A novel cell line (Caco-2) for the study of intestinal lipoprotein synthesis

Lipoprotein synthesis by the colonic adenocarcinoma cell line Caco-2 was investigated to assess the utility of this cell line as a model for the in vitro study of human intestinal lipid metabolism. Electron micrographic analysis of conditioned medium revealed that under basal conditions of culture post-confluent Caco-2 cells synthesize and secrete lipoprotein particles. Lipoproteins of density (d) less than 1.063 g/ml consist of a heterogeneous population of particles (diameter from 10 to 90 nm). This fraction consists of very low density lipoproteins (d less than 1.006 g/ml) and low density lipoproteins (d = 1.019-1.063 g/ml). Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of [35S]methionine-labeled Caco-2 lipoproteins revealed that very low density lipoproteins contain apolipoprotein E (apoE) and C apolipoproteins, while low density lipoproteins contained apoB-100, apoE, apoA-I, and C apolipoproteins. The 1.063-1.21 g/ml density fraction contained two morphological entities, discoidal (diameter 15.6 +/- 3.9 nm) and round high density lipoprotein particles (diameter 10.2 +/- 2.3 nm). The high density lipoproteins contained apoA-I, apoB-100, apoB-48, apoE, and the C apolipoproteins. Using isoelectric focusing polyacrylamide gel electrophoresis newly secreted apoA-I was identified as pro-apoA-I. ApoE and apoC-III released by Caco-2 cells were highly sialylated. mRNA species for apoA-I, apoC-III, and apoE, but not apoA-IV were identified by Northern blot analysis. ApoA-I, apoB, and apoE were visualized in Caco-2 cells by immunolocalization analysis. This intestinal cell line may be useful for in vitro studies of nutritional and hormonal regulation of lipoprotein synthesis.     

Human crypt intestinal epithelial cells are capable of lipid production, apolipoprotein synthesis, and lipoprotein assembly

The recent availability of spontaneously proliferating, non-transformed human crypt intestinal epithelial cells (HIEC) affords an opportunity to investigate lipid metabolism in undifferentiated enterocytes. The major purpose of this study was to explore the capability of undifferentiated crypt cells to synthesize, assemble, and secrete lipids and apolipoproteins. HIEC were cultured in medium with 5% fetal bovine serum for 5 to 21 d. The cells were clearly able to incorporate [(14)C]oleic acid (dpm/mg protein) into triglycerides (128,279 +/- 16,988), phospholipids (30, 278 +/- 2,107), and cholesteryl esters (2,180 +/- 207). Although improvement in lipid secretion was noted with prolongation of cell culture periods, low efficiency of lipid export (10.3 +/- 2.2% of intracellular content) characterized the HIEC. All phospholipid classes were elaborated, with phosphatidylcholine accounting for 79. 3 +/- 1.3% of cellular phospholipids. Chylomicrons were the dominant (46.4%) lipoproteins secreted, followed by high, low, and very low density lipoproteins (HDL, LDL, and VLDL) comprising 22.5, 20.2, and 10.8% of the total, respectively. HIEC elaborated most of the major apolipoprotein (apo) classes (A-I, A-IV, B-100, C, and E), but were less efficient in producing apoB-48. In contrast to the production of apoA-I and C as early as 5 days after confluence, apoA-I and A-IV were maximally expressed at 11 d. Culture media accumulated much more apoB-100 than apoB-48 (B-48/B-100 ratio 0.21 +/- 0.03), reflecting limited apoB mRNA editing. HIEC demonstrated both endogenous cholesterol synthesis and LDL receptor expression. Cholesterol synthesis was sensitive to 25-hydroxycholesterol and mevinolin, but unresponsive to LDL treatment, suggesting independent regulation pathways. In contrast, LDL inhibited receptor activity. The present findings provide the first solid evidence that immature HIEC are capable of key fat absorptive functions of well-differentiated enterocytes. The intracellular mechanisms required for lipid and apolipoprotein synthesis as well as for lipoprotein assembly are already present in intestinal crypt cells. These cells also retain the capacity for sterol enzyme and receptor expression. However, certain limitations, especially apoB-48 production and lipoprotein secretion as well as unresponsiveness of cholesterol synthesis to LDL, may be ascribed to the lack of differentiation.     

