Phosphatidylcholine Synthesis In Type Research Articles

Adaptation of rat type II pneumocytes to NO2: effects of NO2 application mode on phosphatidylcholine metabolism.

Previous studies have shown that nitrogen dioxide (NO2) inhalation affects the extracellular surfactant as well as the structure and function of type II pneumocytes. Since in these studies there were great variabilities in oxidant concentration, duration of exposure, and mode of NO2 application, we evaluated the influence of the NO2 application mode on the phospholipid metabolism of type II pneumocytes. Rats were exposed to identical NO2 body doses (720 ppm x h), which were applied continuously (10 ppm for 3 d), intermittently (10 ppm for 8 h per day, for 9 d), and repeatedly (10 ppm for 3 d, 28 d rest, and then 10 ppm for 3 d). Immediately after exposure, type II cells were isolated and evaluated for cell yield, vitality, phosphatidylcholine (PC) synthesis, and secretion. Type II pneumocyte cell yield from animals that had been continuously exposed to NO2 was significantly increased, whereas intermittently and repeatedly treated rats exhibited cell yields that were nonsignificantly enhanced. Vitality of the isolated type II pneumocytes was not affected by the NO2 exposure modes. Continuous application of 720 ppm x h NO2 resulted in increased activity of the cytidine-5-diphosphate (CDP)-choline pathway. After continuous NO2 application, specific activity of choline kinase, cytidine triphosphate (CTP):cholinephosphate cytidylyltransferase, uptake of choline, and pool sizes of CDP-choline and PC were significantly increased over those of controls. Intermittent application of this NO2 body dose also provoked an increase in PC synthesis, but this increase was less prominent than after continuous exposure. After repeated exposure, the synthesis parameters were comparable to those for cells from control animals. Whereas PC synthesis in type II cells was obviously stimulated by NO2, the secretory activity of the cells was reduced. Continuous exposure reduced this activity most, whereas intermittent exposure nonsignificantly reduced this activity as compared with that of controls. The repeated application of NO2 produced no differences. We conclude that type II pneumocytes adapt to NO2 atmospheres depending on the mode of its application, at least for the metabolism of PC and its secretion from isolated type II pneumocytes. Further studies are necessary to determine whether additional metabolic activities will also adapt to NO2 atmospheres, and if these observations are specific for NO2 or represent effects generally due to oxidants.

Pre-translational regulation by glucocorticoid of fatty acid and phosphatidylcholine synthesis in type II cells from fetal rat lung

Exposure to fibroblast-conditioned cortisol-containing medium increased fatty acid synthase activity and fatty acid synthase, acetyl-CoA carboxylase and ATP citrate lyase mRNA abundance in fetal type II alveolar epithelial cells. Both fibroblast conditioning and cortisol in the medium were required for maximal effect on the mRNA levels, indicating involvement of mesenchymal—epithelial interaction in the cortisol effects. The observed effects provide evidence for an earlier hypothesis that increased activity of CTP:phosphocholine cytidylyltransferase in lung tissue caused by glucocorticoid is due to increased fatty acid synthesis. However, evidence suggesting pre-translational regulation of the enzyme by glucocorticoid was also found.

Enhancement of Disaturated Phosphatidylcholine Synthesis by Epidermal Growth Factor in Cultured Fetal Lung Cells Involves a Fibroblast-Epithelial Cell Interaction

Epidermal growth factor (EGF) increases the rate of choline incorporation into disaturated phosphatidylcholine in cultured fetal rat type II cells via an indirect mechanism. Whereas-EGF has no effect on the rate of disaturated phosphatidylcholine synthesis when added directly to type II pneumocytes, the growth factor is effective if it is present during preliminary conditioning of the media by lung fibroblasts. This effect is concentration dependent with a maximal effect at 20 ng/ml. When lung fibroblasts are incubated with both glucocorticoids and EGF, there is no significant effect of the growth factor over and above that seen with the steroid alone. This suggests that the two agents might act via a similar mechanism. This is supported by the observation that each inducer leads to the production by lung fibroblasts of a stimulatory factor that has a similar, if not identical, chromatographic elution profile. We conclude that EGF may contribute significantly to the normal onset of lung maturation by elaborating a fibroblast-derived factor that stimulates phosphatidylcholine synthesis in type II pneumocytes.

Temporal linkage of glycogen and saturated phosphatidylcholine in fetal lung type II cells.

The developmental profiles of glycogen and surfactant-associated saturated phosphatidylcholine were investigated in type II cells isolated from fetal rat lung. Incorporation of radiolabeled glucose into glycogen and type II cell unlabeled glycogen content decreased as a function of gestational age. Conversely, an increase was noted in radioactive choline incorporation into saturated phosphatidylcholine and in the content of unlabeled saturated phosphatidylcholine as a function of gestational age. Type II cells from days 19 and 21 of gestation were also studied by electron microscopy. Temporal relationships similar to those noted biochemically were observed by morphometric analysis. A decrease in glycogen content and an increase in lamellar bodies (the storage organelles for the pulmonary surfactant) were noted as gestation progressed. These studies biochemically and morphologically demonstrate a temporal relationship between glycogen degradation and saturated phosphatidylcholine synthesis in type II cells isolated from fetal rat lung. These findings provide further support for the use of such type II cell preparations for studies of development at the cellular level.

