Chapter 4 The Biology of Caveolae

Dynamic remodeling of TRPC5 channel-caveolin-1-eNOS protein assembly potentiates the positive feedback interaction between Ca2+ and NO signals

The cell signaling molecules nitric oxide (NO) and Ca2+ regulate diverse biological processes through their closely coordinated activities directed by signaling protein complexes. However, it remains unclear how dynamically the multicomponent protein assemblies behave within the signaling complexes upon the interplay between NO and Ca2+ signals. Here we demonstrate that TRPC5 channels activated by the stimulation of G-protein-coupled ATP receptors mediate Ca2+ influx, that triggers NO production from endothelial NO synthase (eNOS), inducing secondary activation of TRPC5 via cysteine S-nitrosylation and eNOS in vascular endothelial cells. Mutations in the caveolin-1-binding domains of TRPC5 disrupt its association with caveolin-1 and impair Ca2+ influx and NO production, suggesting that caveolin-1 serves primarily as the scaffold for TRPC5 and eNOS to assemble into the signal complex. Interestingly, during ATP receptor activation, eNOS is dissociated from caveolin-1 and in turn directly associates with TRPC5, which accumulates at the plasma membrane dependently on Ca2+ influx and calmodulin. This protein reassembly likely results in a relief of eNOS from the inhibitory action of caveolin-1 and an enhanced TRPC5 S-nitrosylation by eNOS localized in the proximity, thereby facilitating the secondary activation of Ca2+ influx and NO production. In isolated rat aorta, vasodilation induced by acetylcholine was significantly suppressed by the TRPC5 inhibitor AC1903. Thus, our study provides evidence that dynamic remodeling of the protein assemblies among TRPC5, eNOS, caveolin-1, and calmodulin determines the ensemble of Ca2+ mobilization and NO production in vascular endothelial cells.

A study to investigate genetic factors associated with weight gain in people with diabetes: analysis of polymorphisms in four relevant genes

BACKGROUND

Weight change is often seen in people with diabetes. We investigated the effects of genes associated with weight change/glucose handling/insulin-signalling.

MATERIALS/METHODS

DNA from diabetes individuals and non-diabetes individuals, plus clinical data, were available from the DARE study (n = 379 individuals: T1D n = 111; T2D n = 222; controls n = 46). Weight gain was assessed by temporal change of Body Mass Index (BMI). Genotyping was performed for CAV1rs926198, LEPRrs1137101, BDNFrs6265 and FTOrs9939609.

RESULTS

No differences in genotype distributions were observed for the four SNPs in all groups un-stratified by weight gain. Following stratification differences in genotype distribution were observed. For those BMI relatively stable; controls showed a difference in genotype distributions versus T1D (CAV1rs926198, LEPRrs1137101). In T2D vs controls, significant differences were observed in genotype distribution for all four genes. For BMI increase, the only difference by category was LEPRrs1137101 (bothT1D/T2D vs controls). In BMI-stable groups, CAV1rs926198, T1D individuals showed lower T allele frequency (p=0.004) vs non-diabetes and for LEPRrs1137101 a higher G allele frequency versus controls (p=0.002). For T2D, CAV1rs926198, T allele frequency was lower in T2D than controls (p=0.005). For LEPR rs1137101, the G allele frequency was higher than in controls (p=0.004). In those with BMI increase, LEPRrs1137101 T1D individuals had higher G allele frequency versus controls (p=0.002) as did T2D vs controls (p=0.03).

CONCLUSION

Differences in allele frequency were seen between diabetes individuals and non-diabetes diagnosed at baseline in relation to the likelihood of BMI increase of >10%. It is established that the G allele of LEPRrs1137101 is associated with weight gain/obesity. However, this is the first report of CAV1rs926198 polymorphism being associated with weight stability/gain in diabetes.

LDL Transcytosis by the Arterial Endothelium-Atherosclerosis by a Thousand Cuts?

PURPOSE OF REVIEW

The accumulation of LDL in the arterial intima is an initiating event in atherosclerosis. After decades of controversy, it is now clear that transcytosis of LDL across an intact endothelial monolayer contributes to its intimal deposition. We review recent observations in this field and address the question of whether LDL transcytosis can be manipulated therapeutically.

RECENT FINDINGS

The development of a live-cell imaging method for studying transcytosis using total internal reflection fluorescence (TIRF) microscopy has catalyzed recent discoveries. LDL transcytosis is mediated by SR-BI and ALK1. Estrogen down-regulates SR-BI and inhibits LDL transcytosis, while the nuclear structural protein HMGB1 promotes LDL transcytosis. LDL transcytosis by ALK1 is independent of the receptor's kinase activity and is antagonized by BMP9, ALK1's canonical ligand. Inflammation stimulates LDL transcytosis. Identifying the function and mechanisms of LDL transcytosis may ultimately permit its therapeutic manipulation.

Aquaporins in Fetal Development

Water homeostasis is essential for fetal growth, and it depends on the successful development of the placenta. Many aquaporins (AQPs) were identified from blastocyst stages to term placenta. In the last years, cytokines, hormones, second messengers, intracellular pH, and membrane proteins were found to regulate their expression and function in the human placenta and fetal membranes. Accumulated data suggest that these proteins may be involved not only in the maintenance of the amniotic fluid volume homeostasis but also in the development of the placenta and fetal organs. In this sense, dysregulation of placental AQPs is associated with gestational disorders. Thus, current evidence shows that AQPs may collaborate in cellular events including trophoblast migration and apoptosis. In addition, aquaglyceroporins are involved in energy metabolism as well as urea elimination across the placenta. In the last year, the presence of AQP9 in trophoblast mitochondria opened new hypotheses about its role in pregnancy. However, much further work is needed to understand the importance of these proteins in human pregnancies.

Increased Caveolin-2 Expression in Brain Endothelial Cells Promotes Age-Related Neuroinflammation

Aging is a major risk factor for common neurodegenerative diseases. Although multiple molecular, cellular, structural, and functional changes occur in the brain during aging, the involvement of caveolin-2 (Cav-2) in brain ageing remains unknown. We investigated Cav-2 expression in brains of aged mice and its effects on endothelial cells. The human umbilical vein endothelial cells (HUVECs) showed decreased THP-1 adhesion and infiltration when treated with Cav-2 siRNA compared to control siRNA. In contrast, Cav-2 overexpression increased THP-1 adhesion and infiltration in HUVECs. Increased expression of Cav-2 and iba-1 was observed in brains of old mice. Moreover, there were fewer iba-1-positive cells in the brains of aged Cav-2 knockout (KO) mice than of wild-type aged mice. The levels of several chemokines were higher in brains of aged wild-type mice than in young wild-type mice; moreover, chemokine levels were significantly lower in brains of young mice as well as aged Cav-2 KO mice than in their wild-type counterparts. Expression of PECAM1 and VE-cadherin proteins increased in brains of old wild-type mice but was barely detected in brains of young wild-type and Cav-2 KO mice. Collectively, our results suggest that Cav-2 expression increases in the endothelial cells of aged brain, and promotes leukocyte infiltration and age-associated neuroinflammation.

