Inhibition of surfactant release from isolated type II cells by compound 4880

Interactions of surfactant protein A with epithelial cells and phagocytes

Surfactant protein A (SP-A) has been shown to bind to and regulate the functions of both alveolar type II cells and immune cells including alveolar macrophages. The interaction of SP-A with type II cells has been shown in vitro to inhibit lipid secretion and to promote the uptake of lipid by these cells and these observations led to the hypothesis that SP-A plays an important role in regulating surfactant turnover and metabolism. The finding that mice made deficient in SP-A by homologous recombination (SP-A -/- mice) have relatively normal surfactant pool sizes has raised the possibility that either redundant mechanisms function in vivo to keep pool sizes normal in the absence of SP-A or that the in vitro findings are not significant in the context of the whole, unstressed animal. The interaction of SP-A with immune cells has been shown to affect a variety of responses which, in general, function to promote host defense against infection. Although SP-A receptors have been identified, additional studies will be required to elucidate the mechanism of interaction of SP-A with these cells and the relative importance of the different receptors in SP-A mediated regulation of cell function.

Evidence for G beta gamma-mediated cross-talk in primary cultures of lung alveolar cells. Pertussis toxin-sensitive production of cAMP

In the presence of activated Gs alpha, the beta gamma complex of heterotrimeric G proteins (beta gamma) stimulates adenylyl cyclase (AC) in membranes prepared from cells expressing recombinant AC II or AC IV. Conditional stimulation of AC by beta gamma has been hypothesized to integrate cross-talk between Gs- and non-Gs-coupled regulation of cellular cAMP (Tang, W. J., and Gilman, A. G. (1991) Science 254, 1500-1503). Although observations in cells expressing recombinant receptors, G alpha s, and AC support this hypothesis (Federman, A. D., Conklin, B. R., Schrader, K. A., Reed, R. R., and Bourne, H. R. (1992) Nature 356, 159-161), this mechanism has not been investigated in differentiated cells. Expression of AC II has been reported only in lung, olfactory, and brain tissue. We found that rat lung alveolar type II cells express AC II and IV. Therefore, we hypothesized that beta gamma conditionally stimulates AC in type II cells. Consistent with this hypothesis, we found that the alpha 2-adrenergic agonist UK14304 did not affect basal cAMP in type II cells but potentiated terbutaline-stimulated cAMP accumulation. Treatment of cells with pertussis toxin partially inhibited terbutaline-stimulated cAMP accumulation and completely inhibited the effects of UK14304. In type II cell membranes, purified beta gamma tripled the terbutaline-stimulated increase in AC activity. In contrast, beta gamma inhibited AC activity in the absence of terbutaline. The addition of purified Go alpha blocked beta gamma-induced effects. In summary, type II cells expressing endogenous AC II and IV regulate cAMP accumulation and AC activity in a manner consistent with conditional stimulation by beta gamma. These observations support the overall hypothesis that conditional stimulation of AC by beta gamma integrates cross-talk between signal transduction systems in differentiated cells.

Phospholipid content of bronchoalveolar lavage fluid in granite workers with silicosis in Quebec

BACKGROUND

Some of the prominent features of silicosis are hyperplasia and hypertrophy of epithelial type II cells, which in experimental animals are often accompanied by accumulation of phospholipids in the lung.

METHODS

The total phospholipid content of lung lavage fluid and its composition in 28 granite stone cutters with long term exposure to silica dust (23 with radiological silicosis) was compared with that of lavage fluid in 15 normal volunteers, 15 patients with untreated idiopathic pulmonary fibrosis, and 19 patients with untreated stage 2 or 3 sarcoidosis. All lavage fluid was obtained at the time of first pulmonary investigation, which also included lung function tests.

