Visualization of caveolin-1, a caveolar marker protein, in living cells using green fluorescent protein (GFP) chimeras. The subcellular distribution of caveolin-1 is modulated by cell-cell contact

Caveola mechanotransduction reinforces the cortical cytoskeleton to promote epithelial resilience

As physical barriers, epithelia must preserve their integrity when challenged by mechanical stresses. Cell-cell junctions linked to the cortical cytoskeleton play key roles in this process, often with mechanotransduction mechanisms that reinforce tissues. Caveolae are mechanosensitive organelles that buffer tension via disassembly. Loss of caveolae, through caveolin-1 or cavin1 depletion, causes activation of PtdIns(4, 5)P2 signaling, recruitment of FMNL2 formin, and enhanced-cortical actin assembly. How this equates to physiological responses in epithelial cells containing endogenous caveolae is unknown. Here we examined the effect of mechanically inducing acute disassembly of caveolae in epithelia. We show that perturbation of caveolae, through direct mechanical stress, reinforces the actin cortex at adherens junctions. Increasing interactions with membrane lipids by introducing multiple phosphatidylserine-binding undecad cavin1 (UC1) repeat domains into cavin1 rendered caveolae more stable to mechanical stimuli. This molecular stabilization blocked cortical reinforcement in response to mechanical stress. Cortical reinforcement elicited by the mechanically induced disassembly of caveolae increased epithelial resilience against tensile stresses. These findings identify the actin cortex as a target of caveola mechanotransduction that contributes to epithelial integrity.

Mechanical Stretch Redefines Membrane Gαq-Calcium Signaling Complexes

Muscle cells are routinely subjected to mechanical stretch but the impact of stretch on the organization of membrane domains is unknown. In this study, we characterize the effect of stretch on GPCR-Gαq protein signaling. Activation of this pathway leads to an increase in intracellular calcium. In muscle cells, GPCR-Gαq signals are enhanced when these proteins are localized in caveolae membrane domains whose curved structure can flatten with stretch. When we statically stretch rat aortic smooth muscle A10 cells by 1-5%, cellular calcium appears unperturbed as indicated by a calcium indicator. However, when we activate the bradykinin type 2 receptor (B2R)/Gαq pathway, we observe a loss in calcium that appears to be mediated through perturbations in calcium-activated stretch receptors. In contrast, if we apply oscillating stretch, calcium levels are enhanced. We tested whether the observed changes in B2R-Gαq calcium signals were caused by stretch-induced disruption of caveolae using a combination of silencing RNA technology and growth conditions. We find that stretch changes the ability of monoclonal caveolin antibodies to bind caveolae indicating a change in configuration of the domains. This change is seen by the inability of cells to survive stretch cycles when the level of caveolae is significantly reduced. Our studies show that the effect of calcium signals by mechanical stretch is mediated by the type of stretch and the amount of caveolae.

Effect of caveolin-1 on Stat3-ptyr705 levels in breast and lung carcinoma cells

We recently demonstrated that Cav1 (caveolin-1) is a negative regulator of Stat3 (signal transducer and activator of transcription-3) activity in mouse fibroblasts and human lung carcinoma SHP77 cells. We now examined whether the cellular context may affect their levels as well as the relationship between them, by assessing Cav1 and Stat3-ptyr705 amounts in different cell lines. In MDA-MB-231, A549, and HaCat cells, Cav1 levels were high and Stat3-ptyr705 levels were low, consistent with the notion of a negative effect of endogenous Cav1 on Stat3-ptyr705 levels in these lines. In addition, manipulation of Cav1 levels revealed a negative effect in MCF7 and mouse fibroblast cells, while Cav1 upregulation induced apoptosis in MCF7 cells. In contrast, however, line MRC9 had high Cav1 and high Stat3-ptyr705 levels, indicating that high Cav1 is insufficient to reduce Stat3-ptyr705 levels in this line. MCF7 and LuCi6 cells had very low Cav1 and Stat3-ptyr705 levels, indicating that the low Stat3-ptyr705 can be independent from Cav1 levels altogether. Our results reveal a further level of complexity in the relationship between Cav1 and Stat3-ptyr705 than previously thought. In addition, we demonstrate that in a feedback loop, Stat3 inhibition upregulates Cav1 in HeLa cells but not in other lines tested.

Cell number in mesenchymal stem cell aggregates dictates cell stiffness and chondrogenesis

Background

Although mesenchymal stem/stromal cell (MSC) chondrogenic differentiation has been thoroughly investigated, the rudiments for enhancing chondrogenesis have remained largely dependent on external cues. Focus to date has been on extrinsic variables such as soluble signals, culture conditions (bioreactors), and mechanical stimulation. However, the role of intrinsic mechanisms of MSC programming-based mechanobiology remains to be explored. Since aggregation of MSCs, a prerequisite for chondrogenesis, generates tension within the cell agglomerate, we inquired if the initial number of cells forming the aggregate (aggregate cell number (ACN)) can impact chondrogenesis.

Methods

Aggregates of varying ACN were formed using well-established centrifugation approach. Progression of chondrogenic differentiation in the aggregates was assessed over 3 weeks in presence and absence of transforming growth factor-beta 1 (TGF-β1). Mechanical properties of the cells were characterized using high-throughput real-time deformability cytometry (RT-DC), and gene expression was analyzed using Affymetrix gene array. Expression of molecular markers linked to chondrogenesis was assessed using western blot and immunofluorescence.

Results

Reducing ACN from 500 k to 70 k lead to activation and acceleration of the chondrogenic differentiation, independent of soluble chondro-inductive factors, which involves changes to β-catenin-dependent TCF/LEF transcriptional activity and expression of anti-apoptotic protein survivin. RT-DC analysis revealed that stiffness and size of cells within aggregates are modulated by ACN. A direct correlation between progression of chondrogenesis and emergence of stiffer cell phenotype was found. Affymetrix gene array analysis revealed a downregulation of genes associated with lipid synthesis and regulation, which could account for observed changes in cell stiffness. Immunofluorescence and western blot analysis revealed that increasing ACN upregulates the expression of lipid raft protein caveolin-1, a β-catenin binding partner, and downregulates the expression of N-cadherin. As a demonstration of the relevance of these findings in MSC-based strategies for skeletal repair, it is shown that implanting aggregates within collagenous matrix not only decreases the necessity for high cell numbers but also leads to marked improvement in the quality of the deposited tissue.

Conclusions

This study presents a simple and donor-independent strategy to enhance the efficiency of MSC chondrogenic differentiation and identifies changes in cell mechanics coincident with MSC chondrogenesis with potential translational applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1103-y) contains supplementary material, which is available to authorized users.

BioID identifies proteins involved in the cell biology of caveolae

The mechanisms controlling the abundance and sub-cellular distribution of caveolae are not well described. A first step towards determining such mechanisms would be identification of relevant proteins that interact with known components of caveolae. Here, we applied proximity biotinylation (BioID) to identify a list of proteins that may interact with the caveolar protein cavin1. Screening of these candidates using siRNA to reduce their expression revealed that one of them, CSDE1, regulates the levels of mRNAs and protein expression for multiple components of caveolae. A second candidate, CD2AP, co-precipitated with cavin1. Caveolar proteins were observed in characteristic and previously un-described linear arrays adjacent to cell-cell junctions in both MDCK cells, and in HeLa cells overexpressing an active form of the small GTPase Rac1. CD2AP was required for the recruitment of caveolar proteins to these linear arrays. We conclude that BioID will be useful in identification of new proteins involved in the cell biology of caveolae, and that interaction between CD2AP and cavin1 may have an important role in regulating the sub-cellular distribution of caveolae.

Caveolin-1 is an aggresome-inducing protein

Caveolin-1 (Cav1) drives the formation of flask-shaped membrane invaginations known as caveolae that participate in signaling, clathrin-independent endocytosis and mechanotransduction. Overexpression or mutations of Cav1 can lead to its mistrafficking, including its accumulation in a perinuclear compartment previously identified as the Golgi complex. Here, we show that in the case of overexpressed Cav1-GFP, this perinuclear compartment consists of cytoplasmic inclusion bodies generated in response to the accumulation of aggregates of misfolded proteins, known as aggresomes. Aggresomes containing Cav1-GFP are encased within vimentin cages, form in a microtubule-dependent manner, and are enriched in a number of key regulators of protein turnover, including ubiquitin, VCP/p97 and proteasomes. Interestingly, aggresome induction was cell-type dependent and was observed for many but not all Cav1 constructs tested. Furthermore, endogenous Cav1 accumulated in aggresomes formed in response to proteosomal inhibition. Our finding that Cav1 is both an aggresome-inducing and aggresome-localized protein provides new insights into how cells handle and respond to misfolded Cav1. They also raise the possibility that aggresome formation may contribute to some of reported phenotypes associated with overexpressed and/or mutant forms of Cav1.