Shear stress enhances anoikis resistance of cancer cells through ROS and NO suppressed degeneration of Caveolin-1

Circulating tumor cells (CTCs) acquire enhanced anti-anoikis abilities after experiencing flow shear stress in the circulatory system. Our previous study demonstrated that low shear stress (LSS) promotes anoikis resistance of human breast carcinoma cells via caveolin-1 (Cav-1)-dependent extrinsic and intrinsic apoptotic pathways. However, the underlying mechanism how LSS enhanced Cav-1 expression in suspended cancer cells remains unclear. Herein, we found that LSS induced redox signaling was involved in the regulation of Cav-1 level and anoikis resistance in suspension cultured cancer cells. Exposure of human breast carcinoma MDA-MB-231 cells to LSS (2 dyn/cm²) markedly induced ROS and ^•NO generation, which promoted the cell viability and reduced the cancer cell apoptosis. Furthermore, ROS and ^•NO scavenging inhibited the upregulation of Cav-1 by interfering ubiquitination, and suppressed the anoikis resistance of suspended tumor cells. These findings provide new insight into the mechanism by which LSS-stimulated ROS and ^•NO generation increases Cav-1 stabilization in suspended cancer cells through inhibition of ubiquitination and proteasomal degradation, which could be a potential target for therapy of metastatic tumors.

Caveolae-Associated Molecules, Tumor Stroma, and Cancer Drug Resistance: Current Findings and Future Perspectives

Simple Summary

Cell membranes contain small invaginations called caveolae. They are a specialized lipid domain and orchestrate cellular signaling events, mechanoprotection, and lipid homeostasis. Formation of the caveolae depends on two classes of proteins, the caveolins and cavins, which form large complexes that allow their self-assembly into caveolae. Loss of either of these two proteins leads to distortion of the caveolae structure and disruption of many physiological processes that affect diseases of the muscle, metabolic states governing lipids, and the glucose balance as well as cancers. In cancers, the expression of caveolins and cavins is heterogenous, and they undergo alterations both in the tumors and the surrounding tumor microenvironment stromal cells. Remarkably, their expression and function has been associated with resistance to many cancer drugs. Here, we summarize the current knowledge of the resistance mechanisms and how this knowledge could be applied into the clinic in future.

Abstract

The discovery of small, “cave-like” invaginations at the plasma membrane, called caveola, has opened up a new and exciting research area in health and diseases revolving around this cellular ultrastructure. Caveolae are rich in cholesterol and orchestrate cellular signaling events. Within caveola, the caveola-associated proteins, caveolins and cavins, are critical components for the formation of these lipid rafts, their dynamics, and cellular pathophysiology. Their alterations underlie human diseases such as lipodystrophy, muscular dystrophy, cardiovascular disease, and diabetes. The expression of caveolins and cavins is modulated in tumors and in tumor stroma, and their alterations are connected with cancer progression and treatment resistance. To date, although substantial breakthroughs in cancer drug development have been made, drug resistance remains a problem leading to treatment failures and challenging translation and bench-to-bedside research. Here, we summarize the current progress in understanding cancer drug resistance in the context of caveola-associated molecules and tumor stroma and discuss how we can potentially design therapeutic avenues to target these molecules in order to overcome treatment resistance.

Cancer driver gene and non-coding RNA alterations as biomarkers of brain metastasis in lung cancer: A review of the literature

Brain metastasis (BM) is the most common event in patients with lung cancer. Despite multimodal treatments and advances in systemic therapies, development of BM remains one of the main factors associated with poor prognosis and mortality in patients with lung cancer. Therefore, better understanding of mechanisms involved in lung cancer brain metastasis (LCBM) is of great importance to suppress cancer cells and to improve the overall survival of patients. Several cancer-related genes such as EGFR and KRAS have been proposed as potential predictors of LCBM. In addition, there is ample evidence supporting crucial roles of non-coding RNAs (ncRNAs) in mediating LCBM. In this review, we provide comprehensive information on risk assessment, predictive, and prognostic panels for early detection of BM in patients with lung cancer. Moreover, we present an overview of LCBM molecular mechanisms, cancer driver genes, and ncRNAs which may predict the risk of BM in lung cancer patients. Recent clinical studies have focused on determining mechanisms involved in LCBM and their association with diagnosis, prognosis, and treatment outcomes. These studies have shown that alterations in EGFR, KRAS, BRAF, and ALK, as the most frequent coding gene alterations, and dysregulation of ncRNAs such as miR-423, miR-330-3p, miR-145, piR-651, and MALAT1 can be considered as potential biomarkers of LCBM.

Caveolin-1, a novel player in cognitive decline

Cognitive decline (CD), which related to vascular dementia, Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and diabetes mellitus, is a growing health concern that has a great impact on the patients' quality of life. Although extensive efforts, the mechanisms of CD are still far from being clarified, not to mention the effective treatment and prevention strategies. Caveolin-1 (Cav-1), a trans-membrane protein, is a major component of the caveolae structure and scaffolding proteins. Recently, ample evidence depicts a strong correlation between Cav-1 and CD, however, the specific role of Cav-1 in CD has not been clearly examined and how they might be connected have yet to be identified. This review seeks to provide a comprehensive overview about how Cav-1 modulates pathogeneses of CD-associated diseases. In summary, Cav-1 can promote structural and functional plasticity of neurons, improve neurogenesis, relieve mitochondrial dysfunction, inhibit inflammation and suppress oxidative stress, which have shed light on the idea that Cav-1 may be an efficacious therapeutic target to treat CD.

Structural Interplays in the Flexible N-Terminus and Scaffolding Domain of Human Membrane Protein Caveolin 3

Caveolins are critical for the formation of caveolae, which are small invaginations of the plasma membrane involved in a variety of biological processes. Caveolin 3 (Cav3), one of three caveolin isoforms, is an integral membrane protein mainly expressed in muscle tissues. Although various human diseases associated with Cav3 have been reported, structural characterization of Cav3 in the membrane has not been investigated in enough depth to understand the structure–function relationship. Here, using solution NMR, we characterized membrane association, structural communications, and molecular dynamics of the monomeric Cav3 in detergent micelle environment, particularly focused on the whole N-terminal part that is composed of the flexible N-terminus and the scaffolding domain. The results revealed a complicated structural interplay of the individual segments composing the whole N-terminal part, including the pH-dependent helical region, signature motif-like region, signature motif, and scaffolding domain. Collectively, the present study provides novel structural insights into the whole N-terminal part of Cav3 that plays important biological roles in cellular processes and diseases. In particular, given that several disease-related mutations are located at the whole N-terminal part of Cav3, the sophisticated communications in the whole N-terminal segments are likely to have relevance to the molecular basis of Cav3-related disease.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Lifestyle intervention in individuals with impaired glucose regulation affects Caveolin-1 expression and DNA methylation

Aims: We investigated whether a lifestyle intervention could influence expression and DNA methylation of diabetes-related genes in patients with impaired glucose regulation (IGR), the results were compared to bariatric surgery, considering it an intensive change. Methods: Twenty participants with IGR had adipose tissue biopsy and blood collected pre- and post-lifestyle (6 months) intervention; 12 obese patients had subcutaneous fat taken before and after bariatric surgery. RNA/DNA was extracted from all samples and underwent qPCR. DNA was bisulphite converted and 12 CpG sites of Caveolin-1 (CAV1) promoter were pyrosequenced. Results: lifestyle intervention resulted in opposite direction changes in fat tissue and blood for CAV1 expression and DNA methylation and these changes were correlated between tissues, while no significative differences were found in CAV1 expression after bariatric surgery. Conclusions: Our findings suggest a role for CAV1 in modulating adipocyte function as a consequence of lifestyle changes, as exercises and diet. These results may provide insights into new therapeutic targets for diabetes prevention.

