Role of mitogen-activated protein kinases (MAPK) in cell injury and proliferation by environmental particulates

Cell signaling pathways may be initiated by environmental particulates by indirect mechanisms such as elaboration of reactive oxygen or nitrogen species (ROS/RNS) or directly upon contact of particulates with the plasma membrane and uptake by epithelial or mesothelial cells. Research in the last few years has mainly addressed cell signaling cascades leading to activation of the redox-sensitive transcription factors, nuclear factor kappa-B (NF-kappaB), and activator protein-1 (AP-1). The activation of these transcription factors may be linked to increases in gene expression controlling cell injury or apoptosis, proliferation and/or cell survival, and inflammatory cytokines. Here, we provide an overview of the MAPK signaling pathways and their activation by asbestos, specifically the role of ROS, receptor-dependent and independent activation via the epidermal growth factor receptor (EGFR), and strategies for proving causal relationships between these pathways and changes in epithelial cell phenotype linked to disease causation.

Activation of NF-kappaB-dependent gene expression by silica in lungs of luciferase reporter mice

Occupational exposure to crystalline silica is associated with the development of pulmonary inflammation and silicosis, yet how silica initiates pulmonary fibrosis and which cell types are involved are unclear. In studies here, we hypothesized that silica particles interact initially with pulmonary epithelial cells and alveolar macrophages (AMs) to cause transcriptional activation of nuclear factor (NF)-kappaB-regulated genes encoding inflammatory cytokines. Exposure of NF-kappaB luciferase reporter mice intratracheally to silica or lipopolysaccharide (LPS), but not the nonfibrogenic particle titanium dioxide (TiO(2)), increased immunoreactivity of luciferase protein in bronchiolar epithelial cells and AMs. Ribonuclease protection assays revealed significant (P < or = 0.05) increases in mRNA levels of inducible nitric oxide synthase, tumor necrosis factor-alpha, macrophage inflammatory protein-2, macrophage chemotactic protein-1 (MCP-1), interferon-gamma, interleukin (IL)-6, and IL-12 in lung homogenates of reporter mice after exposures to silica or LPS. Immunoreactivity of MCP-1 in these animals was localized to AMs and epithelial cells. These data are the first to show activation of NF-kappaB in situ by fibrogenic particles in pulmonary epithelial cells and AMs. Increased expression of NF-kappaB-related inflammatory cytokines by these cell types, which first encounter silica after inhalation, may be critical to the initiation of silica-associated lung diseases, thus providing a rationale for focusing on NF-kappaB in preventive and therapeutic strategies.

Ultrafine airborne particles cause increases in protooncogene expression and proliferation in alveolar epithelial cells

Exposure to ambient particulate matter (PM) is linked to increases in respiratory morbidity and exacerbation of cardiopulmonary diseases. However, the important components of PM and their mechanisms of action in lung disease are unclear. We demonstrate the development of dose-related proliferation and apoptosis after exposure of an alveolar epithelial cell line (C10) to PM or to ultrafine carbon black (ufCB), a component of PM. Ribonuclease protection assays demonstrated that increases in mRNA levels of the early response protooncogenes c-jun, junB, fra-1, and fra-2 accompanied cell proliferation at low concentrations of PM whereas apoptotic concentrations of PM caused transient increases in expression of fos and jun family members and dose responsive increases in mRNA levels of receptor-interacting protein, Fas-associated death domain, and caspase-8. Significant increases in steady-state mRNA levels of protooncogenes and apoptosis-associated genes, TNFR-associated death domain, and Fas were also observed after exposure of epithelial cells to ufCB, but not fine carbon black or glass beads, respectively, suggesting that the ultrafine particulate component of PM is critical to its biological activity.

Diseases caused by asbestos: mechanisms of injury and disease development

Asbestos is a ubiquitous, naturally occurring fiber that has been linked to the development of malignant and fibrotic diseases of the lung and pleura. These diseases may be initiated by injury to epithelial cells and mesothelial cells by asbestos fibers through the formation of reactive oxygen intermediates. Elaboration of oxidants are also a consequence of inflammation, a hallmark of exposure to asbestos after inhalation or injection of asbestos fibers into animals. The type, size, and durability of asbestos fibers may be important in toxicity and pathogenicity of asbestos types. This review discusses the pathways of oxidant generation by asbestos fibers, cell-cell interaction that may initiate and perpetuate inflammation, cytokine release and proliferative responses to asbestos, and cell signaling pathways implicated in these events.

