Intermediate-density Lipoprotein apoB Research Articles

Contribution of intestinal triglyceride-rich lipoproteins to residual atherosclerotic cardiovascular disease risk in individuals with type 2 diabetes on statin therapy

Aims/hypothesisThis study explored the hypothesis that significant abnormalities in the metabolism of intestinally derived lipoproteins are present in individuals with type 2 diabetes on statin therapy. These abnormalities may contribute to residual CVD risk.MethodsTo investigate the kinetics of ApoB-48- and ApoB-100-containing lipoproteins, we performed a secondary analysis of 11 overweight/obese individuals with type 2 diabetes who were treated with lifestyle counselling and on a stable dose of metformin who were from an earlier clinical study, and compared these with 11 control participants frequency-matched for age, BMI and sex. Participants in both groups were on a similar statin regimen during the study. Stable isotope tracers were used to determine the kinetics of the following in response to a standard fat-rich meal: (1) apolipoprotein (Apo)B-48 in chylomicrons and VLDL; (2) ApoB-100 in VLDL, intermediate-density lipoprotein (IDL) and LDL; and (3) triglyceride (TG) in VLDL.ResultsThe fasting lipid profile did not differ significantly between the two groups. Compared with control participants, in individuals with type 2 diabetes, chylomicron TG and ApoB-48 levels exhibited an approximately twofold higher response to the fat-rich meal, and a twofold higher increment was observed in ApoB-48 particles in the VLDL1 and VLDL2 density ranges (all p < 0.05). Again comparing control participants with individuals with type 2 diabetes, in the latter, total ApoB-48 production was 25% higher (556 ± 57 vs 446 ± 57 mg/day; p < 0.001), conversion (fractional transfer rate) of chylomicrons to VLDL was around 40% lower (35 ± 25 vs 82 ± 58 pools/day; p=0.034) and direct clearance of chylomicrons was 5.6-fold higher (5.6 ± 2.2 vs 1.0 ± 1.8 pools/day; p < 0.001). During the postprandial period, ApoB-48 particles accounted for a higher proportion of total VLDL in individuals with type 2 diabetes (44%) compared with control participants (25%), and these ApoB-48 VLDL particles exhibited a fivefold longer residence time in the circulation (p < 0.01). No between-group differences were seen in the kinetics of ApoB-100 and TG in VLDL, or in LDL ApoB-100 production, pool size and clearance rate. As compared with control participants, the IDL ApoB-100 pool in individuals with type 2 diabetes was higher due to increased conversion from VLDL2.Conclusions/interpretationAbnormalities in the metabolism of intestinally derived ApoB-48-containing lipoproteins in individuals with type 2 diabetes on statins may help to explain the residual risk of CVD and may be suitable targets for interventions.Trial registrationClinicalTrials.gov NCT02948777.Graphical

ANGPTL3 Inhibition With Evinacumab Results in Faster Clearance of IDL and LDL apoB in Patients With Homozygous Familial Hypercholesterolemia—Brief Report

The mechanism by which evinacumab, a fully human monoclonal antibody directed against ANGPTL3 (angiopoietin-like 3 protein) lowers plasma LDL (low-density lipoprotein) cholesterol levels in patients with homozygous familial hypercholesterolemia is unknown. We investigated apoB (apolipoprotein B) containing lipoprotein kinetic parameters in patients with homozygous familial hypercholesterolemia, before and after treatment with evinacumab. Approach and Results: Four patients with homozygous familial hypercholesterolemia underwent apoB kinetic analyses in 2 centers as part of a substudy of a trial evaluating the efficacy and safety of evinacumab in patients with homozygous familial hypercholesterolemia. The enrichment of apoB with the stable isotope (5,5,5-2H3)-Leucine was measured in VLDL (very LDL), IDL (intermediate-density lipoprotein), and LDL at different time points before and after intravenous administration of 15 mg/kg evinacumab. Evinacumab lowered LDL-cholesterol by 59±2% and increased IDL apoB and LDL apoB fractional catabolic rate in all 4 homozygous familial hypercholesterolemia subjects, by 616±504% and 113±14%, respectively. VLDL-apoB production rate decreased in 2 of the 4 subjects. In this small study, ANGPTL3 inhibition with evinacumab is associated with an increase in the fractional catabolic rate of IDL apoB and LDL apoB, suggesting that evinacumab lowers LDL-cholesterol predominantly by increasing apoB-containing lipoprotein clearance from the circulation. Additional studies are needed to unravel which factors are determinants in this biological pathway. URL: https://www.clinicaltrials.gov; Unique identifier: NCT04722068.

Factorial Effects of Evolocumab and Atorvastatin on Lipoprotein Metabolism.

