Preparation of large quantities of synchronized mammalian cells in late G1 in the pre-DNA replicative phase of the cell cycle

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Tools used to assay genomic instability in cancers and cancer meiomitosis

Genomic instability is a defining characteristic of cancer and the analysis of DNA damage at the chromosome level is a crucial part of the study of carcinogenesis and genotoxicity. Chromosomal instability (CIN), the most common level of genomic instability in cancers, is defined as the rate of loss or gain of chromosomes through successive divisions. As such, DNA in cancer cells is highly unstable. However, the underlying mechanisms remain elusive. There is a debate as to whether instability succeeds transformation, or if it is a by-product of cancer, and therefore, studying potential molecular and cellular contributors of genomic instability is of high importance. Recent work has suggested an important role for ectopic expression of meiosis genes in driving genomic instability via a process called meiomitosis. Improving understanding of these mechanisms can contribute to the development of targeted therapies that exploit DNA damage and repair mechanisms. Here, we discuss a workflow of novel and established techniques used to assess chromosomal instability as well as the nature of genomic instability such as double strand breaks, micronuclei, and chromatin bridges. For each technique, we discuss their advantages and limitations in a lab setting. Lastly, we provide detailed protocols for the discussed techniques.

Strengths and Weaknesses of Cell Synchronization Protocols Based on Inhibition of DNA Synthesis

Synchronous cell populations are commonly used for the analysis of various aspects of cellular metabolism at specific stages of the cell cycle. Cell synchronization at a chosen cell cycle stage is most frequently achieved by inhibition of specific metabolic pathway(s). In this respect, various protocols have been developed to synchronize cells in particular cell cycle stages. In this review, we provide an overview of the protocols for cell synchronization of mammalian cells based on the inhibition of synthesis of DNA building blocks-deoxynucleotides and/or inhibition of DNA synthesis. The mechanism of action, examples of their use, and advantages and disadvantages are described with the aim of providing a guide for the selection of suitable protocol for different studied situations.

Overview of Cell Synchronization

The widespread interest in cell synchronization is maintained by the studies of control mechanism involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

A SCARECROW-RETINOBLASTOMA protein network controls protective quiescence in the Arabidopsis root stem cell organizer

Ben Scheres and colleagues report that in the growing tip of plant roots, a gene regulatory network that includes the plant homologue of Retinoblastoma regulates the divisions of long-term stem cells to replenish tissue and to protect the root stem cell niche.

Quiescent long-term somatic stem cells reside in plant and animal stem cell niches. Within the Arabidopsis root stem cell population, the Quiescent Centre (QC), which contains slowly dividing cells, maintains surrounding short-term stem cells and may act as a long-term reservoir for stem cells. The RETINOBLASTOMA-RELATED (RBR) protein cell-autonomously reinforces mitotic quiescence in the QC. RBR interacts with the stem cell transcription factor SCARECROW (SCR) through an LxCxE motif. Disruption of this interaction by point mutation in SCR or RBR promotes asymmetric divisions in the QC that renew short-term stem cells. Analysis of the in vivo role of quiescence in the root stem cell niche reveals that slow cycling within the QC is not needed for structural integrity of the niche but allows the growing root to cope with DNA damage.

In the plant Arabidposis thaliana, root meristems (in the growing tip of the root) contain slowly dividing cells that act as an organizing center for the root stem cells that surround them. This centre is called the quiescent centre (QC). In this study, we show that the slow rate of division in the QC is regulated by the interaction between two proteins: Retinoblastoma homolog (RBR) and SCARECROW (SCR), a transcription factor that controls stem cell maintenance. RBR and SCR regulate quiescence in the QC by repressing an asymmetric cell division that generates short-term stem cells. Here we genetically manipulate the cells in the QC to alter their quiescence by regulating the RBR/SCR interaction to demonstrate that quiescence is not needed for the organizing capacity of the QC but instead provides cells with a higher resistance to genotoxic stress, allowing stem cells in the QC to survive even if more rapidly cycling stem cells are damaged. A role for mitotic quiescence has been reported in animal stem cells, in which Rb has been implicated. These findings indicate that it might serve a similar role in plant stem cells.

Overview of cell synchronization

Widespread interest in cell synchronization is maintained by the studies of control mechanisms involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

Mutant huntingtin alters cell fate in response to microtubule depolymerization via the GEF-H1-RhoA-ERK pathway

Cellular responses to drug treatment show tremendous variations. Elucidating mechanisms underlying these variations is critical for predicting therapeutic responses and developing personalized therapeutics. Using a small molecule screening approach, we discovered how a disease causing allele leads to opposing cell fates upon pharmacological perturbation. Diverse microtubule-depolymerizing agents protected mutant huntingtin-expressing cells from cell death, while being toxic to cells lacking mutant huntingtin or those expressing wild-type huntingtin. Additional neuronal cell lines and primary neurons from Huntington disease mice also showed altered survival upon microtubule depolymerization. Transcription profiling revealed that microtubule depolymerization induced the autocrine growth factor connective tissue growth factor and activated ERK survival signaling. The genotype-selective rescue was dependent upon increased RhoA protein levels in mutant huntingtin-expressing cells, because inhibition of RhoA, its downstream effector, Rho-associated kinase (ROCK), or a microtubule-associated RhoA activator, guanine nucleotide exchange factor-H1 (GEF-H1), all attenuated the rescue. Conversely, RhoA overexpression in cells lacking mutant huntingtin conferred resistance to microtubule-depolymerizer toxicity. This study elucidates a novel pathway linking microtubule stability to cell survival and provides insight into how genetic context can dramatically alter cellular responses to pharmacological interventions.

