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The apoptosis of HEL cells induced by hydroxyurea* †
Abstract
Hydroxyurea has been used to synchronize cultured cells to S-phase and used to treat patients with sickle-cell anemia. Recently, we found that hydroxyurea can induce the apoptosis of HEL (human erythroleukemia) cells. The induced HEL cells showed ultrastructurally chromatin condensation with regular crescents at the nuclear edges and apoptotic bodies. However, the cells of K562, another human erythroleukemia cell line, did not show such morphological changes. Under fluoroscope, the HEL cells after induction often displayed a clear reduction in nuclear diameter and nuclear chromatin cleavage and condensation and the presence of nuclear ring and apoptotic bodies. Analysis with flow cytometry showed that the percentage of apoptotic cells is about 30-40% after HEL cells were induced by hydroxyurea for 3 days. DNA ladder can be observed by electrophoretic analysis.
... To synchronize CSCs and MSCs in the transition from G 1 to S phase, cells were incubated for 12 h with mimosine (MIM; 500 μM), an inhibitor of the cell cycle which induces arrest in G 1 by chelating iron ions and inhibiting DNA replication in mammalian cells [41] or hydroxyurea (HU; 2 mM), which blocks the synthesis of deoxynucleotides inhibiting DNA synthesis, inducing synchronization in S- phase [42]. In order to synchronize cells in the transition from G 2 to M, cells were treated with demecolcine (DC; 20 ng/ml), which arrests cells in metaphase [43]. ...
... We also used other cell cycle inhibitors (HU and NOC) to verify if the shift in the total number of cells presenting spontaneous Ca 2+ oscillations was due to a reaction to DC and MIM that is unrelated to control of the cell cycle (Fig. 1). To synchronize cells in the G 1 /S transition, we used 2 mM hydroxyurea (HU), a drug that arrests cells in the S phase by blocking deoxynucleotides synthesis [42]. In order to arrest the cells in the transition from G 2 to M, cells were incubated with NOC (100 ng/ml), a drug that disrupts mitotic spindles [44,48]. ...
... Some authors speculate that the calcium signals that control mitotic progression may occur in calcium microdomains so localized as to be undetectable in global calcium images. Thus, it is possible that undetectable calcium increases in microdomains initiate mitosis in reports which suggest that global calcium changes are not relevant for mitosis [42,76,77], although future studies are need the clarify this issue. In the present study we have demonstrated that undifferentiated CSCs and adult MSCs exhibit IP 3 R-mediated [Ca 2+ ] i oscillations, while in NPCs both IP 3 Rs and RyRs mediate these oscillations. ...
Spontaneous Ca(2+) events have been observed in diverse stem cell lines, including carcinoma and mesenchymal stem cells. Interestingly, during cell cycle progression, cells exhibit Ca(2+) transients during the G(1) to S transition, suggesting that these oscillations may play a role in cell cycle progression. We aimed to study the influence of promoting and blocking calcium oscillations in cell proliferation and cell cycle progression, both in neural progenitor and undifferentiated cells. We also identified which calcium stores are required for maintaining these oscillations. Both in neural progenitor and undifferentiated cells calcium oscillations were restricted to the G1/S transition, suggesting a role for these events in progression of the cell cycle. Maintenance of the oscillations required calcium influx only through inositol 1,4,5-triphosphate receptors (IP(3)Rs) and L-type channels in undifferentiated cells, while neural progenitor cells also utilized ryanodine-sensitive stores. Interestingly, promoting calcium oscillations through IP(3)R agonists increased both proliferation and levels of cell cycle regulators such as cyclins A and E. Conversely, blocking calcium events with IP(3)R antagonists had the opposite effect in both undifferentiated and neural progenitor cells. This suggests that calcium events created by IP(3)Rs may be involved in cell cycle progression and proliferation, possibly due to regulation of cyclin levels, both in undifferentiated cells and in neural progenitor cells.
