Phosphatidylinositol 4-phosphate synthesis in immunoisolated caveolae-like vesicles and low buoyant density non-caveolar membranes

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.

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.

Characterization of type II phosphatidylinositol 4-kinase isoforms reveals association of the enzymes with endosomal vesicular compartments

Phosphorylation of phosphatidylinositol (PI) to PI 4-phosphate is one of the key reactions in the production of phosphoinositides, lipid regulators of several cellular functions. This reaction is catalyzed by multiple enzymes that belong either to the type II or the type III family of PI 4-kinases. Type III enzymes are structurally similar to PI 3-kinases and are sensitive to PI 3-kinase inhibitors. In contrast, the recent cloning of the first type II PI 4-kinase enzyme defined a novel enzyme family. Here we characterize a new member of this family, the type IIbeta enzyme that has been identified in the NCBI data base based on its homology to the first-cloned type IIalpha enzyme. The type IIbeta enzyme has a primary transcript size of approximately 3.8 kb and shows wide tissue distribution. It contains an open reading frame of 1.4 kb, encoding a protein of approximately 54 kDa. Sequence comparison reveals a high degree of similarity to the type IIalpha enzyme within the C-terminal catalytic domain but significantly lower homology within the N-terminal region. Expression of both enzyme yields increased PI 4-kinase activity that is associated with the microsomal membrane fractions and is significantly lower for the type IIbeta than the type IIalpha form. Both enzymes use PI as their primary substrate and have no detectable activity on PI monophosphates. Epitope-tagged as well as green fluorescent protein-tagged forms of both enzymes localize primarily to intracellular membranes and show prominent co-localization with early endosomes and recycling endosomes but not with the Golgi. These compartments participate in the processing of both the transferrin receptor and the G protein-coupled AT(1A) angiotensin receptor. Our data indicate the existence of multiple forms of type II PI 4-kinase in mammalian cells and suggest that their functions are related to the endocytic pathway.

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.

Lipid rafts and little caves. Compartmentalized signalling in membrane microdomains

Lipid rafts are liquid-ordered membrane microdomains with a unique protein and lipid composition found on the plasma membrane of most, if not all, mammalian cells. A large number of signalling molecules are concentrated within rafts, which have been proposed to function as signalling centres capable of facilitating efficient and specific signal transduction. This review summarizes current knowledge regarding the composition, structure, and dynamic nature of lipid rafts, as well as a number of different signalling pathways that are compartmentalized within these microdomains. Potential mechanisms through which lipid rafts carry out their specialized role in signalling are discussed in light of recent experimental evidence.

Multiple roles for phosphatidylinositol 4-kinase in biosynthetic transport in polarized Madin-Darby canine kidney cells

Phosphatidylinositols (PI) play important roles in regulating numerous cellular processes including cytoskeletal organization and membrane trafficking. The control of PI metabolism by phosphatidylinositol kinases has been the subject of extensive investigation; however, little is known about how phosphatidylinositol kinases regulate traffic in polarized epithelial cells. Because phosphatidylinositol 4-kinase (PI4K)-mediated phosphatidylinositol 4-phosphate (PI(4)P) production has been suggested to regulate biosynthetic traffic in yeast and mammalian cells, we have examined the role of PI4Kbeta in protein delivery in polarized MDCK cells, at different levels of the biosynthetic pathway. Expression of wild type PI4Kbeta had no effect on the rate of transport of influenza hemagglutinin (HA) through the Golgi complex, but inhibited the rate of trans-Golgi network (TGN)-to-cell surface delivery of this protein. By contrast, expression of dominant-negative, kinase-dead PI4Kbeta (PI4Kbeta(D656A)) inhibited intra-Golgi transport but stimulated TGN-to-cell surface delivery of HA. Moreover, expression of PI4Kbeta(D656A) significantly increased the solubility in cold Triton X-100 of HA staged in the TGN, suggesting that altered association of HA with lipid rafts may be responsible for the enhanced transport rate. Both wild type and kinase-dead PI4Kbeta inhibited basolateral delivery of vesicular stomatitis virus G protein, suggesting an effector function for PI4Kbeta in the regulation of basolateral traffic. Thus, by contrast with the observed requirement for PI4Kbeta activity and PI(4)P for efficient transport in yeast, our data suggest that changes in PI(4)P levels can stimulate and inhibit Golgi to cell surface delivery in mammalian cells.

