Tumor gangliosides enhance alpha2 beta1 integrin-dependent platelet activation

Brain-derived gangliosides prime human platelet aggregation and induce platelet-leukocyte aggregate formation

BACKGROUND

Platelet activation and interaction with leukocytes are crucial in inflammation. Gangliosides, sialic acid-containing glycosphingolipids, have been linked to different inflammatory conditions related to cardio- and neurodegenerative disorders. The role of gangliosides in platelet and leukocyte function, although reported, still needs further investigation.

OBJECTIVES

We aimed to study the role of gangliosides in platelet activation and platelet-leukocyte interaction in vitro.

METHODS

Platelet activation was studied through aggregometry in platelet-rich plasma from apparently healthy human volunteers. Signaling protein phosphorylation was analyzed by immunoblotting. Platelet P-selectin expression and platelet-leukocyte aggregate formation were measured by flow cytometry.

RESULTS

The gangliosides monosialoganglioside GM1, disialoganglioside GD1a, and trisialoganglioside GT1b did not induce by themselves any platelet aggregation. Conversely, when preincubated with platelets, they potentiate platelet aggregation induced by submaximal adenosine diphosphate and collagen concentrations and increased P-selectin expression. Incubation of platelets with free sialic acid and the soluble part of monosialoganglioside GM1 induced a similar potentiating effect on platelet aggregation but not on platelet P-selectin expression. Consistently, analyzing the signaling protein phosphorylation, only the entire gangliosides activated extracellular stimuli-responsive kinase 1/2 suggesting that a complete ganglioside is crucial for its action on platelets. Both the priming effect on platelet aggregation and ERK1/2 activation were prevented by aspirin. Moreover, incubation of citrated whole blood with gangliosides induced platelet-leukocyte aggregate formation accompanied by increased expression of granulocyte and monocyte CD11b compared with untreated blood, suggesting a primary leukocyte activation.

CONCLUSION

Gangliosides may act in vitro both on platelet and leukocyte activation and on their interaction. The observed effects might contribute to inflammatory processes in clinical conditions.

The Extracellular Matrix and Neuroblastoma Cell Communication-A Complex Interplay and Its Therapeutic Implications

Neuroblastoma (NB) is a pediatric neuroendocrine neoplasm. It arises from the sympatho-adrenal lineage of neural-crest-derived multipotent progenitor cells that fail to differentiate. NB is the most common extracranial tumor in children, and it manifests undisputed heterogeneity. Unsatisfactory outcomes of high-risk (HR) NB patients call for more research to further inter-relate treatment and molecular features of the disease. In this regard, it is well established that in the tumor microenvironment (TME), malignant cells are engaged in complex and dynamic interactions with the extracellular matrix (ECM) and stromal cells. The ECM can be a source of both pro- and anti-tumorigenic factors to regulate tumor cell fate, such as survival, proliferation, and resistance to therapy. Moreover, the ECM composition, organization, and resulting signaling networks are vastly remodeled during tumor progression and metastasis. This review mainly focuses on the molecular mechanisms and effects of interactions of selected ECM components with their receptors on neuroblastoma cells. Additionally, it describes roles of enzymes modifying and degrading ECM in NB. Finally, the article gives examples on how the knowledge is exploited for prognosis and to yield new treatment options for NB patients.

Cerebral Thromboembolism after Lobectomy for Lung Cancer: Pathological Diagnosis and Mechanism of Thrombus Formation

