The caveolin proteins

Selective COX-2 inhibitors modulate cellular senescence in human dermal fibroblasts in a catalytic activity-independent manner

It has been recently proposed that pro-inflammatory genes such as cyclooxygenase-2 (COX-2) play a key role in the aging process. However, it remains unclear whether the pro-inflammatory activity of COX-2 is involved in the aging process and whether COX-2 inhibitors prevent aging. We therefore examined the effect of COX-2 inhibitors on aging in the cellular senescence model of human dermal fibroblasts (HDFs). While the catalytic activity of COX-2 was observed to increase in the senescence process, we found that among three selective COX-2 inhibitors studied, only NS-398 inhibited the senescence whereas celecoxib and nimesulide accelerated the senescence. Non-selective COX inhibitors including aspirin, ibuprofen and flurbiprofen accelerated the senescence. The senescence-regulating effect of selective COX-2 inhibitors had no correlation with cellular reactive oxygen species levels, NF-kappaB activities or protein levels of p53 and p21. We instead found that selective COX-2 inhibitors regulate caveolin-1 expression at transcriptional levels, which was closely associated with the inhibitors' effect on the senescence. Collectively, these results suggest that COX-2 catalytic activity does not mediate HDF senescence and that selective COX-2 inhibitors modulate HDF senescence by a catalytic activity-independent mechanism.

Caveolae in fibroblast-like synoviocytes: static structures associated with vimentin-based intermediate filaments

The fibroblast-like synoviocyte is a CD13-positive cell-type containing numerous caveolae, both single and interconnected clusters. In unstimulated cells, all single caveolae at the cell surface and the majority of those localized deeper into the cytoplasm were freely accessible from the medium, as judged from electron microscopy of synoviocytes exposed to the membrane impermeable marker Ruthenium Red. Caveolar internalization could be induced by a CD13 antibody or by cholera toxin B subunit (CTB). Thus, in experiments using sequential labeling with Alexa 488- and 594-conjugated CTB, about 50% of CTB-positive caveolae were internalized by 5 min of chase, and these remained inaccessible from the cell surface for periods up to 24 h. No colocalization with an endosomal marker, EEA1, or Lysotracker was observed, indicating that internalized caveolae clusters represent a static compartment. Vimentin was identified as the most abundant protein in detergent resistant membranes (DRM's), and by immunogold electron microscopy caveolae were seen in intimate contact with intermediate-size filaments. These observations indicate that vimentin-based filaments are responsible for the spatio-temporal fixation of caveolae clusters. RECK, a glycosylphosphatidylinositol-anchored protein acting as a negative regulator of cell surface metalloproteinases, was also localized to the caveolae clusters. We propose that these clusters function as static reservoirs of specialized lipid raft domains where proteins involved in cell-cell interactions, such as CD13, can be sequestered by binding to RECK in a regulatory manner.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Filamin A is a novel caveolin-1-dependent target in IGF-I-stimulated cancer cell migration

Caveolin-1 is an essential structural constituent of caveolae which is involved in regulation of mitogenic signaling and oncogenesis. Caveolin-1 has been implicated in cell migration but its exact role and mechanism of action in this process remained obscure. We have previously reported that expression of caveolin-1 in stably transfected MCF-7 human breast cancer (MCF-7/Cav1) cells up-regulates phosphorylation of a putative Akt substrate protein, designated pp340 [D. Ravid, S. Maor, H. Werner, M. Liscovitch, Caveolin-1 inhibits cell detachment-induced p53 activation and anoikis by upregulation of insulin-like growth factor-I receptors and signaling, Oncogene 24 (2005) 1338-1347.]. We now show, using differential detergent extraction, SDS-PAGE and mass spectrometry, that the major protein in the pp340 band is the actin filament cross-linking protein filamin A. The identity of pp340 as filamin A was confirmed by immunoprecipitation of pp340 with specific filamin A antibodies. RT-PCR, flow cytometry and Western blot analyses show that filamin A mRNA and protein levels are respectively 3.5- and 2.5-fold higher in MCF-7/Cav1 cells than in MCF-7 cells. Basal filamin A phosphorylation on Ser-2152, normalized to total filamin A levels, is 7.8-fold higher in MCF-7/Cav1 than in MCF-7 cells. Insulin-like growth factor-I (IGF-I) stimulates phosphorylation of filamin A on Ser-2152 in MCF-7 cells and further enhances Ser-2152 phosphorylation over its already high basal level in MCF-7/Cav1 cells. The effect of IGF-I is inhibited by the PI3K inhibitor wortmannin, indicating that IGF-I-stimulated phosphorylation of filamin A occurs via the PI3K/Akt pathway. Co-immunoprecipitation experiments have confirmed a previous report showing that filamin A and caveolin-1 co-exist in a complex and have revealed the presence of active phospho-Akt in this complex. Ser-2152 phosphorylation of filamin A has been implicated in cancer cell migration. Accordingly, caveolin-1 expression dramatically enhances IGF-I-dependent MCF-7 cell migration. These data indicate that caveolin-1 specifies filamin A as a novel target for Akt-mediated filamin A Ser-2152 phosphorylation thus mediating the effects of caveolin-1 on IGF-I-induced cancer cell migration.

Potential role of caveolin-1-positive domains in the regulation of the acetylcholine receptor's activatable pool: implications in the pathogenesis of a novel congenital myasthenic syndrome

Cholesterol modulates the plasmalemma's biophysical properties and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is how the plasmalemma's lipid composition regulates the activatable pool of membrane receptors. An outstanding model to study this phenomenon is the nicotinic acetylcholine receptor (nAChR), since the nAChR activatable pool has been estimated to be but a small fraction of the receptors present in the plasmalemma. Studies on the effect of cholesterol depletion in the function of the Torpedo californica nAChR, using the lipid-exposed nAChR mutation (alpha C418W) that produces a congenital myasthenic syndrome (CMS), demonstrated that cholesterol depletion causes a remarkable increase in the alpha C418W nAChR's macroscopic current whereas not in the wild-type (WT). A variety of approaches were used to define the mechanism responsible for the cholesterol depletion mediated-increase in the alpha C418W nAChR's macroscopic current. The present study suggests that a substantial fraction of the alpha C418W nAChRs is located in caveolin-1-positive domains, "trapped" in a non-activatable state, and that membrane cholesterol depletion results in the relocation of these receptors to the activatable pool. Co-fractionation and co-immunoprecipitation of the alpha C418W nAChR and the membrane raft protein caveolin-1 (cav1) support the notion that interactions at lipid-exposed domains regulate the partition of the receptor into membrane raft microdomains. These results have potential implications as a novel mechanism to fine-tune cholinergic transmission in the nervous system and in the pathogenesis associated to the alpha C418W nAChR.

Caveolin-1: an ambiguous partner in cell signalling and cancer

Caveolae are small plasma membrane invaginations that have been implicated in a variety of functions including transcytosis, potocytosis and cholesterol transport and signal transduction. The major protein component of this compartment is a family of proteins called caveolins. Experimental data obtained in knockout mice have provided unequivocal evidence for a requirement of caveolins to generate morphologically detectable caveolae structures. However, expression of caveolins is not sufficient per seto assure the presence of these structures. With respect to other roles attributed to caveolins in the regulation of cellular function, insights are even less clear. Here we will consider, more specifically, the data concerning the ambiguous roles ascribed to caveolin-1 in signal transduction and cancer. In particular, evidence indicating that caveolin-1 function is cell context dependent will be discussed.

