The synthesis of phosphatidylcholine by adult rat lung alveolar type II epithelial cells in primary culture

1. The formation of phosphatidylcholine from radioactive precursors was studied in adult rat lung alveolar type II epithelial cells in primary culture. 2. The incorporation of [Me-14C]choline into total lipids and phosphatidylcholine was stimulated by addition of palmitate, whereas the incorporation of [U-14C]glucose into phosphatidylcholine and disaturated phosphatidylcholine was stimulated by addition of choline. Addition of glucose decreased the absolute rate of incorporation of [1(3)-3H]glycerol into total lipids, phosphatidylcholine and disaturated phosphatidylcholine, decreased the percentage [1(3)-3H]glycerol recovered in phosphatidylcholine, but increased the percentage phosphatidylcholine label in the disaturated species. 3. At saturating substrate concentrations, the percentages of phosphatidylcholine radioactivity found in disaturated phosphatidylcholine after incubation with [1-(14)C]acetate (in the presence of glucose) [1-(14)C]palmitate (in the presence of glucose), [Me-14C]choline (in the presence of glucose and palmitate) and [U-14C]glucose (in the presence of choline and palmitate) were 78, 75, 74 and 90%, respectively. 4. Fatty acids stimulated the incorporation of [U-14C]glucose into the glycerol moiety of phosphatidylcholine. The degree of unsaturation of the added fatty acids was reflected in the distribution of [U-14C]glucose label among the different molecular species of phosphatidylcholine. It is suggested that the glucose concentration in the blood as related to the amount of available fatty acids and their degree of unsaturation may be factors governing the synthesis of surfactant lipids.     

A study of the molecular species of diacylglycerol, phosphatidylcholine and phosphatidylethanolamine and of cholinephosphotransferase and ethanolaminephosphotransferase activities in the type II pneumocyte

The percent distributions of the molecular species of diacylglycerol, phosphatidylcholine and phosphatidylethanolamine in rat whole lung and type II pneumocytes were found to differ significantly. Diacylglycerol from the type II pneumocyte is enriched in the disaturated species and diminished in the polyenoic species compared to whole lung. Type II pneumocyte phosphatidylcholine is enriched in the disaturated species and diminished in all other species compared to whole lung. Relative to whole lung, type II pneumocyte phosphatidylethanolamine is greatly enriched in monoenoic and depleted in polyenoic fatty acid species. Analysis of the fatty acid composition of the molecular species in general indicated differences in relative amounts of fatty acids which were most pronounced in palmitic, palmitoleic, stearic and oleic acids, both within and between type II pneumocyte and whole lung glycerolipids. Significant differences between molecular species also existed within type II pneumocyte glycerolipids. In this cell type, phosphatidylcholine is enriched in disaturated and diminished in monoenoic species compared to diacylglycerol. Phosphatidylethanolamine is enriched in monoenoic and polyenoic species relative to diacylglycerol. In order to determine whether differences observed in type II pneumocyte glycerolipid molecular species were attributable to differences in the specificities of cholinephosphotransferase and ethanolaminephosphotransferase, the selectivity of these enzymes was examined. While cholinephosphotransferase showed diminished activity towards 1-stearoyl-2-linoleoyl-sn-glycerol, neither enzyme showed selectivity towards other tested diacylglycerols under a variety of conditions. Therefore, while in the type II pneumocyte significant amounts of phosphatidylcholine (particularly the disaturated species) and phosphatidylethanolamine may be synthesized de novo, enzymes responsible for remodeling (phospholipase A2 and acyltransferases) may play an important role in establishing the final molecular species composition of both phosphatidylcholine and phosphatidylethanolamine.     

