Stimulation of cholinephosphate cytidylyltransferase activity by estrogen in fetal rabbit lung is mediated by phospholipids

Nucleoplasmic Reticulum Formation in Human Endometrial Cells is Steroid Hormone Responsive and Recruits Nascent Components

The nuclei of cells may exhibit invaginations of the nuclear envelope under a variety of conditions. These invaginations form a branched network termed the nucleoplasmic reticulum (NR), which may be found in cells in pathological and physiological conditions. While an extensive NR is a hallmark of cellular senescence and shows associations with some cancers, very little is known about the formation of NR in physiological conditions, despite the presence of extensive nuclear invaginations in some cell types such as endometrial cells. Here we show that in these cells the NR is formed in response to reproductive hormones. We demonstrate that oestrogen and progesterone are sufficient to induce NR formation and that this process is reversible without cell division upon removal of the hormonal stimulus. Nascent lamins and phospholipids are incorporated into the invaginations suggesting that there is a dedicated machinery for its formation. The induction of NR in endometrial cells offers a new model to study NR formation and function in physiological conditions.

Regulation of phosphatidylcholine synthesis in maturing type II cells: increased mRNA stability of CTP:phosphocholine cytidylyltransferase

Pulmonary surfactant phosphatidylcholine synthesis increases in fetal lung type II cells with advancing gestation. This increase is accompanied by an increase in gene and protein expression of CTP:phosphocholine cytidylyltransferase (CT; EC 2.7.7.15), which catalyses a regulatory step in de novo phosphatidylcholine synthesis by fetal type II cells. In the present study we investigated the role of transcriptional and post-transcriptional mechanisms in the developmental induction of CT mRNA in maturing type II cells. We found that CT mRNA increased 2-fold from days 18 to 21 of fetal rat gestation (term 22 d). This increase in CT mRNA was not accompanied by a developmental increase in CT gene transcription. However, CT mRNA was more stable on day 21 (t1/2 48 h) compared with that on day 18 (t1/2 17 h). Glucocorticoids have been shown to enhance surfactant phosphatidylcholine synthesis in fetal type II cells. Therefore we also examined the effect of maternal glucocorticoid administration to pregnant rats at 19 d of gestation on CT mRNA expression in fetal type II cells isolated 24 h later. Glucocorticoid treatment did not increase type II cell CT mRNA. As reported previously, however, glucocorticoids increased CT activity in the microsomal membrane fraction of fetal type II cells, whereas no differences in cytosolic CT activity were observed. We conclude that the developmental increase in CT mRNA in fetal type II cells is due to a decreased breakdown of the CT transcript and that glucocorticoids regulate fetal type II cell CT activity at a post-translational level.

Ca(+2)-phosphatidylserine-dependent protein kinase C activity in fetal, neonatal and adult rabbit lung and isolated lamellar bodies

Evidence indicates that Ca(+2)-phosphatidylserine-dependent protein kinase C (PKC) is involved in regulation of surfactant secretion. This study was done to examine PKC activity in lung as surfactant synthesis and secretion is initiated, and at birth and to compare these enzyme levels with those in the adult lung. NZW rabbits were used. Fetal and adult lungs were fractionated into subcellular compartments including a lamellar body fraction, which represents intracellular surfactant. The time course for microsomal enzyme activity was compared between 24th gestational day and adult rabbit lung. The reactivity appeared similar in both fractions. PKC specific activity displayed a prominent peak between the 27th and 30th gestational days in total homogenate and lamellar bodies. Specific activity was also high in nuclear, mitochondrial and microsomal fractions the day prior to birth. Adult levels were similar or higher. Total PKC activity was high during late gestation but declined sharply the day prior to birth. A marked increase was present on the first postnatal day. In contrast lamellar bodies displayed a peak in activity between the 27th and 30th gestational days followed by a decline to adult levels. Delipidation of lamellar body fraction indicated that the high enzyme activity in this fraction on the 27th gestational day was not artifactual. The changes observed in PKC in fetal, neonatal and adult lung indicate this enzyme activity changes in lung during the period of onset of surfactant synthesis and secretion during late gestation and may be associated with lamellar bodies, in 27th gestational day fetal lung.

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.