Klotho antagonizes pulmonary fibrosis through suppressing pulmonary fibroblasts activation, migration, and extracellular matrix production: a therapeutic implication for idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) has been widely accepted as an aging-related fatal lung disease with a therapeutic impasse, largely a consequence of the complex and polygenic gene architecture underlying the molecular pathology of IPF. Here, by conducting an integrative network analysis on the largest IPF case-control RNA-seq dataset to date, we attributed the systems-level alteration in IPF to disruptions in a handful of biological processes including cell migration, transforming growth factor-β (TGF-β) signaling and extracellular matrix (ECM), and identified klotho (KL), a typical anti-aging molecule, as a potential master regulator of those disease-relevant processes. Following experiments showed reduced Kl in isolated pulmonary fibroblasts from bleomycin-exposed mice, and demonstrated that recombinant KL effectively mitigated pulmonary fibrosis in an ex vivo model and alleviated TGF-β-induced pulmonary fibroblasts activation, migration, and ECM production in vitro, which was partially ascribed to FOXF1 and CAV1, two highly co-expressed genes of KL in the IPF. Overall, KL appears to be a vital regulator during pulmonary fibrosis. Given that administration of exogenous KL is a feasible treatment strategy, our work highlighted a promising target gene that could be easily manipulated, leaving the field well placed to further explore the therapeutic potential of KL for IPF.

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.

Moxifloxacin suppresses airway inflammation and modulates expression of caveolin-1 and flotillin-1 in airway smooth muscle cells of asthmatic rats

Background

Moxifloxacin (MXF) possesses anti-inflammatory properties on asthmatic airway smooth muscle cells (ASMCs) beyond their antimicrobial effects, but the mechanisms are still unknown. This study was to investigate effects of MXF on expression of caveolin-1 (Cav-1) and flotillin-1 (FLOT1) in ASMCs in asthmatic rats.

Methods

ASMCs were collected from the airway and cultured in vitro. Cells from normal rats were treated with normal saline (Group N); cells from asthmatic rats were incubated with normal saline (Group A) or MXF (20 mg/L) (Group M); Cav-1 expression was up-regulated by transferring Cav-1 expressing lentivirus (Group L) and FLOT1 expression down-regulated by using siRNA in cells from asthmatic rats (Group S). The expressions of Cav-1, FLOT1 and p65 NF-κB were measured by Western blotting and quantificational real-time polymerase chain reaction (qRT-PCR), and interleukin-8 (IL-8) and eotaxin contents were measured by enzyme-linked immunosorbent assay (ELISA).

Results

Compared with normal control, Cav-1 expression significantly decreased in asthmatic groups (P<0.01); MXF up-regulated Cav-1 expression in asthmatic groups (P<0.01). However, compared with normal control, the expression of FLOT1 and p65 NF-κB dramatically increased in asthmatic groups (P<0.01); MXF down-regulated the expression of FLOT1 and p65 NF-κB in asthmatic groups (P<0.01); meanwhile, the expressions of FLOT1 and p65 NF-κB decreased after up-regulation of Cav-1 expression in asthmatic groups (P=0.01). Compared with asthmatic groups, the IL-8 and eotaxin contents significantly decreased in MXF Groups, Cav-1 up-regulation asthmatic groups and FLOT1 down-regulation asthmatic groups (P<0.01).

Conclusions

MXF can modulate the airway inflammation, upregulate Cav-1 expression, downregulate the expression of FLOT1 and p65 NF-κB in asthmatic rat ASMCs, which may be related to the anti-inflammatory effects of MXF in asthmatic ASMCs.

Novel finding of caveolin-2 in apical membranes of proximal tubule and first detection of caveolin-2 in urine: A promising biomarker of renal disease

Caveolin-2 (Cav-2) is expressed in a variety of cell tissue, and it has also been found in renal tissue. The expression of Cav-2 in proximal tubules is still unclear. The aim of this study was to carry out a complete evaluation of the expression pattern of Cav-2 in rat renal cortex to clarify and deepen the knowledge about the localization of Cav-2 in the proximal tubules and also to evaluate its presence in urine. Male Wistar rats were used to assess Cav-2 expression by Western blot analysis in homogenates, apical, and basolateral membranes from kidney cortex, in lysates and total plasma membranes from renal cortical cell suspensions, in urine, and in urinary exosomes. Cav-2 was clearly expressed in renal cortex homogenates and in both apical and basolateral membranes isolated from kidney cortex, with a greater expression on the former membranes. It was also observed in lysates and in plasma membranes from cortical cell suspensions. Moreover, Cav-2 was found in urine and in its exosomal fraction. These results confirmed the presence of Cav-2 in proximal tubule cells in the kidney of healthy rats, and showed for the first time its expression at the apical membrane of these cells and in urine. Besides, urinary exosomal pathway could be involved in Cav-2 urinary excretion under normal conditions. We observed an increase in the urinary abundance of Cav-2 in two models of acute kidney injury, and thus we proposed the urinary excretion of Cav-2 as a potential biomarker of kidney injury.

Exosomes and exosomal miRNAs from muscle-derived fibroblasts promote skeletal muscle fibrosis

Exosomes, natural carriers of mRNAs, non-coding RNAs and proteins between donor and recipient cells, actively contribute to cell-cell communication. We investigated the potential pro-fibrotic role of exosomes released by muscle-derived fibroblasts of Duchenne muscular dystrophy (DMD) patients, and of miRNAs carried by exosomes. By fibrosis focused array analysis we found that exosomes from DMD fibroblasts, had significantly higher levels of miR-199a-5p, a miRNA up-regulated in fibrotic conditions, compared to control exosomes, while levels in myoblast-derived exosomes were not increased. In control fibroblasts, exposure to DMD fibroblast-derived exosomes induced a myofibroblastic phenotype with increase in α-smooth actin, collagen and fibronectin transcript and protein expression, soluble collagen production and deposition, cell proliferation, and activation of Akt and ERK signaling, while exposure to control exosomes did not. Transfecting control fibroblasts or loading control exosomes with miR-199a-5p mimic or inhibitor induced opposing effects on fibrosis-related mRNAs and proteins, on collagen production and Akt and ERK pathways. Finally, injection of DMD fibroblast-derived exosomes into mouse tibialis anterior muscle after cardiotoxin-induced necrosis, produced greater fibrosis than control exosomes. Our findings indicate that exosomes produced by local fibroblasts in the DMD muscle are able to induce phenotypic conversion of normal fibroblasts to myofibroblasts thereby increasing the fibrotic response. This conversion is related to transfer of high levels of miR-199a-5p and to reduction of its target caveolin-1; both, therefore, are potential therapeutic targets in muscle fibrosis.

BMPR2 mutations and endothelial dysfunction in pulmonary arterial hypertension (2017 Grover Conference Series)

Despite the discovery more than 15 years ago that patients with hereditary pulmonary arterial hypertension (HPAH) inherit BMP type 2 receptor ( BMPR2) mutations, it is still unclear how these mutations cause disease. In part, this is attributable to the rarity of HPAH and difficulty obtaining tissue samples from patients with early disease. However, in addition, limitations to the approaches used to study the effects of BMPR2 mutations on the pulmonary vasculature have restricted our ability to determine how individual mutations give rise to progressive pulmonary vascular pathology in HPAH. The importance of understanding the mechanisms by which BMPR2 mutations cause disease in patients with HPAH is underscored by evidence that there is reduced BMPR2 expression in patients with other, more common, non-hereditary form of PAH, and that restoration of BMPR2 expression reverses established disease in experimental models of pulmonary hypertension. In this paper, we focus on the effects on endothelial function. We discuss some of the controversies and challenges that have faced investigators exploring the role of BMPR2 mutations in HPAH, focusing specifically on the effects different BMPR2 mutation have on endothelial function, and whether there are qualitative differences between different BMPR2 mutations. We discuss evidence that BMPR2 signaling regulates a number of responses that may account for endothelial abnormalities in HPAH and summarize limitations of the models that are used to study these effects. Finally, we discuss evidence that BMPR2-dependent effects on endothelial metabolism provides a unifying explanation for the many of the BMPR2 mutation-dependent effects that have been described in patients with HPAH.

