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Overexpression of Collagenase (MMP-1) and Stromelysin (MMP-3) by Pterygium Head Fibroblasts
De-Quan Li, MD, PhD;
Sao-Bing Lee, MD;
Zeenat Gunja-Smith, PhD;
Yunqi Liu;
Abraham Solomon, MD;
Daniel Meller, MD;
Scheffer C. G. Tseng, MD, PhD
Arch Ophthalmol. 2001;119:71-80.
ABSTRACT
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Background The balance between matrix metalloproteinases (MMPs) and the tissue
inhibitors of metalloproteinases (TIMPs) determines the extent of connective
tissue degradation and remodeling.
Objective To determine whether pterygium, characterized by fibrovascular invasion
into the cornea, may in part be mediated by an increased activity of MMPs.
Materials and Methods Expression of transcripts and proteins of MMPs, TIMPs, and urokinase
plasminogen activator (uPA) by cultured human pterygium head, body, and subconjunctival
fibroblasts, and normal corneal and conjunctival fibroblasts were determined
by Northern hybridization, enzyme-linked immunosorbent assay, Western blotting,
zymography, and quantitative collagenase assay, respectively.
Results Compared with normal conjunctival fibroblasts from 6 subjects, the expression
of MMP-1 and MMP-3 transcripts was dramatically increased in pterygium head
fibroblasts of 8 patients, but not in pterygium body fibroblasts of 6 patients.
The protein levels and collagenolytic and caseinolytic activities of MMP-1
and/or MMP-3 were also markedly increased in pterygium head fibroblasts. The
MMP-1 and MMP-3 proteins and activity decreased in order from pterygium head
to body to subconjunctival fibroblasts. There was no difference in the transcript
and protein expression of MMP-2, TIMP-1, TIMP-2, and uPA among these groups.
Conclusion Pterygium head fibroblasts express increased mRNA, protein, and activities
of MMP-1 and MMP-3.
Clinical Relevance Overexpression of MMP-1 and MMP-3, a phenotype previously linked with
UV exposure in dermal fibroblasts to explain the pathologic finding of elastotic
degeneration, suggests that pterygium head fibroblasts might be intrinsically
altered by UV, which might be responsible for corneal invasion.
INTRODUCTION
PTERYGIUM represents one of the most common external eye diseases in
countries with relatively high exposure to UV irradiation.1-2
Patients with an early pterygium frequently complain of increased redness
and irritation, and their vision is reduced when the pterygium advances into
the cornea. Although UV irradiation is causatively linked with the formation
of pterygium, the underlying mechanism leading to corneal invasion remains
obscure. Clinically, pterygium is characterized by a wing-shaped overgrowth
of conjunctival tissue into the cornea and can be grossly subdivided into
2 portions, ie, the head and the body. One histopathological feature of pterygium
overgrowth is excessive fibrovascular proliferation.3-5
The extent of such fibrovascular proliferation has recently been regarded
as a reliable morphologic index for predicting recurrence following surgical
removal.6 The other pertinent feature is the
loss of the basement membrane, the Bowman membrane, and the superficial corneal
stroma at the area invaded by the fibrovascular tissue.7
It has thus been speculated that such a tissue loss is a result of destruction
by the invading pterygium,8 although direct
evidence to support such a hypothesis is lacking.
Matrix metalloproteinases (MMPs) are a family of enzymes that act to
modify or degrade the extracellular matrix .9-11
These enzymes are synthesized and secreted by a variety of cell types including
fibroblasts. At least 19 members of the MMP family have been identified and
categorized into 5 groups: collagenases (MMP-1, MMP-8, and MMP-13), gelatinases
(MMP-2 and MMP-9), stromelysins (MMP-3, MMP-10, and MMP-11), membrane-type
MMPs, and others. Matrix metalloproteinases are normally coexpressed with
a family of tissue inhibitors of metalloproteinases (TIMPs), which inhibit
active forms of MMPs. At least 4 inhibitors, 28-kd TIMP-1, 21-kd TIMP-2, 23-kd
TIMP-3 and 24-kd TIMP-4 have been characterized and are also produced by many
cell types including fibroblasts.9-10
The balance between the activity of MMPs and that of TIMPs determines the
extent of proteolysis linked with tissue remodeling or degradation.10-11 For example, an increased ratio of
MMPs to TIMPs is correlated with how tumor cells invade the host stroma.12 Besides MMPs and TIMPs, the other proteolytic cascade
leading to tissue degradation and remodeling involves urokinase plasminogen
activator (uPA), a serine protease. For example, overexpression of uPA is
also correlated with the invasiveness of human cancer cells.13
Herein, we provide experimental evidence that pterygium head fibroblasts in
culture overexpress MMP-1 and MMP-3 without changes in MMP-2, TIMP-1, TIMP-2,
and uPA when compared with fibroblasts derived from other portions of the
pterygium, the normal cornea, and conjunctiva.
MATERIALS AND METHODS
MATERIALS
Dulbecco-Eagle minimum essential medium (DMEM), fetal bovine serum (FBS),
fungizone, phenol, DNA or RNA size marker, and random primers DNA labeling
kit were purchased from GIBCO-BRL (Grand Island, NY). Cell culture dishes,
6-well plates, and 15-mL centrifuge tubes were from Becton Dickinson (Lincoln
Park, NJ). BCA protein assay kit was from Pierce Chemical (Rockford, Ill).
