Lipid rafts/caveolae are essential for insulin-like growth factor-1 receptor signaling during 3T3-L1 preadipocyte differentiation induction

Lipid rafts/caveolae are found to be essential for insulin-like growth factor (IGF)-1 receptor signaling during 3T3-L1 preadipocyte differentiation induction. In 3T3-L1 cells, IGF-1 receptor is located in lipid rafts/caveolae of the plasma membrane and can directly interact with caveolin-1, the major protein component in caveolae. Disruption of lipid rafts/caveolae by depleting cellular cholesterol with cholesterol-binding reagent, beta-methylcyclodextrin or filipin, blocks the IGF-1 receptor signaling in 3T3-L1 preadipocyte. Both hormonal induced adipocyte differentiation and mitotic clonal expansion are inhibited by lipid rafts/caveolae disruption. However, a nonspecific lipid binding reagent, xylazine, does not affect adipocyte differentiation or mitotic clonal expansion. Further studies indicate that lipid rafts/caveolae are required only for IGF-1 receptor downstream signaling and not the activation of receptor itself by ligand. Thus, our results suggest that localization in lipid rafts/caveolae and association with caveolin enable IGF-1 receptor to have a close contact with downstream signal molecules recruited into lipid rafts/caveolae and transmit the signal through these signal molecule complexes.     

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review).

Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.     

脂筏与精子膜信号传导

脂筏是细胞膜内由特殊脂质与蛋白质构成的微域。小窝是脂筏的一种形式,小窝标记蛋白有小窝蛋白和小窝舟蛋白。脂筏或小窝与生物信号传导、细胞蛋白转运和胆固醇平衡有关。最近实验证实哺乳动物精子膜具有脂筏结构,脂筏与膜胆固醇外逸对于启动受精的信号传导具有重要作用。     

Sterol carrier protein-2 selectively alters lipid composition and cholesterol dynamics of caveolae/lipid raft vs nonraft domains in L-cell fibroblast plasma membranes

Although the functional significance of caveolae/lipid rafts in cellular signaling and cholesterol transfer is increasingly recognized, almost nothing is known regarding the lipids, cholesterol dynamics, and factors regulating these properties in caveolae/lipid rafts as opposed to nonlipid raft domains of the plasma membrane. The present findings demonstrate the utility of con-A affinity chromatography for simultaneous isolation of caveolae/lipid raft and nonlipid raft domains from plasma membranes of L-cell fibroblasts. These domains differed markedly in both protein and lipid constituents. Although caveolae/lipid rafts were enriched in total lipid, cholesterol, and phospholipid as well as other markers for these domains, the cholesterol/phospholipid ratio of caveolae/lipid rafts did not differ from that of nonlipid rafts. Nevertheless, spontaneous sterol transfer was 7-12-fold faster from caveolae/lipid raft than nonlipid raft domains of the plasma membrane. This was largely due to the near absence of exchangeable sterol in the nonlipid rafts. SCP-2 dramatically and selectively enhanced sterol transfer from caveolae/lipid rafts, but not from nonlipid rafts. Finally, overexpression of SCP-2 significantly altered the sterol dynamics of caveolae/lipid rafts to facilitate retention of cholesterol within the cell. These results established for the first time that (i) caveolae/lipid rafts, rather than the nonlipid raft domains, contain significant levels of rapidly transferable sterol, consistent with their role in spontaneous sterol transfer from and through the plasma membrane, and (ii) SCP-2 selectively regulates how caveolae/lipid rafts, but not nonlipid raft domains, mediate cholesterol trafficking through the plasma membrane.     

Spatial segregation of transport and signalling functions between human endothelial caveolae and lipid raft proteomes

