The role of caveolae and caveolin in vesicle-dependent and vesicle-independent trafficking

Entry of the Varicellovirus Canid herpesvirus 1 into Madin-Darby canine kidney epithelial cells is pH-independent and occurs via a macropinocytosis-like mechanism but without increase in fluid uptake

Canid herpesvirus 1 (CHV-1) is a Varicellovirus that causes self-limiting infections in adult dogs but morbidity and mortality in puppies. Using a multipronged approach, we discovered the CHV-1 entry pathway into Madin-Darby canine kidney (MDCK) epithelial cells. We found that CHV-1 triggered extensive host cell membrane lamellipodial ruffling and rapid internalisation of virions in large, uncoated vacuoles, suggestive of macropinocytosis. Treatment with inhibitors targeting key macropinocytosis factors, including inhibitors of Na⁺ /H⁺ exchangers, F-actin, myosin light-chain kinase, protein kinase C, p21-activated kinase, phosphatidylinositol-3-kinase and focal adhesion kinase, significantly reduced viral replication. Moreover, the effect was restricted to exposure to the inhibitors early in infection, confirming a role for the macropinocytic machinery during entry. The profile of inhibitors also suggested a role for signalling via integrins and receptor tyrosine kinases in viral entry. In contrast, inhibitors of clathrin, caveolin, microtubules and endosomal acidification did not affect CHV-1 entry into MDCK cells. We found that the virus colocalised with the fluid-phase uptake marker dextran; however, surprisingly, CHV-1 infection did not enhance the uptake of dextran. Thus, our results indicate that CHV-1 uses a macropinocytosis-like, pH-independent entry pathway into MDCK cells, which nevertheless is not based on stimulation of fluid uptake. TAKE AWAYS: CHV-1 enters epithelial cells via a macropinocytosis-like mechanism. CHV-1 induces extensive lamellipodial ruffling. CHV-1 entry into MDCK cells is pH-independent.

Dependence of SARS-CoV-2 infection on cholesterol-rich lipid raft and endosomal acidification

Coronavirus disease 2019 is a kind of viral pneumonia caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, the mechanism whereby SARS-CoV-2 invades host cells remains poorly understood. Here we used SARS-CoV-2 pseudoviruses to infect human angiotensin-converting enzyme 2 (ACE2) expressing HEK293T cells and evaluated virus infection. We confirmed that SARS-CoV-2 entry was dependent on ACE2 and sensitive to pH of endosome/lysosome in HEK293T cells. The infection of SARS-CoV-2 pseudoviruses is independent of dynamin, clathrin, caveolin and endophilin A2, as well as macropinocytosis. Instead, we found that the infection of SARS-CoV-2 pseudoviruses was cholesterol-rich lipid raft dependent. Cholesterol depletion of cell membranes with methyl-β-cyclodextrin resulted in reduction of pseudovirus infection. The infection of SARS-CoV-2 pseudoviruses resumed with cholesterol supplementation. Together, cholesterol-rich lipid rafts, and endosomal acidification, are key steps of SARS-CoV-2 required for infection of host cells. Therefore, our finding expands the understanding of SARS-CoV-2 entry mechanism and provides a new anti-SARS-CoV-2 strategy.

α-Hemolysin suppresses osteogenesis by inducing lipid rafts accumulation in bone marrow stromal cells

α-hemolysin (Hla) is considered an essential virulent factor for Staphylococcus aureus (S. aureus) toxicity, the mechanism by which Hla affect bone metabolism is poorly understood. In this study, 2-month-old C57BL/6 mice were treated with Hla (40 μg/kg, i.p.) or S. aureus (1 × 10⁶ CFU/ml, 100 μl, i.v.) with the presence or absence of methyl-β-cyclodextrin (MβCD) (300 mg/kg, i.p.). MicroCT analysis showed progressive bone loss from week 2 to week 4 after Hla treatment, accompanied by a decreased osteoblasts and increased osteoclasts in femoral metaphysis in mice. Further, Hla stimulated the expression of Caveolin-1 in vivo and in vitro, activated lipid rafts accumulation in cell membrane of bone marrow stromal cells (BMSCs), and suppressed osteogenesis of BMSCs. Destruction of lipid rafts with MβCD or inhibition of Caveolin-1 with Daidzein blocked the detrimental effect of Hla on osteogenesis of BMSCs. Importantly, treating mice with MβCD rescued the loss of osteoblasts and increased osteoclastogenesis induced by Hla as well as the bone loss induced by S. aureus infection. Together, we demonstrate that Hla induces bone destruction directly by suppressing osteogenesis and indirectly by stimulating osteoclastogenesis, and that lipid rafts may mediate the detrimental effect of Hla and S. aureus on osteogenesis and bone formation.

Gold nanoparticles-biomembrane interactions: From fundamental to simulation

Gold Nanoparticles (AuNPs) are a class of promising nanomaterial for biomedical applications ranging from bioimaging, drug delivery to phototherapy because of their biocompatibility, easily tunable size and shape, and versatile surface modifications. In recent years, the rapid development of AuNPs in nanomedicine has made it imperative to seek fundamental understanding on their nano-biointeractions to minimize adverse effects and improve targeting/imaging efficiency. In this review, we summarize the different pathways of NPs-biomembrane interactions with a focus on AuNPs, follow by an analysis on how the physiochemical properties (size, surface charge, shape, surface ligands, and hydrophobicity etc.) of AuNPs can be involved in the mechanisms of cellular uptake. Finally, some recent advances on simulation modelling of AuNPs-biomembrane interactions and a brief outlook in the field are discussed.

3D rotational motion of an endocytic vesicle on a complex microtubule network in a living cell

The transport dynamics of endocytic vesicles in a living cell contains essential biomedical information. Although the movement mechanism of a vesicle by motor proteins has been revealed, understanding the precise movement of vesicles on the cytoskeleton in a living cell has been considered challenging, due to the complex 3D network of cytoskeletons. Here, we specify the shape of the 3D interaction between the vesicle and microtubule, based on the theoretically estimated location of the microtubule and the vesicle trajectory data acquired at high spatial and temporal precision. We detected that vesicles showed more frequent direction changes with either in very acute or in obtuse angles than right angles, on similar time scales in a microtubule network. Interestingly, when a vesicle interacted with a relatively longer (> 400 nm) microtubule filament, rotational movement along the axis of the microtubule was frequently observed. Our results are expected to give in-depth insight into understanding the actual 3D interactions between the intracellular molecule and complex cytoskeletal network.

