Lipoprotein Metabolism1 1In preparation of this study, Dr. Eisenberg was supported in part through the Special Foreign Currency Program of the National Library of Medicine, National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare, Bethesda, Maryland, under an agreement with the Israel Journal of Medical Sciences, Jerusalem, Israel

Varied effects of dietary sucrose and cholesterol on serum lipids, lipoproteins and apolipoproteins in rhesus monkeys

Serum lipid, lipoproteins, apolipoproteins and plasma insulin and glucose were studied in rhesus monkeys (Macaca mulatta) fed high sucrose diets (69%, w/w), with and without added cholesterol. When compared to basal diet, a high sucrose diet with no added cholesterol fed for 6 weeks increased serum total cholesterol and triglycerides by factors of 1.2 and 2.8, respectively. Cholesterol supplementation of sucrose diets increased the serum total cholesterol levels by a factor of 2.2 and decreased the serum triglycerides by 0.47. The serum cholesterol response to experimental diets was reflected predominantly in beta-lipoprotein and to a lesser extent in alpha-lipoprotein. Sucrose diets without cholesterol enriched the beta- and pre-beta-lipoproteins with triglycerides and protein at the expense of cholesterol. On the same diet, the protein content of alpha-lipoprotein increased at the expense of cholesterol and triglycerides. In contrast, dietary cholesterol decreased the triglyceride content and increased the cholesterol content of all the lipoprotein classes. Sucrose feeding seems to increase ApoB more than non-ApoB proteins. The proportion of ApoC-II relative to ApcoC-III increased in each animal on a sucrose diet; exogenous cholesterol further increased this trend. While sucrose diet decreased ApoA-I/ApoA-II ratios, cholesterol supplementation reversed this trend. Dietary sucrose increased the plasma glucose, insulin, and insulin-glucose ratios. The addition of cholesterol also tended to decrease plasma glucose and insulin levels. These observations indicate varied responses of serum lipoproteins and apoproteins to dietary sucrose with and without cholesterol supplementation.

Serum alpha-lipoprotein responses to variations in dietary cholesterol, protein and carbohydrate in different non-human primate species

Serum alpha-lipoprotein responses to variations in dietary cholesterol, protein, and carbohydrate were studied in different nonhuman primate species. Chimpanzee, rhesus, green, patas, squirrel and spider monkeys all showed significant interspecies differences in serum total cholesterol responses to 1.84 mg/kcal exogenous cholesterol. Dietary cholesterol significantly increased the alpha-lipoprotein cholesterol in all species except rhesus and chimpanzee. Among these species, there was no relationship between the basal serum lipoprotein profile and subsequent lipoprotein responses to dietary cholesterol. Although the level of dietary protein at 6%, 12%, and 37% of calories had no appreciable main effect on serum total cholesterol in spider monkeys, very low protein diet (6% of calories) produced a significant elevation in alpha-lipoprotein cholesterol. Serum alpha-lipoprotein responses to exogenous cholesterol (1.84 mg/kcal) was highest for the very low protein diet and lowest for low protein diet (12% of calories). Diets with high sucrose (76.5% of calories) and low saturated fat (12.5% of calories) containing no added cholesterol were tested in squirrel and spider monkeys and produced a consistent serum total cholesterol response; the alpha-lipoprotein response was significantly higher in squirrel monkeys than in spider monkeys. The above findings have implications in experimentally induced and comparative atherogenesis.

Species differences in the activity of a serum triglyceride transferring factor

Low density lipoproteins (LDL), endogenously labelled with 3H in the triglyceride moiety, were isolated from rabbit serum and subsequently incubated in vitro at 37 degrees C with unlabelled preparations of rabbit high density lipoproteins (HDL) or very low density lipoproteins (VLDL). In incubations performed in the presence of phosphate buffer, there was no significant transfer of [3H]triglyceride from LDL to either HDL or VLDL; but when rabbit lipoprotein-free serum (the dialysed 1.21 g/ml infranatant) was added, transfer was apparent to both HDL and VLDL. The triglyceride transferring activity of the lipoprotein-free serum was abolished by heating at 85 degrees C for 10 min; all the transferring activity was found in the fraction which precipitated with ammonium sulphate at a concentration of less than 50% saturation. In direct contrast to the rabbit studies, rat serum failed to show a comparable process of triglyceride transfer. In subsequent experiments, mixtures of labelled LDL and unlabelled VLDL isolated either from rabbits or from rats were incubated with lipoprotein-free rabbit, rat or human serum. The lipoprotein-free serum of both the rabbit and man was effective in promoting transfer of 30--50% of LDL [3H]triglyceride into VLDL, regardless of the species origin of the lipoproteins. By contrast the lipoprotein-free serum of rats was only slightly more effective than buffer alone in promoting such transfers. It has been concluded that rabbit and human serum contains a triglyceride transferring factor of far greater activity than that in rat serum.

