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Phosphatidylethanolamine N-methyltransferase (PEMT) is a small integral membrane protein that converts phosphatidylethanolamine (PE) into phosphatidylcholine (PC). It has been previously reported that, unexpectedly, PEMT deficiency protected from high-fat diet (HFD)-induced obesity and insulin resistance, pointing to a possible role of this enzyme in the regulation of adipose cell metabolism. Using mouse 3T3-L1 preadipocytes as a biological system, we demonstrate that PEMT expression is strongly increased during the differentiation of preadipocytes into mature adipose cells. Knockdown of PEMT reduced the expression of early and late adipogenic markers, inhibited lipid droplet formation, reduced triacylglycerol content and decreased the levels of leptin release from the adipocytes, suggesting that PEMT is a novel and relevant regulator of adipogenesis. Investigation into the mechanisms whereby PEMT regulates adipocyte differentiation revealed that extracellularly regulated kinases (ERK1/2) and AKT are essential factors in this process. Specifically, the activities of ERK1/2 and AKT, which are decreased during adipocyte differentiation, were elevated upon Pemt knockdown. Moreover, treatment of cells with exogenous ceramide 1-phosphate (C1P), which we reported to be a negative regulator of adipogenesis, decreased PEMT expression, suggesting that PEMT is also a relevant factor in the anti-adipogenic action of C1P. Altogether, the data presented here identify PEMT as a novel regulator of adipogenesis and a mediator of the anti-adipogenic action of C1P.
Phospholipid metabolism, including phosphatidylcholine (PC) biosynthesis, is crucial for various biological functions and is associated with longevity. Phosphatidylethanolamine N-methyltransferase (PEMT) is a protein that catalyzes the biosynthesis of PC, the levels of which change in various organs such as the brain and kidneys during aging. However, the role of PEMT for systemic PC supply is not fully understood. To address how PEMT affects aging-associated energy metabolism in tissues responsible for nutrient absorption, lipid storage, and energy consumption, we employed NMR-based metabolomics to study the liver, plasma, intestine (duodenum, jejunum, and ileum), brown/white adipose tissues (BAT and WAT), and skeletal muscle of young (9-10 weeks) and old (91-132 weeks) wild-type (WT) and PEMT knockout (KO) mice. We found that the effect of PEMT-knockout was tissue-specific and age-dependent. A deficiency of PEMT affected the metabolome of all tissues examined, among which the metabolome of BAT from both young and aged KO mice was dramatically changed in comparison to the WT mice, whereas the metabolome of the jejunum was only slightly affected. As for aging, the absence of PEMT increased the divergence of the metabolome during the aging of the liver, WAT, duodenum, and ileum and decreased the impact on skeletal muscle. Overall, our results suggest that PEMT plays a previously underexplored, critical role in both aging and energy metabolism.
Although obesity is undoubtedly major risk for non-alcoholic steatohepatitis (NASH), the presence of lean NASH patients with normal body mass index has been recognized. Here, we report that the insufficiency of phosphatidylethanolamine N-methyltransferase (PEMT) is a risk for the lean NASH. The Pemt-/- mice fed high fat-high sucrose (HFHS) diet were protected from diet-induced obesity and diabetes, while they demonstrated prominent steatohepatitis and developed multiple liver tumors. Pemt exerted inhibitory effects on p53-driven transcription by forming the complex with clathrin heavy chain and p53, and Pemt-/- mice fed HFHS diet demonstrated prominent apoptosis of hepatocytes. Furthermore, hypermethylation and suppressed mRNA expression of F-box protein 31 and hepatocyte nuclear factor 4α resulted in the prominent activation of cyclin D1. PEMT mRNA expression in liver tissues of NASH patients was significantly lower than those with simple steatosis and we postulated the distinct clinical entity of lean NASH with insufficiency of PEMT activities.
Phosphatidylethanolamine N-methyltransferase (PEMT) plays a critical role in breast cancer progression. However, the epigenetic mechanism regulating PEMT transcription remains largely unknown. Here, we show that the first promoter-specific transcript 1 is the major PEMT mRNA species, and methylation of the -132 site is a key regulatory element for the PEMT gene in BRCA1-mutated breast cancer. Mechanistically, hypermethylated -132 site-mediated loss of active histone marks H3K9ac and increase of repressive histone marks H3K9me enrichment synergistically inhibited PEMT transcription. Clinicopathological data indicated that a hypermethylated -132 site was associated with histological grade (P = 0.031) and estrogen receptor status (P = 0.004); univariate survival and multivariate analyses demonstrated that lymph node metastasis was an independent and reliable prognostic factor for BRCA1-mutated breast cancer patients. Our findings imply that genetic (e.g., BRCA1 mutation) and epigenetic mechanisms (e.g., DNA methylation and histone modifications) are jointly involved in the malignant progression of PEMT-related breast cancer.
