Very Low Density Lipoprotein Catabolism Research Articles

The Differential Effects of HDL Subpopulations on Lipoprotein Lipase (LPL)-Mediated VLDL Catabolism.

High-density lipoprotein (HDL) subpopulations functional assessment is more relevant for HDL anti-atherogenic activity than cholesterol level. The aim of the study was to assess the impact of HDL-2 and HDL-3 on lipoprotein lipase (LPL)-mediated very-low-density lipoprotein (VLDL) catabolism related to hypertriglyceridemia development. VLDL and HDLs were isolated from serum by ultracentrifugation. VLDL was incubated with LPL in the absence and presence of total HDL or HDL subpopulations. Next, VLDL remnants were separated, and their composition and electrophoretic mobility was assessed. Both HDL subpopulations increased the efficiency of triglyceride lipolysis and apolipoprotein CII and CIII removal from VLDL up to ~90%. HDL-3 exerted significantly greater impact than HDL-2 on apolipoprotein E (43% vs. 18%, p < 0.001), free cholesterol (26% vs. 18%, p < 0.05) and phospholipids (53% vs. 43%, p < 0.05) removal from VLDL and VLDL remnant electrophoretic mobility (0.18 vs. 0.20, p < 0.01). A greater release of these components was also observed in the presence of total HDL with a low HDL-2/HDL-3 cholesterol ratio. Both HDL subpopulations affect VLDL composition during lipolysis, but HDL-3 exhibited a greater effect on this process. Altered composition of HDL related to significant changes in the distribution between HDL-2 and HDL-3 can influence the VLDL remnant features, affecting atherosclerosis progression.

Impaired Very-Low-Density Lipoprotein catabolism links hypoglycemia to hypertriglyceridemia in Glycogen Storage Disease typeIa.

Prevention of hypertriglyceridemia is one of the biomedical targets in Glycogen Storage Disease type Ia (GSD Ia) patients, yet it is unclear how hypoglycemia links to plasma triglyceride (TG) levels. We analyzed whole‐body TG metabolism in normoglycemic (fed) and hypoglycemic (fasted) hepatocyte‐specific glucose‐6‐phosphatase deficient (L‐G6pc −/−) mice. De novo fatty acid synthesis contributed substantially to hepatic TG accumulation in normoglycemic L‐G6pc −/− mice. In hypoglycemic conditions, enhanced adipose tissue lipolysis was the main driver of liver steatosis, supported by elevated free fatty acid concentrations in GSD Ia mice and GSD Ia patients. Plasma very‐low‐density lipoprotein (VLDL) levels were increased in GSD Ia patients and in normoglycemic L‐G6pc −/− mice, and further elevated in hypoglycemic L‐G6pc −/− mice. VLDL‐TG secretion rates were doubled in normo‐ and hypoglycemic L‐G6pc −/− mice, while VLDL‐TG catabolism was selectively inhibited in hypoglycemic L‐G6pc −/− mice. In conclusion, fasting‐induced hypoglycemia in L‐G6pc −/− mice promotes adipose tissue lipolysis and arrests VLDL catabolism. This mechanism likely contributes to aggravated liver steatosis and dyslipidemia in GSD Ia patients with poor glycemic control and may explain clinical heterogeneity in hypertriglyceridemia between GSD Ia patients.

Beneficial Effects of Adiponectin on Glucose and Lipid Metabolism and Atherosclerotic Progression: Mechanisms and Perspectives.

