Differences in the regulation of protein synthesis, cyclin B accumulation, and cellular growth in response to the inhibition of DNA synthesis in Chinese hamster ovary and HeLa S3 cells

Optimizing Cell Synchronization Using Nocodazole or Double Thymidine Block

Cell synchronization is crucial when studying events that take place at specific points of the cell cycle. Several chemical agents can be used to achieve the cell culture synchronization but not all type of cells respond equally to a given concentration of these drugs. Here we describe a simple optimization method to select concentrations and timings for nocodazole or thymidine treatments using fluorescence staining. In addition, we provide detailed protocols to arrest an asynchronous culture of either suspension or adherent cells in G₁/S or in G₂/M.

Stability of cytoplasmic nanoviscosity during cell cycle of HeLa cells synchronized with Aphidicolin

Nanoviscosity of the cytoplasm is a key factor affecting diffusion of biomolecules and – as a consequence – rates of biochemical reactions in a cell. Nanoviscosity is an outcome of variable chemical and structural factors, which can temporarily change with cell-cycle associated changes of intracellular architecture. Thus, the question arises, whether rates of biochemical reactions depend on the point of cell cycle. In this paper we address this topic by constant observation of nanoviscosity of HeLa cells cytoplasm during S, G2 and G1 phases after Aphidicolin synchronization. For this purpose we measured diffusion rates of EGFP molecules using fluorescence correlation spectroscopy (FCS). To our surprise, a counter-intuitive stability of cytoplasmic viscosity was observed during the cell cycle. Our results hint at possible existence of robust mechanism maintaining stable physiological viscosity of the cytoplasm, despite huge structural changes during cell cycle.

Restriction of protein synthesis abolishes senescence features at cellular and organismal levels

Cellular senescence or its equivalence is induced by treatment of cells with an appropriate inducer of senescence in various cell types. Mild restriction of cytoplasmic protein synthesis prevented induction of all aspects of cellular senescence in normal and tumor-derived human cells. It allowed the cells to continuously grow with no sign of senescent features in the presence of various inducers. It also delayed replicative senescence in normal human fibroblasts. Moreover, it allowed for growth of the cells that had entered a senescent state. When adult worms of the nematode C. elegans were grown under protein-restricted conditions, their average and maximal lifespans were significantly extended. These results suggest that accumulation of cytoplasmic proteins due to imbalance in macromolecule synthesis is a fundamental cause of cellular senescence.

Centrosomes and cancer: revisiting a long-standing relationship

Over a century ago, centrosome aberrations were postulated to cause cancer by promoting genome instability. The mechanisms governing centrosome assembly and function are increasingly well understood, allowing for a timely reappraisal of this postulate. This Review discusses recent advances that shed new light on the relationship between centrosomes and cancer, and raise the possibility that centrosome aberrations contribute to this disease in different ways than initially envisaged.

Inter- and intrachromosomal asynchrony of cell division cycle events in root meristem cells of Allium cepa: possible connection with gradient of cyclin B-like proteins

Alternate treatments of Allium cepa root meristems with hydroxyurea (HU) and caffeine give rise to extremely large and highly elongated cells with atypical images of mitotic divisions, including internuclear asynchrony and an unknown type of interchromosomal asynchrony observed during metaphase-to-anaphase transition. Another type of asynchrony that cannot depend solely on the increased length of cells was observed following long-term incubation of roots with HU. This kind of treatment revealed both cell nuclei entering premature mitosis and, for the first time, an uncommon form of mitotic abnormality manifested in a gradual condensation of chromatin (spanning from interphase to prometaphase). Immunocytochemical study of polykaryotic cells using anti-beta tubulin antibodies revealed severe perturbations in the microtubular organization of preprophase bands. Quantitative immunofluorescence measurements of the control cells indicate that the level of cyclin B-like proteins reaches the maximum at the G2 to metaphase transition and then becomes reduced during later stages of mitosis. After long-term incubation with low doses of HU, the amount of cyclin B-like proteins considerably increases, and a significant number of elongated cells show gradients of these proteins spread along successive regions of the perinuclear cytoplasm. It is suggested that there may be a direct link between the effects of HU-mediated deceleration of S- and G2-phases and an enhanced concentration of cyclin B-like proteins. In consequence, the activation of cyclin B-CDK complexes gives rise to an abnormal pattern of premature mitotic chromosome condensation with biphasic nuclear structures having one part of chromatin decondensed, and the other part condensed.

Centriole reduplication during prolonged interphase requires procentriole maturation governed by Plk1

Supernumerary centrioles lead to abnormal mitosis, which in turn promotes tumorigenesis. Thus, centriole duplication must be coordinated with the cell cycle to ensure that the number of centrioles in the cell doubles precisely during each cell cycle. However, in some transformed cells, centrioles undergo multiple rounds of duplication (reduplication) during prolonged interphase. Mechanisms responsible for centriole reduplication are poorly understood. Here, we report that centrioles reduplicate consistently in cancerous and nontransformed human cells during G2 arrests and that this reduplication requires the activity of Polo-like kinase 1 (Plk1). We also find that a cell's ability to reduplicate centrioles during S arrests depends on the presence of activated (Thr210-phosphorylated) Plk1 at the centrosome. In the absence of activated Plk1, nascent procentrioles remain associated with mother centrioles, which prevents centriole reduplication. In contrast, if Plk1(pT210) appears at the centrosome, procentrioles mature, disengage from mother centrioles, and ultimately duplicate. Plk1 activity is not required for the assembly of procentrioles, however. Thus, the role of Plk1 is to coordinate the centriole duplication cycle with the cell cycle. Activation of Plk1 during late S/G2 induces procentriole maturation, and after this point, the centriole cycle can be completed autonomously, even in the absence of cell-cycle progression.

