An effect of cell-cycle position on ultraviolet-light-induced mutagenesis in Chinese hamster ovary cells

Using synchronous populations obtained by selectively detaching mitotic cells from cultures grown in monolayer, we demonstrate here that Chinese hamster ovary (CHO) cells exhibit a differential sensitivity to mutation induction by UV as a function of position in the cell cycle. When mutation induction to 6-thioguanine (TG) resistance is monitored, several maxima and minima are displayed during cell-cycle traverse, with a major maximum occurring in early S phase. Although cells in S phase are more sensitive to UV-mediated cell lethality than those in G₁ or G₂/M phases, there is not a strict correlation with induced mutation frequency. Fluence-response curves obtained at several times during the cell cycle yield D_q values approximating 6 J/m². The primary survival characteristic which varies with cell cycle position is D₀, ranging from 2.5 J/m² at 6 h after mitotic selection to 5.5 J/m² at 11 h afterward. Based on studies with asynchronous, logarithmically growing populations, as well as those mitotically selected to be synchronous, the optimum phenotypic expression time for induced TG resistance is 7–9 days and is essentially independent of both UV fluence and position in the cell cycle. All isolated mutants have altered hypozanthine—guanine phosphoribosyl transferase (HGPRT) activity, and no difference in the residual level of activity was detected among isolated clones receiving UV radiation during G₁, S, or late S/G₂ phases of the cell cycle. Changes in cellular morphology during cell-cycle traverse do not contribute to the differential susceptibility to UV-induced mutagenesis.     

ADP-ribosylation of Translation Elongation Factor 2 by Diphtheria Toxin in Yeast Inhibits Translation and Cell Separation

Eukaryotic translation elongation factor 2 (eEF2) facilitates the movement of the peptidyl tRNA-mRNA complex from the A site of the ribosome to the P site during protein synthesis. ADP-ribosylation (ADP^R) of eEF2 by bacterial toxins on a unique diphthamide residue inhibits its translocation activity, but the mechanism is unclear. We have employed a hormone-inducible diphtheria toxin (DT) expression system in Saccharomyces cerevisiae which allows for the rapid induction of ADP^R-eEF2 to examine the effects of DT in vivo. ADP^R of eEF2 resulted in a decrease in total protein synthesis consistent with a defect in translation elongation. Association of eEF2 with polyribosomes, however, was unchanged upon expression of DT. Upon prolonged exposure to DT, cells with an abnormal morphology and increased DNA content accumulated. This observation was specific to DT expression and was not observed when translation elongation was inhibited by other methods. Examination of these cells by electron microscopy indicated a defect in cell separation following mitosis. These results suggest that expression of proteins late in the cell cycle is particularly sensitive to inhibition by ADP^R-eEF2.     

BRCA1 Is Required for Maintenance of Phospho-Chk1 and G2/M Arrest during DNA Cross-Link Repair in DT40 Cells

The Fanconi anemia DNA repair pathway is pivotal for the efficient repair of DNA interstrand cross-links. Here, we show that FA-defective (Fancc⁻) DT40 cells arrest in G₂ phase following cross-link damage and trigger apoptosis. Strikingly, cell death was reduced in Fancc⁻ cells by additional deletion of the BRCA1 tumor suppressor, resulting in elevated clonogenic survival. Increased resistance to cross-link damage was not due to loss of toxic BRCA1-mediated homologous recombination but rather through the loss of a G₂ checkpoint. This proapoptotic role also required the BRCA1-A complex member ABRAXAS (FAM175A). Finally, we show that BRCA1 promotes G₂ arrest and cell death by prolonging phosphorylation of Chk1 on serine 345 after DNA damage to sustain arrest. Our data imply that DNA-induced cross-link death in cells defective in the FA pathway is dependent on the ability of BRCA1 to prolong cell cycle arrest in G₂ phase.     

Methylamine facilitates demonstration of specific uptake of diphtheria toxin by CHO cell and toxin-resistant CHO cell mutants

We have measured the specific uptake of ¹²⁵I-labelled diphtheria toxin in the presence of methylamine by a number of cell lines with different sensitivities to diphtheria toxin. The results show a strong correlation between the toxin sensitivities of the cell lines and the amount of specific uptake. The specific association of labelled toxin with cells was clearly demonstrated even with CHO cells, a cell line with relatively low sensitivity. Thus, CHO cell mutants that are resistant to diphtheria toxin could be classified as toxin-binding or non-binding cells by this method.     

