Increases in hyperlipoproteinemia, disturbances in cholesterol metabolism and atherosclerosis induced by dietary restriction in rabbits fed a cholesterol-rich diet

Effect of fish oil supplementation and restricted feeding on body fat distribution and blood lipid profile in a rabbit model of castration-induced obesity

This study investigates the effect of fish oil supplementation and restricted feeding on body fat distribution and blood lipid profile in experimentally induced obesity in rabbits. The trial was carried out with 30 male rabbits, divided into 5 groups of 6 animals each (NC - non-castrated, non-treated, full-diet fed; C100 - castrated, non-treated, full-diet fed; FO100 - castrated, treated with fish oil, full-diet fed; C50 - castrated, non-treated, 50% restricted fed; FO50 - castrated, treated with fish oil, 50% restricted fed). At the end of the experiment, plasma lipids measurement and quantification of fat distribution was performed. The results of this study indicate that fish oil supplementation reduces obesity-associated abnormalities in lipid profile (high-density lipoprotein cholesterol to low-density lipoprotein cholesterol ratio and non-esterified fatty acids) and in body fat distribution in full-diet fed rabbits. Restricted feeding (C50) alone and the combination of restricted feeding and fish oil supplementation (FO50) in particular, has a detrimental effect on the lipid profile despite the marked reduction in intra-abdominal fat.

Dietary restriction slightly affects glucose homeostasis and delays plasma cholesterol removal in rabbits with dietary lipid lowering

Dietary restriction (DR) has been reported to have beneficial effects on atherosclerotic progression as well as lipid and glucose metabolism, but little is known about whether these effects can be enhanced or weakened by dietary lipid lowering. Here, after 12 weeks of high-cholesterol diet feeding, hypercholesterolemic rabbits were fed with either a standard chow diet ad libitum (AL) or a standard chow diet with DR for 16 weeks of dietary lipid lowering. We found that the DR group exhibited a loss of body weight, smaller internal organs, and reduced fat mass, while the AL group accumulated more subcutaneous fat than the baseline group. DR treatment slightly worsened glucose tolerance but enhanced insulin sensitivity, and a slight effect of DR on insulin secretion was also observed. After dietary cholesterol withdrawal, rabbits showed persistent lowering of total cholesterol and triglycerides in plasma. However, the DR group had significantly higher plasma total cholesterol than the AL group at most time points during weeks 7 to 16 of lipid lowering. Although both the AL and DR groups developed more severe atherosclerosis than the baseline group, DR did not improve atherosclerotic progression or the accumulation of macrophages and smooth muscle cells. We conclude that DR affected glucose and lipid metabolism but did not ameliorate atherosclerosis in rabbits when associated with lipid lowering by dietary cholesterol withdrawal.

Effect of behavioral interventions on insulin sensitivity and atherosclerosis in the Watanabe heritable hyperlipidemic rabbit

OBJECTIVE

A previous study suggested that insulin metabolic variables play a role in the progression of atherosclerosis in Watanabe heritable hyperlipidemic (WHHL) rabbits. The present study sought to determine: 1) if young, individually caged WHHLs are insulin-resistant relative to New Zealand white (NZW) rabbits and 2) whether dietary or exercise interventions can improve insulin sensitivity and slow the development of atherosclerosis in these animals.

METHODS

Forty-two WHHLs were assigned to a dietary, exercise, or control condition, and 12 NZWs were used as a comparison control group. The intervention ran from 3 to 7 months of age, and all animals received an intravenous glucose tolerance test at the beginning and end of the intervention.

RESULTS

WHHLs were insulin-resistant relative to NZWs at 3 months of age. Whereas the dietary intervention was effective in controlling insulin resistance, WHHLs in the exercise group without dietary restriction and the control group exhibited significant increases in insulin resistance. No intervention significantly influenced the progression of atherosclerosis.

CONCLUSIONS

Young WHHLs are insulin-resistant during an early period when atherosclerosis is developing rapidly. Dietary restriction, but not exercise without weight control, is effective in controlling insulin metabolic variables in the WHHL model. Although dietary intervention can reduce cardiovascular risk factors such as insulin resistance, it is not effective in slowing the development of atherosclerosis in these genetically dyslipidemic animals. Similarly, exercise training, without dietary control, does not influence the progression of disease in WHHLs.

