Plasmalogen status influences docosahexaenoic acid levels in a macrophage cell line. Insights using ether lipid-deficient variants.
Previously, this laboratory reported the isolation of variants, RAW. 12 and RAW.108, from the macrophage-like cell line RAW 264.7 that are defective in plasmalogen biosynthesis [Zoeller, R.A. et al. 1992. J. Biol. Chem. 267: 8299-8306]. Fatty acid analysis showed significant changes in the mutants in the ethanolamine phospholipids (PE), the only phospholipid class in which the plasmalogen species, plasmenylethanolamine, contributes significantly. Within the PE fraction, docosapentaenoic (DPA; 22:5n-3) and docosahexaenoic (DHA; 22:6n-3) acids were reduced by approximately 50% in the variants while the levels of arachidonic acid (AA; 20:4n-6) remained unaffected. The decrease in DHA was accompanied by a 50% decrease in labeling PE with [3H]DHA over a 90-min period. Restoration of plasmenylethanolamine by supplementing the growth medium with sn -1-hexadecylglycerol (HG) completely reversed these changes in RAW. 108. Pre-existing pools of plasmenylethanolamine were not required for restoration of normal [3H]DHA labeling; addition of HG only during the labeling period was sufficient. Due to the loss of Delta1'-desaturase in RAW.12, HG supplementation resulted in the accumulation of plasmenylethanolamine's immediate biosynthetic precursor, plasmanylethanolamine. Even though this latter phospholipid contained only the ether functionality (lacking the vinyl ether double bond) it was sufficient to restore wild type-like fatty acid composition and DHA labeling of the ethanolamine phospholipids, identifying the ether bond as a structural determinant for this specificity. In summary, we have used these mutants to establish that the plasmalogen status of a cell can influence the levels of certain polyunsaturated fatty acids. These results support the notion that certain polyunsaturated fatty acids, such as DHA, can be selectively targeted to plasmalogens and that this targeting occurs during de novo biosynthesis, or shortly thereafter, through modification of nascent plasmalogen pools. (+info)
Dietary fish oils inhibit early events in the assembly of very low density lipoproteins and target apoB for degradation within the rough endoplasmic reticulum of hamster hepatocytes.
Dietary fish oils inhibited secretion and stimulated intracellular degradation of apolipoprotein (apo)B in hamster hepatocytes, while dietary sunflower oils stimulated secretion and had no effect on degradation of apoB. To investigate the intracellular site at which fish oils act, we have made use of our previous observations that inhibition of degradation by N-acetyl-leucyl-leucyl-norleucinal (ALLN) results in accumulation of apoB in the trans -Golgi membrane and does not stimulate secretion, while inhibition of degradation by o-phenanthroline results in accumulation of apoB in the rough endoplasmic reticulum membrane and stimulates secretion. Thus, ALLN protects apoB which has been diverted from secretion and o -phenanthroline protects apoB which is targetted for secretion. Addition of o -phenantholine to the incubation medium of hepatocytes from fish oil-fed hamsters inhibited degradation of apoB and stimulated its secretion in particles of the density of VLDL, while addition of ALLN had no effect. These observations suggest that dietary fish oils reversibly inhibit early steps in the assembly of very low density lipoprotein precursors and target apoB for degradation in the rough endoplasmic reticulum. (+info)
Carboxyatractyloside increases the effect of oleate on mitochondrial permeability transition.
Addition of a low concentration of carboxyatractyloside (0.075 microM) renders mitochondria susceptible to the opening of the non-specific pore by 5 microM oleate, in a cyclosporin A-sensitive fashion. Matrix Ca2+ efflux as well as collapse of the transmembrane potential reveal permeability transition. The effect of oleate is reached after the titration, by carboxyatractyloside, of 38 pmol of adenine nucleotide translocase per mg mitochondrial protein. We propose that permeability transition may result from an additive action of carboxyatractyloside plus oleate on the ADP/ATP carrier. (+info)
Role of cholesterol ester mass in regulation of secretion of ApoB100 lipoprotein particles by hamster hepatocytes and effects of statins on that relationship.