Giardia intestinalis: aphidicolin influence on the trophozoite cell cycle

This study is a thorough examination of the effects of the DNA polymerase inhibitor aphidicolin on the nuclear cycle and cell cycle progression characteristics, as well as their reversibility, in Giardia intestinalis. Giardia trophozoites are arrested in the G1/S-junction after aphidicolin treatment according to their DNA content. However, cell growth continues and trophozoites arrested with aphidicolin resemble cells in the G2 phase and trophozoites in ageing cultures. Extensive treatment with aphidicolin causes side effects and we detected positive signals for phosphorylated histone H2A, which, in mammalian cells, is involved in a signalling pathway triggered as a reaction to double stranded DNA breaks. These results suggest that aphidicolin causes dissociation of the nuclear and cytoplasmic cycles, a phenomenon that has also been described for other inhibitors in mammalian cell lines. Thus, if aphidicolin is used for synchronization of Giardia trophozoites, this fact must be accounted for, and treatment with aphidicolin must be minimal.

Transient production of recombinant proteins by Chinese hamster ovary cells using polyethyleneimine/DNA complexes in combination with microtubule disrupting anti-mitotic agents

We have developed a simple and robust transient expression system utilizing the 25 kDa branched cationic polymer polyethylenimine (PEI) as a vehicle to deliver plasmid DNA into suspension-adapted Chinese hamster ovary cells synchronized in G2/M phase of the cell cycle by anti-mitotic microtubule disrupting agents. The PEI-mediated transfection process was optimized with respect to PEI nitrogen to DNA phosphate molar ratio and the plasmid DNA mass to cell ratio using a reporter construct encoding firefly luciferase. Optimal production of luciferase was observed at a PEI N to DNA P ratio of 10:1 and 5 mug DNA 10(6) cells(-1). To manipulate transgene expression at mitosis, we arrested cells in G2/M phase of the cell cycle using the microtubule depolymerizing agent nocodazole. Using secreted human alkaline phosphatase (SEAP) and enhanced green fluorescent protein (eGFP) as reporters we showed that continued inclusion of nocodazole in cell culture medium significantly increased both transfection efficiency and reporter protein production. In the presence of nocodazole, greater than 90% of cells were eGFP positive 24 h post-transfection and qSEAP was increased almost fivefold, doubling total SEAP production. Under optimal conditions for PEI-mediated transfection, transient production of a recombinant chimeric IgG4 encoded on a single vector was enhanced twofold by nocodazole, a final yield of approximately 5 microg mL(-1) achieved at an initial viable cell density of 1 x 10(6) cells mL(-1). The glycosylation of the recombinant antibody at Asn297 was not significantly affected by nocodazole during transient production by this method.

Centrosomes split in the presence of impaired DNA integrity during mitosis

A well-established function of centrosomes is their role in accomplishing a successful mitosis that gives rise to a pair of identical daughter cells. We recently showed that DNA replication defects and DNA damage in Drosophila embryos trigger centrosomal changes, but it remained unclear whether comparable centrosomal responses can be provoked in somatic mammalian cells. To investigate the centrosomal organization in the presence of impaired DNA integrity, live and ultrastructural analysis was performed on gamma-tubulin-GFP and EGFP-alpha-tubulin-expressing Chinese hamster ovary cells. We have shown that during mitosis in the presence of incompletely replicated or damaged DNA, centrosomes split into fractions containing only one centriole. This results in the formation of multipolar spindles with extra centrosome-like structures. Despite the extra centrosomes and the multipolarity of the spindles, cells do exit from mitosis, resulting in severe division errors. Our data provide evidence of a novel mechanism showing how numerous centrosomes and spindle defects can arise and how this can lead to the formation of aneuploid cells.

The role of periosteum in cartilage repair

Periosteum, which can be grown in cell and whole tissue cultures, may meet one or more of the three prerequisites for tissue engineered cartilage repair. Periosteum contains pluripotential mesenchymal stem cells with the potential to form either cartilage or bone. Because it can be transplanted as a whole tissue, it can serve as its own scaffold or a matrix onto which other cells and/or growth factors can be adhered. Finally, it produces bioactive factors that are known to be chondrogenic. The chondrocyte precursor cells reside in the cambium layer. These vary in total density and volume with age and in different donor sites. The advantages of whole tissue periosteal transplants for cartilage repair include the fact that this tissue meets the three primary requirements for tissue engineering: a source of cells, a scaffold for delivering and retaining them, and a source of local growth factors. Many growth factors that regulate chondrocytes and cartilage development are synthesized by periosteum in conditions conducive to chondrogenesis. These include transforming growth factor-beta 1, insulinlike growth factor-1, growth and differentiation factor-5, bone morphogenetic protein-2, integrins, and the receptors for these molecules. By additional study of the molecular events in periosteal chondrogenesis, it may be possible to optimize its capacity for articular cartilage repair.

