Analysis of the effects of fatty acids and related compounds on the synthesis of phosphatidylcholine in lymphocytes

The CCT promoter directs high-level transgene expression in distal lung epithelial cell lines

Gene therapy requires the presence of a robust and yet small promoter to drive high-level expression of desired proteins. In comparative analysis, we investigated the promoter strength of the CTP:phosphocholine cytidylyltransferase promoter (CCT alpha) with other commonly used promoters, which were all cloned into a similar background vector (PGL3 basic). Transient promoter-reporter assays in murine lung epithelial (MLE-12) cells revealed that the core CCT alpha promoter (240 bp) was observed to exhibit a 40-fold, 8-fold, and 3-fold higher level of activity compared with the simian virus 40, human cytomegalovirus, and Rous sarcoma virus promoters, respectively. The CCT alpha promoter was significantly more active than the Clara cell 10, thymidine kinase, and phosphoglycerate kinase promoters. This pattern of high-level expression for CCT alpha was detected primarily in cell lines of distal lung epithelial origin (MLE-12, RLE, H441) and was reduced in other cell lines (A549, CHO, HepG 2). CCT alpha promoter-reporter activity, CCT alpha transcript levels, and immunoreactive protein levels increased significantly in the presence of all-trans retinoic acid. The CCT alpha promoter, in a retinoic acid-inducible manner, efficiently directed expression of murine erythropoietin in MLE-12 cells. Collectively, these observations suggest that the CCT alpha construct might be useful to drive high-level, regulatable expression of heterologous proteins in alveolar epithelia.

Studies on the regulation of CTP:phosphocholine cytidylyltransferase using permeabilized HEP G2 cells: evidence that both active and inactive enzyme are membrane-bound

To obtain more insight into the mechanisms regulating CTP:phosphocholine cytidylyltransferase (CT), we determined the effect of oleate treatment on the rate of CT release from permeabilized Hep G2 cells and the distribution of the CT remaining in the permeabilized cells. When we permeabilized untreated cells in pH 7.5 buffer containing 0.15 M KCl, the rate of CT release was much slower than the release of lactate dehydrogenase. Oleate treatment caused a further decrease in CT release from cells. In untreated cells, 70-80% of the CT remaining in cells 10 min after permeabilization was recovered as soluble CT. Oleate treatment increased the amount of bound CT but over 50% of the CT in cells 10 min after permeabilization was recovered as soluble CT. In both control and oleate-treated cells, the increase in CT release with time correlated with a decrease in the amount of CT recovered from permeabilized cells as soluble CT. These results suggested that CT existed in a form that was not immediately available for release from permeabilized cells, but was recovered in the soluble fraction after cell disruption. When cells were permeabilized in 10 mM imidazole-20% glycerol-5 mM Mg2+ pH 6.5, over 80% of CT in control and over 90% of CT in oleate-treated cells was recovered bound to the particulate fraction. Essentially no CT was released from the cells. The recovery of CT in the particulate fraction required Mg2+ to be present when permeabilization was initiated. The addition of Mg2+, after cells were disrupted, did not increase CT in the particulate fraction. In untreated cells, 50% of bound CT was active. Oleate treatment increased the amount of active CT in the particulate fraction to over 70% of total. About 50% of particulate CT in untreated cells but only 15% in oleate-treated cells was extracted with 0.15 M KCl. Inactive CT was preferentially extracted by KCl. The bound CT was recovered in isolated nuclei. Overall, the results suggested that both inactive and active CT are bound to nuclear membranes, and that the activation of CT involves conversion of CT loosely bound to membrane to a form more tightly bound to membranes perhaps by hydrophobic interaction with phospholipids. This model does not involve translocation from a soluble pool.

