Abnormal synthesis of N-methylated phospholipids during calcium paradox of the heart

Phosphatidylethanolamine (PtdEtn) N-methyltransferase activity that synthesizes phosphatidylcholine (PtdCho) via formation of methylated intermediates (phosphatidyl-N-monomethylethanolamine, PtdEtnMe and phosphatidyl-N,N-dimethylethanolamine, PtdEtnMe2) was comparatively studied in rat heart sarcolemmal (SL), sarcoplasmic reticular (SR) and mitochondrial fractions during Ca2+ paradox. Perfusion (5 min) with Ca(2+)-free medium followed by reperfusion (5 min) with Ca(2+)-containing medium produced a marked rise in resting tension without any recovery of contractile force. Methyltransferase catalytic sites I, II and III which synthesize PtdEtnMe, PtdEtnMe2 and PtdCho, respectively, were assayed by measuring the [3H] methyl group incorporation from 0.055, 10 and 150 microM S-adenosyl-L-[3H-methyl] methionine into membrane PtdEtn molecules. Five minutes of perfusion with Ca(2+)-free medium did not affect either SL or SR N-methyltransferase systems. Ca(2+)-readmission for 1 to 5 min induced a selective, time-dependent depression of SL site II and SR site I methyltransferase activities. Individual N-methylated phospholipids specifically formed at the two sites reflected these changes. The above abnormalities were differently influenced by the duration (1-5 min) of Ca(2+)-free perfusion and were characterized by different kinetic alterations. The mitochondrial methylation system was not affected under Ca2+ paradox. The results suggest that reduced synthesis of SL N-methylated phospholipids may contribute to the contractile dysfunction observed in Ca2+ paradox.

Sperm phospholipid methyltransferase activity during preparation for exocytosis

The present report describes experiments to evaluate phospholipid methyltransferase activity in golden hamster spermatozoa incubated under different conditions. Washed cauda epididymal sperm were incubated with taurine, in the presence or absence of epinephrine. At various times, the sperm were separated, and phospholipid methyltransferase activity measured. Also, at each time, aliquots of the sperm suspension were assayed for motility, and acrosome reactions. Some sperm incubated in the presence of taurine and epinephrine were capacitated by 3.5 h, because about 40 per cent of them can undergo the acrosome reaction 10 min after addition of the fusogen lysophosphatidylcholine. In epinephrine-free incubations the fusogen failed to stimulate acrosome reactions. On the other hand, epinephrine stimulated by twofold phospholipid methyltransferase activity from '0 time' incubated sperm, in comparison to that observed in taurine-treated cells. Enzyme activities from both taurine or epinephrine plus taurine-treated cells decreased as the incubation time of the sperm suspension increased. Kinetic properties of the sperm phospholipid methyltransferase activity were modified by the presence of taurine and epinephrine when S-adenosylmethionine was used as the substrate. These results suggest that refined molecular events occur in the sperm cell during the acquisition of fertilizing ability.

A unique mitochondria-associated membrane fraction from rat liver has a high capacity for lipid synthesis and contains pre-Golgi secretory proteins including nascent lipoproteins