Assembly of very low density lipoproteins in mouse liver: evidence of heterogeneity of particle density in the Golgi apparatus

The assembly of very low density lipoproteins (VLDL) by hepatocytes is believed to occur via a two-step process. The first step is the formation of a dense phospholipid and protein-rich particle that is believed to be converted to VLDL by the addition of bulk triglyceride in a second step. Previous studies in our laboratory led us to hypothesize a third assembly step that occurs in route to or in the Golgi apparatus. To investigate this hypothesis, nascent lipoproteins were recovered from Golgi apparatus-rich fractions isolated from mouse liver. The Golgi fractions were enriched 125-fold in galactosyltransferase and contained lipoprotein particles averaging approximately 35 nm in diameter. These lipoproteins were separated by ultracentrifugation into two fractions: d < 1.006 g/ml and d1.006;-1.210 g/ml. The d < 1.006 g/ml fraction contained apolipoprotein B-100 (apoB-100), apoB-48, and apoE, while the d1.006;-1.210 g/ml fraction contained these three apoproteins as well as apoA-I and apoA-IV. Both fractions contained a 21-kDa protein that was isolated and sequenced and identified as major urinary protein. Approximately 50% of the apoB was recovered with the denser fraction. To determine if these small, dense lipoproteins were secreted without further addition of lipid, mice were injected with Triton WR1339 and [(3)H]leucine, and the secretion of apoB-100 and apoB-48 into serum VLDL (d < 1.006 g/ml) and d1.006;-1.210 g/ml fractions was monitored over a 2-h period. More than 80% of the newly synthesized apoB-48 and nearly 100% of the apoB-100 were secreted with VLDL. These studies provide the first characterization of nascent lipoproteins recovered from the Golgi apparatus of mouse liver. We conclude that these nascent hepatic Golgi lipoproteins represent a heterogeneous population of particles including VLDL as well as a population of small, dense lipoproteins. The finding of the latter particles, coupled with the demonstration that the primary secretory product of mouse liver is VLDL, suggests that lipid may be added to nascent lipoproteins within the Golgi apparatus.     

Intestinal apolipoprotein synthesis and secretion in the suckling pig

The present studies report characterization of intestinal apolipoprotein (apoLp) synthesis and secretion in the suckling pig. Lipoproteins (d less than 1.006 g/ml) from mesenteric lymph were found to contain both apoB-100 and B-48, in addition to apoA-IV, E, A-I, and Cs. Lymph low density lipoproteins (LDL) and high density lipoproteins (HDL) contained mainly apoB-100 and apoA-I, respectively. Analysis of core cholesteryl ester fatty acid composition suggested filtration from plasma as the major source of lymph LDL and HDL. Dual radioisotope labeling of intestinal and hepatic apoLps in lymph, as well as immunoprecipitation of radiolabeled intestinal mucosa, demonstrated intestinal synthesis of apoB-48, A-IV, and A-I. There was no evidence for apoB-100 synthesis by intestinal mucosa. By contrast, piglet liver synthesized apoB-100, E, A-I, and Cs, but not apoB-48. Newly synthesized intracellular intestinal apoA-I was mainly (basic) isoform 1 (pI 5.58), while lymph and plasma HDL apoA-I were predominantly isoform 3 (pI 5.33), mature apoA-I. Lymph apoB (P less than 0.001) and apoA-I (P less than 0.04) mass output increased significantly during lipid absorption. Studies were subsequently conducted in fasting, fat-fed, bile-diverted, and sham-operated animals to determine the role of both dietary and biliary lipid in regulating intestinal apoLp biosynthesis. Proximal and distal small intestinal loops were pulse-radiolabeled with [3H]leucine, and apoB-48 and A-I were immunoprecipitated from cytosolic supernatants. Although a proximal to distal gradient in intestinal synthesis rates for both apoB and A-I was noted in all groups, the acute absorption of dietary lipid did not significantly increase apoB or A-I synthesis in either location. Complete removal of biliary lipid for 48 hr did not alter synthesis rates in jejunum or ileum. These studies suggest that mesenteric lymph apoLps in the suckling pig are derived both by filtration from plasma and by direct secretion from the intestine. Physiologic regulation of intestinal apoA-I and B-48 synthesis rates appears to be independent of luminal lipid availability.     

Presence of two forms of apolipoprotein B in patients with dyslipoproteinemia

Current information suggests that the major forms of the human B apolipoproteins, B-100 and B-48, are under separate genetic control and are synthesized by the liver and intestine, respectively. The apolipoprotein B composition of plasma lipoproteins has been determined in order to gain insight into the metabolic defects in patients with dyslipoproteinemias. Patients with type I and type V hyperlipoproteinemia can be separated into two groups based on apolipoprotein composition and triglyceride concentration. The first group had markedly elevated plasma triglycerides with B-48 in the 1.006 g/ml density fraction and only B-100 within IDL and LDL. The second group had plasma triglycerides less than 1200 mg % and only B-100 in all density fractions. Patients with type III hyperlipoproteinemia had B-48 in only the density less than 1.006 g/ml with B-100 in IDL and LDL; the type III hyperlipoproteinemic patient with apolipoprotein E deficiency, however, had B-48 in density less than 1.006 g/ml fraction, IDL, and LDL. Patients with type IIa, IIb, and IV hyperlipoproteinemia had only B-100 in all density fractions. These combined results are interpreted as indicating that B-48 is associated with triglyceride-rich lipoproteins synthesized by the intestine and that patients with phenotypes I, III, and V have defects in chylomicron remnant metabolism. In addition, in patients with types I and V hyperlipoproteinemia, mild hypertriglyceridemia appears to be associated with lipoprotein particles of liver origin.     

