Ganglioside GM3 inhibits the proliferation of cultured keratinocytes

Abnormalities of Sphingolipids Metabolic Pathways in the Pathogenesis of Psoriasis

Psoriasis is immune-mediated skin disorder affecting thousands of people. Sphingolipids (SLs) are bioactive molecules present in the epidermis, involved in the following cellular processes: proliferation, differentiation, and apoptosis of keratinocytes. Alterations in SLs synthesis have been observed in psoriatic skin. To investigate if the imbalance in lipid skin metabolism could be related to psoriasis, we analyzed the gene expression in non-lesioned and lesioned skin of patients with psoriasis available in two datasets (GSE161683 and GSE136757) obtained from National Center for Biotechnology Information (NCBI). The differentially expressed genes (DEGs) were searched for using NCBI analysis, and Gene Ontology (GO) biological process analyses were performed using the Database of Annotation, Visualization, and Integrated Discovery (DAVID) platform. Venn diagrams were done with InteractiVenn tool and heatmaps were constructed using Morpheus software. We observed that the gene expression of cytoplasmic phospholipase A₂ (PLA2G4D), glycerophosphodiester phosphodiesterase domain containing 3 (GDP3), arachidonate 12-lipoxygenase R type (ALOX12B), phospholipase B-like 1 (PLBD1), sphingomyelin phosphodiesterase 3 (SMPD3), ganglioside GM2 activator (GM2A), and serine palmitoyltransferase long chain subunit 2 (SPTLC2) was up-regulated in lesioned skin psoriasis when compared with the non-lesioned skin. These genes are related to lipid metabolism and more specifically to sphingolipids. So, in the present study, the role of sphingolipids in psoriasis pathogenesis is summarized. These genes could be used as prognostic biomarkers of psoriasis and could be targets for the treatment of patients who suffer from the disease.

Mammalian Epidermis: A Compendium of Lipid Functionality

Mammalian epidermis is a striking example of the role of lipids in tissue biology. In this stratified epithelium, highly specialized structures are formed that leverage the hydrophobic properties of lipids to form an impermeable barrier and protect the humid internal environment of the body from the dry outside. This is achieved through tightly regulated lipid synthesis that generates the molecular species unique to the tissue. Beyond their fundamental structural role, lipids are involved in the active protection of the body from external insults. Lipid species present on the surface of the body possess antimicrobial activity and directly contribute to shaping the commensal microbiota. Lipids belonging to a variety of classes are also involved in the signaling events that modulate the immune responses to environmental stress as well as differentiation of the epidermal keratinocytes themselves. Recently, high-resolution methods are beginning to provide evidence for the involvement of newly identified specific lipid molecules in the regulation of epidermal homeostasis. In this review we give an overview of the wide range of biological functions of mammalian epidermal lipids.

Ganglioside GD3 May Suppress the Functional Activities of Benign Skin T Cells in Cutaneous T-Cell Lymphoma

In cutaneous T-cell lymphoma (CTCL), which arises from skin-tropic memory T cells, malignant T cells and benign T cells are confined in the same skin lesions. It is thus difficult to evaluate the phenotypic characteristics and functional activities of benign T cells in CTCL. Disialoganglioside with three glycosyl groups (GD3) is increasingly expressed on the surface of solid malignant tumor cells and takes part in tumor progression and suppression of tumor immunity. However, the role of GD3 in CTCL is not well-understood. In this study, the malignant and benign T cells in CTCL skin lesions were distinguished by flow cytometry and their phenotypic characteristics were compared with those of T cells from control skin specimens. In CTCL skin lesions, the benign T cells included limited resident memory T cells (T_RM), which are sessile in skin and known to exert strong antitumor function. The benign T cells showed diminished Th17 property, and the expression of GD3 was high in the malignant T cells. The expression of GD3 in the malignant T cells inversely correlated with IL-17A production from the benign CD4 T cells. GD3 from the malignant T cells was implied to be involved in suppressing the Th17 activity of the benign T cells independent of the regulation of T_RM differentiation in CTCL. Revealing the role of GD3 in inhibiting the production of IL-17A in CTCL would aid the understanding of the suppressive mechanism of the antitumor activity by malignant tumor cells.

Antigen Specificity Enhances Disease Control by Tregs in Vitiligo

Vitiligo is an autoimmune skin disease characterized by melanocyte destruction. Regulatory T cells (Tregs) are greatly reduced in vitiligo skin, and replenishing peripheral skin Tregs can provide protection against depigmentation. Ganglioside D3 (GD3) is overexpressed by perilesional epidermal cells, including melanocytes, which prompted us to generate GD3-reactive chimeric antigen receptor (CAR) Tregs to treat vitiligo. Mice received either untransduced Tregs or GD3-specific Tregs to test the hypothesis that antigen specificity contributes to reduced autoimmune reactivity in vitro and in vivo. CAR Tregs displayed increased IL-10 secretion in response to antigen, provided superior control of cytotoxicity towards melanocytes, and supported a significant delay in depigmentation compared to untransduced Tregs and vehicle control recipients in a TCR transgenic mouse model of spontaneous vitiligo. The latter findings were associated with a greater abundance of Tregs and melanocytes in treated mice versus both control groups. Our data support the concept that antigen-specific Tregs can be prepared, used, and stored for long-term control of progressive depigmentation.

