Isolation and characterization of hepatic Golgi lipoproteins from hypercholesterolemic rats

Unavailability of liver triacylglycerol increases serum cholesterol concentration induced by dietary cholesterol in exogenously hypercholesterolemic (ExHC) rats

BACKGROUND

Exogenously hypercholesterolemic (ExHC) rats develop hypercholesterolemia and low hepatic triacylglycerol (TAG) levels when dietary cholesterol is loaded. The responsible gene Smek2 was identified via linkage analysis using the original strain Sprague-Dawley (SD) rats. In this study, we compared SD and ExHC rats to investigate a relationship between hypercholesterolemia and the low hepatic TAG levels observed in ExHC rats.

METHODS

Male 4-weeks-old ExHC and SD rats were fed a 1% cholesterol diet for 1 week. Serum and liver parameters were analyzed. Gene expression and enzyme activities related to TAG metabolism were also assessed.

RESULTS

We reproducibly observed higher serum cholesterol and lower hepatic TAG levels in ExHC rats than in SD rats. Golgi apparatus in the livers of ExHC rats secreted β-very-low-density lipoprotein (β-VLDL) that had higher cholesterol ester (CE) and lower TAG content than those in the β-VLDL secreted by SD rats. Gene expression related to fatty acid and TAG synthesis in ExHC rats was lower than that in SD rats. Enzymatic activities for fatty acid synthesis were also relatively lower in ExHC rats. Moreover, the fatty acid composition of hepatic and serum CE in ExHC rats showed that these CEs were not modified after secretion from the liver despite the similar activities of serum lecithin-cholesterol acyltransferase (LCAT) in ExHC rats to those in SD rats.

CONCLUSIONS

Low production of liver TAG and secretion of CE-rich, TAG-poor β-VLDL without modification by LCAT in the circulation contributed to hypercholesterolemia induced by dietary cholesterol in ExHC rats.

Golgi-associated maturation of very low density lipoproteins involves conformational changes in apolipoprotein B, but is not dependent on apolipoprotein E

The major protein component in secreted very low density lipoproteins (VLDL) is apoB, and it is established that these particles can reach sizes approaching 100 nm. We previously employed a cell-free system to investigate the nature of the vesicles in which this large cargo exits the endoplasmic reticulum (ER) (Gusarova, V., Brodsky, J. L., and Fisher, E. A. (2003) J. Biol. Chem. 278, 48051-48058). We found that apoB-containing lipoproteins exit the ER as dense lipid-protein complexes regardless of the final sizes of the particles and that further expansion occurs via post-ER lipidation. Here, we focused on maturation in the Golgi apparatus. In three separate approaches, we found that VLDL maturation (as assessed by changes in buoyant density) was associated with conformational changes in apoB. In addition, as the size of VLDL expanded, apoE concentrated in a subclass of Golgi microsomes or Golgi-derived vesicles that co-migrated with apoB-containing microsomes or vesicles, respectively. A relationship between apoB and apoE was further confirmed in co-localization studies by immunoelectron microscopy. These combined results are consistent with previous suggestions that apoE is required for VLDL maturation. To our surprise, however, we observed robust secretion of mature VLDL when apoE synthesis was inhibited in either rat hepatoma cells or apoE(-/-) mouse primary hepatocytes. We conclude that VLDL maturation in the Golgi involves apoB conformational changes and that the expansion of the lipoprotein does not require apoE; rather, the increase in VLDL surface area favors apoE binding.

