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Short Communication |
From the Departments of Ophthalmology*
and
Dermatology,
Kyoto Prefectural University of
Medicine, Kyoto, Japan
| Abstract |
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| Introduction |
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The general term given to the pathological transition of a nonkeratinized, stratified epithelium into a keratinized epithelium is squamous metaplasia,6,7 and in the human eye, it is a process that is accompanied by the loss of conjunctival goblet cells, an increase in epithelial stratification, and an enlargement of the superficial epithelial cells.8,9 Squamous metaplasia has been described in numerous disorders of the ocular surface,6,7 including dry-eye disorders in which the aqueous layer of the tear film is deficient, eg, Sjögren syndrome,10 as well as disorders such as SJS and ocular cicatricial pemphigoid (OCP) in which the mucous layer is deficient.8 Despite the effort that has gone into understanding squamous metaplasia, the pathogenesis behind the abnormal differentiation of cells remains unknown.
In this study, we first wanted to discover whether or not the
abnormally thick conjunctiva in SJS might be a result of increased
cellular proliferation, as is thought to be the case in squamous
metaplasia associated with OCP.11
To investigate
this we initiated a immunohistochemical study using the murine
monoclonal antibody Ki-67, an antibody that reacts with a human nuclear
antigen that is present in proliferating cells but absent from
quiescent cells.12
Previous work has shown that
the Ki-67 nuclear antigen is expressed in the G1, S, G2, and M phases
but not in the G0 phase.13
Furthermore, when
compared with flow cytometry, 3H-thymidine
labeling and BrdU labeling, Ki-67 immunohistochemistry allows a readier
evaluation of the growth fraction of a given human cell
population.14
The second feature of SJS studied
was prompted by the observation that in severe cases, diseased
conjunctival epithelium covering the cornea often resembles the
keratinized epidermis of skin (Figure 1A)
. This led us to wonder whether
transglutaminase 1 (keratinocyte transglutaminase; TGase1) might be
involved in the pathological keratinization of ocular surface mucosal
epithelia in SJS. TGase1 is an enzyme expressed during the terminal
differentiation of keratinocytes to form the highly insoluble,
cross-linked cell envelope at the periphery of cornified
cells.15
We used in situ hybridization
to investigate the expression of the TGase1 gene in normal and SJS
conjunctiva.
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| Materials and Methods |
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With informed consent we obtained conjunctiva-covering cornea from
five patients with SJS (Table 1)
at the
time of a lamellar keratoplasty (surgical removal of the outer layer of
the cornea) to improve vision. All eyes were in the chronic cicatricial
phase, and the corneal surfaces were totally covered by conjunctival
tissue (Figure 1A)
. In general, the ocular surfaces were relatively
quiet, though some mild episodic inflammation was occasionally seen in
four of the five patients. Impression cytology found very few goblet
cells in the bulbar inferior conjunctiva of the SJS eyes (Figure 1B)
, a
characteristic of squamous metaplasia. Again with proper informed
consent, normal tissue was obtained during ocular surgery from the
bulbar inferior conjunctiva from five age-matched individuals without
any history of ocular surface disease. Impression cytology confirmed
the presence of numerous mucin-producing goblet cells in this tissue
(Figure 1C)
, a normal feature of conjunctiva.
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Samples of conjunctiva were snap frozen, and 7-µm thick sections were cut. To minimize endogenous peroxidase activity, the tissue sections were incubated with 1% H2O2 for 1 hour, and to block nonspecific binding they were incubated with 10% goat serum at room temperature for 1 hour. Subsequently, the sections were incubated at 4°C for 12 hours with the murine monoclonal antibody Ki-67 (Immunotech, Marseille, France), diluted to yield an IgG concentration of 100 µg/ml. Control sections were incubated with the same concentration of normal mouse IgG (DAKO, Kyoto, Japan). Samples were next incubated with a biotinylated antiserum directed against mouse IgG (Vector Laboratories, Burlingame, CA; working dilution 1:200) for 2 hours at room temperature, and then with an avidin-biotin-peroxidase complex (Vector Laboratory; 1:100) for 50 minutes at room temperature. Sections were then exposed to 200 µg/ml of 3,3'-diaminobenzidine in 0.05 mol/L Tris-HCl buffer (pH 7.6) containing 50 µg/ml H2O2 (10 minutes at room temperature). Between each of these steps, sections were rinsed thoroughly with 0.1 mol/L phosphate-buffered saline. As has been done previously,11 to compare the number of proliferating cells in SJS conjunctival epithelium with the number in normal conjunctival epithelium, we counted the Ki-67-labeled cells in all epithelial layers overlying 100 basal epithelial cells. Cells in three sections of each conjunctiva were counted independently by two investigators and the data averaged.