Myristic acid increases dense lipoprotein secretion by inhibiting apoB degradation and triglyceride recruitment

Fatty acids of varying lengths and saturation differentially affect plasma apolipoprotein B-100 (apoB-100) levels. To identify mechanisms at the level of production, rat hepatoma cells, McA-RH7777, were incubated with [(35)S]methionine and either fatty acid-BSA complexes or BSA alone. There were increases in labeled apoB-100 secretion with saturated fatty acids palmitic and myristic (MA) (153 +/- 20% and 165 +/- 11%, respectively, relative to BSA). Incubation with polyunsaturated docosahexaenoic acid (DHA) decreased secretion to 26 +/- 2.0%, while monounsaturated oleic acid (OA) did not change it. In pulse-chase studies, MA treatment resulted in reduced apoB-100 degradation, in agreement with its promotion of secretion. In triglyceride (TG) studies, synthesis was stimulated equally by OA, MA, and DHA, but TG secretion was relatively decreased with MA and DHA. With OA, the majority of newly secreted apoB100-lipoproteins was d < or = 1.006, but with MA, they were much denser (1.063 < d). Furthermore, the relative recruitment of newly synthesized TG to lipoproteins was impaired with MA. We conclude that mechanisms for effects of specific dietary fatty acids on plasma lipoprotein levels may include changes in hepatic production. In turn, hepatic production may be regulated by specific fatty acids at the steps of apoB-100 degradation and the recruitment of nascent TG to lipoprotein particles.     

Direct effects of growth hormone on production and secretion of apolipoprotein B from rat hepatocytes

The aim of this study was to investigate the direct effects of growth hormone (GH) on production and secretion of apolipoprotein B (apoB)-containing lipoproteins from hepatocytes. Bovine GH (5-500 ng/ml) was given for 1 or 3 days to rat hepatocytes cultured on laminin-rich matrigel in serum-free medium. The effects of GH were compared with those of 3 nM insulin and 500 microM oleic acid. GH increased the editing of apoB mRNA, and the proportion of newly synthesized apoB-48 (of total apoB) in the cells and secreted into the medium changed in parallel. GH increased total secretion of apoB-48 (+30%) and apoB-48 in very low density lipoproteins (VLDL) more than twofold. Total apoB-100 secretion decreased 63%, but apoB-100-VLDL secretion was unaffected by GH. Pulse-chase studies indicated that GH increased intracellular early degradation of apoB-100 but not apoB-48. GH had no effect on apoB mRNA or LDL receptor mRNA levels. The triglyceride synthesis, the mass of triglycerides in the cells, and the VLDL fraction of the medium increased after GH incubation. Three days of insulin incubation had effects similar to those of GH. Combined incubation with oleic acid and GH had additive effects on apoB mRNA editing and apoB-48-VLDL secretion. In summary, GH has direct effects on production and secretion of apoB-containing lipoproteins, which may add to the effects of hyperinsulinemia and increased flux of fatty acids to the liver during GH treatment in vivo.     

Very low and low density lipoprotein synthesis and secretion by the human hepatoma cell line Hep-G2: effects of free fatty acid

The liver is a major source of the plasma lipoproteins; however, direct studies of the regulation of lipoprotein synthesis and secretion by human liver are lacking. Dense monolayers of Hep-G2 cells incorporated radiolabeled precursors into protein ([35S]methionine), cholesterol ([3H]mevalonate and [14C]acetate), triacylglycerol, and phospholipid ([3H]glycerol), and secreted them as lipoproteins. In the absence of free fatty acid in the media, the principal lipoprotein secretory product that accumulated had a density maximum of 1.039 g/ml, similar to serum low density lipoprotein (LDL). ApoB-100 represented greater than 95% of the radiolabeled apoprotein of these particles, with only traces of apoproteins A and E present. Inclusion of 0.8 mM oleic acid in the media resulted in a 54% reduction in radiolabeled triacylglycerol in the LDL fraction and a 324% increase in triacylglycerol in the very low density lipoprotein (VLDL) fraction. Similar changes occurred in the secretion of newly synthesized apoB-100. The VLDL contained apoB-100 as well as apoE. In the absence of exogenous free fatty acid, the radiolabeled cholesterol was recovered in both the LDL and the high density lipoprotein (HDL) regions. Oleic acid caused a 50% decrease in HDL radiolabeled cholesterol and increases of radiolabeled cholesterol in VLDL and LDL. In general, less than 15% of the radiolabeled cholesterol was esterified, despite the presence of cholesteryl ester in the cell. Incubation with oleic acid did not cause an increase in the total amount of radiolabeled lipid or protein secreted. We conclude that human liver-derived cells can secrete distinct VLDL and LDL-like particles, and the relative amounts of these lipoproteins are determined, at least in part, by the availability of free fatty acid.     

