Genetic and physiological aspects of cholesterol accumulation in hyperresponding and hyporesponding rabbits

Effects of manipulation of dietary cholesterol on the function of the thoracic aorta from New Zealand white rabbits

Animal studies, while generally showing loss of endothelium-dependent responses after an elevation in plasma cholesterol, have provided conflicting reports with regard to recovery of function after normalisation of cholesterol level. Therefore, we assessed changes in vascular function after a period of hypercholesterolaemia and the subsequent effect of normalisation of cholesterol levels. Contractile responses to phenylephrine (PE) and endothelium-dependent relaxation in response to carbachol were examined in thoracic aorta from New Zealand White rabbits (NZW) fed a 0.3% cholesterol diet for 20 weeks, from NZW fed a 0.3% cholesterol diet for 20 weeks, followed by standard diet for 20 more weeks, and from their respective age-matched controls. Cholesterol levels were increased in rabbits receiving the 0.3% cholesterol diet (12.7 +/- 3.2 mM; 0.5 +/- 0.1 mM control) and returned to normal when standard diet was reintroduced (0.8 +/- 2.0 mM). Contractile responses were not affected by the period of hypercholesterolaemia. Carbachol-induced relaxation of a submaximal PE contraction was impaired after the period of hypercholesterolaemia (Emax 69 +/- 9%; 95 +/- 3% age-matched (control); the effect was reversed after reintroduction of standard diet (Emax 79 +/- 6%; 82 +/- 2% age-matched control). Our results demonstrate that endothelium-dependent relaxation is impaired after a long-term 0.3% cholesterol diet. Furthermore, after reintroduction of a normal diet, there is no further impairment of endothelium-dependent relaxation and endothelium function improves.

Bile acid and very low density lipoprotein production by cultured hepatocytes from hypo- or hyperresponsive rabbits fed cholesterol

Two groups of rabbits, either hyperresponsive or hyporesponsive to dietary cholesterol, were selected after ten weeks of cholesterol feeding (0.2 g cholesterol/kg body weight per day). Bile acids and very low density lipoprotein (VLDL) production were determined in primary hepatocyte cultures from control, hyper- and hyporesponsive rabbits. Free cholesterol and cholesteryl ester contents in hepatocytes of the hyperresponsive rabbits was significantly increased. In contrast, lipid composition in hepatocytes of the hyporesponders was similar to that of control cells. Cholic acid was the predominant bile acid in the culture medium of hepatocytes together with small amounts of chenodeoxycholic and deoxycholic acids. The rate of cholic acid production by hepatocytes in the hyporesponsive group was two times higher than that in the hyperresponsive group. Bile acid production by control hepatocytes was slightly higher than in the hyperresponsive group. In contrast, secretion of VLDL cholesteryl ester was significantly increased by hepatocytes of the hyperresponsive rabbits. Similar differences in bile acid production were found between hypo- and hyperresponsive rabbits selected after five days of cholesterol feeding and subsequent maintenance on a low cholesterol diet for a period of one month. The results suggest that the increased rate of bile acid production could contribute to the apparent resistance of hyporesponders to the atherogenic diet.

The regulation of hepatic lipase and cholesteryl ester transfer protein activity in the cholesterol fed rabbit

Hepatic lipase (HL) and cholesteryl ester transfer protein (CETP) activities are both increased in the rabbit by cholesterol feeding. The in vivo regulation of HL and CETP were explored by examining changes in specific steady-state mRNA levels upon cholesterol feeding. On feeding rabbits cholesterol, HL activity increased 3-fold after 2 days and remained at 2.6-times the control value at 28 days. Specific rabbit HL mRNA levels were assessed by dot blot analysis of liver poly (A)+ RNA hybridized with the human HL cDNA. No significant changes in liver HL mRNA accompanied the increase in activity seen at days 2 and 7. At day 28 a modest rise of 46% was observed. A significant rise in CETP activity, evident 7 days after the commencement of cholesterol feeding, was maintained until day 28 when it was 2.4-times the control value. Using the human CETP cDNA as probe, rabbit liver CETP mRNA was also found to increase by day 7, rising to 3.7-times control by day 28. The strong temporal relationship between the rise in CETP activity and mRNA (r = 0.55, P = 0.02) suggests that the regulation of CETP may be primarily effected by the levels of specific mRNA. In contrast, the discordance between levels of lipase activity and mRNA suggests that post-transcriptional events may be more important in the regulation of HL in the cholesterol fed rabbit.

