Role of circadian clock system in the mitochondrial trans-sulfuration pathway and tissue remodeling

Previous studies from our laboratory revealed that the gaseous molecule hydrogen sulfide (H2S), a metabolic product of epigenetics, involves trans-sulfuration pathway for ensuring metabolism and clearance of homocysteine (Hcy) from body, thereby mitigating the skeletal muscle's pathological remodeling. Although the master circadian clock regulator that is known as brain and muscle aryl hydrocarbon receptor nuclear translocator like protein 1 (i.e., BMAL 1) is associated with S-adenosylhomocysteine hydrolase (SAHH) and Hcy metabolism but how trans-sulfuration pathway is influenced by the circadian clock remains unexplored. We hypothesize that alterations in the functioning of circadian clock during sleep and wake cycle affect skeletal muscle's biology. To test this hypothesis, we measured serum matrix metalloproteinase (MMP) activities using gelatin gels for analyzing the MMP-2 and MMP-9. Further, employing casein gels, we also studied MMP-13 that is known to be influenced by the growth arrest and DNA damage-45 (GADD45) protein during sleep and wake cycle. The wild type and cystathionine β synthase-deficient (CBS-/+) mice strains were treated with H2S and subjected to measurement of trans-sulfuration factors from skeletal muscle tissues. The results suggested highly robust activation of MMPs in the wake mice versus sleep mice, which appears somewhat akin to the "1-carbon metabolic dysregulation", which takes place during remodeling of extracellular matrix during muscular dystrophy. Interestingly, the levels of trans-sulfuration factors such as CBS, cystathionine γ lyase (CSE), methyl tetrahydrofolate reductase (MTHFR), phosphatidylethanolamine N-methyltransferase (PEMT), and Hcy-protein bound paraoxonase 1 (PON1) were attenuated in CBS-/+ mice. However, treatment with H2S mitigated the attenuation of the trans-sulfuration pathway. In addition, levels of mitochondrial peroxisome proliferator-activated receptor-gamma coactivator 1-α (PGC 1-α) and mitofusin-2 (MFN-2) were significantly improved by H2S intervention. Our findings suggest participation of the circadian clock in trans-sulfuration pathway that affects skeletal muscle remodeling and mitochondrial regeneration.

Construction of a novel choline metabolism-related signature to predict prognosis, immune landscape, and chemotherapy response in colon adenocarcinoma

BACKGROUND

Colon adenocarcinoma (COAD) is a common digestive system malignancy with high mortality and poor prognosis. Accumulating evidence indicates that choline metabolism is closely related to tumorigenesis and development. However, the efficacy of choline metabolism-related signature in predicting patient prognosis, immune microenvironment and chemotherapy response has not been fully clarified.

METHODS

Choline metabolism-related differentially expressed genes (DEGs) between normal and COAD tissues were screened using datasets from The Cancer Genome Atlas (TCGA), Kyoto Encyclopedia of Genes and Genomes (KEGG), AmiGO2 and Reactome Pathway databases. Two choline metabolism-related genes (CHKB and PEMT) were identified by univariate and multivariate Cox regression analyses. TCGA-COAD was the training cohort, and GSE17536 was the validation cohort. Patients in the high- and low-risk groups were distinguished according to the optimal cutoff value of the risk score. A nomogram was used to assess the prognostic accuracy of the choline metabolism-related signature. Calibration curves, decision curve analysis (DCA), and clinical impact curve (CIC) were used to improve the clinical applicability of the prognostic signature. Gene Ontology (GO) and KEGG pathway enrichment analyses of DEGs in the high- and low-risk groups were performed. KEGG cluster analysis was conducted by the KOBAS-i database. The distribution and expression of CHKB and PEMT in various types of immune cells were analyzed based on single-cell RNA sequencing (scRNA-seq). The CIBERSORT and ESTIMATE algorithms evaluated tumor immune cell infiltration in the high- and low-risk groups. Evaluation of the half maximal inhibitory concentration (IC50) of common chemotherapeutic drugs based on the choline metabolism-related signature was performed. Small molecule compounds were predicted using the Connectivity Map (CMap) database. Molecular docking is used to simulate the binding conformation of small molecule compounds and key targets. By immunohistochemistry (IHC), Western blot, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) experiments, the expression levels of CHKB and PEMT in human, mouse, and cell lines were detected.

RESULTS

We constructed and validated a choline metabolism-related signature containing two genes (CHKB and PEMT). The overall survival (OS) of patients in the high-risk group was significantly worse than that of patients in the low-risk group. The nomogram could effectively and accurately predict the OS of COAD patients at 1, 3, and 5 years. The DCA curve and CIC demonstrate the clinical utility of the nomogram. scRNA-seq showed that CHKB was mainly distributed in endothelial cells, while PEMT was mainly distributed in CD4+ T cells and CD8+ T cells. In addition, multiple types of immune cells expressing CHKB and PEMT differed significantly. There were significant differences in the immune microenvironment, immune checkpoint expression and chemotherapy response between the two risk groups. In addition, we screened five potential small molecule drugs that targeted treatment for COAD. Finally, the results of IHC, Western blot, and qRT-PCR consistently showed that the expression of CHKB in human, mouse, and cell lines was elevated in normal samples, while PMET showed the opposite trend.

CONCLUSION

In conclusion, we constructed a choline metabolism-related signature in COAD and revealed its potential application value in predicting the prognosis, immune microenvironment, and chemotherapy response of patients, which may lay an important theoretical basis for future personalized precision therapy.

Choline in cystic fibrosis: relations to pancreas insufficiency, enterohepatic cycle, PEMT and intestinal microbiota

BACKGROUND

Cystic Fibrosis (CF) is an autosomal recessive disorder with life-threatening organ manifestations. 87% of CF patients develop exocrine pancreas insufficiency, frequently starting in utero and requiring lifelong pancreatic enzyme substitution. 99% develop progressive lung disease, and 20-60% CF-related liver disease, from mild steatosis to cirrhosis. Characteristically, pancreas, liver and lung are linked by choline metabolism, a critical nutrient in CF. Choline is a tightly regulated tissue component in the form of phosphatidylcholine (Ptd'Cho) and sphingomyelin (SPH) in all membranes and many secretions, particularly of liver (bile, lipoproteins) and lung (surfactant, lipoproteins). Via its downstream metabolites, betaine, dimethylglycine and sarcosine, choline is the major one-carbon donor for methionine regeneration from homocysteine. Methionine is primarily used for essential methylation processes via S-adenosyl-methionine.

CLINICAL IMPACT

CF patients with exocrine pancreas insufficiency frequently develop choline deficiency, due to loss of bile Ptd'Cho via feces. ~ 50% (11-12 g) of hepatic Ptd'Cho is daily secreted into the duodenum. Its re-uptake requires cleavage to lyso-Ptd'Cho by pancreatic and small intestinal phospholipases requiring alkaline environment. Impaired CFTR-dependent bicarbonate secretion, however, results in low duodenal pH, impaired phospholipase activity, fecal Ptd'Cho loss and choline deficiency. Low plasma choline causes decreased availability for parenchymal Ptd'Cho metabolism, impacting on organ functions. Choline deficiency results in hepatic choline/Ptd'Cho accretion from lung tissue via high density lipoproteins, explaining the link between choline deficiency and lung function. Hepatic Ptd'Cho synthesis from phosphatidylethanolamine by phosphatidylethanolamine-N-methyltransferase (PEMT) partly compensates for choline deficiency, but frequent single nucleotide polymorphisms enhance choline requirement. Additionally, small intestinal bacterial overgrowth (SIBO) frequently causes intraluminal choline degradation in CF patients prior to its absorption. As adequate choline supplementation was clinically effective and adult as well as pediatric CF patients suffer from choline deficiency, choline supplementation in CF patients of all ages should be evaluated.

Phenylethanolamine N-methyltransferase gene expression in transplanted human heart

BACKGROUND

Phenylethanolamine N-methyltransferase (PNMT) is an enzyme involved in the epinephrine synthesis. The aim of this study was to investigate PNMT gene expression in the transplanted human heart in relation to the time elapsed from heart transplantation (HTx) and selected clinical characteristics.

PATIENTS AND METHODS

The messenger RNA (mRNA) levels of PNMT in myocardial tissue were determined in 22 (21 males) patients at 0-12 years after HTx. Relative quantification of mRNA levels was performed using reverse transcription and polymerase chain reaction (RT-PCR) for correlation with heart rate (HR), blood pressure (BP), parameters of heart rate variability, and graft systolic and diastolic functions.