Utilization of disaturated and unsaturated phosphatidylcholine and diacylglycerols by cholinephosphotransferase in rat lung microsomes

1. Cholinephosphosphotransferase catalyzes the conversion of diacylglycerol and CDPcholine into phosphatidylcholine and CMP. Incubation of rat lung microsomes containing phosphatidyl[Me-14C]choline with CMP resulted in an increase in water-soluble radioactivity, suggesting that also in rat lung microsomes the cholinephosphotransferase reaction is reversible. 2. Microsomes containing 14C-labeled disaturated and 3H-labeled monoenoic phosphatidylcholine were prepared by incubation of these organelles with [1-14C]palmitate and [9,10-3H2]oleate in the presence of 1-palmitoyl-sn-glycero-3-phosphocholine, ATP, coenzyme A and MgCl2. Incubation of these microsomes with CMP resulted in an equal formation of 14C- and 3H-labeled diacylglycerols, indicating that disaturated and monoenoic phosphatidylcholines were used without preference by the backward reaction of the cholinephosphotransferase. When in a similar experiment the phosphatidylcholine was labeled with [9,10-3H2]palmitate and [1-14C]linoleate, somewhat more 14C- than 3H-labeled diacylglycerol was formed. 3. The backward reaction was used to generate membrane-bound mixtures of [1-14C]palmitate- and [9,10-3H2]oleate- or of [9,10-3H2]palmitate- and [1-14C]linoleate-labeled diacylglycerols. When the microsomes containing diacylglycerols were incubated with CDPcholine, both 3H- and 14C-labeled diacylglycerols were used for the formation of phosphatidylcholine, indicating that there is no absolute discrimination against disaturated diacylglycerols. This observation is in line with our previous findings and indicates that also the CDPcholine pathway may contribute to dipalmitoylphosphatidylcholine synthesis in lung.     

The effect of betamethasone and fetal sex on the synthesis and maturation of lung surfactant phospholipids in rabbits

In the present study we investigated the maturation of the surfactant phospholipids and the role of fetal sex on the effect of betamethasone in male and female rabbit fetuses. Betamethasone was administered to the doe (0.2 mg/kg intramuscularly) 42 and 18 h prior to killing. The fetuses were studied at 27 and 28 days from conception. Results from the alveolar lavage show that male fetuses tended to have a lower disaturated phosphatidylcholine/sphingomyelin ratio and lower levels of phosphatidylinositol. Phosphatidylglycerol was detected in trace amounts. This was apparently due to the high extracellular levels of myo-inositol inhibiting the synthesis of surfactant phosphatidylglycerol while increasing the synthesis of surfactant phosphatidylinositol. Betamethasone increased the recovery of disaturated phosphatidylcholine and phosphatidylinositol from the lung lavage in both sexes. As studied in lung slices in vitro, the betamethasone treatment decreased the incorporation of glucose into phospholipids, including into the fatty acid moiety of disaturated phosphatidylcholine, although it had no significant effect on the incorporation of glucose into the glycerol moiety of disaturated phosphatidylcholine. However, the addition of palmitate increased the incorporation of glucose into the glycerol moiety of disaturated phosphatidylcholine. The betamethasone treatment did not increase the incorporation of [1-14C]pyruvate into disaturated phosphatidylcholine. Following betamethasone administration, the availability of fatty acids may become rate-limiting for the synthesis of surfactant phospholipids. Betamethasone increased the activities of phosphatidic acid phosphohydrolase and phosphatidate cytidyltransferase in a fraction of microsomal membranes. The present evidence suggests that the glucocorticoid-induced lung maturation and the maturation of the normal lung are associated with an increase in the activity of the enzymes which are involved in metabolizing phosphatidic acid to neutral and acidic surfactant secretion of the male fetus was not explained by possible sex-related differences in the biosynthesis of the phospholipids.     

Development of glycogen and phospholipid metabolism in fetal and newborn rat lung.

