Regulation of apoprotein synthesis and secretion in the human hepatoma Hep G2. The effect of exogenous lipoprotein

The regulation of lipoprotein assembly and secretion at a molecular level is incompletely understood. To begin to identify the determinants of apoprotein synthesis and distribution among lipoprotein classes, we have examined the effects of chylomicron remnants which deliver triglyceride and cholesterol, and beta very low density lipoprotein (beta VLDL), which deliver primarily cholesterol, on apolipoprotein synthesis and secretion by the human hepatoma Hep G2. Hep G2 cells were incubated with remnants or beta VLDL for 24 h, the medium was changed and the cells then incubated with [35S]methionine. The secreted lipoproteins were separated by gradient ultracentrifugation and the radiolabeled apoproteins were isolated by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and counted. Remnants caused a 14-fold, and beta VLDL a 7-fold, increase in VLDL apoprotein (apo) secretion; the apoB/apoE ratio in this class was unchanged. Preincubation with either of the lipoproteins also stimulated low density lipoprotein apoB secretion. Preincubation with beta VLDL, but not with remnants, significantly increased apoE and apoA-I secreted in high density lipoprotein (HDL). In addition, the apoE/apoA-I ratio precipitated from the HDL of beta VLDL-treated cells by anti-apoE was 2.2-fold higher than that precipitated by anti-apoA-I. There was no difference in the ratios precipitated from control HDL. This was due to the secretion of a lipoprotein, subsequently isolated by immunoaffinity chromatography, that contained predominantly apoE. When Hep G2 cells were preincubated with oleic acid alone, total apoprotein secretion was not altered. However, cholesterol-rich liposomes stimulated secretion of newly synthesized apoE, but not apoB, while apoA-I secretion was variably affected. Cholesterol-poor liposomes had no effect. Thus, lipid supply is a determinant of apoprotein synthesis and secretion, and cholesterol may be of particular importance in initiating apoprotein synthesis.     

Synthesis, modification, and flotation properties of rat hepatocyte apolipoproteins

We have studied apolipoprotein synthesis, intracellular modification and secretion by primary adult rat hepatocyte cultures using continuous pulse or pulse chase labeling with [35S]methionine, immunoprecipitation and two-dimensional isoelectric focusing/polyacrylamide gel electrophoresis. The flotation properties of the newly secreted apolipoproteins were studied by discontinuous density gradient ultracentrifugation and one- and two-dimensional polyacrylamide gel electrophoresis. These studies showed that rat hepatocyte apoE is modified intracellularly to produce minor isoproteins that differ in size and charge. One of these minor isoproteins represents a monosialated apoE form (apoE3s1). Similarly, apoCIII is modified intracellularly to produce a disialated apoCIII form (apoCIIIs2), whereas newly synthesized apoA-I and apoA-IV are not glycosylated and overlap on two-dimensional gels with the proapoA-I and the plasma apoA-IV form, respectively. Both unmodified and modified apolipoproteins are secreted into the medium. Separation of secreted apolipoproteins by density gradient ultracentrifugation has shown that 50% of apoE, 80% of apoA-I, and more than 90% of apoA-IV and apoCIII are secreted in a lipid-poor form, whereas apoB-100 and apoB-48 are 100% associated with lipids. ApoB-100 floats in the VLDL and IDL regions, whereas apoB-48 is found in all lipoprotein fractions. ApoE and small amounts of apoA-I, apoA-IV and apoCIII float in the HDL region. Small amounts of apoE and apoCIII are also found in the VLDL and IDL regions, and apoE in the LDL region. Ultracentrifugation of nascent lipoproteins in the presence of rat serum promoted flotation of apoA-I and apoA-IV in the HDL fraction and resulted in increased flotation and distribution of apoE and apoCs in VLDL, IDL and LDL regions. These observations are consistent with the hypothesis that intracellular assembly of lipoproteins involves apoB-48 and apoB-100 forms, whereas a large portion of apoA-I, apoCIII and apoA-IV can be secreted in a lipid-poor form, which associates extracellularly with preexisting lipoproteins.     

