New functional cell-culture approach to pulmonary carcinogenesis and toxicology

An improved in vitro model for testing the pulmonary toxicity of complex mixtures such as cigarette smoke

Numerous approaches have been employed for testing the biological activity of cigarette smoke in vitro. None of them has managed to expose cultured lung cells in a realistic manner to the complex gaseous and particulate mixture that constitutes cigarette smoke. We have devised a system that makes this possible. The system presented here enables the direct exposure of human lung cells to native, unmodified cigarette mainstream smoke. It consists of a smoking machine, a dilution device for the smoke, analytical devices for online monitoring and a specially adapted exposure module based on the Cultex** cell cultivation system that is equipped with a gas-exposure top. Due to the special design of the exposure device and the optimised exposure conditions, this equipment allows cultured human lung cells to be exposed to freshly generated cigarette mainstream smoke. Exploratory experiments revealed that the smoke could be diluted over a wide concentration range in a reproducible way with respect to gas and particulate phases, and also demonstrated reproducible particle deposition depending on smoke concentration. Furthermore, it was shown that the exposed cells maintained their viability. Native cigarette mainstream smoke induced dose-dependent cellular effects in exposed cells with respect to cellular viability (viable cell number monitored by tetrazolium salt cleavage) and intracellular parameters (ATP and glutathione content). Therefore, fresh, physically and chemically unmodified cigarette mainstream smoke can be tested using this novel system.

Novel approaches for studying pulmonary toxicity in vitro

The in vitro study of adverse cellular effects induced by inhaled pollutants poses a special problem due to the difficulties of exposing cultured cells of the respiratory tract directly to test atmospheres that can include complex gaseous and particulate mixtures. In general, there is no widely accepted in vitro exposure system. However, in vitro methods offer the unique possibility for use of human cells, developed and validated cell culture and exposure device (CULTEX(1)) using the principle of the air/liquid exposure technique. Cells of the respiratory tract are grown on porous membranes in transwell inserts. After removal of the medium, the cells can be treated on their superficial surfaces with the test atmosphere, and at the same time they are supplied with nutrients through the membrane below. In comparison with other experimental approaches, the goal of our studies is to analyze the biological effects of test atmospheres under environmental conditions, i.e. without humidifying the atmosphere or adding additional CO(2). The system used is small and flexible enough independent of a cultivation chamber and thus offers the opportunity for onsite study of indoor and outdoor atmospheres in the field. The efficacy of the exposure device has already been demonstrated in the analysis of dose-dependent cytotoxic and genotoxic effects of exposure of epithelial lung cells to complex mixtures such as native diesel exhaust and side-stream smoke.

A method for the in vitro exposure of human cells to environmental and complex gaseous mixtures: application to various types of atmosphere

The application of in vitro methods to the analysis of the effects of airborne materials is still limited, because there are no generally accepted concepts and technologies for efficiently exposing adherent growing cells to test atmospheres, especially those comprising complex mixtures of gaseous and particulate phases. The introduction of in vitro research into the field of inhalation toxicology offers a unique possibility for using human cells and tissues for pre-screening studies, thus reducing the necessity for animal experiments, and cutting the numbers of animals used in toxicological testing. We therefore developed a novel experimental concept that uses an exposure device based on the cell cultivation system CULTEX (Patent No. DE 198011763; PCT/EP99/00295). This allowed us to investigate environmental atmospheres, which were chemically and physically unmodified, in an in vitro system, by exposing the target cells directly at the air/liquid interface. The exposure device itself is small and flexible enough to be connected to a variety of aerosol-generating systems without the need for an incubator, as it fulfils all the requirements for maintaining cell viability over a defined period. The general applicability and the sensitivity of this in vitro approach for testing various generated atmospheres under the same cell-exposure conditions were demonstrated by studying dose-dependent cytotoxic effects in human lung epithelial cells exposed to air contaminated with single gases or complex mixtures, such as diesel exhaust fumes and side-stream cigarette smoke.