RESULTS

During the first 3 years after HTx, PNMT mRNA levels were higher (mean +/- SEM) (0.75 +/- 0.06; n = 12) compared with later years (0.33 +/- 0.06; n = 10); (P < .01). HR variability in the low frequency band of the power spectrum (LF) was lower among patients during the first 3 years after HTx (2.73 +/- 0.31 vs 5.67 +/- 0.69 ms; P < .01). A significant negative linear correlation was observed between PNMT mRNA and LF (P = .05; r = -0.47). No association was noted between HR, BP, parameters of systolic or diastolic function, standard deviation of all RR intervals, or high frequency (0.15-0.40 Hz) bands of the 24-hour RR interval power spectrum.

CONCLUSION

This work provides the first evidence of the presence of local PNMT transcription in human heart after HTx. Appearance of PNMT transcription in the myocardium after HTx may reflect autonomous "sympathicotrophy." Decreased PNMT expression with time elapsed after HTx suggests graft reinervation.

Origin of cocaine- and amphetamine-regulated transcript-containing axons innervating hypophysiotropic corticotropin-releasing hormone-synthesizing neurons in the rat

Cocaine- and amphetamine-regulated transcript (CART) has stimulatory effects on the hypothalamic-pituitary-adrenal axis through direct effects on hypophysiotropic CRH neurons. Recently CART-containing axons have been demonstrated to densely innervate the hypophysiotropic CRH neurons. Based on the sources of the CART-immunoreactive (IR) innervation of the paraventricular nucleus, the putative origins of these CART-containing fibers include neurons of the hypothalamic arcuate nucleus that coexpress alphaMSH and medullary adrenaline-producing neurons. To determine whether these cell groups contribute to the CART innervation of the hypophysiotropic CRH neurons, we performed a quadruple-labeling immunofluorescent study using antisera against CRH, CART, alphaMSH, and phenylethanolamine-N-methyl-transferase (PNMT), the latter as a marker for adrenaline. Consistent with previous observations, PNMT- and CART-IR axons densely innervated all CRH neurons, whereas the alphaMSH-IR innervation was sparse. Although approximately 60% of CART-IR varicosities in juxtaposition to CRH neurons cocontained PNMT, only approximately 18% of them were immunopositive for alphaMSH. All alphaMSH-IR boutons and approximately 90% of PNMT-containing varicosities on the surface of CRH neurons were also labeled for CART. The remaining 22% of CART axon varicosities in contact with CRH neurons contained neither alphaMSH nor PNMT. These results indicate that medullary adrenergic/CART neurons are the major source for the CART innervation of CRH neurons in the paraventricular nucleus; however, to a lesser extent the arcuate nucleus also contributes to the CART-IR innervation of these neurons. The observation that nearly 20% of the CART-IR afferents contain neither alphaMSH nor PNMT, however, suggests that additional sources also contribute to the CART-IR input of hypophysiotropic CRH neurons.

Localization-independent regulation of homocysteine secretion by phosphatidylethanolamine N-methyltransferase

Genetic ablation of phosphatidylethanolamine N-methyltransferase (PEMT) in mice causes a 50% reduction in plasma homocysteine (Hcy) levels. Because hyperhomocysteinemia is an independent risk factor for cardiovascular disease, resolution of the molecular basis for this reduction is of significant clinical interest. The PEMT pathway is a metabolically channeled process localized to the endoplasmic reticulum (ER). To assess the importance of PEMT localization for Hcy homeostasis, we identified and ablated the minimal ER targeting motif. Mutagenesis of a conserved, C-terminal lysine residue (197) relocalized the enzyme to the Golgi, demonstrating that Lys-197 is essential for targeting PEMT to the ER. To evaluate the functional significance of PEMT localization, hepatoma cell lines were generated that stably expressed either ER- or Golgi-localized PEMT only. Intriguingly, stable expression of PEMT in either the ER or the Golgi caused increased Hcy secretion. Moreover, PEMT-mediated Hcy secretion correlated with the methyltransferase activity of the enzyme, independently of subcellular localization. Thus, our data suggest that Hcy homeostasis is regulated concomitantly with PEMT activity but independently of PEMT localization.

Early embryonic lethality in mice with targeted deletion of the CTP:phosphocholine cytidylyltransferase alpha gene (Pcyt1a)

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-controlling step in the biosynthesis of phosphatidylcholine (PtdCho). Multiple CCT isoforms, CCTalpha, CCTbeta2, and CCTbeta3, are encoded by two genes, Pcyt1a and Pcyt1b. The importance of CCTalpha in mice was investigated by deleting exons 5 and 6 in the Pcyt1a gene using the Cre-lox system. Pcyt1a-/- zygotes failed to form blastocysts, did not develop past embryonic day 3.5 (E3.5), and failed to implant. In situ hybridization in E11.5 embryos showed that Pcyt1a is expressed ubiquitously, with the highest level in fetal liver, and CCTalpha transcripts are significantly more abundant than transcripts encoding CCTbeta or phosphatidylethanolamine (PtdEtn) N-methyl transferase, two other enzymes capable of producing PtdCho. Reduction of the CCTalpha transcripts in heterozygous E11.5 embryos was accompanied by upregulation of CCTbeta and PtdEtn N-methyltransferase transcripts. In contrast, enzymatic and real-time PCR data revealed that CCTbeta (Pcyt1b) expression is not upregulated to compensate for the reduction in CCTalpha expression in adult liver and other tissues from Pcyt1a+/- heterozygous mice. PtdCho biosynthesis measured by choline incorporation into isolated hepatocytes was not compromised in the Pcyt1a+/- mice. Liver PtdCho mass was the same in Pcyt1a+/+ and Pcyt1a+/- adult animals, but lung PtdCho mass decreased in the heterozygous mice. These data show that CCTalpha expression is required for early embryonic development, but that a 50% reduction in enzyme activity has little detectable impact on the operation of the CDP-choline metabolic pathway in adult tissues.

Relative contribution of brainstem afferents to the cocaine- and amphetamine-regulated transcript (CART) innervation of thyrotropin-releasing hormone synthesizing neurons in the hypothalamic paraventricular nucleus (PVN)

To determine the relative contribution of the brainstem to the CART innervation of the TRH neurons in the PVN, the major ascending brainstem axonal pathways to the PVN were unilaterally transected in the hypothalamus. After 2 weeks survival time, hypothalamic sections were prepared for immunocytochemistry. PNMT-IR axon density decreased 76.0 +/- 3.8% on the side of the knifecut compared to the contralateral side, demonstrating satisfactory disconnection of the ascending brainstem pathways. In contrast, the density of CART-IR axons in the PVN on the lesioned side was reduced by only 26.9 +/- 2.7%. Disconnection of brainstem pathways reduced the total number of TRH neurons contacted by CART from 99.4 +/- 0.9% on the intact side to 74.3 +/- 9.4% on the lesioned side, as well as the number of CART varicosities on the surface of TRH neurons from 6.0 +/- 0.9 to 2.3 +/- 0.4 CART-IR varicosities/cell. These data indicate that CART-IR neurons residing in the brainstem give rise to only approximately one third of the CART input to the PVN as a whole, but serve as a major source of the CART-IR innervation of hypophysiotropic TRH neurons.

In Vivo Evidence for the Specificity of Plasmodium falciparum Phosphoethanolamine Methyltransferase and Its Coupling to the Kennedy Pathway

Unlike humans and yeast, Plasmodium falciparum, the agent of the most severe form of human malaria, utilizes host serine as a precursor for the synthesis of phosphatidylcholine via a plant-like pathway involving phosphoethanolamine methylation. The monopartite phosphoethanolamine methyltransferase, Pfpmt, plays an important role in the biosynthetic pathway of this major phospholipid by providing the precursor phosphocholine via a three-step S-adenosyl-L-methionine-dependent methylation of phosphoethanolamine. In vitro studies showed that Pfpmt has strong specificity for phosphoethanolamine. However, the in vivo substrate (phosphoethanolamine or phosphatidylethanolamine) is not yet known. We used yeast as a surrogate system to express Pfpmt and provide genetic and biochemical evidence demonstrating the specificity of Pfpmt for phosphoethanolamine in vivo. Wild-type yeast cells, which inherently lack phosphoethanolamine methylation, acquire this activity as a result of expression of Pfpmt. The Pfpmt restores the ability of a yeast mutant pem1Deltapem2Delta lacking the phosphatidylethanolamine methyltransferase genes to grow in the absence of choline. Lipid analysis of the Pfpmt-complemented pem1Deltapem2Delta strain demonstrates the synthesis of phosphatidylcholine but not the intermediates of phosphatidylethanolamine transmethylation. Complementation of the pem1Deltapem2Delta mutant relies on specific methylation of phosphoethanolamine but not phosphatidylethanolamine. Interestingly, a mutation in the yeast choline-phosphate cytidylyltransferase gene abrogates the complementation by Pfpmt thus demonstrating that Pfpmt activity is directly coupled to the Kennedy pathway for the de novo synthesis of phosphatidylcholine.

