Elsevier

Toxicology

Volume 263, Issue 1, 1 September 2009, Pages 12-19
Toxicology

Review
Molecular toxicology of sulfur mustard-induced cutaneous inflammation and blistering

https://doi.org/10.1016/j.tox.2009.01.019Get rights and content

Abstract

Sulfur mustard (SM) is a strong alkylating agent, which produces subepidermal blisters, erythema and inflammation after skin contact. Despite the well-described SM-induced gross and histopathological changes, the exact underlying molecular mechanisms of these events are still a matter of research. As part of an international effort to elucidate the components of cellular signal transduction pathways, a large body of data has been accumulated in the last decade of SM research, revealing deeper insight into SM-induced inflammation, DNA damage response, cell death signaling, and wound healing.

SM potentially alkylates nearly every constituent of the cell, leading to impaired cellular functions. However, SM-induced DNA alkylation has been identified as a major trigger of apoptosis. This includes monofunctional SM-DNA adducts as well as DNA crosslinks. As a consequence, DNA replication is blocked, which leads to cell cycle arrest and DNA single and double strand breaks. The SM-induced DNA damage results in poly(ADP-ribose) polymerase (PARP) activation. High SM concentrations induce PARP overactivation, thus depleting cellular NAD+ and ATP levels, which in consequence results in necrotic cell death. Mild PARP activation does not disturb cellular energy levels and allows apoptotic cell death or recovery to occur. SM-induced apoptosis has been linked both to the extrinsic (death receptor, Fas) and intrinsic (mitochondrial) pathway.

Additionally, SM upregulates many inflammatory mediators including interleukin (IL)-1α, IL-1β, IL-6, IL-8, tumor necrosis factor-α (TNF-α) and others. Recently, several investigators linked NF-κB activation to this inflammatory response.

This review briefly summarizes the skin toxicity of SM, its proposed toxicodynamic actions and strategies for the development of improved medical therapy.

Introduction

Sulfur mustard (SM; 2,2′-dichloroethyl sulfide; CASRN: 505-60-2) is a strongly alkylating agent, which can react with all constituents of the skin. Schematically, the process of SM-induced skin pathology can be divided in three overlapping stages: erythema, blister formation and ulceration (Smith et al., 1919). Skin hyperpigmentation is a frequently observed finding accompanying all SM skin lesions (Balali-Mood and Hefazi, 2005). Typical erythema and skin oedema formation occurs several hours after skin contact, which is followed by subepidermal blisters (Aasted et al., 1987). Erythema can frequently be observed 4–8 h after SM exposure at a threshold dose (vapour: 100–300 mg min/m3, liquid: 10–20 μg/cm2) while blister formation occurs at higher doses (vapour: 1000–2000 mg min/m3, liquid: 40–100 μg/cm2) (Kehe and Szinicz, 2005). The blisters are characterized by small vesicles, which coalesce at a later point in time to gross blisters or large bullae (Balali-Mood and Hefazi, 2006). Exposure to higher concentrations of SM results in ulcers penetrating dermal structures of the skin. These three major skin pathologic findings (erythema, blister, and ulcer) have been linked to a variety of molecular mechanisms (Kehe et al., 2008).

The histopathology of SM affected skin shows vasodilatation and neutrophil infiltrate, which indicates that various vasoactive and chemoattractant mediators are produced in the exposed area (Smith et al., 1997).

The aim of this article is to describe the current knowledge of underlying pathophysiological mechanisms of acute epithelial lesions following SM exposure. Based on this concept rational targets for therapeutical intervention are presented.

Section snippets

Tissue destruction (blister and ulcer)

SM-induced blisters are thin-walled and filled with an amber-coloured fluid. A positive Nikolsky sign was frequently described, which means that rubbing of the skin will produce more blistering. Skin blistering may last for several days to weeks after a single SM exposure (Kehe et al., 2004). These blisters are clinical signs of dermal–epidermal separation of skin layers. Histopathologic analysis reveals marked keratinocyte cell death in the basal layer with signs of a massive inflammatory

