Differential effects of ceramide species on exocytosis in rat PC12 cells

Increases in several ceramide species have been shown by non-targeted lipid profiling (lipidomics) of the rat hippocampus after kainate lesions (Guan et al. FASEB J 20:1152-1161, 2006). This study was carried out to examine possible effects of ceramide species on exocytosis. Significant increase in membrane capacitance in voltage-clamped rat pheochromocytoma (PC12) cells, an indication of exocytosis, was detected immediately after external application of C2, C6, and C18 ceramide. In contrast, no increase in capacitance was found after addition of C16 and C20 ceramide, or DMSO vehicle. The effect of ceramide on exocytosis was dependent on the integrity of lipid rafts. Treatment of cells with the cholesterol binding agent/disruptor of lipid rafts, methyl beta cyclodextrin, prior to addition of C18 ceramide suppressed the increase in capacitance induced by this lipid species. The ability of C2, C6 and C18 ceramide to trigger exocytosis was confirmed using total internal reflection fluorescence microscopy (TIRFM) experiments. External application of these species caused an exponential decrease in the number of subplasmalemmal neuropeptide Y (NPY)-enhanced green fluorescence protein (EGFP) labeled vesicles, indicating exocytosis. Interestingly, C18 is also the ceramide species that showed the greatest increase in the rat hippocampus after kainate excitotoxicity. It is postulated that C18 ceramide might facilitate exocytosis of glutamate from damaged neurons, thus propagating neuronal injury.

Mastoparan inhibits beta-adrenoceptor-G(s) signaling by changing the localization of Galpha(s) in lipid rafts

Mastoparan, a wasp venom toxin, has various pharmacological activities, the mechanisms of which are still unknown. To clarify the action of mastoparan on G protein-coupled receptor-mediated signaling, we previously examined the effect of mastoparan on G(q)-mediated signaling and demonstrated that mastoparan binds to gangliosides causing a decrease in Galpha(q/11) content in lipid rafts, and resulting in the inhibition of G(q)-mediated phosphoinositide hydrolysis (Sugama et al., Mol. Pharmacol., 68, 1466, 2005). In the present study, we examined the effect of mastoparan on beta-adrenoceptor-G(s) signaling in 1321N1 human astrocytoma cells. Mastoparan inhibited isoproterenol-induced elevation of cyclic AMP in a concentration-dependent manner. Although mastoparan is known to be an activator of G(i), pertussis toxin only slightly attenuated mastoparan-induced inhibition of cyclic AMP elevation, suggesting that a major part of the inhibition of cyclic AMP elevation induced by mastoparan is not mediated by Galpha(i). By contrast, mastoparan-induced inhibition of cyclic AMP elevation was clearly attenuated by preincubation of the cells with ganglioside mixtures. Moreover, mastoparan changed the localization of Galpha(s) in lipid rafts without disrupting the structure of lipid rafts. Fluorescent staining analysis showed that mastoparan released GFP-Galpha(s) from plasma membranes into the cytosol. These results suggest that the mastoparan-induced suppression of cyclic AMP elevation is mainly caused by changing the localization of Galpha(s) in lipid rafts into a compartment in the cellular interior where it is not available to activate adenylyl cyclase.

B and T lymphocyte attenuator interacts with CD3zeta and inhibits tyrosine phosphorylation of TCRzeta complex during T-cell activation

B and T lymphocyte attenuator (BTLA) is an important negative regulator of T-cell activation. T-cell activation involves partitioning of receptors into discrete membrane compartments known as lipid rafts and the formation of an immunological synapse (IS) between the T cell and antigen-presenting cell (APC). Here we show that after T-cell stimulation, BTLA co-clusters with the CD3zeta and is then involved in IS, as determined by a two-photon microscope. BTLA can interact with the phosphorylated form of T-cell receptor (TCR) within the lipid raft, which is associated with the T-cell signaling complex. Coligation of BTLA with the TCR significantly decreased the amount of phosphorylated TCR-related signal accumulation in the lipid raft during T-cell activation. These results suggest that BTLA functions to regulate T-cell signaling by controlling the phosphorylated form of TCRzeta accumulation in the lipid raft.