Our understanding of the factors that regulate the secretion of apoB100 lipoproteins remains incomplete with considerable debate as to the role, if any, for cholesterol ester in this process. This study examines this issue in primary cultures of hamster hepatocytes, a species in which both cholesterol and apoB100 metabolism are very similar to man. Addition of oleate to medium increased the mass of triglyceride and cholesterol ester within the hepatocyte and also increased the secretion of triglycerides, cholesterol ester, and apoB100 into the medium. Next, the responses of hamster hepatocytes to addition of either an HMG-CoA reductase inhibitor (lovastatin) or an acyl-CoA cholesterol acyltransferase inhibitor (58-035) to the medium, with or without added oleate, were determined. Effects of either agent were only evident in the oleate-supplemented medium in which cholesterol ester mass had been increased above basal. If oleate was not added to the medium, neither agent reduced apoB100 secretion; equally important, over the 24-hour incubation, neither agent, at the concentration used, produced any detectable change in intracellular cholesterol ester mass. However, in contrast to the estimates of mass, which were unchanged, under the same conditions radioisotopic estimates of cholesterol ester synthesis were markedly reduced. Any conclusion as to the relation of cholesterol ester mass to apoB100 secretion would therefore depend on which of the 2 methods was used. Overall, the data indicate a close correlation between the mass of cholesterol ester within the hepatocyte and apoB100 secretion from it and they go far to explain previous apparently contradictory data as to this relation. More importantly, though, taken with other available data, they indicate that the primary response of the liver to increased delivery of lipid is increased secretion rather than decreased uptake. These results point, therefore, to a hierarchy of hepatic responses to increased flux of fatty acids and increased synthesis of cholesterol that in turn suggests a more dynamic model of cholesterol homeostasis in the liver than has been appreciated in the past. (+info)
Prolonged elevation of plasma free fatty acids desensitizes the insulin secretory response to glucose in vivo in rats.
Prolonged exposure of pancreatic islets to free fatty acids (FFAs) inhibits glucose-stimulated insulin secretion (GSIS) in vitro. However, FFA inhibition of GSIS has not been clearly demonstrated in vivo. We examined the in vivo effect of prolonged elevation of plasma FFAs on GSIS using a two-step hyperglycemic clamp in rats treated with a 48-h intravenous infusion of either 20% Intralipid plus heparin (INT) (5 microl/min plus heparin, 0.1 U/min; n = 8), oleate (OLE) (1.3 microEq/min; n = 6), saline (SAL) (n = 6), or bovine serum albumin (BSA) (vehicle for OLE; n = 5). Because there was no difference in any of the parameters between BSA and SAL rats, these groups were combined as control rats (CONT) (n = 11). At the end of the 48-h OLE/INT/CONT infusions, after an overnight fast, plasma glucose was clamped for 2 h at 13 mmol/l and for another 2 h at 22 mmol/l. Preclamp plasma FFAs were elevated twofold (P < 0.01) versus CONT with both INT and OLE (NS, INT vs. OLE). Preclamp glucose, insulin, and C-peptide levels were higher in INT than in CONT rats (P < 0.05), suggesting insulin resistance, but they were not different in OLE and CONT rats. The insulin and C-peptide responses to the rise in plasma glucose from basal to 13 mmol/l were lower in OLE (336 +/- 72 pmol/l and 1.2 +/- 0.1 nmol/l, P < 0.01 and P < 0.05, respectively) than in CONT (552 +/- 54 and 1.9 +/- 0.1) rats, but they were not different between CONT and INT rats (648 +/- 150 and 2.0 +/- 0.4). The insulin and C-peptide responses to the rise in plasma glucose from 13 to 22 mmol/l were lower in both INT (1,188 +/- 204 pmol/l and 3.0 +/- 0.3 nmol/l, P < 0.01 and P < 0.001) and OLE (432 +/- 60 and 1.7 +/- 0.2, P < 0.001 vs. CONT or INT) rats than in CONT rats (1,662 +/- 174 and 5.0 +/- 0.6). In summary, 1) both INT and OLE decreased GSIS in vivo in rats, and 2) the impairing effect of INT on GSIS was less than that of OLE, which might be due to the different type of fatty acid (mostly polyunsaturated in INT versus monounsaturated as OLE) and/or to differential effects of INT and OLE on insulin sensitivity. In conclusion, prolonged elevation of plasma FFAs can desensitize the insulin secretory response to glucose in vivo, thus inducing a beta-cell defect that is similar to that found in type 2 diabetes. (+info)
Fatty acid block of the transient outward current in adult human atrium.