Gestational age dependence of 11 beta-hydroxysteroid dehydrogenase and its relationship to the enzymes of phosphatidylcholine synthesis in lung and liver of fetal rat

Increase in fetal surfactant synthesis and lung maturity is caused by the glucocorticoidal induction of enzymes required for phosphatidylcholine (PC) synthesis towards the end of gestation. The regulation of gestational age-dependent induction of PC synthesis by glucocorticoids is still unclear. Since 11-beta-hydroxysteroid dehydrogenase (11 beta-HSD) activity and its metabolising capacity for glucocorticoids have been suggested to play a central role in this regulation, we measured the gestational age-dependent changes in 11 beta-HSD and PC synthesizing enzymes in lung and liver of fetal rat. The activity of cholinephosphate cytidyltransferase (CCT; key enzyme in PC synthesis), choline phosphotransferase (CPT) and lysolecithin acyltransferase (LAT) were found to increase gradually in the lung towards the end of gestation, reached peak values at term followed by a decrease of activity reaching finally adult levels. Only CK activity exhibited constant levels until term followed by a slight increase after the birth. In comparison with the lung, the liver enzymes followed a similar pattern, but at a higher rate of activity except for CCT which was higher in the lung. The activity of 11 beta-HSD in fetal lung microsomes was detectable from day 20 and increased towards the end of gestation in the lung and liver of the rat. Oxidase activity was always found to exceed the reductase activity. The activity of 11 beta-HSD continued to increase after delivery and reached peak levels in adult animals in both organs. In order to test the hypothesis, whether 11 beta-HSD activity and PC synthesis are induced by increasing endogenous glucocorticoidal levels, we examined on day 19 of gestation the effect of dexamethasone (DEXA) on enzymatic activities (11 beta-HSD, CCT) and on [14C]choline incorporation in phosphatidylcholine in fetal lung organoid cultures. Additionally, changes in CCT activity in fetal lungs after maternal administration of DEXA were measured. DEXA accelerated 11 beta-HSD and CCT activities as well as [14C]choline incorporation. We conclude, that endogenous glucocorticoids induce PC synthesis as well as 11 beta-HSD activity in lung and liver of the fetal rat. Fetal PC synthesis is not altered by increasing 11 beta-HSD levels, because the increase of free serum corticosterone levels apparently exceeds the metabolising capacity of 11 beta-HSD towards term.

Mechanism by which ethanol inhibits phosphatidylcholine biosynthesis in human leukemic monocyte-like U937 cells

A previous study showing that ethanol (ETOH) blocked [3H]choline incorporation into phosphatidylcholine (PC) suggested an inhibition of PC biosynthesis in human leukemic monocyte-like U937 cells. The mechanism of the inhibitory action of ETOH was investigated. Cells were pulsed with [3H]choline for 30 min and chased in the presence or absence of ETOH for up to 6 h. PC biosynthesis was inhibited drastically within 1 h after exposure to ETOH which increased intracellular cAMP appreciably. After a 3-h treatment, ETOH significantly inhibited both choline kinase (CK) and the cytosolic CTP: cholinephosphate cytidylyltransferase (CT). The inactivated CT was no longer stimulated by exogenous phosphatidylglycerol (PG). There was no evidence for redistribution of CT activity between cytosol and microsomes. When cells were exposed to 8-Bromo-cAMP ranging from 100 to 300 microM, PC biosynthesis remained unaffected despite the drastically elevated cAMP. These results seem to suggest that the raised cAMP is not a prerequisite for the inhibition of PC biosynthesis in U937 cells. Following pretreatment with protein kinase inhibitors (H-89 and K-252a), PC biosynthesis was decreased significantly and the inhibitory effect of ETOH was potentiated. Taken together, our results suggest that the inhibition of PC biosynthesis and the inhibitory effect of ETOH are independent of the activation of cAMP-dependent protein kinase. Unlike protein kinase inhibitors, pretreatment with tyrosine kinase inhibitors (erbstatin, genistein and tyrphostin 25) resulted in differential effects on PC biosynthesis and on the inhibitory action of ETOH. Genistein stimulated PC biosynthesis by 30 per cent as well as partially preventing/reversing the ETOH action, while tyrphostin 25 produced a synergistic inhibition. The relevance of tyrosine phosphorylation/dephosphorylation to the regulation of PC biosynthesis and ETOH action remains to be established.

Biphasic modulation of choline uptake and phosphatidylcholine biosynthesis by vasopressin in rat cardiac myocytes

The effect of vasopressin on choline uptake and phosphatidylcholine biosynthesis in isolated rat heart myocytes was investigated. Myocytes were incubated with labelled choline in the presence of 0.05-1.0 microM vasopressin. Uptake of choline was enhanced (25%) by a low concentration (0.2 microM) of vasopressin, but was attenuated (19%) by a higher vasopressin concentration (1.0 microM). The biosynthesis of phosphatidylcholine was also affected by vasopressin in a biphasic manner. At low concentrations of vasopressin, a general increase in cytosine triphosphate:phosphocholine cytidylyltransferase activity was observed that caused an enhanced conversion of phosphocholine to phosphatidylcholine via the cytidine diphosphocholine pathway. At high vasopressin concentrations, a decrease in the activity of cytidylyltransferase was detected, which was caused by the translocation of the enzyme from the microsomal fraction to the cytosolic fraction. The decrease in enzyme activity coincides with a reduction in the conversion of labelled phosphocholine to phosphatidylcholine. In view of the fact that phospholipid biosynthesis in rat hepatocytes is inhibited by vasopressin at all concentrations, the biphasic modulation of phosphatidylcholine biosynthesis in rat heart myocytes illustrates the diverse effects of this hormone in different mammalian tissues.

