Radiolabeling of DNA can induce its fragmentation in HL-60 human promyelocytic leukemic cells

Visualization of DNA Replication in Single Chromosome by Stable Isotope Labeling

Among the inheritance of cellular components during cell division, deoxyribonucleic acid (DNA) and its condensate (chromosome) are conventionally visualized using chemical tag-labeled nucleotide analogs. However, associated mutagenesis with nucleotide analogs in the visualization of chromosomes is cause for concern. This study investigated the efficiency of using stable isotope labels in visualizing the replicating cultured human cell-chromosomes, in the absence of analog labels, at a high spatial resolution of 100 nm. The distinct carbon isotope ratio between sister chromatids reflected the semi-conservative replication of individual DNA strands through cell cycles and suggested the renewal of histone molecules in daughter chromosomes. Thus, this study provides a new, powerful approach to trace and visualize cellular components with stable isotope labeling.Key words: stable isotope, chromosome replication, semi-conservative replication, imaging, mass spectrometry.

Pulse-chase analysis of procollagen biosynthesis by azidohomoalanine labeling

Disruptions in procollagen synthesis, trafficking and secretion by cells occur in multiple connective tissue diseases. Traditionally, these disruptions are studied by pulse-chase labeling with radioisotopes. However, significant DNA damage, excessive accumulation of reactive oxygen species and formation of other free radicals have been well documented in the literature at typical radioisotope concentrations used for pulse-chase experiments. Therefore, it is important to keep in mind that the resulting cell stress response might affect interpretation of the data, particularly with respect to abnormal function of procollagen-producing cells. In this study, we describe an alternative method of pulse-chase procollagen labeling with azidohomoalanine, a noncanonical amino acid that replaces methionine in newly synthesized protein chains and can be detected via highly selective click chemistry reactions. At least in fibroblast culture, this approach is more efficient than traditional radioisotopes and has fewer, if any, unintended effects on cell function. To illustrate its applications, we demonstrate delayed procollagen folding and secretion by cells from an osteogenesis imperfecta patient with a Cys substitution for Gly766 in the triple helical region of the α1(I) chain of type I procollagen.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

Alpha7-nicotinic acetylcholine receptors affect growth regulation of human mesothelioma cells: role of mitogen-activated protein kinase pathway

This study presents data suggesting that both human mesothelioma (cell lines and human mesothelioma biopsies) and human normal mesothelial cells express receptors for acetylcholine and that stimulation of these receptors by nicotine prompted cell growth via activation of nicotinic cholinergic receptors. Thus, these data demonstrate that: (a) human mesothelioma cells and human biopsies of mesothelioma as well as of normal pleural mesothelial cells express functionally alpha-7 nicotinic acethlycholine receptors, evaluated by alpha-bungarotoxin-FITC binding, receptor binding assay, Western blot, and reverse transcription-PCR; (b) choline acetyltransferase immunostaining is present in mesothelioma cells; (c) mesothelioma cell growth is modulated by the cholinergic system in which agonists (i.e., nicotine) has a proliferative effect, and antagonists (i.e., curare) has an inhibitory effect, evaluated by cell cloning, DNA synthesis and cell cycle; (d) nicotine induces Ca(+2) influx, evaluated by [(45)Ca(2+)] uptake, and consequently activation of mitogen-activated protein kinase pathway (extracellular signal-regulated kinase and p90(RSK) phosphorylation), evaluated by Western blot; and (e) apoptosis mechanisms in mesothelioma cells are under the control of the cholinergic system (nicotine antiapoptotic via induction of nuclear factor-kappaB complexes and phosphorylation of Bad at Ser(112); curare proapoptotic via G(0)-G(1) arrest p21(waf-1) dependent but p53 independent). The involvement of the nonneuronal cholinergic system in mesothelioma appears reasonable and open up new therapeutic strategies.