Overlapping functions of the pRb family in the regulation of rRNA synthesis

The "pocket" proteins pRb, p107, and p130 are a family of negative growth regulators. Previous studies have demonstrated that overexpression of pRb can repress transcription by RNA polymerase (Pol) I. To assess whether pRb performs this role under physiological conditions, we have examined pre-rRNA levels in cells from mice lacking either pRb alone or combinations of the three pocket proteins. Pol I transcription was unaffected in pRb-knockout fibroblasts, but specific disruption of the entire pRb family deregulated rRNA synthesis. Further analysis showed that p130 shares with pRb the ability to repress Pol I transcription, whereas p107 is ineffective in this system. Production of rRNA is abnormally elevated in Rb(-/-) p130(-/-) fibroblasts. Furthermore, overexpression of p130 can inhibit an rRNA promoter both in vitro and in vivo. This reflects an ability of p130 to bind and inactivate the upstream binding factor, UBF. The data imply that rRNA synthesis in living cells is subject to redundant control by endogenous pRb and p130.

Overexpression of cdk Inhibitor p16INK4a by adenovirus vector inhibits cardiac hypertrophy in vitro and in vivo: a novel strategy for the gene therapy of cardiac hypertrophy

Cardiac hypertrophy is one of the serious complications which increase mortality due to cardiovascular diseases. However, only a partial reduction of cardiac hypertrophy has been successful using current drug therapy. We demonstrate here reduction of cardiac hypertrophy in vitro and in vivo using an adenovirus vector encoding cyclin-dependent kinase (cdk) inhibitor p16INK4a. Adenovirus-mediated overexpression of cdk inhibitor p16INK4a completely inhibited cardiac myocyte hypertrophy induced by endothelin (ET)-1, as evaluated by [3H]leucine incorporation into the cells and mRNA levels of skeletal alpha -actin (SK-A) or atrial natriuretic peptide (ANP) as well as by morphometric analyses. We then evaluated whether p16INK4a can suppress left-ventricular (LV) hypertrophy induced by aortic banding (AOB) in rats. Catheter-mediated gene transfer of AxCAp16 was performed according to the method reported by Hajjar et al. LV overload was produced by coarctation of the ascending aorta immediately after inoculation of the heart with adenovirus. Two weeks after the procedure, the left ventricular weight/body weight ratio (LVW/BW) increased in the AOB+LacZ group in comparison to that in controls. However, LVW/BW was identical in the AOB+p16 group and controls. Histologic analysis revealed that p16INK4a inhibited hypertrophy of cardiac myocytes. These results suggest that G1 cell cycle regulators may restrict cardiac hypertrophy, and offer a novel strategy for the gene therapy of cardiac hypertrophy.

The potential of iron chelators of the pyridoxal isonicotinoyl hydrazone class as effective antiproliferative agents, IV: The mechanisms involved in inhibiting cell-cycle progression

Some chelators of the pyridoxal isonicotinoyl hydrazone class have antiproliferative activity that is far greater than desferrioxamine (DFO). In this study, DFO was compared with one of the most active chelators (311) on the expression of molecules that play key roles in cell-cycle control. This was vital for understanding the role of iron (Fe) in cell-cycle progression and for designing chelators to treat cancer. Incubating cells with DFO, and especially 311, resulted in a decrease in the hyperphosphorylated form of the retinoblastoma susceptibility gene product (pRb). Chelators also decreased cyclins D1, D2, and D3, which bind with cyclin-dependent kinase 4 (cdk4) to phosphorylate pRb. The levels of cdk2 also decreased after incubation with DFO, and especially 311, which may be important for explaining the decrease in hyperphosphorylated pRb. Cyclins A and B1 were also decreased after incubation with 311 and, to a lesser extent, DFO. In contrast, cyclin E levels increased. These effects were prevented by presaturating the chelators with Fe. In contrast to DFO and 311, the ribonucleotide reductase inhibitor hydroxyurea increased the expression of all cyclins. Hence, the effect of chelators on cyclin expression was not due to their ability to inhibit ribonucleotide reductase. Although chelators induced a marked increase in WAF1 and GADD45 mRNA transcripts, there was no appreciable increase in their protein levels. Failure to translate these cell-cycle inhibitors may contribute to dysregulation of the cell cycle after exposure to chelators. (Blood. 2001;98:842-850)

Technical considerations in periosteal grafting for osteochondral injuries

Periosteum has been used clinically for biologic resurfacing arthroplasty in small series of patients for almost two decades. The author's own experience with this technique in multiple joints, including the knee, has been similar to that already reported in the literature. Observations and considerations are discussed that might help avoid failure in future applications of this technique. Indications and surgical technique, including graft procurement and fixation, and postoperative treatment and possible complications are also described. The rationale for using periosteum as a chondrogenic tissue and the factors affecting its cartilage production are also outlined.

