Pool sizes of precursors for phosphatidylcholine formation in adult rat lung type II cells

Analysis of the regulation of surfactant phosphatidylcholine metabolism using stable isotopes

The pathways and mechanisms that regulate pulmonary surfactant synthesis, processing, secretion and catabolism have been extensively characterised using classical biochemical and analytical approaches. These have constructed a model, largely in experimental animals, for surfactant phospholipid metabolism in the alveolar epithelial cell whereby phospholipid synthesised on the endoplasmic reticulum is selectively transported to lamellar body storage vesicles, where it is subsequently processed before secretion into the alveolus. Surfactant phospholipid is a complex mixture of individual molecular species defined by the combination of esterified fatty acid groups and a comprehensive description of surfactant phospholipid metabolism requires consideration of the interactions between such molecular species. However, until recently, lipid analytical techniques have not kept pace with the considerable advances in understanding of the enzymology and molecular biology of surfactant metabolism. Refinements in electrospray ionisation mass spectrometry (ESI-MS) can now provide very sensitive platforms for the rapid characterisation of surfactant phospholipid composition in molecular detail. The combination of ESI-MS and administration of phospholipid substrates labelled with stable isotopes extends this analytical approach to the quantification of synthesis and turnover of individual molecular species of surfactant phospholipid. As this methodology does not involve radioactivity, it is ideally suited to application in clinical studies. This review will provide an overview of the metabolic processes that regulate the molecular specificity of surfactant phosphatidylcholine together with examples of how the application of stable isotope technologies in vivo has, for the first time, begun to explore regulation of the molecular specificity of surfactant synthesis in human subjects.

Regulation of lung surfactant phospholipid synthesis and metabolism

The alveolar type II epithelial (ATII) cell is highly specialised for the synthesis and storage, in intracellular lamellar bodies, of phospholipid destined for secretion as pulmonary surfactant into the alveolus. Regulation of the enzymology of surfactant phospholipid synthesis and metabolism has been extensively characterised at both molecular and functional levels, but understanding of surfactant phospholipid metabolism in vivo in either healthy or, especially, diseased lungs is still relatively poorly understood. This review will integrate recent advances in the enzymology of surfactant phospholipid metabolism with metabolic studies in vivo in both experimental animals and human subjects. It will highlight developments in the application of stable isotope-labelled precursor substrates and mass spectrometry to probe lung phospholipid metabolism in terms of individual molecular lipid species and identify areas where a more comprehensive metabolic model would have considerable potential for direct application to disease states.

Calmodulin binds and stabilizes the regulatory enzyme, CTP: phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCTalpha) is a proteolytically sensitive enzyme essential for production of phosphatidylcholine, the major phospholipid of animal cell membranes. The molecular signals that govern CCTalpha protein stability are unknown. An NH(2)-terminal PEST sequence within CCTalpha did not serve as a degradation signal for the proteinase, calpain. Calmodulin (CaM) stabilized CCTalpha from calpain proteolysis. Adenoviral gene transfer of CaM in cells protected CCTalpha, whereas CaM small interfering RNA accentuated CCTalpha degradation by calpains. CaM bound CCTalpha as revealed by fluorescence resonance energy transfer and two-hybrid analysis. Mapping and site-directed mutagenesis of CCTalpha uncovered a motif (LQERVDKVK) harboring a vital recognition site, Gln(243), whereby CaM directly binds to the enzyme. Mutagenesis of CCTalpha Gln(243) not only resulted in loss of CaM binding but also led to complete calpain resistance in vitro and in vivo. Thus, calpains and CaM both access CCTalpha using a structurally similar molecular signature that profoundly affects CCTalpha levels. These data suggest that CaM, by antagonizing calpain, serves as a novel binding partner for CCTalpha that stabilizes the enzyme under proinflammatory stress.

Effect of CTP:phosphocholine cytidylyltransferase overexpression on the mouse lung surfactant system

CTP:phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the biosynthesis by type II pneumocytes of phosphatidylcholine (PC), the predominant phospholipid in lung surfactant. Augmentation of endogenous CT activity might therefore result in enhanced surfactant PC production. To test this hypothesis, transgenic mice were created in which rat CT (rCT) was expressed under control of the human surfactant protein C (SP-C) promoter. Transgenic mice were identified by tail-clip PCR analysis and studies of four founder lines were initiated. Lung CT gene expression was enhanced in two transgenic founder lines relative to wild-type controls. These two transgenic lines also exhibited significantly higher levels of immunoreactive CT protein and CT activity in whole-lung homogenates and in cultured type II cell extracts. Disaturated PC (DSPC) content in whole-lung homogenates and the rate of DSPC synthesis in cultured type II cells were significantly increased in one transgenic line. However, neither the incorporation of radiolabeled precursors (choline and palmitate) into DSPC in vivo nor the cellular metabolism of DSPC differed significantly between transgenic and control mice. This transgenic model provides opportunity for further study of factors controlling surfactant phospholipid production in vivo.

