to replicate their DNA in the absence of mitosis. This effect of SP600125 is independent of its suppression of JNK activity. Instead, the inhibitory effect of SP600125 on mitotic entry predominantly occurs upstream of Aurora A kinase and Polo-like kinase 1, resulting in a failure to remove the inhibitory phosphorylation of Cdk1. Importantly, our results directly show that the inhibition of Cdk1 activity and the persistence of Cdk2 activity in G2 cells induces endoreplication without mitosis. Furthermore, endoreplication from G2 phase is independent of p53 control.

The mismatch repair system modulates curcumin sensitivity through induction of DNA strand breaks and activation of G2-M checkpoint

The highly conserved mismatch (MMR) repair system corrects postreplicative errors and modulates cellular responses to genotoxic agents. Here, we show that the MMR system strongly influences cellular sensitivity to curcumin. Compared with MMR-proficient cells, isogenically matched MMR-deficient cells displayed enhanced sensitivity to curcumin. Similarly, cells suppressed for MLH1 or MSH2 expression by RNA interference displayed increased curcumin sensitivity. Curcumin treatment generated comparable levels of reactive oxygen species and the mutagenic adduct 8-oxo-guanine in MMR-proficient and MMR-deficient cells; however, accumulation of gammaH2AX foci, a marker for DNA double-strand breaks (DSB), occurred only in MMR-positive cells in response to curcumin treatment. Additionally, MMR-positive cells showed activation of Chk1 and induction of G(2)-M cell cycle checkpoint following curcumin treatment and inhibition of Chk1 by UCN-01 abrogated Chk1 activation and heightened apoptosis in MMR-proficient cells. These results indicate that curcumin triggers the accumulation of DNA DSB and induction of a checkpoint response through a MMR-dependent mechanism. Conversely, in MMR-compromised cells, curcumin-induced DSB is significantly blunted, and as a result, cells fail to undergo cell cycle arrest, enter mitosis, and die through mitotic catastrophe. The results have potential therapeutic value, especially in the treatment of tumors with compromised MMR function.

Topoisomerase IIalpha-dependent induction of a persistent DNA damage response in response to transient etoposide exposure

Cytotoxicity of the topoisomerase II (topoII) poison etoposide has been ascribed to the persistent covalent trapping of topoII in DNA cleavage complexes that become lethal as cells replicate their DNA. However, short term etoposide treatment also leads to subsequent cell death, suggesting that the lesions that lead to cytotoxicity arise rapidly and prior to the onset DNA replication. In the present study 1h treatment with 25muM etoposide was highly toxic and initiated a double-stranded DNA damage response as reflected by the recruitment of ATM, MDC1 and DNA-PKcs to gammaH2AX foci. While most DNA breaks were rapidly repaired upon withdrawal of the etoposide treatment, the repair machinery remained engaged in foci for at least 24h following withdrawal. TopoII siRNA ablation showed the etoposide toxicity and gammaH2AX response to correlate with the inability of the cell to correct topoIIalpha-initiated DNA damage. gammaH2AX induction was resistant to the inhibition of DNA replication and transcription, but was increased by pre-treatment with the histone deacetylase inhibitor trichostatin A. These results link the lethality of etoposide to the generation of persistent topoIIalpha-dependent DNA defects within topologically open chromatin domains.

Effects of alphavbeta6 gene silencing by RNA interference in PANC-1 pancreatic carcinoma cells

BACKGROUND

Integrin alphavbeta6 is up-regulated in a variety of human carcinomas and plays a crucial role in tumor invasion and metastasis. However, the function of alphavbeta6 in pancreatic carcinoma and its potential role in gemcitabine resistance remain unknown.

MATERIALS AND METHODS

Small interfering RNA (siRNA) targeting alphavbeta6 was constructed and transfected into PANC-1 cells. Effects of alphavbeta6 knockdown on cell proliferation, invasion, cell cycle progression, apoptosis and chemosensitivity to gemcitabine were investigated.

