Alterations in human lymphocyte DNA caused by sulfur mustard can be mitigated by selective inhibitors of poly(ADP-ribose) polymerase

Evaluation of decontamination efficacy of electrolytically generated hypochlorous acid for vesicating agent: A multimodel Study

BACKGROUND

Sulfur Mustard is a strong vesicant and chemical warfare agent that imposes toxicity to the lungs, eyes, and skin after accidental or intended exposure.

OBJECTIVES

The current study was intended to explore in vitro and in vivo decontamination properties of electrolytically generated HOCl (hypochlorous acid) against CEES (2-chloroethyle ethyle sulphide), a known sulfur mustard simulant & vesicating agent.

METHODS

In vitro studies were carried out using UV spectroscopy and GC-MS methods. In vivo studies were perfomred in Strain A and immune compromised mice by subcutaneous as well as prophylactic topical administrion of HOCl pretreated CEES. The blister formation and mortality were considered as end-point. Histopathological study was conducted on skin samples by H & E method. DNA damage studies measuring γ-H2AX and ATM has been carried out in human blood using flow cytometry. Anti-bacterial action was tested by employing broth micro dilution methods. Comparative study was also carried out with known oxidizing agents.

RESULTS

The topical application of pre-treated CEES at 5, 30 min and 1 h time points showed significant (p<0.001) inhibition of blister formation. DNA damage study showed reduced mean flourences intensity of DSBs nearly 17-20 times, suggesting that HOCl plays a protective role against DNA damage. Histopathology showed no sign of necrosis in the epidermis upto 5 min although moderate changes were observed at 30 min. Pretreated samples were analyzed for detection of reaction products with m/z value of 75.04, 69.08, 83.93, 85.95, 123.99, 126.00, and 108.97. HOCl showed strong bactericidal effect at 40 ppm. The absorbance spectra of HOCl treated CEES showed lowered peaks in comparison to CEES alone and other oxidizing agents Conclusion: In a nutshell, our results signify the decontamination role of HOCl for biological surface application.

Sulfur and nitrogen mustards induce characteristic poly(ADP-ribosyl)ation responses in HaCaT keratinocytes with distinctive cellular consequences

Mustard agents are potent DNA alkylating agents with mutagenic, cytotoxic and vesicant properties. They include bi-functional agents, such as sulfur mustard (SM) or nitrogen mustard (mustine, HN2), as well as mono-functional agents, such as "half mustard" (CEES). Whereas SM has been used as a chemical warfare agent, several nitrogen mustard derivatives, such as chlorambucil and cyclophosphamide, are being used as established chemotherapeutics. Upon induction of specific forms of genotoxic stimuli, several poly(ADP-ribose) polymerases (PARPs) synthesize the nucleic acid-like biopolymer poly(ADP-ribose) (PAR) by using NAD(+) as a substrate. Previously, it was shown that SM triggers cellular poly(ADP-ribosyl) ation (PARylation), but so far this phenomenon is poorly characterized. In view of the protective effects of PARP inhibitors, the latter have been proposed as a treatment option of SM-exposed victims. In an accompanying article (Debiak et al., 2016), we have provided an optimized protocol for the analysis of the CEES-induced PARylation response in HaCaT keratinocytes, which forms an experimental basis to further analyze mustard-induced PARylation and its functional consequences, in general. Thus, in the present study, we performed a comprehensive characterization of the PARylation response in HaCaT cells after treatment with four different mustard agents, i.e., SM, CEES, HN2, and chlorambucil, on a qualitative, quantitative and functional level. In particular, we recorded substance-specific as well as dose- and time-dependent PARylation responses using independent bioanalytical methods based on single-cell immuno-fluorescence microscopy and quantitative isotope dilution mass spectrometry. Furthermore, we analyzed if and how PARylation contributes to mustard-induced toxicity by treating HaCaT cells with CEES, SM, and HN2 in combination with the clinically relevant PARP inhibitor ABT888. As evaluated by a novel immunofluorescence-based protocol for the detection of N7-ETE-guanine DNA adducts, the excision rate of CEES-induced DNA adducts was not affected by PARP inhibition. Furthermore, while CEES induced moderate changes in cellular NAD(+) levels, annexin V/PI flow cytometry analysis revealed that these changes did not affect CEES-induced short-term cytotoxicity 24h after treatment. In contrast, PARP inhibition impaired cell proliferation and clonogenic survival, and potentiated micronuclei formation of HaCaT cells upon CEES treatment. Similarly, PARP inhibition affected clonogenic survival of cells treated with bi-functional mustards such as SM and HN2. In conclusion, we demonstrate that PARylation plays a functional role in mustard-induced cellular stress response with substance-specific differences. Since PARP inhibitors exhibit therapeutic potential to treat SM-related pathologies and to sensitize cancer cells for mustard-based chemotherapy, potential long-term effects of PARP inhibition on genomic stability and carcinogenesis should be carefully considered when pursuing such a strategy.

