Biosynthesis and turnover of O-acetyl and N-acetyl groups in the gangliosides of human melanoma cells

Expression of de-N-acetyl-gangliosides in human melanoma cells is induced by genistein or nocodazole

Neuraminic acid is the core structure of most known sialic acids. In natural systems, the amino group at the 5 position of neuraminic acid residues is usually assumed to be acylated. Previously, synthetic de-N-acetyl-gangliosides (with free amino groups at the 5 position of neuraminic acids) have been shown to modulate cellular proliferation and tyrosine phosphokinase reactions. While indirect evidence has suggested that traces of these molecules exist naturally in certain tumor cells, further exploration has been hampered by the lack of a system showing consistent expression at an easily detectable level. Using synthetic compounds as antigens, we have developed highly specific monoclonal antibodies against de-N-acetyl-GM3 and de-N-acetyl-GD3 that require both the free amino group and the exocyclic side chain of sialic acids for recognition. Cultured human melanoma cells showed low but variably detectable levels of reactivity with these antibodies. The ability of various biologically active molecules to stimulate this reactivity was explored. Of many compounds tested, only the tyrosine kinase inhibitor genistein induced reactivity in a dose-dependent manner. Antibody reactivity with ganglioside extracts from genistein-treated cells was abolished by chemical re-N-acetylation and/or truncation of sialic acid side chains by mild periodate oxidation. High performance thin layer chromatography immuno-overlay analysis confirmed the presence of the novel compound de-N-acetyl-GD3 in these extracts. Several other tyrosine kinase inhibitors tested did not give the same increase in de-N-acetyl-ganglioside expression. However, the microtubule inhibitor nocodazole caused a similar accumulation of these molecules, particularly in non-adherent cells expected to be arrested at metaphase. Thus, genistein may induce de-N-acetyl-ganglioside expression by virtue of its known ability to arrest cells in the G2M phase, rather than as a general consequence of tyrosine kinase inhibition. These studies also provide a system in which to analyze the enzymatic basis of de-N-acetyl-ganglioside expression and their potential roles as growth regulating molecules.

Total synthesis of a modified ganglioside, de-N-acetyl GM2

Regio- and stereo-selective glycosylation of a sialyl donor 6 that carries a N-phthaloyl protecting group at C-5 with a lactosyl acceptor 7 armed with a pivaloyl group at O-2a was performed to give the expected glycotrioside 5. Subsequent glycosylation of 5 with 2-azido galactosyl donor 4 gave glycotetraosyl derivatives 18 and 19. After conversion of 18 into imidates 25 and 26, coupling with (2S,3R,4E)-3-O-benzoyl-2-N-tetracosanoylsphingenine (2) was executed to afford completely protected ganglioside analogues 27 and 28. Selective cleavage of the methyl ester and N,O-deprotection gave the target de-N-acetyl GM2 (1).

Activation of human monocyte/macrophage cytotoxicity by IL-2/IFN gamma is linked to increased expression of an antitumor receptor with specificity for acetylated mannose

Spontaneous cytotoxicity of human monocytes (purity: 92-95%) against K562 tumor cells was only observed in 31% healthy donors but, in the presence of rhamnogalacturonan (500 ng/ml), enhanced cytotoxicity was recorded for 79% (n = 14) of the donors. Monocytes activated by culturing with interleukin-2 and/or IFN gamma showed increased antitumor cytotoxicity against K562 tumor cells in 86% (n = 21) of the donors exhibiting additional increases in specific cytotoxicity when the cytotoxicity assays were carried out in the presence of rhamnogalacturonan. Increases of monocyte cytotoxicity achieved by activation with cytokines coincided with increased formation of monocyte/tumor cell conjugates. Similarly, increased monocyte cytotoxicity mediated by rhamnogalacturonan also correlated with increased monocyte/tumor cell conjugate formation most likely due to effector cell/target cell bridging as was originally described for rhamnogalacturonan interacting with CD56+ natural killer or lymphokine-activated killer cells and tumor cells. The chemospecificity of the monocyte-based receptors responsible for cytotoxicity and for monocyte/tumor cell conjugate formation, as well as for their rhamnogalacturonan-mediated enhancements, appears to be identical since all these effects could be inhibited in a dose-dependent manner by partially deacetylated (60%) mannose pentaacetate.

