Abnormal concentrations of CII, CIII, and E apolipoproteins among apolipoprotein B-containing, B-free, and A-I-containing lipoprotein particles in hemodialysis patients

Serum concentrations of total cholesterol, triglycerides, and apolipoproteins (apo) A-I, B, CII, CIII, and E in 36 hemodialysis patients and nine anephric patients were compared with the concentrations in 34 normolipidemic subjects. The dialysis patients displayed a moderate hypertriglyceridemia (1.94 +/- 0.12 vs 1.09 +/- 0.11 mmol/L in controls, mean +/- SEM; P less than 0.001), apo CIII concentrations were also increased (130.2 +/- 2.1 vs 108.4 +/- 0.7 mg/L; P less than 0.001), whereas apo CII (34.5 +/- 0.5 vs 36 +/- 0.5 mg/L; P less than 0.05), apo E (22.7 +/- 0.3 vs 27.9 +/- 0.2 mg/L; P less than 0.001), and apo A-I (1.18 +/- 0.05 vs 1.31 +/- 0.04 g/L; P less than 0.05) were decreased. Concentrations of serum apo B were normal (0.86 +/- 0.03 vs 0.97 +/- 0.07 g/L). In the hemodialysis patients, apo CIII concentrations were increased in apo B-containing lipoproteins (30.1 +/- 0.5 vs 25.0 +/- 0.1 mg/L; P less than 0.001), whereas CII and E were decreased below control values (14.4 +/- 0.2 vs 16.8 +/- 0.1, and 8.2 +/- 0.2 vs 11.4 +/- 0.1 mg/L, respectively; P less than 0.001 each). By calculation, non-B-containing lipoproteins in the hemodialysis group had increased concentrations of apo CIII (100.1 +/- 2.1 vs 83.3 +/- 0.7 mg/L; P less than 0.001) and decreased amounts of apo E (14.5 +/- 0.4 vs 16.4 +/- 0.3 mg/L; P less than 0.001); apo CII content was unchanged (20.1 +/- 0.5 vs 19.3 +/- 0.5 mg/L). Results for apo CII, CIII, and E among apo A-I-containing lipoproteins in both normolipidemic and hemodialysis groups were similar to those in non-B-containing lipoproteins. Finally, the sole significant (P less than 0.01) difference between the anephric and hemodialysis groups was the lower apo E concentrations in the former group. Accumulation of triglyceride-rich lipoproteins in hemodialysis patients may thus be related to the enrichment of apo CIII in apo B-containing lipoproteins and to a marked decrease in the apo CII and E contents.     

Mechanism of hypertriglyceridemia in human apolipoprotein (apo) CIII transgenic mice. Diminished very low density lipoprotein fractional catabolic rate associated with increased apo CIII and reduced apo E on the particles.

Hypertriglyceridemia is common in the general population, but its mechanism is largely unknown. In previous work human apo CIII transgenic (HuCIIITg) mice were found to have elevated triglyceride levels. In this report, the mechanism for the hypertriglyceridemia was studied. Two different HuCIIITg mouse lines were used: a low expressor line with serum triglycerides of approximately 280 mg/dl, and a high expressor line with serum triglycerides of approximately 1,000 mg/dl. Elevated triglycerides were mainly in VLDL. VLDL particles were 1.5 times more triglyceride-rich in high expressor mice than in controls. The total amount of apo CIII (human and mouse) per VLDL particle was 2 and 2.5 times the normal amount in low and high expressors, respectively. Mouse apo E was decreased by 35 and 77% in low and high expressor mice, respectively. Under electron microscopy, VLDL particles from low and high expressor mice were found to have a larger mean diameter, 55.2 +/- 16.6 and 58.2 +/- 17.8 nm, respectively, compared with 51.0 +/- 13.4 nm from control mice. In in vivo studies, radiolabeled VLDL fractional catabolic rate (FCR) was reduced in low and high expressor mice to 2.58 and 0.77 pools/h, respectively, compared with 7.67 pools/h in controls, with no significant differences in the VLDL production rates. In an attempt to explain the reduced VLDL FCR in transgenic mice, tissue lipoprotein lipase (LPL) activity was determined in control and high expressor mice and no differences were observed. Also, VLDLs obtained from control and high expressor mice were found to be equally good substrates for purified LPL. Thus excess apo CIII in HuCIIITg mice does not cause reduced VLDL FCR by suppressing the amount of extractable LPL in tissues or making HuCIIITg VLDL a bad substrate for LPL. Tissue uptake of VLDL was studied in hepatoma cell cultures, and VLDL from transgenic mice was found to be taken up much more slowly than control VLDL (P < 0.0001), indicating that HuCIIITg VLDL is not well recognized by lipoprotein receptors. Additional in vivo studies with Triton-treated mice showed increased VLDL triglyceride, but not apo B, production in the HuCIIITg mice compared with controls. Tissue culture studies with primary hepatocytes showed a modest increase in triglyceride, but not apo B or total protein, secretion in high expressor mice compared with controls. In summary, hypertriglyceridemia in HuCIIITg mice appears to result primarily from decreased tissue uptake of triglyceride-rich particles from the circulation, which is most likely due to increased apo CIII and decreased apo E on VLDL particles. the HuCIIITg mouse appears to be a suitable animal model of primary familial hypertriglyceridemia, and these studies suggest a possible mechanism for this common lipoprotein disorder.     