Acid sphingomyelinase and its redox amplification in formation of lipid raft redox signaling platforms in endothelial cells

This study examined the role of acid sphingomyelinase (ASM) and its redox amplification in mediating the formation of lipid raft (LR) redox signaling platforms in coronary arterial endothelial cells (CAECs). Using small interference RNA (siRNA) of ASM, Fas ligand (FasL)-induced increase in ASM activity, production of ceramide, and LR clustering in CAECs were blocked, and clustered Fas was also substantially reduced in detergent-resistant membrane fractions of CAECs. LR clustering, gp91(phox) aggregation, and p47(phox) translocation to the LR clusters induced by FasL were also blocked in ASM-siRNA transfected CAECs. Corresponding to this reduction of LR clustering with NAD(P)H oxidase subunits in ASM-siRNA transfected CAECs, superoxide (O(2)(-*)) production was significantly decreased as measured by either ESR or fluorescent spectrometry. Interestingly, superoxide dismutase (SOD) not only scavenged (O(2)(-*)), but also markedly attenuated LR clustering. Xanthine/xanthine oxidase, an exogenous (O(2)(-*)) generating system, dramatically increased ASM activity and LR clustering in EC membrane and enhanced FasL-induced LR clustering, which were blocked by SOD. These results suggest that that ASM activates LR clustering to form redox signaling platforms, where (O(2)(-*)) production enhances ASM activity, and thereby results in a forwarding amplification of LR and redox signaling. This ASM-mediated feedforwarding mechanism may be critical for an efficient transmembrane signaling through LRs.

Lipid rafts establish calcium waves in hepatocytes

BACKGROUND & AIMS

Polarity is critical for hepatocyte function. Ca(2+) waves are polarized in hepatocytes because the inositol 1,4,5-trisphosphate receptor (InsP3R) is concentrated in the pericanalicular region, but the basis for this localization is unknown. We examined whether pericanalicular localization of the InsP3R and its action to trigger Ca(2+) waves depends on lipid rafts.

METHODS

Experiments were performed using isolated rat hepatocyte couplets and pancreatic acini, plus SkHep1 cells as nonpolarized controls. The cholesterol depleting agent methyl-beta-cyclodextrin (mbetaCD) was used to disrupt lipid rafts. InsP3R isoforms were examined by immunoblot and immunofluorescence. Ca(2+) waves were examined by confocal microscopy.

RESULTS

Type II InsP3Rs initially were localized to only some endoplasmic reticulum fractions in hepatocytes, but redistributed into all fractions in mbetaCD-treated cells. This InsP3R isoform was concentrated in the pericanalicular region, but redistributed throughout the cell after mbetaCD treatment. Vasopressin-induced Ca(2+) signals began as apical-to-basal Ca(2+) waves, and mbetaCD slowed the wave speed and prolonged the rise time. MbetaCD had a similar effect on Ca(2+) waves in acinar cells but did not affect Ca(2+) signals in SkHep1 cells, suggesting that cholesterol depletion has similar effects among polarized epithelia, but this is not a nonspecific effect of mbetaCD.

CONCLUSIONS

Lipid rafts are responsible for the pericanalicular accumulation of InsP3R in hepatocytes, and for the polarized Ca(2+) waves that result. Signaling microdomains exist not only in the plasma membrane, but also in the nearby endoplasmic reticulum, which in turn, helps establish and maintain structural and functional polarity.

Light-induced recruitment of INAD-signaling complexes to detergent-resistant lipid rafts in Drosophila photoreceptors

Here, we reveal a novel feature of the dynamic organization of signaling components in Drosophila photoreceptors. We show that the multi-PDZ protein INAD and its target proteins undergo light-induced recruitment to detergent-resistant membrane (DRM) rafts. Reduction of ergosterol, considered to be a key component of lipid rafts in Drosophila, resulted in a loss of INAD-signaling complexes associated with DRM fractions. Genetic analysis demonstrated that translocation of INAD-signaling complexes to DRM rafts requires activation of the entire phototransduction cascade, while constitutive activation of the light-activated channels resulted in recruitment of complexes to DRM rafts in the dark. Mutations affecting INAD and TRP showed that PDZ4 and PDZ5 domains of INAD, as well as the INAD-TRP interaction, are required for translocation of components to DRM rafts. Finally, selective recruitment of phosphorylated, and therefore activatable, eye-PKC to DRM rafts suggests that DRM domains are likely to function in signaling, rather than trafficking.

