Glycosylation affects translocation of integrin, Src, and caveolin into or out of GEM

Pathophysiology and molecular mechanism of caveolin involved in myocardial protection strategies in ischemic conditioning

Multiple pathophysiological pathways are activated during the process of myocardial injury. Various cardioprotective strategies protect the myocardium from ischemia, infarction, and ischemia/reperfusion (I/R) injury through different targets, yet the clinical translation remains limited. Caveolae and its structure protein, caveolins, have been suggested as a bridge to transmit damage-preventing signals and mediate the protection of ultrastructure in cardiomyocytes under pathological conditions. In this review, we first briefly introduce caveolae and caveolins. Then we review the cardioprotective strategies mediated by caveolins through various pathophysiological pathways. Finally, some possible research directions are proposed to provide future experiments and clinical translation perspectives targeting caveolin based on the investigative evidence.

N-glycosylation stabilizes MerTK and promotes hepatocellular carcinoma tumor growth

Despite the evidences of elevated expression of Mer tyrosine kinase (MerTK) in multiple human cancers, mechanisms underlying the oncogenic roles of MerTK in hepatocellular carcinoma (HCC) remains undefined. We explored the functional effects of MerTK and N-Glycosylated MerTK on HCC cell survival and tumor growth. Here, we show that MerTK ablation increases reactive oxygen species (ROS) production and promotes the switching from glycolytic metabolism to oxidative phosphorylation in HCC cells, thus suppressing HCC cell proliferation and tumor growth. MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote oncogenic transformation. Moreover, we observed that nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress. Pathologically, tissue microarray (TMA) data indicate that MerTK is a pivotal prognostic factor for HCC. Our data strongly support the roles of MerTK N-glycosylation in HCC tumorigenesis and suggesting N-glycosylation inhibition as a potential HCC therapeutic strategy.

•

MerTK promotes the switching from oxidative phosphorylation to glycolytic metabolism in HCC cells.

•

MerTK is N-glycosylated in HCC cells at asparagine 294 and 454 that stabilizes MerTK to promote HCC tumor growth.

•

The nuclear located non-glycosylated MerTK is indispensable for survival of HCC cells under stress.

•

MerTK is a pivotal prognostic factor for HCC and its N-glycosylation inhibition is a potential HCC therapeutic strategy.

Neuraminidase-3 Is a Negative Regulator of LFA-1 Adhesion

Within the plasma membrane environment, glycoconjugate-receptor interactions play an important role in the regulation of cell-cell interactions. We have investigated the mechanism and activity of the human neuraminidase (NEU) isoenzyme, NEU3, on T cell adhesion receptors. The enzyme is known to prefer glycolipid substrates, and we confirmed that exogenous enzyme altered the glycolipid composition of cells. NEU3 was able to modify the sialic acid content of purified LFA-1 in vitro. Enzymatic activity of NEU3 resulted in re-organization of LFA-1 into large clusters on the membrane. This change was facilitated by an increase in the lateral mobility of LFA-1 upon NEU3 treatment. Changes to the lateral mobility of LFA-1 were specific for NEU3 activity, and we observed no significant change in diffusion when cells were treated with a bacterial NEU (NanI). Furthermore, we found that NEU3 treatment of cells increased surface expression levels of LFA-1. We observed that NEU3-treated cells had suppressed LFA-1 adhesion to an ICAM-1 coated surface using an in vitro static adhesion assay. These results establish that NEU3 can modulate glycoconjugate composition and contribute to the regulation of integrin activity. We propose that NEU3 should be investigated to determine its role on LFA-1 within the inflammatory cascade.

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.

A glycosphingolipid/caveolin-1 signaling complex inhibits motility of human ovarian carcinoma cells

The genetic (stable overexpression of sialyltransferase I, GM3 synthase) or pharmacological (selective pressure by N-(4-hydroxyphenyl)retinamide)) manipulation of A2780 human ovarian cancer cells allowed us to obtain clones characterized by higher GM3 synthase activity compared with wild-type cells. Clones with high GM3 synthase expression had elevated ganglioside levels, reduced in vitro cell motility, and enhanced expression of the membrane adaptor protein caveolin-1 with respect to wild-type cells. In high GM3 synthase-expressing clones, both depletion of gangliosides by treatment with the glucosylceramide synthase inhibitor D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol and silencing of caveolin-1 by siRNA were able to strongly increase in vitro cell motility. The motility of wild-type, low GM3 synthase-expressing cells was reduced in the presence of a Src inhibitor, and treatment of these cells with exogenous gangliosides, able to reduce their in vitro motility, inactivated c-Src kinase. Conversely, ganglioside depletion by D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol treatment or caveolin-1 silencing in high GM3 synthase-expressing cells led to c-Src kinase activation. In high GM3 synthase-expressing cells, caveolin-1 was associated with sphingolipids, integrin receptor subunits, p130(CAS), and c-Src forming a Triton X-100-insoluble noncaveolar signaling complex. These data suggest a role for gangliosides in regulating tumor cell motility by affecting the function of a signaling complex organized by caveolin-1, responsible for Src inactivation downstream to integrin receptors, and imply that GM3 synthase is a key target for the regulation of cell motility in human ovarian carcinoma.

Verotoxin-1 treatment or manipulation of its receptor globotriaosylceramide (gb3) for reversal of multidrug resistance to cancer chemotherapy

A major problem with anti-cancer drug treatment is the development of acquired multidrug resistance (MDR) of the tumor cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the globotriaosylceramide membrane receptor (Gb3), a glycolipid associated with multidrug resistance. Gb3 is overexpressed in many human tumors and tumor cell lines with inherent or acquired MDR. Gb3 is co-expressed and interplays with the membrane efflux transporter P-gp encoded by the MDR1 gene. P-gp could act as a lipid flippase and stimulate Gb3 induction when tumor cells are exposed to cancer chemotherapy. Recent work has shown that apoptosis and inherent or acquired multidrug resistance in Gb3-expressing tumors could be affected by VT-1 holotoxin, a sub-toxic concentration of the holotoxin concomitant with chemotherapy or its Gb3-binding B-subunit coupled to cytotoxic or immunomodulatory drug, as well as chemical manipulation of Gb3 expression. The interplay between Gb3 and P-gp thus gives a possible physiological approach to augment the chemotherapeutic effect in multidrug resistant tumors.

