Very Low Density Lipoprotein Metabolism Research Articles

Effects of Oxidation on Structural Stability and Remodeling of Human Very Low Density Lipoprotein

Very low density lipoproteins (VLDL) are triglyceride-rich precursors of low-density lipoproteins (LDL) and a risk factor for atherosclerosis. The effects of oxidation on VLDL metabolism may be pro- or antiatherogenic. To understand the underlying biophysical basis, we determined the effects of copper (that preferentially oxidizes lipids) and hypochlorite (that preferentially oxidizes proteins) on the heat-induced VLDL remodeling. This remodeling involves VLDL fusion, rupture, and fission of apoE-containing high-density lipoprotein- (HDL-) like particles; HDL with similar size, density, and protein composition are formed upon VLDL remodeling by lipoprotein lipase, a key enzyme in triglyceride metabolism. Circular dichroism, turbidity, and electron microscopy show that mild oxidation promotes VLDL fusion and rupture, while advanced oxidation hampers these reactions. VLDL destabilization upon moderate oxidation results, in part, from the exchangeable apolipoprotein modifications, including proteolysis and limited cross-linking. VLDL stabilization against fusion and rupture upon advanced oxidation probably results from massive protein cross-linking on the particle surface. Electron microscopy and gel electrophoresis reveal that oxidation promotes fission of apoE-containing HDL-size particles; hydrolysis of apolar core lipids probably contributes to this effect. Copper and hypochlorite have similar effects on VLDL remodeling, suggesting that these effects may be produced by other oxidants. In summary, moderate oxidation that encompasses in vivo conditions destabilizes VLDL and promotes fission of HDL-size particles. Consequently, mild oxidation may be synergistic with lipoprotein lipase reaction and, hence, may help to accelerate VLDL metabolism.

Increased Whole‐Body Adiposity Without a Concomitant Increase in Liver Fat is Not Associated With Augmented Metabolic Dysfunction

Aim of this study was to determine whether an increase in adiposity, without a concomitant increase in intrahepatic triglyceride (IHTG) content, is associated with a deterioration in metabolic function. To this end, multiorgan insulin sensitivity, assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusion, and very low-density lipoprotein (VLDL) kinetics, assessed by using stable isotopically labeled tracer infusion and mathematical modeling, were determined in 10 subjects with class I obesity (BMI: 31.6 +/- 0.3 kg/m(2); 37 +/- 2% body fat; visceral adipose tissue (VAT): 1,225 +/- 144 cm(3)) and 10 subjects with class III obesity (BMI: 41.5 +/- 0.5 kg/m(2); 43 +/- 2% body fat; VAT: 2,121 +/- 378 cm(3)), matched on age, sex, and IHTG content (14 +/- 4 and 14 +/- 3%, respectively). No differences between class I and class III obese groups were detected in insulin-mediated suppression of palmitate (67 +/- 3 and 65 +/- 3%, respectively; P = 0.635) and glucose (67 +/- 3 and 73 +/- 5%, respectively; P = 0.348) rates of appearance in plasma, and the insulin-mediated increase in glucose disposal (218 +/- 18 and 193 +/- 30%, respectively; P = 0.489). In addition, no differences between class I and class III obese groups were detected in secretion rates of VLDL-triglyceride (6.5 +/- 1.0 and 6.0 +/- 1.4 micromol/l x min, respectively; P = 0.787) and VLDL-apolipoprotein B-100 (0.40 +/- 0.05 and 0.41 +/- 0.04 nmol/l x min, respectively; P = 0.866), and plasma clearance rates of VLDL-triglyceride (31 (16-59) and 29 (18-46) ml/min, respectively; P = 0.888) and VLDL-apolipoprotein B-100 (15 (11-19) and 17 (11-25) ml/min, respectively; P = 0.608). We conclude that increased adiposity without a concomitant increase in IHTG content does not cause additional abnormalities in adipose tissue, skeletal muscle, and hepatic insulin sensitivity, or VLDL metabolism.

