Simvastatin Downregulates the SARS-CoV-2-Induced Inflammatory Response and Impairs Viral Infection Through Disruption of Lipid Rafts

Coronavirus disease 2019 (COVID-19) is currently a worldwide emergency caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). In observational clinical studies, statins have been identified as beneficial to hospitalized patients with COVID-19. However, experimental evidence of underlying statins protection against SARS-CoV-2 remains elusive. Here we reported for the first-time experimental evidence of the protective effects of simvastatin treatment both in vitro and in vivo. We found that treatment with simvastatin significantly reduced the viral replication and lung damage in vivo, delaying SARS-CoV-2-associated physiopathology and mortality in the K18-hACE2-transgenic mice model. Moreover, simvastatin also downregulated the inflammation triggered by SARS-CoV-2 infection in pulmonary tissue and in human neutrophils, peripheral blood monocytes, and lung epithelial Calu-3 cells in vitro, showing its potential to modulate the inflammatory response both at the site of infection and systemically. Additionally, we also observed that simvastatin affected the course of SARS-CoV-2 infection through displacing ACE2 on cell membrane lipid rafts. In conclusion, our results show that simvastatin exhibits early protective effects on SARS-CoV-2 infection by inhibiting virus cell entry and inflammatory cytokine production, through mechanisms at least in part dependent on lipid rafts disruption.

Role of Lipid Rafts in Pathogen-Host Interaction - A Mini Review

Lipid rafts, also known as microdomains, are important components of cell membranes and are enriched in cholesterol, glycophospholipids and receptors. They are involved in various essential cellular processes, including endocytosis, exocytosis and cellular signaling. Receptors are concentrated at lipid rafts, through which cellular signaling can be transmitted. Pathogens exploit these signaling mechanisms to enter cells, proliferate and egress. However, lipid rafts also play an important role in initiating antimicrobial responses by sensing pathogens via clustered pathogen-sensing receptors and triggering downstream signaling events such as programmed cell death or cytokine production for pathogen clearance. In this review, we discuss how both host and pathogens use lipid rafts and associated proteins in an arms race to survive. Special attention is given to the involvement of the major vault protein, the main constituent of a ribonucleoprotein complex, which is enriched in lipid rafts upon infection with vaccinia virus.

Role of Membrane Lipid Rafts in MRP4 (ABCC4) Dependent Regulation of the cAMP Pathway in Blood Platelets

BACKGROUND

Platelet cytosolic cyclic adenosine monophosphate (cAMP) levels are balanced by synthesis, degradation, and efflux. Efflux can occur via multidrug resistant protein-4 (MRP4; ABCC4) present on dense granule and/or plasma membranes. As lipid rafts have been shown to interfere on cAMP homeostasis, we evaluated the relationships between the distribution and activity of MRP4 in lipid rafts and cAMP efflux.

METHODS

Platelet activation and cAMP homeostasis were analyzed in human and wild-type or MRP4-deleted mouse platelets in the presence of methyl-β-cyclodextrin (MßCD) to disrupt lipid rafts, and of activators of the cAMP signalling pathways. Human platelet MRP4 and effector proteins of the cAMP pathway were analyzed by immunoblots in lipid rafts isolated by differential centrifugation.

RESULTS

MßCD dose dependently inhibited human and mouse platelet aggregation without affecting per se cAMP levels. An additive inhibitory effect existed between the adenylate cyclase (AC) activator forskolin and MßCD that was accompanied by an overincrease of cAMP, and which was significantly enhanced upon MRP4 deletion. Finally, an efflux of cAMP out of resting platelets incubated with prostaglandin E1 (PGE1) was observed that was partly dependent on MRP4. Lipid rafts contained a small fraction (≈15%) of MRP4 and most of the inhibitory G-protein Gi, whereas Gs protein, AC3, and phosphodiesterases PDE2 and PDE3A were all present as only trace amounts.

CONCLUSION

Our results are in favour of part of MRP4 present at the platelet surface, including in lipid rafts. Lipid raft integrity is necessary for cAMP signalling regulation, although MRP4 and most players of cAMP homeostasis are essentially located outside rafts.

Lipid raft-disrupting miltefosine preferentially induces the death of colorectal cancer stem-like cells

BACKGROUND

Lipid rafts (LRs), cholesterol-enriched microdomains on cell membranes, are increasingly viewed as signalling platforms governing critical facets of cancer progression. The phenotype of cancer stem-like cells (CSCs) presents significant hurdles for successful cancer treatment, and the expression of several CSC markers is associated with LR integrity. However, LR implications in CSCs remain unclear.

METHODS

This study evaluated the biological and molecular functions of LRs in colorectal cancer (CRC) by using an LR-disrupting alkylphospholipid (APL) drug, miltefosine. The mechanistic role of miltefosine in CSC inhibition was examined through normal or tumour intestinal mouse organoid, human CRC cell, CRC xenograft and miltefosine treatment gene expression profile analyses.

RESULTS

Miltefosine suppresses CSC populations and their self-renewal activities in CRC cells, a CSC-targeting effect leading to irreversible disruption of tumour-initiating potential in vivo. Mechanistically, miltefosine reduced the expression of a set of genes, leading to stem cell death. Among them, miltefosine transcriptionally inhibited checkpoint kinase 1 (CHEK1), indicating that LR integrity is essential for CHEK1 expression regulation. In isolated CD44high CSCs, we found that CSCs exhibited stronger therapy resistance than non-CSC counterparts by preventing cell death through CHEK1-mediated cell cycle checkpoints. However, inhibition of the LR/CHEK1 axis by miltefosine released cell cycle checkpoints, forcing CSCs to enter inappropriate mitosis with accumulated DNA damage and resulting in catastrophic cell death.

CONCLUSION

Our findings underscore the therapeutic potential of LR-targeting APLs for CRC treatment that overcomes the therapy-resistant phenotype of CSCs, highlighting the importance of the LR/CHEK1 axis as a novel mechanism of APLs.

Force Mapping Study of Actinoporin Effect in Membranes Presenting Phase Domains

Equinatoxin II (EqtII) and Fragaceatoxin C (FraC) are pore-forming toxins (PFTs) from the actinoporin family that have enhanced membrane affinity in the presence of sphingomyelin (SM) and phase coexistence in the membrane. However, little is known about the effect of these proteins on the nanoscopic properties of membrane domains. Here, we used combined confocal microscopy and force mapping by atomic force microscopy to study the effect of EqtII and FraC on the organization of phase-separated phosphatidylcholine/SM/cholesterol membranes. To this aim, we developed a fast, high-throughput processing tool to correlate structural and nano-mechanical information from force mapping. We found that both proteins changed the lipid domain shape. Strikingly, they induced a reduction in the domain area and circularity, suggesting a decrease in the line tension due to a lipid phase height mismatch, which correlated with proteins binding to the domain interfaces. Moreover, force mapping suggested that the proteins affected the mechanical properties at the edge, but not in the bulk, of the domains. This effect could not be revealed by ensemble force spectroscopy measurements supporting the suitability of force mapping to study local membrane topographical and mechanical alterations by membranotropic proteins.

Peiminine Attenuates Acute Lung Injury Induced by LPS Through Inhibiting Lipid Rafts Formation

Acute lung injury (ALI) is a kind of lung serious disease which leads to the damage of alveolar epithelial cells and capillary endothelial. Lipopolysaccharide (LPS) is one of the common factors inducing ALI. The previous study has reported that the anti-inflammatory effect of peiminine, but little is known about its effect on the ALI induced by LPS. The aim of this study is to investigate the therapeutic effect of peiminine on LPS-induced acute lung injury and potential mechanisms. Mice were given LPS through nasal cavity to establish ALI model, and then the peiminine (1, 3, or 5 mg/kg) was injected into the mice as the experimental group. In the present study, we would measure the W/D ratio, activity of MPO, the histopathological changes, and the levels of cytokines. The results showed that peiminine could reduce the W/D ratio and the MPO activity significantly. Furthermore, the histopathological changes and the expression of TNF-α, IL-1β, and IL-6 were inhibited after the peiminine treatment. In vitro, peiminine significantly inhibited LPS-induced IL-8 production in A549 lung epithelial cells. Meanwhile, the activity of NF-κB signaling pathway was suppressed obviously by peiminine with the western blot analysis. Also, peiminine significantly attenuated LPS-induced AKT and PI3K phosphorylation. In addition, peiminine was found to disrupt lipid rafts formation by attenuating the cholesterol content. In conclusion, peiminine could attenuate LPS-induced ALI in mice and it may become a new approach to treat ALI.