Mice lacking phosphatidylethanolamine N-methyltransferase (PEMT) are protected from high-fat diet (HFD)-induced obesity and insulin resistance. However, these mice develop severe nonalcoholic fatty liver disease (NAFLD) when fed the HFD, which is mainly due to inadequate secretion of VLDL particles. Our aim was to prevent NAFLD development in mice lacking PEMT. We treated Pemt-/- mice with either ezetimibe or fenofibrate to see if either could ameliorate liver disease in these mice. Ezetimibe treatment did not reduce fat accumulation in Pemt-/- livers, nor did it reduce markers for hepatic inflammation or fibrosis. Fenofibrate, conversely, completely prevented the development of NAFLD in Pemt-/- mice: hepatic lipid levels, as well as markers of endoplasmic reticulum stress, inflammation, and fibrosis, in fenofibrate-treated Pemt-/- mice were similar to those in Pemt+/+ mice. Importantly, Pemt-/- mice were still protected against HFD-induced obesity and insulin resistance. Moreover, fenofibrate partially reversed hepatic steatosis and fibrosis in Pemt-/- mice when treatment was initiated after NAFLD had already been established. Increasing hepatic fatty acid oxidation can compensate for the lower VLDL-triacylglycerol secretion rate and prevent/reverse fatty liver disease in mice lacking PEMT.
Feline hepatic lipidosis (FHL) is a common cholestatic disease affecting cats of any breed, age and sex. Both choline deficiency and low hepatic phosphatidylethanolamine N-methyltransferase (PEMT) activity are associated with hepatic lipidosis (HL) in humans, mice and rats. The PEMT expression is known to be upregulated by oestrogens, protecting the females in these species from the development of HL when exposed to choline deficient diets. The aim of the present study was to evaluate the influence of sex hormones on choline synthesis via the PEMT pathway in healthy male and female cats before and after spaying/neutering, when fed a diet with recommended dietary choline content.
Phosphatidylethanolamine is converted to phosphatidylcholine in mammalian liver by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT). A form of the enzyme (PEMT2) has been isolated from rat liver, the cDNA cloned and expressed and the murine gene has been characterized and disrupted. Several lines of evidence suggested that PEMT2 might have a role in hepatocyte proliferation and liver cancer. Hence, we decided to investigate the human form of the enzyme. Unexpectedly, we cloned and expressed three novel human cDNAs encoding PEMT2. These forms differ from each other in the 5'-region with the point of divergence being 15 nucleotides upstream of the putative translation initiation codon. The remainder of the three cDNAs was identical. Expression of the coding region of the cDNAs in McArdle rat hepatoma cells resulted in three stable cell lines that showed a 27- to 115-fold elevation of PEMT activity compared to vector-transfected control cell lines. Screening of somatic cell hybrid panels, radiation hybrid panel mapping and fluorescent in situ hybridization mapping localized the human gene for PEMT2 to chromosome 17p11.2. The identification of three different human cDNAs for PEMT2 suggests that understanding the function of PEMT2 will be more complicated than anticipated.
Mice lacking phosphatidylethanolamine N-methyltransferase (PEMT, Pemt(-/-) mice) are resistant to high-fat diet (HFD)-induced obesity (DIO) but develop non-alcoholic steatohepatitis. PEMT expression is strongly induced during differentiation of 3T3-L1 adipocytes. Hence, we hypothesized that white adipose tissue (WAT) might be a key player in the protection against DIO in Pemt(-/-) mice. We fed Pemt(-/-) and Pemt(+/+) mice the HFD for 2 weeks, after which we examined adipocyte differentiation, adipogenesis and lipolysis in WAT. Pemt(-/-) mice gained less body weight, had reduced WAT mass and had smaller adipocytes than Pemt(+/+) mice. The protein levels of adipose differentiation markers FABP4, PPARγ and C/EBPβ were not altered by genotype, but acetyl-CoA carboxylase expression and activation was reduced in the Pemt(-/-) mice. The in vivo conversion of [¹⁴C]acetate to [¹⁴C]TG in WAT was also lower in Pemt(-/-) mice. The release of glycerol from WAT explants was comparable between Pemt(+/+) and Pemt(-/-) mice under basal condition and in the presence of isoproterenol, indicating unaffected lipolytic capacity. Furthermore, the amounts of leptin, cytokines and chemokines in WAT were not altered by genotype in mice fed the HFD for 2 weeks. However, after 10 weeks of HFD, WAT from Pemt(-/-) mice had dramatically lower leptin, inflammatory cytokines (IL-1 and TNF-α) and chemokines (MCP-1 and RANTES), and significantly higher anti-inflammatory cytokine IL-10 than Pemt(+/+) mice. Together, our data show that PEMT deficiency did not affect the capability for differentiation and lipolysis in WAT. Decreased lipogenesis in WAT may contribute to the resistance to DIO in Pemt(-/-) mice.
