Caveolins and cellular cholesterol balance

Interactions between macrophage membrane and lipid mediators during cardiovascular diseases with the implications of scavenger receptors

The onset and progression of cardiovascular diseases with the major underlying cause being atherosclerosis, occur during chronic inflammatory persistence in the vascular system, especially within the arterial wall. Such prolonged maladaptive inflammation is driven by macrophages and their key mediators are generally attributed to a disparity in lipid metabolism. Macrophages are the primary cells of innate immunity, endowed with expansive membrane domains involved in immune responses with their signalling systems. During atherosclerosis, the membrane domains and receptors control various active organisations of macrophages. Their scavenger/endocytic receptors regulate the trafficking of intracellular and extracellular cargo. Corresponding influence on lipid metabolism is mediated by their dynamic interaction with scavenger membrane receptors and their integrated mechanisms such as pinocytosis, phagocytosis, cholesterol export/import, etc. This interaction not only results in the functional differentiation of macrophages but also modifies their structural configurations. Here, we reviewed the association of macrophage membrane biomechanics and their scavenger receptor families with lipid metabolites during the event of atherogenesis. In addition, the membrane structure of macrophages and the signalling pathways involved in endocytosis integrated with lipid metabolism are detailed. This article establishes future insights into the scavenger receptors as potential targets for cardiovascular disease prevention and treatment.

Cholesterol Is a Regulator of CAV1 Localization and Cell Migration in Oral Squamous Cell Carcinoma

Cholesterol plays an important role in cancer progression, as it is utilized in membrane biogenesis and cell signaling. Cholesterol-lowering drugs have exhibited tumor-suppressive effects in oral squamous cell carcinoma (OSCC), suggesting that cholesterol is also essential in OSCC pathogenesis. However, the direct effects of cholesterol on OSCC cells remain unclear. Here, we investigated the role of cholesterol in OSCC with respect to caveolin-1 (CAV1), a cholesterol-binding protein involved in intracellular cholesterol transport. Cholesterol levels in OSCC cell lines were depleted using methyl-β-cyclodextrin and increased using the methyl-β-cyclodextrin-cholesterol complex. Functional analysis was performed using timelapse imaging, and CAV1 expression in cholesterol-manipulated cells was investigated using immunofluorescence and immunoblotting assays. CAV1 immunohistochemistry was performed on surgical OSCC samples. We observed that cholesterol addition induced polarized cell morphology, along with CAV1 localization at the trailing edge, and promoted cell migration. Moreover, CAV1 was upregulated in the lipid rafts and formed aggregates in the plasma membrane in cholesterol-added cells. High membranous CAV1 expression in tissue specimens was associated with OSCC recurrence. Therefore, cholesterol promotes the migration of OSCC cells by regulating cell polarity and CAV1 localization to the lipid raft. Furthermore, membranous CAV1 expression is a potential prognostic marker for OSCC patients.

Caveolae Mechanotransduction at the Interface between Cytoskeleton and Extracellular Matrix

The plasma membrane (PM) is subjected to multiple mechanical forces, and it must adapt and respond to them. PM invaginations named caveolae, with a specific protein and lipid composition, play a crucial role in this mechanosensing and mechanotransduction process. They respond to PM tension changes by flattening, contributing to the buffering of high-range increases in mechanical tension, while novel structures termed dolines, sharing Caveolin1 as the main component, gradually respond to low and medium forces. Caveolae are associated with different types of cytoskeletal filaments, which regulate membrane tension and also initiate multiple mechanotransduction pathways. Caveolar components sense the mechanical properties of the substrate and orchestrate responses that modify the extracellular matrix (ECM) according to these stimuli. They perform this function through both physical remodeling of ECM, where the actin cytoskeleton is a central player, and via the chemical alteration of the ECM composition by exosome deposition. Here, we review mechanotransduction regulation mediated by caveolae and caveolar components, focusing on how mechanical cues are transmitted through the cellular cytoskeleton and how caveolae respond and remodel the ECM.

Caveolae Microdomains Mediate STAT5 Signaling Induced by Insulin in MCF-7 Breast Cancer Cells

Caveolae are small plasma membrane invaginations constituted for membrane proteins namely caveolins and cytosolic proteins termed cavins, which can occupy up to 50% of the surface of mammalian cells. The caveolae have been involved with a variety of cellular processes including regulation of cellular signaling. Insulin is a hormone that mediates a variety of physiological processes through activation of insulin receptor (IR), which is a tyrosine kinase receptor expressed in all mammalian tissues. Insulin induces activation of signal transducers and activators of transcription (STAT) family members including STAT5. In this study, we demonstrate, for the first time, that insulin induces phosphorylation of STAT5 at tyrosine-694 (STAT5-Tyr(P)⁶⁹⁴), STAT5 nuclear accumulation and an increase in STAT5-DNA complex formation in MCF-7 breast cancer cells. Insulin also induces nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴, caveolin-1, and IR in MCF-7 cells. STAT5 nuclear accumulation and the increase of STAT5-DNA complex formation require the integrity of caveolae and microtubule network. Moreover, insulin induces an increase and nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴ in MDA-MB-231 breast cancer cells. In conclusion, results demonstrate that caveolae and microtubule network play an important role in STAT5-Tyr(P)⁶⁹⁴, STAT5 nuclear accumulation and STAT5-DNA complex formation induced by insulin in breast cancer cells.

Roles of Cholesterol and PtdIns(4,5)P2 in the Regulation of STIM1-Orai1 Channel Function

Calcium is one of the most prominent second messengers. It is involved in a wide range of functions at the single-cell level but also in modulating regulatory mechanisms in the entire organism. One process mediating calcium signaling involves hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) by the phospholipase-C (PLC). Thus, calcium and PtdIns(4,5)P₂ are intimately intertwined two second-messenger cascades that often depend on each other. Another relevant lipid associated with calcium signaling is cholesterol. Both PtdIns(4,5)P₂ and cholesterol play key roles in the formation and maintenance of specialized signaling nanodomains known as lipid rafts. Lipid rafts are particularly important in calcium signaling by concentrating and localizing calcium channels such as the Orai1 channel. Depletion of internal calcium stores is initiated by the production of inositol-1,4,5-trisphosphate (IP3). Calcium depletion from the ER induces the oligomerization of STIM1, which binds Orai1 and initiates calcium influx into the cell. In the present review, we analyzed the complex interactions between cholesterol, PtdIns(4,5)P₂, and the complex formed by the Orai1 channel and the signaling molecule STIM1. We explore some of the complex mechanisms governing calcium homeostasis and phospholipid metabolism, as well as the interaction between these two apparently independent signaling cascades.

Caveolin-1 Regulates Cellular Metabolism: A Potential Therapeutic Target in Kidney Disease

The kidney is an energy-consuming organ, and cellular metabolism plays an indispensable role in kidney-related diseases. Caveolin-1 (Cav-1), a multifunctional membrane protein, is the main component of caveolae on the plasma membrane. Caveolae are represented by tiny invaginations that are abundant on the plasma membrane and that serve as a platform to regulate cellular endocytosis, stress responses, and signal transduction. However, caveolae have received increasing attention as a metabolic platform that mediates the endocytosis of albumin, cholesterol, and glucose, participates in cellular metabolic reprogramming and is involved in the progression of kidney disease. It is worth noting that caveolae mainly depend on Cav-1 to perform the abovementioned cellular functions. Furthermore, the mechanism by which Cav-1 regulates cellular metabolism and participates in the pathophysiology of kidney diseases has not been completely elucidated. In this review, we introduce the structure and function of Cav-1 and its functions in regulating cellular metabolism, autophagy, and oxidative stress, focusing on the relationship between Cav-1 in cellular metabolism and kidney disease; in addition, Cav-1 that serves as a potential therapeutic target for treatment of kidney disease is also described.