Cell microvesicles during experimental endotoxemia

The dynamics of microvesicle formation in arterial blood in generalized Schwartzman phenomenon was studied. Successive (with 24-h interval) intravenous injections of endotoxin to rabbits in a dose of 1 mg/kg and 3 mg/kg caused an increase in the content of microvesicles in the blood, some of them containing ecto-5'-nucleotidase. Biphasic changes in arterial blood clotting time and erythrocyte hemolysis were observed.

Lipid rafts are required for Kit survival and proliferation signals

In addition to its physiologic role as central regulator of the hematopoietic and reproductive systems, the Kit receptor tyrosine kinase (RTK) is pathologically overexpressed in some forms of leukemia and constitutively activated by oncogenic mutations in mast-cell proliferations and gastrointestinal stromal tumors. To gain insight into the general activation and signaling mechanisms of RTKs, we investigated the activation-dependent dynamic membrane distributions of wild-type and oncogenic forms of Kit in hematopoietic cells. Ligand-induced recruitment of wild-type Kit to lipid rafts after stimulation by Kit ligand (KL) and the constitutive localization of oncogenic Kit in lipid rafts are necessary for Kit-mediated proliferation and survival signals. KL-dependent and oncogenic Kit kinase activity resulted in recruitment of the regulatory phosphatidylinositol 3-kinase (PI3-K) subunit p85 to rafts where the catalytical PI3-K subunit p110 constitutively resides. Cholesterol depletion by methyl-beta-cyclodextrin prevented Kit-mediated activation of the PI3-K downstream target Akt and inhibited cellular proliferation by KL-activated or oncogenic Kit, including mutants resistant to the Kit inhibitor imatinib-mesylate. Our data are consistent with the notion that Kit recruitment to lipid rafts is required for efficient activation of the PI3-K/Akt pathway and Kit-mediated proliferation.

The tyrosine kinase Fyn determines the localization of TrkB receptors in lipid rafts

Localization of Trk neurotrophin receptors is an important factor in directing cellular communication in developing and mature neurons. One potential site of action is in lipid raft membrane microdomains. Although Trk receptors have been localized to lipid rafts, little is known about how these neurotrophin receptors are directed there or how localization to these membrane microdomains regulates Trk signaling. Here, we report that the TrkB brain-derived neurotrophic factor (BDNF) receptor specifically localized to intracellular lipid rafts in cortical and hippocampal membranes in response to BDNF and that this process was critically dependent on the tyrosine kinase Fyn. BDNF-induced TrkB accumulation at lipid rafts was prevented by blocking the internalization of TrkB. BDNF stimulation also resulted in the association between endogenous TrkB and Fyn. Moreover, in neurons derived from Fyn knock-out mice, the translocation of TrkB to lipid rafts in response to BDNF was compromised, whereas the corticohippocampal region of Fyn mutants displayed lower amounts of TrkB in lipid rafts in vivo. In support of a role for lipid rafts in neurotrophin signaling, inhibiting TrkB translocation to lipid rafts, either by using Fyn knock-out neurons or lipid raft-disturbing agents, prevented the full activation of TrkB and of downstream phospholipase C-gamma. These results indicate that the lipid raft localization of TrkB receptors is regulated by Fyn and represents an important factor in determining the outcome of BDNF signaling in neurons.

