Immunoreactivity to antinucleoside antibodies persists during G2 arrest in X-irradiated HeLa cells

Arrest of HeLa cells in G2 after ionizing radiation is accompanied by persistent nuclear immunoreactivity to antinucleoside antibodies. The reactivity declined to the normal G2 level during escape from arrest and subsequent cell division.     

Fast repair of anoxic radiation damage in HeLa cells detected by antinucleoside antibodies

The influence of anoxia on X-ray damage in HeLa cells was studied by observing effects on nuclear immunoreactivity to antinucleoside antibodies and on the sedimentation in alkaline sucrose gradients of their DNA “complexes”. The fraction of G1 HeLa cells which was immunoreactive to fluorescein labeled antinucleoside antibodies increased from control levels of 11% ± 3.5 S.E. to 71% ± 5.7 S.E. after 1 000 rads in air. In anoxia 1 000 rads increased this fraction to only 42% ± 3.1 S.E. After 1 000 rads in air the return to normal G1 levels of immunoreactivity required 90 min, but it required only 30 min after radiation in anoxia. If cells were held at 0 °C for 35 min before anoxic irradiation the rapid return to control levels of immunoreactivity during postradiation incubation at 37 °C was not observed. Cold shock did not increase the proportion of cells initially made immunoreactive by 1 000 rads in anoxia. Anoxia reduced the effect of 1 000 rads on the sedimentation properties of the DNA complex. Cold shock prior to anoxic radiation retarded the faster reconstitution of the DNA complex otherwise observed after anoxic radiation.     

Crosstalk between elicitor-induced cell death and cell cycle regulation in tobacco BY-2 cells

The molecular links between cell cycle control and the regulation of programmed cell death are largely unknown in plants. Here we studied the relationship between the cell cycle and elicitor-induced cell death using synchronized tobacco BY-2 cells. Flow cytometry and fluorescence microscopy of nuclear DNA, and RNA gel-blot analyses of cell cycle-related genes revealed that the proteinaceous elicitor cryptogein induced cell cycle arrest at the G1 or G2 phase before the induction of cell death. Furthermore, the patterns of cell death induction and defence-related genes were different in different phases of the cell cycle. Constitutive treatment with cryptogein induced cell cycle arrest and cell death at the G1 or G2 phase. With transient treatment for 2 h, cell cycle arrest and cell death were only induced by treatment with the elicitor during the S or G1 phase. By contrast, the elicitor-induced production of reactive oxygen species was observed during all phases of the cell cycle. These results indicate that although recognition of the elicitor signal is cell cycle-independent, the induction of cell cycle arrest and cell death depends on the phase of the cell cycle.     

Simultaneous assessment of chromatin structure, DNA content, and antigen expression by dual wavelength excitation flow cytometry.

7-aminoactinomycin D (7-AMD) efficiently discriminates between cells in the G0 and G1 phases of the cell cycle (Stokke et al., Cancer Res. 48:6708, 1988). The fluorescence and light scatter of cells stained with 7-AMD, Hoechst 33258 (H33258), and fluorescein isothiocyanate (FITC)-labeled antibodies were measured by dual wavelength excitation flow cytometry (488 nm, ultraviolet). The H33258 fluorescence was found to reflect DNA content in the presence of 7-AMD, although energy transfer caused an approximately 50% reduction in H33258 fluorescence intensity. However, energy transfer was more pronounced in dead cells, permitting exclusion of such cells during analysis. The G0, G1, S, and G2 phases of the cell cycle could be identified in the 7-AMD versus H33258 fluorescence histograms, as was demonstrated with mitogen-stimulated B lymphocytes and a mixture of unstimulated B lymphocytes and a proliferating B-cell line. One hour fixation with paraformaldehyde was compatible with prefixation labeling of surface antigens with indirectly FITC-labeled antibodies as well as postfixation labeling of intracellular antigens. Studies of expression of some surface and nuclear activation-associated antigens confirmed that cell cycle-resolved antigen expression and the time course of appearance of such antigens could be assessed accurately. Phycoerythrin could be used to label a second antigen.     