An endoplasmic reticulum-like membrane fraction, termed the "mitochondria-associated membrane" (MAM), that co-isolates with mitochondria from rat liver has been characterized. One potential function of the MAM is as a membrane bridge between the endoplasmic reticulum and mitochondria that may be involved in transfer of phospholipids between these two organelles (Vance, J. E. (1990) J. Biol. Chem. 265, 7248-7256). A polyclonal antibody directed against a peptide corresponding to the C terminus of phosphatidylethanolamine N-methyltransferase-2, a specific marker protein of the MAM (Cui, Z., Vance, J. E., Chen, M. H., Voelker, D. R., and Vance, D. E. (1993) J. Biol. Chem. 268, 16655-16663), was used in immunofluorescence and immunogold electron microscopy localization studies in rat hepatocytes. Immunoreactive protein was clustered in regions of the cell that did not correspond to the bulk of endoplasmic reticulum. A second potential role for the MAM, as a component of the secretory pathway that supplies lipids for assembly into very low density lipoproteins, has been examined. The MAM contains enzymes of similar, or higher, specific activities to those enzymes in the endoplasmic reticulum for the synthesis of phospholipids, triacylglycerols, cholesterol, and cholesteryl esters. Specific activities of diacylglycerol acyltransferase, acyl-coenzyme A:cholesterol acyltransferase, and phosphatidylserine synthase (base exchange enzyme) are enriched 2.2-3.4-fold in the MAM compared with endoplasmic reticulum. In addition, the microsomal triacylglycerol transfer protein, which is required for the assembly/secretion of apolipoprotein B-containing lipoproteins, was present in the MAM. Nascent apolipoprotein B-containing lipoproteins were isolated from the lumen of the MAM. These lipoproteins had the same average density and composition as nascent apolipoprotein B-containing lipoproteins isolated from heavy and light endoplasmic reticulum fractions, from the Golgi, and lipoproteins newly secreted by cultured rat hepatocytes. The MAM is a pre-Golgi compartment of the secretory route, as shown by pulse-chase studies with apolipoprotein B and albumin, as well as the sensitivity of luminal apolipoprotein B to endoglycosidase H.

Relationship between fatty acid accretion, membrane composition, and biologic functions

Dietary fat affects metabolic pathways for phospholipid biosynthesis in tissues in a coordinated fashion. This may be important to aspects of development that concern phosphatidylcholine metabolism or regulatory processes that depend on signals from a changing milieu in the microenvironment of the membrane. Dietary fat influences the phosphatidylethanolamine (PE) composition in many membranes of the brain and retina and may by altered by small changes in the content of 20:4(6) and 22:6(3). Membrane PE fatty acids that contain one, four, or six double bonds and the ratio of 22:5(6) to 22:6(3) in PE that contains four to six double bonds are also affected. An increase in the omega 6 fatty acid content of membranes is associated with increased PE methyltransferase activity and decreased phosphocholine transferase activity, thus indicating a mechanism by which change in an exogenous factor (e.g., dietary fat intake) may alter neural phospholipid biosynthesis. Small changes in the composition of dietary fat intake change the composition of brain membranes during development. It is provocative to ponder whether diet could be used to induce formation of membrane structures that are more resistant to specific insults that cause degeneration of brain structural material, to ensure optimal functional compositions, or to reverse degenerative changes that occur in neural membrane structure and function.

Suppression of rat hepatoma cell growth by expression of phosphatidylethanolamine N-methyltransferase-2

The expression of rat liver phosphatidylethanolamine N-methyltransferase-2 (PEMT2) in McA-RH7777 rat hepatoma cells resulted in the unexpected inhibition of cell growth. There was a strict correlation (r = 0.973) between the level of expression of the enzyme activity and the generation time for hepatoma cell division. Expression of other foreign proteins via the same vector did not inhibit McA-RH7777 cell growth; thus, retardation of cell division was specific for the methyltransferase. Addition of 1 microM 3-deazaadenosine, which causes inhibition of phosphatidylethanolamine methylation, reversed the PEMT2-mediated inhibition of cell division. Transfection of a line of Chinese hamster ovary cells with PEMT2 had no effect on the division of these cells. Induction of hepatic tumors in rats with N-nitrosodiethylamine coincided with a striking decrease in methyltransferase activity and immunoreactive protein in the tumor nodules. Thus, data from studies in cell culture and intact rats suggest a regulatory role for PEMT2 in hepatocyte cell growth and possibly in the development of liver cancer.

