Increased plasma cholesterol esterifying activity during Colestipol resin therapy in man

Lipoprotein cholesteryl ester production, transfer, and output in vivo in humans

Our aim was to identify and quantify the major in vivo pathways of lipoprotein cholesteryl ester transport in humans. Normal (n = 7), bile fistula (n = 5), and familial hypercholesterolemia (FH; n = 1) subjects were studied. Each received isotopic free cholesterol in HDL, LDL, or particulate form, along with another isotope of free or esterified cholesterol or mevalonic acid. VLDL, intermediate density lipoprotein (IDL), LDL, HDL, blood cells, and bile were collected for up to 6 days for analysis of radioactivity and mass of free and esterified cholesterol. These raw data were subjected to compartmental analysis using the SAAM program. Results in all groups corroborated net transport of free cholesterol to the liver from HDL, shown previously in fistula subjects. New findings revealed that 70% of ester was produced from free cholesterol in HDL and 30% from free cholesterol in LDL, IDL, and VLDL. No evidence was found for tissue-produced ester in plasma. There was net transfer of cholesteryl ester to VLDL and IDL from HDL and considerable exchange between LDL and HDL. Irreversible ester output was from VLDL, IDL, and LDL, but very little was from HDL, suggesting that selective and holoparticle uptakes of HDL ester are minor pathways in humans. It follows that 1) they contribute little to reverse transport, 2) very high HDL would not result from defects thereof, and 3) the clinical benefit of high HDL is likely explained by other mechanisms. Reverse transport in the subjects with bile fistula and FH was facilitated by ester output to the liver from VLDL plus IDL.

Study of ABCA1 function in transgenic mice

The ATP-binding cassette transporter A1 (ABCA1), identified in 1999 as the gene defective in Tangier disease, promotes efflux of cellular cholesterol from macrophages and other peripheral tissues to apolipoprotein acceptors. These ABCA1-mediated processes are anticipated to have antiatherogenic properties, prompting the development of pharmacological agents that increase ABCA1 gene expression as well as the establishment of ABCA1-transgenic mouse lines. Preliminary studies of ABCA1-Tg mice seem to validate the selection of this transporter as a therapeutic target for the treatment of low HDL syndromes and cardiovascular disease but have also raised new questions regarding the function of ABCA1. In particular, the relative contribution of hepatic and peripheral ABCA1 to plasma HDL levels and to reverse cholesterol transport, as well as the potential role of ABCA1 in modulating the plasma concentrations of the apolipoprotein B-containing lipoproteins and protecting against atherosclerosis, seem to be promising areas of investigation. The present review summarizes the most recent studies and discusses insights provided by these transgenic mouse models.

Drug control of reverse cholesterol transport

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of excess cholesterol from peripheral tissues to the liver. High-density lipoproteins (HDL) are the vehicle of cholesterol in this reverse transport, a function believed to explain the inverse correlation between plasma HDL levels and atherosclerosis. An attempt to stimulate, by the use of drugs, this transport process may hold promise in the prevention and treatment of arterial disease. Among the agents affecting lipoprotein metabolism, only probucol exerts significant effects on reverse cholesterol transport, by stimulating the activity of the cholesteryl ester transfer protein and, consequently, altering HDL subfraction composition/distribution. Another approach to the stimulation of reverse cholesterol transport consists of raising plasma HDL levels; studies in animals, either by exogenous supplementation or by endogenous overexpression, have shown a consistent benefit in terms of atherosclerosis regression and/or non-progression. Thus, it is time to consider different future treatments of atherosclerosis, combining the classical lipid-lowering treatments with innovative methods to promote cholesterol removal from the arterial wall.

