Evaluation of the size of the proliferative pool and the length of the mitotic cycle according to the accumulation curve of labelled cells]

Abnormal increase of the accumulation curve of H3-thymidine labelled cells for the systems with proliferative pool Pc less than 1 (rat mesothelium and the basal cells of the epithelium of the hamster cheek pouch) is due to stimulation of cell transition from R1 phase to the regulatory G1r phase (the dichophase) within G1 period of the mitotic cycle. The stimulation was assumed to depend on the radiation and transmutation defects in DNA due to H3 disintegration, and to occur when the stream of labelled cells reached the G1r phase. Proliferative pool and the duration of mitotic cycle can be estimated by means of coordinates of the abnormal curve.     

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.     

Hydrogen peroxide inhibits cell cycle progression by inhibition of the spreading of mitotic CHO cells

Hydrogen peroxide (H(2)O(2)) induces a number of events, which are also induced by mitogens. Since the progression through the G1 phase of the cell cycle is dependent on mitogen stimulation, we were interested to study the effect of H(2)O(2) on the cell cycle progression. This study demonstrates that H(2)O(2) inhibits DNA synthesis in a dose-dependent manner when given to cells in mitosis or at different points in the G1 phase. Interestingly, mitotic cells treated immediately after synchronization are significantly more sensitive to H(2)O(2) than cells treated in the G1, and this is due to the inhibition of the cell spreading after mitosis by H(2)O(2). H(2)O(2) reversibly inhibits focal adhesion activation and stress fiber formation of mitotic cells, but not those of G1 cells. The phosphorylation of MAPK is also reversibly inhibited in both mitotic and G1 cells. Taken together, H(2)O(2) is probably responsible for the inhibition of the expression of cyclin D1 and cyclin A observed in cells in both phases. In conclusion, H(2)O(2) inhibits cell cycle progression by inhibition of the spreading of mitotic CHO cells. This may play a role in pathological processes in which H(2)O(2) is generated.     

Cell Cycle Synchronization at the G2/M Phase Border by Reversible Inhibition of CDK1

Chemical agents for cell cycle synchronization have greatly facilitated the study of biochemical events driving cell cycle progression. G1, S and M phase inhibitors have been developed and used widely in cell cycle research. However, currently there are no effective G2 phase inhibitors and synchronization of cultured cells in G2 phase has been challenging. Recently, a selective CDK1 inhibitor, RO-3306, has been identified that reversibly arrests proliferating human cells at the G2/M phase border and provides a novel means for cell cycle synchronization. A single-step protocol using RO-3306 permits the synchronization of >95% of cycling cancer cells in G2 phase. RO-3306 arrested cells enter mitosis rapidly after release from the G2 block thus allowing for isolation of mitotic cells without microtubule poisons. RO-3306 represents a new molecular tool for studying CDK1 function in human cells.     

Characterization of the adaptive response to ionizing radiation induced by low doses of X rays to human lymphocytes

In previous studies we have shown that low doses of radiation from incorporated tritiated thymidine can make human lymphocytes less susceptible to the genetic damage manifested as chromatid breakage induced by a subsequent high dose of X rays. We have also shown that this adaptive response to ionizing radiation can be induced by very low doses of X rays (0.01 Gy; i.e., 1 rad) delivered during S phase of the cell cycle. To see if a low dose of X rays could induce this response in cells at other phases of the cell cycle, human lymphocytes were irradiated with 0.01 or 0.05 Gy before stimulation by phytohemagglutinin (G0) or with 0.01 Gy at various times after stimulation (G1), followed by 1.5 Gy (150 rad) at G2 phase. Although G0 lymphocytes failed to exhibit an adaptive response, G1 cells irradiated as early as 4 h after stimulation did show the response. Experiments were also carried out to determine how long the adaptive response induced by 0.01 Gy could persist. A 0.01-Gy dose was delivered to lymphocytes in the first S phase, followed by 1.5 Gy in the same or subsequent cell cycles. Lymphocytes receiving a 1.5-Gy dose at 40, 48, or 66 h after stimulation exhibited an adaptive response, whereas those receiving a 1.5-Gy dose at 90 or 114 h did not. Duplicate cultures containing bromodeoxyuridine showed that at 40 h all the lymphocytes were in their first cell cycle after stimulation, at 48 h half of the lymphocytes were in their first cell cycle and half in their second, and at 66 h 80% of the lymphocytes were in their third cell cycle. Thus the adaptive response persists for at least three cell cycles after it is induced by 0.01 Gy of X rays. In other experiments, the time necessary for maximal expression of the adaptive response was determined by delivering 0.01 Gy at hourly intervals 1-6 h before the 1.5-Gy dose. While a 4-h interval was enough for expression of the adaptive response, shorter intervals were not.     