Effect of High-Density Lipoprotein from Healthy Subjects and Chronic Kidney Disease Patients on the CD14 Expression on Polymorphonuclear Leukocytes

In uremic patients, high-density lipoprotein (HDL) loses its anti-inflammatory features and can even become pro-inflammatory due to an altered protein composition. In chronic kidney disease (CKD), impaired functions of polymorphonuclear leukocytes (PMNLs) contribute to inflammation and an increased risk of cardiovascular disease. This study investigated the effect of HDL from CKD and hemodialysis (HD) patients on the CD14 expression on PMNLs. HDL was isolated using a one-step density gradient centrifugation. Isolation of PMNLs was carried out by discontinuous Ficoll-Hypaque density gradient centrifugation. CD14 surface expression was quantified by flow cytometry. The activity of the small GTPase Rac1 was determined by means of an activation pull-down assay. HDL increased the CD14 surface expression on PMNLs. This effect was more pronounced for HDL isolated from uremic patients. The acute phase protein serum amyloid A (SAA) caused higher CD14 expression, while SAA as part of an HDL particle did not. Lipid raft disruption with methyl-β-cyclodextrin led to a reduced CD14 expression in the absence and presence of HDL. HDL from healthy subjects but not from HD patients decreased the activity of Rac1. Considering the known anti-inflammatory effects of HDL, the finding that even HDL from healthy subjects increased the CD14 expression was unexpected. The pathophysiological relevance of this result needs further investigation.

Early differences in membrane properties at the neuromuscular junctions of ALS model mice: Effects of 25-hydroxycholesterol

AIMS

Plasma hyperlipidemia is a protective factor in amyotrophic lateral sclerosis (ALS) while cholesterol-lowering drugs aggravate the pathology. We hypothesize that this phenomenon can be linked with membrane lipid alterations in the neuromuscular junctions (NMJs) occurring before motor neuron loss.

METHODS

Neurotransmitter release in parallel with lipid membrane properties in diaphragm NMJs of SOD1G93A (mSOD) mice at nine weeks of age (pre-onset stage) were assessed.

KEY FINDINGS

Despite on slight changes in spontaneous and evoked quantum release of acetylcholine, extracellular levels of choline at resting conditions, an indicator of non-quantum release, were significantly increased in mSOD mice. The use of lipid-sensitive fluorescent probes points to lipid raft disruption in the NMJs of mSOD mice. However, content of cholesterol, a key raft component was unchanged implying another pathway responsible for the loss of raft integrity. In the mSOD mice we found marked increase in levels of raft-destabilizing lipid ceramide. This was accompanied by enhanced ability to uptake of exogenous ceramide in NMJs. Acute and chronic administration of 25-hydroxycholesterol, whose levels increase due to hypercholesterolemia, recovered early alterations in membrane properties. Furthermore, chronic treatment with 25-hydroxycholesterol prevented increase in ceramide and extracellular choline levels as well as suppressed lipid peroxidation of NMJ membranes and fragmentation of end plates.

SIGNIFICANCE

Thus, lipid raft disruption likely due to ceramide accumulation could be early event in ALS which may trigger neuromuscular abnormalities. Cholesterol derivative 25-hydroxycholesterol may serve as a molecule restoring the membrane and functional properties of NMJs at the early stage.

Shiga Toxin (Stx)-Binding Glycosphingolipids of Primary Human Renal Cortical Epithelial Cells (pHRCEpiCs) and Stx-Mediated Cytotoxicity

Human kidney epithelial cells are supposed to be directly involved in the pathogenesis of the hemolytic–uremic syndrome (HUS) caused by Shiga toxin (Stx)-producing enterohemorrhagic Escherichia coli (EHEC). The characterization of the major and minor Stx-binding glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer), respectively, of primary human renal cortical epithelial cells (pHRCEpiCs) revealed GSLs with Cer (d18:1, C16:0), Cer (d18:1, C22:0), and Cer (d18:1, C24:1/C24:0) as the dominant lipoforms. Using detergent-resistant membranes (DRMs) and non-DRMs, Gb3Cer and Gb4Cer prevailed in the DRM fractions, suggesting their association with microdomains in the liquid-ordered membrane phase. A preference of Gb3Cer and Gb4Cer endowed with C24:0 fatty acid accompanied by minor monounsaturated C24:1-harboring counterparts was observed in DRMs, whereas the C24:1 fatty acid increased in relation to the saturated equivalents in non-DRMs. A shift of the dominant phospholipid phosphatidylcholine with saturated fatty acids in the DRM to unsaturated species in the non-DRM fractions correlated with the GSL distribution. Cytotoxicity assays gave a moderate susceptibility of pHRCEpiCs to the Stx1a and Stx2a subtypes when compared to highly sensitive Vero-B4 cells. The results indicate that presence of Stx-binding GSLs per se and preferred occurrence in microdomains do not necessarily lead to a high cellular susceptibility towards Stx.

Lipid Raft Association Stabilizes VEGF Receptor 2 in Endothelial Cells

The binding of vascular endothelial growth factor A (VEGF) to VEGF receptor-2 (VEGFR-2) stimulates angiogenic signaling. Lipid rafts are cholesterol-dense regions of the plasma membrane that serve as an organizational platform for biomolecules. Although VEGFR2 has been shown to colocalize with lipid rafts to regulate its activation, the effect of lipid rafts on non-activated VEGFR2 has not been explored. Here, we characterized the involvement of lipid rafts in modulating the stability of non-activated VEGFR2 in endothelial cells using raft disrupting agents: methyl-β-cyclodextrin, sphingomyelinase and simvastatin. Disrupting lipid rafts selectively decreased the levels of non-activated VEGFR2 as a result of increased lysosomal degradation. The decreased expression of VEGFR2 translated to reduced VEGF-activation of the extracellular signal-regulated protein kinases (ERK). Overall, our results indicate that lipid rafts stabilize VEGFR2 and its associated signal transduction activities required for angiogenesis. Thus, modulation of lipid rafts may provide a means to regulate the sensitivity of endothelial cells to VEGF stimulation. Indeed, the ability of simvastatin to down regulate VEGFR2 and inhibit VEGF activity suggest a potential mechanism underlying the observation that this drug improves outcomes in the treatment of certain cancers.

Genkwadaphnin inhibits growth and invasion in hepatocellular carcinoma by blocking DHCR24-mediated cholesterol biosynthesis and lipid rafts formation

BACKGROUND

The liver is the central organ for cholesterol homoeostasis, and its dysfunction might cause liver pathological alterations including hepatocellular carcinomas (HCCs). 3β-hydroxysteroid-Δ24 reductase (DHCR24), a crucial enzyme of cholesterol biosynthetic pathway, is involved in lipid rafts formation. Genkwadaphnin (GD) is a daphnane diterpene isolated from the flower buds of Daphne genkwa Siebold et Zuccarini (Thymelaeaceae).

METHODS

We evaluated in vitro and in vivo effect of GD using HCC cells and BALB/c nude mice. Microarray assays were used to identify the differential genes by GD. DHCR24 expression and activity, cholesterol level, lipid rafts structure and the role of DHCR24 in human HCC specimens were tested by various molecular biology techniques.

RESULTS

High expression of DHCR24 in human HCC specimens was correlated with poor clinical outcome. Interfering DHCR24 altered growth and migration of HCC cells. GD inhibited growth and metastasis of HCC cells both in vivo and in vitro. GD suppressed DHCR24 expression and activity, as well as DHCR24-mediated cholesterol biosynthesis and lipid rafts formation, then further inhibited HCC cell invasion and migration.

CONCLUSIONS

Our data suggest that DHCR24-mediated cholesterol metabolism might be an effective therapeutic strategy in HCC, and natural product GD might be a promising agent for HCC therapy.