Chylomicronemia due to apolipoprotein CIII overexpression in apolipoprotein E-null mice. Apolipoprotein CIII-induced hypertriglyceridemia is not mediated by effects on apolipoprotein E.

The mechanism of apolipoprotein (apo) CIII-induced hypertriglyceridemia remains uncertain. We crossed apoCIII transgenic and apoE gene knockout (apoE0) mice, and observed severe hypertriglyceridemia with plasma triglyceride levels of 4,521+/-6, 394 mg/dl vs. 423+/-106 mg/dl in apoE0 mice, P < 0.00001 for log(triglycerides [TG]). Cholesterols were 1,181+/-487 mg/dl vs. 658+/-151 mg/dl, P < 0.0001. Lipoprotein fractionation showed a marked increase in triglyceride-enriched chylomicrons+VLDL. This increase was limited to the lowest density (chylomicrons and Sf 100-400) subfractions. Intermediate density lipoproteins (IDL)+LDL increased moderately, and HDL decreased. There was no significant increase in triglyceride production in apoCIII transgenic/apoE0 mice. The clearance of VLDL triglycerides, however, was significantly decreased. Lipoprotein lipase in postheparin plasma was elevated, but activation studies suggested LPL inhibition by both apoCIII transgenic and apoCIII transgenic/apoE0 plasma. ApoCIII overexpression also produced a marked decrease in VLDL glycosaminoglycan binding which was independent of apoE. The predominant mechanism of apoCIII-induced hypertriglyceridemia appears to be decreased lipolysis at the cell surface. The altered lipoprotein profile that was produced also allowed us to address the question of the direct atherogenicity of chylomicrons and large VLDL. Quantitative arteriosclerosis studies showed identical results in both apoCIII transgenic/apoE0 and apoE0 mice, supporting the view that very large triglyceride-enriched particles are not directly atherogenic.     

A rapid methodology for the isolation of intermediate-density lipoprotein: characterization of lipid composition and apoprotein content

BACKGROUND: Intermediate-density lipoprotein (IDL) is a structurally related precursor of low-density lipoprotein (LDL). Although found in significantly lower levels, extensive evidence suggests that IDL shares LDL's capacity to promote atherosclerosis. To assist further investigation into IDL's composition and physiological relevance, we have established a rapid method to extract IDL from plasma. METHODS: IDL was isolated from plasma by sequential floatation ultracentrifugation in 3 h, a significantly shorter isolation time than previously published methods. Apoproteins (apo) B100, CIII, and E, together with the level of albumin contamination, were quantified using single radial immunodiffusion. The lipid composition of IDL was measured using automated enzyme assays. RESULTS: The percent recovery of lipid from all lipoprotein fractions (VLDL+IDL+LDL+HDL) was 97.0+/-4.9% when compared to total plasma lipid. IDL had a reduced concentration of apo CIII, apo E, triglyceride, and free cholesterol, and had a higher concentration of apo B100, cholesterol ester, and phospholipid when compared to VLDL. Pure IDL migrated in advance of LDL during agarose electrophoresis. CONCLUSIONS: This rapid technique minimizes damage to the integrity of IDL and yields sufficient quantities to allow accurate assessment of composition and susceptibility to in vitro oxidation, and thus facilitates further investigation of IDL in the development of atherosclerosis.     

Influence of apolipoprotein E polymorphism on apolipoprotein B-100 metabolism in normolipemic subjects.

