Bimodal distribution of cholesteryl ester transfer protein activities in normotriglyceridemic men with low HDL cholesterol concentrations

Increased plasma activities of cholesteryl ester transfer protein (CETP) theoretically could lower HDL cholesterol levels due to enhanced transfer of cholesteryl esters from HDL to apo B-containing lipoproteins. To determine whether high CETP activities are associated with isolated hypoalphalipoproteinemia, CETP activities were measured in 109 adult men with HDL cholesterol < 35 mg/dL, plasma triglycerides < 200 mg/dL, and LDL cholesterol < 160 mg/dL; the results were compared with those of 50 normolipidemic (HDL cholesterol > 40 mg/dL) male subjects. CETP activities were assayed in vitro and expressed as the percent of [3H]cholesteryl ester transferred from HDL3 to LDL during a 16-hour incubation. In addition, postheparin plasma activities of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) were determined in 71 patients with a low HDL cholesterol level. Distributions of CETP activities were unimodal in control subjects (mean +/- SD, 23.1 +/- 5.0%), but they were bimodal in the low-HDL patients. Among the latter, 27 patients had elevated CETP activities (40.8 +/- 4.6%), whereas 82 patients had CETP activities that overlapped the normal range (26.14 +/- 7.6%). Low-HDL patients with normal CETP activities had 20% lower LPL activities (P = .01), 25% higher HTGL activities (P = .03), and 63% lower LPL/HTGL ratios (P < .001) than those of low-HDL patients with increased CETP activity. Furthermore, mean LPL and HTGL activities in the low-HDL patients with elevated CETP activities were in the normal range. Another important distinction between the two subgroups with low HDL was that the subgroup with high CETP activity had only a 30% prevalence of coronary heart disease compared with a 70% prevalence in the subgroup with normal CETP activity (P < .01). These findings suggest that elevated CETP activity may be a significant factor in causing low HDL cholesterol levels in a distinct subgroup of normolipidemic patients with low HDL cholesterol levels.

Metabolism of low density lipoproteins in nephrotic dyslipidemia: comparison of hypercholesterolemia alone and combined hyperlipidemia

High levels of low-density lipoprotein cholesterol (LDL) (hypercholesterolemia) are commonly present in the nephrotic syndrome. Another pattern of dyslipidemia in nephrotic patients is an elevation of both cholesterol and triglyceride levels (combined hyperlipidemia). It has been postulated that the underlying cause of nephrotic dyslipidemia is an hepatic overproduction of apolipoprotein B (apo B)-containing lipoproteins. To examine this hypothesis, the metabolism of LDL-apo B was compared between nephrotic patients with hypercholesterolemia and with combined hyperlipidemia. Thirteen patients (7 with hypercholesterolemia, and 6 with combined hyperlipidemia) underwent measurements of turnover rates of autologous LDL apo B. The results were compared to normolipidemic controls and to patients with primary combined hyperlipidemia previously studied in our laboratory. Nephrotic patients with hypercholesterolemia generally had: (a) lower fractional catabolic rates of LDL apo B than normolipidemic healthy individuals; (b) LDL particles enriched in cholesterol; but (c) no overproduction of LDL apo B. In contrast, patients with combined hyperlipidemia were found to have: (a) high fractional catabolic rates for LDL apo B compared to normolipidemic controls; (b) cholesterol-poor LDL particles; and (c) markedly elevated production rates for LDL. Also, for the group as a whole, there was a positive correlation between plasma triglyceride levels and fractional catabolic rates. These data indicate that the metabolism of LDL is strikingly different between the two forms of nephrotic dyslipidemia. Although there may be common mechanisms contributing to LDL levels in nephrotic patients, there also appears to be a divergence of mechanisms depending on whether hypertriglyceridemia is associated with hypercholesterolemia.

Relation between cholesterol ester transfer protein activities and lipoprotein cholesterol in patients with hypercholesterolemia and combined hyperlipidemia

