Fluorescence kinetics in HeLa cells after treatment with cell cycle arrest inducers visualized with Fucci (fluorescent ubiquitination-based cell cycle indicator)

Fucci (fluorescent ubiquitination-based cell cycle indicator) is able to visualize dynamics of cell cycle progression in live cells; G1- and S-/G2-/M-phase cells expressing Fucci emit red and green fluorescence, respectively. This system could be applied to cell kinetic analysis of tumour cells in the field of cancer therapy; however, it is still unclear how fluorescence kinetics change after various treatments, including exposure to anticancer agents. To explore this, we arrested live HeLa cells expressing the Fucci probes at various cell cycle stages and observed the fluorescence, in conjunction with flow cytometric analysis. X-irradiation, HU (hydroxyurea) and nocodazole arrest cells at G2/M boundary, early S-phase and early M-phase, respectively. Although X-irradiation and HU treatment induced similar accumulation kinetics of green fluorescent cells, nocodazole treatment induced an abnormal red fluorescence at M phase, followed by accumulation of both red and green fluorescent cells with 4N DNA content. We conclude that certain agents that disrupt normal cell cycle regulation could cause unexpected fluorescence kinetics in the Fucci system.     

Additive effects of radiation and docetaxel on murine SCCVII tumors in vivo: special reference to changes in the cell cycle

The purpose of the present study was to investigate the effects of a combination of docetaxel and irradiation in vivo with special reference to docetaxel-arrested G(2)/M-phase cells. At 24 and 48 h after intraperitoneal administration of docetaxel (90 mg/kg), tumor-bearing mice were irradiated with (60)Co gamma rays. Cell cycle distribution was analyzed by a DNA-Ki-67 double staining method using flow cytometry. An accumulation of cells in the G(2)/M phase of up to approximately 40% was observed 24 h after administration of docetaxel. Between 24 and 72 h, the percentage of cells arrested in G(2)/M phase that expressed Ki-67 decreased from 37.2% to 13.8%, in accordance with the increase in the Ki-67-negative G(2)/M-phase fraction. More than half of the cells arrested in G(2)/M phase lost their expression of Ki-67 protein between 24 and 72 h. The G(1)-phase fraction decreased from 28.4% to 8.6% at 24 h after docetaxel treatment; this remained unchanged at 72 h. These flow cytometry data suggested that docetaxel-arrested G(2)/M-phase cells did not enter the next cell cycle and were killed by docetaxel alone. Our data showed that arrest of cells in G(2)/M phase does not contribute to the synergism that has been reported for combinations of docetaxel and radiation in in vivo tumor models.     

Apoptosis and cell cycle progression in an acidic environment after irradiation

Apoptosis and cell cycle progression in HL60 cells irradiated in an acidic environment were investigated. Apoptosis was determined by TUNEL staining, PARP cleavage, DNA fragmentation, and flow cytometry. The majority of the apoptosis that occurred in HL60 cells after 4 Gy irradiation took place after G(2)/M-phase arrest. When irradiated with 12 Gy, a fraction of the cells underwent apoptosis in G(1) and S phases while the rest of the cells underwent apoptosis in G(2)/M phase. The apoptosis caused by 4 and 12 Gy irradiation was transiently suppressed in medium at pH 7.1 or lower. An acidic environment was found to perturb progression of irradiated cells through the cell cycle, including progression through G(2)/ M phase. Thus it was concluded that the suppression of apoptosis in the cells after 4-12 Gy irradiation in acidic medium was due at least in part to a delay in cell cycle progression, particularly the prolongation of G(2)/M-phase arrest. Irradiation with 20 Gy indiscriminately caused apoptosis in all cell cycle phases, i.e. G(1), S and G(2)/M phases, rapidly in neutral pH medium and relatively slowly in acidic pH medium. The delay in apoptosis in acidic medium after 20 Gy irradiation appeared to result from mechanisms other than prolonged G(2)/ M-phase arrest.     

