Mesothelial cell apoptosis is confirmed in vivo by morphological change in cytokeratin distribution

Malignant pleural mesothelioma: an update

Malignant mesotheliomas are rare types of cancers that affect the mesothelial surfaces, usually the pleura and peritoneum. They are associated with asbestos exposure, but due to a latency period of more than 30 years and difficult diagnosis, most cases are not detected until they reach advanced stages. Treatment options for this tumor type are very limited and survival ranges from 12 to 36 months. This review discusses the molecular physiopathology, current diagnosis, and latest therapeutic options for this disease.

Effect of Ligustrazine on rat peritoneal mesothelial cells treated with lipopolysaccharide

The apoptosis of peritoneal mesothelial cells (PMCs) and peritoneal fibrosis may induce failure of peritoneal membrane function. The study explored the changes of apoptosis and fibrosis in PMCs under lipopolysaccharides (LPS) culture and investigated whether Ligustrazine can affect LPS-induced apoptosis and fibrosis. We found that exposure of rat PMCs to 5 mg·L(-1) LPS for 24 h resulted in a significant induction of apoptosis and increased levels in Reactive oxygen species, and caspase-3 activity. Fibronectin, Collagen I, p-p38, and matrix metalloprotein-9 (MMP-9) levels were also significantly increased by LPS. But superoxide dismutase levels were remarkably decreased. Ligustrazine can restore the changes induced by LPS. The protective effect of Ligustrazine on LPS-induced apoptosis and fibrosis may act through inhibition of oxidative stress and p38/MAPKS, ROS/MMP-9 activation in PMCs.

Inflammatory Cytokines Contribute to Asbestos-Induced Injury of Mesothelial Cells

BACKGROUND

Several diseases have been related to asbestos exposure, including the pleural tumor mesothelioma. The mechanism of pleural injury by asbestos fibers is not yet fully understood. The inflammatory response with release of mediators leading to a dysregulation of apoptosis may play a pivotal role in the pathophysiology of asbestos-induced pleural disease.

OBJECTIVE

To determine whether pro-inflammatory cytokines produced by asbestos-exposed pleural mesothelial cells modify the injury induced by the asbestos.

METHODS

Mouse pleural mesothelial cells (PMC) were exposed to crocidolite or chrysotile asbestos fibers (3.0 μg/cm(2)) for 4, 24, or 48 h and assessed for viability, necrosis and apoptosis, and the production of cytokines IL-1β, IL-6 and macrophage inflammatory protein-2 (MIP-2). Cells exposed to fibers were also treated with antibodies anti-IL-1β, anti-IL-6, anti- IL-1β+anti-IL-6 or anti-MIP-2 or their irrelevant isotypes, and assessed for apoptosis and necrosis. Non-exposed cells and cells treated with wollastonite, an inert particle, were used as controls.

RESULTS

Mesothelial cells exposed to either crocidolite or chrysotile underwent both apoptosis and necrosis and released cytokines IL-1β, IL-6 and MIP-2. In the crocidolite group, apoptosis and the levels of all cytokines were higher than in the chrysotile group, at comparable concentrations. Neutralization of IL-1β andIL-6, but not MIP-2, inhibited apoptosis and necrosis, especially in the cells exposed to crocidolite fibers.

CONCLUSIONS

Both crocidolite and chrysotile asbestos fibers induced apoptosis and produced an acute inflammatory response characterized by elevated levels of IL-1β, IL-6 and MIP-2 in cultured mouse PMC. IL-1β and IL-6, but not MIP-2, were shown to contribute to asbestos-induced injury, especially in the crocidolite group.

