Evidence that acute insulin administration enhances LDL cholesterol susceptibility to oxidation in healthy humans.
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Increased free radical production and hyperinsulinemia are thought to play a role in experimental and human atherosclerosis, but the relation between the 2 abnormalities has not been studied. In 23 healthy volunteers, we measured the susceptibility of circulating low-density lipoprotein (LDL) cholesterol particles to in vitro copper sulfate oxidation (measured as the lag phase) and cell-mediated oxidative modification (measured as malondialdehyde generation in LDL during incubation with human umbilical vein endothelial cells), as well as the vitamin E content of LDL cholesterol at baseline and after 2 hours of physiological hyperinsulinemia (euglycemic insulin clamp). The lag time of LDL oxidation decreased from control values of 108+/-3 and 107+/-3 minutes (at baseline and after 2 hours of saline infusion) to 101+/-3 minutes after 2 hours of clamping (P<0.0001). At corresponding times, cell-mediated malondialdehyde generation in LDL rose from 4.96+/-0.11 and 4.98+/-0.10 to 5.28+/-0.10 nmol/L (P=0. 0006), whereas the LDL vitamin E content decreased from 6.78+/-0.06 and 6.77+/-0.06 to 6.64+/-0.06 microg/mg (P<0.04). The insulin-induced shortening of the lag phase was directly related to the decrement of vitamin E in LDL; furthermore, in subjects with higher baseline serum triglyceride levels, insulin induced a greater shortening of the lag phase than in subjects with low baseline triglycerides. We conclude that in healthy humans acute physiological hyperinsulinemia enhances the oxidative susceptibility of LDL cholesterol particles. This effect may have pathogenic significance for atherogenesis in insulin resistant states. (+info)
Hyperlipidemia of ApoE2(Arg(158)-Cys) and ApoE3-Leiden transgenic mice is modulated predominantly by LDL receptor expression.
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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. (+info)
Overexpression of apolipoprotein E3 in transgenic rabbits causes combined hyperlipidemia by stimulating hepatic VLDL production and impairing VLDL lipolysis.
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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. (+info)
Human apolipoprotein (Apo) B-48 and ApoB-100 kinetics with stable isotopes.
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The kinetics of apolipoprotein (apo) B-100 and apoB-48 within triglyceride-rich lipoproteins (TRLs) and of apoB-100 within IDL and LDL were examined with a primed-constant infusion of (5,5,5-(2)H(3)) leucine in the fed state (hourly feeding) in 19 subjects after consumption of an average American diet (36% fat). Lipoproteins were isolated by ultracentrifugation and apolipoproteins by SDS gels, and isotope enrichment was assessed by gas chromatography/mass spectrometry. Kinetic parameters were calculated by multicompartmental modeling of the data with SAAM II. The pool sizes (PS) of TRL apoB-48, VLDL apoB-100, and LDL apoB-100 were 17+/-10, 273+/-167, and 3325+/-1146 mg, respectively. There was a trend toward a faster fractional catabolic rate (FCR) for VLDL apoB-100 than for TRL apoB-48 (6.73+/-3.48 versus 5.02+/-2.07 pools/d, respectively, P=0.06). The mean FCRs for IDL and LDL apoB-100 were 10.07+/-7.28 and 0.27+/-0.08 pools/d, respectively. The mean production rate (PR) of TRL apoB-48 was 6.5% of VLDL apoB-100 (1. 3+/-0.90 versus 20.06+/-6.53 mg. kg(-1). d(-1), P<0.0001). TRL apoB-48 PS was correlated with apoB-48 PR (r=0.780, P<0.0001) but not FCR (r=-0.1810, P=0.458). VLDL apoB-100 PS was correlated with both PR (r=0.713, P=0.0006) and FCR (r=-0.692, P=0.001) of VLDL apoB-100 and by apoB-48 PR (r=0.728, P=0.0004). LDL apoB-100 PS was correlated with FCR (r=-0.549, P=0.015). These data indicate that (1) the FCRs of TRL apoB-48 and VLDL apoB-100 are similar in the fed state, (2) TRL apoB-48 PS is correlated with TRL apoB-48 PR, (3) VLDL apoB-100 PS is correlated with both PR and FCR of VLDL apoB-100 and PR of TRL apoB-48, and (4) LDL apoB-100 PS is correlated with LDL FCR. (+info)
Dyslipidemia and vascular dysfunction in diabetic pigs fed an atherogenic diet.
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Diabetic patients typically have not only hyperglycemia but also dyslipidemia. Study of the pathogenic components of the diabetic milieu and mechanisms of accelerated atherosclerosis is hindered by inadequate animal models. A potentially suitable animal model for human diabetic dyslipidemia is the pig, because it carries a large fraction of total cholesterol in low-density lipoprotein (LDL), similar to humans. In this study, male Sinclair miniature pigs were made diabetic by destroying the insulin-producing cells of the pancreas with alloxan and then were fed a high fat and high cholesterol diet for comparison with pigs fed a nondiabetic high fat and high cholesterol diet and control pigs. Diabetic pigs exhibited hyperglycemia, but plasma urea nitrogen, creatinine, and transaminase levels were in the normal range, indicating no adverse effects on kidney and liver function. The lipoprotein profile in diabetic pigs was similar to that found in human diabetic patients and was characterized by hypertriglyceridemia (2.8-fold increase versus control and high fat-fed pigs) and a profound shift of cholesterol distribution into the LDL fraction (81%) versus the distribution in high fat-fed (64%) and control (57%) pigs. LDL particles were lipid-enriched and more heterogeneous in diabetic pigs. Apolipoprotein B was distributed among a much broader spectrum of LDL particles, and apolipoprotein E was partially redistributed from high-density lipoprotein to apolipoprotein B-containing lipoproteins in diabetic pigs. There was little change in apolipoprotein A-I distribution. Diabetic pigs showed several early signs of excess vascular disease. In diabetic pigs, 75% of the coronary artery segments showed contractile oscillations in response to prostaglandin F(2alpha) compared with 25% in high fat-fed pigs and 10% in control pigs. Endothelium-dependent relaxation of brachial arteries was nearly abolished in diabetic pigs but unchanged in high fat-fed versus control pigs. Carotid artery Sudan IV staining for fatty streaks was significantly increased only in diabetic pigs. This porcine model should provide insights into the etiology of human diabetic dyslipidemia and facilitate study of peripheral vascular and coronary artery disease in diabetic patients. (+info)
Cyclosporin-induced dyslipoproteinemia is associated with selective activation of SREBP-2.
