Effects of simvastatin on apoB metabolism and LDL subfraction distribution

Metabolism and drug interactions of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in transplant patients: are the statins mechanistically similar?

3-Hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) inhibitors are the most effective drugs to lower cholesterol in transplant patients. However, immunosuppressants and several other drugs used after organ transplantation are cytochrome P4503A (CYP3A, EC 1.14.14.1) substrates. Pharmacokinetic interaction with some of the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, specifically lovastatin and simvastatin, leads to an increased incidence of muscle skeletal toxicity in transplant patients. It is our objective to review the role of drug metabolism and drug interactions of lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, and cerivastatin. In the treatment of transplant patients, from a drug interaction perspective, pravastatin, which is not significantly metabolized by CYP enzymes, and fluvastatin, presumably a CYP2C9 substrate, compare favorably with the other statins for which the major metabolic pathways are catalyzed by CYP3A.

Relationships between insulin resistance and lipoproteins in nondiabetic African Americans, Hispanics, and non-Hispanic whites: the Insulin Resistance Atherosclerosis Study

The study purpose was to explore the association between dyslipidemia and insulin resistance in three ethnic groups. The Insulin Resistance Atherosclerosis Study (IRAS) is a multicenter epidemiologic study conducted at four clinical centers in California, Texas, and Colorado. The study population for this analysis consisted of 931 non-Hispanic white, African American, and Hispanic men and women (aged 45 to 64 years) without diabetes. The IRAS clinical examinations included lipoprotein measures, a 75-g glucose tolerance test, and the frequently sampled intravenous glucose tolerance (FSIGT) test. The results show a consistent relationship between insulin-mediated glucose disposal and dyslipidemia in African American, Hispanic, and non-Hispanic white men and women. Further, LDL size was inversely associated with insulin resistance in all three ethnic groups. These findings indicate that dyslipidemia is a fundamental part of the insulin resistance syndrome in all of the ethnic groups studied.

Evidence based approach for the management of mixed hyperlipidaemia

There is now a large body of evidence from multiple clinical trials confirming that lowering plasma low density lipoprotein (LDL)-cholesterol results in a clinically significant reduction in coronary heart disease (CHD) risk. These include the Scandinavian Simvastatin Survival Study and the West of Scotland Study. However, further analyses of the Helsinki Heart Study (HHS) have provided additional detail on the relationship between other lipid changes and incidence of CHD. In the HHS, the reduction of CHD events was related not only to the LDL-cholesterol lowering ability of the drug used, but also to its HDL-cholesterol raising and triglyceride lowering effects. Furthermore, a recent comparison of the levels of atheroma regression associated with different drug classes reveals that, while statins produce much greater changes in total and LDL-cholesterol, fibrates have the most marked effect on coronary artery minimum lumen diameter changes. In practice, the overall CHD risk of patients should be reduced by a co-ordinated approach to management of all the correctable risk factors. As part of this approach, the lipid profile should be assessed and normalised initially through the instigation of lifestyle changes, and if necessary, the adjunctive use of lipid regulating drugs. While the lowering of LDL-cholesterol is clearly important, the significance of lowering plasma triglyceride should not be overlooked.

Inhibition of cholesterogenesis decreases hepatic secretion of apoB-100 in normolipidemic subjects

We examined the effect of simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on the kinetics of very low-density lipoprotein apolipoprotein B-100 (VLDL apoB) in 13 normolipidemic men in a placebo-controlled crossover study. Simvastatin significantly decreased the plasma concentrations of low-density lipoprotein (LDL) cholesterol by 36%, triglycerides by 26%, mevalonic acid by 34%, and lathosterol by 32%. Hepatic secretion of VLDL apoB was measured using a primed constant intravenous infusion of [1-13C]leucine with monitoring of isotopic enrichment of apoB by gas chromatography-mass spectrometry; fractional turnover rate was derived using a monoexponential function. Simvastatin decreased VLDL apoB pool size by 53% and the hepatic secretion rate of VLDL apoB by 46% but did not significantly alter its fractional catabolism. The change in hepatic VLDL apoB secretion was significantly and independently correlated with changes in plasma mevalonic acid and lathosterol concentrations and the lathosterol-to-cholesterol ratio. The data support the hypothesis that the rate of de novo cholesterol synthesis directly regulates the hepatic secretion of VLDL apoB in normal subjects.