Circulating adiponectin concentrations are reduced in obese individuals, and this reduction has been proposed to have a crucial role in the pathogenesis of atherosclerosis and cardiovascular diseases associated with obesity and the metabolic syndrome. We focus on the effects of adiponectin on glucose and lipid metabolism and on the molecular anti-atherosclerotic properties of adiponectin and also discuss the factors that increase the circulating levels of adiponectin. Adiponectin reduces inflammatory cytokines and oxidative stress, which leads to an improvement of insulin resistance. Adiponectin-induced improvement of insulin resistance and adiponectin itself reduce hepatic glucose production and increase the utilization of glucose and fatty acids by skeletal muscles, lowering blood glucose levels. Adiponectin has also β cell protective effects and may prevent the development of diabetes. Adiponectin concentration has been found to be correlated with lipoprotein metabolism; especially, it is associated with the metabolism of high-density lipoprotein (HDL) and triglyceride (TG). Adiponectin appears to increase HDL and decrease TG. Adiponectin increases ATP-binding cassette transporter A1 and lipoprotein lipase (LPL) and decreases hepatic lipase, which may elevate HDL. Increased LPL mass/activity and very low density lipoprotein (VLDL) receptor and reduced apo-CIII may increase VLDL catabolism and result in the reduction of serum TG. Further, adiponectin has various molecular anti-atherosclerotic properties, such as reduction of scavenger receptors in macrophages and increase of cholesterol efflux. These findings suggest that high levels of circulating adiponectin can protect against atherosclerosis. Weight loss, exercise, nutritional factors, anti-diabetic drugs, lipid-lowering drugs, and anti-hypertensive drugs have been associated with an increase of serum adiponectin level.

Retinol-Binding Protein 4 Is an Independent Factor Associated With Triglycerides and a Determinant of Very Low-Density Lipoprotein–Apolipoprotein B100 Catabolism in Type 2 Diabetes Mellitus

Retinol-binding protein 4 (rbp4) is an adipokine secreted by adipocytes and liver, whose levels are elevated in type 2 diabetes mellitus (T2DM). Plasma levels of rbp4 and triglycerides are strongly correlated in T2DM. However, we do not know whether this association is direct or indirect via liver fat content, and the link between rbp4 and triglyceride metabolism remains unknown. Liver fat measurement by proton spectroscopy was performed in 221 patients with T2DM, and an in vivo kinetic study with stable isotopes was carried out in 14 patients with T2DM. In multivariate analysis, triglycerides were associated positively with rbp4 (β=0.273, P<0.0001), apolipoprotein (apo) B (β=0.258, P<0.0001), and liver fat (β=0.191, P=0.002) and negatively with high-density lipoprotein cholesterol (β=-0.442, P<0.0001). rbp4 was correlated positively with apoB100 very-low-density lipoprotein (VLDL) pool (r=0.62, P=0.017) and negatively with VLDL-apoB100 total fractional catabolic rate (r=-0.66, P=0.001). In multivariate analysis, rbp4 (P=0.015), plasma triglycerides (P=0.024), and sex (P=0.026) were independently associated with VLDL-apoB100 total fractional catabolic rate. In T2DM, plasma rbp4 level is associated with plasma triglycerides, independently of liver fat content. There is a strong independent negative correlation between plasma rbp4 and VLDL-apoB100 total fractional catabolic rate. These data suggest that rbp4 may be involved in the pathophysiology of hypertriglyceridemia in T2DM by reducing VLDL catabolism.

Shedding of syndecan-1 from human hepatocytes alters very low density lipoprotein clearance

We recently showed that the heparan sulfate proteoglycan syndecan-1 mediates hepatic clearance of triglyceride-rich lipoproteins in mice based on systemic deletion of syndecan-1 and hepatocyte-specific inactivation of sulfotransferases involved in heparan sulfate biosynthesis. Here, we show that syndecan-1 expressed on primary human hepatocytes and Hep3B human hepatoma cells can mediate binding and uptake of very low density lipoprotein (VLDL). Syndecan-1 also undergoes spontaneous shedding from primary human and murine hepatocytes and Hep3B cells. In human cells, phorbol myristic acid induces syndecan-1 shedding, resulting in accumulation of syndecan-1 ectodomains in the medium. Shedding occurs through a protein kinase C-dependent activation of ADAM17 (a disintegrin and metalloproteinase 17). Phorbol myristic acid stimulation significantly decreases DiD (1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate)-VLDL binding to cells, and shed syndecan-1 ectodomains bind to VLDL. Although mouse hepatocytes appear resistant to induced shedding in vitro, injection of lipopolysaccharide into mice results in loss of hepatic syndecan-1, accumulation of ectodomains in the plasma, impaired VLDL catabolism, and hypertriglyceridemia. These findings suggest that syndecan-1 mediates hepatic VLDL turnover in humans as well as in mice and that shedding might contribute to hypertriglyceridemia in patients with sepsis.