Zymogram-ready gels containing gelatin or casein, 4% to 15% tris-hydrochloride
polyacrylamide gradient ready gel, sodium dodecyl sulfate, and electrophoresis
equipment were from Bio-Rad (Hercules, Calif). Human MMP-1 and MMP-3 enzyme-linked
immunosorbent assay (ELISA) kits and the monoclonal antibodies against human
MMP-1, MMP-2, MMP-3, MMP-9, TIMP-1, and TIMP-2 were from Oncogene Research
Products of Calbiochem (Cambridge, Mass). Vectastain Elite ABC peroxidase
kit was from Vector Laboratories (Burlingame, Calif). Nitrocellulose membranes
were from Scheicher and Schuell (Keene, NH). GeneAmp RNA–polymerase
chain reaction (PCR) kit was from Perkin-Elmer Cetus (Norwalk, Conn). Wizard
PCR Preps DNA purification kit was from Promega (Madison, Wis).  -Phosphorus
32 deoxycytidine triphosphate ([-32P]-dCTP)was from DuPont
NEN (Boston, Mass). XAR-5 and BioMax MS-1 films and intensifying screens were
from Eastman Kodak (Rochester, NY). All other reagents and chemicals were
from Sigma-Aldrich (St Louis, Mo).
HUMAN CORNEAL, LIMBAL, CONJUNCTIVAL, AND PTERYGIUM FIBROBLAST CULTURES
Human corneas from donors aged 31 to 50 years that were preserved for
less than 72 hours were obtained from the Florida Lions Eye Bank. Normal conjunctiva
and pterygial specimens were obtained from patients receiving cataract and
pterygial removal, respectively, with patient's informed written consent and
followed tenants of the Helsinki declaration. Using the method described below,
a total of 6 strains of healthy conjunctival fibroblasts, 8 of pterygium head
fibroblasts, and 6 of pterygium body fibroblasts were obtained. For this study,
fibroblasts at the third or fourth passage were used. The technique of removing
the pterygium and its surrounding subconjunctival fibrovascular tissue has
previously been described.14 The pterygial
specimen was further subdivided into the head and the body, of which the former
comprised the tip area of 2 x 2 mm, while the latter included the remainder
of the pterygial specimen (for anatomic designation see Figure 1). Normal corneal, limbal, and conjunctival fibroblasts
and pterygial head and body fibroblasts were obtained from explant cultures
using a technique identical to a previously described method.15
In brief, each tissue was cut into explants of approximately 2 x 2 mm2 and placed onto 100-mm tissue culture dishes. Ten to 20 minutes later,
each explant was covered with a drop of DMEM containing 10% FBS (DMEM-FBS),
50 µg/mL of gentamicin, and 1.25 µg/mL of fungizone and placed
overnight in an incubator at 37°C under 95% humidity with 5% carbon dioxide.
On the next day, 10 mL of the same media was added and the media were changed
twice weekly thereafter. These fibroblasts were subcultured with 0.1% trypsin
and 0.02% EDTA in calcium-free Eagle minimum essential medium at 80% to 90%
confluence with 1:3 split for several passages.
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Figure 1. External photograph of a patient
with primary pterygium. A representative photograph showing the pterygium
growing onto the corneal surface. The convergent point is called the "head"
(fleshy tissue within box), and the wing-shaped growth spanning to the fornix
and nasal caruncle is called the "body."
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For Northern blot analysis, fibroblasts were cultured for 7 to 9 days
in 100-mm dishes containing DMEM-FBS until confluence before extraction of
total RNA. For ELISA, Western blot analysis, zymography, and quantitative
collagenase assay, the same passages of fibroblasts from different sources
were seeded in triplicates at the same density (1.5 x 105
cells/well) in 6-well plates and grown for 7 to 9 days until confluence. After
being washed 4 times with phosphate-buffered saline, these cultures were switched
to the same volume (1 mL) of a serum-free DMEM, containing 5 µg/mL of
insulin, 5 µg/mL of transferrin, and 5 ng/mL of sodium selenite (DMEM-ITS)
and incubated for additional 24 and 48 hours. The conditioned media were then
collected, and the adherent cells were trypsinized for cell counting and lysed
in phosphate-buffered saline (pH 7.3) containing 1.5-mol/L sodium chloride
and 0.039% Triton X-100 for BCA protein assay.
PROBE PREPARATION
Five of the human DNA probes, including 185 base pair (bp) fragment
of MMP-1, 480 bp of MMP-2, 155 bp of MMP-3, 551 bp of TIMP-1, and 590 bp of
TIMP-2 were provided by Velidi H. Rao, PhD (University of Nebraska Medical
Center, Omaha). Three complementary DNA (cDNA) probes, 640 bp of MMP-9, 519
bp of uPA, and 498 bp of glyceraldehyde-3-phosphate dehydrogenase were purified
from reverse transcriptase–PCR products by electrophoresis through a
1.2% low melting agarose gel using a DNA purification kit according to the
manufacturer's protocol (Wizard PCR Prep, Promega). The primers used for PCR
were 1502-1531 (sense) and 2111-2140 (antisense) for MMP-9 (accession J05070),
487-506 (sense) and 982-1002 (antisense) for uPA (accession A18397), 541-561
(sense) and 1018-1038 (antisense) for glyceraldehyde-3-phosphate dehydrogenase
(accession M33197). The 32P-labeled cDNA probes (1-2 x 109 cpm/µg DNA) were prepared with [-32P]-dCTP
(1.1 x 1014 Bq/mmol [3000 Ci/mmol]), using a random primers
DNA labeling system.
