Effects of dietary casein and soy protein on metabolism of radiolabelled low density apolipoprotein B in rabbits

Feeds of animal origin in rabbit nutrition – a review

Rabbits are classified as obligate herbivores. However, under natural conditions, some members of the family Leporidae incorporate animal products into their diets. Therefore, it seems biologically justified to supplement the diets of farmed rabbits with feeds of animal origin as sources of protein, fat and minerals. The aim of this review was to describe, from a historical perspective, the use of various feeds of animal origin in rabbit nutrition. The applicability of by-products from mammal, poultry, fish and invertebrate processing for rabbit feeding was evaluated, including the future prospects for their use. A review of the available literature revealed that various animal-based feeds can be valuable protein sources in rabbit diets, but their inclusion levels should not exceed 5-10%. Studies investigating their efficacy have been conducted since the 1970s. In some regions of the world, the use of animal-derived protein in livestock feeds was prohibited due to the risk of spreading bovine spongiform encephalopathy (BSE). However, the interest in animal by-products as protein sources in livestock diets is likely to increase since the above ban has been lifted.

Metabolic profiling of plasma amino acids shows that histidine increases following the consumption of pork

Amino acid (AA) status is determined by factors including nutrition, metabolic rate, and interactions between the metabolism of AA, carbohydrates, and lipids. Analysis of the plasma AA profile, together with markers of glucose and lipid metabolism, will shed light on metabolic regulation. The objectives of this study were to investigate the acute responses to the consumption of meals containing either pork (PM) or chicken (CM), and to identify relationships between plasma AA and markers of glycemic and lipemic control. A secondary aim was to explore AA predictors of plasma zinc concentrations. Ten healthy adults participated in a postprandial study on two separate occasions. In a randomized cross-over design, participants consumed PM or CM. The concentrations of 21 AA, glucose, insulin, triglycerides, nonesterified fatty acids, and zinc were determined over 5 hours postprandially. The meal composition did not influence glucose, insulin, triglyceride, nonesterified fatty acid, or zinc concentrations. Plasma histidine was higher following the consumption of PM (P=0.014), with consistently higher changes observed after 60 minutes (P<0.001). Greater percentage increases were noted at limited time points for valine and leucine + isoleucine in those who consumed CM compared to PM. In linear regression, some AAs emerged as predictors of the metabolic responses, irrespective of the meal that was consumed. The present study demonstrates that a single meal of PM or CM produces a differential profile of AA in the postprandial state. The sustained increase in histidine following the consumption of a PM is consistent with the reported effects of lean pork on cardiometabolic risk factors.

Oral administration of D-003, a mixture of very long chain fatty acids prevents casein-induced endogenous hypercholesterolemia in rabbits

D-003 is a mixture of very long chain saturated fatty acids (VLCSFA) purified from sugar cane wax with cholesterol-lowering effects proven in animal models and healthy volunteers. D-003 inhibits cholesterol biosynthesis through the regulation of HMG-CoA reductase activity. Rabbits fed diets enriched with casein develop endogenous hypercholesterolemia (EH), making them a very useful model for determining the mechanism of action of drugs affecting lipids. We examined whether D-003 prevented EH. Rabbits were fed a casein diet for 4 weeks, administered simultaneously with D-003 (5, 50, and 100 mg·kg–1·day–1). As expected, nontreated rabbits became hipercholesterolemic; however, as early as 15 days following administration, the treated group (50 and 100 mg·kg–1·day–1) had significantly decreased total cholesterol and low-density lipoprotein cholesterol (LDL-C). Triglycerides were not affected; however, at study completion, HDL-C levels significantly increased at all the doses assayed. D-003 inhibited de novo synthesis of cholesterol, since the incorporation of3H2O into sterols in the liver and proximal small bowel was significantly depressed. Also, D-003 significantly raised the rate of removal of [125I]-LDL from serum and significantly elevated [125I]-LDL binding activity to liver homogenates. Taken together, these results show that the efficacy of D-003 in reducing casein-derived hypercholesteromeia could involve, at least partially, an inhibition of hepatic cholesterol bio synthesis, which may elicit a decreased cholesterol concentration in hepatocytes, preventing the loss of hepatic LDL receptors induced by casein administration. However, since casein-induced hypercholesterolemia is also a consequence of a stimulation of cholesterol absorption in the lumen and an increase of the output of cholesterol associated with LDL, the effect of D-003 on cholesterol absorption and LDL synthesis by the liver should be investigated.Key words: D-003, very long chain saturated fatty acids, casein-fed rabbits, LDL-C, cholesterol biosynthesis, LDL clearance, LDL receptor.