Modulation of cell injury and survival by high glucose and advancing age

Old age is associated with a higher prevalence of cardiovascular disease and diabetes mellitus. Vascular smooth muscle cells (VSMC) play a role in the pathogenesis of vascular diseases, often a complication of diabetes mellitus. We examined in explanted aortic VSMC from young vs. older rats glucose-related activation of nuclear factor kappaB (NF-kappaB), a transcription factor induced by many oxidants. Data demonstrate that old age is associated with enhanced NF-kappaB activity in unstimulated VSMC that is further increased after exposure to high glucose medium. Furthermore, VSMC from old animals exhibit increased levels of protein carbonyls, an indicator of oxidative stress, and less apoptosis in response to glucose than VSMC isolated from young animals. These changes are accompanied by increased expression of NF-kappaB-related genes, gamma-glutamylcysteine synthetase, inhibitor of apoptosis protein-1 (IAP-1), and inducible nitric oxide synthase (iNOS). Results suggest that high glucose, a putative oxidative stress, causes apoptosis in VSMC from young animals and is associated with greater induction of NF-kappaB in VSMC from older animals. Increases in IAP-1 and decreased apoptosis implicate NF-kappaB as a survival factor in VSMC.

Laser-based microscopic approaches: application to cell signaling in environmental lung disease

Cell-imaging approaches using new laser-based technologies have a wide applicability to thefields of pathology and cell biology. Here, we present the application of several of these techniques, including confocal scanning laser microscopy (CSLM), laser scanning cytometry (LSC), and laser capture microdissection (LCM), to studies of cell signaling by environmental agents in lung disease. Using both cells in culture and lung tissue, we show that these technologies are powerful tools for understanding signal transduction cascades elicited by toxic agents, such as oxidants and asbestosfibers, and their relationship to the development of cell injury and proliferation, responses leading to lung disease and/or repair.

Silica-induced activation of c-Jun-NH2-terminal amino kinases, protracted expression of the activator protein-1 proto-oncogene, fra-1, and S-phase alterations are mediated via oxidative stress

Crystalline silica has been classified as a group 1 human carcinogen in the lung. However, its mechanisms of action on pulmonary epithelial cells which give rise to lung cancers are unclear. Using a nontransformed alveolar type II epithelial cell line (C10), we show that alpha-quartz silica causes persistent dose-related increases in phosphorylation of c-Jun-NH2-terminal amino kinases (JNKs) that are inhibited by antioxidants (P < or = 0.05). Increases in activator protein-1 (AP-1) binding to DNA and transactivation of AP-1-dependent gene expression by silica were accompanied by increases in steady-state mRNA levels of the AP-1 family members, c-jun, junB, fra-1, and c-fos at 8 h and elevated mRNA levels of fra-1 at 24 h (P < or = 0.05). Addition of tetramethylthiourea inhibited silica-associated increases infra-1 and proportions of cells in S-phase (P < or = .05). Our findings indicate that silica induces JNK activity, AP-1-dependent gene expression, ie., fra-1, and DNA synthesis via oxidative stress. Moreover, they suggest that silica may act mechanistically as a mitogen or tumor promoter, rather than a genotoxic carcinogen, in the development of lung cancers.