Catabolism of very low density lipoproteins in the rabbit. Effect of changing composition and pool size.

To determine the metabolic mechanism of hypercholesterolemia in rabbits produced by feeding cholesterol-rich diets, control and hypercholesterolemic rabbits were injected with I-labelled very low density lipoproteins (VLDL, d 1.006 g/ml) from control and/or hypercholesterolemic donors. Apolipoprotein B in VLDL decayed biphasically. The first phase occurred much more rapid than the second. 95% of the VLDL apolipoprotein B was catabolized via the first phase (t1/2 = 0.55 +/- 0.19 h) in normal rabbit with the immediate appearance of this radioactivity in intermediate density lipoproteins (IDL, d 1.006-1.025 g/ml) and low density lipoproteins (LDL, d 1.025-1.063 g/ml). The apolipoproteins C and E at the same time were transferred to high density lipoproteins where they decayed biphasically. The apolipoprotein B from hypercholesterolemic VLDL in the normal recipient disappeared at a similar rate as from normal VLDL via phase I; however, it was incompletely converted to IDL and LDL. Apolipoprotein B from normal VLDL in cholesterol-fed rabbits disappeared at a normal rate via phase I, but only 82% was catabolized by this phase. Hypercholesterolemic VLDL injected into the hypercholesterolemic recipient was less rapidly catabolized via phase I (T1/2 = 2.5 +/- 0.89 H) and only a small fraction was converted to IDL and LDL.     

Intestinal lipids and lipoproteins in the human fetus: modulation by epidermal growth factor.

The aim of the present investigation was first, to examine the ability of human fetal intestine (17-20 wk) to incorporate fatty acid into esterified lipids; and second, to study in vitro lipoprotein synthesis and secretion by fetal explants, as well as the effect of epidermal growth factor (EGF) on these processes. Cultured fetal jejunal explants were incubated in Leibovitz medium for 42 h with [14C]oleate. Both triglycerides (TG) and phospholipids (PL) were the major labeled products. Whereas TG were predominant (80%) in the culture medium, PL accounted for more than 50% of total tissue lipids. More than 60% of the radioactivity in PL was associated with phosphatidylcholine. Some labeling (< 5%) was also recovered in the cholesteryl ester fraction. Active exocytosis was demonstrated by the accumulation of newly synthesized esterified lipids in the medium and the presence of lipoproteins in the basolateral membrane region and intercellular spaces. Most of the newly synthesized lipids were found in lipoproteins of d < 0.97 g/ml (51.2%) and d < 1.21 g/ml (39.3%), whereas the rest were recovered in d < 1.006 g/ml (9.8%) and 1.063 g/ml (5.6%). A similar trend characterized the lipoprotein secretion. The synthesis of the d < 0.97 g/ml fraction (30,653 +/- 4,122 dpm/mg protein) was significantly greater than the 1.006 g/ml fraction (5,897 +/- 1,734), P < 0.005. The secretion of d < 0.97 g/ml particles into the medium was also five fold higher than that of the d < 1.006 g/ml fraction (P < 0.01). The addition of EGF to the culture medium (25, 50, and 100 ng/ml) significantly enhanced the d < 0.97 g/ml lipoprotein secretion (25-40%) and decreased the d 1.006 g/ml and 1.063 g/ml fraction output. The lipid composition of these lipoprotein fractions was never altered by the presence of EGF, suggesting that the number of lipoprotein particles, rather than size, was modified by the growth factor. The present findings provide the first evidence that the human fetal intestine has the capacity to elaborate lipoprotein fractions for the transport of newly synthesized lipids. Furthermore, our data suggest that EGF, present in significant quantity in saliva, amniotic fluid, and bile, can modulate the release of TG-rich lipoproteins by fetal intestinal explants.     

A density gradient study of the lipoprotein and apolipoprotein distribution in the chicken, Gallus domesticus

