Cigarette smoke and asbestos activate poly-ADP-ribose polymerase in alveolar epithelial cells

Association of serum ferritin and lung function in tobacco-naïve postmenopausal women: Analysis of population-based nationally representative data

INTRODUCTION

Ferritin regulates iron homeostasis, and is involved in the inflammation in the lung, especially in smokers; however, its associations on pulmonary function in nonsmokers remain unclear.

OBJECTIVES

The present study aimed to evaluate the association between serum ferritin and lung function in a tobacco-naïve postmenopausal women.

METHODS

In this study, 25 534 individuals were enrolled, among who 5338 tobacco-naïve individuals were identified; of those, 342 men and 2879 women (742 pre- and 2137 postmenopausal) with data of serum ferritin, lung function and covariates were included. To evaluate the association of ferritin and lung function, multivariable-adjusted linear regression analyses was used including the factors of predicted value of forced expiratory volume in 1 second (FEV1 %) and forced vital capacity (FVC%). Logistic regression analyses were used to measure the relationship between ferritin and restrictive and obstructive lung disease.

RESULTS

In premenopausal women, FEV1 %/FVC was weakly but positively associated with serum ferritin, and after adjusting for covariates, the association was without statistical significance. No significant association between ferritin and obstructive lung disease was observed. In postmenopausal women, predicted FVC% was negatively associated with serum ferritin, and ferritin was dose-dependently related with risk for restrictive lung disease. The odds ratio for restrictive lung disease in postmenopausal women was 2.285 at T3 and 1.560 at T2 relative to that at T1.

CONCLUSIONS

High serum ferritin level was significantly associated with lower FVC% and increased risk of restrictive lung disease in tobacco-naïve postmenopausal women. Further study is needed to determine the mechanism underlying the current findings.

P21-PARP-1 pathway is involved in cigarette smoke-induced lung DNA damage and cellular senescence

Persistent DNA damage triggers cellular senescence, which may play an important role in the pathogenesis of cigarette smoke (CS)-induced lung diseases. Both p21^CDKN1A (p21) and poly(ADP-ribose) polymerase-1 (PARP-1) are involved in DNA damage and repair. However, the role of p21-PARP-1 axis in regulating CS-induced lung DNA damage and cellular senescence remains unknown. We hypothesized that CS causes DNA damage and cellular senescence through a p21-PARP-1 axis. To test this hypothesis, we determined the levels of γH2AX (a marker for DNA double-strand breaks) as well as non-homologous end joining proteins (Ku70 and Ku80) in lungs of mice exposed to CS. We found that the level of γH2AX was increased, whereas the level of Ku70 was reduced in lungs of CS-exposed mice. Furthermore, p21 deletion reduced the level of γH2AX, but augmented the levels of Ku70, Ku80, and PAR in lungs by CS. Administration of PARP-1 inhibitor 3-aminobenzamide increased CS-induced DNA damage, but lowered the levels of Ku70 and Ku80, in lungs of p21 knockout mice. Moreover, 3-aminobenzamide increased senescence-associated β-galactosidase activity, but decreased the expression of proliferating cell nuclear antigen in mouse lungs in response to CS. Interestingly, 3-aminobenzamide treatment had no effect on neutrophil influx into bronchoalveolar lavage fluid by CS. These results demonstrate that the p21-PARP-1 pathway is involved in CS-induced DNA damage and cellular senescence.

Poly(ADP-ribosyl)ation is a survival mechanism in cigarette smoke-induced and hydrogen peroxide-mediated cell death