Cholesterol ester transfer protein (CETP) promotes the transfer of cholesterol esters among different lipoprotein classes-high-density lipoproteins (HDL), very-low-density lipoproteins, intermediate-density lipoproteins, and low-density lipoproteins (LDL). The current study was carried out to determine whether CETP activities are correlated with lipoprotein cholesterol levels in a large number of patients having elevated LDL cholesterol and normal triglycerides (hypercholesterolemia) and elevated LDL cholesterol and high triglycerides (combined hyperlipidemia). Compared with 50 normolipidemic male patients, 113 hypercholesterolemic patients had a 42% higher mean activity of CETP, and approximately 60% of these patients had CETP activities outside the normal range. A similar elevation of CETP was observed in 47 patients with combined hyperlipidemia. Furthermore, in those with combined hyperlipidemia, CETP activities were highly correlated with LDL cholesterol, non-HDL cholesterol, and non-HDL/HDL ratios. Similar high correlations were obtained by combining normotriglyceridemic patients with and without elevated LDL cholesterol. Since patients with elevated LDL cholesterol had a significantly lower mean level of HDL cholesterol, a high CETP activity also was related to a reduced HDL cholesterol level. Our results are consistent with this concept, although they do not constitute final proof that high CETP activities contribute to elevated cholesterol concentrations and reduced HDL cholesterol levels in patients with hypercholesterolemia and in those with combined hyperlipidemia.

Lp(a) lipoprotein is an independent, discriminating risk factor for premature peripheral atherosclerosis among white men

BACKGROUND

Elevated plasma levels of Lp(a) lipoprotein have been linked to the development of premature atherosclerosis in the coronary circulation (coronary artery disease [CAD]). Although Lp(a) lipoprotein has been implicated as a risk factor for premature atherosclerosis in other locations, the patient populations described were not carefully screened for the coexistence of premature CAD. The purpose of this prospective study was to determine whether carefully screened patients with premature peripheral vascular disease (PVD) have elevated plasma levels of Lp(a) lipoprotein and to test the relative strength of Lp(a) lipoprotein level as a risk factor for premature PVD.

METHODS

We studied 55 consecutive white men with premature PVD (onset at 45 years of age or earlier) presenting to our vascular laboratory. Study subjects were substratified into 17 with PVD only and 38 with combined PVD and CAD (PVD + CAD). Two comparison groups included 26 age-matched white men with premature CAD recruited from the Cardiology Service after cardiac catheterization and 32 age-matched white male controls recruited from outpatient clinics.

RESULTS

Mean plasma apolipoprotein B-100 levels were higher in the CAD group than in controls (P = .013). Mean plasma Lp(a) lipoprotein levels were higher among the 17 patients with PVD only than among controls (P = .008). The covariance-adjusted mean Lp(a) lipoprotein levels were higher among all 55 patients with PVD than among controls (P = .014). Logistic regression analysis demonstrated two variables to be significantly related to premature PVD: Lp(a) lipoprotein level greater than 30 mg/dL (odds ratio, 3.9; 95% confidence interval, 1.1 to 13.7) and apolipoprotein B level greater than 95 mg/dL (odds ratio, 3.2; 95% confidence interval, 1.0 to 10.0).

CONCLUSIONS

Lp(a) lipoprotein level is an independent, discriminating risk factor for premature PVD among white men.

Concentrations of apolipoprotein A-I-containing particles in patients with hypoalphalipoproteinemia

This study was performed to determine relations among concentrations of high-density lipoprotein (HDL) apolipoprotein (apo) A-I and apoA-II and lipoproteins with apoA-I only (LpA-I) and with both apoA-I and apoA-II (LpA-I:A-II) in patients with low plasma levels of HDL cholesterol. Seventy-seven middle-aged men with low HDL cholesterol levels (< 40 mg/dL) were compared with 37 middle-aged men with normal HDL cholesterol levels (> 40 mg/dL). Low-HDL patients were divided into those with normotriglyceridemia (triglycerides < 250 mg/dL; n = 49) and hypertriglyceridemia (triglycerides > or = 250 mg/dL; n = 28). Total apoA-I and apoA-II concentrations and apoA-I levels in LpA-I were significantly lower in the two low-HDL groups compared with control subjects. Although low-HDL patients' apoA-I levels were numerically lower in LpA-I:A-II compared with control subjects' levels, the differences were not statistically significant. Thus, there is a preferential reduction in apoA-I levels of LpA-I compared with LpA-I:A-II in patients with low HDL cholesterol. This preferential reduction in LpA-I levels was observed in both normotriglyceridemic and hypertriglyceridemic patients. However, among low-HDL patients levels of apoA-I in LpA-I did not distinguish between those with and without coronary heart disease.

Hypercholesterolemia in postmenopausal women. Metabolic defects and response to low-dose lovastatin

OBJECTIVE

To determine the metabolic mechanisms underlying hypercholesterolemia in postmenopausal women and to determine whether a low dose of lovastatin will correct this abnormality.

DESIGN

In the first part of the study, turnover rates of autologous low-density lipoprotein (LDL) were measured in hypercholesterolemic and control women. In the second part, hypercholesterolemic women participated in a placed-controlled, randomized, double-blind study using lovastatin as the therapeutic agent.

