Effects of low dose oral contraceptives on very low density and low density lipoprotein metabolism.

Oral contraceptives (OC) raise plasma triglyceride and VLDL levels, which may be of concern, since some conditions characterized by elevated triglycerides are associated with atherosclerosis. To identify the responsible mechanism, we studied 11 healthy premenopausal women, 5 of whom were taking OC containing 0.035 mg ethinyl estradiol, and 6 of whom were not. Their rates of VLDL and LDL metabolism were measured by endogenously labeling apoB, the protein component of VLDL and LDL, by an intravenous infusion of deuterated leucine. OC use had the greatest effect on the large, triglyceride-rich VLDL subfraction (Sf 60-400), increasing plasma levels threefold and production rates fivefold (P < 0.05). Among OC users, small VLDL (Sf 20-60) levels were 2.2 times higher, and production rates were 3.4-fold higher (P < 0.05). The fractional catabolic rates of large and small VLDL were similar among OC users and nonusers. LDL levels and metabolic rates were not significantly different between the two groups. Thus, contemporary low dose OC substantially raise VLDL levels by increasing the production rate of large, triglyceride-rich VLDL, and not by slowing VLDL catabolism. Since VLDL catabolism is not impaired, we speculate that the hypertriglyceridemia induced by OC may be less atherogenic than that of hypertriglyceridemia resulting from impaired lipolysis. This may explain why long-term OC use does not appear to promote atherosclerosis.     

Extended worldwide experience. HMG-CoA reductase inhibitors: lovastatin and simvastatin]

Lovastatin and simvastatin are the first licensed compounds of a potent new class of lipid lowering drugs whose mechanism of action is to inhibit HMG-CoA reductase, a rate-limiting enzyme in the cholesterol biosynthetic pathway in the liver. Inhibition of cholesterol synthesis leads to upregulation of LDL-receptors in the liver and subsequent increased LDL clearance. Both agents are pro-drugs (lactones) which undergo extensive first-pass hepatic metabolism to the beta-hydroxyl acid form. These drugs are liver specific and have low systemic exposure as open-acid forms (plasma concentration < 5% of oral dose). Thus, theoretically both drugs have low potential for systemic adverse events. They are potent and effective agents for the treatment of primary hypercholesterolemia. The adverse experiences reported for both drugs demonstrate an excellent safety profile.     

Low density lipoprotein metabolism by human macrophages activated with low density lipoprotein immune complexes. A possible mechanism of foam cell formation

Human macrophages play a key role in atherogenesis and are believed to be the progenitors of the cholesteryl ester (CE)-laden foam cells present in early atherosclerotic lesions. Several mechanisms by which macrophages accumulate CE have been recently described. One involves a perturbation in LDL metabolism subsequent to macrophage activation. Thus, we decided to study the effect of macrophage activation by immune complexes on N-LDL metabolism. Initially, LDL-containing immune complexes (LDL-IC) were chosen, since increased plasma levels of these IC have been reported in patients with coronary heart disease. Human macrophages stimulated for 22 h with LDL-IC (250 micrograms/ml) and incubated afterwards for 20 h with 10 micrograms/ml 125I-N-LDL showed a six- and fourfold increase in the accumulation and degradation, respectively, of 125I-N-LDL over the values observed in nonstimulated cells. Scatchard analysis of 125I-N-LDL-specific binding suggests an increase (20-fold) in the number of LDL receptors in macrophages stimulated with LDL-IC. We studied other immune complexes varying in size and antigen composition. Some of the IC were able to stimulate, although to a lesser degree, the uptake of N-LDL by macrophages. Lipoprotein IC are more efficient and have the greatest capacity to increase N-LDL uptake and CE accumulation. We conclude that human macrophage activation by LDL-IC leads to an increase in LDL receptor activity and promotes in vitro foam cell formation.     

Triglyceride-poor very low density lipoprotein in human serum.

