Patterns of inflammation, cell proliferation, and related gene expression in lung after inhalation of chrysotile asbestos

Biochemical and molecular markers of inflammation, cell proliferation, and pulmonary fibrosis were studied in lungs and bronchoalveolar lavage preparations from Fischer 344 rats at time periods from 3 to 20 days after inhalation of two airborne concentrations (0.18 and 8.2 mg/m3 air) of chrysotile asbestos. Additional groups of animals were examined for lung histopathology and cell proliferation with an antibody to 5-bromo-2'-deoxyuridine after exposure to asbestos for 5 and 20 days and after 20 days of exposure followed by an additional 20 days in room air. Exposure to chrysotile at the higher concentration caused protracted increases in steady-state mRNA levels of manganese-containing superoxide dismutase and elevation in glyceraldehyde-3-phosphate dehydrogenase mRNA at 3 days, but levels of mRNAs encoding copper-zinc-containing superoxide dismutase, ornithine decarboxylase, and the proto-oncogene, c-jun were not statistically elevated from levels occurring in lung homogenates from sham control rats. Differential cell counts in bronchoalveolar lavage revealed an early infiltration of neutrophils that correlated with focal areas of increased cellularity and fibrosis in rat lungs at the higher concentrations of asbestos. However, elevations in lung hydroxyproline were not observed. Significant increases in epithelial cells of the bronchi, the interstitial compartment of the lung, and mesothelial cells incorporating 5-bromo-2'-deoxyuridine, an indication of DNA synthesis, were noted in the higher chrysotile group at 5 days, but labeling in all cell compartments was comparable with that occurring in sham controls at later time points. Indicators of inflammation, increased cell proliferation, and pulmonary fibrosis were not observed in the lungs of rats exposed to the lower concentration of chrysotile. Thus, results indicate that cellular and molecular markers of inflammation and proliferation in lung are dose-related and indicative of the histopathological development of asbestosis.

Asbestos induces nuclear factor kappa B (NF-kappa B) DNA-binding activity and NF-kappa B-dependent gene expression in tracheal epithelial cells

Nuclear factor kappa B (NF-kappa B) is a transcription factor regulating expression of genes intrinsic to inflammation and cell proliferation--features of asbestos-associated diseases. In studies here, crocidolite asbestos caused protracted and dose-responsive increases in proteins binding to nuclear NF-kappa B-binding DNA elements in hamster tracheal epithelial (HTE) cells. This binding was modulated by cellular glutathione levels. Antibodies recognizing p65 and p50 protein members of the NF-kappa B family revealed these proteins in two of the DNA complexes. Transient transfection assays with a construct containing six NF-kappa B-binding DNA consensus sites linked to a luciferase reporter gene indicated that asbestos induced transcriptional activation of NF-kappa B-dependent genes, an observation that was confirmed by northern blot analyses for c-myc mRNA levels in HTE cells. Studies suggest that NF-kappa B induction by asbestos is a key event in regulation of multiple genes involved in the pathogenesis of asbestos-related lung cancers.

Transcriptional activation of the proto-oncogene c-jun by asbestos and H2O2 is directly related to increased proliferation and transformation of tracheal epithelial cells

Asbestos causes persistent increases in c-jun mRNA and AP-1 DNA binding activity in hamster tracheal epithelial (HTE) cells, the progenitor cell type of asbestos-induced bronchogenic carcinoma. Studies here were designed to determine mechanisms of c-jun induction by asbestos and the phenotypic consequences of Jun expression in HTE cells. To examine whether asbestos or H2O2 induced transcription of c-jun, we transiently transfected HTE cells with a plasmid containing a fragment of the c-jun promoter coupled to a luciferase reporter gene. In addition, c-jun was overexpressed in cells using a full-length human c-jun construct, and effects on proliferation and transformation were examined. HTE cells transfected with the jun-luciferase construct showed increased luciferase activity when exposed to crocidolite asbestos or H2O2. These results demonstrate that asbestos and H2O2 activate AP-1-dependent gene transcription. Overexpression of c-jun led to increased proliferation and enhanced ability of HTE cells to grow in soft agar, an indication of cellular transformation. Data suggest that overexpression of c-jun may contribute to asbestos and oxidant-induced proliferation and carcinogenesis.

Induction of c-fos and c-jun proto-oncogene expression by asbestos is ameliorated by N-acetyl-L-cysteine in mesothelial cells

Asbestos fibers cause dose-dependent, persistent increases in mRNA levels of c-jun and c-fos proto-oncogenes in rat pleural mesothelial (RPM) cells, the progenitor cells of asbestos-induced mesothelioma (N. Heintz, Y. M. W. Janssen, and B. T. Mossman. Proc. Natl. Acad. Sci. USA, 90: 3299-3303, 1993). Here we report that addition of N-acetyl-L-cysteine decreases asbestos-mediated induction of c-fos and c-jun mRNA levels in a dose-dependent fashion. Exposure of RPM cells to asbestos causes depletion of total cellular glutathione, a response that can be abolished by pretreatment with N-acetyl-L-cysteine. Pretreatment of cells with buthionine sulfoximine, an agent which diminishes glutathione pools, increases the magnitude of induction of c-fos and c-jun mRNA by asbestos. To determine whether asbestos-induced effects on proto-oncogene expression could be attributed to extracellular generation of active oxygen species (AOS), RPM cells were exposed to H2O2 or xanthine and xanthine oxidase, a generating system of AOS. These oxidant stresses did not decrease cellular glutathione levels nor alter mRNA levels of c-fos or c-jun. However, increased mRNA levels of manganese-containing superoxide dismutase and heme oxygenase were observed, indicating that RPM cells respond to AOS by increased expression of genes encoding antioxidant enzymes. These data indicate that the signaling pathways leading to c-fos/c-jun proto-oncogene induction by asbestos are not triggered directly by formation of extracellular AOS. However, intracellular thiol levels appear to influence the expression of c-fos and c-jun, suggesting a redox-sensitive component in the signaling cascade which modulates gene expression of c-fos and c-jun by asbestos.

Oxygen radicals and asbestos-mediated disease

Asbestos fibers are potent elaborators of active oxygen species whether by reactions involving iron on the surface of the fiber, or by attempted phagocytosis of fibers by cell types resident in the lung. The link between production of active oxygen species and the pathogenesis of asbestos-mediated disease has been highlighted by studies outlined here exploring the use of antioxidant scavengers which inhibit the cytotoxic effects of asbestos both in vitro and in vivo. The use of antioxidant enzymes ameliorates the induction of certain genes necessary for cell proliferation, such as ornithine decarboxylase, implicating oxidants as causative factors in some abnormal cell replicative events. Based on these observations, antioxidant enzymes likely represent an important lung defense mechanism in response to oxidative stress. In addition, their gene expression in lung or in cells from bronchoalveolar lavage might be a valuable biomarker of chronic inflammation and pulmonary disease after inhalation of oxidants.

