Expression of type III hyperlipoproteinemia in apolipoprotein E2 (Arg158 --> Cys) homozygotes is associated with hyperinsulinemia

Type III hyperlipoproteinemia (HLP) is mainly found in homozygous carriers of apolipoprotein E2 (apoE2, Arg158-->Cys). Only a small percentage (< 5%) of these apoE2 homozygotes develops hyperlipidemia, indicating that additional environmental and genetic factors contribute to the expression of type III HLP. In the present study, first, the prevalence of type III HLP among apoE2 homozygotes was estimated in a Dutch population sample of 8888 participants. Second, 68 normocholesterolemic and 162 hypercholesterolemic apoE2 homozygotes (type III HLP patients) were collected to investigate additional factors influencing type III HLP expression. In the Dutch population sample, apoE2 homozygosity occurred with a frequency of 0.6% (57 of 8888 individuals). Among the 57 E2/2 subjects, 10 type III HLP patients were identified (prevalence 18%). Comparison of normocholesterolemic E2/2 subjects and type III HLP patients showed that the latter had a significantly increased body mass index (25.6 +/- 4.0 versus 26.9 +/- 3.8 kg/m(2), respectively; P=0.03) and prevalence of hyperinsulinemia (26% versus 63%, respectively; P<0.001). Multiple linear regression analysis demonstrated that most of the variability in type III HLP expression can be explained by fasting insulin levels (partial correlation coefficient approximately 0.50, P<0.001). In contrast to men, apoE2 homozygous women aged > or = 50 years had significantly higher plasma lipid levels than their counterparts aged < 50 years. These data demonstrate that the expression of type III HLP in E2/2 subjects is elicited to a large extent by hyperinsulinemia. In addition, in female apoE2 homozygotes, the expression increases with age; this increase is most likely due to the loss of estrogen production.     

Lipoprotein and apolipoprotein abnormalities in familial combined hyperlipidemia: a 20-year prospective study.

In order to characterize the lipoprotein abnormalities in familial combined hyperlipidemia (FCHL) and to describe factors associated with the stability of the FCHL phenotype during 20-year follow-up, 287 individuals from 48 families with FCHL originally identified in the early 1970s (baseline) were studied. Hyperlipidemia was defined as lipid-lowering medication use, or > or =age- and sex-specific 90th percentile for triglycerides or cholesterol. Triglyceride, cholesterol and medical history data were obtained at baseline and 20-year follow-up. Additional follow-up measures included HDL-C, LDL-C, LDL particle size, lipoprotein(a), apolipoprotein (apo) A-I, apoB, and apoE polymorphism. Longitudinally, two-thirds of relatives were consistently normolipidemic or hyperlipidemic, and one third were discordant for hyperlipidemic status at baseline and 20-year follow-up. Individuals with hyperlipidemia at baseline and/or follow-up had higher apoB levels than those with consistently normal lipids (P<0.05), whereas small LDL size was associated with concurrent hyperlipidemia. Among individuals who were normolipidemic at baseline, the following variables were independently associated with development of hyperlipidemia over 20 years: older age at baseline, male sex, greater increase in BMI during follow-up, and apoE alleles epsilon 2 or epsilon 4. In conclusion, apoB is associated with hyperlipidemia and apoE polymorphism is associated with later onset of hyperlipidemia in FCHL.     

Overexpression of apolipoprotein E3 in transgenic rabbits causes combined hyperlipidemia by stimulating hepatic VLDL production and impairing VLDL lipolysis