Cigarette smoking can contribute to the development of many human diseases such as cardiovascular disease, lung cancer, asthma, and chronic obstructive pulmonary disease. Thousands of compounds are present in cigarette smoke, including a large number of reactive oxygen species that can cause DNA damage, leading to the activation of poly(ADP-ribose) polymerase (PARP) enzymes. The PAR polymer is degraded by poly(ADP-ribose) glycohydrolase (PARG). Here we have investigated the effects of cigarette smoke extract (CSE) on A549 human lung epithelial cells. CSE induced DNA damage (comet assay), PAR accumulation (immunofluorescence and immunoblotting), impaired proliferation (clonogenic survival assay and electric cell-substrate impedance sensing measurement), and cell death (MTT reduction, propidium iodide uptake, lactate dehydrogenase release). CSE-induced cell death was also characterized by mitochondrial depolarization but massive translocation of apoptosis-inducing factor could not be observed. To investigate the role of PARylation in CSE-induced oxidative stress, PARP-1- and PARG-silenced A549 cells were used. Silencing of both PARP-1 and PARG sensitized cells to CSE-induced toxicity: PARP-1- and PARG-silenced cell lines exhibited reduced clonogenic survival, displayed a delayed repair of DNA breaks, and showed higher levels of cytotoxicity. CSE triggered the production of mitochondrial superoxide and hydrogen peroxide. Addition of superoxide dismutase increased, whereas catalase abolished, CSE-induced PAR formation. In summary, our data show that the superoxide-hydrogen peroxide-DNA breakage pathway activates the PAR cycle by PARP-1 and PARG, which serves as a survival mechanism in CSE-exposed cells. Our data also raise the possibility that the PARP-1/PARG status of smokers may be an important determinant of the efficiency of DNA repair in their lungs and of their susceptibility to CS-induced carcinogenesis.

Role of mutagenicity in asbestos fiber-induced carcinogenicity and other diseases

The cellular and molecular mechanisms of how asbestos fibers induce cancers and other diseases are not well understood. Both serpentine and amphibole asbestos fibers have been shown to induce oxidative stress, inflammatory responses, cellular toxicity and tissue injuries, genetic changes, and epigenetic alterations in target cells in vitro and tissues in vivo. Most of these mechanisms are believe to be shared by both fiber-induced cancers and noncancerous diseases. This article summarizes the findings from existing literature with a focus on genetic changes, specifically, mutagenicity of asbestos fibers. Thus far, experimental evidence suggesting the involvement of mutagenesis in asbestos carcinogenicity is more convincing than asbestos-induced fibrotic diseases. The potential contributions of mutagenicity to asbestos-induced diseases, with an emphasis on carcinogenicity, are reviewed from five aspects: (1) whether there is a mutagenic mode of action (MOA) in fiber-induced carcinogenesis; (2) mutagenicity/carcinogenicity at low dose; (3) biological activities that contribute to mutagenicity and impact of target tissue/cell type; (4) health endpoints with or without mutagenicity as a key event; and finally, (5) determinant factors of toxicity in mutagenicity. At the end of this review, a consensus statement of what is known, what is believed to be factual but requires confirmation, and existing data gaps, as well as future research needs and directions, is provided.

XRCC1 deficiency sensitizes human lung epithelial cells to genotoxicity by crocidolite asbestos and Libby amphibole

BACKGROUND

Asbestos induces DNA and chromosomal damage, but the DNA repair pathways protecting human cells against its genotoxicity are largely unknown. Polymorphisms in XRCC1 have been associated with altered susceptibility to asbestos-related diseases. However, it is unclear whether oxidative DNA damage repaired by XRCC1 contributes to asbestos-induced chromosomal damage.

OBJECTIVES

We sought to examine the importance of XRCC1 in protection against genotoxic effects of crocidolite and Libby amphibole asbestos.

METHODS

We developed a genetic model of XRCC1 deficiency in human lung epithelial H460 cells and evaluated genotoxic responses to carcinogenic fibers (crocidolite asbestos, Libby amphibole) and nongenotoxic materials (wollastonite, titanium dioxide).

RESULTS

XRCC1 knockdown sensitized cells to the clastogenic and cytotoxic effects of oxidants [hydrogen peroxide (H₂O₂), bleomycin] but not to the nonoxidant paclitaxel. XRCC1 knockdown strongly enhanced genotoxicity of amphibole fibers as evidenced by elevated formation of clastogenic micronuclei. Crocidolite induced primarily clastogenic micronuclei, whereas Libby amphibole induced both clastogenic and aneugenic micronuclei. Crocidolite and bleomycin were potent inducers of nuclear buds, which were enhanced by XRCC1 deficiency. Libby amphibole and H₂O₂ did not induce nuclear buds, irrespective of XRCC1 status. Crocidolite and Libby amphibole similarly activated the p53 pathway.

CONCLUSIONS

Oxidative DNA damage repaired by XRCC1 (oxidized bases, single-strand breaks) is a major cause of chromosomal breaks induced by crocidolite and Libby amphibole. Nuclear buds are a novel biomarker of genetic damage induced by exposure to crocidolite asbestos, which we suggest are associated with clustered DNA damage. These results provide mechanistic evidence for the epidemiological association between XRCC1 polymorphisms and susceptibility to asbestos-related disease.