Teaming Up for Trouble: Cancer Cells, Transforming Growth Factor-β1 Signaling and the Epigenetic Corruption of Stromal Naïve Fibroblasts

It is well recognized that cancer cells subvert the phenotype of stromal naïve fibroblasts and instruct the neighboring cells to sustain their growth agenda. The mechanisms underpinning the switch of fibroblasts to cancer-associated fibroblasts (CAFs) are the focus of intense investigation. One of the most significant hallmarks of the biological identity of CAFs is that their tumor-promoting phenotype is stably maintained during in vitro and ex vivo propagation without the continual interaction with the adjacent cancer cells. In this review, we discuss robust evidence showing that the master cytokine Transforming Growth Factor-β1 (TGFβ-1) is a prime mover in reshaping, via epigenetic switches, the phenotype of stromal fibroblasts to a durable state. We also examine, in detail, the pervasive involvement of TGFβ-1 signaling from both cancer cells and CAFs in fostering cancer development, taking colorectal cancer (CRC) as a paradigm of human neoplasia. Finally, we review the stroma-centric anticancer therapeutic approach focused on CAFs—the most abundant cell population of the tumor microenvironment (TME)—as target cells.

Caveolin-1 deficiency induces premature senescence with mitochondrial dysfunction

Paradoxical observations have been made regarding the role of caveolin-1 (Cav-1) during cellular senescence. For example, caveolin-1 deficiency prevents reactive oxygen species-induced cellular senescence despite mitochondrial dysfunction, which leads to senescence. To resolve this paradox, we re-addressed the role of caveolin-1 in cellular senescence in human diploid fibroblasts, A549, HCT116, and Cav-1^-/- mouse embryonic fibroblasts. Cav-1 deficiency (knockout or knockdown) induced cellular senescence via a p53-p21-dependent pathway, downregulating the expression level of the cardiolipin biosynthesis enzymes and then reducing the content of cardiolipin, a critical lipid for mitochondrial respiration. Our results showed that Cav-1 deficiency decreased mitochondrial respiration, reduced the activity of oxidative phosphorylation complex I (CI), inactivated SIRT1, and decreased the NAD⁺ /NADH ratio. From these results, we concluded that Cav-1 deficiency induces premature senescence via mitochondrial dysfunction and silent information regulator 2 homologue 1 (SIRT1) inactivation.

A pH-Mediated Topological Switch within the N-Terminal Domain of Human Caveolin-3

Caveolins mediate the formation of caveolae, which are small omega-shaped membrane invaginations involved in a variety of cellular processes. There are three caveolin isoforms, the third of which (Cav3) is expressed in smooth and skeletal muscles. Mutations in Cav3 cause a variety of human muscular diseases. In this work, we characterize the secondary structure, dynamics, and topology of the monomeric form of the full-length lipidated protein. Cav3 consists of a series of membrane-embedded or surface-associated helical elements connected by extramembrane connecting loops or disordered domains. Our results also reveal that the N-terminal domain undergoes a large scale pH-mediated topological rearrangement between soluble and membrane-anchored forms. Considering that roughly one-third of pathogenic mutations in Cav3 influence charged residues located in this domain, we hypothesize that this transition is likely to be relevant to the molecular basis of Cav3-linked diseases. These results provide insight into the structure of Cav3 and set the stage for mechanistic investigations of the effects of pathogenic mutations.

Epigenetic Regulation of Caveolin-1 Gene Expression in Lung Fibroblasts

Fibrotic disorders are associated with tissue accumulation of fibroblasts. We recently showed that caveolin (Cav)-1 gene suppression by a profibrotic cytokine, transforming growth factor (TGF)-β1, contributes to fibroblast proliferation and apoptosis resistance. Cav-1 has been shown to be constitutively suppressed in idiopathic pulmonary fibrosis (IPF), but mechanisms for this suppression are incompletely understood. We hypothesized that epigenetic processes contribute to Cav-1 down-regulation in IPF lung fibroblasts, and after fibrogenic stimuli. Cav-1 expression levels, DNA methylation status, and histone modifications associated with the Cav-1 promoter were examined by PCR, Western blots, pyrosequencing, or chromatin immunoprecipitation assays in IPF lung fibroblasts, normal fibroblasts after TGF-β1 stimulation, or in murine lung fibroblasts after bleomycin injury. Methylation-specific PCR demonstrated methylated and unmethylated Cav-1 DNA copies in all groups. Despite significant changes in Cav-1 expression, no changes in DNA methylation were observed in CpG islands or CpG island shores of the Cav-1 promoter by pyrosequencing of lung fibroblasts from IPF lungs, in response to TGF-β1, or after bleomycin-induced murine lung injury, when compared with respective controls. In contrast, the association of Cav-1 promoter with the active histone modification mark, H3 lysine 4 trimethylation, correlated with Cav-1 down-regulation in activated/fibrotic lung fibroblasts. Our data indicate that Cav-1 gene silencing in lung fibroblasts is actively regulated by epigenetic mechanisms that involve histone modifications, in particular H3 lysine 4 trimethylation, whereas DNA methylation does not appear to be a primary mechanism. These findings support therapeutic strategies that target histone modifications to restore Cav-1 expression in fibroblasts participating in pathogenic tissue remodeling.

TLR4 and Caveolin-1 in Monocytes Are Associated With Inflammatory Conditions in Diabetic Neuropathy

The purpose of this study was to investigate the expression of TLR4 and caveolin‐1 in monocytes among healthy volunteers as well as those with type‐2 diabetes mellitus (T2DM) and diabetic peripheral neuropathy (DPN). Nineteen healthy control subjects, 18 patients with T2DM, and 20 patients with DPN were enrolled. Toll‐like receptor (TLR)4, caveolin‐1, MyD88, phosphorylated IκB, and plasma TNF‐α and interleukin (IL)‐6 were measured using real‐time polymerase chain reaction, Western blotting, and enzyme‐linked immunosorbent assay. Compared with the other two groups, the DPN group had higher expression of TLR4, MyD88, phosphorylated IκB, TNF‐α, and IL‐6, but significantly lower levels of caveolin‐1 and total IκB in monocytes. Plasma concentrations of TNF‐α and IL‐6 were positively correlated with TLR4 and negatively correlated with caveolin‐1 in patients with DPN. Plasma concentration of TLR4 was negatively correlated with caveolin‐1 in patients with DPN. Reduced expression of caveolin‐1 in monocytes could aggravate the TLR4‐mediated inflammatory cascade.

The prognostic values of caveolin-1 immunoreactivity in peritubular capillaries in patients with kidney transplantation

The low sensitivity of C4d immunoreactivity in peritubular capillaries (PTCs) hinders its use in the diagnosis of chronic active antibody-mediated rejection (CAAMR). C4d-negative CAAMR was defined in the 2013 Banff classification, which included the expression of endothelial-associated transcripts (ENDATs). We previously showed that the ENDAT caveolin-1 (CAV-1) is a distinct feature of CAAMR. In this study, we investigated the prognostic value of CAV-1 immunoreactivity in PTCs in kidney transplant patients. Ninety-eight kidney transplant recipients were included in this study. The prognostic value of CAV-1 immunoreactivity in PTCs was evaluated by double immunostaining for CAV-1 and pathologische Anatomie Leiden endothelium (PAL-E, a PTC marker) in the PTCs of kidney allograft biopsy samples. The patients were divided into two groups: CAV-1/PAL-E<50% and CAV-1/PAL-E≥50%. Kaplan-Meier curves showed that CAV-1/PAL-E≥50% patients had a significantly worse prognosis than that of CAV-1/PAL-E<50% patients (log-rank; P<.001). C4d staining of PTCs was not associated with the development of graft failure (log-rank; P=.345), whereas in a multivariate Cox regression analysis, CAV-1 immunoreactivity in PTCs was independently associated with graft failure (hazard ratio: 11.1; P=.0324). CAV-1 immunoreactivity in PTCs may serve as a prognostic marker for kidney allograft survival.