Different Role of Caveolin-1 Gene in the Progression of Gynecological Tumors

Caveolin-1 (Cav-1), an integral membrane protein, is a principal component of caveolae and has been reported to play a promoting or inhibiting role in cancer progression. Gynecologic tumor is a group of tumors that affect the tissue and organs of the female reproductive system, especially cervical cancer. Cervical cancer, as one of the most common cancers, severely affects female health in developing countries in particular because of its high morbidity and mortality. This review summarizes some mechanisms of Cav-1 in the development and progression of gynecological tumors. The role of Cav-1 in tumorigenesis, including dysregulation of cell cycle, apoptosis and autophagy, adhesion, invasion, and metastasis, such as the formation of invadopodia and matrix metalloproteinase degradation are presented in detail. In addition, Cav-1 modulates autophagy and the formation of invadopodia and target regulated by miRNAs to affect tumor progress. Taken together, we find that, no matter Cav-1 expression in the tumor or stromal cells , Cav-1 has paradoxical role in different types of gynecological tumors in vivo or in vitro and even in the same tumor from the same organ.

Co-disposition of chitosan nanoparticles by multi types of hepatic cells and their subsequent biological elimination: the mechanism and kinetic studies at the cellular and animal levels

Background: The clearance of nanomaterials (NMs) from the liver is essential for clinical safety, and their hepatic clearance is primarily determined by the co-disposition process of various types of hepatic cells. Studies of this process and the subsequent clearance routes are urgently needed for organic NMs, which are used as drug carriers more commonly than the inorganic ones. Materials and methods: In this study, the co-disposition of chitosan-based nanoparticles (CsNps) by macrophages and hepatocytes at both the cellular and animal levels as well as their subsequent biological elimination were investigated. RAW264.7 and Hepa1-6 cells were used as models of Kupffer cells and hepatocytes, respectively. Results: The cellular studies showed that CsNps released from RAW264.7 cells could enter Hepa1-6 cells through both clathrin- and caveolin-mediated endocytosis. The transport from Kupffer cells to hepatocytes was also studied in mice, and it was observed that most CsNps localized to the hepatocytes after intravenous injection. Following the distribution in hepatocytes, the hepatobiliary-fecal excretion route was shown to be the primary elimination route for CsNps, besides the kidney-urinary excretion route. The elimination of CsNps in mice was a lengthy process, with a half time of about 2 months. Conclusion: The demonstration in this study of the transport of CsNps from macrophages to hepatocytes and the subsequent hepatobiliary-fecal excretion provides basic information for the future development and clinical application of NMs.

Endoplasmic reticulum stress enhances endocytosis in calreticulin deficient cells

Calreticulin an endoplasmic reticulum (ER) chaperone that is involved in the quality control process and plays an important role as a regulator of intracellular calcium homeostasis. Previously, we illustrated that loss of calreticulin (crt-/-) results in the activation of ubiquitin-proteasome pathway facilitating the increased resistance to apoptosis. Our preliminary data illustrated a significant increase in the endocytosis in the calreticulin knockout mouse embryonic fibroblast cells (crt-/-). Therefore, we hypothesized that the mechanism for this increased endocytosis in the crt-/- cells is due to onset of ER stress. To test this hypothesis, we measured endocytosis in the wild type (wt) and crt-/- cells using uptake of fluorescent dextran and showed a significant increase in the rate of its uptake in crt-/- cells as compared to wt cells. To determine the endocytic pathway involved we examined both clathrin and caveolin-1 dependent endocytosis. Our results illustrated no change in the expression of clathrin heavy chain while there was a significant increase in the expression of caveolin-1 in the crt-/- cells as compared to the wt cells. Furthermore, using shRNA we illustrated that knockdown of clathrin heavy chain had no effect on endocytosis in the crt-/- cells. While knock-down of caveolin-1 significantly reduced endocytosis in the crt-/- cells. Finally, we illustrated that a chemical chaperone, 4‑phenylbutyrate significantly reduced both the endoplasmic reticulum stress and endocytosis in the crt-/- cells. Our data shows for the first time, that ER stress led to enhanced caveolin-1 mediated endocytosis and reversal of ER stress reduces endocytosis.

Natural Brain Penetration Enhancer-Modified Albumin Nanoparticles for Glioma Targeting Delivery

The unsatisfactory therapeutic outcome for glioma is mainly due to the poor blood-brain barrier (BBB) permeability and inefficient accumulation in the glioma area of chemotherapeutic agents. The existing drug delivery strategies can increase drug transport to the brain but are restricted by side effects and/or poor delivery efficiency. In this study, potent brain penetration enhancers were screened from the active components of aromatic resuscitation drugs used in traditional Chinese medicine. A novel glioma-targeting system based on enhancer-modified albumin nanoparticles was developed to safely and efficiently deliver drugs to the glioma regions in the brain. The nanoparticles improved the transport of nanoparticles across brain capillary endothelial cell (BCEC) monolayer by increasing endocytosis in endothelial cells and causing BBB disruption. In vivo imaging studies demonstrated that the systems could enter the brain and subsequently accumulate in glioma cells with a much higher targeting efficiency than that of transferrin-modified albumin nanoparticles. Of note, the nanoparticles could be captured and penetrate through endothelial cells fenestrae in pineal gland, which is suggestive of an effective way to deliver a nanosystem to the brain by bypassing the BBB. The nanoparticles showed good biocompatibility and negligible cytotoxicity. The results reveal an efficient and safe strategy for brain drug delivery in glioma therapy.

Numerical method for vesicle movement analysis in a complex cytoskeleton network

The detection of the precise movement of a vesicle during transport in a live cell provides key information for the intracellular delivery process. Here we report a novel numerical method for analyzing three-dimensional vesicle movement. Since the vesicle moves along a linear cytoskeleton during the active transport, our method first detects the orientation and position of the cytoskeleton as a linear section based on angle correlation and linear regression, after noise reduction. Then, the precise vesicle movement is calculated using vector analysis, in terms of rotation angle and translational displacement. Using this method, various vesicle trajectories obtained via high spatiotemporal resolution microscopy can be understood..

Caveolin-1 promotes the tumor suppressor properties of oncogene-induced cellular senescence

Oncogene-induced senescence (OIS) is considered a powerful tumor suppressor mechanism. Caveolin-1 acts as a scaffolding protein to functionally regulate signaling molecules. We demonstrate that a lack of caveolin-1 expression inhibits oncogenic K-Ras (K-Ras^G12V)-induced premature senescence in mouse embryonic fibroblasts and normal human bronchial epithelial cells. Oncogenic K-Ras induces senescence by limiting the detoxification function of MTH1. We found that K-Ras^G12V promotes the interaction of caveolin-1 with MTH1, which results in inhibition of MTH1 activity. Lung cancer cells expressing oncogenic K-Ras have bypassed the senescence barrier. Interestingly, overexpression of caveolin-1 restores cellular senescence in both A549 and H460 lung cancer cells and inhibits their transformed phenotype. In support of these findings, our in vivo data demonstrate that overexpression of oncogenic K-Ras (K-Ras^G12D) induces cellular senescence in the lung of wildtype but not caveolin-1-null mice. A lack of K-Ras^G12D-induced premature senescence in caveolin-1-null mice results in the formation of more abundant lung tumors. Consistent with these data, caveolin-1-null mice overexpressing K-Ras^G12D display accelerated mortality. Finally, our animal data were supported by human sample analysis in which we show that caveolin-1 expression is dramatically down-regulated in lung adenocarcinomas from lung cancer patients, both at the mRNA and protein levels, and that low caveolin-1 expression is associated with poor survival. Together, our data suggest that lung cancer cells escape oncogene-induced premature senescence through down-regulation of caveolin-1 expression to progress from premalignant lesions to cancer.

