Effects of X-irradiation and sodium butyrate on cell-cycle traverse of normal and radiosensitive lymphoblastoid cells

We have used a multi-parameter flow-cytometric technique to analyse changes in cell-cycle phase distribution (early and late G1, S and G2+M phases) for normal and X-ray-sensitive (ataxia-telangiectasia, A-T) lymphoblastoid cells exposed to X-irradiation and sodium butyrate (either alone or in combination). Sodium butyrate, an inhibitor of histone deacetylase, is a useful pharmacological tool for determining the proposed role of a histone acetylation-based chromatin surveillance system in controlling cell-cycle responses to DNA damage. We report that X-irradiated A-T cells (acute doses up to 1.5 Gy) demonstrate deficiencies in the capacity to traverse G1 and G2+M phases, although we can find no evidence of the specific involvement of a sodium butyrate-sensitive process in normal cells or abnormalities in the responses of A-T cells to the drug. We conclude that abnormal cellular control of G1 transition in A-T may be the basis of disturbed cellular differentiation in vivo, particularly in non-proliferating tissues under conditions of accumulated environmental or spontaneous DNA damage.     

脾虚证小鼠胸腺细胞周期和免疫细胞化学的实验研究

本文用ＦＣＭ和免疫细胞化学技术检测了小鼠胸腺细胞的活动周期,Ｂｒｄｕ＋细胞和表达Ｔｈｙｌ．２抗原的Ｔ细胞。小鼠包括对照组,利血平诱发的脾虚组,自然恢复组和健脾汤治疗后复健组。ＦＣＭ和免疫细胞化学结果显示,脾虚组小鼠Ｇ１期细胞堆积,Ｓ期和Ｇ２＋Ｍ期细胞减少。同时胸腺内Ｂｒｄｕ＋细胞和Ｔ细胞也相应减少。经过健脾汤治疗后,除Ｇ１期细胞外,Ｓ期和Ｇ２＋Ｍ期细胞,Ｂｒｄｕ＋细胞和Ｔ细胞均显著增加,并且几乎达正常水平。但是自然恢复组在恢复细胞活动周期正常化,以及恢复Ｂｒｄｕ＋细胞和Ｔ细胞方面比复健组缓慢。说明健脾汤对脾虚小鼠的胸腺细胞有促进ＤＮＡ合成和细胞增殖功能。     

An Imperfect G2M Checkpoint Contributes to Chromosome Instability Following Irradiation of S and G2 Phase Cells

DNA double strand break (DSB) repair and checkpoint control represent two major mechanisms that function to reduce chromosomal instability following ionising irradiation (IR). Ataxia telangiectasia (A-T) cells have long been known to have defective checkpoint responses. Recent studies have shown that they also have a DSB repair defect following IR raising the issue of how ATM’s repair and checkpoint functions interplay to maintain chromosomal stability. A-T and Artemis cells manifest an identical and epistatic repair defect throughout the cell cycle demonstrating that ATM’s major repair defect following IR represents Artemis-dependent end-processing. Artemis cells show efficient G2/M checkpoint induction and a prolonged arrest relative to normal cells. Following irradiation of G2 cells, this checkpoint is dependent on ATM and A-T cells fail to show checkpoint arrest. In contrast, cells irradiated during S phase initiate a G2/M checkpoint which is independent of ATM and, significantly, both Artemis and A-T cells show a prolonged arrest at the G2/M checkpoint likely reflecting their repair defect. Strikingly, the G2/M checkpoint is released before the completion of repair when approximately 10-20 DSBs remain both for S phase and G2 phase irradiated cells. This defined sensitivity level of the G2/M checkpoint explains the prolonged arrest in repair-deficient relative to normal cells and provides a conceptual framework for the co-operative phenotype between checkpoint and repair functions in maintaining chromosomal stability.     

Flow cytometric and biochemical analysis of dose-dependent effects of sodium butyrate on human endometrial adenocarcinoma cells.

