From the Division of Biomembrane Research, Pacific
Northwest Research Institute and the Departments of Pathobiology and
Microbiology, University of Washington, Seattle, Washington 98122, the
§ Complex Carbohydrate Research Center, University of
Georgia, Athens, Georgia 30602, and the ¶ Department of Urology,
Tohoku University School of Medicine, Seiryo-machi, Aoba-ku, Sendai
980-8574, Japan
Received for publication, December 29, 2000, and in revised form, February 20, 2001
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ABSTRACT |
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In renal cell carcinoma
(RCC), the level of higher gangliosides is correlated with
degree of metastatic potential, and cell lines derived from
metastatic deposits of RCC are characterized by high expression of
disialogangliosides (Saito, S., Orikasa, S., Ohyama, C., Satoh, M., and
Fukushi, Y. (1991) Int. J. Cancer 49, 329-334 and Saito,
S., Orikasa, S., Satoh, M., Ohyama, C., Ito, A., and Takahashi, T. (1997) Jpn. J. Cancer Res. (Gann) 88, 652-659). We now report two disialogangliosides, G1 and G2,
found in the RCC cell line TOS-1. G1 from TOS-1 cells was characterized as having a novel hybrid structure between ganglio-series (region I as
in Structure TI; same as the terminal structure of ganglioside GM2), and
the lacto-series type 1 (region II). The characterization was based on
reactivity with various monoclonal antibodies (mAbs) with defined
epitope specificity, as well as monosaccharide and fatty acid component
analysis, 1H NMR spectroscopy, and electrospray ionization
mass spectrometry of the intact compound. G1 showed strong reactivity
with mAb RM2, raised originally against TOS-1 cells, and weak
cross-reactivity with anti-GM2 mAb MK-1-8. The antigen is hereby
termed GalNAc disialosyl Lc4Cer
(IV4GalNAcIV3NeuAcIII6NeuAcLc4;
abbreviated GalNAcDSLc4). G2 was identified by
1H NMR and mass spectrometry as having a structure similar
to Structure TI but without the GalNAc1
4 substitution and showed
strong reactivity with mAb FH9 reported previously to be specific for
disialosyl lacto-series type 1 (disialosyl Lc4) having
vicinal
2
3 and
2
6 sialosyl residues, an antigen associated
with human colonic cancer. Clinicopathological studies indicate
that expression of these disialoganglioside antigens in RCC tissue is
correlated with the metastatic potential of RCC.
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INTRODUCTION |
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The expression pattern of glycosphingolipid (GSL)1 at the tumor cell surface may define the ability of the tumor cell to interact with specific target cell where the GSL antigen is recognized and may thus promote distant metastasis through the blood circulatory or lymphatic system. This general idea is supported particularly by tumor cell adhesion to E-selectin expressed on activated endothelial cells. This process is mediated by the E-selectin epitope expressed on tumor cells, which was identified as either sialosyl-Lex (SLex), sialosyl-Lea (SLea), or their analogs (for review, see Refs. 1-3).
An initial study of GSL patterns of renal cell carcinoma (RCC) and its metastatic deposits indicated that enhanced expression of higher gangliosides having TLC mobility similar to or slower than that of GM2 in original RCC tissue is correlated with metastatic potential (4). However, expression of SLex and dimeric Lex in RCC is higher in differentiated than in undifferentiated form and is not correlated with RCC metastasis (5, 6). This trend is opposite to that in other types of cancer (see above).
Two RCC cell lines derived from its metastatic deposits, TOS-1 and ACHN, are characterized by high expression of disialogangliosides in addition to GM2 but do not express SLex or SLea (7). Therefore, an unknown mechanism mediated by glycoepitopes different from SLex, SLea, or their analogs should be considered for RCC metastasis (7). In immunohistological (8) and clinicopathological (9) studies using a series of mAbs directed to di- and monosialogangliosides of RCC, positive staining with these antibodies in original RCC tissue was correlated with later incidence of metastasis.
Disialosylgalactosylgloboside (DSGG) was identified previously as one
of the major disialogangliosides from RCC tissues (10). However,
further studies indicated that two other novel disialogangliosides are
found in RCC tissue as well as TOS-1 cells, and that DSGG is the major
disialoganglioside of ACHN but is absent from TOS-1 (see
"Discussion"). We report here that the two major
disialogangliosides (G1 and G2) present in TOS-1 cells and RCC are not
DSGG but have entirely different properties. Their structures were
characterized by 1H NMR spectroscopy and electrospray
ionization mass spectrometry of intact compounds. G1 is identified as a
novel, hybrid structure between ganglio-series and lacto-series type 1, characterized by strong reactivity with mAb RM2, raised originally
against TOS-1 cells, and by weak cross-reactivity with anti-GM2 mAb
MK-1-8. G2 has the same lacto-series type 1 chain,2 with a vicinal
disialosyl residue, and is identical to the structure described
previously as a colonic cancer-associated antigen defined specifically
by mAb FH9 (11). In this study, we describe isolation and detailed
characterization of G1 and G2 and discuss the possible relationship of
their expression to RCC malignancy.
