'Loop' domain deletional mutant of Bcl-xL is as effective as p29Bcl-xL in inhibiting radiation-induced cytosolic accumulation of cytochrome c (cyt c), caspase-3 activity, and apoptosis

PURPOSE/OBJECTIVE

To investigate the effect of the enforced expression of p29Bcl-xL or its loop deletional mutant, p18Bcl-xLdelta, on irradiation-induced apoptosis and cell-cycle distribution of HL-60 cells.

MATERIALS & METHODS

We compared the irradiation-induced molecular cascade of apoptosis in control human AML HL-60/neo versus Bcl-xL overexpressing (approximately 8-fold) (HL-60/Bcl-xL) and HL-60/Bcl-XLdelta cells that express the loop domain deletional mutant construct (delta26-83 AA) of Bcl-xL. The three cell lines were irradiated with 6MV photons to varying doses up to 20 Gy. Following this, cytosolic cyt c levels, caspase-3 activity, and the Bcl-2 family of proteins were evaluated utilizing Western blot analysis (whole cell lysate or cytosolic S-100 fraction). Apoptosis was assessed by internucleosomal DNA fragmentation, Annexin-V staining and FACS analysis, as well as by morphologic criteria. The cell-cycle effects of radiation were analyzed by flow cytometry.

RESULTS

Eight hours following irradiation (12 Gy) of HL-60/neo cells, a marked increase (approximately 8-fold) in the cytosolic accumulation of cyt c in the S-100 fraction was observed. This was associated with the cleavage of caspase-3, as well as the generation of its poly (ADP-ribose) polymerase (PARP) and DFF (DNA fragmentation factor)-45 cleavage activity. Twenty-four to forty-eight hours after irradiation, internucleosomal DNA fragmentation and positive Annexin-V staining (32.3+/-3.3%) was detected in HL-60/neo cells. In contrast, in both HL-60/Bcl-xL and HL-60/Bcl-xLdelta cells, a significantly lower percentage of apoptotic cells (p<0.05) were detected and internucleosomal DNA fragmentation was not induced. Following irradiation, Western analysis neither demonstrated any significant alteration in Bcl-2, p29Bcl-xL, p18Bcl-xLdelta, or Bax; nor induced CD95 (Fas receptor) or Fas ligand expression in any cell type. However, in all cell types, irradiation produced approximately a 2-fold increase in the percentage of cells in the G2/M phase of the cell cycle.

CONCLUSION

These results demonstrate that an intact loop domain is not necessary for the full antiapoptotic function of Bcl-xL against irradiation-induced cytosolic accumulation of cyt c, caspase activation, and apoptosis of HL-60 cells. Additionally, the cell-cycle effects of ionizing radiation in HL-60 cells are not affected by enforced expression of Bcl-xL or Bcl-xLdelta.

Bcl-2-mediated resistance to apoptosis is associated with glutathione-induced inhibition of AP24 activation of nuclear DNA fragmentation

Studies on the mechanism of apoptosis in this laboratory support a model in which signal transduction involving caspase 3 leads to activation of a serine protease called Mr 24,000 apoptotic protease (AP24), which then induces internucleosomal DNA fragmentation in the nucleus. This study examined the effect of Bcl-2 overexpression on activation of AP24 and the induction of DNA fragmentation by AP24 in isolated nuclei. It was demonstrated that overexpression of Bcl-2 in either HL-60 or PW leukemia cell lines suppressed activation of AP24 induced by either tumor necrosis factor or UV light and protected cells from apoptosis. Furthermore, nuclei isolated from Bcl-2-overexpressing cells were relatively resistant to internucleosomal DNA fragmentation induced by AP24 isolated from apoptotic cells. Bcl-2-overexpressing cells that were nutritionally depleted of glutathione (GSH) became sensitive to tumor necrosis factor- or UV light-induced activation of AP24 and underwent apoptotic cell death. Moreover, nuclei isolated from Bcl-2-overexpressing cells that were depleted of GSH became sensitive to AP24-induced DNA fragmentation. The addition of exogenous GSH blocked the proteolytic activity of AP24, as well as its ability to induce DNA fragmentation in normal isolated nuclei. These results indicate that Bcl-2 can attenuate at least two events in the AP24 apoptotic pathway: activation of AP24 and induction of DNA fragmentation by activated AP24. Furthermore, agents that deplete intracellular levels of GSH may have therapeutic use in the sensitization of Bcl-2-overexpressing cancer cells to apoptotic cell death.

