Inhibition of DNA synthesis by actinomycin D and cycloheximide in synchronized HeLa cells

Cell Cycle Synchronization of HeLa Cells to Assay EGFR Pathway Activation

Progression through the cell cycle causes changes in the cell's signaling pathways that can alter EGFR signal transduction. Here, we describe drug-derived protocols to synchronize HeLa cells in various phases of the cell cycle, including G1 phase, S phase, G2 phase, and mitosis, specifically in the mitotic stages of prometaphase, metaphase, and anaphase/telophase. The synchronization procedures are designed to allow synchronized cells to be treated for EGF and collected for the purpose of Western blotting for EGFR signal transduction components.S phase synchronization is performed by thymidine block, G2 phase with roscovitine, prometaphase with nocodazole, metaphase with MG132, and anaphase/telophase with blebbistatin. G1 phase synchronization is performed by culturing synchronized mitotic cells obtained by mitotic shake-off. We also provide methods to validate the synchronization methods. For validation by Western blotting, we provide the temporal expression of various cell cycle markers that are used to check the quality of the synchronization. For validation of mitotic synchronization by microscopy, we provide a guide that describes the physical properties of each mitotic stage, using their cellular morphology and DNA appearance. For validation by flow cytometry, we describe the use of imaging flow cytometry to distinguish between the phases of the cell cycle, including between each stage of mitosis.

The logic of the Membrane, Magnesium, Mitosis (MMM) model for the regulation of animal cell proliferation

The addition of animal serum or specific protein growth factors to quiescent, serum-deprived vertebrate cells in culture activates a wide variety of biochemical responses within minutes, which are followed in 5-10h by the initiation of DNA synthesis and then by mitosis. The quintessential early and continuing activation step for the increase in DNA synthesis is the increased initiation rate of protein synthesis, which must be continuously maintained throughout the G1 phase for advancement into S. The aggregate of biochemical reactions to growth factors is called the coordinate response, to indicate that many related and unrelated processes are orchestrated to repetitively reproduce cells. It is, however, crucial to recognize that the coordinate response can be induced for one or more rounds of replication by a variety of non-specific and quasi-specific membrane effectors. The logic of considering this framework of events in growth control implied that a single multi-target second messenger plays a central role in coordinating the events of the overall response. The same reasoning suggested that free Mg(2+) is the unifying regulatory element in that response which includes protein kinase pathways, and that the cytoplasmic activity of Mg(2+) increases with the binding of growth factors to their receptors in the cell membrane, or of less specific perturbations of the membrane. Experimental support of this conclusion is presented here and is represented in the MMM model of cell proliferation control.

Central roles of Mg2+ and MgATP2- in the regulation of protein synthesis and cell proliferation: significance for neoplastic transformation

Growth factors are polypeptides that combine with specific membrane receptors on animal cells to stimulate proliferation, but they also stimulate glucose transport, uridine phosphorylation, intermediary metabolism, protein synthesis, and other processes of the coordinate response. There are a variety of nonspecific surface action treatments which stimulate the same set of reactions as the growth factors do, of which protein synthesis is most directly related to the onset of DNA synthesis. Mg(2+) is required for a very wide range of cellular reactions, including all phosphoryl transfers, and its deprivation inhibits all components of the coordinate response that have so far been tested. Growth factors raise the level of free Mg(2+) closer to the optimum for the initiation of protein synthesis. The resulting increase in protein synthesis accelerates progression through G1 to the onset of DNA synthesis and mitosis. None of the other 3 major cellular cations are similarly involved in growth regulation, although internal pH may play an auxiliary role. Almost 10(5) externally bound divalent cations are displaced from membranes for every attached insulin molecule, implying a conformational membrane change that releases enough Mg(2+) from the internal surface of the plasma membrane to account for the increase in free cytosolic Mg(2+). It is proposed that mTOR, the central control point for protein synthesis of the PI 3-K kinase cascade stimulated by insulin, is regulated by MgATP(2-) which varies directly with cytosolic Mg(2+). Other elements of the coordinate response to growth factors such as the increased transport of glucose and phosphorylation of uridine are also dependent upon an increase of Mg(2+). Deprivation of Mg(2+) in neoplastically transformed cultures normalizes their appearance and growth behavior and raises their abnormally low Ca(2+) concentration. Tight packing of the transformed cells at very high saturation density confers the same normalizing effects, which are retained for a few days after subculture at low density. The results suggest that the activity of Mg(2+) within the cell is a central regulator of normal cell growth, and the loss of its membrane-mediated control can account for the neoplastic phenotype.

