Lipid Transport Pathways Research Articles

Intracellular transport of phosphatidic acid and phosphatidylcholine into lipid bodies: use of fluorescent lipids to study lipid-body formation in an oleaginous fungus

Fluorescent phosphatidic acid and phosphatidylcholine were used to characterize lipid-transport pathways into lipid bodies in an oleaginous fungus, Mortierella ramanniana var. angulispora. Several characteristics of the lipid transport such as temperature dependence and ATP dependence were evaluated. The transport depicted by these fluorescent lipids was consistent with metabolism of radiolabelled lipids, indicating that fluorescent lipids are useful to study lipid-body formation in this fungus. The results dissect lipid transport of phosphatidic acid and phosphatidylcholine into lipid bodies and reveal regulatory steps for lipidbody formation in this fungus.

Intracellular transport of phosphatidic acid and phosphatidylcholine into lipid bodies: use of fluorescent lipids to study lipid-body formation in an oleaginous fungus

Fluorescent phosphatidic acid and phosphatidylcholine were used to characterize lipid-transport pathways into lipid bodies in an oleaginous fungus, Mortierella ramanniana var. angulispora. Several characteristics of the lipid transport such as temperature dependence and ATP dependence were evaluated. The transport depicted by these fluorescent lipids was consistent with metabolism of radiolabelled lipids, indicating that fluorescent lipids are useful to study lipid-body formation in this fungus. The results dissect lipid transport of phosphatidic acid and phosphatidylcholine into lipid bodies and reveal regulatory steps for lipid-body formation in this fungus.

Effect of organic anions on acyl chain composition of secreted biliary lecithin in rats: relation to hepatocellular vesicle pathway

Organic anions can uncouple bile salt secretion from the secretion of phospholipid and cholesterol. The uncoupling mechanism appears to be dependent on the type of anion present. To investigate the effect that different uncoupling mechanisms may have on the flow of bile and its composition, rats were depleted of bile salt pool by overnight biliary diversion and reinfused intravenously with sodium taurocholate (STC) at a constant rate (160 nmol/min per 100 g b.w.). After bile salt secretion had achieved a steady state, sulfobromophthalein (BSP) or papaverine was continuously administered at a rate of 100 nmol/min per 100 g b.w. with STC. During the intravenous infusion of BSP or papaverine, the output of biliary lecithin and cholesterol decreased without affecting bile salt output. The degree of fatty acyl chain saturation in biliary lecithin was increased by BSP infusion and decreased by papaverine infusion. The mechanism by which bile salt secretion is uncoupled from the secretion of phospholipid and cholesterol affects the acyl chain composition of secreted lecithin. The changes in composition of lecithin acyl chains may reflect the intervention by different anions at different points in the lipid transport pathway.

Ultrastructural, protein, and lipid changes in liver associated with chlordecone treatment of mice.

Pretreatment of mice with chlordecone (CD) reduced hepatic accumulation of a subsequent dose of [14C]CD without significantly changing [14C]CD biotransformation. To determine if CD-induced changes in hepatic [14C]CD accumulation were coincident with altered cell composition, we examined the effects of CD on hepatic protein and lipid content, on fatty acid profiles of liver and kidney, and on the ultrastructure of hepatocytes. SDS-polyacrylamide gel electrophoresis detected an apparent CD dose-related increase in a microsomal protein with a molecular weight of about 23 kDa. Total liver or kidney lipid contents were not altered by CD but relative amounts of several hepatic fatty acids were changed. CD caused marked hepatic mitochondrial swelling, increased amounts of endoplasmic reticulum, apparently increased numbers of peroxisome-like structures, and decreased numbers of lipid droplets in cytoplasm of hepatocytes. Numbers of lipid droplets were not decreased in perisinusoidal fat storage cells. In addition, the numbers of cytoplasmic lipoprotein vesicles were apparently increased in some hepatocytes. Overall these changes indicated an increased hepatocyte secretory activity and suggested that CD changed hepatocellular lipid transport, storage, and metabolism pathways.

High-Density Lipoprotein Cholesterol, Plasma Triglyceride, and Coronary Heart Disease: Pathophysiology and Management

Publisher Summary Compared with the relation between coronary heart disease (CHD) and its most important risk factor, hypercholesterolemia, the relation between CHD and fasting triglyceride levels is undoubtedly more complex biologically and statistically. This chapter reviews clinically relevant disturbances in the metabolism of triglyceride-rich lipoproteins and high-density lipoprotein (HDL), discusses the evidence linking the metabolism of triglyceride-rich lipoproteins to HDL cholesterol levels and CHD, and describes treatments to correct these disorders. In human, lipoproteins are produced by the intestine and liver. Lipoprotein biosynthesis is a highly specialized process that includes the synthesis of apolipoproteins and lipids, cellular transport of lipoprotein components, and their intracellular assembly and secretion into blood or lymph. Lipoproteins undergo postsecretory processing in the circulation and are eventually removed from the circulation. Exogenous and endogenous lipid transport pathways are distinguished according to lipid origin, even though these pathways overlap extensively. In the plasma, HDL exists as several subfractions that differ in physicochemical and functional properties. Several of these subfractions can be interconverted in the circulation, a process linked to the metabolism of triglyceride rich lipoproteins and involving various lipoprotein-modifying enzymes such as lipoprotein lipase (LPL), Lecithin: cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and hepatic lipase. CETP-mediated transfer of triglyceride from triglyceride-rich lipoproteins to HDL in exchange for cholesteryl ester is central to the inverse relation between plasma triglyceride and HDL cholesterol. Immunological blockage of CETP increases the cholesteryl ester content of HDL and delays HDL clearance from the circulation. The inverse association between plasma triglyceride and HDL cholesterol breaks down in inherited CETP deficiency, in which HDL cholesterol levels are high irrespective of plasma triglyceride concentrations.