Fenofibrate concomitantly decreases serum proprotein convertase subtilisin/kexin type 9 and very‐low‐density lipoprotein particle concentrations in statin‐treated type 2 diabetic patients

Diabetic dyslipidaemia, characterized by hypertriglyceridaemia as a result of elevated serum very-low-density lipoprotein (VLDL) concentrations, contributes to the increased risk of cardiovascular disease (CVD) in type 2 diabetes (T2DM). Proprotein convertase subtilisin/kexin type 9 (PCSK9) may play a role in regulating VLDL metabolism. We investigated the effect of fenofibrate on serum PCSK9 and VLDL particle concentrations in T2DM patients already receiving statin therapy. In a double-blind randomized crossover study, 15 statin-treated T2DM patients (63 +/- 8 years, body mass index (BMI) 29 +/- 3 kg/m(2)) were treated with fenofibrate (145 mg/day) or matching placebo for 12 weeks. Serum PCSK9 concentrations were measured by immunoassay. VLDL particle concentration and size were determined by nuclear magnetic resonance spectroscopy. Fenofibrate decreased serum triglycerides (-23%), VLDL-triglycerides (-51%), total cholesterol (-11%), LDL-cholesterol (-16%), apolipoprotein B-100 (-16%), apolipoprotein C-III (-20%) and PCSK9 (-13%) concentrations compared with placebo (p < 0.05). Fenofibrate also decreased serum concentrations of large (-45%), medium (-66%) and small VLDL (-67%) particles (p < 0.05), without altering VLDL particle size. Serum PCSK9 reduction correlated with decreases in total (r = 0.526, p = 0.044) and small (r = 0.629, p = 0.021) VLDL particle concentrations. Fenofibrate concomitantly decreased serum PCSK9 and VLDL particle concentrations in statin-treated T2DM patients. These findings support a mechanistic link between PCSK9 and VLDL metabolism, possibly through an effect of PSK9 on VLDL receptor degradation.

Expression of Human ApoAII in Transgenic Rabbits Leads to Dyslipidemia

Apolipoprotein AII (apoAII) is the second major apolipoprotein in high-density lipoprotein (HDL). However, the physiological functions of apoAII in lipoprotein metabolism have not been fully elucidated. We generated human apoAII transgenic (Tg) rabbits, a species that normally does not have an endogenous apoAII gene. Plasma levels of human apoAII in Tg rabbits were approximately 30 mg/dL, similar to the plasma levels in healthy humans. The expression of human apoAII in Tg rabbits resulted in increased levels of plasma triglycerides, total cholesterol, and phospholipids accompanied by a marked reduction in HDL-cholesterol levels compared with non-Tg littermates. Analysis of lipoprotein fractions showed that hyperlipidemia exhibited by Tg rabbits was caused by elevated levels of very-low-density lipoproteins (VLDL) and intermediate-density lipoproteins. Furthermore, postheparin lipoprotein lipase activity significantly decreased in Tg rabbits compared with non-Tg rabbits. These results indicate that apoAII plays an important role in both VLDL and HDL metabolism, possibly through the inhibition of lipoprotein lipase activity. ApoAII Tg rabbits may become a new model for the study of human familial combined hyperlipidemia.

Betaine administration corrects ethanol-induced defective VLDL secretion

Our previous studies, demonstrating ethanol-induced alterations in phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine methyltransferase (PEMT) pathway, implicated a defect in very low-density lipoprotein (VLDL) secretion in the pathogenesis of hepatic steatosis. The objective of this study was to determine whether VLDL secretion was reduced by chronic ethanol consumption and whether betaine supplementation, that restores PEMT activity and prevents the development of alcoholic steatosis, could normalize VLDL secretion. The VLDL secretion in rats fed with control, ethanol and the betaine supplemented diets was determined using Triton WR-1339 to inhibit plasma VLDL metabolism. We observed reduced VLDL production rates in chronic alcohol-fed rats compared to control animals. Supplementation of betaine in the ethanol diet increased VLDL production rate to values significantly higher than those observed in the control diet-fed rats. To conclude, chronic ethanol consumption impairs PC generation via the PEMT pathway resulting in diminished VLDL secretion which contributes to the development of hepatic steatosis. By increasing PEMT-mediated PC generation, betaine results in increased fat export from the liver and attenuates the development of alcoholic fatty liver.