The differential effects of overexpression of human apolipoprotein (apo) E3 on plasma cholesterol and triglyceride metabolism were investigated in transgenic rabbits expressing low (<10 mg/dL), medium (10 to 20 mg/dL), or high (>20 mg/dL) levels of apoE3. Cholesterol levels increased progressively with increasing levels of apoE3, whereas triglyceride levels were not significantly affected at apoE3 levels up to 20 mg/dL but were markedly increased at levels of apoE3 >20 mg/dL. The medium expressers had marked hypercholesterolemia (up to 3- to 4-fold over nontransgenics), characterized by an increase in low density lipoprotein (LDL) cholesterol, while the low expressers had only slightly increased plasma cholesterol levels. The medium expressers displayed an 18-fold increase in LDL but also had a 2-fold increase in hepatic very low density lipoprotein (VLDL) triglyceride production, an 8-fold increase in VLDL apoB, and a moderate decrease in the ability of the VLDL to be lipolyzed. However, plasma clearance of VLDL was increased, likely because of the increased apoE3 content. The increase in LDL appears to be due to an enhanced competition of VLDL for LDL receptor binding and uptake, resulting in the accumulation of LDL. The combined hyperlipidemia of the apoE3 high expressers (>20 mg/dL) was characterized by a 19-fold increase in LDL cholesterol but also a 4-fold increase in hepatic VLDL triglyceride production associated with a marked elevation of plasma VLDL triglycerides, cholesterol, and apoB100 (4-, 9-, and 25-fold over nontransgenics, respectively). The VLDL from the high expressers was much more enriched in apoE3 and markedly depleted in apoC-II, which contributed to a >60% inhibition of VLDL lipolysis. The combined effects of stimulated VLDL production and impaired VLDL lipolysis accounted for the increases in plasma triglyceride and VLDL concentrations in the apoE3 high expressers. The hyperlipidemic apoE3 rabbits have phenotypes similar to those of familial combined hyperlipidemia, in which VLDL overproduction is a major biochemical feature. Overall, elevated expression of apoE3 appears to determine plasma lipid levels by stimulating hepatic VLDL production, enhancing VLDL clearance, and inhibiting VLDL lipolysis. Thus, the differential expression of apoE may, within a rather narrow range of concentrations, play a critical role in modulating plasma cholesterol and triglyceride levels and may represent an important determinant of specific types of hyperlipoproteinemia.     

Apolipoprotein-E2 and hyperlipoproteinemia in noninsulin-dependent diabetes mellitus

The association of apolipoprotein-E2 (apoE2) with hyperlipoproteinemia (HLP) was investigated in 23 noninsulin-dependent diabetes mellitus patients with apoE2 and 24 nondiabetic controls with apoE2. The frequency of HLP was significantly higher in diabetic subjects with apoE2 (73.9%) than in nondiabetic controls (37.5%). HLP in nondiabetic subjects with apoE2 included only type IV (n = 9), whereas HLP in diabetic subjects with apoE2 included type IIb (n = 1), type III (n = 7), and type IV (n = 9). Diabetic patients with apoE2 were characterized by increased levels of plasma triglyceride, total cholesterol (chol), very low density lipoprotein (VLDL)-chol, and apoE and an increased VLDL-chol/VLDL-triglyceride ratio, i.e. the accumulation of remnants. In addition, fasting plasma glucose and hemoglobin-A1 levels were significantly higher in hyperlipoproteinemic diabetic patients with apoE2 than in normolipidemic diabetic patients with apoE2. It is concluded that diabetes (poor metabolic control) predisposes apoE2 (epsilon 2)-carrying subjects to HLP (particularly type III) and that apoE2 may be one factor linking diabetes with HLP and cardiovascular disease.     

Effects of estrogen therapy on apolipoprotein E in type II hyperlipoproteinemia.

Type III hyperlipoproteinemia (HLP) is characterized by the accumulation in plasma of an abnormal cholesterol-enriched lipoprotein ($\text{VLDL-Chol}\text{VLDL-TG}\text{ratio > 0.42}$, VLDL-Chol/total TG ratio > 0.30) that floats in the S_f 12–100 range and migrates as β-VLDL on lipoprotein electrophoresis. Type III patients have increased levels of apolipoprotein E and the apoE-III subspecies is deficient, while apoE-I and apoE-II are increased. The role of the apoE abnormalities in the production of abnormal lipoprotein compositions is unknown. Since estrogens exert a hypolipidemic effect, correct the VLDL composition, and improve rates of remnant removal in type III HLP, we investigated whether the administration of estrogen would also correct the apoE-III deficiency. Three type III subjects (two females, one male) were given ethinyl estradiol 1 μg/kg/day for 6–8 wk and fasting plasmas were tested at 2-wk intervals. Lipoprotein lipids were determined chemically, apoA-I, apoC-II, and apoC-III by radioimmunoassay, and the subspecies of apoC and apoE in VLDL by analytic gel isoelectric focusing (IEF). In the women total TG, total Chol, VLDL-Chol, and LDL-Chol decreased whereas HDL-Chol and apoA-I increased. The abnormal lipid ratios reverted to normal and the broad beta lipoprotein decreased or disappeared. Furthermore, the apoE/apoC area ratio (by IEF) in apoVLDL decreased in response to estrogen. This occurred because of a decrease in apoE levels in VLDL. Despite these changes, the apoE-III deficiency remained. In contrast to the females, the male type III exhibited a progressive hyperlipidemia. By the 8th wk, total-TG had risen to 2387 mg/dl. His apoE/apoC area ratio in apoVLDL increased because of an increase in apoE levels in VLDL. Despite the great increases in lipid levels, the relative proportions of the apoE subspecies and apoE deficiency remained unaltered. The dissociation between apoE composition and lipoprotein physiology suggests either that apoE plays no important role in the pathophysiology of the dysbetalipoproteinemia and hyperlipoproteinemia, or if apoE does play an important role in the untreated patient, treatment bypasses any “metabolic blocks” due to the apoE abnormalities.     