The chemical and physical properties of very low density lipoproteins, isolated from the pool of the sera of 60 persons with high pre-beta and normal triglyceride and cholesterol concentrations, have been studied. These very low density lipoproteins, a designated as triglyceride-poor very low density lipoproteins, consist of 20.5% phospholipids, 30.8% free cholesterol, 15% cholesterol esters and 33.7% triglycerides. Their protein content consists of 54.5% apo B, 26% apo A, 11.5% apo E and only 8% apo C, so they differ from any serum lipoprotein described until now. Triglyceride-poor very low density lipoproteins consist of spherical particles 300-450 A in diameter as revealed by electron microscopy.     

HMG-CoA还原酶抑制剂抗肾间质纤维化的作用机制

HMG-CoA还原酶是胆固醇合成过程中的关键性限速酶,他汀类药物作为HMG-CoA还原酶抑制剂是治疗血脂异常的经典药物,其作用不仅局限于此,多效性还表现在可以改善内皮功能,延缓动脉粥样硬化,抗血栓形成等。导致肾间质纤维的作用机制多种多样,往往表现为炎症细胞、因子等的浸润,肾小管上皮细胞的增殖及凋亡,细胞外基质的沉积,肾小管上皮间充质转分化等。而他汀类药物通过减轻炎症细胞、因子的浸润,抗凋亡、抗氧化应激,减轻细胞外基质沉积等作用延缓肾间质纤维化的进程。现将HMG-CoA还原酶抑制剂抗肾间质纤维化的作用机制作一综述。     

The metabolism of low density lipoprotein in endogenous hypertriglyceridaemia.

The metabolism of low density lipoprotein (LDL) was studied in eighteen hypertriglyceridaemic patients by injecting autologous radioiodinated LDL. Over 95% of the label was bound to the protein moiety of LDL and therefore the metabolic data reflect the fate and distribution of LDL apoprotein (apo B). The hypertriglyceridaemic subjects included ten with Type V, five with Type IV, two with Type III and one with Type IIb hyperlipoproteinaemia. For comparison identical studies were carried out in seven normal subjects and five patients with heterozygous familial hyperbetalipoproteinaemia (Type IIa). The groups differed considerably in mean LDL-cholesterol concentration. The patients with Type V lipoprotein pattern had significantly lower LDL-cholesterol concentration (mean 0.754 g/1) than the normal group (mean 1.237 g/1). Raised LDL-cholesterol levels were observed in all patients with heterozygous familial hyperbetalipoproteinaemia. The synthetic rate of LDL-apoprotein was found to be similar in all three groups (hypertriglyceridaemic, normal and hypercholesterolaemic). The highest synthetic rate was observed in the patient with Type IIb pattern. However, the fractional catabolic rate (FCR) of LDL-apoprotein differed significantly. The highest mean FCR was found in the Type V group (0.65 +/- 0.17 day-1) compared with 0.41 +/- 0.09 day-1 in the normal group and 0.185 +/- 0.05 day-1 in the Type IIa group. A strong inverse correlation was found between FCR and LDL apoprotein concentration in the whole series (r = -0.90, p less than 0.001) as well as within the Type V group (r = -0.87, p less than 0.01). These data indicate that the low plasma levels of LDL frequently observed in patients with very high plasma triglyceride levels are due to a high removal rate of LDL in these patients rather than to abnormal LDL synthesis.     

A population-based treat-to-target pharmacoeconomic analysis of HMG-CoA reductase inhibitors in hypercholesterolemia.