Fc gamma RIIB1/SHIP-mediated inhibitory signaling in B cells involves lipid rafts

One type of membrane microdomain, enriched in glycosphingolipids and cholesterol and referred to as lipid rafts, has been implicated in the generation of activating signals triggered by a variety of stimuli. Several laboratories, including ours, have recently demonstrated that the B cell receptor (BCR) inducibly localizes to the rafts upon activation and that functional lipid rafts are important for BCR-mediated "positive" signaling. In the later phases of the immune response, coligation of the BCR and the inhibitory receptor Fc gamma RIIB1 leads to potent inhibition of BCR-induced positive signaling through the recruitment of the inositol phosphatase SHIP to Fc gamma RIIB1. One potential model is that the Fc gamma RIIB1 itself might be excluded from the rafts basally and that destabilization of raft-dependent BCR signaling might be part of the mechanism for the Fc gamma RIIB1-mediated negative regulation. We tested this hypothesis and observed that preventing BCR raft localization is not the mechanism for this inhibition. Surprisingly, a fraction of Fc gamma RIIB1 is constitutively localized in the rafts and increases further after BCR + FcR coligation. SHIP is actively recruited to lipid rafts under negative stimulation conditions, and the majority of Fc gamma RIIB1-SHIP complexes localize to lipid rafts compared with non-raft regions of the plasma membrane. This suggested that this negative feedback loop is also initiated in the lipid rafts. Despite its basal localization to the rafts, Fc gamma RIIB1 did not become phosphorylated after BCR alone cross-linking and did not colocalize with the BCR that moves to rafts upon BCR engagement alone (positive signaling conditions), perhaps suggesting the existence of different subsets of rafts. Taken together, these data suggest that lipid rafts play a role in both the positive signaling via the BCR as well as the inhibitory signaling through Fc gamma RIIB1/SHIP.

The treatment of purified maize oil bodies with organic solvents and exogenous diacylglycerol allows the detection and solubilization of diacylglycerol acyltransferase

In spite of its importance in the biosynthesis of reserve oils in plants, diacylglycerol acyltransferase (DAGAT, EC 2.3.1.20) has not been purified to homogeneity, and its study has remained incomplete. We found that the microsomal preparations from developing maize embryos contained substantial amounts of endogenous diacylglycerol (DAG). A solubilization procedure for extracting DAGAT from the microsomes (D. Little, R. Weselake, K. Pomeroy, S.T. Furukawa, J. Bagu, Biochem. J. 304 (1994)) was ineffective in eliminating the endogenous DAG, even after gel filtration. DAG removal through the preparation of acetone powders from the embryos led to the loss of DAGAT activity. Labelled triacylglycerol (TAG) was produced in the standard DAGAT assay when labelled DAG was supplied in benzene solution to the freeze-dried microsomes and the sample was dried and resuspended in an aqueous buffer. In contrast, no labelled TAG was produced when a similar sample supplied with non-labelled DAG was assayed with emulsified labelled DAG and acyl-CoA. Repeated washing of the microsomal freeze-dried fraction with benzene resulted in a complete loss of DAGAT activity in the standard assay, but the activity was restored by the addition of DAG plus phosphatidylcholine or Tween 20 in benzene. Although DAGAT has been reported to be confined mainly to the endoplasmic reticulum, we found that DAGAT activity was high in the purified oil bodies from both developing and mature maize embryos and was not removed by repeated washing with 6 M urea. The DAGAT activity was restored from delipidated oil bodies and from microsomes after the preparations had been resuspended in methanol/acetic acid/water (1:1:1, v/v). Although most of the proteins in the suspension were eluted as a single peak at the void volume after gel filtration chromatography, DAGAT activity was found in later fractions. SDS-PAGE of the peak activity fraction revealed no protein bands after silver staining, and the finding suggest that DAGAT protein is of low abundance and has a high k(cat).