This study examined apolipoprotein (apo) B metabolism in normolipemic subjects homozygous for the apo E2 (n = 4), apo E3 (n = 5), or apo E4 (n = 5) phenotype. Radioiodinated very low density lipoprotein (VLDL1) (ultracentrifuge flotation rate [Sf] 60-400) and VLDL2 (Sf 20-60) were injected into volunteers and the conversion of apo B was followed through intermediate density lipoprotein (IDL) to low density lipoprotein (LDL). Subjects homozygous for E3 converted approximately 50% of LVDL2 to LDL, the remainder being lost by direct catabolism. Those with the E2 phenotype produced less VLDL1, but converted more of it to VLDL2 (compared to E3 subjects). They displayed a characteristic dyslipidemia with the presence of slowly catabolized VLDL1 and VLDL2 remnants. LDL levels were low owing to increased direct catabolism of VLDL2 and IDL and a reduced efficiency of delipidation; only 25% of VLDL2 apo B was directed to LDL production. In contrast, E4 subjects converted more VLDL2 apo B to LDL than E3 subjects. About 70% of VLDL2 apo B was found in LDL; direct catabolism of VLDL and IDL was reduced as was the fractional catabolic rate of LDL (0.2 vs. 0.26 in E3 subjects). These changes in the VLDL----IDL----LDL metabolic cascade can in part be explained by alterations in hepatic LDL receptors with E2 subjects having higher and E4 subjects lower activities than those in E3 homozygotes.     

Contribution of apo CIII reduction to the greater effect of 12-week micronized fenofibrate than atorvastatin therapy on triglyceride levels and LDL size in dyslipidemic patients

BACKGROUND: Apolipoprotein (apo) CIII plays an important role in the catabolism of triglyceride-rich lipoproteins as it is a potent inhibitor of lipoprotein lipase (LPL). A low LPL activity has been simultaneously associated with hypertriglyceridemia, low HDL cholesterol and with small LDL particles. AIM: To compare the effects of a 12-week treatment with micronized fenofibrate (200 mg) versus atorvastatin (10 mg) on apo CIII and lipoprotein-lipid levels including LDL size. METHOD: After a 4-week washout period, dyslipidemic patients were randomized to either micronized fenofibrate (n = 64) or atorvastatin (n = 72). RESULTS: Both fenofibrate and atorvastatin significantly decreased apo CIII levels by -0.03 +/- 0.03 versus -0.01 +/- 0.03 g/l respectively, and increased LDL size by 4.9 +/- 3.3 versus 1.8 +/- 2.9 A. Improvements in these parameters were significantly greater with fenofibrate (P < 0.0001). Significant relationships were found between changes in triglycerides and changes in apo CIII (r = 0.81 and r = 0.59, P < 0.0001) as well as between changes in LDL size and changes in apo CIII (r = -0.41 and r = -0.45, P < 0.001), in both fenofibrate and atorvastatin groups. respectively. CONCLUSION: The substantial reduction in apo CIII induced by micronized fenofibrate plays an important role in the greater effect of micronized fenofibrate than atorvastatin on plasma triglycerides and LDL size.     

Apolipoprotein B metabolism in subjects with deficiency of apolipoproteins CIII and AI. Evidence that apolipoprotein CIII inhibits catabolism of triglyceride-rich lipoproteins by lipoprotein lipase in vivo.

Previous data suggest that apolipoprotein (apo) CIII may inhibit both triglyceride hydrolysis by lipoprotein lipase (LPL) and apo E-mediated uptake of triglyceride-rich lipoproteins by the liver. We studied apo B metabolism in very low density (VLDL), intermediate density (IDL), and low density lipoproteins (LDL) in two sisters with apo CIII-apo AI deficiency. The subjects had reduced levels of VLDL triglyceride, normal LDL cholesterol, and near absence of high density lipoprotein (HDL) cholesterol. Compartmental analysis of the kinetics of apo B metabolism after injection of 125I-VLDL and 131I-LDL revealed fractional catabolic rates (FCR) for VLDL apo B that were six to seven times faster than normal. Simultaneous injection of [3H]glycerol demonstrated rapid catabolism of VLDL triglyceride. VLDL apo B was rapidly and efficiently converted to IDL and LDL. The FCR for LDL apo B was normal. In vitro experiments indicated that, although sera from the apo CIII-apo-AI deficient patients were able to normally activate purified LPL, increasing volumes of these sera did not result in the progressive inhibition of LPL activity demonstrable with normal sera. Addition of purified apo CIII to the deficient sera resulted in 20-50% reductions in maximal LPL activity compared with levels of activity attained with the same volumes of the native, deficient sera. These in vitro studies, together with the in vivo results, indicate that in normal subjects apo CIII can inhibit the catabolism of triglyceride-rich lipoproteins by lipoprotein lipase.     

Effects of combined estrogen and progestin administration on plasma lipoprotein metabolism in postmenopausal women.

