Compensatory mechanisms governing the concentration of plasma low density lipoprotein

To evaluate factors regulating the concentrations of plasma low density lipoproteins (LDL), apolipoprotein B metabolism was studied in nine Pima Indians (25 +/- 2 yr, 191 +/- 20% ideal wt) with low LDL cholesterol (77 +/- 7 mg/dl) and apoB (60 +/- 4 mg/dl) and in eight age- and weight-matched Caucasians with similar very low density lipoprotein (VLDL) concentrations, but higher LDL (cholesterol = 104 +/- 18; apoB = 82 +/- 10; P less than 0.05). Subjects received autologous 131I-labeled VLDL and 125I-labeled LDL, and specific activities of VLDL-apoB, intermediate density lipoprotein (IDL)-apoB, and LDL-apoB were analyzed using a multicompartmental model. Synthesis of LDL-apoB was similar (1224 +/- 87 mg/d in Pimas vs 1218 +/- 118 mg/d in Caucasians) but in Pimas the fractional catabolic rate (FCR) for LDL-apoB was higher (0.48 +/- 0.02 vs 0.39 +/- 0.04 d-1, P less than 0.05). In the Pimas, a much higher proportion of VLDL-apoB was catabolized without conversion to LDL (47 +/- 3 vs 30 +/- 5%, P less than 0.01). When all subjects were considered together, LDL-apoB concentrations were negatively correlated with both FCR for LDL-apoB (r = -0.79, P less than 0.0001) and the non-LDL pathway (r = -0.43, P less than 0.05). Also, the direct removal (non-LDL) path was correlated with VLDL-apoB production (r = 0.49, P = 0.03), and the direct removal pathway and FCR for LDL-apoB were correlated (r = 0.49, P = 0.03). In conclusion, plasma LDL appear to be regulated by both the catabolism of LDL and the extent of metabolism of VLDL without conversion to LDL; both of these processes may be mediated by the apoB/E receptor, and appear to increase in response to increasing VLDL production.     

Cod and soy proteins compared with casein improve glucose tolerance and insulin sensitivity in rats

The aim of the present study was to determine the effects of feeding various dietary proteins on insulin sensitivity and glucose tolerance in rats. Male Wistar rats were fed for 28 days with isoenergetic diets containing either casein, soy protein, or cod protein. Cod protein-fed and soy protein-fed rats had lower fasting plasma glucose and insulin concentrations compared with casein-fed animals. After intravenous glucose bolus, cod protein- and soy protein-fed rats induced lower incremental areas under glucose curves compared with casein-fed animals. Improved peripheral insulin sensitivity was confirmed by higher glucose disposal rates in cod protein- and soy protein-fed rats (15.2 +/- 0.3 and 13.9 +/- 0.6 mg. kg(-1). min(-1), respectively) compared with casein-fed animals (6.5 +/- 0.7 mg. kg(-1). min(-1), P < 0.05). Moreover, test meal experiments revealed that, in the postprandial state, the lower plasma insulin concentrations in cod protein- and soy protein-fed animals could be also due to decreased pancreatic insulin release and increased hepatic insulin removal. In conclusion, the metabolic responses to three common dietary proteins indicate that cod and soy proteins, when compared with casein, improve fasting glucose tolerance and peripheral insulin sensitivity in rats.     

Lovastatin therapy reduces low density lipoprotein apoB levels in subjects with combined hyperlipidemia by reducing the production of apoB-containing lipoproteins: implications for the pathophysiology of apoB production

We investigated the metabolism of very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), and low density lipoprotein (LDL) apolipoprotein B (apoB) in seven patients with combined hyperlipidemia (CHL), using 125I-labeled VLDL and 131I-labeled LDL and compartmental modeling, before and during lovastatin treatment. Lovastatin therapy significantly reduced plasma levels of LDL cholesterol (142 vs 93 mg/dl, P less than 0.0005) and apoB (1328 vs 797 micrograms/ml, P less than 0.001). Before treatment, CHL patients had high production rates (PR) of LDL apoB. Three-fourths of this LDL apoB flux was derived from sources other than circulating VLDL and was, therefore, defined as "cold" LDL apoB flux. Compared to baseline, treatment with lovastatin was associated with a significant reduction in the total rate of entry of apoB-containing lipoproteins into plasma in all seven CHL subjects (40.7 vs. 25.7 mg/kg.day, P less than 0.003). This reduction was associated with a fall in total LDL apoB PR and in "cold" LDL apoB PR in six out of seven CHL subjects. VLDL apoB PR fell in five out of seven CHL subjects. Treatment with lovastatin did not significantly alter VLDL apoB conversion to LDL apoB or LDL apoB fractional catabolic rate (FCR) in CHL patients. In three patients with familial hypercholesterolemia who were studied for comparison, lovastatin treatment increased LDL apoB FCR but did not consistently alter LDL apoB PR. We conclude that lovastatin lowers LDL cholesterol and apoB concentrations in CHL patients by reducing the rate of entry of apoB-containing lipoproteins into plasma, either as VLDL or as directly secreted LDL.     