Enforced C-Src Activation Causes Compartmental Dysregulation of PI3K and PTEN Molecules in Lipid Rafts of Tongue Squamous Carcinoma Cells by Attenuating Rac1-Akt-GLUT-1-Mediated Sphingolipid Synthesis

Pharmacologic intervention to affect the membrane lipid homeostasis of lipid rafts is a potent therapeutic strategy for cancer. Here we showed that gallic acid (GA) caused the complex formation of inactive Ras-related C3 botulinum toxin substrate 1 (Rac1)-phospho (p)-casein kinase 2 α (CK2α) (Tyr 255) in human tongue squamous carcinoma (TSC) cells, which disturbed the lipid raft membrane-targeting of phosphatidylinositol 3-kinase (PI3K)-Rac1-protein kinase B (Akt) signal molecules by inducing the association of p110α-free p85α with unphosphorylated phosphatase tensin homolog deleted on chromosome 10 (PTEN) in lipid rafts. The effects on induction of inactive Rac1-p-CK2α (Tyr 255) complex formation and attenuation of p-Akt (Ser 473), GTP-Rac1, glucose transporter-1 (GLUT-1) lipid raft membrane-targeting, and cell invasive activity by GA were counteracted either by CK2α short hairpin RNA or cellular-Src (c-Src) inhibitor PP1. PP1 treatment, GLUT-1 or constitutively active Rac1 ectopic-expression blocked GA-induced decreases in cellular glucose, sphingolipid and cholesterol of lipid raft membranes, p85α-p110α-GTP-Rac1 complexes, glucosylceramide synthase activity and increase in ceramide and p110α-free p85α-PTEN complex levels of lipid raft membranes, which reversed the inhibition on matrix metalloproteinase (MMP)-2/-9-mediated cell invasion induced by GA. Using transient ectopic expression of nuclear factor-kappa B (NF-κB) p65, MMP-2/-9 promoter-driven luciferase, and NF-κB-dependent luciferase reporter genes and NF-κB specific inhibitors or Rac1 specific inhibitor NSC23766, we confirmed that an attenuation of Rac1 activity by GA confers inhibition of NF-κB-mediated MMP-2/-9 expression and cell invasion. In conclusion, GA-induced c-Src activation is a key inductive event for the formation of inactive Rac1-p-CK2α (Tyr 255) complexes, which disturbed lipid raft compartment of PI3K and PTEN molecules by impairing Akt-regulated GLUT-1-mediated sphingolipid synthesis, and finally resulting in inhibition of TSC cell invasion.

The antioxidant vitamin E as a membrane raft modulator: Tocopherols do not abolish lipid domains

The antioxidant vitamin E is a commonly used vitamin supplement. Although the multi-billion dollar vitamin and nutritional supplement industry encourages the use of vitamin E, there is very little evidence supporting its actual health benefits. Moreover, vitamin E is now marketed as a lipid raft destabilizing anti-cancer agent, in addition to its antioxidant behaviour. Here, we studied the influence of vitamin E and some of its vitamers on membrane raft stability using phase separating unilamellar lipid vesicles in conjunction with small-angle scattering techniques and fluorescence microscopy. We find that lipid phase behaviour remains unperturbed well beyond physiological concentrations of vitamin E (up to a mole fraction of 0.10). Our results are consistent with a proposed line active role of vitamin E at the domain boundary. We discuss the implications of these findings as they pertain to lipid raft modification in native membranes, and propose a new hypothesis for the antioxidant mechanism of vitamin E.

10-Gingerol Targets Lipid Rafts Associated PI3K/Akt Signaling in Radio-Resistant Triple Negative Breast Cancer Cells

10-gingerol is a major phenolic lipid found in the rhizomes of ginger (Zingiber officinale). Being amphiphilic in nature, phenolic lipids have the ability to incorporate into cell membranes and modulate membrane properties. The purpose of the present study was to evaluate the effects of 10-gingerol on lipid raft/membrane raft modulation in radio-resistant triple negative breast cancer (MDA-MB-231/IR) cells. The effects of 10-gingerol on MDA-MB-231/IR cells' proliferation, clonogenic growth, migration, and invasion were assayed using MTT, colony formation, cell migration, and invasion assays, respectively. Sucrose density gradient centrifugation was used to extract lipid rafts. Western blotting and immunofluorescence were employed to assess the effects of 10-gingerol on lipid raft/membrane raft modulation and lipid rafts-associated PI3K/Akt signaling. Cholesterol measurements were carried out using a commercially available kit. 10-gingerol suppressed the proliferation, migration, invasion, and induced apoptosis through targeting the PI3K/Akt signaling pathway in MDA-MB-231/IR cells. Moreover, 10-gingerol was found to modulate the lipid rafts of MDA-MB-231/IR cells and attenuate the key PI3K/Akt signaling components in lipid rafts. The cholesterol content of the lipid rafts and rafts-resident Akt signaling were also affected by exposure to 10-gingerol. The results of the present study highlight rafts-associated PI3K/Akt signaling as a new target of 10-gingerol in MDA-MB-231/IR cells, thus rationalizing a new rafts-mediated treatment approach for radio-resistant triple negative breast cancer cells.

Fast chemical force microscopy demonstrates that glycopeptidolipids define nanodomains of varying hydrophobicity on mycobacteria

Mycobacterium abscessus is an emerging multidrug-resistant bacterial pathogen causing severe lung infections in cystic fibrosis patients. A remarkable trait of this mycobacterial species is its ability to form morphologically smooth (S) and rough (R) colonies. The S-to-R transition is caused by the loss of glycopeptidolipids (GPLs) in the outer layer of the cell envelope and correlates with an increase in cording and virulence. Despite the physiological and medical importance of this morphological transition, whether it involves changes in cell surface properties remains unknown. Herein, we combine recently developed quantitative imaging (QI) atomic force microscopy (AFM) with hydrophobic tips to quantitatively map the surface structure and hydrophobicity of M. abscessus at high spatiotemporal resolution, and to assess how these properties are modulated by the S-to-R transition and by treatment with an inhibitor of the mycolic acid transporter MmpL3. We discover that loss of GPLs leads to major modifications in surface hydrophobicity, without any apparent change in cell surface ultrastructure. While R bacilli are homogeneously hydrophobic, S bacilli feature unusual variations of nanoscale hydrophobic properties. These previously undescribed cell surface nanodomains are likely to play critical roles in bacterial adhesion, aggregation, phenotypic heterogeneity and transmission, and in turn in virulence and pathogenicity. Our study also suggests that MmpL3 inhibitors show promise in nanomedicine as chemotherapeutic agents to interfere with the highly hydrophobic nature of the mycobacterial cell wall. The advantages of QI-AFM with hydrophobic tips are the ability to map chemical and structural properties simultaneously and at high resolution, applicable to a wide range of biosystems.

Effect of Erufosine on Membrane Lipid Order in Breast Cancer Cell Models

Alkylphospholipids are a novel class of antineoplastic drugs showing remarkable therapeutic potential. Among them, erufosine (EPC3) is a promising drug for the treatment of several types of tumors. While EPC3 is supposed to exert its function by interacting with lipid membranes, the exact molecular mechanisms involved are not known yet. In this work, we applied a combination of several fluorescence microscopy and analytical chemistry approaches (i.e., scanning fluorescence correlation spectroscopy, line-scan fluorescence correlation spectroscopy, generalized polarization imaging, as well as thin layer and gas chromatography) to quantify the effect of EPC3 in biophysical models of the plasma membrane, as well as in cancer cell lines. Our results indicate that EPC3 affects lipid-lipid interactions in cellular membranes by decreasing lipid packing and increasing membrane disorder and fluidity. As a consequence of these alterations in the lateral organization of lipid bilayers, the diffusive dynamics of membrane proteins are also significantly increased. Taken together, these findings suggest that the mechanism of action of EPC3 could be linked to its effects on fundamental biophysical properties of lipid membranes, as well as on lipid metabolism in cancer cells.

Lipid rafts as a therapeutic target

Lipid rafts regulate the initiation of cellular metabolic and signaling pathways by organizing the pathway components in ordered microdomains on the cell surface. Cellular responses regulated by lipid rafts range from physiological to pathological, and the success of a therapeutic approach targeting "pathological" lipid rafts depends on the ability of a remedial agent to recognize them and disrupt pathological lipid rafts without affecting normal raft-dependent cellular functions. In this article, concluding the Thematic Review Series on Biology of Lipid Rafts, we review current experimental therapies targeting pathological lipid rafts, including examples of inflammarafts and clusters of apoptotic signaling molecule-enriched rafts. The corrective approaches include regulation of cholesterol and sphingolipid metabolism and membrane trafficking by using HDL and its mimetics, LXR agonists, ABCA1 overexpression, and cyclodextrins, as well as a more targeted intervention with apoA-I binding protein. Among others, we highlight the design of antagonists that target inflammatory receptors only in their activated form of homo- or heterodimers, when receptor dimerization occurs in pathological lipid rafts. Other therapies aim to promote raft-dependent physiological functions, such as augmenting caveolae-dependent tissue repair. The overview of this highly dynamic field will provide readers with a view on the emerging concept of targeting lipid rafts as a therapeutic strategy.jlr;61/5/687/F1F1f1.