Overexpression of ST6GalNAcV, a ganglioside-specific alpha2,6-sialyltransferase, inhibits glioma growth in vivo

Aberrant cell-surface glycosylation patterns are present on virtually all tumors and have been linked to tumor progression, metastasis, and invasivity. We have shown that expressing a normally quiescent, glycoprotein-specific alpha2,6-sialyltransferase (ST6Gal1) gene in gliomas inhibited invasivity in vitro and tumor formation in vivo. To identify other glycogene targets with therapeutic potential, we created a focused 45-mer oligonucleotide microarray platform representing all of the cloned human glycotranscriptome and examined the glycogene expression profiles of 10 normal human brain specimens, 10 malignant gliomas, and 7 human glioma cell lines. Among the many significant changes in glycogene expression observed, of particular interest was the observation that an additional alpha2,6-sialyltransferase, ST6 (alpha-N-acetyl-neuraminyl-2,3-beta-galactosyl-1,3)-N-acetylgalactosaminide alpha2,6-sialyltransferase 5 (ST6GalNAcV), was expressed at very low levels in all glioma and glioma cell lines examined compared with normal brain. ST6GalNAcV catalyzes the formation of the terminal alpha2,6-sialic acid linkages on gangliosides. Stable transfection of ST6GalNAcV into U373MG glioma cells produced (i) no change in alpha2,6-linked sialic acid-containing glycoproteins, (ii) increased expression of GM2alpha and GM3 gangliosides and decreased expression of GM1b, Gb3, and Gb4, (iii) marked inhibition of in vitro invasivity, (iv) modified cellular adhesion to fibronectin and laminin, (v) increased adhesion-mediated protein tyrosine phosphorylation of HSPA8, and (vi) inhibition of tumor growth in vivo. These results strongly suggest that modulation of the synthesis of specific glioma cell-surface glycosphingolipids alters invasivity in a manner that may have significant therapeutic potential.

Glucosylceramide synthase upregulates MDR1 expression in the regulation of cancer drug resistance through cSrc and beta-catenin signaling

Background

Drug resistance is the outcome of multiple-gene interactions in cancer cells under stress of anticancer agents. MDR1 overexpression is most commonly detected in drug-resistant cancers and accompanied with other gene alterations including enhanced glucosylceramide synthase (GCS). MDR1 encodes for P-glycoprotein that extrudes anticancer drugs. Polymorphisms of MDR1 disrupt the effects of P-glycoprotein antagonists and limit the success of drug resistance reversal in clinical trials. GCS converts ceramide to glucosylceramide, reducing the impact of ceramide-induced apoptosis and increasing glycosphingolipid (GSL) synthesis. Understanding the molecular mechanisms underlying MDR1 overexpression and how it interacts with GCS may find effective approaches to reverse drug resistance.

Results

MDR1 and GCS were coincidently overexpressed in drug-resistant breast, ovary, cervical and colon cancer cells; silencing GCS using a novel mixed-backbone oligonucleotide (MBO-asGCS) sensitized these four drug-resistant cell lines to doxorubicin. This sensitization was correlated with the decreased MDR1 expression and the increased doxorubicin accumulation. Doxorubicin treatment induced GCS and MDR1 expression in tumors, but MBO-asGCS treatment eliminated "in-vivo" growth of drug-resistant tumor (NCI/ADR-RES). MBO-asGCS suppressed the expression of MDR1 with GCS and sensitized NCI/ADR-RES tumor to doxorubicin. The expression of P-glycoprotein and the function of its drug efflux of tumors were decreased by 4 and 8 times after MBO-asGCS treatment, even though this treatment did not have a significant effect on P-glycoprotein in normal small intestine. GCS transient transfection induced MDR1 overexpression and increased P-glycoprotein efflux in dose-dependent fashion in OVCAR-8 cancer cells. GSL profiling, silencing of globotriaosylceramide synthase and assessment of signaling pathway indicated that GCS transfection significantly increased globo series GSLs (globotriaosylceramide Gb3, globotetraosylceramide Gb4) on GSL-enriched microdomain (GEM), activated cSrc kinase, decreased β-catenin phosphorylation, and increased nuclear β-catenin. These consequently increased MDR1 promoter activation and its expression. Conversely, MBO-asGCS treatments decreased globo series GSLs (Gb3, Gb4), cSrc kinase and nuclear β-catenin, and suppressed MDR-1 expression in dose-dependent pattern.

Conclusion

This study demonstrates, for the first time, that GCS upregulates MDR1 expression modulating drug resistance of cancer. GSLs, in particular globo series GSLs mediate gene expression of MDR1 through cSrc and β-catenin signaling pathway.

Involvement of beta1-integrin up-regulation in basic fibroblast growth factor- and epidermal growth factor-induced proliferation of mouse neuroepithelial cells

In neural stem cells, basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) promote cell proliferation and self-renewal. In the bFGF- and EGF-responsive neural stem cells, beta1-integrin also plays important roles in crucial cellular processes, including proliferation, migration, and apoptosis. The cross-talk of the signaling pathways mediated by these growth factors and beta1-integrin, however, has not been fully elucidated. Here we report a novel molecular mechanism through which bFGF or EGF promotes the proliferation of mouse neuroepithelial cells (NECs). In the NECs, total beta1-integrin expression levels and proliferation were dose-dependently increased by bFGF but not by EGF. EGF rather than bFGF strongly induced the increase of beta1-integrin localization on the NEC surface. bFGF- and EGF-induced beta1-integrin up-regulation and proliferation were inhibited after treatment with a mitogen-activated protein kinase kinase inhibitor, U0126, which indicates the dependence on the mitogen-activated protein kinase pathway. Involvement of beta1-integrin in bFGF- and EGF-induced proliferation was confirmed by the finding that NEC proliferation and adhesion to fibronectin-coated dishes were inhibited by knockdown of beta1-integrin using small interfering RNA. On the other hand, apoptosis was induced in NECs treated with RGD peptide, a small beta1-integrin inhibitor peptide with the Arg-Gly-Asp motif, but it was independent of beta1-integrin expression levels. Those results suggest that regulation of beta1-integrin expression/localization is involved in cellular processes, such as proliferation, induced by bFGF and EGF in NECs. The mechanism underlying the proliferation through beta1-integrin would not be expected to be completely identical, however, for bFGF and EGF.

