Divergent effects of d-norgestrel on the metabolism of rat very low density and low density apolipoprotein B

Rats treated with the contraceptive steroid d-norgestrel have lower plasma very low density lipoprotein (VLDL)-triglycerides and higher low density lipoprotein (LDL)-cholesterol than controls. To explain these results, the kinetics of VLDL and LDL turnover were studied by injecting 125I-labeled rat-VLDL and 131I-labeled rat-LDL simultaneously into rats treated with a small dose of d-norgestrel (4 micrograms per day per kg body weight0.75 for 18 days, n = 22) and their untreated controls (n = 22). VLDL- and LDL-apoB specific activity-time curves obtained over 50 hr best conformed to a three-pool model. VLDL-apoB clearance expressed as irreversible catabolic rate (k01) was markedly enhanced in the treated versus control rats (0.57 vs. 0.34 pools hr-1), leading to a marked reduction in VLDL-apoB pool size (270 vs. 420 micrograms). However, VLDL-apoB production rates were similar in the two groups (160 vs. 140 micrograms/hr, respectively). The 125I-labeled apoB specific activity-time curve derived from the catabolism of 125I-labeled VLDL-apoB also showed enhanced clearance in d-norgestrel-treated rats. 125I-Labeled IDL-apoB and 125I-labeled LDL-apoB specific activity-time curves failed to intersect the VLDL-apoB curve at maximal heights, suggesting input of intermediate density lipoprotein (IDL) and LDL independent of VLDL catabolism in both groups. However, the extent of independent LDL-apoB production was similar in both groups. Clearance of 131I-labeled LDL-apoB following injection of 131I-labeled rat-LDL was delayed in the d-norgestrel-treated versus control rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Turnover of 125I-VLDL and 131I-LDL apolipoprotein B in rabbits fed diets containing casein or soy protein

Rabbits fed low-fat, cholesterol-free, semi-purified diets containing casein developed a marked hypercholesterolemia compared to rabbits fed a similar diet containing soy protein (plasma cholesterol 281 +/- 31 vs. 86 +/- 9 mg/dl; P less than 0.05). Turnover studies (three per dietary group) were carried out in which homologous 125I-labeled VLDL and 131I-labeled LDL were injected simultaneously into casein- (n = 8) or soy protein- (n = 9) fed rabbits. ApoB-specific activities were determined in VLDL, IDL and LDL isolated from the pooled plasma of two or three rabbits per dietary group. The production rate of VLDL apoB (1.20 +/- 0.3 vs. 1.09 +/- 0.1 mg/h per kg) was similar for the two dietary groups. The fractional catabolic rate of VLDL apoB was lower for the casein group (0.15 +/- 0.03 vs. 0.23 +/- 0.01.h-1; 0.05 less than P less than 0.10). Although the pool size of VLDL apoB was higher in the casein group (8 +/- 2 vs. 5 +/- 0.3 mg/kg), this value did not reach statistical significance. For LDL apoB, the increased pool size in casein-fed rabbits (30 +/- 5 vs. 5 +/- 1 mg/kg; P less than 0.01) was associated with a decreased fractional catabolic rate (0.03 +/- 0.005 vs. 0.08 +/- 0.008.h-1; P less than 0.01) and a 2-fold increase in the production rate of LDL apoB (1 +/- 0.3 vs. 0.4 +/- 0.06 mg/kg per h; 0.05 less than P less than 0.10) compared to rabbits fed soy protein. Analysis of precursor-product relationships between the various lipoprotein fractions showed that casein-fed rabbits synthesized a higher proportion of LDL apoB (95% +/- 2 vs. 67% +/- 2; P less than 0.001) independent of VLDL catabolism. These results support the concept that the hypercholesterolemia in casein-fed rabbits is associated with impaired LDL removal consistent with a down-regulation of LDL receptors. These changes do not occur when the casein is replaced by soy protein.     

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.     