Sensitivity of a human hybrid cell line (HeLa X skin fibroblast) to radiation-induced neoplastic transformation in G2, M, and mid-G1 phases of the cell cycle

The dependence of gamma-radiation-induced neoplastic transformation frequency on position in the cell cycle was measured for a human hybrid cell line (HeLa X skin fibroblast). The end point used was the induction of a tumor-associated antigen which in these cells correlates with tumorigenicity. Induction was measured in cells at G2, M, and mid-G1 phases and compared with the frequency induced in asynchronous cells. For studies of cells in G2 phase, the cells of an asynchronous population were collected for 3 h post-irradiation using the mitotic shake-off technique. For studies of cells in M and mid-G1 phases, cells were collected by mitotic harvest and then treated at the appropriate time. The data show that cells in G2 and M phase are very radiosensitive in terms of both cell killing and induction of neoplastic transformation compared to cells in mid-G1 or asynchronous populations. At a dose of 1 Gy, the transformation frequency was 10- to 20-fold higher for cells in M and G2 phase than for cells in mid-G1 or for asynchronous cells. However, the data indicate that the transformation frequencies were similar in the different phases of the cell cycle when correlated with surviving fraction. The results indicate that transformation frequency is more sensitive to changes in dose than is cell survival.     

Effect of X-Irradiation at Different Stages in the Cell Cycle on Individual Cell–Based Kinetics in an Asynchronous Cell Population

Using an asynchronously growing cell population, we investigated how X-irradiation at different stages of the cell cycle influences individual cell–based kinetics. To visualize the cell-cycle phase, we employed the fluorescent ubiquitination-based cell cycle indicator (Fucci). After 5 Gy irradiation, HeLa cells no longer entered M phase in an order determined by their previous stage of the cell cycle, primarily because green phase (S and G2) was less prolonged in cells irradiated during the red phase (G1) than in those irradiated during the green phase. Furthermore, prolongation of the green phase in cells irradiated during the red phase gradually increased as the irradiation timing approached late G1 phase. The results revealed that endoreduplication rarely occurs in this cell line under the conditions we studied. We next established a method for classifying the green phase into early S, mid S, late S, and G2 phases at the time of irradiation, and then attempted to estimate the duration of G2 arrest based on certain assumptions. The value was the largest when cells were irradiated in mid or late S phase and the smallest when they were irradiated in G1 phase. In this study, by closely following individual cells irradiated at different cell-cycle phases, we revealed for the first time the unique cell-cycle kinetics in HeLa cells that follow irradiation.     

Condensed chromatin and cell inactivation by single-hit kinetics

Mammalian cells are extremely sensitive to gamma rays at mitosis, the time at which their chromatin is maximally condensed. The radiation-induced killing of mitotic cells is well described by single-hit inactivation kinetics. To investigate if radiation hypersensitivity by single-hit inactivation correlated with chromatin condensation, Chinese hamster ovary (CHO) K1 (wild-type) and xrs-5 (radiosensitive mutant) cells were synchronized by mitotic shake-off procedures and the densities of their chromatin cross sections and their radiosensitivities were measured immediately and 2 h into G1 phase. The chromatin of G1-phase CHO K1 cells was dispersed uniformly throughout their nuclei, and its average density was at least three times less than in the chromosomes of mitotic CHO K1 cells. The alpha-inactivation co-efficient of mitotic CHO K1 cells was approximately 2.0 Gy(-1) and decreased approximately 10-fold when cells entered G1 phase. The density of chromatin in CHO xrs-5 cell chromosomes at mitosis was greater than in CHO K1 cell chromosomes, and the radiosensitivity of mitotic CHO xrs-5 cells was the greatest with alpha = 5.1 Gy(-1). In G1 phase, CHO xrs-5 cells were slightly more resistant to radiation than when in mitosis, but a significant proportion of their chromatin was found to remain in condensed form adjacent to the nuclear membrane. These studies indicate that in addition to their known defects in DNA repair and V(D)J recombination, CHO xrs-5 cells may also be defective in some process associated with the condensation and/or dispersion of chromatin at mitosis. Their radiation hypersensitivity could result, in part, from their DNA remaining in compacted form during interphase. The condensation status of DNA in other mammalian cells could define their intrinsic radiosensitivity by single-hit inactivation, the mechanism of cell killing which dominates at the dose fraction size (1.8-2.0 Gy) most commonly used in radiotherapy.     

