Assessment of Endothelial Cell Migration After Exposure to Toxic Chemicals

Exposure to chemical substances (including alkylating chemical warfare agents like sulfur and nitrogen mustards) cause a plethora of clinical symptoms ...

Bundeswehr Institute of Pharmacology and Toxicology

Sulfur mustard (SM) is a bifunctional alkylating substance being used as chemical warfare agent (vesicant). It is still regarded as a significant threat in chemical warfare and terrorism. Exposure to SM produces cutaneous blisters, respiratory and gastrointestinal tract injury, eye lesions, and bone marrow depression. Victims of World War I as well as those of the Iran-Iraq war have suffered from devastating chronic health impairment. Even decades after exposure, severe long-term effects like chronic obstructive lung disease, lung fibrosis, recurrent corneal ulcer disease, chronic conjunctivitis, abnormal pigmentation of the skin, and different forms of cancer have been diagnosed.

Sulfur mustard research--strategies for the development of improved medical therapy.

The sulphur and nitrogen mustards are strong alkylating agents, which induces inflammations of the skin including blistering right up to ulcerations. Depending on the severity, the wounds may need weeks to heal. In the past it was shown that sulphur mustard has a destructive effect on endothelial precursor cells, which have been shown to play a pivotal role in the wound healing reaction by inducing neovascularisation. However, for these alkylating agents as well as for sulphur mustard nothing is known about their effects on endothelial precursors. Therefore, we investigated and compared the influence of Chlorambucil on proliferation, apoptosis and differentiation of endothelial cells in intact mouse embryoid bodies (EB). EBs were treated at different developmental stages and with different periods of Chlorambucil treatment. It was found that in each developmental stage and under each treatment period's Chlorambucil has an extremely negative effect on the vascularisation with a vessel reduction of around 99%. Of particular importance was the negative effect of treatment around day 3 of the development. On this day we found 377 vessels under control conditions but only 1.6 vessels under 24h treatment of Chlorambucil. At this point in time many endothelial precursors can be found in the EB. Moreover, a negative effect on all stem cells was evident at this point in time, shown by an extreme reduction in EB size with 17.9 mm(2) for the control and only 1.55 mm(2) under Chlorambucil treatment. This negative effect on the vascularisation, on endothelial precursors but also on stem cells in general is of possible importance for impaired wound healing.

Nitrogen mustard (Chlorambucil) has a negative influence on early vascular development.

Whilst the acute effects of sulfur mustard have been relatively well characterised, the chronic effects of short term but significant exposures are still evolving. The approximately 30,000 Iranian victims of CW exposure from the 1980 to 1988 Iran-Iraq war who are currently being followed form a key population who are now 20 years post-exposure. The key chronic findings in this population reflect the common acute effects of sulfur mustard, and are related to the skin, eye and respiratory system. Excluding pruritus, skin changes appear to settle. Eye symptoms are slowly progressive, however a severe, rapid onset form of keratitis is seen to develop in a number of patients after a latent period of 15-20 years. The respiratory tract also shows progressive deterioration, with bronchiolitis obliterans now being considered the main pathological feature of "mustard lung". In addition, there are other potential effects of sulfur mustard exposure which become evident only in the longer term and which are being investigated, including the development of cancer, immunological and neuropsychiatric changes, and reproductive effects. Finally, a chronic effect of sulfur mustard exposure that is now becoming apparent is the wider long-term social and economic effects of these illnesses on individuals and their families.

The chronic effects of sulfur mustard exposure.

Sulfur mustard (SM), an alkylating chemical warfare agent, leads to tissue damage, including inflammation, blister formation, and impaired wound healing. Especially wound healing is of concern because after SM exposure, wound healing is prolonged. In this study, we focused on the effect of SM (30 and 100μM) on endothelial tube formation, apoptosis, and proliferation in mouse embryoid bodies (EBs), which provide an appropriate model for investigating vasculogenesis and angiogenesis. EBs were exposed to SM for 30 min on day 0, 3, or 6 of EBs' growth, were allowed to grow until day 7, then fixed, and immunostained (PECAM-1, Ki67, and activated caspase-3). SM significantly decreased endothelial tube formation compared with unexposed EBs. Additionally, we observed a significant increase of apoptosis. As the formation of reactive oxygen species (ROS) is discussed to be involved in the pathophysiology of SM toxicity, we evaluated the effect of ROS scavengers (α-linolenic acid [ALA] and N-acetyl cysteine [NAC]) in the same experimental setup. Temporary effects of both scavengers could be detected, in particular NAC seemed to have temporary significant positive effects on endothelial tube formation in 100μM SM-exposed EBs. ALA augmented proliferation when administered after 30μM SM exposure on day 3, whereas NAC treatment on day 0 decreased apoptosis induced by 100μM SM. Taken together, our findings pointed to a negative effect of SM on vascularization and endothelial tube formation. ROS scavengers NAC and ALA showed temporary, but not long-lasting, rescuing effects regarding endothelial tube formation after SM exposure.