... The reduction in the pool sizes of dNTP inhibits DNA replication in tumor cells. It has been reported that treatment of chronic myelogenous leukemia lymphocytes [2] and human erythroleukemia cells [10] with HU induces apoptosis. Gottifredi et al. [8] showed that p53 protein accumulates after treatment of human colorectal carcinoma cells with HU. ...
... It has been suggested that the efficacy of many anticancer drugs is related to the propensity of target tumor cells to respond to these drugs by undergoing apoptosis [4,5,15]. Although it has been reported that the treatment of chronic myelogenous leukemia lymphocytes [2] and human erythro- leukemia cells [10] with HU induces apoptosis, HU did not induce apoptosis in a hamster fibrosarcoma cell line [2]. Furthermore, it has been reported that HU-induced cell cycle arrest is highly reversible and that cellular morphologies remain relatively normal [3]. ...
Hydroxyurea (HU), an anticancer drug, inhibits ribonucleoside diphosphate reductase and reduces pool sizes of deoxyribonucleoside triphosphate (dNTP). The reduction of dNTP results in inhibition of DNA replication. The cytotoxic effect of HU was investigated using fibroblast cell lines from LEC rats. LEC rat cells showed significantly higher sensitivity to HU than did cell lines from control WKAH rats. No significant differences were observed between the percentages of apoptotic cells in either LEC or WKAH rat cells that had been treated with HU and those that had not been treated with HU. LEC rat cells also showed significantly higher sensitivity to aphidicolin, which blocks DNA synthesis by inhibiting DNA polymerase alpha, than did WKAH rat cells. In both LEC and WKAH rat cells, intensified bands of p53 protein were observed immediately after treatment with HU. Although the high level of p53 protein persisted in WKAH rat cells until 6 hr post-incubation time after treatment with HU, the level of p53 protein had decreased at 6 hr post-incubation time in LEC rat cells. When the cells were X-irradiated in the absence or presence of HU, the ratio of the surviving fraction without HU to that with HU only slightly increased after X-irradiation in WKAH rat cells. In contrast, the ratio in LEC rat cells significantly increased after X-irradiation in a dose-dependent manner.
... Although by no means as conclusive as the experiments suggested in the previous paragraph, these observations may provide insight as to how hSSB1 functions following replication disruption. The cell death observed following hydroxyurea treatment is likely to occur through an apoptotic response (Gui et al., 1997), most probably due to the formation of doublestrand DNA breaks. Indeed, in a recent publication (Couch et al., 2013), rapid hydroxyurea-induced cell death was observed following inhibition of the ATR kinase, conditions under which double-strand DNA breaks rapidly formed. ...
... To investigate whether apoptosis induction of CPX might be mediated via RR inhibition, apoptosis was measured upon treatment with the RR inhibitor HU. Although some studies demonstrate the apoptosis-inducing effect of HU [280][281][282][283], it was not observed in SiHa cells, suggesting that CPX-induced apoptosis might not be linked to RR inhibition. ...
Oncogenic human papillomavirus (HPV) types are major human carcinogens. Sustained expression of the viral E6 and E7 oncogenes is essential for the malignant growth of HPV-positive cancer cells. Moreover, cancer cells typically exhibit metabolic alterations. Surprisingly little is known about the relation between the HPV oncogenes and the cellular iron metabolism although it is increasingly recognised to be linked to tumourigenesis. Thus, the overall aim of the presented studies was to investigate the crosstalk between oncogenic HPVs and the iron metabolism of their tumourigenic host cells. Interestingly, treatment of HPV-positive cervical cancer cells with the iron chelators deferoxamine (DFO) or ciclopirox (CPX) strongly suppresses expression of the viral E6/E7 oncogenes, suggesting therapeutic potential for these agents. Further detailed work was focused on CPX since (in contrast to DFO) this drug can be applied topically and thus could be suitable for the treatment of HPV-induced (pre)neoplasias. It was found that CPX efficiently blocks cellular proliferation in both 2D and 3D cell culture, induces cell cycle arrest in the G1 and S phase and ultimately leads to cellular senescence. Both E6/E7 repression and senescence are a consequence of iron deprivation since they can be prevented by excess iron. Notably, although mTORC1 signalling is widely believed to be required for senescence induction, the pro-senescent activity of CPX is independent of mTORC1 signalling. Proteome analyses revealed candidate proteins which might be involved in the CPX-induced growth arrest and senescence. Moreover, prolonged treatment of HPV-positive cervical cancer cells with CPX results in apoptosis which is observable under both normoxic and hypoxic conditions. Since CPX exerts pro-apoptotic effects also under hypoxia, it could be used in combination with radio- and chemotherapy which both are typically less active against hypoxic tumour cells. Taken together, these results reveal that HPV oncogene expression is very sensitive to iron deprivation. CPX could be a particularly promising agent for the treatment of HPV-positive cancers since it represses E6/E7, induces senescence and apoptosis under normoxic conditions and can also eliminate hypoxic cells which pose a major problem for the efficacy of anti-cancer drugs in the clinic.