Fish oil decreases hepatic cholesteryl ester secretion but not apoB secretion in African green monkeys

Two groups of African green monkeys were fed diets containing 40% of calories as fat with half of the fat calories as either fish oil or lard. The fish oil-fed animals had lower cholesterol concentrations in blood plasma (33%) and low density lipoproteins (LDL) (34%) than did animals fed lard. Size and cholesteryl ester (CE) content of LDL, strong predictors of coronary artery atherosclerosis in monkeys, were significantly less for the fish oil-fed animals although the apoB and LDL particle concentrations in plasma were similar for both diet groups. We hypothesized that decreased hepatic CE secretion led to the smaller size and reduced CE content of LDL in the fish oil-fed animals. Hepatic CE secretion was studied using recirculating perfusion of monkey livers that were infused during perfusion with fatty acids (85% 18:1 and 15% n-3) at a rate of 0.1 mumol/min per g liver. The rate of cholesterol secretion was less (P = 0.055) for the livers of fish oil versus lard-fed animals (3.3 +/- 0.5 vs. 6.0 +/- 1.2 mg/h per 100 g, mean +/- SEM) but the rate of apoB secretion was similar for both groups (0.92 +/- 0.15 vs. 1.01 +/- 0.13 mg/h per 100 g, respectively). The hepatic triglyceride secretion rate was also less (P less than 0.05) for the fish oil-fed animals (8.3 +/- 2.5 vs. 18.3 +/- 4.4 mg/h per 100 g). Liver CE content was lower (P less than 0.006) in fish oil-fed animals (4.1 +/- 0.8 vs. 7.4 +/- 0.7 mg/g) and this was reflected in a lower (P less than 0.04) esterified to total cholesterol ratio of perfusate VLDL (0.21 +/- 0.045 vs. 0.41 +/- 0.06). The hepatic VLDL of animals fed fish oil had 40-50% lower ratios of triglyceride to protein and total cholesterol to protein. From these data we conclude that livers from monkeys fed fish oil secreted similar numbers of VLDL particles as those of lard-fed animals although the hepatic VLDL of fish oil-fed animals were smaller in size and relatively enriched in surface material and depleted of core constituents. Positive correlations between plasma LDL size and both hepatic CE content (r = 0.87) and hepatic VLDL cholesterol secretion rate (r = 0.84) were also found.(ABSTRACT TRUNCATED AT 400 WORDS)     

Metabolism of apoprotein B in selectively bred baboons with low and high levels of low density lipoproteins

Very low density lipoprotein (VLDL) and low density lipoprotein (LDL) apoprotein (apo)-B turnover rates were measured simultaneously by injecting 131I-labeled VLDL and 125I-labeled LDL into fasting baboons (Papio sp.) selectively bred for high serum cholesterol levels and having either low or high LDL levels. The radioactivities in VLDL, intermediate density lipoprotein (IDL), LDL apoB, and urine were measured at intervals between 5 min and 6 days. Kinetic parameters for apoB were calculated in each baboon fed a chow diet or a high cholesterol, high fat diet (HCHF). VLDL apoB residence times were similar in the two groups of animals fed chow; they were increased by HCHF feeding in high LDL animals, but not in low LDL animals. Production rates of VLDL apoB were decreased by the HCHF diet in both high and low LDL animals. Most of the radioactivity from VLDL apoB was transferred to IDL. However, a greater proportion of radioactivity was removed directly from IDL apoB in low LDL animals than in high LDL animals, and only about one-third appeared in LDL. In high LDL animals, a greater proportion of this radioactivity was converted to LDL (61.4 +/- 7.2% in chow-fed animals and 49.2 +/- 10.9% in animals fed the HCHF diet; mean +/- SEM, n = 5). Production rates for LDL apoB were higher in high LDL animals than those in low LDL animals on both diets. The HCHF diet increased residence times of LDL apoB without changing production rates in both groups. VLDL apoB production was not sufficient to account for LDL apoB production in high LDL animals, a finding that suggested that a large amount of LDL apoB was derived from a source other than VLDL apoB in these animals.     