Phosphoinositides: key players in cell signalling, in time and space

Over the last few years, many reports have extended our knowledge of the inositol lipid metabolism and brought out some exciting information about the location, the variety and the role of phosphoinositides (PIs). Besides the so-called "canonical PI pathway" leading to the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), the precursor of the intracellular second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG), many other metabolic pathways have been identified to produce seven different polyphosphoinositides. Several of these quantitatively minor lipid molecules appear to be specifically involved in the control of cellular events, such as the spatial and temporal organisation of key signalling pathways, the rearrangement of the actin cytoskeleton or the intracellular vesicle trafficking. This is consistent with the fact that many of the enzymes, such as kinases and phosphatases, involved in the tight control of the intracellular level of polyphosphoinositides, are regulated and/or relocated through cell surface receptors for extracellular ligands. The remarkable feature of PIs, which can be rapidly synthesised and degraded in discrete membrane domains or even subnuclear structures, places them as ideal regulators and integrators of very dynamic mechanisms of cell regulation. In this review, we will summarise recent studies on the potential location, the metabolic pathways and the role of the different PIs. Some aspects of the temporal synthesis of D3 PIs will also be discussed.

Cloning of a human type II phosphatidylinositol 4-kinase reveals a novel lipid kinase family

Phosphoinositide lipids regulate numerous cellular processes in all eukaryotes. The versatility of this phospholipid is provided by combinations of phosphorylation on the 3', 4', and 5' positions of the inositol head group. Two distinct structural families of phosphoinositide (PI) kinases have so far been identified and named after their prototypic members, the PI 3-kinase and phosphatidylinositol (PtdIns) phosphate kinase families, both of which have been found to contain structural homologues possessing PI 4-kinase activity. Nevertheless, the prevalent PtdIns 4-kinase activity in many mammalian cell types is conferred by the widespread type II PtdIns 4-kinase, which has so far resisted molecular characterization. We have partially purified the human type II isoform from plasma membrane rafts of human A431 epidermoid carcinoma cells and obtained peptide mass and sequence data. The results allowed the cDNA containing the full open reading frame to be cloned. The predicted amino acid sequence revealed that the type II enzyme is the prototypic member of a novel, third family of PI kinases. We have named the purified protein type IIalpha and a second human isoform, type IIbeta. The type IIalpha mRNA appears to be expressed ubiquitously in human tissues, and homologues appear to be expressed in all eukaryotes.

Phosphatidylinositol transfer proteins couple lipid transport to phosphoinositide synthesis

Phosphatidylinositol transfer proteins (PITPs) are lipid binding proteins that can catalyse the transfer of phosphatidylinositol (PI) from membranes enriched in PI to PI-deficient membranes. Three soluble forms of PITP of 35--38 kDa (PITP alpha, PITP beta and rdgB beta) and two larger integral proteins of 160 kDa (rdgB alpha I and II), which contain a PITP domain, are found in mammalian cells. PITPs are intimately associated with the compartmentalised synthesis of different phosphorylated inositol lipids. PI is the primary inositol lipid that is synthesised at the endoplasmic reticulum and is further phosphorylated in distinct membrane compartments by many specific lipid kinases to generate seven phosphorylated inositol lipids which are required for both signalling and for membrane traffic. PITPs play essential roles in both signalling via phospholipase C and phosphoinositide 3-kinases and in multiple aspects of membrane traffic including regulated exocytosis and vesicle biogenesis.

Evaluation of prototype transmembrane 4 superfamily protein complexes and their relation to lipid rafts

Recent literature suggests that tetraspanin proteins (transmembrane 4 superfamily; TM4SF proteins) may associate with each other and with many other transmembrane proteins to form large complexes that sometimes may be found in lipid rafts. Here we show that prototype complexes of CD9 or CD81 (TM4SF proteins) with alpha(3)beta(1) (an integrin) and complexes of CD63 (a TM4SF protein) with phosphatidylinositol 4-kinase (PtdIns 4-K) may indeed localize within lipid raft-like microdomains, as seen by three different criteria. First, these complexes localize to low density light membrane fractions in sucrose gradients. Second, CD9 and alpha(3) integrin colocalized with ganglioside GM1 as seen by double staining of fixed cells. Third, CD9-alpha3beta1 and CD81-alpha3beta1 complexes were shifted to a higher density upon cholesterol depletion from intact cells or cell lysate. However, CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K complex formation itself was not dependent on localization into raftlike lipid microdomains. These complexes did not require cholesterol for stabilization, were maintained within well solubilized dense fractions from sucrose gradients, were stable at 37 degrees C, and were small enough to be included within CL6B gel filtration columns. In summary, prototype TM4SF protein complexes (CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K) can be solubilized as discrete units, independent of lipid microdomains, although they do associate with microdomains resembling lipid rafts.