Expression of a sterol carrier protein-x gene in the yellow fever mosquito, Aedes aegypti

The sterol carrier protein-x (SCP-x), a peroxisomal thiolase/nonspecific lipid binding protein, was characterized in the yellow fever mosquito, Aedes aegypti. The Aedes aegypti SCP-x (AeSCP-x) has 83% and 75% similarities to Drosophila and mammalian SCP-x, respectively. However, the AeSCP-x gene did not produce multiple transcripts, which is characteristic of the vertebrate SCP-x gene. Levels of AeSCP-x transcription were higher in larvae and pupae. Gut tissue showed the highest level of AeSCP-x mRNA in larvae. In adults, low levels of AeSCP-x transcription were detected in both sexes. Polyclonal antibodies against the sterol carrier protein-2 (SCP-2) domain of AeSCP-x detected two proteins of 62 kDa and 13 kDa. The results indicate that AeSCP-x is proteolytically cleaved after translation to produce a smaller protein that contains only the SCP-2 domain, which is similar to post-translational modification of the vertebrate's SCP-x to produce multiple products.

Hypercholesterolemia in the rabbit induced by feeding graded amounts of low-level cholesterol. Methodological considerations regarding individual variability in response to dietary cholesterol and development of lesion type

While a number of studies have presented detailed examinations of lesion development in the cholesterol-fed rabbit, individual variability in response to cholesterol feeding and type of lesion produced relative to the degree of cholesterol exposure is not well defined. This study analyzed such critical parameters in an attempt to further characterize the model and establish a baseline for future testing of treatments targeted at limiting atherosclerosis. For these experiments, male New Zealand White rabbits were fed atherogenic diets consisting of 0.05%, 0.10%, 0.15%, 0.20%, or 0.25% cholesterol dissolved in 6% peanut oil for 31 to 32 weeks. Raising dietary cholesterol from 0.05% to 0.15% resulted in a less than twofold stepwise increase in total plasma cholesterol (TPC) exposure (area under plasma cholesterol versus time curve), whereas further increases in cholesterol intake resulted in an exponential four- to fivefold increase in TPC exposure. Regression analysis of TPC exposure with aortic sudanophilia demonstrated a threshold of approximately 5000 cholesterol weeks; below this limit lesions were minimal, and above this value the degree of plaque correlated with TPC exposure. Furthermore, a wide biological variability occurred among rabbits with respect to individual responsiveness to dietary cholesterol. In the aorta, various types of plaques, from fatty streaks to atheromatous lesions, were observed, depending on the degree of cholesterol intake. Diets consisting of < 0.15% cholesterol resulted in the development of fatty streak lesions, while transitional lesions and atheromatous plaques were mostly found with higher cholesterol feeding. Coronary artery atherosclerosis was present in > 50% of animals fed diets > or = 0.15% cholesterol. Despite the level of TPC exposure, coronary lesions in epicardial vessels were generally the fibrous type, whereas intramyocardial arteries demonstrated predominantly intimal foam cells. In conclusion, by adjusting dietary cholesterol intake and selecting rabbits with a similar responsiveness to cholesterol, the overall cholesterol exposure can be more closely controlled to minimize the inherent individual variability among animals in this model. The nature of the target lesion must also be carefully considered, because the efficacy of some treatments may depend on the type of atherosclerotic plaque.

Cholesterol-fed and casein-fed rabbit models of atherosclerosis. Part 1: Differing lesion area and volume despite equal plasma cholesterol levels

One-month-old male New Zealand White rabbits were fed either a cholesterol-free casein diet (CAS; n = 10); low-level cholesterol-supplemented (0.125% to 0.5% by weight) chow (CH; n = 10); or standard laboratory rabbit chow (n = 3) for 24 weeks, during which total plasma cholesterol (TPC) levels were matched for the two experimental groups (TPCCAS = 475 +/- 39 mg/dL; TPCCH = 515 +/- 70 mg/dL). The percentage of cholesterol partitioned into each of the lipoprotein fractions except high-density lipoprotein (HDL) was significantly different for the experimental groups: casein-fed rabbits had a primarily low-density lipoprotein (LDL) hypercholesterolemia while cholesterol-fed rabbits had approximately equal levels of very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and LDL cholesterol. Despite matched TPC, lesions in CH animals covered twice the luminal surface area (as detected by oil red O staining; P < .05) and had three times the total volume compared with lesions in the CAS group (P < .05). Lesion volume was positively correlated with TPC and IDL and LDL cholesterol for the CAS group and with TPC and IDL cholesterol for the CH group. When the experimental groups were combined, TPC and VLDL and IDL cholesterol were positively correlated with the lesion volume. Probability of occurrence maps revealed, however, that both groups were virtually identical with respect to the topographic distribution of lesions in the thoracic and abdominal aortas. The data suggested that the differential partitioning of cholesterol into the lipoprotein fractions seen in CAS and CH rabbits influenced lesion area and volume but not topographic distribution.

