cAMP analogues inhibit phosphatidylcholine biosynthesis in cultured rat hepatocytes

The epigenetic drug 5-azacytidine interferes with cholesterol and lipid metabolism

DNA methylation and histone acetylation inhibitors are widely used to study the role of epigenetic marks in the regulation of gene expression. In addition, several of these molecules are being tested in clinical trials or already in use in the clinic. Antimetabolites, such as the DNA-hypomethylating agent 5-azacytidine (5-AzaC), have been shown to lower malignant progression to acute myeloid leukemia and to prolong survival in patients with myelodysplastic syndromes. Here we examined the effects of DNA methylation inhibitors on the expression of lipid biosynthetic and uptake genes. Our data demonstrate that, independently of DNA methylation, 5-AzaC selectively and very potently reduces expression of key genes involved in cholesterol and lipid metabolism (e.g. PCSK9, HMGCR, and FASN) in all tested cell lines and in vivo in mouse liver. Treatment with 5-AzaC disturbed subcellular cholesterol homeostasis, thereby impeding activation of sterol regulatory element-binding proteins (key regulators of lipid metabolism). Through inhibition of UMP synthase, 5-AzaC also strongly induced expression of 1-acylglycerol-3-phosphate O-acyltransferase 9 (AGPAT9) and promoted triacylglycerol synthesis and cytosolic lipid droplet formation. Remarkably, complete reversal was obtained by the co-addition of either UMP or cytidine. Therefore, this study provides the first evidence that inhibition of the de novo pyrimidine synthesis by 5-AzaC disturbs cholesterol and lipid homeostasis, probably through the glycerolipid biosynthesis pathway, which may contribute mechanistically to its beneficial cytostatic properties.

Phosphatidylcholine synthesis for lipid droplet expansion is mediated by localized activation of CTP:phosphocholine cytidylyltransferase

Lipid droplets (LDs) are cellular storage organelles for neutral lipids that vary in size and abundance according to cellular needs. Physiological conditions that promote lipid storage rapidly and markedly increase LD volume and surface. How the need for surface phospholipids is sensed and balanced during this process is unknown. Here, we show that phosphatidylcholine (PC) acts as a surfactant to prevent LD coalescence, which otherwise yields large, lipolysis-resistant LDs and triglyceride (TG) accumulation. The need for additional PC to coat the enlarging surface during LD expansion is provided by the Kennedy pathway, which is activated by reversible targeting of the rate-limiting enzyme, CTP:phosphocholine cytidylyltransferase (CCT), to growing LD surfaces. The requirement, targeting, and activation of CCT to growing LDs were similar in cells of Drosophila and mice. Our results reveal a mechanism to maintain PC homeostasis at the expanding LD monolayer through targeted activation of a key PC synthesis enzyme.

Phosphatidylcholine biosynthesis via CTP:phosphocholine cytidylyltransferase 2 facilitates neurite outgrowth and branching

Hallmarks of neuronal differentiation are neurite sprouting, extension, and branching. We previously showed that increased expression of CTP:phosphocholine cytidylyltransferase beta2 (CTbeta2), an isoform of a key phosphatidylcholine (PC) biosynthetic enzyme, accompanies neurite outgrowth (Carter, J. M., Waite, K. A., Campenot, R. B., Vance, J. E., and Vance, D. E. (2003) J. Biol. Chem. 278, 44988-44994). CTbeta2 mRNA is highly expressed in the brain. We show that CTbeta2 is abundant in axons of rat sympathetic neurons and retinal ganglion cells. We used RNA silencing to decrease CTbeta2 expression in PC12 cells differentiated by nerve growth factor. In CTbeta2-silenced cells, numbers of primary and secondary neurites were markedly reduced, suggesting that CTbeta2 facilitates neurite outgrowth and branching. However, the length of individual neurites was significantly increased, and the total amount of neuronal membrane was unchanged. Neurite branching of PC12 cells is known to be inhibited by activation of Akt and promoted by the Akt inhibitor LY294002. Our experiments showed that LY294002 increases neurite sprouting and branching in control PC12 cells but not in CTbeta2-deficient cells. CTbeta2 was not phosphorylated in vitro by Akt. However, inhibition of Cdk5 by roscovitine blocked CTbeta2 phosphorylation and reduced neurite outgrowth and branching. These results highlight the importance of CTbeta2 in neurons for promoting neurite outgrowth and branching and represent the first identification of a lipid biosynthetic enzyme that facilitates these functions.

