Regulation of DNA synthesis: age-dependent cooperation among G1 cells upon fusion

Second-harmonic generation for studying structural motion of biological molecules in real time and space

SHG and sum-frequency generation (SFG) are surface-selective, nonlinear optical techniques whose ability to measure the average tilt angle of molecules on surfaces is well known in non-biological systems. By labeling molecules with a second-harmonic-active dye probe, SHG detection is extended to any biological molecule. The method has been used in previous work to detect biomolecules at an interface and their ligand-induced conformational changes. Here I demonstrate that SHG can be used to study structural motion quantitatively using a probe placed at a specific site (Cys-77) in adenylate kinase, a protein. The protein is also labeled non-site-specifically via amines. Labeled protein is absorbed to a surface and a baseline SH signal is measured. Upon introducing ATP, AMP or a specific inhibitor, AP(5)A, the baseline signal changes depending on the ligand and the labeling site. In particular, a substantial change in SH intensity is produced upon binding ATP to the amine-labeled protein, consistent with the X-ray crystal structures. In contrast, SHG polarization measurements are used to quantitatively determine that no rotation occurs at site Cys-77, in agreement with the lack of motion observed at this site in the X-ray crystal structures. A method for building a global map of conformational change in real time and space is proposed using a set of probes placed at different sites in a biomolecule. For this purpose, SH-active unnatural amino acids are attractive complements to exogenous labels.

Cell cycle control of DNA replication

The cell cycle is driven by the sequential activation of a family of cyclin-dependent kinases (cdk), which phosphorylate and activate proteins that execute events critical to cell cycle progression. In mammalian cells cdk2-cyclin A has a role in S phase. Many replication proteins are potential substrates for this cdk kinase, suggesting that initiation, elongation and checkpoint control of replication could all be regulated by cdk2. The association of PCNA, a replication protein, with cdk-cyclins during G-1 to S phase transition and with cdk-cyclin inhibitors, adds an interesting complexity to regulation of DNA replication.

p27Kip1, a cyclin-Cdk inhibitor, links transforming growth factor-beta and contact inhibition to cell cycle arrest

Cell-cell contact and TGF-beta can arrest the cell cycle in G1. Mv1Lu mink epithelial cells arrested by either mechanism are incapable of assembling active complexes containing the G1 cyclin, cyclin E, and its catalytic subunit, Cdk2. These growth inhibitory signals block Cdk2 activation by raising the threshold level of cyclin E necessary to activate Cdk2. In arrested cells the threshold is set higher than physiological cyclin E levels and is determined by an inhibitor that binds to cyclin E-Cdk2 complexes. A 27-kD protein that binds to and prevents the activation of cyclin E-Cdk2 complexes can be purified from arrested cells but not from proliferating cells, using cyclin E-Cdk2 affinity chromatography. p27 is present in proliferating cells, but it is sequestered and unavailable to interact with cyclin E-Cdk2 complexes. Cyclin D2-Cdk4 complexes bind competitively to and down-regulate the activity of p27 and may thereby act in a pathway that reverses Cdk2 inhibition and enables G1 progression.

The technique of premature chromosome condensation to study the leukemic process: review and speculations

The technique of premature chromosome condensation involves the fusion of mitotic cells with interphase cells resulting in the immediate condensation of the interphase chromatin into discrete chromosome units, the prematurely condensed chromosomes (PCC). The ability to visualize the interphase chromosomes of bone marrow and blood cells by this technique has proved useful in the study of human leukemia. This article describes how the PCC technique has been used to predict clinical outcome as well as gain insight into the biology of leukemia.