Lung cancer is the leading cause of cancer-related deaths worldwide. Although molecular therapies have emerged as efficacious strategies for the treatment of lung cancer, surgical resection is still recommended as a radical therapeutic option. Currently, lobectomy is regarded as the most reliable radical treatment of primary lung cancer. Among the various complications after lobectomy, cerebral thromboembolism requires attention as a life-threatening complication during the early postoperative period. It occurs in 0.2–1.2% of surgical cases of lung cancer and typically develops following left upper lobectomy with a long pulmonary vein stump (PVS). PVS-associated thrombosis is known to cause cerebral thromboembolism after such procedures; however, distinguishing this specific complication from that caused by postoperative atrial fibrillation is challenging. We summarize herein the diagnostic pathology of thrombus formation in accordance with its thrombogenic mechanism. We focus on the potential utility of the pathological assessment of thrombectomy specimens. The morphological information obtained from these specimens enables the presumption of thrombogenic etiology and provides useful clues to both select an appropriate pharmacotherapy and determine a follow-up treatment for cerebral thromboembolism.

Enhancement of malignant properties of human glioma cells by ganglioside GD3/GD2

Sialic acid-containing glycosphingolipids, gangliosides, are considered as cancer associated antigens in neuro-ectoderm-derived tumors such as melanomas and neuroblastomas. In particular, gangliosides GD3 and GD2 are expressed in human gliomas. It has been reported that their expression levels increase along with increased malignant properties. However, the implication of GD3/GD2 in human glioma cells has never been clarified, at least to the best of our knowledge. In this study, we introduced the cDNA of GD3 synthase (GD3S)(ST8SIA1) into a glioma cell line, U-251MG, that expresses neither GD3 nor GD2, thereby establishing transfectant cells U-251MG-GD3S(+) expressing high levels of GD3 and GD2 on the cell surface. In these U-251MG‑GD3S(+) cell lines, signaling molecules such as Erk1/2, Akt, p130Cas, paxillin and focal adhesion kinase were activated, leading to the enhancement of invasion activity and motility. It was then demonstrated that the U-251MG-GD3S(+) cells could proliferate under culture conditions with low or no serum concentrations without undergoing cell cycle arrest by escaping the accumulation of p16 and p21. All these results suggested that GD3 and GD2 highly expressed in gliomas confer increased invasion and mobility, cell growth abilities under low serum conditions, and increased ratios of the S-G2/M phase in the cell cycle.

Reduced GM1 ganglioside in CFTR-deficient human airway cells results in decreased β1-integrin signaling and delayed wound repair

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function reduces chloride secretion and increases sodium uptake, but it is not clear why CFTR mutation also results in progressive lung inflammation and infection. We previously demonstrated that CFTR-silenced airway cells migrate more slowly during wound repair than CFTR-expressing controls. In addition, CFTR-deficient cells and mouse models have been reported to have altered sphingolipid levels. Here, we investigated the hypothesis that reduced migration in CFTR-deficient airway epithelial cells results from altered sphingolipid composition. We used cell lines derived from a human airway epithelial cell line (Calu-3) stably transfected with CFTR short hairpin RNA (CFTR-silenced) or nontargeting short hairpin RNA (controls). Cell migration was measured by electric cell substrate impedance sensing (ECIS). Lipid analyses, addition of exogenous glycosphingolipids, and immunoblotting were performed. We found that levels of the glycosphingolipid, GM1 ganglioside, were ~60% lower in CFTR-silenced cells than in controls. CFTR-silenced cells exhibited reduced levels of activated β1-integrin, phosphorylated tyrosine 576 of focal adhesion kinase (pFAK), and phosphorylation of Crk-associated substrate (pCAS). Addition of GM1 (but not GM3) ganglioside to CFTR-silenced cells restored activated β1-integrin, pFAK, and pCAS to near control levels and partially restored (~40%) cell migration. Our results suggest that decreased GM1 in CFTR-silenced cells depresses β1-integrin signaling, which contributes to the delayed wound repair observed in these cells. These findings have implications for the pathology of cystic fibrosis, where altered sphingolipid levels in airway epithelial cells could result in a diminished capacity for wound repair after injury.