Sodium butyrate (SB) treatment was previously shown to produce seven-fold increases in estrogen hormone receptor binding sites of human endometrial adenocarcinoma (IK) cells. Flow cytometric analysis and histone gel electrophoresis were used to examine cell cycle, cell metabolism, and nuclear histone fractions in IK cells treated with different concentrations of SB. SB-treated cells stained with fluorochromes specific for DNA, RNA, or general protein were analyzed by flow cytometry (FCM). Changes in accessibility to three DNA stains and gel electrophoresis were used to analyze rearrangements in chromatin structure. SB caused an accumulation of cells in the G1 phase and inhibited DNA synthesis, but not cellular levels of RNA and protein. Hoechst accessibility to A-T rich regions on DNA was dramatically increased after removal of SB. H1 histones were dephosphorylated and core histones were acetylated during SB-treatment. Information obtained in these studies may be useful for correlating cellular and biochemical events with SB-induced increases in nuclear steroid hormone binding sites.     

Comparison of gamma-radiation-induced accumulation of ataxia telangiectasia and control cells in G2 phase

Recent reports from a number of laboratories have linked radiosensitivity in ataxia telangiectasia (A-T) to a large and prolonged block of some cells in G2 phase. Previous results from this laboratory, largely with one Epstein-Barr virus-transformed A-T lymphoblastoid cell line, presented evidence for a dramatic increase in the number of cells in G2 phase over controls during a 24-h period post irradiation. We describe here a study of the effect of gamma-radiation on G2 phase delay in several A-T cell lines. Based on previous results with several cell lines 24 h post irradiation was selected as the optimum time to discriminate between G2 phase delay in control and A-T cells. All A-T homozygotes showed a significantly greater number of cells in G2 phase, 24 h post irradiation, than observed in controls. A more prolonged delay in G2 phase after irradiation was seen in different A-T cell types that included lymphoblastoid cells, fibroblasts and SV40-transformed fibroblasts. At the radiation dose used it was not possible to distinguish A-T heterozygotes from controls.     

Cell cycle perturbations induced by infection with the coronavirus infectious bronchitis virus and their effect on virus replication

In eukaryotic cells, cell growth and division occur in a stepwise, orderly fashion described by a process known as the cell cycle. The relationship between positive-strand RNA viruses and the cell cycle and the concomitant effects on virus replication are not clearly understood. We have shown that infection of asynchronously replicating and synchronized replicating cells with the avian coronavirus infectious bronchitis virus (IBV), a positive-strand RNA virus, resulted in the accumulation of infected cells in the G2/M phase of the cell cycle. Analysis of various cell cycle-regulatory proteins and cellular morphology indicated that there was a down-regulation of cyclins D1 and D2 (G1 regulatory cyclins) and that a proportion of virus-infected cells underwent aberrant cytokinesis, in which the cells underwent nuclear, but not cytoplasmic, division. We assessed the impact of the perturbations on the cell cycle for virus-infected cells and found that IBV-infected G2/M-phase-synchronized cells exhibited increased viral protein production when released from the block when compared to cells synchronized in the G0 phase or asynchronously replicating cells. Our data suggested that IBV induces a G2/M phase arrest in infected cells to promote favorable conditions for viral replication.     

Comparison of cytologic and acridine-orange flow-cytometric detection of malignant cells in human body cavity fluids

