Cholesterol depletion delocalizes phosphatidylinositol bisphosphate and inhibits hormone-stimulated phosphatidylinositol turnover

Constitutive localization of the gonadotropin-releasing hormone (GnRH) receptor to low density membrane microdomains is necessary for GnRH signaling to ERK

Specialized membrane microdomains known as lipid rafts are thought to contribute to G-protein coupled receptor (GPCR) signaling by organizing receptors and their cognate signaling molecules into discrete membrane domains. To determine if the GnRHR, an unusual member of the GPCR superfamily, partitions into lipid rafts, homogenates of alpha T3-1 cells expressing endogenous GnRHR or Chinese hamster ovary cells expressing an epitope-tagged GnRHR were fractionated through a sucrose gradient. We found the GnRHR and c-raf kinase constitutively localized to low density fractions independent of hormone treatment. Partitioning of c-raf kinase into lipid rafts was also observed in whole mouse pituitary glands. Consistent with GnRH induced phosphorylation and activation of c-raf kinase, GnRH treatment led to a decrease in the apparent electrophoretic mobility of c-raf kinase that partitioned into lipid rafts compared with unstimulated cells. Cholesterol depletion of alpha T3-1 cells using methyl-beta-cyclodextrin disrupted GnRHR but not c-raf kinase association with rafts and shifted the receptor into higher density fractions. Cholesterol depletion also significantly attenuated GnRH but not phorbol ester-mediated activation of extracellular signal-related kinase (ERK) and c-fos gene induction. Raft localization and GnRHR signaling to ERK and c-Fos were rescued upon repletion of membrane cholesterol. Thus, the organization of the GnRHR into low density membrane microdomains appears critical in mediating GnRH induced intracellular signaling.

Fluorescence energy transfer reveals microdomain formation at physiological temperatures in lipid mixtures modeling the outer leaflet of the plasma membrane

An approach is described using fluorescence resonance energy transfer (FRET) to detect inhomogeneity in lipid organization, on distance scales of the order of tens of nanometers or greater, in lipid bilayers. This approach compares the efficiency of energy transfer between two matched fluorescent lipid donors, differing in their affinities for ordered versus disordered regions of the bilayer, and an acceptor lipid that distributes preferentially into disordered regions. Inhomogeneities in bilayer organization, on spatial scales of tens of nanometers or greater, are detected as a marked difference in the efficiencies of quenching of fluorescence of the two donor species by the acceptor. Using a novel pair of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled tetraacyl lipids as donor species with a rhodaminyl-labeled acceptor, this strategy faithfully reports homo- versus inhomogeneous mixing in each of several lipid bilayer systems whose organization on the FRET distance scale can be predicted from previous findings. Interestingly, however, the present FRET method reports clear evidence of inhomogeneity in the organization of mixtures combining sphingomyelin or saturated phospholipids with unsaturated phospholipids and physiological proportions of cholesterol, even at physiological temperatures where these systems have been reported to appear homogeneous by fluorescence microscopy. These results indicate that under physiological conditions, lipid mixtures mimicking the lipid composition of the outer leaflet of the plasma membrane can form domains on a spatial scale comparable to that inferred for the dimensions of lipid rafts in biological membranes.

Dynamics of phosphoinositides in membrane retrieval and insertion

The phosphoinositides PtdIns(4,5)P2 and PtdIns(3,4,5)P3 are concentrated in plasma membranes of eukaryotic cells, and excluded from endosomes, whereas PtdIns(3)P is formed in these latter intracellular membranes and is apparently excluded from the plasma membrane. The logic of this asymmetric disposition is now revealed by the nature of the effector proteins that selectively bind these lipids through specific modules and by the processes that they catalyze. PtdIns(3,4,5)P3 has a role in directing exocytosis, in addition to many other signaling events, whereas PtdIns(4,5)P2 directs endocytosis through its ability to anchor several coat proteins to the plasma membrane. Remarkably, the elimination of PtdIns(4,5)P2 from forming endosomes may be required for membrane fission to occur. Thus membrane insertion and retrieval can be regulated by plasma membrane concentrations of PtdIns(3,4,5)P3 and PtdIns(4,5)P2, whereas PtdIns(3)P directs the downstream trafficking and recycling of intracellular membranes through its attraction of proteins that catalyze these processes. The phosphoinositides thereby control many cell features that depend upon protein sorting, including the composition of the plasma membrane itself, which in turn determines the cell's responses to its environment.

From rafts to crafts: membrane asymmetry in moving cells

Many important biological events, including the leukocyte-mediated immune response, wound repair, axon guidance and developmental patterning, involve persistent cell movement towards a directional signal, a process termed chemotaxis. Establishment of functional and spatial cell polarity is an absolute requirement for this response. We propose that redistribution of specific membrane microdomains, termed rafts, during cell migration is a pivotal step in achieving polarity. On the one hand, partitioning of molecules into rafts might help to localize proteins at the front or the rear of moving cells, and on the other hand, rafts might function as platforms for local activation and coordination of the signaling pathways involved in cell migration.

Palmitoylation regulates regulators of G-protein signaling (RGS) 16 function. I. Mutation of amino-terminal cysteine residues on RGS16 prevents its targeting to lipid rafts and palmitoylation of an internal cysteine residue

Regulators of G-protein signaling (RGS) proteins down-regulate signaling by heterotrimeric G-proteins by accelerating GTP hydrolysis on the G alpha subunits. Palmitoylation, the reversible addition of palmitate to cysteine residues, occurs on several RGS proteins and is critical for their activity. For RGS16, mutation of Cys-2 and Cys-12 blocks its incorporation of [3H]palmitate and ability to turn-off Gi and Gq signaling and significantly inhibited its GTPase activating protein activity toward aG alpha subunit fused to the 5-hydroxytryptamine receptor 1A, but did not reduce its plasma membrane localization based on cell fractionation studies and immunoelectron microscopy. Palmitoylation can target proteins, including many signaling proteins, to membrane microdomains, called lipid rafts. A subpopulation of endogenous RGS16 in rat liver membranes and overexpressed RGS16 in COS cells, but not the nonpalmitoylated cysteine mutant of RGS16, localized to lipid rafts. However, disruption of lipid rafts by treatment with methyl-beta-cyclodextrin did not decrease the GTPase activating protein activity of RGS16. The lipid raft fractions were enriched in protein acyltransferase activity, and RGS16 incorporated [3H]palmitate into a peptide fragment containing Cys-98, a highly conserved cysteine within the RGS box. These results suggest that the amino-terminal palmitoylation of an RGS protein promotes its lipid raft targeting that allows palmitoylation of a poorly accessible cysteine residue that we show in the accompanying article (Osterhout, J. L., Waheed, A. A., Hiol, A., Ward, R. J., Davey, P. C., Nini, L., Wang, J., Milligan, G., Jones, T. L. Z., and Druey, K. M. (2003) J. Biol. Chem. 278, 19309-19316) was critical for RGS16 and RGS4 GAP activity.

The direct interaction of phospholipase C-gamma 1 with phospholipase D2 is important for epidermal growth factor signaling

The epidermal growth factor (EGF) receptor has an important role in cellular proliferation, and the enzymatic activity of phospholipase C (PLC)-gamma1 is regarded to be critical for EGF-induced mitogenesis. In this study, we report for the first time a phospholipase complex composed of PLC-gamma1 and phospholipase D2 (PLD2). PLC-gamma1 is co-immunoprecipitated with PLD2 in COS-7 cells. The results of in vitro binding analysis and co-immunoprecipitation analysis in COS-7 cells show that the Src homology (SH) 3 domain of PLC-gamma1 binds to the proline-rich motif within the Phox homology (PX) domain of PLD2. The interaction between PLC-gamma1 and PLD2 is EGF stimulation-dependent and potentiates EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and Ca(2+) increase. Mutating Pro-145 and Pro-148 within the PX domain of PLD2 to leucines disrupts the interaction between PLC-gamma1 and PLD2 and fails to potentiate EGF-induced IP(3) formation and Ca(2+) increase. However, neither PLD2 wild type nor PLD2 mutant affects the EGF-induced tyrosine phosphorylation of PLC-gamma1. These findings suggest that, upon EGF stimulation, PLC-gamma1 directly interacts with PLD2 and this interaction is important for PLC-gamma1 activity.