Recycling of apolipoprotein E in mouse liver

Following the internalization of low density lipoprotein (LDL) by the LDL receptor within cells, both the lipid and the protein components of LDL are completely degraded within the lysosomes. Remnant lipoproteins are also internalized by cells via the LDL receptor as well as other receptors, but the events following the internalization of these complexes, which use apolipoprotein E (apoE) as their ligand for receptor capture, have not been defined. There is evidence that apoE-containing beta-very low density lipoproteins follow differential intracellular routing depending on their size and apoE content and that apoE internalized with lipoproteins can be resecreted by cultured hepatocytes and fibroblasts. In the present studies, we addressed the question of apoE sparing or recycling as a physiologic phenomenon. Remnant lipoproteins (d < 1.019 g/ml) from normal mouse plasma were iodinated and injected into normal C57BL/6 mice. Livers were collected at 10, 30, 60, and 120 min after injection, and hepatic Golgi fractions were prepared for gel electrophoresis analysis. Golgi preparations were analyzed for galactosyltransferase enrichment (>40-fold above cell homogenate) and by appearance of the Golgi stacks and vesicles on electron microscopy. Iodinated apoE was consistently found in the Golgi fractions peaking at 10 min and disappearing by 2 h after injection. Although traces of apoB48 were present in the Golgi fractions, the apoE/apoB ratio in the Golgi was 50-fold higher compared with serum. Quantitatively similar results were obtained when the very low density lipoprotein remnants were injected into mice deficient in either apoE or the LDL receptor, indicating that the phenomenon of apoE recycling is not influenced by the production of endogenous apoE and is not dependent on the presence of LDL receptors. In addition, radioactive apoE in the Golgi fractions was part of d = 1.019-1.21 g/ml complexes, indicating an association of recycled apoE with either newly formed lipoproteins or the internalized complexes. These studies show that apoE recycling is a physiologic phenomenon in vivo and establish the presence of a unique pathway of intracellular processing of apoE-containing remnant lipoproteins.

Effects of diets containing tallow and soybean oil with and without cholesterol on hepatic metabolism of lipids and lipoproteins in the preruminant calf

The effects of long-chain fatty acids (230 g/kg of dietary DM) from tallow and from soybean oil, with or without cholesterol (10 g/kg of dietary DM), on hepatic lipid contents and on in vivo hepatic production rates of lipids and lipoproteins were investigated in 22 preruminant male calves fitted with chronic catheters and with electromagnetic blood flow probes implanted in the hepatic vessels. Diets containing soybean oil and soybean oil with cholesterol led to the development of triglyceride infiltration in the liver and to higher apparent hepatic secretion of very low density lipoproteins than did diets containing tallow or tallow with cholesterol. Addition of cholesterol to diets favored accumulation of low density lipoproteins in plasma and the net apparent secretion of these particles by the liver, especially for the diet containing soybean oil with cholesterol. Regardless of the diet, calf liver clearly removed large high density lipoproteins of type 1 that were rich in cholesteryl esters but secreted heavy high density lipoproteins that were rich in proteins. The intensity of removal of high density lipoproteins of type 1 by the liver depended on the plasma concentration of these particles, probably by mass action. This removal did not prevent the accumulation of high density lipoproteins of type 1 in plasma, such as it did in calves fed soybean oil.

Hepatic processing and biliary secretion of the cholesteryl esters from beta very-low-density lipoproteins in the rat

beta-Migrating very-low-density lipoproteins (beta-VLDL) are cholesteryl-ester-enriched lipoproteins which accumulate in the serum of cholesterol-fed animals or patients with type III hyperlipoproteinemia. In the rat, beta-VLDL are rapidly cleared by the liver and parenchymal liver cells form the major site for uptake. In this investigation, beta-VLDL were labeled with [3H]cholesteryl esters and the hepatic intracellular transport of these esters was followed. 2 min after injection, the major part of the [3H]cholesteryl esters is already associated with the liver and a significant proportion is recovered in endosomes. Up to 25 min after injection, an increase in radioactivity in the lysosomal compartment is noticed. This radioactivity initially represents cholesteryl esters, while from 25 min onward, radioactivity is mainly present in unesterified cholesterol. Between 45 min and 90 min after beta-VLDL injection, specific transfer of unesterified [3H]cholesterol to the endoplasmic reticulum is observed, while by 3 h the majority is located in this fraction. The appearance of radioactivity in the bile was rather slow as compared to the rapid initial uptake and processing, and up to 5 h after injection only 10% of the injected dose had reached the bile (mainly as bile acids). 72 h after injection, the amount of the injected radioactivity recovered in the bile had increased to 50%. Chloroquine treatment of the rats inhibited the hydrolysis of the cholesteryl esters and the appearance of radioactivity in the bile was retarded. It is concluded that beta-VLDL are rapidly processed by parenchymal liver cells and that the cholesteryl esters from beta-VLDL are hydrolyzed in the lysosomal compartment. Unesterified cholesterol remains associated with the endoplasmic reticulum for a prolonged time, although ultimately the majority will be secreted into the bile as bile acids. The effective operation of this pathway will prevent extrahepatic accumulation of cholesteryl esters from beta-VLDL, while the prolonged residence time of unesterified cholesterol in the endoplasmic reticulum might be important for regulation of low-density lipoprotein (LDL) receptors in liver and thus for LDL levels in the blood.