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MATERIALS AND METHODS |
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Cells-- TOS-1 was derived from back metastatic lesion of an RCC patient (8). ACHN was purchased from American Tissue Culture Collection (ATCC) and was originally derived from malignant pleural effusion of a patient with widely metastatic RCC. These cells were maintained in high glucose Dulbecco's modified Eagle's medium (Irvine Scientific, Santa Ana, CA) supplemented with 10% fetal bovine serum (Hyclone, Logan, UT), 1 mM sodium pyruvate, 100 IU/ml penicillin G, and 100 µg/ml streptomycin under a humidified atmosphere with 5% CO2 at 37 °C.
Monoclonal Antibodies-- RM2 was established using TOS-1 cells (10), and FH9 was established in our laboratory (11). The anti-GM2 antibody MK1-8 was kindly donated by Dr. Reiji Kannagi (Molecular Pathology, Aichi Cancer Center, Nagoya, Japan).
Extraction of Glycolipids-- Glycolipids were extracted from packed cells as described previously (12). Briefly, ~130 ml of packed cells were extracted by homogenization and filtration with 15 volumes of isopropyl alcohol/hexane/water (55:25:20, v/v/v). The extraction/filtration procedure was repeated once more. Combined extracts were evaporated and divided into the upper and the lower phases by Folch's partition (13). The upper phase was dialyzed against distilled water, dissolved in chloroform/methanol/water (30:60:8), and fractionated by DEAE-Sephadex A25 column chromatography into neutral glycolipids and gangliosides. Gangliosides were further separated into mono-, di-, and trisialosyl fractions by stepwise elution with 0.03, 0.13, and 0.45 M ammonium acetate in chloroform/methanol/water (30:60:8). The eluted gangliosides were dialyzed against distilled water and lyophilized. Each fraction was spotted onto a HPTLC plate (Whatman HPKF, Clifton, NJ), developed in a solvent system of chloroform-methanol-0.5% aqueous CaCl2 (50:40:10), and visualized by spraying with 0.5% orcinol in 2 N sulfuric acid.
Purification of Gangliosides-- Gangliosides were separated on preparative HPTLC. 50 µl of sample were streaked across a HPTLC plate, developed in a solvent system of chloroform-methanol-0.2% aqueous CaCl2 (50:50:10), and visualized by spraying with 0.001% primulin in acetone/water (4:1). Bands were marked with a pencil under UV light. Marked bands were scraped from the plate using a razor blade, and gangliosides were extracted from the silica by sonicating for 20 min in isopropyl alcohol/hexane/water (55:25:20). The silica was removed by centrifuging at 1000 × g for 10 min, re-extracted as above, and the combined supernatants were dried under N2.
TLC Immunostaining-- Immunostaining was performed on HPTLC plates by a modified version of Magnani's procedure (14). Gangliosides were applied on HPTLC plates for chromatography using a solvent system as described above. Plates were air-dried and immersed in 0.5% poly(isobutyl-metacrylate) in hexane/chloroform (9:1) for 1 min, blocked with 3% bovine serum albumin in PBS for 1 h at room temperature, and reacted for 2 h with mAb at room temperature. Plates were gently washed in PBS, incubated with biotinylated secondary antibody for 1 h, incubated with Vector avidin-biotin solution for 30 min, and stained with 0.05% 3',3-diaminobenzidine and 0.01% H2O2 in 0.05 M Tris-HCl, pH 7.6.
Enzyme-linked Immunosorbent Assay-- Purified gangliosides were dissolved in ethanol (40 nmol/ml), and a 50-µl aliquot or its serially diluted solution was added to each well of 96-well flat bottom polystyrene plate (Falcon 3915, Becton Dickinson, NJ), dried at 37 °C, and washed in PBS. Each well was blocked with 3% bovine serum albumin in PBS for 1 h, and reacted with mAb for 1 h at room temperature. Each well was washed extensively in PBS containing 0.05% Tween 20 (T-PBS), and incubated with peroxidase-linked secondary antibody for 30 min at room temperature. After washing in T-PBS, each well was supplemented with 0.05 M citric-acid, pH 4.0 containing 0.5 mg/ml of 2,2'-Azinobis (3-ethylbenzthiazoline-sulfonic acid) (ABTS) (Sigma) and 0.01% H2O2, and absorbency of the solution at 630 nm was measured with a microplate reader.