Targeting FoxO1 for Hypertriglyceridemia

Hypertriglyceridemia is characterized by increased production and decreased clearance of triglyceride-rich lipoproteins including very low-density lipoprotein (VLDL) and chylomicron. Due to its proatherogenic profile, hypertriglyceridemia contributes to the development of atherosclerosis and coronary artery disease. While the pathophysiology of hypertriglyceridemia remains poorly understood, its close association with obesity and type 2 diabetes implicates insulin resistance in the pathogenesis of hypertriglyceridemia. However, the molecular basis linking insulin resistance to hypertriglyceridemia remains elusive. Preclinical studies show that FoxO1 plays a pivotal role in controlling insulin-dependent regulation of microsomal triglyceride transfer protein (MTP) and apolipoprotein C-III (ApoC-III), two key components that catalyze the rate-limiting steps in the production and clearance of triglyceride-rich lipoproteins. Under physiological conditions, FoxO1 activity is inhibited by insulin. In insulin resistant states, FoxO1 becomes deregulated, contributing to unbridled FoxO1 activity in the liver. This effect contributes to hepatic overproduction of VLDL and impaired catabolism of triglyceride-rich particles, accounting for the pathogenesis of hypertriglyceridemia. These data spur the hypothesis that selective inhibition of FoxO1 activity in the liver would improve triglyceride metabolism and ameliorate hypertriglyceridemia. In this article, we review the role of FoxO1 in insulin action and lipid metabolism, and evaluate the therapeutic potential of targeting FoxO1 for treating hypertriglyceridemia in insulin resistant subjects with obesity and type 2 diabetes.

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.

Genetic epistasis in the VLDL catabolic pathway is associated with deleterious variations on triglyceridemia in obese subjects

Abdominal obesity and hypertriglyceridemia (the hypertriglyceridemic-waist phenotype) increase cardiovascular risk. The very low-density lipoprotein (VLDL) is a triglyceride (TG)-rich particle. Frequent variations in the genes coding for enzymes and proteins involved in the VLDL catabolism have already been documented. The epistatic effect of such variants on the risk profile associated with abdominal obesity remains to be elucidated. This study aims to assess the effect of combinations of frequent single-nucleotide polymorphisms (SNPs) in the VLDL catabolic pathway on the relation between abdominal obesity and fasting TG. Only gene variants in the lipoprotein lipase, apolipoprotein (apo) CIII, hepatic lipase and apo E genes known to be frequent in the general population (allele frequency>5%) were included in this study. The presence of selected SNPs was detected by polymerase chain reaction-restriction fragment length polymorphism in a sample of 640 non-diabetic French Canadians at high cardiovascular risk (405 obese, 235 non-obese). Carrying more than two frequent gene variants involved in the VLDL catabolic pathway significantly increased the risk of hyperTG (odds ratio of TG>1.7 mmol/l=4.15; P=0.001). This effect was proportional to the number of SNPs and genes involved and was significantly amplified by the presence of abdominal obesity defined on the basis of waist circumference. When combined with abdominal obesity, epistasis in the VLDL pathway has a deleterious effect on fasting TG and coronary artery disease risk profile according to the TG threshold (1.7 mmol/l) used in medical guidelines for the assessment of the metabolic syndrome and associated risk.