Human cell membrane oxidative damage induced by single and fractionated doses of ionizing radiation: a fluorescence spectroscopy study

PURPOSE

To investigate the production and repair of lipid oxidative damage in two human cell lines exposed to acute and fractionated dose of ionizing radiation. Radiation dose was in the range from 0.1 to 44 Gy.

MATERIALS AND METHODS

K562 and HL60 human cell lines have been used, 24 and 96 h after seeding. Membrane lipid oxidative damage has been detected by the measurement of the fluorescence decay of 1,6-diphenyl-1,3,5-hexatriene (DPH), its polarization value and the conjugated dienes concentration. The modification of DPH decay has been previously reported to be directly related to the lipid hydroperoxide concentration.

RESULTS

A modification of the DPH decay has been observed as a linear function of the logarithm of the radiation dose and only when the irradiation was performed in the presence of oxygen. The amount of the damage is related to the time after the cell medium change. By exposing the cells to fractionated radiation doses for several days (10 cGy day(-1)), the oxidative damage has been found to be cumulative. After a single acute dose, evidence of repair of the lipid oxidative damage was not obtained.

CONCLUSIONS

Following a previously developed method, the membrane damage was attributed to the production of hydroperoxide residues in the lipid acyl chains with the consequence of water penetration into the external portion of the bilayer, from the aqueous environment to the position of hydroperoxides. This damage is not repaired. The results obtained by measuring the DPH fluorescence decay have been compared with those obtained using other current optical and biochemical methods. None of these techniques could detect membrane oxidative damage at doses < 10 Gy. Finally, the different sensitivity of 'young' and 'old' cells to the oxidative damage can be related to different cholesterol concentrations.

DNA fragmentation induced by a cytoplasmic extract from irradiated cells

Apoptosis is a mode of cell death characterized by distinct morphological features and DNA fragmentation. The program that leads to apoptosis has been considered to be predominantly extranuclear, and a signal transduction pathway to the nucleus exists during apoptosis, while characteristic events occur in the nucleus. As for radiation-induced apoptosis, the signal transduction pathway remains unclear, especially the sites where the primary effect of radiation occurs. In this study, we demonstrate that a cytoplasmic extract prepared from irradiated cells has the ability to cause DNA fragmentation and that caspase-3 is activated in this extract. Normal nuclei of HeLa S3 cells were added to a cytoplasmic extract made from HL60 cells which had been irradiated with 30 Gy of 137Cs gamma rays and were incubated. Agarose gel electrophoresis of the added nuclei showed a characteristic DNA laddering pattern. This reaction was blocked by a caspase-3 inhibitor but not by an ICE inhibitor. These observations suggest that a signal transduction pathway from an unknown target of gamma radiation may exist upstream of caspase-3 during radiation-induced apoptosis.

Explaining differences in sensitivity to killing by ionizing radiation between human lymphoid cell lines

