Activities of the phosphatidylcholine biosynthetic enzymes in rat liver during development

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Phospholipid methylation in mammals: from biochemistry to physiological function

Phosphatidylcholine is made in the liver via the CDP-choline pathway and via the conversion of phosphatidylethanolamine to phosphatidylcholine by 3 transmethylation reactions from AdoMet catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). PEMT is a 22.3kDa integral transmembrane protein of the endoplasmic reticulum and mitochondria-associated membranes. The only tissue with quantitatively significant PEMT activity is liver; however, low levels of PEMT in adipocytes have been implicated in lipid droplet formation. PEMT activity is regulated by the concentration of substrates (phosphatidylethanolamine and AdoMet) as well as the ratio of AdoMet to AdoHcy. Transcription of PEMT is enhanced by estrogen whereas the transcription factor Sp1 is a negative regulator of PEMT transcription. Studies with mice that lack PEMT have provided novel insights into the function of this enzyme. PEMT activity is required to maintain hepatic membrane integrity and for the formation of choline when dietary choline supply is limited. PEMT is required for normal secretion of very low-density lipoproteins. The lack of PEMT protects against diet-induced atherosclerosis in two mouse models. Most unexpectedly, mice that lack PEMT are protected from diet-induced obesity and insulin resistance. Moreover, mice lacking PEMT have increased susceptibility to diet-induced fatty liver and steatohepatitis. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

A role for Sp1 in transcriptional regulation of phosphatidylethanolamine N-methyltransferase in liver and 3T3-L1 adipocytes

Phosphatidylcholine is made in all nucleated mammalian cells via the CDP-choline pathway. Another major pathway for phosphatidylcholine biosynthesis in liver is catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). We have now identified 3T3-L1 adipocytes as a cell culture model that expresses PEMT endogenously. We have found that PEMT mRNA and protein levels increased dramatically in 3T3-L1 cells upon differentiation to adipocytes. 5'-Deletion analysis of the PEMT promoter-luciferase constructs stably expressed in 3T3-L1 adipocytes identified a regulatory region between -471 and -371 bp (relative to the transcriptional start site). Competitive and supershift assays demonstrated binding sites for transcription factors Sp1, Sp3 (-408 to -413), and YY1 (-417 to -420). During differentiation of 3T3-L1 cells to adipocytes, the amount of Sp1 protein decreased by approximately 50% just prior to activation of PEMT. Transduction of 3T3-L1 adipocytes with retrovirus containing Sp1 cDNA demonstrated that Sp1 inhibited PEMT transcriptional activity. Similarly, short hairpin RNA directed against Sp1 in 3T3-L1 adipocytes enhanced PEMT transcriptional activation. Chromatin immunoprecipitation assays confirmed that Sp1 binds to the PEMT promoter, and this interaction decreases upon differentiation to adipocytes. These experiments directly link increased PEMT expression in adipocytes to decreased transcriptional expression of Sp1. In addition, our data established that Sp1 binding was required for tamoxifen-mediated inhibition of Pemt promoter activity.

Regulation of the CDP-choline pathway by sterol regulatory element binding proteins involves transcriptional and post-transcriptional mechanisms

The synthesis of phosphatidylcholine (PtdCho) by the CDP-choline pathway is under the control of the rate-limiting enzyme CTP:phosphocholine cytidylyltransferase (CCT). Sterol regulatory element binding proteins (SREBPs) have been proposed to regulate CCT at the transcriptional level, or via the synthesis of lipid activators or substrates of the CDP-choline pathway. To assess the contributions of these two mechanisms, we examined CCTalpha expression and PtdCho synthesis by the CDP-choline pathway in cholesterol and fatty acid auxotrophic CHO M19 cells inducibly expressing constitutively active nuclear forms of SREBP1a or SREBP2. Induction of either SREBP resulted in increased expression of mRNAs for sterol-regulated genes, elevated fatty acid and cholesterol synthesis (>10-50-fold) and increased PtdCho synthesis (2-fold). CCTalpha mRNA was increased 2-fold by enforced expression of SREBP1a or SREBP2. The resultant increase in CCTalpha protein and activity (2-fold) was restricted primarily to the soluble fraction of cells, and increased CCTalpha activity in vivo was not detected. Inhibition of the synthesis of fatty acids or their CoA esters by cerulenin or triacsin C respectively following SREBP induction effectively blocked the accompanying elevation in PtdCho synthesis. Thus PtdCho synthesis was driven by increased synthesis of fatty acids or a product thereof. These data show that transcriptional activation of CCTalpha is modest relative to that of other SREBP-regulated genes, and that stimulation of PtdCho synthesis by SREBPs in CHO cells is due primarily to increased fatty acid synthesis.