Actin filament nucleation by endosomes, lysosomes and secretory vesicles

Intracellular pathogens such as Listeria monocytogenes and vaccinia virus propel themselves through the cytoplasm of mammalian cells by nucleating actin filaments. Recently, actin assembly has also been shown to power the movement of intracellular vesicles, and this may be a mechanism underlying endomembrane movement in a variety of physiological contexts. Surprisingly, class I myosins have been found to play important roles in both actin nucleation and endomembrane trafficking.

Specific interactions among transmembrane 4 superfamily (TM4SF) proteins and phosphoinositide 4-kinase

In earlier work we established that phosphoinositide 4-kinase (PI 4-kinase) may associate with transmembrane 4 superfamily (TM4SF, tetraspanin) proteins, but critical specificity issues were not addressed. Here we demonstrate that at least five different TM4SF proteins (CD9, CD63, CD81, CD151 and A15/TALLA1) can associate with a similar or identical 55 kDa type II PI 4-kinase. These associations were specific, since we found no evidence for other phosphoinositide kinases (e.g. phosphoinositide 3-kinase and phosphoinositide-4-phosphate 5-kinase) associating with TM4SF proteins, and many other TM4SF proteins (including CD82 and CD53) did not associate with PI 4-kinase. CD63-PI 4-kinase complexes were almost entirely intracellular, and thus are distinct from other TM4SF-PI 4-kinase complexes (e.g. involving CD9), which are largely located in the plasma membrane. These results suggest that a specific subset of TM4SF proteins may recruit PI 4-kinase to specific membrane locations, and thereby influence phosphoinositide-dependent signalling.

Immature CD4+CD8+ thymocytes do not polarize lipid rafts in response to TCR-mediated signals

TCR-mediated stimulation induces activation and proliferation of mature T cells. When accompanied by signals through the costimulatory receptor CD28, TCR signals also result in the recruitment of cholesterol- and glycosphingolipid-rich membrane microdomains (lipid rafts), which are known to contain several molecules important for T cell signaling. Interestingly, immature CD4(+)CD8(+) thymocytes respond to TCR/CD28 costimulation not by proliferating, but by dying. In this study, we report that, although CD4(+)CD8(+) thymocytes polarize their actin cytoskeleton, they fail to recruit lipid rafts to the site of TCR/CD28 costimulation. We show that coupling of lipid raft mobilization to cytoskeletal reorganization can be mediated by phosphoinositide 3-kinase, and discuss the relevance of these findings to the interpretation of TCR signals by immature vs mature T cells.

Activation of phospholipase D by PKC and GTPgammaS in human neuroblastoma cells overexpressing MARCKS

Regulation of phospholipase D (PLD) activity participating in signal transduction involves complex interactions with small G-proteins (ARF, Rho) and protein kinase C isoforms (PKCalpha). In SK-N-MC human neuroblastoma cells, phorbol ester (TPA) activation of PLD was enhanced by overexpressing myristoylated alanine-rich C kinase substrate (MARCKS). To study MARCKS interactions with PLD, we investigated PLD isoform expression and activation by TPA and GTPgammaS in intact and digitonin-permeabilized clones transfected with MARCKS (M22). PLD2 was in both cytosol and membrane fractions while PLD1 was primarily membrane-associated in both vector control and M22 cells; location or quantities were unaltered by TPA treatment. TPA-stimulated PLD activity was higher in both intact and digitonin-permeabilized M22 cells than in vector controls. In contrast, GTPgammaS-stimulated PLD activity was independent of MARCKS expression but was additive with MARCKS-PKC-dependent activation in permeabilized cells. Combinations of PKC inhibition and down-regulation in intact and permeabilized (with GTPgammaS present) cells indicated that a PKC-mediated phosphorylation event was necessary in intact cells without access to GTPgammaS, stimulation of PLD mediated by GTPgammaS was independent of PKC, and PLD activation by PKC in permeabilized cells was kinase-independent. Western blot analysis showed that MARCKS, PKCalpha, PLD1 and PLD2 were present in a detergent-insoluble fraction (DIF); GTPgammaS increased recovery of PLD2 in DIF. Disruption of cholesterol-rich DIFs with digitonin, cyclodextrin or filipin potentiated activation of PLD by TPA. Our studies suggest that activation of PLD by PKC requires MARCKS and can involve both phosphorylation-independent and -dependent processes. As PLD activation by GTPgammaS is PKC-MARCKS-independent, MARCKS may provide a fine tuning component in conjunction with G-protein-mediated mechanisms for regulation of PLD.