... Badania limfocytów uzyskanych od chorych na przewlekłą białaczkę szpikową, inkubowanych z cytostatykami w tym i hydroksymocznikiem wykazały nasilenie apoptozy oraz fragmentację DNA [3]. Podobne wyniki badań otrzymano inkubując komórki ludzkiej białaczki erytroblastycznej (erythroblastosis acuta essentialis) z hydroksymocznikiem [16]. Jednakże, komórki B limfoblastycznej linii transformowanej wirusem Epsteina-Barr inkubowane z HU nie wykazywały fragmentacji DNA ani aktywacji kaspaz. ...
It has been shown that hydroxyurea (HU) induces free radicals which exert cytotoxic effects on cells. However, the molecular mechanism of this effect is still obscure. A possible mechanism for HU-mediated cell death induced by unbalanced growth could be the overproduction, altered accumulation, and progressive leakage of many hydrolytic enzymes through the cell membrane, leading to "lytic" cell death. Protection by antioxidants and radicals scavengers does not change the inhibition of DNA synthesis dependent on ribonucleotide reductase, the main mechanism of the antineoplastic activity of HU. These observations support the hypothesis that membranes are targets for the side effects of HU toxicity. Damage of erythrocytes and granulocytes seems to be an important clinical feature of the HU side toxicity generating free radicals.
The biologic processes of apoptosis and angiogenesis are linked in endothelial biology because some endothelial cell growth factors also exert anti-apoptotic effects. We studied whether apoptosis is occurring in circulating endothelial cells (CEC) that have lost the survival signals derived from anchorage to extracellular matrix. Consistent with this expectation, 64% ± 16% of CEC from normal donors showed evidence of apoptosis (by morphology and TdT-mediated dUTP nick end labeling [TUNEL] assay). However, only 30% ± 15% (P < .001 v normal) of CEC from donors with sickle cell anemia were apoptotic. Vascular endothelial growth factor (VEGF) levels were significantly (P = .001) higher in plasma of sickle donors (120.1 ± 81.4 pg/mL) than that of normal donors (37.6 ± 34.6 pg/mL), and there was an inverse correlation between VEGF and CEC apoptosis (r = .612,P = .001). Consistent with stimulation by VEGF, CEC from sickle donors exhibited increased expression of vβ3. In vitro experiments showed that VEGF inhibits apoptosis for cultured endothelial cells that are kept unanchored and not allowed to re-establish attachment to extracellular matrix, thus demonstrating that VEGF provides survival signals independent of its ability to promote matrix reattachment. These data suggest the hypothesis that sickle cell anemia is a state of enhanced anti-apoptotic tone for endothelial cells. If true, this has implications for disease pathobiology, particularly the development of neovascularizing retinopathy.
