Variant forms of plasma apolipoprotein B. Hepatic and intestinal biosynthesis and heterogeneous metabolism in the rat

The Hsp110 molecular chaperone stabilizes apolipoprotein B from endoplasmic reticulum-associated degradation (ERAD)

Apolipoprotein B (apoB) is the most abundant protein in low density lipoproteins and plays key roles in cholesterol homeostasis. The co-translational degradation of apoB is controlled by fatty acid levels in the endoplasmic reticulum (ER) and is mediated by the proteasome. To define the mechanism of apoB degradation, we employed a cell-free system in which proteasome-dependent degradation is recapitulated with yeast cytosol, and we developed an apoB yeast expression system. We discovered that a yeast Hsp110, Sse1p, associates with and stabilizes apoB, which contrasts with data indicating that select Hsp70s and Hsp90s facilitate apoB degradation. However, the Ssb Hsp70 chaperones have no effect on apoB turnover. To determine whether our results are relevant in mammalian cells, Hsp110 was overexpressed in hepatocytes, and enhanced apoB secretion was observed. This study indicates that chaperones within distinct complexes can play unique roles during ER-associated degradation (ERAD), establishes a role for Sse1/Hsp110 in ERAD, and identifies Hsp110 as a target to lower cholesterol.

Overexpression of human apolipoprotein A-II in mice induces hypertriglyceridemia due to defective very low density lipoprotein hydrolysis

Two lines of transgenic mice, hAIItg-delta and hAIItg-lambda, expressing human apolipoprotein (apo)A-II at 2 and 4 times the normal concentration, respectively, displayed on standard chow postprandial chylomicronemia, large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) but greatly reduced high density lipoprotein (HDL). Hypertriglyceridemia may result from increased VLDL production, decreased VLDL catabolism, or both. Post-Triton VLDL production was comparable in transgenic and control mice. Postheparin lipoprotein lipase (LPL) and hepatic lipase activities decreased at most by 30% in transgenic mice, whereas adipose tissue and muscle LPL activities were unaffected, indicating normal LPL synthesis. However, VLDL-triglyceride hydrolysis by exogenous LPL was considerably slower in transgenic compared with control mice, with the apparent Vmax of the reaction decreasing proportionately to human apoA-II expression. Human apoA-II was present in appreciable amounts in the VLDL of transgenic mice, which also carried apoC-II. The addition of purified apoA-II in postheparin plasma from control mice induced a dose-dependent decrease in LPL and hepatic lipase activities. In conclusion, overexpression of human apoA-II in transgenic mice induced the proatherogenic lipoprotein profile of low plasma HDL and postprandial hypertriglyceridemia because of decreased VLDL catabolism by LPL.

Role of plasma lipoproteins in modifying the biological activity of hydrophobic drugs

The plasma lipoprotein distribution of potential drug candidates is not commonly studied. For some hydrophobic drug candidates, attainment of similar plasma free drug levels has not been associated with uniform production of pharmacological activity in different animal species. It is well-known that plasma lipoprotein lipid profiles vary considerably between different animal species. In addition, human disease states can significantly influence plasma lipoprotein profiles, resulting in altered therapeutic outcomes. Current research has shown that lipoprotein binding of drug compounds can significantly influence not only the pharmacological and pharmacokinetic properties of the drug, but the relative toxicity as well. Elucidation of drug distribution among plasma lipoproteins is expected to yield valuable insight into factors governing the pharmacological activity and potential toxicity of the drug. This paper will present an historical perspective and summarize the latest research in the area of lipoprotein-drug interactions.

Dexamethasone enhanced by insulin, but not by thyroid hormones stimulates apolipoprotein B mRNA editing in cultured rat hepatocytes depending on the developmental stage

The increase of hepatic apolipoprotein B (apoB) mRNA editing during rat development was not affected by hypothyroidism. Furthermore, the addition of 3,3',5'-triiodothyronine (T3) to cultured hepatocytes taken from fetal, neonatal and adult rats had no effect on apoB mRNA editing. In contrast, dexamethasone markedly stimulated apoB mRNA editing in hepatocytes taken from neonates. This effect was enhanced by the addition of insulin. For the first time our data provide evidence that glucocorticoids together with insulin are important for the regulation of apoB mRNA editing during postnatal development, whereas thyroid hormones are not critical for this process.