SETTING

The General Clinical Research Center of the University of Texas Southwestern Medical Center, Dallas, utilizing inpatient and outpatient facilities, and the Veterans Affairs Medical Center, Dallas, Tex.

PATIENTS

For the LDL turnover study, 26 postmenopausal women with moderate hypercholesterolemia (mean +/- SD LDL cholesterol, 4.78 +/- 0.59 mmol/L [185 +/- 23 mg/dL]) and 13 postmenopausal women with normal levels of plasma lipids and lipoproteins (mean +/- SD LDL cholesterol, 3.31 +/- 0.39 mmol/L [128 +/- 15 mg/dL]) were studied. Sixteen postmenopausal women participated in the drug study.

INTERVENTIONS

In the drug study, patients received blindly both lovastatin (10 mg/d) and placebo.

MAIN OUTCOME MEASURES

In the first study, kinetic parameters of LDL metabolism; in the second study, response in lipids and lipoproteins to lovastatin therapy.

RESULTS

In the LDL turnover study, mean (+/- SD) input (production) rates for LDL apolipoprotein B (apo B) were similar for hypercholesterolemic women and control women (12.4 [+/- 3.2] mg/kg per day and 11.1 [+/- 2.2] mg/kg per day, respectively). In contrast, mean (+/- SD) fractional catabolic rates for LDL apo B in hypercholesterolemic women (0.29 [+/- 0.04] pools per day) were significantly lower than those in normolipidemic women (0.35 [+/- 0.03] pools per day). In the drug trial, lovastatin therapy reduced mean (+/- SD) total cholesterol and LDL cholesterol from 7.03 (+/- 1.16) mmol/L (272 [+/- 45] mg/dL) and 4.42 (+/- 0.80) mmol/L (171 [+/- 31] mg/dL, respectively, to 5.70 (+/- 1.03) mmol/L (221 [+/- 40] mg/dL) and 3.46 (+/- 0.85) mmol/L (134 [+/- 33] mg/dL).

CONCLUSIONS

The turnover data suggest that hypercholesterolemia in post-menopausal women is primarily attributable to a reduced activity of LDL receptors. In accord, the hypercholesterolemia in these women was effectively lowered by low doses of lovastatin. Thus, a low dose of lovastatin appears highly effective for treatment of moderate hypercholesterolemia in most postmenopausal women, presumably because it reverses the reduction in LDL receptor activity associated with menopause.

Lipoprotein responses to treatment with lovastatin, gemfibrozil, and nicotinic acid in normolipidemic patients with hypoalphalipoproteinemia

BACKGROUND

The lipoprotein responses to conventional lipid-modifying drugs have not been adequately evaluated in normolipidemic patients with hypoalphalipoproteinemia (low levels of high-density lipoproteins). The purpose of this study was to compare responses to lovastatin, gemfibrozil, and nicotinic acid in such patients.

METHODS

The first phase of the study compared lipoprotein responses to lovastatin and gemfibrozil in 61 middle-aged men with low levels of high-density lipoproteins. In the second phase, 37 patients agreed to take nicotinic acid; 27 patients finished this phase at a dose of 4.5 g/d. Nicotinic acid results were compared with those with lovastatin and gemfibrozil in the same patients.

RESULTS

In the first phase, both drugs effectively lowered triglyceride levels. Gemfibrozil therapy increased high-density lipoprotein cholesterol levels by 10% and lovastatin by 6%, but lovastatin was much more effective for reducing low-density lipoprotein levels. Nicotinic acid did not significantly lower low-density lipoprotein levels in the second phase, but it raised high-density lipoprotein levels by 30%.

CONCLUSIONS

Gemfibrozil therapy produced the least favorable response of the three drugs. Lovastatin markedly lowered low-density lipoprotein levels but only modestly raised levels of high-density lipoprotein, whereas nicotinic acid had the opposite effect. Consequently, the latter two drugs similarly reduced low-density lipoprotein-high-density lipoprotein ratios, although these effects were obtained in different ways. Between these two drugs, lovastatin therapy was more likely to reduce low-density lipoprotein cholesterol levels to below 2.6 mmol/L (100 mg/dL), and in view of recent recommendations, it may be preferable to nicotinic acid for many normolipidemic patients with established coronary heart disease.

