The cell density-dependent growth inhibition of human SK-N-MC neuroblastoma cells is initiated by increased ganglioside sialidase activity leading to elevated cell surface presentation of ganglioside GM1, a ligand of galectin-1. We herein show that the extent of the cell surface expression of the galectin coincides with marked increases of the sialidase activity. Reverse transcriptase-polymerase chain reaction analysis excludes a regulation at the transcriptional level. Exposure of cells to purified galectin-1 reveals its carbohydrate-dependent activity to reduce cell proliferation. Assays to detect DNA fragmentation biochemically and cytometrically and to block caspases render it unlikely that galectin-1 acts as a classical proapoptotic factor on these cells. Because the chimeric galectin-3 shares binding sites and binding parameters with galectin-1 for these cells, we tested whether this galectin will elicit the same response as the homodimeric cross-linking galectin-1. Evidently, galectin-3 fails to affect cell growth by itself but interferes with galectin-1 upon coincubation. Its proteolytically truncated variant, the C-terminal lectin domain with impaired capacity to form aggregates when surface bound, has only weak binding properties. Thus, the way in which the galectin-1 interacts topologically with an apparently common set of ligands relative to galectin-3 is crucial for eliciting post-binding events. We conclude that galectin-1 is a probable effector in the sialidase-dependent growth control in this system. Moreover, the experiments with galectin-3 reveal functional divergence, most probably based on different topologies of presentation of homologous carbohydratebinding sites.

Cell density-dependent inhibition of growth and neural differentiation in the human neuroblastoma cell line SK-N-MC are associated with a ganglioside sialidasemediated increase of GM 1 and lactosylceramide at the cell surface. Because these glycolipids expose galactose residues, we have initiated the study of the potential role of galectins in such cellular events. Using specific antibodies, galectin-1 but not galectin-3 was found to be present at the cell surface. Assessment of carbohydratedependent binding revealed a saturable amount of ligand sites approaching 2.6 ؋ 10 6 galectin-1 molecules bound/cell. Presence during cell culture of the sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid or of the GM 1 -binding cholera toxin B subunit effected a decrease of the presentation of galectin-1 ligands by 30 -50%. The assumption that GM 1 is a major ligand for galectin-1 was reinforced by the correlation between the number of carbohydrate-dependent 125 I-iodinated GM 1 -neoganglioprotein binding sites and the amount of immunoreactive surface galectin-1, the marked sensitivity of probe binding to the presence of anti-galectin-1 antibody, and the inhibition of cell adhesion to surfaceimmobilized GM 1 by the antibody. The results open the possibility that the carbohydrate-dependent interaction between ganglioside GM 1 and galectin-1 may relay sialidase-dependent alterations in this cell system.Gangliosides can exert a variety of cellular functions that include the triggering and modulation of transmembrane signaling and the mediation of recognition of receptor molecules in homotypic and heterotypic associations (1-4). Owing to this versatility in regulatory processes, it is fitting that the presentation of gangliosides at the cell surface is subject to control mechanisms that involve biosynthesis, endocytic uptake, recirculation, and lysosomal degradation (5, 6). Moreover, the structure of the oligosaccharide chains of gangliosides can be remodeled in situ by the action of a cell surface sialidase in the course of transformation, differentiation and cell contact formation (7-12). In human neuroblastoma cells (SK-N-MC), the activity of this sialidase was directed toward gangliosides with terminal sialic acids, yielding a shift from higher sialylated species to GM 1 and a conversion of GM 3 to lactosylceramide (13). Such alterations of the ganglioside profile are apparently of profound impact on the behavior of the neuroblastoma cells, because the selective inhibition of the ganglioside sialidase activity by 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (NeuAc2en) 1 led to marked changes in cellular morphology, a complete release from contact inhibition of growth, and a loss of differentiation markers (14,15). Because the underlying molecular events are unknown, we have now addressed questions on the presence and nature of potential receptors for GM 1 and/or lactosylceramide that are generated by the action of the ganglioside sialidase on the surface of neuroblastoma cells.The current literature prompts inves...

The extracellular functions of galectin-7 (p53-induced gene 1) are largely unknown. On the surface of neuroblastoma cells (SK-N-MC), the increased GM 1 density, a result of upregulated ganglioside sialidase activity, is a key factor for the switch from proliferation to differentiation. We show by solid-phase and cell assays that the sugar chain of this ganglioside is a ligand for galectin-7. In serum-supplemented proliferation assays, galectin-7 reduced neuroblastoma cell growth without the appearance of features characteristic for classical apoptosis. The presence of galectin-3 blocked this effect, which mechanistically resembles that of galectin-1. By virtue of carbohydrate binding, galectin-7 thus exerts neuroblastoma growth control similar to galectin-1 despite their structural differences. In addition to p53-linked proapoptotic activity intracellularly, galectin-7, acting as a lectin on the cell surface, appears to be capable of reducing cancer cell proliferation in susceptible systems.