TGase1 in Situ Hybridization
Paraffin-embedded tissues were sectioned into 4-µm thick sections for in situ hybridization. The RNA probes were synthesized according to our previous reports.16,17 Briefly, a 2-kb KpnI fragment of human TGase1 cDNA was inserted into plasmid pGEM4Z to construct pdM-K2. After linearizing pdM-K2 with EcoRI and BamHI, the antisense and sense cRNAs were transcribed in vitro using digoxigenin (DIG)-labeled UTP by T7 and SP6 RNA polymerase, respectively, according to the manufacturer's manual (DIG RNA Labeling Kit SP6/T7; Boehringer Mannheim, Mannheim, Germany). The RNA probes were fragmented by limited alkaline hydrolysis. In situ hybridization was performed according to our previous protocol.17 First, the sections were deparaffinized in xylene and rehydrated through a graded ethanol series. After proteinase K digestion (18 µg/ml), the sections were postfixed with 4% (w/v) paraformaldehyde in phosphate-buffered saline for 10 minutes and treated with 0.1 mol/L triethanolamine-HCl (pH 8.0) for 1 minute. Following acetylation for 10 minutes, the sections were dehydrated, air-dried then incubated overnight at 45°C in hybridization buffer composed of 50% formamide, 10 mmol/L Tris-HCl (pH 7.5), 1 mg/ml yeast tRNA (Sigma), 1x Denhardt's solution (Sigma), 10% PEG6000, 600 mmol/L NaCl, 0.25% sodium dodecyl sulfate, 1 mmol/L EDTA, and 0.2 µg/ml probe. After hybridization, the sections were washed at 45°C for 1 hour in 50% formamide and 2x SSC, then digested with 20 µg/ml RNase (Sigma) in 10 mmol/L Tris-HCl (pH 8.0), and 500 mmol/L NaCl at 37°C for 10 minutes. Hybridized DIG-labeled probes were visualized with a Nucleic Acid Detection Kit (Boehringer Mannheim).
| Results |
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| Discussion |
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Pathological keratinization of ocular surface mucosal epithelia, the
end stage of squamous metaplasia, is often observed in cicatricial
ocular surface diseases such as SJS and OCP. Where it occurs it
represents a serious clinical problem because it is invariably
associated with a worsening of vision and an instability of the tear
film. At present, the disease mechanism behind this keratinization is
not understood. The present results, however, suggest that, in SJS at
least, a link between TGase1 gene expression and conjunctival
epithelial cell keratinization might exist. TGase1, keratinocyte
transglutaminase, is an enzyme coded for by a gene sublocalized to
chromosome 14q11.219
that catalyzes
-(
-glutamyl) lysine cross-links of proteins to form the cell
envelope at the periphery of cornified cells.15
It is present in the epidermis of skin,15,20,21
and, along with TGase3,22
is involved with
epidermal keratinization. Specifically, TGase3 catalyzes small oligomer
formation of loricrin, a component of the cornified cell envelope, by
intrachain cross-linking,23
whereas TGase1 forms
very large oligomeric loricrin complexes by interchain cross-linking.