Endocytosis and intracellular traffic of cholesterol-PDMAEMA liposome complexes in human epithelial-like cells

Liposomes are generally used as delivery systems, as they are capable of encapsulating a wide variety of molecules (i.e. plasmids, recombinant proteins, therapeutic drugs). However, liposomal drug delivery have to fulfill different requirements, such as the effective internalization by the target cells and avoidance of the degradative activity of the intracellular compartments. The use of polymer lipid complexes (PLCs), by including different polymers in the liposome formulation, could improve internalization and intracellular release of drugs. The aim of the present work is to study the mechanisms of cellular uptaking and the intracellular trafficking of PLCs formed with cholesterol-poly(2-(dimethylamino)ethyl methacrylate) CHO-PDMAEMA and lecithin (LC CHO-PD). Calcein-loaded liposomes were used to determine cellular uptake and intracellular localization by flow cytometry and confocal microscopy. Incorporation of CHO-PDMAEMA to lecithin liposomes enhanced the internalization capacity of PLCs. Internalization of PLCs by human epithelial-like cells (HEK-293) diminished at 4°C, suggesting uptake by endocytosis. PLCs showed no co-localization with acidic compartments after internalization. Experiments with endocytosis inhibitors and co-localization of liposomes and albumin, suggested the caveolae endocytic pathway as the most probable route for intracellular trafficking of PLCs. In this work, we demonstrated an efficient uptake of LC CHO-PDs by human epithelial-like cells (HEK-293) through the non-degradative caveolae endocytic pathway. The mode of internalization and the intracellular fate of liposomes under study, suggest a promising use of LC CHO-PDs as drug delivery systems.

Absorption Mechanism of a Physical Complex of Monomeric Insulin and Deoxycholyl-l-lysyl-methylester in the Small Intestine

Currently, oral administration of insulin still remains the best option to avoid the burden of repeated subcutaneous injections and to improve its pharmacokinetics. The objective of the present investigation was to demonstrate the absorption mechanism of insulin in the physical complexation of deoxycholyl-l-lysyl-methylester (DCK) for oral delivery. The oral insulin/DCK complex was prepared by making a physical complex of insulin aspart with DCK through ion-pair interaction in water. For the cellular uptake study, fluorescein-labeled insulin or DCK were prepared according to a standard protocol and applied to Caco-2 or MDCK cell lines. For the PK/PD studies, we performed intrajejunal administration of different formulation of insulin/DCK complex to diabetic rats. The resulting insulin and DCK complex demonstrated greatly enhanced lipophilicity as well as increased permeation across Caco-2 monolayers. The immunofluorescence study revealed the distribution of the complex in the cytoplasm of Caco-2 cells. Moreover, in the apical sodium bile acid transporter (ASBT) transfected MDCK, the insulin/DCK complex showed interaction with ASBT, and also demonstrated absorption through passive diffusion. We could not find that any evidence of endocytosis in relation to the uptake of insulin complex in vitro. In the rat intestine model, the highest absorption of insulin complex was observed in the jejunum at 1 h and then in the ileum at 2-4 h. In PK/PD study, the complex showed a similar PK profile to that of SC insulin. Overall, the study showed that the effect of DCK on enhancing the absorption of insulin resulted from transcellular processes as well as bile acid transporter activity.

On guanidinium and cellular uptake

Guanidinium-rich scaffolds facilitate cellular translocation and delivery of bioactive cargos through biological barriers. Although impressive uptake has been demonstrated for nonoligomeric and nonpept(o)idic guanidinylated scaffolds in cell cultures and animal models, the fundamental understanding of these processes is lacking. Charge pairing and hydrogen bonding with cell surface counterparts have been proposed, but their exact role remains putative. The impact of the number and spatial relationships of the guanidinium groups on delivery and organelle/organ localization is yet to be established.

C-terminus of human BKca channel alpha subunit enhances the permeability of the brain endothelial cells by interacting with caveolin-1 and triggering caveolin-1 intracellular trafficking

The blood-tumor barrier (BTB) significantly limits the delivery of chemotherapeutic drugs to brain tumors. In this study, we found a significant increase in the permeability of BTB by mediating the association of the C-terminus of alpha subunit of human large-conductance calcium-activated potassium channels (hSlo1c) with caveolin-1 (Cav-1). We present evidence for the first time that hSlo1c associates with Cav-1 in human brain microvascular endothelial cells (HBMECs). A 57-amino acid (966-1022) fragment in hSlo1c was identified to be critical for hSlo1c/Cav-1 interaction. Activation of HBMECs transfected with fusion plasmids of pCMV-hSlo1c containing aa966-1022 by NS1619 selectively enhanced BTB permeability in a BTB model from the co-culture of HBMECs and U87 MG cells but not if the fusion plasmid lacks this fragment. This effect was attenuated by filipin, an agent disrupting caveolae or deletion of the potential interaction fragment, suggesting hSlo1c/Cav-1 association is crucial for regulating the permeability of BTB. Furthermore, we found that hSlo1c/Cav-1 association boosted Cav-1 transferring from the cell membrane to the cytoplasm of HBMECs. Our study indicates that cytoplasmic hSlo1c not only associates with Cav-1 but also has functional consequences on the permeability of BTB by triggering the intracellular trafficking of its interacting protein partner, Cav-1.

Role of fibrinogen in cerebrovascular dysfunction after traumatic brain injury

Traumatic brain injury (TBI) has been associated with various neurological disorders. However, the role of cerebrovascular dysfunction and its mechanisms associated with TBI are still not well understood. Inflammation is the main cause of vascular dysfunction. It affects properties of blood components and the vascular wall leading to changes in blood flow and in interaction of blood components and vascular endothelium exacerbating microcirculatory complications during inflammatory diseases. One of the markers of inflammation is a plasma adhesion protein, fibrinogen (Fg). At elevated levels, Fg can also cause inflammatory responses. One of the manifestations of inflammatory responses is an increase in microvascular permeability leading to accumulation of plasma proteins in the subendothelial matrix and causing vascular remodelling. This has a most devastating effect on cerebral circulation after TBI that is accompanied with an elevation of plasma level of Fg and with an increased cerebrovascular permeability in injury penumbra impairing the normal healing process. This study reviews cerebrovascular alterations after TBI, considers the consequences of increased blood-brain barrier permeability, defines the role of elevated level of Fg and discusses the potential mechanisms of its action leading to vascular dysfunction, which subsequently can cause impairment in neuronal function. Thus, possible mechanisms of vasculo-neuronal dysfunction after TBI are considered.