Independent regulation by sodium butyrate of gonadotropin alpha gene expression and cell cycle progression in HeLa cells

Sodium butyrate alters the growth and gene expression of a variety of differentiating and neoplastic cell types. For example, addition of 5 mM butyrate to HeLa cells is reported to both induce gonadotropin alpha subunit biosynthesis and block cell cycling in G1. We have studied these two actions of butyrate on HeLa cells and found that they are regulated in distinct ways. The induction of alpha subunit synthesis was due to an increase in the rate of transcription of the alpha gene. Using synchronized populations of HeLa cells, we determined that butyrate stimulates alpha transcription throughout the cell cycle. In contrast, treated cells arrest in G1 only if exposed to butyrate for a discrete period during the previous S phase. We conclude that butyrate inhibits DNA synthesis through a cell cycle-specific action that is independent from its direct action to stimulate transcription of the gonadotropin alpha gene.

Events associated with the initiation of mitosis in fused multinucleate HeLa cells

Large multinucleate (LMN) HeLa cells with more than 10-50 nuclei were produced by random fusion with polyethylene glycol. The number of nuclei in a particular stage of the cell cycle at the time of fusion was proportionate to the duration of the phase relative to the total cell cycle. The fused cells did not gain generation time. Interaction of various nuclei in these cells has been observed. The nuclei initially belonging to the G1- or S-phase required a much longer time to complete DNA synthesis than in mononucleate cells. Some of the cells reached mitosis 15 h after fusion, whereas others required 24 h. The cells dividing early, contained a larger number of initially early G1-phase nuclei than those cells dividing late. The former very often showed prematurely condensed chromosome (PCC) groups. In cells with a large number of advanced nuclei the few less advanced nuclei could enter mitosis prematurely. On the other hand, the cells having a large number of nuclei belonging initially to late S- or G2-phase took longer to reach mitosis. These nuclei have been taken out of the normal sequence and therefore failed to synthesize the mitotic factors and depended on others to supply them. Therefore the cells as a whole required a longer period to enter mitosis. Although the nuclei became synchronized at metaphase, the cells revealed a gradation in prophase progression in the different nuclei. At the ultrastructural level the effect of advanced nuclei on the less advanced ones was evident with respect to chromosome condensation and nuclear envelope breakdown. Less advanced nuclei trapped among advanced nuclei showed PCC and nuclear envelope breakdown prematurely, whereas mitotic nuclei near interphase or early prophase nuclei retained their nuclear envelopes for a much longer time. PCC is closely related to premature breakdown of the nuclear envelope. Our observations clearly indicate that chromosome condensation and nuclear envelope breakdown are two distinct events. Kinetochores with attached microtubules could be observed on prematurely condensed chromosomes. Kinetochores of fully condensed chromosomes often failed to become connected to spindle elements. This indicates that the formation of a functional spindle is distinct from the other events and may depend on different factors.

Independent regulation by sodium butyrate of gonadotropin alpha gene expression and cell cycle progression in HeLa cells

Sodium butyrate alters the growth and gene expression of a variety of differentiating and neoplastic cell types. For example, addition of 5 mM butyrate to HeLa cells is reported to both induce gonadotropin alpha subunit biosynthesis and block cell cycling in G1. We have studied these two actions of butyrate on HeLa cells and found that they are regulated in distinct ways. The induction of alpha subunit synthesis was due to an increase in the rate of transcription of the alpha gene. Using synchronized populations of HeLa cells, we determined that butyrate stimulates alpha transcription throughout the cell cycle. In contrast, treated cells arrest in G1 only if exposed to butyrate for a discrete period during the previous S phase. We conclude that butyrate inhibits DNA synthesis through a cell cycle-specific action that is independent from its direct action to stimulate transcription of the gonadotropin alpha gene.

The role of the G1 period in the life cycle of eukaryotic cells

The objective of this study was to test the concept that the G1 period lacks any specific function in the life cycle of mammalian cells and hence could be drastically reduced without any effect on the generation time. HeLa cells were grown in medium containing an optimum dose (60 microM) of hydroxyurea at which the duration of S period was prolonged with little or no increase in generation time. At this concentration of hydroxyurea, we observed a maximum of 3 h (or 28.5%) reduction in the G1 period. We also studied the effects of synchronization in S phase by single and double thymidine blocks on cell size and its relationship to the duration of G1 in the subsequent cycle. By these treatments, we could reduce the G1 period by not more than 2 to 3 h. The reduction in G1 period was not directly proportional to the size (volume) of the G1 cells. These results suggest that G1 period has certain specific functions and cannot be eliminated by alterations in culture conditions.