Hemostasis alterations in metabolic syndrome (Review)

Metabolic syndrome (MS) is characterized by the presence of at least three of the following alterations: enlargement of the waist diameter, higher levels of arterial pressure, low density lipoprotein cholesterol and glycemia, and reduction of high density lipoprotein cholesterol. The prevalence of MS reaches 23% in young adults, a percentage that increases with age. People with MS have a greater risk of suffering from cardiovascular disease (CVD). The physiopathologic alterations now found to exist in MS are diverse; among them is endothelial dysfunction, which triggers atherogenic lesions and hypercoagulability characterized by alterations of the coagulation factors and the regulatory proteins of fibrinolysis such as the plasminogen activator inhibitor (PAI-1). The increase in oxidative stress and/or the reactive oxygen species in patients with MS is partially related to the oxidation state of the lipoproteins, especially of the low density lipoproteins. This fact favors atherogenesis. Moreover, the oxidative stress produces alterations in the production of adipokines, cytokines secreted by the adipose tissues. The abnormality in the transport of lipoprotein diminishes the catabolism of the very low density lipoprotein (VLDL) and increases the catabolism of the high density lipoprotein (HDL), which creates insulin resistance. This process is associated with a lower concentration of adiponectin that in turn regulates the catabolism of VLDL and HDL; consequently increasing the flow of fatty acids from the adipose tissue to the liver and muscles. The proinflammatory cytokines, among them tumor necrosis factor alpha (TNF-alpha), are of great importance in MS regulating different processes and molecules such as PAI-1. PAI-1 is controlled by the group of transcription factors peroxisome proliferator-activated receptor (PPAR), especially by PPAR gamma and alpha ligands. In summary, MS includes multiple alterations related to insulin resistance at several levels: hepatic, muscular, adipose and vascular tissue (endothelium). The exact mechanism that underlies the relationship between MS and CVD are not sufficiently known yet; pathogenic explanations are lacking for the mechanisms relating metabolic factors to insulin resistance and the association with the development of atherosclerosis and thrombosis. MS alterations and the main aspects related to homeostasis alterations are examined in this report.

Apolipoprotein C-III isoforms: kinetics and relative implication in lipid metabolism

Apolipoprotein C-III (apoC-III) production rate (PR) is strongly correlated with plasma triglyceride (TG) levels. ApoC-III exists in three different isoforms, according to the sialylation degree of the protein. We investigated the kinetics and respective role of each apoC-III isoform in modulating intravascular lipid/lipoprotein metabolism. ApoC-III kinetics were measured in a sample of 18 healthy men [mean age (+/-SD) 42.1 +/- 9.5 years, body mass index 29.8 +/- 4.6 kg/m2] using a primed-constant infusion of l-(5,5,5-D3) leucine for 12 h. Mono-sialylated and di-sialylated apoC-III (apo-CIII1 and apoC-III2) exhibited similar PRs (means +/- SD, 1.22 +/- 0.49 mg/kg/day vs. 1.15 +/- 0.59 mg/kg/day, respectively) and similar fractional catabolic rates (FCRs) (0.51 +/- 0.13 pool/day vs. 0.61 +/- 0.24 pool/day, respectively). Nonsialylated apoC-III (apoC-III0) had an 80% lower PR (0.25 +/- 0.12 mg/kg/day) and a 60% lower FCR (0.21 +/- 0.07 pool/day) (P < 0.0001 for comparison with CIII1 and CIII2 isoforms). The PRs of apoC-III1 and apoC-III2 were more strongly correlated with plasma TG levels (r > 0.8, P < 0.0001) than was apoC-III0 PR (r = 0.54, P < 0.05). Finally, the PR of apoC-III2 was strongly correlated with the proportion of LDL <255 A (r = 0.72, P = 0.002). These results suggest that all apoC-III isoforms, especially the predominant CIII1 and CIII2 isoforms, contribute to hypertriglyceridemia and that apoC-III2 may play a significant role in the expression of the small, dense LDL phenotype.

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor α (PPARα)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARα activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid β-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARα target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [ 3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARα-signaling.

