Lipoprotein promotes caveolin-1 and Ras translocation to caveolae: role of cholesterol in endothelial signaling

Postprandial triglyceride-rich lipoproteins promote the adipogenic differentiation of adipose-derived mesenchymal stem cells via the LRP1/caveolin-1/AKT1 pathway

Diet-induced obesity (OB) is usually accompanied by hypertriglyceridemia, which is characterized by the accumulation of triglyceride (TG)-rich lipoprotein (TRL) particles in the circulation. We previously found that postprandial TRL combined with insulin induced the adipogenic differentiation of 3T3-L1 preadipocytes, which may represent a key mechanism underlying obesity. However, the specific mechanism and signaling pathway involved in this process remain to be fully elucidated. In this study, we found that, in the postprandial state, patients with obesity had significantly higher levels of TG and remnant cholesterol (RC) than normal-weight controls. In vitro, we found that postprandial TRL, together with insulin, promoted the adipogenic differentiation of adipose-derived mesenchymal stem cells (AMSCs), as evidenced by the increased expression of lipogenesis-related genes and their protein products, including low-density lipoprotein related protein 1 (LRP1). Besides, caveolin-1 (Cav-1) expression was also significantly upregulated under this condition. Cav-1 and LRP1 were observed to interact, and then led to the activation of the PI3K/AKT1 signaling pathway. Meanwhile, the inhibition of LRP1 or Cav-1 significantly attenuated the adipogenic differentiation of AMSCs and downregulated AKT1 phosphorylation levels. Moreover, treatment with a selective AKT1 inhibitor significantly suppressed postprandial TRL and insulin-induced adipogenesis in AMSCs. Combined, our results demonstrated that, in association with insulin, postprandial TRL can promote the adipogenic differentiation of AMSCs in a manner that is dependent on the LRP1/Cav-1-mediated activation of the PI3K/AKT1 signaling pathway. Our findings indicated that a postprandial increase in TRL content is a critical factor in the pathogenesis of hypertriglyceridemia and diet-induced obesity.

COMP (Cartilage Oligomeric Matrix Protein), a Novel PIEZO1 Regulator That Controls Blood Pressure

BACKGROUND

Vascular endothelial cells are critical for maintaining blood pressure (BP) by releasing biologically active molecules, such as nitric oxide. A non-endothelial cell resident matricellular protein, COMP (cartilage oligomeric matrix protein), plays a pivotal role in maintaining cardiovascular homeostasis, but little is known about its regulatory effect on BP.

METHODS

Mice were infused with AngII (angiotensin II; 450 ng/kg per minute) for 3 days via an osmotic minipump, and BP was monitored by a tail-cuff system. Second-order mesenteric arteries were isolated from mice for microvascular tension measurement. Nitric oxide was detected by an electron paramagnetic resonance technique. Small-interfering RNA transfection, co-immunoprecipitation, bioluminescence resonance energy transfer assays, and patch-clamp electrophysiology experiments were used for further detailed mechanism investigation.

RESULTS

COMP^-/- mice displayed elevated BP and impaired acetylcholine-induced endothelium-dependent relaxation compared with wild-type mice with or without AngII. Inhibition of eNOS (endothelial nitric oxide synthase) abolished the difference in endothelium-dependent relaxation between wild-type and COMP^-/- mice. Furthermore, COMP directly interacted with the C-terminus of Piezo1 via its C-terminus and activated the endogenous Piezo1 currents, which induced intracellular Ca²⁺ influx, Ca²⁺/calmodulin-dependent protein kinase type II and eNOS activation, and nitric oxide production. The Piezo1 activator, Yoda1, reduced the difference in endothelium-dependent relaxation and BP in wild-type and COMP^-/- mice. Moreover, COMP overexpression increased eNOS activation and improved endothelium-dependent relaxation and BP.

CONCLUSIONS

Our study demonstrated that COMP is a novel Piezo1 regulator that plays a protective role in BP regulation by increasing cellular Ca²⁺ influx, eNOS activity, and nitric oxide production.

The role of caveolae in endothelial dysfunction

Caveolae, the specialized cell-surface plasma membrane invaginations which are abundant in endothelial cells, play critical roles in regulating various cellular processes, including cholesterol homeostasis, nitric oxide production, and signal transduction. Endothelial caveolae serve as a membrane platform for compartmentalization, modulation, and integration of signal events associated with endothelial nitric oxide synthase, ATP synthase β, and integrins, which are involved in the regulation of endothelial dysfunction and related cardiovascular diseases, such as atherosclerosis and hypertension. Furthermore, these dynamic microdomains on cell membrane are modulated by various extracellular stimuli, including cholesterol and flow shear stress. In this brief review, we summarize the critical roles of caveolae in the orchestration of endothelial function based on recent findings as well as our work over the past two decades.

ER-anchored CRTH2 antagonizes collagen biosynthesis and organ fibrosis via binding LARP6

Excessive deposition of extracellular matrix, mainly collagen protein, is the hallmark of organ fibrosis. The molecular mechanisms regulating fibrotic protein biosynthesis are unclear. Here, we find that chemoattractant receptor homologous molecule expressed on TH2 cells (CRTH2), a plasma membrane receptor for prostaglandin D2, is trafficked to the endoplasmic reticulum (ER) membrane in fibroblasts in a caveolin-1-dependent manner. ER-anchored CRTH2 binds the collagen mRNA recognition motif of La ribonucleoprotein domain family member 6 (LARP6) and promotes the degradation of collagen mRNA in these cells. In line, CRTH2 deficiency increases collagen biosynthesis in fibroblasts and exacerbates injury-induced organ fibrosis in mice, which can be rescued by LARP6 depletion. Administration of CRTH2 N-terminal peptide reduces collagen production by binding to LARP6. Similar to CRTH2, bumetanide binds the LARP6 mRNA recognition motif, suppresses collagen biosynthesis, and alleviates bleomycin-triggered pulmonary fibrosis in vivo. These findings reveal a novel anti-fibrotic function of CRTH2 in the ER membrane via the interaction with LARP6, which may represent a therapeutic target for fibrotic diseases.

AIBP, Angiogenesis, Hematopoiesis, and Atherogenesis

PURPOSE OF REVIEW

The goal of this manuscript is to summarize the current understanding of the secreted APOA1 binding protein (AIBP), encoded by NAXE, in angiogenesis, hematopoiesis, and inflammation. The studies on AIBP illustrate a critical connection between lipid metabolism and the aforementioned endothelial and immune cell biology.

