QUANTITY PRODUCTION OF SYNCHRONIZED MAMMALIAN CELLS IN SUSPENSION CULTURE

Release from cell cycle arrest with Cdk4/6 inhibitors generates highly synchronized cell cycle progression in human cell culture

Each approach used to synchronize cell cycle progression of human cell lines presents a unique set of challenges. Induction synchrony with agents that transiently block progression through key cell cycle stages are popular, but change stoichiometries of cell cycle regulators, invoke compensatory changes in growth rate and, for DNA replication inhibitors, damage DNA. The production, replacement or manipulation of a target molecule must be exceptionally rapid if the interpretation of phenotypes in the cycle under study is to remain independent of impacts upon progression through the preceding cycle. We show how these challenges are avoided by exploiting the ability of the Cdk4/6 inhibitors, palbociclib, ribociclib and abemaciclib to arrest cell cycle progression at the natural control point for cell cycle commitment: the restriction point. After previous work found no change in the coupling of growth and division during recovery from CDK4/6 inhibition, we find high degrees of synchrony in cell cycle progression. Although we validate CDK4/6 induction synchronization with hTERT-RPE-1, A549, THP1 and H1299, it is effective in other lines and avoids the DNA damage that accompanies synchronization by thymidine block/release. Competence to return to cycle after 72 h arrest enables out of cycle target induction/manipulation, without impacting upon preceding cycles.

Quantitative Studies for Cell-Division Cycle Control

The cell-division cycle (CDC) is driven by cyclin-dependent kinases (CDKs). Mathematical models based on molecular networks, as revealed by molecular and genetic studies, have reproduced the oscillatory behavior of CDK activity. Thus, one basic system for representing the CDC is a biochemical oscillator (CDK oscillator). However, genetically clonal cells divide with marked variability in their total duration of a single CDC round, exhibiting non-Gaussian statistical distributions. Therefore, the CDK oscillator model does not account for the statistical nature of cell-cycle control. Herein, we review quantitative studies of the statistical properties of the CDC. Over the past 70 years, studies have shown that the CDC is driven by a cluster of molecular oscillators. The CDK oscillator is coupled to transcriptional and mitochondrial metabolic oscillators, which cause deterministic chaotic dynamics for the CDC. Recent studies in animal embryos have raised the possibility that the dynamics of molecular oscillators underlying CDC control are affected by allometric volume scaling among the cellular compartments. Considering these studies, we discuss the idea that a cluster of molecular oscillators embedded in different cellular compartments coordinates cellular physiology and geometry for successful cell divisions.

Growth during the cell cycle

During the cell cycle, major bulk parameters such as volume, dry mass, total protein, and total RNA double and such growth is a fundamental property of the cell cycle. The patterns of growth in volume and total protein or RNA provide an "envelope" that contains and may restrict the gear wheels. The main parameters of cell cycle growth were established in the earlier work when people moved from this field to the reductionist approaches of molecular biology, but very little is known on the patterns of metabolism. Most of the bulk properties of cells show a continuous increase during the cell cycle, although the exact pattern of this increase may vary. Since the earliest days, there have been two popular models, based on an exponential increase and linear increase. In the first, there is no sharp change in the rate of increase through the cycle but a smooth increase by a factor of two. In the second, the rate of increase stays constant through much of the cycle but it doubles sharply at a rate change point (RCP). It is thought that the exponential increase is caused by the steady growth of ribosome numbers and the linear pattern is caused by a doubling of the structural genes during the S period giving an RCP--a "gene dosage" effect. In budding yeast, there are experiments fitting both models but on balance slightly favoring "gene dosage." In fission yeast, there is no good evidence of exponential increase. All the bulk properties, except O2 consumption, appear to follow linear patterns with an RCP during the short S period. In addition, there is in wild-type cells a minor RCP in G2 where the rate increases by 70%. In mammalian cells, there is good but not extensive evidence of exponential increase. In Escherichia coli, exponential increase appears to be the pattern. There are two important points: First, some proteins do not show peaks of periodic synthesis. If they show patterns of exponential increase both they and the total protein pattern will not be cell cycle regulated. However, if the total protein pattern is not exponential, then a majority of the individual proteins will be so regulated. If this majority pattern is linear, then it can be detected from rate measurements on total protein. However, it would be much harder at the level of individual proteins where the methods are at present not sensitive enough to detect a rate change by a factor of two. At a simple level, it is only the exponential increase that is not cell cycle regulated in a synchronous culture. The existence of a "size control" is well known and the control has been studied for a long time, but it has been remarkably resistant to molecular analysis. The attainment of a critical size triggers the periodic events of the cycle such as the S period and mitosis. This control acts as a homeostatic effector that maintains a constant "average" cell size at division through successive cycles in a growing culture. It is a vital link coordinating cell growth with periodic events of the cycle. A size control is present in all the systems and appears to operate near the start of S or of mitosis when the cell has reached a critical size, but the molecular mechanism by which size is measured remains both obscure and a challenge. A simple version might be for the cell to detect a critical concentration of a gene product.

