The dually acylated NH2-terminal domain of gi1alpha is sufficient to target a green fluorescent protein reporter to caveolin-enriched plasma membrane domains. Palmitoylation of caveolin-1 is required for the recognition of dually acylated g-protein alpha subunits in vivo.
Here we investigate the molecular mechanisms that govern the targeting of G-protein alpha subunits to the plasma membrane. For this purpose, we used Gi1alpha as a model dually acylated G-protein. We fused full-length Gi1alpha or its extreme NH2-terminal domain (residues 1-32 or 1-122) to green fluorescent protein (GFP) and analyzed the subcellular localization of these fusion proteins. We show that the first 32 amino acids of Gi1alpha are sufficient to target GFP to caveolin-enriched domains of the plasma membrane in vivo, as demonstrated by co-fractionation and co-immunoprecipitation with caveolin-1. Interestingly, when dual acylation of this 32-amino acid domain was blocked by specific point mutations (G2A or C3S), the resulting GFP fusion proteins were localized to the cytoplasm and excluded from caveolin-rich regions. The myristoylated but nonpalmitoylated (C3S) chimera only partially partitioned into caveolin-containing fractions. However, both nonacylated GFP fusions (G2A and C3S) no longer co-immunoprecipitated with caveolin-1. Taken together, these results indicate that lipid modification of the NH2-terminal of Gi1alpha is essential for targeting to its correct destination and interaction with caveolin-1. Also, a caveolin-1 mutant lacking all three palmitoylation sites (C133S, C143S, and C156S) was unable to co-immunoprecipitate these dually acylated GFP-G-protein fusions. Thus, dual acylation of the NH2-terminal domain of Gi1alpha and palmitoylation of caveolin-1 are both required to stabilize and perhaps regulate this reciprocal interaction at the plasma membrane in vivo. Our results provide the first demonstration of a functional role for caveolin-1 palmitoylation in its interaction with signaling molecules. (+info)
The Npc1 mutation causes an altered expression of caveolin-1, annexin II and protein kinases and phosphorylation of caveolin-1 and annexin II in murine livers.
We have previously demonstrated (1) an increased expression of caveolin-1 in murine heterozygous and homozygous Niemann-Pick type C (NPC) livers, and (2) an increased concentration of unesterified cholesterol in a detergent insoluble caveolae-enriched fraction from homozygous livers. To define further the relationship between caveolin-1 function and the cholesterol trafficking defect in NPC, we examined the expression and distribution of additional caveolar and signal transduction proteins. The expression of annexin II was significantly increased in homozygous liver homogenates and the Triton X-100 insoluble floating fraction (TIFF). Phosphoamino acid analysis of caveolin-1 and annexin II from the homozygous TIFF demonstrated an increase in serine and tyrosine phosphorylation, respectively. To determine the basis for increased phosphorylation of these proteins, the expression and distribution of several protein kinases was examined. The expression of PKCalpha, PKCzeta and pp60-src (protein kinases) were significantly increased in both heterozygous and homozygous liver homogenates, while PKCdelta was increased only in homozygous livers. Of the protein kinases analyzed, only CK IIalpha was significantly enriched in the heterozygous TIFF. Finally, the concentration of diacylglycerol in the homozygous TIFF was significantly increased and this elevation may modulate PKC distribution and function. These results provide additional evidence for involvement of a caveolin-1 containing cellular fraction in the pathophysiology of NPC and also suggest that the Npc1 gene product may directly or indirectly, regulate the expression and distribution of signaling molecules. (+info)
Hypercholesterolemia decreases nitric oxide production by promoting the interaction of caveolin and endothelial nitric oxide synthase.