Adenosine stimulation of proliferation of breast carcinoma cell lines: Evaluation of the [3H]thymidine assay system and modulatory effects of the cellular microenvironment in vitro

The purine nucleoside adenosine is produced at increased levels in the tissues of solid cancers as a result of local hypoxia. Adenosine inhibits the cell-mediated anti-tumor immune response, promotes tumor cell migration and angiogenesis, and stimulates the proliferation of tumor cells. We examined the stimulatory effect of adenosine on DNA synthesis, cell cycle progression, and cell proliferation in MCF7 and T-47D breast carcinoma cell lines in culture, and identified factors that modulate the growth response. The ability of adenosine to stimulate DNA synthesis, as measured by the incorporation of [(3)H]thymidine, was independent of the total radioactivity of the [(3)H]thymidine up to 10 microCi/ml, total thymidine concentrations up to 100 microM, and the labeling interval. It was also not affected by the presence of low-molecular-weight compounds (such as thymidine and adenosine) in the serum used to supplement the medium. Adenosine stimulated DNA synthesis and cell proliferation with an EC(50) of 4-6 microM and a maximum response at 30-100 microM, when given as a single addition. The stimulatory effect of adenosine involved progression through the cell cycle and a genuine increase in cell number, in the absence of significant apoptotic or necrotic cell death. The mitogenic effect of adenosine was dependent upon the culture cell density, with an optimum adenosine response at around 50% of confluent density. The response was also highly dependent upon the form of the serum addition to the growth medium, with the best response elicited in the presence of low concentrations of nonfetal bovine serum, although adenosine was mitogenic under standard culture conditions. The effects of serum supplementation and cell density were not due to differences in the rate of adenosine metabolism by either serum or cellular enzymes, but appeared to result from changes in the sensitivity to adenosine of the cell population in response to environmental cues. We, therefore, find that adenosine is consistently mitogenic for human breast carcinoma cells, and that the [(3)H]thymidine incorporation assay is a valid measure of this response. The data are consistent with the stimulatory effect of adenosine on cell proliferation being modulated by the local cellular environment.

Does metabolic radiolabeling stimulate the stress response? Gene expression profiling reveals differential cellular responses to internal beta vs. external gamma radiation

DNA microarray analyses were used to investigate the effect of cell-incorporated 35S-methionine on human colorectal carcinoma cells. This beta-radiation-induced gene expression profile was compared with that induced by external gamma-radiation. The extent of DNA fragmentation was used as a biomarker to determine the external gamma dose that was bioequivalent to that received by cells incubated in medium containing 35S-methionine. Studies showed that 35S-methionine at 100 microCi/mL induced a much more robust transcriptional response than gamma-radiation (2000 cGy) when evaluated 2 h after the labeling or irradiation period. The cellular response to internal beta-radiation was greater not only with respect to the number of genes induced, but also with respect to the level of gene induction. Not surprisingly, the induced genes overlapped with the set of gamma-responsive genes. However, a distinct beta-gene induction profile that included a large number of cell adhesion proteins was also observed. Taken together, these studies demonstrate that metabolic incorporation of a low energy beta-emitter, such as 35S-methionine, can globally influence a diverse set of cellular activities that can, in turn, affect the outcome of many experiments by altering the cell cycle, metabolic, signaling, or redox status (set point) of the cell. Additional studies of the mechanism of beta-induced proliferation arrest and cell death and of the significance of its differential gene induction/repression profile in comparison to pulsed gamma-irradiation may lead to new insights into the ways in which ionizing radiation can interact with cells.

3H-thymidine is a defective tool with which to measure rates of DNA synthesis

Metabolic incorporation of 3H-thymidine into cellular DNA is a widely used protocol to monitor rates of DNA synthesis and cell proliferation. However, this radiochemical has also been reported to induce cell-cycle arrest and apoptosis in addition to DNA damage. Using stable isotope-labeled thymidine, we demonstrate that 3H-thymidine induces dose-dependent inhibition of the rate of DNA synthesis. This inhibition occurred within the first round of replication after addition of the radiolabeled tracer and demonstrates the cytotoxic effects of conventional doses of 3H-thymidine (typically greater than or equal to 1 microCi/ml). These results thus show that stable isotope methods are superior to radioisotopes for determining rates of DNA synthesis and cell replication. Because 3H-thymidine perturbs the very process it was employed to study, experiments using 3H-thymidine to monitor DNA synthesis and cell proliferation should be interpreted with caution.

