Mode of transport of fatty acid to endothelial cells influences intracellular fatty acid metabolism

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Unsaturated fatty acids isolated from human lipoproteins activate protein phosphatase type 2Cβ and induce apoptosis in endothelial cells

Activity of serine/threonine protein phosphatase type 2C is known to be stimulated by certain unsaturated fatty acids and this enzyme dephosphorylates Bad, thus acting on apoptosis. This prompted us to investigate endothelial cell death. Here, we present evidence for the presence of protein phosphatase type 2Cbeta (PP2Cbeta) in human umbilical vein endothelial cells (HUVECs) and report on colocalization of PP2Cbeta and Bad in the cytosol of endothelial cells. Lipophilic compounds that stimulated PP2Cbeta activity in vitro were found to induce cell death of HUVECs. Lipoproteins did neither influence PP2Cbeta activity nor affect cell behaviour. Lipoproteins treated with the lipoprotein lipase, however, stimulated the activity of PP2Cbeta at least 10-fold concomitantly triggering cell death. Analytical methods revealed that both effects - stimulation of PP2Cbeta and apoptosis - were caused by free fatty acids liberated from VLDL, LDL and HDL with oleic acid and linoleic acid as major constituents. The results provide novel insights in endothelial apoptosis and suggest that PP2Cbeta participates in the development and progress of atherosclerosis.

Flux of Fatty Acids through NPC1 Lysosomes

Niemann-Pick type C (NPC) is an autosomal recessive lipid storage disorder characterized by lysosomal accumulation of cholesterol and gangliosides resulting from a defect in intracellular lipid trafficking. The NPC1 gene encodes a 1278-amino acid integral membrane protein involved in the sub-cellular trafficking of lipids. The exact biological function of NPC1 remains unclear. Recent evidence suggests that NPC1 is a eukaryotic member of the RND permease family of transport proteins, which when expressed in bacteria is capable of transporting fatty acids. The goal of this project was to assess the role of NPC1 in the transport of fatty acids in primary human fibroblasts using normal fibroblasts and fibroblasts from patients with three lysosomal storage diseases: NPC, mucolipidosis IV, and Sandhoff disease. If NPC1 is a fatty acid transporter, we expect to find fatty acid accumulation only in NPC fibroblasts. We used three experimental approaches to assess the role of NPC1 as a fatty acid transporter. First, we evaluated the accumulation versus metabolism of low density lipoprotein-derived oleic acid. Second, we assessed the amount of free fatty acid present after growth in lipoprotein-containing media. Third, we assessed the cellular accumulation of acriflavine, a fluorescent substrate for a number of resistance-nodulation-cell division permease transporters. Our results indicate that fatty acid flux through NPC1-deficient lysosomes is normal.

Suppression of Macrophage Eicosanoid Synthesis by Atherogenic Lipoproteins Is Profoundly Affected by Cholesterol-Fatty Acyl Esterification and the Niemann-Pick C Pathway of Lipid Trafficking

Atheroma macrophages internalize large quantities of lipoprotein-derived lipids. While most emphasis has been placed on cholesterol, lipoprotein-derived fatty acids may also play important roles in lesional macrophage biology. Little is known, however, about the trafficking or metabolism of these fatty acids. In this study, we first show that the cholesterol-fatty acyl esterification reaction, catalyzed by acyl-CoA:cholesterol acyltransferase (ACAT), competes for the incorporation of lipoprotein-derived fatty acids into cellular phospholipids. Furthermore, conditions that inhibit trafficking of cholesterol from late endosomes/lysosomes to the endoplasmic reticulum (ER), such as the amphipathic amine U18666A and the Npc1+/- mutation, also inhibit incorporation of lipoprotein-derived fatty acids into phospholipids. The biological relevance of these findings was investigated by studying the suppression of agonist-induced prostaglandin E(2) (PGE(2)) and leukotriene C(4)/D(4)/E(4) production during lipoprotein uptake by macrophages, which has been postulated to involve enrichment of cellular phospholipids with non-arachidonic fatty acids (NAAFAs). We found that eicosanoid suppression was markedly enhanced when ACAT was inhibited and prevented when late endosomal/lysosomal lipid trafficking was blocked. Moreover, PGE(2) suppression depended entirely on acetyl-LDL-derived NAAFAs, not on acetyl-LDL-cholesterol, and was not due to decreased cPLA(2) activity per se. These data support the following model: lipoprotein-derived NAAFAs traffic via the NPC1 pathway from late endosomes/lysosomes to a critical pool of phospholipids. In competing reactions, these NAAFAs can be either esterified to cholesterol or incorporated into phospholipids, resulting in suppression of eicosanoid biosynthesis. In view of recent evidence suggesting dysfunctional cholesterol esterification in late lesional macrophages, these data predict that such cells would have highly suppressed eicosanoid synthesis, thus affecting eicosanoid-mediated cell signaling in advanced atherosclerosis.