Dystrophin associates with caveolae of rat cardiac myocytes: relationship to dystroglycan

The possibility of an interaction between the cytoskeletal protein dystrophin and cell surface caveolae in the mammalian myocardium was investigated by several techniques. Caveolin (cav)-3-enriched, detergent-insoluble membranes isolated from purified ventricular sarcolemma by density-gradient fractionation were found to contain dystrophin and dystroglycan. Further purification of cav-3-containing membranes by immunoprecipitation using anti-cav-3-coated magnetic beads yielded dystrophin but not always dystroglycan. Electron microscopic analysis of precipitated material revealed caveola-sized vesicular profiles that could be double-labeled with anti-dystrophin and anti-cav-3 antibodies. In contrast, immunoprecipitation of membranes with anti-dystrophin-coated beads yielded both cav-3 and dystroglycan. Electron microscopic analysis of this material showed heterogeneous membrane profiles, some of which could be decorated with anti-cav-3 antibodies. To confirm that dystrophin and cav-3 were closely associated in cardiac myocytes, we verified that dystrophin was also present in immunoprecipitated cav-3-containing membranes from detergent extracts, as well as in sonicated extracts of purified ventricular myocytes. Confocal immunofluorescence microscopy of ventricular and atrial cardiac myocytes showed that the cellular distributions of cav-3 and dystrophin partially overlapped. Immuno-electron micrographs of thin sections of rat atrial myocytes revealed a fraction of dystrophin molecules that are in apparently close apposition to caveolae. These results suggest that a subpopulation of dystrophin molecules interacts with cardiac myocyte caveolae in vivo and that some of the dystrophin is engaged in linking cav-3 with the dystroglycan complex.

The EGF receptor: a nexus for trafficking and signaling

Ligand binding to the EGF receptor initiates both the activation of mitogenic signal transduction pathways plus trafficking events that relocalize the receptor on the cell surface and within intracellular compartments. The trafficking compartments include caveolae, clathrin-coated pits, and various endosome populations prior to receptor degradation in lysosomes. Evidence is presented that distinct signaling pathways are initiated from these different compartments. These include the Ras/MAP kinase cascade and the PLC-dependent hydrolysis of PI-4,5 P(2). Multiple tyrosine kinase substrates that facilitate EGF receptor trafficking between these various compartments, as well as the participation of phosphoinositides and Ras-like G proteins in the trafficking pathway are also described.

Sustained elevation in inositol 1,4,5-trisphosphate results in inhibition of phosphatidylinositol transfer protein activity and chronic depletion of the agonist-sensitive phosphoinositide pool

