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At least 343 records · Page 19Linked to original sources
PubMed1982

Poly(ADP-ribosyl)ation of chromosomal proteins in the HeLa S3 cell cycle.

Change in the level of poly(ADP-ribosyl)ation during the cell cycle of HeLa S3 cells was examined in three different in vitro systems: isolated nuclei, chromatin, and poly(ADP-ribose) polymerase extracted from isolated nuclei, and by the direct immunofluorescent antibody technique in situ. In isolated nuclei, after release from thymidine-hydroxyurea double block, the level of poly(ADP-ribosyl)ation was low in the S phase and increased in the late G2 phase with a peak in the M phase. In chromatin, the level began to increase in the mid-late S phase, was maintained at a high level throughout the G2 phase, decreased in the M phase, and became basal during the G1 phase. The activity of poly(ADP-ribose) polymerase extracted from isolated nuclei was relatively high throughout the cell cycle and was highest in the G2 phase. Similar changes during the HeLa S3 cell cycle were observed in the three systems after release from Colcemid block. In synchronously growing HeLa S3 cells in situ, nuclei in the G2-M phase were stained strongest for poly(ADP-ribose) by the immunofluorescent staining. These results were consistent with the peak of poly(ADP-ribosyl)ation in the G2-M phase observed in the nuclear system in vitro.

Cell Cycle↗
PubMed1984

Phase-dependent polymerization and depolymerization of actin in Physarum polycephalum nuclei.

G- and F-actin contents in physarum polycephalum nuclei isolated during the progress of cell cycle were estimated and compared both by the inhibiting activity of deoxyribonuclease I (DNase I) and by the fractionation on DEAE-Toyopearl column chromatography. The inhibition of DNase appeared maximal in late S phase or early G2 phase and decreased to a minimal value in late G2 phase or M phase, while total actin content in a nucleus, which was analyzed by SDS-polyacrylamide gel electrophoresis as well as by DNase inhibition assay in the presence of 0.75 M guanidine-HCl, did not show such phase-dependent dynamics through the cell cycle.

Actins↗
PubMed1984

Comparative studies on the heat-induced thermotolerance of protein synthesis and cell division in synchronized mouse neuroblastoma cells.

Mouse neuroblastoma (N2A) cells react to a heat treatment by inhibition of DNA and protein synthesis and induction of cell cycle progression delay. Mitotic delay of heat-treated G1 cells correlates with reduction of protein synthesis and is due to an extensive delay of entrance into S phase, while the G2 phase of these cells is shortened. Mitotic delay of heat-treated G2 cells is more than in G1 cells and no correlation with protein synthesis reduction is found. In heat-treated G1 phase cells, both protein synthesis and cell cycle progression become thermotolerant to a second incubation at increased temperature. Moreover, the process of DNA synthesis becomes thermotolerant. In contrast, when heat-treated G1 phase cells have progressed into G2 phase and are then incubated at increased temperature, this G2 phase delay is not diminished. Apparently, additional targets for hyperthermia are present in late S and G2 phase cells.

Acclimatization↗
PubMed1994

Identification of a potentially radiosensitive subgroup among patients with breast cancer.

BACKGROUND: The description of genes and genetic syndromes, such as ataxia-telangiectasia, that predispose some women to breast cancer will provide greater insight into the genetic basis of cancer susceptibility. PURPOSE: Our goal was to establish cell lines from patients with breast and bladder cancers, to screen for enhanced levels of radiation-induced arrest in the G2 phase of the cell cycle such as is observed in ataxia-telangiectasia heterozygotes, and to correlate G2 arrest with other prognostic indicators of these cancers and in vivo radiosensitivity. METHODS: Epstein-Barr virus-transformed lymphoblastoid cells were established from 108 female patients with breast cancer and 24 age-matched female control subjects, and from 45 patients with bladder cancer and 18 age-matched control subjects. Cells were exposed to 3 Gy of gamma radiation, and the percentages of cells in G1 and G2 phases were determined at 18 and 24 hours after irradiation by fluorescence-activated cell sorter analysis. Postirradiation delay in G2 phase was determined by calculating the percentage of cells in G2 and by using the ratio G2/G1. RESULTS: When we determined the percentage of cells in G2 phase at 18 hours after irradiation in 108 lymphoblastoid cells from breast cancer patients, we observed an increase of between 3% and 38% in the number of cells in G2 phase in comparison with cells that were not irradiated. Comparison with previous G2-phase arrest data for ataxia-telangiectasia heterozygotes using a cutoff point at 29% delay demonstrated that 20% and 8% of the breast cancer cell lines of the case patients and control subjects, respectively, fell into that category (P < .001). At the same time after irradiation, it was not possible to distinguish between bladder cancer cell lines (7%) and those of the corresponding control group (6%). Assessment of radiation effects by G2/G1 ratio showed that 18% of the breast cancer patients and 8% of the control subjects were in the high range. When G2 arrest was correlated with other prognostic factors, we found that case patients with a greater G2 block were more likely to have had a family history of breast cancer (P < .006) and more aggressive tumors when assessed by number of involved lymph nodes (P < .002) and tumor size (P < .05). Furthermore, an adverse response to radiotherapy was observed in a group of patients with high G2 arrest. IMPLICATIONS: While the postirradiation increase in G2-phase arrest in cells from breast cancer patients observed in this study may indicate genetic heterozygosity for ataxia-telangiectasia, it might also reflect other genetic abnormalities important to breast cancer.