Variability in cholesterol content and physical properties of lipoproteins containing apolipoprotein B-100

The primary objective of this study was to determine the variability in cholesterol carrying capacity of low density lipoproteins (LDLs) and other apolipoprotein B (apo B)-containing lipoproteins in normolipidemic men. One hundred and fifty-nine normolipidemic men, ages 21 to 73 years, were enrolled. In addition to determining plasma lipids and lipoproteins, three primary measurements were made: ratios of cholesterol to apo B in LDL; the electrophoretic pattern of LDL, i.e. pattern A, AB, or B; and levels of cholesterol in all lipoproteins other than high density lipoproteins (nonHDL-cholesterol) along with total apo B. First, the data revealed that about 85% of the variability of LDL-cholesterol levels can be accounted for by LDL-apo B levels, whereas the remaining 15% can be explained by differences in LDL-cholesterol/apo B ratios. Second, LDL electrophoretic pattern A was the predominant pattern in young adult men, but in older men the pattern shifted increasingly to AB and B. And third, there was a high correlation between nonHDL-cholesterol levels and total apo B levels, which suggests that nonHDL-cholesterol can be used as a relatively accurate surrogate for total apo B levels in normolipidemic individuals.

Mechanisms and treatment of dyslipidemia of renal diseases

Dyslipidemia is commonly observed in nephrotic syndrome, in chronic renal failure, and after renal transplantation. The patterns of dyslipidemia, however, differ among these three conditions, and the origins and mechanisms responsible for abnormalities in lipoprotein metabolism in each are not well understood. Whether these dyslipidemias contribute to the development of atherosclerosis and coronary heart disease is uncertain, but it is probable that they do. Important questions are whether an attempt should be made to treat the various renal dyslipidemias, and if so, by what means. Also of current interest are dyslipidemias in the nephrotic syndrome, chronic renal failure (uremia), and the post-renal transplantation state.

Activities of lipoprotein lipase and hepatic triglyceride lipase in postheparin plasma of patients with low concentrations of HDL cholesterol

Previous investigations have shown that abnormalities in the postheparin plasma levels of the lipolytic enzymes, lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL), are correlated with variations in plasma high-density lipoprotein cholesterol (HDL-C) levels. The present study was performed to determine correlations between the postheparin plasma activities of these two enzymes and HDL levels in a sizable number of subjects with low HDL-C levels. Two types of low-HDL subjects were investigated: 159 male subjects with low HDL-C (< 40 mg/dL) and normal triglyceride (< 250 mg/dL) levels (the low-HDL group) and 80 male subjects with low HDL-C (< 40 mg/dL) and elevated triglyceride (> or = 250 mg/dL) levels (the low-HDL/high-TG group). Postheparin plasma activities of LPL and HTGL were determined in these two groups, and these levels were compared with those obtained from 51 normolipidemic (normal-HDL) male subjects. Postheparin LPL activities were significantly lower in the low-HDL and low-HDL/high-TG groups (mean +/- SD, 9.9 +/- 2.9 and 10.4 +/- 3.0 mmol/h per liter, respectively; P < .001 for both) compared with the normal-HDL group (12.5 +/- 3.7 mmol/h per liter). Conversely, postheparin HTGL activities were significantly higher in the low-HDL and low-HDL/high-TG groups (39.3 +/- 16.2 and 44.4 +/- 16.7 mmol/h per liter, respectively; P < .001 for both) compared with the normal-HDL group (29.7 +/- 11.3 mmol/h per liter). Consequently, mean LPL/HTGL ratios were markedly lower in the two low-HDL groups compared with the normal-HDL group.(ABSTRACT TRUNCATED AT 250 WORDS)

Two patterns of LDL metabolism in normotriglyceridemic patients with hypoalphalipoproteinemia

