Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “G1 Phase”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed1991

Identification of a prominent 85-kDa cAMP-dependent phosphoprotein associated with late G1 phase in mitogen-stimulated B lymphocytes.

In order to elucidate late regulatory events which may be involved in the onset of S phase in B lymphocytes, we studied the effect of anti-Ig on phosphorylation of soluble proteins at late G1 phase. Stimulation of murine splenic B lymphocytes with anti-Ig and other mitogens for 18 h was found to be associated with a major increase in phosphorylation of an 85 kDa/pI approximately 5.3 cytosolic protein, conversely, stimulation of the cells with non-mitogenic stimuli did not induce the phosphorylation of pp85. The increase in phosphorylation of pp85 could not be detected after 30 min, was barely detectable after 6 h, but was very prominent after 18 h of stimulation with anti-Ig. Thus, the increase in phosphorylation of pp85 is not an early signal but is rather correlated with the late G1 phase. pp85 could not be detected in the nuclei of either control or stimulated cells. Stimulation of B cells for 30 min with forskolin induced the phosphorylation of pp85, while phorbol ester did not have any effect. The phosphorylation of pp85 was induced by the catalytic subunit of cAMP protein kinase. Comparison of the phosphopeptide map of pp85 phosphorylated by anti-Ig in intact cells to the phosphopeptide map phosphorylated by forskolin or by the catalytic subunit of cAMP protein kinase, showed a striking similarity indicating that cAMP protein kinase may be involved in phosphorylation of pp85 in mitogen-stimulated cells. An increase in intracellular cAMP levels at late G1 phase was found in B cells stimulated by mitogens. These results implicate an important role for cAMP-dependent phosphorylation events, specifically the phosphorylation of pp85/pI 5.3, at late G1 phase during the cell cycle.

Animals↗
PubMed2006

Continuous ERK activation downregulates antiproliferative genes throughout G1 phase to allow cell-cycle progression.

BACKGROUND: The ERK family of MAP kinase plays a critical role in growth factor-stimulated cell-cycle progression from G0/G1 to S phase. It has been suggested that sustained activation, but not transient activation, of ERK is necessary for inducing S phase entry. Although the essential role of ERK MAP kinase in growth factor-stimulated gene expression, especially expression of immediate-early genes, is well established, it has remained unclear how ERK activity duration affects the promotion of G1 phase progression to S phase. RESULTS: We have found that inhibition of ERK activation by the MEK inhibitor or dominant-negative MEK1 even immediately before the onset of S phase leads to the cessation of S phase entry. Our analyses reveal that there are ERK-dependent downregulated genes, whose expression levels return to their original levels rapidly after ERK inactivation, and that their downregulation mostly requires AP-1 activity. Remarkably, microinjection experiments demonstrate that many of the downregulated genes act as antiproliferative genes during G1 phase and that their forced expression to the levels before growth factor stimulation even in late G1 phase blocks S phase entry. CONCLUSIONS: Thus, continuous ERK activation downregulates antiproliferative genes until the onset of S phase to allow successful G1 phase progression. This mechanism may also work as a fail-safe mechanism, which prevents inappropriate stimuli that induce transient ERK activation from causing S phase entry.

Animals↗
PubMed1997

Ras activity late in G1 phase required for p27kip1 downregulation, passage through the restriction point, and entry into S phase in growth factor-stimulated NIH 3T3 fibroblasts.

It is well documented that Ras functions as a molecular switch for reentry into the cell cycle at the border between G0 and G1 by transducing extracellular growth stimuli into early G1 mitogenic signals. In the present study, we investigated the role of Ras during the late stage of the G1 phase by using NIH 3T3 (M17) fibroblasts in which the expression of a dominant negative Ras mutant, p21(Ha-Ras[Asn17]), is induced in response to dexamethasone treatment. We found that delaying the expression of Ras(Asn17) until late in the G1 phase by introducing dexamethasone 3 h after the addition of epidermal growth factor (EGF) abolished the downregulation of the p27kip1 cyclin-dependent kinase (CDK) inhibitor which normally occurred during this period, with resultant suppression of cyclin Ds/CDK4 and cyclin E/CDK2 and G1 arrest. The immunodepletion of p27kip1 completely eliminated the CDK inhibitor activity from EGF-stimulated, dexamethasone-treated cell lysate. The failure of p27kip1 downregulation and G1 arrest was also observed in cells in which Ras(Asn17) was induced after growth stimulation with a phorbol ester or alpha-thrombin and was mimicked by the addition late in the G1 phase of inhibitors for phosphatidylinositol-3-kinase. Ras-mediated downregulation of p27kip1 involved both the suppression of synthesis and the stimulation of the degradation of the protein. Unlike the earlier expression of Ras(Asn17) at the border between G0 and G1, its delayed expression did not compromise the EGF-stimulated transient activation of extracellular signal-regulated kinases or inhibit the stimulated expression of a principal D-type cyclin, cyclin D1, until close to the border between G1 and S. We conclude that Ras plays temporally distinct, phase-specific roles throughout the G1 phase and that Ras function late in G1 is required for p27kip1 downregulation and passage through the restriction point, a prerequisite for entry into the S phase.

