Metabolism of apolipoprotein B-containing lipoproteins in familial hypercholesterolaemia: effects of plasma exchange

The rebound of lipoproteins after LDL-apheresis. Kinetics and estimation of mean lipoprotein levels

We studied the rebound of lipoproteins in 20 hypercholesterolemic men [mean total cholesterol (TC) levels 9.6+/-1.8 mmol/l] after LDL-apheresis (LA) to determine the rate of recovery and the change in cholesterol synthesis, and to find a uniform estimation for time-averaged levels. After 10-20 months on biweekly LA using dextran sulfate cellulose columns and concomitant simvastatin administration, time-averaged levels (+/-SD) measured by integration of the area under the curve were as follows: TC 4.4+/-1.0 mmol/l, LDL cholesterol (LDL-C) 2.5+/-1.0 mmol/l, apolipoprotein B (apo B) 1. 3+/-0.3 g/l, triglycerides (TG) 1.7+/-0.7 mmol/l, HDL-C 1.1+/-0.2 mmol/l, and lipoprotein(a) [Lp(a)] 53.7+/-49.4 mg/dl. Mean acute reductions in TC, LDL-C, apo B, Lp(a), and TG were 61, 77, 75, 76, and 62%, respectively. HDL-C levels were not influenced. Median recovery half times for TC, LDL-C, apo B, and Lp(a) were 3.0, 4.0, 2. 3, and 3.5 days, respectively. The rebound of Lp(a) was identical to LDL-C, in 12 and 13 days post-treatment, respectively, whereas apo B and TC returned to pre-treatment levels in 7.5 and 10 days, respectively, due to the fast rebound of VLDL particles. Notwithstanding these differences, time-averaged levels (C(AVG)) could be estimated uniformly for the four latter parameters with the formula: C(AVG)=C(MIN)+0.73(C(MAX)-C(MIN)), where C(MAX) and C(MIN) are the immediate pre- and post-treatment levels. During long-term treatment the whole-body cholesterol synthesis was increased as measured by the ratio lathosterol to cholesterol of 3.24+/-1.49 mmol/mmol, whereas no further transient increase in the recovery period after LA was found. In conclusion, long-term LA and simvastatin therapy induced acute and chronic changes in lipids and lipoproteins showing the feasibility of biweekly treatment. It was shown that time-averaged levels, as a measure for the effective plasma levels, can be accurately estimated from pre- and post-treatment levels only.

Cholesterol biosynthesis in normocholesterolemic patients after cholesterol removal by plasmapheresis

Plasmapheresis and low-density lipoprotein (LDL)-apheresis are recognized procedures for the treatment of hyperlipidemia resistant to diet and lipid-lowering drugs and provide information on cholesterol synthesis in hypercholesterolemic patients. However, cholesterol synthesis after acute cholesterol removal from plasma has never been investigated in normocholesterolemic patients. In this study, cholesterol synthesis was evaluated in three normocholesterolemic patients by determination of plasma lathosterol, lathosterol-to-cholesterol ratio, and plasma mevalonic acid. In a short-term kinetic study, samples were collected before and after plasmapheresis and every 6 hours during 24 hours. In the second part of the study, cholesterol synthesis was evaluated daily for 3 days. In normocholesterolemic patients, cholesterol returns to basal levels in 3 days. However, cholesterol removal did not result in a significant increase in lathosterol-to-cholesterol ratio or in plasma mevalonic acid, despite a slight increase in lathosterol. In contrast, when repeated plasma exchanges induced a dramatic hypocholesterolemia (< 1 mmol/liter), an acute but transient stimulation of cholesterol synthesis was observed (lathosterol/cholesterol ratio and MVA, respectively, increase from 8.2 to 22.3 and from 28 nmol/liter to 98 nmol/liter). This study shows that cholesterol synthesis is not stimulated by plasmapheresis in normocholesterolemic patients but is enhanced in dramatic hypocholesterolemic patients (< 1 mmol/liter).