Cell cycle dependent distribution of proliferating cell nuclear antigen/cyclin and cdc2-kinase in mouse T-lymphoma cells

The aim of the present study was to investigate bromodeoxyuridine (BrdU) uptake and coordinated distribution of proliferating cell nuclear antigen (PCNA) and p34-cdc2-kinase, two important proteins involved in cell cycle regulation and progression. Flow cytometric analysis of marker proteins in freshly plated mouse T-lymphoma cells (Yac-1 cells), using fluorescein isothiocyanate (FITC)-labeled specific antibodies, showed PCNA distributed throughout the cell cycle with increased intensity in S-phase. PCNA is essential for cells to cycle through S-phase and its synthesis is initiated during late G1-phase before incorporation of BrdU and remains high during active DNA replication. The intensity of PCNA fluorescence increases with the duration of incubation after plating. The cdc2-kinase was detectable in all phases of the cell cycle and the G2-M-phase appears to have the maximum concentrations. The cell cycle analysis of high dose colcemid (2 μg/ml) treated Yac-1 cells showed an aneuploid or hypodiploid population. Although the G2-M-phase seems to be the dominating population in aneuploid cells, the concentrations of cdc2-kinase were variable in this phase of cell cycle. The colcemid treatment at 25 ng/ml arrested 96% of cells in S-phase and G2-M-phase, but PCNA expression was evident in a portion of the cell population in G2-M-phase. Although cells blocked in M-phase seem to have high levels of cdc2-kinase, colcemid renders them inactive. From these data, it appears that the down regulation and/or inactivation of cdc2-kinase could be responsible for the colcemid arrest of cells in M-phase.     

Molecular Cloning of a Murine cDNA Encoding a Novel Protein, p38-2G4, Which Varies with the Cell Cycle

Proliferating cells express genes active in cell cycle control. The modulation of control genes and factors are required to maintain critical cell cycle activities. We used a set of monoclonal antibodies prepared against DNA-binding proteins from Ehrlich ascites tumor cells in immunofluorescent microscopy to screen for proteins showing cell cycle-specific staining patterns. Here, we report cloning and characterizing of a novel mitogen-inducible gene from murine macrophages that predicts a cell cycle-specifically modulated nuclear protein of 38 kDa, designated p38-2G4. p38-2G4 displayed a speckled pattern of varying fluorescence intensity confined to the nucleus, but sparing the nucleoli. Strongly stained granules were observed between G1 and mid S phase, followed by a less abundant punctated arrangement toward the end of S phase, and negative fluorescence at the S/G2 transition. Thereafter, the nuclear staining reappeared. Additionally, p38-2G4 expression vanished in G0-arrested cells and was restored after release from growth arrest. p38-2G4 conserved in vertebrates by means of immunofluorescence data contains a number of putative phosphorylation sites, a cryptic nuclear localization signal, and an amphipathic helical domain. Our cDNA and its deduced amino acid sequence is related to a Schizosaccharomyces pombe gene encoding a 42-kDa protein that associates with curved DNA, suggesting that we have cloned the murine homologue of the S. pombe gene which defines a novel cell cycle-specifically modified and proliferation-associated nuclear protein in mammals.     

Flow cytometric fluorescence pulse width analysis of etoposide-induced nuclear enlargement in HCT116 cells

Fluorescence pulse width can provide size information on the fluorescence-emitting particle, such as the nuclei of propidium iodide-stained cells. To analyze nuclear size in the present study, rather than perform the simple doublet discrimination approach usually employed in flow cytometric DNA content analyses, we assessed the pulse width of the propidium iodide fluorescence signal. The anti-cancer drug etoposide is reportedly cytostatic, can induce a strong G2/M arrest, and results in nuclear enlargement. Based on these characteristics, we used etoposide-treated HCT116 cells as our experimental model system. The fluorescence pulse widths (FL2-W) of etoposide-treated (10 μM, 48 h) cells were distributed at higher positions than those of vehicle control, so the peak FL2-W value of etoposide-treated cells appeared at 400 while those of vehicle control cells appeared at 200 and 270. These results were consistent with our microscopic observations. This etoposide-induced increase in FL2-W was more apparent in G2/M phase than other cell cycle phases, suggesting that etoposide-induced nuclear enlargement preferentially occurred in G2/M phase cells rather than in G0/G1 or S phase cells.     