Acute effects of low density lipoprotein apheresis on metabolic parameters of apolipoprotein B

Apheresis is a treatment option for patients with severe hypercholesterolemia and coronary artery disease. It is unknown whether such therapy changes kinetic parameters of lipoprotein metabolism, such as apolipoprotein B (apoB) secretion rates, conversion rates, and fractional catabolic rates (FCR). We studied the acute effect of apheresis on metabolic parameters of apoB in five patients with drug-resistant hyperlipoproteinemia, using endogenous labeling with D(3)-leucine, mass spectrometry, and multicompartmental modeling. Patients were studied prior to and immediately after apheresis therapy. The two tracer studies were modeled simultaneously, taking into account the non-steady-state concentrations of apoB. The low density lipoprotein (LDL)-apoB concentration was 120+/-32 mg dl(-1) prior to and 52+/-18 mg dl(-1) immediately after apheresis therapy. The metabolic studies indicate that no change in apoB secretion (13.9+/- 4.9 mg kg(-1) day(-1)) is required to fit the tracer and apoB mass data obtained before and after apheresis and that in four of the five patients the LDL-apoB FCR (0.21+/-0.02 day(-1)) was not altered after apheresis. In one subject the LDL-apoB FCR temporarily increased from 0.22 day(-1) to 0.35 day(-1) after apheresis. The conversion rate of very low density lipoprotein (VLDL)-apoB to LDL-apoB is temporarily decreased from 76 to 51% after apheresis and thus less LDL-apoB is produced after apheresis. We conclude that an acute reduction of LDL-apoB concentration does not affect apoB secretion or LDL-apoB FCR, but that apoB conversion to LDL is temporarily decreased. Thus, in most patients the decreased rate of delivery of neutral lipids or apoB to the liver does not result in an upregulation of LDL receptors or in decreased 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.     

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.     

Determination of kinetic parameters of apolipoprotein B metabolism using amino acids labeled with stable isotopes.

The use of amino acids labeled with stable isotopes represents a relatively new approach for determining kinetic parameters of apolipoprotein metabolism; thus, several aspects of experimental protocols need to be defined. The aims of the present study were to determine whether a) different amino acid tracers or b) different methods of tracer administration affected apolipoprotein (apo) B kinetic parameters obtained by multicompartmental modeling, and c) to compare very low density lipoprotein (VLDL)-apoB metabolic parameters determined by multicompartmental modeling with those estimated by linear regression or by monoexponential analysis. [1-13C]leucine and [15N]glycine were given either as bolus injections or as primed constant infusions. A bolus of one amino acid was administered simultaneously with a primed constant infusion (8 h) of the other amino acid into four healthy normolipidemic subjects (age 23.0 +/- 1.4 yr; BMI 20.9 +/- 0.9 kg.m-2). VLDL-, intermediate density lipoprotein (IDL)-, and low density lipoprotein (LDL)-apoB enrichments were followed over 110 h. For subsequent analysis these values were converted to tracer/tracee ratios. Using the multicompartmental model, the fractional catabolic rate (FCR) for VLDL-apoB was estimated to be 0.36 +/- 0.09 h-1 after the administration of the tracer as a primed constant infusion and 0.35 +/- 0.07 h-1 when the tracer was administered as a bolus. The values for VLDL-apoB production were 14.6 +/- 6.5 mg.kg-1.d-1 and 14.1 +/- 5.4 mg.kg-1.d-1, respectively. The corresponding values for LDL-apoB were 0.027 +/- 0.016 h-1 (0.026 +/- 0.018 h-1) for the FCR and 10.5 +/- 3.7 mg.kg-1.d-1 (10.4 +/- 3.8 mg.kg-1.d-1) for the production following administration of the tracer as a primed constant infusion and a bolus, respectively. Approximately 47% of VLDL-apoB ultimately reached the LDL fraction via the VLDL-IDL-LDL pathway. Thirty-five percent of LDL-apoB did not originate from this cascade pathway, but was shunted from a rapidly turning over VLDL compartment directly into the LDL fraction. While there was some variation between individuals, VLDL-apoB and LDL-apoB parameters derived from the bolus and the primed constant infusions showed no significant differences and were closely correlated. Metabolic parameters were also independent of the two amino acids tested. Although values for FCRs of VLDL-apoB obtained from linear regression (0.36 +/- 0.19 h-1) or monoexponential analysis (0.50 +/- 0.36 h-1) did not differ significantly from those obtained by the multicompartmental model, there was considerable variation and no significant correlation in a given individual.(ABSTRACT TRUNCATED AT 400 WORDS)     