The objective of this study was to determine whether normotriglyceridemic patients with low levels of high density lipoprotein (HDL) cholesterol have concomitant defects in the metabolism of low density lipoproteins (LDLs). To address this question, measurements of turnover rates of apolipoprotein A-I (apo A-I) and LDL apolipoprotein B (apo B) were made in 36 middle-aged men with low HDL cholesterol (< 40 mg/dL), normal triglyceride (< 250 mg/dL), and normal total cholesterol (< or = 90th percentile) levels. Similar measurements were made in eight hypertriglyceridemic men having low HDL levels. For control, turnover rates of LDL apo B were measured in 24 healthy, normolipidemic men, and apo A-I kinetics were determined in 20 other healthy men with normal HDL cholesterol levels. In all patients with low HDL levels, fractional catabolic rates (FCRs) for apo A-I were increased compared with control subjects; in contrast, input rates for apo A-I in low-HDL patients were similar to control. Hypertriglyceridemic patients had significantly higher FCRs for LDL (0.463 +/- 0.040 pool/day, [mean +/- SEM]) than control subjects (0.328 +/- 0.008 pool/day, p < 0.001). In normolipidemic patients having low HDL, a bimodal pattern of LDL-apo B kinetics was observed. For 23 low-HDL patients, FCRs for LDL apo B averaged 0.450 +/- 0.017 pool/day and were significantly higher than control values. Additionally, in these patients, levels of very low density lipoprotein plus intermediate density lipoprotein (VLDL+IDL) cholesterol and VLDL+IDL apo B were higher than in control subjects (54 +/- 3 versus 32 +/- 3 mg/dL and 25 +/- 2 versus 18 +/- 1 mg/dL, respectively). The remaining 13 low-HDL patients had lower and essentially normal FCRs for LDL (0.300 +/- 0.009 pool/day); these patients also had relatively low levels of cholesterol and apo B in VLDL+IDL. Thus, two patterns of LDL kinetics were present in normotriglyceridemic patients with low HDL levels. One pattern was indistinguishable from that typically present in patients with hypertriglyceridemia, whereas the other was similar to normal control subjects. These two patterns of LDL-apo B kinetics may reflect different mechanisms for the causation of low HDL cholesterol concentrations.

Effects of lovastatin on the levels, structure, and atherogenicity of VLDL in patients with moderate hypertriglyceridemia

The purpose of this study was to determine whether lovastatin treatment reduced very low density lipoprotein (VLDL) abnormalities in hypertriglyceridemic subjects. Lovastatin reduced plasma triglyceride levels and the levels of total VLDL, intermediate density lipoprotein (IDL), and low density lipoprotein (LDL) cholesterol. The numbers of VLDL particles of Sf 100-400 and Sf 60-100 but not Sf 20-60 particles were reduced by lovastatin, as was the amount of cholesteryl ester per particle. All VLDL subspecies bound to the LDL receptor of cultured human fibroblasts with similar, high affinities on both placebo and lovastatin, but VLDL Sf 100-400 and VLDL Sf 60-100 caused less suppression of 3-hydroxy-3-methyl glutaryl coenzyme A reductase activity after lovastatin therapy, indicating reduced LDL receptor-mediated cholesterol delivery. The average decrease in reductase suppression by VLDL Sf 100-400 after lovastatin was 32%, similar to the 34% average decrease in cholesteryl ester content of VLDL Sf 100-400 after lovastatin. Although statistical significance was not achieved, there was a trend toward decreased VLDL Sf 100-400-induced rapid, receptor-mediated triglyceride accumulation in P388D1 macrophages after lovastatin. Taken together, these observations suggest that lovastatin may be of potential benefit in decreasing the atherosclerotic complications of hypertriglyceridemia.

Four new mutations in the apolipoprotein B gene causing hypobetalipoproteinemia, including two different frameshift mutations that yield truncated apolipoprotein B proteins of identical length

Familial hypobetalipoproteinemia can be caused by mutations in the apolipoprotein (apo)B gene that interfere with the translation of a full-length apoB molecule. Frequently, a truncated apoB molecule can be detected in the plasma lipoproteins of affected subjects. In this report, we characterize four different apoB gene mutations causing hypobetalipoproteinemia that are associated with the synthesis of truncated apoB proteins. Two of the mutations are nonsense mutations caused by single nucleotide substitutions; these mutations are associated with the production of apoB-32.5 (1473 amino acids) and apoB-82 (3733 amino acids). The other two mutations are single nucleotide deletions (of apoB cDNA nucleotides 7295 and 7359, respectively). The altered reading frames created by these different frameshift mutations terminated with the same stop codon, and both therefore yielded a truncated protein of identical size: apoB-52.8 (2395 amino acids). The two apoB-52.8 proteins differ, however, in the number of novel carboxyl-terminal amino acids introduced by the frameshift. The buoyant density of lipoproteins containing the truncated apoBs was inversely related to the length of the truncated apoB. ApoB-32.5 was present only in high density lipoproteins (HDL) and the d > 1.21 g/ml fraction, whereas apoB-82 was present almost exclusively in very low density lipoproteins (VLDL). ApoB-52.8 was present primarily in VLDL, intermediate density lipoproteins (IDL), and low density lipoproteins (LDL); trace amounts were observed in the HDL.