Sp-1 Binds Promoter Elements That Are Regulated by Retinoblastoma and Regulate CTP:Phosphocholine Cytidylyltransferase-α Transcription

The retinoblastoma (Rb) protein is implicated in transcriptional regulation of at least five cellular genes. Co-transfection of Rb and truncated promoter constructs has defined a discrete element (retinoblastoma control element (RCE)) within the promoters of each of these genes as being necessary for Rb-mediated transcriptional control. In the present report we demonstrate that two RCEs identified within the CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) proximal promoter are essential to promote transcription. Mutations that abolished each RCE markedly decreased CTalpha transcription. Co-transfection of Rb and truncated promoter constructs demonstrated that Rb regulates CTalpha expression by different mechanisms depending on the phase of the cell cycle. The regulation of CTalpha expression by Rb required both the Sp1 and the RCEs. Maximal expression occurred when both Rb and Sp1 were overexpressed. Moreover, RCEs were required for Rb association with the DNA. This regulatory mechanism alters CTalpha activity and thereafter changes PC availability and cell physiology. This is the first report demonstrating not only that surrounding Sp1 binding sites alter regulation mediated by Rb, but also that the expression of a gene involved in PC biosynthesis shares a common regulatory pathway with genes responsible for cell growth and differentiation.

Choline redistribution during adaptation to choline deprivation

Choline is an important nutrient for mammals. Choline can also be generated by the catabolism of phosphatidylcholine synthesized in the liver by the methylation of phosphatidylethanolamine by phosphatidylethanolamine N-methyltransferase (PEMT). Complete choline deprivation is achieved by feeding Pemt(-)(/)(-) mice a choline-deficient diet and is lethal due to liver failure. Mice that lack both PEMT and MDR2 (multiple drug-resistant protein 2) successfully adapt to choline deprivation via hepatic choline recycling. We now report another mechanism involved in this adaptation, choline redistribution. Normal levels of choline-containing metabolites were maintained in the brains of choline-deficient Mdr2(-)(/)(-)/Pemt(-)(/)(-) mice for 90 days despite continued choline consumption via oxidation. Choline oxidase activity had not been previously detected in the brain. Plasma levels of choline were also maintained for 90 days, whereas plasma phosphatidylcholine levels decreased by >60%. The injection of [(3)H]choline into Mdr2(-)(/)(-)/Pemt(-)(/)(-) mice revealed a redistribution of choline among tissues. Although CD-Pemt(-)(/)(-) mice failed to adapt to choline deprivation, choline redistribution was also initiated in these mice. The data suggest that adaptation to choline deprivation is not restricted to liver via choline recycling but also occurs in the whole animal via choline redistribution.

Phosphatidylcholine homeostasis and liver failure

In mammals, the only endogenous pathway for choline biosynthesis is the methylation of phosphatidylethanolamine to phosphatidylcholine (PC) by phosphatidylethanolamine N-methyltransferase (PEMT) coupled to PC degradation. Complete choline deprivation in mice by feeding Pemt(-/-) mice a choline-deficient (CD) diet decreases hepatic PC by 50% and is lethal within 5 days. PC secretion into bile is mediated by a PC-specific flippase, multiple drug-resistant protein 2 (MDR2). Here, we report that mice that lack both PEMT and MDR2 and are fed a CD diet survive for >90 days. Unexpectedly, the amount of PC also decreases by 50% in the livers of Mdr2(-/-)/Pemt(-/-) mice. The Mdr2(-/-)/Pemt(-/-) mice adapt to the severe choline deprivation via choline recycling by induction of phospholipase A(2), choline kinase, and CTP:phosphocholine cytidylyltransferase activities and by a strikingly decreased expression of choline oxidase. The ability of Mdr2(-/-)/Pemt(-/-) mice to survive complete choline deprivation suggests that acute lethality in CD-Pemt(-/-) mice results from rapid depletion of hepatic PC via biliary secretion.

A choline-deficient diet in mice inhibits neither the CDP-choline pathway for phosphatidylcholine synthesis in hepatocytes nor apolipoprotein B secretion

Phosphatidylcholine is a major component of very low density lipoproteins (VLDLs) secreted by the liver. Hepatic phosphatidylcholine is synthesized from choline via the CDP-choline pathway and from the phosphatidylethanolamine N-methyltransferase pathway. Elimination of the methyltransferase in male mice reduces hepatic VLDL secretion. Our objective was to determine whether inhibition of the CDP-choline pathway for phosphatidylcholine synthesis (by restricting the supply of choline) also impaired VLDL secretion. In mice fed a choline-deficient (CD), compared with a choline-supplemented, diet for 21 days, the amounts of plasma apolipoproteins (apo) B100 and B48 were reduced and the liver triacylglycerol content was increased. Hepatocytes were isolated from male mice that had been fed the CD diet for 3 or 21 days, and the cells were incubated with or without choline. The secretion of apoB100 and B48 from CD hepatocytes was not reduced, and triacylglycerol secretion was only modestly decreased, compared with that from cells supplemented with choline. Remarkably, in light of widely held assumptions, the rate of phosphatidylcholine synthesis from the CDP-choline pathway was not decreased in CD hepatocytes. Rather, there was a trend toward increased phosphatidylcholine synthesis that might be explained by enhanced CTP:phosphocholine cytidylyltransferase activity. Although the concentration of phosphocholine in CD hepatocytes was reduced, the size of the phosphocholine pool remained well above the K for the cytidylyltransferase. Moreover, the amount and m activity of the cytidylyltransferase and methyltransferase were increased. The reduction in plasma apoB in mice deprived of dietary choline cannot, therefore, be attributed to decreased apoB secretion.

