Plasma apolipoprotein CII levels in hypertriglyceridemia

Analysis of 6-month effect of orlistat administration, alone or in combination with fenofibrate, on triglyceride-rich lipoprotein metabolism in overweight and obese patients with metabolic syndrome

BACKGROUND

Orlistat significantly reduced serum triglycerides (TG) in most clinical trials. Orlistat-induced TG reduction has not been studied to determine the factors contributing to TG alterations in clinical settings.

OBJECTIVE

We examined the factors influencing TG reduction during orlistat administration, alone or in combination with fenofibrate, and we investigated the effects of these treatments on apolipoprotein C-II (ApoC-II) and C-III (ApoC-III) levels.

METHODS

Patients with the metabolic syndrome were randomly allocated to receive orlistat 120 mg three times daily (n = 28, O group), micronized fenofibrate 200 mg/day (n = 28, F group), or both (n = 27, OF group) for 6 months. Plasma ApoC-II and ApoC-III were determined by an immunoturbidimetric assay.

RESULTS

In the O group, we observed reductions of plasma ApoC-III (P < 0.05) and ApoC-II (P = NS) levels. Fenofibrate administration significantly reduced concentrations of ApoC-II and ApoC-III, whereas the combination of orlistat and fenofibrate had an additive effect on these apolipoproteins. There were significant in-group reductions in serum TG levels in all treatment groups. Multivariate analysis showed that in O group's baseline TG levels were independently positively correlated, whereas the baseline ApoC-II levels were negatively correlated with TG-lowering. In the F group, baseline TG levels and ApoC-III reduction were significantly and independently correlated with TG reduction. OF group's baseline TG levels and ApoC-III reduction were independently positively correlated and baseline ApoC-II levels were negatively correlated with TG-lowering.

CONCLUSIONS

Orlistat-mediated TG-lowering is independently associated with baseline TG and ApoC-II levels. When orlistat is combined with fenofibrate, ApoC-III reduction is another independent contributor to TG alterations.

Developmental and pharmacological regulation of apolipoprotein C-II gene expression. Comparison with apo C-I and apo C-III gene regulation

Increased plasma triglyceride concentrations are often observed in metabolic disorders predisposing to coronary heart disease. Among the major determinants of plasma triglyceride metabolism are the apolipoproteins (apos) of the C class, C-I, C-II, and C-III. Whereas physiological concentrations of apo C-II are required for lipolysis of triglycerides by lipoprotein lipase (LPL), overexpression of all 3 C apolipoproteins leads to hypertriglyceridemia. In the present study, we investigated apo C-II gene regulation under conditions associated with profound changes in plasma triglyceride metabolism, ie, during postnatal development and after treatment with the triglyceride-lowering fibrate drugs, and compared its expression to that of apo C-I and apo C-III. Whereas the expression of both apo C-I and apo C-III is low in fetal liver, increases gradually after birth, and attains maximal levels after weaning, apo C-II gene expression is already detectable in the fetal liver, increases rapidly immediately after birth, and remains elevated throughout suckling. Thus, the increased ingestion of lipids during suckling is met by an earlier induction of apo C-II, the obligatory activator for LPL, compared with apo C-III and apo C-I, which antagonize triglyceride catabolism. Treatment of rats with fibrates decreased apo C-II gene expression in the liver, but not in the intestine, whereas apo C-I gene expression did not change. The decrease of liver apo C-II mRNA levels after fenofibrate occurred in a time- and dose-dependent manner and was reversible but appeared less pronounced than the decrease of apo C-III mRNA. Apo C-II mRNA levels were not affected after treatment with BRL49653, a peroxisome proliferator-activated receptor (PPAR)gamma-specific ligand, suggesting that fibrates act on apo C-II expression via PPARalpha. Addition of fenofibric acid to primary rat and human hepatocytes resulted in a decrease of apo C-II expression. In conclusion, fibrates decrease gene expression of apo C-II and apo C-III, but not apo C-I, in rat and human hepatocytes. This decrease of apo C-II and apo C-III gene expression, together with a lowered apo C-III to apo C-II ratio, should result in an improved clearance of triglyceride-rich remnant lipoproteins from plasma, without hampering triglyceride lipolysis by LPL.

Overexpression of apolipoprotein CII causes hypertriglyceridemia in transgenic mice

We have generated transgenic mice expressing the human apolipoprotein CII (apoCII) gene under the transcriptional control of the human cytochrome P-450 IA1 (CYPIA1) promoter. Human apoCII transgenic (HuCIITg) mice exhibited significant basal expression of the transgene (plasma apoCII level = 26.1 +/- 4 mg/dl) and showed further induction of transgene expression after treatment with beta-naphthoflavone. Unexpectedly, HuCIITg mice were hypertriglyceridemic and human apoCII levels correlated strongly to triglyceride levels (R = 0.89, P < 0.0001). Triglyceride levels (mg/dl +/- SEM) were elevated compared to controls in both the fed (804 +/- 113 vs 146 +/- 18, P < 0.001) and fasted (273 +/- 39 vs 61 +/- 4, P < 0.001) states. HuCIITg mice accumulated triglyceride-rich very low density lipoproteins (VLDL) with an increased apoC/apoE ratio. Tracer kinetic studies indicated delayed clearance of VLDL-triglyceride, and studies using Triton inhibition of VLDL clearance showed no increase in VLDL production. Plasma from these mice activated mouse lipoprotein lipase normally and radiolabeled VLDL were normally hydrolyzed. However, HuCIITg VLDL showed markedly decreased binding to heparin-Sepharose, suggesting that apoCII-rich, apoE-poor lipoprotein may be less accessible to cell surface lipases or receptors within their glycosaminoglycan matrices. HuCIITg mice are a promising model of hypertriglyceridemia that suggests a more complex role for apoCII in the metabolism of plasma triglycerides.