Gene expression profiling in phosphatidylethanolamine N-methyltransferase knockout mice

Choline is derived from the diet as well as from de novo methylation of phosphatidylethanolamine catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). Pemt knockout mice have no endogenous synthesis of choline molecules. We previously reported that these mice have excess S-adenosylmethionine and hypermethylated DNA in brain, as well as increased mitosis in neural progenitor cells of the hippocampus in embryonic day 17 (E17) brain. In the present study, E17 fetal brains and adult brains were harvested and total RNA was extracted. In fetal brain, using gene expression profiling and Significance Analysis of Microarrays, we identified 107 significant genes with increased expression and 379 significant genes with decreased expression. In adult brain, we identified 381 significant genes with increased expression and 1037 significant genes with decreased expression. We observed significant changes in expression of genes regulating cell cycle (such as TP53, Fgf4, and Ing1), differentiation and neurogenesis (such as S100A4 and D14Ws), and phospholipid metabolism (such as Pip5k1a, Pitpn, and Pla2g6) as well as in a number of methyltransferase genes (including Gnmt). Some genes with expression known to be regulated by promoter methylation were suppressed in Pemt knockout brain (such as S100a4 and TP53). These findings are consistent with the biochemical changes that we previous reported in fetal brains from Pemt knockout mice. This is the first report of gene profiling in Pemt(-/-) mouse brain.

A comparison of the effects of betaine and S-adenosylmethionine on ethanol-induced changes in methionine metabolism and steatosis in rat hepatocytes

Previous studies showed that chronic ethanol administration alters methionine metabolism in the liver, resulting in increased intracellular S-adenosylhomocysteine (SAH) levels and increased homocysteine release into the plasma. We showed further that these changes appear to be reversed by betaine administration. This study compared the effects of betaine and S-adenosylmethionine (SAM), another methylating agent, on ethanol-induced changes of methionine metabolism and hepatic steatosis. Wistar rats were fed ethanol or control Lieber-Decarli liquid diet for 4 wk and metabolites of the methionine cycle were measured in isolated hepatocytes. Hepatocytes from ethanol-fed rats had a 50% lower intracellular SAM:SAH ratio and almost 2-fold greater homocysteine release into the media compared with controls. Supplementation of betaine or SAM in the incubation media increased this ratio in hepatocytes from both control and ethanol-fed rats and attenuated the ethanol-induced increased hepatocellular triglyceride levels by approximately 20%. On the other hand, only betaine prevented the increase in generation of homocysteine in the incubation media under basal and methionine-loaded conditions. SAM can correct only the ratio and the methylation defects and may in fact be detrimental after prolonged use because of its propensity to increase homocysteine release. Both SAM and betaine are effective in increasing the SAM:SAH ratio in hepatocytes and in attenuating hepatic steatosis; however, only betaine can effectively methylate homocysteine and prevent increased homocysteine release by the liver.

Lysophosphatidylcholine Decreases Locomotor Activities and Dopamine Turnover Rate in Rats

Lysophosphatidylcholine (lyso-PTC), a secondary product of S-adenosylmethionine (SAM)-dependent phosphatidylethanolamine (PTE) methylation, is a potent cytotoxin and might be involved in the pathogenesis of Parkinson's disease (PD). Our previous studies showed that the injection of SAM into the brain caused PD-like changes in rodents. Moreover, 1-methyl-4-phenylpyridinium (MPP+), a Parkinsonism-inducing agent, increased lyso-PTC formation via the stimulation of PTE methylation pathway. These results indicate a possible role of lyso-PTC in the PD-like changes seen following the injection of SAM or MPP+. In the present study, lyso-PTC was injected into the lateral ventricle of rats and locomotor activities and the biogenic amine levels were measured to evaluate the effects of lyso-PTC on the dopaminergic system. Quinacrine, a phospholipase A2 (PLA2) inhibitor, was employed to determine its protective effect on SAM-induced PD-like changes by the inhibition of lyso-PTC formation. The results showed that 1 h after the injection, 0.4 and 0.8 micromol of lyso-PTC increased striatal dopamine (DA) by 20 and 24%, decreased 3,4-dihydroxyphenylacetic acid (DOPAC) by 37 and 45% and decreased homovanilic acid (HVA) by 24 and 13%, respectively. Consequently, dopamine turnover rate, (DOPAC + HVA)/DA, was significantly reduced by 44 and 48% in the rat striatum. Meanwhile, the administration of 0.4 or 0.8 micromol of lyso-PTC decreased movement time by 52 and 63%, total distance by 44 and 48% and the number of movements by 43 and 64%, respectively. Quinacrine attenuated SAM-induced hypokinesia without affecting SAM metabolism prior to its action on rat brain. The results obtained indicate that the hypokinesia observed following the administration of lyso-PTC might be related to the decline in DA turnover in the striatum in response to lyso-PTC exposure. The present study suggests that inhibitory effects of lyso-PTC on dopaminergic neurotransmission is one of the contributing factors in SAM and MPP+-induced PD-like changes.

Nicotine Stimulates Expression of the PNMT Gene Through a Novel Promoter Sequence

Transcription of the gene encoding the epinephrine-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT, E.C. 2.1.1.28) accelerates in response to hormonal and neural stimuli. Cholinergic stimulation through neuronal nicotinic receptors constitutes the primary means for neural regulation of PNMT expression in the adrenal medulla (AM). Therefore, the regulatory sequence conveying responsiveness of the PNMT gene to nicotinic stimuli has been characterized in the 5' upstream region of the rat PNMT promoter. Functional analyses using nested deletion and substitution mutations of the PNMT promoter map the nicotine responsive region to a sequence spanning -633 to -595 bp, designated the PNMT nicotine-responsive element (NicRE). Sequences at the 5' (-633 to -620) and 3' (-599 to -595) ends of this region are essential to convey nicotine responsiveness to PNMT promoter constructs expressed in primary bovine chromaffin cells and in selected lines derived from mouse pheochromocytomas and human neuroblastomas. Profiles of nuclear proteins associating with PNMT promoter sequences also change following nicotine treatment of these cells. Electrophoretic mobility shift and DNase I footprinting analyses distinguish multiple sites of DNA-protein interactions within the NicRE region. Because the PNMT promoter does not contain a cAMP responsive element (the site through which nicotine stimulation is mediated for other catecholamine-synthesizing and AM genes), the NicRE of the PNMT gene must therefore be distinct. Thus, nicotinic cholinergic stimuli appear to regulate expression of the epinephrine-synthesizing gene PNMT through a previously uncharacterized regulatory element.

Phosphatidylcholine and N-Methylated Phospholipids Are Nonessential in Saccharomyces cerevisiae

Phosphatidylcholine (PtdCho) is the most abundant phospholipid in numerous eukaryotes and is generally thought to be essential for membrane structure and cellular function. We designed a specific test of this idea by using genetic and biochemical manipulation of yeast. Yeast mutants (pem1 pem2Delta) lacking the phosphatidylethanolamine (PtdEtn) methyltransferase enzymes require choline for growth and cannot make N-methylated phospholipids. When these strains are grown on a glucose carbon source supplemented with 20 mm propanolamine (Prn), the PtdCho level declines precipitously to the limits of detection (<0.6%), and the hexagonal phase-forming, primary amine-containing lipids, PtdEtn and PtdPrn, constitute approximately 60% of the total phospholipid content of the cell. When the lipids were analyzed by mass spectrometry, there was no compensatory shift in unsaturation of the PtdEtn and PtdPrn toward more bilayer-forming species. Thus the majority of the cellular amino phospholipids remained hexagonal phase-forming. The pem1 pem2Delta cells will also grow without choline, in the presence of Prn, on nonfermentable carbon sources (requiring functional mitochondria) and accumulate nearly 70% of their phospholipid as hexagonal phase-forming types. These data provide compelling evidence that the functions of PtdCho and N-methylated lipids in membranes are nonessential in Saccharomyces cerevisiae.

