Dietary fish oil plus lovastatin decreases both VLDL and LDL apo B production in miniature pigs

JF, Lichtenstein AH. The Ossabaw Pig Is a Suitable Translational Model to Evaluate Dietary Patterns and Coronary Artery Disease Risk

Background

Animal models that mimic diet-induced human pathogenesis of chronic diseases are of increasing importance in preclinical studies. The Ossabaw pig is an established model for obesity-related metabolic disorders when fed extreme diets in caloric excess.

Objective

To increase the translational nature of this model, we evaluated the effect of diets resembling 2 human dietary patterns, the Western diet (WD) and the Heart Healthy Diet (HHD), without or with atorvastatin (-S or +S) therapy, on cardiometabolic risk factors and atherosclerosis development.

Methods

Ossabaw pigs (n = 32; 16 boars and 16 gilts, aged 5-8 wk) were randomized according to a 2 × 2 factorial design into 4 groups (WD-S, WD+S, HHD-S, and HHD+S) and were fed the respective diets for 6 mo. The WD (high in saturated fat, cholesterol, and refined grain) and the HHD (high in unsaturated fat, whole grain, and fruit and vegetables) were isocaloric [38% of energy (%E) from fat, 47%E from carbohydrate, and 15%E from protein]. Body composition was determined by using dual-energy X-ray absorptiometry, serum fatty acid (FA) profiles by gas chromatography, cardiometabolic risk profile by standard procedures, and degree of atherosclerosis by histopathology.

Results

Serum FA profiles reflected the predominant dietary FA. Pigs fed the WD had 1- to 4-fold higher concentrations of LDL cholesterol, non-HDL cholesterol, HDL cholesterol, high-sensitivity C-reactive protein (hs-CRP), tumor necrosis factor α (TNF-α), alkaline phosphatase (ALP), and alanine aminotransferase (ALT) compared with HHD-fed pigs (all P-diet < 0.05). Statin therapy significantly lowered concentrations of LDL cholesterol (-39%), non-HDL cholesterol (-38%), and triglycerides (-6%) (P-statin < 0.02). A greater degree of atheromatous changes (macrophage infiltration, foam cells, fatty streaks) and lesion incidence was documented in the coronary arteries (P-diet < 0.05), as well as 2- to 3-fold higher lipid deposition in the aortic arch or thoracic aorta of WD- compared with HHD-fed pigs (P-diet < 0.001).

Conclusions

Ossabaw pigs manifested a dyslipidemic and inflammatory profile accompanied by early-stage atherosclerosis when fed a WD compared with an HHD, which was moderately reduced by atorvastatin therapy. This phenotype presents a translational model to examine mechanistic pathways of whole food-based dietary patterns on atherosclerosis development.

Dietary fish oil exerts hypolipidemic effects in lean and insulin sensitizing effects in obese LDLR-/- mice

Obesity is often associated with dyslipidemia, insulin resistance, and hypertension. Together, these metabolic perturbations greatly increase the risk of developing cardiovascular disease and diabetes. Although fish oil is a well-established hypolipidemic agent, the mechanisms by which it mediates its lipid-lowering effects are not clear. In addition, it has not been established whether dietary fish oil has different effects in lean and obese mice. LDL receptor deficient (LDLR-/-) and leptin deficient mice on a LDLR-/- background (ob/ob;LDLR-/-) were fed a high fat diet (39% total fat) supplemented with 6% olive oil or fish oil for 6 wk. Fish oil supplementation resulted in lower concentrations of plasma total cholesterol (P < 0.01), triglycerides (P < 0.01), and free fatty acids (P < 0.001) in lean LDLR-/- mice, but not in ob/ob;LDLR-/- mice. In contrast, a fish oil diet did not modulate insulin sensitivity in lean LDLR-/- mice, but it improved insulin sensitivity in ob/ob;LDLR-/- mice (P < 0.05) compared with olive oil fed ob/ob;LDLR-/- mice. Interestingly, plasma adiponectin concentrations were significantly higher and hepatic steatosis was reduced in both mouse models upon fish oil feeding. Finally, fish oil fed LDLR-/- mice exhibited higher hepatic AMP activated protein kinase (AMPK) phosphorylation (P < 0.05), whereas AMPK phosphorylation was not elevated by fish oil feeding in ob/ob;LDLR-/- mice. Taken together, our data suggest that fish oil reduces hepatic steatosis in both lean and obese mice, has potent plasma lipid lowering effects in lean mice, and exerts insulin sensitizing effects in obese mice.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