Specific reduction of plasma large, light low-density lipoprotein by a bile acid sequestering resin, cholebine (MCI-196) in type II hyperlipoproteinemia

The effect of a bile acid sequestrant, cholebine (3 g/day), on plasma lipoprotein subfractions was investigated in 16 patients with type II hyperlipoproteinemia. Activities of low density lipoprotein (LDL)-receptor and activities of lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) were assayed to address the mechanism of cholebine-induced changes in plasma lipoprotein subfractions. Twelve weeks of treatment with cholebine reduced plasma levels of total cholesterol (TC) and LDL-cholesterol (C) by 8.3 +/- 8.1% (mean +/- S.D.) and 14.4 +/- 11.9%, respectively (P < 0.001), but did not affect plasma levels of high density lipoprotein (HDL)-C. Cholebine significantly reduced plasma levels of LDL1-C (1.019 < d < 1.045) by 22.9 +/- 18.9% (P < 0.001) but did not affect plasma levels of very low density lipoprotein (VLDL)-C, intermediate density lipoprotein (IDL)-C, LDL2-C (1.045 < d < 1.063), HDL2-C, and HDL3-C (d > 1.125). Gradient polyacrylamide gel electrophoresis (PAGE) revealed that cholebine reduced large LDL in plasma but had almost no effects on small LDL and HDL subfractions. Cholebine did not alter the activities of LCAT and CETP. LDL-receptor activities of cultured lymphocytes negatively correlated with the reduction in plasma levels of LDL-C (r = -0.500, P < 0.05), IDL-C (r = -0.581, P < 0.02), and LDL1-C (r = -0.610, P < 0.01), respectively. Thus, cholebine seems to reduce further the plasma levels of IDL and large, light LDL in patients with lower LDL-receptor activities. We conclude that cholebine only reduces plasma levels of large, light LDL. This may be due to the stimulation of hepatic LDL-receptor activity.

Clinical pharmacokinetics of 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is the key enzyme of cholesterol synthesis. HMG-CoA reductase inhibitors are potent reversible inhibitors of this enzyme, which act by competing for the substrate HMG-CoA. This review is mainly devoted to the 4 main HMG-CoA reductase inhibitors used today: lovastatin, simvastatin, pravastatin and fluvastatin. Depending upon the dosage, these drugs are able to reduce plasma cholesterol levels by more than 40%. After absorption, each undergoes extensive hepatic first-pass metabolism. Up to 5 primary metabolites are formed, some of which are active inhibitors. The elimination half-lives vary from 0.5 to 3.5 hours and excretion is mainly via the faeces. A limited number of drug interactions has been reported. Increases in liver enzymes and muscle creatine kinase activity are among the most severe adverse effects. These powerful drugs should be reserved for patients with high plasma cholesterol levels and/or those with cardiovascular disease. New therapeutic approaches to atherosclerosis are currently under investigation. HMG-CoA reductase inhibitors are the cornerstone of this research.

The role of stable isotopes in the investigation of plasma lipoprotein metabolism

The last 5 years have seen promising beginnings of the application of stable-isotope-based methods to the study of lipoprotein metabolism. Many aspects of plasma lipid transport make it an attractive system for investigating by this means. While early efforts borrowed from standard techniques of generating and interpreting kinetic parameters (FSRs), the shortcomings of these procedures as applied to lipoproteins are now appreciated. Lipoprotein heterogeneity requires that multicompartmental analysis and relatively long-term studies be employed if the information obtained is going to be a useful adjunct to that produced by radio-iodinated lipoprotein tracers. Both approaches-stable-isotope-labeled endogenous tracer and ex vivo radio-iodinated lipoprotein experiments--must be considered complementary. The first provides direct information on lipoprotein synthesis pathways while the latter is superior at following interconversions and catabolic events. Excellent agreement has been demonstrated where the methods have been used simultaneously to estimate the same kinetic parameter. Many of the questions that have arisen from decades of radioactive tracer studies relate to the nature and rate of lipoprotein synthesis, e.g. what kinds of particles are produced by the liver when different diets are taken, and how are the synthetic pathways altered in dyslipidaemic states? Endogenous tracers can address these issues, and methods are presently available which provide the means for measuring the production of all apolipoproteins and for estimating cholesterol and fatty acid biosynthesis. When techniques are developed to examine triglyceride, cholesteryl ester and phospholipid production then a much clearer picture of how lipoproteins are assembled in vivo will emerge. This kind of information will be essential to an understanding of regulation of plasma lipid transport and the subtle changes that occur in those at risk for coronary heart disease.

Low-density lipoprotein heterogeneity

As a key feature of the ALP, a raised level of small, dense LDL forms part of what is potentially the most common collection of lipoprotein abnormalities to influence the risk of CHD in the general population. The consistency of the association between a prevalence of small, dense LDL and increased risk of CHD is impressive, though the practical constraints of our current methods would limit the clinical application of this information for screening purposes. This highlights the need to elucidate the underlying metabolic and genetic determinants of LDL heterogeneity and to develop alternative forms of analysis based on the structural and functional properties of LDL subclasses and their genotypes. The relationship between enhanced post-prandial lipaemia and raised VLDL is of particular relevance in providing insight into the way in which diet and drugs may target LDL heterogeneity via the insulin-dependent regulation of the post-prandial response. In alluding to the impact of genetic influences on LDL subclasses, twin studies identify a major role for environmental factors as determinants of LDL heterogeneity and, more importantly, as modulators of environmentally susceptible genes. The potential for interactions between dietary factors alone and putative LDL heterogeneity genes is considerable and yet poorly understood, as, for example, the reduced penetrance of the 'ATHS' gene caused through variation in dietary fat and carbohydrate (Nishina et al, 1992). Finally, small, dense LDL has been implicated in at least two major steps of the atherogenic process. Elucidation of the molecular basis of these interactions will be crucial for the identification of genetically susceptible individuals and in the design of appropriate diets and treatment to reduce the risk of CHD mediated through LDL.