Impaired spreading of surfactant phospholipids in the lungs of newborn rats with pulmonary hypoplasia as a model of congenital diaphragmatic hernia induced by nitrofen

In order to clarify the pathological outcome of congenital diaphragmatic hernia (CDH), we devised an animal model of CDH by administration of 2,4-dichlorophenyl-p-nitrophenyl ether (nitrofen) to pregnant rats, and determined the level and distribution of lung surfactant using the monoclonal antibody toward sphingomyelin and disaturated phosphatidylcholine (disat-PC). In control rats, the concentration of disat-PC was found to increase greatly from 16 to 18 days of gestation. Intragastric administration of nitrofen to pregnant rats at day 9 of gestation resulted in CDH in 42.7% of fetuses delivered after 20 days of gestation. In nitrofen-treated fetuses, the concentration of disat-PC in the lungs was lower than those in control fetuses, and surfactant apoprotein SP-A was similarly reduced in nitrofen-treated fetuses. However, the concentration of disat-PC in nitrofen-treated fetuses was higher than that in control fetuses at 18 days of gestation, indicating a synthetic potential of surfactant in nitrofen-treated fetuses comparable to that at the late stage of normal gestation. Immunohistochemical study with the antibody revealed that surfactant phospholipid was mainly in the form of intracellular granules in nitrofen-treated fetuses, probably causing the hypoplastic lungs and then CDH, in contrast to the uniform distribution on the pulmonary alveolar surface in control fetuses.

Effects of intratracheal instillation of TNF-alpha on surfactant metabolism

Tumor necrosis factor-alpha (TNF-alpha) has been shown to play an integral role in the pathogenesis of the acute respiratory distress syndrome. This disorder is characterized by a deficiency of alveolar surfactant, a surface-active material that is composed of key hydrophobic proteins and the major lipid disaturated phosphatidylcholine (DSPC). We investigated how TNF-alpha might alter DSPC content in rat lungs by instilling the cytokine (2.5 microg) intratracheally for 10 min and then assaying parameters of DSPC synthesis and degradation in alveolar type II epithelial cells, which produce surfactant. Cells isolated from rats given TNF-alpha had 26% lower levels of phosphatidylcholine compared with control. TNF-alpha treatment also decreased the ability of these cells to incorporate [(3)H]choline into DSPC by 45% compared with control isolates. There were no significant differences in the levels of choline substrate or choline transport between the groups. However, TNF-alpha produced a 64% decrease in the activity of cytidylyltransferase, the rate-regulatory enzyme required for DSPC synthesis. TNF-alpha administration in vivo also tended to stimulate phospholipase A(2) activity, but it did not alter other parameters for DSPC degradation such as activities for phosphatidylcholine-specific phospholipase C or phospholipase D. These observations indicate that TNF-alpha decreases the levels of surfactant lipid by decreasing the activity of a key enzyme involved in surfactant lipid synthesis. The results do not exclude stimulatory effects of the cytokine on phosphatidylcholine breakdown.

Effect of phosphocholine cytidylyltransferase overexpression on phosphatidylcholine synthesis in alveolar type II cells and related cell lines

Disaturated phosphatidylcholine (DSPC) is the predominate phospholipid component of lung surfactant. In the alveolar type II cell, the cytidine diphosphocholine (CDP-choline) pathway is the major biosynthetic pathway for DSPC. To investigate the hypothesis that phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway, rat alveolar type II cells or lung tumor-derived cell lines (A549 or H441) with type II cell features were transfected with CT complementary DNA (cDNA). Cell fractions were subsequently assayed for CT protein and activity, and cell rates of DSPC synthesis were determined. In all cases, cell CT protein and activity were increased after transfection with CT cDNA but not after control transfection. Rat type II cells, but not A549 or H441 cells, increased the rate of DSPC synthesis after transfection with CT cDNA. Exposure of type II cells transfected with CT cDNA to palmitic acid resulted in a further increase in CT protein and activity. Exposure to dexamethasone resulted in increased CT protein and activity and increased synthesis of DSPC. The results confirm that CT has a rate-limiting and regulatory role in the synthesis of type II cell DSPC, and raise possibilities for novel therapeutic interventions.

Identification of sex-specific differences in surfactant synthesis in rat lung

Delayed lung maturation and lower levels of surfactant phosphatidylcholine have been previously identified in male fetuses compared with female fetuses in several species. We investigated the mechanisms for sex differences in surfactant content by examining parameters of phosphatidylcholine turnover and biosynthesis; the latter was evaluated by measuring metabolic steps within the biosynthetic pathway. Compared with male lung cells, freshly isolated lung cells from female fetuses contained higher levels of disaturated phosphatidylcholine, a marker of surfactant lipid. Female mixed monolayer cultures exhibited a 71% increase in choline incorporation into disaturated phosphatidylcholine compared with male cultures. Male cultures exhibited significantly greater release of [3H]-arachidonic acid into the medium compared with females, suggesting sex differences in phospholipase activity. However, pulse-chase studies showed no sex differences in degradation of disaturated phosphatidylcholine, which was confirmed by assays of phospholipase A2, phosphatidylcholine-specific phospholipase C, and phospholipase D. Female mixed lung cells, however, had greater rates of cellular choline transport and activity of cytidylyltransferase, the rate-regulatory enzyme for phosphatidylcholine synthesis. Separate studies showed that exposure of sex-specific pretype II cell cultures to cortisol-stimulated fibroblast-conditioned medium plus transforming growth factor-beta-neutralizing antibody stimulated cytidylyltransferase activity to a greater extent in male cells compared with female cells. These studies indicate that sex differences in surfactant phospholipid content are not due to differences in phospholipid turnover, but rather differential regulation of specific metabolic steps within the surfactant biosynthetic pathway. The data also support a role for transforming growth factor-beta as a negative regulator of a key surfactant biosynthetic enzyme within male lungs.