Quantitative and qualitative characterization of apolipoprotein B containing lipoproteins produced by the visceral rat yolk sac in two different in vitro systems: organ culture and isolated epithelial cells in suspension culture

Tissue pieces as well as isolated epithelial cells taken from visceral rat yolk sacs at the 18th day of gestation were able to synthesize and to secrete apo B containing lipoproteins floating in the density ranges of VLDL, IDL and LDL. In all three density classes only the high molecular weight apo B was detectable. VLDL secreted from yolk sac tissue or isolated epithelial cells lacked apo E and apo C. Studies with cycloheximide revealed that about 77% of the particles was delivered from a pool of preformed lipoproteins. Evidence was given that also de novo-synthesis of apo B containing lipoproteins took place in the tissue segments and the isolated epithelial cells. Most of apo B mass (90%) and of apo B radioactivity (60%) secreted by the tissue pieces of the visceral yolk sac floated in the density range 1.020-1.064 g/ml, the remainder being found in the d < or = 1.020 g/ml fraction. In contrast, only 40% of apo B mass and 45% of apo B radioactivity delivered from isolated epithelial cells belonged to the d = 1.020-1.064 g/ml fraction. LDL released from yolk sacs and isolated epithelial cells contained more triacylglycerols (41% vs. 25%) and had a larger mean diameter (24 nm vs. 21.8 nm) than those obtained from fetal rat serum, whereas the comparatively small VLDL produced in vitro (mean diameter = 34 nm) contained less triacylglycerols (46% vs. 60.5%) and more protein (20% vs. 10.2%) in comparison with fetal serum VLDL (mean diameter = 42.3 nm). Incubation experiments with [125I]VLDL led to the conclusion that the lipase secreted by yolk sac tissue into the medium could not be responsible for the conversion of VLDL into LDL, thus supporting our view of a direct LDL secretion by visceral rat yolk sacs.

Liver regrowth and apolipoprotein B secretion by rat hepatocytes following partial hepatectomy

Apolipoprotein (apo) B is an essential component for the assembly and secretion of lipoproteins. The current report examines apo B production using primary cultures of hepatocytes derived from rats 3 to 21 days after partial hepatectomy (PH) to determine the effects of liver regrowth on apo B. Studies indicate that hepatocytes stimulated by PH have a two-thirds reduction in net apo B production 3 to 7 days after surgery, which coincides with the period of maximum rate of liver regrowth. Both higher (apo BH)- and lower-molecular-weight (apo BL) apo B are synthesized and secreted after PH, indicating the presence of edited apo B mRNA in hepatocytes. Hepatocytes derived from PH rats are more sensitive to insulin inhibition of apo B secretion compared with controls, suggesting an enhanced effect of insulin on newly replicated hepatocytes. Epidermal growth factor (EGF), a key regulator of liver regrowth following PH, potentiates the inhibitory action of insulin on apo B secretion in control hepatocytes and those derived from rats 2 to 3 weeks after PH. However, the potentiating effect of EGF on insulin inhibition of apo B is not discernible in hepatocytes 3 to 7 days after PH. The short-term in vitro hormonal effects occurring even with decreased apo B production suggest that this pathway remains available following PH to balance lipoprotein secretion with lipid and energy requirements necessary for liver regeneration.