Tumor-Associated Glycans and Immune Surveillance

Changes in cell surface glycosylation are a hallmark of the transition from normal to inflamed and neoplastic tissue. Tumor-associated carbohydrate antigens (TACAs) challenge our understanding of immune tolerance, while functioning as immune targets that bridge innate immune surveillance and adaptive antitumor immunity in clinical applications. T-cells, being a part of the adaptive immune response, are the most popular component of the immune system considered for targeting tumor cells. However, for TACAs, T-cells take a back seat to antibodies and natural killer cells as first-line innate defense mechanisms. Here, we briefly highlight the rationale associated with the relative importance of the immune surveillance machinery that might be applicable for developing therapeutics.

Ganglioside GD3 enhances adhesion signals and augments malignant properties of melanoma cells by recruiting integrins to glycolipid-enriched microdomains

Ganglioside GD3 is widely expressed in human malignant melanoma cell lines and tumors. Previously, we reported that GD3+ cells show stronger tyrosine phosphorylation of focal adhesion kinase (FAK), p130(Cas), and paxillin when treated with fetal calf serum than GD3- cells. In this study, we analyzed the changes in the signals mediated by the interaction between integrins and extracellular matrices (ECM) to clarify how GD3 enhances cell signals in the vicinity of the cell membrane. An adhesion assay with a real time cell electronic sensing system revealed that GD3+ cells had stronger adhesion to all extracellular matrices examined. In particular, GD3+ cells attached more strongly to collagen type I and type IV than controls. Correspondingly, they showed stronger tyrosine phosphorylation of FAK and paxillin during adhesion to collagen type I. In the floating pattern of detergent extracts, a high level of integrin beta1 was found in glycolipid-enriched microdomain (GEM)/rafts in GD3+ cells before adhesion, whereas a smaller amount of integrin beta1 was detected in the GEM/rafts of controls. Some phosphorylated forms of FAK as well as total FAK were found in GEM/rafts during cell adhesion only in GD3+ cells. Another signal consisting of integrin-linked kinase/Akt was also activated during adhesion more strongly in GD3+ cells than in controls. In double stained GD3+ cells, GD3 and integrin beta1 co-localized at the focal adhesion with a punctate pattern. All these results suggested that integrins assembled and formed a cluster in GEM/rafts, leading to the enhanced signaling and malignant properties under GD3 expression.

Gangliosides promote platelet adhesion and facilitate neuroblastoma cell adhesion under dynamic conditions simulating blood flow

BACKGROUND

Aberrant gangliosides are produced and shed by some tumors into the extracellular milieu. Their concentration is related to disease progression in children with neuroblastoma and in experimental models. The mechanism for this tumor promoting effect is not known.

PURPOSE

Here, we investigated the effect of gangliosides on platelet and tumor cell adhesion under shear forces simulating venous and arterial flow.

RESULTS

High shear force increased platelet adhesion 2.5-fold compared to low force. Pre-incubation of platelets with gangliosides increased adhesion at low shear to a level comparable to high shear alone. The addition of gangliosides to platelets perfused at high shear did not further increase adhesion. LAN1 neuroblastoma cells are adherent to collagen in static assays. No effect of either shear or gangliosides was observed on cell adhesion under dynamic conditions. However, when perfused in the presence of platelets, an increase in binding of tumor cells was observed at both shear forces compared to cells alone.

CONCLUSIONS

These results demonstrate that shear and gangliosides increase dynamic platelet adhesion to collagen. In addition, platelets facilitate tumor cell binding. Therefore, by acting as a mediator of platelet activation, gangliosides may promote blood borne metastasis by increasing platelet binding to the vessel wall and in turn facilitate tumor cell arrest in circulation.