Influence of Atorvastatin on Apolipoprotein E and AI Kinetics in Patients with Type 2 Diabetes

Atorvastatin reduces both plasma cholesterol and triglyceride concentrations in patients with type 2 diabetes, but mechanisms underlying triglyceride decrease and the effect of atorvastatin on high density lipoprotein (HDL) still remain unclear. Apolipoprotein (apo) E plays a crucial role in modulating production and clearance of triglyceride-rich very low density lipoprotein (VLDL). The main effect of apoAI is to modulate HDL metabolism. The aim of this work was to study the influence of atorvastatin on apoAI and apoE kinetics and to determine whether its hypocholesterolemic and hypotriglyceridemic effects could be related to changes in this apolipoprotein metabolism. Plasma VLDL-apoE, HDL-apoE, and HDL-apoAI were studied in seven patients with diabetes with mixed hyperlipidemia using a stable isotope labeling technique ([(2)H3]leucine-primed constant infusion) and monocompartmental model before and after 2 months of treatment with 40 mg/day of atorvastatin. Plasma apoE concentration was significantly reduced (44.1 +/- 19.1 versus 32 +/- 11.6 mg/l, p < 0.05) after treatment. This decrease was associated with a diminution of HDL-apoE concentration (17.46 +/- 6.71 versus 13.37 +/- 6.05 mg/l, p < 0.05) and production rate (0.202 +/- 0.085 versus 0.119 +/- 0.047 mg/kg/day, p < 0.05), whereas an increase in VLDL-apoE concentration (6.44 +/- 2.16 before versus 9.23 +/- 4.02 mg/l after, p < 0.05) and production rate (0.827 +/- 0.367 versus 1.524 +/- 0.664 mg/kg/day, p < 0.05) was observed. No significant difference was observed after treatment for apoAI parameters. We conclude that atorvastatin treatment promotes different apoE distribution between HDL and VLDL, favoring VLDL apoE content. The increased number of apoE per VLDL particle suggests that atorvastatin could enhance the direct catabolism of triglyceride-rich VLDL through apoE receptor pathways.

Effects of obesity on lipid profiles in neutered male and female cats.

To examine whether obese cats, compared with lean cats, have alterations in lipoprotein metabolism that might lead to a decrease in glucose metabolism and insulin secretion. 10 lean and 10 obese adults cats (5 neutered males and 5 neutered females each). Intravenous glucose tolerance tests with measurements of serum glucose, insulin, and nonesterified fatty acid (NEFA) concentrations were performed. Lipoprotein fractions were examined in serum by isopycnic density gradient ultracentrifugation. Obese cats had insulin resistance. Plasma triglyceride and cholesterol concentrations were significantly increased in obese cats, compared with lean cats. Very low density lipoprotein (VLDL) concentrations were increased in obese cats, compared with lean cats; however, the composition of various fractions remained unchanged between obese and lean cats, indicating greater synthesis and catabolism of VLDL in obese cats. Serum high density lipoprotein (HDL) cholesterol concentrations were increased in obese cats, compared with lean cats. Serum NEFA concentrations were only significantly different between obese and lean cats when separated by sex; obese male cats had higher baseline serum NEFA concentrations and greater NEFA suppression in response to insulin, compared with lean male cats. Lipid metabolism changes in obese cats, compared with lean cats. The increase in VLDL turnover in obese cats might contribute to insulin resistance of glucose metabolism, whereas the increase in serum HDL cholesterol concentration might reflect a protective effect against atherosclerosis in obese cats.

Ex vivo measurement of lipoprotein lipase-dependent very low density lipoprotein (VLDL)-triglyceride hydrolysis in human VLDL: an alternative to the postheparin assay of lipoprotein lipase activity?

