Caveolins, a family of scaffolding proteins for organizing "preassembled signaling complexes" at the plasma membrane

A Role for Caveolin-3 in the Pathogenesis of Muscular Dystrophies

Caveolae are the cholesterol-rich small invaginations of the plasma membrane present in many cell types including adipocytes, endothelial cells, epithelial cells, fibroblasts, smooth muscles, skeletal muscles and cardiac muscles. They serve as specialized platforms for many signaling molecules and regulate important cellular processes like energy metabolism, lipid metabolism, mitochondria homeostasis, and mechano-transduction. Caveolae can be internalized together with associated cargo. The caveolae-dependent endocytic pathway plays a role in the withdrawal of many plasma membrane components that can be sent for degradation or recycled back to the cell surface. Caveolae are formed by oligomerization of caveolin proteins. Caveolin-3 is a muscle-specific isoform, whose malfunction is associated with several diseases including diabetes, cancer, atherosclerosis, and cardiovascular diseases. Mutations in Caveolin-3 are known to cause muscular dystrophies that are collectively called caveolinopathies. Altered expression of Caveolin-3 is also observed in Duchenne’s muscular dystrophy, which is likely a part of the pathological process leading to muscle weakness. This review summarizes the major functions of Caveolin-3 in skeletal muscles and discusses its involvement in the pathology of muscular dystrophies.

Caveolin-1 stabilizes ATP7A, a copper transporter for extracellular SOD, in vascular tissue to maintain endothelial function

Caveolin-1 (Cav-1) is a scaffolding protein and a major component of caveolae/lipid rafts. Previous reports have shown that endothelial dysfunction in Cav-1-deficient (Cav-1-/-) mice is mediated by elevated oxidative stress through endothelial nitric oxide synthase (eNOS) uncoupling and increased NADPH oxidase. Oxidant stress is the net balance of oxidant generation and scavenging, and the role of Cav-1 as a regulator of antioxidant enzymes in vascular tissue is poorly understood. Extracellular SOD (SOD3) is a copper (Cu)-containing enzyme that is secreted from vascular smooth muscle cells/fibroblasts and subsequently binds to the endothelial cells surface, where it scavenges extracellular [Formula: see text] and preserves endothelial function. SOD3 activity is dependent on Cu, supplied by the Cu transporter ATP7A, but whether Cav-1 regulates the ATP7A-SOD3 axis and its role in oxidative stress-mediated vascular dysfunction has not been studied. Here we show that the activity of SOD3, but not SOD1, was significantly decreased in Cav-1-/- vessels, which was rescued by re-expression of Cav-1 or Cu supplementation. Loss of Cav-1 reduced ATP7A protein, but not mRNA, and this was mediated by ubiquitination of ATP7A and proteasomal degradation. ATP7A bound to Cav-1 and was colocalized with SOD3 in caveolae/lipid rafts or perinucleus in vascular tissues or cells. Impaired endothelium-dependent vasorelaxation in Cav-1-/- mice was rescued by gene transfer of SOD3 or by ATP7A-overexpressing transgenic mice. These data reveal an unexpected role of Cav-1 in stabilizing ATP7A protein expression by preventing its ubiquitination and proteasomal degradation, thereby increasing SOD3 activity, which in turn protects against vascular oxidative stress-mediated endothelial dysfunction.

Lateral heterogeneity and domain formation in cellular membranes

As early as the development of the fluid mosaic model for cellular membranes, researchers began observing the telltale signs of lateral heterogeneity. Over the decades this has led to the development of the lipid raft hypothesis and the ensuing controversy that has unfolded, as a result. Here, we review the physical concepts behind domain formation in lipid membranes, both of their structural and dynamic origins. This, then leads into a discussion of coarse-grained, phenomenological approaches that describe the wide range of phases associated with lipid lateral heterogeneity. We use these physical concepts to describe the interaction between raft-lipid species, such as long-chain saturated lipids, sphingomyelin, and cholesterol, and non-raft forming lipids, such as those with short acyl chains or unsaturated fatty acids. While debate has persisted on the biological relevance of lipid domains, recent research, described here, continues to identify biological roles for rafts and new experimental approaches have revealed the existence of lipid domains in living systems. Given the recent progress on both the biological and structural aspects of raft formation, the research area of membrane lateral heterogeneity will not only expand, but will continue to produce exciting results.

Acute Statin Treatment Improves Antibody Accumulation in EGFR- and PSMA-Expressing Tumors

PURPOSE

Statins are cholesterol-depleting drugs used to treat patients with hypercholesterolemia. Preclinically, statins disrupt trafficking of receptors present at the cell membrane. Membrane receptors, defined as tumor biomarkers and therapeutic targets, are often internalized by an endocytic pathway. Indeed, receptor endocytosis and recycling are dynamic mechanisms that often affect receptor density at the cell surface. In therapies using monoclonal antibodies (mAb), a downregulation in receptor density at the cell surface decreases antibody binding to the extracellular domain of the membrane receptor. Here, we determined the potential of lovastatin, simvastatin, and rosuvastatin in preclinically modulating epidermal growth factor receptor (EGFR) and prostate-specific membrane antigen (PSMA) receptor density at the tumor cell surface.

EXPERIMENTAL DESIGN

Small-animal PET was used to study the binding of ⁸⁹Zr-labeled antibodies in ectopic xenografts. Ex vivo analyses were performed to determine changes in endocytic proteins, EGFR, and PSMA surface levels.

RESULTS

Acute statin treatment using lovastatin, simvastatin, or rosuvastatin enhanced tumors' avidity for the mAbs panitumumab, cetuximab, and huJ591. Statins temporarily modulated caveolin-1, cavin-1, endophilin, clathrin, and dynamin proteins in EGFR- and PSMA-overexpressing xenografts.

CONCLUSIONS

These data show the potential of statins as pharmacologic modulators of endocytic proteins for improved tumors' accumulation of mAbs. The translational significance of these findings lies in the potential of statins to temporarily modulate the heterogeneous presence of receptors at the cell membrane, a characteristic often associated with poor response in tumors to therapeutic antibodies.

Caveolae and Lipid Rafts in Endothelium: Valuable Organelles for Multiple Functions

Caveolae are flask-shaped invaginations of the plasma membrane found in numerous cell types and are particularly abundant in endothelial cells and adipocytes. The lipid composition of caveolae largely matches that of lipid rafts microdomains that are particularly enriched in cholesterol, sphingomyelin, glycosphingolipids, and saturated fatty acids. Unlike lipid rafts, whose existence remains quite elusive in living cells, caveolae can be clearly distinguished by electron microscope. Despite their similar composition and the sharing of some functions, lipid rafts appear more heterogeneous in terms of size and are more dynamic than caveolae. Following the discovery of caveolin-1, the first molecular marker as well as the unique scaffolding protein of caveolae, we have witnessed a remarkable increase in studies aimed at investigating the role of these organelles in cell functions and human disease. The goal of this review is to discuss the most recent studies related to the role of caveolae and caveolins in endothelial cells. We first recapitulate the major embryological processes leading to the formation of the vascular tree. We next discuss the contribution of caveolins and cavins to membrane biogenesis and cell response to extracellular stimuli. We also address how caveolae and caveolins control endothelial cell metabolism, a central mechanism involved in migration proliferation and angiogenesis. Finally, as regards the emergency caused by COVID-19, we propose to study the caveolar platform as a potential target to block virus entry into endothelial cells.