Epidermal growth factor is a positive in vivo regulator of CTP:cholinephosphate cytidylyltransferase

CTP:cholinephosphate cytidylyltransferase (CT) is a key enzyme required for surfactant phosphatidylcholine synthesis, and its activity is regulated by lung lipids. This study evaluated the effect of epidermal growth factor (EGF) on the phospholipid content and the expression of CT in the lung following direct in vivo administration to the newborn rat. EGF caused an increase in cytidylyltransferase activity by 58% in lung cytosol. The increase in cytosolic activity was not mediated by a corresponding increase in enzyme mass. Further, these changes in cytidylyltransferase activity were associated with a significant increase in total lung phospholipid and phosphatidylcholine content. The results suggest that EGF may have important maturational effects on lung surfactant metabolism.

Hormonal regulation of cholinephosphate cytidylyltransferase in human fetal lung

Cytidylyltransferase (CTP: cholinephosphate cytidylyltransferase, EC 2.7.7.15, CYT) is a regulatory enzyme for synthesis of pulmonary surfactant phosphatidylcholine (PC). The effects of glucocorticoid, T3, and cAMP on CYT activity were studied in explants of human fetal lung (18-22 weeks gestation) cultured for 1-6 days in serum-free medium. Dexamethasone (Dex, 10 nM) treatment for 5 days increased homogenate CYT activity (+115%, P < 0.02) when assayed in the presence of added lipid co-factor (L-alpha-phosphatidylglycerol, PG, 1.1 mM) and tended to increase activity in its absence (+77%, P = 0.12). Cytosolic activity was also significantly elevated in the presence of added co-factor (+124%, P < 0.01), but there was no effect of Dex on microsomal specific activity. Dex increased the recovery of CYT activity in the cytosolic fraction (75% vs. 43% (control) of the homogenate activity), but not in the microsomal, nuclear or mitochondrial fractions. Assayed in the presence of added co-factor, stimulation of CYT by Dex was apparent after 48 h exposure and maximal by 5-6 days exposure to < or = 30 nM concentration. T3 or agents that increase endogenous cAMP stimulated cytosolic activity by 40% and 36-74%, respectively, after 4-6 days exposure, but none produced an additive increase in the presence of Dex. We conclude that stimulation of CYT activity contributes to hormonal induction of surfactant lipids by each of these hormones. Glucocorticoids may increase the amount of CYT enzyme as well as activate the enzyme via increased synthesis of lipid co-factor.

In vitro induction of type II pneumocyte-related differentiation in a clonal fetal bronchiolo-alveolar epithelial cell line (M3E3/C3)

The aim of the present study is to investigate the differentiation of a cloned fetal Syrian hamster lung epithelial cell line, M3E3/C3, to assume morphological and biochemical features of Type II pneumocytes (phospholipid synthesis). The use of a soft agar overlay and a differentiation medium, based on RPMI 1640 combined with hormone supplements, increased the cellular content of phosphatidylcholine (PC) from 48.6% in the conventional culture without any of these factors (referred to as 'control') to 64.7% (p < 0.02). The other cell membrane-associated components, phosphatidylethanolamine (p < 0.05), sphingomyelin (p < 0.001), phosphatidylserine (n. s.), phosphatidic acid (p < 0.02) and phosphatidylinositol (p < 0.02) decreased. The content of phosphatidylglycerol showed no essential change (from 11.2% to 8.4%) and the content of disaturated phospholipids decreased from 32.0 to 23.4 micrograms/10(6) cells (p < 0.002). The phospholipid pattern of these differentiated cells is in rough accordance with that of primary isolated Type II pneumocytes. They incorporated 3H-choline over a period of four hours at a higher rate in the Type II pneumocyte-specific phospholipids, PC and dipalmitoyl-phosphatidylcholine (DPPC), than the undifferentiated control. The radiolabelling of PC and DPPC in the differentiated cells, after 3 hours of incubation with 3H-choline, was about 3.2-fold and 2.2-fold, respectively, higher than that in the control cells (p < 0.001). Intracytoplasmatic phospholipid granules were evident in the differentiated cells by light and fluorescence microscopy (modified Papanicolaou stain, Phosphin 3 R fluorescence). Furthermore, the differentiated cells had a high activity of alkaline phosphatase, whereas the control cells showed only little activity of this enzyme. Ultrastructurally, many concentric multilayered osmiophilic bodies, well developed Golgi apparatuses and many cytoplasmic protrusions comparable to microvilli, were detectable in the cuboidal shaped differentiated cells. The control cells remained wide and flattened on the plastic surface and produced a fibrillar extracellular matrix. In the simultaneously studied fetal lung fibroblasts none of these specific features were noted. These results indicate a specific differentiation capacity of the clonal fetal cell line, M3E3/C3, by closely resembling Type II pneumocytes.