Tests for Establishing Compatibility of an Observed Genotype Distribution with Hardy-Weinberg Equilibrium in the Case of a Biallelic Locus

The classical chi(2)-procedure for the assessment of genetic equilibrium is tailored for establishing lack rather than goodness of fit of an observed genotype distribution to a model satisfying the Hardy-Weinberg law, and the same is true for the exact competitors to the large-sample procedure, which have been proposed in the biostatistical literature since the late 1930s. In this contribution, the methodology of statistical equivalence testing is adopted for the construction of tests for problems in which the assumption of approximate compatibility of the genotype distribution actually sampled with Hardy-Weinberg equilibrium (HWE) plays the role of the alternative hypothesis one aims to establish. The result of such a construction highly depends on the choice of a measure of distance to be used for defining an indifference zone containing those genotype distributions whose degree of disequilibrium shall be considered irrelevant. The first such measure proposed here is the Euclidean distance of the true parameter vector from that of a genotype distribution with identical allele frequencies being in strict HWE. The second measure is based on the (scalar) parameter of the distribution first introduced into the present context by Stevens (1938, Annals of Eugenics 8, 377-383). The first approach leads to a nonconditional test (which nevertheless can be carried out in a numerically exact way), the second to an exact conditional test shown to be uniformly most powerful unbiased (UMPU) for the associated pair of hypotheses. Both tests are compared in terms of the exact power attained against the class of those specific alternatives under which HWE is strictly satisfied.

Phosphatidylethanolamine-N-methyltransferase activity and dietary choline regulate liver-plasma lipid flux and essential fatty acid metabolism in mice

Phosphatidylethanolamine-N-methyltransferase (PEMT) catalyzes the methylation of phosphatidylethanolamine to form phosphatidylcholine (PC) and represents one of the two major pathways for PC biosynthesis. Mice with a homozygous disruption of the PEMT gene are dependent on the 1,2-diacylglycerol cholinephosphotransferase (CDP-choline) pathway for the synthesis of PC and develop severe liver steatosis when fed a diet deficient in choline. The present study used quantitative lipid metabolite profiling to characterize lipid metabolism in PEMT-deficient mice fed diets containing varying concentrations of choline. Choline supplementation restored liver, but not plasma PC concentrations of PEMT-deficient mice to levels commensurate with control mice. Choline supplementation also restored plasma triglyceride concentrations to normal levels, but did not restore plasma cholesterol ester concentrations in the PEMT-deficient mice to those equal to control mice. PEMT-deficient mice also had substantially diminished concentrations of docosahexaenoic acid [22:6(n-3)] and arachidonic acid [20:4(n-6)] in plasma, independent of choline status. Thus, choline supplementation rescued some but not all of the phenotypes induced by the knockout. These findings indicate that PEMT activity functions beyond its recognized role as a compensatory pathway for PC biosynthesis and that, in contrast, PEMT activity is involved in many physiologic processes including the flux of lipid between liver and plasma and the delivery of essential fatty acids to blood and peripheral tissues via the liver-derived lipoproteins.

The xipotl mutant of Arabidopsis reveals a critical role for phospholipid metabolism in root system development and epidermal cell integrity

Phosphocholine (PCho) is an essential metabolite for plant development because it is the precursor for the biosynthesis of phosphatidylcholine, which is the major lipid component in plant cell membranes. The main step in PCho biosynthesis in Arabidopsis thaliana is the triple, sequential N-methylation of phosphoethanolamine, catalyzed by S-adenosyl-l-methionine:phosphoethanolamine N-methyltransferase (PEAMT). In screenings performed to isolate Arabidopsis mutants with altered root system architecture, a T-DNA mutagenized line showing remarkable alterations in root development was isolated. At the seedling stage, the mutant phenotype is characterized by a short primary root, a high number of lateral roots, and short epidermal cells with aberrant morphology. Genetic and biochemical characterization of this mutant showed that the T-DNA was inserted at the At3g18000 locus (XIPOTL1), which encodes PEAMT (XIPOTL1). Further analyses revealed that inhibition of PCho biosynthesis in xpl1 mutants not only alters several root developmental traits but also induces cell death in root epidermal cells. Epidermal cell death could be reversed by phosphatidic acid treatment. Taken together, our results suggest that molecules produced downstream of the PCho biosynthesis pathway play key roles in root development and act as signals for cell integrity.

Deletion of the Pemt gene increases progenitor cell mitosis, DNA and protein methylation and decreases calretinin expression in embryonic day 17 mouse hippocampus

Choline is a required nutrient and is derived from the diet as well as from de novo synthesis catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). We previously reported that choline availability during pregnancy alters mitosis and neuronal protein expression during fetal brain development in wild-type mice and rats, and that Pemt-/- mice become choline deficient. In this study, we examined brain development in these knockout mice. Pregnant Pemt-/- and wild-type mice were fed AIN-76A diet until gestation day 17 (E17) when the fetal brains were harvested. Phosphorylation of histone H3 (a measure of mitosis) and calretinin (a GABAergic neuronal marker) were assessed in hippocampal regions. We observed increased numbers of phosphorylated histone H3 positive cells in the Pemt-/- mice (up 54% compared to wild-type mice; p<0.01). We also found decreased calretinin labeling in Pemt-/- (down to 43% compared to wild-type mice; p<0.01). Thus, there was increased stem cell proliferation in the neuroepithelium and decreased GABAergic neuronal differentiation of these animals on E17. These results are opposite to what would have been expected in choline-deficient mice. The concentrations of S-adenosylmethionine (up 21%; p<0.05) and methylation of DNA (up 46%) and proteins (up 12%; p<0.01) in hippocampus were significantly increased in Pemt-/- mice, suggesting that increased S-adenosylmethionine availability may mediate the observed developmental changes. This is the first report of altered brain development in Pemt-/- mice.

A pathway for phosphatidylcholine biosynthesis in Plasmodium falciparum involving phosphoethanolamine methylation

Plasmodium falciparum is the causative agent of the most severe form of human malaria. The rapid multiplication of the parasite within human erythrocytes requires an active production of new membranes. Phosphatidylcholine is the most abundant phospholipid in Plasmodium membranes, and the pathways leading to its synthesis are attractive targets for chemotherapy. In addition to its synthesis from choline, phosphatidylcholine is synthesized from serine via an unknown pathway. Serine, which is actively transported by Plasmodium from human serum and readily available in the parasite, is subsequently converted into phosphoethanolamine. Here, we describe in P. falciparum a plant-like S-adenosyl-l-methionine-dependent three-step methylation reaction that converts phosphoethanolamine into phosphocholine, a precursor for the synthesis of phosphatidylcholine. We have identified the gene, PfPMT, encoding this activity and shown that its product is an unusual phosphoethanolamine methyltransferase with no human homologs. P. falciparum phosphoethanolamine methyltransferase (Pfpmt) is a monopartite enzyme with a single catalytic domain that is responsible for the three-step methylation reaction. Interestingly, Pfpmt activity is inhibited by its product phosphocholine and by the phosphocholine analog, miltefosine. We show that miltefosine can also inhibit parasite proliferation within human erythrocytes. The importance of this enzyme in P. falciparum membrane biogenesis makes it a potential target for malaria chemotherapy.

Regulation of phospholipid synthesis in Saccharomyces cerevisiae by zinc

Zinc is an essential nutrient required for the growth and metabolism of eukaryotic cells. In this work, we examined the effects of zinc depletion on the regulation of phospholipid synthesis in the yeast Saccharomyces cerevisiae. Zinc depletion resulted in a decrease in the activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. In contrast, the activity of phosphatidylinositol synthase was elevated in response to zinc depletion. The level of Aut7p, a marker for the induction of autophagy, was also elevated in zinc-depleted cells. For the CHO1-encoded phosphatidylserine synthase, the reduction in activity in response to zinc depletion was controlled at the level of transcription. This regulation was mediated through the UAS(INO) element and by the transcription factors Ino2p, Ino4p, and Opi1p that are responsible for the inositol-mediated regulation of UAS(INO)-containing genes involved in phospholipid synthesis. Analysis of the cellular composition of the major membrane phospholipids showed that zinc depletion resulted in a 66% decrease in phosphatidylethanolamine and a 29% increase in phosphatidylinositol. A zrt1Delta zrt2Delta mutant (defective in the plasma membrane zinc transporters Zrt1p and Zrt2p) grown in the presence of zinc exhibited a phospholipid composition similar to that of wild type cells depleted for zinc. These results indicated that a decrease in the cytoplasmic levels of zinc was responsible for the alterations in phospholipid composition.

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.

