Synthesis Of Apolipoprotein Research Articles

Beyond alcohol: Beverage consumption and cardiovascular mortality

This paper reviews epidemiological investigations which have identified an inverse relationship between alcohol consumption and death from coronary heart disease: evidence from studies of mixed populations as well as of single-sex populations have, on the whole, demonstrated that this relationship is independent of sex or age. This ‘cardioprotective effect’ of alcohol can be explained, at least in part, by ethanol-related increases in high density lipoprotein cholesterol and reduced platelet coagulability. With certain beverages, especially red wine, phenolic compounds may provide additional protection by altering eicosanoid metabolism in favour of increased prostacyclin and decreased thromboxane synthesis, as well as antioxidant functions which prevent the peroxidation of low-density lipoprotein. Trans-resveratrol, a tri-hydroxy stilbene present in the skins of specific grape cultivars, is a constituent of certain red wines which may play a crucial role in modulating lipoprotein metabolism, eicosanoid synthesis, oxidation and coagulation. Preliminary studies using the human hepatoma cell line HepG2 are described, demonstrating that this compound has no effect upon cell viability or overall protein synthesis in these cells, and at high concentrations DNA synthesis as measured by radioactive thymidine incorporation is enhanced. Reduced intracellular concentration and secretion of apolipoprotein B have been shown to occur in response to resveratrol although a clear dose-dependency has not yet been demonstrated. The mechanisms underlying these changes as well as the effects upon the synthesis and secretion of other apolipoproteins are under active investigation in our laboratory.

Structure and metabolism of lipoprotein (a).

An understanding of the complex structural and metabolic features of lipoprotein (a) is essential because this lipoprotein particle, with poorly recognized physiological functions, has been implicated in both thrombogenesis and atherogenesis. Studies aimed at investigating the structure-function relationship of apolipoprotein (a) have generated new useful information. In-vivo and in-vitro studies have provided insights into the synthesis, secretion, and association of apolipoprotein (a) with apolipoprotein B100 and the mechanisms regulating the lipoprotein (a) concentration in the bloodstream.

New in-vivo techniques for measuring lipoprotein metabolism.

Stable, nonradioactive tracer techniques have provided a safe means of studying in-vivo lipoprotein metabolism, and endogenous labelling with amino acids has allowed apolipoprotein synthesis to be analysed directly. Recent studies have dealt with experimental design, mass spectrometry techniques and the validity of simple formulae used to derive primary kinetic data. Concerns that stable isotopes give different results to the traditional radiolabelling studies have been allayed, in the case of apolipoprotein A-I, by published work. Studies conducted during the review period have also examined the mechanisms responsible for the low plasma concentrations of apolipoprotein A-I and apolipoprotein A-II in lecithin:cholesterol acyltransferase deficiency and the increased concentrations of HDL-apolipoprotein A-I caused by transdermal oestrogen hormone replacement therapy. The first analysis of lipoprotein-lipid rather than lipoprotein-apolipoprotein metabolism using stable isotopes has also been reported.

Local synthesis of apolipoprotein J in the eye

Apolipoprotein J (apoJ), a secretory glycoprotein known to transport lipids and to regulate terminal complement function, is present in the human eye in both aqueous and vitreous, as well as in the retina. Ocular apoJ is the product of local synthesis, rather than plasma contamination, as demonstrated by its distinct structural properties and the presence of abundant apoJ mRNA in retina and retina pigment epithelium. ApoJ mRNA is also present in mouse eye, with a developmentally regulated pattern of expression. In fetal mouse, apoJ mRNA is present in retina, lens and cornea. In contrast, adult eye apoJ mRNA is present in retina and ciliary body. We propose that apoJ is important in tissue remodeling and in stabilizing hydrophobic molecules which are required for vision and/or which would otherwise be deleterious and membrane-active.

Fibroblasts that reside in mouse and frog injured peripheral nerves produce apolipoproteins.

