Expression of phosphatidylethanolamine N-methyltransferase in Yoshida ascites hepatoma cells and the livers of host rats

Metabolomics of Esophageal Squamous Cell Carcinoma Tissues: Potential Biomarkers for Diagnosis and Promising Targets for Therapy

Background

The incidence of esophageal squamous cell carcinoma in China ranks first in the world. The early diagnosis technology is underdeveloped, and the prognosis is poor, which seriously threatens the quality of life of the Chinese people. Epidemiological findings are related to factors such as diet, living habits, and age. The specific mechanism is not clear yet. Metabolomics is a kind of omics that simultaneously and quantitatively analyzes the comprehensive profile of metabolites in living systems. It has unique advantages in the study of the diagnosis and pathogenesis of tumor-related diseases, especially in the search for biomarkers. Therefore, it is desirable to perform metabolic profiling analysis of cancer tissues through metabolomics to find potential biomarkers for the diagnosis and treatment of esophageal squamous cell carcinoma.

Methods

HPLC-TOF-MS/MS technology and Illumina Hiseq Xten Sequencing was used for the analysis of 210 pairs of matched esophageal squamous cell carcinoma tissues and normal tissues in Zhenjiang City, Jiangsu Province, a high-incidence area of esophageal cancer in China. Bioinformatics analysis was also performed.

Results

Through metabolomic and transcriptomic analysis, this study found that a total of 269 differential metabolites were obtained in esophageal squamous cell carcinoma and normal tissues, and 48 differential metabolic pathways were obtained through KEGG enrichment analysis. After further screening and identification, 12 metabolites with potential biomarkers to differentiate esophageal squamous cell carcinoma from normal tissues were obtained.

Conclusions

From the metabolomic data, 4 unknown compounds were found to be abnormally expressed in esophageal squamous cell carcinoma for the first time, such as 9,10-epoxy-12,15-octadecadienoate; 3 metabolites were found in multiple abnormal expression in another tumor, but upregulation or downregulation was found for the first time in esophageal cancer, such as oleoyl glycine; at the same time, it was further confirmed that five metabolites were abnormally expressed in esophageal squamous cell carcinoma, which was similar to the results of other studies, such as PE.

Targeting Phospholipid Metabolism in Cancer

All cancers tested so far display abnormal choline and ethanolamine phospholipid metabolism, which has been detected with numerous magnetic resonance spectroscopy (MRS) approaches in cells, animal models of cancer, as well as the tumors of cancer patients. Since the discovery of this metabolic hallmark of cancer, many studies have been performed to elucidate the molecular origins of deregulated choline metabolism, to identify targets for cancer treatment, and to develop MRS approaches that detect choline and ethanolamine compounds for clinical use in diagnosis and treatment monitoring. Several enzymes in choline, and recently also ethanolamine, phospholipid metabolism have been identified, and their evaluation has shown that they are involved in carcinogenesis and tumor progression. Several already established enzymes as well as a number of emerging enzymes in phospholipid metabolism can be used as treatment targets for anticancer therapy, either alone or in combination with other chemotherapeutic approaches. This review summarizes the current knowledge of established and relatively novel targets in phospholipid metabolism of cancer, covering choline kinase α, phosphatidylcholine-specific phospholipase D1, phosphatidylcholine-specific phospholipase C, sphingomyelinases, choline transporters, glycerophosphodiesterases, phosphatidylethanolamine N-methyltransferase, and ethanolamine kinase. These enzymes are discussed in terms of their roles in oncogenic transformation, tumor progression, and crucial cancer cell properties such as fast proliferation, migration, and invasion. Their potential as treatment targets are evaluated based on the current literature.

Functional analysis of two isoforms of phosphatidylethanolamine N-methyltransferase