Induction of c-fos and c-jun proto-oncogenes in target cells of the lung and pleura by carcinogenic fibers

To study mechanisms of cell proliferation by asbestos and nonasbestos fibers, we examined the effects of these agents on the mRNA levels of c-fos and c-jun and ornithine decarboxylase (ODC) in hamster tracheal epithelial (HTE) cells and rat pleural mesothelial (RPM) cells, the progenitor cells of bronchogenic carcinoma and mesothelioma, respectively. In comparison with crocidolite asbestos, increases in c-jun mRNA were less striking in HTE cells after exposure to man-made vitreous fiber-10 (MMVF-10) or refractory ceramic fiber-1 (RCF-1). No c-fos mRNA was detected in HTE cells after exposure to particulates, but exposure of HTE cells to H2O2 caused striking increases in c-fos and c-jun, which preceded increases in ODC mRNA. Increases in ODC mRNA were also observed in HTE cells after exposure to nonasbestos fibers, whereas only crocidolite asbestos caused elevations in ODC mRNA in RPM cells. In RPM cells, crocidolite and chrysotile asbestos caused increases in mRNA levels of both c-fos and c-jun. No increases in proto-oncogene induction were observed using MMVF-10 or RCF-1 at nontoxic concentrations (< or = 5 micrograms/cm2 dish). Moreover, erionite, a fiber extremely potent in the causation of mesothelioma in humans, caused more dramatic elevations in c-fos and c-jun. Nonfibrous particles (riebeckite, polystyrene beads) did not alter proto-oncogene expression in these cell types, suggesting that the fibrous geometry of particulates is critical in the induction of c-fos and c-jun.

Carcinogenesis and related cell and tissue responses to asbestos: a review

A review of the literature reveals that chrysotile asbestos has a mulitiplicity of effects on cells and tissues which can provide a framework for assessment of its role(s) in initiation, promotion, or as a co-carcinogen which acts in concert with chemical carcinogens found in cigarette smoke in the development of lung cancer. Several caveats important in the interpretation of these data include the general lack of dose-response protocols both for in vivo and for in vitro studies as well as the absence, in many studies, of minerals which are appropriate positive or negative controls based on epidemiological data in man. Other factors which may account for disparities in results between studies include the use of different preparations of chrysotile fibres with distinct chemical and physical compositions and different cell types and species. Whether chrysotile is an initiator of lung cancer or mesothelioma in human cells is unclear, as evidence of chromosomal aberrations in human bronchial epithelial cells are for the most part negative (Kodoma et al., 1993; see Mossman, B., this Workshop): only one study employing pleural mesothelial cells from four individuals, two of whom exhibited chromosomal abnormalities before exposure to asbestos, has documented chromosomal changes with chrysotile, crocidolite and amosite asbestos, and this study did not use a non-pathogenic dust as a negative control. Studies using human lymphocytes show chromosomal changes after exposure to latex beads or chrysotile at equal weight concentrations (Korkina et al., 1992; see Mossman, B., this Workshop). Lastly, although both chrysotile and crocidolite asbestos demonstrated dose-dependent increase in aberrant anaphases in an SV40 T antigen-transformed human mesothelial cell line (Pelin et al., 1992, see Mossman, B., this Workshop), erionite, a potent mesotheliomagenic fibre in rodents and humans, caused no aberrations in this bioassay. Several studies have been performed to determine the interactions of chrysotile with rodent cells or isolated bacterial DNA. Results in a number of bioassays have been positive, but chrysotile is less potent on a fibre number comparative basis than crocidolite and no-observed-effect-levels (NOELs) have been observed in several systems. Cell proliferation by asbestos may be a more relevant phenomenon to tumour development and promotion, and the ability of chrysotile to stimulate cell proliferation, using a number of biomarkers, has been demonstrated both in vitro and after inhalation by rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose-responsive increases in pulmonary fibrosis after inhalation of asbestos

We focused here on steady-state mRNA levels of genes involved in antioxidant defense, i.e., manganese superoxide dismutase and copper-zinc superoxide dismutase, and in cell proliferation, i.e., ornithine decarboxylase, c-jun, and glyceraldehyde-3-phosphate-dehydrogenase in whole-lung homogenates from Fischer 344 rats at 3 h to 20 d after exposure to crocridolite asbestos. Changes in gene expression were correlated with histopathologic findings, total and differential cell counts in bronchoalveolar lavage, and levels of hydroxyproline in lung. Dosage-dependent increases in mRNA levels of antioxidant enzymes and proliferation-related genes were observed. Differential cell counts revealed a dose-related infiltration of neutrophils that preceded elevations in gene expression. Neutrophil infiltration into lung and focal lesions of fibrosis as well as increased levels of hydroxyproline were observed only at high concentrations of asbestos. These results indicate that high airborne concentrations of asbestos cause molecular changes in lung that may be related to antioxidant defense and the triggering of cell proliferation, a feature of asbestosis and lung cancer.

Regulation of antioxidant enzymes in lung after oxidant injury

Studies have implicated active oxygen species (AOS) in the pathogenesis of various lung diseases. Many chemical and physical agents in the environment are potent generators of AOS, including ozone, hyperoxia, mineral dusts, paraquat, etc. These agents produce AOS by different mechanisms, but frequently the lung is the primary target of toxicity, and exposure results in damage to lung tissue to varying degrees. The lung has developed defenses to AOS-mediated damage, which include antioxidant enzymes, the superoxide dismutases [copper-zinc (CuZnSOD) and manganese-containing (MnSOD)], catalase, and glutathione peroxidase (GPX). In this review, antioxidant defenses to environmental stresses in the lung as well as in isolated pulmonary cells following exposure to a number of different oxidants, are summarized. Each oxidant appears to induce a different pattern of antioxidant enzyme response in the lung, although some common trends, i.e., induction of MnSOD following oxidants inducing inflammation or pulmonary fibrosis, in responses to oxidants occur. Responses may vary between the different cell types in the lung as a function of cell-cycle or other factors. Increases in MnSOD mRNA or immunoreactive protein in response to certain oxidants may serve as a biomarker of AOS-mediated damage in the lung.

Asbestos: facts and fiction

Images

Oxidant stress responses in human pleural mesothelial cells exposed to asbestos

The generation of oxidants is a proposed mechanism of cell injury by asbestos fibers. To determine whether human pleural mesothelial cells (HMC) respond to asbestos and active oxygen species (AOS) by induction of antioxidant enzymes, cells obtained from pleural effusion were exposed to crocidolite or chrysotile asbestos or xanthine/xanthine oxidase (X/XO), a chemical-generating system of AOS. Gene expression of manganese-containing superoxide dismutase (MnSOD) and heme oxygenase (HO), endogenous enzymes involved in cell defense against oxidant stresses, was then determined. Dosage-dependent increases in steady-state mRNA levels of MnSOD and HO were observed in HMC exposed to asbestos or X/XO. However, increases in gene expression of MnSOD or HO did not occur in HMC after exposure to particulates such as polystyrene beads or riebeckite, the nonfibrous analog of crocidolite asbestos. Comparative experiments with human adult lung fibroblasts (HAL) showed less striking increases in mRNA levels of MnSOD and HO in response to asbestos, but steady-state mRNA levels for HO were increased more than fivefold in response to X/XO. To determine whether increases in mRNA levels of MnSOD were translated into protein, Western blot analyses were performed on HMC and HAL cells exposed to asbestos or X/XO. Slight increases in MnSOD immunoreactive protein were observed in HMC in response to both agents. In contrast, X/XO caused striking elevations in MnSOD protein levels in HAL cells. These results suggest that certain antioxidant enzymes are inducible in HMC after exposure to asbestos and other oxidants.

