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PubMed1998

Differential effects of caffeine on DNA damage and replication cell cycle checkpoints in the fission yeast Schizosaccharomyces pombe.

Caffeine potentiates the lethal effects of ultraviolet and ionising radiation on wild-type Schizosaccharomyces pombe cells. In previous studies this was attributed to the inhibition by caffeine of a novel DNA repair pathway in S. pombe that was absent in the budding yeast Saccharomyces cerevisiae. Studies with radiation-sensitive S. pombe mutants suggested that this caffeine-sensitive pathway could repair ultraviolet radiation damage in the absence of nucleotide excision repair. The alternative pathway was thought to be recombinational and to operate in the G2 phase of the cell cycle. However, in this study we show that cells held in G1 of the cell cycle can remove ultraviolet-induced lesions in the absence of nucleotide excision repair. We also show that recombination-defective mutants, and those now known to define the alternative repair pathway, still exhibit the caffeine effect. Our observations suggest that the basis of the caffeine effect is not due to direct inhibition of recombinational repair. The mutants originally thought to be involved in a caffeine-sensitive recombinational repair process are now known to be defective in arresting the cell cycle in S and/or G2 following DNA damage or incomplete replication. The gene products may also have an additional role in a DNA repair or damage tolerance pathway. The effect of caffeine could, therefore, be due to interference with DNA damage checkpoints, or inhibition of the DNA damage repair/tolerance pathway. Using a combination of flow cytometric analysis, mitotic index analysis and fluorescence microscopy we show that caffeine interferes with intra-S phase and G2 DNA damage checkpoints, overcoming cell cycle delays associated with damaged DNA. In contrast, caffeine has no effect on the DNA replication S phase checkpoint in response to inhibition of DNA synthesis by hydroxyurea.

Caffeine↗
PubMed1995

Changes in network topology during the replication of kinetoplast DNA.

Kinetoplast DNA of Crithidia fasciculata is a network containing several thousand topologically interlocked DNA minicircles. In the prereplicative Form I network, each of the 5000 minicircles is intact and linked to an average of three neighbors (i.e. the minicircle valence is 3). Replication involves the release of minicircles from the interior of the network, the synthesis of nicked or gapped progeny minicircles and the attachment of the progeny to the network periphery. The ultimate result is a Form II network of 10,000 nicked or gapped minicircles. Our measurements of minicircle valence and density, and the network's surface area, revealed striking changes in network topology during replication. During the S phase, the peripheral newly replicated minicircles have a density twice that of minicircles in Form I networks, which suggests that the valence might be as high as 6. Most of the holes in the central region that occur from the removal of intact minicircles are repaired so that the central density and valence remain the same, as in prereplicative networks. When minicircle replication is complete at the end of the S phase, the isolated network has the surface area of a prereplicative network, despite having twice the number of minicircles. During the G2 phase, the Form II network undergoes a remodeling in which the area doubles and the valence is reduced to 3. Finally, the interruptions in the minicircles are repaired and the double-sized network splits in two.

Animals↗
PubMed1991

Cell kinetic perturbations after irradiation and caffeine in the BG-1 ovarian carcinoma cell line.

The treatment of ovarian carcinoma includes maximum surgical removal of the tumor tissue followed by irradiation or chemotherapy. In this study, the effects of caffeine on cell cycle traverse have been studied over a 168-hr period after X irradiation in BG-1 cells, an ovarian carcinoma cell line. The results were obtained with dual-parameter flow cytometric measurements of DNA and nuclear protein, using propidium iodide and fluorescein isothiocyanate. After radiation alone, a dose-related arrest of cells in G2 phase and cell kill were observed. Irradiating BG-1 cells with 5 Gy produced an accumulation of the cells in G2 at 24-72 hr postirradiation. When G2 was divided into low nuclear protein (G2A) and high nuclear protein (G2B) compartments, there was a G2A peak accumulation at 24 hr and a G2B peak accumulation at 48-72 hr. The addition of 1 mM caffeine to the culture media, starting immediately postirradiation, prevented G2 arrest, promoting a rapid traverse of cells through G2A to G2B to G1, which was associated with diminished survival.