3T3 Cells↗
PubMed1999

G1-checkpoint function including a cyclin-dependent kinase 2 regulatory pathway as potential determinant of 7-hydroxystaurosporine (UCN-01)-induced apoptosis and G1-phase accumulation.

7-Hydroxystaurosporine (UCN-01), which was originally identified as a protein kinase C selective inhibitor, is currently in clinical trials as an anti-cancer drug. We previously showed that UCN-01 induced preferential G1-phase accumulation in tumor cells and this effect was associated with the retinoblastoma (Rb) protein and its regulatory factors, such as cyclin-dependent kinase 2 (CDK2) and CDK inhibitors p21Cip1/WAF1 and p27Kip1. We demonstrate here that G1-phase accumulation was induced by UCN-01 in Rb-proficient cell lines (WiDr and HCT116 human colon carcinomas and WI-38 human lung fibroblast), and it was accompanied by dephosphorylation of Rb. In addition, UCN-01-induced G1-phase accumulation was also demonstrated in a Rb-defective cell line (Saos-2 human osteosarcoma), but not in a simian virus 40 (SV40)-transformed cell line (WI-38 VA13). Apoptosis was induced by UCN-01 in the two Rb-deficient cell lines, but not in the other Rb-proficient cell lines. These observations suggest that G1-checkpoint function might be important for cell survival during UCN-01 treatment. In addition, there may be a UCN-01-responsive factor in the G1-checkpoint machinery other than Rb which is targeted by SV40. Further studies revealed a correlation between UCN-01-induced G1-phase accumulation and reduction of cellular CDK2 kinase activity. This reduction was strictly dependent on down-regulation of the Thr160-phosphorylated form of CDK2 protein, and coincided in part with up-regulation of p27Kip1, but it was independent of the level of the p21Cip1/WAF1 protein. These results suggest that G1-checkpoint function, including a CDK2-regulatory pathway, may be a significant determinant of the sensitivity of tumor cells to UCN-01.

Alkaloids↗
PubMed1997

G1-phase arrest is not a prerequisite for encystment in Physarum.

Amoebae of the Myxomycete Physarum polycephalum form resistant, walled cysts when the food bacteria in a culture have been consumed. No G1 phase has been detected in the vegetative amoebal cell cycle, most of which comprises the G2 phase. Mature cysts are also in G2, but it has been reported that a G1 phase of roughly 24 h, followed by an S phase, is obligatory prior to encystment. We used flow cytometry to determine the distribution of DNA contents in amoebal cultures at intervals during vegetative growth and encystment. In all cultures, the cells were predominantly in G2 phase, and the percentage of cells with G1 DNA content remained very low. We conclude that an extended G1 phase of 24 h did not occur in our cultures and cannot be a prerequisite for encystment.

Animals↗
PubMed1996

Cellular expression of human centromere protein C demonstrates a cyclic behavior with highest abundance in the G1 phase.

Centromere proteins are localized within the centromere-kinetochore complex, which can be proven by means of immunofluorescence microscopy and immunoelectron microscopy. In consequence, their putative functions seem to be related exclusively to mitosis, namely to the interaction of the chromosomal kinetochores with spindle microtubules. However, electron microscopy using immune sera enriched with specific antibodies against human centromere protein C (CENP-C) showed that it occurs not only in mitosis but during the whole cell cycle. Therefore, we investigated the cell cycle-specific expression of CENP-C systematically on protein and mRNA levels applying HeLa cells synchronized in all cell cycle phases. Immunoblotting confirmed protein expression during the whole cell cycle and revealed an increase of CENP-C from the S phase through the G2 phase and mitosis to highest abundance in the G1 phase. Since this was rather surprising, we verified it by quantifying phase-specific mRNA levels of CENP-C, paralleled by the amplification of suitable internal standards, using the polymerase chain reaction. The results were in excellent agreement with abundant protein amounts and confirmed the cyclic behavior of CENP-C during the cell cycle. In consequence, we postulate that in addition to its role in mitosis, CENP-C has a further role in the G1 phase that may be related to cell cycle control.