Increased expression of manganese-containing superoxide dismutase in rat lungs after inhalation of inflammatory and fibrogenic minerals

Steady-state mRNA levels and immunoreactive protein for manganese-containing superoxide dismutase (MnSOD) were assayed in rat lungs after subchronic inhalation of the fibrogenic silicon dioxide, cristobalite, or preparations of titanium dioxide (TiO2) of different inflammatory and fibrogenic potential. Total and differential cell counts recoverable by bronchoalveolar lavage (BAL) were also measured to ascertain whether induction of certain antioxidant enzymes (AOE) correlated with inflammatory responses. Inhalation of cristobalite and ultra-fine TiO2, a particle causing pulmonary inflammation and fibrosis, caused dramatic increases in MnSOD mRNA levels in rat lung which correlated with increases in MnSOD immunoreactive protein. Increases in gene expression of other AOE [catalase, glutathione peroxidase (GPX), copper-zinc containing superoxide dismutase (CuZnSOD)] were less striking and did not correlate precisely with inflammatory potential of minerals. Inflammatory changes in BAL correlated directly with steady-state MnSOD mRNA levels in lung. Inhalation of TiO2-F, a noninflammatory, nonfibrogenic mineral, failed to induce MnSOD or mRNAs for other AOE. Our data suggest that particles causing inflammation and pulmonary fibrosis increase expression of AOE in lung, most notably MnSOD. Thus, elevations of MnSOD mRNA levels in lung or BAL may be predictive of lung disease.

Effects of Aramid, a high strength synthetic fiber, on respiratory cells in vitro

Industry continues to develop synthetic fibers for new technologies and as replacements for asbestos, a toxic and carcinogenic fiber. To determine whether the in vitro effects of the aromatic polyamide fiber, Aramid (Kevlar, Twaron), resembled those induced by asbestos, fibers were surveyed for (1) cytotoxicity as measured by total cell protein, and (2) proliferative capacity as measured by [3H]thymidine incorporation, colony forming efficiency (CFE), and ornithine decarboxylase (ODC) activity in two target cells of mineral dust-induced lung damage, hamster tracheal epithelial (HTE) cells and rat lung (RL90) fibroblasts. Results of cytotoxicity tests indicated that Aramid was as toxic to HTE and RL90 cells as were crocidolite and chrysotile asbestos when expressed on both an equal mass and equal fiber number basis. In HTE cells, Aramid caused a statistically significant increase in [3H]thymidine incorporation and CFE and produced a dose-dependent induction of ODC enzyme activity. Proliferative effects by asbestos or Aramid were not observed in RL90 fibroblasts. Thus, when tested over a respirable size range, Aramid exhibited many of the same effects on epithelial cells in vitro as did asbestos, including increased radiolabeled nucleotide incorporation into DNA and induction of ODC enzyme activity.

Cell and tissue responses to oxidative damage

A broad array of oxidative stresses modulates gene expression in a variety of mammalian cells. One goal of this review was to characterize cellular responses to oxidative injury, how these processes are regulated, and the outcome for a particular cell or tissue. Many genes induced in response to specific oxidant stresses have been identified and include transcription factors, replication proteins, proteases, protease inhibitors, proteins affecting cell proliferation and various antioxidants, i.e. heme oxygenase, MT, and MnSOD. The latter enzyme is induced after a number of cytokines and oxidant stresses including hyperoxia and mineral dusts causing inflammation. Moreover, increases in mRNA levels of TNF and IL-1, cytokines inducing MnSOD, are observed after exposure to UV and ionizing radiation. Since increased electron flow could lead to generation of more AOS within mitochondria, increased levels of MnSOD might be necessary to maintain normal functioning of the mitochondria after oxidative stress. Alterations in cell growth are intrinsically related to the pathogenesis of many diseases. Paradoxically, some of the responses of cells to oxidative stress reflect cytotoxicity and cytostasis, whereas others result in increased cell proliferation. For example, induction of gadd genes observed after oxidative stress is related to growth arrest of cells, a response which might enable the cell to repair oxidative damage prior to replication. This phenomenon might prevent fixation of mutations associated with oxidative DNA damage. On the other hand, increased mRNA expression and activity of ODC, observed after exposure of cells to UV or asbestos is associated with increased cell proliferation. In addition, increased mRNA expression of cellular proto-oncogenes observed after exposure to oxidants could also be related to increased DNA synthesis or proliferation. Figure 5 provides a general scheme of cell responses to oxidative stress and possible ramifications. AOS can react with a number of target molecules including proteins, lipids, and DNA. These interactions elicit a number of signals including activation of gene regulatory factors (transcription factors) which in turn activate oxidative stress-responsive genes or regulons. Consequently, a number of proteins are produced with distinctive functions including DNA repair enzymes, antioxidants, proteases inhibitors, cytokines and proteins affecting cell proliferation. These cellular responses to AOS can lead to restoration of normal cellular function and adaptation to oxidative stress, cell death or aberrant proliferation. It is the latter two responses which can lead to a variety of disease states including cancer.(ABSTRACT TRUNCATED AT 400 WORDS)

Persistent induction of c-fos and c-jun expression by asbestos

To investigate the mechanisms of asbestos-induced carcinogenesis, expression of c-fos and c-jun protooncogenes was examined in rat pleural mesothelial cells and hamster tracheal epithelial cells after exposure to crocidolite or chrysotile asbestos. In contrast to phorbol 12-myristate 13-acetate, which induces rapid and transient increases in c-fos and c-jun mRNA, asbestos causes 2- to 5-fold increases in c-fos and c-jun mRNA that persist for at least 24 hr in mesothelial cells. The induction of c-fos and c-jun mRNA by asbestos in mesothelial cells is dose-dependent and is most pronounced with crocidolite, the type of asbestos most pathogenic in the causation of pleural mesothelioma. Induction of c-jun gene expression by asbestos occurs in tracheal epithelial cells but is not accompanied by a corresponding induction of c-fos gene expression. In both cell types, asbestos induces increases in protein factors that bind specifically to the DNA sites that mediate gene expression by the AP-1 family of transcription factors. The persistent induction of AP-1 transcription factors by asbestos suggests a model of asbestos-induced carcinogenesis involving chronic stimulation of cell proliferation through activation of the early response gene pathway that includes c-jun and/or c-fos.

