Cholesterol kinetics and faecal steroid excretion in subjects with primary hyperalphalipoproteinaemia

LDL-cholesterol lowering or HDL-cholesterol raising for cardiovascular prevention

A number of reports have indicated that both lowering low density lipoprotein (LDL)-cholesterol and raising high density lipoprotein (HDL)-cholesterol can result in significant cardiovascular benefit, both in terms of reduction of events and also, to a variable extent, of atheromatous lesions. LDL and HDL have opposite roles in body cholesterol regulation and, in theory, both reduced deposition (LDL reduction) and increased removal (raised HDL) can improve vascular disease. A number of reports over the last 30 years have attempted to quantitate with cholesterol balance/turnover studies, the correlations between LDL and HDL levels and body cholesterol pool sizes. More recently, these studies have evaluated the effects of LDL or HDL changes on cholesterol elimination. Data have, at times, been fully consistent with theoretical expectations, whereas at others they have not. Evaluation of these, at times, historical data provides, however, an important clue to the understanding of current results with different medications for the management of lipoprotein disorders.

Chronic administration of ethanol with high vitamin A supplementation in a liquid diet to rats does not cause liver fibrosis. 2. Biochemical observations

The inability of the 'ethanol/high vitamin A Lieber-DeCarli diet' to induce liver fibrosis in two different rat strains was further evaluated by determining changes in parameters of liver cell damage and of retinoid and lipid metabolism. In the ethanol/vitamin A-treated group, slight but constant hepatic cell damage, as indicated by elevated alanine aminotransferase, aspartate aminotransferase and glutamate dehydrogenase activities in blood, was already observed at 6 months and maintained until the time of death at 16 months. Serum gamma-glutamyl transaminase activities were not raised. Moderate parenchymal liver cell damage was not accompanied by fibrosis. Hypertriglyceridemia or hypercholesterolemia were observed at 6-16 months of chronic alcohol administration. This response was strain dependent. In ethanol-treated rats of both strains, total liver retinoids and serum retinol concentrations were not altered. Therefore, the hypothesis that interaction between alcohol and retinoids is a major factor in the pathogenesis of alcoholic liver disease, needs to be reconsidered.

A familial hyperalphalipoproteinemia with low uptake of high density lipoproteins into peripheral lymphocytes

A patient with an extremely high level of high density lipoprotein (HDL)-cholesterol and HDLc-like particles in the serum is discussed. The patient was a 46-year-old female with a serum total cholesterol concentration of 382 mg/dl and HDL-cholesterol level of 214 mg/dl. The HDL-cholesterol levels of her mother, brother, sister and 2 of her daughters were 82 mg/dl, 82 mg/dl, 74 mg/dl, 82 mg/dl and 82 mg/dl, respectively (mean HDL-cholesterol levels of control subjects: 52 +/- 6 mg/dl in males and 55 +/- 8 mg/dl in females). Her serum apolipoprotein A-I and E levels were elevated. Zonal ultracentrifugal analysis of her serum lipoproteins showed that the increased level of HDL-cholesterol was mainly due to HDL2; HDLc-like particles were also recognized between the LDL and HDL fractions. The incorporation of the patient's HDL and HDLc-like particles into cultured HepG2 cells was almost the same as that of HDL (1.063 less than d less than 1.21) from normal control serum. The incorporation of normal control HDL into the patient's peripheral blood lymphocytes was markedly less than that into lymphocytes from normal controls. These findings are discussed in terms of the reason for hyperalphalipoproteinemia in this patient.

