The primary sequence of murine caveolin reveals a conserved consensus site for phosphorylation by protein kinase C

Mechanisms regulating endothelial permeability

The endothelial monolayer partitioning underlying tissue from blood components in the vessel wall maintains tissue fluid balance and host defense through dynamically opening intercellular junctions. Edemagenic agonists disrupt endothelial barrier function by signaling the opening of the intercellular junctions leading to the formation of protein-rich edema in the interstitial tissue, a hallmark of tissue inflammation that, if left untreated, causes fatal diseases, such as acute respiratory distress syndrome. In this review, we discuss how intercellular junctions are maintained under normal conditions and after stimulation of endothelium with edemagenic agonists. We have focused on reviewing the new concepts dealing with the alteration of adherens junctions after inflammatory stimulus.

Caveolin-1 expression in different types of psoriatic lesions: analysis of 66 cases

Background:

Caveolin-1 is a key structural and functional protein. Caveolin-1 is known to modulate multiple signal-transducing pathways involved in cell differentiation and proliferation. Psoriasis is viewed as a multifactorial pathology characterized by keratinocyte hyperproliferation and abnormal cell maturation.

Objectives:

To examine the expression of caveolin-1 in skin biopsies from normal subjects, patients, and subjects with the three respective isoforms of psoriasis (psoriasis vulgaris, localized pustular psoriasis, and erythrodermic psoriasis). The expression level of caveolin-1 was compared among psoriasis vulgaris, localized pustular psoriasis, erythrodermic psoriasis, and normal subjects.

Materials and Methods:

Using immunohistochemical methods, caveolin-1 protein expression was assayed in four groups. An analysis was conducted on skin samples obtained from 22 normal subjects and 28 patients with psoriasis vulgaris, 22 patients with localized pustular psoriasis, and 16 patients with erythrodermic psoriasis. The statistical analysis of the scoring criteria reflecting the level of Caveolin-1 immunostaining between different groups was determined using the Mann–Whitney U-test.

Results:

In the normal skin, intense and consistent caveolin-1 staining was present in 22 cases. The Caveolin-1 protein was significantly reduced and showed very weak or absent staining within the tissues of psoriasis vulgaris, localized pustular psoriasis, and erythrodermic psoriasis (respective P < 0.001). Caveolin-1 protein expression in psoriasis vulgaris was higher than that in localized pustular psoriasis and erythrodermic psoriasis (respective P < 0.05). Caveolin-1 protein expression was no different in localized pustular psoriasis and erythrodermic psoriasis (P > 0.05).

Conclusion:

The finding of this study was consistent with a downregulation of Caveolin-1, which might serve as an etiological factor in the development of psoriasis vulgaris, localized pustular psoriasis, and erythrodermic psoriasis. Further mechanistic investigations are required to prove that Caveolin-1 protein has the potential and may be a novel target for therapy of psoriasis vulgaris, localized pustular psoriasis, and erythrodermic psoriasis.

Involvement of caveolin-1 in the Jak-Stat signaling pathway and infectious spleen and kidney necrosis virus infection in mandarin fish (Siniperca chuatsi)

Caveolae, the major source of caveolin-1 protein, are specialized invaginated microdomains of the plasma membrane that act as organizing centers for signaling molecules in the immune system. In the present study, we report the cloning and characterization of caveolin-1 (mCav-1) from mandarin fish (Siniperca chuatsi) and study on the roles of mCav-1 in the fish Jak–Stat signaling pathway and in virus infection. The cDNA sequence of mCav-1 was 707 bp in size, encoding a protein of 181 amino acids, which was different from the mammalian protein (178 amino acids). The deduced amino acid sequence of mCav-1 shared similar architecture with vertebrate caveolin-1 proteins, but mCav-1 lacked a phosphorylation site (y14). The major subcellular location of mCav-1 was in the caveolae, where the protein appeared to have major functions. Real-time PCR revealed that the expression of the mandarin fish Mx, IRF-1, SOCS1, and SOCS3 genes involved in the poly(I:C)-induced Jak–Stat signaling pathway was impaired by the mCav-1 scaffolding domain peptide (mSDP). In mandarin fish fry (MFF-1) cells, the protein levels of mCav-1 were markedly up-regulated at 12 and 24 h post-infection with ISKNV (infectious spleen and kidney necrosis virus). In addition, ISKNV entry into MFF-1 cells was significantly inhibited by mSDP, and the inhibition was dose-dependent. Thus, ISKNV infection was apparently associated with mCav-1 protein and may utilize the caveolae-related endocytosis pathway. The findings reported here further our understanding of the function of caveolin-1 in the complex signal transduction network in fish immune systems and in the cellular entry mechanism of iridoviruses.

Up-regulation of caveolin-1 and blood-brain barrier breakdown are attenuated by N-acetylcysteine in thiamine deficiency

Wernicke's encephalopathy is a cerebral metabolic disorder caused by thiamine (vitamin B1) deficiency (TD). Neuropathologic consequences of TD include region-selective neuronal cell loss and blood-brain barrier (BBB) breakdown. Caveolin-1 is involved in the regulation of tight junction proteins and BBB permeability, and is modulated by oxidative stress, a feature of vulnerable brain regions in TD. We hypothesized that TD-related oxidative stress alters BBB integrity via induction of the caveolin-1 pathway. TD was induced in C57BL6 mice by treatment with a thiamine-deficient diet and administration of the thiamine antagonist pyrithiamine, in the absence or presence of the antioxidant N-acetylcysteine (NAC). A significant and focal increase in both caveolin-1 gene and protein expression was detected in the thalamus of thiamine-deficient mice, concomitant with IgG extravasation. Reduction of oxidative stress by NAC, as shown by normalization of reduced glutathione levels and attenuation of endothelial heme oxygenase-1 and nitric oxide synthase expression, resulted in prevention of the up-regulation of caveolin-1 in TD. Normalization of caveolin-1 levels by NAC was accompanied by a reduction in BBB breakdown, indicated by decreased IgG extravasation, normalization of occludin levels and prevention of matrix metalloproteinase-9 up-regulation. These findings demonstrate a role for caveolin-1 in TD pathogenesis, and suggest that oxidative stress contributes to BBB alterations in TD via modulation of this pathway.

Molecular determinants of endothelial transcytosis and their role in endothelial permeability

Caveolae transcytosis with its diverse mechanisms-fluid phase, adsorptive, and receptor-mediated-plays an important role in the continuous exchange of molecules across the endothelium. We will discuss key features of endothelial transcytosis and caveolae that have been studied recently and have increased our understanding of caveolae function in transcytosis at the molecular level. During transcytosis, caveolae "pinch off" from the plasma membrane to form discrete vesicular carriers that shuttle to the opposite front of endothelial cells, fuse with the plasma membrane, and discharge their cargo into the perivascular space. Endothelial transcytosis exhibits distinct properties, the most important being rapid and efficient coupling of endocytosis to exocytosis on opposite plasma membrane. We address herein the membrane fusion-fission reactions that underlie transcytosis. Caveolae move across the endothelial cells with their cargo predominantly in the fluid phase through an active process that bypasses the lysosomes. Endothelial transcytosis is a constitutive process of vesicular transport. Recent studies show that transcytosis can be upregulated in response to pathological stimuli. Transcytosis via caveolae is an important route for the regulation of endothelial barrier function and may participate in different vascular diseases.