Nerve growth factor stimulates the concentration of TrkA within lipid rafts and extracellular signal-regulated kinase activation through c-Cbl-associated protein

Nerve growth factor (NGF) acts through its receptor, TrkA, to elicit the neuronal differentiation of PC12 cells through the action of extracellular signal-regulated kinase 1 (ERK1) and ERK2. Upon NGF binding, TrkA translocates and concentrates in cholesterol-rich membrane microdomains or lipid rafts, facilitating formation of receptor-associated signaling complexes, activation of downstream signaling pathways, and internalization into endosomes. We have investigated the mechanisms responsible for the localization of TrkA within lipid rafts and its ability to activate ERK1 and ERK2. We report that NGF treatment results in the translocation of activated forms of TrkA to lipid rafts, and this localization is important for efficient activation of the ERKs. TrkA is recruited and retained within lipid rafts through its association with flotillin, an intrinsic constituent of these membrane microdomains, via the adapter protein, c-Cbl associated protein (CAP). Mutant forms of CAP that lack protein interaction domains block TrkA localization to lipid rafts and attenuate ERK activation. Importantly, suppression of endogenous CAP expression inhibited NGF-stimulated neurite outgrowth from primary dorsal root ganglion neurons. These data provide a mechanism for the lipid raft localization of TrkA and establish the importance of the CAP adaptor protein for NGF activation of the ERKs and neuronal differentiation.

Membrane cholesterol modulates Kv1.5 potassium channel distribution and function in rat cardiomyocytes

Membrane lipid composition is a major determinant of cell excitability. In this study, we assessed the role of membrane cholesterol composition in the distribution and function of Kv1.5-based channels in rat cardiac membranes. In isolated rat atrial myocytes, the application of methyl-beta-cyclodextrin (MCD), an agent that depletes membrane cholesterol, caused a delayed increase in the Kv1.5-based sustained component, I(kur), which reached steady state in approximately 7 min. This effect was prevented by preloading the MCD with cholesterol. MCD-increased current was inhibited by low 4-aminopyridine concentration. Neonatal rat cardiomyocytes transfected with Green Fluorescent Protein (GFP)-tagged Kv1.5 channels showed a large ultrarapid delayed-rectifier current (I(Kur)), which was also stimulated by MCD. In atrial cryosections, Kv1.5 channels were mainly located at the intercalated disc, whereas caveolin-3 predominated at the cell periphery. A small portion of Kv1.5 floated in the low-density fractions of step sucrose-gradient preparations. In live neonatal cardiomyocytes, GFP-tagged Kv1.5 channels were predominantly organized in clusters at the basal plasma membrane. MCD caused reorganization of Kv1.5 subunits into larger clusters that redistributed throughout the plasma membrane. The MCD effect on clusters was sizable 7 min after its application. We conclude that Kv1.5 subunits are concentrated in cholesterol-enriched membrane microdomains distinct from caveolae, and that redistribution of Kv1.5 subunits by depletion of membrane cholesterol increases their current-carrying capacity.

Doxorubicin enhances TRAIL-induced cell death via ceramide-enriched membrane platforms

Previous studies indicated that signalling via CD95 and DR5 is greatly enhanced by the formation of ceramide-enriched membrane platforms. Here, we employed this concept to convert doses of subtherapeutic TRAIL that were unable to release ceramide and kill leukemic B-cells or ex vivo T lymphocytes, into a very effective apoptotic stimulus. Ceramide production was induced by application of sub-toxic doses of doxorubicin that resulted in an activation of the acid sphingomyelinase (ASM), release of ceramide and formation of ceramide-enriched membrane platforms. The latter served DR5 to cluster after application of very low doses of TRAIL in combination with doxorubicin. Genetic deficiency of the ASM abrogated doxorubicin-induced ceramide release, as well as clustering of DR5 and apoptosis induced by the combined treatment of doxorubicin and TRAIL. These data show that local release of ceramide potentiates very low, otherwise inactive doses of TRAIL that may represent a novel therapeutic concept to treat tumors.

Ochratoxin A displaces claudins from detergent resistant membrane microdomains

Ochratoxin A (OchA) is a food-borne mycotoxin with multiple effects in vivo. Previously, we have demonstrated that the toxin can significantly impair the barrier function of the gut epithelial cell line, Caco-2. Barrier disruption involved loss of claudins 3 and 4, but not claudin 1 from the tight junction complex. In this study, we demonstrate for the first time, that OchA is able to remove claudins 3 and 4 from the detergent insoluble membrane microdomains associated with the tight junctions. However, cholesterol distribution within the microdomain was unaffected by the toxin. In addition, the thiol antioxidant, N-acetyl cysteine, preserved the microdomain localisation of claudins and also the barrier function of Caco-2 cells. This work suggests that OchA-mediated barrier toxicity is due to removal of claudins from detergent insoluble membrane microdomains. Moreover, loss of microdomain association may be due to oxidative events.

