Differential uptake of cholesterol and plant sterols by rat erythrocytes in vitro

How unsaturated fatty acids and plant stanols affect sterols plasma level and cellular membranes? Review on model studies involving the Langmuir monolayer technique

The Langmuir monolayer technique has long been known for its usefulness to study the interaction between molecules and mimic cellular membranes to understand the mechanism of action of biologically relevant molecules. In this review we summarize the results that provided insight into the potential mechanism for lowering the plasma level of cholesterol by hypocholesterolemic substances (unsaturated fatty acids (UFAs) and phytocompounds) - in the aspect of prevention of atherosclerosis - and their effects on model biomembranes. The results on UFAs/cholesterol (oxysterols) interactions indicate that these systems are miscible and strongly interacting, contrary to immiscible systems containing saturated fatty acids. Lowering of cholesterol plasma level by UFAs was attributed to the strong affinity between UFAs and sterols, resulting in the formation of high stability complexes, in which sterols were bound and eliminated from the body. Studies on the effect of UFAs and plant sterols/stanols on simplified biomembranes (modeled as cholesterol/DPPC system) indicated that the studied hypocholesterolemic substances modify the biophysical properties of model membrane, affecting its fluidity and interactions between membrane components. Both UFAs and plant sterols/stanols were found to loosen interactions between DPPC and cholesterol and decrease membrane rigidity caused by the excess cholesterol in biomembrane, thus compensating strong condensing effect of cholesterol and restoring proper membrane fluidity, which is of utmost importance for normal cells functioning. The agreement between model - in vitro - studies and biological results prove the usefulness of the Langmuir monolayer technique, which helps in understanding the mode of action of biologically relevant substances.

Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia

Sitosterolemia is a rare, autosomal recessive disease caused by mutations in the adenosine triphosphate-binding cassette transporter genes ABCG5 or ABCG8 that result in accumulation of xenosterols in the body. Clinical manifestations include tendon xanthomas, premature coronary artery disease, hemolytic anemia, macrothrombocytopenia, and bleeding. Although the effect of sterol accumulation on the predisposition for atherosclerosis is evident, how xenosterol accumulation leads to defects in platelet physiology is unknown. Sitosterolemia induced in Abcg5- and Abcg8-deficient mice fed a high plant sterol diet resulted in accumulation of free sterols in platelet plasma membranes, leading to hyperactivatable platelets characterized by constitutive binding of fibrinogen to its αIIbβ3 integrin receptor, internalization of the αIIbβ3 complex, generation of platelet-derived microparticles, and changes in the quantity and subcellular localization of filamin. The latter was associated with macrothrombocytopenia, shedding of GPIbα, impaired platelet adhesion to von Willebrand factor, and inability to form stable thrombi. Plasma levels of soluble GPIbα were strongly correlated with plasma sitosterol levels in samples from human sitosterolemic patients, implicating a similar mechanism of sterol-induced platelet passivation in the human disease. Intercalation of plant sterols into the plasma membrane therefore results in dysregulation of multiple platelet activation pathways, leading to macrothrombocytopenia and bleeding.

Effect of plant sterols on the lipid profile of patients with hypercholesterolaemia. Randomised, experimental study

BACKGROUND

Studies have been conducted on supplementing the daily diet with plant sterol ester-enriched milk derivatives in order to reduce LDL-cholesterol levels and, consequently, cardiovascular risk. However, clinical practice guidelines on hypercholesterolaemia state that there is not sufficient evidence to recommend their use in subjects with hypercholesterolaemia. The main objective of this study is to determine the efficacy of the intake of 2 g of plant sterol esters a day in lowering LDL-cholesterol levels in patients diagnosed with hypercholesterolaemia. The specific objectives are: 1) to quantify the efficacy of the daily intake of plant sterol esters in lowering LDL-cholesterol, total cholesterol and cardiovascular risk in patients with hypercholesterolaemia; 2) to evaluate the occurrence of adverse effects of the daily intake of plant sterol esters; 3) to identify the factors that determine a greater reduction in lipid levels in subjects receiving plant sterol ester supplements.

METHODS/DESIGN

Randomised, double-blind, placebo controlled experimental trial carried out at family doctors' surgeries at three health centres in the Health Area of Albacete (Spain). The study subjects will be adults diagnosed with "limit" or "defined" hypercholesterolaemia and who have LDL cholesterol levels of 130 mg/dl or over. A dairy product in the form of liquid yoghurt containing 2 g of plant sterol ester per container will be administered daily after the main meal, for a period of 24 months. The control group will receive a daily unit of yogurt not supplemented with plant sterol esters that has a similar appearance to the enriched yoghurt. The primary variable is the change in lipid profile at 1, 3, 6, 12, 18 and 24 months. The secondary variables are: change in cardiovascular risk, adherence to the dairy product, adverse effects, adherence to dietary recommendations, frequency of food consumption, basic physical examination data, health problems, lipid-lowering medication, physical activity, smoking habits and socio-demographic variables.