Different susceptibilities of postmitotic checkpoint-proficient and -deficient Balb / 3T3 cells to ICRF-193, a catalytic inhibitor of DNA topoisomerase II

Two distinct types of Balb / 3T3 cells were isolated which exhibit either 4 N DNA or both 4 N and 8 N DNA after exposure to colcemid for 48 h. They were found to differ with respect to the postmitotic checkpoint, but not the mitotic checkpoint. Firstly, the checkpoint-proficient and -deficient cells exhibited the same accumulation and subsequent decrease in the number of mitotic cells following exposure to microtubule inhibitors. Secondly, after exit from abnormal mitosis in the presence of ICRF (Imperial Cancer Research Fund)-193, the checkpoint-proficient cells were arrested in the next cycle G1, while the checkpoint-deficient cells progressed into S and G2 phase. When either mitotic or asynchronous cells were exposed to ICRF-193, the checkpoint-proficient cells proved more sensitive to the cytotoxic effect of this agent than the checkpoint-deficient cells. The different susceptibilities of the two types of cells to ICRF-193 were not caused by variation in topoisomerase (topo) II function since both the biochemical activity of this enzyme and chromosome segregation were inhibited by similar concentrations of ICRF-193 in both checkpoint-proficient and -deficient cells. We propose that the inhibition of chromosome segregation by ICRF-193 is monitored by the next G1 checkpoint, resulting in an irreversible G1 block in the case of postmitotic checkpoint-proficient cells. As the checkpoint-deficient cells can escape this G1 block, these cells have an increased survival capacity. In summary, ICRF-193 may prove to be a very useful drug for examination of the postmitotic checkpoint.

Signal transduction pathways that contribute to increased protein synthesis during T-cell activation

Protein synthesis rates were maximally stimulated in human lymphocytes by ionomycin and the phorbol ester PMA (I+P), which promotes proliferation, whereas PMA alone, which does not promote proliferation, stimulated protein synthesis to a lesser degree. Three translation-associated activities, eIF4E phosphorylation, eIF2B activity and 4E-BP1 phosphorylation also increased with stimulation by I+P and PMA, but only 4E-BP1 phosphorylation was differentially stimulated by these conditions. Correspondingly, signaling pathways activated in T cells were probed for their connection to these activities. Immunosuppressants FK506 and rapamycin partially blocked the protein synthesis rate increases by I+P stimulation. FK506 had less of an inhibitory effect with PMA stimulation suggesting that its mechanism mostly affected ionomycin-activated signals. I+P and PMA equally stimulated phosphorylation of ERK1/2, but I+P more strongly stimulated Akt, and p70(S6K) phosphorylation. An inhibitor that blocks ERK1/2 phosphorylation only slightly reduced protein synthesis rates stimulated by I+P or PMA, but greatly reduced eIF4E phosphorylation and eIF2B activity. In contrast, inhibitors of the PI-3 kinase and mTOR pathways strongly blocked early protein synthesis rate stimulated by I+P and PMA and also blocked 4E-BP1 phosphorylation and release of eIF4E suggesting that these pathways regulate protein synthesis activities, which are important for proliferation in T cells.

Pneumocystis carinii uses a functional cdc13 B-type cyclin complex during its life cycle

Pneumocystis carinii causes severe pneumonia in immunocompromised patients. Recent studies indicate that P. carinii uses a Cdc2 cyclin-dependent kinase to control its proliferation. To further study the regulation of the life cycle of P. carinii, we characterized the P. carinii B-type cyclin termed Cdc13, whose binding to Cdc2 is necessary for kinase activity. Antibodies to B-type cyclins (Cdc13) specifically immunoprecipitated Cdc2/ Cdc13 complexes with associated kinase activity from P. carinii extracts. To clone P. carinii cdc13, degenerate polymerase chain reaction was undertaken using primers generated from amino-acid motifs conserved in fungal Cdc13 proteins. This amplicon was used to obtain full-length genomic and complementary DNA (cDNA) clones. A specific synthetic peptide antibody generated to P. carinii Cdc13 further demonstrated differential Cdc2/Cdc13 activity over the life cycle of P. carinii, with greater activity in cysts compared with trophic forms of the organism. Finally, P. carinii cdc13 cDNA was used to rescue mutant Schizosaccharomyces pombe strains containing temperature-sensitive deficiencies of endogenous Cdc13 activity, thus verifying function of the P. carinii Cdc13 protein. Therefore, P. carinii contains a Cdc13 cyclin, which is variably active over its life cycle and which promotes fungal proliferation.

Articular cartilage regeneration using periosteum

Periosteum has chondrogenic potential that makes it possible to repair or regenerate cartilage in damaged joints. Whole periosteal explants also can be cultured in vitro for the purpose of studying chondrogenesis. This chondrogenic potential arises because the cambium layer of periosteum contains chondrocyte precursor cells that form cartilage during limb development and growth in utero, and does so once again during fracture healing. The advantages of whole tissue periosteal transplants for cartilage repair include the fact that this tissue meets the three primary requirements for tissue engineering: a source of cells, a scaffold for delivering and retaining them, and a source of local growth factors. Data from in vivo studies show that periosteum transplanted into osteochondral articular defects produce cartilage that can restore the articular cartilage and be replaced by bone in the subchondral region. This capacity is determined by surgical factors such as the orientation of the cambium layer, postoperative factors such as the use of continuous passive motion, and the age and maturity of the experimental animal. In vitro studies have shown that the chondrogenic potential of periosteal explants is determined by culture, donor conditions, and technical factors. Chondrogenesis is optimized by suspension of the explants in agarose under aerobic conditions, with supplementation of the media using fetal calf serum and growth factors, particularly transforming growth factor-beta 1. The role of physical factors currently is being investigated, but studies show that the mechanical environment is important. Donor factors that are important include the harvest site, the size of the periosteal explant, and most importantly the age of the donor. Periosteal chondrogenesis follows a specific time course of events, with proliferation preceding differentiation. The current challenge is to clarify the process of periosteal chondrogenesis and its regulation at the cellular and molecular levels, so that it can be controlled intelligently and optimized for the purpose of cartilage repair and regeneration.