Inhibitory phosphorylation of PP1alpha catalytic subunit during the G(1)/S transition.

We have shown earlier that, in cells expressing the retinoblastoma protein (pRB), a protein phosphatase (PP) 1alpha mutant (T320A) resistant to inhibitory phosphorylation by cyclin-dependent kinases (Cdks) causes G(1) arrest. In this study, we examined the cell cycle-dependent phosphorylation of PP1alpha in vivo using three different antibodies. PP1alpha was phosphorylated at Thr-320 during M-phase and again in late G(1)- through early S-phase. Inhibition of Cdk2 led to a small increase in PP1 activity and also prevented PP1alpha phosphorylation. In vitro, PP1alpha was a substrate for Cdk2 but not Cdk4. In pRB-deficient cells, phosphorylation of PP1alpha occurred in M-phase but not at G(1)/S. G(1)/S phosphorylation was at least partially restored after reintroduction of pRB into these cells. Consistent with this result, PP1alpha phosphorylated at Thr-320 co-precipitated with pRB during G(1)/S but was found in extracts immunodepleted of pRB in M-phase. In conjunction with earlier studies, these results indicate that PP1alpha may control pRB function throughout the cell cycle. In addition, our new results suggest that different subpopulations of PP1alpha regulate the G(1)/S and G(2)/M transitions and that PP1alpha complexed to pRB requires inhibitory phosphorylation by G(1)-specific Cdks in order to prevent untimely reactivation of pRB and permit transition from G(1)- to S-phase and/or complete S-phase.     

Non-invasive analysis of cell cycle dynamics in single living cells with Raman micro-spectroscopy

Raman micro-spectroscopy is a laser-based technique which enables rapid and non-invasive biochemical analysis of cells and tissues without the need for labels, markers or stains. Previous characterization of the mammalian cell cycle using Raman micro-spectroscopy involved the analysis of suspensions of viable cells and individual fixed and/or dried cells. Cell suspensions do not provide cell-specific information, and fixing/drying can introduce artefacts which distort Raman spectra, potentially obscuring both qualitative and quantitative analytical results. In this article, we present Raman spectral characterization of biochemical changes related to cell cycle dynamics within single living cells in vitro. Raman spectra of human osteosarcoma cells synchronized in G(0)/G(1), S, and G(2)/M phases of the cell cycle were obtained and multivariate statistics applied to analyze the changes in cell spectra as a function of cell cycle phase. Principal components analysis identified spectral differences between cells in different phases, indicating a decrease in relative cellular lipid contribution to Raman spectral signatures from G(0)/G(1) to G(2)/M, with a concurrent relative increase in signal from nucleic acids and proteins. Supervised linear discriminant analysis of spectra was used to classify cells according to cell cycle phase, and exhibited 97% discrimination between G(0)/G(1)-phase cells and G(2)/M-phase cells. The non-invasive analysis of live cell cycle dynamics with Raman micro-spectroscopy demonstrates the potential of this approach to monitoring biochemical cellular reactions and processes in live cells in the absence of fixatives or labels.     