The transcriptional repressor Kaiso localizes at the mitotic spindle and is a constituent of the pericentriolar material

Kaiso is a BTB/POZ zinc finger protein known as a transcriptional repressor. It was originally identified through its in vitro association with the Armadillo protein p120ctn. Subcellular localization of Kaiso in cell lines and in normal and cancerous human tissues revealed that its expression is not restricted to the nucleus. In the present study we monitored Kaiso's subcellular localization during the cell cycle and found the following: (1) during interphase, Kaiso is located not only in the nucleus, but also on microtubular structures, including the centrosome; (2) at metaphase, it is present at the centrosomes and on the spindle microtubules; (3) during telophase, it accumulates at the midbody. We found that Kaiso is a genuine PCM component that belongs to a pericentrin molecular complex. We analyzed the functions of different domains of Kaiso by visualizing the subcellular distribution of GFP-tagged Kaiso fragments throughout the cell cycle. Our results indicate that two domains are responsible for targeting Kaiso to the centrosomes and microtubules. The first domain, designated SA1 for spindle-associated domain 1, is located in the center of the Kaiso protein and localizes at the spindle microtubules and centrosomes; the second domain, SA2, is an evolutionarily conserved domain situated just before the zinc finger domain and might be responsible for localizing Kaiso towards the centrosomal region. Constructs containing both SA domains and Kaiso's aminoterminal BTB/POZ domain triggered the formation of abnormal centrosomes. We also observed that overexpression of longer or full-length Kaiso constructs led to mitotic cell arrest and frequent cell death. Knockdown of Kaiso accelerated cell proliferation. Our data reveal a new target for Kaiso at the centrosomes and spindle microtubules during mitosis. They also strongly imply that Kaiso's function as a transcriptional regulator might be linked to the control of the cell cycle and to cell proliferation in cancer.

Methods for synchronizing cells at specific stages of the cell cycle

Exponentially growing cells are asynchronous with respect to the cell cycle stage. Detection of cell cycle-related events is improved by enriching the culture for cells at the stage during which the particular event occurs. Methods for synchronizing cells are provided here, including those based on morphological features of the cell (mitotic shake-off), cellular metabolism (thymidine inhibition, isoleucine depravation), and chemical inhibitors of cell progression in G1 (lovastatin), S (aphidicolin, mimosine), and G2/M (nocodazole). Applications of these methods and the advantages and disadvantages of each are described.

Visualization of early chromosome condensation: a hierarchical folding, axial glue model of chromosome structure

Current models of mitotic chromosome structure are based largely on the examination of maximally condensed metaphase chromosomes. Here, we test these models by correlating the distribution of two scaffold components with the appearance of prophase chromosome folding intermediates. We confirm an axial distribution of topoisomerase IIα and the condensin subunit, structural maintenance of chromosomes 2 (SMC2), in unextracted metaphase chromosomes, with SMC2 localizing to a 150–200-nm-diameter central core. In contrast to predictions of radial loop/scaffold models, this axial distribution does not appear until late prophase, after formation of uniformly condensed middle prophase chromosomes. Instead, SMC2 associates throughout early and middle prophase chromatids, frequently forming foci over the chromosome exterior. Early prophase condensation occurs through folding of large-scale chromatin fibers into condensed masses. These resolve into linear, 200–300-nm-diameter middle prophase chromatids that double in diameter by late prophase. We propose a unified model of chromosome structure in which hierarchical levels of chromatin folding are stabilized late in mitosis by an axial “glue.”

Cell cycle regulation by galectin-12, a new member of the galectin superfamily

Galectins are a family of beta-galactoside-binding animal lectins with conserved carbohydrate recognition domains (CRDs). Here we report the identification and characterization of a new galectin, galectin-12, which contains two domains that are homologous to the galectin CRD. The N-terminal domain contains all of the sequence elements predicted to form the two beta-sheets found in other galectins, as well as conserved carbohydrate-interacting residues. The C-terminal domain shows considerable divergence from the consensus sequence, and many of these conserved residues are not present. Nevertheless, the protein has lactose binding activity, most likely due to the contribution of the N-terminal domain. The mRNA for galectin-12 contains features coding for proteins with growth-regulatory functions. These include start codons in a context that are suboptimal for translation initiation and AU-rich motifs in the 3'-untranslated region, which are known to confer instability to mRNA. Galectin-12 mRNA is sparingly expressed or undetectable in many tissues and cell lines tested, but it is up-regulated in cells synchronized at the G(1) phase or the G(1)/S boundary of the cell cycle. Ectopic expression of galectin-12 in cancer cells causes cell cycle arrest at the G(1) phase and cell growth suppression. We conclude that galectin-12 is a novel regulator of cellular homeostasis.