Secretion of cholesteryl-ester-rich lipoproteins by the perfused livers of rabbits fed a wheat-starch-casein diet

Rabbits fed a cholesterol-free semi-synthetic wheat-starch-casein diet had a high plasma cholesterol concentration; most of the cholesterol was associated with low-density lipoproteins (LDL). Chemical analyses of plasma lipoproteins revealed that very-low-density lipoproteins (VLDL), intermediate lipoproteins and LDL from casein-fed rabbits contained more cholesteryl ester than that of lipoproteins isolated from chow-fed animals. The fatty acid composition of cholesteryl esters of plasma lipoproteins showed that there were higher contents of oleic acid than linoleic acids in lipoproteins from casein-fed rabbits. Lipoproteins isolated from liver perfusates of casein-fed rabbits had higher cholesteryl oleate content than lipoproteins from chow-fed rabbit liver perfusates. There was a marked increase in secretion of apolipoproteins from perfused livers of casein-fed rabbits. We conclude that the high levels of plasma cholesterol in casein-fed rabbits are of hepatic origin and that one of the hypercholesterolemic actions of dietary casein in rabbits is the induction of hepatic synthesis and secretion of cholesteryl-ester-rich lipoproteins.     

Role of liver endothelial and Kupffer cells in clearing low density lipoprotein from blood in hypercholesterolemic rabbits.

The role of liver endothelial and Kupffer cells in the hepatic uptake of cholesterol-rich low density lipoprotein (LDL) was studied in rabbits fed a diet containing 2% (w/w) cholesterol for 3 weeks. 125I-labeled tyramine cellobiose-labeled cholesterol-rich LDL was injected intravenously into rabbits, and parenchymal and nonparenchymal liver cells were isolated 24 h after injection. The hepatic uptake was 9 +/- 3% of injected dose in cholesterol-fed rabbits 24 h after injection, as compared to 36 +/- 9% in control-fed rabbits (n = 6 in each group; significant difference, P less than 0.005). Endothelial and Kupffer cells took up 2.7 +/- 0.5% and 1.2 +/- 0.8% of injected dose in the hypercholesterolemic rabbits, as compared to 1.9 +/- 0.8% and 0.8 +/- 0.3% in control animals. The amount accounted for by the parenchymal cells was markedly reduced in the cholesterol-fed rabbits to 7.3 +/- 2.7% of injected dose, as compared to 32.8 +/- 7.6% in controls (P less than 0.02). On a per cell basis, the nonparenchymal cells of cholesterol-fed rabbits took up as much LDL as the parenchymal cells (0.6 +/- 0.2, 0.7 +/- 0.1, and 0.6 +/- 0.4% of injected dose per 10(9) parenchymal, endothelial, and Kupffer cells, respectively). This is in marked contrast to the control animals, in which parenchymal cells took up about 6 times more LDL per cell than endothelial and Kupffer cells (3.2 +/- 0.9, 0.7 +/- 0.3, and 0.5 +/- 0.1% of injected dose per 10(9) cells). Thus, 30% of the hepatic uptake of LDL in the cholesterol-fed rabbits took place in nonparenchymal cells, as compared to 6% in controls. Consistent with these data, the concentrations of cholesteryl ester in endothelial and Kupffer cells in rabbits fed the high cholesterol diet were about twofold higher than in parenchymal cells (428 +/- 74 and 508 +/- 125 micrograms/mg protein, respectively, vs. 221 +/- 24 micrograms/mg protein in parenchymal cells). In contrast to cells from normal rabbits, Kupffer and endothelial cells from cholesterol-fed rabbits accumulated significant amounts of Oil Red O-positive material (neutral lipids). Electron microscopic examination of these cells in situ as well as in culture revealed numerous intracellular lipid droplets. Slot blot hybridization of RNA from liver parenchymal, endothelial, and Kupffer cells showed that cholesterol feeding reduced the level of mRNA specific for the apoB,E receptor to a small and insignificant extent in all three cell types (to 70-80% of that observed in control animals).(ABSTRACT TRUNCATED AT 400 WORDS)     

Inhibition of ACAT by avasimibe decreases both VLDL and LDL apolipoprotein B production in miniature pigs.

An orally bioavailable acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor, avasimibe (CI-1011), was used to test the hypothesis that inhibition of cholesterol esterification, in vivo, would reduce hepatic very low density (VLDL) apolipoprotein (apo) B secretion into plasma. ApoB kinetic studies were carried out in 10 control miniature pigs, and in 10 animals treated with avasimibe (10 mg/kg/d, n = 6; 25 mg/kg/d, n = 4). Pigs were fed a diet containing fat (34% of calories) and cholesterol (400 mg/d; 0.1%). Avasimibe decreased the plasma concentrations of total triglyceride, VLDL triglyceride, and VLDL cholesterol by 31;-40% 39-48%, and 31;-35%, respectively. Significant reductions in plasma total cholesterol (35%) and low density lipoprotein (LDL) cholesterol (51%) concentrations were observed only with high dose avasimibe. Autologous 131I-labeled VLDL, 125I-labeled LDL, and [3H]leucine were injected simultaneously into each pig and apoB kinetic data were analyzed using multicompartmental analysis (SAAM II). Avasimibe decreased the VLDL apoB pool size by 40;-43% and the hepatic secretion rate of VLDL apoB by 38;-41%, but did not alter its fractional catabolism. Avasimibe decreased the LDL apoB pool size by 13;-57%, largely due to a dose-dependent 25;-63% in the LDL apoB production rate. Hepatic LDL receptor mRNA abundances were unchanged, consistent with a marginal decrease in LDL apoB FCRs. Hepatic ACAT activity was decreased by 51% (P = 0.050) and 68% (P = 0.087) by low and high dose avasimibe, respectively. The decrease in total apoB secretion correlated with the decrease in hepatic ACAT activity (r = 0.495; P = 0.026).We conclude that inhibition of hepatic ACAT by avasimibe reduces both plasma VLDL and LDL apoB concentrations, primarily by decreasing apoB secretion.     