Inhibition of nuclear factor-erythroid 2-related factor (Nrf2) by caveolin-1 promotes stress-induced premature senescence

Reactive oxygen species can induce premature senescence. Caveolin-1 promotes oxidative stress–induced activation of the p53/p21Waf1/Cip1 pathway and development of premature senescence by acting as an endogenous inhibitor of the transcription factor Nrf2.

Reactive oxygen species (ROS) can induce premature cellular senescence, which is believed to contribute to aging and age-related diseases. The nuclear erythroid 2 p45–related factor-2 (Nrf2) is a transcription factor that mediates cytoprotective responses against stress. We demonstrate that caveolin-1 is a direct binding partner of Nrf2, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82–101) to the caveolin-binding domain of Nrf2 (amino acids 281–289). Biochemical studies show that Nrf2 is concentrated into caveolar membranes in human and mouse fibroblasts, where it colocalizes with caveolin-1, under resting conditions. After oxidative stress, caveolin-1 limits the movement of Nrf2 from caveolar membranes to the nucleus. In contrast, Nrf2 is constitutively localized to the nucleus before and after oxidative stress in caveolin-1–null mouse embryonic fibroblasts (MEFs), which do not express caveolin-1. Functional studies demonstrate that caveolin-1 acts as an endogenous inhibitor of Nrf2, as shown by the enhanced up-regulation of NQO1, an Nrf2 target gene, in caveolin-1–null MEFs and the activation or inhibition of a luciferase construct carrying an antioxidant responsive element (ARE) after down-regulation of caveolin-1 by small interfering RNA or overexpression of caveolin-1, respectively. Expression of a mutant form of Nrf2 that cannot bind to caveolin-1 (Φ→A-Nrf2) hyperactivates ARE and inhibits oxidative stress–induced activation of the p53/p21^Waf1/Cip1 pathway and induction of premature senescence in fibroblasts. Finally, we show that overexpression of caveolin-1 in colon cancer cells inhibits oxidant-induced activation of Nrf2-dependent signaling, promotes premature senescence, and inhibits their transformed phenotype. Thus, by inhibiting Nrf2-mediated signaling, caveolin-1 links free radicals to the activation of the p53/senescence pathway.

Initiation of BMP2 signaling in domains on the plasma membrane

Bone morphogenetic protein 2 (BMP2) is a potent growth factor crucial for cell fate determination. It directs the differentiation of mesenchymal stem cells into osteoblasts, chondrocytes, adipocytes, and myocytes. Initiation of BMP2 signaling pathways occurs at the cell surface through type I and type II serine/threonine kinases housed in specific membrane domains such as caveolae enriched in the caveolin-1 beta isoform (CAV1β, caveolae) and clathrin-coated pits (CCPs). In order for BMP2 to initiate Smad signaling it must bind to its receptors on the plasma membrane resulting in the phosphorylation of the BMP type Ia receptor (BMPRIa) followed by activation of Smad signaling. The current model suggests that the canonical BMP signaling pathway, Smad, occurs in CCPs. However, several recent studies suggested Smad signaling may occur outside of CCPs. Here, we determined; (i) The location of BMP2 binding to receptors localized in caveolae, CCPs, or outside of these domains using AFM and confocal microscopy. (ii) The location of phosphorylation of BMPRIa on the plasma membrane using membrane fractionation, and (iii) the effect of down regulation of caveolae on Smad signaling. Our data indicate that BMP2 binds with highest force to BMP receptors (BMPRs) localized in caveolae. BMPRIa is phosphorylated in caveolae and the disruption of caveolae-inhibited Smad signaling in the presence of BMP2. This suggests caveolae are necessary for the initiation of Smad signaling. We propose an extension of the current model of BMP2 signaling, in which the initiation of Smad signaling is mediated by BMPRs in caveolae.

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.

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.

A self-assembled nanodelivery system enhances the oral bioavailability of daidzein: in vitro characteristics and in vivo performance

AIM

A self-assembled nano-based delivery system was designed and developed to increase the oral bioavailability of poor hydrophilic and lipophilic daidzein.

METHODS

Daidzein was firstly combined with lecithin to form the daidzein-lecithin complex, then self-assembled into micelles (DLMs) with lecithin and sodium bile. The physiochemical properties and intestinal absorption of DLMs were characterized, and the pharmacokinetic behavior was evaluated in rats.

RESULTS

DLMs exhibited nanometer-sized particles. DLMs were mainly distributed in the stomach and proximal intestine after oral administration. The intestinal absorption of DLMs was significantly improved, and DLMs could be absorbed via both endocytosis and passive transport. The AUC(0-t) value of daidzein in rats treated with DLMs was ninefold greater than that of free daidzein suspension.

CONCLUSION

The presented delivery system could provide a new promising strategy for enhancing the oral bioavailability of drugs with poor hydrophilicity and lipophilicity.

Evaluation of cellular uptake and intracellular trafficking as determining factors of gene expression for amino acid-substituted gemini surfactant-based DNA nanoparticles

Background

Gene transfer using non-viral vectors offers a non-immunogenic and safe method of gene delivery. Cellular uptake and intracellular trafficking of the nanoparticles can impact on the transfection efficiency of these vectors. Therefore, understanding the physicochemical properties that may influence the cellular uptake and the intracellular trafficking can aid the design of more efficient non-viral gene delivery systems. Recently, we developed novel amino acid-substituted gemini surfactants that showed higher transfection efficiency than their parent compound. In this study, we evaluated the mechanism of cellular uptake of the plasmid/gemini surfactant/helper lipid nanoparticles and their effect on the transfection efficiency.

Results

Nanoparticles were incubated with Sf 1 Ep cells in the presence of different endocytic inhibitors and gene expression (interferon-γ) was measured using ELISA. Clathrin-mediated and caveolae-mediated uptake were found to be equally contributing to cellular internalization of both P/12-7NH-12/L (parent gemini surfactant) and P/12-7NGK-12/L (amino acid-substituted gemini surfactant) nanoparticles. The plasmid and the helper lipid were fluorescently tagged to track the nanoparticles inside the cells, using confocal laser scanning microscopy. Transmission electron microscopy images showed that the P/12-7NGK-12/L particles were cylindrical while the P/12-7NH-12/L particles were spherical which may influence the cellular uptake behaviour of these particles. Dye exclusion assay and pH-titration of the nanoparticles suggested that high buffering capacity, pH-dependent increase in particle size and balanced DNA binding properties may be contributing to a more efficient endosomal escape of P/12-7NGK-12/L compared to the P/12-7NH-12/L nanoparticles, leading to higher gene expression.