Glucose, a major metabolic substrate for the mammalian fetus, probably makes significant contributions to surface active phospholipid synthesis in adult lung. We examined the developmental patterns of glycogen content, glycogen synthase activity, glycogen phosphorylase activity and glucose oxidation in fetal and newborn rat lung. These patterns were correlated with the development of phosphatidylcholine synthesis, content and the activities of enzymes involved in phosphatidylcholine synthesis. Fetal lung glycogen concentration increased until day 20 of gestation (term is 22 days) after which it declined to low levels. Activity of both glycogen synthase I and total glycogen synthase (I + D) in fetal lung increased late in gestation. Increased lung glycogen concentration preceded changes in enzyme activity. Glycogen phosphorylase a and total glycogen phosphorylase (a + b) activity in fetal lung increased during the period of prenatal glycogen depletion. The activity of the pentose phosphate pathway, as measured by the ratio of CO2 derived from oxidation of C1 and C6 of glucose, declined after birth. Fetal lung total phospholipid, phosphatidycholine and disaturated phosphatidylcholine content increased by 60, 90 and 180%, respectively, between day 19 of gestation and the first postnatal day. Incorporation of choline into phosphatidylcholine and disaturated phosphatidylcholine increased 10-fold during this time. No changes in phosphatidylcholine enzyme activities were noted during gestation, but both choline phosphate cytidylyltransferase and phosphatidate phosphatase activity increased after birth. The possible contributions of carbohydrate derived from fetal lung glycogen to phospholipid synthesis are discussed.     

The species of acyl-CoA in subcellular fractions of type II cells isolated from adult rat lung and their incorporation into phosphatidic acid

Microsomes and cytosol were prepared from type II cells isolated from adult rat lung. Upon determination of the acyl-CoA composition in the microsomes, we found 49% palmitoyl-CoA, 2% myristoyl-CoA, 21% stearoyl-CoA, 5% palmitoleoyl-CoA, 16% oleoyl-CoA, 5% linoleoyl-CoA and 2% arachidonoyl-CoA. The acyl-CoA composition of the cytosol was very similar. Upon incubation of type II cell microsomes with [U-14C]glycerol 3-phosphate and with acyl-CoA species mixed in the proportions in which they were found in this cell fraction, approx. 40% of the synthesized phosphatidic acid was disaturated. Of the two quantitatively most important acyl-CoA species, the palmitoyl species was incorporated 4-times faster into total and disaturated phosphatidic acid than the stearoyl species. These two species were distributed very similarly among the phosphatidic acid species synthesized de novo. In newly formed disaturated phosphatidic acid, the palmitoyl groups were distributed approximately equally between the 1- and the 2-position. From these data, it can be estimated that of the phosphatidic acid molecules synthesized by type II cell microsomes, approx. 26% contain two palmitoyl moieties. Assuming that both phosphatidic acid phosphatase and cholinephosphotransferase are non-selective with regard to the substrate species that they convert, this would mean that 26% of the phosphatidylcholine molecules synthesized de novo would be dipalmitoylphosphatidylcholine. As in surfactant, approx. 60% of the phosphatidylcholine is constituted by the dipalmitoyl species, this would mean that approx. 45% of the surfactant dipalmitoylphosphatidylcholine would be made via de novo synthesis.     

Diacylglycerol synthesized in vitro from sn-glycerol 3-phosphate and the endogenous diacylglycerol are different substrate pools for the biosynthesis of phosphatidylcholine in rat lung microsomes

In microsomes of rat lung, labeled diacylglycerol was synthesized from sn-[3H]glycerol 3-phosphate, which had been added, and from the endogenous free fatty acids. In these microsomes containing biosynthesized [3H]diacylglycerol as well as endogenous nonlabeled diacylglycerol, the synthesis of phosphatidylcholine was measured from added [14C]CDPcholine. The incorporation of [methyl-14C]choline and of [3H]diacylglycerol into phosphatidylcholine showed an entirely different progress in the time-course of incubation. The 14C label of phosphatidylcholine increased continuously, whereas the 3H label remained constant after 2 min up to the end of the incubation period of 20 min. From this result we concluded that the diacylglycerols, synthesized in vitro from glycerol 3-phosphate over an incubation period of 20 min, constitute a separate substrate pool for the biosynthesis of phosphatidylcholine, and are not mixed with the endogenous diacylglycerol pool.     