Occurrence of species of low-density lipoprotein with defective clearance in patients with primary moderate hypercholesterolaemia

Recent studies have shown that one cause of primary moderate hypercholesterolaemia is familial defective apolipoprotein B-100 (FDB), a condition in which a mutation in apolipoprotein B-100 (apo B-100) causes low-density lipoproteins (LDL) to bind poorly to LDL receptors. One specific mutation, a glutamine-for-arginine transformation at position 3500 of apo B-100, has been reported to produce FDB. However, other mutations in apo B-100 might also cause FDB. The present study was designed to determine whether some patients with hypercholesterolaemia, who do not have the 3500 defect, may have a slowly cleared subfraction of LDL compatible with other forms of FDB. It was postulated that slowly removed LDL should accumulate excess cholesterol ester and hence be less dense than normal LDL. If so, in patients who are heterozygous for FDB, two forms of LDL might be separable by ultracentrifugation. To test this hypothesis, less-dense (d = 1.030 g ml-1) and more-dense (d = 1.040 g ml-1) subfractions of LDL were isolated from a patient with proven FDB (3500 mutation); the two forms of LDL were labelled with different isotopes of radioiodine and re-injected into the patient. The less-dense form was removed much more slowly (0.285 pools day-1) than more-dense LDL (0.570 pools day-1). This finding appeared to confirm the validity of the approach. The same procedure was then applied to 18 other patients having elevated LDL cholesterol but not the 3500 mutation. In 13 patients, the two forms of LDL were removed at essentially identical rates, suggesting that they did not have an abnormal form of LDL. In the other five, less-dense LDL were removed at a significantly slower rate than more-dense LDL; this finding suggests that a significant portion of patients with moderate hypercholesterolaemia have an abnormal LDL species, which is not the 3500 mutation, but delays clearance of LDL from the circulation.

Physiologic mechanisms for reduced apolipoprotein A-I concentrations associated with low levels of high density lipoprotein cholesterol in patients with normal plasma lipids

Low plasma concentrations of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I) are major risk factors for coronary heart disease (CHD). Low HDL levels are common in patients with hypertriglyceridemia, but they also occur in those with normal plasma lipids; the latter include obese patients and cigarette smokers, though other patients with low HDL levels are neither obese nor smokers. The present study was designed to define metabolic causes of low apoA-I levels in normal-weight, normolipidemic patients. ApoA-I tracer studies were carried out in two groups of normolipidemic patients having low HDL levels to determine input rates and residence times for ApoA-I; these patients included 11 nonobese nonsmokers and 11 nonobese cigarette smokers. Their results were compared to those of 20 normal-weight, normolipidemic controls with normal HDL levels and 12 obese nonsmokers also having low HDL. In all three groups manifesting low HDL-cholesterol and low apoA-I levels, residence times for plasma apoA-I were reduced by approximately 30%, compared to control subjects with normal HDL levels. In contrast, average input rates for apoA-I were similar among the three low-HDL patients and control subjects. No differences in apoA-I kinetics were observed among any of the three groups with low apoA-I concentrations. Within each of the four groups of the study, however, input rates for apoA-I were highly correlated with plasma concentrations of apoA-I. Thus, for individuals with normal levels of plasma lipids, both residence times and input rates for apoA-I appeared to be important determinants of apoA-I levels. Residence times for apoA-I were reduced in almost all patients with low apoA-I levels, regardless of concomitant factors, whereas input rates were highly variable among individuals.

Persistence of abnormalities in metabolism of apolipoproteins B-100 and A-I after weight reduction in patients with primary hypertriglyceridemia

Obesity commonly accompanies hypertriglyceridemia, and weight reduction is widely recommended for treatment of elevated triglyceride levels. To determine whether weight reduction will normalize lipoprotein metabolism in overweight, hypertriglyceridemic patients, 10 such male patients underwent weight loss until their body weights were within the desirable range. After reestablishment of a steady state in body weight at the lower level, measurements were made of plasma lipid, lipoprotein, and apolipoprotein levels and the kinetics of low density lipoprotein (LDL) apolipoprotein B-100 (apo B) and apolipoprotein A-I (apo A-I). The patients lost an average of 10.6 +/- 2.1 kg (mean +/- SEM). Plasma triglyceride concentrations fell from 431 +/- 42 mg/dl to 248 +/- 27 mg/dl (p less than 0.001), whereas concentrations of total cholesterol, LDL cholesterol, total apo B, and high density lipoprotein (HDL) cholesterol were unchanged after weight loss. On average, the fractional catabolic rates (FCRs) for LDL were much higher in the patients after weight loss than in 16 normal control subjects (0.55 +/- 0.06 versus 0.31 +/- 0.06 pool/day), and input rates for LDL also were higher for hypertriglyceridemic patients after weight loss (22.2 +/- 2.4 versus 12.8 +/- 2.3 mg/kg.day). Compared with 20 normal control subjects, hypertriglyceridemic patients after weight reduction had persistent low HDL cholesterol levels (32 +/- 2 versus 54 +/- 3 mg/dl) as well as low apo A-I levels (99 +/- 5 versus 122 +/- 4 mg/dl).(ABSTRACT TRUNCATED AT 250 WORDS)