Increased manganese superoxide dismutase protein in type II epithelial cells of rat lungs after inhalation of crocidolite asbestos or cristobalite silica

Manganese-containing superoxide dismutase (Mn-SOD) is a mitochondrial enzyme implicated in cellular defense from oxidative damage. We investigated the immunocytochemical distribution and protein concentration of Mn-SOD in rat lungs in response to aerosolized crocidolite asbestos or cristobalite silica, fibrogenic minerals eliciting generation of oxidants by cellular and acellular pathways. Rats were exposed to 7-10 mg/m3 dust for 6 hours a day for 10 days. Experimental and sham control rats were euthanized 10 days after cessation of exposure, and lungs prepared for immunocytochemistry and determination of amounts of Mn-SOD protein. Quantitation of Western blots showed that the amount of immunodetectable Mn-SOD increased in lungs exposed to asbestos or silica by approximately 1.3- and 2.4-fold, respectively, when compared with sham controls. Immunoelectron microscopy using the protein A-gold technique showed that Mn-SOD was located predominantly in mitochondria of type II epithelial cells. Fibroblasts contained little immunodetectable Mn-SOD, whereas type I epithelial cells, polymorphonuclear leukocytes (PMNs), and endothelial cells contained no detectable protein. Some alveolar macrophages (AMs) were found with labeled mitochondria, whereas most interstitial macrophages had no detectable protein. Quantitative analysis of type II cells showed that the number of immunogold particles per unit of mitochondrial area increased in the terminal airways of lungs exposed to asbestos or silica by 2.2-fold and 3.6-fold, respectively, over controls. Morphometric analyses indicated that the size of type II cells, as well as the number of interstitial macrophages and PMNs, increased in the terminal respiratory tissue of silica-exposed lungs. Less pronounced histopathologic changes were evident in asbestos-exposed lungs. These results indicate that the relative concentration of Mn-SOD increases preferentially in type II epithelial cells subsequent to inhalation of silica or asbestos. The magnitude of induction of Mn-SOD protein in these cells and whole lung correlated with the inflammatory potential of these minerals.

Expression of antioxidant enzymes in rat lungs after inhalation of asbestos or silica

Several studies indicate that active oxygen species play an important role in the development of pulmonary disease (asbestosis and silicosis) after exposure to mineral dust. The present study was conducted to determine if inhaled fibrogenic minerals induced changes in gene expression and activities of antioxidant enzymes (AOE) in rat lung. Two different fibrogenic minerals were compared, crocidolite, an amphibole asbestos fiber, and cristobalite, a crystalline silicon dioxide particle. Steady-state mRNA levels, immunoreactive protein, and activities of selected AOE were measured in lungs 1-10 days after initiation of exposure and at 14 days after cessation of a 10-day exposure period. Exposure to asbestos resulted in significant increases in steady-state mRNA levels of manganese-containing superoxide dismutase (MnSOD) at 3 and 9 days and of glutathione peroxidase at 6 and 9 days. An increase in steady-state mRNA levels of copper, zinc-containing superoxide dismutase (CuZnSOD), was observed at 6 days. Exposure to asbestos also resulted in overall increased enzyme activities of catalase, glutathione peroxidase and total superoxide dismutase in lung. In contrast, silica caused a dramatic increase in steady-state levels of MnSOD mRNA at all time periods and an increase in glutathione peroxidase mRNA levels at 9 days. Activities of AOE remained unchanged in silica-exposed lungs. In both models, increases in gene expression of MnSOD correlated with increased amounts of MnSOD immunoreactive protein in lung and the pattern and extent of inflammation. These data indicate that the profiles of AOE are dissimilar during the development of experimental asbestosis or silicosis and suggest different mechanisms of lung defense in response to these minerals.

Differential regulation of antioxidant enzymes in response to oxidants

We have demonstrated the selective induction of manganese superoxide dismutase (MnSOD) or catalase mRNA after exposure of tracheobronchial epithelial cells in vitro to different oxidant stresses. Addition of H2O2 caused a dose-dependent increase in catalase mRNA in both exponentially growing and confluent cells. A 3-fold induction of catalase mRNA was seen at a nontoxic dose of 250 microM H2O2. Increase in the steady-state mRNA levels of glutathione peroxidase (GPX) and MnSOD were less striking. Expression of catalase, MnSOD, and GPX mRNA was highest in confluent cells. In contrast, constitutive expression of copper and zinc SOD (CuZnSOD) mRNA was greatest in dividing cells and was unaffected by H2O2 in both exponentially growing and confluent cells. MnSOD mRNA was selectively induced in confluent epithelial cells exposed to the reactive oxygen species-generating system, xanthine/xanthine oxidase, while steady-state levels of GPX, catalase, and CuZnSOD mRNA remained unchanged. The 3-fold induction of MnSOD mRNA was dose-dependent, reaching a peak at 0.2 unit/ml xanthine oxidase. MnSOD mRNA increases were seen as early as 2 h and reached maximal induction at 24 h. Immunoreactive MnSOD protein was produced in a corresponding dose- and time-dependent manner. Induction of MnSOD gene expression was prevented by addition of actinomycin D and cycloheximide. These data indicate that epithelial cells of the respiratory tract respond to different oxidant insults by selective induction of certain antioxidant enzymes. Hence, gene expression of antioxidant enzymes does not appear to be coordinately regulated in these cell types.

Development and characterization of a rapid-onset rodent inhalation model of asbestosis for disease prevention

A short-term inhalation model of asbestosis was developed in rodents to examine possible preventive approaches to lung disease. Fischer 344 (F344) rats were exposed for 10 and 20 days to National Institute of Environmental Health Sciences (NIEHS) crocidolite asbestos while sham controls were exposed to air only. To determine quantitative biochemical indicators of asbestos-induced lung disease, bronchoalveolar lavage (BAL) fluids were analyzed for lactic dehydrogenase (LDH), alkaline phosphatase, angiotensin-converting enzyme (ACE), and protein. Total and differential cell counts were performed on cell pellets from BAL. Lungs from additional rats were processed for histopathology, measurement of hydroxyproline, and autoradiography after injection of rats with 3H-thymidine. Exposure to asbestos for 10 and 20 days caused increases in LDH, alkaline phosphatase, and protein in BAL. In contrast, ACE was undetectable in BAL fluids from sham or asbestos-exposed rats. At both time periods, the percentages of polymorphonuclear leukocytes (PMNs) and lymphocytes in BAL were increased in asbestos-exposed rats. Total cell numbers in BAL were increased significantly at 20 days in animals inhaling asbestos. Exposure to asbestos for 10 and 20 days caused elevated amounts of hydroxyproline in lung and the development of fibrotic lesions. Asbestos-exposed rats exhibited increased numbers of interstitial cells and airspace epithelial cells incorporating 3H-thymidine, whereas labeled bronchiolar epithelial cells were not elevated significantly. The quantitative changes in asbestos-associated enzyme levels, cell types and protein in BAL, as well as increases in hydroxyproline and morphologic evidence of fibrosis, are useful indices of asbestos-related lung injury which enable preventive and therapeutic approaches to disease.

Activation of protein kinase C by crocidolite asbestos in hamster tracheal epithelial cells

To determine the mechanisms of cell signalling by asbestos in epithelial cells of the respiratory tract, the activity of protein kinase C (PKC) was examined in hamster tracheal epithelial (HTE) cells exposed to mitogenic concentrations of crocidolite asbestos. In the histone phosphorylation assay, asbestos significantly increased activity of PKC associated with the membrane fraction of HTE cells. However, in contrast to 12-O-tetradecanoylphorbol-13-acetate, which caused redistribution of almost all PKC activity from the cytosolic to the membrane fraction, the majority of the PKC activity was associated with the cytosolic fraction at all time periods examined. Asbestos did not inhibit binding of [3H]phorbol-12,13-dibutyrate to intact HTE cells, whereas binding was inhibited by the phorbol compounds phorbol dibutyrate and phorbol dibenzoate. Thus, crocidolite-induced activation of PKC does not appear to be mediated through the same mechanism as classical phorbol ester tumor promoters, compounds which activate PKC by structurally resembling diacylglycerol.