Loss of Caveolin-1 Is Associated with a Decrease in Beta Cell Death in Mice on a High Fat Diet

Elevated free fatty acids (FFAs) impair beta cell function and reduce beta cell mass as a consequence of the lipotoxicity that occurs in type 2 diabetes (T2D). We previously reported that the membrane protein caveolin-1 (CAV1) sensitizes to palmitate-induced apoptosis in the beta pancreatic cell line MIN6. Thus, our hypothesis was that CAV1 knock-out (CAV1 KO) mice subjected to a high fat diet (HFD) should suffer less damage to beta cells than wild type (WT) mice. Here, we evaluated the in vivo response of beta cells in the pancreatic islets of 8-week-old C57Bl/6J CAV1 KO mice subjected to a control diet (CD, 14% kcal fat) or a HFD (60% kcal fat) for 12 weeks. We observed that CAV1 KO mice were resistant to weight gain when on HFD, although they had high serum cholesterol and FFA levels, impaired glucose tolerance and were insulin resistant. Some of these alterations were also observed in mice on CD. Interestingly, KO mice fed with HFD showed an adaptive response of the pancreatic beta cells and exhibited a significant decrease in beta cell apoptosis in their islets compared to WT mice. These in vivo results suggest that although the CAV1 KO mice are metabolically unhealthy, they adapt better to a HFD than WT mice. To shed light on the possible signaling pathway(s) involved, MIN6 murine beta cells expressing (MIN6 CAV) or not expressing (MIN6 Mock) CAV1 were incubated with the saturated fatty acid palmitate in the presence of mitogen-activated protein kinase inhibitors. Western blot analysis revealed that CAV1 enhanced palmitate-induced JNK, p38 and ERK phosphorylation in MIN6 CAV1 cells. Moreover, all the MAPK inhibitors partially restored MIN6 viability, but the effect was most notable with the ERK inhibitor. In conclusion, our results suggest that CAV1 KO mice adapted better to a HFD despite their altered metabolic state and that this may at least in part be due to reduced beta cell damage. Moreover, they indicate that the ability of CAV1 to increase sensitivity to FFAs may be mediated by MAPK and particularly ERK activation.

Muscle Cell Morphogenesis, Structure, Development and Differentiation Processes Are Significantly Regulated during Human Ovarian Granulosa Cells In Vitro Cultivation

Granulosa cells (GCs) have many functions and are fundamental for both folliculogenesis and oogenesis, releasing hormones and communicating directly with the oocyte. Long-term in vitro cultures of GCs show significant stem-like characteristics. In the current study, RNA of human ovarian granulosa cells was collected at 1, 7, 15 and 30 days of long-term in vitro culture. Understanding the process of differentiation of GCs towards different cell lineages, as well as the molecular pathways underlying these mechanisms, is fundamental to revealing other possible stemness markers of this type of cell. Identifying new markers of GC plasticity may help to understand the aetiology and recurrence of a wide variety of diseases and health conditions and reveal possible clinical applications of the ovarian tissue cells, affecting not only the reproductive ability but also sex hormone production. Granulosa cells were the subject of this study, as they are readily available as remnant material leftover after in vitro fertilisation procedures and exhibit significant stem-like characteristics in culture. The change in gene expression was investigated through a range of molecular and bioinformatic analyses. Expression microarrays were used, allowing the identification of groups of genes typical of specific cellular pathways. This candidate gene study focused on ontological groups associated with muscle cell morphogenesis, structure, development and differentiation, namely, “muscle cell development”, “muscle cell differentiation”, “muscle contraction”, “muscle organ development”, “muscle organ morphogenesis”, “muscle structure development”, “muscle system process” and “muscle tissue development”. The results showed that the 10 most upregulated genes were keratin 19, oxytocin receptor, connective tissue growth factor, nexilin, myosin light chain kinase, cysteine and glycine-rich protein 3, caveolin 1, actin, activating transcription factor 3 and tropomyosin, while the 10 most downregulated consisted of epiregulin, prostaglandin-endoperoxide synthase 2, transforming growth factor, interleukin, collagen, 5-hydroxytryptmine, interleukin 4, phosphodiesterase, wingless-type MMTV integration site family and SRY-box 9. Moreover, ultrastructural observations showing heterogeneity of granulosa cell population are presented in the study. At least two morphologically different subpopulations were identified: large, light coloured and small, darker cells. The expression of genes belonging to the mentioned ontological groups suggest the potential ability of GCs to differentiate and proliferate toward muscle lineage, showing possible application in muscle regeneration and the treatment of different diseases.

Membrane Compartmentalization and Scaffold Proteins in Leukocyte Migration

Leukocyte migration across vessels into and within peripheral and lymphoid tissues is essential for host defense against invading pathogens. Leukocytes are specialized in sensing a variety of guidance cues and to integrate environmental stimuli to navigate in a timely and spatially controlled manner. These extracellular signals must be transmitted across the leukocyte’s plasma membrane in a way that intracellular signaling cascades enable directional cell movement. Therefore, the composition of the membrane in concert with proteins that influence the compartmentalization of the plasma membrane or contribute to delineate intracellular signaling molecules are key in controlling leukocyte navigation. This becomes evident by the fact that mislocalization of membrane proteins is known to deleteriously affect cellular functions that may cause diseases. In this review we summarize recent advances made in the understanding of how membrane cholesterol levels modulate chemokine receptor signaling and hence leukocyte trafficking. Moreover, we provide an overview on the role of membrane scaffold proteins, particularly tetraspanins, flotillins/reggies, and caveolins in controlling leukocyte migration both in vitro and in vivo.

An automated data processing and analysis pipeline for transmembrane proteins in detergent solutions

The application of small angle X-ray scattering (SAXS) to the structural characterization of transmembrane proteins (MPs) in detergent solutions has become a routine procedure at synchrotron BioSAXS beamlines around the world. SAXS provides overall parameters and low resolution shapes of solubilized MPs, but is also meaningfully employed in hybrid modeling procedures that combine scattering data with information provided by high-resolution techniques (eg. macromolecular crystallography, nuclear magnetic resonance and cryo-electron microscopy). Structural modeling of MPs from SAXS data is non-trivial, and the necessary computational procedures require further formalization and facilitation. We propose an automated pipeline integrated with the laboratory-information management system ISPyB, aimed at preliminary SAXS analysis and the first-step reconstruction of MPs in detergent solutions, in order to streamline high-throughput studies, especially at synchrotron beamlines. The pipeline queries an ISPyB database for available a priori information via dedicated services, estimates model-free SAXS parameters and generates preliminary models utilizing either ab initio, high-resolution-based, or mixed/hybrid methods. The results of the automated analysis can be inspected online using the standard ISPyB interface and the estimated modeling parameters may be utilized for further in-depth modeling beyond the pipeline. Examples of the pipeline results for the modelling of the tetrameric alpha-helical membrane channel Aquaporin0 and mechanosensitive channel T2, solubilized by n-Dodecyl β-D-maltoside are presented. We demonstrate how increasing the amount of a priori information improves model resolution and enables deeper insights into the molecular structure of protein-detergent complexes.

Circular RNA circCCDC9 acts as a miR-6792-3p sponge to suppress the progression of gastric cancer through regulating CAV1 expression

Background

As a novel type of noncoding RNAs, covalently closed circular RNAs (circRNAs) are ubiquitously expressed in eukaryotes. Emerging studies have related dysregulation of circRNAs to tumorigenesis. However, the biogenesis, regulation, function and mechanism of circRNAs in gastric cancer (GC) remain largely unclear.

Methods

The expression profile of circRNAs in 6 pairs of GC tissues and adjacent non-tumor tissues was analyzed by RNA-sequencing. Quantitative real-time PCR was used to determine the expression level of circCCDC9 in GC tissues and cell lines. Then, functional experiments in vitro and in vivo were employed to explore the effects of circCCDC9 on tumor growth and metastasis in GC. Mechanistically, dual luciferase reporter, fluorescence in situ hybridization (FISH), RNA immunoprecipitation (RIP) and RNA pull-down assays were performed to confirm that circCCDC9 directly sponged miR-6792-3p and alleviated suppression on target CAV1 expression.

Results

Evidently down-regulated expression of circCCDC9 was observed in both GC tissues and cell lines. Expression of circCCDC9 was negatively correlated with tumor size, lymph node invasion, advanced clinical stage and overall survival in GC patients. Functionally, overexpression of circCCDC9 significantly inhibited the proliferation, migration and invasion of GC cell lines in vitro and tumor growth and metastasis in vivo, whereas miR-6792-3p mimics counteracted these effects. Mechanistic analysis demonstrated that circCCDC9 acted as a “ceRNA” of miR-6792-3p to relieve the repressive effect of miR-6792-3p on its target CAV1, then suppressed the tumorigenesis of GC.