Stimulation of cholesterol esterification in hepatic microsomes by lipoproteins from normal and hypercholesterolemic rabbit serum

Incubation of plasma lipoproteins with rabbit hepatic microsomes enriched the microsomes with free cholesterol and stimulated cholesterol esterification. The rate of cholesterol esterification correlated well (r = 0.96) with the concentration of microsomal free cholesterol. Lipoproteins from normal and hypercholesterolemic serum varied in their propensity to stimulate cholesterol esterification. Among the normal lipoproteins, low density lipoproteins was more stimulatory than either high density lipoproteins or intermediate density lipoproteins. However, the intermediate density lipoproteins fraction from hypercholesterolemic serum was consistently more stimulatory than any of the normal lipoproteins. The augmentation of cholesterol content, when microsomes were exposed to mixed hyperlipidemic lipoproteins, was proportionately much greater than augementation of phospholipid or protein concentration.

A review of the unique features of HDL apoproteins

The human plasma high density lipoproteins (HDL) are a heterogeneous ensemble of five proteins associated with both neutral and polar lipids. The sequence of all five proteins are known. ApoA-I and apoA-II are the major protein components; apoC-I, apoC-II and apoC-III are the minor protein components. All these apoproteins spontaneously recombine with phospholipids to give stable lipid-protein complexes and freely exchange between the two major HDL subclasses, HDL2 and HDL3. In addition, ApoC-I, apoC-II, and apoC-III exchange between HDL and very low density lipoproteins. Furthermore, certain HDL apoproteins are activators for plasma enzymes that are important in lipid metabolism. ApoA-I and apoC-I activate lecithin/cholesterol acyltransferase; apoC-II is an activator of lipoprotein lipase. The regions of apoC-I and apoC-II that are involved in the activation of these enzymes have been localized with synthetic peptides. Studies of synthetic and native fragments of apoA-II, apoC-I, apoC-II, and apoC-III as well as model lipid-binding peptides have identified specific regions with structural features common to lipid-binding proteins. These special properties, which include helical potential, sequences with a critical amphipathic length, and high hydrophobicity of the nonpolar side of the amphipathic helix, are the determinants of HDL structure and metabolism.

Metabolism of apolipoprotein B-containing lipoproteins in familial hypercholesterolaemia: effects of plasma exchange

The turnover of apolipoprotein B (apo B) in very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) was investigated in 2 homozygous and 3 heterozygous patients with familial hypercholesterolaemia. The effects of a marked reduction in plasma LDL concentration, brought about by plasma exchange, upon apo B turnover were studied in 4 patients. Specific activity-time curves for the the plasma apo B after intravenous radioactive VLDL before plasma exchange indicated that in the heterozygotes all IDL-apo B was derived from VLDL and all LDL-apo B was derived from IDL, but the curves from the homozygotes showed that a significant fraction of the LDL in the plasma was not derived from IDL. Plasma exchange did not increase the rates of synthesis of LDL-apo B or VLDL-apo B and had no significant effect on the precursor--product relationship between IDL-apo B and LDL-apo B in heterozygous or homozygous patients. These findings provide no support for the hypothesis that apo B synthesis is controlled by the plasma LDL.