Genetic markers for quantitative mutagenesis studies in Chinese hamster ovary cells Characteristics of some recently developed selective systems

Selection conditions have been optimized in the Chinese hamster ovary (CHO) cell system for a number of genetic markers. The genetic systems studied include resistance to the protein-synthesis inhibitors emetine (Emt^r) and diphtheria toxin (Dip^r), resistance to methylglyoxalbisguanylhydrazone (Mbg^r) which affects polyamine transport, resistance to the nucleoside analogs toyocamycin and tubercidin (Toy^r), and resistance to thioguanine (Thg^r) and ouabain (Oua^R). The optimal expression time following mutagenesis for various markers was between 2 and 6 days. A linear dose-response relationship between the concentration of mutagen (ethyl methanesulfonate) and mutation frequency has been observed over the range of 10–700 μm/ml, for all of the above markers except Toy^r. The response of these markers to other mutagens such as tritium (³H) decay and ICR-191 show some specificity. Since the response of a number of genetic markers can be studied simultaneously in the CHO system, it should prove very useful for studies of quantitative mutagenesis and in assay systems for mutagen detection.     

Targeting head and neck squamous cell carcinoma using a novel fusion toxin-diphtheria toxin/HN-1

The current treatment strategies, chemotherapy and radiation therapy being used for the management of cancer are deficient in targeted approach leading to treatment related toxicities and relapse. Contrarily, fusion toxins exhibit remarkable tumor specificity thus emerging as an alternative therapy for the treatment of cancer. Diphtheria toxin-HN-1 peptide (DT/HN-1) is a fusion toxin designed to target the head and neck squamous cell carcinoma (HNSCC). The aim of this study was to construct, characterize, and evaluate the cytotoxicity and specificity of DT/HN-1 fusion toxin against the HNSCC cells. The purified DT/HN-1 fusion toxin was characterized by SDS-PAGE and western blotting. Refolding of purified fusion toxins was monitored by fluorescence spectra and circular dichroism spectra. The activity of DT/HN-1 fusion toxin was demonstrated on various HNSCC cell lines by cell viability assay, cell proliferation assay, protein synthesis inhibition assay, apoptosis and cell cycle analysis. The fusion toxin DT/HN-1 demonstrated remarkably high degree of cytotoxicity specific to the HNSCC cells. The IC₅₀ of DT/HN-1 fusion toxin was ~1 to 5 nM in all the three HNSCC cell lines. The percentage apoptotic cells in DT/HN-1 treated UMB-SCC-745 cells are 16% compared to 4% in untreated. To further demonstrate the specific toxicity of DT/HN-1 fusion toxin towards the HNSCC cells we constructed, characterized and evaluated the efficacy of DT protein. The DT protein coding for only a fragment of diphtheria toxin without its native receptor binding domain failed to exhibit any cytotoxicity on all the cell lines used in this study thus establishing the importance of a ligand in achieving targeted toxicity. To evaluate the translocation ability of HN-1 peptide, an additional construct DTΔT/HN-1 was constructed, characterized and evaluated for its cytotoxic activity. The fusion toxin DTΔT/HN-1 deficient of the translocation domain of diphtheria toxin showed no cytotoxicity on all the cell lines clearly indicating the inability of HN-1 peptide to translocate catalytic domain of the toxin into the cytosol.     

An in situ assay for induced diphtheria-toxin-resistant mutants of diploid human fibroblasts

The sensitivity of diploid human fibroblasts to the cytotoxic effects of diphtheria toxin (DT) depended on the cell growth status. Exponentially growing cells treated with 10^?3-1 lethal flocculating units (LF) of DT/ml for 4 days survived with a frequency of 4 × 10^?4. However, the DT-resistant phenotype of colonies isolated under these conditions was not stable. When the growth of the cells had been arrested by confluence or deprivation of serum growth factors prior to treatment with DT (4 days, 10^?3-0.6 LF/ml), the survival decreased to 2 × 10^?6 and the resistance of isolated colonies was stable. An in situ assay for induced DT-resistant mutants was developed in order to avoid problems associated with the possible reduced viability of the mutants relative to that of wild-type cells. A reproducible and linear dose response was obtained for the induction of DT-resistant mutants by ethylnitrosourea. The mutants were induced with high frequency by this compound (e.g., 10^?3 mutants/viable cell at a 37% survival dose); complete expression of the mutant phenotype occurred after 6 generations of growth under nonselective conditions. Isolated mutant colonies showed stable resistance to DT and were cross-resistant to Pseudomonas aeruginosa exotoxin A.     