Squalene synthase, a determinant of Raft-associated cholesterol and modulator of cancer cell proliferation

Several cues for cell proliferation, migration, and survival are transmitted through lipid rafts, membrane microdomains enriched in sphingolipids and cholesterol. Cells obtain cholesterol from the circulation but can also synthesize cholesterol de novo through the mevalonate/isoprenoid pathway. This pathway, however, has several branches and also produces non-sterol isoprenoids. Squalene synthase (SQS) is the enzyme that determines the switch toward sterol biosynthesis. Here we demonstrate that in prostate cancer cells SQS expression is enhanced by androgens, channeling intermediates of the mevalonate/isoprenoid pathway toward cholesterol synthesis. Interestingly, the resulting increase in de novo synthesis of cholesterol mainly affects the cholesterol content of lipid rafts, while leaving non-raft cholesterol levels unaffected. Conversely, RNA interference-mediated SQS inhibition results in a decrease of raft-associated cholesterol. These data show that SQS activity and de novo cholesterol synthesis are determinants of membrane microdomain-associated cholesterol in cancer cells. Remarkably, SQS knock down also attenuates proliferation and induces death of prostate cancer cells. Similar effects are observed when cancer cells are treated with the chemical SQS inhibitor zaragozic acid A. Importantly, although the anti-tumor effect of statins has previously been attributed to inhibition of protein isoprenylation, the present study shows that specific inhibition of the cholesterol biosynthesis branch of the mevalonate/isoprenoid pathway also induces cancer cell death. These findings significantly underscore the importance of de novo cholesterol synthesis for cancer cell biology and suggest that SQS is a potential novel target for antineoplastic intervention.

Targeting of voltage-gated K+ and Ca2+ channels and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins to cholesterol-rich lipid rafts in pancreatic alpha-cells: effects on glucagon stimulus-secretion coupling

Pancreatic alpha-cells secrete glucagon in response to low glucose to counter insulin actions, thereby maintaining glucose homeostasis. The molecular basis of alpha-cell stimulus-secretion coupling has not been fully elucidated. We investigated the expression of voltage-gated K(+) (K(V)) and Ca(2+) (Ca(V)) channels, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in pancreatic alpha-cells and examined their targeting to specialized cholesterol-rich lipid rafts. In alpha-cells, we detected the expression of K(V)4.1/4.3 (A-type current), K(V)3.2/3.3 (delayed rectifier current), Ca(V)1.2 (L-type current), Ca(V)2.2 (N-type current), and the SNARE (synaptosomal-associated protein of 25 kDa, syntaxin 1A, and vesicle-associated membrane protein 2) and SNARE-associated proteins (Munc-13-1 and Munc-18a). We also detected caveolin-2, a structural protein of cholesterol-rich lipid rafts. Of these proteins, caveolin-2, K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins (syntaxin 1A, synaptosomal-associated protein of 25 kDa, and vesicle-associated membrane protein 2) target to lipid raft domains on alpha-cell plasma membranes. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin decreased the association of K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins with lipid rafts. This resulted in inhibition of A-type K(V) currents and enhancement of glucagon secretion from alpha-cells. Consistently, capacitance measurements of exocytosis of single alpha-cells showed enhanced exocytosis after membrane cholesterol depletion. Taken together, our results demonstrate the association of K(V)4, Ca(V)1.2, and SNARE proteins with lipid rafts in pancreatic alpha-cells. Glucagon secretion from alpha-cells is regulated by lipid rafts, and the dissociation of SNARE proteins from cholesterol-rich lipid raft domains enhances glucagon secretion.