Differential regulation of adipocyte PDE3B in distinct membrane compartments by insulin and the beta3-adrenergic receptor agonist CL316243: effects of caveolin-1 knockdown on formation/maintenance of macromolecular signalling complexes

In adipocytes, PDE3B (phosphodiesterase 3B) is an important regulatory effector in signalling pathways controlled by insulin and cAMP-increasing hormones. Stimulation of 3T3-L1 adipocytes with insulin or the beta3-adrenergic receptor agonist CL316243 (termed CL) indicated that insulin preferentially phosphorylated/activated PDE3B associated with internal membranes (endoplasmic reticulum/Golgi), whereas CL preferentially phosphorylated/activated PDE3B associated with caveolae. siRNA (small interfering RNA)-mediated KD (knockdown) of CAV-1 (caveolin-1) in 3T3-L1 adipocytes resulted in down-regulation of expression of membrane-associated PDE3B. Insulin-induced activation of PDE3B was reduced, whereas CL-mediated activation was almost totally abolished. Similar results were obtained in adipocytes from Cav-1-deficient mice. siRNA-mediated KD of CAV-1 in 3T3-L1 adipocytes also resulted in inhibition of CL-stimulated phosphorylation of HSL (hormone-sensitive lipase) and perilipin A, and of lipolysis. Superose 6 gel-filtration chromatography of solubilized membrane proteins from adipocytes stimulated with insulin or CL demonstrated the reversible assembly of distinct macromolecular complexes that contained 32P-phosphorylated PDE3B and signalling molecules thought to be involved in its activation. Insulin- and CL-induced macromolecular complexes were enriched in cholesterol, and contained certain common signalling proteins [14-3-3, PP2A (protein phosphatase 2A) and cav-1]. The complexes present in insulin-stimulated cells contained tyrosine-phosphorylated IRS-1 (insulin receptor substrate 1) and its downstream signalling proteins, whereas CL-activated complexes contained beta3-adrenergic receptor, PKA-RII [PKA (cAMP-dependent protein kinase)-regulatory subunit] and HSL. Insulin- and CL-mediated macromolecular complex formation was significantly inhibited by CAV-1 KD. These results suggest that cav-1 acts as a molecular chaperone or scaffolding molecule in cholesterol-rich lipid rafts that may be necessary for the proper stabilization and activation of PDE3B in response to CL and insulin.

Regulation of tumor phenotypes by caveolin-1 and sphingolipid-controlled membrane signaling complexes

Aberrant (glyco)sphingolipid expression deeply affects several properties of tumor cells that are involved in tumor progression and metastasis formation: cell adhesion (to the extracellular matrix or to the endothelium of blood vessels), motility, recognition and invasion of host tissues. In particular, (glyco)sphingolipids might contribute to the modulation of integrin-dependent interactions of tumor cells (determining their adhesion, motility and invasiveness) with the extracellular matrix as well as with host cells present in the stromal compartment of the tumor. A model based on solid experimental evidence has been proposed: (glyco)sphingolipids at the cell surface interact with plasma membrane receptors (e.g., integrin receptors and growth factor receptors) and adapter molecules (including tetraspanins) forming signaling complexes that are able to influence the activity of signal transduction molecules oriented at the cytosolic surface of the plasma membrane (mainly the Src kinases pathway members). The function of these signaling complexes appears to be strictly dependent on their (glyco)sphingolipid composition, and likely on specific sphingolipid-protein interactions. From this point of view, particularly intriguing is the connection between (glyco)sphingolipids and caveolin-1, a membrane protein that plays multiple roles as a suppressor of tumor growth and metastasis in ovarian, breast and colon human carcinomas.

Overexpression of caveolin-1 in a human melanoma cell line results in dispersion of ganglioside GD3 from lipid rafts and alteration of leading edges, leading to attenuation of malignant properties

Caveolin-1 is a component of lipid rafts, and is considered to be a tumor suppressor molecule. However, the mechanisms by which caveolin-1 functions in cancer cells are not well understood. We generated caveolin-1 transfectant cells (Cav-1(+) cells) using a human melanoma cell line (SK-MEL-28) and investigated the effects of caveolin-1 overexpression on the GD3-mediated malignant properties of melanomas. Cav-1(+) cells had decreased cell growth and motility, and reduced phosphorylation levels of p130Cas and paxillin relative to controls. In floatation analysis, although GD3 was mainly localized in glycolipid-enriched microdomain (GEM)/rafts in control cells, it was dispersed from GEM/rafts in Cav-1(+) cells. Correspondingly, GD3 in Cav-1(+) cells stained uniformly throughout the membrane, whereas control cells showed partial staining of the membrane, probably at the leading edge. p130Cas and paxillin were stained in the leading edges and colocalized with GD3 in the control cells. In contrast, these molecules were diffusely stained and no definite leading edges were detected in Cav-1(+) cells. These results suggest that caveolin-1 regulates GD3-mediated malignant signals by altering GD3 distribution and leading edge formation. These results reveal one of the mechanisms by which caveolin-1 curtails the malignant properties of tumor cells.

Characterization of the Epstein–Barr virus glycoprotein BMRF-2

Epstein-Barr virus (EBV) BMRF-2 protein interaction with the beta1 family of integrins plays an important role in EBV infection of polarized oral epithelial cells. In this work, we characterized BMRF-2 protein expression in EBV-infected B lymphoblastoid and polarized oral epithelial cells, and in hairy leukoplakia (HL) epithelium. BMRF-2 expression in B cells and polarized oral epithelial cells was associated with the EBV lytic infection. In these cells, BMRF-2 is efficiently transported to the cell membrane and its integrin binding Arg-Gly-Asp (RGD) motif is exposed on the cell surface. BMRF-2 is highly expressed in HL epithelium and accumulates at the lateral border of oral keratinocytes. In EBV-infected polarized oral epithelial cells, this protein is transported to the basolateral membranes and co-localized with beta1 integrin. These data suggest that BMRF-2 may play an important role in cell-to-cell spread of EBV within the oral epithelium. BMRF-2 is glycosylated through O-linked oligosaccharides; it forms oligomers and is associated with the virion envelope. Its C-terminal tail is localized in the cytoplasm. We found that beta1, alpha5, and alpha3 integrins are present in purified EBV virions. We show that BMRF-2 is a ligand for beta1, alpha5, alpha3, and alphav integrins and our data are consistent with a role for BMRF-2 in viral lytic infection.