The inhibitory effect of ECG and EGCG dimeric procyanidins on colorectal cancer cells growth is associated with their actions at lipid rafts and the inhibition of the epidermal growth factor receptor signaling

Colorectal cancer (CRC) is one of the most common cancers worldwide. Epidemiological studies indicate that consumption of fruits and vegetables containing procyanidins is associated with lower CRC risk. This study investigated the capacity of two dimeric procyanidins composed of epicatechin gallate (ECG) or epigallocatechin gallate (EGCG) isolated from persimmons, to inhibit CRC cell growth and promote apoptosis, characterizing the underlying mechanisms. ECG and EGCG dimers reduced the growth of five human CRC cell lines in a concentration (10-60 μM)- and time (24-72 h)-dependent manner, with a 72 h-IC50 value in Caco-2 cells of 10 and 30 μM, respectively. ECG and EGCG dimers inhibited Caco-2 cell proliferation by arresting the cell cycle in G2/M phase and by inducing apoptosis via the mitochondrial pathway. In addition, ECG and EGCG dimers inhibited cell migration, invasion, and adhesion, decreasing the activity of matrix metalloproteinases (MMP-2/9). Mechanistically, ECG and EGCG dimers inhibited the activation of lipid raft-associated epidermal growth factor (EGF) receptor (EGFR), without affecting its localization at lipid rafts. In particular, ECG and EGCG dimers reduced EGFR phosphorylation at Tyr1068 residue, prevented EGFR dimerization and activation upon stimulation, and induced EGFR internalization both in the absence and presence of EGF. Furthermore, ECG and EGCG dimers increased EGFR phosphorylation at Tyr1045 residue, providing a docking site for ubiquitin ligase c-Cbl and induced EGFR degradation by the proteasome. Downstream of EGFR, ECG and EGCG dimers inhibited the activation of the MEK/ERK1/2 and PI3K/AKT signaling pathways, downregulating proteins involved in the modulation of cell survival. In conclusion, ECG and EGCG dimers reduced CRC cell growth by inhibiting EGFR activation at multiple steps, including the disruption of lipid rafts integrity and promoting EGFR degradation. These results shed light on a potential molecular mechanism on how procyanidins-rich diets may lower CRC risk.

Role of Death Receptors-associated Lipid Rafts in Oxaliplatin-induced Death Mode Regulation of HepG2 Cells

BACKGROUND/AIM

We previously showed that oxaliplatin induces necrotic-like cell death in hepatocarcinomas, and combination with ursodexoycholic acid (UDCA) significantly shifts the necrotic-like death to apoptosis. Since cell death mode is crucial on inflammatory responses and chemotherapeutic efficacy, the mechanism underlying determination of cell death mode by UDCA was investigated in this study.

MATERIALS AND METHODS

Apoptosis or necrosis was determined by apoptotic body formation, caspase-8 activity, LDH release and PI inclusion. The involvement of lipid rafts and death receptors was examined by rafts fractionation, confocal microscopy and gene silencing assays.

RESULTS

UDCA combination enhanced recruitment of death receptors and adaptors into cholesterol-enriched lipid rafts, and induced a stronger raft clustering. Lipid raft disruption decreased the UDCA/oxaliplatin-mediated apoptosis and increased necrotic-like death.

CONCLUSION

UDCA promotes lipid raft localization of multiple death receptors, thereby contributing to a shift of cell death mode from oxaliplatin-induced necrotic death to apoptosis in HepG2 cells.

Omega-3 polyunsaturated fatty acids impinge on CD4+ T cell motility and adipose tissue distribution via direct and lipid mediator-dependent effects

AIMS

Adaptive immunity contributes to the pathogenesis of cardiovascular metabolic disorders (CVMD). The omega-3 polyunsaturated fatty acids (n-3PUFA) are beneficial for cardiovascular health, with potential to improve the dysregulated adaptive immune responses associated with metabolic imbalance. We aimed to explore the mechanisms through which n-3PUFA may alter T cell motility and tissue distribution to promote a less inflammatory environment and improve lymphocyte function in CVMD.

METHODS AND RESULTS

Using mass spectrometry lipidomics, cellular, biochemical, and in vivo and ex vivo analyses, we investigated how eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the main n-3PUFA, modify the trafficking patterns of activated CD4+ T cells. In mice subjected to allogeneic immunization, a 3-week n-3PUFA-enriched diet reduced the number of effector memory CD4+ T cells found in adipose tissue, and changed the profiles of eicosanoids, octadecanoids, docosanoids, endocannabinoids, 2-monoacylglycerols, N-acyl ethanolamines, and ceramides, in plasma, lymphoid organs, and fat tissues. These bioactive lipids exhibited differing chemotactic properties when tested in chemotaxis assays with activated CD4+ T cells in vitro. Furthermore, CD4+ T cells treated with EPA and DHA showed a significant reduction in chemokinesis, as assessed by trans-endothelial migration assays, and, when implanted in recipient mice, demonstrated less efficient migration to the inflamed peritoneum. Finally, EPA and DHA treatments reduced the number of polarized CD4+ T cells in vitro, altered the phospholipid composition of membrane microdomains and decreased the activity of small Rho GTPases, Rhoα, and Rac1 instrumental in cytoskeletal dynamics.

CONCLUSIONS

Our findings suggest that EPA and DHA affect the motility of CD4+ T cells and modify their ability to reach target tissues by interfering with the cytoskeletal rearrangements required for cell migration. This can explain, at least in part, the anti-inflammatory effects of n-3PUFA supporting their potential use in interventions aiming to address adipocyte low-grade inflammation associated with cardiovascular metabolic disease.

Impairment of Membrane Lipid Homeostasis by Bichalcone Analog TSWU-BR4 Attenuates Function of GRP78 in Regulation of the Oxidative Balance and Invasion of Cancer Cells

The specialized cholesterol/sphingolipid-rich membrane domains termed lipid rafts are highly dynamic in the cancer cells, which rapidly assemble effector molecules to form a sorting platform essential for oncogenic signaling transduction in response to extra- or intracellular stimuli. Density-based membrane flotation, subcellular fractionation, cell surface biotinylation, and co-immunoprecipitation analyses of bichalcone analog ((E)-1-(4-Hydroxy-3-((4-(4-((E)-3-(pyridin-3-yl)acryloyl)phenyl)piperazin-1-yl)methyl)phenyl)-3-(pyridin-3-yl)prop-2-en-1-one (TSWU-BR4)-treated cancer cells showed dissociation between GRP78 and p85α conferring the recruitment of PTEN to lipid raft membranes associated with p85α. Ectopic expression of GRP78 could overcome induction of lipid raft membrane-associated p85α–unphosphorylated PTEN complex formation and suppression of GRP78−PI3K−Akt−GTP-Rac1-mediated and GRP78-regulated PERK−Nrf2 antioxidant pathway and cancer cell invasion by TSWU-BR4. Using specific inducer, inhibitor, or short hairpin RNA for ASM demonstrated that induction of the lipid raft membrane localization and activation of ASM by TSWU-BR4 is responsible for perturbing homeostasis of cholesterol and ceramide levels in the lipid raft and ER membranes, leading to alteration of GRP78 membrane trafficking and subsequently inducing p85α–unphosphorylated PTEN complex formation, causing disruption of GRP78−PI3K−Akt−GTP-Rac1-mediated signal and ER membrane-associated GRP78-regulated oxidative stress balance, thus inhibiting cancer cell invasion. The involvement of the enrichment of ceramide to lipid raft membranes in inhibition of NF-κB-mediated MMP-2 expression was confirmed through attenuation of NF-κB activation using C2-ceramide, NF-κB specific inhibitors, ectopic expression of NF-κB p65, MMP-2 promoter-driven luciferase, and NF-κB-dependent reporter genes. In conclusion, localization of ASM in the lipid raft membranes by TSWU-BR4 is a key event for initiating formation of ceramide-enriched lipid raft membrane platforms, which causes delocalization of GRP78 from the lipid raft and ER membranes to the cytosol and formation of p85α–unphosphorylated PTEN complexes to attenuate the GRP78-regulated oxidative stress balance and GRP78−p85α−Akt−GTP-Rac1−NF-κB−MMP-2-mediated cancer cell invasion.

The Interaction between Amyloid Prefibrillar Oligomers of Salmon Calcitonin and a Lipid-Raft Model: Molecular Mechanisms Leading to Membrane Damage, Ca2+-Influx and Neurotoxicity

To investigate the interaction between amyloid assemblies and “lipid-rafts”, we performed functional and structural experiments on salmon calcitonin (sCT) solutions rich in prefibrillar oligomers, proto- and mature-fibers interacting with liposomes made of monosialoganglioside-GM1 (4%), DPPC (48%) and cholesterol (48%). To focus on the role played by electrostatic forces and considering that sCT is positive and GM1 is negative at physiologic pH, we compared results with those relative to GM1-free liposomes while, to assess membrane fluidity effects, with those relative to cholesterol-free liposomes. We investigated functional effects by evaluating Ca²⁺-influx in liposomes and viability of HT22-DIFF neurons. Only neurotoxic solutions rich in unstructured prefibrillar oligomers were able to induce Ca²⁺-influx in the “lipid-rafts” model, suggesting that the two phenomena were correlated. Thus, we investigated protein conformation and membrane modifications occurring during the interaction: circular dichroism showed that “lipid-rafts” fostered the formation of β-structures and energy filtered-transmission electron microscopy that prefibrillar oligomers formed pores, similar to Aβ did. We speculate that electrostatic forces between the positive prefibrillar oligomers and the negative GM1 drive the initial binding while the hydrophobic profile and flexibility of prefibrillar oligomers, together with the membrane fluidity, are responsible for the subsequent pore formation leading to Ca²⁺-influx and neurotoxicity.