Conclusions

CircCCDC9 functions as a tumor suppressor in inhibiting the progression of GC through miR-6792-3p/CAV1 axis, which has provided an exploitable biomarker and therapeutic target for patients with GC.

Src-family kinase inhibitors block early steps of caveolin-1-enhanced lung metastasis by melanoma cells

In advanced stages of cancer disease, caveolin-1 (CAV1) expression increases and correlates with increased migratory and invasive capacity of the respective tumor cells. Previous findings from our laboratory revealed that specific ECM-integrin interactions and tyrosine-14 phosphorylation of CAV1 are required for CAV1-enhanced melanoma cell migration, invasion and metastasis in vivo. In this context, CAV1 phosphorylation on tyrosine-14 mediated by non-receptor Src-family tyrosine kinases seems to be important; however, the effect of Src-family kinase inhibitors on CAV1-enhanced metastasis in vivo has not been studied. Here, we evaluated the effect of CAV1 and c-Abl overexpression, as well as the use of the Src-family kinase inhibitors, PP2 and dasatinib (more specific for Src/Abl) in lung metastasis of B16F10 melanoma cells. Overexpression of CAV1 and c-Abl enhanced CAV1 phosphorylation and the metastatic potential of the B16F10 murine melanoma cells. Alternatively, treatment with PP2 or dasatinib for 2 h reduced CAV1 tyrosine-14 phosphorylation and levels recovered fully within 12 h of removing the inhibitors. Nonetheless, pre-treatment of cells with these inhibitors for 2 h sufficed to prevent migration, invasion and trans-endothelial migration in vitro. Importantly, the transient decrease in CAV1 phosphorylation by these kinase inhibitors prevented early steps of CAV1-enhanced lung metastasis by B16F10 melanoma cells injected into the tail vein of mice. In conclusion, this study underscores the relevance of CAV1 tyrosine-14 phosphorylation by Src-family kinases during the first steps of the metastatic sequence promoted by CAV1. These findings open up potential options for treatment of metastatic tumors in patients in which Src-family kinase activation and CAV1 overexpression favor dissemination of cancer cells to secondary sites.

Caveolin-1 Scaffolding Domain Peptide Regulates Colon Endothelial Cell Survival through JNK Pathway

It has been reported that pathological angiogenesis contributes to both experimental colitis and inflammatory bowel disease. Recently, we demonstrated that endothelial caveolin-1 plays a key role in the pathological angiogenesis of dextran sodium sulfate (DSS) colitis. However, the molecular mechanism of caveolin-1 regulation of endothelial function is unknown. In this study, we examined how the antennapedia- (AP-) conjugated caveolin-1 scaffolding domain (AP-Cav) modulates vascular endothelial growth factor- (VEGF-) dependent colon endothelial cell angiogenic responses, as seen during colitis. We used mouse colon endothelial cells and found that AP-Cav significantly inhibited VEGF-mediated bromodeoxyuridine (BrdU) incorporation into colon microvascular endothelial cells. AP-Cav significantly blunted VEGF-dependent extracellular signal-regulated kinase 1/2 (ERK 1/2) phosphorylation at 10 minutes and 2 hours after stimulation, compared with the AP control peptide. AP-Cav + VEGF-A treatment also significantly increased c-Jun N-terminal kinase (JNK) phosphorylation at 2 hours. AP-Cav + VEGF-A treatment significantly downregulated retinoblastoma (Rb) protein levels, upregulated cleaved caspase-3 protein levels at 4 hours, and induced apoptosis. Thus, our study suggests that disruption of endothelial caveolin-1 function via the AP-Cav diverts VEGF signaling responses away from endothelial cell proliferation and toward apoptosis through the inhibition of mitogen-activated protein (MAP) kinase signaling and the induction of JNK-associated apoptosis.

Caveolin proteins electrochemical oxidation and interaction with cholesterol

Caveolae consist in lipid raft domains composed of caveolin proteins, cholesterol, glycosphingolipids, and GPI-anchored proteins. Caveolin proteins present three different types, caveolin 1 (CAV-1), caveolin 2 (CAV-2) and caveolin 3 (CAV-3), with a very similar structure and amino acid composition. The native caveolin proteins oxidation mechanism was investigated for the first time, at a glassy carbon electrode, using cyclic, square wave and differential pulse voltammetry. The three native caveolin proteins oxidation mechanism presented only one tyrosine and tryptophan amino acid residues oxidation peak. Denatured caveolin proteins presented also the tyrosine, tryptophan and cysteine amino acid residues oxidation peaks. The reverse cholesterol transport is related to caveolae and caveolin proteins, and CAV-1 is directly connected to cholesterol transport. The influence of cholesterol on the three caveolin proteins electrochemical behaviour was evaluated. In the absence and in the presence of cholesterol, significant differences in the CAV-1 oxidation peak current were observed.

The importance of caveolins and caveolae to dermatology: Lessons from the caves and beyond

Caveolae are flask-shaped invaginations of the cell membrane rich in cholesterol and sphingomyelin, with caveolin proteins acting as their primary structural components that allow compartmentalization and orchestration of various signalling molecules. In this review, we discuss how pleiotropic functions of caveolin-1 (Cav1) and its intricate roles in numerous cellular functions including lipid trafficking, signalling, cell migration and proliferation, as well as cellular senescence, infection and inflammation, are integral for normal development and functioning of skin and its appendages. We then examine how disruption of the homeostatic levels of Cav1 can lead to development of various cutaneous pathophysiologies including skin cancers, cutaneous fibroses, psoriasis, alopecia, age-related changes in skin and aberrant wound healing and propose how levels of Cav1 may have theragnostic value in skin physiology/pathophysiology.

Alteration in memory cognition due to activation of caveolin-1 and oxidative damage in a model of dementia of Alzheimer's type

OBJECTIVE

The present study aims to investigate the role of caveolin-1 in dementia of Alzheimer's type using intracerebroventricular streptozotocin (ICV-STZ)-induced neurodegeneration model in rats.

MATERIALS AND METHODS

Male Wistar rats (220-260 g) were employed. STZ 3 mg/kg via ICV route was given once to cause neuronal injury. Daidzein - a caveolin inhibitor at 0.2, 0.4, and 0.6 mg/kg s.c. were given daily whereas minoxidil - a caveolin activator was given at 0.45 mg/kg, i.p. on alternate days for 28 days. STZ was also given at its submaximal dose 1.5 mg/kg to minoxidil group only.

RESULTS

ICV-STZ control animals exhibited cognitive and neurological deficits on the Morris water maze, elevated plus maze, and balance beam tests (P < 0.0001). Treatment with daidzein significantly restored memory impairments and decreased oxidative damage whereas minoxidil potentiates the effect of STZ causing significant impairment in memory. Significant oxidative stress such as lipid peroxidation and glutathione (P < 0.0001) were also observed due to ICV-STZ administration resulting in neuronal damage which was significantly prevented by treatment with daidzein in brain tissues.

CONCLUSION

The findings from the present investigation may conclude that the caveolin-1 from caveolae at the cell membrane induces memory deficits and oxidative stress phenotype that resemble the neurological phenotype of Alzheimer's disease. Further studies are warranted to gauge the effect of caveolin dyshomeostasis on the amyloidogenic cascade.

