Compositional and metabolic heterogeneity of alpha 2- and beta-very-low-density lipoproteins in subjects with broad beta disease and endogenous hypertriglyceridemia

Effects of extended-release niacin on the postprandial metabolism of Lp(a) and ApoB-100-containing lipoproteins in statin-treated men with type 2 diabetes mellitus

OBJECTIVE

The effects of extended-release niacin (ERN; 1-2 g/d) on the metabolism of lipoprotein(a) (Lp(a)) and apolipoprotein (apo) B-100-containing lipoproteins were investigated in 11 statin-treated white men with type 2 diabetes mellitus in a randomized, crossover trial of 12-weeks duration.

APPROACH AND RESULTS

The kinetics of Lp(a) and very low-density lipoprotein (VLDL), intermediate-density lipoprotein, and low-density lipoprotein (LDL) apoB-100 were determined following a standardized oral fat load (87% fat) using intravenous administration of D3-leucine, gas chromatography-mass spectrometry, and compartmental modeling. ERN significantly decreased fasting plasma total cholesterol, LDL cholesterol, and triglyceride concentrations. These effects were achieved without significant changes in body weight or insulin resistance. ERN significantly decreased plasma Lp(a) concentration (-26.5%) and the production rates of apo(a) (-41.5%) and Lp(a)-apoB-100 (-32.1%); the effect was greater in individuals with elevated Lp(a) concentration. ERN significantly decreased VLDL (-58.7%), intermediate-density lipoprotein (-33.6%), and LDL (-18.3%) apoB-100 concentrations and the corresponding production rates (VLDL, -49.8%; intermediate-density lipoprotein, -44.7%; LDL, -46.1%). The number of VLDL apoB-100 particles secreted increased in response to the oral fat load. Despite this, total VLDL apoB-100 production over the 10-hour postprandial period was significantly decreased with ERN (-21.9%).

CONCLUSIONS

In statin-treated men with type 2 diabetes mellitus, ERN decreased plasma Lp(a) concentrations by decreasing the production of apo(a) and Lp(a)-apoB-100. ERN also decreased the concentrations of apoB-100-containing lipoproteins by decreasing VLDL production and the transport of these particles down the VLDL to LDL cascade. Our study provides further mechanistic insights into the lipid-regulating effects of ERN.

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.

Extended-release niacin alters the metabolism of plasma apolipoprotein (Apo) A-I and ApoB-containing lipoproteins

OBJECTIVE

Extended-release niacin effectively lowers plasma TG levels and raises plasma high-density lipoprotein (HDL) cholesterol levels, but the mechanisms responsible for these effects are unclear.

METHODS AND RESULTS

We examined the effects of extended-release niacin (2 g/d) and extended-release niacin (2 g/d) plus lovastatin (40 mg/d), relative to placebo, on the kinetics of apolipoprotein (apo) A-I and apoA-II in HDL, apoB-100 in TG-rich lipoproteins (TRL), intermediate-density lipoproteins (IDL) and low-density lipoproteins (LDL), and apoB-48 in TRL in 5 men with combined hyperlipidemia. Niacin significantly increased HDL cholesterol and apoA-I concentrations, associated with a significant increase in apoA-I production rate (PR) and no change in fractional catabolic rate (FCR). Plasma TRL apoB-100 levels were significantly lowered by niacin, accompanied by a trend toward an increase in FCR and no change in PR. Niacin treatment significantly increased TRL apoB-48 FCR but had no effect on apoB-48 PR. No effects of niacin on concentrations or kinetic parameters of IDL and LDL apoB-100 and HDL apoA-II were noted. The addition of lovastatin to niacin promoted a lowering in LDL apoB-100 attributable to increased LDL apoB-100 FCR.

CONCLUSIONS

Niacin treatment was associated with significant increases in HDL apoA-I concentrations and production, as well as enhanced clearance of TRL apoB-100 and apoB-48.

In vitro production of beta-very low density lipoproteins and small, dense low density lipoproteins in mildly hypertriglyceridemic plasma: role of activities of lecithin:cholester acyltransferase, cholesterylester transfer proteins and lipoprotein lipase