Hexadecylphosphocholine inhibits phosphatidylcholine biosynthesis and the proliferation of HepG2 cells

Hexadecylphosphocholine (HePC) is a synthetic lipid representative of a new group of antiproliferative agents, alkylphosphocholines (APC), which are promising candidates in anticancer therapy. Thus we have studied the action of HePC on the human hepatoblastoma cell line HepG2, which is frequently used as a model for studies into hepatic lipid metabolism. Non-toxic, micromolar concentrations of HePC exerted an antiproliferative effect on this hepatoma cell line. The incorporation into phosphatidylcholine (PC) of the exogenous precursor [methyl-14C]choline was substantially reduced by HePC. This effect was not due to any alteration in choline uptake by the cells, the degradation rate of PC or the release of PC into the culture medium. As anaccumulation of soluble choline derivatives points to CTP:phosphocholine cytidylyltransferase (CT) as the target of HePC activity we examined its effects on the different enzymes involved in the biosynthesis of PC via CDP-choline. Treatment with HePC altered neither the activity of choline kinase (CK) nor that of diacylglycerol cholinephosphotransferase (CPT), but it did inhibit CT activity in HepG2 cells. In vitro HePC also inhibited the activity of cytosolic but not membrane-bound CT. Taken together our results suggest that HePC interferes specifically with the biosynthesis of PC in HepG2 cells by depressing CT translocation to the membrane, which may well impair their proliferation.

Perturbations in choline metabolism cause neural tube defects in mouse embryos in vitro

A role for choline during early stages of mammalian embryogenesis has not been established, although recent studies show that inhibitors of choline uptake and metabolism, 2-dimethylaminoethanol (DMAE), and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3), produce neural tube defects in mouse embryos grown in vitro. To determine potential mechanisms responsible for these abnormalities, choline metabolism in the presence or absence of these inhibitors was evaluated in cultured, neurulating mouse embryos by using chromatographic techniques. Results showed that 90%-95% of 14C-choline was incorporated into phosphocholine and phosphatidylcholine (PtdCho), which was metabolized to sphingomyelin. Choline was oxidized to betaine, and betaine homocysteine methyltransferase was expressed. Acetylcholine was synthesized in yolk sacs, but 70 kDa choline acetyltransferase was undetectable by immunoblot. DMAE reduced embryonic choline uptake and inhibited phosphocholine, PtdCho, phosphatidylethanolamine (PtdEtn), and sphingomyelin synthesis. ET-18-OCH3 also inhibited PtdCho synthesis. In embryos and yolk sacs incubated with 3H-ethanolamine, 95% of recovered label was PtdEtn, but PtdEtn was not converted to PtdCho, which suggested that phosphatidylethanolamine methyltransferase (PeMT) activity was absent. In ET-18-OCH3 treated yolk sacs, PtdEtn was increased, but PtdCho was still not generated through PeMT. Results suggest that endogenous PtdCho synthesis is important during neurulation and that perturbed choline metabolism contributes to neural tube defects produced by DMAE and ET-18-OCH3.