RESULTS

In the normal subjects total phospholipid content was 1.13 (0.16) micrograms phosphorus/ml of lung lavage, in the patients with idiopathic pulmonary fibrosis 0.52 (0.07) microgram/ml (p < 0.05), and in the patients with sarcoidosis 1.02 (0.20) microgram/ml composition being in the range reported in humans. In the patients with silicosis total phospholipid content was significantly decreased to an average of 0.46 (0.08) microgram/ml compared with the findings in normal subjects and patients with sarcoidosis. Within the group exposed to silica changes in total phospholipid content did not correlate with the severity of the radiographic disease, changes in lung function, the cellularity of lung lavage fluid, or hyaluronate concentrations. The secretory capacity of rat epithelial type II cells was not significantly different when cultured with bronchoalveolar lavage fluid from all four groups of subjects.

CONCLUSIONS

Total phospholipid content in lung lavage fluid was significantly reduced in granite workers with radiological evidence of lung disease, but showed no correlation with radiological or functional markers of disease severity.

Inhibition of phosphatidylcholine secretion by stilbene disulfonates in alveolar type II cells

Various agents stimulate the secretion of lung surfactant from alveolar type II cells by increasing intracellular Ca2+, cyclic adenosine-3':5'-monophosphate (cAMP), or diacylglycerol. A few agents, including the purified surfactant protein A, are known to inhibit the secretion by an unknown mechanism. In the present study, we demonstrated that stilbene disulfonic acids, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), are potent but reversible inhibitors of lung surfactant secretion. The inhibition was concentration dependent, and the EC50 was 5 microM for DIDS and 50 microM for SITS. The inhibition was not specific to agonists for any one type of receptor, and was also observed for secretion stimulated by 8-bromo-cAMP, or tetradecanoyl phorbol acetate, suggesting that the site of inhibition was distal to the generation of intracellular second messengers. This was also supported by the failure of DIDS to block the stimulus-mediated increase in diacylglycerol content of type II cells. Further, DIDS and SITS were also inhibitory for basal secretion. Based on the reversibility of inhibition and the fact that inhibition was observed with both basal and stimulated secretion, we suggest that stilbene disulfonic acids affect a component of the exocytosis process that occurs at or near the plasma membrane.

Linear arrays of homogeneous mast cells in the dura mater of the rat

Using fluorescence histochemistry, 5-HT, histamine and heparin were colocalized in a large population of cells in the dura mater thereby identifying them as mast cells. In addition, because these cells were highly sensitive to compound 48/80 and were densely packed with granules of a consistent density, they were identified specifically as 'connective tissue' mast cells. Other types of mast cells, i.e. 'mucosal' or 'neurolipomastocytes', were not present in the rat dura mater. 5-HT immunohistochemistry was the best technique for demonstrating that there were populations of mast cells, one associated with each of the two layers of dura. Although shaped differently the type of mast cell in each layer was the same. It was observed that mast cell shape is dependent on the contiguity, density and orientation of its surrounding elements, not its type. In general, mast cells in the outer layer were aligned parallel to the middle meningeal artery and those in the inner layer were parallel to trigeminal nerve branches that coursed obliquely across the middle meningeal artery. Examination of cross-sections of dura revealed that most mast cells also were aligned at the interface between the two dural layers. The linear orientation of mast cells in two planes of each layer suggests a programmed lamellar seeding of these cells during development of the dura. This study also demonstrated that the majority of dural mast cells were more closely related to other connective tissue elements than to blood vessels and nerves. These results (1) are compatible with the suggestion that dural mast cells play a non-obligatory role in the neuroinflammatory response, (2) leave open to question the role of the dural mast cell in headache or the regulation of blood flow, and (3) support evidence that dural mast cells play an important role in connective tissue related functions, e.g. development, inflammatory response to injury and wound repair.

Stimulation of pulmonary surfactant secretion by activating neutrophils in rat type II pneumocytes culture

The influence of activating neutrophils on the secretion of phosphatidylcholine (PC), the predominant component of pulmonary surfactant, was examined using primary culture of rat type II pneumocytes. Simultaneous addition of neutrophils and opsonized zymosan, but not neutrophils or opsonized zymosan alone, to type II pneumocytes caused a significant increase in PC secretion without affecting the release of lactate dehydrogenase, a marker of cytotoxicity. The increase in PC secretion was dependent on the number of activating neutrophils. In addition, pretreatment of culture with the combination of superoxide dismutase and catalase inhibited the increase in PC secretion. These findings indicate that activating neutrophils stimulate the secretion of pulmonary surfactant and that the stimulation is mediated by oxygen radicals.