PARP signaling

Poly(ADP-ribosyl)ation of cellular proteins in combination with marked depletion of nicotine adenine dinucleotide (NAD+) and adenosine triphosphate (ATP) has been observed after SM exposure (Bhat et al., 2000, Meier et al., 1987) and depends on SM-induced activation of poly(ADP-ribose) polymerases (PARPs) (Fig. 2). PARPs modulate SM-induced cell death and thereby blister formation. This mechanism has been extensively reviewed in this issue of Toxicology (Debiak et al., 2009). PARP-1 and PARP-2

Apoptosis

SM induces concentration-dependent necrotic and apoptotic cell death (Rosenthal et al., 1998, Rosenthal et al., 2001). Necrosis causes cytoplasmatic swelling and cellular rupture with release of intracellular contents which results in damage of surrounding cells and a massive inflammatory response. Even though recent findings also describe necrosis as a well orchestrated process (Festjens et al., 2006), it remains distinct from apoptosis, which needs longer time to be completed, follows a

Calcium signaling and calmodulin

Intracellular Ca++ is mainly stored in the endoplasmatic reticulum (ER). Ca++ release from this store is essential in many cellular signaling pathways. Toxicants can cause significant ER stress with changes in Ca++ homeostasis and induction of cell death e.g. apoptosis (Berridge et al., 2000). Interestingly, SM induces a rise of intracellular levels of free Ca++ in adult and neonatal keratinocytes (Mol and Smith, 1996, Sawyer and Hamilton, 2000). The rise in free Ca++ was moderate in these

Nitric oxide signaling and oxidative stress

Ca++ and the calmodulin proteins have been identified to play an essential role in the formation of nitric oxide. Interestingly, reactive nitrogen species (RNS) and peroxynitrite (ONOO–) have recently been proposed as key mediators of SM-induced cytotoxicity (Korkmaz et al., 2006, Yaren et al., 2007, Sawyer et al., 1996). Nitric oxide is produced by nitrogen oxide synthases (NOSs), which convert the amino acid l-arginine into NOradical dot and l-citrullin. There are three types of NOSs: endothelial NOS

Inflammation

The histopathology of SM damaged skin showed a marked inflammatory response (Smith et al., 1998), which indicates the production or release of various vasoactive and chemoattractant mediators in the affected area. Keratinocytes as the first cells in contact with SM are believed to have a central role in the first phase of initiating this response. As of now, several pathways have been identified to be involved in the release and regulation of gene expression of proinflammatory mediators.

NF-κB pathway and MAPKs

The transcription factor NF-κB (nuclear factor kappa-B) family members share structural homology with the retroviral oncoprotein v-Rel (Rel). NF-κB is composed of the five NF-κB/Rel family members p50 (NF-κB1), p52 (NF-κB2), RelA (p65), RelB and c-Rel, which form homo- or heterodimers. NF-κB, most often composed of p50 and p65/RelA, is involved in several cellular responses related to cellular stress and is a crucial mediator of inflammatory processes (Karin and Greten, 2005) (Fig. 4). NF-κB

Matrix-metalloproteases

Large blister formation after SM injury shares some similarities with epidermolysis bullosa (Monteiro-Riviere et al., 1999). A recent study revealed that 24 h after SM exposure epidermal–dermal separation was associated with a discontinuous pattern of laminin 5 and type VII collagen (Greenberg et al., 2006). Hemidesmosomes contain the two proteins BP230 and BP180 that can be used to characterize the blister plane. The intracellular protein BP230 (also known as BPAG1) associates the

Conclusions

In summary, a huge body of data has accumulated in the past century of research to understand the pathophysiology of SM poisoning (Fig. 5). Several pathways have been identified as playing major roles in signaling SM mediated cytotoxic effects. However, many questions are still open and coordinated research is needed to fill the gaps. With respect to our present knowledge about SM-induced pathophysiology anti-inflammatory drugs (e.g. inhibitors of p38), anti-oxidants (e.g. N-acetylcysteine),

Conflict of interest

The authors declare that there are no competing interests.

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      Commonly, cells initiate repair procedures once DNA damage is monitored. However, a high dose of SM can directly result in cell death via necrosis due to overactivation of poly (ADP-ribose) polymerase-1 (PARP-1) and depletion of nicotinamide adenine dinucleotide (NAD+) without recruitment of the repair system (Kehe et al., 2009). It was reported that SM doses above 500 μM cause both apoptosis and necrosis, while doses below 250 μM produce only apoptosis (Dabrowska et al., 1996).

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