Ganglioside GM2-tetraspanin CD82 complex inhibits met and its cross-talk with integrins, providing a basis for control of cell motility through glycosynapse

Glycosphingolipids (GSLs) at the cell surface membrane are associated or complexed with signal transducers (Src family kinases and small G-proteins), tetraspanins, growth factor receptors, and integrins. Such organizational framework, defining GSL-modulated or -dependent cell adhesion, motility, and growth, is termed "glycosynapse" (Hakomori, S., and Handa, K. (2002) FEBS Lett. 531, 88-92; Hakomori, S. (2004) Ann. Braz. Acad. Sci. 76, 553-572). We describe here the functional organization of the glycosynaptic microdomain, and the mechanisms for control of cell motility and invasiveness, in normal bladder epithelial HCV29 cells versus highly invasive bladder cancer YTS1 cells, both derived from transitional epithelia. (i) Ganglioside GM2, but not GM3 or globoside, interacted specifically with tetraspanin CD82, and such a complex inhibited hepatocyte growth factor (HGF)-induced activation of Met tyrosine kinase in a dose-dependent manner. (ii) Depletion of GM2 in HCV29 cells by treatment with D-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4), or reduction of CD82 expression by RNA interference, significantly enhanced HGF-induced Met tyrosine kinase and cell motility. (iii) In contrast, YTS1 cells, lacking CD82, displayed HGF-independent activation of Met tyrosine kinase and high cell motility. Transfection of the CD82 gene to YTS1 inhibited HGF dose-dependent Met tyrosine kinase activity and cell motility, due to formation of the GM2-CD82 complex. (iv) Adhesion of YTS1 or YTS1/CD82 cells to laminin-5-coated plates, as compared with noncoated plates, strongly enhanced Met activation, and the degree of activation was further increased in association with GSL depletion by P4. Laminin-5-dependent Met activation was minimal in HCV29 cells. These findings indicate that GSL, particularly GM2, forms a complex with CD82, and that such complex interacts with Met and thereby inhibits HGF-induced Met tyrosine kinase activity, as well as integrin to Met cross-talk.

Mechanisms for the apoptosis of small cell lung cancer cells induced by anti-GD2 monoclonal antibodies: roles of anoikis

Anti-GD2 ganglioside antibodies could be a promising, novel therapeutic approach to the eradication of human small cell lung cancers, as anti-GD2 monoclonal antibodies (mAbs) induced apoptosis of small cell lung cancer cells in culture. In this study, we analyzed the mechanisms for the apoptosis of these cells by anti-GD2 mAbs and elucidated the mechanisms by which apoptosis signals were transduced via reduction in the phosphorylation levels of focal adhesion kinase (FAK) and the activation of a MAPK family member, p38, upon the antibody binding. Knock down of FAK resulted in apoptosis and p38 activation. The inhibition of p38 activity blocked antibody-induced apoptosis, indicating that p38 is involved in this process. Immunoprecipitation-immunoblotting analysis of immune precipitates with anti-FAK or anti-integrin antibodies using an anti-GD2 mAb revealed that GD2 could be precipitated with integrin and/or FAK. These results suggested that GD2, integrin, and FAK form a huge molecular complex across the plasma membrane. Taken together with the fact that GD2+ cells showed marked detachment from the plate during apoptosis, GD2+ small cell lung cancer cells seemed to undergo anoikis through the conformational changes of integrin molecules and subsequent FAK dephosphorylation.

Effect of ganglioside and tetraspanins in microdomains on interaction of integrins with fibroblast growth factor receptor

The functional interaction ("cross-talk") of integrins with growth factor receptors has become increasingly clear as a basic mechanism in cell biology, defining cell growth, adhesion, and motility. However, no studies have addressed the microdomains in which such interaction takes place nor the effect of gangliosides and tetraspanins (TSPs) on such interaction. Growth of human embryonal WI38 fibroblasts is highly dependent on fibroblast growth factor (FGF) and its receptor (FGFR), stably associated with ganglioside GM3 and TSPs CD9 and CD81 in the ganglioside-enriched microdomain. Adhesion and motility of these cells are mediated by laminin-5 ((LN5) and fibronectin (FN) through alpha3beta1 and alpha5beta1 integrin receptors, respectively. When WI38 cells or its transformant VA13 cells were adhered to LN5 or FN, alpha3beta1 or alpha5beta1 were stimulated, giving rise to signaling to activate FGFR through tyrosine phosphorylation and inducing cell proliferation under serum-free conditions without FGF addition. Types and intensity of signaling during the time course differed significantly depending on the type of integrin stimulated (alpha3beta1 versus alpha5beta1), and on cell type (WI38 versus VA13). Such effect of cross-talk between integrins and FGFR was influenced strongly by the change of GM3 and TSPs. (i) GM3 depletion by P4 caused enhanced tyrosine phosphorylation of FGFR and Akt followed by MAPK activation, without significant change of ceramide level. GM3 depletion also caused enhanced co-immunoprecipitation of FGFR with alpha3/alpha5/beta1 and of these integrins with CD9/CD81. (ii) LN5- or FN-dependent proliferation of both WI38 and VA13 was strongly enhanced by GM3 depletion and by CD9/CD81 knockdown by siRNA. Thus, integrin-FGFR cross-talk is strongly influenced by GM3 and/or TSPs within the ganglioside-enriched microdomain.