Tagging strategies strongly affect the fate of overexpressed caveolin-1

Caveolin-1 (Cav1) is the primary scaffolding protein of caveolae, flask-shaped invaginations of the plasma membrane thought to function in endocytosis, mechanotransduction, signaling and lipid homeostasis. A significant amount of our current knowledge about caveolins and caveolae is derived from studies of transiently overexpressed, C-terminally tagged caveolin proteins. However, how different tags affect the behavior of ectopically expressed Cav1 is still largely unknown. To address this question, we performed a comparative analysis of the subcellular distribution, oligomerization state and detergent resistance of transiently overexpressed Cav1 labeled with three different C-terminal tags (EGFP, mCherry and myc). We show that addition of fluorescent protein tags enhances the aggregation and/or degradation of both wild-type Cav1 and an oligomerization defective P132L mutant. Strikingly, complexes formed by overexpressed Cav1 fusion proteins excluded endogenous Cav1 and Cav2, and the properties of native caveolins were largely preserved even when abnormal aggregates were present in cells. These findings suggest that differences in tagging strategies may be a source of variation in previously published studies of Cav1 and that overexpressed Cav1 may exert functional effects outside of caveolae. They also highlight the need for a critical re-evaluation of current knowledge based on transient overexpression of tagged Cav1.

A role for G-proteins in directing G-protein-coupled receptor-caveolae localization

Caveolae are membrane domains that may influence cell signaling by sequestering specific proteins such as G-protein-coupled receptors (GPCRs). While previous reports largely show that Gα_q subunits, but not other G-proteins, interact strongly with the caveolae protein, Caveolin-1 (Cav1), the inclusion of GPCRs in caveolae is controversial. Here, we have used fluorescence methods to determine the effect of caveolae on the physical and functional properties of two GPCRs that have been reported to reside in caveolae, bradykinin receptor type 2 (B₂R), which is coupled to Gα_q, and the μ-opioid receptor (μOR), which is coupled to Gα_i. While caveolae do not affect cAMP signals mediated by μOR, they prolong Ca²⁺ signals mediated by B₂R. In A10 cells that endogenously express B₂R and Cav1, downregulation of Cav1 ablates the prolonged recovery seen upon bradykinin stimulation in accord with the idea that the presence of caveolae prolongs Gα_q activation. Immunofluorescence and Förster resonance energy transfer (FRET) studies show that a significant fraction of B₂R resides at or close to caveolae domains while none or very little μOR resides in caveolae domains. The level of FRET between B₂R and caveolae is reduced by downregulation of Gα_q or by addition of a peptide that interferes with Gα_q–Caveolin-1 interactions, suggesting that Gα_q promotes localization of B₂R to caveolae domains. Our results lead to the suggestion that Gα_q can localize its associated receptors to caveolae domains to enhance their signals.

Three dimensional neuronal cell cultures more accurately model voltage gated calcium channel functionality in freshly dissected nerve tissue

It has been demonstrated that neuronal cells cultured on traditional flat surfaces may exhibit exaggerated voltage gated calcium channel (VGCC) functionality. To gain a better understanding of this phenomenon, primary neuronal cells harvested from mice superior cervical ganglion (SCG) were cultured on two dimensional (2D) flat surfaces and in three dimensional (3D) synthetic poly-L-lactic acid (PLLA) and polystyrene (PS) polymer scaffolds. These 2D- and 3D-cultured cells were compared to cells in freshly dissected SCG tissues, with respect to intracellular calcium increase in response to high K⁺ depolarization. The calcium increases were identical for 3D-cultured and freshly dissected, but significantly higher for 2D-cultured cells. This finding established the physiological relevance of 3D-cultured cells. To shed light on the mechanism behind the exaggerated 2D-cultured cells’ functionality, transcriptase expression and related membrane protein distributions (caveolin-1) were obtained. Our results support the view that exaggerated VGCC functionality from 2D cultured SCG cells is possibly due to differences in membrane architecture, characterized by uniquely organized caveolar lipid rafts. The practical implication of use of 3D-cultured cells in preclinical drug discovery studies is that such platforms would be more effective in eliminating false positive hits and as such improve the overall yield from screening campaigns.

A high precision survey of the molecular dynamics of mammalian clathrin-mediated endocytosis

The molecular dynamics of clathrin-mediated endocytosis in living cells has been mapped with an approximately ten-fold improvement in temporal accuracy, yielding new insights into the molecular mechanism.

Dual colour total internal reflection fluorescence microscopy is a powerful tool for decoding the molecular dynamics of clathrin-mediated endocytosis (CME). Typically, the recruitment of a fluorescent protein–tagged endocytic protein was referenced to the disappearance of spot-like clathrin-coated structure (CCS), but the precision of spot-like CCS disappearance as a marker for canonical CME remained unknown. Here we have used an imaging assay based on total internal reflection fluorescence microscopy to detect scission events with a resolution of ∼2 s. We found that scission events engulfed comparable amounts of transferrin receptor cargo at CCSs of different sizes and CCS did not always disappear following scission. We measured the recruitment dynamics of 34 types of endocytic protein to scission events: Abp1, ACK1, amphiphysin1, APPL1, Arp3, BIN1, CALM, CIP4, clathrin light chain (Clc), cofilin, coronin1B, cortactin, dynamin1/2, endophilin2, Eps15, Eps8, epsin2, FBP17, FCHo1/2, GAK, Hip1R, lifeAct, mu2 subunit of the AP2 complex, myosin1E, myosin6, NECAP, N-WASP, OCRL1, Rab5, SNX9, synaptojanin2β1, and syndapin2. For each protein we aligned ∼1,000 recruitment profiles to their respective scission events and constructed characteristic “recruitment signatures” that were grouped, as for yeast, to reveal the modular organization of mammalian CME. A detailed analysis revealed the unanticipated recruitment dynamics of SNX9, FBP17, and CIP4 and showed that the same set of proteins was recruited, in the same order, to scission events at CCSs of different sizes and lifetimes. Collectively these data reveal the fine-grained temporal structure of CME and suggest a simplified canonical model of mammalian CME in which the same core mechanism of CME, involving actin, operates at CCSs of diverse sizes and lifetimes.

The molecular machinery of clathrin-mediated endocytosis concentrates receptors at the cell surface in a patch of membrane that curves into a vesicle, pinches off, and internalizes membrane cargo and a tiny volume of extracellular fluid. We know that dozens of proteins are involved in this process, but precisely when and where they act remains poorly understood. Here we used a fluorescence imaging assay to detect the moment of scission in living cells and used this as a reference point from which to measure the characteristic recruitment signatures of 34 fluorescently tagged endocytic proteins. Pair-wise comparison of these recruitment signatures allowed us to identify seven modules of proteins that were recruited with similar kinetics. For the most part the recruitment signatures were consistent with what was previously known about the proteins' structure and their binding affinities; however, the recruitment signatures for some components (such as some BAR and F-BAR domain proteins) could not have been predicted from existing structural or biochemical data. This study provides a paradigm for mapping molecular dynamics in living cells and provides new insights into the mechanism of clathrin-mediated endocytosis.

Interleukin-6 inhibits endothelial nitric oxide synthase activation and increases endothelial nitric oxide synthase binding to stabilized caveolin-1 in human vascular endothelial cells

OBJECTIVE

We hypothesized a possible mechanism for atherosclerosis in which interleukin-6 (IL-6) might affect the endothelial nitric oxide synthase (eNOS)-caveolin-1 interaction and result in decreased nitric oxide bioavailability in the setting of low-grade inflammation.

METHODS

Because eNOS and caveolin-1 are crucial for vascular tone control, we studied the effects of IL-6 on the expression and activation of eNOS and caveolin-1 in human vascular endothelial cells.

RESULTS

IL-6 inhibited the phosphorylation of eNOS at Ser1177 and the bradykinin-stimulated nitric oxide production; however, eNOS protein expression was not changed. In addition, IL-6 inhibited bradykinin-stimulated Akt phosphorylation at Ser473 and Thr 308 without affecting the Akt protein expression. IL-6 did not alter the mRNA level of caveolin-1; however, the caveolin-1 protein level was significantly increased dose-dependently. The binding of eNOS and caveolin-1 in endothelial cells, as demonstrated by coimmunoprecipitation assay, was increased by IL-6 treatment. IL-6 treatment was found to stabilize caveolin-1 protein and its half-life was estimated to prolong from 7.5 h to longer than 12 h. Furthermore, treatment with PD98059 and short interference RNA of extracellular signal-regulated kinase gene reversed the effects of IL-6 on eNOS and caveolin-1.

