Apoptosis in HL-60 cells induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX)

Arsenic induces DNA damage via reactive oxygen species in human cells

To elucidate arsenic-induced oxidative DNA damage, the genotoxicity of arsenic in human cells was comparatively studied with single cell gel electrophoresis (SCGE) assay in combination with the observation of the protective effects of dimethyl sulfoxide (DMSO) and catalase. Arsenic, at the concentration of 2.4 μM by coincubation for 24 hours, significantly induced DNA damage in HL60, a human promyelocytic leukemia cell line. In contrast, significant DNA damage was found in human mononucleocytes at the concentration of 4.8 μM or above. The cells were incubated separately with DMSO (12 mM/l), a well-known hydroxyl radical (OH(-)) scavenger, and catalase (1,300 U/ml), a hydrogen peroxide (H(2)O(2)) scavenger, for 6 hours and then further coincubated with various concentrations of arsenic for 24 hours at 37°C and 5% CO(2). The findings showed that both DMSO and catalase significantly reduced the arsenic-induced tail moment, a parameter of total damaged DNA, in HL60 and mononucleocytes. Hence our findings indicate that arsenic, with micromolar concentrations, induces typical and various extents of DNA damage in human cells via reactive oxygen species in a dose-dependent manner.

Carcinogenicity of the chlorination disinfection by-product MX

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone, better known by its historical name 'mutagen X' or MX, is a chlorination disinfection byproduct that forms from the reaction of chlorine and humic acids in raw water. MX has been measured in drinking water samples in several countries at levels that ranged from non-detectable to 310 ng/L. Although the concentration of MX in drinking water is typically 100- to 1000-fold lower than other common chlorinated by-products of concern (e.g., trihalomethanes), some have hypothesized that MX might play a role in the increased cancer risks that have been associated with the consumption of chlorinated water. This hypothesis is based on observations that MX, in some test systems, is extremely potent relative to trihalomethanes in inducing DNA damage and altering pathways involved in cell growth, and that in some epidemiological studies increased cancer rates are associated with the bacterial mutagenicity of disinfected water of which MX contributes a significant portion. MX also appears to be more potent than other chlorination by-products in causing cancer in animals. This article reviews the available evidence on the carcinogenicity of MX. MX induced cancer at multiple sites in male and female rats, acted as a tumor initiator and promoter, enhanced tumor yields in genetically modified rodents, induced a myriad of genotoxic effects in numerous in vitro and in vivo test systems, and was a potent inhibitor of gap junction intercellular communication. Although the precise mechanism of MX-induced DNA damage is not known, MX is able to cause DNA damage through an unusual mechanism of ionizing DNA bases due to its extremely high reductive potential. MX may also cause mutations through DNA adduction. This article develops a mean cancer potency estimate for MX of 2.3 (mg/kg-d)(-1) and an upper 95% percentile estimate of 4.5 (mg/kg-d)(-1), and examines the potential health risks posed by this chlorination contaminant in drinking water. A discussion of additional data that would be desirable to better characterize the risks posed by MX and other halogenated hydroxyfuranones follows.

Effects of asbestos on initiation of DNA damage, induction of DNA-strand breaks, P53-expression and apoptosis in primary, SV40-transformed and malignant human mesothelial cells

Human mesothelial cells (HMC), the progenitor cells of asbestos-induced mesothelioma, are particularly sensitive to the genotoxic effects of asbestos, although the molecular mechanisms by which asbestos induces injury in HMC are not well known. The high susceptibility of HMC to simian virus 40 (SV40)-mediated transformation is assumed to play a causative role in the pathogenesis of mesothelioma. The aim of this study was to investigate the asbestos-induced DNA damage in cultured HMC and SV40-transformed HMC (MeT-5A) compared with their malignant counterparts, i.e. human mesothelioma cells (MSTO). The time-dependent initiation of DNA-strand breaks as well as the induction of oxidative DNA base modifications were key factors for investigation. HMC, MeT-5A and MSTO cells were exposed to chrysotile and crocidolite asbestos (3 microg/cm2) during different time periods (1-72 h). DNA damage was investigated by use of the Comet assay and alkaline unwinding, the latter in combination with the Fpg protein. The P53 level was analyzed by immunofluorescence, and measurement of apoptosis was conducted by flow cytometry. We found a significant induction of DNA damage in asbestos-treated HMC already after an exposure time of 1.5 h. This effect could not be observed in treated MeT-5A and MSTO cells. Also, a time-dependent significant increase in DNA-strand breaks was observed by alkaline unwinding in asbestos-treated HMC, but not in treated MeT-5A and MSTO cells. In none of the three cell lines we could detect oxidative DNA damage recognized by the Fpg protein (e.g. 8-oxo-guanine), up to 24 h after exposure to asbestos. In contrast to what was found in HMC, P53 was over-expressed in untreated MeT-5A and MSTO. The induction of apoptosis by asbestos fibers was suppressed in MeT-5A and MSTO cells. Crocidolite fibers induced the higher genotoxic effects and chrysotile the more pronounced apoptotic effects. We conclude that asbestos induces DNA damage in HMC already after a very short exposure time in the absence of 8-oxo-guanine formation. The presence of SV40-Tag in MeT-5A and MSTO cells results in an increased expression of P53, but not in additive genotoxic effects after exposure to asbestos. The deregulation of the apoptotic pathway may lead to proliferation of genomically damaged cells and finally to the development of mesothelioma.