Alterations in keratin expression in hamster tracheal epithelial cells exposed to benzo[a]pyrene

Changes in keratin expression were documented in cultures of hamster tracheal epithelial (HTE) cells exposed to vitamin A (retinol 10(-6) M) and benzo[a]pyrene (B[a]P), two agents known to affect the differentiation of tracheal epithelial cells in vivo. Keratin protein patterns were determined after 10 days in culture in the presence and absence of B[a]P in order to determine whether the expression of these proteins was altered by this carcinogen. HTE cells maintained in the presence of vitamin A expressed four simple epithelial keratins (7,8,18 and 19) while vitamin A deficient HTE cells expressed four additional cytokeratins (5,6,14 and 17). No effect of B[a]P on keratin expression was observed in vitamin A treated cells. However, vitamin A deficient HTE cells exposed to B[a]P (0.05-10 micrograms/ml) demonstrated a decrease in the expression of the four differentiation-related keratins while the simple epithelial keratins appeared to be unaffected. These observations were verified at the RNA level employing Northern blot analysis using cDNA probes for human keratins 5,6 and 14. Results demonstrate that B[a]P alters the expression of differentiation-related genes, the cytokeratins, in cell types known to develop into tumors of the respiratory tract.

Role of asbestos and active oxygen species in activation and expression of ornithine decarboxylase in hamster tracheal epithelial cells

Induction of ornithine decarboxylase (ODC) enzyme activity occurs after exposure of hamster tracheal epithelial (HTE) cells to asbestos and the soluble tumor promoter 12-O-tetradecanoylphorbol-13-acetate. Since active oxygen species are implicated as mediators of asbestos-induced biological responses studies here were designed to examine whether active oxygen species generated by asbestos or oxidants caused increased ODC activity. In confluent HTE cells, significant blockage of chrysotile or crocidolite asbestos-stimulated ODC activity occurred with simultaneous addition of catalase, but not superoxide dismutase, to medium. The addition of xanthine plus xanthine oxidase caused a dose-dependent increase in ODC activity, which was inhibited significantly after addition of catalase or mannitol, indicating that H2O2 was the principal oxidant produced in that reaction. Addition of phenazine methosulfate, a redox reagent used to generate superoxide, resulted in significant elevation of ODC, which was inhibited by addition of superoxide dismutase but not catalase. Hydrogen peroxide added to culture medium also caused a potent increase in ODC activity inhabitable by catalase. Hypochlorous acid caused increases in ODC activity, although the magnitude of this response was less than that observed with other oxidants. Therefore, although all active oxygen species examined triggered ODC, less reduced species (O2- and H2O2) were more proficient than OH or a halogenated oxidant. All oxidants, except HOCl, caused a significant increase in [3H] thymidine incorporation at 24 or 48 h after their addition to HTE cells. In comparative studies, exposure of HTE cells to either asbestos or xanthine plus xanthine oxide increased the level of ODC mRNAs proportionate to oxidant concentration and the extent of enzyme induction. Thus, data indicate that H2O2 plays a major role in asbestos-stimulated ODC induction and proliferation of epithelial cells of the respiratory tract by altering the regulation of a gene critical to proliferation.

Enhanced lipid peroxidation in lung lavage of rats after inhalation of asbestos

Exposure of phagocytic cells to asbestos in vitro results in an augmented production of reactive oxygen metabolites and increased peroxidation of lipids. The aim of this investigation was to assess the extent of lipid peroxidation both in cells and fluid obtained from bronchoalveolar lavage (BAL), and in lungs of rats exposed to crocidolite asbestos or titanium dioxide (TiO2), a nonfibrous particulate control. In comparison to sham and TiO2-exposed rats, the BAL fluid and cells of crocidolite-exposed animals contained significantly elevated levels of malondialdehyde (MDA), a breakdown product of lipid peroxidation detected using high-pressure liquid chromatography (HPLC). In contrast, no significant differences in MDA were detected in lavaged lung tissue from these animals. Inhalation of crocidolite caused an early inflammatory response characterized by elevated numbers of polymorphonuclear leukocytes and lymphocytes, as well as enhanced total protein in BAL. Pulmonary fibrosis and increased lung hydroxyproline also were observed after 20 days of exposure. Exposure to TiO2 did not cause inflammation, pulmonary fibrosis, or elevated amounts of hydroxyproline in the lung. Our results show that exposure to the fibrogenic and inflammatory mineral, crocidolite, results in an enhanced lipid peroxidation in BAL cells and fluid not observed after inhalation of the particulate TiO2. These novel observations suggest that MDA in BAL may be useful as a biomarker of exposure to inhaled asbestos or other oxidants.

In vitro studies on the biologic effects of fibers: correlation with in vivo bioassays

In vitro studies employing organ cultures, primary cell cultures, cell lines, and bacterial systems have been used to assess the toxicity, mutagenicity, and carcinogenic potential of asbestos and nonasbestos fibers. These experiments have been useful in defining mechanisms contributing to the causation of fiber-associated lung diseases. Long (greater than 8 microns), thin asbestos fibers are more active in vitro than short (less than or equal to 2 microns) fibers or nonfibrous particles, an observation supporting the importance of fiber dimension in disease. Although in vitro bioassays cannot evaluate characteristics such as clearance and/or durability of fibers which may be critical determinants of fiber toxicity in lung, they can be used both to address dosimetry at the cellular level (i.e., number of fibers per cell that elicit a measurable biologic end point) and to evaluate preventive approaches to fiber-induced cell injury. Development of in vitro models employing target cells of the lung, i.e., mesothelial cells, tracheobronchial epithelial cells, and lung fibroblasts, as well as carefully characterized preparations of fibers and particles, will be necessary to evaluate whether in vitro bioassays are amenable to predicting the pathogenic potential of synthetic and naturally occurring fibers comparatively.