RESULTS

Expression of alphavbeta6 in PANC-1 cells was markedly suppressed by siRNA. Silencing of alphavbeta6 expression significantly inhibited cell proliferation and invasiveness, resulted in cell cycle arrest, and induced cell apoptosis. More importantly, alphavbeta6 knockdown enhanced chemosensitivity to gemcitabine and increased gemcitabine-induced caspase-mediated apoptosis.

CONCLUSION

These findings suggest a novel mechanism by which alphavbeta6 contributes to pancreatic carcinoma progression. The combination of alphavbeta6 silencing and gemcitabine treatment may provide an effective therapeutic strategy for highly resistant pancreatic carcinoma.

Wip1 confers G2 checkpoint recovery competence by counteracting p53-dependent transcriptional repression

Activation of the DNA damage checkpoint causes a cell-cycle arrest through inhibition of cyclin-dependent kinases (cdks). To successfully recover from the arrest, a cell should somehow be maintained in its proper cell-cycle phase. This problem is particularly eminent when a cell arrests in G2, as cdk activity is important to establish a G2 state. Here, we identify the phosphatase Wip1 (PPM1D) as a factor that maintains a cell competent for cell-cycle re-entry during an ongoing DNA damage response in G2. We show that Wip1 function is required throughout the arrest, and that Wip1 acts by antagonizing p53-dependent repression of crucial mitotic inducers, such as Cyclin B and Plk1. Our data show that the primary function of Wip1 is to retain cellular competence to divide, rather than to silence the checkpoint to promote recovery. Our findings uncover Wip1 as a first in class recovery competence gene, and suggest that the principal function of Wip1 in cellular transformation is to retain proliferative capacity in the face of oncogene-induced stress.

Overexpression of eukaryotic elongation factor eEF2 in gastrointestinal cancers and its involvement in G2/M progression in the cell cycle

A high level protein synthesis is one of the characteristics of cancer cells. The aim of this study is to show the contribution of eukaryotic elongation factor 2 (eEF2), which plays an essential role in the polypeptide chain elongation step, in the tumorigenesis of gastrointestinal cancers. In the present study, we demonstrated by using immunohistochemistry that eEF2 protein was overexpressed in 92.9% (13 of 14) of gastric and 91.7% (22 of 24) of colorectal cancers. No mutations were found in any of the exons of the eEF2 gene in six gastric and six colorectal cancers. Knockdown of eEF2 by eEF2-specific short-hairpin RNA (shEF2) inhibited cancer cell growth in two gastric cancer cell lines, AZ-521 and MKN28, and one colon cancer cell line, SW620. Flow cytometric analysis showed that knockdown of eEF2 induced G2/M arrest and resulted in inactivation of Akt and cdc2 (a G2/M regulator) and activation of eEF2 kinase (a negative regulator of eEF2) in these cancer cells. Conversely, forced expression of eEF2 in AZ-521 cells significantly enhanced the cell growth through promotion of G2/M progression in cell cycle, activated Akt and cdc2, and inactivated eEF2 kinase. Furthermore, forced expression of eEF2 in these cancer cells enhanced in vivo tumorigenicity in a mouse xenograft model. These results showed that overexpressed eEF2 in gastrointestinal cancers promoted G2/M progression and enhanced their cell growth in vitro and in vivo. These results also suggested a novel linkage between translational elongation and cell cycle mechanisms, implying that the linkage might play an important role to orchestrate the deregulated translation and cell cycle mechanisms for promotion of the development of gastrointestinal cancers.

B-MYB is required for recovery from the DNA damage-induced G2 checkpoint in p53 mutant cells

In response to DNA damage, several signaling pathways that arrest the cell cycle in G(1) and G(2) are activated. The down-regulation of mitotic genes contributes to the stable maintenance of the G(2) arrest. The human LINC or DREAM complex, together with the B-MYB transcription factor, plays an essential role in the expression of G(2)-M genes. Here, we show that DNA damage results in the p53-dependent binding of p130 and E2F4 to LINC and the dissociation of B-MYB from LINC. We find that B-MYB fails to dissociate from LINC in p53 mutant cells, that this contributes to increased G(2)-M gene expression in response to DNA damage in these cells, and, importantly, that B-MYB is required for recovery from the G(2) DNA damage checkpoint in p53-negative cells. Reanalysis of microarray expression data sets revealed that high levels of B-MYB correlate with a p53 mutant status and an advanced tumor stage in primary human breast cancer. Taken together, these data suggest that B-MYB/LINC plays an important role in the DNA damage response downstream of p53.

