In-vitro-Rekonstitution des vesikulären Transportes zu Recycling-Endosomen und Analyse von sekretiertem Caveolin-1 aus humanen Prostatakrebszellen

Der Transferrin-Zyklus ist ein wichtiges Modell für denintrazellulären Transport, daher sollten in der vorliegendenArbeit einzelne, immer noch unverstandene Prozesse desvesikulären, intrazellulären Transportes durch dieCharakterisierung einen in vitro-Transportassay untersuchtwerden. Der Ansatz eines in vitro-Systems wurde deshalbgewählt, um mit Experimenten in denen einzelne Faktoren undbestimmte Konditionen untersucht werden sollten, diese unterdefinierten, reproduzierbaren Konditionen durchzuführen, diein einem in vivo-System kaum zu gewährleisten sind. Ohne denEinfluss von „störenden“, weil unkontrollierten Faktoren,wie es bei in vivo-Systemen der Fall ist, konnte imvorliegende Ansatz der Transport zu immunisoliertenRecycling-Endosomen (die Isolierung erfolgte hierbei mitanti-Rab11-Antikörpern, einem Marker fürRecycling-Endosomen) unter bestimmten Bedingungen untersuchtwerden. Dabei wurde als Marker Acridinium-markiertesTransferrin gewählt, welches in Zellen internalisiert wurde.Die Spezifität des Transportes in dem zellfreien System warhierbei sehr hoch, wie Kontrollexperimente inImmunisolierungsansätzen ohne Rab11-Antikörper zeigten. ImRahmen einer ersten Charakterisierung des Transportassayswurden essentielle, für den in vivo-Transport essentielleParameter auch in den in vitro-Experimenten untersucht.Hierbei wurde zum einen der Faktor Temperatur gewählt, daTransport in Zellen bei 4°C in der Regel zum Erliegen kommt.Dies konnte auch in dem vorgestellten System gezeigt werden.Ein weiterer, essentieller Faktor ist Energie in Form vonATP. ATP-Depletion wurde in den Experimenten durch Hinzugabeeines ATP-erschöpfenden Systems erzielt. Auch hier zeigteder Transport von Ac-Tfn zu Recycling-Endosomen eine starkeInhibierung. Mit Hilfe des so charakterisierten Assayskonnten anschließend weitere Experimente durchgeführtwerden, die den Einfluss von bestimmten Reagenzien undKonditionen auf den Transport untersuchten. So zeigte derTransport in Zeitverlaufsexperimenten einen Anstieg desTransportes bis 30 Minuten, bei 30 Minuten wurde ein Maximumerreicht. Nach Erreichen dieses Maximums war nachfolgendeine leichte Abnahme des Transfers von Ac-Tfn zu denRecycling-Endosomen zu beobachten. Da Rab-Proteine alsSchlüsselregulatoren für den intrazellulären, vesikulärenTransport gelten, und die Immunisolierungen mitanti-Rab11-Antikörpern durchgeführt wurden, wurde somit auchder Einfluss dieser GTPasen auf das Transportsystemuntersucht. Zugegebenes GDI, welches in der Lage istRab-Proteine in GDP-gebundener Form von spezifischenMembranen zu extrahieren, und daher ein gut untersuchterInhibitor von Rab-Funktionen ist, konnte auch in diesemTransportassay den Transport von Transferrin inhibieren. Einweiterer Aspekt war die Rolle des Cytoskelettes imintrazellulären Transport. Da in früheren Untersuchungen(Trischler et al., 1999) Aktin auf Recycling-Endosomengefunden wurde, erfolgte in diesen Arbeiten eineKonzentration auf die Rolle des Aktin in diesenTransportprozessen. Durch die Zugabe von Cytochalasin D, daseinen Aufbau von Aktingerüsten verhindert, wurde derTransport ebenfalls inhibiert. Durchaffinitätschromatographische Aufreinigungen konnte einestarke Interaktion von Aktin an immobilisiertes Rab11gezeigt werden. Die eluierten Fraktionen, die neben Aktinnoch weitere, jedoch unbekannte Proteine enthielten, konntenin dem in vitro-Fusionsassay eingesetzt werden und führtendort zu einer Stimulation des Transportes. Neben demgefundenen Aktin, könnten somit noch weitere, unbekannteProteine in dem Proteingemisch wichtige Funktionen imintrazellulären, vesikulären Transport übernehmen. EineIdentifizierung dieser Proteine ist dabei für weiterführendeArbeiten essentiell.Caveolin-1, Markerprotein für die Caveolae-Membrandomänewird überraschenderweise von verschiedenen Zellensekretiert. Da Caveolin-1 normalerweise ein integralesMembranprotein ist, wird von einer Sekretion alsLipoproteinpartikel ausgegangen. Die Rolle diesessekretierten Partikels ist unbekannt, wobei einige Autoreneine Funktion als autokrinen/ parakrinen Faktor vorschlagen(Tahir et al., 2001). In der vorliegenden Arbeit solltendiese Partikel daher aufgereinigt und erstmalscharakterisiert werden. Die Partikel wurden aus transienttransfizierten LNCaP-Zellen gewonnen, die Cav-1 in dasserumfreie Medium abgaben. In einer erstenGrößenuntersuchung durch FPLC konnte ein Molekulargewichtzwischen 2.000.000 Da und 660.000 Da bestimmt werden. DieseResultate konnten durch den Ansatz der nativenBlau-Gelelektrophorese bestätigt werden. In einem weiterenAnsatz, der die Dichte der Partikel charakterisieren sollte,wurde in zwei unterschiedlichen Ansätzen (CsCl-, sowieOptiprep Dichtezentrifuagtion) eine ähnliche Dichte desPartikels wie HDL ermittelt. Um eine stärkere Aufreinigungder Partikel zu erzielen, wurde eine Aufreinigung mit Hilfevon Ni-NTA-Agarose durchgeführt. Dies war möglich, denn diebei der Transfektion verwendete C-DNA trug einen His6-tag.Die so aufgereingten Partikel verloren auch nach derNi-NTA-Chromatographie nicht ihre biochemischenEigenschaften, wie in überprüfenden CsCl-Gradienten zu sehenwar. Die Partikel konnten anschließend zum ersten Mal inelektronenmikroskopischen Aufnahmen (Negativkontrastierung)visualisiert werden. Ein weiteres Ziel dieser Arbeit war es,zu untersuchen ob auf Cav-1 Lipoproteinpartikeln nochweitere Proteine zu finden waren. Durch eine kombinierteAufreinigung über Ni-NTA Chromatographie undCsCl-Dichtezentrifugation und im Vergleich mit demAusgangsmaterial konnten in der Silberfärbung Proteinbandenerkannt werden, die wie Cav-1 in den Fraktionen angereichertvorlagen. Eine massenspektroskopische Identifikation einerder Banden ergab, dass es sich hierbei um nm 23(Nukleosid-diphosphat-kinase) handelte, einem Protein dasebenfalls von verschiedenen Tumoren sekretiert wird.