Adult↗
PubMed2001

DNA cytofluorometric analysis of chondrocytes in human articular cartilages under normal aging or arthritic conditions.

OBJECTIVE: Since most chondrocytes in articular cartilage are in the resting phase (G0) of the cell cycle, it has been difficult to investigate their cell kinetics using 3H-thymidine autoradiography, or immunohistochemistry. In the present study, DNA cytofluorometry, which is useful to analyse the cell kinetics even for such inactive cell populations as in the G0 phase, was applied to human chondrocytes of the articular cartilages under normal aging and pathologic conditions such as osteoarthritis (OA), rheumatoid arthritis (RA), and aseptic necrosis (AN). DESIGN: The human articular cartilages for the study were obtained from autopsy and surgical materials. Fifty joints were used for the study of aging, 54 for the study of OA, 20 for studying RA, and 10 for AN study. The isolated chondrocytes were quickly prepared from fresh articular cartilages, using a combination method of enzymatic digestion with papain and collagenase, followed by mechanical cell separation by churning and homogenization. RESULTS: The DNA histograms obtained by cytofluorometry with propidium-iodide staining showed that most chondrocytes had diploid DNA content (2c) in all cartilages studied, suggesting that they were in the G0 phase. However, there were a few chondrocytes having tetraploid DNA content (4c) in the normally aged articular cartilages, and there were some cells having DNA content between 2c and 4c in the diseased cartilages. The former cells were considered to be G0-phase cells of the 4c chondrocytes, while the latter cells were considered to be in the DNA synthetic (S) phase or G2-phase of the 2c chondrocytes. The frequency of 4c chondrocytes in aged cartilage was significantly increased, compared to that in the young cartilage. In contrast to the normal cartilage, the frequency of S- and G2-phase cells, which was expressed as the S- G2 index, in diseased cartilages (OA, RA and AN) was significantly high (P< 0.0001). In OA cartilage, the S-G2 index was much higher in the severe or moderate stage than in the mild stage, suggesting that the chondrocytes in clusters may actively proliferate. CONCLUSION: These results showed that in normal articular cartilages most chondrocytes are in the G0 phase, while some became 4c polyploid cells, and that these G0-phase chondrocytes had a potential to proliferate under diseased conditions.

Adolescent↗
PubMed1999

Chromosome radiosensitivity in human G2 lymphocytes and cell-cycle progression.

PURPOSE: To investigate the possibility that the differential G2-phase radiosensitivity of human peripheral blood lymphocytes, found in normal individuals using the 'G2-phase chromosome radiosensitivity assay', could be attributed to heterogeneity in cellular progression to mitosis rather than differences in radiosensitivity. MATERIALS AND METHODS: Human peripheral blood lymphocytes, from four different donors, were exposed to 50 cGy X-rays and sampled at different times. The progression of cells into mitosis was monitored by 5-bromo 2'-deoxyuridine (BrdUrd) incorporation. RESULTS: The heterogeneous G2-phase chromosome radiosensitivity among different donors was abolished when homogeneous G2-phase cell populations were scored; they contained similar frequencies of cells in early or late G2-phase. CONCLUSIONS: The heterogeneous G2-phase chromosome radiosensitivity, usually found in different normal donors, is caused by the analysis of different cell populations rather than reflecting intrinsic differences in radiosensitivity.

Adult↗
PubMed1986

Comparison of MHC antigen expression on PHA- and MLC-induced T cell lines with that on T and B lymphoblastoid cell lines by cell cycle dependency.

