CYCLIC CHANGES IN THE CELL SURFACE : I. Change in Thymidine Transport and Its Inhibition by Cytochalasin B in Chinese Hamster Ovary Cells

Cytochalasin B (CB) shows a marked concentration-dependent inhibition of the incorporation of [³H]thymidine into Chinese hamster ovary cells. This inhibition was shown to result from an inhibition of thymidine uptake, not from an inhibition of DNA synthesis. Cells normally acquire the capacity to transport thymidine as they move from the G1 stage of the cell cycle into the S phase. If CB is added to cells while they are in G1, they do not acquire the ability to transport thymidine as they enter S. However, the addition of CB to cells that are already in S has no effect on their ability to transport thymidine. These results are discussed in terms of a model in which elements involved in thymidine transport enter the cell surface membrane as the cells move from G1 to S. It is proposed that CB prevents this structural transition by binding to the cell surface.     

Aryl hydrocarbon hydroxylase induction in mouse liver cells--relationship to position in the cell cycle

The inducibility of aryl hydrocarbon hydroxylase (AHH) by benzo[a]-pyrene (BaP) has been studied in synchronously grown cultures of mouse liver cells. These cells (NMuLi cl 8) have low basal levels of AHH which can be induced greater than 100-fold by BaP. Cells were synchronized in G1(G0) by serum starvation and in S by release from serum starvation in combination with excess thymidine. When released from G1(G0) by replating at lower cell density in fresh medium with 20% serum, cells began entering S with a lag of 12 h. Addition of BaP (1 microgram/ml) 8 h before serum stimulation, at the time of stimulation or 7.5 h after stimulation all gave similar induction kinetics: the AHH activity peaked as the cells began entering S regardless of when the BaP was added. Cells blocked in various parts of S by excess thymidine were inducible for AHH activity as efficiently as cells moving through S and into G2. These results indicate that the inducibility of AHH is greater when cells are actively proliferating and may be a contributing factor to why growing cells are more sensitive to mutagenesis and transformation than quiescent cells.     

Glucose-stimulated DNA synthesis through mammalian target of rapamycin (mTOR) is regulated by KATP channels: effects on cell cycle progression in rodent islets

The aim of this study was to define metabolic signaling pathways that mediate DNA synthesis and cell cycle progression in adult rodent islets to devise strategies to enhance survival, growth, and proliferation. Since previous studies indicated that glucose-stimulated activation of mammalian target of rapamycin (mTOR) leads to [3H]thymidine incorporation and that mTOR activation is mediated, in part, through the K(ATP) channel and changes in cytosolic Ca2+, we determined whether glyburide, an inhibitor of K(ATP) channels that stimulates Ca2+ influx, modulates [3H]thymidine incorporation. Glyburide (10-100 nm) at basal glucose stimulated [3H]thymidine incorporation to the same magnitude as elevated glucose and further enhanced the ability of elevated glucose to increase [3H]thymidine incorporation. Diazoxide (250 microm), an activator of KATP channels, paradoxically potentiated glucose-stimulated [3H]thymidine incorporation 2-4-fold above elevated glucose alone. Cell cycle analysis demonstrated that chronic exposure of islets to basal glucose resulted in a typical cell cycle progression pattern that is consistent with a low level of proliferation. In contrast, chronic exposure to elevated glucose or glyburide resulted in progression from G0/G1 to an accumulation in S phase and a reduction in G2/M phase. Rapamycin (100 nm) resulted in an approximately 62% reduction of S phase accumulation. The enhanced [3H]thymidine incorporation with chronic elevated glucose or glyburide therefore appears to be associated with S phase accumulation. Since diazoxide significantly enhanced [3H]thymidine incorporation without altering S phase accumulation under chronic elevated glucose, this increase in DNA synthesis also appears to be primarily related to an arrest in S phase and not cell proliferation.     

Synchronization of the human promyelocytic cell line HL 60 by thymidine

Cultures of the promyelocytic cell line HL 60 were synchronized with thymidine. A concentration of 0.05 mM thymidine and an exposure time of 24 hr was found optimal for blocking about 90% of the cells in S phase. Following release from the thymidine block the cell cultures were followed intermittently over 40 hr for fluctuation in cell numbers, labelling with radioactive thymidine and nuclear DNA distributions. Mathematical evaluation of the results revealed a cycling time of 18.6 hr and a duration of specific cell phases of 8.6 hr, 7.1 hr and 2.9 hr for G1, S and G2 + M, respectively. The doubling time was 26 hr and the growth fraction was estimated as 1.     