Simvastatin. A reappraisal of its pharmacology and therapeutic efficacy in hypercholesterolaemia

Simvastatin is an HMG-CoA reductase inhibitor used in the treatment of patients with hypercholesterolaemia. Since the time simvastatin was previously reviewed in Drugs, a number of large clinical trials have confirmed its clinical efficacy. Thus, reductions from baseline were approximately 20 to 40% for serum levels of total cholesterol, 35 to 45% for low density lipoprotein (LDL)-cholesterol and 10 to 20% for triglycerides in patients with primary hypercholesterolaemia receiving simvastatin 10 to 40 mg/day. High density lipoprotein (HDL)-cholesterol levels were increased modestly by about 5 to 15%. Recent data from long term studies indicate that little or no attenuation of these changes in serum lipid and lipoprotein levels occurred with administration of simvastatin for 3 to 5.4 years. Comparative studies with other HMG-CoA reductase inhibitors (lovastatin, pravastatin and fluvastatin), which were lacking at the time of the previous review of simvastatin, demonstrated greater reductions in serum levels of total cholesterol and LDL-cholesterol with simvastatin than equal dosages of lovastatin or pravastatin. Reductions in serum levels of total cholesterol and LDL-cholesterol were similar between agents only when lovastatin or pravastatin were administered at a total daily dosage twice that of simvastatin and when fluvastatin was administered at a total daily dosage approximately 8 times that of simvastatin. In general, simvastatin 10 to 40 mg/day was also more effective than standard dosages of bile acid sequestrants, fibrates or probucol in lowering serum levels of total cholesterol and LDL-cholesterol; however, fibrates usually produced greater reductions in serum triglycerides and greater elevations in HDL-cholesterol levels. The Scandinavian Simvastatin Survival Study (4S), a large secondary prevention study in patients with coronary heart disease and concomitant hypercholesterolaemia, demonstrated that simvastatin 20 to 40 mg/day for a median of 5.4 years significantly reduced overall mortality (the primary end-point of the study) by 30% compared with placebo, which was attributed to a 42% relative reduction in coronary mortality. Coronary morbidity was also significantly reduced by simvastatin in the 4S trial. The tolerability profile of simvastatin appears to be comparable to that of other HMG-CoA reductase inhibitors. The most frequently reported adverse events are gastrointestinal disturbances, which are generally mild and tend to occur less frequently than with cholestyramine. In conclusion, simvastatin is among the most effective agents available for treating patients with hypercholesterolaemia.(ABSTRACT TRUNCATED AT 400 WORDS)

Simvastatin decreases the hepatic secretion of very-low-density lipoprotein apolipoprotein B-100 in heterozygous familial hypercholesterolaemia: pathophysiological and therapeutic implications

We studied six patients with heterozygous familial hypercholesterolaemia (FH) before and after 8 weeks of treatment with simvastatin (40 mg day-1), an inhibitor of 3-hydroxy-3-methyl-glutaryl-Coenzyme A. Simvastatin decreased plasma low-density lipoprotein (LDL) cholesterol by 43% (P = 0.002), triglycerides by 15% [corrected] (P = 0.05) and mevalonic acid (a measure of in vivo cholesterol synthesis) by 20% (P = 0.002); high-density lipoprotein cholesterol increased by 17% (P = 0.02). The hepatic secretion rate of very-low-density lipoprotein apolipoprotein B-100 (VLDL apoB) was measured directly using a primed, constant intravenous infusion of 1-[13C]-leucine with monitoring of the isotopic enrichment of apoB by gas chromatography-mass spectrometry; fractional secretion rate (FSR) was derived using a monoexponential function. Simvastatin decreased the FSR, ASR and pool size of VLDL apoB by 17% (14.3 (SEM 3.6)) vs. (11.9 (SEM 3.5) pools day-1, P = 0.10), 83% (51.4 (SEM 17.9) vs. (8.6 (SEM 1.4), P = 0.007 mg kg-1 day-1) and 65% (234.2 (SEM 30.4) vs. 82.6 (SEM 24.0) mg, P = 0.02), respectively. The change in the ASR of VLDL apoB was significantly correlated with the change in plasma LDL cholesterol concentration (P = 0.04), but not with the change of triglyceride or mevalonic acid. We conclude that the hepatic secretion of VLDL apoB in FH is decreased by simvastatin, which may partly explain the fall in plasma cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of simvastatin on plasma lipoprotein subfractions, cholesterol esterification rate, and cholesteryl ester transfer protein in type II hyperlipoproteinemia