In vitro genotoxicity assay of sidestream smoke using a human bronchial epithelial cell line

Genotoxic effects of air contaminants, such as gaseous or particulate compounds, have been difficult to investigate due to inefficient methods for exposing cell cultures directly to these substances. New cultivation and exposure techniques enable treatment of epithelial cells with sample atmospheres with subsequent in vitro assays, as demonstrated by a new system called CULTEX (CULTEX: patent No. DE 19801763; PCT/EP99/00295), which uses a transwell membrane technique for direct exposure of complex mixtures, for example sidestream cigarette smoke, at the air/liquid interface. The sensitivity and susceptibility of human bronchial epithelial cells to this complex mixture have already been shown for cytotoxic endpoints. In this study, genotoxic effects of sidestream cigarette smoke at different concentrations were assessed using the alkaline comet assay. HFBE 21 cells were exposed for 1 h to clean air, nitrogen dioxide or sidestream smoke. Exposure of the cells to sidestream cigarette smoke induced DNA strand breaks in a dose-dependent manner. The combination of gas phase exposure and the comet assay provides a realistic and efficient model for sensitive detection of DNA strand breaks induced by airborne and inhalable compounds.

Exposure of human lung cells to native diesel motor exhaust--development of an optimized in vitro test strategy

To investigate the effects of native diesel motor exhaust on human lung cells in vitro, a new experimental concept was developed using an exposure device on the base of the cell cultivation system CULTEX (Patent No. DE19801763.PCT/EP99/00295) to handle the cells during a 1-h exposure period independent of an incubator and next to an engine test rig. The final experimental set-up allows the investigation of native (chemically and physically unmodified) diesel exhaust using short distances for the transportation of the gas to the target cells. The analysis of several atmospheric compounds as well as the particle concentration of the exhaust was performed by online monitoring in parallel. To validate the complete system we concentrated on the measurement of two distinct viability parameters after exposure to air and undiluted, diluted and filtered diesel motor exhaust generated under different engine operating conditions. Cell viability was not influenced by the exposure to clean air, whereas dose-dependent cytotoxicity was found contingent on the dosage of exhaust. Additionally, the quality of exhaust, represented by two engine operating conditions (idling, higher load), also showed well-distinguishable cytotoxicity. In summary, the experimental set-up allows research on biological effects of native engine emissions using short exposure times.

In vitro exposure of isolated cells to native gaseous compounds--development and validation of an optimized system for human lung cells

An exposure system for adherent growing cells to native gaseous compounds was developed using air/liquid culture techniques on the basis of the Cultex system'. In contrast to other exposure systems the reproducible testing of native environmentally relevant gases without changing their physical or chemical properties including heating, CO2- content and humidity is possible. Specially designed systems for medium flow and gas support guarantee the nutrification and humidification as well as the direct gas contact of the exposed cells which are cultivated on microporous membranes (0.4 microm pore size). The system works independently of a cell culture incubator offering the possibility to analyze any relevant gas mixture directly under indoor or outdoor conditions. Several experimental approaches were carried out to characterize the properties of the system. In exploratory experiments without cells, the reproducibility and quality of the gas/membrane contact could be demonstrated. Exposures of human lung fibroblasts (Lk004 cells) and human lung epithelial cells (HFBE-21 cells) to synthetic air, ozone (202 ppb, 510 ppb) and nitrogen dioxide (75 ppb to 1,200 ppb) established that cells could be treated for 120 minutes without significant loss of cellular viability. At the same time, the experiments confirmed that such exposure times are long enough to detect biological effects of environmentally relevant gas mixtures. The analysis of viability (viable cell number, tetrazoliumsalt cleavage) and intracellular end-points (oxidized/reduced glutathione, ATP/ADP) showed that both gases induced relevant cellular changes. In summary, the efficiency and practicability of this newly developed exposure system for adherent human lung cells could be clearly demonstrated.

A method for in vitro analysis of the biological activity of complex mixtures such as sidestream cigarette smoke

Studies of the cytotoxicity of air contaminants such as gaseous or particulate compounds and complex mixtures have traditionally used in animal experiments because of the difficulties in exposing cell cultures directly to these substances. New cultivation and exposure techniques enhance the efficiency of in vitro methods, as demonstrated by a new system called CULTEX* which uses a transwell membrane technique for direct exposure of complex mixtures like sidestream cigarette smoke at the air/liquid interface. The factors influencing the susceptibility of human bronchial epithelial cells (e.g. gas flow rate or duration of exposure) were studied and the cells were finally exposed for one hour to clean air or different concentrations of sidestream smoke. The biological parameters estimated were number of cells, metabolic activity and glutathione concentration. After exposure of the cells to sidestream cigarette smoke, dose-dependent effects were measured. Thus, the introduction of these cultivation and exposure techniques offers new testing strategies for the toxicological evaluation of a broad range of airborne and inhalable compounds.