The role of genotoxicity in asbestos-induced mesothelioma: an explanation for the differences in carcinogenic potential among fiber types

The mechanism(s) underlying asbestos toxicity associated with the pathogenesis of mesothelioma has been a challenge to unravel for more than 60 years. A significant amount of research has focused on the characteristics of different fiber types and their potential to induce mesothelioma. These mechanistic studies of fiber toxicity have proceeded along two lines: those demonstrating biochemical mechanisms by which fibers induce disease and those investigating human susceptibility. Most recent studies focused on in vitro genotoxic effects induced by asbestos as the mechanism responsible for asbestos-induced disease. Although asbestos exerts a genotoxic effect at certain concentrations in vitro, a positive response in these tests does not indicate that the chemical is likely to produce an increased risk of carcinogenesis in exposed human populations. Thus far, findings from studies on the effects of fiber type in mesothelial cells are seriously flawed by a lack of a dose response relationship. The common limitation of these in vitro experiments is the lack of attention paid to the complexities of the human anatomy, biochemistry and physiology, which make the observed effects in these experimental systems difficult to extrapolate to persons in the workplace. Mechanistic differences between carcinogenic and genotoxic processes indicate why tests for genotoxicity do not provide much insight regarding the ability to predict carcinogenic potential in workers exposed to asbestos doses in the post-Occupational Safety and Health Administration era. This review discusses the existing literature on asbestos-induced genotoxicity and explains why these studies may or may not likely help characterize the dose-response curve at low dose.

Pathways responsible for apoptosis resulting from amadori-induced oxidative and nitrosative stress in human mesothelial cells

BACKGROUND

Apoptosis and inflammatory/oxidative stress have been associated with hyperglycemia in human peritoneal mesothelial cells (HPMCs) and other cell types. We and others have highlighted the role of early products of non-enzymatic protein glycation in inducing proinflammatory conditions and increasing apoptotic rates in HPMCs. Loss of HPMCs seems to be a hallmark of complications associated with peritoneal membrane dysfunction. The aim of this work is to elucidate the mechanisms by which Amadori adducts may act upon HPMC apoptosis.

METHODS

HPMCs isolated from different patients were exposed to different Amadori adducts, i.e. highly glycated hemoglobin (10 nM) and glycated bovine serum albumin (250 μg/ml), to study cell death and several proapoptotic markers by different experimental approaches.

RESULTS

Amadori adducts, but not their respective controls, impaired cell proliferation and cell viability by means of apoptosis in a time-dependent manner. They regulated the intrinsic mitochondrial cell death signaling pathway and modulated activation of caspases, Bax, iNOS, p53, NF-κB, and mitogen-activated protein kinases (p38 and JNK) through different reactive oxygen and nitrosative species.

CONCLUSIONS

Our data strongly support the idea that long-term hyperglycemia could act as an inducer of apoptosis in HPMCs through Amadori adducts, involving different oxidative and nitrosative reactive species.

Non-neoplastic and neoplastic pleural endpoints following fiber exposure

Exposure to asbestos fibers is associated with non-neoplastic pleural diseases including plaques, fibrosis, and benign effusions, as well as with diffuse malignant pleural mesothelioma. Translocation and retention of fibers are fundamental processes in understanding the interactions between the dose and dimensions of fibers retained at this anatomic site and the subsequent pathological reactions. The initial interaction of fibers with target cells in the pleura has been studied in cellular models in vitro and in experimental studies in vivo. The proposed biological mechanisms responsible for non-neoplastic and neoplastic pleural diseases and the physical and chemical properties of asbestos fibers relevant to these mechanisms are critically reviewed. Understanding mechanisms of asbestos fiber toxicity may help us anticipate the problems from future exposures both to asbestos and to novel fibrous materials such as nanotubes. Gaps in our understanding have been outlined as guides for future research.

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.

A nanoconjugate Apaf-1 inhibitor protects mesothelial cells from cytokine-induced injury

Background

Inflammation may lead to tissue injury. We have studied the modulation of inflammatory milieu-induced tissue injury, as exemplified by the mesothelium. Peritoneal dialysis is complicated by peritonitis episodes that cause loss of mesothelium. Proinflammatory cytokines are increased in the peritoneal cavity during peritonitis episodes. However there is scarce information on the modulation of cell death by combinations of cytokines and on the therapeutic targets to prevent desmesothelization.

Methodology

Human mesothelial cells were cultured from effluents of stable peritoneal dialysis patients and from omentum of non-dialysis patients. Mesothelial cell death was studied in mice with S. aureus peritonitis and in mice injected with tumor necrosis factor alpha and interferon gamma.