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.     

Reduced inhibition of DNA synthesis and G2 arrest during the cell cycle of resistant HeLa cells in response tocis-diamminedichloroplatinum

The influence of cisplatin, an anticancer agent, on DNA synthesis and cell cycle progression of a cisplatin-resistant cell line was investigated. Cell cycle analysis using flow cytometry showed that cytotoxic concentrations of cisplatin caused a transient inhibition of parental HeLa cells at S phase, followed by accumulation at G₂ phase. In contrast, the resistant cells progressed through the cell cycle without being affected by the same treatment. However, cell cycle distributions were the same in the resistant and the parental cells at IC₅₀, the drug concentration inhibiting cell growth by 50%. Studies using a [³H]thymidine incorporation technique also demonstrated a transient inhibition of DNA synthesis in HeLa cells by cisplatin; such inhibition was greatly reduced in the resistant cells. These data argue for the hypothesis that the inhibition of DNA synthesis is important in determining cisplatin-induced cytotoxicity. In addition, the accumulation of cells at G₀/G₁ by serum starvation was not effective in the resistant cells compared to the parental cells, suggesting that the control of cell cycle exiting is also altered in the resistant cells. Taken together, these results support the notion that alterations in cell cycle control, in particular G₂ arrest, are important in determining the sensitivity or resistance of mammalian cells to cisplatin and may have a role in clinical protocols.     

Induction of Cell Death by Cytokines in Cell Cycle-Synchronous Tumor Cell Populations Restricted to G1and G2

In the present work, cytokine-mediated induction of cell death was investigated by flow cytometry in cell cycle-synchronous human tumor cell populations gained by centrifugal elutriation or by cell cycle blockade with mimosine and aphidicolin. Attention was payed to the question of whether the effector phase of cell death takes place in the same phase of the cell cycle in which the death signal is received. Another point of interest was the question whether synchronization of cell populations with respect to the cell cycle leads to increased synchronicity of the death phase. The results demonstrate that supernatants from monocyte/tumor cell interaction cultures containing tumor necrosis factor-α, interferons, and interleukins-1 and -6 or appropriate combinations of pure cytokines cause cell cycle arrest predominantly in G₁and to a lesser extent in G₂. Cell death is initiated from both arrest points. Cytokine-treated G₁cells do not enter S phase. They die within the same G₁phase in which they receive the death signal. In contrast, a high proportion of cytokine-treated G₂cells pass through mitosis and are arrested and die in the subsequent G₁phase, whereas only a smaller proportion of cells are arrested and die in G₂. The synchronicity of the death phase cannot be increased by the diverse methods of cell cycle synchronization applied. Interestingly, aurin-tricarboxylic acid, an agent known for inhibitory effects on nucleolytic activities and other protein/nucleic acid interactions, not only prevents cell death, but also cell cycle arrest.     

Parthenolide and DMAPT induce cell death in primitive CML cells through reactive oxygen species

Tyrosine kinase inhibitors (TKI) have become a first‐line treatment for chronic myeloid leuakemia (CML). TKIs efficiently target bulk CML cells; however, they are unable to eliminate the leukaemic stem cell (LSC) population that causes resistance and relapse in CML patients. In this study, we assessed the effects of parthenolide (PTL) and dimethyl amino parthenolide (DMAPT), two potent inhibitors of LSCs in acute myeloid leukaemia (AML), on CML bulk and CML primitive (CD34⁺lin^?) cells. We found that both agents induced cell death in CML, while having little effect on the equivalent normal hematopoietic cells. PTL and DMAPT caused an increase in reactive oxygen species (ROS) levels and inhibited NF‐κB activation. PTL and DMAPT inhibited cell proliferation and induced cell cycle arrest in G₀ and G₂ phases. Furthermore, we found cell cycle inhibition to correlate with down‐regulation of cyclin D1 and cyclin A. In summary, our study shows that PTL and DMAPT have a strong inhibitory effect on CML cells. Given that cell cycle arrest was not dependent on ROS induction, we speculate that this effect could be a direct consequence of NF‐κB inhibition and if this mechanism was to be evaded, PTL and DMAPT induced cell death would be potentiated.     