Desialosylation of Ganglioside G2-- A 70-µg aliquot of ganglioside G2 was desialosylated by heating in 1 ml of isopropyl alcohol/hexane/water (55:25:20, v/v/v; upper phase removed) containing 10% acetic acid in a sealed tube at 100 °C for 4 h. After cooling and drying under a N2 stream at 35-40 °C, the products were taken up in chloroform/methanol/water (30:60:8 v/v/v; Solvent A) and passed through a ~1-ml column of DEAE-Sephadex A-25 (acetate form, pre-equilibrated with solvent A), washing with 5-6 ml of the same solvent. The combined eluent, free of sialic acid and residual ganglioside, was dried under a N2 stream and prepared for NMR analysis as described below.
One- and Two-dimensional 1H NMR Spectroscopy-- Purified gangliosides (as well as the product from desialosylation of G2) were deuterium exchanged by dissolving in CDCl3-CD3OD 2:1, evaporating thoroughly under dry nitrogen (repeating 2×), and then dissolved in 0.5 ml of Me2SO-d6/2% D2O (15) for NMR analysis. One-dimensional 1H NMR spectra were acquired at 800 MHz (temperature, 308 K) on a Varian Unity Inova spectrometer, with suppression of the residual HOD signal by a presaturation pulse during the relaxation delay. NMR data were interpreted with comparison to published spectra of related glycosphingolipids (10, 16-23) or to spectra of relevant glycosphingolipid standards acquired under comparable conditions. Two-dimensional 1H-1H-dqCOSY (24, 25), -TOCSY (26, 27), and -NOESY (28) experiments were performed at either 600 or 800 MHz on Varian Unity Inova spectrometers using standard acquisition software available in the Varian VNMR software package.
Glycosyl and Fatty N-Acyl Composition Analysis-- Analysis of monosaccharide and fatty acid components were performed by GC/MS (as per-O-trimethylsilyl methyl glycosides and methyl esters, respectively) following methanolysis of gangliosides and derivatization according to protocols described in detail elsewhere (29).
Electrospray Ionization Mass
Spectrometry--
ESI-MS was performed in the positive ion mode on a
PE-Sciex (Concord, Ontario, Canada) API-III spectrometer, with a
standard IonSpray source, using direct infusion (3-5 µl/min) of
ganglioside samples dissolved (~20 ng/µl) in 100% MeOH
(orifice-to-skimmer voltage [OR], 130 V; Ionspray voltage, 5 kV;
interface temperature, 45 °C) (29). Fragment nomenclature is after
Costello and co-workers (30, 31) as modified by Adams and Ann (32).
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RESULTS |
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Disialoganglioside Pattern of RCC Cell Lines TOS-1 and
ACHN--
Four major disialoganglioside component bands (i, ii, iii,
iv) were separated from TOS-1 cells, and two components (v, vi) were
separated from ACHN cells (Fig. 1). mAb
RM2 reacted strongly with iii and iv from TOS-1, which are termed G1.
The ganglioside corresponding to components i and ii barely reacted
with mAb RM2 but reacted strongly with FH9 and is termed G2. Similarly,
components v and vi did not react with RM2, but reacted strongly with
mAb 5F3, and are termed G3. Thus, components i and ii showed the same reactivity with anti-carbohydrate mAbs and were assumed to have the
same carbohydrate structure, as was also the case for iii and iv, and v
and vi, respectively. These assumptions were confirmed by further
studies of each component, which were purified by repeated preparative
HPTLC, and subjected to (a) detailed immunochemical analysis
using various mAbs, and (b) detailed chemical analysis by
1H NMR spectroscopy and mass spectrometry, as described
under "Materials and Methods." Immunostaining patterns with mAbs
RM2, MK-1-8, FH9, and 5F3 of purified G1, G2, and G3 are shown in Fig.
2.
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Immunochemical Analysis of Purified G1, G2, and G3--
G1 as
above was strongly TLC immunostained with mAb RM2, but G2 was barely
reactive. G1, but not G2, was stained with anti-GM2 mAb MK-1-8. On the
other hand, G2 was strongly stained with mAb FH9, which defines
disialosyl type 1 chain (disialosyl Lc4Cer, i.e.