Reversible translocation of CTP:phosphocholine cytidylyltransferase from cytosol to membranes in the adult bovine liver around parturition

The key regulatory enzyme of phosphatidylcholine (PC) synthesis, CTP:phosphocholine cytidylyltransferase (CT), is known to be activated in vitro by translocation from soluble to particulate fractions of the cell. In the present study the periparturient cow was chosen as a model to investigate whether translocation of CT can contribute to the regulation of PC synthesis in vivo. Between parturition and 1.5 weeks post-partum, the cytosolic CT activity in the liver of the adult animal decreased 1.9-fold, and this correlated with a 1.8-fold increase in microsomal CT activity. At that time, microsomal CT activity started to decline again whereas the cytosolic activity rose concomitantly until both activities reached their pre-partum values at 8 weeks post-partum. The activities of soluble and membrane-bound CTP:phosphoethanolamine cytidylyltransferase (ET), the analogous enzyme in the CDP-ethanolamine pathway, did not change significantly throughout this period. Whereas hepatic PC concentrations declined until about 2 weeks post-partum and thereafter gradually returned to pre-partum levels, the PC levels in very-low-density-lipoproteins, started to rise 2 weeks after the partus reaching a maximum of 219% of the original value at 8 weeks post-partum. These results strongly suggest that there is a reversible redistribution of CT between cytosol and membranes in a physiologically relevant animal model, supporting the concept that translocation of CT is occurring in vivo.

Activators of protein kinase A decrease the levels of free arachidonic acid in osteoblasts via stimulation of phosphatidylcholine and phosphatidylethanolamine synthesis

In order to examine the role of protein kinase A (PKA) in the regulation of arachidonic acid availability, the interaction between cAMP agonists and the G protein activator AIF4- in their effects on phospholipid metabolism were measured in MC3T3-E1 osteoblasts. We show that forskolin and 8-brcAMP, activators of PKA, amplify the AIF4(-)-induced stimulation of phosphatidylinositol-specific phospholipase C (phosphatidylinositol inositolphosphohydrolase; EC 3.1.4.3), measured by the formation of [3H]inositol phosphates in prelabeled cells. However, the AIF4(-)-stimulated production of 1,2-diacylglycerols and the release of [3H]arachidonic acid ([3H]AA) were inhibited 50-75% by forskolin and 8-bromocAMP. Furthermore, pretreatment with PKA activators prevented much of the AIF4(-)-induced loss of [3H]AA from phosphatidylcholine and phosphatidylethanolamine in prelabeled osteoblasts. In addition, in the absence of AIF4-, forskolin was found to stimulate the incorporation of [3H]AA and [32P]orthophosphoric acid selectively into these two major phospholipids and selectively increased their mass. The effects of forskolin and 8-BrcAMP on the levels of free [3H]AA were completely reversed by pretreatment with the PKA inhibitor H-89. Therefore, our findings suggest that the activation of cAMP-dependent protein kinase can reduce the availability of free arachidonic acid for prostaglandin synthesis in osteoblast cells by stimulating its reesterification via phospholipid resynthesis.

Effects of altered phosphorylation sites on the properties of CTP:phosphocholine cytidylyltransferase

To investigate the role of phosphorylation and dephosphorylation in modulating the activity and location of CTP:phosphocholine cytidylyltransferase, we used site-directed mutagenesis to construct four mutant forms of cytidylyltransferase. These forms were 5SP-->AP, in which five of the seven Ser-Pro sequences were converted to Ala-Pro; 7SP-->AP, in which all of the seven Ser-Pro sequences converted to Ala-Pro; 16S-->A, in which all sixteen Ser residues that can be phosphorylated in wild type cytidylyltransferase were converted to Ala; and 16S-->E, in which all sixteen Ser residues were converted to Glu. The mutant enzymes were expressed in the strain 58 Chinese hamster ovary cell line, which is temperature-sensitive for growth and cytidylyltransferase activity. All mutant enzyme forms were enzymatically as active as the wild type when assayed under optimal conditions. In untreated cells, more of the Ser-->Ala mutants were membrane-associated than in cells expressing wild type enzyme, consistent with the phosphorylation state of the enzyme affecting its affinity for membranes. About half of the 16S-->A mutant remained soluble, however, indicating that dephosphorylation alone does not trigger membrane association. Although the amount of membrane-associated enzyme in the 16S-->A mutant was about 10-fold greater than that of wild type, phosphatidylcholine synthesis was increased by only about 75%, suggesting that membrane association does not necessarily cause full activation. All mutant forms, including the 16S-->E mutant, translocated to the particulate fraction upon oleate treatment, indicating that a high negative charge in the phosphorylation region does not preclude association of cytidylyltransferase with membranes. All mutant enzymes were able to support growth of strain 58 at 40 degrees C, and the rate of phosphatidylcholine synthesis was not greatly altered in the cell lines expressing mutant cytidylyltransferase forms. These results are consistent with a role for phosphorylation in the equilibrium distribution of cytidylyltransferase but suggest that changes in enzyme activity and location are not triggered exclusively by changes in the phosphorylation state.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Regulation of CTP: phosphocholine cytidylyltransferase in HepG2 cells: effect of choline depletion on phosphorylation, translocation and phosphatidylcholine levels

