Effect of a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor on triglyceride kinetics in chronically streptozotocin-diabetic rats

Acute Statin Withdrawal Does not Interfere With the Improvements of a Session of Exercise in Postprandial Metabolism

BACKGROUND

The risk for atherogenic plaque formation is high after ingestion of meals in individuals with high blood lipid levels (ie, dyslipidemia). Statins and exercise reduce the rise of blood triglyceride concentrations after a meal, but the effect of their combination is unclear.

METHODS

In a randomized crossover design, 11 individuals with dyslipidemia and metabolic syndrome treated with statins underwent a mixed-meal (970 ± 111 kcal, 24% fat, and 34% carbohydrate) tolerance test. Plasma lipid concentrations, fat oxidation, glucose, and glycerol kinetics were monitored immediately prior and during the meal test. Trials were conducted with participants under their habitual statin treatment and 96 hours after blinded statin withdrawal. Trials were duplicated after a prolonged bout of low-intensity exercise (75 minutes at 53 ± 4% maximal oxygen consumption) to study the interactions between exercise and statins.

RESULTS

Statins reduced postprandial plasma triglycerides from 3.03 ± 0.85 to 2.52 ± 0.86 mmol·L-1 (17%; P = .015) and plasma glycerol concentrations (ie, surrogate of whole-body lipolysis) without reducing plasma free fatty acid concentration or fat oxidation. Prior exercise increased postprandial plasma glycerol levels (P = .029) and fat oxidation rates (P = .024). Exercise decreased postprandial plasma insulin levels (241 ± 116 vs 301 ± 172 ρmol·L-1; P = .026) but not enough to increase insulin sensitivity (P = .614). Neither statins nor exercise affected plasma glucose appearance rates from exogenous or endogenous sources.

CONCLUSIONS

In dyslipidemic individuals, statins reduce blood triglyceride concentrations after a meal, but without limiting fat oxidation. Statins do not interfere with exercise lowering the postprandial insulin that likely promotes fat oxidation. Last, statins do not restrict the rates of plasma incorporation or oxidation of the ingested glucose.

Atorvastatin plus omega-3 fatty acid ethyl ester decreases very-low-density lipoprotein triglyceride production in insulin resistant obese men

AIM

To test the effect of atorvastatin (ATV) and ATV plus ω-3 FAEEs on VLDL-TG metabolism in obese, insulin resistant men.

METHODS

We carried out a 6-week randomized, placebo-controlled study to examine the effect of ATV (40 mg/day) and ATV plus ω-3 FAEEs (4 g/day) on VLDL-TG metabolism in 36 insulin resistant obese men. VLDL-TG kinetics were determined using d5 -glycerol, gas chromatography-mass spectrometry and compartmental modelling.

RESULTS

Compared with the placebo, ATV significantly decreased VLDL-TG concentration (-40%, p < 0.001) by increasing VLDL-TG fractional catabolic rate (FCR) (+47%, p < 0.01). ATV plus ω-3 FAEEs lowered VLDL-TG concentration to a greater degree compared with placebo (-46%, p < 0.001) or ATV monotherapy (-13%, p = 0.04). This was achieved by a reduction in VLDL-TG production rate (PR) compared with placebo (-32%, p = 0.008) or ATV (-20%, p = 0.03) as well as a reciprocal increase in VLDL-TG FCR (+42%, p < 0.05) compared with placebo.

CONCLUSION

In insulin resistant, dyslipidaemic, obese men, ATV improves VLDL-TG metabolism by increasing VLDL-TG FCR. The addition of 4 g/day ω-3 FAEE to statin therapy provides further TG-lowering by lowering VLDL-TG PR.