Preferential up-regulation of G2/M phase-specific genes by overexpression of the hyperactive form of NtmybA2 lacking its negative regulation domain in tobacco BY-2 cells

Many G2/M phase-specific genes in plants contain mitosis-specific activator (MSA) elements, which act as G2/M phase-specific enhancers and bind with R1R2R3-Myb transcription factors. Here, we examined the genome-wide effects of NtmybA2 overexpression, one of the R1R2R3-Myb transcription factors in tobacco (Nicotiana tabacum). We used a custom-made 16-K cDNA microarray for comparative transcriptome analysis of transgenic tobacco BY-2 cell lines that overexpress NtmybA2 or its truncated hyperactive form. The microarray was also used to determine the transcript profile during the cell cycle in synchronized cultures of BY-2 cells. Combined microarray data from transgenic lines and synchronized cells revealed that overexpression of the truncated hyperactive form of NtmybA2, but not its full-length form, preferentially up-regulated many G2/M phase-specific genes in BY-2 cells. We determined promoter sequences of several such up-regulated genes and showed that all contain MSA-like motifs in the proximal regions of their promoters. One of the up-regulated genes, NtE2C, encoding for cyclin-specific ubiquitin carrier proteins, contained a single functional MSA-like motif, which specifically controlled the expression of a reporter gene in the G2/M phase in BY-2 cells. Furthermore, a genomic footprint experiment showed that the MSA element in the NtE2C promoter interacted with nuclear proteins in vivo. Therefore, we propose that the transcription of many G2/M phase-specific genes in tobacco is positively regulated by NtmybA2, in most cases through direct binding to the MSA elements.

DNA damage mediated s and g(2) checkpoints in human embryonal carcinoma cells

For mouse embryonic stem (ES) cells, the importance of the S and G(2) cell cycle checkpoints for genomic integrity is increased by the absence of the G(1) checkpoint. We have investigated ionizing radiation (IR)-mediated cell cycle checkpoints in undifferentiated and retinoic acid-differentiated human embryonal carcinoma (EC) cells. Like mouse ES cells, human EC cells did not undergo G(1) arrest after IR but displayed a prominent S-phase delay followed by a G(2)-phase delay. In contrast, although differentiated EC cells also failed to arrest at G(1)-phase after IR, they quickly exited S-phase and arrested in G(2)-phase. In differentiated EC cells, the G(2)-M-phase cyclin B1/CDC2 complex was upregulated after IR, but the G(1)-S-phase cyclin E and the cyclin E/CDK2 complex were expressed at constitutively low levels, which could be an important factor distinguishing DNA damage responses between undifferentiated and differentiated EC cells. S-phase arrest and expression of p21 could be inhibited by 7-hydroxystaurosporine, suggesting that the ataxia-telangiectasia and Rad-3-related-checkpoint kinase 1 (ATR-CHK1), and p21 pathways might play a role in the IR-mediated S-phase checkpoint in EC cells. IR-mediated phosphorylation of ataxia-telangiectasia mutated, (CHK1), and checkpoint kinase 2 were distinctly higher in undifferentiated EC cells compared with differentiated EC cells. Combined with the prominent S and G(2) checkpoints and a more efficient DNA damage repair system, these mechanisms operate together in the maintenance of genome stability for EC cells.

Effect of diallyl disulfide on cell cycle arrest of human colon cancer SW480 cells

BACKGROUND AND OBJECTIVE

Our previous study revealed that diallyl disulfide (DADS) significantly inhibited cell proliferation and induced cell cycle arrest at G(2)/M phase of human colon cancer SW480 cells. However, the molecular mechanism of cell cycle arrest remains unclear. This study was to investigate the role and the molecular mechanism of DADS in the induction of cell cycle arrest of human colon cancer cell line SW480.

