Interaction of the extracellular domain of the epidermal growth factor receptor with gangliosides

Harnessing potential role of gangliosides in immunomodulation and cancer therapeutics

Gangliosides represent glycolipids containing sialic acid residues, present on the cell membrane with glycan residues exposed to the extracellular matrix (ECM), while the ceramides are anchored within the membrane. These molecules play a critical role in pathophysiological processes such as host-pathogen interactions, cell-cell recognition, signal transduction, cell adhesion, motility, and immunomodulation. Accumulated evidence suggests the overexpression of gangliosides on tumor tissues in comparison to healthy human tissues. These tumor-associated gangliosides have been implicated in various facets of tumor biology, including cell motility, differentiation, signaling, immunosuppression, angiogenesis, and metastasis. Consequently, these entities emerge as attractive targets for immunotherapeutic interventions. Notably, the administration of antibodies targeting gangliosides has demonstrated cytotoxic effects on cancer cells that exhibit an overexpression of these glycolipids. Passive immunotherapy approaches utilizing murine or murine/human chimeric anti-ganglioside antibodies have been explored as potential treatments for diverse cancer types. Additionally, vaccination strategies employing tumor-associated gangliosides in conjunction with adjuvants have entered the realm of promising techniques currently undergoing clinical trials. The present comprehensive review encapsulates the multifaceted roles of gangliosides in tumor initiation, progression, immunosuppression, and metastasis. Further, an overview is provided of the correlation between the expression status of gangliosides in normal and tumor cells and its impact on cancer patient survival. Furthermore, the discussion extends to ongoing and completed clinical trials employing diverse strategies to target gangliosides, elucidating their effectiveness in treating cancers. This emerging discipline is expected to supply substantial impetus for the establishment of novel therapeutic strategies.

T7 phage display reveals NOLC1 as a GM3 binding partner in human breast cancer MCF-7 cells

Ganglioside GM3 is a simple monosialoganglioside (NeuAc-Gal-Glc-ceramide) that modulates cell adhesion, proliferation, and differentiation. Previously, we reported isolation of GM3-binding vascular endothelial growth factor receptor and transforming growth factor-β receptor by the T7 phage display method (Chung et al., 2009; Kim et al., 2013). To further identify novel proteins interacting with GM3, we extended the T7 phage display method in this study. After T7 phage display biopanning combined with immobilized biotin-labeled 3'-sialyllactose prepared on a streptavidin-coated microplate, we isolated 100 candidate sequences from the human lung cDNA library. The most frequently detected clones from the blast analysis were the human nucleolar and coiled-body phosphoprotein 1 (NOLC1) sequences. We initially identified NOLC1 as a molecule that possibly binds to GM3 and confirmed this binding ability using the glutathione S-transferase fusion protein. Herein, we report another GM3-interacting protein, NOLC1, that can be isolated by the T7 phage display method. These results are expected to be helpful for elucidating the functional roles of ganglioside GM3 with NOLC1. When human breast cancer MCF-7 cells were examined for subcellular localization of NOLC1, immunofluorescence of NOLC1 was observed in the intracellular region. In addition, NOLC1 expression was increased in the nucleolus after treatment with the anticancer drug doxorubicin. GM3 and NOLC1 levels in the doxorubicin-treated MCF-7 cells were correlated, indicating possible associations between GM3 and NOLC1. Therefore, direct interactions between carbohydrates and cellular proteins can pave the path for new signaling phenomena in biology.

Neural-specific alterations in glycosphingolipid biosynthesis and cell signaling associated with two human ganglioside GM3 synthase deficiency variants

GM3 Synthase Deficiency (GM3SD) is a neurodevelopmental disorder resulting from pathogenic variants in the ST3GAL5 gene, which encodes GM3 synthase, a glycosphingolipid (GSL)-specific sialyltransferase. This enzyme adds a sialic acid to the terminal galactose of lactosylceramide (LacCer) to produce the monosialylated ganglioside GM3. In turn, GM3 is extended by other glycosyltransferases to generate nearly all the complex gangliosides enriched in neural tissue. Pathogenic mechanisms underlying the neural phenotypes associated with GM3SD are unknown. To explore how loss of GM3 impacts neural-specific glycolipid glycosylation and cell signaling, GM3SD patient fibroblasts bearing one of two different ST3GAL5 variants were reprogrammed to induced pluripotent stem cells (iPSCs) and then differentiated to neural crest cells (NCCs). GM3 and GM3-derived gangliosides were undetectable in cells carrying either variant, while LacCer precursor levels were elevated compared to wildtype (WT). NCCs of both variants synthesized elevated levels of neutral lacto- and globo-series, as well as minor alternatively sialylated GSLs compared to WT. Ceramide profiles were also shifted in GM3SD variant cells. Altered GSL profiles in GM3SD cells were accompanied by dynamic changes in the cell surface proteome, protein O-GlcNAcylation, and receptor tyrosine kinase abundance. GM3SD cells also exhibited increased apoptosis and sensitivity to erlotinib-induced inhibition of epidermal growth factor receptor signaling. Pharmacologic inhibition of O-GlcNAcase rescued baseline and erlotinib-induced apoptosis. Collectively, these findings indicate aberrant cell signaling during differentiation of GM3SD iPSCs and also underscore the challenge of distinguishing between variant effect and genetic background effect on specific phenotypic consequences.

Sialidase-Resistant Ganglioside GM3 Analogues: Evaluation of Biological Activity

Glycolipids play important biological roles mainly in biological membranes, but their functions at the molecular level remain to be fully established. A chemical biology approach using exogenously added glycolipid probes would be promising, but the possibility of cleavage by cellular glycohydrolases complicates the interpretation of results. Thus, there is a need for non-hydolyzable analogues. In the present study, we designed and synthesized GM3 analogues resistant to GM3-degrading sialidase by replacing the O-sialoside linkage with a C-sialoside linkage. The bioactivity of the analogues was also investigated.