ApoE: crossroads between Alzheimer's disease and atherosclerosis

Apolipoprotein E (apoE) is a major constituent of lipoproteins in the plasma and in the brain. There are three common apoE isoforms, termed E2, E3, and E4. By virtue of its ability to bind to lipoprotein receptors, apoE plays a key role in the metabolism of triglyceride-rich lipoproteins in the plasma. Homozygous carriers of apoE2 have an increased risk to develop type III hyperlipoproteinemia, whereas apoE4 is associated with elevated levels of low-density lipoprotein cholesterol. In the brain, apoE is associated with cholesterol-rich lipoproteins and is involved in the transport of cholesterol to neurons. The genetic polymorphism of apoE is among the strongest determinants of the risk and mean age of onset of Alzheimer's disease. The mechanism by which apoE isoforms differentially contribute to disease expression is not known.     

The apolipoprotein E polymorphism is associated with circulating C-reactive protein (the Ludwigshafen risk and cardiovascular health study).

BACKGROUND: Statins and cholesterol absorption inhibitors lower the concentration of C-reactive protein (CRP). The genetic polymorphism of apolipoprotein (apo) E is a strong endogenous determinant of sterol homeostasis. We therefore examined the relationship of CRP to the apoE polymorphism. METHODS AND RESULTS: We studied 739 and 570 subjects with or without stable angiographic coronary artery disease (CAD), respectively. In carriers of apoE2, apoB was lower (P<0.001) than in apoE3/3 homozygotes; in individuals with apoE3/4 and apoE4/4, it was higher (P<0.001). Both in the presence and absence of CAD, CRP was higher in carriers of apoE2 (P=0.002) and apoE3/3 homozygotes (P=0.032) than in individuals with apoE3/4 or apoE4/4. Fibrinogen and white cell count were not related to the apoE genotype. CRP was associated with CAD. Compared to the lowest tertile, crude odds ratios were 1.87 (95% confidence interval (CI), 1.43-2.45, P<0.001) and 2.24 (95% CI, 1.71-2.94, P<0.001) in the second and third tertile. In carriers of apoE2, the use of tertiles defined in controls with apoE2 only diminished the odds ratios for CAD. In apoE3/4 heterozygotes or apoE4/4 homozygotes, the use of tertiles specific for this group only slightly increased the odds ratios. CONCLUSIONS:: The concentration of CRP, but not fibrinogen nor white blood cells is associated with the apoE polymorphism. The activity of the mevalonate pathway in the liver may be related to the metabolism of CRP. The predictive value of CRP for CAD may be modified by the apoE polymorphism.     