Lecithin: cholesterol acyltransferase activity in familial hypercholesterolemia treated with simvastatin and simvastatin plus low-dose colestipol

In 19 patients with heterozygous familial hypercholesterolemia (FH), the effects of simvastatin (S) 20 mg/d, 40 mg/d, and 40 mg/d plus low-dose colestipol (10 g/d) on plasma lipids, plasma lipoproteins, and plasma lecithin: cholesterol acyltransferase (LCAT) activity were investigated after an original dose-range escalation/descalation design. The drug regimen was changed every 8 weeks. A significant reduction in total cholesterol and LDL-cholesterol was observed, reaching 39% and 54% for the drug combination (week 28), and total apoprotein B and LDL-apoprotein B were reduced by 39% and 50%, respectively. Triglycerides were significantly lowered by S alone (up to 29% with 40 mg/d). HDL-cholesterol increased during therapy but the cholesterol content in HDL2-HDL3 fractions (isolated by ultracentrifugation) did not change significantly during the different steps. The ratio LDL-C/HDL-C fell by 57% at week 28. Plasma LCAT activity expressed as FER was significantly enhanced by S alone (+33%), and a further increase on drug combination regimen (+58%) was observed. This effect could be considered as a consequence of the increased fractional clearance of LDL-C. It tended to be sustained during the descalation part of the study. Biochemical adverse effects were scarce and transient. In conclusion, the combination therapy increased the plasma LCAT/FER activity without a preferential enhancement in HDL2-C concentration. This original design allowed to define the most appropriate individual cholesterol-lowering drug dosage in FH patients.

Reverse cholesterol transport: physiology and pharmacology

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of cholesterol from peripheral tissues to the liver and plays a major role in maintaining cholesterol homeostasis in the body. High density lipoproteins (HDL) are the vehicle of cholesterol in this reverse transport, a function believed to explain the inverse correlation between plasma HDL levels and atherosclerosis. An attempt to stimulate, by the use of drugs, this transport process seems to be of great promise in the prevention and treatment of arterial disease. Only few drugs are now known that can modify the activity of the various factors involved in the process. Clofibrate reduces cholesterol esterification, but the newer fibric acids are generally ineffective as anion-exchange resins. Probucol directly increases the activity and mass of cholesteryl ester transfer protein, thus possibly improving the physiological process of cholesterol removal from tissues. The few available data on the effects of drugs on reverse cholesterol transport should stimulate the search for new agents specifically stimulating this antiatherogenic process.

Low-dose colestipol plus fenofibrate: effects on plasma lipoproteins, lecithin:cholesterol acyltransferase, and postheparin lipases in familial hypercholesterolemia

Effects on plasma lipoproteins, lecithin:cholesterol acyltransferase (LCAT), and postheparin lipase (LPL and HTGL) activities were studied in 18 patients with familial hypercholesterolemia during 8-week treatment periods with colestipol (15 g/d), fenofibrate (0.25 g/d), and colestipol plus fenofibrate. Lipoprotein lipids and apolipoproteins were determined by standard procedures, LCAT by a self-substrate method, and lipases by nonradioisotopic methods. Colestipol and fenofibrate, each given independently, caused similar percentage decreases in LDL cholesterol and apolipoprotein B: -18.4% and -8.6% v -17.4% and -10.6% Colestipol increased the VLDL cholesterol concentration, whereas fenofibrate reduced this parameter but increased HDL cholesterol and apolipoprotein A-I levels. The combination of both drugs led to a substantial fall in LDL cholesterol (-36.8%) and in apolipoprotein B (-28.3%) and maintained the other effects of fenofibrate on VLDL and HDL. Colestipol, given independently or with fenofibrate, produced an increase of the fractional esterification rate of the LCAT enzyme (+25.3% and +36.2%). Fenofibrate stimulated the postheparin LPL enzyme by +16.1% and +21.7%, respectively. This study indicates the complementarity in effectiveness when both drugs were administered together. The appropriate reduction in LDL was combined with the favorable effects on HDL in familial hypercholesterolemia.

