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1 From the Department of Ophthalmology, University of Tokyo School of Medicine, Japan; the 2 Department of Ophthalmology, University of Kyushu School of Medicine, Fukuoka, Japan; and the 3 Department of Biochemistry, University of Kumamoto School of Medicine, Japan.
| Abstract |
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METHODS. An anti-AGE monoclonal antibody (6D12), which recognizes a
N
-carboxymethyl lysine (CML)protein adduct as an
epitope, was prepared. Immunohistochemical localization of CML was
examined in human age-matched diabetic and nondiabetic corneas (8 of
each). In vitro, type I collagen, type IV collagen, or
laminin-coated 96-well plates were glycated by glucose-6-phosphate. In
some experiments, aminoguanidine was present in the incubation mixture.
The amounts of CML-protein adducts in the extracellular matrix (ECM)
were determined by enzyme-linked immunosorbent assay using 6D12.
SV40immortalized human corneal epithelial cells were seeded
onto modified or unmodified ECM in 96-well plates and allowed to attach
for 3 hours. Attached cells were fixed, and the areas of attached cells
in each condition were measured. Attached cells without fixation were
removed, and cell number was counted.
RESULTS. In all of the 8 diabetic corneas, CML immunoreactivity was observed in the epithelial basement membrane, whereas CML immunoreactivity was not found in the corresponding area in 7 of 8 nondiabetic corneas. In vitro, nonenzymatic glycation of laminin on the culture dish attenuated adhesion and spreading of corneal epithelial cells. The presence of aminoguanidine in the incubation mixture during glycation inhibited CML formation and promoted the adhesion and spreading of corneal epithelial cells in a dose-dependent manner.
CONCLUSIONS. The accumulation of AGEs on the basement membrane, particularly on laminin, may play a causative role in the corneal epithelial disorders of diabetic patients.
| Introduction |
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The accumulation of advanced glycation end products (AGEs) has been
suggested as a pathogenic mechanism underlying diabetic complications
such as nephropathy,14
retinopathy,15
and
cataract.16
17
Chronic hyperglycemia leads, through
early-stage products of nonenzymatic glycation such as Schiff bases and
Amadori products, to the formation of AGEs on proteins. Among several
structures reported so far, N
-(carboxymethyl)
lysine (CML) has been identified as a dominant AGE antigen in tissue
proteins.18
19
Previous reports10 20 indicated that corneas in diabetic patients are exposed to increased glucose concentration despite the corneas avascular property. This condition may contribute to the accumulation of AGEs in diabetic corneas. However, the existence and the distribution of AGEs in diabetic corneas have not been examined. We prepared an anti-AGE monoclonal antibody and examined the role of AGEs in the pathogenesis of diabetic keratopathy.
| Materials and Methods |
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Antibody
An anti-AGE monoclonal antibody (6D12) was prepared as previously
described.21
Briefly, mice were immunized with
AGE-modified albumin, and the splenic lymphocytes from the immunized
mouse were fused to myeloma cells. Culture supernatant of hybrid cells
was screened by its reactivity to AGEs. The selected cell lines were
injected into Balb/c mice for production of ascites. The antibody was
purified by protein GSepharose DEAEcellulose chromatography. Our
recent study revealed that this antibody recognizes a CML-protein
adduct as an epitope.19
Immunohistochemical Localization of CML
Formalin-fixed and paraffin-embedded corneas were cut into
3-µm-thick sections, deparaffinized by xylene, and dehydrated by
graded ethanol. The sections were treated with 3% hydrogen peroxide in
phosphate-buffered saline (PBS) for 15 minutes and then rinsed in PBS.
To increase the immunoreactivity of AGEs, the sections were placed in
500 ml of 0.01 M of citric acidbuffered solution (pH 7.0) and applied
to microwave (500 W) for 5 minutes.22
23
After thorough
washing, the sections were incubated with the normal rabbit serum for
20 minutes at room temperature to avoid nonspecific binding of the
antibodies. The sections were then incubated overnight at 4°C with 3
µg/ml of the anti-AGE monoclonal antibody (6D12) in PBS containing
1% of bovine serum albumin. Immunoreactivity was detected by the
streptavidinbiotinperoxidase method using a Histofine SAB-PO (M)
kit (Nichirei, Tokyo, Japan) according to the manufacturers protocol.
The final reaction product was visualized with 3,3'-diaminobenzidine
tetrahydrochloride (DAB). We repeated the experiments three times.