Apolipoprotein E gene polymorphism is related to metabolic abnormalities, but does not influence erythrocyte membrane lipid composition or sodium-lithium countertransport activity in essential hypertension

The aim of this study was to analyze the influence of the apolipoprotein E (apoE) gene polymorphism on insulin resistance and plasma lipid composition of essential hypertensive patients. A secondary objective was to analyze if differences regarding plasma lipids had an effect on the erythrocyte membrane lipid composition and the activity of the erythrocyte membrane sodium-lithium countertransport. We studied 128 untreated nondiabetic essential hypertensive patients enrolled from our outpatient clinic. We considered as hyperinsulinemic all subjects having more than 80 mU/L of plasma insulin 120 minutes after a 75-g oral glucose intake. The number of hyperinsulinemic subjects among carriers of the epsilon4 allele was higher that in epsilon4 noncarrier subjects (13 of 19 v45 of 109, P < .05; odds ratio [OR], 3.08; confidence interval [CI], 0.99-10.57). Plasma insulin at baseline and plasma insulin and glucose at 120 minutes after overload was higher in carriers of the epsilon4 allele (respectively, 17.5 +/- 6.9 v 12.4 +/- 4.9 mU/L, P < .01; 111.9 +/- 39.9 v 88.7 +/- 48.2, P < .05; and 143.8 +/- 29.3 v 121.2 +/- 30.8 mg/dL, P < .005). Subjects with the epsilon4 allele had a plasma lipid profile more atherogenic than those without this allele. This profile was mainly characterized by higher levels of low-density lipoprotein (LDL) cholesterol (150.1 +/- 31.2 v 133.0 +/- 34.3 mg/dL, P < .05) and very-low-density lipoprotein (VLDL) triglycerides (134.7 +/- 85.5 v 99.2 +/- 68.8 mg/dL, P < .05) and by lower levels of high-density lipoprotein (HDL) cholesterol (41.8 +/- 10.7 v 50.0 +/- 14.7 mg/dL, P < .05). There were no differences between groups regarding erythrocyte membrane cholesterol or phospholipids composition and sodium-lithium countertransport (SLC) activity.     

Apolipoprotein E and familial dysbetalipoproteinemia: clinical, biochemical, and genetic aspects

In humans, apolipoprotein E (apoE) is a polymorphic protein of which three common isoforms can be distinguished, designated apoE2, apoE3, and apoE4. This genetic variation is associated with different plasma lipoprotein levels, different response to diet and lipid-lowering therapy, and a variable risk for cardiovascular disease and Alzheimer's disease. An example of an apoE-mediated, autosomal recessive, lipid disorder is familial dysbetalipoproteinemia (FD), caused by mutations in the apolipoprotein E gene. Homozygosity for APOE*2 (1 in 170 persons) causes FD or type III hyperlipoproteinemia in less than 20% of the adult APOE*2 homozygotes. Less common, dominant negative mutations may also cause the disorder. The patients may present with typical skin lesions and elevated plasma levels of cholesterol and triglycerides, mainly in very-low-density lipoprotein remnants and intermediate-density lipoproteins. The disorder is associated with peripheral and coronary artery disease. Additional gene and environmental factors are necessary for the expression of this hyperlipoproteinemia. Hyperinsulinemia and defects in genes involved in the hydrolysis of triglycerides are associated with this lipid disorder. Diet and weight reduction are effective but usually not sufficient to normalize the lipid levels. Additional therapy with statins or fibrates is necessary and effective in most patients.     

Associations of leptin with body fat distribution and metabolic parameters in non-insulin-dependent diabetic patients: no effect of apolipoprotein E polymorphism

Leptin levels have been shown previously to be associated with anthropometric parameters such as the body mass index (BMI), total body fat, and subcutaneous fat. Since apolipoprotein E (apoE) polymorphism is known to be a genetic marker affecting the relationship between certain anthropometric and metabolic parameters, we evaluated whether the leptin level and/or associations between the leptin level and body composition in non-insulin-dependent diabetic patients could be determined by apoE polymorphism. In 171 type 2 diabetic patients (105 male and 66 female), body composition (BMI, waist to hip ratio [WHR], fat mass, and visceral fat) was measured and fasting blood samples were obtained to determine the apoE genotype, leptin, glucose, and insulin levels, and the lipid profile. The mean leptin level for the whole group was 11.7 +/- 9.3 ng/mL, with a significant difference (P < .001) between men (7.1 +/- 4.9 ng/mL) and women (19.0 +/- 10.1 ng/mL). No difference was found for leptin levels or anthropometric variables between the 3 different apoE genotypes (E3/E3 homozygotes, E2 carriers, and E4 carriers). Only low-density lipoprotein (LDL) cholesterol was significantly different between the 3 apoE subgroups. The correlations of leptin with anthropometric variables, especially visceral fat, tended to be different between the 3 apoE groups, but this was not independent and no effect was found after controlling for the other parameters in the model. A multiple regression model containing gender, subcutaneous fat, fasting glucose, triglycerides, and high-density lipoprotein (HDL) cholesterol explained 81% of the variance in leptin levels. We conclude that apoE polymorphism has no effect on the leptin level or its associations with other anthropometric and metabolic parameters.     