Flow cytometrically (FCM) derived DNA and RNA profiles were studied in acridine orange (AO)-stained body cavity fluid (BCF) specimens obtained from 78 patients with various solid tissue and hematologic malignancies. The ploidy (DNA index), RNA content (RNA index), proliferative activity (% S + G2M) and DNA and RNA scattergram patterns were tested "double-blind" against the cytologic scoring of specimens as malignant, benign or reactive. It was determined that expression of an "abnormal" RNA index (greater than or equal to 2.8) and an elevated proliferative activity (% S + G2M greater than or equal to 7.4) was dependent on the presence of malignancy; 21 of 22 specimens having those abnormal indices had DNA aneuploidy and were cytologically scored as positive. The AO FCM sensitivity and specificity for detecting malignant cells (when measured against cytology scoring) were 61% and 90%, respectively, using the "abnormal" RNA index and % S + G2M cut-offs together with the cellular DNA aneuploidy marker. By supplementing the cytologic scoring with AO FCM DNA and RNA features, the sensitivity for detecting malignant cells was 94%, as compared to 72% for cytology alone. Two specimens gave false-positive FCM results: a tuberculous effusion with a tetraploid subpopulation and a reactive mesothelial proliferation that was diploid and negative cytologically. Scoring for malignancy based on the visual pattern of the DNA and RNA FCM scattergrams, while showing good correlation for aneuploid specimens, in some cases failed to identify diploid disease. The results demonstrate the usefulness of FCM DNA and RNA analysis for supplementing cytologic examination of BCF specimens for the purpose of detecting malignant cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

A new method for rapid and sensitive detection of bromodeoxyuridine in DNA-replicating cells

A new flow cytometric technique, involving differential fluorescence analysis of two DNA-binding fluorochromes, was used to quantify cellular incorporation of the base analog, bromodeoxyuridine (BrdU), into DNA over short time periods. During analysis of stained cells, the blue fluorescence signal of Hoechst 33342, which is quenched by BrdU-substituted DNA, was subtracted, on a cell by cell basis, from the green-yellow fluorescence signal of mithramycin, which remained stoichiometric to cellular DNA content. Bivariate contour profiles obtained for CHO cells pulse-labeled for 30 min showed that fluorescence quenching of Hoechst 33342 in BrdU-labeled, S phase cells produced fluorescence difference signals that were significantly greater than the difference signals from G1 and G2 + M phase cells. Analysis of L1210 cells demonstrated that the amount of BrdU detected was proportional to the length of the labeling period. The novel technique is simple, rapid, and mild; it produces minimal cell loss and does not significantly affect cellular moieties such as DNA, chromatin, or RNA.     

A new method to discriminate G1, S, G2, M, and G1 postmitotic cells

A new flow cytometric method combining light scattering measurements, detection of bromodeoxyuridine (BrdU) incorporation via fluorescent antibody, and quantitation of cellular DNA content by propidium iodide (PI) allows identification of additional compartments in the cell cycle. Thus, while cell staining with BrdU-antibodies and PI reveals the G1, S, and G2 + M phases of the cell cycle, differences in light scattering allow separation of G2 phase cells from M phase cells and subdivision of G1 phase into two compartments, i.e., G1A representing postmitotic cells which mature to G1B cells ready to initiate DNA synthesis. The method involves fixation of cells in 70% ethanol, extraction of histones with HC1, and thermal denaturation of DNA. This treatment appears to enhance the differences in chromatin structure of cells in the various phases of the cell cycle to the extent that cells could be separated on the basis of the 90 degrees scatter. Mitotic cells show much lower scatter than G2 phase cells, and G1 postmitotic cells (G1A) show lower scatter than G1 cells about to enter the S phase (G1B). Light scattering is correlated with chromatin condensation, as judged by microscopic evaluation of cells sorted on the basis of light scatter. The method has the advantage over the parental BrdU/DNA bivariate analysis in allowing the G2 and M phases of the cell cycle to be separated and the G1 phase to be analyzed in more detail. The method may also allow separation of unlabeled S phase cells from mitotic cells and distinguish between labeled and unlabeled mitotic cells.     