Differential effects of unsaturated fatty acids on phospholipid synthesis in human leukemia monocytic U937 cells

The biosynthesis of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in monocyte-like leukemia U937 cells was monitored by adding [3H]choline, [14C]ethanolamine or [14C]glycerol to the culture media; incorporation into phospholipid (PL) increased with time. The effect of unsaturated fatty acids (UFA) on PC and PE synthesis was investigated by pretreating U937 cells for 72h with 10 microM 18:1 (n - 9), 18:2 (n - 6), 18:3 (n - 3), 20:4 (n - 6) and 20:5 (n - 3). The UFA caused no alteration in cell growth, as evidenced by light microscopy and the incorporation of [3H]thymidine and [3H]leucine. Total cellular uptake of radioactive precursors remained unaffected by all the treatments. Pretreatment with 20:5 resulted in approximately 25 per cent reduction in the incorporation of [3H]choline into PL, while no significant effect was detected with the other UFAs. 18:3, 20:4 and 20:5 depressed the incorporation of [14C]ethanolamine into PL by 34 per cent, 28 per cent and 49 per cent respectively. However, there was no redistribution of label with any of the treatments. 18:3, 20:4 and 20:5 also antagonized the stimulatory effect of endotoxin (LPS) on PC and PE synthesis. In addition, the incorporation from [14C]glycerol into PC and PE was reduced by 18:3, 20:4 and 20:5. Although the PL composition of the cells remained essentially unaffected, our study shows that chronic treatment of U937 cells with n - 3 PUFA (20:5) depressed PC and PE synthesis, and 18:3 and 20:4 also caused inhibition of PE synthesis.

Phosphatidylcholine metabolism in rat liver after partial hepatectomy. Evidence for increased activity and amount of CTP:phosphocholine cytidylyltransferase

The effect of partial (70%) hepatectomy on phosphatidylcholine (PC) synthesis in rat liver was investigated during the first 4 post-operative days. Between 4 and 96 h after partial hepatectomy, the mass of PC increased from 30% to 80% of sham-operation values, being comparable with the restoration of total liver mass after partial hepatectomy. Relative to control (sham-operation), the incorporation in vivo of [3H]choline into PC was stimulated 2.6-fold at 22 h after partial hepatectomy. Moreover, CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15) activity was significantly enhanced, and the pool size of phosphocholine decreased at 22 and 48 h after partial hepatectomy, whereas the activity of choline kinase (EC 2.7.1.32) was augmented at a later stage of liver regeneration (48 and 96 h). Stimulation of CTP:phosphocholine cytidylyltransferase activity by partial hepatectomy occurred in both the microsomal and cytosolic fractions. The stimulatory effect in the cytosolic fraction was mainly due to an increase in the number of enzyme molecules, as demonstrated by immunotitration of the amount of cytosolic cytidylyltransferase protein.

CTP:phosphocholine cytidylyltransferase in rat lung: relationship between cytosolic and membrane forms

The purpose of these studies was to determine the properties of the membrane-bound cytidylyltransferase in adult lung and to assess the relationship between the microsomal enzyme and the two forms of cytidylyltransferase in cytosol. Microsomes, isolated by glycerol density centrifugation, contained significantly less cytidylyltransferase than microsomes isolated by differential centrifugation (11.6 +/- 3.2 vs. 30 +/- 11 nmol/min per g lung). The released activity was recovered as H-form cytidylyltransferase. Cytidylyltransferase activity was not removed from microsomes by washing of the microsomal pellet with homogenizing buffer. Triton X 100 extracted all of the cytidylyltransferase from microsomes. The extracted activity was similar to H-form. Chlorpromazine dissociated microsomal enzyme to L-form. Chlorpromazine has been shown previously to dissociate H-form to L-form. These results suggested that microsomal cytidylyltransferase existed in a form similar if not identical to cytosolic H-form. In vitro translocation experiments demonstrated that the L-form of cytidylyltransferase was the species which binds to microsomal membranes. Triton X 100 extraction of microsomes from translocations experiments removed the bound enzyme activity. Glycerol density fractionation indicated that the activity in the Triton extract was H-form cytidylyltransferase. We concluded that the active lipoprotein form of cytidylyltransferase (H-form) is the membrane-associated form of cytidylyltransferase in adult lung; that it is formed after the L-form binds to microsomal membranes and that cytosolic H-form is released from the membrane.