Na(+)-K (+) pump location and translocation during muscle contraction in rat skeletal muscle

Muscle contraction may up-regulate the number of Na(+)-K(+) pumps in the plasma membrane by translocation of subunits. Since there is still controversy about where this translocation takes place from and if it takes place at all, the present study used different techniques to characterize the translocation. Electrical stimulation and biotin labeling of rat muscle revealed a 40% and 18% increase in the amounts of the Na(+)-K(+) pump alpha(2) subunit and caveolin-3 (Cav-3), respectively, in the sarcolemma. Exercise induced a 36% and 19% increase in the relative amounts of the alpha(2) subunit and Cav-3, respectively, in an outer-membrane-enriched fraction and a 41% and 17% increase, respectively, in sarcolemma giant vesicles. The Na(+)-K(+) pump activity measured with the 3-O-MFPase assay was increased by 37% in giant vesicles from exercised rats. Immunoprecipitation with Cav-3 antibody showed that 17%, 11% and 14% of the alpha(1) subunits were associated with Cav-3 in soleus, extensor digitorum longus, and mixed muscles, respectively. For the alpha(2), the corresponding values were 17%, 5% and 16%. In conclusion; muscle contraction induces translocation of the alpha subunits, which is suggested to be caused partly by structural changes in caveolae and partly by translocation from an intracellular pool.

Caveolins in glial cell model systems: from detection to significance

Glial cells prevail in number and in diversity of cellular phenotypes in the nervous system. They have also gained prominence due to their multiple physiological and pathophysiological roles. Our current knowledge of the asymmetry and heterogeneity of the plasma membrane demands an in depth analysis of the diverse array of membrane microdomains postulated to exist in the context of glial cells. This review focuses and analyzes the studies reported to date on the detection of caveolae membrane rafts and the caveolin family members in glial cell model systems, the conditions leading to changes in their level of expression, and their functional and clinical significance. Outstanding in this work emerge the ubiquitous expression of caveolins, including the typically regarded 'muscle-specific' cav3, in diverse glial cell model systems, their participation in reactive astrogliosis, cancer, and their key relevance to calcium signaling. The knowledge obtained to date demands incorporation of the caveolins and caveolae membrane rafts in our current models on the role of glial cells in heath and neurological disease.

Caveolin-1 and -2 interact with connexin43 and regulate gap junctional intercellular communication in keratinocytes

Connexin43 (Cx43) has been reported to interact with caveolin (Cav)-1, but the role of this association and whether other members of the caveolin family bind Cx43 had yet to be established. In this study, we show that Cx43 coimmunoprecipitates and colocalizes with Cav-1 and Cav-2 in rat epidermal keratinocytes. The colocalization of Cx43 with Cav-1 was confirmed in keratinocytes from human epidermis in vivo. Our mutation and Far Western analyses revealed that the C-terminal tail of Cx43 is required for its association with Cavs and that the Cx43/Cav-1 interaction is direct. Our results indicate that newly synthesized Cx43 interacts with Cavs in the Golgi apparatus and that the Cx43/Cavs complex also exists at the plasma membrane in lipid rafts. Using overexpression and small interfering RNA approaches, we demonstrated that caveolins regulate gap junctional intercellular communication (GJIC) and that the presence of Cx43 in lipid raft domains may contribute to the mechanism modulating GJIC. Our results suggest that the Cx43/Cavs association occurs during exocytic transport, and they clearly indicate that caveolin regulates GJIC.

Caveolin regulates endocytosis of the muscle repair protein, dysferlin

Dysferlin and Caveolin-3 are plasma membrane proteins associated with muscular dystrophy. Patients with mutations in the CAV3 gene show dysferlin mislocalization in muscle cells. By utilizing caveolin-null cells, expression of caveolin mutants, and different mutants of dysferlin, we have dissected the site of action of caveolin with respect to dysferlin trafficking pathways. We now show that Caveolin-1 or -3 can facilitate exit of a dysferlin mutant that accumulates in the Golgi complex of Cav1(-/-) cells. In contrast, wild type dysferlin reaches the plasma membrane but is rapidly endocytosed in Cav1(-/-) cells. We demonstrate that the primary effect of caveolin is to cause surface retention of dysferlin. Caveolin-1 or Caveolin-3, but not specific caveolin mutants, inhibit endocytosis of dysferlin through a clathrin-independent pathway colocalizing with internalized glycosylphosphatidylinositol-anchored proteins. Our results provide new insights into the role of this endocytic pathway in surface remodeling of specific surface components. In addition, they highlight a novel mechanism of action of caveolins relevant to the pathogenic mechanisms underlying caveolin-associated disease.

Caveolin-3 expression and caveolae are required for isoflurane-induced cardiac protection from hypoxia and ischemia/reperfusion injury

Volatile anesthetics protect the heart from ischemia/reperfusion injury but the mechanisms for this protection are poorly understood. Caveolae, sarcolemmal invaginations, and caveolins, scaffolding proteins in caveolae, localize molecules involved in cardiac protection. We tested the hypothesis that caveolae and caveolins are essential for volatile anesthetic-induced cardiac protection using cardiac myocytes (CMs) from adult rats and in vivo studies in caveolin-3 knockout mice (Cav-3(-/-)). We incubated CM with methyl-beta-cyclodextrin (MbetaCD) or colchicine to disrupt caveolae formation, and then exposed the myocytes to the volatile anesthetic isoflurane (30 min, 1.4%), followed by simulated ischemia/reperfusion (SI/R). Isoflurane protected CM from SI/R [23.2+/-1.6% vs. 71.0+/-5.8% cell death (assessed by trypan blue exclusion), P<0.001] but this protection was abolished by MbetaCD or colchicine (84.9+/-5.5% and 64.5+/-6.1% cell death, P<0.001). Membrane fractionation by sucrose density gradient centrifugation of CM treated with MbetaCD or colchicine revealed that buoyant (caveolae-enriched) fractions had decreased phosphocaveolin-1 and caveolin-3 compared to control CM. Cardiac protection in vivo was assessed by measurement of infarct size relative to the area at risk and cardiac troponin levels. Isoflurane-induced a reduction in infarct size and cardiac troponin relative to control (infarct size: 26.5%+/-2.6% vs. 45.3%+/-5.4%, P<0.01; troponin: 27.7+/-4.4 vs. 77.7+/-11.8 ng/ml, P<0.05). Isoflurane-induced cardiac protection was abolished in Cav-3(-/-) mice (infarct size: 53.4%+/-6.1% vs. 53.2%+/-3.5%, P<0.01; troponin: 102.1+/-22.3 vs. 105.9+/-8.2 ng/ml, P<0.01). Isoflurane-induced cardiac protection is thus dependent on the presence of caveolae and the expression of caveolin-3. We conclude that caveolae and caveolin-3 are critical for volatile anesthetic-induced protection of the heart from ischemia/reperfusion injury.