Separation of histone variants and post-translationally modified isoforms by triton/acetic acid/urea polyacrylamide gel electrophoresis

Due to their similarities in size and charge, complete resolution of histones by electrophoresis poses a considerable challenge. The addition of nonionic detergents to the traditional acetic acid/urea (AU) polyacrylamide gel electrophoresis (PAGE) system has afforded an excellent method to separate not only the different modified forms of histones, but also the primary sequence variant subtypes of selected histone species; it is widely used to separate histones with varying levels of acetylation. This unit describes the use of gels containing the nonionic detergent Triton X-100, referred to as Triton/acetic acid/urea (TAU) polyacrylamide gels, for analysis of histones. Also included are support protocols detailing several accessory techniques: assembly of gel plates for the TAU gel, preparation of histones from isolated nuclei in a solubilized form amenable to electrophoresis, and electrophoretic transfer of proteins from these gels to PVDF membranes.

Increased globotriaosylceramide on plasma membranes of synchronized familial dysautonomia cells. Verotoxin binding studies

Familial dysautonomia is an autosomal recessive genetic disease found almost exclusively among Ashkenazi Jews, characterized by deficits in autonomic, sensory, and central functions. Although the gene has been localized to chromosome 9, the biochemical defect remains elusive. We previously reported an increase in globotriaosylceramide in dysautonomic fibroblasts and lymphoblasts, and unusual fibroblast growth patterns suggesting plasma membrane abnormalities. Globotriaosylceramide is a plasma membrane component, and the natural receptor for verotoxin derived from E. coli. In Vero and HeLa cells, which are susceptible to verotoxin, the expression of globotriaosylceramide on the cell surface is maximal at the G1/S boundary of the cell cycle. Measurement of toxin binding at 0 degrees C at this boundary is indicative of the amount of globotriaosylceramide exposed on the cell surface. Above 0 degrees C, verotoxin enters, and is toxic to, the cell. We analyzed verotoxin-globotriaosylceramide interactions in synchronized FD and normal cells at this boundary. 125I-toxin binding was much more marked to lymphoblasts from patients than from controls. When cells were grown in the presence of verotoxin, at 10(-2)-10(-7) micrograms/mL, 70% of dysautonomic lymphoblasts died, compared to 25% of controls. The CD50 was 10 ng/mL for dysautonomic fibroblasts vs 450 for controls. These results may be exploited to create a biological assay to differentiate between FD and normal cells.

Aminoguanidine inhibits cell proliferation by prolongation of the mitotic phase

Aminoguanidine has been reported to increase mitotic cells in the liver and to retard the development in chick embryos. In this study, in vitro effects of aminoguanidine on mammalian cells, BRL-3A (from normal rat liver) and HeLa-S3 (from human cervical cancer), were examined. When cells were incubated with varying concentrations of aminoguanidine for 48 h, the cell growth was inhibited at high concentrations of aminoguanidine, without cell death; the 50% growth-inhibitory concentrations (IC50) against BRL-3A and HeLa-S3 cells were 4.6 and 2.2 mg/ml, respectively. After culture with aminoguanidine for 24 h, the proportion in the G2/M-phase in both cell lines was increased, but the proportion in the G1-phase was decreased, and the rate in the S-phase was slightly increased at low concentrations of this agent and decreased at high concentrations. The effects of aminoguanidine on the increase in the G2/M-phase in HeLa cells were dose-dependent. In synchronized HeLa cells, aminoguanidine clearly prolonged the M-phase. From these results, it is confirmed that aminoguanidine inhibits cell proliferation by prolonging the M-phase in the cell cycle in mammalian cells as well as chick embryo liver cells.