Basal adipose tissue and hepatic lipid kinetics are not affected by a single exercise bout of moderate duration and intensity in sedentary women.

Hypertriacylglycerolaemia is an important risk factor for cardiovascular disease. In men, we have shown that the effects of evening exercise on basal VLDL (very-low-density lipoprotein) metabolism are dose-dependent: a single prolonged bout of aerobic exercise [2 h at 60% of VO(2 peak) (peak oxygen consumption)] reduces fasting plasma TAG [triacylglycerol (triglyceride)] concentrations, via enhanced clearance of VLDL-TAG from the circulation, whereas the same exercise performed for 1 h has no effect on VLDL-TAG metabolism and concentration. We hypothesized that women are more sensitive to the TAG-lowering effect of exercise because they reportedly use more intramuscular TAG as an energy source during exercise, and depletion of muscle TAG stores has been linked to reciprocal changes in skeletal muscle LPL (lipoprotein lipase) activity. To test our hypothesis, we measured basal VLDL-TAG and VLDL-apoB-100 (apolipoprotein B-100), and plasma NEFA [non-esterified fatty acid ('free fatty acid')] kinetics, by using stable isotope-labelled tracer techniques, on the morning after a single session of evening exercise of moderate duration and intensity (1 h at 60% of VO(2 peak)) in eight sedentary pre-menopausal women (age, 28+/-3 years; body mass index, 27+/-2 kg/m(2); body fat, 34+/-3%; values are means+/-S.E.M.). Compared with an equivalent period of evening rest, exercise had no effect on post-absorptive NEFA concentrations and the rate of appearance in plasma, VLDL-TAG and VLDL-apoB-100 concentrations, hepatic VLDL-TAG and VLDL-apoB-100 secretion and plasma clearance rates (all P>0.05). We conclude that, in women, as in men, a single session of exercise of moderate intensity and duration is not sufficient to bring about the alterations in VLDL metabolism that have been linked to post-exercise hypotriacylglycerolaemia.

Leptin Augments the Acute Suppressive Effects of Insulin on Hepatic Very Low-Density Lipoprotein Production in Rats

It is well established that leptin increases the sensitivity of carbohydrate metabolism to the effects of insulin. Leptin and insulin also have potent effects on lipid metabolism. However, the effects of leptin on the regulation of liver lipid metabolism by insulin have not been investigated. The current study addressed the effects of leptin on insulin-regulated hepatic very low-density lipoprotein (VLDL) metabolism in vivo in rats. A 90-min hyperinsulinemic/euglycemic clamp (4 mU/kg x min(-1)) reduced plasma VLDL triglyceride (TG) by about 50% (P < 0.001 vs. saline control). Importantly, a leptin infusion (0.2 microg/kg x min(-1)) in combination with insulin reduced plasma VLDL-TG by about 80% (P < 0.001 vs. insulin alone). These effects did not require altered skeletal muscle lipoprotein lipase activity but did include differential effects of insulin and leptin on liver apolipoprotein (apo) B and TG metabolism. Thus, insulin decreased liver and plasma apoB100/B48 levels (approximately 50%, P < 0.01), increased liver TGs (approximately 20%, P < 0.05), and had no effect on fatty acid oxidation. Conversely, leptin decreased liver TGs (approximately 50%, P < 0.01) and increased fatty acid oxidation (approximately 50%, P < 0.01) but had no effects on liver or plasma apoB levels. Importantly, the TG-depleting and prooxidative effects of leptin were maintained in the presence of insulin. We conclude that leptin additively increases the suppressive effects of insulin on hepatic and systemic VLDL metabolism by stimulating depletion of liver TGs and increasing oxidative metabolism. The net effect of the combined actions of insulin and leptin is to decrease the production and TG content of VLDL particles.