Hyperlipidemia in streptozocin-diabetic hamsters as a model for human insulin-deficient diabetes: comparison to streptozocin-diabetic rats

Characteristics of the lipoprotein profile and metabolism of triglyceride-rich lipoproteins in diabetic hamsters were investigated to assess their suitability as a model for human diabetic hyperlipidemia. Diabetes was induced in the hamsters by intraperitoneal injection of streptozocin (30 mg/kg) for 3 days and compared with the results in streptozocin-diabetic rats (50 mg/kg intravenously). Similar degrees of hyperglycemia and hypoinsulinemia were observed 8 to 10 days after the final streptozocin injection in both groups. Fasting plasma lipid concentrations were about 2.5 times greater in hamsters than in rats. Plasma cholesterol was principally associated with high-density lipoprotein (HDL) in both rodents, although the distribution in very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) was significantly greater in hamsters (44%) than in rats (13%). Diabetes increased the concentrations of triglyceride, cholesterol, and phospholipid 5.6- to 7.8-fold in hamsters, whereas it increased them only 1.3- to 1.6-fold in rats. Diabetic hamsters have a plasma lipoprotein profile similar to that of diabetic man, ie, triglyceride-rich lipoproteins are increased and HDL cholesterol is decreased. The concentration of HDL cholesterol was inversely correlated with the severity of hypertriglyceridemia (r = .76, P < .005). This combination of events does not occur in diabetic rats. Hamsters had a low level of apoprotein B-48-containing triglyceride-rich lipoproteins, although diabetes increased the estimated concentration by fourfold. In rats apoprotein B-48 is the predominant form, but diabetes did not alter the relative proportion of apoprotein B isoforms.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

Liver, lipoproteins and disease: I. Biochemistry of lipoprotein metabolism

Cholesterol is a structural component of biological membranes and an immediate precursor for steroid hormones and bile acids. The liver is central to the production and removal of cholesterol-rich lipoproteins and bile acids. The basic biochemical aspects of hepatic lipoprotein and cholesterol metabolism and how abnormalities in liver function impair these metabolic pathways are reviewed.

Effect of mild diabetes and dietary fructose on very-low-density lipoprotein triglyceride turnover in rats

Very-low-density lipoprotein (VLDL) triglyceride turnover was examined in mildly streptozotocin (25 mg/kg)-diabetic rats, using Triton WR1339. Diabetic rats fed standard rat chow showed mild hyperglycemia and suppressed levels of plasma insulin. Their triglyceride secretion was significantly suppressed despite an elevated level of plasma free fatty acids. However, the plasma triglyceride level of these diabetic rats was significantly elevated compared with nondiabetic controls. This suggested that the removal of triglyceride from the circulation, as well as its entry into the circulation, was impaired in mildly insulin-deficient rats. Glucose or fructose supplementation (10% in drinking water for 14 days) significantly increased the triglyceride secretion rate of diabetic rats. Especially, fructose supplementation increased plasma insulin to normal levels, but resulted in markedly elevated plasma triglyceride levels (three times higher than glucose-supplemented or chow-fed diabetic rats) despite similar triglyceride secretion rates between the two types of sugar-supplemented diabetic rat groups. This suggested an impairment of triglyceride removal by dietary fructose. The result obtained from chow-fed diabetic rats indicates that mild but significant insulin deficiency resulted in mild hypertriglyceridemia, linked to impaired triglyceride removal rather than to an overproduction of VLDL-triglyceride, despite elevated levels of plasma free fatty acids. Furthermore, fructose feeding induced prominent hypertriglyceridemia not only by stimulating triglyceride secretion, but also by suppressing triglyceride removal from the circulation of mildly diabetic rats.

The isolation, characterisation and quantification of the equine plasma lipoproteins