Caveolin elastin-like polypeptide fusions mediate temperature-dependent assembly of caveolar microdomains

Caveolae are membrane organelles formed by submicron invaginations in the plasma membrane, and are involved in mechanosensing, cell signaling, and endocytosis. Although implicated broadly in physiology and pathophysiology, better tools are required to elucidate the precise role of caveolar processes through selective activation and inactivation of their trafficking. Our group recently reported that thermally-responsive elastin-like polypeptides (ELPs) can trigger formation of 'genetically engineered protein microdomains (GEPMs)' functionalized with either Clathrin-light chain or the epidermal growth factor receptor. This manuscript is the first report of this strategy to modulate caveolin-1 (CAV1). By attaching different ELP sequences to CAV1, mild heating can be used to self-assemble CAV1-ELP microdomains inside of cells. The temperature of self-assembly can be controlled by tuning the ELP sequence. The formation of CAV1-ELP microdomains internalizes Cholera Toxin Subunit B, a commonly used marker of caveolae mediated endocytosis. CAV1-ELPs also colocalize with Cavin 1, an essential component of functional caveolae biogenesis. With the emerging significance of caveolae in health and disease and the lack of specific probes to rapidly and reversibly affect caveolar function, CAV1-ELP microdomains are a new tool to rapidly probe caveolae associated processes in endocytosis, cell signaling, and mechanosensing.

Involvement of caveolin-1 in neurovascular unit remodeling after stroke: Effects on neovascularization and astrogliosis

Complex cellular and molecular events occur in the neurovascular unit after stroke, such as blood-brain barrier (BBB) dysfunction and inflammation that contribute to neuronal death, neurological deterioration and mortality. Caveolin-1 (Cav-1) has distinct physiological functions such as caveolae formation associated with endocytosis and transcytosis as well as in signaling pathways. Cav-1 has been proposed to be involved in BBB dysfunction after brain injury; however, its precise role is poorly understood. The goal of this study was to characterize the expression and effect of Cav-1 deletion on outcome in the first week in a transient Middle Cerebral Artery Occlusion stroke model. We found increased Cav-1 expression in new blood vessels in the lesion and in reactive astrocytes in the peri-lesion areas. In Cav-1 KO mice, the lesion volume was larger and the behavioral outcome worse than in WT mice. Cav-1 KO mice exhibited reduced neovascularization and modified astrogliosis, without formation of a proper glial scar around the lesion at three days post injury, coinciding with aggravated outcomes. Altogether, these results point towards a potential protective role of endogenous Cav-1 in the first days after ischemia by promoting neovascularization, astrogliosis and scar formation.

Cultured hippocampal neurons of dystrophic mdx mice respond differently from those of wild type mice to an acute treatment with corticosterone

Duchenne muscular dystrophy is a lethal genetic disease characterised by progressive degeneration of skeletal muscles induced by deficiency of dystrophin, a cytoskeletal protein expressed in myocytes and in certain neuron populations. The severity of the neurological disorder varies in humans and animal models owing to dysfunction in numerous brain areas, including the hippocampus. Cyclic treatments with high-dose glucocorticoids remain a major pharmacological approach for treating the disease; however, elevated systemic levels of either stress-induced or exogenously administered anti-inflammatory molecules dramatically affect hippocampal activity. In this study, we analysed and compared the response of hippocampal neurons isolated from wild-type and dystrophic mdx mice to acute administration of corticosterone in vitro, without the influence of other glucocorticoid-regulated processes. Our results showed that in neurons of mdx mice, both the genomic and intracellular signalling-mediated responses to corticosterone were affected compared to those in wild-type animals, evoking the characteristic response to detrimental chronic glucocorticoid exposure. Responsiveness to glucocorticoids is, therefore, another function of hippocampal neurons possibly affected by deficiency of Dp427 since embryonic development. Knowing the pivotal role of hippocampus in stress hormone signalling, attention should be paid to the effects that prolonged glucocorticoid treatments may have on this and other brain areas of DMD patients.

Mevastatin promotes healing by targeting caveolin-1 to restore EGFR signaling

Diabetic foot ulcers (DFUs) are a life-threatening disease that often results in lower limb amputations and a shortened life span. Current treatment options are limited and often not efficacious, raising the need for new therapies. To investigate the therapeutic potential of topical statins to restore healing in patients with DFUs, we performed next-generation sequencing on mevastatin-treated primary human keratinocytes. We found that mevastatin activated and modulated the EGF signaling to trigger an antiproliferative and promigratory phenotype, suggesting that statins may shift DFUs from a hyperproliferative phenotype to a promigratory phenotype in order to stimulate healing. Furthermore, mevastatin induced a migratory phenotype in primary human keratinocytes through EGF-mediated activation of Rac1, resulting in actin cytoskeletal reorganization and lamellipodia formation. Interestingly, the EGF receptor is downregulated in tissue biopsies from patients with DFUs. Mevastatin restored EGF signaling in DFUs through disruption of caveolae to promote keratinocyte migration, which was confirmed by caveolin-1 (Cav1) overexpression studies. We conclude that topical statins may have considerable therapeutic potential as a treatment option for patients with DFUs and offer an effective treatment for chronic wounds that can be rapidly translated to clinical use.

Depletion of Caveolin-1 in Type 2 Diabetes Model Induces Alzheimer's Disease Pathology Precursors

Type 2 diabetes mellitus (T2DM) is a risk factor for the development of late-onset Alzheimer's disease (AD). However, the mechanism underlying the development of late-onset AD is largely unknown. Here we show that levels of the endothelial-enriched protein caveolin-1 (Cav-1) are reduced in the brains of T2DM patients compared with healthy aging, and inversely correlated with levels of β-amyloid (Aβ). Depletion of Cav-1 is recapitulated in the brains of db/db (Leprdb ) diabetic mice and corresponds with recognition memory deficits as well as the upregulation of amyloid precursor protein (APP), BACE-1, a trending increase in β-amyloid Aβ42/40 ratio and hyperphosphorylated tau (p-tau) species. Importantly, we show that restoration of Cav-1 levels in the brains of male db/db mice using adenovirus overexpressing Cav-1 (AAV-Cav-1) rescues learning and memory deficits and reduces pathology (i.e., APP, BACE-1 and p-tau levels). Knocking down Cav-1 using shRNA in HEK cells expressing the familial AD-linked APPswe mutant variant upregulates APP, APP carboxyl terminal fragments, and Aβ levels. In turn, rescue of Cav-1 levels restores APP metabolism. Together, these results suggest that Cav-1 regulates APP metabolism, and that depletion of Cav-1 in T2DM promotes the amyloidogenic processing of APP and hyperphosphorylation of tau. This may suggest that depletion of Cav-1 in T2DM underlies, at least in part, the development of AD and imply that restoration of Cav-1 may be a therapeutic target for diabetic-associated sporadic AD.SIGNIFICANCE STATEMENT More than 95% of the Alzheimer's patients have the sporadic late-onset form (LOAD). The cause for late-onset Alzheimer's disease is unknown. Patients with Type 2 diabetes mellitus have considerably higher incidence of cognitive decline and AD compared with the general population, suggesting a common mechanism. Here we show that the expression of caveolin-1 (Cav-1) is reduced in the brain in Type 2 diabetes mellitus. In turn, reduced Cav-1 levels induce AD-associated neuropathology and learning and memory deficits. Restoration of Cav-1 levels rescues these deficits. This study unravels signals underlying LOAD and suggests that restoration of Cav-1 may be an effective therapeutic target.

Current advances in idiopathic pulmonary fibrosis: the pathogenesis, therapeutic strategies and candidate molecules

Idiopathic pulmonary fibrosis (IPF) is a type of chronic, progressive lung disease with unknown cause, which is characterized by increasing dyspnea and destruction of lung function with a high mortality rate. Evolving evidence demonstrated that the pathogenesis of IPF involved multiple signaling pathways such as inflammation, oxidative stress and fibrosis. However, drug discovery to prevent or revert IPF has been insufficient to cope with the development. Drug discovery targeting multiple links should be considered. In this review, we will brief the pathogenesis of IPF and discuss several small chemical entities toward the pathogenesis for IPF studied in animal models and clinical trials. The field of novel anti-IPF agents and the future directions for the prevention and treatment of IPF are detailed thoroughly discussed.