Hepatocytic lipoprotein receptors and intracellular lipoprotein catabolism

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

Induction of long-lasting hypercholesterolemia in the rat fed a cystine-enriched diet

The influence of dietary excess (5%) of L-cystine on rat plasma lipoproteins was examined. After only one week of cystine feeding, an increase in the plasma cholesterol level and a decrease in triglyceride levels were observed. The increase in cholesterol level became greater when the duration of cystine-enriched diet increased until eight weeks (+131% after eight weeks), but no further increase occurred between 8 and 20 weeks. This change was essentially due to the progressive increase in cholesterol levels in high density lipoproteins (HDL) and in lipoproteins isolated between 1.040 and 1.063 g/ml, i.e., certain low density lipoproteins (LDL2), and containing mainly apoE-rich lipoproteins (HDL1). The decrease in plasma triglycerides resulted from that of chylomicrons and very low density lipoproteins (VLDL). The effects observed after four or eight weeks of cystine feeding were maintained for eight weeks after replacing the cystine diet by the standard diet. Ingestion of the standard diet containing either cholestyramine (2%) or probucol (0.25%) following eight weeks of cystine feeding significantly decreased plasma cholesterol levels. It is concluded that cystine-fed rats are a useful tool of investigation for understanding mechanisms leading to increased plasma cholesterol level and for hypocholesterolemic drug trials.

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.

Metabolism of canine beta-very low density lipoproteins in normal and cholesterol-fed dogs

Cholesteryl ester-rich beta-very low density lipoproteins (beta-VLDL) are beta-migrating lipoproteins that accumulate in the plasma of cholesterol-fed animals and of patients with type III hyperlipoproteinemia. There are two distinct fractions: fraction I beta-VLDL are chylomicron remnants of intestinal origin, and fraction II beta-VLDL are cholesterol-rich VLDL of hepatic origin. The liver rapidly clears fraction I beta-VLDL from the plasma of both normal and cholesterol-fed dogs. The liver also clears fraction II beta-VLDL rapidly and efficiently from the plasma of normal dogs by receptor-mediated uptake. In cholesterol-fed dogs the clearance is biphasic: an initial rapid die-away of about 30% to 40% of the injected dose within 5 minutes, followed by a slow clearance of plasma radioactivity (a half-life of more than 20 hours). The rapid, initial phase of fraction II beta-VLDL clearance appears to be related to sequestration of the lipoproteins presumably on endothelial cells and is apparently associated with lipolytic processing. Treatment of the fraction II beta-VLDL with lipoprotein lipase abolishes this rapid phase. In the cholesterol-fed dog, the slow, late phase of clearance corresponds to the conversion of fraction II beta-VLDL to the smaller, denser intermediate and low density lipoproteins (IDL and LDL), which are slowly cleared from the plasma. It is concluded that fraction II beta-VLDL are catabolized in the normal dog by rapid uptake mediated at least in part by the apo B,E(LDL) receptor of hepatic parenchymal cells. In cholesterol-fed dogs, in which these receptors are markedly down-regulated, fraction II beta-VLDL are apparently initially bound to endothelial cells and converted to IDL and LDL by lipolytic processing.

The biochemistry of lipoproteins

Lipids are transported in the blood in four major classes of lipoproteins. The triacylglycerol-rich lipoproteins are chylomicrons and very-low-density lipoproteins (VLDL) which are produced by the small intestine and liver, respectively. These lipoproteins mainly carry fatty acids to adipose tissue and muscle where the triacylglycerol is hydrolysed by lipoprotein lipase. The resulting particles that remain in the blood are chylomicron remnants and low-density lipoprotein (LDL), respectively. The remnant is taken up by the liver via endocytosis which is mediated by a specific receptor for apolipoprotein E (apoE). LDL, which are rich in cholesterol, can also be taken up by the liver or extrahepatic tissues by a receptor-mediated endocytosis that specifically recognises apoB or apoE. 'Nascent' high-density lipoprotein (HDL) particles are secreted by the liver and intestine and then undergo modification to become HDL3 and then HDL2 as they acquire cholesterol ester. They facilitate the reverse transport of cholesterol back to the liver. Little is known of the hormonal regulation of lipoprotein uptake by the liver. Recently, we have shown that insulin and tri-iodothyronine (T3) increase the specific binding of LDL to cultured hepatocytes whereas dexamethasone (a synthetic glucocorticoid) has the opposite effect. The changes in binding produced by insulin and dexamethasone are paralleled by alterations in the rate of degradation of apoB. These findings may in part explain the hypercholesterolaemia and increased risk of premature atherosclerosis that can be associated with poorly controlled diabetes or hypothyroidism.

