Decreased expression of caveolin 1 in patients with systemic sclerosis: crucial role in the pathogenesis of tissue fibrosis.

OBJECTIVE: Recent studies have implicated caveolin 1 in the regulation of transforming growth factor beta (TGFbeta) downstream signaling. Given the crucial role of TGFbeta in the pathogenesis of systemic sclerosis (SSc), we sought to determine whether caveolin 1 is also involved in the pathogenesis of tissue fibrosis in SSc. We analyzed the expression of CAV1 in affected SSc tissues, studied the effects of lack of expression of CAV1 in vitro and in vivo, and analyzed the effects of restoration of caveolin 1 function on the fibrotic phenotype of SSc fibroblasts in vitro. METHODS: CAV1 expression in tissues was analyzed by immunofluorescence and confocal microscopy. The extent of tissue fibrosis in Cav1-knockout mice was assessed by histologic/histochemical analyses and quantified by hydroxyproline assays. Cav1-null and SSc fibroblast phenotypes and protein production were analyzed by real-time polymerase chain reaction, immunofluorescence, Western blot, and multiplexed enzyme-linked immunosorbent assay techniques. The effects of restoration of caveolin 1 function in SSc fibroblasts in vitro were also examined using a cell-permeable recombinant CAV1 peptide. RESULTS: CAV1 was markedly decreased in the affected lungs and skin of SSc patients. Cav1-knockout mice developed pulmonary and skin fibrosis. Down-regulation of caveolin 1 was maintained in cultured SSc fibroblasts, and restoration of caveolin 1 function in vitro normalized their phenotype and abrogated TGFbeta stimulation through inhibition of Smad3 activation. CONCLUSION: Caveolin 1 appears to participate in the pathogenesis of tissue fibrosis in SSc. Restoration of caveolin 1 function by treatment with a cell-permeable peptide corresponding to the CAV1 scaffolding domain may be a novel therapeutic approach in SSc

Caveolin-1 interacts with the chaperone complex TCP-1 and modulates its protein folding activity

Abstract.: We report that caveolin-1, one of the major structural protein of caveolae, interacts with TCP-1, a hetero-oligomeric chaperone complex present in all eukaryotic cells that contributes mainly to the folding of actin and tubulin. The caveolin-TCP-1 interaction entails the first 32 amino acids of the N-terminal segment of caveolin. Our data show that caveolin-1 expression is needed for the induction of TCP-1 actin folding function in response to insulin stimulation. Caveolin-1 phosphorylation at tyrosine residue 14 induces the dissociation of caveolin-1 from TCP-1 and activates actin folding. We show that the mechanism by which caveolin-1 modulates TCP-1 activity is indirect and involves the cytoskeleton linker filamin. Filamin is known to bind caveolin-1 and to function as a negative regulator of insulin-mediated signaling. Our data support the notion that the caveolin-filamin interaction contributes to restore insulin-mediated phosphorylation of caveolin, thus allowing the release of active TCP-

Caveolin-1, TGF-β receptor internalization, and the pathogenesis of systemic sclerosis

PURPOSE OF REVIEW: To review the scientific literature supporting the participation of caveolin-1 in the pathogenesis of tissue fibrosis and the notion that modulation of the caveolin-1 pathway may represent a novel treatment for systemic sclerosis and other fibrotic diseases. RECENT FINDINGS: Caveolin-1 plays an important role in the regulation of transforming growth factor-beta (TGF-beta) signaling owing to its participation in TGF-beta receptor internalization. TGF-beta receptor internalized through caveolin-1 lipid rafts undergoes rapid degradation, effectively decreasing TGF-beta signaling. Studies have shown that caveolin-1 knockdown in vitro markedly increased collagen gene expression in normal human lung fibroblasts. Caveolin-1 was reduced in affected systemic sclerosis lungs and skin and in idiopathic pulmonary fibrosis lung tissues and fibroblasts. Increasing caveolin-1 expression markedly improved bleomycin-induced pulmonary fibrosis. Restoration of caveolin bioavailability employing penetratin, a cell-permeable peptide carrier for a bioactive caveolin-1 fragment, abrogated TGF-beta activation of cultured human dermal fibroblasts. Systemic administration of penetratin-caveolin-1 peptide to mice with bleomycin-induced lung fibrosis reduced fibrosis. SUMMARY: Caveolin-1 plays an important role in the regulation of TGF-beta signaling and participates in the pathogenesis of systemic sclerosis and idiopathic pulmonary fibrosis. Restoration of caveolin function employing active caveolin-1 fragments coupled to cell-permeable carrier peptides may represent a novel approach for their treatment

Expression of Caveolin-1 Is Required for the Transport of Caveolin-2 to the Plasma Membrane RETENTION OF CAVEOLIN-2 AT THE LEVEL OF THE GOLGI COMPLEX