Apolipoprotein distribution in human lipoproteins separated by polyacrylamide gradient gel electrophoresis

The heterogeneity of serum lipoproteins (excluding very low density (VLDL) and intermediate density (IDL) lipoproteins) and that of lipoproteins secreted by HepG2 cells has been studied by immunoblot analysis of the apolipoprotein composition of the particles separated by polyacrylamide gradient gel electrophoresis (GGE) under nondenaturing conditions. The reactions of antibodies to apoA-I, apoA-II, apoE, apoB, apoD, and apoA-IV have revealed discrete bands of particles which differ widely in size and apolipoprotein composition. GGE of native serum lipoproteins demonstrated that apoA-II is present in lipoproteins of limited size heterogeneity (apparent molecular mass 345,000 to 305,000) and that apoB is present in low density lipoproteins (LDL) and absent from all smaller or denser lipoproteins. In contrast, serum apoA-I, E, D, and A-IV are present in very heterogeneous particles. Serum apoA-I is present mainly in particles of 305 to 130 kDa where it is associated with apoA-II, and in decreasing order of immunoreactivity in particles of 130-90 kDa, 56 kDa, 815-345 kDa, and finally within the size range of LDL, all regions where there is little detectable apoA-II. Serum apoE is present in three defined fractions, one within the size range of LDL, one containing heterogeneous particles between 640 and 345 kDa, and one defined fraction at 96 kDa. Serum apoD is also present in three defined fractions, one comigrating with LDL, one containing heterogeneous particles between 390 and 150 kDa, and one band on the migration front. Most of serum apoA-IV is contained in a band comigrating with albumin. GGE of centrifugally prepared LDL shows the presence of apoB, apoE, and apoD, but not that of apoA-I. However, the particles containing apoA-I, which, in serum, migrated within the LDL size range and as bands of 815 to 345 kDa, were recovered upon centrifugation in the d greater than 1.21 g/ml fraction. GGE of high density lipoproteins (HDL) indicated that most of apoA-I, A-II, and A-IV were present in lipoproteins of the same apparent molecular mass (390-152 kDa). ApoD tended to be associated with large HDL, and this was also significant for HDL apoE, which is present in lipoproteins ranging from 640 to 275 kDa. GGE of very high density lipoproteins (VHDL) presented some striking features, one of which was the occurrence of apolipoproteins in very discrete bands of different molecular mass. ApoA-II was bimodally distributed at 250-175 kDa and 175-136 kDa, the latter fraction also containing apoA-I.(ABSTRACT TRUNCATED AT 400 WORDS)     

Lipoprotein composition of human suction-blister interstitial fluid

Interstitial fluid (IF) was obtained in 27 apparently healthy subjects (12 males, 15 females) by applying mild suction (200-250 mm Hg) on the skin either on the midvolar forearm or on the paraumbilical region of the abdomen. The IF concentrations of lipids and apolipoproteins (apo) were studied and compared with those of serum (S). The mean ratio between interstitial fluid and serum (IF/S ratio) varied from 0.14 for forearm apoE to 0.29 for apoA-II on the abdomen. This ratio was consistently lower for apoE, C-II, C-III, and B than for apoA-I and A-II, and significantly lower on the arm than on the abdomen for all apolipoproteins studied. The IF/S ratios showed marked variations among individuals. However, interstitial fluid apolipoprotein concentrations at different blister sites were highly correlated within each individual. Studies with agarose gel electrophoresis and density gradient ultracentrifugation revealed that large triglyceride-rich particles were virtually lacking in interstitial fluid and that the relation between the low density lipoproteins (LDL) and high density lipoproteins (HDL) was shifted towards a greater proportion of HDL. The lipoprotein distribution in the HDL range of interstitial fluid differed from that of serum showing one maximum at a density of about 1.070 g/ml (serum HDL2 about 1.090 g/ml) and one at a density of 1.130-1.140 g/ml (serum HDL3, 1.110-1.120 g/ml). The former subfraction contained most of the lipoprotein-bound apoE while the latter contained the major part of apoA-I and apoA-II. Studies of the lipoproteins of interstitial fluid may add to our understanding of the development of atherosclerosis and xanthomatosis and may also provide valuable information on the permeability of the capillary membrane in normo- and pathophysiological states.     

Isolation and characterization of lipoproteins produced by human hepatoma-derived cell lines other than HepG2