Gb3-cSrc complex in glycosphingolipid-enriched microdomains contributes to the expression of p53 mutant protein and cancer drug resistance via β-catenin-activated RNA methylation

Glucosylceramide synthase (GCS) is a key enzyme catalyzing ceramide glycosylation to generate glucosylceramide (GlcCer), which in turn serves as the precursor for cells to produce glycosphingolipids (GSLs). In cell membranes, GSLs serve as essential components of GSL-enriched microdomains (GEMs) and mediate membrane functions and cell behaviors. Previous studies showed that ceramide glycosylation correlates with upregulated expression of p53 hotspot mutant R273H and cancer drug resistance. Yet, the underlying mechanisms remain elusive. We report herewith that globotriaosylceramide (Gb3) is associated with cSrc kinase in GEMs and plays a crucial role in modulating expression of p53 R273H mutant and drug resistance. Colon cancer cell lines, either WiDr homozygous for missense-mutated TP53 (R273H+/+) or SW48/TP53-Dox bearing heterozygous TP53 mutant (R273H/+), display drug resistance with increased ceramide glycosylation. Inhibition of GCS with Genz-161 (GENZ 667161) resensitized cells to apoptosis in these p53 mutant-carrying cancer cells. Genz-161 effectively inhibited GCS activity, and substantially suppressed the elevated Gb3 levels seen in GEMs of p53-mutant cells exposed to doxorubicin. Complex formation between Gb3 and cSrc in GEMs to activate β-catenin was detected in both cultured cells and xenograft tumors. Suppression of ceramide glycosylation significantly decreased Gb3-cSrc in GEMs, β-catenin, and methyltransferase-like 3 for m6A RNA methylation, thus altering pre-mRNA splicing, resulting in upregulated expression of wild-type p53 protein, but not mutants, in cells carrying p53 R273H. Altogether, increased Gb3-cSrc complex in GEMs of membranes in response to anticancer drug induced cell stress promotes expression of p53 mutant proteins and accordant cancer drug resistance.

Increased Expression of GM1 Detected by Electrospray Mass Spectrometry in Rat Primary Embryonic Cortical Neurons Exposed to Glutamate Toxicity

Neurons within different brain regions have varying levels of vulnerability to external stress and respond differently to injury. A potential reason to explain this may lie within a key lipid class of the cell's plasma membrane called gangliosides. These glycosphingolipid species have been shown to play various roles in the maintenance of neuronal viability. The purpose of this study is to use electrospray ionization mass spectrometry (ESI-MS) and immunohistochemistry to evaluate the temporal expression profiles of gangliosides during the course of neurodegeneration in rat primary cortical neurons exposed to glutamate toxicity. Primary embryonic (E18) rat cortical neurons were cultured to DIV (days in vitro) 14. Glutamate toxicity was induced for 1, 3, 6, and 24 h to injure and kill neurons. Immunofluorescence was used to stain for GM1 and GM3 species, and ESI-MS was used to quantify the ganglioside species expressed within these injured neurons. ESI-MS data revealed that GM1, GM2, and GM3 were up-regulated in neurons exposed to glutamate. Interestingly, using immunofluorescence, we demonstrated that the GM1 increase following glutamate exposure occurred in viable neurons, possibly indicating a potential intrinsic neuroprotective response. To test this potential neuroprotective property, neurons were pretreated with GM1 for 24 h prior to glutamate exposure. Pretreatment with GM1 conferred significant neuroprotection against glutamate-induced cell death. Overall, work from this study validates the use of ESI-MS for cell-derived gangliosides and supports the further development of lipid based strategies to protect against neuron cell death.

Ganglioside GM3 depletion reverses impaired wound healing in diabetic mice by activating IGF-1 and insulin receptors

Background

Ganglioside GM3 mediates adipocyte insulin resistance, but the role of GM3 in diabetic wound healing, a major cause of morbidity, is unclear.

Purpose

Determine whether GM3 depletion promotes diabetic wound healing and directly activates keratinocyte insulin pathway signaling.

Results

GM3 synthase (GM3S) expression is increased in human diabetic foot skin, ob/ob and diet-induced obese diabetic mouse skin, and mouse keratinocytes exposed to increased glucose. GM3S knockout in diet-induced obese mice prevents the diabetic wound healing defect. Keratinocyte proliferation, migration, and activation of insulin receptor (IR) and insulin growth factor-1 receptor (IGF-1R) are suppressed by excess glucose in wild type cells, but increased in GM3S ^−/− keratinocytes with supplemental glucose. Co-immunoprecipitation of IR, IR substrate-1 (IRS-1), and IGF-1R, and increased IRS-1 and Akt phosphorylation accompany receptor activation. GM3 supplementation or inhibition of IGF-1R or PI3K reverses the increased migration of GM3S^−/− keratinocytes, whereas IR knockdown only partially suppresses migration.

Conclusions

Cutaneous GM3 accumulation may participate in the impaired wound healing of diet-induced diabetes by suppressing keratinocyte insulin/IGF-1 axis signaling. Strategies to deplete GM3S/GM3 may improve diabetic wound healing.

Ceramide signaling in mammalian epidermis

Ceramide, the backbone structure of all sphingolipids, as well as a minor component of cellular membranes, has a unique role in the skin, by forming the epidermal permeability barrier at the extracellular domains of the outermost layer of the skin, the stratum corneum, which is required for terrestrial mammalian survival. In contrast to the role of ceramide in forming the permeability barrier, the signaling roles of ceramide and its metabolites have not yet been recognized. Ceramide and/or its metabolites regulate proliferation, differentiation, and apoptosis in epidermal keratinocytes. Recent studies have further demonstrated that a ceramide metabolite, sphingosine-1-phosphate, modulates innate immune function. Ceramide has already been applied to therapeutic approaches for treatment of eczema associated with attenuated epidermal permeability barrier function. Pharmacological modulation of ceramide and its metabolites' signaling can also be applied to cutaneous disease prevention and therapy. The author here describes the signaling roles of ceramide and its metabolites in mammalian cells and tissues, including the epidermis. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Suppression of epidermal growth factor receptor signaling by protein kinase C-alpha activation requires CD82, caveolin-1, and ganglioside

Activation of protein kinase C (PKC)-alpha decreases normal and neoplastic cell proliferation by inhibiting epidermal growth factor receptor (EGFR)-related signaling. The molecular interactions upstream to PKC-alpha that influence its suppression of EGFR, however, are poorly understood. We have found that caveolin-1, tetraspanin CD82, and ganglioside GM3 enable the association of EGFR with PKC-alpha, ultimately leading to inhibition of EGFR signaling. GM3- and CD82-induced inhibition of EGFR signaling requires PKC-alpha translocation and serine/threonine phosphorylation, which eventually triggers EGFR Thr654 phosphorylation and receptor internalization. Within this ordered complex of signaling molecules, the ability of CD82 to associate with PKC-alpha requires the presence of caveolin-1, whereas the interaction of caveolin-1 or PKC-alpha with EGFR requires the presence of CD82 and ganglioside GM3. Disruption of the membrane with methyl-beta-cyclodextrin dissociates the EGFR/GM3/caveolin-1/CD82/PKC-alpha complex and prevents the inhibitory effect of PKC-alpha on EGFR phosphorylation, suggesting that caveolin-1, CD82, and ganglioside interact with EGFR and PKC-alpha within intact cholesterol-enriched membrane microdomains. Given the role of these membrane molecules in suppressing EGFR signaling, up-regulation of GM3, caveolin-1, and CD82 function may be an effective adjunctive therapy for treating epithelial cell malignancies.