Monoclonal antibodies against proprotein convertase subtilisin kexin type 9 (PCSK9), such as evolocumab, lower plasma low-density lipoprotein (LDL)-cholesterol concentrations. Evolocumab is under investigation for its effects on cardiovascular outcomes in statin-treated, high-risk patients. The mechanism of action of PCSK9 monoclonal antibodies on lipoprotein metabolism remains to be fully evaluated. Stable isotope tracer kinetics can effectively elucidate the mode of action of new lipid-regulating pharmacotherapies. We conducted a 2-by-2 factorial trial of the effects of atorvastatin (80 mg daily) and subcutaneous evolocumab (420 mg every 2 weeks) for 8 weeks on the plasma kinetics of very-low-density lipoprotein (VLDL)-apolipoprotein B-100 (apoB), intermediate-density lipoprotein-apoB, and LDL-apoB in 81 healthy, normolipidemic, nonobese men. The kinetics of apoB in these lipoproteins was studied using a stable isotope infusion of D3-leucine, gas chromatography/mass spectrometry, and multicompartmental modeling. Atorvastatin and evolocumab independently accelerated the fractional catabolism of VLDL-apoB (P<0.001 and P.032, respectively), intermediate-density lipoprotein-apoB (P=0.021 and P=.002, respectively), and LDL-apoB (P<0.001, both interventions). Evolocumab but not atorvastatin decreased the production rate of intermediate-density lipoprotein-apoB (P=0.043) and LDL-apoB (P<0.001), which contributed to the reduction in the plasma pool sizes of these lipoprotein particles. The reduction in LDL-apoB and LDL-cholesterol concentrations was significantly greater with combination versus either monotherapy (P<0.001). Whereas evolocumab but not atorvastatin lowered the concentration of free PCSK9, atorvastatin lowered the lathosterol/campesterol ratio (a measure of cholesterol synthesis/absorption) and apoC-III concentration. Both interventions decreased plasma apoE, but neither significantly altered lipoprotein lipase and cholesteryl ester protein mass or measures of insulin resistance. In healthy, normolipidemic subjects, evolocumab decreased the concentration of atherogenic lipoproteins, particularly LDL, by accelerating their catabolism. Reductions in intermediate-density lipoprotein and LDL production also contributed to the decrease in LDL particle concentration with evolocumab by a mechanism distinct from that of atorvastatin. These kinetic findings provide a metabolic basis for understanding the potential benefits of PCSK9 monoclonal antibodies incremental to statins in on-going clinical end point trials. URL: http://www.clinicaltrials.gov. Unique identifier: NCT02189837.

In vivo kinetic studies to further understand pathogenesis of abnormal lipoprotein metabolism in chronic kidney disease

Patients undergoing hemodialysis (HD) have been shown to be at increased risk for cardiovascular disease (CVD) morbidity and mortality which are, at least in part, due to uremic dyslipidemia including increased triglyceride-rich lipoproteins, in particular remnants, decreased high-density lipoprotein (HDL), and increased lipoprotein(a) [Lp(a)]. In vivo kinetic studies using stable isotope revealed that apolipoprotein (apo)A-I, a primary apoprotein constitute of HDL, was catabolized at a faster rate in HD patients, leading to decreased apoA-I, and therefore reduced HDL cholesterol concentrations. Likewise, apoB catabolic rates were significantly lower in intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL) apoB; the latter is also accompanied by a decreased production rate. In HD patients, IDL apoB levels were elevated, but LDL apoB levels remained within the normal range. Nonetheless, a prolonged residence time for LDL apoB of 2-5 days, made LDL more atherogenic. Atorvastatin completely ameliorated impaired LDL apoB catabolism. With regard to Lp(a) metabolism, both apoB and apo(a) were found to be slowly catabolized, indicating roles of normal kidney function on Lp(a) catabolism. Finally, a compartmental model suggests intracellular, rather than extracellular, assembly of Lp(a). This in vivo kinetic evidence will uncover the underlying mechanism for uremic dyslipidemia and provide strategies to reduce CVD in HD patients.