Whole genome expression analysis in primary bronchial epithelial cells after exposure to sulphur mustard

Sulphur mustard (SM) is a highly toxic chemical agent and poses a current threat to both civilians and military personnel in the event of a deliberate malicious release. Acute SM toxicity develops over the course of several hours and mainly affects the skin and mucosal surfaces of the eyes and respiratory system. In cases of acute severe exposure, significant lung injury can result in respiratory failure and death. Systemic levels of SM can also be fatal, frequently due to immunodepletion and the subsequent development of secondary infections. Whilst the physical effects associated with SM exposure are well documented, the molecular mechanisms mediating these changes are poorly understood, hindering the development of an effective therapeutic strategy. To gain a better understanding of the mechanism of SM toxicity, this study investigated whole genome transcriptional changes after SM in primary human bronchial epithelial cells, as a model for inhalation exposure. The analysis revealed >400 transcriptional changes associated with SM exposure. Pathways analysis confirmed the findings of previous studies suggesting that DNA damage, cell cycle arrest, cell death and inflammation were important components of SM toxicity. In addition, several other interesting observations were made, suggesting that protein oxidation as well as effects on the mitotic apparatus may contribute to SM toxicity.

Protective effect of liposome-encapsulated glutathione in a human epidermal model exposed to a mustard gas analog

Sulfur mustard or mustard gas (HD) and its monofunctional analog, 2-chloroethyl ethyl sulfide (CEES), or "half-mustard gas," are alkylating agents that induce DNA damage, oxidative stress, and inflammation. HD/CEES are rapidly absorbed in the skin causing extensive injury. We hypothesize that antioxidant liposomes that deliver both water-soluble and lipid-soluble antioxidants protect skin cells from immediate CEES-induced damage via attenuating oxidative stress. Liposomes containing water-soluble antioxidants and/or lipid-soluble antioxidants were evaluated using in vitro model systems. Initially, we found that liposomes containing encapsulated glutathione (GSH-liposomes) increased cell viability and attenuated production of reactive oxygen species (ROS) in HaCaT cells exposed to CEES. Next, GSH-liposomes were tested in a human epidermal model, EpiDerm. In the EpiDerm, GSH-liposomes administered simultaneously or 1 hour after CEES exposure (2.5 mM) increased cell viability, inhibited CEES-induced loss of ATP and attenuated changes in cellular morphology, but did not reduce caspase-3 activity. These findings paralleled the previously described in vivo protective effect of antioxidant liposomes in the rat lung and established the effectiveness of GSH-liposomes in a human epidermal model. This study provides a rationale for use of antioxidant liposomes against HD toxicity in the skin considering further verification in animal models exposed to HD.

Sodium Pyruvate Modulates Cell Death Pathways in HaCaT Keratinocytes Exposed to Half-Mustard Gas

2-Chloroethyl ethyl sulfide (CEES) or half-mustard gas, a sulfur mustard (HD) analog, is a genotoxic agent that causes oxidative stress and induces both apoptotic and necrotic cell death. Sodium pyruvate induced a necrosis-to-apoptosis shift in HaCaT cells exposed to CEES levels ≤ 1.5 mmol/L and lowered markers of DNA damage, oxidative stress, and inflammation. This study provides a rationale for the future development of multicomponent therapies for HD toxicity in the skin. We hypothesize that a combination of pyruvates with scavengers/antioxidants encapsulated in liposomes for optimal local delivery should be therapeutically beneficial against HD-induced skin injury. However, the latter suggestion should be verified in animal models exposed to HD.

