Alteration of neuroblastoma ganglioside metabolism by retinoic acid

Low complex ganglioside expression characterizes human neuroblastoma cell lines

Low (< or = 35%) or absent expression of the complex 'b' pathway gangliosides GD1b, GT1b and GQ1b (CbG) correlates with an aggressive biological phenotype in human neuroblastoma tumors. To develop an in vitro model to probe mechanisms by which CbG may contribute to neuroblastoma behavior, we have comprehensively evaluated ganglioside expression in nine well-established human neuroblastoma cell lines, all derived from poor prognosis tumors. Total cellular ganglioside content ranged from 8 to 69 nmol/10(8) cells. High performance thin layer chromatography revealed that the simple disialoganglioside GD2 was prominent in eight of the cell lines (up to 60% of total gangliosides), whereas CbG were low (1-21%) in all nine cell lines. The structurally most complex 'b' pathway species, GQ1b, was not detected in any of the cell lines. The prominence of GD2 in neuroblastoma cell lines mirrors the high expression of GD2 that characterizes human neuroblastoma tumors, and the low CbG expression in the cell lines is analogous to that found in clinically and biologically unfavorable neuroblastoma tumors, thus establishing these neuroblastoma cell lines as valuable model systems for study of the role of CbG in the pathobiology of human neuroblastoma.

Alterations in neuroblastoma ganglioside synthesis by induction of GD1b synthase by retinoic acid

Recent findings link increased expression of the structurally complex 'b' pathway gangliosides GD1b, GT1b, GQ1b (CbG) to a favourable clinical and biological behaviour in human neuroblastoma (NB). Seeking a model to probe these observations, we evaluated four human NB cell lines. Very low CbG content (4-10%) in three of the four cell lines (LAN-5, LAN-1, SMS-KCNR) reflected the ganglioside pattern observed in the most aggressive NB tumours. Pharmacological alterations of complex ganglioside synthesis in vitro by a 5-7 day exposure to 5-10 microM retinoic acid, which is employed in maintenance therapy of disseminated NB, included markedly increased (i) relative expression of CbG (6.6+/-2.0-fold increase, P=0.037), (ii) relative expression of the analogous 'a' pathway gangliosides, termed CaG (6.4+/-1.4-fold increase in GM1a and GD1a; P=0.010), and (iii) total cellular ganglioside content (2.0-6.3-fold), which in turn amplified the accumulation of structurally complex gangliosides. Substantial increases (2.7-2.9-fold) in the activity of GD1b/GM1a synthase (beta-1,3-galactosyltransferase), which initiates the synthesis of CbG and CaG, accompanied the all-trans retinoic acid (ATRA)-induced ganglioside changes. Thus, increased CbG synthesis in NB cell lines is attributable to a specific effect of ATRA, namely induction of GD1b/GM1a synthase activity. Since the shift towards higher expression of CbG and CaG during retinoic acid-induced cellular differentiation reflects a ganglioside pattern found in clinically less-aggressive tumours, our studies suggest that complex gangliosides may play a role in the biological and clinical behaviour of NB.

Retinoic acid-induced changes in polysialyltransferase mRNA expression and NCAM polysialylation in human neuroblastoma cells

Polysialic acid (PSA) is a dynamically regulated carbohydrate modification of the neural cell adhesion molecule NCAM, which is implicated in neural differentiation and cellular plasticity. The cloning and characterization of two polysialyltransferases, termed ST8SiaII (STX) and ST8SiaIV (PST), opened up new perspectives in the search for factors that control this unique cell surface glycosylation. In vitro and transfection approaches revealed that ST8SiaII and ST8SiaIV are independently capable of synthesizing PSA on NCAM with slightly different specificities towards the major NCAM isoforms and glycosylation sites. Their overlapping but distinct expression patterns during brain development point towards an independent transcriptional regulation. However, the factors driving their joint or distinct expression, as well as the significance of divergent expression patterns in vivo, are not yet understood. In the present study, the mRNA expression of ST8SiaII and ST8SiaIV was comparatively analyzed in neuronal differentiation of PSA-positive human neuroblastoma cell lines induced by retinoic acid (RA), phorbolester, or growth factors. Using a semiquantitative RT-PCR strategy, we demonstrated a general decrease in the mRNA level of ST8SiaII upon differentiation of SH-SY5Y and LAN-5 cells. In contrast, a drastic increase of ST8SiaIV was specifically induced by RA-treatment of SH-SY5Y cells. To explore the significance of these changes, the cellular capacity to perform PSA synthesis and the degree of NCAM polysialylation were analyzed. Our data indicate that the increased expression of ST8SiaIV enables an accelerated polysialylation of NCAM, which, however, is not converted into higher amounts of PSA.