DISCUSSION

If plant sterol ester supplements were effective a sounder recommendation for the consumption of plant sterols in subjects with hypercholesterolaemia could be made.

Phytosterols: physiologic and metabolic aspects related to cholesterol-lowering properties

The aim of this review is to give a general contemporary overview of the physiologic effects of phytosterols and their role in cholesterol uptake in the intestinal tract. The mechanism of phytosterols action is based on its ability to reduce cholesterol absorption. Doses of 0.8 to 4.0 g/d of phytosterols were used to reduce low-density lipoprotein cholesterol concentrations by 10% to 15%, although most of the studies described used 2 g/d of phytosterol to achieve a reduction of 10% in low-density lipoprotein cholesterol concentrations. Although some studies point to the possibility that elevated plasma phytosterol concentrations could contribute to the development of premature coronary artery diseases, extensive safety evaluation studies have been conducted for these compounds, and they have been considered safe.

Plant stanol and sterol esters in the control of blood cholesterol levels: mechanism and safety aspects

Incorporation of plant stanol esters into margarine is among the first examples of a functional food with proven low-density lipoprotein (LDL) cholesterol-lowering effectiveness. Recently, there have been many studies on the effects of plant stanols/sterols on cholesterol metabolism. It has been found that the serum LDL cholesterol-lowering effect of plant stanols/sterols originates from reduced intestinal cholesterol absorption, a process in which changes in micellar composition are thought to play a major role. However, recent findings suggest that there is an additional process in which plant stanols/sterols actively influence cellular cholesterol metabolism within intestinal enterocytes. Furthermore, in response to the reduced supply of exogenous cholesterol, receptor-mediated lipoprotein cholesterol uptake is probably enhanced, as shown by increased LDL receptor expression. At recommended intakes of about 2 to 2.5 g/day, products enriched with plant stanol/sterol esters lower plasma LDL cholesterol levels by 10% to 14% without any reported side effects. Thus, plant stanols/sterols can be considered to be effective and safe cholesterol-lowering functional food ingredients.

Metabolic effects of plant sterols and stanols (Review)

High serum LDL cholesterol concentration is a major risk factor for cardiovascular complications. This risk can be lowered by diet. In this respect foods containing plant sterol or stanol esters can be useful for mildly- and hypercholesteraemic subjects. Plant sterols and stanols, which are structurally related to cholesterol, decrease the incorporation of dietary and biliary cholesterol into micelles. This lowers cholesterol absorption. Furthermore, these components increase ABC-transporter expression, which may also contribute to the decreased cholesterol absorption. Consequently, cholesterol synthesis and LDL receptor activity increase, which ultimately leads to decreased serum LDL cholesterol concentrations. Animal studies have further shown that these dietary components may also lower atherosclerotic lesion development. Plant sterols and stanols also lower plasma lipid-standardized concentrations of the hydrocarbon carotenoids, but not those of the oxygenated cartenoids and tocopherols. Also, vitamin A and D concentrations are not affected. Although absorption of plant sterols and stanols (0.02-3.5%) is low compared to cholesterol (35-70%), small amounts are found in the circulation and may influence other physiological functions. However, there is no consistent evidence that plant sterols or stanols can change the risk of colon or prostate cancer, or immune status. In conclusion, plant sterols and stanols effectively reduce serum LDL cholesterol and atherosclerotic risk. In addition potential effects of plant sterols and stanols on other metabolic processes remain to be elucidated.

Red cell and plasma plant sterols are related during consumption of plant stanol and sterol ester spreads in children with hypercholesterolemia

OBJECTIVE

To show whether the ratios of squalene and cholesterol precursor sterols to cholesterol and cholestanol and plant sterols to cholesterol change differently in plasma and especially in the red cells of hypercholesterolemic children during consumption of plant stanol and sterol ester spreads.

STUDY DESIGN

In a randomized, double-blind, crossover study, hypercholesterolemic children (n = 23) consumed low-fat plant stanol and sterol ester spreads for 5-week periods separated by a 5-week washout period. Plasma and red cell lipids, squalene, and noncholesterol sterols were measured before and at the end of each period.

