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From the W.M. Keck Center for Transgene Research and the Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana
| Abstract |
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| Introduction |
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Using a skin wound healing model and PAI-1-/- mice, we have investigated the effects of alterations in the expression of PAI-1 on skin wound healing processes. The results of this investigation are reported herein.
| Materials and Methods |
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The generation of mice homozygous for a total PAI-1 deficiency
(PAI-1-/-) has been described.8
PAI-1-/- mice were
back-crossed to at least the F7 generation in strain C57Bl/6J (
99%
C57Bl/6J background). The animals were housed in micro-isolation cages
on a constant 12 hours light/dark cycle with controlled temperature and
humidity and given access to food and water ad libitum.
C57Bl/6J animals (Jackson Laboratories, Bar Harbor, ME) were used as
wild-type (WT) controls. All mice used in these
studies were between 8 and 12 weeks of age and of mixed gender. All
animal experiments were performed in accordance with protocols approved
by the Institutional Animal Care and Use Committee.
Induction of the Skin Wound
Mice were anesthetized by intraperitoneal injection of rodent cocktail (0.015 mg xylazine/0.075 mg ketamine/0.0025 mg aceprozamine per gram body weight). The backs of the mice were shaved and sterilized with alcohol, followed by 1% iodine solution. A full thickness wound, approximately 8 mm in diameter, was made using a dermal biopsy punch, down, but not through, the muscle fascia. Mice were singly caged, without bedding, for the first several days until a provisional matrix had formed. Wound areas (width x length) were measured every other day. Mice were sacrificed at various time points during healing or at the time when the wound appeared closed (endpoint). At least three mice were sacrificed for each time point. The wounded tissues and the surrounding skin were carefully excised, pinned to a corkboard, and fixed flat in 10% neutral buffered formalin (NBF) for 3 hours before alcoholic dehydration and paraffinization. Wounds were bisected and embedded in paraffin. Microtomy was performed at 4 µm.
Histology, Histochemistry, and Immunohistochemistry
Sections were stained with hematoxylin and eosin (H&E) to examine general tissue and cellular morphology, and with Massons Trichrome for identification of collagen IV.9 The Ayoub-Shklar method was used for identification of keratin and prekeratin structures.10 The periodic acid-Schiff procedure (PAS) was used for identification of basement membranes.11
A number of immunohistochemical stains were performed.
Fibrin was identified with a polyclonal goat-anti-mouse fibrin(ogen)
antibody (Accurate Chemicals, Westbury, NY). Antigen retrieval was
performed under high temperature and pressure with citrate buffer
(BioGenex, San Ramon, CA), followed by endogenous peroxidase blocking
with Peroxoblock (Zymed, South San Francisco, CA). Subsequent to
incubation, first with rabbit serum and then with the primary antibody,
the slides were incubated with the secondary rabbit anti-goat IgG
(Dako, Carpinteria, CA), followed by goat peroxidase anti-peroxidase
(Dako). Peroxidase activity was detected with the substrate
3-amino-9-ethylcarbazole (AEC), (Biomeda, Foster City, CA). PAI-1
antigen was detected by citrate buffer antigen retrieval and a rabbit
anti-rat PAI-1 IgG (American Diagnostica, Greenwich, CT). Slides were
then incubated with a swine anti-rabbit biotin
F(ab)2 IgG, followed by streptavidin-HRP
(BioGenex) and AEC detection. A similar procedure was used for
detection of smooth muscle cell
-actin with an anti-
-actin
monoclonal antibody (Sigma Chemical Co., St Louis, MO) and vascular
endothelial growth factor (VEGF) with a mouse anti-human monoclonal
antibody (Oncogene Research Products, Boston, MA). CD45-positive
leukocytes were detected using citrate buffer antigen retrieval and a
biotinylated rat anti-mouse CD45 monoclonal antibody (PharMingen, San
Diego, CA). Following a streptavidin incubation (NEN, Boston, MA),
detection was performed with diaminobenzidine (DAB) (Dako). Urokinase
was identified using a rabbit anti-rodent antibody (American
Diagnostica), followed by biotin-conjugated swine anti-rabbit
F(ab)2 IgG and then streptavidin conjugated HRP.
