Chylomicron Clearance Research Articles

CD36 deficiency impairs intestinal lipid secretion and clearance of chylomicrons from the blood

CD36 mediates the transfer of fatty acids (FAs) across the plasma membranes of muscle and adipose cells, thus playing an important role in regulating peripheral FA metabolism in vivo. In the proximal intestine, CD36 is localized in abundant quantities on the apical surface of epithelial cells, a pattern similar to that of other proteins implicated in the uptake of dietary FAs. To define the role of CD36 in the intestine, we examined FA utilization and lipoprotein secretion by WT and CD36-null mice in response to acute and chronic fat feeding. CD36-null mice given a fat bolus by gavage or fed a high-fat diet accumulated neutral lipid in the proximal intestine, which indicated abnormal lipid processing. Using a model in which mice were equipped with lymph fistulae, we obtained evidence of defective lipoprotein secretion by directly measuring lipid output. The secretion defect appeared to reflect an impaired ability of CD36-null enterocytes to efficiently synthesize triacylglycerols from dietary FAs in the endoplasmic reticulum. In the plasma of intact mice, the reduced intestinal lipid secretion was masked by slow clearance of intestine-derived lipoproteins. The impaired clearance occurred despite normal lipoprotein lipase activity and likely reflected feedback inhibition of the lipase by FAs due to their defective removal from the plasma. We conclude that CD36 is important for both secretion and clearance of intestinal lipoproteins. CD36 deficiency results in hypertriglyceridemia both in the postprandial and fasting states and in humans may constitute a risk factor for diet-induced type 2 diabetes and cardiovascular disease.

CD36 deficiency impairs intestinal lipid secretion and clearance of chylomicrons from the blood

CD36 mediates the transfer of fatty acids (FAs) across the plasma membranes of muscle and adipose cells, thus playing an important role in regulating peripheral FA metabolism in vivo. In the proximal intestine, CD36 is localized in abundant quantities on the apical surface of epithelial cells, a pattern similar to that of other proteins implicated in the uptake of dietary FAs. To define the role of CD36 in the intestine, we examined FA utilization and lipoprotein secretion by WT and CD36-null mice in response to acute and chronic fat feeding. CD36-null mice given a fat bolus by gavage or fed a high-fat diet accumulated neutral lipid in the proximal intestine, which indicated abnormal lipid processing. Using a model in which mice were equipped with lymph fistulae, we obtained evidence of defective lipoprotein secretion by directly measuring lipid output. The secretion defect appeared to reflect an impaired ability of CD36-null enterocytes to efficiently synthesize triacylglycerols from dietary FAs in the endoplasmic reticulum. In the plasma of intact mice, the reduced intestinal lipid secretion was masked by slow clearance of intestine-derived lipoproteins. The impaired clearance occurred despite normal lipoprotein lipase activity and likely reflected feedback inhibition of the lipase by FAs due to their defective removal from the plasma. We conclude that CD36 is important for both secretion and clearance of intestinal lipoproteins. CD36 deficiency results in hypertriglyceridemia both in the postprandial and fasting states and in humans may constitute a risk factor for diet-induced type 2 diabetes and cardiovascular disease.

W10-P-025 Delayed postprandial clearance of chylomicrons and decreased lipoprotein lipase (LPL) activity in persons with echolucent carotid plaques

W10-P-025 Delayed postprandial clearance of chylomicrons and decreased lipoprotein lipase (LPL) activity in persons with echolucent carotid plaques