Although it is well known that the expression of major histocompatibility complex (MHC) antigens on the surface of lymphoblastoid cell lines are cell cycle dependent, the way in which the MHC antigen expression on activated T cells varies with cell cycle phase has not previously been described. Using 11 lymphoblastoid cell lines from malignant and nonmalignant tissues (B cells, T cells, and myeloid cells) and five activated T cell lines (two cell lines activated by phytohemagglutinin and three alloreactive T cell clones), MHC antigen expression was quantitatively studied by dual-beam flow cytometry. Correlated measurements of surface antigen quantity (immunofluorescence), DNA content (Hoechst 33342), and cell size (light scatter), uninfluenced by induction synchrony and cell fixation, were performed. The data indicate that cell surface antigen quantity and cell surface area demonstrate specific values at each phase of the cell cycle when the cells are in logarithmic growth. Examining cells in logarithmic growth, it was confirmed, for all lymphoblastoid cell lines, that the quantity of MHC antigens on G2 (S + G2 + M) cells was greater than that on G1 cells. In addition, it was found, by analyzing antigen quantity and surface area, that class I antigen density in the G2 phase is 17% less than that in the G1 phase in leukemic T cell lines, and that both class I and class II antigen densities in the G2 phase were 21% less than that in the G1 phase in lymphoblastoid B cell lines. In activated T cells, class I antigen density in the G2 phase was 11% less than that in the G1 phase, while class II antigen density in the G2 phase was 12% greater than that in the G1 phase. We describe four important observations in this report. In both G1 and G2 phases, activated T cells express: quantitatively fewer class I antigens than lymphoblastoid B cell lines; similar quantity of class I antigens as that of leukemic T cell lines; and similar quantity of class II antigens as that of lymphoblastoid B cell lines. Also, class II antigens are expressed in greater density in the G2 phase than in the G1 phase in activated T cells. In contrast, lymphoblastoid B cell lines express greater density of class II antigens in the G1 phase than in the G2 phase of the cell cycle. These findings differ from previous reports, suggesting that G1 phase cells may have a more significant role than G2 phase cells as target cells for MHC restricted cytotoxic cells.

B-Lymphocytes↗
PubMed1997

Cyclin/Cdk-dependent initiation of DNA replication in a human cell-free system.

We describe a cell-free system from HeLa cells that initiates DNA replication under cell cycle control. G1 but not G2 phase nuclei initiate replication when coincubated with S phase nuclei in cytosolic extracts from S phase but not from G1 or G2 phase HeLa cells. S phase nuclei or an S phase nuclear extract are required for the initiation of semiconservative DNA replication in G1 nuclei but not for elongation in S phase nuclei. S phase nuclear extract could be replaced by recombinant human cyclins A and E complexed to Cdk2 but not by Cdk2 alone or by human cyclin B1 complexed to Cdc2. In S phase cytosol, cyclins A/Cdk2 and E/Cdk2 triggered initiation synergistically.

CDC2-CDC28 Kinases↗
PubMed1999

Cell cycle block at G1-S or G2-M phase correlates with differentiation of Caco-2 cells: effect of constitutive insulin-like growth factor II expression.

BACKGROUND & AIMS: We have previously shown that autocrine insulin-like growth factor (IGF)-II synthesis through IGF-I receptor stimulates proliferation and inhibits differentiation of Caco-2 cells. To demonstrate whether differentiation of Caco-2 cells is dependent on cell growth status, we analyzed the effect of cell cycle arrest on differentiation of wild-type and IGF-II-overexpressing cells. METHODS: Cells were treated with drugs that inhibit the progression either to S phase (l-b-D-arabinofuranosylcytosine or M phase (nocodazole). Cell differentiation was analyzed by assessing apolipoprotein A-1 and sucrase-isomaltase expression. Cell proliferation and DNA content were assessed by thymidine incorporation and fluorescence-activated cell sorter analysis, respectively. Cell cycle regulatory molecules were analyzed by assessing p21 and retinoplasma protein (pRb) expression and pRb phosphorylation. RESULTS: Cell cycle block at G1-S phase was associated with increased expression of differentiation markers in both parental and IGF-II-transfected cells. On the contrary, cell cycle arrest at G2-M phase correlated with the expression of differentiation markers in parental but not in IGF-II-transfected cells. Constitutive IGF-II-expressing cells actively incorporated thymidine and showed an increase in the proportion of cells with >4N DNA ploidy in the presence of nocodazole. Nocodazole treatment of constitutive IGF-II-expressing cells stimulated p21 expression in the presence of hyperphosphorylated pRb. CONCLUSIONS: The data show that cell cycle arrest increases differentiation of Caco-2 cells. IGF-II-mediated proliferation may prevent cell differentiation through effects on control cell checkpoint proteins.