On the associations of body cholesterol pool size with age, HDL cholesterol and plasma total cholesterol concentration in humans

Data from 17 subjects, in whom cholesterol kinetics had been measured by two-pool analysis of medium-term plasma cholesterol specific activity-time curves, were examined by multiple linear regression to explore the determinants of the size of the slowly exchanging cholesterol pool (MBmin) in humans. Pool size was independently and positively related to body weight (regression coefficient, 0.94 g per kg; P = 0.05) and age (1.77 g per year; P = 0.02). After allowance for these effects, MBmin retained a significant negative association with the plasma high density lipoprotein (HDL) cholesterol concentration (-0.56 g per mg/dl; P = 0.03), but was unrelated to plasma total cholesterol. This result is consistent with published data on the composition of those human tissues whose cholesterol is known to be largely a component of the slowly exchanging pool. It differs, however, from that of a recent study of cholesterol turnover [Blum et al, J. Lipid Res., 1985; 26: 1079-1088] in which pool size, measured by three-pool analysis of long-term decay curves, was unrelated to HDL and directly related to plasma total cholesterol. On the basis of other published data, it is considered that this discrepancy is unlikely to be a consequence of the difference between our respective studies in the duration and method of analysis of the specific activity decay curves. Differences in the variances of HDL cholesterol and plasma total cholesterol concentration that were examined, and in the biochemical-genetic factors underlying these variances, provide a more likely explanation. The overall weight of evidence favours the view that the pool of slowly exchangeable cholesterol in many human tissues expands during ageing at a rate which is increased in the presence of severe hypercholesterolemia, and which under some, but not all, circumstances also varies inversely with HDL cholesterol. The critical components of HDL metabolism which affect this process remain to be identified.

High-density lipoprotein: a major risk factor for coronary atherosclerosis

The high-density lipoproteins (HDL) are a polydisperse family of lipid--protein complexes whose principal functions in lipid transport are: (1) to act as a reservoir of C apoproteins required for triglyceride transport; (2) to act as a 'scavenger' of surplus cholesterol and phospholipid liberated from lipolysed triglyceride-rich lipoproteins; and (3) to transport surplus cholesterol from peripheral tissues to the liver for excretion and catabolism (reverse cholesterol transport), both directly and indirectly via other lipoproteins and the lipid transfer protein. The concentration of HDL cholesterol (mostly cholesteryl ester) has been found to be a strong risk factor for coronary atherosclerosis, and its clinical complications in most industrialized communities have been studied. The association with disease risk is independent of other lipoproteins and risk factors, has been found in both sexes, and persists following reduction of plasma lipids by diet and certain drugs. It is not yet clear whether or not certain HDL subclasses and/or apoproteins are better predictors of risk than HDL cholesterol. Indirect evidence from clinical studies and data from animal experiments suggests that certain pharmacologically induced increases in HDL cholesterol concentration are associated with a reduction of atherogenesis. However, the mechanism of the link between HDL and atherogenesis is not yet clear: although the original suggestion that it reflects the function of HDL in reverse cholesterol transport remains plausible, alternative mechanisms are possible. These include effects of HDL on platelet function and prostacyclin synthesis. Alternatively, the association might be indirect, reflecting an atherogenic effect of triglyceride-rich lipoproteins and/or their remnants, the plasma concentrations of which are correlated with HDL cholesterol.

A hyperalphalipoproteinaemic family with normal cholesteryl ester transfer/exchange activity

Reports of two independent studies suggest that familial hyperalphalipoproteinaemia (FHALP) may be caused by a deficiency of cholesteryl ester transfer/exchange activity (CETP). We also have studied CETP in the plasma of an Italian FHALP kindred. The study group was divided into blood relatives with greater than 1.70 mM high-density-lipoprotein cholesterol (HDL-C) (group I, n = 9), with less than 1.70 mM-HDL-C (group II, n = 12) and in spouses (group III, n = 6). Two different assays were performed to measure CETP activity. In method A the interfering endogenous lipoproteins in the plasma samples were removed by poly(ethylene glycol) precipitation or by ultracentrifugation at a relative density (d) of 1.180. The CETP-activity of these samples was measured in a system consisting of fixed amounts of HDL and cholesteryl [1-14C]oleate-labelled low-density lipoproteins (LDL). In method B, trace amounts of HDL (radiolabelled with cholesteryl [1-14C]oleate) were incubated with plasma for 3 h at 37 degrees C and the distribution of the label among lipoproteins was measured (CET activity). The results can be summarized as follows. The mean CETP activities measured by method A were 187, 213 and 243 nmol/h per ml in groups I, II and III respectively. The proband with the highest HDL-C (4.98 mM) had a CETP activity of 231 nmol/h per ml. The corresponding CET activities measured by method B and expressed as percentage transfer/h were 4.3, 8.0 and 11.2 in groups I-III. The proband with HDL-C = 4.98 mM had a value of only 1.7%/h. There was a strong negative correlation between percentage CE transfer and HDL-C concentration. Calculating these data in terms of CE exchange (nmol/h per ml), groups I, II and III exhibited mean activities of 86, 124 and 110 nmol/h per ml respectively; for the proband this value was 80 nmol/h per ml. Only a slight correlation was found between these values and the HDL-C value. Thus by both methods, (A), measuring the CETP activity per se and (B), measuring the activity in whole plasma (reflecting the activity of the protein and the concentration and composition of lipoproteins), no major differences could be found between the three groups. In our family, therefore, no connection between FHALP and CETP deficiency could be found. It is concluded that, for hyper- and dys-lipoproteinaemic samples, a careful selection of the assay procedure as well as the mode of calculating results is essential. Since this may not hold the previous studies, the supposed connection between FHALP and CETP deficiency is challenged.