Graded Effects of Proteinuria on HDL Structure in Nephrotic Rats

Nephrotic syndrome is characterized by increased triglycerides resulting from decreased clearance of VLDL and chylomicrons. These triglyceride-rich lipoproteins are structurally altered by interaction with HDL derived from animals with proteinuria and not as a consequence of hypoalbuminemia. HDL isolated from rats with massive proteinuria is depleted in apolipoprotein E (apoE). It is unknown at what threshold of urinary albumin loss HDL structure is altered, and it is unknown what effects proteinuria has on apolipoproteins other than apoE. Two models of albuminuria were used in Sprague-Dawley rats: Adriamycin and passive Heymann nephritis (HN). The adriamycin group was divided into minimal albumin excretion (MAE) and intermediate albumin excretion (MAE, 1 to 40; intermediate albumin excretion, 60 to 210 mg/d per 100 g body wt). Urinary albumin excretion exceeded 300 mg/d per 100 g body wt in the HN rats. HDL apolipoprotein composition was analyzed with SDS-PAGE densitometry and liquid chromatography-time of flight mass spectrometer mass spectrometry. HDL apoA-IV content relative to apoA-I was reduced at all levels of albuminuria (P < 0.0001). ApoE was not reduced in MAE but was significantly reduced in IAE (72%; P < 0.001). By contrast, apoA-II and apoC-III were each significantly increased with increasing UAE. ApoA-IV and apoE were decreased to approximately 10% of control in HDL isolated from rats with HN, whereas apoA-II, apoC-II, and apoC-III were each significantly increased relative to apoA-I. HDL is structurally altered by levels of albuminuria that are insufficient to change serum albumin levels and is progressively altered as albuminuria increases.

High cholesterol absorption efficiency and rapid biliary secretion of chylomicron remnant cholesterol enhance cholelithogenesis in gallstone-susceptible mice

The study of chylomicron pathway through which it exerts its metabolic effects on biliary cholesterol secretion is crucial for understanding how high dietary cholesterol influences cholelithogenesis. We explored a relationship between cholesterol absorption efficiency and gallstone prevalence in 15 strains of inbred male mice and the metabolic fate of chylomicron and chylomicron remnant cholesterol in gallstone-susceptible C57L and gallstone-resistant AKR mice. Our results show a positive and significant ( P<0.0001, r=0.87) correlation between percent cholesterol absorption and gallstone prevalence rates. Compared with AKR mice, C57L mice displayed significantly greater absorption of cholesterol from the small intestine, more rapid plasma clearance of chylomicrons and chylomicron remnants, higher activities of lipoprotein lipase and hepatic lipase, greater hepatic uptake of chylomicron remnants, and faster secretion of chylomicron remnant cholesterol from plasma into bile. All of these increased susceptibility to cholesterol gallstone formation in C57L mice. We conclude that genetic variations in cholesterol absorption efficiency are associated with cholesterol gallstone formation in inbred mice and cholesterol absorbed from the intestine provides an important source for biliary hypersecretion. Differential metabolism of the chylomicron remnant cholesterol between C57L and AKR mice clearly plays a crucial role in the formation of lithogenic bile and gallstones.

The VLDL receptor plays a major role in chylomicron metabolism by enhancing LPL-mediated triglyceride hydrolysis

The VLDL receptor (VLDLr) is involved in tissue delivery of VLDL-triglyceride (TG)-derived FFA by facilitating the expression of lipoprotein lipase (LPL). However, vldlr-/- mice do not show altered plasma lipoprotein levels, despite reduced LPL expression. Because LPL activity is crucial in postprandial lipid metabolism, we investigated whether the VLDLr plays a role in chylomicron clearance. Fed plasma TG levels of vldlr-/- mice were 2.5-fold increased compared with those of vldlr+/+ littermates (1.20 +/- 0.37 mM vs. 0.47 +/- 0.18 mM; P < 0.001). Strikingly, an intragastric fat load led to a 9-fold increased postprandial TG response in vldlr-/- compared with vldlr+/+ mice (226 +/- 188 mM/h vs. 25 +/- 11 mM/h; P < 0.05). Accordingly, the plasma clearance of [3H]TG-labeled protein-free chylomicron-mimicking emulsion particles was delayed in vldlr-/- compared with vldlr+/+ mice (half-life of 12.0 +/- 2.6 min vs. 5.5 +/- 0.9 min; P < 0.05), with a 60% decreased uptake of label into adipose tissue (P < 0.05). VLDLr deficiency did not affect the plasma half-life and adipose tissue uptake of albumin-complexed [14C]FFA, indicating that the VLDLr facilitates postprandial LPL-mediated TG hydrolysis rather than mediating FFA uptake. We conclude that the VLDLr plays a major role in the metabolism of postprandial lipoproteins by enhancing LPL-mediated TG hydrolysis.