Caffeine↗
PubMed1989

[A cell population analysis of the initial period of hepatocyte proliferation in the regenerating mouse liver].

Morphological autoradiography was used to determine the hepatocyte percentage in the regenerating liver of mice in S-, G-, M-, post-M phases at various times after partial hepatectomy, namely at 24, 30, 36 and 42 hours. The total number of cells in the above mentioned phases turned out to be very little at 24 and 30 hours. At 36 h., the percentage of cells in S phase (labelled nuclei) as well as in G2 phase (unlabelled morphologically obvious premitotic nuclei) appeared to be high. The two alternative explanation of G2 hepatocytes emerging at 36 hours were verified to find out if they are a) that is called "a reserved G2 population", or b) a population characterized by a shorter (down to 2 or 3 h) time of S phase.

Animals↗
PubMed2002

Increased expression of T-fimbrin gene after DNA damage in CHO cells and inactivation of T-fimbrin by CpG methylation in human colorectal cancer cells.

When DNA damage is induced by unprogrammed extrinsic events, activating-cell-cycle checkpoints delay cell-cycle progression in the G1 or G2 phases and allow repair of a damaged template. In this study, we evaluated changes in gene expression upon radiation-induced G2 cell-cycle arrest using Chinese hamster ovary (CHO) cells. T-fimbrin, an actin-binding protein, was overexpressed in CHO cells in which G2 arrest had been induced by X-radiation. Northern blot analysis revealed that T-fimbrin gene expression was induced not only by X-radiation but also by a topoisomerase II inhibitor, etoposide. Transfection of CHO cells with a vector encoding T-fimbrin antisense RNA demonstrated that reduced T-fimbrin expression induced alterations in cell-cycle control; radiation-induced G2 arrest was short and decreased in cells transfected with antisense T-fimbrin. Additionally, T-fimbrin gene expression was suppressed in a human colorectal cancer cell line, SW948, because of promoter-specific DNA methylation. These results suggest that downregulation of T-fimbrin may be involved in cancer development through G2/M cell-cycle control in mammalian cells.

Animals↗
PubMed1992

Herpes simplex virus immediate early gene expression in the absence of transinduction by Vmw65 varies during the cell cycle.

The requirement for the herpes simplex virus type 1 (HSV-1) protein Vmw65 (VP16) for activation of immediate early (IE) gene expression was examined in synchronized HeLa cells. Analyses of IE RNA levels were conducted during infection with a viral Vmw65 mutant, in1814. The results revealed an increased requirement for Vmw65 when cultures reached G2 phase of the cell cycle. The levels of IE RNAs 1, 2, and 4 were reduced 5-10 times more in G2 than G1/S for in1814-infected cells when compared to cells infected with wild-type virus or 1814R (a rescued virus), and similar but smaller effects were observed on IE RNA 3 levels. The relative decrease at G2 was reversed by resynchronization of cells to G1/S. Mutant in1814 formed plaques less efficiently on cells at G2 than on cells synchronized at G1/S. The results show that, in the absence of functional Vmw65, HSV-1 IE gene expression and replication vary during the cell cycle.

Base Sequence↗
PubMed1995

Isolation of a cDNA encoding a growth-arrest associated gene and characterization of its regulation.