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyses the signalling molecules inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4, 5)P(4)) in a signal-terminating reaction. We have utilised cell lines that stably underexpress the 43 kDa 5-phosphatase, as a model system to investigate whether Ins(1,4,5)P(3) can control the rate of its own formation by regulating the resupply of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). A sustained 2.6-fold elevation in the basal concentration of Ins(1,4,5)P(3), in cell lines underexpressing the 43 kDa 5-phosphatase, correlated with a 32% reduction in the total cellular mass of PtdIns(4,5)P(2). The depletion in cellular PtdIns(4,5)P(2) was confined to a Triton-insoluble cell compartment, enriched in caveolin. In resting cells with elevated Ins(1,4,5)P(3) concentrations resulting from underexpression of the 43 kDa 5-phosphatase, phosphatidylinositol (PtdIns) and phosphatidylinositol 4-phosphate (PtdIns(4)P) were depleted by 50% and PtdIns(4,5)P(2) by 61% in the caveolin-enriched Triton-insoluble compartment. Agonist stimulation resulted in the rapid turnover of phosphoinositides in the caveolin-enriched Triton-insoluble fraction of vector-transfected cells, but not in cells with high basal Ins(1,4,5)P(3) concentrations. Depletion of phosphoinositides from the caveolin-enriched Triton-insoluble pool in cells underexpressing the 43 kDa 5-phosphatase did not result from activation of phospholipase C isoenzymes, or inhibition of PtdIns 4-kinase or PtdIns(4)P 5-kinase activities. Significant inhibition of phosphatidylinositol transfer protein (PITP) activity (up to 70%) was observed in cells with elevated basal Ins(1,4,5)P(3) concentrations; however, no reduction in PITP(α) protein expression was detected. These studies indicate that chronic elevation in cellular Ins(1,4,5)P(3) concentrations decreases the PITP-mediated resupply of phosphoinositides in the caveolin-enriched agonist-sensitive pool.

Heterogeneity of detergent-insoluble membranes from human intestine containing caveolin-1 and ganglioside G(M1)

In intestinal epithelia, cholera and related toxins elicit a cAMP-dependent chloride secretory response fundamental to the pathogenesis of toxigenic diarrhea. We recently proposed that specificity of cholera toxin (CT) action in model intestinal epithelia may depend on the toxin's cell surface receptor ganglioside G(M1). Binding G(M1) enabled the toxin to elicit a response, but forcing the toxin to enter the cell by binding the closely related ganglioside G(D1a) rendered the toxin inactive. The specificity of ganglioside function correlated with the ability of G(M1) to partition CT into detergent-insoluble glycosphingolipid-rich membranes (DIGs). To test the biological plausibility of these hypotheses, we examined native human intestinal epithelia. We show that human small intestinal epithelia contain DIGs that distinguish between toxin bound to G(M1) and G(D1a), thus providing a possible mechanism for enterotoxicity associated with CT. We find direct evidence for the presence of caveolin-1 in DIGs from human intestinal epithelia but find that these membranes are heterogeneous and that caveolin-1 is not a structural component of apical membrane DIGs that contain CT.

A requirement for lipid rafts in B cell receptor induced Ca(2+) flux

Although the major biochemical events triggered by ligation of the B-cell receptor (BCR) have been well defined [1] [2], little is known about the spatio-temporal organization of BCR signaling components within the cell membrane and the mechanisms by which signaling specificity is achieved. Partitioning of signaling complexes into specialized domains in the plasma membrane may provide a mechanism for channeling specific stimuli into distinct signaling pathways. Here, we report that multiple tyrosine-phosphorylated proteins accumulate transiently upon BCR activation in detergent-insoluble membrane microdomains known as lipid rafts. We found an activation-dependent translocation to the rafts of the BCR itself, as well as phospholipase Cgamma2 (PLCgamma2), an enzyme critical for BCR-induced Ca(2+) flux in B cells. An intact raft structure was required for BCR-induced tyrosine phosphorylation of PLCgamma2 and the induction of Ca(2+) flux. Taken together, these data provide a functional role for lipid rafts in BCR signaling.

Functional roles of glycosphingolipids in signal transduction via lipid rafts

The formation of glycosphingolipid (GSL)-cholesterol microdomains in cell membranes has been proposed to function as platforms for the attachment of lipid-modified proteins, such as glycosylphosphatidylinositol (GPI)-anchored proteins and src-family tyrosine kinases. The microdomains are postulated to be involved in GPI-anchored protein signaling via src-family kinase. Here, the functional roles of GSLs in signal transduction mediated by the microdomains are discussed. Antibodies against GSLs co-precipitate GPI-anchored proteins, src-family kinases and several components of the microdomains. Antibody-mediated crosslinking of GSLs, as well as that of GPI-anchored proteins, induces a rapid activation of src-family kinases and a transient increase in the tyrosine phosphorylation of several substrates. Enzymatic degradation of GSLs reduces the activation of src-family kinase and tyrosine phosphorylation by antibody-mediated crosslinking of GPI-anchored protein. Furthermore, GSLs can also modulate signal transduction of immunoreceptors and growth factor receptors in the microdomains. Thus, GSLs have important roles in signal transduction mediated by the microdomains.