Overexpressed GM1 Suppresses Nerve Growth Factor (NGF) Signals by Modulating the Intracellular Localization of NGF Receptors and Membrane Fluidity in PC12 Cells

Ganglioside GM1 has been considered to have a neurotrophic factor-like activity. To analyze the effects of endogenously generated GM1, the rat pheochromocytoma cell line PC12 was transfected with the GM1/GD1b/GA1 synthase gene and showed increased expression levels of GM1. To our surprise, GM1+-transfectant cells (GM1+ cells) showed no neurite formation after stimulation with nerve growth factor (NGF). Autophosphorylation of NGF receptor TrkA and activation of ERK1/2 after NGF treatment were scarcely detected in GM1+ cells. Binding of 125I-NGF to PC12 cells was almost equivalent between GM1+ cells and controls. However, dimer formation of TrkA upon NGF treatment was markedly suppressed in GM1+ cells in both cross-linking analysis with Bis(sulfosuccinimidyl)suberate 3 and 125I-NGF binding assay. The sucrose density gradient fractionation of the cell lysate revealed that TrkA primarily located in the lipid raft fraction moved to the non-raft fraction in GM1+ cells. p75NTR and Ras also moved from the raft to non-raft fraction in GM1+ cells, whereas flotillin and GM1 persistently resided in the lipid raft. TrkA kinase activity was differentially regulated when GM1 was added to the kinase assay system in vitro, suggesting suppressive/enhancing effects of GM1 on NGF signals based on the concentration. Measurement of fluorescence recovery after photobleaching revealed that the membrane fluidity was reduced in GM1+ cells. These results suggested that overexpressed GM1 suppresses the differentiation signals mediated by NGF/TrkA by modulating the properties of the lipid raft and the intracellular localization of NGF receptors and relevant signaling molecules.

Sigma-1 receptors potentiate epidermal growth factor signaling towards neuritogenesis in PC12 cells: potential relation to lipid raft reconstitution

We previously demonstrated that overexpression of sigma-1 receptors (sigma-1R) potentiated neurite sprouting caused by nerve growth factor in PC12 cells (Takebayashi et al. 2002 J Pharmacol Exp Ther 202:1227-1237). In this study we examined if sigma-1R may be involved in the action of epidermal growth factor (EGF). EGF is conventionally recognized as a mitogenic factor that stimulates only the proliferation of various types of cells, including PC12 cells. We found here that in sigma-1 receptor-overexpressing PC12 cells (sigma-1R OE cells), EGF markedly stimulates neuritogenesis without affecting cellular proliferation. EGF receptors (EGFR) are largely reduced in lipid rafts and are enriched in non-raft regions in sigma-1R OE cells. The enrichment of EGFR in the non-raft region is correlated with enhanced downstream signaling of EGFR including the phosphorylation of both EGFR and extracellular signal-regulated kinases (ERKs). Destruction of cholesterol-containing rafts by treating cells with methyl-beta-cyclodextrin also causes a reduction of EGFR in lipid rafts, a concomitant increase in the phosphorylation of both EGFR and ERK, and an increase in the EGF-induced neurite sprouting in wildtype cells. Furthermore, while overexpression of sigma-1R increases the level of lipid raft-associated cholesterol, the overexpression alters the levels of gangliosides in lipid rafts: GM1 and GM2 are decreased, whereas GD1a is increased. We conclude that sigma-1R cause the remodeling of lipid rafts, at least by increasing the level of lipid raft-associated cholesterol and by altering the levels of certain critical lipid raft-forming gangliosides. sigma-1R may thus play an important role in directing EGF signaling towards neuritogenesis, perhaps by shifting EGFR from the lipid raft into non-raft regions.

Membrane distribution of epidermal growth factor receptors in cells expressing different gangliosides

Gangliosides have been found to reside in glycosphingolipid-enriched microdomains (GEM) of the plasma membrane and to be involved in the regulation of epidermal growth factor receptor (EGFr or ErbB1) activity. To gain further insight into the mechanisms involved in EGFr modulation by gangliosides, we investigated the distribution of EGFr family members in the plasma membrane of CHO-K1 cells, which were genetically modified to express different ganglioside molecules or depleted of glycolipids. Our data demonstrate that at least four different sets of endogenously expressed gangliosides, including GD3, did not have a significant effect on EGFr distribution in the plasma membrane. In addition, using confocal microscopy analysis we clearly demonstrated that the EGFr co-localizes only to a minor extent with GD3. We also explored the endogenous expression, in wild-type CHO-K1 cells, of the orphan receptor ErbB2 (which is the preferred heteroassociation partner of all other ErbB proteins) and the effect of GD3 expression on its membrane distribution. Our results showed that CHO-K1 cells endogenously express ErbB2 and that expression of the GD3 affected, to some extent, the membrane distribution of endogenous ErbB2. Finally, our findings support the notion that most EGFr are excluded from GEM, while an important fraction of ErbB2 is found to be associated with these microdomains in membranes from CHO-K1 cells.

Cell growth regulation through GM3-enriched microdomain (glycosynapse) in human lung embryonal fibroblast WI38 and its oncogenic transformant VA13

Cell growth control mechanisms were studied based on organization of components in glycosphingolipid-enriched microdomain (GEM) in WI38 cells versus their oncogenic transformant VA13 cells. Levels of fibroblast growth factor receptor (FGFR) and cSrc were 4 times and 2-3 times higher, respectively, in VA13 than in WI38 GEM, whereas the level of tetraspanin CD9/CD81 was 3-5 times higher in WI38 than in VA13 GEM. Csk, the physiological inhibitor of cSrc, was present in WI38 but not in VA13 GEM. Functional association of GEM components in control of cell growth in WI38 is indicated by several lines of evidence. (i) Confluent, growth-inhibited WI38 showed a lower degree of FGF-induced MAPK activation than actively growing cells in sparse culture. (ii) The level of inactive cSrc (with Tyr-527 phosphate) was higher in confluent cells than in actively growing cells. Both processes i and ii were inhibited by GM3 since they were enhanced by GM3 depletion with d-threo-1-phenyl-2-palmitoylamino-3-pyrrolidino-1-propanol (P4). (iii) The high level of inactive cSrc associated with growth-inhibited cells was caused by coexisting Csk in WI38 GEM. (iv) Interaction of GM3 with FGFR was demonstrated by binding of GM3 to FGFR in the GEM fraction, as probed with GM3-coated beads, and by confocal microscopy. In contrast to WI38, both cSrc and MAPK in VA13 were strongly activated regardless of FGF stimulation or GM3 depletion by P4. Continuous, constitutive activation of both cSrc and MAPK was due to (i) a much higher level of cSrc and FGFR in VA13 than in WI38 GEM, (ii) their close association/interaction in VA13 GEM as indicated by clear coimmunoprecipitation between cSrc and FGFR, and (iii) the absence of Csk in VA13 GEM, making GEM incapable of inhibiting cSrc activation.