Inhaled particulate matter causes expression of nuclear factor (NF)-kappaB-related genes and oxidant-dependent NF-kappaB activation in vitro

High levels of ambient air pollution are associated with exacerbation of asthma and respiratory morbidity, yet little is known concerning the mechanisms of inflammation and toxicity by components of inhaled particulate matter (PM). Brief inhalation of PM(2.5) (particles of an aerodynamic diameter of < 2.5 microns) (300 microg/m(3) air for 6 h followed by a period of 24 h in clean air) by either C3H/HeJ or C57/BL6 mice caused significant (P </= 0.05) increases in steady-state messenger RNA (mRNA) levels of a number of nuclear factor (NF)-kappaB-associated and/ or -regulated genes, including tumor necrosis factor-alpha and -beta, interleukin-6, interferon-gamma, and transforming growth factor-beta. Lung mRNA levels of lymphotoxin-beta and macrophage migration inhibitory factor were unchanged. In murine C10 alveolar cells and an NF-kappaB-luciferase reporter cell line, exposure to PM(2.5) at noncytotoxic concentrations resulted in increases in transcriptional activation of NF-kappaB-dependent gene expression which were inhibited in the presence of catalase. Early and persistent increases in intracellular oxidants, as measured by flow cytometry and cell imaging using the oxidant probe 2'-7'-dichlorofluoroscin diacetate, were observed in epithelial cells exposed to PM(2.5) and ultrafine carbon black particles. Studies here are the first to show NF-kappaB-related inflammatory and cytokine gene expression after inhalation of PM(2.5) and oxidant-dependent induction of NF-kappaB activity by PM(2.5) in pulmonary epithelial cells.

Silica, silicosis, and lung cancer: a response to a recent working group report

The relationship between crystalline silica and lung cancer has been the subject of many recent publications, conferences, and regulatory considerations. An influential, international body has determined that there was sufficient evidence to conclude that quartz and cristobalite are carcinogenic in humans. The present authors believe that the results of these studies are inconsistent and, when positive, only weakly positive. Other, methodologically strong, negative studies have not been considered, and several studies viewed as providing evidence supporting the carcinogenicity of silica have significant methodological weaknesses. Silica is not directly genotoxic and is a pulmonary carcinogen only in the rat, a species that seems to be inappropriate for assessing particulate carcinogenesis in humans. Data on humans demonstrate a lack of association between lung cancer and exposure to crystalline silica. Exposure-response relationships have generally not been found. Studies in which silicotic patients were not identified from compensation registries and in which enumeration was complete did not support a causal association between silicosis and lung cancer, which further argues against the carcinogenicity of crystalline silica.

Asbestos and cigarette smoke cause increased DNA strand breaks and necrosis in bronchiolar epithelial cells in vivo

Coexposures to asbestos and cigarette smoke cause increased risks of lung cancer in asbestos workers. Although these carcinogens cause DNA damage to epithelial cells in vitro via generation of reactive oxygen species (ROS), it is unclear whether they cause injury to bronchiolar epithelial cells (i.e., the target cells of lung cancers in vivo). We exposed rats to amosite asbestos, cigarette smoke, and the two agents in combination for 1, 2, and 14 d. Numbers of cells exhibiting DNA strand breaks in comparison to sham rats were then evaluated in lungs using the terminal deoxynucleotidyl transferase (TDT)-mediated dUTP-biotin nick end labeling (TUNEL) method and by transmission electron microscopy (TEM). Increases in TUNEL-positive, necrotic epithelial cells occurred after exposure to asbestos alone and in an additive fashion after smoke and asbestos in combination. These results indicate that DNA strand breakage and necrosis are prominent mechanisms of injury by asbestos fibers and cigarette smoke in vivo to epithelial cells of the respiratory tract, thus validating in vitro observations from a number of laboratories.

Increased phosphorylated extracellular signal-regulated kinase immunoreactivity associated with proliferative and morphologic lung alterations after chrysotile asbestos inhalation in mice

Activation of extracellular signal-regulated kinases (ERK) has been associated with the advent of asbestos-associated apoptosis and proliferation in mesothelial and alveolar epithelial cells and may be linked to the development of pulmonary fibrosis. The objective of studies here was to characterize the development of inflammation, cellular proliferation, and fibrosis in asbestos-exposed C57Bl/6 mice in relationship to patterns of ERK phosphorylation. Inflammation occurred after 10 and 20 days of asbestos exposure as evidenced by increases in total protein and neutrophils in bronchoalveolar lavage fluid. Increases in cell proliferation were observed at 30 days in bronchiolar epithelia and at 4, 14, and 30 days in the alveolar compartment of the lung. Trichrome-positive focal lesions of pulmonary fibrosis developed at 30 days in the absence of elevations in lung hydroxyproline or procollagen mRNA levels. Striking increases in ERK phosphorylation were observed within pulmonary epithelial cells at sites of developing fibrotic lesions after 14 and 30 days of inhalation. In addition to characterizing a murine inhalation model of asbestosis, we provide the first evidence showing activation of ERK signaling within lung epithelium in vivo, following inhalation of asbestos fibers.