Inhibition of lung injury, inflammation, and interstitial pulmonary fibrosis by polyethylene glycol-conjugated catalase in a rapid inhalation model of asbestosis

Several in vitro studies suggest the involvement of active oxygen metabolites in cell damage caused by asbestos. To determine if lung injury, inflammation, and asbestosis could be inhibited in vivo in a rapid-onset, inhalation model of disease, a novel method of chronic administration of antioxidant enzymes was developed. In brief, Fischer 344 rats were treated with polyethylene glycol-conjugated (PEG-) superoxide dismutase or catalase in osmotic pumps over a 10-day (5 days/wk for 2 wk) or 20-day (5 days/wk for 2 wk) period of exposure to crocidolite asbestos. Control rats included sham-exposed animals and those exposed to asbestos but receiving chemically inactivated enzymes. After 10 days of exposure to asbestos, lactic dehydrogenase (LDH), alkaline phosphatase, and total protein in bronchoalveolar lavage (BAL) were measured in one group of rats. Total and differnetial cell counts in BAL also were assessed. After 20 days of exposure, lungs of an additional group of rats were evaluated by histopathology and by measurement of hydroxyproline. Asbestos-associated elevations in LDH, protein, and total cell numbers in BAL were reduced in rats receiving PEG-catalase. Decreases in numbers of alveolar macrophages, polymorphonuclear leukocytes, and lymphocytes occurred in these animals. Exposure to asbestos for 20 days caused significant increases in both the amount of hydroxyproline in lung and the severity and extent of fibrotic lesions as determined by histopathology. These indicators of asbestosis were inhibited in a dosage-dependent fashion in rats receiving PEG-catalase. Use of inactivated PEG-catalase failed to boost serum levels of catalase and did not inhibit asbestos-induced elevation of hydroxyproline in lung.(ABSTRACT TRUNCATED AT 250 WORDS)

Brief inhalation of asbestos compromises superoxide production in cells from bronchoalveolar lavage

Production of superoxide (O-.2) was measured in alveolar macrophages (AM) exposed to asbestos in vitro and in cells obtained from bronchoalveolar lavage (BAL) of rats inhaling asbestos. Steady state levels of O-.2 released by AM in vitro were dose and time dependent in response to crocidolite, chrysotile, and opsonized zymosan, a particulate used to trigger O-.2 generation. In contrast, an inhalation exposure for 1 h to crocidolite or for 6 days to either crocidolite or chrysotile asbestos resulted in a decreased production of O-.2 by BAL cells. Likewise, BAL cells from rats inhaling chrysotile for 1 h or crocidolite for 9 days exhibited a diminished capacity to secrete O-.2 when challenged with the particulate opsonized zymosan. Diminished generation of O-.2 by asbestos occurred in BAL cell populations containing either significantly increased numbers of polymorphonuclear leukocytes and lymphocytes (6- and 9-day exposures) or 99% AM (1-h exposure). Thus, these novel observations suggest that short-term inhalation of asbestos compromises the ability of BAL cells to produce O-.2 in the presence or absence of an additional phagocytic stimulus.

Asbestos: scientific developments and implications for public policy

Asbestos is a commercial term for a group of fibrous minerals often associated with the development of pulmonary interstitial fibrosis (asbestosis), lung cancer, and malignant mesothelioma in occupationally exposed individuals. The pathogenicity of different forms of asbestos varies--long, thin amphibole fibers are most pathogenic, particularly in the induction of mesothelioma. Available data do not support the concept that low-level exposure to asbestos is a health hazard in buildings and schools. The concentration of asbestos fibers in air, type of asbestos, and size of fibers must be considered in evaluation of potential health risks.

Increases in endogenous antioxidant enzymes during asbestos inhalation in rats

Although the pathogenesis of asbestos-induced pulmonary damage is still not completely understood, an important role has been attributed to active oxygen species. In the present paper we present results of a study investigating the effect of crocidolite asbestos inhalation on different lung antioxidant enzymes in rats. During the development of pulmonary fibrosis induced by crocidolite asbestos, lung superoxide dismutase, catalase and selenium-dependent glutathione peroxidase activities increased, indicating an adaptive response to increased pulmonary oxidant stress. However, this adaptive response obviously is not sufficient to protect the lung from asbestos-induced pulmonary damage. Considering the role of active oxygen species in both the fibrotic process and tumor promotion, it is hypothesized that antioxidants may also protect the lung from chronic asbestos-induced pulmonary damage such as bronchogenic carcinoma.

Detection of lipid peroxidation in lung and in bronchoalveolar lavage cells and fluid

Inhalation of toxic materials such as asbestos, silica, 100% oxygen, ozone, or nitrogen dioxide may lead to an increased production of reactive oxygen metabolites which may initiate lipid peroxidation. Measurement of lipid peroxidation in cells and fluid obtained by bronchoalveolar lavage (BAL), as well as in lung tissue, may aid in monitoring the development and extent of pulmonary damage after inhalation of a toxic substance. In this study, we employed a sensitive assay for detection of malondialdehyde (MDA), a breakdown product of lipid peroxidation. By separation of the adduct with thiobarbituric acid, using a reverse phase high pressure liquid chromatographic technique, we accurately and sensitively measured the content of MDA in BAL cells, lavage fluid, and lavaged lung tissue homogenates of rats. The amounts of sample required for detection of MDA were small enough possibly to be applied to use with human specimens; in addition, recovery of added MDA was acceptable with all types of samples. Inclusion of a metal chelator in the preparation of samples appeared necessary to prevent metal-catalyzed propagation of lipid peroxidation during the assay. Overall, the method described here using samples from rats may be applicable to detecting lipid peroxidation in BAL samples from humans.

In vitro assays to predict the pathogenicity of mineral fibers

A number of mineral dusts are associated with the development of inflammation in lung and pulmonary interstitial fibrosis. In an effort to find alternative approaches to animal testing, cells (rat alveolar macrophages, hamster tracheal epithelial cells, rat lung fibroblasts) of the respiratory tract have been evaluated for cytotoxic and metabolic changes after exposure to fibers (defined as a greater than 3:1 length to diameter ratio) and particles. In all bioassays, fibrous materials provoked greater biological responses in cells in comparison to non-fibrous, chemically similar particulates at identical concentrations. For example, release of superoxide (O2-.) from alveolar macrophages (AMs) was increased (in comparison to that observed in untreated cells) after exposure to the fibers, crocidolite asbestos, erionite, Code 100 fiberglass and sepiolite. Riebeckite, mordenite and glass particles elicited minimal generation of O2-. at similar concentrations of dusts. In hamster tracheal epithelial (HTE) cells, fibers such as chrysotile asbestos, crocidolite asbestos, and Code 100 fiberglass caused increased release of 51Chromium in comparison to the particles antigorite, riebeckite and glass. Another area of exploration is the measurement of collagen and non-collagen protein in a cell line (RL-82) of rat lung fibroblasts as an indication of the fibrogenic potential of minerals. Crocidolite asbestos caused an increase in the ratio of cell-associated collagen to non-collagen protein in these cell types whereas glass beads did not affect biosynthesis of collagen. Results suggest that a battery of in vitro assays can be used to screen the capability of minerals to elicit cell damage and inflammatory changes in the respiratory tract.

Regulation of differentiation and keratin protein expression by vitamin A in primary cultures of hamster tracheal epithelial cells

Hamster tracheal epithelial (HTE) cells maintained in primary culture show the induction of specific keratin species under vitamin A-deficient conditions. A comparison was made between the morphology and the expression of keratins in HTE cells in vivo and in primary culture with and without vitamin A. HTE cells cultured in serum-free, vitamin A-supplemented medium formed a simple cuboidal, ciliated monolayer and produced four simple epithelial keratins (7, 8, 18, and 19). In contrast, vitamin A-deficient HTE cells, which were squamous-like and stratified in culture, produced a more complex keratin pattern, with the induction of four additional keratin species (5, 6, 14, and 17). A keratin pair whose expression serves as a marker of stratified epithelia was induced, as well as a single keratin species unique to lesions of squamous metaplasia in vitamin A-deficient hamster tracheal organ cultures. Thus it appears that HTe cells retain the ability to respond to a deficiency in vitamin A through squamous differentiation and increased keratin production when removed from the intact organ and maintained in primary culture in a chemically defined medium. This system may be useful for the study of mechanisms underlying the squamous differentiation of respiratory epithelial cells in the development of bronchogenic tumors.