Secondary structure of caveolins: a mini review

Caveolae are 50–100 nm invaginations found within the plasma membrane of cells. Caveolae are involved in many processes that are essential for homeostasis, most notably endocytosis, mechano-protection, and signal transduction. Within these invaginations, the most important proteins are caveolins, which in addition to participating in the aforementioned processes are structural proteins responsible for caveolae biogenesis. When caveolin is misregulated or mutated, many disease states can arise which include muscular dystrophy, cancers, and heart disease. Unlike most integral membrane proteins, caveolin does not have a transmembrane orientation; instead, it is postulated to adopt an unusual topography where both the N- and C-termini lie on the cytoplasmic side of the membrane, and the hydrophobic span adopts an intramembrane loop conformation. While knowledge concerning the biology of caveolin has progressed apace, fundamental structural information has proven more difficult to obtain. In this mini-review, we curate as well as critically assess the structural data that have been obtained on caveolins to date in order to build a robust and compelling model of the caveolin secondary structure.

The TGF-β1/p53/PAI-1 Signaling Axis in Vascular Senescence: Role of Caveolin-1

Stress-induced premature cellular senescence is a significant factor in the onset of age-dependent disease in the cardiovascular system. Plasminogen activator inhibitor-1 (PAI-1), a major TGF-β1/p53 target gene and negative regulator of the plasmin-based pericellular proteolytic cascade, is elevated in arterial plaques, vessel fibrosis, arteriosclerosis, and thrombosis, correlating with increased tissue TGF-β1 levels. Additionally, PAI-1 is necessary and sufficient for the induction of p53-dependent replicative senescence. The mechanism of PAI-1 transcription in senescent cells appears to be dependent on caveolin-1 signaling. Src kinases are upstream effectors of both FAK and caveolin-1 activation as FAK^Y577,Y861 and caveolin-1^Y14 phosphorylation are not detected in TGF-β1-stimulated src family kinase (pp60^c-src, Yes, Fyn) triple-deficient (SYF^−/−/−) cells. However, restoration of pp60^c-src expression in SYF-null cells rescued both caveolin-1^Y14 phosphorylation and PAI-1 induction in response to TGF-β1. Furthermore, TGF-β1-initiated Src phosphorylation of caveolin-1^Y14 is critical in Rho-ROCK-mediated suppression of the SMAD phosphatase PPM1A maintaining and, accordingly, SMAD2/3-dependent transcription of the PAI-1 gene. Importantly, TGF-β1 failed to induce PAI-1 expression in caveolin-1-null cells, correlating with reductions in both Rho-GTP loading and SMAD2/3 phosphorylation. These findings implicate caveolin-1 in expression controls on specific TGF-β1/p53 responsive growth arrest genes. Indeed, up-regulation of caveolin-1 appears to stall cells in G₀/G₁ via activation of the p53/p21 cell cycle arrest pathway and restoration of caveolin-1 in caveolin-1-deficient cells rescues TGF-β1 inducibility of the PAI-1 gene. Although the mechanism is unclear, caveolin-1 inhibits p53/MDM2 complex formation resulting in p53 stabilization, induction of p53-target cell cycle arrest genes (including PAI-1), and entrance into premature senescence while stimulating the ATM→p53→p21 pathway. Identification of molecular events underlying senescence-associated PAI-1 expression in response to TGF-β1/src kinase/p53 signaling may provide novel targets for the therapy of cardiovascular disease.

Targeting the Mitochondrial Potassium Channel Kv1.3 to Kill Cancer Cells: Drugs, Strategies, and New Perspectives

Cancer is the consequence of aberrations in cell growth or cell death. In this scenario, mitochondria and ion channels play a critical role in regard to cell proliferation, malignant angiogenesis, migration, and metastasis. In this review, we focus on Kv1.3 and specifically on mitoKv1.3, which showed an aberrant expression in cancer cells compared with healthy tissues and which is involved in the apoptotic pathway. In recent years, mitoKv1.3 has become an oncological target since its pharmacological modulation has been demonstrated to reduce tumor growth and progression both in vitro and in vivo using preclinical mouse models of different types of tumors.

Cell Intrinsic and Extrinsic Mechanisms of Caveolin-1-Enhanced Metastasis

Caveolin-1 (CAV1) is a scaffolding protein with a controversial role in cancer. This review will initially discuss earlier studies focused on the role as a tumor suppressor before elaborating subsequently on those relating to function of the protein as a promoter of metastasis. Different mechanisms are summarized illustrating how CAV1 promotes such traits upon expression in cancer cells (intrinsic mechanisms). More recently, it has become apparent that CAV1 is also a secreted protein that can be included into exosomes where it plays a significant role in determining cargo composition. Thus, we will also discuss how CAV1 containing exosomes from metastatic cells promote malignant traits in more benign recipient cells (extrinsic mechanisms). This ability appears, at least in part, attributable to the transfer of specific cargos present due to CAV1 rather than the transfer of CAV1 itself. The evolution of how our perception of CAV1 function has changed since its discovery is summarized graphically in a time line figure.

Caveolin-1 enhances brain metastasis of non-small cell lung cancer, potentially in association with the epithelial-mesenchymal transition marker SNAIL

Background

Caveolin-1 (Cav-1) plays an important role in the development of various human cancers. We investigated the relationship between Cav-1 expression and non-small cell lung cancer (NSCLC) progression in the context of brain metastasis (BM).

Methods

Cav-1 expression was investigated in a series of 102 BM samples and 49 paired primary NSCLC samples, as well as 162 unpaired primary NSCLC samples with (63 cases) or without (99 cases) metastasis to distant organs. Human lung cancer cell lines were used for in vitro functional analysis.

Results

High Cav-1 expression in tumor cells was observed in 52% (38/73) of squamous cell carcinomas (SQCs) and 33% (45/138) of non-SQCs. In SQC, high Cav-1 expression was increased after BM in both paired and unpaired samples of lung primary tumors and BM (53% vs. 84% in paired samples, P = 0.034; 52% vs. 78% in unpaired samples, P = 0.020). Although the difference in median overall survival in patients NSCLC was not statistically significant, high Cav-1 expression in tumor cells (P = 0.005, hazard ratio 1.715, 95% confidence index 1.175–2.502) was independent prognostic factors of overall survival on multivariate Cox regression analyses, in addition to the presence of BM and non-SQC type. In vitro assays revealed that Cav-1 knockdown inhibited the invasion and migration of lung cancer cells. Genetic modulation of Cav-1 was consistently associated with SNAIL up- and down-regulation. These findings were supported by increased SNAIL and Cav-1 expression in BM samples of SQC.

Conclusions

Cav-1 plays an important role in the BM of NSCLC, especially in SQC. The mechanism may be linked to SNAIL regulation.

Electronic supplementary material

The online version of this article (10.1186/s12935-019-0892-0) contains supplementary material, which is available to authorized users.

Association of caveolin-1 protein expression with hepatocellular carcinoma: a meta-analysis and literature review

Background: Aberrant expression of caveolin-1 (CAV-1) is involved in the pathogenesis of hepatocellular carcinoma (HCC); however, the results have been inconsistent due to the small size of sample in the individual study.

Methods: We performed a meta-analysis and evaluated the association of CAV-1 protein overexpression and clinicopathological significance by using Review Manager 5.2. Pooled ORs and HR with corresponding CIs were calculated.

Results: Nine studies were included in the meta-analysis with 810 HCC and 172 cirrhosis patients. CAV-1 protein overexpression was correlated with the risk of cirrhosis; OR was 3.25, p=0.01. Furthermore, the rate of CAV-1 protein overexpression was significantly higher in HCC with cirrhosis than HCC without cirrhosis, suggesting that the CAV-1 protein overexpression likely initiated carcinogenesis in liver with cirrhosis and subsequently contributed to the progression of HCC. In addition, CAV-1 protein overexpression was strongly associated with poor differentiated HCC and invasion; ORs were 2.61 and 2.71, respectively. CAV-1 protein overexpression was strongly correlated with poor overall survival in patients with HCC; HR was 0.4, p=0.03.

Conclusions: In summary, CAV-1 protein overexpression is at risk for liver cirrhosis and HCC derived from cirrhosis, and CAV-1 is also a promising prognostic predictor in HCC.