We surveyed five human hematopoietic cell lines (HSB-2, MOLT-4, Reh, CEM, and HL-60) to determine whether any simple correlates with sensitivity to killing by gamma-irradiation might be revealed. The clonogenic survival gamma-ray dose-response curves for these cell lines cover a wide range of sensitivities. Consistent with previous results for murine hematopoietic cell lines, there was a clear correlation between the rapidity with which irradiation induced apoptosis and clonogenic radiosensitivity of a cell line, although the relationship between timing of apoptosis and radiosensitivity differed between human and murine cell lines. Flow cytometric determination of cell cycle distribution after irradiation showed that differences between human hematopoietic cell lines, in the rate of induction of apoptosis, were generally related to the functioning of cell cycle checkpoints. Whereas the rapidly dying and radiosensitive HSB-2 cell line underwent apoptosis from different points in the cell cycle, the more slowly dying cell lines showed a variety of cell cycle arrest profiles and initiated apoptosis after accumulation of cells in the G2 phase. The lag-phase between arrest in G2 and induction of apoptosis was comparable for MOLT-4, Reh, and CEM; however, HL-60 cells showed a markedly longer G2 arrest that correlated with their greater radioresistance. The results suggest that the total length of time available for DNA damage repair (irrespective of whether this time accrues as blockage in G1, S, or G2), prior to potential activation of apoptosis, is a critical determinant of radiosensitivity in human hematopoietic cell lines. Comparison of the p53 status of these cell lines suggested that mutations in the TP53 gene are contributing to the delay of induction of apoptosis seen in the more radioresistant cell lines. The sensitivity of MOLT-4 and HL-60 cells to killing by DNA-associated 125I decays was determined and was found to correlate with the relative sensitivity of these lines to gamma-irradiation. The highly localized deposition of energy by 125I decays argues that DNA damage is a potent initiator of apoptosis in these cell lines. The results presented suggest that differences in the radiosensitivity of the cell lines examined reflect differences in the rapidity of induction of apoptosis and that radiation-induced cell death in hematopoietic cells can be explained as a response to DNA damage.

Radiosensitivity in ataxia telangiectasia fibroblasts is not associated with deregulated apoptosis

Ataxia telangiectasia (AT) is an autosomal recessive human disorder featuring diverse clinical abnormalities including proneness to cancer and extreme sensitivity to ionizing radiation. Although cells from AT patients exhibit faulty activation of the p53 signal transduction pathway at early times after radiation exposure, it has been proposed that high levels of DNA damage persisting in AT cells may up-regulate p53 through an ATM-independent mechanism at late times after irradiation, leading to cell death by apoptosis. In this study we demonstrate that diploid skin fibroblast strains homozygous for the AT mutation fail to up-regulate p53 protein at late times (< or = 48 h) after irradiation with 60Co gamma rays. Moreover, exposure of normal and AT fibroblasts to a dose of 8 Gy does not result in a significant increase in the fraction of apoptotic cells. Since this treatment reduces the clonogenic potential of human cells by at least two orders of magnitude, we conclude that apoptosis is not the primary mechanism of cell death induced by ionizing radiation in human normal and AT fibroblast cultures. Therefore, our results are not in accordance with the current hypothesis suggesting that increased radiosensitivity of AT cells is associated with deregulated apoptosis.

Resistance of human multidrug-resistant neoplastic cell lines to paclitaxel-induced-radiosensitization is reduced by the non-immunosuppressive cyclosporine analog SDZ PSC 833

The non-immunosuppressive cyclosporine analog SDZ PSC 833 abolished the resistance of human multidrug resistant (MDR-1, P-gp) human promyelocyte leukemia HL-60/VCR cells in vitro to paclitaxel-induced cell cycle- and viability alterations, as well as resistance to paclitaxel-induced radiosensitization. Furthermore, SDZ PSC 833 abolished also the resistance of human multidrug-resistant ovarian A2780/ADR cells to paclitaxel-induced cell cycle alterations and reduced its resistance to paclitaxel-induced radiosensitization. In these multidrug-resistant ovarian carcinoma cells the supra-additive interaction between paclitaxel and SDZ PSC 833 pre-exposure and subsequent irradiation appeared at slightly higher paclitaxel concentrations (40-100 nM) compared to those required for a similar interaction in the parental drug sensitive A2780 cells (40-80 nM paclitaxel).