Anti-Bacterial Agents↗
PubMed1985

Chromosomal radiosensitivity during the G2 cell-cycle period of skin fibroblasts from individuals with familial cancer.

We reported previously that human cells after neoplastic transformation in culture had acquired an increased susceptibility to chromatid damage induced by x-irradiation during the G2 phase of the cell cycle. Evidence suggested that this results from deficient DNA repair during G2 phase. Cells derived from human tumors also showed enhanced G2-phase chromosomal radiosensitivity. Furthermore, skin fibroblasts from individuals with genetic diseases predisposing to a high risk of cancer, including ataxia-telangiectasia, Bloom syndrome, Fanconi anemia, and xeroderma pigmentosum exhibited enhanced G2-phase chromosomal radiosensitivity. The present study shows that apparently normal skin fibroblasts from individuals with familial cancer--i.e., from families with a history of neoplastic disease--also exhibit enhanced G2-phase chromosomal radiosensitivity. This radiosensitivity appears, therefore, to be associated with both a genetic predisposition to cancer and a malignant neoplastic state. Furthermore, enhanced G2-phase chromosomal radiosensitivity may provide the basis for an assay to detect genetic susceptibility to cancer.

Adolescent↗
PubMed2005

Destabilization of cyclin D1 message plays a critical role in cell cycle exit upon mitogen withdrawal.

Cyclin D1 is critical for entry into, continuation of, and exit from the cell division cycle. Mitogen stimulation of quiescent cells induces cyclin D1 expression in a transcription-dependent manner. In actively cycling cells, on the other hand, fluctuation of cyclin D1 protein levels through the cell cycle is post-transcriptionally regulated. Cyclin D1 is expressed at low levels during S phase to allow efficient DNA synthesis, and induced to high levels in G2 phase through Ras activity to commit the cells to continuing cell cycle progression. Once induced in G2 phase, cyclin D1 expression becomes Ras independent through the next G1 phase, where it promotes G1/S transition. When mitogenic signaling is abrogated, however, cyclin D1 fails to increase during G2 phase and the cell becomes arrested in the next G1 phase. In this way, the expression levels of cyclin D1 in G2 phase determine the fate of the next cell cycle. Despite its importance of the mechanism of cyclin D1 suppression upon mitogen withdrawal is unknown. Using both quantitative fluorescence microscopy and biochemical analyses, we have found that, upon serum deprivation, cyclin D1 mRNA is downmodulated without any decline in its rate of transcription. Furthermore, cyclin D1 mRNA half-life becomes shorter when serum is removed. These results demonstrate that cyclin D1 message destabilization plays a critical role in cyclin D1 suppression during G2 phase of serum-deprived cultures, and therefore in the withdrawal from the cell cycle.

Animals↗
PubMed2001

Iron deficiency alters the progression of mitogen-treated murine splenic lymphocytes through the cell cycle.

The influence of iron deficiency on the progression of mitogen-treated splenic lymphocytes through the cell cycle was studied in 16 control, 16 pair-fed, 15 iron-deficient (ID) and 16 ID mice that were repleted for up to 3 d (R3). The test and control diets differed only in iron concentrations (0.09 vs. 0.9 mmol/kg). When mice were killed (68 d of feeding), the hemoglobin concentration and liver iron stores of ID and R3 mice were <50% those of control mice (P < 0.05). Iron deficiency did not reduce the percentage of CD3(+) cells, but decreased CD3(+) cells/mg spleen (P < 0.05). In concanavalin A-treated and nonactivated cultures, there were no significant differences among groups in the percentages of cells in resting phase of the cell cycle (G0) to cell cycle initiation phase (G1), DNA synthesis phase (S) and exit from the S phase (G2) to mitosis phase (M) phases. In anti-CD3 and anti-CD3/anti-CD28-treated cultures, higher percentages of lymphocytes from ID and R3 mice than those from control and pair-fed mice were in the G0--G1 phase (P < 0.05). Conversely, lower percentages of activated cells from ID and R mice than those from control and pair-fed mice were in S and G2--M phases (P < 0.05). Incubation of lymphocytes with mitogens decreased the percentages of cells in G0--G1 phase from 90% to 80% in control and pair-fed but not in ID and R3 mice (P < 0.05). In activated cells, indices of iron status negatively correlated with the percentages of cells in G0--G1 (r = -0.306 to -0.597) but positively with those in S (r = 0.166--0.511) and G2--M phases (r = 0.265-0.59; P < 0.05). Data suggest that altered cell cycle progression likely contributes to impaired lymphocyte proliferation usually associated with iron deficiency.