Effect of taurine and methionine on sarcoplasmic reticular Ca2+ transport and phospholipid methyltransferase activity

Perfusion of rat hearts with buffer containing 300 microM L-methionine led to a decrease in Ca(2+)-induced Ca2+ release in sarcoplasmic reticulum (SR) but an increase in calcium-independent Ca2+ release from junctional SR. These effects of L-methionine were not altered by exposure of the hearts to high levels of extracellular taurine, but changes in the size of the intracellular taurine pool appear to modulate calcium transport in SR through two mechanisms. First, millimolar concentrations of taurine can directly promote release of calcium from 45Ca(2+)-loaded junctional SR vesicles. Second, taurine serves as an inhibitor of SR phospholipid methyltransferase, an enzyme that appears to be responsible for methionine-mediated loss in Ca(2+)-induced Ca2+ release activity and promotion of Ca(2+)-independent Ca2+ release. The data imply that modulation of the intracellular taurine pool may affect cellular calcium homeostasis and myocardial contractile function. This may be important in development of the cardiomyopathy linked to taurine deficiency.

Hepatic phosphatidylethanolamine methyltransferase activity is decreased by ethanol and increased by phosphatidylcholine

Phosphatidylethanolamine N-methyltransferase participates in the synthesis of membrane phosphatidylcholine. Its activity was reported to be decreased in patients with alcoholic cirrhosis, but it is not known whether this is a consequence of the cirrhosis or precedes it. This question was studied in a baboon model of alcohol-induced fibrosis. Phosphatidylethanolamine N-methyltransferase activity was measured in sequential percutaneous needle liver biopsies by the conversion of phosphatidylethanolamine to phosphatidylcholine, using radioactive S-adenosylmethionine as a methyl donor. Chronic alcohol consumption (1-6 years) significantly decreased hepatic phospholipid and phosphatidylcholine levels and reduced phosphatidyl-ethanolamine N-methyltransferase activity even before the development of fibrosis. These effects were prevented or attenuated by supplementing the diet with 2.8 g/1000 kcal of a preparation rich in dilinoleoyl phosphatidylcholine, a highly bioavailable phosphatidylcholine species. There were significant (p < 0.001) correlations between phosphatidylethanolamine N-methyltransferase activity and both hepatic phosphatidylcholine (r = 0.678) and total phospholipid (r = 0.662).

CONCLUSIONS

1. Alcohol consumption diminishes phosphatidylethanolamine N-methyltransferase activity prior to the development of cirrhosis and decreases the hepatic content of its product, namely phosphatidylcholine, a key component of cell membranes. This may promote hepatic injury and possibly trigger fibrosis. 2. Phosphatidylcholine administration ameliorates the ethanol-induced decrease in phosphatidylethanolamine N-methyltransferase activity and corrects phospholipid and phosphatidylcholine depletions, thereby possibly contributing to the protection against alcoholic liver injury.

Isolation and characterization of a mutant of Saccharomyces cerevisiae with pleiotropic deficiencies in transcriptional activation and repression

The isolation of the dep1 mutant of Saccharomyces cerevisiae is reported. The mutant was identified by its disability to regulate expression of structural genes involved in phospholipid biosynthesis, INO1, CHO1 and OPI3, in response to supplementation with soluble lipid precursors. Expression of the INO1, CHO1 and OPI3 genes was not fully derepressed in the absence of soluble lipid precursors, inositol and choline in the dep1 mutant, as compared to wild type. The mutant also exhibited incomplete repression of these same genes in the presence of inositol and choline. Repression by phosphate of the PHO5 gene was reduced in the mutant, as was derepression of this gene in the absence of phosphate. In addition, we show that expression of INO1 and OPI3 structural genes is strongly dependent on the growth phase both in wild-type and dep1 mutant strains. However, in the mutant, elevated basal steady-state mRNA levels were reached in the late stationary growth phase, independent of supplementation conditions. The dep1 mutation represents a new complementation group with respect to phospholipid synthesis and was mapped to a position of about 12 cM distal from the centromere on the left arm of chromosome I. Deficiencies in transcription activation and repression of metabolically unrelated genes, as well as reduced mating efficiency and lack of sporulation of homozygous diploid dep1/dep1 mutants indicate a pleiotropic regulatory function of the DEP1 gene product. Thus, Dep1p appears to be a new member of a class of transcriptional modulators, including Rpd1p/Sin3p/Ume4p/Sdi1p/Gam3p, Rpd3p, Spt10p and Spt21p.