Nutrition and biogenesis of plasma lipoproteins in nonhuman primates

In this article we examine the production of very low-density lipoprotein (VLDL) by perfused livers obtained from chow- and cholesterol-fed nonhuman primates. These data illustrate two important features of VLDL production. First, VLDL is secreted from the liver in a form very close to that of its plasma counterpart. Thus for chow-fed animals, plasma VLDL and liver perfusate VLDL have similar lipid compositions. Second, the composition of VLDL can be modified significantly by diet in each of two primate species, the Rhesus monkey and the baboon. Rhesus monkey livers uniformly secrete larger quantities of VLDL and show more dramatic dietary effects than do baboon livers. Nevertheless, perfused livers from both species reveal qualitatively similar responses to dietary peanut oil and to lard fed in combination with cholesterol. Both fat-containing diets induce the livers to secrete VLDL enriched in cholesteryl ester compared with control perfusates yet still cholesteryl ester deficient compared with the animals' plasma VLDL. Peanut oil diet reduces the hepatic output of VLDL-associated apoprotein B and triglyceride, whereas lard increases hepatic secretion of VLDL-associated lipids and apoprotein E. We conclude that the nature of dietary fat plays an important role in determining the profile and composition of lipoproteins formed and secreted by the primate liver. We have also briefly reviewed the production of high-density lipoprotein, which is probably formed in the plasma from many sources, with special emphasis on the possible role of newly secreted lipid-poor apolipoprotein A-I and A-II.

Hypo- and hyperresponders: individual differences in the response of serum cholesterol concentration to changes in diet

The feeding of cholesterol-rich diets to random-bred animals results in marked interindividual differences in the response of serum cholesterol. Certain animals show only small responses (hyporesponders), whereas others develop high degrees of hypercholesterolemia (hyperresponders). Inbred strains of rabbits, rats, and mice differing in their sensitivity to dietary cholesterol are available. In these animals, and also in monkeys, the responsiveness to high-cholesterol diets has a strong genetic basis. The existence of hyper- and hyporesponders also holds in humans, though not as pronounced as in laboratory animals. Repeated trials with the same subjects have shown that persons exist with a consistently low or high response to increased intakes of cholesterol. However, "spontaneous," diet-independent within-person variations in the level of serum cholesterol markedly inflate the between-person variation in the response of serum cholesterol; both variations are of the same order of magnitude. Hypo- and hyperresponsiveness to dietary cholesterol extends to other hypercholesterolemic components of the diet. In humans and rabbits hyperresponsiveness to dietary cholesterol is associated with responsiveness to dietary saturated fatty acids. The mechanisms underlying hypo- and hyperresponsiveness to dietary cholesterol have not yet been unraveled. On the basis of available data, we propose that in hyperresponders, compared with hyporesponders, there is a higher hepatic efflux of cholesterol in low-density lipoproteins (LDL), or its precursors, after cholesterol consumption. This may be caused by insufficient inhibition of cholesterol biosynthesis and/or the high capacity of cholesterol absorption in the hyperresponders. The stimulation of LDL production accounts for the increase in LDL cholesterol in serum. The number of hepatic LDL receptors, which may be already decreased in hyperresponders, will decrease further through down-regulation. The receptor-mediated LDL clearance decreases, but the absolute amount of LDL cholesterol taken up by the cells via the receptor and by the receptor-independent pathway increases because of the increased level of LDL cholesterol. In this way a new equilibrium is reached in which LDL production equals LDL catabolism. The phenomenon of hypo- and hyperresponsiveness may have implications for counseling subjects who attempt to lower their serum cholesterol by diet. However, identification of true hyper- and hyporesponders is greatly hampered by within-person fluctuations of the level of serum cholesterol. No simple test is available to discriminate hypo- from hyperresponders.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of dietary cholesterol on biliary cholesterol content and bile flow in the hypothyroid rat

