Tyrosine-phosphorylated caveolin is a physiological substrate of the low M(r) protein-tyrosine phosphatase

Isolation and biochemical characterization of novel acid phosphatase and zinc-dependent acid phosphatase from the chicken's brain

The AcPase exhibits a specific activity of 31.32 U/mg of protein with a 728-fold purification, and the yield of the enzyme is raised to 3.15 %. The Zn2+-dependent AcPase showed a purification factor of 1.34 specific activity of 14 U/mg of proteins and a total recovery of 5.14. The SDS-PAGE showed a single band corresponding to a molecular weight of 18 kDa of AcPase and 29 kDa of Zn2+-dependent AcPase. The AcPase enzyme has shown a wide range of substrate specificity for p-NPP, phenyl phosphate and FMN, while in the case of ZnAcPase α and β-Naphthyl phosphate and p-NPP were proved to be superior substrates. The divalent metal ions like Mg2+, Mn2+, and Ca2+ increased the activity, while other substrates decreased the enzyme activity. The Km (0.14 mM) and Vmax (21 μmol/min/mg) values of AcPase were higher than those of Zn2+-AcPase (Km = 0.5 mM; Vmax = 9.7 μmol/min/mg). The Zn2+ ions activate the Zn2+-AcPase while Fe3+, Al3+, Pb2+, and Hg2+ showed inhibition on enzyme activity. Molybdate, vanadate and phosphate were found to be competitive inhibitors of AcPase with Ki values 316 μM, 185 μM, and 1.6 mM, while in Zn2+-AcPase tartrate and phosphate also showed competitive inhibition with Ki values 3 mM and 0.5 mM respectively.

Dephosphorylation of Fas-ligand and caveolin-1 is a prerequisite step in Fas-ligand - caveolin-1 complex formation and cell death stimulation

Fas-ligand/CD178 belongs to the TNF family proteins and is the well-characterized inducer of cell death. We showed previously that the interaction of Fas-ligand and caveolin-1 is necessary for Fas-ligand translocation to rafts, and the subsequent induction of Fas-ligand-dependent cell death. Both molecules can undergo phosphorylation, however the role of the phosphorylation state of Fas-ligand and caveolin-1 in their physical association, and consequently in of Fas - mediated cell death induction is currently unknown. In this study, we show that in control cells Fas-ligand interaction with caveolin-1 is not observed, and both molecules are phosphorylated. The intracellular part of Fas-ligand was shown to form a complex with p59Fyn-kinase. Upon cell death activation, the expression and activity of p59Fyn-kinase decreases substantially, leading to the disruption of Fas-ligand - p59Fyn-kinase association, dephosphorylation of Fas-ligand and caveolin-1, and formation of a complex between them (Fas-ligand - caveolin-1). The analysis of the effects of kinase and phosphatase inhibitors revealed that phosphorylation of Fas-ligand and caveolin-1 at tyrosine residues suppressed Fas-mediated cell death. Thus, dephosphorylation of Fas-ligand and caveolin-1 is critical for triggering Fas-ligand-mediated apoptotic pathway and cell death execution.

Low molecular weight protein tyrosine phosphatase: Multifaceted functions of an evolutionarily conserved enzyme

Originally identified as a low molecular weight acid phosphatase, LMW-PTP is actually a protein tyrosine phosphatase that acts on many phosphotyrosine-containing cellular proteins that are primarily involved in signal transduction. Differences in sequence, structure, and substrate recognition as well as in subcellular localization in different organisms enable LMW-PTP to exert many different functions. In fact, during evolution, the LMW-PTP structure adapted to perform different catalytic actions depending on the organism type. In bacteria, this enzyme is involved in the biosynthesis of group 1 and 4 capsules, but it is also a virulence factor in pathogenic strains. In yeast, LMW-PTPs dephosphorylate immunophilin Fpr3, a peptidyl-prolyl-cis-trans isomerase member of the protein chaperone family. In humans, LMW-PTP is encoded by the ACP1 gene, which is composed of three different alleles, each encoding two active enzymes produced by alternative RNA splicing. In animals, LMW-PTP dephosphorylates a number of growth factor receptors and modulates their signalling processes. The involvement of LMW-PTP in cancer progression and in insulin receptor regulation as well as its actions as a virulence factor in a number of pathogenic bacterial strains may promote the search for potent, selective and bioavailable LMW-PTP inhibitors.