Plasma membrane rafts of rainbow trout are subject to thermal acclimation

Rafts are cholesterol- and sphingolipid-enriched microdomains of the plasma membrane (PM) that organize many signal transduction pathways. Interactions between cholesterol and saturated lipids lead to patches of liquid-ordered membrane (rafts) phase-separating from the remaining PM. Phase behavior is temperature sensitive, and acute changes in temperature experienced by poikilotherms would be expected to perturb raft structure, necessitating an acclimatory response. Therefore, with thermal acclimation, we would expect compositional changes in the raft directed to offset this perturbation. Using differential and density gradient centrifugation, we separated PM from the livers of rainbow trout acclimated to 5 degrees C and 20 degrees C into raft-enriched (raft) and raft-depleted PM (RDPM). Compared with RDPM, the raft fractions were enriched in cholesterol, the beta(2)-adrenergic receptor and adenylyl cyclase, which are commonly used markers for this microdomain. Furthermore, cholesterol was enriched in all fractions from warm-compared with cold-acclimated animals, but this increase was 3.4 times greater in raft than in PM. We developed a novel approach for measuring membrane molecular interaction strength (and thus the tendency to stabilize raft structure) based on the susceptibility of membranes to detergent. Specifically, studies with model vesicles demonstrated that the capacity of a membrane to accommodate detergent prior to solubilization (saturation point) was a good index of this property. The saturation point of the isolated membrane preparations was temperature sensitive and was significantly different in 5 degrees C- and 20 degrees C-acclimated RDPM when assayed at 5 degrees C and 20 degrees C, respectively. By contrast, this comparison in rafts was not significantly different, suggesting compensation of this property. These data suggest that compositional changes made in the PM during thermal acclimation act to offset thermal perturbation of the raft but not the RDPM structural integrity.

Caveolar compartmentation of caspase-3 in cardiac endothelial cells

Endothelial cell apoptosis is intimately involved in the balance between blood vessel growth and regression and is promoted by numerous stimuli including angiostatin and endostatin, reactive oxygen species (ROS) released during inflammatory processes, and chronic use of drugs of abuse such as cocaine. Apoptosis is characterized by many biological signalling events, including the activation of caspases. Caveolar domains have been hypothesized to mediate apoptotic signalling. We have addressed this hypothesis in cardiac endothelial cells and here we show that caspase-3 proenzyme (32 kDa) and its activated counterpart (17 kDa) co-purify with low-density, caveolin-enriched microdomains and that caspase-3 can be localized with caveolae in intact cells using fluorescent microscopy. Disruption of caveolae results in temporal and spatial changes in enzyme activity. While caspase-3 has been associated with mitochondrial, cytosolic, and high-density regions, the co-purification of activated caspase-3 and caveolar domains reported here suggests the possibility that sarcolemmal caspase-3 may be targeted to plasma-membrane associated substrates.

Imaging metabolism of phosphatidylinositol 4,5-bisphosphate in T-cell GM1-enriched domains containing Ras proteins

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and Ras proteins are involved in signalling pathways originating at the plasma membrane. The localisation and metabolism of PI(4,5)P(2) was studied in Jurkat T cells using fluorescence microscopic imaging with EGFP-tagged and antibody probes. Software was developed to objectively quantitate colocalisation and was used to show that plasma membrane PI(4,5)P(2) was enriched in lipid raft-containing patches of GM1 ganglioside, formed by crosslinking cholera toxin B-subunit (CT-B). The PI(4,5)P(2) metabolites phosphatidylinositol 3,4,5-trisphosphate and diacylglycerol appeared in plasma membrane CT-B-GM1 patches upon induction of signalling. Transferrin receptor and the CD45 tyrosine phosphatase did not colocalise with CT-B-GM1 patches, whereas the tyrosine kinase Lck, the scaffolding protein LAT, and endogenous Ras proteins did partially colocalise with CT-B-GM1 patches as did transfected EGFP-K-Ras(4B) and EGFP-H-Ras. The results demonstrate that T-cell PI(4,5)P(2) metabolism is occurring in GM1-enriched domains and that Ras proteins are present in these domains in vivo.

Lipid rafts: bringing order to chaos

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids. They exist as distinct liquid-ordered regions of the membrane that are resistant to extraction with nonionic detergents. Rafts appear to be small in size, but may constitute a relatively large fraction of the plasma membrane. While rafts have a distinctive protein and lipid composition, all rafts do not appear to be identical in terms of either the proteins or the lipids that they contain. A variety of proteins, especially those involved in cell signaling, have been shown to partition into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction. Experimental evidence suggests that there are probably several different mechanisms through which rafts control cell signaling. For example, rafts may contain incomplete signaling pathways that are activated when a receptor or other required molecule is recruited into the raft. Rafts may also be important in limiting signaling, either by physical sequestration of signaling components to block nonspecific interactions, or by suppressing the intrinsic activity of signaling proteins present within rafts. This review provides an overview of the physical characteristics of lipid rafts and summarizes studies that have helped to elucidate the role of lipid rafts in signaling via receptor tyrosine kinases and G protein-coupled receptors.

Cholesterol attenuates linoleic acid-induced endothelial cell activation

Vascular endothelial cell activation and dysfunction are critical early events in atherosclerosis. Even though very low or high levels of cholesterol can compromise cellular functions, cholesterol is a critical membrane component and may protect the vascular endothelium from oxidative stress and polyunsaturated fatty acid-mediated inflammatory responses. We have previously shown that the parent omega-6 fatty acid linoleic acid can markedly activate vascular endothelial cells. We now propose that membrane cholesterol can modify and inhibit linoleic acid-mediated endothelial cell dysfunction. To test this hypothesis, pulmonary artery endothelial cells were incubated with cholesterol (0 to 100 micromol/L) for 24 hours and then treated with 90 micromol/L of linoleic acid (18:2n-6) for 6 to 24 hours. In control cells, treatment with linoleic acid reduced intracellular glutathione levels and induced the DNA binding activity of nuclear factor-kappaB (NF-kappaB) leading to the upregulation of interleukin-6 (IL-6). In addition, the expression of endothelial nitric oxide synthase (eNOS) was altered, with linoleic acid increasing eNOS activity. In contrast, enrichment with cholesterol enhanced glutathione levels and reduced the linoleic acid-induced activation of NF-kappaBand the production of IL-6. Prior exposure to 50 micromol/L cholesterol also prevented the fatty acid-induced increase in eNOS activation. Cholesterol loading activated peroxisome proliferator-activated receptor-gamma (PPAR-gamma), a nuclear receptor that can decrease inflammatory responses. Furthermore, the PPAR-gamma agonist thiazolidinedione markedly downregulated the NF-kappaB activation mediated by linoleic acid. Our data suggest that signaling pathways linked to endothelial cell activation by prooxidant and proinflammatory insults may be influenced by cellular cholesterol levels.

The adipocyte plasma membrane caveolin functional/structural organization is necessary for the efficient endocytosis of GLUT4

It is well established that insulin stimulation of glucose uptake requires the translocation of intracellular localized GLUT4 protein to the cell surface membrane. This plasma membrane-redistributed GLUT4 protein was partially co-localized with caveolin as determined by confocal fluorescent microscopy but was fully excluded from lipid rafts based upon Triton X-100 extractability. Cholesterol depletion with methyl-beta-cyclodextrin, filipin, or cholesterol oxidase resulted in an insulin-independent increase in the amount of plasma membrane-localized GLUT4 that was fully reversible by cholesterol replenishment. This basal accumulation of cell surface GLUT4 occurred due to an inhibition of GLUT4 endocytosis. However, this effect was not specific since cholesterol extraction also resulted in a dramatic inhibition of clathrin-mediated endocytosis as assessed by transferrin receptor internalization. To functionally distinguish between caveolin- and clathrin-dependent endocytic processes, we took advantage of a dominant-interfering caveolin 1 mutant (Cav1/S80E) that specifically disrupts caveolae organization. Expression of Cav1/S80E, but not the wild type (Cav1/WT) or Cav1/S80A mutant, inhibited cholera toxin B internalization without any significant effect on transferrin receptor endocytosis. In parallel, Cav1/S80E expression increased the amount of plasma membrane-localized GLUT4 protein in an insulin-independent manner. Although Cav1/S80E also decreased GLUT4 endocytosis, the extent of GLUT4 internalization was only partially reduced ( approximately 40%). In addition, expression of Cav1/WT and Cav1/S80A enhanced GLUT4 endocytosis by approximately 20%. Together, these data indicate that the endocytosis of GLUT4 requires clathrin-mediated endocytosis but that the higher order structural organization of plasma membrane caveolin has a significant influence on this process.