Expression of GPC3, an X-linked recessive overgrowth gene, is silenced in malignant mesothelioma

Gene expression changes in rat asbestos-induced malignant mesothelioma (MM) cells were investigated by differential mRNA display. A mRNA transcript identified by this approach was abundant in normal rat mesothelial cells but not expressed in rat MM cell lines. Northern blot analysis confirmed that this transcript is uniformly silenced in rat MM cell lines and primary tumors. Nucleotide sequence analysis revealed that this transcript is encoded by the rat glypican 3 gene (GPC3), whose human homolog is mutated in the Simpson-Golabi-Behmel overgrowth syndrome. Allelic loss at the GPC3 locus was infrequent (6.9%) in MM cell lines, and no mutations were found. GPC3 transcript levels were markedly decreased in 16 of 18 primary tumors and 17 of 22 human MM cell lines. Most of the cell lines were shown to have aberrant methylation of the GPC3 promoter region. In two of four human MM cell lines tested, GPC3 expression was restored after 2-deoxy 5-azacytidine (DAC)-mediated demethylation of its promoter region. Ectopic expression of GPC3 inhibited in vitro colony formation of human MM cells. Collectively, these data suggest that down-regulation of GPC3 is a common occurrence in MM and that GPC3, an X-linked recessive overgrowth gene, may encode a negative regulator of mesothelial cell growth.

Strategies for evaluation of signaling pathways and transcription factors altered in aging

Aging is characterized by an accumulation of oxidative injury to DNA, RNA, proteins, lipids, and carbohydrates. In addition to damage, oxidative stress can initiate cell signaling cascades that modulate cell function, growth, and death. Aging and two common age-related diseases, diabetes mellitus and atherosclerosis, may share common oxidant-related signaling pathways that lead to abnormal transcription factor activation and ultimately to cellular dysfunction, degeneration, or death. This review will focus on approaches to evaluate key redox-sensitive signaling pathways and the transcription factors altered by diabetes, atherosclerosis, and aging.

Mechanisms of action of poorly soluble particulates in overload-related lung pathology

For reasons that are unclear, poorly soluble particulates are associated with the development of inflammation, fibrogenesis, and carcinogenesis in the rat. The pathogenesis of these changes may be triggered by distinct or species-specific cellular responses to inhaled particulates in a manner similar to known fibrogenic and carcinogenic fibers, such as asbestos. Data reviewed here suggest that generation of oxidants by poorly soluble particulates is a key factor in the initiation of inflammation and generation of chemokines and cytokines in the rat. These substances then cause hyperplasia of epithelial cells and fibroblasts. The diminished or lack of proliferative responses by poorly soluble particulates in mice and primates, in comparison to rats, may be reflected by intrinsic differences in their oxidant-generating capacities or repair after oxidant injury or DNA damage.

Measurement of oxidant-induced signal transduction proteins using cell imaging

In addition to their capacity to damage macromolecules, oxidants play important roles in initiation of a number of signal transduction pathways. These include phosphorylation and dephosphorylation of members of the extracellular-regulated kinase (ERK) family of the mitogen-activated protein kinase (MAPK) cascade and events leading to activation of the transcription factor nuclear factor-kappaB (NF-kappaB). These cascades are key to transcriptional upregulation of genes important for cell survival, apoptosis, proliferation, transformation, and inflammation. To complement biochemical assays, cell-imaging approaches are necessary to detect the phosphorylated proteins of these cascades and their nuclear translocation, i.e., activation in cells. Protocols for these studies are presented, and the advantages of in situ microscopy-based techniques to detect oxidant-induced signaling pathways are discussed.