Enhanced expression and activation of CTP:phosphocholine cytidylyltransferase beta2 during neurite outgrowth

During differentiation neurons increase phospholipid biosynthesis to provide new membrane for neurite growth. We studied the regulation of phosphatidylcholine (PC) biosynthesis during differentiation of two neuronal cell lines: PC12 cells and Neuro2a cells. We hypothesized that in PC12 cells nerve growth factor (NGF) would up-regulate the activity and expression of the rate-limiting enzyme in PC biosynthesis, CTP:phosphocholine cytidylyltransferase (CT). During neurite outgrowth, NGF doubled the amount of cellular PC and CT activity. CTbeta2 mRNA increased within 1 day of NGF application, prior to the formation of visible neurites, and continued to increase during neurite growth. When neurites retracted in response to NGF withdrawal, CTbeta2 mRNA, protein, and CT activity decreased. NGF specifically activated CTbeta2 by promoting its translocation from cytosol to membranes. In contrast, NGF did not alter CTalpha expression or translocation. The increase in both CTbeta2 mRNA and CT activity was inhibited by U0126, an inhibitor of mitogen-activated kinase/extracellular signal-regulated kinase kinase 1/2 (MEK1/2). In Neuro2a cells, retinoic acid significantly increased CT activity (by 54%) and increased CTbeta2 protein, coincident with neurite outgrowth but did not change CTalpha expression. Together, these data suggest that the CTbeta2 isoform of CT is specifically up-regulated and activated during neuronal differentiation to increase PC biosynthesis for growing neurites.

Hepatocyte growth factor is elevated in chronic lung injury and inhibits surfactant metabolism

Adult respiratory distress syndrome may incorporate in its pathogenesis the hyperplastic proliferation of alveolar epithelial type II cells and derangement in synthesis of pulmonary surfactant. Previous studies have demonstrated that hepatocyte growth factor (HGF) in the presence of serum is a potential mitogen for adult type II cells (R. J. Panos, J. S. Rubin, S. A. Aaronson, and R. J. Mason. J. Clin. Invest. 92: 969-977, 1993) and that it is produced by fetal mesenchymal lung cells (J. S. Rubin, A. M.-L. Chan, D. P. Botarro, W. H. Burgess, W. G. Taylor, A. C. Cech, D. W. Hirschfield, J. Wong, T. Miki, P. W. Finch, and S. A. Aaronson. Proc. Natl. Acad. Sci. USA 88: 415-419, 1991). In these studies, we expand on this possible involvement of HGF in chronic lung injury by showing the following. First, normal adult lung fibroblasts transcribe only small amounts of HGF mRNA, but the steady-state levels of this message rise substantially in lung fibroblasts obtained from animals exposed to oxidative stress. Second, inflammatory cytokines produced early in the injury stimulate the transcription of HGF in isolated fibroblasts, providing a plausible mechanism for the increased amounts of HGF seen in vivo. Third, HGF is capable of significantly inhibiting the synthesis and secretion of the phosphatidylcholines of pulmonary surfactant. Fourth, HGF inhibits the rate-limiting enzyme in de novo phosphatidylcholine synthesis, CTP:choline-phosphate cytidylyltransferase (EC 2.7.7.15). Our data indicate that fibroblast-derived HGF could be partially responsible for the changes in surfactant dysfunction seen in adult respiratory distress syndrome, including the decreases seen in surfactant phosphatidylcholines.

CTP:phosphocholine cytidylyltransferase: insights into regulatory mechanisms and novel functions

A key regulatory enzyme in phosphatidylcholine biosynthesis, CTP:cholinephosphate cytidylyltransferase (CCT), catalyzes the formation of CDP-choline. This review discusses the essential features of CCT and addresses intriguing new insights into the catalytic and regulatory properties of this complex enzyme. Characterization of a lipid-binding segment in rat CCT is described and the role of lipids in CCT activation is discussed. An analysis of the phosphorylation domain is presented and possible physiological rationales for reversible phosphorylation of CCT are discussed. The nuclear localization of CCT is examined in the context of multiple CCT isoforms, as is recent evidence establishing a potential link between CCT activity and vesicular transport.

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.

CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes the synthesis of CDP-choline and is regulatory for phosphatidylcholine biosynthesis. This review focuses on recent developments in understanding the catalytic and regulatory mechanisms of this enzyme. Evidence for the nuclear localization of the enzyme is discussed, as well as evidence suggesting cytoplasmic localization. A comparison of the catalytic domains of CCTs from a wide variety of organisms is presented, highlighting a large number of completely conserved residues. Work implying a role for the conserved HXGH sequence in catalysis is described. The membrane-binding domain in rat CCT has been defined, and the role of lipids in activating the enzyme is discussed. The identification of the phosphorylation domain is described, as well as approaches to understand the role of phosphorylation in enzyme activity. Other possible control mechanisms such as enzyme degradation and gene expression are presented.