Plasma lipoproteins from 5-week old male chickens were separated over the density range 1.006-1.172 g/ml into 22 subfractions by isopycnic density gradient ultracentrifugation, in order to establish the distribution of these particles and their constituent apolipoproteins as a function of density. Lipoprotein subfractions were characterized by electrophorectic, chemical and morphological analyses, and their protein moieties were defined according to net charge at alkaline pH, molecular weight and isoelectric point. These analyses have permitted us to reevaluate the density limits of the major chicken lipoprotein classes and to determine their main characteristics, which are as follows: (1) very-low-density lipoproteins (VLDL), isolated at d less than 1.016 g/ml, were present at low concentrations (less than 0.1 mg/ml) in fasted birds; their mean diameter determined by gradient gel electrophoresis and by electron microscopy was 20.5 and 31.4 nm respectively; (2) as the the density increased from VLDL to intermediate density lipoproteins (IDL), d 1.016-l.020 g/ml) and low-density lipoproteins (LDL, d 1.020-1.046 g/ml), the lipoprotein particles contained progressively less triacylglycerol and more protein, and their Stokes diameter decreased to 20.0 nm; (3) apolipoprotein B-100 was the major apolipoprotein in lipoproteins of d less than 1.046 g/ml, with an Mr of 350000; small amounts of apolipoprotein B-100 were detectable in HDL subfractions of d less than 1.076 g/ml; urea-soluble apolipoproteins were present in this density range as minor components of Mr 38000-39000, 27000-28000 (corresponding to apolipoprotein A-1) and Mr 11000-12000; (4) high density lipoprotein (HDL, d 1.052-1.130 g/ml) was isolated as a single band, whose protein content increased progressively with increase in density; the chemical composition of HDL resembled that of human HDL2, with apolipoprotein A-1 (M 27000-28000) as the major protein component, and a protein of Mr 11000-12000 as a minor component; (5) heterogeneity was observed in the particle size and apolipoprotein distribution of HDL subfractions: two lipoprotein bands which additional apolipoproteins of Mr 13000 and 15000 were detected. These studies illustrate the inadequacy in the chicken of the density limits applied to fractionate the lipoprotein spectrum, and particularly the inappropriateness of the 1.063 g/ml density limit as the cutoff for LDL and HDL particle populations in the species.     

ApoB-75, a truncation of apolipoprotein B associated with familial hypobetalipoproteinemia: genetic and kinetic studies.

We have identified a mutation of apolipoprotein B (apoB) in a kindred with hypobetalipoproteinemia. Four affected members had plasma concentrations of total cholesterol of 115 +/- 14, low density lipoprotein (LDL)-C of 48 +/- 11, and apoB of 28 +/- 9 (mg/dl mean +/- SD). The values correspond to approximately 30% the values for unaffected relatives. Triglyceride and high density lipoprotein (HDL)-C concentrations were 92 +/- 50 and 49 +/- 4, respectively, neither significantly different from unaffected relatives. Western blots of plasma apoB of affected subjects showed two major bands: apoB-100 and an apoB-75 (mol wt of approximately 418,000). DNA sequencing of the appropriate polymerase chain reaction (PCR)-amplified genomic DNA segment revealed a deletion of the cytidine at nucleotide position 10366, resulting in a premature stop codon at amino acid residue 3387. In apoB-75/apoB-100 heterozygotes, two LDL populations containing either apoB-75 or apoB-100 could be distinguished from each other by gel permeation chromatography and by immunoblotting of nondenaturing gels using monoclonal antibodies B1B3 (epitope between apoB amino acid residues 3506-3635) and C1.4 (epitope between residues 97-526). ApoB-75 LDL were smaller and more dense than apoB-100 LDL. To determine whether the low concentration of apoB-75 was due to its enhanced LDL-receptor-mediated removal, apoB-75 LDL were isolated from the proband's d 1.063-1.090 g/ml fraction (which contained most of the apoB-75 in his plasma) by chromatography on anti-apoB and anti-apoA-I immunoaffinity columns. The resulting pure apoB-75 LDL fraction interacted with the cells 1.5-fold more effectively than apoB-100 LDL (d 1.019-1.063 g/ml). To determine the physiologic mechanism responsible for the hypobetalipoproteinemia, in vivo kinetic studies were performed in two affected subjects, using endogenous labeling of apoB-75 and apoB-100 with [13C]leucine followed by multicompartmental kinetic analyses. Fractional catabolic rates of apoB-75 VLDL and LDL were 2- and 1.3-fold those of apoB-100 very low density lipoprotein (VLDL) and LDL, respectively. Production rates of apoB-75 were approximately 30% of those for apoB-100. This differs from the behavior of apoB-89, a previously described variant, whose FCRs were also increased approximately 1.5-fold relative to apoB-100, but whose production rates were nearly identical to those of apoB-100. Thus, in contrast to the apoB-89 mutation, the apoB-75 mutation imparts two physiologic defects to apoB-75 lipoproteins that account for the hypobetalipoproteinemia, diminished production and increased catabolism.     

Measurement of apolipoprotein B concentration in plasma lipoproteins by combining selective precipitation and mass spectrometry

The measurement of apolipoprotein B (apoB) in purified lipoproteins by immunological assays is subject to criticism because of denatured epitopes or immunoreactivity differences between purified lipoproteins and standard. Chemical methods have therefore been developed, such as the selective precipitation of apoB followed by quantification of the precipitate. In this study, we present the measurement of apoB concentration in lipoproteins purified by ultracentrifugation by combining isopropanol precipitation and gas chromatography/mass spectrometry. Very low density lipoprotein (VLDL; d < 1.006 g/mL); VLDL plus intermediate density lipoprotein (VLDL + IDL; d < 1.019 g/mL); and VLDL, IDL, and low density lipoprotein (VLDL + IDL + LDL; d < 1.063 g/mL) were purified by ultracentrifugation. Apolipoprotein B-100 was selectively precipitated by isopropanol. The leucine content of the pellet was then determined by gas chromatography/mass spectrometry, using norleucine as internal standard. Knowledge of the number of leucine molecules in one apoB-100 molecule makes it possible to calculate the plasma concentration of apoB in the various lipoprotein fractions. ApoB in IDL (d 1.006-1.019 g/mL) and LDL (d 1.019-1.063 g/mL) were then determined by subtracting VLDL-apoB from apoB in lipoproteins d < 1.019 and apoB in lipoproteins d < 1.019 g/mL from apoB in lipoproteins d < 1.063 g/mL, respectively. The isopropanol precipitate was verified as pure apoB (>97%) in lipoprotein fractions isolated from normo- and hyperlipidemic plasma and the method appeared reproducible.The combination of isopropanol precipitation and the GC/MS method appears therefore to be a precise and reliable method for kinetic and epidemiological studies.     