Periosteum responds to dynamic fluid pressure by proliferating in vitro

Periosteum provides a source of undifferentiated chondrocyte precursor cells for fracture healing that can also be used for cartilage repair. The quantity of cartilage that can be produced, which is a determining factor in fracture healing and cartilage repair, is related to the number of available stem cells in the cambium layer. Cartilage formation during both of these processes is enhanced by motion of the fracture or joint in which periosteum has been transplanted. The effect of dynamic fluid pressure on cell proliferation in periosteal tissue cultures was determined in 452 explants from 60 immature (2-month-old) New Zealand White rabbits. The explants were cultured in agarose suspension for 1-14 days. One group was subjected to cyclic hydrostatic pressure, which is referred to as dynamic fluid pressure, at 13 kPa and a frequency of 0.3 Hz. Control explants were cultured in similar chambers without application of pressure. DNA synthesis ([3H]thymidine uptake) and total DNA were measured. The temporal pattern and distribution of cell proliferation in periosteum were evaluated with autoradiography and immunostaining with proliferating cell nuclear antigen. Dynamic fluid pressure increased proliferation of periosteal cells significantly, as indicated by a significant increase in [3H]thymidine uptake at all time points and a higher amount of total DNA compared with control values. On day 3, when DNA synthesis reached a peak in periosteal explants, [3H]thymidine uptake was 97,000+/-5,700 dpm/microg DNA in the group exposed to dynamic fluid pressure and 46,000+/-6,000 dpm/microg in the controls (p < 0.001). Aphidicolin, which blocks DNA polymerase alpha, inhibited [3H]thymidine uptake in a dose-dependent manner in the group subjected to dynamic fluid pressure as well as in the positive control (treated with 10 ng/ml of transforming growth factor-beta1) and negative control (no added growth factors) groups, confirming that [3H]thymidine measurements represent proliferation and dynamic fluid pressure stimulates DNA synthesis. Total DNA was also significantly higher in the group exposed to dynamic fluid pressure (5,700+/-720 ng/mg wet weight) than in the controls (3,700+/-630) on day 3 (p < 0.01). Autoradiographs with [3H]thymidine revealed that one or two cell cycles of proliferation took place in the fibrous layer prior to proliferation in the cambium layer (where chondrocyte precursors reside). Proliferating cell nuclear antigen immunophotomicrographs confirmed the increased proliferative activity due to dynamic fluid pressure. These findings suggest either a paracrine signaling mechanism between the cells in these two layers of the periosteum or recruitment/migration of proliferating cells from the fibrous to the cambium layer. On the basis of the data presented in this study, we postulate that cells in the fibrous layer respond initially to mechanical stimulation by releasing growth factors that induce undifferentiated cells in the cambium layer to divide and differentiate into chondrocytes. These data indicate that cell proliferation in the early stages of chondrogenesis is stimulated by mechanical factors. These findings are important because they provide a possible explanation for the increase in cartilage repair tissue seen in joints subjected to continuous passive motion. The model of in vitro periosteal chondrogenesis under dynamic fluid pressure is valuable for studying the mechanisms by which mechanical factors might be involved in the formation of cartilage in the early fracture callus and during cartilage repair.

HPV16-E7 expression causes fluorodeoxyuridine-mediated radiosensitization in SW620 human colon cancer cells

We have reported that HT29 colon cancer cells, which are radiosensitized by fluorodeoxyuridine (FdUrd), exhibit a greater increase in cyclin E-dependent kinase activity and progress further into S phase in the presence of FdUrd than do SW620 colon cancer cells, which are only minimally sensitized by this drug (Cancer Res 56: 3203, 1996). Although these findings suggested that the ability to progress into S phase in the presence of FdUrd permits cells to be radiosensitized, we wished to test this hypothesis by attempting to drive SW620 human colon cells into S phase by transducing them with the HPV16-E7 gene. Two-parameter flow cytometry showed that E7-transduced cells progressed through S phase after radiation and FdUrd treatment more rapidly than SW620 parental cells. We found that E7-transduced SW620 cells were significantly radiosensitized by FdUrd (100 nmol/L, 14 hours) with an enhancement ratio for 2 clones of 1.47 +/- 0.03 and 1.51 +/- 0.14, compared with 1.24 +/- 0.04 in SW620 parental cells. These data strongly support the hypothesis that dysregulation of S-phase progression is an important factor in FdUrd-mediated radiosensitization.