Membrane lipid organization is critical for human neutrophil polarization

In response to chemoattractants neutrophils extend an actin-rich pseudopod, which imparts morphological polarity and is required for migration. Even when stimulated by an isotropic bath of chemoattractant, neutrophils exhibit persistent polarization and continued lamellipod formation at the front, suggesting that the cells establish an internal polarity. In this report, we show that perturbing lipid organization by depleting plasma membrane cholesterol levels reversibly inhibits cell polarization and migration. Among other receptor-mediated responses, beta(2) integrin up-regulation was unaffected, and initial calcium mobilization was only partially reduced by cholesterol depletion, indicating that this treatment did not abrogate initial receptor-mediated signal transduction. Interestingly, cholesterol depletion did not prevent initial activation of the GTPase Rac or an initial burst of actin polymerization, but rather it inhibited prolonged activation of Rac and sustained actin polymerization. Collectively, these findings support a model in which the plasma membrane is organized into domains that aid in amplifying the chemoattractant gradient and maintaining cell polarization.

Role of cholesterol in lipid raft formation: lessons from lipid model systems

Biochemical and cell-biological experiments have identified cholesterol as an important component of lipid 'rafts' and related structures (e.g., caveolae) in mammalian cell membranes, and membrane cholesterol levels as a key factor in determining raft stability and organization. Studies using cholesterol-containing bilayers as model systems have provided important insights into the roles that cholesterol plays in determining lipid raft behavior. This review will discuss recent progress in understanding two aspects of lipid-cholesterol interactions that are particularly relevant to understanding the formation and properties of lipid rafts. First, we will consider evidence that cholesterol interacts differentially with different membrane lipids, associating particularly strongly with saturated, high-melting phospho- and sphingolipids and particularly weakly with highly unsaturated lipid species. Second, we will review recent progress in reconstituting and directly observing segregated raft-like (liquid-ordered) domains in model membranes that mimic the lipid compositions of natural membranes incorporating raft domains.

Lipid rafts are critical membrane domains in blood platelet activation processes

Among the various hematopoi;etic cells, platelets are critical for maintaining the integrity of the vascular system. They must be rapidly activated by sequential and coordinated mechanisms in order to efficiently prevent haemorrhage upon vascular injury. Several signal transduction pathways lead to platelet activation in vitro and in vivo, among them, several are initiated via receptors or co-receptors containing immuno-receptor tyrosine-based activation motifs (ITAM) which trigger downstream signalling like the immune receptors in lymphocytes. However, in contrast to immune cells for which the role of lipid rafts in signalling has largely been described, the involvement of laterally segregated membrane microdomains in platelet activation has been investigated only recently. The results obtained until now strongly suggest that early steps of platelet activation via the collagen receptor GpVI or via FcgammaRIIa occur preferentially in these microdomains where specific proteins efficiently organize key downstream signalling pathways. In addition, lipid rafts also contribute to platelet activation via heterotrimeric G-protein-coupled receptors. They are sites where the phosphoinositide (PI) metabolism is highly active, leading to a local generation of lipid second messengers such as phosphatidylinositol 3,4,5-trisphosphate. Here, evidence is accumulating that cholesterol-enriched membrane microdomains are part of a general process that contributes to the efficiency and the coordination of platelet activation mechanisms. Here we will discuss the biochemical and functional characterizations of human platelet rafts and their potential impact in platelet physiopathology.

pp90RSK- and protein kinase C-dependent pathway regulates p42/44MAPK-induced LDL receptor transcription in HepG2 cells

We have previously shown that different extracellular stimuli require signaling through the Raf/MEK/p42/44MAPK cascade to induce LDL receptor expression. The present studies were designed to delineate the molecular mechanisms underlying p42/44MAPK-induced LDL receptor transcription in HepG2-Delta Raf-1:ER cells, a modified HepG2 cell line in which the Raf-1/MEK/p42/44MAPK cascade can be specifically activated by anti-estradiol ICI182,780 in an agonist-specific manner. Using these cells, we show that: a) LDL receptor induction was reduced in reporter constructs containing mutation in either Sp1 or sterol-regulatory element-1 (SRE-1) sites, whereas inactivation of both sites abolished the induction; b) E1A, which inhibits CREB binding protein (CBP), a common activator of SRE-1 binding protein and Sp1, strongly repressed the induction; c) intracellular inhibition of the 90 kDa ribosomal S6 kinase (pp90RSK) cascade reduced LDL receptor induction; d) highly selective protein kinase C (PKC) inhibitors effectively abrogated the induction without affecting activation of pp90RSK; and e) overexpression of PKC beta significantly induced LDL receptor promoter activity. Taken together, these results demonstrate that pp90RSK and PKC beta are downstream effectors and Sp1, SRE-1 binding protein, and CBP are part of the transcriptional complex resulting in induction of LDL receptor expression in response to activation of the Raf/MEK/p42/44MAPK cascade. These findings identify for the first time a role for PKC beta in determining the specificity of p42/44MAPK signaling by participating with pp90RSK in regulating gene expression.

Phosphorylation of FcgammaRIIA is required for the receptor-induced actin rearrangement and capping: the role of membrane rafts

Activation of Fcgamma receptor II (FcgammaRII) induces rearrangement of the actin-based cytoskeleton that serves as a driving force for FcgammaRII-mediated phagocytosis and FcgammaRII capping. To get insight into the signaling events that lead to the actin reorganization we investigated the role of raft-associated Src family tyrosine kinases in capping of FcgammaRII in U937 cells. After crosslinking, FcgammaRII was found to be recruited to detergent-resistant membrane domains (DRMs), rafts, where it coexisted with Lyn kinase and underwent tyrosine phosphorylation. Lyn was displaced from DRMs under the influence of DL-alpha-hydroxymyristic acid and 2-bromopalmitic acid, agents blocking N-terminal myristoylation and palmitoylation of proteins, respectively, and after disruption of DRM integrity by depletion of plasma membrane cholesterol with beta-cyclodextrin. Under these conditions, phosphorylation of the crosslinked FcgammaRII was diminished and assembly of FcgammaRII caps was blocked. The similar reduction of FcgammaRII cap formation correlated with inhibition of receptor phosphorylation was achieved with the use of PP1 and herbimycin A, specific inhibitors of Src family tyrosine kinases. Phosphorylation of FcgammaRIIA expressed in BHK cells, lacking endogenous FcgammaRs, was abolished by substitution of tyrosine 298 by phenylalanine in the ITAM of the receptor. The mutant receptor did not undergo translocation towards cap-like structures and failed to promote the receptor-mediated spreading of the cells, as compared to BHK cells transfected with the wild-type FcgammaRIIA. On the basis of these data, we suggest that tyrosine phosphorylation of activated FcgammaRIIA by raft-residing tyrosine kinases of the Src family triggers signaling pathways that control the rearrangement of the actin cytoskeleton required for FcgammaRII-mediated motility.

Loss of lipopolysaccharide-induced nitric oxide production and inducible nitric oxide synthase expression in scrapie-infected N2a cells

In scrapie-infected cells, the conversion of the cellular prion protein to the pathogenic prion has been shown to occur in lipid rafts, which are suggested to function as signal transduction platforms. Neuronal cells may respond to bacterial lipopolysaccharide (LPS) treatment with a sustained and elevated nitric oxide (NO) release. Because prions and the major LPS receptor CD14 are colocalized in lipid rafts, the LPS-induced NO production in scrapie-infected neuroblastoma cells was studied. This study shows that LPS induces a dose- and time-dependent increase in NO release in the murine neuroblastoma cell line N2a, with a 50-fold increase in NO production at 1 microg/ml LPS after 96 hr, as measured by nitrite in the medium. This massive NO release was not caused by activation of the neuronal NO synthase (nNOS), but by increased expression of the inducible NOS (iNOS) mRNA and protein. However, in scrapie-infected N2a cells (ScN2a), the LPS-induced NO production was completely abolished. The absence of LPS-induced NO production in ScN2a was due not to abolished enzymatic activity of iNOS but to a complete inhibition of the LPS-induced iNOS gene expression as measured by Western blot and RT-PCR. These results indicate that scrapie infection inhibits the LPS-mediated signal transduction upstream of the transcriptional step in the signaling cascade and may reflect the important molecular and cellular changes induced by scrapie infection.