Interaction between variant apolipoproteins C-II and E that affects plasma lipoprotein concentrations

The genes for apolipoprotein (apo) C-II, a cofactor for activation of lipoprotein lipase, and apo E, a ligand for receptor-mediated uptake of triglyceride-rich lipoproteins, are physically linked on chromosome 19q13.1. In a large Caribbean Caucasian family, several individuals had clinical features of the complete absence of lipoprotein lipase activity and were homozygous for a DNA frameshift mutation of apo C-II, imparting functional inactivity to the mutant protein. Plasma from heterozygous carriers of this mutation, when compared with plasma from relatives who were noncarriers, had significantly diminished capacity to activate lipoprotein lipase in vitro. We also observed in heterozygotes for this mutation a wide range of serum lipid and lipoprotein levels. When age and sex were taken into account, the presence of a single apo E allele encoding the E4 isoform occurring in individuals with a single mutant apo C-II allele was strongly associated with higher levels of cholesterol, triglycerides, very low density lipoprotein cholesterol, and non-high density lipoprotein cholesterol when compared with those of relatives who carried neither or only one variant allele. This suggests that a single genetic mutation that usually has a recessive effect on lipoprotein metabolism can have an interactive effect on lipid phenotype when it is coinherited with a single mutation at another gene whose product affects the same metabolic pathway.

A neonatal case of apolipoprotein C-II deficiency

A neonatal case of apo C-II deficiency with hypertriglyceridaemia and xanthomas is presented. The patient responded well to a special diet formula containing medium-chain triglycerides (MCT). This is the first case of apo C-II deficiency to be discovered during the neonatal period.

Long-term treatment of hypercholesterolemic non-insulin dependent diabetics (NIDDM) with pravastatin (CS-514)

We examined the long-term effect of pravastatin, a new potent inhibitor of endogenous cholesterol biosynthesis, on glucose and lipid metabolism in hyperlipidemic NIDDM. Ten patients (5 on sulfonylurea, 5 on diet) were studied over 12 months. Five were WHO type IIa and 5 were type IIb. Blood was taken before and then 1, 6 and 12 months after initiating 10 or 20 mg daily of pravastatin. The cholesterol concentration in whole plasma and very low density lipoprotein (VLDL), plasma triglyceride and apolipoprotein (apo) B were all significantly decreased within the first month. These changes lasted for 1 year. High density lipoprotein (HDL)-cholesterol increased in the first month but returned to base line thereafter. Low density lipoprotein (LDL)-cholesterol tended to decrease in the first month, and was suppressed significantly from the 6th month (11%) to the 12th month (16%). The effect of pravastatin on LDL-cholesterol in NIDDM was slower and weaker than that published for non-diabetic hypercholesterolemia. Therefore, the mechanism by which pravastatin suppresses plasma cholesterol levels in these two conditions may differ. After 1 year, no adverse effects were noted on hematopoietic, hepatic or renal function. Blood glucose level, hemoglobin A1c and the insulin response to oral glucose were unchanged. In addition, serum creatine phosphokinase showed no abnormal increase. Careful ophthalmological examinations before and after pravastatin treatment revealed no development of new lenticular opacities. Thus, pravastatin appears to be a safe and effective drug for the long-term treatment of NIDDM with hypercholesterolemia.

Effects of CS-514 on plasma lipids and lipoprotein composition in hypercholesterolemic subjects

The effect of CS-514 (eptastatin, Sankyo Co., Tokyo), a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, was investigated in 47 patients with hypercholesterolemia (WHO type IIa: 27, IIb: 20). Ten or 20 mg of CS-514 was administered daily for 3 months. In both types of patient, total cholesterol and phospholipid levels were significantly reduced by CS-514. The triglyceride, cholesterol and phospholipid content of low density lipoprotein (LDL) and the plasma levels of apolipoprotein B were also decreased in both groups. In contrast, total triglyceride, very low density lipoprotein (VLDL)-triglyceride and apolipoprotein C-II were decreased only in type IIb subjects. Also the levels of high density lipoprotein (HDL)-cholesterol and apolipoproteins A-I and A-II were increased by CS-514 in IIb but not in IIa patients. In both groups, no change occurred in either the cholesterol/triglyceride or phospholipid ratio in any lipoprotein fraction, nor in the ratio of HDL-cholesterol to apolipoprotein A-I or A-II, respectively. Therefore, CS-514 suppresses plasma levels of cholesterol in hypercholesterolemic patients without modifying lipoprotein composition. Moreover, this drug has different effects on the levels of plasma triglyceride and HDL-cholesterol of type IIa and IIb patients.