Metabolic radiolabeling: experimental tool or Trojan horse? (35)S-Methionine induces DNA fragmentation and p53-dependent ROS production

Despite the general assumption that widely used radiolabeled metabolites such as [(35)S]methionine and (3)H-thymidine do not adversely affect or perturb cell function, we and others have shown that such low-energy beta-emitters can cause cell cycle arrest and apoptosis of proliferating cells. The goal of the present study was to elucidate the targets and mechanisms of [(35)S]methionine-induced cellular toxicity. Comet analyses (single-cell electrophoresis) demonstrated dose-dependent DNA fragmentation in rabbit smooth muscle cells within a time frame (1-4 h) well within that of most radiolabeling protocols, whereas fluorescence analyses using a peroxide/hydroperoxide-sensitive dye revealed production of reactive oxygen species (ROS). Although ROS generation was inhibitable by antioxidants, DNA fragmentation was not inhibited and was in fact observed even under hypoxic conditions, suggesting that beta-radiation-induced DNA damage can occur independently of ROS formation. Studies with p53(+/+) and p53(-/-) human colorectal carcinoma cells further demonstrated the dissociation of early DNA damage from ROS formation in that both cell types exhibited DNA fragmentation in response to radiolabeling whereas only the p53(+/+) cells exhibited significant increases in ROS formation, which occurred well after significant DNA damage was observed. These findings demonstrate that metabolically incorporated low-energy beta-emitters such as [(35)S]methionine and (3)H-thymidine can induce DNA damage, thereby initiating cellular responses leading to cell cycle arrest or apoptosis. The results of this study require a reevaluation using low-energy beta-emitters to follow not only experimental protocols in vivo processes, but also acceptable exposure levels of these genotoxic compounds in the workplace and environment.

Stable labeled microspheres to measure perfusion: validation of a neutron activation assay technique

Neutron activation is an accurate analytic method in which trace quantities of isotopes of interest in a sample are activated and the emitted radiation is measured with high-resolution detection equipment. This study demonstrates the application of neutron activation for the measurement of myocardial perfusion using stable isotopically labeled microspheres. Stable labeled and standard radiolabeled microspheres (15 microm) were coinjected in an in vivo rabbit model of myocardial ischemia and reperfusion. Radiolabeled microspheres were detected with a standard gamma-well counter, and stable labeled microspheres were detected with a high-resolution Ge detection after neutron activation of the myocardial and reference blood samples. Regional myocardial blood flow was calculated from the deposition of radiolabeled and stable labeled microspheres. Both sets of microspheres gave similar measurements of regional myocardial blood flow over a wide range of flow with a high linear correlation (r = 0.95-0.99). Neutron activation is capable of detecting a single microsphere in an intact myocardial sample while providing simultaneous quantitative measurements of multiple isotope labels. This high sensitivity and capability for measuring perfusion in intact tissue are advantages over other techniques, such as optical detection of microspheres. Neutron activation also can provide an effective method for reducing the production of low-level radioactive waste generated from biomedical research. Further applications of neutron activation offer the potential for measuring other stable labeled compounds, such as fatty acids and growth factors, in conjunction with microsphere measured flow, providing the capability for simultaneous measurement of regional metabolism and perfusion.

Radiolabeling revisited: metabolic labeling with (35)S-methionine inhibits cell cycle progression, proliferation, and survival

Metabolic labeling of cells with low-energy beta-emitting radioisotopes such as [(35)S]methionine is often used to follow the biosynthesis, maturation, and degradation of proteins in vivo. Such techniques have generally been assumed to be relatively nonperturbing to the cell. The results presented here indicate that metabolic labeling of cells with [(35)S]methionine under standard experimental conditions can inhibit cell progression into mitosis, cause cell cycle arrest, inhibit cell proliferation in both short-term and colony-forming assays, alter cell morphology, and induce apoptosis over the course of several days. These results thus suggest the need for caution in interpretation of studies using such methods, especially if the experiments rely on the normal progression of the cell cycle or are intended to monitor events occurring in a normally proliferating cell.

Evaluation of cell death in EBV-transformed lymphocytes using agarose gel electrophoresis, light microscopy and electron microscopy. II. Induction of non-classic apoptosis ("para-apoptosis") by tritiated thymidine