Treatment with high-dose simvastatin reduces secretion of apolipoprotein B-lipoproteins in patients with diabetic dyslipidemia

HMG-CoA reductase inhibitors (statins) are effective lipid-altering drugs for the treatment of dyslipidemia in patients with type 2 diabetes mellitus. We conducted a randomized, double-blind, placebo-controlled, crossover design trial to determine the effects of simvastatin, 80 mg/day, on plasma lipid and lipoprotein levels and on the metabolism of apolipoprotein B (apoB) in VLDL, intermediate density lipoprotein (IDL), and LDL and of triglycerides (TGs) in VLDL. Simvastatin therapy decreased TG, cholesterol, and apoB significantly in VLDL, IDL, and LDL. These effects were associated with reduced production of LDL-apoB, mainly as a result of reduced secretion of apoB-lipoproteins directly into the LDL density range. Statin therapy also reduced hepatic production of VLDL-TG. There were no effects of simvastatin on the fractional catabolic rates of VLDL-apoB or -TG or LDL-apoB. The basis for decreased VLDL-TG secretion during simvastatin treatment is not clear, but recent studies suggest that statins may activate peroxisomal proliferator-activated receptor alpha (PPARalpha). Activation of PPARalpha could lead to increased hepatic oxidation of fatty acids and less synthesis of TG for VLDL assembly.     

A study of the metabolism of apolipoprotein B100 in relation to insulin resistance in African American males.

The purpose of this study was to determine the relationship between insulin resistance and apoB100 metabolism in African American males. Fifteen subjects, 33 +/- 7.6 years old, were divided into two groups, insulin-resistant (IR) or insulin-sensitive (IS), based on the sum of the plasma insulin concentrations during an oral glucose tolerance test. The IR group (n = 8) differed significantly from the IS group (n = 7) with respect to body mass index (BMI) (30.1 vs 23.1 kg/m2; P = 0.0003), fasting triglycerides, (118 vs 54 mg/dl, P = 0. 013), and total plasma apolipoprotein B100 (80 vs 59 mg/dl, P = 0.014). Significantly elevated apoB100 levels in the IR group were seen in very low density lipoprotein (VLDL) (5.1 vs 3.4 mg/dl, P = 0.045) and intermediate density lipoprotein (IDL) (18 vs 12 mg/dl, P = 0.017) but not in low density lipoprotein (LDL) (57 vs 46 mg/dl, P = 0.19). Total cholesterol, high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), apolipoprotein A-I, and blood pressure were not significantly different between the two groups. There was a high correlation between the sum of insulins during the oral glucose tolerance test and the BMI (rho = 0.88, P = 0.0001). In five IR and five IS subjects, apoB100 kinetics were determined in the fasting state using a bolus dose of deuteroleucine and multicompartmental modeling. IR subjects had significantly lower fractional catabolic rates (FCR) in the larger VLDL1 (-70%), the smaller VLDL2 (-71%), and the IDL (-53%) fractions. No significant differences in production rates were observed for any lipoprotein class. There was a significant correlation between the sum of insulins and the FCR of the apoB100 of VLDL1 (rho = -0.65, P = 0.05) and of IDL (rho = -0.85, P = 0.004). The correlation coefficient of the sum of insulins and the FCR of VLDL2 was -0.61 with P = 0.067. We conclude that in this population of African American males, IR is correlated with a decreased FCR of apoB100 in VLDL and IDL and elevated plasma levels of apoB and triglycerides (TG). These changes might be explained by decreased clearance of the TG-rich lipoproteins. We postulate that this may reflect decreased lipoprotein and/or hepatic lipase activity related to insulin resistance and its association with obesity.     

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.     