A Detergent-Free Method for Preparation of Lipid Rafts for the Shotgun Lipidomics Study

Lipid rafts are microdomains on plasma membrane that contain high levels of cholesterol and sphingolipids. Because of the detergent-resistant property of lipid rafts, lipid rafts isolated by methods that use detergents frequently yield different results. Artifacts can also be introduced through the use of detergents. These limitations could be overcome with a detergent-free method which eliminates possible artificial influences. Importantly, lipid rafts prepared with a detergent-free method is more compatible to mass spectrometric analysis since the ion suppression effect is largely reduced.This chapter describes a detergent-free two-step method for preparation of lipid rafts. Firstly, a purified plasma membrane fraction is prepared from cells by sedimentation of the postnuclear supernatant (PNS) in a Percoll gradient. Secondly, the as-prepared plasma membranes are sonicated to release lipid rafts which are further isolated by flotation in a continuous gradient of Optiprep solution. Then, we introduce a typical shotgun lipidomics workflow that can be used as a cost-effective and relatively high throughput method to determine the lipidomes of lipid rafts.The method also makes an easy start for lipidomics studies.

Caveolae and lipid sorting: Shaping the cellular response to stress

Caveolae are an abundant and characteristic surface feature of many vertebrate cells. The uniform shape of caveolae is characterized by a bulb with consistent curvature connected to the plasma membrane (PM) by a neck region with opposing curvature. Caveolae act in mechanoprotection by flattening in response to increased membrane tension, and their disassembly influences the lipid organization of the PM. Here, we review evidence for caveolae as a specialized lipid domain and speculate on mechanisms that link changes in caveolar shape and/or protein composition to alterations in specific lipid species. We propose that high membrane curvature in specific regions of caveolae can enrich specific lipid species, with consequent changes in their localization upon caveolar flattening. In addition, we suggest how changes in the association of lipid-binding caveolar proteins upon flattening of caveolae could allow release of specific lipids into the bulk PM. We speculate that the caveolae-lipid system has evolved to function as a general stress-sensing and stress-protective membrane domain.

Unidirectional Regulation of Vimentin Intermediate Filaments to Caveolin-1

Both the mechanosensitive vimentin cytoskeleton and endocytic caveolae contribute to various active processes such as cell migration, morphogenesis, and stress response. However, the crosstalk between these two systems has remained elusive. Here, we find that the subcellular expression between vimentin and caveolin-1 is mutual exclusive, and vimentin filaments physically arrest the cytoplasmic motility of caveolin-1 vesicles. Importantly, vimentin depletion increases the phosphorylation of caveolin-1 on site Tyr14, and restores the compromised cell migration rate and directionality caused by caveolin-1 deprivation. Moreover, upon hypo-osmotic shock, vimentin-knockout recovers the reduced intracellular motility of caveolin-1 vesicles. In contrary, caveolin-1 depletion shows no effect on the expression, phosphorylation (on sites Ser39, Ser56, and Ser83), distribution, solubility, and cellular dynamics of vimentin filaments. Taken together, our data reveals a unidirectional regulation of vimentin to caveolin-1, at least on the cellular level.

Endocytosis in proliferating, quiescent and terminally differentiated cells

Endocytosis mediates nutrient uptake, receptor internalization and the regulation of cell signaling. It is also hijacked by many bacteria, viruses and toxins to mediate their cellular entry. Several endocytic routes exist in parallel, fulfilling different functions. Most studies on endocytosis have used transformed cells in culture. However, as the majority of cells in an adult body have exited the cell cycle, our understanding is biased towards proliferating cells. Here, we review the evidence for the different pathways of endocytosis not only in dividing, but also in quiescent, senescent and terminally differentiated cells. During mitosis, residual endocytosis is dedicated to the internalization of caveolae and specific receptors. In non-dividing cells, clathrin-mediated endocytosis (CME) functions, but the activity of alternative processes, such as caveolae, macropinocytosis and clathrin-independent routes, vary widely depending on cell types and functions. Endocytosis supports the quiescent state by either upregulating cell cycle arrest pathways or downregulating mitogen-induced signaling, thereby inhibiting cell proliferation. Endocytosis in terminally differentiated cells, such as skeletal muscles, adipocytes, kidney podocytes and neurons, supports tissue-specific functions. Finally, uptake is downregulated in senescent cells, making them insensitive to proliferative stimuli by growth factors. Future studies should reveal the molecular basis for the differences in activities between the different cell states.

Vimentin intermediate filaments function as a physical barrier during intracellular trafficking of caveolin-1

Both the cytoskeletal intermediate filaments (IFs) and cytoplasmic caveolae contribute to active processes such as cell migration, morphogenesis and vesicular trafficking, but the interplay between these two systems has remained elusive. Here, we find that vimentin and nestin IFs interact with caveolae central component caveolin-1 (CAV-1) and importantly, restrain the intracellular trafficking of CAV-1 positive vesicles by serving as a physical barrier. Consequently, CAV-1 vesicles show less density and mobility in vimentin IFs enriched region, which is a substrate stiffness independent process. Moreover, depletion of vimentin IFs releases the slow movement proportion of CAV-1 positive vesicles and thus increases their cytoplasmic dynamics, whereas the expression of caveolae-associated protein CAV-1, CAV-2 and cavin-1 were unaffected. Collectively, these results reveal a negative role of IFs in regulating the trafficking of intracellular CAV-1 vesicles in live cells.

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.

Simvastatin Promotes Cardiac Myocyte Relaxation in Association with Phosphorylation of Troponin I

The number of people taking statins is set to increase across the globe due to recent changes in prescription guidelines. For example, half the US population over 40 is now eligible for these drugs, whether they have high serum cholesterol or not. With such development in policy comes a stronger need for understanding statins’ myriad of effects. Surprisingly little is known about possible direct actions of statins on cardiac myocytes, although claims of a direct myocardial toxicity have been made. Here, we determine the impact of simvastatin administration (40 mg/kg/day) for 2 weeks in normocholesterolemic rats on cardiac myocyte contractile function and identify an underlying mechanism. Under basal conditions, statin treatment increased the time to half (t_0.5) relaxation without any effect on the magnitude of shortening, or the magnitude/kinetics of the [Ca²⁺]_i transient. Enhanced myocyte lusitropy could be explained by a corresponding increase in phosphorylation of troponin I (TnI) at Ser^23,24. Statin treatment increased expression of eNOS and Ser¹¹⁷⁷ phosphorylated eNOS, decreased expression of the NOS-inhibitory proteins caveolins 1 and 3, and increased (P = 0.06) NO metabolites, consistent with enhanced NO production. It is well-established that NO stimulates protein kinase G, one of the effectors of TnI phosphorylation at Ser^23,24. Trends for parallel changes in phospho-TnI, phospho-eNOS and caveolin 1 expression were seen in atrial muscle from patients taking statins. Our data are consistent with a mechanism whereby chronic statin treatment enhances TnI phosphorylation and myocyte lusitropy through increased NO bioavailability. We see no evidence of impaired function with statin treatment; the changes we document at the level of the cardiac myocyte should facilitate diastolic filling and cardiac performance.

Synthesis and Live-Cell Imaging of Fluorescent Sterols for Analysis of Intracellular Cholesterol Transport

Cellular cholesterol homeostasis relies on precise control of the sterol content of organelle membranes. Obtaining insight into cholesterol trafficking pathways and kinetics by live-cell imaging relies on two conditions. First, one needs to develop suitable analogs that resemble cholesterol as closely as possible with respect to their biophysical and biochemical properties. Second, the cholesterol analogs should have good fluorescence properties. This interferes, however, often with the first requirement, such that the imaging instrumentation must be optimized to collect photons from suboptimal fluorophores, but good cholesterol mimics, such as the intrinsically fluorescent sterols, cholestatrienol (CTL) or dehydroergosterol (DHE). CTL differs from cholesterol only in having two additional double bonds in the ring system, which is why it is slightly fluorescent in the ultraviolet (UV). In the first part of this protocol, we describe how to synthesize and image CTL in living cells relative to caveolin, a structural component of caveolae. In the second part, we explain in detail how to perform time-lapse experiments of commercially available BODIPY-tagged cholesterol (TopFluor-cholesterol^®; TF-Chol) in comparison to DHE. Finally, using two-photon time-lapse imaging data of TF-Chol, we demonstrate how to use our imaging toolbox SpatTrack for tracking sterol rich vesicles in living cells over time.