The association of alanine aminotransferase within the normal and mildly elevated range with lipoproteins and apolipoproteins: the Insulin Resistance Atherosclerosis Study

Markers of liver injury, such as alanine aminotransferase (ALT), have been associated with atherogenic lipoprotein changes. We examined the extent to which this association was explained by insulin resistance, adiposity, glucose tolerance and chronic inflammation. In this analysis we included 824 non-diabetic participants (age 40-69 years) in the Insulin Resistance Atherosclerosis Study. No participants reported excessive alcohol intake or treatment with lipid-lowering medications. Lipoproteins and apolipoproteins were measured by conventional methods and lipoprotein heterogeneity by nuclear magnetic resonance (NMR) spectroscopy. ALT had a positive relationship with triacylglycerols, LDL-to-HDL-cholesterol ratio and apolipoprotein B (ApoB) after adjusting for demographic variables (p < 0.001 for all three relationships). ALT was also associated with the following NMR lipoproteins: positively with large VLDL (p < 0.001), intermediate-density lipoprotein (IDL) (p < 0.001) and small LDL subclass particles (p < 0.001), and VLDL particle size (p < 0.001); and negatively with large LDL subclass particles (p < 0.05) and LDL (p < 0.001) and HDL particle sizes (p < 0.01). ALT remained associated with IDL and small LDL subclass particles and ApoB after adjusting for glucose tolerance, adiposity, directly measured insulin sensitivity and C-reactive protein. ALT is associated with a wide range of atherogenic lipoprotein changes, which are partially explained by insulin resistance, adiposity, glucose tolerance and chronic inflammation. Because of the significant variability in the relationship between ALT and liver fat, further studies are needed to assess the extent of the lipoprotein changes using a direct measure of liver fat.

Effect of apolipoprotein E genotype on apolipoprotein B-100 metabolism in normolipidemic and hyperlipidemic subjects

The effect of apolipoprotein (apo) E genotype on apoB-100 metabolism was examined in three normolipidemic apoE2/E2, five type III hyperlipidemic apoE2/E2, and five hyperlipidemic apoE3/E2 subjects using simultaneous administration of (131)I-VLDL and (125)I-LDL, and multi-compartmental modeling. Compared with normolipidemic apoE2/E2 subjects, type III hyperlipidemic E2/E2 subjects had increased plasma and VLDL cholesterol, plasma and VLDL triglycerides, and VLDL and intermediate density lipoprotein (IDL) apoB concentrations (P < 0.05). These abnormalities were chiefly a consequence of decreased VLDL and IDL apoB fractional catabolic rate (FCR). Compared with hyperlipidemic E3/E2 subjects, type III hyperlipidemic E2/E2 subjects had increased IDL apoB concentration and decreased conversion of IDL to LDL particles (P < 0.05). In a pooled analysis, VLDL cholesterol was positively associated with VLDL and IDL apoB concentrations and the proportion of VLDL apoB in the slowly turning over VLDL pool, and was negatively associated with VLDL apoB FCR after adjusting for subject group. VLDL triglyceride was positively associated with VLDL apoB concentration and VLDL and IDL apoB production rates after adjusting for subject group. A defective apoE contributes to altered lipoprotein metabolism but is not sufficient to cause overt hyperlipidemia. Additional genetic mutations and environmental factors, including insulin resistance and obesity, may contribute to the development of type III hyperlipidemia.

Chronic kidney disease delays VLDL-apoB-100 particle catabolism: potential role of apolipoprotein C-III

To determine the relative contribution of obesity and/or insulin resistance (IR) in the development of dyslipidemia in chronic kidney disease (CKD), we investigated the transport of apolipoprotein (apo) B-100 in nonobese, nondiabetic, nonnephrotic CKD subjects and healthy controls (HC). We determined total VLDL, VLDL(1), VLDL(2), intermediate density lipoprotein (IDL), and LDL-apoB-100 using intravenous D3-leucine, GC-MS, and multicompartmental modeling. Plasma apoC-III and apoB-48 were immunoassayed. In this case control study, we report higher plasma triglyceride, IDL-, VLDL-, VLDL(1)-, and VLDL(2)-apoB-100 concentrations in CKD compared with HC (P < 0.05). This was associated with decreased fractional catabolic rates [FCRs (pools/day)] [IDL:CKD 3.4 (1.6) vs. HC 5.0 (3.2), P < 0.0001; VLDL:CKD 4.8 (5.2) vs. HC 7.8 (4.8), P = 0.038; VLDL(1):CKD 10.1 (8.5) vs. HC 29.5 (45.1), P = 0.007; VLDL(2):CKD 5.4 (4.6) vs. HC 10.4 (3.4), P = 0.001] with no difference in production rates. Plasma apoC-III and apoB-48 were significantly higher in CKD (P < 0.001) and both correlated with impaired FCRs of VLDL, VLDL(1), and VLDL(2) apoB-100 (P < 0.05). In CKD, apoC-III concentration was the only independent predictor of clearance defects in VLDL and its subfractions. Moderate CKD in the absence of central adiposity and IR is associated with mild hypertriglyceridemia due to delayed catabolism of triglyceride rich lipoproteins, IDL, and VLDL, without changes in production rate. Altered apoC-III metabolism may contribute to dyslipidemia in CKD, and this requires further investigation.