Positive correlation between cytosolic free calcium and surfactant secretion in cultured rat alveolar type II cells

To determine whether increases in the cytosolic free Ca2+ concentration ([Ca2+]i) accompany agonist-stimulated surfactant secretion by cultured alveolar type II cells, we measured the [Ca2+]i of quin2-loaded cells isolated from adult rats before and after cells were stimulated with ionomycin, terbutaline or tetradecanoylphorbol acetate (TPA). To determine whether increases in [Ca2+]i are necessary for stimulated surfactant secretion to occur, we measured secretion in cells after [Ca2+]i had been reduced by loading cells with quin2 in medium containing low [Ca2+]. Ionomycin increased [Ca2+]i and stimulated surfactant secretion in a dose-dependent manner. Reductions in [Ca2+]i correlated with reductions in secretion stimulated by ionomycin, terbutaline or TPA. Ionomycin-stimulated secretion was most sensitive to reductions in [Ca2+]i; terbutaline-stimulated secretion was more sensitive than TPA-stimulated secretion. When [Ca2+]i was less than 65 nM, all stimulated secretion was blocked. Restoration of [Ca2+]i to greater than 100 nM restored ionomycin-stimulated secretion. We conclude that ionomycin increases [Ca2+]i and stimulates surfactant secretion in cultured alveolar type II cells, and that increased [Ca2+]i appears to be necessary for ionomycin-stimulated secretion to occur. Terbutaline-stimulated surfactant secretion seems to be more easily inhibited by a reduction in [Ca2+]i than does TPA-stimulated secretion.

Regulation of surfactant phospholipid secretion from isolated rat alveolar type II cells by lectins

The major surfactant-associated protein is a potent inhibitor of surfactant phospholipid secretion from isolated type II cells. Since the major surfactant-associated protein contains a carboxy terminal polypeptide domain which is homologous to the lectin-like liver mannose-binding protein, we tested whether lectins inhibit surfactant phospholipid secretion from rat alveolar type II cells. Concanavalin A, wheat germ agglutinin and Maclura pomifera agglutinin were potent inhibitors of surfactant phospholipid secretion. When adenosine 5'-triphosphate (ATP) was utilized as a secretagogue, the IC50 values for inhibition of surfactant phospholipid secretion were 5.10(-7) (wheat germ agglutinin), 1.10(-6) (concanavalin A) and 2.5.10(-5) M (M. pomifera agglutinin). Similar results were obtained when 12-O-tetradecanoylphorbol 13-acetate was utilized as a secretagogue: IC50 values of 1.10(-6) M for concanavalin A and wheat germ agglutinin and 2.5.10(-5) M for M. pomifera agglutinin. Hapten sugars were utilized to antagonize the inhibitory effect of the lectins. N-Acetyl-D-glucosamine significantly reversed inhibition of phospholipid secretion by wheat germ agglutinin in a dose-dependent fashion and methyl alpha-D-mannoside significantly reversed inhibition of phospholipid secretion by concanavalin A. N-Acetyl-D-galactosamine had no significant effect on inhibition of secretion produced by any of the lectins. The inhibitory effect of the lectins did not appear to be due to cytotoxicity since lactate dehydrogenase was not released above control levels and the inhibition of the surfactant phospholipid secretion by wheat germ agglutinin could be reversed after treatment of cells with wheat germ agglutinin by washing the lectin from the cells followed by treatment of the cells with ATP. These studies demonstrate a direct inhibitory effect of plant lectins on phospholipid secretion from type II cells in vitro.