E‐SAR‐94010 (LipoEsar®): A Pleiotropic Lipoprotein Compound with Powerful Anti‐atheromatous and Lipid Lowering Effects

E-SAR-94010 (LipoEsar) is a natural product extracted from the marine species S. pilchardus, by means of non-denaturing biotechnological procedures. The main chemical ingredient of LipoEsar is a lipoprotein (60–80%) whose micelle structure probably mimics that of physiological lipoproteins involved in lipid metabolism. In preclinical studies LipoEsar has shown to be effective in (a) reducing blood cholesterol (Cho), triglyceride (TG), uric acid (UA), and glucose (Glu) levels, as well as liver alanine aminotransferase (ALT), and aspartate aminotransferase (AST) activity; (b) enhancing immunological function by regulating both lymphocyte and microglia activity; (c) inducing antioxidant effects mediated by superoxide dismutase activity; and (d) improving cognitive function. Clinical studies have revealed that LipoEsar reduces blood total cholesterol (T-Cho) (20–30%), Glu (5–10%), UA (10–15%), TG (30–50%), ALT and AST, after 1–3 months of treatment at a daily dose of 250–500 mg (t.i.d.). The effect on T-Cho is the result of decreasing LDL-Cho levels and increasing HDL-Cho levels in parallel with an improvement in hepatic protection reflected by reduction in ALT, AST, and GGT activity. Most of these therapeutic effects on the regulation of lipid metabolism tend to show an age-dependent pattern and are also associated with specific genomic profiles in the population. In addition, LipoEsar diminishes the size of xanthelasma plaques by 30–60% after 6–9 months of treatment. Similar effects can be observed on atheromatous plaques on the aortic wall of patients with familial and sporadic hypercholesterolemia. LipoEsar is the first marine biotechnological product with lipoprotein structure displaying hypolipemic activity in blood and tissues acting as a potential neuroprotectant in cerebrovascular disorders and arteriosclerosis. Preliminary studies indicate that the biological activity of LipoEsar is genotype-dependent. Since genetic defects in the APOE gene result in familial dysbetalipoproteinemia or type III hyperlipoproteinemia (HLP-III) with increased plasma Cho and TG as a consequence of impaired clearance of chylomicron and VLDL remnants, in the present paper we have investigated the influence of different APOE genotypes on the therapeutic response of LipoEsar in patients with chronic dyslipemia. The study has been performed in 419 patients (age: 55.24 ± 17.71 years; range: 9–96 years) of both sexes (females: N = 212; age: 57.04 ± 17.68 years; range: 15–96 years; males: N = 207; age: 53.38 ± 17.58

Elevated Remnant-Like Particle Cholesterol Concentration

Patients at increased risk of coronary artery disease (CAD) frequently exhibit an atherogenic lipoprotein phenotype characterized by elevated plasma levels of both triglyceride-rich lipoproteins (TRL) and small, dense LDL and low concentrations of HDL cholesterol. Recently, in a large observational study, the calculated non-HDL plasma cholesterol concentration (the sum of the cholesterol contents of LDL, intermediate-density lipoprotein [IDL], and very-low-density lipoprotein [VLDL]) was a stronger predictor of cardiovascular events than plasma cholesterol alone.1–3 Improvement in the predictability of CAD on inclusion of VLDL and IDL cholesterol emphasizes the proatherogenic nature of TRL and their remnant particles. The atherogenic lipoprotein phenotype has been defined by Austin et al4 as the presence of a predominance of small, dense LDL particles, elevated plasma triglyceride (TG) levels, and low plasma HDL cholesterol levels in the lipoprotein profile, which is associated with an approximately 3-fold increased risk of atherosclerotic disease.5–8 It is now commonly accepted that small, dense LDL particles are the products of the intravascular remodeling of TG-rich VLDL particles after interaction primarily with lipoprotein lipase, hepatic lipase, and cholesterol ester transfer protein9,10 (Figure). The atherogenic lipoprotein phenotype is strongly linked to obesity, insulin resistance, familial combined hyperlipidemia (FCHL), hypertension, and abnormalities in postprandial lipid metabolism.7,11,12 Epidemiological data from the Framingham study have already revealed that plasma TG concentration is an important independent risk indicator of CAD in women13; additional evidence supporting this observation was obtained by Yarnell et al14 in a 10-year follow-up study and confirmed by others.15,16 In the PROCAM (Prospective Cardiovascular Munster) study,17 this relationship was dependent on plasma HDL cholesterol concentration. Criqui et al,18 however, could not demonstrate an independent relationship between plasma TG and cardiovascular mortality in a North American population participating in the Lipid Research Clinics Follow-up. …