CONCLUSION

In addition to decreasing Akt phosphorylation, the results of this study demonstrate, for the first time, the molecular mechanism underlying the effect of IL-6 to decrease the nitric oxide bioavailability by increasing the half-life and, therefore, the protein levels of caveolin-1. The increased caveolin-1 proteins bind more eNOS and consequently decrease eNOS activation by reducing the Ser1177 phosphorylation.

Oncosome formation in prostate cancer: association with a region of frequent chromosomal deletion in metastatic disease

Oncosomes have recently been described as membrane-derived microvesicles secreted by cancer cells, which transfer oncogenic signals and protein complexes across cell boundaries. Here, we show the rapid formation and secretion of oncosomes from DU145 and LNCaP human prostate cancer cells. Oncosome formation was stimulated by epidermal growth factor receptor activation and also by overexpression of membrane-targeted Akt1. Microvesicles shed from prostate cancer cells contained numerous signal transduction proteins and were capable of activating rapid phospho-tyrosine and Akt pathway signaling, and stimulating proliferation and migration, in recipient tumor cells. They also induced a stromal reaction in recipient normal cells. Knockdown of the actin nucleating protein Diaphanous Related Formin 3 (DRF3/Dia2) by RNA interference enhanced rates of oncosome formation, indicating that these structures resemble, and may be identical to, nonapoptotic membrane blebs, a feature of the amoeboid form of cell motility. Analysis of primary and metastatic human prostate tumors using 100K single nucleotide polymorphism arrays revealed a significantly higher frequency of deletion of the locus encoding DRF3 (DIAPH3) in metastatic tumors (P = 0.001) in comparison with organ-confined tumors. Fluorescence in situ hybridization confirmed increased chromosomal loss of DIAPH3 in metastatic tumors in a different cohort of patients (P = 0.006). These data suggest that microvesicles shed from prostate cancer cells can alter the tumor microenvironment in a manner that may promote disease progression. They also show that DRF3 is a physiologically relevant protein that seems to regulate this process.

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.

Cytoplasmic provenance of STAT3 and PY-STAT3 in the endolysosomal compartments in pulmonary arterial endothelial and smooth muscle cells: implications in pulmonary arterial hypertension

Lung vascular lesions in pulmonary arterial hypertension (PAH) are characterized by enlarged, vacuolated ("megalocytotic") pulmonary arterial endothelial (PAEC) and smooth muscle cells (PASMC). We have recently proposed that dysfunction of vesicle tethers, soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs), and SNAP receptors (SNAREs), leading to disruptions of intracellular trafficking in the Golgi to plasma membrane (centrifugal) and the plasma membrane to cell interior (centripetal) directions is a key causal mechanism in this disease. In PAH, there was a reciprocal relationship between loss of caveolin-1 (cav-1) in PAECs and increased expression of "activated" tyrosine-phosphorylated STAT3 (PY-STAT3) associated with a block in centrifugal trafficking to/through the Golgi organelle. In the present study, we investigated 1) whether centripetal trafficking of STAT3 and PY-STAT3 in PAECs and PASMCs was membrane-associated, and 2) whether this might be affected in PAH. Immunofluorescence and live cell imaging studies showed that, in both PAEC and PASMC, STAT3 was associated with cytoplasmic vesicles partially colocalizing with markers of the endolysosomal compartments (clathrin, EEA1, Rab5, Rab11, and LAMP1). Overexpression of cav-1 increased the targeting of STAT3 to lysosomes and inhibited STAT3 transcriptional activity. Exposure of PAECs to monocrotaline (MCT) pyrrole, which causes PAH in the rat, led to a loss of caveolar STAT3 with increased sequestration of STAT3 and PY-STAT3 in endosomes. In vivo, marked cytoplasmic sequestration of activated PY-STAT3 was a common feature in PAEC in the rat/MCT model and in cells in the proliferative arterial and plexiform lesions in PAH in humans. These data highlight the epigenetic regulation of centripetal cytokine and growth-factor signaling pathways and its modulation in PAH.

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.

Immunohistochemical study on caveolin-1alpha in regenerating process of tubular cells in gentamicin-induced acute tubular injury in rats

Caveolin-1, a principal component of caveolae, modulates growth signaling, endocytosis, and intracellular transport. We examined the expression of caveolin-1alpha and its relation to cell cycle and caveolin-interacting growth factor receptors in regenerating proximal tubules (PTs) after gentamicin-induced acute renal failure in rats. Caveolin-1alpha appeared in regenerating PTs as early as day 4 after last gentamicin, peaked at days 6 to 8, and showed cytoplasmic pattern after day 8. Immunoelectron microscopy revealed caveolin-1alpha-positive caveolae on the cell membrane and in cytoplasms in regenerating PTs at days 4 to 8 and caveolin-positivity confined to cytoplasms after day 10. The number of PT cells with proliferation markers peaked at day 6 and decreased afterwards as expression of cyclin-dependent kinase inhibitors increased. Platelet-derived growth factor receptor-beta (PDGFR-beta) and epidermal growth factor receptor (EGFR) were colocalized with caveolin-1alpha in proliferating PTs as early as day 4. Phosphorylated EGFR increased at day 8 and afterwards when caveolins dissociated from EGFR or decreased. In case of PDGFR-beta, phosphorylation seemed to be associated with the increase and association of caveolins to the receptors. Our results suggest that transient expression of caveolin-1alpha in early regenerating PTs might contribute to the regenerating process of PTs through modulating growth factor receptors.

Binding of nuclear caveolin-1 to promoter elements of growth-associated genes in ovarian carcinoma cells

Caveolin-1 (cav-1), a member of a protein family associated mainly with cell membrane microdomains in many cell types, acts as a tumor suppressor in ovarian carcinoma cells. Biochemical analyses demonstrated that cav-1 was also localized in the nuclei of ovarian carcinoma cells, endogenously (SKOV3) or ectopically (IGtC3) expressing cav-1. By confocal analyses, the same cell lines as well as IGROV1 and SKOV3 cells transiently transfected with green fluorescent protein-cav-1 fusion protein showed nuclear punctate speckled pattern. Subnuclear distribution analysis revealed cav-1 mainly associated with the nuclear matrix, but also slightly with chromatin. Cav-1 was found in nuclear high-molecular weight complexes and by confocal analysis was found to co-localized with the inner nuclear membrane protein emerin. Cyclin D1 and folate receptor promoters were modulated by cav-1 in SKOV3 cells as demonstrated by transient transfection with or silencing of cav-1. Chromatin immunoprecipitation and supershift assays indicated that nuclear cav-1 can bind in vitro and in vivo to promoter sequences of both cyclin D1 and folate receptor genes. These data suggest that in ovarian carcinoma cells cav-1, localized in transcriptionally inactive chromatin, exerts a functional activity mediated, at least in part, by directly binding to sequences of genes involved in proliferation.

Caveolin-1 and Lipid Rafts in Confluent BeWo Trophoblasts: Evidence for Rock-1 Association with Caveolin-1