Potent inhibition of gap junctional intercellular communication by 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) in BALB/c 3T3 cells

The chlorohydroxyfuranones (CHFs) MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone], MCA [3,4-dichloro-5-hydroxy-2(5H)-furanone], CMCF [3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone], and MCF [3-chloro-4-methyl-5-hydroxy-2(5H)-furanone] are genotoxic disinfection by-products of drinking water chlorination. MX, MCA, and MCF also promote foci formation in the two-stage cell transformation assay. The cellular mechanisms underlying this apparent promotional effect are not known. In the present study, the effects of MX, MCA, CMCF, and MCF on gap junctional intercellular communication (GJIC) were measured in BALB/c 3T3 cells using the scrape loading dye technique. The effect of MX on apoptosis in the same cell line was explored by assaying caspase-3-like protease activity. All the four CHFs inhibited GJIC after 30 min exposure in a dose-dependent fashion but there was a marked difference in the ranges of their active concentrations. MX was almost as potent an inhibitor of GJIC (inhibition at nanomolar concentrations) as 12-O-tetradecanoylphorbol-13-acetate (TPA) (positive control), while MCA was 10 times weaker, CMCF 10,000 times weaker, and MCF 20,000 times weaker than MX. After prolonged exposure periods (up to 6 h), GJIC recovered somewhat upon MX and MCA exposures, the inhibition of GJIC by MCF remained constant but CMCF showed an irreversible increasing inhibitory effect. MX caused apoptosis as a "window" effect at concentrations 2000-4000-fold higher than those needed to inhibit GJIC. The results indicate that MX is a potent inhibitor of GJIC in BALB/c 3T3 cells and this inhibition might be one mechanism by which MX can promote malignant foci formation. MCA also has a specific potential to inhibit GJIC whereas MCF and CMCF affected GJIC at concentrations, similar to those evoking genotoxicity in vitro.

Fluoride-induced apoptosis in human epithelial lung cells (A549 cells): role of different G protein-linked signal systems

In the present study, possible mechanisms involved in fluoride-induced apoptosis in a human epithelial lung cell line (A549) were examined. Sodium fluoride (NaF) induced apoptosis in the A549 cells, with a maximum at 5-7.5 mM after 20 hours of exposure. The number of cells with plasma membrane damage (PI-positive cells) increased moderately up to 5 mM, but markedly at 7.5 mM. Deferoxamine (an Al3+ chelator) almost completely prevented these NaF-induced responses, which may suggest a role for G protein activation. The apoptotic effect was partially reduced by the PKA inhibitor H89. NaF induced a weak but sustained increase in PKC activity, whereas the PKC activator TPA induced a transient effect. TPA, which enhanced the NaF-induced PKC activity, was not apoptotic when added alone, but facilitated the NaF-induced apoptosis and the increase in PI-positive cells. PKC downregulation induced by TPA pretreatment almost completely prevented the NaF-induced apoptosis and the increase in PI-positive cells. Pretreatment with the PKC inhibitor GF109203X, which abolished the PKC activity after 3 hours, enhanced the NaF-induced apoptosis. KN93 (a CaM kinase II inhibitor) and W7 (a calmodulin inhibitor) seem to reduce the apoptotic effect of NaF, whereas BAPTA-AM (a Ca2+ chelator) was without effect. The tyrosine kinase inhibitor genistein also markedly reduced the NaF-induced apoptosis, whereas the PI-3 kinase inhibitor wortmannin augmented the response. In conclusion, the present results suggest that NaF induces an apoptotic effect and an increase in PI-positive A549 cells via similar mechanisms, involving PKC, PKA, tyrosine kinase and Ca2+-linked enzymes, whereas PI-3 kinase seems to exert a counteracting effect.