Classical homocystinuria (HCU) due to inactivating mutation of cystathionine β-synthase (CBS) is a poorly understood life-threatening inborn error of sulfur metabolism. A previously described cbs-/- mouse model exhibits a semi-lethal phenotype due to neonatal liver failure. The transgenic HO mouse model of HCU exhibits only mild liver injury and recapitulates multiple aspects of the disease as it occurs in humans. Disruption of the methionine cycle in HCU has the potential to impact multiple aspect of phospholipid (PL) metabolism by disruption of both the Kennedy pathway and phosphatidylethanolamine N-methyltransferase (PEMT) mediated synthesis of phosphatidylcholine (PC). Comparative metabolomic analysis of HO mouse liver revealed decreased levels of choline, and choline phosphate indicating disruption of the Kennedy pathway. Alterations in the relative levels of multiple species of PL included significant increases in PL degradation products consistent with enhanced membrane PL turnover. A significant decrease in PC containing 20:4n6 which primarily formed by the methylation of phosphatidylethanolamine to PC was consistent with decreased flux through PEMT. Hepatic expression of PEMT in both the cbs-/- and HO models is post-translationally repressed with decreased levels of PEMT protein and activity that inversely-correlates with the scale of liver injury. Failure to induce further repression of PEMT in HO mice by increased homocysteine, methionine and S-adenosylhomocysteine or depletion of glutathione combined with examination of multiple homocysteine-independent models of liver injury indicated that repression of PEMT in HCU is a consequence rather than a cause of liver injury. Collectively, our data show significant alteration of a broad range of hepatic PL and choline metabolism in HCU with the potential to contribute to multiple aspects of pathogenesis in this disease.
ATP8B1 is a phospholipid flippase that is deficient in patients with progressive familial intrahepatic cholestasis type 1 (PFIC1). PFIC1 patients suffer from severe liver disease but also present with dyslipidemia, including low plasma cholesterol, of yet unknown etiology. Here we show that ATP8B1 knockdown in HepG2 cells leads to a strong increase in the mitochondrial oxidative phosphorylation (OXPHOS) without a change in glycolysis. The enhanced OXPHOS coincides with elevated low-density lipoprotein receptor protein and increased mitochondrial fragmentation and phosphatidylethanolamine levels. Furthermore, expression of phosphatidylethanolamine N-methyltransferase, an enzyme that catalyzes the conversion of mitochondrial-derived phosphatidylethanolamine to phosphatidylcholine, was reduced in ATP8B1 knockdown cells. We conclude that ATP8B1 deficiency results in elevated mitochondrial PE levels that stimulate mitochondrial OXPHOS. The increased OXPHOS leads to elevated LDLR levels, which provides a possible explanation for the reduced plasma cholesterol levels in PFIC1 disease.