Interphase cell cycle dynamics of a late-replicating, heterochromatic homogeneously staining region: precise choreography of condensation/decondensation and nuclear positioning

Recently we described a new method for in situ localization of specific DNA sequences, based on lac operator/repressor recognition (Robinett, C.C., A. Straight, G. Li, C. Willhelm, G. Sudlow, A. Murray, and A.S. Belmont. 1996. J. Cell Biol. 135:1685-1700). We have applied this methodology to visualize the cell cycle dynamics of an approximately 90 Mbp, late-replicating, heterochromatic homogeneously staining region (HSR) in CHO cells, combining immunostaining with direct in vivo observations. Between anaphase and early G1, the HSR extends approximately twofold to a linear, approximately 0.3-mum-diam chromatid, and then recondenses to a compact mass adjacent to the nuclear envelope. No further changes in HSR conformation or position are seen through mid-S phase. However, HSR DNA replication is preceded by a decondensation and movement of the HSR into the nuclear interior 4-6 h into S phase. During DNA replication the HSR resolves into linear chromatids and then recondenses into a compact mass; this is followed by a third extension of the HSR during G2/ prophase. Surprisingly, compaction of the HSR is extremely high at all stages of interphase. Preliminary ultrastructural analysis of the HSR suggests at least three levels of large-scale chromatin organization above the 30-nm fiber.

Adenine-induced arrest of mammalian cells in early S-phase is related to the prevention of DNA synthesis inhibition caused by gamma-irradiation

Adenine prevents the down-regulation of DNA synthesis in gamma-irradiated EAT and HL60 cells. This protective effect is related to the initiation of DNA replication. The effectiveness of this protection increases with the concentration of adenine in both cell lines. The maximum protective effect of adenine induced in EAT cells occurs at about 10 times lower concentration as compared with HL60 cells; however, the level of this effect is almost the same in both cell lines. The results indicate that the prevention of the down-regulation of DNA synthesis in gamma-irradiated EAT and HL60 cells is conditioned mainly by the arrest of the cells in the early S-phase. It is evident that DNA synthesis in cells beyond G1/S boundary is insensitive to gamma-radiation.

The dual effect of mimosine on DNA replication

The plant amino acid, mimosine, is an extremely effective inhibitor of DNA replication in mammalian cells, but the mechanism by which this inhibition is achieved is unknown. The drug has been proposed either to inhibit initiation at origins of replication or to inhibit chain elongation by lowering nucleotide pool levels. In an attempt to determine which mode of action is correct, we have analyzed its effects on SV40 DNA replication. Using a two-dimensional gel replicon mapping technique, we show that mimosine completely inhibits incorporation of [3H]thymidine into viral DNA, but only after approximately 4 h. Qualitative analysis of replication intermediates during this interval suggests that the drug partially inhibits both initiation and elongation, and pulse-chase experiments support this contention. The drug has no effect when added directly to an SV40 in vitro replication extract. However, extracts prepared from cells pretreated with mimosine are compromised in their ability to support replication in vitro in the presence of a full complement of nucleotides. Thus, although mimosine may alter nucleotide pool levels in vivo, it also appears to affect one or more essential replication proteins.

The requirements for G1 checkpoint progression of Trypanosoma brucei S 427 clone 1

Trypanosoma brucei S 427 clone 1 accumulated in G1 when incubated under growth-limiting conditions. Further incubation of the G1-restricted organisms in medium containing 10% fetal bovine serum (FBS) and 2 mM hydroxyurea resulted in their reversible arrest after a G1 checkpoint beyond which serum was not required for progress into and through S. Progress of the G1-restricted T. brucei through the G1 checkpoint was linear and required continuous incubation with exogenous serum growth factors. These were principally low and high density lipoproteins; both lipoproteins triggered G1 progression in a dose- and time-dependent manner whilst their removal by immunoaffinity chromatography severely reduced the capacity of FBS to stimulate G1 progression. Serum-induced progress of T. brucei through G1 was Ca(2+)-independent, but required gene transcription, protein synthesis, and continuous kinase activity that was inhibited by tyrphostin 51 and DAPH 1 which typically inhibit epidermal growth factor receptor protein tyrosine kinase activity. The tyrphostin 51-sensitive catalytic activity was not required for T. brucei protein synthesis, glycolysis, or S phase progression but was required for tyrosine phosphorylation of several polypeptides, none of which was specifically associated with serum-induced G1 progression.

Mimosine targets serine hydroxymethyltransferase

The plant amino acid, mimosine, is an extremely effective inhibitor of DNA replication in mammalian cells (Mosca, P. J., Dijkwel, P. A., and Hamlin, J. L. (1992) Mol. Cell. Biol. 12, 4375-4383). Mimosine appears to prevent the formation of replication forks at early-firing origins when delivered to mammalian cells approaching the G1/S boundary, and blocks DNA replication when added to S phase cells after a lag of approximately 2.5 h. We have shown previously that [3H]mimosine can be specifically photocross-linked both in vivo and in vitro to a 50-kDa polypeptide (p50) in Chinese hamster ovary (CHO) cells. In the present study, six tryptic peptides (58 residues total) from p50 were sequenced by tandem mass spectrometry and their sequences were found to be at least 77.5% identical and 96.5% similar to sequences in rabbit mitochondrial serine hydroxymethyltransferase (mSHMT). This assignment was verified by precipitating the [3H]mimosine-p50 complex with a polyclonal antibody to rabbit cSHMT. The 50-kDa cross-linked product was almost undetectable in a mimosine-resistant CHO cell line and in a CHO gly- cell line that lacks mitochondrial, but not cytosolic, SHMT activity. The gly- cell line is still sensitive to mimosine, suggesting that the drug may inhibit both the mitochondrial and the cytosolic forms. SHMT is involved in the penultimate step of thymidylate biosynthesis in mammalian cells and, as such, is a potential target for chemotherapy in the treatment of cancer.