Dimethylethanolamine does not prevent liver failure in phosphatidylethanolamine N-methyltransferase-deficient mice fed a choline-deficient diet

Mice that lack phosphatidylethanolamine-N-methyltransferase (PEMT) and are fed a choline-deficient (CD) diet suffer severe liver damage and do not survive. Since phosphatidyldimethylethanolamine (PDME) has physical properties similar to those of phosphatidylcholine (PC), we hypothesized that dimethylethanolamine (DME) would be converted into PDME that might substitute for PC, and therefore abrogate the liver damage in the Pemt -/- mice fed a CD diet. We fed Pemt -/- mice either a CD diet, a CD diet supplemented with choline, or a CD diet supplemented with DME (CD + DME). Pemt -/- mice fed the CD diet developed severe liver failure by 4 days while CD + DME-fed mice developed severe liver failure by 5 days. The hepatic PC level in choline-supplemented (CS) mice was 67 +/- 4 nmol/mg protein, whereas the PC content was reduced in CD- and CD + DME-fed mice (49 +/- 3 and 30 +/- 3 nmol/mg protein, respectively). Upon supplementation of the CD diet with DME the amount of hepatic PDME was 81 +/- 9 nmol/mg protein so that the hepatic content of PC + PDME combined was 111 nmol/mg protein. Moreover, plasma apolipoprotein B100 and Al levels were markedly lower in mice fed the CD + DME diet compared to mice fed the CS diet, as was the plasma content of PC. Thus, despite replacement of the deficit in hepatic PC with PDME in Pemt -/- mice fed a CD diet, normal liver function was not restored. We conclude that although PC and PDME exhibit similar physical properties, the three methyl groups of choline are required for hepatic function in mice.

Methylation demand: a key determinant of homocysteine metabolism

Elevated plasma homocysteine is a risk factor for cardiovascular disease and Alzheimer's disease. To understand the factors that determine the plasma homocysteine level it is necessary to appreciate the processes that produce homocysteine and those that remove it. Homocysteine is produced as a result of methylation reactions. Of the many methyltransferases, two are, normally, of the greatest quantitative importance. These are guanidinoacetate methyltransferase (that produces creatine) and phosphatidylethanolamine N-methyltransferase (that produces phosphatidylcholine). In addition, methylation of DOPA in patients with Parkinson's disease leads to increased homocysteine production. Homocysteine is removed either by its irreversible conversion to cysteine (transsulfuration) or by remethylation to methionine. There are two separate remethylation reactions, catalyzed by betaine:homocysteine methyltransferase and methionine synthase, respectively. The reactions that remove homocysteine are very sensitive to B vitamin status as both the transsulfuration enzymes contain pyridoxal phosphate, while methionine synthase contains cobalamin and receives its methyl group from the folic acid one-carbon pool. There are also important genetic influences on homocysteine metabolism.

Insights into the requirement of phosphatidylcholine synthesis for liver function in mice

Phosphatidylcholine (PC) is made in the liver by the CDP-choline pathway and via phosphatidylethanolamine N-methyltransferase (PEMT), which catalyzes the conversion of phosphatidylethanolamine to PC. Unexpectedly, hepatic apolipoprotein B-100 secretion is inhibited in male, but not female, Pemt-/- mice (Noga, A. A., Y. Zhao, and D. E. Vance. 2002. J. Biol. Chem. 277: 42358-42365; Noga, A. A., and D. E. Vance. 2003. J. Biol. Chem. 278: 21851-21859). To gain further insight into this process, we compared PC metabolism in male and female mice fed chow or a high-fat/high-cholesterol (HF/HC) diet. Immunoblot analyses demonstrated that twice as much PEMT2 was present in livers from female compared with male mice. In contrast, assays of CTP:phosphocholine cytidylyltransferase from livers of Pemt+/+ mice demonstrated more active cytidylyltransferase in male than in female mice. Secretion of PEMT-derived PC into lipoproteins was examined in vivo by injection of mice with [methyl-3H]methionine in the presence of Triton WR1339. The PEMT-derived PC shifts to smaller-sized particles in response to a HF/HC diet, but only in male mice. Secretion of PEMT-derived PC into bile was enhanced in mice fed a HF/HC diet. These results demonstrate that the synthesis and targeting of PC produced by the PEMT pathway in the livers of mice differs in a gender- and diet-specific manner.

Expression of phosphatidylethanolamine N-methyltransferase in human hepatocellular carcinomas

OBJECTIVE

Hepatic phosphatidylethanolamine is converted into phosphatidylcholine by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT) when the dietary choline supply is inadequate. Our previous reports implicated PEMT in the regulation of hepatocyte growth and transformation. In the present study, we analyzed PEMT activity, PEMPT gene status and its mRNA expression in 29 human hepatocellular carcinomas (HCC).

METHODS

The status of the PEMPT gene and PEMT2 mRNA expression were evaluated with Southern and Northern blotting, respectively, in HCC and the noninvolved liver. PEMT activity was assessed by biochemical assay. Cell proliferation markers were defined by immunohistochemical or histoautoradiographic methods.

RESULTS

PEMT activity was lower in HCC than in the noninvolved liver and it was negligible in 62% of the tumors. No deletions or mutations of the PEMPT gene were found and PEMT2 mRNA expression was absent or reduced in HCC compared with peritumoral liver tissue. PEMT2 mRNA expression was inversely related to tumor proliferation and to histologic grade. Patients whose HCC did not express PEMT2 mRNA showed poorer outcomes for cancer-related survival than those with PEMT2-positive HCC.

CONCLUSIONS

The present findings suggest that (1). clones lacking PEMT2 expression may have been selected during liver tumorigenesis and progression, and (2). PEMT2 expression seems to be associated with clinical progression.

Molecular dissection of the S-adenosylmethionine-binding site of phosphatidylethanolamine N-methyltransferase

Phosphatidylethanolamine N-methyltransferase (PEMT) is a quatrotopic membrane protein that catalyzes the conversion of phosphatidylethanolamine to phosphatidylcholine through three sequential methylation reactions. Analysis of mice lacking a functional PEMT gene revealed a severe reduction in plasma homocysteine levels. Homocysteine is generated by the hydrolysis of S-adenosylhomocysteine, which is also a product of the PEMT reaction. To gain insight into the PEMT transmethylation reaction and the mechanism by which PEMT regulates homocysteine levels, we sought to define residues that are required for binding of the methyl group donor, S-adenosylmethionine (AdoMet). Bioinformatic analysis of the predicted amino acid sequence of human PEMT identified two putative AdoMet-binding motifs (98GXG100 and 180EE181). Site-directed mutagenesis experiments demonstrated the requirement for the conserved motifs in PEMT specific activity. Analysis of the AdoMet binding ability of mutant recombinant PEMT derivatives established that residues Gly100 and Glu180 are essential for binding of the AdoMet moiety. A significantly elevated KD with respect to AdoMet is observed following conservative mutagenesis of residues Gly98 (400 pmol) and Glu181 (666.7 pmol), relative to the unmodified enzyme (303.1 pmol), suggesting that these residues also participate in AdoMet binding. A model positions two separate AdoMet-binding motifs of PEMT in close proximity at the external leaflet of the endoplasmic reticulum membrane.

Betaine lowers elevated s-adenosylhomocysteine levels in hepatocytes from ethanol-fed rats

Previous studies showed that chronic ethanol administration inhibits methionine synthase activity, resulting in impaired homocysteine remethylation to form methionine. This defect in homocysteine remethylation was shown to increase plasma homocysteine and to interfere with the production of hepatic S-adenosylmethionine (SAM) in ethanol-fed rats. These changes were shown to be reversed by the administration of betaine, an alternative methylating agent. This study was undertaken to determine additional effects of ethanol on methionine metabolism and their functional consequences. The influences of methionine loading and betaine supplementation were also evaluated. Adult Wistar rats were fed ethanol or a control Lieber-DeCarli liquid diet for 4 wk, and metabolites of the methionine cycle were measured in vitro in isolated hepatocytes under basal and methionine-supplemented conditions. S-Adenosylhomocysteine (SAH) concentrations were elevated in hepatocytes isolated from ethanol-fed rats compared with controls and in hepatocytes from both groups when supplemented with methionine. The addition of betaine to the methionine-supplemented incubation media reduced the elevated SAH levels. The decrease in the intracellular SAH:SAM ratio due to ethanol consumption inhibited the activity of the liver-specific SAM-dependent methyltransferase, phosphatidylethanolamine methyltransferase. Our data indicate that betaine, by remethylating homocysteine and removing SAH, overcomes the detrimental effects of ethanol consumption on methionine metabolism and may be effective in correcting methylation defects and treating liver diseases.