Dexamethasone increases the activity but not the amount of choline-phosphate cytidylyltransferase in fetal rat lung

The activity of choline-phosphate cytidylyltransferase is increased by glucocorticoids in late gestation fetal lung in association with increased phosphatidylcholine biosynthesis. Previous indirect data had suggested that the stimulatory effect of the hormone was due to activation of existing enzyme rather than synthesis of new cytidylyltransferase protein. Using a rabbit antibody raised against purified rat liver choline-phosphate cytidylyltransferase, we have now quantitated the amount of the enzyme in fetal rat lung explants cultured with and without dexamethasone. Our results show that the hormone increased the activity of the enzyme but not the amount of cytidylyltransferase protein. Thus the stimulatory effect of dexamethasone on cytidylyltransferase is due to activation of existing enzyme rather than induction of enzyme synthesis.

Glucocorticoid induction of fatty-acid synthase mediates the stimulatory effect of the hormone on choline-phosphate cytidylyltransferase activity in fetal rat lung

Fetal lung fatty-acid synthase and choline-phosphate cytidylyltransferase activities are increased by glucocorticoids. There is evidence that the hormone increases synthesis of fatty-acid synthase but only increases the catalytic activity of the cytidylyltransferase. Free fatty acids and a number of phospholipids have been reported to stimulate cytidylyltransferase activity in several organs, including the lung. We have addressed the question of whether glucocorticoid induction of fatty-acid synthase mediates the stimulatory effect of the hormone on choline-phosphate cytidylyltransferase activity. Explants of 18-day fetal rat lung were cultured for 48 h with dexamethasone and inhibitors of de novo fatty acid biosynthesis (agaric acid and hydroxycitric acid) being included in the medium for the final 20 h. Dexamethasone increased the activities of fatty acid synthase and choline-phosphate cytidylyltransferase by 84% and 60%, respectively. Agaric acid and hydroxycitric acid completely abolished the stimulatory effect of the hormone on cytidylyltransferase but not on fatty-acid synthase. The inhibitors had no effect on cytidylyltransferase activity in control cultures. Fetal lung choline-phosphate cytidylyltransferase can be maximally stimulated by inclusion of phosphatidylglycerol in the assay mixture and under this condition, cytidylyltransferase activity in control and dexamethasone-treated cultures in the presence and absence of the inhibitors were all increased to the same level. Therefore, the inhibitors did not diminish the capacity of cytidylyltransferase to be fully activated. We suggest that the glucocorticoid induction of fatty-acid synthase in fetal lung results in increased synthesis of fatty acids which in turn, either as free acids or after incorporation into phospholipids, activate choline-phosphate cytidylyltransferase.

Characterization of cytosolic forms of CTP: choline-phosphate cytidylyltransferase in lung, isolated alveolar type II cells, A549 cell and Hep G2 cells

The subcellular forms of cytidylyltransferase (EC 2.7.7.15) in rat lung, rat liver, Hep G2 cells, A549 cells and alveolar Type II cells from adult rats were separated by glycerol density centrifugation. Cytosol prepared from lung, Hep G2 cells, A549 cells and alveolar Type II cells contained two forms of the enzyme. These species were identical to the L-Form and H-Form isolated previously from lung cytosol by gel filtration. Liver cytosol contained only the L-Form. Rapid treatment of Hep G2 cells with digitonin released all of the cytoplasmic cytidylyltransferase activity. The released activity was present in both H-Form and L-Form. The molecular weight of L-Form was determined from sedimentation coefficients and Stokes radius values to be 97,690 +/- 10,175. Thus, the L-Form appears to be a dimer of the Mr 45,000 catalytic subunit. The f/f degrees value of 1.5 indicated that the protein molecule has an axial ratio of 10, assuming a prolate ellipsoid shape. The estimated molecular weight of the H-Form was 284,000 +/- 25,000. The H-Form was dissociated into L-Form by incubation of cytosol at 37 degrees C. Triton X-100 (0.1%) and chlorpromazine (1.0 mM) also dissociated the H-Form into L-Form. Western blot analysis indicated that both forms contained the catalytic subunit. An increase in Mr 45,000 subunit coincided with the increase in cytidylyltransferase activity in L-Form, which resulted from the dissociated of H-Form. The L-Form was dependent on phospholipid for activity. The H-Form was active without lipid. Phosphatidylinositol was present in the H-Form isolated from Hep G2 cells. The phosphatidylinositol dispersed when the H-Form was dissociated into L-Form. Phosphatidylinositol and phosphatidylglycerol cause L-Form to aggregate into a form similar to H-Form. Phosphatidylcholine/oleic acid (1:1 molar ratio) and oleic acid also aggregated the L-Form. Phosphatidylcholine did not produce aggregation. We conclude that the H-Form is the active form of cytidylyltransferase in cytoplasm. The H-Form appears to be a lipoprotein consisting of an apoprotein (L-Form dimer of the Mr 45,000 subunit) complexed with lipids. A change in the relative distribution of H-Form and L-Form in cytosol would alter the cellular activity and thus may be important in the regulation of phosphatidylcholine synthesis.