We are interested in understanding the molecular events associated with the growth-arrest of vascular SMCs. We constructed a subtracted cDNA library enriched in nucleotide sequences associated with quiescent SMCs. This library was screened with similarly subtracted 32P-labeled cDNAs to identify growth-arrest associated cDNA clones. Characterization of 19 of these cDNA clones revealed that 9 hybridized to mRNAs that exhibited a 2-3-fold increase in growth-arrested SMCs. In addition, two other cDNAs hybridized to a 5 Kb mRNA that was elevated approximately 10-fold in high density growth-arrested SMCs. Genomic Southern blot hybridization and DNA sequencing analysis indicated that these cDNAs encoded the same gene (LG7) and that this gene may be a member of a multigene family or that it may contain a sequence shared by other unrelated genes. Augmented expression of LG7 was associated with both high cell density and serum deprivation induced growth-arrest. LG7 mRNA expression was down-regulated when SMCs were incubated with FBS or with reagents that arrest cells in early S-phase. Additional analysis with cell cycle specific inhibitors indicated that LG7 mRNA levels were also low when cells were blocked at the G2 phase of the cell cycle but blockage at mitosis resulted in an elevated level of LG7 mRNA. We further demonstrated that the expression of LG7 was dependent on the presence of a relatively labile protein since protein synthesis inhibitors specifically blocked the expression of this mRNA but not the mRNA expression of alpha 1(III) collagen or ferritin H-chain. Finally, we demonstrated that Bt2cAMP was able to induce mRNA expression of LG7 within 2 h, suggesting that this gene may be directly regulated via the cyclic-AMP-dependent protein kinase pathway.

Animals↗
PubMed2005

G2 damage checkpoints: what is the turn-on?

Cells mount a coordinated response to DNA damage, activating DNA repair pathways and cell-cycle checkpoint pathways to allow time for DNA repair to occur. In human cells, checkpoint responses can be divided into p53-dependent and p53-independent pathways, the latter being predominant in G2 phase of the cell cycle. The p53-independent pathway involves a phosphorylation cascade that activates the Chk1 effector kinase and induces G2 arrest through inhibitory tyrosine phosphorylation of Cdc2. At the top of this cascade are the ATR and ATM kinases. How ATM and ATR recognize DNA damage and activate this checkpoint pathway is only beginning to emerge. Single-stranded DNA, a result of stalled DNA replication or processing of chromosomal lesions, appears to be central to the activation of ATR. The recruitment of replication protein A to single-stranded DNA facilitates the recruitment of several complexes of checkpoint proteins. In this context, ATR is activated and then phosphorylates the C-terminus of Chk1, activating it to enforce a block to mitotic entry.

Ataxia Telangiectasia Mutated Proteins↗
PubMed2005

Good timing in the cell cycle for precise DNA repair by BRCA1.

It is now clear that large DNA-binding proteins have evolved in mammals to orchestrate the relatively ancient process of DNA recombinational repair. These proteins are recruited to accurately repair DNA double strand breaks (DSBs)--the frequent, potentially lethal and mutagenic lesions in the genomes of all organisms. An essential mammalian regulator of DSB repair is BRCA1. Heterozygous BRCA1 mutations predispose individuals to breast, ovarian and other secondary cancers. BRCA1-defective cells exhibit reduced DSB repair, sensitivity to a wide range of DNA damaging agents, genomic instability and defects in the S-phase checkpoint, transcription and chromatin remodelling. DSBs can be repaired by RAD51/RPA-dependent homologous recombination (HR) or DNA-PK-dependent non-homologous end-joining (NHEJ). Both of these pathways can be imprecise and mutagenic. BRCA1 plays a central role in promoting accurate repair by both HR and NHEJ. Consistent with recent evidence, we have assembled a novel cell-cycle-dependent model in which DNA-PK inhibits RPA in S-phase of the cell cycle, while BRCA1 inhibits the exonuclease processivity of the MRE11/RAD50/NBS1 (MRN) complex and facilitates the removal of RPA in S and G2 phase. This model provides an explanation for how BRCA1 promotes accurate DSB repair during various phases of the cell cycle and also accounts for the dual effects that BRCA1 and MRN activity have upon DNA repair and S-phase arrest.

Acid Anhydride Hydrolases↗
PubMed1983

Effects of exogenous cyclic AMP on growth characteristics and radiation response of Reuber H35 hepatoma cells.