Molecular cloning of mouse ganglioside sialidase and its increased expression in Neuro2a cell differentiation

Ganglioside sialidases have been implicated in neuronal differentiation processes, including neurite outgrowth. To understand further the roles and regulation mechanisms of the sialidase in neuronal systems, we have cloned mouse ganglioside sialidase cDNA and observed its expression in Neuro2a cell differentiation. A 3339-base pair cDNA, cloned based on the sequence information of previously cloned enzymes, encodes 418 amino acids containing three Asp boxes characteristic of sialidases. Northern blot analysis revealed a 3.4-kilobase transcript expressed highly in heart but also in several other tissues including brain. In situ hybridization of mouse brain demonstrated the mRNA to be present in the cerebral cortex, as well as in the granule cell layer, Purkinje cells, and deep cerebellar nucleus of the cerebellum. Transient expression of the cDNA in COS-1 cells resulted in over 300-fold increase in sialidase activity toward gangliosides compared with the control level, with a preference for ganglioside substrate. During 5-bromodeoxyuridine-induced Neuro2a cell differentiation, the expression of the sialidase was increased as assessed by activity assays and quantitative reverse transcription-polymerase chain reaction analyses. Stable transfection of the sialidase in Neuro2a cells resulted in accelerated neurite arborization following 5-bromodeoxyuridine treatment, indicating the direct participation of this ganglioside sialidase in neuronal cell differentiation.

Anti-GM2 IgM antibodies: clinical correlates and reactivity with a human neuroblastoma cell line

Anti-GM2 IgM antibodies have been reported in some patients with dysimmune neuropathy or lower motor neuron syndrome, in whom they were often associated with a concomitant reactivity with GM1. To investigate the possible clinical and pathogenetic relevance of these antibodies we measured serum anti-GM2 IgM titers by ELISA in 224 patients with different neuropathies and motor neuron disease and examined their binding to SK-N-SH neuroblastoma cells by indirect immunofluorescence (IIF). High titers of anti-GM2 IgM antibodies were found in eight patients with dysimmune neuropathies including two with multifocal motor neuropathy (MMN), two with purely motor demyelinating neuropathy without conduction block (MN) and four with Guillain-Barré syndrome (GBS). In two MMN patients reactivity with GM2 was associated with anti-GM1 reactivity and in one MN patient with anti-GM1, -GD1a and -GD1b reactivity. All but one patient had a concomitant reactivity with GalNAc-GD1a. Serum IgM from all positive patients intensely stained by IIF the surface of SK-N-SH neuroblastoma cells. This reactivity was blocked by serum pre-incubation with GM2, was not observed with sera from patients without anti-GM2 antibodies including those with high anti-GM1 or other anti-glycolipid antibodies, and correlated with the presence of GM2 in the SK-N-SH neuroblastoma cells. These findings indicate that anti-GM2 antibodies, though infrequent, are strictly associated with dysimmune neuropathies and suggest that SK-N-SH neuroblastoma cells can be a suitable in vitro model to study the functional and biological effects of these antibodies.

Nerve growth factor-induced neurite formation in PC12 cells is independent of endogenous cellular gangliosides

The PC12 rat pheochromocytoma cell line is an established model for nerve growth factor (NGF)-induced neurite formation. It has been shown that when gangliosides are added to the culture medium of PC12 cells, NGF-induced neurite formation of PC12 cells is enhanced. To determine the role of endogenous cellular gangliosides themselves in NGF-elicited neurite formation, we depleted cellular gangliosides using the new specific glucosylceramide synthase inhibitor, d, l-threo-1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol.HCl (PPPP). 0.5-2 microM PPPP rapidly inhibited ganglioside synthesis and depletedcellular gangliosides. Nonetheless, over a concentration range of 5-100 ng/ml NGF, in both low serum and serum-free medium, neurite formation was normal. Even pretreatment of PC12 cells for up to 6 days with 1 microM PPPP followed by cotreatment with PPPP and NGF for 10 days, still did not inhibit neurite formation. The conclusion that ganglioside depletion did not block neurite formation stimulated by NGF was supported by the lack of effect of PPPP, under these same conditions, on cellular acetylcholine esterase activity, a neuronal differentiation marker (73.8 +/- 12.1 versus 67.2 +/- 4.6 nmol/min/mg protein at 50 ng/ml NGF; control versus 1 microM PPPP). These findings, together with previous studies showing enhancement of NGF-induced neurite formation by exogenous gangliosides, underscore the vastly different effects that exogenous gangliosides and endogenous gangliosides may have upon cellular functions.