Visualization of G1 chromosomes: a folded, twisted, supercoiled chromonema model of interphase chromatid structure

We have used light microscopy and serial thin-section electron microscopy to visualize intermediates of chromosome decondensation during G1 progression in synchronized CHO cells. In early G1, tightly coiled 100-130-nm "chromonema" fibers are visualized within partially decondensed chromatin masses. Progression from early to middle G1 is accompanied by a progressive uncoiling and straightening of these chromonema fibers. Further decondensation in later G1 and early S phase results in predominantly 60-80-nm chromonema fibers that can be traced up to 2-3 microns in length as discrete fibers. Abrupt transitions in diameter from 100-130 to 60-80 nm along individual fibers are suggestive of coiling of the 60-80-nm chromonema fibers to form the thicker 100-130-nm chromonema fiber. Local unfolding of these chromonema fibers, corresponding to DNA regions tens to hundreds of kilobases in length, reveal more loosely folded and extended 30-nm chromatin fibers. Kinks and supercoils appear as prominent features at all observed levels of folding. These results are inconsistent with prevailing models of chromosome structure and, instead, suggest a folded chromonema model of chromosome structure.

A general protocol for evaluating the specific effects of DNA replication inhibitors

Inhibitors of DNA replication in mammalian cells are of great interest because of their potential use in chemotherapy and in cell synchronizing protocols in the laboratory. We have used a combination of isotopic labelling protocols and a two-dimensional gel replicon mapping procedure to determine the specific effects of five different replication inhibitors in cultured cells. Utilizing this protocol, we show that hydroxyurea, aphidicolin, and cytosine arabinoside, three known chain elongation inhibitors, are rather ineffective at preventing fork progression even at relatively high concentrations. In contrast, two related compounds that have been suggested to be G1/S inhibitors (mimosine and ciclopyrox olamine [CPX]) actually appear to inhibit initiation at origins. One of these agents (CPX) appears also to inhibit replication in yeast, opening the possibility that the gene encoding the target (initiator?) protein can first be identified in yeast by genetic approaches and can then be used to isolate the mammalian homologue.

The plant amino acid mimosine may inhibit initiation at origins of replication in Chinese hamster cells

An understanding of replication initiation in mammalian cells has been hampered by the lack of mutations and/or inhibitors that arrest cells just prior to entry into the S period. The plant amino acid mimosine has recently been suggested to inhibit cells at a regulatory step in late G1. We have examined the effects of mimosine on cell cycle traverse in the mimosine [corrected]-resistant CHO cell line CHOC 400. When administered to cultures for 14 h after reversal of a G0 block, the drug appears to arrest the population at the G1/S boundary, and upon its removal cells enter the S phase in a synchronous wave. However, when methotrexate is administered to an actively dividing asynchronous culture, cells are arrested not only at the G1/S interface but also in early and middle S phase. Most interestingly, two-dimensional gel analysis of replication intermediates in the initiation locus of the amplified dihydrofolate reductase domain suggests that mimosine may actually inhibit initiation. Thus, this drug represents a new class of inhibitors that may open a window on regulatory events occurring at individual origins of replication.

The plant amino acid mimosine may inhibit initiation at origins of replication in Chinese hamster cells

An understanding of replication initiation in mammalian cells has been hampered by the lack of mutations and/or inhibitors that arrest cells just prior to entry into the S period. The plant amino acid mimosine has recently been suggested to inhibit cells at a regulatory step in late G1. We have examined the effects of mimosine on cell cycle traverse in the mimosine [corrected]-resistant CHO cell line CHOC 400. When administered to cultures for 14 h after reversal of a G0 block, the drug appears to arrest the population at the G1/S boundary, and upon its removal cells enter the S phase in a synchronous wave. However, when methotrexate is administered to an actively dividing asynchronous culture, cells are arrested not only at the G1/S interface but also in early and middle S phase. Most interestingly, two-dimensional gel analysis of replication intermediates in the initiation locus of the amplified dihydrofolate reductase domain suggests that mimosine may actually inhibit initiation. Thus, this drug represents a new class of inhibitors that may open a window on regulatory events occurring at individual origins of replication.

Cell cycle synchronization: reversible induction of G2 synchrony in cultured rodent and human diploid fibroblasts

In accord with a set of prespecified principles of cell synchrony induction, a three-step procedure was developed to arrest cells reversibly in the G2 phase of the cell cycle. Cultures of Chinese hamster ovary (CHO) cells were presynchronized in early S phase by sequential treatment with isoleucine deficiency and hydroxyurea blockades; then they were switched to medium supplemented with either of two agents that inhibit DNA topoisomerase II activity by different mechanisms, Hoechst 33342 at 7.5 micrograms/ml for 12 hr or VM-26 at 0.5 micrograms/ml for 8 hr. Up to 95% of the cells accumulated in G2 phase under those conditions. After switch of Hoechst 33342-treated cells to drug-free medium, the cells divided as a highly synchronized cohort of cells within 3 hr. Up to 85% of the cells in a culture of human diploid dermal fibroblasts (HSF-55 cells) could be accumulated in G2 phase by placing cells presynchronized in early-S phase in medium containing Hoechst 33342 at 0.1 micrograms/ml for 10 hr. Reversal of G2 arrest in the HSF-55 cultures resulted in cells dividing synchronously over 3.5 hr. By varying the concentration of Hoechst 33342 and the duration of the treatment period, it was possible to alter the position within G2 phase at which cells accumulated. This synchronization protocol should greatly facilitate study of G2/M biochemical events in mammalian cells, in particular, those associated with cdc2 gene regulation of the onset of mitosis.

