Lipoprotein lipase modulates net secretory output of apolipoprotein B in vitro. A possible pathophysiologic explanation for familial combined hyperlipidemia

How do elevated triglycerides and low HDL-cholesterol affect inflammation and atherothrombosis?

This review article summarizes recent research into the mechanisms as to how elevated levels of triglyceride (TG) and low levels of high- density- lipoprotein cholesterol (HDL-C) contribute to inflammation and atherosclerosis. Evidence supports the role of TG-rich lipoproteins in signaling mechanisms via apolipoproteins C-III and free fatty acids leading to activation of NFKβ, VCAM-1 and other inflammatory mediators which lead to fatty streak formation and advanced atherosclerosis. Moreover, the cholesterol content in TG-rich lipoproteins has been shown to predict CAD risk better than LDL-C. In addition to reverse cholesterol transport, HDL has many other cardioprotective effects which include regulating immune function. The "functionality" of HDL appears more important than the level of HDL-C. Insulin resistance and central obesity underlie the pathophysiology of elevated TG and low HDL-C in metabolic syndrome and type 2 diabetes. Lifestyle recommendations including exercise and weight loss remain first line therapy in ameliorating insulin resistance and the adverse signaling processes from elevated levels of TG-rich lipoproteins and low HDL-C.

Presecretory oxidation, aggregation, and autophagic destruction of apoprotein-B: a pathway for late-stage quality control

Hepatic secretion of apolipoprotein-B (apoB), the major protein of atherogenic lipoproteins, is regulated through posttranslational degradation. We reported a degradation pathway, post-ER pre secretory proteolysis (PERPP), that is increased by reactive oxygen species (ROS) generated within hepatocytes from dietary polyunsaturated fatty acids (PUFA). We now report the molecular processes by which PUFA-derived ROS regulate PERPP of apoB. ApoB exits the ER; undergoes limited oxidant-dependent aggregation; and then, upon exit from the Golgi, becomes extensively oxidized and converted into large aggregates. The aggregates slowly degrade by an autophagic process. None of the oxidized, aggregated material leaves cells, thereby preventing export of apoB-lipoproteins containing potentially toxic lipid peroxides. In summary, apoB secretory control via PERPP/autophagosomes is likely a key component of normal and pathologic regulation of plasma apoB levels, as well as a means for remarkably late-stage quality control of a secreted protein.

Practical guidelines for familial combined hyperlipidemia diagnosis: an up-date

Familial combined hyperlidemia (FCH) is a common metabolic disorder characterized by: (a) increase in cholesterolemia and/or triglyceridemia in at least two members of the same family, (b) intra-individual and intrafamilial variability of the lipid phenotype, and (c) increased risk of premature coronary heart disease (CHD). FCH is very frequent and is one of the most common genetic hyperlipidemias in the general population (prevalence estimated: 0.5%-2.0%), being the most frequent in patients affected by CHD (10%) and among acute myocardial infarction survivors aged less than 60 (11.3%). This percentage increases to 40% when all the myocardial infarction survivors are considered without age limits. However, because of the peculiar variability of laboratory parameters, and because of the frequent overlapping with the features of metabolic syndrome, this serious disease is often not recognized and treated. The aim of this review is to define the main characteristics of the disease in order to simplify its detection and early treatment by all physicians by mean of practical guidelines.

Oxidation, lipoproteins, and atherosclerosis: which is wrong, the antioxidants or the theory?

PURPOSE OF REVIEW

Paradoxically, many well-established components of the heart-healthy lifestyle are pro-oxidant, including polyunsaturated fat and moderate alcohol consumption. Moreover, antioxidant supplements have failed to decrease cardiovascular risk in extensive human clinical trials to date. Recent progress in understanding the roles of oxidants in regulating VLDL secretion and as essential signaling molecules supports the concept that oxidation may be beneficial in certain circumstances but damaging in others. We summarize recent data on the roles played by oxidative metabolism in different tissues and pathways, and address whether it is currently advisable to use antioxidant supplements to reduce cardiovascular risk.

RECENT FINDINGS

Our recent study reported that in liver cells, polyunsaturated fatty acids increased reactive oxygen species, which in turn lowered the secretion of the atherogenic lipoprotein, VLDL, in vitro and in vivo. Antioxidant treatments prevented VLDL-lowering effects of polyunsaturated fatty acids in vitro, suggesting that supplemental antioxidants could either raise apolipoprotein-B-lipoprotein plasma levels in vivo, or impair the response to lipid-lowering therapies. The failure of antioxidants to decrease cardiovascular disease risk in many trials is also discussed in the context of current models for atherosclerosis progression and regression.

SUMMARY

Oxidation includes distinct biochemical reactions, and it is overly simplistic to lump them into a unitary process that affects all cell types and metabolic pathways adversely. Guidelines for diet should adhere closely to what has been clinically proved, and by this standard there is no basis to recommend antioxidant use, beyond what is inherent to the 'heart healthy' diet in order to benefit cardiovascular health.

Phospholipid transfer protein deficiency impairs apolipoprotein-B secretion from hepatocytes by stimulating a proteolytic pathway through a relative deficiency of vitamin E and an increase in intracellular oxidants

Genetic deficiency of the plasma phospholipid transfer protein (PLTP) in mice unexpectedly causes a substantial impairment in liver secretion of apolipoprotein-B (apoB), the major protein of atherogenic lipoproteins. To explore the mechanism, we examined the three known pathways for hepatic apoB secretory control, namely endoplasmic reticulum (ER)/proteasome-associated degradation (ERAD), post-ER pre-secretory proteolysis (PERPP), and receptor-mediated degradation, also known as re-uptake. First, we found that ERAD and cell surface re-uptake were not active in PLTP-null hepatocytes. Moreover, ER-to-Golgi blockade by brefeldin A, which enhances ERAD, equalized total apoB recovery from PLTP-null and wild-type cells, indicating that the relevant process occurs post-ER. Second, because PERPP can be stimulated by intracellular reactive oxygen species (ROS), we examined hepatic redox status. Although we found previously that PLTP-null mice exhibit elevated plasma concentrations of vitamin E, a lipid anti-oxidant, we now discovered that their livers contain significantly less vitamin E and significantly more lipid peroxides than do livers of wild-type mice. Third, to establish a causal connection, the addition of vitamin E or treatment with an inhibitor of intracellular iron-dependent peroxidation, desferrioxamine, abolished the elevation in cellular ROS as well as the defect in apoB secretion from PLTP-null hepatocytes. Overall, we conclude that PLTP deficiency decreases liver vitamin E content, increases hepatic oxidant tone, and substantially enhances ROS-dependent destruction of newly synthesized apoB via a post-ER process. These findings are likely to be broadly relevant to hepatic apoB secretory control in vivo.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Coronary artery disease risk in familial combined hyperlipidemia and familial hypertriglyceridemia: a case-control comparison from the National Heart, Lung, and Blood Institute Family Heart Study

BACKGROUND

Conventional wisdom suggests that a diagnosis of familial combined hyperlipidemia (FCHL) carries a substantially greater risk of premature coronary artery disease (CAD) than a diagnosis of familial hypertriglyceridemia (FHTG). However, no population-based studies have critically addressed this issue.