Abnormal metabolism of S-adenosyl-L-methionine in hypoxic rat liver. Similarities to its abnormal metabolism in alcoholic cirrhosis

S-Adenosylmethionine (AdoMet) is an important biologic methylating agent for nucleic acids, phospholipids, biologic amines, and proteins. Previous studies indicated that hepatic AdoMet synthetase and hepatic levels of AdoMet are subnormal in patients with alcoholic cirrhosis. This abnormality limits the patients' capacity to convert phosphatidylethanolamine to phosphatidylcholine by way of phosphatidylethanolamine-N-methyltransferase (PEMT). Because alcoholic consumption appears to be associated with hepatic hypoxia, we previously measured AdoMet concentration in liver cells under acute hypoxia and found the level to be decreased substantially. In the present study, we determined whether a similar metabolic abnormality was also observed in rats maintained under physiologic hypoxia for 9 days and administered standard rat chow. The study showed that AdoMet levels in hypoxic rat (ave +/- SD) were significantly lower than those in the control (36.8 +/- 11.6 vs. 60.4 +/- 2.3 nmol/g liver; P < 0.05). Also significantly lower in the hypoxic group were the activities of AdoMet synthetase (0.60. +/- 0.07 vs. 0.97 +/- 0.20 U; P < 0.05) and PEMT (26.2 +/- 4.2 vs. 35.6 +/- 5.8 U; P < 0.02). The mRNA levels of AdoMet synthetase also declined in hypoxia indicating that hypoxia may modulate the gene expression of hepatic AdoMet synthetase. Thus, in vivo hypoxia may have an important effect on 1-carbon metabolism.

S-adenosyl-L-methionine-mediated enzymatic methylations in the rat retinal membranes

Enzymatic step-wise methylation of membrane phosphatidylethanolamine (PE) to phosphatidyl-N-methylethanolamine (PME) and then phosphatidyl-choline (PC) has been known to alter membrane properties and responsiveness of cells for activation of receptors by chemical transmitters. Conversion of PE to PME and PME to PC in the presence of S-adenosyl-L-methionine (SAM) are catalyzed by two phospholipid N-methyltransferases, PMT I and PMT II, of which PMT I is the rate limiting enzyme. Retina is a good neuronal model for chemical transmission. However, retina was not studied for PMT activity. Therefore, we studied the rat retina for PMT I activity. Methylation of PE in the rat retinal sonicates was assayed using 3H-SAM (2 microM) at 37 degrees C in Tris-glycylglycine buffer (50 mM, pH 8.0) and methylated phospholipids were extracted with chloroform/methanol/HCl (2/1/0.02, v/v) and separated by thin layer chromatography on Silica Gel G plates. Chromatograms were developed in a solvent system of propionic acid/n-propyl alcohol/chloroform/water (2/2/1/1, v/v). This study gave the following results: (a) the total methylated phospholipids were (M +/- SE, N = 5) 19.90 +/- 4.03 fmol/mg protein/min; (b) the major methylated phospholipid was PME (4.21 +/- 0.68 fmol/mg protein/min; (c) the fatty acid methylesters formed by fatty acid carboxymethylase (FACM) which accumulated in the solvent front amounted to 18.82 +/- 2.84 fmol/mg protein/min. Both PMT I and FACM were inhibited by S-adenosyl-L-homocysteine (I50, 1.2-5 microM). These observations indicate that rat retina contains both PMTs and FACM.