Caveolin proteins are essential for distinct effects of membrane estrogen receptors in neurons

It has become widely accepted that along with its ability to directly regulate gene expression, estradiol also influences cell signaling and brain function via rapid membrane-initiated events. Many of these novel signaling processes are dependent on estrogen receptors (ERs) localized to the neuronal membrane. However, the mechanism(s) by which ERs are able to trigger cell signaling when targeted to the neuronal membrane surface has yet to be determined. In hippocampal neurons, we find that caveolin proteins are essential for the regulation of CREB (cAMP response element-binding protein) phosphorylation after estradiol activation of metabotropic glutamate receptor (mGluR) signaling. Furthermore, caveolin-1 (CAV1) and CAV3 differentially regulate the ability of estradiol to activate two discrete signaling pathways. ER alpha activation of mGluR1a is dependent on CAV1, whereas CAV3 is necessary for ER alpha and ER beta activation of mGluR2/3. These results are consistent with previous reports in non-neuronal cells, implicating the importance of caveolin proteins in rapid estrogen signaling. In addition, the functional isolation of distinct estrogen-sensitive signaling pathways by different caveolin proteins suggests novel mechanisms through which the membrane-initiated effects of estradiol are orchestrated.

The Membrane Topology of RTN3 and Its Effect on Binding of RTN3 to BACE1

Reticulon 3 (RTN3) has recently been shown to modulate Alzheimer BACE1 activity and to play a role in the formation of dystrophic neurites present in Alzheimer brains. Despite the functional importance of this protein in Alzheimer disease pathogenesis, the functional correlation to the structural domain of RTN3 remained unclear. RTN3 has two long transmembrane domains, but its membrane topology was not known. We report here that the first transmembrane domain dictates membrane integration and its membrane topology. RTN3 adopts a omega-shape structure with two ends facing the cytosolic side. Subtle changes in RTN3 membrane topology can disrupt its binding to BACE1 and its inhibitory effects on BACE1 activity. Thus, the determination of RTN3 membrane topology may provide an important structural basis for our understanding of its cellular functions.

Participation of macrophage membrane rafts in Trypanosoma cruzi invasion process

Membrane rafts are small and dynamic regions enriched in sphingolipids, cholesterol, ganglioside GM1 and protein markers like flotillins, forming the flatter domains or caveolins, which are characterized as stable flask-shape invaginations. We explored whether membrane rafts participate in the entry of Trypanosoma cruzi's trypomastigotes into murine macrophages through transient depletion of macrophage membrane cholesterol with methyl-beta-cyclodextrin and treatment with filipin. These treatments led to a decrease in the trypomastigote invasion process. Macrophage pre incubated with increasing concentrations of cholera toxin B, that binds GM1, inhibited the adhesion and invasion of trypomastigote and amastigote forms. Immunofluorescence analysis demonstrated a colocalization of GM1, flotillin 1 and caveolin 1 in the T. cruzi parasitophorous vacuole. Taken together these data suggest that membrane rafts, including caveolae, are involved in the process of T. cruzi invasion of macrophages.

CD147 immunoglobulin superfamily receptor function and role in pathology

The immunoglobulin superfamily member CD147 plays an important role in fetal, neuronal, lymphocyte and extracellular matrix development. Here we review the current understanding of CD147 expression and protein interactions with regard to CD147 function and its role in pathologic conditions including heart disease, Alzheimer's disease, stroke and cancer. A model linking hypoxic conditions found within the tumor microenvironment to upregulation of CD147 expression and tumor progression is introduced.

Regulation of tumor phenotypes by caveolin-1 and sphingolipid-controlled membrane signaling complexes

Aberrant (glyco)sphingolipid expression deeply affects several properties of tumor cells that are involved in tumor progression and metastasis formation: cell adhesion (to the extracellular matrix or to the endothelium of blood vessels), motility, recognition and invasion of host tissues. In particular, (glyco)sphingolipids might contribute to the modulation of integrin-dependent interactions of tumor cells (determining their adhesion, motility and invasiveness) with the extracellular matrix as well as with host cells present in the stromal compartment of the tumor. A model based on solid experimental evidence has been proposed: (glyco)sphingolipids at the cell surface interact with plasma membrane receptors (e.g., integrin receptors and growth factor receptors) and adapter molecules (including tetraspanins) forming signaling complexes that are able to influence the activity of signal transduction molecules oriented at the cytosolic surface of the plasma membrane (mainly the Src kinases pathway members). The function of these signaling complexes appears to be strictly dependent on their (glyco)sphingolipid composition, and likely on specific sphingolipid-protein interactions. From this point of view, particularly intriguing is the connection between (glyco)sphingolipids and caveolin-1, a membrane protein that plays multiple roles as a suppressor of tumor growth and metastasis in ovarian, breast and colon human carcinomas.

Caveolins and intracellular calcium regulation in human airway smooth muscle

Regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is a key factor in airway smooth muscle (ASM) tone. In vascular smooth muscle, specialized membrane microdomains (caveolae) expressing the scaffolding protein caveolin-1 are thought to facilitate cellular signal transduction. In human ASM cells, we tested the hypothesis that caveolae mediate Ca(2+) responses to agonist stimulation. Fluorescence immunocytochemistry with confocal microscopy, as well as Western blot analysis, was used to determine that agonist receptors (M(3) muscarinic, bradykinin, and histamine) and store-operated Ca(2+) entry (SOCE)-regulatory mechanisms colocalize with caveolin-1. Although caveolin-2 coexpressed with caveolin-1, caveolin-3 was absent. In fura 2-loaded ASM cells, [Ca(2+)](i) responses to 1 microM ACh, 10 microM histamine, and 10 nM bradykinin, as well as SOCE, were attenuated (each to a different extent) after disruption of caveolae by the cholesterol-chelating drug methyl-beta-cyclodextrin. Transfection of ASM cells with 50 nM caveolin-1 small interfering RNA significantly weakened caveolin-1 expression and blunted [Ca(2+)](i) responses to bradykinin and histamine, as well as SOCE, but the response to ACh was less intense. These results indicate that caveolae are present in ASM and that caveolin-1 contributes to regulation of [Ca(2+)](i) responses to agonist.

Lipid rafts are triage centers for multimeric and monomeric thyrotropin receptor regulation

The TSH receptor (TSHR), a heptahelical G protein-coupled receptor on the surface of thyrocytes, is a major autoantigen and physiological regulator of the thyroid gland. Unlike other G protein-coupled receptors, the TSHR undergoes posttranslational cleavage of its ectodomain, leading to the existence of several forms of the receptor on the plasma membrane. We previously hypothesized that to achieve high fidelity and specificity of TSH ligand or TSHR autoantibody signaling, the TSHR may compartmentalize into microdomains within the plasma membrane. In support of this hypothesis we have shown previously that TSHRs reside in GM1 ganglioside-enriched lipid rafts in the plasma membrane of TSHR-expressing cells. In this study, we further explored the different forms of TSHRs that reside in lipid rafts. We studied both TSHR-transfected cells and rat thyrocytes, using both nondetergent biochemical analyses and receptor-lipid raft colocalization. Using the biochemical approach, we observed that monomeric receptors existed in both raft and nonraft fractions of the cell surface in the steady state. We also demonstrated that the multimeric forms of the receptor were preferentially partitioned into the lipid microdomains. Different TSHR forms, including multimers, were dynamically regulated both by receptor-specific and postreceptor-specific modulators. TSH ligand and TSHR antibody of the stimulating variety induced a decrease of multimeric forms in the raft fractions. In addition, multimeric and monomeric forms of the receptor were both associated with Gsalpha within and without the rafts. Although failure to achieve total lipid raft disruption prevented a conclusion regarding the relative power of TSHR signaling within and without the raft domains, these data showed clearly that not only were a significant proportion of TSHRs residing within lipid microdomains but that constitutive multimerization of TSHRs was actually regulated within the lipid rafts.