Effects of continuous conjugated estrogen and micronized progesterone therapy upon lipoprotein metabolism in postmenopausal women

The effects of continuously administering both conjugated equine estrogens (CEE) and micronized progesterone (MP) on the concentration, composition, production and catabolism of very low density (VLDL) and low density lipoproteins (LDL) have not previously been reported. The mechanism of the hormonally induced reductions of plasma LDL cholesterol of S(f) 0;-20 (mean 16%, P < 0.005) and LDL apoB (mean 6%, P < 0.025) were investigated by studying the kinetics of VLDL and LDL apolipoprotein (apo) B turnover after injecting autologous (131)I-labeled VLDL and (125)I-labeled LDL into each of the 6 moderately hypercholesterolemic postmenopausal subjects under control conditions and again in the fourth week of a 7-week course of therapy (0.625 mg/d of CEE + 200 mg/d of MP). The combined hormones significantly lowered plasma LDL apoB by increasing the mean fractional catabolic rate of LDL apoB by 20% (0. 32 vs. 0.27 pools/d, P < 0.03). Treatment also induced a significant increase in IDL production (6.3 vs. 3.7 mg/kg/d, P = 0.028). However, this did not result in an increase in LDL production because of an increase in IDL apoB direct catabolism (mean 102%, P = 0.033). VLDL kinetic parameters were unchanged and the concentrations of plasma total triglycerides (TG), VLDL-TG, VLDL-apoB did not rise as often seen with estrogen alone. Plasma HDL-cholesterol rose significantly (P < 0.02). Our major conclusion is that increased fractional catabolism of LDL underlies the LDL-lowering effect of the combined hormones.     

Low-density lipoprotein catabolism in WHHL rabbits after partial ileal bypass surgery

The effect of partial ileal bypass surgery (PIB) on lipoprotein concentrations and compositions and on the catabolism of low-density lipoproteins (LDL) was studied in Watanabe heritable hyperlipidemic (WHHL) rabbits. After PIB, total serum cholesterol was 65% lower (6.22 +/- 1.58 vs. 17.24 +/- 3.22 mmol/l) and LDL cholesterol 81% lower (2.02 +/- 0.95 vs. 10.90 +/- 3.60 mmol/l) than in control WHHL rabbits; cholesteryl esters, expressed as percentage of mass, were 55% lower in the very-low and intermediate-density lipoprotein (VLDL + IDL) fractions, and 45% lower in LDL, whereas triacylglycerols were 89% higher in VLDL + IDL and 121% higher in LDL. The fractional catabolic rate (FCR) of LDL protein (apoLDL) from operated animals was 10% higher than that from controls in all animals (0.55 +/- 0.10 vs. 0.50 +/- 0.10 pools/day; P less than 0.01). The FCR of autologous apoLDL in PIB rabbits was 50% higher than that of autologous apoLDL in control rabbits (0.63 +/- 0.05 vs. 0.42 +/- 0.06 pools/day); this was not caused by induction of receptor-mediated clearance of LDL. The production rate of apoLDL after PIB in PIB rabbits was 50% lower compared to control apoLDL in controls (26.0 +/- 6.7 vs. 51.7 +/- 16.4 mg/kg per day). We conclude that PIB lowers LDL cholesterol in WHHL rabbits by a decreased production of LDL, by an increased non-specific clearance of LDL and by compositional changes, which lead to LDL particles containing less cholesterol.     

Differential binding of triglyceride-rich lipoproteins to lipoprotein lipase.