Soya phytoestrogens, genistein and daidzein, decrease apolipoprotein B secretion from HepG2 cells through multiple mechanisms

Diets containing the soya-derived phytoestrogens, genistein and daidzein, decrease plasma cholesterol in humans and experimental animals. The mechanisms responsible for the hypocholesterolaemic effects of these isoflavones are unknown. The present study was conducted to determine if genistein and daidzein regulate hepatocyte cholesterol metabolism and apolipoprotein (apo) B secretion in cultured human hepatoma (HepG2) cells. ApoB secretion was decreased dose-dependently by up to 63% and 71% by genistein and daidzein (100 microM; P<0.0001) respectively. In contrast, no effect on apoAI secretion was observed. Cellular cholesterol synthesis was inhibited 41% by genistein (100 microM; P<0.005) and 18% by daidzein (100 microM; P<0.05), which was associated with significant increases in 3-hydroxy-3-methylglutaryl-CoA reductase mRNA. Cellular cholesterol esterification was decreased 56% by genistein (100 microM; P<0.04) and 29% by daidzein (100 microM; P<0.04); however, mRNA levels for acyl-CoA:cholesterol acyltransferase (ACAT) 1 and ACAT2 were unaffected. At 100 microM, both isoflavones equally inhibited the activities of both forms of ACAT in cells transfected with either ACAT1 or ACAT2. Genistein (100 microM) and daidzein (100 microM) significantly decreased the activity of microsomal triacylglycerol transfer protein (MTP) by 30% and 24% respectively, and significantly decreased MTP mRNA levels by 35% and 55%. Both isoflavones increased low-density lipoprotein (LDL)-receptor mRNA levels by 3- to 6-fold (100 microM; P<0.03) and significantly increased the binding, uptake and degradation of (125)I-labelled LDL, suggesting that enhanced reuptake of newly secreted apoB-containing lipoproteins contributed to the net decrease in apoB secretion. These results indicate that genistein and daidzein inhibit hepatocyte apoB secretion through several mechanisms, including inhibition of cholesterol synthesis and esterification, inhibition of MTP activity and expression and increased expression of the LDL-receptor.

The effect of supplementation with isoflavones on plasma lipids and oxidisability of low density lipoprotein in premenopausal women

Results of recent clinical studies have lead to the hypothesis that isoflavones are cardioprotective. The aims of this trial were to determine the effect of supplementation with isoflavonoid phytoestrogens on plasma cholesterol concentrations and its distribution among lipoproteins and whether supplementation with isoflavones influences oxidisability of low density lipoprotein (LDL) ex vivo. Fourteen healthy premenopausal women participated in a randomised cross-over trial lasting four menstrual cycles (approximately 4 months). The subjects were asked to consume 86 mg of isoflavones daily for the duration of two menstrual cycles followed by placebo for an equivalent period, or vice versa. Venous blood samples were collected initially and at the end of the second and fourth menstrual cycles for the determination of plasma lipid concentrations and the resistance of LDL to copper-induced oxidation ex vivo. Accustomed dietary intake of isoflavones and lignans during the placebo period were 6.87+/-3.0 and 1.80+/-0.22 mg/day (mean+/-S.E.M.), respectively, and these did not change during the supplementation period. The intake of other dietary components remained constant during the trial. Supplementation resulted in a 5-fold increase in urinary isoflavone excretion (12.2+/-14.2 versus 70.1+/-10.3 micromol/24 h, placebo and isoflavone periods, respectively, P=0.0001). No changes in the oxidisability of LDL (lag time of 32.9+/-3.1 versus 30.4+/-2.9 min) or the plasma concentrations of total cholesterol (4.03+/-0.21 versus 4.11+/-0.18 mmol/l) or triacylglycerol (0.67+/-0.04 versus 0.73+/-0.06 mmol/l) were observed following supplementation. However a significant period effect (P=0.024) was observed and a trend towards a carryover effect (P=0.086) was noted for the concentration of HDL(3) cholesterol. Further studies are required to clarify the potential effect of isoflavones on HDL metabolism and the interaction with plasma steroid hormones during the menstrual cycle.