Effect of IR laser on myoblasts: a proteomic study

Laser therapy is used in physical medicine and rehabilitation to accelerate muscle recovery and in sports medicine to prevent damages produced by metabolic disturbances and inflammatory reactions after heavy exercise. The aim of this research was to get insight into possible benefits deriving from the application of an advanced IR laser system to counteract deficits of muscle energy metabolism and stimulate the recovery of hypotrophic tissue. We studied the effect of IR laser treatment on proliferation, differentiation, cytoskeleton organization and global protein expression in C2C12 myoblasts. We found that laser treatment induced a decrease in the cell proliferation rate without affecting cell viability, while leading to cytoskeletal rearrangement and expression of the early differentiation marker MyoD. The differential proteome analysis revealed the up-regulation and/or modulation of many proteins known to be involved in cell cycle regulation, cytoskeleton organization and differentiation.

Differential timing of defense-related responses induced by cerato-platanin and cerato-populin, two non-catalytic fungal elicitors

The cerato-platanin (CP) family consists of fungal-secreted proteins involved in various stages of the host-fungus interaction and acting as phytotoxins and elicitors of defense responses. The founder member of this family is CP, a non-catalytic protein with a six-stranded double-ψβ-barrel fold. Cerato-populin (Pop1) is an ortholog showing low sequence identity with CP. CP is secreted by Ceratocystis platani, the causal agent of the canker stain of plane. Pop1 is secreted by Ceratocystis populicola, a pathogen of poplar. CP and Pop1 have been suggested to act as PAMPs (pathogen-associated molecular patterns) because they induce phytoalexin synthesis, transcription of defense-related genes, restriction of conidia growth and cell death in various plants. Here, we treated plane leaves with CP or Pop1, and monitored defense responses to define the role of these elicitors in the plant interactions. Both CP and Pop1 were able to induce mitogen-activated protein kinases (MAPKs) phosphorylation, production of reactive oxygen species and nitric oxide, and overexpression of defense related genes. The characteristic DNA fragmentation and the cytological features indicate that CP and Pop1 induce cell death by a mechanism of programmed cell death. Therefore, CP and Pop1 can be considered as two novel, non-catalytic fungal PAMPs able to enhance primary defense. Of particular interest is the observation that CP showed faster activity compared to Pop1. The different timing in defense activation could potentially be due to the structural differences between CP and Pop1 (i.e. different hydrophobic index and different helix content) therefore constituting a starting point in unraveling their structure-function relationships.

Caveolae-dependent internalization and homologous desensitization of VIP/PACAP receptor, VPAC₂, in gastrointestinal smooth muscle

The main membrane proteins of caveolae (caveolin-1, -2 and -3) oligomerize within lipid rich domains to form regular invaginations of smooth muscle plasma membrane and participate in receptor internalization and desensitization independent of clathrin-coated vesicle endocytosis. We have previously shown that Gs-coupled VIP/PACAP receptors, VPAC2, predominantly expressed in smooth muscle cells of the gut, are exclusively phosphorylated by GRK2 leading to receptor internalization and desensitization. Herein, we characterized the role of caveolin-1 in VPAC2 receptor internalization and desensitization in gastric smooth muscle using three approaches: (i) methyl β-cyclodextrin (MβCD) to deplete cholesterol and disrupt caveolae in dispersed muscle cells, (ii) caveolin-1 siRNA to suppress caveolin-1 expression in cultured muscle cells, and (iii) caveolin-1 knockout mice (caveolin-1(-/-)). Pretreatment of gastric muscle cells with VIP stimulated tyrosine phosphorylation of caveolin-1, and induced VPAC2 receptor internalization (measured as decrease in (125)I-VIP binding after pretreatment) and desensitization (measured as decrease in VIP-induced cAMP formation after pretreatment). Caveolin-1 phosphorylation, and VPAC2 receptor internalization and desensitization were blocked by disruption of caveolae with MβCD, suppression of caveolin-1 with caveolin-1 siRNA or inhibition of Src kinase activity by PP2. Pretreatment with VIP significantly inhibited adenylyl cyclase activity and muscle relaxation in response to subsequent addition of VIP in freshly dispersed muscle cells and in muscle strips isolated from wild type and caveolin-1(-/-) mice; however, the inhibition was significantly attenuated in caveolin-1(-/-) mice. These results suggest that caveolin-1 plays an important role in VPAC2 receptor internalization and desensitization.