Bleomycin induces caveolin-1 and -2 expression in epithelial lung cancer A549 cells

BACKGROUND

Bleomycin induces apoptosis in alveolar epithelial cells. The expression of caveolin-1 and -2 in lung epithelial-derived A549 cells was analysed in terms of apoptosis after exposure to bleomycin.

MATERIALS AND METHODS

Apoptosis was investigated using flow cytometry, ELISA, immunohistochemistry and Western blot analysis. Caveolin-1 and -2 were determined at the protein level (Western blot). Intracellular caveolin-1 distribution was studied with immunofluorescence, as well as sucrose density gradient centrifugation.

RESULTS

Caveolin-1 and -2 were up-regulated 1 h after exposure to bleomycin and preceding the occurrence of caspase-8, and of caspase-3 and caspase-9 cleavage products. Sucrose density gradient centrifugation revealed that bleomycin exposure led to a partial translocation of caveolin-1 from caveolin-rich membrane fractions to non-raft fractions. Successful inhibition of bleomycin-induced apoptosis by the broad-spectrum caspase inhibitor zVAD-fmk did not influence the amount of caveolin-1 and -2.

CONCLUSION

The early up-regulation of caveolin-1 and -2 following bleomycin exposure is a rather apoptosis-independent event related to other unknown mechanisms of bleomycin-mediated cell injury.

Inhibition of cell death by a novel 16.2 kD heat shock protein predominantly via Hsp90 mediated lipid rafts stabilization and Akt activation pathway

AlphaB-crystallin homology, heat stress induction and chaperone activity suggested that a previously encloned gene product is a novel small heat shock protein (Hsp16.2). Suppression of Hsp16.2 by siRNA sensitized cells to hydrogen peroxide or taxol induced cell-death. Over-expressing of Hsp16.2 protected cells against stress stimuli by inhibiting cytochrome c release from the mitochondria, nuclear translocation of AIF and endonuclease G, and caspase 3 activation. Recombinant Hsp16.2 protected mitochondrial membrane potential against calcium induced collapse in vitro indicating that Hsp16.2 stabilizes mitochondrial membrane systems. Hsp16.2 formed self-aggregates and bound to Hsp90. Inhibition of Hsp90 by geldanamycin diminished the cytoprotective effect of Hsp16.2 indicating that this effect was Hsp90-mediated. Hsp16.2 over-expression increased lipid rafts formation as demonstrated by increased cell surface labeling with fluorescent cholera toxin B, and increased Akt phosphorylation. The inhibition of PI-3-kinase-Akt pathway by LY-294002 or wortmannin significantly decreased the protective effect of the Hsp16.2. These data indicate that the over-expression of Hsp16.2 inhibits cell death via the stabilization of mitochondrial membrane system, activation of Hsp90, stabilization of lipid rafts and by the activation of PI-3-kinase-Akt cytoprotective pathway.

Functional expression and microdomain localization of prestin in cultured cells

INTRODUCTION

Prestin is an essential component of the molecular motor of cochlear outer hair cells that contribute to frequency selectivity and sensitivity of mammalian hearing. A model system to study prestin employs its transfection into cultured HEK 293 cells. Our goal was to characterize prestin's trafficking pathway and localization in the plasma membrane.

METHODS

We used immuno-colocalization of prestin with intracellular and plasma membrane markers and sucrose density fractionation to analyze prestin in membrane compartments. Voltage clamping was used to measure nonlinear capacitance (NLC), prestin's electrical signature.

RESULTS & DISCUSSION

Prestin targets to the membrane by 24 hours post-transfection when NLC is measurable. Prestin then concentrates into membrane foci that colocalize and fractionate with membrane microdomains. Depleting membrane cholesterol content altered prestin localization and NLC.

CONCLUSION

Prestin activity in HEK 293 cells results from expression in the plasma membrane and altering membrane lipid content affects prestin localization and activity.

Modulation of tissue factor-factor VIIa signaling by lipid rafts and caveolae

OBJECTIVE

Coagulation factor VIIa (VIIa) binding to its cellular receptor, tissue factor (TF), not only initiates the coagulation cascade but also induces cell signaling by activating G-protein coupled protease-activated receptors. The objective of the present study is to investigate the role of lipid rafts and caveolae in modulating TF-VIIa signaling and coagulant functions.

