Epidermal growth factor receptors are localized to lipid rafts that contain a balance of inner and outer leaflet lipids: a shotgun lipidomics study

Arginine-Rich Cell-Penetrating Peptides Induce Lipid Rearrangements for Their Active Translocation across Laterally Heterogeneous Membranes

Arginine-rich cell-penetrating peptides (CPPs) have emerged as valuable tools for the intracellular delivery of bioactive molecules, but their membrane perturbation during cell penetration is not fully understood. Here, nona-arginine (R9)-mediated membrane reorganization that facilitates the translocation of peptides across laterally heterogeneous membranes is directly visualized. The electrostatic binding of cationic R9 to anionic phosphatidylserine (PS)-enriched domains on a freestanding lipid bilayer induces lateral lipid rearrangements; in particular, in real-time it is observed that R9 fluidizes PS-rich liquid-ordered (Lo) domains into liquid-disordered (Ld) domains, resulting in the membrane permeabilization. The experiments with giant unilamellar vesicles (GUVs) confirm the preferential translocation of R9 through Ld domains without pore formation, even when Lo domains are more negatively charged. Indeed, whenever R9 comes into contact with negatively charged Lo domains, it dissolves the Lo domains first, promoting translocation across phase-separated membranes. Collectively, the findings imply that arginine-rich CPPs modulate lateral membrane heterogeneity, including membrane fluidization, as one of the fundamental processes for their effective cell penetration across densely packed lipid bilayers.

Confinement energy landscape classification reveals membrane receptor nano-organization mechanisms

The cell membrane organization has an essential functional role through the control of membrane receptor confinement in micro- or nanodomains. Several mechanisms have been proposed to account for these properties, although some features have remained controversial, notably the nature, size, and stability of cholesterol- and sphingolipid-rich domains or lipid rafts. Here, we probed the effective energy landscape acting on single-nanoparticle-labeled membrane receptors confined in raft nanodomains- epidermal growth factor receptor (EGFR), Clostridium perfringens ε-toxin receptor (CPεTR), and Clostridium septicum α-toxin receptor (CSαTR)-and compared it with hop-diffusing transferrin receptors. By establishing a new analysis pipeline combining Bayesian inference, decision trees, and clustering approaches, we systematically classified single-protein trajectories according to the type of effective confining energy landscape. This revealed the existence of only two distinct organization modalities: confinement in a quadratic energy landscape for EGFR, CPεTR, and CSαTR (A), and free diffusion in confinement domains resulting from the steric hindrance due to F-actin barriers for transferrin receptor (B). The further characterization of effective confinement energy landscapes by Bayesian inference revealed the role of interactions with the domain environment in cholesterol- and sphingolipid-rich domains with (EGFR) or without (CPεTR and CSαTR) interactions with F-actin to regulate the confinement energy depth. These two distinct mechanisms result in the same organization type (A). We revealed that the apparent domain sizes for these receptor trajectories resulted from Brownian exploration of the energy landscape in a steady-state-like regime at a common effective temperature, independently of the underlying molecular mechanisms. These results highlight that confinement domains may be adequately described as interaction hotspots rather than rafts with abrupt domain boundaries. Altogether, these results support a new model for functional receptor confinement in membrane nanodomains and pave the way to the constitution of an atlas of membrane protein organization.

Lysophospholipid acyltransferase-mediated formation of saturated glycerophospholipids maintained cell membrane integrity for hypoxic adaptation

Adaptation to hypoxia has attracted much public interest because of its clinical significance. However, hypoxic adaptation in the body is complicated and difficult to fully explore. To explore previously unknown conserved mechanisms and key proteins involved in hypoxic adaptation in different species, we first used a yeast model for mechanistic screening. Further multi-omics analyses in multiple species including yeast, zebrafish and mice revealed that glycerophospholipid metabolism was significantly involved in hypoxic adaptation with up-regulation of lysophospholipid acyltransferase (ALE1) in yeast, a key protein for the formation of dipalmitoyl phosphatidylcholine [DPPC (16:0/16:0)], which is a saturated phosphatidylcholine. Importantly, a mammalian homolog of ALE1, lysophosphatidylcholine acyltransferase 1 (LPCAT1), enhanced DPPC levels at the cell membrane and exhibited the same protective effect in mammalian cells under hypoxic conditions. DPPC supplementation effectively attenuated growth restriction, maintained cell membrane integrity and increased the expression of epidermal growth factor receptor under hypoxic conditions, but unsaturated phosphatidylcholine did not. In agreement with these findings, DPPC treatment could also repair hypoxic injury of intestinal mucosa in mice. Taken together, ALE1/LPCAT1-mediated DPPC formation, a key pathway of glycerophospholipid metabolism, is crucial for cell viability under hypoxic conditions. Moreover, we found that ALE1 was also involved in glycolysis to maintain sufficient survival conditions for yeast. The present study offers a novel approach to understanding lipid metabolism under hypoxia and provides new insights into treating hypoxia-related diseases.

Methoxychalcones as potential anticancer agents for colon cancer: Is membrane perturbing potency relevant?

Chalcones are naturally produced by many plants, and constitute precursors for the synthesis of flavons and flavanons. They were shown to possess antibacterial, antifungal, anti-cancer, and anti- inflammatory properties. The goal of the study was to assess the suitability of three synthetic methoxychalcones as potential anticancer agents. In a panel of colon cancer cell lines they were demonstrated to be cytotoxic, proapoptotic, causing cell cycle arrest, and increasing intracellular level of reactive oxygen species. Anticancer activity of the compounds was not diminished in the presence of stool extract containing microbial enzymes that could change the structure of chalcones. Moreover, methoxychalcones interacted strongly with model phosphatidylcholine membranes as detected by differential scanning calorimetry. Metohoxychalcones particularly affected the properties of lipid domains in giant unilamellar liposomes formed from raft-mimicking lipid composition. This may be of importance since many molecular targets for therapy of metastatic colon cancer are raft-associated receptors (e.g., receptor tyrosine kinases). The importance of membrane perturbing potency of methoxychalcones for their biological activity was additionally corroborated by the results obtained by molecular modelling.

Research Advances on the Role of Lipids in the Life Cycle of Human Coronaviruses

Coronaviruses (CoVs) are emerging pathogens with a significant potential to cause life-threatening harm to human health. Since the beginning of the 21st century, three highly pathogenic and transmissible human CoVs have emerged, triggering epidemics and posing major threats to global public health. CoVs are enveloped viruses encased in a lipid bilayer. As fundamental components of cells, lipids can play an integral role in many physiological processes, which have been reported to play important roles in the life cycle of CoVs, including viral entry, uncoating, replication, assembly, and release. Therefore, research on the role of lipids in the CoV life cycle can provide a basis for a better understanding of the infection mechanism of CoVs and provide lipid targets for the development of new antiviral strategies. In this review, research advances on the role of lipids in different stages of viral infection and the possible targets of lipids that interfere with the viral life cycle are discussed.

The expanding organelle lipidomes: current knowledge and challenges

Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed.

