Triglyceride-rich Lipoprotein Fractions Research Articles

The cholesterol-lowering effect of guar gum is not the result of a simple diversion of bile acids toward fecal excretion.

The effects of partially hydrolyzed, nonviscous, guar gum (PHGG) on cholesterol metabolism and digestive balance have been compared with those of native guar gum (GUAR) in rats adapted to 0.4% cholesterol diets. Both types of guar gum elicited acidic fermentations in the large intestine, but only GUAR effectively lowered plasma cholesterol (P < 0.001), chiefly in the triglyceride-rich lipoprotein fraction. The biliary bile acid excretion was significantly enhanced in rats fed GUAR (P < 0.05), as well as the intestinal and cecal bile acid pool (P < 0.001). In rats fed GUAR and to a lesser extent in those fed PHGG, the fecal excretion of bile acids and neutral sterol was higher than in controls (P < 0.01). The digestive balance (cholesterol intake-steroid excretion) was positive in control rats (+47 mumol/d), whereas it was negative in rats fed GUAR (-20 mumol/d), which could involve a higher rate of endogenous cholesterol synthesis. In rats fed PHGG, the steroid balance remained slightly positive. Liver 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase activity was very low (22 pmol/min/mg protein), owing to cholesterol supplementation, in control rats or in rats fed PHGG, whereas it was markedly higher (+463%) in rats fed GUAR. In conclusion, even if PHGG does alter some parameters of the enterohepatic cycle of cholesterol and bile acids, its effects are not sufficient to elicit a significant cholesterol-lowering effect. The intestinal (ileal or cecal) reabsorption of bile acids was not reduced, but rather increased, by GUAR; nevertheless the intestinal capacities of reabsorption were overwhelmed by the enlargement of the digestive pool of bile acids. In the present model, induction of HMG-CoA reductase probably takes place in the presence of elevated portal bile acid concentrations.

Association of Lp(a) rather than integrally-bound apo(a) with triglyceride-rich lipoproteins of human subjects

The majority of apolipoprotein (a) [apo(a)] in plasma is characteristically associated with Lipoprotein (a) [Lp(a)], having a buoyant density (1.05–1.08 g/ml) intermediate between low density lipoproteins (LDL) and high density lipoproteins (HDL). In the fed (postprandial) state or in the presence of fasting (endogenous) hypertriglyceridemia, a small proportion of plasma apo(a) is found in the density <1.006 g/ml fraction of plasma, associated with larger and less dense triglyceride-rich lipoproteins (TRL). In order to further characterize the presence of apo(a) in ultracentrifugally-separated TRL (UTC-TRL), this lipoprotein fraction was isolated from plasma obtained in the fed state (three hours after an oral fat load) from healthy normolipidemic subjects (Lp(a): 38±8 mg/dl (mean±S.E.), n=4) and also from plasma obtained after an overnight fast from hypertriglyceridemic patients (plasma TG: 8.16±2.00 mmol/l, Lp(a): 41±3 mg/dl, n=18). Apo(a) in 3 h-postprandial UTC-TRL (5±2% of total plasma apo(a)) and in hypertriglyceridemic UTC-TRL (8±2% total apo(a)) was separable by electrophoresis and/or gel chromatography (FPLC) from the majority of UTC-TRL lipid. Apo(a) in UTC-TRL fractions had slow pre- β electrophoretic mobility and was isolated in a lipoprotein size-range smaller than VLDL and larger than LDL, consistent with it being Lp(a). Recentrifugation of UTC-TRL resulted in the majority of apo(a) being recovered in the density>1.006 g/ml fraction. Addition of proline to plasma samples before ultracentrifugation (final concentration: 0.1 M) substantially reduced the amount of Lp(a) in UTC-TRL. TRL separated from plasma by FPLC contained less apo(a) (2–5% of total plasma apo(a)), but this apo(a) was also readily dissociable from TRL lipid, had slow pre- β electrophoretic mobility, and was associated with a lipoprotein with the size of Lp(a). Our data suggest that apo(a) in the TRL fraction of subjects with postprandial triglyceridemia or endogenous hypertriglyceridemia is not an integral component of plasma VLDL or chylomicrons, but represents the presence of non-covalently bound Lp(a).