CD14-dependent internalization and metabolism of extracellular phosphatidylinositol by monocytes

We report that membrane CD14 (mCD14), a cell surface receptor found principally on leukocytes, can mediate the uptake and metabolism of extracellular phosphatidylinositol (PtdIns). mCD14 facilitates PtdIns internalization, targeting it to intracellular sites where, following stimulation with a calcium ionophore, it can be acted upon by cytosolic phospholipase A(2). The [(14)C]arachidonate released from mCD14-acquired [(14)C]arachidonyl-PtdIns is either esterified to triacylglycerol and retained in the cell or secreted as free arachidonate or leukotrienes. Although less than 10% of the arachidonate-derived lipids secreted from endogenous cellular stores are 5-lipoxygenase metabolites, over one-half of the secreted (14)C-lipids derived from mCD14-acquired PtdIns are hydroxyeicosatetraenoic acids or leukotriene B(4). mCD14 may allow these highly active blood cells to acquire and use extracellular PtdIns as a source of arachidonate for leukotriene synthesis.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Changes in membrane microdomains and caveolae constituents in multidrug-resistant cancer cells

Cancer chemotherapy often fails because of the development of tumors which are resistant to most commonly used cytotoxic drugs. This phenomenon, multidrug resistance (MDR), is usually mediated by overexpression of P-glycoprotein (P-gp), an ATPase that pumps out the drugs used in chemotherapy, thereby preventing their accumulation in cancer cells and greatly reducing their cytotoxic efficacy. A large body of work indicates that MDR is associated also with marked changes in membrane lipid composition. Most notably, elevated levels of cholesterol, glycosphingolipids (e.g., glucosylceramide), and sphingomyelin have been reported. These lipids are enriched in caveolae and in membrane microdomains termed detergent-insoluble glycosphingolipid-enriched complexes (DIGs). Recently we demonstrated that in multidrug-resistant tumor cells there is a dramatic increase in the number of caveolae and in the level of caveolin-1, an essential structural constituent of caveolae. Another constituent of membrane microdomains, phospholipase D, is also elevated in MDR cells. These findings may be related to the fact that a significant fraction of cellular P-gp is associated with caveolin-rich membrane domains. The possible role of DIGs and caveolae in the acquisition and/or maintenance of the multidrug resistant phenotype is discussed.

T-cell activation and the dynamic world of rafts

Recent evidence, indicates that T-cell receptor (TCR) triggering and T-cell activation are dynamic processes that involve various aspects of T-cell organization. In addition to the interaction between the TCR molecule and its ligand, T-cell activation depends on a combination of many other events involving coreceptor molecules, actin cytoskeleton and plasma membrane lipids. Altogether, these cell structures organize the formation of a specialized junction between the T cell and the antigen-presenting cell (APC), that plays a critical role in sustaining and amplifying TCR signalling.

Regulation of G protein-coupled receptor kinases by caveolin

G protein-coupled receptor kinases (GRKs) have been principally characterized by their ability to phosphorylate and desensitize G protein-coupled receptors. However, recent studies suggest that GRKs may have more diverse protein/protein interactions in cells. Based on the identification of a consensus caveolin binding motif within the pleckstrin homology domain of GRK2, we tested the direct binding of purified full-length GRK2 to various glutathione S-transferase-caveolin-1 fusion proteins, and we discovered a specific interaction of GRK2 with the caveolin scaffolding domain. Interestingly, analysis of GRK1 and GRK5, which lack a pleckstrin homology domain, revealed in vitro binding properties similar to those of GRK2. Maltose-binding protein caveolin and glutathione S-transferase-GRK fusion proteins were used to map overlapping regions in the N termini of both GRK2 and GRK5 that appear to mediate conserved GRK/caveolin interactions. In vivo association of GRK2 and caveolin was suggested by co-fractionation of GRK2 with caveolin in A431 and NIH-3T3 cells and was further supported by co-immunoprecipitation of GRK2 and caveolin in COS-1 cells. Functional significance for the GRK/caveolin interaction was demonstrated by the potent inhibition of GRK-mediated phosphorylation of both receptor and peptide substrates by caveolin-1 and -3 scaffolding domain peptides. These data reveal a novel mode for the regulation of GRKs that is likely to play an important role in their cellular function.