The plasma lipolysis of triglyceride (TG)-rich lipoproteins is mainly due to the activity of lipoprotein lipase (LPL). Albeit important for our analysis of certain physiopathological situations, the determination of the magnitude of LPL-dependent lipolysis is not easy to perform. This essentially results from the binding of LPL to the luminal surface of vascular endothelium. The measurements of the whole putative LPL activity have been achieved after injection of heparin, a procedure that releases LPL from endothelium. However, the physiopathological relevance of this postheparin lipolysis assay (PHLA) remains questionable because it has never been demonstrated that the bulk of endothelium-bound LPL was active. It has been recently shown that a small part of LPL is associated to circulating lipoproteins in nonheparinized plasma, raising the possibility that the lipolysis mediated by this circulating LPL might reflect the overall LPL-dependent TG hydrolysis in plasma. To address this question, we developed a new lipolysis assay in which the very low density lipoprotein (VLDL)-bound LPL-dependent VLDL-TG hydrolysis (LVTH) was directly determined through the measurement of nonesterified fatty acid (NEFA) release during in vitro incubations. LVTH measurements were performed in control subjects, in type 2 diabetics, and in either heterozygous or homozygous LPL-deficient patients. In the latter group, LVTH values were extremely low. Those of heterozygous patients and of diabetics were similarly decreased by about 40% with respect to control group. Plasma TG concentrations exhibited an inverse relationship with LVTH level. In a subgroup of subjects, LVTH and PHLA were positively correlated and the inverse correlation of LVTH with plasma or VLDL-TG concentration was stronger than that obtained with PHLA. To further study the validity of this new assay, we measured LVTH in nine subjects who were studied for their catabolism of VLDL labeled with stable isotope. No relation was observed between the direct hepatic removal of VLDL and LVTH, whereas the latter was strikingly correlated with the rate of conversion of VLDL to intermediary density lipoprotein. Collective consideration of these findings strongly suggests that LVTH is a physiologically relevant index which could advantageously replace the measurements of PHLA in numerous physiopathological situations.

Overexpression of Human Apolipoprotein A-II in Mice Induces Hypertriglyceridemia Due to Defective Very Low Density Lipoprotein Hydrolysis

Two lines of transgenic mice, hAIItg-delta and hAIItg-lambda, expressing human apolipoprotein (apo)A-II at 2 and 4 times the normal concentration, respectively, displayed on standard chow postprandial chylomicronemia, large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) but greatly reduced high density lipoprotein (HDL). Hypertriglyceridemia may result from increased VLDL production, decreased VLDL catabolism, or both. Post-Triton VLDL production was comparable in transgenic and control mice. Postheparin lipoprotein lipase (LPL) and hepatic lipase activities decreased at most by 30% in transgenic mice, whereas adipose tissue and muscle LPL activities were unaffected, indicating normal LPL synthesis. However, VLDL-triglyceride hydrolysis by exogenous LPL was considerably slower in transgenic compared with control mice, with the apparent Vmax of the reaction decreasing proportionately to human apoA-II expression. Human apoA-II was present in appreciable amounts in the VLDL of transgenic mice, which also carried apoC-II. The addition of purified apoA-II in postheparin plasma from control mice induced a dose-dependent decrease in LPL and hepatic lipase activities. In conclusion, overexpression of human apoA-II in transgenic mice induced the proatherogenic lipoprotein profile of low plasma HDL and postprandial hypertriglyceridemia because of decreased VLDL catabolism by LPL.

Effects of a cardioselective β-blocker on postprandial triglyceride-rich lipoproteins, low density lipoprotein particle size and glucose-insulin homeostasis in middle-aged men with modestly increased cardiovascular risk

β-adrenergic receptor-blocking agents are commonly used for treatment of hypertension, angina pectoris and arrhythmias and as secondary prevention after myocardial infarction. The modest protection against myocardial infarction conferred by these compounds in primary-preventive studies has suggested that beneficial effects of β-blockade are counteracted by known adverse influences on lipid and glucose metabolism. As most β-blockers increase plasma triglycerides and decrease the high density lipoprotein (HDL) cholesterol concentration, a randomized, double-blind, cross-over study was conducted to evaluate whether a 12-week treatment with metoprolol (100 mg o.d.) or placebo affected the metabolism of postprandial triglyceride-rich lipoproteins in 15 middle-aged men with a modestly increased cardiovascular risk. Metoprolol treatment significantly increased the postprandial responses of very low density lipoprotein (VLDL) and VLDL remnants to a mixed meal-type of oral fat tolerance test. The effect was particularly prominent for larger (Svedberg flotation rate (Sf)>400 and Sf 60–400) particle species ( P<0.001 in repeated measures ANOVA), whereas the smaller (Sf 20–60) particles were less affected ( P<0.05). The changes in the postprandial responses of the different VLDL species were mainly related to an effect on the fasting plasma concentrations, with limited or no influences on VLDL catabolism during the postprandial state. In contrast, metoprolol treatment did not significantly influence the postprandial responses of chylomicrons and chylomicron remnants. Notably, the enhanced fasting and postprandial triglyceridaemia during metoprolol treatment was neither accompanied by a rise in fasting or postprandial free fatty acid concentrations, nor by alterations of the glucose and insulin responses to a standard oral glucose challenge. The ensuing shift in the LDL particle size distribution towards smaller particles was limited (fraction small LDL: metoprolol 22.8±15.7% versus placebo 19.3±15.0%, P<0.05). In conclusion, metoprolol treatment primarily enhances fasting and postprandial triglyceridaemia in middle-aged men by increasing the basal hepatic production of VLDL.