Lipid rafts and caveolae are biochemically similar, specialized domains of the PM (plasma membrane) that cluster specific proteins. However, they are morphologically distinct, implying different, possibly complementary functions. Two-dimensional gel electrophoresis preceding identification of proteins by MS was used to compare the relative abundance of proteins in DRMs (detergent-resistant membranes) isolated from HUVEC (human umbilical-vein endothelial cells), and caveolae immunopurified from DRM fractions. Various signalling and transport proteins were identified and additional cell-surface biotinylation revealed the majority to be exposed, demonstrating their presence at the PM. In resting endothelial cells, the scaffold of immunoisolated caveolae consists of only few resident proteins, related to structure [CAV1 (caveolin-1), vimentin] and transport (V-ATPase), as well as the GPI (glycosylphosphatidylinositol)-linked, surface-exposed protein CD59. Further quantitative characterization by immunoblotting and confocal microscopy of well-known [eNOS (endothelial nitric oxide synthase) and CAV1], less known [SNAP-23 (23 kDa synaptosome-associated protein) and BASP1 (brain acid soluble protein 1)] and novel [C8ORF2 (chromosome 8 open reading frame 2)] proteins showed different subcellular distributions with none of these proteins being exclusive to either caveolae or DRM. However, the DRM-associated fraction of the novel protein C8ORF2 (approximately 5% of total protein) associated with immunoseparated caveolae, in contrast with the raft protein SNAP-23. The segregation of caveolae from lipid rafts was visually confirmed in proliferating cells, where CAV1 was spatially separated from eNOS, SNAP-23 and BASP1. These results provide direct evidence for the previously suggested segregation of transport and signalling functions between specialized domains of the endothelial plasma membrane.     

Long-chain fatty acid uptake into adipocytes depends on lipid raft function

This study investigates the role of lipid rafts and caveolae, a subclass of lipid raft microdomains, in the binding and uptake of long-chain fatty acids (LCFA) by 3T3-L1 cells during differentiation. Disruption of lipid rafts by beta-cyclodextrin (betaCD) or selective inhibition of caveolae by overexpression of a dominant-negative mutant of caveolin-3 (Cav(DGV)) resulted in disassembly of caveolae structures at the cell surface, as assessed by electron microscopy. While in 3T3-L1 fibroblasts, which express few caveolae, Cav(DGV) or betaCD had no effect on LCFA uptake, in 3T3-L1 adipocytes the same treatments decreased the level of [(3)H]oleic acid uptake by up to 55 +/- 8 and 49 +/- 7%, respectively. In contrast, cholesterol loading of 3T3-L1 adipocytes resulted in a 4-fold increase in the extent of caveolin-1 expression and a 1.7-fold increase in the level of LCFA uptake. Both the inhibitory and enhancing effects of these treatments were constantly increasing with the [(3)H]oleic acid incubation time up to 5 min. Incubation of 3T3-L1 adipocytes with [(3)H]stearate followed by isolation of a caveolin-1 positive detergent-resistant membrane (DRM) fraction revealed that [(3)H]stearate binds to caveolae. Fatty acid translocase (FAT/CD36) was found to be present in this DRM fraction as well. Our data thus strongly indicate a critical involvement of lipid rafts in the binding and uptake of LCFA into 3T3-L1 adipocytes. Furthermore, our findings suggest that caveolae play a pivotal role in lipid raft-dependent LCFA uptake. This transport mechanism is induced in conjunction with cell differentiation and might be mediated by FAT/CD36.     

Intracellular retention of caveolin 1 in presenilin-deficient cells

Mutations in genes encoding presenilins (PS1 and PS2) are responsible for the majority of early onset familial Alzheimer's disease. PS, a critical component of gamma-secretase, is responsible for the intramembranous cleavage of amyloid precursor protein and Notch. Other physiological functions have been assigned to PS without any clear identification of the mechanisms underlying these multiple biological roles. The early embryonic lethality of PS1 and PS2 double knock-out (PS1/2 null) mice prevents the evaluation of physiological roles of PS. To investigate new functions for presenilins, we performed a proteomic approach by using cells derived from PS1/2 null blastocysts and wild type controls. We identified a presenilin-dependent cell-surface binding of albumin. Binding of albumin depends on intact caveolae on the cellular surface. Abnormal caveolin 1 localization in PS1/2 null cells was associated with a loss of caveolae and an absence of caveolin 1 expression within lipid rafts. Expressing PS1 or PS2 but not the intracellular form of Notch1 in PS1/2 null cells restored normal caveolin 1 localization, demonstrating that presenilins are required for the subcellular trafficking of caveolin 1 independently from Notch activity. Despite an expression of both caveolin 1 and PS1 within lipid raft-enriched fractions after sucrose density centrifugation in wild type cells, no direct interaction between these two proteins was detected, implying that presenilins affect caveolin 1 trafficking in an indirect manner. We conclude that presenilins are required for caveolae formation by controlling transport of intracellular caveolin 1 to the plasma membrane.     