PARP-1 inhibition does not restore oxidant-mediated reduction in SIRT1 activity

Sirtuin1 (SIRT1) deacetylase and poly(ADP-ribose)-polymerase-1 (PARP-1) respond to environmental cues, and both require NAD(+) cofactor for their enzymatic activities. However, the functional link between environmental/oxidative stress-mediated activation of PARP-1 and SIRT1 through NAD(+) cofactor availability is not known. We investigated whether NAD(+) depletion by PARP-1 activation plays a role in environmental stimuli/oxidant-induced reduction in SIRT1 activity. Both H(2)O(2) and cigarette smoke (CS) decreased intracellular NAD(+) levels in vitro in lung epithelial cells and in vivo in lungs of mice exposed to CS. Pharmacological PARP-1 inhibition prevented oxidant-induced NAD(+) loss and attenuated loss of SIRT1 activity. Oxidants decreased SIRT1 activity in lung epithelial cells; however increasing cellular NAD(+) cofactor levels by PARP-1 inhibition or NAD(+) precursors was unable to restore SIRT1 activity. SIRT1 was found to be carbonylated by CS, which was not reversed by PARP-1 inhibition or selective SIRT1 activator. Overall, these data suggest that environmental/oxidant stress-induced SIRT1 down-regulation and PARP-1 activation are independent events despite both enzymes sharing the same cofactor.

Effects of mainstream cigarette smoke on the global metabolome of human lung epithelial cells

Metabolomics is a technology for identifying and quantifying numerous biochemicals across metabolic pathways. Using this approach, we explored changes in biochemical profiles of human alveolar epithelial carcinoma (A549) cells following in vitro exposure to mainstream whole smoke (WS) aerosol as well as to wet total particulate matter (WTPM) or gas/vapor phase (GVP), the two constituent phases of WS from 2R4F Kentucky reference cigarettes. A549 cells were exposed to WTPM or GVP (expressed as WTPM mass equivalent GVP volumes) at 0, 5, 25, or 50 microg/mL or to WS from zero, two, four, and six cigarettes for 1 or 24 h. Cell pellets were analyzed for perturbations in biochemical profiles, with named biochemicals measured, analyzed, and reported in a heat map format, along with biochemical and physiological interpretations (mSelect, Metabolon Inc.). Both WTPM and GVP exposures likely decreased glycolysis (based on decreased glycolytic intermediaries) and increased oxidative stress and cell damage. Alterations in the Krebs cycle and the urea cycle were unique to WTPM exposure, while induction of hexosamines and alterations in lipid metabolism were unique to GVP exposure. WS altered glutathione (GSH) levels, enhanced polyamine and pantothenate levels, likely increased beta-oxidation of fatty acids, and increased phospholipid degradation marked by an increase in phosphoethanolamine. GSH, glutamine, and pantothenate showed the most significant changes with cigarette smoke exposure in A549 cells based on principal component analysis. Many of the changed biochemicals were previously reported to be altered by cigarette exposure, but the global metabolomic approach offers the advantage of observing changes to hundreds of biochemicals in a single experiment and the possibility for new discoveries. The metabolomic approach may thus be used as a screening tool to evaluate conventional and novel tobacco products offering the potential to reduce risks of smoking.

Oxidative stress and accelerated vascular aging: implications for cigarette smoking

Cigarette smoking is the major cause of preventable morbidity and mortality in the United States and constitutes a major risk factor for atherosclerotic vascular disease, including coronary artery disease and stroke. Increasing evidence supports the hypothesis that oxidative stress and inflammation provide the pathophysiological link between cigarette smoking and CAD. Previous studies have shown that cigarette smoke activates leukocytes to release reactive oxygen and nitrogen species (ROS/RNS) and secrete pro-inflammatory cytokines, increases the adherence of monocytes to the endothelium and elicits airway inflammation. Here we present an overview of the direct effects of water-soluble cigarette smoke constituents on endothelial function, vascular ROS production and inflammatory gene expression. The potential pathogenetic role of peroxynitrite formation, and downstream mechanisms including poly(ADP-ribose) polymerase (PARP) activation in cardiovascular complications in smokers are also discussed.