RNA ISOLATION AND NORTHERN HYBRIDIZATION
Total RNA isolation and Northern hybridization were performed using
a previously described method.15 Briefly, total
RNA was isolated from various types of fibroblast cultures by acid guanidium
thiocyanate-phenol-chloroform extraction. Total RNA at 25 µg per lane
was electrophoresed through 1.2% agarose containing formaldehyde, transferred
to nitrocellulose membranes, and hybridized with 32P-labeled cDNA
probes at 2 x 106 to 4 x 106 cpm per 3 to
8 ng/mL in the hybridization solution. After visualization of the hybridization
product in the x-ray film, the 32P label on the membrane was stripped
by washing the membranes at 65°C for 1 hour twice in 5-mmol/L trishydrochloride
(pH 8.0), 0.2-mmol/L EDTA, 0.05% sodium pyrophosphate, and 0.1 x Denhardt
solution, and rehybridized with other 32P-labeled probes. The relative
amount of each messenger RNA (mRNA) of interest was determined by scanning
its autoradiofluorogram with a laser scanner (Densitometer Model FB910; Fisher
Scientific, Pittsburgh, Pa) and normalized as a ratio to that of the glyceraldehyde-3-phosphate
dehydrogenase mRNA band.
MMP-1 AND MMP-3 ELISA
Human MMP-1 or MMP-3 double-sandwiched ELISA was performed using commercial
ELISA kits according to the manufacturer's protocol. In brief, 100 µL
of standard dilutions of recombinant human MMP-1 or MMP-3 and experimental
conditioned media were dispensed into a 96-well microtiter plate coated with
mouse anti-MMP-1 or MMP-3 monoclonal antibody, respectively. The plate was
sealed, incubated at room temperature for 2 hours or at 4°C for 1 hour,
respectively, and washed 4 times with phosphate-buffered saline containing
0.033% Tween 20. After addition of 100 µL of diluted rabbit anti–MMP-1
serum into each well and incubation for 2 hours followed by 4 washes, 100
µL of diluted donkey antirabbit horseradish peroxidase conjugates was
added and incubated for 1 hour. For MMP-3, 100 µL of diluted rabbit
anti–MMP-3 horseradish peroxidase was added into each well and incubated
at 4°C for 2 hours. Aliquots of 100 µL of the color reagent 3,3',5,5'-tetramethylbenzidine
were then applied for 30 minutes to develop a blue color, and the reaction
was stopped by adding 100 µL of 1-mol/L sulfuric acid. Absorbance was
read at 450 nm by an automatic plate reader with a reference wavelength of
570 nm.
WESTERN BLOT ANALYSIS
To identify MMP and TIMP proteins present in each fibroblast-conditioned
medium, Western blot analysis was performed using their specific antibodies.
Conditioned media from different fibroblast cultures were adjusted to a final
volume of 20 to 25 µL to represent the same quantity of cellular protein
(8.3 µg) or cell number (5000 cells) and electrophoresed under reducing
condition at 4°C in a 4% to 15% gradient polyacrylamide gel. After electrophoretic
transfer to a nitrocellulose membrane at 4°C, the membrane was immersed
with 0.1% (vol/vol) Tween 20 in tris-buffered saline (100-mmol/L tris, 0.9%
sodium chloride, pH 7.5) (TTBS) for 30 minutes with agitation. The primary
antibody, ie, 1 µg/mL of mouse monoclonal antibody against human MMP-1,
MMP-2, MMP-3, MMP-9, TIMP-1, or TIMP-2, in TTBS containing 1% horse serum
was placed on each membrane and incubated at roomtemperature for 60 minutes
with agitation. After being washed with 3 to 4 changes of TTBS over 15 minutes,
each membrane was transferred to a 1:200 diluted solution of biotinylated
second antibody (goat antimouse IgG from Vectastain Elite ABC kit) in TTBS
containing 1% horse serum and incubated for 30 minutes. After 3 to 4 washes
with the same solution, the membranes were incubated to 1:50 diluted Vectastain
Elite ABC reagent conjugated with peroxidase for 30 minutes and processed
for color development in 0.5 µg/mL of diaminobenzidine in 50-mmol/L
tris-hydrochloride (pH 7.2) containing 0.05% hydrogen peroxide for 10 to 20
minutes.
ZYMOGRAPHY OF METALLOPROTEINASE ACTIVITY
To determine gelatinolytic and stromelysin activities of the various
fibroblast cultures, zymography was performed using a method similar to that
previously described.16 Each conditioned medium
(12-15 µL), after being adjusted to represent the same quantity of cellular
protein (5 µg) or cell number (3000 cells), was treated with sample
buffer without boiling or reduction. Sodium dodecyl sulfate–polyacrylamide
gel electrophoresis was performed using a 10% polyacrylamide gel containing
0.1% gelatin or a 12% gel containing 0.1% casein. The gels were soaked in
2.5% Triton X-100 for 30 minutes at room temperature to remove the sodium
dodecyl sulfate and incubated in a reaction buffer (50-mmol/L tris-hydrochloride
[pH 7.5], 200-mmol/L sodium chloride, 5-mmol/L calcium chloride, and 0.02%
23 lauryl ether [Brij-35]) at 37°C overnight to allow proteinase digestion
of its substrate. Gels were rinsed again in distilled water, stained with
0.5% Coomassie brilliant blue R-250 in 40% methanol and 10% acetic acid for
1 hour, and destained with 40% methanol and 10% acetic acid. Proteolytic activities
appeared as clear bands of lysis against a dark background of stained gelatin
or casein. To verify that the detected gelatinolytic and caseinolytic activities
were specifically derived from metalloproteinases, the gels were treated with
the Triton X-100 solution and the tris–sodium chloride/calcium chloride
reaction buffer containing 5-mmol/L phenylmethylsulfonyl fluoride with or
without 10-mmol/L EDTA in the parallel experiments.