A total of six established human hepatoma-derived cell lines, including Hep3B, NPLC/PRF/5 (NPLC), Tong/HCC, Hep 10, huH1, and huH2, were screened for their ability to accumulate significant quantities of lipoproteins in serum-free medium. Only two cell lines, Hep3B and NPLC, secreted quantitatively significant amounts of lipoproteins. In a 24-h period the accumulated mass of apolipoproteins (apo) A-I, A-II, B, and E and albumin for Hep3B cells was 1.96, 1.01, 1.96, 1.90, and 53.2 micrograms/mg cell protein per 24 h, respectively. NPLC cells secreted no detectable albumin but the 24-h accumulated mass for apolipoproteins A-I, A-II, B, and E was 0.45, 0.05, 0.32, and 0.68 micrograms/mg cell protein per 24 h, respectively. Twenty four-hour serum-free medium of Hep3B cells contained lipoproteins corresponding to the three major density classes of plasma; percent protein distribution among the lipoprotein classes was 4%, 41%, and 56% for very low density lipoprotein ("VLDL"), low density lipoprotein ("LDL"), and high density lipoprotein ("HDL"), respectively. NPLC was unusual since most of the lipoprotein mass was in the d 1.063-1.235 g/ml range. Hep3B "LDL", compared with plasma LDL, contained elevated triglyceride, phospholipid, and free cholesterol. Nondenaturing gradient gel electrophoresis revealed that Hep3B "LDL" possessed a major component at 25.5 nm and a minor one at 18.3 nm. Immunoblots showed that the former contained only apoB while the latter possessed only apoE. Like plasma VLDL, Hep3B "VLDL" particles (30.5 nm diameter) isolated from serum-free medium contained apoB, apoC, and apoE. "HDL" harvested from Hep3B and NPLC medium were enriched in phospholipid and free cholesterol and poor cholesteryl ester which is similar to the composition of HepG2 "HDL." "HDL" from Hep3B and NPLC culture medium on gradient gel electrophoresis had peaks at 7.5, 10, and 11.9 nm which were comparable to major components found in HepG2 cell medium. Hep3B cells, in addition, possessed a particle that banded at 8.2 nm which appeared to be an apoA-II without apoA-I particle by Western blot analysis. The cell line also produced a subpopulation of larger-sized "HDL" not found in HepG2 medium. NPLC "HDL" had a distinct peak at 8.3 nm which by Western blot was an apoE-only particle. Electron microscopy revealed that "HDL" harvested from Hep3B and NPLC medium consisted of discoidal and small, spherical particles like those of HepG2. The "HDL" apolipoprotein content of each cell line was distinct from that of HepG2. ApoA-II at 35% of apolipoprotein distinguishes Hep3B "HDL" from HepG2, which contains only 10%.(ABSTRACT TRUNCATED AT 400 WORDS)     

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

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

Rapid quantitative apolipoprotein analysis by gradient ultracentrifugation and reversed-phase high performance liquid chromatography

A new methodology for the analysis of lipoprotein composition using a combination of gradient ultracentrifugation and high performance liquid chromatography was used to determine the differences in lipoprotein composition between non-hyperlipidemic men and women. Lipoproteins from each subject were separated into six subfractions: VLDL, IDL, LDL, and three subfractions of HDL by a single gradient ultracentrifugation spin of less than 5 hr. The HDL subfractions were designated HDL-L (the lightest density subfraction, rich in apoCs and poor in apoA-II), HDL-M (the middle subfraction, rich in apoA-II), and HDL-D (the most dense, relatively poor in both the apoCs and apoA-II). The concentrations of the water-soluble apolipoproteins in each subfraction were determined using reversed-phase HPLC. The concentrations of apoB and the lipid components of the lipoproteins were determined by chemical and enzymatic methods. This methodology proved to be highly reproducible when performed on fresh plasma samples and we were able to identify many sex-associated differences in lipoprotein composition. This methodology is the only nonimmunological technique available for analyzing lipoprotein composition that offers such a combination of accuracy, speed, and completeness.     

High density lipoprotein accumulation in perfusates of isolated livers of African green monkeys. Effects of saturated versus polyunsaturated dietary fat

To determine whether altered hepatic secretion of HDL is part of the mechanism by which polyunsaturated fat lowers plasma HDL concentration, we have studied HDL secretion in the isolated perfused livers of African green monkeys fed an atherogenic diet containing either safflower oil as the polyunsaturated fat or butter as the saturated fat. During recirculating perfusion with a lipoprotein-free medium, livers from safflower oil-fed animals produced 21% less HDL mass on the average than those from butter-fed animals. Newly secreted hepatic HDL were characterized after their isolation and subfractionation by a combination of agarose column chromatography and density gradient ultracentrifugation. In both diet groups the HDL were heterogeneous in size, morphology, and composition and consisted of discoidal particles ranging in diameter from greater than 200 A to as little as 50 A. Large, discoidal particles that were rich in apoE and apoA-I were separated from small particles that were poor in apoE but rich in apoA-I. All hepatic HDL subfractions contained only small amounts of cholesteryl ester and triglyceride. The hepatic particles resembled in composition and structure the large variety of HDL particles found in the plasma of patients with the familial deficiency of lecithin:cholesterol acyltransferase. Accordingly, perfusate LCAT activity was measured and found to be 2% or less than that in monkey plasma. We conclude that the perfused monkey liver produces a variety of nascent HDL that are relatively unmodified by the post-secretory metabolic events which normally occur in blood plasma in vivo, and that livers of polyunsaturated fat-fed monkeys secrete fewer plasma HDL precursor particles than do those of saturated fat-fed monkeys.     