Regulation of serum-induced lipid accumulation in human monocyte-derived macrophages by interferon-γ. Correlations with apolipoprotein E production, lipoprotein lipase activity and LDL receptor-related protein expression

The demonstration of lipid loaded macrophages in atherosclerotic tissue has led to the development of in vitro systems to elucidate the mechanisms involved in lipid accumulation. Here we have characterised the changes which occur in human monocyte-derived macrophage (MDM) lipids during culture in either human serum (HS) or foetal calf serum (FCS). MDM cultured in HS were rapidly converted to lipid filled foam cells, as assessed using HPLC analysis and oil red-O staining and compared with the same cells grown in FCS. However, the lipids which accumulated were predominantly triglycerides with smaller amounts of unesterified cholesterol (UC) and only traces of cholesteryl esters (CE). α-Tocopherol (α-TocH) was present at higher levels in MDM cultured in HS compared to the same cells grown in FCS. MDM lipid accumulation was dependent on the triglyceride-rich lipoprotein (TGRL) fraction of human serum; accordingly, supplementation of FCS with human TGRL also induced MDM lipid accumulation. The relationships between cellular lipid accumulation and secretion of apolipoprotein E (apo E) and lipoprotein lipase (LPL) as well as expression of the low density lipoprotein receptor-related protein (LRP) were also examined. MDM lipid accumulation was associated with increased apo E secretion but did not alter extracellular LPL activity. The lipid accumulation which was induced by HS was potently inhibited (but not reversed) by the inflammatory cytokine interferon-γ (IFNγ), and this was associated with decreased apo E production, LPL secretion and expression of LRP. These studies reveal striking differences in the lipid composition of MDM cultured in either HS or FCS, and indicate that oil red-O staining is not necessarily associated with cholesteryl ester accumulation in human macrophages. Furthermore, the effect that serum-induced lipid accumulation has on the specific MDM functions studied should be appreciated when developing in vitro macrophage models.

Elevated triglyceride-rich lipoproteins in diabetes

The role of the intestine in cholesterol metabolism in human diabetes is unclear, although abnormalities have been demonstrated in cholesterol synthesis and absorption in diabetic animals. This study examines the relationship between fasting and post-prandial apolipoprotein B-48 in type 2 (non-insulin-dependent) diabetic and non-diabetic subjects. Eight type 2 diabetic patients and ten healthy non-diabetic control subjects were given a high-fat meal (1300 kcal), and the triglyceride-rich lipoprotein fraction was isolated by ultracentrifugation (d < 1.006 g/ml) from fasting and post-prandial plasma. Apolipoprotein B-48 and apo B-100 were separated on 4%-15% gradient gels and quantified by densitometric scanning with reference to a purified low-density lipoprotein (LDL) apo B-100 preparation. Diabetic patients had significantly higher concentrations of apo B-48 and apo B-100 in both the fasting (P < 0.05) and post-prandial (P < 0.001) triglyceride-rich lipoprotein samples compared with non-diabetic subjects. The diabetic patients also exhibited a significantly different post-prandial profile for apo B-48 and apo B-100, with a prolonged increase and a later post-prandial peak, than the non-diabetic subjects (P < 0.01). These results suggest that the raised fasting triglyceride-rich lipoproteins, often found in diabetes, are associated with apo B-48 and may be derived from increased intestinal chylomicron production. The post-prandial pattern suggests an abnormality in intestinal production as well as hepatic clearance of apo B-48 in type 2 diabetes.

