Expression of proliferationg cell nuclear antigen during the cell cycle of human diploid fibroblasts

Summary Proliferating cell nuclear antigen mRNA levels were determined in human diploid fibroblasts as they progressed through the cell cycle. PCNA message levels were low at G0, gradually increased following entrance into G1, peaked at G1/S, and declined during S phase. PCNA mRNA was determined to have a half life of 12 hours when cells were blocked at the G1/S interface. PCNA protein levels increased two- to three-fold as cells moved from G0 to S phase.     

The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.     

桦木酸对H22荷瘤小鼠肿瘤细胞凋亡及细胞周期的影响

研究桦木酸对H22荷瘤小鼠生命延长率、肿瘤细胞凋亡及细胞周期的影响。结果表明桦木酸能够明显延长H22荷瘤小鼠生生存时间,其中低、中剂量效果显著（P〈0．05）;利用DNA结合性荧光探针直接对细胞DNA染色后FCM分析,桦木酸可能是通过影响H22肿瘤细胞S期而诱导肿瘤细胞凋亡。     

EGF induces cell cycle arrest of A431 human epidermoid carcinoma cells

The human carcinoma cell line A431 is unusual in that physiologic concentrations of epidermal growth factor (EGF) inhibit proliferation. In the presence of 5-10 nM EGF proliferation of A431 cells is abruptly and markedly decreased compared to the untreated control cultures, with little loss of cell viability over a 4-day period. This study was initiated to examine how EGF affects the progression of A431 cells through the cell cycle. Flow cytometric analysis of DNA in EGF-treated cells reveals a marked change in the cell cycle distribution. The percentage of cells in late S/G2 increases and early S phase is nearly depleted. Since addition of the mitotic inhibitor vinblastine causes accumulation of cells in mitosis and prevents reentry of cells into G1, it is possible to distinguish between slow progression through G1 and G2 and blocks in those phases. When control cells, not treated with EGF, are exposed to vinblastine, the cells accumulate mitotic figures, as expected, and show progression into S, thus diminishing the number of cells in G1. In contrast, no mitotic figures are found among the EGF-treated cells in the presence or absence of vinblastine, and progression from G1 into S is not observed, as the number of cells in G1 remains constant. These results suggest that there are two EGF-induced blocks in cell cycle transversal; one is in late S and/or G2, blocking entry into mitosis, and the other is in G1, blocking entry into S phase. After 24 hours of EGF treatment, DNA synthesis is reduced to less than 10% compared to untreated controls as measured by the incorporation of [3H]thymidine or BrdU. In contrast, protein synthesis is inhibited by about twofold. Although inhibition of protein synthesis is less extensive, it occurs 6 hours prior to an equivalent inhibition of DNA synthesis. The rapid decrease in protein synthesis may result in the subsequent cell cycle arrest which occurs several hours later.     

Cell cycle analysis of cultured porcine mammary cells

One of the major points of debate in determining the effectiveness of nuclear transfer technology has been the phase of the cell cycle of the donor cell at the time of nuclear transfer. Here, a primary mammary cell line has been isolated and various treatments for synchronization of the cell cycle have been tested. The cells were then simultaneously stained for DNA content and protein content and the percentages of cells in G1, G0, S, and G2 + M were estimated. In the first experiment, cells were either cycling, grown to confluence, or serum-starved for 5 days. Serum starvation increased (p < 0.05) the percentage of cells in G0 compared to confluent or cycling cells from 3% to 8% to 22%. By using forward scatter to determine the size of the cells it was determined that if small cells (7-15 microm) were selected from the serum-starved group 43.9% will be in G(0) as compared to 4.5% of cycling cells and 9.9% of confluent cells. Dimethyl sulfoxide (DMSO) treatment (0%, 0.5%, or 1.0%) for 72 hours (shown to synchronize some cell types in G0) had no effect on the percentage of cells in G0, G1, S, or G2 + M. Treatment with mimosine (0 microM, 0.4 microM, 0.8 microM or 1.2 microM), a compound that should synchronize the cells in G1, increased (p < 0.05) the percentage of cells in G1 from 66.7% (0 microM mimosine) to 79.0% to 82.0%. Finally, treatment with colchicine for 24 hours (shown to synchronize some cell types in G2 + M) increased (p < 0.05) the percentage of cells in G2 + M (0 microM colchicine) from 13.3% to 27.2% to 31.6%. It is concluded that many cell cycle synchronization techniques are effective in porcine mammary cell lines, but none of the techniques are 100% effective. Such results should help elucidate the mechanisms involved in nuclear transfer.     