Lipid activation of CTP:phosphocholine cytidylyltransferase is regulated by the phosphorylated carboxyl-terminal domain

The role of the phosphorylated carboxyl-terminal domain of CTP:phosphocholine cytidylyltransferase (CT) in the regulation of enzyme activity was investigated by comparing the catalytic properties of wild-type CT to two mutant proteins with altered carboxyl-terminal phosphorylation domains. CT isolated from a baculovirus expression system was extensively phosphorylated at multiple sites in the carboxyl-terminal domain. The CT[S315A] mutant lacked a major CT phosphorylation site, and the carboxyl-terminal deletion mutant, CT[delta 312-367], was not phosphorylated. The higher activities of CT[delta 312-367] and CT[S315A] relative to CT were attributed to differences in the sensitivities of the enzymes to lipid activators. The rank order of the apparent Km values for activation by either phosphatidylcholine/oleic acid or phosphatidylcholine/diacylglycerol was CT > CT[S315A] > CT[delta 312-367]. In addition, CT exhibited negative cooperativity in its activation by phosphatidylcholine/oleic acid (nH = 0.64) and phosphatidylcholine/diacylglycerol (nH = 0.74) vesicles, whereas CT[delta 312-367] and CT[S315A] did not. These data support the concept that the phosphorylation of the CT carboxyl-terminal domain interferes with the activation of CT by lipid regulators.

Effects of sodium butyrate on the transfer of arachidonic acid to phosphatidylcholine in a clonal oligodendrocyte cell line (CB-II)

The effect of sodium butyrate on membrane phospholipid metabolism in a neonate rat cerebellum derived clonal oligodendrocyte cell line (CB-II) was investigated. Sodium butyrate is an agent known to induce cell differentiation and morphological transformations. A comparison of the in vivo phospholipid labeling patterns obtained by incubating CB-II cells with [3H]choline, [14C]myristic acid or [3H]arachidonic acid indicated that butyrate altered the route of acylation-deacylation in phosphatidylcholine (PC) biosynthesis. Using an in vitro incubation system containing homogenates of CB-II cells, the largest proportion of radioactivity was found in PC, and addition of sodium butyrate resulted in a further increase in the transfer of arachidonic acid to PC, but not to phosphatidylinositol. Similar results were obtained when this in vitro acylation activity was tested using homogenates from sodium butyrate pretreated cells. The butyrate effect was observed regardless of whether or not exogenous lysophosphatidylcholine (LPC) was added to the incubation system. Addition of butyrate did not result in a change in the activity of LPC:acyl-CoA (coenzyme A) acyltransferase (EC 2.3.1.23) in CB-II cells upon incubating cell homogenates with [1-14C]arachidonoyl-CoA and LPC. However, when cell homogenates were incubated with [3H]arachidonic acid in the presence of 2.5-10 mM sodium butyrate, arachidonoyl-CoA synthesis was stimulated. A time course study demonstrated that significant stimulation occurred after three minutes. Taken together, the results suggest that in CB-II cells, sodium butyrate stimulates the transfer of arachidonic acid into PC and that this effect is at least partially due to a stimulation of arachidonoyl-CoA ligase (EC 6.2.1.3).

Cyclic AMP-dependent protein kinase does not regulate CTP:phosphocholine cytidylyltransferase activity in maturing type II cells

CTP:phosphocholine cytidylyltransferase catalyses a rate regulatory step in the de novo synthesis of surfactant phosphatidylcholine in alveolar type II cells. To investigate if cytidylyltransferase can be regulated by cAMP-dependent protein kinase, we first studied the ontogeny of cAMP-dependent protein kinase activity in type II cells of fetal rat lung. Total cAMP-dependent protein kinase activity, measured in the presence of 10 microM cAMP, as well as endogenous activity, measured without cAMP, increased with advancing gestation. Phosphocholine cytidylyltransferase activity showed a similar developmental profile. This temporal relationship between cAMP-dependent protein kinase and cytidylyltransferase supports a potential role for cAMP-dependent protein kinase in regulating cytidylyltransferase phosphorylation. Cytidylyltransferase purified from adult rat lung was, indeed, phosphorylated in vitro by cAMP-dependent protein kinase. Despite the phosphorylation, however, no change in cytidylyltransferase activity was noted. Pre-incubation of fetal type II cell cytosol with ATP and Mg2+ did not affect cytidylyltransferase activity. Addition of either cAMP, dibutyryl-cAMP or the catalytic subunit of cAMP-dependent protein kinase to the pre-incubation medium did also not alter cytidylyltransferase activity. Furthermore, neither cAMP-dependent protein kinase inhibitor peptide, nor H8, a cyclic nucleo-dependent protein kinase inhibitor, affected cytidylyltransferase activity in fetal type II cell cytosol. Treatment of intact fetal type II cells with either cAMP, dibutyryl-cAMP or 8-[4-chlorophenylthio]cAMP activated cAMP-dependent protein kinase activity but did not alter cytidylyltransferase activity. We conclude that the increase in cytidylyltransferase activity in fetal type II cells at late gestation is not regulated by the developmental activation of cAMP-dependent protein kinase.

