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At least 163 records · Page 9Linked to original sources
PubMed1993

Cytogenetic evidence for differences in DNA incision activity in xeroderma pigmentosum group A, C and D cells after X-irradiation during G2 phase.

The capacity of cells to incise DNA to remove altered sites after DNA damage can be determined from the rate of DNA-strand break accumulation in the presence of an inhibitor of DNA-repair synthesis, such as 1-beta-D-arabinofuranosylcytosine (ara-C). Because each chromatid contains a single continuous molecule of double-stranded DNA, chromatid breaks and gaps, i.e., non-displaced breaks, represent unrepaired DNA-strand breaks. The accumulation of chromatid breaks and gaps after X-irradiation in the presence of ara-C thus provides a measure of DNA incision activity. Addition of ara-C to skin fibroblasts or stimulated blood lymphocytes from normal individuals at intervals after X-irradiation significantly increased frequencies of chromatid breaks and/or gaps. In contrast, addition of ara-C to XP cells of complementation groups A and D had a negligible effect and a significant but less than normal effect on XP cells of complementation group C and one sample of blood lymphocytes of undetermined complementation group. The results thus show negligible incision activity after G2 phase X-irradiation in XP-A and XP-D cells and a level higher but less than normal in XP-C cells.

Cell Line↗
PubMed2006

PML nuclear bodies are highly organised DNA-protein structures with a function in heterochromatin remodelling at the G2 phase.

We have recently demonstrated that heterochromatin HP1 proteins are aberrantly distributed in lymphocytes of patients with immunodeficiency, centromeric instability and facial dysmorphy (ICF) syndrome. The three HP1 proteins accumulate in one giant body over the 1qh and 16qh juxtacentromeric heterochromatins, which are hypomethylated in ICF. The presence of PML (promyelocytic leukaemia) protein within this body suggests it to be a giant PML nuclear body (PML-NB). The structural integrity of PML-NBs is of major importance for normal cell functioning. Nevertheless, the structural organisation and the functions of these nuclear bodies remain unclear. Here, we take advantage of the large size of the giant body to demonstrate that it contains a core of satellite DNA with proteins being organised in ordered concentric layers forming a sphere around it. We extend these results to normal PML-NBs and propose a model for the general organisation of these structures at the G2 phase. Moreover, based on the presence of satellite DNA and the proteins HP1, BRCA1, ATRX and DAXX within the PML-NBs, we propose that these structures have a specific function: the re-establishment of the condensed heterochromatic state on late-replicated satellite DNA. Our findings that chromatin-remodelling proteins fail to accumulate around satellite DNA in PML-deficient NB4 cells support a central role for PML protein in this cellular function.

Cell Line, Tumor↗
PubMed1978

Size distribution and maturation of newly replicated DNA through the S and G2 phases of Physarum polycephalum.

The size distribution of newly made DNA and the dynamics of size maturation of progeny DNA molecules were studied in the synchronous S and G2 phases of Physarum polycephalum. Pulse labeling of DNA and analysis of the products on alkaline sucrose gradients showed that synthesis of primary replication units (which will also be referred to as "Okazaki" fragments) occurred throughout the S period. Pulse and pulse-chase experiments revealed a distinct pattern of size maturation. An apparently linear increase in molecular weight of progeny DNA molecules during the first hour of the S phase occurred at a rate of approximately 4-5 X 10(5) daltons per min at 26 degrees C, corresponding to the joining of 6-8 Okazaki fragments. The resulting 35-45S (1.1-2.2 X 10(7) daltons) DNA molecules may correspond to the Physarum "replicon." The further size increases of the newly made DNA appear to occur in steps, possibly reflecting a clustering of isochronous replicons along the chromatide. These observations are discussed with regard to mechanisms of DNA replication and size maturation.

Cell Cycle↗
PubMed1996

An inhibitor of the chymotrypsin-like activity of the multicatalytic proteinase complex (20S proteasome) induces arrest in G2-phase and metaphase in HeLa cells.

HeLa cells were treated with different concentrations of an inhibitor of the proteasome chymotrypsin-like activity, the peptidyl aldehyde N-benzyloxycarbonyl-Ile-Glu(O-t-butyl)-Ala-leucinal (PSI). A detailed analysis, which included flow cytometry, cell counting and morphological assessment, was performed. PSI treatment induces a significant reduction of mitotic activity, accompanied by metaphase arrest of the mitotic cells. DNA flow cytometry shows an accumulation of the cells in G2+M phases of the cell cycle, which indicates the existence of a proteasome-mediated step in the G2-phase of the cell cycle. After removal of the inhibitor and supplementation with fresh medium, the cell cycle is resumed, but the mitotic cells show increased misalignment of chromosomes in the metaphase plate. PSI also induces HeLa cells to acquire a fibroblastoid phenotype.