Caveolin-1 promotes tumor growth and metastasis via autophagy inhibition in hepatocellular carcinoma

BACKGROUND

Caveolin-1 is a member of the caveolae family of membrane proteins. Although some researchers have investigated the function of Caveolin-1 in hepatocellular carcinoma, the mechanism of Caveolin-1 action and its prognostic value in hepatocellular carcinoma remain unclear.

METHODS

Caveolin-1 expression was measured in hepatocellular carcinoma cell lines and tissues using quantitative reverse transcription-polymerase chain reaction, western blot, and immunofluorescence assays. In in vitro experiments, Caveolin-1 was depleted using a short hairpin RNA lentiviral vector, and tumor cell behavior was analyzed. The effect of Caveolin-1 on hepatocellular carcinoma cell autophagy was investigated. Prognostic value of Caveolin-1 was analyzed by immunohistochemistry in two cohorts that included a total of 721 hepatocellular carcinoma patients.

RESULTS

We found that Caveolin-1 was overexpressed in highly metastatic hepatocellular carcinoma cell lines and tumor tissues. Moreover, Caveolin-1 promoted hepatocellular carcinoma cell proliferation, migration, and angiogenesis and inhibited autophagy. Finally, Caveolin-1 expression in hepatocellular carcinoma tissues was inversely correlated with patient overall survival and time to recurrence.

CONCLUSION

Our data obtained from cell lines suggest an oncogenic role for Caveolin-1 in hepatocellular carcinoma, Caveolin-1 contributed to hepatocellular carcinoma cell autophagy deficiency. Furthermore, Caveolin-1 may function as a novel prognostic indicator for hepatocellular carcinoma patients after curative resection, and combination of targeted therapy aimed at Caveolin-1 and autophagy modulation may represent an effective way to treat hepatocellular carcinoma.

Topologically Diverse Human Membrane Proteins Partition to Liquid-Disordered Domains in Phase-Separated Lipid Vesicles

The integration of membrane proteins into “lipid raft” membrane domains influences many biochemical processes. The intrinsic structural properties of membrane proteins are thought to mediate their partitioning between membrane domains. However, whether membrane topology influences the targeting of proteins to rafts remains unclear. To address this question, we examined the domain preference of three putative raft-associated membrane proteins with widely different topologies: human caveolin-3, C99 (the 99 residue C-terminal domain of the amyloid precursor protein), and peripheral myelin protein 22. We find that each of these proteins are excluded from the ordered domains of giant unilamellar vesicles containing coexisting liquid-ordered and liquid-disordered phases. Thus, the intrinsic structural properties of these three topologically distinct disease-linked proteins are insufficient to confer affinity for synthetic raft-like domains.

CAVEOLIN-1 expression in brain metastasis from lung cancer predicts worse outcome and radioresistance, irrespective of tumor histotype

Brain metastases develop in one-third of patients with non-small-cell lung cancer and are associated with a dismal prognosis, irrespective of surgery or chemo-radiotherapy. Pathological markers for predicting outcomes after surgical resection and radiotherapy responsiveness are still lacking. Caveolin 1 has been associated with chemo- and radioresistance in various tumors, including non-small-cell lung cancer. Here, caveolin 1 expression was assessed in a series of 69 brain metastases from non-small-cell lung cancer and matched primary tumors to determine its role in predicting survival and radiotherapy responsiveness. Only caveolin 1 expression in brain metastasis was associated with poor prognosis and an increased risk of death (log rank test, p = 0.015). Moreover, in the younger patients (median age of <54 years), caveolin 1 expression neutralized the favorable effect of young age on survival compared with the older patients. Among the radiotherapy-treated patients, an increased risk of death was detected in the group with caveolin 1-positive brain metastasis (14 out of 22 patients, HR=6.839, 95% CI 1.849 to 25.301, Wald test p = 0.004). Overall, caveolin 1 expression in brain metastasis from non-small-cell lung cancer is independently predictive of worse outcome and radioresistance and could become an additional tool for personalized therapy in the critical subset of brain-metastatic non-small-cell lung cancer patients.

The Mammalian Blood-Testis Barrier: Its Biology and Regulation

Spermatogenesis is the cellular process by which spermatogonia develop into mature spermatids within seminiferous tubules, the functional unit of the mammalian testis, under the structural and nutritional support of Sertoli cells and the precise regulation of endocrine factors. As germ cells develop, they traverse the seminiferous epithelium, a process that involves restructuring of Sertoli-germ cell junctions, as well as Sertoli-Sertoli cell junctions at the blood-testis barrier. The blood-testis barrier, one of the tightest tissue barriers in the mammalian body, divides the seminiferous epithelium into 2 compartments, basal and adluminal. The blood-testis barrier is different from most other tissue barriers in that it is not only comprised of tight junctions. Instead, tight junctions coexist and cofunction with ectoplasmic specializations, desmosomes, and gap junctions to create a unique microenvironment for the completion of meiosis and the subsequent development of spermatids into spermatozoa via spermiogenesis. Studies from the past decade or so have identified the key structural, scaffolding, and signaling proteins of the blood-testis barrier. More recent studies have defined the regulatory mechanisms that underlie blood-testis barrier function. We review here the biology and regulation of the mammalian blood-testis barrier and highlight research areas that should be expanded in future studies.

Are caveolae a cellular entry route for non-viral therapeutic delivery systems?

The development of novel therapies increasingly relies on sophisticated delivery systems that allow the drug or gene expression-modifying agent of interest entry into cells. These systems can promote cellular targeting and/or entry, and they vary in size, charge, and functional group chemistry. Their optimization requires an in depth knowledge of the cellular routes of entry in normal and pathological states. Caveolae are plasma membrane invaginations that have the potential to undergo endocytosis. We critically review the literature exploring whether drug or nucleic acid delivery systems exploit and/or promote cellular entry via caveolae. A vast majority of studies employ pharmacological tools, co-localization experiments and very few make use of molecular tools. We provide clarification on how results of such studies should be interpreted and make suggestions for future studies.

Blocking VEGF/Caveolin-1 signaling contributes to renal protection of fasudil in streptozotocin-induced diabetic rats

AIM

RhoA/ROCK signaling plays an important role in diabetic nephropathy, and ROCK inhibitor fasudil exerts nephroprotection in experimental diabetic nephropathy. In this study we investigated the molecular mechanisms underlying the protective actions of fasudil in a rat model of diabetic nephropathy.

METHODS

Streptozotocin (STZ)-induced diabetic rats, to which fasudil or a positive control drug enalapril were orally administered for 8 months. Metabolic parameters and blood pressure were assessed during the treatments. After the rats were euthanized, kidney samples were collected for histological and molecular biological studies. VEGF, VEGFR1, VEGFR2 and fibronectin expression, and Src and caveolin-1 phosphorylation in the kidneys were assessed using RT-PCR, Western blot and immunohistochemistry assays. The association between VEGFR2 and caveolin-1 was analyzed with immunoprecipitation.