Serum lipids and lipoprotein composition in spontaneously diabetic BB Wistar rats

Serum lipid and lipoprotein composition in spontaneously diabetic BB Wistar rats, nondiabetic littermates, and control Wistar rats was studied to elucidate diabetes-related abnormalities of lipoprotein composition. Serum total triglycerides and pre-beta-lipoprotein concentrations of insulin-treated spontaneously diabetic BB and nondiabetic littermate rats were significantly higher than those of control Wistar rats. Serum cholesterol and HDL cholesterol concentrations of spontaneously diabetic BB and nondiabetic littermate rats did not differ from controls. Concentrations of very low density lipoproteins (VLDL), low density lipoproteins (LDL), and high density lipoproteins (HDL) of spontaneously diabetic BB and nondiabetic littermate rats were higher than those of normal rats. With sodium dodecylsulfate-polyacrylamide gel electrophoresis it was observed that the spontaneously diabetic BB and nondiabetic littermate rat VLDL contained higher percentages of apoE relative to total apoC when compared with control Wistar rats. With isoelectric focusing, apoC-II relative percentages in VLDL and HDL of both spontaneously diabetic BB and nondiabetic littermate rats were higher than apoC-II proportions in VLDL and HDL of controls. Apolipoprotein A-I of the control rat HDL showed four isoforms that focused at pI 5.8 (17.3%), 5.75 (30.6%), 5.65 (31.8%), and 5.55 (20.5%); however, the spontaneously diabetic BB and nondiabetic littermate rat HDL apoA-I was mainly represented by two isoforms that focused at pI 5.8 and 5.75. VLDL of both diabetic and nondiabetic BB rats contained higher levels of acidic apoE isoforms compared to their counterparts in control Wistar rats. Although HDL cholesterol concentrations of spontaneously diabetic BB rats remained normal, protein concentrations were higher resulting in a low cholesterol/protein ratio in HDL suggesting that the cholesterol-carrying capacity of spontaneously diabetic BB rat HDL could be less than normal and may be due to an abnormal apoA-I composition. Quantitative alterations of lipid and lipoprotein composition appear in the BB Wistar rat when compared to the Wistar rat, but some of the changes are more pronounced in the spontaneously diabetic BB Wistar rat.     

Apolipoprotein changes associated with the plasma lipid-regulating activity of gemfibrozil in cholesterol-fed rats

Gemfibrozil (Lopid) is a new plasma lipid-regulating drug that decreases very low and low density lipoprotein (VLD/LDL) and increases high density lipoprotein (HDL) concentrations in man. The present experiments tested the effects of gemfibrozil on plasma lipoproteins and apolipoproteins in rats fed high fat/high cholesterol diets. Compared to chow-fed rats, cholesterol feeding for 2 weeks (20% olive oil/2% cholesterol) produced the expected increases in VLDL and intermediate density lipoprotein (IDL) while lowering plasma HDL. This was documented by using three methods of lipoprotein isolation: sequential ultracentrifugation, density gradient ultracentrifugation, and agarose gel filtration. Gemfibrozil gavaged at 50 mg/kg per day for 2 weeks during cholesterol feeding prevented these changes such that lipoprotein patterns were similar to those in chow-fed animals. Whole plasma apoE and apoA-I concentrations were decreased and apoB increased due to cholesterol feeding as determined by electroimmunoassay, but again gemfibrozil treatment prevented these diet-induced alterations. Gradient polyacrylamide gel electrophoresis patterns of the total d less than 1.21 g/ml lipoprotein fractions reflected the changes in apolipoprotein concentrations and further demonstrated a greater increase of apoBl compared to apoBh in cholesterol-fed rats. Gemfibrozil lowered the concentration of both apoB variants and prevented the shift of apoE from HDL to lower density lipoproteins. Changes in the distribution of apoE were confirmed using agarose gel column chromatography followed by electroimmunoassay. These methods also revealed a shift of apoA-IV from HDL to the d greater than 1.21 g/ml, lipoprotein-free fraction with gemfibrozil treatment when blood was taken from fasted or postabsorptive animals. Since it was also noted that in chow-fed rats more apoA-IV was present in the d greater than 1.21 g/ml fraction in the postabsorptive or fed state compared to fasted animals, it could be postulated that the shift of apoA-IV into this fraction in gemfibrozil-treated rats is related to an accelerated clearance of chylomicrons. It is concluded that gemfibrozil largely prevents the accumulation of abnormal lipoproteins in this model of dyslipoproteinemia, and that apoE may play a critical role in this normalization process.     