Study of cholesterol and vitamin E levels in broiler meat from different feeding regimens by TOF-SIMS

The quality of chicken meat, which is one of the most widely consumed meats in the world, has been the subject of research and studies for many years. There are several ways to improve the quality of this type of meat, including changing the concentrations of individual molecular components. Such important components of meat are inter alia, cholesterol, vitamin E, and some fatty acids such as ω-3 and ω-6. Manipulation of ingredient levels may be achieved by enriching chicken feed with elements of different types such as vegetable oils, garlic, or selenium. Thus far, various biochemical and biophysical methods have been used to study quality of different meat types, especially broiler meat. Here, the authors demonstrate the use of high-resolution time-of-flight secondary ion mass spectrometry (TOF-SIMS) mass spectrometry to assess how variations in animal nutrition affect concentrations of specific lipids in the meat, such as cholesterol and vitamin E. In the presented experiment, there were four different dietary treatments. Feed for animals in the first group was supplemented with soy oil in 50%, the second group's feed was supplemented with linseed oil in 50%, a combination of these two oils in the proportion of 44%:56% was used for the third group, and in the reference group, animals were fed with beef tallow. From each group, four individuals were selected for further analysis. Positive and negative ion mass spectra were generated from the pectoralis superficialis muscle tissue of the left carcass side of each one animal. Using TOF-SIMS with a bismuth cluster ion source (Bi3 (+)), and based on characteristic peaks for cholesterol in the positive mode and vitamin E in the negative mode, the authors have illustrated the relationship of these lipids levels to the various feeding regimens. Simultaneously, the authors characterized the varying dependences on the concentrations of measured lipids in fat and muscle fibers. The cholesterol concentration in muscle fibers was the lowest in the group fed with soybean oil and the highest in reference group IV (tallow feed). In the fatty region, the highest level of cholesterol was found in the third group. The highest concentrations of vitamin E were found in the fibers of the first group and the fat region of the second group. The obtained results show that SIMS imaging is a useful approach for assessing changes in lipid concentrations in the meat tissue from animals on different diets and provides a foundation for future research.

Caveolin-1 regulates corneal wound healing by modulating Kir4.1 activity

The expression of caveolin-1 (Cav1) in corneal epithelium is associated with regeneration potency. We used Cav1(-/-) mice to study the role of Cav1 in modulating corneal wound healing. Western blot and whole cell patch clamp were employed to study the effect of Cav1 deletion on Kir4.1 current density in corneas. We found that Ba(2+)-sensitive K(+) currents in primary cultured murine corneal epithelial cells (pMCE) from Cav1(-/-) were dramatically reduced (602 pA) compared with those from wild type (WT; 1,300 pA). As a consequence, membrane potential was elevated in pMCE from Cav1(-/-) compared with that from WT (-43 ± 7.5 vs. -58 ± 4.0 mV, respectively). Western blot showed that either inhibition of Cav1 expression or Ba(2+) incubation stimulated phosphorylation of the EGFR. The transwell migration assay showed that Cav1 genetic inactivation accelerated cell migration. The regrowth efficiency of human corneal epithelial cells (HCE) transfected with siRNA-Cav1 or negative control was evaluated by scrape injury assay. With the presence of mitomycin C (10 μg/ml) to avoid the influence of cell proliferation, Cav1 inhibition with siRNA significantly increased migration compared with control siRNA in HCE. This promoting effect by siRNA-Cav1 could not be further enhanced by cotransfection with siRNA-Kcnj10. By using corneal debridement, we found that wound healing was significantly accelerated in Cav1(-/-) compared with WT mice (70 ± 10 vs. 36 ± 3%, P < 0.01). Our findings imply that the mechanism by which Cav-1 knockout promotes corneal regrowth is, at least partially, due to the inhibition of Kir4.1 which stimulates EGFR signaling.

Imaging approaches for analysis of cholesterol distribution and dynamics in the plasma membrane

Cholesterol is an important lipid component of the plasma membrane (PM) of mammalian cells, where it is involved in control of many physiological processes, such as endocytosis, cell migration, cell signalling and surface ruffling. In an attempt to explain these functions of cholesterol, several models have been put forward about cholesterol's lateral and transbilayer organization in the PM. In this article, we review imaging techniques developed over the last two decades for assessing the distribution and dynamics of cholesterol in the PM of mammalian cells. Particular focus is on fluorescence techniques to study the lateral and inter-leaflet distribution of suitable cholesterol analogues in the PM of living cells. We describe also several methods for determining lateral cholesterol dynamics in the PM including fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single particle tracking (SPT) and spot variation FCS coupled to stimulated emission depletion (STED) microscopy. For proper interpretation of such measurements, we provide some background in probe photophysics and diffusion phenomena occurring in cell membranes. In particular, we show the equivalence of the reaction-diffusion approach, as used in FRAP and FCS, and continuous time random walk (CTRW) models, as often invoked in SPT studies. We also discuss mass spectrometry (MS) based imaging of cholesterol in the PM of fixed cells and compare this method with fluorescence imaging of sterols. We conclude that evidence from many experimental techniques converges towards a model of a homogeneous distribution of cholesterol with largely free and unhindered diffusion in both leaflets of the PM.

Promotion of human mesenchymal stem cell osteogenesis by PI3-kinase/Akt signaling, and the influence of caveolin-1/cholesterol homeostasis

Introduction

Stem cells are considered an important resource for tissue repair and regeneration. Their utilization in regenerative medicine will be aided by mechanistic insight into their responsiveness to external stimuli. It is likely that, similar to all other cells, an initial determinant of stem cell responsiveness to external stimuli is the organization of signaling molecules in cell membrane rafts. The clustering of signaling molecules in these cholesterol-rich membrane microdomains can affect the activity, specificity, cross-talk and amplification of cell signaling. Membrane rafts fall into two broad categories, non-caveolar and caveolar, based on the absence or presence, respectively, of caveolin scaffolding proteins. We have recently demonstrated that caveolin-1 (Cav-1) expression increases during, and knockdown of Cav-1 expression enhances, osteogenic differentiation of human bone marrow derived mesenchymal stem cells (MSCs). The increase in Cav-1 expression observed during osteogenesis is likely a negative feedback mechanism. We hypothesize that focal adhesion signaling pathways such as PI3K/Akt signaling may be negatively regulated by Cav-1 during human MSC osteogenesis.

Methods

Human bone marrow MSCs were isolated from femoral heads obtained after total hip arthroplasty. MSCs were incubated in standard growth medium alone or induced to osteogenically differentiate by the addition of supplements (β-glycerophosphate, ascorbic acid, dexamethasone, and 1,25-dihydroxyvitamin D₃). The activation of and requirement for PI3K/Akt signaling in MSC osteogenesis were assessed by immunoblotting for phosphorylated Akt, and treatment with the PI3K inhibitor LY294002 and Akt siRNA, respectively. The influences of Cav-1 and cholesterol membrane rafts on PI3K/Akt signaling were investigated by treatment with Cav-1 siRNA, methyl-β-cyclodextrin, or cholesterol oxidase, followed by cellular sub-fractionation and/or immunoblotting for phosphorylated Akt.

Results

LY294002 and Akt siRNA inhibited MSC osteogenesis. Methyl-β-cyclodextrin, which disrupts all membrane rafts, inhibited osteogenesis. Conversely, Cav-1 siRNA and cholesterol oxidase, which displaces Cav-1 from caveolae, enhanced Akt signaling induced by osteogenic supplements. In control cells, phosphorylated Akt began to accumulate in caveolae after 10 days of osteogenic differentiation.

Conclusions

PI3K/Akt signaling is a key pathway required for human MSC osteogenesis, and it is likely that localization of active Akt in non-caveolar and caveolar membrane rafts positively and negatively contributes to osteogenesis, respectively.