Oxygen alters caveolin-1 and nitric oxide synthase-3 functions in ovine fetal and neonatal lung microvascular endothelial cells

We determined the effect of oxygen [∼100 Torr (normoxia) and ∼30–40 Torr (hypoxia)] on functions of endothelial nitric oxide (NO) synthase (NOS-3) and its negative regulator caveolin-1 in ovine fetal and neonatal lung microvascular endothelial cells (MVECs). Fetal NOS-3 activity, measured as NO production with 0.5–0.9 μM 4-amino-5-methylamino-2,7-difluorofluorescein, was decreased in hypoxia by 14.4% ( P < 0.01), inhibitable by the NOS inhibitor N-nitro-l-arginine, and dependent on extracellular arginine. Caveolar function, assessed as FITC-BSA (160 μg/ml) endocytosis, was decreased in hypoxia by 13.5% in fetal and 22.8% in neonatal MVECs ( P < 0.01). NOS-3 and caveolin-1 were physically associated, as demonstrated by coimmunoprecipitation and colocalization, and functionally associated, as shown by cross-activation of endocytosis, by their specific antibodies and activation of NOS by albumin. Caveolin peptide, containing the sequence for the PKC phosphorylation site of caveolin, and caveolin antiserum against the site increased NO production and endocytosis by 12.3% ( P < 0.05) and 16% ( P < 0.05), respectively, in normoxia and increased endocytosis by 25% ( P < 0.001) in hypoxia. PMA decreased NO production in normoxia and hypoxia by 19.32% ( P < 0.001) and 11.8% ( P < 0.001) and decreased endocytosis in normoxia by 20.35% ( P < 0.001). PKC kinase activity was oxygen sensitive, and threonine phosphorylation was enhanced in hypoxia. Pertussis toxin increased caveolar and NOS functions. These data support our hypothesis that increased Po2at birth promotes dissociation of caveolin-1 and NOS-3, with an increase in their activities, and that PKC and an oxygen-sensitive cell surface G protein-coupled receptor regulate caveolin-1 and NOS-3 interactions in fetal and neonatal lung MVECs.

CO as a cellular signaling molecule

Many biological functions of heme oxygenase (HO), such as cytoprotection against oxidative stress, vasodilation, neurotransmission in the central or peripheral nervous systems, and anti-inflammatory, anti-apoptotic, or anti-proliferative potential, have been attributed to its enzymatic byproduct carbon monoxide (CO), although roles for biliverdin/bilirubin and iron have also been proposed. In addition to these well-characterized effects, recent findings reveal that HO-derived CO may act as an oxygen sensor and circadian modulator of heme biosynthesis. In lymphocytes, CO may participate in regulatory T cell function. A number of the known signaling effects of CO depend on stimulation of soluble guanylate cyclase and/or activation of mitogen-activated protein kinases (MAPK). Furthermore, modulation of caveolin-1 status may serve as an essential component of certain aspects of CO action, such as growth control. In this review, we summarize recent findings of the beneficial or detrimental effects of endogenous CO with an emphasis on the signaling pathways and downstream targets that trigger the action of this gas.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Bile acid induces hydrophobicity-dependent membrane alterations

Elevated concentrations of fecal bile acids are a known risk factor for colon cancer, owing to alterations in cellular signaling. In colonic cells, where bile acid uptake is minimal, the hydrophobicity-induced membrane perturbation and alterations have been proposed, but these membrane alterations are largely uncharacterized. In this study, we examined the determinants and characteristics of bile acid-induced membrane alterations, utilizing PKCalpha activation and cholesterol up-regulation as model indicators. We found that bile acid-induced PKCalpha activation is a function of hydrophobicity and correlated with alteration in membrane lipid composition, as evident by the significant up-regulation in membrane cholesterol and phospholipid. We found that bile acid do not cause cell membrane disruption at a concentration sufficient to activate PKCalpha, but do induce drastic alterations in membrane composition. Bile acid also induced the modification and up-regulation of caveolin-1 in a hydrophobicity-dependent manner, implying widespread receptor dysregulation. Similarly, ERK1/2 activation was observed only in response to hydrophobic bile acids, suggesting hydrophobicity-induced caveolar or membrane stress. Experiments with sodium lauryl sarcosine and cholesteryl hemisuccinate showed that bile acid-induced membrane alterations can be mimicked by hydrophobic molecules unrelated to bile acids, strongly implicating hydrophobicity as an important determinant of bile acid signaling.

The use of time-resolved fluorescence imaging in the study of protein kinase C localisation in cells

Background

Two-photon-excitation fluorescence lifetime imaging (2P-FLIM) was used to investigate the association of protein kinase C alpha (PKCα) with caveolin in CHO cells. PKCα is found widely in the cytoplasm and nucleus in most cells. Upon activation, as a result of increased intracellular Ca²⁺ and production of DAG, through G-protein coupled-phospholipase C signalling, PKC translocates to a variety of regions in the cell where it phosphorylates and interacts with many signalling pathways. Due to its wide distribution, discerning a particular interaction from others within the cell is extremely difficult

Results

Fluorescence energy transfer (FRET), between GFP-PKCα and DsRed-caveolin, was used to investigate the interaction between caveolin and PKC, an aspect of signalling that is poorly understood. Using 2P-FLIM measurements, the lifetime of GFP was found to decrease (quench) in certain regions of the cell from ~2.2 ns to ~1.5 ns when the GFP and DsRed were sufficiently close for FRET to occur. This only occurred when intracellular Ca²⁺ increased or in the presence of phorbol ester, and was an indication of PKC and caveolin co-localisation under these conditions. In the case of phorbol ester stimulated PKC translocation, as commonly used to model PKC activation, three PKC areas could be delineated. These included PKCα that was not associated with caveolin in the nucleus and cytoplasm, PKCα associated with caveolin in the cytoplasm/perinuclear regions and probably in endosomes, and PKC in the peripheral regions of the cell, possibly indirectly interacting with caveolin.

Conclusion

Based on the extent of lifetime quenching observed, the results are consistent with a direct interaction between PKCα and caveolin in the endosomes, and possibly an indirect interaction in the peripheral regions of the cell. The results show that 2P-FLIM-FRET imaging offers an approach that can provide information not only confirming the occurrence of specific protein-protein interactions but where they occur within the cell.

Ocadaic acid treatment causes tyrosine phosphorylation of caveolin-2 and induces internalization of caveolae in rat peritoneal macrophages

In this paper, we provide evidences that protein phosphatases could regulate the internalization cycle of caveolae in rat peritoneal cells. Ocadaic acid (OA)--a serine/threonine phosphatase inhibitor--was used in various concentrations (4 and 100 nM) to study the internalization of horseradish peroxidase (HRP) in resident and elicited macrophages. We have found that OA in both concentrations has significantly decreased HRP uptake in resident and elicited cells. The results of our morphometrical analysis showed that in OA-treated cells, the number of surface-connected caveolae has been dramatically decreased. Simultaneously large, endosome-like vacuoles containing small vesicles appeared in the cytoplasm. The membrane of these small vesicles was labeled with anti-caveolin-1 antibody. Immunoprecipitation and Western blot analysis revealed that in OA-treated cells an approximately 29 kDa protein identified as caveolin-2 in macrophages was phosphorylated on tyrosine residues. These data support the idea that there is a close correlation between the phosphorylation of caveolin-2 and endocytosis of caveolae: the tyrosine phosphorylation of this approximately 29 kDa protein can drive caveolae to pinch off from the plasma membrane and causes accumulation of caveolae in a multivesicular body-like cellular compartment, which was never found to contain lysosomal enzymes. As a result of OA treatment caveolin-2 remains phosphorylated and the phosphorylation of these protein might inhibit the recycling of caveolae.

