The replacement of cholesterol by phytosterols and the increase of total sterol content in model erythrocyte membranes

Advantages of the classical thermodynamic analysis of single-and multi-component Langmuir monolayers from molecules of biomedical importance-theory and applications

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

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.

Ezetimibe reduces plant sterol accumulation and favorably increases platelet count in sitosterolemia

OBJECTIVE

To assess if ezetimibe (EZE), a sterol-absorption inhibitor, improves platelet (PLT) count and size relative to its effect on plasma plant sterol (PS) in patients with sitosterolemia (STSL).

STUDY DESIGN

Patients with STSL (5 males, 3 females, 16-56 years of age) receiving EZE intervention as part of their routine care participated in this study. EZE was discontinued for 14 weeks (off) and then resumed for another 14 weeks (on). Hematology variables along with plasma and red blood cells (RBC) PS and total cholesterol (TC) levels were measured at the end of each phase.

RESULTS

EZE increased PLT count (23% ± 9%) and decreased mean PLT volume (MPV; 10% ± 3%, all P < .05). In patients off EZE, PLT counts inversely correlated (r = -0.96 and r = -0.91, all P < .01) with plasma and RBC PS to TC ratio (PS/TC), and MPV positively correlated (r = 0.91, P = .03 and r = 0.93, P = .02) with plasma and RBC PS/TC. EZE reduced plasma and RBC sitosterol (-35% ± 4% and -28% ± 3%), total PS (-37% ± 4% and -28% ± 3%, all P < .0001) levels, and PS/TC (-27% ± 4% and -28% ± 4%, P < .01).

CONCLUSIONS

EZE reduces plasma and RBC PS levels, while increasing PLT count and decreasing MPV, and thereby may reduce the risk for bleeding in STSL. Plasma PS levels and ABCG5/ABCG8 genes should be analyzed in patients with unexplained hematologic abnormalities.

Miscibility and interactions of animal and plant sterols with choline plasmalogen in binary and multicomponent model systems

In this work miscibility and interactions of sterols with choline plasmalogen (PC-plasm) in Langmuir monolayers were studied. Moreover, the properties of cholesterol/phosphatidylcholine/plasmalogen mixtures of different PC-plasm concentration were investigated. The foregoing systems were treated as a model of cancer cell membranes, which are of higher plasmalogen level than normal cells. Finally, the influence of β-sitosterol and stigmasterol (phytosterols differing in anticancer potency) on these mixtures was verified. The properties of monolayers were analyzed based on the parameters derived from the surface pressure-area isotherms and images taken with Brewster Angle Microscope. It was found that at 30% of sterol in sterol/plasmalogen monolayer the lipids are immiscible and 3D crystallites are formed within the film. Cholesterol molecules mix favorably with PC-plasm at Xchol ≥ 0.5, while the investigated phytosterols only at their prevailing proportion in binary system. The increase of choline plasmalogen in cholesterol/phosphatidylcholine monolayer causes destabilization of the system. Moreover, the incorporation of phytosterols into cholesterol/phosphatidylcholine+PC-plasm mixtures disturbed membrane morphology and this effect was stronger for β-sitosterol as compared to stigmasterol. It was concluded that the presence of vinyl ether bond at sn-1 position in PC-plasm molecule strongly affects miscibility of choline plasmalogen with sterols. The comparison of the collected data with those reported in literature allowed one to conclude that miscibility and interactions of sterols with PC-plasm are less favorable than those with phosphatidylcholine. It was also suggested that overexpression of plasmalogens in cancer cell membranes may be a factor differentiating sensitivity of cells to anticancer effect of phytosterols.

Impaired hepatic and intestinal ATP-binding cassette transporter G5/8 was associated with high exposure of β-sitosterol and the potential risks to blood-brain barrier integrity in diabetic rats

OBJECTIVES

Plant sterols are thought to treat hypercholesterolemia via inhibiting intestinal cholesterol absorption. The aim of this study was to evaluate the contribution of impaired ATP-binding cassette transporter G5/8 (ABCG5/8) expression by diabetes to the increased β-sitosterol (BS) exposure and impact of increased BS on integrity of blood-brain barrier (BBB).

METHODS

Basal BS level in tissues of streptozotocin-inducted rats and ABCG5/8 protein levels in liver and intestine were investigated; pharmacokinetics of BS was studied following oral dose; and primarily cultured rat brain microvessel endothelial cells (rBMECs) were used to study BS transportation across BBB and effect of BS on BBB integrity.