Fetal lung disaturated phosphatidylcholine. Ostensible increase following exposure to dexamethasone

Fetal rat lung removed at 15 days gestation and placed in organ culture incorporates choline into phosphatidylcholine. Addition of 10(-9) M dexamethasone resulted in increased rates of choline incorporation per micrograms protein after both 6 and 12 days culture. This concentration of dexamethasone did not increase tissue phosphatidylcholine or disaturated phosphatidylcholine. Thus, at a culture time when dexamethasone had a significant effect on choline incorporation, there was no change in either the total phospholipid or disaturated phosphatidylcholine content of the lung tissue. The transplacental administration of dexamethasone decreased fetal lung DNA and phospholipid content. At the mid-range dosage tested (400 micrograms), dexamethasone depressed DNA (51%) appreciably more than total phosphatidylcholine (28%) and disaturated phosphatidylcholine (33%). These results show that the hormone does not increase the total amount of surfactant per lung. The increased disaturated phosphatidylcholine per mg DNA results in an ostensible beneficial effect of dexamethasone on surfactant and may reflect an increased proportion of Type II cells in fetal lung both in vitro and in vivo following hormone exposure. Disaturated phosphatidylcholine per Type II alveolar cell is no doubt increased but the trade-off is fewer total cells in the lung.     

The development of surfactant synthesis in fetal rabbit lung organ culture exhibits a sex dimorphism

Sex differences in amniotic fluid and lung lavage surfactant have been found. Although these studies suggest that augmented fetal surfactant synthesis occurs earlier in the female fetus, there is little direct evidence for a sex difference in fetal surfactant synthesis. We studied the synthesis of surfactant by evaluating the appearance of labelled phospholipids in lamellar bodies recovered from sex-specific organ culture of fetal rabbit lungs. Furthermore, we studied the ability of dexamethasone to stimulate surfactant synthesis in male and female fetal lungs. Organ culture was begun on day 21 of gestation. After 5 days the incorporation of [1,3-14C]glycerol into phosphatidylcholine (PC), disaturated phosphatidylcholine, phosphatidylinositol (PI), and phosphatidylglycerol was studied. Female lungs in organ culture synthesized more disaturated PC per milligram protein than male lungs. In the presence of dexamethasone (10(-8) M) and dihydrotestosterone (10(-8) M) an increased synthesis was noted in the female cultures of PC (270%), disaturated PC (234%), PI (281%), and phosphatidylglycerol (754%). No significant increase in the synthesis of PC or disaturated PC was observed in the male cultures. However in the male cultures smaller increases in the synthesis of PI (193%) and of phosphatidylglycerol (360%) were observed. Overall, dexamethasone stimulated synthesis in females but not in males such that significant differences in the synthesis of all phospholipids were found in the presence of 10(-8) M dexamethasone. These studies show that the synthesis of surfactant in the fetal lung is sexually dimorphic, as is the ability of dexamethasone to regulate synthesis. An understanding of the mechanism which causes these differences may provide important insight into the control of the developmental clock which regulates the orderly progression of development.     

On the source of the vasopressin-induced increases in diacylglycerol in hepatocytes

In hepatocytes pre-labelled with [3H]glycerol, vasopressin increased by 20% the amount of radioactivity present in diacylglycerols. The effect of vasopressin was partially dependent on Ca2+. The magnitude of the increase in [3H]diacylglycerol was 5-times the sum of the radioactivity present in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. No stimulation by vasopressin of the initial rate of incorporation of radioactivity into diacylglycerols was observed in cells incubated in the presence of 10 mM [3H]glycerol. Treatment of hepatocytes labelled with either [3H]ethanolamine or [3H]choline with vasopressin, ionophore A23187 or phospholipase C increased the amount of radioactivity present in trichloroacetic acid extracts of the cells. The effect of vasopressin was dependent on extracellular Ca2+. It is concluded that in hepatocytes vasopressin increases diacylglycerols by a process which does not principally involve the conversion of phosphoinositides to diacylglycerol or the de novo synthesis of diacylglycerol from glycerol 3-phosphate, but does involve the Ca2+-dependent conversion of phosphatidylethanolamine and phosphatidylcholine to diacylglycerol.     