Caveolins-1 and -2 are normally co-expressed, and they form a hetero-oligomeric complex in many cell types. These caveolin hetero-oligomers are thought to represent the assembly units that drive caveolae formation in vivo. However, the functional significance of the interaction between caveolins-1 and -2 remains unknown. Here, we show that caveolin-1 co-expression is required for the transport of caveolin-2 from the Golgi complex to the plasma membrane. We identified a human erythroleukemic cell line, K562, that expresses caveolin-2 but fails to express detectable levels of caveolin-1. This allowed us to stringently assess the effects of recombinant caveolin-1 expression on the behavior of endogenous caveolin-2. We show that expression of caveolin-1 in K562 cells is sufficient to reconstitute the de novo formation of caveolae in these cells. In addition, recombinant expression of caveolin-1 allows caveolin-2 to form high molecular mass oligomers that are targeted to caveolae-enriched membrane fractions. In striking contrast, in the absence of caveolin-1 expression, caveolin-2 forms low molecular mass oligomers that are retained at the level of the Golgi complex. Interestingly, we also show that expression of caveolin-1 in K562 cells dramatically up-regulates the expression of endogenous caveolin-2. Northern blot analysis reveals that caveolin-2 mRNA levels remain constant under these conditions, suggesting that the expression of caveolin-1 stabilizes the caveolin-2 protein. Conversely, transient expression of caveolin-2 in CHO cells is sufficient to up-regulate endogenous caveolin-1 expression. Thus, the formation of a hetero-oligomeric complex between caveolins-1 and -2 stabilizes the caveolin-2 protein product and allows caveolin-2 to be transported from the Golgi complex to the plasma membrane

Caveolin-1 recruitment to the trailing edge of motile cells results in focal adhesion disassembly and nascent interaction with actin stress fibers

The protein caveolin-1 has been shown to positively affect angiogenesis and vascular remodeling in vivo via studies using knockout mice. In fact, defects in these two processes are among the major hallmarks of an otherwise benign caveolin-null phenotype. Current dogma on the function of caveolin-1 does not predict or account for these deficits. The overall objective of the following studies was to uncover the role of caveolin-1 in angiogenesis and vascular remodeling through study of the protein in cell-substratum remodeling during cell motility in vitro.;In the first study, caveolin-1 and its parent organelle, caveolae, conspicuously polarize to the rear of migrating human umbilical vein endothelial cells. Moreover, caveolin-1 localizated at the cell rear is mutually exclusive with focal adhesion staining and lamellipodial protrusion. Acute caveolin-1 knockdown by small, interfering RNA diminished the ability of endothelial cells to polarize and migrate toward a chemotactic stimulus.;In the second study, live cell imaging was used to study the dynamics between caveolin-1, focal adhesions, and the actin cytoskeleton. Caveolin-1 recruitment and transient association with focal adhesions at the trailing edge resulted in adhesion sliding and disassembly, concomitant with recoil of the trailing edge into the cell body proper. Moreover, association of caveolin-1 with actin stress fibers previously associated with adhesions in the collapsing trailing edge was observed. Mouse embryonic fibroblasts from caveolin-1 null mice demonstrated defects in trailing edge recoil compared to control cells with no decrease in cell contractility, suggesting a specific deficit in adhesion disassembly. Furthermore, caveolin-null cells displayed a decrease in overall chemokinetic motility and an increase in directional persistence, an indication that caveolin-1 contributes to movement plasticity via trailing edge focal adhesion disassembly.;In the final study, the interaction of polarized caveolin-1 with actin stress fibers at the cell rear was characterized. Caveolin-1 predictably associated with the cell perimeter depending on the direction of cell migration. Importantly, inhibition on non-muscle myosin by blebbistatin treatment abrogated initial polarization of caveolin-1, but did not affect caveolin-1 that had already polarized. Using live cell imaging in conjunction with photobleaching, actin-associated caveolin-1 was found to be extremely static upon polarization to the cell rear. In contrast, the initial polarization of caveolin-1 to retracting areas was highly dynamic. Furthermore, GM1 internalization at the cell rear was negligible, confirming that polarized caveolae are highly static. Forced disruption of the actin cytoskeleton by cytochalasin D treatment resulted in caveolin-1 depolarization and disaggregation into small puncta displaying frenetic, kiss-and-run movement. Furthermore, cytoskeletal remodeling in response to change in direction of a cell resulted in similar caveolin depolarization.;In summary, stress fibers associate with and exert traction on trailing edge focal adhesions during cell motility. This traction force is prerequisite for caveolin-1 recruitment. Arrival and transient association of caveolin-1 with focal adhesions results in adhesion disassembly and stable interaction of caveolin with actin stress fibers. Thus, a novel mechanism in cellular mechanotransduction can be described, whereby cells utilize caveolin-1 recruitment to relieve strain generated at the cell perimeter by the actin cytoskeleton during movement. This novel function of caveolin-1 may analogously occur in vivo, beyond the context of endothelial cell migration. The deficits in angiogenesis and vascular remodeling seen in caveolin-1 null mice might thus be explained by the role of caveolin-1 in cell-substratum remodeling in response to strain