MTP inhibitor decreases plasma cholesterol levels in LDL receptor-deficient WHHL rabbits by lowering the VLDL secretion

To examine whether a microsomal triglyceride transfer protein (MTP)-inhibitor is effective in patients with homozygous familial hypercholesterolemia, we administered (2S)-2-cyclopentyl-2-[4-[(2,4-dimethyl-9H-pyrido[2,3-b]indol-9-yl)methyl]phenyl]-N-[(1S)-2-hydroxy-1-phenylethyl]ethanamide (Implitapide), a new MTP inhibitor, to low-density lipoprotein (LDL)-receptor-deficient Watanabe heritable hyperlipidemic (WHHL) rabbits at doses of 3, 6, and 12 mg/kg for 4 weeks. In the 12 mg/kg group, the plasma cholesterol and triglyceride levels were decreased by 70% and 45%, respectively, and the very low-density lipoprotein (VLDL) secretion rate was decreased by 80%. The composition of newly secreted VLDL was similar in each group. This suggests that Implitapide diminished the number of VLDL particles secreted from the liver. Although the ratio of vitamin E/LDL was not altered by Implitapide, triglyceride accumulation and a decrease in vitamin E were observed in the liver. In conclusion, an inhibition of VLDL secretion led to a decrease of plasma LDL in WHHL rabbits, and MTP inhibitors should have hypolipidemic effects against homozygous familial hypercholesterolemia.

Age-associated decrease in plasma cholesterol and changes in cholesterol metabolism in homozygous Watanabe heritable hyperlipidemic rabbits

We examined the cholesterol metabolism of homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits, an animal model deficient in low-density lipoprotein (LDL) receptors, to clarify the mechanism of the age-associated decrease of plasma total cholesterol, one of the properties of WHHL rabbits. The rabbits were examined at several ages: after weaning at 3 months, at sexual maturation at 6 months, at 12 months, and at 24 months, equivalent to about 35 years of age in humans. Plasma total cholesterol, triglyceride, and phospholipid levels decreased with aging by about 45%. These reductions were mainly dependent on a decrease in the LDL fraction. In the liver microsomal fraction, although there were no age-related changes in the cholesterol concentration and cholesterol 7alpha-hydroxylase (C7H) activity, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity increased and acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity decreased with aging. The lipolytic activity varied with aging. The secretion rate of very-low-density lipoprotein (VLDL) cholesterol as determined by injection of Triton WR-1339 decreased significantly with aging, while the catabolic rate of VLDL cholesterol was about 2-fold higher in the oldest group versus the young groups. From these results, we conclude that the age-associated decrease in plasma cholesterol in WHHL rabbits is related not only to a decrease in the secretion rate of VLDL cholesterol but also to an increase in the catabolic rate.

Effects of statins on triglyceride metabolism

Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or "statins," have been extremely efficacious in decreasing low-density lipoprotein (LDL) cholesterol levels in patients with hypercholesterolemia and in treating patients with dysbetalipoproteinemia, a relatively rare form of hyperlipidemia. Clinical trials have indicated that statins can significantly lower very-low-density lipoprotein (VLDL) triglyceride levels, although the central mechanism of action of statins-i.e., increasing the number of LDL receptors-would appear to suggest that they would have no significant effect on VLDL levels. Through a review of published data from animal and human studies, this article addresses the important clinical question of how drugs that inhibit the rate-limiting enzyme for cholesterol can affect triglyceride metabolism.

Triglyceride metabolism in heterozygote of Watanabe heritable hyperlipidemic rabbit

The present study was conducted in order to examine the role of low-density lipoprotein (LDL)-receptor activity in very-low-density lipoprotein (VLDL) triglyceride metabolism in vivo. Fructose-feeding (10% in drinking water) for 2 weeks resulted in elevated plasma triglyceride in heterozygote of Watanabe heritable hyperlipidemic (WHHL) rabbit (WHHLH) associated with suppressed fractional catabolic rate (FCR) of plasma triglyceride, whereas Japanese white (JW) rabbit with normal LDL receptor activity showed no remarkable change in plasma triglyceride turnover after fructose-feeding, suggesting an involvement of LDL receptor activity on triglyceride metabolism. Thereafter, in order to stimulate cellular LDL receptor activity, fluvastatin, a new 3-hydroxy-3-methylglutaryl-coenzyme-A (HMG-CoA) reductase inhibitor, was administered orally (1.52 +/- 0.26 mg/kg) to fructose-fed WHHLH. Significant suppression of triglyceride secretion rate (TGSR) was observed after treatment. However, since plasma triglyceride level was markedly suppressed, FCR of plasma triglyceride was significantly elevated by fluvastatin. Thus, it is speculated from the present data that LDL receptor activity is significantly involved in VLDL triglyceride metabolism in rabbits.