Premenopausal advantages in postprandial lipid metabolism are lost in women with type 2 diabetes.

Women with type 2 diabetes appear to lose the protection against cardiovascular disease afforded by estrogens. We examined the effects of menopausal status on postprandial clearance of dietary fat in healthy and diabetic women. Fasting subjects (premenopausal and postmenopausal control subjects, premenopausal and postmenopausal diabetic women, all n = 8) were given a meal containing the stable isotope 1,1,1-(13)C-tripalmitin, with blood and breath sampled for 6 and 24 h, respectively, in the postprandial period. Lower levels of (13)C-palmitic acid ((13)C-PA) in the triglyceride fraction implies more efficient chylomicron clearance, lower levels of (13)C-PA in the nonesterified fatty acid (NEFA) fraction implies improved dietary NEFA entrapment, and higher levels of (13)CO(2) in the breath denote more efficient of oxidation of dietary-derived lipid. In diabetic women, there were no differences between the pre- and postmenopausal groups for any of these parameters. In contrast, premenopausal control subjects, compared with postmenopausal control subjects, had lower (13)C-PA in the triglyceride fraction area under the curve (AUC) (premenopausal median [range] 25.2 [12.1-49.4 mmol/l] per 6 h, postmenopausal 48.5 [15.5-77.2 mmol/l] per 6 h; P < 0.01) and higher (13)CO(2) levels in the breath AUC (premenopausal 22.5 [18.0-31.5%] of administered dose, postmenopausal 17.2 [11.2-31.5%] of administered dose; P < 0.01) with no difference between groups in levels of (13)C-PA in the NEFA fraction AUC. The premenopausal advantage in clearance of dietary lipid is not seen in premenopausal diabetic women. This is likely to promote an atherogenic lipoprotein profile and may contribute to the loss of cardiovascular disease protection seen in diabetic women.

The apolipoprotein AV gene and diurnal triglyceridaemia in normolipidaemic subjects.

The newly recognised apolipoprotein (apo) AV gene (APOAV) has been linked to fasting plasma triglyceride (TG) concentrations with some polymorphisms associated with elevated fasting TGs. Since fasting plasma TGs are mainly determined by the hepatic production of TG-rich particles (very low density lipoprotein; VLDL), and fasting TGs are the major determinants of postprandial lipaemia, we have evaluated the effects of an APOAV polymorphism on postprandial triglyceridaemia, which is largely determined by the intestinal production and clearance of chylomicrons. For this purpose, diurnal capillary triglyceridaemia (reflecting postprandial lipaemia) was determined in a cohort of 88 healthy volunteers (48 males and 40 females) in relation with a -1131T>C variant in the promoter of APOAV. Thirteen of these subjects (7 males and 6 females) were carriers of the -1131C allele, which has been associated with higher fasting plasma TG levels. The carriers had higher fasting capillary TG concentrations, although plasma TGs were not significantly different from non-carriers in this cohort. Surprisingly, total diurnal triglyceridaemia calculated as the area under the capillary TG curve was similar in carriers compared to non-carriers but after correction for fasting capillary TG levels, incremental diurnal triglyceridaemia was significantly lower in carriers (1.74 (5.27) mmol/h/l) than in non-carriers (4.91 (4.90) mmol/h/l; p = 0.036). The same trends were found for both males and females when analysed separately. Since dietary intake, which is a major determinant of incremental diurnal triglyceridaemia, did not differ between the two groups, we believe that these differences are at least partly explained by the APOAV. In summary, the APOAV assessed by means of the -1131T>C variant seemed to have a paradoxical effect on postprandial lipaemia when compared to fasting TG levels.