Killing lung cancer cells at cell-cycle phase by a new indium-111-bleomycin complex

The efficacy of killing small cell lung cancer (SCLC) cells at the G1, S, and G2-M phase of the cell-cycle by a new 111In-bleomycin complex (111In-BLMC) was investigated. SCLC cells (N417, H526, H209) were synchronized by double thymidine block and assessed by DNA content with flow cytometry, and the period for the maximal accumulation of cells in S, G1, or G2-M phase was determined. Cells in different cell cycle phases were exposed to 0.9% NaCl, BLM, or 111In-BLMC for 1 hour and observed for colony formation. The survival of H526 cells treated with 111In-BLMC was 71% (for enriched S phase), 46% (G1), and 31% (G2-M). For N417 cells, it was 25% (S), 20% (G1), and 8% (G2-M) for 111In-BLMC and 18% (S), 33% (G1), and 10% (G2-M) for BLM. These results indicated that SCLC cells in G2-M were most sensitive and those in S phase were least sensitive to 111In-BLMC; cells in G1 phase were the least sensitive to BLM.

Accumulation of adrenocorticotropin secretory granules in the midbody of telophase AtT20 cells: evidence that secretory granules move anterogradely along microtubules

During the cell cycle the distribution of the ACTH-containing secretory granules in AtT20 cells, as revealed by immunofluorescence labeling and electron microscopy of thin sections, undergoes a cycle of changes. In interphase cells the granules are concentrated in the Golgi region, where they form, and also at the tips of projections from the cells, where they accumulate. These projections contain many microtubules extending to their tips. During metaphase and anaphase the granules are randomly distributed in the cytoplasm of the rounded-up mitotic cells. On entry into telophase there is a rapid and striking redistribution of the granules, which accumulate in large numbers in the midbody as it develops during cytokinesis. This accumulation of secretory granules in the midbody is dependent upon the presence of microtubules. The changing pattern of distribution of the secretory granules during the cell cycle fulfills the predictions of a model envisaging first that secretory granules associate with and move along interphase microtubules in a net anterograde direction away from the centrioles, and secondly that they do not associate with microtubules of the mitotic spindle during metaphase and anaphase.

Recycling of transferrin receptors in A431 cells is inhibited during mitosis

There is a marked reduction in the number of surface transferrin receptors as A431 cells enter mitosis which persists until telophase when receptors reappear to a level that exceeds the original interphase value. This is most simply explained by assuming that recycling of receptors back to the cell surface is inhibited as cells enter mitosis but that internalisation continues for a short while, causing surface receptor depletion. In telophase recycling would resume before internalisation giving a temporary excess of surface transferrin receptors.

The interleukin-2 T-cell system: a new cell growth model

Synchronized interleukin-2 receptor-positive T cells, homogeneous immunoaffinity-purified interleukin-2, and a monoclonal antibody to interleukin-2 receptors were used to show that only three factors are critical for T-cell cycle progression: interleukin-2 concentration, interleukin-2 receptor density, and the duration of the interleukin-2 receptor interaction. Since the proliferative characteristics of T cells are identical to those of both prokaryotic and all other eukaryotic cells, these findings provide a new model that accounts fully for the variables that determine cell cycle progression.

Active genes are sensitive to deoxyribonuclease I during metaphase

The active exogenous murine leukemia virus sequences of mouse cells growing in culture are preferentially digested by deoxyribonuclease I in metaphase chromosomes. As determined by nuclear nick translation, all of the gene sequences of these cells active during interphase are in a deoxyribonuclease I-sensitive conformation during metaphase. This method of nick translation can therefore be used to label chromosomes in situ in order to visualize the active regions of the genome.