Integrin {alpha}1{beta}1 promotes caveolin-1 dephosphorylation by activating T cell protein-tyrosine phosphatase

Integrin α1β1 is a collagen receptor that down-regulates collagen and reactive oxygen species (ROS) production, and mice lacking this receptor show increased ROS levels and exacerbated glomerular sclerosis following injury. Caveolin-1 (Cav-1) is a multifunctional protein that is tyrosine-phosphorylated in response to injury and has been implicated in ROS-mediated injury. Cav-1 interacts with integrins, and integrin α1β1 binds/activates T cell protein-tyrosine phosphatase (TCPTP), which is homologous to the tyrosine phosphatase PTP1B known to dephosphorylate Cav-1. In this study, we analyzed whether phosphorylated Cav-1 (pCav-1) is a substrate of TCPTP and if integrin α1β1 is essential for promoting TCPTP-mediated Cav-1 dephosphorylation. We found that Cav-1 phosphorylation is significantly higher in cells lacking integrin α1β1 at base line and following oxidative stress. Overexpression of TCPTP leads to reduced pCav-1 levels only in cells expressing integrin α1β1. Using solid phase binding assays, we demonstrated that 1) purified Cav-1 directly interacts with TCPTP and the integrin α1 subunit, 2) pCav-1 is a substrate of TCPTP, and 3) TCPTP-mediated Cav-1 dephosphorylation is highly increased by the addition of purified integrin α1β1 or an integrin α1 cytoplasmic peptide to which TCPTP has been shown to bind. Thus, our results demonstrate that pCav-1 is a new substrate of TCPTP and that integrin α1β1 acts as a negative regulator of Cav-1 phosphorylation by activating TCPTP. This could explain the protective function of integrin α1β1 in oxidative stress-mediated damage and why integrin α1-null mice are more susceptible to fibrosis following injury.

Concerted regulation of focal adhesion dynamics by galectin-3 and tyrosine-phosphorylated caveolin-1

Both tyrosine-phosphorylated caveolin-1 (pY14Cav1) and GlcNAc-transferase V (Mgat5) are linked with focal adhesions (FAs); however, their function in this context is unknown. Here, we show that galectin-3 binding to Mgat5-modified N-glycans functions together with pY14Cav1 to stabilize focal adhesion kinase (FAK) within FAs, and thereby promotes FA disassembly and turnover. Expression of the Mgat5/galectin lattice alone induces FAs and cell spreading. However, FAK stabilization in FAs also requires expression of pY14Cav1. In cells lacking the Mgat5/galectin lattice, pY14Cav1 is not sufficient to promote FAK stabilization, FA disassembly, and turnover. In human MDA-435 cancer cells, Cav1 expression, but not mutant Y14FCav1, stabilizes FAK exchange and stimulates de novo FA formation in protrusive cellular regions. Thus, transmembrane crosstalk between the galectin lattice and pY14Cav1 promotes FA turnover by stabilizing FAK within FAs defining previously unknown, interdependent roles for galectin-3 and pY14Cav1 in tumor cell migration.

Internalization of caveolae and their relationship with endosomes in cultured human and mouse endothelial cells

Treatment of cells with pervanadate or vanadate induces the phosphorylation of caveolin-1 and its internalization from the cell surface, but the intracellular fate of caveolae has not been fully elucidated. In the present study, we examined the fate of endocytosed caveolae in human umbilical vein endothelial cells and mouse endothelial KOP2.16 cells. The localization of internalized caveolae and their relationship with the endosomes were examined by immunofluorescence microscopy as well as by immunoprecipitation and chasing of biotinylated transferrin. In untreated cells, caveolin-1 was mostly confined to the cell surface. When cells were treated with either pervanadate for 30 min or vanadate for 3 h, many caveolin-1-labeled vesicles were formed inside the cells, some of which were colocalized with Rab5 or Rab4. The internalized caveolin-1 was colocalized with the endocytosed transferrin in the Rab5-, Rab4- or early endosome antigen-1-labeled compartment where caveolin-1 was phosphorylated. It then moved to the Rabl 1-associated compartment. Immunogold electron microscopy revealed that internalized caveolin-1 colocalized with Rab5 or Rab4 in vesicles larger than caveolae. These results suggest that the internalized caveolae interact with early endosomes.

