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

The lipid homeostasis regulation study of arenobufagin in zebrafish HepG2 xenograft model and HepG2 cells using integrated lipidomics-proteomics approach

ETHNOPHARMACOLOGICAL RELEVANCE

Arenobufagin (ArBu) is an important anti-tumor ingredient of Chan'su which has long been used as traditional Chinese medicine in clinic for tumor therapy in China.

AIM OF THE STUDY

The purpose of our study is to investigate the lipid homeostasis regulation effects of ArBu on zebrafish model of liver cancer and hepatoma cells, and to provide a reference for further clarifying its active mechanisms.

MATERIALS AND METHODS

The zebrafish xenograft model was established by injecting HepG2 cells stained with CM-Dil red fluorescent dye. Both the xenograft model and HepG2 cells were used to evaluate the anti-hepatoma activity of ArBu. High performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was the main method to study lipidomics, proteomics and the semiquantification of endogenous metabolites. Bioinformatics was used as an assistant tool to further explore the antitumor mechanism of ArBu.

RESULTS

The lipidomics analysis revealed that ArBu caused differential lipids changes in a dose-dependent manner, including PCs, PEs, TGs, SMs, DGs, Cer and PA. PCs, PEs, SMs and TGs were markedly altered in both two models. The influence of glycerophospholipid metabolism was the major and commonly affected pathway. Notably, DGs and Cer were significantly changed only in HepG2 cells. Furthermore, the proteomics research in HepG2 cells fished the target proteins related to lipid homeostasis abnormalities and tumor suppression. ArBu reduced the expression of 65 differential proteins associated with the lipid metabolism, apoptosis and autophagy, such as LCLAT1, STAT3, TSPO and RPS27. Meanwhile, 7 amino acids of 29 determined metabolites were significantly changed, including tyrosine, glutamate, glutamine, leucine, threonine, arginine and isoleucine.

CONCLUSION

ArBu has a significant anti-hepatoma effect in vitro and a therapeutic effect on zebrafish xenograft model. It regulated the lipid homeostasis. Activated SM synthase and arginine deiminase, inhibited sphingomyelinase, amino acid supply and JAK-STAT3 signaling pathway, and the affected glycerophospholipid metabolism might explain these results.

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.

Pemt deficiency ameliorates endoplasmic reticulum stress in diabetic nephropathy

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.

Protective effect of phosphatidylcholine on restoration of ethanol-injured hepatocytes related with caveolin-1

The absorption of phospholipid may improve the fluidity of membrane and enzyme activities. Phospholipids also play a role in promoting Caveolae formation and membrane synthesis. Caveolin-1 has a significant effect on signaling pathways involved in regulating cell proliferation and stress responsiveness. Thus, we can speculate that Caveolin-1 could affect the sense of environmental stress. We use Chang liver cell line to investigate the ability of Caveolin-1 to modulate the cellular response to ethanol injury. Caveolin-1 downregulate cells (Cav-1(-/-)) were established by stable transfecting with psiRNA-CAV1 plasmids, which were more sensitive to toxic effects of ethanol than the untransfected parental cells (WT). Releasing of ALT and electric conductivity were changed significantly in Cav-1(-/-) cells compared with WT. Caveolin-1 gene silencing could obviously down-regulate the activities of protein kinase C-α (PKC-α) and phospho-p42/44 MAP kinase, indicating cell proliferation and self-repairing abilities were inhibited. However, the levels of Caveolin-1 and PKC-α were increased by phosphatidylcholine administration. The results indicated that the inhibition of lipid peroxidation by phosphatidylcholine could lead to the prevention of membrane disruption, which closely correlated with the level of Caveolin-1. Since the protective effects of phosphatidylcholine against ethanol-induced lipid peroxidation might be regulated by phospholipid-PKC-α signaling pathway, related with Caveolin-1, the potential effects of phosphatidylcholine on membranes need to be verified.