Cholesterol-lowering effect of policosanol on rabbits with hypercholesterolaemia induced by a wheat starch-casein diet

The effect of policosanol, a mixture of high-molecular-weight aliphatic alcohols isolated from sugarcane wax, on casein-induced hypercholesterolaemia in rabbits was studied. When policosanol was administered by the oral route once daily for 30 d (50 mg/kg) the increases in plasma total cholesterol and LDL-cholesterol (LDC-C) were significantly reduced when compared with the control group. The incorporation of 3H2O into sterols in the liver was significantly depressed, suggesting inhibition of hepatic cholesterol biosynthesis. The oral administration of policosanol raised the rate of removal of 125I-labelled LDL from serum. Kinetic parameters calculated following injection of [125I]LDL showed than in casein-fed rabbits, the terminal half-life (t1/2) was significantly decreased after policosanol treatment. The hepatic LDL-binding activity was increased after policosanol administration which suggested that the enhanced clearance was due, at least in part, to increased receptor-mediated uptake of LDL by the liver. Considered together, these results suggest that policosanol can significantly reduce the increase of plasma LDL-C in rabbits fed on a wheat starch-casein diet by reducing cholesterol biosynthesis in the liver. Such an effect could account for the enhancement of LDL catabolism through the receptor-mediated pathway.

The effect of individual amino acids on ApoB100 and Lp(a) secretion by HepG2 cells

The rate at which HepG2 cells secrete apoB100 lipoproteins is inversely related to the concentration of amino acids in the medium (Zhang, Z., Sniderman, A. D., Kalant, D., Vu, H., Monge, J. C., Tao, Y., and Cianflone, K. (1993) J. Biol. Chem. 268, 26920-26926). The purpose of the present study was to determine the effect of individual amino acids on apoB100 and lipoprotein secretion. Asparagine was associated with modestly increased secretion. The branched chain amino acids (leucine, isoleucine, and valine) and lysine had minor inhibitory effects. The other amino acids, by contrast, decreased apoB secretion, although the magnitude of the effect varied considerably, the most potent being tyrosine, cysteine, phenylalanine, tryptophan, methionine, and glutamine. Although the effect on Lp(a) generally paralleled that on apoB100, it was usually much less pronounced. No amino acid caused a marked decrease in albumin, apoAI, or total protein secreted from the HepG2 cells. The amino acid effect on apoB was paralleled by similar decreases in secreted cholesterol ester (CE) primarily in the low density lipoprotein density range (d < 1.006-1.063 g/ml), although there was no significant change in intracellular CE. Neither intracellular nor secreted triglycerides (TG) or free cholesterol changed, resulting in a slightly larger TG-enriched particle being secreted. The effect was confirmed in cultured primary hamster hepatocytes, where a mixture of amino acids also caused a decrease in apoB secretion (up to 40%). ApoAI appeared to increase as with the HepG2 cells. Secreted CE paralleled apoB . There was no change in intracellular or secreted TG or free cholesterol, resulting in a substantially larger TG-rich particle being secreted. mRNA for apoB100 increased with asparagine, decreased moderately with branched chain amino acids, and decreased further with glutamine, as shown by dot blot and Northern blotting. Pulse-chase studies indicated that there was no change in apoB secretion efficiency under any condition. These results extend our previous observations by demonstrating specificity of the amino acid effect on apoB100 secretion. Although an effect on transcription is the likely mechanism, the exact basis for this remains to be determined.

Hypolipidaemic properties of a potent and bioavailable alkylsulphinyl-diphenylimidazole ACAT inhibitor (RP 73163) in animals fed diets low in cholesterol