METHODS AND RESULTS

FCHL and FHTG were diagnosed in 10.2% and 12.3% of 334 random control families and in 16.7% and 20.5% of 293 families with at least one case of premature CAD. The diagnosis of either FCHL or FHTG in an individual was associated with an odds ratio for CAD of 2.0 (P=0.003 and 0.002, respectively). However, odds ratios for premature CAD associated with both lipid disorders decreased substantially and identically with further adjustment for hypertension, diabetes, and especially HDL cholesterol, triglycerides, or apolipoprotein B. Similar results were found for differences in carotid intima-medial thickness and ankle-brachial index. Metabolic syndrome was identified in 65% of FCHL and 71% of FHTG patients compared with 19% in controls without FCHL or FHTG and was associated with an odds ratio of 3.3 (P<0.0001). The increased prevalence of the metabolic syndrome alone could account for the elevated CAD risk associated with both FCHL and FHTG.

CONCLUSIONS

FCHL and FHTG appear more alike than dissimilar. Further, the risk of CAD in FCHL and FHTG was strongly related to features of the metabolic syndrome. These findings suggest that the hypertriglyceridemia in FHTG is not benign and may warrant a change in epidemiological, genetic, and clinical approaches to these lipid disorders.

An observational study of severe hypertriglyceridemia, hypertriglyceridemic acute pancreatitis, and failure of triglyceride-lowering therapy when estrogens are given to women with and without familial hypertriglyceridemia

BACKGROUND

We assessed severe hypertriglyceridemia, hypertriglyceridemic acute pancreatitis, and failure of triglyceride-lowering therapy when estrogens were given to 56 women with and without familial hypertriglyceridemia. The 56 women had been consecutively referred to our center over a 3-year period because of triglycerides >400 mg/dl despite diet-drug treatment and/or a history of hypertriglyceridemic acute pancreatitis (AP). Of the 56 women, 17 had received estrogen replacement therapy (ERT), hormone replacement (HRT, n=6), or selective estrogen receptor modulators (SERM, n=1).

METHODS

After study at entry, in 56 women (median age, 52 years), 36 with familial hypertriglyceridemia, to lower triglycerides, estrogens and SERMs (hormone treatment, HT) were stopped; a very low fat diet (<15% of calories), gemfibrozil (1.2-1.5 mg/day), and omega-3-fatty acid (4-12 g/day) were started, with restudy 2-4 weeks later.

RESULTS

Of the 56 women, 24 (43%) were taking HT at entry, with median fasting triglycerides 1270 mg/dl in the HT group and 1087 mg/dl in the no-HT group. Seventeen women (30%) had a history of AP, nine of whom (53%) were/had been on HT at the development of AP. Significant positive correlates of triglycerides at entry in a stepwise regression model were hemoglobin A(1C) (partial r(2)=10.7%, p<0.05) and an interaction between estrogen use and familial hypertriglyceridemia (partial r(2)=15%, p=0.017). After 2-4 weeks on therapy, median triglycerides in the previous-HT group fell from 1270 to 284 mg/dl (p<0.0001) and in the no-HT group from 1087 to 326 mg/dl (p<0.0001).

CONCLUSIONS

Before starting HT, to avoid HT induced hypertriglyceridemic AP and exacerbation of overt or covert familial hypertriglyceridemia, triglycerides must be measured. HT is contraindicated in women with preexisting hypertriglyceridemia (triglycerides> or =500 mg/dl). Triglyceride-lowering diets and drugs often fail in the presence of HT and/or poorly controlled diabetes mellitus, but commonly succeed when HT is stopped and diabetes mellitus is tightly controlled.

Myristic acid increases dense lipoprotein secretion by inhibiting apoB degradation and triglyceride recruitment

Fatty acids of varying lengths and saturation differentially affect plasma apolipoprotein B-100 (apoB-100) levels. To identify mechanisms at the level of production, rat hepatoma cells, McA-RH7777, were incubated with [(35)S]methionine and either fatty acid-BSA complexes or BSA alone. There were increases in labeled apoB-100 secretion with saturated fatty acids palmitic and myristic (MA) (153 +/- 20% and 165 +/- 11%, respectively, relative to BSA). Incubation with polyunsaturated docosahexaenoic acid (DHA) decreased secretion to 26 +/- 2.0%, while monounsaturated oleic acid (OA) did not change it. In pulse-chase studies, MA treatment resulted in reduced apoB-100 degradation, in agreement with its promotion of secretion. In triglyceride (TG) studies, synthesis was stimulated equally by OA, MA, and DHA, but TG secretion was relatively decreased with MA and DHA. With OA, the majority of newly secreted apoB100-lipoproteins was d < or = 1.006, but with MA, they were much denser (1.063 < d). Furthermore, the relative recruitment of newly synthesized TG to lipoproteins was impaired with MA. We conclude that mechanisms for effects of specific dietary fatty acids on plasma lipoprotein levels may include changes in hepatic production. In turn, hepatic production may be regulated by specific fatty acids at the steps of apoB-100 degradation and the recruitment of nascent TG to lipoprotein particles.

Indinavir inhibits sterol-regulatory element-binding protein-1c-dependent lipoprotein lipase and fatty acid synthase gene activations

BACKGROUND

A syndrome characterized by hypertriglyceridaemia, hypercholesterolaemia, hyperinsulinaemia, and lipodystrophy has been found to be associated with highly active antiretroviral treatment (HAART) including protease inhibitors. A marker predicting this syndrome has been previously identified in the gene encoding the sterol-regulatory element-binding protein (SREBP)-1c, a regulator of triglycerides, cholesterol, insulin, and adipocytes.

OBJECTIVE

A possible inhibition of SREBP-1c-dependent genes by the protease inhibitor indinavir and its possible reversal by the lipid-lowering drug simvastatin were studied.

METHODS

The effects of indinavir and simvastatin on the inhibition/activation of SREBP-1c-dependent genes were compared with the effects of indinavir and simvastatin on the inhibition/activation of SREBP-1c-independent genes.

RESULTS

Indinavir inhibited the SREBP-1c-dependent genes encoding the lipoprotein lipase (103 nmol/l resulted in an inhibition of 12.4%; P = 0.0051) and the fatty acid synthase (103 nmol/l resulted in an inhibition of 30.3%; P = 0.036) in a dose-dependent fashion but not the SREBP-1c-independent gene encoding the low-density lipoprotein receptor. Simvastatin antagonized the indinavir-induced SREBP-1c-inhibition.

CONCLUSIONS

Indinavir inhibits important effector genes of the SREBP-1c pathway, explaining major HAART-related adverse effects.

Effect of IGF-I therapy on VLDL apolipoprotein B100 metabolism in type 1 diabetes mellitus

Abnormal lipid metabolism may be related to the increased cardiovascular risk in type 1 diabetes. Secretion and clearance rates of very low density lipoprotein (VLDL) apolipoprotein B100 (apoB) determine plasma lipid concentrations. Type 1 diabetes is characterized by increased growth hormone (GH) secretion and decreased insulin-like growth factor (IGF) I concentrations. High-dose IGF-I therapy improves the lipid profile in type 1 diabetes. This study examined the effect of low-dose (40 microg.kg(-1).day(-1)) IGF-I therapy on VLDL apoB metabolism, VLDL composition, and the GH-IGF-I axis during euglycemia in type 1 diabetes. Using a stable isotope technique, VLDL apoB kinetics were estimated before and after 1 wk of IGF-I therapy in 12 patients with type 1 diabetes in a double-blind, placebo-controlled trial. Fasting plasma triglyceride (P < 0.03), VLDL-triglyceride concentrations (P < 0.05), and the VLDL-triglyceride-to-VLDL apoB ratio (P < 0.002) significantly decreased after IGF-I therapy, whereas VLDL apoB kinetics were not significantly affected by IGF-I therapy. IGF-I therapy resulted in a significant increase in IGF-I and a significant reduction in GH concentrations. The mean overnight insulin concentrations during euglycemia decreased by 25% after IGF-I therapy. These results indicate that low-dose IGF-I therapy restores the GH-IGF-I axis in type 1 diabetes. IGF-I therapy changes fasting triglyceride concentrations and VLDL composition probably because of an increase in insulin sensitivity.