METHODS AND RESULTS

TF-VIIa coagulant function was measured in factor X activation assay and the signaling function was evaluated in phosphoinositide hydrolysis and IL-8 gene induction. Buoyant density gradient centrifugation and immunofluorescence confocal microscopy were used to determine cellular localization of TF and protease-activated receptor 2. The data show that a substantial fraction of TF and protease-activated receptor 2 resides in lipid rafts/caveolae, and disruption of lipid rafts by cholesterol depletion or modification reduced TF-VIIa-induced cell signaling. Disruption of caveolae with caveolin-1 silencing had no effect on the TF-VIIa coagulant activity but inhibited the TF-VIIa-induced cell signaling.

CONCLUSION

Overall our data show that lipid raft/caveolae play a selective role in modulating the TF-VIIa signaling function without affecting the TF-VIIa coagulant activity.

Heparin suppresses lipid raft-mediated signaling and ligand-independent EGF receptor activation

Heparin is well known to suppress vascular smooth muscle cell (VSMC) proliferation, and attempts to exploit this therapeutically have led to recognition of multiple pathways for heparin's anti-mitogenic actions. At low concentrations (ca. 1 microg.ml(-1)), these suppressive effects may reflect physiological activities of endogenous heparan sulfates, and appear to be rapid responses to extracellular or cell surface-associated heparin. Because heparin has been shown to influence expression of caveolin proteins, and caveolae/lipid rafts are critical structures modulating cell signaling, we examined the effect of heparin on signaling involving cholesterol-rich membrane microdomains. The VSMC line PAC-1 activates the MAP kinase Erk in response to the cholesterol-sequestering agents methyl-beta-cyclodextrin and nystatin. This follows a temporal sequence that involves Ras-GTP activation of MEK, and is independent of PKC, Src, and PI3 kinase. However, ligand-independent phosphorylation of the EGF receptor (EGFR) by removal of cholesterol precedes Ras activation, and the EGFR kinase inhibitor AG1478 blocks Erk phosphorylation, supporting occurrence of the signaling sequence EGFR-Ras-MEK-Erk. Phosphorylation of EGFR occurs predominantly in caveolin-rich microdomains as identified by Western blotting of fractions from density gradient centrifugation of membranes prepared under detergent-free conditions. In these situations, heparin inhibits phosphorylation of EGFR on the Src-dependent site Tyr(845), but not the autophosphorylation of Tyr(1173), and decreases Ras activation and Erk phosphorylation. We conclude that heparin can suppress Erk signaling in VSMC with effects on site-specific phosphorylation of EGFR localized in caveolin-enriched lipid rafts.

Shed gangliosides provide detergent-independent evidence for Type-3 glycosynapses

Membrane microdomains, or rafts, at the plasma membrane have been invoked to explain many cellular processes. Protein-protein interactions within such microdomains including, for example, the tetraspanin web are reported to provide a scaffold for signal transduction. However, the nature of such protein-protein interactions is not fully elucidated. Hakomori [S.I. Hakomori, The glycosynapse, Proc. Natl. Acad. Sci. USA 99 (2002) 225-232] has advanced the concept that glycosphingolipids, particularly gangliosides, provide the intermediary link between transmembrane receptors and signal transducers and has redefined membrane rafts as Type-1, -2 or -3 glycosynapses. Here, using simple immunofluorescent analysis of the ganglioside complexes naturally released from cellular microprocesses (termed "footprints") we show that the ganglioside can determine the nature of protein-protein associations. Specifically, we demonstrate that CD36 and the tetraspanin CD151, both of which interact with beta1 integrins, associate together only in the presence of the gangliosides GD2/GD3. These results substantiate the glycosynapse hypothesis and suggest that the nature of the tetraspanin web may be determined by gangliosides.