Staphylococcus aureus adherence to nasal epithelial cells in a physiological in vitro model

Nasal carriage of Staphylococcus aureus represents a risk factor for subsequent invasive infections and interpatient transmission of strains. No physiological in vitro model of nasal epithelial cells is available to study both patient- and bacteria-related characteristics and their interaction, leading to adherence and colonization. Starting with tissues from human nasal polyps, a confluent, squamous, nonkeratinized epithelium in collagen-coated 96-well microtiter plates was obtained after 14 d. This in vitro cell-layer was characterized histologically, ultrastructurally, and immunohistochemically and showed features that were indistinguishable from those observed in the squamous nonkeratinized epithelium found in the posterior part of the vestibulum nasi. Adherence experiments were performed with four different 3H-thymidine-labeled Staphylococcus aureus strains. The effect of bacterial inoculum size, temperature of incubation, and incubation medium were studied. The adherence results were found to be reproducible, reliable and sensitive, allowing detection of small quantitative differences in adherence between the Staphylococcus aureus strains. There was no significant difference in adherence at 23 degrees C and 37 degrees C, nor between the incubation medium M199 and phosphate-buffered saline. Plastic adherence could be reduced and standardized with use of siliconized tips and a constant bacterial inoculum volume of 100 microl/well. This physiological and reliable in vitro cell-culture model offers a unique opportunity to study Staphylococcus aureus adherence to squamous, nonkeratinized nasal epithelial cells and both patient and bacterial characteristics involved in this interaction.

Stem cells of the respiratory epithelium and their in vitro cultivation

Parenchymal (epithelial or mesenchyma) stem cells are rapidly drawing both scientific and clinical attention in solid organs like the liver, skin, intestine and abdominal mesothelium, just as has been the case in the hematopoietic system. For the stem cells of these organs various definitions, markers for identification, methods of isolation and in vitro cultivation, and lineage mechanisms have been proposed and some of them are now proven to be valid and useful. In this article attempts will be made to explore whether there are stem cells in the lower respiratory system (from the trachea to the lung periphery) and what they look like. Because of its anatomical and functional complexity the stem cell concept for the respiratory system has been developing rather slowly. Nevertheless, the data available seem to indicate that in analogy to the above mentioned organs there is only one type of epithelial stem cells throughout all sections of the lower respiratory system during fetal through adult stages. They are multipotent for cell differentiation and able to yield lineage progenitors for ciliated, goblet, basal. Clara neuroendocrine, alveolar type 1 and alveolar type 2 cells.

Sensitivity of a hamster lung cell line to direct and indirect acting carcinogens

Cytotoxicity of benzo(a)pyrene (B(a)P), 7,12-dimethylbenz(a)anthracene (DMBA), aflatoxin B1 (AB1), and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was estimated in vitro by using a hamster lung cell line. The studies were conducted to assess the usefulness of an organ-specific cell culture system for demonstrating differences in the cytotoxic potency of diverse chemical carcinogens. Cytotoxicity was determined by using the succinate dehydrogenase assay (MTT assay) after different incubation times and concentrations with the corresponding carcinogens. The effective concentration EC50 as well as the slope of the regression line were used as parameters for the biological effects. The results from these studies indicate a clear dose-dependent reaction after incubation of the cells with aflatoxin B1 (EC50: 2.3 microM) and MNNG (EC50: 4.0 microM). For the polycyclic hydrocarbons benzo(a)pyrene and DMBA, a dose-independent reaction was found. These results indicate that consideration of the EC50 values only might not be sufficient to characterize differences in the cytotoxic activity of different substances. Chemicals can lead to equal values in the EC50, but cells can differ significantly in their biological sensitivity, meaning that the extent of reduction in cell proliferation depends on the chemical used. By considering the two above-mentioned parameters, a ranking for the analyzed substances will be possible in the following way: AB1, MNNG, DMBA and B(a)P. Taken together, our experiments show that it is possible to reveal differences in the cytotoxic potency of chemicals by using in vitro methods.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Neuroendocrine phenotype differentiation in a hamster lung epithelial cell line under low oxygen pressure or after transformation by diethylnitrosamine

Oxygen pressure as low as 5% in the gas phase or 87 mmHg in the liquid phase induced various neuroendocrine cell (NEC) phenotypes in more than 80% of cells of a cloned fetal Syrian hamster lung epithelial cell line (M3E3/C3). Further, cells from a number of colonies transformed in an anchorage-independent manner by diethylnitrosamine (DEN) demonstrated a NEC phenotype. Since the cell line used is of a pluripotent stem cell type, both hypoxia and DEN appear to possess a potency for NEC phenotype induction.