QUANTITATIVE COLLAGENASE ACTIVITY ASSAY
Collagenase activity was verified and quantified by incubation with
soluble, telopeptide-free collagen extracted from rat skin and labeled with
[3H]-acetic anhydride.17 For assay
of collagenase with labeled substrate, it was necessary to destroy the TIMPs
in media (500 µL) from various fibroblast cultures by reduction in 2-mmol/L
dithiothreitol at 37°C for 30 minutes, followed by alkylation in 5-mmol/L
iodoacetamide at 37°C for 30 minutes. This step also destroys any 2-macroglobulin. The samples were chilled on ice and dialyzed against
the assay buffer (50-mmol/L tris-hydrochloride, 200-mmol/L sodium chloride,
10-mmol/L calcium chloride, and 0.005% Brij-35, pH 7.5) for 4 hours before
use. Each sample was then tested in triplicate at different volumes (10, 30,
and 90 µL), and the assay was performed twice for accuracy. One of each
triplicate sample was added with aminophenylmercuric acetate to a final concentration
of 0.5 mmol/L to activate latent procollagenase. Another was added with 1,10-phenanthroline
to a final concentration of 2.0 mmol/L in the presence of 0.5-mmol/L aminophenylmercuric
acetate to chelate the zinc and inactivate the collagenase. The blanks were
prepared by replacing the conditioned medium with an equivalent volume of
the assay buffer. [3H]-acetic collagen (116 000 cpm per 16.6
µg/5 µL) was added to each sample and the final volume was adjusted
to 110 µL with the assay buffer. The incubation was performed at 30°C
for 18 hours, and the reaction was stopped by placing the tubes in an ice
bath. After adding with 120 µL of the assay buffer containing 200 µg
acid-soluble intact collagen as a cold carrier, 20 µg of trypsin, 20
µg of chymotrypsin, and 30-mmol/L EDTA, the second digestion was performed
at 31.5°C for 90 minutes. The soluble digested products were separated
from the undigested collagen by precipitating with an equal volume of ice-cold
20% trichloroacetic acid. After centrifugation at 13 000 rpm for 5 minutes,
triplicate aliquots (100 µL each) of the supernatant (representing trichloroaceticacid–soluble
peptides) were counted by liquid scintillation for 3 minutes. The collagenase
activity was reported as units per milliliter (1 U of enzyme digests 1 µg
of collagen per minute at 30°C).
STATISTICAL ANALYSIS
The t test was used for statistical comparison
for the data of Northern hybridization, ELISA, and collagenase activity assay.
RESULTS
TRANSCRIPT EXPRESSION OF MMPs AND TIMPs IN 5 TYPES OF CULTURED HUMAN
FIBROBLASTS
Northern blot analysis showed that the transcripts of 3 MMPs, ie, 2.2
kilobase (kb) of MMP-1, 3.1 kb of MMP-2, and 1.9 kb of MMP-3, and of 2 TIMPs,
ie, 0.9 kb of TIMP-1 and 3.5 kb of TIMP-2, were expressed by all 5 types of
fibroblasts cultured from normal cornea, limbus, conjunctiva, pterygium head,
and pterygium body (Figure 2). The
sizes of these 5 transcripts were consistent with those previously reported.13, 18-19 No MMP-9 transcript
was detected in any of these 5 fibroblasts. Among the 3 MMPs expressed, the
MMP-2 transcript did not show any difference in quantity among the 5 types
of fibroblasts (Figure 2). In contrast,
corneal fibroblasts expressed much more MMP-1 and MMP-3 transcripts than limbal
fibroblasts, and a marked increase in the expression of MMP-1 and MMP-3 transcripts
by pterygium head fibroblasts was noted (Figure 2). Transcripts of both TIMP-1 and TIMP-2 were expressed
by all 5 fibroblasts. TIMP-1 transcript was more predominantly expressed by
human corneal fibroblast. Expression of TIMP-1 by pterygium head fibroblasts
was slightly lower than that by normal conjunctival fibroblasts. There was
no significant difference in the expression of TIMP-2 transcript by these
5 types of fibroblasts (Figure 2).
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Figure 2. Northern hybridization. Transcripts
of matrix metalloproteinase (MMP)-1, MMP-2, MMP-3, tissue inhibitor of metalloproteinase
(TIMP)-1, and TIMP-2 were expressed by human corneal (C), limbal (L), conjunctival
(J) and pterygium head (H), and pterygium body (B) fibroblasts with glyceraldehyde-3-phosphate
dehydrogenase (GAPDH), as a loading control. All 5 fibroblast types were grown
to confluence in Dulbecco-Eagle minimum essential medium–fetal bovine
serum and subjected to total RNA extraction and Northern blotting. The RNA
blots were then individually hybridized with phosphorus 32–labeled specific
complementary DNA probes. kb indicates kilobase. See the "Materials and Methods"
section for more details.