Modulation of biosynthesis of phosphatidylcholine via CDP-choline in rat liver: influence of ethanol on the microsomal cholinephosphotransferase activity

We have studied in vitro the effects of ethanol on the different enzymes involved in the biosynthesis of phosphatidylcholine (PC) via CDP-choline. Ethanol alters neither choline kinase (CK) nor CTP:phosphocholine cytidylyltransferase (CT) activities but, at levels higher than 50 mM, it does significantly inhibit microsomal cholinephosphotransferase (CPT) activity concomitantly with an increase in the ethanol concentration. A study of the kinetics of the reaction catalysed by CPT shows that ethanol decreases Vmax without altering Km, indicating a non-competitive inhibitory effect. An analysis of the thermodependence of CPT activity in the absence of ethanol reveals a break in the Arrhenius plot and thus a straight relationship between enzyme activity and the physico-chemical state of the microsomal membrane. Incubation of microsomes in the presence of ethanol increased the transition temperature from 25.8-28.2 degrees C. Microsomes were also incubated with n-alkanols with chain-lengths of fewer than five carbon atoms at concentrations which, according to their partition coefficients, produce equimolar levels in the membrane. Under these conditions all the alkanols caused the same inhibitory effect. All these results demonstrate that ethanol modulate the PC biosynthesis at the level of CPT activity and does not affect the CT enzyme. The inhibition found on CPT is clearly dependent on the alteration produced by ethanol on the hepatic microsomal membrane.

Identification of three novel cDNAs for human phosphatidylethanolamine N-methyltransferase and localization of the human gene on chromosome 17p11.2

Phosphatidylethanolamine is converted to phosphatidylcholine in mammalian liver by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT). A form of the enzyme (PEMT2) has been isolated from rat liver, the cDNA cloned and expressed and the murine gene has been characterized and disrupted. Several lines of evidence suggested that PEMT2 might have a role in hepatocyte proliferation and liver cancer. Hence, we decided to investigate the human form of the enzyme. Unexpectedly, we cloned and expressed three novel human cDNAs encoding PEMT2. These forms differ from each other in the 5'-region with the point of divergence being 15 nucleotides upstream of the putative translation initiation codon. The remainder of the three cDNAs was identical. Expression of the coding region of the cDNAs in McArdle rat hepatoma cells resulted in three stable cell lines that showed a 27- to 115-fold elevation of PEMT activity compared to vector-transfected control cell lines. Screening of somatic cell hybrid panels, radiation hybrid panel mapping and fluorescent in situ hybridization mapping localized the human gene for PEMT2 to chromosome 17p11.2. The identification of three different human cDNAs for PEMT2 suggests that understanding the function of PEMT2 will be more complicated than anticipated.

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.

Histological changes caused by the rc mutation in chickens

The rc gene represents a recessive mutation in chickens, known to cause retinal degeneration and blindness, as well as abnormal sarcolemmal membranes of cardiac myocytes associated with reduced choline transport. In this study, the visceral organs from "old" (aged 12 months) and "young" (aged 5 months) homozygous blind (rc/rc), heterozygous (Rc+/rc) and normal (Rc+/Rc+) chickens were examined histologically to investigate whether the primary effect of the mutation was on cellular structure. Homozygous birds showed enlarged thyroids with acidophilic colloids in enlarged and often ruptured follicles, macrovesicular lipid accumulation in the liver, increased numbers of nuclei in the myocardial fibres, hypertrophy of the lobular structure of the medullary portion of the thymus, cloudy swelling of the tubular epithelium of the kidney and slow maturation (in young birds) and degeneration (in old birds) of the gonads. All lesions, except for those of the thymus, were more severe in old than in young birds. Some heterozygous chickens were mildly affected and none of the normal (Rc+/Rc+) birds exhibited these abnormalities.

Lysophosphatidylcholine and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine inhibit the CDP-choline pathway of phosphatidylcholine synthesis at the CTP:phosphocholine cytidylyltransferase step

The regulation of the CDP-choline pathway of phosphatidylcholine synthesis at the CTP:phosphocholine cytidylyltransferase (CT) step by lysophosphatidylcholine (LPC) and the nonhydrolyzable LPC analog, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3), was investigated in a colony-stimulating factor 1-dependent murine macrophage cell line. LPC inhibited phosphatidylcholine synthesis in vivo and led to the accumulation of choline and phosphocholine coupled to the disappearance of CDP-choline pointing to CT as the intracellular target. LPC neither inhibited cell growth nor decreased the cellular content of CT or altered the distribution of CT between soluble and particulate subcellular fractions. The inhibition of phosphatidylcholine synthesis was specific for LPC since lysophospholipids lacking the choline headgroup were not inhibitors. ET-18-OCH3 was a more potent inhibitor of phosphatidylcholine synthesis than LPC and caused the translocation of CT from the soluble compartment to the particulate compartment. Both LPC and ET-18-OCH3 were inhibitors of CT activity in vitro and kinetic analysis showed competitive inhibition with respect to the lipid activator. These data point to LPC as a negative regulator of de novo phosphatidylcholine synthesis that acts at the CT step and establish the mechanism for the inhibition of phosphatidylcholine biosynthesis by antineoplastic phospholipids.