Increased production of apolipoprotein B and its lipoproteins by oleic acid in Caco-2 cells

The production of lipids, apolipoproteins (apo), and lipoproteins induced by oleic acid has been examined in Caco-2 cells. The rates of accumulation in the control medium of 15-day-old Caco-2 cells of triglycerides, unesterified cholesterol, and cholesteryl esters were 102 +/- 8, 73 +/- 5, and 11 +/- 1 ng/mg cell protein/h, respectively; the accumulation rates for apolipoproteins A-I, B, C-III, and E were 111 +/- 9, 53 +/- 4, 13 +/- 1, and 63 +/- 4 ng/mg cell protein/h, respectively. Whereas apolipoproteins A-IV and C-II were detected by immunoblotting, apoA-II was absent in most culture media. In contrast to an early production of apolipoproteins A-I and E occurring 2 days after plating, the apoB expression appeared to be differentiation-dependent and was not measurable in the medium until the sixth day post-confluency. In the control medium, very low density lipoproteins (VLDL), low density lipoproteins (LDL), high density lipoproteins (HDL), and lipid-poor very high density lipoproteins (VHDL) accounted for 12%, 46%, 18%, and 24% of the total lipid and apolipoprotein contents, respectively. The triglyceride-rich VLDL contained mainly apoE (75%) and apoB (23%), while the protein moiety of LDL was composed of apoB (59%), apoE (20%), apoA-I (15%), and apoC-III (6%). The cholesterol-rich HDL contained mainly apoA-I (69%) and apoE (27%). In the control medium, major portions of apolipoproteins B and C-III (93-97%) were present in LDL, whereas the main parts of apoA-I (92%) and apoE (76%) were associated with HDL and VHDL. Oleate increased the production of triglycerides 10-fold, cholesteryl esters 7-fold, and apoB 2- to 4-fold. There was also a moderate increase (39%) in the production of apoC-III but no significant changes in those of apolipoproteins A-I and E. These increases were reflected mainly in a 55-fold elevation in the concentration of VLDL, and a 2-fold increase in the level of LDL; there were no significant changes in HDL and VHDL. VLDL contained the major parts of total neutral lipids (74-86%), apoB (65%), apoC-III (81%) and apoE (58%). In the presence of oleate, the VLDL, LDL, HDL, and VHDL accounted for 76%, 15%, 3%, and 6% of the total lipoproteins, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Coupling of photoactivatable glycolipid probes to apolipoproteins A-I and A-II in human high density lipoproteins 2 and 3

High density lipoprotein (HDL) from human serum was subfractionated into HDL2 and HDL3 by rate-zonal density gradient ultracentrifugation. The orientation of apoproteins (apo) A-I and A-II in these subfractions was investigated by use of the photosensitive glycolipid probes, 2-(4-azido-2-nitrophenoxy)-palmitoyl[1-14C]glucosamine (compound A) and 12-(4-azido-2-nitrophenoxy)-stearoyl[1-14C]glucosamine (compound B). Both probes were added to the HDL-structures in a ratio of two or three probe molecules per particle and were photoactivated by irradiation at a wavelength above 340 nm. After delipidation the probe-apoprotein adducts were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both the "shallow" probe (compound A) and the "depth" probe (compound B) were coupled for 10-14% (of the label added) to apoA-I and apoA-II from HDL3 and for about 6% to apoA-I and apoA-II from HDL2. By taking into account the relative amounts of apoA-I and apoA-II, it was estimated that the "shallow" probe labeled apoA-I 40% more effectively than apoA-II in both HDL2 and HDL3; the "depth" probe labeled apoA-I and apoA-II equally well in both subfractions. The data suggest that towards the surface HDL2 and HDL3 contain a relatively larger portion of apoA-I than apoA-II, whilst towards the core both subfractions are occupied by an equal portion of apoA-I and apoA-II. Application of these photolabels has failed to point out differences in the structural organization of HDL2 and HDL3.     