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

Perlecan heparan sulfate proteoglycan: a novel receptor that mediates a distinct pathway for ligand catabolism

Cell surface heparan sulfate proteoglycans (HSPGs) participate in the catabolism of many physiologically important ligands. We previously reported that syndecan HSPGs directly mediate endocytosis, independent of coated pits. We now studied perlecan, a major cell surface HSPG genetically distinct from syndecans. Cells expressing perlecan but no other proteoglycans bound, internalized, and degraded atherogenic lipoproteins enriched in lipoprotein lipase. Binding was blocked by heparitinase, and degradation by chloroquine. Antibodies against beta(1) integrins reduced initial ligand binding, consistent with their roles as cell surface attachment sites for perlecan. By several criteria, catabolism via perlecan was distinct from either coated pits or the syndecan pathway. The kinetics of internalization (t(12) = 6 h) and degradation (t(12) approximately 18 h) were remarkably slow, unlike the other pathways. Blockade of the low density lipoprotein receptor-related protein did not slow perlecan-dependent internalization. Internalization via perlecan was inhibited by genistein but unaffected by cytochalasin D, a pattern distinct from coated pits or syndecan-mediated endocytosis. Finally, we examined cooperation between perlecan and low density lipoprotein receptors and found limited synergy. Our results demonstrate that perlecan mediates internalization and lysosomal delivery that is kinetically and biochemically distinct from other known uptake pathways and is consistent with a very slow component of HSPG-dependent ligand processing found in vitro and in vivo.

Hypolipidemic effect of NK-104, a potent HMG-CoA reductase inhibitor, in guinea pigs

The hypolipidemic effect of NK-104 and its mechanisms of action (effects on hepatic sterol synthesis, low density lipoprotein (LDL)-receptor expression and very low density lipoprotein (VLDL) secretion) were studied in guinea pigs using simvastatin as a reference substance. There was a dose-dependent and significant reduction of both plasma total cholesterol (17.4, 24.5 and 45.3% at 0.3, 1 and 3 mg/kg, respectively) and triglycerides (21.1 and 32.2% at 1 and 3 mg/kg, respectively) after 14-day administration of NK-104. Simvastatin at 30 mg/kg lowered plasma total cholesterol (25.0%) but not triglyceride levels. NK-104 (3 mg/kg) and simvastatin (30 mg/kg) inhibited hepatic sterol synthesis by approximately 80%, 3 h after dosing, and enhanced LDL receptor binding-capacity of liver membranes 1.5-fold after 14-day dosing. The former group accelerated LDL clearance somewhat more markedly than the latter, and increased fractional catabolic rate 1.8-fold (vs. 1.4-fold). Furthermore, only the NK-104 (3 mg/kg) suppressed VLDL secretion into the liver perfusate (triglyceride. 19.9%; apoB, 24.2%) with extensive reduction of hepatic sterol synthesis caused by prolonged action. These results indicate that NK-104 and simvastatin at 10 times the dosage of the former, similarly enhances hepatic LDL receptor; however, only NK-104 with prolonged action suppresses VLDL secretion to show higher cholesterol-lowering potency and triglyceride-reducing effect.

The syndecan family of proteoglycans. Novel receptors mediating internalization of atherogenic lipoproteins in vitro

Cell-surface heparan sulfate proteoglycans have been shown to participate in lipoprotein catabolism, but the roles of specific proteoglycan classes have not been examined previously. Here, we studied the involvement of the syndecan proteoglycan family. First, transfection of CHO cells with expression vectors for several syndecan core proteins produced parallel increases in the cell association and degradation of lipoproteins enriched in lipoprotein lipase, a heparan-binding protein. Second, a chimeric construct, FcR-Synd1, that consists of the ectodomain of the IgG Fc receptor Ia linked to the highly conserved transmembrane and cytoplasmic domains of syndecan-1 directly mediated efficient internalization, in a process triggered by ligand clustering. Third, internalization of lipase-enriched lipoproteins via syndecan-1 and of clustered IgGs via the chimera showed identical kinetics (t1/2 = 1 h) and identical dose-response sensitivities to cytochalasin B, which disrupts microfilaments, and to genistein, which inhibits tyrosine kinases. In contrast, internalization of the receptor-associated protein, which proceeds via coated pits, showed a t1/2 < 15 min, limited sensitivity to cytochalasin B, and complete insensitivity to genistein. Thus, syndecan proteoglycans can directly mediate ligand catabolism through a pathway with characteristics distinct from coated pits, and might act as receptors for atherogenic lipoproteins and other ligands in vivo.

Lipoprotein lipase-enhanced binding of lipoprotein(a) [Lp(a)] to heparan sulfate is improved by apolipoprotein E (apoE) saturation: secretion-capture process of apoE is a possible route for the catabolism of Lp(a)

Recently, it has been recognized that cell-bound heparan sulfate (HS) proteoglycans (HSPG) are able to bind and subsequently initiate degradation of lipoproteins. Two mediators of lipoprotein catabolism, both with HS binding capacity, lipoprotein lipase (LPL) and apolipoprotein E (apoE), are involved in this process. This mechanism is known as the secretion-capture process of apoE. Lipoprotein(a) [Lp(a)] was shown to have a strong binding capacity to cell-associated HSPG. This binding capacity was increased by LPL addition. We investigated the effects of recombinant apoE (r-apoE) enrichment of Lp(a) on the binding to HS. Lp(a), isolated by ultracentrifugation and gel filtration, was incubated with r-apoE and reisolated by ultracentrifugation, resulting in r-apoE-enriched Lp(a). ApoE-enriched Lp(a) and control Lp(a) were coated to microtiter plates. The capacity to bind biotin-conjugated HS (b-HS) in the presence or absence of inactivated bovine LPL was studied. R-apoE-enriched Lp(a) showed increased b-HS binding capacity versus control Lp(a). Addition of LPL resulted in an increased b-HS binding capacity of both control and r-apoE-enriched Lp(a). To investigate whether binding of Lp(a) to endothelial cell HSPG occurred in vivo, 39 volunteers were injected with heparin (50 U/kg) and plasma lipid and Lp(a) levels were determined before and 20 minutes after heparin injection. No significant increase in plasma Lp(a) concentrations was found. The results showed that Lp(a) can be enriched with apoE and that this resulted in increased LPL-enhanced binding to HSPG. From the in vitro studies, it can be concluded that the secretion-capture process of apoE is a possible catabolic route for Lp(a). However, whether this also occurs in vivo remains to be confirmed.