Phosphatidylcholine de novo synthesis and modification are carried out sequentially in HL60 cells: evidence from mass isotopomer distribution analysis

The traditional (parallel) model of molecular species synthesis of phosphatidylcholine is based on the substrate specificity of two glycerolphosphate acyltransferases. Preformed molecular species of diacylglycerols are then converted to phosphatidylcholine. In this investigation, we used [1,2,3,4-(13)C(4)]palmitate as a tracer to determine the turnover rates of diacylglycerols and phosphatidylcholines. In HL60 cells, the fractional turnover rate is 34.1 +/- 16.6%/h for 1,2-dipalmitoylglycerophosphocholine (16:0,16:0-GPC), which accounts for approximately 10% of total diacylglycerol turnover. The turnover rates of other phosphotidylcholines reflect the primary event of 16:0,16:0-GPC turnover. In addition, the distribution of mass isotopomers is used to study the biosynthesis of diacylglycerols and phosphatidylcholines. On the basis of precursor-product enrichments, we propose a sequential model to account for the synthesis of phosphatidylcholine molecular species. In this model, 1,2-dipalmitoylglycerol is the only molecular species used for the synthesis of phosphatidylcholine. This precursor is converted to 1,2-dipalmitoylglycerophosphocholine, which is then deacylated to provide substrates for chain elongation and/or desaturation. These modified acyl substrates are then reacylated back to form other molecular species. This sequential model is consistent with palmitate being the dominant fatty acid product derived from mammalian fatty acid synthase. It has the advantage of protecting cells from acyl modification by exogenous substrates. Furthermore, this sequence generates only inert 1,2-dipalmitoylglycerol instead of the active diacylglycerol molecular species that contain unsaturated fatty acids.     

The molecular species of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from sn-glycerol 3-phosphate in rat lung microsomes

The species pattern of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from [14C]glycerol 3-phosphate was measured using a newly developed HPLC technique yielding 13 molecular species. A direct comparison of these species patterns presupposes determination of the lipolytic activity of lung microsomes. The lipolytic activity was quantitatively determined by measuring the changes of the endogenous concentration of diacylglycerol, triacylglycerol and free fatty acids. The species pattern of endogenous diacylglycerol measured in the time-course of lipolysis did not show any changes up to an incubation period of 20 min, suggesting that the lipolytic activity showed only a very low selectivity for individual substrate species. Diisopropylfluorophosphate (5 mumol/mg microsomal protein) strongly decreased the lipolytic activities as well as the microsomal phosphatidate phosphohydrolase activity, as measured by means of exogenous phosphatidic acid, and also the generation of phosphatidic acid from [14C]glycerol 3-phosphate. In lung microsomes, labeled phosphatidic acid and diacylglycerols were synthesized from the endogenous free fatty acids and sn-[14C]glycerol 3-phosphate, which had previously been added. By addition of CDPcholine to the prelabeled microsomes the synthesis of phosphatidylcholine was measured. After hydrolysis of phosphatidic acid and phosphatidylcholine with cytoplasmatic phosphatidate phosphohydrolase or phospholipase C, respectively, the de novo synthesized species patterns of these two lipids and of the diacylglycerol were determined. Comparison of the species pattern of de novo synthesized phosphatidic acid with that of diacylglycerol largely showed the same distribution of radioactivity among the individual species, except that the relative proportion of label was higher in the 16:0/16:0 and 16:0/18:0 species of phosphatidic acid and lower in the 16:0/20:4 and 18:0/20:4 species than in the corresponding species of diacylglycerol. The species pattern of de novo-synthesized diacylglycerol showed no differences from that of the phosphatidylcholine synthesized from it. From this result we concluded that the cholinephosphotransferase of lung microsomes is nonselective for individual species of the diacylglycerol substrate. The 16:0/18:1 and 16:0/18:2 species of phosphatidic acid, diacylglycerol and phosphatidylcholine showed a higher synthesis rate than their 18:0 counterparts, whereas the 16:0 or 18:0 analogues of species containing 20:4 and 22:6 fatty acids showed nearly the same synthesis rates.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synthesis of phosphatidylcholine and phosphatidylethanolamine in relation to the concentration of membrane-bound diacylglycerols of rat lung microsomes