Effect of hyperpnea on enzymes of the CDPcholine path for phosphatidylcholine synthesis in rat lung

We have examined the activity of three enzymes in pulmonary surfactant phosphatidylcholine synthesis following the hyperpnea induced by having rats either inspire 5%CO2/13%O2/82%N2 for 24 hr or swim in thermoneutral water for 30 min. Both stimuli markedly increase frequency and tidal volume of breathing and promote the release of surfactant. Lungs were perfused to remove blood, lavaged, and then homogenized in 1 mM Hepes, 0.15M KCl at pH 7.0. The homogenate was centrifuged at 9,000 g (av) for 10 min to sediment the mitochondria and lamellar bodies and at 100,000 g (av) for 60 min to obtain the microsomal and cytosol fractions. Incubations were carried out under determined optimal conditions and zero order kinetics. Choline kinase (CK), cholinephosphate cytidylyltransferase (CP-cyT) and choline phosphotransferase (CPT) were assayed by the incorporation of [methyl-14C]choline chloride into phosphocholine, [methyl-14C]phosphocholine into CDPcholine, and [14C]CDPcholine into phosphatidylcholine, respectively. The incubation products were separated by thin-layer chromatography. Whereas both forms of hyperpnea increased the activity of CP-cyT in the microsomal fraction, they had no effect on the activity of either cytosolic CP-cyT and CK, or microsomal CPT. A similar increase in tidal volume in an isolated perfused rat lung had no effect. We conclude that, in vivo, hyperpnea increases the activity of CP-cyT, the rate-limiting enzyme in phosphatidylcholine synthesis. Whether this is due to an increase in the amount of enzyme, or of a cofactor, is unknown.

Glucose-6-phosphate phosphohydrolase activity in guinea pig liver microsomes is influenced by phosphatidylcholine. Interaction with cholesterol-enriched membranes

Guinea pig liver microsomal membranes were cholesterol-enriched by feeding guinea pigs a high-cholesterol diet. Cholesterol enrichment as well as partial lipid removal of normal native microsomes by acetone-butanol extraction resulted in 40-50% loss in activity of the glucose-6-phosphate phosphohydrolase (G-6-Pase) (EC 3.1.3.9) enzyme system. The activity was restored by supplementation of microsomal total phospholipid (PL) and its phosphatidylcholine (PC) species but not with microsomal neutral lipids, cholesterol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, sphingomyelin or diphosphatidylglycerol (cardiolipin). The activity was decreased by sodium deoxycholate but enhanced by dimethylsulfoxide. Egg-yolk PC and asolectin influenced the activity of the enzyme to the same extent as microsomal PC did. Lipid depletion and cholesterol produced an increase in Km while the Vmax was lowered. The non-linearity in the Arrhenius plot of the native microsomes was lost on lipid removal and cholesterol enrichment. The energy of activation (Ea) calculated from the continuous line was found to be lowered to the level that was observed above the break points in intact microsomes. Addition of microsomal PC to the assay system decreased the Km of the enzymatic reaction in native membranes, in partially lipid-depleted and cholesterol-enriched membranes, but did not alter the Vmax values and only marginally influenced the non-linear relationship of the Arrhenius expression of temperature dependence. The ability of immature rat liver phospholipid exchange protein to introduce alien PL into microsomal membrane was used to study the lipid dependence of G-6-Pase. Protein-catalyzed and detergent (cholate)-mediated membrane PL exchange for egg-yolk PC from the PC/cholesterol unilamellar liposomes resulted in substantial loss of enzyme activity. The discrepancies in the influence of PC on G-6-Pase were interpreted by assuming that the enzyme was a two-component system, a surface-located substrate transporter unit and a membrane integral catalytic phosphohydrolase unit. The lipid microenvironment and PL requirement in particular, could be different for the two components, although they represented a single functional unit at the time of enzymatic reaction.

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.

