Apolipoprotein(a) is present in the triglyceride-rich fraction in type IV hypertriglyceridemia

Differential influence of LDL cholesterol and triglycerides on lipoprotein(a) concentrations in diabetic patients

OBJECTIVE

To evaluate the relationship between plasma lipid profiles and lipoprotein(a) [Lp(a)] concentrations in diabetic patients, taking into account the Lp(a) phenotype.

RESEARCH DESIGN AND METHODS

We included 191 consecutive diabetic outpatients (69 type 1 and 122 type 2 diabetic patients) in a cross-sectional study Serum Lp(a) was determined by enzyme-linked immunosorbent assay, and Lp(a) phenotypes were assessed by SDS-PAGE followed by immunoblotting. The statistical methods included a stepwise multiple regression analysis using the Lp(a) serum concentration as the dependent variable. The lipid profile consisted of total cholesterol, HDL cholesterol, LDL cholesterol, corrected LDL cholesterol, triglycerides, and apolipoproteins AI and B.

RESULTS

In the multiple regression analysis, LDL cholesterol (positively) and triglycerides (negatively) were independently related to the Lp(a) concentration, and they explained the 6.6 and 7.8% of the Lp(a) variation, respectively. After correcting LDL cholesterol, the two variables explained 3.8 and 6.4% of the Lp(a) variation, respectively. In addition, we observed that serum Lp(a) concentrations were significantly lower in patients with type IV hyperlipidemia (mean 1.0 mg/dl [range 0.5-17], n = 16) than in normolipidemic patients (6.5 mg/dl [0.5-33.5], n = 117) and in type II hyperlipidemic patients (IIa 15.5 mg/dl [3.5-75], n = 13; IIb 9 mg/dl [1-80], n = 45); P < 0.001 by analysis of variance.

CONCLUSIONS

Lp(a) concentrations were directly correlated with LDL cholesterol and negatively correlated with triglyceride levels in diabetic patients. Therefore, our results suggest that the treatment of diabetic dyslipemia may indirectly affect Lp(a) concentrations.

Quantification of lipoprotein(a) and apolipoprotein(a) in plasma and lipoprotein fractions in the hypertriglyceridemic state

Lipoprotein(a) [Lp(a)] is a risk factor for coronary heart disease (CHD) in particular in association with high low density lipoprotein (LDL) cholesterol concentrations. Hypertriglyceridemia on the other hand has been found to be associated with low Lp(a) values. This observation could be confirmed in 851 patients of the outpatient lipid clinic. Lp(a) median levels were 2.7-fold higher in patients with triglycerides below 200 mg/dl as compared with patients expressing triglyceride levels above 200 mg/dl (19 vs 7 mg/dl, P < 0.0001). In contrast to these data apolipoprotein(a) [apo(a)] has been detected in triglyceride-rich lipoproteins (TRL). To find out whether the presence of apo(a) in TRL is determined by the concentration of these particles, apo(a) concentrations were measured in TRL in fasting plasma of ten hypertriglyceridemic patients and ten normal controls with Lp(a) serum levels above 25 mg/dl. The apo(a) concentration in TRL did not show statistically significant differences between controls and patients (2.0+/-0.9 vs 1.8+/-1.6 mg/dl). In the second part of the study apo(a) levels in TRL were measured before and after fat feeding in eight healthy volunteers. Again no significant differences were observed in the apo(a) concentrations of the d < 1.006 a ml fraction before and after fat feeding (1.03+/-1.06 vs 0.81+/-0.63 mg/dl). In summary, this study fails to show an association of apo(a) with TRL for different states of hypertriglyceridemia. This negative finding is shown for constant particle numbers but might not be true if the particle number in TRL increases.

Association of Lp(a) rather than integrally-bound apo(a) with triglyceride-rich lipoproteins of human subjects