Low-density lipoprotein-apolipoprotein B metabolism following apheresis: simulation studies of mass changes and tracer kinetics

Low-density lipoprotein (LDL) apheresis is an effective method to treat severe hyperlipoproteinemia such as heterozygous familial hypercholesterolemia (FH). It is unknown whether apheresis induces changes in metabolic parameters of LDL-apolipoprotein B (apoB) such as the fractional catabolic rate (FCR) or production rate. We performed simulation studies to determine the effect of potential changes in the LDL FCR on LDL-apoB mass and on exogenous and endogenous tracer studies. For these studies, we assumed a two-compartment LDL model and the following metabolic parameters: plasma LDL-apoB, 180 mg.dL(-1); LDL-apoB production rate, 36 mg.dL(-1).d(-1) (approximately 14.4 mg.kg(-1).d(-1)); and LDL-apo FCR, 0.2 d(-1). It was also assumed that apheresis instantaneously decreased the LDL-apoB concentration to 60 mg.dL(-1) and that LDL-apoB production was not perturbed. The simulations examined three possible outcomes: (1) no change in FCR, (2) a temporary doubling in FCR, and (3) a temporary tripling in FCR. Monoexponential models were fit to the rebound of LDL-apoB mass data generated using the different FCRs. In no instance did the FCR determined from the fit match the FCR used to generate the data; FCRs were either higher or lower than the original FCR used to generate the data. Simulations of the kinetics of exogenously labeled LDL showed that if apheresis was performed on day 7 of a turnover study, it would be possible to detect large changes in LDL-apoB FCR. In contrast, during an endogenous labeling study, potential increases in FCR induced by apheresis may not be detected. However, our simulations do show that endogenous labeling studies performed before and after apheresis should yield data that will permit detection of changes in the FCR. Thus, these studies indicate that large differences in the LDL-apoB FCR induced by apheresis can be detected by either an exogenous tracer experiment perturbed by apheresis or by endogenous labeling experiments performed before and after apheresis. Small changes in the FCR that may be induced by apheresis will probably be indistinguishable from experimental noise.

No evidence for feedback inhibition of hepatic apolipoprotein B (apo B) production after extracorporeal low density lipoprotein precipitation as determined by [1-13C]leucine infusion in normal volunteers

To determine the impact of an acute reduction of the circulating mass of apolipoprotein B (apo B) on apo B metabolism we studied six healthy male volunteers before (day 0), 1 day after (day 2), and 7 days after (day 8) an LDL apheresis treatment which reduced apo B mass by 59%. Appearance of newly synthesized apo B in plasma VLDL and LDL was studied using a primed-constant infusion of [1-13C]-leucine. VLDL apo B pool size and fractional VLDL apo B production rate calculated using a one-compartment model were similar on all 3 study days. Absolute VLDL apo B production was not statistically different throughout the study (19.7 +/- 12.3, 19.5 +/- 7.5, 29.1 +/- 17.7 mg kg-1 day-1). LDL apo B fractional production rate was increased on day 2 (0.38 +/- 0.17, 0.68 +/- 0.08, 0.37 +/- 0.06 pools day-1 on days 0, 2, and 8; P < 0.01). Absolute LDL apo B production, however, remained constant throughout the study (10.8 +/- 3.3, 11.0 +/- 1.9, 10.8 +/- 3.1 mg kg-1 day-1). We conclude that in healthy male volunteers acute reduction of the circulating apo B mass by LDL apheresis does not affect apo B metabolism significantly.