Reuber H35 rat hepatoma cells, clone KRC, were used to study the effect of cyclic AMP on radiation-induced cell death. Treatment of logarithmically growing cultures with 0.5 mM cAMP for 17 hr prior to irradiation resulted in a decreased cell survival. Similar results were obtained with cultures irradiated after treatment with Bt2cAMP. Treatment of H35 cells with cAMP or Bt2cAMP caused inhibition of their proliferation and resulted in an accumulation of cells in early S phase and a depletion of G2-phase cells. In synchronized cultures cells were relatively radioresistant during their S phase. In addition to single-dose treatment with X rays, the effect of Bt2cAMP on radiation-induced cell death was studied during fractionated irradiation with 2.5 Gy per day. This fractionated irradiation resulted in a dose-reduction factor of 1.6 at the 10% survival level and a 10-fold decrease in the surviving cell population due to the cooperative effects of Bt2cAMP on growth rate and radiation survival. The effect of cAMP on radiation-induced mitotic delay was also studied. It appeared that whereas cAMP had no effect on the progression of G2 cells into mitosis, it prevented cells from recovery from the X-ray mitotic delay in G2.

Animals↗
PubMed2003

Mitotic G2-arrest is required for neural cell fate determination in Drosophila.

In the wing discs of Drosophila, the mecanosensory precursor cells are singled out from clusters of cells blocked at the G2 phase of the cell cycle. This mitotic quiescence and the selection of the precursors are under strict spatio-temporal control. We forced G2 cells to enter mitosis by overexpression of string, the Drosophila homologue of the cdc25 gene. Premature entrance in the cell cycle is associated to a loss of precursor cells. Precursors are lost consecutively to a transcriptional down-regulation of the determinant proneural achaete/scute genes. This down-regulation results from an over-activation of the Enhancer of Split genes, known as effectors of the Notch signalling pathway. We conclude that exit from the cell cycle is required for proper neural cell fate determination.

Animals↗
PubMed2005

Cross-talk between DNA damage and cell survival checkpoints during G2 and mitosis: pharmacologic implications.

In this study, we wanted to clarify the role of survivin-mediated survival signaling during G2 and M in tumor cells treated with DNA-damaging agents. As a cellular model, we selected MOLT-4 human T-cell lymphoblastic leukemia cells that overexpress survivin and nonfunctional p53. Treatment with melphalan, a classic DNA-damaging agent, led to the induction of the DNA damage checkpoint and growth arrest in the G2 phase of the cell cycle. Checkpoint abrogation by caffeine was accompanied by mitotic entry and rapid apoptotic cell death, whereas cells remaining in G2 remained viable during the same time interval. Unexpectedly, when the spindle checkpoint was activated following G2 abrogation, two different effects could be observed. If the microtubules of the melphalan-treated cells were destabilized by nocodazole, cells became arrested in prometaphase with low survivin levels and entered apoptosis. In contrast, if the microtubules of the melphalan-treated cells were stabilized by taxol, cells were still arrested in prometaphase, but apoptotic execution was inhibited. This effect is, most likely, directly mediated by survivin itself given its well-established antiapoptotic functions. In conclusion, depending on the way the spindle checkpoint was activated in cells with damaged DNA, cells could be either protected by survivin or die during mitosis. We suggest that the efficacy of DNA damage checkpoint abrogators used in combination with DNA-damaging agents may critically depend on whether DNA damage is able to invoke spindle checkpoint response and to activate survivin-associated survival signaling during mitosis.

Antineoplastic Agents, Alkylating↗
PubMed1996

The Xenopus origin recognition complex is essential for DNA replication and MCM binding to chromatin.