Selective induction of leukemia-associated fusion genes by high-dose ionizing radiation

There is strong clinical and epidemiological evidence that ionizing radiation can cause leukemia by inducing DNA damage. This crucial initiation event is believed to be the result of random DNA breakage and misrepair, whereas the subsequent steps, promotion and progression, must rely on mechanisms of selective pressure to provide the expanding leukemic population with its proliferative/renewal advantage. To investigate the susceptibility of human cells to external agents at the genetic recombination stage of leukemogenesis, we subjected two hematopoietic cell lines, KG1 and HL60, to high doses of gamma-irradiation. The irradiation induced the formation of fusion genes characteristic of leukemia in both cell lines, but at a much higher frequency in KG1 than in HL60. In KG1 cells, the AML1-ETO hybrid gene [associated with the t(8;21) translocation of acute myeloid leukemia] occurred significantly more often than the BCR-ABL [associated with t(9;22) chronic myeloid leukemia] or the DEK-CAN [associated with t(6;9) acute myeloid leukemia] fusion genes. These findings support the notion that ionizing radiation can directly generate leukemia-specific fusion genes but emphasize the differing susceptibility of different cell populations and the differing frequency with which the various fusion genes are formed. The selectivity observed at the primary level of gene fusion formation may explain at least in part the differential risk for development of some but not other forms of leukemia after high-dose radiation exposure.

Early changes in cell cycle kinetics after ionizing irradiation below 1 GY

The present study describes a procedure for quantifying cell cycle alterations within 15 h after radiation with doses below 1 Gy. For detection and assessment of the relevant changes, 5-bromo-2'-deoxyuridine (BrdUrd)-labelling and flow cytometry were used. Using this approach, as early as 6 h after exposure of radiosensitive leukemic HL-60 cells, radiation-induced changes in cell cycle progression could be measured even with radiation doses as low as 0.25 Gy. As a result, a method to define transition rates for a single cell cycle phase or from one phase to another is described. Even minor changes can be described. Moreover, the BrdUrd assay allows for discrimination of cells irradiated in different phases of the cell cycle. Thus, it is possible to follow the progression in the cell cycle of cells either irradiated in G1, S, or G2 + M phase, respectively. Radiation effects on single cell cycle phases can be analysed separately. A detailed evaluation of the cellular response to irradiation regarding dose, time, and effect is described. The value of cell cycle parameters for assessment of various biological indicators of radiation effect is discussed.

Reduction in the radiation-induced late S phase and G2 blocks in HL-60 cell populations by amiloride, an efficient inhibitor of the Na+/H+ transporter

Recent investigations that showed that amiloride delayed or inhibited apoptosis indicated it might also attenuate cell cycle checkpoints activated by ionizing radiation. In this report, single- and dual-parameter flow cytometry were used to investigate the effects of amiloride on cell cycle progression, and the effectiveness of amiloride to attenuate the S and G2 phase checkpoint responses induced by 2.5, 5.0, and 7.5 Gy of gamma radiation. The late S-phase delay, noted at 8 h following irradiation, and a radiation-induced G2 block, which was maximum at 16 h after irradiation, were both significantly reduced in amiloride-treated samples. Attenuation of the radiation-induced late S phase and G2 blocks resulted in cell division without apparent apoptosis or necrosis over a 24-h period. Results presented indicate that amiloride reduces the radiation-induced G2 block in HL-60 cell populations almost equally well as caffeine and to a greater extent than staurosporine. Immunofluorescent detection and quantitation of cyclin B1 expression demonstrated that amiloride only significantly reduced cyclin B1 expression following 5.0 Gy, when there was a notable induction of a significant G2 delay, followed by a relatively rapid recovery in cycling potential. The results suggest that amiloride affects the radiation-triggered signaling cascades to alter the kinase activity of proteins associated with mitotic progression, particularly the cyclin B1-p34cdc2 complex. Alternatively, alterations in intracellular ion concentrations induced by amiloride may lead to changes in Ca2+-dependent signaling cascades and thereby decrease the radiation-mediated cell cycle perturbations.

Taxol-enhanced cytotoxic effect of radiation in human promyelocytic leukemia cells: relative resistance of multidrug-resistant HL-60 cells in vitro

Cytotoxic effects of sequential taxol (paclitaxel) and X-irradiation on drug-sensitive human cultured promyelocytic leukemia (HL-60) cell line and its multidrug-resistant sublines were examined using photometric MTT test and flow cytometry. Paclitaxel (at concentrations 1-10 nmol) stimulated the cytotoxic effect of irradiation in HL-60 and to a lesser extent also in HL-60/ADR, but not in HL-60/VCR cells. The stimulation of radiation-induced cytotoxic effect by paclitaxel correlated with its potential to induce cell cycle and viability alterations identified with flow cytometric analysis (i.e. increased propidium iodide staining, increased side scatter, decreased forward angle scatter, accumulation of necrotic cell detritus, apoptotic pre-G0 cells and cells in the G2/M phase of the cell cycle).