Periodic mitotic events induced in the absence of DNA replication

We have discovered and report here a means of separating a mitotic "subcycle" from the G1- and S-phase events of the mammalian cell cycle. Time-lapse videomicroscopy of Syrian hamster fibroblast (BHK) cells revealed that caffeine could induce multiple entries into mitosis while cells were blocked in DNA synthesis. As with normal mitoses, the abundance of mitosis-specific phosphoproteins was coupled with the condensation of chromatin. The BHK temperature-sensitive mutant tsBN2 also completed multiple entries into mitosis while arrested during DNA replication and raised to the restrictive temperature. Periodic mitotic events occurred even when BHK cells were exposed to low concentrations of serum or cycloheximide, conditions that prevent the cycling of BHK cells by blocking their entry into S phase. These results suggest that an oscillation governing the activation and inactivation of mitotic factors can be generated in mammalian cells and uncoupled from the G1 and DNA replication events of the normal cell cycle. This system will be useful for examining the molecular nature of mitotic factors.

Exposure to caffeine and suppression of DNA replication combine to stabilize the proteins and RNA required for premature mitotic events

Caffeine had been shown to induce mitotic events in Syrian hamster fibroblast (BHK) cells that were arrested during DNA replication (Schlegel and Pardee, Science 232:1264-1266, 1986). Inhibition of protein synthesis blocked these caffeine-induced events, while inhibition of RNA synthesis showed little effect. We now report that the protein(s) that are required for inducing mitosis in these cells were synthesized shortly after caffeine addition, the activity was very labile in the absence of caffeine, and the activity was lost through an ATP-dependent mechanism. Caffeine dramatically increased the stability of these putative proteins while having no effect on overall protein degradation. Experiments with an inhibitor of RNA synthesis indicated that mitosis-related RNA had accumulated during the suppression of DNA replication, and this RNA was unstable when replication was allowed to resume. These results suggest that the stability of RNA needed for mitosis is regulated by the DNA replicative state of the cell and that caffeine selectively stabilizes the protein product(s) of this RNA. Conditions can therefore be selected that permit mitotic factors to accumulate in cells at inappropriate times in the cell cycle. Two-dimensional gel electrophoresis has demonstrated several protein changes resulting from caffeine treatment; their relevance to mitosis-inducing activity remains to be determined.

Toxicity of oxidized low-density lipoprotein to cultured fibroblasts is selective for S phase of the cell cycle

Oxidized LDL (o-LDL) is toxic to a variety of cultured cells. Preliminary results suggested that susceptibility is enhanced by cell proliferation. As a step toward determining the mechanism of cytotoxicity, we chose to identify the cell cycle phase(s) during which exposure of cultured human fibroblasts to o-LDL leads to death. Cytochalasin B, which blocks cell migration and proliferation, and irradiation, which prevents mitosis but not migration, both blocked cytotoxicity. Colchicine, which arrests cells in mitosis but does not inhibit DNA synthesis, did not block cytotoxicity. Treatment of cells with hydroxyurea, which blocks cells prior to S phase, prevented cell death. Addition of o-LDL to cells immediately after S phase allowed mitosis without death. The above results coupled with results using cells synchronized by three different means indicate that cell death is selective for proliferating cells and occurs after exposure to o-LDL during S phase. Understanding the mechanism of o-LDL-induced death may have implications for tissue damage in vivo in the numerous instances of pathology in which oxidized lipoproteins or lipids are present.

Effects of human interferon and alpha-difluoromethylornithine on T47D cells

The effect of human interferon (IFN) and alpha-difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of eukaryotic ornithine decarboxylase, on the rate of DNA synthesis and the increase of ornithine decarboxylase activity of T47D cells was examined. It was found that IFN or DFMO alone causes little or appreciable inhibition of the [3H]thymidine incorporation, respectively. Each of the drugs alone has a significant inhibitory effect on ornithine decarboxylase activity. Combination of the two drugs has a synergistic effect and eliminates completely the [3H]thymidine incorporation and the activity of ornithine decarboxylase. The biological implication of IFN and DFMO is discussed with regard to the regulation of ornithine decarboxylase activity and the antiproliferative effects of the two drugs.

5-Azacytidine-induced conversion to cadmium resistance correlates with early S phase replication of inactive metallothionein genes in synchronized CHO cells

Previous studies have shown both hypermethylation and late replication of DNA sequences to be associated with gene inactivity. To determine whether there is a causal relationship between patterns of DNA methylation and replication timing during S phase, we have examined the timing of replication of the inactive, hypermethylated metallothionein (MT) I and II genes in synchronized, cadmium-sensitive (Cds) CHO cells. The time of S-phase replication of the MT genes was ascertained by determining the period of S phase wherein cadmium-resistant (Cdr) cells could be induced with highest frequency by pulse treatment of synchronized Cds cells with the hypomethylating drug 5-azacytidine (5-aza-CR), and by analyzing Southern blots of density fractionated DNAs isolated from synchronized cells pulse-labeled with BrdU during different intervals after release from hydroxyurea blockade. Southern filter hybridization analyses demonstrated replication of both MTI and II gene sequences within the first half of S phase. Consistent with this result, phenotypic conversion of Cds to Cdr was maximal immediately after hydroxyurea release and decreased abruptly within three hours. The replication of inactive hypermethylated MT genes in early S phase argues that transcriptional inactivity and gene-specific hypermethylation are not sufficient conditions for late DNA replication.