We investigated the effects of simvastatin on plasma levels of lipoprotein subfractions, cholesterol esterification rates and activities of cholesteryl ester transfer protein in 28 patients with type II hyperlipoproteinemia (i.e., nonfamilial hyperlipoproteinemia type IIa and type IIb, and heterozygous familial hypercholesterolemia (FH)). Plasma levels of VLDL-cholesterol (C) and VLDL-triglyceride (TG) were significantly reduced overall by 12.9 +/- 58.0% (mean +/- S.D.; P < 0.05) and 4.2 +/- 54.2% (P < 0.05) respectively, but not in FH. Plasma levels of IDL-C and IDLT-G were decreased overall by 23.2 +/- 47.5% (P < 0.001) and 12.3 +/- 49.7% (P < 0.05), respectively, again mainly due to decreases seen in nonfamilial type II hyperlipoproteinemia. Plasma levels of LDL1 (1.019 < d < 1.045)-C and LDL1-TG were significantly reduced by 33.1 +/- 12.9% (P < 0.001) and 23.3 +/- 24.7% (P < 0.001), respectively. Plasma levels of LDL2 (1.045 < d < 1.063)-C were significantly reduced by 22.9 +/- 18.1% (P < 0.001) overall but not in FH. Gradient PAGE showed no consistent changes in the distribution of LDL particles. Thus, plasma levels of all apo B-containing lipoprotein subfractions were reduced by simvastatin, but its effects varied among the three subgroups. Cholesterol esterification rates were suppressed by 9.3 +/- 19.7% (P < 0.01) and activities of cholesteryl ester transfer protein were reduced by 30.6 +/- 21.5% (P < 0.001). Changes in CETP activity and in plasma levels of cholesterol in lipoprotein subfractions were not correlated. Thus, the changes in distribution of lipoprotein subfractions were not due mainly to CETP suppression.

Fasting and postprandial determinants for the occurrence of small dense LDL species in non-insulin-dependent diabetic patients with and without hypertriglyceridaemia: the involvement of insulin, insulin precursor species and insulin resistance

We have studied low density lipoprotein (LDL) subclass distribution in a group of male patients with non-insulin-dependent diabetes mellitus (NIDDM) and investigated its relationships to fasting and postprandial triglyceride (TG)-rich lipoproteins, insulin resistance, lipoprotein lipase (EC 3.1.1.3; LPL), hepatic lipase (EC 3.1.1.34; HL), lecithin:cholesterol acyl transferase (EC 2.3.1.43; LCAT) and cholesteryl ester transfer protein (CETP) activities. LDL was subfractionated by density gradient ultracentrifugation. Postprandial lipoproteins were measured after an oral fat load using retinyl palmitate as a marker for intestinal TG-rich lipoproteins. Hypertriglyceridaemic NIDDMs (HTG) had a preponderance of small dense LDL particles present in the plasma and reduced amounts of large buoyant species when compared to normotriglyceridaemic patients (NTG) and controls. Both groups of diabetics were more insulin resistant than the controls (P < 0.05) and had raised concentrations of proinsulin (P < 0.05), although insulin content did not differ significantly. 32-33 split proinsulin (SPI) was the major insulin-like molecule present in HTG and was present in significantly higher amounts in these patients (P < 0.05) than either NTG or control subjects and correlated significantly with the presence of small dense LDL particles. After a test meal, the postprandial chylomicron response was greater in HTG than either NTG diabetics or controls (P < 0.05). Chylomicron remnants were present to a greater extent in HTG than in NTG and controls (P < 0.05), although in this case NTG also contained more chylomicron remnants than control subjects (P < 0.05). There was no difference in the LPL activity, CETP and LCAT between diabetics and controls, whereas an increase in hepatic lipase activity was seen in the HTG diabetics (P < 0.05). Both CETP and LCAT activities increased postprandially. Multivariate analysis showed that TG, HDL content and HL activity were the most important determinants of small dense LDL concentration in the fasting state (R2 = 67%). Postprandially, chylomicron remnant clearance, HL and insulin resistance were the major determinants (R2 = 61%) of LDL-III.