METHODS

Proliferation of SW480 cells after DADS treatment was measured by MTT assay and cell counting. Phase distribution of cell cycle was analyzed by flow cytometry. Expressions of PCNA, p53, p21(WAF1) and cyclin B1 were detected by immunohistochemistry and western blot.

RESULTS

DADS (30-70 microg/mL) significantly inhibited proliferation and retarded the population doubling time of colonies in SW480 cells. Compared with the control group, SW480 cells were markedly accumulated at G(2)/M phase after the treatment with DADS (p < 0.05). Moreover, DADS remarkably decreased the protein contents of PCNA, p53 and cyclin B1, but increased the expression of p21(WAF1) in a time- and dose-dependent manner.

CONCLUSION

DADS induces G(2)/M arrest in human colon cancer SW480 cells, probably through the downregulation of PCNA, p53 and cyclin B1 and upregulation of p21(WAF1).

Analysis of the topoisomerase II-dependent decatenation G2 checkpoint and checkpoint kinases in human cells

Inhibition of Topo II function using poisons and catalytic inhibitors triggers checkpoints that act to delay progression of G2 cells into mitosis. Topo II poisons induce Topo II-associated DNA double-strand breaks that activate ATM and the DNA damage G2 checkpoint. Topo II catalytic inhibitors do not induce DNA double-strand breaks but block decatenation of intertwined daughter chromatids. Complete decatenation before anaphase of mitosis is required for chromatid segregation. G2 cells appear to sense the degree of chromatid arm catenations and actively delay the onset of mitosis by sustaining the inhibition of mitosis-promoting factor (MPF) and polo-like kinase 1 (Plk-1) kinase activities that normally propel G2 cells into mitosis. This chapter details the methods for assay of decatenation G2 checkpoint function and checkpoint kinase activities.

Gli1 induces G2/M arrest and apoptosis in hippocampal but not tumor-derived neural stem cells

Sonic hedgehog (Shh) is necessary for sustaining the proliferation of neural stem cells (NSCs), yet little is known about its mechanisms. Whereas Gli1, Gli2, and Gli3, the primary mediators of Shh signaling, were all expressed in hippocampal neural progenitors, Shh treatment of NSCs induced only Gli1 expression. Acute depletion of Gli1 in postnatal NSCs by short-hairpin RNA decreased proliferation, whereas germline deletion of Gli1 did not affect NSC proliferation, suggesting a difference in mechanisms of Gli1 compensation that may be developmentally dependent. To determine whether Gli1 was sufficient to enhance NSC proliferation, we overexpressed this mitogen and were surprised to find that Gli1 resulted in decreased proliferation, accumulation of NSCs in the G2/M phase of cell cycle, and apoptosis. In contrast, Gli1-expressing lineage-restricted neural precursors demonstrated a 4.5-fold proliferation enhancement. Expression analyses of Gli1-expressing NSCs identified significant induction of Gadd45a and decreased cyclin A2 and Stag1 mRNA, genes involved in the G2-M transition and apoptosis. Furthermore, Gadd45a overexpression was sufficient to partially recapitulate the Gli1-induced G2/M accumulation and cell death of NSCs. In contrast to normal stem cells, tumor-derived stem cells had markedly higher basal Gli1 expression and did not undergo apoptosis with further elevation of Gli1. Our data suggest that Gli1-induced apoptosis may serve as a protective mechanism against premature mitosis and may give insight into mechanisms by which nonmalignant stem cells restrain hyperproliferation in the context of potentially transforming mitogenic signals. Tumor-derived stem cells apparently lack these mechanisms, which may contribute to their unrestrained proliferation and malignant potential.