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.

Very low density lipoproteins stimulate surfactant lipid synthesis in vitro

Surfactant synthesis is critically dependent on the availability of fatty acids. One fatty acid source may be circulating triglycerides that are transported in VLDL, and hydrolyzed to free fatty acids by lipoprotein lipase (LPL). To evaluate this hypothesis, we incubated immortalized or primary rat alveolar pre-type II epithelial cells with VLDL. The cells were observed to surface bind, internalize, and degrade VLDL, a process that was induced by exogenous LPL. LPL induction of lipoprotein uptake significantly increased the rates of choline incorporation into phosphatidylcholine (PC) and disaturated PC, and these effects were associated with a three-fold increase in the activity of the rate-regulatory enzyme for PC synthesis, cytidylyltransferase. Compared with native LPL, a fusion protein of glutathione S-transferase with the catalytically inactive carboxy-terminal domain of LPL did not activate CT despite inducing VLDL uptake. A variant of the fusion protein of glutathione S-transferase with the catalytically inactive carboxy-terminal domain of LPL that partially blocked LPL-induced catabolism of VLDL via LDL receptors also partially blocked the induction of surfactant synthesis by VLDL. Taken together, these observations suggest that both the lipolytic actions of LPL and LPL-induced VLDL catabolism via lipoprotein receptors might play an integral role in providing the fatty acid substrates used in surfactant phospholipid synthesis.

Betamethasone modulation of sphingomyelin hydrolysis up-regulates CTP:cholinephosphate cytidylyltransferase activity in adult rat lung

Glucocorticoids appear to play an integral role in stimulating surfactant synthesis by activating the rate-regulatory enzyme for phosphatidylcholine synthesis, CTP:cholinephosphate cytidylyltransferase (CT). The activity of liver CT, in vitro, has been shown to be inhibited by the sphingomyelin hydrolysis product, sphingosine. In order to investigate the mechanisms by which glucocorticoids alter CT activity, in vivo, we administered betamethasone (1 mg/kg intraperitoneally) sequentially to adult male rats for 5 days. Betamethasone increased CT activity 2-fold relative to control in whole lung. The hormone also increased membrane-bound activity, but did not affect cytosolic enzyme activity. Betamethasone modestly increased CT mRNA as determined by the reverse-transcription PCR and Southern analysis of PCR products, but did not alter the levels of immunoreactive enzyme in lung membranes as demonstrated by Western blotting. The hormone did, however, produce a nearly 3-fold increase in membrane-associated sphingomyelin, and co-ordinately a substantial decrease in the levels of sphingosine in lung membranes. Sphingosine, but not sphinganine, was a competitive, reversible inhibitor of lung CT with respect to the enzyme activator, phosphatidylglycerol. Betamethasone decreased the activities of the sphingomyelin hydrolases: acid sphingomyelinase by 33% and of alkaline ceramidase by 21%. The hormone also inhibited the generation of sphingosine from lysosphingomyelin in lung membranes. There was no significant effect of the hormone on serine palmitoyltransferase activity, the first committed enzyme for sphingolipid biosynthesis. Further, administration of L-cycloserine, an inhibitor of sphingosine formation, was shown to stimulate CT activity by 74% and increase disaturated phosphatidylcholine in alveolar lavage by 52% relative to control. These observations suggest that glucocorticoids up-regulate surfactant synthesis at the level of a key regulatory enzyme by significantly altering the availability of inhibitory metabolites resulting from sphingomyelin hydrolysis.

Betamethasone activation of CTP:cholinephosphate cytidylyltransferase is mediated by fatty acids

The purpose of the present study was to determine the mechanisms by which glucocorticoids increase the activity of CTP:cholinephosphate cytidylyltransferase, a key enzyme required for the synthesis of surfactant phosphatidylcholine. Lung cytidylyltransferase exists as an inactive, light form low in lipids (L-form) and an active, heavy form high in lipid content (H-form). In vitro, fatty acids stimulate and aggregate the inactive L-form to the active H-form. In vivo, betamethasone increases the amount of H-form while decreasing the amount of L-form in fetal lung. There is also a coordinate increase in total free fatty acids in the H-form. In the present study, we used gas chromatography-mass spectrometry to measure the fatty acid species associated with the H-forms in fetal rat lung after the mothers were treated with betamethasone (1 mg/kg). In vivo, betamethasone increased the total amount of free fatty acids associated with the H-form by 62%. Further, the hormone selectively increased the mass of myristic and oleic acids in H-form by 52 and 82%, respectively. However, betamethasone produced the greatest increase in the amount of H-form linoleic acid, which increased fourfold relative to control. In vitro, each of the fatty acids increased L-form activity in a dose-dependent manner; however, linoleic acid was the most potent. Linoleic and oleic acids also effectively increased L-form aggregations. These observations suggest that in vivo glucocorticoids elevate the level of specific fatty acids which convert cytidylyltransferase to the active form.