Regulation of signal transduction by gangliosides in lipid rafts: focus on GM3-IR and GM1-TrkA interactions

The interactions between gangliosides and proteins belonging to the same or different lipid domains and their influence on physiological and pathological states have been analysed in detail. A well-known factor impacting on lipid-protein interactions and their biological outcomes is the dynamic composition of plasma membrane. This review focuses on GM1 and GM3 gangliosides because they are an integral part of protein-receptor complexes and dysregulation of their concentration shows a direct correlation with the onset of pathological conditions. We first discuss the interaction between GM3 and insulin receptor in relation to insulin responses, with an increase in GM3 correlating with the onset of metabolic dysfunction. Next, we describe the case of the GM1-TrkA interaction, relevant to nerve-cell differentiation and homeostasis as deficiency in plasma-membrane GM1 is known to promote neurodegeneration. These two examples highlight the fact that interactions between gangliosides and receptor proteins within the plasma membrane are crucial in controlling cell signalling and pathophysiological cellular states.

Lipid Signaling During Gamete Maturation

Cell lipids are differentially distributed in distinct organelles and within the leaflets of the bilayer. They can further form laterally defined sub-domains within membranes with important signaling functions. This molecular and spatial complexity offers optimal platforms for signaling with the associated challenge of dissecting these pathways especially that lipid metabolism tends to be highly interconnected. Lipid signaling has historically been implicated in gamete function, however the detailed signaling pathways involved remain obscure. In this review we focus on oocyte and sperm maturation in an effort to consolidate current knowledge of the role of lipid signaling and set the stage for future directions.

Glycosphingolipids in human embryonic stem cells and breast cancer stem cells, and potential cancer therapy strategies based on their structures and functions

Expression profiles of glycosphingolipids (GSLs) in human embryonic stem cell (hESC) lines and their differentiated embryoid body (EB) outgrowth cells, consisting of three germ layers, were surveyed systematically. Several globo- and lacto-series GSLs were identified in undifferentiated hESCs and during differentiation of hESCs to EB outgrowth cells, and core structure switching of these GSLs to gangliosides was observed. Such switching was attributable to altered expression of key glycosyltransferases (GTs) in GSL biosynthetic pathways, reflecting the unique stage-specific transitions and mechanisms characteristic of the differentiation process. Lineage-specific differentiation of hESCs was associated with further GSL alterations. During differentiation of undifferentiated hESCs to neural progenitor cells, core structure switching from globo- and lacto-series to primarily gangliosides (particularly GD3) was again observed. During differentiation to endodermal cells, alterations of GSL profiles were distinct from those in differentiation to EB outgrowth or neural progenitor cells, with high expression of Gb₄Cer and low expression of stage-specific embryonic antigen (SSEA)-3, -4, or GD3 in endodermal cells. Again, such profile changes resulted from alterations of key GTs in GSL biosynthetic pathways. Novel glycan structures identified on hESCs and their differentiated counterparts presumably play functional roles in hESCs and related cancer or cancer stem cells, and will be useful as surface biomarkers. We also examined GSL expression profiles in breast cancer stem cells (CSCs), using a model of epithelial-mesenchymal transition (EMT)-induced human breast CSCs. We found that GD2 and GD3, together with their common upstream GTs, GD3 synthase (GD3S) and GD2/GM2 synthase, maintained stem cell phenotype in breast CSCs. Subsequent studies showed that GD3 was associated with epidermal growth factor receptor (EGFR), and activated EGFR signaling in breast CSCs and breast cancer cell lines. GD3S knockdown enhanced cytotoxicity of gefitinib (an EGFR kinase inhibitor) in resistant MDA-MB468 cells, both in vitro and in vivo. Our findings indicate that GD3S contributes to gefitinib resistance in EGFR-positive breast cancer cells, and is a potentially useful therapeutic target in drug-resistant breast cancers.

Structure and function of glycosphingolipids on small extracellular vesicles

Extracellular vesicles (EVs) are membrane-delineated particles secreted by most types of cells under both normal and pathophysiological conditions. EVs are believed to mediate intercellular communication by serving as carriers of different bioactive ingredients, including proteins, nucleic acids and lipids. Glycoconjugates are complex molecules consisting of covalently linked carbohydrate with proteins or lipids. These glycoconjugates play essential roles in the sorting of vesicular protein and the uptake of small extracellular vesicles (30–100 nm, sEVs) into recipient cells. Glycosphingolipids (GSLs), one subtype of glycolipids, which are ubiquitous membrane components in almost all living organisms, are also commonly distributed on sEVs. However, the study of functional roles of GSLs on sEVs are far behind than other functional cargos. The purpose of this review is to highlight the importance of GSLs on sEVs. Initially, we described classification and structure of GSLs. Then, we briefly introduced the essential functions of GSLs, which are able to interact with functional membrane proteins, such as growth factor receptors, integrins and tetraspanins, to modulate cell growth, adhesion and cell motility. In addition, we discussed analytical methods for studying GSLs on sEVs. Finally, we focused on the function of GSLs on sEVs, including regulating the aggregation of extracellular α-synuclein (α-syn) or extracellular amyloid-β (Aβ) and influencing tumor cell malignancy.

Role of glycosyltransferases in carcinogenesis; growth factor signaling and EMT/MET programs

The glycosylation of cell surface receptors has been shown to regulate each step of signal transduction, including receptor trafficking to the cell surface, ligand binding, dimerization, phosphorylation, and endocytosis. In this review we focus on the role of glycosyltransferases that are involved in the modification of N-glycans, such as the effect of branching and elongation in signaling by various cell surface receptors. In addition, the role of those enzymes in the EMT/MET programs, as related to differentiation and cancer development, progress and therapy resistance is discussed.