Caffeine-induced uncoupling of mitosis from the completion of DNA replication in mammalian cells

Caffeine was shown to induce mitotic events in mammalian cells before DNA replication (S phase) was completed. Synchronized BHK cells that were arrested in early S phase underwent premature chromosome condensation, nuclear envelope breakdown, morphological "rounding up," and mitosis-specific phosphoprotein synthesis when they were exposed to caffeine. These mitotic responses occurred only after the cells had entered S phase and only while DNA synthesis was inhibited by more than 70 percent. Inhibitors of protein synthesis blocked these caffeine-induced events, while inhibitors of RNA synthesis had little effect. These results suggest that caffeine induces the translation or stabilizes the protein product (or products) of mitosis-related RNA that accumulates in S-phase cells when DNA replication is suppressed. The ability to chemically manipulate the onset of mitosis should be useful for studying the regulation of this event in mammalian cells.

Two functionally distinct classes of growth arrest states in human prokeratinocytes that regulate clonogenic potential

Rapidly growing normal human neonatal prokeratinocytes (HPK) cultured in serum-free medium can be induced to undergo either reversible or irreversible growth arrest at distinct cell cycle states. Reversible G1 arrest was induced by culture of low-density cells in human lymphocyte conditioned medium, by culture in high-density stationary phase conditioned medium, and by culture in isoleucine-deficient medium. Irreversible arrest of HPK growth predominantly in G1 was induced by culture in growth factor-deficient medium. Irreversible arrest of HPK growth in G1 and G2 was also induced by culture in suspension in methylcellulose prepared in complete MCDB 153 medium or by culture in serum-containing medium. Finally, the drug razoxane was employed to induce irreversible arrest of HPK in G2. These data establish that there are 2 distinct classes of growth arrest states for HPK and suggest that each arrest mechanism may serve a unique role in the control of keratinocyte differentiation in normal cells. It is also possible that the development of selective defects in either of these processes could be of etiologic significance in certain epidermal disease states.

Chloroplast DNA synthesis during the cell cycle in cultured cells of Nicotiana tabacum: inhibition by nalidixic acid and hydroxyurea

The effects of nalidixic acid and hydroxyurea on nuclear and chloroplast DNA formation in cultured cells of Nicotiana tabacum were investigated. At low concentrations (5 and 20 micrograms/ml) nalidixic acid, an inhibitor of DNA gyrase, exhibited a greater inhibitory effect on plastid DNA synthesis than on nuclear DNA formation. Since the plastid genome is a circular double-stranded DNA, this is consistent with the proven involvement of a DNA gyrase in the replication of closed circular duplex DNA genomes in procaryotic cells. At a high concentration of nalidixic acid (50 micrograms/ml), DNA synthesis in both the plastid and nuclear compartment was rapidly inhibited. Removal of the drug from the culture medium led to the resumption of DNA synthesis in 8 h. Hydroxyurea, an inhibitor of ribonucleoside diphosphate reductase, also depresses nuclear as well as plastid DNA formation. Removal of hydroxyurea from the blocked cells leads to a burst of nuclear DNA synthesis, suggesting that the cells had been synchronized at the G1/S boundary. The recovery of plastid DNA synthesis occurs within the same time frame as that of nuclear DNA. However, whereas plastid DNA formation is then maintained at a constant rate, nuclear DNA synthesis reaches a peak and subsequently declines. These results indicate that the synthesis of plastid DNA is independent of the cell cycle events governing nuclear DNA formation in cultured plant cells.

Cell cycle variations in chromatin structure detected by DNase I

We have recently developed a reproducible method for the use of DNase I as a sensitive probe of chromatin structure (Prentice, D A & Gurley, L R, Biochim biophys acta 740 (1983) 134) [12] and have used this probe to investigate chromatin structure during the interphase of the cell cycle. Chinese hamster cells (line CHO) were synchronized by: (1) mitotic detachment, to obtain M-phase cells; (2) isoleucine deprivation, to obtain G1-phase cells; and (3) sequential use of isoleucine deprivation followed by release into the presence of hydroxyurea, to obtain cells blocked at the start of S phase. The cells were released from the various blocking schemes and nuclei were isolated and digested with DNase I at various times. The digestion kinetics were monitored to detect possible changes in chromatin condensation through the cell cycle. The chromatin was much more accessible to DNase I in G1 phase than in S or G2 phase, with only small variations in structure detected in late G1 and very early S phase. From early S phase up to mitosis, the chromatin became increasingly condensed and inaccessible to DNase I action. These results support the concept of a chromatin condensation cycle during interphase as well as during mitosis.

Inducibility of metallothionein throughout the cell cycle

Synchronized Chinese hamster cells were induced with ZnCl2 at multiple stages of the cell cycle and labeled with [35S]cysteine, and the 35S-labeled proteins were isolated and separated into metallothionein and nonmetallothionein fractions. Metallothionein was found to be inducible in all stages of the cell cycle and in G1-arrested cells.