Genomics and proteomics in chemical warfare agent research: recent studies and future applications

Medical research on the effects of chemical warfare agents (CWAs) has been ongoing for nearly 100 years, yet these agents continue to pose a serious threat to deployed military forces and civilian populations. CWAs are extremely toxic, relatively inexpensive, and easy to produce, making them a legitimate weapon of choice for terrorist organizations. While the mechanisms of action for many CWAs have been known for years, questions about their molecular effects following acute and chronic exposure remain largely unanswered. Global approaches that can pinpoint which cellular pathways are altered in response to CWAs and characterize long-term toxicity have not been widely used. Fortunately, innovations in genomics and proteomics technologies now allow for thousands of genes and proteins to be identified and subsequently quantified in a single experiment. Advanced bioinformatics software can also help decipher large-scale changes observed, leading to mapping of signaling pathways, functional characterization, and identification of potential therapeutic targets. Here we present an overview of how genomics and proteomics technologies have been applied to CWA research and also provide a series of questions focused on how these techniques could further our understanding of CWA toxicity.

Ex vivo supplementation with nicotinic acid enhances cellular poly(ADP-ribosyl)ation and improves cell viability in human peripheral blood mononuclear cells

Poly(ADP-ribosyl)ation is a posttranslational modification of proteins, which is mainly catalyzed by poly(ADP-ribose) polymerase-1 (PARP-1) by using NAD(+) as substrate and is directly triggered by DNA strand breaks. Under mild genotoxic stress poly(ADP-ribose) (PAR) formation plays an important role in DNA repair whereas severe genotoxic stress and the ensuing overactivation of PARP-1 induce cellular NAD(+) depletion, energy failure and ultimately cell death. We are interested in studying the consequences of moderately enhanced enzymatic activity under conditions of DNA damage. Here we chose supplementation of cells with the NAD(+) precursor nicotinic acid (NA) as a strategy. In order to reliably assess PAR accumulation in living cells we first developed a novel, sensitive flow-cytometric method for the rapid analysis of poly(ADP-ribose) accumulation (RAPARA). Our data showed that ex vivo supplementation of human peripheral blood mononuclear cells (PBMC) with low concentrations of NA significantly raised cellular NAD(+) levels by 2.1-fold. Upon X-irradiation or exposure to hydrogen peroxide or N-methyl-N'-nitro-N-nitrosoguanidine, PAR accumulation was significantly increased and sustained in NA-supplemented cells. Furthermore, NA-supplemented PBMC displayed significantly higher cell viability due to a lower rate of necrotic cell death. In summary, ex vivo supplementation of human PBMC with NA increases cellular NAD(+) levels, boosts the cellular poly(ADP-ribosyl)ation response to genotoxic treatment, and protects from DNA-damage-induced cell death.

Genotoxicants target distinct molecular networks in neonatal neurons

BACKGROUND

Exposure of the brain to environmental agents during critical periods of neuronal development is considered a key factor underlying many neurologic disorders.

OBJECTIVES

In this study we examined the influence of genotoxicants on cerebellar function during early development by measuring global gene expression changes.

METHODS

We measured global gene expression in immature cerebellar neurons (i.e., granule cells) after treatment with two distinct alkylating agents, methylazoxymethanol (MAM) and nitrogen mustard (HN2). Granule cell cultures were treated for 24 hr with MAM (10-1,000 microM) or HN2 (0.1-20 microM) and examined for cell viability, DNA damage, and markers of apoptosis.

RESULTS

Neuronal viability was significantly reduced (p < 0.01) at concentrations > 500 microM for MAM and > 1.0 microM for HN2; this correlated with an increase in both DNA damage and markers of apoptosis. Neuronal cultures treated with sublethal concentrations of MAM (100 microM) or HN2 (1.0 microM) were then examined for gene expression using large-scale mouse cDNA microarrays (27,648). Gene expression results revealed that a) global gene expression was predominantly up-regulated by both genotoxicants; b) the number of down-regulated genes was approximately 3-fold greater for HN2 than for MAM; and c) distinct classes of molecules were influenced by MAM (i.e, neuronal differentiation, the stress and immune response, and signal transduction) and HN2 (i.e, protein synthesis and apoptosis).