Loss of parkin promotes lipid rafts-dependent endocytosis through accumulating caveolin-1: implications for Parkinson's disease

Background

Parkinson’s disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor dysfunctions. While most PD is sporadic in nature, a significant subset can be linked to either autosomal dominant or recessive mutations. PARK2, encoding the E3 ubiquitin ligase, parkin, is the most frequently mutated gene in autosomal recessive early onset PD. It has recently been reported that PD-associated gene products such as PINK1, α-synuclein, LRRK2, and DJ-1, as well as parkin associate with lipid rafts, suggesting that the dysfunction of these proteins in lipid rafts may be a causal factor of PD. Therefore here, we examined the relationship between lipid rafts-related proteins and parkin.

Results

We identified caveolin-1 (cav-1), which is one of the major constituents of lipid rafts at the plasma membrane, as a substrate of parkin. Loss of parkin function was found to disrupt the ubiquitination and degradation of cav-1, resulting in elevated cav-1 protein level in cells. Moreover, the total cholesterol level and membrane fluidity was altered by parkin deficiency, causing dysregulation of lipid rafts-dependent endocytosis. Further, cell-to-cell transmission of α-synuclein was facilitated by parkin deficiency.

Conclusions

Our results demonstrate that alterations in lipid rafts by the loss of parkin via cav-1 may be a causal factor of PD, and cav-1 may be a novel therapeutic target for PD.

Comparative analysis of caveolins in mouse and tammar wallaby: role in regulating mammary gland function

Recent studies using the mouse showed an inverse correlation between the Caveolin 1 gene expression and lactation, and this was regulated by prolactin. However, current study using mammary explants from pregnant mice showed that while insulin (I), cortisol (F) and prolactin (P) resulted in maximum induction of the β-casein gene, FP and IFP resulted in the downregulation of Caveolin 1. Additionally, IF, FP and IFP resulted in the downregulation of Caveolin 2. Immunohistochemistry confirmed localisation of Caveolin 1 specific to myoepithelial cells and adipocytes. Comparative studies with the tammar wallaby showed Caveolin 1 and 2 had 70-80% homology with the mouse proteins. However, in contrast to the mouse, Caveolin 1 and 2 genes showed a significantly increased level of expression in the mammary gland during lactation. The regulation of tammar Caveolin 1 and 2 gene expression was examined in mammary explants from pregnant tammars, and no significant difference was observed either in the absence or in the presence of IFP.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

The less-often-traveled surface of stem cells: caveolin-1 and caveolae in stem cells, tissue repair and regeneration

Stem cells are an important resource for tissue repair and regeneration. While a great deal of attention has focused on derivation and molecular regulation of stem cells, relatively little research has focused on how the subcellular structure and composition of the cell membrane influences stem cell activities such as proliferation, differentiation and homing. Caveolae are specialized membrane lipid rafts coated with caveolin scaffolding proteins, which can regulate cholesterol transport and the activity of cell signaling receptors and their downstream effectors. Caveolin-1 is involved in the regulation of many cellular processes, including growth, control of mitochondrial antioxidant levels, migration and senescence. These activities are of relevance to stem cell biology, and in this review evidence for caveolin-1 involvement in stem cell biology is summarized. Altered stem and progenitor cell populations in caveolin-1 null mice suggest that caveolin-1 can regulate stem cell proliferation, and in vitro studies with isolated stem cells suggest that caveolin-1 regulates stem cell differentiation. The available evidence leads us to hypothesize that caveolin-1 expression may stabilize the differentiated and undifferentiated stem cell phenotype, and transient downregulation of caveolin-1 expression may be required for transition between the two. Such regulation would probably be critical in regenerative applications of adult stem cells and during tissue regeneration. We also review here the temporal changes in caveolin-1 expression reported during tissue repair. Delayed muscle regeneration in transgenic mice overexpressing caveolin-1 as well as compromised cardiac, brain and liver tissue repair and delayed wound healing in caveolin-1 null mice suggest that caveolin-1 plays an important role in tissue repair, but that this role may be negative or positive depending on the tissue type and the nature of the repair process. Finally, we also discuss how caveolin-1 quiescence-inducing activities and effects on mitochondrial antioxidant levels may influence stem cell aging.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging reveals cholesterol overload in the cerebral cortex of Alzheimer disease patients

Although cholesterol has been involved in the pathophysiology of Alzheimer disease (AD), its distribution in the cerebral cortex over the course of AD is unknown. We describe an original method to quantify cholesterol distribution using time-of-flight secondary ion mass spectrometry imaging. Cholesterol was unevenly distributed along the cortical thickness, being more abundant close to the white matter, in both control and AD cases. However, the mean cholesterol signal was significantly higher in the lower half of the cortex in AD samples compared to controls. This increase, when converted into cortical layers, was statistically significant for layers III and IV and did not reach significance in layers V + VI, the variability being too high at the interface between grey and white matter. The density of neurofibrillary tangles and of senile plaques was not statistically linked to the abundance of cholesterol. Cholesterol overload thus appears a new and independent alteration of AD cerebral cortex. The structure in which cholesterol accumulates and the mechanism of this accumulation remain to be elucidated.

Forced exercise enhances functional recovery after focal cerebral ischemia in spontaneously hypertensive rats

Caveolin is the principal protein of caveolae and has been implicated in the pathogenesis of cerebral ischemia. To investigate whether changed expression of caveolins has a pivotal role in focal cerebral ischemia, we induced middle cerebral artery occlusion (MCAo)-reperfusion and examined expression of caveolins, inflammatory activation markers, and mediators of autophagic cell death. We also treated MCAo rats with forced exercise to determine its effects on neurological outcome. Particularly, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were used to compare the effects of hypertension on focal cerebral ischemia. All MCAo groups showed neurological deficiencies, motor dysfunction, and disruption of balancing ability; however, these pathological changes were more severe in SHR than WKY rats. Expression of caveolins was decreased in MCAo brain tissue, whereas the levels of iNOS and glial fibrillary acidic protein (GFAP) increased. Additionally, LC3-II and beclin-1 levels were elevated in the MCAo groups. Forced exercise attenuated both molecular and behavioral changes in MCAo animals, but SHR rats showed delayed functional recovery and residual molecular changes when compared to WKY rats. These results suggest that forced exercise may be beneficial for promoting functional recovery following cerebral ischemia through caveolin-dependent mechanisms or interactions between caveolins and these signaling molecules in ischemic brain regions.

Characterization of the molecular architecture of human caveolin-3 and interaction with the skeletal muscle ryanodine receptor

Caveolin-3 (cav-3), an integral membrane protein, is a building block of caveolae as well as a regulator of a number of physiological processes by facilitating the formation of multiprotein signaling complexes. We report that the expression of cav-3 in insect (Sf9) cells induces caveola formation, comparable in size with those observed in native tissue. We have also purified the recombinant cav-3 determining that it forms an oligomer of ∼220 kDa. We present the first three-dimensional structure for cav-3 (using transmission electron microscopy and single particle analysis methods) and show that nine cav-3 monomers assemble to form a complex that is toroidal in shape, ∼16.5 nm in diameter and ∼ 5.5 nm in height. Labeling experiments and reconstitution of the purified cav-3 into liposomes have allowed a proposal for the orientation of the protein with respect to the membrane. We have identified multiple caveolin-binding motifs within the ryanodine receptor (RyR1) sequence employing a bioinformatic analysis. We have then shown experimentally that there is a direct interaction between recombinant cav-3 nonamers and purified RyR1 homotetramers that would imply that at least one of the predicted cav-3-binding sites is exposed within the fully assembled RyR1 structure. The cav-3 three-dimensional model provides new insights as to how a cav-3 oligomer can bind multiple partners in close proximity to form signaling complexes. Furthermore, a direct interaction with RyR1 suggests a possible role for cav-3 as a modifier of muscle excitation-contraction coupling and/or for localization of the receptor to regions of the sarcoplasmic reticulum.

Caveolin-1 reduces HIV-1 infectivity by restoration of HIV Nef mediated impairment of cholesterol efflux by apoA-I

Background

HIV infection results in inhibited cholesterol efflux by apolipoprotein A-I (apoA-I) in macrophages, and this impairment involves Nef mediated down-regulation and redistribution of ATP-binding cassette transporter A1 (ABCA-1). We investigated the effect of caveolin-1 (Cav-1) on the cholesterol efflux by apoA-I in HIV infected primary and THP-1 cell-differentiated macrophages as well as astrocyte derived glioblastoma U87 cells.