Differences in the concentration and composition of low-density lipoprotein subfraction particles between sedentary and trained hypercholesterolemic men

There is evidence that a low-density lipoprotein (LDL) subfraction profile of increased concentrations of small, dense LDL particles is less common among trained than among sedentary normocholesterolemic men, but it is still uncertain whether there is a similar association in hypercholesterolemia also. Therefore, we determined the lipid and apolipoprotein concentration and composition of six LDL subfractions (density gradient ultracentrifugation) in 20 physically fit, regularly exercising (>three times per week) hypercholesterolemic men and 20 sedentary hypercholesterolemic controls. Trained (maximal oxygen consumption [VO2max], 57.3 +/- 7.4 mL/kg/min) and sedentary (VO2max, 37.5 +/- 8.8 mL/kg/min) individuals (aged 35 +/- 11 years; body mass index [BMI], 23.9 +/- 2.7 kg/m2) were matched for LDL apolipoprotein (apo) B levels (108 +/- 23 and 112 +/- 36 mg/dL, respectively). Trained subjects had significantly lower serum triglyceride (P < .05) and very-low-density lipoprotein (VLDL) cholesterol levels (P < .05) and higher high-density lipoprotein 2 (HDL2) cholesterol levels (P < .01) than sedentary controls. LDL particle distribution showed that trained individuals had significantly less small, dense LDL (d = 1.040 to 1.063 g/mL) and more large LDL (d = 1.019 to 1.037 g/mL) subfraction particles than sedentary controls, despite equal total LDL particle number. Analysis of LDL composition showed that LDL particles of hypercholesterolemic trained men had a higher free cholesterol content than LDL of untrained hypercholesterolemic men. Small, dense LDL in hypercholesterolemic trained men were richer in phospholipids than those in sedentary controls. These data demonstrate the significant influence of aerobic fitness on lipoprotein subfraction concentration and composition, thereby emphasizing the role of exercise in the treatment and risk reduction of hypercholesterolemia.

The Asp9 Asn mutation in the lipoprotein lipase gene is associated with increased progression of coronary atherosclerosis. REGRESS Study Group, Interuniversity Cardiology Institute, Utrecht, The Netherlands. Regression Growth Evaluation Statin Study

BACKGROUND

Many patients suffering from premature coronary artery disease report a family history for such events. A mutation in a particular gene, which confers susceptibility for atherosclerosis, will be found more frequently in individuals suffering from coronary atherosclerosis than in the general population. We have recently reported the identification of an Asp9 Asn substitution in the lipoprotein lipase (LPL) enzyme. We analyzed the impact of this mutation on the progression of coronary atherosclerosis and the effect of pravastatin in both carriers and noncarriers.

METHODS AND RESULTS

All patients were enrolled in the quantitative coronary angiographic clinical trial REGRESS, which studied the impact of pravastatin therapy on coronary atherosclerosis. The Asp9 Asn mutation was identified in 38 of 819 (4.8%) patients. Carriers of the mutation more often had a positive family history of cardiovascular disease and lower HDL cholesterol levels than noncarriers. In the placebo group, carriers showed more progression of coronary atherosclerosis than noncarriers: mean reduction of the minimum obstruction diameter of -0.25 mm versus -0.12 mm (P = .029) and increase of percentage diameter stenosis of 6.4% versus 1.4% (P = .004). Moreover, the adjusted relative risk for a clinical event for carriers was calculated at 2.16 (95% CI, 1.09 to 4.29; P = .027). Although the lipid-lowering effect of pravastatin was attenuated in carriers, it appeared that these patients showed a response similar to noncarriers in terms of less progression of atherosclerosis and event-free survival.

CONCLUSIONS

This study shows that heterozygosity for a mutation in the LPL gene, which causes only subtle changes in fasting plasma lipids, may promote the progression of coronary atherosclerosis and diminish clinical event-free survival.

Lipoprotein lipase gene mutations D9N and N291S in four pedigrees with familial combined hyperlipidaemia

The role of the lipoprotein lipase (LPL) gene in familial combined hyperlipidaemia (FCH) is unclear at present. We screened a group of 28 probands with familial combined hyperlipidaemia and a group of 91 population controls for two LPL gene mutations, D9N and N291S. LPL-D9N was found in two probands and one normolipidaemic population control. LPL-N291S was found in four probands and four population controls. Subsequently, two pedigrees from probands with the D9N mutation and two pedigrees from probands with the N291S mutation were studied, representing a total of 24 subjects. Both LPL gene mutations were associated with a significant effect on plasma lipids and apolipoproteins. Presence of the D9N mutation (n = 7) was associated with hypertriglyceridaemia [2.69 +/- 1.43 (SD) mmol L-1] and reduced plasma high-density lipoprotein cholesterol (HDL-C) concentrations (0.92 +/- 0.21 mmol L-1) compared with 11 non-carriers (triglyceride 1.75 +/- 0.64 mmol L-1; HDL-C 1.23 +/- 0.30 mmol L-1, P = 0.03 and P = 0.025 respectively). LPL-D9N carriers had higher diastolic blood pressures than non-carriers. LPL-N291S carriers (n = 6) showed significantly higher (26%) apo B plasma concentrations (174 +/- 26 mg dL-1) than non-carriers (138 +/- 26 mg dL-1; P = 0.023), with normal post-heparin plasma LPL activities. Linkage analysis revealed no significant relationship between the D9N or N291S LPL gene mutations and the FCH phenotype (hypertriglyceridaemia, hypercholesterolaemia or increased apo B concentrations). It is concluded that the LPL gene did not represent the major single gene causing familial combined hyperlipidaemia in the four pedigrees studied, but that the LPL-D9N and LPL-N291S mutations had significant additional effects on lipid and apolipoprotein phenotype.

Transcriptional control of triglyceride metabolism: fibrates and fatty acids change the expression of the LPL and apo C-III genes by activating the nuclear receptor PPAR

The development of atherosclerosis is often associated with altered concentrations of systemic lipoproteins, which are determined by the concentration and/or activity of three groups of different proteins, i.e. apolipoproteins (apo), enzymes, and receptors. The effects of diet or therapeutic interventions on lipid metabolism are mediated by changes in activity or concentrations of these three components. Fibrates have been shown to activate nuclear receptors belonging to the steroid hormone receptor super-family, termed peroxisome proliferator activated receptor (PPAR). These activated PPARs are potent transcription factors which influence the expression of several target genes implicated in lipoprotein homeostasis, e.g. LPL, apo C-III and apo A-1. Fibrates decrease apo C-III transcription and increase LPL production via these PPARs resulting in a profound hypotriglyceridaemic effect. Apolipoproteins and enzymes are important in governing lipid metabolism, thus therapeutically altering the expression of these genes constitutes an efficient therapeutic option.