BACKGROUND: The origin recognition complex (ORC) and the minichromosome maintenance (MCM) protein complex were initially discovered in yeast and shown to be essential for DNA replication. Homologues of ORC and MCM proteins exist in higher eukaryotes, including Xenopus. The Xenopus MCM proteins and the Xenopus homologues of Saccharomyces cerevisiae Orc 1p and Orc2p (XOrc1 and XOrc2) have recently been shown to be essential for DNA replication. Here, we describe the different but interdependent functions of the ORC and MCM complexes in DNA replication in Xenopus egg extracts. RESULTS: The XOrc1 and XOrc2 proteins are present in the same multiprotein complex in Xenopus egg extracts. Immunodepletion of ORC inhibits DNA replication of Xenopus sperm nuclei. Mixing MCM-depleted and ORC-depleted extracts restores replication capacity. ORC does not co-localize with sites of DNA replication during elongation. However, at initiation the two staining patterns overlap. In contrast to MCMs, which are displaced from chromatin during S phase, XOrc1 and XOrc2 are nuclear chromatin-bound proteins throughout interphase and move to the cytoplasm in mitosis. Permeable HeLa G1- and G2-phase nuclei can replicate in ORC-depleted extract, consistent with the presence of chromatin-bound ORC in both pre-replicative and post-replicative nuclei. Interestingly, the binding of ORC to chromatin does not require the presence of MCMs; however, the binding of MCM proteins to chromatin is dependent on the presence of ORC. CONCLUSIONS: The Xenopus ORC and the MCM protein complex perform essential, non-redundant functions in DNA replication. Xenopus ORC is bound to chromatin throughout interphase but, in contrast to S. cerevisiae ORC, it appears to be, at least partly, displaced from chromatin during mitosis. The binding of MCM proteins requires the presence of ORC. Thus, the assembly of replication-competent chromatin involves the sequential binding of ORC and MCMs to DNA.

Animals↗
PubMed1981

DNA synthesis in human duodenal biopsies maintained in organ culture.

Incorporation of 3H-thymidine during organ culture was studied in duodenal biopsies from 14 patients. Pulse-label at various intervals disclosed active incorporations during the first 2 h in culture. Labelling index declined to low levels at 3-4 h. Thereafter incorporation increased again and persisted throughout the rest of the culture period of 11 h. The DNA synthesis rate of crypt cells between 4 and 11 h in culture was calculated in 5 patients after pulse-label and continuous labelling of explants in parallel culture. The rate of entry into DNA synthesis was about 24 cells per 1,000 crypt cells per hour in flat, coeliac biopsies, versus 9-13 in controls, Gluten did not influence DNA synthesis rate, whereas wheat germ lectin inhibited DNA synthesis. Counting of the total number of mitoses and labelled fraction of mitoses disclosed active crypt cell renewal in flat, coeliac biopsies. In normal-appearing biopsies no mitoses were labelled, indicating delayed exit from S-phase or long duration of G2-phase in these explants.

Atrophy↗
PubMed1981

Separation of AKR mouse thymus lymphoma from normal thymic cells by centrifugal elutriation.

The rapid separation of large numbers of viable thymus cells from AKR mice bearing transplanted or spontaneous thymic lymphomas was achieved by centrifugal elutriation. Separation of more than 3 x 10(8) cells from either a transplanted lymphoma, designated 720, or from a spontaneous thymic lymphoma, required only 15 min. Unfractionated thymus cells obtained from mice bearing the transplanted lymphoma consisted of 80% lymphoma cells (by immunofluorescence for the viral protein, gp70) and 20% normal cells. Fractions of slowly sedimenting cells consisted almost exclusively of normal cells (95%) with modal volumes of 95 micrometers cubed. Fractions of rapidly sedimenting cells consisted of 95% tumor cells with volumes of 150-400 micrometers cubed. The slowly sedimenting cells were almost exclusively (98%) in the GO- or G1-phase. Fractions of rapidly sedimenting cells contained up to 55% S-phase and up to 30% G2-phase cells. Intermediate fractions contained mixtures of normal cells and small GO- or G1-phase tumor cells. Thymidine uptake by the separated cells was determined. The fractions containing normal cells showed little thymidine uptake after 4 and 48 h in culture, while the fractions of tumor cells showed high levels of incorporation. In contrast to the high levels of thymidine uptake by the tumor cell fractions after 48 h in culture, there was little uptake by the unseparated cell suspension, suggesting a possible interaction between normal and tumor cells during the culture period.