Very Low Density Lipoprotein Metabolism and Plasma Adiponectin as Predictors of High-Density Lipoprotein Apolipoprotein A-I Kinetics in Obese and Nonobese Men

Hypercatabolism of high-density lipoprotein (HDL) apolipoprotein (apo) A-I results in low plasma apoA-I concentration. The mechanisms regulating apoA-I catabolism may relate to alterations in very low density lipoprotein (VLDL) metabolism and plasma adiponectin and serum amyloid A protein (SAA) concentrations. We examined the associations between the fractional catabolic rate (FCR) of HDL-apoA-I and VLDL kinetics, plasma adiponectin, and SAA concentrations. The kinetics of HDL-apoA-I and VLDL-apoB were measured in 50 obese and 37 nonobese men using stable isotopic techniques. In the obese group, HDL-apoA-I FCR was positively correlated with insulin, homeostasis model of assessment for insulin resistance (HOMA-IR) score, triglycerides, VLDL-apoB, and VLDL-apoB production rate (PR). In the nonobese group, HDL-apoA-I FCR was positively correlated with triglycerides, apoC-III, VLDL-apoB, and VLDL-apoB PR and negatively correlated with plasma adiponectin. Plasma SAA was not associated with HDL-apoA-I FCR in either group. In multiple regression analyses, VLDL-apoB PR and HOMA-IR score, and VLDL-apoB PR and adiponectin were independently predictive of HDL-apoA-I FCR in the obese and nonobese groups, respectively. HDL-apoA-I FCR was positively and strongly associated with HDL-apoA-I PR in both groups. Variation in VLDL-apoB production, and hence plasma triglyceride concentrations, exerts a major effect on the catabolism of HDL-apoA-I. Insulin resistance and adiponectin may also contribute to the variation in HDL-apoA-I catabolism in obese and nonobese subjects, respectively. We also hypothesize that apoA-I PR determines a steady-state, lowered plasma of apoA-I, which may reflect a compensatory response to a primary defect in the catabolism of HDL-apoA-I due to altered VLDL metabolism.

Overproduction of Very Low–Density Lipoproteins Is the Hallmark of the Dyslipidemia in the Metabolic Syndrome

Insulin resistance is a key feature of the metabolic syndrome and often progresses to type 2 diabetes. Both insulin resistance and type 2 diabetes are characterized by dyslipidemia, which is an important and common risk factor for cardiovascular disease. Diabetic dyslipidemia is a cluster of potentially atherogenic lipid and lipoprotein abnormalities that are metabolically interrelated. Recent evidence suggests that a fundamental defect is an overproduction of large very low-density lipoprotein (VLDL) particles, which initiates a sequence of lipoprotein changes, resulting in higher levels of remnant particles, smaller LDL, and lower levels of high-density liporotein (HDL) cholesterol. These atherogenic lipid abnormalities precede the diagnosis of type 2 diabetes by several years, and it is thus important to elucidate the mechanisms involved in the overproduction of large VLDL particles. Here, we review the pathophysiology of VLDL biosynthesis and metabolism in the metabolic syndrome. We also review recent research investigating the relation between hepatic accumulation of lipids and insulin resistance, and sources of fatty acids for liver fat and VLDL biosynthesis. Finally, we briefly discuss current treatments for lipid management of dyslipidemia and potential future therapeutic targets.

Rapid turnover of apolipoprotein C-III-containing triglyceride-rich lipoproteins contributing to the formation of LDL subfractions

The atherogenicity theory for triglyceride-rich lipoproteins (TRLs; VLDL + intermediate density lipoprotein) generally cites the action of apolipoprotein C-III (apoC-III), a component of some TRLs, to retard their metabolism in plasma. We studied the kinetics of multiple TRL and LDL subfractions according to the content of apoC-III and apoE in 11 hypertriglyceridemic and normolipidemic persons. The liver secretes mainly two types of apoB lipoproteins: TRL with apoC-III and LDL without apoC-III. Approximately 45% of TRLs with apoC-III are secreted together with apoE. Contrary to expectation, TRLs with apoC-III but not apoE have fast catabolism, losing some or all of their apoC-III and becoming LDL. In contrast, apoE directs TRL flux toward rapid clearance, limiting LDL formation. Direct clearance of TRL with apoC-III is suppressed among particles also containing apoE. TRLs without apoC-III or apoE are a minor, slow-metabolizing precursor of LDL with little direct removal. Increased VLDL apoC-III levels are correlated with increased VLDL production rather than with slow particle turnover. Finally, hypertriglyceridemic subjects have significantly greater production of apoC-III-containing VLDL and global prolongation in residence time of all particle types. ApoE may be the key determinant of the metabolic fate of atherogenic apoC-III-containing TRLs in plasma, channeling them toward removal from the circulation and reducing the formation of LDLs, both those with apoC-III and the main type without apoC-III.