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To verify that pterygium head fibroblasts indeed carried the phenotype
of MMP-1 and MMP-3 overexpression, normal conjunctival fibroblasts from 6
patients, pterygium head fibroblasts from 8 patients, and pterygium body fibroblasts
from 6 patients were compared. As shown in Figure 3, the amount of the MMP-1 transcript expressed by 6 strains
of normal conjunctival fibroblasts was low to undetectable. In contrast, the
amount of the MMP-1 transcript was dramatically increased from 11 to 94 folds
(mean ± SD, 38.6 ± 28.3 folds) in all 8 strains of pterygium
head fibroblasts when compared with that of normal conjunctival fibroblasts
(P<.01; n = 8). Except 1 (No. 6), the amount of
the MMP-1 transcript expressed by 6 strains of pterygium body fibroblasts
was also low to undetectable and was not significantly different from that
of normal conjunctival fibroblasts (mean ± SD, 2.3 ± 2.2 folds, P>.1; n = 6). A similar trend was noted for the expression
pattern of the MMP-3 transcript. The amount of the MMP-3 transcript was also
dramatically increased in all 8 pterygium head fibroblasts except 1 (No. 3)
from 1.6 to 37 folds (mean ± SD, 12.6 ± 13.6 folds) when compared
with that of normal conjunctival fibroblasts (P<.05;
n = 8). Although the amount of the MMP-3 transcript expressed by 2 of 6 pterygium
body fibroblasts (Nos. 2 and 5) was higher than, but, as a group, was not
statistically different from that of normal conjunctival fibroblasts (mean
± SD, 1.28 ± 1.33 folds; P>.5; n =
6). There was no significant difference in the transcript expression of MMP-2
and uPA (Figure 3) and TIMP-1 and
TIMP-2 (not shown) among these 3 groups of fibroblasts.
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Figure 3. Northern hybridization. The pattern
of transcript expression of matrix metalloproteinase (MMP)-1, MMP-2, MMP-3
and urokinase plasminogen activator (uPA) were compared among 6 strains of
normal conjunctival fibroblasts (HJF), 8 strains of pterygium head fibroblasts
(PHF), and 6 strains of pterygium body fibroblasts (PBF). All 20 strains of
above 3 group fibroblasts were grown to confluence in Dulbecco-Eagle minimum
essential medium–fetal bovine serum and subjected to total RNA extraction
and Northern blotting. The RNA blots were then individually hybridized with
phosphorus 32–labeled specific complementary DNA probes with glyceraldehyde-3-phosphate
dehydrogenase (GAPDH) as a loading control. kb indicates kilobase. See the
"Materials and Methods" section for more details.
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ELISA OF MMP-1 AND MMP-3 SECRETED IN MEDIA FROM 5 TYPES OF CULTURED
HUMAN FIBROBLASTS
The protein level of MMP-1 and MMP-3 was determined by its respective
ELISA in their serum-free conditioned media (Figure 4). The amount of MMP-1 and MMP-3 in the conditioned media
from corneal fibroblasts was 48.4 ± 16.2 and 370.4 ± 36.1 ng/mL
(n = 3), respectively, which were 10.4-fold and 9.0-fold higher (P<.05 and P<.01; n = 3) than that (4.7
± 3.7 and 41.2 ± 12.9 ng/mL; n = 3) from conjunctival fibroblasts,
respectively. Similar to the aforementioned Northern data, the amount of MMP-1
and MMP-3 in the conditioned media from pterygium head fibroblasts was the
highest among the 5 types of fibroblasts (332.0 ± 52.2 and 1448.5 ±
67.2 ng/mL, respectively; n = 3), which were 71-fold and 35-fold higher (P<.001 and P<.001; n = 3)
than that from normal conjunctival fibroblasts, respectively. MMP-1 and MMP-3
protein levels decreased in order from pterygium head to body to subconjunctival
fibroblasts, of which the latter two were slightly but not significantly higher
(all P>.1; n = 3) than that of normal conjunctival
fibroblasts, respectively.
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Figure 4. Results of enzyme-linked immunosorbent
assay. Protein levels (mean ± SD) of matrix metalloproteinase (MMP)-1
and MMP-3 secreted in the serum-free conditioned media from normal corneal
(C), limbal (L), and conjunctival (J) fibroblast and from pterygial head (H),
pterygial body (B), and subconjunctival tissue (S) fibroblasts. See the "Materials
and Methods" section for more details.
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PROTEIN EXPRESSION OF MMPs AND TIMPs
Western blot was performed to identify and compare the protein expression
of MMPs and TIMPs in serum-free conditioned media derived from normal conjunctiva
and pterygium head, body, and subconjunctival fibroblasts using their specific
monoclonal antibodies. Monoclonal antibodies to MMPs used in this study recognize
both latent and active forms. As shown in Figure 5, the protein amount of each MMP and TIMP expressed by the
first 3 fibroblasts was consistent with their mRNA expression. In other words,
the amounts of a 54-kd MMP-1 band and a 57-kd MMP-3 band secreted by pterygium
head fibroblasts were markedly increased when compared with those from normal
conjunctival fibroblasts and pterygium body and subconjunctival fibroblasts.