The HELP-LDL-apheresis multicentre study, an angiographically assessed trial on the role of LDL-apheresis in the secondary prevention of coronary heart disease. I. Evaluation of safety and cholesterol-lowering effects during the first 12 months. HELP Study Group

Fifty-one patients with coronary heart disease (CHD) and LDL-cholesterol levels greater than or equal to 200 mg dl-1 despite diet and drug therapy have been recruited into an angiographically controlled, multicentre, two-year study to evaluate HELP-LDL-apheresis in the secondary prevention of CHD. There were five drop-outs in the first year and 46 patients completed one year of therapy. An average of 2.791 of plasma was treated per patient every 7.7 days. Treatment was well tolerated and the incidence of side effects was small (2.9% of treatments). Mean pre-/post-apheresis LDL-cholesterol levels decreased from 283/120 mg dl-1 at baseline to 207/78 mg dl-1 and 203/76 mg dl-1 after 6 and 12 months, respectively. Mean pre-/post-apheresis HDL-cholesterol levels rose significantly over the course of therapy from 40.5/36.6 mg dl-1 to 44.8/39.7 mg dl-1 and 48.2/41.3 mg dl-1 after 0, 6 and 12 months, respectively. No major derangement of pre-apheresis haemostasis nor of haematological or clinical chemical parameters had occurred after 12 months of treatment. The data from this study support the feasibility of HELP-LDL-apheresis as an adjunctive therapy for lowering cholesterol levels in CHD patients refractory to cholesterol-lowering drugs while substantially improving the HDL/LDL ratio.

LDL-apheresis: results of longterm treatment and vascular outcome

LDL-apheresis (immunoabsorption, heparin precipitation (HELP), dextran sulfate cellulose binding (DSC) or filtration) is a potent therapeutic tool in familial hypercholesterolemia (FH) to eliminate LDL-cholesterol, Lp(a) or fibrinogen from the circulation and improve blood rheology. Repetitive use can deplete the cholesterol pool between 40 and 80%. As first reports showed, progression of coronary atherosclerosis can be stopped and sometimes regression can be induced. So far the domain of plasmapheresis was homozygous familial hypercholesterolemia. With several apheresis methods now available, it seems timely to define the indication of plasma therapy for heterozygous FH and the place of this potent therapeutic tool in primary and secondary prevention of atherosclerotic coronary heart disease in patients suffering from severe hypercholesterolemia resistant to diet and/or drug therapy.

In vitro remodelling of plasma lipoproteins in whole plasma by lipoprotein lipase in primary and secondary hypertriglyceridaemia

In patients with familial lipoprotein lipase deficiency (FLPL-d) and glycogen storage disease type I (GSD-I), hypertriglyceridaemia (1445 +/- 247 and 1082 +/- 312 mg dl-1, n = 5 per group) was associated primarily with reduced extrahepatic lipoprotein lipase (LPL) activity (0.33 +/- 0.33 and 1.69 +/- 0.38 mumol FFA ml-1 h-1) when compared with controls (4.83 +/- 0.90). Hypercholesterolaemia was characterized by elevated LDL cholesterol (191 +/- 30 and 344 +/- 34 vs. 115 +/- 5 mg dl-1 in controls P less than 0.01) and low HDL cholesterol (12 +/- 2 and 22 +/- 2 vs. 56 +/- 3 in controls, P less than 0.001). In order to ascertain the role of LPL in the interconversion and remodelling of lipoproteins in these disorders, we analysed lipid and lipoprotein profiles before and following in vitro incubation of patient plasma with purified milk LPL (EC 3.1.1.34) for 6 h at 37 degrees C. The efficiency of exogenous LPL in vitro was demonstrated by the extent of hydrolysis of chylomicrons and of VLDL-TG in both groups. Concomitant with the disappearance of TG-rich lipoprotein particles, a consistent per cent increment of IDL (99.2 +/- 30.8 and 43.9 +/- 70.5), LDL (152.8 +/- 36.2 and 137.0 +/- 36.1) and of HDL2 (144.8 +/- 29.4 and 99.8 +/- 18.7) was observed in both groups of patients. The enhancement of the latter fractions contrasted with the decline of HDL3 mass concentration (25.4 +/- 7.7 and 51.4 +/- 5.8%), suggesting that a major shift of HDL3----HDL2 occurs following in vitro lipolysis by LDL. Simultaneous compositional and morphological changes of individual lipoprotein particles were noted, confirming the dynamic movement and exchange of neutral lipids and proteins. Specificity of LPL results was demonstrated by experiments in which incubation of the whole plasma at 37 degrees C without exogenous lipolytic enzyme did not cause any substantial changes. The present study, therefore, demonstrates a correction of the major lipoprotein abnormalities associated with FLPL-d and GSD-I by exogenous LPL. No substantial difference was noted between primary (FLPL-d) and secondary (GSD-I) hyperlipidaemias. These studies allow us to conclude that a simple in vitro system, utilizing an exogenous source of LPL and plasma from patients, may serve as a suitable model for the study of the metabolic relationships of lipoproteins. However, in view of the fact that the extent of lipolysis achieved in vitro did not differ between FLPL-d and GSD-I, it may not be able to separate primary from secondary hyperlipaemias.