Results

Our results reveal that Cav-1 restores the Nef -mediated impairment of cholesterol efflux by apoA-I in both cell types. Co-immunoprecipitation studies indicate a physical association of Cav-1 and Nef. The level of ABCA-1 expression remains the same whether Cav-1 is over-expressed or not. In addition, we examined the cholesterol composition of HIV particles released from Cav-1 treated cells and identified that the cholesterol content is dramatically reduced. The infectivity level of these virus particles is also significantly decreased.

Conclusions

These observations suggest that the interplay of Cav-1 with Nef and cholesterol subsequently counters Nef induced impairment of cholesterol efflux by apoA-l. The findings provide a cellular mechanism by which Cav-1 has an ability to restore HIV mediated impairment of cholesterol efflux in macrophages. This subsequently influences the cholesterol content incorporated into virus particles thereby inhibiting HIV infectivity and contributing to HIV’s persistent infection of macrophages.

Compartmentalization of endocannabinoids into lipid rafts in a microglial cell line devoid of caveolin-1

BACKGROUND AND PURPOSE

N-acyl ethanolamines (NAEs) and 2-arachidonoyl glycerol (2-AG) are endogenous cannabinoids and along with related lipids are synthesized on demand from membrane phospholipids. Here, we have studied the compartmentalization of NAEs and 2-AG into lipid raft fractions isolated from the caveolin-1-lacking microglial cell line BV-2, following vehicle or cannabidiol (CBD) treatment. Results were compared with those from the caveolin-1-positive F-11 cell line.

EXPERIMENTAL APPROACH

BV-2 cells were incubated with CBD or vehicle. Cells were fractionated using a detergent-free continuous OptiPrep density gradient. Lipids in fractions were quantified using HPLC/MS/MS. Proteins were measured using Western blot.

KEY RESULTS

BV-2 cells were devoid of caveolin-1. Lipid rafts were isolated from BV-2 cells as confirmed by co-localization with flotillin-1 and sphingomyelin. Small amounts of cannabinoid CB(1) receptors were found in lipid raft fractions. After incubation with CBD, levels and distribution in lipid rafts of 2-AG, N-arachidonoyl ethanolamine (AEA), and N-oleoyl ethanolamine (OEA) were not changed. Conversely, the levels of the saturated N-stearoyl ethanolamine (SEA) and N-palmitoyl ethanolamine (PEA) were elevated in lipid raft fractions. In whole cells with growth medium, CBD treatment increased AEA and OEA time-dependently, while levels of 2-AG, PEA and SEA did not change.

CONCLUSIONS AND IMPLICATIONS

Whereas levels of 2-AG were not affected by CBD treatment, the distribution and levels of NAEs showed significant changes. Among the NAEs, the degree of acyl chain saturation predicted the compartmentalization after CBD treatment suggesting a shift in cell signalling activity.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Oligodendroglial process formation is differentially affected by modulating the intra- and extracellular cholesterol content

Cholesterol is an essential component of eukaryotic plasma membranes and plays an important role in membrane organization and signaling processes. It is the major lipid component of detergent resistant caveolin-1 containing rafts which previously had been reported as a platform for nerve growth factor (NGF) signaling in oligodendrocytes (OL). Surprisingly, a knockdown of caveolin-1 attenuated the process formation of OL (Schmitz et al. J Neurosci Res 88:572–588, 2010), for which a loss of cholesterol could be responsible. In the present report, we could show that a caveolin-1 knockdown resulted in an elevation of cellular cholesterol level; it may indicate an important role of caveolin-1 in cholesterol trafficking to the plasma membrane. Treatment with exogenous PEG cholesterol, which was incorporated to the plasma membrane, supported oligodendroglial process formation, in particular when OL were stimulated by NGF. In this context we have found that OL express NPC1L1 (Niemann–Pick disease type C1-Like 1) which could modulate cholesterol uptake. In contrast, depletion of membrane-bound cholesterol diminished NGF-induced process formation concomitant with a reduced activity of p42/44 mitogen-activated protein kinases.

Caveolin-1 suppresses human immunodeficiency virus-1 replication by inhibiting acetylation of NF-κB

Caveolin-1 is an integral membrane protein primarily responsible for the formation of membrane structures known as caveolae. Caveolae are specialized lipid rafts involved in protein trafficking, cholesterol homeostasis, and a number of signaling functions. It has been demonstrated that caveolin-1 suppresses HIV-1 protein expression. We found that co-transfecting cells with HIV-1 and caveolin-1 constructs, results in a marked decrease in the level of HIV-1 transcription relative to cells transfected with HIV-1 DNA alone. Correspondingly, reduction of endogenous caveolin-1 expression by siRNA-mediated silencing resulted in an enhancement of HIV-1 replication. Further, we observed a loss of caveolin-mediated suppression of HIV-1 transcription in promoter studies with reporters containing mutations in the NF-κB binding site. Our analysis of the posttranslational modification status of the p65 subunit of NF-κB demonstrates hypoacetylation of p65 in the presence of caveolin-1. Since hypoacetylated p65 has been shown to inhibit transcription, we conclude that caveolin-1 inhibits HIV-1 transcription through a NF-κB-dependent mechanism.

Elucidation of the Rotavirus NSP4-Caveolin-1 and -Cholesterol Interactions Using Synthetic Peptides

Rotavirus (RV) NSP4, the first described viral enterotoxin, is a multifunctional glycoprotein that contributes to viral pathogenesis, morphogenesis, and replication. NSP4 binds both termini of caveolin-1 and is isolated from caveolae fractions that are rich in anionic phospholipids and cholesterol. These interactions indicate that cholesterol/caveolin-1 plays a role in NSP4 transport to the cell surface, which is essential to its enterotoxic activity. Synthetic peptides were utilized to identify target(s) of intervention by exploring the NSP4-caveolin-1 and -cholesterol interactions. NSP4_112–140 that overlaps the caveolin-1 binding domain and a cholesterol recognition amino acid consensus (CRAC) motif and both termini of caveolin-1 (N-caveolin-1_2–20,  _19–40 and C-caveolin-1_161–180) were synthesized. Direct fluorescence-binding assays were employed to determine binding affinities of the NSP4-caveolin-1 peptides and cholesterol. Intracellular cholesterol alteration revealed a redistribution of NSP4 and disintegration of viroplasms. These data further imply interruption of NSP4_112–140-N-caveolin-1_19–40 and cholesterol interactions may block NSP4 intracellular transport, hence enterotoxicity.

Caveolin targeting to late endosome/lysosomal membranes is induced by perturbations of lysosomal pH and cholesterol content

Caveolin-1 traffics to late endosomal/lysosomal membranes in response to manipulations of the cholesterol content of cells, suggesting that caveolin functions in the egress of cholesterol from this organelle. Cavicles associate with the periphery of the lysosome as they do with caveosomes, but these are separate organelles.

Caveolin-1 is an integral membrane protein of plasma membrane caveolae. Here we report that caveolin-1 collects at the cytosolic surface of lysosomal membranes when cells are serum starved. This is due to an elevation of the intralysosomal pH, since ionophores and proton pump inhibitors that dissipate the lysosomal pH gradient also trapped caveolin-1 on late endosome/lysosomes. Accumulation is both saturable and reversible. At least a portion of the caveolin-1 goes to the plasma membrane upon reversal. Several studies suggest that caveolin-1 is involved in cholesterol transport within the cell. Strikingly, we find that blocking cholesterol export from lysosomes with progesterone or U18666A or treating cells with low concentrations of cyclodextrin also caused caveolin-1 to accumulate on late endosome/lysosomal membranes. Under these conditions, however, live-cell imaging shows cavicles actively docking with lysosomes, suggesting that these structures might be involved in delivering caveolin-1. Targeting of caveolin-1 to late endosome/lysosomes is not observed normally, and the degradation rate of caveolin-1 is not altered by any of these conditions, indicating that caveolin-1 accumulation is not a consequence of blocked degradation. We conclude that caveolin-1 normally traffics to and from the cytoplasmic surface of lysosomes during intracellular cholesterol trafficking.