Deficiency of serum cholesteryl-ester transfer activity in patients with familial hyperalphalipoproteinaemia

Lipoprotein patterns and cholesteryl ester transfer activity (CETA) were examined in 2 patients with familial hyperalphalipoproteinaemia (FHALP). The proband was a healthy 58-year-old Japanese male who had an HDL cholesterol of 7.83 mmol/l (301 mg/dl). His sister's HDL cholesterol was 4.52 mmol/l (174 mg/dl), which suggested that both were homozygous carriers of FHALP. In both subjects HDL showed a high cholesterol/apo A-I ratio and appeared to be a larger-sized particle than normal HDL on agarose gel chromatography. Two of the proband's children showed higher HDL cholesterol levels (1.74 mmol/l, 2.16 mmol/l) than normal, but another 2 children showed normal levels (1.48 mmol/l, 1.40 mmol/l). However, the ratios of HDL cholesterol to total cholesterol and to apo A-I in all children were higher than normal. These data suggest, but do not prove, that all his children were heterozygotes. Apo B levels in all of the family members studied were lower than normal (47-80 mg/dl). Deceased members of the same family had not died from cardiovascular disease. Cholesteryl-ester transfer activity was studied in both patients. When serum or lipoprotein deficient serum (d greater than 1.21) and [3H]cholesteryl ester labelled HDL3 were incubated in the presence of an LCAT inhibitor, there was no evidence of cholesteryl ester transfer from HDL to VLDL and/or LDL, unlike normal subjects. The deficiency of CETA in these patients with FHALP presumably accounted for the increase in particle size and cholesterol enrichment of HDL.

Plasma lipoprotein levels and in vitro cholesterol synthesis by cells in human blood

This study has measured plasma lipoprotein lipid levels and the in vitro rate of cholesterol synthesis from [2(-14)C]acetate by mononuclear leukocytes in blood from normolipidemic subjects and two patients with Tangier disease. The rate of cholesterol synthesis in blood was related inversely to plasma levels of HDL cholesterol in the normolipidemic subjects. This relationship was mainly due to a similar correlation in the women. The rate of blood cholesterol synthesis was raised in a woman with Tangier disease, which is consistent with the above correlation, but not in a man with this disease. We suggest that this correlation reflects an association between plasma HDL cholesterol levels and whole-body sterol synthesis.

Effects of chronic ethanol intake on mobilization and excretion of cholesterol in baboons

To investigate the effects of chronic ethanol administration on the mobilization and excretion of cholesterol, turnover and balance studies were carried out in baboons pair-fed cholesterol-free diets containing 50% of energy either as ethanol or as additional carbohydrate for several years. Ethanol feeding increased free cholesterol in all plasma lipoprotein fractions, and esterified cholesterol in very low density lipoprotein, intermediate density lipoprotein, and high density lipoprotein (HDL). The major increase occurred in HDL, mainly as esterified cholesterol. The latter was associated with decreased transfer of esterified cholesterol from HDL to low density lipoprotein. By contrast, the smaller increase in HDL-free cholesterol was associated with increased turnover in the plasma, increased splanchnic uptake, and increased fecal excretion of plasma cholesterol, mainly as neutral steroids. Cholesterol extraction predominated over release in the splanchnic vascular bed, suggesting that the excess of cholesterol excreted in the feces originated in extrasplanchnic tissues. Thus, these findings indicate that alcohol consumption favors mobilization of tissue free cholesterol for hepatic removal and excretion. By contrast the increase in HDL-cholesterol (mainly esterified) appears to be a poor indicator of cholesterol mobilization.