Flow cytometric discrimination of mitotic nuclei by right-angle light scatter

Flow cytometry has been used to demonstrate alterations in protein, RNA, and DNA content of cells as they traverse the cell cycle. Employing fluorescein isothiocyanate (FITC) to stain protein and propidium iodide (PI) to stain nucleic acids, multiple regions within the G1 and G2 phases of the cell cycle, in addition to the M phase, can be distinguished. In this study, cytograms of the 90 degree light scatter signal vs. PI fluorescence were remarkably similar to those of FITC fluorescence vs. PI fluorescence, suggesting a relationship between 90 degree light scatter and protein content. M-phase nuclei can be distinguished from G2-phase nuclei on cytograms of 90 degree light scatter vs. PI fluorescence. However, the percentage of mitotic nuclei obtained by this technique is less than that found by light microscopic analysis. Flow cytometric parameters of nuclei prepared by nonionic detergent (NP40) lysis in Dulbecco's PBS, Vindelov's buffer, or Pollack's hypotonic EDTA/Tris buffer were compared. The best resolution of mitotic nuclei was obtained in Pollack's buffer. However, the stainability of the M-phase nuclei is reduced, and the nuclei are located in the late S/G2 region of the single-parameter histogram.     

Cell cycle-dependent DHFR and t-PA production in cotransfected, MTX-amplified CHO cells revealed by dual-laser flow cytometry

The cell cycle-dependent regulation of the cellular dihydrofolate reductase content (DHFR) and tissue plasminogen activator (t-PA) production and secretion in plasmid-amplified cells was investigated in the DHFR-negative CHO cells transfected with the plasmid pSV-tPA.dhfr. This plasmid, carrying the dhfr and t-PA gene under control of different promotors, was amplified by serial passages in 5 microM methotrexate (MTX) for dhfr gene amplification. The intracellular amount of DHFR was quantitated in viable cells by MTX-FITC labeling and flow cytometric analysis of the FITC fluorescence. In comparison with the original CHO cells, the pSVtPA.dhfr-amplified cells showed a greater than 230-fold increase in MTX-FITC fluorescence. Using dual laser flow cytometry (uv: vital cell cycle with Hoechst 33342; 488 nm: DHFR with MTX-FITC), we show a maximum increase in the intracellular DHFR content during G1 and/or at G1/S transition (100 to 157%), followed by a continuous increase to 200% during S and G2/M. To determine t-PA production CHO cells were sorted from G1-, early/late S-, and G2/M-phase. After 1-, 2-, and 4-h incubation periods, t-PA production was quantitated using a sensitive t-PA ELISA technique. We found that t-PA production and secretion (2-h assay), unlike the expression of DHFR, increased continuously from relatively 100% in G1 to 127% in early S and reached its maximum of 159% in late S, whereas in G2/M-phase it decreased to 118%. Our results show that in pSVtPA.dhfr-coamplified CHO cells gene products DHFR and t-PA both exhibit different cell cycle-correlated accumulation and secretion, respectively, indicating that the brightest MTX-FITC-positive cells (G2/M) do not display the highest t-PA secretion rate.     

Monitoring differential synthesis of RNA in individual cells by capillary electrophoresis

A capillary electrophoresis (CE)-based technique is reported here to monitor differential RNA synthesis in individual Chinese hamster ovary cells at distinct stages of the cell proliferation cycle. Cell synchronization was achieved by the shake-off method, in which mitotic (M) cells were dislodged, and cells at G(1), S, and G(2) phases were harvested 2.5, 10, and 13 h, respectively, after synchronizing the mitotic cells. Thirty-two cells (eight from each phase) were analyzed by injecting each cell into the capillary, lysing it with dilute surfactant, separating the RNA by capillary electrophoresis, and detecting the peaks with laser-induced fluorescence. The results from single cells show that the total amount of RNA increased at each successive stage (from G(1) to M), while the relative synthetic rates of different RNA fractions varied with progression through the cycle. There was a threefold increase in the synthetic rate of total RNA from S to G(2), compared with G(1) to S. In addition, differential accumulation of specific RNA fractions was observed, with the low-molecular-mass fraction exhibiting a much higher synthetic rate from G(2) to M, relative to the rates of the larger ribosomal RNA (rRNA) fractions. Comparison of the large rRNA fractions with one another reveals that at S phase more 28S rRNA was accumulated than 18S rRNA, and at G(1) and M phases, the synthetic rate of 28S rRNA was slowed compared with that of 18S. Minimal sample preparation, combined with the separation power of CE and single-cell detection sensitivity of laser-induced fluorescence, results in a simple method for assessing differential accumulation of RNA from distinct individual cells.     