Caveolin-1-mediated apolipoprotein A-I membrane binding sites are not required for cholesterol efflux

Caveolin-1 (Cav1), a structural protein required for the formation of invaginated membrane domains known as caveolae, has been implicated in cholesterol trafficking and homeostasis. Here we investigated the contribution of Cav1 to apolipoprotein A-I (apoA-I) cell surface binding and intracellular processing using mouse embryonic fibroblasts (MEFs) derived from wild type (WT) or Cav1-deficient (Cav1(-/-)) animals. We found that cells expressing Cav1 have 2.6-fold more apoA-I binding sites than Cav1(-/-) cells although these additional binding sites are not associated with detergent-free lipid rafts. Further, Cav1-mediated binding targets apoA-I for internalization and degradation and these processes are not correlated to cholesterol efflux. Despite lower apoA-I binding, cholesterol efflux from Cav1(-/-) MEFs is 1.7-fold higher than from WT MEFs. Stimulation of ABCA1 expression with an LXR agonist enhances cholesterol efflux from both WT and Cav1(-/-) cells without increasing apoA-I surface binding or affecting apoA-I processing. Our results indicate that there are at least two independent lipid binding sites for apoA-I; Cav1-mediated apoA-I surface binding and uptake is not linked to cholesterol efflux, indicating that membrane domains other than caveolae regulate ABCA1-mediated cholesterol efflux.

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

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

Inhibition of renal caveolin-1 reduces natriuresis and produces hypertension in sodium-loaded rats

Renal dopamine receptor function and ion transport inhibition are impaired in essential hypertension. We recently reported that caveolin-1 (CAV1) and lipid rafts are necessary for normal D(1)-like receptor-dependent internalization of Na-K-ATPase in human proximal tubule cells. We now hypothesize that CAV1 is necessary for the regulation of urine sodium (Na(+)) excretion (U(Na)V) and mean arterial blood pressure (MAP) in vivo. Acute renal interstitial (RI) infusion into Sprague-Dawley rats of 1 μg·kg⁻¹·min⁻¹ fenoldopam (FEN; D(1)-like receptor agonist) caused a 0.46 ± 0.15-μmol/min increase in U(Na)V (over baseline of 0.29 ± 0.04 μmol/min; P < 0.01). This increase was seen in Na(+)-loaded rats, but not in those under a normal-sodium load. Coinfusion with β-methyl cyclodextrin (βMCD; lipid raft disrupter, 200 μg·kg⁻¹·min⁻¹) completely blocked this FEN-induced natriuresis (P < 0.001). Long-term (3 day) lipid raft disruption via continuous RI infusion of 80 μg·kg⁻¹·min⁻¹ βMCD decreased renal cortical CAV1 expression (47.3 ± 6.4%; P < 0.01) and increased MAP (32.4 ± 6.6 mmHg; P < 0.001) compared with vehicle-infused animals. To determine whether the MAP rise was due to a CAV1-dependent lipid raft-mediated disruption, Na(+)-loaded rats were given a bolus RI infusion of CAV1 siRNA. Two days postinfusion, cortical CAV1 expression was decreased by 73.6 ± 8.2% (P < 0.001) and the animals showed an increase in MAP by 17.4 ± 2.9 mmHg (P < 0.01) compared with animals receiving scrambled control siRNA. In summary, acute kidney-specific lipid raft disruption decreases CAV1 expression and blocks D(1)-like receptor-induced natriuresis. Furthermore, chronic disruption of lipid rafts or CAV1 protein expression in the kidney induces hypertension.

Topography of lipid droplet-associated proteins: insights from freeze-fracture replica immunogold labeling

Lipid droplets are not merely storage depots for superfluous intracellular lipids in times of hyperlipidemic stress, but metabolically active organelles involved in cellular homeostasis. Our concepts on the metabolic functions of lipid droplets have come from studies on lipid droplet-associated proteins. This realization has made the study of proteins, such as PAT family proteins, caveolins, and several others that are targeted to lipid droplets, an intriguing and rapidly developing area of intensive inquiry. Our existing understanding of the structure, protein organization, and biogenesis of the lipid droplet has relied heavily on microscopical techniques that lack resolution and the ability to preserve native cellular and protein composition. Freeze-fracture replica immunogold labeling overcomes these disadvantages and can be used to define at high resolution the precise location of lipid droplet-associated proteins. In this paper illustrative examples of how freeze-fracture immunocytochemistry has contributed to our understanding of the spatial organization in the membrane plane and function of PAT family proteins and caveolin-1 are presented. By revisiting the lipid droplet with freeze-fracture immunocytochemistry, new perspectives have emerged which challenge prevailing concepts of lipid droplet biology and may hopefully provide a timely impulse for many ongoing studies.

Src and caveolin-1 reciprocally regulate metastasis via a common downstream signaling pathway in bladder cancer

In bladder cancer, increased caveolin-1 (Cav-1) expression and decreased Src expression and kinase activity correlate with tumor aggressiveness. Here, we investigate the clinical and functional significance, if any, of this reciprocal expression in bladder cancer metastasis. We evaluated the ability of tumor Cav-1 and Src RNA and protein expression to predict outcome following cystectomy in 257 patients enrolled in two independent clinical studies. In both, high Cav-1 and low Src levels were associated with metastasis development. We overexpressed or depleted Cav-1 and Src protein levels in UMUC-3 and RT4 human bladder cancer cells and evaluated the effect of this on actin stress fibers, migration using Transwells, and lung metastasis following tail vein inoculation. Cav-1 depletion or expression of active Src in metastatic UMUC-3 cells decreases actin stress fibers, cell migration, and metastasis, while Cav-1 overexpression or Src depletion increased the migration of nonmetastatic RT4 cells. Biochemical studies indicated that Cav-1 mediates these effects via its phosphorylated form (pY14), whereas Src effects are mediated through phosphorylation of p190RhoGAP and these pathways converge to reduce activity of RhoA, RhoC, and Rho effector ROCK1. Treatment with a ROCK inhibitor reduced UMUC-3 lung metastasis in vivo, phenocopying the effect of Cav-1 depletion or expression of active Src. Src suppresses whereas Cav-1 promotes metastasis of bladder cancer through a pharmacologically tractable common downstream signaling pathway. Clinical evaluation of personalized therapy to suppress metastasis development based on Cav-1 and Src profiles seems warranted.

Annexin A6-Linking Ca(2+) signaling with cholesterol transport

Annexin A6 (AnxA6) belongs to a conserved family of Ca(2+)-dependent membrane-binding proteins. Like other annexins, the function of AnxA6 is linked to its ability to bind phospholipids in cellular membranes in a dynamic and reversible fashion, in particular during the regulation of endocytic and exocytic pathways. High amounts of AnxA6 sequester cholesterol in late endosomes, thereby lowering the levels of cholesterol in the Golgi and the plasma membrane. These AnxA6-dependent redistributions of cellular cholesterol pools give rise to reduced cytoplasmic phospholipase A2 (cPLA(2)) activity, retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. In addition to regulating cholesterol and caveolin distribution, AnxA6 acts as a scaffold/targeting protein for several signaling proteins, the best characterized being the Ca(2+)-dependent membrane targeting of p120GAP to downregulate Ras activity. AnxA6 also stimulates the Ca(2+)-inducible involvement of PKC in the regulation of HRas and possibly EGFR signal transduction pathways. The ability of AnxA6 to recruit regulators of the EGFR/Ras pathway is likely potentiated by AnxA6-induced actin remodeling. Accordingly, AnxA6 may function as an organizer of membrane domains (i) to modulate intracellular cholesterol homeostasis, (ii) to create a scaffold for the formation of multifactorial signaling complexes, and (iii) to regulate transient membrane-actin interactions during endocytic and exocytic transport. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Phosphotyrosine protein dynamics in cell membrane rafts of sphingosine-1-phosphate-stimulated human endothelium: role in barrier enhancement