Apolipoproteins C-III and A-V as Predictors of Very-Low-Density Lipoprotein Triglyceride and Apolipoprotein B-100 Kinetics

We investigated the associations between plasma very-low-density lipoprotein (VLDL)-apolipoprotein (apo)C-III and apoA-V concentrations and the kinetics of VLDL-apoB-100 and VLDL triglycerides in 15 men. We also explored the relationship between these parameters of VLDL metabolism and VLDL-apoC-III kinetics. ApoC-III, apoB, and triglyceride kinetics in VLDL were determined using stable isotopes and multicompartmental modeling to estimate production rate (PR) and fractional catabolic rate (FCR). Plasma VLDL-apoC-III concentration was significantly and inversely associated with the FCRs of VLDL triglycerides (r=-0.610) and VLDL-apoB (r=-0.791), and positively correlated with the PR of VLDL-apoC-III (r=0.842). However, apoA-V concentration was not significantly associated with any of the kinetic variables. There was a significant association (P<0.01) between the PRs of VLDL triglycerides and VLDL-apoB (r=0.641), and between the FCRs of VLDL triglycerides and VLDL-apoB (r=0.737). In multiple regression analysis, plasma VLDL-apoC-III concentration was a significant predictor of VLDL triglyceride FCR (beta-coefficient=-0.575) and VLDL-apoB FCR (beta-coefficient=-0.839). Our findings suggest that increased VLDL-apoC-III concentrations resulting from an overproduction of VLDL-apoC-III are strongly associated with the delayed catabolism of triglycerides and apoB in VLDL. We also demonstrated that the kinetics of VLDL triglycerides and apoB are closely coupled. Our data do not support a role for plasma apoA-V in regulating VLDL kinetics.

Type III hyperlipoproteinemia with xanthomas and multiple myeloma

Type III hyperlipoproteinemia usually results from an inherited defect in the composition of apolipoprotein E and is associated with atherosclerosis. An acquired form of the type III phenotype may rarely be associated with myeloma and immunoglobulin-lipoprotein complexes. We present the case of a 72-year-old man with a history of well-controlled, unclassified hypercholesterolemia and hypertriglyceridemia, without evidence of atherosclerotic disease. He subsequently developed refractory dyslipidemia, palmar crease, and tuberous xanthomas. Type III hyperlipoproteinemia was confirmed, and nonclassic defective apolipoprotein E. Common secondary causes of hyperlipidemia were ruled out. A workup for malignancy revealed monoclonal IgA gammopathy. Immunostaining confirmed IgA antibodies complexed to the patient's very low-density lipoprotein (VLDL) fraction, causing gross impairment of VLDL metabolism. Conventional therapy for type III hyperlipoproteinemia was attempted but ineffective. Thus, chemotherapy was initiated for his myeloma, with subsequent lowering of his IgA, cholesterol, and triglyceride levels, and improvement of his xanthomas. There are several unusual features to this case. Planar xanthomas can be associated with myelomas, but usually in the setting of normal lipids. Type III hyperlipoproteinemias are not usually refractory to standard therapy and are only rarely associated with IgA myeloma. IgA antibodies complexed to the patient's VLDL caused gross impairment of VLDL metabolism. The patient's apolipoprotein E genotype (heterozygote E2/E3) is not typical for expression of the heritable type III phenotype (homozygote E2/E2). These features support a causal relationship between this patient's multiple myeloma and type III hyperlipoproteinemia rather than two independent, coexistent conditions.