Genetic transmission of isoapolipoprotein E phenotypes in a large kindred: Relationship to dysbetalipoproteinemia and hyperlipidemia

The largest reported kindred of a proband with type III hyperlipoproteinemia was investigated by assessment of lipid and lipoprotein levels and very low density lipoprotein (VLDL) isoapolipoprotein E distributions in all accessible family members (56% of the 124 living blood relatives and 59% of the 37 spouses). The results confirm in this kindred a trimodal distribution of apoE₃/E₂ ratios, and segregation analysis of 16 informative matings classified according to $\text{E}{}_3\text{E}{}_2$ ratio demonstrated classical Mendelian inheritance of the autosomal codominant type: the $\text{E}{}_3\text{E}{}_2$ ratio is determined by two alleles, apoE₃^d and apoE₃ⁿ, which produce three phenotypes apoE₃-D, apoE₃-ND, and apoE₃-N, corresponding to the low, intermediate, and high modes, respectively. Vertical transmission of the apoE₃-D phenotype occurred in two branches of the second generation. In both instances this represented pseudodominance; i.e., products of heterozygous (apoE₃-ND) × homozygous (apoE₃-D) matings. Hyperlipidemia (defined as a low density lipoprotein cholesterol and/or plasma triglyceride level exceeding the respective age-, sex-, and sex-steroid-specific 95th percentiles derived from Lipid Research Clinics population studies) was present in 15 blood relatives in multiple lipoprotein patterns, consistent with the presence of familial combined hyperlipidemia in this kindred. Eight of nine members with the apoE₃-D phenotype had either type III hyperlipoproteinemia or, in the absence of hyperlipidemia, β-VLDL and at least marginally cholesterol-rich VLDL (VLDL-cholesterol/plasma triglyceride >0.25) (defined as dysbetalipoproteinemia). The ninth such member, the only child with this phenotype, was normal. β-VLDL and marginally cholesterol-rich VLDL was seen in but one of six hyperlipidemic family members of phenotype apoE₃-ND, in none of seven hyperlipidemic blood relatives of phenotype apoE₃-N, in no normolipidemic family members of phenotype apoE₃-ND or apoE₃-N, and in no spouses (three of whom were hyperlipidemic and nine of phenotype apoE₃-ND). Thus, among adult members of the O'D kindred the apo₃-D phenotype was nearly specifically associated with dysbetalipoproteinemia or, when hyperlipidemia was present, type III hyperlipoproteinemia.     

Apolipoprotein E polymorphism influences lipid phenotypes in Chinese families with familial combined hyperlipidemia.

BACKGROUND: Apolipoprotein E (apoE) polymorphism is associated with changes in the lipoprotein profile of individuals with familial combined hyperlipidemia (FCHL), but its effects on the lipoprotein profiles of members of Chinese families with FCHL remain uncertain. METHODS AND RESULTS: 43 FCHL families (n=449) and 9 normolipidemic families (n=73) were recruited to assess the influence of apoE polymorphism on plasma lipids. The relative frequency of the epsilon4 allele in affected and unaffected FCHL relatives, spouses and normolipidemic members was 13.8%, 5.3%, 9.1% and 6.8%, respectively, with a significantly higher frequency in affected FCHL relatives, compared with unaffected FCHL relatives or normolipidemic members (p=0.0002 or p=0.029). In FCHL relatives, the apoE4 subset (E4/4 and E4/3) exhibited significantly higher levels of apoB, total cholesterol and low-density lipoprotein-cholesterol (LDL-C) than did the apoE3 (E3/3) subset, especially in women (all p<0.05), and there was significant elevation of LDL-C concentrations in men only (p<0.05). In men, the apoE2 (E3/2) subset indicated a decreased level of apoB and increased apoA1 compared with those in the apoE3 subset (p<0.05). Conclusions: ApoE polymorphism appears to be associated with variance of the lipoprotein phenotype in Chinese families with FCHL.     