Metabolism of apoB-100 in lipoproteins separated by density gradient ultracentrifugation in normal and Watanabe heritable hyperlipidemic rabbits

We have examined the capability of a previously developed compartmental model to explain the kinetics of radioiodinated apolipoprotein (apo) B-100 in very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL) separated by density gradient ultracentrifugation after intravenous injection of radioiodinated VLDL into New Zealand white (NZW) and Watanabe heritable hyperlipidemic (WHHL) rabbits. Our model was developed primarily from kinetics in whole blood plasma of apoB-100 in particles with and without apoE after intravenous injection of large VLDL, total VLDL, IDL, and LDL. When the initial conditions for this model were assumed to be an intravenous injection of radiolabeled VLDL, the plasma VLDL and LDL simulations for NZW rabbits and the VLDL, IDL, and LDL simulations for WHHL rabbits were found to be inconsistent with the observed density gradient data. By adding a new pathway in the VLDL portion of the model for NZW rabbits and a new compartment in VLDL for WHHL rabbits, and by assuming some cross-contamination in the density gradient ultracentrifugal separations, it was possible to bring our model, which was based upon measurements of 125I-labeled apoB-100 in whole plasma, into conformity with the data obtained by density gradient ultracentrifugation. The relatively modest changes required in the model to fit the gradient ultracentrifugation data support the suitability of our approach to the kinetic analysis of the metabolism of apoB-100 in VLDL and its conversion to IDL and LDL based upon measurements of 125I-labeled apoB-100 in whole plasma after injection of radiolabeled VLDL, IDL, and LDL. Furthermore, the differences in kinetics observed by us between data from whole plasma and data from plasma submitted to ultracentrifugal separation from the same or similar animals highlight the fact that small variations that can occur in the separation of lipoprotein classes by buoyant density can lead to confusing results.     

Intracellular apoA-I and apoB distribution in rat intestine is altered by lipid feeding

Intracellular forms of chylomicrons, very low density lipoprotein (VLDL) and high density lipoprotein (HDL) have previously been isolated from the rat intestine. These intracellular particles are likely to be nascent precursors of secreted lipoproteins. To study the distribution of intracellular apolipoprotein among nascent lipoproteins, a method to isolate intracellular lipoproteins was developed and validated. The method consists of suspending isolated enterocytes in hypotonic buffer containing a lipase inhibitor, rupturing cell membranes by nitrogen cavitation, and isolating lipoproteins by sequential ultracentrifugation. ApoB and apoA-I mass are determined by radioimmunoassay and newly synthesized apolipoprotein characterized following [3H]leucine intraduodenal infusion. Intracellular chylomicron, VLDL, low density lipoprotein (LDL), and HDL fractions were isolated and found to contain apoB, and apoA-IV, and apoA-I. In the fasted animal, less than 10% of total intracellular apoB and apoA-I was bound to lipoproteins and 7% of apoB and 35% of apoA-I was contained in the d 1.21 g/ml infranatant. The remainder of intracellular apolipoprotein was in the pellets of centrifugation. Lipid feeding doubled the percentage of intracellular apoA-I bound to lipoproteins and increased the percentage of intracellular apoB bound to lipoproteins by 65%. Following lipid feeding, the most significant increase was in the chylomicron apoB and HDL apoA-I fractions. These data suggest that in the fasting state, 90% of intracellular apoB and apoA-I is not bound to lipoproteins. Lipid feeding shifts intracellular apolipoprotein onto lipoproteins, but most intracellular apolipoprotein remains non-lipoprotein bound. The constant presence of a large non-lipoprotein-bound pool suggests that apolipoprotein synthesis is not the rate limiting step in lipoprotein assembly or secretion.     

Synthesis of plasma lipoproteins by the isolated perfused liver from the fasted and fed pig.

Livers from fasted or fed pigs were perfused for 5 h with Krebs-Ringer bicarbonate buffer containing human erythrocytes, bovine serum albumin, glucose, and amino acids. Liver viability was estimated by color, consistency, portal pressure, bile flow, electrolyte changes, and glucose levels in the perfusate, urea synthesis, [1-14C]leucine incorporation into protein, oxygen uptake, and histological examination. It was shown that the liver was maintained in good condition throughout the perfusions. The apolipoprotein B (apoB) and apolipoprotein A-I (apoA-I) in the perfusate were measured by solid phase radioimmunoassay. In the fasted state, the amount of apoB released was greatest in the low density lipoprotein (LDL) fraction and the amount was especially high during the 1st h. There was no increase of apoB in this fraction by feeding. The apoB in the very low density lipoprotein (VLDL) fraction was less than that in the LDL fraction in the fasted state, and it increased more than 2-fold in the fed animals. The amount of apoA-I was greatest in the 1.21 bottom fraction and was relatively small in the high density lipoprotein (HDL) fraction. The HDL fraction contained approximately one-twentieth as much apoA-I as the 1.21 bottom fraction in the fasted condition. In the fed state, apoA-I in the HDL fraction increased markedly, although the amount was still less than in the 1.21 bottom fraction.     