The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors have become the drugs of choice for the treatment of patients with hypercholesterolemia. However, one of the major concerns with these drugs is cost. In an attempt to develop a cost-effective treatment strategy for patients referred to our lipid clinic, we conducted a meta-analysis to estimate the lipid-lowering efficacy of the various HMG-CoA reductase inhibitors alone or in combination with niacin or cholestyramine. Based on cholesterol-lowering efficacy estimates derived from a literature-based meta-analysis, we performed a population-based treat-to-target analysis. Fifty-six trials with 101 monotherapy cohorts and 20 trials with 31 combination-therapy cohorts (573 patients) were included in the meta-analysis. Based on reduction in low-density lipoprotein cholesterol (LDL-C), the most effective monotherapy was atorvastatin and the least effective monotherapy was fluvastatin. Combination therapy was more effective in reducing LDL-C than monotherapy with the respective HMG-CoA reductase inhibitor. However, on the basis of dollars spent per percentage of LDL-C reduction, combination therapy was frequently less cost-effective than monotherapy. In addition, combination therapy was associated with a higher rate of noncompliance and a greater risk of drug-drug interactions. As a result, we based our treat-to-target analysis on the use of monotherapy as first-line treatment, with combination therapy reserved for patients failing to achieve the target LDL-C levels of the US National Cholesterol Education Program Adult Treatment Panel II (NCEP ATP-II) with monotherapy. In the population-based treat-to-target analysis, atorvastatin was the most cost-effective drug for high-risk patients (those with coronary heart disease [CHD]), whereas fluvastatin was the most cost-effective agent for low-risk patients (<2 risk factors for CHD) and moderate-risk patients (> or =2 risk factors for CHD). If 1 drug is chosen to treat all patients (i.e., in cases of formulary restriction), atorvastatin would be the most cost-effective agent. In adapting the findings on cholesterol-lowering efficacy from this analysis to our lipid clinic, we concluded that the most cost-effective treatment approach is to individualize the selection of an HMG-CoA reductase inhibitor based on both coronary risk and the LDL-C reduction required to achieve NCEP ATP-II goals. Based on our results, 2 agents--atorvastatin and fluvastatin--should be available on the formulary.     

Turnover of plasma total and very low density lipoprotein triglyceride in man.

Basal plasma total triglyceride and very low density lipoprotein (VLDL) triglyceride turnover rates were determined in 110 subjects whose triglyceride concentrations ranged from low normal to markedly elevated values. The mean total triglyceride turnover rate was 13.7 mg - kg-1- hr-1, whereas the mean VLDL triglyceride turnover rate was 13.2 mg - kg-1 - hr-1. A highly significant correlation was present between the two turnover rates (r equal + 0.75). The endogenous serum triglyceride transported in the other lipoproteins (LDL and HDL) may account for more than half of the circulating triglyceride mass, but its significance in the total triglyceride transport is small. In a selected subgroup of 31 healthy subjects the plasma VLDL triglyceride concentration did not exceed 160 mg/100 ml. The range of this group's triglyceride turnover rate was completely comparable with most data reported in the literature for total serum or VLDL triglyceride transport in normal human subjects. When the turnover rate was plotted against the VLDL triglyceride concentration, three kinetic subgroups could be separated in accordance with the earlier experience on total serum triglyceride transport kinetics.     

Suppression by diets rich in fish oil of very low density lipoprotein production in man.

The highly polyunsaturated fatty acids in fish oils lower the plasma triglyceride concentration. We have studied the effect of a diet rich in fish oil on the rate of production of the triglyceride-transporting very low density lipoprotein (VLDL). Seven subjects, five normal and two with hypertriglyceridemia received up to 30% of daily energy needs from a fish oil preparation that was rich in eicosapentaenoic acid and docosahexaenoic acid, omega-3 fatty acids with five and six double bonds, respectively. Compared with a diet similarly enriched with safflower oil (in which the predominant fatty acid is the omega-6 linoleic acid, with two double bonds), the fish oil diet lowered VLDL lipids and B apoprotein concentrations profoundly. High density lipoprotein lipids and A1 apoprotein were also lowered, but the effect on low density lipoprotein (LDL) concentration was inconsistent. The daily production or flux of VLDL apoprotein B, calculated from reinjected autologous 125I-labeled lipoprotein, was substantially less in six subjects studied after 3 wk of fish oil, compared with after safflower oil. This effect on flux was more consistent than that on the irreversible fractional removal rate, which was increased in the four normolipidemic but inconsistent in the hypertriglyceridemic subjects. This suggests that fish oil reduced primarily the production of VLDL. The daily production of VLDL triglyceride, calculated from the kinetics of the triglyceride specific radioactivity-time curves after [3H]glycerol was injected, also showed very substantial reductions in five subjects studied. The marked suppression in VLDL apoprotein B and VLDL triglyceride formation was found not to be due to diminished plasma total free fatty acid or plasma eicosapentaenoic flux, calculated during constant infusions of [14C]eicosapentaenoic acid and [3H]oleic acid in four subjects. In two subjects there was presumptive evidence for substantial independent influx of LDL during the fish oil diet, based on the precursor-product relationship between the intermediate density lipoprotein and LDL apoprotein B specific radioactivity-time curves.     