Animals↗
PubMed1982

Cell cycle control in synchronously dividing antheridial filaments of Chara vulgaris L. as revealed by cycloheximide pulse treatment.

In antheridial filaments of Chara vulgaris L. (cell cycle S+G2+M type) cycloheximide (2.5 micrograms/ml) totally inhibits protein synthesis after 2 h. The protein synthesis is resumed after the removal of the inhibitor from the medium. The inhibition of protein synthesis in the second half of G2 phase causes supercontraction of mitotic chromosomes, whereas in the late G2 phase it induces diminishing of tubulin content and the inhibition of the initiation of DNA synthesis in the next generation of antheridial filaments. The inhibition of protein synthesis in G2 phase does not affect the mitotic index. The inhibition of protein synthesis during S phase results in a lower rate of DNA replication and the prolongation of S phase. The inhibition of protein synthesis in the second half of G2 phase of the previous generation and in the S phase of the given generation causes mitotic delay which increases in proportion to the duration of G2 phase in the given generation.

Cycloheximide↗
PubMed1999

Human cyclin A is required for mitosis until mid prophase.

We have used microinjection and time-lapse video microscopy to study the role of cyclin A in mitosis. We have injected purified, active cyclin A/cyclin-dependent kinase 2 (CDK2) into synchronized cells at specific points in the cell cycle and assayed its effect on cell division. We find that cyclin A/CDK2 will drive G2 phase cells into mitosis within 30 min of microinjection, up to 4 h before control cells enter mitosis. Often this premature mitosis is abnormal; the chromosomes do not completely condense and daughter cells fuse. Remarkably, microinjecting cyclin A/CDK2 into S phase cells has no effect on progress through the following G2 phase or mitosis. In complementary experiments we have microinjected the amino terminus of p21(Cip1/Waf1/Sdi1) (p21N) into cells to inhibit cyclin A/CDK2 activity. We find that p21N will prevent S phase or G2 phase cells from entering mitosis, and will cause early prophase cells to return to interphase. These results suggest that cyclin A/CDK2 is a rate-limiting component required for entry into mitosis, and for progress through mitosis until late prophase. They also suggest that cyclin A/CDK2 may be the target of the recently described prophase checkpoint.

CDC2-CDC28 Kinases↗
PubMed1987

Confirmation of Fanconi's anemia and detection of a chromosomal aberration (1Q12-32 triplication) via BrdU/Hoechst flow cytometry.

Peripheral blood lymphocyte cultures of a patient with clinical evidence of Fanconi's anemia (FA) were investigated by means of BrdU/Hoechst flow cytometry. This technique can be used to differentiate cycling from noncycling cells after PHA stimulation; it also permits the quantitative assessment of cell cycle progression and cell-cycle arrest. A characteristic cell kinetic pattern of combined G2 phase prolongation and G2 phase arrest was observed in the present patient, confirming the clinical diagnosis of FA. An additional finding was the presence of a small subpopulation of peripheral blood cells, refractory to PHA stimulation, and displaying a DNA content 5% greater than that of the diploid cells. BrdU/Hoechst flow cytometry was crucial to the unequivocal demonstration of this subpopulation, which would not have been detected without enrichment by sequential depletion of PHA-responsive cells from the G0/G1 compartment during a 96-h stimulation experiment. The parallel finding of a presumptive triplication affecting the q12-32 segment of chromosome 1 in rare spontaneous divisions of a 40-h peripheral blood culture is consistent with the flow-cytometric detection of elevated DNA content if, as is common in FA, the abnormal cell lineage represents a preleukemic clone of monocytic origin.

Adolescent↗
PubMed1997

Pleomorphic xanthoastrocytoma: DNA flow cytometry and outcome analysis of 12 patients.