NADH dehydrogenase deficiency in an apoptosis-resistant mutant isolated from a human HL-60 leukemia cell line

An apoptosis-resistant mutant (VC-33) was selected from HL-60 by alternating exposure to camptothecin and etoposide. VC-33 cells demonstrated resistance to apoptosis as induced not only by camptothecin and etoposide but by a variety of other agents as well, including 1-beta-D-arabinofuranosylcytosine, hydroxyurea, calcium ionophore (A23187), cycloheximide, and UV irradiation. In an effort to identify the mechanism of such apoptosis resistance, a mRNA differential display analysis was used. Among a total of 12 bands with reduced expression in VC-33 cells, 1 cDNA clone was isolated that was hybridized to the wild-type transcript but not to the VC-33 transcript on Northern blotting. Partial sequence of this gene revealed 98% homology to mitochondrial NADH dehydrogenase subunit 5. When cell growth and intracellular ATP levels under glucose starvation were measured, VC-33 cells were found to be more sensitive than wild-type cells. Thus, NADH dehydrogenase deficiency may contribute, at least in part, to the mechanism of resistance to apoptosis in VC-33 cells.

Modulation of radiation-induced apoptosis by thiolamines

Exposure to the thiolamine radioprotector N-(2-mercaptoethyl)-1,3-propanediamine (WR-1065) induced apoptosis in the mouse TB8.3 hybridoma after a 60-min (LD50 = 4.5 mM) or during a 20-h (LD50 = 0.15 mM) exposure. In contrast, a 20-h exposure to 17 mM L-cysteine or 10 mM cysteamine was required to induce 50% apoptosis within 20 h. Apoptosis was not induced by either a 60-min or 20-h exposure to 10 mM of the thiazolidine prodrugs ribose-cysteine (RibCys) or ribose-cysteamine (RibCyst). Thiolamine-induced apoptosis appeared to be a p53-independent process since it was induced by WR-1065 exposure in human HL60 cells. Exposure to WR-1065 (4 mM for 15 min) or cysteine (10 mM for 60 min) before and during irradiation protected cells against the induction of both DNA double-strand breaks and apoptosis, while exposure to RibCys (10 mM for 3 h) did not. Treatment with either WR-1065, cysteine, RibCys or RibCyst for 60 min beginning 60 min after irradiation did not affect the level of radiation-induced apoptosis. In contrast, treatment with either cysteine, cysteamine or RibCys for 20 h beginning 60 min after irradiation enhanced radiation-induced apoptosis. Similar experiments could not be conducted with WR-1065 because of its extreme toxicity. Our results indicate that thiolamine enhancement of radiation-induced apoptosis is not involved in their previously reported capacity to reduce radiation-induced mutations.

Role for E2F in DNA damage-induced entry of cells into S phase

Mammalian cells respond to ionizing radiation (IR) with transient cell cycle arrest and induction of apoptosis. Here we show that IR increases the expression of the E2F-1 transcription factor and the entry of cells into S phase. E2F-1 transactivation function is inhibited by cyclin A-kinase to ensure orderly progression through S phase. However, in contrast to proliferating cells, IR treatment results in down-regulation of cyclin A-kinase. Expression of a dominant negative form of the E2F heterodimeric partner DP-1 confirmed the involvement of E2F in IR-induced S-phase entry. These findings also support opposing signals involving the induction of E2F and the down-regulation of cyclin A-kinase in the IR response.