VEGF signaling through NADPH oxidase-derived ROS

Angiogenesis is a key process involved in normal development and wound repair, as well as ischemic heart and limb diseases, and atherosclerosis. Vascular endothelial growth factor (VEGF), a potent angiogenesis factor, stimulates proliferation, migration, and tube formation of endothelial cells (ECs), primarily through the VEGF receptor type2 (VEGFR2). Reactive oxygen species (ROS) function as signaling molecules to mediate biological responses. In ECs, NADPH oxidase is one of the major sources of ROS and consists of catalytic subunits (Nox1, Nox2, and Nox4), p22phox, p47phox, p67phox, and the small GTPase Rac1. VEGF stimulates ROS production via activation of gp91phox (Nox2)-based NADPH oxidase, and ROS are involved in VEGFR2-mediated signaling linked to EC migration and proliferation. Moreover, ROS derived from NADPH oxidase are involved in postnatal angiogenesis. Localizing NADPH oxidase and its regulators at the specific subcellular compartment is an important mechanism for activating specific redox signaling events. This review focuses on a role of NADPH oxidase-derived ROS in angiogenesis and critical regulators involved in generation of spatially and temporally restricted ROS-dependent VEGF signaling at leading edge, focal adhesions/complexes, caveolae/lipid rafts, and cell-cell junctions in ECs. Understanding these mechanisms should facilitate the development of new therapeutic strategies to modulate new blood vessel formation.

Low Molecular Weight Protein Tyrosine Phosphatase and Caveolin-1: Interaction and Isoenzyme-Dependent Regulation

Low molecular weight protein tyrosine phosphatases (LMW-PTPs) are small enzymes that are ubiquitous in many organisms. They are important in biological processes such as cell proliferation, adhesion, migration, and invasiveness. LMW-PTP is expressed in mammalian cells as two isoforms (IF1 and IF2) originating through alternative splicing. We have previously shown that IF2 targets lipid rafts called caveolae and interacts with caveolin-1, their major structural protein. Caveolae are cholesterol- and sphingolipid-rich membrane microdomains that have been implicated in a variety of cellular functions, including signal transduction events. Caveolin-1 contains a scaffolding region that contributes to the binding of the protein to the plasma membrane and mediates protein omo- and etero-oligomerization. Interaction of many signaling molecules with the scaffolding domain sequesters them into caveolae and inhibits or suppresses their activities. Caveolin-interacting proteins usually have a typical sequence motif, also present in all the LMW-PTPs, which is characterized by aromatic or large hydrophobic residues in specific positions. We have examined here the interaction of the LMW-PTP isoforms with caveolin-1 and its molecular mechanism, together with the consequences for their tyrosine phosphatase activities. We found that IF1 and IF2 are both capable of interacting with defined regions of caveolin-1 and that their putative caveolin binding sequence motif is not responsible for the association. The formation of LMW-PTP/caveolin-1 complexes is accompanied by modulation of the enzyme activities, and the inhibitory effect elicited against IF1 is stronger than that against IF2. The caveolin scaffolding domain is directly involved in the observed phenomena.

White Matter Rafting––Membrane Microdomains in Myelin

The myelin membrane comprises a plethora of regions that are compositionally, ultrastructurally, and functionally distinct. Biochemical dissection of oligodendrocytes, Schwann cells, and central and peripheral nervous system myelin by means such as cold-detergent extraction and differential fractionation has led to the identification of a variety of detergent-resistant membrane assemblies, some of which represent putative signalling platforms. We review here the different microdomains that have hitherto been identified in the myelin membrane, particularly lipid rafts, caveolae, and cellular junctions such as the tight junctions that are found in the radial component of the CNS myelin sheath.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Structure of caveolae

The introduction of the electron microscope to the study of the biological materials in the second half of the last century has dramatically expanded our view and understanding of the inner workings of cells by enabling the discovery and study of subcellular organelles. A population of flask-shaped or spherical invaginations of the plasma membrane were described and named plasmalemmal vesicles or caveolae. Until the discovery of caveolin-1 as their first molecular marker in early 1990s, the study of caveolae was the exclusive domain of electron microscopists that demonstrated caveolae at different surface densities in most mammalian cells with few exceptions. Electron microscopy techniques in combination with other approaches have also revealed the structural features of caveolae as well as some of their protein and lipid residents. This review summarizes the data on the structure and components of caveolae and their stomatal diaphragms.