IV3NeuAcIII6NeuAcLc4Cer) (11),
whereas G1 was not stained. Comparative reactivity of various
gangliosides and GSLs with RM2 is shown in Fig.
3. These results are consistent with
chemical analysis, indicating that G2 is identical to the FH9 antigen
and G1 is GalNAc 1
4 linked to the terminal Gal of FH9
antigen.
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Disialoganglioside bands v and vi from ACHN cells, defined by mAb 5F3, were identified as DSGG. These results will be described elsewhere, together with reactivity and tissue distribution pattern of the antigen defined by 5F3.
Glycosyl and Fatty N-Acyl Composition Analysis of TOS-1 Gangliosides-- Monosaccharides were identified by GC-MS analysis of the per-O-trimethylsilyl derivatives of methyl glycosides produced from G1 and G2 following methanolysis and re-N-acetylation (data not shown). In the analysis of both G1 and G2, derivatives of Glc, Gal, GlcNAc, and NeuAc were identified in 1:2:1:2 ratio, but G1 was distinct from G2 in yielding an additional peak corresponding to a residue of GalNAc. Fatty acids in the ceramides of G1 and G2 were then identified as their methyl esters by GC-MS of hexane extracts of the methanolysates (data not shown). The predominant fatty acids from ganglioside G1 were 16:0, 24:1 (mostly nervonate), and 24:0, together with significant amounts of 18:0 and 22:0, and much smaller amounts of various other species, including 22:1, 18:1 (oleate), 16:1, 26:1, 14:0, 15:0, 23:0, and 26:0. The distribution from G2 was very similar, but with a much higher proportion of 16:0 fatty acid, and with some of the minor species near or below the limits of detection.
One and Two-dimensional 1H NMR Spectroscopy--
For
structural characterization, high resolution one-dimensional
1H and two-dimensional 1H-1H
correlation NMR spectra were acquired at 800 MHz on both TOS-1 gangliosides in Me2SO-d6/2%
D2O. One-dimensional 1H NMR spectra of G2 and
G1 are reproduced in Fig. 4, panels
A and B, respectively; two-dimensional TOCSY and NOESY
spectra of G1 are represented in Fig. 5,
panels A and B; chemical shift data (and
3J1,2 coupling constants) are
compiled in Table I. The one-dimensional NMR spectrum of the simpler compound, G2, clearly displays two characteristic -NeuAc H-3eq resonances at 2.769 and 2.727 ppm, as
well as four
-anomeric resonances
(3J1,2 = 7-9 Hz) in the downfield
region, of which that at 4.512 ppm could be tentatively assigned as the
-GlcNAc H-1, that at 4.160 ppm most likely as the
-Glc H-1, and
those at 4.248 and 4.117 ppm as two
-Gal H-1, in agreement with the
glycosyl composition analysis. Based on analogy to published NMR data,
the rather upfield chemical shift of one of the
-Gal H-1 resonances
suggested the possibility of a type 1 chain core, as only the terminal
-Gal H-1 of Lc4Cer has ever been observed upfield of
4.15 ppm (18). Three NAc methyl singlets (1.888, 1.875, and 1.759 ppm)
are likewise consistent with the presence of two
-NeuAc and one
-GlcNAc residue in the ganglioside; moreover, the chemical shifts of
the two more downfield singlets suggested the presence of NeuAc in both
2
3 and
2
6 linkages, respectively (10, 17, 20). The
possibility that the two NeuAc residues are linked together
(i.e. as a NeuAc
2
8NeuAc
2
X disaccharide
structure) appears unlikely, as data previously published for
disialosyl versus parent monosialosyl gangliosides (16, 33)
shows that attachment of a second NeuAc
2
8 to the first results
in substantial shift changes for key resonances of the newly
internalized residue, upfield for H-3eq and downfield for H-3ax and
H-4. For example, in comparing these resonances in NeuAc A of GD3
versus GM3,
=
0.41, +0.33, and +0.28 ppm,
respectively; these changes result in chemical shifts of 2.34, 1.69, and 3.83 ppm, respectively, for these signals in GD3 (33). There are no
indications from the data for H-3eq, H-3ax, or H-4 of NeuAc in
ganglioside G2 for such shielding/deshielding interactions affecting
either of the NeuAc residues. The presence of two other resonances in
the downfield region, a broad doublet at 4.091 ppm, and a narrow
signal, resembling a
-Gal H-4 in its splitting pattern, but
unusually deshielded at 4.293 ppm, are unique features compared with
any previously published spectral data.