Correlation between cell cycle duration and RNA content.

The metachromatic fluorochrome acridine orange was used to differentially stain DNA and RNA in Chinese hamster ovary (CHO) cells and in mitogen-stimulated human lymphocytes during their progression through the cell cycle. Green and red fluorescence of individual cells, representing cellular DNA and RNA, respectively, was measured by flow cytometry. CHO cells were synchronized by selective detachment at mitosis. Their rate of progression through G1 and subsequently through S phase correlated with the content of stainable RNA. The mean duration of the G1 phase was 5.2 hours for cells with high RNA content (highest 25 percentile population) and 8.1 hours for cells with low RNA (lowest 25 percentile). The duration of S phase was 5.9 and 7.5 hours for high- and low-RNA, 25 percentile subpopulations, respectively. Lymphocytes synchronized at the G1/S boundary by hydroxyurea or 5-fluorodeoxyuridine showed extremely high intercellular variation with respect to content of stainable RNA. After release from the block they traversed S phase at rates linearly proportional to the content of stainable RNA. The duration of S phase was five hours for cells with high RNA-, six to nine hours for cells with moderate RNA- and up to 27 hours for cells with minimal RNA-content. The data suggest that the rate of progression through the cell cycle of individual cells within a population may be correlated with the number of ribosomes per cell.     

Membrane transport in synchronized Ehrlich ascites tumor cells: uptake of amino acids by the A and L system during the cell cycle.

Using the double thymidine block technique. Ehrlich ascites tumor cells (ELD) carried in continuous spinner culture have been synchronized. Simultaneous monitoring of 3H-thymidine incorporation, cell number and mitotic index yielded a cell cycle time of approximately 13.5 hours. This is composed of an S period of 3-4 hours. G2 of 6-8 hours and M of 1-2 hours. No appreciable G1 is present. Ehrlich cells synchronized in this manner were used to investigate the characteristics of two neutral amino acid transport systems during progression through the cell cycle. Unidirectional influx via the Na-dependent system A was studied using C14-alpha-aminoisobutyrate (AIB) as substrate. The Na-independent system L was monitored using 3H-leucine and 14C-cycloleucine as substrates. Transport by the A system was minimal in M and early S. It underwent a three-fold increase during late S and early G2. In mid G2 the transport via this system rapidly dropped and remained low again through M and early S. The intracellular/extracellular ratios of AIB indicate that the system is actively transporting AIB thoughout the cell cycle. The minimum ratios of approximately 3 were achieved during early M and the maximum ratios of approximately 9 were achieved in late S, early G2. The uptake of leucine and cycloleucine by the L system was quite different during the cell cycle. Maximal unidirectional influx by this system occurred during early and mid S period. Upon progression into G2 the transport rate dropped and remained reduced throughout M. Intracellular/extracellular ratios of leucine or cycloleucine were near unity at the peak of the transport activity (early S) and dropped to values of 0.5 to 0.6 throughout the remainder of the cycle. This result indicates that inward transport by the L system is, for the most part, non-active in growing cells.     

Cellular Ras and Cyclin D1 Are Required during Different Cell Cycle Periods in Cycling NIH 3T3 Cells

Novel techniques were used to determine when in the cell cycle of proliferating NIH 3T3 cells cellular Ras and cyclin D1 are required. For comparison, in quiescent cells, all four of the inhibitors of cell cycle progression tested (anti-Ras, anti-cyclin D1, serum removal, and cycloheximide) became ineffective at essentially the same point in G1 phase, approximately 4 h prior to the beginning of DNA synthesis. To extend these studies to cycling cells, a time-lapse approach was used to determine the approximate cell cycle position of individual cells in an asynchronous culture at the time of inhibitor treatment and then to determine the effects of the inhibitor upon recipient cells. With this approach, anti-Ras antibody efficiently inhibited entry into S phase only when introduced into cells prior to the preceding mitosis, several hours before the beginning of S phase. Anti-cyclin D1, on the other hand, was an efficient inhibitor when introduced up until just before the initiation of DNA synthesis. Cycloheximide treatment, like anti-cyclin D1 microinjection, was inhibitory throughout G1 phase (which lasts a total of 4 to 5 h in these cells). Finally, serum removal blocked entry into S phase only during the first hour following mitosis. Kinetic analysis and a novel dual-labeling technique were used to confirm the differences in cell cycle requirements for Ras, cyclin D1, and cycloheximide. These studies demonstrate a fundamental difference in mitogenic signal transduction between quiescent and cycling NIH 3T3 cells and reveal a sequence of signaling events required for cell cycle progression in proliferating NIH 3T3 cells.     