Addition of arginine but not glycine to lysine plus methionine-enriched diets modulates serum cholesterol and liver phospholipids in rabbits

Previous experiments from our laboratory showed that in rabbits fed an amino acid diet corresponding to 30% casein, enrichment of the diet with L-lysine and L-methionine caused a marked increase in serum total and LDL cholesterol levels as well as a substantial body weight loss. Both effects were partially prevented by supplementation with L-arginine. The present studies were designed to extend this earlier observation by assessing the role of different dietary amino acids in modulation of cholesterolemic responses and body weights. In the first experiment, the original lysine and methionine-enriched diet was supplemented with glycine in an attempt to modify methionine metabolism, and thus to reduce body weight loss. In addition, the mechanism of action of lysine and methionine was investigated by quantitation of major liver phospholipids. The results showed that glycine addition had no effect on weight loss or hypercholesterolemia, nor did it alter plasma levels of homocyst(e)ine, an intermediate in methionine metabolism. However, enrichment of the diet with lysine and methionine (with or without glycine) significantly increased liver levels of phosphatidylcholine and the ratio of phosphatidylcholine to phosphatidylethanolamine, apparently through increased enzymatic conversion. These changes were consistent with higher lipoprotein levels and thus may explain the hypercholesterolemia. A second experiment showed that similar effects on body weights and serum cholesterol could be obtained by adding lysine and methionine to a diet containing amino acids equivalent to only 15% casein, or 15% intact casein. This approach is more physiologic and also reduces the expense of experiments designed to study the effects of lysine and methionine in more detail.

Role of dietary lysine, methionine, and arginine in the regulation of hypercholesterolemia in rabbits

These experiments were conducted to see whether the hypercholesterolemia produced by a diet enriched in lysine (Lys) and methionine (Met) can be reproduced by feeding these amino acids separately, and whether dietary arginine (Arg) counteracts their hypercholesterolemic effects. Another aim was to investigate the mechanisms involved in modulations of serum cholesterol levels by these amino acids. The results of this study, which were in agreement with the results of earlier experiments in our laboratory, showed that feeding a low-fat, cholesterol-free, semipurified amino acid diet enriched with Lys + Met to rabbits caused a marked increase in serum total and low density lipoprotein cholesterol and apolipoprotein B levels, whereas a similar diet enriched in essential ketogenic amino acids (EketoAA) resulted in a more moderate increase in these parameters. Supplementing the diet with either Lys or Met alone was also less effective in inducing hypercholesterolemia than increasing levels of both amino acids. Dietary Arg partially counteracted the hypercholesterolemic effect of Lys + Met but not that of the EketoAA or of Lys and Met fed separately. The growth performance of rabbits fed the Lys + Met diet was inferior to that of those fed the other diets. Liver total phospholipid levels and the ratio of phosphatidylcholine to phosphatidylethanolamine were higher in rabbits fed the Lys + Met-enriched diet than in those animals fed a diet in which Arg was supplemented. In conclusion, our results indicate that high levels of both Lys and Met are needed to cause a maximum elevation of serum cholesterol and that the moderately antihypercholesterolemic effect of Arg is seen only when both amino acids are supplemented. They also suggest that these essential amino acids may affect cholesterol metabolism partly through alteration of liver phospholipids.

Inhibition of cholesterol esterification by DuP 128 decreases hepatic apolipoprotein B secretion in vivo: effect of dietary fat and cholesterol