Glutathione redox system in oxidative lung injury

Glutathione (GSH) is a ubiquitous intracellular thiol present in all tissues, including lung. Besides maintaining cellular integrity by creating a reduced environment, GSH has multiple functions, including detoxification of xenobiotics, synthesis of proteins, nucleic acids, and leukotrienes. Present in high concentrations in bronchoalveolar lavage fluid (BALF), GSH provides protection to the lung from oxidative injury induced by different endogenous or exogenous pulmonary toxicants. Its depletion in the lung has been associated with the increased risk of lung damage and disease. The redox system of GSH consists of primary and secondary antioxidants, including glutathione peroxidase (GPx), glutathione reductase (GR), glutathione S-transferase (GST), and glucose 6-phosphate dehydrogenase (G6PD). Alterations in the activities of these enzymes may reflect reduced cellular defense and may serve as surrogate markers of many lung diseases. As GSH is also involved in the regulation of expression of protooncogenes and apoptosis (programmed cell death), the development of diseases such as cancer and human immune deficiency may be affected by depleting or elevating cellular GSH levels. Exogenous delivery of GSH or its precursor N-acetyl cysteine (NAC) is being used as chemotherapeutic approach.

Asbestos-induced phosphorylation of epidermal growth factor receptor is linked to c-fos and apoptosis

We examined the mechanisms of interaction of crocidolite asbestos fibers with the epidermal growth factor (EGF) receptor (EGFR) and the role of the EGFR-extracellular signal-regulated kinase (ERK) signaling pathway in early-response protooncogene (c-fos/c-jun) expression and apoptosis induced by asbestos in rat pleural mesothelial (RPM) cells. Asbestos fibers, but not the nonfibrous analog riebeckite, abolished binding of EGF to the EGFR. This was not due to a direct interaction of fibers with ligand, inasmuch as binding studies using fibers and EGF in the absence of membranes showed that EGF did not adsorb to the surface of asbestos fibers. Exposure of RPM cells to asbestos caused a greater than twofold increase in steady-state message and protein levels of EGFR (P < 0.05). The tyrphostin AG-1478, which inhibits the tyrosine kinase activity of the EGFR, but not the tyrphostin A-10, which does not affect EGFR activity, significantly ameliorated asbestos-induced increases in mRNA levels of c-fos but not of c-jun. Pretreatment of RPM cells with AG-1478 significantly reduced apoptosis in cells exposed to asbestos. Our findings suggest that asbestos-induced binding to EGFR initiates signaling pathways responsible for increased expression of the protooncogene c-fos and the development of apoptosis. The ability to block asbestos-induced elevations in c-fos mRNA levels and apoptosis by small-molecule inhibitors of EGFR phosphorylation may have therapeutic implications in asbestos-related diseases.

Cooperativity between oxidants and tumor necrosis factor in the activation of nuclear factor (NF)-kappaB: requirement of Ras/mitogen-activated protein kinases in the activation of NF-kappaB by oxidants

The transcription factor nuclear factor (NF)-kappaB is activated by oxidative stress or cytokines and is critical to the activation of inflammatory genes. Here, we report that hydrogen peroxide or 3-morpholinosydnonimine, which simultaneously releases nitric oxide and superoxide, synergize with the cytokine tumor necrosis factor (TNF)-alpha to activate NF-kappaB in rat lung epithelial cells, suggesting that signaling pathways elicited by reactive oxygen species (ROS)/reactive nitrogen species (RNS) are different from TNF-induced signaling. These findings were substantiated by observations that levels of IkappaB-alpha did not change after exposure to ROS/RNS, whereas a rapid depletion of IkappaB-alpha was observed in cells exposed to TNF. In addition, the proteosome inhibitor MG132 did not affect activation of NF-kappaB by ROS/RNS, whereas it abolished the TNF response. Transfection of a dominant negative Ras construct prevented the activation of NF-kappaB by ROS/RNS, demonstrating the requirement for Ras in the activation of NF-kappaB by oxidants. In contrast, TNF activated NF-kappaB in a Ras-independent fashion. Evaluation of members of the mitogen-activated protein kinase (MAPK) family as downstream effectors of Ras revealed the requirement of MAPK/ extracellular-regulated kinase (ERK) kinase kinase (MEKK)1 and c-Jun N-terminal kinases in the induction of NF-kappaB by both oxidants and TNF, whereas the MEK-ERK pathway negatively regulates NF-kappaB. Our findings demonstrate that cytokines and oxidants cooperate in the activation of transcription factors through distinct pathways, and suggest that anti-inflammatory and antioxidant therapies may be required in concert to prevent the activation of NF-kappaB-regulated genes important in the development of inflammatory diseases.