The enzyme catalysing the conversion of PE (phosphatidylethanolamine) into PC (phosphatidylcholine), PEMT (PE N-methyltransferase), exists as two isoforms, PEMT-L (longer isoform of PEMT) and PEMT-S (shorter isoform of PEMT). In the present study, to compare the functions of the two isoforms of PEMT, we established HEK (human embryonic kidney)-293 cell lines stably expressing PEMT-L and PEMT-S. Both PEMT-L and PEMT-S were localized in the ER (endoplasmic reticulum). PEMT-L, but not PEMT-S, was N-glycosylated with high-mannose oligosaccharides. The enzymatic activity of PEMT-S was much higher than that of PEMT-L. By using novel enzymatic assays for measuring PC and PE, we showed that PEMT-L and PEMT-S expression remarkably increased the cellular PC content, whereas the PE content was decreased by PEMT-S expression, but was hardly affected by PEMT-L expression. The cellular content of phosphatidylserine was also reduced by the expression of PEMT-L or PEMT-S. MS analyses demonstrated that the expression of PEMT-S led to more increases in the molecular species of PC and PC-O (ether-linked PC) with longer polyunsaturated chains than that of PEMT-L, whereas the PC-O species with shorter chains were increased more by PEMT-L expression than by PEMT-S expression, suggesting a difference in the substrate specificity of PEMT-L and PEMT-S. On the other hand, various PE and PE-O species were decreased by PEMT-S expression. In addition, PEMT-L and PEMT-S expression promoted the proliferation of HEK-293 cells. Based upon these findings, we propose a model in which the enzymatic activity and substrate specificity are regulated by the glycosylated N-terminal region of PEMT-L localized in the ER lumen.

Over-expression of pemt2 into rat hepatoma cells contributes to the mitochondrial apoptotic pathway

We previously established a line of phosphatidylethanolamine N-methyltransferase 2 (pemt2) -stably transfected CBRH-7919 hepatoma cells, and showed that pemt2 over-expression inhibited cell proliferation and induced apoptosis. This study was aimed to further elucidate the cellular mechanisms leading to this apoptosis in these cells. Fatty acid compositions of phosphatidylcholine (PC) in pemt2 over-expressed cells and control cells, and the location of PC synthesized by PEMT2 pathway were analyzed with lipid extraction, high-performance thin layer chromatography, high-performance gas chromatography (HPGC), and [(3)H]-ethanolamine tracing. The effects of pemt2 over-expression on the mitochondrial membrane fluidity, the release of cytochrome C from mitochondria, and the activity of caspases were determined by Western blot. Newly synthesized PC by PEMT2 contained more acyl groups of oleic acid (P < 0.01) and was mainly located in mitochondria; pemt2 over-expression increased the mitochondrial membrane fluidity and the release of cytochrome C from the mitochondria into the cytoplasma, which in turn activated caspase-9 and caspase-3, the key molecules in the mitochondrial apoptotic pathway. We demonstrated that, in rat hepatoma cells, PEMT2-induced apoptosis proceeds through mitochondria.

Inactivation of PEMT2 in hepatocytes initiated by DENA in fasted/refed rats

Phosphatidylethanolamine N-methyltransferase (PEMT) is the enzyme that converts phosphatidylethanolamine (PE) into phosphatidylcholine. We have previously shown that PEMT suppressed hepatoma growth by triggering apoptosis. We investigate whether PEMT controlled cell death and cell proliferation triggered by fasting/refeeding and whether it is a marker of early preneoplastic lesions. The induction of programmed cell death and suppression of cell proliferation by fasting were associated with enhanced PEMT expression and activity, and with a decrease in CTP:phosphocholine cytidylyltransferase expression. Refeeding returned the liver growth and expression of CTP:phosphocholine cytidylyltransferase to control levels, while the expression of PEMT decreased to below control values. After DENA administration, PEMT protein, evaluated by Western blotting, slightly increased, but it remained below control levels. The treatment with 20 mg/kg DENA to refed rats induced the appearance of initiated hepatocytes that were negative for PEMT expression. Present findings indicate that PEMT is a novel tumour marker for early liver preneoplastic lesions.

Mice heterozygous for the Mdr2 gene demonstrate decreased PEMT activity and diminished steatohepatitis on the MCD diet

BACKGROUND/AIMS

The administration of a methionine and choline deficient (MCD) diet to mice serves as an animal model of NASH. The multidrug resistant 2 (Mdr2) P-glycoprotein encodes for the canalicular phospholipid transporter, and Mdr2 (+/-) mice secrete 40% less phosphatidylcholine than wild-type mice. We have hypothesized that phosphatidylethanolamine-N-methyl transferase (PEMT) up-regulation is a consequence of MCD diet administration, and is important for the pathogenesis of steatohepatitis in this model. However, the effect of decreased phosphatidylcholine secretion and modulation of PEMT on the development of diet-induced steatohepatitis in Mdr2 (+/-) mice has not been explored. Thus, the purpose of the study is to examine the effects of the MCD diet on Mdr2 (+/-) mice.