Cell division cycle in mammalian cells. VIII. Mapping of G1 into six segments using temperature-sensitive cell cycle mutants

The G1 blocks in three temperature-sensitive (ts) Syrian hamster cell-cycle mutants have been mapped in relation to other G1 landmarks. Two mutants reported here, ts-559 and ts-694, show defective progression only in G1. When shifted from the permissive temperature of 33 degrees C to the non-permissive temperature of 39 degrees C, G1 cells of these two mutants show no further cell cycle progression, while cells in S, G2 and mitosis progress through the cell cycle but become blocked after entering G1. The two mutants complement each other, and also complement the previously reported mutant ts-550C with blocks in both G1 and G2 of the cell cycle. The locations of the G1 blocks in both ts-559 and ts-694 are before the hydroxyurea arrest point. The G1 ts point in ts-694 is prior to the isoleucine deprivation and serum starvation points, while the G1 block in ts-559 is after the serum starvation point but before the isoleucine block. Other G1 block points which have been reported are in mutants of different species and isolated in different laboratories, causing difficulties for relative positioning of the blocks in G1. The mutants for mapping in this study have been isolated from the same cell line. The G1 ts arrest points of ts-559 and ts-694, and that found in ts-550C, together with nutritional deprivations and metabolic inhibitors, provide seven reference points which divide G1 into six segments, each of which is bracketed by two adjacent points: mitosis, ts-694 block, serum starvation arrest point, ts-559 block, isoleucine deprivation arrest point, ts-550C block, hydroxyurea or excess-thymidine arrest segment.

Inducibility of metallothionein throughout the cell cycle

Synchronized Chinese hamster cells were induced with ZnCl2 at multiple stages of the cell cycle and labeled with [35S]cysteine, and the 35S-labeled proteins were isolated and separated into metallothionein and nonmetallothionein fractions. Metallothionein was found to be inducible in all stages of the cell cycle and in G1-arrested cells.

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.

DNAase I and cellular factors that affect chromatin structure

The use of DNAase I as a probe of chromatin structure is frequently fraught with problems of irreproducibility. We have recently evaluated this procedure, documented the sources of the problems, and standardized the method for reproducible results (Prentice and Gurley (1983) Biochim. Biophys. Acta 740, 134-144). We have now used this probe to detect differences in chromatin structure between cells blocked (1) in G1 phase by isoleucine deprivation, or (2) in early S phase by sequential use of isoleucine deprivation followed by release into the presence of hydroxyurea. The cells blocked in G1 phase have easily-digestible chromatin, while cells blocked in early S phase have chromatin which is much more resistant to DNAase I. These differences were found to be the result of diffusible factors found in the cytoplasm and nuclei of G1- and S-phase cells, respectively. The G1 cells contained a cytoplasmic factor which modulates the chromatin structure of S-phase nuclei to a more easily digestible state, while cells blocked in S phase contain a nuclear factor which modulates the chromatin structure of G1 nuclei to a state more resistant to digestion. DNAase I is much more sensitive to these cell cycle-specific chromatin changes than is micrococcal nuclease. The results indicate that, under controlled conditions, DNAase I should be a valuable probe for detecting chromatin structural changes associated with cell cycle traverse, differentiation, development, hormone action and chemical toxicity.

Chinese hamster ovary cells cultured in low concentrations of fetal bovine serum: cloning efficiency, growth in suspension, and selection of drug-resistant mutant phenotypes

Reducing serum concentrations in media provides a potential cost advantage. To determine whether such media could be used for applied mutagenesis assays, we measured cloning efficiency and growth parameters in suspension of Chinese hamster ovary cells cultured in reduced serum with or without additives (1 microgram/ml insulin, 3 X 10(-7) M linoleic acid, 1 X 10(-8) M H2SeO3) or bovine serum albumin (BSA, 1% wt/vol). With the additives and less than or equal to 0.5% fetal bovine serum (FBS), Ham's F12 medium (without hypoxanthine and thymidine) was more optimal than alpha Eagle's minimum essential medium (MEM) (without ribosides and deoxyribosides) for low density cloning and high density suspension growth. Acceptable cloning efficiencies were obtained with 2% FBS plus BSA without additives in either medium; the addition of BSA resulted in improved colony size and more compact colony morphology. In alpha MEM, satisfactory growth rates and maximum saturation densities in suspension culture were obtained only with 5% FBS; in Ham's F12, 1% FBS + deoxycytidine + BSA yielded satisfactory suspension growth. Spontaneous mutant frequencies were compared for each medium containing 10% dialyzed FBS (DFBS), 1% FBS plus BSA, or 2% FBS plus BSA. The spontaneous frequency of azaadenine-resistant phenotypes (mutant at the aprt locus) in 1% FBS plus BSA was significantly lower than the frequency observed in 2% FBS plus BSA or 10% DFBS. Frequencies of spontaneous mutants resistant to thioguanine (hgprt locus) or fluorodeoxyuridine (tk locus) were similar with 10% DFBS, 1% FBS plus BSA, or 2% FBS plus BSA. Compared to alpha MEM with 10% DFBS, frequencies of drug-resistant mutants induced by ethyl methanesulfonate or mitomycin C (MMC) were not significantly lower in alpha MEM with 2% FBS plus BSA; observed mutant frequencies induced by dimethylnitrosamine or benzo(a)pyrene seemed to be decreased at lower survival levels.