Pravastatin sodium, a competitive inhibitor of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase, decreases the cholesterol content of newly secreted very-low-density lipoprotein in Watanabe heritable hyperlipidemic rabbits

We examined the secretion of very-low-density lipoprotein (VLDL) when hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme of cholesterol biosynthesis, was inhibited. To inhibit HMG-CoA reductase in liver, pravastatin sodium, a competitive inhibitor of HMG-CoA reductase, was administered to homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits, a low-density lipoprotein receptor-deficient animal model, at a dosage of 50 mg/kg per day for 5 weeks. Although triglyceride levels were not changed, total cholesterol levels of sera and each atherogenic lipoprotein were decreased by approximately 30%. As a result, the percentage of cholesterol concentration in newly secreted VLDL was significantly decreased by 24%. The VLDL secretion rate was determined by intravenous injection of Triton WR-1339. The VLDL secretion rate was significantly decreased by 23% using cholesterol as an index, but it did not change using triglyceride, phospholipid, or protein as an index. It is concluded that one of the mechanisms of serum total cholesterol decrease due to reduction of the putative cholesterol pool of the liver in homozygous WHHL rabbits is caused by a decrease of cholesterol content in newly secreted VLDL particles.

In vivo regulation of hepatic lipase activity and mRNA levels by diets which modify cholesterol influx to the liver

The aim of this study was to assess whether diets enriched in cholesterol, sodium cholate and drugs known to modify liver cholesterol biosynthesis can modulate hepatic lipase (H-TGL) expression and activity in vivo. Female lean Zucker rats, known to be good responders to cholesterol, were fed for 7 days with a control C diet or the C diet supplemented (w/w) with either 2% cholesterol, 0.5% sodium cholate, 2% cholestyramine or simvastatin (0.1%) added to the cholestyramine diet or given by gavage (10 mg/rat) for 3 days. H-TGL activity decreased by 34% with cholesterol, and by 27% when both cholesterol and cholate were administered to the rats. Under these conditions, H-TGL mRNA decreased by 34% and 87%, respectively. The sharp decrease in H-TGL expression was associated with a strong increase in cholesteryl ester in total liver and in the liver microsome fraction. H-TGL activity decreased by 33% with cholestyramine and the mRNA level decreased by 47%. Simvastatin lowered H-TGL activity by 55% when added to the cholestyramine diet, probably because of a reduction in food intake. When administrated by gavage, simvastatin increased both the H-TGL activity (by 28%) and mRNA (by 23%). These variations may be linked to the availability of mevalonate-derived sterol and non-sterol products.

Effect of simvastatin treatment on plasma apolipoproteins and hepatic apolipoprotein mRNA levels in the genetically hypercholesterolemic rat (RICO)

The effects of long-term treatment with simvastatin on plasma lipoproteins, plasma apolipoproteins, and on hepatic apolipoprotein gene expression were evaluated in genetically hypercholesterolemic (RICO) rats. Simvastatin administration caused a decrease in plasma triglyceride and phospholipid concentrations. Plasma cholesterol concentration was not changed by simvastatin, but cholesterol distribution among plasma lipoproteins was altered. Plasma apo B, apo A-I, and apo A-IV concentrations were lowered by simvastatin treatment whereas plasma apo E concentration was not affected by this drug. In the liver, simvastatin treatment induced a significant decrease of apo E mRNA level but had no effect on apo B, apo A-I, and apo A-IV mRNA abundances. It appears that simvastatin may modify plasma apolipoprotein concentrations by influencing their hepatic synthesis at both pre- and posttranscriptional levels.