Host lactosylceramide enhances Edwardsiella tarda infection

Edwardsiella tarda is a Gram-negative bacterium causing economic damage in aquaculture. The interaction of E. tarda with microdomains is an important step in the invasion, but the target molecules in microdomains remain undefined. Here, we found that intraperitoneal injection of E. tarda altered splenic glycosphingolipid patterns in the model host medaka (Oryzias latipes) accompanied by alteration of glycosphingolipid metabolism-related gene expressions, suggesting that glycosphingolipid levels are involved in E. tarda infection. To ascertain the significance of glycosphingolipids in the infection, fish cell lines, DIT29 cells with a high amount of lactosylceramide (LacCer) and glucosylceramide (GlcCer), and GAKS cells with a low amount of these lipids, were treated with methyl-β-cyclodextrin to disrupt the microdomain. E. tarda infection was suppressed in DIT29 cells, but not in GAKS cells, suggesting the involvement of microdomain LacCer and GlcCer in the infection. DL-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, an inhibitor of glycosphingolipid-synthesis, attenuated the infection in DIT29 cells, while Neu3-overexpressing GAKS cells, which accumulated LacCer, enhanced the infection. E. tarda possessed binding ability towards LacCer, but not GlcCer, and LacCer preincubation declined the infection towards fish cells, possibly due to the masking of binding sites. The present study suggests that LacCer may be a positive regulator of E. tarda invasion.

A Comprehensive Review: Sphingolipid Metabolism and Implications of Disruption in Sphingolipid Homeostasis

Sphingolipids are a specialized group of lipids essential to the composition of the plasma membrane of many cell types; however, they are primarily localized within the nervous system. The amphipathic properties of sphingolipids enable their participation in a variety of intricate metabolic pathways. Sphingoid bases are the building blocks for all sphingolipid derivatives, comprising a complex class of lipids. The biosynthesis and catabolism of these lipids play an integral role in small- and large-scale body functions, including participation in membrane domains and signalling; cell proliferation, death, migration, and invasiveness; inflammation; and central nervous system development. Recently, sphingolipids have become the focus of several fields of research in the medical and biological sciences, as these bioactive lipids have been identified as potent signalling and messenger molecules. Sphingolipids are now being exploited as therapeutic targets for several pathologies. Here we present a comprehensive review of the structure and metabolism of sphingolipids and their many functional roles within the cell. In addition, we highlight the role of sphingolipids in several pathologies, including inflammatory disease, cystic fibrosis, cancer, Alzheimer’s and Parkinson’s disease, and lysosomal storage disorders.

The structure of gangliosides hides a code for determining neuronal functions

Gangliosides are particularly abundant in the central nervous system, where they are mainly associated with the synaptic membranes. Their structure underlies a specific role in determining several cell physiological processes of the nervous system. The high number of different gangliosides available in nature suggests that their structure, related to both the hydrophobic and hydrophilic portion of the molecule, defines a code, although not completely understood, that through hydrophobic interactions and hydrogen bonds allows the transduction of signals starting at the plasma membranes. In this short review, we describe some structural aspects responsible for the role played by gangliosides in maintaining and determining neuronal functions.

FRET detects lateral interaction between transmembrane domain of EGF receptor and ganglioside GM3 in lipid bilayers

Ganglioside GM3 in the plasma membranes suppresses cell growth by preventing the autophosphorylation of the epidermal growth factor receptor (EGFR). Biological studies have suggested that GM3 interacts with the transmembrane segment of EGFR. Further biophysical experiments are particularly important for quantitative evaluation of the peptide-glycolipid interplay in bilayer membranes using a simple reconstituted system. To examine these interactions in this way, we synthesized the transmembrane segment of EGFR bearing a nitrobenzoxadiazole fluorophore (NBD-TM) at the N-terminus. The affinity between EGFR and GM3 was evaluated based on Förster resonance energy transfer (FRET) between NBD-TM and ATTO594-labeled GM3 in bilayers where their non-specific interaction due to lateral proximity was subtracted by using NBD-labeled phospholipid. This method for selectively detecting the specific lipid-peptide interactions in model lipid bilayers disclosed that the lateral interaction between GM3 and the transmembrane segment of EGFR plays a certain role in disturbing the formation of active EGFR dimers.

Ganglioside GM3 Analogues Containing Monofluoromethylene-Linked Sialoside: Synthesis, Stereochemical Effects, Conformational Behavior, and Biological Activities

Glycoconjugates are an important class of biomolecules that regulate numerous biological events in cells. However, these complex, medium-size molecules are metabolically unstable, which hampers detailed investigations of their functions as well as their potential application as pharmaceuticals. Here we report sialidase-resistant analogues of ganglioside GM3 containing a monofluoromethylene linkage instead of the native O-sialoside linkage. Stereoselective synthesis of CHF-linked disaccharides and kinetically controlled Au(I)-catalyzed glycosylation efficiently furnished both stereoisomers of CHF-linked as well as CF ₂ - and CH ₂ -linked GM3 analogues. Like native GM3, the C-linked GM3 analogues inhibited the autophosphorylation of epidermal growth factor (EGF) receptor induced by EGF in vitro. Assay of the proliferation-enhancing activity toward Had-1 cells together with NMR-based conformational analysis showed that the (S)-CHF-linked GM3 analogue with exo-gauche conformation is the most potent of the synthesized compounds. Our findings suggest that exo-anomeric conformation is important for the biological functions of GM3.

Glycosphingolipids

Glycosphingolipids are amphiphilic plasma membrane components formed by a glycan linked to a specific lipid moiety. In this chapter we report on these compounds, on their role played in our cells to maintain the correct cell biology.In detail, we report on their structure, on their metabolic processes, on their interaction with proteins and from this, their property to modulate positively in health and negatively in disease, the cell signaling and cell biology.

Role of NEU3 Overexpression in the Prediction of Efficacy of EGFR-Targeted Therapies in Colon Cancer Cell Lines

The epidermal growth factor receptor (EGFR), through the MAP kinase and PI3K-Akt-mTOR axis, plays a pivotal role in colorectal cancer (CRC) pathogenesis. The membrane-associated NEU3 sialidase interacts with and desialylates EGFR by promoting its dimerization and downstream effectors’ activation. Among the targeted therapies against EGFR, the monoclonal antibody cetuximab is active only in a subgroup of patients not carrying mutations in the MAP kinase pathway. In order to better understand the EGFR-NEU3 interplay and the mechanisms of pharmacological resistance, we investigated the role of NEU3 deregulation in cetuximab-treated CRC cell lines transiently transfected with NEU3 using Western blot analysis. Our results indicate that NEU3 overexpression can enhance EGFR activation only if EGFR is overexpressed, indicating the existence of a threshold for NEU3-mediated EGFR activation. This enhancement mainly leads to the constitutive activation of the MAP kinase pathway. Consequently, we suggest that the evaluation of NEU3 expression cannot entirely substitute the evaluation of EGFR because EGFR-negative cases cannot be stimulated by NEU3. Furthermore, NEU3-mediated hyperactivation of EGFR is counterbalanced by the administration of cetuximab, hypothesizing that a combined treatment of NEU3- and EGFR-targeted therapies may represent a valid option for CRC patients, which must be investigated in the future.