Effect of fasting on the composition of plasma lipoproteins in cholesterol-fed diabetic rabbits

Cholesterol-fat feeding is associated with unusual alterations in the composition of plasma lipoproteins in alloxan-diabetic rabbits. In the present study plasma lipoprotein lipid and apoprotein composition was studied before and after 48 hr of fasting in cholesterol-fed diabetic and control rabbits in order to further characterize these alterations. Compared with control rabbits, the diabetic rabbits had similar plasma cholesterol levels, but 100-fold higher triglyceride levels prior to fasting. These plasma lipids were distributed mainly to large, Sf greater than 400 plasma lipoproteins in the diabetic rabbits, and to beta-VLDL in control rabbits. Sf greater than 400 lipoproteins, VLDL, IDL, LDL, and HDL from diabetic rabbits had triglyceride as the predominant lipoprotein core lipid. Sf greater than 400 lipoproteins and VLDL from diabetic rabbits had lesser amount of apoprotein E, and greater amounts of apoproteins A-I, A-IV, and B-48 as percent of total apoprotein mass in comparison with control rabbits. Fasting reduced plasma triglyceride levels by 55% in diabetic rabbits. Sf greater than 400 lipoprotein and VLDL triglyceride content decreased but remained a major core lipid. Fasting eliminated apoproteins A-I and A-IV from Sf greater than 400 lipoproteins and VLDL, but had no significant effect on apoB-48 content. Insulin treatment of the diabetic rabbits reduced plasma triglyceride by approximately 90% resulting in cholesteryl ester-rich particles reassembling beta-VLDL both in the Sf greater than 400 lipoprotein and VLDL fractions. These results indicate that the alterations in plasma lipoproteins in cholesterol-fed diabetic rabbits result from the presence in the d less than 1.006 g/ml plasma lipoprotein class of partially metabolized, intestinally derived particles.     

Early changes in plasma lipoprotein structure and biosynthesis in cholesterol-fed rabbits

Plasma lipoproteins of d < 1.063 g/ml from rabbits fed a diet containing 1% cholesterol for 4 days showed changes in concentration and rates of flotation as determined by analytical ultracentrifugation. A marked increase in cholesteryl ester content of lipoprotein with d < 1.019 g/ml was the most prominent change in rabbits fed the diet for 21 days. Gel electrophoresis and immunochemical procedures demonstrated that in control and hypercholesterolemic rabbits there were some common apolipoproteins found in all lipoproteins with density < 1.063 g/ml. In control rabbits, there were also apolipoproteins specific to the lipoprotein fraction with d < 1.019 and to the fraction with d 1.019-1.063 g/ml. However, in rabbits fed the hypercholesterolemic diet for 21 days, the apolipoproteins characteristic of fraction 1.019-1.063 were the most abundant in the fraction with d < 1.019 g/ml. Liver slices from rabbits fed the high cholesterol diet for 7 and 21 days incorporated more l-[(14)C]leucine into very low density and low density lipoproteins than controls. The results suggest that cholesterol feeding leads to an increase in biosynthesis of lipoproteins with d < 1.063 g/ml. The newly synthesized lipoprotein contains apolipoproteins similar to those found in controls but with a higher lipid-to-protein ratio. From the apoprotein composition, it is concluded that the very low density fraction present in cholesterol-fed animals is more structurally related to low density lipoproteins than to the very low density lipoproteins isolated from control animals.     