The majority of apolipoprotein (a) [apo(a)] in plasma is characteristically associated with Lipoprotein (a) [Lp(a)], having a buoyant density (1.05-1.08 g/ml) intermediate between low density lipoproteins (LDL) and high density lipoproteins (HDL). In the fed (postprandial) state or in the presence of fasting (endogenous) hypertriglyceridemia, a small proportion of plasma apo(a) is found in the density < 1.006 g/ml fraction of plasma, associated with larger and less dense triglyceride-rich lipoproteins (TRL). In order to further characterize the presence of apo(a) in ultracentrifugally-separated TRL (UTC-TRL), this lipoprotein fraction was isolated from plasma obtained in the fed state (three hours after an oral fat load) from healthy normolipidemic subjects (Lp(a): 38 +/- 8 mg/dl (mean +/- S.E.), n = 4) and also from plasma obtained after an overnight fast from hypertriglyceridemic patients (plasma TG: 8.16 +/- 2.00 mmol/l, Lp(a): 41 +/- 3 mg/dl, n = 18). Apo(a) in 3 h-postprandial UTC-TRL (5 +/- 2% of total plasma apo(a)) and in hypertriglyceridemic UTC-TRL (8 +/- 2% total apo(a)) was separable by electrophoresis and/or gel chromatography (FPLC) from the majority of UTC-TRL lipid. Apo(a) in UTC-TRL fractions had slow pre-beta electrophoretic mobility and was isolated in a lipoprotein size-range smaller than VLDL and larger than LDL, consistent with it being Lp(a). Recentrifugation of UTC-TRL resulted in the majority of apo(a) being recovered in the density > 1.006 g/ml fraction. Addition of proline to plasma samples before ultracentrifugation (final concentration: 0.1 M) substantially reduced the amount of Lp(a) in UTC-TRL. TRL separated from plasma by FPLC contained less apo(a) (2-5% of total plasma apo(a)), but this apo(a) was also readily dissociable from TRL lipid, had slow pre-beta electrophoretic mobility, and was associated with a lipoprotein with the size of Lp(a). Our data suggest that apo(a) in the TRL fraction of subjects with postprandial triglyceridemia or endogenous hypertriglyceridemia is not an integral component of plasma VLDL or chylomicrons, but represents the presence of non-covalently bound Lp(a).

Plasma lipoprotein(a) distribution in the Framingham Offspring Study as determined with a commercially available immunoturbidimetric assay

The purpose of our research was to evaluate a commercially available, automated, immunoturbidimetric assay for lipoprotein(a) (Lp(a)), to determine the distribution of Lp(a) in the Framingham Offspring Study population, and to determine Lp(a) levels that may be useful for assessing coronary heart disease risk. The mean between-run coefficient of variation for this assay was 5.65%. Lp(a) concentration was slightly, but significantly, higher in 1949 white women (mean +/- S.D. 214 +/- 195 mg/l, median 150 mg/l) than in 1884 white men (mean +/- S.D. 200 +/- 193 mg/l, median 130 mg/l) participating in Cycle 4 of the Framingham Offspring Study (P = 0.0015). Lp(a) values of 300 mg/l and 500 mg/l corresponded to approximately the 75th and 90th percentiles, respectively, for both men and women, and subjects with concentrations greater than or equal to 500 mg/l were more likely to have coronary heart disease than subjects with an Lp(a) concentration less than 300 mg/l (P < 0.05 for men).

Reduction of lipoprotein(a) following treatment with lovastatin in patients with unremitting nephrotic syndrome

Pharmocologic treatment of the hyperlipidemia associated with the nephrotic syndrome with lovastatin has been previously shown to be safe and effective. However, there is no information on the effect of lovastatin treatment on plasma lipoprotein(a) [Lp(a)] levels in patients with the nephrotic syndrome. We administered lovastatin (40 to 80 mg/day) to 20 adult patients with unremitting nephrotic syndrome for 8 weeks to assess its effect on plasma Lp(a) and other plasma lipid concentrations. Apoprotein(a) (apo(a)) phenotype was determined in all patients. Patients were grouped according to their plasma Lp(a) levels. Those with elevated plasma Lp(a) (> or = 30 mg/dL) were placed in group I and those with normal Lp(a) levels (< 30 mg/dL) were placed in group II. Mean total cholesterol and LDL cholesterol were similarly and significantly reduced in groups I and II (-35.9% and -43.3%, P < 0.0005, P < 0.0005 group I, and -31.0% and -42.0%, P < 0.02, P < 0.03 group II, respectively). The median reduction in plasma Lp(a) was -32% (P < 0.003) in nephrotic patients in group I, whereas the median decline in plasma Lp(a) levels in nephrotic patients in group II was only -8.0% (P = 0.052). The overall frequency of the high molecular weight (M(r)) apo(a) phenotype S4 was 70% in nephrotic patients. There was no correlation between plasma Lp(a) and apo(a) phenotype. Treatment with lovastatin results in a favorable response in terms of total and low-density lipoprotein cholesterol lowering in patients with the nephrotic syndrome; however, plasma Lp(a) levels are uniformly and significantly reduced only in nephrotic patients with elevated baseline plasma Lp(a) concentrations. There was no correlation between plasma Lp(a) concentration and other lipid and biochemical parameters.