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The spectrum of G1 clearly displays several similar
features, in particular the latter two resonances essentially
unaffected at 4.091 and 4.294 ppm, respectively. These similarities
suggest a close structural relationship between G1 and G2, with the
former most likely representing addition of a single monosaccharide
residue to the latter. The additional residue is manifested by the
appearance of a second downfield -anomeric resonance tentatively
assigned as
-GalNAc H-1. Significant glycosylation-induced shift
changes are observable involving the
-Gal H-1 resonance at 4.117 ppm in G2 (apparently shifted to 4.207 ppm in G1), the
-GlcNAc H-1 resonance at 4.512 ppm (shifted to 4.590 ppm in G1), and the
-NeuAc H-3eq resonance at 2.769 (shifted to 2.571 ppm in G1). Changes in
chemical shifts of 2 of 3 NAc methyl singlets are also apparent, with
those at 1.888 and 1.759 ppm, shifting to 1.871 and 1.793 ppm
(coincident with the NAc signal for the
-GalNAc residue), respectively. These suggest the presence in G2 of a terminal
NeuAc
2
3Gal
1
3GlcNAc
1
trisaccharide to which, in G1, a
-GalNAc has been added in a position to affect resonances of these
three residues most strongly. Comparison of the data on G2 and G1
available from two-dimensional NMR analysis shows that the
1H resonance undergoing the largest change is that for H-4
of the
-Gal correlating with the H-1 at 4.117 in G2 and 4.207 ppm in G1 (
H-4 = 0.3 ppm), suggesting attachment of the
GalNAc
1
4 to this
-Gal in G1. Consistent with this tentative
assessment, the chemical shifts of resonances associated with the
terminal disaccharide, i.e.
-NeuAc H-3eq, H-3ax, H-4, and
NAc methyl,
-Gal H-1, along with the newly added
-GalNAc H-1 and
NAc methyl, are very similar in G1 to those found for the
NeuAc
2
3(GalNAc
1
4)Gal
1
3 trisaccharide of GM2
ganglioside (16, 21, 34). This supports by analogy that the structural
relationship between G2 and G1 is the addition of GalNAc
1
4 to
-Gal of the terminal NeuAc
2
3Gal
1
3 disaccharide of the
former, consistent with the reactivity of G1 but not G2 with anti-GM2
MAb MK-1-8.
Because neither reactivity with antibodies nor analysis of NMR spectra
by analogy and chemical shift changes are completely unambiguous
indicators of structure, and especially in view of the complexity and
novelty of the one-dimensional NMR spectra, further glycosyl
connectivity analysis was carried out by acquisition of
1H-1H NOESY data (shown for G1 only in Fig. 5,
panel B), which is an indicator of spatial proximity between
nuclei. With few published exceptions (see Ref. 35), the magnitudes of
interresidue NOEs are greatest between those nuclei directly connected
by glycosyl linkages. In the case of G2, strong interresidue NOEs can
be clearly observed between -Gal H-1 and
-GlcNAc H-3; between
-GlcNAc H-1 and H-3 of the other
-Gal; and between
-Glc H-1
and Cer H-1a/H-1b (an NOE between the internal
-Gal H-1 and the
-Glc residue is somewhat ambiguous, but this linkage is unlikely to be anything but Gal
1
4). These data are consistent with a type 1 chain, Lc4Cer core structure for G2. Unfortunately, no NOEs were observed between either of the NeuAc residues and those of the
core glycan. The reactivity of G2 with MAb FH9, previously shown to
react with a disialosyl-Lc4Cer having NeuAc residues linked
2
3 to Gal
1
3 IV and
2
6 to GlcNAc
1
3 III, suggests the same structure for this ganglioside, and is certainly consistent with the NMR data. In the case of G1, the same interresidue NOEs can be observed (including one between the internal
-Gal H-1 and
-Glc H-4); an additional strong NOE is observable between
-GalNAc H-1 and
-Gal IV H-4, again consistent with the
GalNAc
1
4Gal
1
linkage (Fig. 5, panel B).