Nascent lipoproteins from recirculating and nonrecirculating liver perfusions and from the hepatic Golgi apparatus of African green monkeys

Perfusate apoB-100-containing lipoproteins from the isolated, perfused livers of African green monkeys consist of significant amounts of d greater than 1.006 g/ml particles in addition to very low density lipoproteins (VLDL). Distinguishing characteristics of these perfusate lipoproteins are the relative abundance of surface lipids and deficiency of core lipids. The present studies were performed to determine the likelihood that the d greater than 1.006 g/ml perfusate lipoproteins are secretion products instead of products of post-secretory modification (e.g., lipolysis) of secreted VLDL. [14C]Leucine from the perfusate became incorporated into the apoB of each of the perfusate lipoprotein classes to a similar extent in both recirculating and nonrecirculating perfusions. When endogenously radiolabeled perfusate VLDL from one liver was recirculated through a second liver, only about 15% of the radiolabeled protein appeared in the d greater than 1.006 g/ml fraction. The particle morphology and the cholesterol and apoB distribution between VLDL and d greater than 1.006 g/ml fractions were similar in recirculating and nonrecirculating perfusions. A Golgi apparatus-rich fraction was isolated from the homogenates of fresh liver samples and the isolated Golgi VLDL and d greater than 1.006 g/ml lipoproteins exhibited morphologic evidence of extra surface material analogous to that seen in perfusate. Taken together, these data support the possibility that significant amounts of d greater than 1.006 g/ml lipoproteins, many with surface-rich properties, are nascent, secretory products of the primate liver. The low level of lecithin:cholesterol acyltransferase (LCAT) in this perfusion system appears to permit detection of these secretion products and it is significant to note that the perfusate lipoprotein profile, which is unlike that of normal plasma, is similar to that of LCAT-deficient patients.     

Effect of portacaval anastomosis on rat lipoproteins

The distribution of cholesterol (C), triglycerides (TG), phospholipids (PL) and protein in the different lipoproteins was studied in male Wistar rats under 2 conditions: control and 2 months after portacaval anastomosis (PCA). PCA decreased the levels of cholesterol and the other components in chylomicrons (-90%), very low density lipoproteins (-65 to -78%), LDL2 (1.040 less than d less than 1.063 g/ml; -51 to -61%) and HDL (1.063 less than d less than 1.21 g/ml), whereas no change was observed in LDL1 (1.006 less than d less than 1.040 g/ml). Apoprotein C contents were decreased in all lipoproteins. The relative proportions of C, TG, PL and proteins in lipoproteins were essentially unchanged by the shunt, suggesting a reduced number of lipoprotein particles in plasma after PCA. It was concluded that PCA reduced the levels of all lipoproteins secreted by liver and/or the intestine without modifying those of intraplasmatic origin (LDL1).     

Mechanism of the HDL2 stimulation of progesterone secretion in cultured placental trophoblast

Lipoprotein cholesterol (C) supports the high rate of progesterone production by the human placenta as endogenous cholesterol synthesis is low. To study underlying mechanisms whereby lipoproteins, including high density lipoprotein-2 (HDL2), stimulate progesterone secretion, trophoblast cells were isolated from human term placentas and maintained in primary tissue culture. Lipoproteins were added at several concentrations and medium progesterone secretion was determined. HDL2 (d 1.063-1.125 g/ml) as well as low density lipoproteins (LDL) (d 1.019-1.063 g/ml) but not HDL3 (d 1.125-1.21 g/ml) stimulated progesterone secretion in a dose-dependent manner, with HDL2 cholesterol entering the cell and serving as substrate for progesterone synthesis. Conversely, LDL and HDL2 produced a significant decrease in [2-14C]acetate incorporation into cell cholesterol. Cholesterol-depleted lipoproteins did not stimulate progesterone secretion. The stimulating effect of LDL was abolished by apolipoprotein modification by cyclohexanedione or reductive methylation and by the addition of anti-LDL receptor antibody or 10 microM chloroquine to the medium. [14C]acetate conversion into cholesterol was accelerated by these procedures. However, HDL2 stimulation of progesterone secretion and reduction of [14C]acetate incorporation into cholesterol was not blocked by chemical modification of apolipoproteins, anti-LDL receptor antibody, or chloroquine. Treatment of HDL2 with tetranitromethane or dimethylsuberimidate also did not block the stimulation of progesterone. To determine whether the capacity of HDL2 to deliver cholesterol to the trophoblast cells was restricted to subfractions differing in apoE content, HDL2 was chromatographed on heparin-Sepharose and three fractions (A, B, and C) were obtained. Fraction A was poorest in apoE and free cholesterol, fraction B contained the majority of cholesterol, and fraction C was the richest in apoE and free cholesterol. When added to trophoblast cells, fraction A stimulated little progesterone secretion, fraction B stimulated moderately, and fraction C did so greatly. Modification of these subfractions with cyclohexanedione or reductive methylation did not inhibit these effects. In conclusion, HDL2 stimulated progesterone secretion in human trophoblast cell culture. Contrary to LDL, the HDL effect was not mediated by apolipoproteins or the LDL receptor pathway. The ability of HDL2 to stimulate progesterone secretion is consistent with the passive transfer of free cholesterol to the cell membrane from a physicochemically specific subfraction of HDL. This mechanism may be an auxiliary source of cholesterol for human steroidogenic cells.     