Lipid rafts are detergent-insoluble, low-density membrane domains that are rich in cholesterol and sphingolipids; caveolae are a subdomain of the biochemically defined glycolipid raft whose expression is associated with the protein caveolin-1. This protein associates with numerous signalling molecules, regulating their activity by holding them inactive. Human villous cytotrophoblasts contain caveolin-1, but levels reduce greatly during their differentiation into syncytiotrophoblast. Since caveolin-1 is a known regulator of apoptosis and trophoblast syncytialisation involves the apoptotic cascade, we hypothesised that cytotrophoblast caveolin-1 may also play a role in regulating fusion events involved in syncytium formation. The BeWo choriocarcinoma cell line has previously proved valuable for studying trophoblast syncytialisation, hence the present work was carried out to determine whether BeWo cells could be used as a model for the exploration of caveolin-1's role in regulating the syncytialisation process. Undifferentiated BeWo cells were found to express caveolin-1 in similar amounts to villous cytotrophoblasts isolated from term placenta. Lipid raft fractions prepared from these BeWo cells at confluence contained the raft-associated proteins caveolin-1 and -2, flotillin-1 and -2, stomatin and the heterotrimeric G protein, Galphaq. Confocal immunofluorescence studies revealed that caveolin-1 is internalized to the mitochondria, but not to the Golgi or endoplasmic reticulum, in subconfluent BeWo and that the protein relocates to the plasma membrane upon confluence, an observation confirmed by caveolin-1 and cytochrome c Western blotting of lipid raft fractions and mitochondria purified from confluent and subconfluent cells. Western blotting and immunofluorescence experiments comparing undifferentiated cells and those induced to differentiate using the cAMP analogue, dibutyryl cAMP, showed that BeWo syncytialisation was accompanied by a reduction in caveolin-1 levels, similar to the situation in primary villous cytotrophoblasts. Confluent, undifferentiated BeWo cultures were then used to investigate the cellular localisation of Rock-1, a protein which promotes cytoskeletal re-organisation important for syncytialisation and apoptosis. Its association with caveolin-1 was evidenced by the demonstration that the 160kDa proenzyme form of Rock-1 co-immunoprecipitates with caveolin-1 and vice versa, as well as by the co-localisation of the two proteins at the plasma membrane, as shown in immunofluorescence studies. A proportion of the total cell Rock-1 content was found in BeWo lipid raft fractions, confirming its membrane presence in confluent cells. This close association of plasmalemmal caveolin-1 with Rock-1 protein raises the possibility that caveolin-1 may regulate Rock-1 in these trophoblasts. We conclude that cell-cell contact is required for BeWo trophoblast to exhibit plasmalemmal caveolin-1; BeWo cells at confluence offer a useful model for the study of trophoblast raft behaviour during syncytialisation and for the exploration of the potential Rock-1-regulating role of caveolin-1 in this process.

Caveolin-1 interacts with the Gag precursor of murine leukaemia virus and modulates virus production

Background

Retroviral Gag determines virus assembly at the plasma membrane and the formation of virus-like particles in intracellular multivesicular bodies. Thereby, retroviruses exploit by interaction with cellular partners the cellular machineries for vesicular transport in various ways.

Results

The retroviral Gag precursor protein drives assembly of murine leukaemia viruses (MLV) at the plasma membrane (PM) and the formation of virus like particles in multivesicular bodies (MVBs). In our study we show that caveolin-1 (Cav-1), a multifunctional membrane-associated protein, co-localizes with Gag in a punctate pattern at the PM of infected NIH 3T3 cells. We provide evidence that Cav-1 interacts with the matrix protein (MA) of the Gag precursor. This interaction is mediated by a Cav-1 binding domain (CBD) within the N-terminus of MA. Interestingly, the CBD motif identified within MA is highly conserved among most other γ-retroviruses. Furthermore, Cav-1 is incorporated into MLV released from NIH 3T3 cells. Overexpression of a GFP fusion protein containing the putative CBD of the retroviral MA resulted in a considerable decrease in production of infectious retrovirus. Moreover, expression of a dominant-negative Cav-1 mutant affected retroviral titres significantly.

Conclusion

This study demonstrates that Cav-1 interacts with MLV Gag, co-localizes with Gag at the PM and affects the production of infectious virus. The results strongly suggest a role for Cav-1 in the process of virus assembly.

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.

Translocation of endothelial nitric-oxide synthase involves a ternary complex with caveolin-1 and NOSTRIN

Recently, we characterized a novel endothelial nitric-oxide synthase (eNOS)-interacting protein, NOSTRIN (for eNOS-trafficking inducer), which decreases eNOS activity upon overexpression and induces translocation of eNOS away from the plasma membrane. Here, we show that NOSTRIN directly binds to caveolin-1, a well-established inhibitor of eNOS. Because this interaction occurs between the N terminus of caveolin (positions 1-61) and the central domain of NOSTRIN (positions 323-434), it allows for independent binding of each of the two proteins to eNOS. Consistently, we were able to demonstrate the existence of a ternary complex of NOSTRIN, eNOS, and caveolin-1 in Chinese hamster ovary (CHO)-eNOS cells. In human umbilical vein endothelial cells (HUVECs), the ternary complex assembles at the plasma membrane upon confluence or thrombin stimulation. In CHO-eNOS cells, NOSTRIN-mediated translocation of eNOS involves caveolin in a process most likely representing caveolar trafficking. Accordingly, trafficking of NOSTRIN/eNOS/caveolin is affected by altering the state of actin filaments or cholesterol levels in the plasma membrane. During caveolar trafficking, NOSTRIN functions as an adaptor to recruit mediators such as dynamin-2 essential for membrane fission. We propose that a ternary complex between NOSTRIN, caveolin-1, and eNOS mediates translocation of eNOS, with important implications for the activity and availability of eNOS in the cell.

Simultaneous expression of caveolin-1 and E-cadherin in ovarian carcinoma cells stabilizes adherens junctions through inhibition of src-related kinases

Cadherin-mediated adhesion plays an important role in maintaining cell-cell contacts and reducing tumor metastasis. However, neo-expression of E-cadherin in ovarian carcinoma does not prevent the release and spread of cells from the primary tumor. Because caveolin-1 is down-regulated concomitantly with E-cad expression, we investigated whether the stability of adherens junctions in ovarian carcinoma was affected by caveolin-1 expression. We used IGROV1 cells transfected with caveolin-1 (IGtC3), mock-transfected control cells (IGtM87), and SKOV3 cells that endogenously express caveolin-1. Simultaneous expression of caveolin-1 and E-cadherin favored membrane distribution of E-cadherin and its associated catenin (p120ctn), even when caveolin-1 was only focally associated with adherens junctions. Silencing of caveolin-1 induced intracellular E-cadherin redistribution in IGtC3 and SKOV3 cells. Treatment with the specific src kinase inhibitor PP1 increased E-cadherin expression in IGtM87 and SKOV3 cells and enhanced membrane localization of both E-cadherin and p120ctn. However, PP1 could not completely reverse the detrimental effects on cell-cell adhesion induced by Ca2+ depletion in IGtM87 cells. Together, our data suggest that caveolin-1 expression indirectly promotes cell-cell adhesion in ovarian carcinoma cells by a mechanism involving inhibition of src-related kinases. Thus, down-regulation or loss of caveolin-1 might contribute significantly to the spread of tumor cells from the primary tumor.

Caveolins, caveolae, and lipid rafts in cellular transport, signaling, and disease

Caveolae were initially described some 50 years ago. For many decades, they remained predominantly of interest to structural biologists. The identification of a molecular marker for these domains, caveolin, combined with the possibility to isolate such cholesterol- and sphingolipid-rich regions as detergent-insoluble membrane complexes paved the way to more rigorous characterization of composition, regulation, and function. Experiments with knock-out mice for the caveolin genes clearly demonstrate the importance of caveolin-1 and -3 in formation of caveolae. Nonetheless, detergent-insoluble domains are also found in cells lacking caveolin expression and are referred to here as lipid rafts. Caveolae and lipid rafts were shown to represent membrane compartments enriched in a large number of signaling molecules whose structural integrity is essential for many signaling processes. Caveolin-1 is an essential structural component of cell surface caveolae, important for regulating trafficking and mobility of these vesicles. In addition, caveolin-1 is found at many other intracellular locations. Variations in subcellular localization are paralleled by a plethora of ascribed functions for this protein. Here, more recent data addressing the role of caveolin-1 in cellular signaling and the development of diseases like cancer will be preferentially discussed.

Endocytosis of influenza viruses

Receptor-mediated endocytosis is known to play an important role in the entry of many viruses into host cells. However, the exact internalization mechanism has, until recently, remained poorly understood for many medically important viruses, including influenza. Developments in real-time imaging of single viruses as well as the use of dominant-negative mutants to selectively block specific endocytic pathways have improved our understanding of the influenza infection process.

Translocation of caveolin regulates stretch-induced ERK activity in vascular smooth muscle cells

Mechanical stress contributes to vascular disease related to hypertension. Activation of ERK is key to mediating cellular proliferation and vascular remodeling in response to stretch stress. However, the mechanism by which stretch mediates ERK activation in the vascular tissue is still unclear. Caveolin, a major component of a flasklike invaginated caveolae, acts as an adaptor protein for an integrin-mediated signaling pathway. We found that cyclic stretch transiently induced translocation of caveolin from caveolae to noncaveolar membrane sites in vascular smooth muscle cells (VSMCs). This translocation of caveolin was determined by detergent solubility, sucrose gradient fractionation, and immunocytochemistry. Cyclic stretch induced ERK activation; the activity peaked at 5 min (the early phase), decreased gradually, but persisted up to 120 min (the late phase). Disruption of caveolae by methyl-beta-cyclodextrin, decreasing the caveolar caveolin and accumulating the noncaveolar caveolin, enhanced ERK activation in both the early and late phases. When endogenous caveolins were downregulated, however, the late-phase ERK activation was subsided completely. Caveolin, which was translocated to noncaveolar sites in response to stretch, is associated with beta1-integrins as well as with Fyn and Shc, components required for ERK activation. Taken together, caveolin in caveolae may keep ERK inactive, but when caveolin is translocated to noncaveolar sites in response to stretch stress, caveolin mediates stretch-induced ERK activation through an association with beta1-integrins/Fyn/Shc. We suggest that stretch-induced translocation of caveolin to noncaveolar sites plays an important role in mediating stretch-induced ERK activation in VSMCs.