Role of caveolin-1 in the proliferation of solid tumours in vitro

Caveolin-1 (Cav-1), the essential structural constituent of caveolae, which are flask-shaped invaginations of the plasma membrane, has been found to play a key role in the modulation of cell proliferation and cancer development. It seems to act as an oncosuppressor or a promoter of growth, depending on the histotype, stage and grade of each tumour. The aim of this study was to analyze the effects of Caveolin-1 gene silencing on the proliferation of human lung cancer and osteosarcoma in vitro. Our data show that Cav-1 silencing blocks the growth in both metastatic lung cancer cell lines analyzed, suggesting a proliferation promoting action of the protein in these cells. A marked decrease of phospho-Akt, phospho-ERK, STAT3, cyclin D1, CDK4 and consequently of phospho-Rb expression was evident in the cells treated with Cav-1 siRNA. With regards to osteosarcoma, we demonstrated that the suppression of Cav-1 results in the blocking of MG-63 and in the slowing down of HOS proliferation, suggesting a role for Cav-1 as a promoter of tumour growth in these cell lines. A marked decrease of phospho-Akt, cyclin E, CDK2 and phospho-Rb and an increase of p21 expression levels were evident in the cells treated with Cav-1 siRNA. Our results suggest two new cell cycle inhibiting pathways, mediated by Cav-1 knock-down, and provide new insights into the molecular mechanisms underlying the tumour-promoting role of Cav-1 in lung cancer and osteosarcoma. In this work we also investigated the role of estrogens in lung cancer and the functional cross-talk between Cav-1 and estrogens/estrogen receptors in it. Our results show that 17β-estradiol induces proliferation either in RAL or in SCLC-R1 cells and that both cell lines are sensitive to 4-OHT antiproliferative effect. The sensitivity to estrogen stimulation seems to be gender- and/or histological type-independent in metastatic lung cancer in vitro.