The effect of cell cycle on GFPuv gene expression in the baculovirus expression system.

The effects of cell cycle on recombinant protein production and infection yield in the baculovirus-insect cell expression system (BES) were investigated. When, at any cell cycle phase, the host cell was infected by baculovirus, the cell cycle was finally arrested at the S or G(2)/M phase with 4n DNA. In the case of G(1) or S phase-infection, cell cycle of virus-infected cells began to be arrested at S phase from 8 h post-infection or at G(2)/M phase from 4 h post-infection, respectively; while, in the case of M phase-infection, cell cycle was arrested at S phase after 12 h post-infection. When the host cell was infected at the G(1) phase, average intracellular GFPuv fluorescence intensity was 1.3-fold higher than that at G(2)/M phase at 24 h post-infection. The GFPuv expression corresponded to the profile of the G(1) cell cycle in the BES. Infection yield was measured by detection of intracellular DNA binding protein using immunohistochemical method within 7 h post-infection. The infection yield at G(1) or S phase-infection was 1.5-1.8-fold higher than that at G(2)/M phase-infection.     

BCL-2 is phosphorylated and inactivated by an ASK1/Jun N-terminal protein kinase pathway normally activated at G(2)/M

Multiple signal transduction pathways are capable of modifying BCL-2 family members to reset susceptibility to apoptosis. We used two-dimensional peptide mapping and sequencing to identify three residues (Ser70, Ser87, and Thr69) within the unstructured loop of BCL-2 that were phosphorylated in response to microtubule-damaging agents, which also arrest cells at G(2)/M. Changing these sites to alanine conferred more antiapoptotic activity on BCL-2 following physiologic death signals as well as paclitaxel, indicating that phosphorylation is inactivating. An examination of cycling cells enriched by elutriation for distinct phases of the cell cycle revealed that BCL-2 was phosphorylated at the G(2)/M phase of the cell cycle. G(2)/M-phase cells proved more susceptible to death signals, and phosphorylation of BCL-2 appeared to be responsible, as a Ser70Ala substitution restored resistance to apoptosis. We noted that ASK1 and JNK1 were normally activated at G(2)/M phase, and JNK was capable of phosphorylating BCL-2. Expression of a series of wild-type and dominant-negative kinases indicated an ASK1/Jun N-terminal protein kinase 1 (JNK1) pathway phosphorylated BCL-2 in vivo. Moreover, the combination of dominant negative ASK1, (dnASK1), dnMKK7, and dnJNK1 inhibited paclitaxel-induced BCL-2 phosphorylation. Thus, stress response kinases phosphorylate BCL-2 during cell cycle progression as a normal physiologic process to inactivate BCL-2 at G(2)/M.     

Radiation-induced apoptosis and cell cycle progression in Jurkat T cells.

The purpose of this study was to explore the connection between radiation-induced apoptosis and progression of cells through the phases of the cell cycle. Cells of the human T-cell line Jurkat were separated by centrifugal elutriation into populations enriched in G(1)-, S- and G(2)/M-phase cells before irradiation. After a dose of 20 Gy, the onset of massive apoptosis occurred at about 6 h in all populations regardless of the phase of the cell cycle in which they were irradiated. In contrast, after 2 Gy, cells died at various times after a pronounced G(2)/M-phase arrest. These results indicate that radiation-induced apoptosis can occur independently of cell cycle arrest and that the time for onset of apoptosis may be dependent on the radiation dose.     

c-Fos/activator protein-1 transactivates wee1 kinase at G(1)/S to inhibit premature mitosis in antigen-specific Th1 cells