Choline plasmalogens isolated from swine liver inhibit hepatoma cell proliferation associated with caveolin-1/Akt signaling

Plasmalogens play multiple roles in the structures of biological membranes, cell membrane lipid homeostasis and human diseases. We report the isolation and identification of choline plasmalogens (ChoPlas) from swine liver by high performance thin layer chromatography (HPTLC) and high performance liquid chromatography (HPLC)/MS. The growth and viability of hepatoma cells (CBRH7919, HepG2 and SMMC7721) was determined following ChoPlas treatment comparing with that of human normal immortal cell lines (HL7702). Result indicated that ChoPlas inhibited hepatoma cell proliferation with an optimal concentration and time of 25 μmol/L and 24 h. To better understand the mechanism of the ChoPlas-induced inhibition of hepatoma cell proliferation, Caveolin-1 and PI3K/Akt pathway signals, including total Akt, phospho-Akt(pAkt) and Bcl-2 expression in CBRH7919 cells, were determined by western blot. ChoPlas treatment increased Caveolin-1 expression and reduced the expression of phospho-Akt (pAkt) and Bcl-2, downstream targets of the PI3K/Akt pathway. Further cell cycle analysis showed that ChoPlas treatment induced G1 and G1/S phase transition cell cycle arrest. The expression of essential cell cycle regulatory proteins involved in the G1 and G1/S phase transitions, cyclin D, CDK4, cyclin E and CDK2, were also analyzed by western blot. ChoPlas reduced CDK4, cyclin E and CDK2 expression. Taken together, the results indicate that swine liver-derived natural ChoPlas inhibits hepatoma cell proliferation associated with Caveolin-1 and PI3K/Akt signals.

The role of phosphatidylcholine and choline metabolites to cell proliferation and survival

The reorganization of metabolic pathways in cancer facilitates the flux of carbon and reducing equivalents into anabolic pathways at the expense of oxidative phosphorylation. This provides rapidly dividing cells with the necessary precursors for membrane, protein and nucleic acid synthesis. A fundamental metabolic perturbation in cancer is the enhanced synthesis of fatty acids by channeling glucose and/or glutamine into cytosolic acetyl-CoA and upregulation of key biosynthetic genes. This lipogenic phenotype also extends to the production of complex lipids involved in membrane synthesis and lipid-based signaling. Cancer cells display sensitivity to ablation of fatty acid synthesis possibly as a result of diminished capacity to synthesize complex lipids involved in signaling or growth pathways. Evidence has accrued that phosphatidylcholine, the major phospholipid component of eukaryotic membranes, as well as choline metabolites derived from its synthesis and catabolism, contribute to both proliferative growth and programmed cell death. This review will detail our current understanding of how coordinated changes in substrate availability, gene expression and enzyme activity lead to altered phosphatidylcholine synthesis in cancer, and how these changes contribute directly or indirectly to malignant growth. Conversely, apoptosis targets key steps in phosphatidylcholine synthesis and degradation that are linked to disruption of cell cycle regulation, reinforcing the central role that phosphatidylcholine and its metabolites in determining cell fate.

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.

Selective tropism of liver stem cells to hepatocellular carcinoma in vivo

AIM: To investigate the selective tropism of liver stem cells to hepatocellular carcinoma (HCC) in an animal model and its feasibility as a vector to deliver therapeutic genes for targeted therapy of HCC.

METHODS: WB-F344, a kind of rat liver stem cell, was infected with recombinant virus to establish a cell line with stable, high-level expressing enhanced green fluorescent protein (EGFP). An animal model of HCC in Wistar rats was established by implanting HCC cells (CBRH7919) combined with an immunosuppressive drug. EGFP labeled liver stem cells were injected into caudal veins of the animals and distribution was observed at different time points after injection. SDF-1 and c-kit expression in non-tumor liver and tumor tissue were analysed by immunohistochemistry for the relationshiop between the expression and migration of liver stem cells. Furthermore, hepatic stem cells were injected via the portal vein, hepatic artery, caudal vein, or directly into the pericancerous liver tissue, respectively, and effects on migration, localization, and proliferation of the hepatic stem cells within the tumor tissue were observed and analyzed.