Plasma lipoproteins were isolated from eight Thoroughbred horses and eight Shetland ponies on the basis of particle size by gel filtration chromatography and according to density using rate-zonal ultracentrifugation. Three major classes corresponding to very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL) were identified and characterised by their lipid and apolipoprotein compositions. The particle size distributions of each class were determined by electron microscopy and non-denaturing polyacrylamide gradient gel electrophoresis. HDL was found to dominate the equine lipoprotein spectrum, accounting for 61 per cent of the total plasma lipoprotein mass (VLDL 24 per cent, LDL 15 per cent). The VLDL class was isolated as a single population of particles that were triglyceride rich and cholesterol, phospholipid and protein poor. Equine LDL was characteristically cholesterol rich and was found to be polydisperse comprising three subfractions that were discrete with respect to particle size and lipid composition. The HDL class was composed of homogeneous particles that were typically protein rich. Apolipoprotein (apo) B was the major protein of VLDL and LDL, and presented two components on polyacrylamide gel electrophoresis with molecular weights in the region of human apoB-100 and a third in VLDL similar to that of apoB-48. ApoA-I was the predominant protein in equine HDL. Although there were no breed differences in the physical or chemical properties of each lipoprotein class, the Shetland ponies had higher plasma triglyceride and VLDL concentrations than their Thoroughbred counterparts.

Isolation and characterization of plasma lipoproteins of common marmoset monkey. Comparison of effects of control and atherogenic diets

This study examines the potential of the common marmoset monkey (Callithrix jacchus) to serve as a model for human lipoprotein metabolism and atherosclerosis. The lipoproteins of animals fed a low-fat, low-cholesterol diet and a high-fat (12% wt/wt lard), high-cholesterol (0.34% wt/wt) diet were characterized by the combination of sequential ultracentrifugation and Pevikon block electrophoresis. Based on chemical and physical properties, equivalents of human very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL), including and HDL-with apolipoprotein E subclass, were demonstrated. In control animals, whose plasma cholesterol concentration was 140.1 +/- 20.2 mg/dl (means +/- SD), approximately 40% of the plasma cholesterol was transported by LDL as compared with approximately 70% in humans. The cholesterol-fed marmosets segregated into two groups: hypo- and hyper-responders. The hyper-responders had plasma cholesterol levels of 450 to 970 mg/ml. The hypercholesterolemia was associated with elevated concentrations of VLDL, intermediate density lipoproteins, and LDL; in addition, these lipoproteins were enriched in cholesteryl esters relative to lipoproteins isolated from control animals. The HDL (d greater than 1.09 g/ml) levels did not change in response to cholesterol feeding, although the HDL-with apolipoprotein E found in the d = 1.02 to 1.09 g/ml fraction increased approximately fivefold. Based on immunological characteristics and sialic acid content, the common marmoset appeared to lack a lipoprotein(a) equivalent. The results of a short-term feeding study (11 months) suggest that this monkey was susceptible to the development of diet-induced atherosclerosis. The hyper-responsive animals developed foam cell lesions and moderately proliferative intimal lesions, predominantly within the thoracic aorta. In summary, the results of our studies indicate that the common marmoset monkey potentially is a useful model for the study of both lipoprotein metabolism and diet-induced atherosclerosis.

The effect of diet on tumor necrosis factor stimulation of hepatic lipogenesis

Previous studies have demonstrated that tumor necrosis factor (TNF) acutely increases serum triglyceride levels and stimulates hepatic lipid synthesis. In this study, we determined the effects of TNF on serum lipid levels and hepatic lipid synthesis in animals whose diets and feeding conditions were varied to induce changes in baseline serum lipid levels and/or rates of hepatic lipid synthesis. In animals studied at both the nadir and peak of the diurnal cycle of hepatic lipid synthesis, TNF acutely increases serum triglyceride levels, stimulates hepatic fatty acid synthesis, and increases the quantity of newly synthesized fatty acids found in the serum. Similarly, in animals ingesting either high-sucrose or cholesterol-enriched diets, TNF induces the characteristic rapid increase in serum triglyceride levels, hepatic fatty acid synthesis, and quantity of labeled fatty acids in the serum. In animals fed a diet high in triglycerides, using either corn oil or lard, TNF stimulates hepatic fatty acid synthesis and increases the quantity of newly synthesized fatty acids in the serum, but serum triglyceride levels do not change. However, TNF inhibits gastric emptying, which results in a marked decrease in fat absorption in TNF-treated animals. It is likely that a decrease in the dietary contribution to serum triglyceride levels during high-triglyceride feeding counterbalances the increased hepatic contribution induced by TNF treatment. In animals fasted before TNF administration there was no acute change in either serum lipid levels, hepatic fatty acid synthesis, or the quantity of labeled fatty acids in the serum. Thus, TNF stimulates hepatic fatty acid synthesis and increases serum triglyceride levels under many diverse dietary conditions, suggesting that there is a strong linkage between the immune system and lipid metabolism that is independent of most dietary manipulations and may be of fundamental importance in the body's response to infection.