Cell selective glucocorticoid induction of caveolin-1 and caveolae in differentiating pulmonary alveolar epithelial cell cultures

Increased caveolin-1 expression is a marker of the differentiation of lung alveolar epithelial type II cells into a type I phenotype. Here, we show in both a primary differentiating rat alveolar culture, and a human alveolar cell line (A549) that caveolae formation and caveolin-1 expression are dependent upon dexamethasone Dex, and is inhibited by the glucocorticoid receptor (GR) antagonist, mifepristone. Study of a panel of 20 different cell types showed the effect of (Dex) upon caveolin-1 expression to be highly cell selective for lung alveolar epithelial cells. The actions of glucocorticoid upon caveolin-1 appear indirect acting via intermediary genes as evidenced by cycloheximide (CHX) abolition of Dex-induced increases in caveolin-1 mRNA and by recombinant transfection studies using the caveolin-1 promoter cloned upstream of a reporter gene. Treatment with actinomycin D (ACD) revealed that the effects of Dex are also, at least in part, mediated by stabilisation of caveolin-1 mRNA. Collectively, these results indicate that glucocorticoids modulate the expression of caveolin-1 and caveolae biogenesis within alveolar epithelial cells via both transcriptional and translational modifications. The cell-selective effects of glucocorticoid upon caveolin may represent a previously unrecognised mechanism by which glucocorticoids affect lung development.

Status of caveolin-1 in various membrane domains of the bovine lens

Recent studies of the distribution and relative concentration of caveolin-1 in fractions of bovine lens epithelial and fiber cells have led to the novel concept that caveolin-1 may largely exist as a peripheral membrane protein in some cells. Caveolin-1 is typically viewed as a scaffolding protein for caveolae in plasma membrane. In this study, membrane from cultured bovine lens epithelial cells and bovine lens fiber cells were divided into urea soluble and insoluble fractions. Cytosolic lipid vesicles were also recovered from the lens epithelial cells. Lipid-raft domains were recovered from fiber cells following treatment with detergents and examined for caveolin and lipid content. Aliquots of all fractions were Western blotted for caveolin-1. Fluorescence microscopy and double immunofluorescence labeling were used to examine the distribution of caveolin-1 in cultured epithelial cells. Electron micrographs revealed an abundance of caveolae in plasma membrane of cultured lens epithelial cells. About 60% of the caveolin-1 in the epithelial-crude membrane was soluble in urea, a characteristic of peripheral membrane proteins. About 30% of the total was urea-insoluble membrane protein that likely supports the structure of caveolae. The remaining caveolin was part of cytosolic lipid vesicles. By contrast, most caveolin in the bovine lens fiber cell membrane was identified as intrinsic protein, being present at relatively low concentrations in caveolae-free lipid raft domains enriched in cholesterol and sphingomyelin. We estimate that these domains occupied 25-30% of the fiber cell membrane surface. Thus, the status of caveolin-1 in lens epithelial cells appears markedly different from that in fiber cells.

Arsenic trioxide (As2O3) reduces the invasive and metastatic properties of cervical cancer cells in vitro and in vivo

OBJECTIVES

Arsenic trioxide (As(2)O(3)) was found to induce apoptosis in certain types of cancer cells including acute promyelocytic leukemia, and recently in solid tumors. We have previously demonstrated that As(2)O(3) has a therapeutic effect on cervical cancer by apoptosis promotion in vitro and in vivo. Here we further our study on the role of arsenic trioxide in regulating invasive activity of cervical cancer cells in vitro and in vivo.

METHODS

The effects of As(2)O(3) on human cervical cancer cell lines (HeLa, SiHa, Caski) adhesion, migration and invasion were observed by means of cell adhesion test, cell migration test and cell invasion test. The effects of As(2)O(3) on p-IkappaB, MMP-2, E-cadherin, caveolin-1 and beta-catenin protein expressions of tumor cells were determined by Western blot. In addition, the effects of As(2)O(3) on NF-kappaB activity of tumor cells were analyzed by immunoblot in whole lysates, cytosol and nucleus, respectively. In animal experiments, cervical cancer cells TC-1 were injected into tail veins of C57BL/6 mice and then the mice were treated by intraperitoneal injection of different doses As(2)O(3). Lung weights and the foci on the surface of lungs were measured.

RESULTS

As(2)O(3) inhibited attachment of tumor cells to Fibronectin and Matrigel, reduced cell motility and inhibited tumor invasion potential. As(2)O(3) treatment also resulted in a positive regulation of caveolin-1, upregulation of E-cadherin and decreased activity of beta-catenin, NF-kappaB and NF-kappaB-regulated gene MMP-2. In animal experiments, lung weights in PBS group (0.31+/-0.07 g) were significantly elevated compared with those in As(2)O(3)-treated groups (0.21+/-0.03 g and 0.17+/-0.03 g) also As(2)O(3) reduced number of metastatic lesions of lungs (15.4+/-3.5 vs. 8.3+/-2.0 and 6.3+/-2.3) in a dose-dependent manner.

CONCLUSIONS

This study is the first to report the effectiveness of As(2)O(3) as an inhibitor of cervical cancer invasion both in vitro and in vivo, suggesting a potential clinical application of As(2)O(3) in cervical cancer therapies combining apoptosis induction and metastasis inhibition.

Rotavirus NSP4 interacts with both the amino- and carboxyl-termini of caveolin-1

Rotavirus NSP4 plays multiple roles in viral pathogenesis, morphogenesis and replication. We previously reported a direct interaction between full-length NSP4 and the enterotoxic peptide composed of NSP4 residues 114-135 with full-length caveolin-1, the structural protein of caveolae. Caveolin-1 forms a hairpin loop in the cytoplasmic leaflet of plasma membrane caveolae. This unique orientation results in both termini of caveolin-1 exposed to the cytoplasm. The goal of this study was to map the caveolin-1 residues that interact with NSP4 to obtain a more complete picture of this binding event. Utilizing reverse yeast two-hybrid analyses and direct peptide binding assays, the NSP4 binding site was localized to caveolin-1 residues 2-22 and 161-178, at the amino- and carboxyl-termini, respectively. However, NSP4 binding to one of the termini was sufficient for the interaction.

Severely altered cholesterol homeostasis in macrophages lacking apoE and SR-BI

Mice deficient in scavenger receptor class B type I (SR-BI) and apolipoprotein E (apoE) [double knockout (DKO) mice] develop dyslipidemia, accelerated atherosclerosis, and myocardial infarction, and die prematurely. We examined effects of apoE and SR-BI deficiency on macrophage cholesterol homeostasis. DKO macrophages had increased total cholesterol (TC) stores (220-380 microg/mg protein) compared with apoE-/- cells (40 microg/mg), showed significant lysosomal lipid engorgement, and increased their TC by 34% after exposure to HDL. DKO macrophages from apoE-/- mice reconstituted with DKO bone marrow showed less cholesterol accumulation (89 microg/mg), suggesting that the dyslipidemia of DKO mice explains part of the cellular cholesterol defect. However, analyses of DKO and apoE-/- macrophages from transplanted apoE-/- mice revealed a role for macrophage SR-BI, inasmuch as the TC in DKO macrophages increased by 10% in the presence of HDL, whereas apoE-/- macrophage TC decreased by 33%. After incubation with HDL, the free cholesterol (FC) increased by 29% in DKO macrophages, and decreased by 8% in apoE-/- cells, and only DKO cells had FC in large peri-nuclear pools. Similar trends were observed with apoA-I as an acceptor. Thus, the abnormal cholesterol homeostasis of DKO macrophages is due to the plasma lipid environment of DKO mice and to altered trafficking of macrophage cholesterol. Both factors are likely to contribute to the accelerated atherosclerosis in DKO mice.