Hypercholesterolemia is a central pathogenic factor of endothelial dysfunction caused in part by an impairment of endothelial nitric oxide (NO) production through mechanisms that remain poorly characterized. The activity of the endothelial isoform of NO synthase (eNOS) was recently shown to be modulated by its reciprocal interactions with the stimulatory Ca2+-calmodulin complex and the inhibitory protein caveolin. We examined whether hypercholesterolemia may reduce NO production through alteration of this regulatory equilibrium. Bovine aortic endothelial cells were cultured in the presence of serum obtained from normocholesterolemic (NC) or hypercholesterolemic (HC) human volunteers. Exposure of endothelial cells to the HC serum upregulated caveolin abundance without any measurable effect on eNOS protein levels. This effect of HC serum was associated with an impairment of basal NO release paralleled by an increase in inhibitory caveolin-eNOS complex formation. Similar treatment with HC serum significantly attenuated the NO production stimulated by the calcium ionophore A23187. Accordingly, higher calmodulin levels were required to disrupt the enhanced caveolin-eNOS heterocomplex from HC serum-treated cells. Finally, cell exposure to the low-density lipoprotein (LDL) fraction alone dose-dependently reproduced the inhibition of basal and stimulated NO release, as well as the upregulation of caveolin expression and its heterocomplex formation with eNOS, which were unaffected by cotreatment with antioxidants. Together, our data establish a new mechanism for the cholesterol-induced impairment of NO production through the modulation of caveolin abundance in endothelial cells, a mechanism that may participate in the pathogenesis of endothelial dysfunction and the proatherogenic effects of hypercholesterolemia. (+info)
Regulation of G protein-coupled receptor kinases by caveolin.
G protein-coupled receptor kinases (GRKs) have been principally characterized by their ability to phosphorylate and desensitize G protein-coupled receptors. However, recent studies suggest that GRKs may have more diverse protein/protein interactions in cells. Based on the identification of a consensus caveolin binding motif within the pleckstrin homology domain of GRK2, we tested the direct binding of purified full-length GRK2 to various glutathione S-transferase-caveolin-1 fusion proteins, and we discovered a specific interaction of GRK2 with the caveolin scaffolding domain. Interestingly, analysis of GRK1 and GRK5, which lack a pleckstrin homology domain, revealed in vitro binding properties similar to those of GRK2. Maltose-binding protein caveolin and glutathione S-transferase-GRK fusion proteins were used to map overlapping regions in the N termini of both GRK2 and GRK5 that appear to mediate conserved GRK/caveolin interactions. In vivo association of GRK2 and caveolin was suggested by co-fractionation of GRK2 with caveolin in A431 and NIH-3T3 cells and was further supported by co-immunoprecipitation of GRK2 and caveolin in COS-1 cells. Functional significance for the GRK/caveolin interaction was demonstrated by the potent inhibition of GRK-mediated phosphorylation of both receptor and peptide substrates by caveolin-1 and -3 scaffolding domain peptides. These data reveal a novel mode for the regulation of GRKs that is likely to play an important role in their cellular function. (+info)
Analysis of the CAVEOLIN-1 gene at human chromosome 7q31.1 in primary tumours and tumour-derived cell lines.
We identified CAVEOLIN-1 as a candidate for a tumour suppressor gene mapping to human chromosome 7q31.1. A number of studies suggest that caveolin could function as a tumour suppressor. Expression of caveolin, and in turn the number of caveolae within a cell, are inversely correlated with the transforming ability of numerous oncoproteins, including H-ras, v-abl, and bcr-abl, and caveolin is a major transformation-dependent substrate of v-src. Heterologous expression of caveolin has been shown to abrogate anchorage-independent growth and induce apoptosis in transformed fibroblasts and also to suppress anchorage-independent growth in human mammary carcinoma cells. We have analysed the status and expression of the human CAVEOLIN-1 gene in primary tumours and tumour-derived cell lines. We found no evidence for mutation of CAVEOLIN-1 in human cancers. Additionally, we found that while the first two exons of CAVEOLIN-1 are associated with a CpG island, this is not methylated in either primary tumours or in tumour-derived cell lines in which Caveolin-1 expression is low or undetectable. The level of expression of Caveolin-1 does not correlate with loss of heterozygosity at the CAVEOLIN-1 locus in these same cell lines. Contrary to other published studies, we have shown that CAVEOLIN-1 is not expressed in normal breast ductal epithelial cells in vivo. CAVEOLIN-1 is however highly expressed in breast myoepithelial cells and its expression is retained in tumours derived from breast myoepithelium. Together our data refute a role for CAVEOLIN-1 as a breast tumour suppressor gene in vivo. (+info)
A role for caveolin and the urokinase receptor in integrin-mediated adhesion and signaling.