The caveolin-3 P104L mutation in LGMD-1C patients inhibits non-insulin-stimulated glucose metabolism and growth but promotes myocyte proliferation

The caveolin-3 (CAV3) protein is known to be specifically expressed in various myocytes, and skeletal muscle consumes most of the blood glucose as an energy source to maintain normal cell metabolism and function. The P104L mutation in the coding sequence of the human CAV3 gene leads to autosomal dominant disease limb-girdle muscular dystrophy type 1C (LGMD-1C). We previously reported that C2C12 cells transiently transfected with the P104L CAV3 mutant exhibited decreased glucose uptake and glycogen synthesis after insulin stimulation. The present study aimed to examine whether the P104L mutation affects C2C12 cell glucose metabolism, growth, and proliferation without insulin stimulation. C2C12 cells stably transfected with CAV3-P104L were established, and biochemical assays, western blot analysis and confocal microscopy were used to observe glucose metabolism as well as cell growth and proliferation and to determine the effect of the P104L mutation on the PI3K/Akt signaling pathway. Without insulin stimulation, C2C12 cells stably transfected with the P104L CAV3 mutant exhibited decreased glucose uptake and glycogen synthesis, decreased CAV3 expression and reduced localization of CAV3 and GLUT4 on the cell membrane. The P104L mutant significantly reduced the cell diameters, but accelerated cell proliferation. Akt phosphorylation was inhibited, and protein expression of GLUT4, p-GSK3β, and p-p70s6K, which are molecules downstream of Akt, was significantly decreased. The CAV3-P104L mutation inhibits glycometabolism and cell growth but accelerates C2C12 cell proliferation by reducing CAV3 protein expression and cell membrane localization, which may contribute to the pathogenesis of LGMD-1C.

Utility of cytokeratin 7, S100A1 and caveolin-1 as immunohistochemical biomarkers to differentiate chromophobe renal cell carcinoma from renal oncocytoma

Background

Differentiation of chromophobe renal cell carcinoma (chRCC) from benign renal oncocytoma (RO) can be challenging especially when there are overlapping histological and morphological features. In this study we have investigated immunohistochemical biomarkers (cytokeratin 7/CK7, Caveolin-1/Cav-1 and S100 calcium-binding protein A1/S100A1) to aid in this difficult differentiation and attempted to validate their use in human renal tumour tissue to assess their discriminatory ability, particularly for chRCC and RO, in an Australian cohort of patients.

Methods

Retrospective study was carried out of archived formalin-fixed paraffin-embedded renal tumours from tumour nephrectomy specimens of 75 patients: 30 chRCC, 15 RO and 30 clear cell RCC (ccRCC). Sections were cut and immunostained with specific polyclonal antibodies of CK7, Cav-1 and S100A1. Morphometry was used to determine expression patterns of the biomarkers using Aperio ImageScope. Results were assessed with student t-test and ANOVA with significance at P<0.05.

Results

From this cohort, male-to-female ratio was 1.9:1. Median age was 64 (45-88 years) and median tumour size was 3.8 cm (range, 1.2-18 cm). There were 47 (62.7%) T1, 7 T2, 20 T3 and one T4 stage of RCC; with 2 patients presenting with M1 stage. There was significantly higher CK7 expression in chRCC compared to RO (P=0.03), and chRCC also had a different staining pattern and higher expression of Cav-1 compared to RO. There was higher expression of S100A1 in RO compared to chRCC.

Conclusions

Immunohistochemical staining and standard morphometry of CK7, Cav-1 and S100A1 can aid in the differentiation of chRCC and RO. This may guide clinicians in management of patients when faced with difficult diagnostic histological distinction between the two tumour subtypes.

Blood-brain barrier receptors and transporters: an insight on their function and how to exploit them through nanotechnology

INTRODUCTION

The blood-brain barrier (BBB) is a highly limiting barrier that prevents the brain from contacting with several circulating molecules, including harmful agents. However, certain systemic nutrients and macromolecules are able to cross the BBB and reach the brain parenchyma, involving the interaction with multiple receptors and/or transporters at the BBB surface. Nanotechnology allows the creation of drug vehicles, functionalized with targeting ligands for binding specific BBB receptors and/or transporters, hence triggering the transport through this biobarrier.

AREAS COVERED

This review focuses the BBB receptors/transporters to be exploited in regard to their overall structure and biologic function, as well as their role in the development of strategies envisaging drug delivery to the brain. Then, the interplay between the targeting of these BBB receptors/transporters and nanotechnology is explored, as they can increase by several-fold the effectiveness of brain-targeted therapies.

EXPERT OPINION

Nanomedicine may be particularly useful in brain drug delivery, mainly due to the possibility of functionalizing nanoparticles to target specific receptors/transporters. Since the BBB is endowed with numerous receptors and transporters responsible for regulating the proper metabolic activity of the brain, their targeting can be a promising bypass strategy to circumvent the hurdle that the BBB represents for brain drug delivery.

Role of the Endocytosis of Caveolae in Intracellular Signaling and Metabolism

Caveolae are 60-80 nm invaginated plasma membrane (PM) nanodomains, with a specific lipid and protein composition, which assist and regulate multiple processes in the plasma membrane-ranging from the organization of signalling complexes to the mechanical adaptation to changes in PM tension. However, since their initial descriptions, these structures have additionally been found tightly linked to internalization processes, mechanoadaptation, to the regulation of signalling events and of endosomal trafficking. Here, we review caveolae biology from this perspective, and its implications for cell physiology and disease.

The ERα membrane pool modulates the proliferation of pituitary tumours

The molecular mechanisms underlying the ERα nuclear/cytoplasmic pool that modulates pituitary cell proliferation have been widely described, but it is still not clear how ERα is targeted to the plasma membrane. The aim of this study was to analyse ERα palmitoylation and the plasma membrane ERα (mERα) pool, and their participation in E2-triggered membrane-initiated signalling in normal and pituitary tumour cell growth. Cell cultures were prepared from anterior pituitaries of female Wistar rats and tumour GH3 cells, and treated with 10 nM of oestradiol (E2). The basal expression of ERα was higher in tumour GH3 than in normal pituitary cells. Full-length palmitoylated ERα was observed in normal and pituitary tumour cells, demonstrating that E2 stimulation increased both, ERα in plasma membrane and ERα and caveolin-1 interaction after short-term treatment. In addition, the Dhhc7 and Dhhc21 palmitoylases were negatively regulated after sustained stimulation of E2 for 3 h. Although the uptake of BrdU into the nucleus in normal pituitary cells was not modified by E2, a significant increase in the GH3 tumoural cell, as well as ERK1/2 activation, with this effect being mimicked by PPT, a selective antagonist of ERα. These proliferative effects were blocked by ICI 182780 and the global inhibitor of palmitoylation. These findings indicate that ERα palmitoylation modulated the mERα pool and consequently the ERK1/2 pathway, thereby contributing to pituitary tumour cell proliferation. These results suggest that the plasma membrane ERα pool might be related to the proliferative behaviour of prolactinoma and may be a marker of pituitary tumour growth.

Proteins Regulating Microvesicle Biogenesis and Multidrug Resistance in Cancer

Microvesicles (MV) are emerging as important mediators of intercellular communication. While MVs are important signaling vectors for many physiological processes, they are also implicated in cancer pathology and progression. Cellular activation is perhaps the most widely reported initiator of MV biogenesis, however, the precise mechanism remains undefined. Uncovering the proteins involved in regulating MV biogenesis is of interest given their role in the dissemination of deleterious cancer traits. MVs shed from drug-resistant cancer cells transfer multidrug resistance (MDR) proteins to drug-sensitive cells and confer the MDR phenotype in a matter of hours. MDR is attributed to the overexpression of ABC transporters, primarily P-glycoprotein and MRP1. Their expression and functionality is dependent on a number of proteins. In particular, FERM domain proteins have been implicated in supporting the functionality of efflux transporters in drug-resistant cells and in recipient cells during intercellular transfer by vesicles. Herein, the most recent research on the proteins involved in MV biogenesis and in the dissemination of MV-mediated MDR are discussed. Attention is drawn to unanswered questions in the literature that may prove to be of benefit in ongoing efforts to improve clinical response to chemotherapy and circumventing MDR.