The glycosphingolipid, lactosylceramide, regulates beta1-integrin clustering and endocytosis

Glycosphingolipids are known to play roles in integrin-mediated cell adhesion and migration; however, the mechanisms by which glycosphingolipids affect integrins are unknown. Here, we show that addition of the glycosphingolipid, C8-lactosylceramide (C8-LacCer), or free cholesterol to human fibroblasts at 10 degrees C causes the formation of glycosphingolipid-enriched plasma membrane domains as shown by visualizing a fluorescent glycosphingolipid probe, BODIPY-LacCer, incorporated into the plasma membrane of living cells. Addition of C8-LacCer or cholesterol to cells initiated the clustering of beta1-integrins within these glycosphingolipid-enriched domains and the activation of the beta1-integrins as assessed using a HUTS antibody that only binds activated integrin. On warming to 37 degrees C, beta1-integrins were rapidly internalized via caveolar endocytosis in cells treated with C8-LacCer or cholesterol, whereas little beta1-integrin was endocytosed in untreated fibroblasts. Incubation of cells with C8-LacCer or cholesterol followed by warm-up caused src activation, a reorganization of the actin cytoskeleton, translocation of RhoA GTPase away from the plasma membrane as visualized using total internal reflection fluorescence microscopy, and transient cell detachment. These studies show that LacCer can regulate integrin function both by modulating integrin clustering in microdomains and by regulating integrin endocytosis via caveolae. Our findings suggest the possibility that aberrant levels of glycosphingolipids found in cancer cells may influence cell attachment events by direct effects on integrin clustering and internalization.

Differential modulation of human melanoma cell metalloproteinase expression by alpha2beta1 integrin and CD44 triple-helical ligands derived from type IV collagen

Tumor cell binding to components of the basement membrane is well known to trigger intracellular signaling pathways. Signaling ultimately results in the modulation of gene expression, facilitating metastasis. Type IV collagen is the major structural component of the basement membrane and is known to be a polyvalent ligand, possessing sequences bound by the alpha1beta1, alpha2beta1, and alpha3beta1 integrins, as well as cell surface proteoglycan receptors, such as CD44/chondroitin sulfate proteoglycan (CSPG). The role of alpha2beta1 integrin and CD44/CSPG receptor binding on human melanoma cell activation has been evaluated herein using triple-helical peptide ligands incorporating the alpha1(IV)382-393 and alpha1(IV)1263-1277 sequences, respectively. Gene expression and protein production of matrix metalloproteinases-1 (MMP-1), -2, -3, -13, and -14 were modulated with the alpha2beta1-specific sequence, whereas the CD44-specific sequence yielded significant stimulation of MMP-8 and lower levels of modulation of MMP-1, -2, -13, and -14. Analysis of enzyme activity confirmed different melanoma cell proteolytic potentials based on engagement of either the alpha2beta1 integrin or CD44/CSPG. These results are indicative of specific activation events that tumor cells undergo upon binding to select regions of basement membrane collagen. Based on the present study, triple-helical peptide ligands provide a general approach for monitoring the regulation of proteolysis in cellular systems.

Gangliosides regulate tumor cell adhesion to collagen

The ability of tumor cells to adhere to extracellular matrix proteins is critical for migration and invasion. The factors that regulate tumor cell adhesion are poorly characterized. Gangliosides promote platelet adhesion and may also play a role in the adhesion of other cell types. We hypothesized that pharmacological depletion of membrane gangliosides from adherent cells would abrogate adhesion to collagen and promote migration and invasion. To test these hypotheses, LA-N1 neuroblastoma cells, which avidly adhere to collagen and are rich with membrane gangliosides (43.69 nmol/10(8) cells), were cultured in the presence of D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol-HCl. Endogenous gangliosides were reduced by 98% (0.76 nmol/10(8) cells) and adhesion to collagen decreased by 67%. There were no changes in cell morphology, viability, proliferation rate or apoptosis. Pre-incubation of ganglioside-depleted cells in conditioned medium from control cells restored adhesion to collagen (0.45 +/- 0.002), comparable to that of control cells (0.49 +/- 0.035). Similarly, pre-incubation of ganglioside-depleted cells with purified GD2 completely restored adhesion in a concentration-dependent manner. When LA-N1 cells were cultured with retinoic acid, a biological response modifier known to increase endogenous gangliosides, adhesion to collagen increased. Next, we questioned whether changes in adhesion would be reflected as changes in migration and invasion. Cells depleted of endogenous cellular gangliosides migrated more than control cells. Finally, control cells replete with their endogenous gangliosides demonstrated less invasive potential than control cells. The data demonstrate that endogenous tumor gangliosides increase neuroblastoma cell adhesion to collagen and reduce migration and invasion in vitro.