Synchronous DNA synthesis and mitosis in multinucleate cells with one chromosome in each nucleus

Multinucleate cells were induced by colcemid treatment in PtK1 cells in culture. DNA synthesis and mitotic behavior of those cells in which each nucleus contained a single chromosome were studied. More than 80% of such cells showed synchronous DNA synthesis and mitosis in all nuclei. As these genetically different nuclei respond identically to the molecules that initiate DNA synthesis and mitosis, intranuclear control of initiation of DNA synthesis and induction of mitosis by genes on individual chromosomes can be excluded. The occasional occurrence of asynchronous division in multinucleate cells is assumed to result from unequal availability of the inducer molecules to the individual nuclei.

Analysis of muscle protein expression in polyethylene glycol-induced chicken: rat myoblast heterokaryons

Heterokaryons derived from polyethylene glycol-mediated fusion of myoblasts at different stages of development were used to investigate the transition of cells in the skeletal muscle lineage from the determined to the differentiated state. Heterokaryons were analyzed by immunofluorescence, using rabbit antibodies against the skeletal muscle isoforms of chicken creatine kinase and myosin, and a mouse monoclonal antibody that cross-reacts with chicken and rat skeletal muscle myosin. When cytochalasin B-treated rat L8(E63) myocytes (Konieczny S.F., J. McKay, and J. R. Coleman, 1982, Dev. Biol., 91:11-26) served as the differentiated parental component and chicken limb myoblasts from stage 23-26 or 10-12-d embryos were used as the determined, undifferentiated parental cell, heterokaryons exhibited a progressive extinction of rat skeletal muscle myosin during a 4-6-d culture period, and no precocious expression of chicken differentiated gene products was detected. In the reciprocal experiment, 85-97% of rat myoblast X chicken myocyte heterokaryons ceased expression of chicken skeletal muscle myosin and the M subunit of chicken creatine kinase within 7 d of culture. Extinction was not observed in heterokaryons produced by fusion of differentiated chicken and differentiated rat myocytes and thus is not due to species incompatibility or to the polyethylene glycol treatment itself. The results suggest that, when confronted in a common cytoplasm, the regulatory factors that maintain myoblasts in a proliferating, undifferentiated state are dominant over those that govern expression of differentiated gene products.

Enhanced synthesis and stabilization of Mr 68,000 protein in transformed BALB/c-3T3 cells: candidate for restriction point control of cell growth

We have proposed that transformation of cells to tumorigenicity by chemical carcinogens can depend upon stabilization of a protein responsible for growth regulation. Cell kinetic experiments in which normal and benzo[a]pyrene-transformed BALB/c-3T3 cells were pulsed with cycloheximide indicated this protein should have a half-life of a few hours in normal cells and be considerably more stable in transformed cells [Campisi, J., Medrano, E. E., Morreo, G. & Pardee, A. B. (1982) Proc. Natl. Acad. Sci. USA 79, 436-440]. A protein with these properties has not yet been reported. We have searched for such a protein using two-dimensional electrophoresis to resolve protein from cells labeled with [35S]methionine. Among approximately 1,000 proteins that were resolved in these gels, we have found one that has a greater rate of synthesis and stability in benzo[a]pyrene-transformed than in untransformed cells. This result satisfies a necessary prediction of our labile protein hypothesis. We suggest that this protein could be important in determining the loss of growth regulation in these tumor cells.

