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

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.

CTP:cholinephosphate cytidylyltransferase in human and rat lung: association in vitro with cytoskeletal actin

CTP:cholinephosphate cytidylyltransferase activities were compared in saline homogenates of immature fetal (15-16 weeks gestation) and adult human lung. There were no differences in subcellular enzyme distribution, in Vmax activity, or in the phosphatidylglycerol-mediated stimulation of soluble enzyme activity. These results provide no support for a developmental translocation of cytidylyltransferase from a cytosolic to a microsomal location in human lung, such as that proposed to accompany the maturation of pulmonary surfactant phosphatidylcholine biosynthesis in rat. Soluble cytidylyltransferase activity from human but not rat lung was increased after manipulation in vitro. Resolution of human H form (greater than 10(3) kDa) and L form (200 kDa) enzyme by gel filtration led to an activity increase of 200%. Incubation at 37 degrees C for 2 h increased soluble enzyme recovery, although prior centrifugal removal of generated actin-rich aggregates was necessary in adult lung fractions. In contrast, 85% of soluble rat lung cytidylyltransferase was actin aggregate-associated after incubation. The apparent heteroassociation of rat and human lung enzyme with actin in the presence of poly(ethylene glycol) at 4 degrees C strongly suggested close in vitro and potential in vivo linkage. A partial co-purification of adult human lung cytidylyltransferase with actin was also consistent with this idea. We propose that some reported cytidylyltransferase translocation phenomena may be mediated by cytoskeletal interactions in vitro.

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.

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

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

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.

Cytosol-stimulated remodeling of phosphatidylcholine in rat lung microsomes

When 600 X g supernatants of 10% (w/v) rat lung homogenates were incubated with CDP[Me-14C]choline both saturated and unsaturated species of phosphatidylcholine were formed from endogenous diacylglycerols. The percentage radioactivity in the disaturated species of total phosphatidylcholine increased with time from 12% after 5 min to 30% after 60 min incubation. In similar experiments with 20 000 X g supernatants, the increase in the disaturated species of microsomal phosphatidylcholine was from 25 to 37% over the same time period. In incubations of isolated microsomes in buffer, the percent of 14C label in disaturated phosphatidylcholine remained constant at a level of 25%. To investigate a possible role of cytosolic factor(s) in the increase in the percentage of disaturated phosphatidylcholine with time, microsomes were prelabeled by incubation in buffer with CDP[Me-14C]choline to give a fixed ratio of radioactive saturated and unsaturated phosphatidylcholine species. When the reisolated microsomes were incubated in buffer, the distribution of radioactivity over saturated and unsaturated species remained constant. In contrast, incubation of prelabeled microsomes in the presence of cytosol caused an increase in the percent radioactivity in saturated phosphatidylcholines from a starting value of 18 to 30% after 60 min incubation, while leaving total phosphatidylcholine radioactivity unaffected. These results indicate a remodeling of phosphatidylcholine under the influence of a cytosolic factor(s). Evidence is presented that suggests that Ca2+-independent-cytosolic phospholipase A2 activity as well as a microsomal ATP-independent CoA-mediated acyltransferase activity might contribute to this remodeling. The cytosol donates the necessary CoA for this acyl transfer as well as saturated acyl-CoA for the reacylation of lysophosphatidylcholine.

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.