Analysis of peroxynitrite-mediated post-translational modifications of caveolin-1

Caveolin-1 is an important protein in caveolae, which plays a role in cholesterol transport, signal transduction, and transcytosis and tumor suppression. Caveolin-1 is found in endothelial cells, smooth muscle cells and adipocytes. The main focus of this study is to investigate the peroxynitrite-mediated in vitro post-translational modifications (PTMs) of caveolin-1. Bovine brain was used to isolate caveolin-1 as an initial step for isolation method development. Density gradient centrifugation was used to isolate caveolin-1 from bovine brain. From the isolate, caveolin-1α, caveolin-1β isomers and caveolin dimer were identified by western blotting with anti-caveolin monoclonal antibody. Glutathione S-transferase (GST)-caveolin fusion protein was used to isolate caveolin-1 and used for in vitro experiments in this study. During normal and pathological conditions, endothelial cells are subjected to locally generated reactive oxygen species such as peroxynitrite. Peroxynitrite is capable of modifying amino acids such as tyrosine, cysteine, tryptophan and methionine. Peroxynitrite mediated tyrosine nitration of caveolin-1 was detected by SDS-PAGE followed by western blotting with anti-nitrotyrosine monoclonal antibody. The approach used to identify potentially modified peptide sequences of caveolin-1 was ESI-MS/MS. Fluorometry was used to detect formation of dityrosine. Caveolin-1 was treated with different concentrations of peroxynitrite in caveolin- under the physiological conditions and found that caveolin-1 form dimer and oligomer under the physiological conditions. The stability of caveolin-1 dimer and oligomer suggests that the coupling mechanism could most likely be occurred via a covalent bond. Western blotting with anti-nitrotyrosine monoclonal antibody revealed the formation of nitrotyrosine upon the exposure to peroxynitrite. In this study, we report the nitration of specific tyrosine residues of caveolin-1 for the first time. ESI-MS/MS analysis revealed that peroxynitrite can selectively nitrate Tyr6 and Tyr14 located in the tryptic peptide YVDSEGHLYTVPIR under physiological conditions. Caveolin-1 can form dityrosine upon exposure to peroxynitrite as shown by fluorometry. Oxidative and nitrative modifications due to the reaction of peroxynitrite with caveolin-1 may lead to several pathological conditions. Our study can provide authentic standards of modified proteins, which will be used to determine post-translational modifications of caveolin-1 in vivo

Caveolin-1 and -2 in the Exocytic Pathway of MDCK Cells

Abstract. We have studied the biosynthesis and transport of the endogenous caveolins in MDCK cells. We show that in addition to homooligomers of caveolin-1, heterooligomeric complexes of caveolin-1 and -2 are formed in the ER. The oligomers become larger, increasingly detergent insoluble, and phosphorylated on caveolin-2 during transport to the cell surface. In the TGN caveolin-1/-2 heterooligomers are sorted into basolateral vesicles, whereas larger caveolin-1 homooligomers are targeted to the apical side. Caveolin-1 is present on both the apical and basolateral plasma membrane, whereas caveolin-2 is enriched on the basolateral surface where caveolae are present. This suggests that caveolin-1 and -2 heterooligomers are involved in caveolar biogenesis in the basolateral plasma membrane. Anti–caveolin-1 antibodies inhibit the apical delivery of influenza virus hemagglutinin without affecting basolateral transport of vesicular stomatitis virus G protein. Thus, we suggest that caveolin-1 homooligomers play a role in apical transport

Ocadaic acid treatment alters the intracellular localization of caveolin-1 and caveolin-2 in HepG2 cells