Conclusion

Amino-acid substitution in the spacer of gemini surfactant did not alter the cellular uptake pathway, showing similar pattern to the unsubstituted parent gemini surfactant. Glycyl-lysine substitution in the gemini spacer improved buffering capacity and imparted a pH-dependent increase of particle size. This property conferred to the P/12-7NGK-12/L nanoparticles the ability to escape efficiently from clathrin-mediated endosomes. Balanced binding properties (protection and release) of the 12-7NGK-12 in the presence of polyanions could contribute to the facile release of the nanoparticles internalized via caveolae-mediated uptake. A more efficient endosomal escape of the P/12-7NGK-12/L nanoparticles lead to higher gene expression compared to the parent gemini surfactant.

The potential role of caveolin-1 in inhibition of aquaporins during the AVD

BACKGROUND INFORMATION

During apoptosis, the first morphological change is a distinct cell shrinkage known as the AVD (apoptotic volume decrease). This event is driven by a loss of intracellular K(+), which creates an osmotic gradient, drawing water out of the cell through AQPs (aquaporins). Loss of water in balance with K(+) would create a shrunken cell with an equivalent intracellular concentration of K(+) ([K(+)](i) = 140 mM). However, we have previously shown that the [K(+)](i) of the shrunken apoptotic cell is 35 mM, and this level is absolutely essential for the activation of apoptotic enzymes. We have recently found that AQPs are inactivated following the AVD, so that continued loss of K(+) will reduce the intracellular concentration to this critical level. Using thymocytes, we have investigated the expression profile and regulation of the AQP family members.

RESULTS

In the present study, we have found that AQP1, AQP8 and AQP9 are present in non-apoptotic thymocytes and localized primarily to the plasma membrane. Expression and localization did not change when these cells were induced to undergo apoptosis by growth factor withdrawal for 24 h. To explore other possible mechanisms by which these water channels are inactivated, we investigated their association with CAV-1 (caveolin-1), binding to which is known to inactivate a variety of proteins. We found that CAV-1 is present in thymocytes and that this protein co-localizes with a portion of AQP1 in normal (non-apoptotic) thymocytes. However, thymocytes induced to undergo apoptosis greatly increase their AQP1/CAV-1 association.

CONCLUSIONS

Taken together, these results indicate that AQPs are localized to the plasma membrane of shrunken apoptotic thymocytes where increased binding to CAV-1 potentially inactivates them. AQP inactivation, coupled with continued K(+) efflux, then allows the [K(+)](i) to decrease to levels conducive for the activation of downstream apoptotic enzymes and the completion of the apoptotic cascade.

Caveolin-1, cellular senescence and age-related diseases

According to the "free radical theory" of aging, normal aging occurs as the result of tissue damages inflicted by reactive oxygen species (ROS) when ROS production exceeds the antioxidant capacity of the cell. ROS induce cellular dysfunctions such as stress-induced premature senescence (SIPS), which is believed to contribute to normal organismal aging and play a role in age-related diseases. Consistent with this hypothesis, increased oxidative damage of DNA, proteins, and lipids have been reported in aged animals and senescent cells accumulate in vivo with advancing age. Caveolin-1 acts as a scaffolding protein that concentrates and functionally regulates signaling molecules. Recently, great progress has been made toward understanding of the role of caveolin-1 in stress-induced premature senescence. Data show that caveolin-mediated signaling may contribute to explain, at the molecular level, how oxidative stress promotes the deleterious effects of cellular senescence such as aging and age-related diseases. In this review, we discuss the cellular mechanisms and functions of caveolin-1 in the context of SIPS and their relevance to the biology of aging.

Polymerase I and transcript release factor (PTRF)/cavin-1 is a novel regulator of stress-induced premature senescence

According to the "free radical theory" of aging, premature senescence induced by oxidative stress contributes to organismal aging. Polymerase I and transcript release factor (PTRF)/cavin-1 is a structural protein component of caveolae, invaginations of the plasma membrane involved in signal transduction. We show that oxidative stress up-regulates PTRF/cavin-1 protein expression and promotes the interaction between PTRF/cavin-1 and caveolin-1, another structural protein component of caveolae. Consistent with these data, the number of caveolae is dramatically increased in cells subjected to oxidative stress. We demonstrate that down-regulation of PTRF/cavin-1 by shRNA significantly inhibits oxidative stress-induced premature senescence. Mechanistically, we found that PTRF/cavin-1 expression is necessary for the oxidant-induced sequestration of Mdm2, a negative regulator of p53, into caveolar membranes, away from p53, and activation of the p53/p21(Waf1/Cip1) pathway. Expression of a mutant form of PTRF/cavin-1, which fails to localize to caveolar membranes after oxidative stress, inhibits oxidative stress-induced activation of p53 and induction of premature senescence. Thus, PTRF/cavin-1 is a novel regulator of oxidative stress-induced premature senescence by acting as a link between free radicals and activation of the p53/p21(Waf1/Cip1) pathway.

Epigallocatechin-gallate stimulates NF-E2-related factor and heme oxygenase-1 via caveolin-1 displacement

Flavonoids, such as the tea catechin epigallocatechin-gallate (EGCG), can protect against atherosclerosis by decreasing vascular endothelial cell inflammation. Heme oxygenase-1 (HO-1) is an enzyme that plays an important role in vascular physiology, and its induction may provide protection against atherosclerosis. Heme oxygenase-1 can be compartmentalized in caveolae in endothelial cells. Caveolae are plasma microdomains important in vesicular transport and the regulation of signaling pathways associated with the pathology of vascular diseases. We hypothesize that caveolae play a role in the uptake and transport of EGCG and mechanisms associated with the anti-inflammatory properties of this flavonoid. To test this hypothesis, we explored the effect of EGCG on the induction of NF-E2-related factor (Nrf2) and HO-1 in endothelial cells with or without functional caveolae. Treatment with EGCG activated Nrf2 and increased HO-1 expression and cellular production of bilirubin. In addition, EGCG rapidly accumulated in caveolae, which was associated with caveolin-1 displacement from the plasma membrane towards the cytosol. Similar to EGCG treatment, silencing of caveolin-1 by siRNA technique also resulted in up-regulation of Nrf2, HO-1 and bilirubin production. These data suggest that EGCG-induced caveolin-1 displacement may reduce endothelial inflammation.

Decreased expression of ARV1 results in cholesterol retention in the endoplasmic reticulum and abnormal bile acid metabolism

Endoplasmic reticulum (ER) membrane cholesterol is maintained at an optimal concentration of ∼5 mol % by the net impact of sterol synthesis, modification, and export. Arv1p was first identified in the yeast Saccharomyces cerevisiae as a key component of this homeostasis due to its probable role in intracellular sterol transport. Mammalian ARV1, which can fully complement the yeast lesion, encodes a ubiquitously expressed, resident ER protein. Repeated dosing of specific antisense oligonucleotides to ARV1 produced a marked reduction of ARV1 transcripts in liver, adipose, and to a lesser extent, intestine. This resulted in marked hypercholesterolemia, elevated serum bile acids, and activation of the hepatic farnesoid X receptor (FXR) regulatory pathway. Knockdown of ARV1 in murine liver and HepG2 cells was associated with accumulation of cholesterol in the ER at the expense of the plasma membrane and suppression of sterol regulatory element-binding proteins and their targets. These studies indicate a critical role of mammalian Arv1p in sterol movement from the ER and in the ensuing regulation of hepatic cholesterol and bile acid metabolism.