Expression and isotopic labeling of structural domains of the human protein DEK

The 375 amino acid human protein DEK has been expressed in two functional, structured domains. DEK is an abundant nuclear protein that associates with chromatin and alters its topology by introducing positive supercoiling in DNA, which results in lower replication efficiency. DEK has clinical importance as transfection of the cDNA of the C-terminal region of DEK can partially reverse the abnormal DNA-mutagen sensitivity in fibroblasts derived from ataxia-telangiectasia (A-T) patients, and elevated levels of DEK mRNA are observed in various forms of cancer. Because high-level expression of full-length DEK has proved elusive, we sought an alternative for structural studies that would provide insights on DEK's function. We have discovered that DEK contains two structured domains and have expressed these domains at a high level in Escherichia coli in M9 minimal media. The N-terminal domain (amino acids 68-226) includes the region responsible for introducing supercoils into DNA, and the C-terminal domain (amino acids 309-375) includes the region that can reverse the abnormal DNA-mutagen sensitivity of A-T cells. 1H-15N correlation nuclear magnetic resonance spectra of these two fragments reveal the characteristic signature of folded proteins.     

A critical role for Pin2/TRF1 in ATM-dependent regulation. Inhibition of Pin2/TRF1 function complements telomere shortening, radiosensitivity, and the G(2)/M checkpoint defect of ataxia-telangiectasia cells.

Cells derived from patients with the human genetic disorder ataxia-telangiectasia (A-T) display many abnormalities, including telomere shortening, premature senescence, and defects in the activation of S phase and G(2)/M checkpoints in response to double-strand DNA breaks induced by ionizing radiation. We have previously demonstrated that one of the ATM substrates is Pin2/TRF1, a telomeric protein that binds the potent telomerase inhibitor PinX1, negatively regulates telomere elongation, and specifically affects mitotic progression. Following DNA damage, ATM phosphorylates Pin2/TRF1 and suppresses its ability to induce abortive mitosis and apoptosis (Kishi, S., Zhou, X. Z., Nakamura, N., Ziv, Y., Khoo, C., Hill, D. E., Shiloh, Y., and Lu, K. P. (2001) J. Biol. Chem. 276, 29282-29291). However, the functional importance of Pin2/TRF1 in mediating ATM-dependent regulation remains to be established. To address this question, we directly inhibited the function of endogenous Pin2/TRF1 in A-T cells by stable expression of two different dominant-negative Pin2/TRF1 mutants and then examined their effects on telomere length and DNA damage response. Both the Pin2/TRF1 mutants increased telomere length in A-T cells, as shown in other cells. Surprisingly, both the Pin2/TRF1 mutants reduced radiosensitivity and complemented the G(2)/M checkpoint defect without inhibiting Cdc2 activity in A-T cells. In contrast, neither of the Pin2/TRF1 mutants corrected the S phase checkpoint defect in the same cells. These results indicate that inhibition of Pin2/TRF1 in A-T cells is able to bypass the requirement for ATM in specifically restoring telomere shortening, the G(2)/M checkpoint defect, and radiosensitivity and demonstrate a critical role for Pin2/TRF1 in the ATM-dependent regulation of telomeres and DNA damage response.     

Mean diameter of nucleolar bodies in cultured human leukemic myeloblasts is mainly related to the S and G2 phase of the cell cycle

Mean diameter of nucleolar bodies (nucleoli without the perinucleolar chromatin) per cell was studied in human leukemic myeloblasts represented by K 562 and Kasumi 1 cell lines which originated from chronic and acute myeloid leukaemia. The measurement of mean diameter of nucleolar bodies in specimens stained for RNA was very simple. Such approach eliminated the variability of the perinucleolar chromatin discontinuous shell which might influence the measured nucleolar size as suggested by earlier studies. Ageing of K 562 myeloblasts produced a significant decrease of cells in S+G2 phase of the cell cycle accompanied by a significant reduction of mean diameter of nucleolar bodies (MDNoBs) per cell. In contrast, treatment of Kasumi 1 myeloblasts with histone deacetylase inhibitor - Trichostatin A - produced a large incidence of resistant cells in S+G2 phase which were characterised by a large increase of MDNoBs. Thus, MDNoBs in leukemic myeloblasts might be a helpful tool to estimate the incidence of cells in the S+G2 phase at the single cell level in smear preparations when the number of cells is very small.     