Dietary and mechanically induced rabbit iliac-femoral atherosclerotic lesions: a chemical and morphologic evaluation

The effect of combining chronic endothelial injury and intermittent meal feeding of a high and low cholesterol, coconut oil, peanut oil diet on plama lipid and lipoprotein content and on the formation of atherosclerotic lesions within the iliac-femoral artery of rabbits was studied. Alternate feeding of a 1 or 0.1% cholesterol, 3% coconut oil, 3% peanut oil diet for 3 to 14 weeks resulted in a 4- to 11-fold increase in plasma cholesterol with 59 to 79% of the plasma cholesterol eluting in a molecular weight fraction comparable to human low density lipoproteins (LDL). In the iliac-femoral artery, an atherosclerotic intimal lesion with an average cross-sectional area of 0.452 mm2 was present in 98% of the animals. The lesion was typically eccentric in location and contained both superficial- and deep-intimal lipid-filled monocyte-macrophages, extracellular lipid, smooth muscle cells, and extracellular connective tissue matrix. The relative percent lipid composition of the iliac-femoral lesion was 62% cholesteryl ester, 21% free cholesterol, and 17% phospholipid. Thus, we conclude that the combination of meal feeding a cholesterol/fat diet, dietary regimen and chronic mild endothelial injury in the rabbit results in (1) a diet-induced hypercholesterolemia in which LDL appear to be the predominant lipoprotein; and (2) a lesion within the iliac-femoral artery comparable in histologic and chemical composition to a human fatty streak.

Relative effects of feeding saturated fats and cholesterol on fluidity of rabbit lipoproteins

1. The effects of saturated fat and cholesterol on lipoprotein fluidity were tested in New Zealand white rabbits fed diets containing corn oil (CO) or cocoa butter (CB) with and without added 0.2% cholesterol. 2. Saturated fats had little effect on fluidity in any lipoprotein fraction. 3. Cholesterol feeding dramatically reduced fluidity in VLDL and LDL, but minimal change was noted in HDL. 4. Cholesterol-fed rabbits were hypercholesteroloemic throughout the 10-month study. 5. The rabbits became adapted to cholesterol feeding as VLDL became more fluid with time.

Influence of high density lipoprotein (HDL), prepared from human blood, on prostanoid formation, serum and tissue lipids and development of arteriosclerosis in cholesterol rich fed rabbits

The i.v. administration of high density lipoprotein (HDL) into cholesterol fed rabbits decreased statistically significantly the serum level of total cholesterol and of low density lipoprotein cholesterol after a feeding period of 8 weeks. These diminished levels of cholesterol were associated with a statistically significant reduction in the levels of cholesterol esters in kidneys and platelets but not in hepatic tissue or in aorta. Macroscopically detectable arteriosclerosis was not statistically significantly diminished. The formation of prostanoids by the aorta remained unchanged. The atherogenic role of immunologic factors acting against the heterologous HDL may have compensated for the antiatherogenic HDL action on plasma and tissue lipids.

Low-density lipoprotein catabolism in WHHL rabbits after partial ileal bypass surgery

The effect of partial ileal bypass surgery (PIB) on lipoprotein concentrations and compositions and on the catabolism of low-density lipoproteins (LDL) was studied in Watanabe heritable hyperlipidemic (WHHL) rabbits. After PIB, total serum cholesterol was 65% lower (6.22 +/- 1.58 vs. 17.24 +/- 3.22 mmol/l) and LDL cholesterol 81% lower (2.02 +/- 0.95 vs. 10.90 +/- 3.60 mmol/l) than in control WHHL rabbits; cholesteryl esters, expressed as percentage of mass, were 55% lower in the very-low and intermediate-density lipoprotein (VLDL + IDL) fractions, and 45% lower in LDL, whereas triacylglycerols were 89% higher in VLDL + IDL and 121% higher in LDL. The fractional catabolic rate (FCR) of LDL protein (apoLDL) from operated animals was 10% higher than that from controls in all animals (0.55 +/- 0.10 vs. 0.50 +/- 0.10 pools/day; P less than 0.01). The FCR of autologous apoLDL in PIB rabbits was 50% higher than that of autologous apoLDL in control rabbits (0.63 +/- 0.05 vs. 0.42 +/- 0.06 pools/day); this was not caused by induction of receptor-mediated clearance of LDL. The production rate of apoLDL after PIB in PIB rabbits was 50% lower compared to control apoLDL in controls (26.0 +/- 6.7 vs. 51.7 +/- 16.4 mg/kg per day). We conclude that PIB lowers LDL cholesterol in WHHL rabbits by a decreased production of LDL, by an increased non-specific clearance of LDL and by compositional changes, which lead to LDL particles containing less cholesterol.