We studied the effect of choline depletion on the biosynthesis of phosphatidylcholine (PC) and the distribution and phosphorylation of cytidylyltransferase (CT) in HepG2 cells. Phosphocholine concentrations decreased within 24 h of choline depletion to values less than 2% of controls. The incorporation of [3H]glycerol into PC was reduced in choline-depleted (CD) cells. The apparent turnover of PC was similar in CD and choline-supplemented (CS) cells (T1/2 = 20 h). The methylation pathway for PC synthesis increased nearly 10-fold in CD cells. Cell growth was similar in CD and CS cells. Over 95% of CT activity in CS cells was in the soluble pool. Choline depletion resulted in a progressive decrease in CT activity and immunodetected enzyme in the soluble pool and a corresponding increase in membrane CT over a 48-h period. Choline supplementation of CD cells caused a rapid release of membrane CT (complete release by 3 h). Two phosphorylated forms of CT were identified. One form contained a higher level of phosphorylation (HPCT) than the other form (LPCT). HPCT migrated slightly slower than LPCT on SDS gels. CD cells contained only LPCT in both soluble and membrane pools. CS cells contained only HPCT. During choline depletion PC content decreased nearly 20% but CT binding did not occur until LPCT was generated in cytosol. Conversely, choline supplementation released LPCT into cytosol and HPCT was formed only after the release. We conclude that both the induction of binding sites, perhaps by depletion of PC and dephosphorylation of HPCT to LPCT, are required for CT translocation to membranes. The release of CT from membranes is initiated by changes in membrane binding sites followed by trapping of the CT in the soluble pool by phosphorylation of LPCT to HPCT.

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.

Reversible translocation of cytidylyltransferase between cytosol and endoplasmic reticulum occurs within minutes in whole cells

Addition of oleic acid to Krebs II cells induced a rapid incorporation of [3H]choline into phosphatidylcholine, since 500 microM of the fatty acid stimulated choline incorporation by 5-fold over the control after 5 min of incubation. In fact, a noticeable increase in phosphatidylcholine labelling could be monitored immediately after 1 min of cell incubation with [3H]choline, at which time 50% of cytosolic cytidylyltransferase activity (EC 2.7.7.15), the regulatory enzyme of phosphatidylcholine synthesis, was translocated on to membranes. Non-esterified [3H]oleic acid content was also increased in the same range of time in the particulate fraction. Subcellular fractionation indicated that endoplasmic reticulum was the unique binding site for cytidylyltransferase even after 1 min of incubation. Also, [3H]oleic acid accumulated mainly in the same internal membrane. Addition of exogenous albumin to cells prelabelled with [3H]oleic acid induced the release of 50% of membrane-bound cytidylyltransferase activity within 1 min, together with a decrease in unesterified oleic acid in the same membrane. Although total depletion of oleic acid was obtained, total release of membrane-bound cytidylyltransferase was not. The remaining minor pool of membrane-bound cytidylyltransferase was not affected by cell incubation with dibutyryl cyclic AMP, suggesting that this pool was neither regulated by fatty acid nor modulated by cyclic-AMP-dependent protein phosphorylation. Addition of [3H]oleic acid directly to an homogenate led to a less specific accumulation of the fatty acid in the endoplasmic reticulum, but cytidylyltransferase remained exclusively associated with this membrane. We concluded that in vivo translocation of cytidylyltransferase provoked by oleic acid concerns one specific pool of the enzyme distinct from the enzyme firmly bound to endoplasmic reticulum, but other factor(s) than fatty acid seem to be required to explain the specificity of endoplasmic reticulum for cytidylyltransferase binding.