Composition of very low density lipoproteins and in vitro effect of lipoprotein lipase.

In order to clarify the relationship between composition and lipolytic responses to lipoprotein lipase (LPL), very low density lipoproteins (VLDL) from rats or humans were incubated with a commercially available LPL or with a partially purified LPL from postheparin human plasma and fatty acids released from VLDL were determined in vitro. VLDL from rats fed a diet containing 0.25% cholesterol for 6 months were rich in cholesterol and poor in triglycerides, and released less fatty acids from incubation with LPL than those from control rats. VLDL from normo-and hypertriglyceridemic human subjects were incubated with LPL. The fatty acid release poorly correlated with the apoprotein ratios of VLDL, apo C-III/C-II, B/E, and C/E with the exception of apo B/C, but it correlated well with the ratio of triglyceride/either one of the surface components including total apoproteins, free cholesterol and phospholipids in VLDL or the ratio of the triglyceride/total sum of the surface components. The correlation coefficients between fatty acid release and a ratio of triglyceride/total surface components were 0.774 (using the commercially available LPL) and 0.786 (using the partially purified human LPL). The fatty acid release increased after pretreatment of VLDL with phospholipase A2. The phospholipid content of VLDL was reduced without significant changes in other VLDL components. Thus, the responses of VLDL to LPL treatment may depend mainly upon the surface: core relationship of VLDL rather than its apoprotein composition except in rare clinical cases such as apo C-II deficiency.     

Separation of two serum very low density lipoprotein fractions using starch block electrophoresis.

A quantitative electrophoretic method has been developed in order to differentiate very low density (VLDL) pre-beta lipoproteins from late pre-beta lipoproteins using starch as a supporting medium. It was possible to obtain a bimodal distribution of lipoprotein lipids from VLDL which on agarose gel electrophoresis had a pre-beta band and a late pre-beta band. Optimal conditions were: ammonium carbonate buffer, mu = 0.025, dialysis prior to electrophoresis. Agarose gel electrophoresis demonstrated that the fast and slow components obtained on starch block electrophoresis corresponded to the pre-beta and late pre-beta band respectively. With increasing migration towards the anode the ratio of cholesterol to triglycerides decreased continuously. It is suggested that the fast triglyceride rich component represent newly secreted VLDL species and the slower component mainly postlipolytic particles. The pre-beta band on agarose gel electrophoresis might represent more newly secreted VLDL than the late pre-beta band. However, it cannot be excluded that part of late pre-beta lipoproteins may be secreted de novo.     

Comparison of glucosylated low density lipoprotein with methylated or cyclohexanedione-treated low density lipoprotein in the measurement of receptor-independent low density lipoprotein catabolism.