Magnesium: The missing element in molecular views of cell proliferation control

The quantitative study of regulation of cell growth and proliferation began with the development of the technique for monolayer culture of vertebrate cells in the late 1960s. The basic parameters were defined in the early physiological studies, which continued through the next decade. These included specific and non-specific growth factors and the requirement for continuous exposure to such factors through most of the G1 period for progression to S. In the course of this work, the diversity of biochemical responses and the critical role of increased protein synthesis and accumulation for the onset of DNA synthesis were elucidated. In particular, a central role of free cytosolic Mg2+ in direct regulation of protein synthesis and in ancillary processes as a response to membrane perturbation was established. Eventually, the physiological era was superseded by the molecular era beginning in the 1980s. This work focussed on specific receptors for growth factors that entrained a protein kinase cascade, which terminated in a higher frequency of initiation of protein synthesis. However, the molecular studies virtually ignored the key results of the physiological era. Recent studies of the penultimate molecular steps in the regulatory pathway of protein synthesis, however, have supported a model of growth regulation involving membrane perturbation and MgATP2- concentration, results that integrate the findings of the physiological and molecular eras. The resulting relatively simple "membrane, magnesium mitosis" (MMM) model of proliferation control can explain the seeming paradox of the variety of specific and non-specific growth-enhancing treatments that are mediated by the plasma membrane and which bring about a shared, complex but coordinated growth response that drives cell proliferation.

Mg2+ as activator of uridine phosphorylation in coordination with other cellular responses to growth factors

The divalent cation ionophore A23187 facilitates the manipulation of intracellular Mg2+ without increasing the general permeability of the cell. The uptake of uridine into cells is limited by its rate of intracellular phosphorylation that increases within minutes after the addition of growth factors. In the experiments described here, the rate of uridine uptake in ionophore-treated cells stimulated by either serum or insulin depended on the extracellular and intracellular concentrations of Mg2+ and was independent of the extracellular Ca2+ concentration. In very high concentrations of Mg2+ (50 mM), ionophore-treated cells take up uridine as fast, in the absence of growth factors as in their presence, demonstrating that Mg2+ can replace the growth factor requirement for the stimulation of uridine uptake. In contrast, thymidine uptake, which also is limited by its rate of intracellular phosphorylation, showed no early response to either growth factors or Mg2+ concentration, which is consistent with the 10-fold lower Mg2+ requirement of thymidine kinase compared with uridine kinase. The feedback inhibition of uridine kinase by UTP and CTP in cell-free extracts was alleviated by increased Mg2+ concentration. The results support the thesis that the increased uptake of uridine in cells treated with growth factors is determined by a membrane-induced increase in intracellular free Mg2+. Such increase would also accelerate the rate of translation-initiation and other coordinate responses that, unlike increased uridine uptake, are essential for cell proliferation. The rate of uridine uptake is suggested as a direct indicator of free cytosolic Mg2+ that drives the shift from quiescence to proliferation.

Progress through G1 and S in relation to net protein accumulation in human NHIK 3025 cells

We have investigated whether human NHIK 3025 cells are dependent upon a net increase in cellular protein content in order to traverse G1 and S. The increase in DNA and protein content was studied by means of two-parameter flow cytometry using populations of cells synchronized by mitotic selection. By adding 1 microM cycloheximide to the medium protein synthesis was partially inhibited, resulting in negligible net accumulation of protein. The cells were able to enter S and progress through S under such conditions. The latter was the case whether the cells had been accumulating protein during G1 or not. The results further indicate that the larger cells enter S earlier and traverse S at a higher rate than the smaller cells. Our conclusion is that net accumulation of protein does not seem to be a prerequisite for traverse through G1 and S, i.e. DNA replication may be dissociated from the general growth of cell mass.

The role of protein accumulation in the cell cycle control of human NHIK 3025 cells

The cell cycle kinetics of NHIK 3025 cells, synchronized by mitotic selection, was studied in the presence of cycloheximide at concentrations (0.125-1.25 microM) which inhibited protein synthesis partially and slowed down the rate of cell cycle traverse. The median cell cycle duration was equal to the protein doubling time in both the control cells and in the cycloheximide-treated cultures at all drug concentrations. This conclusion was valid whether protein synthesis was continuously depressed by cycloheximide throughout the entire cell cycle, or temporarily inhibited during shorter periods at various stages of the cell cycle. These results may indicate that cell division does not take place before the cell has reached a critical size, or has completed a protein accumulation-dependent sequence of events. When present throughout the cell cycle, cycloheximide increased the median G1 duration proportionally to the total cell cycle prolongation. However, the entry of cells into S, once initiated, proceeded at an almost unaffected rate even at cycloheximide concentrations which reduced the rate of protein synthesis 50%. The onset of DNA synthesis seemed to take place in the cycloheximide-treated cells at a time when the protein content was lower than in the control cells. This might suggest that DNA synthesis in NHIK 3025 cells is not initiated at a critical cell mass.