There is an extensive literature dating back to the late 1950's, on the damaging biological effects of radiolabeling DNA in vivo. Nonetheless, tritiated thymidine has often been used to label DNA in studies of programmed cell death (apoptosis). In the present study, we have investigated the effects of incorporation of tritiated thymidine into the DNA of an Epstein-Barr virus-transformed cell line (NC-37) in the absence of any other apoptosis-inducing agent. Cells were incubated in media containing 1-20 microCi/ml [methyl-3H]-thymidine ([3H]-TdR). At each concentration of tritiated thymidine used, cell proliferation ceased within 12 hours of incubation. The mode of cell death caused by tritiated thymidine incorporation was evaluated using DNA degradation patterns and cellular morphology. DNA degradation, in the absence of a "ladder" pattern, was shown by agarose gel electrophoresis. Electron microscopy was used as the "gold standard" to evaluate the specific morphologic type of cell death that accompanied the DNA degradation. Although some of the features of apoptosis were present, the cells lacked the early margination of the chromatin within an intact nucleus and surface blebbing leading to apoptotic body formation, two characteristic morphological features of apoptosis. We, therefore, coined the term "para-apoptosis" to be more precise about the morphologic type of cell death. The percent of para-apoptotic cells was quantitated by light microscopy using whole mount preparations (cytospins). The morphologic criteria of chromatin condensation, nuclear fragmentation, increase in cell density and cytoplasmic vacuolization were used for the evaluation of para-apoptosis by light microscopy of cytospin preparations. In the absence of tritiated thymidine, < 2% of the cells became apoptotic/para-apoptotic after 43 hours of incubation. However, at all concentrations of tritiated thymidine used in the incubation medium (1-20 microCi/ml), the number of para-apoptotic cells increased. In addition, we detected perturbations in the timing of the cell cycle of the surviving cells and an increase in the number of micronuclei after only one division cycle. The induction of para-apoptosis and micronuclei formation represent two distinct modes of cell death caused by tritiated thymidine incorporation. These studies emphasize the necessity for morphological examination in characterizing the induction of cell death in a new experimental system.

Elevated levels of wild-type p53 induced by radiolabeling of cells leads to apoptosis or sustained growth arrest

BACKGROUND

The tumor suppressor protein p53 regulates progression through the checkpoint between the G1 and S phases of the cell cycle in response to radiation- or drug-induced DNA damage. We have examined potential p53-mediated effects of metabolically labeling cultured mammalian cells with [35S]methionine and [3H]thymidine, methods that are commonly used to study the biochemical properties, synthesis, processing and degradation of proteins and the replication of DNA in proliferating cells.

RESULTS

Wild-type p53 protein concentrations rapidly increased to high levels following metabolic radiolabeling of cells, as determined by four distinct assays. The increased concentration of wild-type p53 resulted in apoptosis of normal human peripheral blood lymphocytes and of murine T-cell acute lymphoblastic leukemia cells. In leukemia cells containing no p53, or only mutant p53 alleles, p53 protein levels were not increased and the cells did not undergo apoptosis in response to radioactive labeling. Radiolabeling of human diploid fibroblasts resulted in a prolonged growth arrest that was maintained for nearly three weeks.

CONCLUSIONS

The results of experiments employing radiolabeling techniques to characterize various aspects of cellular physiology may be seriously influenced by the induction of aberrant cell-cycle arrest and/or apoptosis mediated by wild-type p53. Furthermore, our observations suggest that stabilization of wild-type p53 in response to irradiation may not act primarily to facilitate the repair of DNA damage by inducing a transient G1-phase arrest, but rather to ensure genetic stability through sustained cell-cycle arrest or apoptotic death of the damaged cells.

Apoptosis induced by DNA topoisomerase I and II inhibitors in human leukemic HL-60 cells

The induction of apoptosis following topoisomerase inhibitors proceeds in at least three distinct steps: (1) induction of cleavable complexes (potentially lethal damage), (2) topoisomerase-induced DNA damage, and (3) a presently unknown sequence of events that must either lead to cell cycle arrest (G2-block, differentiation) or apoptosis. DNA degradation provides a convenient way to quantify apoptosis in HL-60 cells. Extensive apoptosis can be induced rapidly in undifferentiated HL-60 cells without prevention by cycloheximide or actinomycin D. Therefore, HL-60 cells appear to express constitutively the apoptotic machinery that may be kept under control of a yet unknown repressor. The absence of the tumor suppressor p53 and the presence of bcl-2 are in contrast with the sensitivity of these cells to apoptosis. Agents that modify chromatin structure (zinc, poly[ADPribose] inhibitors, spermine) can block DNA fragmentation without affecting cell survival. By contrast macrophage-like differentiation by phorbol esters suppresses apoptosis without affecting topoisomerase-induced DNA damage. Better understanding of the apoptotic regulation in the widely used and characterized HL-60 cell line should allow the identification of new mechanisms and parameters of cellular sensitivity and resistance to the cytotoxic activity of anticancer agents.