Loss of caveolin-1 expression is associated with disruption of muscarinic cholinergic activities in the urinary bladder

Caveolin-1 (Cav1), a structural protein of caveolae, plays cell- and context-dependent roles in signal transduction pathway regulation. We have generated a knockout mouse homozygous for a null mutation of the Cav1 gene. Cav1 knockout mice exhibited impaired urinary bladder contractions in vivo during cystometry. Contractions of male bladder strips were evoked with electric and pharmacologic stimulation (5-40 Hz, 1-10 microM carbachol, 10 mM alpha,beta-methylene ATP, 100 mM KCl). Acetylcholine (ACh) and norepinephrine (NE) release from bladder strips were measured with a radiochemical method by incubating the strips with 14C-choline and 3H-NE prior to electric stimulation, whereas ATP release was measured using the luciferin-luciferase assay with a luminometer. A 60-75% decline in contractility was observed when Cav1 knockout muscle strips were stimulated with electric current or carbachol, compared to wildtype muscle strips. No difference in contractility was noted when contractions were evoked either by the purinergic agonist alpha,beta-methylene ATP, or by extracellular potassium. To investigate the relative contribution of non-cholinergic activity to bladder contractility, the amplitude of the electric stimulation-evoked contractions was compared in the presence of the muscarinic antagonist atropine (1 microM). While the non-muscarinic (purinergic) response was unaltered, muscarinic cholinergic response was principally disrupted in Cav1 knockout mice. The loss of Cav1 gene expression was also associated with a 70% reduction in ACh release. NE and ATP release was not altered. It is concluded that the loss of caveolin-1 is associated with disruption of M3 muscarinic cholinergic activity in the bladder. Both pre-junctional (acetylcholine neurotransmitter release from neuromuscular junctions) and post-junctional (M3 receptor-mediated signal transduction in bladder smooth muscles) mechanisms are disrupted, resulting in impaired bladder contraction.

N-terminal processing and modifications of caveolin-1 in caveolae from human adipocytes

Caveolin, the principal structural protein of caveolae membrane domains, has a cytosol-exposed N-terminal part that was cleaved off by trypsin treatment of caveolae vesicles isolated from primary human adipocytes. Sequencing of the released tryptic peptides by nanospray quadrupole time-of-flight mass spectrometry revealed that both caveolin-1alpha and caveolin-1beta were processed by excision of the starting methionines. The N-terminus of the mature caveolin-1alpha was acetylated, while caveolin-1beta was found in acetylated as well as in non-acetylated forms. Fractional phosphorylation of serine-36 in the mature caveolin-1alpha and of the homologous serine-5 in caveolin-1beta was identified. This is the first experimental evidence for in vivo phosphorylation of caveolin-1 at the consensus site for phosphorylation by protein kinase C. The phosphorylation was found in both the acetylated and non-acetylated variants of caveolin-1beta. This variability in modifications is consistent with critical involvement of the N-terminal domain of caveolin in the regulation of caveolae.

Escherichia coli K1 internalization via caveolae requires caveolin-1 and protein kinase Calpha interaction in human brain microvascular endothelial cells

The morbidity and mortality associated with Escherichia coli K1 meningitis during the neonatal period have remained significant over the last decade and are once again on the rise. Transcytosis of brain microvascular endothelial cells (BMEC) by E. coli within an endosome to avoid lysosomal fusion is crucial for dissemination into the central nervous system. Central to E. coli internalization of BMEC is the expression of OmpA (outer membrane protein A), which interacts with its receptor for the actin reorganization that leads to invasion. However, nothing is known about the nature of the signaling events for the formation of endosomes containing E. coli K1. We show here that E. coli K1 infection of human BMEC (HBMEC) results in activation of caveolin-1 for bacterial uptake via caveolae. The interaction of caveolin-1 with phosphorylated protein kinase Calpha (PKCalpha) at the E. coli attachment site is critical for the invasion of HBMEC. Optical sectioning of confocal images of infected HBMEC indicates continuing association of caveolin-1 with E. coli during transcytosis. Overexpression of a dominant-negative form of caveolin-1 containing mutations in the scaffolding domain blocked the interaction of phospho-PKCalpha with caveolin-1 and the E. coli invasion of HBMEC, but not actin cytoskeleton rearrangement or the phosphorylation of PKCalpha. The interaction of caveolin-1 with phospho-PKCalpha was completely abrogated in HBMEC overexpressing dominant-negative forms of either focal adhesion kinase or PKCalpha. Treatment of HBMEC with a cell-permeable peptide that represents the scaffolding domain, which was coupled to an antennapedia motif of a Drosophila transcription factor significantly blocked the interaction of caveolin-1 with phospho-PKCalpha and E. coli invasion. These results show that E. coli K1 internalizes HBMEC via caveolae and that the scaffolding domain of caveolin-1 plays a significant role in the formation of endosomes.

Expression of caveolin by bovine lymphocytes and antigen-presenting cells

Caveolin is a generic term for a family of proteins that include caveolin-1, -2 and -3. Although the distribution of these proteins varies between cells, caveolin-1 and -2 are commonly found coating membrane invaginations known as caveolae. Studies on human and murine cells suggest that caveolin/caveolae can be found in neutrophils, macrophages and mast cells, in which they are involved in the uptake of pathogens, but not in lymphocyte cell lines. Expression of caveolin-1, -2 and -3 in bovine immune cells was investigated using confocal microscopy and Western blotting. Staining for caveolin-1 was evident in all peripheral blood mononuclear cells (PBMC) and in CD4+, CD8+ and CD21+ lymphocytes, monocytes, macrophages and monocyte-derived dendritic cells (DC). In addition, the caveolin-1 antibody detected a protein with a molecular weight of approximately 22[?]000 in all PBMC, macrophages and DC, as well as in bovine aortic endothelial (BAE)-1 cells and human endothelial cells by Western blotting. In macrophages and DC, caveolin co-localized with the endoplasmic reticulum--Golgi intermediate compartment (ERGIC) and to a lesser extent with Golgi, but not with endoplasmic reticulum. Staining was not seen on the plasma membrane in any bovine immune cells, suggesting the absence of caveolae, while in BAE-1 cells staining was predominantly on the cell membrane. Caveolin-2 could not be detected in any bovine cells by confocal microscopy or Western blotting, while caveolin-3 was detected in all bovine cells by Western blotting, but not by confocal microscopy. These data provide evidence for the presence of caveolin in bovine lymphocytes and antigen-presenting cells.

