Diamine uptake by rat lung type II cells in vitro

Foetal rat lung epithelial (FRLE) cells: partial characterisation and response to pneumotoxins

Cultured cell lines are routinely used for in vitro toxicity screens, reducing the requirement for animal studies during the development of new pharmaceutical, agrochemical and cosmetic products. The foetal rat lung epithelial (FRLE) cell line was originally derived from alveolar type II cells (ATII) of the lung. The aims of this study were to further characterise FRLE cells and investigate their potential for screening for pneumotoxins. The cells were found to have retained some of the features of their progenitor cells, namely the expression of cytokeratin proteins, specifically cytokeratin 18, and the ability to actively accumulate the non-selective contact herbicide paraquat. However, the cells have lost the ability to synthesise surfactant protein mRNA and no longer contain multiple lamellar bodies. Toxins that damage ATII cells in vivo (cadmium chloride, cobalt chloride and paraquat) were found to induce cytotoxicity in FRLE cells, as did the non-specific pneumotoxin nitrofurantoin, and hydrogen peroxide. However, the cells were less sensitive to the effects of compounds that require metabolic activation (1-nitronaphthalene, coumarin and butylated hydroxytoluene) and the hepatotoxin bromobenzene. Thus, FRLE cells appear to be a good in vitro model for monitoring the potential toxicity to ATII cells and could be used as an initial screen for pneumotoxicity.

Polyamines in the lung: polyamine uptake and polyamine-linked pathological or toxicological conditions

The natural polyamines putrescine, cadaverine, spermidine, and spermine are found in all cells. These (poly)cations exert interactions with anions, e.g., DNA and RNA. This feature represents their best-known direct physiological role in cellular functions: cell growth, division, and differentiation. The lung and, more specifically, alveolar epithelial cells appear to be endowed with a much higher polyamine uptake system than any other major organ. In the lung, the active accumulation of natural polyamines in the epithelium has been studied in various mammalian species including rat, hamster, rabbit, and human. The kinetic parameters (Michaelis-Menten constant and maximal uptake) of the uptake system are the same order of magnitude regardless of the polyamine or species studied and the in vitro system used. Also, other pulmonary cells accumulate polyamines but never to the same extent as the epithelium. Although different uptake systems exist for putrescine, spermidine, and spermine in the lung, neither the nature of the carrier protein nor the reason for its existence is known. Some pulmonary toxicological and/or pathological conditions have been related to polyamine metabolism and/or polyamine content in the lung. Polyamines possess an important intrinsic toxicity. From in vitro studies with nonpulmonary cells, it has been shown that spermidine and spermine can be metabolized to hydrogen peroxide, ammonium, and acrolein, which can all cause cellular toxicity. In hyperoxia or after ozone exposure, the increased polyamine synthesis and polyamine content of the rat lung is correlated with survival of the animals. Pulmonary hypertension induced by monocrotaline or hypoxia has also been linked to the increased polyamine metabolism and polyamine content of the lung. In a small number of studies, it has been shown that polyamines can contribute to the suppression of immunologic reactions in the lung.

Putrescine uptake by alveolar epithelial cell monolayers exhibiting differing transepithelial electrical resistances

Rat alveolar type II cells were isolated following elastase digestion and cultured on polycarbonate filters at various densities and in different media. Two days after seeding, the cells formed a monolayer on the filters which consisted predominantly of type II cells, these then de-differentiated to a alveolar type I-like cell monolayer by day 6. The seeding density and media utilized affected the transepithelial electrical resistance (TEER) generated by the monolayer. Only certain culture conditions allowed the production of a monolayer that mimics, putatively, the in vivo alveolar epithelium (TEER greater than 1000 omega cm2). Vmax and K(m) values for the uptake of putrescine by monolayers exhibiting low and high TEERs on day 6 were determined. The capacity of the putrescine uptake mechanisms was greater in cell monolayers exhibiting a high TEER than those exhibiting a low TEER, suggesting that the TEER does not only measure the "tightness" of the monolayer but contains an element representative of the viability of the cell monolayer. The selection of appropriate TEERs for cell culture investigations is discussed.

Role of gamma-glutamyltransferase in putrescine uptake by rat type II pneumocytes

Putrescine uptake in type II pneumocytes is a carrier-mediated active process. Our hypothesis was that oligoamines might be taken up into the cell at least in part by gamma-glutamyltransferase (gamma-GT). This was investigated in rat type II pneumocytes 24 hr after their isolation. Preexposure to 125 microM L-buthionine-[SR]-sulfoximine (BSO) or 100 microM diethylmaleate (DEM), both of which affect intracellular glutathione (GSH) only, were found to decrease GSH by 85% (p < 0.05) and 62%, respectively (p < 0.05), without change in [3H]-putrescine uptake. Preexposure to 20 microM N-ethylmaleimide (NEM), which affects intra- and extracellular GSH, decreased intracellular GSH by 79% (p = 0.015) and putrescine uptake by 39% (p = 0.03). Selective extracellular GSH depletion by 10 microM copper-o-phenanthroline complex (CuP) led to a decrease in putrescine uptake of 41% (p = 0.001), while intracellular GSH remained unchanged. Specific inhibition of gamma-GT by 5-20 mM serine-borate or 5 mM acivicin gave similar degrees of putrescine uptake inhibition (39.5% and 40.5%). The kinetic properties of the putrescine uptake system in the presence of acivicin and serine-borate indicated that the Vmax decreased by 25%, while Km remained unchanged. In experiments with pure gamma-GT, the oligoamines putrescine, spermidine and spermine, and cystamine proved to be acceptor substrates for gamma-GT, all having similar efficiencies (Vmax/Km); methylglyoxal-bis-(guanyl-hydrazone) and paraquat were not accepted. As extracellular GSH is required for gamma-GT, and because its extracellular depletion inhibits putrescine uptake as much as specific inhibition of gamma-GT, we suggest that 30-40% of the putrescine uptake in type II pneumocytes occurs by gamma-GT and that, therefore, at least two systems are involved in the uptake of putrescine.