Cholesterol metabolism in hypercholesterolemia-resistant rabbits

Normal rabbits typically respond to a diet high in cholesterol with a large increase in the concentration of plasma cholesterol. We have previously described the breeding and partial characterization of a variant rabbit which does not respond to a high cholesterol diet with changes in plasma cholesterol concentration. In the present report we have characterized three components involved in cholesterol homeostasis: the B/E (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity (HMG-CoA reductase, EC 1.1.1.34) and acyl-coenzyme A: cholesterol acyltransferase activity (ACAT, EC 2.3.1.26) in the livers of the hypercholesterolemia-resistant rabbits. Using normal cholesterol-fed rabbit [125I] beta-VLDL as a ligand, liver membranes prepared from resistant rabbits fed a low-cholesterol diet had 70% higher binding capacity than membranes from normal rabbits fed the same diet. Similar experiments demonstrated that the resistant rabbits had a 240% higher B/E receptor binding capacity compared to normal animals when liver membranes were prepared from animals fed a 0.25% cholesterol-enriched diet. No difference in the binding affinity of [125I]beta-VLDL was detected in membranes prepared from normal or resistant animals. When fed a low-cholesterol diet, the resistant rabbits had approximately 2-fold higher hepatic HMG-CoA reductase activity (97.4 +/- 3.5 pmol product/mg/min in resistant animals compared to 45 +/- 1.1 pmol product/min/mg in normal animals). The difference was exaggerated in animals fed the 0.25% cholesterol-enriched diet, 73.3 +/- 5.5 vs 2.4 +/- 0.56 pmol product/min/mg for resistant and normal membranes respectively. The basal activity of ACAT in hepatic membranes was significantly lower in the resistant rabbits compared to normal rabbits (138 +/- 11 vs 268 +/- 19 pmol cholesteryl ester/min/mg in resistant and normal rabbits respectively); when fed a 0.25% cholesterol-enriched diet, the enzyme was induced 6-fold in normal animals but was increased only 2-fold in the resistant animal. These biochemical data suggested that the resistant rabbit maintained low intracellular cholesterol even when fed a cholesterol-enriched diet. Direct measurement of cellular cholesterol and cholesteryl esters demonstrated that the concentration of these lipids was significantly lower in the resistant animal than in normal animals with the largest differences found in the cytoplasmic rather than the membrane compartment. These studies demonstrate that the resistant rabbit manifests several quantitative differences in cholesterol metabolism and in the regulation of cholesterol metabolism; but these studies do not directly explain the underlying cause of the resistance to hypercholesterolemia in the resistant rabbit.

Impaired triacylglycerol catabolism in hypertriglyceridemia of the diabetic, cholesterol-fed rabbit: a possible mechanism for protection from atherosclerosis

The etiology of the hypertriglyceridemia in alloxan-diabetic rabbits was studied by two independent methods. Production and removal rates of VLDL triacylglycerol were measured in diabetic rabbits by injection of [3H]palmitate-labelled VLDL. Similarly, triacylglycerol total removal rates were determined in non-diabetic rabbits which were infused with Intralipid to mimic the plasma triacylglycerol concentrations of diabetic rabbits. Compared to nondiabetic rabbits, triacylglycerol removal rats were decreased in diabetic rabbits, particularly at higher levels of plasma triacylglycerol. During cholesterol and triacylglycerol supplementation of the diet, post-heparin plasma lipoprotein lipase activity of diabetic rabbits with severe hypertriglyceridemia averaged 36% of that of nondiabetics, suggesting an impaired triacylglycerol removal capacity. Furthermore, plasma triacylglycerol was inversely related to post-heparin plasma lipoprotein lipase activity among diabetic rabbits. VLDL triacylglycerol production increased with increasing plasma triacylglycerol concentration among diabetic cholesterol-fed rabbits with moderately severe hypertriglyceridemia, but reached an apparent plateau among rabbits with plasma triacylglycerol concentrations from approx. 2000-9000 mg/dl. Thus, severe hypertriglyceridemia in this model of insulin deficiency can be attributed only partially to VLDL hypersecretion, whereas a removal defect, resulting in saturation of the triacylglycerol removal mechanism, appears to be largely responsible. The impaired removal of plasma triacylglycerol is also related to the presence of cholesterol predominantly in lipoproteins of increased size. The data support the hypothesis that protection against atherosclerosis in cholesterol-fed diabetic rabbits results from exclusion of very large cholesterol-containing lipoproteins from the arterial wall.