RECENT FINDINGS

AIBP dictates both developmental processes such as angiogenesis and hematopoiesis, and pathological events such as inflammation, tumorigenesis, and atherosclerosis. Although cholesterol efflux dictates AIBP-mediated lipid raft disruption in many of the cell types, recent studies document cholesterol efflux-independent mechanism involving Cdc42-mediated cytoskeleton remodeling in macrophages. AIBP disrupts lipid rafts and impairs raft-associated VEGFR2 but facilitates non-raft-associated NOTCH1 signaling. Furthermore, AIBP can induce cholesterol biosynthesis gene SREBP2 activation, which in turn transactivates NOTCH1 and supports specification of hematopoietic stem and progenitor cells (HSPCs). In addition, AIBP also binds TLR4 and represses TLR4-mediated inflammation. In this review, we summarize the latest research on AIBP, focusing on its role in cholesterol metabolism and the attendant effects on lipid raft-regulated VEGFR2 and non-raft-associated NOTCH1 activation in angiogenesis, SREBP2-upregulated NOTCH1 signaling in hematopoiesis, and TLR4 signaling in inflammation and atherogenesis. We will discuss its potential therapeutic applications in angiogenesis and inflammation due to selective targeting of activated cells.

Atherosclerosis: integration of its pathogenesis as a self-perpetuating propagating inflammation: a review

This review proposes that the development of the atherosclerotic plaque is critically dependent on its inflammatory components forming a self-perpetuating and propagating positive feedback loop. The components involved are: (1) LDL oxidation, (2) activation of the endothelium, (3) recruitment of inflammatory monocytes, (4) macrophage accumulation, which induces LDL oxidation, and (5) macrophage generation of inflammatory mediators, which also activate the endothelium. Through these stages, the positive feedback loop is formed, which generates and promotes expansion of the atherosclerotic process. To illustrate this dynamic of lesion development, the author previously produced a computer simulation, which allowed realistic modelling. This hypothesis on atherogenesis can explain the existence and characteristic focal morphology of the atherosclerotic plaque. Each of the components contributing to the feedback loop is discussed. Many of these components also contain subsidiary positive feedback loops, which could exacerbate the overall process.

Integrative functional analysis of super enhancer SNPs for coronary artery disease

Clinical research in coronary artery disease (CAD) primarily focused on genetic variants located in protein-coding regions. Recently, mutations fall within non-coding regions have been suggested to be essential to the pathogenesis of human complex disease. Super enhancer is a densely spaced cluster of transcriptional enhancers located in non-coding regions, which is critical for regulating cell-type specific gene expression. However, the underlying mechanism of the super enhancer single-nucleotide polymorphisms (SNPs) affecting the risk of CAD remains unclear. By integrating genome-wide association study (GWAS) meta-analysis of CAD and cell/tissue-specific histone modification data set, we identified 366 potential CAD-associated super enhancer SNPs in 67 loci, including 94 SNPs that are involved in regulating chromatin interactive and/or affecting the transcription factors binding affinity. Interestingly, we found 7 novel functional loci (CBFA2T3, ZMIZ1, DIP2B, SCNN1D/ACAP3, TMEM105, CAMK2G, and MAPK1) that CAD-associated super enhancer SNPs were clustered into the same or neighboring super enhancers. Pathway analysis showed a significant enrichment in several well-known signaling and regulatory processes, e.g., cAMP signaling pathway and ErbB signaling pathway, which play a key role in CAD metabolism. Our results highlight the potential functional importance of CAD-associated super enhancer SNPs and provide the targets for further insights on the pathogenesis of CAD.

MiR-6835 promoted LPS-induced inflammation of HUVECs associated with the interaction between TLR-4 and AdipoR1 in lipid rafts

Background

High mortality rate of critically-ill patients could be induced by sepsis and septic shock, which is the extremely life threatening. The purpose of this work is to identify and evaluate the potential regulatory mechanism of LPS-induced inflammation associated with miR-6835 and lipid rafts in HUVECs.

Methods

The 3’ UTR luciferase activity of AdipoR1 was detected, which was predicted the potential target gene of miR-6835. Moreover, the treated HUVECs with or without inhibitors or mimics of miR-6835 were used. Furthermore, the bio-functions of HUVECs were explored. The protein expression levels of SIRT-1, AMPK, and AdipoR1 were assessed, which were involved in the AdipoR1 signaling pathway. Then, the interaction between TLR-4 and AdipoR1 in lipid rafts and its mediation role on LPS-induced inflammation was investigated in HUVECs.

Results

MiR-6835 targeted directly on AdipoR1, and suppressed its expression in mRNA (mimics of miR-6835: 0.731±0.016 vs control: 1.527±0.015, P<0.001) and proteins levels, then regulated protein expression of SIRT-1 and AMPK, which were the downstream target genes of AdipoR1 signaling pathway. MiR-6835 enhanced LPS-induced inflammation process in HUVECs (TNF-α: LPS+mimics of miR-6835: 1638.51±78.43 vs LPS: 918.73±39.73, P<0.001; IL-6: LPS+mimics of miR-6835: 1249.35±69.51 vs LPS: 687.52±43.64, P<0.001), which was associated with the interaction between TLR-4 and AdipoR1 in lipid rafts.

Conclusions

MiR-6835 is the key regulator of LPS-induced inflammation process in HUVECs. The interaction between TLR-4 and AdipoR1 mediated by lipid rafts at membrane of HUVECs with inflammation process induced by miR-6835. Our results demonstrated a hopeful strategy for treatment on sepsis by aiming at lipid rafts and miR-6835.

Lipid raft-mediated miR-3908 inhibition of migration of breast cancer cell line MCF-7 by regulating the interactions between AdipoR1 and Flotillin-1

Background

The mechanisms of lipid raft regulation by microRNAs in breast cancer are not fully understood. This work focused on the evaluation and identification of miR-3908, which may be a potential biomarker related to the migration of breast cancer cells, and elucidates lipid-raft-regulating cell migration in breast cancer.