To further test the hypothesis that newly synthesized cholesteryl esters regulate hepatic apolipoprotein B (apoB) secretion into plasma, apoB kinetic studies were carried out in seven control miniature pigs and in seven animals after 21 days intravenous administration of the acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor DuP 128 (2.2 mg/kg/day). Pigs were fed a fat (34% of calories; polyunsaturated/monounsaturated/saturated ratio, 1:1:1) and cholesterol (400 mg/day; 0.1%; 0.2 mg/kcal) containing pig chow based diet. DuP 128 significantly reduced total plasma triglyceride and very low density lipoprotein (VLDL) triglyceride concentrations by 36 and 31%, respectively (P<0.05). Autologous 131I-VLDL and 125I-LDL were injected simultaneously into each pig and apoB kinetic data was analyzed using multicompartmental analysis (SAAM II). The VLDL apoB pool size decreased by 26% (0.443 vs. 0.599 mg/kg; P<0. 001) which was due entirely to a 28% reduction in VLDL apoB production or secretion rate (1.831 vs. 2.548 mg/kg/h; P=0.006). The fractional catabolic rate (FCR) for VLDL apoB was unchanged. The LDL apoB pool size and production rate were unaffected by DuP 128 treatment. Hepatic microsomal ACAT activity decreased by 51% (0.44 vs. 0.90 nmol/min/mg; P<0.001). Although an increase in hepatic free cholesterol and subsequent decrease in both LDL receptor expression and LDL apoB FCR might be expected, this did not occur. The concentration of hepatic free cholesterol decreased 12% (P=0.008) and the LDL apoB FCR were unaffected by DuP 128 treatment. In addition, DuP 128 treatment did not alter the concentration of hepatic triglyceride or the activity of diacylglycerol acyltransferase, indicating a lack of effect of DuP 128 on hepatic triglyceride metabolism. In our previous studies, DuP 128 treatment of miniature pigs fed a low fat, cholesterol free diet, decreased VLDL apoB secretion by 65% resulting in a reduction in plasma apoB of 60%. We conclude that in miniature pigs fed a high fat, cholesterol containing diet, the inhibition of hepatic cholesteryl ester synthesis by DuP 128 decreases apoB secretion into plasma, but the effect is attenuated relative to a low fat, cholesterol free diet.

Inhibition of cholesterogenesis decreases hepatic secretion of apoB-100 in normolipidemic subjects

We examined the effect of simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on the kinetics of very low-density lipoprotein apolipoprotein B-100 (VLDL apoB) in 13 normolipidemic men in a placebo-controlled crossover study. Simvastatin significantly decreased the plasma concentrations of low-density lipoprotein (LDL) cholesterol by 36%, triglycerides by 26%, mevalonic acid by 34%, and lathosterol by 32%. Hepatic secretion of VLDL apoB was measured using a primed constant intravenous infusion of [1-13C]leucine with monitoring of isotopic enrichment of apoB by gas chromatography-mass spectrometry; fractional turnover rate was derived using a monoexponential function. Simvastatin decreased VLDL apoB pool size by 53% and the hepatic secretion rate of VLDL apoB by 46% but did not significantly alter its fractional catabolism. The change in hepatic VLDL apoB secretion was significantly and independently correlated with changes in plasma mevalonic acid and lathosterol concentrations and the lathosterol-to-cholesterol ratio. The data support the hypothesis that the rate of de novo cholesterol synthesis directly regulates the hepatic secretion of VLDL apoB in normal subjects.

Simvastatin decreases the hepatic secretion of very-low-density lipoprotein apolipoprotein B-100 in heterozygous familial hypercholesterolaemia: pathophysiological and therapeutic implications

We studied six patients with heterozygous familial hypercholesterolaemia (FH) before and after 8 weeks of treatment with simvastatin (40 mg day-1), an inhibitor of 3-hydroxy-3-methyl-glutaryl-Coenzyme A. Simvastatin decreased plasma low-density lipoprotein (LDL) cholesterol by 43% (P = 0.002), triglycerides by 15% [corrected] (P = 0.05) and mevalonic acid (a measure of in vivo cholesterol synthesis) by 20% (P = 0.002); high-density lipoprotein cholesterol increased by 17% (P = 0.02). The hepatic secretion rate of very-low-density lipoprotein apolipoprotein B-100 (VLDL apoB) was measured directly using a primed, constant intravenous infusion of 1-[13C]-leucine with monitoring of the isotopic enrichment of apoB by gas chromatography-mass spectrometry; fractional secretion rate (FSR) was derived using a monoexponential function. Simvastatin decreased the FSR, ASR and pool size of VLDL apoB by 17% (14.3 (SEM 3.6)) vs. (11.9 (SEM 3.5) pools day-1, P = 0.10), 83% (51.4 (SEM 17.9) vs. (8.6 (SEM 1.4), P = 0.007 mg kg-1 day-1) and 65% (234.2 (SEM 30.4) vs. 82.6 (SEM 24.0) mg, P = 0.02), respectively. The change in the ASR of VLDL apoB was significantly correlated with the change in plasma LDL cholesterol concentration (P = 0.04), but not with the change of triglyceride or mevalonic acid. We conclude that the hepatic secretion of VLDL apoB in FH is decreased by simvastatin, which may partly explain the fall in plasma cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of apolipoprotein B secretion in hepatocytes from Watanabe heritable hyperlipidemic rabbit, an animal model for familial hypercholesterolemia