Treatment of postmenopausal women with low doses of estradiol-17 beta (1 mg/d) and dl-norgestrel (0.075 [corrected] mg/d) significantly reduced fasting serum levels of low density lipoprotein (LDL) cholesterol and lowered very low density lipoprotein (VLDL) triglycerides in four of five subjects. To explain these results, the kinetics of VLDL and LDL apolipoprotein (apo) B turnover were studied by injecting autologous 125I-labeled VLDL and 131I-labeled LDL into subjects before discontinuing long-term (4-yr) treatment with the estradiol-17 beta and dl-norgestrel and again 7 wk after stopping treatment. The 24% mean decrease in VLDL apo B pool size during treatment was associated with a significant increase in VLDL apo B fractional catabolic rate (15 +/- 1 vs. 11 +/- 1 pools/d), whereas production rate was similar to control (24 +/- 3 vs. 21 +/- 2 mg/kg per d). There was a significant 25% mean decrease in LDL apo B pool size (27 +/- 2 vs. 36 +/- 3 mg/kg) due to a significant decrease in total (8.3 +/- 0.3 vs. 11 +/- 1 mg/kg per d) and independent (3.3 +/- 0.5 vs. 6.6 +/- 0.8 mg/kg per d, P less than 0.05) LDL apo B production. Estradiol-17 beta together with dl-norgestrel lowered plasma VLDL by enhancing their clearance and LDL by reducing their production.     

Effect of a high-carbohydrate, low-saturated-fat diet on apolipoprotein B and triglyceride metabolism in Pima Indians.

The mechanisms by which high-carbohydrate, low-saturated-fat diets lower LDL cholesterol (LDLC) concentrations are unknown. In this study, kinetics of VLDL, intermediate density lipoprotein (IDL), and LDL apoprotein B and VLDL triglyceride were determined in seven nondiabetic (ND) and seven non-insulin-dependent diabetic (NIDDM) Pima Indian subjects on high-fat and high-carbohydrate (HICHO) diets. Metabolic changes were similar in ND and NIDDM. On the HICHO diet, LDLC decreased (131 +/- 8 vs. 110 +/- 7 mg/dl, P less than 0.0001) in all subjects. Mean fasting and 24-h triglyceride (TG) concentrations were unchanged, as were mean production rates and fractional clearance rates (FCR) of VLDL apoB and VLDL TG. The mean VLDL apoB pool size (303 +/- 20 vs. 371 +/- 38 mg, P = 0.01) increased owing to a decrease in the mean transport rate (10.7 +/- 1.1 vs. 8.4 +/- 0.9 mg/kg fat-free mass (ffm) per day, P less than 0.0001) and the mean rate constant (2.3 +/- 0.2 vs. 1.5 +/- 0.2, P less than 0.001) for the VLDL apoB to IDL apoB conversion pathway. The mean transport rate of VLDL apoB to LDL apoB via IDL (10.2 +/- 0.9 vs. 8.0 +/- 0.8 mg/kg ffm per day, P less than 0.001) decreased. Mean LDL apoB concentrations decreased (70 +/- 5 vs. 61 +/- 5 mg/dl, P less than 0.001) on the HICHO diet. Means for total LDL apoB transport rate, LDL apoB FCR, and LDLC/apoB ratios were unchanged. In summary, the HICHO diet decreased the activity of mechanisms that convert VLDL to LDL, which contributed to the decrease in LDLC in all subjects. There was also evidence in some subjects for increased activity of LDL apoB clearance mechanisms, and a decrease in the LDLC to apoB ratio.     

Automated immunoturbidimetric analysis of six plasma apolipoproteins: correlation with radial immunodiffusion assays

We measured six apolipoproteins (apo AI, AII, B, CII, CIII, and E) by turbidimetric method using an automatic discrete biochemical analyzer and commercially available antisera. The turbidimetric method was compared with the single radial immunodiffusion method. Linearity for serum apolipoprotein assay by the automated turbidimetric method was better than by the single immuno-diffusion method. The linearity by the turbidimetric method was 2.5 G/L for AI, 1.0 G/L for AII, 4.5 G/L for B, 0.12 G/L for CII, 0.3 G/L for CIII, and 0.12 G/L for E. The presence of high concentrations of bilirubin (up to 0.15 G/L) and hemoglobin (up to 50 G/L) interfered with apolipoprotein measurement. Comparison of the immunoturbidimetric and the single radial immunodiffusion (SRID) methods showed excellent coefficients of correlation, r = 0.963, 0.896, 0.846, 0.936, 0.972, and 0.937 for apo AI, AII, B, CII, CIII, and E, respectively. Reference ranges for the six apolipoproteins were determined by using sera from 450 healthy subjects and were 1.4 +/- 0.3 G/L for AI and 0.3 +/- 0.01 for E. The observed levels of AII (P less than 0.001), B (P less than 0.01), and CIII (P less than 0.01) were significantly higher in males. The serum levels of apo B, CII, and E showed a gradual increase with age which was more prominent in females than in males. The levels of apo AI, AII decreased significantly over an 11 day period in 22 patients with myocardial infarction.     