In Saccharomyces cerevisiae an unbalanced level of tyrosine phosphorylation down-regulates the Ras/PKA pathway

The role of tyrosyl phosphorylation/dephosphorylation in the budding yeast Saccharomyces cerevisiae, whose genome does not encode typical tyrosine kinases, has long remained elusive. Nevertheless, several protein kinases phosphorylating poly(TyrGlu) substrates have been identified. In this work, we use the expression of the low molecular weight tyrosine phosphatase Stp1 from the distantly related yeast Schizosaccharomyces pombe, as a tool to investigate whether an unbalanced level of protein tyrosine phosphorylation affects S. cerevisiae growth and metabolism. We correlate the previously reported down-regulation of the phosphotyrosine level brought about by overexpression of Stp1 with a large number of phenotypes indicative of down-regulation of the Ras pathway. These phenotypes include reduction in both glucose- and acidification-induced GTP loading of the Ras2 protein and cAMP signaling, impaired growth on a non-fermentable carbon source, alteration of cell cycle parameters, delayed recovery from nitrogen starvation, increased heat-shock resistance, attenuated pseudohyphal and invasive growth. Genetic data suggest that Stp1 acts either at, or above, the level of Ras2, possibly on the Ira proteins. Consistently, Stp1 was found to bind to immunoprecipitated Ira2. Since a catalytically inactive mutant form of Stp1 (Stp1(C11S)) effectively binds to Ira2 without producing any effect on yeast physiology, we conclude that down-regulation of the Ras pathway by Stp1 requires its phosphatase activity. In conclusion, our data suggest a possible cross-talk between tyrosine phosphorylation and the Ras pathway in yeast.

Novel role of ARF6 in vascular endothelial growth factor-induced signaling and angiogenesis

Vascular endothelial growth factor (VEGF) stimulates endothelial cell (EC) migration and proliferation primarily through the VEGF receptor-2 (VEGFR2). We have shown that VEGF stimulates a Rac1-dependent NAD(P)H oxidase to produce reactive oxygen species (ROS) that are involved in VEGFR2 autophosphorylation and angiogenic-related responses in ECs. The small GTPase ARF6 is involved in membrane trafficking and cell motility; however, its roles in VEGF signaling and physiological responses in ECs are unknown. In this study, we show that overexpression of dominant-negative ARF6 [ARF6(T27N)] almost completely inhibits VEGF-induced Rac1 activation, ROS production, and VEGFR2 autophosphorylation in ECs. Fractionation of caveolae/lipid raft membranes demonstrates that ARF6, Rac1, and VEGFR2 are localized in caveolin-enriched fractions basally. VEGF stimulation results in the release of VEGFR2 from caveolae/lipid rafts and caveolin-1 without affecting localization of ARF6, Rac1, or caveolin-1 in these fractions. The egress of VEGFR2 from caveolae/lipid rafts is contemporaneous with the tyrosine phosphorylation of caveolin-1 (Tyr14) and VEGFR2 and with their association with each other. ARF6(T27N) significantly inhibits both VEGF-induced responses. Immunofluorescence studies show that activated VEGFR2 and phosphocaveolin colocalize at focal complexes/adhesions after VEGF stimulation. Both overexpression of ARF6(T27N) and mutant caveolin-1(Y14F), which cannot be phosphorylated, block VEGF-stimulated EC migration and proliferation. Moreover, ARF6 expression is markedly upregulated in association with an increase in capillary density in a mouse hindlimb ischemia model of angiogenesis. Thus, ARF6 is involved in the temporal-spatial organization of caveolae/lipid rafts- and ROS-dependent VEGF signaling in ECs as well as in angiogenesis in vivo.