Electron-microscopy study of the ciliary epithelium structure of the mollusk Lymnaea stagnalis was carried out under the action of hydroxyurea. By the method of radioautography, a high proliferative activity of the ciliary epithelium was established as the norm; a cluster distribution of cells, including the label, was noted. The presence of hydroxyurea in the mollusk organism was shown to inhibit proliferation. Scanning electron microscopy of the molluskan foot surface revealed clusters of nonciliated cells and of cells with short villi in control epithelial folds. Under hydroxyurea treatment for 24 h, such sites disappeared completely and ciliary epithelium looked uniform and was composed of cells with long cilia. By transmission electron microscopy, it was established that hydroxyurea did not affect the formation of the basal body and course of ciliogenesis. It has been suggested that hydroxyurea not only inhibits proliferative activity of epithelial cells, but also induces differentiation of unciliated into the ciliated cells.
The biologic processes of apoptosis and angiogenesis are linked in endothelial biology because some endothelial cell growth factors also exert anti-apoptotic effects. We studied whether apoptosis is occurring in circulating endothelial cells (CEC) that have lost the survival signals derived from anchorage to extracellular matrix. Consistent with this expectation, 64% +/- 16% of CEC from normal donors showed evidence of apoptosis (by morphology and TdT-mediated dUTP nick end labeling [TUNEL] assay). However, only 30% +/- 15% (P <.001 v normal) of CEC from donors with sickle cell anemia were apoptotic. Vascular endothelial growth factor (VEGF) levels were significantly (P =.001) higher in plasma of sickle donors (120.1 +/- 81.4 pg/mL) than that of normal donors (37.6 +/- 34.6 pg/mL), and there was an inverse correlation between VEGF and CEC apoptosis (r =. 612, P =.001). Consistent with stimulation by VEGF, CEC from sickle donors exhibited increased expression of alphavbeta3. In vitro experiments showed that VEGF inhibits apoptosis for cultured endothelial cells that are kept unanchored and not allowed to re-establish attachment to extracellular matrix, thus demonstrating that VEGF provides survival signals independent of its ability to promote matrix reattachment. These data suggest the hypothesis that sickle cell anemia is a state of enhanced anti-apoptotic tone for endothelial cells. If true, this has implications for disease pathobiology, particularly the development of neovascularizing retinopathy.
Hydroxyurea (HU) is an inhibitor of nucleotide synthesis extensively used to control the chronic phase of myeloid leukemia. This antimetabolite has been employed in the clinic for several decades but in recent years the leukemogenic potential of HU has been suspected. In the present study, a B-lymphoblastoid cell line transformed by the Epstein-Barr virus was used to investigate the apoptotic effects of HU and delineate some of the molecular pathways implicated in the cytotoxic action. The cell line, characterized by immunophenotyping, cytogenetic and fluorescence in situ hybridization (FISH) studies, showed no chromosomal abnormalities, even after a prolonged exposure to HU. Different flow cytometry assays were used to measure HU-induced impairment of the cell cycle, inhibition of DNA synthesis, and the occurrence of apoptosis. The treatment with HU leads to the appearance of a hypo-diploid DNA content peak (sub-G1) characteristic of the apoptotic cell population. The drug also induces a cell block in S phase as measured by 5-bromo-2'-deoxyuridine (BrdU) incorporation. Inhibition of DNA synthesis precedes induction of apoptosis by HU. A drug-induced loss of plasma membrane asymmetry was characterized by flow cytometry using annexin V-FITC to stain phosphatidylserine residues. The implication of the antiapoptotic protein Bcl-2 and the tumor suppressor p53 in the development of HU-mediated apoptosis was also evidenced. The drug appears to promote cell death by regulating the expression levels of these two proteins. Different criteria define the apoptotic response of the lymphoblastoid cells to the treatment with HU. However, the extent of drug-induced cell death is limited, and no DNA fragmentation and no activation of the caspase cascade was observed in this model. Beyond the specific interest in HU-induced apoptosis, the work reported here illustrates the utility of the EBV immortalization process to investigate the pharmacological activity of specific drugs from clinical samples.