Metabolism of VLDL and LDL subclasses

The recognition of lipoprotein heterogeneity has given new depth to our understanding of lipoprotein metabolism and the role of plasma triglyceride as a determinant of lipoprotein subclass distribution and potential atherogenicity. It has also provided insight into the relationship between exogenous and endogenous lipid transport pathways, the roles of endothelial lipases and lipid transfer proteins in the remodelling of lipoproteins, and the link between insulin resistance, lipids and coronary disease.

Tunicamycin does not inhibit transport of phosphatidylinositol toXenopus rod outer segments

Tunicamycin inhibits the dolichol pathway for N-linked glycosylation of proteins, including photoreceptor opsin, and causes a buildup of tubulo-vesicular profiles in the intersegmental space between photoreceptor rod inner and outer segments associated with disruption of new disc assembly. We tested the hypothesis that a tunicamycin lesion in photoreceptors would block lipid transport into the outer segment. Adult Xenopus retinas were preincubated in dim red light with 20 micrograms ml-1 of tunicamycin for one hour followed by incubation in the light for 2-6 h with tunicamycin plus either [3H]mannose, [3H]leucine, [2-(3)H]glycerol or [3H]myo-inositol. Tunicamycin caused accumulation of tubulo-vesicular membranes in the intersegmental space and significantly reduced both [3H]leucine and [3H]mannose incorporation into the basal region of rod outer segments. However, tunicamycin had no effect on [3H]glycerol incorporation into the rod outer segment phospholipids. After 5 h incubation with [3H]glycerol, radiolabel in outer segment fractions was associated primarily with phosphatidylinositol in both control and tunicamycin treated retinas. Quantitative light microscope autoradiography of both [3H]glycerol and [3H]inositol labelled retinas showed diffuse labelling over the entire rod outer segment in both control and tunicamycin treated retinas with no accumulation of radioactivity in the basal discs of control retinas or in the tubulo-vesicular structures in the intersegmental space of tunicamycin treated retinas. Our results indicate that despite the morphological disruption and inhibition of glycoprotein transport to outer segments after tunicamycin treatment, transport of labelled phosphatidylinositol occurs normally. These data add to a growing body of evidence separating the lipid and protein transport pathways to the outer segment.

Synthesis of phosphatidylinositol 4,5-bisphosphate in the endoplasmic reticulum of Chinese hamster ovary cells.

We have investigated the intracellular localization and synthesis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in Chinese hamster ovary (CHO) cells by analyzing membrane fractions that were obtained by sucrose density gradient centrifugation. After labeling the cells for 24 h with [3H]inositol, the bulk of [3H] PtdInsP2 was found in the plasma membrane fraction, yet this lipid was also distinctly present in the microsomal fraction (20% of total cellular [3H]PtdInsP2). To determine the origin of this microsomal PtdInsP2, gradient fractions from unlabeled CHO cells were incubated with [3H]inositol in the presence of an ATP-generating system. Under these conditions of labeling, [3H]PtdIns was exclusively present in the microsomal fractions and found to be partially converted to [3H] phosphatidylinositol 4-phosphate ([3H]PtdInsP) and phosphatidylinositol 4-phosphate ([3H]PtdInsP) and [3H]PtdInsP2. The ability of microsomes to synthesize PtdInsP and PtdInsP2 was confirmed by assaying the gradient fractions for PtdIns and PtdInsP kinase activity using endogenous substrate and [gamma-32P]ATP. In the presence of exogenous substrate and Triton X-100, PtdInsP kinase activity was particularly high in the plasma membrane fractions. When phosphoinositide synthesis was studied in permeabilized CHO cells under conditions of sustained membrane vesicle flow (Helms, J. B., Karrenbauer, A., Wirtz, K. W. A., Rothman, J. E., and Wieland, F. T. (1990) J. Biol. Chem. 265, 20027-20032), no lag-time could be detected between the synthesis of [3H]PtdIns and the formation of [3H]PtdInsP2. Moreover, when lipid transport pathways were blocked in these permeabilized cells either by omission of membrane-free cytosol, addition of GTP gamma S and brefeldin A, or temperature block, PtdInsP2 formation still occurred at normal levels. These results strongly suggest that PtdInsP2 can be formed at the site of PtdIns synthesis, i.e. the endoplasmic reticulum (ER). The relationship between PtdInsP2, generated in the ER, and PtdInsP2 present in the plasma membrane, remains to be established.

Lipid transport pathways in mammalian cells

A major deficit in our understanding of membrane biogenesis in eukaryotes is the definition of mechanisms by which the lipid constituents of cell membranes are transported from their sites of intracellular synthesis to the multiplicity of membranes that constitute a typical cell. A variety of approaches have been used to examine the transport of lipids to different organelles. In many cases the development of new methods has been necessary to study the problem. These methods include cytological examination of cells labeled with fluorescent lipid analogs, improved methods of subcellular fractionation, in situ enzymology that demonstrates lipid translocation by changes in lipid structure, and cell-free reconstitution with isolated organelles. Several general patterns of lipid transport have emerged but there does not appear to be unifying mechanism by which lipids move among different organelles. Significant evidence now exists for vesicular and metabolic energy-dependent mechanisms as well as mechanisms that are clearly independent of cellular ATP content.

Defining lipid transport pathways in animal cells.

A new technique for studying the metabolism and intracellular transport of lipid molecules in living cells based on the use of fluorescent lipid analogs is described. The cellular processing of various intermediates (phosphatidic acid and ceramide) and end products (phosphatidylcholine and phosphatidylethanolamine) in lipid biosynthesis is reviewed and a working model for compartmentalization during lipid biosynthesis is presented.