Effects of Gangliosides on Spermatozoa, Oocytes, and Preimplantation Embryos

Gangliosides are sialic acid-containing glycosphingolipids, which are the most abundant family of glycolipids in eukaryotes. Gangliosides have been suggested to be important lipid molecules required for the control of cellular procedures, such as cell differentiation, proliferation, and signaling. GD1a is expressed in interstitial cells during ovarian maturation in mice and exogenous GD1a is important to oocyte maturation, monospermic fertilization, and embryonic development. In this context, GM1 is known to influence signaling pathways in cells and is important in sperm-oocyte interactions and sperm maturation processes, such as capacitation. GM3 is expressed in the vertebrate oocyte cytoplasm, and exogenously added GM3 induces apoptosis and DNA injury during in vitro oocyte maturation and embryogenesis. As a consequence of this, ganglioside GT1b and GM1 decrease DNA fragmentation and act as H2O2 inhibitors on germ cells and preimplantation embryos. This review describes the functional roles of gangliosides in spermatozoa, oocytes, and early embryonic development.

Ganglioside GM3 and Its Role in Cancer

Ganglioside GM3 is strongly related with human tumors, such as lung, brain cancers and melanomas, and more and more evidences have revealed that GM3 possesses powerful effects on cancer development and progression. GM3 is over expressed on several types of cancers, and can be as a tumor-associated carbohydrate antigen, used for immunotherapy of cancers. GM3 can also inhibit tumor cells growth by anti-angiogenesis or motility and so on. Especially, GM3 has effects on the EGFR tyrosine kinase signaling, uPAR-related signaling and glycolipid-enriched microdomains, which are essential for cancer signaling conduction. It is obvious that GM3 will be a promising target for cancer treatment.

Ganglioside GM1 promotes contact inhibition of growth by regulating the localization of epidermal growth factor receptor from glycosphingolipid-enriched microdomain to caveolae

Objectives

Accumulating data show that gangliosides are involved in regulation of cell proliferation. Specific changes in gangliosides expression associated with growth density of cells have been documented in several cell lines. However, the function and the potential mechanism of ganglioside GM1 in contact inhibition of growth are not clear.

Materials and Methods

EdU incorporation assay and western blot were applied to detect the contact inhibition of growth in human mammary epithelial cells. GM1 manipulation of cell proliferation and epidermal growth factor receptor (EGFR) activation was investigated by immunoprecipitation, OptiPrep density gradient centrifugation and immunofluorescence. The function of GM1 on contact inhibition of growth was further studied by using GM1 stably knockdown and overexpression cells.

Results

MCF‐10A, MCF‐7 and MDA‐MB‐231 cells showed contact inhibition of growth in high‐density condition. Exogenous addition of GM1 to high‐density cells clearly inhibited cell growth and deactivated EGFR signalling. Compared to normal‐density cells, distribution of EGFR in high‐density cells was decreased in glycosphingolipid‐enriched microdomain (GEM), but more concentrated in caveolae, and incubation with GM1 obviously promoted this translocation. Furthermore, the cell growth and EGFR activation were increased in GM1 stably knockdown cells and decreased in GM1 stably overexpression cells when cultured in high density.

Conclusions

Our results demonstrated that GM1 suppressed EGFR signalling and promoted contact inhibition of growth by changing the localization of EGFR from GEM to caveolae.

Overexpression of HexCer and LacCer containing 2-hydroxylated fatty acids in cholangiocarcinoma and the association of the increase of LacCer (d18:1-h23:0) with shorter survival of the patients

Alteration of glycosphingolipid (GSL) synthesis is observed in many types of cancer. In this study, we have analyzed the expression of sphingolipids and GSLs in cholangiocarcinoma (CCA) tissues and adjacent normal liver tissues. Neutral lipids were extracted from tissue samples using mild-alkaline treatment method followed by TLC and LC-MS analysis. The expression of ceramides, hexosylceramides (HexCer), and lactosylceramides (LacCer) was altered in CCA tissues, 61.1% (11/18) of them showing an increase whereas 38.9% (7/18) showing a decrease, compared with the adjacent normal tissue. Cers and GSLs containing 2-hydroxylated fatty acids except one LacCer molecular species were overexpressed in CCA tissues, and the increase of LacCer (d18:1-h23:0) was correlated with shorter survival of CCA patients, suggesting the involvement of GSL synthesis and fatty acid hydroxylation in progression of CCA. Taken together, we have demonstrated in this study the increase of GSL synthesis and fatty hydroxylation in CCA, which probably be used as a target for CCA treatment.

Sialic acid as a target for the development of novel antiangiogenic strategies

Sialic acid is associated with glycoproteins and gangliosides of eukaryotic cells. It regulates various molecular interactions, being implicated in inflammation and cancer, where its expression is regulated by sialyltransferases and sialidases. Angiogenesis, the formation of new capillaries, takes place during inflammation and cancer, and represents the outcome of several interactions occurring at the endothelial surface among angiogenic growth factors, inhibitors, receptors, gangliosides and cell-adhesion molecules. Here, we elaborate on the evidences that many structures involved in angiogenesis are sialylated and that their interactions depend on sialic acid with implications in angiogenesis itself, inflammation and cancer. We also discuss the possibility to exploit sialic acid as a target for the development of novel antiangiogenic drugs.