The Bile Acid-Phospholipid Conjugate Ursodeoxycholyl-Lysophosphatidylethanolamide (UDCA-LPE) Disintegrates the Lipid Backbone of Raft Plasma Membrane Domains by the Removal of the Membrane Phospholipase A2

The bile acid-phospholipid conjugate ursodeoxycholyl-lysophosphatidylethanolamide (UDCA-LPE) was shown to have anti-inflammatory, antisteatotic, and antifibrotic properties, rendering it as a drug targeting non-alcoholic steatohepatitis (NASH). On a molecular level, it disrupted the heterotetrameric fatty acid uptake complex localized in detergent-resistant membrane domains of the plasma membrane (DRM-PM). However, its mode of action was unclear. Methodologically, UDCA-LPE was incubated with the liver tumor cell line HepG2 as well as their isolated DRM-PM and all other cellular membranes (non-DRM). The membrane cholesterol and phospholipids were quantified as well as the DRM-PM protein composition by Western blotting. The results show a loss of DRM-PM by UDCA-LPE (50 µM) with a 63.13 ± 7.14% reduction of phospholipids and an 81.94 ± 8.30% reduction of cholesterol in relation to mg total protein. The ratio of phospholipids to cholesterol changed from 2:1 to 4:1, resembling those of non-DRM fractions. Among the members of the fatty acid uptake complex, the calcium-independent membrane phospholipase A2 (iPLA₂β) abandoned DRM-PM most rapidly. As a consequence, the other members of this transport system disappeared as well as the DRM-PM anchored fibrosis regulating proteins integrin β-1 and lysophospholipid receptor 1 (LPAR-1). It is concluded that UDCA-LPE executes its action by iPLA₂β removal from DRM-PM and consequent dissolution of the raft lipid platform.

Effects of Omega-3 Fatty Acids on Immune Cells

Alterations on the immune system caused by omega-3 fatty acids have been described for 30 years. This family of polyunsaturated fatty acids exerts major alterations on the activation of cells from both the innate and the adaptive immune system, although the mechanisms for such regulation are diverse. First, as a constitutive part of the cellular membrane, omega-3 fatty acids can regulate cellular membrane properties, such as membrane fluidity or complex assembly in lipid rafts. In recent years, however, a new role for omega-3 fatty acids and their derivatives as signaling molecules has emerged. In this review, we describe the latest findings describing the effects of omega-3 fatty acids on different cells from the immune system and their possible molecular mechanisms.

Peripheral Antimicrobial Peptide Gomesin Induces Membrane Protrusion, Folding, and Laceration

Optical microscopy shows that the peripheral antimicrobial peptide (AMP) gomesin does not disrupt the bacterial membrane by forming stable transmembrane pores but induces lipid accumulation domains, which is followed by a sudden burst near the domains. The molecular action mechanisms of gomesin on vesicle and planar bilayer membranes are investigated in this work using coarse-grained molecular dynamics simulations. By comparing the membrane morphology and property changes induced by gomesin and the pore-forming AMP melittin, we determined that the amphiphilic shape of the AMPs is a key factor affecting the mechanism of cell death. The binding of wedge-shaped gomesin, with a small hydrophobic surface, onto the membrane induces protrusion and folding of the outer monolayer followed by sudden membrane lacerations at the axillae of the protuberances. Alternatively, cylinder-shaped melittins with comparable hydrophilic and hydrophobic surfaces destroy membranes by forming stable pores coexisting with exocytosis-like buddings and endocytosis-like invaginations. The multiple actions of AMPs on the bacterial membrane suggest diverse paradigms for designing molecular carriers for delivering drugs to the cell.

Allicin Inhibited Staphylococcus aureus -Induced Mastitis by Reducing Lipid Raft Stability via LxRα in Mice

Mastitis, inflammation of the mammary gland, occurs in both humans and animals. Staphylococcus aureus is the most common infectious bacterial pathogen associated with mastitis. We investigated the effects of allicin on S. aureus-induced mastitis in mice. Pathological histology revealed that allicin inhibited S. aureus-induced pathological damage and myeloperoxidase activity in mammary tissues. Enzyme-linked immunosorbent assays demonstrated that allicin reduced the production of IL-1β and TNF-α as well as inhibited the NF-κB and mitogen-activated protein kinase pathway by reducing phosphorylation of p65, IκBα, p38, JNK, and ERK. Western blotting revealed that allicin reduced TLR2 and TLR6 expression in mammary tissues and cells but not in HEK293 cells. The lipid raft content was reduced by allicin, which inhibited signaling downstream of TLR2 and TLR6. Liver X receptor α (LXRα) luciferase reporter assays and LXRα interference experiments showed that allicin improved the LXRα activity and adenosine 5'-triphosphate-binding cassette G and A1 (ABCG and ABCA1) expression, thereby reducing the cholesterol level, lipid raft formation, and downstream TLR2 and TLR6 pathway activity. These results demonstrated that allicin exerted anti-inflammatory effects against S. aureus mastitis by improving the LXRα activity and reducing lipid raft formation.

AIMP1 regulates TCR signaling and induces differentiation of regulatory T cells by interfering with lipid raft association

In addition to a role in translation, AIMP1 is secreted to affect various immune cells, such as macrophages, dendritic cells, B cells, and natural killer cells. However, the direct effects of AIMP1 on T cells have not yet been reported. In this study, we investigated whether AIMP1 could modulate T cell responses directly. Results revealed that AIMP1 significantly inhibited T cell receptor (TCR)-dependent activation and proliferation of CD4 T cells, as well as decreased TCR stimuli-induced Ca²⁺ influx in CD4 T cells. In addition, microscopic analysis revealed that lipid raft association in response to TCR engagement was significantly reduced in the presence of AIMP1, and the phosphorylation of PLCγ and PI3K was also down-regulated in CD4 T cells by AIMP1. Furthermore, AIMP1 specifically enhanced the differentiation of regulatory T (Treg) cells, while it had no effect on T helper type 1 (Th1), type 2 (Th2), and type 17 (Th17) cell differentiation. Collectively, these results indicate that AIMP1 affects T cells directly by down-regulating TCR signaling complex formation and inducing Treg cell differentiation in CD4 T cells.

Shenmai injection maintains blood-brain barrier integrity following focal cerebral ischemia via modulating the expression and trafficking of occludin in lipid rafts

ETHNOPHARMACOLOGICAL RELEVANCE

Shenmai injection (SMI), a traditional Chinese herbal medicine is widely used for the clinical treatment of cerebral infarction in China.

AIM OF THE STUDY

Tight junctions (TJs) are major components of the blood-brain barrier (BBB) that physically restrict the paracellular diffusion of blood-borne substances between endothelial cells into the CNS. TJ proteins are associated with cholesterol-enriched regions of plasma membrane known as lipid rafts, which are critical for the trafficking, positioning and function of TJ proteins. In this study, we investigated the effect of SMI on the expression and trafficking of the key TJ-associated protein, occludin, in lipid rafts.

MATERIALS AND METHODS

Using a neutral pH, rat cerebral microvessels were subjected to detergent-free density-gradient fractionation to isolate lipid rafts containing occludin. Transmission electron microscopy (TEM) was performed to study the effects of drug administration on ultrastructural changes to TJs. Western blotting (WB), immunofluorescence (IF), and co-immunoprecipitation (COIP) were used to observe the localization and function of TJ-associated proteins.

RESULTS

We successfully isolated cerebral microvessels and separated lipid rafts from plasma membranes. With SMI treatment, extravasation of FITC-albumin decreased around the cerebral vessels by IF, the tight junctions were found to still be intact and the basement membrane appeared to be of uniform thickness in TEM. Compared with the untreated group, the co-expression of flotillin-1 and occludin in microvascular endothelial cells was increased and distributed continuously in SMI treatment as shown in double label IF. SMI significantly increased the translocation of occludin to lipid raft fractions by WB and COIP.

CONCLUSIONS

SMI helps maintain the proper assembly of the TJ multiprotein complex in lipid rafts, thereby helping to preserve BBB functional integrity during focal cerebral ischemic insult. Our findings enhance our understanding of the mechanisms underlying the neuroprotective effect of SMI in cerebral ischemia.