Failure to reabsorb the primary cilium induces cellular senescence

Aurora kinase A (AURKA) is necessary for proper primary cilium disassembly before mitosis. We found that depletion of caveolin-1 expression promotes primary cilia formation through the proteasomal-dependent degradation of aurora kinase A and induces premature senescence in human fibroblasts. Down-regulation of intraflagellar transport-88, a protein essential for ciliogenesis, inhibits premature senescence induced by the depletion of caveolin-1. In support of these findings, we showed that alisertib, a pharmacological inhibitor of AURKA, causes primary cilia formation and cellular senescence by irreversibly arresting cell growth. Suppression of primary cilia formation limits cellular senescence induced by alisertib. The primary cilium must be disassembled to free its centriole to form the centrosome, a necessary structure for mitotic spindle assembly and cell division. We showed that the use of the centriole to form primary cilia blocks centrosome formation and mitotic spindle assembly and prevents the completion of mitosis in cells in which cellular senescence is caused by the inhibition of AURKA. We also found that AURKA is down-regulated and primary cilia formation is enhanced when cellular senescence is promoted by other senescence-inducing stimuli, such as oxidative stress and UV light. Thus, we propose that impaired AURKA function induces premature senescence by preventing reabsorption of the primary cilium, which inhibits centrosome and mitotic spindle formation and consequently prevents the completion of mitosis. Our study causally links the inability of the cell to disassemble the primary cilium, a microtubule-based cellular organelle, to the development of premature senescence, a functionally and pathologically relevant cellular state.-Jeffries, E. P., Di Filippo, M., Galbiati, F. Failure to reabsorb the primary cilium induces cellular senescence.

A review of the role of cav-1 in neuropathology and neural recovery after ischemic stroke

Ischemic stroke starts a series of pathophysiological processes that cause brain injury. Caveolin-1 (cav-1) is an integrated protein and locates at the caveolar membrane. It has been demonstrated that cav-1 can protect blood–brain barrier (BBB) integrity by inhibiting matrix metalloproteases (MMPs) which degrade tight junction proteins. This article reviews recent developments in understanding the mechanisms underlying BBB dysfunction, neuroinflammation, and oxidative stress after ischemic stroke, and focuses on how cav-1 modulates a series of activities after ischemic stroke. In general, cav-1 reduces BBB permeability mainly by downregulating MMP9, reduces neuroinflammation through influencing cytokines and inflammatory cells, promotes nerve regeneration and angiogenesis via cav-1/VEGF pathway, reduces apoptosis, and reduces the damage mediated by oxidative stress. In addition, we also summarize some experimental results that are contrary to the above and explore possible reasons for these differences.

Interplay between Membrane Curvature and Cholesterol: Role of Palmitoylated Caveolin-1

Caveolin-1 (cav-1) is an important player in cell signaling and endocytosis that has been shown to colocalize with cholesterol-rich membrane domains. Experimental studies with varying cav-1 constructs have suggested that it can induce both cholesterol clustering and membrane curvature. Here, we probe the molecular origin of membrane curvature and cholesterol clustering by cav-1 by using coarse-grain molecular dynamics simulations. We have performed a series of simulations of a functionally important cav-1 construct, comprising the membrane-interacting domains and a C-terminal palmitoyl tail. Our results suggest that cav-1 is able to induce cholesterol clustering in the membrane leaflet to which it is bound as well as the opposing leaflet. A positive membrane curvature is observed upon cav-1 binding in cholesterol-containing bilayers. Interestingly, we observe an interplay between cholesterol clustering and membrane curvature such that cav-1 is able to induce higher membrane curvature in cholesterol-rich membranes. The role of the cav-1 palmitoyl tail is less clear and appears to increase the membrane contacts. Further, we address the importance of the secondary structure of cav-1 domains and show that it could play an important role in membrane curvature and cholesterol clustering. Our work is an important step toward a molecular picture of caveolae and vesicular endocytosis.

Biochemical Characterization of Aspergillus fumigatus AroH, a Putative Aromatic Amino Acid Aminotransferase

The rise in the frequency of nosocomial infections is becoming a major problem for public health, in particular in immunocompromised patients. Aspergillus fumigatus is an opportunistic fungus normally present in the environment directly responsible for lethal invasive infections. Recent results suggest that the metabolic pathways related to amino acid metabolism can regulate the fungus-host interaction and that an important role is played by enzymes involved in the catabolism of L-tryptophan. In particular, in A. fumigatus L-tryptophan regulates Aro genes. Among them, AroH encodes a putative pyridoxal 5'-phosphate-dependent aminotransferase. Here we analyzed the biochemical features of recombinant purified AroH by spectroscopic and kinetic analyses corroborated by in silico studies. We found that the protein is dimeric and tightly binds the coenzyme forming a deprotonated internal aldimine in equilibrium with a protonated ketoenamine form. By setting up a new rapid assay method, we measured the kinetic parameters for the overall transamination of substrates and we demonstrated that AroH behaves as an aromatic amino acid aminotransferase, but also accepts L-kynurenine and α-aminoadipate as amino donors. Interestingly, computational approaches showed that the predicted overall fold and active site topology of the protein are similar to those of its yeast ortholog, albeit with some differences in the regions at the entrance of the active site, which could possibly influence substrate specificity. Should targeting fungal metabolic adaptation be of therapeutic value, the results of the present study may pave the way to the design of specific AroH modulators as potential novel agents at the host/fungus interface.

Caveolae: Structure, Function, and Relationship to Disease

The plasma membrane of eukaryotic cells is not a simple sheet of lipids and proteins but is differentiated into subdomains with crucial functions. Caveolae, small pits in the plasma membrane, are the most abundant surface subdomains of many mammalian cells. The cellular functions of caveolae have long remained obscure, but a new molecular understanding of caveola formation has led to insights into their workings. Caveolae are formed by the coordinated action of a number of lipid-interacting proteins to produce a microdomain with a specific structure and lipid composition. Caveolae can bud from the plasma membrane to form an endocytic vesicle or can flatten into the membrane to help cells withstand mechanical stress. The role of caveolae as mechanoprotective and signal transduction elements is reviewed in the context of disease conditions associated with caveola dysfunction.

Downregulated caveolin-1 expression serves a potential role in coronary artery spasm by inducing nitric oxide production in vitro

The present study aimed to investigate the effects of downregulated caveolin-1 (Cav-1) expression on nitric oxide (NO) production in lipopolysaccharide (LPS)-damaged primary human umbilical vein endothelial cells (HUVECs) in a model of coronary artery spasm (CAS) microenvironment induced by acetylcholine (ACh) treatment. Small interfering RNA (siRNA)-mediated Cav-1 downregulation in HUVECs was confirmed by western blotting. The cell viability and superoxide dismutase (SOD) inhibition in HUVECs incubated with LPS (0, 10, 25, 50, 75 and 100 µg/ml) were measured by cell counting kit-8 assay and a SOD kit, respectively. Intracellular Ca2+ [(Ca2+)i] in Fluo4-acetoxymethyl ester-loaded cells was detected by fluorescence microscopy. NO levels in the cell culture supernatants were measured by the nitrate reductase method. The results indicated that transfection with Cav-1 siRNA, in particular siCav-1 (2), downregulated the Cav-1 protein expression. LPS at a dose of 75 µg/ml induced a significant decrease in HUVECs/si-NC and HUVECs/siCav-1 viability compared with the other concentrations of LPS. Compared with the effects of untreated cells, SOD inhibition in HUVECs/si-NC and HUVECs/siCav-1 was significantly decreased by LPS (75 µg/ml). In addition, ACh stimulation caused a greater increase in [Ca2+]i in HUVECs/si-NC as compared with LPS-treated HUVECs/si-NC. ACh stimulation also induced significantly higher NO levels in LPS-treated HUVECs/siCav-1 compared with LPS-treated HUVECs/si-NC cells (P<0.05). In conclusion, the downregulated Cav-1 expression served a key role in NO production in the in vitro model of CAS induced by ACh stimulation of LPS-damaged HUVECs.