Direct determination of human and rabbit apolipoprotein B selectively precipitated with butanol-isopropyl ether

A method is described for the rapid, selective, and quantitative precipitation of apolipoprotein B from isolated hypercholesterolemic rabbit and human very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL). Lipoprotein samples are heat-treated at 100 degrees C in 1% SDS. The denatured apoprotein solutions are then mixed briefly with two volumes of butanol-isopropyl ether 45:55 (v/v) to precipitate the apoB. The supernatant solutions, containing the non-apoB proteins and lipids, are removed and the apoB pellet is washed once with water. To determine apoB specific activity, the apoB pellet is resolubilized in 0.5 M NaOH by heating for 30 min at 120 degrees C. The hydrolyzed apoB protein is quantitated by fluorescence of a fluorescamine derivative. The precipitation of apoB is quantitative and selective: 99.5% of rabbit 125I-labeled LDL-apoB and 97.5% of human 125I-labeled LDL-apoB is precipitated and less than 5% of 125I-labeled HDL added to unlabeled VLDL, IDL, or LDL is precipitated. Triglyceride and cholesteryl ester contamination of the apoB pellet is less than 2% of their original radioactivities.     

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)     

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.     

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.     

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.     

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.     

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.     

Human hepatic low-density lipoprotein receptors: associations of receptor activities in vitro with plasma lipid and apolipoprotein concentrations in vivo

The relationships of plasma lipid and apolipoprotein (apo) concentrations to hepatic low-density lipoprotein (LDL) receptor activity were examined in 21 subjects (16 females, 5 males), who were undergoing laparotomy for non-neoplastic disease (cholecystectomy in 16). None had familial hypercholesterolemia, or renal, endocrine or hepatic disease. Ages were 37-77 years (mean, 58 years), plasma cholesterol concentrations 4.09-6.72 mmol/l (5.38) and plasma triacylglycerol concentrations 0.75-2.35 mmol/l (1.36). Receptor activity was quantified in vitro as the total saturable binding and EDTA-suppressible binding (representing apoB,E receptors) of 125I-labelled human LDL (15 micrograms protein/ml) by liver homogenate at 37 degrees C. There were no significant differences between men and women in 125I-labeled LDL binding. In the pooled data, EDTA-suppressible binding averaged 50 ng 125I-LDL protein/mg cell protein (S.D., 15). Total saturable binding averaged 2-fold greater (mean, 101 ng/mg; S.D., 32). Plasma cholesterol, LDL cholesterol and apoB concentrations were negative functions of both EDTA-suppressible binding and total saturable binding, but the correlations with EDTA-suppressible binding were stronger (cholesterol: r = -0.59, P less than 0.01; LDL cholesterol: r = -0.48, P less than 0.05; apoB: r = -0.61, P less than 0.01). Plasma triacylglycerol, high-density lipoprotein cholesterol and apoA-I concentrations were not related to either measure of receptor activity. These results provide evidence that the activity of apoB,E receptors in the liver is a major determinant of the plasma LDL concentration in middle-aged and elderly humans.     

Low density lipoprotein metabolism in hypertriglyceridemic and normolipidemic patients with coronary heart disease

The turnover rates of low density lipoprotein-apolipoprotein B (LDL-apoB) were determined in 32 men with coronary heart disease (CHD) and 11 control men with normal plasma lipids. Thirty patients with CHD had normal levels of LDL-cholesterol (LDL-C); of these patients, 9 had hypertriglyceridemia and 21 had normal plasma lipids. Mean concentrations of total cholesterol and LDL-C were similar among the control subjects and CHD patients, although the latter had significantly lower HDL-C. In control subjects, transport rates and fractional catabolic rates (FCR) of LDL-B were 10.6 +/- 0.5 (SEM) mg/kg-day and 0.31 +/- 0.01 pools/day, respectively. In 10 hypertriglyceridemic patients with CHD, transport rates were 21.7 +/- 1.7 mg/kg-day, and FCRs averaged 0.56 +/- 0.06 pools/day; both were significantly higher than normal (P less than 0.05). Six normolipidemic patients also had abnormally high transport rates of LDL-apoB (19.4 +/- 2.8 mg/kg-day) and FCRs (0.51 +/- 0.03 pools/day); again both were higher than normal. The remaining 16 normolipidemic patients with CHD had normal transport rates (9.9 +/- 0.6 mg/kg-day) and FCRs (0.28 +/- 0.01 pools/day). Thus, hypertriglyceridemic patients with CHD and a portion of normolipidemic patients with CHD were characterized by increases in both transport and fractional catabolic rate of LDL-apoB; these abnormalities in LDL metabolism may have contributed to their coronary heart disease. However, the majority of normolipidemic patients with CHD did not show a distinct defect in their LDL metabolism.