Methods

To confirm the prediction that miR-3908 is matched with AdipoR1, we used 3’-UTR luciferase activity of AdipoR1 to assess this. Then, human breast cancer cell line MCF-7 was cultured in the absence or presence of the mimics or inhibitors of miR-3908, after which the biological functions of MCF-7 cells were analyzed. The protein expression of AdipoR1, AMPK, and SIRT-1 were examined. The interaction between AdipoR1 and Flotillin-1, or its effects on lipid rafts on regulating cell migration of MCF-7, was also investigated.

Results

AdipoR1 is a direct target of miR-3908. miR-3908 suppresses the expression of AdipoR1 and its downstream pathway genes, including AMPK, p-AMPK, and SIRT-1. miR-3908 enhances the process of breast cancer cell clonogenicity. miR-3908 exerts its effects on the proliferation and migration of MCF-7 cells, which are mediated by lipid rafts regulating AdipoR1’s ability to bind Flotillin-1.

Conclusions

miR-3908 is a crucial mediator of the migration process in breast cancer cells. Lipid rafts regulate the interactions between AdipoR1 and Flotillin-1 and then the migration process associated with miR-3908 in MCF-7 cells. Our findings suggest that targeting miR-3908 and the lipid raft, may be a promising strategy for the treatment and prevention of breast cancer.

LDL-Cholesterol Increases the Transcytosis of Molecules through Endothelial Monolayers

Cholesterol has been identified as a causative factor in numerous pathologies including atherosclerosis and cancer. One of the frequent effects of elevated cholesterol levels in humans is the compromise of endothelial function due to activation of pro-inflammatory signalling pathways. While the mechanisms involved in endothelial activation by cholesterol during an inflammatory response are well established, less is known about the mechanisms by which cholesterol may affect endothelial barrier function, which were the subject of the present study. Here we show that low density lipoprotein (LDL) increases the permeability of endothelial monolayers to high molecular weight dextrans in an LDL receptor and cholesterol-dependent manner. The increased permeability seen upon LDL treatment was not caused by disruption of cell-to-cell junctions as determined by a normal localization of VE-Cadherin and ZO-1 proteins, and no major alterations in transendothelial electrical resistance or permeability to fluorescein. We show instead that LDL increases the level of high molecular weight transcytosis and that this occurs in an LDL receptor, cholesterol and caveolae-dependent way. Our findings contribute to our understanding of the systemic pathological effects of elevated cholesterol and the transport of cargo through endothelial monolayers.

Activation of integrin α5 mediated by flow requires its translocation to membrane lipid rafts in vascular endothelial cells

Local flow patterns determine the uneven distribution of atherosclerotic lesions. Membrane lipid rafts and integrins are crucial for shear stress-regulated endothelial function. In this study, we investigate the role of lipid rafts and integrin α5 in regulating the inflammatory response in endothelial cells (ECs) under atheroprone versus atheroprotective flow. Lipid raft proteins were isolated from ECs exposed to oscillatory shear stress (OS) or pulsatile shear stress, and then analyzed by quantitative proteomics. Among 396 proteins redistributed in lipid rafts, integrin α5 was the most significantly elevated in lipid rafts under OS. In addition, OS increased the level of activated integrin α5 in lipid rafts through the regulation of membrane cholesterol and fluidity. Disruption of F-actin-based cytoskeleton and knockdown of caveolin-1 prevented the OS-induced integrin α5 translocation and activation. In vivo, integrin α5 activation and EC dysfunction were observed in the atheroprone areas of low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice, and knockdown of integrin α5 markedly attenuated EC dysfunction in partially ligated carotid arteries. Consistent with these findings, mice with haploinsufficency of integrin α5 exhibited a reduction of atherosclerotic lesions in the regions under atheroprone flow. The present study has revealed an integrin- and membrane lipid raft-dependent mechanotransduction mechanism by which atheroprone flow causes endothelial dysfunction.

Vascular endothelial dysfunction and pharmacological treatment

The endothelium exerts multiple actions involving regulation of vascular permeability and tone, coagulation and fibrinolysis, inflammatory and immunological reactions and cell growth. Alterations of one or more such actions may cause vascular endothelial dysfunction. Different risk factors such as hypercholesterolemia, homocystinemia, hyperglycemia, hypertension, smoking, inflammation, and aging contribute to the development of endothelial dysfunction. Mechanisms underlying endothelial dysfunction are multiple, including impaired endothelium-derived vasodilators, enhanced endothelium-derived vasoconstrictors, over production of reactive oxygen species and reactive nitrogen species, activation of inflammatory and immune reactions, and imbalance of coagulation and fibrinolysis. Endothelial dysfunction occurs in many cardiovascular diseases, which involves different mechanisms, depending on specific risk factors affecting the disease. Among these mechanisms, a reduction in nitric oxide (NO) bioavailability plays a central role in the development of endothelial dysfunction because NO exerts diverse physiological actions, including vasodilation, anti-inflammation, antiplatelet, antiproliferation and antimigration. Experimental and clinical studies have demonstrated that a variety of currently used or investigational drugs, such as angiotensin-converting enzyme inhibitors, angiotensin AT1 receptors blockers, angiotensin-(1-7), antioxidants, beta-blockers, calcium channel blockers, endothelial NO synthase enhancers, phosphodiesterase 5 inhibitors, sphingosine-1-phosphate and statins, exert endothelial protective effects. Due to the difference in mechanisms of action, these drugs need to be used according to specific mechanisms underlying endothelial dysfunction of the disease.

Effects of selenoprotein S on oxidative injury in human endothelial cells

Background

Selenoprotein S (SelS) is an important endoplasmic reticulum and plasma membrane-located selenoprotein implicated in inflammatory responses and insulin resistance. However, the effects of SelS on endothelial cells (ECs) have not been reported. In the present study, the role of SelS in oxidative stress and the underlying mechanism were investigated in human ECs.

Methods

A SelS over-expression plasmid (pc-SelS) and a SelS-siRNA plasmid were transfected into human umbilical vein endothelial cells (American Type Culture Collection, USA). The cells were divided into four groups: control, SelS over-expression (transfected with pc-SelS), vector control, and SelS knockdown (transfected with siRNA-SelS). After treating the cells with H₂O₂, the effects of oxidative stress and the expression of caveolin-1 (Cav-1) and protein kinase Cα (PKCα) were investigated.