Comparison of gemfibrozil versus simvastatin in familial combined hyperlipidemia and effects on apolipoprotein-B-containing lipoproteins, low-density lipoprotein subfraction profile, and low-density lipoprotein oxidizability

We evaluated in a double-blind, placebo-controlled, randomized trial of 45 well-defined patients with familial combined hyperlipidemia, the effect of gemfibrozil (1,200 mg/day) or simvastatin (20 mg/day) on apolipoprotein-B (apo-B)-containing lipoproteins, low-density lipoprotein (LDL) subfraction profile, and LDL oxidizability. Although both drugs reduced plasma cholesterol and triglyceride concentrations, gemfibrozil reduced plasma triglycerides more effectively and simvastatin reduced plasma cholesterol more effectively. LDL cholesterol was reduced with simvastatin. With both drugs, total serum apo-B concentration decreased. With gemfibrozil, this was due to an exclusive reduction (-46%) of very low/intermediate-density lipoprotein (VLDL + IDL) apo-B, whereas simvastatin decreased apo-B in both VLDL + IDL and LDL (34% and 15%, respectively). Initially, a dense LDL subfraction profile was present in all patients. The decrease in LDL cholesterol with simvastatin was due to a decrease in all isolated LDL subfractions except LDL2; gemfibrozil increased LDL1 and LDL2 cholesterol (p = 0.001) and reduced LDL4 cholesterol, resulting in a more buoyant LDL subfraction profile compared with simvastatin. In both groups, a predominance of small dense LDL remained despite therapy. LDL fatty acid composition showed a shift from oleic acid to linoleic acid after gemfibrozil; arachidonic acid increased after simvastatin. Vitamin E was lower after gemfibrozil. In the measurements of LDL oxidation, only the oxidation rate was significantly reduced with simvastatin. Thus, quantitative and qualitative changes of LDL cholesterol had only a small effect on total in vitro LDL oxidizability in this population with familial combined hyperlipidemia.

Pravastatin effectively lowers LDL cholesterol in familial combined hyperlipidemia without changing LDL subclass pattern

Familial combined hyperlipidemia (FCHL) is the most common genetic lipid disorder among young survivors of myocardial infarction. Elevations of plasma total and low-density lipoprotein (LDL) cholesterol and the prevalence of small, dense LDL particles are both involved in the high coronary risk of FCHL patients. We investigated the ability of pravastatin to favorably correct plasma lipid and lipoprotein levels and LDL structure in FCHL patients. Twelve patients with FCHL, documented by studies of first-degree relatives, received pravastatin (40 mg/d) for 12 weeks. Pravastatin significantly lowered plasma total and LDL cholesterol levels by 21% and 32%, respectively. Triglyceride levels did not change, and apolipoprotein B (apoB) concentrations decreased by 9% (P = NS). High-density lipoprotein (HDL) cholesterol increased by 6% because of a significant 73% rise of HDL2 cholesterol. LDL were smaller (diameter, 24.5 +/- 0.5 nm), less buoyant, and apoB-rich (cholesteryl ester-apoB ratio, 1.64 +/- 0.46) in the selected patients compared with patients with familial hypercholesterolemia or healthy control subjects. LDL became even smaller (23.8 +/- 0.6 nm) and richer in apoB (cholesteryl ester-apoB ratio, 1.27 +/- 0.52) after pravastatin treatment. Although pravastatin favorably altered plasma lipid and lipoprotein levels in FCHL patients, the abnormal LDL particle distribution and composition were not affected. Because of the apparent resistance of the small, dense LDL to drug-induced modifications, a maximal lipid-lowering effect is needed to reduce coronary risk in FCHL patients.

Inhibition of hepatic ACAT decreases ApoB secretion in miniature pigs fed a cholesterol-free diet

To test the hypothesis that hepatic cholesteryl ester is involved in the regulation of apolipoprotein (apo) B secretion into plasma, apoB kinetic studies were performed in six control miniature pigs and in six pigs after a 21-day administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg.kg-1.d-1 i.v.). Pigs were fed low-fat, cholesterol-free diets. Total plasma cholesterol, triglyceride, very-low-density lipoprotein (VLDL) triglyceride, and low-density lipoprotein (LDL) cholesterol decreased 18%, 29%, 40%, and 26% respectively (P < .03). 131I-VLDL and 125I-LDL were injected simultaneously into each animal, and apoB kinetics were analyzed by using multi-compartmental analysis (SAAM30). VLDL apoB pool size decreased significantly by 60% (0.32 versus 0.84 mg/kg), which was due to a 65% reduction in the VLDL apoB production or secretion rate (1.03 versus 2.94 mg.kg-1.h-1). The fractional catabolic rate was unchanged. LDL apoB pool size decreased nonsignificantly by 18% (5.61 versus 6.90 mg/kg) due entirely to a 24% decrease in production rate (0.26 versus 0.34 mg.kg-1.h-1). At necropsy, hepatic microsomal ACAT activity decreased by 68% (0.28 versus 0.88 nmol.min-1.mg-1; P < .0002). Although an increase in hepatic free cholesterol leading to a decreased LDL receptor expression might be expected, this did not occur. The concentration of hepatic cholesterol and the LDL apoB fractional catabolic rate were unaffected by DuP 128. In addition, the concentration of hepatic triglyceride and the activity of diacylglycerol acyltransferase were not altered by DuP 128, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. We conclude that inhibition of hepatic cholesteryl ester synthesis in vivo decreases apoB secretion into plasma.