Caveolae and caveolin isoforms in rat peritoneal macrophages

Caveolea are special (highly hydrophobic) plasma membrane invaginations with a diameter of 50-100 nm. Their characteristic features are the flask- or omega-shape and the lack of basket-like coat composed of clathrin. Caveolin-an integral membrane protein-is the principal component of caveolae membranes in vivo. Multiple forms of caveolin have been identified: caveolin-1alpha, caveolin-1beta, caveolin-2 and caveolin-3. They differ in their specific properties and tissue distribution. In this paper we summarize the morphological and biochemical data providing strong evidence about the existence and function of caveolae in rat peritoneal macrophages. When studied electron microscopically, the surface of both resident and elicited macrophages exhibited omega- or flask-shaped plasma membrane invaginations. There was a significant difference, however, in the number of these profiles: whereas in resident cells only a small amount of them was found on the cell surface, in elicited cells they were abundantly present on the plasma membrane. Using an antibody against the VIP21/caveolin-1 isoform we showed that these plasma membrane pits were indeed caveolae. The number and the appearance of caveolae were found to be in close correlation with the functional activity of these phagocytotic cells, indicating that the formation of caveolae is a highly regulated process. Using Western blot analysis two different proteins ( approximately 29 and approximately 20 kDa)-both labelled with anti-caveolin antibodies-were identified in resident and elicited macrophages that have been isolated from rat peritoneal cavity. The approximately 20 kDa protein was labelled specifically only by anti-VIP21/caveolin-1, while the approximately 29 kDa protein was labelled by both anti-VIP21/caveolin-1 and anti-caveolin-2 antibodies. The presence of the approximately 29 kDa protein was highly characteristic of resident cells, and only a small amount of approximately 20 kDa protein was detected in these cells. Elicitation has resulted in a significant increase in the amount of approximately 20 kDa protein labeled only with anit-VIP21/caveolin-1. Our morphological (confocal and electron microscopical) studies have shown that in resident cells caveolin was present in the cytoplasm, in smaller vesicles and multivesicular bodies around the Golgi area. Only a very small amount of caveolae was found on the cell surface of these cells. In elicited macrophages, caveolae (labelled with anti-VIP21/caveolin-1 antibody) appeared in large numbers on the cell surface, but caveolin detected by anti-caveolin-2 was also found in small vesicles and multivesicular bodies. These data support the idea that the expression of the approximately 29 kDa (caveolin-related) protein is insufficient for caveolae formation in resident cells, it can function as a modified, macrophage-specific caveolin-2 isoform. Our results strongly suggest that caveolin-1 plays a crucial role in the formation of caveolae: it is the amount of caveolin-1 that regulates the appearance of caveolae on the plasma membrane. Studying the endocytotic processes of resident and elicited macrophages we have found that elicited macrophages bound and internalized significantly larger amounts of fluid phase marker (HRP) and immune complex (peroxidase-antiperoxidase-PAP) than resident cells. Serial section analysis, double labelled immunocytochemistry, and filipin treatment were used to demonstrate that caveolae can pinch off from the plasma membrane and can take part in endocytotic processes as alternative carriers in elicited macrophages.

Influence of caveolin-1 on cellular cholesterol efflux mediated by high-density lipoproteins

Caveolin-1 is a principal structural component of caveolae membranes. These membrane microdomains participate in the regulation of signaling, transcytosis, and cholesterol homeostasis at the plasma membrane. In the present study, we determined the effect of caveolin-1 expression on cellular cholesterol efflux mediated by high-density lipoprotein (HDL). We evaluated this effect in parental NIH/3T3 cells as well as in two transformed NIH/3T3 cell lines in which caveolin-1 protein levels are dramatically downregulated. Compared with parental NIH/3T3 cells, these two transformed cell lines effluxed cholesterol more rapidly to HDL. In addition, NIH/3T3 cells harboring caveolin-1 antisense also effluxed cholesterol more rapidly to HDL. However, this effect was not due to changes in total cellular cholesterol content. We further showed that chronic HDL exposure reduced caveolin-1 protein expression in NIH/3T3 cells. HDL exposure also inhibited caveolin-1 promoter activity, suggesting a direct negative effect of HDL on caveolin-1 gene transcription. Moreover, we showed that HDL-induced downregulation of caveolin-1 prevents the uptake of oxidized low-density lipoprotein in human endothelial cells. These data suggest a novel proatherogenic role for caveolin-1, i.e., regarding the uptake and/or transcytosis of modified lipoproteins.

Ovine caveolin-1: cDNA cloning, E. coli expression, and association with endothelial nitric oxide synthase

Caveolin-1 (Cav-1), the principal coat protein of caveolae, plays an obligatory role in regulating the activity of endothelial nitric oxide (NO) synthase (eNOS). We propose that Cav-1 may be critical to eNOS-NO mediated uterine vasodilatation during pregnancy and estrogen replacement therapy. To test this hypothesis in the sheep model, we isolated the full-length cDNA of ovine Cav-1 (oCav-1) from a Lambda ZAP cDNA library of ovine placental artery endothelial cells. Thirty-two positive oCav-1 clones were recognized by a partial oCav-1 cDNA from this library, of which eight were sequenced. Restriction digestion of these clones revealed that the cDNAs of oCav-1 ranged from approximately 2.1 to 2.7 kb. Northern analysis of Cav-1 mRNAs in ovine uterine artery endothelial cells (UAEC) showed two transcripts of approximately 2.1 and 2.7 kb, respectively. Immunoreactive Cav-1 protein, but not caveolin-2 or caveolin-3, was detected in UAEC. Sequence analysis revealed that in addition to a 537-bp open reading frame encoding a 178 amino acid oCav-1 protein, full-length oCav-1 cDNAs apparently possess a approximately 1.6-2.1 kb 3'-untranslated region. Database searches with oCav-1 cDNA revealed that the coding region of mammalian Cav-1 genes is highly conserved. We prepared a recombinant full-length oCav-1 protein in which six consecutive histidine residues were tagged at the end of its COOH-terminus and developed a [His]6-tagged oCav-1 'pull-down assay' for studying the association of eNOS with Cav-1. Incubation of exogenous [His]6-tagged oCav-1 with resting UAEC extracts led to the formation of a [His]6-tagged oCav-1-eNOS complex. In the presence of a synthetic caveolin-scaffolding domain (CSD, aa 82-101) peptide, but not a mutated CSD peptide, [His]6-tagged oCav-1 associated eNOS was dose (0-10 microM)-dependently inhibited. eNOS association with Cav-1 in UAEC was further confirmed by the facts that eNOS co-immunoprecipitated with Cav-1 and vice versa, and that eNOS co-existed with Cav-1 during the isolation of caveolae membranes. Because dissociation of eNOS from Cav-1 is required for the activation of eNOS, eNOS association with Cav-1 in UAEC suggests an important role of Cav-1 in regulating UAEC production of NO and possibly NO-mediated uterine vasodilatation.

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.