High resolution analysis of the timing of replication of specific DNA sequences during S phase of mammalian cells

A new method, utilizing selective photodegradation of 5-bromo-deoxyuridine (BUdR)-substituted DNA and flow cytometry, has been developed for analyzing the timing of replication of specific DNA sequences. Chemically synchronized Chinese hamster ovary cells were given a pulse of the deoxythymidine analogue, BUdR, at different times during S phase, and flow sorted according to DNA content, before DNA isolation. Newly-replicated, unifilarly BUdR-substituted DNA was selectively degraded by treatment with 33258 Hoechst plus near UV light followed by S1 nuclease digestion; the resistant DNA was analyzed for its content of 18s and 28s rDNA or dihydrofolate reductase (DHFR) sequences via Southern blot analysis. Both the rDNA and DHFR sequences were found to replicate almost entirely during the first quarter of S phase. The approach described should have general utility for analyzing replication kinetics of specific DNA sequences in mammalian cells.

Studies on cell division in mammalian cells: VI. A temperature-sensitive mutant blocked in both G1 and G2 phases of the cell cycle

A temperature-sensitive mammalian cell cycle mutant with blocks in G1 and G2 phases of the cell cycle has been isolated in culture. When shifted from the permissive temperature of 33 degrees C to the nonpermissive temperature of 39 degrees C, the fraction of cells initiating DNA synthesis as well as the fraction of cells entering mitosis decreased rapidly. Combined cytophotometric and autoradiographic analysis on the cells at 39 degrees C showed that G1 cells, with the exception of those in late G1, were arrested in that phase. Cells is S phase at the time of temperature shift, together with the late g1 cells which subsequently entered S, continued through S into G2, but were blocked in that phase of the cell cycle and unable to initiate mitosis. Those cells already in mitosis completed cell division at 39 degrees C. The G1 block point of ts-550C was found to be located after the serum starvation and isoleucine deprivation arrest points, approximately 3 h before initiation of DNA synthesis.

Cell cycle kinetics of Chinese hamster (CHO) cells treated with the iron-chelating agent picolinic acid

Spontaneously transformed (tumorigenic) Chinese hamster cells (line CHO) do not exhibit picolinic acid-sensitive G1 and G2 cell cycle arrest points observed in normal and virus-transformed cells. Rather, picolinic acid arrests CHO cells in S phase only and produces culture growth behaviour similar to that produced by hydroxyurea. Prolonged treatment with picolinic acid permits a slow but significant traverse of cells through S phase. Thus, like hydroxyurea, picolinic acid is not a useful agent for synchronizing exponential CHO cells, but it can be used to resynchronize cultures in early S phase if a previous synchronization procedure (such as isoleucine deprivation) is used. The iron chelating properties of picolinic acid, and the similarities of its effects on cultured cells to those of hydroxyurea and the iron-chelating drug desferrioxamine, suggest that picolinic acid inhibits DNA synthesis by interfering with the iron-dependent production of a stable free organic radical which is essential for the ribonucleotide reductase formation of deoxyribonucleotides.

Replication pattern of a large homogenously staining chromosome region in antifolate-resistant Chinese hamster cell lines

We have investigated the replication pattern of a large, homogenously staining chromosome region (HSR) in two antifolate-resistant Chinese hamster cell lines. This region is believed to be the location of an amplified genetic sequence which includes at least the gene coding for dihydrofolate reductase and which may be present in as many as 200 copies. It is shown that the HSR in both cell lines is among the first chromosome regions to begin DNA synthesis after reversal of an early G1 block. In cells synchronized in the S period with hydroxyurea, it is also clear that the HSR in both cell lines begins replication at many sites within its length in early S. The replicons comprising the HSR therefore may respond to a common initiation signal in early S. In on cell line (A3), replication of the HSR requires, at most, 3 hours of a 7-hour S period; in a second line (MQ19), replication proceeds for approximately 5 hours. In neither line does replication of the HSR occur concomitantly with synthesis of characteristic late replicating regions. These results were confirmed in exponential cultures using a retroactive labeling technique. The significance of these findings is discussed with reference to the possible origin and arrangement of the amplified sequence in these two cell lines.

Mitochondrial protein synthesis in Chinese hamster cells (line CHO) synchronized by isoleucine deprivation

Mitochondrial protein synthesis was measured in line CHO cells after phases of the cell cycle were synchronized by isoleucine deprivation or mitotic selection. Maximum incorporation of [3H] leucine into mitochondrial polypeptides occurred within 2 hours after isoleucine was added to initiate G1 traverse. In cells synchronized in G1 by mitotic selection, the rate of mitochondrial protein synthesis was fairly constant throughout the cell cycle. SDS-polyacrylamide gel electrophoretic profiles of labeled mitochondrial polypeptides were similar in cells synchronized by either isoleucine deprivation or mitotic selection. Obvious changes in the distribution of polypeptides were not detected during various phases of the cell cycle. The increased rate of incorporation of [3H] leucine into mitochondrial polypeptides after reversal of G1-arrest may indicate that mitochondrial protein synthesis and possibly mitochondrial biogenesis are synchronized in CHO cells deprived of isoleucine.