Lipoprotein(a) is increased in triglyceride-rich lipoproteins in men with coronary heart disease, but does not change acutely following oral fat ingestion

Association of apo(a)/Lp(a) with triglyceride-rich lipoproteins (TGR-Lps) is determined by different factors that are poorly understood. Some previous studies suggested that apo(a) in TGR-Lps may affect the atherogenicity of the TGR particles. To study whether there are any peculiarities in postprandial (pp) Lp(a) metabolism, we have determined apo(a) phenotypes and Lp(a) concentrations in 46 subjects with coronary heart disease (CHD) and in six normolipidemic individuals at different time points (4, 6 and 8 h) following an oral fat tolerance test. While mean triglyceride concentration reached its maximum 6 h after a standardized fat meal, no change in total cholesterol and in mean Lp(a) plasma concentration was detected at any time point after the fat load. In 6 normolipidemic probands and in 8 patients with CHD, who were matched for apo(a) phenotype, lipoprotein levels, age and body weight, we followed the distribution of apo(a) in plasma density gradient fractions in the fasting and pp state. In the CHD patients a significant larger percentage of apo(a) reactivity was detected in TGR-Lps in the pre- as well as in the postprandial state, compared to control subjects. The fat intake did not induce a significant change of apo(a) reactivity in the TGR-Lp fractions in both groups. The apo(a) isoform-size and the Lp(a) plasma concentration in the fasting state had no influence on the individual variation of the Lp(a) concentration in pp TGR-Lp fractions. Our results provide evidence that TGR-Lp fractions of CHD patients are enriched in apo(a) reactivity compared to healthy controls, but do not support the hypothesis that Lp(a) acts atherogenically through a pp increase of its plasma concentration.

Intestinal beta-carotene absorption and cleavage in men: response of beta-carotene and retinyl esters in the triglyceride-rich lipoprotein fraction after a single oral dose of beta-carotene

Postprandial response curves of beta-carotene and retinyl esters in a triglyceride-rich lipoprotein (TRL) fraction were evaluated as a potential measure of beta-carotene uptake and cleavage. beta-Carotene, retinyl ester, and triglyceride concentrations in the TRL fraction (density < 1.006 kg/L) and plasma were measured in 10 men for 8 or 16 h after an oral dose of 15 mg beta-carotene. The beta-carotene response, unlike the triglyceride and retinyl ester response, can be evaluated in the TRL fraction but not in plasma. Intraindividual variations in the triglyceride-adjusted response of beta-carotene and retinyl palmitate in TRL fractions were 23% and 20% and interindividual variations were 42% and 36%, respectively. A low beta-carotene response was associated with a high ratio between retinyl palmitate and beta-carotene responses (r = -0.56, P = 0.013). In conclusion, the measurement of beta-carotene and retinyl esters in the TRL fraction after a dose of beta-carotene with a vitamin A-free meal may be an appropriate method to study beta-carotene uptake and cleavage.

Alterations in apolipoprotein B-48 in the postprandial state in NIDDM

The intestine is a major site of cholesterol synthesis and produces apolipoprotein B-48, which is critical for intestinal cholesterol absorption and secretion. The purpose of this study was to examine postprandial changes in apolipoprotein B-48 in diabetes. Six non-insulin-dependent diabetic patients and six non-diabetic control subjects were given a high-fat meal (1300 kcal) and blood samples were taken pre- and postprandially, from which the triglyceride-rich lipoprotein fraction was isolated by ultracentrifugation (density < 1.006 g/ml). Apolipoprotein B-48 was separated on 4-15% gradient gels and quantified as a percentage of the fasting concentration by densitometric scanning. Total protein, triglyceride and cholesterol in the triglyceride-rich lipoprotein fraction, blood glucose, and serum insulin were also measured. Diabetic patients exhibited a postprandial triglyceride-rich apolipoprotein B-48 profile significantly different from that of control subjects (p < 0.05). The triglyceride and total protein concentration in the triglyceride-rich lipoprotein fraction mirrored the post-prandial profile and apolipoprotein B-48 in both groups. Significantly different patterns for triglyceride (p < 0.02) and total protein (p < 0.05) following the fat-rich meal were observed in the two groups. Fasting and postprandial triglyceride-rich lipoprotein cholesterol and total apolipoprotein B were significantly higher in diabetic patients than in control subjects (p < 0.05). Since apolipoprotein B-48 is the structural protein of intestinally-derived lipoprotein particles, these studies suggest an abnormality in intestinal lipoprotein metabolism in diabetes.