Selective cholesterol dynamics between lipoproteins and caveolae/lipid rafts

Although low-density lipoprotein (LDL) receptor-mediated cholesterol uptake through clathrin-coated pits is now well understood, the molecular details and organizing principles for selective cholesterol uptake/efflux (reverse cholesterol transport, RCT) from peripheral cells remain to be resolved. It is not yet completely clear whether RCT between serum lipoproteins and the plasma membrane occurs primarily through lipid rafts/caveolae or from non-raft domains. To begin to address these issues, lipid raft/caveolae-, caveolae-, and non-raft-enriched fractions were resolved from purified plasma membranes isolated from L-cell fibroblasts and MDCK cells by detergent-free affinity chromatography and compared with detergent-resistant membranes isolated from the same cells. Fluorescent sterol exchange assays between lipoproteins (VLDL, LDL, HDL, apoA1) and these enriched domains provided new insights into supporting the role of lipid rafts/caveolae and caveolae in plasma membrane/lipoprotein cholesterol dynamics: (i) lipids known to be translocated through caveolae were detected (cholesteryl ester, triacylglycerol) and/or enriched (cholesterol, phospholipid) in lipid raft/caveolae fractions; (ii) lipoprotein-mediated sterol uptake/efflux from lipid rafts/caveolae and caveolae was rapid and lipoprotein specific, whereas that from non-rafts was very slow and independent of lipoprotein class; and (iii) the rate and lipoprotein specificity of sterol efflux from lipid rafts/caveolae or caveolae to lipoprotein acceptors in vitro was slower and differed in specificity from that in intact cells-consistent with intracellular factors contributing significantly to cholesterol dynamics between the plasma membrane and lipoproteins.     

Perfringolysin O, a cholesterol-binding cytolysin, as a probe for lipid rafts

Gaining an understanding of the structural and functional roles of cholesterol in membrane lipid rafts is a critical issue in studies on cellular signaling and because of the possible involvement of lipid rafts in various diseases. We have focused on the potential of perfringolysin O (theta-toxin), a cholesterol-binding cytolysin produced by Clostridium perfringens, as a probe for studies on membrane cholesterol. We prepared a protease-nicked and biotinylated derivative of perfringolysin O (BCtheta) that binds selectively to cholesterol in cholesterol-rich microdomains of cell membranes without causing membrane lesions. Since the domains fulfill the criteria of lipid rafts, BCtheta can be used to detect cholesterol-rich lipid rafts. This is in marked contrast to filipin, another cholesterol-binding reagent, which binds indiscriminately to cell cholesterol. Using BCtheta, we are now searching for molecules that localize specifically in cholesterol-rich lipid rafts. Recently, we demonstrated that the C-terminal domain of perfringolysin O, domain 4 (D4), possesses the same binding characteristics as BCtheta. BIAcore analysis showed that D4 binds specifically to cholesterol with the same binding affinity as the full-size toxin. Cell-bound D4 is recovered predominantly from detergent-insoluble, low-density membrane fractions where raft markers, such as cholesterol, flotillin and Src family kinases, are enriched, indicating that D4 also binds selectively to lipid rafts. Furthermore, a green fluorescent protein-D4 fusion protein (GFP-D4) was revealed to be useful for real-time monitoring of cholesterol in lipid rafts in the plasma membrane. In addition, the expression of GFP-D4 in the cytoplasm might allow the investigations of intracellular trafficking of lipid rafts. The simultaneous visualization of lipid rafts in plasma membranes and inside cells might help in gaining a total understanding of the dynamic behavior of lipid rafts.     

Insulin-like growth factor-1 receptor signaling in 3T3-L1 adipocyte differentiation requires lipid rafts but not caveolae

Previously, we have found that lipid rafts/caveolae were essential for insulin-like growth factor-1 (IGF-1) receptor signaling during 3T3-L1 preadipocytes differentiation induction. However, it was not identified as to which of the membrane lipid-ordered microdomains mediates the receptor signal. Using small double-stranded RNA-mediated interference (RNAi), we successfully suppressed the caveolin-1 protein expression. In cells stably transfected with vector expressing small interfering RNA (siRNA) fragment, no caveolin-1 protein or caveola was detected. On the other hand, removal of caveolin-1 did not affect the caveolinless lipid rafts or the localization of IGF-1 receptor in lipid rafts on plasma membrane. IGF-1 receptor signal transduction and induced cellular differentiation were normal in RNAi cells with only lipid rafts. Furthermore, these IGF-1 receptor signaling events were still sensitive to the cholesterol-binding reagents. Thus, our results suggest that lipid rafts are sufficient for IGF-1 receptor signaling and the recruitment of signal molecules by caveolin-1 is not essential for IGF-1 receptor signaling.     