In normal conjunctival fibroblasts, MMP-1 and MMP-3 proteins were secreted
much less than MMP-2. In pterygium head fibroblasts, these 2 proteins were
secreted much more than MMP-2. In pterygium body fibroblasts, MMP-1 and MMP-3
were secreted at a level similar to MMP-2. In contrast, MMP-1 and MMP-3 proteins
secreted by pterygium subconjunctival fibroblasts were less than MMP-2 protein,
a pattern similar to that of normal conjunctival fibroblasts. Because the
level of 72-kd MMP-2 protein was similar among all 4 fibroblasts, it is concluded
that the ratios of MMP-1 and MMP-3 to MMP-2 increased from subconjunctival
fibroblasts to body fibroblasts and head fibroblasts. No MMP-9 was detected
by Western blot, similar to the result of Northern hybridization. The protein
levels of 28-kd TIMP-1 and 21-kd TIMP-2 did not show any notable difference
in the conditioned media secreted by these 4 types of fibroblasts.
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Figure 5. Western blot. Proteins of matrix
metalloproteinase (MMP)-1, MMP-2, MMP-3, tissue inhibitor of metalloproteinase
(TIMP)-1, and TIMP-2 expressed in the serum-free conditioned media of human
conjunctival (J), pterygium head (H), pterygium body (B), and subconjunctival
tissue (S) fibroblasts. See the "Materials and Methods" section for more details.
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ZYMOGRAPHY FOR GELATINOLYTIC AND CASEINOLYTIC ACTIVITIES OF MMP-2 AND
MMP-3
Zymography was performed to verify the gelatinolytic and caseinolytic
activities of MMPs detected by Western blot in serum-free conditioned media.
For comparison, all 6 types of fibroblasts were subcultured at the same density
until confluence and switched to the same volume of serum-free DMEM-ITS for
24 or 48 hours. The 48-hour media contained slightly higher MMP activity than
that of 24-hour media. As shown in Figure
6A, the strong gelatinolytic activity of a 72-kd clear band noted
on the gelatin zymogram corresponded to MMP-2. This band was similarly detected
in 48-hour conditioned media from all 6 fibroblasts. Both latent (predominantly)
and active forms of MMP-2 existed. This gelatinolytic activity of 72-kd MMP-2
was completely inhibited by incubating the gel with solutions containing 10-mmol/L
EDTA (not shown). The gelatinolytic activity of 92-kd MMP-9 was not detected
in any of these conditioned media.
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Figure 6. Zymograms. The gelatinolytic activity
of matrix metalloproteinase (MMP)-2 (A) and the caseinolytic activity of MMP-3
(B) were demonstrated in the serum-free conditioned medium secreted by human
corneal (C), limbal (L), conjunctival (J), pterygium head (H), pterygium body
(B), and subconjunctival tissue (S) fibroblasts. MW indicates molecular weight;
DMEM-ITS, Dulbecco-Eagle minimum essential medium containing insulin, transferrin,
and sodium selenite. See the "Materials and Methods" section for more details.
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The casein zymogram also disclosed a strong caseinolytic activity of
57-kd MMP-3 and a 70-kd proteinase in these conditioned media (Figure 6B and Figure 7A).
With respect to MMP-3, the enhanced activity was primarily produced by pterygium
head fibroblasts (Figure 6B), which
was significantly more than body fibroblasts. In contrast, both subconjunctival
fibroblasts and normal conjunctival fibroblasts produced little caseinolytic
activity of MMP-3 (Figure 7A). The
2 bands of MMP-3 with molecular weights close to each other might be of the
glycosylated and unglycosylated forms of MMP-3. For comparison, the caseinolytic
activity of a 70-kd proteinase band was produced in a similar amount by these
4 fibroblasts. Its identity might be a serine proteinase, because its caseinolytic
activity disappeared after treatment with 5-mmol/L phenylmethylsulfonyl fluoride,
one of the serine proteinase inhibitors (Figure 7B). In contrast, the caseinolytic activity of 57-kd MMP-3
withstood such a treatment (Figure 7B)
but disappeared after incubating the gel with the solution containing 10-mmol/L
EDTA, one of the metalloproteinase inhibitors (Figure 7C). These results also confirmed that the markedly increased
caseinolytic activity by pterygium head fibroblasts was indeed that of a metalloproteinase.
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Figure 7. Zymograms. The caseinolytic activities
of matrix metalloproteinase 3 (MMP-3) and a serine proteinase were demonstrated
in serum-free conditioned media secreted by human conjunctival (J, pterygium
head (H), pterygium body (B), and subconjunctival tissue (S) fibroblasts.
The activities were displayed under normal conditions (panel A), and with
an additional treatment of phenylmethylsulfonyl fluoride (PMSF) without (panel
B) or with EDTA (panel C). MW indicates molecular weight. See the "Materials
and Methods" section for more details.
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QUANTITATIVE COLLAGENASE ACTIVITY ASSAY FOR MMP-1
The collagenolytic activity of MMP-1 was investigated with a soluble,
telopeptide-free collagen extracted from rat skin and labeled with [3H]-acetic anhydride. We noted that such an activity was mainly present
in a latent form in the serum-free conditioned media of different fibroblasts
and could be activated by p-aminophenylmercuric acetate (APMA). As shown in Figure 8, the activity after activation produced
by pterygium head fibroblasts was 0.27 ± 0.01 U/mL, ie, 4.8-fold higher
than that produced by normal conjunctival fibroblasts (0.056 ± 0.007
U/mL; P<.005; n = 3). The activities produced
by pterygium body and subconjunctival fibroblasts were 0.04 ± 0.01
and 0.07 ± 0.002 U/mL, respectively, both of which were similar to
that of normal conjunctival fibroblasts (P>.05; n
= 3). This finding was consistent with the transcript and protein expression
of MMP-1, indicating that pterygium head fibroblasts produced more MMP-1 than
other fibroblasts.
