Interactions among dietary fat, protein, and cholesterol in atherosclerosis-susceptible pigeons. Effects on serum cholesterol and aortic atherosclerosis

Cardiovascular benefits of Momordica charantia in cholesterol-fed Wistar rats

Background

Momordica. charantia is popularly used as a medicinal herb in ethnomedicine for the management of cardiovascular diseases. In this study, we evaluated the beneficial effects of the ethanolic extract of M. charantia (Linn.) in experimentally induced cardiovascular disorders using cholesterol-fed Wistar rat.

Methods

Seventy-two experimental rats were randomly assigned into nine 9 groups of 8 rats each and treated as follows: Rats in group A (control) were given distilled water only; Rats in group B were given 30 mg/kg of cholesterol dissolved in coco-nut oil (cholesterol solution); Rats in group C were given cholesterol solution and 100 mg/kg Atorvastatin; Rats in group D were given 250 mg/kg of M. charantia and cholesterol solution; Rats in group E were given 500 mg/kg of M. charantia and cholesterol solution; Rats in group F were given 250 mg/kg. M. charantia; Rats in group G were given 500 mg/kg M. charantia; Rats in group H were given 1 ml of coconut oil; Rats in group I were given 100 mg/kg of Atorvastatin.

Results

Mean LDL-cholesterol was significantly (P < 0.05) lower in groups F, E and H as compared with the control groups. Histological analysis of the heart and aortic branch of the experimental rats show that cholesterol administration induced myocardial degeneration, vascular ulceration and stenosis in the aorta and heavy perivascular infiltrates of inflammatory cells. However, these deleterious effects were ameliorated upon treatment with Momordica charantia and Atorvastatin as compared with the control groups.

Conclusion

Our findings indicate the possible cardiovascular benefits of M. charantia.

The role of casein in the development of hypercholesterolemia

Atherosclerosis remains the leading cause of severe cardiovascular complications such as cardio- and cerebrovascular events. Given that prevention and early intervention play important roles in the reduction of cardiovascular complications associated with atherosclerosis, it is critical to better understand how to target the modifiable risk factors, such as diet, in order to best minimize their contributions to the development of the disease. Studies have shown that various dietary sources of protein can affect blood lipid levels, a modifiable risk factor for atherosclerosis, either positively or negatively. This clearly highlights that not all proteins are "created equal." For example, consumption of diets high in either animal- or vegetable-based sources of protein have resulted in varied and inconsistent effects on blood cholesterol levels, often depending on the amino acid composition of the protein and the species investigated. Careful consideration of the source of dietary protein may play an important role in the prevention of atherosclerosis and subsequent cardiovascular complications. Given the recent focus on high protein diets, an emphasis on controlled studies in the area is warranted. The goal of this review is to present the current state of the literature that examines the effects of casein, a commonly utilized animal-based protein, on blood cholesterol levels and the varying effects noted in both animals and humans.

Influence of dietary cholesterol and fat source on atherosclerosis in the Japanese quail (Coturnix japonica)

The Japanese quail has been used as a model of human atherosclerosis to investigate the mechanisms underlying the development of vascular lesions, i.e. hyperlipoproteinaemia and impaired endogenous antioxidant status. In the present study, Japanese quail were fed on semi-purified diets containing butter, beef tallow or soyabean-oil blends, with either 0.5 or 5 g cholesterol/kg for 9 weeks to examine the effects of dietary fat blends varying in fatty acid composition and cholesterol intake on plasma lipids and aortic atherosclerotic plaque and sterol composition. These findings were related to possible diet-induced changes in antioxidant status of selected tissues. Hypercholesterolaemia was confirmed (P < 0.001) in birds fed on high-cholesterol (HC) diets. Plasma total cholesterol concentration and cholesterol content of lipoprotein fractions in hypercholesterolaemic birds were lower (P < 0.05) in quail fed on the soyabean-oil blend. Plasma triacylglycerol content was increased (P < 0.001) in HC-fed birds. Dietary fat blends did not influence plasma triacylglycerol levels. Tissue antioxidant status (catalase (EC 1.11.1.6), glutathione peroxidase (EC 1.11.1.9), glutathione reductase (EC 1.6.4.1) and superoxide dismutase (EC 1.15.1.1) activities and glutathione content) was generally not greatly affected by dietary fat blend or cholesterol treatment. Birds fed on HC diets exhibited severe (P < 0.001) atherosclerotic plaque in aortas which was not influenced by the source of dietary fat blend. Scanning electron microscopy confirmed results of visual aortic plaque scoring using dissecting light microscopy. Several cholesterol oxides were identified and quantified in aortic plaque from HC-fed birds (5,6 alpha-epoxy-5 alpha-cholesterol, 7(beta-hydroxycholesterol and 7-ketocholesterol) regardless of dietary fat blend. The results indicate that dietary fat blends varying in polyunsaturated:saturated fatty acid ratios only marginally influence the degree of hypercholesterolaemia in atherosclerosis-susceptible quail fed on atherogenic diets only, and are not a factor, compared with sterol feeding, in modulating the degree of atherosclerosis or the aortic oxysterol content in these same birds. Moreover, diet-induced hyperlipoproteinaemia had only a small effect on antioxidant status of selected tissues examined.