Demonstration of recovery from the potentially mutagenic effects of ultraviolet light by replication-inhibited Chinese hamster V79 cells

The effects of replication-inhibiting conditions on the ability of Chinese hamster (V79) cells to recover from the potentially mutagenic effects of ultraviolet (UV) light were investigated. V79 cells were synchronized by a new technique using a low concentration of hydroxyurea (0.2 mM), which provided a mildly-toxic, nonmutagenic method for producing large quantities of synchronized cells needed for these studies. The protocol developed for this study involved UV-irradiation of synchronized V79 cells which were blocked at the Gl/S boundary. Following UV-irradiation, the cells were either allowed to enter S phase immediately or were blocked for increasing periods of time by the addition of more hydroxyurea. The former cells contained the highest frequencies of ouabain-resistant mutants, while cells whose replication was blocked following UV-irradiation showed decreasing mutation frequencies with respect to time. Therefore, V79 cells are able to demonstrate liquid holding recovery from potentially mutagenic UV-lesions. Since the UV-induced mutation frequency was reduced by almost 50% following 6 h of 'liquid holding', the mutagenic lesions seem to be removed at a faster rate than has previously been reported for the removal of pyrimidine dimers from these cells.

On the distribution of cell cycle generation times

The problem of whether the cell cycle is a deterministic or probabilistic process is widely discussed in the current literature (P. Nurse, Nature, 286, pp. 9-10, 1980). In this report the question of fluctuations of cell cycle period is treated in the limits of the membrane model of cell division regulation. The parametric analysis of the equations set both for normal and tumour cells is carried out. We describe the bifurcation parameters in the neighbourhood of which the system can amplify the small fluctuations. The presence of white noise in parameters describing the lipids and antioxidants influxes into membrane is examined by methods of Marcovian processes and also by direct stochastic computer simulation. The equation for the distribution function of generation times is obtained and the increase of dispersion and mean cycle time during the changes of those parameters which would be connected with cell culture density is calculated. The influence of parameter fluctuations upon the cycle period for both normal and tumour cells is compared in the framework of model assumptions. The ratio of dispersion of generation time distribution to mean period value for an ensemble of tumour cells is shown to be several times greater than that for normal ones. In the discussion the problem of the presence of a premitotical (G02) resting state and of the possibility of its experimental detection is considered.

Cell cycle variability: the oscillator model of the cell cycle yields transition probability alpha and beta type curves

The limit cycle concept of cell replication attributes cell cycle variability to continuous random modulation of the rates of reactions forming the intracellular control system believed to be responsible for replication processes. It is shown that this model can yield frequency histograms and both alpha and beta type accumulative distribution curves (with respect to generation times and also to cycle phases) which are of the various forms seen experimentally. The results thus provide additional support for this concept.

Clocked cell cycle clocks

The cell division cycle of both mammalian cells and microorganisms, which apparently has both deterministic and probabilistic features, is a clock of sorts in that the sequence of events that comprise it measures time under a given set of environmental conditions. The cell division cycle may itself be regulated by a programmable clock that, under certain conditions, can generate circadian periodicities by interaction with a circadian pacemaker. These clocks must insert time segments into the cell division cycle in order to generate the observed variability in cellular generation times.

Variation in G1 transit time relative to the cycloheximide and actinomycin D drug restriction points

The transit time distribution at various points in the cell cycle of synchronized Chinese hamster ovary cells was determined from the mitotic index, [3H]thymidine labeling index and increase in cell number monitored at regular intervals after mitotic selection. Variation in G1 transit time compared with that for the total cell cycle indicates that variation in cell cycle transit time occurs mainly during G1 phase. The cycloheximide (5.0 microgram/ml) and actinomycin D (3.0 microgram/ml) restriction points occur 0.2 and 1.7 hr prior to entry into S phase, respectively. The transit time distributions are further characterized by the moments of the distributions. The variance (2nd moment about the mean) of the transit time distribution at the actinomycin D restriction point is similar to the variance of the transit time distribution at the G1/S border, thus variation in cell cycle transit time originates earlier than 1.7 hr prior to entry into S phase (i.e., the first 3/4 of G1). If G1 transit time variability and cell cycle control are related, then the results presented here indicate that the major regulatory events do not occur during late G1 phase.