Effect of heparin-induced lipolysis on the distribution of apolipoprotein e among lipoprotein subclasses. Studies with patients deficient in hepatic triglyceride lipase and lipoprotein lipase

In normal subjects, apolipoprotein E (apo E) is present on very low density lipoproteins (VLDL) (fraction I) and on particles of a size intermediate between VLDL and low density lipoproteins (LDL) (fraction II). The major portion of apo E is, however, on particles smaller than LDL but larger than the average high density lipoproteins (HDL) (fraction III). To investigate the possible role of the vascular lipases in determining this distribution of apo E among the plasma lipoproteins, we studied subjects with primary deficiency of either hepatic lipase or of lipoprotein lipase and compared them with normal subjects. Subjects with familial hepatic triglyceride lipase deficiency (n = 2) differ markedly from normal in that fraction II is the dominant apo E-containing group of lipoproteins. When lipolysis of VLDL was enhanced in these subjects upon release of lipoprotein lipase by intravenous heparin, a shift of the apo E from VLDL into fractions II and III was observed. In contrast, apolipoproteins CII and CIII (apo CII and CIII, respectively) did not accumulate in intermediate-sized particles but were shifted markedly from triglyceride rich lipoproteins to HDL after treatment with heparin. In subjects with primary lipoprotein lipase deficiency (n = 4), apo E was confined to fractions I and III. Release of hepatic triglyceride lipase by heparin injection in these subjects produced a shift of apo E from fraction I to III with no significant increase in fraction II. This movement of apo E from large VLDL and chylomicron-sized particles occurred with little hydrolysis of triglyceride and no significant shift of apo CII or CIII into HDL from triglyceride rich lipoproteins. When both lipoprotein lipase and hepatic triglyceride lipase were released by intravenous heparin injection into normal subjects (n = 3), fraction I declined and the apo E content of fraction III increased by an equivalent amount. Either moderate or no change was noted in the intermediate sized particles (fraction II). These data strongly support the hypothesis that fraction II is the product of the action of lipoprotein lipase upon triglyceride rich lipoproteins and is highly dependent on hepatic triglyceride lipase for its further catabolism. In addition, the hydrolysis by hepatic triglyceride lipase of triglyceride rich lipoproteins in general results in a preferential loss of apo E and its transfer to a specific group of large HDL.     

Regulation of the production and catabolism of plasma low density lipoproteins in hypertriglyceridemic subjects. Effect of weight loss.

In subjects with hypertriglyceridemia, plasma concentrations of low density lipoprotein (LDL) cholesterol are often normal or reduced. Perturbations that alter plasma very low density lipoprotein (VLDL) concentrations are associated with opposite changes in plasma LDL levels. To determine the mechanisms regulating plasma LDL levels, we used 131I-VLDL and 125I-LDL to measure the fractional catabolic rates (FCR), production rates (PR), and rates of interconversion of apoprotein B (apo B) in VLDL, intermediate density lipoprotein, and LDL in six hypertriglyceridemic subjects pre- and post-weight reduction. [2-3H]glycerol was used to quantitate VLDL triglyceride PR. All data are presented as mean +/- SD. Percent ideal body weight fell from 132 +/- 17.9 to 119 +/- 15.9% in the group, P less than 0.05. After weight loss, plasma VLDL triglyceride (486.0 +/- 364.1 vs. 191.3 +/- 65.4 mg/dl, P less than 0.05) and VLDL apo B (32.2 +/- 12.0 vs. 14.8 +/- 6.8 mg/dl, P less than 0.05) concentrations were reduced. VLDL triglyceride PR also fell after weight reduction (56.6 +/- 39.0 vs. 28.6 +/- 23.1 mg/kg per h, P less than 0.05), as did VLDL apo B PR (47.9 +/- 41.4 vs. 19.0 +/- 14.1 mg/kg per d, P less than 0.05). Pre-weight loss, plasma LDL cholesterol and apo B levels were low-normal or reduced (64.0 +/- 12.6 and 58.4 +/- 11.9 mg/dl, respectively) despite normal or elevated LDL apo B PR (17.4 +/- 7.2 mg/kg per d). The reduced cholesterol and apo B levels were associated with increased FCRs (0.68 +/- 0.29 d-1) and reduced cholesterol/protein ratios (1.01 +/- 0.18) in LDL. The plasma levels of LDL cholesterol and apo B rose after weight reduction (84.8 +/- 24.9, P less than 0.05; and 69.5 +/- 14.3 mg/dl, P less than 0.05, respectively, vs. base line). These increased concentrations resulted from a combination of events. First, the FCR for LDL apo B fell in five of six subjects with a significant reduction for the group as a whole (0.48 +/- 0.11 d-1, P less than 0.05 vs. base line). Second, the cholesterol/protein ratio increased in all six subjects with a significantly greater mean after weight loss (1.25 +/- 0.27, P less than 0.05 vs. base line). In contrast, the LDL apo B PR fell or was essentially unchanged in the six subjects after weight loss (mean, 14.4 +/- 2.8 mg/kg per d; NS vs. pre-weight loss). The changes in LDL catabolism and composition were associated with changes in the source of LDL apo B. Pre-weight loss, 73.3% of LDL was derived from VLDL, while 26.7% was directly secreted into plasma. Post-weight reduction, VLDL-derived LDL fell to 46.8% of total, while direct secretion accounted for 53.2% of LDL production. These changes were significant; P < 0.95. Thus, all subjects had direct secretion of LDL apo B and the magnitude of this source of VLDL triglyceride secretion. These results indicate that the regulation of plasma LDL levels in hypertriglyceridemic subjects is quite complex and that the rise in LDL levels after weight loss results from reduction in the fractional catabolism of this lipoprotein. The fall in the FCR is associated with changes in the source of LDL and in its composition.     