Lipids in Extracellular Vesicles: What Can Be Learned about Membrane Structure and Function?

Extracellular vesicles, such as exosomes, can be used as interesting models to study the structure and function of biological membranes as these vesicles contain only one membrane (i.e., one lipid bilayer). In addition to lipids, they contain proteins, nucleic acids, and various other molecules. The lipid composition of exosomes is here compared to HIV particles and detergent-resistant membranes, which also have a high content of sphingolipids, cholesterol, and phosphatidylserine (PS). We discuss interactions between the lipids in the two bilayers, and especially those between PS 18:0/18:1 in the inner leaflet and the very-long-chain sphingolipids in the outer leaflet, and the importance of cholesterol for these interactions. We also briefly discuss the involvement of ether-linked phospholipids (PLs) in such lipid raft-like structures, and the possible involvement of these and other lipid classes in the formation of exosomes. The urgent need to improve the quality of quantitative lipidomic studies is highlighted.

Mutant APC reshapes Wnt signaling plasma membrane nanodomains by altering cholesterol levels via oncogenic β-catenin

Although the role of the Wnt pathway in colon carcinogenesis has been described previously, it has been recently demonstrated that Wnt signaling originates from highly dynamic nano-assemblies at the plasma membrane. However, little is known regarding the role of oncogenic APC in reshaping Wnt nanodomains. This is noteworthy, because oncogenic APC does not act autonomously and requires activation of Wnt effectors upstream of APC to drive aberrant Wnt signaling. Here, we demonstrate the role of oncogenic APC in increasing plasma membrane free cholesterol and rigidity, thereby modulating Wnt signaling hubs. This results in an overactivation of Wnt signaling in the colon. Finally, using the Drosophila sterol auxotroph model, we demonstrate the unique ability of exogenous free cholesterol to disrupt plasma membrane homeostasis and drive Wnt signaling in a wildtype APC background. Collectively, these findings provide a link between oncogenic APC, loss of plasma membrane homeostasis and CRC development.

Src family kinases engage differential pathways for encapsulation into extracellular vesicles

Extracellular vesicles (EVs) are heterogeneous biological nanoparticles secreted by all cell types. Identifying the proteins preferentially encapsulated in secreted EVs will help understand their heterogeneity. Src family kinases including Src and Fyn are a group of tyrosine kinases with fatty acylation modifications and/or multiple lysine residues (contributing charge interaction) at their N-terminus. Here, we demonstrate that Src and Fyn kinases were preferentially encapsulated in EVs and fatty acylation including myristoylation and palmitoylation facilitated their encapsulation. Genetic loss or pharmacological inhibition of myristoylation suppressed Src and/or Fyn kinase levels in EVs. Similarly, loss of palmitoylation reduced Fyn levels in EVs. Additionally, mutation of lysine at sites 5, 7, and 9 of Src kinase also inhibited the encapsulation of myristoylated Src into EVs. Knockdown of TSG101, which is a protein involved in the endosomal sorting complexes required for transport (ESCRT) protein complex mediated EVs biogenesis and led to a reduction of Src levels in EVs. In contrast, filipin III treatment, which disturbed the lipid raft structure, reduced Fyn kinase levels, but not Src kinase levels in EVs. Finally, elevated levels of Src protein were detected in the serum EVs of host mice carrying constitutively active Src-mediated prostate tumors in vivo. Collectively, the data suggest that different EVs biogenesis pathways exist and can regulate the encapsulation of specific proteins into EVs. This study provides an understanding of the EVs heterogeneity created by different EVs biogenesis pathways.

Effect of Cholesterol on Biomimetic Membrane Curvature and Coronavirus Fusion Peptide Encapsulation

Biomimetic cubic phases can be used for protein encapsulation in a variety of applications such as biosensors and drug delivery. Cubic phases with a high concentration of cholesterol and phospholipids were obtained herein. It is shown that the cubic phase structure can be maintained with a higher concentration of biomimetic membrane additives than has been reported previously. Opposing effects on the curvature of the membrane were observed upon the addition of phospholipids and cholesterol. Furthermore, the coronavirus fusion peptide significantly increased the negative curvature of the biomimetic membrane with cholesterol. We show that the viral fusion peptide can undergo structural changes leading to the formation of hydrophobic α-helices that insert into the lipid bilayer. This is of high importance, as a fusion peptide that induces increased negative curvature as shown by the formation of inverse hexagonal phases allows for greater contact area between two membranes, which is required for viral fusion to occur. The cytotoxicity assay showed that the toxicity toward HeLa cells was dramatically decreased when the cholesterol or peptide level in the nanoparticles increased. This suggests that the addition of cholesterol can improve the biocompatibility of the cubic phase nanoparticles, making them safer for use in biomedical applications. As the results, this work improves the potential for the biomedical end-use applications of the nonlamellar lipid nanoparticles and shows the need of systematic formulation studies due to the complex interplay of all components.

Lipid Raft Facilitated Receptor Organization and Signaling: A Functional Rheostat in Embryonic Development, Stem Cell Biology and Cancer

Molecular views of plasma membrane organization and dynamics are gradually changing over the past fifty years. Dynamics of plasma membrane instigate several signaling nexuses in eukaryotic cells. The striking feature of plasma membrane dynamics is that, it is internally transfigured into various subdomains of clustered macromolecules. Lipid rafts are nanoscale subdomains, enriched with cholesterol and sphingolipids, reside as floating entity mostly on the exoplasmic leaflet of the lipid bilayer. In terms of functionality, lipid rafts are unique among other membrane subdomains. Herein, advances on the roles of lipid rafts in cellular physiology and homeostasis are discussed, precisely, on how rafts dynamically harbor signaling proteins, including GPCRs, catalytic receptors, and ionotropic receptors within it and orchestrate multiple signaling pathways. In the developmental proceedings signaling are designed for patterning of overall organism and they differ from the somatic cell physiology and signaling of fully developed organisms. Some of the developmental signals are characteristic in maintenance of stemness and activated during several types of tumor development and cancer progression. The harmony between extracellular signaling and lineage specific transcriptional programs are extremely important for embryonic development. The roles of plasma membrane lipid rafts mediated signaling in lineage specificity, early embryonic development, stem cell maintenance are emerging. In view of this, we have highlighted and analyzed the roles of lipid rafts in receptor organization, cell signaling, and gene expression during embryonic development; from pre-implantation through the post-implantation phase, in stem cell and cancer biology.

Rafting on the Plasma Membrane: Lipid Rafts in Signaling and Disease

The plasma membrane is not a uniform phospholipid bilayer; it has specialized membrane nano- or microdomains called lipid rafts. Lipid rafts are small cholesterol and sphingolipid-rich plasma membrane islands. Although their existence was long debated, their presence in the plasma membrane of living cells is now well accepted with the advent of super-resolution imaging techniques. It is interesting to note that lipid rafts function to compartmentalize receptors and their regulators and substantially modulate cellular signaling. In this review, we will examine the role of lipid rafts and caveolae-lipid raft-like microdomains with a distinct 3D morphology-in cellular signaling. Moreover, we will investigate how raft compartmentalized signaling regulates diverse physiological processes such as proliferation, apoptosis, immune signaling, and development. Also, the deregulation of lipid raft-mediated signaling during tumorigenesis and metastasis will be explored.