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The use of cyclosporin A has contributed greatly to the success of organ transplantation. However, cyclosporin-associated side effects of hypertension, nephrotoxicity, and dyslipoproteinemia have tempered these benefits. Cyclosporin-induced dyslipoproteinemia may be an important risk factor for the accelerated atherosclerosis observed posttransplantation. Using a mouse model, we treated Swiss-Webster mice for 6 days with a daily dose of 20 microg/g body wt of cyclosporin and observed significant elevations of plasma cholesterol, triglyceride, and apolipoprotein B (apoB) levels relative to vehicle-alone treated control animals. Measurement of the rate of secretion of very low-density lipoprotein (VLDL) by the liver in vivo showed that cyclosporin treatment led to a significant increase in the rate of hepatic VLDL triglyceride secretion. Total apoB secretion was unaffected. Northern analysis showed that cyclosporin A treatment increased the abundance of hepatic mRNA levels for a number of key genes involved in cholesterol biosynthesis relative to vehicle-alone treated animals. Two key transcriptional factors, sterol regulatory element-binding protein (SREBP)-1 and SREBP-2, also showed differential expression; SREBP-2 expression was increased at the mRNA level, and there was an increase in the active nuclear form, whereas the mRNA and the nuclear form of SREBP-1 were reduced. These results show that the molecular mechanisms by which cyclosporin causes dyslipoproteinemia may, in part, be mediated by selective activation of SREBP-2, leading to enhanced expression of lipid metabolism genes and hepatic secretion of VLDL triglyceride. (+info)
A locus conferring resistance to diet-induced hypercholesterolemia and atherosclerosis on mouse chromosome 2.
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Dietary cholesterol is known to raise total and low density lipoprotein cholesterol concentrations in humans and experimental animals, but the response among individuals varies greatly. Here we describe a mouse strain, C57BL/6ByJ (B6By), that is resistant to diet-induced hypercholesterolemia, in contrast to the phenotype seen in other common strains of mice including the closely related C57BL/6J (B6J) strain. Compared to B6J, B6By mice exhibit somewhat lower basal cholesterol levels on a chow diet, and show a relatively modest increase in absolute levels of total and LDL/VLDL cholesterol in response to an atherogenic diet containing 15% fat, 1.25% cholesterol, and 0.5% cholate. Correspondingly, B6By mice are also resistant to diet-induced aortic lesions, with less than 15% as many lesions as B6J. Food intake and cholesterol absorption are similar between B6By and B6J mice. To investigate the gene(s) underlying the resistant B6By phenotype, we performed genetic crosses with the unrelated mouse strain, A/J. A genome-wide scan revealed a locus, designated Diet1, on chromosome 2 near marker D2Mit117 showing highly significant linkage (lod = 9.6) between B6By alleles and hypo-response to diet. Examination of known genes in this region suggested that this locus represents a novel gene affecting plasma lipids and atherogenesis in response to diet. (+info)
Serum, biliary, and fecal cholesterol and plant sterols in colectomized patients before and during consumption of stanol ester margarine.
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BACKGROUND: Cholesterol metabolic studies are simplified in colectomized patients because of rapid intestinal passage and reduced bacterial action. OBJECTIVE: Our objective was to study the effect on cholesterol and plant sterol metabolism of feeding a margarine containing stanol ester to 11 colectomized patients. DESIGN: A margarine containing 2 g stanol was consumed for 7-18 d. Serum, biliary, and fecal lipids were measured before and during consumption of the margarine. RESULTS: Serum cholesterol concentrations and the ratio of plant sterol to cholesterol decreased after 1 d of consumption of stanol esters (P < 0.05). After 7 d, serum cholesterol decreased by 16% (P < 0.01), cholesterol absorption efficiency decreased by approximately 40%, and fecal output of cholesterol as neutral sterols (but not as bile acids) increased by 36%. Biliary bile acid composition and the molar percentage of biliary cholesterol were unchanged. Increased ratios of cholesterol precursor sterols in serum and bile indicated enhanced cholesterol synthesis during consumption of stanol esters; the percentage absorption of plant sterols and the ratios of plant sterols to cholesterol decreased, whereas serum and biliary plant stanols and their biliary secretion gradually increased. In feces, 95% of cholesterol and 90% of plant stanols were in unesterified form. CONCLUSIONS: In colectomized patients, effective inhibition of cholesterol absorption and lowering of serum cholesterol concentrations and plant sterol ratios occurs within 1 d of the start of consumption of stanol esters. The composition of major bile lipids is unchanged, indicating that gallstone formation is unlikely. Small amounts of plant stanols are recovered in serum and bile during consumption of stanol esters but effectively are secreted through bile, thereby balancing the intake-induced increase in their absorption. (+info)