| For medical practitioners and the general public - Cholesterol Journal Article Catalog. |
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| Caveolin mRNA levels are up-regulated by free cholesterol and down-regulated by oxysterols in fibroblast monolayers Fielding, C. J., A. Bist, et al. (1997), Proc Natl Acad Sci U S A 94(8): 3753-8. Abstract: In confluent fibroblast monolayers, an increase in the selective uptake of free cholesterol (FC) from plasma low density lipoprotein (LDL) was accompanied by an increase in FC efflux. The rate of FC efflux was proportional to the FC content of the cell surface caveolae and to mRNA levels of caveolin, an FC-binding protein of caveolae. Inhibitors of LDL-FC internalization reduced the increase in caveolin mRNA levels and FC efflux. Oxysterols reduced caveolin mRNA levels, as well as transport of FC to the cell surface and FC efflux. DNA antisense to caveolin reduced caveolin mRNA and inhibited FC efflux. These data suggest that regulation of FC efflux can contribute to cellular FC homeostasis when LDL levels are modified over the physiological range, and they link this regulation to caveolin. |
| Caveolin scaffolding region and cholesterol-rich domains in membranes Epand, R. M., B. G. Sayer, et al. (2005), J Mol Biol 345(2): 339-50. Abstract: A protein that constitutes a good marker for a type of cholesterol-rich domain in biological membranes is caveolin. A segment of this protein has a sequence that corresponds to a cholesterol recognition/interaction amino acid consensus (CRAC) motif; this motif has been suggested to cause the incorporation of proteins into cholesterol-rich domains. We have studied the interaction of two peptides containing the CRAC motif of caveolin-1 by differential scanning calorimetry, fluorescence, circular dichroism and magic angle spinning NMR. These peptides promote the segregation of cholesterol into domains from mixtures of the sterol with phosphatidylcholine, as shown by depletion of cholesterol from a portion of the membrane and enrichment of cholesterol in another domain. Cholesterol passes its solubility limit in the cholesterol-rich domain, resulting in the formation of cholesterol crystallites, suggesting that not all of the cholesterol recruited to this domain is bound to the peptide. NMR studies show that the peptides insert somewhat more deeply into membranes when cholesterol is present, but their strongest interaction takes place with the interfacial region of the membrane. We conclude that the peptides we studied containing CRAC sequences are more effective in promoting the formation of cholesterol-rich domains than are shorter peptides of this region of caveolin, which although they contain several aromatic amino acids, they have no CRAC motif. The presence or absence of a CRAC motif, however, is not a sufficient criterion to determine the extent to which a protein will promote the segregation of cholesterol in membranes. |
| Caveolin, cholesterol and Ras signalling Sternberg, P. W. and S. L. Schmid (1999), Nat Cell Biol 1(2): E35-7. |
| Caveolin, cholesterol, and lipid bodies Martin, S. and R. G. Parton (2005), Semin Cell Dev Biol 16(2): 163-74. Abstract: In mammalian cells a complex interplay regulates the distribution of cholesterol between intracellular membrane compartments. One important aspect of cholesterol regulation is intracellular cholesterol storage in neutral lipid storage organelles called lipid droplets or lipid bodies (LBs). Recent work has thrust the LB into the limelight as a complex and dynamic cellular organelle. LBs play a crucial role in maintaining the cellular levels of cholesterol by regulating the interplay between lipid storage, hydrolysis and trafficking. Studies of caveolins, caveolar membrane proteins linked to lipid regulation, are providing new insights into the role of LBs in regulating cholesterol balance. |
| Caveolin, cholesterol, and lipid droplets? van Meer, G. (2001), J Cell Biol 152(5): F29-34. |