Analysis of Lipids, Fatty Acid, and Cholesterol in Membrane Microdomains

The original concept that lipid and protein components are randomly distributed in cellular membranes has been challenged by evidence of compartmentalization of such components into discrete membrane microdomains (known as lipid rafts). The lipid microdomain hypothesis has generated significant controversy and rigorous inquiry to test the idea that such domains concentrate machinery to mediate cellular processes such as signaling, synaptic plasticity, and endocytosis. As such, a large number of studies have used biochemical, cell biological, and biophysical methodologies to define the composition of membrane microdomains in experimental contexts. Although biochemical preparation strategies are not without limitations (as discussed herein), the isolation of detergent-resistant and detergent-free membrane domains can provide important information about the segregation of lipids and proteins in membranes. In this chapter, we describe methodologies to isolate membranes from cell or tissue sources with biophysical/biochemical properties of membrane microdomains and also provide methods for subsequent classical or mass spectrometry-based lipid analytical approaches.

Alteration of cholesterol distribution at the plasma membrane of cancer cells: From evidence to pathophysiological implication and promising therapy strategy

Cholesterol-enriched domains are nowadays proposed to contribute to cancer cell proliferation, survival, death and invasion, with important implications in tumor progression. They could therefore represent promising targets for new anticancer treatment. However, although diverse strategies have been developed over the years from directly targeting cholesterol membrane content/distribution to adjusting sterol intake, all approaches present more or less substantial limitations. Those data emphasize the need to optimize current strategies, to develop new specific cholesterol-targeting anticancer drugs and/or to combine them with additional strategies targeting other lipids than cholesterol. Those objectives can only be achieved if we first decipher (i) the mechanisms that govern the formation and deformation of the different types of cholesterol-enriched domains and their interplay in healthy cells; (ii) the mechanisms behind domain deregulation in cancer; (iii) the potential generalization of observations in different types of cancer; and (iv) the specificity of some alterations in cancer vs. non-cancer cells as promising strategy for anticancer therapy. In this review, we will discuss the current knowledge on the homeostasis, roles and membrane distribution of cholesterol in non-tumorigenic cells. We will then integrate documented alterations of cholesterol distribution in domains at the surface of cancer cells and the mechanisms behind their contribution in cancer processes. We shall finally provide an overview on the potential strategies developed to target those cholesterol-enriched domains in cancer therapy.

Membrane lipid rafts are required for AMPA receptor tyrosine phosphorylation

Membrane lipid rafts are sphingolipids and cholesterol-enriched membrane microdomains, which form a center for the interaction or assembly of palmitoylated signaling molecules, including Src family non-receptor type protein tyrosine kinases. Lipid rafts abundantly exist in neurons and function in the maintenance of synapses. Excitatory synaptic strength is largely controlled by the surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the mammalian brain. AMPA receptor endocytosis from the synaptic surface is regulated by phosphorylation of the GluA2 subunit at tyrosine 876 by Src family kinases. Here, I revealed that tyrosine phosphorylated GluA2 is concentrated in the lipid rafts fraction. Furthermore, stimulation-induced upregulation of GluA2 tyrosine phosphorylation is disrupted by the treatment of neurons with a cholesterol-depleting compound, filipin III. These results indicate the importance of lipid rafts as enzymatic reactive sites for AMPA receptor tyrosine phosphorylation and subsequent AMPA receptor internalization from the synaptic surface.

Effect of reversible osmotic stress on live cell plasma membranes, probed via Laurdan general polarization measurements

Here we seek to gain insight into changes in the plasma membrane of live cells upon the application of osmotic stress using Laurdan, a fluorescent probe that reports on membrane organization, hydration, and dynamics. It is known that the application of osmotic stress to lipid vesicles causes a decrease in Laurdan's generalized polarization (GP), which has been interpreted as an indication of membrane stretching. In cells, we see the opposite effects, as GP increases when the osmolarity of the solution is decreased. This increase in GP is associated with the presence of caveolae, which are known to disassemble and flatten in response to osmotic stress.

Direct Regulation of Hyperpolarization-Activated Cyclic-Nucleotide Gated (HCN1) Channels by Cannabinoids

Cannabinoids are a broad class of molecules that act primarily on neurons, affecting pain sensation, appetite, mood, learning, and memory. In addition to interacting with specific cannabinoid receptors (CBRs), cannabinoids can directly modulate the function of various ion channels. Here, we examine whether cannabidiol (CBD) and Δ9-tetrahydrocannabinol (THC), the most prevalent phytocannabinoids in Cannabis sativa, can regulate the function of hyperpolarization-activated cyclic-nucleotide-gated (HCN1) channels independently of CBRs. HCN1 channels were expressed in Xenopus oocytes since they do not express CBRs, and the effects of cannabinoid treatment on HCN1 currents were examined by a two-electrode voltage clamp. We observe opposing effects of CBD and THC on HCN1 current, with CBD acting to stimulate HCN1 function, while THC inhibited current. These effects persist in HCN1 channels lacking the cyclic-nucleotide binding domain (HCN1ΔCNBD). However, changes to membrane fluidity, examined by treating cells with TX-100, inhibited HCN1 current had more pronounced effects on the voltage-dependence and kinetics of activation than THC, suggesting this is not the primary mechanism of HCN1 regulation by cannabinoids. Our findings may contribute to the overall understanding of how cannabinoids may act as promising therapeutic molecules for the treatment of several neurological disorders in which HCN function is disturbed.

It Takes More than Two to Tango: Complex, Hierarchal, and Membrane-Modulated Interactions in the Regulation of Receptor Tyrosine Kinases

The search for an understanding of how cell fate and motility are regulated is not a purely scientific undertaking, but it can also lead to rationally designed therapies against cancer. The discovery of tyrosine kinases about half a century ago, the subsequent characterization of certain transmembrane receptors harboring tyrosine kinase activity, and their connection to the development of human cancer ushered in a new age with the hope of finding a treatment for malignant diseases in the foreseeable future. However, painstaking efforts were required to uncover the principles of how these receptors with intrinsic tyrosine kinase activity are regulated. Developments in molecular and structural biology and biophysical approaches paved the way towards better understanding of these pathways. Discoveries in the past twenty years first resulted in the formulation of textbook dogmas, such as dimerization-driven receptor association, which were followed by fine-tuning the model. In this review, the role of molecular interactions taking place during the activation of receptor tyrosine kinases, with special attention to the epidermal growth factor receptor family, will be discussed. The fact that these receptors are anchored in the membrane provides ample opportunities for modulatory lipid-protein interactions that will be considered in detail in the second part of the manuscript. Although qualitative and quantitative alterations in lipids in cancer are not sufficient in their own right to drive the malignant transformation, they both contribute to tumor formation and also provide ways to treat cancer. The review will be concluded with a summary of these medical aspects of lipid-protein interactions.