Glycosphingolipid expression in acute nonlymphocytic leukemia: common expression of shiga toxin and parvovirus B19 receptors on early myeloblasts

Glycosphingolipids (GSLs) are complex macromolecules on cell membranes that have been shown to play a role in neutrophil differentiation, activation, phagocytosis, and adhesion to both microorganisms and vascular endothelium. Because GSLs are often cryptic antigens on cell membranes, little is known regarding GSL expression in early myelopoiesis. To study the latter, myeloblasts were collected from patients with acute nonlymphocytic leukemia (ANLL) who required therapeutic leukocytopheresis for hyperleukocytosis. The neutral GSLs were isolated and identified by high-performance thin-layer chromatography (HPTLC), HPTLC immunostaining, gas chromatography, nuclear magnetic resonance, and fast atom bombardment-mass spectrometry. Like mature peripheral blood neutrophils, myeloblasts expressed glucosylceramide, lactosylceramide, and the neolacto-family GSLs, lactotriaosylceramide and neolactotetraosylceramide. Unlike neutrophils and chronic myeloid leukemia, most ANLL samples also expressed the globo-series GSLs, globotriaosylceramide and globotetraosylceramide. Globo GSL expression was strongly associated with a myeloblastic (ANLL M0-M2) and monoblastic phenotype (M5). A weak association was also noted with expression of either lymphoid (P <.10) or early hematopoietic markers (terminal deoxynucleotidyl transferase [TdT], CD34; P <.10). Globo-positive ANLL samples bound both shiga toxin and parvovirus B19 on HPTLC immunostaining. Based on these findings, we propose that neolacto and globo GSLs are expressed during early myeloid differentiation. Globotriaosylceramide expression on myeloblasts, and possibly myeloid stem cells, may have important implications for the use of shiga toxin as an ex vivo purging agent in autologous stem cell transplantation. Expression of globotetraosylceramide, the parvovirus B19 receptor, on myeloblasts may also explain the association between B19 infection, aplastic anemia, and chronic neutropenia of childhood.

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.

Inhibition of human platelet aggregation by gangliosides

The content and composition of gangliosides is modified upon platelet stimulation, suggesting that these lipids may play functional roles in platelet physiology. Therefore, the effect of exogenously added gangliosides on human platelet aggregation was evaluated. The pretreatment of platelets with a mixture of total gangliosides from bovine brain and a series of purified mono-, di- and tri-sialogangliosides partially inhibit the collagen-induced aggregation process and ATP release and completely block the generation of the second aggregation wave when ADP is used as agonist. The inhibition was exerted at around 100 microM by G(TOT) as well as purified G(M1), G(M3), G(D1a), and G(T1b) gangliosides, whereas asialoG(M1) and sulphatide did not show a significant influence on platelet aggregation. Thrombin, Ca(2+) ionophores (A23187 and Ionomycin), arachidonic acid, and U46619 were unable to bypass the inhibitory effect exerted by gangliosides, suggesting that gangliosides inhibit platelet aggregation by inhibiting the synthesis or action of prostaglandins. Gangliosides inhibited U46619-induced aggregation, thus suggesting that they block the action of thromboxane A(2). Epinephrine induces a partial aggregation on gangliosides-treated platelets, similar to fluoroaluminate and phorbol myristate acetate, indicating that these platelets are still functional. To summarize, these results indicate that the major pathway(s), but not all, driving to the aggregation process following the interaction of ligand-receptor may be blocked by pretreatment of human platelets with gangliosides.