A gender-specific role for phosphatidylethanolamine N-methyltransferase-derived phosphatidylcholine in the regulation of plasma high density and very low density lipoproteins in mice

Phosphatidylethanolamine N-methyltransferase (PEMT)is involved in a secondary pathway for production of phosphatidylcholine (PC) in liver. We fed Pemt-/-mice a high fat/high cholesterol diet for 3 weeks to determine whether or not PC derived from PEMT is required for very low density lipoprotein secretion. Lipid analyses of plasma and liver indicated that male Pemt-/- mice accumulated triacylglycerols in their livers and were unable to secrete the same amount of triacylglycerols from the liver as did Pemt+/+ mice. Plasma levels of triacylglycerol and both apolipoproteins B100 and B48 were significantly decreased only in male Pemt-/- mice. Experiments in which mice were injected with Triton WR1339 showed that, whereas hepatic apoB100 secretion was decreased in male Pemt-/- mice, the decrease in plasma apoB48 in male Pemt-/- mice was not due to reduced secretion. Moreover, female and, to a lesser extent, male Pemt-/- mice showed a striking 40% decrease in plasma PC and cholesterol in high density lipoproteins. These results suggest that, even though the content of hepatic PC was normal in PEMT-deficient mice, plasma lipoprotein levels were profoundly altered in a gender-specific manner.

Phosphatidylethanolamine N-methyltransferase (PEMT) knockout mice have hepatic steatosis and abnormal hepatic choline metabolite concentrations despite ingesting a recommended dietary intake of choline

Choline is an essential nutrient for humans and is derived from the diet as well as from de novo synthesis involving methylation of phosphatidylethanolamine catalysed by the enzyme phosphatidylethanolamine N -methyltransferase (PEMT). This is the only known pathway that produces new choline molecules. We used mice with a disrupted Pemt-2 gene (which encodes PEMT; Pemt (-/-)) that have previously been shown to possess no hepatic PEMT enzyme. Male, female and pregnant Pemt (-/-) and wild-type mice ( n =5-6 per diet group) were fed diets of different choline content (deficient, control, and supplemented). Livers were collected and analysed for choline metabolites, steatosis, and apoptotic [terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labelling (TUNEL)] positive cells. We found that, in livers of Pemt (-/-) mice fed any of the diets, there was hepatic steatosis and significantly higher occurrence of TUNEL positive cells compared with wild-type controls. In male, female and pregnant mice, liver phosphatidylcholine concentrations were significantly decreased in Pemt (-/-) choline deficient and in Pemt (-/-) choline control groups but returned to normal in Pemt (-/-) choline supplemented groups. Phosphocholine concentrations in liver were significantly diminished in knockout mice even when choline was supplemented to above dietary requirements. These results show that PEMT normally supplies a significant portion of the daily choline requirement in the mouse and, when this pathway is knocked out, mice are unable to attain normal concentrations of all choline metabolites even with a supplemental source of dietary choline.

Plasma homocysteine is regulated by phospholipid methylation

Mild hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Homocysteine, a non-protein amino acid, is formed from S-adenosylhomocysteine and partially secreted into plasma. A potential source for homocysteine is methylation of the lipid phosphatidylethanolamine to phosphatidylcholine by phosphatidylethanolamine N-methyltransferase in the liver. We show that mice that lack phosphatidylethanolamine N-methyltransferase have plasma levels of homocysteine that are approximately 50% of those in wild-type mice. Hepatocytes isolated from methyltransferase-deficient mice secrete approximately 50% less homocysteine. Rat hepatoma cells transfected with phosphatidylethanolamine N-methyltransferase secrete more homocysteine than wild-type cells. Thus, phosphatidylethanolamine N-methyltransferase is an important source of plasma homocysteine and a potential therapeutic target for hyperhomocysteinemia.

Characterization of antibody responses to endogenous and exogenous antigen in the nonobese diabetic mouse

It is suggested that a T-helper cell 2 (Th2) shift and Th2 spreading of autoimmunity following immunization with beta-cell antigen causes diabetes protection. To address this, antibody titer and subclass to insulin, glutamic acid decarboxylase (GAD)65, IA-2, and IA-2beta proteins were measured by radiobinding assays in untreated or immunized female nonobese diabetic mice. Untreated nonobese diabetic mice developed autoantibodies to insulin (IAA), but not GAD or IA-2/IA-2beta, and IAA-positive mice had increased diabetes risk (P < 0.001). IAA were IgG1 and IgG2b. In immunized mice, IgG1 and lesser IgG2b insulin antibodies were promoted by subcutaneous injection of insulin plus incomplete Freund's adjuvant, insulin plus Montanide ISA 720, and glucagon plus incomplete Freund's adjuvant, but not by incomplete Freund's adjuvant plus GAD65, IA-2beta, or phenylethanolamine N-methyltransferase, or adjuvant alone. Diabetes incidence was significantly reduced in immunized groups with elevated insulin antibody (IA) responses. Spreading of antibody responses to GAD or IA-2/IA-2beta following immunization was rare, and antibody epitope spreading was only detected in IA-2beta immunized mice. Humoral autoimmunity in nonobese diabetic mice is, therefore, limited to IAA with Th2 subclass phenotype and is associated with increased diabetes risk. This contrasts the diabetes protection provided by immunization protocols that promote this response and suggests that Th2 immunity may not be the principal regulator of beta-cell destruction in autoimmune diabetes.

Membrane topography of human phosphatidylethanolamine N-methyltransferase

In liver, phosphatidylethanolamine is converted to phosphatidylcholine through a series of three sequential methylation reactions. Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes each transmethylation reaction, and S-adenosylmethionine is the methyl group donor. Biochemical analysis of human liver revealed that the methyltransferase activity is primarily localized to the endoplasmic reticulum and mitochondria-associated membranes. Bioinformatic analysis of the predicted amino acid sequence suggested that the enzyme adopts a polytopic conformation in those membranes. To elucidate the precise membrane topography of PEMT and thereby provide the basis for in-depth functional characterization of the enzyme, we performed endoproteinase-protection analysis of epitope-tagged, recombinant protein. Our data suggest a topographical model of PEMT in which four transmembrane regions span the membrane such that both the N and C termini of the enzyme are localized external to the ER. Two hydrophilic connecting loops protrude into the luminal space of the microsomes whereas a corresponding loop on the cytosolic side remains proximate to the membrane. Further support for this model was obtained following endoproteinase-protection analysis of mutant recombinant PEMT derivatives in which specific protease cleavage sites had been genetically engineered or ablated.

Phosphatidylethanolamine N-methyltransferase activity is increased in rat intestinal brush-border membrane by chronic ethanol ingestion

AIMS

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyses the synthesis of phosphatidylcholine from phosphatidylethanolamine. The aim of this study was to evaluate the effect of chronic ethanol ingestion on PEMT activity in the jejunal brush-border membrane (BBM) of adequately nourished rats.

METHODS

For this purpose, rats were fed a liquid diet containing ethanol [ethanol-fed group (EFG)] or an isocaloric liquid diet without ethanol [pair-fed group (PFG)] for 4 weeks. Diet ingestion, body weight, nitrogen balance and urinary creatinine excretion were monitored during the experimental period, and serum transferrin levels were determined at the end. BBM was isolated for the determination of PEMT activity.

RESULTS

PEMT activity was significantly increased in the jejunal BBM of the EFG. Nutritional parameters, however, did not differ between groups.

CONCLUSIONS

The increase in PEMT activity may be attributed exclusively to chronic ethanol ingestion, since a major nutritional deficit was excluded.

An unexpected requirement for phosphatidylethanolamine N-methyltransferase in the secretion of very low density lipoproteins

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes the conversion of phosphatidylethanolamine to phosphatidylcholine (PC). We investigated whether there was diminished secretion of lipoproteins from hepatocytes derived from mice that lacked PEMT (Pemt(-/-)) compared with Pemt(+/+) mice. Hepatocytes were incubated with 0.75 mm oleate, the media were harvested, and triacylglycerol (TG), PC, apolipoprotein (apo) B100, and apoB48 were isolated and quantified. Compared with hepatocytes from Pemt(+/+) mice, hepatocytes from Pemt(-/-) mice secreted 50% less TG, whereas secretion of PC was unaffected. Fractionation of the secreted lipoproteins on density gradients demonstrated that the decrease in TG was in the very low density lipoprotein (VLDL)/low density lipoprotein fractions. The secretion of apoB100 was decreased by approximately 70% in VLDLs/low density lipoproteins, whereas there was no significant decrease in apoB48 secretion in any fraction. Transfection of McArdle hepatoma cells (that lack PEMT) with PEMT cDNA enhanced secretion of TG in the VLDLs. Because the levels of PC in the hepatocytes and hepatoma cells were unaffected by the lack of PEMT expression, there appears to be an unexpected requirement for PEMT in the secretion of apoB100-containing VLDLs.

