Phospholipid dependence of membrane-bound phospholipase A2 in ras-transformed NIH 3T3 fibroblasts

Alteration in lipid and protein profiles of ovarian cancer: similarity to breast cancer

This study was undertaken to evaluate protein and lipid profiles of ovarian cancer tissue samples. Twenty-three frozen ovarian cancer samples and 6 adjacent normal samples were analyzed using histology-directed, matrix-assisted laser desorption/ionization mass spectrometry. Sinapinic acid and 2, 5-dihydroxybenzoic acid/α-cyano-4-hydroxycinnamic acid were manually deposited on areas of each tissue section enriched in epithelial cells to identify protein and lipid profiles respectively, and mass spectra were acquired using a matrix-assisted laser desorption/ionization-time of flight instrument. Protein and lipid profiles classify 11 cancer and 3 adjacent normal samples in 100 random test sets with 92.9% median accuracy. Phosphatidylcholines {32:3} [M + Na] (m/z = 750.66), {34:1} [M + K] (m/z = 798.60), and {36:2} [M + K] (m/z = 824.56) were found to be increased in ovarian cancer. Interestingly, breast cancer-associated changes in lipid and protein profiles were also found in ovarian cancer. Thus, protein and lipid profiles accurately distinguish ovarian cancer from adjacent normal tissue samples. Common cancer-associated alterations in lipid and protein profiles were identified between ovarian and breast cancers.

Protein and lipid MALDI profiles classify breast cancers according to the intrinsic subtype

Background

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has been demonstrated to be useful for molecular profiling of common solid tumors. Using recently developed MALDI matrices for lipid profiling, we evaluated whether direct tissue MALDI MS analysis on proteins and lipids may classify human breast cancer samples according to the intrinsic subtype.

Methods

Thirty-four pairs of frozen, resected breast cancer and adjacent normal tissue samples were analyzed using histology-directed, MALDI MS analysis. Sinapinic acid and 2,5-dihydroxybenzoic acid/α-cyano-4-hydroxycinnamic acid were manually deposited on areas of each tissue section enriched in epithelial cells to identify lipid profiles, and mass spectra were acquired using a MALDI-time of flight instrument.

Results

Protein and lipid profiles distinguish cancer from adjacent normal tissue samples with the median prediction accuracy of 94.1%. Luminal, HER2+, and triple-negative tumors demonstrated different protein and lipid profiles, as evidenced by permutation P values less than 0.01 for 0.632+ bootstrap cross-validated misclassification rates with all classifiers tested. Discriminatory proteins and lipids were useful for classifying tumors according to the intrinsic subtype with median prediction accuracies of 80.0-81.3% in random test sets.

Conclusions

Protein and lipid profiles accurately distinguish tumor from adjacent normal tissue and classify breast cancers according to the intrinsic subtype.

The plasma membrane lipid composition affects fusion between cells and model membranes

Investigations were carried out on the effect of plasma membrane lipid modifications on the fusogenic capacity of control and ras-transformed fibroblasts. The plasma membrane lipid composition was modified by treatment of cells with exogenous phospholipases C and D, sphingomyelinase and cyclodextrin. The used enzymes hydrolyzed definite membrane lipids thus inducing specific modifications of the lipid composition while cyclodextrin treatment reduced significantly the level of cholesterol. The cells with modified membranes were used for assessment of their fusogenic capacity with model membranes with a constant lipid composition. Treatment with phospholipases C and D stimulated the fusogenic potential of both cell lines whereas the specific reduction of either sphingomyelin or cholesterol induced the opposite effect. The results showed that all modifications of the plasma membrane lipid composition affected the fusogenic capacity irrespective of the initial differences in the membrane lipid composition of the two cell lines. These results support the notion that the lipid composition plays a significant role in the processes of membrane-membrane fusion. This role could be either direct or through modulation of the activity of specific proteins which regulate membrane fusion.