Different concentrations of membrane-bound diacylglycerol were generated in vitro in rat lung microsomes by treatment with CMP. Diacylglycerol concentrations of between 16 (endogenous content) and 48 nmol/mg of microsomal protein were obtained. The relative proportion of the disaturated species of diacylglycerol remained constant at all diacylglycerol concentrations. Choline- and ethanolaminephosphotransferase activity was determined in relation to the diacylglycerol concentrations of microsomes. The activity of both phosphotransferases increased. The relative proportion of disaturated phosphatidylcholine synthesized at each diacylglycerol concentration was nearly the same and corresponded to the relative proportion of the disaturated species in the diacylglycerol. Disaturated phosphatidylethanolamine was not formed. The affinities of the choline- and ethanolaminephosphotransferases for the diacylglycerol substrate were different. We conclude that the cholinephosphotransferase is generally non-selective for the diacylglycerol substrate. The available diacylglycerol pattern seems to govern the species pattern of phosphatidylcholine and phosphatidylethanolamine. The kinetics of the phosphotransferases regulate the mass proportion of these phospholipids.     

Regulation of fetal lung disaturated phosphatidylcholine synthesis by de novo palmitate supply

Lung surfactant disaturated phosphatidylcholine (PC) is highly dependent on the supply of palmitate as a source of fatty acid. The purpose of this study was to investigate the importance of de novo fatty acid synthesis in the regulation of disaturated PC production during late prenatal lung development. Choline incorporation into disaturated PC and the rate of de novo fatty acid synthesis was determined by the relative incorporation of [14C]choline and 3H2O, respectively, in 20-day-old fetal rat lung explants and in 18-day-old explants which were cultured 2 days. Addition of exogenous palmitate (0.15 mM) increased (26%) choline incorporation into disaturated PC but did not inhibit de novo fatty acid synthesis, as classically seen in other lipogenic tissue. Even in the presence of exogenous palmitate, de novo synthesis accounted for 87% of the acyl groups for disaturated PC. Inhibition of fatty acid synthesis by agaric acid or levo-hydroxycitrate decreased the rate of choline incorporation into disaturated PC. When explants were subjected to both exogenous palmitate and 60% inhibition of de novo synthesis, disaturated PC synthesis was below control values and 75% of disaturated PC acyl moieties were still provided by de novo synthesis. These data show that surfactant disaturated PC synthesis is highly dependent on the supply of palmitate from de novo fatty acid synthesis.     

Influence of dexamethasone on the lipid distribution of newly synthesized fatty acids in fetal rat lung

There is a developmental increase in fatty acid biosynthesis and surfactant production in late-gestation fetal lung and both are accelerated by glucocorticoids. We have examined the distribution of the newly synthesized fatty acids to determine whether they are preferentially incorporated into surfactant. Explants of 18 day fetal rat lung were cultured with and without dexamethasone for 48 h and then with [3H]acetate for 4 h after which labeled fatty acids were measured. Incorporation of radioactivity from acetate was considered a measure of newly synthesized fatty acids. Phospholipids contained 86% of the newly synthesized fatty acids of which approx. 80% were in phosphatidylcholine. Phosphatidylcholine and disaturated phosphatidylcholine contained a much greater percentage of the labeled fatty acids than of the phospholipid mass determined by phosphorus assay while phosphatidylethanolamine, phosphatidylserine and sphingomyelin contained less. Dexamethasone increased the rate of acetate incorporation into total lipid fatty acids but it had little effect on fatty acid distribution, except that it increased the percentages in phosphatidylglycerol and disaturated phosphatidylcholine. The hormone also increased the mass of these two phospholipids to a greater extent than that of the total. These data suggested that the newly synthesized fatty acids are preferentially incorporated into surfactant phospholipids and that this process is accelerated by dexamethasone. However, since phosphatidylcholine and phosphatidylglycerol are not exclusive to surfactant, we compared isolated lamellar bodies with a residual fraction not enriched in surfactant. The rate of acetate incorporation into fatty acids in lamellar body phosphatidylcholine as well as its specific activity (radioactivity per unit phosphorus) were both increased by dexamethasone. Specific activity, however, was no greater in the lamellar bodies than in the residual fraction in both control and dexamethasone-treated cultures. Therefore, there is no preferential incorporation of newly synthesized fatty acids into phospholipids in surfactant as opposed to those in other components of the lung.     