| Caveolin-1 does not affect SR-BI-mediated cholesterol efflux or selective uptake of cholesteryl ester in two cell lines Wang, L., M. A. Connelly, et al. (2003), J Lipid Res 44(4): 807-15. Abstract: Free cholesterol (FC) has been reported to efflux from cells through caveolae, which are 50-100 nm plasma membrane pits. The 22 kDa protein caveolin-1 is concentrated in caveolae and is required for their formation. The HDL scavenger receptor BI (SR-BI), which stimulates both FC efflux and selective uptake of HDL-derived cholesteryl ester (CE), has been reported to be concentrated in caveolae, suggesting that this localization facilitates flux of FC and CE across the membrane. However, we found that overexpression of caveolin-1 in Fischer rat thyroid (FRT) cells, which lack caveolin-1 and caveolae, or HEK 293 cells, which normally express very low levels of caveolin-1, did not affect FC efflux to HDL or liposomes. Transient expression of SR-B1 did not affect this result. Similarly, caveolin-1 expression did not affect selective uptake of CE from labeled HDL particles in FRT or HEK 293 cells transfected with SR-BI. We conclude that basal and SR-BI-stimulated FC efflux to HDL and liposomes and SR-BI-mediated selective uptake of HDL CE are not affected by caveolin-1 expression in HEK 293 or FRT cells. |
| Caveolin-1 is not required for murine intestinal cholesterol transport Valasek, M. A., J. Weng, et al. (2005), J Biol Chem 280(30): 28103-9. Abstract: Caveolin-1 (CAV1) is the structural protein of the filamentous coat that decorates the cytoplasmic surface of each caveola. Cell culture studies have implicated CAV1 in playing an important role in intracellular cholesterol trafficking. In addition, it has been reported that CAV1 forms a detergent-resistant protein complex with Annexin-2 in enterocytes that can be disrupted by the cholesterol absorption inhibitor ezetimibe, suggesting a possible role for CAV1 in cholesterol absorption. In this report, we have evaluated cholesterol homeostasis in Cav1 knock-out mice. Deletion of CAV1 does not result in either a compensatory increase of CAV2 or CAV3 in intestine. In addition, Cav1 knock-out mice display normal mRNA and protein levels of Annexin-2 or the putative cholesterol transport protein Niemann-Pick C1-like 1 (NPC1L1) in proximal intestinal mucosa. Fractional cholesterol absorption and fecal neutral sterol excretion are statistically similar in Cav1 knock-out mice and their wild-type littermates. Moreover, oral administration of ezetimibe is equally effective in decreasing cholesterol absorption in Cav1 null mice and wild-type controls. The mRNA expression levels of genes sensitive to intracellular cholesterol concentration (ATP-binding cassette transporters ABCA1 and ABCG5, hydroxymethylglutaryl-CoA synthase and the LDL receptor) are similarly altered in the proximal intestinal mucosa of Cav1 null and wild-type mice following ezetimibe treatment. These results demonstrate that CAV1 is not required for cholesterol absorption or ezetimibe sensitivity in the mouse. |
| Caveolins and cellular cholesterol balance Ikonen, E. and R. G. Parton (2000), Traffic 1(3): 212-7. Abstract: Caveolins are major integral membrane components of caveolae. Over the last few years, evidence has accumulated for a close link between caveolin, caveolae, and the regulation of cellular cholesterol levels. However, the exact role of caveolin in this process, the intracellular trafficking routes followed by caveolin/cholesterol complexes, and the relationship of caveolin-cholesterol to other caveolin-mediated processes such as signal transduction have remained unclear. Recent findings from a number of systems suggest that specific signaling pathways require precise regulation of cellular cholesterol. Here we review evidence for caveolin regulation of cholesterol transport and consider how this may relate to signal transduction. |
| Caveolins and membrane cholesterol Ikonen, E., S. Heino, et al. (2004), Biochem Soc Trans 32(Pt 1): 121-3. Abstract: Caveolae (small plasma membrane invaginations) and their coat proteins, caveolins, have attracted the attention of researchers in diverse fields, including cell biology, cardiovascular and cancer research. The tight association between caveolin and cholesterol governs the biochemical behaviour of caveolae and is emerging as an important characteristic in a number of processes assigned to these multifunctional organelles. In this review, selected aspects of the caveolin-cholesterol association and its potential functional implications are discussed. |