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Figure 8. Collagenase activity. The collagenase
activity of matrix metalloproteinase 1 (MMP-1) was measured by a quantitative
collagenase assay in serum-free conditioned media secreted by human conjunctival
(J) and pterygium head (H), pterygium body (B), and subconjunctival tissue
(S) fibroblasts. One unit of enzyme digests 1 µg of collagen per minute
at 30°C. See the "Materials and Methods" section for more details. Data
are mean ± SD.
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COMMENT
Matrix metalloproteinases and their inhibitors, TIMPs, play a vital
role in connective tissue degradation and remodeling.10-11
In the human eye, studies of MMPs and TIMPs have been performed in the aqueous
humor,20-21 vitreous,22-23 keratoconus corneas,24-25
myopic sclera,26 and corneas during wound healing.27-28 These studies have focused primarily
on gelatinase A (MMP-2), gelatinase B (MMP-9), TIMP-1, and TIMP-2. Our study
concludes that under the same culture condition with respect to the cell passage,
seeding density, and presence or absence of FBS, the expression and activity
of MMP-1 and MMP-3, but not those of MMP-2 or MMP-9, are significantly increased
in pterygium head fibroblasts, while the expression of TIMP-1 and TIMP-2 is
not changed compared with fibroblasts from other regions of the pterygium
and from the normal bulbar conjunctiva. These data provide for the first time
direct evidence supporting the notion that invasion of the pterygium may in
part be facilitated by an unusual population of head fibroblasts. Although
uPA, a serine protease, is found to be responsible for tissue degradation
and tumor cell invasion,13 its expression is
not changed among normal conjunctival and pterygial head and body fibroblasts.
Up-regulation of MMP-1 and MMP-3 transcript and protein expression by
pterygium head fibroblasts was demonstrated by Northern hybridization (Figure 2 and Figure 3), ELISA (Figure 4),
and Western blot analysis (Figure 5),
respectively. For comparison, although transcripts of TIMP-1 was slightly
down-regulated (Figure 2), their
protein levels were not changed (Figure 5). Taken together, these data suggest that the ratio between MMPs
and TIMPs produced by pterygium head fibroblasts is higher than those produced
by other fibroblasts tested. This notion is further supported by a higher
caseinolytic activity of MMP-3 (Figure 6B
and Figure 7) and a significantly
higher collagenolytic activity of MMP-1 produced by pterygium head fibroblasts
(Figure 8). The nature of their
being a genuine MMP was further confirmed by enzyme inhibitors (Figure 7). Expression of MMP-1 by cultured fibroblasts is mediated
by activating an interleukin 1 autocrine feedback loop.29
Therefore, future studies are needed to address whether overexpression of
MMP-1 and MMP-3 by pterygium head fibroblasts is indeed a phenotype in vivo
or an artifact created by cell culture. Furthermore, it may be informative
to investigate whether pterygium head fibroblasts may have a defect leading
to the activation of this interleukin 1 autocrine feedback loop.
MMP-1, an interstitial collagenase, can degrade native fibrillar collagen
types I, II, III, IX, and XI.10-11,30
MMP-3, or stromelysin-1, has a broad substrate specificity that includes casein,
proteoglycans, fibronectin, elastin, laminin, as well as collagen types III,
IV, V, IX, and IX.9-11,31
Cooperative actions of MMP-1 and MMP-3 further augment the final proteolytic
action. Therefore, it is conceivable that overproduction of MMP-1 and MMP-3
relative to their TIMPs facilitates the invasion of head fibroblasts into
the cornea by degrading the basement membrane, the Bowman membrane, and the
superficial corneal stroma, ie, histopathological findings well recognized
in pterygia.7 Such higher expression of MMP-1
and MMP-3 with no change in TIMPs has also been proposed as the basis for
tumor cell metastasis,12 where cell invasion
is a common feature.32-33
Compared with head pterygium fibroblasts, body fibroblasts and subconjunctival
fibroblasts adjacent to the pterygium expressed gradual decreases of MMP-1
and MMP-3 protein levels (Figure 4 and Figure 5) and activities (Figure 7) without notable changes in TIMP-1
and TIMP-2. It has been reported that MMP-1 and TIMP-1 proteins are expressed
by human conjunctival fibroblasts in in vivo wound healing and in culture.34 Herein we noted that normal human conjunctival fibroblasts
express additional MMP-3 and TIMP-2. In addition, both MMP-1 and MMP-3 were
also expressed by cultured corneal fibroblasts, and the levels of their transcripts
and proteins were higher than those of conjunctival fibroblasts (Figure 2 and Figure 4). It has been reported that MMP-1 and MMP-3 proteins are
produced by explant cultures of human corneas35
and rabbit corneal fibroblasts,36 and that
MMP-1 and MMP-3 are not expressed by normal uninjured corneal fibroblasts
but are expressed during remodeling of corneal stroma wounds in rabbits.37 Because there exists a gradient of decreasing MMP-1
and MMP-3 expression from the head to the normal conjunctiva, resembling that
from the repairing corneal stroma,37 we speculate
that the primary abnormality of pterygium exists in the head region (also
see Figure 1). Because the level
of such expression by pterygium subconjunctival fibroblasts was similar to
that produced by normal conjunctival fibroblasts, we also speculate that the
mass built up in adjacent subconjunctival fibroblasts of a pterygium is a
secondary effect. Future studies are needed to understand how the pterygium
itself attracts and drags the surrounding normal conjunctiva into the well-known
"wing-shaped" growth during the process of corneal invasion.