Coexpression of two thermosensitive defects in a Chinese hamster cell line : Manifestation of a G1 block associated with a mitosis defect

Asynchronous cultures of ts12, an anchorage-dependent derivative of the thermosensitive Chinese hamster cell line ts111, show a rapid drop in [3H]thymidine incorporation with accumulation of the cells in the G1 and in the G2 phases of the cycle, when shifted from 34.5 to 39.4 degrees C. Shift-up experiments carried out after either isoleucine deprivation or synchronization at 39.4 degrees C, locate the execution point of a ts function in late G1 (2.5-3 h before S). However, stimulation of proliferation of a high density-arrested population allows a fraction of the cells to enter S. In addition to the G1 ts defect, ts12 expresses a slight cytokinesis defect at 39.4 degrees C (8-15% binucleate cells). The results suggest that altered processes are taking place at a post-metaphasic stage during the first hours after the shift-up. When populations are synchronized by a thymidine block and released at 39.4 degrees C, multinucleate cells in addition to binucleate cells are observed. Part of these multinucleate cells result from abnormal karyokinesis without inhibition of cytokinesis. Evidence is presented suggesting that excess thymidine allows the re-expression of the multinucleation phenotype of ts111.     

Terminal differentiation of human promyelocytic leukaemia cells, HL-60, during the G1 period

Human promyelocytic leukaemic cells, HL-60, arrested in mitosis by nocodazole were released in the presence of 1alpha,25-dihydroxyvitamin D3 and thymidine or hydroxyurea. Cells moved from early G1 period to the G1/S boundary and differentiated. Furthermore, cells arrested at the G1/S boundary by double thymidine block were released, with 1alpha,25-dihydroxyvitamin D3 being added at the end of DNA synthesis. Under the latter conditions, differentiated cells developed, indicating that DNA synthesis is not required for cell differentiation.     

A biochemical investigation of the adenovirus-induced G1 to S phase progression: Thymidine kinase,ornithine decarboxylase,and inhibitors of polyamine biosynthesis

Biochemical events were investigated in the G1 to S phase progression induced in quiescent rodent cells by human adenovirus type 5 (Ad5) and by serum. Thymidine kinase activity increased after infection of cells with Ad5 or addition of 10% serum. These stimulations were additive. An early viral gene was respnsible for induction by Ad5, but the early mutants ts36, ts37, and ts125 induced thymidine kinase at the permissive and nonpermissive temperatures. Several differences were found between cells stimulated by serum compared with Ad5. Induction of thymidine kinase was delayed in Ad5- infected cells, insensitive to 0.01 μ/ml actinomycin D and relatively resistant to reduced Ca²⁺ compared with induction by serum. Ornithine decarboxylase was induced by serum, but not by Ad5. α-Methylornithine had little effect on the induction of thymidine kinase by Ad5, but reduced the induction of thymidine kinase by serum, suggesting that Ad5-induced entry into S phase is uncoupled from polyamine biosynthesis. Methylglyoxal bis(guanylhydrazone), however, prevented the induction of thymidine kinase by both serum and Ad5. Adenovirus infection appears to induce cellular DNA synthesis and thymidine kinase in G1-arrested cells by a mechanism different from serum, and by passes events in the normal G1 to S phase progression.     

Location of the isoleucine arrest point in CHO and 3T3 cells

An isoleucine arrest point in G1 was determined by two methods for CHO and 3T3 cells. In the first method the fraction of cells entering S after isoleucine deprivation was assessed by [³H]thymidine labelling and autoradiography. In the second method cells entering S after isoleucine deprivation were identified by double-label autoradiography using [³H] and [¹⁴C]thymidine. From the fraction of cells entering S, determined by the two methods, the arrest point in G1 (and entry into G0) is located within the last 40 min of G1.     

The origin of variability in cell cycle durations of NHIK 3025 cells

The origin of cell cycle variability was investigated in NHIK 3025 cells synchronized by mitotic selection from an exponentially growing population. The variability in G1 durations was measured by flow cytometric analysis of the fraction of cells in G1 as a function of time after mitotic selection. Immediately before the first cells entered S, medium containing 2.0 mM thymidine was added to the cells, and removed when all the cells had reached S. Since the cells had approximately the same DNA content upon removal of the thymidine, the variability in the durations of S+G2+M was measured by counting the fraction of undivided cells as a function of time after removing the thymidine. Such a thymidine treatment did not affect the naturally occurring variability in cell cycle durations generated after the start of S. The results indicate that the cell cycle variability of NHIK 3025 cells can be adequately described by a cell cycle model consisting of at least two compartments, which the cells leave according to first order kinetics. The model accounts for the initial shoulder of the curve representing the fraction of undivided cells as a function of time after mitotic selection. Furthermore, it accounts for the reduction in the rate of entry into the subsequent cell cycle compared to the rate of entry into S. Both rate constants were equally reduced after serum stepdown.     