Mechanisms of cardiac protection from ischemia/reperfusion injury: a role for caveolae and caveolin-1

Caveolae, small invaginations in the plasma membrane, contain caveolins (Cav) that scaffold signaling molecules including the tyrosine kinase Src. We tested the hypothesis that cardiac protection involves a caveolin-dependent mechanism. We used in vitro and in vivo models of ischemia-reperfusion injury, electron microscopy (EM), transgenic mice, and biochemical assays to address this hypothesis. We found that Cav-1 mRNA and protein were expressed in mouse adult cardiac myocytes (ACM). The volatile anesthetic, isoflurane, protected ACM from hypoxia-induced cell death and increased sarcolemmal caveolae. Hearts of wild-type (WT) mice showed rapid phosphorylation of Src and Cav-1 after isoflurane and ischemic preconditioning. The Src inhibitor PP2 reduced phosphorylation of Src (Y416) and Cav-1 in the heart and abolished isoflurane-induced cardiac protection in WT mice. Infarct size (percent area at risk) was reduced by isoflurane in WT (30.5+/-4 vs. 44.2+/-3, n=7, P<0.05) but not Cav-1(-/-) mice (46.6+/-5 vs. 41.7+/-3, n=7). Cav-1(-/-) mice exposed to isoflurane showed significant alterations in Src phosphorylation and recruitment of C-terminal Src kinase, a negative regulator of Src, when compared to WT mice. The results indicate that isoflurane modifies cardiac myocyte sarcolemmal membrane structure and composition and that activation of Src and phosphorylation of Cav-1 contribute to cardiac protection. Accordingly, therapies targeted to post-translational modification of Src and Cav-1 may provide a novel approach for such protection.

The trick of the tail: protein-protein interactions of metabotropic glutamate receptors

It was initially believed that G-protein-coupled receptors, such as metabotropic glutamate receptors, could simply be described as individual proteins that are associated with intracellular signal cascades via G-proteins. This view is no longer tenable. Today we know that metabotropic glutamate receptors (mGluRs) can dimerize and bind to a variety of proteins in addition to trimeric G-proteins. These newly identified protein interactions led to the discovery of new regulatory mechanisms that are independent of and sometimes synergistic with the classical G-protein-coupled second messenger pathways. Notably, several of these mechanisms connect mGluR-mediated signaling to other receptor classes, thereby creating a network of different receptor types and associated signal cascades. The intracellular C-termini of mGluRs play a key role in the regulation of these networks, and various new protein interactions of these domains were described recently. Because mGluRs are involved in a variety of physiological and pathophysiological processes, some of the proteins interacting with this receptor class have potential as valuable pharmaceutical targets. This review will give a comprehensive overview of proteins interacting with mGluR C-termini, highlight new evolving regulatory mechanisms for glutamatergic signal transduction and discuss possibilities for future drug development.

Detergent and detergent-free methods to define lipid rafts and caveolae

Lipid rafts and their related membrane vesicular structures, caveolae, are cholesterol- and sphingolipid-rich microdomains of the plasma membrane that have attracted considerable interest because of their ability to concentrate numerous signaling proteins. Efforts to define the proteins that reside in lipid rafts and caveolae as well as investigations into the functional role of these microdomains in signaling, endocytosis, and other cellular processes have led to the hypothesis that they compartmentalize or prearrange molecules involved in regulating these pathways. This chapter describes biochemical approaches for defining lipid rafts and caveolae. Included are detergent- and nondetergent-based fractionations on sucrose-density gradients that isolate buoyant lipid rafts and caveolae as well as caveolin antibody-based immunoisolation of detergent-insoluble membranes that selectively isolates caveolae and not lipid rafts. Also, a general method to disrupt lipid rafts and caveolae using beta-cyclodextrin that is useful for probing the role of these microdomains in cellular processes is described. The advantages and disadvantages of the respective approaches are discussed. Taken together, these methods are useful for defining the role of lipid rafts and caveolae in cell signaling.

Calcium misregulation and the pathogenesis of muscular dystrophy

Although the exact nature of the relationship between calcium and the pathogenesis of Duchenne muscular dystrophy (DMD) is not fully understood, this is an important issue which has been addressed in several recent reviews (Alderton and Steinhardt, 2000a, Gailly, 2002, Allen et al., 2005). A key question when trying to understand the cellular basis of DMD is how the absence or low level of expression of dystrophin, a cytoskeletal protein, results in the slow but progressive necrosis of muscle fibres. Although loss of cytoskeletal and sarcolemmal integrity which results from the absence of dystrophin clearly plays a key role in the pathogenesis associated with DMD, a number of lines of evidence also establish a role for misregulation of calcium ions in the DMD pathology, particularly in the cytoplasmic space just under the sarcolemma. A number of calcium-permeable channels have been identified which can exhibit greater activity in dystrophic muscle cells, and exIsting evidence suggests that these may represent different variants of the same channel type (perhaps the transient receptor potential channel, TRPC). In addition, a prominent role for calcium-activated proteases in the DMD pathology has been established, as well as modulation of other intracellular regulatory proteins and signaling pathways. Whether dystrophin and its associated proteins have a direct role in the regulation of calcium ions, calcium channels or intracellular calcium stores, or indirectly alters calcium regulation through enhancement of membrane tearing, remains unclear. Here we focus on areas of consensus or divergence amongst the existing literature, and propose areas where future research would be especially valuable.

Stromal and epithelial caveolin-1 both confer a protective effect against mammary hyperplasia and tumorigenesis: Caveolin-1 antagonizes cyclin D1 function in mammary epithelial cells

Here, we investigate the role of caveolin-1 (Cav-1) in breast cancer onset and progression, with a focus on epithelial-stromal interactions, ie, the tumor microenvironment. Cav-1 is highly expressed in adipocytes and is abundant in mammary fat pads (stroma), but it remains unknown whether loss of Cav-1 within mammary stromal cells affects the differentiated state of mammary epithelia via paracrine signaling. To address this issue, we characterized the development of the mammary ductal system in Cav-1-/- mice and performed a series of mammary transplant studies, using both wild-type and Cav-1-/- mammary fat pads. Cav-1-/- mammary epithelia were hyperproliferative in vivo, with dramatic increases in terminal end bud area and mammary ductal thickness as well as increases in bromodeoxyuridine incorporation, extracellular signal-regulated kinase-1/2 hyperactivation, and up-regulation of STAT5a and cyclin D1. Consistent with these findings, loss of Cav-1 dramatically exacerbated mammary lobulo-alveolar hyperplasia in cyclin D1 Tg mice, whereas overexpression of Cav-1 caused reversion of this phenotype. Most importantly, Cav-1-/- mammary stromal cells (fat pads) promoted the growth of both normal mammary ductal epithelia and mammary tumor cells. Thus, Cav-1 expression in both epithelial and stromal cells provides a protective effect against mammary hyperplasia as well as mammary tumorigenesis.