Involvement of mitochondrial contact sites in the subcellular compartmentalization of phospholipid biosynthetic enzymes

The concerted synthesis of phospholipids derived from serine involving two microsomal enzymes (phosphatidylserine synthase and phosphatidylethanolamine N-methyltransferase) and a mitochondrial one (phosphatidylserine decarboxylase) occurs in reconstituted cell-free systems. Subfractionation of crude mitochondria after swelling and separating on a sucrose density gradient resulted in the isolation of two contact site-enriched fractions from total outer membranes and inner membranes, respectively. Estimation of marker enzyme activities shows a high recovery of glucose-6-phosphate phosphatase (a marker for the endoplasmic reticulum) associated with contact site-enriched fractions. Accordingly, the linked synthesis of phosphatidylserine, phosphatidylethanolamine, and at a lesser extent phosphatidylcholine can occur. This biosynthetic pathway was absent from purified contact site-enriched fractions correlative with the absence of glucose-6-phosphate phosphatase activity. Reconstitution experiments, including contact site-enriched fractions incubated with endoplasmic reticulum-rich fraction, led to the restoration of the linked synthesis of phospholipids, thereby demonstrating that a reversible association between these two fractions can occur. These functional interactions between the endoplasmic reticulum and mitochondria are confirmed at the ultrastructural level using either chemical or physical fixation before resin embedding. These results show that the interorganelle trafficking of lipids may involve only highly specialized microdomains of both membranes, thereby allowing the maintenance of a specific lipid composition and distribution within membranes.

Cloning and expression of a novel phosphatidylethanolamine N-methyltransferase. A specific biochemical and cytological marker for a unique membrane fraction in rat liver

Phosphatidylethanolamine N-methyltransferase catalyzes the synthesis of phosphatidylcholine from phosphatidylethanolamine and is most active in liver. A cDNA for this enzyme from a rat liver cDNA library has been cloned, sequenced, and expressed in COS-1 cells, McArdle-RH7777 rat hepatoma cells, and Sf9 insect cells. The expressed protein was capable of converting phosphatidylethanolamine into phosphatidylcholine in intact COS-1 cells, which normally have very low methyltransferase activity. The calculated molecular mass of the methyltransferase protein is 22.3 kDa, which is equivalent to that of the pure protein isolated from rat liver. Comparison of the sequence of the cloned rat liver methyltransferase with the yeast phosphatidylethanolamine methyltransferase PEM2 gene product revealed 44% identical amino acids and 68% similarity in the two predicted protein sequences. A polyclonal antibody was raised against a synthetic peptide corresponding to the carboxyl-terminal region of the enzyme and was affinity purified. The antibody recognized a single protein with a molecular mass of approximately 20 kDa when either rat liver proteins or proteins derived from the transfected COS-1 cells were electrophoresed on polyacrylamide gels containing sodium dodecyl sulfate. Surprisingly, the antibody exhibited no reactivity with endoplasmic reticulum proteins, even though the major phosphatidylethanolamine methyltransferase activity resides on this subcellular organelle. Instead, the antibody specifically recognized a protein in a unique subcellular membrane fraction purified from a crude mitochondrial preparation on a Percoll gradient. Immunocytochemical examination by electron microscopy showed positive labeling only in unique regions of the hepatocytes. The data suggest that this phosphatidylethanolamine methyltransferase is a specific marker for this unique membrane fraction.