A truncated species of apolipoprotein B, B-83, associated with hypobetalipoproteinemia

Familial hypobetalipoproteinemia, a syndrome associated with low plasma cholesterol levels, can be caused by apoB gene mutations. We identified a healthy 42-year-old man whose total plasma cholesterol level was 80 mg/dl. His plasma very low density lipoprotein (VLDL) contained a unique truncated apoB species, apoB-83, in addition to the normal B apolipoproteins, apoB-100 and apoB-48. Virtually no apoB-83 was detectable in his low density lipoprotein (LDL). From the subject's kindred, we identified nine other hypocholesterolemic subjects whose VLDL contained apoB-83. A tendency for cholelithiasis was noted in the apoB-83 heterozygotes, particularly in the older individuals. From the apparent size of apoB-83 on SDS-polyacrylamide gels and its reactivity with apoB-specific monoclonal antibodies, we estimated that it would contain approximately 3700-3800 amino acids. DNA sequencing of apoB genomic clones from two affected individuals revealed that apoB-83 was caused by a C----A transversion in exon 26 of the apoB gene (apoB cDNA nucleotide 11458). This mutation converts Ser-3750 (TCA) into a premature stop codon (TAA) and creates a unique MseI restriction endonuclease site. Thus, a single nucleotide transversion in the apoB gene results in a unique truncated apoB species, apoB-83, and the clinical syndrome of familial hypobetalipoproteinemia.

Two different views of the relationship of hypertriglyceridemia to coronary heart disease. Implications for treatment

Hypertriglyceridemia is commonly found in patients with coronary heart disease. The reason for this connection, however, is not well understood, and two different views have been put forth to explain the link. First, triglyceride-rich lipoproteins, particularly very-low-density lipoproteins, may be directly atherogenic. Or second, the metabolic consequences of hypertriglyceridemia may account for the triglyceride-coronary heart disease relationship. These consequences include an increase in postprandial lipoproteins, large very-low-density lipoprotein particles, small, dense low-density lipoprotein particles, low levels of high-density lipoprotein cholesterol, and possibly a procoagulant state. The appropriate treatment of hypertriglyceridemia depends on which of these views is nearer the truth. If triglyceride-rich lipoproteins are directly atherogenic, then the preferred therapy would be hepatic hydroxymethylglutaryl coenzyme A reductase inhibitors, which lower both very-low-density lipoprotein and low-density lipoprotein levels. On the other hand, if the link to atherogenesis is through the metabolic consequences of hypertriglyceridemia, the appropriate therapy would be to directly lower serum triglyceride levels, as with niacin or a fibric acid. Thus, discovery of the mechanism of the connection between triglycerides and coronary heart disease is crucial for developing a rational therapy.

Influence of lovastatin therapy on metabolism of low density lipoproteins in mixed hyperlipidaemia

To determine the mechanisms whereby HMG-CoA reductase inhibitors lower the levels of low density lipoproteins (LDL) in patients with mixed hyperlipidaemia, LDL turnover studies were conducted in 12 such patients during placebo and then during treatment with lovastatin. Drug therapy reduced total cholesterol and triglyceride concentrations by 33% and 32%, respectively. During lovastatin therapy, LDL-cholesterol levels fell by 37%, and LDL-apo B concentrations decreased by an average of 29%. The decrease in LDL-apo B concentrations on lovastatin therapy was largely due to an increase in fractional catabolic rates (FCRs) for LDL apo B. The average increase in FCRs was 34%, whereas transport rates (production rates) for LDL apo B remained unchanged. These results strongly suggest that an increase in LDL-receptor activity is the major mechanism whereby LDL levels are lowered during lovastatin therapy. The data do not indicate that this drug inhibited the input of apo B-containing lipoproteins, which would have been expected to result in a decrease in the rate of production of LDL.