Plasma cholesterol esterification rate in hyperlipoproteinaemia: relation to cholesterol elimination

The present study was undertaken to determine the relationship between plasma lecithin: cholesterol acyl transfer (LCAT) rate and cholesterol and bile acid turnover in hyperlipidaemia. Nineteen healthy controls, 19 patients with hyperlipoproteinaemia (HLP) type IIa and 12 patients with HLP type IV were studied under standardized dietary conditions. Bile acid kinetics was determined with the aid of [14C]labelled cholic acid and chenodeoxycholic acid. In the hyperlipidaemic patients, cholesterol balance was calculated as the sum of bile acid synthesis plus daily faecal excretion of neutral C27 steroids minus dietary intake of cholesterol. The plasma LCAT rate was determined simultaneously. The mean values of bile acid formation, cholesterol balance, and LCAT rate in HLP type IV patients exceeded those in HLP type IIa patients or in the controls. An increased plasma LCAT rate was found among HLP type IV patients with and without evidence of cardiovascular disease. Plasma LCAT rate correlated positively with bile acid formation (Rs = +0.78, p less than 0.01) and cholesterol balance (Rs = +0.88, p less than 0.002) in HLP type IV. No such relationships were obtained in the controls or in HLP type IIa. It is suggested that an increased production and/or flux of VLDL in HLP type IV is linked to an enhanced plasma LCAT rate and to an increased formation and metabolism of cholesterol in the liver.

Simvastatin and bezafibrate: effects on serum lipoproteins and lecithin: cholesterol acyltransferase activity in familial hypercholesterolaemia

Sixteen subjects with familial hypercholesterolaemia were randomly assigned to treatment with simvastatin 20-40 mg/day (an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase) or with bezafibrate 600 mg/day (a clofibrate analogue) for 12 weeks. Both drugs produced significant reductions in serum and LDL cholesterol; mean percentage fall -30.5% and -38.1% (simvastatin) and -17.8% and -20.6% (bezafibrate), respectively. Both drugs also caused a decrease in VLDL cholesterol, while only bezafibrate decreased the serum and VLDL triglyceride levels and increased HDL cholesterol and serum apolipoprotein A-I and A-II levels. Serum apolipoprotein B fell by 33.3% (simvastatin) and 15.7% (bezafibrate). Simvastatin and bezafibrate produced significant increases in the mean fractional esterification rate of LCAT, by +124.1% and +20.6%, respectively. Thus simvastatin was clearly more effective than bezafibrate in lowering LDL by enhancing its turnover, but bezafibrate had specific effects on VLDL and HDL that might be favourable in combined treatment regimens.

[Relation between serum lipoprotein metabolism and biliary lipid metabolism]

This review concern with recent results regarding interrelationships between serum lipoprotein and bile acid metabolism. First, changes in bile acid metabolism in primary hyperlipoproteinaemia type II and type IV are described. In addition, influences of lipid-lowering drugs such as sitosterol, cholestyramine/colestipol, neomycin, nicotinic acid, clofibrate and thyroxin on serum lipoproteins and biliary lipid metabolism are discussed. Changes in lipoprotein metabolism in disorders of bile acid metabolism and effects of primary and secundary bile acids on lipoprotein metabolism, hepatic cholesterol metabolism and intestinal cholesterol absorption are described. In the final discussion interrelationships between very low density lipoprotein - triglycerides and cholic acid metabolism as well as low density lipoprotein and chenodeoxycholic acid metabolism are stressed. The positive correlation between serum triglycerides and lithogenicity of bile is underlined and the possible significance of high density lipoprotein-cholesterol as precursor of biliary cholesterol is discussed.

Effects of colestipol, clofibrate, and placebo on plasma lipoproteins of patients with hypercholesterolemia

To evaluate the effectiveness of colestipol and clofibrate in patients with hypercholesterolemia and normal concentrations of triglycerides, 27 patients were randomized into three groups, and colestipol, clofibrate, and avicel powder placebo were compared for effects on concentration of total plasma lipid and lipoprotein cholesterol and triglyceride in a single blind protocol over 8 mo. Mean pretreatment values for low density lipoprotein (LDL)-cholesterol and very low density lipoprotein (VLDL) triglyceride were 250 mg/dl and 68 mg/dl, respectively. Colestipol (30 g/day) lowered total- and LDL-cholesterol by 25% and 31%k, respectively, while VLDL-triglyceride rose. Overall clofibrate lowered total- and LDL-cholesterol by 13% and 12% while lowering VLDL-triglyceride 21% and VLDL cholesterol by 50%. For clofibrate, certain patients showed a more pronounced effect than others: in seven of nine patients clofibrate lowered both mean total- and LDL-cholesterol by 17% (range 8% to 31%) and 19% (range 10%--44%) respectively, whereas two patients did not respond to clofibrate. High density lipoproteins were not affected by either colestipol or clofibrate in these patients. Thus, while colestipol was more consistently effective, certain hypercholesterolemic patients responded equally well to clofibrate with substantial lowering of total-, LDL-, and VLDL-cholesterol.