Effect of Trolox, a synthetic analog of alpha-tocopherol, on cytotoxicity induced by UV irradiation and antioxidants

The addition of Trolox, a synthetic analog of alpha-tocopherol, significantly reduced the cytotoxicity induced by UV irradiation and antioxidants, such as ascorbate, gallate and caffeate. Ascorbate and gallate, but not UV irradiation, stimulated the oxidation of methionine to methionine sulfoxide in the culture medium, possibly because of their prooxidant actions. Trolox slightly, but significantly reduced the methionine oxidation. On the other hand, alpha-tocopherol showed a much lower protective effect against ascorbate and gallate-induced cytotoxicity, and failed to reduce the methionine oxidation induced by these agents. ESR spectroscopy showed that both Trolox and alpha-tocopherol did not significantly change the radical intensity of ascorbate and gallate. The present study suggests that the antioxidative efficacy of Trolox surpasses that of alpha-tocopherol.

Induction of apoptotic cell death in human leukemic cell line, HL-60, by extremely low frequency electric magnetic fields: analysis of the possible mechanisms in vitro

The influence of extremely low frequency electric magnetic fields. (ELF EMFs) on apoptotic cell death was examined using a human leukemic cell line, HL-60 and normal human peripheral blood leukocytes. When HL-60 cells were exposed to 45 mT ELF EMFs, apoptotic cell death, characterized by cell shrinkage, nuclear fragmentation and cleavage of internucleosomal DNA to yield fragments that were multiples of 180-200 base pairs, were induced. The minimum periods required apoptotic HL-60 cell death was 1.0 hour. However, exposure to ELF EMFs could not produce detectable DNA fragmentation in human peripheral blood leukocytes. Static magnetic fields could not induce apoptotic cell death in HL-60 cells, even when the cells were exposed to 180 mT of magnetism for 3.5 hours. We further examined whether hyperthermia induced by induction current in ELF EMFs produced apoptosis in HL-60 cells. Induction current in ELF EMFs enhanced temperature of culture medium to 40.3 degrees C at 3.0 hours of exposure. However, this level of temperature could not induced apoptotic cell death in HL-60 cells, even when cells were cultured for 3.5 hours. These results suggest that induction current produced by ELF EMFs may be one of main mediator in apoptosis in HL-60 cells.

Csa-19, a radiation-responsive human gene, identified by an unbiased two-gel cDNA library screening method in human cancer cells

A novel polymerase chain reaction (PCR)-based method was used to identify candidate genes whose expression is altered in cancer cells by ionizing radiation. Transcriptional induction of randomly selected genes in control versus irradiated human HL60 cells was compared. Among several complementary DNA (cDNA) clones recovered by this approach, one cDNA clone (CL68-5) was downregulated in X-irradiated HL60 cells but unaffected by 12-O-tetradecanoyl phorbol-13-acetate, forskolin, or cyclosporin-A. DNA sequencing of the CL68-5 cDNA revealed 100% nucleotide sequence homology to the reported human Csa-19 gene. Northern blot analysis of RNA from control and irradiated cells revealed the expression of a single 0.7-kilobase (kb) messenger RNA (mRNA) transcript. This 0.7-kb Csa-19 mRNA transcript was also expressed in a variety of human adult and corresponding fetal normal tissues. Moreover, when the effect of X- or fission neutron-irradiation on Csa-19 mRNA was compared in cultured human cells differing in p53 gene status (p53-/- versus p53+/+), downregulation of Csa-19 by X-rays or fission neutrons was similar in p53-wild type and p53-null cell lines. Our results provide the first known example of a radiation-responsive gene in human cancer cells whose expression is not associated with p53, adenylate cyclase or protein kinase C.

Effect of alpha-tocopherol on cytotoxicity induced by UV irradiation and antioxidants

The addition of DL-alpha-tocopherol (vitamin E) at the time of UV irradiation only marginally protects cells from UV-induced cytotoxicity. However, a protective effect of alpha-tocopherol emerged when it was added to the cells before UV irradiation, alpha-Tocopherol was progressively and dose-dependently incorporated into the cells. Washout experiments showed that the intracellular concentration of alpha-tocopherol decreased with an approximate half-life of 14-20 hours, due to the release from the cells and dilution by cell proliferation. Pretreatment of the cells with alpha-tocopherol significantly increased the resistancy against the cytotoxic action of UV irradiation and antioxidants such as sodium ascorbate, gallic acid, n-propyl gallate and caffeic acid. ESR spectroscopy showed that alpha-tocopherol enhanced the ascorbyl radical intensity, whereas it reduced caffeic acid radical intensity, without affecting the radical intensity of gallic acid and n-propyl gallate. Both control and treated cell lysates scavenged superoxide anion (generated by xanthine-xanthine oxidase reaction) and hydroxyl radical (generated by Fenton reaction) to a comparable extent. The present study suggests that the protective effect of alpha-tocopherol might be derived from its incorporation into the cell membranes rather than its scavenging activity.