In comparison to very low density lipoprotein (VLDL), chylomicrons are cleared quickly from plasma. However, small changes in fasting plasma VLDL concentration substantially delay postprandial chylomicron triglyceride clearance. We hypothesized that differential binding to lipoprotein lipase (LPL), the first step in the lipolytic pathway, might explain these otherwise paradoxical relationships. Competition binding assays of different lipoproteins were performed in a solid phase assay with purified bovine LPL at 4 degrees C. The results showed that chylomicrons, VLDL, and low density lipoprotein (LDL) were able to inhibit specific binding of (125)I-labeled VLDL to the same extent (85.1% +/- 13.1, 100% +/- 6.8, 90.7% +/- 23.2% inhibition, P = NS), but with markedly different efficiencies. The rank order of inhibition (K(i)) was chylomicrons (0.27 +/- 0.02 nm apoB) > VLDL (12.6 +/- 3.11 nm apoB) > LDL (34.8 +/- 11.1 nm apoB). By contrast, neither triglyceride (TG) liposomes, high density lipoprotein (HDL), nor LDL from patients with familial hypercholesterolemia were efficient at displacing the specific binding of (125)I-labeled VLDL to LPL (30%, 39%, and no displacement, respectively). Importantly, smaller hydrolyzed chylomicrons had less affinity than the larger chylomicrons (K(i) = 2.34 +/- 0.85 nm vs. 0.27 +/- 0.02 nm apoB respectively, P < 0.01). This was also true for hydrolyzed VLDL, although to a lesser extent. Chylomicrons from patients with LPL deficiency and VLDL from hypertriglyceridemic subjects were also studied. Taken together, our results indicate an inverse linear relationship between chylomicron size and K(i) whereas none was present for VLDL. We hypothesize that the differences in binding affinity demonstrated in vitro when considered with the differences in particle number observed in vivo may largely explain the paradoxes we set out to study.     

Soy protein with or without isoflavones, soy germ and soy germ extract, and daidzein lessen plasma cholesterol levels in golden Syrian hamsters

Dietary isolated soy protein (ISP, containing approximately equal amounts of daidzein and genistein), ethanol-extracted ISP (ISP (-)), soygerm or soygerm extract (containing large amounts of daidzein and glycitein and little genistein) and the isoflavone, daidzein, were hypothesized to lessen plasma cholesterol in comparison with casein. Sixty male and 60 female golden Syrian hamsters (6-8 weeks of age) were randomly assigned to six treatments fed for 10 weeks. Four of the experimental diets (ISP, daidzein, soygerm, and soygerm extract) contained 1.3 mmol total isoflavones/kg. The ISP (-) diet contained 0.013 mmol isoflavone/kg, whereas the casein diet contained no isoflavones. Hamsters fed ISP, ISP (-), daidzein, soygerm, and soygerm extract had significantly less plasma total cholesterol (by 16%-28%), less non-HDL cholesterol (by 15%-50%) and less non-HDL/HDL cholesterol ratios compared with hamsters fed casein (P < 0.01). For male hamsters, there were no differences among treatments in plasma HDL concentrations. Female hamsters fed ISP (-) had significantly greater HDL levels (P < 0.01) than females fed casein or daidzein. Triglyceride concentration was significantly less in hamsters fed ISP (-) compared with the casein-fed females. Because soy protein with or without isoflavones, soygerm and soygerm extract, and daidzein lessened plasma cholesterol to an approximately equal extent, soy protein alone, varying mixtures of isoflavones, and other extractable components of soy are responsible for cholesterol-lessening effects of soy foods, mainly due to their effects to lessen LDL cholesterol.     

Apolipoprotein B overproduction by the perfused liver of the St. Thomas' mixed hyperlipidemic (SMHL) rabbit

The St. Thomas' mixed hyperlipidemic (SMHL) rabbit (previously St. Thomas' Hospital rabbit) is a putative model of familial combined hyperlipidemia (FCH). When fed a low (0.08%) cholesterol diet, it exhibits elevations in both plasma cholesterol and triglyceride compared to New Zealand White (NZW) controls. To determine the mechanism for this hyperlipidemia we studied the secretion of apolipoprotein B (apoB)-containing lipoproteins from perfused livers of both young and mature rabbits. During a 3-h perfusion we measured the total cholesterol and triglyceride content of the medium and the cholesterol, triglyceride, and apoB content of very low density lipoprotein (VLDL)(1) (S(f) 60;-400), VLDL(2) (S(f) 20;-60), intermediate (S(f) 12;-20), and low (S(f) 0;-12) density lipoproteins (IDL, LDL). Lipoprotein concentrations increased linearly throughout the perfusion period. The rate of cholesterol output was 3-fold higher (459 vs. 137 ng/g liver/min, P = 0.003) in SMHL versus NZW rabbits whilst that of triglyceride was similar (841 vs. 662 ng/g liver/min, NS). VLDL(1) cholesterol output was elevated 2-fold (232 vs. 123 ng/g liver/min, P < 0.05) and VLDL(2) + IDL + LDL cholesterol output, 4.5-fold (106 vs. 23 ng/g liver/min, P < 0. 005) in SMHL versus NZW rabbits. ApoB output in VLDL1 was 38 ng/g liver per min in SMHL and 14 ng/g liver per min in NZW (NS). In SMHL VLDL(2) + IDL + LDL apoB was increased 9-fold at 53 versus 6 ng/g liver per min in NZW (P < 0.001). We conclude that the SMHL rabbit overproduces apoB-containing lipoproteins particularly in the VLDL(2) + IDL + LDL fraction, a characteristic consistent with its use as a model of FCH.     