Isolation and functional expression in Escherichia coli of a gene encoding phosphatidylethanolamine methyltransferase (EC 2.1.1.17) from Rhodobacter sphaeroides

Phosphatidylcholine is a major component of membranes in most eukaryotes, but it is found only in a small number of bacteria, where it is synthesized by N-methylation of phosphatidylethanolamine. In yeast and other fungi the methylation of phosphatidylethanolamine to phosphatidylcholine proceeds in two steps: the methylation of phosphatidylethanolamine by phosphatidylethanolamine methyltransferase followed by the methylation of monomethylphosphatidylethanolamine by phospholipid methyltransferase. Here we describe the isolation of two allelic phosphatidylcholine-deficient mutants of Rhodobacter sphaeroides which are unable to methylate phosphatidylethanolamine, monomethylphosphatidylethanolamine, or dimethylphosphatidylethanolamine. A DNA fragment containing a gene designated pmtA, which encodes a 22.9-kDa protein, was found to complement both mutants. Expression of this gene in Escherichia coli, which normally lacks phosphatidylcholine or methylated derivatives of phosphatidylethanolamine, resulted in the formation of phosphatidylcholine. A protein extract derived from the E. coli strain expressing the pmtA gene was able to convert phosphatidylethanolamine, mono- and dimethylphosphatidylethanolamine into phosphatidylcholine. Based on these data we conclude that the product of the pmtA gene catalyzes a sequence of three chemically distinct, methylation reactions beginning with phosphatidylethanolamine and leading to the formation of phosphatidylcholine in R. sphaeroides.

Molecular cloning of the yeast OPI3 gene as a high copy number suppressor of the cho2 mutation

By functional complementation of the auxotrophic requirements for choline of a cdg1, cho2 double-mutant, by transformation with a genomic DNA library in a high copy number plasmid, two different types of complementing DNA inserts were identified. One type of insert was earlier shown to represent the CHO2 structural gene. In this report we describe the molecular and biochemical chemical characterization of the second type of complementing activity. The transcript encoded by the cloned gene was about 1000-nt in length and was regulated in response to the soluble phospholipid precursors, inositol and choline. A gene disruption resulted in no obvious growth phenotype at 23 degrees C or 30 degrees C, but in a lack of growth at 37 degrees C in the presence of monomethylethanolamine. Null-mutants exhibited an inositol-secretion phenotype, indicative of mutations in the lipid biosynthetic pathway. Complementation analysis, biochemical analysis of the phospholipid methylation pathway in vivo, and comparison of the restriction pattern of the cloned gene to published sequences, unequivocally identified the cloned gene as the OPI3 gene, encoding phospholipid-N-methyltransferase in yeast. When present in multiple copies the OPI3 gene efficiently suppresses the phospholipid methylation defect of a cho2 mutation. As a result of impaired synthesis of phosphatidylcholine, the INO1-deregulation phenotype is abolished in cho2 mutants transformed with the OPI3 gene on a high copy number plasmid.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of molecular species of phosphatidylethanolamine and phosphatidylcholine in rat hepatocytes during prolonged inhibition of phosphatidylethanolamine N-methyltransferase

The metabolism of the molecular species of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) derived from [3H]ethanolamine has been studied in rat hepatocytes during prolonged inhibition of phosphatidylethanolamine-N-methyltransferase (PEMT) with 3-deazaadenosine (DZA). After an initial pulse of radioactivity for 1 h and a chase for up to 24 h in the presence or absence of DZA, the cells were harvested and the incorporation of label into the various molecular species of PE and PC was determined. Prolonged inhibition of PEMT did not affect the mol% distribution of either PE or PC molecular species. Thus, PE methylation is not required for the maintenance of cellular PE and PC molecular species composition. While the overall catabolism of PE was not affected by DZA treatment, inhibition of PEMT resulted in the selective degradation of 16:0-22:6-PE. The major catabolic products of PE in the hepatocytes and the medium were glycerophosphoethanolamine and ethanolamine-phosphate. PC, derived from PE, was remodeled at both the sn-1 and sn-2 positions and this process was not affected by the inhibition of PEMT. The major species being remodeled was 16:0-22:6-PC. The rapid turnover of 16:0-22:6-PE and PC species compared to other PE and PC species may be due to the presence of a 16:0-22:6 selective phospholipase.