Characterization of functionally active interleukin‐18/eGFP fusion protein expression during cell cycle phases in recombinant chicken DF1 Cells

The dependence of foreign gene expression on cell cycle phases in mammalian cells has been described. In this study, a DF1/chIL‐18a cell line that stably expresses the fusion protein chIL‐18 was constructed and the enhanced green fluorescence protein connected through a (G₄S)₃ linker sequence investigated the relationship between cell cycle phases and fusion protein production. DF1/chIL‐18a cells (1 × 10⁵) were inoculated in 60‐mm culture dishes containing 5 mL of media to achieve 50%–60% confluence and were cultured in the presence of the cycle‐specific inhibitors 10058‐F4, aphidicolin, and colchicine for 24 and 48 h. The percentage of cell density and mean fluorescence intensity in each cell cycle phase were assessed using flow cytometry. The inhibitors effectively arrested cell growth. The fusion protein production rate was higher in the S phase than in the G0/G1 and G2/M phases. When cell cycle progression was blocked in the G0/G1, S, and G2/M phases by the addition of 10058‐F4, aphidicolin, and colchicine, respectively, the aphidicolin‐induced single cells showed higher fusion protein levels than did the 10058‐F4‐ or colchicine‐induced phase cells and the uninduced control cells. Although the cells did not proliferate after the drug additions, the amount of total fusion protein accumulated in aphidicolin‐treated cells was similar to that in the untreated cultures. Fusion protein is biologically active because it induces IFN‐γ production in splenocyte cultures of chicken. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:581–591, 2016     

X-ray Sensitivity of HeLa S3 Cells in the G2 Phase: Comparison of Two Methods of Synchronization

The sensitivity of HeLa S3 cells to 220 kv X-rays was measured in terms of cell survival (colony development) during the G2 phase of the cell generation cycle, employing two procedures designed to free G2 cultures from contaminating cells from other phases of the cycle. Treatment of synchronous cultures (obtained initially by mitotic selection) with high specific activity tritiated thymidine (HSA-³HTdR) selectively eliminated S phase cells, while addition of vinblastine permitted removal of cells as they entered mitosis. It was found that HeLa S3 cells become increasingly sensitive as they progress through G2. The pattern of sensitivity fluctuations observed in synchronous HeLa S3 populations selected by the foregoing method was compared with that found in synchronous cultures prepared by the HSA-³HTdR method of Whitmore. The latter method had been used previously with mouse L cells, which were found to undergo a different pattern of sensitivity fluctuations. The two methods yield similar results for HeLa cells in the S and G2 phases of the cycle. It may be concluded, therefore, that the discrepancies between HeLa and mouse L cells do not arise from methodological factors, but represent fundamental differences between the cell types.     

Alteration of the Cloudman melanoma cell cycle by prostaglandins E1 and E2 determined by using a 5-bromo-2'-deoxyuridine method of DNA analysis

Prostaglandins (PGs) E1 and E2 stimulate tyrosinase activity and suppress the proliferation of Cloudman S91 melanoma cells by altering their progression through the cell cycle. Prostaglandin E1 and PGE2 have prolonged or residual effects on melanoma cells. Cells treated for 5 or 24 hours with 10 micrograms/ml PGE1 or cells treated for 8 or 24 hours with 10 micrograms/ml PGE2 demonstrated decreased proliferation and increased tyrosinase activity for 48 hours after removal of the PGs. The effects of PGs on the cell cycle were investigated by determining total DNA content in cells stained with propidium iodide (PI) and analyzed by a fluorescence activated cell sorter (FACS). Prostaglandin E1 blocked cells in G2 phase after 5 hours of treatment, corresponding to when inhibition of proliferation was first evident. Similarly, after 9 hours of treatment with PGE2, more cells were in late S, early G2 phase and less in G1 than their control counterparts. Also, melanoma cells were pulse-labeled with 5-bromo-2'-deoxyuridine (BrdUrd) prior to or at the end of PG treatment and then stained with a fluoresceinated monoclonal antibody to BrdUrd, and with PI. This allows one to observe how BrdUrd-labeled S-phase cells cycle with time. Both PGE1 and PGE2 inhibit proliferation by blocking cells in G2 phase of the cell cycle. The PG-induced block in G2 may be required by melanoma cells to synthesize mRNA and proteins that are essential for stimulation of tyrosinase activity. Ultrastructurally, only a subpopulation of the cells treated with PGE1 or PGE2 contained more mature melanosomes than control cells.     