Results

Following treatment with H₂O₂, over-expression of SelS significantly increased cell viability and superoxide dismutase (SOD) activity, and decreased malondialdehyde (MDA) production and Cav-1 gene and protein expression. However, no effects on PKCα were observed. In contrast, knockdown of SelS significantly decreased cell viability, SOD activity, and PKCα gene and protein expression, and increased MDA production and Cav-1 gene and protein expression.

Conclusions

SelS protects ECs from oxidative stress by inhibiting the expression of Cav-1 and PKCα.

Loss of caveolin-1 promotes endothelial-mesenchymal transition during sepsis: a membrane proteomic study

Sepsis is a clinical syndrome that reflects an uncontrolled systemic inflammatory response to microbial infections. Endothelial cells, which are highly responsive to their extracellular environment, are the primary targets of sepsis-induced damage. Endothelial-mesenchymal transition (EndMT), characterized by loss of endothelial cell markers and gain of mesenchymal cell markers, contributes to embryonic cardiac formation as well as development of various diseases. We showed that incubation with septic serum induced a rapid loss of endothelial barrier integrity in a human umbilical vein endothelial cell (HUVEC) monolayer. Notably, exposure to septic serum triggered EndMT in HUVECs, companied by increased cell motility and invasion. Furthermore, membrane proteomic analysis was applied to analyze the key mediators in septic serum-induced EndMT. A total of 29 proteins with altered expression level were positively identified. Expression of four proteins with the most significant alteration, including caveolin-1, S100A4, α-enolase and galectin-1, were validated by western blotting. Finally, we showed that restoration of caveolin-1 expression markedly attenuated septic serum-induced EndMT in HUVEC cells. This is the first report showing that endothelial cells undergo EndMT during sepsis. The present study may lead to a better understanding of the biological role of endothelial cells and caveolin-1 during sepsis.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Proteomic analysis of endothelial lipid rafts reveals a novel role of statins in antioxidation

As inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase, statins have pleiotropic vascular-protective effects, such as anti-inflammatory and antioxidative effects. We investigated the short-term beneficial effects of statins on modulating the translocation of lipid-raft-related proteins in endothelial cells (ECs). Human umbilical vein ECs were treated with atorvastatin for 30 min or 2 h; lipid-raft proteins were isolated and examined by quantitative proteome assay. Functional classification of identified proteins in lipid rafts revealed upregulated antioxidative proteins; downregulated proteins were associated with inflammation and cell adhesion. Among proteins verified by Western blot analysis, endoplasmic reticulum protein 46 (ERp46) showed increased level in lipid rafts with atorvastatin. Further, atorvastatin inhibited the activation of membrane-bound NADPH oxidase in both untreated and angiotensin II-treated ECs, as shown by reduced reactive oxygen species production. Co-immunoprecipitation and immunofluorescence experiments revealed that atorvastatin increased the association of ERp46 and Nox2, an NADPH oxidase isoform, in lipid rafts, thereby inhibiting Nox2 assembly with its regulatory subunits, such as p47phox and p67phox. Our results reveal a novel antioxidative role of atorvastatin by promoting the membrane translocation of ERp46 and its binding with Nox2 to inhibit Nox2 activity in ECs, which may offer another insight into the pleiotropic functions of statins.

Regulation of Cell Signaling and Function by Endothelial Caveolins: Implications in Disease

Caveolae are cholesterol- and glycosphingolipid-rich omega-shaped invaginations of the plasma membrane that are very abundant in vascular endothelial cells and present in most cell types. Caveolins are the major coat protein components of caveolae. Multiple studies using knockout mouse, small interfering RNA, and cell-permeable peptide delivery approaches have significantly enhanced our understanding of the role of endothelial caveolae and caveolin-1 in physiology and disease. Several postnatal pulmonary and cardiovascular pathologies have been reported in caveolin-1 knockout mice, many of which have been recently rescued by selective re-expression of caveolin-1 in endothelium of these mice. A large body of experimental evidence mostly using caveolin-1 knockout mice suggests that, depending on the disease model, endothelial caveolin-1 may play either a protective or a detrimental role. For instance, physiological or higher expression levels of caveolin-1 in endothelium might be beneficial in such diseases as pulmonary hypertension, cardiac hypertrophy, or ischemic injury. On the other hand, endothelial caveolin-1 might contribute to acute lung injury and inflammation, atherosclerosis or pathological angiogenesis associated with inflammatory bowel disease. Moreover, depending on the specific model, endothelial caveolin-1 may either promote or suppress tumor-induced angiogenesis. In addition to overwhelming evidence for the role of endothelial caveolin-1, more recent studies also suggest that endothelial caveolin-2 could possibly play a role in pulmonary disease. The purpose of this review is to focus on how caveolin-1 expressed in endothelial cells regulates endothelial cell signaling and function. The review places particular emphasis on relevance to disease, including but not limited to Pulmonary and cardiovascular disorders as well as cancer. In addition to caveolin-1, possible importance of the less-studied endothelial caveolin-2 in pulmonary diseases will be also discussed.

Low-density lipoprotein receptor gene transfer in hypercholesterolemic mice improves cardiac function after myocardial infarction

Left ventricular (LV) function post-myocardial infarction (MI) is adversely influenced by hypercholesterolemia independent of the severity of coronary atherosclerosis. The objective of this study was to evaluate whether lipid lowering by adenoviral low-density lipoprotein (LDL) receptor (AdLDLr) gene transfer in C57BL/6 LDL receptor (LDLr)-deficient mice beneficially affects ventricular remodeling and cardiac function post-MI independent of effects on the coronary circulation. AdLDLr transfer reduced plasma cholesterol by 77% (P<0.0001). Survival 28 days post-MI was higher in AdLDLr-treated mice (95%) compared with control mice (80%) (P<0.05) (hazard ratio for mortality 0.26, 95% confidence interval 0.11–0.84). Infarct size was not significantly different at day 1 and day 7 but was reduced by 18% (P<0.05) at day 28 in AdLDLr MI mice compared with control MI mice. Cardiomyocyte hypertrophy and interstitial fibrosis were reduced and neovascularization was increased in AdLDLr MI mice. LDLr gene transfer had beneficial effects on endothelial progenitor cell (EPC) number and ex vivo EPC function. LV contractility and relaxation were better preserved in AdLDLr MI mice compared with control MI mice. In conclusion, lipid lowering in hypercholesterolemic mice exerts direct cardioprotective effects resulting in enhanced survival, reduced infarct size, decreased ventricular remodeling and better cardiac function.