Poly(adenosine diphosphate-ribose) polymerase 1 expression in malignant melanomas from photoexposed areas of the head and neck region

The family of the poly(adenosine diphosphate-ribose) polymerase (PARP) proteins is directly involved in genomic stability, DNA repair, and apoptosis by DNA damage. In this study, we evaluated the role of PARP-1 in melanoma and its prognostic importance. We studied by immunohistochemistry and Western blot analysis PARP-1 expression in a selected series of 80 primary melanoma of the head and neck region. The results were correlated with tumor thickness and patient's outcome. A follow-up of at least 3 years was available. Fifteen cases of benign melanocytic nevi were used as controls. Normal melanocytes showed only scattered, focal nuclear positivity and were considered as negative for PARP-1 expression by immunohistochemistry (score, 0). Thirty cases of melanoma (37.5%) showed nuclear expression of PARP-1 in both radial and vertical growth phases. Western blot analysis showed the presence of a high signal for full-length PARP-1 only in the cases with high immunohistochemical (nuclear) expression of protein (score, ++/+++) in both radial and vertical growth phase. A significant correlation was present between PARP-1 expression in vertical growth phase and the thickness of tumor lesion (P = .014); all but one tumor measuring less than 0.75 mm showed no or low PARP-1 expression. No correlation was found between PARP-1 expression in radial growth phase and tumor thickness (P = .38, data not shown). These data suggest that PARP-1 overexpression is a potential novel molecular marker of aggressive cutaneous malignant melanoma and a direct correlation between PARP-1-mediated inhibition of the apoptosis and biologic behavior of cutaneous malignant melanoma.

Poly(ADP-ribosyl)ation in asthma and other lung diseases

Inhibition of poly(ADP-ribosyl)ation in oxidative stress-related pathologies has recently emerged as a very effective anti-inflammatory intervention in animal models of arthritis, colitis, diabetes and shock. Recent data from three laboratories also support the role of poly(ADP-ribose) polymerase-1 (PARP-1) activation in asthma. Similarly to other inflammatory conditions, the protective effects of PARP inhibition and the PARP-1 knock out phenotype in asthma models have been attributed to inhibition of inflammatory signal transduction (mainly via NF-kappaB) and of oxidative stress-induced cell dysfunction and tissue injury. Here I discuss the complex role of poly(ADP-ribosyl)ation in the regulation of inflammatory cell migration, chemokine and cytokine production and expression of other inflammatory mediators (inducible nitric oxide synthase, matrix metalloproteinases) in asthma. The role of PARP-1 in other oxidative stress-related lung diseases such as asbestosis, silicosis, acute respiratory distress syndrome and ischemia-reperfusion injury is also reviewed.

Role of asbestos in etiology of malignant pleural mesothelioma

The evidence presented in this article demonstrates that asbestos fibers may be genotoxic to mesothelial cells through their distinctive structure and chemistry and through their interactions with complex cellular response mechanisms. Reactive oxygen and nitrogen species play a key role. Understanding the balance between these complex mechanisms that permit neoplastic transformation and facilitate the proliferation of tumor cells is the focus of current investigation in the development of mesothelial malignancy. In human disease, the persistence of asbestos fibers in the lung and pleural tumor is a critical feature that links the exposure to asbestos with the development of disease.

Inhaled particles and lung cancer. Part A: Mechanisms

Both occupational and environmental exposure to particles is associated with an increased risk of lung cancer. Particles are thought to impact on genotoxicity as well as on cell proliferation via their ability to generate oxidants such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). For mechanistic purposes, one should discriminate between a) the oxidant-generating properties of particles themselves (i.e., acellular), which are mostly determined by the physicochemical characteristics of the particle surface, and b) the ability of particles to stimulate cellular oxidant generation. Cellular ROS/RNS can be generated by various mechanisms, including particle-related mitochondrial activation or NAD(P)H-oxidase enzymes. In addition, since particles can induce an inflammatory response, a further subdivision needs to be made between primary (i.e., particle-driven) and secondary (i.e., inflammation-driven) formation of oxidants. Particles may also affect genotoxicity by their ability to carry surface-adsorbed carcinogenic components into the lung. Each of these pathways can impact on genotoxicity and proliferation, as well as on feedback mechanisms involving DNA repair or apoptosis. Although abundant evidence suggests that ROS/RNS mediate particle-induced genotoxicity and mutagenesis, little information is available towards the subsequent steps leading to neoplastic changes. Additionally, since most of the proposed molecular mechanisms underlying particle-related carcinogenesis have been derived from in vitro studies, there is a need for future studies that evaluate the implication of these mechanisms for in vivo lung cancer development. In this respect, transgenic and gene knockout animal models may provide a useful tool. Such studies should also include further assessment of the relative contributions of primary (inflammation-independent) and secondary (inflammation-driven) pathways.