Tumor necrosis factor mediates hypertriglyceridemia during thermal injury in mice genetically susceptible to lipopolysaccharides

A rise in plasma triglycerides has been noted after thermal injury in a number of animal species including humans. In this study we identified a factor, tumor necrosis factor, which was responsible for increased plasma triglycerides during thermal injury that was induced by scalding. Two strains of mice that differed genetically in susceptibility to lipopolysaccharides were used. These were CH3HEB/HeJ (LPS-) and CH3HEB/FeJ (LPS+). A 15% total body surface area was burned; this resulted in an increase of plasma triglycerides of 126% of preburn levels in the LPS+ strain 24 hours after burn injury. No change in triglycerides was noted in the LPS- mice at any time after burn injury. Sera from LPS+ mice at 1 to 2 hours after burn injury was injected into nonburned animals of the same strain; this caused a 62% +/- 5% increase in plasma triglycerides 24 hours after injection. When thermally injured LPS+ mice were injected with anti-tumor necrosis factor-alpha at 1 hour after injury, they did not show a rise in plasma triglycerides at any time between 24 to 72 hours after injury. Hepatic secretion of triglycerides was also measured 1 day after burn; the average secretion of triglycerides was significantly reduced (2.69 +/- 0.36 mg/kg/hr, compared with 3.83 +/- 0.15 mg/kg/hr for the control). We conclude that tumor necrosis factor, a cytokine that inhibits lipoprotein lipase, causes hypertriglyceridemia during thermal injury in spite of a decreased secretion of triglycerides. This is the first report that demonstrates that hypertriglyceridemia that is secondary to thermal injury is induced by tumor necrosis factor.

Metabolic basis of primary hypercholesterolemia

BACKGROUND

Hypercholesterolemia is a well-established risk factor for coronary heart disease. However, the mechanisms underlying hypercholesterolemia, elevated low density lipoprotein (LDL) in particular, are not well understood. To determine these mechanisms, we studied LDL kinetics in a group of men with primary hypercholesterolemia.

METHODS AND RESULTS

LDL kinetics in 134 middle-aged men with high-risk levels of LDL cholesterol (more than 160 mg/dl) were compared with kinetics in 16 men with borderline high-risk levels of LDL cholesterol (120-159 mg/dl) and 14 men with heterozygous familial hypercholesterolemia (FH). Patients with primary hypercholesterolemia (non-FH) were further divided into moderate hypercholesterolemia (LDL cholesterol, 160-210 mg/dl; n = 108) and severe hypercholesterolemia groups (LDL cholesterol, more than 210 mg/dl; n = 26). Four factors contributed to increasing LDL cholesterol concentrations above the borderline range to moderately elevated levels: 37 patients had no increase in LDL apolipoprotein (apo) B levels but had abnormally high LDL cholesterol-to-apo B ratios; 14 patients had very low fractional catabolic rates (FCRs) for LDL, similar to FH patients; 35 patients had FCRs for LDL in the borderline range but high production rates for LDL; and 22 patients had a high flux of LDL (high production rates and high FCRs). In general, patients with severe hypercholesterolemia resembled those with moderate LDL elevations, except that their LDL particles were enriched with cholesterol.

CONCLUSIONS

Data from the present study reveal that there are several distinct patterns of LDL metabolism responsible for primary hypercholesterolemia. These patterns can serve as the basis for further investigation to determine the molecular defects responsible for each pattern.

Low density lipoprotein kinetics in a family having defective low density lipoprotein receptors in which hypercholesterolemia is suppressed

Heterozygous familial hypercholesterolemia (FH) usually presents with severe elevations of low density lipoprotein (LDL) cholesterol. Recently, a family with FH was described in which several members heterozygous for a mutation in the LDL receptor gene had normal LDL cholesterol levels. Kinetic studies of LDL apolipoprotein B (apo B) were conducted to determine the metabolic differences between the normolipidemic and hypercholesterolemic FH heterozygotes in the family. Studies were performed in 14 family members including the proband (who has homozygous FH), four FH heterozygotes with high LDL levels, four FH subjects with normolipidemia, and five healthy relatives without FH. The proband had a very low fractional catabolic rate (FCR) for LDL (0.15 pool/day). All the FH and non-FH subjects studied, excluding the FH homozygote, had higher than expected FCRs for LDL. The average FCRs for LDL of hypercholesterolemic and normocholesterolemic subjects were not significantly different (0.39 +/- 0.06 versus 0.37 +/- 0.02 pool/day), and these values were 70-80% of those in unaffected relatives. Compared with hypercholesterolemic FH heterozygotes, normolipidemic heterozygotes had much lower input rates for LDL (17.1 +/- author query macros2.6 versus 8.7 +/- 0.9 pools/day, respectively). These low input rates, together with the higher than usual FCRs for LDL, are responsible for the normal concentrations of LDL cholesterol in some of the FH heterozygotes. The low input of LDL could be due to either a decreased secretion of apo B-containing lipoproteins or an enhanced clearance of LDL precursor lipoproteins.