AEC chromogen was used for visualization and antigen retrieval was
accomplished using Tris-HCl, pH 8.0. All immunohistochemical slides
were counterstained with hematoxylin (Biomeda).
| Results |
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After dermal incision, wound areas were measured every other
day for 15 days and expressed as percent wound remaining as a function
of time. The rate of wound healing was significantly increased in
PAI-1-/- mice relative to
WT mice, day 39 comparisons P < 0.05
(Figure 1)
. By day 7, approximately 90%
of the wound was closed in
PAI-1-/- mice compared to
50% in WT mice. Between days 11 and 13, the wound was
completely sealed in
PAI-1-/- mice while in
WT mice wound closure was still incomplete.
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At day 5, it was already evident that wound healing was
accelerated in PAI-1-/- mice
relative to WT mice. The newly formed neoepithelial layer
from the marginal edges of the wound were thickened and connected
underneath the provisional matrix in
PAI-1-/- mice (Figure 2a)
. This is in contrast to WT
mice, at this time interval, where the neoepithelial layer was still
associated with the incisional edges of the wound, marginal to the
matrix (Figure 2b)
. Additionally, weak PAI-1 immunoreactivity was
observed in WT wounds at the marginal edges and no
differences in uPA localization were observed in wounds from
PAI-1-/- and WT
mice (data not shown). While the underlying dermal layer in
PAI-1-/- mice was not yet
developed, organization of collagen fiber formation was initiated at
this time in PAI-1-/- mice,
with little evidence of this occurring in WT mice (Figure 2, c and d)
. A small amount of bleeding was still observed in
PAI-1-/- mice wounds in
unconnected areas under the matrix but was much more evident in
WT mice wounds where there was a larger open area of injury
(Figure 2, e and f)
. While wound-associated fibrin(ogen) was observed
to be diffuse in PAI-1-/-
animals, it was much more evident in wounds from WT mice
(Figure 3, a and b)
. This supports
observations made in PG-/-
mice and
PG-/-/FG-/-
double deletions, where it was demonstrated that a lack of Pm activity
resulted in diminished skin wound healing which was resolved with a
fibrinogen deficiency.3,4
Inflammation appeared to be much
more prominent in WT wounds at day 5 than in
PAI-1-/- wounds consistent
with the different stages of wound healing in these mice (Figure 3, c and d)
. VEGF, a growth factor known to be up-regulated during skin
wound healing,12
was evident in the granulation tissue in
WT mice wounds but very poorly expressed in
PAI-1-/- skin wounds.
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Healed wounds in
PAI-1-/- mice demonstrated a
thickened epidermal layer within the newly healed lesion that is not
seen in uninjured skin (Figure 4, a and b)
. There is also evidence of hair follicle structure development that
is represented by areas of invagination of the epidermis (Figure 4a)
.
The dermal layer contains diffuse deposition of collagen fibers that
are not organized as in uninjured skin (Figure 4, c and d)
. A thickened
keratinized layer is more evident above the epidermal layer of the
healed lesion relative to non-wounded skin (Figure 4, e and f)
.
Reticular fiber structures of a developing basement membrane are
unorganized and, for the most part, cellular in the healed lesion as
compared to that observed in non-wounded skin (Figure 4, g and h)
. The
finding of localized
-actin positive cells within the developing
basement membrane indicates that this region is fibroblast-rich (Figure 5,a and b)
. Additionally, the lack of
significant accumulation of fibrin(ogen) (Figure 5, c and d)
or
inflammatory cells (Figure 5, e and f)
within the lesion is indicative
of a resolved wound and is similar in appearance to nonwounded skin.
These findings are similar to those observed in healed WT
skin lesions.