As a model for the formation of beta-very low density lipoproteins (VLDL) and small, dense LDL by the intraplasma metabolic activities in vivo, lipoproteins in fresh plasma were interacted in vitro with endogenous lecithin:cholesterol acyltransferase (LCAT) and cholesterylester transfer proteins (CETP) and subsequently with purified lipoprotein lipase (LpL). The LCAT and CETP reactions in a mildly hypertriglyceridemic (HTG) plasma at 37 degrees C for 18 h resulted in (1) esterification of about 45% plasma unesterified cholesterol (UC), (2) a marked increase in cholesterylester (CE) (+129%) and a decrease in triglyceride (TG) (-45%) in VLDL, and (3) a marked increase of TG (+ 341%) with a small net decrease of CE (-3.6%) in LDL, causing a significant alteration in the TG/CE of VLDL (from 8.0 to 1.9) and of LDL (from 0.20 to 0.93). The LDL in LCAT and CETP-reacted plasma is larger and more buoyant than that in control plasma. In vitro lipolysis of control and LCAT and CETP-reacted plasma by LpL, which hydrolyzed >90% of VLDL-TG and about 50-60% of LDL-TG, converted most of VLDL in control plasma (>85%) but less than half (40%) of VLDL in LCAT and CETP-reacted plasma into the IDL-LDL density fraction and transformed the large, buoyant LDL in the LCAT and CETP-reacted plasma into particles smaller and denser than those in the control plasma. The remnants that accumulated in the VLDL density region of the postlipolysis LCAT and CETP-reacted plasma contained apo B-100 and E but little or no detectable apo Cs and consisted of particles having pre-beta and beta-electrophoretic mobilities. The inhibition of LCAT during incubation of plasma, which lessened the extent of alteration in VLDL and LDL core lipids, increased the extent of lipolytic removal of VLDL from the VLDL density region but lowered the extent of alteration in the size and density of LDL. The LCAT, CETP and/or LpL-mediated alterations in the density of LDL in normolipidemic fasting plasma were less pronounced than that in mildly HTG plasma, but they became highly pronounced upon increase of its TG-rich lipoprotein level by the addition of preisolated VLDL or by the induction of postprandial lipemia. Although the effect of LCAT, CETP and LpL reactions in non-circulating plasma in vitro may be different from that in vivo, the above data suggests that the plasma TG-rich lipoprotein level and the extent of intraplasma LCAT, CETP, LpL and likely hepatic lipase (HL) reactions in vivo may play a role in determining the LDL phenotype.

Different hydrolytic efficiencies of adipose tissue lipoprotein lipase on very-low-density lipoprotein subfractions separated by heparin-Sepharose chromatography

Human very-low-density lipoproteins (VLDL) were subfractionated by heparin-Sepharose chromatography into an unbound (A) and three bound (B, C and D) populations at increasing ionic strengths. Subfractions were characterized regarding their chemical composition and efficiency of triacylglycerol hydrolysis by rat adipose tissue LPL. The triacylglycerol content decreased, whereas the cholesterol and protein contents increased from subfractions A and B to subfraction D. VLDL-D showed the highest apo E/apo C ratio, though all the subfractions contained appreciable apo E. Appearance of VLDL-A resulted from exceeding the binding capacity of the column, since practically all its particles eluted at positions of bound VLDL under re-chromatography. Subfractions B, C and D stimulated LPL activity on emulsified tri[14C]oleoylglycerol to a similar extent, indicating that their apo C-II content was equally effective activating LPL. Incubation of tri[14C]oleoylglycerol labeled VLDL subfractions with fat pad pieces in the presence or absence of heparin resulted in greater hydrolysis and fatty acid uptake for VLDL-B and -C than for VLDL-D, a pattern observed over a wide range of LPL activities in the media. We conclude: (1) any VLDL particle can interact with heparin, which is consistent with the presence of apo E in all the subfractions, and (2) triacylglycerols in apo E-rich VLDL are less efficiently hydrolyzed by LPL than those in apo E-poor particles. We propose that richness in apo E impairs LPL action upon VLDL and decreases the rate of delivery of fatty acids to peripheral tissues.

Metabolism of apolipoprotein B in members of a family with accelerated atherosclerosis: influence of apolipoprotein E-3/E-2 pattern

Familial combined hyperlipidemia (FCHL) appears to be the most common, simply inherited hyperlipidemia strongly associated with coronary heart disease. In the family examined in this study, two of the siblings who met diagnostic criteria for FCHL had extensive clinical atherosclerosis before age 30, unusually premature for this form of hyperlipidemia. Lipoproteins and low-density lipoprotein (LDL) apolipoprotein (apo) B metabolism were characterized in these siblings in an attempt to gain insight into the cause of the rapid atherosclerosis in the two siblings so affected. LDL apo B production rates were very high in all three siblings (25 to 30 mg/kg/d), consistent with FCHL. beta-Very-low-density lipoprotein-beta (beta-VLDL) was present in the plasma of both siblings with accelerated atherosclerosis. The isoapolipoprotein E pattern in both of these siblings was E-3/E-2. In the third sibling, who was free of premature clinical atherosclerosis and lacked plasma beta-VLDL, the pattern was E-3/E-3. Thus, the heterozygote apo E-3/E-2 pattern may be related to the accumulation of beta-VLDL in persons with a very high apo B production rate. The abnormal accumulation of beta-VLDL may be one of the possible explanations for the rapid, premature atherosclerosis in the two siblings with FCHL in this kindred. Both male members in this kindred also had low levels of high-density lipoproteins, and thus may have had an additional risk of developing atherosclerosis due to this lipoprotein abnormality as well.