Midblastula transition (MBT) of the cell cycles in the yolk and pigment granule-free translucent blastomeres obtained from centrifuged Xenopus embryos

We obtained translucent blastomeres free of yolk and pigment granules from Xenopus embryos which had been centrifuged at the beginning of the 8-cell stage with cellular integrity. They divided synchronously regardless of their cell size until they had decreased to 37.5 microm in radius; those smaller than this critical size, however, divided asynchronously with cell cycle times inversely proportional to the square of the cell radius after midblastula transition (MBT). The length of the S phase was determined as the time during which nuclear DNA fluorescence increased in Hoechst-stained blastomeres. When the cell cycle time exceeded 45 min, S and M phases were lengthened; when the cell cycle times exceeded 70 min, the G2 phase appeared; and after cell cycle times became longer than 150 min, the G1 phase appeared. Lengths of G1, S and M phases increased linearly with increasing cell cycle time. Enhanced green fluorescent protein (EGFP)-tagged proliferating cell nuclear antigen (PCNA) expressed in the blastomeres appeared in the S phase nucleus, but suddenly dispersed into the cytoplasm at the M phase. The system developed in this study is useful for examining the cell cycle behavior of the cell cycle-regulating molecules in living Xenopus blastomeres by fluorescence microscopy in real time.     

IQGAP1 translocates to the nucleus in early S-phase and contributes to cell cycle progression after DNA replication arrest

IQGAP1 is a plasma membrane-associated protein and an important regulator of the actin cytoskeleton, contributing to cell migration, polarity and adhesion. In this study, we demonstrate the nuclear translocation of IQGAP1 using confocal microscopy and cell fractionation. Moreover, we identify a specific pool of IQGAP1 that accumulates in the nucleus during late G1-early S phase of the cell cycle. The nuclear targeting of IQGAP1 was facilitated by N- and C-terminal sequences, and its ability to slowly shuttle between nucleus and cytoplasm/membrane was partly regulated by the CRM1 export receptor. The inhibition of GSK-3β also stimulated nuclear localization of IQGAP1. The dramatic nuclear accumulation of IQGAP1 observed when cells were arrested in G1/S phase suggested a possible role in cell cycle regulation. In support of this, we used immunoprecipitation assays to show that the nuclear pool of IQGAP1 in G1/S-arrested cells associates with DNA replication complex factors RPA32 and PCNA. More important, the siRNA-mediated silencing of IQGAP1 significantly delayed cell cycle progression through S phase and G2/M in NIH 3T3 cells released from thymidine block. Our findings reveal an unexpected regulatory pathway for IQGAP1, and show that a pool of this cytoskeletal regulator translocates into the nucleus in late G1/early S phase to stimulate DNA replication and progression of the cell cycle.     

DNA synthesis in the second and third cell cycles of mouse preimplantation development. A cytophotometric study

Female Swiss mice were sacrificed at 2 h intervals between 16–30 and 40–56 h after insemination. One-, 2- and 4-cell embryos were stained by the Feulgen method and cytophotometric measurement of their nuclear DNA content was carried out. The cells with 2C and 4C DNA content were assumed to be in G1 and G2 phase and those with intermediate DNA content in S phase of the cell cycle. The fractions of cells which had passed a given phase of the cell cycle were calculated for various times after insemination and utilized for measurements of the second and third cell cycle timing. Results of measurements for the second cell cycle: G1 phase 1.3 h, S phase 6.1 h, G2 phase 15.4 h, whereas for the third cell cycle: G1 phase 1.6 h, S phase 7.4 h, G2 phase 0.5 h. The first cleavage division was calculated as 1.6 h, the second as 1.3 h and the third as 1.2 h. Complete intra-embryonic synchronization of the DNA-synthesizing nuclei was preserved during the entire synthesis phase of 2-cell embryos, while in 4-cell embryos they were slightly asynchronized. Among mitotic cells of the first cleavage division and G1 cells of 2-cell embryos a slight interembryonic asynchronization was found which deepened during subsequent cell cycle phases.     