Ras Multimers on the Membrane: Many Ways for a Heart-to-Heart Conversation

Formation of Ras multimers, including dimers and nanoclusters, has emerged as an exciting, new front of research in the 'old' field of Ras biomedicine. With significant advances made in the past few years, we are beginning to understand the structure of Ras multimers and, albeit preliminary, mechanisms that regulate their formation in vitro and in cells. Here we aim to synthesize the knowledge accrued thus far on Ras multimers, particularly the presence of multiple globular (G-) domain interfaces, and discuss how membrane nanodomain composition and structure would influence Ras multimer formation. We end with some general thoughts on the potential implications of Ras multimers in basic and translational biology.

Cholesterol-Rich Lipid Rafts as Platforms for SARS-CoV-2 Entry

Since its appearance, the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2), the causal agent of Coronavirus Disease 2019 (COVID-19), represents a global problem for human health that involves the host lipid homeostasis. Regarding, lipid rafts are functional membrane microdomains with highly and tightly packed lipid molecules. These regions enriched in sphingolipids and cholesterol recruit and concentrate several receptors and molecules involved in pathogen recognition and cellular signaling. Cholesterol-rich lipid rafts have multiple functions for viral replication; however, their role in SARS-CoV-2 infection remains unclear. In this review, we discussed the novel evidence on the cholesterol-rich lipid rafts as a platform for SARS-CoV-2 entry, where receptors such as the angiotensin-converting enzyme-2 (ACE-2), heparan sulfate proteoglycans (HSPGs), human Toll-like receptors (TLRs), transmembrane serine proteases (TMPRSS), CD-147 and HDL-scavenger receptor B type 1 (SR-B1) are recruited for their interaction with the viral spike protein. FDA-approved drugs such as statins, metformin, hydroxychloroquine, and cyclodextrins (methyl-β-cyclodextrin) can disrupt cholesterol-rich lipid rafts to regulate key molecules in the immune signaling pathways triggered by SARS-CoV-2 infection. Taken together, better knowledge on cholesterol-rich lipid rafts in the SARS-CoV-2-host interactions will provide valuable insights into pathogenesis and the identification of novel therapeutic targets.

Stearic acid blunts growth-factor signaling via oleoylation of GNAI proteins

Covalent attachment of C16:0 to proteins (palmitoylation) regulates protein function. Proteins are also S-acylated by other fatty acids including C18:0. Whether protein acylation with different fatty acids has different functional outcomes is not well studied. We show here that C18:0 (stearate) and C18:1 (oleate) compete with C16:0 to S-acylate Cys3 of GNAI proteins. C18:0 becomes desaturated so that C18:0 and C18:1 both cause S-oleoylation of GNAI. Exposure of cells to C16:0 or C18:0 shifts GNAI acylation towards palmitoylation or oleoylation, respectively. Oleoylation causes GNAI proteins to shift out of cell membrane detergent-resistant fractions where they potentiate EGFR signaling. Consequently, exposure of cells to C18:0 reduces recruitment of Gab1 to EGFR and reduces AKT activation. This provides a molecular mechanism for the anti-tumor effects of C18:0, uncovers a mechanistic link how metabolites affect cell signaling, and provides evidence that the identity of the fatty acid acylating a protein can have functional consequences.

The Protein Toxins Ricin and Shiga Toxin as Tools to Explore Cellular Mechanisms of Internalization and Intracellular Transport

Protein toxins secreted by bacteria and found in plants can be threats to human health. However, their extreme toxicity can also be exploited in different ways, e.g., to produce hybrid toxins directed against cancer cells and to study transport mechanisms in cells. Investigations during the last decades have shown how powerful these molecules are as tools in cell biological research. Here, we first present a partly historical overview, with emphasis on Shiga toxin and ricin, of how such toxins have been used to characterize processes and proteins of importance for their trafficking. In the second half of the article, we describe how one can now use toxins to investigate the role of lipid classes for intracellular transport. In recent years, it has become possible to quantify hundreds of lipid species using mass spectrometry analysis. Thus, it is also now possible to explore the importance of lipid species in intracellular transport. The detailed analyses of changes in lipids seen under conditions of inhibited toxin transport reveal previously unknown connections between syntheses of lipid classes and demonstrate the ability of cells to compensate under given conditions.

Membrane therapy using DHA suppresses epidermal growth factor receptor signaling by disrupting nanocluster formation

Epidermal growth factor receptor (EGFR) signaling drives the formation of many types of cancer, including colon cancer. Docosahexaenoic acid (DHA, 22∶6Δ4,7,10,13,16,19), a chemoprotective long-chain n-3 polyunsaturated fatty acid suppresses EGFR signaling. However, the mechanism underlying this phenotype remains unclear. Therefore, we used super-resolution microscopy techniques to investigate the mechanistic link between EGFR function and DHA-induced alterations to plasma membrane nanodomains. Using isogenic in vitro (YAMC and IMCE mouse colonic cell lines) and in vivo (Drosophila, wild type and Fat-1 mice) models, cellular DHA enrichment via therapeutic nanoparticle delivery, endogenous synthesis, or dietary supplementation reduced EGFR-mediated cell proliferation and downstream Ras/ERK signaling. Phospholipid incorporation of DHA reduced membrane rigidity and the size of EGFR nanoclusters. Similarly, pharmacological reduction of plasma membrane phosphatidic acid (PA), phosphatidylinositol-4,5-bisphosphate (PIP2) or cholesterol was associated with a decrease in EGFR nanocluster size. Furthermore, in DHA-treated cells only the addition of cholesterol, unlike PA or PIP2, restored EGFR nanoscale clustering. These findings reveal that DHA reduces EGFR signaling in part by reshaping EGFR proteolipid nanodomains, supporting the feasibility of using membrane therapy, i.e., dietary/drug-related strategies to target plasma membrane organization, to reduce EGFR signaling and cancer risk.

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

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

Advances in understanding and in multi-disciplinary methodology used to assess lipid regulation of signalling cascades from the cancer cell plasma membrane

The lipid bilayer is a functional component of cells, forming a stable platform for the initiation of key biological processes, including cell signalling. There are distinct changes in the lipid composition of cell membranes during oncogenic transformation resulting in aberrant activation and inactivation of signalling transduction pathways. Studying the role of the cell membrane in cell signalling is challenging, since techniques are often limited to by timescale, resolution, sensitivity, and averaging. To overcome these limitations, combining 'computational', 'wet-lab' and 'semi-dry' approaches offers the best opportunity to resolving complex biological processes involved in membrane organisation. In this review, we highlight analytical tools that have been applied for the study of cell signalling initiation from the cancer cell membranes through computational microscopy, biological assays, and membrane biophysics. The cancer therapeutic potential of extracellular membrane-modulating agents, such as cholesterol-reducing agents is also discussed, as is the need for future collaborative inter-disciplinary research for studying the role of the cell membrane and its components in cancer therapy.