Role of peroxide and superoxide anion during tumour cell apoptosis

Apoptosis or programmed cell death was induced in the human promyelocytic leukemia cell line HL-60 by UV irradiation or treatment with cytotoxic drugs (etoposide, camptothecin, melphalan or chlorambucil). These treatments caused a rapid increase in intracellular peroxide levels. Preincubation of HL-60 cells with the hydrogen peroxide-scavenging enzyme catalase (500 U/ml) inhibited apoptosis due to UV irradiation or low concentrations of camptothecin, etoposide or melphalan, but did not protect against higher concentrations. In contrast, superoxide anion levels in the cells remained unchanged upon treatment with cytotoxic drugs, while UV irradiation led to a transient doubling in superoxide levels. Exogenous superoxide dismutase (400 U/ml) provided modest protection against UV irradiation and had no effect on cytotoxic drug-induced apoptosis. The results suggest that both hydrogen peroxide and superoxide anion may be involved in the induction of apoptosis by UV irradiation. On the other hand, while exposure to cytotoxic drugs induces a large increase in intracellular peroxide levels, catalase is able to protect the cells from apoptosis only when low concentrations of these compounds are used, thus indicating the involvement of other factors in this process, particularly at higher drug concentrations.

Rodent cell transformation and immediate early gene expression following 60-Hz magnetic field exposure

Some epidemiological studies suggest that exposure to power frequency magnetic fields (MFs) may be associated with an elevated risk of human cancer, but the experimental database remains limited and controversial. We investigated the hypothesis that 60-Hz MF action at the cellular level produces changes in gene expression that can result in neoplastic transformation. Twenty-four hour 200 microT continuous MF exposure produced negative results in two standard transformation systems (Syrian hamster embryo cells and C3H/10T1/2 murine fibroblasts) with or without postexposure to a chemical promoter. This prompted a reexamination of previously reported MF-induced changes in gene expression in human HL60 cells. Extensive testing using both coded and uncoded analyses was negative for an MF effect. Using the same exposure conditions as in the transformation studies, no MF-induced changes in ornithine decarboxylase expression were observed in C3H/10T1/2 cells, casting doubt on a promotional role of MF for the tested cells and experimental conditions.

Alterations in p53 do not correlate with radioresistant DNA synthesis

The radiation hypersensitivity disorder ataxia-telangiectasia (A-T) is uniquely characterized by a failure to immediately inhibit DNA synthesis in response to low dose ionizing radiation which is referred to as radioresistant DNA synthesis (RDS). As it remains controversial as to whether p53 abnormalities are central to RDS and the A-T radiation hypersensitivity disorder, post-irradiation p53 responses and cell cycle alterations were investigated in EBV immortalized B cells (LCL's) from normal individuals (N LCL's) and A-T patients (A-T LCL's). Here it is shown that mutations in p53 are insufficient to give cells such as HL-60 cells RDS potential. Furthermore, RDS which is demonstrable at 2 Gy, does not correlate with radiation induced alterations in p53 or with alterations in the Gt/S block. However, at 10 Gy, abnormalities in p53 and cell cycle changes were noted for A-T LCL's. Although the results suggest that p53 abnormalities are not central to RDS or the A-T ionizing radiation hypersensitivity disorder, the demonstration of a threshold effect for secondary abnormalities in p53 and cell cycle changes post-irradiation, may help resolve conflicting reports on the involvement of p53 downstream of the central defect in A-T.