Apolipoprotein synthesis and secretion is upregulated in wallerian degenerating peripheral nerves. A commonly expressed view has been that macrophages are solely responsible for their production. In the present study we provide evidence that (1) nerve-derived fibroblasts contribute to apolipoprotein production, (2) apolipoprotein production is confined to regions where myelin destruction and phagocytosis occur, and (3) some experimental procedures are detrimental for the production of apolipoproteins. Apolipoprotein production was studied in C57BL/6/NHSD (N) and C57/BL/6-WLD/OLA/NHSD (W) mice that display, respectively, rapid and slow progression of wallerian degeneration. In N nerves, apolipoprotein E (apo-E) is produced during in vitro and in vivo degeneration, and in vivo after freeze damage. In W nerves, apo-E is produced at the injury region where degeneration occurs but not farther distally where degeneration fails to develop. Apo-E is also produced in W nerves during in vitro degeneration and in vivo after freeze damage. In culture, N and W mice nerve-derived fibroblasts, but neither macrophages nor Schwann cells produced apo-E. Two apolipoproteins are produced in in vivo wallerian degenerating and freeze-damaged frog nerves, i.e., apo-39 and apo-29. Only apo-39 is produced in in vitro degenerating nerves. Neither apo-39 nor apo-29 is produced during in vivo degeneration in diffusion chambers. In culture, apo-39 is produced by nerve-derived fibroblasts and macrophages but not by Schwann cells.

Stimulation of proximal jejunal synthesis of apolipoprotein A-IV by ileal lipid infusion is independent of the presence of jejunal lipid

Stimulation of proximal jejunal synthesis of apolipoprotein A-IV by ileal lipid infusion is independent of the presence of jejunal lipid

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.

Variable expression of familial heterozygous hypobetalipoproteinemia: transient malabsorption during infancy.

Rare instances of symptomatic fat malabsorption have been reported in patients with heterozygous hypobetalipoproteinemia, but with an unclear pathogenesis. An 8-month-old boy with chronic diarrhea and failure to thrive was found to have abnormally low plasma total cholesterol (85 mg/dl), LDL-cholesterol (48 mg/dl), apoB (52 mg/dl), apoA-I (53 mg/dl), and vitamin E (0.22 mg/dl). Decreased plasma LDL-C and apoB were noted in the father (34 and 40 mg/dl, respectively), as well as several other family members. Fasting triglycerides were normal but did not increase normally in response to a fat meal test. Lipoprotein composition showed an abnormal profile of very low density (VLDL, d 1.006 g/ml), low density (LDL, d 1.063 g/ml), and high density (HDL, d 1.21 g/ml) lipoproteins. A fasting jejunal biopsy revealed lipid-laden enterocytes. Electron microscopy of the jejunal biopsy revealed the absence of lipid particles in the intercellular spaces after a fat meal. Jejunal explants cultured with [14C]palmitate and [3H]leucine showed limited synthesis of triglycerides and apolipoproteins (36 and 42% of controls, respectively), whereas the father's results were close to normal. At 1 year of age, improvement in intestinal fat absorption was accompanied by the presence of chylomicrons in the intercellular space, concomitant with the enhanced synthesis of lipids and apoB by jejunal explants. These data provide evidence that heterozygous hypobetalipoproteinemia may present early in life as transient, symptomatic lipid malabsorption. The mechanisms responsible for improved lipid transport despite persistent hypobetalipoproteinemia remain to be established.

Oxidative-modified and acetylated low-density lipoproteins differ in their effects on cholesterol synthesis and stimulate synthesis of apolipoprotein E in rat peritoneal macrophages by different mechanisms

Apolipoprotein (apo) E plays an important role in the recognition of lipoproteins by cellular lipoprotein receptors. Unlike other apolipoproteins, apo E is expressed by many extrahepatic tissue including macrophages (MΦ). Resident MΦ express low levels of apo E. However, their synthesis of apo E is substantially increased after MΦ have been incubated with acetylated low-density lipoprotein (LDL). But acetylation of LDL is not known to occur in vivo. On the other hand, modification of LDL by oxidation and by enzymatic action is believed to happen physiologically. In this report, we compared the effects of various modified LDLs on the synthesis of apo E by MΦ. Freshly isolated human LDL was modified by (1) repeated addition of acetic anhydride (Ac-LDL); (2) incubation with 20 μmol/L CuSO 4 at 37°C for 24 hours (Ox-LDL); and (3) incubation with phospholipase C at 37°C for 1 hour (PI-LDL). Resident peritoneal MΦ were collected by lavage from rats and allowed to attach to plastic culture dishes. Although native LDL had no effect, treatment with Ac-, Ox-, and PI-LDL (50 μg/mL each) was found to increase medium apo E by (-fold) 4.19 ± 0.26, 4.20 ± 0.34, and 2.02 ± 0.20 (mean ± SEM, n = 5), respectively, as compared with untreated cells. Northern blot analysis revealed that cellular apo E mRNA was increased in parallel to apo E protein by Ac-LDL and PI-LDL. However, increases of apo E protein and mRNA by Ox-LDL were not equal. Ac-, Ox-, and PI-LDL-treated cells each contained 2.66-, 1.95- and 2.10-fold more total cholesterol than untreated cells (20.5 ± 2.1 μg/mg cell protein, n = 11). Interestingly, newly synthesized cholesterol esters in MΦ were increased (-fold) 19.20 ± 2.50 (mean ± SEM, n = 4) by Ac-LDL and 5.16 ± 0.19 by PI-LDL, but only 1.81 ± 0.19 by Ox-LDL. It is concluded that (1) similar to Ac-LDL, Ox- and PI-LDL also stimulate MΦ apo E synthesis; (2) all three modified LDLs are effective in loading cholesterol into macrophages, but vary greatly in their abilities to stimulate cholesterol synthesis; (3) stimulation of apo E synthesis in rat peritoneal MΦ by modified LDLs does not parallel cellular cholesterol contents nor correspond to de novo cholesterol synthesis; and (4) Ox-LDL and Ac-LDL stimulate MΦ apo E synthesis by different mechanisms.