The brucellae are facultative intracellular bacteria with a cell envelope rich in phosphatidylcholine (PC). PC is abundant in eukaryotes but rare in prokaryotes, and it has been proposed that Brucella uses PC to mimic eukaryotic-like features and avoid innate immune responses in the host. Two PC synthesis pathways are known in prokaryotes: the PmtA-catalyzed trimethylation of phosphatidylethanolamine and the direct linkage of choline to CDP-diacylglycerol catalyzed by the PC synthase Pcs. Previous studies have reported that B. abortus and B. melitensis possess non-functional PmtAs and that PC is synthesized exclusively via Pcs in these strains. A putative choline transporter ChoXWV has also been linked to PC synthesis in B. abortus. Here, we report that Pcs and Pmt pathways are active in B. suis biovar 2 and that a bioinformatics analysis of Brucella genomes suggests that PmtA is only inactivated in B. abortus and B. melitensis strains. We also show that ChoXWV is active in B. suis biovar 2 and conserved in all brucellae except B. canis and B. inopinata. Unexpectedly, the experimentally verified ChoXWV dysfunction in B. canis did not abrogate PC synthesis in a PmtA-deficient mutant, which suggests the presence of an unknown mechanism for obtaining choline for the Pcs pathway in Brucella. We also found that ChoXWV dysfunction did not cause attenuation in B. suis biovar 2. The results of these studies are discussed with respect to the proposed role of PC in Brucella virulence and how differential use of the Pmt and Pcs pathways may influence the interactions of these bacteria with their mammalian hosts.
Folate and choline are interconnected metabolically. The MTHFD1 R653Q SNP is a risk factor for birth defects and there are concerns that choline deficiency may interact with this SNP and exacerbate health risks. 80-90% of women do not meet the Adequate Intake (AI) for choline. The objective of this study was to assess the effects of choline deficiency on maternal one-carbon metabolism and reproductive outcomes in the MTHFD1-synthetase deficient mouse (Mthfd1S), a model for MTHFD1 R653Q. Mthfd1S+/+ and Mthfd1S+/- females were fed control (CD) or choline-deficient diets (ChDD; 1/3 the amount of choline) before mating and during pregnancy. Embryos were evaluated for delays and defects at 10.5 days gestation. Choline metabolites were measured in the maternal liver, and total folate measured in maternal plasma and liver. ChDD significantly decreased choline, betaine, phosphocholine, and dimethylglycine in maternal liver (p < 0.05, ANOVA), and altered phosphatidylcholine metabolism. Maternal and embryonic genotype, and diet-genotype interactions had significant effects on defect incidence. Mild choline deficiency and Mthfd1S+/- genotype alter maternal one-carbon metabolism and increase incidence of developmental defects. Further study is required to determine if low choline intakes contribute to developmental defects in humans, particularly in 653QQ women.
During fasting, the liver increases lipid storage as a mean to reserve and provide energy for vital cellular functions. After re-feeding, hepatocytes rapidly decrease the amount of triacylglycerol that is stored in lipid droplets (LDs), visible as the size of hepatic LDs significantly decreases after re-feeding. Little is known about the changes in the liver LD proteome that occur during the fasting/re-feeding transition. This study aimed to investigate the hepatic LD proteome in fasted and re-fed conditions in the mouse. Using label-free LC-MS/MS analysis the relative abundance of 817 proteins was determined in highly purified LDs. Comparative analysis for differential protein abundance with respect to feeding states revealed 130 with higher abundance in LDs from fasted mice and 31 in LDs from re-fed mice. Among proteins observed to have higher abundance on LDs in the fasted state we found perilipin-5, and several mitochondrial and peroxisomal marker proteins, supporting the role of LDs in the provision of substrates for fatty acid oxidation. Proteins of higher abundance upon re-feeding included several peroxisomal and mitochondrial marker proteins and expand our understanding of the dynamic nature of the hepatic LD proteome according to the energetic requirements of the cell.
Folate, vitamin B-12, and vitamin B-6 are essential nutritional components in one-carbon metabolism and are required for methylation capacity. The availability of these vitamins may therefore modify methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) by PE-N-methyltransferase (PEMT) in the liver. It has been suggested that PC synthesis by PEMT plays an important role in the transport of polyunsaturated fatty acids (PUFAs) like docosahexaenoic acid (DHA) from the liver to plasma and possibly other tissues. We hypothesized that if B-vitamin supplementation enhances PEMT activity, then supplementation could also increase the concentration of plasma levels of PUFAs such as DHA. To test this hypothesis, we determined the effect of varying the combined dietary intake of these three B-vitamins on plasma DHA concentration in rats.