Effect of N-acetyl cysteine and alpha-linolenic acid on sulfur mustard caused impairment of in vitro endothelial tube formation.

Sulfur mustard (SM) is a vesicating chemical warfare agent causing skin blistering, ulceration, impaired wound healing, prolonged hospitalization and permanent lesions. Silibinin, the lead compound from Silybum marianum, has also been discussed as a potential antidote to SM poisoning. However, its efficacy has been demonstrated only with regard to nitrogen mustards. Moreover, there are no data on the efficacy of the water-soluble prodrug silibinin-bis-succinat (silibinin-BS). We investigated the effect of SIL-BS treatment against SM toxicity in HaCaT cells with regard to potential reduction of necrosis, apoptosis and inflammation including dose-dependency of any protective effects. We also demonstrated the biotransformation of the prodrug into free silibinin. HaCaT cells were exposed to SM (30, 100, and 300μM) for 30min and treated thereafter with SIL-BS (10, 50, and 100μM) for 24h. Necrosis and apoptosis were quantified using the ToxiLight BioAssay and the nucleosome ELISA (CDDE). Pro-inflammatory interleukins-6 and -8 were determined by ELISA. HaCaT cells, incubated with silibinin-BS were lysed and investigated by LC-ESI MS/MS. LC-ESI MS/MS results suggest that SIL-BS is absorbed by HaCaT cells and biotransformed into free silibinin. SIL-BS dose-dependently reduced SM cytotoxicity, even after 300μM exposure. Doses of 50-100μM silibinin-BS were required for significant protection. Apoptosis and interleukin production remained largely unchanged by 10-50μM silibinin-BS but increased after 100μM treatment. Observed reductions of SM cytotoxicity by post-exposure treatment with SIL-BS suggest this as a promising approach for treatment of SM injuries. While 100μM SIL-BS is most effective to reduce necrosis, 50μM may be safer to avoid pro-inflammatory effects. Pro-apoptotic effects after high doses of SIL-BS are in agreement with findings in literature and might even be useful to eliminate cells irreversibly damaged by SM. Further investigations will focus on the protective mechanism of silibinin and its prodrug and should establish an optimum concentration for treatment.

Silibinin as a potential therapeutic for sulfur mustard injuries.

Sulfur Mustard (SM) is a vesicant chemical warfare agent, which is acutely toxic to a variety of organ systems including skin, eyes, respiratory system and bone marrow. The underlying molecular pathomechanism was mainly attributed to the alkylating properties of SM. However, recent studies have revealed that cellular responses to SM exposure are of more complex nature and include increased protein expression and protein modifications that can be used as biomarkers. In order to confirm already known biomarkers, to detect potential new ones and to further elucidate the pathomechanism of SM, we conducted large-scale proteomic experiments based on a human keratinocyte cell line (HaCaT) exposed to SM. Surprisingly, our analysis identified glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) as one of the up-regulated proteins after exposure of HaCaT cells to SM. In this paper we demonstrate the sulfur mustard induced nuclear translocation of GAPDH in HaCaT cells by 2D gel-electrophoresis (2D GE), immunocytochemistry (ICC), Western Blot (WB) and a combination thereof. 2D GE in combination with MALDI-TOF MS/MS analysis identified GAPDH as an up-regulated protein after SM exposure. Immunocytochemistry revealed a distinct nuclear translocation of GAPDH after exposure to 300μM SM. This finding was confirmed by fractionated WB analysis. 2D GE and subsequent immunoblot staining of GAPDH demonstrated two different spot locations of GAPH (pI 7.0 and pI 8.5) that are related to cytosolic or nuclear GAPDH respectively. After exposure to 300μM SM a significant increase of nuclear GAPDH at pI 8.5 occurred. Nuclear GAPDH has been associated with apoptosis, detection of structural DNA alterations, DNA repair and regulation of genomic integrity and telomere structure. The results of our study add new aspects to the pathophysiology of sulfur mustard toxicity, yet further studies will be necessary to reveal the specific function of nuclear GAPDH in the pathomechanism of sulfur mustard.

Sulfur mustard induced nuclear translocation of glyceraldehyde-3-phosphate-dehydrogenase (GAPDH).