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

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

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.

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.

Effects of age-dependent or liposome-induced alterations in the phospholipid composition on sphingomyelin biosynthesis in rat liver microsomal and plasma membranes

1. The effects of age-dependent or liposome-induced alterations in the phospholipid composition of rat liver plasma and microsomal membranes on the phosphatidylethanolamine:ceramide-phosphoethanolamine (PE:Cer-PEt) and phosphatidylcholine:ceramide-phosphocholine (PC:Cer-PCh) transferase activities were studied. 2. In all cases under study the PE:Cer-PEt transferase activity was found to be several times higher than that of PC:Cer-PCh transferase in both plasma and microsomal rat liver membranes. 3. The presence of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) in plasma membranes was observed to enhance the PE:Cer-PEt transferase activity, while phosphatidylserine (PS) inhibited it.

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.

Fatty acid synthesis in the fetal lung: relationship to surfactant lipids

The aims of this study were to investigate the control of fatty acid synthesis and its relationship to surfactant production in the fetal lung during alteration of hormonal and substrate conditions. Lung explants from 18 day fetuses (term = 22 days) which were cultured 2 days in the presence of 10 mM lactate showed parallel acceleration of de novo fatty acid synthesis (3H2O incorporation) and [14C]choline incorporation into disaturated phosphatidylcholine (DSPC) compared to culture of explants in glucose. Both the cultured and fresh explants were resistant to the classical short term (4 h) cAMP inhibition of fatty acid synthesis with 3 mM dibutyryl cAMP or 0.5 mM aminophylline. In the cultured explants short term cAMP elevation increased DSPC production, and long term (2 day) cAMP elevation caused a further increase in DSPC synthesis and also stimulated fatty acid synthesis. In cultured explants from 17 day fetuses, dexamethasone (0.1 microM) caused a synergistic increase with aminophylline in both fatty acid synthesis and DSPC production whereas, in explants from 18 day fetuses, dexamethasone inhibited both processes and reduced the level of stimulation of DSPC and fatty acid synthesis seen with aminophylline alone. Dexamethasone also reduced the stimulation of both DSPC and fatty acid synthesis produced in the culture of 18 day explants with bacitracin (0.5 mg/ml), whereas the combination of bacitracin and aminophylline resulted in a synergistic increase in DSPC production. Culture with glucagon (0.1 microM) also stimulated DSPC synthesis but at physiological levels insulin had no effect on either DSPC or fatty acid synthesis. These data show that lung fatty acid synthesis exhibits unique features of fatty acid synthesis regulation compared to other lipogenic tissues and also suggest a link between de novo fatty acid synthesis and surfactant production during the critical period of accelerated lung maturation.

Developmental changes in the activities of enzymes related to methyl group metabolism in sheep tissues

The activities of choline oxidase and betaine-homocysteine methyltransferase increased markedly in pre-ruminant lamb liver after birth and subsequently decreased when the lambs reached the ruminant state, while the developmental changes in hepatic 5-methyl-H4folate-homocysteine methyltransferase were negatively correlated with those of betaine-homocysteine methyltransferase. Hepatic phospholipid methyltransferase was elevated almost four-fold by the 10th postnatal day, but declined thereafter. Hepatic glycine methyltransferase in one-day-old lambs increased 55-fold, compared with that of fetuses, and thereafter decreased dramatically with age. Guanidoacetate methyltransferase, glycine methyltransferase and betaine-homocysteine methyltransferase in sheep pancreas increased markedly with age and were many times higher than the hepatic enzymes in adult sheep. Choline oxidase, betaine-homocysteine methyltransferase, cystathionine beta-synthase and glycine methyltransferase in adult sheep liver were much lower than those in rat. These results illustrate the conservative features of methyl group metabolism in postruminant sheep.