The crosstalk between glycosphingolipids and neural stem cells

Until a few years ago, the majority of cell functions were envisioned as the result of protein and DNA activity. The cell membranes were considered as a mere structure of support and/or separation. In the last years, the function of cell membranes has, however, received more attention and their components of lipid nature have also been depicted as important cell mediators and the membrane organization was described as an important determinant for membrane-anchored proteins activity. In particular, because of their high diversity, glycosphingolipids offer a wide possibility of regulation. Specifically, the role of glycosphingolipids, in the fine-tuning of neuron activity, has recently received deep attention. For their pivotal role in vertebrate and mammals neural development, neural stem cells regulation is of main interest especially concerning their further functions in neurological pathology progression and treatment. Glycosphingolipids expression present a developmental regulation. In this view, glycosphingolipids can hold an important role in neural stem cells features because of their heterogeneity and their consequent capacity for eclectic interaction with other cell components.

GD3 ganglioside-enriched extracellular vesicles stimulate melanocyte migration

Melanomas often accumulate gangliosides, sialic acid-containing glycosphingolipids found in the outer leaflet of plasma membranes, as disialoganglioside G_D3 and its derivatives. Here, we have transfected the G_D3 synthase gene (ST8Sia I) in a normal melanocyte cell line in order to evaluate changes in the biological behavior of non-transformed cells. G_D3-synthase expressing cells converted G_M3 into G_D3 and accumulated both G_D3 and its acetylated form, 9-O-acetyl-G_D3. Melanocytes were rendered more migratory on laminin-1 surfaces. Cell migration studies using the different transfectants, either treated or not with the glucosylceramide synthase inhibitor d-1-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (PPPP), allowed us to show that while G_M3 is a negative regulator of melanocyte migration, G_D3 increases it. We showed that gangliosides were shed to the matrix by migrating cells and that G_D3 synthase transfected cells shed extracellular vesicles (EVs) enriched in G_D3. EVs enriched in G_D3 stimulated cell migration of G_D3 negative cells, as observed in time lapse microscopy studies. Otherwise, EVs shed by G_M3^+veG_D3^-ve cells impaired migration and diminished cell velocity in cells overexpressing G_D3. The balance of antimigratory G_M3 and promigratory G_D3 gangliosides in melanocytes could be altered not only by the overexpression of enzymes such as ST8Sia I, but also by the horizontal transfer of ganglioside enriched extracellular vesicles. This study highlights that extracellular vesicles transfer biological information also through their membrane components, which include a variety of glycosphingolipids remodeled in disease states such as cancer.

Single Site Fluorination of the GM4 Ganglioside Epitope Upregulates Oligodendrocyte Differentiation

Relapsing multiple sclerosis is synonymous with demyelination, and thus, suppressing and or reversing this process is of paramount clinical significance. While insulating myelin sheath has a large lipid composition (ca. 70-80%), it also has a characteristically large composition of the sialosylgalactosylceramide gangliosde GM₄ present. In this study, the effect of the carbohydrate epitope on oligodendrocyte differentiation is determined. While the native epitope had no impact on oligodendroglial cell viability, a single site OH → F substitution is the structural basis of a significant increase in ATP production that is optimal at 50 μg/mL. From a translational perspective, this subtle change increases the amount of MBP+ oligodendrocytes compared to the control studies and may open up novel therapeutic remyelination strategies.

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.

N-glycans of growth factor receptors: their role in receptor function and disease implications

Numerous signal-transduction-related molecules are secreted proteins or membrane proteins, and the mechanism by which these molecules are regulated by glycan chains is a very important issue for developing an understanding of the cellular events that transpire. This review covers the functional regulation of epidermal growth factor receptor (EGFR), ErbB3 and the transforming growth factor β (TGF-β) receptor by N-glycans. This review shows that the N-glycans play important roles in regulating protein conformation and interactions with carbohydrate recognition molecules. These results point to the possibility of a novel strategy for controlling cell signalling and developing novel glycan-based therapeutics.

Role of Cytokine-Induced Glycosylation Changes in Regulating Cell Interactions and Cell Signaling in Inflammatory Diseases and Cancer

Glycosylation is one of the most important modifications of proteins and lipids, and cell surface glycoconjugates are thought to play important roles in a variety of biological functions including cell-cell and cell-substrate interactions, bacterial adhesion, cell immunogenicity and cell signaling. Alterations of glycosylation are observed in number of diseases such as cancer and chronic inflammation. In that context, pro-inflammatory cytokines have been shown to modulate cell surface glycosylation by regulating the expression of glycosyltransferases involved in the biosynthesis of carbohydrate chains. These changes in cell surface glycosylation are also known to regulate cell signaling and could contribute to disease pathogenesis. This review summarizes our current knowledge of the glycosylation changes induced by pro-inflammatory cytokines, with a particular focus on cancer and cystic fibrosis, and their consequences on cell interactions and signaling.

Integrin-mediated cell migration is blocked by inhibitors of human neuraminidase

Integrins are critical receptors in cell migration and adhesion. A number of mechanisms are known to regulate the function of integrins, including phosphorylation, conformational change, and cytoskeletal anchoring. We investigated whether native neuraminidase (Neu, or sialidase) enzymes which modify glycolipids could play a role in regulating integrin-mediated cell migration. Using a scratch assay, we found that exogenously added Neu3 and Neu4 activity altered rates of cell migration. We observed that Neu4 increased the rate of migration in two cell lines (HeLa, A549); while Neu3 only increased migration in HeLa cells. A bacterial neuraminidase was able to increase the rate of migration in HeLa, but not in A549 cells. Treatment of cells with complex gangliosides (GM1, GD1a, GD1b, and GT1b) resulted in decreased cell migration rates, while LacCer was able to increase rates of migration in both lines. Importantly, our results show that treatment of cells with inhibitors of native Neu enzymes had a dramatic effect on the rates of cell migration. The most potent compound tested targeted the human Neu4 isoenzyme, and was able to substantially reduce the rate of cell migration. We found that the lateral mobility of integrins was reduced by treatment of cells with Neu3, suggesting that Neu3 enzyme activity resulted in changes to integrin-co-receptor or integrin-cytoskeleton interactions. Finally, our results support the hypothesis that inhibitors of human Neu can be used to investigate mechanisms of cell migration and for the development of anti-adhesive therapies.

