Post-secretory acquisition of apolipoprotein E by nascent rat hepatic very-low-density lipoproteins in the absence of cholesteryl ester transfer

Isolation of very low density lipoprotein phospholipids enriched in ethanolamine phospholipids from rats injected with Triton WR 1339

Phospholipids carried by very low density lipoprotein (VLDL) are hydrolysed in circulation by lipoprotein and hepatic lipases and lecithin-cholesterol acyltransferase. We have previously demonstrated [J.J. Agren, A. Ravandi, A. Kuksis, G. Steiner, Structural and compositional changes in very low density lipoprotein triacylglycerols during basal lipolysis, Eur. J. Biochem. 269 (2002) 6223-6232] that the infusion of Triton WR 1339 (TWR), which inhibits these lipases, leads in 2 h to five-fold increase in VLDL triacylglycerol concentration along with major differences in the composition of their molecular species. The present study demonstrates that the accumulation of triacylglycerols is accompanied by major changes in the content of the VLDL phospholipids, of which the most significant is the enrichment of phosphatidylethanolamine (PtdEtn). This finding coincides with the enrichment in PtdEtn demonstrated in the VLDL of a hepatocytic Golgi fraction but it had not been demonstrated that the Golgi VLDL, along with its unusual phospholipid composition, can be directly transferred to plasma. Aside from providing an easy access to nascent plasma VLDL, the TWR infusion demonstrates that lipoprotein and hepatic lipases are also responsible for the degradation of plasma VLDL PtdEtn, as independently demonstrated for plasma phosphatidylcholine. Our results indicate also, with the exception of lysophosphatidylcholine, that preferential basal hydrolysis no dot lead to major differences in molecular species composition between circulating and newly secreted VLDL phospholipids. The comparison of the molecular species composition of VLDL and liver phospholipids suggests a selective secretion of PtdEtn and sphingomyelin molecular species during VLDL secretion.

Effect of simvastatin on the synthesis and secretion of lipoproteins in relation to the metabolism of cholesterol in cultured hepatocytes

In primary culture of rat hepatocytes, simvastatin, a powerful HMGCoA reductase inhibitor, inhibited acetate incorporation into cellular and secreted cholesterol and cholesteryl-esters, without any significant effect on triacylglycerol synthesis and secretion. When applied to the culture for 24 h at 10(-7) M, a concentration shown to inhibit cholesterol synthesis by 61%, simvastatin increased apolipoprotein BH and BL synthesis and secretion and strongly decreased apolipoprotein AI synthesis and secretion whereas apolipoprotein AIV remained unaffected. The synthesis and secretion of apolipoprotein E was only slightly affected in contrast with other situations where cholesterol synthesis decreased. All of these modifications occurred at a post-transcriptional level, as the corresponding messenger RNAs of the apolipoproteins did not vary. These results suggest that either the drug itself or variations in cholesterol synthesis might be involved in apo B and apo AI synthesis and secretion.

Distribution of apolipoprotein E between free and A-II complexed forms in very-low- and high-density lipoproteins: functional implications

The stability of apolipoprotein E/lipoprotein associations has been examined as a function of apolipoprotein E phenotype. Visualisation by immunoblotting showed plasma apolipoprotein E to be present in two forms; the free form and, as previously described, an E-A-II complex. In very low density lipoproteins isolated by gel filtration from subjects with E3/3 and E4/3 phenotypes, apolipoprotein E was present essentially in the free form (ratio free: complex of 12.2 and 37.5, respectively). Exploiting ultracentrifugation as the disruptive agent, very-low-density lipoproteins thus isolated were shown to have substantially lower ratios (5.6 and 5.4, respectively) reflecting preferential loss of free apolipoprotein E. In high-density lipoproteins isolated by gel filtration from E3/3 phenotypes, apolipoprotein E was largely present as an E-A-II complex (80.3%). In contrast, the majority of apolipoprotein E in high-density lipoproteins from E4/3 phenotypes was present in the free form (58.7%). In both phenotypes, the content of free apolipoprotein E was markedly reduced by ultracentrifugation. The results confirm the notion that the formation of the E-A-II complex is a major determinant of the stability of apolipoprotein E-high-density lipoprotein associations. Moreover, that the predominant, ancestral isoform, apolipoprotein E3, exists largely as an E-A-II complex in higher density lipoproteins has important functional implications for this plasma source of apolipoprotein E.