Differential regulations of phosphatidylcholine biosynthesis in U937 cells by inhibitors of protein and tyrosine kinases

1. The differential effects of inhibitors of protein kinase (PK) or tyrosine kinase (TK) on phosphatidylcholine (PC) biosynthesis in monocyte-like U937 cells were compared in pulse-chase-studies in which the cells prelabelled with [3H]choline for 30 min were chased in the absence or presence of kinase inhibitors. 2. PKA inhibitor (H-89) decreased the label incorporation into PC, while PKA activator (8-BrcAMP) had no effect. 3. PKC inhibitors (chelerythrine and hypericin) inhibited PC biosynthesis; on the other hand, PKC activator (SC-10) was stimulatory. 4. The inhibition of PC biosynthesis by H-89 and chelerythrine was accompanied by the inactivation of CTP: cholinephosphate cytidylyltransferase (CT). 5. In contrast, TK inhibitor (genistein) markedly stimulated CT and PC biosynthesis, while erbstatin and tyrphostin No. 25 showed no effect.

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.

Evidence that the major membrane lipids, except cholesterol, are made in axons of cultured rat sympathetic neurons

Membrane lipids and proteins required for axonal growth and regeneration are generally believed to be synthesized in the cell bodies of neurons and transported into the axons. However, we have demonstrated recently that, in cultured rat sympathetic neurons, axons themselves have the capacity to synthesize phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine. In these experiments, we employed a compartment model of neuron culture in which pure axons grow in a fluid environment separate from that containing the cell bodies. In the present study, we again used compartmented cultures to confirm and extend the previous results. We have shown that three enzymes of phosphatidylcholine biosynthesis via the CDP-choline pathway are present in axons. We have also shown that the rate-limiting step in the biosynthesis of phosphatidylcholine by this route in neurons, and locally in axons, is catalyzed by the enzyme CTP:phosphocholine cytidylytransferase. The biosynthesis of other membrane lipids, such as phosphatidylserine, phosphatidylethanolamine derived by decarboxylation of phosphatidylserine, phosphatidylinositol, and fatty acids, also occurs in axons. However, the methylation pathway for the conversion of phosphatidylethanolamine into phosphatidylcholine appears to be a quantitatively insignificant route for phosphatidylcholine synthesis in neurons. Moreover, our data provided no evidence for the biosynthesis of another important membrane lipid, cholesterol, in axons.

Evidence that CTP:choline-phosphate cytidylyltransferase is regulated at a pretranslational level in rat liver after partial hepatectomy

Regulation of CTP:choline-phosphate cytidylyltransferase activity was studied in regenerating rat liver. The formation of phosphatidylcholine from [14C]choline in hepatocytes isolated from regenerating liver at 22 h after surgery was increased 1.9-fold when compared with hepatocytes from sham-operated animals. This effect was accompanied by a 1.4-fold increase in cytosolic cytidylyltransferase activity as well as by a 1.5-fold increase in the amount of immunoreactive cytidylyltransferase protein, and a 1.7-fold increase in [35S]methionine incorporation into cytidylyltransferase protein. Northern blot analysis of cytidylyltransferase mRNA showed two signals at 1.5 and 5.0 kb. Partial hepatectomy caused a significant 2-3-fold increase in the 1.5-kb and 5.0-kb messengers at 12 h after surgery. During the next 10 h after partial hepatectomy cytidylyltransferase mRNA levels slightly decreased, although they were still elevated in comparison with sham-operated rats 20-22 h after surgery. In contrast to the elevated cytidylyltransferase mRNA levels, the amount of acetyl-CoA carboxylase mRNA did not increase between 12 and 22 h after surgery, which is in line with the unchanged activity of this enzyme. In conclusion, our data demonstrate that in regenerating liver phosphatidylcholine biosynthesis and cytidylyltransferase activity are regulated at a pretranslational level.