Effects of different doses of atorvastatin on human apolipoprotein B-100, B-48, and A-I metabolism

Nine hypercholesterolemic and hypertriglyceridemic subjects were enrolled in a randomized, placebo-controlled, double-blind, crossover study to test the effect of atorvastatin 20 mg/day and 80 mg/day on the kinetics of apolipoprotein B-100 (apoB-100) in triglyceride-rich lipoprotein (TRL), intermediate density lipoprotein (IDL), and LDL, of apoB-48 in TRL, and of apoA-I in HDL. Compared with placebo, atorvastatin 20 mg/day was associated with significant reductions in TRL, IDL, and LDL apoB-100 pool size as a result of significant increases in fractional catabolic rate (FCR) without changes in production rate (PR). Compared with the 20 mg/day dose, atorvastatin 80 mg/day caused a further significant reduction in the LDL apoB-100 pool size as a result of a further increase in FCR. ApoB-48 pool size was reduced significantly by both atorvastatin doses, and this reduction was associated with nonsignificant increases in FCR. The lathosterol-campesterol ratio was decreased by atorvastatin treatment, and changes in this ratio were inversely correlated with changes in TRL apoB-100 and apoB-48 PR. No significant effect on apoA-I kinetics was observed at either dose of atorvastatin. Our data indicate that atorvastatin reduces apoB-100- and apoB-48-containing lipoproteins by increasing their catabolism and has a dose-dependent effect on LDL apoB-100 kinetics. Atorvastatin-mediated changes in cholesterol homeostasis may contribute to apoB PR regulation.

Evidence of increased secretion of apolipoprotein B-48-containing lipoproteins in subjects with type 2 diabetes

Patients with type 2 diabetes have high levels of triglyceride-rich lipoproteins (TRLs), including apolipoprotein B-48 (apoB-48)-containing TRLs of intestinal origin, but the mechanism leading to overaccumulation of these lipoproteins remains to be fully elucidated. Therefore, the objective of this study was to examine the in vivo kinetics of TRL apoB-48 and VLDL, intermediate density lipoprotein (IDL), and LDL apoB-100 in type 2 diabetic subjects (n = 11) and nondiabetic controls (n = 13) using a primed-constant infusion of l-[5,5,5-D(3)]leucine for 12 h in the fed state. Diabetic subjects had significantly higher fasting glycemia, higher fasting insulinemia, higher plasma triglyceride, and lower HDL-cholesterol levels than controls. Compared with controls, diabetic subjects had increased TRL apoB-48, VLDL apoB-100, and IDL apoB-100 pool sizes as a result of increased production rates (PRs) and reduced fractional catabolic rates of these lipoprotein subfractions. Furthermore, multiple linear regression analyses revealed that the diabetic/control status was an independent predictor of TRL apoB-48 PR and represented nearly 35% of its variance. These results suggest that the overaccumulation of TRLs seen in patients with type 2 diabetes is attributable to increased PRs of both intestinally derived apoB-48-containing lipoproteins and TRL apoB-100 of hepatic origin and to decreased catabolism of these subfractions.

Dose‐Dependent Effects of Docosahexaenoic Acid Supplementation on Blood Lipids in Statin‐Treated Hyperlipidaemic Subjects

The objective of the study was to evaluate potential benefits of docosahexaenoic acid (DHA) rich fish oil supplementation as an adjunct to statin therapy for hyperlipidaemia. A total of 45 hyperlipidaemic patients on stable statin therapy with persistent elevation of plasma triglycerides (averaging 2.2 mmol/L) were randomised to take 4 g/day (n = 15) or 8 g/day (n = 15) of tuna oil or olive oil (placebo, n = 15) for 6 months. Plasma lipids, blood pressure and arterial compliance were assessed initially and after 3 and 6 months in 40 subjects who completed the trial. Plasma triglycerides were reduced 27% by 8 g/day DHA-rich fish oil (P < 0.05) but not by 4 g/day when compared with the placebo and this reduction was achieved by 3 months and was sustained at 6 months. Even though total cholesterol was already well controlled by the statin treatment (mean initial level 4.5 mmol/L), there was a further dose-dependent reduction with fish oil supplementation (r = -0.344, P < 0.05). The extent of total cholesterol reduction correlated (r = -0.44) with the initial total cholesterol levels (P < 0.005). In the subset with initial plasma cholesterol above 3.8 mmol/L, plasma very low density lipoprotein (VLDL), intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL) were isolated and assayed for cholesterol and apolipoprotein B (apoB) at the commencement of the trial and at 3 months of intervention. Fish oil tended to lower cholesterol and apoB in VLDL and raise both in LDL. There were no changes in IDL cholesterol, IDL apoB and high-density lipoprotein cholesterol. The results demonstrate that DHA-rich fish oil supplementation (2.16 g DHA/day) can improve plasma lipids in a dose-dependent manner in patients taking statins and these changes were achieved by 3 months. Fish oil in addition to statin therapy may be preferable to drug combinations for the treatment of combined hyperlipidaemia.