We previously showed that glucosylation of lysine residues of low density lipoproteins (LDL) blocks high-affinity degradation by cultured human fibroblasts, and markedly slows LDL turnover in guinea pigs. The present studies were done to evaluate glucosylated (GLC) LDL as a tracer of receptor-independent LDL catabolism, and to compare it with two other modified LDL, methylated (MET) LDL, and cyclohexanedione (CHD)-treated LDL, which have been used previously for this purpose. Glucosylation of LDL did not affect receptor-independent degradation in vivo, as the turnover of GLC-LDL and native LDL were similar in the LDL receptor-deficient, Watanabe heritable hyperlipidemic rabbit. Each modified radiolabeled LDL preparation was injected into eight guinea pigs, and fractional catabolic rates (FCR) determined. The FCR of GLC-LDL (0.024 +/- 0.005 h-1; SD) was similar to that of MET-LDL (0.023 +/- 0.006 h-1), and approximately 22% of that of native LDL (0.105 +/- 0.02 h-1). The FCR of CHD-LDL was greater than that of the other modified LDL, and it varied depending on how soon after preparation the CHD-LDL was injected: when used within 2 h of preparation, the mean FCR was 0.044 +/- 0.007 h-1 (n = 4); when used after overnight dialysis at 4 degrees C, the mean FCR was 0.082 +/- 0.03 h-1 (n = 4). This suggests that CHD-LDL overestimates the amount of LDL degraded by receptor-independent pathways, perhaps because the CHD modification is spontaneously reversible. The present studies indicate that GLC-LDL is a useful tracer of receptor-independent LDL catabolism in animals.     

Electrophoretic mobility in agarose of very low density lipoprotein subfractions in type III hyperlipoproteinaemia.

The electrophoretic mobilities in agarose gel of the very low density lipoprotein (VLDL) subfractions of Sf greater than 100, 60-100 and 20-60 from six subjects with type III hyperlipoproteinaemia (HLP) have been compared with those from eight normal volunteers. In type III HLP beta or near-beta (slower VLDL) electrophoretic mobility was not necessarily confined to the VLDL fraction of Sf 20-60 in which it may normally be detected.     

Rabbit beta-migrating very low density lipoprotein increases endothelial macromolecular transport without altering electrical resistance.

Rabbit aortic endothelial cells (RAEC) were grown on micropore filters in a new device. This system allowed in situ measurement of transendothelial electrical resistance (TEER). The monolayers demonstrated a TEER of 14 +/- 1 omega X cm2 at confluence. No difference was seen in the transport of low density lipoproteins (LDL) across endothelial cell monolayers obtained from normal or Watanabe heritable hyperlipidemic rabbits, indicating that the LDL receptor was not involved in the LDL transport. TEER was inversely correlated with 22Na transport (r2 = 0.93, P = less than 0.001) but not with 125I-LDL transport. The amount of LDL transported at 15 degrees C or across glutaraldehyde-fixed monolayers was half that of the controls at 37 degrees C. Preincubation of the monolayers with rabbit beta-migrating very low density lipoproteins (beta-VLDL) increased cholesterol content by 65%, and the transport of albumin and LDL doubled without a change in TEER. Removal of beta-VLDL from the culture medium resulted in the return of cellular cholesterol content and LDL transport to control values. We conclude that preincubation of RAEC with beta-VLDL resulted in an increased permeability to LDL and albumin, and that beta-VLDL may promote increased transendothelial transport of macromolecules in cholesterol-fed rabbits.     

Mechanism of regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by low density lipoprotein in human lymphocytes

Abstract. The mechanism of action of low density lipoprotein (LDL) on the rate of sterol synthesis and on the activity of its rate-determining enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase-(HMG-CoA reductase) was studied in lymphocytes freshly isolated from normal subjects and patients heterozygous for familial hypercholesterolaemia. Incubation of normal cells in lipid-depleted serum led to a substantial rise in the rate of sterol synthesis from [¹⁴C]acetate. Addition of cycloheximide (20 μg/ml), a translational inhibitor, reduced sterol synthesis; addition of LDL (100 μg LDL-cholesterol/ml) had a similar effect as cycloheximide in reducing sterol synthesis with a half-life of about 3 h. Cordycepin (50 μg/ml) inhibited messenger RNA synthesis by more than 50% but had no inhibitory effect on the induction of sterol synthesis suggesting that the induction of the pathway is independent of newly synthesized messenger RNA. Lymphocytes from patients heterozygous for familial hypercholesterolaemia behaved similarly to cells from normal subjects in that cycloheximide reduced the incorporation of [¹⁴C]acetate into sterols, while cordycepin had no effect on the induction of sterol synthesis mediated by lipid-depleted serum. Since sterol synthesis from [¹⁴C]acetate can be taken as a measure of HMG-CoA reductase activity under our experimental conditions, the results suggest that: (i) the increase HMG-CoA reductase activity in cells incubated in the presence of lipid-depleted serum is due to increased de novo synthesis of the enzyme which can rapidly be inhibited by the addition of LDL, and (ii) in lymphocytes from normal subjects and patients heterozygous for familial hypercholesterolaemia the induction of HMG-CoA reductase by lipid-depleted serum and, by implication, the subsequent repression of the enzyme by LDL cannot be accounted for by a corresponding increase or decrease in the synthesis of messenger RNA.     