CONCLUSIONS

These studies demonstrate that individual genotoxicants induce distinct gene expression signatures. Further study of these molecular networks may explain the variable response of the developing brain to different types of environmental genotoxicants.

Calmodulin mediates sulfur mustard toxicity in human keratinocytes

Sulfur mustard (SM) causes blisters in the skin through a series of cellular changes that we are beginning to identify. We earlier demonstrated that SM toxicity is the result of induction of both death receptor and mitochondrial pathways of apoptosis in human keratinocytes (KC). Because of its importance in apoptosis in the skin, we tested whether calmodulin (CaM) mediates the mitochondrial apoptotic pathway induced by SM. Of the three human CaM genes, the predominant form expressed in KC was CaM1. RT-PCR and immunoblot analysis revealed upregulation of CaM expression following SM treatment. To delineate the potential role of CaM1 in the regulation of SM-induced apoptosis, retroviral vectors expressing CaM1 RNA in the antisense (AS) orientation were used to transduce and derive stable CaM1 AS cells, which were then exposed to SM and subjected to immunoblot analysis for expression of apoptotic markers. Proteolytic activation of executioner caspases-3, -6, -7, and the upstream caspase-9, as well as caspase-mediated PARP cleavage were markedly inhibited by CaM1 AS expression. CaM1 AS depletion attenuated SM-induced, but not Fas-induced, proteolytic processing and activation of caspase-3. Whereas control KC exhibited a marked increase in apoptotic nuclear fragmentation after SM, CaM1 AS cells exhibited normal nuclear morphology up to 48h after SM, indicating that suppression of apoptosis in CaM1 AS cells increases survival and does not shift to a necrotic death. CaM has been shown to activate the phosphatase calcineurin, which can induce apoptosis by Bad dephosphorylation. Interestingly, whereas SM-treated CaM1-depleted KC expressed the phosphorylated non-apoptotic sequestered form of Bad, Bad was present in the hypophosphorylated apoptotic form in SM-exposed control KC. To determine if pharmacological CaM inhibitors could attenuate SM-induced apoptosis via Bad dephosphorylation, KC were pretreated with the CaM-specific antagonist W-13 or its less active structural analogue W-12. Following SM exposure, KC exhibited Bad dephosphorylation, which was inhibited in the presence of W-13, but not with W-12. Consequently, W-13 but not W-12 markedly suppressed SM-induced proteolytic processing and activation of caspase-3, as well as apoptotic nuclear fragmentation. Finally, while the CaM antagonist W-13 and the calcineurin inhibitor cyclosporin A attenuated SM-induced caspase-3 activation, inhibitors for CaM-dependent protein kinase II (KN62 and KN93) did not. These results indicate that CaM, calcineurin, and Bad also play a role in SM-induced apoptosis, and may therefore be targets for therapeutic intervention to reduce SM injury.

Inhibition of caspase-dependent mitochondrial permeability transition protects airway epithelial cells against mustard-induced apoptosis

In the present study, the toxicity of yperite, SM, and its structural analogue mechlorethamine, HN2, was investigated in a human bronchial epithelial cell line 16HBE. Cell detachment was initiated by caspase-2 activation, down-regulation of Bcl-2 and loss of mitochondrial membrane potential. Only in detached cells, mustards induced apoptosis associated with increase in p53 expression, Bax activation, decrease in Bcl-2 expression, opening of the mitochondrial permeability transition pore, release of cytochrome c, caspase-2, -3, -8, -9 and -13 activation and DNA fragmentation. Apoptosis, occurring only in detached cells, could be recognized as anoikis and the mitochondrion, involved both in cell detachment and subsequent cell death, appears to be a crucial checkpoint. Based on our understanding of the apoptotic pathway triggered by mustards, we demonstrated that inhibition of the mitochondrial pathway by ebselen, melatonin and cyclosporine A markedly prevented mustard-induced anoikis, pointing to these drugs as interesting candidates for the treatment of mustard-induced airway epithelial lesions.