Sucrose in a lipid-rich meal amplifies the postprandial excursion of serum and lipoprotein triglyceride and cholesterol concentrations by decreasing triglyceride clearance

Nineteen young male normolipidemic volunteers sequentially consumed three test meals consisting of cream only, sucrose only, or cream with sucrose. These oral fat-tolerance tests showed an amplification of the postprandial excursion of serum triglyceride and cholesterol concentrations when sucrose was included in a lipid-rich meal compared with both the cream-only meal and the sucrose-only meal. The triglyceride concentration increase occurred only in the late postprandial phase whereas the cholesterol concentration was increased for the entire 8 h studied. The increased triglyceride and cholesterol concentrations in the triglyceride-rich lipoprotein (TRL) fraction accounted for most of the increase. The clearance of an intravenous lipid emulsion was measured before and 2 and 4 h after a sucrose meal. The two postprandial clearance rates were 34% slower than the fasting value. These data indicate that sucrose-induced postprandial hypertriglyceridemia may be induced by an inhibition of the clearance of triglyceride. The slower rate of lipolysis may cause the accumulation of cholesterol in TRL.

Contribution of apoB-48 and apoB-100 triglyceride-rich lipoproteins (TRL) to postprandial increases in the plasma concentration of TRL triglycerides and retinyl esters.

After the ingestion of a fat-rich meal, there is a postprandial increase in the plasma concentration of both apolipoprotein B-48- and apoB-100-containing triglyceride-rich lipoproteins (apoB-48 and apoB-100 TRL). In order to determine the contribution of these lipoproteins to postprandial lipemia, the concentration of triglycerides (TG) and retinyl esters (RE) was measured in apoB-48 and apoB-100 TRL after an oral fat load. Six normolipidemic male subjects were fed heavy cream (1 g fat per kg body weight) containing vitamin A (3000 retinol equivalents). TRL were isolated by ultracentrifugation from plasma samples obtained at regular intervals after the meal, and apoB-100 TRL were separated from apoB-48 TRL by affinity chromatography using monoclonal antibodies. Postprandial increase in plasma TG concentration was due to an increase in TG in the TRL fraction, which in turn was predominantly (82 +/- 4%) due to an increase in TG in apoB-48 TRL. Contribution of apoB-100 TRL to postprandial increase in TRL TG was 3-27% in individual subjects. ApoB-100 TRL remained a significant carrier of total plasma triglyceride in the fed state, as reflected by similar apoB-100 and apoB-48 TRL TG concentrations at 2, 4, and 6 h after the fat meal. Retinyl esters were regularly detected in apoB-100 TRL. Seventy-five (+/- 9) percent of the increase in TRL-RE was due to RE in apoB-48 TRL and 25 +/- 9% was due to RE in apoB-100. These data suggest that RE in plasma are not always associated with apoB-48-containing lipoproteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Postprandial changes in apolipoprotein(a) concentration of triglyceride-rich lipoproteins can be reproduced by in vitro incubation: implications for underlying mechanism

We investigated the reciprocal changes in apolipoprotein(a) concentration in the lipoprotein of d > 1.006 and the triglyceride-rich lipoprotein (TRL) fractions of plasma which occur in vivo following fat ingestion. Twenty fasting subjects were studied before and 4 h after a fat-rich meal. In 75% of cases, in vitro incubation of the postprandial (4-h) TRL with autologous fasting (0-h) lipoproteins of d > 1.006 resulted in further substantial (>5% total) reciprocal changes in apo(a) concentration of the 2 fractions. The increase in re-isolated postprandial TRL apo(a) was 25.1% ± 5.1% of the total apo(a), compared to the insignificant increase (0.1% ± 0.1%) in re-isolated fasting TRL. Most of the further increase in 4-h TRL apo(a) (94% ± 4%) could be achieved by incubation with the corresponding 4-h chylomicron fraction (CM) alone. The re-isolated 4-h TRL apo(a) concentration correlated positively with 4-h plasma TG concentration ( r = 0.65, P < 0.01) and other indices of postprandial lipaemia. In vitro incubation of pooled serum lipoproteins of d > 1.006 with serial dilutions of nascent CM obtained from chylous ascitic fluid revealed that the reciprocal changes in apo(a) concentration exhibit a curvilinear relationship with the concentration of CM triglyceride which plateaued round 7 mmol/1 in this instance. We conclude that the reciprocal changes in apo(a) concentration between TRL and lipoproteins of d > 1.006 which occur in the postprandial phase are quantitatively significant and largely represent a redistribution process rather than de novo synthesis because they can be reproduced by in vitro incubation.