Mechanism of platelet-derived growth factor-dependent caveolin-1 phosphorylation: relationship to sterol binding and the role of serine-80

In human vascular smooth muscle cells, inhibitors of protein kinase C activity reduced serine phosphorylation of caveolin-1 and increased sterol binding by this protein. This was measured after immunoprecipitation of caveolin-1 from cells labeled with tritiated cholesterol or the photoactivable cholesterol analogue FCBP [Fielding et al. (2002) Biochemistry 41, 4929-4937]. At the same time cellular sterol efflux was inhibited. Mutagenesis within a caveolin-1 central domain (residues 80-104) suggested a major role for serine-80 in mediating both of these effects. To perturb sterol binding, platelet-derived growth factor was added to the cells, leading to a transient loss of caveolin-1-associated sterol. Under these conditions, sterol efflux was stimulated, and caveolin-1 phosphorylation at tyrosine(14), assayed with a selective antibody, was substantially increased above baseline levels. These changes were also blocked by inhibitors of protein kinase C activity. Selective inhibitors of the platelet-derived growth factor receptor and downstream kinases were used to show that loss of sterol from caveolin-1 preceded tyrosine phosphorylation, but relipidation was dependent on phosphotyrosine hydrolysis.

Ouabain assembles signaling cascades through the caveolar Na+/K+-ATPase

Based on the observation that the Na(+)/K(+)-ATPase alpha subunit contains two conserved caveolin-binding motifs, we hypothesized that clustering of the Na(+)/K(+)-ATPase and its partners in caveolae facilitates ouabain-activated signal transduction. Glutathione S-transferase pull-down assay showed that the Na(+)/K(+)-ATPase bound to the N terminus of caveolin-1. Significantly, ouabain regulated the interaction in a time- and dose-dependent manner and stimulated tyrosine phosphorylation of caveolin-1 in LLC-PK1 cells. When added to the isolated membrane fractions, ouabain increased tyrosine phosphorylation of proteins from the isolated caveolae but not other membrane fractions. Consistently, ouabain induced the formation of a Na(+)/K(+)-ATPase-Src-caveolin complex in the isolated caveolae preparations as it did in live cells. Finally, depletion of either cholesterol by methyl beta-cyclodextrin or caveolin-1 by siRNA significantly reduced the caveolar Na(+)/K(+)-ATPase and Src. Concomitantly, cholesterol depletion abolished ouabain-induced recruitment of Src to the Na(+)/K(+)-ATPase signaling complex. Like depletion of caveolin-1, it also blocked the effect of ouabain on ERKs, which was restored after cholesterol repletion. Clearly, the caveolar Na(+)/K(+)-ATPase represents the signaling pool of the pump that interacts with Src and transmits the ouabain signals.

Relationship between cholesterol trafficking and signaling in rafts and caveolae

Caveolae and lipid rafts are two distinct populations of free cholesterol, sphingolipid (FC/SPH)-rich cell surface microdomains. They differ in stability, shape, and the presence or absence of caveolin (present in caveolae) or GPI-anchored proteins (enriched in lipid rafts). In primary cells, caveolae and rafts support the assembly of different signaling complexes, though signal transduction from both is strongly dependent on the presence of FC. It was initially thought that FC promoted the formation of inactive reservoirs of signaling proteins. Recent data supports the concept of a more dynamic role for FC in caveolae and probably, also lipid rafts. It is more likely that the FC content of these domains is actively modulated as protein complexes are formed and, following signal transduction, disassembled. In transformed cell lines with few caveolae, little caveolin and a preponderance of rafts, complexes normally assembled on caveolae may function in rafts, albeit with altered kinetics. However, caveolae and lipid rafts appear not to be interconvertible. The presence of non-caveolar pools of caveolin in recycling endosomes (RE), the trans-Golgi network (TGN) and in mobile chaperone complexes is now recognized. A role in the uptake of microorganisms by cells ascribed to caveolae now seems more likely to be mediated by cell surface rafts.

Regulation of vascular endothelial growth factor receptor-2 activity by caveolin-1 and plasma membrane cholesterol

The stimulation of vascular endothelial growth factor receptor-2 (VEGFR-2) by tumor-derived VEGF represents a key event in the initiation of angiogenesis. In this work, we report that VEGFR-2 is localized in endothelial caveolae, associated with caveolin-1, and that this complex is rapidly dissociated upon stimulation with VEGF. The kinetics of caveolin-1 dissociation correlated with those of VEGF-dependent VEGFR-2 tyrosine phosphorylation, suggesting that caveolin-1 acts as a negative regulator of VEGF R-2 activity. Interestingly, we observed that in an overexpression system in which VEGFR-2 is constitutively active, caveolin-1 overexpression inhibits VEGFR-2 activity but allows VEGFR-2 to undergo VEGF-dependent activation, suggesting that caveolin-1 can confer ligand dependency to a receptor system. Removal of caveolin and VEGFR-2 from caveolae by cholesterol depletion resulted in an increase in both basal and VEGF-induced phosphorylation of VEGFR-2, but led to the inhibition of VEGF-induced ERK activation and endothelial cell migration, suggesting that localization of VEGFR-2 to these domains is crucial for VEGF-mediated signaling. Dissociation of the VEGFR-2/caveolin-1 complex by VEGF or cyclodextrin led to a PP2-sensitive phosphorylation of caveolin-1 on tyrosine 14, suggesting the participation of Src family kinases in this process. Overall, these results suggest that caveolin-1 plays multiple roles in the VEGF-induced signaling cascade.