Regulation of phosphatidylcholine biosynthesis in Plasmodium-infected erythrocytes

Plasmodium knowlesi-infected erythrocytes efficiently incorporated choline and metabolize it into phosphatidylcholine via the de novo Kennedy pathway. No formation of either betaine or acetylcholine was detected. At physiological concentrations of external choline, isotopic equilibrium between intracellular choline and phosphocholine was reached in less than 1 h, whereas labeled phosphatidylcholine accumulated constantly, until at least 210 min. During this time, intracellular CDP-choline remained quite low compared to phosphocholine, which suggests that choline-phosphate cytidylyltransferase (EC 2.7.7.15) is the rate-limiting step of the Kennedy pathway. However, this activity was probably not saturated in situ by phosphocholine, since the external choline concentration, up to 100 microM, can regulate phosphatidylcholine biosynthesis via the level of intracellular phosphocholine. This was corroborated by the respective velocities and affinity characteristics of the three enzymatic steps involved in the Kennedy pathway. These results, together with the localization of both choline metabolites and enzyme activities, provide a precise scheme of the dynamics of de novo phosphatidylcholine biosynthesis. Concerning the alternative pathway for phosphatidylcholine biosynthesis via the methylation of phosphatidylethanolamine, we show that an increase in de novo phosphatidylcholine biosynthesis could instigate a concomitant decrease in the steps of phosphatidylethanolamine methylation, indicating that the parasite is able to modulate its phosphatidylcholine biosyntheses.

Activities of enzymes of phospholipid and fatty acid synthesis in fetal and adult rat type II pneumocytes

Although differentiated fetal and adult type II pneumocytes are ultrastructurally similar, it is not known whether there are metabolic differences between them. We measured the activities of selected enzymes of phospholipid and fatty acid synthesis in fetal and adult rat type II cells, in late gestation fetal rat lung explants and in intact lung from rat fetuses of comparable gestational age. The activity of 1-acylglycerophosphocholine acyltransferase was significantly greater in adult type II cells than in fetal type II cells, fetal explants or intact fetal lung. The activity of CDP diacylglycerol:glycerol-3-phosphate 3-phosphatidyltransferase was similar in fetal and adult type II cells, but significantly lower in explants and intact fetal lung. There was a significant positive correlation between the percentage of alveolar epithelial cells in the cultures and tissue studied and CDP diacylglycerol:glycerol-3-phosphate 3-phosphatidyltransferase activity. This suggests that the previously reported correlation between phosphatidylglycerol synthesis and the percentage of alveolar epithelial cells in various lung culture systems may be related to the activity of this enzyme. Phosphatidylglycerol synthesis and CDP diacylglycerol:glycerol-3-phosphate 3-phosphatidyltransferase activity may be metabolic markers of type II cells, whereas the acyltransferase activity may be an indicator of type II cell maturation.

Effect of 1-oleoyl-2-acetylglycerol and other lipids on phosphatidylcholine synthesis and cholinephosphate cytidylyltransferase activity in cultured type II pneumocytes

We previously reported that addition of phosphatidylglycerol to the culture medium stimulates phosphatidylcholine synthesis and cholinephosphate cytidylyltransferase activity in type II pneumocytes. In view of the known biological effects of diacylglycerols and since phosphatidylglycerol could be metabolized to diacylglycerol, we now examined the effects of diacylglycerols on the same parameters. The rate of choline incorporation into phosphatidylcholine was increased 30-60% by 10 microM phosphatidylglycerol, diolein, mixed diacylglycerols and 1-oleoyl-2-acetylglycerol (OAG). The effects of phosphatidylglycerol and OAG were not additive, suggesting a similar mechanism of action. The diacylglycerols and phosphatidylglycerol increased the activity of cholinephosphate cytidylyltransferase in type II cell sonicates by 35-50%, but had no effect on the activities of choline kinase, cholinephosphotransferase or 1-acylglycerophosphocholine acyltransferase. Again, the effects of OAG and phosphatidylglycerol on cytidylyltransferase were not additive. It is known that addition of lipids to the assay mixture increases the activity of cholinephosphate cytidylyltransferase in vitro and inclusion of the above lipids (1.1 mM) in the in vitro assay mixture increased cytidylyltransferase activity in type II cell sonicates. In addition, the stimulatory effects of OAG and of diolein, as well as of phosphatidylglycerol as reported previously, in the culture medium on cytidylyltransferase activity in type II cells were diminished or abolished when the assay was carried out in the presence of sufficient amounts of the same lipids to stimulate maximally the activity in vitro. These data show that lipids in the culture medium stimulate phosphatidylcholine biosynthesis in type II cells by direct activation of cholinephosphate cytidylyltransferase.