12-O-tetradecanoylphorbol-13-acetate induces an immediate, but transient increase in mitotic activity uncoupled from DNA synthesis in the monolayer cultures of rat keratinocyte

A single wave of mitotic activity was observed in a monolayer culture of rat keratinocytes immediately after exposure to 12-O-tetradecanoylphorbol-13-acetate. A peak for cells in prophase, observed at 10 min after the exposure, was followed by a peak for metaphase at 20 min, for anaphase at 25 min and telophase at 30 min after the exposure. Thereafter, the mitotic activity began to subside. This transient stimulation of mitotic activity resulted in an increase of population density in the monolayer culture. There was neither a stimulation of DNA synthesis during this period nor a change of the DNA content after the mitotic activity was completed. This single burst of synchronous mitotic activity which did not require a substantial stimulation of DNA synthesis suggests that the effect was on the initiation process of mitosis among a subpopulation of cells, presumably cells delayed in the G2 phase of the cell cycle.     

Cell-cycle studies by multiparametric automatic scanning of topographically preserved cells in culture

The authors have developed a new methodology for characterizing in situ the cell cycle of human mammary epithelial cell lines. Using a SAMBA 200 cell image processor (scanning cytometry), 15 densitometric and textural parameters were computed on each Feulgen-stained nucleus. Parameters computed from the grey level cooccurrence and run-length section matrices allowed assessment of the chromatin pattern. Multiparametric analysis of data defined: 1) the relative position of each cell; 2) the relative positions of groups of cells, each group corresponding to a definite phase of the cell cycle; and 3) the function of these parameters best separating these phases. Files then were constructed for each phase: G0/G1, S, G2/ and M. Using these three files as a reference to classify cells, it was possible to ascertain the phase of the cell cycle for each cell of a population. The MDA AG human cell line synchronized by mitotic selection was used as a model to develop this method. The criteria used to assign cells to G0/G1, S, or G2 was DNA content. Classification in M phase was achieved by visual identification of mitotic cells. This method was checked on unsynchronized MDA AG and then applied to other human cell lines (MDA MB231, MCF-7, T47D C111). Comparison of results obtained by scanning cytometry and flow cytometry showed the proportion of cells assigned to G0/G1, S, and G2/M by the two methods to be similar. This new method removes some of the limitations of flow cytometry by 1) allowing visual verification of each cell analyzed; 2) lowering the number of cells required for study; 3) discriminating between G2 and M; and 4) preserving cell topography.     

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.     

Restriction of CEA synthesis to the stationary phase of growth of cultured human colon carcinoma cells

CEA production by cultured human colon carcinoma cells was measured as a function of proliferative status. Both intra- and extracellular CEA levels were low for exponentially growing cells, but increased several fold when cells entered stationary phase of growth. It is suggested that CEA production may be limited to the G1 phase of the mitotic cycle.     

Synchronization of goat fibroblast cells at quiescent stage and determination of their transition from G0 to G1 by detection of cyclin D1 mRNA

A number of studies have reported that donor cells consisting of serum starved cells, which are assumed to be at quiescence (G0), or non-starved confluent cells or mitotic cells obtained by shake-off, both of which are assumed to be at G1 phase, give better results in nuclear transfer (NT) than cells at other phases of the cell cycle. Whether G0 or G1 cells function better as donor cells is yet to be determined by detailed studies. The aims of this study were to analyze the cell cycle of goat transfected fibroblasts and determine the timing of transition from G0 to G1 by detecting G1-specific marker, cyclin D1 mRNA. Fluorescent-activated cell sorting (FACS) analyses of cells after 4 days of serum starvation showed that more that 90% of cells were in G0/G1. Additionally, detection of cyclin D1 mRNA by northern blot analysis showed that 4-day serum starved quiescent cells started entering G1 a few hours after addition of 10% serum to the medium. Taken together, the data indicated that serum starved transfected primary fibroblasts of adult goats experienced the G0 to G1 transition within 5 h of serum stimulation and were at the mid-G1 stage within 10 h of serum stimulation.     