Measurement of very low density and low density lipoprotein apolipoprotein (Apo) B-100 and high density lipoprotein Apo A-I production in human subjects using deuterated leucine. Effect of fasting and feeding.

Six normolipidemic male subjects, after an 8-h overnight fast, were given a bolus injection and then a 15-h constant intravenous infusion of [D3]L-leucine. Subjects were studied in the fasted state and on a second occasion in the fed state (small, physiological meals were given every hour for 15 h). Apolipoproteins were isolated by preparative gradient gel electrophoresis from plasma lipoproteins separated by sequential ultracentrifugation. Incorporation of [D3]L-leucine into apolipoproteins was monitored by negative ionization, gas chromatography-mass spectrometry. Production rates were determined by multiplying plasma apolipoprotein pool sizes by fractional production rates (calculated as the rate of isotopic enrichment [IE] of each protein as a fraction of IE achieved by VLDL (d less than 1.006 g/ml) apo B-100 at plateau. VLDL apo B-100 production was greater, and LDL (1.019 less than d less than 1.063 g/ml) apo B-100 production was less in the fed compared with the fasted state (9.9 +/- 1.7 vs. 6.4 +/- 1.7 mg/kg per d, P less than 0.01, and 8.9 +/- 1.2 vs. 13.1 +/- 1.2 mg/kg per d, P less than 0.05, respectively). No mean change was observed in high density lipoprotein apo A-I production. We conclude that: (a) this stable isotope, endogenous-labeling technique, for the first time allows for the in vivo measurement of apolipoprotein production in the fasted and fed state; and (b) since LDL apo B-100 production was greater than VLDL apo B-100 production in the fasted state, this study provides in vivo evidence that LDL apo B-100 can be produced independently of VLDL apo B-100 in normolipidemic subjects.     

Effects of low dosage progestin-only administration upon plasma triglycerides and lipoprotein metabolism in postmenopausal women.

Oral administration to five postmenopausal women of dl-norgestrel (0.075 mg/d for 7 wk) reduced mean fasting plasma levels of triglycerides by 29% (P < 0.001), VLDL triglycerides by 39% (P < 0.01), and VLDL apo B by 26% (P < 0.05), while lowering mean total cholesterol by 7% (P < 0.06). To explain these observations the kinetics of VLDL and LDL apo B turnover were studied by injecting autologous 125I-labeled VLDL and 131I-labeled LDL under control conditions and again in the fourth week of a 7-wk course of dl-norgestrel. VLDL apo B pool size fell by an average of 27% (1.2 vs 1.7 mg/kg, P < 0.06) and production of apo B by 18% (18 vs 22 mg/kg per d, P < 0.05) with unchanged fractional catabolic rate. Production of LDL apo B increased 36% with dl-norgestrel (12 vs 9.4 mg/kg per d, P < 0.05), but this was compensated by a 36% increase in fractional catabolic rate of LDL apo B (0.33 vs 0.25 pools/d, P < 0.005), thereby maintaining pool size. Lipoprotein (a) fell by an average of 12% (16 vs 18 mg/dl, P < 0.06). dl-Norgestrel reduced VLDL triglycerides (40 vs 64 mg/dl, P < 0.05), intermediate density lipoprotein cholesterol (14 vs 19 mg/dl, P < 0.02), IDL apo B (5.3 vs 7.2 mg/dl, P < 0.05), and VLDL cholesterol (3.1 vs 5.1 mg/dl, 0.10 > P > 0.05), in parallel with the reductions in VLDL apo B production and pool size. dl-Norgestrel significantly lowered the production rate of VLDL apo B, thereby decreasing plasma VLDL and intermediate density lipoprotein concentrations.     