The rabbit as an animal model of hepatic lipase deficiency

A natural deficiency of hepatic lipase in rabbits has been exploited to gain insights into the physiological role of this enzyme in the metabolism of plasma lipoproteins. A comparison of human and rabbit lipoproteins revealed obvious species differences in both low-density lipoproteins (LDL) and high-density lipoproteins (HDL), with the rabbit lipoproteins being relatively enlarged, enriched in triacylglycerol and depleted of cholesteryl ester. To test whether these differences related to the low level of hepatic lipase in rabbits, whole plasma or the total lipoprotein fraction from rabbits was either kept at 4 degrees C or incubated at 37 degrees C for 7 h in (i) the absence of lipase, (ii) the presence of hepatic lipase and (iii) the presence of lipoprotein lipase. Following incubation, the lipoproteins were recovered and subjected to gel permeation chromatography to determine the distribution of lipoprotein components across the entire lipoprotein spectrum. An aliquot of the lipoproteins was subjected also to gradient gel electrophoresis to determine the particle size distribution of the LDL and HDL. Both hepatic lipase and lipoprotein lipase hydrolysed lipoprotein triacylglycerol and to a much lesser extent, also phospholipid. There were, however, obvious differences between the enzymes in terms of substrate specificity. In incubations containing hepatic lipase, there was a preferential hydrolysis of HDL triacylglycerol and a lesser hydrolysis of VLDL triacylglycerol. By contrast, lipoprotein lipase acted primarily on VLDL triacylglycerol. When more enzyme was added, both lipases also acted on LDL triacylglycerol, but in no experiment did lipoprotein lipase hydrolyse the triacylglycerol in HDL. Coincident with the hepatic lipase-induced hydrolysis of LDL and HDL triacylglycerol, there were marked reductions in the particle size of both lipoprotein fractions, which were now comparable to those of human LDL and HDL3, respectively.

Esterases in inbred strains of mice with differential cholesterolemic responses to a high-cholesterol diet

Specific esterase isoenzyme patterns in plasma may be associated with responsiveness of serum cholesterol to dietary cholesterol. In rabbits and rats the presence and absence of a high-mobility, anodal esterase band on electrophoresis have been shown to be associated with hypo- and hyperresponsiveness, respectively. We fed for 28 days male mice of 7 inbred strains either a low-cholesterol, commercial diet or a diet containing 2% (w/w) cholesterol, 0.5% cholic acid and 5% olive oil. Feeding the high-cholesterol diet revealed marked inter-strain differences in the responses of plasma and liver cholesterol; the increases ranged from 21 to 129% and from 10 to 80-fold, respectively. There was no association between esterase isoenzyme patterns in plasma and the sensitivity to the high-cholesterol diet. The mean baseline plasma total esterase activity tended to be positively associated with the absolute response of plasma cholesterol to the high-cholesterol diet (r = 0.56; n = 7), but the positive relationship between the baseline concentration of the ES-1 component in plasma and the cholesterolemic response was stronger (r = 0.84; n = 7; P less than 0.05). The high-cholesterol diet caused a significant increase in plasma total esterase activities in 6 out of the 7 strains. Evidence is presented that the increase in plasma total esterase activity, which was associated with an increase in the activity and concentration of the so-called ES-2 isoenzyme, is the result of an enhanced release of esterases from the intestine, rather than from the liver. A significant, positive correlation was found between the baseline intestinal esterase activity and the cholesterolemic response after cholesterol feeding (r = 0.83; n = 7; P less than 0.05).

Hypo- and hyperresponders: individual differences in the response of serum cholesterol concentration to changes in diet

The feeding of cholesterol-rich diets to random-bred animals results in marked interindividual differences in the response of serum cholesterol. Certain animals show only small responses (hyporesponders), whereas others develop high degrees of hypercholesterolemia (hyperresponders). Inbred strains of rabbits, rats, and mice differing in their sensitivity to dietary cholesterol are available. In these animals, and also in monkeys, the responsiveness to high-cholesterol diets has a strong genetic basis. The existence of hyper- and hyporesponders also holds in humans, though not as pronounced as in laboratory animals. Repeated trials with the same subjects have shown that persons exist with a consistently low or high response to increased intakes of cholesterol. However, "spontaneous," diet-independent within-person variations in the level of serum cholesterol markedly inflate the between-person variation in the response of serum cholesterol; both variations are of the same order of magnitude. Hypo- and hyperresponsiveness to dietary cholesterol extends to other hypercholesterolemic components of the diet. In humans and rabbits hyperresponsiveness to dietary cholesterol is associated with responsiveness to dietary saturated fatty acids. The mechanisms underlying hypo- and hyperresponsiveness to dietary cholesterol have not yet been unraveled. On the basis of available data, we propose that in hyperresponders, compared with hyporesponders, there is a higher hepatic efflux of cholesterol in low-density lipoproteins (LDL), or its precursors, after cholesterol consumption. This may be caused by insufficient inhibition of cholesterol biosynthesis and/or the high capacity of cholesterol absorption in the hyperresponders. The stimulation of LDL production accounts for the increase in LDL cholesterol in serum. The number of hepatic LDL receptors, which may be already decreased in hyperresponders, will decrease further through down-regulation. The receptor-mediated LDL clearance decreases, but the absolute amount of LDL cholesterol taken up by the cells via the receptor and by the receptor-independent pathway increases because of the increased level of LDL cholesterol. In this way a new equilibrium is reached in which LDL production equals LDL catabolism. The phenomenon of hypo- and hyperresponsiveness may have implications for counseling subjects who attempt to lower their serum cholesterol by diet. However, identification of true hyper- and hyporesponders is greatly hampered by within-person fluctuations of the level of serum cholesterol. No simple test is available to discriminate hypo- from hyperresponders.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of dietary cholesterol on adipose tissue lipoprotein lipase in the baboon