Corneal Epithelial Cell Line and Culture Condition
We used immortalized human corneal epithelial cells with a
recombinant SV40-adenovirus vector.24
This cell line
exhibits a cobblestonelike appearance similar to that of basal cells
of normal corneal epithelium in culture, and can differentiate in a
multilayered fashion when grown at air-liquid interface on collagen
gel. They were grown in supplemented hormone epithelial medium as
previously described.24
The culture was maintained in a
humidified atmosphere (95% air and 5% CO2) at
37°C.
In Vitro Glycation of Extracellular Matrix Components
Type I collagen, type IV collagen, or laminin-coated 96-well
plates were purchased (Falcon, Oxnard, CA). According to the
manufacturers information, the laminin was produced by mouse
EngelbrethHolmSwarm tumor and mainly had laminin-1. To glycate
extracellular matrix (ECM) components of cultured dishes, 200 µl of
50 mM glucose-6-phosphate (G-6-P; Wako, Osaka, Japan) in 0.2 M
NaHPO4buffered solution (pH 7.8) was added in each well
under sterile conditions, and dishes were maintained at 37°C for 3
weeks. In some experiments, aminoguanidine (220 mM) was present in
the incubation mixture. The control wells were incubated without G-6-P.
Measurement of CML Contents in ECM Components
The amounts of CML-protein adducts in ECM were determined by
enzyme-linked immunosorbent assay (ELISA) using 6D12. The glycated and
nonglycated wells were washed 3 times with PBS and blocked with 0.5%
of gelatin hydrolysate (Sigma, St. Louis, MO) in 0.05 M
carbonatebuffered solution (pH 9.6) for 1 hour. The wells were washed
3 times with washing buffer containing 0.05% of Tween-20 in PBS and
were incubated for 1 hour with 100 µl/well of 0.1 mg/ml of the
anti-AGE antibody (6D12) in dilution buffer containing 0.3% bovine
serum albumin (Sigma) in washing buffer. The wells were then washed
with washing buffer 3 times and incubated for 1 hour with 100 µl/well
of horseradish peroxidaseconjugated anti-mouse IgG diluted 1:10,000.
After a thorough washing, the wells were developed with 200 µl/well
of substrate solution containing 0.55 mg/ml of ortho-phenylenediamine
and 0.002% of hydrogen peroxide in 50 mM of citric acidbuffered
solution (pH 5.0). The reaction was stopped by 50 µl/well of 1 M
H2SO4, and the absorbance
at 490 nm was measured using DigiScan Reader (ASYS Hitech GmbH,
Eugendorf, Austria). The experiments were repeated 8 times.
Cell Spreading and Adhesion on ECM Components
Human corneal epithelial cells were seeded at 10,000 cells/0.25 ml
onto modified or unmodified ECM in 96-well plates and allowed to attach
for 3 hours. The cultured media and unattached cells were aspirated,
and the wells were gently washed with PBS. Attached cells were fixed
and stained with Diff-Quick staining set (International Reagents, Kobe,
Japan), and random pictures of attached cells were taken under light
microscope (Nikon, Tokyo, Japan) at the magnification of 200. By
analyzing the pictures using NIH image, the areas of 50 attached cells
in each condition were measured. Attached cells without fixation were
removed by exposing each well to 0.25 ml of trypsinEDTA
(GIBCO, Grand Island, NY) for 20 minutes. Isoton solution
(American Scientific Products, McGraw, IL) was added to the cell
suspension to a final volume of 10 ml, and cell number was counted with
a Coulter counter (type Z1). The experiments were repeated 8 times.
Statistical Analysis
The effect of glycation and aminoguanidine was analyzed by ANOVA.
Additionally, the Scheffé multiple comparison test was used to
compare the differences between all possible pairs of means within each
experiment.
| Results |
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| Discussion |
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Considering that laminin is a major component in the human corneal basement membrane26 27 28 and that the turnover rate of laminin is as slow as that of other components,29 30 it is likely that laminin in the basement membrane of diabetic cornea contains CML.
The mechanisms by which increased glucose concentration results in altered cell behavior might be multiple. We have examined the effects of one of these mechanisms, glycation of the matrix and its influence on the cell behavior. The presence of aminoguanidine during glycation inhibited the formation of CML on the matrix and regained adhesion and spreading of corneal epithelial cells on the matrix in a dose-dependent manner. These results indicate that glycation of the matrix plays a certain role in altered cell behavior induced by increased glucose concentration.