Some protein tyrosine phosphatases target in part to lipid rafts and interact with caveolin-1

A profile-based search of the SWISS-PROT database reveals that most protein tyrosine phosphatases (PTPs) contain at least one caveolin-1-binding motif. To ascertain if the presence of caveolin-binding motif(s) in PTPs corresponds to their actual localization in caveolin-1-enriched membrane fractions, we performed subcellular fractionating experiments. We found that all tested PTPs (PTP1B, PTP1C, SHPTP2, PTEN, and LAR) are actually localized in caveolin-enriched membrane fractions, despite their distribution in other subcellular sites, too. More than 1/2 of LAR and about 1/4 of SHPTP2 and PTP-1C are localized in caveolin-enriched membrane fractions whereas, in these fractions, PTP-1B and PTEN are poorly concentrated. Co-immunoprecipitation experiments with antibodies specific for each tested PTP demonstrated that all five phosphatases form molecular complexes with caveolin-1 in vivo. Collectively, our findings propose that particular PTPs could perform some of their cellular actions or are regulated by recruitment into caveolin-enriched membrane fractions.

Respiratory syncytial virus assembly occurs in GM1-rich regions of the host-cell membrane and alters the cellular distribution of tyrosine phosphorylated caveolin-1

We have previously shown that respiratory syncytial virus (RSV) assembly occurs within regions of the host-cell surface membrane that are enriched in the protein caveolin-1 (cav-1). In this report, we have employed immunofluorescence microscopy to further examine the RSV assembly process. Our results show that RSV matures at regions of the cell surface that, in addition to cav-1, are enriched in the lipid-raft ganglioside GM1. Furthermore, a comparison of mock-infected and RSV-infected cells by confocal microscopy revealed a significant change in the cellular distribution of phosphocaveolin-1 (pcav-1). In mock-infected cells, pcav-1 was located at regions of the cell that interact with the extracellular matrix, termed focal adhesions (FA). In contrast, RSV-infected cells showed both a decrease in the levels of pcav-1 associated with FA and the appearance of pcav-1-containing cytoplasmic vesicles, the latter being absent in mock-infected cells. These cytoplasmic vesicles were clearly visible between 9 and 18 h post-infection and coincided with the formation of RSV filaments, although we did not observe a direct association of pcav-1 with mature virus. In addition, we noted a strong colocalization between pcav-1 and growth hormone receptor binding protein-7 (Grb7), within these cytoplasmic vesicles, which was not observed in mock-infected cells. Collectively, these findings show that the RSV assembly process occurs within specialized lipid-raft structures on the host-cell plasma membrane, induces the cellular redistribution of pcav-1 and results in the formation of cytoplasmic vesicles that contain both pcav-1 and Grb7.

Localization of neutral ceramidase in caveolin-enriched light membranes of murine endothelial cells

Sphingomyelinase (SMase) and ceramidase (CDase) activities participate in sphingomyelin (SM) metabolism and have a role in the signal transduction of a variety of ligands. In this study evidence is presented that caveolin-enriched light membranes (CELMs) of murine endothelial cells, characterized by high SM, ceramide (Cer) and cholesterol content, bear acid and neutral SMase as well as neutral CDase activities. Localization of neutral CDase in CELMs was confirmed by Western analysis. Notably, cell treatment with cyclodextrin, which depleted cell cholesterol, did not affect acid or neutral SMase activities but significantly enhanced neutral CDase activity in CELMs, indicating a negative role for cholesterol in CDase regulation. These findings suggest that neutral CDase is implicated, together with SMase activities, in the control of caveolar Cer content that may be critical for caveola dynamics.