Increased level of apolipoprotein B mRNA in the liver of ventromedial hypothalamus lesioned obese rats

The mRNA level of apolipoprotein B (apoB), which is a principal protein component of nascent very low density lipoprotein (VLDL), was determined in parallel with the measurement of acetyl-coenzyme A (Ac-CoA) carboxylase activity in the liver of ventromedial hypothalamus (VMH) lesioned obese rats. Eight weeks after the electrolysis of the bilateral VMH, the level of apoB mRNA in the VMH-lesioned rats was about 1.5-fold higher than that in the sham-operated rats, indicating increased apoB synthesis in the liver of the VMH-lesioned obese rats. The activity of Ac-CoA carboxylase, which is a rate-limiting enzyme for the fatty acid biosynthesis, was about 1.8-fold higher in the VMH-lesioned rats. These observations indicated that VLDL synthesis is increased in the liver of VMH-lesioned obese rats.

The low density lipoprotein receptor

The study of familial hypercholesterolemia at the molecular level has led to its advancement from a clinical syndrome to a fascinating experimental system. FH was first described 50 years ago by Carl Müller who concluded that the disease produces high plasma cholesterol levels and myocardial infarctions in young people, and is transmitted as an autosomal dominant trait determined by a single gene. The existence of two forms of FH, namely heterozygous and homozygous, was recognized by Khachadurian and Fredrickson and Levy much later. The value of FH as an experimental model system lies in the availability of homozygotes, because mutant genes can be studied without interference from the normal gene. The first and most important breakthrough was the realization that the defect underlying FH could be studied in cultured skin fibroblasts. Rapidly, the LDL receptor pathway was conceptualized and its dysfunction in cells from FH homozygotes was demonstrates. Isolation of the normal LDL receptor protein and studies on the biosynthesis and structure of abnormal receptors in mutant cell lines provided essential groundwork for elucidation of defects at the DNA level. The power of the experimental system, FH, became nowhere more obvious than in work that correlated structural information at the protein level with the elucidation of defined defects in the LDL receptor gene. In addition to revealing important structure-function relationships in the LDL receptor polypeptide and delineating mutational events, studies of FH have established several more general concepts. First, the tight coupling of LDL binding to its internalization suggested that endocytosis was not a non-specific process as suggested from early observations. The key finding was that LDL receptors clustered in coated pits, structures that had been described by Roth and Porter 10 years earlier. These investigators had demonstrated, in electron microscopic studies on the uptake of yolk proteins by mosquito oocytes, that coated pits pinch off from the cell surface and form coated vesicles that transport extracellular fluid into the cell. Studies on the LDL receptor system showed directly that receptor clustering in coated pits is the essential event in this kind of endocytosis, and thus established receptor-mediated endocytosis as a distinct mechanism for the transport of macromolecules across the plasma membrane. Subsequently, many additional systems of receptor-mediated endocytosis have been defined, and variations of the overall pathway have been described.(ABSTRACT TRUNCATED AT 400 WORDS)