Regulation of caveolae through cholesterol-depletion-dependent tubulation mediated by PACSIN2

The membrane-shaping ability of PACSIN2 (also known as syndapin II), which is mediated by its F-BAR domain, has been shown to be essential for caveolar morphogenesis, presumably through the shaping of the caveolar neck. Caveolar membranes contain abundant cholesterol. However, the role of cholesterol in PACSIN2-mediated membrane deformation remains unclear. Here, we show that the binding of PACSIN2 to the membrane can be negatively regulated by cholesterol. We prepared reconstituted membranes based on the lipid composition of caveolae. The reconstituted membrane with cholesterol had a weaker affinity for the F-BAR domain of PACSIN2 than a membrane without cholesterol. Consistent with this, upon depletion of cholesterol from the plasma membrane, PACSIN2 localized at tubules that had caveolin-1 at their tips, suggesting that cholesterol inhibits membrane tubulation mediated by PACSIN2. The tubules induced by PACSIN2 could be representative of an intermediate of caveolae endocytosis. Consistent with this, the removal of caveolae from the plasma membrane upon cholesterol depletion was diminished in the PACSIN2-deficient cells. These data suggest that PACSIN2-mediated caveolae internalization is dependent on the amount of cholesterol, providing a mechanism for cholesterol-dependent regulation of caveolae.This article has an associated First Person interview with the first author of the paper.

Elevating EGFR-MAPK program by a nonconventional Cdc42 enhances intestinal epithelial survival and regeneration

The regulatory mechanisms enabling the intestinal epithelium to maintain a high degree of regenerative capacity during mucosal injury remain unclear. Ex vivo survival and clonogenicity of intestinal stem cells (ISCs) strictly required growth response mediated by cell division control 42 (Cdc42) and Cdc42-deficient enteroids to undergo rapid apoptosis. Mechanistically, Cdc42 engaging with EGFR was required for EGF-stimulated, receptor-mediated endocytosis and sufficient to promote MAPK signaling. Proteomics and kinase analysis revealed that a physiologically, but nonconventionally, spliced Cdc42 variant 2 (V2) exhibited stronger MAPK-activating capability. Human CDC42-V2 is transcriptionally elevated in some colon tumor tissues. Accordingly, mice engineered to overexpress Cdc42-V2 in intestinal epithelium showed elevated MAPK signaling, enhanced regeneration, and reduced mucosal damage in response to irradiation. Overproducing Cdc42-V2 specifically in mouse ISCs enhanced intestinal regeneration following injury. Thus, the intrinsic Cdc42-MAPK program is required for intestinal epithelial regeneration, and elevating this signaling cascade is capable of initiating protection from genotoxic injury.

The roles of the diversity of amphipathic lipids in shaping membranes by membrane-shaping proteins

Lipid compositions of cells differ according to cell types and intracellular organelles. Phospholipids are major cell membrane lipids and have hydrophilic head groups and hydrophobic fatty acid tails. The cellular lipid membrane without any protein adapts to spherical shapes, and protein binding to the membrane is thought to be required for shaping the membrane for various cellular events. Until recently, modulation of cellular lipid membranes was initially shown to be mediated by proteins recognizing lipid head groups, including the negatively charged ones of phosphatidylserine and phosphoinositides. Recent studies have shown that the abilities of membrane-deforming proteins are also regulated by the composition of fatty acid tails, which cause different degrees of packing defects. The binding of proteins to cellular lipid membranes is affected by the packing defects, presumably through modulation of their interactions with hydrophobic amino acid residues. Therefore, lipid composition can be characterized by both packing defects and charge density. The lipid composition regarding fatty acid tails affects membrane bending via the proteins with amphipathic helices, including those with the ArfGAP1 lipid packing sensor (ALPS) motif and via membrane-deforming proteins with structural folding, including those with the Bin-Amphiphysin-Rvs167 (BAR) domains. This review focuses on how the fatty acid tails, in combination with the head groups of phospholipids, affect protein-mediated membrane deformation.

The role of cholesterol and cholesterol-driven membrane raft domains in prostate cancer

Membrane rafts are heterogeneous and dynamic domains that are characterized by tight packing of lipids. They are enriched in cholesterol, sphingolipids, and certain types of proteins. Among these are various cell signaling proteins, which indicate that rafts play an important role in cell signal transduction pathways, including some involved in cancer development, progression, and invasiveness. Due to their increased cholesterol content, raft domains exhibit lower fluidity than the surrounding membrane. The cell membranes of some solid tumors, such as breast and prostate cancer, contain higher levels of cholesterol, which means larger raft domain can form in those membranes. This may stimulate signaling pathways to promote tumor growth and progression. This review focuses on the known raft-dependent regulatory mechanisms that promote prostate cancer progression.

Impact statement

Prostate cancer remains the most common malignancy and second most frequent cause of cancer-related death in men. Cholesterol levels are usually higher in prostate cancer cells. This affects the cell membrane composition, with cholesterol and sphingolipid-containing raft membrane domains becoming a greater component. In addition to polar lipids, these domains recruit and regulate certain types of protein, including various cell signaling proteins that are critical to cancer cell survival and invasiveness. This suggests that membrane rafts have a regulatory role in tumor progression, making them a potential target in prostate cancer treatment.

The role of lipid species in membranes and cancer-related changes

Several studies have demonstrated interactions between the two leaflets in membrane bilayers and the importance of specific lipid species for such interaction and membrane function. We here discuss these investigations with a focus on the sphingolipid and cholesterol-rich lipid membrane domains called lipid rafts, including the small flask-shaped invaginations called caveolae, and the importance of such membrane structures in cell biology and cancer. We discuss the possible interactions between the very long-chain sphingolipids in the outer leaflet of the plasma membrane and the phosphatidylserine species PS 18:0/18:1 in the inner leaflet and the importance of cholesterol for such interactions. We challenge the view that lipid rafts contain a large fraction of lipids with two saturated fatty acyl groups and argue that it is important in future studies of membrane models to use asymmetric membrane bilayers with lipid species commonly found in cellular membranes. We also discuss the need for more quantitative lipidomic studies in order to understand membrane function and structure in general, and the importance of lipid rafts in biological systems. Finally, we discuss cancer-related changes in lipid rafts and lipid composition, with a special focus on changes in glycosphingolipids and the possibility of using lipid therapy for cancer treatment.

Lipid rafts as signaling hubs in cancer cell survival/death and invasion: implications in tumor progression and therapy: Thematic Review Series: Biology of Lipid Rafts

Cholesterol/sphingolipid-rich membrane domains, known as lipid rafts or membrane rafts, play a critical role in the compartmentalization of signaling pathways. Physical segregation of proteins in lipid rafts may modulate the accessibility of proteins to regulatory or effector molecules. Thus, lipid rafts serve as sorting platforms and hubs for signal transduction proteins. Cancer cells contain higher levels of intracellular cholesterol and lipid rafts than their normal non-tumorigenic counterparts. Many signal transduction processes involved in cancer development (insulin-like growth factor system and phosphatidylinositol 3-kinase-AKT) and metastasis [cluster of differentiation (CD)44] are dependent on or modulated by lipid rafts. Additional proteins playing an important role in several malignant cancers (e.g., transmembrane glycoprotein mucin 1) are also being detected in association with lipid rafts, suggesting a major role of lipid rafts in tumor progression. Conversely, lipid rafts also serve as scaffolds for the recruitment and clustering of Fas/CD95 death receptors and downstream signaling molecules leading to cell death-promoting raft platforms. The partition of death receptors and downstream signaling molecules in aggregated lipid rafts has led to the formation of the so-called cluster of apoptotic signaling molecule-enriched rafts, or CASMER, which leads to apoptosis amplification and can be pharmacologically modulated. These death-promoting rafts can be viewed as a linchpin from which apoptotic signals are launched. In this review, we discuss the involvement of lipid rafts in major signaling processes in cancer cells, including cell survival, cell death, and metastasis, and we consider the potential of lipid raft modulation as a promising target in cancer therapy.