Hepatic monoacylglycerol acyltransferase activity in HA1 and HA7 hepatoma/hepatocyte hybrid cells: regulation by insulin and dexamethasone and by cell density

Hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) is a developmentally-expressed enzyme that catalyzes the stereospecific synthesis of sn-1,2-diacylglycerol from sn-2-monoacylglycerol and long-chain fatty acyl-CoA. In order to study the regulation of MGAT, we developed a rapid assay that can be performed directly on permeabilized HA rat hepatocyte/hepatoma hybrid cells, a line that expresses levels of hepatic MGAT activity and a lipogenic program characteristic of fetal hepatocytes. In permeabilized HA cells, MGAT activity was proportional to the time of incubation and was highly dependent on added sn-2-monoacylglycerol and palmitoyl-CoA. The apparent Km values were 16.6 and 12.7 microM for palmitoyl-CoA and 2-monooleoylglycerol, respectively. Activity was low with the 1(3)- and sn-2-ether analogs of monooleoylglycerol, supporting the conclusion that the cells express the hepatic isoenzyme of MGAT. MGAT activity increased directly with cell density and was unrelated to the number of days in culture. Long-term incubation (2-4 days) of HA cells with various hormones (including triiodothyronine, human placental lactogen, epidermal growth factor, glucagon and growth hormone) showed that only a combination of dexamethasome and insulin resulted in significantly decreased MGAT activity. None of these hormones affected MGAT activity in short-term (0.5-4 h) incubations. These studies suggest that the developmental decline in rat hepatic MGAT activity may be regulated by glucocorticoids and insulin, hormones that increase during and after the second postnatal week.

A selective inhibitor of cyclic AMP-dependent protein kinase, N-[2-bromocinnamyl(amino)ethyl]-5-isoquinolinesulfonamide (H-89), inhibits phosphatidylcholine biosynthesis in HeLa cells

In this study, we report that the potent and selective inhibitor of cyclic AMP-dependent protein kinase, N-[2-bromocinnamyl(amino)ethyl]-5-isoquinolinesulfonamide (H-89) interferes with the incorporation of choline into phosphatidylcholine in HeLa cells. Treatment of cells with 10 microM H-89 for 1 h decreases the phosphatidylcholine biosynthesis by 50%. This inhibition is prevented by simultaneous addition of 10 microM forskolin, while the choline uptake itself is not affected by H-89.

Correlation between intracellular cAMP levels and phospholipids of Microsporum gypseum

Atropine, a modulator of cAMP has been used to examine the relationship between phospholipids and intracellular levels of cAMP in Microsporum gypseum. A decreased phospholipid content was observed in atropine grown cells as a result of reduced levels of intracellular cAMP. This decline was caused by the inhibitory effect of atropine on adenylate cyclase. Lowered phospholipid content was supported by decreased [14C]acetate incorporation as well as reduced activities of key enzymes of phospholipid biosynthesis. In vitro supplementation of atropine in control cells also caused inhibition in lipid synthesis indicating similar effects of atropine and its metabolites. These results in conjunction with our previous report, in which enhanced levels of cAMP resulted in increased phospholipid synthesis, suggest a direct correlation between phospholipid biosynthesis and intracellular levels of cAMP in M. gypseum.

Regulation of CTP:phosphocholine cytidylyltransferase activity and phosphorylation in rat hepatocytes: Lack of effect of elevated cAMP levels

Immunoprecipitation of 32P-labeled CTP:phosphocholine cytidylyltransferase from freshly isolated rat hepatocytes followed by trypsin digestion and two-dimensional peptide mapping revealed multiple phosphorylation sites. Treatment of the hepatocytes with 0.5 mM of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-monophosphate or elevation of intracellular cAMP levels by cholera toxin activated the cAMP-dependent protein kinase activity in intact cells. Despite the activation of cAMP-dependent protein kinase no change in the rate of [3H]choline incorporation into phosphatidylcholine was detected. In addition, the activity of cytidylyltransferase in total cell homogenates and its distribution between soluble and particulate fractions remained unchanged. Comparison of peptide maps of 32P-labeled cytidylyltransferase obtained from control and cholera-toxin-treated hepatocytes did not reveal any differences in the phosphorylation state of cytidylyltransferase. Furthermore, only [32P]phosphoserine residues were detected following phosphoamino acid analysis. We conclude that cytidylyltransferase activity is not altered solely by the activation of the cAMP-dependent kinase in fresh hepatocytes.

Phospholipid turnover and ultrastructural correlates during spontaneous germinal vesicle breakdown of the bovine oocyte: effects of a cyclic AMP phosphodiesterase inhibitor