Toxin binding site of the diphtheria toxin receptor: loss and gain of diphtheria toxin binding of monkey and mouse heparin-binding, epidermal growth factor-like growth factor precursors by reciprocal site-directed mutagenesis

The transmembrane precursor of the monkey (Mk) heparin-binding, epidermal growth factor-like growth factor (proHB-EGF) functions as a diphtheria toxin (DT) receptor, whereas the mouse (Ms) precursor does not. Previously, using chimeric Ms/Mk precursors, we have shown that DT resistance of cells bearing Ms proHB-EGF may be accounted for by several amino acid substitutions between residues 122 and 148 within the EGF-like domain and that Glu-141 is an important amino acid residue for DT binding. In this study, reciprocal site-directed mutagenesis was performed on the major non-conserved residues in the region of 122–148, alone or in combination, between Mk and Ms precursors to identify more precisely which amino acid residues are important for DT binding. Two approaches were used. The first, more traditional approach was to destroy DT sensitivity and binding of Mk proHB-EGF by substitution(s) with the corresponding Ms residue(s). From the single mutations, the greatest loss of DT sensitivity was observed with Mk/Glu-141His (approximately 4000-fold) and the next greatest with Mk/Ile-133Lys (approximately fourfold). The double mutations Mk/Leu-127Phe/Glu-141His, Mk/Ile-133Lys/Glu-141His and Mk/His-135Leu/Glu-141His resulted in complete toxin resistance (> 100 000-fold). The second approach, both novel and complementary, was to gain DT binding and sensitivity of Ms proHB-EGF by substitution(s) with the corresponding Mk residue(s). Surprisingly, the single mutation Ms/His-141Glu resulted in the gain of moderate DT sensitivity (> 260-fold). The double mutation Ms/Lys-133Ile/His-141Glu and the triple mutation Ms/Lys-133Ile/Leu-135His/His-141Glu resulted in a progressive gain in toxin sensitivity (> 4700-fold and > 16 000-fold respectively) and affinity. This triple mutant cell line is essentially as sensitive (IC₅₀ = 3.1 ng ml^?1) as the highly toxin-sensitive monkey Vero cell line (IC₅₀ = 4 ng ml^?1), indicating that these three Mk residues enable the Ms proHB-EGF to act as a fully functional DT receptor. Taken together, these results indicate that Glu-141 plays the most critical role in DT binding and sensitivity and that two additional amino acid residues, Ile-133 and His-135, also play significant roles.     

The cytotoxic and mutagenic effects of 5-bromodeoxyuridine-plus-black-light treatment in cultured mammalian cells

The cytotoxic and mutagenic effects of the incorporation of 5-bromodeoxyuridine (BrdU)_followed by exposure to black light were investigated with Chinese hamster ovary (CHO) cells in cell culture. Mutation induction at the hypoxanthine-guanine phosphoribosyl transferase (hgprt) locus was determined by selection for 6-thioguanine resistant (TG^r) mutants (CHO/HGPRT system). BrdU alone has been shown to be mutagenic only at concentrations of 50 μM or greater. This study was performed in an effort to determine whether BrdU is actually incorporated into the hgprt gene when lower, nonmutagenic concentrations are employed. Neither BrdU (1–20 μM) nor exposure to black light alone was mutagenic, but the combined treatment did result in the induction of TG^r mutants. The mutant frequency increased with increasing light exposure at constant BrdU and with inreasing BrdU at constant light exposure. These results show that BrdU is incorporated into the hgprt gene, but that this does not result in mutation induction in the absence of light exposure. Such a BrdU-plus-light procedure might be applied to studies of DNA repair at this locus, since mutation induction requires both BrdU incorporation and subsequent exposure to black light.     

Ornithine decarboxylase induction and the cytoskeleton in normal and transformed cells

ODC induction by fresh medium added to stationary, medium-depleted, confluent cultures has been studied in transformed HeLa and CHO cells, and in normal human fibroblasts as an indicator of the resumption of cell multiplication. The transformed HeLa cell displays a more easily reversed G₁ block, a higher peak ODC level, and a shorter time period for achievement of the peak ODC value than does the normal fibroblast. Low concentrations of microtubule depolymerizing agents like colchicine suppress ODC induction almost completely in the normal fibroblast, but hardly at all in the HeLa or CHO cells. Both transformed cells occasionally reveal a superinduction of ODC at very low colchicine levels (10^?8-10^?7 M) and a more variable response to such agents than does the normal fibroblast. Higher concentrations of colchicine suppress ODC induction in all cells. Experiments with actinomycin D and cycloheximide indicate that the principal colchicine action involves inhibition at the level of protein or mRNA synthesis, rather than inactivation of the already synthesized enzyme. These experiments are provisionally interpreted as an indication that a microtubular system is needed to reinitiate certain steps associated with growth in G₁-blocked, normal cells, and that a second microtubular action terminating enzyme biosynthesis may exist. This microtubular control is defective in the transformed cells here studied. Specific microtubular actions necessary for initiation and termination of protein syntheses may occur throughout the cell reproductive cycle, and in the course of normal differentiation processes.     