beta-VLDL and acetylated-LDL binding to pigeon monocyte macrophages

Blood-derived monocytes are an important source of foam cells in atherosclerotic lesions of White Carneau pigeons. Based upon studies with cultured blood monocytes (monocyte macrophages) and peritoneal macrophages from a variety of mammalian species, it has been proposed that these cells become loaded with cholesteryl esters through the uptake of lipoproteins including beta-migrating very low density lipoproteins (beta-VLDL) and low density lipoproteins that have been chemically modified in a manner analogous to experimental acetylation (Ac-LDL). The purpose of this study was to determine whether similar mechanisms functioned in pigeon monocyte macrophages. Radioiodinated pigeon beta-VLDL and Ac-LDL were incubated with White Carneau pigeon monocyte macrophages that had been maintained in culture for 7 days. Scatchard analysis of the specific binding data revealed the presence of specific and saturable receptors for both beta-VLDL and Ac-LDL. beta-VLDL receptors had both low and high affinity binding components, whereas Ac-LDL receptors displayed a single class of high affinity binding sites. beta-VLDL binding remained relatively constant from 3 to 10 days in culture while Ac-LDL binding increased with time in culture. Competition studies demonstrated a high degree of binding specificity for 125I-Ac-LDL, but less for 125I-beta-VLDL. Binding of 125I-beta-VLDL was not competed for by Ac-LDL, but was by beta-VLDL and by low density lipoproteins from both normal and hypercholesterolemic pigeons. Following binding of beta-VLDL and Ac-LDL, the lipoproteins were rapidly internalized and degraded. Although the majority of degradation was secondary to internalization by the monocyte macrophages, approx. 5% of the degradation resulted from enzymatic activity in the culture medium, presumably due to secretion of proteolytic enzymes by the cells. As measured by esterification of [14C]oleate to cholesterol, it was shown that the cholesterol liberated from the degradation of both beta-VLDL and Ac-LDL stimulated cholesteryl ester synthesis in pigeon monocyte macrophages. These studies confirm the existence of specific beta-VLDL and Ac-LDL receptors on the surface of pigeon monocyte macrophages which facilitate both internalization of the lipoproteins and subsequent stimulation of cholesteryl ester synthesis. This is the first demonstration of beta-VLDL and Ac-LDL receptors on monocyte macrophages from an avian species, and the findings support the potential role for the receptor-mediated uptake of a variety of abnormal lipoproteins in the formation of monocyte-derived foam cells in the arterial wall of White Carneau pigeons during the development of atherosclerosis.

The effect of semipurified diets containing different proportions of either casein or soybean protein on the concentration of cholesterol in whole serum, serum lipoproteins and liver in male and female rats

Male and female lean Zucker strain rats were fed cholesterol-enriched semipurified diets containing 2 levels (20% and 50%, w/w) of either casein or soybean protein for a period of 14 weeks. In the female rats, the feeding of casein diets resulted in significantly higher levels of serum cholesterol than when diets containing soybean protein were fed. In addition, the hypercholesterolemic effect of dietary casein could be enhanced by increasing the proportion of this protein in the diet. Modulations in the proportion of dietary soybean protein did not significantly affect the serum cholesterol levels. In the male rats, however, no such differential effects were observed, indicating a difference between male and female rats in susceptibility to the induction of changes in serum cholesterol levels by dietary means. Upon feeding casein diets, both the male and female rats exhibited a shift of cholesterol from the high density lipoproteins to the lipoproteins with a lower density. This effect was more pronounced in the female than in the male rats. Liver cholesterol concentrations were markedly affected by modulations both in the type and proportion of dietary protein in both sexes. The concentration of cholesterol in the liver of the rats was highest in those fed the 50% casein diet and progressively lower in the animals on diets containing 20% casein, 20% soybean protein and 50% soybean protein.

Vegetable protein and atherosclerosis

A number of studies purport to demonstrate that animal protein (usually casein) is more cholesteremic and atherogenic than vegetable protein (usually soy protein). These findings are generally true, but the effect of any single dietary component may be influenced by any other one. Thus, when the carbohydrate is dextrose, casein is more cholesteremic in rabbits than is soy protein, but when the carbohydrate is raw potato starch, the two proteins are equivalent. Similarly, a casein‐cellulose diet is more cholesteremic and atherogenic for rabbits than is one containing soy protein and cellulose. Substitution of alfalfa for cellulose renders the two proteins virtually the same. In man, too, vegetable protein appears to be less cholesteremic than animal protein. The difference persists even in the face of saturated fat. In seeking a mechanism to explain the differences in atherogenicity, we have hypothesized that a high ratio of lysine to arginine may be important. The lysine/arginine ratio of casein is 2.0 and that of soy protein is 0.90. Addition of enough lysine to a soy protein diet to raise the lysine/arginine ratio to 2.0 also increases the atherogenicity of that diet.