Cycloheximide resistance of Physarum polycephalum

In the presence of cycloheximide, wild-type plasmodia of Physarum polycephalum exhibit an immediate decrease in deoxyribonucleic acid synthesis, a reduction in the incorporation of [3H]thymidine into thymidine triphosphate, and an increase in the level of thymidine triphosphate, as well as a decrease in protein synthesis. In this study, we have utilized a cycloheximide-resistant (Cycr) amoebic strain selected from a population of cells mutagenized with nitrosoguanidine. Segregation data indicate that the resistance is due to a single mutation. We have used this Cycr mutant to construct Cycr plasmodial strains. Ribosomes isolated from such Cycr plasmodia showed resistance to cycloheximide in vitro, in contrast to ribosomes isolated from wild-type plasmodia. The Cycr plasmodia showed none of the cycloheximide-induced biochemical effects. Plasmodia heterozygous for the resistance marker were sensitive to cycloheximide with regard to growth but showed an intermediate response in the biochemical parameters. Heterokaryons formed by fusion of various proportions of the sensitive and resistant plasmodia showed a resistance with regard to both growth and biochemical parameters which was directly related to the fraction of Cycr plasmodia present in the heterokaryons. The data are consistent with the hypothesis that the effects of cycloheximide on deoxyribonucleic acid synthesis and nucleoside metabolism are secondary to the effect of the drug on protein synthesis in this organism.

The development of alkaline phosphatase activity in limb buds of mouse embryos in vitro and its relation to chondrogenesis

The development of alkaline phosphatase (aPh) activity and chondrogenesis were studied in the limb buds of mouse embryos (day 11 p.c.) that had been grown in an organ culture. During a 12-day culture period an increase in aPh activity to more than 40 mU/limb bud was measured from day 2 in vitro onward. Depending on the time of application, aPh formation can be inhibited by certain substances. Cytosine-arabinoside inhibits aPh activity when the substance is added on day 2, 3, or 4. Chondrogenesis, on the other hand, is affected on days 1, 2, and 3 and to a lesser degree on day 4. Actinomycin D interferes with aPh activity after its addition on day 1, 2, 3, or 4. Chondrogenesis is only inhibited when the drug is applied on the 1st, 2nd, or to a lesser degree on the 3rd day. Cycloheximide inhibits aPh formation on all days of treatment, but to a lesser degree on days 5 and 6; chondrogenesis is most influenced on days 2, 3, and 4. On day 6 of the culture period, aPh activity can be demonstrated histochemically only in the region of humerus and proximal parts of radius and ulna. Alterations in the distal cartilage skeleton, therefore, do not influence the activity data. A prerequisite for an increase in aPh activity is cartilage growth in the proximal part of the limb buds and subsequent induction of a perichondral cell population to proliferation and differentiation.

Control of DNA synthesis in tissue culture cells

Eukaryotic DNA is functionally divided into thousands of replicons, each of which may be duplicated at a characteristic time within the DNA synthetic (S) period. Our approach toward an understanding of the molecular mechanisms which control orderly eukaryotic DNA synthesis has been: (a) to devise a method of cell synchrony in a suitable tissue culture system wherein all cells in the population enter and traverse the S period with a high degree of synchrony; (b) to determine, utilizing this system, precisely when during the S period critical events and macromolecular syntheses occur; and (c) to examine, by polyacrylamide-gel electrophoresis, the spectrum of proteins which become associated with chromatin during the S period in such a way as to suggest their involvement with DNA synthesis. Possible mechanisms for control are discussed based on the results presented here.

Premature replication of late S period DNA regions in early S nuclei transferred to late S cytoplasm by fusion in Physarum polycephalum

Fusion of a late S period plasmodium of Physarum polycephalum to an early S period plasmodium causes premature replication of late S replicating regions in the nuclei of the early S plasmodium. The extent of ahead-of-schedule replication of late S replicating regions in early S period nuclei increases to a plateau of 16-20% for fusions with 40-70 min of phase difference, then declines for larger phase differences. The stimulatory factors for late S replicative units are present only in late S plasmodia and appear to act only on late S regions. Once replicated, early S replicating regions are not stimulated to replicate again by fusion to a plasmodium entering the S period. Our data do not discriminate between anti-termination of replication by factors of stop sites on long replicons, and a sequential initiation of replication on new, possibly non-adjacent regions, but does provide evidence that the stimulatory factors are distinct from one another and specific for certain target replicative units.