The hypothyroid rat model was used to investigate the effect of dietary-induced hypercholesterolemia on biliary cholesterol content and bile flow. Rats were divided into four dietary groups--diet A: Rat Chow; diet B: Rat Chow plus 0.1% propylthiouracil; diet C: Rat Chow plus 0.1% propylthiouracil, 0.3% taurocholate, 5% lard; diet D: Rat Chow plus 0.1% propylthiouracil, 0.3% taurocholate, 5% lard, and 1% cholesterol. After 6 wk, bile was collected and livers were excised for the preparation of membranes. In cholesterol-fed animals, biliary cholesterol content was increased. However, because of a significant decrease in the rate of bile flow that occurred in these animals, biliary cholesterol output was unchanged from the cholesterol output observed in control animals. Dietary cholesterol also caused a threefold increase in liver membrane cholesterol content and a 64% decrease in the activity of sodium-potassium-stimulated adenosine triphosphatase (Na+,K+-ATPase). In a separate group of animals, microsomes prepared from livers of control rats were incubated with phosphatidylserine liposomes, liposomes containing cholesterol, or buffer. The activity of Na+,K+-ATPase was increased in microsomes incubated with phosphatidylserine liposomes. However, when the cholesterol content of the microsomes was increased twofold by incubating the membranes with liposomes containing cholesterol, the stimulation of Na+,K+-ATPase activity was significantly decreased. The data suggest that in the cholesterol-fed hypothyroid rat, biliary cholesterol content is significantly increased; however, because of a decrease in the rate of bile flow, biliary cholesterol output is not changed. The decrease in bile flow is associated with an accumulation of cholesterol and a decrease in the activity of Na+,K+-ATPase in hepatic membranes.

Effects of high cholesterol high fat diet on plasma lipoproteins in familial hypercholesterolemia

Heterozygous individuals with familial hypercholesterolemia possess about half of the normal numbers of functioning receptors on their cells. This is thought to be responsible for their hypercholesterolemia. In normals, dietary cholesterol increases LDL production and decreases LDL receptor-related LDL clearance, resulting in elevations in LDL cholesterol levels of approximately 30 mg/dL. To assess the effects of high fat and high cholesterol diets on the lipoproteins of individuals with diminished LDL receptors, three kinds of diets, including ones high in cholesterol, were fed to four patients with familial hypercholesterolemia, in the expectation that diet effects on apoB- or apoE-containing lipoproteins would be exaggerated. The basal diet consisted of 15% protein, 30% fat, 55% carbohydrate, 300 mg/d cholesterol, P/S ratio 0.4; the high fat diet was identical except that fat calories were 55% and carbohydrate 30%; the high fat-high cholesterol diet was identical with the high fat diet except approximately 750 or approximately 1,500 mg/d of cholesterol were added. Each diet was eaten for five weeks at home and for the sixth week at the general Clinical Research Center. Fasting (12-14 hours) plasmas were collected every two weeks for lipoprotein-lipid and apoprotein quantitation. At the end of each period, fasting and 4-hour postprandial samples were analyzed also by zonal ultracentrifugation and gel permeation chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of multivesicular bodies from rat hepatocytes: an organelle distinct from secretory vesicles of the Golgi apparatus

Hepatocytes of estradiol-treated rats, which express many low density lipoprotein receptors, rapidly accumulate intravenously injected low density lipoprotein in multivesicular bodies (MVBs). We have isolated MVBs and Golgi apparatus fractions from livers of estradiol-treated rats. MVB fractions were composed mainly of large vesicles, approximately 0.55 micron diam, filled with remnantlike very low density lipoproteins, known to be taken up into hepatocytes by receptor-mediated endocytosis. MVBs also contained numerous small vesicles, 0.05-0.07 micron in diameter, and had two types of appendages: one fingerlike and electron dense and the other saclike and electron lucent. MVBs contained little galactosyltransferase or arylsulfatase activity, and content lipoproteins were largely intact. Very low density lipoproteins from Golgi fractions, which are derived to a large extent from secretory vesicles, were larger than those of MVB fractions and contained newly synthesized triglycerides. Membranes of MVBs contained much more cholesterol and less protein than did Golgi membranes. We conclude that two distinct lipoprotein-filled organelles are located in the bile canalicular pole of hepatocytes. MVBs, a major prelysosomal organelle of low density in the endocytic pathway, contain remnants of triglyceride-rich lipoproteins, whereas secretory vesicles of the Golgi apparatus contain nascent very low density lipoproteins.