Metabolism of apoB lipoproteins of intestinal and hepatic origin during constant feeding of small amounts of fat

We aimed to identify mechanisms by which apolipoprotein B-48 (apoB-48) could have an atherogenic role by simultaneously studying the metabolism of postprandial apoB-48 and apoB-100 lipoproteins. The kinetics of apoB-48 and apoB-100, each in four density subfractions of VLDL and intermediate density lipoprotein (IDL), were studied by stable isotope labeling in a constantly fed state with half-hourly administration of almond oil in five postmenopausal women. A non-steady-state, multicompartmental model was used. Despite a much lower production rate, VLDL and IDL apoB-48 shared a similar secretion pattern with apoB-100: both were directly secreted into all fractions with similar percentage mass distributions. Fractional catabolic rates (FCRs) of apoB-48 and apoB-100 were similar in VLDL and IDL. We identified a fast turnover compartment of light VLDL that had a residence time of <30 min for apoB-48 and apoB-100. Finally, a high secretion rate of apoB-48 was associated with a slow FCR of VLDL and IDL apoB-100. In conclusion, the intestine secretes a spectrum of apoB lipoproteins, similar to what the liver secretes, albeit with a much lower secretion rate. Once in plasma, intestinal and hepatic triglyceride-rich lipoproteins have similar rates of clearance and participate interactively in similar metabolic pathways, with high apoB-48 production inhibiting the clearance of apoB-100.

Effect of Ezetimibe on the In Vivo Kinetics of ApoB-48 and ApoB-100 in Men With Primary Hypercholesterolemia

To examine the impact of ezetimibe, a selective inhibitor of intestinal cholesterol absorption, on the in vivo kinetics of apolipoproteins (apo) B-48 and B-100 in humans. Kinetics of triglyceride-rich lipoprotein (TRL) apoB-48 and very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) apoB-100 labeled with a stable isotope were assessed at baseline and at the end of 8 weeks of treatment with 10 mg/d of ezetimibe in 8 men with moderate primary hypercholesterolemia. Data were fit to a multicompartmental model using SAAMII to calculate fractional catabolic rate (FCR) and production rate (PR). Ezetimibe significantly decreased total and LDL cholesterol concentrations by -14.5% and -22.0% (P=0.004), respectively, with no significant change in plasma triglyceride and high-density lipoprotein (HDL) cholesterol levels. Ezetimibe had no significant effect on TRL apoB-48 kinetics and pool size (PS). However, VLDL and IDL apoB-100 FCRs were significantly increased (+31.2%, P=0.02 and +20.8%, P=0.04, respectively) with a concomitant elevation of VLDL apoB-100 PR (+20.9%, P=0.04). Furthermore, LDL apoB-100 PS was significantly reduced by -23.2% (P=0.004), caused by a significant increase in FCR of this lipoprotein fraction (+24.0%, P=0.04). These results indicate that reduction of plasma LDL cholesterol concentration after treatment with ezetimibe is associated with an increase in FCR of apoB-100-containing lipoproteins.

Microsomal triglyceride transfer protein −493T variant reduces IDL plus LDL apoB production and the plasma concentration of large LDL particles

The microsomal triglyceride transfer protein (MTP) is essential for the synthesis and secretion of apolipoprotein B (apoB)-containing lipoproteins. We investigated the role the MTP -493G/T gene polymorphism in determining the apoB-100 secretion pattern and LDL heterogeneity in healthy human subjects. Groups of carriers of the T and the G variants (n = 6 each) were recruited from a cohort of healthy 50-yr-old men. Kinetic studies were performed by endogenous [(2)H(3)]leucine labeling of apoB and subsequent quantification of the stable isotope incorporation. apoB production rates, metabolic conversions, and eliminations were calculated by multicompartmental modeling (SAAM-II). LDL subfraction distribution was analyzed in the entire cohort (n = 377). Carriers of the MTP -493T allele had lower plasma LDL apoB and lower concentration of large LDL particles [LDL-I: 136 +/- 57 (TT) vs. 175 +/- 55 (GG) mg/l, P < 0.01]. Kinetic modeling suggested that MTP -493T homozygotes had a 60% lower direct production rate of intermediate-density lipoprotein (IDL) plus LDL compared with homozygotes for the G allele (P < 0.05). No differences were seen in production rates of large and small VLDL, nor were there any differences in metabolic conversion or elimination rates of apoB between the genotype groups. This study shows that a polymorphism in the MTP gene affects the spectrum of endogenous apoB-containing lipoprotein particles produced in humans. Reduced direct production of LDL plus IDL appears to be related to lower plasma concentrations of large LDL particles.