Disease-associated glycans on cell surface proteins

Most of membrane molecules including cell surface receptors and secreted proteins including ligands are glycoproteins and glycolipids. Therefore, identifying the functional significance of glycans is crucial for developing an understanding of cell signaling and subsequent physiological and pathological cellular events. In particular, the function of N-glycans associated with cell surface receptors has been extensively studied since they are directly involved in controlling cellular functions. In this review, we focus on the roles of glycosyltransferases that are involved in the modification of N-glycans and their target proteins such as epidermal growth factor receptor (EGFR), ErbB3, transforming growth factor β (TGF-β) receptor, T-cell receptors (TCR), β-site APP cleaving enzyme (BACE1), glucose transporter 2 (GLUT2), E-cadherin, and α5β1 integrin in relation to diseases and epithelial-mesenchymal transition (EMT) process. Above of those proteins are subjected to being modified by several glycosyltransferases such as N-acetylglucosaminyltransferase III (GnT-III), N-acetylglucosaminyltransferase IV (GnT-IV), N-acetylglucosaminyltransferase V (GnT-V), α2,6 sialyltransferase 1 (ST6GAL1), and α1,6 fucosyltransferase (Fut8), which are typical N-glycan branching enzymes and play pivotal roles in regulating the function of cell surface receptors in pathological cell signaling.

Structure-Activity Relationship Study of the Neuritogenic Potential of the Glycan of Starfish Ganglioside LLG-3 (‡)

LLG-3 is a ganglioside isolated from the starfish Linchia laevigata. To clarify the structure-activity relationship of the glycan of LLG-3 toward rat pheochromocytoma PC12 cells in the presence of nerve growth factor, a series of mono- to tetrasaccharide glycan derivatives were chemically synthesized and evaluated in vitro. The methyl group at C8 of the terminal sialic acid residue was crucial for neuritogenic activity, and the terminal trisaccharide moiety was the minimum active motif. Furthermore, the trisaccharide also stimulated neuritogenesis in human neuroblastoma SH-SY5Y cells via mitogen-activated protein kinase (MAPK) signaling. Phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 was rapidly induced by adding 1 or 10 nM of the trisaccharide. The ratio of phosphorylated ERK to ERK reached a maximum 5 min after stimulation, and then decreased gradually. However, the trisaccharide did not induce significant Akt phosphorylation. These effects were abolished by pretreatment with the MAPK inhibitor U0126, which inhibits enzymes MEK1 and MEK2. In addition, U0126 inhibited the phosphorylation of ERK 1/2 in response to the trisaccharide dose-dependently. Therefore, we concluded that the trisaccharide promotes neurite extension in SH-SY5Y cells via MAPK/ERK signaling, not Akt signaling.

Role of lipid rafts in neuronal differentiation of dental pulp-derived stem cells

Human dental pulp-derived stem cells (hDPSCs) are characterized by a typical fibroblast-like morphology. They express specific markers for mesenchymal stem cells and are capable of differentiation into osteoblasts, adipoblasts and neurons in vitro. Previous studies showed that gangliosides are involved in the induction of early neuronal differentiation of hDPSCs. This study was undertaken to investigate the role of lipid rafts in this process. Lipid rafts are signaling microdomains enriched in glycosphingolipids, cholesterol, tyrosine kinase receptors, mono- or heterotrimeric G proteins and GPI-anchored proteins. We preliminary showed that established cells expressed multipotent mesenchymal stromal-specific surface antigens. Then, we analyzed the distribution of lipid rafts, revealing plasma membrane microdomains with GM2 and EGF-R enrichment. Following stimulation with EGF/bFGF, neuronal differentiation was observed. To analyze the functional role of lipid rafts in EGF/bFGF-induced hDPSCs differentiation, cells were preincubated with lipid raft affecting agents, i.e. [D]-PDMP or methyl-β-cyclodextrin. These compounds significantly prevented neuronal-specific antigen expression, as well as Akt and ERK 1/2 phosphorylation, induced by EGF/bFGF, indicating that lipid raft integrity is essential for EGF/bFGF-induced hDPSCs differentiation. These results suggest that lipid rafts may represent specific chambers, where multimolecular signaling complexes, including lipids (gangliosides, cholesterol) and proteins (EGF-R), play a role in hDPSCs differentiation.

Two-Dimensional Trap for Ultrasensitive Quantification of Transient Protein Interactions

We present an ultrasensitive technique for quantitative protein-protein interaction analysis in a two-dimensional format based on phase-separated, micropatterned membranes. Interactions between proteins captured to lipid probes via an affinity tag trigger partitioning into the liquid-ordered phase, which is readily quantified by fluorescence imaging. Based on a calibration with well-defined low-affinity protein-protein interactions, equilibrium dissociation constants >1 mM were quantified. Direct capturing of proteins from mammalian cell lysates enabled us to detect homo- and heterodimerization of signal transducer and activator of transcription proteins. Using the epidermal growth factor receptor (EGFR) as a model system, quantification of low-affinity interactions between different receptor domains contributing to EGFR dimerization was achieved. By exploitation of specific features of the membrane-based assay, the regulation of EGFR dimerization by lipids was demonstrated.

Integrin α5 Suppresses the Phosphorylation of Epidermal Growth Factor Receptor and Its Cellular Signaling of Cell Proliferation via N-Glycosylation

Integrin α5β1-mediated cell adhesion regulates a multitude of cellular responses, including cell proliferation, survival, and cross-talk between different cellular signaling pathways. Integrin α5β1 is known to convey permissive signals enabling anchorage-dependent receptor tyrosine kinase signaling. However, the effects of integrin α5β1 on cell proliferation are controversial, and the molecular mechanisms involved in the regulation between integrin α5β1 and receptor tyrosine kinase remain largely unclear. Here we show that integrin α5 functions as a negative regulator of epidermal growth factor receptor (EGFR) signaling through its N-glycosylation. Expression of WT integrin α5 suppresses the EGFR phosphorylation and internalization upon EGF stimulation. However, expression of the N-glycosylation mutant integrin α5, S3-5, which contains fewer N-glycans, reversed the suppression of the EGFR-mediated signaling and cell proliferation. In a mechanistic manner, WT but not S3-5 integrin α5 forms a complex with EGFR and glycolipids in the low density lipid rafts, and the complex formation is disrupted upon EGF stimulation, suggesting that the N-glycosylation of integrin α5 suppresses the EGFR activation through promotion of the integrin α5-glycolipids-EGFR complex formation. Furthermore, consistent restoration of those N-glycans on the Calf-1,2 domain of integrin α5 reinstated the inhibitory effects as well as the complex formation with EGFR. Taken together, these data are the first to demonstrate that EGFR activation can be regulated by the N-glycosylation of integrin α5, which is a novel molecular paradigm for the cross-talk between integrins and growth factor receptors.