Ganglioside GM3 promotes cell migration by regulating MAPK and c-Fos/AP-1

Gangliosides have been proposed as modulators of transmembrane signaling. Recently, GM3, a glycosphingolipid containing monosaialic acids, is thought to be one of the key molecules of signal transduction in mammalian cells. In this study, we used mouse embryonic fibroblast cell lines (MEFs) established from sialyltransferase-I knockout mice (GM3 synthase KO mice) to evaluate the regulation of mitogenic signals by gangliosides. Cell proliferation assay revealed a higher growth potential of GM3 KO MEFs. Immunoblots showed upregulation of Ras/Raf/MEK/ERK pathway in GM3 KO MEFs, and these signals resulted in enhanced translocation of ERK into the nuclei. Further, both exogenous and endogenous add-back of GM3 decreased the activities of MAPK in GM3 KO MEFs. In addition, GM3 KO MEFs formed foci in high-density culture condition, and analyses of cell cycle modulators revealed the resistance of GM3 KO MEFs for entering cell cycle arrest. Finally, sustained expressions of c-Fos in GM3 KO MEFs were shown to correlate with DNA-binding activity between c-Fos and AP-1. These results demonstrate that the deletion of sialyltransferase-I changes the character of MEFs to a highly activated state of the MAPK pathway, indicating the critical role of GM3 as a regulator of membrane-transmitted signals.

Endogenous GD3 ganglioside induces apoptosis in U-1242 MG glioma cells

GD3 ganglioside induces apoptosis in several cell types, but the molecular events through which this occurs are largely unknown. We investigated the apoptotic effects of GD3 expression using U-1242 MG glioblastoma cells, as these cells synthesize almost exclusively GM3 and GM2 but not GD3. To express GD3 under the control of the TetOn system with minimum leakage, we modified the system by constructing a single tri-cistronic retrovirus vector containing three genes separated by two internal ribosome entry sites: (a) transcriptional silencer, tTS; (b) mutant of reverse transcriptional activator, rtTA2(S)-M2 (provided by H. Bujard, Heidelberg, Germany); and (c) enhanced green fluorescent protein (EGFP), as an indicator of the tri-cistronic gene expression. Using flow cytometry, we selected glioma cells (U1242MG-GD3 clone) that express high levels of GD3 in response to doxycycline. Expression of GD3 was associated with apoptosis as verified by annexin-V binding, TdT-mediated dUTPnick end-labelling assay (TUNEL), and EGFP degradation. GD3-induced apoptosis occurred via caspase-8 activation, as GD3 caused cleavage of caspase-8 and inhibition of caspase-8 activation by zlETD-fmk minimized GD3-induced apoptosis.

Surface Adhesins of Staphylococcus aureus

An important facet in the interaction between Staphylococcus aureus and its host is the ability of the bacterium to adhere to human extracellular matrix components and serum proteins. In order to colonise the host and disseminate, it uses a wide range of strategies, the molecular and genetic basis of which are multifactorial, with extensive functional overlap between adhesins. Here, we describe the current knowledge of the molecular features of the adhesive components of S. aureus, mechanisms of adhesion and the impact that these have on host-pathogen interaction.

Pathophysiological implication of ganglioside GM3 in early mouse embryonic development through apoptosis

Apoptosis may occur in early embryos where the execution of essential developmental events has failed, and gangliosides, sialic acid-conjugated glycosphingolipids, are proposed to regulate cell differentiation and growth. To evaluate the regulatory roles of ganglioside GM3 in early embryonic development, this study examined its expressional patterns in apoptotic cells during early embryonic development in mice. Pre-implanted embryos were obtained by in vitro fertilization, which were treated at the 4-cell stage with three the apoptosis inducers, actinomycin D, camptothecin and cycloheximide, for 15 h. All three inducers significantly increased the percentage of apoptotic cells, as measured using a TUNEL method, but remarkably reduced the total cell numbers. The numbers of morula and blastocyst stages were significantly decreased by treatment of the embryos with the three apoptosis inducers compared with the control, with a similar result also observed in the number of blastomeres. Staining of early embryos with Hoechst 33342 revealed a significant percentage of apoptotic nuclei. Prominent immunofluorescence microscopy revealed a significant difference in the ganglioside GM3 expression in apoptotic embryos compared with the control, and RT-PCR also demonstrated a dramatic increase in ganglioside GM3 synthase mRNA in the apoptotic embryos. These results suggest that ganglioside GM3 may be pathophysiologically implicated in the regulation of early embryonic development through an apoptotic mechanism.

Ganglioside GM3 Blocks the Activation of Epidermal Growth Factor Receptor Induced by Integrin at Specific Tyrosine Sites

The epidermal growth factor receptor (EGFR) can be activated by both direct ligand binding and cross-talk with other molecules, such as integrins. This integrin-mediated cross-talk with growth factor receptors participates in regulating cell proliferation, survival, migration, and invasion. Previous studies have shown that ligand-dependent EGFR activation is inhibited by GM3, the predominant ganglioside of epithelial cells, but the effect of GM3 on ligand-independent, integrin-EGFR cross-talk is unknown. Using a squamous carcinoma cell line we show that endogenous accumulation of GM3 disrupts the ligand-independent association of the integrin beta1 subunit with EGFR and results in inhibition of cell proliferation. Consistently, endogenous depletion of GM3 markedly increases the association of EGFR with tyrosine-phosphorylated integrin beta1 and promotes cell proliferation. The ligand-independent stimulation of EGFR does not require focal adhesion kinase phosphorylation or cytoskeletal rearrangement. Stimulation of EGFR and mitogen-activated protein kinase signaling by GM3 depletion involves the phosphorylation of EGFR at tyrosine residues 845, 1068, and 1148 but not 1086 or 1173. The specific blockade of phosphorylation at Tyr-845 with Src family kinase inhibition and at Tyr-1148 with phosphatidylinositol 3-kinase inhibition suggests that GM3 inhibits integrin-induced, ligand-independent EGFR phosphorylation (cross-talk) through suppression of Src family kinase and phosphatidylinositol 3-kinase signaling.