Arabidopsis RETINOBLASTOMA-RELATED PROTEIN 1 is involved in G1 phase cell cycle arrest caused by sucrose starvation

Although sucrose availability is crucial for commitment to plant cell division during G1 phase by controlling the expression of D-type cyclins, it has remained unclear how these factors mediate entry into the cell cycle. Here we show that Arabidopsis RETINOBLASTOMA-RELATED PROTEIN 1 (AtRBR1) is involved in G1-phase cell cycle arrest caused by sucrose starvation. We generated estrogen-inducible AtRBR1 RNA interference (RNAi) Arabidopsis suspension MM2d cells, and found that downregulation of AtRBR1 leads to a higher frequency of arrest in G2 phase, instead of G1-phase arrest in the uninduced control, after sucrose starvation. Synchronization experiments confirmed that downregulation of AtRBR1 leads to a prolonged G2 phase and delayed activation of G2/M marker genes. Downregulation of AtRBR1 also stimulated the activation of E2F-regulated genes when these genes were repressed in the uninduced cells under the limited sucrose conditions. We conclude that AtRBR1 is a key effector for the ability of sucrose to modulate progression from G1 phase.

Cell-cycle regulation of cohesin stability along fission yeast chromosomes

Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.

Lymphocyte radiosensitivity in BRCA1 and BRCA2 mutation carriers and implications for breast cancer susceptibility

There is conflicting evidence as to whether individuals who are heterozygous for germ-line BRCA1 or BRCA2 mutations have an altered phenotypic cellular response to irradiation. To investigate this, chromosome breakage and apoptotic response were measured after irradiation in peripheral blood lymphocytes from 26 BRCA1 and 18 BRCA2 mutation carriers without diagnosed breast cancer, and 38 unaffected age, ethnically and sex-matched controls. To assess the role of BRCA1 and BRCA2 in homologous recombination, an S phase enrichment chromosome breakage assay was used. BrdUrd incorporation studies allowed verification of the correct experimental settings. We found that BRCA1 mutation carriers without cancer had increased chromosome breaks as well as breaks and gaps per cell post irradiation using the classical G2 assay (p = 0.01 and 0.004, respectively) and the S phase enrichment assay (p = 0.01 and 0.01, respectively) compared to age-matched unaffected controls. BRCA2 mutation carriers without cancer had increased breaks as well as breaks and gaps per cell post irradiation using the S phase enrichment assay (p = 0.045 and 0.012, respectively). No difference was detected using the G2 assay (p = 0.88 and 0.40 respectively). BRCA1 and BRCA2 mutation carriers had normal cell cycle kinetics and apoptotic response to irradiation compared to age-matched controls. Our results show a demonstrable impairment in irradiation induced DNA repair in women with heterozygous germline BRCA1 and BRCA2 mutations prior to being diagnosed with breast cancer.

In vivo significance of the G2 restriction point

Loss of activity of the retinoblastoma pathway is a common event in human cancer. Mouse models have revealed that tumorigenesis by loss of Rb was accelerated by concomitant loss of the cell cycle inhibitor p27KIP1. This has been attributed to reduced apoptosis and weakening of the G1 checkpoint. However, the role of p27KIP1 in a recently identified G2 restriction point may offer an alternative explanation for this synergy. Here, we have investigated the significance of the G2 restriction point in Rb-deficient pituitaries. We show that Rb loss in the pituitary gland activated the G2 restriction point, as evidenced by the appearance of cyclin B1-p27KIP1 complexes. Somewhat unexpectedly, these complexes remained present in Rb-deficient tumors. These results indicate that the G2 restriction point does operate in vivo. However, in the pituitary gland, this mechanism seems to retard rather than to prevent tumor growth.

p53 arrests growth and induces differentiation of v-Myb-transformed monoblasts

The p53 protein can control cell cycle progression, programmed cell death, and differentiation of many cell types. Ectopic expression of p53 can resume capability of cell cycle arrest, differentiation, and apoptosis in various leukemic cell lines. In this work, we expressed human p53 protein in v-Myb-transformed chicken monoblasts. We found that even this protein possessing only 53% amino acid homology to its avian counterpart can significantly alter morphology and physiology of these cells causing the G2-phase cell cycle arrest and early monocytic differentiation. Our results document that the species-specific differences of the p53 molecules, promoters/enhancers, and co-factors in avian and human cells do not interfere with differentiation- and cell cycle arrest promoting capabilites of the p53 tumor suppressor even in the presence of functional v-Myb oncoprotein. The p53-induced differentiation and cell cycle arrest of v-Myb-transformed monoblasts are not associated with apoptosis suggesting that the p53-driven pathways controlling apoptosis and differentiation/proliferation are independent.

