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1 From the Department of Ophthalmology and the 2 Laboratory of Molecular Genetics, University of Burgundy, Dijon; the 3 Biochemical Chemistry Laboratory, UMR 8576 CNRS, University of Science and Technology of Lille; and the 4 U-482 INSERM, Saint Antoine Hospital, Paris, France.
| Abstract |
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2,3-sialyltransferase (ST3Gal III), sialyltransferase necessary to
build the sialyl-Lea (Lewisa) antigen, were
studied. METHODS. Immunoperoxidase staining was performed on 28 pterygia using different monoclonal antibodies: anti-M1 (against the peptidic core of gastric mucins encoded by MUC 5AC gene), anti-Lea (7LE), antisialyl Lea (NS 19-9), and anti-Leb (2-25LE). A serologic Lewis determination was done in 18 patients. ST3Gal III sialyltransferase expression was also studied in 10 healthy conjunctiva and 10 pterygia by reverse transcriptasepolymerase chain reaction (RTPCR). Glyceraldehyde-3-phosphate-dehydrogenase was used as an endogenous internal control.
RESULTS. First, Lea, sialyl Lea, and Leb immunoreactivities either decreased or were no longer detectable in pterygium goblet cells as opposed to normal conjunctiva. Second, unlike in pterygium, the Lewis immunoreactivity, which is mainly located in the surface epithelial cells in the normal conjunctiva, was occasionally restricted to the epithelial cells of the deep layers. However, M1 mucins did show an identical pattern expression in a normal conjunctiva and pterygium. ST3Gal III expression was significantly lower in pterygium (0.20 ± 0.02 AU [arbitrary units]) than in normal conjunctiva (0.95 ± 0.12 AU).
CONCLUSIONS. ST3Gal III gene is less expressed in pterygium than in normal conjunctiva. This observation could explain the decrease of sialyl Lea expression observed in pterygium by immunohistology.
| Introduction |
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Pterygium (from the diminutive of 




, small wing), is
considered to be an invasion of the cornea, with dissolution of the
Bowmans membrane by a triangular segment of bulbar
conjunctiva.7
It shows basophilic degeneration with
actinic or senile elastosis of the subepithelial substantia propria.
The fully developed pterygium is covered by flat or cylindrical
epithelial cells that are comparable to the conjunctival epithelium.
The epithelium of pterygium also contains goblet cells whose morphology
is similar to those found in the conjunctiva, but their densities are
increased.8
Human conjunctival mucins consist of a peptidic core bearing a large
number of oligosaccharide side chains9
showing alternating
poorly (naked) and highly glycosylated (T domain)
regions.10
Mucins may be considered as a mosaic of
epitopes,11
some of which are associated with the peptidic
core of the naked regions (M1 epitopes encoded by MUC 5AC
gene)12
and others, with the peripheral parts of the
oligosaccharide chains located in T domains displaying blood
grouprelated epitopes (ABO, H and Lewis).13
These histo-blood group antigens are built by the addition of
monosaccharides onto precursor glycoprotein by specific
glycosyltransferases such as galactosyltransferases,
sialyltransferases, and fucosyltransferases.14
These
glycosyltransferases are encoded by the genes of ABH,
Secretor (Se), and Lewis (Le)
systems.13
Secretor individuals show an active
1-2
fucosyltransferase encoded by the secretor
gene.15
Mucin alteration can be observed in several ocular diseases: aqueous tear deficiency, ocular rosacea,16 atopic conjunctivitis,17 and others. To the best of our knowledge, mucin expression in pterygium has only been studied using lectins. Nongoblet cells of pterygium were labeled with ulex europaeus agglutinin-1 (UEA-1), doliches biflorus agglutinin (DBA), and peanut agglutinin (PNA), but those of normal conjunctiva were not. Abnormal distribution of the UEA-1 staining was noted in the superficial layers of epithelial cells in pterygium.8
The aim of this study was to characterize the immunohistologic mucin modifications in the pterygium. First, we used monoclonal antibodies (Mabs) directed against the peptidic core and Mabs against saccharide moieties of mucin, which are known to be more specific than lectins. Second, in addition, we evaluated a sialyltransferase (ST3Gal III)18 expression in pterygium and healthy conjunctiva to try to explain the mucin immunohistologic modification in the pterygium. We observed different patterns in sialyl Lea immunoreactivity, which is confirmed by a decrease of the ST3Gal III gene expression.
| Materials and Methods |
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Mabs against Saccharide Moieties of Mucin.