Very low density and low density lipoprotein subfractions in type III and type IV hyperlipoproteinemia. Chemical and physical properties

Subfractions of CLDL (VLDL), Sf 100-400; CLDL2, Sf 60--100; VLDL3, Sf 20--60) and LDL (LDL), Sf 12--20; LDL2, Sf 6--12; LDL3, Sf 3--6) were isolated from the plasma of three normal, three type III and four type IV hyperlipoproteinemic subjects. In the type IV group, all VLDL subspecies were of normal composition but were increased in concentration in the order VLDL1 greater than VLDL2 greater than VLDL3. In the same subjects, although LDL2 was lowered and LDL3 increased, the total plasma LDL concentration was normal. All VLDL subfractions were elevated in the type III group, but in this case VLDL3 predominated. These subfractions were enriched in cholesteryl esters and depleted in triglyceride. In the LDL density range there was a shift of mass towards the least dense fraction, LDL1, which was of normal composition. EPR studies of the VLDL and LDL subfractions in a type IV subject demonstrated a decrease in fluidity with increasing density. The major change occurred between VLDL3 and LDL1 and was attributed to a substantial alteration in the cholesteryl ester : triglyceride ratio in the particle. A similar argument was used to explain thction in normal or type IV subjects. Particle diameters, determined by laser light-scattering spectroscopy were in good agreement with the values obtained by electron microscopy. This study provides a baseline for the examination of the relationship between the physical and metabolic properties of VLDL and LDL subfractions in type III and IV hyperlipoproteinemia.

Reversible abnormalities of low density lipoprotein composition in familial hypercholesterolaemia

Low density lipoprotein (LDL) composition was analysed in twenty-one patients with familial hypercholesterolaemia, including six homozygotes. By comparison with nineteen controls the patients' LDL had an increased ratio of cholesterol:phospholipid and a decreased ratio of lecithin:sphingomyelin; these changes were more marked in homozygotes than in heterozygotes. Treatment of sevel patients with plasma exchange temporarily resulted in near-normalization of the composition of their LDL. These results suggest that abnormalities of LDL composition in familial hypercholesterolaemia are secondary to hypocatabolism of LDL, which prolongs the half-life of LDL and thus increases the mean age of the population of particles circulating in plasma.

Lipodystrophy with hyperlipidaemia: the role of insulin in very low density lipoprotein over-synthesis

A patient with partial lipodystrophy is described in whom hypertriglyceridaemia was accompanied by marked hyperinsulinaemia. The hyperlipidaemia was due to increased plasma levels of very low density lipoprotein (VLDL). Kinetic studies, performed after injection of autologous radioiodinated VLDL, indicated that the raised VLDL levels were associated with over-production of this lipoprotein. Administration of diazoxide led to a substantial fall in serum insulin levels, accompanied by reduction in VLDL production and in serum triglyceride concentration. The possible role of insulin in inducing hyperlipidaemia by causing over-production of VLDL is discussed.

[Tangier-disease (author's transl)]

Tangier disease is a rare autosomal recessive lipid transport disease characterized by the absence of the usual high density lipoproteins from plasma and cholesteryl ester storage in many organs. 25 cases of Tangier disease have been described so long. The predominant clinical symptoms include tonsilar hypertrophy, splenomegaly and peripheral neuropathy. The cholesteryl ester storage is limited to macrophages, Schwann's cells and intestinal smooth muscle cells. Hypocholest erolemia (less than 80mg/dl), hypertriglyceridemia (greater than 200 mg/dl), and the absence of high density lipoproteins in agarose electrophoresis are the major plasma abnormalities. The protein moiety of normal high density lipoprotein consists of apoprotein A-I and apoprotein A-II. In Tangier disease, serum concentrations of these apoproteins are reduced to less than 1% and 5-10%, respectively. Theories concerning the pathogenesis of Tangier disease are only incomplete and unproved up to now; however, a structural abnormality of apoprotein A-I causing an inability to bind to lipid or other proteins (apoprotein A-II) is consistent with several of the recent biochemical findings. The imbalance of cellular cholesterol metabolism caused by the absence of high density lipoproteins as well as the presumed role of these lipoproteins in cholesterol removal from cells are discussed in this article.