Plasma lipoprotein distribution of apolipoprotein(a) in the fed and fasted states

In order to quantitate the contribution of triglyceride-rich lipoprotein (TRL) apolipoprotein(a) to total plasma apo(a) concentration in the fed and fasted states, we have studied a group of 20 male subjects (age 49 ± 3 years) with fasting apo(a) concentrations varying from 39 to 1385 U/1. After a 12-h overnight fast, each subject was given a fat-rich meal (1 g fat/kg body weight) and venous blood samples were obtained at hourly intervals for 10 h. TRL were isolated from bihourly plasma samples by ultracentrifugation (d < 1.006g/ml) and apo(a) was measured by radioiummunoassay. Total plasma apo(a) concentration did not change after the meal. However, TRL apo(a) increased significantly (0 h: 3 ± 1, 4 h: 30 ± 7 U/1; p < 0.001) and‘d > 1.006’ apo(a) decreased (0 h: 267 ± 56, 4 h: 231 ± 50 U/1;P < 0.05). Similar postprandial changes were observed in apoB concentration (TRL apo B at 0 h: 10.3 ± 1.5, 4 h: 13.6 ± 1.7 g/l,P < 0.001, ‘itd > 1.006’ apoB at 0 h: 118 ± 7, 4 h: 110 ± 7 g/l,P < 0.001). In the fasted state 2.0 ± 1.0% and in the fed state (4 h postprandially) 16.0 ± 4.6% of total plasma apo(a) was found in the TRL fraction. Eleven subjects had less than 10% of total apo(a) in TRL, 5 had 25% or more apo(a) in TRL in the fed state. Postprandial increase in TRL apo(a) was significantly correlated (r = 0.75, P < 0.001) with increase in plasma triglycerides. TRL apo(a) concentration in the fed state was not correlated with total fasting cholesterol, triglyceride, apo(a) or HDL cholesterol concentration. We conclude that in some individuals, TRL apo(a) makes a significant contribution to total plasma apo(a) concentration in the fed state.

Postprandial Changes in the Plasma Concentration of α- and γ-Tocopherol in Human Subjects Fed a Fat-Rich Meal Supplemented with Fat-Soluble Vitamins

The plasma concentrations of alpha (alpha)- and gamma (gamma)-tocopherol in 10 male and 15 female subjects (n = 14) received 1 g fat/kg body wt as soybean oil, and the meal was supplemented with 100% of the RDA for fat-soluble vitamins. In expt. 2, the subjects (n = 11) received 1 g fat/kg body wt as 50% soybean oil + 50% cream, and the meal was supplemented with 200% of the RDA for fat-soluble vitamins. The ratio of gamma- :alpha-tocopherol given in the test meal of expt. 1 was 2.8:1 and in expt. 2 was 0.9:1. Blood samples were obtained 0, 3, 6, 9 and 12 h after the meal. Tocopherol concentration was measured in plasma and lipoprotein fractions. In both studies, plasma triglyceride concentration increased significantly after the meal (P less than 0.001). Mean plasma cholesterol and alpha-tocopherol concentrations were unchanged, but plasma gamma-tocopherol concentration was significantly increased at 6, 9 and 12 h after the meal (P less than 0.05). The increase in plasma gamma-tocopherol was due to increases within the triglyceride-rich lipoprotein (TRL) fraction (d less than 1.006 g/ml) at earlier timepoints, followed by a sustained increase within low density lipoprotein (LDL) and high density lipoprotein (HDL) fractions at later timepoints. In contrast, alpha-tocopherol in LDL and HDL decreased postprandially, concomitant with a rise in TRL alpha-tocopherol. Our results are consistent with the concept that there are differences in the distribution of alpha- and gamma-tocopherol in postprandial lipoproteins.

Correlation between apolipoprotein A-IV and triglyceride concentrations in human sera.