The following Mabs were used: anti-Lea
(7LE),19
20
anti-Leb
(2-25LE),21
22
antiH type 2 (19-0LE) from our
laboratory,19
20
and antisialyl
Lea (NS 19-9).23
The biochemical
structures recognized by these Mabs are shown in Table 1
. They all strongly reacted using immunoperoxidase methods with mucus
cells of gastroduodenal mucosae according to the phenotype of the
tissue donor: Anti-Lea Mab reacted on nonsecretor
individuals and antisialyl Lea Mab on mucosae
of Lewis-positive individuals.
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Preparation of Tissues.
Pterygium was fixed in 95% ethanol overnight, routinely processed, and
then embedded in paraffin wax. Serial sections (3-µg thick) were cut
with an Autocut (ReichertJung, Heidelberg, Germany).
Immunoperoxidase.
The sections were deparaffinized with three successive baths of xylene
and ethanol of 10 minutes each. The sections were rinsed with water for
10 minutes, preincubated for 3 minutes in phosphate-buffered saline
containing 0.1% Tween-20 (PBSTween), and then incubated for 30
minutes with the Mabs (undiluted hybridoma supernatants). After 3
rinses in PBSTween, the sections were incubated for 30 minutes with
mouse anti-Ig antibodies (diluted 1/200) linked to peroxidase
(Amersham, Aylesbury, UK). The sections were washed three times with
PBSTween and incubated for 4 minutes with amino-ethylcarbazole
(Sigma, St. Louis, MO) containing
H2O2. Before microscopic
examination, the cell nuclei were stained with 1% hematein for 2
minutes. The specificity of the immunoreactivity was controlled by the
inhibition of the staining after incubation of the hybridoma
supernatant with the gastric mucin preparation (100 µg/ml
supernatant), red blood cells, or tissue extracts containing M1 or
Lewis antigens.
ABO and Lewis Phenotype.
Blood samples were obtained from the 28 patients to determine ABO and
Lewis erythrocyte phenotype by hemagglutination.
RTPCR
Tissue Samples.
The guidelines of the Declaration of Helsinki were followed.
Conjunctival biopsy specimens were collected with fully informed
written consent and authorization of the local ethical committee in 10
patients undergoing cataract surgery (age range, 3565 years).
Pterygia were obtained from 10 patients who ranged in age from 26 to 68
years. After removal, these specimens were immediately frozen in liquid
nitrogen and stored at -80°C before further analysis.
Extraction of RNA and Analysis of ST3Gal III Transcripts by
RTPCR.