There has been an intense focus to uncover the molecular mechanisms by which fasting triggers the adaptive cellular responses in the major organs of the body. Here, we show that in mice, hepatic S-adenosylmethionine (SAMe)-the principal methyl donor-acts as a metabolic sensor of nutrition to fine-tune the catabolic-fasting response by modulating phosphatidylethanolamine N-methyltransferase (PEMT) activity, endoplasmic reticulum-mitochondria contacts, β-oxidation, and ATP production in the liver, together with FGF21-mediated lipolysis and thermogenesis in adipose tissues. Notably, we show that glucagon induces the expression of the hepatic SAMe-synthesizing enzyme methionine adenosyltransferase α1 (MAT1A), which translocates to mitochondria-associated membranes. This leads to the production of this metabolite at these sites, which acts as a brake to prevent excessive β-oxidation and mitochondrial ATP synthesis and thereby endoplasmic reticulum stress and liver injury. This work provides important insights into the previously undescribed function of SAMe as a new arm of the metabolic adaptation to fasting.
The loss of liver glycine N-methyltransferase (GNMT) promotes liver steatosis and the transition to hepatocellular carcinoma (HCC). Previous work showed endogenous glucose production is reduced in GNMT-null mice with gluconeogenic precursors being used in alternative biosynthetic pathways that utilize methyl donors and are linked to tumorigenesis. This metabolic programming occurs before the appearance of HCC in GNMT-null mice. The metabolic physiology that sustains liver tumor formation in GNMT-null mice is unknown. The studies presented here tested the hypothesis that nutrient flux pivots from glucose production to pathways that incorporate and metabolize methyl groups in GNMT-null mice with HCC.
Non-alcoholic steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1) is overexpressed in NASH. Importantly, GLS1 inhibition reduces lipid content in choline and/or methionine deprivation-induced steatotic mouse primary hepatocytes, in human hepatocyte cell lines, and in NASH mouse livers. We suggest that under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprogramming of serine metabolism, wherein serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. The restored methionine cycle can induce phosphatidylcholine synthesis from the phosphatidylethanolamine N-methyltransferase-mediated and CDP-choline pathways as well as by base-exchange reactions between phospholipids, thereby restoring hepatic phosphatidylcholine content and very-low-density lipoprotein export. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.
Choline is an important nutrient for humans and animals. Animals obtain choline from the diet and from the catabolism of phosphatidylcholine made by phosphatidylethanolamine N-methyltransferase (PEMT). The unique model of complete choline deprivation is Pemt(-/-) mice that are fed a choline-deficient diet. This model, therefore, can be used for the examination of choline substitutes in mammalian systems. Recently, propanolamine was found to be a replacement for choline in yeast. Thus, we tested to see whether or not choline can be replaced by propanolamine in mice. Mice were fed a choline-deficient diet and supplemented with either methionine, 2-amino-propanol, 2-amino-isopropanol and 3-amino-propanol. We were unable to detect the formation of any of the possible phosphatidylpropanolamines. Moreover, none of them prevented liver damage, reduction of hepatic phosphatidylcholine levels or fatty liver induced in choline-deficient-Pemt(-/-) mice. These results suggest that choline in mice cannot be replaced by any of the three propanolamine derivatives.
Phosphatidylethanolamine N-methyltransferase (Pemt) catalyzes the methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) mainly in the liver. Under an obese state, the upregulation of Pemt induces endoplasmic reticulum (ER) stress by increasing the PC/PE ratio in the liver. We targeted the Pemt gene in mice to explore the therapeutic impact of Pemt on the progression of diabetic nephropathy and diabetes, which was induced by the injection of streptozotocin (STZ). Although the blood glucose levels were similar in STZ-induced diabetic Pemt+/+ and Pemt-/-mice, the glomerular hypertrophy and albuminuria in Pemt-/- mice were significantly reduced. Pemt deficiency reduced the intraglomerular F4/80-positive macrophages, hydroethidine fluorescence, tubulointerstitial fibrosis and tubular atrophy. The expression of glucose-regulated protein-78 (GRP78) was enriched in the renal tubular cells in STZ-induced diabetic mice, and this was ameliorated by Pemt deficiency. In mProx24 renal proximal tubular cells, the treatment with ER-stress inducers, tunicamycin and thapsigargin, increased the expression of GRP78, which was reduced by transfection of a shRNA lentivirus for Pemt (shRNA-Pemt). The number of apoptotic cells in the renal tubules was significantly reduced in Pemt-/- diabetic mice, and shRNA-Pemt upregulated the phosphorylation of Akt and decreased the cleavage of caspase 3 and 7 in mProx24 cells. Taken together, these findings indicate that the inhibition of Pemt activity ameliorates the ER stress associated with diabetic nephropathy in a model of type 1 diabetes and corrects the functions of the three major pathways downstream of ER stress, i.e. oxidative stress, inflammation and apoptosis.
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