苜蓿银纹夜蛾核型多角体病毒的感染对Sf9细胞周期的影响

病毒的感染导致细胞内部发生一系列变化。应用流式细胞仪FACS的荧光检测 ,测出Sf9细胞完成整个周期循环大约需要 18h ,G1、S、G2 /M各时相的时间间隔约为 6h ;AcNPV感染Sf9细胞 12 18h ,细胞被抑制于G2 /M期 ;Sf9细胞同步于G1/S期后释放细胞并用AcNPV感染 ,12h后 ,2 / 3的细胞处于G2 /M期 ,1/ 3的细胞处于S期     

Life Cycle Analysis of Mammalian Cells: I. A Method for Localizing Metabolic Events within the Life Cycle, and Its Application to the Action of Colcemide and Sublethal Doses of X-Irradiation

Equations are presented describing the accumulation of cells at any part of the life cycle as a result of addition of specific blocking agents. An experimental methodology using these relationships is described which makes possible analysis with relatively high resolution of the distribution of cells throughout the life cycle in normal cultures or those treated with various agents. The action of colcemide on S3 HeLa cells studied by this method revealed that colcemide has no effect on the G1, S, or G2 stages; it blocks cells quantitatively at the metaphase-anaphase region; but it accumulates mitotic figures only from the cells which have not yet entered mitosis at the time of its addition. The technique was also applied to study the efficiency of x-irradiation in delaying the entrance of G2 cells into mitosis. A definite lag was found at the lowest dose studied which was 9 rads. Only the cells confined to a central region of G2 at the time of irradiation are affected by this dose.     

Identification of G1-Regulated Genes in Normally Cycling Human Cells

Background

Obtaining synchronous cell populations is essential for cell-cycle studies. Methods such as serum withdrawal or use of drugs which block cells at specific points in the cell cycle alter cellular events upon re-entry into the cell cycle. Regulatory events occurring in early G1 phase of a new cell cycle could have been overlooked.

Methodology and Findings

We used a robotic mitotic shake-off apparatus to select cells in late mitosis for genome-wide gene expression studies. Two separate microarray experiments were conducted, one which involved isolation of RNA hourly for several hours from synchronous cell populations, and one experiment which examined gene activity every 15 minutes from late telophase of mitosis into G1 phase. To verify synchrony of the cell populations under study, we utilized methods including BrdU uptake, FACS, and microarray analyses of histone gene activity. We also examined stress response gene activity. Our analysis enabled identification of 200 early G1-regulated genes, many of which currently have unknown functions. We also confirmed the expression of a set of genes candidates (fos, atf3 and tceb) by qPCR to further validate the newly identified genes.

Conclusion and Significance

Genome-scale expression analyses of the first two hours of G1 in naturally cycling cells enabled the discovery of a unique set of G1-regulated genes, many of which currently have unknown functions, in cells progressing normally through the cell division cycle. This group of genes may contain future targets for drug development and treatment of human disease.     

Variations in DNA Methylation Patterns During the Cell Cycle of HeLa Cells

DNA methylation has been viewed as a stable component of the epigenome, which is established during development and fixed thereafter. We show here using nearest neighbor analysis, immunocytochemistry, and high performance capillary electrophoresis that the DNA methylation pattern varies in HeLa cells during a single cell cycle. Immunocytochemical analysis in primary human fibroblasts shows similar variations. The global levels of DNA methylation decreased in G1 and increase during the S phase of the cell cycle. Since there was little change in the DNA methylation levels in repetitive sequences throughout the cell cycle, we examined the DNA methylation pattern of unique sequences using a human CpG island microarray. Hybridization with methylated DNA from G1 and S phase of the cell cycle revealed that 174 CG-containing sequences were differentially methylated between G1 and S. 75% of all the variations in DNA methylation detected in unique sequences represented hypomethylation at G0, with changes occurring in both CpG islands and non-CpG islands. Bisulfite mapping confirmed these changes in methylation in the regions identified by the microarray. This is the first demonstration of a dynamic DNA methylation pattern within a single cell cycle of a mature somatic cell. These data are important for our understanding of the stability of DNA methylation patterns in somatic cells.     