IKKalpha shields 14-3-3sigma, a G(2)/M cell cycle checkpoint gene, from hypermethylation, preventing its silencing

We recently reported that a large proportion of aggressive squamous cell carcinomas of humans and mice express markedly reduced IKKalpha. However, the role of IKKalpha in maintaining genomic stability is unknown. Here we reported that IKKalpha-deficient keratinocytes had a defect in the G(2)/M cell-cycle arrest in response to DNA damage due to downregulated 14-3-3sigma, a cell cycle checkpoint protein. Trimethylated histone H3 lysine 9 (H3-K9) was found to associate with the histone trimethyltransferase Suv39h1 and DNA methyltransferase Dnmt3a in the methylated 14-3-3sigma locus. Reintroduction of IKKalpha restored the expression of 14-3-3sigma. IKKalpha was found to associate with H3 in 14-3-3sigma, which prevented access of Suv39h1 to H3, thereby preventing hypermethylation of 14-3-3sigma. IKKalpha mutants that failed to bind to H3 did not restore the expression of 14-3-3sigma. Thus, IKKalpha protects the 14-3-3sigma locus from hypermethylation, which serves as a mechanism of maintaining genomic stability in keratinocytes.

Human T-lymphotropic virus type-1 p30 alters cell cycle G2 regulation of T lymphocytes to enhance cell survival

Background

Human T-lymphotropic virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is linked to a number of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13 and p30, whose roles are still being defined in the virus life cycle and in HTLV-1 virus-host cell interactions. Proviral clones of HTLV-1 with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. p30 expressed exogenously differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and while acting as a repressor of many genes including Tax, in part by blocking tax/rex RNA nuclear export, selectively enhances key gene pathways involved in T-cell signaling/activation.

Results

Herein, we analyzed the role of p30 in cell cycle regulation. Jurkat T-cells transduced with a p30 expressing lentivirus vector accumulated in the G2-M phase of cell cycle. We then analyzed key proteins involved in G2-M checkpoint activation. p30 expression in Jurkat T-cells resulted in an increase in phosphorylation at serine 216 of nuclear cell division cycle 25C (Cdc25C), had enhanced checkpoint kinase 1 (Chk1) serine 345 phosphorylation, reduced expression of polo-like kinase 1 (PLK1), diminished phosphorylation of PLK1 at tyrosine 210 and reduced phosphorylation of Cdc25C at serine 198. Finally, primary human lymphocyte derived cell lines immortalized by a HTLV-1 proviral clone defective in p30 expression were more susceptible to camptothecin induced apoptosis. Collectively these data are consistent with a cell survival role of p30 against genotoxic insults to HTLV-1 infected lymphocytes.

Conclusion

Collectively, our data are the first to indicate that HTLV-1 p30 expression results in activation of the G2-M cell cycle checkpoint, events that would promote early viral spread and T-cell survival.

TPD52 and NFKB1 gene expression levels correlate with G2 chromosomal radiosensitivity in lymphocytes of women with and at risk of hereditary breast cancer

PURPOSE

To evaluate a transcriptomic approach to identify healthy women at increased risk of breast cancer due to G2-radiosensitivity and look at transcripts that are differentially expressed between individuals.

MATERIALS AND METHODS

We perform the first study to assess the association of G2 radiosensitivity with basal gene expression in cultured T-lymphocytes from 11 women with breast cancer and 12 healthy female relatives using Affymetrix GeneChips.