Ethanol metabolism and transcription factor activation in pancreatic acinar cells in rats

BACKGROUND & AIMS

Ethanol metabolism by pancreatic acinar cells and the role of its metabolites in ethanol toxicity to the pancreas remain largely unknown. Here, we characterize ethanol metabolism in pancreatic acinar cells and determine the effects of ethanol metabolites on nuclear factor kappa B (NF-kappa B) and activator protein (AP)-1, transcription factors that are activated in pancreatitis and mediate expression of inflammatory molecules critical for this disease.

METHODS

We measured activities of fatty acid ethyl ester (FAEE) synthase and alcohol dehydrogenase (ADH), as well as accumulation of ethanol metabolites. We measured the effects of ethanol and its metabolites on NF-kappa B and AP-1 activation by using a gel shift assay.

RESULTS

Pancreas metabolizes ethanol via both oxidative and nonoxidative pathways. Acinar cells are the main source of ethanol metabolism in the pancreas. Compared with the liver, FAEE synthase activity in the pancreas is greater, whereas that of ADH is much less. FAEEs activated NF-kappa B and AP-1, whereas acetaldehyde inhibited NF-kappa B activation. Ethanol decreased NF-kappa B binding activity in acinar cells, which was potentiated by cyanamide.

CONCLUSION

Oxidative and nonoxidative ethanol metabolites regulate transcription factors differently in pancreatic acinar cells. Ethanol may regulate NF-kappa B and AP-1 positively or negatively, depending on which metabolic pathway's effect predominates. These regulatory mechanisms may play a role in ethanol toxicity to the pancreas.

Lipid remodeling in mouse liver and plasma resulting from delta6 fatty acid desaturase inhibition

Electrospray/tandem mass spectrometry was used to quantify lipid remodeling in mouse liver and plasma during inhibition of polyunsaturated fatty acid synthesis by the delta6 fatty acid desaturase inhibitor, SC-26196. SC-26196 caused increases in linoleic acid and corresponding decreases in arachidonic acid and docosahexaenoic acid in select molecular species of phosphatidylcholine, phosphatidylethanolamine, and cholesterol esters but not in phosphatidylserine, phosphatidylinositol, or triglycerides. For linoleic acid-, arachidonic acid-, and docosahexaenoic acid-containing phospholipid species, this difference was, in part, determined by the fatty acid at the sn-1 position, namely, palmitic or stearic acid. An understanding of phospholipid remodeling mediated by delta6 desaturase inhibition should aid in clarifying the contribution of arachidonic acid derived via de novo synthesis or obtained directly in the diet during inflammatory responses.

The kinetics of copper-induced LDL oxidation depend upon its lipid composition and antioxidant content

Copper promotes oxidation of human low-density lipoprotein (LDL) through molecular mechanisms that are still under investigation. We employed native human LDL, phospholipid-containing delipidated LDL ghosts, or trilinolein-reconstituted, phospholipid-containing LDL to investigate both LDL oxidation and the associated process of copper reduction. Both LDL ghosts and trilinolein-reconstituted LDL were devoid of antioxidants and were extremely susceptible to AAPH-induced oxidation but, paradoxically, were rather resistant to copper-mediated oxidation. The dynamic reduction of Cu(II) to Cu(I) was quantitatively decreased in LDL ghosts and in trilinolein-reconstituted LDL, also lacking the initial rapid reduction and the subsequent inhibition phases, due to the absence of endogenous antioxidants. Conversely, the rate of copper reduction was linear and likely due to lipid peroxides, either already present or formed during copper-induced oxidation. We suggest that copper undergoes redox transitions in LDL by utilizing reducing equivalents originating from endogenous antioxidants and/or from lipid peroxides in the LDL lipid core.