Clinical aspects of glycoprotein biosynthesis

Glycoproteins are widely distributed among species in soluble and membrane-bound forms, associated with many different functions. The heterogenous sugar moieties of glycoproteins are assembled in the endoplasmic reticulum and in the Golgi and are implicated in many roles that require further elucidation. Glycoprotein-bound oligosaccharides show significant changes in their structures and relative occurrences during growth, development, and differentiation. Diverse alterations of these carbohydrate chains occur in diseases such as cancer, metastasis, leukemia, inflammatory, and other diseases. Structural alterations may correlate with activities of glycosyltransferases that assemble glycans, but often the biochemical origin of these changes remains unclear. This suggests a multitude of biosynthetic control mechanisms that are functional in vivo but have not yet been unraveled by in vitro studies. The multitude of carbohydrate alterations observed in disease states may not be the primary cause but may reflect the growth and biochemical activity of the affected cell. However, knowledge of the control mechanisms in the biosynthesis of glycoprotein glycans may be helpful in understanding, diagnosing, and treating disease.

Use of influenza C virus for detection of 9-O-acetylated sialic acids on immobilized glycoconjugates by esterase activity

An overlay and a solid-phase assay are presented which allow the specific detection of 9-O-acetylated sialic acids on sialoglycoconjugates immobilized on microtiter plates, nitrocellulose or separated on thin-layer chromatograms. The assay takes advantage of two different biological properties of influenza C virus, its high-affinity binding to 9-O-acetylated sialic acids and its sialate 9-O-acetylesterase that is used for detection of bound virus with fluorogenic or chromogenic substrates. Though simple and rapid, the assay is highly sensitive with a detection limit of 65 fmol 9-O-acetylated sialic acid in 9-O-acetylated ganglioside GD1a. Influenza C virus is able to bind to a wide spectrum of sialoglycoconjugates like mucins, serum glycoproteins or gangliosides containing naturally or synthetically O-acetylated sialic acids. 9-O-Acetyl-N-glycoloylneuraminic acid can also function as a high-affinity receptor determinant for influenza C virus. While the acetyl ester at the 9 position is essential for virus binding in all cases, a 4-O-acetyl group is not recognized. In addition to alpha(2.3) or alpha(2.6) bonds, 9-O-acetyl-N-acetylneuraminic acid in alpha(2.8) linkage to N-acetylneuraminic acid is also functionally active.

Alteration in keratinocyte ganglioside content in basal cell carcinomas

We examined the ganglioside content of normal human keratinocytes and basal cell carcinomas (BCC). The total ganglioside content of the epidermis was 0.098 +/- 0.01 microgram lipid-bound sialic acid/mg dry weight. GM3 was the predominant ganglioside of epidermis. GM2 and GD3 were also found in significant amounts. Polysialylated gangliosides were identified in only small amounts. In contrast to all other body locations, breast epidermis showed large amounts of GM1. The total ganglioside content of nodular and sclerosing facial BCC was approximately 3.5 times that of normal facial epidermis. This marked elevation of total ganglioside was not affected by dermal ganglioside contamination, because the total ganglioside content of the dermis was similar to that of the epidermis. The relative percentage of GM2 was significantly decreased, whereas the relative percentage of GM3 was slightly decreased in BCC. 9-O-acetyl-GD3 was present in the BCC, but not in normal epidermis or dermis. 9-O-acetyl-GD3 may be a surface marker for BCC. Furthermore, the alterations in amount and composition of individual gangliosides on neoplastic membranes may lead to novel therapeutic interventions.

A novel O-acetylated ganglioside detected by anti-GD2 monoclonal antibodies

Murine monoclonal antibody (MAb) 3F8 was previously shown to react with disialoganglioside GD2, but not with GD3, GT1b, GD1b, GD1a, GM1, GM3 and GM4. However, when the base-treatment step was ommitted from the standard neuroblastoma ganglioside extraction procedure, immuno-thin-layer-chromatography (ITLC) using 3F8 and other anti-GD2 MAbs revealed a new ganglioside band, abbreviated as NG (Rf 0.342) besides GD2 (R 0.183). It migrated below GD3 (Rf 0.358) on high-performance (HP) TLC plate and its binding to 3F8 on ITLC could be inhibited by rat anti-3F8 idiotypic antibody Idio-2, while the binding of GD2 to MAb 3F8 was not affected. Immunochemical analysis showed that this new neuroblastoma ganglioside contained alkali-sensitive O-acetylated sialic-acid residues recognized by MAb DI.I. After base treatment, its subsequent identity on ITLC was confirmed to be GD2. Lactonization of GD2 yielded 2 major bands, with Rf values (0.401, 0.583) distinct from that of the new ganglioside band. In addition, MAb DI.I did not bind to any of these GD2 lactones. Of 15 anti-GD2 MAbs studied, 13 reacted strongly with the novel ganglioside NG. By ITLC, this NG was found in ganglioside extracts of fresh surgical tumor specimens (4/4 neuroblastomas, I/I schwannoma and I/I anaplastic astrocytoma), and nude mice/rat xenografts (2/2 neuroblastomas, 2/2 osteogenic sarcomas). These data provided the first evidence that O-acetylated GD2 is a naturally occurring ganglioside derivative in human tumors and that it could cross-react with most anti-GD2 antibodies.