Cyclin-dependent kinase control of centrosome duplication

Centrosomes nucleate microtubules and duplicate once per cell cycle. This duplication and subsequent segregation in mitosis results in maintenance of the one centrosome/cell ratio. Centrosome duplication occurs during the G1/S transition in somatic cells and must be coupled to the events of the nuclear cell cycle; failure to coordinate duplication and mitosis results in abnormal numbers of centrosomes and aberrant mitoses. Using both in vivo and in vitro assays, we show that centrosome duplication in Xenopus laevis embryos requires cyclin/cdk2 kinase activity. Injection of the cdk (cyclin-dependent kinase) inhibitor p21 into one blastomere of a dividing embryo blocks centrosome duplication in that blastomere; the related cdk inhibitor p27 has a similar effect. An in vitro system using Xenopus extracts carries out separation of the paired centrioles within the centrosome. This centriole separation activity is dependent on cyclin/cdk2 activity; depletion of either cdk2 or of the two activating cyclins, cyclin A and cyclin E, eliminates centriole separation activity. In addition, centriole separation is inhibited by the mitotic state, suggesting a mechanism of linking the cell cycle to periodic duplication of the centrosome.

Connections between growth and the cell cycle

To maintain a constant size during cellular proliferation, a cell's growth rate must match its rate of division. Factors that govern proliferation must therefore coordinately regulate two distinct processes: the cellular biosynthesis that drives accumulation of mass, and progression through the cell division cycle. Recent work has identified several mechanisms which couple cell division to growth. Different mechanisms are used at different times during development to coordinate growth, cell division, and patterning.

Distinct alterations in mitochondrial mass and function characterize different models of apoptosis

Recent studies have shown that reduction in mitochondrial membrane potential (delta psi m) and generation of reactive oxygen species are early events in apoptosis. In this study, we present two different models of apoptotic cell death, Chinese hamster ovary (CHO) cells treated with aphidicolin and dexamethasone-treated 2B4 T-cell hybridoma cells, which display opposing mitochondrial changes. CHO cells arrested at G1/S with aphidicolin have a progressive increase in mitochondria mass and number, assessed by flow cytometry and fluorescent microscopy with mitochondria-specific probes. The increase in mitochondrial mass was not accompanied by a gain in net cellular mitochondrial membrane potential, consistent with an accumulation of relatively depolarized mitochondria. Fluorescent microscopy demonstrated an increased content of low delta psi m mitochondria in aphidicolin-treated CHO cells, but high delta psi m mitochondria were also present and remained stable in number. Mitochondrial mass correlated with decreased clonogenicity of aphidicolin-treated CHO cells. Cycloheximide prevented both the proliferation of mitochondria and subsequent cell death. In contrast, dexamethasone treatment of 2B4 T-cell hybridoma cells caused a decrease in delta psi m without mitochondrial proliferation. Cycloheximide and Bcl-2 overexpression inhibited the loss of delta psi m, as well as apoptosis. In both models, cell death was associated with a decrease in mitochondrial potential relative to mitochondrial mass, suggesting that an accumulation of damaged or dysfunctional mitochondria had occurred.

Mitochondrial proliferation and paradoxical membrane depolarization during terminal differentiation and apoptosis in a human colon carcinoma cell line

Herbimycin A, a tyrosine kinase inhibitor, induces cellular differentiation and delayed apoptosis in Colo-205 cells, a poorly differentiated human colon carcinoma cell line. Cell cycle analysis in conjunction with end labeling of DNA fragments revealed that G2 arrest preceded apoptotic cell death. Ultrastructural examination of herbimycin-treated cells demonstrated morphologic features of epithelial differentiation, including formation of a microvillar apical membrane and lateral desmosome adhesions. A marked accumulation of mitochondria was also observed. Fluorometric analysis using the mitochondrial probes nonyl-acridine orange and JC-1 confirmed a progressive increase in mitochondrial mass. However these cells also demonstrated a progressive decline in unit mitochondrial transmembrane potential (DeltaPsim) as determined by the DeltaPsim-sensitive fluorescent probes rhodamine 123 and JC-1 analyzed for red fluorescence. In concert with these mitochondrial changes, Colo-205 cells treated with herbimycin A produced increased levels of reactive oxygen species as evidenced by oxidation of both dichlorodihydrofluorescein diacetate and dihydroethidium. Cell-free assays for apoptosis using rat-liver nuclei and extracts of Colo-205 cells at 24 h showed that apoptotic activity of Colo-205 lysates requires the early action of mitochondria. Morphological and functional mitochondrial changes were observed at early time points, preceding cleavage of poly (ADP-ribose) polymerase. These results suggest that apoptosis in differentiated Colo-205 cells involves unrestrained mitochondrial proliferation and progressive membrane dysfunction, a novel mechanism in apoptosis.