Alkalosis- and ATP-induced increases in the diacyglycerol pool in alveolar type II cells are derived from phosphatidylcholine and phosphatidylinositol

Alkalosis and ATP increase surfactant secretion in alveolar type II cells, possibly via non-receptor- and receptor-mediated mechanisms respectively. We compared the effects of these two agonists on phosphatidylinositol (PI) and 1,2-diacylglycerol (DAG) pools and on phosphatidylcholine (PC) hydrolysis in alveolar type II cells. Alkalosis, caused by transfer of cells from 5% (control) to 0% CO2 in air, and ATP increased the secretion of surfactant compared with the controls. The stimulated secretion was inhibited by staurosporine, a protein kinase C inhibitor. DAG and PI contents of control cells were 50 +/- 1.1 (mean +/- S.E.M., n = 8) and 14 +/-0.8 nmol/mg phospholipid (n = 7) respectively. The DAG content increased by approximately 50 nmol (100%) within 5 s of treatment with both alkalosis and ATP, returned to control levels by 1 min, and increased again at 5 min by approximately 20 nmol. The PI content decreased maximally by approximately 6 nmol (40%) at 5 s and returned to control levels by 30 s with both alkalosis and ATP, but was unchanged thereafter. Mass-balance analysis of net changes in DAG and PI pools suggests that additional sources, possibly PC, must also contribute to the DAG increase. ATP or alkalosis also increased the hydrolysis of PC. The labelling of phosphocholine was increased (approximately 60%) at as early as 5 s and remained elevated at subsequent time points, whereas labelling of choline was higher only with ATP at 50 s and later, suggesting activation of phospholipase C by both agonists, and of phospholipase D by only ATP. Our studies demonstrate that ATP and alkalosis stimulate rapid hydrolysis of inositol and choline phospholipids to increase the DAG mass in type II cells, and that phospholipase C-stimulated PC hydrolysis is the major pathway for DAG formation.

Effect of nitrogen dioxide inhalation on surfactant phosphatidylcholine synthesis in rat alveolar type II cells

After exposure of rats to NO2 (10 ppm, 72 h) type II pneumocytes were isolated and compared to cells from control animals in order to determine whether nitrogen dioxide inhalation affects surfactant phospholipid synthesis. (1) Exposed cells contained more DNA, protein and phospholipid than type II cells from controls. (2) Choline kinase, CTP: cholinephosphate cytidylyltransferase, and cholinephosphotransferase showed higher specific activities in the exposed cells. (3) In correspondence with this finding, the incorporation rates of choline into intermediate metabolic products were also higher in the NO2-exposed cells. (4) The pool sizes of the intermediate metabolic products of the CDP-choline-pathway for the synthesis of phosphatidylcholine were also higher in the cells isolated from exposed animals. This suggests that acute nitrogen dioxide exposure leads to an enhanced phospholipid synthesis that may be responsible for the higher amount of phospholipid detectable in lung lavage.

Modulation of phosphatidylcholine biosynthesis by peroxisome proliferating fatty acid analogues

The modulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthesis by sulfur-substituted fatty acid analogues has been investigated in rats. We have compared the effects of two non-beta-oxidizable fatty acid analogues, 3-thiadicarboxylic acid and tetradecylthioacetic acid, which induce proliferation of peroxisomes, with those of the analogue tetradecylthiopropionic acid, which is a weak peroxisome proliferator. Repeated administration of 3-thiadicarboxylic acid for seven days resulted in increased hepatic concentrations of both PC and PE, but the PC/PE ratio was decreased. PC synthesis was increased, as evidenced by increased incorporation of [3H]choline into PC and an increased activity of cytidinetriphosphate (CTP): phosphocholine cytidylyltransferase. This was accompanied by a reduction in the pool sizes of choline and phosphocholine. The S-adenosylmethione/S-adenosylhomocysteine ratio (AdoMet/AdoHcy) was marginally affected, indicating no increase in the rate of methylation of PE to PC. Administration of tetradecylthioacetic acid also resulted in increased hepatic phospholipid levels, increased AdoMet/AdoHcy ratios and in slightly elevated activity of CTP:phosphocholine cytidylyltransferase. The most striking effect observed after tetradecylthiopropionic acid treatment was the development of fatty liver. The activity of CTP:phosphocholine cytidylyltransferase and the incorporation of [3H]choline into PC was reduced compared to 3-thiadicarboxylic acid treatment. Although the rate of methylation of PE seemed to be increased at an elevated AdoMet/AdoHcy ratio, this resulted in only minor changes in the hepatic PC and PE levels, and the PC/PE ratio remained unchanged. Furthermore, the hepatic levels of choline and phosphocholine were reduced in these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Choline transport by lung epithelium

The uptake of [3H]choline was investigated using isolated perfused rat lungs and primary cultures of granular pneumocytes isolated by tryptic digestion of rat lungs. Metabolic products were separated from free choline by chloroform:methanol extraction and column chromatography. Tissue-associated [3H]choline increased progressively in the perfused lung, and estimated mean intracellular concentration at 2 h was 12 times the extracellular concentration (5 microM). Choline uptake was inhibited by ventilation with CO and by perfusion with the choline analog, hemicholinium-3 (HC-3). Isolated granular pneumocytes also accumulated choline against a concentration gradient by an energy-dependent process. The concentration for half-maximal uptake, after correction for the diffusion component, was estimated at 18 +/- 4 microM (mean +/- SE; n = 3), and the estimated maximal rate of uptake was 213 +/- 44 pmol/min/microliter cell water. HC-3 inhibited uptake by approximately 50% at a concentration of 10(-4) M. There was no effect on uptake when Na+ in the medium was replaced by Li+ or N-methylglucamine+. These results indicate that granular pneumocytes possess a transport system that results in accumulation of choline against a concentration gradient. The characteristics of uptake indicate that this system is similar to the low affinity choline transport system of other organs.