RESULTS

The plant stanol and sterol ester spreads lowered plasma total (-9% and -6%, respectively) and low-density lipoprotein (-12% and -9%) cholesterol but had no effect on red cell cholesterol, high-density lipoprotein cholesterol, or plasma triglycerides. The ratios of plasma and red cell sitosterol and campesterol to cholesterol decreased by 32% to 36% (P <.001) with the plant stanol ester and increased by 40% to 52% (P <.001) with the sterol ester spread.

CONCLUSIONS

Consumption of plant sterols increases and consumption of plant stanols decreases the ratios of plant sterols to cholesterol in red cells of hypercholesterolemic children proportionately to the respective changes in plasma.

Dietary sitostanol reduces plaque formation but not lecithin cholesterol acyl transferase activity in rabbits

The effects of graded amounts of dietary sitostanol (0.01, 0.2 and 0.8% (w/w)) were examined on plasma lipid-profile, coronary artery plaque development and lecithin:cholesterol acyl transferase activity in male New Zealand White rabbits given semi-purified diets for 10 weeks. All diets provided < 10% energy in the form of fat and contained 0.5% (w/w) cholesterol (C). Rabbits fed the semi-purified diet with 0.8% (w/w) (0.64 g/day) sitostanol had lower plasma total cholesterol (TC) (p = 0.006) (15.2 +/- 4.80 mmol/l) and very low-density lipoprotein-cholesterol (VLDL-C) (p = 0.007) (6.31 +/- 3.11 mmol/l) levels compared to the atherogenic control group (n = 6) (29.6 +/- 5.52 and 17.16 +/- 7.43 mmol/l, respectively). Dietary sitostanol at 0.8% (w/w) depressed plaque accretion in coronary arteries (p = 0.0014) and ascending aorta (p = 0.0004) compared with the atherogenic control, 0.01 and 0.2% (w/w) sitostanol-fed groups. No differences (p = 0.24) in the activity of lecithin:cholesterol acyl transferase (LCAT) were observed across groups, although plasma cholesterol fractional esterification rate was higher (p = 0.004) in the 0.8% (w/w) sitostanol fed animals compared with the atherogenic control. Significant negative correlations were demonstrated between sitostanol intake and plasma TC, LDL-C and VLDL-C levels. Hepatic campesterol levels were correlated (r = 0.3, p = 0.03) with plasma but not hepatic TC concentrations. These results demonstrate that dietary sitostanol at a concentration of 0.8% (w/w) or 0.64 g/day lowered plasma cholesterol levels and depressed atherosclerosis development in rabbits, but did not alter LCAT activity.

Incorporation of liposomal phytosterols into human cells in culture: a potential in vitro model for investigating pathological effects of phytosterolemia

A potential in vitro cell culture model was developed for studies concerning the pathological effect of phytosterolemia in which liposomal phytosterols were incorporated into human skin fibroblasts and hepatoblastoma (HepG2) cells. After incubation with phytosterols, fibroblasts and HepG2 cells contained a significant amount (20-27%) of phytosterols (campesterol and beta-sitosterol). Phytosterol accumulation caused a significant reduction in the cholesterol content of cells. Labeled sitosterol and cholesterol showed similar uptake with lower esterification of sitosterol when compared to cholesterol. Labeled sitosterol incorporated into LDL was esterified to a greater extent than sitosterol added as straight liposome. About 23% of the labeled sitosterol was converted into acidic products and 5.6% was present as 5 alpha-stanols in HepG2 cells.

Association of the intestinal brush-border membrane phospholipase A2 and lysophospholipase activities (phospholipase B) with a stalked membrane protein

We have attempted to determine the size and membrane orientation of a recently described rat jejunal brush-border protein possessing phospholipase A2 and lysophospholipase activities (phospholipase B) (Pind, S. and Kuksis, A. [1988] Biochim, Biophys. Acta 938, 211-221). The phospholipase A2 and lysophospholipase activities were renatured following nonreducing sodium dodecyl sulphate polyacrylamide gel electrophoresis of the total membrane proteins and were shown to migrate as a component of a protein band having a relative molecular mass of 170 kDa. This band accounted for approximately 1% of the total Coomassie Blue staining proteins. Phospholipase B was also shown to be solubilized from the membranes, in an active form, by a proteolytic digestion with papain. Papain solubilization resulted in a loss of the hydrophobic properties observed for the intact phospholipase. These results suggest that the active site of the phospholipase projects from the luminal surface of the membrane vesicles. In support of this, phospholipase activity towards exogenous, detergent-solubilized phosphatidylcholine was demonstrated under conditions in which the membranes remained intact. We conclude that the phospholipase B has the characteristics of a stalked, brush-border membrane protein and may be considered as another digestive enzyme anchored in this membrane.