SH2-containing inositol phosphatase (SHIP-1) transiently translocates to raft domains and modulates CD16-mediated cytotoxicity in human NK cells

Membrane recruitment of the SH2-containing 5' inositol phosphatase 1 (SHIP-1) is responsible for the inhibitory signals that modulate phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. Here we have investigated the molecular mechanisms underlying SHIP-1 activation and its role in CD16-mediated cytotoxicity. We initially demonstrated that a substantial fraction of SHIP-1-mediated 5' inositol phosphatase activity associates with CD16 zeta chain after receptor cross-linking. Moreover, CD16 stimulation on human primary natural killer (NK) cells induces the rapid and transient translocation of SHIP-1 in the lipid-enriched plasma membrane microdomains, termed rafts, where it associates with tyrosine-phosphorylated zeta chain and shc adaptor protein. As evaluated by confocal microscopy, CD16 engagement by reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly induces SHIP-1 redistribution toward the area of NK cell contact with target cells and its codistribution with aggregated rafts where CD16 receptor also colocalizes. The functional role of SHIP-1 in the modulation of CD16-induced cytotoxicity was explored in NK cells infected with recombinant vaccinia viruses encoding wild-type or catalytic domain-deleted mutant SHIP-1. We found a significant SHIP-1-mediated decrease of CD16-induced cytotoxicity that is strictly dependent on its catalytic activity. These data demonstrate that CD16 engagement on NK cells induces membrane targeting and activation of SHIP-1, which acts as negative regulator of ADCC function.

Ganglioside induces caveolin-1 redistribution and interaction with the epidermal growth factor receptor

Although caveolin-1 is thought to facilitate the interaction of receptors and signaling components, its role in epidermal growth factor receptor (EGFR) signaling remains poorly understood. Ganglioside GM3 inhibits EGFR autophosphorylation and may thus affect the interaction of caveolin-1 and the EGFR. We report here that endogenous overexpression of GM3 leads to the clustering of GM3 on the cell membrane of the keratinocyte-derived SCC12 cell line and promotes co-immunoprecipitation of caveolin-1 and GM3 with the EGFR. Overexpression of GM3 does not affect EGFR distribution but shifts caveolin-1 to the detergent-soluble, EGFR-containing region; consistently, caveolin-1 is retained in the detergent-insoluble membrane when ganglioside is depleted. GM3 overexpression inhibits EGFR tyrosine phosphorylation and receptor dimerization and concurrently increases both the content and tyrosine phosphorylation of EGFR-associated caveolin-1, providing evidence that tyrosine phosphorylation of caveolin-1 inhibits EGFR signaling. Consistently, depletion of ganglioside both increases EGFR phosphorylation and prevents the EGF-induced tyrosine phosphorylation of caveolin-1. GM3 also induces delayed serine phosphorylation of EGFR-unassociated caveolin-1, suggesting a role for serine phosphorylation of caveolin-1 in regulating EGFR signaling. These studies suggest that GM3 modulates the caveolin-1/EGFR association and is critical for the EGF-induced tyrosine phosphorylation of caveolin-1 that is associated with its inhibition of EGFR activation.

Analysis of the catalytic domain of phosphatidylinositol 4-kinase type II

Phosphatidylinositol (PtdIns) 4-kinases catalyze the conversion of PtdIns to PtdIns 4-phosphate, the major precursor of phosphoinositides that regulates a vast array of cellular processes. Based on enzymatic differences, two classes of PtdIns 4-kinase have been distinguished termed Types II and III. Type III kinases, which belong to the phosphatidylinositol (PI) 3/4-kinase family, have been extensively characterized. In contrast, little is known about the Type II enzymes (PI4KIIs), which have been cloned and sequenced very recently. PI4KIIs bear essentially no sequence similarity to other protein or lipid kinases; hence, they represent a novel and distinct branch of the kinase superfamily. Here we define the minimal catalytic domain of a rat PI4KII isoform, PI4KIIalpha, and identify conserved amino acid residues required for catalysis. We further show that the catalytic domain by itself determines targeting of the kinase to membrane rafts. To verify that the PI4KII family extends beyond mammalian sources, we expressed and characterized Drosophila PI4KII and its catalytic domain. Depletion of PI4KII from Drosophila cells resulted in a severe reduction of PtdIns 4-kinase activity, suggesting the in vivo importance of this enzyme.

Localization of phospholipase D1 to caveolin-enriched membrane via palmitoylation: implications for epidermal growth factor signaling

Phospholipase D (PLD) has been suggested to mediate epidermal growth factor (EGF) signaling. However, the molecular mechanism of EGF-induced PLD activation has not yet been elucidated. We investigated the importance of the phosphorylation and compartmentalization of PLD1 in EGF signaling. EGF treatment of COS-7 cells transiently expressing PLD1 stimulated PLD1 activity and induced PLD1 phosphorylation. The EGF-induced phosphorylation of threonine147 was completely blocked and the activity of PLD1 attenuated by point mutations (S2A/T147A/S561A) of PLD1 phosphorylation sites. The expression of a dominant negative PKCalpha mutant by adenovirus-mediated gene transfer greatly inhibited the phosphorylation and activation of PLD1 induced by EGF in PLD1-transfected COS-7 cells. EGF-induced PLD1 phosphorylation occurred primarily in the caveolin-enriched membrane (CEM) fraction, and the kinetics of PLD1 phosphorylation in the CEM were strongly correlated with PLD1 phosphorylation in the total membrane. Interestingly, EGF-induced PLD1 phosphorylation and activation and the coimmunoprecipitation of PLD1 with caveolin-1 and the EGF receptor in the CEM were significantly attenuated in the palmitoylation-deficient C240S/C241S mutant, which did not localize to the CEM. Immunocytochemical analysis revealed that wild-type PLD1 colocalized with caveolin-1 and the EGF receptor and that phosphorylated PLD1 was localized exclusively in the plasma membrane, although some PLD1 was also detected in vesicular structures. Transfection of wild-type PLD1 but not of C240S/C241S mutant increased EGF-induced raf-1 translocation to the CEM and ERK phosphorylation. This study shows, for the first time, that EGF-induced PLD1 phosphorylation and activation occur in the CEM and that the correct localization of PLD1 to the CEM via palmitoylation is critical for EGF signaling.

New advances on structural and biological functions of ceramide in apoptotic/necrotic cell death and cancer

Recent data on the cellular ceramide functions and its involvement in the apoptotic/necrotic cell death as well as its anticarcinogenic properties are presented. The emphasis is on the connections between the ceramide and caspase signaling pathways during the apoptotic cell death process. Notably, the experimental strategies and pharmacological tools used for establishment of the role of ceramide in triggering cell death are described. Moreover, the importance of a compartmentation of endogenous ceramide within the plasma membrane microdomains, lysosomes and mitochondria is discussed. Information on the deregulated functions of ceramide and caspase signaling pathways in several metastatic cancer types is also presented.

The mechanism of docosahexaenoic acid-induced phospholipase D activation in human lymphocytes involves exclusion of the enzyme from lipid rafts

Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid that inhibits T lymphocyte activation, has been shown to stimulate phospholipase D (PLD) activity in stimulated human peripheral blood mononuclear cells (PBMC). To elucidate the mechanisms underlying the DHA-induced PLD activation, we first characterized the PLD expression pattern of PBMC. We show that these cells express PLD1 and PLD2 at the protein and mRNA level and are devoid of oleate-dependent PLD activity. DHA enrichment of PBMC increased the DHA content of cell phospholipids, which was directly correlated with the extent of PLD activation. The DHA-induced PLD activation was independent of conventional protein kinase C but inhibited by brefeldin A, which suggests ADP-ribosylation factor (ARF)-dependent mechanism. Furthermore, DHA enrichment dose-dependently stimulated ARF translocation to cell membranes. Whereas 50% of the guanosine 5'-3-O-(thio)triphosphate plus ARF-dependent PLD activity and a substantial part of PLD1 protein were located to the detergent-insoluble membranes, so-called rafts, of non-enriched PBMC, DHA treatment strongly displaced them toward detergent-soluble membranes where ARF is present. Collectively, these results suggest that the exclusion of PLD1 from lipid rafts, due to their partial disorganization by DHA, and its relocalization in the vicinity of ARF, is responsible for its activation. This PLD activation might be responsible for the immunosuppressive effect of DHA because it is known to transmit antiproliferative signals in lymphoid cells.