The receptor-bound N-terminal ectodomain of the amyloid precursor protein is associated with membrane rafts

The soluble N-terminal ectodomain of amyloid precursor protein (sAPP), resulting from alpha-secretase-mediated proteolytic processing, has been shown to function as a growth factor for epithelial cells, including keratinocytes and thyrocytes. Extracellularly applied sAPP binds to a cell surface receptor and exhibits a patchy binding pattern reminiscent of that observed for raft proteins. Here we show that (i) the receptor-bound sAPP resides in a detergent-insoluble membrane microdomain which cofractionates in density gradients with cholesterol-rich membrane rafts and caveolae; (ii) the sAPP-binding microdomains are different from caveolae; and (iii) sAPP is capable of binding to isolated rafts and inducing tyrosine phosphorylation of some raft proteins. These observations suggest that a novel type of membrane raft is involved in sAPP signaling.     

Caveolae and non-caveolae lipid raft microdomains of human umbilical vein endothelial cells contain utrophin-associated protein complexes

Several studies have shown the importance of dystrophin-associated protein complex in the development of muscular dystrophies and dilated cardiomyopathy associated to vascular dysfunction. In vascular endothelium, dystrophin is substituted for utrophin (autosomal homolog of dystrophin); however, its role in this tissue is unknown. Therefore, it is important to obtain a more extensive knowledge of utrophin and its associated proteins in endothelial cells. In a previous study, we demonstrated the presence of utrophin-associated protein complex (UAPC) in human umbilical vein endothelial cells HUVEC, which interacts with caveolin-1 (Cav-1) and endothelial nitric oxide synthase (eNOS). Also, some of our observations suggested the presence of this complex in distinct membrane domains. Therefore, the aim of this study was to analyze the presence of the UAPC in caveolae and non-caveolae lipid rafts domains of HUVEC at baseline and with a mechanical stimulus. It was demonstrated, by subcellular fractionation and co-immunoprecipitation assays, the association of UAPC with Cav-1 and eNOS in caveolae domains, as well as its interaction with eNOS in non-caveolae lipid raft domains. Additionally, it was also observed that mechanical stress on endothelial cells induced activation and release of eNOS from both caveolae and non-caveolae lipid raft associated to UAPC. Together these results suggest that UAPC located in caveolae and non-caveolae lipid raft domains of HUVECs may have a mechanosensory function that could participate in the control of eNOS activity.     

Endocytosis of oxidized low density lipoprotein through scavenger receptor CD36 utilizes a lipid raft pathway that does not require caveolin-1

The scavenger receptor CD36 binds a diverse array of ligands, including thrombospondin-1, oxidized low density lipoprotein (OxLDL), fatty acids, anionic phospholipids, and apoptotic cells. CD36 has been reported to be present in lipid rafts/caveolae, but little is known about the membrane trafficking of this protein at baseline or following ligand binding. Here, we determined that expression of CD36 in Chinese hamster ovary (CHO) cells and endogenous expression of CD36 in C32 cells led to a homogeneous distribution of the protein on the plasma membrane, as judged by confocal fluorescence microscopy. This homogeneous pattern was observed both by anti-CD36 antibody staining and by live cell imaging of CHO cells expressing a chimeric CD36-green fluorescent protein construct. In contrast, caveolin-1 displayed its usual punctate surface distribution. Correspondingly, dual labeling of CD36 and caveolin-1 showed essentially no overlap, neither by immunofluorescence light microscopy nor by immunogold electron microscopy. Furthermore, isolation of lipid rafts by sucrose gradient ultracentrifugation of cold Triton X-100 cell lysates yielded both CD36 and caveolin-1, but immunoprecipitates of caveolin-1 did not contain CD36. Binding of Ox-LDL led to internalization of CD36 and OxLDL into endosomal structures that did not contain caveolin-1 or transferrin but that co-internalized the glycosyl-phosphatidylinositol-anchored protein decay accelerating factor, a lipid raft protein. Furthermore, expression of CD36 in the caveolin-1-negative KB cell line is sufficient for OxLDL-induced internalization of CD36, indicating that caveolin-1 is not required for this endocytic process. Taken together, these data demonstrate that at steady state, CD36 is localized in lipid rafts but not in caveolae, and that binding of OxLDL to CD36 leads to endocytosis through a lipid raft pathway that is distinct from the clathrin-mediated or caveolin internalization pathways.     