KEY FINDINGS

Diabetic rats showed greatly upgraded basal levels of BS in plasma, intestine, cerebral and hippocampus, accompanied by impairment of ABCG5/8 protein expression in liver and intestine. Pharmacokinetics studies demonstrated higher AUC0-48 and Cmax , and lower faecal recoveries of BS after oral administration, indicating enhancement of absorption or efflux impairment. In-vitro data showed increased ratio of BS/cholesterol in high levels BS-treated rBMECs was associated with increased BBB permeability of some biomarkers including BS itself.

CONCLUSIONS

Impaired ABCG5/8 protein expression by diabetes led to increase in BS exposure, which may be harmful to BBB function.

Molecular organization of bacterial membrane lipids in mixed systems--A comprehensive monolayer study combined with Grazing Incidence X-ray Diffraction and Brewster Angle Microscopy experiments

To properly design and investigate new antibacterial drugs a detailed description of the organization of bacterial membrane is highly important. Therefore in this work we performed a comprehensive characteristic of the Langmuir monolayers composed of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) mixed in a wide range of composition and treated as an artificial cytoplasmic layer of bacterial membrane. To obtain detailed information on the properties of these films we combined the analysis of the surface pressure-area curves with the surface potential measurements, Brewster Angle Microscopy studies and Grazing Incidence X-ray Diffraction experiments. It was found that the investigated phospholipids mix nonideally in the monolayers and that the most favorable packing of molecules occurs at their equimolar proportion. This is directly connected with the formation of hydrogen bonds between both types of molecules in the system. All the collected experimental data evidenced that dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidylglycerol (DPPG) form highly ordered associates of fixed (DPPE:DPPG 1:1) stoichiometry. The obtained results allow one to conclude a nonuniform distribution of lipids in bacterial membranes and the existence of domains composed of the investigated phospholipids. The latter seems to be of great importance in the perspective of further studies on the mechanism of action of antibacterial agents.

The effect of β-sitosterol on the properties of cholesterol/phosphatidylcholine/ganglioside monolayers--the impact of monolayer fluidity

In this paper the influence of one of phytosterols, namely β-sitosterol on cholesterol (Chol)/phosphatidylcholine (PC)/ganglioside (GM3) monolayers was examined to find the correlation between the properties of model system and the effect of phytocompound. The studied monolayers differed in condensation and fluidity, which were modified by the structure of phosphatidylcholine. It was found that the incorporation of β-sitosterol into cholesterol/phosphatidylcholine/ganglioside films changes their morphology, condensation and interactions between the lipids. The substitution of cholesterol more strongly decreased the condensation and stability of the film containing PC molecules having monounsaturated chains than more densely packed monolayer composed of saturated phosphatidylcholine. However, thorough analysis of data obtained so far suggests that the magnitude of β-sitosterol effect is determined by the composition of the system rather than its fluidity itself. Moreover, the results collected herein correlate well with the findings that phytosterol more strongly inhibits the growth of cancer cells, which at a given proportion of cholesterol to phospholipids in membranes, have more unsaturated fatty acids within phospholipids molecules.

Effects of sterol-binding agent nystatin on wheat roots: the changes in membrane permeability, sterols and glycoceramides

Plant sterols are important multifunctional lipids, which are involved in determining membrane properties. Biophysical characteristics of model lipid and isolated animal membranes with altered sterol component have been intensively studied. In plants however, the precise mechanisms of involvement of sterols in membrane functioning remain unclear. In present work the possible interactions between sterols and other membrane lipids in plant cells were studied. A useful experimental approach for elucidating the roles of sterols in membrane activity is to use agents that specifically bind with endogenous sterols, for example the antibiotic nystatin. Membrane characteristics and the composition of membrane lipids in the roots of wheat (Triticum aestivum L.) seedlings treated with nystatin were analyzed. The application of nystatin greatly increased the permeability of the plasma membrane for ions and SH-containing molecules and decreased the total sterol level mainly as a consequence of a reduction in the amount of β-sitosterol and campesterol. Dynamic light-scattering was used to confirm the in vitro formation of stable complexes between nystatin and β-sitosterol or cholesterol. Sterol depletion was accompanied by a significant rise in total glycoceramide (GlCer) content after 2h treatment with nystatin. Analysis of the GlCer composition using mass spectrometry with electrospray ionization demonstrated that nystatin induced changes in the ratio of molecular species of GlCer. Our results suggest that changes in the sphingolipid composition can contribute to the changes in plasma membrane functioning induced by sterol depletion.