Live imaging of the Dictyostelium cell cycle reveals widespread S phase during development, a G2 bias in spore differentiation and a premitotic checkpoint

The regulation of the Dictyostelium cell cycle has remained ambiguous owing to difficulties in long-term imaging of motile cells and a lack of markers for defining cell cycle phases. There is controversy over whether cells replicate their DNA during development, and whether spores are in G1 or G2 of the cell cycle. We have introduced a live-cell S-phase marker into Dictyostelium cells that allows us to precisely define cycle phase. We show that during multicellular development, a large proportion of cells undergo nuclear DNA synthesis. Germinating spores enter S phase only after their first mitosis, indicating that spores are in G2. In addition, we demonstrate that Dictyostelium heterochromatin is copied late in S phase and replicates via accumulation of replication factors, rather than recruitment of DNA to pre-existing factories. Analysis of variability in cycle times indicates that regulation of the cycle manifests at a single random transition in G2, and we present the first identified checkpoint in Dictyostelium, which operates at the G2-M transition in response to DNA damage.     

Intranuclear localization of proliferating cell nuclear antigen during the cell cycle in renal cell carcinoma

OBJECTIVE: To investigate, with laser scanning cytometry (LSC), proliferating cell nuclear antigen (PCNA) expression during the cell cycle in renal cell carcinoma. STUDY DESIGN: DNA ploidy and intracellular localization of PCNA in renal cell carcinoma were determined using LSC and immunohistochemistry. The subjects were nine patients who had received surgery for renal cell carcinoma. After DNA ploidy analysis, the glass slides were restained by immunohistochemistry of PCNA. LSC allowed direct observation of PCNA localization during the cell cycle because we could obtain immunohistochemical staining of PCNA as a function of cell cycle phase for individual cells. RESULTS: PCNA was not demonstrated in the nuclei of G0/G1 cells. PCNA expression increased from the S phase of the cell cycle. PCNA rapidly degraded at the end of the G2 phase. In the late G2 and M phase, PCNA was not detected in almost any nucleus. CONCLUSION: LSC allows morphologic observation of the intracellular distribution of PCNA during the cell cycle in renal cell carcinoma.     

Intracellular distribution of DNA methyltransferase during the cell cycle

The intracellular distribution of DNA methyltransferase has been analyzed in synchronously proliferating human cells. The localization of DNA methyltransferase was determined immunocytochemically using monoclonal antibodies directed against this enzyme. DNA methyltransferase was found to accumulate predominantly in nuclei with weak cytoplasmic staining. The DNA methyltransferase antigen was absent in early G1 phase, appeared in late G1 prior to the onset of DNA synthesis and persisted throughout S and G2 phases of the cell cycle. Mitotic cells showed a particularly strong staining intensity. These results show that DNA methyltransferase levels fluctuate during the cell cycle. This has possible implications on the stability of the DNA methylation pattern.     

S-phase-dependent cell cycle disturbances caused by Aleutian mink disease parvovirus.

We examined replication of the autonomous parvovirus Aleutian mink disease parvovirus (ADV) in relation to cell cycle progression of permissive Crandell feline kidney (CRFK) cells. Flow cytometric analysis showed that ADV caused a composite, binary pattern of cell cycle arrest. ADV-induced cell cycle arrest occurred exclusively in cells containing de novo-synthesized viral nonstructural (NS) proteins. Production of ADV NS proteins, indicative of ADV replication, was triggered during S-phase traverse. The NS+ cells that were generated during later parts of S phase did not undergo cytokinesis and formed a distinct population, termed population A. Formation of population A was not prevented by VM-26, indicating that these cells were arrested in late S or G2 phase. Cells in population A continued to support high-level ADV DNA replication and production of infectious virus after the normal S phase had ceased. A second, postmitotic, NS+ population (termed population B) arose in G0/G1, downstream of population A. Population B cells were unable to traverse S phase but did exhibit low-level DNA synthesis. Since the nature of this DNA synthesis was not examined, we cannot at present differentiate between G1 and early S arrest in population B. Cells that became NS+ during S phase entered population A, whereas population B cells apparently remained NS- during S phase and expressed high NS levels postmitosis in G0/G1. This suggested that population B resulted from leakage of cells with subthreshold levels of ADV products through the late S/G2 block and, consequently, that the binary pattern of ADV-induced cell cycle arrest may be governed merely by viral replication levels within a single S phase. Flow cytometric analysis of propidium iodide fluorescence and bromodeoxyuridine uptake showed that population A cells sustained significantly higher levels of DNA replication than population B cells during the ADV-induced cell cycle arrest. Therefore, the type of ADV-induced cell cycle arrest was not trivial and could have implications for subsequent viral replication in the target cell.     