Effect of HELP-LDL-apheresis on serum concentrations of human lipoprotein(a): kinetic analysis of the post-treatment return to baseline levels

In addition to LDL, Lp(a) can be quantitatively eliminated from plasma in an extracorporeal LDL-apheresis procedure based on precipitation with heparin at pH 5.12. The rates of return of post-apheresis Lp(a) and LDL-cholesterol concentrations to baseline levels were investigated in six individuals with familial hypercholesterolaemia (one homozygote, five heterozygote) and one normolipaemic individual. The first-order disappearance constants (kappa) were derived for LDL and Lp(a) according to Apstein et al. The kappa values for LDL in the homozygous FH and the normolipaemic individual were 0.082 and 0.43 respectively while the heterozygous FH patients had kappa values intermediate between the two (median 0.231; range 0.116-0.261). The first-order disappearance constants of Lp(a) did not correlate with those of LDL. The homozygous FH and normolipaemic individuals had Lp(a) kappa values of 0.158 and 0.199 respectively; the corresponding values in the heterozygous FH patients were: median 0.142; range 0.045-0.179.

Plasma lipid and lipoprotein response to fat feeding in alcoholic liver disease

Serum lipids and lipoproteins were analyzed after an overnight fast, and following a fatty meal in 10 patients with cirrhosis, 5 with fatty liver, and 5 normal subjects. Cirrhotic patients were divided into two groups of five on the basis of serum lecithin-cholesterol acyltransferase activity. Fasting triglyceride levels were similar in all four groups. In all but cirrhotic patients with low lecithin-cholesterol acyltransferase activity, most fasting triglyceride was found in very low density lipoproteins; in the latter group, most of it was found in low density lipoproteins. We confirmed that patients with fatty liver have a higher serum triglyceride response to fat feeding than normal subjects, but we did not find higher levels in cirrhotic patients. Cirrhotic patients with "normal" lecithin-cholesterol acyltransferase activity had a normal triglyceride response to dietary fat. In patients with cirrhosis and low lecithin-cholesterol acyltransferase activity, the increase in triglyceride was less than in normal subjects. In this group, most of the extra triglyceride was carried in low density lipoprotein, and not in chylomicrons and very low density lipoprotein, as in the other groups.

Role of insulin in lipoprotein secretion by cultured rat hepatocytes

To study the effect of insulin on lipoprotein synthesis and secretion by the liver, apoprotein and lipid levels were measured in primary rat liver cell cultures grown on fibronectin-coated dishes. Triglycerides, phospholipids, apoprotein (apo) B, apo-E, and apo-C-III3 all accumulated in culture media linearly for periods up to 20 h. During incubations, cellular triglyceride contents increased slightly, while cellular apoprotein and phospholipid contents remained constant. In the absence of insulin, rates of accumulation in media of triglycerides, apo-B, apo-C-III3, and apo-E were 2.5 +/- 0.3 micrograms/mg and 33 +/- 5, 24 +/- 3, and 162 +/- 32 ng/mg cell protein per h, respectively. On gel permeation chromatography and density gradient ultracentrifugation, the majority of apoproteins in media were found to be associated with very low density lipoproteins (VLDL) and very little eluted or sedimented with albumin. Incubations in the presence of 50-800 microU/ml of insulin resulted in dose-dependent decreases of triglyceride, phospholipid, apo-B, and apo-E accumulation in the media, paralleled by increases in the cellular contents of these lipoprotein components. The inhibitory effects of insulin on secretion were reversible. Levels of apo-C-III3 and albumin were not affected by insulin. In addition to decreasing secretory rates, the proportion of apo-B, apo-E, and apo-C-III3 associated with VLDL also decreased after the addition of insulin. Concomitantly, the proportion of apo-B eluting with LDL and apo-C-III3, and apo-E eluting near albumin increased. Control experiments, in which exogenous 125I-VLDL or endogenously labeled [14C]VLDL were added to cultures, revealed that the insulin-induced differences in VLDL accumulation and the lipid association of media apoproteins were not due to differences in the processing of VLDL by cells cultured in the presence or absence of insulin. Therefore, it appears that insulin may inhibit the secretion of VLDL perhaps by reducing the intracellular association of lipids and apoproteins.