Expression of MMP-2 has been reported in human,24, 35
rabbit,27, 36 and rat 28 corneal fibroblasts. Herein, we noted that the expression
of MMP-2 transcript and protein and an as yet unknown 70-kd serine proteinase
was not changed among different types of fibroblasts (Figure 2, Figure 3, and Figure 7). The finding the MMP-2 expression
was unaltered is consistent with the view that MMP-2 expression tends to be
constitutive and is thought to perform a surveillance function.27
This unique feature is due to the unusual promoter structure of MMP-2, which
does not have a TATA box or AP-1 elements commonly found and critical for
gene activation in the promoters of MMP-1, MMP-3, and other inducible MMPs.9 With participation of the constitutively expressed
serine proteinase and MMP-2, ie, gelatinase A, which can digest types IV,
V, and VII collagens and denatured fibrillar collagens,10-11
corneal invasion by pterygium head fibroblasts can be enhanced.
Expression of MMP-9 transcript and protein was not detected in all the
types of cultured fibroblasts tested (not shown). In rabbits, MMP-9 is expressed
by corneal stromal fibroblasts and epithelial cells after wounding.27 In rats, however, MMP-9 is expressed by migrating
basal epithelial cells but not by corneal fibroblasts.28
Therefore, our human data resemble those of rats but not those of rabbits.
Recently, a preliminary report showed that protein expression of MMP-2 and
MMP-9 is higher in pterygial tissues.38 The
discrepancy in MMP-9 expression of the latter finding may be explained by
their inclusion of epithelial cells in homogenized pterygium samples.
Damage to skin collagen and elastin, leading to the pathologic sign
of "elastotic degeneration," is the hallmark of long-term exposure to solar
UV irradiation in photo-aged skin.39 A similar
finding is also observed in the pterygium.40
Such damage in the dermis has long been regarded as a result of proteolytic
actions on the connective tissue extracellular matrix. This hypothesis has
been supported by experimental data showing that cultured normal dermal fibroblasts
increase expression of MMP-141-44
and MMP-344 after UV irradiation. Because normal
cultured dermal fibroblasts were used in these dermatological studies, overexpression
of MMP-1 and MMP-3 has been regarded as a phenotype extrinsically induced
by UV. Extending from these dermatological data, our report reveals for the
first time that such an altered phenotype is still retained in fibroblasts
grown out of the pterygium head and to a lesser extent out of the pterygium
body, suggesting that chronic UV irradiation may have induced an intrinsic
genotypic change. Future studies to explore the mechanism leading to such
intrinsic overexpression of MMP-1 and MMP-3 should allow us to map pathways
linking long-term UV exposure with the pterygium formation.
The histopathological feature of pterygium resembles that seen in corneal
diseases with limbal epithelial stem cell deficiency,45-46
leading one to speculate that limbal stem cells may have been destroyed in
pterygium. This hypothetical view has been suggested by theoretical calculation
of the albedo (indirect) light projected from the temporal sclera. This light
source is concentrated and focused at the nasal limbus.47
Supporting this view are the findings that pterygial-basal epithelial cells
expressed an oncogene, p53,48-49
and vimentin,50 which is normally found in
mesenchymal cells and migrating epithelial cells, and that pterygia frequently
coexist with conjunctival intraepithelial neoplasia or carcinoma.49 Recently, overexpression of MMP-1 and MMP-2 has been
noted in pterygial epithelial cells.51 This
new piece of data, along with those reported herein, suggests that both epithelial
cells and stromal fibroblasts may contribute to the dissolution of the basement
membrane and the Bowman layer. Future studies are also needed to discern the
relative contribution between pterygial epithelial cells and fibroblasts and
whether interactions between these 2 cell types may actually activate fibrovascular
invasion into the cornea.
Although not directly relevant to this work, we have previously reported
that expression of a smooth muscle actin, shown by Northern hybridization,
is higher in serum-free condition (DMEM + ITS) than in serum-containing medium
(DMEM + 10% FBS), and is dramatically up-regulated by transforming growth
factor- 1, but suppressed by amniotic membrane in cultured human corneal
and limbal fibroblasts.52 A similar finding
was also noted in cultured human conjunctival and pterygium body fibroblasts.53
AUTHOR INFORMATION
Accepted for publication June 22, 2000.
This work was supported in part by Public Health Service Research Grant
#EY 06819 (SCGT) from Department of Health and Human Services, National Eye
Institute, National Institutes of Health, Bethesda, Md, and in part by an
unrestricted grant from Research to Prevent Blindness, Inc, New York, NY.
Presented in part as an abstract at the annual meeting of the Association
for Research in Vision and Ophthalmology, May 10, 1999, Fort Lauderdale, Fla.
Corresponding author and reprints: Scheffer C. G. Tseng, MD, PhD,
Bascom Palmer Eye Institute, William L. McKnight Vision Research Center, 1638
NW 10th Ave, Miami, FL 33136 (e-mail: stseng{at}bpei.med.miami.edu).
From the Ocular Surface and Tear Center, Department of Ophthalmology,
Bascom Palmer Eye Institute (Drs Li, Lee, Solomon, Meller and Tseng), and
Departments of Medicine (Dr Gunja-Smith and Ms Liu) and Cell Biology and Anatomy
(Dr Tseng), University of Miami School of Medicine, Miami, Fla.
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