Plasma lipid concentrations and lecithin:cholesterol acyltransferase activity in normolipidemic subjects given fenofibrate and colestipol

Plasma lipids and lipoprotein cholesterol concentrations and lecithin:cholesterol acyltransferase activity were measured in 7 normolipidemic subjects before, and 7 days after, the administration of fenofibrate (300 mg daily) and colestipol (15 g daily) taken separately or simultaneously. Fenofibrate provoked a significant decrease in the mean plasma triglycerides (26%) and cholesterol (10%) concentration; only plasma cholesterol concentrations were significantly lowered by colestipol (26%). The cholesterol lowering effects of the two drugs were additive as was observed when colestipol was added to fenofibrate. The mean plasma high density lipoprotein cholesterol (HDL-C) concentration was significantly increased by fenofibrate (10%) and when colestipol was added to fenofibrate (15%), but not by colestipol alone. Both fenofibrate and colestipol caused significant reduction of the mean plasma low density lipoprotein cholesterol (LDL-C) concentration and the mean plasma LDL-C/HDL-C ratio (13% and 18%, respectively, with fenofibrate, 44% and 52% with colestipol, and 53% and 62% with colestipol added to fenofibrate). The mean plasma fractional esterification rate was significantly increased by 25% and 12%, respectively, with fenofibrate and colestipol when taken separately, and still more (91%) when colestipol was added to fenofibrate. The mean plasma molar esterification rate was significantly lowered by colestipol, but remained unchanged with the other drug regimens. This study shows that fenofibrate and colestipol given to normolipidemic subjects can induce in a very short period of time (7 days) marked changes in lipoprotein metabolism. Interpretations of the findings in relation to lipoprotein metabolism are discussed.

Colestipol: a review of its pharmacological properties and therapeutic efficacy in patients with hypercholesterolaemia

Colestipol is an anion exchange resin with bile acid sequestering properties resembling those of cholestyramine, another lipid-lowering binding resin. In daily doses of 15 to 30g colestipol reduces total plasma cholesterol concentrations (primarily low density lipoprotein cholesterol) by about 15 to 30%, but plasma triglyceride concentrations may be unchanged or in some patients increased. Thus, like cholestyramine, colestipol is of benefit in patients with primary hypercholesterolaemia without associated hypertriglyceridaemia (type IIa hyperlipoproteinaemia). Colestipol is odourless and tasteless, and is said by some to be more readily tolerated by patients than cholestyramine, leading to improved compliance, but such data has not been documented in most studies. Side effects of colestipol treatment are primarily gastrointestinal in nature since the drug is essentially unabsorbed. As with cholestyramine, colestipol may bind with other concomitantly administered drugs reducing their absorption or enterohepatic recirculation; dosage intervals of other concurrent medications should be adjusted to minimise the potential for such an interaction.

Similar behaviour of lecithin:cholesterol acyltransferase and pseudocholinesterase in liver disease and hyperlipoproteinemia

Using exogenous substrate for its assay, lecithin:cholesterol acyltransferase (LCAT) was found to be decreased in liver disease and higher than normal in endogenous hypertriglyceridemia. LCAT activity was positively correlated with serum cholesterol and triglyceride. However in the six patients with excessive hypertriglyceridemia (type V), LCAT activity was lower than in type IV hyperlipoproteinemia. LCAT activity was not changed significantly in type II-a hyperlipoproteinemia. A striking parallel was noted between plasma LCAT and serum pseudocholinesterase activity. It suggested that both these liver secretion enzymes might be induced by an accelerated turnover of serum lipids and lipoproteins. Pathogenical implications of these findings are briefly discussed.