Caveolin-1 and -2 expression is differentially regulated in cultured keratinocytes and within the regenerating epidermis of cutaneous wounds

Keratinocyte growth factor (KGF) and its receptor are involved in various types of epithelial repair processes. To gain insight into the molecular mechanisms of KGF action in the healing skin wound, we searched for genes which are regulated by this factor in cultured keratinocytes. Using the PCR-select technology we constructed a subtractive cDNA library. One of the KGF-regulated genes that we identified was shown to encode caveolin-1, a major component of caveolar membranes. Caveolin-1 is involved in a wide variety of cellular processes, particularly in the regulation of various signal transduction pathways. Caveolin-1 mRNA levels increased in cultured keratinocytes after KGF treatment. By in situ hybridization and immunohistochemistry we found a strong expression of caveolin-1 in the KGF-responsive basal keratinocytes of the epidermis and the hyperproliferative epithelium of the wound as well as in endothelial cells and in other cells of the granulation tissue. In 13-day wounds expression of caveolin-1 mRNA was restricted to the regenerated dermis. In addition to caveolin-1, the mRNA expression of caveolin-2, a second member of the caveolin family, was also induced in keratinocytes after stimulation with KGF but also with other growth factors and cytokines. In contrast to caveolin-1, caveolin-2 protein was expressed in all layers of the normal epidermis and in the suprabasal layers of the hyperproliferative wound epithelium. These results demonstrate a differential expression of caveolin-1 and -2 in proliferating versus differentiating keratinocytes.

Caveolin isoforms in resident and elicited rat peritoneal macrophages

Caveolin--an integral membrane protein--is the principal component of caveolae membranes in vivo. Multiple forms of caveolin have been identified: caveolin-1alpha, caveolin-1beta, caveolin-2 and caveolin-3. They differ in their specific properties and tissue distribution. When we studied the lysate of resident and elicited macrophages isolated from rat peritoneal cavity by Western blot analysis, we identified two different proteins (approximately 29 kDa and approximately 20 kDa) which were labelled with anti-caveolin antibodies. The approximately 20-kDa protein was labelled specifically only by anti-VIP21/caveolin-1, while the approximately 29-kDa protein was labelled by anti-VIP21/caveolin-1 and anti-caveolin-2. The presence of the approximately 29-kDa protein was characteristic of resident macrophages, and only a small amount of the approximately 20-kDa protein was detected in these cells. Elicitation resulted in a significant increase in the amount of the approximately 20-kDa protein labelled by anti-VIP21/caveolin-1 only. According to its molecular mass and antibody-specificity, this protein might be identical with the caveolin-1beta isoform. Our morphological (confocal and electron microscopical) studies have shown that in resident cells caveolin was present in the cytoplasm, in smaller vesicles and multivesicular bodies around the Golgi area. Only a very small amount of caveolae was found on the surface of these cells. In elicited macrophages, caveolae (labelled with the anti-VIP21/caveolin-1 antibody) appeared in large numbers on the cell surface, but caveolin detected by anti-caveolin-2 was also found in small vesicles and multivesicular bodies in the cytoplasm. According to these results, the absence of caveolae in resident cells can be explained by the absence of caveolin-1. The expression of the approximately 29-kDa (caveolin-related) protein in resident macrophages seems to be insufficient for caveolae formation. Elicitation significantly increased the expression of caveolin-1, and the increased amount of caveolin-1 resulted in caveolae formation on the cell surface.

Novel mechanisms of estrogen action in the brain: new players in an old story

Estrogen elicits a selective enhancement of the growth and differentiation of axons and dendrites (neurites) in the developing brain. Widespread colocalization of estrogen and neurotrophin receptors (trk) within estrogen and neurotrophin targets, including neurons of the cerebral cortex, sensory ganglia, and PC12 cells, has been shown to result in differential and reciprocal transcriptional regulation of these receptors by their ligands. In addition, estrogen and neurotrophin receptor coexpression leads to convergence or cross-coupling of their signaling pathways, particularly at the level of the mitogen-activated protein (MAP) kinase cascade. 17beta-Estradiol elicits rapid (within 5-15 min) and sustained (at least 2 h) tyrosine phosphorylation and activation of the MAP kinases, extracellular-signal regulated kinase (ERK)1, and ERK2, which is successfully inhibited by the MAP kinase/ERK kinase 1 inhibitor PD98059, but not by the estrogen receptor (ER) antagonist ICI 182,780 and also does not appear to result from estradiol-induced activation of trk. Furthermore, the ability of estradiol to phosphorylate ERK persists even in ER-alpha knockout mice, implicating other estrogen receptors such as ER-beta in these actions of estradiol. The existence of an estrogen receptor-containing, multimeric complex consisting of hsp90, src, and B-Raf also suggests a direct link between the estrogen receptor and the MAP kinase signaling cascade. Collectively, these novel findings, coupled with our growing understanding of additional signaling substrates utilized by estrogen, provide alternative mechanisms for estrogen action in the developing brain which could explain not only some of the very rapid effects of estrogen, but also the ability of estrogen and neurotrophins to regulate the same broad array of cytoskeletal and growth-associated genes involved in neurite growth and differentiation. This review expands the usually restrictive view of estrogen action in the brain beyond the confines of sexual differentiation and reproductive neuroendocrine function. It considers the much broader question of estrogen as a neural growth factor with important influences on the development, survival, plasticity, regeneration, and aging of the mammalian brain and supports the view that the estrogen receptor is not only a ligand-induced transcriptional enhancer but also a mediator of rapid, nongenomic events.

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.

Identification of caveolin-1 as a fatty acid binding protein

In an attempt to identify high affinity, fatty acid binding proteins present in 3T3-L1 adipocytes plasma membranes, we labeled proteins in purified plasma membranes with the photoreactive fatty acid analogue, 11-m-diazirinophenoxy[11-3H]undecanoate. A single membrane protein of 22 kDa was covalently labeled after photolysis. This protein fractionated with caveolin-1 containing caveolae and was immunoprecipitated by an anti-caveolin-1 monoclonal antibody. Furthermore, 2D-PAGE analysis revealed that both the alpha and beta isoforms of caveolin-1 could be labeled by the photoreactive fatty acid upon photolysis, indicating that both bind fatty acids. The saturable binding of the photoreactive fatty acid suggests caveolin-1 has a lipid binding site that may either operate during intracellular lipid traffic or regulate caveolin-1 function.

Targeted downregulation of caveolin-1 is sufficient to drive cell transformation and hyperactivate the p42/44 MAP kinase cascade

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 downregulation of caveolin-1 is sufficient to mediate cell transformation or tumorigenicity. Here, we employ an antisense approach to derive stable NIH 3T3 cell lines that express dramatically reduced levels of caveolin-1 but contain normal amounts of caveolin-2. NIH 3T3 cells harboring antisense caveolin-1 exhibit anchorage-independent growth, form tumors in immunodeficient mice and show hyperactivation of the p42/44 MAP kinase cascade. Importantly, transformation induced by caveolin-1 downregulation is reversed when caveolin-1 protein levels are restored to normal by loss of the caveolin-1 antisense vector. In addition, we show that in normal NIH 3T3 cells, caveolin-1 expression levels are tightly regulated by specific growth factor stimuli and cell density. Our results suggest that upregulation of caveolin-1 may be important in mediating contact inhibition and negatively regulating the activation state of the p42/44 MAP kinase cascade.