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Wound healing is a multifactorial process involving 3 distinct yet interrelated phases that temporally overlap. The inflammatory phase, or exudative phase, is involved in removal of the damaged tissue and cleansing of the wound site. The proliferative phase involves formation of the granulation tissue which acts as a provisional replacement tissue. Finally, the differentiation or regeneration phase supports scar formation and reepithelialization of the wound site. Components of the fibrinolytic system have been implicated in a number of the events associated with wound healing and direct studies in PG-/- mice have indicated a significantly attenuated rate of wound healing which is resolved in a fibrinogen deficient state. While wound healing is impaired in PG-/- mice, it eventually occurs, indicating the involvement of other proteolytic pathways in these events. Additional studies support a functional overlap between the fibrinolytic system and the MMP family of matrix degrading proteases in skin wound healing processes13 and have shown that inhibition of both pathways leads to a complete arrest of wound healing and closure. Therefore, imbalances in either proteolytic system could manifest itself in altered wound healing events.
An important physiological regulator for the generation of plasmin is PAI-1. While PAI-1 is not normally expressed by keratinocytes in the epidermis, it has been shown to be increased in expression following in vitro and in vivo wound injury1,14 and localized to the hyperproliferative migrating zone of epithelial cells. Clinically, it has been demonstrated that elevated PAI-1 levels occur in diabetic patients who also suffer from delayed wound healing responses resulting in skin ulcerations and other serious clinical complications.15,16 The current study supports a role for PAI-1 in wound healing and indicates that a loss of PAI-1 function results in accelerated wound healing. Interestingly, as observed in other models in our laboratory7,17, PAI-1 expression appears to regulate VEGF expression. Further studies to characterize the mechanisms associated with this observation are ongoing.
PAI-2 is also expressed in the epidermis and could play a role in skin wound healing.18 Surprisingly, a wound healing model using PAI-2-/- and PAI-2-/-/PAI-1-/- double deficient mice failed to show an effect on the healing process.19 PAI-2 is a member of the ovalbumin family of serpins and it has been demonstrated that mice produce a much larger array of these serpins relative to humans.20 Perhaps a loss of PAI-2 expression is compensated for by increased expression of one or more of these other inhibitors. Indeed, the ovalbumin serpin, proteinase inhibitor 6, PI-6, is up-regulated during keratinocyte differentiation21 and hurpin, proteinase inhibitor 13 (PI13), is up-regulated in keratinocytes during psoriasis.22
Additional studies using artificially wounded keratinocytes, in cell culture, have indicated that healing events are impaired after transfection with an antisense PAI-1 vector.2 However, these studies were performed on human transformed cells and with a single population of cell type, keratinocytes. Normal wound healing involves a number of different cells, i.e., leukocytes, endothelial cells, and fibroblasts.
PAI-1, through its ability to alter the fibrinolytic environment,
could manifest its effects during wound healing by temporally
regulating the extent of the fibrin-rich provisional matrix allowing
for fibroblast migration into the wound and eventual replacement of
fibrin with collagen. Other studies have indicated that vitronectin
receptor and plasminogen activators are increased in expression in
migrating cells during wound healing.1,23-25
PAI-1 has
been shown to inhibit cell migration by blocking the vitronectin
receptor,
Vß3, interaction with vitronectin.26
PAI-1
interaction with plasminogen activators, in this setting, results in a
loss of affinity of PAI-1 for vitronectin, which restores cell
migration. In this context, PAI-I exerts its effects by regulating cell
adhesion and detachment during cell migratory events of wound healing.
A role for vitronectin in wound healing was confirmed using a skin
wound healing model in vitronectin deficient animals in which it was
demonstrated that healing was delayed.27
This would
indicate that vitronectin/receptor interactions facilitate cell
migration during wound healing and that blocking that interaction,
through PAI-I, could alter the healing process.
| Acknowledgements |
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| Footnotes |
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Supported in part by an American Heart Association Midwest Postdoctoral Fellowship (to J. C. Y. C.), National Institutes of Health grants HL-13423 (to F. J. C.) and HL-63682 (to V. A. P.), a grant from the W. M. Keck Foundation (to F. J. C.), and by the Kleiderer-Pezold Family Endowed Professorship (to F. J. C.).
Accepted for publication July 23, 2001.
| References |
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