Effect of Fenofibrate on Plasma Lipoprotein Composition and Kinetics in Patients With Complete Hepatic Lipase Deficiency

The goal of this study was to characterize the effect of microcoated fenofibrate (160 mg/day for 6 months) on plasma lipoprotein composition and kinetics in 2 patients with complete hepatic lipase (HL) deficiency. Fenofibrate treatment normalized the plasma lipoprotein profile of patients with complete HL deficiency, as evidenced by significant reductions in the plasma concentration of cholesterol (-49%) and triglycerides (-82%) and a significant increase in low-density lipoprotein (LDL) size (251.5+/-1.8 versus 263.5+/-0.7 A). The in vivo kinetics of very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and LDL apolipoprotein (apo)B-100 and plasma apoA-I and apoA-II were studied using a primed-constant infusion of L-[5,5,5-D3]-leucine for 12 hours in the fasted state. Fenofibrate treatment in complete HL-deficient patients substantially decreased plasma concentrations of VLDL, IDL, and LDL apoB-100 attributable to important increases in VLDL (+325%), IDL (+129%), and LDL (+218%) apoB-100 fractional catabolic rates (FCR). IDL apoB-100 FCR nevertheless remained 60% lower after treatment compared with values obtained in controls (n=5). The kinetics of plasma apoA-I and apoA-II as well as the capacity of total plasma and of high-density lipoprotein particles to efflux cellular cholesterol from normal human skin fibroblasts was not altered by fenofibrate. Fenofibrate therapy exerts a pronounced antiatherogenic effect on triglyceride-rich lipoproteins even in the complete absence of HL.

Enhanced Conversion of Triglyceride-Rich Lipoproteins and Increased Low-Density Lipoprotein Removal in LPLS447X Carriers

Lipoprotein lipase (LPL) exerts 2 principal actions, comprising enzymatic hydrolysis of triglyceride-rich lipoproteins (TRLs) and nonenzymatic ligand capacity for enhancing lipoprotein removal. The common LPLS447X variant has been associated with cardiovascular protection, for which the mechanism is unknown. We therefore evaluated enzymatic and nonenzymatic consequences of this LPL variant on TRL metabolism. TRL apolipoprotein B100 (apoB100) metabolism was determined in 5 homozygous LPLS447X carriers and 5 controls. Subjects were continuously fed and received infusion of stable isotope l-[1-(13C)]-valine. Results were analyzed by SAAMII modeling. Also, preheparin and postheparin LPL concentration and activity were measured. Compared with controls, carriers presented increased very low-density lipoprotein 1 (VLDL1) to VLDL2 apoB100 flux (P=0.04), increased VLDL2 to intermediate-density lipoprotein (IDL) apoB100 flux (P=0.02), increased IDL to low-density lipoprotein (LDL) apoB100 flux (P=0.049), as well as an increased LDL clearance (P=0.04). Additionally, IDL apoB100 synthesis was attenuated (P=0.05). Preheparin LPL concentration was 4-fold higher compared with controls (P=0.01), and a correlation was observed between preheparin LPL concentration and LDL clearance (r2=0.92; P=0.01). Enhanced TRL conversion and enhanced LDL removal combined with increased preheparin LPL concentration suggest increased enzymatic consequences as well as increased nonenzymatic consequences of LPL in LPLS447X carriers, which might both contribute to the cardiovascular benefit of this LPL variant.

Antiretroviral Treatment Reduces Very-Low-Density Lipoprotein and Intermediate-Density Lipoprotein Apolipoprotein B Fractional Catabolic Rate in Human Immunodeficiency Virus-Infected Patients with Mild Dyslipidemia