Effects of simvastatin on plasma lipoproteins and response to arterial injury in wild-type and apolipoprotein-E-deficient mice

OBJECTIVE: To test the non-lipid-lowering effects of simvastatin on the response to injury in normolipidemic and hyperlipidemic mice. METHODS AND RESULTS: Wild-type (WT) mice (n = 40) and hyperlipidemic apolipoprotein-E-deficient (apoE(-/-)) mice (n = 40) received normal chow or chow containing simvastatin 100 mg/kg/day prior to bilateral femoral artery wire injury. Intimal hyperplasia and plasma cholesterol concentration were quantified after 4 weeks. Plasma cholesterol in WT mice treated or untreated with simvastatin was similar (100.9 +/- 6.6 vs. 94.3 +/- 17.5 mg/dl). Simvastatin did not affect intimal hyperplasia. In apoE(-/-) mice, intimal hyperplasia was increased 2.3-fold relative to WT mice (17090 +/- 4998 vs. 39490 +/- 16190; p < 0.001). In apoE(-/- )mice, simvastatin caused a paradoxical increase in plasma cholesterol (1094 +/- 60.3 vs. 658 +/- 66.8 mg/dl; p < 0.001), confirmed by FPLC. This was associated with a further increase in intimal area (39490 +/- 16190 vs. 55420 +/- 22590 mm(2); p < 0.01). CONCLUSIONS: (1). Simvastatin had no effect on plasma cholesterol or the response to arterial injury in normolipidemic WT mice; (2). hyperlipidemia was associated with markedly increased intimal hyperplasia, and (3). simvastatin treatment of apoE(-/-) mice caused paradoxical hyperlipidemia and increased intimal hyperplasia.     

Hyperlipidemia of ApoE2(Arg(158)-Cys) and ApoE3-Leiden transgenic mice is modulated predominantly by LDL receptor expression

To investigate the relative roles of the LDL receptor- and non-LDL receptor-mediated pathways in the clearance of apolipoprotein E (apoE) variants in vivo, we have generated apoE2(Arg(158)-Cys) (apoE2) and apoE3-Leiden transgenic mice deficient for the endogenous mouse Apoe and Ldl receptor genes (Apoe-/-.Ldlr-/- mice). Unexpectedly, on the Apoe-/-.Ldlr-/- background, expression of neither apoE2 nor apoE3-Leiden results in a decrease of the hyperlipidemia. In contrast, serum cholesterol levels are increased by the introduction of apoE2 and apoE3-Leiden in Apoe-/-.Ldlr-/- mice (to 39.1+/-7.1 and 37.6+/-7.6 mmol/L, respectively, from 25. 9+/-6.5 mmol/L). In addition, in these transgenic mice, the serum triglyceride levels are substantially increased (to 9.6+/-7.0 and 5. 8+/-2.8 mmol/L, respectively, from 0.7+/-0.5 mmol/L), which is associated with a decreased efficiency of in vitro LPL-mediated lipolysis of circulating VLDL. The VLDL-triglyceride secretion rate is not affected by the expression of apoE2 or apoE3-Leiden on the Apoe-/-.Ldlr-/- background. These results indicate that in the absence of the LDL receptor, clearance of triglyceride-rich apoE2 and apoE3-Leiden-containing lipoproteins via alternative hepatic receptors, such as the LDL receptor-related protein (LRP) is inefficient. Although apoE2 and apoE3-Leiden are disturbed in binding to the LDL receptor in vitro, expression of 1 or 2 mouse Ldlr alleles in an apoE2.Apoe-/- or apoE3-Leiden.Apoe-/- background results in a gene dose-dependent decrease of the hyperlipidemia. Furthermore, overexpression of the LDL receptor via adenovirus-mediated gene transfer rescues the hyperlipidemia associated with apoE2 and apoE3-Leiden expression. These data indicate that in apoE2 and apoE3-Leiden transgenic mice, the LDL receptor constitutes the predominant route for clearance of VLDL remnants, carrying even poorly binding apoE variants, and that this pathway is functional despite an apoE-mediated disturbance in VLDL triglyceride lipolysis.     