Analysis and subcellular localization of lipid in alcoholic liver disease

The nature and subcellular distribution of lipids in alcoholic liver disease have been little studied. Micro-methods for lipid analysis were applied to needle biopsy homogenates and their subcellular fractions. Alcoholic fatty liver was accompanied by a major increase (up to 50 fold) in triglyceride and a smaller (2-3 fold) increase in cholesteryl ester: there was no significant change in the free cholesterol, free fatty acid or phospholipid content. Homogenates were fractionated into macro- and micro-droplets and membrane fractions by differential centrifugation. The subcellular location of the membrane lipids were determined by sucrose density gradient centrifugation in a vertical pocket reorientating rotor. In fatty liver, although there was a 2-3 fold increase in macro-droplet and micro-droplet (tentatively identified as VLDL) lipid, the major increase was in the membrane-bound triglyceride (8-10 fold). Sucrose density gradient centrifugation demonstrated that these membranes had an equilibrium density of 1.12 g/ml, clearly separated from droplet lipid, density less than 1.04 g/ml. The membrane fraction was tentatively identified as Golgi in origin and it is suggested that alcoholic fatty liver in man is due to impaired Golgi secretion of triglyceride-rich lipid complexes.

Studies on the production of low density lipoproteins by perfused livers from nonhuman primates. Effect of dietary cholesterol

Nonhuman primates consuming diets containing cholesterol develop coronary artery atherosclerosis that we have found to be highly correlated with an increase in the size and cholesteryl ester content of plasma low density lipoproteins (LDL). The present studies were designed to determine whether the enlarged plasma LDL are produced directly by the liver of cholesterol-fed monkeys. African green monkeys were fed a diet containing 40% of calories as butter fat and either 0.16 mg cholesterol/kcal (control diet) or 0.78 mg cholesterol/kcal (test diet). The livers of these monkeys were perfused by recirculation with a lipoprotein-free medium for 4 h. The rate of accumulation of perfusate cholesterol was linear and greater in liver perfusates from test diet-fed vs. control diet-fed monkeys and was positively correlated with both the plasma cholesterol concentration and LDL size in the donor animal. All perfusate d less than 1.063 g/ml lipoprotein subfractions from livers of test diet-fed monkeys were enriched in cholesteryl ester severalfold over the corresponding subfractions from control diet-fed monkeys and contained only the larger form of apolipoprotein B typical of plasma LDL. However, the perfusate lipoproteins in the LDL density range did not have an average size or composition typical of LDL from plasma. Rather, they were relatively enriched in phospholipid and unesterified cholesterol and were deficient in cholesteryl esters. In addition, perfusate high density lipoproteins were discoidal particles. These data show that the enzyme lecithin:cholesterol acyltransferase (LCAT) was essentially inactive in these perfusates and, as a result, the dietary cholesterol-induced enrichment of perfusate d less than 1.063 g/ml lipoproteins with cholesteryl esters probably resulted from increased hepatic secretion of cholesteryl esters and not from modification of lipoproteins by LCAT during recirculating perfusion. In spite of this increase, enlarged cholesteryl ester-rich LDL were not found in the perfusate, suggesting that large molecular weight plasma LDL are not directly secreted by the liver but instead probably result from further intravascular metabolism of cholesteryl ester-enriched hepatic precursor lipoproteins.

Preliminary report increased transport of intermediate density lipoprotein (LDL) with cholesterol loading

Although cholesterol loading in man generally leads to an increase in plasma cholesterol and low density lipoprotein cholesterol in particular, the mechanism for this is unknown. Because IDL formation rises in cholesterol-fed rats we have measured the transport of IDL apoprotein B in 8 men during two dietary periods containing either 200 mg or 1700 mg cholesterol per day. Radioiodinated IDL (Sf 12-20 or Sf 12-60) were reinjected and transport calculated from 48 hr specific radioactivity-time curves of apoprotein B in reisolated IDL. With cholesterol loading the IDL cholesterol and apoprotein B concentrations rose in 7 of 8 subjects (substantially in only 2). However IDL transport was stimulated more consistently and significantly (p less than 0.01), rising by at least 50% in 5 subjects. Furthermore the change in plasma total cholesterol concentration was significantly correlated with the change in IDL transport (r = +0.70, p less than 0.05). Since LDL are derived from IDL, the cholesterol-induced rise in IDL formation may explain the increase in LDL concentration with dietary cholesterol.