Sensitivity to DNA-damaging agents and mutation induction by UV light in UV-sensitive CHO cells

Three UV-sensitive (UVs) mutants isolated from a CHO cell line were analyzed for survival after exposure to H2O2, EMS, MMC, CCNU, X-rays and for mutation induction after UV-irradiation. The UVs mutants showed normal sensitivities to EMS and H2O2, whereas they were hypersensitive to the bifunctional alkylating agents MMC and CCNU and to hypoxic X-irradiation. Compared to parental cells, one of the UV-sensitive clones showed approximately 3- and 7-fold enhancement in the mutagenic response per unit UV dose for 6-thioguanine and ouabain resistance, respectively.     

Hamster and rat fetal cells have low spontaneous mutation frequencies and rates

Somatic cells of whole Syrian hamster fetuses (gestation day 13) were isolated and tested by an in vivo/in vitro mutation assay for spontaneous mutation frequencies using independent 6-thioguanine (6-TG), diphtheria toxin (DT), and ouabain mutation selection systems. Optimum conditions were ascertained. For 6-TG mutants, a total of 21 mutants were found in cells from 24 litters on 1993 plates, for an overall mutant frequency of 1.8 x 10(-7) per viable cell with 12 positive litters. In all, 26 litters were tested using DT; 77 mutants were found in 840 plates, yielding an overall mutant frequency of 2.6 x 10(-7), with 20 positive litters. No correlations or familial effects were found among 23 litters tested for both DT and 6-TG. Of 14 litters which were tested for ouabain mutants, 4 were positive, with a total of 5 mutants found on 988 plates, for an overall mutant frequency of 7.6 x 10(-8). For 14 F344 rat fetuses, the overall 6-TG spontaneous mutation frequency was determined to be 1.6 x 10(-7). From the data, estimates of mutation rates were calculated. For mutation to 6-TG resistance the rate was 8.3 x 10(-8), for mutation to DT resistance the rate was 8.1 x 10(-8) and for ouabain, the spontaneous mutation rate was 5.7 x 10(-8). For F344 rat, the spontaneous mutation rate was 1.1 x 10(-7). Induced mutant frequencies after in utero exposure to 1 mmol/kg N-ethyl-N-nitrosourea (ENU) were 311, 135 and 200 times the spontaneous value for 6-TG, DT and ouabain, respectively, for Syrian hamster fetal cells and 125 times the spontaneous 6-TG value for fetal F344 rat cells. Both spontaneous mutation frequencies and underlying spontaneous mutation rates are low, consistent with the view that fetal cells exercise extremely tight control over DNA fidelity.     

p53 and Translation Attenuation Regulate Distinct Cell Cycle Checkpoints during Endoplasmic Reticulum (ER) Stress

Cell cycle checkpoints ensure that proliferation occurs only under permissive conditions, but their role in linking nutrient availability to cell division is incompletely understood. Protein folding within the endoplasmic reticulum (ER) is exquisitely sensitive to energy supply and amino acid sources because deficiencies impair luminal protein folding and consequently trigger ER stress signaling. Following ER stress, many cell types arrest within the G₁ phase, although recent studies have identified a novel ER stress G₂ checkpoint. Here, we report that ER stress affects cell cycle progression via two classes of signal: an early inhibition of protein synthesis leading to G₂ delay involving CHK1 and a later induction of G₁ arrest associated both with the induction of p53 target genes and loss of cyclin D1. We show that substitution of p53/47 for p53 impairs the ER stress G₁ checkpoint, attenuates the recovery of protein translation, and impairs induction of NOXA, a mediator of cell death. We propose that cell cycle regulation in response to ER stress comprises redundant pathways invoked sequentially first to impair G₂ progression prior to ultimate G₁ arrest.     