| Caveolins in cholesterol trafficking and signal transduction: implications for human disease Schlegel, A., R. G. Pestell, et al. (2000), Front Biosci 5: D929-37. Abstract: Caveolins are a family of proteins that coat the cytoplasmic face of caveolae, vesicular invaginations of the plasma membrane. These proteins are central to the organization of the proteins and lipids that reside in caveolae. Caveolins transport cholesterol to and from caveolae, and they regulate the activity of signaling proteins that reside in caveolae. Through studying the genes encoding the caveolae coat proteins, we have learned much about how they perform these multiple functions. |
| Cavity depth and width effects on cyclophane-steroid recognition: molecular complexation of cholesterol and progesterone in aqueous solution Peterson, B. R., T. Mordasini-Denti, et al. (1995), Chem Biol 2(3): 139-46. Abstract: BACKGROUND: Recent X-ray crystal structures show that steroid-binding proteins contain deep hydrophobic cavities defined by aromatic amino-acid side chains which encapsulate steroid molecules. These cavities resemble the binding site of a synthetic macrotricyclic cyclophane receptor which we recently reported to form complexes with cholesterol in aqueous solution. The binding affinity of the cyclophane-cholesterol complex (Ka approximately 10(6) M-1, 295 K) is similar to that measured for the cholesterol complex of steroid-transport proteins such as sterol carrier protein-2 (SCP-2). Here we describe synthesis and binding studies of a related receptor with a cavity that is wider and 2 A deeper than that of the previous cyclophane, and a comparison of the steroid-binding affinity and selectivity of the two synthetic receptors. RESULTS: A new tricyclic cyclophane receptor with a 13 A deep cavity was synthesized to study the effect of increased cavity depth on receptor selectivity for steroids. NMR analysis demonstrated that this receptor provided increased steroidal side-chain encapsulation with a corresponding gain in binding free energy of 0.9 kcal mol-1 (in d4-methanol) as compared to our previously reported 11 A deep receptor. An unexpected consequence of the increase in cavity depth was a corresponding enlargement of the cavity width, as indicated both by steroid-binding studies and molecular modeling. This enlargement in cavity width increases binding affinity for saturated steroids while decreasing the association strength of unsaturated steroids such as cholesterol. In water, cholesterol binds to the new receptor with Ka approximately 1.5 x 10(5) M-1 and exhibits a significant complexation-mediated solubility increase. CONCLUSIONS: Small changes in steroid receptor dimensions have resulted in large differences in steroid selectivity and binding affinity. These results indicate that potentially large gains in steroid-binding free energy may be obtainable from complete hydrophobic encapsulation of the flexible aliphatic steroidal side chain. These results have implications for the design of synthetic receptor mimics of natural steroid binding proteins. |
| CCK receptor dysfunction in muscle membranes from human gallbladders with cholesterol stones Xiao, Z. L., Q. Chen, et al. (1999), Am J Physiol 276(6 Pt 1): G1401-7. Abstract: Human gallbladders with cholesterol stones exhibit impaired muscle contraction induced by agonists that act on transmembrane receptors, increased membrane cholesterol content, and abnormal cholesterol-to-phospholipid ratio compared with those with pigment stones. The present study was designed to investigate the functions of the CCK receptor of gallbladder muscle membranes by radioreceptor assay and cross-linking. 125I-labeled CCK-8 binding was time-dependent, competitive, and specific. Scatchard analysis showed that the maximum specific binding (Bmax) was significantly decreased in cholesterol compared with pigment stone gallbladders (0.18 +/- 0. 