MT1-MMP shedding involves an ADAM and is independent of its localization in lipid rafts

The membrane type 1-matrix metalloproteinase (MT1-MMP) is a membrane-anchored protease that its entire ectodomain is shed from the cell surface. Here we show that in HT1080 cells MT1-MMP is shed as two soluble forms of approximately 52 and approximately 50kDa. Analyses in purified HT1080 plasma membranes show that release of these species is a two-step time-dependent process that is mediated by integral membrane metalloprotease(s). Differential sensitivity to TIMP-3 inhibition of the shedding process suggests that the second cleavage step leading to the formation of the 50-kDa soluble species is mediated by an ADAM. We also show that shedding of MT1-MMP is independent of its partition into lipid rafts because both wild type and glycosylphosphatidylinositol (GPI)-anchored MT1-MMP are shed. These studies provide new insights into the process of MT1-MMP ectodomain shedding, which may regulate pericellular proteolysis.

Is caveolin-1 a viable therapeutic target to reduce cancer metastasis?

Caveolin-1 is the major structural protein in caveolae; small Omega-shaped invaginations within the plasma membrane. Caveolae are involved in signal transduction, wherein caveolin-1 acts as a scaffold to organise multiple molecular complexes regulating a variety of cellular events. Caveolin-1 has both tumour suppressor and oncogenic activities. However, recent evidence suggests a role for caveolin-1 in promoting cancer cell migration and metastasis with both loss and overexpression of caveolin-1 being described as a marker for progression in a variety of tumour types. Further studies are beginning to determine the molecular mechanisms by which caveolin-1 acts in promoting a metastatic phenotype. Targeting caveolin-1 expression may present a novel means of preventing metastasis. The purpose of this review is twofold: firstly, to survey the current knowledge of the contribution of caveolin-1 in promoting a metastasis, and secondly, to explore the viability of targeting caveolin-1 with novel therapeutics.

A functional interaction between sprouty proteins and caveolin-1

Growth factor-mediated signal transduction cascades can be regulated spatio-temporally by signaling modulators, such as Sprouty proteins. The four mammalian Sprouty family members are palmitoylated phosphoproteins that can attenuate or potentiate numerous growth factor-induced signaling pathways. Previously, we have shown that Sprouty-1 and Sprouty-2 associate with Caveolin-1, the major structural protein of caveolae. Like Sprouty, Caveolin-1 inhibits growth factor-induced mitogen-activated protein kinase activation. Here, we demonstrate that all four mammalian Sprouty family members physically interact with Caveolin-1. The C terminus of Caveolin-1 is the major Sprouty-binding site, whereas Sprouty binds Caveolin-1 via its conserved C-terminal domain. A single point mutation in this domain results in loss of Caveolin-1 interaction. Moreover, we demonstrate that the various Sprouty isoforms differ dramatically in their cooperation with Caveolin-1-mediated inhibition of mitogen-activated protein kinase activation and that such cooperation is also highly dependent on the type of growth factor investigated and on cell density. Together, the data suggest that the Sprouty/Caveolin-1 interaction modulates signaling in a growth factor- and Sprouty isoform-specific manner.

Separation and characterization of caveolae subclasses in the plasma membrane of primary adipocytes; segregation of specific proteins and functions

Caveolae are nearly ubiquitous plasma membrane domains that in adipocytes vary in size between 25 and 150 nm. They constitute sites of entry into the cell as well as platforms for cell signalling. We have previously reported that plasma membrane-associated caveolae that lack cell surface access can be identified by electron microscopy. We now report the identification, after density gradient ultracentrifugation, of a subclass of very high-density apparently closed caveolae that were not labelled by cell surface protein labelling of intact cells. These caveolae contained caveolin-1 and caveolin-2. Another class of high-density caveolae contained caveolin-1, caveolin-2 and specifically fatty acid transport protein-1, fatty acid transport protein-4, fatty acyl-CoA synthetase, hormone-sensitive lipase, perilipin, and insulin-regulated glucose transporter-4. This class of caveolae was specialized in fatty acid uptake and conversion to triacylglycerol. A third class of low-density caveolae contained the insulin receptor, class B scavenger receptor-1, and insulin-regulated glucose transporter-4. Small amounts of these proteins were also detected in the high-density caveolae. In response to insulin, the insulin receptor autophosphorylation and the amount of insulin-regulated glucose transporter-4 increased in these caveolae. The molar ratio of cholesterol to phospholipid in the three caveolae classes varied considerably, from 0.4 in very high-density caveolae to 0.9 in low-density caveolae. There was no correlation between the caveolar contents of caveolin and cholesterol. The low-density caveolae, with the highest cholesterol concentration, were particularly enriched with the cholesterol-rich lipoprotein receptor class B scavenger receptor-1, which mediated cholesteryl ester uptake from high-density lipoprotein and generation of free cholesterol in these caveolae, suggesting a specific role in cholesterol uptake/metabolism. These findings demonstrate a segregation of functions in caveolae subclasses.

Divergent expression and roles for caveolin-1 in mouse hepatocarcinoma cell lines with varying invasive ability

Caveolin-1 is the major component protein of caveolae and associated with a lot of cellular events such as endocytosis, cholesterol homeostasis, signal transduction, and tumorigenesis. The majority of results suggest that caveolin-1 might not only act as a tumor suppressor gene but also a promoting metastasis gene. In this study, the divergent expression and roles of caveolin-1 were investigated in mouse hepatocarcinoma cell lines Hca-F, Hca-P, and Hepa1-6, which have high, low, and no metastatic potential in the lymph nodes, as compared with normal mouse liver cell line IAR-20. The results showed that expression of caveolin-1 mRNA and protein along with the amount of caveolae number in Hca-F cells was higher than that in Hca-P cells, but was not detectable in Hepa1-6 cells. When caveolin-1 expression in Hca-F cells was down-regulated by RNAi approach, Hca-F cells proliferation rate in vitro declined and the expression of lymphangiogenic factor VEGFA in Hca-F decreased as well. Furthermore, in vivo implantation assay indicated that reduction of caveolin-1 expression in Hca-F prevented the lymphatic metastasis tumor burden of Hca-F cells in 615 mice. These results suggest that caveolin-1 facilities the lymphatic metastasis ability of mouse hepatocarcinoma cells via regulation tumor cell growth and VEGFA expression.

Protein kinase C and downstream signaling pathways in a three-dimensional model of phorbol ester-induced angiogenesis

Angiogenesis, a critical process in both health and disease, is mediated by a number of signaling pathways. Although proangiogenic stimuli, including vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and the phorbol ester phorbol-12 myristate-13 acetate (PMA) are known to promote blood vessel formation, their downstream targets are ill defined. We sought to investigate the signaling pathways required for vessel assembly by utilizing a three-dimensional collagen matrix in which human umbilical vein endothelial cells (HUVECs) form tubular structures. Our data show that PMA is sufficient for the induction of angiogenesis, and that protein kinase C (PKC) is necessary for this process. Evaluation of PKC isoforms alpha and sigma revealed that these proteins are uniquely regulated. Characterization of an additional PMA target, protein kinase D (PKD) demonstrated that this enzyme becomes phosphorylated in HUVECs, and may therefore be involved in proangiogenic signaling. Further examination of downstream effectors of PKC showed that extracellular signal-regulated kinase (ERK) is critical for angiogenesis, and is accordingly phosphorylated in response to PMA. Surprisingly however, phosphorylation of ERK is independent of PKC activity. In addition, we show that the PKC target sphingosine kinase (SPK) is required for vessel formation. These findings illustrate the complexities of blood vessel formation, and suggest that activators utilize multiple independent pathways to invoke a complete angiogenic response.