Remodelling of membrane rafts expression in lung cells as an early sign of mechanotransduction-signalling in pulmonary edema

Membrane rafts (MRs) are clusters of lipids, organized in a "quasicrystalline" liquid-order phase, organized on the cell surface and whose pattern of molecules and physicochemical properties are distinct from those of the surrounding plasma membrane. MRs may be considered an efficient and fairly rapid cell-activated mechanism to express or mask surface receptors aimed at triggering specific response pathways. This paper reports observations concerning the role of MRs in the control of lung extravascular water that ought to be kept at minimum to assure gas diffusion, supporting the hypothesis that MRs expression is a potential mechanism of sensing minor changes in the volume of extravascular water. We present the evidence that MRs expression specifically relates to signal-transduction processes evoked by mechanical stimuli arising in the interstitial lung compartment when a small increase in extravascular volume occurs. We further hypothesize that a differential expression of MRs might also reflect the damage to precise components of the extracellular matrix caused by the perturbation in water balance and thus can trigger a molecule-oriented specific matrix remodelling.

A novel mechanism of γ/δ T-lymphocyte and endothelial activation by shear stress: the role of ecto-ATP synthase β chain

RATIONALE

Endothelial cells (ECs) have distinct mechanotransduction mechanisms responding to laminar versus disturbed flow patterns. Endothelial dysfunction, affected by imposed flow, is one of the earliest events leading to atherogenesis. The involvement of γ/δ T lymphocytes in endothelial dysfunction under flow is largely unknown.

OBJECTIVE

To investigate whether shear stress regulates membrane translocation of ATP synthase β chain (ATPSβ) in ECs, leading to the increased γ/δ T-lymphocyte adhesion and the related functions.

METHOD AND RESULTS

We applied different flow patterns to cultured ECs. Laminar flow decreased the level of membrane-bound ATPSβ (ecto-ATPSβ) and depleted membrane cholesterol, whereas oscillatory flow increased the level of ecto-ATPSβ and membrane cholesterol. Incubating ECs with cholesterol or depleting cellular cholesterol with β-cyclodextrin mimicked the effect of oscillatory or laminar flow, respectively. Knockdown caveolin-1 by small interfering RNA prevented ATPSβ translocation in response to laminar flow. Importantly, oscillatory flow or cholesterol treatment elevated the number of γ/δ T cells binding to ECs, which was blocked by anti-ATPSβ antibody. Furthermore, the incubation of γ/δ T cells with ECs increased tumor necrosis fact α and interferon-γ secretion from T cells and vascular cell adhesion molecule-1 expression in ECs. In vivo, γ/δ T-cell adhesion and ATPSβ membrane translocation was elevated in the aortic inner curvature and disturbed flow areas in partially ligated carotid arteries of ApoE(-/-) mice fed a high-fat diet.

CONCLUSIONS

This study provides evidence that disturbed flow and hypercholesterolemia synergistically promote γ/δ T-lymphocyte activation by the membrane translocation of ATPSβ in ECs and in vivo in mice, which is a novel mechanism of endothelial activation.

Interleukin-6 inhibits endothelial nitric oxide synthase activation and increases endothelial nitric oxide synthase binding to stabilized caveolin-1 in human vascular endothelial cells

OBJECTIVE

We hypothesized a possible mechanism for atherosclerosis in which interleukin-6 (IL-6) might affect the endothelial nitric oxide synthase (eNOS)-caveolin-1 interaction and result in decreased nitric oxide bioavailability in the setting of low-grade inflammation.

METHODS

Because eNOS and caveolin-1 are crucial for vascular tone control, we studied the effects of IL-6 on the expression and activation of eNOS and caveolin-1 in human vascular endothelial cells.

RESULTS

IL-6 inhibited the phosphorylation of eNOS at Ser1177 and the bradykinin-stimulated nitric oxide production; however, eNOS protein expression was not changed. In addition, IL-6 inhibited bradykinin-stimulated Akt phosphorylation at Ser473 and Thr 308 without affecting the Akt protein expression. IL-6 did not alter the mRNA level of caveolin-1; however, the caveolin-1 protein level was significantly increased dose-dependently. The binding of eNOS and caveolin-1 in endothelial cells, as demonstrated by coimmunoprecipitation assay, was increased by IL-6 treatment. IL-6 treatment was found to stabilize caveolin-1 protein and its half-life was estimated to prolong from 7.5 h to longer than 12 h. Furthermore, treatment with PD98059 and short interference RNA of extracellular signal-regulated kinase gene reversed the effects of IL-6 on eNOS and caveolin-1.

CONCLUSION

In addition to decreasing Akt phosphorylation, the results of this study demonstrate, for the first time, the molecular mechanism underlying the effect of IL-6 to decrease the nitric oxide bioavailability by increasing the half-life and, therefore, the protein levels of caveolin-1. The increased caveolin-1 proteins bind more eNOS and consequently decrease eNOS activation by reducing the Ser1177 phosphorylation.

Lung ischemia: a model for endothelial mechanotransduction

Endothelial cells in vivo are constantly exposed to shear associated with blood flow and altered shear stress elicits cellular responses (mechanotransduction). This review describes the role of shear sensors and signal transducers in these events. The major focus is the response to removal of shear as occurs when blood flow is compromised (i.e., ischemia). Pulmonary ischemia studied with the isolated murine lung or flow adapted pulmonary microvascular endothelial cells in vitro results in endothelial generation of reactive oxygen species (ROS) and NO. The response requires caveolae and is initiated by endothelial cell depolarization via K(ATP) channel closure followed by activation of NADPH oxidase (NOX2) and NO synthase (eNOS), signaling through MAP kinases, and endothelial cell proliferation. These physiological mediators can promote vasodilation and angiogenesis as compensation for decreased tissue perfusion.