Deoxyribonucleoside triphosphate metabolism and the mammalian cell cycle: Effects of thymidine on wild-type and dCMP deaminase-deficient mouse S49 T-lymphoma cells

The size of the dCTP pool has been implicated as a possible regulator of DNA synthesis. In this investigation we correlate large intracellular variations in deoxyribonucleoside triphosphate levels to the growth rates and cell-cycle kinetics of mouse S49 T-lymphoma cells. Wild-type and a mutant line AzidoC-100-5, lacking dCMP-deaminase activity resulting in a 10-fold expanded dCTP pool were studied and compared using flow cytometry, centrifugal elutriation and nucleoside triphosphate determinations. An increase in the dCTP pool was closely correlated to the passage of cells from G1 to S phase in both cell types. Addition of thymidine to wild-type and mutant cells resulted in an accumulation of cells in early S phase, concomitant with a decreased dCTP level. Mutant cells excreted large amounts of deoxycytidine into the medium which partially protected the cells from thymidine inhibition. The doubling times for the mutant and wild-type cells were very similar but the mutant had a somewhat prolonged S phase and shortened G1 phase compared with the wild-type cells. Large changes in the DNA precursor levels were produced by addition of thymidine to mutant cultures. This gave no change in the growth rate but a somewhat shortened S phase and prolonged G1. The biochemical background for these effects is discussed.     

Relationship between the presence of microfilaments and the cell cycle of HeLa cells in synchronus culture]

Some cytoplasmic organelles have showed characteristic variations which are related to the different cell cycle phases, in thymidine synchonized HeLa cells in culture. In these cells, the most modified organelles were intracytoplasmic membranes (endoplasmic reticulum) and microfilament arrangements. Microfilaments were numerous under the cell membrane, but also some of them were dispersed in dense bundles. These structures were seen around the nucleus, 12-14 h after removal of excess thymidine (G1). They migrated to the periphery of the cell during S and G2. During mitosis, they were directly under superficial membrane-associated microfilaments.     

Cyclic AMP-dependent and -independent protein phosphokinase activity in subcellular fractions of synchronously growing leydig I-10 cells.

The Leydig I-10 tumor cell line was synchronized by the double thymidine block method using 1.0 mM thymidine. Protein phosphokinase activity of subcellular fractions was determined at various times throughout the cell cycle. Microsomal cAMP-independent kinase activity increased in G2 and decreased during the S and G1 phases. Except for relatively small increases during the G1 and late S phases, microsomal cAMP-dependent kinase activity remained unchanged throughout most of the cycle. In the lysosomal-mitochondrial fraction, cAMP-dependent and cAMP-independent protein kinase activity increased during the S phase. Independent kinase activity peaked again during G1, while the dependent kinase became depressed. Phosphokinase activity increased in the nuclear fraction in late G2 and during mitosis, and was due to increases in both cAMP-independent and cAMP-dependent kinase activity. Cytosol cAMP-dependent kinase activity increased in G2 and during mitosis; cAMP-independent kinase activity showed some increased activity during late G2 and mitosis. These temporal variations in the subcellular kinase activities throughout the cell cycle may act to phosphorylate subcellular protein substrates in a cell cycle-specific fashion.     

Lack of effect of butyrate on S phase DNA synthesis despite presence of histone hyperacetylation

The effects of sodium butyrate on [³H]thymidine incorporation and cell growth characteristics in randomly growing and synchronized HeLa S₃ cells have been examined in an attempt to determine what effects, if any, butyrate has on S phase cells. Whereas 5 mM sodium butyrate rapidly inhibits [⁵H]thymidine incorporation in a randomly growing cell populations, it has no effect on incorporation during the S phase in cells synchronized by double thymidine block techniques. This lack of effect does not result from an impaired ability of the S phase cells to take up butyrate, since butyrate administration during this period leads to histone hyperacetylation that is identical with that seen with butyrate treatment of randomly growing cells. Furthermore, the ability to induce such hyperacetylation with butyrate during an apparently normal progression through S phase indicates that histone hyperacetylation probably has no effect on the overall process of DNA replication. Temporal patterns of [³H]thymidine incorporation and cell growth following release from a 24-h exposure to butyrate confirm blockage of cell growth in the G1 phase of the cell cycle. Thus, the inhibition by butyrate of [³H]thymidine incorporation in randomly growing HeLa S₃ cell populations can be accounted for solely on the basis of a G1 phase block, with no inhibitory effects on cells already engaged in DNA synthesis or cells beyond the G1 phase block at the time of butyrate administration.     