Lecithin: cholesterol acyltransferase activity and cholestyramine resin therapy in man

The effect of the bile-acid-sequestering agent cholestyramine on esterification of plasma cholesterol has been studied in vitro. No change in the activity of plasma lecithin: cholesterol acyltransferase was found by either of the two assay methods employed. Similarly the resin did not produce any significant change in the ability of plasma to act as substrate for a given lecithin:cholesterol acyltransferase source. It is concluded that the frequently reported relationship between plasma cholesterol concentration and lecithin:cholesterol acyltransferase activity does not represent cause and effect. Also other variables influenced by bile-acid-sequestering agents, such as hepatic cholesterol synthesis, are unlikely to be major determinants of plasma lecithin: cholesterol acyltransferase activity.

Influence of fat ingestion on lecithin:cholesterol acyl transfer rate in plasma of normal persons

Lecithin:cholesterol acyl transfer (LCAT) rate in plasma and lipid concentrations in total plasma and high density lipoproteins (HDL) were determined before and after oral fat loads in healthy human subjects. The changes of LCAT rate after fat loading were compared to the effect of chylomicrons or lipid emulsions added in vitro to fasting plasma. After the fat loads there was an increase of mean molar LCAT rate simultaneous with an increase of mean phospholipid (PL) and HDL-PL concentration but not simultaneous with the increase of mean triglyceride (TG) concentration. Individual changes of molar LCAT rate correlated positively with changes of PL, HDL-PL, and unesterified cholesterol (UC) concentration but not with changes of TG concentration in the separate plasma samples after the fat loads. If only the maximal changes in each subject in any sample at any time after the loads were taken into account, the maximal increase of molar LCAT rate correlated positively with the maximal increase of TG concentration. Molar LCAT rate was not influenced by addition of chylomicrons in vitro but increased after addition of a PL emulsion. It is suggested that LCAT rate is stimulated by an excess of PL in plasma and substrate lipoproteins. This excess of PL may be created in vivo temporarily during chylomicron catabolism. The stimulation of LCAT rate by fat ingestion emphasizes the importance of LCAT as a connecting link between triglyceride and cholesterol metabolism.

On the interpretation and potential diagnostic value of the measurements related to lecithin:cholesterol acyltransferase activity

At least one reliable method is available for measuring LCAT activity in human subjects(25), and this assay may help to probe even subtle changes in plasma lipids and lipoproteins. Since plasma lipids and lipoproteins are known to be present in abnormal amounts in a large number of metabolic diseases, the method of measuring the rate of cholesterol esterification in plasma could assume a substantial role in the diagnosis and (or) monitoring of treatment of pathologic states.

Turnover of plasma cholesteryl esters and its relationship to other parameters of lipid metabolism in man

Studies on plasma cholesteryl esters were made in 32 normal and hyperlipidaemic subjects in an attempt to elucidate further the metabolic relationships between cholesterol and other plasma lipids and lipoproteins. Fractional and net turnover rates of plasma cholesteryl esters were measured using two independent (in vivo and in vitro) methods, both of which determined the prevailing activity of plasma lecithin: cholesterol acyltransferase in the ambiance of the patients own plasma. The results obtained by the two methods were in excellent agreement. The turnover rates of plasma cholesteryl esters in hypertriglyceridaemic patients were significantly greater than in patients with only hypercholesterolaemia and in those with normal plasma lipids. In agreement with these observations, the esterified to free cholesterol ratios in hypertriglyceridaemic patients were significantly greater than in those with normal plasma triglycerides. Patients who had only hypercholesterolaemia had normal esterified to free cholesterol ratios. The rates of esterification of plasma free cholesterol correlated well with triglyceride concentrations as well as with the net turnover of plasma triglycerides and also with the turnover or synthesis of endogenous cholesterol. Since the turnover of plasma triglycerides in most patients with modest to moderate hypertriglyceridaemia reflects the turnover of their plasma very low density lipoproteins, it is suggested that it is the turnover of these lipoproteins which correlates with the hepatic synthesis of cholesterol and with the esterification of plasma cholesteryl esters.