Variant glycosylation: an underappreciated regulatory mechanism for β1 integrins

Although it has been known for many years that beta1 integrins undergo differential glycosylation in accordance with changes in cell phenotype, the potential role of N-glycosylation as a modulator of integrin function has received little attention. One reason for the relatively limited interest in this topic likely relates to the fact that much of the prior research was correlative in nature. However, new results now bolster the hypothesis that there is a causal relationship between variant glycosylation and altered integrin activity. In this review, the evidence for variant glycosylation as a regulatory mechanism for beta1 integrins are summarized, with particular emphasis on: (1). outlining the instances in which cell phenotypic variation is associated with differential beta1 glycosylation, (2). describing the specific alterations in glycan structure that accompany phenotypic changes and (3). presenting potential mechanisms by which variant glycosylation might regulate integrin function.

Transfection of 2,6 and 2,3‐sialyltransferase genes and GlcNAc‐transferase genes into human glioma cell line U‐373 MG affects glycoconjugate expression and enhances cell death

Human glioma cell line U-373 MG expresses CMP-NeuAc : Galbeta1,3GlcNAc alpha2,3-sialyltransferase [EC No. 2.4.99.6] (alpha2,3ST), UDP-GlcNAc : beta-d-mannoside beta1,6-N-acetylglucosaminyltransferase V [EC 2.4.1.155] (GnT-V) and UDP-GlcNAc3: beta-d-mannoside beta1,4-N-acetylglucosaminyltransferase III [EC 2.4.1.144] (GnT-III) but not CMP-NeuAc : Galbeta1,4GlcNAc alpha2,6-sialyltransferase [EC 2.4.99.1] (alpha2,6ST) under normal culture conditions. We have previously shown that transfection of the alpha2,6ST gene into U-373 cells replaced alpha2,3-linked sialic acids with alpha2,6 sialic acids, resulting in a marked inhibition of glioma cell invasivity and a significant reduction in adhesivity. We now show that U-373 cells, which are typically highly resistant to cell death induced by chemotherapeutic agents (< 10% death in 18 h), become more sensitive to apoptosis following overexpression of these four glycoprotein glycosyltransferases. U-373 cell viability showed a three-fold decrease (from 20 to 60% cell death) following treatment with staurosporine, C2-ceramide or etoposide, when either alpha2,6ST and GnT-V genes were stably overexpressed. Even glycosyltransferases typically raised in cancer cells, such as alpha2,3ST and GnT-III, were able to decrease viability two-fold (from 20 to 40% cell death) following stable overexpression. The increased susceptibility of glycosyltransferase-transfected U-373 cells to pro-apoptotic drugs was associated with increased ceramide levels in Rafts, increased caspase-3 activity and increased DNA fragmentation. In contrast, the same glycosyltransferase overexpression protected U-373 cells against a different class of apoptotic drugs, namely the phosphatidylinositol 3-kinase inhibitor LY294002. Thus altered surface protein glycosylation of a human glioblastoma cell line can lead to lowered resistance to chemotherapeutic agents.

Caveolin-1 regulates matrix metalloproteinases-1 induction and CD147/EMMPRIN cell surface clustering

CD147, a regulator of matrix metalloproteinase (MMP) production, showed highly specific association with caveolin-1 on the surface of multiple cell types. CD147-caveolin-1 complex formation was temperature and cholesterol dependent, reminiscent of associations seen within caveolae/lipid rafts. However, the subset of caveolin-1 associated with CD147 appeared exclusively within intermediate density sucrose gradient fractions, rather than in the low density fractions containing the bulk of caveolin-1. Mutagenesis experiments revealed that CD147 Ig domain 2 was required for caveolin-1 association. In contrast to CD147-caveolin-1 complexes, CD147-alpha(3) integrin association was not disrupted upon cholesterol depletion, occurred in high density sucrose fractions, and did not involve CD147 Ig domain 2. Overexpression of caveolin-1 caused a specific decrease in clustering of cell surface CD147, as detected by "cluster specific" mAb M6/13. Conversely, a mutant CD147 deficient in caveolin-1 association showed enhanced spontaneous cell surface clustering (detected by mAb M6/13), and did not show decreased clustering in response to caveolin-1 overexpression. Furthermore, the same CD147 mutant yielded an elevated induction of MMP-1. In conclusion, caveolin-1 associates with CD147, in a complex distinct from CD147-alpha(3) integrin complexes, thereby diminishing both CD147 clustering and CD147-dependent MMP-1-inducing activity.

Suppression of lung metastasis of mouse Lewis lung cancer P29 with transfection of the ganglioside GM2/GD2 synthase gene

Ganglioside functions in tumor metastasis were analyzed by carbohydrate remodeling of a mouse Lewis lung cancer (subline P29) by introducing beta1,4GalNAc-T cDNA. Although P29 was originally a low-metastatic subline in the s.c. injection system, it showed high potential in lung metastasis when i.v.-injected via the tail vein. Two lines of GM(2)(+) transfectants showed markedly reduced metastatic potential to the lung compared to 2 control lines. However, cell proliferation rates and expression levels of various cell adhesion molecules, e.g., integrin family members, SLe(x) and CD44, were essentially unchanged after transfection of the cDNA. Then, cell adhesion to fibronectin-coated dishes was examined, showing that GM(2) (+) transfectants attached to the plates much more slowly than controls, suggesting functional modulation of integrins with newly expressed GM(2). Phosphorylation of the FAK located at downstream of integrin molecules was markedly reduced in GM(2)(+) transfectants, suggesting that GM(2) suppressed cell adhesion signals via fibronectin-integrin interaction.

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.

A signaling role for the uncleaved form of alpha 6 integrin in differentiating lens fiber cells

Many alpha integrin subunits are cleaved during their processing to yield heavy and light chains, which remain associated by disulfide bonds. While uncleaved alpha integrin subunits can form functional receptors that sometimes have distinct signaling roles from their better-characterized endoproteolytically cleaved counterparts, their expression at the cell surface and their association with signaling complexes have yet to be determined in vivo. In this study, we demonstrate that, in differentiating lens fiber cells, the uncleaved form of alpha 6 integrin was expressed at the cell surface. This form of alpha 6 integrin coimmunoprecipitated with both the signaling adaptor molecule Shc and its downstream effector Grb2, suggesting that, in lens fiber cells, uncleaved alpha 6 integrin was associated with a Shc-mediated signaling complex. We show that expression of the cleaved form of alpha 6 integrin progressively decreased relative to uncleaved alpha 6 integrin as the state of lens cell differentiation increased, resulting in the predominance of uncleaved alpha 6 integrin in the lens fiber cell zones. Interestingly, we previously have shown that alpha 6 integrin is localized principally along the extensive cell-cell interfaces of these lens fiber cells, in the absence of its extracellular matrix ligand laminin. While we found that the cleaved form of alpha 6 integrin contained both high mannose and complex sugars, the uncleaved form of alpha 6 integrin contained only high mannose sugars. These properties suggest that the uncleaved form of alpha 6 integrin may have a unique role in the embryonic lens. Its high association with Shc and Grb2 in the differentiating cortical fiber cell zone indicates that alpha 6 integrin may provide a cell survival signal in the presence of the apoptotic-like processes that are initiated in this region of the embryonic lens to clear the lens cells of their organelles.

