Phospholipid methylation in MOPC-31C cell membranes with modified phospholipid composition

Chemotherapy induced PRL3 expression promotes cancer growth via plasma membrane remodeling and specific alterations of caveolae-associated signaling

BACKGROUND

The outcome of cancer therapy is greatly defined by the ability of a tumor cell to evade treatment and re-establish its bulk mass after medical interventions. Consequently, there is an urgent need for the characterization of molecules affecting tumor reoccurrence. The phosphatase of regenerating liver 3 (PRL3) protein was recently emerged among the targets that could affect such a phenomenon.

METHODS

The expression induction of PRL3 in melanoma cells treated with chemotherapeutic agents was assessed by western blotting. The effect of PRL3 expression on cancer growth was investigated both in vitro and in vivo. The association of PRL3 with the caveolae structures of the plasma membrane was analyzed by detergent free raft purification. The effect of PRL3 expression on the membrane organization was assayed by electron microscopy and by membrane biophysical measurements. Purification of the plasma membrane fraction and co-immunoprecipitation were used to evaluate the altered protein composition of the plasma membrane upon PRL3 expression.

RESULTS

Here, we identified PRL3 as a genotoxic stress-induced oncogene whose expression is significantly increased by the presence of classical antitumor therapeutics. Furthermore, we successfully connected the presence of this oncogene with increased tumor growth, which implies that tumor cells can utilize PRL3 effects as a survival strategy. We further demonstrated the molecular mechanism that is connected with the pro-growth action of PRL3, which is closely associated with its localization to the caveolae-type lipid raft compartment of the plasma membrane. In our study, PRL3 was associated with distinct changes in the plasma membrane structure and in the caveolar proteome, such as the dephosphorylation of integrin β1 at Thr788/Thr789 and the increased partitioning of Rac1 to the plasma membrane. These alterations at the plasma membrane were further associated with the elevation of cyclin D1 in the nucleus.

CONCLUSIONS

This study identifies PRL3 as an oncogene upregulated in cancer cells upon exposure to anticancer therapeutics. Furthermore, this work contributes to the existing knowledge on PRL3 function by characterizing its association with the caveolae-like domains of the plasma membrane and their resident proteins.

The incorporation of monomethylethanolamine and dimethylethanolamine in fetal brain aggregating cell culture

Fetal rat brain aggregating cell cultures were exposed to varying concentrations of [3H]monomethylethanolamine (MME) and [3H] dimethylethanolamine (DME). The rate of labeling of water-soluble compounds was more rapid and the amount of radioactivity present was greater than in the lipids. After a 72 hour incubation in the presence of millimolar concentrations of these nitrogenous bases, the major water-soluble products were the phosphorylated form of the bases. Little label was associated with the free bases or their cytidyl derivative. In the phospholipids, 97% of the radioactivity was recovered in phosphatidylmonomethylethanolamine (PMME) and 3% in phosphatidyldimethylethanolamine (PDME) or 95% in PDME and 5% in phosphatidylcholine (PC) after growth in presence of [3H]MME and [3H]DME respectively. The rate of formation of the radioactive products increased as function of the concentration of the nitrogenous base added up to 4 mM, the highest concentration employed. There was no significant difference in the pattern of labeling with cells grown in media devoid of methionine or choline. The turnover of the water-soluble metabolites was more rapid than in the phospholipids where an apparent half-life of 24 hours was calculated.

The effect of polar head group substitution on phospholipid methylation and the beta-adrenergic response in C6 glial cells

The membranes of intact C6 cells were enriched with phosphatidyldimethylethanolamine or phosphatidylmonomethylethanolamine. These cells showed enhanced rates of phospholipid methylation but this was not accompanied by an increased beta-adrenergic response. We conclude that phospholipid methylation is not coupled to the activation of adenylate cyclase in the beta-adrenergic response of C6 glial cells.