Chenodeoxycholic acid normalizes elevated lipoprotein secretion and catabolism in cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX) is a rare inherited lipid storage disease caused by a defect in bile acid synthesis in which cholesterol and its product cholestanol are deposited in neurological and vascular tissue. Therapy with chenodeoxycholic acid but not with the 7 beta-epimeric ursodeoxycholic acid is usually successful. In an untreated patient, total and low density lipoprotein (LDL) cholesterol were found to be low (134 +/- 11 and 78 +/- 8 mg/dl, respectively). The production rate (PR) and fractional catabolic rate (FCR) of very low density (VLDL) apolipoprotein B (apoB) were, however, both markedly increased (34.7 mg/kg per day and 13.7 pools/day, respectively vs. 15.1 +/- 5.0 mg/kg per day and 6.2 +/- 3.8 pools/day in controls) while the PR and FCR of LDL apoB were moderately elevated (16.3 mg/kg per day and 0.65 pools/day, respectively vs. 12.9 +/- 1.2 mg/kg per day and 0.52 +/- 0.10 pools/day in controls). After 1 month of 750 mg/day of chenodeoxycholic acid, the FCR and PR of both VLDL and LDL apoB became normal while total plasma cholesterol increased significantly to 145 +/- 18 mg/dl. In a second patient who had been receiving 750 mg/day of chenodeoxycholic acid for 6 months lipoprotein kinetics were normal. These parameters did not change when the subject was switched to 750 mg/day ursodeoxycholic acid. We postulate that cholesterol biosynthesis in CTX is derepressed by a diminished hepatic pool of chenodeoxycholic acid and that the elevated secretion of apoB is a response to the increased rate of cholesterol production.     

54Mn absorption and excretion in rats fed soy protein and casein diets

Rats were fed diets containing either soy protein or casein and different levels of manganese, methionine, phytic acid, or arginine for 7 days and then fed test meals labeled with 2 microCi of 54Mn after an overnight fast. Retention of 54Mn in each rat was measured every other day for 21 days using a whole-body counter. Liver manganese was higher (P less than 0.0001) in soy protein-fed rats (8.8 micrograms/g) than in casein-fed rats (5.2 micrograms/g); manganese superoxide dismutase activity also was higher in soy protein-fed rats than in casein-fed rats (P less than 0.01). There was a significant interaction between manganese and protein which affected manganese absorption and biologic half-life of 54Mn. In a second experiment, rats fed soy protein-test meals retained more 54Mn (P less than 0.001) than casein-fed rats. Liver manganese (8.3 micrograms/g) in the soy protein group was also higher than that (5.7 micrograms/g) in the casein group (P less than 0.0001), but manganese superoxide dismutase activity was unaffected by protein. Supplementation with methionine increased 54Mn retention from both soy and casein diets (P less than 0.06); activity of manganese superoxide dismutase increased (P less than 0.05) but liver manganese did not change. The addition of arginine to casein diets had little effect on manganese bioavailability. Phytic acid affected neither manganese absorption nor biologic half-life in two experiments, but it depressed liver manganese in one experiment. These results suggest that neither arginine nor phytic acid was the component in soy protein which made manganese more available from soy protein diets than casein diets.     

Regulation of hepatic receptor-dependent degradation of LDL by mevinolin in rabbits with hypercholesterolemia induced by a wheat starch-casein diet