Visceral obesity and hyperinsulinemia modulate the impact of the microsomal triglyceride transfer protein -493G/T polymorphism on plasma lipoprotein levels in men

The dyslipidemic state of visceral obesity is characterized by increased plasma triglyceride levels, low high-density lipoprotein-cholesterol concentration and alterations in low-density lipoprotein (LDL) composition and concentration. A functional, non-coding microsomal triglyceride transfer protein (MTP) −493G/T polymorphism of the microsomal triglyceride transfer protein gene has been related to variations in LDL-cholesterol levels. To study the effect of the MTP −493G/T polymorphism on lipoprotein levels in visceral obesity and hyperinsulinemia, a total of 227 men were assigned into two groups on the basis of their MTP −493G/T polymorphism, including 121 GG homozygotes and 105 carriers of the T allele (92 GT and 13 TT). The two genotypic groups did not differ for their physiological characteristics nor for lipoprotein–lipid profiles, before and after adjustment for age. However, GG homozygotes were characterized by higher fasting insulin levels than carriers of the T allele (P<0.05). When the two genotypic groups were further divided on the basis of their visceral adipose tissue (AT) accumulation, assessed by computed tomography, we observed that T allele carriers with low levels of visceral AT (<130 cm²) had decreased plasma total cholesterol and LDL-apolipoprotein B (LDL-apoB) levels compared to viscerally obese men (P=0.035 and P=0.0001, respectively). Among GG homozygotes, no significant difference were observed. Although not significant, T allele carriers characterized by visceral obesity tended to have smaller, denser LDL particles than T allele carriers characterized by a low accumulation of visceral AT. When subjects were divided on the basis of their fasting insulin levels, it appears that hyperinsulinemic men were characterized by a deteriorated lipoprotein–lipid profile when they were carriers of the T allele compared to normoinsulinemic men. In summary, visceral obesity and hyperinsulinemia modulate the impact of the MTP −493G/T polymorphism on plasma total cholesterol and LDL-apoB levels, as well as on LDL peak particle diameter.     

Effects of a new single-nucleotide polymorphism in the Acyl-CoA:cholesterol acyltransferase-2 gene on plasma lipids and apolipoproteins in patients with hyperlipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes cholesterol esterification in mammalian cells. Two isoforms of ACAT have been reported to date (ACAT-1 and ACAT-2). ACAT-1 protein is ubiquitously expressed in tissues, including macrophages, hepatocytes, adrenal glands, and intestines. In contrast, ACAT-2 is expressed mainly in the intestine in humans. However, the roles of ACAT-1 and ACAT-2 in lipoprotein metabolism in humans have not yet been reported. This study was carried out to clarify the relationship between ACAT-2 gene mutations and hyperlipidemia in humans. To identify gene mutations, we screened 30 subjects with hyperlipidemia (TC > 220 mg/dl or TG >150 mg/dl) by direct sequencing. As a result, we found a new single-nucleotide polymorphism (SNP; a point mutation in intron 1, IVS1 -8 G-->C) in the ACAT-2 gene. To investigate the relationship between this SNP and both plasma lipids and apolipoproteins, 91 unrelated hyperlipidemic subjects (40 males and 51 females), and 92 unrelated normolipidemic subjects (46 males and 46 females) were screened by direct sequencing. The frequencies of the IVS1 - 8G-->C allele in normolipidemic and hyperlipidemic subjects were 0.131 and 0.125, respectively. IVS1 -8 G-->C did not affect plasma concentrations of lipids or apolipoproteins in either normolipidemic or hyperlipidemic subjects. Although further studies are needed, our data suggest that the ACAT-2 gene may not affect lipid levels in humans.     

Recent insights into the pathogenesis of type III hyperlipoproteinemia

Type III hyperlipoproteinemia is a genetic disorder of lipoprotein metabolism characterized by high plasma cholesterol and triglycerides, the presence of a characteristic lipoprotein termed β-very low density lipoprotein (gb-VLDL), xanthomatosis, and premature atherosclerosis. Defects in the gene for apolipoprotein (apo) E represent the molecular cause of this disorder. ApoE serves as a ligand for the receptor-mediated clearance of the triglyceride-rich lipoproteins from the circulation. All defects in apoE cause impaired lipoprotein clearance through the same mechanism, that is, the disruption, either direct or indirect, of the receptor-binding region of the apoE molecule. However, the receptor-binding defect alone does not fully explain the pathogenesis of β-VLDL formation, the development of type III hyperlipoproteinemia, and its mode of inheritance, which can be either recessive or dominant, depending on the apoE mutation. Recent studies suggest that other functional properties of apoE might play a role in the formation of β-VLDL. Modulation of the receptor-binding defect, association with VLDL, heparin-binding affinity, and interaction with lipolytic enzymes could all contribute to the expression of the disease phenotype and determine its mode of inheritance.     