Structural and Functional Determinants of AC8 Trafficking, Targeting and Responsiveness in Lipid Raft Microdomains

The fidelity of cAMP in controlling numerous cellular functions rests crucially on the precise organization of cAMP microdomains that are sustained by the scaffolding properties of adenylyl cyclase. Earlier studies suggested that AC8 enriches in lipid rafts where it interacts with cytoskeletal elements. However, these are not stable structures and little is known about the dynamics of AC8 secretion and its interactions. The present study addresses the role of the cytoskeleton in maintaining the AC8 microenvironment, particularly in the context of the trafficking route of AC8 and its interaction with caveolin1. Here, biochemical and live-cell imaging approaches expose a complex, dynamic interaction between AC8 and caveolin1 that affects AC8 processing, targeting and responsiveness in plasma membrane lipid rafts. Site-directed mutagenesis and pharmacological approaches reveal that AC8 is processed with complex N-glycans and associates with lipid rafts en route to the plasma membrane. A dynamic picture emerges of the trafficking and interactions of AC8 while travelling to the plasma membrane, which are key to the organization of the AC8 microdomain.

Inhibition of the IGF-1-PI3K-Akt-mTORC2 pathway in lipid rafts increases neuronal vulnerability in a genetic lysosomal glycosphingolipidosis

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper.

Rosmarinic acid and its esters inhibit membrane cholesterol domain formation through an antioxidant mechanism based, in nonlinear fashion, on alkyl chain length

BACKGROUND

Under conditions of oxidative stress, cholesterol aggregates into discrete membrane bilayer domains that precipitate the formation of extracellular crystals, a feature of advanced atheroma in cardiovascular disease. Therapeutic interventions using membrane-directed antioxidants, such as polyphenolic esters, may reduce cholesterol domains and crystal formation. In this study, the effects of rosmarinic acid (RC0) and rosmarinic esters, with alkyl chain lengths ranging from 4 to 16‑carbons (RC4-RC16), on membrane lipid oxidation and cholesterol domain formation were investigated.

METHODS

Model membranes were prepared with 1,2-dilinoleoyl-sn-glycero-3-phosphocholine and cholesterol at different cholesterol-to-phospholipid mole ratios (0.3:1, 0.9:1, and 1.2:1), in the absence or presence of each molecule and exposed to 72 h of oxidation. Changes in lipid hydroperoxide (LOOH) and cholesterol domain formation were measured using iodometric and small angle x-ray diffraction approaches, respectively.

RESULTS

Rosmarinic acid and its esters had differential effects on LOOH formation based on alkyl chain length. RC8 exhibited the greatest antioxidant effect, reducing LOOH levels by 82%, and inhibited cholesterol domain formation. By contrast, RC0 and RC16 failed to inhibit either LOOH formation or cholesterol domain formation.

CONCLUSION

These data indicate that the membrane antioxidant and cholesterol domain inhibition activities of rosmarinic acid esters are dependent, nonlinearly, on alkyl chain length. The mechanism for this effect is attributed to the influence of alkyl chain length on the optimal depth of the polyphenols into the lipid bilayer for trapping free radicals.

GENERAL SIGNIFICANCE

These findings provide insight into novel atheroprotective benefits of polyphenol esters that are dependent on their membrane location.

Cellular and Molecular Mechanisms Mediated by recPrPC Involved in the Neuronal Differentiation Process of Mesenchymal Stem Cells

Human Dental Pulp Stem Cells (hDPSCs) represent a type of adult mesenchymal stem cells that have the ability to differentiate in vitro in several lineages such as odontoblasts, osteoblasts, chondrocytes, adipocytes and neurons. In the current work, we used hDPSCs as the experimental model to study the role of recombinant prion protein 23⁻231 (recPrPC) in the neuronal differentiation process, and in the signal pathway activation of ERK 1/2 and Akt. We demonstrated that recPrPC was able to activate an intracellular signal pathway mediated by extracellular-signal-regulated kinase 1 and 2 (ERK 1/2) and protein kinase B (Akt). Moreover, in order to understand whether endogenous prion protein (PrPC) was necessary to mediate the signaling induced by recPrPC, we silenced PrPC, demonstrating that the presence of endogenous PrPC was essential for ERK 1/2 and Akt phosphorylation. Since endogenous PrPC is a well-known lipid rafts component, we evaluated the role of these structures in the signal pathway induced by recPrPC. Our results suggest that lipid rafts integrity play a key role in recPrPC activity. In fact, lipid rafts inhibitors, such as fumonisin B1 and MβCD, significantly prevented ERK 1/2 and Akt phosphorylation induced by recPrPC. In addition, we investigated the capacity of recPrPC to induce hDPSCs neuronal differentiation process after long-term stimulation through the evaluation of typical neuronal markers expression such as B3-Tubulin, neurofilament-H (NFH) and growth associated protein 43 (GAP43). Accordingly, when we silenced endogenous PrPC, we observed the inhibition of neuronal differentiation induced by recPrPC. The combined data suggest that recPrPC plays a key role in the neuronal differentiation process and in the activation of specific intracellular signal pathways in hDPSCs.

The actions of volatile anesthetics: a new perspective

This article reviews recent work in applying neutron and X-ray scattering towards the elucidation of the molecular mechanisms of volatile anesthetics. Experimental results on domain mixing in ternary lipid mixtures, and the influence of volatile anesthetics and hydrostatic pressure are placed in the contexts of ion-channel function and receptor trafficking at the postsynaptic density.

Docoxahexaenoic Acid Induces Apoptosis of Pancreatic Cancer Cells by Suppressing Activation of STAT3 and NF-κB

The ω3-polyunsaturated fatty acid docosahexenoic acid (DHA) is known to induce apoptosis of cancer cells. In this study, DHA was shown to reduce viability of pancreatic cancer cells (PANC-1) by inducing DNA fragmentation, activating caspase-3, and increasing the ratio of Bax/Bcl-2. To determine the DHA mechanism of action, the impact of DHA on the activation of the key signaling proteins epidermal growth factor receptor (EGFR), signal transducer and activator of transcription factor 3 (STAT3), nuclear transcription factor-κB (NF-κB), and IκBα in PANC-1 cells was probed. The observed DHA suppression of NF-κB DNA-binding activity was found to result from reduced IκBα phosphorylation. The observed DHA-induced suppression of STAT3 activation was found to be the result of suppressed EGFR activation, which derives from the inhibitory effect of DHA on the integrity of localization of EGFR to cell membrane lipid rafts. Since the activation of STAT3 and NF-κB mediates the expression of survival genes cyclin D1 and survivin, DHA induced apoptosis by suppressing the STAT3/NF-κB-cyclin D1/survivin axis. These results support the proposal that DHA-induced apoptosis of pancreatic cells occurs via disruption of key pro-cell survival signaling pathways. We suggest that the consumption of DHA-enriched foods could decrease the incidence of pancreatic cancer.

Nuclear Lipid Microdomains Regulate Daunorubicin Resistance in Hepatoma Cells

Daunorubicin is an anticancer drug, and cholesterol is involved in cancer progression, but their relationship has not been defined. In this study, we developed a novel experimental model that utilizes daunorubicin, cholesterol, and daunorubicin plus cholesterol in the same cells (H35) to search for the role of nuclear lipid microdomains, rich in cholesterol and sphingomyelin, in drug resistance. We find that the daunorubicin induces perturbation of nuclear lipid microdomains, localized in the inner nuclear membrane, where active chromatin is anchored. As changes of sphingomyelin species in nuclear lipid microdomains depend on neutral sphingomyelinase activity, we extended our studies to investigate whether the enzyme is modulated by daunorubicin. Indeed the drug stimulated the sphingomyelinase activity that induced reduction of saturated long chain fatty acid sphingomyelin species in nuclear lipid microdomains. Incubation of untreated-drug cells with high levels of cholesterol resulted in the inhibition of sphingomyelinase activity with increased saturated fatty acid sphingomyelin species. In daunodubicin-treated cells, incubation with cholesterol reversed the action of the drug by acting via neutral sphingomyelinase. In conclusion, we suggest that cholesterol and sphingomyelin-forming nuclear lipid microdomains are involved in the drug resistance.