Intestinal lipoprotein synthesis in control and hypercholesterolemic rats

Previous studies in our laboratory have shown that very-low-density lipoproteins (VLDL) synthesized by the intestine of the diet-induced hypercholesterolemic rat are enriched in cholesteryl esters and unesterified cholesterol compared with intestinal VLDL from control rats. In these studies, we isolated and characterized nascent intestinal Golgi intermediate-density lipoproteins (IDL, d 1.006-1.040 g/ml) and studied isotope incorporation into apoliproteins of Golgi VLDL from control and hypercholesterolemic rats. IDL were triacylglycerol-rich lipoproteins but contained more cholesteryl ester and protein than the corresponding Golgi VLDL fractions. IDL from hypercholesterolemic rats were enriched in cholesteryl esters to a greater extent than IDL from control rats. The apolipoprotein patterns of IDL fractions were the same as those of intestinal Golgi VLDL, consisting of apolipoproteins (apo) B-48, A-I and A-IV. Time-course isotope incorporation curves for apo A-I and A-IV in Golgi VLDL were similar, but they differed from curves for apo B-48. None of these curves was markedly altered in the hypercholesterolemic rat. We conclude that the major effect of increased dietary cholesterol on intestinal lipoprotein biosynthesis is to increase the percentage of cholesteryl esters in Golgi lipoproteins. Dietary cholesterol does not alter the apolipoprotein composition of Golgi lipoproteins, nor does it have a significant effect on the pattern of isotope incorporation into apolipoproteins of Golgi VLDL. The effect of cholesteryl ester enrichment on the subsequent metabolism of these particles in the circulation and the effect of these particles on hepatic lipoprotein production remain to be determined.

Radioimmunoassay of rat apolipoprotein B peptides in lipoproteins and tissues

Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE) separates rat apolipoprotein B (apoB) into one lower and two higher molecular weight components. Of the latter, peptide I (PI) corresponds to human B-100, while the slightly faster-migrating peptide II (PII) lacks a human counterpart; the smaller species peptide III (PIII) corresponds to human B-48. We describe here a competitive radioimmunoassay which separately measures the amounts of total (i.e., PI + PII + PIII) and larger (i.e., PI + PII) rat apoB peptides, with the amounts of PIII obtained by difference. Standard rat PIII and combined PI + PII (PI,II) were isolated by high-pressure gel filtration liquid chromatography in the presence of SDS, and the PI,II was used as an immunogen to raise rabbit antisera which were capable of binding all three forms of rat apoB. However, Scatchard analysis showed this binding to represent two distinct types of antibodies: one high-affinity class which bound only PI,II and a second class which bound all apoB peptides with equal but lower affinity. Thus, since 125I-labeled PIII was displaced equally effectively by PI,II and PIII, but 125I-labeled PI,II was displaced only by PI,II, the unabsorbed antiserum could be used to measure either total apoB or PI,II alone, depending on the choice of labeled ligand. The validity of the assay for apoB peptides in very-low-density and low-density lipoproteins and in liver microsomes was verified by comparison with peptide determinations by SDS-PAGE.

The production and utility of monoclonal antibodies to rat apolipoprotein B lipoproteins

Murine hybridomas were prepared by fusion of azaguanine-resistant plasmacytoma cells and spleen cells from BALB/c mice immunized with intact rat lipoproteins (Lp). Seventeen stable hybridomas from 5 fusions producing antibody against rat apolipoprotein B (apo B)-containing Lp were prepared. The distribution of antigenic determinants of rat apo B variants defined by these antibodies was similar to that found for human apo B. Thirteen mAbs were reactive to epitopes found on both the higher (apo BH) and lower (apo BL) molecular weight forms of rat apo B; two were reactive to apo BH-specific epitopes and two were reactive to apolipoprotein E. No antibodies reactive to unique apo BL epitopes were found. One anti-rat apo B mAb (1.34.3) of high affinity and avidity was reactive to denatured and intact Lp-apo B. Reactivity towards Lp-apo B was independent of variant composition and density class of Lp. The mAb was subsequently used to develop a radioimmunoassay (RIA) which was sensitive down to 5 ng apo B. The assay was used successfully to measure total nascent Lp-apo B secreted by hepatocytes in culture.

