Beta-VLDL increases endothelial cell plasma membrane cholesterol

Omega-3 fatty acids, membrane remodeling and cancer prevention

Proteins are often credited as the macromolecule responsible for performing critical cellular functions, however lipids have recently garnered more attention as our understanding of their role in cell function and human health becomes more apparent. Although cellular membranes are the lipid environment in which many proteins function, it is now apparent that protein and lipid assemblies can be organized to form distinct micro- or nanodomains that facilitate signaling events. Indeed, it is now appreciated that cellular function is partly regulated by the specific spatiotemporal lipid composition of the membrane, down to the nanosecond and nanometer scale. Furthermore, membrane composition is altered during human disease processes such as cancer and obesity. For example, an increased rate of lipid/cholesterol synthesis in cancerous tissues has long been recognized as an important aspect of the rewired metabolism of transformed cells. However, the contribution of lipids/cholesterol to cellular function in disease models is not yet fully understood. Furthermore, an important consideration in regard to human health is that diet is a major modulator of cell membrane composition. This can occur directly through incorporation of membrane substrates, such as fatty acids, e.g., n-3 polyunsaturated fatty acids (n-3 PUFA) and cholesterol. In this review, we describe scenarios in which changes in membrane composition impact human health. Particular focus is placed on the importance of intrinsic lipid/cholesterol biosynthesis and metabolism and extrinsic dietary modification in cancer and its effect on plasma membrane properties.

Direct visualization of the arterial wall water permeability barrier using CARS microscopy

The artery wall is equipped with a water permeation barrier that allows blood to flow at high pressure without significant water leak. The precise location of this barrier is unknown despite its importance in vascular function and its contribution to many vascular complications when it is compromised. Herein we map the water permeability in intact arteries, using coherent anti-Stokes Raman scattering (CARS) microscopy and isotopic perfusion experiments. Generation of the CARS signal is optimized for water imaging with broadband excitation. We identify the water permeation barrier as the endothelial basolateral membrane and show that the apical membrane is highly permeable. This is confirmed by the distribution of the AQP1 water channel within endothelial membranes. These results indicate that arterial pressure equilibrates within the endothelium and is transmitted to the supporting basement membrane and internal elastic lamina macromolecules with minimal deformation of the sensitive endothelial cell. Disruption of this pressure transmission could contribute to endothelial cell dysfunction in various pathologies.

Angiotensin II impairs endothelial nitric-oxide synthase bioavailability under free cholesterol-enriched conditions via intracellular free cholesterol-rich membrane microdomains

Vascular endothelial function is impaired in hypercholesterolemia partly because of injury by modified LDL. In addition to modified LDL, free cholesterol (FC) is thought to play an important role in the development of endothelial dysfunction, although the precise mechanisms remain to be elucidated. The aim of this study was to clarify the mechanisms of endothelial dysfunction induced by an FC-rich environment. Loading cultured human aortic endothelial cells with FC induced the formation of vesicular structures composed of FC-rich membranes. Raft proteins such as phospho-caveolin-1 (Tyr-14) and small GTPase Rac were accumulated toward FC-rich membranes around vesicular structures. In the presence of these vesicles, angiotensin II-induced production of reactive oxygen species (ROS) was considerably enhanced. This ROS shifted endothelial NOS (eNOS) toward vesicle membranes and vesicles with a FC-rich domain trafficked toward perinuclear late endosomes/lysosomes, which resulted in the deterioration of eNOS Ser-1177 phosphorylation and NO production. Angiotensin II-induced ROS decreased the bioavailability of eNOS under the FC-enriched condition.

alpha 7-type acetylcholine receptor localization and its modulation by nicotine and cholesterol in vascular endothelial cells

The neuronal-type α7 nicotinic acetylcholine receptor (α7AChR) is also found in various non-neural tissues, including vascular endothelium, where its peculiar ionotropic properties (high Ca(2+) permeability) and its supervening Ca(2+) -mediated intracellular cascades may play important roles in physiology (angiogenesis) and pathology (inflammation and atherogenesis). Changes in molecular (up-regulation, affinity, and conformational states) and cellular (distribution, association with membranes) properties of the α7AChR related to angiogenesis (wound-repair cell migration) and atherogenesis (alterations in cholesterol content) were studied in living endothelial cells, with the aim of determining whether such changes constitute early markers of inflammatory response. The combination of pharmacological, biochemical, and fluorescence microscopy tools showed that α7AChRs in rat arterial endothelial (RAEC) and human venous endothelial (HUVEC) cells occur at extremely low expression levels (∼50 fmol/mg protein) but undergo agonist-induced up-regulation at relatively high nicotine concentrations (∼300-fold with 50 µM ligand), increasing their cell-surface exposure. When analyzed in terms of cold Triton X-100 solubility and subcellular distribution, α7AChRs occur in the "non-raft" subcellular membrane fractions. Acute cholesterol depletion reduced not only cholesterol levels but also the number of cell-surface α7AChRs. Nicotine exposure markedly stimulated cell migration and accelerated wound repair, which drastically diminished in cells deprived of the sterol. The angiogenic effect of nicotine appears to be synergistic with cholesterol content. Finally, the apparent K(D) of α7AChRs for the open-channel blocker crystal violet was found to be ∼600-fold lower in receptor-enriched membranes obtained from up-regulated HUVEC.