Rabbits fed a wheat starch-casein diet develop a marked hypercholesterolemia and have a slower rate of removal of rabbit 125I-labeled low density lipoproteins (LDL) from plasma. Treating rabbits with mevinolin, a highly potent competitive inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase, at a daily dose of 20 mg per animal prevents the increase in plasma and LDL cholesterol. The mevinolin effect is mediated through an increased rate of removal of rabbit 125I-labeled LDL from plasma. To study the role of mevinolin on the regulation of the hepatic LDL receptor in rabbits, the binding of 125I-labeled LDL and 125I-labeled beta-VLDL (beta-migrating very-low-density lipoproteins) to liver membranes prepared from rabbits fed the wheat starch-casein diet with or without mevinolin was investigated. Liver membranes from wheat starch-casein-fed rabbits have no demonstrable EDTA-sensitive binding activity of 125I-labeled LDL and low (37 ng/mg protein) binding activity of 125I-labeled beta-VLDL. Treatment of the wheat starch-casein fed rabbits with mevinolin results in high levels of specific EDTA-sensitive binding of 125I-labeled LDL (28.7 ng/mg protein) and 125I-labeled beta-VLDL (120 ng/mg protein). To assess the functional role of the hepatic LDL receptor in response to mevinolin, the catabolism of 125I-labeled LDL by perfused rabbit livers was studied. Perfused livers from mevinolin-treated rabbits show a 3.3-fold increase in the rate of receptor-dependent catabolism of 125I-labeled LDL (4.6% X h-1) when compared with that of livers from rabbits not treated with mevinolin (1.4% X h-1). Thus, these studies demonstrate that mevinolin prevents the increase of plasma LDL cholesterol level in rabbits fed a wheat starch-casein diet by regulating the levels of hepatic LDL-binding sites and the rate of receptor-dependent catabolism of LDL by the liver.     

Role of insulin in regulation of high density lipoprotein metabolism

The effect of alloxan-induced insulin deficiency on high density lipoprotein (HDL) metabolism was studied in rabbits. Rabbits with alloxan-induced diabetes had significantly higher (P less than 0.001, mean +/- SEM) plasma concentrations of glucose (541 +/- 13 vs. 130 +/- 2 mg/dl), triglyceride (2851 +/- 332 vs. 101 +/- 10 mg/dl), and total plasma cholesterol (228 +/- 55 vs. 42 +/- 4 mg/dl) than did normal control rabbits. However, diabetic rabbits had lower plasma HDL-cholesterol (7.2 +/- 1 vs. 51.3 +/- 1.3 mg/dl, P less than 0.001) and HDL apoA-I (38.3 +/- 6.0 vs. 87.2 +/- 4.3 mg/dl, P less than 0.001) concentrations. HDL kinetics were compared in diabetic and normal rabbits, using either 125I-labeled HDL or HDL labeled with 125I-labeled apoA-I, and it was demonstrated that HDL fractional catabolic rate (FCR) was slower and residence time was longer in the diabetic rabbits when either tracer was used. The slow FCR and the low apoA-I pool size led to reduced apoA-I/HDL synthetic rate in diabetic rabbits (0.97 +/- 0.11 vs. 0.34 +/- 0.07 mg per kg per hr). Thus, the reduced plasma HDL-cholesterol concentrations seen in rabbits with alloxan-induced insulin deficiency was associated with a lower total apoA-I/HDL synthetic rate. Since insulin treatment restored to normal all of the changes in plasma lipoprotein concentration and kinetics seen in diabetic rabbits, it is unlikely that the phenomena observed were secondary to a nonspecific toxic effect of alloxan. These data strongly support the view that insulin plays an important role in regulation of HDL metabolism.     

Effect of fish oil diet on hepatic lipid metabolism in nonhuman primates: lowering of secretion of hepatic triglyceride but not apoB

African green monkeys were fed diets containing either 11% (by weight) fish oil or lard for 2.5 yr. To test the hypothesis that fish oil decreases hepatic secretion of triglyceride (TG) and apoB, livers from these animals were perfused with a fatty acid mixture [85% (w/w) oleate containing [14C]oleate and 15% n-3 containing [3H]eicosapentaenoic acid (EPA)] at a rate of 0.1 mumol fatty acid/min per g liver. Liver perfusate was sampled every 30 min during 4 h of recirculating perfusion. The concentration of triglyceride was similar for livers of animals of both groups and there was no difference between groups in the extent of incorporation of [3H]EPA or [14C]oleate into hepatic TG. While the secretion rate for the mass of TG was less in the fish oil-fed group (8.3 +/- 2.5 vs 18.3 +/- 4.4 mg/h per 100 g liver, P less than 0.05), the apoB secretion rate was similar (0.92 +/- 0.15 vs 1.01 +/- 0.13 mg/h per 100 g liver). Significantly less [3H]EPA was incorporated into secreted TG in the fish oil group (0.4 +/- 0.1 vs 1.0 +/- 0.1% infused dose/h; P less than 0.01). The rate of secretion of [14C]TG was similar for both groups (1.3 +/- 0.3 vs 1.4 +/- 0.1% infused dose/h for fish oil and lard groups, respectively). No significant diet-related differences in [3H]TG or [14C]TG fatty acid specific activity were observed for perfusate TG or hepatic TG. After perfusion, livers from fish oil-fed monkeys contained significantly more [3H]EPA in hepatic phospholipid than livers from lard-fed monkeys (19.5 +/- 1.8 vs 11.4 +/- 1.7% infused dose; P less than 0.01) although hepatic phospholipid mass concentrations were similar. The liver phospholipids of the fish oil group were enriched in n-3 fatty acid mass and were relatively depleted of oleate and linoleate. We conclude that although apoB secretion was unaffected, dietary fish oil significantly decreased hepatic TG secretion through relatively poor utilization of EPA for the synthesis of TG destined for secretion in VLDL; at the same time, increased incorporation of [3H]EPA into hepatic phospholipid accompanied the decreased incorporation into secreted TG and these events may be coupled.(ABSTRACT TRUNCATED AT 400 WORDS)     