Increased VLDL in nephrotic patients results from a decreased catabolism while increased LDL results from increased synthesis

Increased VLDL in nephrotic patients results from a decreased catabolism while increased LDL results from increased synthesis

Regulation of apolipoprotein B-containing lipoproteins by dietary soluble fiber in guinea pigs

Dietary soluble-fiber sources such as pectin, guar gum, or psyllium decrease plasma concentrations of low-density-lipoprotein (LDL) cholesterol in guinea pigs by distinct mechanisms, including increases in LDL apolipoprotein (apo) B turnover and/or decreases in LDL apo B flux (J Lipid Res 1995; 36:2394-404). The present studies were undertaken to test whether changes in the rates of very-low-density lipoprotein (VLDL) apo B secretion, VLDL conversion to LDL, and hepatic uptake of VLDL were related to the cholesterol-lowering actions of these soluble fibers. Guinea pigs were fed (by wt) 12.5% pectin, 12.5% guar gum, 7.5% psyllium, or a control diet containing cellulose as the fiber source. Plasma cholesterol concentrations were significantly lower in guinea pigs fed pectin, guar gum, and psyllium by 42%, 46%, and 35%, respectively (P < 0.001), compared with those animals fed the control diet, whereas plasma triacylglycerol concentrations were lower only with guar gum intake. The secretion rate of triacylglycerol, determined after Triton was injected to block VLDL catabolism, was not different among dietary treatment groups whereas the secretion rate of apo B was lower with pectin, guar gum, and psyllium intakes (P < 0.01). In addition, pectin, guar gum, and psyllium significantly altered the composition of newly secreted VLDLs by increasing the number of triacylglycerol and phospholipid molecules in the secreted lipoprotein, indicating the presence of larger nascent VLDLs. In contrast, the average particle diameter of mature VLDLs as determined by electron microscopy was smaller in the dietary soluble-fiber groups in the following order: pectin < psyllium < guar gum. Plasma lecithin-cholesteryl acyltransferase and cholesteryl ester transfer protein activities were lower with intake of pectin, guar gum, and psyllium (P < 0.01). Injection of radiolabeled lipoproteins indicated that pectin, guar gum, and psyllium intakes resulted in more rapid VLDL and LDL apo B turnover and lower LDL apo B flux in guinea pigs fed guar gum and psyllium. These results suggest that the secretion rate of apo B, compositional changes in lipoproteins occurring in the intravascular compartment, and up-regulation of LDL receptors-associated with faster LDL apo B turnover rates-are secondary metabolic responses induced by dietary fiber that contribute to the lowering of plasma cholesterol concentrations.

Efficient adenovirus-mediated ectopic gene expression of human lipoprotein lipase in human hepatic (HepG2) cells.