The metabolic basis of familial hypercholesterolemia

Familial hypercholesterolaemia (FH) is a dominantly inherited error of metabolism characterised by a raised plasma low-density lipoprotein (LDL) concentration, xanthomas of skin and tendons, and a tendency to premature heart disease due to atherosclerosis of the coronary arteries. The clinical and biochemical abnormalities are more marked in homozygotes than in heterozygotes. Other biochemical changes include an increased concentration of very-low-density lipoprotein (VLDL) remnants and of a minor subfraction of high-density lipoproteins. Measurement of plasma lipoprotein turnover shows reduced fractional rates of catabolism of LDL and VLDL remnants, and increased production of LDL. Similar abnormalities are found in Watanabe rabbits, an inbred strain carrying a mutation similar to that responsible for FH. Cultured cells from human and animal tissues express surface receptors with high binding affinity for LDL. Binding of LDL to LDL receptors is followed by endocytosis and lysosomal digestion of the lipoprotein. Cultured cells from FH heterozygotes express only half the normal number of LDL receptors; those from homozygotes have little or no receptor activity and are therefore unable to degrade significant amounts of LDL by the LDL-receptor pathway. The LDL receptor has been isolated from cell membranes; it has a molecular weight of about 160 kd. Several different mutant forms of the receptor have been identified in the cells of FH homozygotes. The LDL-receptor pathway for the catabolism of LDL accounts for at least 1/3 of the total LDL catabolised by normal human subjects in vivo and almost none of that catabolised by FH homozygotes. Deficiency of LDL receptors accounts for the increased plasma concentrations of LDL and VLDL remnants in FH. The increased plasma concentration in these lipoproteins is the cause of deposition of lipid in xanthomas and arterial wall, but the mechanism by which lipoprotein enters the cells in which lipid accumulates is not yet understood.

The metabolism of very low density and intermediate density lipoproteins in patients with familial hypercholesterolaemia

The metabolism of apolipoprotein B (apoB) in very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL) was studied in normal subjects and in patients with familial hypercholesterolaemia (FH) after an intravenous injection of autologous VLDL labelled with 125I. There were no significant differences in half life, pool size and turnover rate (mg/kg/h) of VLDL-apoB between the normal subjects, the FH heterozygotes and the FH homozygotes. IDL-apoB metabolism in the FH patients differed significantly from that in the normal subjects. In the FH patients, the rise to the maximum of the specific activity curve was slower, the half life of the descending limb of the specific-activity curve was longer, the fractional rate of turnover was lower and the plasma concentration was higher than in the normals. The effect of cholestyramine on IDL-apoB metabolism in the normal subjects did not differ from that in the FH heterozygotes and homozygotes, though cholestyramine is known to stimulate hepatic uptake of low density lipoprotein (LDL) by the LDL receptor. It is suggested that in normal human subjects the LDL receptor makes some contribution to the hepatic uptake of IDL-apoB derived from VLDL, but that IDL uptake is mediated partly by a separate receptor that recognizes apolipoprotein E but not apoB.