Potentiation of epidermal growth factor-mediated oncogenic transformation by sialidase NEU3 leading to Src activation

We previously demonstrated that sialidase NEU3, a key glycosidase for ganglioside degradation, is up-regulated in various human cancers, leading to increased cell invasion, motility and survival of cancer cells possibly through activation of EGF signaling. Its up-regulation is also important for promotion of the stage of colorectal carcinogenesis in vivo in human NEU3 transgenic mice treated with azoxymethane for the induction of aberrant crypt foci in the colon mucosa, accompanied by enhanced phosphorylation of EGF receptor (EGFR). To address whether the activation of EGF signaling by the sialidase is associated with oncogenic transformation, we here analyzed the effects of overexpression of NEU3 and EGFR in NIH-3T3 cells. When NEU3 was stably transfected with or without EGFR, it was associated with significant increases in clonogenic growth, clonogenicity on soft agar and in vivo tumor growth in nude mice either with or without the receptor overexpression in the presence of EGF, compared with the levels in their vector controls. Despite the fact that the endogenous level of EGFR is known to be extremely low in these cells, NEU3 significantly enhanced the phosphorylation of Akt and ERK, as well as that of the receptor. The NEU3-mediated activation was largely abrogated by the EGFR inhibitor AG1478 or PD153035, but significant clonogenic growth still remained. NEU3 was then found to activate Src kinase, and the clonogenicity was completely suppressed by an Src inhibitor, PP2. The activity-null mutants failed to activate Src and EGFR, indicating that ganglioside modulation by NEU3 may be necessary for the activation. NEU3 and Src were co-immunoprecipitated with EGFR in NEU3- and EGFR- transfected cells. These findings identify NEU3 as an essential participant in tumorigenesis through the EGFR/Src signaling pathway and a potential target for inhibiting EGFR-mediated tumor progression.

N-Glycosylation as determinant of epidermal growth factor receptor conformation in membranes

The epidermal growth factor receptor (EGFR) regulates several critical cellular processes and is an important target for cancer therapy. In lieu of a crystallographic structure of the complete receptor, atomistic molecular dynamics (MD) simulations have recently shown that they can excel in studies of the full-length receptor. Here we present atomistic MD simulations of the monomeric N-glycosylated human EGFR in biomimetic lipid bilayers that are, in parallel, also used for the reconstitution of full-length receptors. This combination enabled us to experimentally validate our simulations, using ligand binding assays and antibodies to monitor the conformational properties of the receptor reconstituted into membranes. We find that N-glycosylation is a critical determinant of EGFR conformation, and specifically the orientation of the EGFR ectodomain relative to the membrane. In the absence of a structure for full-length, posttranslationally modified membrane receptors, our approach offers new means to structurally define and experimentally validate functional properties of cell surface receptors in biomimetic membrane environments.

Mimicking/extracting structure and functions of natural products: synthetic approaches that address unexplored needs in chemical biology

Natural products are often attractive and challenging targets for synthetic chemists, and many have interesting biological activities. However, synthetic chemists need to be more than simply suppliers of compounds to biologists. Therefore, we have been seeking ways to actively apply organic synthetic methods to chemical biology studies of natural products and their activities. In this personal review, I would like to introduce our work on the development of new biologically active compounds inspired by, or extracted from, the structures of natural products, focusing on enhancement of functional activity and specificity and overcoming various drawbacks of the parent natural products.

Molecular recognition of gangliosides and their potential for cancer immunotherapies

Gangliosides are sialic-acid-containing glycosphingolipids expressed on all vertebrate cells. They are primarily positioned in the plasma membrane with the ceramide part anchored in the membrane and the glycan part exposed on the surface of the cell. These lipids have highly diverse structures, not the least with respect to their carbohydrate chains, with N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc) being the two most common sialic-acid residues in mammalian cells. Generally, human healthy tissue is deficient in NeuGc, but this molecule is expressed in tumors and in human fetal tissues, and was hence classified as an onco-fetal antigen. Gangliosides perform important functions through carbohydrate-specific interactions with proteins, for example, as receptors in cell–cell recognition, which can be exploited by viruses and other pathogens, and also by regulating signaling proteins, such as the epidermal growth factor receptor (EGFR) and the vascular endothelial growth factor receptor (VEGFR), through lateral interaction in the membrane. Through both mechanisms, tumor-associated gangliosides may affect malignant progression, which makes them attractive targets for cancer immunotherapies. In this review, we describe how proteins recognize gangliosides, focusing on the molecular recognition of gangliosides associated with cancer immunotherapy, and discuss the importance of these molecules in cancer research.

Efficient synthesis of chloro-derivatives of sialosyllactosylceramide, and their enhanced inhibitory effect on epidermal growth factor receptor activation

Glycosphingolipids are components of essentially all mammalian cell membranes and are involved in a variety of significant cellular functions, including proliferation, adhesion, motility and differentiation. Sialosyllactosylceramide (GM3) is known to inhibit the activation of epidermal growth factor receptor (EGFR). In the present study, an efficient method for the total chemical synthesis of monochloro- and dichloro-derivatives of the sialosyl residue of GM3 was developed. The structures of the synthesized compounds were fully characterized by high-resolution mass spectrometry and nuclear magnetic resonance. In analyses of EGFR autophosphorylation and cell proliferation ([³H]-thymidine incorporation) in human epidermoid carcinoma A431 cells, two chloro-derivatives exhibited stronger inhibitory effects than GM3 on EGFR activity. Monochloro-GM3, but not GM3 or dichloro-GM3, showed a significant inhibitory effect on ΔEGFR, a splicing variant of EGFR that lacks exons 2-7 and is often found in human glioblastomas. The chemical synthesis of other GM3 derivatives using approaches similar to those described in the present study, has the potential to create more potent EGFR inhibitors to block cell growth or motility of a variety of types of cancer that express either wild-type EGFR or ΔEGFR.