Animals↗
PubMed1992

Effect of anticancer agents on cell cycle of regenerating hepatocytes in rats.

The effect of three anticancer agents, cisplatin, doxorubicin, and mitomycin, on liver regeneration after 70% partial hepatectomy in rats was investigated by total DNA content of the liver and flow cytometric analysis of hepatocyte nuclei using two-color staining of anti-bromodeoxyuridine monoclonal antibody and propidium iodide. Total DNA content of regenerating liver 7 days after hepatectomy showed significant suppression of regeneration by these agents (P less than 0.01). The inhibitory effects of the agents on the cell cycle of hepatocytes by flow cytometric analysis were (1) a delay of the peak or a decrease in the proportion of S phase nuclei and/or (2) polyploidization of the nuclei, demonstrated by accumulation of 8c and occasionally 16c nuclei, of which the DNA contents were four and eight times as much as that of diploid (2c) cell nuclei, respectively. The former (1) suggests G0 or G1 phase block, and the latter (2) G2 phase block. In terms of total DNA content of regenerating liver, the inhibitory effect was most prominent in the cisplatin-administered groups. The polyploidization of nuclei was most remarkable in the mitomycin-administered groups. Although the total DNA content recovered to the level of control at 6 weeks after hepatectomy, the polyploidization effect persisted in the drug-administered groups. These agents had no cytocidal action on proliferating hepatocytes as can be seen from the aspartate aminotransferase and alanine aminotransferase serum levels. We conclude that in a short-term observation, the anti-cancer agents significantly inhibit liver regeneration, although the inhibitory effect on DNA synthesis does not last long.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

G2 repair and aging: influence of donor age on chromosomal aberrations in human lymphocytes.

The caffeine effects on chromosomal aberration frequency and mean G2 duration were studied in human lymphocytes in vitro from three age groups of normal donors (I: 1-5 years old; II: 30-40 years old; III: 60-70 years old). Under control conditions, the three age groups showed a similar frequency of chromosomal aberrations. All three age groups exhibited a linear dose response for aberrations with caffeine treatments. However, lymphocytes from aged individuals (groups II and III) showed higher chromosomal aberration frequencies and longer G2 duration than cells from young individuals (group I). The caffeine effect in reducing G2 length was rather similar in every age group. The reversion of caffeine effects by adenosine or niacinamide in lymphocytes from older individuals was higher than in cells from group I. The different caffeine effects and G2 values between lymphocytes from old and young individuals are most likely due to a higher number of DNA lesions reaching G2 phase and/or a decrease of the G2 repair capability of lymphocytes from older individuals.

Adenosine↗
PubMed1998

Comparison between radiation-induced cell cycle delay in lymphocytes and radiotherapy response in head and neck cancer.

A study was made evaluating the use of radiation-induced cell cycle delay in lymphocytes to predict tumour response to radiotherapy. Peripheral blood lymphocytes were isolated from whole blood from 49 patients with head and neck cancer before treatment with radiotherapy and from 25 healthy donors. The clinical response to radiotherapy was assessed at 0-2 months after treatment. The level of radiation-induced cell cycle delay was measured using flow cytometry after mitogen stimulation of lymphocytes. The analysis of ten normal donors gave no significant difference in variability between the intra-assay and the intra-donor samples. However, the cell cycle data for lymphocytes from these healthy donors showed significant inter-individual differences in G2 phase accumulation. Patients showing no response to radiotherapy had a high level of S-phase cells compared with partial (P < 0.001) and complete responders (P = 0.016). An inverse relationship was found when analysing the fraction of cells in G2 (P = 0.009 and 0.034 respectively). In general, healthy donors had similar cell cycle kinetics compared with the non-responders. In conclusion, the result indicates that radiation-induced cell cycle delay in lymphocytes is inversely correlated with tumour response to radiotherapy in head and neck cancer patients. However, the value of the present test for predicting individual tumour response is limited, because of assay variability and overlap between groups.

Adult↗