Microarray analysis of gene expression in murine skin exposed to sulfur mustard

The chemical warfare agent sulfur mustard [bis-(2-chloroethyl)-sulfide; SM] produces a delayed inflammatory response followed by blister formation in skin of exposed individuals. Studies are underway evaluating the efficacy of pharmacological compounds to protect against SM skin injury. Microarray analysis provides the opportunity to identify multiple transcriptional biomarkers associated with SM exposure. This study examined SM-induced changes in gene expression in skin from mice cutaneously exposed to SM using cDNA microarrays. Ear skin from five mice, paired as SM-exposed right ear and dichloromethane vehicle-exposed left ear at six dose levels (0.005, 0.01, 0.02, 0.04, 0.08, and 0.16 mg; 6 mM to 195 mM range), was harvested at 24 h post-exposure. SM-induced gene expression was analyzed using cDNA microarrays that included 1,176 genes. Genes were selected on the basis of all mice (N=5) in the same dose group demonstrating a > or =2-fold increase or decrease in gene expression for the SM-exposed tissue compared to the dichloromethane vehicle control ear tissue at all six SM doses. When skin exposed to all six concentrations of SM was compared to controls, a total of 19 genes within apoptosis, transcription factors, cell cycle, inflammation, and oncogenes and tumor suppressors categories were found to be upregulated; no genes were observed to be downregulated. Differences in the number and category of genes that were up- or down-regulated in skin exposed to low (0.005-0.01 mg) and high (0.08-0.16 mg) doses of SM were also observed. The results of this study provide a further understanding of the molecular responses to cutaneous SM exposure, and enable the identification of potential diagnostic markers and therapeutic targets for treating SM injury.

Expression of dominant-negative Fas-associated death domain blocks human keratinocyte apoptosis and vesication induced by sulfur mustard

DNA damaging agents up-regulate levels of the Fas receptor or its ligand, resulting in recruitment of Fas-associated death domain (FADD) and autocatalytic activation of caspase-8, consequently activating the executioner caspases-3, -6, and -7. We found that human epidermal keratinocytes exposed to a vesicating dose (300 microm) of sulfur mustard (SM) exhibit a dose-dependent increase in the levels of Fas receptor and Fas ligand. Immunoblot analysis revealed that the upstream caspases-8 and -9 are both activated in a time-dependent fashion, and caspase-8 is cleaved prior to caspase-9. These results are consistent with the activation of both death receptor (caspase-8) and mitochondrial (caspase-9) pathways by SM. Pretreatment of keratinocytes with a peptide inhibitor of caspase-3 (Ac-DEVD-CHO) suppressed SM-induced downstream markers of apoptosis. To further analyze the importance of the death receptor pathway in SM toxicity, we utilized Fas- or tumor necrosis factor receptor-neutralizing antibodies or constructs expressing a dominant-negative FADD (FADD-DN) to inhibit the recruitment of FADD to the death receptor complex and block the Fas/tumor necrosis factor receptor pathway following SM exposure. Keratinocytes pretreated with Fas-blocking antibody or stably expressing FADD-DN and exhibiting reduced levels of FADD signaling demonstrated markedly decreased caspase-3 activity when treated with SM. In addition, the processing of procaspases-3, -7, and -8 into their active forms was observed in SM-treated control keratinocytes, but not in FADD-DN cells. Blocking the death receptor complex by expression of FADD-DN additionally inhibited SM-induced internucleosomal DNA cleavage and caspase-6-mediated nuclear lamin cleavage. Significantly, we further found that altering the death receptor pathway by expressing FADD-DN in human skin grafted onto nude mice reduces vesication and tissue injury in response to SM. These results indicate that the death receptor pathway plays a pivotal role in SM-induced apoptosis and is therefore a target for therapeutic intervention to reduce SM injury.