The retinoblastoma gene product regulates progression through the G1 phase of the cell cycle.

The RB gene product is a nuclear phosphoprotein that undergoes cell cycle-dependent changes in its phosphorylation status. To test whether RB regulates cell cycle progression, purified RB proteins, either full-length or a truncated form containing the T antigen-binding region, were injected into cells. Injection of either protein early in G1 inhibits progression into S phase. Co-injection of anti-RB antibodies antagonizes this effect. Injection of RB into cells arrested at G1/S or late in G1 has no effect on BrdU incorporation, suggesting that RB does not inhibit DNA synthesis in S phase. These results indicate that RB regulates cell proliferation by restricting cell cycle progression at a specific point in G1 and establish a biological assay for RB activity. Neither co-injection of RB with a T antigen peptide nor injection into cells expressing T antigen prevents cells from progressing into S phase, which supports the hypothesis that T antigen binding has functional consequences for RB.     

Human cytomegalovirus infection inhibits G1/S transition.

Cell cycle progression during cytomegalovirus infection was investigated by fluorescence-activated cell sorter (FACS) analysis of the DNA content in growth-arrested as well as serum-stimulated human fibroblasts. Virus-infected cells maintained in either low (0.2%) or high (10%) serum failed to progress into S phase and failed to divide. DNA content analysis in the presence of G1/S (hydroxyurea and mimosine) and G2/M (nocodazole and colcemid) inhibitors demonstrated that upon virus infection of quiescent (G0) cells, the cell cycle did not progress beyond the G1/S border even after serum stimulation. Proteins which normally indicate G1/S transition (proliferating cell nuclear antigen [PCNA]) or G2/M transition (cyclin B1) were elevated by virus infection. PCNA levels were induced in infected cells and exhibited a punctate pattern of nuclear staining instead of the diffuse pattern observed in mock-infected cells. Cyclin B1 was induced in infected cells which exhibited a G1/S DNA content by FACS analysis, suggesting that expression of this key cell cycle function was dramatically altered by viral functions. These data demonstrate that contrary to expectations, cytomegalovirus inhibits normal cell cycle progression. The host cell is blocked prior to S phase to provide a favorable environment for viral replication.     

The relationship between thrombomodulin expression and cell cycle stages in cultured rabbit endothelial cells

Cultured rabbit endothelial cells have significant but variable amounts of thrombomodulin (TM), both on their surface as well as inside the cell. To determine if variations in TM antigen is cell cycle related, cells with very high levels of TM antigen were identified and staged according to the intracellular distribution and relative amounts of the antigen, using immunofluorescence techniques. After staging, the nuclear DNA content of each of these cells was determined by measuring the propidium iodide (PI) fluorescence intensity cytophotometrically. Stages 1, 2, and 3, which exhibited TM immunofluorescence in the golgi area, clustered to the G1 phase of the cell cycle. Cells without discernible golgi fluorescence (stages 4 and 5) but with variable amounts of cytoplasmic and surface fluorescence appeared to have little or no relationship to the cell cycle.     

Tissue-specific G1-phase cell-cycle arrest prior to terminal differentiation in Dictyostelium

The cell cycle status of developing Dictyostelium cells remains unresolved because previous studies have led to conflicting interpretations. We propose a new model of cell cycle events during development. We observe mitosis of about 50% of the cells between 12 and 18 hours of development. Cellular DNA content profiles obtained by flow cytometry and quantification of extra-chromosomal and chromosomal DNA suggest that the daughter cells have half the chromosomal DNA of vegetative cells. Furthermore, little chromosomal DNA synthesis occurs during development, indicating that no S phase occurs. The DNA content in cells sorted by fluorescent tissue-specific reporters indicates that prespore cells divide before prestalk cells and later encapsulate as G1-arrested spores. Consistent with this, germinating spores have one copy of their chromosomes, as judged by fluorescence in situ hybridization and they replicate their chromosomes before mitosis of the emergent amoebae. The DNA content of mature stalk cells suggests that they also attain a G1 state prior to terminal differentiation. As prestalk cells appear to be in G2 up to 22 hours of development, our data suggest that they divide just prior to stalk formation. Our results suggest tissue-specific regulation of G1 phase cell cycle arrest prior to terminal differentiation in Dictyostelium.