Sphingolipid trafficking and protein sorting in epithelial cells

Sphingolipids represent a minor, but highly dynamic subclass of lipids in all eukaryotic cells. They are involved in functions that range from structural protection to signal transduction and protein sorting, and participate in lipid raft assembly. In polarized epithelial cells, which display an asymmetric apical and basolateral membrane surface, rafts have been proposed as a sorting principle for apical resident proteins, following their biosynthesis. However, raft-mediated trafficking is ubiquitous in cells. Also, sphingolipids per se, which are strongly enriched in the apical domain, are subject to sorting in polarity development. Next to the trans Golgi network, a subapical compartment called SAC or common endosome appears instrumental in regulating these sorting events.

Hsp90 interactions and acylation target the G protein Galpha 12 but not Galpha 13 to lipid rafts

The heterotrimeric G proteins, G(12) and G(13), are closely related in their sequences, signaling partners, and cellular effects such as oncogenic transformation and cytoskeletal reorganization. Yet G(12) and G(13) can act through different pathways, bind different proteins, and show opposing actions on some effectors. We investigated the compartmentalization of G(12) and G(13) at the membrane because other G proteins reside in lipid rafts, membrane microdomains enriched in cholesterol and sphingolipids. Lipid rafts were isolated after cold, nonionic detergent extraction of cells and gradient centrifugation. Galpha(12) was in the lipid raft fractions, whereas Galpha(13) was not associated with lipid rafts. Mutation of Cys-11 on Galpha(12), which prevents its palmitoylation, partially shifted Galpha(12) from the lipid rafts. Geldanamycin treatment, which specifically inhibits Hsp90, caused a partial loss of wild-type Galpha(12) and a complete loss of the Cys-11 mutant from the lipid rafts and the appearance of a higher molecular weight form of Galpha(12) in the soluble fractions. These results indicate that acylation and Hsp90 interactions localized Galpha(12) to lipid rafts. Hsp90 may act as both a scaffold and chaperone to maintain a functional Galpha(12) only in discrete membrane domains and thereby explain some of the nonoverlapping functions of G(12) and G(13) and control of these potent cell regulators.

Functional compartmentation of endothelial P2Y receptor signaling

The presence of multiple receptors for disparate nucleotides on endothelial cells makes it unclear how the endothelium differentiates among these signals. We propose that endothelial P2Y receptors are organized into cholesterol-rich signaling domains, such as caveolae and respond to nucleotide agonists by mobilizing intracellular calcium. Treatment of endothelial cells with 5 mmol/L beta-methyl-cyclodextrin prevents calcium release in response to the nucleotide receptor agonists 2-methylthio-ATP, ATP, ADP, and UTP, but not the kinin receptor agonist bradykinin, suggesting that depletion of membrane cholesterol disrupts signaling at P2Y receptors and that bradykinin receptors are not prelocalized to cholesterol microdomains in these cells. Direct measurement of cholesterol content after beta-methyl-cyclodextrin treatment of aortic rings reveals a concentration-dependent depletion of cholesterol that parallels functional antagonism of P2Y-mediated relaxation. Nucleotide- and bradykinin-mediated relaxation is disrupted by 5 to 15 mmol/L beta -methyl-cyclodextrin treatment or 1 to 10 microg/mL filipin III in a concentration-dependent fashion. Norepinephrine contracted aorta treated with A23187 relaxes in an endothelium-dependent fashion despite depletion of 84% of membrane-extractable cholesterol. These data indicate that in the basal state, P2Y receptors but not the kinin receptor may be compartmented to cholesterol-dependent signaling domains in guinea pig endothelium and that cholesterol-rich microdomains in these cells can respond to intracellular calcium in an agonist-specific manner. We suggest that the functional organization of cholesterol-rich signaling microdomains allows agonist-specific responses to increases in intracellular calcium and that this property may be a general phenomenon that permits cells to respond disparately to agonists that may signal through common calcium release pathways.

PIP(2) and proteins: interactions, organization, and information flow

We review the physical properties of phosphatidylinositol 4,5-bisphosphate (PIP2) that determine both its specific interactions with protein domains of known structure and its nonspecific electrostatic sequestration by unstructured domains. Several investigators have postulated the existence of distinct pools of PIP2 within the cell to account for the myriad functions of this lipid. Recent experimental work indicates certain regions of the plasma membrane-membrane ruffles and nascent phagosomes-do indeed concentrate PIP2. We consider two mechanisms that could account for this phenomenon: local synthesis and electrostatic sequestration. We conclude by considering the hypothesis that proteins such as MARCKS bind a significant fraction of the PIP2 in a cell, helping to sequester it in lateral membrane domains, then release this lipid in response to local signals such as an increased concentration of Ca(++)/calmodulin or activation of protein kinase C.

Membrane ruffling and macropinocytosis in A431 cells require cholesterol

Cholesterol is important for the formation of caveolea and deeply invaginated clathrin-coated pits. We have now investigated whether formation of macropinosomes is dependent on the presence of cholesterol in the plasma membrane. Macropinocytosis in A431 cells was induced by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, a potent activator of protein kinase C (PKC). When cells were pretreated with methyl-β-cyclodextrin to extract cholesterol, the phorbol ester was unable to induce the increased endocytosis of ricin otherwise seen, although PKC could still be activated. Electron microscopy revealed that extraction of cholesterol inhibited the formation of membrane ruffles and macropinosomes at the plasma membrane. Furthermore, cholesterol depletion inhibited the phorbol ester-induced reorganization of filamentous actin at the cell periphery, a prerequisite for the formation of membrane ruffles that close into macropinosomes. Under normal conditions the small GTPase Rac1 is activated by the phorbol ester and subsequently localized to the plasma membrane, where it induces the reorganization of actin filaments required for formation of membrane ruffles. Cholesterol depletion did not inhibit the activation of Rac1. However,confocal microscopy showed that extraction of cholesterol prevented the phorbol ester-stimulated localization of Rac1 to the plasma membrane. Thus,our results demonstrate that cholesterol is required for the membrane localization of activated Rac1, actin reorganization, membrane ruffling and macropinocytosis.

Regulation of phospholipase C-gamma2 and phosphoinositide 3-kinase pathways by adaptor proteins in B lymphocytes

The importance of phosphoinositide 3-kinase (PI3K) and phospholipase C (PLC)-gamma2 in B cell function and development has been highlighted by gene targeting experiments in mice. In fact, these knockout mice exhibit a profound inhibition of proliferative responses upon B cell receptor (BCR) engagement. The molecular connections between these effectors and upstream tyrosine kinases such as Syk have been studied intensively in the past few years. This mechanism involves the action of cytoplasmic adaptor molecules, which participate in forming multicomponent signaling complexes, thereby directing the appropriate subcellular localization of effector enzymes. In addition to these cytoplasmic adaptor proteins, cell surface coreceptors can be viewed as transmembrane adaptor proteins, because coreceptors can also change the localization of effector enzymes, which in turn modulates the BCR-initiated signals.

Hypercholesterolemia promotes a CD36-dependent and endothelial nitric-oxide synthase-mediated vascular dysfunction

Numerous studies have implicated either the presence or absence of CD36 in the development of hypertension. In addition, hypercholesterolemia is associated with the loss of nitric oxide-induced vasodilation and the subsequent increase in blood pressure. In the current study, we tested the hypothesis that diet-induced hypercholesterolemia promotes the disruption of agonist-stimulated nitric oxide generation and vasodilation in a CD36-dependent manner. To test this, C57BL/6, apoE null, CD36 null, and apoE/CD36 null mice were maintained on chow or high fat diets. In contrast to apoE null mice fed a chow diet, apoE null mice fed a high fat diet did not respond to acetylcholine with a decrease in blood pressure. Caveolae isolated from in vivo vessels did not contain endothelial nitric-oxide synthase and were depleted of cholesterol. Age-matched apoE/CD36 null mice fed a chow or high fat diet responded to acetylcholine with a decrease in blood pressure. The mechanism underlying the vascular dysfunction was reversible because vessels isolated from apoE null high fat-fed mice regained responsiveness to acetylcholine when incubated with plasma obtained from chow-fed mice. Further analysis demonstrated that the plasma low density lipoprotein fraction was responsible for depleting caveolae of cholesterol, removing endothelial nitric-oxide synthase from caveolae, and preventing nitric oxide production. In addition, the pharmacological removal of caveola cholesterol with cyclodextrin mimicked the effects caused by the low density lipoprotein fraction. We conclude that the ablation of CD36 prevented the negative impact of hypercholesterolemia on agonist-stimulated nitric oxide-mediated vasodilation in apoE null mice. These studies provide a direct link between CD36 and the early events that underlie hypercholesterolemia-mediated hypertension and mechanistic linkages between CD36 function, nitric-oxide synthase activation, caveolae integrity, and blood pressure regulation.