Oxidative Stress, Cell Death, and Other Damage to Alveolar Epithelial Cells Induced by Cigarette Smoke

Cigarette smoking is a major risk factor in the development of various lung diseases, including pulmonary emphysema, pulmonary fibrosis, and lung cancer. The mechanisms of these diseases include alterations in alveolar epithelial cells, which are essential in the maintenance of normal alveolar architecture and function. Following cigarette smoking, alterations in alveolar epithelial cells induce an increase in epithelial permeability, a decrease in surfactant production, the inappropriate production of inflammatory cytokines and growth factors, and an increased risk of lung cancer. However, the most deleterious effect of cigarette smoke on alveolar epithelial cells is cell death, i.e., either apoptosis or necrosis depending on the magnitude of cigarette smoke exposure. Cell death induced by cigarette smoke exposure can largely be accounted for by an enhancement in oxidative stress. In fact, cigarette smoke contains and generates many reactive oxygen species that damage alveolar epithelial cells. Whether apoptosis and/or necrosis in alveolar epithelial cells is enhanced in healthy cigarette smokers is presently unclear. However, recent evidence indicates that the apoptosis of alveolar epithelial cells and alveolar endothelial cells is involved in the pathogenesis of pulmonary emphysema, an important cigarette smoke-induced lung disease characterized by the loss of alveolar structures. This review will discuss oxidative stress, cell death, and other damage to alveolar epithelial cells induced by cigarette smoke.

Oxidative stress, cell death, and other damage to alveolar epithelial cells induced by cigarette smoke

Cigarette smoking is a major risk factor in the development of various lung diseases, including pulmonary emphysema, pulmonary fibrosis, and lung cancer. The mechanisms of these diseases include alterations in alveolar epithelial cells, which are essential in the maintenance of normal alveolar architecture and function. Following cigarette smoking, alterations in alveolar epithelial cells induce an increase in epithelial permeability, a decrease in surfactant production, the inappropriate production of inflammatory cytokines and growth factors, and an increased risk of lung cancer. However, the most deleterious effect of cigarette smoke on alveolar epithelial cells is cell death, i.e., either apoptosis or necrosis depending on the magnitude of cigarette smoke exposure. Cell death induced by cigarette smoke exposure can largely be accounted for by an enhancement in oxidative stress. In fact, cigarette smoke contains and generates many reactive oxygen species that damage alveolar epithelial cells. Whether apoptosis and/or necrosis in alveolar epithelial cells is enhanced in healthy cigarette smokers is presently unclear. However, recent evidence indicates that the apoptosis of alveolar epithelial cells and alveolar endothelial cells is involved in the pathogenesis of pulmonary emphysema, an important cigarette smoke-induced lung disease characterized by the loss of alveolar structures. This review will discuss oxidative stress, cell death, and other damage to alveolar epithelial cells induced by cigarette smoke.

Asbestos-induced pulmonary toxicity: role of DNA damage and apoptosis

Asbestos causes asbestosis and various malignancies by mechanisms that are not clearly defined. Here, we review the accumulating evidence showing that asbestos is directly genotoxic by inducing DNA strand breaks (DNA-SB) and apoptosis in relevant lung target cells. Although the exact mechanisms by which asbestos causes DNA damage and apoptosis are not firmly established, some of the implicated mechanisms include the generation of iron-derived reactive oxygen species (ROS) as well as reactive nitrogen species (RNS), alteration in the mitochondrial function, and activation of the death receptor pathway. We focus on the accumulating evidence implicating ROS. DNA repair mechanisms have a key role in limiting the extent of DNA damage. Recent studies show that asbestos activates DNA repair enzymes such as apurinic/apyrimidinic endonuclease (APE) and poly (ADP-ribose) polymerase (PARP). Asbestos-induced neoplastic transformation may result in the setting where DNA damage overwhelms DNA repair in the face of a persistent proliferative signal. Strategies aimed at limiting asbestos-induced oxidative stress may reduce DNA damage and, as such, prevent malignant transformation.