The turnover of [32P]orthophosphate in bovine oocyte phospholipids was studied during the early stages of spontaneous meiotic maturation, and during inhibition of this process by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). Radioactive lipids were separated by TLC and the meiotic stage was determined cytogenetically. Ultrastructure of the nuclear membrane was examined using transmission EM. During the commitment period to meiotic resumption, which precedes germinal vesicle breakdown (GVBD), small localized convolutions appeared in the intact nuclear membrane. This was accompanied by a decrease in [32P]phosphatidic acid (PA) and an increase in [32P]-phosphatidylcholine (PC). This was followed by extensive convolutions, and subsequent dissociation, of the nuclear membrane, concomitant with a tremendous surge in [32P]PC and [32P]phosphatidylethanolamine (PE). The cAMP-mediated maintenance of meiotic arrest involved retention of entire nuclear envelope integrity and total inhibition of the surge in [32P]PC and [32P]PE which accompanied GVBD. The increase in [32P]phosphatidylinositol (PI) associated with all stages of early meiotic resumption was unaffected by IBMX. Microinjection of heparin inhibited GVBD, and injection of inositol 1,4,5-trisphosphate (IP3) overrode IBMX-maintained meiotic arrest in almost 40% of the oocytes. The results suggest that there may be several functions for phospholipid turnover in the regulation of spontaneous meiotic resumption in the bovine oocyte. The first precedes the commitment period, and involves IP3 generation to serve as the primary signal for meiotic resumption. The second occurs concomitant with the commitment period, is unaffected by the level of intracellular cAMP, and is associated with the general turnover of phospholipid. The third is associated with GVBD, and is cAMP-sensitive, and may represent stimulation of de novo synthesis of phospholipid, thereby permitting disruption of the nuclear membrane.

The protein phosphatase inhibitor, okadaic acid, inhibits phosphatidylcholine biosynthesis in isolated rat hepatocytes

There is evidence that phosphatidylcholine (PC) biosynthesis in hepatocytes is regulated by a phosphorylation-dephosphorylation mechanism. The phosphatases involved have not been identified. We, therefore, investigated the effect of okadaic acid, a potent protein phosphatase inhibitor, on PC biosynthesis via the CDP-choline pathway in suspension cultures of isolated rat hepatocytes. Okadaic acid caused a 15% decrease (P less than 0.05) in [Me-3H]choline uptake in continuous-pulse labeling experiments. After 120 min of treatment, the labeling of PC was decreased 46% (P less than 0.05) with a corresponding 20% increase (P less than 0.05) in labeling of phosphocholine. Cells were pulsed with [Me-3H]choline for 30 min and subsequently chased for up to 120 min with choline in the absence or presence of okadaic acid. The labeling of phosphocholine was increased 86% (P less than 0.05) and labeling of PC decreased 29% (P less than 0.05) by 120 min of chase in okadaic acid-treated hepatocytes. The decrease of label in PC was quantitatively accounted for in the phosphocholine fraction. Incubation of hepatocytes with both okadaic acid and CPT-cAMP did not produce an additive inhibition in labeling of PC. Choline kinase and cholinephosphotransferase activities were unaltered by treatment with okadaic acid. Hepatocytes were incubated with digitonin to cause release of cytosolic components. Cell ghost membrane cytidylyltransferase (CT) activity was decreased 37% (P less than 0.005) with a concomitant 33% increase (P less than 0.05) in released cytosolic cytidylyltransferase activity in okadaic acid-treated hepatocytes. We postulate that CT activity and PC biosynthesis are regulated by protein phosphatase activity in isolated rat hepatocytes.

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 NaF and okadaic acid on the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity in rat liver

The effect of preincubation of rat liver post-mitochondrial supernatant with NaF and okadaic acid on the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity was investigated. NaF (20 mM) inhibited the time-dependent activation of cytidylyltransferase activity in post-mitochondrial supernatant. Subcellular fractionation of the post-mitochondrial supernatant revealed that cytidylyltransferase activity in the microsomal fraction was decreased and activity in the cytosolic fraction increased with time of preincubation with NaF compared to controls. Okadaic acid is a specific and potent inhibitor of type 1 and 2A phosphoprotein phosphatases. Preincubation of cytosol with 5 microM okadaic acid inhibited the time-dependent activation of cytosolic cytidylyltransferase activity. Preincubation of post-mitochondrial supernatants with 5 microM okadaic acid inhibited the time-dependent activation of cytidylyltransferase activity by 13% at 45 min and 16% at 60 min of preincubation compared to controls. Microsomal cytidylyltransferase activity was decreased 27% at 45 min and 31% at 60 min with a corresponding retention of cytosolic cytidylyltransferase activity of 21% at 45 min and 37% at 60 min of preincubation with okadaic acid compared to controls. We postulate that the activity of the type 1 and/or type 2A phosphoprotein phosphatases affect the subcellular distribution of CTP: phosphocholine cytidylyltransferase activity in rat liver.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by incubation of rat hepatocytes with phospholipase A2