RESULTS

Chronic administration of fasudil (30 and 100 mg·kg(-1)·d(-1)) or enalapril (10 mg/kg, bid) significantly attenuated the glomerular sclerosis and albuminuria in the diabetic rats. Furthermore, fasudil treatment prevented the upregulation of VEGF, VEGFR1, VEGFR2 and fibronectin, and the increased association between VEGFR2 and caveolin-1 in the renal cortices, and partially blocked Src activation and caveolin-1 phosphorylation on tyrosine 14 in the kidneys, whereas enalapril treatment had no effects on the VEGFR2/Src/caveolin-1 signaling pathway.

CONCLUSION

Fasudil exerts protective actions in STZ-induced diabetic nephropathy by blocking the VEGFR2/Src/caveolin-1 signaling pathway and fibronectin upregulation. Thus, VEGFR2 may be a potential therapeutic target for the treatment of diabetic nephropathy.

Caveolae: molecular insights and therapeutic targets for stroke

INTRODUCTION

Caveolae are specialized plasma membrane micro-invaginations of most mammalian cell types. The organization and function of caveolae are carried out by their coat proteins, caveolins and adaptor proteins, cavins. Caveolae/caveolins physically interact with membrane-associated signaling molecules and function in cholesterol incorporation, signaling transduction and macromolecular transport/permeability.

AREAS COVERED

Recent investigations have implicated a check-and-balance role of caveolae in the pathophysiology of cerebral ischemia. Caveolin knockout mice displayed exacerbated ischemic injury, whereas caveolin peptide exerted remarkable protection against ischemia/reperfusion injury. This review attempts to provide a comprehensive synopsis of how caveolae/caveolins modulate blood-brain barrier permeability, pro-survival signaling, angiogenesis and neuroinflammation, and how this may contribute to a better understanding of the participation of caveolae in ischemic cascade. The role of caveolin in the preconditioning-induced tolerance against ischemia is also discussed.

EXPERT OPINION

Caveolae represent a novel target for cerebral ischemia. It remains open how to manipulate caveolin expression in a practical way to recapitulate the beneficial therapeutic outcomes. Caveolin peptides and associated antagomirs may be efficacious and deserve further investigations for their potential benefits for stroke.

Caveolae control the anti-inflammatory phenotype of senescent endothelial cells

Senescent endothelial cells (EC) have been identified in cardiovascular disease, in angiogenic tumour associated vessels and in aged individuals. We have previously identified a novel anti-inflammatory senescent phenotype of EC. We show here that caveolae are critical in the induction of this anti-inflammatory senescent state. Senescent EC induced by either the overexpression of ARHGAP18/SENEX or by H₂O₂ showed significantly increased numbers of caveolae and associated proteins Caveolin-1, cavin-1 and cavin-2. Depletion of these proteins by RNA interference decreased senescence induced by ARHGAP18 and by H₂O₂. ARHGAP18 overexpression induced a predominantly anti-inflammatory senescent population and depletion of the caveolae-associated proteins resulted in the preferential reduction in this senescent population as measured by neutrophil adhesion and adhesion protein expression after TNFα treatment. In confirmation, EC isolated from the aortas of CAV-1(-/-) mice failed to induce this anti-inflammatory senescent cell population upon expression of ARHGAP18, whereas EC from wild-type mice showed a significant increase. NF-κB is one of the major transcription factors mediating the induction of E-selectin and VCAM-1 expression, adhesion molecules responsible for leucocyte attachment to EC. TNFα-induced activation of NF-κB was suppressed in ARHGAP18-induced senescent EC, and this inhibition was reversed by Caveolin-1 knock-down. Thus, out results demonstrate that an increase in caveolae and its component proteins in senescent ECs is associated with inhibition of the NF-kB signalling pathway and promotion of the anti-inflammatory senescent pathway.

Heterozygous null bone morphogenetic protein receptor type 2 mutations promote SRC kinase-dependent caveolar trafficking defects and endothelial dysfunction in pulmonary arterial hypertension

Hereditary pulmonary arterial hypertension (HPAH) is a rare, fatal disease of the pulmonary vasculature. The majority of HPAH patients inherit mutations in the bone morphogenetic protein type 2 receptor gene (BMPR2), but how these promote pulmonary vascular disease is unclear. HPAH patients have features of pulmonary endothelial cell (PEC) dysfunction including increased vascular permeability and perivascular inflammation associated with decreased PEC barrier function. Recently, frameshift mutations in the caveolar structural protein gene Caveolin-1 (CAV-1) were identified in two patients with non-BMPR2-associated HPAH. Because caveolae regulate endothelial function and vascular permeability, we hypothesized that defects in caveolar function might be a common mechanism by which BMPR2 mutations promote pulmonary vascular disease. To explore this, we isolated PECs from mice carrying heterozygous null Bmpr2 mutations (Bmpr2(+/-)) similar to those found in the majority of HPAH patients. We show that Bmpr2(+/-) PECs have increased numbers and intracellular localization of caveolae and caveolar structural proteins CAV-1 and Cavin-1 and that these defects are reversed after blocking endocytosis with dynasore. SRC kinase is also constitutively activated in Bmpr2(+/-) PECs, and localization of CAV-1 to the plasma membrane is restored after treating Bmpr2(+/-) PECs with the SRC kinase inhibitor 3-(4-chlorophenyl)-1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine (PP2). Late outgrowth endothelial progenitor cells isolated from HPAH patients show similar increased activation of SRC kinase. Moreover, Bmpr2(+/-) PECs have impaired endothelial barrier function, and barrier function is restored after treatment with PP2. These data suggest that heterozygous null BMPR2 mutations promote SRC-dependent caveolar trafficking defects in PECs and that this may contribute to pulmonary endothelial barrier dysfunction in HPAH patients.

Differential effects of caveolin-1 and -2 knockdown on aqueous outflow and altered extracellular matrix turnover in caveolin-silenced trabecular meshwork cells

PURPOSE

A single nucleotide polymorphism (SNP) identified between caveolin-1 (CAV1) and caveolin-2 (CAV2) on chromosome 7 is associated with glaucoma. One function of CAVs is endocytosis and recycling of extracellular matrix (ECM) components. Here, we generated CAV-silencing lentivirus to evaluate the effects on ECM turnover by trabecular meshwork (TM) cells and to measure the effect on outflow facility in anterior segment perfusion culture.

METHODS

Short hairpin CAV1 and CAV2 silencing and control lentivirus were generated, characterized, and applied to anterior segments in perfusion culture. Colocalization of CAVs with various ECM molecules in TM cells was investigated using immunofluorescence and confocal microscopy. Western immunoblotting and fluorogenic-based enzyme activity assays were used to investigate ECM protein levels and degradation, respectively.

RESULTS

Endogenous CAVs colocalized with cortactin at podosome- or invadopodia-like structures (PILS), which are areas of focal ECM degradation. In perfusion culture, outflow rates increased significantly in CAV1-silenced anterior segments, whereas outflow significantly decreased in CAV2-silenced anterior segments. Matrix metalloproteinase (MMP)2 and MMP14, and a disintegrin and metalloproteinase with thrombospondin motifs-4 (ADAMTS4) colocalized with both CAVs in TM cells. Protein levels and enzyme activities of MMP/ADAMTS4, fibronectin protein levels, actin stress fibers, and α-smooth muscle actin were all increased in CAV-silenced cells.

CONCLUSIONS

Caveolin-mediated endocytosis is one mechanism by which TM cells can alter the physiological catabolism of ECM in order to change the composition of the outflow channels in the TM to regulate aqueous outflow resistance. Dysregulation of CAV function could contribute to the pathological changes in ECM that are observed in glaucoma.