Caveolin-1 facilitated KCNA5 expression, promoting breast cancer viability

Potassium voltage-gated channel subfamily A member 5 (KCNA5) is a voltage-gated potassium channel protein encoded by the KCNA5 gene. A large number of studies have shown that KCNA5 is associated with the survival of malignant tumors, including breast cancer, but the detailed mechanism remains inconclusive. Our previous study found that KCNA5 is co-expressed with a scaffolding protein, caveolin-1 in MCF-10A-neoT non-tumorigenic epithelial cell. In the present study, KCNA5 and caveolin-1 were expressed in breast cancer tissues and cell lines. Exposing MCF-10A-neoT to 2 mM of methyl-β-cyclodextrin, an agent to disrupt caveolae and lipid rafts led to a downregulation of caveolin-1 that reduced the expression of KCNA5. Furthermore, following caveolin-1 knockdown, the expression of KCNA5 was decreased in MDA-MB-231 human breast cancer and MCF-10A-neoT non-tumorigenic epithelial cell lines. In subsequent experiments, the MTT assay showed that increased caveolin-1 and KCNA5 expression promoted the survival of MCF-7 human breast cancer cells, but cell survival was not affected following KCNA5 overexpression alone. Using small interfering RNA technology, KCNA5-silenced MCF-10A-neoT cells were established and a decreased level of phosphorylated-AKT serine/threonine kinase (AKT) was observed in the cells compared with the parental cells. Overall, these results suggested that caveolin-1 facilitated KCNA5 expression and may be associated with AKT activation.

Nongenomic cortisol signaling in fish

Glucocorticoids are critical regulators of the cellular processes that allow animals to cope with stressors. In teleosts, cortisol is the primary circulating glucocorticoid and this hormone mediates a suite of physiological responses, most importantly energy substrate mobilization that is essential for acute stress adaptation. Cortisol signaling has been extensively studied and the majority of work has been on the activation of the glucocorticoid receptor (GR), a ligand-bound transcription factor, and the associated downstream transcriptional and protein responses. Despite the role of this hormone in acute stress adaptation, very few studies have examined the rapid effects of this hormone on cellular function. The nongenomic corticosteroid effects, which are rapid (seconds to minutes) and independent of transcription and translation, involve changes to second-messenger pathways and effector proteins, but the primary receptors involved in this pathway activation remain elusive. In teleosts, a few studies suggested the possibility that GR located on the membrane may be initiating a rapid response based on the abrogation of this effect with RU486, a GR antagonist. However, studies have also proposed other signaling mechanisms, including a putative novel membrane receptor and changes to membrane biophysical properties as initiators of rapid signaling in response to cortisol stimulation. Emerging evidence suggests that cortisol activates multiple signaling pathways in cells to bring about rapid effects, but the underlying physiological implications on acute stress adaptation are far from clear.

Insights into the key determinants of membrane protein topology enable the identification of new monotopic folds

Monotopic membrane proteins integrate into the lipid bilayer via reentrant hydrophobic domains that enter and exit on a single face of the membrane. Whereas many membrane-spanning proteins have been structurally characterized and transmembrane topologies can be predicted computationally, relatively little is known about the determinants of membrane topology in monotopic proteins. Recently, we reported the X-ray structure determination of PglC, a full-length monotopic membrane protein with phosphoglycosyl transferase (PGT) activity. The definition of this unique structure has prompted in vivo, biochemical, and computational analyses to understand and define key motifs that contribute to the membrane topology and to provide insight into the dynamics of the enzyme in a lipid bilayer environment. Using the new information gained from studies on the PGT superfamily we demonstrate that two motifs exemplify principles of topology determination that can be applied to the identification of reentrant domains among diverse monotopic proteins of interest.

The caveolae-associated coiled-coil protein, NECC2, regulates insulin signalling in Adipocytes

Adipocyte dysfunction in obesity is commonly associated with impaired insulin signalling in adipocytes and insulin resistance. Insulin signalling has been associated with caveolae, which are coated by large complexes of caveolin and cavin proteins, along with proteins with membrane-binding and remodelling properties. Here, we analysed the regulation and function of a component of caveolae involved in growth factor signalling in neuroendocrine cells, neuroendocrine long coiled-coil protein-2 (NECC2), in adipocytes. Studies in 3T3-L1 cells showed that NECC2 expression increased during adipogenesis. Furthermore, NECC2 co-immunoprecipitated with caveolin-1 (CAV1) and exhibited a distribution pattern similar to that of the components of adipocyte caveolae, CAV1, Cavin1, the insulin receptor and cortical actin. Interestingly, NECC2 overexpression enhanced insulin-activated Akt phosphorylation, whereas NECC2 downregulation impaired insulin-induced phosphorylation of Akt and ERK2. Finally, an up-regulation of NECC2 in subcutaneous and omental adipose tissue was found in association with human obesity and insulin resistance. This effect was also observed in 3T3-L1 adipocytes exposed to hyperglycaemia/hyperinsulinemia. Overall, the present study identifies NECC2 as a component of adipocyte caveolae that is regulated in response to obesity and associated metabolic complications, and supports the contribution of this protein as a molecular scaffold modulating insulin signal transduction at these membrane microdomains.

Vaccination with the Conserved Caveolin-1 Binding Motif in Human Immunodeficiency Virus Type 1 Glycoprotein gp41 Delays the Onset of Viral Infection and Provides Partial Protection in Simian/Human Immunodeficiency Virus-Challenged Cynomolgus Macaques

We have previously reported that the CBD1 peptide (SLEQIWNNMTWMQWDK), corresponding to the consensus caveolin-1 binding domain in human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp41, elicits peptide-specific antibodies. Here, we have investigated the cellular immune response and the protective efficacy against a simian/human immunodeficiency virus (SHIV_162P3) challenge. In addition to the CBD1 peptide, peptides overlapping the caveolin-binding-motif (CBM) (₆₂₂IWNNMTWMQW₆₃₁ or ₆₂₂IWNNMTW₆₂₈) were fused to a Gag-p24 T helper epitope for vaccination. All immunized cynomolgus macaques responded to a cocktail peptide immunization by inducing specific T cells and the production of high-titer CBD1/CBM peptide-specific antibodies. Six months after the fourth vaccine boost, six control and five vaccinated animals were challenged weekly by repeated exposure to SHIV_162P3 via the mucosal rectal route. All control animals were infected after 1 to 3 challenges with SHIV, while among the five vaccinated monkeys, three became infected after a delay compared to control; one was infected after the eighth viral challenge, and one remained uninfected even after the ninth SHIV challenge. Immunized animals maintained a CD4 T cell count, and their central memory CD4 T cells were less depleted than in the control group. Furthermore, SHIV challenge stimulates antigen-specific memory T cell response in vaccinated macaques. Our results indicate that peptides derived from the CBM region can be immunogenic and provide protection against SHIV infection in cynomolgus monkeys.IMPORTANCE In HIV-1-producing cells, gp41 exists in a complexed form with caveolin-1, an interaction most probably mediated by the caveolin-1 binding motif. This sequence is highly conserved in every single HIV-1 isolate, thus suggesting that there is constant selective pressure to preserve this sequence for a specific function in the HIV infectious cycle. Consequently, the CBM sequence may represent the "Achilles' heel" of HIV-1 in the development of an efficient vaccine. Our results demonstrate that macaques immunized with the CBM-based peptides displayed a delay in the onset of viral infection and CD4 depletion, as well as a significant induction of antigen-specific memory T cell response, which is essential for the control of HIV/SIV infections. Finally, as HIV-infected individuals lack anti-CBM immune responses, CBM-based vaccines could have applications as a therapeutic vaccine in AIDS patients.