Effect of caffeine on gamma-ray-induced G2 delay in ataxia telangiectasia

Exposure of normal control and ataxia-telangiectasia (A-T) lymphoblastoid cell lines to ionizing radiation gives rise to an increase in the proportion of G2 phase cells. The size and extent of the G2 phase block is greater in A-T cells than in normal cells. Caffeine has a similar overall effect in control and A-T cell lines in reducing the G2 arrest observed after ionizing radiation. While the proportion of cells accumulated in G2 in A-T cells is considerably greater than in controls, addition of caffeine at the time of maximal G2 block brings about a return of G2 phase cell numbers to unirradiated values in 3 hours in both cell types. In normal control cells the caffeine-mediated decrease in G2 cells is reflected by an increase in mitotic cells. These mitotic cells have a higher frequency of chromosome aberrations compared to cells harvested in the absence of caffeine. Similarly in A-T cells addition of caffeine to irradiated cultures, delayed in G2 phase, increased the number of mitotic cells and the frequency of chromosome aberrations.     

Visualization of radiation-induced cell cycle-associated events in tumor cells expressing the fusion protein of Azami Green and the destruction box of human Geminin

Ionizing radiation (IR) influences cell cycle-associated events in tumor cells. We expressed the fusion protein of Azami Green (AG) and the destruction box plus nuclear localization signal of human Geminin, an inhibitor of DNA replication licensing factor, in oral tumor cells. This approach allowed us to visualize G2 arrest in living cells following irradiation. The combination of time-lapse imaging analysis allowed us to observe the nuclear envelope break down (NEBD) at early M phase, and disappearance of fluorescence (DF) at the end of M phase. The duration from NEBD to DF was not much affected in irradiated cells; however, most of daughter cells harbored double-strand breaks. Complete DF was also observed in cells exhibiting abnormal mitosis or cytokinesis. We conclude that the fluorescent Geminin probe could function as a stable cell cycle indicator irrespective of genome integrity.     

On the role of sulfhydryl groups in the structure and function of the Azotobacter vinelandii RNA polymerase.

Exposure of sulfhydryl groups as indicated by titration kinetics is decreased under conditions where RNA polymerase exists as a dimer or higher aggregate (low salt), in the presence of Mn2+, or when bound to d(A-T). Incubation of phenylmercurisulfonate with RNA polymerase above pH 9.0 results in loss of d(A-T) binding ability. Poly(U) binding is more sensitive to sulfhydryl modification and is lost as pH's above 8.0. The presence of 4 mM Mn2+ has an obvious effect in stabilizing the polymerase-poly(U) complex when incubated with 10 muM phenylmercurisulfonate + 1 M urea. Incubation of the enzyme with the mercurial and urea results in disaggregation to subprotomeric forms and release of the alpha subunit. Similar treatment in the presence of 4 mM MnSO4 stabilizes the protomeric structure of the enzyme. During chain elongation the enzyme exists as a ternary d(A-T)n-enzyme-r(U-A)n complex in which the bound d(A-T)n is refractory to the destabilizing effect of the mercurial; however, further phosphodiester bond formation is inhibited. The results are defined in terms of a role which reflects the involvement of polymerase sulfhydryl groups in the various conformations necessary for subunit-subunit interaction, tight template binding and catalytic activity.     