Phospholipid metabolism as a new target for malaria chemotherapy. Mechanism of action of D-2-amino-1-butanol

A number of choline and ethanolamine analogs were evaluated as inhibitors of P. falciparum growth in vitro. 1-Aziridineethanol, DL-2-amino-1,3-propranediol and D- or L-2-amino-1-butanol were the most efficient inhibitors of parasite multiplication, with an IC50 of 50-80 microM, whereas numerous other analogs were less active. The effect of D-2-amino-1-butanol on various metabolisms of P. knowlesi-infected simian erythrocytes was studied by incubating these cells with different labeled precursors of phospholipids, nucleic acids, proteins, and with radioactive glucose. In the presence of radioactive glycerol, oleate or lysophosphatidylcholine, the appearance of radioactivity in an unnatural phospholipid indicated that 2-aminobutanol was incorporated into a new PL which accounted for up to 30-40% of the total biosynthesized lipids. This new phospholipid accumulated primarily at the expense of PE biosynthesis and decreased the decarboxylation of phosphatidylserine. These effects were not accompanied, over a large range of concentrations, by any parallel change in nucleic or protein synthesis, nor in glucose metabolism. These data demonstrate that the incorporation of analogs, instead of the natural polar head groups, into cellular phospholipids, and/or modification of phospholipid composition have a deleterious impact on the growth of Plasmodium. It follows that PL metabolism is a crucial process for Plasmodium growth and may constitute a potentially fruitful chemotherapeutic approach to malaria.

Effect of changes in the phospholipid composition on the enzymatic activity of D-beta-hydroxybutyrate dehydrogenase in rat hepatocytes

The phospholipid composition of primary rat hepatocytes was manipulated by supplementing the medium with choline analogues. The unnatural analogue l-2-amino-1-butanol was incorporated into membrane phospholipids to the largest extent, whereas the natural choline analogues ethanolamine, N-methylethanolamine, and N,N-dimethyl-ethanolamine were methylated to yield phosphatidylcholine. When cells were supplemented with [14C]ethanolamine, greater than 25% of the total phosphatidylcholine contained radiolabel in the polar head group after 2 days of supplementation. The extent of phospholipid methylation was reduced by depriving the cells of serine and methionine. Under these conditions, N-methylethanolamine and N,N-dimethylethanolamine were incorporated into phospholipids and were not further metabolized to phosphatidylcholine. After 3 days of supplementation with N-methylethanolamine, the content of phosphatidyl-methylethanolamine went from essentially 0 to 40% of the total phospholipids and surpassed the extent of incorporation of all other analogues. The formation of the new phospholipid species was primarily at the expense of phosphatidylcholine and phosphatidylethanolamine. D-beta-Hydroxybutyrate dehydrogenase, which requires phosphatidylcholine for activity, was assayed in submitochondrial membranes isolated from supplemented cells. For cells supplemented with either l-2-amino-1-butanol or N-methylethanolamine, the Km for NADH increased relative to choline-supplemented cells while the Km for acetoacetate remained the same. For example, after 3 days of supplementation with N-methylethanolamine, the Km for NADH was 3-fold higher than the value for the choline-supplemented control cells. The change in the Km was due to the change in the lipid environment with no alteration in the enzyme itself. The results suggest that the phosphatidylcholine molecules necessary to activate the enzyme exchange with the other phospholipids in the membrane so that the Km of the enzyme reflects the overall content of phosphatidylcholine as well as other properties of the membrane phospholipids.

Phospholipid modification retards intracellular transport and secretion of immunoglobulin G1 by mouse MOPC-31c plasmacytoma cells