In this paper we provide evidences that protein phosphatases could regulate the intracellular localization of caveolin isoforms in a hepatoma cell line (HepG2). Ocadaic acid (OA) - a serine/threonine phosphatase inhibitor – was used in various concentrations (4nM and 100nM) to study the localization of caveolin-1 and caveolin-2 in HepG2 cells. Using fluorescent and confocal immunocytochemistry we have found that OA in both concentrations has significantly altered the intracellular localization and distribution of the caveolin-1 and caveolin-2 as well. In control (-OA treatment) the caveolin-1 was present in discrete punctate structures in the cytoplasm and also on the cell membrane. Caveolin-2 has partly overlapped with caveolin-1, but a significant amount caveolin-2 was detected around the nucleus. After OA (4 and 100 nM) treatment caveolin-1 has disappeared from the cell membrane, it was present mainly in the cytoplasm in larger vesicle or vacuole-like structures that were arranged along the cables of the cytoskeleton. In many cases caveolin-2 was found to colocalize with caveolin-1, but there was always a significant amount of caveolin-2 present around the nucleus. Immunoprecipitation and Western blot analysis revealed that in OA-treated cells a ~24 kDa protein identified as caveolin-2 was strongly phosphorylated on tyrosine residues. The effect of OA was not reversible, since the removal of OA has not resulted in the dephosphorylation of caveolin-2 and the perinuclear localization of caveolin-2 remained. Our data indicate that phophorylation of caveolin-2 can alter not only the intracellular localization of caveolin isoforms but also the distribution of caveolae. The cytoskeleton seems to play an important role in the normal and altered distribution of caveolae, and the tyrosine phosphorylation or the absence of dephosphorylation of caveolin-2 isoform can inhibit the recycling of caveolae

Caveolin-2 associates with intracellular chlamydial inclusions independently of caveolin-1

BACKGROUND: Lipid raft domains form in plasma membranes of eukaryotic cells by the tight packing of glycosphingolipids and cholesterol. Caveolae are invaginated structures that form in lipid raft domains when the protein caveolin-1 is expressed. The Chlamydiaceae are obligate intracellular bacterial pathogens that replicate entirely within inclusions that develop from the phagocytic vacuoles in which they enter. We recently found that host cell caveolin-1 is associated with the intracellular vacuoles and inclusions of some chlamydial strains and species, and that entry of those strains depends on intact lipid raft domains. Caveolin-2 is another member of the caveolin family of proteins that is present in caveolae, but of unknown function. METHODS: We utilized a caveolin-1 negative/caveolin-2 positive FRT cell line and laser confocal immunofluorescence techniques to visualize the colocalization of caveolin-2 with the chlamydial inclusions. RESULTS: We show here that in infected HeLa cells, caveolin-2, as well as caveolin-1, colocalizes with inclusions of C. pneumoniae (Cp), C. caviae (GPIC), and C. trachomatis serovars E, F and K. In addition, caveolin-2 also associates with C. trachomatis serovars A, B and C, although caveolin-1 did not colocalize with these organisms. Moreover, caveolin-2 appears to be specifically, or indirectly, associated with the pathogens at the inclusion membranes. Using caveolin-1 deficient FRT cells, we show that although caveolin-2 normally is not transported out of the Golgi in the absence of caveolin-1, it nevertheless colocalizes with chlamydial inclusions in these cells. However, our results also show that caveolin-2 did not colocalize with UV-irradiated Chlamydia in FRT cells, suggesting that in these caveolin-1 negative cells, pathogen viability and very likely pathogen gene expression are necessary for the acquisition of caveolin-2 from the Golgi. CONCLUSION: Caveolin-2 associates with the chlamydial inclusion independently of caveolin-1. The function of caveolin-2, either in the uninfected cell or in the chlamydial developmental cycle, remains to be elucidated. Nevertheless, this second caveolin protein can now be added to the small number of host proteins that are associated with the inclusions of this obligate intracellular pathogen

Caveolin-1-Enhanced Motility and Focal Adhesion Turnover Require Tyrosine-14 but Not Accumulation to the Rear in Metastatic Cancer Cells

Caveolin-1 is known to promote cell migration, and increased caveolin-1 expression is associated with tumor progression and metastasis. In fibroblasts, caveolin-1 polarization and phosphorylation of tyrosine-14 are essential to promote migration. However, the role of caveolin-1 in migration of metastatic cells remains poorly defined. Here, caveolin-1 participation in metastatic cell migration was evaluated by shRNA targeting of endogenous caveolin-1 in MDA-MB-231 human breast cancer cells and ectopic expression in B16-F10 mouse melanoma cells. Depletion of caveolin-1 in MDA-MB-231 cells reduced, while expression in B16-F10 cells promoted migration, polarization and focal adhesion turnover in a sequence of events that involved phosphorylation of tyrosine-14 and Rac-1 activation. In B16-F10 cells, expression of a non-phosphorylatable tyrosine-14 to phenylalanine mutant failed to recapitulate the effects observed with wild-type caveolin-1. Alternatively, treatment of MDA-MB-231 cells with the Src family kinase inhibitor PP2 reduced caveolin-1 phosphorylation on tyrosine-14 and cell migration. Surprisingly, unlike for fibroblasts, caveolin-1 polarization and re-localization to the trailing edge were not observed in migrating metastatic cells. Thus, expression and phosphorylation, but not polarization of caveolin-1 favor the highly mobile phenotype of metastatic cells