Lipoproteins and apolipoproteins in intestinal lymph of the preruminant calf, Bos spp., at peak lipid absorption

We have recently evaluated the in vivo role of the liver in lipoprotein homeostasis in the preruminant calf (Bauchart, D., D. Durand, P. M. Laplaud, P. Forgez, S. Goulinet, and M. J. Chapman, 1989. J. Lipid Res. 30: 1499-1514). We now present the partial characterization of lipoprotein particles in postprandial intestinal lymph at peak lipid absorption (i.e., 10 h after a meal) in the preruminant calf fed a curdled milk replacer. Intestinal lymph from four male preruminant calves was analyzed for its content of lipids and fractionated by sequential and density gradient ultracentrifugation into chylomicrons (Sf greater than 400), very low density lipoproteins (VLDL) (Sf less than 400; d less than 1.006 g/ml), and a series of lipoprotein subfractions with d greater than 1.006 g/ml. Postprandial lymph contained predominantly triglycerides (1099 +/- 611 mg/100 ml), with lesser amounts of phospholipids (197 +/- 107 mg/100 ml) and cholesterol (52 +/- 30 mg/100 ml). The most abundant particles were triglyceride-rich chylomicrons and VLDL which accounted for approximately 76% and approximately 19%, respectively, of total d less than 1.21 g/ml lipoproteins. As judged by negative stain electron microscopy, chylomicron particle diameters ranged from 650 to 2400 A, while VLDL were smaller and distributed over a distinct size range (340-860 A). These two lipoprotein classes each presented protein components with Mr comparable to those of human apoB-48, apoA-I, and C apoproteins, together with an Mr 52,000 protein resembling human beta 2-glycoprotein-I. In addition, VLDL exhibited a polypeptide with Mr approximately 61,000. Lymph lipoproteins with d greater than 1.006 g/ml consisted primarily (approximately 81% of total) of particles distributed over the 1.053-1.119 g/ml density range. Electrophoretic analysis of the latter lipoprotein fraction showed it to be heterogeneous, including particles with the migration characteristics of low and of high density lipoproteins, respectively. Subfractions in the d 1.053-1.076 g/ml range were dominated by particles with Stokes diameters typical of high density lipoproteins (HDL), but also contained three different populations of low density lipoprotein-like particles. The high molecular weight apolipoproteins in these same cholesteryl ester-rich (greater than 30% of lipoprotein mass) subfractions comprised components with Mr resembling those of human apoB-100 and apoB-48, respectively, and with the latter protein predominating to a varying degree. A counterpart to human apoA-I was the major protein component over the entire density range from d 1.053 to 1.119 g/ml.(ABSTRACT TRUNCATED AT 400 WORDS)     

Characterization of lipoproteins produced by the human liver cell line, Hep G2, under defined conditions

Confluent monolayers of the human hepatoblastoma-derived cell line, Hep G2, were incubated in serum-free medium. Conditioned medium was ultracentrifugally separated into d less than 1.063 g/ml and d 1.063-1.20 g/ml fractions since very little VLDL was observed. The d less than 1.063 g/ml fraction was examined by electron microscopy; it contained particles of 24.5 +/- 2.3 nm diameter, similar in size to plasma LDL; a similar size was demonstrated by nondenaturing gradient gel electrophoresis. These particles possessed apoB-100 only. The d less than 1.063 g/ml fraction had a lipid composition unlike that of plasma LDL; unesterified cholesterol was elevated, there was relatively little cholesteryl ester, and triglyceride was the major core lipid. The d 1.063-1.20 g/ml fraction was heterogeneous in size and morphology. Electron microscopy revealed discoidal particles (14.9 +/- 3.2 nm long axis and 4.5 +/- 0.2 nm short axis) as well as small spherical ones (7.6 +/- 1.4 nm diameter). Nondenaturing gradient gel electrophoresis consistently showed the presence of peaks at 13.4 11.9, 9.7, and 7.4 nm. The latter peak was conspicuous and probably corresponded to the small spherical structures seen by electron microscopy. Unlike plasma HDL, Hep G2 d 1.063-1.20 g/ml lipoproteins contained little or no stainable material in the (HDL3a)gge region by gradient gel electrophoresis. Hep G2 d 1.063-1.20 g/ml lipoproteins differed significantly in composition from their plasma counterparts; unesterified cholesterol and phospholipid were elevated and the mole ratio of unesterified cholesterol to phospholipid was 0.8. Cholesteryl ester content was extremely low. ApoA-I was the major apolipoprotein, while apoE was the next most abundant protein; small quantities of apoA-II and apoCs were also present. Immunoblot analysis of the d 1.063-1.20 g/ml fraction after gradient gel electrophoresis showed that apoE was localized in the larger pore region of the gel (apparent diameter greater than 12.2 nm); the apoA-I distribution in this fraction was very broad (7.1-12.2 nm), and included a distinct band at 7.4 nm. Immunoblotting after gradient gel electrophoresis of concentrated medium revealed that a significant fraction of apoA-I in the uncentrifuged medium was in a lipid-poor or lipid-free form. This cell line may be a useful model for investigating the metabolism of newly formed HDL.