Sex-specific differences in rabbit fetal lung maturation in response to epidermal growth factor.

Males and females exhibit different stages of lung development at the same gestation with males lagging behind. We hypothesized that one of the mechanisms responsible for the sex-specific difference in fetal lung maturation is a delay in the onset of epidermal growth factor (EGF) activity in the male fetal lung. EGF influences growth and differentiation during development. We studied the effects of EGF on the incorporation of glycerol into lamellar body disaturated phosphatidylcholine (DSPC) in sex-specific fetal rabbit lung explants prepared at 21 and 24 days gestation (term 31 days). The explants were maintained in Waymouth's media + 10% stripped fetal calf serum with or without EGF (10 ng/ml). The incorporation of [1,3-14C]glycerol into lamellar body DSPC was assessed after 3, 5, or 7 days of culture. Female lung explants prepared at 21 days of gestation had increased incorporation of glycerol into DSPC over time in response to EGF treatment. Male lung explants prepared at 21 days did not respond to EGF treatment. In explants prepared at 24 days gestation, baseline glycerol incorporation into DSPC was higher in female as compared to male fetal lung explants. EGF-responsiveness was also sex-specific in these more mature explants, with the male explants now responding to EGF with a consistent increase in the incorporation of glycerol into lamellar body DSPC. We conclude that one of the mechanisms responsible for the lag in male fetal lung development is a delay in the onset of EGF activity.     

Further evidence for the existence of different diacylglycerol pools of the phosphatidylcholine synthesis in microsomes

Endogenous diacylglycerol and diacylglycerol, synthesized in vitro by glycerol 3-phosphate acylation, are not mixed and represent different substrate pools for the biosynthesis of phosphatidylcholine in microsomes of rat muscle, liver and lung. Freshly isolated lung microsomes contain 12-18 nmol diacylglycerol per mg protein, and incubation with CDPcholine showed a biphasic curve for the synthesis of phosphatidylcholine as lung microsomes enriched in diacylglycerol through the glycerol phosphate pathway. With respect to the synthesis of phosphatidylcholine, a part of this endogenous diacylglycerol (0.4-0.8 nmol/mg) was comparable with diacylglycerol de novo formed in vitro by glycerol 3-phosphate acylation. An increase in the relative proportion of de novo-formed diacylglycerol in the total amount of diacylglycerol caused an increase in phosphatidylcholine synthesis by nearly the same factor. The apparent Km of the de novo-formed diacylglycerol substrate for the choline phosphotransferase was 10-times higher than the pool size of this diacylglycerol substrate in freshly isolated lung microsomes. The results supported the idea that the availability of this substrate type may be rte limiting for the de novo synthesis of phosphatidylcholine. As shown by use of the proteolytic technique measuring the mannose-6-phosphatase as lumenal control activity, the phosphatidylcholine synthesis from de novo-formed diacylglycerol and endogenous as well as exogenous diacylglycerol seems to be located on the cytoplasmic leaflet of the microsomal vesicles isolated from rat lung.     