Differential stimulation of cholesterol and unsaturated fatty acid biosynthesis in cells expressing individual nuclear sterol regulatory element-binding proteins

Three sterol regulatory element-binding proteins (SREBP-1a, -1c, and -2) stimulate transcription of genes involved in synthesis and receptor-mediated uptake of cholesterol and fatty acids. Here, we explore the individual roles of each SREBP by preparing lines of Chinese hamster ovary (CHO) cells that express graded amounts of nuclear forms of each SREBP (designated nSREBPs) under control of a muristerone-inducible nuclear receptor system. The parental hamster cell line (M19 cells) lacks its own nSREBPs, owing to a deletion in the gene encoding the Site-2 protease, which releases nSREBPs from cell membranes. By varying the concentration of muristerone, we obtained graded expression of individual nSREBPs in the range that restored lipid synthesis to near physiologic levels. The results show that nSREBP-2 produces a higher ratio of synthesis of cholesterol over fatty acids than does nSREBP-1a. This is due in part to a selective ability of low levels of nSREBP-2, but not nSREBP-1a, to activate the promoter for squalene synthase. nSREBP-1a and -2 both activate transcription of the genes encoding stearoyl-CoA desaturase-1 and -2, thereby markedly enhancing the production of monounsaturated fatty acids. nSREBP-1c was inactive in stimulating any transcription at the concentrations achieved in these studies. The current data support the emerging view that the nSREBPs act in complementary ways to modulate the lipid composition of cell membranes.

Endothelial Ca2+ waves preferentially originate at specific loci in caveolin-rich cell edges

Stimulation of endothelial cells (ECs) with ATP evoked an increase in intracellular Ca2+ concentration ([Ca2+]i). In a single bovine aortic EC, the [Ca2+]i rise started at a specific peripheral locus and propagated throughout the entire cell as a Ca2+ wave. The initiation locus was constant upon repeated stimulation with ATP or other agonists (bradykinin and thrombin). The Ca2+ wave was unaffected by the removal of extracellular Ca2+, demonstrating its dependence on intracellular Ca2+ release. Microinjection of heparin into the cell inhibited the ATP-induced Ca2+ responses, indicating that the Ca2+ wave is at least partly mediated by the inositol 1,4, 5-trisphosphate receptor. Immunofluorescence staining revealed that caveolin, a marker protein for caveolae, is distributed heterogeneously in the cell and that Ca2+ waves preferentially originate at caveolin-rich cell edges. In contrast to caveolin, internalized transferrin and subunits of the clathrin-associated adaptor complexes such as adaptor protein-1 and -2 were diffusely distributed. Disruption of microtubules by Colcemid led to redistribution of caveolin away from the edges into the perinuclear center of the cell, and the ATP-induced [Ca2+]i increase was initiated on the rim of the centralized caveolin. Thus, caveolae may be involved in the initiation of ATP-induced Ca2+ waves in ECs.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.

Caveolin interaction with protein kinase C. Isoenzyme-dependent regulation of kinase activity by the caveolin scaffolding domain peptide

Caveolar localization of protein kinase C and the regulation of caveolar function by protein kinase C are well known. This study was undertaken to examine whether caveolin subtypes interact with various protein kinase C isoenzymes using the caveolin scaffolding domain peptide. When protein kinase C-alpha, -epsilon, and -zeta were overexpressed in COS cells followed by subcellular fractionation using the sucrose gradient method, all the isoenzymes (alpha, epsilon, and zeta) were detected in the same fraction as caveolin. The scaffolding domain peptide of caveolin-1 and -3, but not -2, inhibited the kinase activity and autophosphorylation of protein kinase C-alpha and -zeta, but not of protein kinase C-epsilon, overexpressed in insect cells. Truncation mutation studies of the caveolin-1 and -3 peptides demonstrated that a minimum of 16 or 14 amino acid residues of the peptide were required for the inhibition or direct binding of protein kinase C. Thus, the caveolin peptide physically interacted with protein kinase C and regulated its function. Further, this regulation occurred in a protein kinase C isoenzyme-dependent manner. Our results may provide a new mechanism regarding the regulation of protein kinase C isoenzyme activity and the molecular interaction of protein kinase C with its putative binding proteins.

Direct interaction of endothelial nitric-oxide synthase and caveolin-1 inhibits synthase activity

Endothelial nitric-oxide synthase (eNOS) and caveolin-1 are associated within endothelial plasmalemmal caveolae. It is not known, however, whether eNOS and caveolin-1 interact directly or indirectly or whether the interaction affects eNOS activity. To answer these questions, we have cloned the bovine caveolin-1 cDNA and have investigated the eNOS-caveolin-1 interaction in an in vitro binding assay system using glutathione S-transferase (GST)-caveolin-1 fusion proteins and baculovirus-expressed bovine eNOS. We have also mapped the domains involved in the interaction using an in vivo yeast two-hybrid system. Results obtained using both in vitro and in vivo protein interaction assays show that both N- and C-terminal cytosolic domains of caveolin-1 interact directly with the eNOS oxygenase domain. Interaction of eNOS with GST-caveolin-1 fusion proteins significantly inhibits enzyme catalytic activity. A synthetic peptide corresponding to caveolin-1 residues 82-101 also potently and reversibly inhibits eNOS activity by interfering with the interaction of the enzyme with Ca2+/calmodulin (CaM). Regulation of eNOS in endothelial cells, therefore, may involve not only positive allosteric regulation by Ca2+/CaM, but also negative allosteric regulation by caveolin-1.

Identification of peptide and protein ligands for the caveolin-scaffolding domain. Implications for the interaction of caveolin with caveolae-associated proteins

Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolae membranes. We have suggested that caveolin functions as a scaffolding protein to organize and concentrate certain caveolin-interacting proteins within caveolae membranes. In this regard, caveolin co-purifies with a variety of lipid-modified signaling molecules, including G-proteins, Src-like kinases, Ha-Ras, and eNOS. Using several independent approaches, it has been shown that a 20-amino acid membrane proximal region of the cytosolic amino-terminal domain of caveolin is sufficient to mediate these interactions. For example, this domain interacts with G-protein alpha subunits and Src-like kinases and can functionally suppress their activity. This caveolinderived protein domain has been termed the caveolin-scaffolding domain. However, it remains unknown how the caveolin-scaffolding domain recognizes these molecules. Here, we have used the caveolin-scaffolding domain as a receptor to select random peptide ligands from phage display libraries. These caveolin-selected peptide ligands are rich in aromatic amino acids and have a characteristic spacing in many cases. A known caveolin-interacting protein, Gi2alpha, was used as a ligand to further investigate the nature of this interaction. Gi2alpha and other G-protein alpha subunits contain a single region that generally resembles the sequences derived from phage display. We show that this short peptide sequence derived from Gi2alpha interacts directly with the caveolin-scaffolding domain and competitively inhibits the interaction of the caveolin-scaffolding domain with the appropriate region of Gi2alpha. This interaction is strictly dependent on the presence of aromatic residues within the peptide ligand, as replacement of these residues with alanine or glycine prevents their interaction with the caveolin-scaffolding domain. In addition, we have used this interaction to define which residues within the caveolin-scaffolding domain are critical for recognizing these peptide and protein ligands. Also, we find that the scaffolding domains of caveolins 1 and 3 both recognize the same peptide ligands, whereas the corresponding domain within caveolin-2 fails to recognize these ligands under the same conditions. These results serve to further demonstrate the specificity of this interaction. The implications of our current findings are discussed regarding other caveolin- and caveolae-associated proteins.