Fate of fatty acids taken up as cholesteryl ester by rat hepatocytes in primary culture from high-density lipoprotein

Primary cultures of rat hepatocytes were incubated in the presence of high-density lipoproteins (HDL) labelled with [1-14C]oleyl or [1-14C]linoleyl cholesteryl ester. Labelled HDL were prepared by selective delipidation with heptane, relipidation and sequential ultracentrifugations. Hepatocytes took up cholesteryl esters and cholesteryl ether their non-hydrolizable analog, at the same rate. The uptake increased with time, the cholesteryl ester/protein ratio and the amount of added HDL. It was not dependent on the nature of acyl chain or on the nature of the bond. The uptake did not depend on a specific interaction between HDL and cell membranes, since cholesteryl ester was taken up from HDL to the same extent as from albumin complexes. Linoleic and oleic acids released from cholesteryl esters taken up by hepatocytes were mainly reesterified into phosphatidylcholine and triacylglycerols. Linoleic acid was preferentially channelled into PC. A portion of these lipids were secreted by hepatocytes during a 24-h reincubation in a medium devoid of lipoprotein. Nearly the same amount of radioactivity was recovered in secreted phospholipids as in secreted triacylglycerols, in contrast with hepatocytes labelled with free fatty acids which secreted very little radioactivity as phospholipids. From these results and the high content in polyunsaturated fatty acids of cholesteryl esters, one can hypothesize that hepatic cholesteryl ester uptake may contribute to biliary phosphatidylcholine production, and therefore to polyunsaturated fatty acid sparing.

Hepatocytic lipoprotein receptors and intracellular lipoprotein catabolism

Hepatocytes, as the major site of synthesis and terminal catabolism of plasma lipoproteins, exert the major regulatory influence on the concentration of atherogenic lipoproteins in blood plasma and may thereby influence the rate of atherogenesis. The LDL receptor on the microvillous sinusoidal surface of hepatocytes mediates the catabolism of remnants of triglyceride-rich lipoproteins and LDL. Binding of VLDL remnants to the receptor, mediated by apo E, is of very high affinity and presumably multivalent, whereas binding of LDL, mediated by apo B-100, is monovalent and of lower affinity, accounting for the much longer residence time of the latter in the blood. The magnitude of the influx of lipoprotein particles into hepatocytic endosomal compartments dwarfs that of other macromolecules undergoing receptor-mediated endocytosis and terminal catabolism in lysosomes of these cells. The intracellular compartments and processing steps in hepatocytic lipoprotein uptake and degradation are essentially the same as those described for other ligands in the liver and other cells. Receptors with bound lipoproteins migrate into coated pits which become coated vesicles. These vesicles uncoat and fuse to form CURL vesicles and tubules near the cell surface where most receptors are recycled, presumably via receptor-rich appendages that become separated from the vesicles. CURL vesicles become mature MVBs as they migrate to the Golgi/bile canalicular pole of hepatocytes, where they fuse with putative Golgi-derived primary lysosomes and are transformed into heterophagic secondary lysosomes. MVBs also contain a receptor-rich appendage that may recycle some receptors directly to the cell surface or through adjacent Golgi compartments. Dilated ends of trans-Golgi cisternae contain nascent VLDL undergoing packaging for secretion following their synthesis and assembly in the endoplasmic reticulum. Because these "forming secretory vesicles" resemble remnant-filled MVBs, occur in a similar location in the Golgi area of hepatocytes and coisolate in centrifugal fractions of liver homogenates, there has been considerable confusion about the identity of these compartments. With the aid of specific endocytic and exocytic markers, highly purified and morphologically intact endosomal and Golgi compartments can now be obtained from rat liver homogenates. The availability of these and similar fractions of defined purity should facilitate investigation of the hepatocytic processing of endocytosed and secreted macromolecules. Although chylomicron remnants are also taken up by receptor-mediated endocytosis, the nature of the hepatocytic remnant receptor remains elusive.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that during very low density lipoprotein assembly in rat hepatocytes most of the triacylglycerol and phospholipid are packaged with apolipoprotein B in the Golgi complex