07 vs. 0.38 +/- 0.05 pmol/mg protein, P < 0.05). In contrast, the affinity for CCK was higher in cholesterol than pigment stone gallbladders (0.18 +/- 0.06 vs. 1.2 +/- 0.23 nM). Similar results were observed in binding studies with the CCK-A receptor antagonist 3HL-364,718. Cross-linking and saturation binding studies also showed significantly less CCK binding in gallbladders with cholesterol stones. These abnormalities were reversible after incubation with cholesterol-free liposomes. The Bmax increased (P < 0.01) and the dissociation constant decreased (P < 0.001) after incubation with cholesterol-free liposomes. In conclusion, human gallbladders with cholesterol stones have impaired CCK receptor binding compared with those with pigment stones. These changes are reversed by removal of the excess membrane cholesterol. These receptor alterations may contribute to the defective contractility of the gallbladder muscle in patients with cholesterol stones. |
| CD2 rocking modes as quantitative infrared probes of one-, two-, and three-bond conformational disorder in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine/cholesterol mixtures Mendelsohn, R., M. A. Davies, et al. (1991), Biochemistry 30(35): 8558-63. Abstract: The use of CD2 rocking modes in the IR spectrum as quantitative probes of phospholipid conformational disorder has recently been described for aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol mixtures Mendelsohn et al. (1989) Biochemistry 28, 8934-8939; Davies et al. (1990) Biochemistry 29, 4368-4373. Initial studies focused at the 4, 6, and 10 acyl chain positions of DPPC. In the current work, the method is extended to the 2, 3, 12, and 13 positions. Conformational disorder in the L alpha phase is approximately the same (about 20% gauche) at positions 4, 10, and 13, but an unexpected higher value is observed (about 30%) at the 6 position. Cholesterol (33 mol%) restricts gauche rotamer formation by factors ranging from 6 to 9 at positions 4 and 6, respectively, to 1.5-2 at positions 10, 12, and 13. Quantitative analysis for the DPPC/cholesterol "liquid-ordered" phase indicates the occurrence of 1.2 gauche bonds/chain, a marked reduction from the 3.6-4.2 gauche bonds/chain for DPPC alone. Proximity to the ester moiety at acyl chain position 3 perturbs the vibrational coupling patterns of the CD2 rocking modes and eliminates their sensitivity to conformational change. In addition, the feasibility of a method based on the conformation-dependent coupling between CD2 rocking frequencies of two successive CD2 groups for the quantitative detection of specific, position-dependent king (gtg') and isolated gauche (gtt) conformers is demonstrated. Finally, comparisons between IR measurements and explicit theoretical predictions of acyl chain conformational order are presented. |
| CD36 LIMPII analogous-1, a human homolog of the rodent scavenger receptor B1, provides the cholesterol ester for steroidogenesis in adrenocortical cells Imachi, H., K. Murao, et al. (1999), Metabolism 48(5): 627-30. Abstract: CD36 and LIMPII analogous-1 (CLA-1), a human homolog of the rodent scavenger receptor B1 (SR-B1), binds high-density lipoprotein (HDL) and mediates the selective uptake of HDL cholesterol ester (CE) by cultured transfected cells. CLA-1 is strongly expressed in steroidogenic tissues, including the adrenal gland, suggesting that CLA-1 plays a role in providing substrates for steroidogenesis. To address this, we established an adrenocortical cell line that highly expresses CLA-1. These cells increased CE uptake from HDL to 140.5% of the level in mock-transfected cells. After incubation of the transfected cells with HDL, corticosterone secretion from CLA-1-transfected cells increased to about two times the level in mock-transfected cells. These results indicate the possibility that CLA-1 (a close structural homolog of SR-B1)-mediated uptake of HDL CE may be a significant source of precursor cholesterol for steroidogenesis in humans as it is in mice. |