Synchronization of 9L rat brain tumor cells by centrifugal elutriation

Asynchronous 9L cells were separated into relatively homogeneously-sized populations using centrifugal elutriation with both a conventional collection method and a long collection method. A substantial increase in the homogeneity of the volume distributions and in the degree of synchrony of the separated fractions was obtained using the long collection method. Autoradiographic data indicated that fractions containing greater than or equal to 97% G1 cells, greater than or equal to 80% S cells, and 70-75% G2 cells could be routinely recovered with this procedure. Recovery in these fractions varied from 5 to 8% of the total number of cells elutriated. The colony forming efficiency (CFE) of cells from fractions representing each phase of the cell cycle was a constant 60-70%, which was comparable to the 60-80% usually found for asynchronous 9L cells. The percentage of cells in the G1, S, and G2 phases in the elutriated fractions was more accurately determined from the volume distribution than from computer fits of the DNA histogram obtained from flow cytometry. In genereal, the degree of synchrony was related to the coefficient of variation (CV) of the volume distributions of the elutriated fractions. The CV was about 14% for all elutriated fractions. When the greater than or equal to 97% G1 population was allowed to progress to S and G2, the CVs were about 17 and 20.2%, respectively. Thus, the best nonperturbing method for obtaining synchronous 9L cells in the S or G2 phases was direct elutriation with the long collection method.

Effect of cell synchronization techniques on polyamine content of HeLa cells

Synchronized cultures of mitotic HeLa cells were obtained by different protocols and the polyamine content of these cells determined. It was found that the method of synchronization can significantly change the polyamine content of the mitotic cells, and can also alter the time course of polyamine accumulation during the subsequent cell cycle.

Influence of poliovirus infection on S-phase and mitosis of the host cell

Hep-2 cells were synchronized by a double thymidine block and infected with poliovirus type I (Mahoney) in hourly intervals after release from the second thymidine block. The S-phase is not prevented by a poliovirus infection but, with cells infected 0-4 hours after release, an increase of its duration is found. With an infection 5 hours and later after release, the duration of the S-phase is not different from that of an uninfected, synchronized control culture. DNA synthesis itself is slower early in S-phase and gets inhibited up to 75 per cent late in S-phase. All cultures show the first signs of a CPE 3.5 hours p.i. and, in spite of CPE, the cells continue to synthesize DNA. In infected cells a slightly higher peak of mitotic cells compared to control cultures is found. The time point of the mitotic peak is dependant of the time of infection and seems no longer controlled by the cell cycle. The mitotic indices are similar for all cultures infected at different times after release. When the cells are infected early after release CPE appears before mitosis and prevents the cells from entering mitosis. Cells which are infected towards the end of the S-phase finish mitosis normally before they exhibit characteristics of CPE. Extent and kinetics of poliovirus RNA synthesis and yield of virus progeny are not altered by the cell cycle.

Inhibition of lymphocyte proliferation by polyamines requires ruminant-plasma polyamine oxidase

Spermine and spermidine in vitro are potent inhibitors of proliferation of phytohaemagglutinin-stimulated rat thymic lymphocytes, lymphoma cells and human lymphoblastic leukaemia cells, but only in media supplemented by foetal calf serum. This inhibition is shown to be due to a bovine plasma polyamine oxidase, with a high specificity for these polyamines. Spontaneously dividing lymphocytes are not subject to this inhibition. This, plus direct evidence from synchronous cultures of EB2 cells demonstrates that the inhibition is expressed in the late G1 or G1/S interface of the cell cycle. Putrescine was not an inhibitor in the presence of foetal calf serum but became so in the presence of human pregnancy serum, possibly due to the action of diamine oxidase.

Role of nuclear size in cell growth initiation

Swiss 3T3 cells arrested in B0 (quiescent state) by reducing serum content of the medium all contain the same amount of DNA but vary in nuclear volume over approximately a twofold range. By use of flow microfluorimetry, scatterplots of nuclear volume versus DNA content were obtained in intervals after serum stimulation. The earliest cells to enter DNA synthesis were those with the largest nuclei, whereas cells with the smallest nuclei were among the latest. Regulation of cellular transit from G0 to the S phase was therefore, at least in part, deterministic, since all G0 cells did not have equal probabilities of entry into S at a given moment. All cells having the same nuclear volume did not initiate DNA synthesis at the same moment; therefore, factors other than nuclear volume must also influence this timing. Nuclear volume correlated with the maximum rate at which cells could enter S. The kinetic model of the cell cycle postulating a probabilistic event as solely responsible for entry into S thus appears too simple.