Effects of bezafibrate on insulin secretion and peripheral insulin sensitivity in hyperlipidemic patients with and without diabetes

Although it has been reported that bezafibrate influences carbohydrate metabolism, this possibility has never been properly evaluated in a controlled clinical trial. In this study we attempted to evaluate the effects of bezafibrate on plasma lipoproteins, glucose tolerance, insulin secretion and peripheral insulin sensitivity in a group of hypertriglyceridemic patients with and without diabetes. Sixteen hyperlipidemic patients (10 males and 6 females) participated in the study. Eight had type IIB and 8 type IV hyperlipoproteinemia; 6 of them also had non-insulin dependent diabetes mellitus. The study was performed according to a double blind, crossover design: after 1 month wash-out period in which patients were on diet alone, they underwent, in a random order, a period of placebo therapy and another period in which they received a single daily dose of a long-acting bezafibrate preparation (400 mg) administered in the evening. Each treatment lasted 2 months. Total plasma and VLDL triglyceride concentrations were consistently reduced by bezafibrate (-46%, P less than 0.001; and -50%, P less than 0.001). Total and VLDL-cholesterol were also reduced by bezafibrate. The effects of bezafibrate on lipoproteins were similar in diabetic and non-diabetic subjects. Bezafibrate treatment did not influence fasting blood glucose concentration, glucose tolerance, peripheral insulin sensitivity or insulin secretion. In conclusion, the results of this controlled trial clearly indicate that bezafibrate can be successfully employed to lower plasma lipid levels in patients with non-insulin dependent diabetes mellitus and hyperlipidemia.     

ACAT activity in freshly isolated human mononuclear cell homogenates from hyperlipidemic subjects.

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) catalyzes the esterification of cholesterol in human mononuclear cells (MNC). In order to assess the relationship between lipid levels and ACAT activity in circulating MNC, we measured the rate of [14C]oleoyl-CoA incorporation into cholesterol ester in freshly isolated MNC homogenates from hyperlipidemic subjects. Baseline, off-treatment results obtained in 14 hypertriglyceridemic subjects (eight type IV and six type III) and seven subjects with familial hypercholesterolemia (FH) due to the same deletion of greater than 10 kb on the low-density lipoprotein (LDL)-receptor gene were compared with values determined in 12 healthy normolipidemic subjects. The rate of cholesterol esterification was 45 +/- 28 pmol/5 min/mg cell protein in healthy normolipidemic controls. This rate was significantly higher in type IV subjects (84 +/- 52 pmol/5 min/mg cell protein, P less than .05) and FH subjects (67 +/- 25 pmol/5 min/mg cell protein, P less than .05). The values were more dispersed in type III subjects; the mean value for the group (72 +/- 46 pmol/5 min/mg cell protein) was not statistically different from the control. Hypertriglyceridemic patients were then treated with 6 g/d of omega-3 fatty acids. This resulted in a significant reduction in plasma total triglycerides and very-low-density lipoprotein (VLDL)-cholesterol in both type III subjects (-57% and -51%, P less than .05) and type IV subjects (-62% and -62%, P less than .01). The reduction in VLDL concentration was associated with a significantly lower ACAT activity in MNC homogenates from type IV subjects (from 84 +/- 52 to 60 +/- 36 pmol/5 min/mg cell protein, P less than .05), but not from type III hypertriglyceridemic subjects (from 72 +/- 46 to 73 +/- 36 pmol/5 min/mg cell protein). In conclusion, we found that cholesterol esterification in human MNC is elevated in hyperlipidemic subjects and can be decreased with normalization of lipid levels. However, ACAT activity changes occurring with treatment are heterogeneous among hyperlipidemic subjects, suggesting that factors other than plasma lipid level reduction affect ACAT activity in vivo.