METHODS

Mdr2 (+/-) and Mdr2 (+/+) mice were treated with an MCD or control diet for up to 30 days, and the severity of steatohepatitis, PEMT activity and hepatic S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) levels were measured.

RESULTS

Serum ALT levels, hepatic inflammation, and PEMT activity were significantly lower, and hepatic SAM:SAH ratios were significantly higher in Mdr2 (+/-) mice at 7 and 30 days on the MCD diet.

CONCLUSIONS

Mdr2 (+/-) mice have diminished susceptibility to MCD diet-induced NASH, which is associated with a relative decrease in PEMT activity and increased SAM:SAH ratios.

Methionine-dependence phenotype of tumors: Metabolite profiling in a melanoma model usingL-[methyl-13C]methionine and high-resolution magic angle spinning1H–13C nuclear magnetic resonance spectroscopy

Tumors frequently have abnormal L-methionine (Met) metabolism, the so-called Met-dependence phenotype that refers to the inability to proliferate in the absence of Met. However, the origin of this phenotype is still unknown and may arise from one of several pathways of Met metabolism. To help characterize the metabolic features of Met-dependent/independent phenotypes, the fate of the methyl carbon of L-[methyl-13C]Met was chased in a murine model of malignant melanoma (B16-F1) in vitro and in vivo. Growth curves under Met restriction showed that melanoma cells in vitro were Met-independent, whereas implanted melanoma tumors in vivo were Met-dependent. Label-assisted high-resolution magic angle spinning 1H-13C NMR spectroscopy metabolite profiling showed that, in vitro, creatine and phosphatidylcholine 13C-enrichments were poor, but S-adenosyl-Met and posttranslationally N-methylated protein signals were strong. In contrast, in vivo, creatine and phosphatidylcholine enrichments were strong but S-adenosyl-Met and N-methylated protein signals were poor. In addition, in vivo, transsulfuration was very efficient, consumed one-carbon units originating from the methyl carbon of Met, and yielded taurine labeling. From these data, the Met-dependent/independent phenotypes appear closely related to the source of one-carbon units. Thus, L-[methyl-13C]Met-assisted NMR spectroscopy metabolite profiling allowed the discrimination between Met-dependence and Met-independence and provided novel mechanistic information on their origin.

Hepatic phospholipids in alcoholic liver disease assessed by proton-decoupled 31P magnetic resonance spectroscopy

BACKGROUND/AIMS

Alteration of the phospholipid composition of hepatic biomembranes may be one mechanism of alcoholic liver disease (ALD). We applied proton-decoupled (31)P magnetic resonance spectroscopic imaging ({(1)H}-(31)P MRSI) to 40 patients with ALD and to 13 healthy controls to confirm that metabolic alterations in hepatic phospholipid intermediates could be detected non-invasively.

METHODS

All patients underwent liver biopsy. Specimens were scored in non-cirrhosis [fatty liver (n=3), alcoholic hepatitis (n=2), fibrosis (n=4), alcoholic hepatitis plus fibrosis (n=16)], and cirrhosis (n=15). {(1)H}-(31)P spectra were collected on a clinical 1.5-Tesla MR system and were evaluated by calculating signal intensity ratios of hepatic phosphomonoester (PME), phosphodiester (PDE), phosphoethanolamine (PE), phosphocholine (PC), glycerophosphorylethanolamine (GPE), and glycerophosphorylcholine (GPC) resonances.

RESULTS

The signal intensity ratio GPE/GPC was significantly elevated in cirrhotic (1.19+/-0.22; P=0.002) and non-cirrhotic ALD patients (1.01+/-0.13; P=0.006) compared to healthy controls (0.68+/-0.04), while PE/PC and PME/PDE were significantly elevated in cirrhotic ALD patients compared to controls (1.68+/-0.60 vs. 0.97+/-0.31; P=0.02, and 0.38+/-0.02 vs. 0.25+/-0.01; P=0.002, respectively) and non-cirrhotic patients.