Mutational analysis of the properties of caveolin-1. A novel role for the C-terminal domain in mediating homo-typic caveolin-caveolin interactions

Caveolin is a principal structural component of caveolae membranes in vivo. Recently, a family of caveolin-related proteins has been identified; caveolin has been retermed caveolin-1. Caveolin family members share three characteristic properties: (i) detergent insolubility at low temperatures; (ii) self-oligomerization; and (iii) incorporation into low density Triton-insoluble fractions enriched in caveolae membranes. Here, we have used a deletion mutagenesis approach as a first step toward understanding which regions of caveolin-1 contribute to its unusual properties. Two caveolin-1 deletion mutants were created that lack either the C-terminal domain (Cav-1DeltaC) or the N-terminal domain (Cav-1DeltaN); these mutants were compared with the behavior of full-length caveolin-1 (Cav-1FL) expressed in parallel. Our results show that the N-terminal domain and membrane spanning segment are sufficient to form high molecular mass oligomers of caveolin-1. However, a complete caveolin-1 molecule is required for conveying detergent insolubility and incorporation into low density Triton-insoluble complexes. These data indicate that homo-oligomerization and an intact transmembrane are not sufficient to confer detergent insolubility, suggesting an unknown role for the C-terminal domain in this process. To better understand the role of the C-terminal domain, this region of caveolin-1 (residues 135-178) was expressed as a glutathione S-transferase fusion protein in Escherichia coli. Purified recombinant glutathione S-transferase-C-Cav-1 was found to stably interact with full-length caveolin-1 but not with the two caveolin-1 deletion mutants. These results suggest that the C-terminal domain interacts with both the N-terminal and C-terminal domains of an adjacent caveolin-1 homo-oligomer. This appears to be a specific homo-typic interaction, because the C-terminal domain of caveolin-1 failed to interact with full-length forms of caveolin-2 and caveolin-3. Homo-typic interaction of the C-terminal domain with an adjacent homo-oligomer could provide a mechanism for clustering caveolin-1 homo-oligomers while excluding other caveolin family members. This type of lateral segregation event could promote caveolae membrane formation and contribute to the detergent insolubility of caveolins-1, -2, and -3.

Activation of protein phosphorylation by oxidants in vascular endothelial cells: identification of tyrosine phosphorylation of caveolin

Oxidants play a significant role in endothelial cell dysfunction through modulation of diverse biochemical reactions and signal transduction pathways. Towards understanding the role of oxidants in vascular injury, we studied the effect of hydrogen peroxide (H2O2), vanadate, and pervanadate (V(4+)-OOH) on [32Pi] uptake and protein phosphorylation in bovine pulmonary artery endothelial cells (BPAEC). The incorporation of labelled [32Pi] into BPAEC was dependent on the concentration of the oxidant employed and time of incubation. Of the oxidants tested, pervanadate (10 microM) induced maximum incorporation of [32Pi] into cells (two- to threefold over control) followed by H2O2 (1 mM) and vanadate (100 microM) and clear differences in labeled protein profiles were noticed between control and oxidant treated cells. The proteins, analyzed by SDS-PAGE, showed distinct increases in labeling patterns ranging from 21-205 kDa, as evidenced by autoradiography. While the majority of the incorporated [32Pi] was in serine/threonine residues, immunoprecipitation and immunoblotting of cell lysates, using an antiphosphotyrosine antibody, revealed that oxidant treatment resulted in significant increases in total protein tyrosine phosphorylation. Most significantly, immunoprecipitation of cell lysates, from pervanadate treatment showed distinct tyrosine phosphorylation of 22 kDa protein, which was identified as caveolin, a marker of caveolae. Pervanadate-mediated phosphorylation was effectively inhibited by staurosporine (5 microM), while genistein showed only partial attenuation. Furthermore, H2O2 treatment resulted in enhanced phosphorylation of 24 kDa protein, which was attenuated by genistein. In addition, oxidant-treated cells exhibited increased tyrosine kinase activity and decreased phosphatase activity. These data show differences in labeling profiles of proteins in response to different oxidants, suggesting differential modulation of distinct protein kinases/phosphatases.

Src tyrosine kinases, Galpha subunits, and H-Ras share a common membrane-anchored scaffolding protein, caveolin. Caveolin binding negatively regulates the auto-activation of Src tyrosine kinases

Caveolae are plasma membrane specializations present in most cell types. Caveolin, a 22-kDa integral membrane protein, is a principal structural and regulatory component of caveolae membranes. Previous studies have demonstrated that caveolin co-purifies with lipid modified signaling molecules, including Galpha subunits, H-Ras, c-Src, and other related Src family tyrosine kinases. In addition, it has been shown that caveolin interacts directly with Galpha subunits and H-Ras, preferentially recognizing the inactive conformation of these molecules. However, it is not known whether caveolin interacts directly or indirectly with Src family tyrosine kinases. Here, we examine the structural and functional interaction of caveolin with Src family tyrosine kinases. Caveolin was recombinantly expressed as a glutathione S-transferase fusion. Using an established in vitro binding assay, we find that caveolin interacts with wild-type Src (c-Src) but does not form a stable complex with mutationally activated Src (v-Src). Thus, it appears that caveolin prefers the inactive conformation of Src. Deletion mutagenesis indicates that the Src-interacting domain of caveolin is located within residues 82-101, a cytosolic membrane-proximal region of caveolin. A caveolin peptide derived from this region (residues 82-101) functionally suppressed the auto-activation of purified recombinant c-Src tyrosine kinase and Fyn, a related Src family tyrosine kinase. We further analyzed the effect of caveolin on c-Src activity in vivo by transiently co-expressing full-length caveolin and c-Src tyrosine kinase in 293T cells. Co-expression with caveolin dramatically suppressed the tyrosine kinase activity of c-Src as measured via an immune complex kinase assay. Thus, it appears that caveolin structurally and functionally interacts with wild-type c-Src via caveolin residues 82-101. Besides interacting with Src family kinases, this cytosolic caveolin domain (residues 82-101) has the following unique features. First, it is required to form multivalent homo-oligomers of caveolin. Second, it interacts with G-protein alpha-subunits and down-regulates their GTPase activity. Third, it binds to wild-type H-Ras. Fourth, it is membrane-proximal, suggesting that it may be involved in other potential protein-protein interactions. Thus, we have termed this 20-amino acid stretch of caveolin residues the caveolin scaffolding domain.