Bovine parvovirus uses clathrin-mediated endocytosis for cell entry

Entry events of bovine parvovirus (BPV) were studied. Transmission electron micrographs of infected cells showed virus particles in cytoplasmic vesicles. Chemical inhibitors that block certain aspects of the cellular machinery were employed to assess viral dependency upon those cellular processes. Chlorpromazine, ammonium chloride, chloroquine and bafilamicin A1 were used to inhibit acidification of endosomes and clathrin-associated endocytosis. Nystatin was used as an inhibitor of the caveolae pathway. Cytochalasin D and ML-7 were used to inhibit actin and myosin functions, respectively. Nocodazole and colchicine were employed to inhibit microtubule activity. Virus entry was assessed by measuring viral transcription using real-time PCR, synthesis of capsid protein and assembly of infectious progeny virus in the presence of inhibitor blockage. The results indicated that BPV entry into embryonic bovine trachael cells utilizes endocytosis in clathrin-coated vesicles, is dependent upon acidification, and appears to be associated with actin and microtubule dependency. Evidence for viral entry through caveolae was not obtained. These findings provide a fuller understanding of the early cell-entry events of the replication cycle for members of the genus Bocavirus.

Mutual effects of caveolin and nerve growth factor signaling in pig oligodendrocytes

Signaling of growth factors may depend on the recruitment of their receptors to specialized microdomains. Previous reports on PC12 cells indicated an interaction of raft-organized caveolin and TrkA signaling. Because porcine oligodendrocytes (OLs) respond to nerve growth factor (NGF), we were interested to know whether caveolin also plays a role in oligodendroglial NGF/TrkA signaling. OLs expressed caveolin at the plasma membrane but also intracellularly. This was partially organized in the classically Omega-shaped invaginations, which may represent caveolae. We could show that caveolin and TrkA colocalize by using a discontinuous sucrose gradient (Song et al. [1996] J. Biol. Chem. 271:9690-9697), MACS technology, and immunoprecipitation. However, differential extraction of caveolin and TrkA with Triton X-100 at 4 degrees C indicated that caveolin and TrkA are probably not exclusively present in detergent-resistant, caveolin-containing rafts (CCRs). NGF treatment of OLs up-regulated the expression of caveolin-1 (cav-1) and stimulated tyrosine-14 phosphorylation of cav-1. Furthermore, OLs were transfected with cav-1-specific small interfering RNA (siRNA). A knockdown of cav-1 resulted in a reduced activation of downstream components of the NGF signaling cascade, such as p21Ras and mitogen-activated protein kinase (MAPK) after NGF exposure of OLs. Subsequently, increased oligodendroglial process formation via NGF was impaired. The present study indicates that CCRs/caveolin could play a modulating role during oligodendroglial differentiation and regeneration.

Gene Transfer by Means of Lipo- and Polyplexes: Role of Clathrin and Caveolae-Mediated Endocytosis

In this paper we address the contribution of different endocytic pathways to the intracellular uptake and processing of differently sized latex particles and of plasmid DNA complexes by means of fluorescence microscopy and FACS analysis. By using a number of specific inhibitors of either clathrin-dependent or caveolae-dependent endocytosis we were able to discriminate between these two pathways. Latex particles smaller than 200 nm were internalized exclusively by clathrin-mediated endocytosis, whereas larger particles entered the cells via a caveolae-dependent pathway.The route of uptake of plasmid DNA complexes appears strongly dependent on the nature of the complexes. Thus, lipoplexes containing the cationic lipid DOTAP, were exclusively internalized by a clathrin-dependent mechanism, while polyplexes prepared from the cationic polymer polyethyleneimine (PEI) were internalized in roughly equal proportions by both pathways. Upon incubation of cells with lipoplexes containing the luciferase gene abundant luciferase expression was observed, which was effectively blocked by inhibitors of clathrin-dependent endocytosis but not by inhibitors of the caveolae-dependent uptake mechanism. By contrast, luciferase transfection of the cells with polyplexes was unaffected by inhibition of clathrin-mediated endocytosis, but was nearly completely blocked by inhibitors interfering with the caveolae pathway. The results are discussed with respect to possible differences in the mechanism by which plasmid DNA is released from lipoplexes and polyplexes into the cytosol and to the role of size in the uptake and processing of the complexes. Our data suggest that improvement of non-viral gene transfection could very much benefit from controlling particle size, which would allow targeting of particle internalization via a non-degradative pathway, involving caveolae-mediated endocytosis.

The role of fatty acids and caveolin-1 in tumor necrosis factor alpha-induced endothelial cell activation

Hypertriglyceridemia and associated high circulating free fatty acids are important risk factors for atherosclerosis. In contrast to omega-3 fatty acids, linoleic acid, the major omega-6 unsaturated fatty acid in the American diet, may be atherogenic by amplifying an endothelial inflammatory response. We hypothesize that omega-6 and omega-3 fatty acids can differentially modulate tumor necrosis factor alpha (TNF-alpha)-induced endothelial cell activation and that functional plasma membrane microdomains called caveolae are required for endothelial cell activation. Caveolae are particularly abundant in endothelial cells and play a major role in endothelial trafficking and the regulation of signaling pathways associated with the pathology of vascular diseases. To test our hypothesis, endothelial cells were preenriched with either linoleic acid or alpha-linolenic acid before TNF-alpha-induced endothelial activation. Measurements included oxidative stress and nuclear factor kappaB-dependent induction of cyclooxygenase-2 (COX-2) and prostaglandin E(2) (PGE(2)) under experimental conditions with intact caveolae and with cells in which caveolin-1 was silenced by small interfering RNA. Exposure to TNF-alpha induced oxidative stress and inflammatory mediators, such as p38 mitogen-activated protein kinase (MAPK), nuclear factor kappaB, COX-2, and PGE(2), which were all amplified by preenrichment with linoleic acid but blocked or reduced by alpha-linolenic acid. The p38 MAPK inhibitor SB203580 blocked TNF-alpha-mediated induction of COX-2 protein expression, suggesting a regulatory mechanism through p38 MAPK signaling. Image overlay demonstrated TNF-alpha-induced colocalization of TNF receptor type 1 with caveolin-1. Caveolin-1 was significantly induced by TNF-alpha, which was further amplified by linoleic acid and blocked by alpha-linolenic acid. Furthermore, silencing of the caveolin-1 gene completely blocked TNF-alpha-induced production of COX-2 and PGE(2) and significantly reduced the amplified response of linoleic acid plus TNF-alpha. These data suggest that omega-6 and omega-3 fatty acids can differentially modulate TNF-alpha-induced inflammatory stimuli and that caveolae and its fatty acid composition play a regulatory role during TNF-alpha-induced endothelial cell activation and inflammation.