Serum cholesterol and cholesterol and lipoprotein metabolism in hypercholesterolaemic NIDDM patients before and during sitostanol ester-margarine treatment

Cholesterol absorption and metabolism and LDL and HDL kinetics were investigated in 11 hypercholesterolaemic non-insulin-dependent diabetic men off and on a hypolipidaemic treatment with sitostanol ester, (3 g sitostanol daily) dissolved in rapeseed oil margarine, by a double-blind crossover study design. Serum total, VLDL and LDL cholesterol and apoprotein B fell significantly by 6 +/- 2, 12 +/- 6, 9 +/- 3 and 6 +/- 2%, mean +/- SEM, and HDL cholesterol was increased by 11 +/- 4% (p < 0.05) by sitostanol ester. LDL cholesterol and apoprotein B were significantly decreased in the dense (1.037-1.055 g/ml), but not light, LDL subfraction due to a significantly diminished transport rate for LDL apoprotein B, while the fractional catabolic rate was unchanged. HDL kinetics, measured with autologous apoprotein A I, was unaffected by sitostanol ester. Cholesterol absorption efficiency was markedly reduced from 25 +/- 2 to 9 +/- 2% (p < 0.001) during sitostanol ester followed by proportionately decreased serum plant sterol proportions. Cholesterol precursor sterol proportions in serum, fecal neutral sterol excretion, and cholesterol synthesis, cholesterol transport, and biliary secretion were all significantly increased by sitostanol ester. We conclude that the sitostanol ester-induced decrease in cholesterol absorption compensatorily stimulated cholesterol synthesis, had no effect on fractional catabolic rate, but decreased transport rate for LDL apoprotein B so that serum total, VLDL and LDL cholesterol levels were decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and environmental influences on LDL subclass phenotypes

There is accumulating evidence that subclasses of low-density lipoproteins (LDL) are important in atherosclerosis. Several case-control studies have demonstrated that a predominance of small, dense LDL (LDL subclass phenotype B) is associated with increased risk of coronary heart disease (CHD). Phenotype B is also consistently characterized by an atherogenic lipoprotein phenotype, including increased levels of plasma triglyceride and decreased high-density lipoprotein cholesterol. Family studies and genetic linkage studies demonstrate that LDL subclasses are influenced by a single major gene effect, although this locus (or loci) remain to be definitively mapped. Twin studies confirm the presence of genetic effects, but also show that non-genetic influences are important. Hypolipidemic drugs, beta-blockers, diet and exercise, in particular, appear to influence the expression of LDL subclass phenotypes. This combination of genetic and environmental influences may provide opportunities to develop targeted intervention strategies to reduce CHD risk among genetically susceptible individuals.

Determinants of LDL subfraction distribution and concentrations in young normolipidemic subjects

Human low-density lipoproteins (LDLs) comprise a spectrum of particles that vary in size, density, chemical composition, metabolic behavior, and atherogenicity. To identify determinants of this heterogeneity, we measured the percent distribution and plasma concentration of the three major LDL subfractions in 34 young healthy subjects. These parameters were correlated in univariate and multivariate analyses with various body and lifestyle factors; plasma lipids and lipoprotein; and the activities of cholesteryl ester transfer protein, lipoprotein lipase, and hepatic lipase (HL). Women (n = 15) had significantly more large, buoyant LDL (LDL-I; density, 1.025 to 1.034 g/mL) and high-density lipoprotein2 (HDL2) than men (n = 19). Both the percentage and concentration of LDL-I were correlated negatively with very-low-density lipoprotein triglycerides (VLDL-TG) and HL; they were correlated positively with HDL-cholesterol (HDL-C) and HDL2. In addition, percent LDL-I was negatively correlated with plasma triglycerides, VLDL-C, LDL-C, and apo-lipoprotein (apo) B concentrations. The concentrations of intermediate and small, dense LDL (LDL-II and LDL-III; density, 1.034 to 1.044 and 1.044 to 1.060 g/L, respectively) were positively correlated with LDL-C. LDL-III concentrations were also related to plasma cholesterol and apoB concentrations and HL activity. On multivariate analyses, approximately one third of the variability in LDL-I was explained by HL and plasma triglycerides. More than 80% of the variation in LDL-II was accounted for by a model that combined LDL-C and plasma apoB with body mass index and VLDL-TG.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of plasma triglyceride in the regulation of plasma low density lipoprotein (LDL) subfractions: relative contribution of small, dense LDL to coronary heart disease risk