M-phase promoting factor is a complex of cdc2 and cyclin B that is regulated positively by cdc25 phosphatase and negatively by wee1 kinase. We isolated the wee1 gene promoter and found that it contains one AP-1 binding motif and is directly activated by the immediate early gene product c-Fos at cellular G(1)/S phase. In antigen-specific Th1 cells stimulated by antigen, transactivation of the c-fos and wee1 kinase genes occurred sequentially at G(1)/S, and the substrate of wee1 kinase, cdc2-Tyr15, was subsequently phosphorylated at late G(1)/S. Under prolonged expression of the c-fos gene, however, the amount of wee1 kinase was increased and its target cdc2 molecule was constitutively phosphorylated on its tyrosine residue, where Th1 cells went into aberrant mitosis. Thus, an immediate early gene product, c-Fos/AP-1, directly transactivates the wee1 kinase gene at G(1)/S. The transient increase in c-fos and wee1 kinase genes is likely to be responsible for preventing premature mitosis while the cells remain in the G(1)/S phase of the cell cycle.     

Alterations of cell cycle kinetics and vimentin expression in TPA-treated, asynchronous MPC-11 mouse plasmacytoma cells

Vimentin expression throughout the cell cycle has been analyzed at the single-cell level in asynchronously growing MPC-11 cells using multiparameter flow cytometry. We have previously shown that these cells normally lack detectable amounts of intermediate filament proteins. In the presence of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), cell proliferation ceases and large quantities of the intermediate filament protein vimentin are synthesized and accumulate in most of the cells. In the present study, the short-term effect of TPA on distribution of cells within the cell cycle was a depletion in early S phase followed by a depletion in mid- and late S phase. In parallel, the G1-phase fraction increased significantly. In addition, a delay in progression through G2/M phase was observed. These data strongly suggest an inhibition of progression of cells through the cell cycle in G1 phase as the primary event on cell cycle kinetics elicited by TPA. Vimentin accumulation could be detected by flow cytometry as early as 2 h after TPA addition; at this time, the percentage of vimentin-positive cells was highest in G2/M phase. Prolonged TPA treatment induced vimentin accumulation in cells of all cell cycle phases. However, even at later times, the G1-phase population consisted of two subpopulations with low and high vimentin content, respectively. The fraction of cells which displayed a higher level of vimentin probably represents those G1-phase cells which previously had undergone cell division in the presence of TPA. Our data indicate that TPA-induced vimentin synthesis is regulated in a cell cycle-dependent manner and is maximally induced in cells which have passed a putative cell cycle restriction point in G1 phase.     

Cell cycle analysis of synchronized chinese hamster cells using bromodeoxyuridine labeling and flow cytometry

Summary Chinese hamster ovary cells were synchronized into purified populations of viable G1-, S-, G2-, and M-phase cells by a combination of methods, including growth arrest, aphidicolin block, cell cycle progression, mitotic shake-off, and centrifugal elutriation. The DNA content and bromodeoxyuridine (BrdUrd) labeling index were measured in each purified fraction by dual-parameter flow cytometry. The cell cycle distributions determined from the DNA measurements alone (single parameter) were compared with those calculated from both DNA and BrdUrd data (dual parameter). The results show that highly purified cells can be obtained using these methods, but the assessed purity depends on the method of cell cycle analysis. Using the single versus dual parameter measurement to determine cell cycle distributions gave similar results for most phases of the cell cycle, except for cells near the transition from G1- to S-phase and S- to G2-phase. There the BrdUrd labeling index determined by flow cytometry was more sensitive for detecting small amounts of DNA synthesis. As an alternative to flow cytometry, a simple method of measuring BrdUrd labeling index on cell smears was used and gave the same result as flow cytometry. Measuring both DNA content and DNA synthesis improves characterization of synchronized cell populations, especially at the transitions in and out of S-phase, when cells are undergoing dramatic shifts in biochemical activity.