Ganglioside GM3 inhibits matrix metalloproteinase-9 activation and disrupts its association with integrin

Gangliosides GM3 and GT1b both inhibit epithelial cell adhesion and migration on fibronectin. GT1b binds to integrin alpha5beta1 and blocks the integrin-fibronectin interaction; GM3 does not interact with integrin, and its effect is poorly understood. We evaluated the effects of endogenous modulation of GM3 expression on epithelial cell motility on several matrices and the mechanism of these effects. Endogenous accumulation of GM3 decreased cell migration on fibronectin, types I, IV, and VII collagen matrices; depletion of GM3 dramatically increased cell migration, regardless of matrix. GM3 overexpression and depletion in vitro correlated inversely with the expression and activity of matrix metalloproteinase-9; consistently, the cell migration stimulated by GM3 depletion is reversed by inhibition of matrix metalloproteinase-9 activity. Accumulation and depletion of GM3 in epithelial cells grown on fibronectin also correlated inversely with epidermal growth factor receptor and mitogen activated protein kinase phosphorylation and with Jun expression. Ganglioside depletion facilitated the co-immunoprecipitation of matrix metal-loproteinase-9 and integrin alpha5beta1, while endogenous accumulation of GM3, but not GT1b, blocked the co-immunoprecipitation. These data suggest modulation of epidermal growth factor receptor signaling and dissociation of integrin/matrix metalloproteinase-9 as mechanisms for the GM3-induced effects on matrix metalloproteinase-9 function.

Ganglioside induces caveolin-1 redistribution and interaction with the epidermal growth factor receptor

Although caveolin-1 is thought to facilitate the interaction of receptors and signaling components, its role in epidermal growth factor receptor (EGFR) signaling remains poorly understood. Ganglioside GM3 inhibits EGFR autophosphorylation and may thus affect the interaction of caveolin-1 and the EGFR. We report here that endogenous overexpression of GM3 leads to the clustering of GM3 on the cell membrane of the keratinocyte-derived SCC12 cell line and promotes co-immunoprecipitation of caveolin-1 and GM3 with the EGFR. Overexpression of GM3 does not affect EGFR distribution but shifts caveolin-1 to the detergent-soluble, EGFR-containing region; consistently, caveolin-1 is retained in the detergent-insoluble membrane when ganglioside is depleted. GM3 overexpression inhibits EGFR tyrosine phosphorylation and receptor dimerization and concurrently increases both the content and tyrosine phosphorylation of EGFR-associated caveolin-1, providing evidence that tyrosine phosphorylation of caveolin-1 inhibits EGFR signaling. Consistently, depletion of ganglioside both increases EGFR phosphorylation and prevents the EGF-induced tyrosine phosphorylation of caveolin-1. GM3 also induces delayed serine phosphorylation of EGFR-unassociated caveolin-1, suggesting a role for serine phosphorylation of caveolin-1 in regulating EGFR signaling. These studies suggest that GM3 modulates the caveolin-1/EGFR association and is critical for the EGF-induced tyrosine phosphorylation of caveolin-1 that is associated with its inhibition of EGFR activation.

Ganglioside modulation regulates epithelial cell adhesion and spreading via ganglioside-specific effects on signaling

Gangliosides are implicated in regulating cell adhesion and migration on fibronectin by binding with the alpha(5) subunit of alpha(5)beta(1) integrin. However, the effects of gangliosides on cell spreading and related signaling pathways are unknown. Increases in gangliosides GT1b and GD3 inhibited spreading on fibronectin, concurrent with inhibition of Src and focal adhesion kinase. Although antibody blockade of GT1b or GD3 function and gene-modulated ganglioside depletion stimulated spreading and activated Src and focal adhesion kinase, the augmented spreading by disruption of GT1b function, but not by disruption of GD3 function, was inhibited by blockade of Src and focal adhesion kinase activation. In contrast, inhibitors of protein kinase C prevented the stimulation of spreading by GD3 functional inhibition, but not by GT1b functional blockade. Modulation of either GT1b or GD3 content affected phosphoinositol 3-kinase activation, and inhibition of this activation reversed the stimulation of cell spreading by anti-GD3 antibody, anti-GT1b antibody, and ganglioside depletion, suggesting that phosphoinositol 3-kinase is an intermediate in both the FAK/Src and protein kinase C pathways that lead to cell spreading. These studies demonstrate that epithelial cell ganglioside GT1b modulates cell spreading through alpha(5)beta(1)/FAK and phosphoinositol 3-kinase signaling, whereas GD3-modulated spreading appears to involve phosphoinositol 3-kinase-dependent protein kinase C signaling.

Characterization of a 500 kb region on 17q25 and the exclusion of candidate genes as the familial Tylosis Oesophageal Cancer (TOC) locus

The locus for a syndrome of focal palmoplantar keratoderma (Tylosis) associated with squamous cell oesophageal cancer (TOC) has been mapped to chromosome 17q25, a region frequently deleted in sporadic squamous cell oesophageal tumours. Further haplotype analysis described here, based on revised maps of marker order, has reduced the TOC minimal region to a genetic interval of 2 cM limited by the microsatellite markers D17S785 and D17S751. Partial sequence data and complete physical maps estimate the actual size of this region to be only 0.5 Mb. This analysis allowed the exclusion of proposed candidate tumour suppressor genes including MLL septin-like fusion (MSF), survivin, and deleted in multiple human cancer (DMC1). Computer analysis of sequence data from the minimal region identified 13 candidate genes and the presence of 50-70 other 'gene fragments' as ESTs and/or predicted exons and genes. Ten of the characterized genes were assayed for mutations but no disease-specific alterations were identified in the coding and promoter sequences. This region of chromosome 17q25 is, therefore, relatively gene-rich, containing 13 known and possibly as many as 50 predicted genes. Further mutation analysis of these predicted genes, and others possibly residing in the region, is required in order to identify the elusive TOC locus.