Caveolin-1 and regulation of cellular cholesterol homeostasis

Caveolae are 50- to 100-nm cell surface plasma membrane invaginations present in terminally differentiated cells. They are characterized by the presence of caveolin-1, sphingolipids, and cholesterol. Caveolin-1 is thought to play an important role in the regulation of cellular cholesterol homeostasis, a process that needs to be properly controlled to limit and prevent cholesterol accumulation and eventually atherosclerosis. We have recently generated caveolin-1-deficient [Cav-1(-/-)] mice in which caveolae organelles are completely eliminated from all cell types, except cardiac and skeletal muscle. In the present study, we examined the metabolism of cholesterol in wild-type (WT) and Cav-1(-/-) mouse embryonic fibroblasts (MEFs) and mouse peritoneal macrophages (MPMs). We observed that Cav-1(-/-) MEFs are enriched in esterified cholesterol but depleted of free cholesterol compared with their wild-type counterparts. Similarly, Cav-1(-/-) MPMs also contained less free cholesterol and were enriched in esterified cholesterol on cholesterol loading. In agreement with this finding, caveolin-1 deficiency was associated with reduced free cholesterol synthesis but increased acyl-CoA:cholesterol acyl-transferase (ACAT) activity. In wild-type MPMs, we observed that caveolin-1 was markedly upregulated on cholesterol loading. Despite these differences, cellular cholesterol efflux from MEFs and MPMs to HDL was not affected in the Cav-1-deficient cells. Neither ATP-binding cassette transporter G1 (ABCG1)- nor scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux was affected. Cellular cholesterol efflux to apolipoprotein A-I was not significantly reduced in Cav-1(-/-) MPMs compared with wild-type MPMs. However, ABCA1-mediated cholesterol efflux was clearly more sensitive to the inhibitory effects of glyburide in Cav-1(-/-) MPMs versus WT MPMs. Taken together, these findings suggest that caveolin-1 plays an important role in the regulation of intracellular cholesterol homeostasis and can modulate the activity of other proteins that are involved in the regulation of intracellular cholesterol homeostasis.

Cholesterol and the interaction of proteins with membrane domains

Cholesterol is not uniformly distributed in biological membranes. One of the factors influencing the formation of cholesterol-rich domains in membranes is the unequal lateral distribution of proteins in membranes. Certain proteins are found in cholesterol-rich domains. In some of these cases, it is as a consequence of the proteins interacting directly with cholesterol. There are several structural features of a protein that result in the protein preferentially associating with cholesterol-rich domains. One of the best documented of these is certain types of lipidations. In addition, however, there are segments of a protein that can preferentially sequester cholesterol. We discuss two examples of these cholesterol-recognition elements: the cholesterol recognition/interaction amino acid consensus (CRAC) domain and the sterol-sensing domain (SSD). The requirements for a CRAC motif are quite flexible and predict that a large number of sequences could recognize cholesterol. There are, however, certain proteins that are known to interact with cholesterol-rich domains of cell membranes that have CRAC motifs, and synthetic peptides corresponding to these segments also promote the formation of cholesterol-rich domains. Modeling studies have provided a rationale for certain requirements of the CRAC motif. The SSD is a larger protein segment comprising five transmembrane domains. The amino acid sequence YIYF is found in several SSD and in certain other proteins for which there is evidence that they interact with cholesterol-rich domains. The CRAC sequences as well as YIYF are generally found adjacent to a transmembrane helical segment. These regions appear to have a strong influence of the localization of certain proteins into domains in biological membranes. In addition to the SSD, there is also a domain found in soluble proteins, the START domain, that binds lipids. Certain proteins with START domains specifically bind cholesterol and are believed to function in intracellular cholesterol transport. One of these proteins is StAR-D1, that also has a mitochondrial targeting sequence and plays an important role in delivering cholesterol to the mitochondria of steroidogenic cells.

Caveolin-1 expression is associated with a basal-like phenotype in sporadic and hereditary breast cancer

The role of caveolin 1 (CAV1), a structural component of caveolae in breast cancer is controversial, although most studies suggest that it functions as a tumor-suppressor gene. In addition, some studies have identified CAV1 as a marker of myoepithelial cells. Since myoepithelial markers are frequently expressed in breast carcinomas with a basal-like phenotype, which are frequently occurring tumors in women with BRCA1 germline mutations, we evaluated whether CAV1 was associated with a basal-like phenotype in 509 sporadic and 47 hereditary BRCA1-/BRCA2-associated carcinomas. Immunohistochemistry was performed on tissue microarrays and cases were classified as having a basal-like-phenotype if they were estrogen-receptor- and HER2-negative but cytokeratin 5/6- and/or epidermal growth factor receptor-positive. In sporadic carcinomas, CAV1 expression was found in 21 out of 496 valuable cases (4.2%). A basal-like-phenotype was found in 53 out of 498 (10.6%) cases. A strong association was found between CAV1 expression and a basal-like-phenotype, since 52% of tumors that expressed CAV1 had this phenotype, compared with only 9% of CAV1-negative carcinomas (p<0.001). CAV1 was expressed in six (12.8%) familial cases, five of which had a basal-like-phenotype (p = 0.009). Moreover, these six CAV1-positive cases were BRCA1 tumors. The difference in the frequency of CAV1 expression between BRCA1- and BRCA2-associated tumors was statistically significant (p = 0.024). In conclusion, this study reports for the first time CAV1 expression in BRCA1 and BRCA2 hereditary breast cancer and identifies CAV1 as a marker associated with a basal-like-phenotype in both hereditary and sporadic breast cancer.

Caveolin regulates microtubule polymerization in the vascular smooth muscle cells

Microtubule and caveolin have common properties in intracellular trafficking and the regulation of cellular growth. Overexpression of caveolin in vascular smooth muscle cells increased the polymer form of microtubule without changing in the total amount of tubulin, and downregulation of caveolin decreased the polymer form of microtubule. Fractionation of cellular proteins followed by immunodetection as well as immunostaining of caveolin and microtubule revealed that caveolin and a portion of microtubule were co-localized in caveolar fractions. A caveolin scaffolding domain peptide, which mimics caveolin function, did not alter the polymerization of microtubule in vitro, but dramatically inhibited the depolymerization of microtubule induced by stathmin, a microtubule destabilizing protein, which was also found in caveolar fractions. Accordingly, it is most likely that caveolin increased the polymer form of microtubule through the inhibition of a microtubule destabilizer, stathmin, suggesting a novel role of caveolin in regulating cellular network and trafficking.