Critical role of diacylglycerol- and phospholipid-regulated protein kinase C epsilon in induction of low-density lipoprotein receptor transcription in response to depletion of cholesterol

Induction of low-density lipoprotein (LDL) receptor transcription in response to depletion of cellular sterols in animal cells is well established. The intracellular signal or signals involved in regulating this process, however, remain unknown. Using a specific inhibitor of protein kinase C (PKC), calphostin C, we show the requirement of this kinase in the induction process in human hepatoma HepG2 cells. Overexpression of PKC epsilon, but not PKC alpha, -gamma, -delta, or -zeta was found to dramatically induce (approximately 18-fold) LDL receptor promoter activity. Interestingly, PKC epsilon-mediated induction was found to be sterol resistant. To further establish that PKC epsilon is involved in the sterol regulation of LDL receptor gene transcription, endogenous PKC epsilon was specifically inhibited by transfection with antisense PKC epsilon phosphorothionate oligonucleotides. Antisense treatment decreased endogenous PKC epsilon protein levels and completely blocked induction of LDL receptor transcription following sterol depletion. PKC epsilon-induced LDL receptor transcription is independent of the extracellular signal-regulated kinase 1 and 2 (p42/44(MAPK)) cascade, because the MEK-1/2 inhibitor, PD98059 did not inhibit, even though it blocked p42/44(MAPK) activation. Finally, photoaffinity labeling studies showed an isoform-specific interaction between PKC epsilon and sterols, suggesting that sterols may directly modulate its function by hampering binding of activators. This was confirmed by PKC activity assays. Altogether, these results define a novel signaling pathway leading to induction of LDL receptor transcription following sterol depletion, and a model is proposed to account for a new function for PKC epsilon as part of a sterol-sensitive signal transduction pathway in hepatic cells.

Detergent-insoluble glycosphingolipid/cholesterol microdomains of the myelin membrane

Glycosphingolipids and cholesterol form lateral assemblies, or lipid 'rafts', within biological membranes. Lipid rafts are routinely studied biochemically as low-density, detergent-insoluble complexes (in non-ionic detergents at 4 degrees C; DIGs, detergent-insoluble glycosphingolipid/cholesterol microdomains). Recent discrepancies recommended a re-evaluation of the conditions used for the biochemical analysis of lipid rafts. We have investigated the detergent insolubility of several known proteins present in the glycosphingolipid/cholesterol-rich myelin membrane, using four detergents representing different chemical classes (TX-100, CHAPS, Brij 96 and TX-102), under four conditions: detergent extraction of myelin either at (i) 4 degrees C or (ii) 37 degrees C, or at 4 degrees C after pre-extraction with (iii) saponin or (iv) methyl-beta-cyclodextrin (MbetaCD). Each detergent was different in its ability to solubilize myelin proteins and in the density of the DIGs produced. Brij 96 DIGs floated to a lower density than other detergents tested, possibly representing a subpopulation of DIGs in myelin. DIGs pre-extracted with saponin were denser than DIGs pre-extracted with MbetaCD. Furthermore, pre-extraction with MbetaCD solubilized proteolipid protein (known to associate with cholesterol), whereas pre-extraction with saponin did not, suggesting that saponin is less effective as a cholesterol-perturbing agent than is MbetaCD. These results demonstrate that DIGs isolated by different detergents are not necessarily comparable, and that these detergent-specific DIGs may represent distinct biochemical, and possibly physiological, entities based on the solubilities of specific lipids/proteins in each type of detergent.

Sequestration of epidermal growth factor receptors in non-caveolar lipid rafts inhibits ligand binding

Cholesterol depletion has been shown to increase mitogen-activated protein kinase activation in response to stimulation with epidermal growth factor (EGF) (Furuchi, T., and Anderson, R. G. W. (1998) J. Biol. Chem. 273, 21099-21104). However, the underlying mechanisms are unknown. We show that cholesterol depletion increases EGF binding, whereas cholesterol loading lowers EGF binding. Based on binding analyses, we demonstrate that the observed changes in EGF binding are caused by alterations in the number of EGF receptors available for ligand binding, whereas the affinity of the receptor for EGF remains unaltered. We also show by immunofluorescence that in unstimulated cells the EGF receptor is localized in non-caveolar lipid rafts containing the ganglioside GM1 and that patching of these rafts by cholera toxin B-chain causes co-patching of EGF receptors. Experiments with solubilization in different detergents at 4 degrees C show that the association of the EGF receptor with these rafts is sensitive to Triton X-100 extraction but insensitive to extraction with another non-ionic detergent, Brij 58. Furthermore, experiments with cholesterol-depleted cells show that the association is cholesterol-dependent. We propose that non-caveolar lipid rafts function as negative regulators of EGF receptor signaling by sequestering a fraction of the EGF receptors in a state inaccessible for ligand binding.

Regulation of phospholipase C-gamma activity by glycosphingolipids

Glycosphingolipid-enriched domains are hot spots for cell signaling within plasma membranes and are characterized by the enrichment of glycosphingolipids. A role for glucosylceramide-based glycosphingolipids in phospholipase C-mediated inositol 1,4,5-trisphosphate formation has been previously documented. These earlier studies utilized a first generation glucosylceramide synthase inhibitor to deplete cells of their glycosphingolipids. Recently, more active and specific glucosylceramide synthase inhibitors, including d-threo-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidinopropanol (d-t-EtDO-P4), have been designed. d-t-EtDO-P4 has the advantage of blocking glucosylceramide synthase at low nanomolar concentrations but does not cause secondary elevations in cell ceramide levels. In the present study, d-t-EtDO-P4 depleted cellular glucosylceramide and lactosylceramide in cultured ECV304 cells at nanomolar concentrations without obvious cellular toxicity. The expression of several signaling proteins was evaluated in glycosphingolipid-depleted ECV304 cells to study the role of glycosphingolipids in phospholipase C-mediated signaling. No difference was observed in the cellular expression of phospholipase C-gamma between controls and glycolipid-depleted cells. Western blot analysis, however, revealed that depletion of endogenous glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 induced tyrosine phosphorylation of phospholipase C-gamma in a concentration-dependent manner with maximum induction at 100 nm. The phosphorylation of phospholipase C-gamma induced by d-t-EtDO-P4 was abolished by exogenously added glucosylceramide, consistent with a specific glycosphingolipid-phospholipase C-gamma interaction. The phospholipase C-gamma phosphorylation was maximally enhanced by bradykinin when cells were exposed to 100 nm d-t-EtDO-P4. The measurement of cellular activity of phospholipase C-gamma, by myo-inositol 1,4,5-trisphosphate radioreceptor assay, demonstrated that depletion of glucosylceramide-based glycosphingolipids in cultured ECV304 cells with d-t-EtDO-P4 resulted in significantly increased formation of inositol 1,4,5-trisphosphate above base line, and an increased sensitivity of phospholipase C-gamma to bradykinin stimulation. Thus, the activation of phospholipase C-gamma is negatively regulated by membrane glycosphingolipids in ECV304 cells.