Modulation in cell cycle and cyclin B1 expression in irradiated HeLa cells and normal human skin fibroblasts treated with staurosporine and caffeine

The comparative effects of staurosporine or caffeine on G2-phase arrest and cyclin B1 expression in human skin fibroblasts (HSF) and transformed HeLa cells following gamma-irradiation were examined by flow cytometry. Contrary to some earlier reports with HeLa cells, the arrest in G2 after irradiation was accompanied by an increase in cyclin B1 levels in both asynchronous and synchronized HeLa cells irradiated in early S phase. Caffeine and staurosporine were equally effective in attenuating both the radiation-induced increase in cyclin B1 expression and the prolongation of G2 in synchronous and asynchronous HeLa cell populations. Staurosporine treatment was less effective in down-regulating cyclin B1 expression in asynchronous HeLa cells at earlier time points following irradiation when compared to caffeine-treated cells. In synchronized HeLa cells, down-regulation of an irradiation-induced increase in cyclin B1 expression was similar to either staurosporine or caffeine treatments, with caffeine being more effective at later time points. An increase in cyclin B1 expression was also observed in irradiated HSF cells (synchronous and asynchronous), which decreased when the cells were treated with staurosporine or caffeine. However, staurosporine was ineffective in attenuating the radiation-induced prolongation of G2 in synchronous and asynchronous HSF cells, whereas treatment of irradiated synchronous or asynchronous HSF cells with caffeine significantly reduced the prolongation of G2. These results suggest that both staurosporine and caffeine treatments act on different pathways of cell cycle control in normal and transformed cells, in terms of attenuation of G2 block and diminution of elevated levels of cyclin B1 expression, in response to radiation.

ICRF-193, a catalytic inhibitor of DNA topoisomerase II, delays the cell cycle progression from metaphase, but not from anaphase to the G1 phase in mammalian cells

We have shown previously that ICRF-193, a catalytic inhibitor of DNA topoisomerase II (topo II), delays cell cycle progression in HeLa S3 cells. We report here that the delay of the transition in M phase is observed when HeLa S3 cells were treated with ICRF-193 during metaphase, but not thereafter. ICRF-193 also delayed the degradation of cyclin B in the transition from M to G1 phase, while in Chinese hamster ovary (CHO) cells the drug did not delay the progression in M phase. Since HeLa S3 and CHO cells are 'stringent' and 'relaxed' in mitotic control, respectively, it is suggested that under topo II inhibition, the M phase checkpoint operates through an inability for chromosome segregation.

Cytometry of cyclin proteins

Cyclins are key components of the cell cycle progression machinery. They activate their partner cyclin-dependent kinases (CDKs) and possibly target them to respective substrate proteins within the cell. CDK-mediated phosphorylation of specific sets of proteins drives the cell through particular phases or checkpoints of the cell cycle. During unperturbed growth of normal cells, the timing of expression of several cyclins is discontinuous, occurring at discrete and well-defined periods of the cell cycle. Immunocytochemical detection of cyclins in relation to cell cycle position (DNA content) by multiparameter flow cytometry has provided a new approach to cell cycle studies. This approach, like no other method, can be used to detect the unscheduled expression of cyclins, namely, the presentation of G1 cyclins by cells in G2/M and of G2/M cyclins by G1 cells, without the need for cell synchronization. Such unscheduled expression of cyclins B1 and A was seen when cell cycle progression was halted, e.g., after synchronization at the G1/S boundary by inhibitors of DNA replication. The unscheduled expression of cyclins B1 or E, but not of A, was also observed in some tumor cell lines even when their growth was unperturbed. Likewise, whereas the expression of cyclins D1 or D3 in nontumor cells was restricted to an early section of G1, the presentation of these proteins in many tumor cell lines also was seen during S and G2/M. This suggests that the partner kinase CDK4 (which upon activation by D-type cyclins phosphorylates pRB committing the cell to enter S) is perpetually active throughout the cell cycle in these tumor lines. Expression of cyclin D also may serve to discriminate G0 vs. G1 cells and, as an activation marker, to identify the mitogenically stimulated cells entering the cell cycle. Differences in cyclin expression make it possible to discriminate between cells having the same DNA content but residing at different phases such as in G2 vs. M or G2/M of a lower DNA ploidy vs. G1 cells of a higher ploidy. The expression of cyclins D, E, A and B1 provides new cell cycle landmarks that can be used to subdivide the cell cycle into several distinct subcompartments. The point of cell cycle arrest by many antitumor agents can be estimated with better accuracy in relation to these compartments compared to the traditional subdivision into four cell cycle phases. The latter applications, however, pertain only to normal cells or to tumor cells whose phenotype is characterized by scheduled expression of cyclins. As sensitive and specific indicators of the cell's proliferative potential, the cyclins, in particular D-type cyclins, are expected to be key prognostic markers in neoplasia.

The kinetics of the B cyclin p56cdc13 and the phosphatase p80cdc25 during the cell cycle of the fission yeast Schizosaccharomyces pombe

The levels of the B cyclin p56cdc13 and the phosphatase p80cdc25 have been followed in selection-synchronised cultures of Schizosaccharomyces pombe wild-type and wee1 mutant cells. p56cdc13 has also been followed in induction-synchronised cells of the mutant cdc2-33. The main conclusions are: (1) cdc13 levels in wild-type cells start to rise from base line at about mid-G2, reach a peak before mitosis and then fall slowly through G1. Cells exit mitosis with appreciable levels of cdc13. (2) cdc13 levels in wee1 cells fall to zero in interphase. They also start to rise at the beginning of G2, which may be related to the absence of a mitotic size control. (3) cdc25 starts to rise later and reaches a peak after mitosis. This is not what would be expected from a simple mitotic inducer and suggests that cdc25 has an important function at the end of mitosis. (4) An upper (heavier) band of cdc25 peaks at the same time as the main band but rises and falls more rapidly. If this is a hyperphosphorylated form, its timing shows that it is most unlikely to function in the ways shown for such a form in eggs and mammalian cells. (5) Experiments with the mutant cdc10-129 and with hydroxyurea show that the initial signal to begin synthesis of cdc13 originates at Start. (6) In induction synchrony, where G2 spans across cell division, there is evidence that some events in one cycle cannot start in the previous one. (7) Revised timings are given for the times of mitosis in these cultures.