Apolipoprotein Synthesis in Human Fetal Intestine: Regulation by Epidermal Growth Factor

In this investigation, human fetal intestine (17-20 wk) was used to study the synthesis of apolipoproteins (apo) A-I and B and its modulation by epidermal growth factor. Cultured jejunal explants were incubated with [35S]-methionine, homogenized, immunoprecipitated and subjected to gel analysis. Radioactivity was detected mostly in apo A-I and, to a lesser extent, in both apo B-48 and apo B-100. However, apo B-48 was always the predominant form. The addition of EGF (100 ng/ml) resulted in a simultaneous decrease in apo B-100 synthesis and increase in apo B-48 production without a marked effect on apo A-I. Moreover, the incubation of jejunal explants with [14C] oleic acid documented the ability of fetal intestine to produce chylomicrons, VLDL, LDL and HDL. Therefore, the human fetal intestine has an efficient lipoprotein-lipid transport system and EGF is able to modulate levels of both apo B species as well as lipoprotein fractions.

Apolipoprotein A‐I, A‐IV and E synthesis in the liver of copper‐deficient rats

Copper deficiency induces hypercholesterolemia in the rat. This hypercholesterolemia is mainly due to an increase in apo E-rich high density lipoproteins (HDL1). The present study was undertaken to determine whether the HDL increase could be explained by altered low-molecular weight apolipoprotein (apo) synthesis in the liver. The effect of copper deficiency on apo A-I, apo A-IV and apo E concentrations in plasma, as well as on respective mRNA levels and synthesis in the liver, were therefore investigated. We observed that the increased HDL1 levels in the plasma of copper-deficient rats were associated with a significant rise in plasma apo E concentrations; however, plasma apo A-I and apo A-IV concentrations remained unchanged. Liver apo synthesis and respective apo mRNA levels were not significantly altered in copper-deficient animals when compared to control rats. No changes in apo E mRNA levels in various tissues from copper-deficient, as compared to control rats, were noted. Based on the data obtained, it was concluded that the observed changes in plasma lipoprotein and apo concentrations are not related to changes in low-molecular weight apo synthesis in the liver. The mechanisms of the impaired catabolism of HDL1 should be further evaluated to possibly explain the observed increase in this fraction in copper-deficient rats.

Apolipoprotein synthesis in human fetal intestine: regulation by epidermal growth factor

Apolipoprotein synthesis in human fetal intestine: regulation by epidermal growth factor

The Modulation of Apolipoprotein E Gene Expression by 3,3′‐5‐triiodothyronine in HepG2 Cells Occurs at Transcriptional and Post‐transcriptional Levels

The regulation of the synthesis and secretion of apolipoprotein E (apoE) is incompletely understood. This study examines the mechanisms responsible for regulating apoE gene expression in HepG2 cells by thyroid hormone (3,3'-5-triiodothyronine). The secretion rate of apoE was by thyroid hormone increased (1.5-1.8-fold) in pulse/chase experiments. Thyroid hormone doubled apoE mRNA concentration as determined by Northern-blot analysis. Inhibition of protein synthesis by cycloheximide increased the thyroid-hormone-induced stimulation of apoE mRNA. This suggests that the synthesis of new protein is not required for thyroid hormone to stimulate apoE mRNA. Actinomycin D was used to inhibit new transcription; there was a more rapid degradation of mature apoE mRNA in thyroid hormone-treated HepG2 cells than in control cells, suggesting that thyroid hormone acts post-transcriptionally to regulate apoE gene expression. Cycloheximide blocked the action of thyroid hormone, suggesting that thyroid hormone regulates the turnover of apoE mRNA via the synthesis of de novo protein. Nuclear run-on transcription assays demonstrated that thyroid hormone stimulated apoE gene transcription threefold in 24 h. These findings indicate that the expression of the apoE gene is controlled at both transcriptional and post-transcriptional loci by the thyroid hormone.