Inhibition of phosphatidylcholine synthesis via the phosphatidylethanolamine methylation pathway impairs incorporation of bulk lipids into VLDL in cultured rat hepatocytes

Inhibition of phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine (PE) methylation pathway was shown to decrease the secretion of VLDL from primary rat hepatocytes (Nishimaki-Mogami et al. 1996. BIOCHIM: Biophys. Acta. 1304: 21-31). To understand further the role of PE methylation, we determined the effect of bezafibrate, an inhibitor of PE methylation, on VLDL assembly within the microsomal lumen. Bezafibrate was shown to decrease VLDL (triacylglycerol) secretion only when cellular PE methylation was active in the presence of methionine. Pulse-chase experiments showed that bezafibrate treatment did not impair the movement of [(35)S]apolipoprotein (apo)B-48 from microsomal membranes into the lumen. However, bezafibrate treatment resulted in reduced VLDL-[(35)S]apoB-48 and increased [(35)S]apoB-48-containing particles in the HDL density range (HDL-[(35)S]apoB-48) within the lumen. Inhibition of PE methylation by bezafibrate or 3-deazaadenosine after the completion of HDL-[(35)S]apoB-48 assembly effectively decreased VLDL-[(35)S]apoB-48 secretion with a concomitant increase in HDL-[(35)S]apoB-48 secretion. These findings suggest that inhibition of PC synthesis via the PE methylation pathway impairs the stage of bulk triacylglycerol incorporation during the assembly of VLDL.

Overexpression of PEMT2 downregulates the PI3K/Akt signaling pathway in rat hepatoma cells

Phosphatidylethanolamine N-methyltransferase 2 (PEMT2) is an isoform of PEMT that converts phosphatidylethanolamine to phosphatidylcholine in mammalian liver. Overexpression of PEMT2 led to inhibition of proliferation of hepatoma cells [J. Biol. Chem. 269 (1994) 24531]. The present study aims to unravel the molecular mechanism of the reduced proliferation, especially the signaling transducer proteins involved in this process. Thus, we chose PI3K/Akt pathway that is initiated by growth factors and leads to cell survival and proliferation. Rat hepatoma CBRH-7919 cells transfected with pemt2-cDNA showed that: (1) signaling proteins including c-Met, PDGF receptor, PI3K, Akt and Bcl-2 all had reduced expression as shown by Western blotting studies; (2) flow cytometric and DNA ladder assays showed that 22.9% of the pemt2-transfected cells were undergoing apoptosis; (3) the activity of Akt was decreased as shown by Western blotting using antibody directed against p-Akt (Thr308); (4) wortmannin and PD98059, inhibitors of PI3K and MEK, respectively, both inhibited Akt activity, indicating that PI3K and MAPK pathways were merging at Akt in CBRH-7919 cells. The above results suggest that overexpression of PEMT2 strongly downregulated the PI3K/Akt signaling pathway at multiple sites and induced apoptosis. This, at least partly, explains the molecular mechanism of impaired proliferation induced by pemt2 transfection.

Kinetic analyses of liver phosphatidylcholine and phosphatidylethanolamine biosynthesis using (13)C NMR spectroscopy

Choline and ethanolamine are substrates for de novo synthesis of phosphatidylcholine (PtdC) and phosphatidylethanolamine (PtdE) through the CDP-choline and CDP-ethanolamine pathways. In liver, PtdE can also be converted to PtdC by PtdE N-methyltransferase (PEMT). We investigated these kinetics in rat liver during a 60 min infusion with (13)C-labeled choline and ethanolamine. NMR analyses of liver extracts provided concentrations and (13)C enrichments of phosphocholine (Pcho), phosphoethanolamine (Peth), PtdC, and PtdE. Kinetic models showed that the de novo and PEMT pathways are 'channeled' processes. The intermediary metabolites directly derived from exogenous choline and ethanolamine do not completely mix with the intracellular pools, but are preferentially used for phospholipid synthesis. Of the newly synthesized PtdC, about 70% was derived de novo and 30% was by PEMT. PtdC and PtdE de novo syntheses displayed different kinetics. A simple model assuming constant fluxes yielded a modest fit to the data; allowing upregulated fluxes significantly improved the fit. The ethanolamine-to-Peth flux exceeded choline-to-Pcho, and the rate of PtdE synthesis (1.04 micromol/h/g liver) was 2-3 times greater than that of PtdC de novo synthesis. The metabolic pathway information provided by these studies makes the NMR method superior to earlier radioisotope studies.

Cooperative activity of phospholipid-N-methyltransferases localized in different membranes

The possibility that the phospholipid-N-methyltransferases from yeast are capable of acting upon a phospholipid substrate, localized in a different membrane than in which the enzymes reside ('trans-catalysis' hypothesis), was investigated using cho2 and opi3 gene disruptant strains, which are defective in phosphatidylethanolamine transferase (PEMT) and phospholipid methyltransferase (PLMT), respectively. When cell homogenates or microsomes of the two disruptant strains are mixed, the combined methyltransferase activity, measured as the incorporation of [(3)H]methyl label from S-adenosyl methionine, exceeds that expected based on the separate activities of PEMT and PLMT. The increased incorporation implies that monomethylphosphatidylethanolamine generated by PEMT becomes available for PLMT, as evidenced by increased synthesis of dimethylphosphatidylethanolamine and phosphatidylcholine. The kinetics of the cooperativity suggest a collision-based process, enabling either transport of substrate or 'trans-catalysis'.

Identification of 197 genetic variations in six human methyltranferase genes in the Japanese population

Methylation is an important event in the biotransformation pathway for many drugs and xenobiotic compounds. We screened DNA from 48 Japanese individuals for single-nucleotide polymorphisms (SNPs) in six methyltransferase (MT) genes (catechol-O-MT, COMT; guanidinoacetate N-MT, GAMT; histamine N-MT, HNMT; nicotinamide N-MT, NNMT; phosphatidylethanolamine N-MT, PEMT; and phenylethanolamine N-MT, PNMT) by direct sequencing of their entire genomic regions except for repetitive elements. This approach identified 190 SNPs and seven insertion/deletion polymorphisms among the six genes. Of the 190 SNPs, 33 were identified in the COMT gene, 6 in GAMT, 41 in HNMT, 8 in NNMT, 98 in PEMT, and 4 in PNMT. Nine were located in 5' flanking regions, 156 in introns, 10 in exons, and 15 in 3' flanking regions. These variants may contribute to a more precise understanding of possible correlations between genotypes and disease-susceptibility phenotypes or risk for side effects from drugs.

Choline deficiency-induced liver damage is reversible in Pemt(-/-) mice

Hepatic tissue has two pathways for phosphatidylcholine (PC) synthesis, i.e., the cytidinediphosphocholine (CDP-choline) pathway and the methylation pathway, which utilizes phosphatidylethanolamine-N-methyltransferase (PEMT). Fatal liver damage occurs in Pemt(-/-)mice fed a choline-deficient (CD) diet. We investigated whether liver damage can be reversed by the addition of dietary choline. Mice (8 wk old) were fed the CD purified diet for 4 d, a choline-supplemented (CS) diet (CD diet + 0.4% choline chloride) for 4 d, or the CD diet for 3 d and a CS diet for 1 d (CD/CS). Pemt(-/-)mice fed the CD diet for 3 d exhibited liver damage as assayed by plasma aminotransferase levels. The livers appeared normal after subsequent feeding of the CS diet for 1 d (CD/CS). The activities of plasma aminotransferases of CD/CS fed mice were comparable to Pemt(-/-)mice fed the CS diet. Hepatic PC and triacylglycerol levels as well as plasma PC levels in the CD/CS-fed Pemt(-/-)mice were lower than those of mice fed the CD diet and began to approach normal levels. Although the CD diet induces liver damage in Pemt(-/-)mice, this damage can be rapidly reversed by the addition of dietary choline.

Encephalitozoon cuniculi (Microspora): characterization of a phospholipid metabolic pathway potentially linked to therapeutics

Phospholipid metabolism of the microsporidian Encephalitozoon cuniculi, an obligate intracellular parasite, has been investigated. Labeled precursor incorporation experiments have shown that phosphatidylserine decarboxylase and phosphatidylethanolamine N-methyltransferase are more active in cells infected by E. cuniculi than in uninfected cells. In contrast, no difference was observed in the activity of Kennedy pathway's enzymes, the mammalian pathway. This suggests the occurrence in microsporidia of a bacteria- and fungi-typical pathway for phospholipid synthesis, which is supported by the identification of two genes implicated in this pathway, the cds gene encoding the key enzyme CDP-diacylglycerol synthase (E.C. 2.7.7.41) and the pss gene for CDP-alcohol phosphatidyltransferase. The pss gene could encode phosphatidylserine synthase (E.C. 2.7.8.8.), which catalyses the de novo synthesis of phosphatidylserine in bacteria and fungi. The complete CDP-diacylglycerol synthase messenger has been isolated and shows very short 5' and 3' untranslated regions. This is strong evidence for the functionality of a metabolic pathway which could be a potential target against microsporidia which infect humans.