Endocytosis without clathrin coats

Endocytosis is involved in an enormous variety of cellular processes. To date, most studies on endocytosis in mammalian cells have focused on pathways that start with uptake through clathrin-coated pits. Recently, new techniques and reagents have allowed a wider range of endocytic pathways to begin to be characterized. Various non-clathrin endocytic mechanisms have been identified, including uptake through caveolae, macropinosomes and via a separate constitutive pathway. Many markers for clathrin-independent endocytosis are found in detergent-resistant membrane fractions, or lipid rafts. We will discuss these emerging new findings and their implications for the nature of lipid rafts themselves, as well as for the potential roles of non-clathrin endocytic pathways in remodeling of the plasma membrane and in regulating the membrane composition of specific intracellular organelles.     

Plasma membrane microdomains regulate TACE-dependent TNFR1 shedding in human endothelial cells

Upon stimulation by histamine, human vascular endothelial cells (EC) shed a soluble form of tumour necrosis factor receptor 1 (sTNFR1) that binds up free TNF, dampening the inflammatory response. Shedding occurs through proteolytic cleavage of plasma membrane-expressed TNFR1 catalysed by TNF-α converting enzyme (TACE). Surface expressed TNFR1 on EC is largely sequestered into specific plasma membrane microdomains, the lipid rafts/caveolae. The purpose of this study was to determine the role of these domains in TACE-mediated TNFR1 shedding in response to histamine. Human umbilical vein endothelial cells derived EA.hy926 cells respond to histamine via H1 receptors to shed TNFR1. Both depletion of cholesterol by methyl-β-cyclodextrin and small interfering RNA knockdown of the scaffolding protein caveolin-1 (cav-1), treatments that disrupt caveolae, reduce histamine-induced shedding of membrane-bound TNFR1. Moreover, immunoblotting of discontinuous sucrose gradient fractions show that TACE, such as TNFR1, is present within low-density membrane fractions, concentrated within caveolae, in unstimulated EA.hy926 endothelial cells and co-immunoprecipitates with cav-1. Silencing of cav-1 reduces the levels of both TACE and TNFR1 protein and displaces TACE, from low-density membrane fractions where TNFR1 remains. In summary, we show that endothelial lipid rafts/caveolae co-localize TACE to surface expressed TNFR1, promoting efficient shedding of sTNFR1 in response to histamine.     

Sterol carrier protein-2 directly interacts with caveolin-1 in vitro and in vivo

HDL-mediated reverse-cholesterol transport as well as phosphoinositide signaling are mediated through plasma membrane microdomains termed caveolae/lipid rafts. However, relatively little is known regarding mechanism(s) whereby these lipids traffic to or are targeted to caveolae/lipid rafts. Since sterol carrier protein-2 (SCP-2) binds both cholesterol and phosphatidylinositol, the possibility that SCP-2 might interact with caveolin-1 and caveolae was examined. Double immunolabeling and laser scanning fluorescence microscopy showed that a small but significant portion of SCP-2 colocalized with caveolin-1 primarily at the plasma membrane of L-cells and more so within intracellular punctuate structures in hepatoma cells. In SCP-2 overexpressing L-cells, SCP-2 was detected in close proximity to caveolin, 48 +/- 4 A, as determined by fluorescence resonance energy transfer (FRET) and immunogold electron microscopy. Cell fractionation of SCP-2 overexpressing L-cells and Western blotting detected SCP-2 in purified plasma membranes, especially in caveolae/ lipid rafts as compared to the nonraft fraction. SCP-2 and caveolin-1 were coimmunoprecipitated from cell lysates by anti-caveolin-1 and anti-SCP-2. Finally, a yeast two-hybrid assay demonstrated that SCP-2 directly interacts with caveolin-1 in vivo. These interactions of SCP-2 with caveolin-1 were specific since a functionally related protein, phosphatidyinositol transfer protein (PITP), colocalized much less well with caveolin-1, was not in close proximity to caveolin-1 (i.e., >120 A), and was not coimmunoprecipitated by anti-caveolin-1 from cell lysates. In summary, it was shown for the first time that SCP-2 (but not PITP) selectively interacted with caveolin-1, both within the cytoplasm and at the plasma membrane. These data contribute significantly to our understanding of the role of SCP-2 in cholesterol and phosphatidylinositol targeted from intracellular sites of synthesis in the endoplasmic reticulum to caveolae/lipid rafts at the cell surface plasma membrane.     