PARP determines the mode of cell death in skin fibroblasts, but not keratinocytes, exposed to sulfur mustard

Sulfur mustard is cytotoxic to dermal fibroblasts as well as epidermal keratinocytes. We demonstrated that poly(ADP-ribose) polymerase (PARP) modulates Fas-mediated apoptosis, and other groups and we have shown that PARP plays a role in the modulation of other types of apoptotic and necrotic cell death. We have now utilized primary dermal fibroblasts, immortalized fibroblasts, and keratinocytes derived from PARP(-/-) mice and their wildtype littermates (PARP(+/+)) to determine the contribution of PARP to sulfur mustard toxicity. Following sulfur mustard exposure, primary skin fibroblasts from PARP-deficient mice demonstrated increased internucleosomal DNA cleavage, caspase-3 processing and activity, and annexin V positivity, compared to those derived from PARP(+/+) animals. Conversely, propidium iodide staining, PARP cleavage patterns, and random DNA fragmentation revealed a dose-dependent increase in necrosis in PARP(+/+) but not PARP(-/-) cells. Using immortalized PARP(-/-) fibroblasts stably transfected with the human PARP cDNA or with empty vector alone, we show that PARP inhibits markers of apoptosis in these cells as well. Finally, primary keratinocytes were derived from newborn PARP(+/+) and PARP(-/-) mice and immortalized with the E6 and E7 genes of human papilloma virus. In contrast to fibroblasts, keratinocytes from both PARP(-/-) and PARP(+/+) mice express markers of apoptosis in response to sulfur mustard exposure. The effects of PARP on the mode of cell death in different skin cell types may determine the severity of vesication in vivo, and thus have implications for the design of PARP inhibitors to reduce sulfur mustard pathology.

TANK2, a new TRF1-associated poly(ADP-ribose) polymerase, causes rapid induction of cell death upon overexpression

Tankyrase (TANK1) is a human telomere-associated poly(ADP-ribose) polymerase (PARP) that binds the telomere-binding protein TRF1 and increases telomere length when overexpressed. Here we report characterization of a second human tankyrase, tankyrase 2 (TANK2), which can also interact with TRF1 but has properties distinct from those of TANK1. TANK2 is encoded by a 66-kilobase pair gene (TNKS2) containing 28 exons, which express a 6.7-kilobase pair mRNA and a 1166-amino acid protein. The protein shares 85% amino acid identity with TANK1 in the ankyrin repeat, sterile alpha-motif, and PARP catalytic domains but has a unique N-terminal domain, which is conserved in the murine TNKS2 gene. TANK2 interacted with TRF1 in yeast and in vitro and localized predominantly to a perinuclear region, similar to the properties of TANK1. In contrast to TANK1, however, TANK2 caused rapid cell death when highly overexpressed. TANK2-induced death featured loss of mitochondrial membrane potential, but not PARP1 cleavage, suggesting that TANK2 kills cells by necrosis. The cell death was prevented by the PARP inhibitor 3-aminobenzamide. In vivo, TANK2 may differ from TANK1 in its intrinsic or regulated PARP activity or its substrate specificity.

DNA fragmentation pattern induced in thymocytes by sulphur mustard

Sulphur mustard (HD) is a blister agent for which no specific therapy exists. The mechanism of cell injury caused by HD is not well understood. This study examined DNA damage in thymocytes exposed to a range of HD concentrations over a time course of 1-24 h. Thymocytes incubated with HD showed an increase in the production of DNA fragments of the type frequently associated with apoptosis, namely, initial formation of large fragments of 30-50, 200-300 and > 700 kilobase pairs (kbp), followed by further degradation to produce an internucleosomal 'ladder' of oligomers of approximately 180 base pairs (bp). Pulsed field electrophoresis analysis of thymocytes incubated with HD detected breakdown of the chromatin up to 3 h before a corresponding increase in the low molecular weight (MW) oligonucleosomal fragments could be seen on conventional agarose gels. These results suggest that cells damaged by HD poisoning may be irretrievably committed to cell death sooner after exposure than previous studies suggested. The nature of the DNA fragments produced suggested that apoptosis may represent a component of the pathway of cell death induced by HD. These aspects may have implications for the search for specific therapeutic reagents effective in the prevention or treatment of HD poisoning.