Radiosensitization of HL-60 human leukaemia cells by bryostatin-1 in the absence of increased DNA fragmentation or apoptotic cell death

Ionizing radiation produced a dose-dependent reduction in the proliferative capacity of HL-60 human promyelocytic leukaemia cells. A small percentage of the cell population demonstrated morphological evidence of apoptosis at 24h following radiation doses of > or = 5 Gy (i.e. 8% at 5 Gy and 16% at 10 Gy respectively) and produced a laddered oligonucleosomal pattern of DNA fragments by static-field gel electrophoresis. The antiproliferative effects of 1 and 2.5 Gy ionizing radiation were significantly enhanced by preincubating cells with bryostatin-1 at a concentration (10 nM) and time frame (24h) associated with down-regulation of total cellular protein kinase C (PKC) activity. Potentiation by bryostatin-1 of the radiation effect on proliferation was not associated with a concomitant increase in internucleosomal DNA fragmentation, in the fraction of cells exhibiting apoptotic morphology, or in the extent of radiation-induced single- or double-strand breaks in bulk DNA. Staurosporine, a potent but nonspecific inhibitor of PKC, was ineffective in altering the radiosensitivity of HL-60 cells or the degree of DNA fragmentation induced by ionizing radiation. These findings indicate that bryostatin 1 increases the sensitivity of human myeloid leukaemic cells to low radiation doses without enhancing DNA fragmentation or apoptosis, and that this capacity may involve factors other than, or in addition to, down-modulation of PKC activity.

Evidence for a G2 checkpoint in p53-independent apoptosis induction by X-irradiation

The p53 tumor suppressor gene is thought to be required for the induction of programmed cell death (apoptosis) initiated by DNA damage. We show here, however, that the human promyelocytic leukemia cell line HL-60, which is known to be deficient in p53 because of large deletions in the p53 gene, can be induced to undergo apoptosis following X-irradiation. We demonstrate that the decision to undergo apoptosis in this cell line appears to be made at a G2 checkpoint. In addition, we characterize an HL-60 variant, HCW-2, which is radioresistant. HCW-2 cells display DNA damage induction and repair capabilities identical to those of the parental HL-60 cell line. Thus, the difference between the two cell lines appears to be that X-irradiation induces apoptosis in HL-60, but not in HCW-2, cells. Paradoxically, HCW-2 cells display high levels of expression of bax, which enhances apoptosis, and no longer express bcl-2, which blocks apoptosis. HCW-2 cells' resistance to apoptosis may be due to the acquisition of expression of bcl-XL, a bcl-2-related inhibitor of apoptosis. In summary, apoptosis can be induced in X-irradiated HL-60 cells by a p53-independent mechanism at a G2 checkpoint, despite the presence of endogenous bcl-2. The resistance shown by HCW-2 cells suggests that bcl-XL can block this process.

Expression of apoptosis-related oncoproteins and modulation of apoptosis by caffeine in human leukemic cells

We investigated the modulation of radio- and chemoresistance by caffeine and mechanisms of resistance in human leukemic cell lines and mononuclear cells from 18 leukemic patients. Caffeine synergistically potentiated cytotoxicity and apoptosis induced by ionizing radiation or carboplatin (CPt), but attenuated induction of apoptosis by daunorubicin (DNR) in KG-1a cells. Since caffeine released irradiated as well as DNR-treated KG-1a cells from G2M cell cycle arrest and CPt-treated cells from S-phase arrest, this release does not fully explain the different effects of caffeine. Caffeine synergistically reduced the level of the apoptosis inhibitor glutathione after irradiation or CPt treatment. In contrast, treatment with DNR plus caffeine diminished glutathione levels to a lesser extent than DNR alone. We conclude that the effect of caffeine on glutathione depletion represents a mechanism of action by which caffeine can modulate apoptosis. Caffeine increased CPt cytotoxicity in K562 cells and its doxorubicin-resistant subline (K562/ADM), but little effect was seen in HL-60 cells or mononuclear cells from leukemic patients. Multivariate cluster analysis revealed an association of CPt resistance with the expression of c-Fos, c-N-Ras, and p53 oncoproteins and with proliferative activity (S-phase of cell cycle), but not with Bcl-2 expression.