Manchette-acrosome disorders and testicular efficiency decline observed in hypercholesterolemic rabbits are recovered with olive oil enriched diet

High-fat diet is associated with hypercholesterolemia and seminal alterations in White New Zealand rabbits. We have previously reported disorders in the development of the manchette-acrosome complex during spermiogenesis and decreased testicular efficiency in hypercholesterolemic rabbits. On the other hand, olive oil incorporated into the diet improves cholesterolemia and semen parameters affected in hypercholesterolemic rabbits. In this paper, we report the recovery—with the addition of olive oil to diet—from the sub-cellular mechanisms involved in the shaping of the sperm cell and testicular efficiency altered in hypercholesterolemic rabbits. Using morphological (structural, ultra-structural and immuno-fluorescence techniques) and cell biology techniques, a reorganization of the manchette and related structures was observed when olive oil was added to the high-fat diet. Specifically, actin filaments, microtubules and lipid rafts—abnormally distributed in hypercholesterolemic rabbits—were recovered with dietary olive oil supplementation. The causes of the decline in sperm count were studied in the previous report and here in more detail. These were attributed to the decrease in the efficiency index and also to the increase in the apoptotic percentage in testis from animals under the high-fat diet. Surprisingly, the addition of olive oil to the diet avoided the sub-cellular, efficiency and apoptosis changes observed in hypercholesterolemic rabbits. This paper reports the positive effects of the olive oil addition to the diet in the recovery of testicular efficiency and normal sperm shaping, mechanisms altered by hypercholesterolemia.

Membrane Remodeling as a Key Player of the Hepatotoxicity Induced by Co-Exposure to Benzo[a]pyrene and Ethanol of Obese Zebrafish Larvae

The rise in prevalence of non-alcoholic fatty liver disease (NAFLD) constitutes an important public health concern worldwide. Including obesity, numerous risk factors of NAFLD such as benzo[a]pyrene (B[a]P) and ethanol have been identified as modifying the physicochemical properties of the plasma membrane in vitro thus causing membrane remodeling—changes in membrane fluidity and lipid-raft characteristics. In this study, the possible involvement of membrane remodeling in the in vivo progression of steatosis to a steatohepatitis-like state upon co-exposure to B[a]P and ethanol was tested in obese zebrafish larvae. Larvae bearing steatosis as the result of a high-fat diet were exposed to ethanol and/or B[a]P for seven days at low concentrations coherent with human exposure in order to elicit hepatotoxicity. In this condition, the toxicant co-exposure raised global membrane order with higher lipid-raft clustering in the plasma membrane of liver cells, as evaluated by staining with the fluoroprobe di-4-ANEPPDHQ. Involvement of this membrane’s remodeling was finally explored by using the lipid-raft disruptor pravastatin that counteracted the effects of toxicant co-exposure both on membrane remodeling and toxicity. Overall, it can be concluded that B[a]P/ethanol co-exposure can induce in vivo hepatotoxicity via membrane remodeling which could be considered as a good target mechanism for developing combination therapy to deal with steatohepatitis.

Dynamic cycling of t-SNARE acylation regulates platelet exocytosis

Platelets regulate vascular integrity by secreting a host of molecules that promote hemostasis and its sequelae. Given the importance of platelet exocytosis, it is critical to understand how it is controlled. The t-SNAREs, SNAP-23 and syntaxin-11, lack classical transmembrane domains (TMDs), yet both are associated with platelet membranes and redistributed into cholesterol-dependent lipid rafts when platelets are activated. Using metabolic labeling and hydroxylamine (HA)/HCl treatment, we showed that both contain thioester-linked acyl groups. Mass spectrometry mapping further showed that syntaxin-11 was modified on cysteine 275, 279, 280, 282, 283, and 285, and SNAP-23 was modified on cysteine 79, 80, 83, 85, and 87. Interestingly, metabolic labeling studies showed incorporation of [3H]palmitate into the t-SNAREs increased although the protein levels were unchanged, suggesting that acylation turns over on the two t-SNAREs in resting platelets. Exogenously added fatty acids did compete with [3H]palmitate for t-SNARE labeling. To determine the effects of acylation, we measured aggregation, ADP/ATP release, as well as P-selectin exposure in platelets treated with the acyltransferase inhibitor cerulenin or the thioesterase inhibitor palmostatin B. We found that cerulenin pretreatment inhibited t-SNARE acylation and platelet function in a dose- and time-dependent manner whereas palmostatin B had no detectable effect. Interestingly, pretreatment with palmostatin B blocked the inhibitory effects of cerulenin, suggesting that maintaining the acylation state is important for platelet function. Thus, our work shows that t-SNARE acylation is actively cycling in platelets and suggests that the enzymes regulating protein acylation could be potential targets to control platelet exocytosis in vivo.

The cellular uptake of angiogenin, an angiogenic and neurotrophic factor is through multiple pathways and largely dynamin independent

Angiogenin (ANG), a member of the RNase superfamily (also known as RNase 5) has neurotrophic, neuroprotective and angiogenic activities. Recently it has also been shown to be important in stem cell homeostasis. Mutations in ANG are associated with neurodegenerative diseases such as Amyotrophic Lateral Sclerosis (ALS) and Fronto-temporal dementia (FTD). ANG is a secreted protein which is taken up by cells and translocated to the nucleus. However, the import pathway/s through which ANG is taken up is/are still largely unclear. We have characterised the uptake of ANG in neuronal, astrocytic and microglial cell lines as well as primary neurons and astrocytes using pharmacological agents as well as dominant negative dynamin and Rab5 to perturb uptake and intracellular trafficking. We find that uptake of ANG is largely clathrin/dynamin independent and microtubule depolymerisation has a marginal effect. Perturbation of membrane ruffling and macropinocytosis significantly inhibited ANG uptake suggesting an uptake mechanism similar to RNase A. Our findings shed light on why mutations which do not overtly affect RNase activity but cause impaired localization are associated with neurodegenerative disease.

Dynamic membrane interactions of antibacterial and antifungal biomolecules, and amyloid peptides, revealed by solid-state NMR spectroscopy

A variety of biomolecules acting on the cell membrane folds into a biologically active structure in the membrane environment. It is, therefore, important to determine the structures and dynamics of such biomolecules in a membrane environment. While several biophysical techniques are used to obtain low-resolution information, solid-state NMR spectroscopy is one of the most powerful means for determining the structure and dynamics of membrane bound biomolecules such as antibacterial biomolecules and amyloidogenic proteins; unlike X-ray crystallography and solution NMR spectroscopy, applications of solid-state NMR spectroscopy are not limited by non-crystalline, non-soluble nature or molecular size of membrane-associated biomolecules. This review article focuses on the applications of solid-state NMR techniques to study a few selected antibacterial and amyloid peptides. Solid-state NMR studies revealing the membrane inserted bent α-helical structure associated with the hemolytic activity of bee venom melittin and the chemical shift oscillation analysis used to determine the transmembrane structure (with α-helix and 310-helix in the N- and C-termini, respectively) of antibiotic peptide alamethicin are discussed in detail. Oligomerization of an amyloidogenic islet amyloid polypeptide (IAPP, or also known as amylin) resulting from its aggregation in a membrane environment, molecular interactions of the antifungal natural product amphotericin B with ergosterol in lipid bilayers, and the mechanism of lipid raft formation by sphingomyelin studied using solid state NMR methods are also discussed in this review article. This article is part of a Special Issue entitled "Biophysical Exploration of Dynamical Ordering of Biomolecular Systems" edited by Dr. Koichi Kato.

The potent effect of mycolactone on lipid membranes

Mycolactone is a lipid-like endotoxin synthesized by an environmental human pathogen, Mycobacterium ulcerans, the causal agent of Buruli ulcer disease. Mycolactone has pleiotropic effects on fundamental cellular processes (cell adhesion, cell death and inflammation). Various cellular targets of mycolactone have been identified and a literature survey revealed that most of these targets are membrane receptors residing in ordered plasma membrane nanodomains, within which their functionalities can be modulated. We investigated the capacity of mycolactone to interact with membranes, to evaluate its effects on membrane lipid organization following its diffusion across the cell membrane. We used Langmuir monolayers as a cell membrane model. Experiments were carried out with a lipid composition chosen to be as similar as possible to that of the plasma membrane. Mycolactone, which has surfactant properties, with an apparent saturation concentration of 1 μM, interacted with the membrane at very low concentrations (60 nM). The interaction of mycolactone with the membrane was mediated by the presence of cholesterol and, like detergents, mycolactone reshaped the membrane. In its monomeric form, this toxin modifies lipid segregation in the monolayer, strongly affecting the formation of ordered microdomains. These findings suggest that mycolactone disturbs lipid organization in the biological membranes it crosses, with potential effects on cell functions and signaling pathways. Microdomain remodeling may therefore underlie molecular events, accounting for the ability of mycolactone to attack multiple targets and providing new insight into a single unifying mechanism underlying the pleiotropic effects of this molecule. This membrane remodeling may act in synergy with the other known effects of mycolactone on its intracellular targets, potentiating these effects.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.