Development of an integrated model for analysis of the kinetics of apolipoprotein B in plasma very low density lipoproteins, intermediate density lipoproteins, and low density lipoproteins.

To quantify more precisely the metabolism of apolipoprotein B (apo B) in human beings, an integrated model was developed for the analysis of the isotope kinetics of apo B in very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL). The experimental basis for model development was a series of 30 triple-isotope studies in which patients received autologous 131I-VLDL, 125I-IDL, and [3H]glycerol as a precursor of VLDL triglycerides. The currently proposed model contains the following components: (a) a VLDL delipidation cascade that has a variable number of subcompartments, (b) a slowly catabolized pool of VLDL, (c) an IDL compartment consisting of two closely connected subcompartments, one of which is outside the immediate circulation, and (d) a two-compartment subsystem for LDL. Because mass data indicate that not all VLDL were converted to LDL, the model allows for irreversible removal of apo B from VLDL (or IDL) subsystems. It accounts for apparent "direct" input of LDL by postulating an early, rapidly metabolized compartment of VLDL that is converted directly to IDL. The model appears to be consistent with specific activity curves from the current triple-isotope studies and with present concepts of lipoprotein physiology; it also can be used to quantify pathways of lipoprotein apo B transport in normal and abnormal states.     

Plasma lipoprotein metabolism in transgenic mice overexpressing apolipoprotein E. Accelerated clearance of lipoproteins containing apolipoprotein B.

We have reported that transgenic mice overexpressing rat apo E shows marked reduction of plasma cholesterol and triglyceride levels due to the disappearance of VLDL and LDL. In this study, we investigated the metabolism of plasma lipoproteins in transgenic mice. After intravenous injection, the rates of clearance of 125I-VLDL and 125I-LDL were 3.0- and 2.4-fold greater in transgenic mice than in controls, respectively. Furthermore, clearance of chylomicron remnants estimated by oral retinyl palmitate-loading test was markedly enhanced in transgenic mice. The hepatic expression of LDL receptors by immunoblot analysis was similar in both groups. These data suggest that elimination of lipoproteins containing apo B was due to enhanced clearance of these lipoproteins enriched with apo E through hepatic LDL receptors. When fed a high cholesterol diet, controls showed twofold elevation of plasma cholesterol levels with marked increases in VLDL and LDL cholesterol on gel filtration chromatography. In contrast, cholesterol-fed transgenic mice showed resistance against these increases. High cholesterol feeding decreased the activity of hepatic LDL receptors and had no effect on enhancement of chylomicron remnant clearance in transgenic mice. Thus, overexpression of apo E facilitates metabolism of lipoproteins containing apo B presumably primarily via the LDL receptor pathway and possibly through an interaction with the chylomicron remnant receptor.     

Variable expression of familial dysbetalipoproteinemia in apolipoprotein E*2 (Lys146-->Gln) Allele carriers.

Genetic and biochemical studies were carried out in 96 relatives of six independently ascertained probands with familial dysbetalipoproteinemia (FD) carrying the APOE*2 (Lys146-->Gln) allele. Compared to noncarriers, the 40 heterozygous APOE*2 (Lys146-->Gln) allele carriers exhibited markedly increased mean levels of cholesterol and triglyceride in the very low density lipoproteins (VLDL) (1.89 +/- 0.37 vs 0.30 +/- 0.27 and 1.86 +/- 0.37 vs 0.68 +/- 0.27 mmol/liter, respectively) and plasma apolipoprotein (apo) E levels (28.1 +/- 1.6 vs 4.6 +/- 1.1 mg/dl), which is characteristic for FD. By means of a pedigree-based maximum likelihood method we calculated that carrier-status accounted for 57% and 71%, respectively, of the total variance of the ratio (VLDL + IDL)-cholesterol/plasma triglyceride and plasma apoE levels. APOE*2 (Lys146-->Gln) and APOE*3-Leiden allele carriers were found to differ significantly in: (a) plasma apoE levels, (b) in the amounts of triglycerides in the VLDL and VLDL + IDL fraction, and (c) in the amount of cholesterol in the VLDL and VLDL + IDL fraction relative to the amount of triglyceride in these fractions. In the APOE*2 (Lys146-->Gln) allele carriers the VLDL and VLDL + IDL fraction is relatively rich in triglycerides as compared with that in APOE*3-Leiden carriers. We hypothesize that these two rare mutations of apoE both lead to dominantly inherited forms of FD along different underlying metabolic defects.     

Interaction between free fatty acids and insulin in the acute control of very low density lipoprotein production in humans.