The cellular mechanism of glucocorticoid acceleration of fetal lung maturation. Fibroblast-pneumonocyte factor stimulates choline-phosphate cytidylyltransferase activity.

The cellular mechanism by which glucocorticoids stimulate phosphatidylcholine biosynthesis has been studied in the fetal rat lung in vivo and in cultured fetal rat lung cells of varying levels of complexity. Administration of dexamethasone to pregnant rats at 18 days gestation resulted in a significant increase in saturated phosphatidylcholine content in fetal lung 24 h after injection. Dexamethasone administration increased the activity of fetal lung choline-phosphate cytidylyltransferase by 34%. It had no effect on the activities of fetal lung choline kinase and choline phosphotransferase. Exposure of fetal lung type II cells in organotypic cultures (which contain both type II cells and fibroblasts) to cortisol resulted in a 1.6-fold increase in the incorporation of [Me-3H]choline into saturated phosphatidylcholine. The activities of the enzymes in the choline pathway for the de novo biosynthesis of phosphatidylcholine were not significantly altered except for a 105% increase in choline-phosphate cytidylyltransferase activity. Treatment of monolayer cultures of fetal type II cells with cortisol-conditioned medium from fetal lung fibroblasts resulted in a 1.5-fold increase in saturated phosphatidylcholine production. This effect correlated with a doubling of choline-phosphate cytidylyltransferase activity. Additional evidence that this stimulatory action is mediated by fibroblast-pneumonocyte factor, produced by fetal lung fibroblasts in response to cortisol, was obtained. The factor was partially purified from cortisol-conditioned medium of fetal lung fibroblasts by gel filtration and affinity chromatography. Based on biological activity, a 3000-fold purification was obtained. Stimulation of saturated phosphatidylcholine synthesis in type II cells by fibroblast-pneumonocyte factor was maximal within 60 min of incubation. Pulse-chase experiments indicated that the stimulatory effect was correlated with an increased conversion of choline phosphate into CDP choline. Moreover, the enhanced phosphatidylcholine formation by fetal type II cells in response to fibroblast-pneumonocyte factor was accompanied by decreased levels of cellular choline phosphate. These findings further support the concept that glucocorticoid action on surfactant-associated phosphatidylcholine synthesis occurs ultimately at the level of the alveolar type II cell and involves fibroblast-pneumonocyte factor which stimulates the activity of choline-phosphate cytidylyltransferase.

Phosphatidylglycerol stimulates cholinephosphate cytidylyltransferase activity and phosphatidylcholine synthesis in type II pneumocytes

Phosphatidylcholine synthesis in type II pneumocytes is stimulated by inclusion of phosphatidylglycerol and other phospholipids in the culture medium (Gilfillan, A.M., Chu, A.J. and Rooney, S.A. (1984) Biochim. Biophys. Acta 794, 269-273). We have now examined the effect of phosphatidylglycerol in the medium on enzymes of de novo phosphatidylcholine synthesis in adult rat type II cells. Activities of choline kinase, cholinephosphate cytidylyltransferase and cholinephosphotransferase in homogenates of whole lung and type II cells were generally similar. Phosphatidate phosphatase activity in type II cells, however, was only 16% that in whole lung. Addition of phosphatidylglycerol (10 microM) to the culture medium had no effect on choline kinase, cholinephosphotransferase or phosphatidate phosphatase activities in type II cells but it increased the activity of cholinephosphate cytidylyltransferase by 56%. Since it is known that cholinephosphate cytidylyltransferase is stimulated in vitro by addition of phospholipids to the assay mixture, we also measured its activity in the presence of sufficient phosphatidylglycerol (1.1 mM) to maximally stimulate in vitro. Even under these conditions cholinephosphate cytidylyltransferase activity in type II cells cultured in the presence of phosphatidylglycerol was 32% greater than in control cells. These data show that the stimulatory effect of phospholipid in the culture medium on phosphatidylcholine synthesis in type II cells is mediated by increased cholinephosphate cytidylyltransferase activity. The mechanism of increased cytidylyltransferase activity remains to be elucidated but it is not due to direct in vitro activation by the phospholipid.

Correlation between phosphatidylcholine labeling and hormone receptor levels in alveolar type II epithelial cells: Effects of dexamethasone and epidermal growth factor

Phosphatidylcholine labeling was studied in freshly isolated adult rat alveolar type II epithelial cells exposed to dexamethasone and epidermal growth factor. Dexamethasone at a medium concentration of 10 −8 m, enhanced phosphatidylcholine labeling in type II cells by about 25%. In lung fibroblast controls, dexamethasone had no effect. Phosphatidylcholine secretion into the culture medium was not observed in either cell type. Quantitation of dexamethasone receptors revealed a twofold greater number of receptors in type II cells than in control fibroblasts. In contrast, the addition of epidermal growth factor to the medium of type II cells or lung fibroblasts had no effect on phosphatidylcholine labeling or secretion into culture medium. Lung fibroblasts were found to have 11-fold more surface receptors for epidermal growth factor than isolated type II cells. These results indicate that dexamethasone significantly increases phosphatidylcholine synthesis in type II cells and thus, may also effect the production of surfactant by these cells.