BACKGROUND: Pleomorphic xanthoastrocytoma (PXA) is an astrocytic tumor occurring primarily in childhood and adolescence with some malignant histologic features but a relatively slow clinical course. However, some tumors progress more rapidly and can undergo malignant degeneration. The authors attempted to determine whether various histologic features or tumor cell proliferative indices might help identify lesions at risk for early progression and distinguish PXAs from malignant gliomas. METHODS: In a retrospective study of 12 patients with PXA, the tumor's histologic features and DNA flow cytometric parameters were compared with their clinical course. DNA flow cytometry values for the S- and G2-phase of the PXAs also were compared with control group samples of malignant and low grade astrocytomas. RESULTS: Of the 12 tumors at initial diagnosis, 5 were considered typical PXAs whereas 7 had some atypical features (4 with paucity of reticulin fibers, 1 with focal necrosis, and 2 with both atypical reticulin and focal necrosis). During the follow-up period (range, 3.75-11 years; mean, 6.8 years), 2 patients had recurrences; 1 atypical reticulin PXA progressed to glioblastoma after 6.5 years and the 1 tumor with focal necrosis recurred at 6 months and again at 2 years with typical histologic features. DNA flow cytometry parameters of the typical PXA group were similar to values for malignant astrocytoma and significantly higher than values for control specimens of low grade astrocytomas. There were no distinctive DNA flow cytometric features that could distinguish this last tumor from others with a more benign clinical course. CONCLUSIONS: Measurements of the S-phase and G2-phase obtained from DNA flow cytometry and atypical histologic features cannot reliably identify PXA patients at risk for early progression and overall are significantly higher than values obtained for low grade gliomas. Therefore, frequent (i.e., two to three times per year) postoperative clinical and radiologic examinations are necessary to judge the appropriateness of adjuvant therapy in patients with PXA. The paradox of slow growth but DNA flow cytometry consistent with aggressive malignant lesions may represent a cell-cycle arrest mechanism in these lesions that could be verified in subsequent studies.

Adolescent↗
PubMed1994

Nitric oxide blocks the cell cycle of mouse macrophage-like cells in the early G2+M phase.

The effects of nitric oxide produced by macrophage-like cells (Mm1) on the cell cycle were investigated. Mm1 cells lost proliferative activity in the presence of interleukin-6 (IL-6) and a subpopulation accumulated in the G2+M phase. This level increased in proportion to the incubation time. The DNA content of the cells was slightly lower than that of Mm1 cells treated with vinblastine or demecolcine, drugs which block the cell cycle in the M phase. The peak of the early G2+M phase was reduced by treatment with NG-mono-methyl-L-arginine. However, after treatment with exogenous nitric oxide or sodium nitroprusside, the G0/G1 phase increased, but the early-G2+M and the S phase decreased. The flow cytometry pattern in IL-6-treated Mm1 was the same as that of cytochalasin B-treated Mm1. These data suggest that endogenous nitric oxide affects the microfilament system of IL-6-treated Mm1 cells and blocks the cell cycle in the early G2+M phase.

Animals↗
PubMed1987

Cell cycle parameters of adult rat hepatocytes in a defined medium. A note on the timing of nucleolar DNA replication.

Hepatocytes, isolated from adult (250-350 g) rats, attached and survived well in primary culture on highly diluted (less than 1 microgram/cm2) collagen gel in a synthetic medium without serum or hormones. About 20% of the cells "spontaneously" entered S phase during the first 4 days of culturing, and mitoses were easily demonstrated at the near physiological concentration (1.25 mM) of Ca++ prevailing in the medium. Cultures given 9 nM epidermal growth factor (EGF) and 20 nM insulin 20 h after inoculation showed vigorous DNA synthesis and mitotic activity. Autoradiography of such cells exposed to [3H]thymidine allowed the determination of the following cell cycle parameters: Lag period from EGF/insulin stimulation till onset of increased DNA synthesis, 17 h; rate of entry into S phase (kG1/S), 0.028/h; duration of S phase, 8.4 h; duration of G2 phase, 2.7 h. The peak DNA synthesis (pulse labelling index, 24%) and peak mitotic activity (mitotic index, 1.7%) occurred 35 and 43 h, respectively, after the stimulation with EGF/insulin. These values are comparable to those reported during the in vivo compensatory hyperplasia following partial hepatectomy of adult rats. A marked variation of the intranuclear [3H]thymidine pulse labelling pattern was noted: During the first 1.5 h of the S phase, the labelling was extranucleolar and during the last 1.5 h chiefly nucleolar. The cells survived well in the absence of glucocorticoid, whose effect on cell cycle parameters therefore could be studied. Dexamethasone (25-250 nM) did not appreciably affect the durations of S phase and G2 phase or the pattern of preferential extranucleolar and nucleolar DNA synthesis within the S phase.

Animals↗