Final support for the core structure was obtained by acquisition of a
one-dimensional NMR spectrum of completely desialosylated G2. The
spectrum (not shown) clearly exhibited four -anomeric resonances at
4.785 ppm (3J1,2 = 8.4 Hz), 4.268 ppm (3J1,2 = 7.7 Hz), 4.168 ppm
(3J1,2 = 7.7 Hz), and 4.137 ppm
(3J1,2 = 7.2 Hz); additional
resonances corresponded to
-Gal II H-4 (3.850 ppm;
3J3,4 = 3 Hz)) and
-GlcNAc III
NAc (1.815 ppm). These chemical shifts are virtually indistinguishable
(
±0.005 ppm) from those observed previously for H-1 of
-GlcNAc III,
-Gal II,
-Glc I,
-Gal IV, H-4 of
-Gal II,
and NAc of
-GlcNAc III of Lc4Cer under identical
conditions (at 4.781, 4.263, 4.166, 4.136, 3.851, and 1.815 ppm,
respectively, Ref. 36; see also Ref. 19), and distinct from spectral
data for any other known core structure. Taken together, these data
strongly support the structures of G2 and G1 as IV3NeuAc,
III6NeuAc-Lc4Cer and
IV4(
-GalNAc), IV3NeuAc,
III6NeuAc-Lc4Cer, respectively.
Electrospray Ionization Mass Spectrometry--
Low resolution ESI
mass spectra of both TOS-1 gangliosides were acquired by direct
infusion as described previously (29). Single quadrupole mass spectra
of G2 and G1 are reproduced in Fig. 6,
panels A and B, respectively; assignments for
fragments are summarized in Scheme 1,
a and b (nominal, monoisotopic
m/z are used throughout). The most abundant
molecular adduct ions [M2H+3Na]+ are observed for G2
(panel A) at m/z 1875, 1985, and 1987, corresponding to a glycan formula
Hex3·HexNAc·NeuAc2 in combination with
ceramides consisting of d18:1 sphingosine predominantly
N-acylated with 16:0, 24:1, and 24:0 fatty acids. In the
spectrum of G1 (panel B), the most abundant molecular adduct
ions are observed at m/z 2078, 2188, and 2190, corresponding to a glycan formula
Hex3·HexNAc2·NeuAc2 in
combination with the same predominant ceramides. The additional HexNAc
residue observed for G1 is consistent with the glycosyl composition and
NMR analysis, and the relative abundances of the molecular species are
essentially consistent with the fatty acid analyses, including the
greater proportion of the 16:0 species in G2. As often is the case with
ESI-MS, the higher molecular weight ganglioside G1 also produces a more
abundant set of dicationized doubly charged molecular ions,
[M
2H+4Na]2+, observed at m/z
1050.5, 1105.5, and 1106.5 (panel B).
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Both gangliosides readily lose 1 and 2 residues of NeuAc (as
NeuAcH+Na), giving abundant sets of Y fragments at
m/z 1562/1672/1674 and 1249/1359/1361 for G2, and
at m/z 1765/1875/1877 and 1452/1562/1564 for G1.
The latter sets in each case represent double cleavage ions, which
might be expected to be of low abundance, but in fact they are more
abundant than the losses of single NeuAc residues. An additional loss
of 1 HexNAc residue from G1 yields a set of Y fragments identical to
that at 1249/1359/1361 observed for G2, and below this mass, the
fragmentation patterns are very similar, except for the higher
abundance of the 16:0 species in G2. Thus Y2
(Hex2Cer) and Y1 (HexCer) fragments are
observable at m/z 884/994/996 and 722/832/834 in
both spectra, whereas a set of Y3
fragments, which could
be expected at m/z 1400/1510/1512, is not
observed significantly above noise level in either spectrum.
Y3
,3
fragments at
m/z 1087/1197/1199 are similarly not observed,
although these represent a double cleavage not expected to be very
abundant. An interesting double cleavage of one NeuAc residue and the
sphingosine moiety at the C2-C3 bond ("G" fragmentation) yields
sets of fragments at m/z 1325/1435/1437 for G2
(panel A) and at m/z 1528/1638/1640 for G1 (panel B). Such cleavages of the sphingenine C2-C3
bond have been observed previously in positive ion ESI-MS of
glycosphingolipids (37, 38).
The fragmentation data are consistent with the proposed structures for
G1 and G2, but in the absence of certain ions as mentioned above, they
are not unambigous with respect to key parts of their putative sugar
sequences. For example, in the absence of an observable set of
Y3 fragments, these data would not by themselves rule out the possibility of a disialosyl disaccharide in the structure. However, the presence of such a disialosyl linkage is not consistent with the observed chemical shifts for key NeuAc resonances in the NMR
spectra of G2 and G1. Other features of the proposed structures are
supported with very high confidence level by the NMR data and are
consistent also with the pattern of monoclonal antibody reactivities
(G2 with mAb FH9 and G1 with MK-1-8).
