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Short Communications |



From the Departments of Pathology and Laboratory
Medicine *
and
Otorhinolaryngology,
University of
Pennsylvania, Philadelphia, Pennsylvania; the Vision Research
Laboratories,
New England Eye Center, Tufts
University School of Medicine, Boston, Massachusetts; and the
Departments of Biology and Pathology,§
Institute for Molecular Biology and Biotechnology, McMaster University,
Hamilton, Ontario, Canada
| Abstract |
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| Introduction |
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Matrix metalloproteinase 9 (MMP-9) is a member of the family of matrix metalloproteinases.5 Like other metalloproteinases, MMP-9 has a catalytic region that includes a putative metal binding domain. It is secreted as a latent proenzyme and is frequently isolated in a complex with its natural inhibitor, tissue inhibitor of metalloproteinase-1 (TIMP-1). An inhibitory propeptide must be cleaved to release an active enzyme. There is some evidence that the secreted molecule can bind to the surface of cells via CD44 or a chain of Type IV collagen.6,7 This binding may be critical for its biological action. MMP-9 has been shown to be required for metastasis by both murine prostate carcinoma cells and transformed rat embryo fibroblasts.8,9
MMP-9 is induced in tissue culture cells by a variety of cytokines and
oncogenes.10
This induction is strongly based on
transcriptional mechanisms. The 5' upstream region of MMP-9 has been
cloned from the human, rabbit, and murine genome and in each case
directs transcription both constitutively and after induction by
TPA or other cytokines.10-13
The MMP-9 region
upstream from each of these species is highly similar with identical
transcriptional factor binding consensus sites including a proximal
activated protein-1 (AP-1) site that is necessary, but not
sufficient, for consitituitive expression or for TPA or tumor necrosis
factor-
(TNF-
) induction.11
This element is
also required for v-src-activated transcription, but, surprisingly, is
not required for transcription in SK-N-SH
cells.14,15
There is a GT-rich region that also makes a
significant contribution to transcriptional control up-regulated by
src, ras, or in SK-N-SH cells.11,14-19
The MMP-9 promoter
from each species also contains a more distal activator
protein-1 (AP-1) site, a nuclear factor
B (NF
B), an ets,
and SP-1 recognition sites, all of which contribute to the activity of
this promoter and its stimulation by the ras oncogene or TPA. The
NF
B site is required for stimulation of MMP-9 transcription by
cytokines in fibroblasts.20
The ets site was also required
for maximal stimulation by ras, TNF, cell contact, and epidermal
growth factor (EGF).11,16-19
Fini et al have
identified activator protein-2 (AP-2) as a critical motif in the rat
promoter for induction in wound healing.13
The MMP-9 promoter linked to the reporter gene ß-galactosidase has been used to generate transgenic mice.12,21 Fini et al used a 510-bp upstream fragment from the rabbit promoter to generate such a mouse. The promoter fragment they used was sufficient to drive ß-galactosidase expression during development, with strong staining at the endochondrial plates during bone development and in the ventricular lining and developing neuroblasts of the central nervous system in a temporal pattern similar to that seen with in situ hybridization. This transgenic mouse also showed the expected pattern of MMP-9 expression during wound healing, with the migrating epithelium showing significant ß-galactosidase expression.21 This fragment of the promoter may not be sufficient to direct all expression during development. Munaut et al12 found that the murine region homologous to that of the rabbit used by Fini et al failed to stimulate ß-galactosidase expression in alveolar macrophages, cells of which a subset usually show very strong MMP-9 expression. Munaut et al also failed to see expression in osteoclasts. A 2.8-kb upstream fragment, however, led to expression in alveolar macrophages and osteoclasts. Thus, the rabbit promoter recapitulates much, but perhaps not all, of the expression expected to be directed by an MMP-9 promoter.
| Materials and Methods |
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Mice bearing the rabbit MMP-9 promoter (from -522 to +12) linked to the reporter ß-galactosidase were developed and described by Fini et al (derived from founder 3445).21 This founder had identical patterns of expession to a second founder, but overall levels of expression were higher. Hence, we chose to use this strain for these experiments.21 Mice bearing the transgene linking the promoter from the mouse mammary tumor virus to the oncogene polyoma virus middle T antigen were developed and characterized by Mullers lab.1-3 Female mice homozygous for the rabbit MMP-9 promoter (from -522 to +12) linked to the reporter ß-galactosidase were mated to males homozygous for murine mammary tumor virus (MMTV) polyoma middle T antigen. All of the resultant offspring were expected to carry both transgenes. Presence of the ß-galactosidase transgene was confirmed in all offspring by polymerase chain reaction of DNA extracted from the tails of 2-week-old mice. (Primers were 5'ACTCGGCGTTTCATCTGTGG and 5' AGCGACATCCAGAGGCACTT, which yield a characteristic 1579-bp band.) All animals were sacrificed as soon as or before they developed tumors 3 cm in diameter.
Mice with the MMP-9 promoter linked to the ß-galactosidase reporter were maintained by mating the homozygous mice to each other. Presence of the transgene was confirmed by polymerase chain reaction analysis for ß-galactosidase and was found in all offspring as expected.
Carcinogenesis
Skin carcinogenesis by application of initiator and promoter was performed on 7 mice with the MMP-9 ß-galactosidase transgene.22 Fifty micrograms of 7,12-dimethylbenz[a]anthracene (DMBA) in 50 µl acetone were applied to the shaved skin of each 4- to 8-week-old mouse. Phorbol 12 myristate 13-acetate (TPA) (20 nmol) in 200 µl acetone was then applied after 10 days and twice weekly for 20 to 25 weeks. One animal died from no apparent cause without any visible tumors. Multiple papillomas were observed on 3 mice after 8 to 12 weeks and a carcinoma was observed in a fourth mouse. The remaining 2 animals had no visible tumors. Animals were sacrificed at 16 weeks.