Chymotrypsin↗
PubMed1999

Estimation of kinetic cell-cycle-related gene expression in G1 and G2 phases from immunofluorescence flow cytometry data.

BACKGROUND: Flow cytometry of immunofluorescence and DNA content provides measures of cell-cycle-related gene expression (protein and/or epitope levels) for asynchronously growing cells. From these data, time-related expression through S phase can be directly measured. However, for G1, G2, and M phases, this information is unavailable. We present an objective method to model G1 and G2 kinetic expression from an estimate of a minimum biological unit of positive immunofluorescence derived from the distribution of specific immunofluorescence of mitotic cells. METHODS: DU 145 cells were stained for DNA, cyclin B1, and a mitotic marker (p105) and analyzed by flow cytometry. The cyclin B1 immunofluorescence (B1) distribution of p105-positive cells was used to model the B1 distribution of G2 and G1 cells. The G1/S and S/G2 interface measurements were used to calculate expression in S phase and test the validity of the approach. RESULTS: B1 at S/G2 closely matched the earliest modeled estimate of B1 in G2. B1 increased linearly through G1 and S but exponentially through G2; mitotic levels were equivalent to the highest G2 levels. G1 modeling of B1 was less certain than that of G2 due to low levels of expression but demonstrated general feasibility. CONCLUSIONS: By this method, the upper and lower bounds of cyclin B1 expression could be estimated and kinetic expression through G1, G2, and M modeled. Together with direct measurements in S phase, expression of B1 throughout the entire cell cycle of DU 145 cells could be modeled. The method should be generally applicable given model-specific assumptions.

Cell Cycle↗
PubMed1999

Association between G2-phase block and repair of radiation-induced chromosome fragments in human lymphocytes.

We have studied the induction of chromosomal aberrations in human lymphocytes exposed in G0 to X rays or carbon ions. Aberrations were analyzed in G0, G1, G2 or M phase. Analysis during the interphase was performed by chemically induced premature chromosome condensation, which allows scoring of aberrations in G1, G2 and M phase; fusion-induced premature chromosome condensation was used to analyze the damage in G0 cells after incubation for repair; M-phase cells were obtained by conventional Colcemid block. Aberrations were scored by Giemsa staining or fluorescence in situ hybridization (chromosomes 2 and 4). Similar yields of fragments were observed in G1 and G2 phase, but lower yields were scored in metaphase. The frequency of chromosomal exchanges was similar in G0 (after repair), G2 and M phase for cells exposed to X rays, while a lower frequency of exchanges was observed in M phase when lymphocytes were irradiated with high-LET carbon ions. The results suggest that radiation-induced G2-phase block is associated with unrejoined chromosome fragments induced by radiation exposure during G0.

Adult↗
PubMed2005

Inhibition of mixed-lineage kinase (MLK) activity during G2-phase disrupts microtubule formation and mitotic progression in HeLa cells.

The mixed-lineage kinases (MLK) are serine/threonine protein kinases that regulate mitogen-activated protein (MAP) kinase signaling pathways in response to extracellular signals. Recent studies indicate that MLK activity may promote neuronal cell death through activation of the c-Jun NH2-terminal kinase (JNK) family of MAP kinases. Thus, inhibitors of MLK activity may be clinically useful for delaying the progression of neurodegenerative diseases, such as Parkinson's. In proliferating non-neuronal cells, MLK may have the opposite effect of promoting cell proliferation. In the current studies we examined the requirement for MLK proteins in regulating cell proliferation by examining MLK function during G2 and M-phase of the cell cycle. The MLK inhibitor CEP-11004 prevented HeLa cell proliferation by delaying mitotic progression. Closer examination revealed that HeLa cells treated with CEP-11004 during G2-phase entered mitosis similar to untreated G2-phase cells. However, CEP-11004 treated cells failed to properly exit mitosis and arrested in a pro-metaphase state. Partial reversal of the CEP-11004 induced mitotic arrest could be achieved by overexpression of exogenous MLK3. The effects of CEP-11004 treatment on mitotic events included the inhibition of histone H3 phosphorylation during prophase and prior to nuclear envelope breakdown and the formation of aberrant mitotic spindles. These data indicate that MLK3 might be a unique target to selectively inhibit transformed cell proliferation by disrupting mitotic spindle formation resulting in mitotic arrest.

Animals↗
PubMed1990

Analysis of events associated with cell cycle arrest at G2 phase and cell death induced by cisplatin.