Isolation and characterization of a larval lipoprotein from the hemolymph of Manduca sexta

1. The larval high density lipoprotein (HDL) from the hemolymph of Manduca sexta, isolated by density gradient centrifugation, contains 61% protein, 37% lipid and 2% carbohydrate. 2. The molecular weight of HDL is 6 x 10(5), with two apoproteins of 2.85 x 10(5) and 8.1 x 10(4) daltons. 3. The large apoprotein is destroyed by trypsin treatment of the particle, while the small one is not. 4. Calculations based upon size and composition show that this particle is very different in structure from mammalian lipoproteins. It is proposed that a portion of the apoprotein occupies the central core region.

Apoprotein A-I synthesis in normal intestinal mucosa and in Tangier disease

To determine whether human small intestine synthesizes apoA-I, the major apoprotein of plasma high-density lipoproteins, we used immunofluorescence technics and monospecific antiserums to visualize apoA-I within intestinal epithelial cells from four normal subjects and one patient with Tangier disease. Biopsies from all subjects during fasting showed limited fluorescence. After lipid feeding intracellular apoA-I markedly increased in both normal subjects and the patient. During alimentary lipemia, mean plasma apoA-I levels (milligrams per deciliter) increased in four normal subjects from 161 +/- 12 (+/- S.E.M.) to 180 +/- 15 (P less than 0.05) and in the patient from 1.9 to 6.8. Normal plasma chylomicrons contained apoB, apoE and the C peptides but not apoA-I. The patient's chylomicrons contained ap0A-I. Normal and Tangier-disease intestinal-mucosa cells increase their content of apoA-I during chylomicron formation and subsequently contribute to plasma apoA-I levels. The low levels of apoA-I in Tangier disease are not due to a failure of intestinal synthesis but might be due to abnormal metabolism of chylomicron apoproteins.

Metabolism of esterified cholesterol in the plasma very low density lipoproteins of the rabbit

The in vivo metabolism of esterified cholesterol and triglyceride in plasma very low density lipoproteins (VLDL) has been studied in postabsorptive rabbits injected with endogenously 3H-labelled preparations of VLDL. The rates at which the injected esterified [3H]cholesterol and [3H]triglyceride were removed from the recipient VLDL fraction were remarkably similar. But, whereas most of the [3H]triglyceride was rapidly lost from the plasma compartment, more than 60% of the esterified [3H]cholesterol removed from VLDL was recovered in the plasma low density lipoprotein (LDL) and high density lipoprotein (HDL) fractions, an observation in direct contrast to previous findings in rats and guinea pigs in which hepatic uptake accounted for the major proportion of esterified cholesterol removed from VLDL. The appearance of VLDL-esterified [3H]cholesterol in rabbit LDL was quite compatible with the well-documented catabolic conversion of VLDL to LDL. The appearance in HDL, on the other hand, could not be so explained and was further investigated in studies performed in vitro. It was found that, concurrent with a net mass transfer of esterified cholesterol from HDL to VLDL, there was a much more rapid transfer of esterified [3H]cholesterol in the reverse direction, implying the existence of a process of molecular exchange of esterified cholesterol between the two fractions, analogous to that recently described between VLDL and LDL.

Electron-microscopic evidence for particles smaller than 250 A in very low density lipoproteins of human plasma

Particle size distributions of very low density lipoproteins (VLDL) in man were examined by electron microscopy. A comparison was made between normolipidemic and hyperlipidemic subjects. The results showed that the VLDL fraction frequently contained particles between 100--250 A in diameter and in some cases particles less than 100 A were observed.

Impaired very low density lipoprotein and triglyceride removal in broad beta disease: comparison with endogenous hypertriglyceridemia

The removal rate of apoprotein-B (apo B) in very low density lipoprotein (VLDL) was decreased in individuals with broad beta disease when compared with endogenous hypertriglyceridemia. Following the injection of 125I-VLDL isolated from individuals with endogenous hypertriglyceridemia, both VLDL apo B fractional catabolic rate (0.058 +/- 0.029 hr-1) and VLDL apo-B turnover rate (0.300 +/- 0.070 mg/kg/hr) were lower in broad beta disease than in endogenous hypertriglyceridemia (fractional catabolic rate 0.112 +/- 0.046, p less than .05; turnover rate 0.640 +/- 0.199, p less than .005) despite equivalent plasma concentrations of VLDL-apo-B. Furthermore, conversion of VLDL apo-B to LDL was impaired in broad beta disease relative to endogenous hypertriglyceridemia. Differences in the kinetics of lipoprotein lipase-related triglyceride removal during a maximal heparin infusion were also demonstrated between these two disorders. These differences suggest an abnormality in the interaction of lipoprotein lipase with the lipoproteins of unusual composition in broad beta disease. This is further supported by the normalization of lipoprotein composition in broad beta disease by estrogen therapy, with a simultaneous change in the kinetics of lipoprotein lipase-related triglyceride removal towards those seen in endogenous hypertriglyceridemia.