Effect of clofibrate on the composition of very low and low density lipoprotein subfractions in type III hyperlipoproteinaemia.

The effect of clofibrate on the lipid and protein composition of very low and low density lipoprotein subfractions (VLDL of SF greater than 100, 60-100 and 20-60; LDL of Sf 10.4-20, 5.7-12 and 3.5-6.5), was investigated in 6 patients with type III hyperlipoproteinaemia (HLP). After four weeks of therapy significant reductions occurred in the concentration of cholesterol in each VLDL fraction, and of triglycerides in Sf greater than 100 and Sf 60-100 VLDL. No changes were found in the concentrations of apolipoprotein B or of the total tetramethylurea (TMU) soluble proteins, but in four patients in whom polyacrylamide disc gel electrophoresis of the TMU soluble proteins was carried out, it was found that arginine-rich peptide (ARP) had largely disappeared on therapy. These findings would be in keeping with increased catabolism of VLDL in response to clofibrate. No significant changes were observed in LDL lipid or protein concentrations.     

The metabolism of low density lipoprotein in familial type II hyperlipoproteinemia

The metabolism of low density lipoprotein (LDL, beta lipoprotein) was studied in 10 normal individuals and 10 patients with familial type II hyperlipoproteinemia using purified radioiodinated LDL. Over 97% of the label was bound to the protein moiety of LDL and therefore the turnover data reflect the fate and distribution of LDL-apoprotein. Comparison of the metabolic behavior of biologically screened and unscreened labeled LDL preparations in dogs as well as the analysis of the urinary excretion of radioiodide derived from labeled LDL degradation in humans indicated that no significant denaturation resulted from the isolation, purification, and labeling techniques.The plasma concentration of LDL-cholesterol in normals was 105+/-21 mg/100 ml (mean +/-1 SD) in contrast to 254+/-47 mg/100 mg in patients with type II hyperlipoproteinemia; these values corresponded to LDL-apoprotein concentrations of 63+/-13 mg/100 ml and 153+/-30 mg/100 ml, respectively. Despite these differences in concentration, the synthetic rate of LDL-apoprotein in both groups was not significantly different (14.43+/-1.75 mg/kg per day in normals vs. 15.01+/-1.71 mg/kg per day in type II) nor was there any difference in the fraction of the total exchangeable LDL which was in the intravascular space (68.4+/-4.3% vs. 73.3+/-5.2%). However, the fractional catabolic rate of LDL in normal individuals differed significantly from that of patients with type II hyperlipoproteinemia (0.462+/-0.077/day in normals vs. 0.237+/-0.044/day in type II) and correspondingly the biological half-life of LDL was significantly prolonged (3.08+/-0.35 days normals vs. 4.68+/-0.44 days in type II).These data indicate that the pathologic elevation of plasma LDL concentration in the individuals with type II hyperlipoproteinemia studied here is due to a decreased fractional rate of LDL degradation rather than to an abnormality of LDL synthesis. This defect of catabolism may be the primary defect in type II hyperlipoproteinemia or, alternatively, may be secondary to an underlying abnormality in lipid metabolism.