Assembly of endocytic machinery around individual influenza viruses during viral entry

Most viruses enter cells via receptor-mediated endocytosis. However, the entry mechanisms used by many of them remain unclear. Also largely unknown is the way in which viruses are targeted to cellular endocytic machinery. We have studied the entry mechanisms of influenza viruses by tracking the interaction of single viruses with cellular endocytic structures in real time using fluorescence microscopy. Our results show that influenza can exploit clathrin-mediated and clathrin- and caveolin-independent endocytic pathways in parallel, both pathways leading to viral fusion with similar efficiency. Remarkably, viruses taking the clathrin-mediated pathway enter cells via the de novo formation of clathrin-coated pits (CCPs) at viral-binding sites. CCP formation at these sites is much faster than elsewhere on the cell surface, suggesting a virus-induced CCP formation mechanism that may be commonly exploited by many other types of viruses.

Distinct mechanisms of agonist-induced endocytosis for human chemokine receptors CCR5 and CXCR4

Desensitization of the chemokine receptors, a large class of G protein-coupled receptors, is mediated in part by agonist-driven receptor endocytosis. However, the exact pathways have not been fully defined. Here we demonstrate that the rate of ligand-induced endocytosis of CCR5 in leukocytes and expression systems is significantly slower than that of CXCR4 and requires prolonged agonist treatment, suggesting that these two receptors use distinct mechanisms. We show that the C-terminal domain of CCR5 is the determinant of its slow endocytosis phenotype. When the C-tail of CXCR4 was exchanged for that of CCR5, the resulting CXCR4-CCR5 (X4-R5) chimera displayed a CCR5-like trafficking phenotype. We found that the palmitoylated cysteine residues in this domain anchor CCR5 to plasma membrane rafts. CXCR4 and a C-terminally truncated CCR5 mutant (CCR5-KRFX) lacking these cysteines are not raft associated and are endocytosed by a clathrin-dependent pathway. Genetic inhibition of clathrin-mediated endocytosis demonstrated that a significant fraction of ligand-occupied CCR5 trafficked by clathrin-independent routes into caveolin-containing vesicular structures. Thus, the palmitoylated C-tail of CCR5 is the major determinant of its raft association and endocytic itineraries, differentiating it from CXCR4 and other chemokine receptors. This novel feature of CCR5 may modulate its signaling potential and could explain its preferential use by HIV for person-to-person transmission of disease.

Peroxisome proliferator-activated receptor-gamma upregulates caveolin-1 and caveolin-2 expression in human carcinoma cells

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear receptor for eicosanoids that promotes differentiation of human epithelial and mesenchymal cells in vitro and in vivo. PPARgamma was proposed as a target for drug-induced differentiation therapy of cancer. Caveolin-1 is a constituent of plasma membrane caveolae in epithelial cells that is often downregulated upon oncogenic transformation. Caveolin-1 has growth-inhibitory activities and its disruption is sufficient to induce transformation in fibroblasts. Herein we have tested the hypothesis that caveolins are transcriptional target genes for PPARgamma. In human HT-29 colon carcinoma cells, thiazolidinedione PPARgamma ligands increased the levels of caveolin-1 and caveolin-2 proteins two to fivefold in a concentration-dependent manner within 24 h. In human MCF-7 breast adenocarcinoma cells, nonthiazolidinedione PPARgamma ligands elevated caveolin-2 protein three to fourfold, while the thiazoli-dinediones were less effective. Caveolin-1 mRNA levels were found to be upregulated by PPARgamma ligands already after 3 h in both the cell lines. Ectopic expression of a dominant-negative PPARgamma construct attenuated ligand-induced upregulation of caveolins in both HT-29 and HEK-293T cells, indicating that ligand action is mediated by PPARgamma. Ligand-treated MCF-7 cells exhibited a differentiated phenotype, as evinced by analysis of cell-specific differentiation markers: protein levels of maspin were elevated and perinuclear lipid droplets accumulated. In contrast, in HT-29 cells, caveolin expression was not correlated with differentiation. Interestingly, PPARgamma partially cofractionated in lipid rafts and could be coimmunoprecipitated from cell lysates with caveolin-1, indicating that PPARgamma and caveolin-1 may coexist in a complex. Our data indicate that PPARgamma participates in the regulation of caveolin gene expression in human carcinoma cells and suggest that caveolin-1 may mediate some of the phenotypic changes induced by this nuclear receptor in cancer cells. These findings may have potentially important functional implications in the context of cancer differentiation therapy and multidrug resistance.

Caveolin: a key target for modulating nitric oxide availability in health and disease

The endothelial layer is a key component of the cardiovascular system. Recent evidence indicates that strategies aimed at preserving the endothelium may have important implications in the battle against cardiovascular disease. Nitric oxide remains the critical factor determinant of endothelial function. Understanding the regulatory components involved in nitric oxide production may elucidate novel targets for improving compromised vascular function. The caveolae/caveolin system has recently become of interest due to its ability to regulate endothelial nitric oxide synthase activity. The caveolae/caveolin system is a multifaceted structure in the plasma membrane, which plays an integral role in cellular signaling. Recognizing the potential of this specialized domain may provide the fundamental knowledge to target the endothelium in disease.

Dual control of caveolar membrane traffic by microtubules and the actin cytoskeleton

Live cell, time-lapse microscopy was used to study trafficking of caveolin-1-GFP in stably expressing CHO cells. Multiple cytological and biochemical tests verified that caveolin-1-GFP was a reliable marker for endogenous caveolin-1. At steady state, most caveolin-1-GFP was either at the cell surface associated with invaginated caveolae or near the centrosome in caveosomes. Live cell fluorescence imaging indicated that while much of the caveolin-1-GFP in caveolae at the cell surface was relatively sessile, numerous, highly motile caveolin-1-GFP-positive vesicles were present within the cell interior. These vesicles moved at speeds ranging from 0.3-2 microm/second and movement was abolished when microtubules were depolymerized with nocodazole. In the absence of microtubules, cell surface invaginated caveolae increased more than twofold and they became organized into linear arrays. Complete depolymerization of the actin cytoskeleton with latrunculin A, by contrast, triggered rapid and massive movements of caveolin-positive structures towards the centrosomal region of the cell. The caveolar membrane system of CHO cells therefore appears to be comprised of three caveolin-1-containing compartments. These include caveolae that are confined to the cell surface by cortical actin filaments, the peri-centrosomal caveosomes and caveolar vesicles, which we call 'cavicles', that move constitutively and bi-directionally along microtubules between the cell surface and caveosomes. The behavior of cavicles suggests that they function as transport intermediates between caveolae and caveosomes.

Caveolin-1 mutations (P132L and null) and the pathogenesis of breast cancer: caveolin-1 (P132L) behaves in a dominant-negative manner and caveolin-1 (-/-) null mice show mammary epithelial cell hyperplasia

Caveolin-1 (Cav-1) is the principal structural protein of caveolae membranes that are found in most cells types, including mammary epithelial cells. Recently, we mapped the human CAV1 gene to a suspected tumor suppressor locus (7q31.1/D7S522) that is deleted in a variety of human cancers, as well as mammary tumors. In addition, the CAV1 gene is mutated (P132L) in up to approximately 16% of human breast cancers. The mechanism by which deletion or mutation of the Cav-1 gene contributes to mammary tumorigenesis remains unknown. To understand the role of the Cav-1 (P132L) mutation in the pathogenesis of human breast cancers, we generated the same mutation in wild-type (WT) Cav-1 and studied its behavior in cultured cells. Interestingly, the P132L mutation leads to formation of misfolded Cav-1 oligomers that are retained within the Golgi complex and are not targeted to caveolae or the plasma membrane. To examine whether the Cav-1 (P132L) mutant behaves in a dominant-negative manner, we next co-transfected cells with Cav-1 (P132L) and WT Cav-1, and evaluated their caveolar targeting. Our results indicate that Cav-1 (P132L) behaves in a dominant-negative manner, causing the mislocalization and intracellular retention of WT Cav-1. Virtually identical results were obtained when Cav-1 (P132L) was stably expressed at physiological levels in a nontransformed human mammary epithelial cell line (hTERT-HME1). These data provide a molecular explanation for why only a single mutated CAV1 allele is found in patients with breast cancer. Thus, we next investigated if functional inactivation of Cav-1 gene expression leads to mammary tumorigenesis in vivo. For this purpose, we performed mammary gland analysis on Cav-1-deficient mice (-/-) that harbor a targeted disruption of the Cav-1 gene (a null mutation). Interestingly, we show that inactivation of Cav-1 gene expression leads to mammary epithelial cell hyperplasia, even in 6-week-old virgin female mice. These data clearly implicate loss of functional Cav-1 in the pathogenesis of mammary epithelial cell hyperplasia, and suggest that Cav-1-null mice represent a novel animal model to study premalignant mammary disease.