Di(2-ethylhexyl)phthalate enhances hepatic phospholipid synthesis in rats

The effects of di(2-ethylhexyl)phthalate, a typical peroxisomal proliferator, on the activities of key enzymes in the glycerophospholipid synthetic pathway and the incorporation of lipid precursors into liver lipids in vitro were studied periodically in rats. When di(2-ethylhexyl)phthalate was fed at the 1% level to rats, glycerol-3-phosphate acyltransferase activity increased 2-3-fold in liver homogenates and microsomes in 2-4 days. The specific activity of microsomal CTP:phosphocholine cytidylyltransferase increased by 1.5-fold, whereas the cytosolic activity was depressed. The microsomal CDPcholine:diacylglycerol cholinephosphotransferase specific activity decreased, whereas the activity in the homogenates increased, suggesting the proliferation of the hepatic endoplasmic reticulum in di(2-ethylhexyl)phthalate-treated rats. The incorporation of [1(3)-3H]glycerol or [1-14C]acetate into liver phospholipids in vitro increased in 2 days and stayed at a high level up to 12 days. The present study confirmed that di(2-ethylhexyl)phthalate induced an enhancement of phospholipid synthesis in the liver. The increase in hepatic phospholipid synthesis by this drug is presumably linked to the proliferation of peroxisomes and other intracellular membranes.

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.

Presence of apocrine epithelial antigen (AEA) in type II pneumocytes and in hyaline membranes of neonatal RDS

We have investigated the distribution of an apocrine membrane antigen (AEA) in pulmonary tissue using a rabbit antiserum raised against fat globule glycoproteins isolated from human milk. In indirect immunostaining (PAP, IF) of sections from normal lung tissue, the membranes facing the alveolar lumen of cells corresponding to the type II pneumocytes in the alveolar walls were decorated. The selective distribution of AEA to the membranes of type II pneumocytes was confirmed in double immunostaining by identification of these cells with rat antibodies against surfactant apoprotein. In fetal lung tissue, the AEA antigen was detected by the 9th week of gestation. In lung samples from newborns which had died of respiratory distress syndrome (RDS) the intra-alveolar hyaline membranes stained for the AEA antigen. SDS-PAGE of the immunoprecipitate obtained with anti-AEA serum from radiolabelled glycoprotein fraction of normal lung tissue revealed a single band of 79,000 dalton apparent molecular weight. These findings indicate that the AEA constitutes a membrane marker of the type II pneumocytes and might be involved in the secretory process of surfactant. Immunohistological evidence for the presence of AEA in the hyaline membranes of neonatal RDS is also presented.

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.

Inhibition of foetal pulmonary choline-phosphate cytidylyltransferase under conditions favouring protein phosphorylation

Choline-phosphate cytidylyltransferase (EC 2.7.7.15) activity from 25- and 29-day-foetal rabbit lungs was inhibited in both the cytosolic and the microsomal fractions by preincubation with MgATP. The inhibition of the cytosolic enzyme was greater when measured with added phosphatidylglycerol (PG) than without (78-89% versus 50-55%), whereas the inhibition of the microsomal enzyme did not exhibit this distinction (66-72% versus 60-70%). When preincubated with the buffer alone, the cytosolic enzyme was activated to a greater extent by added PG than was the microsomal enzyme (13-14-fold versus 2-3-fold). However, after preincubation with MgATP, the cytosolic enzyme was activated to a smaller extent by added PG (3-6-fold). The inhibition of the enzyme by MgATP required a preincubation and was absent when ADP or AMP was substituted for ATP. Moreover, ATP analogues such as adenosine 5'-[beta, gamma-methylene]triphosphate and adenosine 5'-[gamma-thio]triphosphate also failed to inhibit the enzyme when substituted for ATP in the preincubation. The inhibition by MgATP was not affected by including cyclic AMP in the preincubation, but Ca2+ ions alone or plus diacylglycerol in the preincubation increased the inhibition slightly. The inhibition was abolished by including an inhibitor of cyclic-AMP-dependent protein kinase in the preincubation. These observations, taken collectively, point to the inhibition of foetal pulmonary cytidylyltransferase through the phosphorylation of a protein and suggest that this key enzyme in lung surfactant production may be regulated through this mechanism.

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.

Developmental differences in activation of cholinephosphate cytidylyltransferase by lipids in rabbit lung cytosol

Lung cytosolic cholinephosphate cytidylyltransferase is activated by lipids. We examined the lipid activation pattern as a function of development in rabbit lung from 27 days gestation through term (31 days) and in the adult. The enzyme in both the fetal and adult cytosol was dependent on lipids for activity. Extraction of the cytosol with acetone/butanol virtually abolished cytidylyltransferase activity, but the activity could be restored on addition of lipids extracted with chloroform/methanol from additional cytosol. Cytosolic phospholipids from the fetal lung reactivated cytidylyltransferase but both neutral lipids and phospholipids from the adult were required. The lipids had the same effect on cytidylyltransferase activity in delipidated cytosol from either the fetus or adult so the difference in activation pattern was attributable to the lipids rather than the protein. There was a shift from the fetal to the adult lipid activation pattern as development progressed. Further, there was a significant correlation between cytidylyltransferase activities in intact cytosols from developing lung and activities in delipidated cytosol in the presence of lipids from the same animals. Although these data suggest that lipids regulate cytosolic cytidylyltransferase activity in developing lung their physiological significance remains to be established.