Effect of secretagogues on cytoplasmic free calcium in alveolar type II epithelial cells

Pulmonary surfactant is synthesized and secreted by alveolar type II epithelial cells. Although intracellular calcium and other second messengers have been implicated in secretion by type II cells, this is the first report on measurement of cytoplasmic free calcium concentration ([Ca2+]i). Known secretagogues, 12-O-tetradecanoylphorbol-13-acetate (TPA) and terbutaline, were tested to see if they caused rapid increases in cytoplasmic calcium. Ionomycin, a calcium ionophore, was used to increase cytoplasmic free calcium concentration, to determine if a rapid increase in cytoplasmic free calcium would stimulate secretion, and to measure interactions with other secretagogues. Ionomycin increased both [Ca2+]i and pulmonary surfactant secretion from alveolar type II cells. A low concentration of ionomycin (100 nM) greatly enhanced secretion stimulated by terbutaline or by 8-bromo-cAMP but only had an additive effect on secretion stimulated by TPA. Terbutaline transiently increased [Ca2+]i by 24% over control basal condition, and the increase in [Ca2+]i produced by terbutaline occurred in the absence of extracellular calcium. TPA itself did not change [Ca2+]i. However, TPA completely inhibited the terbutaline-induced increase of [Ca2+]i but not the increase due to ionomycin. When alveolar type II cells were loaded with 2-(2-bis-[carboxymethyl]-amino-5-methyl-phenoxy)-methyl-6-methoxy-8-bis carboxymethylaminoquinoline (quin2) in calcium-free buffer, [Ca2+]i decreased from 143 +/- 10 to 31 +/- 8 nM. Lowering [Ca2+]i inhibited TPA- or terbutaline-induced secretion by 22 and 40%, respectively. Although the precise role of cytoplasmic free calcium on surfactant secretion cannot be established on the basis of current data, our results indicate that an increase in cytoplasmic free calcium produced by ionomycin stimulates secretion and that an increase in [Ca2+]i affects cAMP-induced secretion more than protein kinase C-mediated secretion in alveolar type II cells.

Regulation of surfactant secretion from isolated Type II pneumocytes by substance P

Substance P, an eleven amino acid neuropeptide, significantly inhibited release of [3H]phosphatidylcholine from pulmonary Type II epithelial cells in vitro. Basal release and release in response to the beta-adrenergic agonist, terbutaline and 12-O-tetradecanoylphorbol 13-acetate (TPA) were significantly decreased in the presence of substance P. Inhibitory effects of substance P were noted following a 1 h exposure of primary cultures of Type II cells in vitro and persisted up to 3 h in the presence of the secretagogues, TPA and terbutaline. The IC50 values for substance P inhibition of [3H]PC release were 10 microM for basal release, 40 microM for TPA-induced release and 50 microM for terbutaline-induced release. The related neuropeptide, physalaemin and the stable active analog of substance P, [pGlu5, MePhe8, MeGly9]substance P [5-11], had no significant inhibitory effects on surfactant release whether in the presence or absence of TPA or terbutaline. These data support the hypothesis that NH2-terminal basic groups of substance P are necessary for inhibition of surfactant secretion from isolated Type II cells and support the concept that an inhibitory system contributes to mediation of surfactant secretion from Type II epithelial cells.

The role of calcium in the secretion of surfactant by rat alveolar type II cells

Beta adrenergic agonists, tetradecanoylphorbol acetate, and the ionophore A23187 all stimulate surfactant secretion in type II cells isolated from rats. We found that combinations of these agonists cause augmented secretion, suggesting that the agonists may effect different steps in the secretory process. Previous studies have shown that cAMP is likely to be an intracellular 'second messenger' in type II cells. A23187, which has been reported to increase cAMP in some cell systems, did not increase the cAMP content of type II cells. We investigated the possible role of Ca2+ as another 'second messenger' by studying cellular 45Ca fluxes and the effect of extracellular calcium depletion on secretion. Depletion of extracellular calcium for as long as 3 h did not alter stimulated secretion, although basal secretion was increased. Secretagogues did not stimulate 45Ca influx from extracellular sources. A23187 and, to a lesser extent, terbutaline caused an acceleration of 45Ca efflux from type II cells. The addition of terbutaline or tetradecanoylphorbol acetate to A23187 further accelerated 45Ca efflux, suggesting that these agonists may act on separate calcium pools or by different mechanisms on the same calcium pool. Although secretion from type II cells is not inhibited by extracellular calcium depletion, the studies on 45Ca efflux suggest that Ca2+ plays a role in the regulation of surfactant secretion from isolated type II cells.