RESULTS

Transcripts associated with radiosensitivity and breast cancer risk were predominantly involved in innate immunity and inflammation, such as interleukins and chemokines. Genes differentially expressed in radiosensitive individuals were more similarly expressed in close family members than in un-related individuals, suggesting heritability of the trait. The expression of tumour protein D52 (TPD52), a gene implicated in cell proliferation, apoptosis, and vesicle trafficking was the most strongly correlated with G2 score while nuclear factor (kappa)-B (NFKB1) was highly inversely correlated with G2 score. NFKB1 is known to be activated by irradiation and its inhibition has been previously shown to increase radiosensitivity.

CONCLUSIONS

Gene expression analysis of lymphocytes may provide a quantitative measure of radiation response potential and is a promising marker of breast cancer susceptibility.

The benzene metabolite hydroquinone enhances G2-chromosomal radiosensitivity by inducing a less-efficient G2-M-checkpoint in irradiated lymphocytes

The hypothesis tested is that a 24-h pre-irradiation-exposure of peripheral blood lymphocytes (PBL) to the benzene metabolite hydroquinone (HQ), at doses that are non-acutely toxic (5 microM), induces a less efficient G2-M-checkpoint and enhances the G2-chromosomal radiosensitivity in a statistically significant manner (p<0.01). A less efficient G2-M-checkpoint may allow the transition of damaged cells from G2- to M-phase and experimental data in the present work support this hypothesis. In fact HQ sensitizes lymphocytes obtained from healthy donors, as they exhibit increased G2-chromosomal radiosensitivity which interestingly is similar to that observed in cases of radiosensitive cancer-prone individuals. This finding is important since a deficiency in cell cycle checkpoints and an increase in G2-chromosomal radiosensitivity are linked to chromosomal instability, cancer proneness and the development of leukemia. The observed chromosome radiosensitization may be a consequence either of an effect of HQ on the initial induction of radiation-induced chromosomal aberrations, or on the DNA repair capacity of the cells, or it may be linked to HQ-induced alterations in the cell cycle and feedback control mechanism during the G2- to M-phase transition. In order to elucidate which is the mechanism involved, conventional cytogenetics and premature chromosome condensation (PCC) methodologies were applied. The experimental data obtained support the hypothesis that HQ increases G2-chromosomal radiosensitivity in human peripheral blood lymphocytes by inducing a less efficient G2-M-checkpoint, facilitating thus the transition of damaged cells from G2- to M-phase.

Deficiency in the response to DNA double-strand breaks in mouse early preimplantation embryos

DNA double-strand breaks (DSBs) are caused by various environmental stresses, such as ionizing radiation and DNA-damaging agents. When DSBs occur, cell cycle checkpoint mechanisms function to stop the cell cycle until all DSBs are repaired; the phosphorylation of H2AX plays an important role in this process. Mouse preimplantation-stage embryos are hypersensitive to ionizing radiation, and X-irradiated mouse zygotes are arrested at the G2 phase of the first cell cycle. To investigate the mechanisms responding to DNA damage at G2 in mouse preimplantation embryos, we examined G2/M checkpoint and DNA repair mechanisms in these embryos. Most of the one- and two-cell embryos in which DSBs had been induced by gamma-irradiation underwent a delay in cleavage and ceased development before the blastocyst stage. In these embryos, phosphorylated H2AX (gamma-H2AX) was not detected in the one- or two-cell stages by immunocytochemistry, although it was detected after the two-cell stage during preimplantation development. These results suggest that the G2/M checkpoint and DNA repair mechanisms have insufficient function in one- and two-cell embryos, causing hypersensitivity to gamma-irradiation. In addition, phosphorylated ataxia telangiectasia mutated protein and DNA protein kinase catalytic subunits, which phosphorylate H2AX, were detected in the embryos at one- and two-cell stages, as well as at other preimplantation stages, suggesting that the absence of gamma-H2AX in one- and two-cell embryos depends on some factor(s) other than these kinases.