Sensitivity of hamster tracheal epithelial cells to asbestiform minerals modulated by serum and by transforming growth factor beta 1

A number of proliferative and cytotoxic responses similar to those induced by classical tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate occur in tracheal epithelial cells and organ cultures exposed to asbestiform minerals. In these studies, cloned diploid hamster tracheal epithelial cells were examined in a number of short term (3-24 h) and long term (greater than 24 h) biological assays for their responsiveness to crocidolite asbestos in comparison to other fibers (chrysotile asbestos, Code 100 fiberglass) and particulates (riebeckite, antigorite, glass beads). In addition, the influence of serum on the biological effects of crocidolite were assessed. In 10% serum-containing medium, low concentrations of crocidolite (1.0 micrograms/cm2 dish) stimulated cellular proliferation (measured as increased [3H]thymidine incorporation), and concentrations greater than 5 micrograms/cm2 dish were cytotoxic (measured by decreased colony forming efficiency). In 2% serum-containing medium, lower concentrations of fibers (0.1-0.5 micrograms/cm2 dish) caused increases in [3H]thymidine incorporation and colony forming efficiency whereas higher concentrations of fibers were required for responses equitoxic to those observed in 10% serum. Crocidolite-induced [3H]thymidine incorporation was inhibited when 2% serum-containing medium was supplemented with transforming growth factor type beta at concentrations (3 ng/ml) which give rise to altered morphology of hamster tracheal epithelial cells. Furthermore, the profiles of colony forming efficiency and 51Cr release of cells grown in the presence of transforming growth factor beta resembled those of cells grown in 10% serum. Results suggest that the phenotypic changes induced by serum factors such as transforming growth factor beta influence the sensitivity of hamster tracheal epithelial cells to crocidolite asbestos.

Evidence supporting a role for active oxygen species in asbestos-induced toxicity and lung disease

Asbestos is an important occupational and environmental toxicant that affects several cell types in the respiratory tract. In an effort to understand how asbestos causes cell injury and/or altered proliferation and differentiation of cells, this laboratory has focused on reactive oxygen species as mediators of asbestos-induced biological effects. A compendium of experimental results reported by this laboratory and others supports this hypothesis. For example, scavengers of reactive oxygen metabolites and iron chelators (i.e., desferroxamine) prevent cytotoxicity after addition of asbestos to a variety of cell lines and macrophages in vitro. DNA strand breakage associated with toxicity of crocidolite asbestos in C3H10T 1/2 cells also is ameliorated with use of desferroxamine. All types of asbestos cause lipid peroxidation in mammalian cells and artificial membranes, a phenomenon that can be prevented by removal of catalytic iron. Last, asbestos causes generation of active oxygen species after interaction with leukocytes or by reduction of oxygen on the surface of the fibers.

Mechanisms of asbestos-induced squamous metaplasia in tracheobronchial epithelial cells

Within 1 to 4 weeks after exposure to asbestos, differentiated rodent and human tracheobronchial epithelial cells in organ culture undergo squamous metaplasia, a putative preneoplastic lesion characterized by conversion of mucociliary cell types to keratinizing cells. The exogenous addition of retinal acetate (RA) to culture medium of hamster tracheal organ cultures reverses preestablished, asbestos-induced squamous metaplasia, although data suggest that the effectiveness of RA decreases as the length of time between exposure to asbestos and initial application of RA increases. alpha-Difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC), inhibits squamous metaplasia caused by asbestos or vitamin A deficiency, whereas addition of methylglyoxal bis(guanylhydrazone) (MGBG), a structural analog of spermidine and inhibitor of S-adenosylmethionine decarboxylase, causes an enhancement of metaplasia under both circumstances. Basal cell hyperplasia and increased incorporation of 3H-thymidine by tracheal epithelial cells also are seen after addition of the polyamines, putrescine or spermidine, to tracheal organ cultures, an observation supporting the importance of polyamines in the development of this lesion. The use of retinoids and inhibitors of ODC could be promising as preventive and/or therapeutic approaches for individuals at high risk for development of asbestos-associated diseases.

Mechanisms of induction of ornithine decarboxylase activity in tracheal epithelial cells by asbestiform minerals

Asbestos induces a constellation of biological responses in cells of the respiratory tract that are similar to those of classical tumor promoters. In this regard, induction of ornithine decarboxylase (ODC) activity and increased incorporation of [3H]thymidine have been documented after addition of crocidolite and chrysotile asbestos to a hamster tracheal epithelial cell line (J. M. Landesman and B. T. Mossman, Cancer Res., 42:3669-3675, 1982). The objectives of studies here were to determine: (a) the importance of geometry, size, and/or chemical composition of asbestos fibers on induction of ODC activity; and (b) the possible involvement of calcium and/or protein kinase C in asbestos-induced ODC activity. After addition for 24 h to confluent hamster tracheal epithelial cells, fibers of crocidolite, chrysotile, and glass in medium containing fresh serum caused a significant increase in ODC activity. Stimulation of ODC was not observed when nonfibrous analogues (riebeckite, antigorite, and glass particles) were used. Sized preparations of long (greater than 10-micron length) chrysotile fibers were more potent in enhancing ODC activity than shorter (less than or equal to 2-micron length) fibers at similar concentrations. The mechanisms of ODC induction by asbestos were probed by adding the calcium channel blockers (verapamil and nifedipine) and inhibitors [10(-5) to 10(-7)M of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, of N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, of TMB-8, and of palmitoyl carnitine] of protein kinase C simultaneously with chrysotile asbestos. These agents inhibited ODC activity by chrysotile in a dosage-dependent fashion. Results suggest that the fibrous geometry and length of asbestos fibers are critical in initiating ODC activity in airway epithelial cells. Moreover, they implicate the importance of calcium and protein kinase C in asbestos-induced mitogenic responses.

Generation of superoxide (O2-.) from alveolar macrophages exposed to asbestiform and nonfibrous particles

Active oxygen species are implicated causally in tumor promotion by 12-O-tetradecanoylphorbol-13-acetate and in cell damage by asbestos, a putative tumor promoter of the respiratory tract. To determine the properties of asbestos important in generation of the oxygen free radical, superoxide (O2-.), hamster and rat alveolar macrophages were exposed in vitro to nontoxic concentrations of fibrous (crocidolite, erionite, Code 100 fiberglass, sepiolite) and nonfibrous (riebeckite, mordenite, glass) particulates. The amount of O2-. released by cells in response to dusts was determined by measuring the reduction of cytochrome c in the presence and absence of superoxide dismutase. Results showed that all fibrous (defined as a greater than 3:1 length:diameter ratio) dusts caused a significant increase in both release of O2-. from rat macrophages and enhancement of zymosan-triggered O2-. from hamster macrophages. Nonfibrous particles were less active than fibers at comparable concentrations. These results suggest that the geometry of particulates is of critical importance in the generation of O2-. from cells of the respiratory tract.