The results of the current in vitro and in vivo studies raise the following presumption: nonenzymatic glycation of ECM (particularly laminin) in the corneal epithelial basement membrane decreases adhesion and spreading of the corneal epithelial cells, leading to corneal epithelial disorders in diabetic patients. This presumption is compatible with the clinical observation that in corneal epithelial defects of diabetic patients, corneal epithelial cells proliferate onto the underlying stroma but often easily detach from the stroma.31 32 Previous studies have shown that nonenzymatic glycation of ECM weakens adhesion and spreading of mesangial cells, glomerular epithelial cells, and vascular endothelial cells and that it probably causes chronic diabetic complications such as nephropathy and atherosclerosis.33 34 35 36 Our results support the hypothesis that AGEs may be responsible for diabetic corneal epitheliopathy. However, we have not shown that the specific inhibition of AGEs formation normalizes corneal epithelial behavior in diabetic corneas. Additional experiments are needed to prove that AGEs are responsible for diabetic corneal epitheliopathy.
A recent study reported that the immunoreactivity for entactin/nidogen and for chains of laminin-1 and laminin-10 was very weak or absent in the corneal epithelial basement membrane of diabetics with diabetic retinopathy, whereas corneas from nondiabetic individuals or diabetics without diabetic retinopathy showed such alterations much less frequently.37 As they stated in their report, their results suggest that, as retinal diabetic disease worsens, concomitant alterations of the corneal epithelial basement membrane occur in parallel. They suggested that these alterations might be due to the action of growth factors or proteinases abnormally expressed in the eyes with diabetic retinopathy. In addition to those mechanisms, it is possible that advanced accumulation of AGEs concomitant with a long history of diabetes mellitus altered the immunoreactivity of the ECM components in the corneal epithelial basement membrane. This hypothesis should be confirmed in the future study using a greater number of corneas from diabetes, with or without diabetic retinopathy, and nondiabetes.
CML immunoreactivity was not observed in the Descemets membrane of diabetic or nondiabetic corneas, which is the basement membrane of the corneal endothelial cells. This result is somewhat puzzling because Descemets membrane should be exposed to a higher concentration of glucose than the epithelial basement membrane (being closer to the aqueous humor, which is the source of most of the corneal glucose). Why did Descemets membrane contain no CML immunoreactivity? One possibility is that high concentrations of oxygen may accelerate the formation of CML in the epithelial basement membrane. Considerable evidence shows that oxidative mechanism is involved in the process of AGEs formation.38 39 40 Because oxygen in the cornea is supplied primarily by diffusion from the tear fluid, the epithelial basement membrane should be exposed to a higher concentration of oxygen than Descemets membrane. Further experiments are necessary to determine the actual mechanisms of the discrepancy of CML immunoreactivity between epithelial basement membrane and Descemets membrane.
In both diabetic and nondiabetic corneas, CML immunoreactivity was found in endothelial cells. Human corneal endothelial cells do not proliferate in vivo, and the number of endothelial cells gradually decreases with aging.41 Because the correlation between the accumulation of AGEs and the aging process has been shown in other tissues,17 23 42 43 the accumulation of AGEs might cause the decrease of corneal endothelial cells with aging. Further studies are necessary to elucidate these points.
In summary, the present study indicates that CML immunoreactivity is increased in the epithelial basement membrane of diabetic corneas and that nonenzymatic glycation of laminin reduces adhesion and spreading of corneal epithelial cells. Thus, the accumulation of AGEs on the basement membrane, particularly on laminin, may play a causative role in diabetic corneal epitheliopathy. With the relative accessibility of the cornea to the local drug delivery, the potential for the development of anti-AGE drugs for diabetic corneal disorders appears promising.
| Acknowledgements |
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| Footnotes |
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Commercial relationships policy: N.
Corresponding author: Shiro Amano, Department of Ophthalmology, University of Tokyo School of Medicine, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. amanos-tky{at}umin.ac.jp
| References |
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-(carboxymethyl) lysine in lens proteins Biochemistry 28,9464-9468[Medline][Order article via Infotrieve]
-(carboxymethyl) lysine is a dominant advanced glycation end product (AGE) antigen in tissue proteins Biochemistry 34,10872-10878[Medline][Order article via Infotrieve]
-(carboxymethyl) lysine protein adduct is a major immunological epitope in proteins modified with advanced glycation end products of the Maillard reaction Biochemistry 35,8075-8083[Medline][Order article via Infotrieve]
-(carboxymethyl) lysine formation from Amadori product Biochem Biophys Res Commun 234,167-172[Medline][Order article via Infotrieve]
-(carboxymethyl) lysine in lens proteins Biochemistry 28,9464-9468
-(carboxymethyl) lysine and N
-(carboxymethyl) hydroxylysine in human skin collagen Biochemistry 30,1205-1210[Medline][Order article via Infotrieve]
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