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

Previous work has shown that nascent hepatic very-low-density lipoproteins (VLDL) in the rat are biosynthesized without the obligatory co-factor (apolipoprotein C-II) for lipoprotein lipase-mediated hydrolysis of their core triacylglycerols. Upon secretion, apolipoproteins C-II and C-III are rapidly transferred to the particles from high-density lipoprotein (HDL) within the space of Disse and upon the entry into the plasma. Here we extend those studies to include observations on the apolipoprotein E content and lipid composition of nascent hepatic VLDL before and after exposure to plasma components. We have elected to use hepatic secretory vesicle VLDL rather than liver perfusate VLDL as truly representative of the nascent lipoproteins. Nascent VLDL from fed rats has an apolipoprotein B/E ratio of 6.6 +/- 0.5, whereas that from fasted animals is 13.9 +/- 2.3. Incubation of nascent VLDL from fed and fasted rats with d greater than 1.063 g/ml rat serum, HDL or the d greater than 1.21 g/ml fraction resulted in a mass transfer of apolipoprotein E to the VLDL such that the apolipoprotein B/E ratio decreased to at least that of serum VLDL (3.4 +/- 0.3). The d greater than 1.21 g/ml fraction appeared to contain a species of apolipoprotein E which most actively transferred to VLDL. The acquisition of apolipoprotein E by nascent secretory vesicle VLDL was attended by a loss of phospholipids, particularly the C40 (stearoylarachidonyl) molecular species, and an increase in the cholesterol-to-phospholipid ratio from 0.11 +/- 0.01 to 0.18 +/- 0.03. No evidence was obtained to suggest a simultaneous acquisition of cholesteryl esters upon incubation of nascent VLDL with VLDL-free serum. We conclude that nascent hepatic VLDL is modified after secretion by acquisition of apolipoproteins C-II, C-III and E with a concomitant loss of phospholipids.

Complete response to vasopressin requires epithelial organization in A6 cells in culture

Arginine vasopressin (AVP) stimulates adenylate cyclase activity in A6 epithelia grown on filters but not in A6 epithelia grown on plastic culture dishes. When A6 cells are subcultured from culture dishes and seeded at high density on filters, stimulation of adenylate cyclase by AVP is not evident for approximately 3 days. Peak stimulation by AVP occurs between 5 and 10 days. The time course for development of responsiveness to AVP corresponds to the development of an ordered epithelium with polarized cells, tight junctions, and a high transepithelial resistance. A similar correlation is seen when mature filter-grown epithelia that respond to AVP are dissociated by chelation of calcium and subcultured as single cells onto another filter. Stimulation of adenylate cyclase is not evident until an epithelium with significant electrical resistance is reformed. Filters seeded at 1.6 X 10(6) cells/cm2 require 1-2 days to show AVP sensitivity and electrical resistance; filters seeded at 0.4 X 10(6) cells/cm2 require 4 days. Experiments to test the function of the subunits of adenylate cyclase with forskolin (catalytic subunit) and with cholera toxin (Ns, guanine nucleotide-binding subunit) indicate that they are functional during the entire time and that it is the receptor for AVP that is not functional until an ordered epithelium has been formed.

Human apolipoprotein B: structure of carboxyl-terminal domains, sites of gene expression, and chromosomal localization

Apolipoprotein (apo-) B is the ligand responsible for the receptor-mediated catabolism of low density lipoproteins, the principal cholesterol-transporting lipoproteins in plasma. The primary structure of the carboxyl-terminal 30 percent (1455 amino acids) of human apo-B (apo-B100) has been deduced from the nucleotide sequence of complementary DNA. Portions of the protein structure that may relate to its receptor binding function and lipid binding properties have been identified. The apo-B100 messenger RNA is about 19 kilobases in length. The apo-B100 gene is expressed primarily in liver and, to a lesser extent, in small intestine, but in no other tissues. The gene for apo-B100 is located in the p24 region (near the tip of the short arm) of chromosome 2.