Mechanismen der Caveolin-Genregulation in neuralen Zellsystemen

Caveolae sind vesikuläre Invaginationen der eukaryontischen Zellmembran, die bei einer Vielzahl zellbiologischer Prozesse eine bedeutende Rolle spielen. Die strukturellen und funktionellen Hauptbestandteile der Caveolae sind die Caveolin-Proteine, welche von drei homologen Genen (Caveolin-1,-2,-3) kodiert werden. Die Caveoline stellen die Struktur-Organisatoren der Caveolae dar, und regulieren direkt die Aktivität von zahlreichen Caveolae-assoziierten Rezeptorproteinen und Signalmolekülen. Oftmals werden die pleiotropen Effekte der Caveoline über eine Veränderung der Caveolin-Genexpressionsstärke moduliert. In der vorliegenden Arbeit wurden drei unterschiedliche biologische Steuerfaktoren identifiziert, unter deren Kontrolle die Caveolin-Genexpression in neuralen Zellsystemen steht. Bei diesen Faktoren handelt es sich um das Steroidhormon Oestrogen und seine Rezeptoren, den Wachstumsfaktor TGFa und den sekundären Botenstoff zyklisches AMP (cAMP). Oestrogen wirkt über die Aktivierung von Oestrogen-Rezeptoren (ERs) im zentralen Nervensystem in der Regel als neurotropher Faktor. In der vorliegenden Arbeit konnte erstmalig gezeigt werden, daß in humanen Neuroblastom-Zellen (SK-N-MC) die stabile, rekombinante Expression des ERa-Subtyps zu einer drastischen Reduktion der Caveolin-1/-2-Transkription führt, und daß in der Folge die zelluläre Caveolin-Biosynthese eingestellt wird. Eine Analyse des Caveolin-1-Gens ergab, daß einhergehend mit der Inaktivierung der Caveolin-1-Transkription eine Vielzahl der im Promoter enthaltenen CpG-Dinukleotide methyliert vorliegen. Durch pharmakologische Inhibition der nukleären DNA-Methyltransferasen sowie der Histon-Deacetylasen konnte die Caveolin-1-Transkription teilweise wiederhergestellt werden. Diese Befunde lassen auf die Existenz eines DNA-Methylierungs-abhängigen Stilllegungsmechanismus der Caveolin-Genexpression durch ERa schließen. Dagegen führte die Überexpression des ERb-Subtyps in SK-N-MC-Zellen zu keiner Veränderung der Caveolin-1/-2-Expression. Interessanterweise wurde die supprimierende Wirkung des ERa durch die gleichzeitige Überexpression des ERb vollständig aufgehoben. Der mitogene Wachstumsfaktor TGFa wurde als zweites extrazelluläres Signalmolekül identifiziert, welches eine Reduktion der Caveolin-1/-2-Genexpression bewirkt. In primären kortikalen Astrozyten konnte gezeigt werden, daß TGFa seine supprimierende Wirkung auf die Caveolin-1-Expression partiell über die Aktivierung des PI3-Kinase-abhängigen Signalweges vermittelt. Zudem wurde die supprimierende Wirkung von TGFa durch einen Inhibitior der Histon-Deacetylasen relativiert. Daher scheinen sowohl für den ERa als auch für TGFa epigenetische Prozesse bei der Suppression der Caveolin-1-Genexpression eine entscheidende Rolle zu spielen. Intrazellulär wirkte neben der PI3-Kinase auch der Botenstoff cAMP in kortikalen Astrozyten als Suppressor der Caveolin-Genexpression. Es wäre denkbar, daß die Caveolin-Suppression funktioneller Bestandteil des seit langem etablierten Effekts der cAMP-induzierten Astrozyten-Differenzierung ist. Desweiteren wiesen der cAMP- und TGFa-abhängige Signalweg ein überlappendes, Gehirnregion-spezifisches Regulationsprofil der Caveolin-Expression in Astrozyten auf: während in Kortex und Striatum eine Regulation durch cAMP und TGFa erfolgte, blieb diese in Klein- und Zwischenhirn aus. Somit bewirken drei zentrale regulatorische Faktoren der Proliferation und Differenzierung neuraler Zellen eine Reduktion in der Konzentration der pleiotrop funktionellen Caveoline. Zukünftige Studien müssen zeigen, inwieweit die reduzierte Caveolin-Expression für die morphologischen und biochemischen Primärwirkungen dieser Faktoren während der Entwicklung und im Zuge der Tumorgenese mitverantwortlich ist. Außerdem könnten über die Beobachtungen der zellbiologischen Auswirkungen reduzierter Caveolin-Spiegel neue Erkenntnisse über die Funktion dieser Proteine gewonnen werden.