Tumor necrosis factor alpha and interferon gamma alone do not induce apoptosis in cultured mesothelial cells. By contrast, the cytokine combination increased the rate of apoptosis 2 to 3-fold over control. Cell death was associated with the activation of caspases and a pancaspase inhibitor prevented apoptosis. Specific caspase-8 and caspase-3 inhibitors were similarly effective. Co-incubation with both cytokines also impaired mesothelial wound healing in an in vitro model. However, inhibition of caspases did not improve wound healing and even impaired the long-term recovery from injury. By contrast, a polymeric nanoconjugate Apaf-1 inhibitor protected from apoptosis and allowed wound healing and long-term recovery. The Apaf-1 inhibitor also protected mesothelial cells from inflammation-induced injury in vivo in mice.

Conclusion

Cooperation between tumor necrosis factor alpha and interferon gamma contributes to mesothelial injury and impairs the regenerative capacity of the monolayer. Caspase inhibition attenuates mesothelial cell apoptosis but does not facilitate regeneration. A drug targeting Apaf-1 allows protection from apoptosis as well as regeneration in the course of inflammation-induced tissue injury.

Apoptosis in the lung: induction, clearance and detection

Apoptosis and other forms of programmed cell death are important contributors to lung pathophysiology. In this brief review, we discuss some of the implications of finding apoptotic cells in the lung and methods for their detection. The balance between induction of apoptosis and the normally highly efficient clearance of such cells shows that these are highly dynamic processes and suggests that abnormalities of apoptotic cell clearance may be an alternative explanation for their detection. Because recognition of apoptotic cells by other lung cells has additional effects on inflammation, immunity, and tissue repair, local responses to the dying cells may also have important consequences in addition to the cell death itself.

Pleural mesothelial cells mediate inflammatory and profibrotic responses in talc-induced pleurodesis

Intrapleural talc is used to produce pleurodesis in malignant pleural effusions. Prior in vivo studies have documented an acute inflammatory response to talc in the pleural space but the cellular source of cytokines has not been identified. The aim of this study was to investigate the acute response of rabbit pleural mesothelial cells challenged with talc used for pleurodesis and compare it to prior studies of the response to talc in the rabbit pleural space. Cultured rabbit pleural mesothelial cells (PMC) were exposed to talc (25 mug/cm(2)) for 6, 24, or 48 h and assessed for viability, necrosis, and apoptosis by flow cytometry, Trypan Blue exclusion, and immunocytochemistry, and for the production of interleukin-8 (IL-8), vascular endothelial growth factor (VEGF), and transforming growth factor-beta(1) (TGF-beta(1)) by ELISA. More than 50% of the PMC remained viable 48 h after talc stimulation. The PMC that were nonviable were identified as either apoptotic or necrotic, with roughly 20% in each category over the 48 h. At 6 h, the IL-8, VEGF, and TGF-beta(1) levels produced by talc-exposed PMC increased significantly and remained elevated for up to 48 h. These cytokine levels rose at similar times and at the same or higher levels than have been measured in the rabbit pleural space in prior studies. We report that viable, talc-exposed, pleural mesothelial cells may actively mediate the primary inflammatory pleural response in talc-induced pleurodesis.

Bitumen products alter bax, bcl-2 and cytokeratin expression: an in vivo study of chronically exposed road pavers

BACKGROUND

The skin of road pavers is chronically exposed to bitumen fumes, a mixture of volatile compounds and particles, containing several carcinogenic and non-carcinogenic polycyclic aromatic hydrocarbons. Bitumen fumes can alter cutaneous barrier integrity in various ways and induce skin diseases. The present study was devised to investigate the expression of apoptosis proteins (bax and bcl-2) and the cytokeratin pattern in skin specimens from road paving workers exposed to bitumen fumes using immunohistochemical techniques.

METHODS

Skin forearm punch biopsies from 16 occupationally exposed workers and an unexposed control group were processed for immunohistochemistry using a broad-spectrum anti-cytokeratin antibody and monoclonal antibodies for bax and bcl-2 immunostaining. Urinary 1-hydroxypyrene (1-OHP) was also determined.