Inducers of DNA synthesis: levels higher in transformed cells than in normal cells

The objective of this study was to determine whether transformed cells have greater DNA synthesis-inducing ability (DSIA) than normal cells when fused with G1 phase cells. HeLa cells synchronized in G1 phase, prelabeled with large latex beads, were fused separately with (a) quiescent human diploid fibroblasts (HDF), (b) HDF partially synchronized in late G1, and random populations of (c) HeLa, (d) WI-38, (e) SV-40 transformed WI-38, (f) CHO, (g) chemically transformed mouse cells (AKR-MCA), and (h) T98G human glioblastoma cells (all prelabeled with small latex beads) using UV-inactivated Sendai virus. The fusion mixture was incubated with [3H] thymidine, sampled at regular intervals, and processed for radioautography. Among the heterodikaryons, the frequency of those with a labeled and an unlabeled nuclei (L/U) were scored as a function of time after fusion. The faster the induction of DNA synthesis in HeLa G1, the steeper the drop in the L/U class and hence the higher DSIA in the S phase cells. The DSIA, which is indicative of the intracellular levels of the inducers of DNA synthesis, was the highest in HeLa and virally transformed WI-38 cells and the lowest in normal human diploid fibroblasts (HDF) while those of chemically and spontaneously transformed cells are intermediate between these two extremes. Higher level of DNA synthesis inducers appears to be one of the pleotropic effects of transformation by DNA tumor viruses. These studies also revealed that initiation of DNA synthesis per se is regulated by the presence of inducers and not by inhibitors.

The effects of growth factors on tRNALys modification

The tRNALys population from tissue culture cells contains several isoaccepting species which are not present in the tRNALys population from tissue sources. These isoacceptors were isolated from mouse LM cells and tested for their coding properties in ribosomal binding assays and their ability to incorporate lysine into protein in a reticulocyte lysate. tRNALys5A and tRNALys6 eluted in the area of tRNALys5. All three species coded preferentially for AAA and bound with equal efficiency. tRNALys1, tRNALys3, tRNALys4, and tRNALys6 all transferred lysine into protein at a slower rate than tRNALys2 and tRNALys5, which are the native species. Several purified growth factors were tested for their ability to affect the levels of tRNALys4, a tRNA possibly associated with cell division. When Balb/c 3T3 cells were grown in medium containing plasma instead of serum, there was a decrease in tRNALys2, tRNALys3, tRNALys4 and an increase in tRNALys5 and tRNALys6. The addition of either FGF or PDGF returned the tRNALys profile to normal. The extent of the tRNALys changes depended upon the concentration of growth factor added. FGF was able to cause a 35% decrease in the tRNALys5 peak with a corresponding increase in tRNALys2 within 1 h of the addition of the factor. These data suggest that competence factors have the ability to stimulate the modification of specific tRNALys isoacceptors.

Cytoplasts can transfer factor(s) that stimulate quiescent fibroblasts to enter S phase

Cytoplasts prepared from fibroblasts arrested by hydroxyurea were fused with serum-arrested, quiescent fibroblasts. In contrast to unfused mono-nucleated cells in the same culture, a sizable fraction of these cybridoids entered S phase in the absence of extracellular serum stimulation. Because DNA synthesis commenced only after a considerable lag following fusion, it was concluded that the cytoplasts contain factors which initiate the progression of quiescent cells toward S phase.

Block of glioma cell line in S by interferon

The human glioma cell line U-251 MG, with a well-characterized defect in growth control, was sensitive to the antiproliferative effects of human (fibroblast) interferon (IFN). IFN inhibited exponentially growing cells by increasing the number of cells in the S stage of the cell cycle. At the same time the number of cells in Go/G1 diminished. The rate of thymidine incorporation was decreased during the first cell cycle, with no prolongation of S. However, in synchronized cultures, the wave of cells with a S-phase content did not decrease over a time period several hours longer than the length of S measured by pulse labelling. Thus we conclude that, at a sufficient dose, the cells were unable to accomplish cell division as they prematurely stopped synthesizing DNA.