Regulation of phosphatidylcholine biosynthesis in activated alveolar type II cells

The biosynthesis of phosphatidylcholine was studied in a population of activated Type II cells isolated from the lungs of rats treated with silica. Type II cells were separated by centrifugal elutriation into two populations, designated Type IIA and Type IIB. The Type IIB or activated population consisted of Type II cells that were larger than normal cells; Type IIA cells were morphologically similar to normal Type II cells. Type IIB cells incorporated more [Me-14C]choline into both total phosphatidylcholine and disaturated phosphatidylcholine than did Type IIA or control Type II cells. Measurement of the pool sizes of the choline-containing precursors to phosphatidylcholine indicated that the biosynthesis of phosphatidylcholine was increased 4- to 5-fold in Type IIB cells. Increased conversion of cholinephosphate to CDP-choline was associated with increased phosphatidylcholine biosynthesis in Type IIB cells. Cholinephosphate cytidylyltransferase activity was increased approximately threefold in Type IIB cells. Subcellular fractionation indicated that essentially all of the increase in cytidylyltransferase activity was associated with the particulate fraction (100,000 x g pellet). In Type IIB cells, the particulate fraction contained 83% of the total cellular cytidylyltransferase activity; in control cells, this fraction contained 67% of the total activity. The specific activity of the cytidylyltransferase associated with the particulate fraction was increased twofold in Type IIB cells. The specific activity of the cytosolic enzyme was similar to that in control cells. Cholinephosphotransferase specific activity was increased approximately twofold in the activated Type II cells. The specific activity of choline kinase was the same as that in control Type II cells. These results demonstrate that the increased biosynthesis of phosphatidylcholine in Type IIB cells is a result of stimulation of the CDP-choline pathway. This study indicates that both cholinephosphate cytidylyltransferase and cholinephosphotransferase may be involved in regulating the de novo biosynthesis of phosphatidylcholine in alveolar Type II cells.

Cholinephosphate cytidylyltransferase in fetal rat lung cells: activity and subcellular distribution in response to dexamethasone, triiodothyronine, and fibroblast-conditioned medium

The initiation of pulmonary surfactant synthesis during fetal development has been shown to be under hormonal control. Using cultured lung cells isolated from 19-day-gestation fetal rats, we evaluated the effects of various hormones on the activity and subcellular distribution of cholinephosphate cytidylyltransferase, a rate-controlling enzyme in phosphatidylcholine synthesis. The cells were incubated in medium containing 10% carbon-stripped fetal bovine serum to which dexamethasone, triiodothyronine, and/or conditioned medium from dexamethasone-treated fetal rat lung fibroblasts were added for 48 h. Dexamethasone and fibroblast-conditioned medium increased microsomal enzyme activity 169% +/- 6% (mean +/- SE, p less than 0.01) and 150% +/- 2% (p less than 0.05) over control levels, respectively. Further, dexamethasone increased cytosolic specific activity 160% +/- 17% (p less than 0.05). Addition of T3 to the fibroblast-conditioned medium caused a further increase in microsomal activity, but T3 alone had no effect. Increased microsomal cytidylyltransferase activity correlated with an increased rate of [3H]choline incorporation into disaturated phosphatidylcholine. Hormonal induced increases in enzyme activity were not adequately explained by simple translocation of enzyme from cytosol to microsomes. Cycloheximide (5 micrograms/ml) inhibited enzyme stimulation by dexamethasone and fibroblast-conditioned medium, suggesting that protein synthesis of new enzyme or regulatory proteins is involved. We conclude that hormones modulate cytidylyltransferase activity of isolated fetal lung cells. Dexamethasone and fibroblast-conditioned medium exert their major effects by stimulating microsomal activity.

Fatty acids stimulate phosphatidylcholine synthesis and CTP:choline-phosphate cytidylyltransferase in type II pneumocytes isolated from adult rat lung

The regulation by cyclic AMP and fatty acids of phosphatidylcholine (PC) synthesis in rat alveolar type II cells was studied. In contrast with results with hepatocytes, cyclic AMP and its potent chlorophenylthio analogue had no inhibitory effect on [Me-14C]choline incorporation into PC in pulse-chase studies with alveolar type II cells. The inclusion of the fatty acids palmitate, oleate or linoleate in the chase incubation medium stimulated the incorporation of [Me-14C]choline into PC by type II cells. The effect of palmitate, which was more pronounced than that of the other fatty acids, appeared to be concentration-dependent. Increased [Me-14C]choline incorporation into PC was paralleled by an accelerated disappearance of the radiolabel from choline phosphate, which is consistent with an activation of CTP:choline-phosphate cytidylyltransferase. This enzyme is considered to be rate-limiting in the synthesis of PC de novo by type II cells. As fatty acids are also substrate for PC synthesis, their effect could also be due to compensation for a fatty acid deficiency. To test this possibility, fatty acid synthesis in the type II cells was stimulated by addition of lactate. Even then, an additional stimulation of PC synthesis by palmitate was observed, which supports the regulatory influence of exogenous fatty acids. Incubation of type II cells in the presence of 0.2 mM-palmitate resulted in a 45% increase in the membrane-bound CTP:choline-phosphate cytidylyltransferase activity, whereas the soluble activity remained unchanged. Choline kinase activity in the soluble fraction increased by 48%. However, the increase in choline kinase is unlikely to be responsible for the increased metabolic flux through the choline phosphate pathway, because there is a relatively large pool of choline phosphate in type II cells. Therefore it is suggested that the microsomal CTP:choline-phosphate cytidylyltransferase is the form of this enzyme which is active in surfactant PC synthesis, and possibly has a regulatory role in this pathway.