The concentration of plasma LDL subfractions is described in four groups of normocholesterolaemic (total plasma cholesterol < 6.5 mmol/l) male subjects consisting of men with and without coronary artery disease (CAD+/-), as determined by angiography, post-myocardial infarct survivors (PMI) and normal, healthy controls. The CAD(+) and PMI groups were distinguished from the CAD(-) and controls by raised concentrations of plasma triglyceride, very low density lipoprotein (VLDL) cholesterol, small, dense LDL (LDL-III density (d) 1.044-1.060 g/ml) and lower concentrations of high density lipoprotein (HDL) cholesterol and large, buoyant LDL (LDL-I d 1.025-1.034 g/ml). In all groups, a subfraction of intermediate density, LDL-II (d 1.034-1.044 g/ml), was the predominant LDL species but was not related to coronary heart disease risk. Plasma triglyceride showed a positive association with LDL-II (r = 0.51, P < 0.001) below a triglyceride level of 1.5 mmol/l. Above this threshold of 1.5 mmol/l, LDL-II and LDL-I showed significant negative associations with triglyceride (LDL-II r = -0.5, P < 0.001; LDL-I r = -0.45, P < 0.001). Small, dense LDL-III showed a weak positive association with triglyceride that became highly significant above the 1.5 mmol/l threshold (r = 0.54, P < 0.001). While age was positively related to LDL-II within the control subjects (r = 0.3, P < 0.05), there was no difference in the percentage abundance or concentration of LDL-III within control and CAD(-) subjects above and below the age of 40 years. Smoking was associated with a relative deficiency of the LDL-I subfraction (LDL-I to LDL-III ratio in smokers = 0.77, in ex-smokers = 0.95, in non-smokers = 1.89; P < 0.01), as was beta-blocker medication (% LDL-I, users vs. non-users, P < 0.05). Both of these effects could be explained by their primary influence on plasma triglyceride. Analysis of the frequency distributions for the three LDL subfractions revealed the concentration of small, dense LDL-III to be bimodal around a concentration of 100 mg (lipoprotein mass)/100 ml plasma. The calculation of odds ratios based on this figure indicated relative risk estimates of 4.5 (chi 2: P < 0.01) for the presence of coronary artery disease and 6.9 (chi 2: P < 0.001) for myocardial infarction.(ABSTRACT TRUNCATED AT 400 WORDS)

ApoB metabolism in familial hypercholesterolemia. Inconsistencies with the LDL receptor paradigm

The biology of the low-density lipoprotein (LDL) receptor has been examined in detail, and a paradigm for LDL metabolism has evolved from comparative studies of cholesterol metabolism in a variety of cells cultured from normal individuals and subjects with familial hypercholesterolemia (FH). Cultured cells from patients with homozygous FH lack a functional LDL receptor and show diminished LDL clearance, induction of the enzyme hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase, increased cholesterol synthesis, decreased cholesterol ester production, and depleted cholesterol ester stores. The observed decrease in the fractional catabolic rate (FCR) of LDL is attributed to the mutated LDL receptor gene. However, in the experimental animal model of this disease, the Watanabe heritable hyperlipidemic (WHHL) rabbit, cholesterol ester stores are increased, while hepatic cholesterol synthesis is decreased. Furthermore, in humans HMG-CoA reductase is suppressed, and the LDL apolipoprotein (apo) B production rate is increased in patients with FH. These findings raise questions about the adequacy of the paradigm in understanding hepatic cholesterol metabolism in vivo. In humans, apoB metabolism is believed to be principally determined by the liver, where apoB is both synthesized and catabolized. Assuming the neutral lipid content of the liver is the major determinant of apoB metabolism, we postulated that the changes in apoB metabolism in FH are predictable when based on the assumption of an increase in hepatic cholesterol and cholesterol ester content, as observed both in the WHHL rabbit and in humans. We examined this hypothesis in vivo in patients with heterozygous FH by using tracer kinetic methodology and have used similar data from normal and hypertriglyceridemic (HTG) subjects as controls. Whereas normal and HTG subjects secrete apoB primarily as large, triglyceride-enriched very-low-density lipoprotein (VLDL), heterozygous FH patients have an absolute decrease in apoB production and secrete almost 40% of apoB as smaller intermediate-density lipoprotein (IDL)/LDL. In normal humans, about half of secreted apoB is catabolized rather than being converted to LDL. In HTG subjects two thirds of apoB follows this same route, by which VLDL remnants remaining after triglyceride hydrolysis are largely returned to the liver. In contrast, in FH subjects secreted apoB is fully converted to LDL. Thus, although total apoB secretion is reduced in FH subjects, total LDL production is greater than in either normal or HTG subjects. Under basal conditions the elevated LDL in heterozygous FH is due to both decreased LDL receptor-mediated catabolism and increased LDL production. However, the number of LDL receptors actually expressed is suppressed below the number of potentially functional receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholesterol-lowering drug therapy in a patient with receptor-negative homozygous familial hypercholesterolaemia