Atherosclerosis and apoproteins B and A-I

The lipid hypothesis stipulates that the risk of developing CAD is related to the cholesterol levels of various lipoprotein fractions, the risk increasing with either a higher LDL cholesterol level or a lower HDL cholesterol level. The data reviewed here indicate that the measurement of the plasma level of the major apoproteins of LDL and HDL, apoB and apoA-I, respectively, provide additional information in the assessment of a patient at risk for CAD. In the case of LDL B, two "normocholesterolemic" groups with CAD are detected, those with normotriglyceridemic HyperapoB and those with hypertriglyceridemic HyperapoB . In all of these syndromes associated with premature CAD, HyperapoB , FCH, and FH, the common denominator is an increased number of LDL particles. A low level of apoA-I may indicate that one of the subfractions of HDL (HDL2) is decreased. HDL2 is generally decreased in disorders where LDL B is elevated, a combination that may be particularly atherogenic. Conversely, elevated apoA-I and HDL cholesterol levels, or decreased LDL cholesterol and LDL B protein levels, are associated with a low prevalence of CAD and longevity. Thus, LDL and HDL levels may be metabolically linked, a relation which is more evident if apoproteins are measured and which may be obscured if apoproteins are not determined. The assessment of dyslipoproteinemia in a patient at risk for CAD might optimally include measurement of LDL B and apoA-I levels, in addition to LDL cholesterol and HDL cholesterol levels.

Relation of high-density lipoproteins to coronary artery disease

Low mean concentrations of high-density lipoprotein (HDL) cholesterol have long been recognized as a characteristic of patients with coronary heart disease, and the measurement of this fraction is a relatively strong discriminator between patients with coronary heart disease and those without. When subjects are ranked by the severity of coronary atherosclerosis determined angiographically, levels of HDL cholesterol, particularly of its HDL2 subclass, are consistently lower in subjects with extensive disease than in those with minimal atheroma. HDL cholesterol is derived from a number of sources, mobilization from peripheral tissues being but one. Generally, longitudinal studies have confirmed that a low HDL cholesterol level is potently and independently predictive of a high risk of coronary heart disease, one exception being a study of subjects with hypercholesterolemia. Despite the strength of these epidemiologic associations, there is no evidence from experimental studies or clinical trials to establish that low HDL levels are causally important in atherogenesis.

Prevention of coronary heart disease: the role of high density lipoproteins

There is now strong evidence that the risk of developing clinical coronary heart disease (CHD) in apparently healthy middle-aged men is inversely related to the plasma high density lipoprotein (HDL) cholesterol concentration. This reflects an underlying relationship between HDL cholesterol concentration and the severity of coronary atherosclerosis. This new information represents a significant development in atherosclerosis research. In the author's opinion, however, it is not yet justifiable, or indeed possible, to utilize this knowledge in CHD prevention programmes for 3 reasons. Firstly, there is still little prospective information on HDL and CHD in women, in younger subjects and in patients with existing clinical disease. Secondly, although there are at least 2 working hypotheses, a causal relationship between HDL metabolism and atherogenesis has not yet been established. Thirdly, there is still a relative paucity of information on the environmental determinants of HDL concentration and metabolism. Thus, premature attempts at intervention in an uncontrolled manner, and particularly the use of drugs for an HDL-raising effect, might only confuse the issue. While further research is being undertaken, attention should continue to be directed towards other reversible coronary risk factors (including hypercholesterolaemia, hypertension, and cigarette smoking) in CHD prevention programmes.