Stimulation of surfactant phospholipid biosynthesis in the lungs of rats treated with silica

The effects of intratracheally instilled silica (10 mg/rat) on the biosynthesis of surfactant phospholipids was investigated in the lungs of rats. The sizes of the intracellular and extracellular pools of surfactant phospholipids were measured 7, 14 and 28 days after silica exposure. The ability of lung slices to incorporate [14C]choline and [3H]palmitate into surfactant phosphatidylcholine (PC) and disaturated phosphatidylcholine (DSPC) was also investigated. Both intra- and extra-cellular pools of surfactant phospholipids were increased by silica treatment. The intracellular pool increased linearly over the 28-day time period, ultimately reaching a size 62-fold greater than controls. The extracellular pool also increased, but showed a pattern different from that of the intracellular pool. The extracellular pool increased non-linearly up to 14 days, and then declined. At its maximum, the extracellular pool was increased 16-fold over the control. The ability of lung slices to incorporate phospholipid precursors into surfactant-associated PC and DSPC was elevated at all time periods. The rate of incorporation of [14C]choline into surfactant PC and DSPC was maximal at 14 days and was nearly 3-fold greater than the rate in controls. The rate of incorporation of [3H]palmitate was also maximal at 14 days, approx. 5-fold above controls for PC and 3-fold for DSPC. At this same time point, the microsomal activity of cholinephosphate cytidylyltransferase was increased 4.5-fold above controls, but cytosolic activity was not significantly affected by silica treatment. These data indicate that biosynthesis of surfactant PC is elevated after treatment of lungs with silica and that this increased biosynthesis probably underlies the expansion of the intra- and extra-cellular pools of surfactant phospholipids.

Effects of dietary conditions on the pool sizes of precursors of phosphatidylcholine and phosphatidylethanolamine synthesis in rat liver

We developed a new method for the determination of choline- and ethanolamine-containing precursors of phosphatidylcholine and phosphatidylethanolamine after their separation by HPLC and we have studied the effects of different dietary conditions on the pool sizes of these metabolites in rat liver. Fasting for 48 h induced only a small decrease in the amounts of phosphatidylethanolamine and its water-soluble precursors. Upon refeeding with a high-sucrose, fat-free diet for 72 h, the levels of ethanolamine-containing compounds were only slowly restored. The effects of various dietary conditions on the amounts of phosphatidylcholine and its water-soluble precursors were much more pronounced. Fasting induced a sharp decrease, especially of the amount of cholinephosphate. However, the levels of phosphatidylcholine and the choline-containing precursors were rapidly restored upon refeeding for 24 h. Continued refeeding for an additional 48 h enhanced the cholinephosphate pool size to a level more than double that found in normally fed rats. The latter effect was accompanied by an inhibition of the enzyme CTP:choline-phosphate cytidylyltransferase. The results are discussed in view of a possible regulatory mechanism that may balance the amounts of phosphatidylcholine and phosphatidylethanolamine.

Choline-phosphate cytidyltransferase activity and phosphatidylcholine synthesis in rat granular pneumocytes are increased with exogenous fatty acids

We investigated the effect of exogenous fatty acids on phosphatidylcholine (PC) and disaturated phosphatidylcholine (DSPC) synthesis by rat granular pneumocytes in primary culture. Synthesis of PC and DSPC from [3H-methyl]choline, as evaluated by increasing specific activity (pmol choline incorporated/microgram phosphorus), was linear for 3 h. Exogenous palmitic, oleic, linoleic, or linolenic acid (100 microM each) increased the synthesis of PC by approx. 50% during incubation for 3 h. In contrast, synthesis of DSPC was increased only by palmitic acid. The increase in DSPC synthesis was approx. 150% after 3 h. Conversion of choline phosphate to PC was increased in the presence of palmitic or oleic acid as indicated by pulse-chase studies with [3H-methyl]choline in the intact cells. Cells incubated for 3 h with either oleic or palmitic acid showed increased choline-phosphate cytidyltransferase activity in the cells and the microsomal fraction. In addition, oleic acid increased the activity of this enzyme in the cytosolic fraction. The distribution of this enzyme in cytosolic and microsomal fraction was 24 and 76% in the cells incubated with palmitic acid and 32 and 68% in control cells. These results suggest that exogenous fatty acids stimulate the de novo pathway of PC synthesis in granular pneumocytes by increasing the microsomal choline-phosphate cytidyltransferase activity.

Synthesis of phosphatidylcholines in ozone-exposed alveolar type II cells isolated from adult rat lung: is glycerolphosphate acyltransferase a rate-limiting enzyme?