Qualitative and quantitative changes in hepatic lipoprotein particles following acute injury of the rat liver induced by thioacetamide

Acute intoxication of the rat liver with a single dose of 100 mg thioacetamide (TAA)/kg body weight causes within 48 h a fatty liver and a heterogeneous reaction in the hepatocytes. This affects principally the centrilobular liver parenchymal cells (zone 3) and to a lesser extent the periportal ones (zone 1). Ultrastructural analysis was performed to determine to what extent the formation of lipid-carrying particles of the very low density type (VLDL) is changed in affected hepatocytes in zones 1 and 3. Being morphologically the most conspicuous site of VLDL processing, the Golgi complex was chosen for quantitation by measuring its volume, VLDL content and particle size. The concentration and composition of the liver lipids were determined, biochemically. After TAA treatment of the liver the number of Golgi-VLDL particles is significantly reduced to about 50% in both the lobular zones examined. In addition, distinct classes of size-modified Golgi-VLDL particles appear which show an abnormally wide size distribution pattern. In periportal hepatocytes the size distribution of Golgi-VLDL particles shows a clear shift towards smaller particles homogeneous in size (mean diameter 39 nm). In contrast, centrilobular hepatocytes contain particles of very heterogeneous size, the mean diameter of which is nearly doubled (77 nm). The decrease in VLDL particle number and their size modification induced by TAA is not accompanied by significant changes in the volume of the Golgi complex. Biochemical analysis showed that the accumulation of lipids in the TAA-treated liver, mainly evident morphologically as drop-like deposits in the central area of the liver lobules, is due to an increase in triglycerides (TG) by 23 mumol/g liver wet weight, which represents nearly 95% of the accumulated lipids. Despite the striking elevation of the absolute cholesterol ester (CHOL-E) content (2 mumol/g liver wet weight), this corresponds to only 5% of the newly accumulated lipids. Our electron optical and biochemical results support the suggestion that, in spite of the markedly different intralobular reaction of TAA-intoxicated hepatocytes, the formation of triglyceride-carrying particles is altered significantly in both lobular zones examined.

Effects of dietary cholesterol and fatty acids on plasma lipoproteins

The effects of dietary cholesterol and fatty acids on low density and high density lipoproteins (LDL and HDL) were studied in 20 young men. After 2-3 wk of evaluations on ad lib. diets, basal diets, which consisted of 15% protein, 45% carbohydrates, 40% fat, and 300 mg/day of cholesterol, were given for 4-5 wk (Basal). The ratio of dietary polyunsaturated to saturated fatty acids (P/S) for different groups of subjects were 0.25, 0.4, 0.8, or 2.5. 750 and 1,500 mg/d of cholesterol were added to the basal diets as 3 and 6 eggs, respectively. Total cholesterol and LDL cholesterol were lower in all subjects on the basal diets than on the ad lib. diets. Addition of 750 mg cholesterol to the diet with P/S = 0.25-0.4 raised LDL cholesterol by 16 +/- 14 mg/dl to 115% of basal diet values (n = 11, P less than 0.01); 1,500 mg increased LDL cholesterol by 25 +/- 19 mg/dl to 125% (n = 9, P less than 0.01). On the diet with P/S = 0.8, 750 mg produced insignificant increases in LDL cholesterol, but 1,500 mg produced increases of 17 +/- 22 mg/dl to 115% of basal (n = 6, P less than 0.02). On the P/S = 2.5 diet, neither 750 nor 1,500 mg produced significant changes. Thus, both the cholesterol contents and P/S ratios of diets were important in determining LDL levels. The lipid and apoprotein compositions, flotation rates, molecular weights, and binding by cellular receptors of LDL were virtually unchanged by the addition of cholesterol to the diets high in saturated fat. These diets, therefore, caused an increase in the number of LDL particles of virtually unchanged physical and biological properties. On the diet with low P/S ratio, HDL2 rose, whereas this effect was absent on diets with high P/S ratios. The response of LDL to dietary manipulations is consonant with epidemiologic data relating diets high in cholesterol and saturated fat to atherogenesis. The response of HDL2, however, is opposite to that of its putative role as a negative risk factor. Further work is needed to clarify this interesting paradox.