Coplanar polychlorinated biphenyl-induced CYP1A1 is regulated through caveolae signaling in vascular endothelial cells

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that can induce inflammatory processes in the vascular endothelium. We hypothesize that the plasma membrane microdomains called caveolae are critical in endothelial activation and toxicity induced by PCBs. Caveolae are particularly abundant in endothelial cells and play a major role in endothelial trafficking and the regulation of signaling pathways associated with the pathology of vascular diseases. We focused on the role of caveolae and their major protein component, caveolin-1 (Cav-1), on aryl hydrocarbon receptor (AhR)-mediated induction of cytochrome P450 1A1 (CYP1A1) by coplanar PCBs. Endothelial cell exposure to PCB77 increased both caveolin-1 and CYP1A1 levels in a time-dependent manner in total cell lysates, with a maximum increase at 6h. Furthermore, PCB77 accumulated mainly in the caveolae-rich fraction, as determined by gas chromatograph-mass spectrometry. Immunoprecipitation analysis revealed that PCB77 increased AhR binding to caveolin-1. Silencing of caveolin-1 significantly attenuated PCB77-mediated induction of CYP1A1 and oxidative stress. Similar effects were observed in caveolin-1 null mice treated with PCB77. These data suggest that caveolae may play a role in regulating vascular toxicity induced by persistent environmental pollutants such as coplanar PCBs. This may have implications in understanding mechanisms of inflammatory diseases induced by environmental pollutants.

Origins of intestinal ABCA1-mediated HDL-cholesterol

The origins of cholesterol utilized by intestinal ABCA1 were investigated in the human intestinal cell line Caco-2. Influx of apical membrane cholesterol increases ABCA1 mRNA and mass, resulting in enhanced efflux of HDL-cholesterol. Luminal (micellar) cholesterol and newly synthesized cholesterol are not transported directly to ABCA1 but reach the ABCA1 pool after incorporation into the apical membrane. Depleting the apical or the basolateral membrane of cholesterol by cyclodextrin attenuates the amount of cholesterol transported by ABCA1 without altering ABCA1 expression. Filipin added to the apical side but not the basal side attenuates ABCA1-mediated cholesterol efflux, suggesting that apical membrane "microdomains," or rafts, supply cholesterol for HDL. Preventing cholesterol esterification increases the amount of cholesterol available for HDL. Ezetimibe, a Niemann-Pick C1-like 1 protein inhibitor, does not alter ABCA1-mediated cholesterol efflux. U18666A and imipramine, agents that mimic cholesterol trafficking defects of Neimann-Pick type C disease, attenuate cholesterol efflux without altering ABCA1 expression; thus, intestinal NPC1 may facilitate cholesterol movement to ABCA1. ABCA1-mediated cholesterol efflux is independent of cholesterol synthesis. The results suggest that following incorporation into plasma membrane and rafts of the apical membrane, dietary/biliary and newly synthesized cholesterol contribute to the ABCA1 pool and HDL-cholesterol. NPC1 may have a role in this process.

Role of caveolin-1 in EGCG-mediated protection against linoleic-acid-induced endothelial cell activation

Flavonoids can protect against inflammatory diseases such as atherosclerosis by decreasing vascular endothelial cell activation. Plasma microdomains called caveolae may be critical in regulating endothelial activation. Caveolae are particularly abundant in endothelial cells and play a major role in endothelial trafficking and the regulation of signaling pathways associated with the pathology of vascular diseases. We hypothesize that flavonoids can down-regulate endothelial inflammatory parameters by modulating caveolae-regulated cell signaling. We focused on the role of caveolae and its major protein, caveolin-1, in mechanisms of linoleic-acid-induced endothelial cell activation and protection by the catechin epigallocatechin-3-gallate (EGCG). Exposure to linoleic acid for 6 h induced expression of both caveolin-1 and cyclooxygenase (COX)-2. Pretreatment with EGCG blocked fatty-acid-induced caveolin-1 and COX-2 expression in a time- and concentration-dependent manner. Similar results were observed with nuclear factor-kappa B DNA binding activity, which was also reduced by caveolin-1 silencing. Exposure to linoleic acid rapidly increased phosphorylation of several kinases, including p38 MAPK, extracellular signal regulated kinase 1/2 (ERK1/2) and amino kinase terminal (Akt), with maximal induction at about 10 min. Inhibitors of ERK1/2 and Akt down-regulated the linoleic-acid-induced increase in COX-2 protein, which also occurred after pretreatment with EGCG. Caveolin-1 silencing blocked linoleic-acid-induced phosphorylation of ERK1/2 and protein expression of COX-2, suggesting that specific MAPK signaling is caveolae dependent. Our data provide evidence that caveolae may play a critical role in regulating vascular endothelial cell activation and protection by flavonoids such as EGCG.

Caveolae structure and function

Studies on the structure and function of caveolae have revealed how this versatile subcellular organelle can influence numerous signalling pathways. This brief review will discuss a few of the key features of caveolae as it relates to signalling and disease processes.

A novel IRQ ligand-modified nano-carrier targeted to a unique pathway of caveolar endocytic pathway

In the present study, the cellular uptake and subsequent intracellular trafficking of liposomes was investigated, in which a novel peptide (IRQ), identified with in vivo phage display, was modified on the surface. Since the novel peptide IRQRRRR is rich in arginine, the cellular uptake mechanism was compared with octaarginine (R8)-modified liposomes, which are known to be taken up by cells via macropinocytosis. The uptake mechanism and intracellular trafficking of peptide-modified liposomes was determined by confocal laser scanning microscopy and flow cytometry analysis. Modification of the liposomal surface with the IRQ peptide (IRQ-Lip), induced internalization via a novel pathway-caveolar endocytosis-in parallel with clathrin-mediated endocytosis. Furthermore, the IRQ peptide stimulated escape from endocytic vesicles, leading to efficient gene silencing. When siRNA was condensed and encapsulated in an IRQ-modified multifunctional envelope-type nano-device (IRQ-MEND), transgene expression was reduced 52% with the fusogenic lipid, DOPE/CHEMS. This result shows that the novel IRQ can be utilized for cytoplasmic delivery of macromolecules. Moreover, the IRQ has the potential to be useful for delivery therapeutic agents to parenchymal cells via caveolar endocytosis, as this uptake pathway also plays an important role in transcytosis.