Osmotic properties of Ehrlich ascites tumor cells during the cell cycle

Ehrlich ascites tumor cells were grown and maintained in continuous spinner culture. The population of dividing cells was synchronized by a double thymidine block technique. Cell cycle phases were determined graphically by plotting mitotic index, cell number, and DNA synthesis against time. Changes in the osmotic properties of Ehrlich ascites tumor cells during the cell cycle are described. Permeability to water is highest at the initiation of S and progressively decreases to its lowest value just after mitosis. Heats of activation for water permeability vary during the cell cycle, ranging from 9-14 kcal/mole. Results may imply changes in the state of water in the membrane during the cycle. The volume of osmotically active cell water is highest during S and early G2 and decreases during the mitotic phase, as cells undergo division. Total water content remains stable at 82% (w/w) during the cycle. Total concentration of the three major ions (Na, K, Cl), expressed as mEq/liter total cell volume, does not change. The fraction of total cell water which is osmotically active (Ponder's R) decreased gradually from 0.75 at S to about 0.56 following mitosis. Findings suggest that a fraction of the total water within the cell exists in a "bound" form and is, therefore, incapable of being shifted under the driving force of osmotic pressure. This fraction of bound water increases during the cell cycle. Possible alterations in membrane fluidity and the state of water in the cell are discussed.

Characterization of HeLa 5'-nucleotidase: a stable plasma membrane marker

5'-Nucleotidase, assayed as 5'-AMPase, has been extensively characterized and established as a stable, quantitative plasma membrane marker in HeLa S3 cells. The membrane 5'-AMPase has a Km of 7.0 microM. Relative affinities of the other 5'-mononucleotides for the enzyme are 5'-GMP > 5'-TMP > 5'-UMP > 5'-CMP. There are activity optima at pH7 and 10; the latter is Mg(2+)-dependent. The membrane preparations have a small amount of acid phosphatase activity that is distinct from 5'-AMPase activity but no alkaline phosphatase. AOPCP, ADP, and ATP are strongly inhibitory. Mg2+, Ca2+, or Co2+ additions do not affect the pH 7.0 activity; Mn2+ activates slightly, whereas Zn2+, Cu2+, and Ni2+ are inhibitory. EDTA slowly inactivates, but removal of the EDTA without the addition of divalent cations restores activity. The inactivation is also substantially reversed by Co2+ or Mn2+, but reactivability by divalent cations decreases with time in EDTA. ConA strongly inhibits, and alpha-methyl-D-mannoside or glucose (the latter much less efficiently) relieves the inhibition, indicating that the 5'-AMPase is a glycoprotein. Histidine is also inhibitory. Ouabain, phloretin, cytochalasin B, cysteine, phenyl-alanine, MalNEt, and IAA are without effect. 5'-AMPase activity codistributes with pulse-bound [3H]ouabain when either of two cell fractionation procedures are used. The 5'-AMPase activity per cell is constant at different cell densities in exponentially growing cells, and activity per unit cell volume remains constant throughout the cell cycle. These properties, together with its absence in other organelles, its stability to storage, its insensitivity to certain experimental manipulations, and its general insensitivity to inhibitors of specific transport systems, make 5'-AMPase a useful quantitative marker in studies on the regulation of HeLa membrane transport systems. Key Words: HeLa, 5'-nucleotidase, plasma membrane marker, non-specific phosphatases, divalent ions, ConA, AOPCP, cell cycle, mitochondria, transport inhibitors.

Selective lethal effect of thymidine on human and mouse tumor cells

Human cell lines derived from a melanoma and a colon carcinoma, and cultures of human melanocytes and intestinal epithelial cells, as well as a mouse mesenchymal non-neoplastic cell line and a malignant subline of the same have been quantitatively studied in tissue culture for their sensitivity to thymidine. All three tumor lines produced solid tumors when injected into nude thymus-deficient mice. No tumors were obtained by injecting cells of the human normal long-term cultures or the non-neoplastic mouse line. The tumor-producing lines showed a greater sensitivity to the lethal effects of high concentrations of thymidine than their non-tumor-producing counterparts. Less than 23% of the tumor cells survived 72 hours in the presence of 1 mg/ml of thymidine, in contrast to 60% or more of the non-tumor cells. Colony formation was much more inhibited by thymidine and the differential between normal and tumor cells was even more pronounced. Tumor cells which also were treated for 72 hours with 1 mg/ml of thymidine and then plated in fresh medium formed very few colonies. If the plating efficiency of the untreated controls is considered as 100%, 4.3% or less of the treated tumor cells formed colonies, in contrast to 33% or more of the non-tumor cells.