Evidence that two synthetic pathways contribute to the apolipoprotein B pool of the low density lipoprotein fraction of rabbit plasma

The plasma specific activity of apolipoprotein B was determined in normal rabbit injected with homologous radioiodinated very low and intermediate density lipoproteins (VLDL and IDL). The specific activity was followed in the same density range of the injected lipoproteins and in two low density lipoprotein subfractions (LDL1 and LDL2). VLDL B (d less than 1.006 g/ml) is cleared from plasma mainly in IDL (d = 1.006-1.019 g/ml), with minor radioactivity recovery in LDL1 (d = 1.019-1.040 g/ml) and LDL2 (d = 1.040-1.063 g/ml) fractions. The main catabolic product of IDL B is, on the other hand, LDL1 B. LDL2 B, which represent more than 20% of the whole LDL B plasma pool in this animal species, is not derived to a significant extent from either VLDL or IDL and, conceivably, is synthesized independently.

Regulation of plasma cholesterol by lipoprotein receptors

The lipoprotein transport system holds the key to understanding the mechanisms by which genes, diet, and hormones interact to regulate the plasma cholesterol level in man. Crucial components of this system are lipoprotein receptors in the liver and extrahepatic tissues that mediate the uptake and degradation of cholesterol-carrying lipoproteins. The number of lipoprotein receptors, and hence the efficiency of disposal of plasma cholesterol, can be increased by cholesterol-lowering drugs. Regulation of lipoprotein receptors can be exploited pharmacologically in the therapy of hypercholesterolemia and atherosclerosis is man.

Selective removal of apolipoprotein B-containing serum lipoproteins from blood plasma

Studies were undertaken to determine the applicability and effectiveness of immunoadsorption chromatography on anti-low density lipoprotein (LDL) columns as a plasma-cholesterol-lowering procedure. Mass production and isolation of monospecific antibodies against swine LDL (rho = 1.006-1.063 g/ml) was carried out by immunization of sheep with swine LDL and selective antibody adsorption from their antisera by chromatography on LDL-Sepharose. The isolated LDL antibodies were then covalently linked to Sepharose CL-4B. Pig plasma LDL was effectively removed in vitro by the anti-LDL-Sepharose beads. In vivo studies were performed in pigs with (i) a plasma-separator membrane permeable to solutes below Mr 2,000,000 or (ii) a blood centrifuge interposed into an arteriovenous shunt to separate the corpuscular elements of blood from the plasma. In either case, the plasma was passed through the anti-LDL-Sepharose column and recirculated into the venous part of the shunt. Plasma cholesterol levels were reduced 70-80% by this procedure and rebounded 3-4 days after the extracorporeal immunoadsorption procedure. This continuous plasma separation-immunoadsorption procedure may have broad applicability for the elimination of any plasma component with antigenic properties.

Assessment of long-term plasma exchange for familial hypercholesterolaemia

The effectiveness of repeated plasma exchange with 2 to 4 litres of plasma protein fraction as long-term treatment for familial hypercholesterolaemia has been evaluated in six severely affected patients receiving conventional cholesterol lowering treatment. Cell-separator mediated exchange at monthly intervals for one to two years reduced mean serum cholesterol levels from 18.5 mmol/l (715 mg/dl) to 12.4 mmol/l (480 mg/dl) in two female homozygotes but failed to influence xanthomata or prevent a two- to threefold increase in their left ventricular aortic systolic pressure gradients. More effective reduction of mean serum cholesterol levels from 15.7 mmol/l (608 mg/dl) to 8.6 mmol/l (333 mg/dl) in two male homozygotes by plasma exchange at fortnightly intervals for two to three years was accompanied by resolution of xanthomata and by stabilisation of aortocoronary lesions. In two male heterozygotes with angina, coronary angiographic appearances were unaltered or improved after one to two years of thrice-monthly plasma exchange, which reduced mean serum cholesterol levels from 6.4 mmol/l (248 mg/dl) to 4.7 mmol/l (182 mg/dl). We conclude that plasma exchange every one to two weeks, combined with oral nicotinic acid and/or cholestyramine, retards the rate of progression of atheroma in homozygotes and possibly induces regression in heterozygotes.