DNA and RNA Syntheses by Intraerythrocytic Stages of Plasmodium knowlesi*

Incorporation of the nucleic acid precursors, orotic acid, adenosine, thymidine, and uridine, was studied in various stages of intraerythrocytic Plasmodium knowlesi from infected rhesus monkeys. Incubation of the parasitized erythrocytes with the precursors was for 3 hr periods using a plasma-free culture medium. The samples containing primarily rings, early trophozoites, or late trophozoites incorporated orotic acid, adenosine, and uridine into RNA; however, these stages exhibited negligible or very low levels of incorporation of any of the precursors into DNA. The sample containing late trophozoite and schizont stages incorporated orotic acid, adenosine, and uridine into RNA, and orotic acid, adenosine, and very low levels of thymidine into DNA. These results indicate that DNA synthesis (the S phase of the cell cycle) occurs very close to the time of nuclear division, and that either the G1 or G2 phase is very short in P. knowlesi. It was also observed that adenosine and orotic acid, 2 precursors which are incorporated into both DNA and RNA, are utilized differently by the intraerythrocytic parasites. Incorporation of orotic acid into RNA and DNA and adenosine incorporation into DNA were continuous for the entire incubation period, whereas incorporation of adenosine into RNA was very low during the last 2 hr of each period. It was further demonstrated that the parasites utilized exogenous uridine for synthesis of RNA, and that the older parasite stages incorporated thymidine into DNA.     

EGF-dependent growth inhibition in MDA-468 human breast cancer cells is characterized by late G1 arrest and altered gene expression.

The MDA-468 human breast cancer cell line displays the unusual phenomenon of growth inhibition in response to pharmacological concentrations of EGF. This study was initiated with the objective of elucidating the cellular mechanisms involved in EGF-induced growth inhibition. Following EGF treatment the percentage of MDA-468 cells in G1 phase increased, together with a concomitant depletion in S and G2/M phase populations, as revealed by flow cytometry of DNA content. The apparent G1 block in the cell cycle was confirmed by treating the cells with vinblastine. DNA synthesis was reduced to about 35% of that measured in control, untreated cells after 48 h of EGF treatment, as measured by the incorporation of [3H]thymidine. DNA synthesis returned to normal following the removal of EGF from the growth-arrested cells. In order to locate the EGF-induced event responsible for the G1 arrest more precisely, we examined the expression of certain cell cycle-dependent genes by Northern blot analysis. EGF treatment did not alter either the induction of the early G1 marker, c-myc, or the expression of the late G1 markers, proliferating cell nuclear antigen, and thymidine kinase. However, EGF-treated cells revealed down regulation of p53 and histone 3.2 expression, which are expressed at the G1/S boundary and in S phase, respectively. These results indicate that EGF-induced growth inhibition in MDA-468 human breast cancer cells is characterized by a reversible cell cycle block at the G1/S boundary.     

Assembly of bacteriophage phi X174: identification of a virion capsid precursor and proposal of a model for the functions of bacteriophage gene products during morphogenesis.

A capsomeric structure sedimenting with an S value of 108 in sucrose gradients was isolated from Escherichia coli infected with bacteriophage phi X174. The 108S material contained viral proteins F, G, H, and D, and the relative amounts of these proteins in the 108S material were similar to those in the infectious 132S particle, which has previously been described as a possible intermediate in the assembly of 114S phage particles. Electron micrographs indicated that the size and shape of the 108S material resemble those of the 132S particle. The 108S material contained no DNA, and its formation occurred independently of DNA synthesis. The 108S material accumulated in infected cells when viral DNA replication was prevented either by mutation in phage genes A or C or by removal of thymidine from a culture infected with wild-type phage or with a lysis gene E mutant. Upon restoration of thymidine to cells infected with the lysis gene E mutant and then starved of thymidine, the accumulated 108S material was converted to 132S particles and to 114S phage particles, implying that the 108S material is a precursor of phage particles. A model that proposes possible functions for the products of phi X174 genes A, B, C, D, F, and G during viral replication and phage maturation is described.