Ganglioside loss promotes survival primarily by activating integrin-linked kinase/Akt without phosphoinositide 3-OH kinase signaling

Keratinocyte gangliosides influence cellular functions, including proliferation, adhesion, migration, and differentiation. The effects of endogenous depletion of membrane gangliosides by gene transfection of a human ganglioside-specific sialidase on cell survival were investigated. Ganglioside depletion promotes survival of the human keratinocyte-derived SCC12 cell line through upregulated phosphorylation of beta1 integrin, and increased phosphorylation and activity of integrin-linked kinase, protein kinase B/Akt, and Bad, with resultant inhibition of caspase-9 activation. Ganglioside deficiency also increases expression of cyclins D1 and E, promoting cell cycle progression from G1 phase to S phase. Inhibition of either protein kinase B/Akt or integrin-linked kinase activity renders the ganglioside-deficient cells susceptible to triggers of apoptosis. Both serine-473 and threonine-308 sites of protein kinase B/Akt show increased phosphorylation in ganglioside-deficient cells, but the cell survival correlates with increased phosphorylation of the serine-473 site of Akt, not with increased phosphorylation of the threonine-308 site. Consistently, blockade of ganglioside GT1b function activates integrin-linked kinase and only the serine-473 site of protein kinase B/Akt. In contrast, antibody-induced blockade of GM3 function increases only threonine-308 phosphorylation of ganglioside-deficient cells. Whereas blockade of phosphoinositide 3-OH kinase function suppresses threonine-308 phosphorylation, it neither inhibits serine-473 phosphorylation nor triggers apoptosis. These data suggest that ganglioside depletion modulates cell survival primarily through protein kinase B/Akt stimulation by a pathway that does not require phosphoinositide 3-OH kinase and epidermal growth factor receptor signaling.

Inhibition of human neuroblastoma cell proliferation and EGF receptor phosphorylation by gangliosides GM1, GM3, GD1A and GT1B

The inhibitory action of gangliosides GT1B, GD1A, GM3 and GM1 on cell proliferation and epidermal growth factor receptor (EGFR) phosphorylation was determined in the N-myc amplified human neuroblastoma cell line NBL-W. The IC50 of each ganglioside was estimated from concentration-response regressions generated by incubating NBL-W cells with incremental concentrations (5-1000 microm) of GT1B, GD1A, GM3 or GM1 for 4 days. Cell proliferation was quantitatively determined by a colourimetric assay using tetrazolium dye and spectrophotometric analysis, and EGFR phosphorylation by densitometry of Western blots. All gangliosides assayed, with the exception of GM1, inhibited NBL-W cell proliferation in a concentration-dependent manner. The IC50s for gangliosides GT1B [molecular weight (MW) 2129], GM3 (MW 1236), and GD1A (MW 1838) were (mean +/- SEM) 117 +/- 26, 255 +/- 29, and 425 +/- 44 m, respectively. In contrast, the IC50 for GM1 (MW 1547) could not be determined. Incubation of NBL-W cells with epidermal growth factor (EGF) concentrations ranging from 0.1 to 1000 ng/ml progressively increased cell proliferation rate, but it plateaued at concentrations above 10 ng/ml. EGFR tyrosine phosphorylation, however, was incrementally stimulated by EGF concentrations from 1 to 100 ng/ml. The suppression of EGF-induced EGFR phosphorylation differed for each ganglioside, and their respective inhibitory potencies were as follows: EGFR phosphorylation [area under curve (+ EGF)/area under curve (- EGF)]: control (no ganglioside added) = 8.2; GM1 = 8.3; GD1A = 6.7; GM3 = 4.87, and GT1B = 4.09. The lower the ratio, the greater the inhibitory activity of the ganglioside. Gangliosides GD1A and GT1B, which have terminal N-acetyl neuraminic acid moieties, as well as one and two N-acetyl neuraminic acid residues linked to the internal galactose, respectively, both inhibited cell proliferation and EGFR phosphorylation. However, GD1A was a more potent suppressor of cell proliferation and GT1B most effective against EGFR phosphorylation. GM3, which only has a terminal N-acetyl neuraminic acid, inhibited cell proliferation and EGFR phosphorylation almost equivalently. These data suggest that gangliosides differ in their potency as inhibitors of NBL-W neuroblastoma cell proliferation and EGFR tyrosine phosphorylation, and that perturbations in the differential expression of membrane glycosphingolipids may play a role in modulating neuroblastoma growth.

Differential expression of receptors for Shiga and Cholera toxin is regulated by the cell cycle

Cholera and Shiga toxin bind to the cell surface via glycolipid receptors GM1 and Gb3, respectively. Surprisingly, the majority of Vero cells from a non-synchronized population bind either Cholera or Shiga toxin but not both toxins. The hypothesis that the differential expression of toxin receptors is regulated by the cell cycle was tested. We find that Cholera toxin binds preferentially in G0/G1, with little binding through S-phase to telophase,whereas Shiga toxin binds maximally through G2 to telophase but does not bind during G0/G1 and S-phase. The changes result from the corresponding changes in Gb3 and GM1 synthesis, not from variations of receptor transport to the cell surface. The changes do not reflect competition of Gb3 and GM1 synthesis for lactosylceramide. Cells as diverse as Vero cells, PC12 cells and astrocytes show the same cell-cycle-dependent regulation of glycosphingolipid receptors,suggesting that this novel phenomenon is based on a conserved regulatory mechanism.