RP 73163 ((S)-2-[5-(3,5-dimethyl-l-pyrazolyl)pent-l-yl)-sulphinyl]-5, 6-diphenylimidazole) has been shown to be a potent and specific inhibitor of acyl-CoA:cholesterol acyltransferase (EC 2.3.1.26; ACAT) in vitro using the tissues of experimental animals as sources of the enzyme. The concentrations of RP 73163 required to produce 50% inhibition of ACAT activity (IC50 values) in microsomal preparations ranged from 86 nM for rat liver to 370 nM for rabbit intestine. In whole cell assays using human hepatic (HepG2), intestinal (Caco2), and monocytic (THP-1) cell lines, RP 73163 inhibited ACAT activity with IC50 values of 266, 158, and 314 nM, respectively. The addition of RP 73163 (0.03-1.0 microM) to the medium of cultured HepG2 cells produced a concentration-dependent decrease in apolipoprotein B (apoB) secretion. The compound has high systemic bioavailability. Using a bioassay, a concentration of active inhibitor equivalent to 29 microM of parent compound was present in plasma 1 hr after oral administration of RP 73163 (50 mg.kg-1). In rats that had been fed a basal diet ad libitum or starved for 18 hr prior to blood sampling, the administration of RP 73163 (50 mg.kg-1 b.i.d. for 7 days) reduced plasma triglyceride levels by 50% without affecting the concentration of cholesterol. This hypotriglyceridaemic effect was associated with reductions in plasma very-low-density-lipoprotein (VLDL) and low-density-lipoprotein (LDL) levels. RP 73163 decreased the rate of VLDL secretion by 24% in Triton WR-1339-treated rats that had been fasted overnight but did not affect the secretion rate in animals fed ad libitum, indicating that ACAT was only important in regulating VLDL secretion under certain nutritional conditions. RP 73163 reduced the accumulation of intraperitoneally administered [3H]leucine into the plasma VLDL-apoB pool in both fed and fasted states. The results suggest that, in fed animals at least, an increase in the clearance of VLDL from the bloodstream may contribute to the hypolipidaemic activity of the compound. In rabbits with casein-induced endogenous hypercholesterolaemia, RP 73163 specifically reduced the levels of cholesterol carried by LDL. In conclusion, the hypolipidaemic actions of RP 73163, a potent and systemically bioavailable ACAT inhibitor, are consistent with a reduction in the secretion of apoB containing lipoproteins by hepatic tissue and possibly with an increase in the clearance of these particles.

Comparative effects of HMG-CoA reductase inhibitors on apo B production in the casein-fed rabbit: atorvastatin versus lovastatin

Rabbits fed a diet enriched in casein develop an endogenous hypercholesterolemia (EH) due both to an increased low density lipoprotein (LDL) synthetic rate and decreased LDL receptor activity. Pre-established EH in this model was used to assess the ability and mechanism by which atorvastatin lowers total plasma cholesterol (TPC) compared to the reference agent lovastatin. Rabbits were fed a casein diet for 6 weeks, obtaining average TPC levels above 200 mg/dl. To ensure equivalent mean cholesterol concentrations, animals were randomized into treatment groups based on the 6-week TPC levels, and fed the casein diet alone or in combination with either atorvastatin or lovastatin for an additional 6 weeks. Under these conditions, new steady-state cholesterol values were established. Lipoprotein concentrations and distributions were determined at this point. Compared to pretreatment values, TPC were similar in untreated animals. Atorvastatin, however, significantly reduced TPC by 38%, 45%, and 54% at the 1, 3, and 10 mg/kg doses, respectively. Statistically significant lowering of TPC (35%) by lovastatin was only achieved at the 10 mg/kg dose. To determine the mechanism by which atorvastatin lowered TPC in the EH rabbits, kinetic studies using human [125I]-LDL were performed in a subset of animals maintained on the casein diet alone (n = 5), or those treated with 3 mg/kg of atorvastatin (n = 5) or lovastatin (n = 7). In this set of studies, atorvastatin significantly lowered TPC compared to control and lovastatin-treated rabbits by 57% and 46%, respectively. Lovastatin treatment resulted in a 20% decrease in TPC as compared to untreated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoprotein metabolism in the preruminant calf: effect of a high fat diet supplemented with L-methionine

The effects of dietary lipid and L-Met supplementation on plasma lipids and lipoproteins were investigated in 16-wk-old preruminant calves. Four calves received the basal milk diet (2.0 g of lipid/kg of BW per meal) for 8 d followed by the same diet supplemented with L-Met (2.6 g/kg of dietary DM) for 5 d. Similarly, seven calves received successively the basal diet supplemented with cream (2.7 g of lipid/kg of BW per meal) and the same diet supplemented with L-Met. The diet with cream induced higher triglyceridemia than the basal diet because of a marked increase in chylomicra and in very low density lipoproteins, which suggested stimulation of intestinal lipoprotein secretion. Moreover, this lipid-enriched diet stimulated the formation of very light high density lipoproteins to the detriment of heavy high density lipoproteins. These particles, the bovine counterpart of mammalian high density lipoproteins of type 1, were distributed within the density range of low density lipoproteins. Addition of L-Met in the diets increased plasma concentrations of chylomicra and very low density lipoproteins, suggesting direct stimulation of the intestinal secretion of both of these lipoproteins and of the hepatic very low density lipoproteins. No effect of L-Met was observed on the concentrations and the physicochemical properties of low and light high density lipoproteins.