Tetraspanin CD9 is a "proteolipid," and its interaction with alpha 3 integrin in microdomain is promoted by GM3 ganglioside, leading to inhibition of laminin-5-dependent cell motility

GM3 ganglioside inhibits tetraspanin CD9-facilitated cell motility in various cell lines (Ono, M., Handa, K., Sonnino, S., Withers, D. A., Nagai, H., and Hakomori, S. (2001) Biochemistry 40, 6414-6421). We now report the following: (i) CD9 has the novel feature of being soluble in chloroform/methanol, and classifiable as "proteolipid"; (ii) CD9 and alpha(3) integrin were concentrated together in the low-density glycolipid-enriched microdomain (GEM) of ldlD/CD9 cells, and the alpha(3) expression ratio (value for cells grown under +Gal condition divided by the value for cells grown under -Gal condition) in GEM of ldlD/CD9 cells was higher than that in control ldlD/moc cells, suggesting that CD9 recruits alpha(3) in GEM under +Gal condition, whereby GM3 is present. (iii) Chemical levels of alpha(3) and CD9 in the total extract or membrane fractions from cells grown under +Gal versus -Gal condition were nearly identical, whereas alpha(3) expressed at the cell surface, probed by antibody binding in flow cytometry, was higher under -Gal than +Gal condition. These results suggest that GM3 synthesized under +Gal condition promotes interaction of alpha(3) with CD9, which restricts alpha(3) binding to its antibody. A concept of the alpha(3)/CD9 interaction promoted by GM3 was further supported by (i) co-immunoprecipitation of CD9 and alpha(3) under +Gal but not -Gal condition, (ii) enhanced co-immunoprecipitation of CD9 and alpha(3) when GM3 was added exogenously to cells under -Gal condition, and (iii) the co-localization images of CD9 with alpha(3) and of GM3 with CD9 in fluorescence laser scanning confocal microscopy. Based on the promotion of alpha(3)/CD9 interaction by GM3 and the status of laminin-5 as a true ligand for alpha(3), the laminin-5/alpha(3)-dependent motility of ldlD/CD9 cells was found to be greatly enhanced under -Gal condition, but strongly inhibited under +Gal condition. Such a motility difference under +Gal versus -Gal condition was not observed for ldlD/moc cells. The inhibitory effect observed in ldlD/CD9 cells under +Gal condition was reversed upon addition of anti-alpha(3) antibody and is therefore based on interaction between alpha(3), CD9, and GM3 in GEM.

Hyposialylation of integrins stimulates the activity of myeloid fibronectin receptors

Despite numerous reports suggesting that beta(1) integrin receptors undergo differential glycosylation, the potential role of N-linked carbohydrates in modulating integrin function has been largely ignored. In the present study, we find that beta(1) integrins are differentially glycosylated during phorbol ester (PMA)-stimulated differentiation of myeloid cells along the monocyte/macrophage lineage. PMA treatment of two myeloid cell lines, U937 and THP-1, induces a down-regulation in expression of the ST6Gal I sialyltransferase. Correspondingly, the beta(1) integrin subunit becomes hyposialylated, suggesting that the beta(1) integrin is a substrate for this enzyme. The expression of hyposialylated beta(1) integrin isoforms is temporally correlated with enhanced binding of myeloid cells to fibronectin, and, importantly, fibronectin binding is inhibited when the Golgi disrupter, brefeldin A, is used to block the expression of the hyposialylated form. Consistent with the observation that cells with hyposialylated integrins are more adhesive to fibronectin, we demonstrate that the enzymatic removal of sialic acid residues from purified alpha(5)beta(1) integrins stimulates fibronectin binding by these integrins. These data support the hypothesis that unsialylated beta(1) integrins are more adhesive to fibronectin, although desialylation of alpha(5) subunits could also contribute to increased fibronectin binding. Collectively our results suggest a novel mechanism for regulation of the beta(1) integrin family of cell adhesion receptors.

Characterization of the acid stability of glycosidically linked neuraminic acid: use in detecting de-N-acetyl-gangliosides in human melanoma

The glycosidic linkage of sialic acids is much more sensitive to acid hydrolysis than those of other monosaccharides in vertebrates. The commonest sialic acids in nature are neuraminic acid (Neu)-based and are typically N-acylated at the C5 position. Unsubstituted Neu is thought to occur on native gangliosides of certain tumors and cell lines, and synthetic de-N-acetyl-gangliosides have potent biological properties in vitro. However, claims for their natural existence are based upon monoclonal antibodies and pulse-chase experiments, and there have been no reports of their chemical detection. Here we report that one of these antibodies shows nonspecific cross-reactivity with a polypeptide epitope, further emphasizing the need for definitive chemical proof of unsubstituted Neu on naturally occurring gangliosides. While pursuing this, we found that alpha2-3-linked Neu on chemically de-N-acetylated G(M3) ganglioside resists acid hydrolysis under conditions where the N-acetylated form is completely labile. To ascertain the generality of this finding, we investigated the stability of glycosidically linked alpha- and beta-methyl glycosides of Neu. Using NMR spectroscopy to monitor glycosidic linkage hydrolysis, we find that only 47% of Neualpha2Me is hydrolyzed after 3 h in 10 mm HCl at 80 degrees C, whereas Neu5Acalpha2Me is 95% hydrolyzed after 20 min under the same conditions. Notably, Neubeta2Me is hydrolyzed even slower than Neualpha2Me, indicating that acid resistance is a general property of glycosidically linked Neu. Taking advantage of this, we modified classical purification techniques for de-N-acetyl-ganglioside isolation using acid to first eliminate conventional gangliosides. We also introduce a phospholipase-based approach to remove contaminating phospholipids that previously hindered efforts to study de-N-acetyl-gangliosides. The partially purified sample can then be N-propionylated, allowing acid release and mass spectrometric detection of any originally existing Neu as Neu5Pr. These advances allowed us to detect covalently bound Neu in lipid extracts of a human melanoma tumor, providing the first chemical proof for naturally occurring de-N-acetyl-gangliosides.