RESULTS

Morphological specimen evaluation showed epidermal thinning of exposed skin and flattened dermal papillae. In sections from exposed subjects, grade 3 bax overexpression and cytokeratin immunoreaction was detected in all layers, while bcl-2 expression was downregulated (grade 1) and confined to the basal layer. There was a significant difference in 1-OHP values between road pavers and the control group (p < 0.05).

CONCLUSION

Overexpression of the cytokeratin pattern and bax and underexpression of bcl-2 in chronically bitumen-exposed skin suggest that bitumen fumes induce activation of apoptosis as a defense mechanism.

A novel in vitro model of human mesothelioma for studying tumor biology and apoptotic resistance

Like many tumors, malignant mesothelioma exhibits significant chemoresistance and resistance to apoptosis in vivo that is not seen in current in vitro models. To study the mechanisms of this multicellular resistance, biologically relevant in vitro models are necessary. Therefore, we characterized and tested human mesothelioma tissue grown in vitro as tumor fragment spheroids. After 5-10 d in culture, fragments from each of 15 human mesothelioma tumors rounded into spheroids. The tumor fragment spheroids maintained multiple characteristics of the original tumors for up to 3 mo including the presence of viable mesothelioma cells, macrophages, and a collagen-rich stroma. In 14-d-old spheroids, mesothelioma cells showed the same proliferation rate and expression of a death receptor, DR5, as in the original tumor. To determine responses to treatment, we treated tumor fragment spheroids grown from three separate tumors with agents, TNF-related apoptosis-inducing ligand (TRAIL) plus cycloheximide, that induced near total apoptosis in three human mesothelioma cell lines (M28, REN, MS-1) grown as monolayers (94 +/- 6% apoptosis; mean +/- SEM). Compared with mesothelioma cells in monolayers, mesothelioma cells in the spheroids were resistant to TRAIL plus cycloheximide (32 +/- 4% apoptosis; mean +/- SEM). Apoptotic resistance of mesothelioma cells was significantly reduced by inhibiting either the PI3K/Akt pathway with LY294002 (47 +/- 6% apoptosis) or the mTOR pathway with rapamycin (50 +/- 17% apoptosis). We conclude that human mesothelioma can be maintained in vitro in a biologically relevant model that exhibits apoptotic resistance, thereby permitting study of its tumor biology and of novel approaches to therapy.

Mesothelial cell proliferation and apoptosis

Mesothelial cells line the pleural and peritoneal surfaces, where under normal conditions they proliferate and undergo cell death at a slow rate, thereby maintaining a constant number of cells. These tightly regulated processes are disrupted in malignancy. By developing a better understanding of the mechanisms that regulate cell proliferation and apoptosis in mesothelial and mesothelioma cells, we may be able to develop more effective therapeutic agents that target specific steps in these pathways to induce apoptosis more efficiently. This paper reviews our current knowledge of the signaling pathways involved in the regulation of mesothelial cell proliferation and apoptosis. The latest advancements in identifying proteins that play key roles in the resistance to apoptosis are highlighted.

Management of malignancy-associated pleural effusion: current and future treatment strategies

Management of recurrent malignant pleural effusion, a common complication of malignancy, poses a challenge to clinicians. Although almost one century has elapsed since the introduction of the pleurodesis procedure, the ideal approach and best agent are still to be defined. Optimally, pleurodesis should be done at the bedside with a minimally invasive procedure, and suitable agents to achieve pleural symphysis should be inexpensive, available worldwide and free of adverse effects. To date, no substance completely fulfills these requirements. Silver nitrate should be considered for pleurodesis because of its low cost and ease of handling. Although talc has been used most frequently to induce pleurodesis, reports of death due to acute respiratory failure have raised concerns about the safety of this agent. Tetracycline, an effective alternative used in the past, is no longer commercially available. This agent has been substituted with derivatives of tetracycline, such as minocycline and doxycycline with success rates similar to those with tetracycline. Several antineoplastic agents have been injected into the pleural space with the aim of producing pleural symphysis, the most representative of this group being bleomycin. Recent knowledge of the molecular mechanisms involved in pleural inflammation has brought into focus new substances, such as transforming growth factor beta and vascular endothelial growth factor, which may be used as pleurodesis agents in the future. Nevertheless, more studies are necessary to better define the potential of these substances in the induction of pleural symphysis.Ideally, a sclerosing agent should be cost-effective, available worldwide and easily administered. Talc will probably stand as the preferred agent to be used for pleurodesis in malignant pleural effusion because of its efficacy, easy manipulation and handling. However, further investigation is necessary to minimize adverse effects related to talc.