p16INK4A Sensitizes Human Leukemia Cells to FAS- and Glucocorticoid-induced Apoptosis via Induction of BBC3/Puma and Repression of MCL1 and BCL2

Loss of CDKN2A/p16^INK4A in hematopoietic stem cells is associated with enhanced self-renewal capacity and might facilitate progression of damaged stem cells into pre-cancerous cells that give rise to leukemia. This is also reflected by the frequent loss of the INK4A locus in acute lymphoblastic T-cell leukemia. T-cell acute lymphoblastic leukemia cells designed to conditionally express p16^INK4A arrest in the G₀/G₁ phase of the cell cycle and show increased sensitivity to glucocorticoid- and tumor necrosis factor receptor superfamily 6-induced apoptosis. To investigate the underlying molecular mechanism for increased death sensitivity, we interfered with specific steps of apoptosis signaling by expression of anti-apoptotic proteins. We found that alterations in cell death susceptibility resulted from changes in the composition of pro- and anti-apoptotic BCL2 proteins, i.e. repression of MCL1, BCL2, and PMAIP1/Noxa and the induction of pro-apoptotic BBC3/Puma. Interference with Puma induction by short hairpin RNA technology or retroviral expression of MCL1 or BCL2 significantly reduced both glucocorticoid- and FAS-induced cell death in p16^INK4A-reconstituted leukemia cells. These results suggest that Puma, in concert with MCL1 and BCL2 repression, critically mediates p16^INK4A-induced death sensitization and that in human T-cell leukemia the deletion of p16^INK4A confers apoptosis resistance by shifting the balance of pro- and anti-apoptotic BCL2 proteins toward apoptosis protection.     

In vitro effects of 2‐methoxyestradiol on cell numbers,morphology, cell cycle progression,and apoptosis induction in oesophageal carcinoma cells

The influence of 2‐methoxyestradiol (2‐ME) was investigated on cell numbers, morphology, cell cycle progression, and apoptosis induction in an oesophageal carcinoma cell line (WHCO3). Dose‐dependent studies (1 × 10^?9M–1 × 10^?6M) revealed that 2‐ME significantly reduced cell numbers to 60% in WHCO3 after 72 h of exposure at a concentration of 1 × 10^?6M compared to vehicle‐treated cells. Morphological studies entailing light‐, fluorescent‐, as well as transmission electron microscopy (TEM) confirmed 2‐ME's antimitotic effects. These results indicated hallmarks of apoptosis including cell shrinkage, hypercondensation of chromatin, cell membrane blebbing, and apoptotic bodies in treated cells. Flow cytometric analyses demonstrated an increase in the G₂/M‐phase after 2‐ME exposure; thus preventing cells from proceeding through the cell cycle. β‐tubulin immunofluorescence revealed that 2‐ME caused spindle disruption. In addition, increased expression of death receptor 5 protein was observed further supporting the proposed mechanism of apoptosis induction via the extrinsic pathway in 2‐ME‐exposed oesophageal carcinoma cells. Copyright © 2009 John Wiley & Sons, Ltd.     

SYNCHRONIZATION OF MITOCHONDRIAL DNA SYNTHESIS IN CHINESE HAMSTER CELLS (LINE CHO) DEPRIVED OF ISOLEUCINE

Mitochondrial DNA (mit-DNA) synthesis was compared in suspension cultures of Chinese hamster cells (line CHO) whose cell cycle events had been synchronized by isoleucine deprivation or mitotic selection. At hourly intervals during cell cycle progression, synchronized cells were exposed to tritiated thymidine ([³H]TdR), homogenized, and nuclei and mitochondria isolated by differential centrifugation. Mit-DNA and nuclear DNA were isolated and incorporation of radioisotope measured as counts per minute ([³H]TdR) per microgram DNA. Mit-DNA synthesis in cells synchronized by mitotic selection began after 4 h and continued for approximately 9 h. This time-course pattern resembled that of nuclear DNA synthesis. In contrast, mit-DNA synthesis in cells synchronized by isoleucine deprivation did not begin until 9–12 h after addition of isoleucine and virtually all [³H]TdR was incorporated during a 3-h interval. We have concluded from these results that mit-DNA synthesis is inhibited in CHO cells which are arrested in G₁ because of isoleucine deprivation and that addition of isoleucine stimulates synchronous synthesis of mit-DNA. We believe this method of synchronizing mit-DNA synthesis may be of value in studies of factors which regulate synthesis of mit-DNA.