Intake of 25 g of soybean protein with or without soybean fiber alters plasma lipids in men with elevated cholesterol concentrations

Twenty-one mildly hypercholesterolemic men consumed a diet that was low in fat (< 30% of energy) and cholesterol (300 mg/d) and were given muffins containing 25 g protein + 20 g dietary fiber daily from either isolated soybean protein + soybean cotyledon fiber, isolated soybean protein + cellulose, casein + soybean cotyledon fiber or casein + cellulose. All subjects progressed through the low fat, low cholesterol baseline period, lasting 2 wk, and then through all four dietary treatments, lasting 4 wk each, according to a Latin square design. Plasma concentrations of total, LDL, HDL and VLDL cholesterol, total and VLDL triacylglycerols, and apolipoprotein A-I and B were measured at the end of each period. When data from all subjects were analyzed, dietary treatments did not influence lipemia; however, in subjects with initial total cholesterol concentrations > 5.7 mmol/L, both isolated soybean protein treatments resulted in significantly lower total cholesterol compared with the two casein treatments (P < 0.05). In addition, a negative linear relationship was observed when a subject's total or LDL cholesterol change after each of the soybean treatments was regressed against the subject's baseline cholesterol concentration (P < 0.05). Apolipoprotein A-I varied dependent on baseline cholesterol with no apparent pattern, whereas apolipoprotein B levels were not affected. Results indicate that consumption of 25 g soybean protein/d is associated with lower total cholesterol concentrations in individuals with initial cholesterol concentrations > 5.7 mmol/L.

Interaction between dietary proteins and lipids in the regulation of serum and liver lipids in the rabbit. Effect of fish protein

Purified diets varying in dietary protein, namely casein (CA), soy protein (SP), fish protein (FP), and lipid origin (corn oil (CN), coconut oil (CO)) were fed to rabbits to evaluate the effects of protein and fat source, as well as protein-lipid interactions, on serum total, lipoprotein and hepatic lipid levels. Dietary proteins and lipids exerted a separate effect on serum total cholesterol (C), very low-density lipoprotein cholesterol (VLDL-C), and low-density lipoprotein cholesterol to high density lipoprotein cholesterol (LDL-C/HDL-C) ratio. Hence, CA increased serum cholesterol compared to SP, while coconut oil enhanced serum and VLDL-C, and decreased LDL-C/HDL-C compared to corn oil. Dietary proteins interacted with dietary lipids to modulate HDL-C levels. Thus, FP maintained a high level of HDL-C regardless of lipid origin, compared to CA and SP whose HDL-C levels were decreased by corn oil, compared to coconut oil. A dietary protein-lipid interaction was also observed in the regulation of liver cholesterol levels. Coconut oil, compared to corn oil, decreased liver cholesterol in rabbits fed FP, whereas hepatic cholesterol concentration was unaltered by dietary lipid source in CA- and SP-fed rabbits. These results demonstrate that dietary proteins act synergistically with dietary lipids to regulate cholesterol metabolism in the rabbit.

Compositional changes in serum lipoproteins during developing hypercholesterolemia induced in rabbits by cholesterol-free, semipurified diets

Changes in the concentration and composition of serum very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) were studied in rabbits transferred from chow diet to cholesterol-free, semipurified diets containing casein or isolated soy protein. The fat and fibre content of these diets was similar to that of chow but a higher protein level was used to enhance the hypercholesterolemia. During the first week on the casein diet, there was a marked increased in LDL-cholesterol, protein and phospholipids, and these higher levels were maintained during the subsequent 3 weeks of the study. Similar but less marked changes were obtained with the soy protein diet. The components of VLDL showed relatively little change after introduction of the diets to the animals. In both VLDL and LDL, the proportion of cholesterol increased and that of triglycerides decreased after 1 week on the casein diet and a similar trend was seen in HDL. The concentration of HDL-cholesterol showed little change but triglycerides, protein and phospholipids all tended to decline on both casein and soy protein diets.