Localization of phospholipase D in detergent-insoluble, caveolin-rich membrane domains. Modulation by caveolin-1 expression and caveolin-182-101

The activation of cellular phospholipase D (PLD) is implicated in vesicular trafficking and signal transduction. Two mammalian PLD forms, designated PLD1 and PLD2, have been cloned, but their cellular localization and function are not fully understood. Here, we report that in HaCaT human keratinocytes, as well as other cell lines, PLD activity is highly enriched in low density, Triton X-100-insoluble membrane domains that contain the caveolar marker protein caveolin-1. Similar to other PLDs, the PLD activity in these membrane domains is stimulated by phosphatidylinositol 4, 5-bisphosphate and is inhibited by neomycin. Immunoblot analysis indicated that caveolin-rich membrane domains do not contain the PLD1 isoform. Stable transfection of mouse PLD2 in Chinese hamster ovary cells greatly increased PLD activity in these domains compared with PLD activity in control Chinese hamster ovary cells transfected with vector alone. PLD activity is enriched in low density Triton-insoluble membrane domains also in U937 promonocytes, even though these cells do not express caveolin-1. In U937 cells, also, PLD1 is largely excluded from low density Triton-insoluble membrane domains. Expression of recombinant caveolin-1 in v-Src-transformed NIH-3T3 cells resulted in up-regulation of PLD activity in the caveolin-containing membrane domains. The caveolin scaffolding peptide (caveolin-182-101) modulated the caveolar PLD activity, causing stimulation at concentration of 1-10 microM and inhibition at concentrations >10 microM. We conclude that a PLD activity, which is likely to represent PLD2, is enriched in low density Triton-insoluble membrane domains. The effects of caveolin-1 expression and of the caveolin scaffolding peptide suggest that in cells that express caveolin-1, PLD may be targeted to caveolae. The possible functions of PLD in the dynamics of caveolae and related domains and in signal transduction processes are discussed.

Separation of "glycosphingolipid signaling domain" from caveolin-containing membrane fraction in mouse melanoma B16 cells and its role in cell adhesion coupled with signaling

Two membrane subfractions, one enriched in GM3 ganglioside and the other containing caveolin, were separated from low density detergent-insoluble membrane fraction prepared by sucrose density gradient centrifugation of postnuclear fraction of mouse melanoma B16 cells. The GM3-enriched subfraction, separated by anti-GM3 monoclonal antibody DH2, contained sphingomyelin, cholesterol, c-Src, and Rho A but not caveolin. In contrast, the caveolin-containing subfraction, separated by anti-caveolin antibody, contained neither GM3, c-Src, nor Rho A but did contain glucosylceramide, Ras, a very small quantity of sphingomyelin, and a very large quantity of cholesterol. The GM3/c-Src-enriched membrane subfraction was characterized by (i) maintenance of GM3-dependent adhesion and (ii) susceptibility to being activated for signal transduction through GM3. 32P-Phosphorylation of c-Src (Mr 60,000) together with two other components (Mr 45,000 and 29,000) was enhanced in the fraction bound to dishes coated with asialo-GM2 (Gg3) or with anti-GM3 monoclonal antibody DH2, detected by incubation with [gamma-32P]ATP at 37 degreesC for 5 min. GM3-dependent adhesion of B16 cells to Gg3-coated dishes and associated signaling were not reduced or abolished in the presence of either filipin or nystatin, which are cholesterol-binding reagents known to abolish caveolae structure and function. B16 melanoma cells incubated with filipin (0.16-0.3 micrograms/ml) or with nystatin (25 micrograms/ml) for 30 min showed depletion of cholesterol in detergent-insoluble membrane fraction but were still capable of binding to Gg3-coated plate and capable of the associated signaling. Thus, the GM3-enriched subfraction, involved in cell adhesion and capable of sending signals through GM3, represents a membrane domain distinguishable from caveolin-containing subfraction or caveolae. This microdomain is hereby termed the "glycosphingolipid signaling domain" or "glycosignaling domain".