RESULTS: Recombinant adenovirus could deliver the EGFP gene to hepatic stem cells. A new stem cell line, named WB-EGFP, was established that stably expressed EGFP. WB-EGFP cells still showed selective tropism towards HCC and EGFP expression was stable in vivo. According to immunohistochemistry results, SDF-1 may not be related to the mechanisms of tropism of hepatic stem cells. Different application sites affected the distribution of liver stem cells. Injection via the portal vein was superior with regard to selective migration, localization, and proliferation of the hepatic stem cells within the tumor tissue.

CONCLUSION: Liver stem cells have the biological behavior of selective migration to HCC in vivo and they could localize and proliferate within HCC tissue stably expressing the target gene. Liver stem cells are a potential tool for a targeted gene therapy of HCC.

Increase of phosphatidylethanolamine N-methyltransferase activity in the small bowel brush-border membrane after massive intestinal distal resection in rats

Phosphatidylcholine plays an important role for the structure and function of the cell membrane, and its synthesis from phosphatidylethanolamine is catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). This study investigates changes in PEMT activity in the intestinal brush border membrane after extensive distal enterectomy (60%) in 40 Wistar rats. Four groups, each of 10 rats, were killed immediately after surgery (day 0) and on the 7th, 14th and 28th day postoperatively. Samples from jejunum were collected for histomorphometry and PEMT activity was determined by measuring the incorporation of [(3)H]-methyl groups from S-adenosyl-L-(methyl-(3)H)-methionine into phospholipids. Enterectomy induced 30%, 48% and 21% increases in the jejunum villus cell population, and 32%, 81%, and 32% in the crypt cell population at postoperative days 7, 14, and 28, respectively. PEMT activity increased 41% at day 14, suggesting functional differentiation, remaining at this level until day 28, when a reduction in the epithelial cell population was observed, thus indicating that adaptation was completed. The observed increase in PEMT-specific activity in the residual intestine suggests that extensive enterectomy stimulates the synthesis of phosphatidylcholine by the PEMT-controlled pathway.

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.

Liver tumors escape negative control of proliferation via PI3K/Akt-mediated block of C/EBP alpha growth inhibitory activity

Liver tumor cells arise from normal hepatocytes that escape negative control of proliferation. The transcription factor C/EBPalpha maintains quiescence of hepatocytes through two pathways: inhibition of cdks and repression of E2F. Nevertheless, liver tumors and cultured hepatoma cell lines proliferate in the presence of C/EBPalpha. In this paper, we present evidence that the activation of the PI3K/Akt pathway in liver tumor cells blocks the growth inhibitory activity of C/EBPalpha through the PP2A-mediated dephosphorylation of C/EBPalpha on Ser 193, leading to a failure of C/EBPalpha to interact with and inhibit cdks and E2F. Mutation of Ser 193 to Ala also abolishes the ability of C/EBPalpha to cause growth arrest because of a lack of interactions with cdk2 and E2F-Rb complexes. These data provide a molecular basis for the development of liver tumors in which the activation of PI3K/Akt pathway neutralizes C/EBPalpha growth inhibitory activity.

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.

Effects of ganglioside GM3 on phospholipid turnover of human leukemic J6-2 cells

Ganglioside GM3 was reported to induce the differentiation of HL-60 cells to differentiate along the macrophage-monocytic route. We used human monocytoid leukemia J6-2 cells and successfully induced differentiation by GM3. Because differentiation is accompanied by retarded growth rate and cell cycle is intimately related to phospholipid metabolism, so we explored how GM3 was related to phospholipid metabolism. By using [32P]Pi, [3H-CH3]choline, [3H-CH3]SAM, and [3H]inositol as radioactive tracers, we studied the turnover changes of phospholipids and their metabolites induced by GM3. For the morphological changes of differentiation to occur, the cells had to be treated with GM3 at a concentration of 50 microM for 5-6 days, but the phospholipid changes occurred at a very early stage of GM3 treatment (only 1 h). Our results indicate that GM3 stimulated PE methylation pathway inhibited both CDP-choline pathway and PI cycle. The phospholipid changes may constitute the early events in differentiation induced by GM3.