CONCLUSIONS

The data support that {(1)H}-(31)P MRSI appears to distinguish cirrhotic from non-cirrhotic ALD patients and confirms changes in hepatic phospholipid metabolism observed in an animal model.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Identification of Ets-1 as an important transcriptional activator of CTP:phosphocholine cytidylyltransferase alpha in COS-7 cells and co-activation with transcriptional enhancer factor-4

Phosphatidylcholine biosynthesis via the CDP-choline pathway is primarily regulated by CTP:phosphocholine cytidylyltransferase (CT). Transcriptional enhancer factor-4 (TEF-4) enhances the transcription of CTalpha in COS-7 cells by interactions with the basal transcription machinery (Sugimoto, H., Bakovic, M., Yamashita, S., and Vance, D.E. (2001) J. Biol. Chem. 276,12338-12344). To identify the most important transcription factor involved in basal CTalpha transcription, we made CTalpha promoter-deletion and -mutated constructs linked to a luciferase reporter and transfected them into COS-7 cells. The results indicate that an important site regulating basal CTalpha transcription is -53/-47 (GACTTCC), which is a putative consensus-binding site of Ets transcription factors (GGAA) in the opposite orientation. Gel shift analyses indicated the existence of a binding protein for -53/-47 (GACTTCC) in nuclear extracts of COS-7 cells. When anti-Ets-1 antibody was incubated with the probe in gel shift analyses, the intensity of the binding protein was decreased. The binding of endogenous Ets-1 to the promoter probe was increased when TEF-4 was expressed; however, the amount of Ets-1 detected by immunoblotting was unchanged. When cells were transfected with Ets-1 cDNA, the luciferase activity of CTalpha promoter constructs was greatly enhanced. Co-transfection experiments with Ets-1 and TEF-4 showed enhanced expression of reporter constructs as well as CTalpha mRNA. These results suggest that Ets-1 is an important transcriptional activator of the CTalpha gene and that Ets-1 activity is enhanced by TEF-4.

Phosphatidylcholine and cell death

Phosphatidylcholine (PC) constitutes a major portion of cellular phospholipids and displays unique molecular species in different cell types and tissues. Inhibition of the CDP-choline pathway in most mammalian cells or overexpression of the hepatic phosphatidylethanolamine methylation pathway in hepatocytes leads to perturbation of PC homeostasis, growth arrest or even cell death. Although many agents that perturb PC homeostasis and induce cell death have been identified, the signaling pathways that mediate this cell death have not been well defined. This review summarizes recent progress in understanding the relationship between PC homeostasis and cell death.

Disruption of choline methyl group donation for phosphatidylethanolamine methylation in hepatocarcinoma cells

Despite being widely hypothesized, the actual contribution of choline as a methyl source for phosphatidylethanolamine (PE) methylation has never been demonstrated, mainly due to the inability of conventional methods to distinguish the products from that of the CDP-choline pathway. Using a novel combination of stable-isotope labeling and tandem mass spectrometry, we demonstrated for the first time that choline contributed to phosphatidylcholine (PC) synthesis both as an intact choline moiety via the CDP-choline pathway and as a methyl donor via PE methylation pathway. When hepatocytes were labeled with d(9)-choline containing three deuterium atoms on each of the three methyl groups, d(3)-PC and d(6)-PC were detected, indicating that newly synthesized PC contained one or more individually mobilized methyl groups from d(9)-choline. The synthesis of d(3)-PC and d(6)-PC was sensitive to the general methylation inhibitor 3-deazaadenosine and were specific products of PE methylation using choline as a one-carbon donor. While the contribution to the CDP-choline pathway remained intact in hepatocarcinoma cells, contribution of choline to PE methylation was completely disrupted. In addition to a previously identified lack of PE methyltransferase, hepatocarcinoma cells were found to lack the abilities to oxidize choline to betaine and to donate the methyl group from betaine to homocysteine, whereas the usage of exogenous methionine as a methyl group donor was normal. The failure to use choline as a methyl source in hepatocarcinoma cells may contribute to methionine dependence, a widely observed aberration of one-carbon metabolism in malignancy.

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.