The assembly of signaling molecules surrounding the integrin family of adhesion receptors remains poorly understood. Recently, the membrane protein caveolin was found in complexes with beta1 integrins. Caveolin binds cholesterol and several signaling molecules potentially linked to integrin function, e.g., Src family kinases, although caveolin has not been directly implicated in integrin-dependent adhesion. Here we report that depletion of caveolin by antisense methodology in kidney 293 cells disrupts the association of Src kinases with beta1 integrins resulting in loss of focal adhesion sites, ligand-induced focal adhesion kinase (FAK) phosphorylation, and adhesion. The nonintegrin urokinase receptor (uPAR) associates with and stabilizes beta1 integrin/caveolin complexes. Depletion of caveolin in uPAR-expressing 293 cells also disrupts uPAR/integrin complexes and uPAR-dependent adhesion. Further, beta1 integrin/caveolin complexes could be disassociated by uPAR-binding peptides in both uPAR-transfected 293 cells and human vascular smooth muscle cells. Disruption of complexes by peptides in intact smooth muscle cells blocks the association of Src family kinases with beta1 integrins and markedly impairs their migration on fibronectin. We conclude that ligand-induced signaling necessary for normal beta1 integrin function requires caveolin and is regulated by uPAR. Caveolin and uPAR may operate within adhesion sites to organize kinase-rich lipid domains in proximity to integrins, promoting efficient signal transduction. (+info)
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.
Caveolin-1, a suspected tumor suppressor, is a principal protein component of caveolae in vivo. Recently, we have shown that NIH 3T3 cells harboring anti-sense caveolin-1 exhibit a loss of contact inhibition and anchorage-independent growth. These observations may be related to the ability of caveolin-1 expression to positively regulate contact inhibition. In order to understand the postulated role of caveolin-1 in contact inhibition, it will be necessary to follow the distribution of caveolins in living cells in response to a variety of stimuli, such as cell density. Here, we visualize the distribution of caveolin-1 in living normal NIH 3T3 cells by creating GFP-fusion proteins. In many respects, the behavior of these GFP-caveolin-1 fusion proteins is indistinguishable from endogenous caveolin-1. These GFP-caveolin-1 fusion proteins co-fractionated with endogenous caveolin-1 using an established protocol that separates caveolae-derived membranes from the bulk of cellular membranes and cytosolic proteins, and co-localized with endogenous caveolin-2 in vivo as seen by immunofluorescence microscopy. We show here that as NIH 3T3 cells become confluent, the distribution of GFP-caveolin-1 and endogenous caveolin-1 shifts to areas of cell-cell contact, coincident with contact inhibition. However, unlike endogenous caveolin-1, the levels of GFP-caveolin-1 expression are unaffected by changes in cell density, serum starvation, or growth factor stimulation. These results are consistent with the idea that the levels of endogenous caveolin-1 are modulated by either transcriptional or translational control, and that this modulation is separable from density-dependent regulation of the distribution of caveolin-1. These studies provide a new living-model system for elucidating the dynamic mechanisms underlying the density-dependent regulation of the distribution of caveolin-1 and how this relates to contact inhibition. (+info)
Tyrosine-phosphorylated caveolin-1: immunolocalization and molecular characterization.
Caveolin-1 was discovered as a major substrate for v-Src, but the effect of its tyrosine phosphorylation has not been known. We generated a specific antibody (PY14) to caveolin-1 phosphorylated at tyrosine 14 and studied the significance of the modification. By Western blotting of lysates of v-Src-expressing cells, PY14 recognized not only a 22-kDa band (the position of nonphosphorylated caveolin-1) but bands at 23-24 and 25 kDa. Bands of slower mobility were diminished by dephosphorylation and were also observed for mutant caveolin-1 lacking tyrosine 14. By immunofluorescence microscopy, PY14 did not label normal cells but detected large dots in v-Src-expressing cells. Immunoelectron microscopy revealed that the dots corresponded to aggregated caveolae and/or vesicles of various sizes; besides, the label was observed in intramembrane particle-free areas in the plasma membrane, which appeared to have been formed by fusion of flattened caveolae. A positive reaction with PY14 was found in normal cells after vanadate or pervanadate treatment; it occurred mainly at 22 kDa by Western blotting and was not seen as large dots by immunofluorescence microscopy. Detergent solubility, oligomerization, and association with caveolin-2 were observed similarly for caveolin-1 in normal and v-Src-expressing cells. The results indicate that phosphorylation of caveolin-1 in v-Src-expressing cells occurs at multiple residues and induces flattening, aggregation, and fusion of caveolae and/or caveolae-derived vesicles. (+info)