Dietary restriction amplifies the metabolic disturbances of very-low-density lipoproteins in cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on VLDL turnover was investigated in cholesterol-fed rabbits. Rabbits on standard, cholesterol and restricted cholesterol diets were injected with homologous 125I-labelled VLDL. Accompanying the amplification of hypercholesterolemia, additional disturbances of VLDL turnover were observed when cholesterol feeding was associated with dietary restriction. Cholesterol-fed rabbits with normal caloric ration exhibited delayed clearance of 125I-labelled apolipoprotein B component of VLDL compared to control rabbits. This was markedly accentuated in underfed rabbits, indicating further down-regulation of apolipoprotein B,E receptors in these animals. Furthermore, a reduced proportion of radiolabelled apolipoprotein B was converted from VLDL to intermediate-density lipoprotein (IDL) and LDL in both groups receiving cholesterol-rich diets. Thus, the combination of further impairment in plasma clearance of VLDL and the poor conversion into IDL and LDL could account for the massive increase of beta-VLDL in underfed animals on cholesterol-rich diets.

Effects of a low-energy diet associated with egg supplementation on plasma cholesterol and lipoprotein levels in normal subjects: results of a cross-over study

1. The influence of a low-energy diet when associated with high-cholesterol intake was investigated in seventeen normal men during an 8-week cross-over study. The subjects were given a daily supplement of two whole eggs and two egg yolks (approximately 1 g cholesterol) either with their usual diet for 4 weeks or with a low-energy diet for 4 weeks. Each subject took part randomly in both dietary periods. 2. During the first part of the study, no changes occurred in the plasma cholesterol of the subjects with egg supplementation of the usual diet. 3. In contrast, the low-energy diet and associated weight loss markedly decreased tolerance to high-cholesterol intake resulting in increased plasma cholesterol. The mean rise was 22.7% but with wide individual variations in the response. This was almost completely normalized when the subjects returned to their usual energy intake indicating the involvement of weight reduction in the increase observed. 4. Changes in low-density-lipoprotein (LDL) cholesterol were parallel to those of total plasma cholesterol with an increase following the low-energy diet and normalization after body-weight recovery. 5. The opposite effect was shown with the low-energy diet after previous adaptation to the consumption of four eggs per day. This dietary regimen resulted in a decrease in plasma cholesterol although it was not significant. Moreover, the lipoprotein profile was improved with a decrease in very-low-density-lipoprotein (VLDL) cholesterol and an increase in high-density-lipoprotein (HDL) cholesterol. 6. High-cholesterol intake induced significant changes in lipoprotein composition whatever the energy ration. LDL and HDL were enriched in cholesterol esters as early as the 1st month of egg supplementation of the diet. 7. Taken together, the results emphasize the possible adverse effect of slimming diets when associated with high-cholesterol intake. The existence of 'high-responders' to these dietary conditions calls for special attention to be paid to the cholesterol content of restricted diets.

Effect of dietary restriction on the plasma apolipoprotein pattern in cholesterol-fed rabbits

Dietary restriction (half of the control ration) was performed in rabbits given either standard or cholesterol-rich diets. The plasma apolipoproteins were studied on the total, d less than 1.21, lipoprotein fraction using polyacrylamide gel electrophoresis. A marked rise in the amount of both apo-B and apo-E appeared in cholesterol-fed rabbits and was enhanced by dietary restriction. These results reflect the aggravation of hypercholesterolemia when cholesterol feeding is carried out in underfed rabbits. In all groups only one molecular species of apo-B, identified as apo-B-100, was present after overnight fasting. Thus, lipoproteins which accumulate in the plasma following cholesterol feeding, associated with dietary restriction or not, probably are remnants of hepatogenous triglyceride-rich lipoproteins.