Uncoupling of the cholera toxin-G(M1) ganglioside receptor complex from endocytosis, retrograde Golgi trafficking, and downstream signal transduction by depletion of membrane cholesterol

To induce toxicity, cholera toxin (CT) must first bind ganglioside G(M1) at the plasma membrane, enter the cell by endocytosis, and then traffic retrograde into the endoplasmic reticulum. We recently proposed that G(M1) provides the sorting motif necessary for retrograde trafficking into the biosynthetic/secretory pathway of host cells, and that such trafficking depends on association with lipid rafts and lipid raft function. To test this idea, we examined whether CT action in human intestinal T84 cells depends on membrane cholesterol. Chelation of cholesterol with 2-hydroxypropyl beta-cyclodextrin or methyl beta-cyclodextrin reversibly inhibited CT-induced chloride secretion and prolonged the time required for CT to enter the cell and induce toxicity. These effects were specific to CT, as identical conditions did not alter the potency or toxicity of anthrax edema toxin that enters the cell by another mechanism. We found that endocytosis and trafficking of CT into the Golgi apparatus depended on membrane cholesterol. Cholesterol depletion also changed the density and specific protein content of CT-associated lipid raft fractions but did not entirely displace the CT-G(M1) complex from these lipid raft microdomains. Taken together these data imply that cholesterol may function to couple the CT-G(M1) complex with raft domains and with other membrane components of the lipid raft required for CT entry into the cell.

Partitioning of lipid-modified monomeric GFPs into membrane microdomains of live cells

Many proteins associated with the plasma membrane are known to partition into submicroscopic sphingolipid- and cholesterol-rich domains called lipid rafts, but the determinants dictating this segregation of proteins in the membrane are poorly understood. We suppressed the tendency of Aequorea fluorescent proteins to dimerize and targeted these variants to the plasma membrane using several different types of lipid anchors. Fluorescence resonance energy transfer measurements in living cells revealed that acyl but not prenyl modifications promote clustering in lipid rafts. Thus the nature of the lipid anchor on a protein is sufficient to determine submicroscopic localization within the plasma membrane.

Metabolic labelling of membrane microdomains/rafts in Jurkat cells indicates the presence of glycerophospholipids implicated in signal transduction by the CD3 T-cell receptor

Cell membranes contain sphingolipids and cholesterol, which cluster together in distinct domains called rafts. The outer-membrane leaflet of these peculiar membrane domains contains glycosylphosphatidylinositol-anchored proteins, while the inner leaflet contains proteins implicated in signalling, such as the acylated protein kinase p56(lck) and the palmitoylated adaptator LAT (linker for activation of T-cells). We present here an approach to study the lipid composition of rafts and its change upon T-cell activation. Our method is based on metabolic labelling of Jurkat T-cells with different precursors of glycerophospholipid synthesis, including glycerol and fatty acids with different lengths and degrees of saturation as well as phospholipid polar head groups. The results obtained indicate that lipid rafts isolated by the use of sucrose density-gradient centrifugation after Triton X-100 extraction in the cold, besides sphingolipids and cholesterol, contain unambiguously all classes of glycerophospholipids: phosphatidylserine, phosphatidylinositol, phosphatidylethanolamine and phosphatidylcholine. Fatty acid labelling shows that lipid rafts are labelled preferentially with saturated fatty acids while the rest of the plasma membrane incorporates mostly long-chained polyunsaturated fatty acids. To see whether the raft composition as measured by metabolic labelling of phospholipids is involved in T-cell activation, we investigated the production of sn-1,2-diacylglycerol (DAG) in CD3-activated cells. DAG production occurs within rafts, confirming previous demonstration of protein kinase C translocation into membrane microdomains. Our data demonstrate that raft disorganization by methyl-beta-cyclodextrin impairs both CD3-induced DAG production and changes in cytosolic Ca(2+) concentration. These lines of evidence support the conclusion that the major events in T-cell activation occur within or due to lipid rafts.

Regulation of B-cell signal transduction by adaptor proteins

An important role has emerged for adaptor molecules in linking cell-surface receptors, such as the B-cell antigen receptor, with effector enzymes. Adaptor proteins direct the appropriate subcellular localization of effectors and regulate their activity by inducing conformational changes, both of which, in turn, contribute to the spatio-temporal precision of B-cell signal-transduction events. In addition, adaptor molecules participate in establishing negative- or positive-feedback regulatory loops in signalling networks, thereby fine-tuning the B-cell response.

Subcellular localization of phosphatidylinositol 4,5-bisphosphate using the pleckstrin homology domain of phospholipase C delta1

Ptd(4,5)P(2) is thought to promote and organize a wide range of cellular functions, including vesicular membrane traffic and cytoskeletal dynamics, by recruiting functional protein complexes to restricted locations in cellular membranes. However, little is known about the distribution of PtdIns(4,5)P(2) in the cell at high resolution. We have used the pleckstrin homology (PH) domain of phospholipase delta(1) (PLCdelta(1)), narrowly specific for PtdIns(4,5)P(2), to map the distribution of the lipid in astrocytoma and A431 cells. We applied the glutathione S-transferase-tagged PLCdelta(1) PH domain (PLCdelta(1)PH-GST) in an on-section labelling approach which avoids transfection procedures. Here we demonstrate PtdIns(4,5)P(2) labelling in the plasma membrane, and also in intracellular membranes, including Golgi (mainly stack), endosomes and endoplasmic reticulum, as well as in electron-dense structures within the nucleus. At the plasma membrane, labelling was more concentrated over lamellipodia, but not in caveolae, which contained less than 10% of the total cell-surface labelling. A dramatic decrease in signal over labelled compartments was observed on preincubation with the cognate headgroup [Ins(1,4,5)P(3)], and plasma-membrane labelling was substantially decreased after stimulation with thrombin-receptor-activating peptide (SFLLRN in the one-letter amino acid code), a treatment which markedly diminishes PtdIns(4,5)P(2) levels. Thus we have developed a highly selective method for mapping the PtdIns(4,5)P(2) distribution within cells at high resolution, and our data provide direct evidence for this lipid at key functional locations.

Myristoylated alanine-rich C kinase substrate (MARCKS) sequesters spin-labeled phosphatidylinositol 4,5-bisphosphate in lipid bilayers

The myristoylated alanine-rich protein kinase C substrate (MARCKS) may function to sequester phosphoinositides within the plane of the bilayer. To characterize this interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), a novel spin-labeled derivative, proxyl-PIP(2), was synthesized and characterized. In the presence of molecules known to bind PI(4,5)P(2) the EPR spectrum of this label exhibits an increase in line width because of a decrease in label dynamics, and titration of this probe with neomycin yields the expected 1:1 stoichiometry. Thus, this probe can be used to quantitate the interactions made by the PI(4,5)P(2) head group within the bilayer. In the presence of a peptide comprising the effector domain of MARCKS the EPR spectrum broadens, but the changes in line shape are modulated by both changes in label correlation time and spin-spin interactions. This result indicates that at least some proxyl-PIP(2) are in close proximity when bound to MARCKS and that MARCKS associates with multiple PI(4,5)P(2) molecules. Titration of the proxyl-PIP(2) EPR signal by the MARCKS-derived peptide also suggests that multiple PI(4,5)P(2) molecules interact with MARCKS. Site-directed spin labeling of this peptide shows that the position and conformation of this protein segment at the membrane interface are not altered significantly by binding to PI(4,5)P(2). These data are consistent with the hypothesis that MARCKS functions to sequester multiple PI(4,5)P(2) molecules within the plane of the membrane as a result of interactions that are driven by electrostatic forces.

Phosphoinositides regulate membrane-dependent actin assembly by latex bead phagosomes

Actin assembly on membrane surfaces is an elusive process in which several phosphoinositides (PIPs) have been implicated. We have reconstituted actin assembly using a defined membrane surface, the latex bead phagosome (LBP), and shown that the PI(4,5)P(2)-binding proteins ezrin and/or moesin were essential for this process (). Here, we provide several lines of evidence that both preexisting and newly synthesized PI(4,5)P(2), and probably PI(4)P, are essential for phagosomal actin assembly; only these PIPs were routinely synthesized from ATP during in vitro actin assembly. Treatment of LBP with phospholipase C or with adenosine, an inhibitor of type II PI 4-kinase, as well as preincubation with anti-PI(4)P or anti-PI(4,5)P(2) antibodies all inhibited this process. Incorporation of extra PI(4)P or PI(4,5)P(2) into the LBP membrane led to a fivefold increase in the number of phagosomes that assemble actin. An ezrin mutant mutated in the PI(4,5)P(2)-binding sites was less efficient in binding to LBPs and in reconstituting actin assembly than wild-type ezrin. Our data show that PI 4- and PI 5-kinase, and under some conditions also PI 3-kinase, activities are present on LBPs and can be activated by ATP, even in the absence of GTP or cytosolic components. However, PI 3-kinase activity is not required for actin assembly, because the process was not affected by PI 3-kinase inhibitors. We suggest that the ezrin-dependent actin assembly on the LBP membrane may require active turnover of D4 and D5 PIPs on the organelle membrane.