The effects of feeding suet-enriched chow on site-specific differences in the composition of triacylglycerol fatty acids in adipose tissue and its interactions in vitro with lymphoid cells

The effects of diet on the composition and properties of adipose tissue in relation to lymph nodes were studied in adult guinea-pigs. The proportions of monoenoic triacylglycerol fatty acids were constant in all sites in adipose tissue of similarly fed guinea-pigs, but were substantially greater in samples from guinea-pigs fed on suet-enriched chow. Triacylglycerols in adipose tissue from near nodes contained significantly fewer saturated fatty acids, and significantly more 18:2n-6 and 18:3n-3 than those in samples from sites remote from nodes within the same depot. Depots that interact most strongly with lymphoid cells in vitro had the largest and most consistent within-depot differences. The gradients of triacylglycerol fatty acid composition with distance from lymph nodes in two small intermuscular depots were similar in guinea-pigs fed on plain or suet-enriched chow. These findings are consistent with the hypothesis that adipose tissue around lymph nodes is specialized for local interactions with the lymphoid cells therein, and help to explain the variability of serial or duplicate measurements of adipose tissue composition. When cultured alone, lipopolysaccharide-stimulated lymph node lymphoid cells from suet-fed guinea-pigs incorporated as much labelled thymidine as the controls. Adipose tissue explants from suet-fed guinea-pigs inhibited lymphocyte proliferation much less than those of the controls, although the site-specific differences were similar. The pattern of site-specific differences in glycerol released from explants incubated alone was generally similar for both dietary groups, but except in the popliteal depot, the increases following co-culturing with lymphoid cells were smaller for samples from suet-fed guinea-pigs. These experiments show that minor changes in the fatty acid composition of the diet can substantially alter the interactions between adipose tissue and lymphoid cells.

Albumin prevents metabolism of 12-hydroxyeicosatetraenoic acid by leukocytes in vitro

In the present paper we studied the influence of albumin on the in vitro metabolism of 12-hydroxyeicosatetraenoic acid (12-HETE) and arachidonic acid in leukocytes and aspirin-treated platelets. In the presence of physiological concentrations of albumin, the metabolism of both 12-HETE and arachidonic acid was substantially altered, implicating the importance fatty acid binding proteins might have on the profile of products formed both in vitro and in vivo. The results clearly showed that albumin effectively withdraws arachidonic acid and 12-HETE from further metabolism by the leukocytes but does not influence the conversion of arachidonic acid to 12-HETE by the platelets. Thus, some of the hypotheses concerning transcellular metabolism raised from in vitro data within the eicosanoid field might have little relevance for the in vivo situation.

Linoleic acid esterified in low density lipoprotein serves as substrate for increased arachidonic acid synthesis in differentiating monocytic cells

The cellular metabolism of albumin- and lipoprotein-bound 18:2(n - 6) during monocytic differentiation was examined in the human premonocytic U937 and Mono Mac 6 cells. Differentiation for 72 h of U937 cells with retinoic acid (RA, 1 microM) or 1,25-(OH)2-vitamin D3 (1,25-D3, 10 nM) and of Mono Mac 6 cells with RA (1 microM) or lipopolysaccharide (LPS, 10 ng/ml) increased the desaturation and elongation of [1-14C]18:2(n - 6) to [1-14C]20:4(n - 6). In undifferentiated U937 and Mono Mac 6 cells, incubations with human LDL (100 micrograms/ml, 18 h) resulted in a 2.5-fold increase in 18:2(n - 6) levels in the cellular phospholipids. Differentiation of U937 cells with RA or or of Mono Mac 6 cells with LPS prior to LDL addition. Significantly reduced 18:2(n - 6) and elevated 20:4(n - 6) levels in cellular phospholipids. This increase in 20:4(n - 6) was likely not due to an increased incorporation of preformed 20:4(n - 6) esterified in LDL, as the receptor-specific degradation of [125I]LDL was reduced in both the RA-treated U937 and LPS-treated Mono Mac 6 cells. In U937 cells incubated with [1-14C]18:2(n - 6), the synthesis of TXB2, PGE2 and HHT could be detected after differentiation with RA. suggesting the availability of [1-14C]20:4(n - 6), derived from [1-14C]18:2(n - 6), for cyclooxygenase metabolism. Our results show that the conversion of 18:2(n - 6) to 20:4(n - 6) increases during monocyte differentiation. The 18:2(n - 6) supplied to the cells via the receptor-mediated uptake of LDL was utilized as substrate for the increased 20:4(n - 6) synthesis.