The Ras inhibitors caveolin-1 and docking protein 1 activate peroxisome proliferator-activated receptor ? through spatial relocalization at helix 7 of its ligand-binding domain

Peroxisome proliferator-activated receptor ? (PPAR?) is a transcription factor that promotes differentiation and cell survival in the stomach. PPAR? upregulates and interacts with caveolin-1 (Cav1), a scaffold protein of Ras/mitogen-activated protein kinases (MAPKs). The cytoplasmic-to-nuclear localization of PPAR? is altered in gastric cancer (GC) patients, suggesting a so-far-unknown role for Cav1 in spatial regulation of PPAR? signaling. We show here that loss of Cav1 accelerated proliferation of normal stomach and GC cells in vitro and in vivo. Downregulation of Cav1 increased Ras/MAPK-dependent phosphorylation of serine 84 in PPAR? and enhanced nuclear translocation and ligand-independent transcription of PPAR? target genes. In contrast, Cav1 overexpression sequestered PPAR? in the cytosol through interaction of the Cav1 scaffolding domain (CSD) with a conserved hydrophobic motif in helix 7 of PPAR?'s ligand-binding domain. Cav1 cooperated with the endogenous Ras/MAPK inhibitor docking protein 1 (Dok1) to promote the ligand-dependent transcriptional activity of PPAR? and to inhibit cell proliferation. Ligand-activated PPAR? also reduced tumor growth and upregulated the Ras/MAPK inhibitors Cav1 and Dok1 in a murine model of GC. These results suggest a novel mechanism of PPAR? regulation by which Ras/MAPK inhibitors act as scaffold proteins that sequester and sensitize PPAR? to ligands, limiting proliferation of gastric epithelial cells.

Caveolin-1 opens endothelial cell junctions by targeting catenins

A fundamental phenomenon in inflammation is the loss of endothelial barrier function, in which the opening of endothelial cell junctions plays a central role. However, the molecular mechanisms that ultimately open the cell junctions are largely unknown.