Measurements of target activation in cells or tissues are key indicators of efficacy during drug development. In contrast to established methods that require reagents and multiple preprocessing steps, reagent-free in situ analysis of engaged drug targets or target-proximal pharmacodynamic signatures in solid tumors remains challenging. Here, we demonstrate that label-free quantification of histone acetylation-specific mass shifts by matrix-assisted laser desorption ionization (MALDI) mass spectrometry biotyping can be used for measurement of cellular potency of histone deacetylase inhibitors in intact cells. Furthermore, we employ MALDI mass spectrometry imaging of these mass shifts to visualize the spatiotemporal distribution of acetylated histones and thus the tumor-selective pharmacodynamic responses in a mouse model of gastrointestinal cancer. Taken together, our study suggests that the monitoring of drug-induced mass shifts in protein ion intensity fingerprints or images may be a powerful analytical tool in pharmacology and drug discovery.

Gangliosides of the plasma membrane are important modulators of cellular functions. Previous work from our laboratory had suggested that a plasma membrane sialidase was involved in growth control and differentiation in cultured human neuroblastoma cells (SK-N-MC), but its substrates had remained obscure. We now performed sialidase specificity studies in subcellular fractions and found ganglioside GM3 desialylating activity in presence of Triton X-100 to be associated with the plasma membrane, but absent in lysosomes. This Triton-activated plasma membrane enzyme desialylated also gangliosides GD1a, GD1b, and GT1b, thereby forming GM1; cleavage of GM1 and GM2, however, was not observed. Sialidase activity towards the glycoprotein fetuin with modified C-7 sialic acids and towards 4-methylumbelliferyl neuraminate was solely found in lysosomal, but not in plasma membrane fractions. The role of the plasma membrane sialidase in gangliosides desialylation of living cells was examined by following the fate of [3H]galactose-labelled individual gangliosides in pulse-chase experiments in absence and presence of the extracellular sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid. When the plasma membrane sialidase was inhibited, radioactivity of all gangliosides chased at the same rate. In the absence of inhibitor, GM3, GD1a, GD1b, GD2, GD3 and GT1b were degraded at a considerably faster rate in confluent cultures, whereas the GM1-pool seemed to be filled by the desialylation of higher gangliosides. The results thus suggest that the plasma membrane sialidase causes selective ganglioside desialylation, and that such surface glycolipid modification triggers growth control and differentiation in human neuroblastoma cells.

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Intact cell mass spectrometry biotyping, a collection of methods for classification of cells based on mass spectrometric fingerprints, is an established method in clinical and environmental microbiology. It has recently also been applied to the investigation of mammalian cells including primary blood cells and cultured cells. However, few automated procedures suitable for higher throughput and little analytical standardization of mammalian biotyping approaches have been reported so far. Here, we present a novel automated method that robustly classifies as few as 250 cells per spot. Automatically acquired cell fingerprints from cultured and primary cells show high technical (R > 0.95) and biological reproducibility (R = 0.83-0.96), with a median peak variance below 12 %. Ion suppression is shown to be a major concern at higher cell numbers and needs to be carefully monitored. We demonstrate that intact cell mass spectrometric signatures of different cell lines start to resemble each other at higher trifluoroacetic acid (TFA) concentrations and that therefore low concentrations of TFA in the matrix solution are preferred. We show that in vitro differentiation of HL-60 cells into a neutrophil-like phenotype can be rapidly and robustly monitored. We utilize the method for global analysis of person-to-person differences in mass spectral signatures of intact polymorphonuclear neutrophils and monocytes obtained from healthy volunteers. Our data suggest that automated MALDI mass spectrometry cell biotyping could be a useful complementary approach in clinical cell analysis.

Gangliosides located in the outer leaflet of the plasma membrane are important modulators of cellular functions. Our previous work has shown that in cultured human SK-N-MC neuroblastoma cells a sialidase residing in the same membrane selectively desialylates gangliosides with terminal sialic acid residues, causing a shift from higher species to GM1 and a conversion of GM3 to lactosylceramide. Inhibition of this sialidase by 2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid (NeuAc2en) resulted in increased cell proliferation and a loss of differentiation markers. In this study, we examined the occurrence and function of this ganglioside sialidase in other neuronal cells. Subcellular fractionation showed the sialidase to be located in the plasma membrane of all cell lines studied. The presence of the inhibitor NeuAc2en led to a profound decrease in the amount of the differentiation marker 200 kDa/70 kDa neurofilaments and an increase in cell proliferation in the cholinergic SK-N-MC and mixed cholinergic/adrenergic SK-N-FI and SK-N-DZ neuroblastoma lines, but had little or no effect in the human adrenergic SK-N-SH and SK-N-AS and the adrenergic/cholinergic PC12 cells from rat. The influence of the inhibitor on cell behaviour was paralleled by a diminished number of cholera toxin B-binding GM1 sites. The findings demonstrate that the plasma membrane ganglioside sialidase is an important element of proliferation and differentiation control in some, but not all, neuroblastoma cells and suggest that there might be a relationship between plasma membrane sialidase activity and cholinergic differentiation.