Huangqi (Astragalus membranaceus), a traditional Chinese medicine, has been used to ameliorate side effects of cancer chemotherapy in China. However, little
is known about its molecular mechanisms. Here we show that induction of K562 or HEL cells with 1.5 mg/ml of Huangqi (Hex)
(Components extracted from Huangqi) for 3–5 d results in the expression of β-globin gene in both cell lines and leads to terminal
differentiation. Moreover, the apoptosis in HEL cells can be induced by increasing concentration of Huangqi (Hex) to 4.5 mg/ml
for 3–5 d. Upregulation of Apaf-1, caspase-3 and acetylcholinesterase (AChE) in HEL cells may play a crucial role in the process
of apoptosis. The prospect of inducing expression of adult (β) globin gene and apoptosis selectively in cancer cells is obviously
attractive from a therapeutic point of view.
The rapid hypotonic staining procedure developed by Krishan for DNA determinations by flow cytofluorometry has been proven accurate for in vivo cell samples and for cell lines growing in suspension culture. We show that the unmodified procedure may produce distorted DNA histograms when used for staining cells growing in monolayer cultures, however. To eliminate these distortions, it was necessary to avoid the use of trypsin by staining the attached cells directly, using a hypotonic fluorochrome solution to which nonionic detergent was added. Two sublines of HeLa S3 cells are shown to exhibit major differences in their staining characteristics. By using our revised staining procedure, the two sublines appear to produce very satisfactory DNA histograms. However, in only one subline does the S phase fraction calculated from the histograms agree with the autoradiographical labeling index. Mitotic cells remain intact under these staining conditions, and the principal observed effect of nonionic detergents in this case is to decrease the coefficient of variation of fluorescence intensity.
Because fetal hemoglobin contains gammaglobin chains instead of beta chains, it is not affected by the genetic defect that causes sickle cell disease. Increased levels of fetal hemoglobin decrease the tendency toward intracellular polymerization of sickle hemoglobin that characterizes this disease. Hydroxyurea is one of several cytostatic agents that have been shown to increase the production of fetal hemoglobin in some patients with sickle cell disease. We studied the effects of hydroxyurea administration in 10 hospitalized patients with sickle cell disease, each of whom was treated for three months. Seven patients responded with a 2- to 10-fold increase in fetal hemoglobin, from a mean (+/- SD) of 1.6 +/- 1.6 percent of total hemoglobin to 6.8 +/- 4.7 percent; three patients had fetal-hemoglobin levels of 10 to 15 percent of total hemoglobin. Three did not respond to treatment. Four of the patients who responded were retreated with hydroxyurea after one to four months without treatment and were found to have larger increases in fetal-hemoglobin levels. In most patients, levels were still rising at the end of the study, even after 90 days of therapy. Fetal-hemoglobin levels tended to peak at dosages of hydroxyurea that were myelosuppressive. In the patients who responded to treatment, there were significant increases in the percentage of reticulocytes and erythrocytes containing fetal hemoglobin and in the amount of fetal hemoglobin within these cells. The percentage of dense red cells decreased in the patients who responded to treatment. The tendency toward intracellular polymerization at physiologic oxygen saturation was reduced by about 33 percent in the cells containing fetal hemoglobin, whereas there was no change in the other cells. We conclude that hydroxyurea is effective in increasing the production of fetal hemoglobin, which in this study was found to be associated with a small decrease in hemolysis and an increase in hemoglobin levels despite myelosuppression. Controlled, prospective trials are necessary to establish whether these effects will lead to clinical benefit.