Sphingosine-1-phosphate (S1P), a lipid growth factor, is critical to the maintenance and enhancement of vascular barrier function via processes highly dependent upon cell membrane raft-mediated signaling events. Anti-phosphotyrosine 2 dimensional gel electrophoresis (2-DE) immunoblots confirmed that disruption of membrane raft formation (via methyl-beta-cyclodextrin) inhibits S1P-induced protein tyrosine phosphorylation. To explore S1P-induced dynamic changes in membrane rafts, we used 2-D techniques to define proteins within detergent-resistant cell membrane rafts which are differentially expressed in S1P-challenged (1microM, 5min) human pulmonary artery endothelial cells (EC), with 57 protein spots exhibiting >3-fold change. S1P induced the recruitment of over 20 cell membrane raft proteins exhibiting increasing levels of tyrosine phosphorylation including known barrier-regulatory proteins such as focal adhesion kinase (FAK), cortactin, p85alpha phosphatidylinositol 3-kinase (p85alphaPI3K), myosin light chain kinase (nmMLCK), filamin A/C, and the non-receptor tyrosine kinase, c-Abl. Reduced expression of either FAK, MLCK, cortactin, filamin A or filamin C by siRNA transfection significantly attenuated S1P-induced EC barrier enhancement. Furthermore, S1P induced cell membrane raft components, p-caveolin-1 and glycosphingolipid (GM1), to the plasma membrane and enhanced co-localization of membrane rafts with p-caveolin-1 and p-nmMLCK. These results suggest that S1P induces both the tyrosine phosphorylation and recruitment of key actin cytoskeletal proteins to membrane rafts, resulting in enhanced human EC barrier function.

Epidermal growth factor receptor-mediated membrane type 1 matrix metalloproteinase endocytosis regulates the transition between invasive versus expansive growth of ovarian carcinoma cells in three-dimensional collagen

The epidermal growth factor receptor (EGFR) is overexpressed in ovarian carcinomas and promotes cellular responses that contribute to ovarian cancer pathobiology. In addition to modulation of mitogenic and motogenic behavior, emerging data identify EGFR activation as a novel mechanism for rapid modification of the cell surface proteome. The transmembrane collagenase membrane type 1 matrix metalloproteinase (MT1-MMP, MMP-14) is a major contributor to pericelluar proteolysis in the ovarian carcinoma microenvironment and is subjected to extensive posttranslational regulation. In the present study, the contribution of EGFR activation to control of MT1-MMP cell surface dynamics was investigated. Unstimulated ovarian cancer cells display caveolar colocalization of EGFR and MT1-MMP, whereas EGFR activation prompts internalization via distinct endocytic pathways. EGF treatment results in phosphorylation of the MT1-MMP cytoplasmic tail, and cells expressing a tyrosine mutated form of MT1-MMP (MT1-MMP-Y(573)F) exhibit defective MT1-MMP internalization. As a result of sustained cell surface MT1-MMP activity, a phenotypic epithelial-mesenchymal transition is observed, characterized by enhanced migration and collagen invasion, whereas growth within three-dimensional collagen gels is inhibited. These data support an EGFR-dependent mechanism for regulation of the transition between invasive and expansive growth of ovarian carcinoma cells via modulation of MT1-MMP cell surface dynamics.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Cholesterol-dependent anaplasma phagocytophilum exploits the low-density lipoprotein uptake pathway

In eukaryotes, intracellular cholesterol homeostasis and trafficking are tightly regulated. Certain bacteria, such as Anaplasma phagocytophilum, also require cholesterol; it is unknown, however, how this cholesterol-dependent obligatory intracellular bacterium of granulocytes interacts with the host cell cholesterol regulatory pathway to acquire cholesterol. Here, we report that total host cell cholesterol increased >2-fold during A. phagocytophilum infection in a human promyelocytic leukemia cell line. Cellular free cholesterol was enriched in A. phagocytophilum inclusions as detected by filipin staining. We determined that A. phagocytophilum requires cholesterol derived from low-density lipoprotein (LDL), because its replication was significantly inhibited by depleting the growth medium of cholesterol-containing lipoproteins, by blocking LDL uptake with a monoclonal antibody against LDL receptor (LDLR), or by treating the host cells with inhibitors that block LDL-derived cholesterol egress from late endosomes or lysosomes. However, de novo cholesterol biosynthesis is not required, since inhibition of the biosynthesis pathway did not inhibit A. phagocytophilum infection. The uptake of fluorescence-labeled LDL was enhanced in infected cells, and LDLR expression was up-regulated at both the mRNA and protein levels. A. phagocytophilum infection stabilized LDLR mRNA through the 3′ UTR region, but not through activation of the sterol regulatory element binding proteins. Extracellular signal–regulated kinase (ERK) was up-regulated by A. phagocytophilum infection, and inhibition of its upstream kinase, MEK, by a specific inhibitor or siRNA knockdown, reduced A. phagocytophilum infection. Up-regulation of LDLR mRNA by A. phagocytophilum was also inhibited by the MEK inhibitor; however, it was unclear whether ERK activation is required for LDLR mRNA up-regulation by A. phagocytophilum. These data reveal that A. phagocytophilum exploits the host LDL uptake pathway and LDLR mRNA regulatory system to accumulate cholesterol in inclusions to facilitate its replication.

Maintenance of the cholesterol amount and transport within cells are essential for healthy human cell functions. Most bacteria do not need cholesterol, but certain bacteria that infect human cells are dependent on host cell cholesterol for their infection. How infected human cells deal with these cholesterol-robbing bacteria, and in turn how these bacteria hijack host cholesterol, are intriguing questions. Anaplasma phagocytophilum is a bacterium that lives inside white blood cells, and causes the disease human granulocytic anaplasmosis (HGA). A. phagocytophilum needs host cholesterol to live. Here, we discovered that A. phagocytophilum infection increases the amount of cholesterol in host cells and sequesters the majority of cholesterol in A. phagocytophilum inclusions inside host cells. Human cells acquire cholesterol from two sources: receptor-mediated endocytosis of cholesterol-containing low-density lipoprotein (LDL) from the circulating blood, and synthesis of cholesterol inside the cells. Since A. phagocytophilum depends on cholesterol derived from LDL, it coaxes the host cell to take up more LDL by increasing LDL receptor, through inhibition of LDL receptor mRNA degradation. A. phagocytophilum infection may serve as a model to improve our understanding of the cellular cholesterol regulation in white blood cells, and may provide insight regarding new therapeutic target for treatment of HGA.

Endolysosomal phospholipidosis and cytosolic lipid droplet storage and release in macrophages

This review summarizes the current knowledge of endolysosomal and cytoplasmic lipid storage in macrophages induced by oxidized LDL (Ox-LDL), enzymatically degraded LDL (E-LDL) and other atherogenic lipoprotein modifications, and their relation to the adapter protein 3 (AP-3) dependent ABCA1 and ABCG1 cellular lipid efflux pathways. We compare endolysosomal lipid storage caused either through drug induced phospholipidosis, inheritable endolysosomal and cytosolic lipid storage disorders and Ox-LDL or E-LDL induced phagosomal uptake and cytosolic lipid droplet storage in macrophages. Ox-LDL is resistant to rapid endolysosomal hydrolysis and is trapped within the endolysosomal compartment generating lamellar bodies which resemble the characteristics of phospholipidosis. Various inherited lysosomal storage diseases including sphingolipidosis, glycosphingolipidosis and cholesterylester storage diseases also present a phospholipidosis phenotype. In contrast E-LDL resembling coreless unesterified cholesterol enriched LDL-particles, with a multilamellar, liposome-like structure, lead to rapid phagosomal degradation and cytosolic lipid droplet accumulation. As a consequence the uptake of E-LDL through type I and type II phagocytosis leads to increased lipid droplet formation and moderate upregulation of ABCA1 and ABCG1 while uptake of Ox-LDL leads to a rapid expansion of the lysosomal compartment and a pronounced upregulation of the ABCA1/ABCG1/AP-3 lipid efflux pathway.