Growth factor receptors, lipid rafts and caveolae: an evolving story

Growth factor receptors have been shown to be localized to lipid rafts and caveolae. Consistent with a role for these cholesterol-enriched membrane domains in growth factor receptor function, the binding and kinase activities of growth factor receptors are susceptible to regulation by changes in cholesterol content. Furthermore, knockouts of caveolin-1, the structural protein of caveolae, have confirmed that this protein, and by implication caveolae, modulate the ability of growth factor receptors to signal. This article reviews the findings pertinent to the relationship between growth factor receptors, lipid rafts and caveolae and presents a model for understanding the disparate observations regarding the role of membrane microdomains in the regulation of growth factor receptor function.     

Depletion of plasma membrane cholesterol dampens hydrostatic pressure and shear stress-induced mechanotransduction pathways in osteoblast cultures

The preferential association of cholesterol and sphingolipids within plasma membranes forms organized compartments termed lipid rafts. Addition of caveolin proteins to this lipid milieu induces the formation of specialized invaginated plasma membrane structures called caveolae. Both lipid rafts and caveolae are purported to function in vesicular transport and cell signaling. We and others have shown that disassembly of rafts and caveolae through depletion of plasma membrane cholesterol mitigates mechanotransduction processes in endothelial cells. Because osteoblasts are subjected to fluid-mechanical forces, we hypothesize that cholesterol-rich plasma membrane microdomains also serve the mechanotransduction process in this cell type. Cultured human fetal osteoblasts were subjected to either sustained hydrostatic pressure or laminar shear stress using a pressure column or parallel-plate apparatus, respectively. We found that sustained hydrostatic pressure induced protein tyrosine phosphorylation, activation of extracellular signal-regulated kinase (ERK)1/2, and enhanced expression of c-fos in both time- and magnitude-dependent manners. Similar responses were observed in cells subjected to laminar shear stress. Both sustained hydrostatic pressure- and shear stress-induced signaling were significantly reduced in osteoblasts pre-exposed to either filipin or methyl--cyclodextrin. These mechanotransduction responses were restored on reconstitution of lipid rafts and caveolae, which suggests that cholesterol-rich plasma membrane microdomains participate in the mechanotransduction process in osteoblasts. In addition, mechanical force-induced phosphoproteins were localized within caveolin-containing membranes. These data support the concept that lipid rafts and caveolae serve a general function as cell surface mechanotransduction sites within the plasma membrane. lipid rafts; caveolae; extracellular signal-regulated kinase     

Cross-talk between caveolae and glycosylphosphatidylinositol-rich domains.

Most mammalian cells have in their plasma membrane at least two types of lipid microdomains, non-invaginated lipid rafts and caveolae. Glycosylphosphatidylinositol (GPI)-anchored proteins constitute a class of proteins that are enriched in rafts but not caveolae at steady state. We have analyzed the effects of abolishing GPI biosynthesis on rafts, caveolae, and cholesterol levels. GPI-deficient cells were obtained by screening for resistance to the pore-forming toxin aerolysin, which uses this class of proteins as receptors. Despite the absence of GPI-anchored proteins, mutant cells still contained lipid rafts, indicating that GPI-anchored proteins are not crucial structural elements of these domains. Interestingly, the caveolae-specific membrane proteins, caveolin-1 and 2, were up-regulated in GPI-deficient cells, in contrast to flotillin-1 and GM1, which were expressed at normal levels. Additionally, the number of surface caveolae was increased. This effect was specific since recovery of GPI biosynthesis by gene recomplementation restored caveolin expression and the number of surface caveolae to wild type levels. The inverse correlation between the expression of GPI-anchored proteins and caveolin-1 was confirmed by the observation that overexpression of caveolin-1 in wild type cells led to a decrease in the expression of GPI-anchored proteins. In cells lacking caveolae, the absence of GPI-anchored proteins caused an increase in cholesterol levels, suggesting a possible role of GPI-anchored proteins in cholesterol homeostasis, which in some cells, such as Chinese hamster ovary cells, can be compensated by caveolin up-regulation.