Type II cells were exposed to ozone by gas diffusion through the thin Teflon bottom of culture dishes. The rate of phosphatidylcholine synthesis by type II cells, monitored by the incorporation of [Me-14C]choline, was impaired by ozone at concentrations that did not affect other cellular parameters. The enzymes choline kinase and cholinephosphate cytidylyltransferase were not susceptible to inactivation by ozone at concentrations at which the activity of glycerolphosphate acyltransferase was decreased. The enzyme activity of lactate dehydrogenase increased after ozone exposure. The specific activity of choline kinase in the cytosolic fraction of type II cells was fivefold that in whole lung. The metabolism of [Me-14C]choline was studied as a function of the choline concentration. Maximal rates of phosphatidylcholine synthesis were already attained at a concentration of 20 microM choline. Exposure of type II cells to ozone did not affect the recovery of label from [Me-14C]choline in choline phosphate and CDP choline. However, the maximal rate of phosphatidylcholine synthesis decreased after ozone exposure, which indicates that the decreased apparent activity of glycerolphosphate acyltransferase limits the supply of diacylglycerols and thereby the rate of phosphatidylcholine synthesis. If the flux through the diacylglycerol pathway was stimulated by the addition of palmitic acid, a higher maximal rate of phosphatidylcholine synthesis was observed. The uptake of [Me-14C]choline and the recovery of label in CDPcholine were not altered by the addition of different concentrations of palmitate. It is concluded that type II cells take up choline very efficiently, probably due to the high specific activity of choline kinase. At low choline concentrations the rate of phosphatidylcholine synthesis is determined by the supply of CDPcholine. At concentrations of choline in the upper physiological range, the rate of phosphatidylcholine synthesis is determined by the availability of diacylglycerols, which in turn is limited by the apparent activity of glycerolphosphate acyltransferase.

Maternal administration of dexamethasone stimulates choline-phosphate cytidylyltransferase in fetal type II cells

Administration of dexamethasone to pregnant rats at 19 days gestation increased phosphatidylcholine synthesis (45%) from radioactive choline in type II cells. This enhanced synthesis of phosphatidylcholine was accompanied by an increased conversion of choline phosphate into CDP-choline. Similar results were obtained by incubating organotypic cultures of 19-day-fetal rat lung with cortisol. The increased conversion of choline phosphate into CDP-choline correlated with an enhanced choline-phosphate cytidylyltransferase activity (31% after dexamethasone treatment; 47% after cortisol exposure) in the cell homogenates. A similar increase (26% after dexamethasone treatment; 39% after cortisol exposure) was found in the microsomal-associated enzyme. No differences in cytosolic enzyme activity were observed. The specific activity of the microsomal enzyme was 3-4 times that of the cytosolic enzyme. Most of the enzyme activity was located in the microsomal fraction (58-65%). The treatments had no effect on the total amount of enzyme recovered from the cell homogenates. These results, taken collectively, are interpreted to indicate that the active form of cytidylyltransferase in type II cells is the membrane-bound enzyme and that cytidylyltransferase activation in type II cells from fetal rat lung after maternal glucocorticoid administration occurs by binding of inactive cytosolic enzyme to endoplasmic reticulum.

Glucocorticoid stimulation of choline-phosphate cytidylyltransferase activity in fetal rat lung: receptor-response relationships

A number of previous studies using in vivo and cultured fetal lung models have shown that the activity of choline-phosphate cytidylyltransferase, the enzyme which catalyzes a rate-limiting reaction in de novo phosphatidylcholine synthesis, is increased by glucocorticoids and other hormones which accelerate fetal lung maturation. To examine the mechanism of this glucocorticoid action further, we examined the effect of dexamethasone on cytidylyltransferase activity in cultured fetal rat lung explants and related it to specific dexamethasone binding. Dexamethasone stimulated cytidylyltransferase activity in the homogenate, microsomal and 105,000 X g supernatant fractions. The hormone did not alter the subcellular distribution of the enzyme, however; the bulk of the activity was in the supernatant fraction in both the control and dexamethasone-treated cultures. The dose-response curves for stimulation of cytidylyltransferase activity in the supernatant fraction and specific nuclear binding of dexamethasone were similar and both plateaued at approx. 20 nM. The EC50 for cytidylyltransferase stimulation was 6.6 nM and the Kd for dexamethasone binding was 6.8 nM. The relative potencies of various steroids for stimulating choline-phosphate cytidylyltransferase and for specific nuclear glucocorticoid binding were the same: dexamethasone greater than cortisol = corticosterone = dihydrocorticosterone greater than progesterone. The stimulation by dexamethasone of cytidylyltransferase activity and of choline incorporation into phosphatidylcholine were both abolished by actinomycin D. These data show that the stimulatory effect of dexamethasone on fetal rat lung choline-phosphate cytidylyltransferase activity is largely on the enzyme in the supernatant fraction and does not involve enzyme translocation to the microsomes as has been reported for cytidylyltransferase activation in some other systems. This effect of dexamethasone is a receptor-mediated process dependent on RNA and protein synthesis.

Stimulation of phosphatidylcholine synthesis by fatty acids in fetal rabbit type II pneumocytes