Microsomal CTP:choline phosphate cytidylyltransferase: kinetic mechanism of fatty acid stimulation

Fatty acids are known to cause an increase in the incorporation of radioactive choline into phosphatidylcholine. A coincident increase in membrane cytidylyltransferase activity is well documented. The purpose of the present studies was to determine the direct effects of oleic acid on the kinetic properties of membrane cytidylyltransferase. An examination of the reaction characteristics of membrane cytidylyltransferase revealed that membranes from adult rat lung contained high CTPase activity. This activity prevented the determination of reaction velocities at low CTP concentrations. The CTPase activity was blocked by the addition of ADP or ATP to the reaction. The addition of 6.0 mM ADP to the assay mixture enabled us to determine the effect of oleate on the CTP Km. Oleate (122 microM) caused a significant decrease in CTP Km for microsomal cytidylyltransferase (0.99 mM to 0.33 mM) and H-Form cytidylyltransferase (1.04 mM to 0.27 mM). Oleate did not decrease the CTP Km for L-Form cytidylyltransferase. Oleate had no effect on the choline phosphate Km in microsomal, H-Form or L-Form cytidylyltransferase. Oleate also increased the Vmax for cytidylyltransferase. The increase was dependent upon the concentration of oleate with a maximal increase of 50-60% at 100-130 microM oleate. We conclude that oleate has a direct stimulatory effect on cytidylyltransferase when it is in the active form (membrane bound or H-Form lipoprotein complex). We suggest that the kinetic effects operate synergistically with other regulatory mechanisms such as translocation or conversion of inactive to active species. The direct effect of oleate on the cytidylyltransferase may be an important regulatory mechanism when CTP concentrations are limiting.

Cytidylyltransferase translocation onto endoplasmic reticulum and increased de novo synthesis without phosphatidylcholine accumulation in Krebs-II ascite cells

Addition of oleic acid to Krebs-II cells stimulated by 9-fold [3H]choline incorporation into choline glycerophospholipids without affecting the selective incorporation of the precursor into diacyl subclass (90% of total [3H]choline glycerophospholipids). The total activity of cytidylyltransferase (E.C. 2.7.7.15), the regulatory enzyme of choline glycerophospholipid synthesis, was increased in the particulate fraction at the expense of cytosol. Free [3H]oleic acid was also associated with the particulate fraction. Subcellular fractionation of membranes on Percoll gradient, indicated that the endoplasmic reticulum, which contained 90% of total cell free oleic acid, was the unique target for the translocation of cytidylyltransferase. [3H]oleic acid was incorporated almost exclusively into phosphatidylcholine and corresponded to a synthesis of 9 nmol/h per 10(6) cells. Based on [3H]choline incorporation a net synthesis of 22 nmol/h per 10(6) cells was determined. However, oleic acid treatment did not change the total amount of phosphatidylcholine (45 nmol/10(6) cells) and other phospholipids; also no modification in the subcellular distribution of phospholipids was observed. It is concluded that the stimulation of the de novo synthesis of phosphatidylcholine which involves translocation of cytidylyltransferase onto the endoplasmic reticulum, is accompanied by a renewal of their polar head group. Also exogenous oleic acid induces an enhanced fatty acid turnover, highly specific for phosphatidylcholine. Therefore, Krebs-II cells exhibited a high degree of regulation of their phosphatidylcholine content, suggesting a parallel stimulation of both synthesis and degradation.

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.

Effect of hypoxia on phosphatidylcholine biosynthesis in the isolated hamster heart