Rat liver lipids were labelled by an intraportal injection of [3H]palmitic acid followed by isolation of rough and smooth endoplasmic reticulum or 'cis' or 'trans'-enriched Golgi fractions. The preparations were separated into membrane and contents and the apolipoprotein B of the content fractions was immunoprecipitated. More than 90% of the labelled triacylglycerol and phospholipid secreted into the blood immunoprecipitated with apolipoprotein B. Under the same experimental conditions 8, 12, 27 and 59% of the lipids of the rough, smooth, 'cis-Golgi' and 'trans-Golgi' contents, respectively, were immunoprecipitated. Thus, the 'trans-Golgi' region appears to be the major intracellular site of assembly of apolipoprotein B with triacylglycerol and phospholipid.

Effects of oleic acid on the biosynthesis of lipoprotein apoproteins and distribution into the very-low-density lipoprotein by the isolated perfused rat liver

The effects of oleic acid on the biosynthesis and secretion of VLDL (very-low-density-lipoprotein) apoproteins and lipids were investigated in isolated perfused rat liver. Protein synthesis was measured by the incorporation of L-[4,5-3H]leucine into the VLDL apoproteins (d less than 1.006) and into apolipoproteins of the whole perfusate (d less than 1.21). Oleate did not affect incorporation of [3H]leucine into total-perfusate or hepatic protein. The infusion of oleate, however, increased the mass and radioactivity of the VLDL apoprotein in proportion to the concentration of oleate infused. Uptake of oleate was similar with livers from fed or fasted animals. Fasting itself (24 h) decreased the net secretion and incorporation of [3H]leucine into total VLDL apoprotein and decreased the output of VLDL protein by the liver. A linear relationship existed between the output of VLDL triacylglycerol (mumol/h per g of liver) and secretion and/or synthesis of VLDL protein. Net output of VLDL cholesterol and phospholipid also increased linearly with VLDL-triacylglycerol output. Oleate stimulated incorporation of [3H]leucine into VLDL apo (apolipoprotein) E and apo C by livers from fed animals, and into VLDL apo Bh, B1, E and C by livers from fasted rats. The incorporation of [3H]leucine into individual apolipoproteins of the total perfusate lipoprotein (d less than 1.210 ultracentrifugal fraction) was not changed significantly by oleate during perfusion of livers from fed rats, suggesting that the synthesis de novo of each apolipoprotein was not stimulated by oleate. This is in contrast with that observed with livers from fasted rats, in which the synthesis of the total-perfusate lipoprotein (d less than 1.210 fraction) apo B, E and C was apparently stimulated by oleate. The observations with livers from fed rats suggest redistribution of radioactive apolipoproteins to the VLDL during or after the process of secretion, rather than an increase of apoprotein synthesis de novo. It appears, however, that the biosynthesis of apo B1, Bh, E and C was stimulated by oleic acid in livers from fasted rats. Since the incorporations of [3H]leucine into the VLDL and total-perfusate apolipoproteins were increased in fasted-rat liver when the fatty acid was infused, part of the apparent stimulated synthesis of the VLDL apoprotein may be in response to the increased formation and secretion of VLDL lipid.

Lipids and their constituents

Many tissues, cells and body fluids possess characteristic lipid composition that can be readily recognized without full resolution and quantitation of individual molecular species. Various chromatographic methods have been adopted for this purpose and are extensively employed in biomedical research. Although lipid profiles are known to change with disease and lipid profiling holds considerable potential for clinical diagnosis, few routines have been established for this purpose. This is partly due to the laborious nature of the simpler methods and the high cost of automated systems. A combination of thin-layer or liquid chromatography with universal detection systems promises to provide more attractive analytical routines for clinical application in the future. At present thin-layer chromatography is the simplest and most rapid qualitative assay for both neutral and polar lipids. Low-temperature gas chromatography is still the method of choice for fatty acid analyses, while high-temperature gas chromatography is eminently suited for quantitative analysis of intact neutral lipids. The availability of the flame ionization and mass detectors now makes high-performance liquid chromatography more useful for profiling both neutral and polar lipids. Combinations of gas or liquid chromatography with mass spectrometry must remain of specialized interest only because of the prohibitive costs of operation.