Atorvastatin enhances the plasma clearance of chylomicron-like emulsions in subjects with atherogenic dyslipidemia: relevance to the in vivo metabolism of triglyceride-rich lipoproteins

Delayed chylomicron clearance is a characteristic of patients with coronary artery disease. In vivo study of the clearance of labeled chylomicron-like emulsions constitutes a valid model system for evaluation of chylomicron catabolism. The effects of atorvastatin at low (10 mg) and high (40 mg) dose upon the intravascular metabolism and plasma kinetics of chylomicron-like emulsions were evaluated in fasting hyperlipidemic subjects ( n=45). Subjects were randomized to a 6-week treatment period with placebo ( n=15), low dose or high dose atorvastatin (10 mg/day, n=17 and 40 mg/day, n=13). The chylomicron-like emulsion, double-labeled with 14C-Cholesteryl oleate ( 14C-CE) and 3H-triolein ( 3H-TG), was injected in a bolus after a 12-h fast, and blood samples were collected up to 60 min. Plasma decay curves were determined for labeled emulsion CE and TG and residence times (RT) calculated by the occupancy principle. The 14C-CE RT was decreased by 50% after low dose atorvastatin and by 73% after atorvastatin at high dose in comparison to placebo ( P<0.05). The 3H-TG RT was significantly reduced (−55%) after high dose atorvastatin, but in contrast was not significantly reduced after placebo or low dose statin. By compartmental analysis, both doses of atorvastatin led to marked elevation in the slow removal component of emulsion remnant particles (10 mg/day=107%; 40 mg/day=195%, P=0.01). Equally, the rapid removal component was increased (+99%) at high dose ( P=0.015). Recirculation of 3H-fatty acids was significantly reduced at both statin doses (43 and 83%, respectively) in comparison to placebo ( P=0.01). In conclusion, atorvastatin treatment accelerates the plasma clearance of chylomicron-like emulsions and reduces recirculation of fatty acids in subjects with atherogenic hyperlipidemia. Such effect might implicate in reduction of cardiovascular risk.

Effect of dietary cholesterol oxidation products on the plasma clearance of chylomicrons in the rat.

Oxidized cholesterols in the diet have been shown to exacerbate arterial cholesterol deposition and the development of atherosclerosis in animal models. Dietary oxidized cholesterols are absorbed through the intestine and incorporated into lymph chylomicrons. The aim of this study was to investigate the effect of oxidized cholesterols on the metabolism of nascent chylomicrons in vivo. It was shown that oxidized cholesterols markedly delay the clearance of chylomicrons from plasma compared to rats given TG alone. However, there was no difference in the clearance of chylomicrons containing oxidized cholesterols vs. purified cholesterol, although the presence of oxysterols did appear to exacerbate the removal of these particles from circulation. The impaired clearance of chylomicrons containing oxidized cholesterols was not due to impaired lipolysis and slower conversion to the remnant form. Moreover, the incorporation of oxidized cholesterols did not alter the hepatic or splenic uptake of chylomicrons compared to chylomicrons isolated from rats given purified cholesterol or TG alone. Collectively, the results of this study suggest that the exacerbated delay in clearance of chylomicron remnants enriched with oxysterols may be due to impaired uptake by tissues other than the liver and spleen. Apolipoprotein (apo) analysis showed that oxysterol incorporation reduced the apoE content and altered the apoC phenotype of chylomicrons, which may have an impact on the removal of chylomicron remnants from plasma. In conclusion, dietary oxysterols appear to have the potential to adversely affect chylomicron metabolism. Therefore, further investigations in humans are required to determine whether dietary oxidized cholesterols found in cholesterol-rich processed foods delay the clearance of postprandial remnants, which may contribute to and exacerbate the development of atherosclerosis.