Conjugated linoleic acid suppresses the secretion of atherogenic lipoproteins from human HepG2 liver cells

Studies in healthy humans have shown that consumption of conjugated linoleic acid (CLA) significantly reduced very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) blood concentrations. We propose that decreased concentrations are due to the inhibition of VLDL production and secretion [measured by apolipoprotein B100 (apoB100)] from the liver. To investigate the effects of a mixture of CLA isomers on VLDL metabolism, HepG2 liver cells were incubated for 24 h with 50 micromol/L of the different fatty acids. Effects of CLA were compared to a saturated fatty acid (palmitic acid), an n-6 fatty acid (linoleic acid) and no treatment (control). HepG2-cell apoB100 levels were measured using Western blotting. ApoB100 secretion was significantly decreased in cells treated with CLA (44%, p<0.005) compared to control cells and those enriched with palmitic acid. Treatment of cells with CLA also decreased intracellular cholesterol levels. Collectively, these results demonstrate that CLA reduces apoB100 production and secretion compared to saturated and polyunsaturated fatty acids, possibly by limiting the availability of free cholesterol (required for apoB100 production). A reduction in apoB100 production in the body would decrease the levels of VLDL and atherogenic LDL and thus reduce the risk of developing cardiovascular disease.

Metabolism of cholesterol ester of apolipoprotein B100-containing lipoproteins in dogs: evidence for disregarding cholesterol ester transfer.

It has been shown that dogs exhibit no cholesterol ester transfer protein (CETP) activity in vitro, in contrast to humans. The aim of our study was to determine modalities of in vivo plasma cholesterol ester turnover in this species, using a kinetic approach with stable isotopes. Kinetics of very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) were studied in seven adult male Beagle dogs using a dual isotope approach through endogenous labelling of both their cholesterol moiety and their protein moiety. A primed constant infusion of both [1,2(13)C]acetate and [5,5,5-2H3]leucine enabled us to obtain measurable deuterium enrichments by gas chromatography-mass spectrometry for plasma leucine and apoB100, as well as measurable 13C enrichment by gas chromatography-combustion-isotopic ratio mass spectrometry for unesterified cholesterol and cholesterol ester in the VLDL and LDL. Two identical multicompartmental models (SAAM II) were used together for the analysis of tracer kinetics' data of proteins and cholesterol. Characterization of the apoB100-containing lipoprotein cholesterol ester model allowed determination of kinetic parameters of VLDL and LDL cholesterol ester metabolism. We succeeded in modelling VLDL and LDL cholesterol ester metabolism and apoB100 metabolism simultaneously. Fractional catabolic rate (FCR) of apoB100 and CE had the same values. Introducing cholesterol ester transfer between lipoproteins in the model did not significantly improve the fit. Total VLDL FCR was 2.97 +/- 01.47 h(-1). Approximately one-quarter corresponded to the direct removal of VLDL (0.81 +/- 00.34 h(-1)) and the remaining three-quarters corresponded to the fraction of VLDL converted to LDL, which represented a conversion of VLDL into LDL of 2.16 +/- 01.16 h(-1). Low-density lipoproteins were produced exclusively from VLDL conversion and were then removed (0.031 +/- 0.004 h(-1)) from plasma. These kinetic data showed that VLDL cholesterol ester and LDL cholesterol ester metabolism followed VLDL and LDL apoB100 metabolism, and that consequently there is no in vivo transfer of cholesterol ester in dogs.

Defective VLDL metabolism and severe atherosclerosis in mice expressing human apolipoprotein E isoforms but lacking the LDL receptor

Differences in affinity of human apolipoprotein E (apoE) isoforms for the low density lipoprotein receptor (LDLR) are thought to result in the differences in lipid metabolism observed in humans with different APOE genotypes. Mice expressing three common human apoE isoforms, E2, E3, and E4, in place of endogenous mouse apoE were used to investigate the relative roles of apoE isoforms in LDLR- and non-LDLR-mediated very low density lipoprotein (VLDL) clearance. While both VLDL particles isolated from mice expressing apoE3 and apoE4 bound to mouse LDLR with affinity and Bmax similar to VLDL containing mouse apoE, VLDL with apoE2 bound with only half the Bmax. In the absence of the LDLR, all lines of mice expressing human apoE showed dramatic increases in VLDL cholesterol and triglycerides (TG) compared to LDLR knockout mice expressing mouse apoE. The mechanism of the hyperlipidemia in mice expressing human apoE isoforms is due to impairment of non-LDL-receptor-mediated VLDL clearance. This results in the severe atherosclerosis observed in mice expressing human apoE but lacking the LDLR, even when fed normal chow diet. Our data show that defects in LDLR independent pathway(s) are a potential factor that trigger hyperlipoproteinemia when the LDLR pathway is perturbed, as in E2/2 mice.