Caveolin-1 expression enhances endothelial capillary tubule formation

The level of caveolin-1 expression closely correlates with the oncogenic transformation of NIH 3T3 cells, the proliferation of human cancer cells, and the differentiation of adipocytes and muscle cells. However, the role of caveolin-1 in endothelial cell proliferation and differentiation remains unknown. Here, we have shown that angiogenic growth factors that stimulate endothelial cell proliferation lead to dramatic reductions in caveolin-1 expression. In addition, using an in vitro Matrigel assay system, we studied the potential role of caveolin-1 in capillary-like tubule formation (i.e. endothelial cell differentiation) using human microvascular endothelial cells (HMEC-1). We showed that the level of endogenous caveolin-1 expression increased in a time-dependent manner when endothelial cells underwent differentiation and that the maximum level of caveolin-1 expression occurred just prior to the formation of capillary-like tubules. Interestingly, overexpression of caveolin-1, via an adenoviral gene delivery system, clearly accelerated endothelial cell differentiation/tubule formation and led to a dramatic approximately 3-fold increase in the number of capillary-like tubular structures. Conversely, down-regulation of caveolin-1 expression, via an antisense adenoviral approach, reduced the number of capillary-like tubules formed by >10-fold. Consistent with the unique function of caveolin-1 in interacting with key signaling molecules, delivery of the caveolin-1 scaffolding domain into the cytoplasm of living endothelial cells was also sufficient to enhance capillary-like tubule formation. Taken together, these results clearly demonstrate that caveolin-1 and the caveolin-1 scaffolding domain play an important positive role in the regulation of endothelial cell differentiation, a prerequisite step in the process of angiogenesis.

Dual-colour imaging of membrane protein targeting directed by poa semilatent virus movement protein TGBp3 in plant and mammalian cells

The movement function of poa semilatent hordeivirus (PSLV) is mediated by the triple gene block (TGB) proteins, of which two, TGBp2 and TGBp3, are membrane proteins. TGBp3 is localized to peripheral bodies in the vicinity of the plasma membrane and is able to re-direct TGBp2 from the endoplasmic reticulum (ER) to the peripheral bodies. For imaging of TGBp3-mediated protein targeting, PSLV TGBp3 tagged with a red fluorescent protein (DsRed) was used. Coexpression of DsRed-TGBp3 with GFP targeted to the ER lumen (ER-GFP) demonstrated that ER-GFP was contained in typical ER structures and peripheral bodies formed by TGBp3 protein, suggesting an ER origin for these bodies. In transient coexpression with viral membrane proteins tagged with GFP, DsRed-TGBp3 directed to the peripheral bodies the homologous TGBp2 protein and two unrelated membrane proteins, the 6 kDa movement protein of beet yellows closterovirus and the putative movement protein encoded by the genome component 4 of faba bean necrotic yellows nanovirus. However, coexpression of TGBp3 with GFP derivatives targeted to the ER membranes by artificial hydrophobic tail sequences suggested that targeting to the ER membranes per se was not sufficient for TGBp3-directed protein trafficking to peripheral bodies. TGBp3-induced targeting of TGBp2 also occurred in mammalian cells, indicating the universal nature of the protein trafficking signals and the cotargeting mechanism.

Sites of Ca2+ wave initiation move with caveolae to the trailing edge of migrating cells

The caveola is a membrane domain that compartmentalizes signal transduction at the cell surface. Normally in endothelial cells, groups of caveolae are found clustered along stress fibers or at the lateral margins in all regions of the cell. Subsets of these clusters appear to contain the signaling machinery for initiating Ca2+ wave formation. Here we report that induction of cell migration, either by wounding a cell monolayer or by exposing cells to laminar shear stress, causes caveolae to move to the trailing edge of the cell. Concomitant with the relocation of the caveolae,sites of Ca2+ wave initiation move to the same location. In as much as the relocated caveolae contain elements of the signaling machinery required for ATP-stimulated release of Ca2+ from the ER, these results suggest that caveolae function as containers that carry this machinery to different cellular locations.

Endothelial nitric oxide synthase activity is linked to its presence at cell-cell contacts

The enzyme endothelial nitric oxide synthase (eNOS) is essential for vascular integrity. Many studies have demonstrated a link between the localization and activity of eNOS. Here, we studied the influence of cell-cell contact on this link in the microvascular endothelial bEnd.3 cell line. By immunofluorescence microscopy, eNOS localization at the plasma membrane was found to be dependent on cell-cell contact. In particular, eNOS was highly enriched at the intercellular contact sites. Further analysis showed that the pattern of eNOS localization at the plasma membrane resembled that of PECAM-1 (platelet endothelial cell adhesion molecule 1), but not that of the adherens junction proteins VE (vascular endothelial)-cadherin and plakoglobin. eNOS that was localized at the contact sites was, in part, Triton X-100-insoluble, in contrast with eNOS at the Golgi complex, which may indicate an association of eNOS with the actin cytoskeleton. Interestingly, eNOS activity was up-regulated in confluent monolayers compared with subconfluent cells, while there was no difference in eNOS expression. This correlation between cell confluence and eNOS activity was also found when primary bovine aortic endothelial cells were studied. These data imply that cell-cell contact induces the localization of eNOS at intercellular junctions, which is required for agonist-induced eNOS activation.

Caveolin-1 null mice are viable but show evidence of hyperproliferative and vascular abnormalities

Caveolin-1 is the principal structural protein of caveolae membranes in fibroblasts and endothelia. Recently, we have shown that the human CAV-1 gene is localized to a suspected tumor suppressor locus, and mutations in Cav-1 have been implicated in human cancer. Here, we created a caveolin-1 null (CAV-1 -/-) mouse model, using standard homologous recombination techniques, to assess the role of caveolin-1 in caveolae biogenesis, endocytosis, cell proliferation, and endothelial nitric-oxide synthase (eNOS) signaling. Surprisingly, Cav-1 null mice are viable. We show that these mice lack caveolin-1 protein expression and plasmalemmal caveolae. In addition, analysis of cultured fibroblasts from Cav-1 null embryos reveals the following: (i) a loss of caveolin-2 protein expression; (ii) defects in the endocytosis of a known caveolar ligand, i.e. fluorescein isothiocyanate-albumin; and (iii) a hyperproliferative phenotype. Importantly, these phenotypic changes are reversed by recombinant expression of the caveolin-1 cDNA. Furthermore, examination of the lung parenchyma (an endothelial-rich tissue) shows hypercellularity with thickened alveolar septa and an increase in the number of vascular endothelial growth factor receptor (Flk-1)-positive endothelial cells. As predicted, endothelial cells from Cav-1 null mice lack caveolae membranes. Finally, we examined eNOS signaling by measuring the physiological response of aortic rings to various stimuli. Our results indicate that eNOS activity is up-regulated in Cav-1 null animals, and this activity can be blunted by using a specific NOS inhibitor, nitro-l-arginine methyl ester. These findings are in accordance with previous in vitro studies showing that caveolin-1 is an endogenous inhibitor of eNOS. Thus, caveolin-1 expression is required to stabilize the caveolin-2 protein product, to mediate the caveolar endocytosis of specific ligands, to negatively regulate the proliferation of certain cell types, and to provide tonic inhibition of eNOS activity in endothelial cells.