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DISCUSSION |
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Many studies indicate that certain GSLs and gangliosides in tumor cells define their malignancy, in terms of metastatic potential (1). Our initial, comparative study on GSL and ganglioside patterns showed enhanced expression of GM2 or gangliosides higher than GM2 (including slow-migrating disialoganglioside species) in RCC having metastatic deposits and in the deposits themselves compared with RCC without metastatic deposits (4). Our subsequent studies were focused on the following: (a) establishment of RCC cell lines, and their tumor cell biological properties (8); (b) biochemical characterization of major gangliosides showing enhanced expression in RCC tissues, identified as monosialosyl- and disialosylgalactosylgloboside (MSGG and DSGG) (10); (c) production of mAb RM2 directed to TOS-1 cells, mAb RM1 directed to RCC (10), and mAb 5F3 directed to RCC gangliosides;3 and (d) application of these antibodies to compare expression of defined epitopes in RCC having metastatic deposits, the deposits themselves, and RCC without metastatic deposits. Expression of each of the antigens defined by RM2, RM1, and 5F3 was correlated with metastatic potential (9).3 This series of studies indicated that in addition to previously identified DSGG two other disialogangliosides are found as major components of RCC and TOS-1 cells: G1 (defined by mAb RM2) and G2 (defined by mAb FH9). Expression of SLex or dimeric Lex in RCC was found to be correlated with degree of differentiation, but not with metastatic potential (5, 6). In contrast, expression of SLex and SLea in colorectal carcinoma was highly correlated with metastatic potential (1-3).
Our results clearly identify one major disialoganglioside (G1)
expressed in RCC tissue and a cell line derived therefrom (TOS-1) as
GalNAc 1
4 linked to terminal Gal of disialosyl
Lc4Cer, and the other disialoganglioside (G2) as disialosyl
Lc4Cer, i.e. FH9 antigen reported previously as
colonic cancer-associated antigen defined by mAb FH9.
Disialoganglioside fraction of RCC tissue in some cases was identified
as DSGG, which shows nearly identical TLC mobility, and co-migration in
various solvent systems, to that of a novel antigen now identified as
GalNAcDSLc4. This novel antigen is a hybrid-type composed
of ganglio-series with the same structure as GM2 (region I), and
lacto-series type 1 with a vicinal disialosyl residue (region II in
Structure TI in the Abstract, and Fig. 7).
Ganglio-series structures are characterized by
1
4GalNAc linked to
Gal, whereas lacto-series are characterized by
1
3GlcNAc linked to
Gal. These two structures are usually mutually exclusive, and hybrids
are rare in humans and mammals. There are two types of
ganglio/lacto-series hybrids, branched and linear. The first branched
hybrid structure was found in undifferentiated murine leukemia cells,
with the structure below, where R1 could be
Gal
1
3Gal
1
4 (39).
|
R1GlcNAc
1
3
Gal1
4Glc
1
1Cer
R2GalNAc
1
4
The level of this hybrid structure declined upon
differentiation of leukemia cells, and the structure appears to
represent undifferentiated state of the cells. A similar branched
ganglio/lacto hybrid with R1 equal to GM2 structure or with
R2 equal to Gal1
3 structure was isolated from brain
of patients with amyotrophic lateral sclerosis-like disorder (22). The
first linear-type hybrid GSL was found in the liver of English sole
(40), and a similar one was found later in roe of striped mullet (23). GalNAcDSLc4 from G1 fraction as described in the present
study is also a linear-type hybrid but is derived from human tissue. Expression of ganglio/lacto hybrid structure is apparently repressed in
normal mammalian or human tissues, but increases (i.e. is
"de-repressed") under pathological conditions. It is possible that
specificities of
1
4GalNAc or
1
3GlcNAc transferases are
mutually restricted to the respective substrate structures under normal
conditions but lose their restriction under pathological conditions,
including cancer. This phenomenon could be caused by subtle changes in
the transferase genes, or post-translational modification of the transferases.
Three disialogangliosides are present in RCC tissue. One, expressed in TOS-1 cells, is now identified as GalNAcDSLc4, defined by and reacting strongly with mAb RM2. Another, expressed in ACHN cells is now identified as DSGG, which was previously described as one of the major gangliosides of RCC tissue and is now defined by another mAb (5F3) but not by RM2. The third disialoganglioside, expressed in TOS-1 cells, is now identified as disialosyl Lc4 and is identical to the antigen of mAb FH9, described previously as a colonic cancer-associated antigen (11).