Caveolin-1 is required for signaling and membrane targeting of EphB1 receptor tyrosine kinase

Eph receptor tyrosine kinases are key players during the development of the embryonic vasculature; however, their role and regulation in adult angiogenesis remain to be defined. Caveolae are flask-shaped invaginations of the cell membrane; their major structural protein, caveolin-1, has been shown to regulate signaling molecules localized in these micro-domains. The interaction of caveolin-1 with several of these proteins is mediated by the binding of its scaffolding domain to a region containing hydrophobic amino acids within these proteins. The presence of such a motif within the EphB1 kinase domain prompted us to investigate the caveolar localization and regulation of EphB1 by caveolin-1. We report that EphB1 receptors are localized in caveolae, and directly interact with caveolin-1 upon ligand stimulation. This interaction, as well as EphB1-mediated activation of extracellular-signal-regulated kinase (ERK), was abrogated by overexpression of a caveolin-1 mutant lacking a functional scaffolding domain. Interaction between Ephs and caveolin-1 is not restricted to the B-subclass of receptors, since we show that EphA2 also interacts with caveolin-1. Furthermore, we demonstrate that the caveolin-binding motif within the kinase domain of EphB1 is primordial for its correct membrane targeting. Taken together, our findings establish caveolin-1 as an important regulator of downstream signaling and membrane targeting of EphB1.

TRPC1 is regulated by caveolin-1 and is involved in oxidized LDL-induced apoptosis of vascular smooth muscle cells

Oxidized low-density lipoprotein (oxLDL) induced-apoptosis of vascular cells may participate in plaque instability and rupture. We have previously shown that vascular smooth muscle cells (VSMC) stably expressing caveolin-1 were more susceptible to oxLDL-induced apoptosis than VSMC expressing lower level of caveolin-1, and this was correlated with enhanced Ca(2+) entry and pro-apoptotic events. In this study we aimed to identify the molecular events involved in oxLDL-induced Ca(2+) influx and their regulation by the structural protein caveolin-1. In VSMC, transient receptor potential canonical-1 (TRPC1) silencing by ARN interference, prevents the Ca(2+) influx and reduces the toxicity induced by oxLDL. Moreover, caveolin-1 silencing induces concomitant decrease of TRPC1 expression and reduces oxLDL-induced-apoptosis of VSMC. OxLDL enhanced the cell surface expression of TRPC1, as shown by biotinylation of cell surface proteins, and induced TRPC1 translocation into caveolar compartment, as assessed by subcellular fractionation. OxLDL-induced TRPC1 translocation was dependent on actin cytoskeleton and associated with a dramatic rise of 7-ketocholesterol (a major oxysterol in oxLDL) into caveolar membranes, whereas the caveolar content of cholesterol was unchanged. Altogether, the reported results show that TRPC1 channels play a role in Ca(2+) influx and Ca(2+) homeostasis deregulation that mediate apoptosis induced by oxLDL. These data also shed new light on the role of caveolin-1 and caveolar compartment as important regulators of TRPC1 trafficking to the plasma membrane and apoptotic processes that play a major role in atherosclerosis.

Caveolin-1 sensitizes vascular smooth muscle cells to mildly oxidized LDL-induced apoptosis

Oxidized low-density lipoprotein (oxLDL)-induced apoptosis of vascular cells may participate to plaque instability and rupture. Caveolin-1 has emerged as an important regulator of several signal transduction pathways and processes that play a role in atherosclerosis. In this study we examined the potential role of caveolin-1 in the regulation of oxLDL-induced Ca(2+) signaling and apoptosis in vascular smooth muscle cells (VSMC). Cells expressing caveolin-1 were more susceptible to oxLDL-induced apoptosis, and this was correlated with enhanced Ca(2+) entry and pro-apoptotic events. Moreover, caveolin-1 silencing by small interfering RNA decreased the level of apoptotic cells after oxLDL treatment. These findings provide new insights about the potential role of caveolin-1 in the regulation of oxLDL-induced apoptosis in vascular cells and its contribution to the instability of the plaque.