Comparable DNA and chromosome damage in Chinese hamster ovary cells by chlorohydroxyfuranones

Chlorinated drinking water contains several chlorohydroxyfuranone (CHF) by-products whose contribution to cancer risk is not presently known. 3,4-Dichloro-5-hydroxy-2(5H)-furanone (MCA), 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF), and 3- chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF) were studied for the induction of DNA damage, using the alkaline single-cell gel (SCG)/comet assay, and for chromosome damage, using sister-chromatid exchange (SCE) and chromosome aberration (CA) tests, in Chinese hamster ovary (CHO) cells. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), the known genotoxic chlorination by-product and a rat carcinogen, was used as a reference chemical. The SCG analyses were done using concentrations that caused little or no cytotoxicity compared to that of the concurrent control cultures. In the cytogenetic tests, the CHFs were tested up to maximum cytotoxicity. MX and MCA were the most cytotoxic of the compounds in CHO cells followed by CMCF and MCF. All of the CHFs induced DNA damage, SCEs and CAs (mainly chromatid-type breaks and exchanges) in a concentration-related manner, with the exception that MCA was a weak inducer of SCEs. There were no significant differences between the lowest concentration of MX, MCA, and CMCF to cause DNA damage (SCG assay). Based on comparisons of the slopes of regression lines, MX was somewhat more potent than either MCA or CMCF, and MCF was clearly less potent than the other three compounds in the assay. The order of potency was MX > CMCF > MCA > MCF in inducing SCEs and MX > MCA > CMCF > MCF in inducing CAs. The data show that there are differences in the potency of genotoxicity among the CHFs tested. In many cases, however, the extent of maximum effect observed was comparable between the compounds. The results suggest that besides MX other CHFs should be considered in the evaluation of genotoxic risks associated with the consumption of chlorinated drinking water.

Comet assay and DNA flow cytometry analysis of staurosporine-induced apoptosis

BACKGROUND

The ability of the comet assay to quantify DNA strand breaks and alkali labile sites has been widely demonstrated. In this study, this assay was tested for its ability to identify DNA fragmentation occurring during apoptosis in comparison with standard DNA flow cytometry analysis.

METHODS

Staurosporine-induced apoptosis in CHO cells is an adequate model to study a rapid time- and dose-dependent appearance of this process.

RESULTS

Nuclear staining with DAPI confirmed the induction of apoptosis with a typical chromatin condensation and fragmentation. Analysis of propidium-iodide- (PI) stained DNA by flow cytometry showed the presence of a pre-G1 peak, characteristic of apoptotic cells, 6 h after drug treatment. The detection of highly damaged cells (HDC) by the comet assay after 3 h treatment occurred earlier than the detection of apoptotic cells by flow cytometry. However, HDC were missed when the DNA fragmentation was too high, preventing accurate quantification of late apoptotic cells.

CONCLUSIONS

The comet assay is more sensitive than standard DNA flow cytometry to detect early DNA fragmentation events occurring during apoptosis. However, the comet assay modified by omitting electrophoresis was necessary to quantify apoptotic fraction at later stages.

Single cell gel electrophoresis assay: methodology and applications

The single cell gel electrophoresis or Comet assay is a sensitive, reliable, and rapid method for DNA double- and single-strand breaks, alkali-labile sites and delayed repair site detection, in eukaryotic individual cells. Given its overall characteristics, this method has been widely used over the past few years in several different areas. In this paper we review the studies published to date about the principles, the basic methodology with currently used variations. We also explore the applications of this assay in: genotoxicology, clinical area, DNA repair studies, environmental biomonitoring and human monitoring.

DNA damage induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in HL-60 cells and purified DNA in vitro

Chlorinated tap water often contains 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), which is a potent directly acting bacterial mutagen. We have investigated the induction of DNA damage by MX in a promyelocytic human leukaemia cell line (HL-60 cells). Exposure of HL-60 cells to 100-300 microM MX resulted in increased levels of DNA single-strand breaks and/or alkali-labile sites (SSBs) as detected by alkaline filter elution. When adding inhibitors of DNA break repair (AraC plus hydroxyurea), increased levels of DNA SSBs were observed at very low concentrations (1-3 microM) of MX, as observed by both alkaline filter elution and the single-cell gel electrophoresis assay. Increased DNA SSBs could also be observed if DNA repair inhibitors were added immediately after exposure to 10 microM MX, indicating that low concentrations of MX cause a relatively stable modification of DNA that may be recognized and incised by DNA repair enzyme activities. Further studies with DNA break repair inhibitors indicated that HL-60 cells exposed to 10 microM MX for 1 h repaired 50% of their initial DNA damage during a 2-h period and the repair appeared to be complete at 22 h. Analysis of MX-treated DNA by sequencing methods indicated that MX preferentially reacts with guanines in DNA.