Effect of dietary restriction on cholesterol biosynthesis in the liver and the intestine of cholesterol-fed rabbits

The effect of dietary restriction (half of the control ration) on cholesterol biosynthesis was investigated in rabbits fed either standard or cholesterol-rich diets. Accompanying the amplification of hypercholesterolemia, additional disturbances of cholesterol metabolism were observed when cholesterol feeding was associated with dietary restriction. In the intestine, underfed rabbits showed a more marked inhibition of duodenal cholesterol biosynthesis from [14C]acetate following cholesterol feeding than rabbits on normal caloric ration. In contrast liver cholesterogenesis was equally suppressed in both groups receiving cholesterol-rich diets. Cholesterol biosynthesis from [14C]mevalonate was also inhibited by cholesterol feeding particularly in the duodenum of underfed rabbits. In addition cholesterol feeding induced a marked increase of the labeled esterified: free cholesterol ratio in the liver, demonstrating intensive esterification, this was enhanced by dietary restriction. The additional cholesterol which accumulates in the plasma and in various tissues in underfed rabbits is of dietary origin since the feedback control of cholesterogenesis by exogenous cholesterol was shown to be very effective in these animals.

Digestibilities of casein and soya-bean protein in relation to their effects on serum cholesterol in rabbits

Diets containing isolated soya-bean protein induce lower levels of serum cholesterol in animals than diets containing casein. Experiments were conducted to investigate whether differences in digestibility of the proteins might explain this effect. At pH 8 with pancreatic enzymes or intestinal peptidase, soya-bean protein was hydrolysed in vitro much less rapidly than casein. However, with pepsin (EC 3.4.23.1) at acid pH, soya-bean protein was hydrolysed more rapidly than casein. These differences in digestibility may be due to pH-dependent changes in solubility of the proteins. Casein and soya-bean protein were most soluble at alkaline and acid pH respectively. Heat treatment of the proteins resulted in lower solubilities and digestibilities. Sonication of soya-bean protein at pH 7.8 increased solubility but only slightly raised digestibility. When fed to rabbits, enzymically hydrolysed soya-bean protein induced a 2.3-fold higher concentration of serum cholesterol than did intact soya-bean protein. The hypocholesterolaemic effect of soya-bean protein may be partly attributable to its low solubility and digestibility at alkaline pH.

Effects of beef, soy and conventional diets on body composition and plasma lipids of young pigs fed restricted or liberal amounts of diet

Effects were determined of restricted and liberal feeding of beef-based, soy-based and conventional diets on growth rate, nutrient absorption, body composition and plasma lipid and urea concentrations of young pigs. Beef and soy diets contained more fat (40-50% of calories vs. 8-9%) and cholesterol (0.09 vs. 0%) than did conventional diets; calorie and protein intakes were equal across diets. Beef-fed pigs had greater average daily gain and absorption of fat and gross energy than did conventionally fed pigs; soy-fed pigs had intermediate growth rates but the greatest absorption of fat and gross energy. With restricted intakes, percentage of body fat was greater in soy-fed pigs than in pigs fed a conventional diet or beef and with liberal feeding, greater in soy- and beef-fed pigs than in pigs provided with a conventional diet. Plasma free fatty acid, triglyceride and cholesterol concentrations were greater in soy- and beef-fed pigs than in conventionally fed pigs. These results indicate that absorption and subsequent partitioning of nutrients toward body fat are influenced by composition as well as the amount of diet consumed. Our results also demonstrate that the pig can successfully be fed diets resembling human diets (i.e., high fat) and because of its physiological similarities to humans our understanding of nutrition and lipid metabolism of humans may be advanced.

Influence of cholesterol feeding and dietary restriction on cholesterol absorption in rabbits

Plasma cholesterol and cholesterol absorption were measured in rabbits given either a standard or cholesterol-rich diet which were fed either ad libitum or reduced to 50% of the control ration. The results confirmed the aggravating effect of dietary restriction on the plasma cholesterol response to cholesterol feeding. Hypercholesterolemia was doubled when cholesterol feeding was associated with reduced dietary intake. The percentage of cholesterol absorbed increased significantly in cholesterol-fed rabbits on normal caloric ration, while dietary restriction had no effect on this parameter either with the standard or the cholesterol-rich diet. These data indicate that the mechanisms by which plasma cholesterol increases in response to cholesterol feeding involve increased cholesterol absorption. Nevertheless the aggravating effect of dietary restriction cannot be attributed to increased cholesterol absorption.