Effect of excess thymidine on the growth of human melanoma cells transplanted in thymus deficient nude mice

The effect of thymidine (TdR) on the growth of a human melanoma transplanted in nude mice has been studied. It was found that the injection of 1 g/kg/h of TdR for at least 72 h is sufficient to suppress the growth of the melanoma cells. This inhibition lasts for the duration of the treatment, and causes no apparent toxicity to the host. Nude mice treated for 6--9 days with TdR survived 158 days after melanoma transplant versus 126 days for the controls.

A model of cell cycle control: effects of thymidine on synchronous cell cultures

Further evidence is presented in support of a model for growth control in which commitment for cell division is determined by an event in the preceding cell cycle. A study was made of conditions affecting synchronous growth following treatment of murine mastocytoma cells with excess thymidine at different phases of the cell cycle. Cells were synchronized by a physical procedure involving velocity sedimentation in a zonal rotor. Pulse treatment of such cultures with thymidine at times corresponding to the S, G2, and M periods had no effect on further growth. However, addition at G1, although having no immediate effect, arrested cell growth in the next cell cycle. This temporal effect may account for the decay of synchrony observed during double thymidine blockade or thymidine-FUdR blockade. When the time interval between two such blocks was 7 hr or less, P815Y cells were arrested after one synchronous division. At this critical time a majority of cells were at, or near, G1. It is suggested that thymidine exerts a hitherto unrecognized effect at the G1 interval.

Differential accumulation of virus-specific RNA during the cell cycle of adenovirus-transformed rat embyro cells

In adenovirus type 2-transformed rat embryo cells there is a threefold greater incorporation of [3-H]uridine into virus-specific RNA early in S phase than in late S or G2. This heightened accumulation of labeled RNA is true for both nuclear and cytoplasmic virus-specific labeling. Inhibition of DNA synthesis decreases the virus-specific RNA labeling, whereas reversal of inhibition again allows the elevated level of virus-specific RNA labeling.

Cell-cycle dependent appearance of murine leukemia-sarcoma virus antigens

Normal rat kidney (NRK) cells, NRK cells infected with Rauscher murine leukemia virus, and NRK cells infected with Kirsten murine sarcoma-leukemia virus (NRK-K) were synchronized by a double thymidine block. At intervals after release from thymidine blockage, the cells were examined for the presence of viral antigens in the cytoplasm and on the cell surface by immunofluorescent microscopy by using goat anti-Rauscher murine leukemia virus and goat anti-Moloney leukemia virus (Tween-ether disrupted) sera. Detection of viral antigens in the cytoplasm was periodic during the cell cycle. Antigens were detected first during the S phase, increased during the G2 phase, and disappeared during the M and G1 phases. A similar pattern of surface immunofluorescence was observed. Infectious virus was detected in culture fluids from synchronized cells during the M phase. Surface immunofluorescence was detected in NRK-K cells with anti-Rauscher murine leukemia virus and may represent the presence of group-specific antigens on the cell surface. Control, uninfected NRK cells, which did not normally fluoresce, showed weak immunofluorescence during the S and G2 phases after synchronization. Synchronization can be used to amplify latent oncornavirus expression.

Do cells cycle?

We propose that a cell's life is divided into two fundamentally different parts. Some time after mitosis all cells enter a state (A) in which their activity is not directed towards replication. A cell may remain in the A-state for any lenght of time, throughout which its probability of leaving A-state remains constant. On leaving A-state, cells enter B-phase in which their activities are deterministic, and directed towards replication. Initiation of cell replication processes is thus random, in the sense that radioactive decay is random. Cell population growth rates are determined by the probability with which cells leave the A-state, the duration of the B-phase, and the rate of cell death. Knowledge of these parameters permits precise calculation of the distribution of intermitotic times within populations, the behavior of synchronized cell cultures, and the shape of labeled mitosis curves.