Transmembrane signaling pathway mediates oxidized low-density lipoprotein-induced expression of plasminogen activator inhibitor-1 in vascular endothelial cells

Atherosclerotic cardiovascular disease is the number one cause of death for adults in Western society. Plasminogen activator inhibitor-1 (PAI-1), the major physiological inhibitor of plasminogen activators, has been implicated in both thrombogenesis and atherogenesis. Previous studies demonstrated that copper-oxidized low-density lipoprotein (C-oLDL) stimulated production of PAI-1 in vascular endothelial cells (EC). The present study examined the involvement of lectin-like oxidized LDL receptor-1 (LOX-1) and Ras/Raf-1/ERK1/2 pathway in the upregulation of PAI-1 in cultured EC induced by oxidized LDLs. The results demonstrated that C-oLDL or FeSO(4)-oxidized LDL (F-oLDL) increased the expression of PAI-1 or LOX-1 in human umbilical vein EC (HUVEC) or coronary artery EC (HCAEC). Treatment with C-oLDL significantly increased the levels of H-Ras mRNA, protein, and the translocation of H-Ras to membrane fraction in EC. LOX-1 blocking antibody, Ras farnesylation inhibitor (FTI-277), or small interference RNA against H-Ras significantly reduced C-oLDL or LDL-induced expression of H-Ras and PAI-1 in EC. Incubation with C-oLDL or F-oLDL increased the phosphorylation of Raf-1 and ERK1/2 in EC compared with LDL or vehicle. Treatment with Raf-1 inhibitor blocked Raf-1 phosphorylation and the elevation of PAI-1 mRNA level in EC induced by C-oLDL or LDL. Treatment with PD-98059, an ERK1/2 inhibitor, blocked C-oLDL or LDL-induced ERK1/2 phosphorylation or PAI-1 expression in EC. The results suggest that LOX-1, H-Ras, and Raf-1/ERK1/2 are implicated in PAI-1 expression induced by oxidized LDLs or LDL in cultured EC.

A high-cholesterol diet increases the association between caveolae and insulin receptors in rat liver

Caveolin-1, a component of caveolae, regulates signaling pathway compartmentalization by interacting with tyrosine (Tyr) kinase receptors and their substrates. Perturbations in caveolae lipid composition have been shown in vitro to displace proteins from lipid microdomains, thereby altering their functionality and subsequent downstream signaling. The role of caveolin-1 in insulin receptor (IR) signaling has been widely investigated in vitro mainly in 3T3-L1 adipocyte cells. However, in vivo experiments investigating this connection in liver tissue have not been carried out. The objective of the present study was to investigate the effects of a high-cholesterol diet on caveolin-1 expression and IR localization and activity in the rat liver. Compared with a standard diet, rats fed with diet rich in cholesterol significantly altered liver caveolae by increasing both caveolin-1 (66%, P < 0.05) and caveolin-2 (55%, P < 0.05) expression while caveolin-1 mRNA levels were reduced. Concomitantly, a 25% increase in localization of the caveolae-resident signaling protein IR was observed. The distribution of caveolar and noncaveolar phosphorylated IR was unaffected but insulin-induced IR activation was significantly enhanced following consumption of the high-cholesterol diet (120%, P < 0.001). However, the downstream molecules IRS-1 and Akt have shown impaired activity in cholesterol-fed rats suggesting insulin resistance condition. Insulin stimulation failed to induce Tyr phosphorylation of caveolin-1 in cholesterol-fed rats. These findings suggest a mechanism by which a high-cholesterol diet altered caveolin-1 expression in vivo accompanied by altered IR localization and activity.

Adenoviral low density lipoprotein receptor attenuates progression of atherosclerosis and decreases tissue cholesterol levels in a murine model of familial hypercholesterolemia

Familial hypercholesterolemia is an autosomal codominant disease characterized by high concentrations of pro-atherogenic lipoproteins and premature atherosclerosis secondary to low density lipoprotein receptor (LDLr) deficiency. In the current study, the effects of gene transfer with 5 x 10(10) particles of E1E3E4-deleted adenoviral vectors expressing the LDLr (AdLDLr) or VLDLr (AdVLDLr) under control of the hepatocyte-specific human alpha(1)-antitrypsin promoter and 4 copies of the human apo E enhancer in C57BL/6 LDLr(-/-) mice were investigated. Evaluation was performed in both sexes and in mice fed either standard chow or an atherogenic diet containing 0.2% cholesterol and 10% coconut oil. Compared to control mice, AdLDLr and AdVLDLr persistently decreased plasma non-HDL cholesterol in both sexes and on both diets. Six months after LDLr gene transfer in mice fed the atherogenic diet, average intimal area was 2.5-fold (p<0.01) and 3.2-fold (p<0.001) lower in male and female mice, respectively, compared to controls. In mice fed standard chow, intimal area was reduced 22-fold (p<0.001) and 21-fold (p<0.001) after LDLr gene transfer in male and female mice, respectively. We show that non-HDL lipoproteins are more atherogenic in female mice, independent of sex differences of plasma HDL cholesterol levels, and that saturated fat does not have an effect on atherosclerosis independent of plasma cholesterol levels. Finally, quantification of tissue cholesterol levels indicates that AdLDLr does not induce cholesterol accumulation in the liver and reduces cholesterol content in the myocardium, quadriceps muscle and kidney. In conclusion, hepatocyte-specific LDLr gene transfer significantly improves cholesterol homeostasis in LDLr(-/-) mice.

Essential role for membrane lipid rafts in interleukin-1beta-induced nitric oxide release from insulin-secreting cells: potential regulation by caveolin-1+

We recently reported that the activation of H-Ras represents one of the signaling steps underlying the interleukin-1beta (IL-1beta)-mediated metabolic dysfunction of the islet beta-cell. In the present study, we examined potential contributory roles of membrane-associated, cholesterol-enriched lipid rafts/caveolae and their constituent proteins (e.g., caveolin-1 [Cav-1]) as potential sites for IL-1beta-induced nitric oxide (NO) release in the isolated beta-cell. Disruption of lipid rafts (e.g., with cyclodextrin) markedly reduced IL-1beta-induced gene expression of inducible NO synthase (iNOS) and NO release from beta-cells. Immunologic and confocal microscopic evidence also suggested a transient but significant stimulation of tyrosine phosphorylation of Cav-1 in beta-cells briefly (for 15 min) exposed to IL-1beta that was markedly attenuated by three structurally distinct inhibitors of protein tyrosine phosphorylation. Overexpression of an inactive mutant of Cav-1 lacking the tyrosine phosphorylation site (Y14F) or an siRNA-mediated Cav-1 knock down also resulted in marked attenuation of IL-1beta-induced iNOS gene expression and NO release from these cells, thus further implicating Cav-1 in this signaling cascade. IL-1beta treatment also increased (within 20 min) the translocation of H-Ras into lipid rafts. Here we provide the first evidence to suggest that tyrosine phosphorylation of Cav-1 and subsequent interaction among members of the Ras signaling pathway within the membrane lipid microdomains represent early signaling mechanisms of IL-1beta in beta-cells.