Regulation of Polo-like kinase 1 by DNA damage in mitosis. Inhibition of mitotic PLK-1 by protein phosphatase 2A

DNA damage triggers multiple checkpoint pathways to arrest cell cycle progression. Polo-like kinase 1 (Plk1) is an important regulator of several events during mitosis. In addition to Plk1 functions in cell cycle, Plk1 is involved in DNA damage check-point in G2 phase. Normally, ataxia telangiectasia-mutated kinase (ATM) is a key enzyme involved in G2 phase cell cycle arrest following DNA damage, and inhibition of Plk1 by DNA damage during G2 occurs in a ATM/ATR-dependent manner. However, it is still unclear how Plk1 is regulated in response to DNA damage in mitosis in which Plk1 is already activated. Here, we show that treatment of mitotic cells with doxorubicin and gamma-irradiation inhibits Plk1 activity through dephosphorylation of Plk1, and cells were arrested in G2 phase. Treatments of the phosphatase inhibitors and siRNA experiments suggested that PP2A pathway might be involved in regulating mitotic Plk1 activity in mitotic DNA damage. Finally, we propose a novel pathway, which is connected between ATM/ATR/Chk and protein phosphatase-Plk1 in DNA damage response in mitosis.     

Transformation of human embryonic cells by the Rous sarcoma virus and the polyomavirus depending on the mitotic cycle phase]

The whole cycle of skin-muscle embryonic human tissue culture is 18 hours, with phases S, G1, G2 and M being 7, 6, 4 and 1 hour, respectively. The mitotic index of this culture is 28%. The maximum sensitivity of these synchronized cell cultures to transforming activity of the Rous and Sindai viruses was observed in phase S. The infection of synchronized primary embryonic human fibroblasts in phase S with the polyoma virus together with the Sindai virus has resulted in single cases of transformation. Similar results were obtained with non-synchronized human cultures.     

Vincristine induces somatic segregation,via mitotic crossing-over,in diploid cells of Aspergillus nidulans

Vincristine is an alkaloid widely used as an antineoplastic agent. In eukaryotic cells the drug causes blockage in the G2 phase of the cell cycle and an increase in the frequency of sister chromatid exchanges. Due to the fact that germinating Aspergillus nidulans cells spend most of their cycle in G2 phase, they provide an excellent system for the study of mitotic crossing-over. Taking into account that mitotic crossing-over occurs during G2 period, the evaluation of recombinagenic and aneugenic potential of vincristine is provided with regard to two diploid strains of A. nidulans: a wild strain (uvsH+//uvsH+) and a defective one in DNA repair (uvsH//uvsH). Drug toxicity and its effect on the asexual cycle of A. nidulans has been evaluated as well. Treatment of both strains with vincristine did not change colony growth in the culture, however cytological analyses showed aberrant conidiophores. Recombinagenic potential of vincristine was evaluated by induction of gene homozygosis originally present in heterozygosity diploid strains (Homozygotization Index). Results show that vincristine induces mitotic crossing-over and higher frequency of aneuploid mitotic segregants. The results also show the recombinagenic and aneuploidogenic potential of vincristine and suggest its participation in the induction of secondary malignancies.     

Human cytomegalovirus infection inhibits cell cycle progression at multiple points, including the transition from G1 to S.

Human cytomegalovirus inhibits the growth of human foreskin fibroblast cells by 12 h after infection. Analysis of the cellular DNA content of infected cells by flow cytometry demonstrated that cytomegalovirus does not arrest cell cycle progression at a single point. At least two blockages occur, one of which is in the G1 phase of the cell cycle. The G1 arrest introduced by cytomegalovirus infection blocks S-phase entry after serum stimulation.     

Cell cycle-specific glucocorticoid growth regulation of a human cell line (NHIK 3025)

It has been reported that the human cell line NHIK 3025 has a specific cytoplasmic glucocorticoid receptor. When these cells were exposed to glucocorticoids, the cell cycle time was prolonged. Cells, synchronized by mitotic selection, were subjected to the synthetic glucocorticoid dexamethasone throughout the cell cycle. Only cells exposed in the first half of G1 phase had a lengthened cell cycle time. Most of the prolongation was also located within the G1 phase. The dexamethasone growth inhibition was reversible and could be detected only in the cell cycle where the cells were exposed to the steroid. DNA-histograms of asynchronous cells were recorded by flowcytometry at various times after steroid exposure. These histograms also showed G1 phase sensitivity and G1 phase prolongation after exposure to dexamethasone. Our results thus indicate that these cells have a dexamethasone-sensitive restriction point in mid-G1 phase of the cell cycle.     