Effect of hypothyroidism on kinetics of metabolism of very-low-density lipoprotein in mares.

To compare kinetics of the metabolism of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) before and after thyroidectomy in mares. 5 healthy adult mares. Thyroidectomy was performed in euthyroid mares. Kinetics of VLDL apoB metabolism were measured before and after thyroidectomy by use of a bolus IV injection of 5,5,5-2H3 (98%) leucine (5 mg/kg) and subsequent isolation of labeled amino acid from plasma and VLDL. Labeled leucine was quantified by use of gas chromatography-mass spectrometry. Production rate (PR), delay time, and fractional catabolic rate (FCR) were calculated for the 2 forms of equine VLDL, apoB-48 VLDL, and apoB-100 VLDL. Plasma lipid concentrations were measured, and VLDL composition was determined. Physical appearance of horses was not altered by thyroidectomy. Significantly lower mean blood concentrations of thyroid hormones and non-esterified fatty acids were detected following thyroidectomy. Mean percentage of free cholesterol in VLDL was significantly higher after thyroidectomy. Mean plasma VLDL concentration or kinetics of apoB-48 or apoB-100 were not significantly altered by thyroidectomy. Mean +/- SEM PR was significantly lower (8.70 +/- 1.61 mg/kg/d) and mean delay time significantly longer (1.58 +/- 0.12 hours) for apoB-48 VLDL in euthyroid mares, compared with values for thyroidectomized mares (16.15 +/- 2.24 mg/kg/d and 0.93 +/- 0.10 hours, respectively). Hypothyroidism did not significantly alter plasma VLDL concentrations or kinetics of VLDL apoB metabolism. Metabolism of apoB-48 VLDL differed significantly from that of apoB-100 VLDL in euthyroid mares.

Differential metabolism of human VLDL according to content of ApoE and ApoC-III.

We studied the metabolism of very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL) particles that did or did not have apolipoprotein E (apoE) in 12 normolipidemic women by endogenously labeling plasma apolipoprotein B. The plasma was separated into bound (E+) and unbound (E-) fractions by use of a monoclonal antibody (1D7), and the fractions were ultracentrifuged to yield E+ and E- subfractions of light and dense VLDL and IDL. VLDL E+ and IDL E+ were produced mainly by the liver. VLDL E+ and IDL E+ had lower fractional catabolic rates and much higher apolipoprotein C-III (apoC-III) content than did the corresponding E- particles. Most light VLDL apoE+ underwent lipolysis to dense VLDL E+ with reduced apoC-III content, which was removed from the circulation without conversion to IDL. In contrast, most light VLDL apoE-, poor in apoC-III, was removed from the circulation, and a smaller proportion underwent lipolysis to dense VLDL E-. Most dense VLDL E- underwent lipolysis to IDL E-. The rate constant for lipolysis of dense VLDL to IDL was greater for E- than for E+, and the rate constant for clearance from plasma was greater for dense VLDL E+ than for E-. In conclusion, metabolism of human VLDL particles is influenced by their content of apoE, further modulated by the coexistence of apoC-III.