Protection of adult rat cardiac myocytes from ischemic cell death: role of caveolar microdomains and delta-opioid receptors

The role of caveolae, membrane microenvironments enriched in signaling molecules, in myocardial ischemia is poorly defined. In the current study, we used cardiac myocytes prepared from adult rats to test the hypothesis that opioid receptors (OR), which are capable of producing cardiac protection in vivo, promote cardiac protection in cardiac myocytes in a caveolae-dependent manner. We determined protein expression and localization of delta-OR (DOR) using coimmunohistochemistry, caveolar fractionation, and immunoprecipitations. DOR colocalized in fractions with caveolin-3 (Cav-3), a structural component of caveolae in muscle cells, and could be immunoprecipitated by a Cav-3 antibody. Immunohistochemistry confirmed plasma membrane colocalization of DOR with Cav-3. Cardiac myocytes were subjected to simulated ischemia (2 h) or an ischemic preconditioning (IPC) protocol (10 min ischemia, 30 min recovery, 2 h ischemia) in the presence and absence of methyl-beta-cyclodextrin (MbetaCD, 2 mM), which binds cholesterol and disrupts caveolae. We also assessed the cardiac protective effects of SNC-121 (SNC), a selective DOR agonist, on cardiac myocytes with or without MbetaCD and MbetaCD preloaded with cholesterol. Ischemia, simulated by mineral oil layering to inhibit gas exchange, promoted cardiac myocyte cell death (trypan blue staining), a response blunted by SNC (37 +/- 3 vs. 59 +/- 3% dead cells in the presence and absence of 1 muM SNC, respectively, P < 0.01) or by use of the IPC protocol (35 +/- 4 vs. 62 +/- 3% dead cells, P < 0.01). MbetaCD treatment, which disrupted caveolae (as detected by electron microscopy), fully attenuated the protective effects of IPC or SNC, resulting in cell death comparable to that of the ischemic group. By contrast, SNC-induced protection was not abrogated in cells incubated with cholesterol-saturated MbetaCD, which maintained caveolae structure and function. These findings suggest a key role for caveolae, perhaps through enrichment of signaling molecules, in contributing to protection of cardiac myocytes from ischemic damage.

Exploring the interaction between the protein kinase A catalytic subunit and caveolin-1 scaffolding domain with shotgun scanning, oligomer complementation, NMR, and docking

The techniques of phage-displayed homolog shotgun scanning, oligomer complementation, NMR secondary structure analysis, and computational docking provide a complementary suite of tools for dissecting protein-protein interactions. Focusing these tools on the interaction between the catalytic sub-unit of protein kinase A (PKAcat) and caveolin-1 scaffolding domain (CSD) reveals the first structural model for the interaction. Homolog shotgun scanning varied each CSD residue as either a wild-type or a homologous amino acid. Wild-type to homolog ratios from 116 different homologous CSD variants identified side-chain functional groups responsible for precise contacts with PKAcat. Structural analysis by NMR assigned an alpha-helical conformation to the central residues 84- 97 of CSD. The extensive mutagenesis data and NMR secondary structure information provided constraints for developing a model for the PKAcat-CSD interaction. Addition of synthetic CSD to phage-displayed CSD resulted in oligomer complementation, or enhanced binding to PKAcat. Together with previous experiments examining the interaction between CSD and endothelial nitric oxide synthase (eNOS), the results suggest a general oligomerization-dependent enhancement of binding between signal transducing enzymes and caveolin-1.

Interaction of synthetic peptides corresponding to the scaffolding domain of Caveolin-3 with model membranes

Caveolin-1 and -3 are among the few proteins in which the functional domains are contiguous and modular. The interaction of synthetic peptides spanning the scaffolding domain of caveolin-3 with model membranes has been investigated. The peptides include the scaffolding domain, the aromatic and positively charged residues at the C-terminal end of this domain as well as deletion of three amino acids TFT, observed in certain patients with limb girdle muscular dystrophy. All of the peptides appear to be peripherally bound to the bilayer surface. However, no preferential binding to sphingomyelin and cholesterol-containing lipid vesicles was observed. Deletion of TFT appears to affect the association with lipid vesicles compared with the native sequence. Association with lipids decreases considerably when TFT as well as the aromatic-rich segment YWFYR, which occurs at the extreme C-terminus of the scaffolding domain, are deleted.

Induction of alpha-caveolin-1 (αCAV1) expression in bovine granulosa cells in response to an ovulatory dose of human chorionic gonadotropin

Caveolins are implicated in endocytosis, cholesterol trafficking and signal transduction. A cDNA fragment corresponding to caveolin-1 (CAV1) was identified in a mRNA profiling expression study in bovine granulosa cells (GC) following human chorionic gonadotropin (hCG)-induced ovulation. Thus, we have characterized CAV1 cDNA and studied its spatio-temporal expression pattern in bovine ovarian follicles. The full-length bovine alphaCAV1 cDNA was cloned and encodes a putative 22 kDa protein. Expression of alphaCAV1 was studied in bovine GC obtained from follicles at different developmental stages: small follicles (SF: 2-4 mm), dominant follicles (DF), ovulatory follicles (OF: 24 hr post-hCG), and corpus luteum (CL). Semiquantitative RT-PCR analysis showed a 6.5-fold increase in alphaCAV1 mRNA in GC of OF versus DF (P < 0.0001), whereas CAV2 mRNA was increased by only twofold (P < 0.0007). Temporal expression of alphaCAV1 mRNA from OF recovered at 0, 6, 12, 18, and 24 hr after hCG injection showed an 8.5-fold increase of alphaCAV1 mRNA after 24 hr compared to 0 hr (P < 0.0018) whereas no significant variation was detected for CAV2. Immunoblot demonstrated an initial increase in alphaCAV1 protein level 12 hr post-hCG, reaching a maximum at 24 hr. Immunohistochemical localization of CAV1 was observed in GC of OF isolated 18 and 24 hr after hCG injection, whereas no signal was detected in GC of DF and SF. The induction of alphaCAV1 in GC of OF suggests that alphaCAV1 likely contributes to control the increase in membrane signaling that occurs at the time of ovulation and luteinization.

Colocalization between caveolin isoforms in the intestinal smooth muscle and interstitial cells of Cajal of the Cav1(+/+) and Cav1 (-/-) mouse

Confocal microscopic images were obtained from the immunohistochemical sections of jejeunum to determine the localization/colocalization between caveolin-1, caveolin-2 and caveolin-3 in intestinal smooth muscle cells (SMCs) and interstitial cells of Cajal (ICC) of Cav1(+/+) and Cav1(-/-) mouse. Intestinal regions were segmented [inner circular muscle (icm), outer circular muscle (ocm), myenteric plexus region (mp), and longitudinal muscle (lm)] by LSM 5 and analyzed by ImageJ to show Pearson's correlation (r (p)) and overlap coefficient (r) of colocalization. In the intestine of Cav1(+/+), caveolin-1 (cav1) was colocalized with caveolin-2 (cav2) and caveolin-3 (cav3). Cav2 also was well colocalized with cav3. In the intestine of Cav1(-/-), cav1 and cav2 were absent in all images, but reduced cav3 was expressed in ocm. Caveolae were present in cell types with cav1 in Cav1(+/+), and present with cav3 in ocm of Cav1(-/-). C-kit occurred in deep muscular plexus (ICC-DMP) and myenteric plexus (ICC-MP), in both Cav1(+/+) and Cav1(-/-), and colocalized with cav1 and cav2 in the intestine of Cav1(+/+). Cav3 was absent/present at low immunoreactivity in ICC-DMP and ICC-MP of the intestines of Cav1(+/+) and Cav1(-/-). To conclude, cav1 is necessary for the expression of cav2 in SMC and ICC of intestine and facilitates, but is not necessary for the expression of cav3.

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.