PTRF/Cavin-1 decreases prostate cancer angiogenesis and lymphangiogenesis

Caveolae are specialized plasma membrane subdomains implicated in cellular functions such as migration, signalling and trafficking. Caveolin-1 and polymerase I and transcript release factor (PTRF)/cavin-1 are essential for caveola formation. Caveolin-1 is overexpressed and secreted in prostate tumors and promotes aggressiveness and angiogenesis. In contrast, a lack of PTRF expression is reported in prostate cancer, and ectopic PTRF expression in prostate cancer cells inhibits tumor growth and metastasis. We experimentally manipulated PTRF expression in three prostate cancer cell lines, namely the caveolin-1 positive cells PC3 and DU145 and the caveolin-1-negative LNCaP cells, to evaluate angiogenesis- and lymphangiogenesis-regulating functions of PTRF. We show that the conditioned medium of PTRF-expressing prostate cancer cells decreases ECs proliferation, migration and differentiation in vitro and ex vivo. This can occur independently from caveolin-1 expression and secretion or caveola formation, since the anti-angiogenic effects of PTRF were detected in caveolin-1-negative LNCaP cells. Additionally, PTRF expression in PC3 cells significantly decreased blood and lymphatic vessel densities in orthotopic tumors in mice. Our results suggest that the absence of PTRF in prostate cancer cells contributes significantly to tumour progression and metastasis by promoting the angiogenesis and lymphangiogenesis potential of the cancer cells, and this could be exploited for therapy.

Modest effects of lipid modifications on the structure of caveolin-3

Caveolin-3 (Cav3) is an unconventional membrane protein that serves as a critical scaffolding hub in caveolae and is genetically linked to various muscle disorders. In this work, we report the expression, purification, and characterization of full-length human Cav3. To mimic the palmitoylation of endogenous Cav3, we developed a generally applicable approach to covalently attached thioalkyl chains at natively modified cysteine residues. Nuclear magnetic resonance measurements indicate that lipidation exerts only a modest and local effect on the Cav3 structure, with little impact on the structures of the N-terminal domain, the scaffolding domain, and the extreme C-terminus.

Endothelial caveolin-1 plays a major role in the development of atherosclerosis

Clinical studies have established the important impact of atherosclerotic disease in Western societies. This disease is characterized by the accumulation of lipids and the migration of various cell types in the sub-endothelial space of blood vessels. As demonstrated by many studies, endothelial cells play an essential role in the development of this disease. The endothelium acts as a gatekeeper of blood vessel integrity and cardiovascular health status. For instance, the transfer of lipids via the transport of lipoproteins in the arterial intima is believed to be mediated by endothelial cells through a process termed transcytosis. In addition, lipoproteins that accumulate in the sub-endothelial space may also be modified, in a process that can direct the activation of endothelial cells. These steps are essential for the initiation of an atherosclerotic plaque and may be mediated, at least in part, by caveolae and their associated protein caveolin-1. In the present study, we evaluate the role of caveolin-1/caveolae in the regulation of these two steps in endothelial cells. Our data clearly demonstrate that caveolin-1 is involved in the regulation of lipoprotein transcytosis across endothelial cells and in the regulation of vascular inflammation.

Role of caveolin-1 in fibrotic diseases

Fibrosis underlies the pathogenesis of numerous diseases and leads to severe damage of vital body organs and, frequently, to death. Better understanding of the mechanisms resulting in fibrosis is essential for developing appropriate treatment solutions and is therefore of upmost importance. Recent evidence suggests a significant antifibrotic potential of an integral membrane protein, caveolin-1. While caveolin-1 has been widely studied for its role in the regulation of cell signaling and endocytosis, its possible implication in fibrosis remains largely unclear. In this review we survey involvement of caveolin-1 in various cellular processes and highlight different aspects of its antifibrotic activity. We hypothesize that caveolin-1 conveys a homeostatic function in the process of fibrosis by (a) regulating TGF-β1 and its downstream signaling; (b) regulating critical cellular processes involved in tissue repair, such as migration, adhesion and cellular response to mechanical stress; and (c) antagonizing profibrotic processes, such as proliferation. Finally, we consider this homeostatic function of caveolin-1 as a possible novel approach in treatment of fibroproliferative diseases.

A potential role for caveolin-1 in VEGF-induced fibronectin upregulation in mesangial cells: involvement of VEGFR2 and Src

VEGF is known to be an endothelial cell mitogen that stimulates angiogenesis by promoting endothelial cell survival, proliferation, migration, and differentiation. Recent studies have suggested that VEGF may play a pivotal role in glomerular sclerosis through extracellular matrix protein (ECM) accumulation, although the signaling mechanism is still unclear. The GTPase RhoA has been implicated in VEGF-induced type IV collagen accumulation in some settings. Here we study the role of different VEGF receptors and membrane microdomain caveolae in VEGF-induced RhoA activation and fibronectin upregulation in mesangial cells (MCs). In primary rat MC, VEGF time and dose dependently increased fibronectin production. Rho pathway inhibition blocked VEGF-induced fibronectin upregulation. VEGF-induced RhoA activation was prevented by disrupting caveolae with cholesterol depletion and rescued by cholesterol repletion. VEGF stimulation led to a markedly increased VEGFR2/caveolin-1 but failed to increase VEGFR1/caveolin-1 association. VEGF also increased caveolin-1/Src association and activated Src, and Src inhibitor blocked RhoA activation and fibronectin upregulation. Src-mediated phosphorylation of caveolin-1 on Y14 has also been implicated in signaling responses. Overexpression of nonphosphorylatable caveolin-1 Y14A prevented VEGF-induced RhoA activation and fibronectin upregulation. In vivo, although VEGFR1 and VEGFR2 protein levels were both increased in the kidney cortices of diabetic rats, VEGFR2/caveolin-1 association but not VEGFR1/caveolin-1 association was significantly increased. In conclusion, VEGF-induced RhoA activation and fibronectin upregulation require caveolae and caveolin-1 interaction with VEGFR2 and Src. Interference with caveolin/-ae signaling may provide new avenues for the treatment of fibrotic renal disease.

Ubiquitination of the N-terminal region of caveolin-1 regulates endosomal sorting by the VCP/p97 AAA-ATPase

Caveolin-1 (CAV1) is the defining constituent of caveolae at the plasma membrane of many mammalian cells. For turnover, CAV1 is ubiquitinated and sorted to late endosomes and lysosomes. Sorting of CAV1 requires the AAA+-type ATPase VCP and its cofactor UBXD1. However, it is unclear in which region CAV1 is ubiquitinated and how ubiquitination is linked to sorting of CAV1 by VCP-UBXD1. Here, we show through site-directed mutagenesis that ubiquitination of CAV1 occurs at any of the six lysine residues, 5, 26, 30, 39, 47, and 57, that are clustered in the N-terminal region but not at lysines in the oligomerization, intramembrane, or C-terminal domains. Mutation of Lys-5-57 to arginines prevented binding of the VCP-UBXD1 complex and, importantly, strongly reduced recruitment of VCP-UBXD1 to endocytic compartments. Moreover, the Lys-5-57Arg mutation specifically interfered with trafficking of CAV1 from early to late endosomes. Conversely and consistently, depletion of VCP or UBXD1 led to accumulation of ubiquitinated CAV1, suggesting that VCP acts downstream of ubiquitination and is required for transport of the ubiquitinated form of CAV1 to late endosomes. These results define the N-terminal region of CAV1 as the critical ubiquitin conjugation site and, together with previous data, demonstrate the significance of this ubiquitination for binding to the VCP-UBXD1 complex and for sorting into lysosomes.