To investigate the mechanisms that ensure the dependency relationships between cell cycle events and to investigate the checkpoints that prevent progression through the cell cycle after DNA damage, we have isolated mutants defective in the checkpoint and feedback control pathways. We report the isolation and characterization of 11 new loci that define distinct classes of mutants defective in one or more of the checkpoint and feedback control pathways. Two mutants, rad26.T12 and rad27.T15, were selected for molecular analysis. The null allele of the rad26 gene (rad26.d) shares the phenotype reported for the "checkpoint rad" mutants rad1, rad3, rad9, rad17, and hus1, which are defective in the radiation checkpoint and in the feedback controls that ensure the order of cell cycle events. The null allele of the rad27 gene (rad27.d) defines a new class of Schizosaccharomyces pombe mutant. The rad27 complementing gene codes for a putative protein kinase that is required for cell cycle arrest after DNA damage but not for the feedback control that links mitosis to the completion of prior DNA synthesis (the same gene has recently been described by Walworth et al. (1993) as chk1). These properties are similar to those of the rad9 gene of Saccharomyces cerevisiae. A comparative analysis of the radiation responses in rad26.d, rad26.T12, and rad27.d cells has revealed the existence of two separable responses to DNA damage controlled by the "checkpoint rad" genes. The first, G2 arrest, is defective in rad27.d and rad26.d but is unaffected in rad26.T12 cells. The second response is not associated with G2 arrest after DNA damage and is defective in rad26.d and rad26.T12 but not rad27.d cells. A study of the radiation sensitivity of these mutants through the cell cycle suggests that this second response is associated with S phase and that the checkpoint rad mutants, in addition to an inability to arrest mitosis after radiation, are defective in an S phase radiation checkpoint.
Hydroxyurea is well absorbed after oral administration, converted to a free radical nitroxide in vivo, and transported by diffusion into cells where it quenches the tyrosyl free radical at the active site of the M2 protein subunit of ribonucleotide reductase, inactivating the enzyme. The entire replitase complex, including ribonucleotide reductase, is inactivated and DNA synthesis is selectively inhibited, producing cell death in S phase and synchronization of the fraction of cells that survive. Repair of DNA damaged by chemicals or irradiation is also inhibited by hydroxyurea, offering potential synergy between hydroxyurea and radiation or alkylating agents. Hydroxyurea renders cells sensitive to bleomycin because the quenched tyrosyl free radical no longer stabilizes the adjacent iron center, making it more susceptible to the chelating properties of bleomycin, which then produces active oxygen. Synergy has also been observed between hydroxyurea and a number of other chemotherapeutic agents, including cytarabine and etoposide. Recently, two new effects of hydroxyurea have been observed: hydroxyurea increases the level of fetal hemoglobin, leading to a reduction in the incidence of vasoocclusive crises in sickle cell anemia, and hydroxyurea selectively reduces the level of episomal DNA and thus potentially may reduce drug resistance associated with duplicated genes retained as episomes. Further exploration of the efficacy of hydroxyurea in combination with other therapeutic agents is warranted.
This chapter discusses the mechanisms of programmed cell death (PCD). In many different examples of PCD, regardless of the cell type involved and regardless of the stimulus, the cells undergo similar biochemical and morphological events, and PCD often follows a final common pathway. Evidence for the existence of programmed cell death and cell death program is discussed. Programmed cell death is an old phenomenon that has awakened new interest. It can be demonstrated in various systems.. They all have in common nuclear disintegration, the morphology of apoptosis, DNA damage, and early recognition by phagocytic cells. It is likely that there is a death program that all cells follow when they embark on this pathway, even if the specific (or nonspecific) triggers are very different. There are various stages in a B cell's life when it risks undergoing a programmed death.
Entry into mitosis in fission yeast is controlled by the p34cdc2 protein kinase, which is activated by cdc25+ and inhibited by wee1+. In "wee" mutants one or the other of these controls is circumvented resulting in advancement of mitosis. We report that dependence of mitosis on DNA synthesis is lost in wee mutants in which cdc25+ control is circumvented either by mutations in cdc2+ or by overproduction of cdc25+. In contrast, dependence is maintained when the wee1+ control is bypassed. We propose that cdc25+ activity requires completion of earlier cell-cycle events such as DNA synthesis, and thus links p34cdc2 kinase activation to completion of these earlier events. Constitutive expression of cdc25+ homologs could explain why mitosis is not dependent on DNA replication in some early embryos.