Two sterol regulatory element-like sequences mediate up-regulation of caveolin gene transcription in response to low density lipoprotein free cholesterol

Caveolae form the terminus for a major pathway of intracellular free cholesterol (FC) transport. Caveolin mRNA levels in confluent human skin fibroblasts were up-regulated following increased uptake of low density lipoprotein (LDL) FC. The increase induced by FC was not associated with detectable change in mRNA stability, indicating that caveolin mRNA levels were mediated at the level of gene transcription. A total of 924 bp of 5′ flanking region of the caveolin gene were cloned and sequenced. The promoter sequence included three G+C-rich potential sterol regulatory elements (SREs), a CAAT sequence and a Sp1 consensus sequence. Deletional mutagenesis of individual SRE-like sequences indicated that of these two (at −646 and −395 bp) were essential for the increased transcription rates mediated by LDL-FC, whereas the third was inconsequential. Gel shift analysis of protein binding from nuclear extracts to these caveolin promoter DNA sequences, together with DNase I footprinting, confirmed nucleoprotein binding to the SRE-like elements as part of the transcriptional response to LDL-FC. A supershift obtained with antibody to SRE-binding protein 1 (SPEBP-1) indicated that this protein binds at −395 bp. There was no reaction at −395 bp with anti-Sp1 antibody nor with either antibody at −646 bp. The cysteine protease inhibitor N-acetyl-leu-leu-norleucinal (ALLN), which inhibits SREBP catabolism, superinhibited caveolin mRNA levels regardless of LDL-FC. This finding suggests that SREBP inhibits caveolin gene transcription in contrast to its stimulating effect on other promoters. The findings of this study are consistent with the postulated role for caveolin as a regulator of cellular FC homeostasis in quiescent peripheral cells, and the coordinate regulation by SREBP of FC influx and efflux.

Tyrosine-phosphorylated Caveolin-1: Immunolocalization and Molecular Characterization

Caveolin-1 was discovered as a major substrate for v-Src, but the effect of its tyrosine phosphorylation has not been known. We generated a specific antibody (PY14) to caveolin-1 phosphorylated at tyrosine 14 and studied the significance of the modification. By Western blotting of lysates of v-Src–expressing cells, PY14 recognized not only a 22-kDa band (the position of nonphosphorylated caveolin-1) but bands at 23–24 and 25 kDa. Bands of slower mobility were diminished by dephosphorylation and were also observed for mutant caveolin-1 lacking tyrosine 14. By immunofluorescence microscopy, PY14 did not label normal cells but detected large dots in v-Src–expressing cells. Immunoelectron microscopy revealed that the dots corresponded to aggregated caveolae and/or vesicles of various sizes; besides, the label was observed in intramembrane particle-free areas in the plasma membrane, which appeared to have been formed by fusion of flattened caveolae. A positive reaction with PY14 was found in normal cells after vanadate or pervanadate treatment; it occurred mainly at 22 kDa by Western blotting and was not seen as large dots by immunofluorescence microscopy. Detergent solubility, oligomerization, and association with caveolin-2 were observed similarly for caveolin-1 in normal and v-Src–expressing cells. The results indicate that phosphorylation of caveolin-1 in v-Src–expressing cells occurs at multiple residues and induces flattening, aggregation, and fusion of caveolae and/or caveolae-derived vesicles.

Segregation of Heterotrimeric G Proteins in Cell Surface Microdomains: Gq Binds Caveolin to Concentrate in Caveolae, whereas Gi and Gs Target Lipid Rafts by Default

Select lipid-anchored proteins such as glycosylphosphatidylinositol (GPI)-anchored proteins and nonreceptor tyrosine kinases may preferentially partition into sphingomyelin-rich and cholesterol-rich plasmalemmal microdomains, thereby acquiring resistance to detergent extraction. Two such domains, caveolae and lipid rafts, are morphologically and biochemically distinct, contain many signaling molecules, and may function in compartmentalizing cell surface signaling. Subfractionation and confocal immunofluorescence microscopy reveal that, in lung tissue and in cultured endothelial and epithelial cells, heterotrimeric G proteins (Gi, Gq, Gs, and Gβγ) target discrete cell surface microdomains. Gq specifically concentrates in caveolae, whereas Gi and Gs concentrate much more in lipid rafts marked by GPI-anchored proteins (5′ nucleotidase and folate receptor). Gq, apparently without Gβγ subunits, stably associates with plasmalemmal and cytosolic caveolin. Gi and Gs interact with Gβγ subunits but not caveolin. Gi and Gs, unlike Gq, readily move out of caveolae. Thus, caveolin may function as a scaffold to trap, concentrate, and stabilize Gq preferentially within caveolae over lipid rafts. In N2a cells lacking caveolae and caveolin, Gq, Gi, and Gs all concentrate in lipid rafts as a complex with Gβγ. Without effective physiological interaction with caveolin, G proteins tend by default to segregate in lipid rafts. The ramifications of the segregated microdomain distribution and the Gq-caveolin complex without Gβγ for trafficking, signaling, and mechanotransduction are discussed.