Introduction of caveolae structural proteins into the protozoan Toxoplasma results in the formation of heterologous caveolae but not caveolar endocytosis

Present on the plasma membrane of most metazoans, caveolae are specialized microdomains implicated in several endocytic and trafficking mechanisms. Caveolins and the more recently discovered cavins are the major protein components of caveolae. Previous studies reported that caveolar invaginations can be induced de novo on the surface of caveolae-negative mammalian cells upon heterologous expression of caveolin-1. However, it remains undocumented whether other components in the transfected cells participate in caveolae formation. To address this issue, we have exploited the protozoan Toxoplasma as a heterologous expression system to provide insights into the minimal requirements for caveogenesis and caveolar endocytosis. Upon expression of caveolin-1, Toxoplasma accumulates prototypical exocytic caveolae 'precursors' in the cytoplasm. Toxoplasma expressing caveolin-1 alone, or in conjunction with cavin-1, neither develops surface-located caveolae nor internalizes caveolar ligands. These data suggest that the formation of functional caveolae at the plasma membrane in Toxoplasma and, by inference in all non-mammalian cells, requires effectors other than caveolin-1 and cavin-1. Interestingly, Toxoplasma co-expressing caveolin-1 and cavin-1 displays an impressive spiraled network of membranes containing the two proteins, in the cytoplasm. This suggests a synergistic activity of caveolin-1 and cavin-1 in the morphogenesis and remodeling of membranes, as illustrated for Toxoplasma.

Co-regulation of cell polarization and migration by caveolar proteins PTRF/Cavin-1 and caveolin-1

Caveolin-1 and caveolae are differentially polarized in migrating cells in various models, and caveolin-1 expression has been shown to quantitatively modulate cell migration. PTRF/cavin-1 is a cytoplasmic protein now established to be also necessary for caveola formation. Here we tested the effect of PTRF expression on cell migration. Using fluorescence imaging, quantitative proteomics, and cell migration assays we show that PTRF/cavin-1 modulates cellular polarization, and the subcellular localization of Rac1 and caveolin-1 in migrating cells as well as PKCα caveola recruitment. PTRF/cavin-1 quantitatively reduced cell migration, and induced mesenchymal epithelial reversion. Similar to caveolin-1, the polarization of PTRF/cavin-1 was dependent on the migration mode. By selectively manipulating PTRF/cavin-1 and caveolin-1 expression (and therefore caveola formation) in multiple cell systems, we unveil caveola-independent functions for both proteins in cell migration.

N-terminal tyrosine phosphorylation of caveolin-2 negates anti-proliferative effect of transforming growth factor beta in endothelial cells

Here we show that tyrosine phosphorylation of caveolin-2 (Cav-2) negatively regulates the anti-proliferative function of transforming growth factor beta (TGF-beta) in endothelial cells. In contrast to wild-type-Cav-2, retroviral re-expression of Y19/27F-Cav-2 in Cav-2 knockout endothelial cells did not affect anti-proliferative effect of TGF-beta compared to empty vector. Conversely, although less effective than wild-type, re-expression of S23/36A-Cav-2 reduced the effect of TGF-beta compared to empty vector. This differential effect of tyrosine and serine phosphorylation mutants of Cav-2 correlated with TGF-beta-induced Smad3 phosphorylation and transcriptional activation of plasminogen activator inhibitor-1. Thus tyrosine-phosphorylated Cav-2 counteracts anti-proliferative effect of TGF-beta in endothelial cells.

Lipid rafts, synaptic transmission and plasticity: impact in age-related neurodegenerative diseases

The synapse is a crowded area. In the last years, the concept that proteins can be organized in different membrane domains according to their structure has emerged. Cholesterol-rich membrane domains, or lipid rafts, form an organized portion of the membrane that is thought to concentrate signaling molecules. Accumulating evidence has shown that both the pre-synaptic and post-synaptic sites are highly enriched in lipid rafts, which are likely to organize and maintain synaptic proteins in their precise localization. Here we review recent studies highlighting the importance of lipid rafts for synaptic function and plasticity, as well as their relevance for age or disease-related cognitive impairment. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Interference in mevalonate pathway ameliorates homocysteine-induced endothelium-dysfunction

Homocysteine is a risk factor for atherosclerosis and hypertension and induces endothelium-dysfunction. Accumulation of cholesterol and reactive oxygen species plays a key role in the endothelium-dysfunction. This study investigated the hypothesis of an involvement of mevalonate pathway and oxidative pathway in homocysteine-induced endothelial damage. Homocysteine induced impairment of the endothelium-dependent vasorelaxation of rat aortic rings by isometric tension, while it also reduced the nitric oxide level and the nitric oxide synthase activity in human umbilical vein endothelial cells, followed by accumulation of superoxide anion and cholesterol. However, the level of asymmetric dimethylarginine remained unaffected by homocysteine. The adverse effect of homocysteine on endothelial function was found to be partially enhanced either by squalestatin-reducing cholesterol or by superoxide dismutase-reducing superoxide anion. Moreover, this effect of homocysteine could be completely ameliorated by simvastatin, very similar to that of cotreatment of squalestatin and superoxide dismutase. Respectively, mevalonolactone partly or squalene fully attenuated the effect of simvastatin or squalestatin on homocysteine-induced endothelial dysfunction. In conclusion, our results suggested that the mevalonate pathway mediates homocysteine-induced endothelium dysfunction besides the oxidative pathway. Interference in the mevalonate pathway and oxidative pathway provides effective protection of endothelial function.

Interaction of caveolin-1 with Ku70 inhibits Bax-mediated apoptosis

Caveolin-1, the structural protein component of caveolae, acts as a scaffolding protein that functionally regulates signaling molecules. We show that knockdown of caveolin-1 protein expression enhances chemotherapeutic drug-induced apoptosis and inhibits long-term survival of colon cancer cells. In vitro studies demonstrate that caveolin-1 is a novel Ku70-binding protein, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82-101) to the caveolin-binding domain (CBD) of Ku70 (amino acids 471-478). Cell culture data show that caveolin-1 binds Ku70 after treatment with chemotherapeutic drugs. Mechanistically, we found that binding of caveolin-1 to Ku70 inhibits the chemotherapeutic drug-induced release of Bax from Ku70, activation of Bax, translocation of Bax to mitochondria and apoptosis. Potentiation of apoptosis by knockdown of caveolin-1 protein expression is greatly reduced in the absence of Bax expression. Finally, we found that overexpression of wild type Ku70, but not a mutant form of Ku70 that cannot bind to caveolin-1 (Ku70 Φ→A), limits the chemotherapeutic drug-induced Ku70/Bax dissociation and apoptosis. Thus, caveolin-1 acts as an anti-apoptotic protein in colon cancer cells by binding to Ku70 and inhibiting Bax-dependent cell death.

Molecular and cellular mechanisms underlying the role of blood vessels in spinal cord injury and repair

Spinal cord injury causes immediate damage of nervous tissue accompanied by the loss of motor and sensory function. The limited self-repair ability of damaged nervous tissue underlies the need for reparative interventions to restore function after spinal cord injury. Blood vessels play a crucial role in spinal cord injury and repair. Injury-induced loss of local blood vessels and a compromised blood-brain barrier contribute to inflammation and ischemia and thus to the overall damage to the nervous tissue of the spinal cord. Lack of vasculature and leaking blood vessels impede endogenous tissue repair and limit prospective repair approaches. A reduction of blood vessel loss and the restoration of blood vessels so that they no longer leak might support recovery from spinal cord injury. The promotion of new blood vessel formation (i.e., angio- and vasculogenesis) might aid repair but also incorporates the danger of exacerbating tissue loss and thus functional impairment. The delicate interplay between cells and molecules that govern blood vessel repair and formation determines the extent of damage and the success of reparative interventions. This review deals with the cellular and molecular mechanisms underlying the role of blood vessels in spinal cord injury and repair.