Kinetic analysis of binding between Shiga toxin and receptor glycolipid Gb3Cer by surface plasmon resonance

Shiga toxin (Stx) binds to the receptor glycolipid Gb3Cer on the cell surface and is responsible for hemolytic uremic syndrome. Stx has two isoforms, Stx1 and Stx2, and in clinical settings Stx2 is known to cause more severe symptoms, although the differences between the mechanisms of action of Stx1 and Stx2 are as yet unknown. In this study, the binding modes of these two isoforms to the receptor were investigated with a surface plasmon resonance analyzer to compare differences by real time receptor binding analysis. A sensor chip having a lipophilically modified dextran matrix or quasicrystalline hydrophobic layer was used to immobilize an amphipathic lipid layer that mimics the plasma membrane surface. Dose responsiveness was observed with both isoforms when either the toxin concentration or the Gb3Cer concentration was increased. In addition, this assay was shown to be specific, because neither Stx1 nor Stx2 bound to GM3, but both bound weakly to Gb4Cer. It was also shown that a number of fitting models can be used to analyze the sensorgrams obtained with different concentrations of the toxins, and the "bivalent analyte" model was found to best fit the interaction between Stxs and Gb3Cer. This shows that the interaction between Stxs and Gb3Cer in the lipid bilayer has a multivalent effect. The presence of cholesterol in the lipid bilayer significantly enhanced the binding of Stxs to Gb3Cer, although kinetics were unaffected. The association and dissociation rate constants of Stx1 were larger than those of Stx2: Stx2 binds to the receptor more slowly than Stx1 but, once bound, is difficult to dissociate. The data described herein clearly demonstrate differences between the binding properties of Stx1 and Stx2 and may facilitate understanding of the differences in clinical manifestations caused by these toxins.

Association of MUC-1 and SPGL-1 with low-density microdomain in T-lymphocytes: a preliminary note

Two mucin-type glycoproteins, MUC-1 and P-selectin glycoprotein ligand-1 (PSGL-1), and glycosphingolipids (GSLs), expressed in human T-cell line HUT78, were highly enriched in low-density buoyant fraction (termed "GEM"), together with CD45, Yes, Fyn, and lck(56). Enrichment of MUC-1, PSGL-1 and GSLs, together with these signal transducer molecules in low-density membrane fraction was observable when fraction was prepared from cells either in nonionic detergent Brij 58 or in hypertonic alkaline conditions (500 mM Na(2)CO(3)). On pretreatment of cells with cholesterol-binding reagent methyl beta-cyclodextrin, levels of MUC-1 and PSGL-1 together with the above signal transducers in GEM was greatly reduced, and they were translocated into high-density membrane fraction. Similar association of lck(56), Yes, Fyn, and cSrc together with MUC-1 was also found in GEM fraction of mouse T-cell lymphoma EL4 cells expressing MUC-1 through transfection of its gene. These findings indicate the presence of another glycosyl cluster ("glycocluster"), in addition to the previously well-established GSL cluster organized with signal transducer molecules in GEM fraction, and its possible functional role in T-cells.

Evaluation of prototype transmembrane 4 superfamily protein complexes and their relation to lipid rafts

Recent literature suggests that tetraspanin proteins (transmembrane 4 superfamily; TM4SF proteins) may associate with each other and with many other transmembrane proteins to form large complexes that sometimes may be found in lipid rafts. Here we show that prototype complexes of CD9 or CD81 (TM4SF proteins) with alpha(3)beta(1) (an integrin) and complexes of CD63 (a TM4SF protein) with phosphatidylinositol 4-kinase (PtdIns 4-K) may indeed localize within lipid raft-like microdomains, as seen by three different criteria. First, these complexes localize to low density light membrane fractions in sucrose gradients. Second, CD9 and alpha(3) integrin colocalized with ganglioside GM1 as seen by double staining of fixed cells. Third, CD9-alpha3beta1 and CD81-alpha3beta1 complexes were shifted to a higher density upon cholesterol depletion from intact cells or cell lysate. However, CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K complex formation itself was not dependent on localization into raftlike lipid microdomains. These complexes did not require cholesterol for stabilization, were maintained within well solubilized dense fractions from sucrose gradients, were stable at 37 degrees C, and were small enough to be included within CL6B gel filtration columns. In summary, prototype TM4SF protein complexes (CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K) can be solubilized as discrete units, independent of lipid microdomains, although they do associate with microdomains resembling lipid rafts.

Glycosylation effect on membrane domain (GEM) involved in cell adhesion and motility: a preliminary note on functional alpha3, alpha5-CD82 glycosylation complex in ldlD 14 cells

Laminin (LN)- or fibronectin (FN)-dependent adhesion in Krieger's ldlD 14 (D14) cells is enhanced significantly in the presence vs absence, of galactose (Gal), whereas LN- or FN-induced haptotactic cell motility is barely affected unless cells express CD82 by its gene transfection (cells termed D14/CD82). The effect of CD82 on LN- or FN-induced motility is based on its ability to associate with alpha3 or alpha5 integrin to form a complex associated with a low-density lipid membrane domain (termed GEM or GSD). Complex formation is greatly affected by N-glycosylation of both integrin and CD82, as well as by concurrent GM3 ganglioside synthesis. The effect of glycosylation on alpha5-CD82 complex was also studied in D14 cells expressing mutant CD82, defective in all three N-glycosylation sites. LN-induced motility was greatly inhibited, whereas FN-induced motility was enhanced, with complete N-glycosylation in D14/CD82 cells in Gal-added medium, whereby alpha5-CD82 complex formation did not occur or occurred at a minimal level. Both LN- and FN-induced motility were inhibited when N-glycosylation was impaired, or N-glycosylation of CD82 was deleted, whereby alpha5-CD82 complex formation occurred strongly. Thus, glycosylation profoundly affects interaction of integrin with CD82, leading to significant inhibition or promotion of cell motility.