Cell Cycle↗
PubMed1999

Regulation of p21(cip1) expression by growth factors and the extracellular matrix reveals a role for transient ERK activity in G1 phase.

We have examined the regulation of p21(cip1) by soluble mitogens and cell anchorage as well as the relationship between the expression of p21(cip1) and activation of the ERK subfamily of MAP kinases. We find that p21(cip1) expression in G1 phase can be divided into two discrete phases: an initial induction that requires growth factors and the activation of ERK, and then a subsequent decline that is enhanced by cell anchorage in an ERK-independent manner. In contrast to the induction of cyclin D1, the induction of p21(cip1) is mediated by transient ERK activity. Comparative studies with wild-type and p21(cip1)-null fibroblasts indicate that adhesion-dependent regulation of p21(cip1) is important for proper control of cyclin E-cdk2 activity. These data lead to a model in which mitogens and anchorage act in a parallel fashion to regulate G1 phase expression of p21(cip1). They also show that (a) growth factors and growth factor/extracellular matrix cooperation can have different roles in regulating G1 phase ERK activity and (b) both transient and sustained ERK signals have functionally significant roles in controlling cell cycle progression through G1 phase.

Animals↗
PubMed1982

Glucocorticoid hormone renders a rat glioma cell line sensitive to a G1 phase block by microtubule disrupting agents.

1. Hydrocortisone, a glucocorticoid hormone, renders C6 rat glioma cells (clone ST1) sensitive to a block by colchicine or nocodazole (microtubule disrupters) at the G1 phase of the cell cycle. Restimulation of DNA synthesis in hydrocortisone-treated glioma cells arrested at G0/G1 phase by serum step-down is inhibited (85%) by colchicine (0.4 microgram/ml) added during the first 6 h of restimulation by serum step-up. 2. Exponentially growing, hydrocortisone-treated glioma cell cultures when subjected to colchicine treatment accumulated mitoses for 16.5 h, resulting in two types of cell cycle blocked cells: mitotic (round, detached or poorly attached) and G1 phase cells (flat and well attached to the solid substrate). The latter reinitiated DNA synthesis 15 h after colchicine withdrawal. Plating efficiency assays showed that while the colchicine block was highly toxic for mitotic cells, the survival of G1 phase arrested cells was not affected. In conclusion, in these rat glioma cells, hydrocortisone reversibly makes G1 phase progress dependent on microtubule integrity. 3. Restimulation of DNA synthesis in "normal" 3T3 fibroblasts arrested at the G0/G1 phase by serum deprivation was not inhibited by colchicine when the drug was added at the time of serum step-up. However, 70% inhibition occurred when colchicine was added at 10 h of serum stimulation.

Animals↗
PubMed2004

Cytochrome P4501A1 promotes G1 phase cell cycle progression by controlling aryl hydrocarbon receptor activity.

The aryl hydrocarbon receptor (AhR) transcription factor is increasingly recognized as functioning in cell cycle control. Several recent reports have shown that AhR activity in the absence of exogenous agonists or presence of the prototypical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin can affect G1 phase progression in cultured cells. Serum release of serum-starved (G0) 5L rat hepatoma cells triggers transient AhR activation and P4501A1 protein expression concomitant with the G0/G1-to-S phase transition. In contrast, sustained AhR activation in response to TCDD treatment increases p27Kip1 expression in addition to P4501A1, resulting in G1 phase cell cycle arrest. Treating serum-released 5L cells with the alkyne metabolism-based P4501A1 inhibitor 1-(1-propynyl)pyrene results in prolonged AhR activation, enhanced p27Kip1 expression, and G1 phase arrest after serum release. The data are consistent with a cell cycle role for P4501A1 because they show that P4501A1 negatively regulates the duration of AhR action through the metabolic removal of the receptor agonist, thereby preventing AhR-mediated G1 phase arrest.

Animals↗
PubMed2001

MCM2-7 proteins are essential components of prereplicative complexes that accumulate cooperatively in the nucleus during G1-phase and are required to establish, but not maintain, the S-phase checkpoint.