Staining with pyronin Y detects changes in conformation of RNA during mitosis and hyperthermia of CHO cells

Cellular RNA in Chinese hamster ovary (CHO) cells synchronized in mitosis (M) or G2 phase, as well as in interphase cells subjected to hyperthermia (42 degrees C, 10 min), was stained with acridine orange (AO), ethidium bromide (EB), or pyronin Y (PY) and the resultant fluorescence was measured by flow cytometry. Total RNA content detected after staining with AO increased in M as compared to G2-phase cells, consistent with continued RNA synthesis during G2 phase. The content of double-stranded RNA, stained with EB (after DNase treatment), was also somewhat higher in M cells. In contrast, the stainability of RNA with PY decreased by 27% in M- compared to G2-phase cells. Furthermore, a decrease in stainability of RNA with PY was observed in G2 cells compared to cells in G1 phase. In separate experiments, RNA stainability with AO or EB was generally unaffected when interphase CHO cells were exposed to 42 degrees C for 10 min, though this same treatment resulted in a 26% decrease in RNA stainability with PY. The decreased PY stainability of cellular RNA in M or heat-treated cells was observed at a relatively narrow range of dye concentration (1.0-2.0 micrograms/ml). The observed hypochromicity of RNA coincides with dissociation of polyribosomes into single ribosomes known to occur during mitosis and following exposure to hyperthermia. It is presumed that the phenomenon involves selective denaturation and condensation of ribosomal (r) RNA by PY in single ribosomes which does not occur in polyribosomes. While the molecular mechanisms responsible for stabilization of rRNA in polyribosomes preventing its denaturation and condensation by PY are unknown, PY appears to be a sensitive probe that can be used to detect and study these changes in rRNA confirmation in situ.     

Cultured human keratinocytes: discrimination of different cell cycle compartments based upon measurement of nuclear RNA or total cellular RNA content

Correlated measurements of total cellular RNA and DNA of cultured human keratinocytes by flow cytometry, followed by multivariate analysis, discriminate three distinct subpopulations of cells differing in RNA content. The first subpopulation is comprised of small cells resembling basal cells of epidermis, with low RNA content and long (100-300 h) generation times. The second subpopulation consists of keratinocytes resembling cells in the spinous layer of epidermis, characterized by increased RNA content and shorter (35-40 h) generation times. The third subpopulation consists of the largest, keratinohyalin-containing cells which remain in G1 and undergo terminal differentiation. In contrast to total cellular RNA, correlated measurements of DNA and nuclear RNA reveal that: (1) entrance of all cultured cells from G1 into S phase occurs only after accumulation of the same, threshold amount of nuclear RNA; hence there is only a single population of S + G2 + M-phase cells; (2) there are two distinct subpopulations in G1, one with minimal nuclear RNA content and another with increased RNA. Stathmokinetic experiments indicate that the G1-phase cells with low nuclear RNA have distinctly longer residence times in G1 compared to cells with high nuclear RNA content. Thus, measurements of the total cellular RNA versus nuclear RNA content reveal kinetically distinct cell subpopulations. Whereas total cellular RNA content correlates more with differentiation, nuclear RNA content reflects primarily the kinetic properties of the cell.     

Cdt1 associates dynamically with chromatin throughout G1 and recruits Geminin onto chromatin

To maintain genome integrity, eukaryotic cells initiate DNA replication once per cell cycle after assembling prereplicative complexes (preRCs) on chromatin at the end of mitosis and during G1. In S phase, preRCs are disassembled, precluding initiation of another round of replication. Cdt1 is a key member of the preRC and its correct regulation via proteolysis and by its inhibitor Geminin is essential to prevent premature re-replication. Using quantitative fluorescence microscopy, we study the interactions of Cdt1 with chromatin and Geminin in living cells. We find that Cdt1 exhibits dynamic interactions with chromatin throughout G1 phase and that the protein domains responsible for chromatin and Geminin interactions are separable. Contrary to existing in vitro data, we show that Cdt1 simultaneously binds Geminin and chromatin in vivo, thereby recruiting Geminin onto chromatin. We propose that dynamic Cdt1-chromatin associations and the recruitment of Geminin to chromatin provide spatio-temporal control of the licensing process.