Regulation of apolipoprotein B-100 (apo B) secretion in the liver in familial hypercholesterolemia (FH) remains largely unknown. In a previous study, we developed a rabbit hepatocyte culture system and investigated a cellular mechanism which regulates apo B secretion from hepatocytes in response to cellular cholesteryl ester contents [18]. Using this system, in the present study, we investigated regulation of apo B secretion in hepatocytes from the Watanabe heritable hyperlipidemic (WHHL) rabbit, an animal model for FH. Incubation with low density lipoproteins (LDL) at concentrations of 50 or 200 micrograms/ml, which increased both cellular cholesteryl ester and apo B secretion significantly in normal rabbit hepatocytes, did not cause such increases in WHHL rabbit hepatocytes. However, when WHHL rabbit hepatocytes were incubated with LDL at a concentration of 500 micrograms/ml, a significant increase in cellular cholesteryl ester and apo B secretion was observed. The effect of the increase in cellular level of cholesteryl ester upon apo B secretion in WHHL rabbit hepatocytes was compatible with that in normal rabbit hepatocytes. Indeed, when WHHL rabbit hepatocytes were incubated with LDL at 1680 micrograms/ml, a concentration comparable to that of WHHL rabbit plasma, the amount of LDL degradation, cellular cholesteryl ester level, and level of apo B secretion were the same as those in normal rabbit hepatocytes that were incubated with LDL at 160 micrograms/ml, a concentration comparable to that of normal rabbit plasma. In summary, our present study suggests that, at a steady state with such a high plasma concentration of LDL, the hepatic cholesterol content in WHHL rabbits could be set at the same level as in normal rabbits. It was also shown that cellular cholesteryl ester contents had the same regulatory effect on apo B secretion in WHHL rabbit hepatocytes as in normal rabbit hepatocytes. Therefore, we conclude that in the presence of the genetic defect of the LDL receptor, plasma cholesterol in WHHL rabbit is maintained at a concentration such that apo B secretion is similar to that in normal rabbit.

Lipoprotein lipases, lipoprotein density gradient profile and LDL receptor activity in miniature pigs fed fish oil and corn oil

The effects of fish oil and corn oil on plasma lipoprotein concentrations, the lipolytic enzymes, lipoprotein lipase and hepatic triacylglycerol lipase, the density distribution of the plasma lipoproteins and LDL receptor activity were studied. These experiments were designed, in part, to define the mechanism(s) responsible for the increased conversion of plasma VLDL apolipoprotein B to LDL and a decreased LDL apolipoprotein B fractional catabolic rate described in previous apolipoprotein B kinetic studies. Miniature pigs were fed diets for 3 to 6 weeks containing supplements of corn oil or fish oil as Maxepa. Triacylglycerol and cholesterol in plasma and VLDL were significantly reduced by the fish oil diet. LDL and HDL cholesterol were not significantly changed. The fish oil diet significantly reduced post-heparin plasma lipoprotein lipase and hepatic triacylglycerol lipase activities, which may be an adaptive response to the low concentration of substrates (triacylglycerol-rich lipoproteins) for these enzymes. No differences were observed in the density of VLDL, LDL or HDL as determined by density gradient ultracentrifugation with the fish oil diet. No major changes in percent lipid composition of VLDL, LDL and HDL were observed. No differences were found with respect to LDL uptake by J774 macrophages. Receptor mediated clearance of LDL in vivo, as assessed by measuring the difference in fractional catabolic rate of native vs. methylated LDL decreased significantly by 17% (P < 0.032). We conclude that the increased conversion of VLDL apolipoprotein B to LDL in miniature pigs fed fish oil is not related to an increase in lipolytic enzymes or density distribution of VLDL, but may be due in part to a decrease in LDL receptor activity.