Cell population heterogeneity in the inducibility of DNA synthesis in human diploid fibroblasts

The initiation of nuclear DNA synthesis has been studied in cytochalasin B (CB)-induced binucleate human diploid fibroblasts (WI-38 cells). Mitotic cells from different passage levels were rendered binucleate by a brief pulse of CB. The cells were then washed free of the drug, and DNA synthesis was studied by [3H]thymidine labeling. The results showed that, in a small percentage of binucleate cells, one nucleus was labeled (S phase) and the other nucleus was unlabeled (G1 phase). There was no significant difference in the percentage of these cells with increasing passage levels. The results of this study suggest that some WI-38 cells retire from the cell cycle at different passage levels, and thereby become refractory to inducers of nuclear DNA synthesis generated by sister cells in S phase.

Differential response of cycling and noncycling cells to inducers of DNA synthesis and mitosis

The objective of this study was to determine whether cells in G(0) phase are functionally distinct from those in G(1) with regard to their ability to respond to the inducers of DNA synthesis and to retard the cell cycle traverse of the G(2) component after fusion. Synchronized populations of HeLa cells in G(1) and human diploid fibroblasts in G(1) and G(0) phases were separately fused using UV-inactivated Sendai virus with HeLa cells prelabeled with [(3)H]ThdR and synchronized in S or G(2) phases. The kinetics of initiation of DNA synthesis in the nuclei of G(0) and G(1) cells residing in G(0)/S and G(1)/S dikaryons, respectively, were studied as a function of time after fusion. In the G(0)/G(2) and G(1)/G(2) fusions, the rate of entry into mitosis of the heterophasic binucleate cells was monitored in the presence of Colcemid. The effects of protein synthesis inhibition in the G(1) cells, and the UV irradiation of G(0) cells before fusion, on the rate of entry of the G(2) component into mitosis were also studied. The results of this study indicate that DNA synthesis can be induced in G(0)nuclei after fusion between G(0)- and S-phase cells, but G(0) nuclei are much slower than G(1) nuclei in responding to the inducers of DNA synthesis because the chromatin of G(0) cells is more condensed than it is in G(1) cells. A more interesting observation resulting from this study is that G(0) cells is more condensed than it is in G(1) cells. A more interesting observation resulting from this study is that G(0) cells differ from G(1) cells with regard to their effects on the cell cycle progression of the G(2) nucleus into mitosis. This difference between G(0) and G(1) cells appears to depend on certain factors, probably nonhistone proteins, present in G(1) cells but absent in G(0) cells. These factors can be induced in G(0) cells by UV irradiation and inhibited in G(1) cells by cycloheximide treatment.

Fluorimetric measurements and chromatin condensation patterns of nuclei from 3T3 cells throughout G1

Using two cytological methods based on nuclear morphology, quinacrine dihydrochloride (QDH) staining and premature chromosome condensation (PCC), it has been possible to identify cell cycle positions within G1 of growing and arrested 3T3 cells. The fluorescent intensity of QDH-stained interphase cells appears to decrease as the cells pass from mitosis to S phase. Likewise, the length and thickness of prematurely condensed chromatids can be related to the cells; position within the G1 period. Data are presented that deal with three interrelated topics: 1) We determined by fluorometric measurements of nuclei from 3T3 cells that the visual observation of the decrease in QDH fluorescence during G1 reflects an actual decrease in total fluorescence and not a dispersion of the fluorescent chromatin in a larger nuclear area. 2) We correlated the results obtained by QDH staining with those of PCC on the same cell samples blocked in G1 by different conditions. Serum-starved and contact-inhibited cell nuclei had the highest intensity, hydroxyurea-treated ones had the lowest intensity, while that of isoleucine-deprived cells was in between. The same relative order of G1 positions was obtained based on PCC morphology. Thus, both methods monitor the state of chromatin condensation and can be used to identify cell cycle position within G1. 3) We showed with both methods that the states of chromatin resulting from the various G1 blocking conditions differ from each other.