Alveolar pre-type II cells from the fetal rabbit lung: effect of confluence on the production of disaturated phosphatidylcholine

Pre-type II alveolar cells isolated from the fetal rabbit lung on the 24th gestational day have been maintained in vitro for 14 days in a chemically defined medium supplemented with hormone-stripped serum. These cells replicate in culture. Measurement of the incorporation of [14C]choline into cellular disaturated phospholipid indicated that those cells grown in vitro under standard conditions for 8 days (pre-confluent) incorporate the radioactive precursor at a similar rate to cells maintained for 14 days (post-confluent). Both dexamethasone and serum-free medium conditioned by monolayer cultures of fetal rabbit lung fibroblasts stimulated [14C]choline incorporation into disaturated phosphatidylcholine (PC) by the pre- and post-confluent cultures after 24 or 48 h of exposure: the conditioned medium was more effective than the steroid. These treatments had little effect on choline incorporation into disaturated phosphatidylcholine of preconfluent cells during the first 12 h. A marked response occurred by 24 h after which the labelling of disaturated phosphatidylcholine plateaued. In contrast, with post-confluent cells labelling of disaturated PC increased in a more linear fashion and only plateaued after 72 h. Determination of the ratio of incorporation of [14C]choline into disaturated versus unsaturated phospholipid indicated that serum-free medium conditioned by monolayer cultures of fetal lung fibroblasts specifically increased the level of radioactive precursor in the disaturated phospholipid in both the pre- and post-confluent cell monolayers.     

SELECTIVE SYNTHESIS OF MOLECULAR CLASSES OF PHOSPHATIDIC ACID, DIACYLGLYCEROL AND PHOSPHATIDYLINOSITOL IN RAT BRAIN

—Phosphatidic acids of rat brain were shown to be predominantly of the monoenoic class while diacylglycerols and phosphatidylinositols were constituted mainly by tetraenes. Metabolic inter-relationships were examined after intraventricular injection of [¹⁴C]glycerol, [³H]arachidonate and [9,10-³H₂]stearate. In each case, diacylglycerols were most highly labelled, although a small pool of arachidonate was located in brain triacylglycerols, mainly esterified to a primary carbinol, with extremely high turnover rate. Fractionation of the lipids showed a preferential synthesis of disaturated, monoenoic and polyenoic classes (>4 double bonds) of phosphatidic acid, diacylglycerol and phosphatidylinositol. The high flux of [³H]stearate through disaturated species of phosphatidic acid and diacylglycerol could be partially suppressed by simultaneous injections of unsaturated fatty acids, both probably consequences of perturbing the very small brain pool of free fatty acids. Kinetics of labelling of phosphatidylinositols were consistent with formation of arachidonoyl-containing species by acyl transfer mechanisms with disaturated and oligoenoic classes serving as precursors. Although the profile of molecular classes of diacylglycerol and phosphatidylinositol strongly suggests a metabolic relation, there was no obvious evidence for this in the kinetic studies of the whole brain lipids. Such relation, however, may have been masked by the rapid flow of radioactivity from phosphatidic acids to diacylglycerols.     

Altered phospholipid biosynthesis in type II pneumocytes isolated from streptozotocin-diabetic rats

To determine whether type II pneumocytes isolated from diabetic animals could serve as a useful model for the study of surfactant phospholipid biosynthesis and its regulation, type II pneumocytes were isolated from adult streptozotocin-diabetic rats and placed in short-term primary culture. On a DNA basis, total cellular disaturated phosphatidylcholine (disaturated PC) and phosphatidylglycerol (PG) were decreased 36 and 66%, respectively, in type II cells from diabetic animals. 7 days of insulin treatment of diabetic rats returned the cellular disaturated PC and PG content to control values and increased the total cellular phosphatidylethanolamine (PE) content by 51%. The rates of glucose and acetate incorporation into disaturated PC per unit DNA were reduced 32 and 38%, respectively, in cells isolated from diabetic rats, while glycerol incorporation was increased by 143%. Insulin treatment of diabetic rats returned the glucose and glycerol incorporation rates to control values and increased acetate incorporation into disaturated PC by 66%. These data suggest that the biosynthesis of surfactant is altered by both diabetes mellitus and in vivo insulin treatment.