ω-3 polyunsaturated fatty acids direct differentiation of the membrane phenotype in mesenchymal stem cells to potentiate osteogenesis

Mammalian cells produce hundreds of dynamically regulated lipid species that are actively turned over and trafficked to produce functional membranes. These lipid repertoires are susceptible to perturbations from dietary sources, with potentially profound physiological consequences. However, neither the lipid repertoires of various cellular membranes, their modulation by dietary fats, nor their effects on cellular phenotypes have been widely explored. We report that differentiation of human mesenchymal stem cells (MSCs) into osteoblasts or adipocytes results in extensive remodeling of the plasma membrane (PM), producing cell-specific membrane compositions and biophysical properties. The distinct features of osteoblast PMs enabled rational engineering of membrane phenotypes to modulate differentiation in MSCs. Specifically, supplementation with docosahexaenoic acid (DHA), a lipid component characteristic of osteoblast membranes, induced broad lipidomic remodeling in MSCs that reproduced compositional and structural aspects of the osteoblastic PM phenotype. The PM changes induced by DHA supplementation potentiated osteogenic differentiation of MSCs concurrent with enhanced Akt activation at the PM. These observations prompt a model wherein the DHA-induced lipidome leads to more stable membrane microdomains, which serve to increase Akt activity and thereby enhance osteogenic differentiation. More broadly, our investigations suggest a general mechanism by which dietary fats affect cellular physiology through remodeling of membrane lipidomes, biophysical properties, and signaling.

Extracellular reactive oxygen species are generated by a plasma membrane oxidative phosphorylation system

Although the oxidative phosphorylation (OXPHOS) system has been found in mitochondria and the plasma membrane of various mammalian cell lines, understanding the physiological functions of the plasma membrane OXPHOS system is challenging. Here, we demonstrated that OXPHOS I, II, III, IV and V subunits were expressed in the plasma membrane of HepG2 cells and primary mouse hepatocytes, as determined by non-permeabilized immunofluorescence, total internal reflection fluorescence (TIRF) microscopy, cell surface-biotin labeling and plasma membrane and lipid raft isolation. Next, we demonstrated that NADH administration generated extracellular superoxide and improved insulin signaling in HepG2 cells and primary mouse hepatocytes. The NADH-dependent generation of extracellular superoxide was prevented by knockdown of NDUFV-1, the first subunit of OXPHOS I receiving electrons from NADH and the NADH-improved insulin signaling was abolished by extracellular catalase. Thus, we conclude that the OXPHOS system in the plasma membrane may be required for the generation of extracellular ROS and the regulation of insulin signaling.

The in vivo structure of biological membranes and evidence for lipid domains

Examining the fundamental structure and processes of living cells at the nanoscale poses a unique analytical challenge, as cells are dynamic, chemically diverse, and fragile. A case in point is the cell membrane, which is too small to be seen directly with optical microscopy and provides little observational contrast for other methods. As a consequence, nanoscale characterization of the membrane has been performed ex vivo or in the presence of exogenous labels used to enhance contrast and impart specificity. Here, we introduce an isotopic labeling strategy in the gram-positive bacterium Bacillus subtilis to investigate the nanoscale structure and organization of its plasma membrane in vivo. Through genetic and chemical manipulation of the organism, we labeled the cell and its membrane independently with specific amounts of hydrogen (H) and deuterium (D). These isotopes have different neutron scattering properties without altering the chemical composition of the cells. From neutron scattering spectra, we confirmed that the B. subtilis cell membrane is lamellar and determined that its average hydrophobic thickness is 24.3 ± 0.9 Ångstroms (Å). Furthermore, by creating neutron contrast within the plane of the membrane using a mixture of H- and D-fatty acids, we detected lateral features smaller than 40 nm that are consistent with the notion of lipid rafts. These experiments-performed under biologically relevant conditions-answer long-standing questions in membrane biology and illustrate a fundamentally new approach for systematic in vivo investigations of cell membrane structure.

The Important Role of Lipid Raft-Mediated Attachment in the Infection of Cultured Cells by Coronavirus Infectious Bronchitis Virus Beaudette Strain

Lipid raft is an important element for the cellular entry of some viruses, including coronavirus infectious bronchitis virus (IBV). However, the exact role of lipid rafts in the cellular membrane during the entry of IBV into host cells is still unknown. In this study, we biochemically fractionated IBV-infected cells via sucrose density gradient centrifugation after depleting plasma membrane cholesterol with methyl-β-cyclodextrin or Mevastatin. Our results demonstrated that unlike IBV non-structural proteins, IBV structural proteins co-localized with lipid raft marker caveolin-1. Infectivity assay results of Vero cells illustrated that the drug-induced disruption of lipid rafts significantly suppressed IBV infection. Further studies revealed that lipid rafts were not required for IBV genome replication or virion release at later stages. However, the drug-mediated depletion of lipid rafts in Vero cells before IBV attachment significantly reduced the expression of viral structural proteins, suggesting that drug treatment impaired the attachment of IBV to the cell surface. Our results indicated that lipid rafts serve as attachment factors during the early stages of IBV infection, especially during the attachment stage.

HS-AFM and SERS Analysis of Murine Norovirus Infection: Involvement of the Lipid Rafts

Studies on human norovirus are severely hampered by the absence of a cell culture system until the discovery of murine norovirus (MNV). The cell membrane domains called lipid rafts have been defined as a port of entry for viruses. This study is conducted to investigate murine norovirus binding on the mouse leukemic monocyte macrophage cell line. Lipid raft related structures are extracted from cells by detergent treatment resulting detergent-resistant membrane (DRMs) domains. The real-time polymerase chain reaction technique is performed to detect the viral genome, thereby the MNV binding on the DRMs. The interactions between MNV and DRMs are investigated by high-speed atomic force microscopy (HS-AFM) combined with surface-enhanced Raman spectroscopy (SERS). The inoculation of the virus onto cells results in the aggregations of detergent-resistant membrane domains significantly. The characteristic Raman band of MNV is found in inoculated samples. To be sure that these results are originated from specific interactions between DRM and MNV, methyl-β-cyclo-dextrin (MβCD) is applied to disrupt lipid rafts. The MNV binding on DRMs is precluded by the MβCD treatment. The cholesterols chains are defined as a key factor in the interactions between norovirus and DRMs. The authors conclude that the MNV binding involves the presence of DRMs and cholesterol dependent.

Inhibition of Cholesterol Biosynthesis Reduces γ-Secretase Activity and Amyloid-β Generation

Amyloid-β (Aβ) is one of major molecules contributing to the pathogenesis of Alzheimer's disease (AD). Aβ is derived from amyloid-β protein precursor (AβPP) through sequential cleavages by β- and γ-secretases. Regulation of these components is thought to be an important factor in Aβ generation during the pathogenesis of AD. AβPP, β-secretase, and γ-secretase reside in lipid rafts, where cholesterol regulates the integrity and flexibility of membrane proteins and Aβ is generated. However, the relationship between cholesterol and Aβ generation is controversial. In this study, we aimed to elucidate the direct effects of cholesterol depletion on AβPP processing using AY9944, which blocks the last step of cholesterol biosynthesis and thus minimizes the unknown side effects of upstream inhibitors, such as HMG-CoA reductase inhibitors. Treatment with AY9944 decreased γ-secretase activity and Aβ generation. These results suggested that changes in membrane composition by lowering cholesterol with AY9944 affected γ-secretase activity and Aβ generation, which is associated with AD pathogenesis.

A Novel Role for Flotillin-Containing Lipid Rafts in Negative-Feedback Regulation of Thyroid-Specific Gene Expression by Thyroglobulin

BACKGROUND

Thyroglobulin (Tg) stored in thyroid follicles regulates follicular function in thyroid hormone (TH) synthesis by suppressing thyroid-specific gene expression in a concentration-dependent manner. Thus, Tg is an intrinsic negative-feedback regulator that can restrain the effect of thyrotropin (TSH) in the follicle. However, the underlying mechanisms by which Tg exerts its prominent autoregulatory effect following recognition by thyrocytes remains unclear.

METHODS

In order to identify potential proteins that recognize and interact with Tg, mass spectrometry was used to analyze immunoprecipitated Tg-bound proteins derived from Tg-treated rat thyroid FRTL-5 cells.

RESULTS

Flotillin 1 and flotillin 2, two homologs that are integral membrane proteins in lipid rafts, were identified as novel Tg-binding proteins with high confidence. Further studies revealed that flotillins physically interact with endocytosed Tg, and together these proteins redistribute from the cell membrane to cytoplasmic vesicles. Treatment with the lipid raft disrupter methyl-β-cyclodextrin abolished both the endocytosis and the negative-feedback effect of Tg on thyroid-specific gene expression. Meanwhile, siRNA-mediated knockdown of flotillin 1 or flotillin 2 also significantly inhibited Tg effects on gene expression.

CONCLUSION

Together these results indicate that flotillin-containing lipid rafts are essential for follicular Tg to be recognized by thyrocytes and exert its negative-feedback effects in the thyroid.