Atherosclerotic aortic gangliosides enhance integrin-mediated platelet adhesion to collagen

Gangliosides, sialic acid-containing glycosphingolipids, accumulate in atherosclerotic vessels. Their role in the pathogenesis of atherosclerosis is unknown. Gangliosides isolated from tumor cells promote collagen-stimulated platelet aggregation and ATP secretion and enhance platelet adhesion to immobilized collagen. These activities are all mediated by ganglioside effects on the platelet integrin collagen receptor alpha2beta1. Therefore, we hypothesized that gangliosides isolated from atherosclerotic plaques would enhance platelet adhesion to immobilized collagen, a major component of the subendothelial matrix of blood vessels. Furthermore, we questioned whether this effect of atherosclerotic gangliosides might play a role in the pathogenesis of atherosclerosis. To test this hypothesis, we isolated the gangliosides from postmortem aortas of patients with extensive atherosclerotic disease and examined their effects on platelet adhesion. Samples of aortic tissue taken from areas involved with atherosclerotic plaque demonstrated accumulation of gangliosides (64.9+/-6.5 nmol/g wet weight) compared with gangliosides isolated from control normal aortic tissue taken from children who died of noncardiac causes (NAGs; 21.1+/-6.4 nmol/g wet weight). Interestingly, samples of tissue taken from diseased aortas but from areas not involved with gross plaque formation also demonstrated ganglioside accumulation (47.6+/-12.8 nmol/g wet weight). Next, the activity of each of these gangliosides on platelet adhesion to immobilized type I collagen was studied. Atherosclerotic aortic gangliosides (AAGs) as well as those isolated from grossly unaffected areas of the same aorta (UAGs) both increased platelet adhesion compared with control NAGs (OD570, 0. 37+/-0.11 and 0.29+/-0.14 versus 0.16+/-0.07, respectively; P<0.01 and P<0.05, respectively). These OD570 values corresponded to 9x10(5), 8x10(4), and 6x10(3) platelets per well after preincubation with 5 micromol/L AAG, UAG, and NAG, respectively. Increased adhesion was observed after preincubation with as little as 0.5 micromol/L AAG, and maximal adhesion was seen at 2.5 micromol/L, with a plateau extending to the highest concentration tested, 10 micromol/L. The effect of AAGs on platelet adhesion to collagen was abrogated by incubation of treated platelets with F-17 anti-alpha2 monoclonal antibody (OD570, 0.13+/-0.02). Finally, the effects of the major individual gangliosides isolated from atherosclerotic tissues, GM3 and GD3, were tested. GM3 increased adhesion to collagen (OD570, 0.415+/-0.06) as did GD3 (0.31+/-0.08). Similar to that of AAGs, the effect of both molecules was blocked by F-17 (0. 09+/-0.04 and 0.13+/-0.06, respectively). These experiments demonstrate that accumulated atherosclerotic gangliosides promote platelet adhesion to collagen, the major component of the subendothelial matrix. Furthermore, this activity is mediated by an effect of the gangliosides on the collagen-binding integrin alpha2beta1. This activity may provide a mechanism for the development of platelet thrombi at sites where atherosclerotic gangliosides accumulate and help to explain the role of platelets in the process of atherosclerotic disease progression.