Pulmonary lipid phosphate phosphohydrolase in plasma membrane signalling platforms

Lipid phosphate phosphohydrolase (LPP) has recently been proposed to have roles in signal transduction, acting sequentially to phospholipase D (PLD) and in attenuating the effects of phospholipid growth factors on cellular proliferation. In this study, LPP activity is reported to be enriched in lipid-rich signalling platforms isolated from rat lung tissue, isolated rat type II cells and type II cell-mouse lung epithelial cell lines (MLE12 and MLE15). Lung and cell line caveolin-enriched domains (CEDs), prepared on the basis of their detergent-insolubility in Triton X-100, contain caveolin-1 and protein kinase C isoforms. The LPP3 isoform was predominantly localized to rat lung CEDs. These lipid-rich domains, including those from isolated rat type II cells, were enriched both in phosphatidylcholine plus sphingomyelin (PC+SM) and cholesterol. Saponin treatment of MLE15 cells shifted the LPP activity, cholesterol, PC+SM and caveolin-1 from lipid microdomains to detergent-soluble fractions. Elevated LPP activity and LPP1/1a protein are present in caveolae from MLE15 cells prepared using the cationic-colloidal-silica method. In contrast, total plasma membranes had a higher abundance of LPP1/1a protein with low LPP activity. Phorbol ester treatment caused a 3.8-fold increase in LPP specific activity in MLE12 CEDs. Thus the activated form of LPP1/1a may be recruited into caveolae/rafts. Transdifferentiation of type II cells into a type I-like cell demonstrated enrichment in caveolin-1 levels and LPP activity. These results indicate that LPP is localized in caveolae and/or rafts in lung tissue, isolated type II cells and type II cell lines and is consistent with a role for LPP in both caveolae/raft signalling and caveolar dynamics.

Caveolin-1 expression negatively regulates cell cycle progression by inducing G(0)/G(1) arrest via a p53/p21(WAF1/Cip1)-dependent mechanism

Caveolin-1 is a principal component of caveolae membranes in vivo. Caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes. Interestingly, the human caveolin-1 gene is localized to a suspected tumor suppressor locus (7q31.1). However, it remains unknown whether caveolin-1 plays any role in regulating cell cycle progression. Here, we directly demonstrate that caveolin-1 expression arrests cells in the G(0)/G(1) phase of the cell cycle. We show that serum starvation induces up-regulation of endogenous caveolin-1 and arrests cells in the G(0)/G(1) phase of the cell cycle. Moreover, targeted down-regulation of caveolin-1 induces cells to exit the G(0)/G(1) phase. Next, we constructed a green fluorescent protein-tagged caveolin-1 (Cav-1-GFP) to examine the effect of caveolin-1 expression on cell cycle regulation. We directly demonstrate that recombinant expression of Cav-1-GFP induces arrest in the G(0)/G(1) phase of the cell cycle. To examine whether caveolin-1 expression is important for modulating cell cycle progression in vivo, we expressed wild-type caveolin-1 as a transgene in mice. Analysis of primary cultures of mouse embryonic fibroblasts from caveolin-1 transgenic mice reveals that caveolin-1 induces 1) cells to exit the S phase of the cell cycle with a concomitant increase in the G(0)/G(1) population, 2) a reduction in cellular proliferation, and 3) a reduction in the DNA replication rate. Finally, we demonstrate that caveolin-1-mediated cell cycle arrest occurs through a p53/p21-dependent pathway. Taken together, our results provide the first evidence that caveolin-1 expression plays a critical role in the modulation of cell cycle progression in vivo.

Caveolae and the caveolins in human disease

There has been an exponential growth in caveolae research since the early 1990s. The caveolae membrane system comprises unique lipid and protein domains, and fulfills a role in a wide range of processes. At the plasma membrane caveolae serve to compartmentalise and integrate a wide range of signal transduction processes. A key structural and functional protein for caveolae is caveolin. Caveolin proteins possess a 'scaffolding' domain which for caveolins-1 and -3 appear central to many of the reported signal regulation functions for caveolae. Caveolae or caveolin protein are increasingly implicated in the molecular pathology of a number of diseases. Opportunities exist for basic and applied investigators working within the pharmaceutical sciences to exploit the caveolae membrane system to identify novel pharmacological targets and therapeutic strategies, including the delivery of pharmacologically active caveolin based peptides.

Palmitoylation of caveolin-1 in endothelial cells is post-translational but irreversible

Caveolin-1 is a palmitoylated protein involved in assembly of signaling molecules in plasma membrane subdomains termed caveolae and in intracellular cholesterol transport. Three cysteine residues in the C terminus of caveolin-1 are subject to palmitoylation, which is not necessary for caveolar targeting of caveolin-1. Protein palmitoylation is a post-translational and reversible modification that may be regulated and that in turn may regulate conformation, membrane association, protein-protein interactions, and intracellular localization of the target protein. We have undertaken a detailed analysis of [(3)H]palmitate incorporation into caveolin-1 in aortic endothelial cells. The linkage of palmitate to caveolin-1 was hydroxylamine-sensitive and thus presumably a thioester bond. However, contrary to expectations, palmitate incorporation was blocked completely by the protein synthesis inhibitors cycloheximide and puromycin. In parallel experiments to show specificity, palmitoylation of aortic endothelial cell-specific nitric-oxide synthase was unaffected by these reagents. Inhibitors of protein trafficking, brefeldin A and monensin, blocked caveolin-1 palmitoylation, indicating that the modification was not cotranslational but rather required caveolin-1 transport from the endoplasmic reticulum and Golgi to the plasma membrane. In addition, immunophilin chaperones that form complexes with caveolin-1, i.e. FK506-binding protein 52, cyclophilin A, and cyclophilin 40, were not necessary for caveolin-1 palmitoylation because agents that bind immunophilins did not inhibit palmitoylation. Pulse-chase experiments showed that caveolin-1 palmitoylation is essentially irreversible because the release of [(3)H]palmitate was not significant even after 24 h. These results show that [(3)H]palmitate incorporation is limited to newly synthesized caveolin-1, not because incorporation only occurs during synthesis but because the continuous presence of palmitate on caveolin-1 prevents subsequent repalmitoylation.

Caveolae/lipid rafts in fibroblast-like synoviocytes: ectopeptidase-rich membrane microdomains

Membrane peptidases play important roles in cell activation, proliferation and communication. Human fibroblast-like synoviocytes express considerable amounts of aminopeptidase N/CD13, dipeptidyl peptidase IV/CD26, and neprilysin/CD10, transmembrane proteins previously proposed to be involved in the regulation of intra-articular levels of neuropeptides and chemotactic mediators as well as in adhesion and cell-cell interactions. Here, we report these peptidases in synoviocytes to be localized predominantly in glycolipid- and cholesterol-rich membrane microdomains known as 'rafts'. At the ultrastructural level, aminopeptidase N/CD13 and dipeptidyl peptidase IV/CD26 were found in caveolae, in particular in intracellular yet surface-connected vesicle-like structures and 'rosettes' made up of several caveolae. In addition, clusters of peptidases were seen at the cell surface in flat patches ranging in size from about 60 to 160 nm. Cholesterol depletion of synoviocytes by methyl-beta-cyclodextrin disrupted >90% of the caveolae and reduced the raft localization of aminopeptidase N/CD13 without affecting Ala-p-nitroanilide-cleaving activity of confluent cell cultures. In co-culture experiments with T-lymphocytes, cholesterol depletion of synoviocytes greatly reduced their capability to induce an early lymphocytic expression of aminopeptidase N/CD13. We propose caveolae/rafts to be peptidase-rich 'hot-spot' regions of the synoviocyte plasma membrane required for functional cell-cell interactions with lymphocytes. The peptidases may act in concert with other types of proteins such as receptors and signal transducers localized in these specialized membrane domains.

Hypermethylation of the caveolin-1 gene promoter in prostate cancer

BACKGROUND

Hypermethylation of CpG islands in the promoter regions of tumor suppressor genes is one mechanism of tumorigenesis. Caveolin-1 (Cav-1), a gene coding for the structural component of cellular caveolae, is involved in cell signaling and has been proposed to be a tumor suppressor gene in several malignancies. This gene maps to 7q31.1, a site known to be deleted in some prostate tumors. We chose to examine the methylation status of the promoter region of Cav-1 to determine whether this gene could function as a tumor suppressor in prostate cancer

METHODS

Genomic DNA from both tumor and normal prostate epithelial cells was obtained from paraffin-embedded prostate sections by laser capture microdissection (LCM). The methylation status of 24 CpG sites at the 5' promoter region of Cav-1 was analyzed by bisulfite-direct-sequencing after amplification by PCR using primers specific for bisulfate modified DNA. Immunohistochemistry staining with a cav-1-specific antibody was also performed to evaluate the expression of the gene

RESULTS

Twenty of the 22 (90.9%) informative cases showed promoter hypermethylation in the tumor cell population when compared with adjacent normal prostate cells with an average Methylation Index (potential frequency of total possible methylated Cs) from tumor cells equal to 0.426 vs. 0.186 for normal cells (P = 0.001). While no association with Gleason grade was found, overall increased methylation correlated with PSA failure (P = 0.016), suggestive of clinical recurrence. Elevated immunoreactivity with a Cav-1 antibody was observed in tumor cells from 7 of 26 prostate samples tested; this was associated with a Gleason score but not correlated with PSA failure or Methylation Index

CONCLUSIONS

CpG sites at the 5' promoter of Cav-1 are more methylated in tumor than in adjacent normal prostate cells. Hypermethylation of the Cav-1 promoter supports the notion that Cav-1 may function as a tumor suppressor gene in prostate cancer and evidence is presented suggesting that methylation status of this gene is not only a marker for cancer but also may be predictive of outcome.