Comparison of biological effects of different polycyclic aromatic hydrocarbons in lung cells of hamster and rat in vitro

The cytotoxicity and frequencies of transformation induced by 5 environmental polycyclic aromatic hydrocarbons (PAH) in hamster (M3E3/C3) and rat (WRB K3) lung cells were compared. Both cell strains investigated here retain major metabolic characteristics of the target cells in vivo and are thus able to effectively metabolize, i.e. activate, PAH. Cytotoxic effects of the carcinogen were determined in colony-forming assays and the PAH tested induced dose-dependent cytotoxic responses in the M3E3/C3 and WRB cells. They could then be classified into strong and weak cytotoxicity. Compared to the hamster cell system, the WRB cells were generally shown to be more sensitive. The transforming capacity of the compounds was determined by a soft agar colony formation assay detecting cells with anchorage independency (AI). All PAH investigated induced transformation to AI growth in both cell systems. The transforming activity of the PAH, relative to benzo[a]pyrene (B[a]P) as a reference substance, was determined to facilitate their ranking. This order of transforming potency appears to be similar to that observed in animal studies.

In vitro induction of type II pneumocyte-related differentiation in a clonal fetal bronchiolo-alveolar epithelial cell line (M3E3/C3)

The aim of the present study is to investigate the differentiation of a cloned fetal Syrian hamster lung epithelial cell line, M3E3/C3, to assume morphological and biochemical features of Type II pneumocytes (phospholipid synthesis). The use of a soft agar overlay and a differentiation medium, based on RPMI 1640 combined with hormone supplements, increased the cellular content of phosphatidylcholine (PC) from 48.6% in the conventional culture without any of these factors (referred to as 'control') to 64.7% (p < 0.02). The other cell membrane-associated components, phosphatidylethanolamine (p < 0.05), sphingomyelin (p < 0.001), phosphatidylserine (n. s.), phosphatidic acid (p < 0.02) and phosphatidylinositol (p < 0.02) decreased. The content of phosphatidylglycerol showed no essential change (from 11.2% to 8.4%) and the content of disaturated phospholipids decreased from 32.0 to 23.4 micrograms/10(6) cells (p < 0.002). The phospholipid pattern of these differentiated cells is in rough accordance with that of primary isolated Type II pneumocytes. They incorporated 3H-choline over a period of four hours at a higher rate in the Type II pneumocyte-specific phospholipids, PC and dipalmitoyl-phosphatidylcholine (DPPC), than the undifferentiated control. The radiolabelling of PC and DPPC in the differentiated cells, after 3 hours of incubation with 3H-choline, was about 3.2-fold and 2.2-fold, respectively, higher than that in the control cells (p < 0.001). Intracytoplasmatic phospholipid granules were evident in the differentiated cells by light and fluorescence microscopy (modified Papanicolaou stain, Phosphin 3 R fluorescence). Furthermore, the differentiated cells had a high activity of alkaline phosphatase, whereas the control cells showed only little activity of this enzyme. Ultrastructurally, many concentric multilayered osmiophilic bodies, well developed Golgi apparatuses and many cytoplasmic protrusions comparable to microvilli, were detectable in the cuboidal shaped differentiated cells. The control cells remained wide and flattened on the plastic surface and produced a fibrillar extracellular matrix. In the simultaneously studied fetal lung fibroblasts none of these specific features were noted. These results indicate a specific differentiation capacity of the clonal fetal cell line, M3E3/C3, by closely resembling Type II pneumocytes.

In vitro investigation on proliferative cystic keratinizing squamous lesions in the rat lung

The proliferative cystic squamous cell lesion inducible by inhalation of particulate matter in the rat lung is characterized by the formation of keratin-filled cystic cavities of various sizes bordered by multiple layers of keratinizing squamous epithelial cells. The current investigation is primarily concerned with two points. One is whether the cells participating in this particular cystic configuration can recapitulate their specific in vivo morphogenetic behavior also in the in vitro circumstances. The other is whether these squamous epithelial cells are neoplastic in nature. Although the currently adopted cell culture system was two-dimensional, the specific morphogenetic pattern was reproduced in vitro in a corresponding manner by the squamous cells derived from the aforementioned rat lung lesions. Exposure of these cells to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) at 100 ng/ml induced a marked polymorphism in them and also stimulated their keratinization. In soft agar culture, both TPA-exposed and unexposed cells developed colonies larger than 0.05 mm in diameter with an incidence of 0.73% and 1.12%, respectively. The lower incidence in TPA-exposed cultures can be interpreted in terms of TPA stimulation of keratinization. However, colonies larger than 0.1 mm in diameter were also developed by the TPA-exposed cells, indicating the presence of a cell population responsive to promotional effects of TPA. These results imply that the cells involved in the formation of cystic squamous epithelial cell lesions are already initiated and possess a potential for autonomous benign growth.