Lipoprotein ApoC-II activation of lipoprotein lipase. Modulation by apolipoprotein A-IV

Lipoprotein lipase (LPL)-mediated hydrolysis of triglycerides (TG) contained in chylomicrons requires the presence of a cofactor, apolipoprotein (apo) C-II. The physiological mechanism by which chylomicrons gain apoC-II necessary for LPL activation in whole plasma is not known. Using a gum arabic stabilized TG emulsion, activation of LPL by lipoprotein apoC-II was studied. Hydrolysis of TG by LPL was greater in the presence of serum than with addition of either high density lipoproteins (HDL) or very low density lipoproteins (VLDL). LPL activation by either VLDL or HDL increased with addition of the lipoprotein-free fraction of plasma. A similar increase in LPL activity by addition of the lipoprotein-free fraction together with HDL or VLDL was observed when another TG emulsion (Intralipid) or TG-rich lipoproteins from an apoC-II deficient subject were used as a substrate. Human apoA-IV, apoA-I, apoE, and cholesteryl ester transfer protein were assessed for their ability to increase LPL activity in the presence of VLDL. At and below physiological concentrations, only apoA-IV increased LPL activity. One hundred percent of LPL activity measured in the presence of serum was achieved using VLDL plus apoA-IV. In the absence of an apoC-II source, apoA-IV had no effect on LPL activity. Removal of greater than 80% of the apoA-IV from the nonlipoprotein-containing fraction of plasma by incubation with Intralipid markedly reduced its ability to activate LPL in the presence of VLDL or HDL. Gel filtration chromatography demonstrated that incubation of the nonlipoprotein-containing fraction of plasma with HDL and the TG emulsion caused increased transfer of apoC-II to the emulsion and association of apoA-IV with HDL. Our studies demonstrate that apoA-IV increases LPL activation in the presence of lipoproteins. We hypothesize that apoA-IV is required for efficient release of apoC-II from either HDL or VLDL, which then allows for LPL-mediated hydrolysis of TG in nascent chylomicrons.     

Characterization of apoA- and apoB-containing lipoprotein particles in a variant of familial apoA-I deficiency with planar xanthoma: the metabolic significance of LP-A-II particles.

This study describes a variant of familial apoA-I deficiency associated with a moderate risk for premature coronary artery disease. The proband, a 25-year-old man of Philippine origin, and his 62-year-old maternal aunt had peripheral corneal opacification, xanthelasma, and planar xanthoma; the aunt had coronary artery bypass surgery at 61 years of age. Proband's parents and three brothers were asymptomatic and apparently healthy. The characteristic apolipoprotein features of affected patients were the immunochemically and chemically undetectable apoA-I, reduced levels of apoA-II, apoC-II, apoC-III, and apoD, and normal levels of apoB and apoE; except for negligible levels of high density lipoprotein (HDL)-cholesterol (2-3 mg/dl), their plasma lipid profile was normal. The apoA-I levels in all five unaffected relatives were more than one SD below the normal mean values for their age and sex; the HDL-cholesterol levels of proband's unaffected brothers were below the 10th percentile of normal control values. Patient's very low density lipoprotein (VLDL), low density lipoprotein (LDL), and HDL contained 1.4, 80.4, and 18.1%, whereas those of control subjects contained 2.7, 28.8, and 68.1% of the total apolipoprotein mass, respectively. In unaffected relatives, the levels of LP-A-I, but not LP-A-I:A-II, were significantly lower than in controls. Neither of the two patients had detectable concentrations of LP-A-I or LP-A-I:A-II. Their HDL only consisted of LP-A-II particles, the levels of which (7-13 mg/dl) were similar to those of unaffected relatives or controls. There was no difference in the lipid composition of LP-A-II between patients and their relatives. However, LP-A-II from patients contained substantial amounts of apoC-peptides and apoE (0.40-0.98 mg/mg apoA-II), whereas those from unaffected relatives were free of these minor apolipoproteins. In patients, among all four major apoB-containing lipoproteins, only the levels of LP-B and LP-B:C were slightly higher than those in controls. Results of this study suggest a genetic cause for this variant of apoA-I deficiency characterized most probably by autosomal recessive inheritance. It appears that patients are likely to be homozygous for a gene present in single dose in the parents and brothers of the affected proband.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dissociation of high density lipoprotein precursors from apolipoprotein B-containing lipoproteins in the presence of unesterified fatty acids and a source of apolipoprotein A-I