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.

Modulation of DNA single-strand breaks by intracellular glutathione in human lung cells exposed to asbestos fibers

We investigated the role of glutathione and nitric oxide synthase (NOS) in fiber-induced cell and DNA toxicity using alkaline (pH 13) single-cell gel electrophoresis (the Comet assay). Transformed cultured human pleural mesothelial (MeT-5A) cells and alveolar epithelial cells (A549) were exposed to crocidolite asbestos fibers (1-10 microg/cm(2)) in the presence of buthionine sulfoximine (BSO) or L-arginine-methyl ester (L-NAME). BSO inhibits gamma-glutamylcysteine synthetase (gamma-GCS) and causes glutathione depletion, and L-NAME inhibits nitric oxide generation. Studies were also conducted to assess the expression of the heavy and light subunits of gamma-GCS in human pleural mesothelium and bronchial epithelium in vivo and the induction of inducible NOS (iNOS) by asbestos fibers. Asbestos fibers caused DNA single-strand breaks, and the process was significantly enhanced by BSO (69% compared to the non-treated cells). A549 cells had a 3.5-fold glutathione content compared to MeT-5A cells, which was consistent with the higher resistance of these cells against oxidants and fibers. Flow cytometry of iNOS showed no change of iNOS by the fibers in either cell type in vitro. L-NAME had no effects on the DNA single-strand breaks in the Comet assay, either. Studies on lung biopsies showed that the immunoreactivities of both gamma-GCS subunits were very low in healthy human mesothelium in vivo. We conclude that glutathione may play an essential role in protecting intact cells against fiber-induced oxidative DNA alterations, and low gamma-GCS reactivity in pleural mesothelium may be associated with the high sensitivity of mesothelial cells to fiber-induced toxicity.

Tumor necrosis factor-related apoptosis-inducing ligand and chemotherapy cooperate to induce apoptosis in mesothelioma cell lines

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis in certain tumor cells. In addition, TRAIL and chemotherapy can act cooperatively, possibly as a result of chemotherapy-induced increases in expression of a TRAIL receptor, DR5. We used cell lines derived from a highly chemoresistant tumor, malignant mesothelioma, to learn whether TRAIL was effective alone or together with chemotherapy and whether cooperativity depended on increases in DR5 expression. TRAIL (codons 95-285) was expressed in a bacterial expression vector and purified by nickel affinity chromatography. TRAIL alone (25 to 500 ng/ml) had little effect on mesothelioma cells. TRAIL plus chemotherapy (doxorubicin, cis-platinum, etoposide, or gemcitabine) acted cooperatively to induce apoptosis in mesothelioma cells (M28, REN, VAMT, and MS-1). For example, in M28 cells treated for 18 h, apoptosis from TRAIL (100 ng/ml) plus doxorubicin (0.6 microg/ml; 71 +/- 11%) greatly exceeded that from TRAIL alone (21 +/- 8%) or from doxorubicin alone (6 +/- 2%) (means +/- standard deviation; P < 0.03). Mesothelioma cells treated with chemotherapy showed no change in DR5 protein by Western analysis or by immunocytochemistry. TRAIL plus chemotherapy was associated with an increase in mitochondrial cytochrome c release and mitochondrial depolarization. We conclude that TRAIL and chemotherapy act cooperatively to kill mesothelioma cell lines, not by increases in DR5 receptor but in association with mitochondrial amplification of apoptotic signals.