Phosphorylation of caveolin by src tyrosine kinases. The alpha-isoform of caveolin is selectively phosphorylated by v-Src in vivo

Caveolae are flask-shaped plasma membrane specializations that are thought to exist in most cell types. A 22-kDa protein, caveolin, is an integral membrane component of caveolae membranes in vivo. Previous studies have demonstrated that caveolin is phosphorylated on tyrosine by oncogenic viral Src (v-Src) and that caveolin is physically associated as a hetero-oligomeric complex with normal cellular Src (c-Src) and other Src family tyrosine kinases. Caveolin contains eight conserved tyrosine residues that may serve as potential substrates for Src. Here, we have begun to study the phosphorylation of caveolin by Src family tyrosine kinases both in vitro and in vivo. Using purified recombinant components, we first reconstituted the phosphorylation of caveolin by Src kinase in vitro. Microsequencing of Src-phosphorylated caveolin revealed that phosphorylation occurs within the extreme N-terminal region of full-length caveolin between residues 6 and 26. This region contains three tyrosine residues at positions 6, 14, and 25. Deletion mutagenesis demonstrates that caveolin residues 1-21 are sufficient to support this phosphorylation event, implicating tyrosine 6 and/or 14. In vitro phosphorylation of caveolin-derived synthetic peptides and site-directed mutagenesis directly show that tyrosine 14 is the principal substrate for Src kinase. In support of these observations, tyrosine 14 is the only tyrosine residue within caveolin that bears any resemblance to the known recognition motifs for Src family tyrosine kinases. In order to confirm or refute the relevance of these in vitro studies, we next analyzed the tyrosine phosphorylation of endogenous caveolin in v-Src transformed NIH 3T3 cells. In vivo, two isoforms of caveolin are known to exist: alpha-caveolin contains residues 1-178 and beta-caveolin contains residues 32-178. Only alpha-caveolin underwent tyrosine phosphorylation in v-Src transformed NIH 3T3 cells, although beta-caveolin is well expressed in these cells. As beta-caveolin lacks residues 1-31 (and therefore tyrosine 14), these in vivo studies directly demonstrate the validity of our in vitro studies. Because alpha- and beta-caveolin are known to assume a distinct but overlapping subcellular distribution within a single cell, v-Src phosphorylation of alpha-caveolin may only affect a subpopulation of caveolae that contain alpha-caveolin.

Expression of caveolin-3 in skeletal, cardiac, and smooth muscle cells. Caveolin-3 is a component of the sarcolemma and co-fractionates with dystrophin and dystrophin-associated glycoproteins

Caveolae are microdomains of the plasma membrane that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal component of the caveolae membrane. Recently, we and others have identified a family of caveolin-related proteins; caveolin has been retermed caveolin-1. Caveolin-3 is most closely related to caveolin-1, but caveolin-3 mRNA is expressed only in muscle tissue types. Here, we examine (i) the expression of caveolin-3 protein in muscle tissue types and (ii) its localization within skeletal muscle fibers by immunofluorescence microscopy and subcellular fractionation. For this purpose, we generated a novel monoclonal antibody (mAb) probe that recognizes the unique N-terminal region of caveolin-3, but not other members of the caveolin gene family. A survey of tissues and muscle cell types by Western blot analysis reveals that the caveolin-3 protein is selectively expressed only in heart and skeletal muscle tissues, cardiac myocytes, and smooth muscle cells. Immunolocalization of caveolin-3 in skeletal muscle fibers demonstrates that caveolin-3 is localized to the sarcolemma (muscle cell plasma membrane) and coincides with the distribution of another muscle-specific plasma membrane marker protein, dystrophin. In addition, caveolin-3 protein expression is dramatically induced during the differentiation of C2C12 skeletal myoblasts in culture. Using differentiated C2C12 skeletal myoblasts as a model system, we observe that caveolin-3 co-fractionates with cytoplasmic signaling molecules (G-proteins and Src-like kinases) and members of the dystrophin complex (dystrophin, alpha-sarcoglycan, and beta-dystroglycan), but is clearly separated from the bulk of cellular proteins. Caveolin-3 co-immunoprecipitates with antibodies directed against dystrophin, suggesting that they are physically associated as a discrete complex. These results are consistent with previous immunoelectron microscopic studies demonstrating that dystrophin is localized to plasma membrane caveolae in smooth muscle cells.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Molecular cloning of caveolin-3, a novel member of the caveolin gene family expressed predominantly in muscle

Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolar membranes in vivo. Caveolin interacts directly with heterotrimeric G-proteins and can functionally regulate their activity. Recently, a second caveolin gene has been identified and termed caveolin-2. Here, we report the molecular cloning and expression of a third member of the caveolin gene gamily, caveolin-3. Caveolin-3 is most closely related to caveolin-1 based on protein sequence homology; caveolin-1 and caveolin-3 are approximately 65% identical and approximately 85% similar. A single stretch of eight amino acids (FED-VIAEP) is identical in caveolin-1, -2, and -3. This conserved region may represent a "caveolin signature sequence" that is characteristic of members of the caveolin gene family. Caveolin-3 mRNA is expressed predominantly in muscle tissue-types (skeletal muscle, diaphragm, and heart) and is selectively induced during the differentiation of skeletal C2C12 myoblasts in culture. In many respects, caveolin-3 is similar to caveolin-1: (i) caveolin-3 migrates in velocity gradients as a high molecular mass complex; (ii) caveolin-3 colocalizes with caveolin-1 by immunofluorescence microscopy and cell fractionation studies; and (iii) a caveolin-3-derived polypeptide functionally suppresses the basal GTPase activity of purified heterotrimeric G-proteins. Identification of a muscle-specific member of the caveolin gene family may have implications for understanding the role of caveolin in different muscle cell types (smooth, cardiac, and skeletal) as previous morphological studies have demonstrated that caveolae are abundant in these cells. Our results also suggest that other as yet unknown caveolin family members are likely to exist and may be expressed in a regulated or tissue-specific fashion.

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

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

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

Caveolin isoforms differ in their N-terminal protein sequence and subcellular distribution. Identification and epitope mapping of an isoform-specific monoclonal antibody probe

Caveolin, an integral membrane protein, is a principal component of caveolae membranes in vivo. Two isoforms of caveolin have been identified: a slower migrating 24-kDa species (alpha-isoform) and a faster migrating 21-kDa species (beta-isoform). Little is known about how these isoforms differ, either structurally or functionally. Here we have begun to study the differences between these two isoforms. Microsequencing of caveolin reveals that both isoforms contain internal caveolin residues 47-77. In a second independent approach, we recombinantly expressed caveolin in a caveolin-negative cell line (FRT cells). Stable transfection of FRT cells with the full-length caveolin cDNA resulted in the expression of both caveolin isoforms, indicating that they can be derived from a single cDNA. Using extracts from caveolin-expressing FRT cells, we fortuitously identified a monoclonal antibody that recognizes only the alpha-isoform of caveolin. Epitope mapping of this monoclonal antibody reveals that it recognizes an epitope within the extreme N terminus of caveolin, specifically residues 1-21. These results suggest that alpha- and beta-isoforms of caveolin differ in their N-terminal protein sequences. To independently evaluate this possibility, we placed an epitope tag at either the extreme N or C terminus of full-length caveolin. Results of these "tagging" experiments clearly demonstrate that (i) both isoforms of caveolin contain a complete C terminus and (ii) that the alpha-isoform contains a complete N terminus while the beta-isoform lacks N-terminal-specific protein sequences. Mutational analysis reveals that these two isoforms apparently derive from the use of two alternate start sites: methionine at position 1 and an internal methionine at position 32. This would explain the approximately 3-kDa difference in their apparent migration in SDS-polyacrylamide electrophoresis gels. In addition, using isoform-specific antibody probes we show that caveolin isoforms may assume a distinct but overlapping subcellular distribution by confocal immunofluorescence microscopy. We discuss the possible implications of these differences between alpha- and beta-caveolin.