Therapeutic targeting of Neu1 sialidase with oseltamivir phosphate (Tamiflu®) disables cancer cell survival in human pancreatic cancer with acquired chemoresistance

Background

Resistance to drug therapy, along with high rates of metastasis, contributes to the low survival rate in patients diagnosed with pancreatic cancer. An alternate treatment for human pancreatic cancer involving targeting of Neu1 sialidase with oseltamivir phosphate (Tamiflu®) was investigated in human pancreatic cancer (PANC1) cells with acquired resistance to cisplatin and gemcitabine. Its efficacy in overcoming the intrinsic resistance of the cell to chemotherapeutics and metastasis was evaluated.

Methods

Microscopic imaging, immunocytochemistry, immunohistochemistry, and WST-1 cell viability assays were used to evaluate cell survival, morphologic changes, and expression levels of E-cadherin, N-cadherin, and VE-cadherin before and after treatment with oseltamivir phosphate in PANC1 cells with established resistance to cisplatin, gemcitabine, or a combination of the two agents, and in archived paraffin-embedded PANC1 tumors grown in RAGxCγ double mutant mice.

Results

Oseltamivir phosphate overcame the chemoresistance of PANC1 to cisplatin and gemcitabine alone or in combination in a dose-dependent manner, and disabled the cancer cell survival mechanism(s). Oseltamivir phosphate also reversed the epithelial-mesenchymal transition characteristic of the phenotypic E-cadherin to N-cadherin changes associated with resistance to drug therapy. Low-dose oseltamivir phosphate alone or in combination with gemcitabine in heterotopic xenografts of PANC1 tumors growing in RAGxCγ double mutant mice did not prevent metastatic spread to the liver and lung.

Conclusion

Therapeutic targeting of Neu1 sialidase with oseltamivir phosphate at the growth factor receptor level disables the intrinsic signaling platform for cancer cell survival in human pancreatic cancer with acquired chemoresistance. These findings provide evidence for oseltamivir phosphate (Tamiflu) as a potential therapeutic agent for pancreatic cancer resistant to drug therapy.

How Do Gangliosides Regulate RTKs Signaling?

Gangliosides, the glycosphingolipids carrying one or several sialic acid residues, are located on the outer leaflet of the plasma membrane in glycolipid-enriched microdomains, where they interact with molecules of signal transduction pathways including receptors tyrosine kinases (RTKs). The role of gangliosides in the regulation of signal transduction has been reported in many cases and in a large number of cell types. In this review, we summarize the current knowledge on the biosynthesis of gangliosides and the mechanism by which they regulate RTKs signaling.

Interaction of ganglioside GD3 with an EGF receptor sustains the self-renewal ability of mouse neural stem cells in vitro

Mounting evidence supports the notion that gangliosides serve regulatory roles in neurogenesis; little is known, however, about how these glycosphingolipids function in neural stem cell (NSC) fate determination. We previously demonstrated that ganglioside GD3 is a major species in embryonic mouse brain: more than 80% of the NSCs obtained by the neurosphere method express GD3. To investigate the functional role of GD3 in neurogenesis, we compared the properties of NSCs from GD3-synthase knockout (GD3S-KO) mice with those from their wild-type littermates. NSCs from GD3S-KO mice showed decreased self-renewal ability compared with those from the wild-type animals, and that decreased ability was accompanied by reduced expression of EGF receptor (EGFR) and an increased degradation rate of EGFR and EGF-induced ERK signaling. We also showed that EGFR switched from the low-density lipid raft fractions in wild-type NSCs to the high-density layers in the GD3S-KO NSCs. Immunochemical staining revealed colocalization of EGFR and GD3, and EGFR could be immunoprecipitated from the NSC lysate with an anti-GD3 antibody from the wild-type, but not from the GD3S-KO, mice. Tracking the localization of endocytosed EGFR with endocytosis pathway markers indicated that more EGFR in GD3S-KO NSCs translocated through the endosomal-lysosomal degradative pathway, rather than through the recycling pathway. Those findings support the idea that GD3 interacts with EGFR in the NSCs and that the interaction is responsible for sustaining the expression of EGFR and its downstream signaling to maintain the self-renewal capability of NSCs.

Differential expression profiles of glycosphingolipids in human breast cancer stem cells vs. cancer non-stem cells

Previous studies demonstrated that certain glycosphingolipids (GSLs) are involved in various cell functions, such as cell growth and motility. Recent studies showed changes in GSL expression during differentiation of human embryonic stem cells; however, little is known about expression profiles of GSLs in cancer stem cells (CSCs). CSCs are a small subpopulation in cancer and are proposed as cancer-initiating cells, have been shown to be resistant to numerous chemotherapies, and may cause cancer recurrence. Here, we analyzed GSLs expressed in human breast CSCs by applying a CSC model induced through epithelial-mesenchymal transition, using mass spectrometry, TLC immunostaining, and cell staining. We found that (i) Fuc-(n)Lc4Cer and Gb3Cer were drastically reduced in CSCs, whereas GD2, GD3, GM2, and GD1a were greatly increased in CSCs; (ii) among various glycosyltransferases tested, mRNA levels for ST3GAL5, B4GALNT1, ST8SIA1, and ST3GAL2 were increased in CSCs, which could explain the increased expression of GD3, GD2, GM2, and GD1a in CSCs; (iii) the majority of GD2+ cells and GD3+ cells were detected in the CD44(hi)/CD24(lo) cell population; and (iv) knockdown of ST8SIA1 and B4GALNT1 significantly reduced the expression of GD2 and GD3 and caused a phenotype change from CSC to a non-CSC, which was detected by reduced mammosphere formation and cell motility. Our results provide insight into GSL profiles in human breast CSCs, indicate a functional role of GD2 and GD3 in CSCs, and suggest a possible novel approach in targeting human breast CSCs to interfere with cancer recurrence.