EXPRESSION OF FLOTILLIN-1 ON EIMERIA TENELLA SPOROZOITES AND ITS ROLE IN HOST CELL INVASION

Lipid rafts are detergent-resistant, liquid-ordered microdomains in plasma membranes that are enriched in cholesterol and sphingolipids and involved in intracellular signal transduction, membrane trafficking, and molecular sorting. In this study, we investigated the possibility that lipid rafts on Eimeria tenella sporozoites may act as platforms for host cell invasion. Flotillin-1, a resident protein of lipid rafts, was identified on E. tenella sporozoites and was prominently expressed at the apex of the cells, a region mediating host cell invasion. Pretreatment of sporozoites with antibody against flotillin-1 blocked parasite invasion. Furthermore, the anticoccidial drug, monensin, disrupted the localization of flotillin-1 within raft structures resulting in loss of invasion. We conclude that Eimeria sporozoites utilize lipid rafts containing flotillin-1 for internalization into host cells.

Quantitative proteomics analysis of human endothelial cell membrane rafts: evidence of MARCKS and MRP regulation in the sphingosine 1-phosphate-induced barrier enhancement

Endothelial cell barrier dysfunction results in the increased vascular permeability observed in inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Sphingosine 1-phosphate (S1P), a biologically active phosphorylated lipid growth factor released from activated platelets, enhances the endothelial cell barrier integrity in vitro and in vivo. To begin to identify the molecular mechanisms mediating S1P induced endothelial barrier enhancement, quantitative proteomics analysis (iTRAQ) was performed on membrane rafts isolated from human pulmonary artery endothelial cells in the absence or presence of S1P stimulation. Our results demonstrated that S1P mediates rapid and specific recruitment (1 microM, 5 min) of myristoylated alanine-rich protein kinase C substrate (MARCKS) and MARCKS-related protein (MRP) to membrane rafts. Western blot experiments confirmed these findings with both MARCKS and MRP. Finally, small interfering RNA-mediated silencing of MARCKS or MRP or both attenuates S1P-mediated endothelial cell barrier enhancement. These data suggest the regulation of S1P-mediated endothelial cell barrier enhancement via the cell specific localization of MARCKS and MRP and validate the utility of proteomics approaches in the identification of novel molecular targets.

TLR4, ethanol, and lipid rafts: a new mechanism of ethanol action with implications for other receptor-mediated effects

Ethanol (EtOH) is the most widely abused substance in the United States, and it contributes to well-documented harmful (at high dosages) and beneficial (at low dosages) changes in inflammatory and immune responses. Lipid rafts have been implicated in the regulation and activation of several important receptor complexes in the immune system, including the TLR4 complex. Many questions remain about the precise mechanisms by which rafts regulate the assembly of these receptor complexes. Results summarized in this review indicate that EtOH acts by altering the LPS-induced redistribution of components of the TLR4 complex within the lipid raft and that this is related to changes in actin cytoskeleton rearrangement, receptor clustering, and subsequent signaling. EtOH provides an example of an immunomodulatory drug that acts at least in part by modifying lipid rafts, and it could represent a model to probe the relationships between rafts, receptor complexes, and signaling.

Inhibition of caveolar uptake, SV40 infection, and beta1-integrin signaling by a nonnatural glycosphingolipid stereoisomer

Caveolar endocytosis is an important mechanism for the uptake of certain pathogens and toxins and also plays a role in the internalization of some plasma membrane (PM) lipids and proteins. However, the regulation of caveolar endocytosis is not well understood. We previously demonstrated that caveolar endocytosis and β1-integrin signaling are stimulated by exogenous glycosphingolipids (GSLs). In this study, we show that a synthetic GSL with nonnatural stereochemistry, β-d-lactosyl-N-octanoyl-l-threo-sphingosine, (1) selectively inhibits caveolar endocytosis and SV40 virus infection, (2) blocks the clustering of lipids and proteins into GSLs and cholesterol-enriched microdomains (rafts) at the PM, and (3) inhibits β1-integrin activation and downstream signaling. Finally, we show that small interfering RNA knockdown of β1 integrin in human skin fibroblasts blocks caveolar endocytosis and the stimulation of signaling by a GSL with natural stereochemistry. These experiments identify a new compound that can interfere with biological processes by inhibiting microdomain formation and also identify β1 integrin as a potential mediator of signaling by GSLs.