Cell cycle diversity involves differential regulation of Cyclin E activity in the Drosophila bristle cell lineage

In the Drosophila bristle lineage, five differentiated cells arise from a precursor cell after a rapid sequence of asymmetric cell divisions (one every 2 hours). We show that, in mitotic cells, this rapid cadence of cell divisions is associated with cell cycles essentially devoid of the G1-phase. This feature is due to the expression of Cyclin E that precedes each cell division, and the differential expression of the S-transition negative regulator, Dacapo. Thus, apart from endocycles (G/S), which occurred in two out of five terminal cells, two other cell cycles coexist in this lineage: (1) an atypical cell cycle (S/G2/M), in which the S-phase is initiated during the preceding telophase; and (2) a canonical cell cycle (G1/S/G2/M) with a brief G1 phase. These two types of cell cycle result from either the absence or very transient expression of Dap, respectively. Finally, we show that the fate determinant factor, Tramtrack, downregulates Cyclin E expression and is probably involved in the exit of the cells from the cell cycle.     

Enhanced Sensitivity of G1 Arrested Human Cancer Cells Suggests a Novel Therapeutic Strategy Using a Combination of Simvastatin and TRAIL

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) preferentially induces apoptosis in tumor cells over normal cells. To study the relationship between cell cycle progression and TRAIL-induced apoptosis, SW480 colon cancer and H460 lung cancer cell lines were examined for their sensitivity to TRAIL after arrest in different cell cycle phases. Cells were synchronized in G0/G1, S, and G2/M phase by serum starvation, aphidicolin, or nocodazole treatment, respectively. We found that arrest of cells in G0/G1 phase confers significantly higher susceptibility to TRAIL-induced apoptosis as compared to cells in late G1, S, or G2/M phase. To determine if cell cycle phase could be harnessed for therapeutic gain in the presence of TRAIL, we used the HMG-CoA reductase inhibitor, Simvastatin and lovastatin, to enrich a cancer cell population in G0/G1. Both simvastatin and lovastatin significantly augmented TRAIL-induced apoptosis in tumor cells, but not in normal keratinocytes. The results indicate that TRAIL, in combination with a HMG-CoA reductase inhibitor, may have therapeutic potential in the treatment of human cancer.

Key Words

TRAIL, Synchronization, Simvastatin, Cancer Therapy, Lovastatin, Cell Cycle, Apoptosis     

中药固真方对UMR106细胞周期蛋白表达及细胞周期的影响

应用流式细胞技术和 Western blot技术 ,观察了中药固真方对大鼠成骨肉瘤细胞 UMR1 0 6细胞的细胞周期及细胞周期蛋白 cyclin E和 cyclin A表达的影响 .结果显示 :固真方处理 UMR1 0 6细胞后 2 4 h,S期细胞百分比达高峰 ,占 49.6% ,而对照组 30 h时 ,S期细胞才增高 ,达 43.2 % .cy-clin E的表达在固真方处理 8h时增高 ,而对照组在 1 6h时 ,cyclin E表达才达高峰 .cyclin A的表达在给药组处理 2 4 h时最高 ,而对照组在 30 h时 ,cyclin A表达才增高 .揭示固真方可缩短细胞周期的时程 ,加速细胞周期的运行 ,从而促进细胞增殖 .     

Differentiation of HL-60 cells: cell volume and cell cycle changes

HL-60 promyelocytic leukemic cells can differentiate into more mature myeloid cells with the addition of dimethylsulfoxide, butyric acid or retinoic acid and can differentiate into macrophages with the addition of phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA). After the addition of an inducer, the HL-60 cell volume shows a daily decrease while the cell number increases at a rate similar to the untreated control cells. Flow cytometry measurements show an increase in G1 cells and a decrease in S cells after day 1. Since the generation time is constant, the data suggest that the length of time spent in the different cell cycle stages has changed during differentiation. Within 3 hours after the addition of TPA to HL-60 cells, selective adhesion of G1 cells occurs. Smaller sized cells are recovered from the flask bottom and larger sized cells are recovered from the supernate. Flow cytometric analysis reveals a G1 and S block in cells obtained from both the supernatant and from the flask bottom. After 1 day of TPA incubation, there is preferential adhesion of G1 and G2 cells with the nonadherent cells being primarily in the S and G2 cell cycle stages and undergoing a cell cycle traverse.