Apolipoprotein B synthesis in humans: liver synthesizes only apolipoprotein B-100

Apolipoprotein (apo) B-100 and B-48 are prominent apolipoproteins in VLDL, IDL, and chylomicrons. Organ cultures of normal adult human liver were established to ascertain the form of apo B synthesized by hepatocytes in humans. Human liver was minced and incubated in 15 mL methionine-free RPMI-1640 medium with 10% dialyzed fetal calf serum plus 250 microCi 35S-methionine for eight hours at 37 degrees C. Lipoproteins secreted by the liver were isolated by ultracentrifugation and the content of newly synthesized apo B determined by quantitation of radioactivity in the apoB-100 and apoB-48 bands after separation by 3% NaDodSO4 gel electrophoresis. In the eight-hour period, 2.5% to 3.2% of added 35S-methionine was secreted in TCA-precipitable protein of which 0.34% was apo B. Ninety-nine percent of the apo B in VLDL, IDL, and LDL was in the apo B-100 electrophoretic band. No significant radioactivity was detected in the apo B-48 electrophoretic band. Eighty-nine percent of the total radioactivity of apo B-100 was in VLDL with 3% and 8% in IDL and LDL, respectively. These results establish that adult human liver in organ culture synthesizes apo B-100 but not apo B-48.

Metabolism of protein-free lipid emulsion models of chylomicrons in rats

Emulsions were prepared by ultrasonication of mixtures of triolein, cholesteryl oleate, phosphatidylcholine and cholesterol in aqueous dispersions, then purified by ultracentrifugation. After injection into rats, the metabolism of the artificial, protein-free emulsions was comparable to the metabolism of chylomicrons collected from rat intestinal lymph during the absorption of fat. Like chylomicrons, the emulsion triacylglycerol was removed from the plasma more quickly than emulsion cholesteryl ester. Also like chylomicrons, much more emulsion cholesteryl ester than triacylglycerol appeared in the liver 10 min after injection, and only trace amounts appeared in the spleen. Because the artificial emulsions gained apolipoproteins when incubated with plasma, their metabolism was probably facilitated by the recipient rat plasma apolipoproteins and so, in rats made apolipoprotein-deficient by treatment with estrogen, the removal of emulsions from the plasma was slowed. Removal was also slowed in hyperlipidemic rats fed a high-fat, high-cholesterol diet to expand the plasma pools of the triacylglycerol-rich lipoproteins and remnants. The results indicate that the metabolism of lymph chylomicrons can be modeled by artificial, protein-free lipid emulsions not only in the initial partial hydrolysis by lipoprotein lipase, but also in the delivery of a remnant-like particle to the liver.

Apolipoprotein B-48 and B-100 very low density lipoproteins. Comparison in dysbetalipoproteinemia (type III) and familial hypertriglyceridemia (type IV)

A protein band having the same migration as apolipoprotein (apo) B-48 was observed by SDS electrophoresis in the plasma very low density lipoprotein (VLDL) from 14 Type IV and three Type III hyperlipoproteinemic subjects and from six normal fasting subjects. The VLDL from five Type IV, three Type III, and one normal subject were separated into two subfractions, retained and nonretained, by immunoaffinity chromatography on monoclonal anti-apo B-100 Sepharose. Based on results of electrophoresis and radioimmunoassay, we have concluded that these two fractions represent apo B-48 and apo B-100 lipoproteins that we have named apo B-48 and apo B-100 VLDL. When compared to their respective apo B-100 VLDL, the apo B-48 VLDL from either Type III or Type IV was principally enriched in total lipids, in apo E, and had an electrophoretic migration similar to chylomicrons. This suggests that apo B-48 VLDL has the same origin (i.e., intestinal) in the two disorders. Both apo B-48 and apo B-100 VLDL were enriched in cholesteryl ester (CE) and depleted in triglyceride (TG) in Type III; however, both fractions were rich in TG and poor in CE in Type IV and in normal subjects. In addition, compared to their respective apo B-100 VLDL, the apo B-48 fraction was enriched in CE in Type III and in TG in Type IV. We conclude that, despite a possible similar origin for apo B-48 VLDL in Type III and in Type IV subjects, the composition of apo B-48 VLDL is variable and the CE/TG ratio is more characteristic of the type of hyperlipidemia than of the particular VLDL subfractions.