Environmental pathology: new directions and opportunities

The National Institute of Environmental Health Sciences (NIEHS) supports a number of training programs for predoctoral and postdoctoral (D.V.M., M.D., Ph.D.) fellows in toxicology, epidemiology and biostatistics, and environmental pathology. At the Experimental Biology meeting in April 1997, the American Society of Investigative Pathology (ASIP) sponsored a workshop including directors, trainees, and other interested scientists from several environmental pathology programs in medical and veterinary colleges. This workshop and a related session on "Novel Cell Imaging Techniques for Detection of Cell Injury" revealed advances in molecular and cell imaging approaches as reviewed below that have a wide applicability to toxicologic pathology.

Crocidolite activates NF-kappa B and MIP-2 gene expression in rat alveolar epithelial cells. Role of mitochondrial-derived oxidants

Nuclear factor kappa B (NF-kappa B) is a transcription factor that regulates expression of several genes coding for inflammatory and immunoregulatory proteins including the neutrophil chemotactic cytokine MIP-2. In previous studies we found that crocidolite asbestos activates the nuclear translocation of NF-kappa B as well as MIP-2 gene expression in rat alveolar type II cells. Here we report that both crocidolite-induced NF-kappa B activation of MIP-2 gene expression can be attenuated by the antioxidant tetramethylthiourea, suggesting the dependence of these responses on oxidative stress. Crocidolite exposure of RLE-TN cells also increased production of H2O2, a response that was inhibited by the mitochondrial respiratory chain inhibitor thenoyltrifluoroacetone (TTFA). TTFA treatment of RLE-6TN cells also inhibited crocidolite-induced nuclear translocation of NF-kappa B and MIP-2 gene expression. These results indicate crocidolite exposure of rat alveolar type II cells results in increased production of mitochondrial-derived hydrogen peroxide and that mitochondrial-derived oxidants contribute to crocidolite activation of NF-kappa B and increases in MIP-2 gene expression.

Modulation of mitochondrial gene expression in pulmonary epithelial cells exposed to oxidants

Oxidants are important in the regulation of signal transduction and gene expression. Multiple classes of genes are transcriptionally activated by oxidants and are implicated in different phenotypic responses. In the present study, we performed differential mRNA display to elucidate genes that are induced or repressed after exposure of rat lung epithelial (RLE) cells to H2O2 or crocidolite asbestos, a pathogenic mineral that generates oxidants. After 8 or 24 hr of exposure, RNA was extracted, reverse transcribed, and amplified by polymerase chain reaction with degenerate primers to visualize alterations in gene expression. The seven clones obtained were sequenced and encoded the mitochondrial genes, NADH dehydrogenase subunits ND5 and ND6, and 16S ribosomal RNA. Evaluation of their expression by Northern blot analysis revealed increased expression of 16S rRNA after 1 or 2 hr of exposure to H2O2. At later time periods (4 and 24 hr), mRNA levels of 16S rRNA and NADH dehydrogenase were decreased in H2O2-treated RLE cells when compared to sham controls. Crocidolite asbestos caused increases in 16S rRNA levels after 8 hr of exposure, whereas after 24 hr of exposure to asbestos, 16S rRNA levels were decreased in comparison to sham controls. In addition to these oxidants, the nitric oxide generator spermine NONOate caused similar decreases in NADH dehydrogenase mRNA levels after 4 hr of exposure. The present data and previous studies demonstrated that all oxidants examined resulted in apoptosis in RLE cells during the time frame where alterations of mitochondrial gene expression were observed. As the mitochondrion is a major organelle that controls apoptosis, alterations in expression of mitochondrial genes may be involved in the regulation of apoptosis.