Incubation of low (LDL), intermediate (IDL), or very low density lipoproteins (VLDL) with palmitic acid and either high density lipoproteins (HDL), delipidated HDL, or purified apolipoprotein (apo) A-I resulted in the formation of lipoprotein particles with discoidal structure and mean particle diameters ranging from 146 to 254 A by electron microscopy. Discs produced from IDL or LDL averaged 26% protein, 42% phospholipid, 5% cholesteryl esters, 24% free cholesterol, and 3% triglycerides; preparations derived from VLDL contained up to 21% triglycerides. ApoA-I was the predominant protein present, with smaller amounts of apoA-II. Crosslinking studies of discs derived from LDL or IDL indicated the presence of four apoA-I molecules per particle, while those derived from large VLDL varied more in size and contained as many as six apoA-I molecules per particle. Incubation of discs derived from IDL or LDL with purified lecithin:cholesterol acyltransferase (LCAT), albumin, and a source of free cholesterol produced core-containing particles with size and composition similar to HDL2b. VLDL-derived discs behaved similarly, although the HDL products were somewhat larger and more variable in size. When discs were incubated with plasma d greater than 1.21 g/ml fraction rather than LCAT, core-containing particles in the size range of normal HDL2a and HDL3a were also produced. A variety of other purified free fatty acids were shown to promote disc formation. In addition, some mono and polyunsaturated fatty acids facilitated the formation of smaller, spherical particles in the size range of HDL3c. Both discoidal and small spherical apoA-I-containing lipoproteins were generated when native VLDL was incubated with lipoprotein lipase in the presence of delipidated HDL. We conclude that lipolysis product-mediated dissociation of lipid-apoA-I complexes from VLDL, IDL, or LDL may be a mechanism for formation of HDL subclasses during lipolysis, and that the availability of different lipids may influence the type of HDL-precursors formed by this mechanism.     

Distribution and characterization of the serum lipoproteins and apoproteins in the mouse, Mus musculus

Murine lipoproteins were separated into nine subfractions by a density gradient ultracentrifugal procedure. They were characterized by electrophoretic, immunological, chemical, and morphological analyses, and their protein moieties were defined according to charge, molecular weight, and isoelectric point. HDL predominated (approximately 500 mg/dl serum), the mode of its distribution being situated in the d 1.09-1.10 g/ml (F 1.21 approximately 4) region. Chemical analysis showed subfractions of d 1.085-1.136 g/ml to resemble human HDL3 closely, including the presence of apoA-I (Mr 25,000-27,000) as their major apolipoprotein. An apoA-II-like protein, of Mr 8400 (in monomeric form), was also tentatively identified. In electrophoretic mobility and chemical composition, the d 1.060-1.085 g/ml subfraction (approximately 10% of total HDL) was distinct and akin to human HDL2. ApoA-I represented approximately 60% of its complement of low molecular weight apoproteins. The density range used for separation of human HDL2 (d 1.066-1.100 g/ml) by gradient ultracentrifugation is inadequate in the mouse, and the d 1.060-1.085 g/ml interval is more appropriate. The 1.063 g/ml boundary for separation of mouse LDL from HDL was unsuitable. Immunological and electrophoretic studies revealed that alpha-migrating lipoproteins were present in the d 1.046-1.060 g/ml range, a finding consistent with their enrichment in apoA-I; apoE-, apoA-II-, and apoC-like proteins were also detected. These findings indicate the presence of HDL1 particles. Murine apoA-I and apoB-like proteins of higher (apoBH) and lower (apoBL) molecular weight were constituents of the d 1.033-1.046 g/ml fraction. Alternative techniques, such as electrophoresis in starch block, are therefore a prequisite for separation of apoB from alpha-migrating, apoA-I-containing lipoproteins in the low density range in mouse serum. The LDL class (d 1.023-1.060 g/ml) amounted to only approximately 20% of the total murine lipoproteins of d less than 1.188 g/ml (65-70 mg/dl serum). Particles were richer In triglyceride, larger in diameter (mean 244 A), and more heterogeneous than typical of man. VLDL (40-80 mg/dl serum) was triglyceride-rich (66% by weight) and similarly heterogeneous in size (mean diameter 494 A; range 270-750 A). ApoBH and apoBL were prominent in murine VLDL, and cross-reacted with an antiserum to human apoB. ApoE- and apoA-I-like proteins were also detectable in apoVLDL, as was a protein of 70,000-75,000 mol wt. The presence of murine apolipoproteins analogous to human apoB and apoE was confirmed by the immunological cross-reactivities of VLDL and LDL with monospecific antisera to the human proteins. The marked similarity of lipoprotein and apolipoprotein profile in the mouse and rat is notable. Since murine VLDL contains apoE and apoBL, this resemblance may extend to the metabolism of chylomicron remnants and hepatic VLDL in the two species.