Association between the LPL-D9N mutation in the lipoprotein lipase gene and plasma lipid traits in myocardial infarction survivors from the ECTIM Study

Using polymerase chain reaction (PCR) based techniques, we have identified individuals in the ECTIM study of myocardial infarction survivors (cases) and healthy matched controls who are carriers for a mutation of the gene for lipoprotein lipase (LPL) which alters amino acid 9 from aspartic acid to asparagine (LPL-D9N). The frequency of carriers in the cases from Belfast and France (3 separate centres) was 2.5 and 3.7%, respectively (mean 3.3%, 95% CI 1.9-4.7) and in the controls 2.0 and 2.9%, respectively (mean 2.7%, 95% CI 1.6-3.8%), but this difference was not statistically significant. In the cases, carriers of the allele for LPL-N9 had higher levels of several plasma lipid traits including total triglycerides (TG) (30%), very low density lipoprotein (VLDL) cholesterol (19%), apo E (24%), apo C-III (17%), lipoprotein particles (Lp) containing both apo E and apo B (LpE:B) (32%), and particles containing both apo C-III and apo B (LpCIII:B) (39%), and this effect was consistent in cases both from Belfast and from the French centres combined. By contrast, in the controls there were no differences in any lipid trait between carriers and non-carriers of the mutation that was consistent between the French centres and Belfast. There were no significant differences in the levels of any measured factor between cases and controls that could explain the different effect on plasma lipid traits associated with the mutation. However, compared to the non-carriers, in both cases and controls who carried the mutation, plasma TG concentrations were higher in those whose body mass index (BMI) was above the mean of the sample (26.0 kg/m2), with statistically significant interaction seen between BMI and genotype and levels of apo C-III, and lipoprotein particles containing both apo C-III and apo B (P < 0.02). The data suggest that carriers for the LPL-N9 mutation have a mild genetic predisposition to developing hyperlipidaemia and an atherogenic lipid profile, but that this requires the presence of other genetic or environmental factors for full expression, one of which appears to be increasing obesity.

Pancreatic carboxyl ester lipase: a circulating enzyme that modifies normal and oxidized lipoproteins in vitro

Pancreatic carboxyl ester lipase (CEL) hydrolyzes cholesteryl esters (CE), triglycerides (TG), and lysophospholipids, with CE and TG hydrolysis stimulated by cholate. Originally thought to be confined to the gastrointestinal system, CEL has been reported in the plasma of humans and other mammals, implying its potential in vivo to modify lipids associated with LDL, HDL (CE, TG), and oxidized LDL (lysophosphatidylcholine, lysoPC). We measured the concentration of CEL in human plasma as 1.2+/-0.5 ng/ml (in the range reported for lipoprotein lipase). Human LDL and HDL3 reconstituted with radiolabeled lipids were incubated with purified porcine CEL without or with cholate (10 or 100 microM, concentrations achievable in systemic or portal plasma, respectively). Using a saturating concentration of lipoprotein-associated CE (4 microM), with increasing cholate concentration there was an increase in the hydrolysis of LDL- and HDL3-CE; at 100 microM cholate, the present hydrolysis per hour was 32+/-2 and 1.6+/-0.1, respectively, indicating that CEL interaction varied with lipoprotein class. HDL3-TG hydrolysis was also observed, but was only approximately 5-10% of that for HDL3-CE at either 10 or 100 microM cholate. Oxidized LDL (OxLDL) is enriched with lysoPC, a proatherogenic compound. After a 4-h incubation with CEL, the lysoPC content of OxLDL was depleted 57%. Colocalization of CEL in the vicinity of OxLDL formation was supported by demonstrating in human aortic homogenate a cholate-stimulated cholesteryl ester hydrolytic activity inhibited by anti-human CEL IgG. We conclude that CEL has the capability to modify normal human LDL and HDL composition and structure and to reduce the atherogenicity of OxLDL by decreasing its lysoPC content.

The influence of age and relative weight on the presentation of familial combined hyperlipidemia in childhood

BACKGROUND

Familial combined hyperlipidemia (FCHL) has been described as the leading cause of familial hyperlipidemia. FCHL is dominantly inherited, occurs in at least 1% of the population, and is responsible for about 10% of premature coronary artery disease (CAD).

OBJECTIVE

Because FCHL in childhood is not well characterized, we evaluated the interrelationships among age, percentage of ideal body weight (%IBW) and plasma lipoprotein levels in FCHL children (age 2-18 years), exploring the possibility that obesity and age may influence the presentation of FCHL in childhood.

METHODS

One hundred and eighty-nine children with FCHL were studied. Significant correlations within this group were further evaluated by examining a subset of 36 FCHL children, each of whom had an unaffected sibling who could serve as a control for comparison.

RESULTS

When the full group was divided into those with TG levels > 90% and those with TG levels < 90%, the correlation with %IBW was stronger in the former (r = 0.45, P < 0.005) as compared with the latter (r = 0.25, P = 0.05). Within the subset of 36 FCHL children and their 36 unaffected siblings (controls), age and sex distributions were similar. Percentage IBW (mean +/- S.D.) (117.3 +/- 29.1 for FCHL and 111.2 +/- 19.4 for controls) was similar and in the overweight range. FCHL children had significantly higher levels of total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), apolipoprotein B (apo B) and triglyceride (TG) levels compared with controls (P < 0.0005 for all comparisons). Of several significant correlations observed in the full group (n = 189), only the correlations of %IBW with plasma TG levels (r = 0.45, P = 0.006), and of age with plasma TG levels (r = 0.48, P = 0.003) persisted with a similar degree of magnitude in the subset of 36 FCHL children. No correlation was significant in the controls. By Fisher's Z-test, the correlation between %IBW and TG in the FCHL children was significantly different from controls.

CONCLUSIONS

These results suggest that TG levels in FCHL children, but not in their unaffected siblings, and sensitive to the presence of obesity, implying an interaction between obesity and the underlying condition, in addition, the association between age and TG level in FCHL children suggests a gradual expression of the hyperlipidermia (i.e. TG) during childhood.

In vitro lipolysis of human VLDL: effect of different VLDL compositions in normolipidemia, familial combined hyperlipidemia and familial hypertriglyceridemia

Suboptimal lipolysis of very low density lipoproteins (VLDL) due to reduced substrate affinity for lipoprotein lipase (LPL) may contribute to the accumulation of apolipoprotein (apo) B in familial combined hyperlipidemia (FCH) or the characteristic increase in triglyceride-rich lipoproteins in familial hypertriglyceridemia (FHTG). To investigate this hypothesis in detail, the VLDL composition and substrate affinity for lipoprotein lipase was determined in 22 normolipidemic controls, 16 FCH probands, and 12 FHTG subjects. VLDL from FCH subjects were enriched in cholesterol and phospholipid. VLDL from FHTG subjects were enriched in triglycerides, cholesterol and phospholipid. Potential apolipoprotein regulators of LPL activity including apo C-II, apo C-III and apo E were not significantly different between FCH and controls when expressed per VLDL apo B. High apo C-III concentrations were present in FHTG-VLDL, and the apo C-III/E-ratio was significantly higher than in FCH- and control-VLDL. An increase of C-III-0, the desialylated isoform, was observed in FHTG-VLDL. The kinetic indicators for in vitro triglyceride hydrolysis by LPL, KM and VMAX, were not significantly different between the groups. KM values measured in vitro were remarkably and consistently high (1.54 mmol VLDL-TG/I), predicting saturation of LPL when VLDL-TG levels exceed 5.5 mmol/l (2 times KM + 2S.D.). In conclusion, VLDL from individuals with FCH or FHTG are normal substrate for lipoprotein lipase in spite of significant differences in lipid and apolipoprotein composition. The high apo C-III content of FHTG-VLDL supports a role in the expression of hypertriglyceridemia.