Cholesterol inhibits M-type K+ channels via protein kinase C-dependent phosphorylation in sympathetic neurons

M-type (KCNQ) potassium channels play an important role in regulating the action potential firing in neurons. Here, we investigated the effect of cholesterol on M current in superior cervical ganglion (SCG) sympathetic neurons, using the patch clamp technique. M current was inhibited in a dose-dependent manner by cholesterol loading with a methyl-beta-cyclodextrin-cholesterol complex. This effect was prevented when membrane cholesterol level was restored by including empty methyl-beta-cyclodextrin in the pipette solution. Dialysis of cells with AMP-PNP instead of ATP prevented cholesterol action on M currents. Protein kinase C (PKC) inhibitor, calphostin C, abolished cholesterol-induced inhibition whereas the PKC activator, PDBu, mimicked the inhibition of M currents by cholesterol. The in vitro kinase assay showed that KCNQ2 subunits of M channel can be phosphorylated by PKC. A KCNQ2 mutant that is defective in phosphorylation by PKC failed to show current inhibition not only by PDBu but also by cholesterol. These results indicate that cholesterol-induced inhibition of M currents is mediated by PKC phosphorylation. The inhibition of M currents by PDBu and cholesterol was completely blocked by PIP(2) loading, indicating that the decrease in PIP(2)-channel interaction underlies M channel inhibition by PKC-mediated phosphorylation. We conclude that cholesterol specifically regulates M currents in SCG neurons via PKC activation.

Cholesterol and Kir channels

To date, most of the major types of Kir channels, Kir2s, Kir3s, Kir4s, and Kir6s, have been found to partition into cholesterol-rich membrane domains and/or to be regulated by changes in the level of membrane cholesterol. Surprisingly, however, in spite of the structural similarities between different Kirs, effects of cholesterol on different types of Kir channels vary from cholesterol-induced decrease in the current density (Kir2 channels) to the loss of channel activity by cholesterol depletion (Kir4 channels) and loss of channel coupling by different mediators (Kir3 and Kir6 channels). Recently, we have gained initial insights into the mechanisms responsible for cholesterol-induced suppression Kir2 channels, but mechanisms underlying cholesterol sensitivity of other Kir channels are mostly unknown. The goal of this review is to present a summary of the current knowledge of the distinct effects of cholesterol on different types of Kir channels in vitro and in vivo.

Use of cyclodextrins to manipulate plasma membrane cholesterol content: evidence, misconceptions and control strategies

The physiological importance of cholesterol in the cell plasma membrane has attracted increased attention in recent years. Consequently, the use of methods of controlled manipulation of membrane cholesterol content has also increased sharply, especially as a method of studying putative cholesterol-enriched cell membrane domains (rafts). The most common means of modifying the cholesterol content of cell membranes is the incubation of cells or model membranes with cyclodextrins, a family of compounds, which, due to the presence of relatively hydrophobic cavity, can be used to extract cholesterol from cell membranes. However, the mechanism of this activity of cyclodextrins is not completely established. Moreover, under conditions commonly used for cholesterol extraction, cyclodextrins may remove cholesterol from both raft and non-raft domains of the membrane as well as alter the distribution of cholesterol between plasma and intracellular membranes. In addition, other hydrophobic molecules such as phospholipids may also be extracted from the membranes by cyclodextrins. We review the evidence for the specific and non-specific effects of cyclodextrins and what is known about the mechanisms for cyclodextrin-induced cholesterol and phospholipid extraction. Finally, we discuss useful control strategies that may help to verify that the observed effects are due specifically to cyclodextrin-induced changes in cellular cholesterol.

New insights into the pathological mechanisms of cerebrotendinous xanthomatosis in the Taiwanese using genomic and proteomic tools

Cerebrotendinous xanthomatosis (CTX) is an autosomal recessive lipid-storage disorder caused by a deficiency of the mitochondrial sterol 27-hydroxylase. Genetic analysis utilizing SSCP and direct DNA sequencing identified a new mutation. One base-pair of cytosine was deleted at codon 326 on exon 2 of CYP27 in all CTX patients while their father was heterozygotic. This novel point deletion predicts a frameshift in mRNA (Pro(102) -->Leu) and results in the appearance of a premature termination codon (TGA) to substitute for Val(106) (GTG). To characterize the pathological mechanism of CTX patients, the protein profiles of serum and leukocytes extracted from these subjects were presented by means of proteomic technologies including 2-DE and MALDI-TOF analysis. According to the results, the amount of vinculin, ABP-280, talin and vimentin in leukocytes of CTX patients had changed significantly, reflecting the changes in membrane dynamics concerning cholestanol accumulation. The expression of target proteins in CTX patients and control was further verified by western blotting which indicated the same tendency as 2-DE data. This is the first paper to integrate both genomic and proteomic concepts for analyzing the possible mechanism of CTX and provides more information for related study in the future.