After 24 h exposure to 0.1 mM oleate or 0.1 mM palmitate there was a 2- and 1.7-fold increase, respectively, in the incorporation of choline into the lipids of type II pneumocytes. Palmitate increased the labeling of disaturated phosphatidylcholine (PC) from 23.0% of total labeled PC in control cultures to 56.6% and oleate decreased labeling of disaturated PC to 9.4%. The percentage of total cellular radioactivity found in the lipid fraction was also markedly higher in the fatty acid-treated cells (83.3% for oleate and 78.7% for palmitate) than in control cultures (64.0%). Radioactivity in water-soluble choline metabolites was correspondingly lower, with phosphocholine representing more than 95% of the label in both control and experimental cultures. After a 3 h pulse-chase period, oleate and palmitate significantly increased the percentage of total cellular radioactivity in PC and decreased the percentage in phosphocholine. Similar results were obtained by adding melittin (1-2 micrograms/ml) or phospholipase C (0.05 U/ml) to the culture medium. The stimulation of PC synthesis by fatty acids was demonstrated as early as 1 h after exposure to oleate or palmitate and at all concentrations from 0.025 to 0.25 mM. Cytidylyltransferase activity in total cell homogenates was also enhanced by long-term exposure to fatty acids and short-term addition of fatty acids or phospholipase C and melittin to the culture medium. A similar increase in cytidylyltransferase activity was found in the 100 000 X g particulate fraction of type II cells exposed to fatty acids, whereas no differences were found between the cytosolic fractions of control and treated cells. These results support the concept that an increase in intracellular level of fatty acids either from an exogenous source or following the activation of endogenous phospholipases regulates PC synthesis in fetal type II pneumocytes.

Regulation of CTP: phosphocholine cytidylyltransferase activity in type II pneumonocytes

Phosphatidylcholine synthesis by rat type II pneumonocytes was altered either by depleting the cells of choline or by exposing the cells to extracellular lung surfactant. Effects of these experimental treatments on the activity of a regulatory enzyme, CTP:phosphocholine cytidylyltransferase, were investigated. Although choline depletion of type II pneumonocytes resulted in inhibition of phosphatidylcholine synthesis, cytidylyltransferase activity (measured in cell homogenates in either the absence or presence of added lipids) was greatly increased. Activation of cytidylyltransferase in choline-depleted cells was rapid and specific, and was quickly and completely reversed when choline-depleted cells were exposed to choline (but not ethanolamine). Choline-dependent changes in enzymic activity were apparently not a result of direct actions of choline on cytidylyltransferase and they were largely unaffected by cyclic AMP analogues, oleic acid, linoleic acid or cycloheximide. The Km value of cytidylyltransferase for CTP (but not phosphocholine) was lower in choline-depleted cells than in choline-repleted cells. Subcellular redistribution of cytidylyltransferase also was associated with activation of the enzyme in choline-depleted cells. When measured in the presence of added lipids, 66.5 +/- 5.0% of recovered cytidylyltransferase activity was particulate in choline-depleted cells but only 34.1 +/- 4.5% was particulate in choline-repleted cells. An increase in particulate cytidylyltransferase also occurred in type II pneumonocytes that were exposed to extracellular surfactant. This latter subcellular redistribution, however, was not accompanied by a change in cytidylyltransferase activity even though incorporation of [3H]choline into phosphatidylcholine was inhibited by approx. 50%. Subcellular redistribution of cytidylyltransferase, therefore, is associated with changes in enzymic activity under some conditions, but can also occur without a resultant alteration in enzymic activity.

Cytidine monophosphate-dependent synthesis of phosphatidylglycerol in permeabilized type II pneumonocytes

Results of previous investigations support the proposition that, in type II pneumonocytes, CMP is involved in integration of the synthesis of phosphatidylcholine and phosphatidylglycerol for lung surfactant. In the present investigation, the amount of CMP in rat type II pneumonocytes was altered directly and resultant changes in the synthesis of phosphatidylglycerol were examined. Type II pneumonocytes were made permeable to CMP by treatment with Ca2+-free medium, and phosphatidylglycerol synthesis was then assessed by measurement of the incorporation of a radiolabelled precursor, [14C]glycerol 3-phosphate, that was not effectively utilized by cells that resisted permeabilization. Incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol (but not into other lipids) was stimulated greatly by CMP (half-maximal stimulation at approx. 0.1 mM). CMP stimulated the incorporation of [14C]glycerol 3-phosphate into both the phosphatidyl moiety and the head group of phosphatidylglycerol. Incorporation of [14C]palmitate into phosphatidylglycerol was also stimulated by CMP. myo-Inositol, at concentrations found in foetal-rat serum (0.2-2.0 mM), inhibited CMP-dependent incorporation of [14C]glycerol 3-phosphate into phosphatidylglycerol and promoted, instead, CMP-dependent incorporation into phosphatidylinositol. These data, when extrapolated to foetal type II pneumonocytes, are consistent with the view that the developmental increase in the synthesis of phosphatidylglycerol for surfactant by foetal lungs is promoted by the increase in intracellular CMP and the declining availability of myo-inositol that were found previously to be associated with this period of development.

The rate-limiting reaction in phosphatidylcholine synthesis by alveolar type II cells isolated from fetal rat lung

The rate-limiting reaction in the formation of phosphatidylcholine by type II cells isolated from fetal rat lung was examined. Studies on the uptake of [Me-3H]choline and its incorporation into its metabolites indicated that in these cells the choline phosphate pool was much larger than both the choline and CDPcholine pools. Chemical measurements of the pool sizes showed that the choline phosphate pool was indeed much larger than the intracellular choline and CDPcholine pools. Pulse-chase studies with [Me-3H]choline revealed that labelled choline taken up by the cells was rapidly phosphorylated to choline phosphate and that the radioactivity lost from choline phosphate during the chase period appeared in phosphatidylcholine. Little change was observed in the labelling of CDPcholine during the chase period. These results indicate that cholinephosphate cytidylyltransferase catalyzes a rate-limiting reaction in phosphatidylcholine formation by fetal rat lung type II cells.