| cDNA cloning of cholesterol 24-hydroxylase, a mediator of cholesterol homeostasis in the brain Lund, E. G., J. M. Guileyardo, et al. (1999), Proc Natl Acad Sci U S A 96(13): 7238-43. Abstract: The turnover of cholesterol in the brain is thought to occur via conversion of excess cholesterol into 24S-hydroxycholesterol, an oxysterol that is readily secreted from the central nervous system into the plasma. To gain molecular insight into this pathway of cholesterol metabolism, we used expression cloning to isolate cDNAs that encode murine and human cholesterol 24-hydroxylases. DNA sequence analysis indicates that both proteins are localized to the endoplasmic reticulum, share 95% identity, and represent a new cytochrome P450 subfamily (CYP46). When transfected into cultured cells, the cDNAs produce an enzymatic activity that converts cholesterol into 24S-hydroxycholesterol, and to a lesser extent, 25-hydroxycholesterol. The cholesterol 24-hydroxylase gene contains 15 exons and is located on human chromosome 14q32.1. Cholesterol 24-hydroxylase is expressed predominantly in the brain as judged by RNA and protein blotting. In situ mRNA hybridization and immunohistochemistry localize the expression of this P450 to neurons in multiple subregions of the brain. The concentrations of 24S-hydroxycholesterol in serum are low in newborn mice, reach a peak between postnatal days 12 and 15, and thereafter decline to baseline levels. In contrast, cholesterol 24-hydroxylase protein is first detected in the brain of mice at birth and continues to accumulate with age. We conclude that the cloned cDNAs encode cholesterol 24-hydroxylases that synthesize oxysterols in neurons of the brain and that secretion of 24S-hydroxycholesterol from this tissue in the mouse is developmentally regulated. |
| Cell binding, uptake and cytosolic partition of HIV anti-gag phosphodiester oligonucleotides 3'-linked to cholesterol derivatives in macrophages LeDoan, T., F. Etore, et al. (1999), Bioorg Med Chem 7(11): 2263-9. Abstract: The purpose of this study is to evaluate the cell interactions of a new class of compounds composed of phosphodiester oligonucleotides linked to the cholesterol group at position 3, 7, or 22 of the steroid structure. The resulting conjugates were assessed for their capacity to bind, penetrate and partition in the cytoplasmic compartment of murine macrophages. The results showed that lipophilic conjugates bind to cells much faster (t(1/2) < or = 10 min) than do underivatized oligomers. Oligomers tethered to the cholesterol at positions 3 and 7 (PO-GEM-3-Chol and PO-GEM-7-Chol) interacted more efficiently with cell membranes and were better internalized than oligomers attached to the cholesterol moiety at position 22 (PO-GEM-22-Chol). The cytosolic fraction of internalized oligomers was studied by a digitonin-based membrane permeabilization method. The recovered fraction of oligomers that can freely diffuse from the cytosol was comparable for GEM-91, a phosphorothioate congener, and for PO-GEM-7-Chol (50-60% of the internalized oligomers), while that of PO-GEM-3-Chol was less (30% of the internalized oligomers) indicating a higher membrane affinity of the latter derivative as compared to the other investigated compounds. Membrane binding and cell internalization correlated well with the hydrophobicity of the conjugates as characterized by their partition coefficients in a water-octanol system. Due to their capacity of rapid binding and cytosolic partition in cells, cholesterol-derivatized oligonucleotides at position 3 or 7 of the steroid molecule appeared as good candidates for systemic delivery of anti-HIV antisense compounds. |
| Cell biology. Disease genes clarify cholesterol trafficking Marx, J. (2000), Science 290(5500): 2227-9. |