LINC, a human complex that is related to pRB-containing complexes in invertebrates regulates the expression of G2/M genes

Here we report the identification of the LIN complex (LINC), a human multiprotein complex that is required for transcriptional activation of G2/M genes. LINC is related to the recently identified dREAM and DRM complexes of Drosophila and C. elegans that contain homologs of the mammalian retinoblastoma tumor suppressor protein. The LINC core complex consists of at least five subunits including the chromatin-associated LIN-9 and RbAp48 proteins. LINC dynamically associates with pocket proteins, E2F and B-MYB during the cell cycle. In quiescent cells, LINC binds to p130 and E2F4. During cell cycle entry, E2F4 and p130 dissociate and LINC switches to B-MYB and p107. Chromatin Immunoprecipitation experiments demonstrate that LINC associates with a large number of E2F-regulated promoters in quiescent cells. However, RNAi experiments reveal that LINC is not required for repression. In S-phase, LINC selectively binds to the promoters of G2/M genes whose products are required for mitosis and plays an important role in their cell cycle dependent activation.

Genetic determination of chromosomal radiosensitivities in G0- and G2-phase human lymphocytes

BACKGROUND AND PURPOSE

The radiosensitivity of human lymphocytes measured using a G0- or G2-assay has been linked with an individual's risk of developing normal tissue complications following radiotherapy. This study was performed to increase basic knowledge of the genetics of the human radiation response, and chromosomal aberration induction in particular.

MATERIALS AND METHODS

The study was carried out with blood samples taken from 15 monozygotic twin pairs. G0-assay was performed for cells irradiated with 6 Gy counting only deletions and G2-assay for cells irradiated with 0.5 Gy scoring only chromatid breaks.

RESULTS

The mean number of deletions measured at 6 Gy for all 30 samples using the G0-assay amounted to 2.96+/-0.37 (means+/-SD), which corresponds to a coefficient of variation (CV) of 13%. There is a highly significant intra-pair correlation for this number among twins (r(2)=0.911) demonstrating that this parameter is mostly determined by genetic factors. According to the mean number of deletions, a theoretical classification based on the definition < or = MV-SD as resistant, MV+/-SD as normal and > or = MV+SD as sensitive was made, identifying two pairs as sensitive or resistant, respectively, while nine were normal and two pairs are intermediate. For chromatid breaks measured at 0.5 Gy with the G2-assay the mean number was 1.35+/-0.42 (means+/-SD) corresponding to a CV of 31%. There was again a strong intra-pair correlation among twins with r(2)=0.837 showing that this sensitivity is also determined mostly by genetic factors. There was, however, no inter-assay correlation between the G0- and G2-sensitivity (r(2)=0.006) demonstrating that these two sensitivities depend on different genetic factors.

CONCLUSION

The chromosomal radiosensitivity of lymphocytes as defined by G0- or G2-assay is largely determined by different genetic factors, which may allow the use of genetic profiling as an indicator of the respective individual radiosensitivity.

Conserved C-terminal domains of mCenp-F (LEK1) regulate subcellular localization and mitotic checkpoint delay

Centromeric Protein-F (Cenp-F) family members have been identified in organisms from yeast to human. Cenp-F proteins are a component of kinetochores during mitosis, bind to the Rb family of tumor suppressors, and have regulatory effects on the cell cycle and differentiation; however, their role in these processes has not been resolved. Here, we provide evidence that the role of murine Cenp-F (mCenp-F, also known as LEK1) remains largely conserved and that the domains within the C-terminus collectively function to regulate the G2/M cell cycle checkpoint. Overexpression of the C-terminal domain of mCenp-F decreases DNA synthesis. Analyses of deletion mutants of mCenp-F reveal that the complete C-terminal domain is required to delay cell cycle progression at G2/M. Signal transduction pathway profiling experiments indicate that the mCenp-F-mediated cell cycle delay does not involve transcriptional activity of key cell cycle regulators such as Rb, E2F, p53, or Myc. However, endogenous mCenp-F colocalizes with pRb and p107, which demonstrates in vivo protein-protein interaction during cell division. These observations suggest that the domains of the C-terminus of mCenp-F have a conserved function in control of mitotic progression through protein-protein interaction with pocket proteins, thus providing a direct connection between cell cycle regulation and mitotic progression.