Glucocorticoid hormones kill immature thymocytes by activating a self-destructive process that involves extensive DNA fragmentation. It has been demonstrated that thymocyte suicide is dependent on an early, sustained increase in cytosolic Ca2+ concentration, and new protein synthesis, but the biochemical lesion that leads to cell death has not been established. To determine whether endonuclease activation or activation of another Ca2+-dependent process could mediate cell killing, we treated thymocytes with the glucocorticoid methylprednisolone in the presence of inhibitors of various Ca2+-dependent degradative enzymes. The role of poly(ADP-ribose) polymerase, an enzyme known to be activated by DNA damage, was also assessed. Glucocorticoid-induced chromatin cleavage and cell killing were blocked by the endonuclease inhibitor aurintricarboxylic acid, whereas inhibitors of other Ca2+-dependent degradative processes or of poly(ADP-ribose) polymerase did not abrogate cell death. In addition, stimulation of thymocyte DNA fragmentation by the Ca2+ ionophore A23187 resulted in cell killing that could be blocked by the endonuclease inhibitor. Together, our results suggest that thymocyte suicide is caused by extensive Ca2+-stimulated DNA fragmentation.
An early event in death of interphase lymphocytes exposed in vivo or in vitro to low doses of gamma-irradiation is the degradation of DNA into nucleosome-sized fragments. Induction of fragmentation required RNA and protein synthesis because actinomycin D and cycloheximide, respectively, are able to inhibit DNA fragmentation in irradiated lymphocytes. Studies adding cycloheximide and actinomycin D at various times postirradiation suggest that once the metabolic process is initiated within an individual cell it proceeds to completion. The reversible RNA synthesis inhibitor, 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole inhibits DNA fragmentation in irradiated thymocytes. When this drug is removed after 6 hr, irradiated thymocytes proceed to fragment their DNA; this suggests that an inducing "signal" that is not simply mRNA persists within the irradiated cell for at least 6 hr after irradiation. In contrast to mitogen-activated T and B lymphoblasts, resting T and B cells show significant DNA fragmentation after exposure to 100 to 500 rad. At 2000 rad, all of the splenic subpopulations die rapidly via a different mechanism. By studying the mechanism of DNA fragmentation induced during the interphase death of lymphocytes, we hope to understand better the extreme sensitivity of resting lymphocytes to radiation and what may be the common final pathway of programmed cell death.
The classification of cell death can be based on morphological or biochemical criteria or on the circumstances of its occurrence. Currently, irreversible structural alteration provides the only unequivocal evidence of death; biochemical indicators of cell death that are universally applicable have to be precisely defined and studies of cell function or of reproductive capacity do not necessarily differentiate between death and dormant states from which recovery may be possible. It has also proved feasible to categorize most if not all dying cells into one or the other of two discrete and distinctive patterns of morphological change, which have, generally, been found to occur under disparate but individually characteristic circumstances. One of these patterns is the swelling proceeding to rupture of plasma and organelle membranes and dissolution of organized structure—termed “coagulative necrosis.” It results from injury by agents, such as toxins and ischemia, affects cells in groups rather than singly, and evokes exudative inflammation when it develops in vivo. The other morphological pattern is characterized by condensation of the cell with maintenance of organelle integrity and the formation of surface protuberances that separate as membrane-bounded globules; in tissues, these are phagocytosed and digested by resident cells, there being no associated inflammation.
Normal eukaryotic cells do not enter mitosis unless DNA is fully replicated and repaired. Controls called ‘check-points’, mediate cell cycle arrest in response to unreplicated or damaged DNA. Two independent Schizosaccharomyces pombe mutant screens, both of which aimed to isolate new elements involved in checkpoint controls, have identified alleles of the hus5+ gene that are abnormally sensitive to both inhibitors of DNA synthesis and to ionizing radiation. We have cloned and sequenced the hus5+ gene. It is a novel member of the E2 family of ubiquitin conjugating enzymes (UBCs).