| Cell cholesterol efflux to reconstituted high-density lipoproteins containing the apolipoprotein A-IMilano dimer Calabresi, L., M. Canavesi, et al. (1999), Biochemistry 38(49): 16307-14. Abstract: The apolipoprotein A-IMilano (apoA-IM) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, resulting in the formation of homodimers A-IM/A-IM. The introduction of the interchain disulfide bridge in the A-IM dimer limits the apolipoprotein conformational flexibility and restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection. To investigate whether the structural changes in A-IM/A-IM affect apoA-I capacity for cell cholesterol uptake, we tested the ability of four reconstituted HDL (rHDL), that contained either apoA-I or A-IM/A-IM, to remove cholesterol from Fu5AH hepatoma cells and cholesterol-loaded murine primary macrophages (MPM). As the HDL particle size is known to affect the rHDL capacity for cell cholesterol uptake, the reconstitution conditions were carefully selected to produce two sets of rHDL particles of small and large size (7.8 and 12.5 nm in diameter). The small A-IM/A-IM rHDL were more efficient than the corresponding apoA-I particles as acceptors of membrane cholesterol from Fu5AH cells and MPM, and as inhibitors of cholesterol esterification in MPM. The large rHDL and the lipid-free apolipoproteins displayed instead similar capacities for cell cholesterol efflux. These results suggest that cell cholesterol efflux to rHDL particles of different size occurs through different mechanisms. Large HDL accommodate and retain the cholesterol molecules that have desorbed from the cell membrane into the extracellular fluid, in a process that is less sensitive to protein conformation. Small HDL accelerate the desorption of cholesterol from the cell membrane, in a process that is influenced by the conformation of the proteins on the surface of the acceptor particle. The enhanced efficiency of small A-IM/A-IM rHDL seems related to the peculiar structure of the protein on the rHDL surface, with a hydrophobic C-terminal domain extending out of the rHDL particle, available for anchoring the acceptor to the plasma membrane. |
| Cell cholesterol efflux: integration of old and new observations provides new insights Rothblat, G. H., M. de la Llera-Moya, et al. (1999), J Lipid Res 40(5): 781-96. Abstract: Numerous studies using a variety of cell/acceptor combinations have demonstrated differences in cholesterol efflux among cells. These studies also show that different acceptors, ranging from simple molecules like cyclodextrins to serum, stimulate efflux through a variety of mechanisms. By combining early observations with data derived from recent studies, it is now possible to formulate a model for cell cholesterol efflux which proposes that an array of different mechanisms, including aqueous diffusion, lipid-free apolipoprotein membrane microsolubilization, and SR-BI-mediated cholesterol exchange contribute to cholesterol flux. In this model the relative importance of each mechanism would be determined both by the cell type and the nature of the extracellular cholesterol acceptor. |
| Cell cholesterol esters and high-density lipoprotein plasma levels during liver hyperplasia in choline-fed male and female rats Tessitore, L., B. Batetta, et al. (2000), Int J Exp Pathol 81(4): 241-8. Abstract: Sexual dimorphism exists in the response of rats to lead nitrate, liver hyperplasia occuring earlier and being more pronounced in males. Excess dietary choline in females shifted the growth pattern towards that of males. To determine whether phosphatidylcholine-induced growth modulations could be related to a derangement of cholesterol metabolism, liver accumulation of cholesterol esters and plasma lipoprotein patterns were investigated. In males, lead-induced liver hyperplasia was associated with increased total cholesterol hepatic content, accumulated cholesterol esters and reduced concentration of plasma High Density Lipoprotein (HDL) cholesterol. Females were less responsive to the liver mitogenic signal of lead nitrate; there was no elevation of cholesterol content nor any marked accumulation of cholesterol esters. This is consistent with the lack of change in the plasma levels of HDL cholesterol. Continuous choline feeding displaced the liver cholesterol ester pattern and plasma HDL cholesterol levels in females, and in parallel that of DNA synthesis, towards those of males. Choline was not observed to have any effect in males. These results suggest that the derangement of phosphatidylcholine metabolism induces growth-related changes in cholesterol turnover; they are consistent with the proposal that the intracellular content of cholesterol esters may have a role in regulating liver growth rates. |