Familial hypercholesterolaemia (FH) is caused by mutations in the gene for the low density lipoprotein (LDL) receptor. It is generally believed that homozygous FH patients do not respond well to lipid-lowering drug therapy with inhibitors of 3-hydroxy-3-methylglutaryl CoA reductase because they cannot respond to an increased demand for hepatic cholesterol by up-regulation of LDL-receptor activity. In this paper we show that serum cholesterol in a homozygous FH patient with a receptor-negative LDL-receptor phenotype was reduced by 30% after treatment with simvastatin alone and by a further 11% with simvastatin in combination with probucol and nicotinic acid. The patient was a true homozygote, with two identical alleles of the LDL receptor gene in which a previously undescribed point mutation in exon 11 introduces a premature termination codon at residue 540 in the protein; the mutant protein is predicted to be truncated in the domain with homology to the epidermal growth factor precursor. Cultured cells from the patient were unable to bind, internalise or degrade LDL by the receptor pathway and there was no immunodetectable LDL receptor protein in the cells. Thus the lipid lowering effect of simvastatin in this individual must involve mechanisms other than stimulation of LDL receptors.

The effects of hypercholesterolaemia, simvastatin and dietary fat on the low density lipoprotein receptor of unstimulated mononuclear cells

The in vivo expression and regulation of the LDL receptor of circulating mononuclear cells was studied using a sensitive spectrophotometric assay with low density lipoproteins conjugated to colloidal gold (LDL-gold). The high plasma cholesterol of familial hypercholesterolemic subjects was shown to be related to a low in vivo LDL receptor activity; cells from a homozygote had virtually no activity and those from 24 heterozygotes expressed 45% of the activity of cells from 35 normals. The average receptor activity of cells from 18 polygenic hypercholesterolemic (PH) subjects was not significantly different from normal but a low expression may have been a factor in six of these subjects. Simvastatin increased the LDL receptor activity of cells from the PH subjects by 70% while lowering their plasma cholesterol by 26%, but reducing the fat intake from 38% to 20% of energy and cholesterol from 239 to 96 mg/day had no effect on the receptor despite a 10% reduction in plasma cholesterol. Upregulation of the LDL receptor may therefore have been involved in the lowering of plasma cholesterol by simvastatin but not by the reduction in dietary fat and cholesterol.

Effects of pravastatin on apolipoprotein-specific high density lipoprotein subpopulations and low density lipoprotein subclass phenotypes in patients with primary hypercholesterolemia

The HMG-CoA reductase inhibitor class of cholesterol-lowering agents reduces very low density lipoproteins (VLDL) and low density lipoproteins (LDL) and slightly increases high density lipoproteins (HDL). However, the effects of these agents on subclasses within the LDL and HDL fractions are not well understood. We have employed an HMG-CoA reductase inhibitor, pravastatin, to determine if LDL subclass phenotypes, as determined by gradient gel electrophoresis, and HDL particles containing both apolipoprotein (apo) A-I and A-II, Lp(AI w AII), and those containing apo A-I but not A-II, Lp(AI w/o AII) are affected by pravastatin (10 mg daily). Twenty-four subjects with LDL-cholesterol (LDL-C) > 160 mg/dl, triglyceride (TG) < 350 mg/dl and no recent myocardial infarction or secondary causes of hypercholesterolemia were enrolled. Compared with an age- and sex-matched normolipidemic reference group (controls), the hypercholesterolemic subjects had reduced levels of Lp(AI w/o AII) and increased levels of Lp(AI w AII) at baseline. In addition, both of their HDL subpopulations had significantly more small (7.0-8.2 nm) particles (P < 0.02 and 0.0001) but significantly fewer large (9.2-11.2 nm) particles (P < 0.002 and 0.0001). Pravastatin induced statistically significant (P < 0.001) reductions in plasma total C (15%), LDL-C (18%), and apo B (16%). While apo A-I and A-II levels increased 5% (P < 0.001) and 6% (P < 0.05), respectively, concentration, composition, and size abnormalities in Lp(AI w AII) and Lp(AI w/o AII) persisted. Lp(a), apo E and cholesteryl ester transfer protein (CETP) levels also did not change. Although changes in LDL subclass phenotypes were observed, all changes involved the intermediate phenotype, and no significant changes in LDL peak particle diameter were seen in either group. Interrelationships between CETP, LDL subclass phenotypes and HDL subpopulations were also seen.

CONCLUSIONS

Although pravastatin decreased plasma apo B and LDL lipid concentrations, no major changes were seen in LDL subclass phenotypes or HDL subpopulations even in the presence of abnormalities associated with arteriosclerosis. Similarly, CETP, which is believed to play a role in HDL and LDL particle size distribution, did not change with pravastatin treatment. Further research is needed to determine the pathophysiological basis of abnormal HDL and LDL subclasses in hypercholesterolemia and explore methods of rectifying the abnormalities.