Cell cholesterol esters and high-density lipoprotein plasma levels during liver hyperplasia in choline-fed male and female rats

Sexual dimorphism exists in the response of rats to lead nitrate, liver hyperplasia occuring earlier and being more pronounced in males. Excess dietary choline in females shifted the growth pattern towards that of males. To determine whether phosphatidylcholine-induced growth modulations could be related to a derangement of cholesterol metabolism, liver accumulation of cholesterol esters and plasma lipoprotein patterns were investigated. In males, lead-induced liver hyperplasia was associated with increased total cholesterol hepatic content, accumulated cholesterol esters and reduced concentration of plasma High Density Lipoprotein (HDL) cholesterol. Females were less responsive to the liver mitogenic signal of lead nitrate; there was no elevation of cholesterol content nor any marked accumulation of cholesterol esters. This is consistent with the lack of change in the plasma levels of HDL cholesterol. Continuous choline feeding displaced the liver cholesterol ester pattern and plasma HDL cholesterol levels in females, and in parallel that of DNA synthesis, towards those of males. Choline was not observed to have any effect in males. These results suggest that the derangement of phosphatidylcholine metabolism induces growth-related changes in cholesterol turnover; they are consistent with the proposal that the intracellular content of cholesterol esters may have a role in regulating liver growth rates.

Inactivation of phosphatidylethanolamine N-methyltransferase-2 in aflatoxin-induced liver cancer and partial reversion of the neoplastic phenotype by PEMT transfection of hepatoma cells

Phosphatidylethanolamine N-methyltransferase(PEMT) is an enzyme in liver that catalyzes the stepwise methylation of phosphatidylethanolamine to phosphatidylcholine, in addition to the main pathway that synthesizes phosphatidylcholine directly from choline. We have reported that PEMT is permanently inactivated in liver cancer induced by the Solt and Farber model. Here we studied, (i) whether similar changes also occur in the progression of hepatocarcinoma triggered by aflatoxin B(1) (AFB(1)) in rats; (ii) whether the hepatoma phenotype could be reversed by over-expression of PEMT2. We found that PEMT2 protein decreased in pre-neoplastic nodules and virtually disappeared in hepatocellular carcinoma induced by AFB(1) due to decreased levels of mRNA without any deletion or mutation of the DNA sequence. PEMT activity, which reflects the function of both PEMT1 and PEMT2, was lower in nodules and negligible in the tumor, consistent with its regulation at the level of gene transcription. McArdle hepatoma cells transfected with PEMT2 failed to form anchorage-independent colonies in soft agar, while the vector-transfected control line grew efficiently. Moreover, PEMT2-transfected cells were also poorly tumorigenic in vivo in athymic mice, as shown by the lower tumor incidence, the longer cancer-free-time and the lower tumor volume and weight. Together, these data indicate that the loss of PEMT function may contribute to malignant transformation of hepatocytes.

NMR spectroscopic analyses of liver phosphatidylcholine and phosphatidylethanolamine biosynthesis in rats exposed to peroxisome proliferators-A class of nongenotoxic hepatocarcinogens

Peroxisome proliferators (PPs) are commercial/industrial chemicals that display tumor promoter activity in rodents. The mechanism is not completely understood, and our ability to predict tumorigenicity a priori is even less developed. Wy-14,643, perfluorooctanoic acid (PFOA), and di(2-ethylhexyl)phthalate (DEHP) are strong, moderate, and weak tumor promoters, respectively, while perfluorodecanoic acid (PFDA) lacks promoter activity. This investigation examined the effects of these PPs on the biosyntheses of phosphatidylcholine (PtdC) and phosphatidylethanolamine (PtdE) in rat liver. After exposure to PPs, rats were administered [1-(13)C]choline + [2-(13)C]ethanolamine and liver extracts were analyzed by (31)P and (13)C NMR. The ratio of choline-derived to ethanolamine-derived phospholipids, R(c/e), was significantly affected by all PPs (p < 0. 05). R(c/e) values were in the order Wy-14,643 > PFOA > DEHP > control > PFDA. The amounts of PtdC derived via the CDP-choline pathway versus PtdE-N-methyltransferase (PEMT) activity was 71 vs 29% in controls. This distribution was significantly affected by treatments with Wy-14,643 (95 vs 5%), DEHP (87 vs 13%), and PFDA (39 vs 61%) (p < 0.02). Data suggest that Wy-14,643, PFOA, and DEHP cause a preference for choline and the CDP-choline pathway for biosynthesis of PtdC. Additionally, Wy-14,643 and DEHP inhibited the PEMT pathway. In contrast, PFDA-treated rats showed a preference for ethanolamine, and PtdC was predominately synthesized through the PEMT pathway. These data corroborate studies by Vance and co-workers which suggest that the pathways for PtdC biosynthesis are important for hepatocarcinogenesis. Further studies to evaluate the potential of these measurements as a biomarker for PP-associated tumorigenesis is warranted.