Total cytoplasmic RNA was extracted from homogenized conjunctiva or
pterygium tissue by guanidiniumphenolchloroform
technique24
with Trizol kit (GibcoBRL, Grand Island,
NY). Three micrograms of total RNA was converted into first-strand cDNA
by a random priming technique25
in 20 µl of RT mixture:
8 µl RNA, 2 µl random hexamers (150 ng/µl), 10 µl RT buffer (40
mM/l TrisHCl, 100 µM/l each dNTP, 6 U Rnasin, 50 nM/l
dithiothreitol, 5 ng/ml bovine serum albumin, 8 mM/l
MgCl2). After RNA denaturation for 10 minutes at
70°C and then cooling on ice, 1 µl of Avian Myeloblastosis Virus
reverse transcriptase was added, and the samples were incubated for 60
minutes at 37°C. All reagents were purchased from Promega (Madison,
WI). Changes in the amount of ST3Gal III mRNAs were monitored by
coamplification in the same tube of the target fragment and of a
glyceraldehyde-3-phosphate dehydrogenase (GAPDH) housekeeping gene
sequence as an internal standard. For reliable quantification, the
concentration of each set of primers and the stringency of PCR
conditions were adjusted so that the amplicons would have similar
amplification kinetics and their exponential phases of amplification
would overlap as previously described.26
Amplification was
done with specific primer target sequences as follows27
:
for ST3Gal III, 5'-CGGATGGCTTCTGGAAATCTGT-3' and
3'-AGTTTCTCAGGACCTGCGTGTT-5' and for GAPDH, 5'-ACCACAGTCCATGCCATCAC-3'
and 3'-TCCACCACCCTGTTGCTGTA-5'. The PCR reaction was set up in
a25-µl mixture consisting of 5 µl cDNA solution diluted in
2.5 µl PCR buffer (1 mM/l TrisHCl, pH 8.3, 1 mM/l KCl, 1 mM/l each
dNTP, and 15 mM/l MgCl2), 2 µl ST3Gal III
primers, 0.2 µl Taq polymerase (Biotaq; Bioprobe,
Montrevil, France), and an adjusted volume of distilled water. After 10
minutes of denaturation at 95°C, 36 cycles of amplification were
carried out on a PHC3 thermocycler (Techne, Cambridge, UK) followed by
a final 10-minute extension at 72°C. The cycling parameters were the
following: denaturation at 94°C for 1 minute, annealing at 63°C for
1 minute, and extension at 72°C for 2 minutes.
After amplification, 10 µl of PCR products was electrophoresed in a 2% ethidium bromidestained agarose gel for quantitation of fluorescence by image analysis under UV light (UVP Transiluminator, Cambridge, UK).
The PCR amplification yield of target sequences was expressed in arbitrary units (AU) as a ratio of ST3Gal III/GAPDH electrophoretic band optical density.
Statistics
All values given were mean ± SEM. An unpaired
t-test was performed to compare the different mean values.
Values were considered significant if P
0.05.
| Results |
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RTPCR
RTPCR indicated that ST3Gal III expression was present in all
the samples of conjunctiva or pterygium. ST3Gal III expression was
0.20 ± 0.02 AU in pterygium, whereas it was 0.95 ± 0.12
A.U. in normal conjunctiva (Fig. 4)
. There was a statistically significant difference between pterygium
and normal conjunctiva (P < 0.0001).
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| Discussion |
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The present study also showed modifications in the carbohydrate structures because pterygium mucins secreted by the goblet cells present variations on the oligosaccharide chains but not on the peptidic core (M1 antigen) encoded by MUC-5AC gene.12 It also indicated a lower expression of the blood grouprelated antigens, suggesting an abnormal expression of the fucosyl- and sialyltransferases involved in the biosynthesis of these antigens. The antiblood grouprelated antigen Mabs we used characterize the alteration of glycosylation associated with glycoconjugates expressed by the conjunctiva including the mucins. Indeed, Mab NS 19-9 recognizes the sialyl Lea tetrasaccharide structure (Table 1) . The absence of the neuraminic acid or fucose on this sialyl Lea structure because of a decrease of sialyl- or fucosyltransferase involves a loss of the sialyl Lea immunoreactivity. Concerning the Leb structure, the absence of one of the two fucoses on the tetrasaccharide Leb molecule (Table 1) , due to a decrease of fucosyltransferases, involves also a loss of Leb immunoreactivity (2-25LE). The decrease of staining suggested that sialyltransferases, fucosyltransferases, or both are less efficient in the goblet cells of pterygium.
In the epithelial cells, sialyl Lea immunostaining is occasionally restricted to the deep layer. Such an abnormal pattern was not observed in the normal conjunctiva. Restriction of abnormal patterns to surface epithelium observed with UEA-1 lectin is not in contradiction with our results. Indeed, on the surface epithelium, the decrease of the sialyltransferase we observed could favor the abnormal fucosyltransferase activity of the Se gene.