Initiation of DNA synthesis in the prematurely condensed chromosomes of G1 cells

The objective of this study was to investigate whether G1 cells could enter S phase after premature chromosome condensation resulting from fusion with mitotic cells. HeLa cell synchronized in early G1, mid-G1, late G1, and G2 and human diploid fibroblasts synchronized in G0 and G1 phases were separately fused by use of UV-inactivated Sendai virus with mitotic HeLa cells. After cell fusion and premature chromosome condensation, the fused cells were incubated in culture medium containing Colcemid (0.05 micrograms/ml) and [3H]thymidine ([3H]ThdR) (0.5 microCi/ml; sp act, 6.7 Ci/mM). At 0, 2, 4, and 6 h after fusion, cell samples were taken to determine the initation of DNA synthesis in the prematurely condensed chromosomes (PCC) on the basis of their morphology and labeling index. The results of this study indicate that PCC from G0, G1, and G2 cells reach the maximum degree of compaction or condensation at 2 h after PCC induction. In addition, the G1-PCC from normal and transformed cells initiated DNA synthesis, as indicated by their "pulverized" appearance and incorporation of [3H]ThdR. Further, the initiation of DNA synthesis in G1-PCC occurred significantly earlier than in the mononucleate G1 cells. Neither pulverization nor incorporation of label was observed in the PCC of G0 and G2 cells. These findings suggest that chromosome decondensation, although not controlling the timing of a cell's entry into S phase, is an important step for the initiation of DNA synthesis. These data also suggest that the entry of a S phase may be regulated by cell cycle phase-specific changes in the permeability of the nuclear envelope to the inducers of DNA synthesis present in the cytoplasm.

Reinitiation of DNA synthesis in postmitotic nuclei of myotubes by virus-mediated fusion with embryonic fibroblasts

Myotubes, whose nuclei have stopped DNA synthesis were fused with replicative embryonic fibroblasts. In heterokaryons the postmitotic muscle nuclei resumed DNA synthesis. Incorporation of radioactive thymidine into muscle, and also into fibroblast nuclei was dependent upon the time elapsed between virus-mediated fusion and administration of radioactive thymidine. Whereas incorporation into fibroblast nuclei diminished with time, there was an early increase of labelling into muscle nuclei followed by a decrease of incorporation of 3H thymidine. DNA synthesis was also dependent upon the ratio of noncycling (muscle) to cycling (fibroblast) nuclei. There was a greater incorporation of 3H thymidine into muscle and fibroblast nuclei in myotubes containing larger numbers of fibroblast nuclei. A model is discussed for the control of DNA synthesis in polykaryocytes derived from fusion of cycling and noncycling cells.

Multinucleation in mouse fibroblasts cultured in methocel medium

3T3-4E cells formed multinucleate cells with high frequency when incubated in methocel medium. The experiment with hydroxyurea and the cytological observation of mitoses showed that multinucleate cells were produced by nuclear division in the absence of cytokinesis. When transferred onto a solid substratum, most of the multinucleate cells divided within seven hours into mononucleate cells through the process of cytoplasmic division, indicating that cell spreading induced cytokinesis. Other mouse fibroblast lines examined so far showed only the low frequency of multinucleation. These findings indicate that in 3T3-4E cells cultivated in methocel, nuclear division occurred independently of cytokinesis, and that cytokinesis was also anchorage-dependent. This system will be available for studying cytoplasmic division of mammalian cells.

Inducers of DNA synthesis present during mitosis of mammalian cells lacking G1 and G2 phases

The cell cycle analysis of Chinese hamster lung fibroblast V79-8 line by the premature chromosome condensation method has confirmed the absence of measurable G1 and G2 periods. Sendai virus-mediated fusion of mitotic V79-8 cells with G1 phase HeLa cells resulted in the induction of both DNA synthesis and premature chromosome condensation in the latter, indicating the presence of the inducers of DNA synthesis above the critical level not only throughout S phase, as it is in HeLa, but also during mitosis of V79-8 cells. No initiation of DNA synthesis was observed when G1 phase HeLa cells were fused with mitotic CHO cells. These results indicate that the presence or absence of a G1 period in the cell cycle depends on the levels of the inducers of DNA synthesis present in the cell during mitosis.