Tetraspanin CD82 interaction with cholesterol promotes extracellular vesicle-mediated release of ezrin to inhibit tumour cell movement

Tumour metastasis suppressor KAI1/CD82 inhibits tumour cell movement. As a transmembrane protein, tetraspanin CD82 bridges the interactions between membrane microdomains of lipid rafts and tetraspanin-enriched microdomains (TEMs). In this study, we found that CD82 and other tetraspanins contain cholesterol recognition/interaction amino-acid consensus (CRAC) sequences in their transmembrane domains and revealed that cholesterol binding of CD82 determines its interaction with lipid rafts but not with TEMs. Functionally, CD82 needs cholesterol binding to inhibit solitary migration, collective migration, invasion and infiltrative outgrowth of tumour cells. Importantly, CD82–cholesterol/–lipid raft interaction not only promotes extracellular release of lipid raft components such as cholesterol and gangliosides but also facilitates extracellular vesicle (EV)–mediated release of ezrin–radixin–moesin (ERM) protein Ezrin. Since ERM proteins link actin cytoskeleton to the plasma membrane, we show for the first time that cell movement can be regulated by EV-mediated releases, which disengage the plasma membrane from cytoskeleton and then impair cell movement. Our findings also conceptualize that interactions between membrane domains, in this case converge of lipid rafts and TEMs by CD82, can change cell movement. Moreover, CD82 coalescences with both lipid rafts and TEMs are essential for its inhibition of tumour cell movement and for its enhancement of EV release. Finally, our study underpins that tetraspanins as a superfamily of functionally versatile molecules are cholesterol-binding proteins.

Abbreviations:Ab: antibody; CBM: cholesterol-binding motif; CCM: cholesterol consensus motif; CRAC/CARC: cholesterol recognition or interaction amino-acid consensus; CTxB: cholera toxin B subunit; ECM: extracellular matrix; ERM: ezrin, radixin and moesin; EV: extracellular vesicles; FBS: foetal bovine serum; mAb: monoclonal antibody; MST: microscale thermophoresis; pAb: polyclonal antibody; and TEM: tetraspanin-enriched microdomain

Distribution, dynamics and functional roles of phosphatidylserine within the cell

Phosphatidylserine (PtdSer), an essential constituent of eukaryotic membranes, is the most abundant anionic phospholipid in the eukaryotic cell accounting for up to 10% of the total cellular lipid. Much of what is known about PtdSer is the role exofacial PtdSer plays in apoptosis and blood clotting. However, PtdSer is generally not externally exposed in healthy cells and plays a vital role in several intracellular signaling pathways, though relatively little is known about the precise subcellular localization, transmembrane topology and intracellular dynamics of PtdSer within the cell. The recent development of new, genetically-encoded probes able to detect phosphatidylserine is leading to a more in-depth understanding of the biology of this phospholipid. This review aims to give an overview of recent developments in our understanding of the role of PtdSer in intracellular signaling events derived from the use of these recently developed methods of phosphatidylserine detection.

Oncogene Amplification in Growth Factor Signaling Pathways Renders Cancers Dependent on Membrane Lipid Remodeling

Advances in DNA sequencing technologies have reshaped our understanding of the molecular basis of cancer, providing a precise genomic view of tumors. Complementary biochemical and biophysical perspectives of cancer point toward profound shifts in nutrient uptake and utilization that propel tumor growth and major changes in the structure of the plasma membrane of tumor cells. The molecular mechanisms that bridge these fundamental aspects of tumor biology remain poorly understood. Here, we show that the lysophosphatidylcholine acyltransferase LPCAT1 functionally links specific genetic alterations in cancer with aberrant metabolism and plasma membrane remodeling to drive tumor growth. Growth factor receptor-driven cancers are found to depend on LPCAT1 to shape plasma membrane composition through enhanced saturated phosphatidylcholine content that is, in turn, required for the transduction of oncogenic signals. These results point to a genotype-informed strategy that prioritizes lipid remodeling pathways as therapeutic targets for diverse cancers.

The role of PS 18:0/18:1 in membrane function

Various studies have demonstrated that the two leaflets of cellular membranes interact, potentially through so-called interdigitation between the fatty acyl groups. While the molecular mechanism underlying interleaflet coupling remains to be fully understood, recent results suggest interactions between the very-long-chain sphingolipids in the outer leaflet, and phosphatidylserine PS18:0/18:1 in the inner leaflet, and an important role for cholesterol for these interactions. Here we review the evidence that cross-linking of sphingolipids may result in clustering of phosphatidylserine and transfer of signals to the cytosol. Although much remains to be uncovered, the molecular properties and abundance of PS 18:0/18:1 suggest a unique role for this lipid.

There are several lines of evidence for interactions between the two membrane leaflets in cells. In this review the authors discuss the transmembrane coupling of lipids, the involvement of phosphatidyl serine species PS 18:0/18:1, and their importance for various cellular processes.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Molecular Targets of Epigallocatechin-Gallate (EGCG): A Special Focus on Signal Transduction and Cancer

Green tea is a beverage that is widely consumed worldwide and is believed to exert effects on different diseases, including cancer. The major components of green tea are catechins, a family of polyphenols. Among them, epigallocatechin-gallate (EGCG) is the most abundant and biologically active. EGCG is widely studied for its anti-cancer properties. However, the cellular and molecular mechanisms explaining its action have not been completely understood, yet. EGCG is effective in vivo at micromolar concentrations, suggesting that its action is mediated by interaction with specific targets that are involved in the regulation of crucial steps of cell proliferation, survival, and metastatic spread. Recently, several proteins have been identified as EGCG direct interactors. Among them, the trans-membrane receptor 67LR has been identified as a high affinity EGCG receptor. 67LR is a master regulator of many pathways affecting cell proliferation or apoptosis, also regulating cancer stem cells (CSCs) activity. EGCG was also found to be interacting directly with Pin1, TGFR-II, and metalloproteinases (MMPs) (mainly MMP2 and MMP9), which respectively regulate EGCG-dependent inhibition of NF-kB, epithelial-mesenchimal transaction (EMT) and cellular invasion. EGCG interacts with DNA methyltransferases (DNMTs) and histone deacetylases (HDACs), which modulates epigenetic changes. The bulk of this novel knowledge provides information about the mechanisms of action of EGCG and may explain its onco-suppressive function. The identification of crucial signalling pathways that are related to cancer onset and progression whose master regulators interacts with EGCG may disclose intriguing pharmacological targets, and eventually lead to novel combined treatments in which EGCG acts synergistically with known drugs.