Thus, we studied the Galß1-3GlcNAc
2,3-sialyltransferase (ST3Gal
III) expression in pterygium. Sialyltransferases form a subclass of
glycosyltransferases that catalyze the transfer of sialic acid (NeuAc)
from CMPNeuAc to galactose, N-acetylgalactosamine, or
another sialic acid residue. Indeed, to date, 13 different
sialyltransferase cDNAs have been cloned,28
and at least 8
different enzymes are involved in the biosynthesis of the sialylated
oligosaccharidic chains that define the mucin sialyl
epitopes.29
Each sialyltransferase can be distinguished
enzymatically by its specificity for the sequence of the acceptor
oligosaccharide and the anomeric linkage formed between the sialic acid
and the sugar to which it is attached.30
The fine
substrate specificity of four
2,3-sialyltransferases (ST3Gals I, II,
III, and IV) was recently defined by the use of soluble recombinant
enzymes.31
This study confirmed that ST3Gal III, which
preferentially transfers sialic acid in
2,3 linkage to the
Galß1-3GlcNAc disaccharidic sequence, was the best candidate for the
biosynthesis of the sialyl Lea epitope. The
recent progress obtained in the molecular cloning of human
sialyltransferase cDNA allowed the development of an RTPCRbased
method that will assay the expression of these enzymes in biological
samples.27
We showed that ST3Gal III expression is
significantly lower in pterygium than in normal conjunctiva. These
findings could explain the decrease of sialyl Lea
expression observed in pterygium by immunohistology.
Regulation mechanisms of sialyltransferases are not well established. Variations in complex carbohydrate structures were observed during development, during differentiation, in disease processes, and between different normal tissues, leading to the conclusion that the expression of terminal sequences was strictly controlled both spatially and temporally.30 The modification of cellular glycosylation is a common phenotypic change in malignancy, but only a limited subset of biosynthetic pathways is frequently altered in cancer. Increased ß1,6 branching, increased sialyl Lea and sialyl Lea epitopes, accumulation of H and Ley antigens, and a general increase in sialylation are commonly observed in N- and O-linked oligosaccharides of carcinoma cells.32 33 These changes in glycosylation are correlated with grade, with invasion and metastasis, and with a poor prognosis. As an example, increased sialyl Lea expression occurred in colon cancer cells and has been associated with the acquisition of a high metastatic capacity.34 35 36
The increase in sialylation of the metastatic tumor cell surface can result in a decreased attachment to basement membrane protein collagen type IV and fibronectin, predisposing the tumor cells to an increased mobility and a decreased growth control by substratum contact.37 Sialyl Lea epitope is also a ligand for E-selectin.38 39 40 It is an attractive hypothesis that some carcinoma cells would use that selectincarbohydrate interaction in the cascade of events involved in metastasis. By contrast, pterygium is a tumor without degenerative processes,41 in which ST3Gal III is less expressed. This could be explained by the fact that this tumor is benign.
These abnormalities also suggest an abnormal expression of the fucosyl- and sialyltransferases and suggest changes during cell differentiation or maturation. Such a change seems specific to pterygium and, as far as we know has not yet been observed in other ocular diseases.
To the best of our knowledge, no study has already tested ST3Gal III expression on the ocular surface. We demonstrated that ST3Gal III expression is lower in pterygium than in normal conjunctiva, which partially explains the modifications of mucin carbohydrate epitopes in pterygium. In fact, glycosyltransferases other than ST3Gal III could be altered in pterygium and be involved in these modifications. Moreover, we showed an abnormal pattern of cell differentiation characterized by sialyl Lea expression, which is specific to pterygium.
| Footnotes |
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Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Ft. Lauderdale, Florida, May 1998.
Submitted for publication June 17, 1998; revised February 11, 1999; accepted March 23, 1999.
Proprietary interest category: N.
| References |
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2,3-sialyltransferases in the assembly of 3'-sialyl-6-sulfo/sialyl Lewis a and x, 3'-sialyl-6'-sulfo Lewis x, and 3'-sialyl-6-sialyl/sulfo blood group T-hapten Biochemistry 34,2925-2936[Medline][Order article via Infotrieve]
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