Pathophysiology of lipoprotein transport

A system for classification of genetic and acquired forms of hyperlipidemia in humans based on lipoprotein physiology is described. Most hyperlipidemia can be accounted for by defects in one of four sites of physiologic regulation: (1) triglyceride-rich lipoprotein production, (2) lipoprotein lipase-mediated triglyceride catabolism, (3) remnant lipoprotein catabolism, and (4) extrahepatic cholesterol-rich lipoprotein catabolism.

Lipoprotein lipase activator deficiency in very low density lipoproteins in rat nephrotic syndrome

Marked urinary loss of lipoprotein lipase activator in experimental rat nephrotic syndrome may be partly responsible for its deficiency in plasma very low density lipoproteins.

The degradation of very low density lipoprotein by the extrahepatic tissues of the rat

The supradiaphragmatic rat was used to investigate the metabolism by the extrahepatic tissues of endogenous plasma VLDL of d less than 1.006 g/ml. The demonstration that, at 20, 30 and 40 min after the isolation of the supradiaphragmatic rat, the VLDL lose respectively 29, 54, and 63% of their triglyceride provides evidence for the suitability of this preparation for the investigation of VLDL degradation. At all time intervals after the isolation of the supradiaphragmatic rat, VLDL triglyceride loss was accompanied by similar losses of cholesterol, protein and phospholipid, with the result that the percentage by weight composition of the residual VLDL remained unaltered. By subfractionation of the VLDL, a group of particles with an Sf range of 20--60 were isolated that, when compared with total VLDL, were enriched in their cholesterol (P less than 0.02), protein (P less than 0.001) and phospholipid (P less than 0.01) content. However, these particles represented only a small percentage of the total VLDL mass. Furthermore, their amount was not increased in the circulation of the supradiaphragmatic rat. The amount of IDL (d = 1.006--1.019 g/m) and of LDL (d = 1.019--1.063 g/ml) was increased in the supradiaphragmatic rat and a part of the total cholesterol and protein lost from the VLDL could be accounted for by the increases in these constituents in the IDL and LDL fractions. It is suggested that, although the liver probably takes up partial degradation products of VLDL in the intact animal, the extrahepatic tissues alone can metabolize VLDL to LDL of d = 1.019--1.063 g/ml. The lipoprotein particles taken up by the liver in the intact animal appear most likely to be those of Sf greater than 100.

Transfer of phosphatidylcholine facilitated by a component of human plasma

A constituent of lipoprotein-free (p greater than 1.21) human plasma from normolipemic donors facilitates the transfer of diacyl phosphatidylcholine from unilamellar egg yolk phosphatidylcholine liposomes to liver mitochondria. The active component is heat labile, has a hydrated density greater than 1.25 and an apparent molecular weight of more than 100 000. The presence of this protein in plasma may facilitate movement of diacylphospholipids between the surfaces of lipid-containing particles such as lipoproteins and erythrocytes. Knowledge of the properties and behavior of this protein are important in designing methods of drug therapy based on encapsulation in biodegradable lipid vesicles.

Proteolysis of very low density lipoprotein in perfused lung

Perfusion of homologous 125I-labeled rat very low density lipoprotein through isolated rat lungs in the presence of heparin resulted in apoprotein proteolysis. At least the apoprotein C was degraded into two peptides smaller than 7500 daltons as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The lung uptake of radioactivity was small and due mainly to the presence of the larger of the two peptides. The lung protease was not active against an 125-I-labeled albumin substrate and was not released into the medium by heparin.