The intracellular transport and secretion of immunoglobulin G1(IgG1) by mouse MOPC-31C plasmacytoma cells were analyzed from the viewpoint of the roles of phospholipids. The membrane phospholipids were modified by culturing cells in a medium supplemented with choline analogues, N,N'-dimethylethanolamine or N-monomethylethanolamine, and accordingly the membranes were enriched in phosphatidyl-N,N'-dimethylethanolamine or phosphatidyl-N-monomethylethanolamine (Maeda, M., Tanaka, Y. and Akamatsu, Y. (1980) Biochem. Biophys. Res. Commun. 96, 876-881). The modified cells were pulse-labeled with L-[35S]methionine and the secretion of labeled IgG1 was chased. Half of the IgG1 was exported to the extracellular medium 1-1.5 h and 2-3 h after synthesis by choline- and dimethylethanolamine-supplemented cells, respectively. However, most of the newly synthesized IgG1 was not secreted by monomethylethanolamine-supplemented cells, even after 5 h; it remained within the cells. The sensitivity of intracellular IgG1 to endoglycosidase H was examined for probing the movement of IgG1 from the rough endoplasmic reticulum to the Golgi complex. Half of the newly synthesized IgG1 acquired resistance to endoglycosidase H after 30-45 min, 1-1.5 h and 2-3 h in choline-, dimethylethanolamine- and monomethylethanolamine-supplemented cells, respectively. Thus, the transport of IgG1 was markedly retarded by the modification with choline analogues, dimethylethanolamine or monomethylethanolamine, at least in the following two processes, from the rough endoplasmic reticulum to the Golgi complex and from the Golgi to the outside of cells. Modification with monomethylethanolamine was more effective than that with dimethylethanolamine in slowing down the transport of IgG1 and appeared to cause accumulation of IgG1 within the cells. A morphological study was also carried out for the three kinds of cell. The roles of phospholipids in the processes of membrane flow are discussed.

Modification of membrane phospholipid composition by choline analogues induces differentiation of cultured mouse myeloid leukemia cells

Mouse myeloid leukemia M1 cells could be induced by various inducers to form Fc receptors, phagocytize, produce lysozyme, and change into forms that were morphologically similar to macrophages and granulocytes. Previous experiments showed that change in phospholipid metabolism was associated with cell differentiation. In the present experiment, culture of M1 cells with choline analogs such as N-monomethyl-ethanolamine and N,N'-dimethylethanolamine resulted in accumulation of phosphatidyl-N-monomethyl-ethanolamine and phosphatidyl-N,N'-dimethylethanolamine in the cell membranes. This change upon treatment with choline analogs was associated with morphological and functional differentiation of the M1 cells into macrophages and granulocytes. These results suggest that phospholipid metabolism is involved in the mechanism of differentiation of M1 cells.

Rapid determination of lipids in healthy human lymphocytes

Lipids of human lymphocytes were determined from 20 ml of heparinized peripheral blood using thin-layer chromatography with a flame ionization detector, and gas chromatography. The weight per cent and microgram per 10(6) lymphocytes for cholesterol ester, triglyceride, free cholesterol and phospholipid were 11.1 and 5.2, 18.1 and 17.9, 15.1 and 8.5, and 55.7 and 44.2, respectively. Phospholipid was the major lipid component in human lymphocytes. Phospholipid was subfractionated into phosphatidylethanolamine, phosphatidylinositol plus phosphatidylserine, phosphatidylcholine and sphingomyelin in amounts of 25.2, 6.1, 46.9 and 22.9%, respectively. Total fatty acid composition was analyzed as: C14:0, 13.3%; C16:0, 20.9%; C16:1, 6.5%; C18:0, 19.6%; C18:1, 18.8%: C18:2, 7.1%; and C20:4, 12.3%. Higher cholesterol ester and triglyceride and lower C14:0 were characteristic of female lymphocytes when compared with male lymphocytes. The lipid composition quantitated by this method corresponded well with previously reported data. Thus, this method can be used clinically because of its simplicity and higher sensitivity.

Presence of the phospholipid methylation pathway in mammalian cultured cells

Stepwise N-methylation of phosphatidylethanolamine to phosphatidylcholine was examined by measuring incorporation of the radioactive methyl group of S-adenosylmethionine into phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N'-dimethylethanolamine and phosphatidylcholine by membranes of five mammalian cultured cell lines (BHK-21 cells, Chang liver cells, PC-12 h cells, MOPC-31C cells and LM cells). The three successive methylation steps were found in all the cells studied, though the total incorporation and the distribution profile of the radioactivity among the products differed with different cells. Furthermore, increase in the amounts of phosphatidyl-N-monomethylethanolamine and phosphatidyl-N,N'-dimethylethanolamine in the amine and N,N'-dimethylethanolamine, respectively, resulted in increased methylation of phospholipids. This remarkable enhancement of methylation seems to be a characteristic effect of such modification of membrane phospholipids in mammalian cultured cells.