Secretory differentiation and cell type identification of a human fetal bronchial epithelial cell line (HFBE)

A human fetal bronchial epithelial cell line (HFBE) grew in an undifferentiated pattern under conventional culture conditions. Despite a somewhat fibroblastic shape the cells maintained immunoreactivity to cytokeratin, carcinoembryonic antigen and epithelial membrane antigen. When grown on a collagen gel in a growth-hormone-supplemented medium, their spindle shape became more conspicuous. With an additional supplement of vitamin A (6 micrograms/ml), most of the cells underwent differentiation by producing many bright inclusion bodies which proved to be strongly positive with periodic acid-Schiff and weakly positive with alcian blue staining. Electron microscopy revealed a well-developed rough endoplasmic reticulum, an enlarged Golgi apparatus and many highly electron-dense secretory granules resembling those of Clara cells. Biochemical analysis demonstrated that HFBE cells cultured on collagen gel with vitamin A secreted hyaluronic acid and neutral glycoproteins containing mainly N-linked glycoproteins whose glycans were of a complex type. A monoclonal antibody (SEC-41) generated against the neutral glycoproteins detected a glycoprotein of approximately 52 kDa in the spent culture medium of differentiated HFBE cells. This antibody also reacted with the intracytoplasmic secretory granules in these cells. When tested on frozen sections of lung tissue, the immunohistochemical reactivity of the SEC-41 antibody was confined to Clara cells, some type II pneumocytes in the adult lung, and respiratory epithelial cells in the fetal lung. Moreover, this antibody could detect secretory glycoprotein in broncho-alveolar lavages from two patients. This paper clearly demonstrates that cells derived from human fetal bronchial epithelium can be cultivated in an undifferentiated precursor state and, under appropriate culture conditions, can be stimulated to undergo differentiation into a Clara cell type.

Differentiation of Clara cell (distal type) antigen in human fetal bronchial epithelial cell line (HFBE)

A human fetal bronchial cell line (HFBE) grew in an undifferentiated pattern under conventional culture conditions. Despite a somewhat fibroblastic shape, however, they still maintained an immuno-reactivity to cytokeratin, carcinoembryonic antigen (CEA) and epithelial membrane antigen (EMA). When grown on a collagen gel in a growth hormone-supplemented medium, their spindle shape became more conspicuous. With the additional supplement of 6 micrograms/ml vitamin A, most of the cells underwent differentiation by producing many bright inclusion bodies which proved to be strongly positive to periodic acid Schiff (PAS) and barely positive to alcian-blue (AB) staining. Electron microscopy revealed well-developed rough endoplasmic reticulum (rER), enlarged Golgi apparatus and many highly electron-dense secretory granules resembling those of Clara cells. Biochemical analysis demonstrated that HFBE cells cultured on collagen gel with vitamin A secreted products containing glycoconjugates of two different molecular weights. The higher molecular weight-product was identified as hyaluronic acid and the lower molecular weight-product as a mixture of neutral glycoproteins containing mainly N-linked glycoproteins whose glycans were of a complex type. While the secretion of hyaluronic acid was inhibited by vitamin A in a dose-dependent manner, that of the neutral glycoproteins was most enhanced by vitamin A in the range from the physiological concentration of 600 ng/ml to 6 micrograms/ml. A monoclonal antibody (SEC-41) generated against the secretory products with the lower molecular weight detected a glycoprotein of approximately 52 kd in the spent culture medium of differentiated HFBE cells. This antibody also reacted with the intra-cytoplasmic secretory granules in these cells. When tested on freeze-sectioned lung tissue, immunohistochemical reactivity of SEC-41 antibody was confined to Clara cells, some type II pneumocytes in the adult lung and respiratory epithelial cells of the fetal lung tissue. Moreover, this antibody could detect the secretory glycoproteins in the broncho-alveolar lavages (BAL) of two human cases. This paper has clearly demonstrated that cells derived from human fetal bronchial epithelium can be cultivated in an undifferentiated precursor state and under appropriate culture conditions they can be stimulated to undergo differentiation into a Clara cell type.