Indeed, experiments with knockout mice have revealed that TGase1 is
essential for the distribution of the cell envelope precursor protein
at the cell periphery and that the function of TGase1 cannot be
compensated for by TGase3 nor other TGase
isozymes.24
In view of this, the present study
used TGase1 as a representative marker of keratinization. We did not
detect TGase1 gene expression in the mucous epithelia of normal
conjunctiva (Figure 3D)
and cornea (not shown). In contrast, TGase1
mRNA was clearly present in suprabasal cells in the conjunctival
epithelium in SJS. The obvious implication is that the pathological
keratinization of the conjunctival epithelium in SJS might be caused by
the expression of TGase1, an enzyme whose mRNA is not ordinarily
present in mucosal ocular surface epithelia.
As stated, this study suggests that conjunctival epithelial hyperplasia
and keratinization in SJS are based on increased proliferation coupled
with TGase1 gene expression. What might cause the hyperproliferation
and unusual expression of the TGase1 gene? After considering possible
disease mechanisms, we suspect that inflammatory processes might be
involved. Previously, other investigators have speculated that
conjunctival inflammation might influence goblet cell loss in diseases
like SJS.9
We now speculate that epithelial
keratinization in SJS might also be related to inflammatory activity.
This is based on the notion that epithelial keratinization in SJS is a
consequence of TGase1 gene expression, along with the hypothesis that
the expression of the TGase1 gene might be induced by cytokines
released by infiltrating cells. In support of this we point out that
TGase1 mRNA expression can be induced by a number of factors, including
Ca2+,25,26
ganglioside
GQ1b,26
12-O-tetradecanoyl-phorobol-13-acetate,19,27
and interferon-
(IFN-
).28
We feel that of
these inducers of the TGase1 gene, IFN-
is the most likely to be
involved with the pathogenesis of SJS, especially in light of our
previous immunohistochemical work (unpublished data) that has indicated
that the subepithelial infiltrating cells in SJS conjunctiva are
immunoreactive with antibodies to IFN-
.
Our proposed pathogenesis for SJS, ie, conjunctival epithelial
hyperfroliferation accompanied by subepithelial cellular infiltration
and the release of IFN-
leading to TGase1 gene expression and
subsequent keratinization, is similar in some respects to the disease
mechanism behind the keratinizing skin disease, psoriasis. For example,
epidermal hyperproliferation and incomplete epidermal differentiation
are hallmarks of psoriasis. We also know that, in psoriasis, TGase1
gene expression is altered,17
and the activity of
the enzyme in the psoriatic epidermis is
increased.29
Moreover, Th1 type CD4-lymphocytes
and numerous inflammatory cytokines are present in the lesional skin in
psoriasis.30
Our previous work has indicated that
the subepithelial infiltrating cells in SJS conjunctiva are also
Th1-type CD4-lymphocytes. The cytokines found in psoriatic epidermis
include IFN-
, as stated earlier, a potent inducer of squamous
differentiation and TGase1 expression28
that we
have found previously in SJS conjunctiva. It is worth pointing out that
if inflammatory events do indeed cause hyperproliferation and TGase1
gene expression in SJS conjunctiva, it is likely that, as is the case
in psoriasis,30
several cytokines, not just
IFN-
, would be involved.
We propose that epithelial hyperproliferation and TGase1 gene
expression underlie conjunctival hyperplasia and pathological
keratinization in SJS, and possibly other forms of dry eye as well.
This identifies the down-regulation of TGase1 gene expression as a
potential treatment. Furthermore, if, as we suspect, inflammatory
cytokines such as IFN-
are found to induce TGase1 gene expression,
anti-inflammatory medication might prove effective in controlling SJS
even in its chronic cicatrical phase.
| Acknowledgements |
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| Footnotes |
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Supported by Grants-in-Aid for Scientific Research (09771460, 09877336, 10470365, and 08457467) from the Ministry of Education, Culture and Science, Japan, by a Health Sciences Research Grant (Research on Eye and Ear Sciences) from the Ministry of Health and Welfare, Japan, by a research grant from the Kyoto Foundation for the Promotion of Medical Science, and by the Intramural Research Fund of Kyoto Prefectural University of Medicine.
Accepted for publication November 3, 1998.
| References |
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. J Biol Chem 1994, 269:2016-2022This article has been cited by other articles:
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