Association between atmospheric ozone levels and damage to human nasal mucosa in Florence, Italy

We evaluated the effects of urban air pollutants on human nasal mucosa over an 8-month period on 102 subjects living in Florence, Tuscany, Italy. A group of subjects living in a city with a lower level of pollution (Sassari, Sardinia, Italy) was also analyzed. Nasal mucosa cells were harvested by brushing, a noninvasive procedure. Half of the cells were used for genotoxicity studies using the alkaline comet assay, and half for morphological studies. The levels of DNA damage in the nasal mucosa were considerably higher (+73%) in the subjects living in Florence than in Sassari. High levels of atmospheric ozone in Florence air correlated with DNA damage, and to the prevalence of inflammatory pathologies of the upper respiratory tract, although the ozone concentrations were below the Italian recommended attention level. Furthermore, higher levels of DNA damage were correlated with a dysfunction in the ability to maintain a normal epithelial cell structure. These data suggest an association between ozone air levels and damage in the upper respiratory tract. It remains unclear whether ozone itself or other associated pollutants are responsible for the observed alterations.

Iron and the redox status of the lungs

Iron is an element essential for the survival of most aerobic organisms. However, when its availability is not adequately controlled, iron, can catalyze the formation of a range of aggressive and damaging reactive oxygen species, and act as a microbial growth promoter. Depending on the concentrations formed such species can cause molecular damage or influence redox signaling mechanisms. This review describes recent knowledge concerning iron metabolism in the lung, during both health and disease. In the lower part of the lung a small redox active pool of iron is required for reasons that are at present unclear, but may be related to antimicrobial functions. When the concentration of iron is increased in the lung (usually because of environmental exposure), iron is deleterious and contributes to a range of chronic and acute respiratory diseases. Moreover, aberrant regulation of iron metabolism, and/or deficient antioxidant protection, is also associated with acute lung diseases, such as the acute respiratory distress syndrome (ARDS). Iron, with the consequent production of reactive oxygen species (ROS), microbial growth promotion, and adverse signaling is strongly implicated as a major contributor to the pathogenesis of numerous disease processes involving the lung. Heme oxgenase, an enzyme that produces reactive iron from heme catabolism, is also briefly discussed in relation to lung disease.

Asbestos causes apoptosis in alveolar epithelial cells: role of iron-induced free radicals

Asbestos causes asbestosis and malignancies by mechanisms that are not fully understood. Alveolar epithelial cell (AEC) injury by iron-induced reactive oxygen species (ROS) is one important mechanism. To determine whether asbestos causes apoptosis in AECs, we exposed WI-26 (human type I-like cells), A549 (human type II-like cells), and rat alveolar type II cells to amosite asbestos and assessed apoptosis by terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate-biotin nick end labeling (TUNEL) staining, nuclear morphology, annexin V staining, DNA nucleosome formation, and caspase 3 activation. In contrast to control medium and TiO2, amosite asbestos and H2O2 each caused AEC apoptosis. A role for iron-catalyzed ROS was suggested by the finding that asbestos-induced AEC apoptosis and caspase 3 activation were each attenuated by either an iron chelator (phytic acid and deferoxamine) or a.OH scavenger (dimethyl-thiourea, salicylate, and sodium benzoate) but not by iron-loaded phytic acid. To determine whether asbestos causes apoptosis in vivo, rats received a single intratracheal instillation of amosite (5 mg) or normal saline solution, and apoptosis in epithelial cells in the bronchoalveolar duct regions was assessed by TUNEL staining. One week after exposure, amosite asbestos caused a 3-fold increase in the percentage of apoptotic cells in the bronchoalveolar duct regions as compared with control (control, 2.1% +/- 0.35%; asbestos, 7.61% +/- 0.15%; n = 3). However, by 4 weeks the number of apoptotic cells was similar to control. We conclude that asbestos-induced pulmonary toxicity may partly be caused by apoptosis in the lung epithelium that is mediated by iron-catalyzed ROS and caspase 3 activation.