Apolipoprotein A-IV concentration was measured by a newly developed competitive enzyme immunoassay in sera from fasted human subjects (n = 105) whose triglyceride concentrations ranged from 20 to 474 mg/dl (total cholesterol below 260 mg/dl) and in which chylomicrons could not be detected. Mean (+/- SD) apolipoprotein A-IV concentration was 13.0 +/- 2.6 mg/dl in sera with triglyceride levels ranging from 20 to 100 mg/dl, 16.9 +/- 3.7 mg/dl in sera with triglyceride levels ranging from 101 to 250 mg/dl, and 22.7 +/- 6.7 mg/dl in sera with triglyceride levels ranging from 251 to 474 mg/dl. The differences among the three groups were highly significant (P less than 0.001). Moreover, variations of apolipoprotein A-IV concentrations according to the triglyceride levels were noted within the normo-triglyceridemic population. Apolipoprotein A-IV concentration was 12.8 +/- 2.1 mg/dl for triglyceride levels ranging from 20 to 75 mg/dl and 16.4 +/- 3.8 mg/dl for triglyceride levels ranging from 76 to 150 mg/dl (P less than 0.01). In the entire population that was studied there was a significant linear correlation (r = 0.61, P less than 0.001) between the concentrations of serum apolipoprotein A-IV and triglyceride. Although the hypothesis of an unknown factor independently influencing both very low density lipoproteins and apolipoprotein A-IV cannot be ruled out, and although no apolipoprotein A-IV was found in the triglyceride-rich lipoprotein fraction after separation by gel filtration, these data suggest that, in fasting subjects, the secretion of very low density lipoproteins could contribute to the plasma apolipoprotein A-IV level.

Estimation of cholesterol loading of the low-density lipoprotein fraction in diabetic subjects without ultracentrifugation

We report here a new formula for estimating apoliporotein (apo) B concentration in the low-density lipoprotein (LDL) fraction from measurements of plasma triglyceride and apoB. ApoB in plasma and in the triglyceride-rich lipoprotein fraction (VLDL, d < 1.019) and plasma triglyceride were measured in 112 subjects, including 56 diabetics. There was a significant correlation between VLDL-apoB and plasma triglyceride (Y = 0.07X + 1, r = 0.73, P < 0.001). We calculated LDL-apoB according to this formula: LDL-apoB = total apoB -(0.07 × total triglyceride + 1). We found an excellent relationship between LDL-apoB (total apoB - VLDL-apoB) and calculated LDL-apoB (Y = 1.0X + 1, r = 0.96, P < 0.001). This new formula will enable us to estimate the apoB concentration in the LDL fraction without ultracentrifugation.

Changes of serum apolipoprotein levels after oral administration of fat in human subjects

Changes of serum apolipoprotein levels were studied every hour for 6 h after the administration of 55 g butter to 8 healthy male subjects. The mean serum apo A-IV level was significantly increased at 4 h ( P < 0.05) after fat ingestion compared to the mean initial level, although no significant changes of levels in other apolipoproteins (apo A-I, A-II, B, C-II, C-III, and E) were observed. The mean apo A-IV level in the triglyceride-rich lipoprotein (TRL) fraction ( d < 1.006) increased progressively over 6 h. In all 8 subjects, the time of peak concentration of apo A–IV in TRL fraction was delayed by 1–2 h compared to that in whole serum. On the other hand, the mean apo B-48 level in the fraction reached a peak at 4 h. These results raise the possibilities that some apo A–IV, newly synthesized or already existing in intestinal cells, may be directly secreted into the venous circulation and that apo A-IV and apo B-48 may distribute differently in different sizes of chylomicron. Alternatively, the amount of each apolipoprotein synthesized may depend upon the content of ingested fat. It is suggested that apo A–IV production by intestinal cells does not appear to be regulated by the rate of fat transport, and that apo A–IV does not play an important role in chylomicron formation compared to apo B-48.