Heterologous desensitization of EGF receptors and PDGF receptors by sequestration in caveolae

Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) receptors have been reported to signal via caveolin-containing membranes called caveolae. In contrast, others report that EGF and PDGF receptors are exclusively associated with caveolin-devoid membranes called rafts. Our subcellular fractionation and coimmunoprecipitation studies demonstrate that, in the absence of ligand, EGF and PDGF receptors are associated with rafts. However, in the presence of ligand, EGF and PDGF receptors transiently associate with caveolae. Surprisingly, pretreatment of cells with EGF prevents PDGF-dependent phosphorylation of PDGF receptors and extracellular signal-regulated kinase (ERK) 1/2 kinase activation. Furthermore, cells pretreated with PDGF prevent EGF-dependent phosphorylation of EGF receptors and ERK1/2 kinase activation. Radioligand binding studies demonstrate that incubation of cells with EGF or PDGF causes both EGF and PDGF receptors to be reversibly sequestered from the extracellular space. Experiments with methyl-beta-cyclodextrin, filipin, and antisense caveolin-1 demonstrate that sequestration of the receptors is dependent on cholesterol and caveolin-1. We conclude that ligand-induced stimulation of EGF or PDGF receptors can cause the heterologous desensitization of the other receptor by sequestration in cholesterol-rich, caveolin-containing membranes or caveolae.

Cholesterol is important in control of EGF receptor kinase activity but EGF receptors are not concentrated in caveolae

We have investigated the localization and function of the epidermal growth factor receptor (EGFR) in normal cells, in cholesterol-depleted cells and in cholesterol enriched cells. Using immunoelectron microscopy we find that the EGFR is randomly distributed at the plasma membrane and not enriched in caveolae. Binding of EGF at 4°C does not change the localization of EGFR,and by immunoelectron microscopy we find that only small amounts of bound EGF localize to caveolae. However, upon patching of lipid rafts, we find that a significant amount of the EGFR is localized within rafts. Depletion of the plasma membrane cholesterol causes increased binding of EGF, increased dimerization of the EGFR, and hyperphosphorylation of the EGFR. Addition of cholesterol was found to reduce EGF binding and reduce EGF-induced EGFR activation. Our results suggest that the plasma membrane cholesterol content directly controls EGFR activation.

Agonist-induced translocation of the kinin B(1) receptor to caveolae-related rafts

The kallikrein-kinin system, activated during inflammatory conditions and the regulation of specific cardiovascular and renal functions, includes two G protein-coupled receptors for bradykinin (BK)-related peptides. The B(1) receptor (B(1)R) subtype is not believed to undergo agonist-induced phosphorylation and endocytosis. A conjugate made of the rabbit B(1)R fused with the yellow variant of green fluorescent protein (YFP) was expressed in mammalian cells. In COS-1 or human embryonic kidney (HEK) 293 cells, the construction exhibited a nanomolar affinity for the agonist radioligand [(3)H]Lys-des-Arg(9)-BK or the antagonist ligand [(3)H]Lys-[Leu(8)]des-Arg(9)-BK and a pharmacological profile virtually identical to that of wild-type B(1)R. Lys-des-Arg(9)-BK stimulation of HEK 293 cells stably expressing B(1)R-YFP but not stimulation of untransfected cells released [(3)H]arachidonate in a phospholipase A(2) assay. B(1)R-YFP was visualized as a continuous labeling of the plasma membranes in stably transfected HEK 293 cells (confocal microscopy). Addition of Lys-des-Arg(9)-BK (1-100 nM) rapidly concentrated the receptor-associated fluorescence into multiple aggregates that remained associated with the plasma membrane (no significant internalization) and colocalized with caveolin-1. This reaction was slowly reversible upon agonist washing at 37 degrees C and prevented pretreatment with a B(1)R antagonist. beta-Cyclodextrin treatment, which extracts cholesterol from membranes and disrupts caveolae-related rafts, prevented agonist-induced redistribution of B(1)R-YFP but not the PLA(2) activation mediated by this receptor. The agonist radioligand copurified with caveolin-1 to a greater extent than the tritiated antagonist in buoyant fractions of HEK 293 cells treated with the ligands. Agonist-induced cellular translocation of the kinin B(1)R to caveolae-related rafts without endocytosis is a novel variation on the theme of G protein-coupled receptor adaptation.

Active EGF receptors have limited access to PtdIns(4,5)P2 in endosomes: implications for phospholipase C and PI 3-kinase signaling

Although prolonged cell signaling is attenuated by internalization and downregulation of active receptors, it is now appreciated that many receptors continue to signal in intracellular compartments. Employing enhanced green fluorescent protein fusion probes, we have investigated the hypothesis that multiple signaling pathways are affected by the differential trafficking of membrane substrates such as PtdIns(4,5)P2. A phosphotyrosine-specific probe, but not a PtdIns(4,5)P2-specific probe, colocalized with internalized EGF as well as transferrin in EGF-stimulated living cells expressing autophosphorylation-competent EGF receptors. Neither probe colocalized with transferrin in the absence of EGF, demonstrating that the reduced level of accessible PtdIns(4,5)P2 in endosomes is constitutive. Finally, a PtdIns(3,4,5)P3-specific probe, which monitors phosphorylation of PtdIns(4,5)P2 by phosphoinositide 3-kinases, was recruited to the plasma membrane but not to EGF- or transferrin-containing endosomes in response to EGF stimulation. These results suggest that while many internalized receptors continue to engage intracellular enzymes, the phospholipase C and phosphoinositide 3-kinase signaling pathways are abrogated by the constitutive lack of accessible PtdIns(4,5)P2 in endosomes.

Tyrosine phosphorylation of B-cell adaptor for phosphoinositide 3-kinase is required for Akt activation in response to CD19 engagement

CD19 is a coreceptor that amplifies signaling initiated by antigen cross-linking of the B-cell antigen receptor (BCR). CD19 can also signal independently of BCR coligation. This study shows that B-cell adaptor for phosphoinositide 3-kinase (BCAP), previously characterized as a substrate of the tyrosine kinases upon BCR engagement, is phosphorylated by cross-linking of CD19. Tyrosine phosphorylation of BCAP, mediated by Lyn, provides binding site(s) for phosphoinositide 3-kinase (PI3K), thereby participating in Akt activation. Thus, these results provide evidence that BCAP serves as an adaptor molecule for CD19 to activate the PI3K pathway in B cells.

CD38 is associated with lipid rafts and upon receptor stimulation leads to Akt/protein kinase B and Erk activation in the absence of the CD3-zeta immune receptor tyrosine-based activation motifs

T lymphocytes can be activated via the T cell receptor (TCR) or by triggering through a number of other cell surface structures, including the CD38 co-receptor molecule. Here, we show that in TCR+ T cells that express a CD3-zeta lacking the cytoplasmic domain, cross-linking with CD38- or CD3-specific monoclonal antibodies induces tyrosine phosphorylation of CD3-epsilon, zeta-associated protein-70, linker for activation of T cells, and Shc. Moreover, in these cells, anti-CD38 or anti-CD3 stimulation leads to protein kinase B/Akt and Erk activation, suggesting that the CD3-zeta-immunoreceptor tyrosine-based activation motifs are not required for CD38 signaling in T cells. Interestingly, in unstimulated T cells, lipid rafts are highly enriched in CD38, including the T cells lacking the cytoplasmic tail of CD3-zeta. Moreover, CD38 clustering by extensive cross-linking with an anti-CD38 monoclonal antibody and a secondary antibody leads to an increased resistance of CD38 to detergent solubilization, suggesting that CD38 is constitutively associated with membrane rafts. Consistent with this, cholesterol depletion with methyl-beta-cyclodextrin substantially reduces CD38-mediated Akt activation while enhancing CD38-mediated Erk activation. CD38/raft association may improve the signaling capabilities of CD38 via formation of protein/lipid domains to which signaling-competent molecules, such as immunoreceptor tyrosine-based activation motif-bearing CD3 molecules and protein-tyrosine kinases, are recruited.