Postprandial chylomicron clearance rate in late teenagers with diabetes mellitus type 1.

A delayed chylomicron (CM) clearance rate, a known risk factor for atherosclerosis, has been described in adults with diabetes type 1 (DM1). We determined the CM clearance rate in late teenagers with DM1, and the relationship between CM clearance rate and elevated plasma lipid concentrations in DM1 teenagers in poor metabolic control (as characterized by HbA(1c) percentage). Plasma lipids and CM clearance were determined in nine patients with DM1 (mean age +/- SD: 17.5 +/- 0.6 y) and four healthy controls (mean age +/- SD: 20.1 +/- 0.8 y), by measuring breath (13)CO(2), plasma triglyceride, retinyl palmitate, and (13)C-labeled oleic acid concentrations, after oral administration of a fat-rich meal together with vitamin A and (13)C-oleic acid. In patients with DM1, fasting triglyceride and cholesterol concentrations were positively correlated with HbA(1c) percentage (p < 0.05). Neither in DM1 patients, nor in controls, was an elevated triglyceride concentration (above 1.7 mmol/L) found. Yet, in 22% of DM1 patients, cholesterol concentration was above 5.2 mmol/L, but not in any of the controls. CM clearance rate in DM1 patients was similar to that in controls and did not significantly correlate with HbA(1c) percentage. Fasting lipid concentrations in DM1 patients were not significantly correlated with CM clearance rate. Present data indicate that elevated lipid concentrations in late teenagers with DM1 are not attributable to a delay in CM clearance rate. A delayed CM clearance rate at late teenager age is not a risk factor contributing to the increased risk for atherosclerosis in DM1.

The impact of hypertriglyceridemia on endothelial function, vascular structure and fibrinolysis in hypercholesterolemic rabbits

The reduced clearance of chylomicrons from plasma results in an exaggerated post prandial lipaemia and fasting hypertriglyceridaemia. This study evaluated whether oral and intravenous fat tolerance tests are appropriate for the in vivo analysis of chylomicron clearance in dogs. Plasma and chylomicron triglyceride concentrations were measured in eight beagles after the administration of a cream-based meal of 2·35 g fat kg−l bodyweight. The changes in each parameter were determined chiefly by the activity of lipoprotein lipase, which was measured in plasma collected after the intravenous injection of heparin and did not appear to be influenced by intestinal fat absorption. The inclusion of retinyl palmitate in the meal provided additional information on the metabolic fate of chylomicron remnants. After the intravenous injection of 0·1 g Intralipid kg−1 bodyweight, there was an initial linear decay in plasma triglyceride concentrations that represented the maximal elimination rate K1. This was followed by a second exponential component so that the plasma triglyceride concentration returned to baseline by 60 minutes. Lipoprotein lipase was the major determinant of K1 and the area under the curve of plasma triglycerides.

Hypoalbuminemia and proteinuria contribute separately to reduced lipoprotein catabolism in the nephrotic syndrome