The effect of phospholipase A2 treatment of rat hepatocytes on CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis was investigated. Cytidylyltransferase is recovered from the cytosol and in a membrane-bound form with the microsomes. Digitonin treatment of cells causes rapid release into the medium of the cytosolic, but not the microsomal form of the cytidylyltransferase. Incubation of hepatocytes for 10 min with phospholipase A2 (0.9 units/dish) in the medium, resulted in a 33% decrease in the cytidylyltransferase activity released by digitonin treatment (2.5 +/- 0.15 nmol/min per mg compared to 3.9 +/- 0.10 nmol/min per mg in the control). In agreement with the digitonin experiments, incubation with 0.9 units/dish of phospholipase A2 resulted in a decrease in the cytidylyltransferase activity in the cytosol (from 4.3 +/- 0.10 nmol/min per mg to 2.6 +/- 0.14 nmol/min per mg) and a corresponding increase in the microsomal fraction (from 0.9 +/- 0.16 nmol/min per mg to 1.8 +/- 0.20 nmol/min per mg). The effect of phospholipase A2 on cytidylyltransferase translocation was concentration- and time-dependent. Incubation of hepatocytes in the presence of phospholipase A2 (0.9 units/dish) for 10 min prior to pulse-chase experiments resulted in an increase in radiolabel incorporation into phosphatidylcholine (from 2.4 +/- 0.02.10(-5) dpm/dish to 3.1 +/- 0.1.10(-5) dpm/dish) and a corresponding decrease in radiolabel associated with the choline (from 2.5 +/- 0.05.10(-5) to 1.4 +/- 0.03.10(-5) dpm) and phosphocholine fractions (from 8.5 +/- 0.07.10(-5) to 6.9 +/- 0.05.10(-5) dpm). We conclude that phospholipase A2 can cause a stimulation of CTP: phosphocholine cytidylyltransferase activity and phosphatidylcholine synthesis in cultured rat hepatocytes.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by calcium in rat hepatocytes

The effects of Ca2+, ionophore A23187, and vasopressin on CTP:phosphocholine cytidylyltransferase were investigated. Cytidylyltransferase is present in the cytosol and in a membrane-bound form on the microsomes. Digitonin treatment caused release of the cytosolic form rapidly. Addition of 7 mM Ca2+ to hepatocyte medium resulted in a 3-fold decrease in cytidylyltransferase released by digitonin treatment (1.7 +/- 0.1 nmol/min per mg compared to 5.1 +/- 0.2 nmol/min per mg in the control). Verapamil, a calcium channel blocker, partially overcame this effect of Ca2+. Ionophore A23187 and vasopressin both mimicked the effect of Ca2+ and resulted in a decrease in cytidylyltransferase release (2.4 +/- 0.1 nmol/min per mg and 2.5 +/- 0.2 nmol/min per mg, respectively) compared to control (3.4 +/- 0.1 nmol/min per mg). In agreement with the digitonin experiments, incubation with 7 mM Ca2+ resulted in a decrease in cytidylyltransferase in the cytosol (from 4.0 to 1.2 mol/min per mg) and a corresponding increase in the microsomes (from 0.6 to 2.4 nmol/min per mg). Verapamil partially blocked this translocation caused by Ca2+. Ionophore A23187 and vasopressin also caused translocation of the cytidylyltransferase from the cytosol to the microsomes. The addition of Ca2+ also resulted in an increase in PC synthesis. With 7 mM Ca2+ in the medium, the label associated with PC increased to 3.8 +/- 0.1.10(6) dpm/dish from 2.7 +/- 0.1.10(6) dpm/dish after 10 min. PC degradation was also affected, since 7 mM Ca2+ in the medium resulted in an increase in LPC formation both in the cell and the medium. We conclude that high concentrations of calcium in the hepatocyte medium can cause a stimulation of CTP:phosphocholine cytidylyltransferase and PC synthesis in cultured hepatocytes.

Effects of ethanol feeding on hepatic lipid synthesis

Rats were fed a high-fat, liquid diet containing either 36% of total calories as ethanol or an isocaloric amount of sucrose, for a period up to 35 days. At different time intervals we measured the effects of ethanol administration on the activities of a number of key enzymes involved in hepatic lipid synthesis. At the start of the experimental period the activities of acetyl-CoA carboxylase and fatty acid synthase, measured in liver homogenates, increased in the control as well as in the ethanol-fed group. After 35 days these enzyme activities were still elevated but there were no significant differences between the two groups. In hepatocytes isolated from controls as well as from ethanol-fed rats, short-term incubations with ethanol induced an increase in the rate of fatty acid synthesis and in the activities of acetyl-CoA carboxylase and fatty acid synthase. However, no alterations in the regulation of these enzymes by short-term modulators of lipogenesis were apparent in hepatocytes isolated from alcohol-treated animals. The results do not indicate a major role for the enzymes of de novo fatty acid synthesis in the development of the alcoholic fatty liver. The amount of liver triacylglycerols increased in ethanol-fed rats during the entire treatment period, whereas the hepatic levels of phosphatidylcholine and phosphatidylethanolamine were not affected by ethanol ingestion. Ethanol administration for less than 2 weeks increased the activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and microsomal phosphocholine cytidylyltransferase, whereas the cytosolic activity of phosphocholine cytidylyltransferase was slightly decreased. Upon prolonged ethanol administration the activities of these enzymes were slowly restored to control values after 35 days, suggesting development of some kind of adaptation. It is interesting that, although the activities of phosphatidate phosphohydrolase and diacylglycerol acyltransferase were restored to the levels found in the control rats, this effect was not accompanied by a stabilization or decrease of the concentration of hepatic triacylglycerols.