Gene therapy to deliver and express a corrective lipoprotein lipase (LPL) gene may improve the lipid profile and reduce the morbidity and potential atherogenic risk from hypertriglyceridemia and dyslipoproteinemia in patients with complete or partial LPL deficiency. We have used an E1-/E3- adenoviral vector, with an RSV-driven human LPL cDNA expression cassette (Ad-RSV-LPL), to achieve high ectopic LPL gene expression in the human hepatoma cell line HepG2, an accepted hepatocellular model of lipoprotein metabolism. Ad-RSV-LPL transduction of HepG2 cells with a multiplicity of infection (moi) between 12.5 and 100 yielded dose-dependent increments in LPL mass and activity. Peak levels of LPL protein of 2,032.1 +/- 274.5 ng/10(5) cells per ml (mol 100) correlated with increased activity of 92.7 +/- 22.6 mU/10(5) cells per ml relative to negligible LPL levels in Ad-RSV-LacZ (beta-galactosidase) controls. Exogenous LPL expression over a 5-day period peaked at day 3. Susceptibility to inhibition by 1 M NaCl and an anti-LPL monoclonal antibody confirmed that lipase activity was indeed derived from human LPL. Hydrolysis, by LPL-overexpressing HepG2 cells, of TG carried in very-low-density lipoprotein (VLDL) showed that greater than 50% of the triglycerides (TG) disappeared after 4 hr of incubation. These results were compatible with FPLC evidence of a marked reduction in VLDL-TG. These results provide strong in vitro evidence that adenoviral-mediated ectopic expression of the human LPL gene could render hepatic cells capable of VLDL catabolism and thus support the possibility for in vivo adenoviral vector-mediated liver-targeted LPL gene therapy.

Structure-Function Relationship of Lipoprotein Lipase-mediated Enhancement of Very Low Density Lipoprotein Binding and Catabolism by the Low Density Lipoprotein Receptor: FUNCTIONAL IMPORTANCE OF A PROPERLY FOLDED SURFACE LOOP COVERING THE CATALYTIC CENTER

We examined the structure-function relationship of human lipoprotein lipase (hLPL) in its ability to enhance the binding and catabolism of very low density lipoproteins (VLDL) in COS cells. Untransfected COS cells did not bind to or catabolize normal VLDL. Expression of wild-type hLPL by transient transfection enhanced binding, uptake, and degradation of the VLDL (a property of LPL that we call bridge function). Heparin pretreatment and a monoclonal antibody ID7 that blocks LDL receptor-binding domain of apoE both inhibited binding, and apoE2/E2 VLDL from a Type III hyperlipidemic subject did not bind. However, LDL did not reduce 125I-VLDL binding to the hLPL-expressing cells, whereas rabbit beta-VLDL was an effective competitor. By contrast, LDL reduced uptake and degradation of 125I-VLDL to the same extent as excess unlabeled VLDL or beta-VLDL. These data suggest that binding occurs by direct interaction of VLDL with LPL but the subsequent catabolism of the VLDL is mediated by the LDL receptor. Mutant hLPLs that were catalytically inactive, S132A, S132D, as well as the partially active mutant, S251T, and S172G, gave normal enhancement of VLDL binding and catabolism, whereas the partially active mutant S172D had markedly impaired capacity for the process; thus, there is no correlation between bridge function and lipolytic activity. A naturally occurring genetic variant hLPL, S447-->Ter, has normal bridge function. The catalytic center of LPL is covered by a 21-amino acid loop that must be repositioned before a lipid substrate can gain access to the active site for catalysis. We studied three hLPL loop mutants (LPL-cH, an enzymatically active mutant with the loop replaced by a hepatic lipase loop; LPL-cP, an enzymatically inactive mutant with the loop replaced by a pancreatic lipase loop; and C216S/C239S, an enzymatically inactive mutant with the pair of Cys residues delimiting the loop substituted by Ser residues) and a control double Cys mutant, C418S/C438S. Two of the loop mutants (LPL-cH and LPL-cP) and the control double Cys mutant C418S/C438S gave normal enhancement of VLDL binding and catabolism, whereas the third loop mutant, C216S/C239S, was completely inactive. We conclude that although catalytic activity and the actual primary sequence of the loop of LPL are relatively unimportant (wild-type LPL loop and pancreatic lipase loops have little sequence similarity), the intact folding of the loop, flanked by disulfide bonds, must be maintained for LPL to express its bridge function.