Phospholipid methyltransferase activity in diabetic rat fat cells: effect of isoproterenol and insulin

The effects of isoproterenol and insulin on phospholipid methyltransferase (PLMT) activity were investigated in adipocytes from control and streptozotocin-diabetic rats. PLMT activity was assayed by measuring the rate of incorporation of 3H-methyl groups from S-adenosyl-L-[methyl-3H] methionine into phospholipids. Basal PLMT activity was higher in adipocytes from diabetic animals. Treatment of adipocytes with isoproterenol induced a concentration-dependent stimulation of PLMT activity. In control adipocytes, the maximal effect was obtained at 100 nM isoproterenol with 2.3 fold increase in PLMT activity and a half maximal effect at 25 nM. In adipocytes from diabetic rats, a lower dose of isoproterenol (10 nM), caused 1.2 fold increase with a half maximal effect at 4 nM. Addition of 100 nM insulin inhibited basal PLMT activity and the stimulatory effect of isoproterenol in both types of adipocytes. The beta-adrenergic blocking agent propranolol inhibited the stimulatory effect of isoproterenol on PLMT activity in control and diabetic adipocytes. Intracellular concentration of cAMP was higher in diabetic adipocytes but decreased to normal values after incubation in the presence of insulin.

Regulation of phosphatidylethanolamine methyltransferase and phospholipid methyltransferase by phospholipid precursors in Saccharomyces cerevisiae

Phosphatidylethanolamine methyltransferase (PEMT) and phospholipid methyltransferase (PLMT), which are encoded by the CHO2 and OPI3 genes, respectively, catalyze the three-step methylation of phosphatidylethanolamine to phosphatidylcholine in Saccharomyces cerevisiae. Regulation of PEMT and PLMT as well as CHO2 mRNA and OPI3 mRNA abundance was examined in S. cerevisiae cells supplemented with phospholipid precursors. The addition of choline to inositol-containing growth medium repressed the levels of CHO2 mRNA and OPI3 mRNA abundance in wild-type cells. The major effect on the levels of the CHO2 mRNA and OPI3 mRNA occurred in response to inositol. Regulation was also examined in cho2 and opi3 mutants, which are defective in PEMT and PLMT activities, respectively. These mutants can synthesize phosphatidylcholine when they are supplemented with choline by the CDP-choline-based pathway but they are not auxotrophic for choline. CHO2 mRNA and OPI3 mRNA were regulated by inositol plus choline in opi3 and cho2 mutants, respectively. However, there was no regulation in response to inositol when the mutants were not supplemented with choline. This analysis showed that the regulation of CHO2 mRNA and OPI3 mRNA abundance by inositol required phosphatidylcholine synthesis by the CDP-choline-based pathway. The regulation of CHO2 mRNA and OPI3 mRNA abundance generally correlated with the activities of PEMT and PLMT, respectively. CDP-diacylglycerol synthase and phosphatidylserine synthase, which are regulated by inositol in wild-type cells, were examined in the cho2 and opi3 mutants. Phosphatidylcholine synthesis was not required for the regulation of CDP-diacylglycerol synthase and phosphatidylserine synthase by inositol.

Identification of the upstream activation sequences responsible for the expression and regulation of the PEM1 and PEM2 genes encoding the enzymes of the phosphatidylethanolamine methylation pathway in Saccharomyces cerevisiae