Apolipoprotein B metabolism and the distribution of VLDL and LDL subfractions

Apolipoprotein B (apoB) metabolism was investigated in 20 men with plasma triglyceride 0.66-2.40 mmol/l and plasma cholesterol 3.95-6. 95 mmol/l. Kinetics of VLDL(1) (S(f) 60-400), VLDL(2) (S(f) 20-60), IDL (S(f) 12-20), and LDL (S(f) 0;-12) apoB were analyzed using a trideuterated leucine tracer and a multicompartmental model which allowed input into each fraction. VLDL(1) apoB production varied widely (from 5.4 to 26.6 mg/kg/d) as did VLDL(2) apoB production (from 0.18 to 8.4 mg/kg/d) but the two were not correlated. IDL plus LDL apoB direct production accounted for up to half of total apoB production and was inversely related to plasma triglyceride (r = -0.54, P = 0.009). Percent of direct apoB production into the IDL/LDL density range (r = 0.50, P < 0.02) was positively related to the LDL apoB fractional catabolic rate (FCR). Plasma triglyceride in these subjects was determined principally by VLDL(1) and VLDL(2) apoB fractional transfer rates (FTR), i.e., lipolysis. IDL apoB concentration was regulated mainly by the IDL to LDL FTR (r = -0.71, P < 0.0001). LDL apoB concentration correlated with VLDL(2) apoB production (r = 0.48, P = 0.018) and the LDL FCR (r = -0.77, P < 0. 001) but not with VLDL(1), IDL, or LDL apoB production. Subjects with predominantly small, dense LDL (pattern B) had lower VLDL(1) and VLDL(2) apoB FTRs, higher VLDL(2) apoB production, and a lower LDL apoB FCR than those with large LDL (pattern A). Thus, the metabolic conditions that favored appearance of small, dense LDL were diminished lipolysis of VLDL, resulting in a raised plasma triglyceride above the putative threshold of 1.5 mmol/l, and a prolonged residence time for LDL. This latter condition presumably permitted sufficient time for the processes of lipid exchange and lipolysis to generate small LDL particles.     

Effect of atorvastatin on plasma apoE metabolism in patients with combined hyperlipidemia

Atorvastatin, a synthetic HMG-CoA reductase inhibitor used for the treatment of hyperlipidemia and the prevention of coronary artery disease, significantly lowers plasma cholesterol and low-density lipoprotein cholesterol (LDL-C) levels. It also reduces total plasma triglyceride and apoE concentrations. In view of the direct involvement of apoE in the pathogenesis of atherosclerosis, we have investigated the effect of atorvastatin treatment (40 mg/day) on in vivo rates of plasma apoE production and catabolism in six patients with combined hyperlipidemia using a primed constant infusion of deuterated leucine. Atorvastatin treatment resulted in a significant decrease (i.e., 30-37%) in levels of total triglyceride, cholesterol, LDL-C, and apoB in all six patients. Total plasma apoE concentration was reduced from 7.4 +/- 0.9 to 4.3 +/- 0.2 mg/dl (-38 +/- 8%, P < 0.05), predominantly due to a decrease in VLDL apoE (3.4 +/- 0.8 vs. 1.7 +/- 0.2 mg/dl; -42 +/- 11%) and IDL/LDL apoE (1.9 +/- 0.3 vs. 0.8 +/- 0.1 mg/dl; -57 +/- 6%). Total plasma lipoprotein apoE transport (i.e., production) was significantly reduced from 4.67 +/- 0.39 to 3.04 +/- 0.51 mg/kg/day (-34 +/- 10%, P < 0.05) and VLDL apoE transport was reduced from 3.82 +/- 0.67 to 2.26 +/- 0.42 mg/kg/day (-36 +/- 10%, P = 0.057). Plasma and VLDL apoE residence times and HDL apoE kinetic parameters were not significantly affected by drug treatment. Percentage decreases in VLDL apoE concentration and VLDL apoE production were significantly correlated with drug-induced reductions in VLDL triglyceride concentration (r = 0.99, P < 0.001; r = 0.88, P < 0.05, respectively, n = 6). Our results demonstrate that atorvastatin causes a pronounced decrease in total plasma and VLDL apoE concentrations and a significant decrease in plasma and VLDL apoE rates of production in patients with combined hyperlipidemia.