Hypercholesterolemia suppresses inwardly rectifying K+ channels in aortic endothelium in vitro and in vivo

Inwardly rectifying K+ (Kir) channels are responsible for maintaining endothelial membrane potential and play a key role in endothelium-dependent vasorelaxation. In this study, we show that endothelial Kir channels are suppressed by hypercholesterolemic levels of lipoproteins in vitro and by serum hypercholesterolemia in vivo. Specifically, exposing human aortic endothelial cells to acetylated low-density lipoprotein or very low density lipoprotein resulted in a time- and concentration-dependent decrease in Kir current that correlated with the degree of cholesterol loading. The suppression was fully reversible by cholesterol depletion. Furthermore, a decrease in Kir current resulted in depolarization of endothelial membrane potential. Most important, the flow sensitivity of Kir currents was also impaired by cholesterol loading. Specifically, flow-induced increase in Kir current was suppressed by 70%, and flow-induced hyperpolarization was almost completely abrogated. Furthermore, we show that hypercholesterolemia in vivo also strongly suppresses endothelial Kir currents and causes a shift in endothelial membrane potential, as determined by comparing the currents in aortic endothelial cells freshly isolated from healthy or hypercholesterolemic pigs. Therefore, we suggest that suppression of Kir current is one of the important factors in hypercholesterolemia-induced endothelial dysfunction.

Vascular endothelial wound closure under shear stress: role of membrane fluidity and flow-sensitive ion channels

Sufficiently rapid healing of vascular endothelium following injury is essential for preventing further pathological complications. Recent work suggests that fluid dynamic shear stress regulates endothelial cell (EC) wound closure. Changes in membrane fluidity and activation of flow-sensitive ion channels are among the most rapid endothelial responses to flow and are thought to play an important role in EC responsiveness to shear stress. The goal of the present study was to probe the role of these responses in bovine aortic EC (BAEC) wound closure under shear stress. BAEC monolayers were mechanically wounded and subsequently subjected to either "high" (19 dyn/cm(2)) or "low" (3 dyn/cm(2)) levels of steady shear stress. Image analysis was used to quantify cell migration and spreading under both flow and static control conditions. Our results demonstrate that, under static conditions, BAECs along both wound edges migrate at similar velocities to cover the wounded area. Low shear stress leads to significantly lower BAEC migration velocities, whereas high shear stress results in cells along the upstream edge of the wound migrating significantly more rapidly than those downstream. The data also show that reducing BAEC membrane fluidity by enriching the cell membrane with exogenous cholesterol significantly slows down both cell spreading and migration under flow and hence retards wound closure. Blocking flow-sensitive K and Cl channels reduces cell spreading under flow but has no impact on cell migration. These findings provide evidence that membrane fluidity and flow-sensitive ion channels play distinct roles in regulating EC wound closure under flow.

Membrane cholesterol content modulates activation of volume-regulated anion current in bovine endothelial cells

Activation of volume-regulated anion current (VRAC) plays a key role in the maintenance of cellular volume homeostasis. The mechanisms, however, that regulate VRAC activity are not fully understood. We have examined whether VRAC activation is modulated by the cholesterol content of the membrane bilayer. The cholesterol content of bovine aortic endothelial cells was increased by two independent methods: (a) exposure to a methyl-beta-cyclodextrin saturated with cholesterol, or (b) exposure to cholesterol-enriched lipid dispersions. Enrichment of bovine aortic endothelial cells with cholesterol resulted in a suppression of VRAC activation in response to a mild osmotic gradient, but not to a strong osmotic gradient. Depletion of membrane cholesterol by exposing the cells to methyl-beta-cyclodextrin not complexed with cholesterol resulted in an enhancement of VRAC activation when the cells were challenged with a mild osmotic gradient. VRAC activity in cells challenged with a strong osmotic gradient were unaffected by depletion of membrane cholesterol. These observations show that changes in membrane cholesterol content shift VRAC sensitivity to osmotic gradients. Changes in VRAC activation were not accompanied by changes in anion permeability ratios, indicating that channel selectivity was not affected by the changes in membrane cholesterol. This suggests that membrane cholesterol content affects the equilibrium between the closed and open states of VRAC channel rather than the basic pore properties of the channel. We hypothesize that changes in membrane cholesterol modulate VRAC activity by affecting the membrane deformation energy associated with channel opening.