DNA is the accepted target for cisplatin, but recent evidence has shed doubt on DNA synthesis as the critical process. L1210/0 cells incubated for 2 hours with cisplatin progress to the G2 phase of the cell cycle and are arrested there for several days. They then either progress in the cell cycle or die. In cells that eventually die, total transcription, polyadenylated [poly(A)+] RNA synthesis, and protein synthesis were markedly inhibited only after 48 hours. Nicotinamide adenine dinucleotide (NAD) and adenosine triphosphate (ATP) levels decreased after 3 days. Cell membrane integrity was lost after 4 days. These results demonstrate that cells can be lethally damaged, yet continue to undergo apparently normal metabolic activities for several days. In a previous study, DNA double-strand breaks were detected after 1 day. We now show that by 2 days, breaks are visible as fragmentation in the nucleosome spacer regions of chromatin. This type of damage is consistent with cell death occurring by the process of apoptosis. Cell shrinkage and morphology were also consistent with this type of cell death. The slow cell death reported here appears to occur at the G2/M transition and may involve events that normally occur at this stage of the cell cycle. These results demonstrate the importance of DNA degradation as an early and possibly essential step in cell death.

Adenosine Triphosphate↗
PubMed1997

Mammalian G1 and G2 phase checkpoints.

This present review explores the mechanisms for DNA damage induced G1 and G2 arrest in mammalian cells. The complexity of the TP53 pathway is attested to by the variety of genes regulated by TP53, many of which require further investigation to bring their importance into focus. One gene intensely studied, p21, has been linked to the G1 arrest mechanism and may, like TP53, be involved in some aspect of DNA repair. The outcome of TP53 activation for cell survival is equally complex and relies much upon cellular context and the type of DNA damaging agent employed. Although TP53 may participate in sensing DNA damage, additional components are likely to be required. Much of the focus on defining the mechanism of G2 arrest in mammalian cells has concentrated on the cyclin B1/CDC2 kinase. Activation of this kinase is suppressed by DNA damage, and this may result from the imposition of inhibitory phosphorylations on the CDC2 kinase as well as downregulation of cyclin B1 levels. The logical point where the G2 checkpoint interacts with the CDC2-CDC25C autocatalytic loop to prevent CDC2 activation remains to be defined and could involve inhibition of CDC25C-CDC2 interaction. It is hoped that moving upstream of CDC2 towards the point where DNA damage is sensed by the cell will uncover homologues of yeast components implicated in G2 checkpoint control. The finding that certain G2 checkpoint abrogators preferentially synergize with DNA damaging agents in cells with defective TP53 provides a potential pharmacological route through which TP53 defective cells might be targeted for destruction. Further exploration of this vulnerability might prove useful for future anti-cancer drug discovery efforts.

Animals↗
PubMed1989

Deficient G2 phase repair of radiation-induced chromatin damage in the SENCAR mouse.

The SENCAR mouse, selectively bred for sensitivity to skin tumor induction by two-stage chemical carcinogenesis, is also genetically hypersensitive to skin carcinogenesis by high doses of UV. Skin fibroblasts from the SENCAR mouse exposed during G2 phase to radiation associated with intracellular hydroxyl radical formation, i.e. X-rays or near UV/visible light, showed a higher frequency of chromatid breaks at 2 h post-irradiation than skin fibroblasts from six other inbred strains of mice including C3H/HeN. However the level was similar to that reported previously for the BALB/cAn mouse known to be resistant to chemical carcinogenesis of skin. When exposed for 2 h to visible light (8 W/m2), chromatid breaks persisted in fibroblasts from the SENCAR mouse during the 4 h post-irradiation period, but showed an abrupt decline in C3H/HeN fibroblasts within 1 h after irradiation. Skin fibroblasts from the tumor-sensitive SENCAR mouse differed from those of the resistant BALB/cAn in response to beta-cytosine arabinoside (ara-C) an inhibitor of the repair polymerase. Ara-C added to cultures of SENCAR and BALB/cAn cells after G2 X-irradiation (68R) increased chromatid gaps 18-fold and breaks 2-fold in BALB/cAn cells but had no effect on SENCAR cells even though incorporated into DNA to the same extent. The present results suggest that the SENCAR mouse is deficient in at least two aspects of DNA repair: (i) ligation or a prior step in the repair process leading to ligation and (ii) repair endonucleolytic incision at DNA lesions induced by irradiation.

Animals↗
PubMed1990

Persistence of chromatid damage after G2 phase X-irradiation in lymphoblastoid cells from Gardner's syndrome.