ApoA-II expression in CETP transgenic mice increases VLDL production and impairs VLDL clearance

Apolipoprotein (apo)A-II is a major high density lipoprotein (HDL) protein; however, its role in lipoprotein metabolism is largely unknown. Transgenic (Tg) mice that overexpress human apoA-II present functional lecithin: cholesterol acyltransferase deficiency, HDL deficiency, hypertriglyceridemia and, when fed an atherogenic diet, increased non-HDL cholesterol and increased susceptibility to atherosclerosis. In contrast to humans, mice do not present cholesteryl ester transfer protein (CETP) activity in plasma. To study the in vivo interaction of these two proteins, we crossbred human apoA-II and CETP-Tg mice. CETP×apoA-II-Tg mice fed an atherogenic diet, compared with CETP-Tg mice presented a 2-fold decrease in HDL cholesterol and a quantitatively similar increase in total plasma cholesterol and percentage of free cholesterol, non-HDL cholesterol, and free fatty acids, together with a remarkable 112-fold increase in plasma triglycerides. Plasma triglycerides in CETP×apoA-II-Tg mice were mainly associated with very low density lipoproteins (VLDL), which were also enriched in protein content, and resulted from a combination of higher production rate compared with both of their progenitors and non-Tg control mice, and decreased catabolism compared only with CETP-Tg mice. These results show CETP×apoA-II-Tg mice to be a good model with which to study mechanisms leading to VLDL overproduction and suggest that CETP and, in particular apoA-II, may play a role in the regulation of VLDL metabolism. —Escolà-Gil, J. C., J. Julve, À. Marzal-Casacuberta, J. Ordóñez-Llanos, F. González-Sastre, and F. Blanco-Vaca. ApoA-II expression in CETP transgenic mice increases VLDL production and impairs VLDL clearance.

Glucose-stimulated insulin secretion suppresses hepatic triglyceride-rich lipoprotein and apoB production.

The current study assessed in vivo the effect of insulin on triglyceride-rich lipoprotein (TRL) production by rat liver. Hepatic triglyceride and apolipoprotein B (apoB) production were measured in anesthetized, fasted rats injected intravenously with Triton WR-1339 (400 mg/kg). After intravascular catabolism was blocked by detergent treatment, glucose (500 mg/kg) was injected to elicit insulin secretion, and serum triglyceride and apoB accumulation were monitored over the next 3 h. In glucose-injected rats, triglyceride secretion averaged 22.5 +/- 2.1 microg.ml(-1).min(-1), which was significantly less by 30% than that observed in saline-injected rats, which averaged 32.1 +/- 1.4 microg.ml(-1).min(-1). ApoB secretion was also significantly reduced by 66% in glucose-injected rats. ApoB immunoblotting indicated that both B100 and B48 production were significantly reduced after glucose injection. Results support the conclusion that insulin acts in vivo to suppress hepatic very low density lipoprotein (VLDL) triglyceride and apoB secretion and strengthen the concept of a regulatory role for insulin in VLDL metabolism postprandially.

Metabolism of VLDL is increased in streptozotocin-induced diabetic rat hearts.

In streptozotocin (STZ)-induced diabetic rats, we previously showed an increased heparin-releasable (luminal) lipoprotein lipase (LPL) activity from perfused hearts. To study the effect of this enlarged LPL pool on triglyceride (TG)-rich lipoproteins, we examined the metabolism of very-low-density lipoprotein (VLDL) perfused through control and diabetic hearts. Diabetic rats had elevated TG levels compared with control. However, fasting for 16 h abolished this difference. When the plasma lipoprotein fraction of density <1.006 g/ml from fasted control and diabetic rats was incubated in vitro with purified bovine or rat LPL, VLDL from diabetic animals was hydrolyzed as proficiently as VLDL from control animals. Post-heparin plasma lipolytic activity was comparable in control and diabetic animals. However, perfusion of control and diabetic rats with heparinase indicated that diabetic hearts had larger amounts of LPL bound to heparan sulfate proteoglycan-binding sites. [(3)H]VLDL obtained from control rats, when recirculated through the isolated heart, disappeared at a significantly faster rate from diabetic than from control rat hearts. This increased VLDL-TG hydrolysis was essentially abolished by prior perfusion of the diabetic heart with heparin, implicating LPL in this process. These findings suggest that the enlarged LPL pool in the diabetic heart is present at a functionally relevant location (at the capillary lumen) and is capable of hydrolyzing VLDL. This could increase the delivery of free fatty acid to the heart, and the resultant metabolic changes could induce the subsequent cardiomyopathy that is observed in the chronic diabetic rat.