Synthesis and lymphatic transport of intestinal apolipoprotein A-IV in response to graded doses of triglyceride.

Factors regulating the intestinal synthesis and secretion of apoA-IV are incompletely understood. Although it is known that apoA-IV is stimulated by dietary lipid, it is not known whether graded doses of triglyceride elicit graded responses in apoA-IV synthesis and secretion. We used the chronic intestinal lymph-fistula rat to examine the effect of graded levels of intestinal triglyceride transport on secretion of apoA-IV into lymph and synthesis of apoA-IV in various regions of intestine. Rats were implanted with chronic duodenal and intestinal lymph duct cannulas and infused with lipid emulsions containing 5, 10, 20, 40, 80 mumol triolein (including [3H]triolein, 1.2 microCi), 8.7 mumol phosphatidylcholine, and 57 mumol sodium taurocholate in 3 ml phosphate-buffered saline (pH 6.4) at ml/h for 8 h. Lymph samples were collected for 1 h prior to and at 2, 4, 5, 6, 7, and 8 h after the start of lipid infusion. Lymphatic output of triglyceride, phospholipid, and apoA-IV was measured. Steady-state (8 h) content of radioactive lipid was also measured in the lumen and the wall of the gut. In separate studies rats were given infusions of either 5% glucose in saline ("fasting") or triolein emulsion (10, 20, 40, 80 mumol/h) for 8 h after which incorporation of [3H]leucine into apoA-IV in isolated, in situ loops of duodenum, proximal jejunum, mid-distal jejunum, and terminal ileum was measured. Lipid output increased dose-dependently in response to triolein infusion, with steady-state lipid transport achieved by 5 h after start of lipid infusion. Total recovery of 3H-labeled lipid was similar for all triolein doses. Increase in lymphatic apoA-IV output lagged that of triglyceride by 3-4 h, but by 8 h showed graded increases with triolein dose. ApoA-IV synthesis (apoA-IV radioactivity immunoprecipitated by an apoA-IV monospecific antibody and expressed as % of trichloroacetic acid-precipitable [3H]leucine radioactivity) in the duodenum and proximal jejunum showed a 2-fold increase (compared with fasting) in response to 10 mumol triolein/h, but no further increase at higher doses. However, apoA-IV synthesis in mid to distal jejunum increased dose-dependently.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytokines decrease apolipoprotein accumulation in medium from Hep G2 cells.

Cytokines, important biochemical mediators of inflammation, cause a rapid fall in the plasma concentration of cholesterol in vivo. One mechanism by which cytokines may cause acquired hypocholesterolemia is by decreasing the hepatic synthesis and secretion of apolipoproteins. To test this hypothesis, we incubated Hep G2 cells with human recombinant tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6. Each of the cytokines resulted in a dose-related reduction in the concentrations of apolipoprotein (apo) A-I, apoB, and lecithin:cholesterol acyltransferase (LCAT) activity in the medium after 24 hours of incubation. The effect of cytokines on apolipoprotein accumulation was not affected by preincubation of Hep G2 cells with fatty acids. Cytokines decreased the concentration of cellular apoA-I mRNA in a dose-related fashion but did not affect cellular concentrations of apoB mRNA. The concentrations of triglyceride and cholesterol were also reduced in the medium of cells incubated with cytokines. Total cell sterol synthesis rates were calculated by [14C]acetate incorporation. Cells incubated with interleukin-6 had a 31% increase in sterol synthesis rate but a 41% decrease in sterol secretion. These data suggest that these cytokines can decrease the hepatic synthesis and/or secretion of apolipoproteins and that this may explain, in part, the acquired hypocholesterolemia seen during acute and chronic inflammation.

Relationship between lipoprotein(a) levels in serum and some indices of protein synthesis in liver cirrhosis

Recent research has demonstrated that the major site of apolipoprotein(a) synthesis, the characteristic protein of lipoprotein(a) (Lp(a)), is the liver and that patients affected by liver cirrhosis have low serum concentrations of Lp(a). Nevertheless, it is still not clear whether Lp(a) behaviour in these patients is related to reduced hepatic protein synthesis, or to decreased serum lipid levels or to both these conditions. In order to investigate further the behaviour of Lp(a) and, in particular, its relationship with some indices of blood lipids and coagulation, 30 patients affected by liver cirrhosis have been studied. Significantly low serum values of Lp(a) were observed in patients with more severe hepatic injury included in classes B and C according to the Child-Pugh score. Lipoprotein(a) was directly correlated with prothrombin plasma activity and with apolipoprotein B-100 and albumin concentrations in serum. This study confirms low serum levels of Lp(a) in cirrhotic patients and suggests that its decrease could be partly due to impaired liver protein synthesis.