Gangliosides in Membrane Organization

Since the structure of GM1 was elucidated 55years ago, researchers have been attracted by the sialylated glycans of gangliosides. Gangliosides head groups, protruding toward the extracellular space, significantly contribute to the cell glycocalyx; and in certain cells, such as neurons, are major determinants of the features of the cell surface. Expression of glycosyltransferases involved in the de novo biosynthesis of gangliosides is tightly regulated along cell differentiation and activation, and is regarded as the main metabolic mechanism responsible for the acquisition of cell-specific ganglioside patterns. The resulting sialooligosaccharides are characterized by a high degree of geometrical complexity and by highly dynamic properties, which seem to be functional for complex interactions with other molecules sitting on the same cellular membrane (cis-interactions) or soluble molecules present in the extracellular environment, or molecules associated with the surface of other cells (trans-interactions). There is no doubt that the multifaceted biological functions of gangliosides are largely dependent on oligosaccharide-mediated molecular interactions. However, gangliosides are amphipathic membrane lipids, and their chemicophysical, aggregational, and, consequently, biological properties are dictated by the properties of the monomers as a whole, which are not merely dependent on the structures of their polar head groups. In this chapter, we would like to focus on the peculiar chemicophysical features of gangliosides (in particular, those of the nervous system), that represent an important driving force determining the organization and properties of cellular membranes, and to emphasize the causal connections between altered ganglioside-dependent membrane organization and relevant pathological conditions.

Sphingolipids: membrane microdomains in brain development, function and neurological diseases

Sphingolipids are highly enriched in the nervous system where they are pivotal constituents of the plasma membranes and are important for proper brain development and functions. Sphingolipids are not merely structural elements, but are also recognized as regulators of cellular events by their ability to form microdomains in the plasma membrane. The significance of such compartmentalization spans broadly from being involved in differentiation of neurons and synaptic transmission to neuronal–glial interactions and myelin stability. Thus, perturbations of the sphingolipid metabolism can lead to rearrangements in the plasma membrane, which has been linked to the development of various neurological diseases. Studying microdomains and their functions has for a long time been synonymous with studying the role of cholesterol. However, it is becoming increasingly clear that microdomains are very heterogeneous, which among others can be ascribed to the vast number of sphingolipids. In this review, we discuss the importance of microdomains with emphasis on sphingolipids in brain development and function as well as how disruption of the sphingolipid metabolism (and hence microdomains) contributes to the pathogenesis of several neurological diseases.

Modulation of key lipid raft constituents in primary rat hepatocytes by fumonisin B1 - Implications for cancer promotion in the liver

Fumonisin B₁ (FB₁), a group 2B natural occurring carcinogenic mycotoxin, modulated lipid and fatty acid (FA) constituents of lipid rafts isolated from primary hepatocytes following exposure to a cytotoxic concentration of FB₁ (250 μM). The major effects observed in rafts, included a significant (p < 0.05) increase in raft cholesterol (CHOL) and glycerophospholipid such as phosphatidylethanolamine (PE), whereas sphingomyelin (SM) decreased (p < 0.05). Changes in lipid constituents resulted in the disruption of important membrane fluidity parameters represented as a decreased (p < 0.05) in the phosphatidylcholine (PC)/PE and PC/(PE+SM) ratios and an increase (p < 0.05) in the CHOL/PL (PL=PC+PE) ratio, suggesting the preservation of lipid raft rigidity and integrity. Observed FA changes in the raft PE fraction included a significant (p < 0.05) increase in C18:2ω-6, C20:3ω-6, C20:4ω-6, C22:4ω-6, C22:5ω-3 and C22:6ω-3, with an increase in total ω-6 and ω-3 polyunsaturated fatty acids (PUFAs). Modulation of the FA content in PE, specifically the C20:4ω-6 PC/PE ratio and PUFA levels, together with changes in CHOL and SM are key determinants regulating the integrity and function of lipid rafts. In primary hepatocytes these changes are associated with the inhibition of cell proliferation and induction of apoptosis. A lipogenic mechanism is proposed whereby FB₁ modulates lipid rafts and differentially target cell survival indices of normal and preneoplastic hepatocytes during cancer promotion in the liver.

Inhibition of SREBP increases gefitinib sensitivity in non-small cell lung cancer cells

The clinical success of EGFR inhibitors in patients with lung cancer is limited by the inevitable development of treatment resistance. Here, we show that inhibition of SREBP increase gefitinib sensitivity in vitro and in vivo. Interference of SREBP1 binding partner MARVELD1 potentiate the therapeutic effect of gefitinib as well. Mechanistically, SREBP inhibition decreases the cell membrane fluidity, results in a decreased tyrosine phosphorylation of EGFR. Therefore, targeting lipid metabolism combined with EGFR-TKIs is potentially a novel therapeutic strategies for cancer treatment.

Alterations in the properties of the cell membrane due to glycosphingolipid accumulation in a model of Gaucher disease

Gaucher disease is a lysosomal storage disease characterized by the malfunction of glucocerebrosidase resulting in the accumulation of glucosylceramide and other sphingolipids in certain cells. Although the disease symptoms are usually attributed to the storage of undigested substrate in lysosomes, here we show that glycosphingolipids accumulating in the plasma membrane cause profound changes in the properties of the membrane. The fluidity of the sphingolipid-enriched membrane decreased accompanied by the enlargement of raft-like ordered membrane domains. The mobility of non-raft proteins and lipids was severely restricted, while raft-resident components were only mildly affected. The rate of endocytosis of transferrin receptor, a non-raft protein, was significantly retarded in Gaucher cells, while the endocytosis of the raft-associated GM1 ganglioside was unaffected. Interferon-γ-induced STAT1 phosphorylation was also significantly inhibited in Gaucher cells. Atomic force microscopy revealed that sphingolipid accumulation was associated with a more compliant membrane capable of producing an increased number of nanotubes. The results imply that glycosphingolipid accumulation in the plasma membrane has significant effects on membrane properties, which may be important in the pathogenesis of Gaucher disease.

Determination of the Relative Efficacy of Eicosapentaenoic Acid and Docosahexaenoic Acid for Anti-Cancer Effects in Human Breast Cancer Models

Epidemiological studies have associated high fish oil consumption with decreased risk of breast cancer (BC). n-3 long chain polyunsaturated fatty acids (n-3 LCPUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) found in fish and fish oils exert anti-cancer effects. However, few studies have examined the relative efficacy of EPA and DHA alone and in mixtures on BC subtypes. This was the objective of the present review, as this research is a necessity for the translation of findings to human health and disease. The literature suggests that DHA has a greater anti-cancer effect in triple negative BC (TNBC). In estrogen positive (ER+) BC, DHA has a greater effect on cell viability, while both fatty acids have similar effects on apoptosis and proliferation. These effects are associated with preferential uptake of DHA into TNBC lipid rafts and EPA in ER+ BC. EPA:DHA mixtures have anti-cancer activity; however, the ratio of EPA:DHA does not predict the relative incorporation of these two fatty acids into membrane lipids as EPA appears to be preferentially incorporated. In summary, DHA and EPA should be considered separately in the context of BC prevention. The elucidation of optimal EPA:DHA ratios will be important for designing targeted n-3 LCPUFA treatments.