Inhibition of integrin-linked kinase/protein kinase B/Akt signaling: mechanism for ganglioside-induced apoptosis

Ganglioside GT1b inhibits keratinocyte attachment to and migration on a fibronectin matrix by binding to alpha(5)beta(1) and preventing alpha(5)beta(1) interaction with fibronectin. The role of gangliosides in triggering keratinocyte apoptosis, however, is unknown. Addition of GT1b to keratinocyte-derived SCC12 cells, grown in serum-free medium but exposed to fibronectin, suppressed Bad phosphorylation, activated caspase-9, and inhibited cyclin D and E expression, resulting in cell cycle arrest at G(1) phase and initiation of apoptosis. The mechanism of GT1b activation of caspase-9 involved inhibition of beta(1) integrin serine/threonine phosphorylation and decreased phosphorylation of both integrin-linked kinase and protein kinase B/Akt at its Ser-473 site, leading to cytochrome c release from mitochondria. Consistently, blockade of GT1b function with anti-GT1b antibody specifically activated the Ser-473 site of Akt, markedly suppressing apoptosis. The ganglioside-induced inhibition of Akt phosphorylation was GT1b-specific and was not observed when cells were treated with other keratinocyte gangliosides, including GD3. These studies suggest that the modulation of keratinocyte cell cycle and survival by GT1b is mediated by its direct interaction with alpha(5)beta(1) and resultant inhibition of the integrin/integrin-linked kinase/protein kinase B/Akt signaling pathway.

Epidermal growth factor receptor glycosylation is required for ganglioside GM3 binding and GM3-mediated suppression [correction of suppresion] of activation

Gangliosides are able to bind to the epidermal growth factor receptor and inhibit its activation, but the mechanism of this inhibition is unknown. To address the role of receptor carbohydrates in facilitating interaction with gangliosides, we examined the ability of GM3 to bind the deglycosylated receptor and inhibit its autophosphorylation. Flow cytometry studies demonstrated that deglycosylation of the receptor did not affect its ability to be transported to the cell membrane. In contrast with the native (fully glycosylated) receptor, GM3 did not coimmunoprecipitate with the deglycosylated receptor. Using a novel colorimetric bead binding assay, GM3 was shown to bind well to the immunoprecipitated native receptor but not at all to the deglycosylated receptor. Finally, the addition of GM3 to cells with deglycosylated epidermal growth factor receptors did not result in significant further inhibition of autophosphorylation of the receptor, despite a 10-fold decrease in phosphorylation of the native epidermal growth factor receptor by 200 microM GM3. These studies suggest that ganglioside affects epidermal growth factor receptor activity through a direct interaction that requires receptor glycosylation, and contribute to our understanding of the role of gangliosides in cell membrane function.

Carbohydrate-carbohydrate binding of ganglioside to integrin alpha(5) modulates alpha(5)beta(1) function

Gangliosides GT1b and GD3, components of keratinocyte membranes, inhibit keratinocyte adhesion to fibronectin. Although ganglioside sialylation is known to be important, the mechanism of inhibition is unknown. Using purified insect recombinant alpha(5) and beta(1) proteins and alpha(5)beta(1) integrin from lysed keratinocyte-derived SCC12 cells, we have shown that GT1b and GD3 inhibit the binding of alpha(5)beta(1) to fibronectin. Co-immunoprecipitation of GT1b and alpha(5)beta(1) from SCC12 cells and direct binding of GT1b and GD3 to affinity-purified alpha(5)beta(1) from SCC12 cells and insect recombinant alpha(5)beta(1), particularly the alpha(5) subunit, further suggest interaction between ganglioside and alpha(5)beta(1). The carbohydrate moieties of integrin appear to be critical since gangliosides are unable to bind deglycosylated forms of alpha(5)beta(1) from SCC12 and insect cells or poorly glycosylated recombinant alpha(5)beta(1) from Escherichia coli cells. The GT1b-alpha(5)beta(1) interaction is inhibited by concanavalin A, suggesting that GT1b binds to mannose structures in alpha(5)beta(1). The preferential binding of GT1b to high mannose rather than reduced mannose ovalbumin further implicates the binding of GT1b to mannose structures. These data provide evidence that highly sialylated gangliosides regulate alpha(5)beta(1)-mediated adhesion of epithelial cells to fibronectin through carbohydrate-carbohydrate interactions between GT1b and the alpha(5) subunit of alpha(5)beta(1) integrin.

Glycosphingolipid changes induced by advanced glycation end-products

The effects of advanced glycation end-products (AGEs) on retinal microvascular cell glycosphingolipids were investigated as a potential pathogenic mechanism of diabetic retinopathy. The results obtained showed that, in microvascular retinal endothelial cells and pericytes, AGEs increased the amount of all glycosphingolipids studied (from 25 to 115% depending on the glycosphingolipid species), except for a specific ganglioside, GD3, which decreased by 35% only in pericytes. Glycosphingolipid profiles and GM3 fatty acid analysis did not show any qualitative differences after incubation with AGEs, suggesting that AGEs only induced quantitative changes in cell glycosphingolipids. These results show a new metabolic effect of AGEs, which could be involved in the microvascular alterations observed in diabetic retinopathy.

Keratinocyte CDw60 expression is modulated by both a Th-1 type cytokine IFN-gamma and Th-2 cytokines IL-4 and IL-13: relevance to psoriasis

SUMMARY

Psoriasis is a chronic skin disease with an immunocytic infiltrate, including activated T lymphocytes, producing multiple cytokines that can influence the phenotype of epidermal keratinocytes. In these studies we examined the effect of the cytokines interferon-gamma and interleukin-13 or interleukin-4 on keratinocytes, alone and in combination, on surface levels of HLA-DR, intercellular adhesion molecule 1, and CDw60, as well as the transcription factors STAT1, STAT6, and BCL-6. As CDw60 is an acetylated form of the GD3 ganglioside and may function as a T cell costimulatory molecule, the modulation of CDw60 expression by keratinocytes in psoriatic lesions was highlighted to gain insight into potentially important T cell-keratinocyte interactions. Interferon-gamma was observed to block the interleukin-4- or interleukin-13-mediated induction of CDw60 on cultured keratinocytes, but not induction of the transcription factor STAT6. Interleukin-13 and interleukin-4 were unable to block interferon-gamma-mediated induction of STAT1 or BCL-6, however, or the upregulation of intercellular adhesion molecule 1 and HLA-DR. In psoriatic plaques, CDw60 was not consistently detected on keratinocytes in acute lesions, but was detected predominantly on basal layer keratinocytes in chronic lesions. In addition we found that BCL-6 levels were increased in psoriatic lesions; in acute lesions BCL-6 was primarily localized in the basal layer keratinocytes, whereas in chronic plaques nuclear BCL-6 was predominantly expressed by keratinocytes in the suprabasal cell layers. These studies highlight the complex modulation of the keratinocyte phenotype by immunocyte-derived cytokines, in which induction of CDw60 involving interleukin-4, or interleukin-13 was antagonized by interferon-gamma. We suggest in psoriatic plaques that the presence or absence of CDw60 expression by keratinocytes may reflect the dynamic interplay between Th-1-type cytokines such as interferon-gamma and Th-2-type cytokines such as interleukin-4 and interleukin-13. The ability of interferon-gamma to induce the transcription repressor BCL-6 may also contribute to the overall immunologic events in skin, including suppression of the intermediates in the synthetic pathway leading to expression of the T cell costimulatory ganglioside CDw60.