Putrescine and paraquat uptake in human lung slices and isolated type II pneumocytes

Paraquat is accumulated into the lungs of various species by an active uptake system which also appears to mediate the uptake of endogenous polyamines, such as putrescine. The accumulation of putrescine in the human lung has been previously shown to be mainly located in the type II cells. In the present study, we have studied the mutually competitive inhibition of putrescine and paraquat in human lung slices and the inhibition of putrescine by paraquat or cystamine in isolated human type II pneumocytes. Peripheral lung tissue taken from patients undergoing pneumectomy or lobectomy was used. The initial steps of the cell isolation procedure differed from the literature in that the tissue was first sliced in 0.7 mm thick slices, which were washed in phosphate buffered saline without calcium and magnesium (PBS-), followed by incubations with trypsin. The type II cells were purified and isolated by differential adherence on plastic followed by Percoll gradient centrifugation. Uptake was determined 48 hr after cell isolation. The accumulation of radiolabelled putrescine showed saturation kinetics, with the following apparent kinetic parameters: Km 6.7 and 6.2-7.6 microM and Vmax 2.7 and 3.0-3.4 mumol/g prot/hr for slices and isolated cells, respectively. In the presence of paraquat, putrescine uptake was reduced, in both systems, in a manner compatible with competitive inhibition, with calculated inhibition constants (Ki) of 549-614 and 659-895 microM paraquat for slices and isolated cells, respectively. The accumulation of putrescine in isolated human pneumocytes was strongly reduced in the presence of cystamine, with calculated Ki of 3.7 microM cystamine. These data indicate that putrescine, paraquat and cystamine accumulate in the human lung by the same uptake system, but that the affinities for the three substrates differ. The presence of an uptake system for putrescine in cultured human pulmonary type II is probably useful as a functional viability test.

Inhibition of calcium signalling in murine splenocytes by polyamines: differential effects on CD4 and CD8 T-cells

Transmembrane Ca2+ influx is recognized as a universal second messenger that transduces T-cell activation signals to cytoplasm and nucleus, thereby stimulating transcription and cell division. To examine the role of endogenous factors that regulate mitogenic Ca2+ signalling of T-cells, we measured the concanavalin (Con) A-induced increase in cytoplasmic free calcium ([Ca2+]i) in spleen cells of BALB/c mice, using flow cytometry with an indicator dye, Indo-1 acetoxymethyl ester (Indo-1/AM). Con A is a polyclonal activator of T-cells. Unstimulated splenocytes had a [Ca2+]i of 100 nM. [Ca2+]i increased with Con A in a dose-dependent manner up to a concentration of 50 micrograms/ml. In the presence of 50 micrograms/ml Con A, [Ca2+]i was 350 nM. Natural polyamines (putrescine, spermidine and spermine) inhibited Con-A-induced Ca2+ influx in a dose-dependent manner. Putrescine was the most effective polyamine in desensitizing the Ca2+ signal, and decreased [Ca2+]i from 350 nM in the absence of putrescine to 250 nM in the presence of 100 microM putrescine. This effect was not mimicked by structurally related homologues or inorganic cations, suggesting a specific structural effect of the polyamine. H.p.l.c. analysis showed that polyamines were internalized during incubation of cells in vitro. In experiments using monoclonal anti-CD4 and anti-CD8 antibodies, we found a differential effect of putrescine on Ca2+ influx in CD4 and CD8 subpopulations of T cells. For CD4+ cells, [Ca2+]i decreased from 625 nM to 420 nM in the presence of 500 microM putrescine, whereas [Ca2+]i was not affected by putrescine in CD8+ cells. These data suggest that natural polyamines have cell-specific effects on mitogen-stimulated Ca(2+)-influx in T-cell subsets.

Competition between paraquat and putrescine for uptake by suspensions of rat alveolar type II cells

Paraquat and the structurally similar polyamines, such as putrescine and spermidine, are accumulated actively and selectively by the alveolar type II cells via the polyamine uptake system. We report the uptake kinetics of paraquat and putrescine and their mutual inhibition in freshly isolated rat type II cell suspensions. The uptake of paraquat by type II cells exhibited saturation kinetics and could be inhibited in a concentration-dependent manner by putrescine. By applying enzyme kinetic analysis to our experimental data it was demonstrated that the uptake of paraquat or putrescine is inhibited in a partially competitive manner by the respective inhibitor. Thus, we postulate that the polyamine uptake pathway in type II cells for paraquat and putrescine has two separate sites, one for each substrate, and that binding of one leads to a conformational change in the other.