Fatty acids represent an essential source of fuel for the heart and play an important role in the mechanical, electrical, and synthetic activities of cardiac cells. Under pathological conditions, such as ischemia followed by reperfusion, the myocardium is exposed to very high levels of fatty acids, in particular the monounsaturated fatty acid, oleic acid. Elevated plasma fatty acids have been linked to an increased risk for cardiac arrhythmias. In other species, fatty acids have been shown to modulate several cardiac ion channels, most notably potassium channels. Virtually nothing is known about the actions of oleic acid on potassium channels in human heart. We therefore characterized the effects of oleic acid on the transient outward current, sustained current, and inwardly rectifying current, some of the major potassium channels present in human atrium, using the whole-cell patch clamp method. Exposure of cells to oleic acid (5 microM) reduced the transient outward potassium current to 3.7 +/- 0.8 pA/pF (n = 4) compared with 7.0 +/- 0.7 pA/pF (n = 4) (P <. 05) for cells not exposed. In contrast, oleic acid had little effect on either the sustained current (4.3 +/- 0.3 pA/pF, n = 4 for oleic acid versus 4.8 +/- 0.5, n = 5 for control) present after the decay of the transient outward current or on the amplitude of IK1 measured at -100 mV (1.4 +/- 0.4 pA/pF, n = 4 for oleic acid versus 1.3 +/- 0. 4 pA/pF, n = 6 for control). In addition, oleic acid significantly slowed the rate of recovery of the transient outward current, which is predicted to result in a use-dependent reduction in current amplitude in the beating heart. These results suggest a possible contributing role for oleic acid block of the transient outward current in the pathological consequences of myocardial ischemia. (+info)
Identification of peroxisomal acyl-CoA thioesterases in yeast and humans.
A computer-based screen of the Saccharomyces cerevisiae genome identified YJR019C as a candidate oleate-induced gene. YJR019C mRNA levels were increased significantly during growth on fatty acids, suggesting that it may play a role in fatty acid metabolism. The YJR019C product is highly similar to tesB, a bacterial acyl-CoA thioesterase, and carries a tripeptide sequence, alanine-lysine-phenylalanineCOOH, that closely resembles the consensus sequence for type-1 peroxisomal targeting signals. YJR019C directed green fluorescence protein to peroxisomes, and biochemical studies revealed that YJR019C is an abundant component of purified yeast peroxisomes. Disruption of the YJR019C gene caused a significant decrease in total cellular thioesterase activity, and recombinant YJR019C was found to exhibit intrinsic acyl-CoA thioesterase activity of 6 units/mg. YJR019C also shared significant sequence similarity with hTE, a human thioesterase that was previously identified because of its interaction with human immunodeficiency virus-Nef in the yeast two-hybrid assay. We report here that hTE is also a peroxisomal protein, demonstrating that thioesterase activity is a conserved feature of peroxisomes. We propose that YJR019C and hTE be renamed as yeast and human PTE1 to reflect the fact that they encode peroxisomal thioesterases. The physical segregation of yeast and human PTE1 from the cytosolic fatty acid synthase suggests that these enzymes are unlikely to play a role in formation of fatty acids. Instead, the observation that PTE1 contributes to growth on fatty acids implicates this thioesterase in fatty acid oxidation. (+info)
Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations.
It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion. (+info)