Identification, sequence, and expression of caveolin-2 defines a caveolin gene family.

Caveolin, a 21- to 24-kDa integral membrane protein, is a principal component of caveolae membranes. Caveolin interacts directly with heterotrimeric guanine nucleotide binding proteins (G proteins) and can functionally regulate their activity. Here, an approximately 20-kDa caveolin-related protein, caveolin-2, was identified through microsequencing of adipocyte-derived caveolin-enriched membranes; caveolin was retermed caveolin-1. Caveolins 1 and 2 are similar in most respects. mRNAs for both caveolin-1 and caveolin-2 are most abundantly expressed in white adipose tissue and are induced during adipocyte differentiation. Caveolin-2 colocalizes with caveolin-1, indicating that caveolin-2 also localizes to caveolae. However, caveolin-1 and caveolin-2 differ in their functional interactions with heterotrimeric G proteins, possibly explaining why caveolin-1 and -2 are coexpressed within a single cell.

VIP21/caveolin is a cholesterol-binding protein.

VIP21/caveolin is localized to both caveolae and apical transport vesicles and presumably cycles between the cell surface and the Golgi complex. We have studied the lipid interactions of this protein by reconstituting Escherichia coli-expressed VIP21/caveolin into liposomes. Surprisingly, the protein reconstituted only with cholesterol-containing lipid mixtures. We demonstrated that the protein binds at least 1 mol of cholesterol per mole of protein and that this binding promotes formation of protein oligomers. These findings suggest that VIP21/caveolin, through its cholesterol-binding properties, serves a specific function in microdomain formation during membrane trafficking.

Oligomeric structure of caveolin: implications for caveolae membrane organization.

A 22-kDa protein, caveolin, is localized to the cytoplasmic surface of plasma membrane specializations called caveolae. We have proposed that caveolin may function as a scaffolding protein to organize and concentrate signaling molecules within caveolae. Here, we show that caveolin interacts with itself to form homooligomers. Electron microscopic visualization of these purified caveolin homooligomers demonstrates that they appear as individual spherical particles. By using recombinant expression of caveolin as a glutathione S-transferase fusion protein, we have defined a region of caveolin's cytoplasmic N-terminal domain that mediates these caveolin-caveolin interactions. We suggest that caveolin homooligomers may function to concentrate caveolin-interacting molecules within caveolae. In this regard, it may be useful to think of caveolin homooligomers as "fishing lures" with multiple "hooks" or attachment sites for caveolin-interacting molecules.

De novo formation of caveolae in lymphocytes by expression of VIP21-caveolin.

Caveolae are plasma membrane invaginations, which have been implicated in endothelial transcytosis, endocytosis, potocytosis, and signal transduction. In addition to their well-defined morphology, caveolae are characterized by the presence of an integral membrane protein termed VIP21-caveolin. We have recently observed that lymphocytes have no detectable VIP21-caveolin and lack plasma membrane invaginations resembling caveolae. Here we transiently express VIP21-caveolin in a lymphocyte cell line using the Semliki Forest virus expression system and show de novo formation of plasma membrane invaginations containing VIP21-caveolin. These invaginations appear homogeneous in size and morphologically indistinguishable from caveolae of nonlymphoid cells. Moreover, the glycosylphosphatidylinositol-anchored protein. Thy1, patched by antibodies, redistributes to the newly formed caveolae. Our results show that VIP21-caveolin is a key structural component required for caveolar biogenesis.