Previous reports showed that skin fibroblasts or peripheral blood lymphocytes from individuals with hereditary cancer or with a genetic disorder predisposing to cancer show an abnormally high frequency of chromatid damage after X-irradiation in G2 phase. The reproducibility of this response suggested that it could provide the basis of an assay for genetic predisposition to cancer. The present blind study tested whether lymphoblastoid cell lines could also be used in this assay. Lymphoblastoid cell lines from patients with Gardner's syndrome (GS) were compared with those from clinically normal controls. In metaphase cells collected during the first 30 min after X-irradiation (58R), frequencies of chromatid breaks and gaps were similar in GS and normal cells. However, in metaphase cells collected from 0.5 to 1.5 h and 1.5 to 2.5 h after X-irradiation, the total unrepaired damage for each GS cell line was greater than that observed in any of the lines from clinically normal controls. The persistence of chromatid damage in the GS cells after X-irradiation suggests a deficiency or imbalance in the repair or processing of the radiation-induced DNA damage. The results show that lymphoblastoid cell lines in early passage can be used in this cytogenetic assay to identify members in a GS family who have the GS gene(s) or other individuals with a genetic predisposition to cancer.

Cells, Cultured↗
PubMedDate not supplied

Trichomonas vaginalis: dominant G2 period and G2 phase arrest in a representative of an early branching eukaryotic lineage.

Eukaryotic mitotic cell cycles have been extensively studied in yeasts and vertebrate cells but little is known about cell cycle mechanisms in early branches of the eukaryotic lineage. Trichomonas vaginalis represents one of the earliest branching eukaryotic lineages available for study. In contrast with most yeasts and vertebrate cells, the T. vaginalis G2 period was prolonged, comprising 50 to 58% of the cell population. Hydroxyurea, aphidicolin, and excess thymidine, all of which arrest yeasts and vertebrate cells at the G1/S phase boundary, had no effect on the T. vaginalis cell cycle, probably due to the known absence of synthetic pathways. The anti-microtubule mitotic inhibitors, colchicine and nocodazole, induced G2 phase synchrony. Metronidazole, a therapeutic reagent, also caused G2 phase arrest. These observations suggest that T. vaginalis is similar to yeasts and vertebrate cells in G2 and M phases, but the parasite's G1/S phase transition is distinctive. The results also suggest potentially therapeutic, anti-trichomonad activity of microtubule inhibitors such as nocodazole. The cultured parasite may prove useful as a model for the mitotic cell cycle in the absence of G1/S phase transitional activities universal in yeasts and vertebrate cells.

Animals↗
PubMed1985

Allocation of the suppressive activity of normal peripheral blood lymphocytes induced by diffusion chamber culture and Con A stimulation to the G2 phase of the cell cycle.

Peripheral blood mononuclear cells stimulated either by diffusion chamber culture or by a high Con A concentration exhibit suppressive activity under conditions where no increase in cell number takes place. Instead an accumulation of large cells is observed which, according to their DNA contents, are classified as cells in the G2 phase of the cell cycle. By elutriation separation the suppressive activity is shown to be confined to this cell cycle phase.

Adult↗
PubMed1983

Chromatid damage after G2 phase x-irradiation of cells from cancer-prone individuals implicates deficiency in DNA repair.

Ten lines of skin fibroblasts from individuals with genetic disorders predisposing to a high risk of cancer were compared with nine lines from normal adult donors with respect to chromatid damage after x-irradiation [25, 50, and 100 rad (0.25, 0.50, and 1 gray)] during G2 phase. The 10 cell lines represented five genetic disorders: Bloom syndrome, familial polyposis, Fanconi anemia, Gardner syndrome, and xeroderma pigmentosum, complementation groups A(XP-A), C(XP-C), E(XP-E), and variant (XP-Va). The incidence of chromatid breaks in all cancer-prone lines except XP-E and XP-A was significantly higher than in the normal lines. The incidence of chromatid gaps in all cancer-prone lines except XP-A and XP-Va was significantly higher than in the normal lines. Because each chromatid apparently contains a single continuous DNA double strand, chromatid breaks and gaps represent unrepaired DNA strand breaks arising directly or indirectly during excision repair of x-ray-induced DNA damage. These cytogenetic data together with results from use of the DNA repair inhibitor arabinofuranosyl cytosine (cytosine arabinoside) suggest that cells from all of these cancer-prone individuals are deficient in some step of DNA repair, predominantly excision repair operative during the G2-prophase period of the cell cycle. It appears that these DNA repair deficiencies are associated with a genetic predisposition to a high risk of cancer.

Abnormalities, Multiple↗
PubMed1985

Radiation sensitivity of Bloom's syndrome lymphocytes during S and G2 phases.

In order to study chromosome sensitivity of Bloom's syndrome (BS) cells in relation to the replication stage, gamma-ray irradiation was performed immediately before adding bromodeoxyuridine (BrdU) to lymphocyte cultures of one BS patient and of one control. It was found that BS cells are much more sensitive to the irradiation than control cells at the end of S and at G2 phases. The rate of induction of chromosome breaks is significantly increased and that of chromatid breaks and exchanges is also increased, though to a lesser degree. Our results also favor the existence of a cell subpopulation in BS characterized by a slow cycle, a high spontaneous chromosome aberration rate, and a high radiation sensitivity.

Bloom Syndrome↗