A prereplicative complex (pre-RC) of proteins is assembled at budding yeast origins of DNA replication during the G1-phase of the cell cycle, as shown by genomic footprinting. The proteins responsible for this prereplicative footprint have yet to be identified but are likely to be involved in the earliest stages of the initiation step of chromosome replication. Here we show that MCM2-7 proteins are essential for both the formation and maintenance of the pre-RC footprint at the origin ARS305. It is likely that pre-RCs contain heteromeric complexes of MCM2-7 proteins, since degradation of Mcm2, 3, 6, or 7 during G1-phase, after pre-RC formation, causes loss of Mcm4 from the nucleus. It has been suggested that pre-RCs on unreplicated chromatin may generate a checkpoint signal that inhibits premature mitosis during S-phase. We show that, although mitosis does indeed occur in the absence of replication if MCM proteins are degraded during G1-phase, anaphase is prevented if MCMs are degraded during S-phase. Our data indicate that pre-RCs do not play a direct role in checkpoint control during chromosome replication.

Cell Cycle Proteins↗
PubMed1989

Photoreactivation of lethal damage and damage leading to chromatid deletions induced in G1 phase hamster x Xenopus hybrid cells by UV.

A86 Xenopus cells, cloned from a Xenopus line that exhibited a high level of photoreactivation of UV-induced lethal damage, and V79M1 hamster cells, cloned from a hamster line that did not exhibit efficient photoreactivation of such damage, were fused to produce the V79M1 x A86 cell line--a hybrid line in which approximately 84% of the cells contained the entire V79M1 and A86 genomes. Ultraviolet and UV plus photoreactivation fluence-survival relations were then determined and compared for hybrid and parental G1 phase cells in a first attempt to elucidate interactions of the parental genetic potentials for photoreactivation in the hybrid. Specifically, it was anticipated that the combined V79M1 and A86 genomes in the hybrid would produce photoreactivating enzymes sufficient to efficiently photoreactivate UV-induced lethal damage in both A86 and V79M1 DNA and little difference would be observed in the levels of photoreactivation exhibited by V79M1 x A86 and A86 G1 phase cells. To the contrary, the level of photoreactivation observed for the hybrid did not closely approach that observed for the A86 line. To assist in the interpretation of this somewhat unexpected observation, three additional studies were performed: (1) comparison of 'optimal' schemes for photoreactivation of UV-induced lethal damage in the hybrid and parental G1 phase cells, (2) comparison of the effects of some different types of growth medium on photoreactivation of UV-induced lethal damage in hybrid and parental G1 phase cells, and (3) comparison of the levels of photoreactivation of UV-induced chromatid deletions in the V79M1 and A86 chromosomes of G1 phase hybrid cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2005

G1 phase arrest of the cell cycle by a ginseng metabolite, compound K, in U937 human monocytic leukamia cells.

We recently reported that the ginseng saponin metabolite, compound K (20-O-beta-D-glucopyranosyl-20(S)-protopanaxadiol, IH901), inhibits the growth of U937 cells through caspase-dependent apoptosis pathway. In this study, we further characterized the effects of compound K on U937 cells and found that, in addition to apoptosis, compound K induced the arrest of the G1 phase. The compound K treated U937 cells showed increased p21 expression; an inhibitory protein of cyclin-cdk complex. The up-regulation of p21 was followed by the inactivation of cyclin D and the cdk4 protein, which act at the early G1 phase, and cyclin E, which acts at the late G1 phase. Furthermore, compound K induced the activation of JNK and the transcription factor AP-1, which is a downstream target of JNK. These findings suggest that the up-regulation of p21 and activation of JNK in the compound K treated cells contribute to the arrest of the G1 phase.

Cyclin D↗
PubMed2002

Reappraisal of G1-phase arrest and synchronization by lovastatin.

It has been proposed that lovastatin arrests cells in the G1-phase of the division cycle, and that release from lovastatin inhibition produces a synchronized culture. A new method of methocel time-lapse-videography has been used to analyse cell division patterns following lovastatin treatment. Release of L1210 cells from lovastatin inhibition failed to produce synchronized divisions. Moreover, contrary to earlier proposals, lovastatin did not arrest cells with a G1-phase amount of DNA. Analysis of previous reports of 'synchronization' and growth-arrest support these findings. It is concluded that lovastatin neither synchronizes cells, nor arrests cells in the G1-phase of the division cycle.

Animals↗
PubMed2005

Integrin-dependent signal transduction regulating cyclin D1 expression and G1 phase cell cycle progression.

Integrins and growth factor receptors coordinately regulate proliferation in nontransformed cells. Coordinate signaling from these receptors controls the activation of the G1 phase cyclin-dependent kinases, largely by regulating levels of cyclin D1 and p27(kip1). Induction of cyclin D1 is one of the best understood examples of an integrin/growth factor receptor-regulated G1 phase target. This review focuses on the integrin-dependent signal transduction events that regulate the expression of cyclin D1 during G1 phase.