In hamster heart, the majority of the phosphatidylcholine is synthesized via the CDP-choline pathway, and the rate-limiting step of this pathway is catalysed by CTP:phosphocholine cytidylyltransferase (EC 2.7.7.15). We have shown previously [Choy (1982) J. Biol. Chem. 257, 10928-10933] that, in the myopathic heart, the level of cardiac CTP was diminished during the development of the disease. In order to maintain the level of CDP-choline, and consequently the rate of phosphatidylcholine biosynthesis, cardiac cytidylyltransferase activity was increased. However, it was not clear if the same compensatory mechanism would occur when the cardiac CTP level was decreased rapidly. In this study, hypoxia of the hamster heart was produced by perfusion with buffer saturated with 95% N2. The heart was pulse-labelled with radioactive choline and then chased with non-radioactive choline for various periods under hypoxic conditions. There was a severe decrease in ATP and CTP levels within 60 min of hypoxic perfusion, with a corresponding fall in the rate of phosphatidylcholine biosynthesis. Analysis of the choline-containing metabolites revealed that the lowered ATP level did not affect the phosphorylation of choline to phosphocholine, but the lower CTP level resulted in the decreased conversion of phosphocholine to CDP-choline. Determination of enzyme activities revealed that hypoxic treatment resulted in the enhanced translocation of cytidylyltransferase from the cytosolic to the microsomal form. This enhanced translocation was probably caused by the accumulation of fatty acids in the heart during hypoxia. We postulate that the enhancement of translocation of the cytidylyltransferase to the microsomal form (a more active form) is a mechanism by which the heart can compensate for the decrease in CTP level during hypoxia in order to maintain phosphatidylcholine biosynthesis.

Synergistic activation of CTP:phosphocholine cytidylyltransferase by phosphatidylethanolamine and oleic acid

CTP:phosphocholine cytidylyltransferase present in rat liver cytosol was activated almost 30-fold when assayed in the presence of liposomes containing 60 mole % dioleoyl phosphatidylethanolamine (DOPE). During the assay, some of the DOPE was degraded to lysoPE and oleic acid. Whereas cytidylyltransferase activity was not affected when assayed in the presence of liposomes containing lysoPE, liposomes containing oleic acid activated the enzyme. Activation by oleic acid could be eliminated by the addition of fatty acid-free bovine serum albumin (BSA) to the assay. When cytidylyltransferase activity was measured in the presence of both BSA and liposomes containing DOPE, enzyme activity was increased almost 20-fold, as compared with assays performed in the absence of added lipid. The 1.5-fold difference in cytidylyltransferase activity observed when cytosol was assayed with DOPE containing liposomes in the absence or presence of BSA (30-fold stimulation vs 20-fold stimulation) cannot be explained by the loss of activation attributable to oleic acid alone. Activation of the enzyme in the presence of liposomes containing DOPE and oleic acid is several-fold greater than the sum of the activations caused by the individual compounds. These data suggest that PE and oleic acid act synergistically in activating the 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.

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.

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.

Concurrent changes in Dunaliella salina ultrastructure and membrane phospholipid metabolism after hyperosmotic shock

Hyperosmotic shock, induced by raising the NaCl concentration of Dunaliella salina medium from 1.71 to 3.42 M, elicited a rapid decrease of nearly one-third in whole cell volume and in the volume of intracellular organelles. The decrease in cell volume was accompanied by plasmalemma infolding without overall loss of surface area. This contrasts with the dramatic increase in plasmalemma surface area after hypoosmotic shock (Maeda, M., and G. A. Thompson. 1986. J. Cell Biol. 102:289-297). Although plasmalemma surface area remained constant after hyperosmotic shock, the nucleus, chloroplast, and mitochondria lost membrane surface area, apparently through membrane fusion with the endoplasmic reticulum. Thus the endoplasmic reticulum serves as a reservoir for excess membrane during hyperosmotic stress, reversing its role as membrane donor to the same organelles during hypoosmotically induced cell expansion. Hyperosmotic shock also induced rapid changes in phospholipid metabolism. The mass of phosphatidic acid dropped to 56% of control and that of phosphatidylinositol 4,5-bisphosphate rose to 130% of control within 4 min. Further analysis demonstrated that within 10 min after hyperosmotic shock, there was 2.5-fold increase in phosphatidylcholine turnover, a twofold increase in lysophosphatidylcholine mass, a four-fold increase in lysophosphatidate mass, and an elevation in free fatty acids to 124% of control, all observations suggesting activation of phospholipase A. The observed biophysical and biochemical phenomena are likely to be causally interrelated in providing mechanisms for successful accommodation to such severe osmotic extremes.

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.