Regulation of sodium pump endocytosis by cardiotonic steroids: Molecular mechanisms and physiological implications

We have previously shown that ouabain and other cardiotonic steroids interact with the plasmalemmal Na/K-ATPase and cause a time and dose dependent endocytosis of the Na/K-ATPase. This endocytosis is demonstrable using fluorescence imaging as well as conventional biochemical and biophysical cell separation methods. In proximal tubule cells, this process appears to regulate the density of basolateral Na/K-ATPase expression directly as well as indirectly modulate transepithelial sodium transport. Work with genetic manipulations, as well as pharmacological agents with cell culture models, have demonstrated that the cardiotonic steroid stimulated endocytosis of the plasmalemmal Na/K-ATPase requires caveolin and clathrin as well as the activation of c-Src, transactivation of the EGFR and activation of PI3K. Interestingly c-Src, EGFR and ERK1/2 all appear to be endocytosed along with the plasmalemmal Na/K-ATPase. These observations suggest a close analogy between a subset of plasmalemmal Na/K-ATPase and signaling companions with conventional receptor tyrosine kinases. While further studies are necessary to delineate the role of this endocytosis in the generation as well as the limit of signal transduction through the Na/K-ATPase signal cascade, we propose that it has an important role in the regulation of renal sodium handling as well as other important processes.

The role of caveolin-1 in PCB77-induced eNOS phosphorylation in human-derived endothelial cells

Polychlorinated biphenyls (PCBs) may contribute to the pathology of atherosclerosis by activating inflammatory responses in vascular endothelial cells. Endothelial nitric oxide synthase (eNOS) is colocalized with caveolae and is a critical regulator of vascular homeostasis. PCBs may be proatherogenic by causing dysfunctional eNOS signaling. The objective of this study was to investigate the role of caveolin-1 in PCB-induced endothelial dysfunction with a focus on mechanisms associated with eNOS signaling. Cells derived from an immortalized human vascular endothelial cell line were treated with PCB77 to study nitrotyrosine formation through eNOS signaling. Phosphorylation studies of eNOS, caveolin-1, and kinases, such as Src, phosphatidylinositol 3-kinase (PI3K), and Akt, were conducted in cells containing either functional or small-interfering RNA-silenced caveolin-1 protein. We also investigated caveolin-1-regulated mechanisms associated with PCB-induced markers of peroxynitrite formation and DNA binding of NF-kappaB. Cellular exposure to PCB77 increased eNOS phosphorylation and nitric oxide production, as well as peroxynitrite levels. A subsequent PCB-induced increase in NF-kappaB DNA binding may have implications in oxidative stress-mediated inflammatory mechanisms. The activation of eNOS by PCB77 treatment was blocked by inhibitors of the Src/PI3K/Akt pathway. PCB77 also increased phosphorylation of caveolin-1, indicating caveolae-dependent endocytosis. Caveolin-1 silencing abolished both the PCB-stimulated Akt and eNOS phosphorylation, suggesting a regulatory role of caveolae in PCB-induced eNOS signaling. These findings suggest that PCB77 induces eNOS phosphorylation in endothelial cells through a Src/PI3K/Akt-dependent mechanism, events regulated by functional caveolin-1. Our data provide evidence that caveolae may play a critical role in regulating vascular endothelial cell activation and toxicity induced by persistent environmental pollutants such as coplanar PCBs.

Cholesterol sensing, trafficking, and esterification

Mammalian cells acquire cholesterol from low-density lipoprotein (LDL) and from endogenous biosynthesis. The roles of the Niemann-Pick type C1 protein in mediating the endosomal transport of LDL-derived cholesterol and endogenously synthesized cholesterol are discussed. Excess cellular cholesterol is converted to cholesteryl esters by the enzyme acyl-coenzyme A:cholesterol acyltransferase (ACAT) 1 or is removed from a cell by cellular cholesterol efflux at the plasma membrane. A close relationship between the ACAT substrate pool and the cholesterol efflux pool is proposed. Sterol-sensing domains (SSDs) are present in several membrane proteins, including NPC1, HMG-CoA reductase, and the SREBP cleavage-activating protein. The functions of SSDs are described. ACAT1 is an endoplasmic reticulum cholesterol sensor and contains a signature motif characteristic of the membrane-bound acyltransferase family. The nonvesicular cholesterol translocation processes involve the START domain proteins and the oxysterol binding protein-related proteins (ORPs). The properties of these proteins are summarized.

A new generation of anticancer, drug-loaded, colloidal vectors reverses multidrug resistance in glioma and reduces tumor progression in rats

By focusing on rat glioma, we elucidated whether new lipid nanocapsules (LNC) were able to improve anticancer hydrophobic drug bioavailability while also overcoming multidrug resistance. Blank LNCs and LNCs loaded with the antineoplastic agent paclitaxel were formulated by an emulsion inversion phase process. Expression of efflux pumps by rat glioma cells was assessed by reverse transcription-PCR, Western blot, and immunohistochemistry, and their activity was followed using the tracer (99)Tc(m)-methoxyisobutylisonitrile. Modalities of LNC action were addressed by using confocal microscopy detection of fluorescently labeled LNCs, fluorescence-activated cell sorting, high-performance liquid chromatography measurement of paclitaxel release, and analysis of tumor cell growth. This revealed an interaction between LNCs and efflux pumps that resulted in an inhibition of multidrug resistance in glioma cells, both in culture and in cell implants in animals. LNCs were able to target the intracellular compartment of glioma cells, a mechanism that was abrogated by using intracellular cholesterol inhibitors but not by clathrin-coated pit or caveolae uptake inhibitors. This result can be correlated to the LNC inhibitory effects on efflux pump activity that is itself known to be stimulated by intracellular cholesterol. In parallel, we showed that paclitaxel-loaded LNCs were active reservoirs from which paclitaxel could be released. Finally, we established that paclitaxel-loaded LNCs were more efficient than the commercially available paclitaxel formulation (Taxol) for clinical use, thus reducing tumor expansion in vitro and in vivo. Considering the physiologically compatible nature of LNC excipients, these data may represent an important step towards the development of new clinical therapeutic strategies against cancers.

Dynamic regulation of caveolin-1 trafficking in the germ line and embryo of Caenorhabditis elegans

Caveolin is the major protein component required for the formation of caveolae on the plasma membrane. Here we show that trafficking of Caenorhabditis elegans caveolin-1 (CAV-1) is dynamically regulated during development of the germ line and embryo. In oocytes a CAV-1-green fluorescent protein (GFP) fusion protein is found on the plasma membrane and in large vesicles (CAV-1 bodies). After ovulation and fertilization the CAV-1 bodies fuse with the plasma membrane in a manner reminiscent of cortical granule exocytosis as described in other species. Fusion of CAV-1 bodies with the plasma membrane appears to be regulated by the advancing cell cycle, and not fertilization per se, because fusion can proceed in spe-9 fertilization mutants but is blocked by RNA interference-mediated knockdown of an anaphase-promoting complex component (EMB-27). After exocytosis, most CAV-1-GFP is rapidly endocytosed and degraded within one cell cycle. CAV-1 bodies in oocytes appear to be produced by the Golgi apparatus in an ARF-1-dependent, clathrin-independent, mechanism. Conversely endocytosis and degradation of CAV-1-GFP in embryos requires clathrin, dynamin, and RAB-5. Our results demonstrate that the distribution of CAV-1 is highly dynamic during development and provides new insights into the sorting mechanisms that regulate CAV-1 localization.