Sterol-responsive element-binding protein (SREBP) 2 down-regulates ATP-binding cassette transporter A1 in vascular endothelial cells: a novel role of SREBP in regulating cholesterol metabolism

ATP-binding cassette transporter A1 (ABCA1) is a pivotal regulator of cholesterol efflux from cells to apolipoproteins, whereas sterol-responsive element-binding protein 2 (SREBP2) is the key protein regulating cholesterol synthesis and uptake. We investigated the regulation of ABCA1 by SREBP2 in vascular endothelial cells (ECs). Our results showed that sterol depletion activated SREBP2 and increased its target, low density lipoprotein receptor mRNA, with a concurrent decrease in the ABCA1 mRNA. Transient transfection analysis revealed that sterol depletion decreased the ABCA1 promoter activity by 50%, but low density lipoprotein receptor promoter- and the sterol-responsive element-driven luciferase activities were increased. Overexpression of the N terminus of SREBP2 (SREBP2(N)), an active form of SREBP2, also inhibited the ABCA1 promoter activity. Functionally adenovirus-mediated SREBP2(N) expression increased cholesterol accumulation and decreased apoA-I-mediated cholesterol efflux. The conserved E-box motif was responsible for the SREBP2(N)-mediated inhibition since mutation of the E-box increased the basal activity of the ABCA1 promoter and abolished the inhibitory effect of SREBP2(N). Furthermore sterol depletion and SREBP2(N) overexpression induced the binding of SREBP2(N) to both consensus and ABCA1-specific E-box. Chromatin immunoprecipitation assay demonstrated that serum starvation enhanced the association of SREBP2 and the ABCA1 promoter in ECs. To correlate this mechanism pathophysiologically, we found that oscillatory flow caused the activation of SREBP2 and therefore attenuated ABCA1 promoter activity in ECs. Thus, this SREBP-regulated mechanism may control the efflux of cholesterol, which is a newly defined function of SREBP2 in ECs in addition to its role in cholesterol uptake and biosynthesis.

n-3 PUFA alter caveolae lipid composition and resident protein localization in mouse colon

Caveolae, by virtue of their unique lipid environment, serve as signaling platforms that regulate cellular events. Perturbations in caveolae lipid composition have been shown in vitro to displace proteins from lipid microdomains, thereby altering their functionality and subsequent downstream signaling. Because membrane remodeling may not be accurately represented by using pharmacological treatments and in vitro models, we investigated the in vivo ability of dietary n-3 polyunsaturated fatty acids (PUFA) to alter caveolae lipid environment and the compartmentalization of resident proteins in mouse colonic mucosa. n-3 PUFA were examined for their chemoprotective, membrane lipid-modifying properties. Colonic caveolae in mice fed n-6 or n-3 PUFA enriched diets were characteristically enriched in cholesterol, sphingomyelin, and caveolin-1. n-3 PUFA feeding, compared with n-6 PUFA, significantly altered colonic caveolae microenvironment by increasing phospholipid n-3 fatty acyl content and reducing both cholesterol (by 46%) and caveolin-1 (by 53%), without altering total cellular levels. Concomitantly, localization of caveolae-resident signaling proteins H-Ras and eNOS in colonic caveolae was decreased by n-3 PUFA, by 45 and 56%, respectively. The distribution of non-caveolae proteins K-Ras and clathrin was unaffected. Moreover, EGF-stimulated H-Ras, but not K-Ras activation was significantly suppressed following n-3 PUFA feeding, in parallel with the selective alterations in their microlocalization. These findings reveal a novel modality by which n-3 PUFA remodel membrane microdomains in vivo and thereby alter caveolae protein localization and functionality.

Low density lipoprotein induces eNOS translocation to membrane caveolae: the role of RhoA activation and stress fiber formation

A decrease in the bioavailability of endothelium-derived nitric oxide (NO) is linked to hypercholesterolemia. However, the mechanism by which low density lipoprotein (LDL) mediates endothelial NO synthase (eNOS) dysfunction remains controversial. We investigate the effect of LDL on eNOS regulation in human endothelial cells (ECs). In cultured ECs, a high level of LDL increased the abundance of eNOS and caveolin-1 (Cav-1) in the membrane caveolae and the association of eNOS with Cav-1. Furthermore, it decreased the basal level of NO and blocked NO production stimulated by the calcium ionophore A23187. LDL exposure also increased the formation of stress fibers and the membrane translocation of eNOS. These effects can be blocked by cytochalasin D, an actin cytoskeleton disruptor. In revealing the mechanism underlying the translocation of eNOS, we found that a high level of LDL increased the level of membrane-associated and GTP-formed RhoA and activated the RhoA downstream kinase ROCK-1 activity. Y-27632, a specific inhibitor of ROCK-1, blocked LDL-induced stress fiber formation, eNOS translocation and NO production. In conclusion, a high level of LDL increases the movement of eNOS to membrane caveolae via the increased stress fibers. The RhoA-mediated pathway may play a crucial role in this process in vascular ECs.

Caveolin interacts with the angiotensin II type 1 receptor during exocytic transport but not at the plasma membrane

The mechanisms involved in angiotensin II type 1 receptor (AT1-R) trafficking and membrane localization are largely unknown. In this study, we examined the role of caveolin in these processes. Electron microscopy of plasma membrane sheets shows that the AT1-R is not concentrated in caveolae but is clustered in cholesterol-independent microdomains; upon activation, it partially redistributes to lipid rafts. Despite the lack of AT1-R in caveolae, AT1-R.caveolin complexes are readily detectable in cells co-expressing both proteins. This interaction requires an intact caveolin scaffolding domain because mutant caveolins that lack a functional caveolin scaffolding domain do not interact with AT1-R. Expression of an N-terminally truncated caveolin-3, CavDGV, that localizes to lipid bodies, or a point mutant, Cav3-P104L, that accumulates in the Golgi mislocalizes AT1-R to lipid bodies and Golgi, respectively. Mislocalization results in aberrant maturation and surface expression of AT1-R, effects that are not reversed by supplementing cells with cholesterol. Similarly mutation of aromatic residues in the caveolin-binding site abrogates AT1-R cell surface expression. In cells lacking caveolin-1 or caveolin-3, AT1-R does not traffic to the cell surface unless caveolin is ectopically expressed. This observation is recapitulated in caveolin-1 null mice that have a 55% reduction in renal AT1-R levels compared with controls. Taken together our results indicate that a direct interaction with caveolin is required to traffic the AT1-R through the exocytic pathway, but this does not result in AT1-R sequestration in caveolae. Caveolin therefore acts as a molecular chaperone rather than a plasma membrane scaffold for AT1-R.