Changes of serum apo A-IV levels in a newborn with chylothorax treated with medium-chain triglyceride milk

Chylothorax in the neonatal period is a very rare condition and its etiology remains uncertain. The pleural effusion in a patient with chylothorax contains fat absorbed from the intestine. Thus it is worthwhile analyzing the effusion to determine the content of chylomicrons formed in intestinal cells during fat absorption, though it is contaminated with plasma components to some degree. It is known that human apo A-IV [l] and B-48 [2] are specifically synthesized in the intestine. Both are major apolipoproteins of chylomicrons. Chylomicrons are formed in intestinal cells after fat absorption and then enter into blood circulation via intestinal lymphatics and the thoracic duct. However, it is not well known how these apolipoproteins participate in the formation of chylo~crons. On the other hand, MCT (medium-Ghan triglyceride) contains fatty acid molecules with a chain length varying from 6 to 12 carbon atoms. Because fatty acids with a carbon length of below 12 are absorbed almost wholly into the portal venous system [3], MCT has been used in the treatment of chylothorax [4]. In this study, we examined the changes of apo A-IV levels in serum and pleural effusion and the apo B-48/total apo B ratio in the TRL (triglyceride-rich lipoprotein) fraction of the effusion during treatment with MCT milk. In addition, we tried to elucidate the influence of the MCT absorption on the levels of apo A-IV in serum and B-48 in the pleural effusion.

Plasma apolipoprotein changes in the triglyceride-rich lipoprotein fraction of human subjects fed a fat-rich meal

Twenty two subjects (9 males, 13 females) were fed a fat-rich meal (1 g of fat/kg body weight). Triglyceride-rich lipoproteins (TRL) were isolated by ultracentrifugation (d less than 1.006 g/ml) from blood drawn 0, 3, 6, 9, and 12 hr after the meal. Plasma triglyceride increased then decreased postprandially, while plasma apoA-I and apoB concentrations decreased. TRL triglyceride, TRL total protein, and TRL apoB concentrations all increased then decreased after the fat-rich meal. Postprandial rise in plasma triglyceride was significantly correlated with fasting plasma triglyceride levels (r = 0.66, P less than 0.001); postprandial rise in TRL triglyceride was significantly correlated with fasting TRL triglyceride levels (r = 0.58, P less than 0.01); postprandial rise in TRL apoB was not, however, significantly correlated with fasting TRL apoB levels (r = 0.37, N.S.). TRL apolipoproteins were separated by polyacrylamide gradient (4-22.5%) gel electrophoresis and protein bands were scanned in two dimensions with a laser densitometer. Relative postprandial changes in the concentration of the TRL apolipoproteins were determined. TRL apoB-100, apoB-48, apoE, and apoC increased then decreased postprandially. The increase in TRL apoB-100 after the fat-rich meal was confirmed in 8 subjects by direct measurement of apoB-100 with a monoclonal antibody ELISA assay. ApoA-I concentration in TRL was unchanged. Albumin in the TRL fraction was significantly increased 12 hr after the meal. Subjects with a greater magnitude of postprandial triglyceridemia had a greater increase in TRL triglyceride and TRL apoB, but their TRL apoB-100/apoB-48 ratios were not different from subjects with less pronounced triglyceridemia. Assuming that plasma TRL containing apoB-100 are predominantly derived from the liver, our data suggest that triglyceride-rich lipoproteins from both the liver and intestine make a significant contribution to postprandial triglyceridemia.

Intestinal apolipoprotein A-I and B-48 metabolism: effects of sustained alterations in dietary triglyceride and mucosal cholesterol flux