Different susceptibility of alkylacyl--versus diacyl--and alkenylacyl--phosphatidylcholine subclasses to stimulation of biosynthesis by phospholipase C

Krebs II ascites cells were incubated with [3H] or [14C] choline in the presence or in the absence of Clostridium welchii phospholipase C (PLC). At enzyme concentrations where cell lysis remained limited, PLC specifically enhanced phosphatidylcholine (PC) biosynthesis, as shown by comparison with [14C] ethanolamine. Further analysis revealed that the stimulating effect of PLC remained limited to 1,2-diacyl-sn-glycero-3-phosphocholine (diacyl-GPC) and 1-alkenyl-2-acyl-GPC, whereas the biosynthesis of 1-alkyl-2-acyl-GPC, the putative precursor of platelet activating factor (PAF-acether) remained unchanged. These differences reflect different localizations of the three PC subclasses in the plasma membrane and are discussed in relation to the regulation mechanism of PC biosynthesis.

Phorbol esters stimulate phosphatidylcholine biosynthesis by translocation of CTP:phosphocholine cytidylyltransferase from cytosol to microsomes

Previous studies have demonstrated that 12-O-tetradecanoyl phorbol 13-acetate (TPA) stimulates phosphatidylcholine biosynthesis in HeLa cells. The stimulation was apparently caused by an acceleration of the reaction catalyzed by CTP:phosphocholine cytidylyltransferase (CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15) (Paddon, H.B. and Vance, D.E. (1980) Biochim. Biophys. Acta 620, 636-640). We now provide evidence that the enzyme activation is due to a translocation of the cytidylyltransferase from the cytosol to the microsomes. The rate of phospho[Me-3H]choline conversion into phosphatidylcholine was approx. 3-fold faster in HeLa cells treated with 100 nM TPA. This stimulation correlated with a 2.3-fold activation (P less than 0.05) of cytidylyltransferase in homogenates from treated cells. There was a 1.7-fold increase in the enzyme associated with microsomes (P less than 0.05) and a corresponding decrease in enzyme recovered from cytosol (P less than 0.01). The total amount of enzyme recovered from the homogenates was unchanged. Further evidence for TPA causing an increased association of cytidylyltransferase with cellular membranes was obtained when cells were treated with digitonin. The release of cytidylyltransferase into the medium was inhibited by 4-fold from cells previously treated with TPA. Similar results on phospho[Me-3H]choline incorporation into phosphatidylcholine were found with cells incubated with phorbol-12,13-dibutyrate, a water-soluble tumor-promoting agent.

Trifluoperazine and chlorpromazine inhibit phosphatidylcholine biosynthesis and CTP:phosphocholine cytidylyltransferase in HeLa cells

The influence of chlorpromazine and trifluoperazine on phosphatidylcholine biosynthesis in HeLa cells was investigated. HeLa cells were prelabeled with [Me-3H]choline for 1 h. The cells were subsequently incubated with various concentrations of drugs. Both compounds were potent inhibitors of phosphatidylcholine biosynthesis, with 50% inhibition by 5 micron of either drug. Analysis of the radioactivity in the soluble precursors indicated a block in the conversion of phosphocholine to CDPcholine catalyzed by CTP:phosphocholine cytidylyltransferase (CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15). Inhibition by these drugs was slowly reversed after incubation for more than 2 h, or was immediately abolished when 0.4 mM oleate was included in the cell medium or when the drug-containing medium was removed. The subcellular location of the cytidylyltransferase was unaffected by either drug, nor did the drugs alter the rate of release of cytidylyltransferase from HeLa cells by digitonin treatment. The drugs had a direct inhibitory effect on cytidylyltransferase activity in HeLa cell postmitochondrial supernatants. Half-maximal inhibition was achieved with 30 microM trifluoperazine and 50 microM chlorpromazine. These drugs did not change the apparent Km of the cytidylyltransferase for CTP or phosphocholine. Inhibition of cytidylyltransferase by these compounds was reversible with exogenous phospholipid or oleate in the enzyme assay. The data indicate that both drugs inhibit phosphatidylcholine synthesis by an effect on the cytidylyltransferase. The mechanism of action remains unknown at this time.