The yeast phosphatidylethanolamine methylation pathway is encoded by two structural genes, PEM1 and PEM2. The abundance of their transcripts was coordinately repressed by myo-inositol and choline. The most upstream transcriptional start sites for PEM1 and PEM2 were mapped at positions -142 and -42 relative to their first ATG codons, respectively. Promoter deletion analysis defined the 5' boundary of the regulatory region of PEM1 between -336 and -332 and that of PEM2 between -177 and -158. The 38-bp sequence between -336 and -299 from PEM1 and the 48-bp sequence between -177 and -130 from PEM2 conferred regulated transcription upon an upstream-activation-sequence-deficient test gene, CYC1-lacZ. Comparison of these two regions revealed the presence of a common octameric sequence, 5-CATRTGAA-3', which occurred twice in the 38-bp PEM1 regulatory region and once, followed by the 5'-AAACCCACACATG-3' GRFI site, in the 48-bp PEM2 regulatory region. When synthesized chemically and placed in front of CYC1-lacZ, a single copy of CATATGAA directed a rather low level of gene expression, but multiple copies produced high-level expression. In both cases, gene expression was sensitive to myo-inositol and choline. The synthesized GRFI site directed considerable but constitute lacZ expression. When used in conjunction with CATATGAA, synergistic, regulated gene expression was obtained. Hence CATRTGAA was concluded to play an important role in the myo-inositol-choline regulation of PEM1 and PEM2. Binding proteins to these sequences were demonstrated by electrophoretic mobility shift assay. Protein binding to CATRTGAA was not competitive with binding to the GRFI sequence, and vice versa. CATRTGAA was also found in the upstream regions of other genes encoding phospholipid-synthesizing enzymes, such as choline kinase, phosphatidylserine synthase, and myo-inositol-1-phosphate synthase, known to be repressed by myo-inositol and choline.

Dietary orotic acid accentuates the hepatic response to phenobarbital in rats

In rats treated with phenobarbital for 3 days and simultaneously fed a semisynthetic diet containing 1.0% orotic acid, the extent of the increases in liver microsomal phosphatidylcholine, phosphatidylethanolamine, total RNA, total protein, and cytochrome P-450 were significantly greater than they were in rats treated identically with phenobarbital but without dietary orotic acid. This is attributed primarily to the stimulation of hepatic phosphatidylcholine synthesis by dietary orotic acid. In the absence of phenobarbital, orotic acid was shown to cause some increase in liver smooth endoplasmic reticulum components, but not cytochrome P-450. Orotic acid also decreased the activity of microsomal phosphatidylethanolamine N-methyltransferase, which may have contributed to the increase in the microsomal content of phosphatidylethanolamine. The hypothesis is advanced that phospholipid availability is a limiting factor in the hepatic response to phenobarbital. When more phospholipid is available to provide the structural framework for biogenesis of endoplasmic reticulum, all of the hepatic actions of phenobarbital, including induction of cytochrome P-450, are amplified.

Alterations in phospholipid N-methylation of cardiac subcellular membranes due to experimentally induced diabetes in rats

Phosphatidylethanolamine N-methylation was examined in cardiac subcellular membranes after inducing chronic experimental diabetes in rats (65 mg streptozotocin/kg, i.v.). The incorporation of radiolabeled methyl groups from S-adenosyl-L-methionine in diabetic sarcolemma was significantly depressed at all three catalytic sites (I, II, and III) of the methyltransferase system. An increase in methyl group incorporation was evident at site I without any changes at sites II and III in diabetic sarcoplasmic reticulum and mitochondria. Similar changes were also seen for the individual N-methylated lipids (monomethyl-, dimethylphosphatidylethanolamine, and phosphatidylcholine) specifically formed at each catalytic site in all cardiac membranes from diabetic animals. These alterations in N-methylation were reversible by a 14-d insulin therapy to the diabetic animals. In the presence of 10 microM ATP and 0.1 microM Ca2+, N-methylation was maximally activated at site I in both control and diabetic sarcolemma and sarcoplasmic reticulum, but not in mitochondria. Incubation of cardiac membranes with of S-adenosyl-L-methionine showed that Ca2(+)-stimulated ATPase activities in both sarcolemma and sarcoplasmic reticulum were augmented; however, the activation of diabetic sarcolemma was lesser and that of diabetic sarcoplasmic reticulum was greater in comparison with the control preparations. These results identify alterations in phosphatidylethanolamine N-methylation in subcellular membranes from diabetic heart, and it is suggested that these defects may be crucial in the development of cardiac dysfunction in chronic diabetes.