In recent studies (1985. J. Lipid Res. 26:368-379 and 1986. J. Lipid Res. 27:30-39) we characterized aspects of synthesis of rat intestinal apolipoproteins (apo) A-I and B-48 in vivo, and their short term regulation by dietary and biliary lipid flux. We now report studies extending these observations to the effects on intestinal apoA-I and apoB-48 metabolism of sustained (3 or 6 weeks) isocaloric intake of diets containing 0-30% (by weight) triglyceride, the latter as either butter fat (saturated) or corn oil (polyunsaturated). Additional studies were conducted to determine, separately, the effects of perturbations of intestinal mucosal cholesterol flux and hypothyroidism on intestinal apoA-I and apoB-48 metabolism. Intestinal synthesis (% total protein) of apoA-I and apoB-48 was not influenced by either dietary triglyceride quantity or quality (saturated vs. polyunsaturated fat); the values that were obtained were strictly comparable to those of both chow-fed animals and animals maintained for 3 weeks on fat-free chow. Intestinal apoA-I synthesis was not influenced by either acute or chronic perturbations of mucosal cholesterol flux. Hypothyroid rats demonstrated a 50% suppression of jejunal apoA-I synthesis. Intestinal synthesis of apoB-48, by contrast, appeared to undergo regulation by chronic (but not acute) perturbations of mucosal cholesterol flux. Maneuvers that augmented intestinal cholesterol uptake (particularly hypothyroidism) appeared to suppress intestinal apoB-48 synthesis by over 40%, while Surfomer (AOMA) administration reduced cholesterol absorption (control, 54 +/- 7%; AOMA, 26 +/- 8%; P less than 0.0005) and resulted in a 24% increase in apoB-48 synthesis by jejunal enterocytes. Intracellular intestinal lipoproteins demonstrated marked cholesteryl ester enrichment of the triglyceriderich lipoprotein fractions in hypercholesterolemic, hypothyroid rats. When all the groups were compared, cholesterol absorption (used as an index of mucosal cholesterol uptake) was negatively correlated with jejunal apoB-48 synthesis (r = -0.92, P less than 0.05). The data suggest that regulation of rat intestinal apoA-I and apoB-48 metabolism is independent of triglyceride flux. It is further concluded that an important regulatory effect of mucosal cholesterol flux can be demonstrated on enterocyte apoB-48 synthesis. Finally, the data suggest the additional possibility that circulating levels of thyroid hormone may exert an independent effect on the expression of rat intestinal apolipoproteins A-I and B-48.

Increased cholesterol concentration in intermediate density lipoprotein fraction of normolipidemic non-insulin-dependent diabetics

There is increasing agreement about the atherogenicity of intermediate density lipoprotein (IDL). In order to determine whether normocholesterolemic diabetics are at a higher risk of atherosclerosis, cholesterol concentrations in three subclasses of triglyceride-rich lipoprotein fraction ( S f 12–400) were examined. Their plasma triglyceride and cholesterol levels were limited to below 150 and 250 mg/dl, respectively. They were divided into 3 groups according to their treatment: insulin injection (group I), sulphonylurea (group S) and diet alone (group D). Age-matched healthy normolipidemic non-obese subjects served as controls (group C). Triglyceride-rich lipoproteins were separated by ultracentrifugation: very low density lipoprotein (VLDL), S f 60–400; intermediate density lipoprotein, (IDL 1), S f 20–60; IDL 2; S f 12–20. Cholesterol concentrations in total plasma, VLDL, IDL 2 and high density lipoprotein (HDL) were all identical in every group. A significant increase in cholesterol concentration was found in IDL 1 of groups S and D. Low density lipoprotein-cholesterol of group I was also increased. These findings indicate an increased risk factor in normolipidemic diabetics.

Apolipoprotein E quantified by enzyme-linked immunosorbent assay.

We developed a sensitive, specific "sandwich"-type enzyme-linked immunosorbent assay in which affinity-purified antibodies are used for coating and also for preparing an antibody-peroxidase conjugate to quantify apolipoprotein E in serum and in its lipoprotein fractions. This technique is rapid (results within 5 h), precise (mean intra- and interassay CVs 4.3 and 8.2%, respectively), accurate, and simple to perform. Sample pretreatment did not enhance apo E immunoreactivity. For 47 normolipidemic subjects, the mean apo E concentration was 38.11 (SD 11.1) mg/L. Above-normal apo E concentrations were measured in all types of hyperlipoproteinemia, especially types III and V. Apo E correlated with triglyceride (r = 0.58) and cholesterol concentrations (r = 0.60). As determined by gel filtration, hypertriglyceridemia was associated with a redistribution of apo E towards the triglyceride-rich lipoprotein fractions, which contained 77.1% (SD 16.8%) of total plasma apo E in Fredrickson type V patients, compared with 12.5% (SD 6.3%) in normal persons.