Alkylating agents (e.g. sulfur and nitrogen mustards) cause a variety of cell and tissue damage including wound healing disorder. Migration of endothelial cells is of utmost importance for effective wound healing. In this study we investigated the effects of chlorambucil (a nitrogen mustard) on early endothelial cells (EEC) with special focus on cell migration. Chlorambucil significantly inhibited migration of EEC in Boyden chamber and wound healing experiments. Cell migration is linked to cytoskeletal organization. We therefore investigated the distribution pattern of the Golgi apparatus as a marker of cell polarity. Cells are polarized under control conditions, whereas chlorambucil caused an encircling perinuclear position of the Golgi apparatus, indicating non-polarized cells. ROS are discussed to be involved in the pathophysiology of alkylating substances and are linked to cell migration and cell polarity. Therefore we investigated the influence of ROS-scavengers (α-linolenic acid (ALA) and N-acetylcysteine (NAC)) on the impaired EEC migration. Both substances, in particular ALA, improved EEC migration. Notably ALA restored cell polarity. Remarkably, investigations of ROS and RNS biomarkers (8-isoprostane and nitrotyrosine) did not reveal a significant increase after chlorambucil exposure when assessed 24h post exposure. A distinct breakdown of mitochondrial membrane potential (measured by TMRM) that recovered under ALA treatment was observed. In conclusion our results provide compelling evidence that the alkylating agent chlorambucil dramatically impairs directed cellular migration, which is accompanied by perturbations of cell polarity and mitochondrial membrane potential. ALA treatment was able to reconstitute cell polarity and to stabilize mitochondrial potential resulting in improved cell migration.

Chlorambucil (nitrogen mustard) induced impairment of early vascular endothelial cell migration - effects of α-linolenic acid and N-acetylcysteine.

Sulfur mustard (SM) is a chemical warfare agent causing skin blistering, ulceration and delayed wound healing. Inflammation and extrinsic apoptosis are known to have an important role in SM-induced cytotoxicity. As immune cells are involved in those processes, they may significantly modulate SM toxicity, but the extent of those effects is unknown. We adapted a co-culture model of immortalized keratinocytes (HaCaT) and immune cells (THP-1) and exposed this model to SM. Changes in necrosis, apoptosis and inflammation, depending on SM challenge, absence or presence and number of THP-1 cells were investigated. THP-1 were co-cultured for 24h prior to SM exposure in order to model SM effects on immune cells continuously present in the skin. Our results indicate that the presence of THP-1 strongly increased necrosis, apoptosis and inflammation. This effect was already significant when the ratio of THP-1 and HaCaT cells was similar to the ratio of Langerhans immune cells and keratinocytes in vivo. Any further increases in the number of THP-1 had only slight additional effects on SM-induced cytotoxicity. In order to assess the effects of immune cells migrating into skin areas damaged by SM, we added non-exposed THP-1 to SM-exposed HaCaT. Those THP-1 had only slight effects on SM-induced cytotoxicity. Notably, in HaCaT exposed to 300μM SM, necrosis and inflammation were slightly reduced by adding intact THP-1. This effect was dependent on the number of immune cells, steadily increasing with the number of unexposed THP-1 added. In summary, we have demonstrated that (a) the presented co-culture is a robust model to assess SM toxicity and can be used to test the efficacy of potential antidotes in vitro; (b) immune cells, damaged by SM strongly amplified cytotoxicity, (c) in contrast, unexposed THP-1 (simulating migration of immune cells into affected areas after exposure in vivo) had no pronounced adverse, but exhibited some protective effects. Thus, protecting immune cells from SM toxicity may help to reduce overall injury.

Development of a co-culture of keratinocytes and immune cells for in vitro investigation of cutaneous sulfur mustard toxicity.

The transient receptor potential ankyrin 1 (TRPA1) cation channel is expressed in different tissues including skin, lung and neuronal tissue. Recent reports identified TRPA1 as a sensor for noxious substances, implicating a functional role in the molecular toxicology. TRPA1 is activated by various potentially harmful electrophilic substances. The chemical warfare agent sulfur mustard (SM) is a highly reactive alkylating agent that binds to numerous biological targets. Although SM is known for almost 200 years, detailed knowledge about the pathophysiology resulting from exposure is lacking. A specific therapy is not available. In this study, we investigated whether the alkylating agent 2-chloroethyl-ethylsulfide (CEES, a model substance for SM-promoted effects) and SM are able to activate TRPA1 channels. CEES induced a marked increase in the intracellular calcium concentration ([Ca(2+)]i) in TRPA1-expressing but not in TRPA1-negative cells. The TRP-channel blocker AP18 diminished the CEES-induced calcium influx. HEK293 cells permanently expressing TRPA1 were more sensitive toward cytotoxic effects of CEES compared with wild-type cells. At low CEES concentrations, CEES-induced cytotoxicity was prevented by AP18. Proof-of-concept experiments using SM resulted in a pronounced increase in [Ca(2+)]i in HEK293-A1-E cells. Human A549 lung epithelial cells, which express TRPA1 endogenously, reacted with a transient calcium influx in response to CEES exposure. The CEES-dependent calcium response was diminished by AP18. In summary, our results demonstrate that alkylating agents are able to activate TRPA1. Inhibition of TRPA1 counteracted cellular toxicity and could thus represent a feasible approach to mitigate SM-induced cell damage.

Frank Balszuweit

Bundeswehr Institute of Pharmacology and Toxicology

Assessment of Endothelial Cell Migration After Exposure to Toxic Chemicals