Inhibition of endogenous ganglioside synthesis does not block neurite formation by retinoic acid-treated neuroblastoma cells

Gangliosides are believed to play a critical role in cellular differentiation. To test this concept, we determined the effect of inhibition of endogenous ganglioside synthesis upon neurite formation induced by retinoic acid in LAN-5 human neuroblastoma cells. Ganglioside synthesis and content of LAN-5 cells exposed for 6 days to 10 microM D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) (an inhibitor of glucosylceramide synthase) were reduced by >90%. However, these ganglioside-depleted cells were not blocked from forming neurites when exposed to 10 microM retinoic acid. Even more extensive treatment of LAN-5 cells with 20 microM D-PDMP (6 day pretreatment followed by 6 days together with 10 microM retinoic acid) still did not block the retinoic acid-induced neurite formation. An element of neuroblastoma tumor cell differentiation, neurite formation, is therefore dependent neither on an intact cellular ganglioside complement nor on new ganglioside synthesis.

Antigen binding of antiganglioside antibodies in vitro is strongly influenced by the ganglioside composition of the sample

The ability of antiganglioside antibodies to detect their respective antigens in various environments was studied. In contrast to sensitive detection of pure ganglioside standards by ELISA, antibody binding to mixtures of gangliosides was drastically reduced. This loss of sensitivity also occurred with immunostaining of gangliosides absorbed to silica gel. Moreover, absorption of antibodies to antigen-containing lipid vesicles failed, if the vesicles were prepared together with other gangliosides. These data indicate that antigen accessibility is strongly influenced by surrounding gangliosides. This should be considered whenever gangliosides are traced by antibodies. The ELISA procedure appears useful to test for such properties.

Structural characterization and in vivo immunosuppressive activity of neuroblastoma GD2

Shedding of neuroblastoma gangliosides is positively correlated with tumour progression in patients with neuroblastoma. In assessing the biological activity of these ganglioside molecules, we recently found that total human neuroblastoma gangliosides inhibit cellular immune responses. Here, we have studied the major neuroblastoma ganglioside, GD2. GD2 was purified by high performance liquid chromatography and structurally characterized by mass spectrometry. Immunoregulatory effects of GD2 in vivo were then determined in an established murine model. GD2 significantly downregulated the local cellular immune response to an allogeneic cell challenge; the usual increase in mass of the lymph node draining the injection site was reduced by 88%, from 1.52 to 0.19 mg (control versus GD2-treated mice; p < 0.01). In parallel, lymphocyte recovery from each node was also reduced from 2.4 to 1.2 x 10(6) cells, and lymphocyte DNA synthesis was reduced to half of the control level. These results show that certain shed tumour gangliosides, such as GD2, function as intercellular signalling molecules, downregulate the cellular immune response, and may thereby enhance tumour formation and progression.

Biosynthesis and shedding of murine lymphoma gangliosides

Ganglioside biosynthesis and subsequent shedding are a potential mechanism contributing to tumor cell escape from the host immune response. As a first step in identifying active molecular species, structural characterization and quantification of the purified individual cellular and shed gangliosides of YAC-1 murine lymphoma cells were undertaken. These studies uncovered three striking changes in ganglioside metabolism in cells passaged in vivo, compared with cells cultured in vitro. (i) Marked inhibition of GalNAcGM1b synthesis: GM1b was present in an equal proportion to its biosynthetic product GalNAcGM1b in vitro, but was present in a 6-fold higher concentration in vivo. (ii) Marked inhibition of NeuGc synthesis: NeuGc, present in vitro in an up to 7-fold higher concentration than its biosynthetic precursor NeuAc, was decreased in relative concentration in vivo (1:1). (iii) Selectivity of shedding: ganglioside shedding in vitro was generalized with respect to both carbohydrate structure and ceramide structure (mainly d18:1-C24:1 and d18:1-C16:0), while in vivo, there was selective shedding of gangliosides containing NeuGc and the shorter chain fatty acid. The reduced synthesis of NeuGc and of GalNAcGM1b in vivo, and the selective shedding of more polar ganglioside species, also in vivo, show that the extracellular environment can markedly affect cellular ganglioside metabolism.