Cholesterol effectively blocks entry of flavivirus

Japanese encephalitis virus (JEV) and dengue virus serotype 2 (DEN-2) are enveloped flaviviruses that enter cells through receptor-mediated endocytosis and low pH-triggered membrane fusion and then replicate in intracellular membrane structures. Lipid rafts, cholesterol-enriched lipid-ordered membrane domains, are platforms for a variety of cellular functions. In this study, we found that disruption of lipid raft formation by cholesterol depletion with methyl-beta-cyclodextrin or cholesterol chelation with filipin III reduces JEV and DEN-2 infection, mainly at the intracellular replication steps and, to a lesser extent, at viral entry. Using a membrane flotation assay, we found that several flaviviral nonstructural proteins are associated with detergent-resistant membrane structures, indicating that the replication complex of JEV and DEN-2 localizes to the membranes that possess the lipid raft property. Interestingly, we also found that addition of cholesterol readily blocks flaviviral infection, a result that contrasts with previous reports of other viruses, such as Sindbis virus, whose infectivity is enhanced by cholesterol. Cholesterol mainly affected the early step of the flavivirus life cycle, because the presence of cholesterol during viral adsorption greatly blocked JEV and DEN-2 infectivity. Flavirial entry, probably at fusion and RNA uncoating steps, was hindered by cholesterol. Our results thus suggest a stringent requirement for membrane components, especially with respect to the amount of cholesterol, in various steps of the flavivirus life cycle.

Potential role of caveolin-1-positive domains in the regulation of the acetylcholine receptor's activatable pool: implications in the pathogenesis of a novel congenital myasthenic syndrome

Cholesterol modulates the plasmalemma's biophysical properties and influences the function and trafficking of membrane proteins. A fundamental phenomenon that remains obscure is how the plasmalemma's lipid composition regulates the activatable pool of membrane receptors. An outstanding model to study this phenomenon is the nicotinic acetylcholine receptor (nAChR), since the nAChR activatable pool has been estimated to be but a small fraction of the receptors present in the plasmalemma. Studies on the effect of cholesterol depletion in the function of the Torpedo californica nAChR, using the lipid-exposed nAChR mutation (alpha C418W) that produces a congenital myasthenic syndrome (CMS), demonstrated that cholesterol depletion causes a remarkable increase in the alpha C418W nAChR's macroscopic current whereas not in the wild-type (WT). A variety of approaches were used to define the mechanism responsible for the cholesterol depletion mediated-increase in the alpha C418W nAChR's macroscopic current. The present study suggests that a substantial fraction of the alpha C418W nAChRs is located in caveolin-1-positive domains, "trapped" in a non-activatable state, and that membrane cholesterol depletion results in the relocation of these receptors to the activatable pool. Co-fractionation and co-immunoprecipitation of the alpha C418W nAChR and the membrane raft protein caveolin-1 (cav1) support the notion that interactions at lipid-exposed domains regulate the partition of the receptor into membrane raft microdomains. These results have potential implications as a novel mechanism to fine-tune cholinergic transmission in the nervous system and in the pathogenesis associated to the alpha C418W nAChR.

Caveolae and endothelial dysfunction: filling the caves in cardiovascular disease

Discovery in the early 1990s of caveolin-1, the structural protein responsible for maintaining the ohm shape of caveolae, greatly enhanced investigations to elucidate the role of these little caves in the plasma membrane. Perhaps one of the most important realizations concerning caveolae and caveolin is that these elements play an important functional role in the modulation of cell signal transduction pathways, including those involved in endothelial nitric oxide synthase (eNOS) function. Their role was confirmed by studies with caveolin-1 knockout mice which lack caveolae and display abnormal endothelial function responses. One limitation of these knockout models, however, is that absence of the caveolin protein not only results in the lack of caveolae as a structure but also in the lack of interaction/modulation of enzymes/molecules (e.g. eNOS) to which caveolin binds (whether in- or outside caveolae). In contrast to caveolin knockout models, recent experimental findings suggest that in certain cardiovascular diseases caveolin may dissociate from caveolae to the cytosol, hence decreasing the number of caveolae without a change in the total amount of caveolin. Therefore, as the importance of defining the role of caveolins both in caveolae and in cellular regions is being highlighted, it seems also important at the same time to further define the role of caveolae per se being present in the plasma membrane as a structural entity. The objective of this review is to make an explorative tour on the role of caveolae in vascular endothelial function based on existing literature together with some preliminary experimental findings. Evidence and arguments are put forward that alterations in endothelial caveolae do occur in cardiovascular disease and may contribute to the observed endothelial dysfunction in these conditions.

Annexin A6-induced alterations in cholesterol transport and caveolin export from the Golgi complex

Annexin A6 (AnxA6) belongs to a family of Ca(2+)-dependent membrane-binding proteins and is involved in the regulation of endocytic and exocytic pathways. We previously demonstrated that AnxA6 regulates receptor-mediated endocytosis and lysosomal targeting of low-density lipoproteins and translocates to cholesterol-enriched late endosomes (LE). As cholesterol modulates the membrane binding and the cellular location of AnxA6, but also affects the intracellular distribution of caveolin, we investigated the localization and trafficking of caveolin in AnxA6-expressing cells. Here, we show that cells expressing high levels of AnxA6 are characterized by an accumulation of caveolin-1 (cav-1) in the Golgi complex. This is associated with a sequestration of cholesterol in the LE and lower levels of cholesterol in the Golgi and the plasma membrane, both likely contributing to retention of caveolin in the Golgi apparatus and a reduced number of caveolae at the cell surface. Further strengthening these findings, knock down of AnxA6 and the ectopic expression of the Niemann-Pick C1 protein in AnxA6-overexpressing cells restore the cellular distribution of cav-1 and cholesterol, respectively. In summary, this study demonstrates that elevated expression levels of AnxA6 perturb the intracellular distribution of cholesterol, which indirectly inhibits the exit of caveolin from the Golgi complex.

Formyl peptide-receptor like-1 requires lipid raft and extracellular signal-regulated protein kinase to activate inhibitor-κB kinase in human U87 astrocytoma cells

Formyl peptide-receptor like-1 (FPRL-1) may possess critical roles in Alzheimer's diseases, chemotaxis and release of neurotoxins, possibly through its regulation of nuclear factor-kappaB (NFkappaB). Here we illustrate that activation of FPRL-1 in human U87 astrocytoma or Chinese hamster ovary cells stably expressing the receptor resulted in the phosphorylations of inhibitor-kappaB kinase (IKK), an onset kinase for NFkappaB signaling cascade. FPRL-1 selective hexapeptide Trp-Lys-Tyr-Met-Val-Met (WKYMVM) promoted IKK phosphorylations in time- and dose-dependent manners while pre-treatment of pertussis toxin abrogated the Galpha(i/o)-dependent stimulations. The FPRL-1-mediated IKK phosphorylation required extracellular signal-regulated protein kinase (ERK), phosphatidylinositol 3-kinase and cellular Src (c-Src), but not c-Jun N-terminal kinase and p38 mitogen-activated protein kinase. Despite its ability to mobilize Ca(2+), WKYMVM did not require Ca(2+) for the modulation of IKK phosphorylation. Activation of FPRL-1 also induced NFkappaB-driven luciferase expression. Interestingly, cholesterol depletion from plasma membrane by methyl-beta-cyclodextrin abolished the FPRL-1-stimulated IKK phosphorylation, denoting the important role of lipid raft integrity in the FPRL-1 to IKK signaling. Furthermore, we demonstrated that in U87 cells, several signaling intermediates in the FPRL-1-IKK pathway including Galpha(i2), c-Src and ERK were constitutively localized at the raft microdomains. WKYMVM administration not only resulted in higher amount of ERK recruitment to the raft region, but also specifically stimulated raft-associated c-Src and ERK phosphorylations. Taken together, these results demonstrate that FPRL-1 is capable of activating NFkappaB signaling through IKK phosphorylation and this may serve as a useful therapeutical target for FPRL-1-related diseases.