Translocation of CTP: phosphocholine cytidylyltransferase from cytosol to membranes in HeLa cells: stimulation by fatty acid, fatty alcohol, mono- and diacylglycerol

Addition of oleate, oleyl alcohol, or palmitate to HeLa cell medium resulted in a rapid stimulation of PC synthesis and activation of CTP: phosphocholine cytidylyltransferase. Stimulation was optimal with 0.35 mM oleate, 0.3 mM oleyl alcohol and 5 mM palmitate, or 1 mM palmitate if EGTA were added to the medium. The cytidylyltransferase was activated by translocation of the inactive cytosolic form to membranes. In untreated cells approx. 30% of the total cytidylyltransferase was membrane bound, while in treated cells, 80-90% was membrane associated. Addition of bovine serum albumin (10 mg/ml) to cells previously treated with oleate (0.35 mM) rapidly removed cellular fatty acid, and the membrane-bound cytidylyltransferase activity returned to approx. 30%. Similar results were obtained by extraction of membranes with albumin in vitro. Although 95% of the free fatty acid was extracted, 30-40% of the membrane cytidylyltransferase remained bound. Translocation of cytidylyltransferase between isolated cytosol and microsomal fractions was promoted by addition of oleate, palmitate, oleyl alcohol, and monoolein. Addition of diacylglycerol, lysophosphatidylcholine, lysophosphatidylethanolamine, calcium palmitate, and detergents such as Triton X-100, cholate or Zwittergent did not stimulate translocation of the enzyme. Addition of oleoyl-CoA promoited translocation, however, 40% of it was hydrolyzed releasing free oleic acid. Cytosolic cytidylyltransferase bound to microsomes pre-treated with phospholipase C, which had 7-fold elevated diacylglycerol content. Fatty acid-promoted translocation was blocked by Triton X-100, but not by 1 M KCl. These results suggest that a variety of compounds with differing head group size and charge, and number of hydrocarbon chains can function as translocators, and that hydrophobic rather than ionic interactions mediate the binding of cytidylyltransferase to membranes.

Binding of CTP: phosphocholine cytidylyltransferase to large unilamellar vesicles

We have studied the binding of CTP: phosphocholine cytidylyltransferase from HeLa cell cytosol to large unilamellar vesicles of egg phosphatidylcholine (PC) or HeLa cell phospholipids that contain various amounts of oleic acid. A fatty acid/phospholipid molar ratio exceeding 10% was required for CTP: phosphocholine cytidylyltransferase binding to liposomes. At a fatty acid/phospholipid molar ratio of 1; 85% of the cytosolic CTP: phosphocholine cytidylyltransferase was bound. The enzyme also bound to liposomes with at least 20 mol% palmitic acid, monoolein, diolein or oleoylacetylglycerol. Oleoyl-CoA did not promote enzyme binding to liposomes. Binding to oleate-PC vesicles was blocked by Triton X-100 but not by 1 M KCl, and was reversed by incubation of the vesicles with bovine serum albumin. Cytidylyltransferase bound to egg PC vesicles that contained 33 mol% oleic acid equally well at 4 degrees C and 37 degrees C. The enzyme also bound to dimyristoyl- and dipalmitoylphosphatidylcholine vesicles containing oleic acid at temperatures below the phase transition for these liposomes. Binding of the cytidylyltransferase to egg PC vesicles containing oleic acid, monoolein, oleoylacetylglycerol or diolein resulted in enzyme activation, as did binding to dipalmitoylPC-oleic acid vesicles. However, binding to egg PC-palmitic acid vesicles did not fully activate the transferase. Various mechanisms for cytidylyltransferase interaction with membranes are discussed.

Role of fatty acid structure in the reversible activation of phosphatidylcholine synthesis in lymphocytes

Fatty acids rapidly accelerate (1.5-7.0-fold) the incorporation of [methyl-3H]choline chloride into the phosphatidylcholine fraction of bovine lymphocyte lipids. This ability of fatty acids to activate choline phospholipid synthesis has been correlated with certain structural features of fatty acids. Mono- and polyenoic unsaturated fatty acids of 18 and 20 carbons in length are highly active, whereas their saturated analogues are nearly inactive. Among the unsaturated fatty acids, the cis-isomers are active, while the trans-isomers are relatively ineffective. The delayed addition of bovine serum albumin (5 mg/ml) and other lipid-binding proteins to activated cells rapidly counteracts the lipid effects. The activated state of the cell membrane thus appears to be a dynamic one, requiring the continued interaction of the fatty acid with a lipid-sensitive target molecule of the cell surface that in turn appears to coordinate the enzymatic components of this pathway.