A possible mechanism of chrysotile asbestos toxicity

In vitro evaluations of heat treated and/or irradiated chrysotile asbestos indicate the importance of electron transfer in determining the biological outcome of asbestos exposure. Regardless of the assay system used, all show the same trends. Namely, heat treatment reduces cytotoxicity while heat treatment in combination with irradiation increases activity over heat treatment alone. Furthermore, studies of BSA and DNA binding consistently indicate a lower molecular adsorption for heated asbestos. BSA and DNA adsorption are similar for untreated, irradiated and heated or irradiated samples. Physical and chemical analyses indicate that the size and elemental composition are unaffected by the treatment processes. X-ray diffraction indicates no change in the crystal structure of chrysotile asbestos. IR spectroscopy suggests that heat treatment affects the external hydroxyl group population while heat treatment and irradiation may repopulate this functional group. A proposed mechanism consistent with the biological and physical characterization is based upon electron transfer from the chrysotile asbestos matrix to biological receptors.

Comparative interactions of streptozotocin and chlorozotocin with DNA of an insulin-secreting cell line (RINr)

Both streptozotocin and chlorozotocin, the 2-chloroethyl analogue of streptozotocin, are diabetogenic chemicals in rodents. Although these chemicals are similar structurally, they appear to act on pancreatic B cells via different mechanisms. In studies here, damage and repair of DNA after exposure of an insulin-secreting cell line to streptozotocin and chlorozotocin were assessed by nucleoid sedimentation and alkaline elution. Equitoxic concentrations of streptozotocin and chlorozotocin caused significant single-strand breakage of DNA (p less than 0.005). These lesions were repaired in a time-dependent manner, with most repair completed by 24-h post-exposure to chemicals. Additionally, chlorozotocin caused DNA-DNA and DNA-protein crosslinks in insulinoma cells. When proteinase K was included in the crosslinking assay, a substantial proportion of the chlorozotocin-associated crosslinks proved to be DNA interstrand in nature. Analysis of the amount of interstrand crosslinking in insulinoma cells after exposure to chlorozotocin for 1 h showed that formation of interstrand crosslinks was slow. Increasing amounts appeared over a 24-h period. These results suggest that the formation of irreversible DNA interstrand crosslinks may be a critical factor in cytotoxicity and diabetogenicity caused by chlorozotocin.

Alteration of superoxide dismutase activity in tracheal epithelial cells by asbestos and inhibition of cytotoxicity by antioxidants

We report here the inhibition of asbestos-induced cytotoxicity in a hamster tracheal epithelial cell line by superoxide dismutase, a scavenger of superoxide (O2-.), and by mannitol and dimethylthiourea, scavengers of the hydroxyl radical (OH.). By using these agents, cell damage was ameliorated in cultures exposed to long (greater than 10 microns in length) fibers of chrysotile and crocidolite asbestos. In contrast, injury to epithelial cells by short (less than or equal to 2 microns) chrysotile or glass fibers was not prevented by scavengers of O2-., OH., H2O2 or 1O2 (singlet oxygen). These results implicate active oxygen species as mediators of injury by long asbestos fibers to cells of the respiratory tract. By using immunocytochemical and biochemical techniques, we detected appreciable amounts of copper-zinc superoxide dismutase in hamster tracheobronchial epithelial cells and alveolar macrophages in vitro and in histologic sections of rat and human respiratory tract. Activity of total endogenous superoxide dismutase (copper-zinc and manganese forms) increased in tracheal epithelial cells exposed for several days in vitro to either crocidolite or chrysotile asbestos but was unchanged in untreated cells and those exposed to comparable amounts of glass fibers. After inhalation of asbestos by rats, or exposure of cells in culture to asbestos, long fibers were observed protruding from both epithelial cells and alveolar macrophages. The unsuccessful phagocytosis of long fibers of asbestos coupled with generation of oxygen free radicals might explain the increased pathogenic potential of long fibers in asbestos-associated diseases of the respiratory tract.

Approaches to prevention of asbestos-induced lung disease using polyethylene glycol (PEG)-conjugated catalase

Asbestos-associated damage to cells of the respiratory tract in vitro can be prevented by the simultaneous addition of scavengers of active oxygen species to cultures. To determine if administration of scavenger enzymes to animals and humans is a plausible approach to the prevention of asbestos-induced lung disease, osmotic pumps were filled with various concentrations of PEG-coupled catalase and implanted subcutaneously into Fischer 344 rats over a 28-day period. At 3, 14, and 28 days after implantation of the pumps, the animals were evaluated for levels of catalase in serum and lung. In addition, lung tissue and lavage fluids were examined at 28 days for biochemical and morphologic indications of cell injury, inflammation, and fibrotic lung disease. At all time points examined, the administration of PEG-catalase caused a dosage-dependent increase in serum levels of catalase. The levels of lung catalase were evaluated at 28 days but not at earlier time periods. In comparison to control rats, the amounts of enzymes (lactic dehydrogenase, alkaline phosphatase), protein, and cells in lavage fluids from treated animals were unaltered. Moreover, the lungs showed no evidence of inflammation or fibrotic disease as determined by differential cell counts in lavage and measurement of hydroxyproline. These studies suggest that administration of PEG-catalase does not cause injury or other alterations in lung tissue and can be pursued as a feasible approach to prevention of asbestosis.

Chlorozocin. A diabetogenic analogue of streptozocin with dissimilar mechanisms of action on pancreatic beta cells

Chlorozotocin (chlorozocin, CLZ), the 2-chloroethyl analogue of streptozocin (STZ), was evaluated in three species of rodents. The drug is currently being used in phase II chemotherapeutic trials in man, and appears to be effective in the treatment of certain tumors. In our studies, hyperglycemia was induced in hamsters as early as 2 days after a single intraperitoneal (i.p.) injection of 30-60 mg/kg and was most striking at 4 days. Greater concentrations of CLZ (greater than or equal to 50 mg/kg) were required to produce hyperglycemia in CD-1 mice. Degranulation and necrosis of beta cells developed in hamsters and mice, whereas alpha and acinar cells of the pancreas revealed no morphologic changes. Hyperglycemia was not induced in rats at any concentration tested; however, animals showed abnormal carbohydrate tolerance after administration of 100 mg/kg CLZ (LD50 dosage). The nature of damage by CLZ to beta cells was investigated both in vivo and in vitro. Pretreatment of hamsters with nicotinamide (500 mg/kg, i.p.) failed to alter the extent of CLZ-induced beta cell injury and associated hyperglycemia, but decreased the amount of beta cell necrosis and hyperglycemia in animals receiving STZ. The nonmetabolizable sugar, 3-O-methylglucose (3-O-MG), and 3-aminobenzamide, an inhibitor of the nuclear enzyme, polyADPribose synthetase, prevented STZ-associated damage to beta cells in islet cell cultures, but only 3-O-MG reduced CLZ-induced toxicity. Thus, in comparison to STZ, CLZ appears to be a diabetogenic agent with different species specificity and alternative mechanisms of cytotoxicity. The glucose moiety of both drugs appears critical in the induction of beta cell damage.