Relationship between cholesterol trafficking and signaling in rafts and caveolae

Caveolae and lipid rafts are two distinct populations of free cholesterol, sphingolipid (FC/SPH)-rich cell surface microdomains. They differ in stability, shape, and the presence or absence of caveolin (present in caveolae) or GPI-anchored proteins (enriched in lipid rafts). In primary cells, caveolae and rafts support the assembly of different signaling complexes, though signal transduction from both is strongly dependent on the presence of FC. It was initially thought that FC promoted the formation of inactive reservoirs of signaling proteins. Recent data supports the concept of a more dynamic role for FC in caveolae and probably, also lipid rafts. It is more likely that the FC content of these domains is actively modulated as protein complexes are formed and, following signal transduction, disassembled. In transformed cell lines with few caveolae, little caveolin and a preponderance of rafts, complexes normally assembled on caveolae may function in rafts, albeit with altered kinetics. However, caveolae and lipid rafts appear not to be interconvertible. The presence of non-caveolar pools of caveolin in recycling endosomes (RE), the trans-Golgi network (TGN) and in mobile chaperone complexes is now recognized. A role in the uptake of microorganisms by cells ascribed to caveolae now seems more likely to be mediated by cell surface rafts.

Endothelin-1 activates mesangial cell ERK1/2 via EGF-receptor transactivation and caveolin-1 interaction

Endothelin-1 (ET-1) stimulates glomerular mesangial cell proliferation and extracellular matrix protein transcription through an ERK1/2-dependent pathway. In this study, we determined whether ET-1 activation of glomerular mesangial cell ERK1/2 is mediated through EGF receptor (EGF-R) transactivation and whether intact caveolae are required. We showed that ET-1 stimulated tyrosine phosphorylation of the EGF-R in primary cultured, growth-arrested rat mesangial cells. In response to ET-1, ERK1/2 phosphorylation was increased by 27 +/- 1-fold and attenuated by AG-1478, a specific EGF-R inhibitor, to 9 +/- 1-fold. Moreover, filipin III and beta-cyclodextrin, two cholesterol-depleting drugs known to disrupt caveolae, significantly reduced ET-1-induced phosphorylation of ERK1/2. In addition, preincubation of mesangial cells with a myristoylated peptide that binds to the caveolin-1 scaffolding domain diminished ET-1 activation of ERK1/2. ET-1 caused interaction of caveolin-1 with phosphorylated ERK1/2 identified by coimmunoprecipitation. Activation of ERK1/2 and its interaction with caveolin-1 were reduced by AG-1478, beta-cyclodextrin, or inhibition of PKC. Phosphorylated ERK1/2 localized at focal adhesion complexes along with phospho-caveolin-1, suggesting specific sites of compartmentalization of these signaling molecules. Hence, ET-1 activates mesangial cell ERK1/2 predominantly through a pathway involving EGF-R transactivation, leading to a mechanism involving attachment to caveolin-1, presumably in caveolae.

Small-sized low-density lipoproteins of subclass B from patients with end-stage renal disease effectively augment tumor necrosis factor-alpha-induced adhesive properties in human endothelial cells

Increased prevalence of small-sized low-density lipoprotein (LDL) subclass B (diameter < 25.5 nm) possibly is involved in the multifactorial process of cardiovascular disease in patients with end-stage renal disease. Given these epidemiological observations, mechanisms underlying the combined effect of a proinflammatory insult and LDL of different subclasses (subclass A, diameter > 25.5 nm, and subclass B) in a cellular model were investigated. For this, human umbilical vein endothelial cells were preexposed to LDL, then stimulated with tumor necrosis factor-alpha (TNF-alpha). Modulatory effects of LDL phenotypes on the activation of adhesion molecules, monocyte adherence, and transcriptional activity of nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) were investigated. Our data show that subclass B LDLs were metabolized through nonspecific scavenger receptors and specific LDL-receptor pathways in endothelial cells. Furthermore, LDL subclass B in comparison to subclass A more effectively enhanced monocyte recruitment and adhesive properties of endothelial cells in response to TNF-alpha. These effects appeared not to be mediated by oxidative stress-responsive NF-kappaB because modulation of this transcription factor by LDL was moderate and similar for both LDL phenotypes. Conversely, effects of LDL subclass B were considered to be caused by augmented AP-1 binding activity. In conclusion, the present model provides new clues in atherogenic mechanisms of small-sized LDLs, which sensitize vascular cells to inflammatory signals more effectively than normal-sized LDLs.

Caveolae and caveolin in immune cells: distribution and functions

Caveolae are small, cholesterol-rich, hydrophobic membrane domains, characterized by the presence of the protein caveolin and involved in several cellular processes, including clathrin-independent endocytosis, the regulation and transport of cellular cholesterol, and signal transduction. Recently, caveolae have been identified as providing a novel route by which several pathogens are internalized by antigen-presenting cells and as centers for signal transduction. Here, we review the distribution and role of caveolae and caveolin in mammalian immune cells.

Caveolae and signaling

Caveolae, cholesterol-rich invaginations of the plasma membrane, have been implicated as scaffolds where signaling complexes are assembled, and as portals to which recycling or newly synthesized free cholesterol is transported prior to efflux or redistribution. New data indicate that these functions may be related; membrane cholesterol content can regulate receptor-mediated signal transduction, while signals responding to membrane cholesterol levels may reach the nucleus via a parallel kinase-dependent pathway. This information from both sources may be integrated at the nuclear level to control complex biological functions such as locomotion and cell division.