In a previous publication (10), we mistakenly assigned the specificity of mAb RM2 as being directed to DSGG (G3), although RM2 was raised against TOS-1 cells. The mistake may have arisen because of the following: (a) disialoganglioside fractions prepared from a few cases of RCC tissue contained DSGG and G1 (GalNAc disialosyl Lc4Cer, abbreviated GalNAcDSLc4) in varying proportions; some RCC tissues contained globo-series structures as major component, whereas others contained ganglio/lacto-series structures as major component; (b) DSGG and G1 (GalNAcDSLc4) show identical TLC mobility, i.e. co-migrate together in various solvent systems; (c) the sample prepared from RCC tissue and used for structural analysis contained DSGG as major component but may have been contaminated with a small quantity of G1 showing reactivity with RM2. We have now clarified that RM2 is directed specifically to G1, whereas another mAB, 5F3, is directed specifically to DSGG.3 It is possible that RCC tissue expresses high level of G1 in some cases, and DSGG in other cases. Cell lines representing the former and latter situations are TOS-1 and ACHN respectively.
From a clinicopathological and histological point of view, RCC cases
expressing either GalNAcDSLc4 or DSGG are highly malignant, and show initiation of lymph node and distant metastasis at an early
stage (9). Preferential adherence of TOS-1 cells to lung tissue,
determined by Stamper-Woodruff assay, is based on interaction of major
disialoganglioside present in TOS-1 with receptor present on
perialveolar endothelial cells. This process is strongly inhibited by
RM2 (7) and is therefore presumed to be mediated by
GalNAcDSLc4, rather than DSGG and its corresponding
perialveolar receptor. Interestingly, SLex,
SLea, or their analogs, which bind to E-selectin, are not
involved in this process. Studies on the identification of the
receptors for both GalNAcDSLc4 and DSGG are essential to
understand mechanisms for RCC metastasis and its possible inhibition.
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ACKNOWLEDGEMENTS |
---|
We thank Theresa Nguyen for her great efforts in growing a large quantity of TOS-1 and ACHN cells, under the supervision of Dr. Kazuko Handa. We also thank Dr. Stephen Anderson for scientific editing and preparation of the manuscript.
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FOOTNOTES |
---|
* This study was supported by NCI, National Institutes of Health (NIH) Grants CA80054 and CA82167 (to S. H.) and the NIH Resource for Complex Biomedical Carbohydrates, P41 RR005351 (to S. B. L.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.:
206-726-1222; Fax: 206-726-1212; E-mail:
hakomori@u.washington.edu.
Published, JBC Papers in Press, February 27, 2001, DOI 10.1074/jbc.M011791200
2 Glycosphingolipids are abbreviated according to the recommendations of the IUPAC-IUB Commission on Biochemical Nomenclature (CBN for lipids: (1977) Eur. J. Biochem. 79, 11-21; (1982) J. Biol. Chem. 257, 3347-3351; and (1987) J. Biol. Chem. 262, 13-18). However, the suffix -OseCer is omitted. Gangliosides are abbreviated according to the extended version of Svennerholm's list (Holmgren et al. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 1947-1950).
3 A ganglioside from RCC tissue showing the same TLC mobility as G1 (i.e. between that of GD1a and GD1b) but not showing reactivity with RM2, was found. mAb 5F3 was prepared against this ganglioside. Immunohistological studies of many cases of RCC, using mAbs RM1, RM2, and 5F3, showed clear correlation between metastatic potential and the epitopes defined by these mAbs (A. Ito, Dr. Med. Sci. (Ph.D. thesis) presented at Tohoku Univ. School of Medicine, Japan, 1997).
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ABBREVIATIONS |
---|
The abbreviations used are:
GSL, glycosphingolipid;
DSGG, disialosylgalactosylgloboside
(NeuAc3Gal
3[NeuAc
6]GalNAc
3Gal
4Gal
4GlcCer);
DSLc4, IV3NeuAcIII6NeuAcLc4;
GalNAcDSLc4, IV4GalNAcIV3NeuAcIII6NeuAcLc4;
mAb, monoclonal antibody;
MSGG, monosialosylgalactosylgloboside
(NeuAc
3Gal
3GalNAc
3Gal
4Gal
4GlcCer);
PBS, Dulbecco's
phosphate-buffered saline;
RCC, renal cell carcinoma (derived mainly
from proximal tubular epithelia of kidney);
SLea, sialosyl-Lea
(NeuAc
3Gal
3[Fuc
4]GlcNAc
3Gal
4Glc);
SLex, sialosyl-Lex
(NeuAc
3Gal
4[Fuc
3]GlcNAc
3Gal
4Glc);
cer, ceramide.
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