To understand the role of hus5+ in cell cycle control we have characterized the phenotypes of the hus5 mutants and the hus5 gene disruption. We find that, whilst the mutants are sensitive to inhibitors of DNA synthesis and to irradiation, this is not due to an inability to undergo mitotic arrest. Thus, the hus5+ gene product is not directly involved in checkpoint control. However, in common with a large class of previously characterized checkpoint genes, it is required for efficient recovery from DNA damage or S-phase arrest and manifests a rapid death phenotype in combination with a temperature sensitive S phase and late S/G2 phase cdc mutants. In addition, hus5 deletion mutants are severely impaired in growth and exhibit high levels of abortive mitoses, suggesting a role for hus5+ in chromosome segregation. We conclude that this novel UBC enzyme plays multiple roles and is virtually essential for cell proliferation.
During the cell cycle, DNA is replicated and segregated equally into two daughter cells. The DNA damage checkpoint ensures
that DNA damage is repaired before mitosis is attempted. Genetic studies of the fission yeast Schizosaccharomyces pombe have
identified two genes, rad24 and rad25, that are required for this checkpoint. These genes encode 14-3-3 protein homologs that
together provide a function that is essential for cell proliferation. In addition, S. pombe rad24 null mutants, and to a lesser
extent rad25 null mutants, enter mitosis prematurely, which indicates that 14-3-3 proteins have a role in determining the
timing of mitosis.
Although the hydroxyurea (HU) has been extensively studied, little is known of its molecular mechanism in controlling the expression of human globin gene and in modulating the progression of cell-cycle in K 562 cell. In the present study, the effect of hydroxyurea on proliferative kinetics of K 562 cells was examined by monitoring the number of cells during a period of 8 day's cell culture. Our results showed that there was a dose related decrease in cell growth when K562 cells were incubated with HU. Moreover, cell-cycle analysis demonstrated that HU had profound effect on cell-cycle distribution. In the case of the induced K 562 cells, there was an increased accumulation of cells in S phase and a decreased fraction of cells in G 1 and G 2 + M phase. Furthermore, HU could induce the expression of human beta-globin gene in the induced K 562 cells. Our results indicate that HU has a potential to inhibit the proliferation of K 562 cells and to stimulate the terminal differentiation of this cell.
The HEL cells, a human erythroleukemia cell line, express mainly the fetal globin gene and small amount of the embryonic (epsilon) globin gene, but not the adult (beta) globin gene. Hydroxyurea, a small organic compound, has been successfully used to treat sickle cell anemia and beta-thalessaemia. Our data demonstrated that the growth rate of HEL cell proliferation was inhibited by different doses of hydroxyurea (from 50 mumol/L to 200 mumol/L). Using both routine RT-PCR and quantitative PCR analyses, we revealed that the expression of beta-globin gene was sharply activated and alpha-globin gene was almost completely silenced when HEL cells were induced for 3 or 5 days. Meanwhile, gamma-globin gene was expressed with no much difference between induced and uninduced HEL cells. We also demonstrated that the expression of GATA-1 and NF-E 2, which were two of the most important transcription factors in erythrocyte development, was activated about 3 folds in the induced cells. We suggested that the induction of GATA-1 and NF-E 2 expression by hydroxyurea might lead to activation of the adult beta-globin gene through some pathways of signal transduction, therefore, hydroxyurea might play a role to induce HEL cells to terminal differentiation.
Schizosaccharomyces pombes 14-3-3 homologues encode an essential function required for DNA damage checkpoint
- J C Ford
- F Al-Khodairy
- E Fotou
- K S Sheldrick
- Djf Griffiths
- A M Carr
- JC Ford
Ford JC, Al-Khodairy F, Fotou E, Sheldrick KS, Griffiths DJF, Carr AM. Schizosaccharomyces
pombes 14-3-3 homologues encode an essential function required for DNA damage checkpoint.
Science 1994; 265:533-5.
The effects of hydroxyurea on cell-cycle distribution and the expression of human β-globin gene in K562 cells
- Jiang Chu
- chu Jiang