The relationship between antiretroviral treatment of HIV infection, body fat distribution, insulin resistance, and very-low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) apolipoprotein-B (apoB) kinetics was investigated in 55 HIV-infected patients taking two nucleoside analogs plus either a protease inhibitor (n = 15) or a nonnucleoside reverse transcriptase inhibitor (n = 25), 15 antiretroviral therapy-naive patients, and 12 HIV-negative controls. Compared with the controls, high-density lipoprotein cholesterol was reduced in all groups (P < 0.01). Plasma triglyceride was increased in patients taking protease inhibitors (P < 0.05). VLDL and IDL apoB fractional catabolic rate (FCR) was lower in all treatment groups (P < 0.05) compared with controls. Trunk fat, VLDL apoB absolute secretion rate, and insulin resistance were not different between groups. Peripheral fat was lower in the treated patients (P < 0.05) and correlated with duration of therapy (r = -0.55; P < 0.001). There was a positive correlation between peripheral fat and VLDL apoB FCR (P = 0.002) and IDL apoB FCR (P = 0.002) and a negative correlation with VLDL apoB pool size, VLDL cholesterol, and triglyceride (P < 0.03; P < 0.01; P < 0.002). These results suggest that mild dyslipidemia resulting from antiretroviral therapy is caused by a decrease in VLDL and IDL apoB FCR, which is associated with a loss of peripheral fat.

Influence of atorvastatin and simvastatin on apolipoprotein B metabolism in moderate combined hyperlipidemic subjects with low VLDL and LDL fractional clearance rates

Subjects with moderate combined hyperlipidemia ( n=11) were assessed in an investigation of the effects of atorvastatin and simvastatin (both 40 mg per day) on apolipoprotein B (apoB) metabolism. The objective of the study was to examine the mechanism by which statins lower plasma triglyceride levels. Patients were studied on three occasions, in the basal state, after 8 weeks on atorvastatin or simvastatin and then again on the alternate treatment. Atorvastatin produced significantly greater reductions than simvastatin in low density lipoprotein (LDL) cholesterol (49.7 vs. 44.1% decrease on simvastatin) and plasma triglyceride (46.4 vs. 39.4% decrease on simvastatin). ApoB metabolism was followed using a tracer of deuterated leucine. Both drugs stimulated direct catabolism of large very low density lipoprotein (VLDL 1) apoB (4.52±3.06 pools per day on atorvastatin; 5.48±4.76 pools per day on simvastatin versus 2.26±1.65 pools per day at baseline (both P<0.05)) and this was the basis of the 50% reduction in plasma VLDL 1 concentration; apoB production in this fraction was not significantly altered. On atorvastatin and simvastatin the fractional transfer rates (FTR) of VLDL 1 to VLDL 2 and of VLDL 2 to intermediate density lipoprotein (IDL) were increased significantly, in the latter instance nearly twofold. IDL apoB direct catabolism rose from 0.54±0.30 pools per day at baseline to 1.17±0.87 pools per day on atorvastatin and to 0.95±0.43 pools per day on simvastatin (both P<0.05). Similarly the fractional transfer rate for IDL to LDL conversion was enhanced 58–84% by statin treatment ( P<0.01) LDL apoB fractional catabolic rate (FCR) which was low at baseline in these subjects (0.22±0.04 pools per day) increased to 0.44±0.11 pools per day on atorvastatin and 0.38±0.11 pools per day on simvastatin (both P<0.01). ApoB-containing lipoproteins were more triglyceride-rich and contained less free cholesterol and cholesteryl ester on statin therapy. Further, patients on both treatments showed marked decreases in all LDL subfractions. In particular the concentration of small dense LDL (LDL-III) fell 64% on atorvastatin and 45% on simvastatin. We conclude that in patients with moderate combined hyperlipidemia who initially have a low FCR for VLDL and LDL apoB, the principal action of atorvastatin and simvastatin is to stimulate receptor-mediated catabolism across the spectrum of apoB-containing lipoproteins. This leads to a substantial, and approximately equivalent, percentage reduction in plasma triglyceride and LDL cholesterol.

Metabolic abnormalities of apolipoprotein B‐containing lipoproteins in non‐insulin‐dependent diabetes: a stable isotope kinetic study

Kinetic abnormalities of apolipoprotein B (apoB)-containing lipoproteins in noninsulin-dependent diabetes mellitus (NIDDM) remain poorly understood. To get further insight into these abnormalities we performed a stable isotope kinetic experiment comparing the metabolism of apoB-containing lipoproteins in moderately severe NIDDM patients and healthy control subjects. The study was performed in the fed state. Subjects underwent a primed infusion of 0.7 mg kg(-1) of L-[1-(13)C]leucine followed by a 16-h constant infusion of 0.7 mg kg(-1) h(-1). [13C]Leucine enrichment in apoB was measured by gas chromatography/combustion/isotope ratio mass spectrometry. In NIDDM patients, we observed a 3.49- and 4.52-fold increase of very-low-density lipoprotein (VLDL) and intermediate-density lipoprotein (IDL) apoB plasma concentrations, respectively (P<0.01). VLDL apoB production was increased by 41% (P<0.05) and fractional catabolic rate towards IDL and low-density lipoprotein (LDL) was decreased by 61% (P<0.05). The increased IDL apoB plasma concentration was also related to a major catabolic defect (-78%; P<0.01). For most patients, plasma LDL apoB concentration was comparable to that of controls. Nevertheless, LDL apoB metabolism was impaired in NIDDM subjects, with both a decreased LDL catabolic rate (-28%; P<0.05) and a trend towards a diminished synthesis. NIDDM is associated with multiple apoB metabolism abnormalities that are potentially atherogenic. In addition to the increased number of circulating VLDL and IDL particles, the increased residence time observed on all apoB-containing lipoproteins may promote the development of atherosclerotic lesions, by potentiating their oxidizability.

VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100

Mutations in the apolipoprotein (apo) B, E (LDL) receptor gene and in the apolipoprotein B-100 gene are the cause of familial hypercholesterolemia (FH) and of familial defective apo B-100 (FDB), respectively. Whether these abnormalities lead to altered production or uptake of very low density lipoprotein (VLDL) or intermediate density lipoprotein (IDL) has not been established previously. Therefore VLDL and IDL apo B-100 kinetics were measured in seven subjects with FH, in six subjects with FDB, and in five normocholesterolemic controls using primed-constant infusions of [1-13C]leucine. Absolute production rates (APR) of VLDL apoB were higher in FH than in controls (27.1 ± 1.9 vs. 17.9 ± 2.1 mg/kg/day P < 0.03). VLDL APR in FDB were between those of FH and controls (24.3 ± 4.8 mg/kg/day), and demonstrated a relatively large inter-individual variability. The increase in VLDL APR in FH resulted in higher fasting serum triglyceride concentrations than in controls (P < 0.05), whereas in FDB triglycerides were between those observed in FH and in controls. A significant correlation was observed between VLDL apoB APR and serum triglycerides in FH and in FDB; the correlation coefficient for all subjects was r = 0.84 (P < 0.0001), indicating that the major determinant of serum triglyceride concentrations was VLDL apoB APR. IDL apoB APR was lower in FH and in FDB compared to controls (P < 0.03 P < 0.02, respectively): and its fractional catabolic rate (FCR) was slightly lower in FH and in FDB, resulting in similar plasma IDL apoB concentrations in all three groups of subjects. IDL apoB APR in FH were negatively correlated with LDL cholesterol concentrations (r = –0.89; P < 0.001); LDL cholesterol concentrations correlated positively with the part of VLDL that did not appear in IDL (r = 0.82 P < 0.02), bypassing therefore the delipidation cascade. In conclusion the data demonstrate increased VLDL apoB production rates in FH. VLDL and IDL kinetics differ when LDL concentrations are elevated either due to a LDL receptor defect or due to defective apolipoprotein B-100.—Zulewski H., R. Ninnis, A. R. Miserez, M. W. Baumstark, and U. Keller. VLDL and IDL apolipoprotein B-100 kinetics in familial hypercholesterolemia due to impaired LDL receptor function or to defective apolipoprotein B-100.

Thyroid replacement therapy and its influence on postheparin plasma lipases and apolipoprotein-B metabolism in hypothyroidism.

T4 replacement at 150 micrograms/day in a group of six hypothyroid subjects led to the development of a euthyroid state and produced a fall in the cholesterol content of plasma and low and high density lipoproteins (LDL and HDL). The effect of T4 on apolipoprotein-B (apoB) metabolism was followed using radioiodinated very low density lipoprotein1 (VLDL1; 60-400 Svedberg units) and VLDL2 (20-60 Svedberg units). The pretreatment plasma concentration of VLDL1 apoB and its rates of synthesis and catabolism were similar to those in normal subjects. VLDL2 apoB was synthesized at a supranormal rate in hypothyroid subjects, and this led to a doubling of its circulating mass. Treatment did not significantly alter the kinetics of apoB in either VLDL1 or VLDL2. The concentration of intermediate density lipoprotein (IDL) apoB in untreated hypothyroids was 170% of normal and fell during T4 treatment due to stimulation of conversion of LDL (from 0.46 +/- 0.14 to 0.91 +/- 0.30 pools/day; mean +/- SD; P < 0.01). Direct IDL apoB clearance was not altered by treatment, whereas the fractional catabolic rate of LDL increased 76% (from 0.17 +/- 0.06 to 0.27 +/- 0.07 pools/day), leading to a 36% decrement in LDL mass. The stimulation of IDL to LDL conversion during therapy was probably due to a 3-fold increase in hepatic lipase activity (P < 0.02). This change together with the known effects of T4 on LDL receptors largely explained the lipoprotein abnormality in hypothyroidism and the effects of replacement therapy.