Effect of surfactants and natural detergents on phosphatidylcholine synthesis in photoreceptor membranes

The synthesis of phosphatidylcholine (PC) in rod outer segments (ROS) catalysed by lysophosphatidylcholine acyltransferase and phosphatidylethanolamine N-methyltransferase (PE N-MTase) was studied and the effects of natural (FA and lysophospholipids) and synthetic (Triton X-100, deoxycholate and CHAPS) surfactants was evaluated. In all experimental conditions used, incorporation of labelled oleate into lysophosphatidylcholine (lysoPC) was at least 40 times greater than oleate incorporation into any other lysophospholipid. Acylation of lysoPC was slightly affected by Triton X-100 and was totally inhibited in the presence of 10 mM sodium deoxycholate (NaDOC) or CHAPS. Below their critical micelle concentration (cmc) Triton X-100 and NaDOC stimulated acylation of all ROS lysophospholipids analysed. The activity of PE N-MTase was stimulated at detergent concentrations below the cmc and inhibited at concentrations above the cmc for all three detergents tested. The effect of FA with differing degree of unsaturation on PC synthesis was evaluated. Oleic acid (10 microM) inhibited methyl group incorporation into total PC, whereas from 100 microM onward, the methylating activity increased with preferential synthesis of PC. Docosahexaenoic acid, in turn, inhibited PE N-MTase activity at every concentration tested. These results suggest that PC synthesis in ROS membranes is modified by bioregulators and surfactants altering the physico-chemical state of the membrane.

Structure, expression profile and alternative processing of the human phosphatidylethanolamine N-methyltransferase (PEMT) gene

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes the conversion of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) in a series of three methylation reactions. Preliminary studies of PEMT in humans led to the cloning of three cDNAs each of which has a different 5' untranslated region (5'UTR). To determine the origin of PEMT splice variants and to investigate expression of the gene in human liver, we isolated a bacterial artificial chromosome (BAC) clone containing the full-length human gene. Each of the three unique untranslated first exons is present in a contiguous array in the gene, confirming the integrity of the cDNAs and alternative processing of PEMT transcripts. Human liver, heart and testis contain the highest levels of PEMT transcripts and of these, liver has the greatest PEMT expression. Furthermore, each of the three PEMT transcripts is present in varying abundance in liver whereas heart and testis contain only one and two transcripts, respectively. Thus, differential promoter usage in the human PEMT gene generates three unique transcripts and confers a tissue-specific expression pattern.

Phosphatidylcholine levels in Bradyrhizobium japonicum membranes are critical for an efficient symbiosis with the soybean host plant

Phosphatidylcholine (PC), the major membrane phospholipid in eukaryotes, is found in only some bacteria including members of the family Rhizobiaceae. For this reason, it has long been speculated that rhizobial PC might be required for a successful interaction of rhizobia with their legume host plants in order to allow the formation of nitrogen-fixing root nodules. A major pathway for PC formation in prokaryotes involves a threefold methylation of the precursor phosphatidylethanolamine (PE). Here, we report on the isolation of a Bradyrhizobium japonicum gene (pmtA) encoding the phospholipid N-methyltransferase PmtA. Upon expression of the bradyrhizobial pmtA gene in Escherichia coli, predominantly monomethylphosphatidylethanolamine was formed from PE. PmtA-deficient B. japonicum mutants still produced low levels of PC by a second methylation pathway. The amount of PC formed in such mutants (6% of total phospholipids) was greatly decreased compared with the wild type (52% of total phospholipids). Root nodules of soybean plants infected with B. japonicum pmtA mutants showed a nitrogen fixation activity of only 18% of the wild-type level. The interior colour of the nodules was beige instead of red, suggesting decreased amounts of leghaemoglobin. Moreover, ultrastructure analysis of these nodules demonstrated a greatly reduced number of bacteroids within infected plant cells. These data suggest that the biosynthesis of wild-type amounts of PC are required to allow for an efficient symbiotic interaction of B. japonicum with its soybean host plant.

A network of yeast basic helix-loop-helix interactions

The Ino4 protein belongs to the basic helix-loop-helix (bHLH) family of proteins. It is known to form a dimer with Ino2p, which regulates phospholipid biosynthetic genes. Mammalian bHLH proteins have been shown to form multiple dimer combinations. However, this flexibility in dimerization had not been documented for yeast bHLH proteins. Using the yeast two-hybrid assay and a biochemical assay we show that Ino4p dimerizes with the Pho4p, Rtg1p, Rtg3p and Sgc1p bHLH proteins. Screening a yeast cDNA library identified three additional proteins that interact with Ino4p: Bck2p, YLR422W and YNR064C. The interaction with Bck2p prompted us to examine if any of the Bck2p-associated functions affect expression of phospholipid biosynthetic genes. We found that hyperosmotic growth conditions altered the growth phase regulation of a phospholipid biosynthetic gene, CHO1. There are two recent reports of initial whole genome yeast two-hybrid interactions. Interestingly, one of these reports identified five proteins that interact with Ino4p: Ino2p, Hcs1p, Apl2p, YMR317W and YNL279W. Ino2p is the only protein in common with the data presented here. Our finding that Ino4p interacts with five bHLH proteins suggests that Ino4p is likely to be a central player in the coordination of multiple biological processes.

[Comparison of the expression and activity of phosphatidylethanolamine N-methyltransferase 2 between primary cultured hepatocytes and hepatoma cells in rats]

OBJECTIVE

To compare the expression and activity of phosphatidyl-ethanolamine N-methyltransferase 2 (PEMT(2)) between the primary cultured hepatocytes and CBRH7919 hepatoma cells in rats.

METHODS

The expression and activity of PEMT(2) were demonstrated by immunochemistry, Western immunoblotting and [(3)H-CH(3)] SAM incorporation. Flow cytometry (FCM) was used to investigate the DNA content of the cells.

RESULTS

PEMT(2) expression and activity were higher in the primary cultured hepatocytes than in hepatoma cells. Cell cycle analysis showed that the proportion of the hepatocytes was higher in G(1) phase and lower in S phase in the primary cultured hepatocytes than hepatoma cell lines.

CONCLUSION

There is an intimate relationship between the expression of PEMT(2) and the proliferation of hepatocytes.

Alternative pathways for phospholipid synthesis in different brain areas during aging

Morphological and biochemical changes take place in the membrane of aged brain. In particular, studies on aged rats report alterations in brain phospholipid synthesis and in phospholipid-specific fatty acid composition. However, no significant changes in main phospholipid class content have been reported in aged brain, possibly owing to alterations in the alternative pathways for phospholipid synthesis during aging. Therefore, the present study was designed to determine the effect of aging on the enzyme activities responsible for phospholipid synthesis by alternative pathways. Indifferent brain areas of adult (3.5-month-old) and aged (28.5-month-old) rats we examined: 1) the activity of base exchange enzymes, which is a calcium-dependent, energy-independent and calcium stimulated enzymatic pathway; 2) phosphatidylethanolamine (PE) synthesis by phosphatidylserine decarboxylase activity (PSD); 3) phosphatidylcholine (PC) synthesis by transfer of methyl groups to endogenous PE by phosphatidylethanolamine N-methyltransferase activity (PEMT); 4) the synthesis of phosphatidylglycerol (PG) through phospholipase D (PLD) activity. Because the dependence on and the stimulation by calcium of base-exchange reactions is a well known mechanism and alterations in calcium levels in rat brain have been reported, we decided to investigate PS synthesis in the presence of endogenous and exogenous calcium (2.5mM). PS synthesis increased in cerebral cortex (CC) and cerebellum (CRBL) of aged rats with respect to adult rats in basal conditions (without the addition of exogenous calcium), but more significant changes were observed in serine base exchange activity during aging when exogenous calcium was added. PEMT activity in aged CC increased by 100%, the principal modification being observed in the first methylated product of the sequential reaction. Furthermore, the transphosphatidyl reaction was higher in aged brain as indicated by the increased PG synthesis. Our findings allow us to conclude that age affects some alternative pathways for phospholipid synthesis in the central nervous system, and indicate the presence of a compensatory mechanism to provide a pool of phospholipid classes for the maintenance of cellular membrane lipid composition during aging.