Hypertriglyceridemia is a result of reduced triglyceride (TG)-rich lipoprotein (TRL) catabolism and occurs in rats with nephrotic syndrome (NS) and in Nagase analbuminemic rats (NARs). While the heparin-releasable lipoprotein lipase (LpL) pool in NAR and in NS is similar, TG levels are significantly greater in NS, suggesting that factors other than reduced LpL alone act in NS but not in NARs. Furthermore, clearance of chylomicrons (CM) and very low-density lipoprotein (VLDL) is normal in vivo in NAR despite low LpL levels. We tested the hypotheses that impaired binding of VLDL and impaired VLDL-high density lipoprotein (HDL) interactions contribute to hyperlipidemia in NS. TG and apoB secretion was measured using Triton WR 1339. Clearance of CMs by perfused hearts from NS and NAR was determined. Binding of VLDL from control, NS and NAR to rat aortic endothelial cells (RAECs) was measured prior to and following incubation with HDL from NS, NARs, and control. ApoE, protein, and TG content was determined. TG levels were greatest in NS (516 +/- 95 mg/dL), intermediate in NAR (193 +/- 20), and least in control (97 +/- 16, P = 0.05), while in contrast, TG secretion was least in NS (178 +/- 33 mg/dL/hour) versus 212 +/- 17 in NAR and 294 +/- 15 in control (P < 0.001 vs. NS). Clearance of CMs by NS and NAR hearts was the same and significantly reduced versus control (P < 0.005). Binding of NS-VLDL to endothelial cells was reduced, while NAR-VLDL binding was increased versus control (P < 0.001). Incubation of NS-VLDL with control or NAR HDL increased VLDL binding compared with binding following incubation with NS HDL (P < 0.001). Increased TG levels in both NS and NAR are the result of decreased TRL clearance. TG levels are greater in NS because of the presence of a combined defect: (1) a decrease in endothelial-bound LpL that occurs as a consequence of reduced serum albumin concentration, and (2) a defect in VLDL binding to endothelial-bound LpL. This latter defect occurs only in the presence of proteinuria and is conferred by HDL.

Mechanism of HDL lowering in insulin resistant states.

Hypertriglyceridemia and low plasma HDL-cholesterol (HDL-c) are the lipid abnormalities that typically occur as a consequence of insulin resistant conditions, including abdominal obesity, impaired fasting glucose and Type 2 diabetes. HDL-c and apolipoprotein A-1 (apo A-1, the major protein moiety of HDL particles) are inversely associated with plasma triglycerides (TG). In fact, the majority of individuals who have HDL-c levels below the 10th percentile for age and gender have some degree of fasting and/or postprandial hypertriglyceridemia, insulin resistance and abdominal obesity. Hypertriglyceridemia is due predominantly to elevated very low density lipoprotein (VLDL) production by the liver in the fasting state and delayed clearance of intestinally-derived chylomicrons and hepatically-derived VLDL in the postprandial state. In hypertriglyceridemic states there is active lipid exchange between TG-rich lipoproteins (VLDL and chylomicrons) and HDL. We hypothesise that TG enrichment of HDL particles, followed by lipolytic modification by HL, enhances the clearance of HDL particles from the circulation, ultimately lowering plasma HDL-c concentrations. Since we have recently reviewed in detail the various lines of evidence in support of this hypothesis,1 the present brief review will summarize only our own recent work in this area.KeywordsCholesteryl Ester Transfer ProteinHepatic LipasePostprandial LipemiaFractional Catabolic RateIntralipid InfusionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Metabolic disorders among HIV-infected patients treated with protease inhibitors: a review.

Currently available protease inhibitors are associated with development of a group of metabolic disorders. These include a peripheral lipodystrophy syndrome in which there is fat wasting in the face, arms, and legs; fat accumulation in the abdomen, dorsocervical region, and/or breasts (women only); as well as hyperlipidemia, hypercholesterolemia, and insulin resistance. A review of 15 observational studies and case reports shows that the incidence of the peripheral lipodystrophy syndrome increases with time of exposure to protease inhibitors, with a >60% incidence seen after 1 year of continuous treatment. Protease inhibitors are hypothesized to cause this syndrome by impairing conversion of retinoic acid to cis-9-retinoic acid (leading to impaired peripheral fat storage, sequestration of body fat to central adipocytes, and hyperlipidemia) and by inhibiting low-density lipoprotein receptor-related protein (LRP), thus preventing postprandial chylomicron clearance and further contributing to hyperlipidemia. Recent in vitro data suggest that more than one pathway contributes to the lipodystrophy syndrome and that pathways may differ among protease inhibitors. Although the central fat accumulation, hyperlipidemia, and insulin resistance components of this syndrome may reverse after discontinuation of protease inhibitor therapy, it is not known whether complete normalization of fast-wasted body regions is possible. Prospective controlled studies are needed to define whether protease inhibitors currently under development are less prone to produce the lipodystrophy syndrome.