Cytogenetic evaluations in human lymphocytes exposed to methyl acetimidate, a lysine-specific protein crosslinking agent

The cytogenetic effects of methyl acetimidate (MAI), a lysine-specific protein crosslinking reagent, were investigated using human peripheral lymphocytes in culture. Lymphocytes were treated with the chemical either prior to PHA exposure or 2-3 days following mitogenic stimulation and assessed for perturbations in cellular proliferation and induction of SCEs. Severe reductions in the mitotic index (MI) and pronounced decreases in the proportion of metaphases proceeding beyond M(1) were observed following G0 exposure to MAI concentrations of as low as 2 mM; with complete suppression of mitotic activity in all cultures exposed to levels of 3 mM MAI or greater. Concentrations resulting in severe depression in MI caused only moderate increases in SCEs. Cells exposed to less than 10 mM MAI during the late S-G2 stages of the cell cycle and harvested at the first metaphase following treatment exhibited profound mitotic delay, impaired prophase to metaphase transitions and abnormal mitotic configurations. These findings demonstrate that protein-specific crosslinking agents may induce a wide spectrum of adverse cytogenetic outcomes in both cycling and noncycling lymphocytes.     

Chalones and the Control of Normal, Regenerative, and Neoplastic Growth of the Skin

SYNOPSIS Chalones,inhibitors of cell dmsion have been isolatedand studied from a number of mammalian tissues, most notably,the epidermis The epidermal rhalone is a glycoprotein It exhibitsconsiderable, but not complete specificity The epidermal chalone decreases mitotic activity by inhibitingcells in the G 2 phase of the cell cycle from entering mitosis,and probably also by inhibiting ceils in the G 1 phase of thecell cycle from entering mitosis To inhibit cells in G 2 fromentering mitosis the chilone requnes adrenalin, and for maximalactivity hydrocortisone It is not known if idrenalin and hydrocortisoneare required for chalone inhibition of cells in G 1 In addition to inhibiting cell division in normal epidermalcells the epidermal chalone can inhibit cell division in regeneratingepidermal cells induced to proliferate by chemical damage Thephase of the cell cycle in which the chalone inhibits legeneratingepidermal cells from entering mitosis is not known Epidermal tumors contain a decreased amount of chalone Mitosisin epidermal tumors is inhibited by treatment with epidermalchalone Tumor cells are inhibitedfrom entering mitosis fromeither the G 1 or G 2 phases of the cell cycle Chalones are said to inhibit mitosis by a negative feedbackmechanism However, experiments which presumably result in adecrease in chalone concentration do not result in an increasein mitotic activity It is suggested that if chalones are physiological controllers of cell division they do not act by a simplenegative feedback mechanism but require the action of a substanceto decrease their concentration     

PPARgamma1 synthesis and adipogenesis in C3H10T1/2 cells depends on S-phase progression, but does not require mitotic clonal expansion

Adipogenesis is typically stimulated in mouse embryo fibroblast (MEF) lines by a standard hormonal combination of insulin (I), dexamethasone (D), and methylisobutylxanthine (M), administered with a fresh serum renewal. In C3H10T1/2 (10T1/2) cells, peroxisome proliferator-activated receptor gamma1 (PPARgamma1) expression, an early phase key adipogenic regulator, is optimal after 36 h of IDM stimulation. Although previous studies provide evidence that mitotic clonal expansion of 3T3-L1 cells is essential for adipogenesis, we show, here, that 10T1/2 cells do not require mitotic clonal expansion, but depend on cell cycle progression through S-phase to commit to adipocyte differentiation. Exclusion of two major mitogenic stimuli (DM without insulin and fresh serum renewal) from standard IDM protocol removed mitotic clonal expansion, but sustained equivalent PPARgamma1 synthesis and lipogenesis. Different S-phase inhibitors (aphidicolin, hydroxyurea, l-mimosine, and roscovitin) each arrested cells in S-phase, under hormonal stimulation, and completely blocked PPARgamma1 synthesis and lipogenesis. However, G2/M inhibitors effected G2/M accumulation of IDM stimulated cells and prevented mitosis, but fully sustained PPARgamma1 synthesis and lipogenesis. DM stimulation with or without fresh serum renewal elevated DNA synthesis in a proportion of cells (measured by BrdU labeling) and accumulation of cell cycle progression in G2/M-phase without complete mitosis. By contrast, standard IDM treatments with fresh serum renewal caused elevated DNA synthesis and mitotic clonal expansion while achieved equivalent level of adipogenesis. At most, one-half of the 10T1/2 mixed cell population differentiated to mature adipocytes, even when clonally isolated. PPARgamma was exclusively expressed in the cells that contained lipid droplets. IDM stimulated comparable PPARgamma1 synthesis and lipogenesis in isolated cells at low cell density (LD) culture, but in about half of the cells and with sensitivity to G1/S, but not G2/M inhibitors. Importantly, growth arrest occurred in all differentiating cells, while continuous mitotic clonal expansion occurred in non-differentiating cells. Irrespective of confluence level, 10T1/2 cells differentiate after progression through S-phase, where adipogenic commitment induced by IDM stimulation is a prerequisite for PPARgamma synthesis and subsequent adipocyte differentiation.