Detection of medulloblastoma and astrocytoma-associated ganglioside GD3 in cerebrospinal fluid

Shedding of gangliosides by tumor cells may enhance tumor development. We recently showed that cells of the human brain tumor, medulloblastoma, shed gangliosides in vitro and have therefore examined ganglioside shedding by pediatric brain tumors into the cerebrospinal fluid (CSF). GD3, a major ganglioside in medulloblastoma and astrocytoma, was the target for detection in the CSF by immunostaining using the monoclonal antibody R24 and enhanced chemiluminescence detection. Mean CSF GD3 levels in patients with medulloblastomas (n = 9) and astrocytomas (n = 10) were significantly higher than those of controls (mean +/- SD 44.7 +/- 8.4 versus 18.2 +/- 1.9 pmol/ml, n = 20, P < 0.0002). Mass spectrometric analysis showed that tumor-derived ganglioside GD3 contained heterogeneous ceramide structures and, interestingly, the ceramide subspecies with shorter fatty acyl chains were selectively shed. The elevated CSF GD3 concentrations in patients with medulloblastoma and astrocytoma support the concept that ganglioside shedding, which may have significant biological consequences, is characteristic of human brain tumors.

Shedding of gangliosides by human medulloblastoma cells

Shedding of immunosuppressive gangliosides is an important characteristic of both experimental and human tumors. Using a medulloblastoma cell line, Daoy, with a very high ganglioside expression (141 +/- 13 nmol/10(8) cells) and a well-characterized ganglioside complement, we have now studied ganglioside shedding by human brain tumor cells. Shedding of gangliosides, quantified by metabolic radiolabeling, was significant (169 pmol/10(8) cells/h) and was generalized with respect to the major ganglioside carbohydrate structures (G(M2), G(M3), and G(D1a)). For each ganglioside, however, shedding was selective for ceramide structures containing shorter fatty acyl chains. Rapid and ceramide-selective shedding was confirmed in two additional human medulloblastoma cell lines, D341 Med and D283 Med (112 and 59 pmol/10(8) cells/h). Significant ganglioside shedding is therefore a common characteristic of human medulloblastoma cells and may influence the biological behavior of this tumor, in view of immunosuppressive and other biological properties of shed gangliosides.

Effects of neuroblastoma tumor gangliosides on platelet adhesion to collagen

Gangliosides, sialic acid-containing glycosphingolipids, enhance tumor formation in experimental animals and are associated with tumor progression and metastasis in humans. The mechanism(s) for this activity is (are) unknown. One possibility is enhanced platelet activation, since the interaction of platelets with tumor cells contributes to tumor cell arrest in the vascular compartment. We have previously shown that neuroblastoma tumor gangliosides (NBTG) enhance platelet adenosine triphosphate (ATP) secretion, aggregation, and adhesion. We determined that these NBTG effects are specific for collagen and are mediated through an alpha2 beta1 integrin-dependent mechanism. This report describes the effects of NBTG on a physiologically relevant model of collagen-alpha2 betal interaction. Platelet adhesion to immobilized native collagen fibers similar to those found in the extracellular matrix of blood vessels was determined. Platelet adhesion is enhanced by NBTG in a concentration-dependent manner. Incubation with concentrations of 1 and 10 microM NBTG increased platelet adhesion by 9% and 52%, respectively, compared to less than 1% in controls not incubated with gangliosides (P = 0.001 and P < 0.0001, respectively). In addition to increasing the number of adherent platelets, NBTG promoted more rapid attachment. In NBTG-incubated platelets, platelet adhesion began after a 5-min lag phase and was maximal at 30 min compared to a 20-min lag phase and maximal adhesion at 60 min for control platelets. At 30 min this difference was significant (P = 0.017); however, by 120 min there was no difference between NBTG and controls (P = 0.259). NBTG also induces platelet adhesion at collagen concentrations (0.1 microg) that failed to support adhesion of control platelets. These effects of NBTG require Mg2+ or Mn2+ ions but are not supported by Zn2+ or Ca2+ ions. Furthermore, preincubation of platelets with a blocking antibody (6F1) to the integrin collagen receptor alpha2 beta1 abrogates all of the effects of NBTG. These results indicate that tumor gangliosides enhance platelet adhesion to extracellular matrix collagen and promote rapid stabilization of the collagen-alpha2 beta1 interaction, the initial steps in platelet activation.