Effect of Pravastatin on plasma removal of a chylomicron-like emulsion in men with coronary artery disease

The speed of the plasma removal of chylomicrons, the lipoproteins that carry dietary lipids absorbed in the intestine, may influence atherogenesis. Thus, the effects of a 30-day pravastatin or placebo treatment on the plasma kinetics of chylomicron-like emulsions were evaluated in 25 patients with coronary artery disease who were not hypertriglyceridemic in a randomized, single-blinded study. Eleven patients (53 ± 4 years, 10 men) received pravastatin 40 mg/day and 14 received placebo (52 ± 3 years, 13 men). Emulsions labeled with triolein ( 3H-TO) and cholesteryl oleate ( 14C-CO) to assess lipolysis and clearance of chylomicron and remnants, respectively, were injected intravenously in a bolus after a 12-hour fast. Blood samples were collected during 60 minutes to determine radio isotope decaying curves and fractional catabolic rates. Subjects were studied at baseline and after the treatment period. Compared with placebo (data expressed as mean ± SEM), pravastatin treatment increased the 14C-CO fractional catabolic rates (70 ± 45% vs 18 ± 10%, p = 0.01), reduced total cholesterol (−21 ± 3% vs −3 ± 2% p = 0.0001), low-density lipoprotein (LDL) cholesterol (−25 ± 5% vs 4 ± 6%, p = 0.0001), and apolipoprotein B levels (−22 ± 3% vs −7 ± 3% p = 0.01). 3H-TO fractional catabolic rates, plasma triglycerides, very-low-density lipoprotein (VLDL) cholesterol and high-density lipoprotein (HDL) cholesterol variations did not differ between the groups. The fractional catabolic rate of 14C-CO was inversely correlated with plasma apolipoprotein B levels (r = −0.7, p = 0.04). This suggests that besides reducing LDL cholesterol, pravastatin also increases chylomicron remnant clearance, with possible antiatherogenic implications.

Decreased Hepatic Expression of the Low-Density Lipoprotein (LDL) Receptor and LDL Receptor-Related Protein in Aging Rats Is Associated With Delayed Clearance of Chylomicrons From the Circulation

Aging in both humans and rats is associated with the development of insulin resistance and the ensuing alterations in the plasma lipoprotein profile. In this study, young (2 months) and old (15 months) Sprague-Dawley (SD) rats were used to investigate age-related alterations in the chylomicron clearance pathway. Clearance from the blood of an intravenously injected bolus of 14C-labeled cholesterol ester (CE) and 3H-labeled triacylglycerol (TAG) lymph chylomicrons was markedly delayed in the old rats (P < .05). Hepatic expression of the two principal receptors of chylomicron remnant removal, the low-density lipoprotein (LDL) receptor and LDL receptor-related protein (LRP), was determined by ligand blotting and immunoblotting. The old rats expressed 43%+/-7% of the level of LDL receptor in the young animals (P < .05) and 45%+/-16% of the corresponding level of LRP (P < .05). The results suggest that the delayed clearance of chylomicron remnants in this animal model of aging and insulin resistance is due, at least in part, to a decrease in the hepatic expression of LDL receptor and LRP.

Apolipoprotein E polymorphism: automated determination of apolipoprotein E2, E3, and E4 isoforms.

Apolipoprotein E (apo E) plays an essential role in lipoprotein metabolism, where it is involved in the clearance of chylomicrons and very low density lipoproteins. Apart from some rare variants, apo E exists in three common isoforms (E2, E3, and E4). The different isoforms have not only been associated with different plasma lipid levels but have also been correlated with certain pathological conditions, such as lipid disorders (dysbetalipoproteinemia, hypercholesterolemia), cardiovascular diseases, and Alzheimer's disease. Here we describe a rapid, automated test for the determination of the most frequent polymorphisms (E2, E3, and E4). This polymerase chain reaction-based test allows the reliable discrimination of all six genotypes. The assay has been developed especially for the nonspecialized routine clinical laboratory by employing an analyzer and chemistry often present in this type of laboratory. Because of its low costs and easy handling, the assay can be performed on a daily basis.