Effect of the interruption of enterohepatic circulation of bile acids by cholestyramine on apolipoprotein gene expression in the rat

The aim of the study was to assess how the interruption of enterohepatic circulation may affect the regulation of apolipoprotein synthesis. For this purpose, apolipoprotein mRNA levels were measured in the liver and small intestine of male Wistar rats fed for 3 weeks diets with or without cholestyramine (5% w/w). In order to modulate intestinal lipid flux, we used diets with 2 or 10% lipid content. Cholestyramine treatment decreased plasma triglyceride and cholesterol concentrations (by 63% and 32%, respectively) in rats fed 10% corn oil containing diet but had no significant effect on plasma lipids in rats fed a low-fat diet. Plasma apo B and apo A-IV concentrations were lowered by cholestyramine treatment in both 10 and 2% oil fed groups; however plasma apo E concentration was only affected in rats receiving 10% corn oil in the diet. Cholestyramine treatment had no significant effect on plasma apo A-I concentration. Hepatic apo B, apo E and apo A-IV mRNA levels were similar among dietary groups. Cholestyramine administration caused an increase in hepatic apo A-I mRNA level only in rats fed low-fat diet. In the intestine cholestyramine treatment decreased apolipoprotein mRNA levels in jejunum but had a few effect on apolipoprotein mRNA levels in ileum. These experiments have shown that long-term decrease in the intraluminal availability of bile acids may alter not only lipid and lipoprotein metabolism but also the regulation of apolipoprotein synthesis.

Extrahepatic lipogenesis contributes to hyperlipidemia in the analbuminemic rat

Hepatic lipid and apolipoprotein synthesis is increased in the nephrotic syndrome. Catabolism of triglyceride-rich lipoproteins is impaired in nephrotic syndrome but not in rats with hereditary analbuminemia (NA), suggesting that lipid synthesis should be increased by analbuminemia in the absence of proteinuria. In this study the rate of cholesterol and fatty acid synthesis in liver and extrahepatic tissue was measured in female NA and control Sprague-Dawley (SD) rats to determine whether lipid synthesis was indeed increased in isolated analbuminemia and to identify the site(s) of increased lipogenesis. We also measured the concentrations of apolipoproteins (apo) AI, B, and E in plasma, as well as the levels of the respective mRNAs in liver. Plasma cholesterol, triglycerides, and apo AI, B, and E were all increased severalfold in the NA rat (P < 0.001). Although liver apolipoprotein mRNA content was significantly increased (P < 0.001) for apo AI (643%), B (273%), and E (299%), 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in liver microsomes and hepatic cholesterol synthesis were not significantly increased in the NA rats. Hepatic fatty acid synthesis and intestinal cholesterol synthesis were not increased in the NA rats. Surprisingly, intestinal fatty acid synthesis was elevated by 60% (P < 0.01). The NA rats demonstrated approximately fourfold increases in the incorporation of 3H2O into circulating cholesterol and fatty acids (P < 0.001). A 56% increase in the synthesis of total nonsaponifiable lipid was found in the extravisceral carcass (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic apolipoprotein B synthesis in copper-deficient rats

The present study was designed to examine if induction of apolipoprotein B synthesis is associated with hypercholesterolemia in copper-deficient rats. This hypercholesterolemia mainly resides in an increase in the HDL-1 and LDL and is associated with a significant increase in plasma apoB concentration. Liver apoB mRNA levels were not significantly modified in deficient animals as compared to control rats. Studies on liver apolipoprotein synthesis indicated that apoB100 synthesis was increased in deficient animals whereas apoB48 synthesis was unchanged. Thus, it appears that the increase in apoB synthesis in the liver of copper-deficient rats occurs at the posttranscriptional level. The selective increase in apoB100 synthesis indicates the possible impact of this deficiency on the editing of apoB. An increase in apoB100 synthesis by the liver in copper-deficient rats may significantly contribute to the increase in plasma concentration of LDL.

Synthesis and secretion of apolipoprotein A-I

Synthesis and secretion of apolipoprotein A-I