Deficiency of joining of Okazaki-type fragments in absence of cellular protein synthesis

The dependence of integration of newly formed DNA chain ( less than 10 S) into larger DNA on concomitant protein synthesis was studied in a special cellular system. Exponentially growing Ehrlich ascites tumor cells in vivo show decreasing rated and finally complete cessation of protein and DNA synthesis upon transfer into an isotonic but non-nutritive environment (Hanks' balanced salt solution). Both protein and DNA synthesis is stimulated in these cells for a period of 30 min when they are placed into fresh Hanks' balanced salt solution; however, stimulation of protein synthesis is completely prevented in Hanks' balanced salt solution containing cycloheximide. This system allowed us to investigate the formation and fate of newly formed DNA chains ( less than 10 S) in dependence of protein synthesis. Analysis of DNA produced in [3H]thymidine pulses showed that DNA chains smaller than 18 S were still formed during the phase of totally delayed protein synthesis and in the presence of cycloheximide, but they were not converted into DNA molecules sedimenting faster than 18 S under these conditions. Stimulation of protein synthesis for a period of 30 min allowed the short DNA pieces to be chased into larger DNA 30 min post stimulation of protein synthesis. The results clearly indicate that DNA chain growth, by sealing of DNA chains smaller than 18 S, is strongly dependent of concomitant protein synthesis. Direct chain elongation by addition of new deoxyribonucleotides is less dependent on concomitant cellular protein synthesis.

A requirement for RNA, protein and DNA synthesis in the establishment of DNA replicase activity in synchronized HeLa cells

DNA replicase activity of synchronized cultures of HeLa cells was assayed by a permeable cell technique during normal S-phase and under conditions of restricted RNA, protein, or DNA synthesis. Inhibition studies with puromycin, cycloheximide, actinomycin D, and 2-mercapto-1-(beta-4-pyridethyl)benzimidazole revealed that the establishment as well as the maintenance of DNA replicase activity in S-phase cells was dependent on the continued synthesis of both RNA and protein. Measurements during limitation of DNA replication by hydroxyurea, cytosine arabinoside, or restricted availability of thymidine indicate that a low level of DNA synthesis is required to activate or assemble some subunits of DNA replicase. Evidence for the existence of short-lived RNA and protein factors essential for DNA replicase activity is discussed.

Regulation of polyribosome formation and cell division in cultured soybean cells by cytokinin

Cytokinin was shown to be required for cell division in cultured cells of Glycine max var. Sodifury. This cytokinin-induced mitotic activity was correlated with a high cellular content of polysomes. Within 24 h after transfer to a cytokinin-containing medium there was a 4”5-fold increase in the percentage of ribosomes bound as polyribosomes, as determined by sucrose density gradient centrifugation of extracted ribosomal material. Relatively high levels of polysomes and mitotic activity were maintained through the first 6 days of the culture period on cytokinin-containing medium. Thereafter, both cell division activity and the percentage of polyribosomes declined progressively with increasing time in culture. A comparatively small increase in polyribosomes occurred within 24 h of transfer to medium lacking cytokinins, followed by the progressive decline of the level of polyribosomes. The time course of cytokinin-induced polyribosome formation was determined by treating cells with cytokinin after they had been cultured for 24 h on a medium lacking cytokinin. Under these conditions there was a rapid increase in polyribosomes over the next 3 h with no detectable lag period, and near maximal levels of polyribosomes after 6 h of treatment. The initial stimulation of polyribosome formation by cytokinin was not blocked by actinomycin D. Cytokinin was shown to have a comparatively small effect on the ribonuclease activity in extracts of these cells.

Analysis of simian virus 40-induced transformation of hamster kidney tissue in vitro. 8. Induction of infectious simian virus 40 from virogenie transformed hamster cells by amino acid deprivation or cycloheximide treatment

Clones of virogenic simian virus 40 (SV40)-transformed hamster kidney cells were exposed to medium deficient in the essential amino acids leucine, arginine, or methionine. Infectious virus was induced after deprivation periods of from 24 to 32 hr. The highest yields of infectious SV40 were obtained from cultures deprived for 3 to 4 days. Infectious virus was also induced in cells that were treated with the metabolic inhibitor cycloheximide. Pulse labeling experiments revealed that both protein synthesis and deoxyribonucleic acid (DNA) synthesis were inhibited by concentrations of cycloheximide which were effective for virus induction. It is suggested that inhibition of protein synthesis by either amino acid deprivation or by cycloheximide was responsible for the induction of infectious virus from virogenic cells. We postulate that the inhibition of protein synthesis caused a temporary inhibition of DNA synthesis which resulted in the induction of infectious virus.