Regulation of triglyceride metabolism by PPARs: fibrates and thiazolidinediones have distinct effects

The molecular mechanism by which hypolipidemic fibrates and antidiabetic thiazolidinediones exert their hypotriglyceridemic action are discussed. Increased activity of lipoprotein lipase (LPL), a key lipolytic enzyme, and decreased levels of apolipoprotein C-III (apo C-III) seem to explain the hypotriglyceridemic effects of compounds. Both fibrates and thiazolidinediones exert their action by activating transcription factors of the peroxisome proliferator activated receptor (PPAR) family, thereby modulating the expression of the LPL and apo C-II genes. First, treatment of rats with PPAR alpha activators, such as fibrates induced LPL mRNA and activity selectively in the liver. In contrast, the thiazolidinediones, which are high affinity ligands for PPAR gamma, have no effect on liver, but induce LPL mRNA and activity levels in adipose tissue. In hepatocytes, fibrates, unlike the thiazolidinediones, induce LPL mRNA levels, whereas in preadipocyte cell lines the PPAR gamma ligand induces LPL mRNA levels much quicker and to a higher extent than fibrates. Second, apo C-III mRNA and protein production strongly decrease in livers of fibrate but not thiazolidinedione-treated animals. Fibrates also reduced apo C-III production in primary cultures of rat and human hepatocytes. The modulation of the expression of the LPL and apo C-III genes by either PPAR alpha or gamma activators, correlates with the tissue-specific distribution of the respective PPARs: PPAR gamma expression is restricted to adipose tissues, whereas PPAR alpha is expressed predominantly in liver. In both the LPL and apo C-III genes, sequence elements responsible for the modulation of their expression by activated PPARs have been identified which supports that the transcriptional regulation of these genes by fibrates and thiazolidinediones contributes significantly to their hypotriglyceridemic effects in vivo. Whereas thiazolidinediones predominantly affect adipocyte LPL production through activation of PPAR gamma, fibrates exert their effects mainly in the liver via a PPAR alpha-mediated reduction in apo C-III production. This tissue specific transcriptional regulation of genes involved in lipid metabolism by PPAR activators and/or ligands might have important therapeutic implications.

Severe hypertriglyceridemia, reduced high density lipoprotein, and neonatal death in lipoprotein lipase knockout mice. Mild hypertriglyceridemia with impaired very low density lipoprotein clearance in heterozygotes

Lipoprotein lipase (LPL)-deficient mice have been created by gene targeting in embryonic stem cells. At birth, homozygous knockout pups have threefold higher triglycerides and sevenfold higher VLDL cholesterol levels than controls. When permitted to suckle, LPL-deficient mice become pale, then cyanotic, and finally die at approximately 18 h of age. Before death, triglyceride levels are severely elevated (15,087 +/- 3,805 vs 188 +/- 71 mg/dl in controls). Capillaries in tissues of homozygous knockout mice are engorged with chylomicrons. This is especially significant in the lung where marginated chylomicrons prevent red cell contact with the endothelium, a phenomenon which is presumably the cause of cyanosis and death in these mice. Homozygous knockout mice also have diminished adipose tissue stores as well as decreased intracellular fat droplets. By crossbreeding with transgenic mice expressing human LPL driven by a muscle-specific promoter, mouse lines were generated that express LPL exclusively in muscle but not in any other tissue. This tissue-specific LPL expression rescued the LPL knockout mice and normalized their lipoprotein pattern. This supports the contention that hypertriglyceridemia caused the death of these mice and that LPL expression in a single tissue was sufficient for rescue. Heterozygous LPL knockout mice survive to adulthood and have mild hypertriglyceridemia, with 1.5-2-fold elevated triglyceride levels compared with controls in both the fed and fasted states on chow, Western-type, or 10% sucrose diets. In vivo turnover studies revealed that heterozygous knockout mice had impaired VLDL clearance (fractional catabolic rate) but no increase in transport rate. In summary, total LPL deficiency in the mouse prevents triglyceride removal from plasma, causing death in the neonatal period, and expression of LPL in a single tissue alleviates this problem. Furthermore, half-normal levels of LPL cause a decrease in VLDL fractional catabolic rate and mild hypertriglyceridemia, implying that partial LPL deficiency has physiological consequences.

COOH-terminal disruption of lipoprotein lipase in mice is lethal in homozygotes, but heterozygotes have elevated triglycerides and impaired enzyme activity

The role of the enzyme lipoprotein lipase (LPL) in atherosclerosis is uncertain. To generate an animal model of LPL deficiency, we targeted the LPL gene in embryonic stem cells with a vector designed to disrupt the COOH terminus of the protein and used these cells to generate LPL-deficient mice. Germ line transmission of the disrupted LPL allele was achieved with two chimeric males, and offspring from each of these animals were phenotypically identical. Pups homozygous (-/-) for LPL deficiency died within 48 h of birth with extreme elevations of serum triglycerides (13,327 mg/dl) associated with essentially absent LPL enzyme activity in heart and carcass. Newborn heterozygous (+/-) LPL-deficient pups had lower LPL enzyme activity and higher triglycerides (370 versus 121 mg/dl) than wild type (+/+) littermates. Adult heterozygotes had higher triglycerides than wild type mice with ad libitum feeding (236 mg/dl for +/- versus 88 mg/dl for +/+) and after fasting for 4 h (98 mg/dl for +/- versus 51 for +/+) or 12 h (109 mg/dl for +/- versus 56 mg/dl for +/+). Triglycerides were present as very low density lipoprotein particles and chylomicrons, but high density lipoprotein cholesterol levels were not decreased in +/- animals. Plasma heparin-releasable LPL activity was 43% lower in +/- versus +/+ adult animals. LPL activity, mRNA, and protein were lower in the tissues of +/- versus +/+ mice. Homozygous LPL deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice. Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels. Heterozygous LPL-deficient mice could be useful for determining if hypertriglyceridemia, independently or in combination with other discrete defects, influences atherosclerosis.

Familial combined hyperlipidemia in children: clinical expression, metabolic defects, and management

The first evidence that elevation of plasma levels of cholesterol is a risk factor for the development of atherosclerosis in children came from the Bogalusa Heart Study in 1986, which reported an association between aortic fatty streaks in 3- to 26-year-old subjects and increased plasma levels of low-density lipoprotein cholesterol (LDL-C). The most compelling evidence of a cause-and-effect relationship has come from the multicenter cooperative study called the Pathobiological Determinants of Atherosclerosis in Youth. When the investigators examined the abdominal aorta and the right coronary artery of adolescents and young adults who had died of trauma, they found a significant relationship between the sum of the very low density lipoprotein (VLDL) plus LDL-C level and both fatty streaks and raised atherosclerotic lesions. They also found an inverse relationship between those lesions and increased high-density lipoprotein cholesterol (HDL-C) levels. In addition, their studies showed that smoking (as assessed by the serum thiocyanate level) promotes atherogenesis in children as young as age 15 years. Thus many pediatricians have now accepted the importance of identifying children with significant hypercholesterolemia so that appropriate dietary and life-style modifications can be recommended. This is especially important because there is often a major genetic component to the hyperlipidemia seen in children.

Analysis of DNA changes in the LPL gene in patients with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by an increase in cholesterol and/or triglyceride levels in multiple individuals of the same family. Prior reports document a decreased activity of lipoprotein lipase (LPL) in FCHL, and studies of the role of LPL in the remodeling of nascent lipoproteins suggest that disturbances in LPL function could underlie FCHL. We studied the LPL gene in 31 unrelated individuals with FCHL. A total of 25 DNA changes (13 "silent" substitutions and 12 DNA changes resulting in amino acid substitutions) were detected in 16 patients. Three new exonic polymorphisms as well as a previously described Ser447-->stop and an Asp9-->Asn substitution were seen with similar frequency on control and FCHL chromosomes. Two novel DNA changes resulting in an Asp21-->Val and an His44-->Tyr substitution were seen in only two FCHL individuals. In vitro studies showed no effect of these mutations on LPL catalytic activity. LPL mutations impairing catalytic activity did not represent a significant factor leading to FCHL in this population. Variations in any portion of the coding region of the LPL gene affecting other functions besides catalysis are not a frequent cause of FCHL.