Ganglioside modulates ligand binding to the epidermal growth factor receptor

Whereas previous investigations have shown that pharmacologic addition of gangliosides inhibits keratinocyte proliferation by downregulating epidermal growth factor receptor phosphorylation, the underlying biochemical basis and physiologic relevance are unknown. Using Scatchard and displacement plots, we have shown that supplemental purified gangliosides decrease the binding of (125)I-labeled epidermal growth factor to keratinocyte-derived SCC12 cells. Conversely, SCC12 cells transfected with sialidase and thus depleted of gangliosides show increased ligand binding to the epidermal growth factor receptor, which is consistent with their increased proliferation in response to epidermal growth factor and transforming growth factor-alpha, and increased phosphorylation of the epidermal growth factor receptor, and downstream signal transduction pathway components. The mechanism of the altered binding appears to involve primarily decreased numbers of available receptors within the intact membrane, but not altered receptor protein expression. These studies provide evidence that the effect of gangliosides on keratinocyte proliferation results, at least in part, from the direct binding of ganglioside to the receptor and disruption of the receptor-ligand interaction. Manipulation of membrane ganglioside content may be a powerful new means to alter epidermal growth factor receptor-dependent cell proliferation.

Expression cloning of mouse cDNA of CMP-NeuAc:Lactosylceramide alpha2,3-sialyltransferase, an enzyme that initiates the synthesis of gangliosides

Expression cloning of a cDNA for the alpha2,3-sialyltransferase (GM3 synthase) (EC 2.4.99.-) gene was performed using a GM3-lacking mouse fibroblast line L cell and anti-GM3 monoclonal antibody. Plasmids from a cDNA library generated with poly(A)+ RNA of a mouse fibrosarcoma line CMS5j and pdl3027 (polyoma T antigen) were co-transfected into L cells. The isolated cDNA clone pM3T-7 predicted a type II membrane protein with 13 amino acids of cytoplasmic domain, 17 amino acids of transmembrane region, and a large catalytic domain with 329 amino acids. Introduction of the cDNA clone into L cells resulted in the neo-synthesis of GM3 and high activity of alpha2,3-sialyltransferase. Among glycosphingolipids, only lactosylceramide showed significant activity as an acceptor, indicating that this gene product is a sialyltransferase specific for the synthesis of GM3. An amino acid sequence deduced from the cloned cDNA showed the typical sialyl motif with common features among alpha2,3-sialyltransferases. Among various mouse tissues, brain, liver, and testis showed relatively high expression of a 2.3-kilobase mRNA, whereas all tissues, more or less, expressed this gene.

Sphingolipids as receptor modulators. An overview

Glycosphingolipids are amphipathic compounds that exist mainly in the plasmalemma with their oligosaccharide portion protruding into the extracellular environment. In this position they are admirably situated for interacting with both ligands and receptors. Binding studies have demonstrated that specific glycolipids function as receptors for some microorganisms and bacterial toxins. Specific oligosaccharides on both glycolipids and glycoproteins bind members of the selection families, and some gangliosides facilitate integrins binding to their ligands. Gangliosides modulate the trophic factor-stimulated dimerization, tyrosine phosphorylation, and subsequent signal transduction events of several tyrosine kinase receptors. GM3 inhibits both the epidermal growth factor receptor and basic fibroblast factor receptor; several gangliosides except GM3 inhibit the platelet-derived growth-factor receptor; GM1 enhances nerve growth-factor-stimulated activation of TrkA; insulin receptor is inhibited to varying degrees by several gangliosides, but 2-->3 sialosylparagloboside is most effective. Activities of the beta(1)-adrenergic and delta-opioid receptors are modulated by GM1. Available information suggests that glycolipids serve as coordinators of multiple receptor functions.

Integrin alpha 5 beta 1 expression is required for inhibition of keratinocyte migration by ganglioside GT1b

Polysialoganglioside GT1b, a keratinocyte membrane glycosphingolipid, inhibits normal keratinocyte adhesion and migration on a fibronectin matrix. The specificity of the inhibition for cells plated on a fibronectin matrix and competition of GT1b inhibition with peptide RGDS suggest that GT1b abrogates the alpha 5 beta 1/fibronectin interaction. We examined the effects of GT1b on the adhesion and migration of keratinocyte-derived cell lines and correlated GT1b responsiveness and alpha 5 beta 1 integrin expression. GT1b (5 nM) significantly inhibited migration of normal human keratinocytes, immortalized keratinocytes, and squamous cell carcinoma SCC12F2 cells on fibronectin, but not on collagen I. Concentrations as high as 5 microM had no effect on SCC13 or HaCaT cells. Likewise, GT1b inhibited fibronectin-dependent cell adhesion of normal human keratinocytes, immortalized keratinocytes, and SCC12F2 cells, but had no effect on SCC13 or HaCaT cells. Flow cytometric and Western immunoblot analysis of integrin expression showed significantly decreased alpha 5 and beta 1 integrin expression in SCC13 and HaCaT cells compared to normal keratinocytes, immortalized keratinocytes, and SCC12F2 cells. Incubation with TGF-beta 1 increased alpha 5 beta 1 integrin expression and induced responsiveness to GT1b in HaCaT cells. These data imply that GT1b "response" requires sufficient expression of alpha 5 beta 1 and further suggest that the mechanism of the inhibitory effect of GT1b involves GT1b/alpha 5 beta 1 interaction.