The effects of infant diet (breast milk or formula containing 2, 30 or 60 mg/dl cholesterol) and subsequent dietary cholesterol (0.02, 1.0 or 1.7 mg/kcal) and fat (saturated or unsaturated) on heparin-releasable lipolytic activity from omental adipose tissue was estimated from 99 baboons of 5-8 years of age. This lipase activity was characterized as lipoprotein lipase based on salt inhibition and apolipoprotein C-II activation. Lipoprotein lipase activity released from adipose tissue by heparin was significantly (P less than 0.002) lower in high cholesterol-fed baboons than in those fed low cholesterol. Most of this difference was due to impaired long-term heparin release of lipoprotein lipase. Adipose tissue lipoprotein lipase increased with increasing fat cell size regardless of diet, but there was no effect of diet on adipocyte size. There were no significant effects of infant cholesterol intake nor adult saturated or unsaturated fat on lipoprotein lipase activity. Adult baboons breast fed as infants had lower adipose tissue lipoprotein lipase activity (P less than 0.07) than adults fed formula as infants.

The use of three esterase kits to measure plasma cholesterol concentration in the rat and three other species

Three enzymatic methods for the determination of cholesterol and its esters were applied to plasma samples obtained from normocholesterolaemic rats, dogs, rabbits and man. A major difference was observed for rat plasma values, where the results obtained with a microbial esterase for the hydrolysis of cholesteryl esters were much lower than those obtained with mammalian esterases. This difference was greater for rats than for the other three species studied and is probably related to the plasma distribution of lipoproteins in the rat.

Influence of cholesterol feeding on the production of eicosanoids, tissue plasminogen activator and superoxide anion (O2-) by rabbit blood monocytes

The metabolism in vitro of exogenous and endogenous arachidonic acid was studied in circulating blood monocytes obtained from control (control group) and cholesterol (0.5%)-fed (cholesterol group) rabbits. The production of superoxide anion (O2-), tissue plasminogen activator (t-PA) and adherence of monocytes were assessed in both groups of animals. The amounts of cyclooxygenase and lipoxygenase products derived from exogenously added [1-14C]AA were not significantly different in monocytes collected from both groups of animals. However, the amounts of PGD2, TXB2 and PGE2 formed from endogenous substrate were decreased significantly in monocytes obtained from the cholesterol group compared to those from the control group. The production of immunoreactive LTB4 was not suppressed significantly in monocytes collected from the cholesterol group. The production of O2- and t-PA was suppressed significantly in monocytes obtained from the cholesterol group and these cells adhered onto glass surfaces more efficiently than control cells. Since the formation of prostanoids from endogenous but not exogenous substrate is reduced, an effect of cholesterol on the liberation of AA from phospholipid pools is implicated.

Correlation between the cholesterolemic responses produced by dietary cholesterol and casein in rabbits

Rabbits were fed three semipurified diets, namely a cholesterol-free soy protein diet, a cholesterol-free casein diet or a diet containing soy protein plus cholesterol (2 g/kg). One group (Chol-Cas) of 24 rabbits was fed successively the diet containing soy protein plus cholesterol (25 days), the soy protein diet (36 days) and the casein diet (20 days). Another group (Cas-Chol) consisting of 25 animals received consecutively the diet containing casein, soy protein and soy protein supplemented with cholesterol. Hypercholesterolemia was induced by the diets containing cholesterol or casein, whereas regression was seen on the soy protein diet. Within the Chol-Cas or Cas-Chol group the cholesterolemic response to casein and that to cholesterol was significantly correlated, the correlation coefficient being as high as 0.66. This correlation was still observed when calculated for animals either hypo- or hyperresponsive to dietary casein or cholesterol. When all animals were pooled, the correlation coefficient for the cholesterolemic response to casein and to cholesterol was only 0.33. The mean response to dietary cholesterol and to casein was significantly higher in the last dietary period than in the first period.