Caveolin-1 binding to endoplasmic reticulum membranes and entry into the regulated secretory pathway are regulated by serine phosphorylation. Protein sorting at the level of the endoplasmic reticulum

Caveolin-1 serves as the main coat protein of caveolae membranes, as an intracellular cholesterol shuttle, and as a regulator of diverse signaling molecules. Of the 12 residues conserved across all caveolin isoforms from all species examined to date, only Ser(80) and Ser(168) could serve as phosphorylation sites. We show here that mimicking chronic phosphorylation of Ser(80) by mutation to Glu (i.e. Cav-1(S80E)), blocks phosphate incorporation. However, Cav-1(S168E) is phosphorylated to the same extent as wild-type caveolin-1. Cav-1(S80E) targets to the endoplasmic reticulum membrane, remains oligomeric, and maintains normal membrane topology. In contrast, Cav-1(S80A), which cannot be phosphorylated, targets to caveolae membranes. Some exocrine cells secrete caveolin-1 in a regulated manner. Cav-1(S80A) is not secreted by AR42J pancreatic adenocarcinoma cells even in the presence of dexamethasone, an agent that induces the secretory phenotype. Conversely, Cav-1(S80E) is secreted to a greater extent than wild-type caveolin-1 following dexamethasone treatment. We conclude that caveolin-1 phosphorylation on invariant serine residue 80 is required for endoplasmic reticulum retention and entry into the regulated secretory pathway.

Nitric oxide attenuates signal transduction: possible role in dissociating caveolin-1 scaffold

Caveolae harbor different serpentine receptors, intracellular components of signaling cascades, and certain enzymes, including endothelial nitric oxide synthase (eNOS). The regulation of eNOS activity by Ca(2+)/calmodulin and caveolin has been described. We have previously demonstrated that nitric oxide (NO) can modulate signaling initiated via receptors localized to caveolae. In the present study, we show that NO donors induced an increase in the monomeric form of this scaffolding protein in cultured endothelial cells, the effect mimicked by 8-bromo cGMP. Proximity imaging of endothelial cells transfected with the thermotolerant green fluorescent protein-caveolin-1 construct demonstrated that sodium nitroprusside resulted in the increased fluorescence ratio of 410:470 nm, consistent with the distancing of fluorescently tagged caveolin-1. Pulse labeling of endothelial cells with cholera toxin B subunit indicated that sodium nitroprusside reversibly decreased its binding. Signaling via G protein-coupled receptors resident to caveolae was inhibited by pretreatment with NO donor. The data demonstrate that NO modulation of cell signaling is accomplished in part by regulating the state of caveolin-1 oligomerization. NO-induced attenuation of signaling involves reversible dissociation of caveolin scaffold, thus providing both spatial and temporal modulation of signal transduction.

Limb-girdle muscular dystrophy (LGMD-1C) mutants of caveolin-3 undergo ubiquitination and proteasomal degradation. Treatment with proteasomal inhibitors blocks the dominant negative effect of LGMD-1C mutanta and rescues wild-type caveolin-3

Caveolin-3 is the principal structural protein of caveolae in striated muscle. Autosomal dominant limb-girdle muscular dystrophy (LGMD-1C) in humans is due to mutations (DeltaTFT and Pro --> Leu) within the CAV3 gene. We have shown that LGMD-1C mutations lead to formation of unstable aggregates of caveolin-3 that are retained intracellularly and are rapidly degraded. The mechanism by which LGMD-1C mutants of caveolin-3 are degraded remains unknown. Here, we show that LGMD-1C mutants of caveolin-3 undergo ubiquitination-proteasomal degradation. Treatment with proteasomal inhibitors (MG-132, MG-115, lactacystin, or proteasome inhibitor I), but not lysosomal inhibitors, prevented degradation of LGMD-1C caveolin-3 mutants. In the presence of MG-132, LGMD-1C caveolin-3 mutants accumulated within the endoplasmic reticulum and did not reach the plasma membrane. LGMD-1C mutants of caveolin-3 behave in a dominant negative fashion, causing intracellular retention and degradation of wild-type caveolin-3. Interestingly, in cells co-expressing wild-type and mutant forms of caveolin-3, MG-132 treatment rescued wild-type caveolin-3; wild-type caveolin-3 was not degraded and reached the plasma membrane. These results may have clinical implications for treatment of patients with LGMD-1C.

Caveolin-1 expression inhibits Wnt/beta-catenin/Lef-1 signaling by recruiting beta-catenin to caveolae membrane domains

Caveolin-1 is a principal component of caveolae membranes. In NIH 3T3 cells, caveolin-1 expression is dramatically up-regulated in confluent cells and localizes at areas of cell-cell contact. However, it remains unknown whether caveolin-1 is involved in cell-cell signaling. Here, we examine the potential role of caveolin-1 in regulating beta-catenin signaling. beta-Catenin plays a dual role in the cell, linking E-cadherin to the actin cytoskeleton and in Wnt signaling by forming a complex with members of the lymphoid enhancing factor (Lef-1) family of transcription factors. We show that E-cadherin, beta-catenin, and gamma-catenin (plakoglobin) are all concentrated in caveolae membranes. Moreover, we demonstrate that activation of beta-catenin/Lef-1 signaling by Wnt-1 or by overexpression of beta-catenin itself is inhibited by caveolin-1 expression. We also show that recombinant expression of caveolin-1 in caveolin-1 negative cells is sufficient to recruit beta-catenin to caveolae membranes, thereby blocking beta-catenin-mediated transactivation. These results suggest that caveolin-1 expression can modulate Wnt/beta-catenin/Lef-1 signaling by regulating the intracellular localization of beta-catenin.

A molecular dissection of caveolin-1 membrane attachment and oligomerization. Two separate regions of the caveolin-1 C-terminal domain mediate membrane binding and oligomer/oligomer interactions in vivo

Caveolins form interlocking networks on the cytoplasmic face of caveolae. The cytoplasmically directed N and C termini of caveolins are separated by a central hydrophobic segment, which is believed to form a hairpin within the membrane. Here, we report that the caveolin scaffolding domain (CSD, residues 82-101), and the C terminus (residues 135-178) of caveolin-1 are each sufficient to anchor green fluorescent protein (GFP) to membranes in vivo. We also show that the first 16 residues of the C terminus (i.e. residues 135-150) are necessary and sufficient to attach GFP to membranes. When fused to the caveolin-1 C terminus, GFP co-localizes with two trans-Golgi markers and is excluded from caveolae. In contrast, the CSD targets GFP to caveolae, albeit less efficiently than full-length caveolin-1. Thus, caveolin-1 contains at least two membrane attachment signals: the CSD, dictating caveolar localization, and the C terminus, driving trans-Golgi localization. Additionally, we find that caveolin-1 oligomer/oligomer interactions require the distal third of the caveolin-1 C terminus. Thus, the caveolin-1 C-terminal domain has two separate functions: (i) membrane attachment (proximal third) and (ii) protein/protein interactions (distal third).

Concentration of caveolin-1 in the cleavage furrow as revealed by time-lapse analysis

Caveolin-1 is a major component of caveolae. Recent studies have suggested a possible role of caveolin-1 in cell transformation and normal cell proliferation. To observe the behavior of caveolin-1 in living mitotic cells, we prepared cDNA constructs encoding the chimeric protein of alpha- or beta-caveolin-1 and green fluorescent protein (GFP) and transfected culture cells with them. Correct targeting of the chimera to the caveolae was confirmed by colocalization with the caveolar inositol 1,4,5-trisphosphate receptor-like protein. By time-lapse observation of mitotic MDCKII cells, the GFP-caveolin-1 chimeras were seen throughout the plasma membrane before cell division, but became markedly concentrated at the cleavage furrow during cytokinesis. Accumulation around the spindle poles was also observed at late telophase. The result showed that caveolin-1 undergoes a drastic distributional change during cell division and suggested that the protein may be involved in the cytokinetic process.