Effects of albumin on lipid synthesis, apo B-100 secretion, and LDL catabolism in HepG2 cells

This study examines the effects of extracellular albumin on hepatic apo B-100 metabolism. To do so, a transformed human liver cell line, HepG2, was used as a hepatocyte model and the concentration of albumin in the medium was varied between 0 and 5 g%. Apo B-100 and apo A1 concentrations in the medium were determined by specific enzyme-linked immunoassay (ELISA) and intracellular synthesis of cholesterol ester and triglyceride were determined by addition of appropriate radiolabels to the medium. The data demonstrate that the reduction of extracellular albumin concentration resulted in increased apo B-100 concentration in the medium. Apo A1 secretion, however, was unaffected. While the differences in apo B-100 concentration in the medium were statistically significant (33% +/- 7%, P < 0.0025, 0 g% albumin compared to 5 g% albumin in the medium), the absolute magnitude of the effect under these conditions was relatively modest. Nevertheless, the changes were consistent and evident over incubation periods as long as 8 days. Of interest, although triglyceride synthesis was unaffected, cholesterol ester synthesis changed such that as albumin concentration decreased, synthesis of cholesterol ester increased paralleling the changes in apo B-100 (170% +/- 9%, P < 0.005). These findings were extended by studying interventions which altered cholesterol ester synthesis. Addition of the compound 58-035 (5 micrograms/ml, a specific inhibitor of acylcholesterol acyltransferase activity) resulted in substantial inhibition of cholesterol ester synthesis (39% to 66%, P < 0.025 and P < 0.005, respectively) and apo B-100 concentrations in the medium which decreased by 20% to 28%, P < 0.025. Triglyceride synthesis, in contrast, increased significantly by 32% P < 0.025. Therefore, addition of 58-035 confirmed the previous findings of a parallel relation between cholesterol ester synthesis and apo B-100 concentration in the medium. Nonetheless, albumin still had an additional inhibitory effect on cholesterol ester and apo B-100 secretion. Of interest, when chylomicron remnants (25 micrograms/ml cholesterol), which cause apo B-100 secretion to increase by more than threefold, were added to the medium, albumin now had a more pronounced absolute effect on apo B-100 secretion with a 48% inhibition observed as albumin was increased from 0 to 5 g% in the medium (P < 0.0125). The effect of extracellular albumin on the low density lipoprotein (LDL) pathway was also examined. No differences in non-specific cell association component were detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

Human apolipoprotein (apo) B-100 is required for the synthesis and secretion of hepatic triacyglycerol-rich lipoproteins. This review summarizes recent developments in understanding the interaction of cis-acting DNA sequences and trans-acting protein factors in regulation of apo B gene expression and apo B mRNA editing, and the role of structural determinants of apo B-100 in the assembly and secretion of hepatic lipoproteins. In particular, experimental results obtained from cell culture studies using techniques of molecular and cellular biology are described and discussed. The relationship between apo B length and its ability to recruit lipids is presented, and the involvement of factors other than apo B in hepatic triacylglycerol-rich lipoprotein production is discussed.

Lipoprotein lipase activity in patients with combined hyperlipidaemia

The aetiology of familial combined hyperlipidaemia remains obscure, with both genetic and environmental factors contributing to the phenotype, which is frequently associated with premature coronary heart disease. We have studied lipoprotein lipase (LPL) activity and hepatic lipase (HL) activity in patients with coronary heart disease to determine whether variation in lipase activities contributes to this phenotype. Forty-one patients (mean age 50 years; 30 male) were selected on the basis of cholesterol levels above 6.5 mmol/l and triglyceride levels above 2.2 mmol/l, with apoprotein B values over the 90th percentile. There was a family history of premature coronary heart disease in 78% and a personal history in 64%, at mean age 44, the patient group therefore predominantly corresponded to the common definition of familial combined hyperlipidaemia, appropriate in the absence of molecular markers. None of the patients was diabetic; hypertension and smoking were not over represented. Blood samples were taken following intravenous administration of heparin (100 IU/kg body wt), and LPL and HL activities were measured. Mean post-heparin LPL was significantly lower in patients than controls 10 min after heparin administration (2.98 +/- 1.04 and 3.86 +/- 0.93 mumol ml-1 h-1, respectively, P = 0.001), and 37% patients had values below the 10th percentile of controls. Both male and female patients had significantly higher HL activities than their respective controls at 5, 10, 20 and 30 minutes post-heparin. As expected, both female patients and controls had lower HL activities than males, although this sex difference did not reach statistical significance in the patient group.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical activity and lipoprotein lipid disorders

Working muscle plays a central role in the control of lipid metabolism. Increased physical activity induces a number of positive changes in the metabolism of lipoproteins: serum triglycerides are lowered by the increased lipolytic activity and the production of native high density lipoprotein (HDL) particles is increased. The increased lecithin: cholesterol acyltransferase activity leads to an increased production of HDL2, which in addition is catabolised more slowly due to a decreased activity of hepatic lipase. The 3 effects explain the increased HDL levels of endurance trained individuals. These effects have been demonstrated in cross-sectional as well as longitudinal studies by different groups, and can be induced by training, independent of changes in bodyweight. The influence of endurance activity on the quality and quantity of low density lipoprotein (LDL) particles is a further reason for the antiatherogenic potential of increased physical activity. It has been shown by several groups that small dense LDL particles represent a particular risk factor for atherosclerosis. Recent studies presented strong evidence that LDL level and composition can be influenced favorably by physical activity. In addition to the direct influence of physical activity on lipids and lipoproteins, physical exercise may improve the disturbances of haemorheological factors, particularly those associated with hypertriglyceridaemia. In conclusion, there is increased evidence that physical activity is able to favourably influence all 3 components of the atherogenic lipoprotein phenotype: the HDL concentration increases, the concentration of small dense LDL decreases, and serum triglycerides are reduced.

Obese men with type IIB hyperlipidemia are insulin resistant

By using the euglycemic clamp technique and indirect calorimetry, we determined the degree of insulin resistance in 12 obese (body mass index > 27.0 kg/m2), normotensive patients with type IIB hyperlipidemia (HLIIB) (total cholesterol > or = 6.5 mmol/L and total triglycerides > or = 2.0 mmol/L) and 17 control subjects (total cholesterol < or = 6.1 mmol/L and total triglycerides < 1.8 mmol/L) who were carefully matched for sex, age, and obesity. Fasting plasma insulin was higher in HLIIB patients than in control subjects (18.4 +/- 4.6 versus 8.9 +/- 1.2 mU/L, respectively; P = .010). The rates of whole-body glucose uptake were significantly lower in HLIIB patients than in control subjects during the last hour of the clamp (42.2 +/- 3.9 versus 54.6 +/- 2.8 mumol/kg per minute, respectively; P = .013). Glucose oxidation during the last 30 minutes of the euglycemic clamp was lower in HLIIB patients than in control subjects (14.6 +/- 0.9 versus 19.0 +/- 1.3 mumol/kg per minute, respectively; P = .017). Nonoxidative glucose disposal during the last 30 minutes of the euglycemic clamp was also lower in HLIIB patients than in control subjects, but the difference was not statistically significant (27.6 +/- 3.3 versus 35.8 +/- 2.8 mumol/kg per minute, respectively; P = .069). Lipid oxidation during the clamp was completely suppressed in control subjects (-0.24 +/- 0.44 mumol/kg per minute) but was significantly less suppressed in the HLIIB patients (0.94 +/- 0.29 mumol/kg per minute, P = .024).(ABSTRACT TRUNCATED AT 250 WORDS)