The catabolism of human and rat very low density lipoproteins by perfused rat hearts

The catabolism of human and rat 125I-labelled very low density lipoproteins (VLDL) was compared by perfusing the lipoproteins through beating rat hearts. Triacylglycerol was removed from the VLDL to a greater extent than the protein moiety, leaving remnants containing relatively more apo-B and less apo-C. The change in apo-C content of the remnants correlated with the loss of triacylglycerol. The extent of removal of triacylglycerol from the rat and human VLDL was similar and in most cases appeared to saturate the heart lipoprotein lipase. The remnants were slightly smaller in size than the VLDL, and included particles which appeared to be partially emptied. In addition to remnants of d less than 1.019 g/ml, iodinated lipoproteins derived from rat and human VLDL were recovered at d 1.019-1.063 and 1.063-1.21 g/ml. The former contained largely cholesterol and cholesteryl esters, while phospholipids were the dominant lipid in the latter. An average of 40% of the 125I-labelled apoprotein lost from the VLDL was associated with the perfused hearts. Very little d 1.019-1.063 g/ml lipoprotein was produced from low (physiological) concentrations of rat VLDL, most of the lipoprotein being removed by the heart. However, lipoproteins of density 1.019-1.063 g/ml were formed from human VLDL at all concentrations in the perfusate, as well as from higher concentrations of the rat VLDL. Agarose gel filtration of lipoproteins following heart perfusion with human VLDL revealed large aggregates containing particles which resemble low density lipoproteins (LDL) in electron microscopic appearance and apoprotein composition, since they contain largely apo-B. These data suggest that at normal concentrations rat VLDL are almost completely catabolised and taken up by the heart without the formation of LDL, while LDL is produced from human VLDL at all concentrations.

Changes in rat serum lipoproteins following ligation of the bile duct

Lipoproteins in the serum of normal and of cholestatic rats have been studied by crossed immunoelectrophoresis coupled with separations by rate and isopycnic density gradient centrifugation and by gel exclusion chromatography. Normal rat serum contained distinct lipoprotein species closely analogous to human VLDL, LDL, HDL2 and HDL3. Three days after ligation of the common bile duct, there were major changes in the lipoproteins of rat serum. The amounts of VLDL, LDL and of a minor HDL component were elevated and several novel types of lipoprotein were detected. Three of these could be identified as the characteristic lipoprotein of cholestasis, LP-X, as an enlarged and modified HDL and as a lipoprotein of density 1.055 g/ml intermediate in size between LDL and VLDL. Some unusually small VLDL particles were also detected. It is concluded that the changes in rat serum lipoproteins following ligation of the common bile duct are very similar to the changes observed in cholestatic disease in human patients.

A comparison of the composition and apolipoprotein content of the lipoproteins isolated from human and ferret (Mustela putorius furo L.) serum

1. The lipoproteins isolated at densities of less than 1.006, 1.006-1.063 and 1.063-1.21 g/ml from human and ferret (Mustela putorius furo L.) serum were compared. 2. Ferret very low density lipoprotein contained proportionately less triglyceride and more phospholipid than human. 3. Ferret low density lipoprotein contained proportionately more triglyceride and less cholesterol than human. 4. High density lipoprotein was the major lipoprotein in ferret serum. 5. The gel electrophoretic patterns of lipoprotein apoproteins and the pattern of apoprotein solubility in tetramethylurea were similar for human and ferret fractions. 6. The ferret may provide a convenient animal for the study of serum lipoprotein structure, function and metabolism.

Peripheral vascular disease in cigarette smokers and impaired hepatic metabolism of lipoprotein remnants

The metabolism of very-low-density lipoproteins and chylomicrons includes the extrahepatic hydrolysis of their triglycerides by lipoprotein lipase. This results in cholesterol-rich "remnants" which are further metabolised by the liver. There is experimental evidence that in both patients with type-III hyperlipoproteinaemia and cigarette smokers hepatic-remnant metabolism may be depressed. In type-III hyperlipoproteinaemia the defect is inherited while in smokers it occurs in response to raised blood concentrations of carboxyhaemoglobin. The striking clinical similarity between type-III hyperlipoproteinaemic patients and smokers--namely, a high incidence of peripheral vascular disease--may be due to a common cause, the accumulation of cholesterol-rich remnants in the plasma.