Glycolipids and myelin proteins in human oligodendrogliomas

We studied myelin proteins and glycolipids in 24 human oligodendrogliomas (16 pure, eight mixed), including two grade I, 13 grade II, five grade III, and four grade IV. Tumours with a 1b ganglioside content (GD1b, GT1b and GQ1b) over 30% of total gangliosides occur more frequently in the WHO grade I and II (47%) and grade III (40%) than in the grade IV (25%) group; there was no difference in the amounts of total ganglioside or individual gangliosides between pure and mixed oligodendrogliomas. The presence of 6'-LM1 correlated with higher grades of tumours (chi 2 P approximately 0.02); however, 3'-LM1 and total neolacto-series gangliosides did not correlated with grade. Immunohistochemical studies of oligodendrocyte and myelin markers (GalCer, sulfatide, 2',3' -cyclic nucleotide phosphodiesterase, myelin basic protein and proteolipid protein) using specific antibodies showed only a very small proportion of tumour cells staining. These data do not support the hypothesis that tumours classified as oligodendrogliomas are derived from mature oligodendrocytes.

Ganglioside GT1b inhibits keratinocyte adhesion and migration on a fibronectin matrix

Highly sialylated gangliosides have been shown to alter cellular adhesion to a fibronectin matrix. The effect of these gangliosides on the adhesion, spreading, and migration of cultured keratinocytes on a fibronectin matrix has not been explored. Ganglioside GT1b significantly prevented attachment of keratinocytes to fibronectin and also detached previously adherent keratinocytes in a concentration-dependent manner without cell toxicity. GT1b did not affect adhesion of keratinocytes to wells coated with laminin, type I or type IV collagen, 804G extracellular matrix, or albumin. GT1b also inhibited keratinocyte migration on fibronectin in a concentration-dependent manner at concentrations as low as 5 nM GT1b, but had no effect on migration of keratinocytes plated on other matrices. GT1b binds to intact fibronectin and to the 120-kD RGDS-containing cell-binding fibronectin fragment, but not to the heparin- or gelatin-binding fragments of fibronectin. Although RGDS competes with GT1b in inhibiting adhesion, GT1b does not diminish binding of keratinocytes to a derivatized RGDS substratum, suggesting that the GT1b effect involves a non-RGDS site in the cell-binding region that modulates RGDS/alpha 5 beta 1 integrin receptor interaction. Through a specific effect on keratinocyte interaction with fibronectin, GT1b may participate in the regulation of cell adhesion and migration on a fibronectin substratum, which are important events during wound healing and the spreading of cutaneous neoplasia.

Involvement of phospholipase D in ganglioside GQ1b-induced biphasic diacylglycerol production in human keratinocytes

Ganglioside IV3 (NeuAc)2, II3 (NeuAc)2-GgOse4Cer (GQ1b), which induces terminal differentiation in keratinocytes, was previously found to enhance the mass content of inositol 1,4,5-trisphosphate and intracellular calcium concentration ([Ca++]i), peaking at 30 seconds. In the present study, the biphasic accumulation of 1,2 diacylglycerol, i.e., the first transient and the second sustained phase, was observed in cultured human keratinocytes stimulated by GQ1b. On the other hand, II3 NeuAc-LacCer (GM3), which inhibits keratinocyte proliferation without inducing differentiation, did not cause diacylglycerol formation. Phosphatidylethanol, produced by transphosphatidylation and a potential marker for phospholipase D activity, was produced by the exposure to GQ1b in the presence of ethanol. The second sustained phase of diacylglycerol was repressed by ethanol, indicating that the diacylglycerol-formation pathway via phospholipase D followed by phosphatidic acid phosphohydrolase would in part account for the second diacylglycerol phase. Furthermore, this second phase of GQ1b-induced diacylglycerol generation was reduced by pretreatment with propranolol, an inhibitor of phosphatidic acid phosphohydrolase. In addition, the levels of [3H]choline, a direct metabolite of the phospholipase D pathway, were elevated within 1 min after GQ1b addition and then sustained for at least 20 min. Taken together, the results suggest that the phospholipase D pathway may contribute to the second phase of diacylglycerol formation, which might be involved in differentiation.

Correlation of ganglioside patterns of primary brain tumors with survival

BACKGROUND

Classification/grading schemes for brain tumors are based mainly on histologic examinations, but these have major limitations, which has led to a search for more objective prognostic markers. Gangliosides have several biologic effects relevant to tumors, and ganglioside compositions of primary brain tumors correlate with diagnosis. This led to the authors' hypothesis that ganglioside patterns of brain tumors might be useful as prognostic indicators.

METHODS

Gangliosides in primary brain tumors of different histologic types from 84 patients were analyzed. Specific ganglioside patterns and several other relevant variables were examined for associations with survival using a Cox proportional hazards model. Kaplan-Meier survival curves were analyzed using the log-rank test.

RESULTS

Patients in whom less than 30% of total tumor gangliosides consisted of 1b pathway gangliosides (GD1b, GT1b, and GQ1b) had significantly higher risk ratios than those with more than 30% 1b gangliosides (P approximately 0.03). The presence of 6'-LM1 (NeuAc alpha 2-->6Gal beta 1-->4Glc-NAc beta 1-->3Gal beta 1-->4Glc beta 1-->1Cer was also associated with a higher risk ratio (P approximately 0.007). Combinations of 1b gangliosides and 6'-LM1 identified three groups of patients regardless of histologic diagnosis. Group A, with less than 30% 1b and the presence of 6'-LM1, had a median survival time of 331 days. Group B, with less than 30% 1b but no 6'-LM1, had a median survival time of more than 698 days. Group C, with more than 30% 1b had a median survival time of more than 776 days.

CONCLUSIONS

The correlation of ganglioside patterns with survival in this initial investigation suggests the potential of 1b gangliosides and 6'-LM1 to be used as prognostic indicators. Continuing research is being conducted to assess this possibility prospectively.