Familial combined hyperlipidemia in children: clinical expression, metabolic defects, and management

Familial combined hyperlipidemia (FCHL) is a dominantly inherited hyperlipidemia that occurs in at least 1% of the adult population and is responsible for 10% of premature coronary artery disease. In families referred for evaluation because of primary hyperlipidemia in a child, FCHL is expressed three times more commonly than familial hypercholesterolemia and half of the siblings are affected. Several metabolic defects apparently are associated with the FCHL phenotype. Most commonly, excess production of very low density lipoprotein apolipoprotein B can be demonstrated. In other families, reduced lipoprotein lipase activity is associated. One allele at a locus influencing apolipoprotein B levels predicts FCHL in a large proportion of families ascertained through affected children. Whether this allele is responsible for the excess of very low density lipoprotein apolipoprotein B detected in metabolic studies has not been elucidated. Management of FCHL in children begins with dietary modification. A bile acid sequestrant may be considered as well if diet cannot reduce the plasma low-density lipoprotein cholesterol level to less than 4.13 mmol/L (160 mg/dl) after the age of 10 years. Although the hydroxymethylglutaryl-coenzyme A reductase inhibitors are not currently recommended for children younger than 19 years of age, we speculate that they will be increasingly utilized for the management of FCHL in teenage boys who continue to have low density lipoprotein cholesterol levels greater than 4.13 mmol/L (160 mg/dl) after dietary modification.

Impaired fatty acid metabolism in familial combined hyperlipidemia. A mechanism associating hepatic apolipoprotein B overproduction and insulin resistance

To establish whether insulin resistance and/or postprandial fatty acid metabolism might contribute to familial combined hyperlipidemia (FCH) we have examined parameters of insulin resistance and lipid metabolism in six FCH kindreds. Probands and relatives (n = 56) were divided into three tertiles on the basis of fasting plasma triglycerides (TG). Individuals in the highest tertile (TG > 2.5 mM; n = 14) were older and had increased body mass index, systolic blood pressure, and fasting plasma insulin concentrations compared with individuals in the lowest tertile (n = 24). The former also presented with decreased HDL cholesterol and increased total plasma cholesterol, HDL-TG, and apoprotein B, E, and CIII concentrations. Insulin concentrations were positively correlated with plasma apo B, apo CIII, apo E, and TG, and inversely with HDL cholesterol. Fasting nonesterified fatty acids (NEFA) were elevated in FCH subjects compared to six unrelated controls and five subjects with familial hypertriglyceridemia. Prolonged and exaggerated postprandial plasma NEFA concentrations were found in five hypertriglyceridemic FCH probands. In FCH the X2 minor allele of the AI-CIII-AIV gene cluster was associated with increased fasting plasma TG, apo CIII, apo AI, and NEFA concentrations and decreased postheparin lipolytic activities. The clustering of risk factors associated with insulin resistance in FCH indicates a common metabolic basis for the FCH phenotype and the syndrome of insulin resistance probably mediated by an impaired fatty acid metabolism.

Substrate delivery as a determinant of hepatic apoB secretion

The evidence that apoB particles secreted by the liver can differ in number and composition has been reviewed. No evidence has yet emerged that changes in apoB100 itself affect the rate of its secretion from the liver. The metabolic role of apoB appears to be the prevention of lipid accumulation within the liver cell: when delivery of lipid to the liver increases, apoB secretion will increase pari passu. This reality in no way detracts from the critical role played by the LDL receptor in determining the number of LDL particles in plasma, nor does it diminish the potential importance of intracellular processes such as 7 alpha-hydroxylase activity to also mediate LDL receptor activity. However, it should be obvious that variation in catabolism by itself cannot explain all that has been observed in physiological and pathological studies. On the contrary, the whole process must be taken into account--the rate at which apoB particles are added to the circulation, the rate at which they are converted to LDL, and the rate at which they are irreversibly removed from plasma--if we are to understand and appreciate this most peculiar and most important of transport systems.

Simvastatin improves chylomicron remnant removal in familial combined hyperlipidemia without changing chylomicron conversion

It is unknown whether the clearance of atherogenic chylomicron remnants and the postprandial lipoprotein metabolism in general can be improved by 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors in subjects with familial combined hyperlipidemia (FCH). Therefore, the postprandial chylomicron remnant clearance was studied in nine normolipidemic untreated controls and seven FCH patients before and after treatment with simvastatin using an oral vitamin A-fat load (24 hours, 50 g/m2). Treatment with simvastatin reduced plasma cholesterol level by 16% (mean +/- SEM, 8.1 +/- 0.8 v 6.8 +/- 0.8 mmol/L; P < .05) and plasma apolipoprotein (apo) B level by 19% (1.6 +/- 0.2 v 1.3 +/- 0.2 g/L; P < .05). Plasma apo E level (89.6 +/- 21.0 mg/L) was reduced by 29% (63.5 +/- 14.1 mg/L; P < .05). High-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) levels did not change; consequently, the reductions seen had been due to a decrease in very-low-density lipoprotein (VLDL) levels. Fasting plasma triglyceride (30% reduction) and plasma apo C-II (31% reduction) levels did not change significantly. Mean postheparin plasma lipoprotein lipase (LPL) activity increased by 13% after treatment (90.4 +/- 19.8 v 102.6 +/- 20.3 mU/mL; P < .05), but hepatic lipase (HL) activity was not altered. The clearance of chylomicrons (Sf > 1,000), expressed as the area under the 24-hour retinyl palmitate curve, did not change with simvastatin (52.8 +/- 12.9 v 51.8 +/- 13.4 h.mg-1/L).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and delayed effects of prolonged exercise on serum lipoproteins. II. Concentration and composition of low-density lipoprotein subfractions and very low-density lipoproteins

To investigate the effects of a single period of prolonged exercise on lipoprotein concentration and composition, 13 healthy endurance-trained men were examined before and after (1 h, 20 h) a cross-country run [30 km, time: 130 (SD 7.4) min]. The data show that following acute exercise, serum triglyceride (TG) concentration were reduced (36%) as a consequence of a reduced number (31%) of very low density lipoprotein (VLDL) particles. Changes in composition of VLDL were present but less evident. In contrast to this, acute exercise did not induce significant changes in the average concentration of individual low-density lipoprotein (LDL) subfractions. However, changes in dense LDL [density (d) > 1.044 g.ml-1] concentration were significantly correlated to changes in serum TG: a reduction of dense LDL occurred in subjects with large reductions in serum TG. In addition, LDL composition changed significantly. Immediately (1 h) after exercise the TG content of all LDL subfractions was reduced. These reductions were significant in large (d = 1.006-1.037 g.ml-1) and small LDL (1.044-1.063 g.ml-1). It can be concluded therefore from our study that acute exercise primarily altered the composition of LDL subfractions while their concentration remained stable.