Apolipoprotein E Promotes Invasion in Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) patients generally have a poor prognosis, because of the invasive nature of these tumors. In comparing transcription profiles between OSCC tumors with a more invasive (worst pattern of tumor invasion 5) versus a less invasive (worst pattern of tumor invasion 3) pattern of invasion, we identified a total of 97 genes that were overexpressed at least 1.5-fold in the more invasive tumor subtype. The most functionally relevant genes were assessed using in vitro invasion assays with an OSCC cell line (UM-SCC-1). Individual siRNA knockdown of 15 of these 45 genes resulted in significant reductions in tumor cell invasion compared to a nontargeting siRNA control. One gene whose knockdown had a strong effect on invasion corresponded to apolipoprotein E (APOE). Both matrix degradation and the number of mature invadopodia were significantly decreased with APOE knockdown. APOE knockdown also resulted in increased cellular cholesterol, consistent with APOE's role in regulating cholesterol efflux. APOE knockdown resulted in decreased levels of phospho-extracellular signal-regulated kinase 1/2, phospho-c-Jun N-terminal kinase, and phospho-cJun, as well as decreased activator protein 1 (AP-1) activity. Expression of matrix metalloproteinase 7 (MMP7), an AP-1 target, was also significantly decreased. Our findings suggest that APOE protein plays a significant role in OSCC tumor invasion because of its effects on cellular cholesterol and subsequent effects on cell signaling and AP-1 activity, leading to changes in the expression of invasion-related proteins, including MMP7.

The effect of sterol structure upon clathrin-mediated and clathrin-independent endocytosis

Ordered lipid domains (rafts) in plasma membranes have been hypothesized to participate in endocytosis based on inhibition of endocytosis by removal or sequestration of cholesterol. To more carefully investigate the role of the sterol in endocytosis, we used a substitution strategy to replace cholesterol with sterols that show various raft-forming abilities and chemical structures. Both clathrin-mediated endocytosis of transferrin and clathrin-independent endocytosis of clustered placental alkaline phosphatase were measured. A subset of sterols reversibly inhibited both clathrin-dependent and clathrin-independent endocytosis. The ability of a sterol to support lipid raft formation was necessary for endocytosis. However, it was not sufficient, because a sterol lacking a 3β-OH group did not support endocytosis even though it had the ability to support ordered domain formation. Double bonds in the sterol rings and an aliphatic tail structure identical to that of cholesterol were neither necessary nor sufficient to support endocytosis. This study shows that substitution using a large number of sterols can define the role of sterol structure in cellular functions. Hypotheses for how sterol structure can similarly alter clathrin-dependent and clathrin-independent endocytosis are discussed.

A-type ECG and EGCG dimers inhibit 3T3-L1 differentiation by binding to cholesterol in lipid rafts

The present study aimed to explore the underlying mechanisms of epicatechin-3-gallate-(4β→8, 2β→O→7)-epicatechin-3-gallate (A-type ECG dimer) and epigallocatechin-3-gallate-(4β→8, 2β→O→7)-epigallocatechin-3-gallate (A-type EGCG dimer) involved in their strong inhibitory effects on 3T3-L1 preadipocytes differentiation. In the synthetic "lipid raft-like" liposome, A-type ECG and EGCG dimers incorporated into the liposome with high affinity and decreased the fluidity of the liposome. In 3T3-L1 preadipocytes, A-type ECG and EGCG dimers possibly bonded to lipid rafts cholesterol and disrupted the integrity of lipid rafts, thus exerting their notable inhibitory effects on 3T3-L1 preadipocytes differentiation by suppressing mitotic clonal expansion process and mRNA levels of PPARγ, C/EBPα and SREBP1C. A highly positive correlation between the cholesterol binding capacity of the two dimers and their inhibitory effect on 3T3-L1 preadipocytes differentiation (R²=0.9328) was observed. Molecular dynamics simulation further verified that A-type ECG and EGCG dimers could bond to cholesterol via hydrogen bonding. The results of this study suggested that the disruption of A-type ECG and EGCG dimers on membrane lipid rafts by targeting cholesterol in the lipid rafts was involved in the underlying mechanisms of their strong inhibitory effects on 3T3-L1 preadipocytes differentiation. This broadens the understanding of the molecular mechanisms of polyphenols on modulating and controlling of metabolic dysregulation, particularly adipocyte differentiation, which is a significant risk factor associated with the development of cardiovascular disease.

The dipole potential correlates with lipid raft markers in the plasma membrane of living cells

The dipole potential generating an electric field much stronger than any other type of membrane potential influences a wide array of phenomena, ranging from passive permeation to voltage-dependent conformational changes of membrane proteins. It is generated by the ordered orientation of lipid carbonyl and membrane-attached water dipole moments. Theoretical considerations and indirect experimental evidence obtained in model membranes suggest that the dipole potential is larger in liquid-ordered domains believed to correspond to lipid rafts in cell membranes. Using three different dipole potential-sensitive fluorophores and four different labeling approaches of raft and nonraft domains, we showed that the dipole potential is indeed stronger in lipid rafts than in the rest of the membrane. The magnitude of this difference is similar to that observed between the dipole potential in control and sphingolipid-enriched cells characteristic of Gaucher's disease. The results established that the heterogeneity of the dipole potential in living cell membranes is correlated with lipid rafts and imply that alterations in the lipid composition of the cell membrane in human diseases can lead to substantial changes in the dipole potential.

Green Tea Catechins for Prostate Cancer Prevention: Present Achievements and Future Challenges

Green tea catechins (GTCs) are a family of chemically related compounds usually classified as antioxidant molecules. Epidemiological evidences, supported by interventional studies, highlighted a more than promising role for GTCs in human prostate cancer (PCa) chemoprevention. In the last decades, many efforts have been made to gain new insights into the mechanism of action of GTCs. Now it is clear that GTCs' anticancer action can no longer be simplistically limited to their direct antioxidant/pro-oxidant properties. Recent contributions to the advancement of knowledge in this field have shown that GTCs specifically interact with cellular targets, including cell surface receptors, lipid rafts, and endoplasmic reticulum, modulate gene expression through direct effect on transcription factors or indirect epigenetic mechanisms, and interfere with intracellular proteostasis at various levels. Many of the effects observed in vitro are dose and cell context dependent and take place at concentrations that cannot be achieved in vivo. Poor intestinal absorption together with an extensive systemic and enteric metabolism influence GTCs' bioavailability through still poorly understood mechanisms. Recent efforts to develop delivery systems that increase GTCs' overall bioavailability, by means of biopolymeric nanoparticles, represent the main way to translate preclinical results in a real clinical scenario for PCa chemoprevention.