Changes in VLDL triglyceride and VLDL apo B production were determined semiquantitatively in healthy young men by examining the effect of altering plasma insulin and/or FFA levels on the change in the slopes of the specific activity of VLDL [3H]triglyceride glycerol or the 131I-VLDL apo B versus time curves. In one study (n = 8) insulin was infused for 5 h using the euglycemic hyperinsulinemic clamp technique. Plasma FFA levels declined by approximately 80% (0.52 +/- 0.01 to 0.11 +/- 0.02 mmol/liter), VLDL triglyceride production decreased by 66.7 +/- 4.2% (P = 0.0001) and VLDL apo B production decreased by 51.7 +/- 10.6% (P = 0.003). In a second study (n = 8) heparin and Intralipid (Baxter Corp., Toronto, Canada) were infused with insulin to prevent the insulin-mediated fall in plasma FFA levels. Plasma FFA increased approximately twofold (0.43 +/- 0.05 to 0.82 + 0.13 mmol/liter), VLDL triglyceride production decreased to a lesser extent than with insulin alone (P = 0.006) (-31.8 +/- 9.5%, decrease from baseline P = 0.03) and VLDL apo B production did not decrease significantly (-6.3 +/- 13.6%, P = NS). In a third study (n = 8) when heparin and Intralipid were infused without insulin, FFA levels rose approximately twofold (0.53 +/- 0.04 to 0.85 +/- 0.1 mmol/liter), VLDL triglyceride production increased by 180.1 +/- 45.7% (P = 0.008) and VLDL apo B production increased by 94.2 +/- 28.7% (P = 0.05). We confirm our previous observation that acute hyperinsulinemia suppresses VLDL triglyceride and VLDL apo B production in healthy humans. In addition, we have demonstrated that elevation of plasma FFA levels acutely stimulates VLDL production in vivo in healthy young males. Elevating plasma FFA during hyperinsulinemia attenuates but does not completely abolish the suppressive effect of insulin on VLDL production, at least with respect to VLDL triglycerides. Therefore, in normal individuals the acute inhibition of VLDL production by insulin in vivo is only partly due to the suppression of plasma FFA, and may also be due to an FFA-independent process.     

Effect of low density lipoprotein receptor deficiency on the metabolism of apolipoprotein B-100 in blood plasma. Kinetic studies in normal and Watanabe heritable hyperlipidemic rabbits

The kinetics of apolipoprotein (apo) B-100 in particles containing apo E (B,E particles) or lacking apo E (B particles) were studied in Watanabe heritable hyperlipidemic (WHHL) rabbits deficient in low density lipoprotein (LDL) receptors, and compared with those of normal rabbits after injection of radioiodinated very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and LDL. In both groups results of kinetic modeling were consistent with the hypothesis that all apo B enters the plasma in VLDL, mainly as B,E particles, followed by delipidation and partial conversion to IDL and LDL, with concomitant conversion of some B,E particles to B particles. In WHHL rabbits, production of VLDL apo B was reduced by 40%, but LDL production was increased threefold. Defective removal of B,E and B particles in all three lipoprotein classes, coupled with preserved processes of delipidation, can account for the observed increases in the concentration of apo B (threefold in VLDL, fivefold in IDL, and twenty-twofold in LDL) in WHHL rabbits.     

Contribution of apo CIII reduction to the greater effect of 12-week micronized fenofibrate than atorvastatin therapy on triglyceride levels and LDL size in dyslipidemic patients

BACKGROUND. Apolipoprotein (apo) CIII plays an important role in the catabolism of triglyceride-rich lipoproteins as it is a potent inhibitor of lipoprotein lipase (LPL). A low LPL activity has been simultaneously associated with hypertriglyceridemia, low HDL cholesterol and with small LDL particles.

AIM. To compare the effects of a 12-week treatment with micronized fenofibrate (200 mg) versus atorvastatin (10 mg) on apo CIII and lipoprotein-lipid levels including LDL size.

METHOD. After a 4-week washout period, dyslipidemic patients were randomized to either micronized fenofibrate (n = 64) or atorvastatin (n = 72).

RESULTS. Both fenofibrate and atorvastatin significantly decreased apo CIII levels by -0.03 ± 0.03 versus -0.01 ± 0.03 g/l respectively, and increased LDL size by 4.9 ± 3.3 versus 1.8 ± 2.9 Å. Improvements in these parameters were significantly greater with fenofibrate (P < 0.0001). Significant relationships were found between changes in triglycerides and changes in apo CIII (r = 0.81 and r = 0.59, P < 0.0001) as well as between changes in LDL size and changes in apo CIII (r = -0.41 and r = -0.45, P < 0.001), in both fenofibrate and atorvastatin groups. respectively.

CONCLUSION. The substantial reduction in apo CIII induced by micronized fenofibrate plays an important role in the greater effect of micronized fenofibrate than atorvastatin on plasma triglycerides and LDL size.