Animals↗
PubMed2003

Spatial distribution and specification of mammalian replication origins during G1 phase.

We have examined the distribution of early replicating origins on stretched DNA fibers when nuclei from CHO cells synchronized at different times during G1 phase initiate DNA replication in Xenopus egg extracts. Origins were differentially labeled in vivo versus in vitro to allow a comparison of their relative positions and spacing. With nuclei isolated in the first hour of G1 phase, in vitro origins were distributed throughout a larger number of DNA fibers and did not coincide with in vivo origins. With nuclei isolated 1 h later, a similar total number of in vitro origins were clustered within a smaller number of DNA fibers but still did not coincide with in vivo origins. However, with nuclei isolated later in G1 phase, the positions of many in vitro origins coincided with in vivo origin sites without further change in origin number or density. These results highlight two distinct G1 steps that establish a spatial and temporal program for replication.

Animals↗
PubMed2006

Unscheduled CDK1 activity in G1 phase of the cell cycle triggers apoptosis in X-irradiated lymphocytic leukemia cells.

Cyclin-dependent kinase 1 (CDK1) is a major component of the cell cycle progression engine. Recently, several investigations provided evidence demonstrating that unscheduled CDK1 activation may also be involved in apoptosis in cancerous cells. In this article, we demonstrate that X-ray irradiation induced G1 arrest in MOLT-4 lymphocytic leukemia cells, the arrest being accompanied by reduction in the activity of CDK2, but increased CDK1 activity and cell apoptosis in the G1 phase. Interestingly, this increase in CDK1 and apoptosis by ionizing radiation was prevented by pretreatment with the CDK1 inhibitor, roscovitine, suggesting that CDK1 kinase activity is required for radiation-induced apoptotic cell death in this model system. Furthermore, cyclin B1 and CDK1 were detected co-localizing and associating in G1 phase MOLT-4 cells, with the cellular lysates from these cells revealing a genotoxic stress-induced increase in CDK1 phosphorylation (Thr-161) and dephosphorylation (Tyr-15), as analyzed by postsorting immunoprecipitation and immunoblotting. Finally, X-irradiation was found to increase Bcl-2 phosphorylation in G1 phase cells. Taken together, these novel findings suggest that CDK1 is activated by unscheduled accumulation of cyclin B1 in G1 phase cells exposed to X-ray, and that CDK1 activation, at the wrong time and in the wrong phase, may directly or indirectly trigger a Bcl-2-dependent signaling pathway leading to apoptotic cell death in MOLT-4 cells.

Apoptosis↗
PubMed1977

Differential chromosomal radiosensitivity within the first G1-phase of the cell cycle of early-dividing human leukocytes in vitro after stimulation with PHA.

Human leukocyte cultures were irradiated with 200 RX-rays before the addition of phytohemagglutinin (PHA) in the Go-stage and at different times up to 25 h within the first G1-phase of the cell cyle after the addition of PHA. The results of the analysis of chromosomal aberrations show that the frequencies of dicentric chromosomes increase significantly when leukocytes leave the Go-stage, reaching a miximum yield of aberrations about halfway through the first G1-phase. After that, toward the end of the G1-phase, the frequencies of dicentric chromosomes decrease again to a level similar to that found in the Go-stage. Different possible explanations for the differential chromosomal radiosensitivity of human leukocytes within the first post-stimulation G1-phase are discussed.

Chromosome Aberrations↗
PubMed1995

A model of the G1 phase of the cell cycle incorporating cyclin E/cdk2 complex and retinoblastoma protein.

A mathematical model of cyclin E, cdk2 and retinoblastoma protein control of the G1 phase of the human cell cycle is proposed. The model includes retinoblastoma (Rb) protein phosphorylation by a cyclin E/cdk2 complex and its subsequent dephosphorylation at the end of the cell cycle. The numerical solutions to this model demonstrates the cyclic behavior of the cyclin E/cdk2 complex, with and without Rb function, cell cycle. This model suggests an inhibition of cyclin E/cdk2 complex formation (or its activation) by hypophosphorylated retinoblastoma protein. The experimental results of cell cycle arrest upon injection of transforming growth factor-beta, alpha-interferon or D-erythro-sphingosine during G1 phase are reproduced. Cell cycle behavior predicted by this model for increasing the concentration of hypophosphorylated retinoblastoma protein during the G1 phase is discussed. Additional results are obtained by numerical simulation.

CDC2-CDC28 Kinases↗