Effect of cell cycle on the regulation of the cell surface and secreted forms of type I and type II human tumor necrosis factor receptors

The cell cycle has been shown to regulate the biological effects of human tumor necrosis factor (TNF), but to what extent that regulation is due to the modulation of TNF receptors is not clear. In the present report we investigated the effect of the cell cycle on the expression of surface and soluble TNF receptors in human histiocytic lymphoma U-937. Exposure to hydroxyurea, thymidine, etoposide, bisbensimide, and demecolcine lead to accumulation of cells primarily in G1/S, S, S/G2/M, G2/M, and M stages of the cell cycle, respectively. While no significant change in TNF receptors occurred in cells arrested in G1/S or S/G2 stages, about a 50% decrease was observed in cells at M phase of the cycle. Scatchard analysis showed a reduction in receptor number rather than affinity. In contrast, cells arrested at S phase (thymidine) showed an 80% increase in receptor number. The decrease in the TNF receptors was not due to changes in cell size or protein synthesis. The increase in receptors, however, correlated with an increase in total protein synthesis (to 3.8-fold of the control levels). A proportional change was observed in the p60 and p80 forms of the TNF receptors. A decrease in the surface receptors in cells arrested in M phase correlated with an increase in the amount of soluble receptors. The cellular response to TNF increased to 8- and 2-fold in cells arrested in G1 and S phase, respectively; but cells at G2/M phase showed about 6-fold decrease in response. In conclusion, our results demonstrate that the cell cycle plays an important role in regulation of cell-surface and soluble TNF receptors and also in the modulation of cellular response.

A cytotoxicity assay utilizing a fluorescent dye that determines accurate surviving fractions of cells

A cytotoxicity assay has been developed based on the measurement of the proliferative activity of surviving cells as quantified by a cell-incorporated fluorescent dye, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The BCECF proliferative assay is fast (the results are obtained within 3-4 days depending on the cell line), accurate, not labor-intensive, does not require the use of radioisotopes or toxic compounds, and is amenable to automation. The BCECF proliferative assay was compared with two other indirect cytotoxicity tests, a trypan blue exclusion test and a BCECF viability test. Neither of these two latter assays reflected in any way the killing of cells by ricin. In contrast, using the BCECF proliferation assay, an optimal period of cell culturing after exposure to a toxin could be found so that the cytotoxicity values produced agreed with the surviving fractions of cells measured in a direct cytotoxicity assay. Under non-optimal conditions, the assay reflected the cell kill only qualitatively. Although it is common practice to conduct indirect cytotoxicity tests without validating them with a direct assay, our experiments demonstrate that the values obtained in such non-optimized indirect cytotoxicity tests may not parallel the cell kill and may, therefore, be meaningless.

The propensity for gene amplification: a comparison of protocols, cell lines, and selection agents

We have studied cell lines of rodent and human origin for their propensity to become resistant to antifolates (methotrexate, trimetrexate), phosphonacetyl-L-aspartate (PALA), and colcemid, resistances associated with amplification of the DHFR, CAD, and MDR1 genes, respectively. We have employed two different methods: (1) a shallow step-wise selection protocol, where time to attain specified resistance is the quantitative measure, (2) the frequency of resistant colonies at specified drug concentrations. Although there are advantages and disadvantages to both methods, the two methods gave the same relative ranking of cell lines. Striking differences in the propensity for gene amplification (resistance) were found: human cell lines were less prone to amplify genes than Chinese hamster ovary (CHO) cells. This ranking was similar with all of the agents employed. Additionally, we observed that whereas PALA resistance in CHO cells is associated with amplification of the CAD gene, PALA resistance in the two human cell lines studied (HeLaS3 and VA13) was not associated with amplification and/or overexpression of the CAD gene, and thus this resistance to PALA occurs by an unknown mechanism.

Participation of initiation factors in the recruitment of mRNA to ribosomes

The step of protein synthesis which is normally rate limiting, formation of the 48S initiation complex, is catalyzed by the group 4 initiation factors. Collectively they recognize the 7-methylguanosine-containing cap of mRNA, unwind mRNA secondary structure, and allow scanning for the initiation codon by the small ribosomal subunit. The activities of the eIF-4 polypeptides are modulated by phosphorylation. Recent studies shed new light on the mechanism of assembly of the 48S initiation complex and the effect of phosphorylation of one of the eIF-4 polypeptides, the cap-binding protein eIF-4E.

Novel actions of inhibitors of DNA topoisomerase II in drug-resistant tumor cells

We review herein current work on the cytotoxic and cellular actions of two classes of inhibitors of DNA topoisomerase II: one represented by etoposide and teniposide, which stabilize DNA-protein complexes, and another represented by merbarone and aclarubicin, which do not stabilize such complexes. We discuss current concepts of protooncogene activation and cell cycle perturbations by some of these inhibitors and summarize recent findings of novel actions of the latter compounds in tumor cells that express a mutant topoisomerase II.