Clofibrate-induced relocation of phosphatidylcholine transfer protein to mitochondria in endothelial cells

The phosphatidylcholine transfer protein (PC-TP) is a specific transporter of phosphatidylcholine (PC) between membranes. To get more insight into its physiological function, we have studied the localization of PC-TP by microinjection of fluorescently labeled PC-TP in foetal bovine heart endothelial (FBHE) cells and by expression of an enhanced yellow fluorescent protein-PC-TP fusion protein in FBHE cells, human umbilical vein endothelial cells, and HepG2 cells. Analysis by confocal laser scanning microscopy showed that PC-TP was evenly distributed throughout the cytosol with an apparently elevated level in nuclei. By measuring the fluorescence recovery after bleaching it was established that PC-TP is highly mobile throughout the cell, with its transport into the nucleus being hindered by the nuclear envelope. Given the proposed function of PC-TP in lipid metabolism, we have tested a number of compounds (phorbol ester, bombesin, A23187, thrombin, dibutyryl cyclic AMP, oleate, clofibrate, platelet-derived growth factor, epidermal growth factor, and hydrogen peroxide) for their ability to affect intracellular PC-TP distribution. Only clofibrate (100 microM) was found to have an effect, with PC-TP moving to mitochondria within 5 min of stimulation. This relocation did not occur with PC-TP(S110A), lacking the putative protein kinase C (PKC)-dependent phosphorylation site, and was restricted to the primary endothelial cells. Relocation did not occur in HepG2 cells, possibly due to the fact that clofibrate does not induce PKC activation in these cells.

The binding of phosphatidylcholine to the phosphatidylcholine transfer protein: affinity and role in folding

Bovine liver phosphatidylcholine transfer protein (PC-TP) has been expressed in Escherichia coli and purified to homogeneity from the cytosol fraction at a yield of 0.45 mg PC-TP per 10 mg total cytosolic protein. In addition, active PC-TP was obtained from inclusion bodies. An essential factor in the activation of PC-TP was phosphatidylcholine (PC) present in the folding buffer. PC-TP from the cytosol contains phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) with a preference for the di-monounsaturated species over the saturated species as determined by fast atom bombardment mass spectrometry (FAB-MS). By incubation with microsomal membranes the endogenous PE and PG were replaced by PC. Relative to the microsomal PC species composition, PC-TP bound preferentially C16:0/C20:4-PC and C16:0/C18:2-PC (twofold enriched) whereas the major microsomal species C18:0/C18:1-PC and C18:0/C18:2-PC were distinctly less bound. PC-TP is structurally homologous to the lipid-binding domain of the steroidogenic acute regulatory protein (Nat. Struct. Biol. 7 (2000) 408). Replacement of Lys(55) present in one of the beta-strands forming the lipid-binding site, with an isoleucine residue yielded an inactive protein. This suggests that Lys(55) be involved in the binding of the PC molecule.

Stimulated nonspecific transport of phospholipids results in elevated external appearance of phosphatidylserine in ras-transformed fibroblasts

The content of phosphatidylserine (PS) was found to be increased three times in the plasma membrane outer leaflet of ras-transformed fibroblasts compared to their nontransformed counterparts. In an attempt to determine the mechanisms responsible for the enhanced external appearance of PS, we investigated the activities of aminophospholipid translocase and the nonspecific lipid scramblase. Both transport systems could separately or in combination contribute to PS accumulation in the extracellular leaflet. Aminophospholipid transfer was assessed by measuring the rate of NBD-PS internalization, and scramblase activity was estimated from the internalization of NBD-PC. The results showed that the aminophospholipid transport was inhibited and the nonspecific transport was stimulated in ras-transformed cells. To assess which of these two transport systems was related to elevation of PS external appearance, each of them was submitted to reversible alterations and the content of PS was measured simultaneously. Aminophospholipid translocase activity was inhibited by pyridyldithioethylamine treatment and reversed by reduction with dithiothreitol. Scramblase activity was modulated by a calcium repletion-depletion procedure. Calcium depletion was performed by cell incubation with BAPTA-AM and EGTA as Ca2+ intracellular and extracellular chelators. Restoration of the intracellular Ca2+ was achieved by cell incubation with Ca2+ and Ca2+-ionophore A23187. The results showed that the changes in PS outer appearance did not correlate with the uptake of NBD-PS but were closely related to NBD-PC internalization, suggesting that the nonspecific bidirectional lipid transfer was the major transport system translocating PS to the outer leaflet in ras-transformed cells.