Tissue Evaluation
After sacrifice, mice were immediately dissected. Tissues were harvested and frozen in OCT (Fisher, Trenton, NJ) on dry ice. Frozen sections (10 µm) were cut on a cryostat. ß-Galactosidase was visualized using the staining kit and protocol from Specialty Media (Phillipsburg, NJ). In brief, air-dried sections were washed in phosphate buffered saline including Ca2+ and Mg2+, and fixed in ice-cold 2% (v/v) formaldehyde and 0.2% (v/v) glutaraldehyde for 1 hour on ice. After washing, X-gal solution was placed on the section at 37°C in the dark overnight. Sections were then fixed in 10% formalin on ice for 10 minutes, washed, and counterstained in 2% hematoxylin for approximately 10 seconds. After washing in H2O, the sections were mounted in Permount. Skin sections were included in each batch for positive controls.
| Results |
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Female mice homozygous for the MMP-9 promoter ß-galactosidase
transgene were mated to male mice bearing the MMTV polyoma middle T
trangene. Mice resulting from this cross were sacrificed at 2 weeks of
age or after development of tumors from 1 to 3 cm in diameter. Tissues
were frozen and stained for ß-galactosidase activity using an
X-gal-based method that results in a light blue stain. Females have
hyperplasia of the mammary ducts at birth. Despite extensive
hyperplasia, no ß-galactosidase was evident in these ducts or the
adjacent fat and stromal tissue (Table 1
and Figure 1B
). Male mice at birth have
histologically normal ducts; these were also negative for
ß-galactosidase (Table 1
and Figure 1A
). The tumors were tested for
ß-galactosidase activity. Only one carcinoma in situ
without any evidence of invasion was found. There was no staining in
this tumor. In addition, 12 adenocarcinomas were sectioned. Three of
these tumors had distinct areas of intraductal carcinoma as well as
areas with invasive carcinoma. These tumors had histologies similar to
those previously described for MMTV-polyoma middle T-induced breast
carcinomas.2
The areas of intraductal carcinoma or
carcinoma in situ had no staining (Figure 1C)
. In all 12,
the areas of invasive tumor had ß-galactosidase staining. Only a
fraction of the cells in the invasive areas were positive; not all
tumor cells were positive. Thus, early neoplastic lesions of the
breast, hyperplasia, and carcinoma in situ did not show
evidence of activation of the MMP-9 promoter, but invasive lesions all
contained some cells with MMP-9 promoter activity (Figure 1D)
.
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Skin Carcinogenesis
To extend these observations indicating that the MMP-9 promoter is
activated during the stages of tumor progression associated with
invasion, we examined the expression of ß-galactosidase in skin
tumors induced by intitiation-promotion with DMBA and TPA in mice
bearing the MMP-9 ß-galactosidase transgene. Skin from an untreated
area is shown in Figure 1E
. The epithelium and subcutaneous tissue were
negative. Cells at the neck of the duct connecting the sebaceous glands
associated with hair follicles to the hair shaft consistently stained.
Skin from mice treated with TPA was also negative, with the exception
of cells in the sebaceous glands (not shown). Seven papillomas were
examined. Four showed dysplasia suggestive of carcinoma in
situ. None of these papillomas or carcinomas in situ
had ß-galactosidase staining (Figure 1, F and G
, and Table 2
). Only 1 animal developed invasive
carcinoma. This carcinoma had extensive staining, but again, only some,
not all, of the tumor cells were positive (Figure 1H)
.
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| Discussion |
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In this study, we used two murine models for tumor progression. One, a model of squamous cell carcinoma of the skin based on carcinogenesis, results in activity of the MMP-9 promoter at the stage when invasive carcinoma can first be seen. Squamous cell carcinomas of the skin as well as at other sites in patients can be shown to express MMP-9 mRNA by in situ hybridization.23-25 MMP-9 expression is also correlated with invasion and poorer prognosis in squamous cell carcinoma.26-28 Its expression tends to be elevated at the invasive fronts, and this can be modeled in three-dimensional tissue culture.29 In melanoma, a tumor type with a clear histological distinction between different stages of tumor progression, cells from vertical growth phase melanomas, the most invasive and aggressive stage of melanoma, express MMP-9 in culture and after stimulation with transforming growth factor-ß, but cells from the earlier stage radial growth phase do not.30 We also used a model of breast cancer. The literature is more contradictory regarding expression patterns of MMP-9 in human breast carcinoma. Although some studies find increased expression in breast carcinonomas, others have failed to find any difference.31-38 However, in several instances, manipulations that led to increased invasiveness or metastasis by breast carcinoma cells were associated with increased expression of MMP-9.39-42 Studies in patient material tend to be difficult because immunohistochemical identification of secreted proteins may not be reliable and the sensitivity of in situ hybridization is variable. Nonetheless, the increase in promoter activity found here may not translate into increased MMP-9 in actual tumors. These experiments would suggest that there is a consistent change in transcriptional activity of an MMP-9 promoter during tumor progression.
The upstream elements of the MMP-9 promoter include two AP-1 sites, one
adjacent to the MMP-9 transcript start site and the other more distal.
In addition to AP-1, the MMP-9 promoters all contain AP-2, ets, NF
B,
and Sp-1 recognition consensus elements. Each of these elements adds
incrementally to MMP-9 expression in cells constitutively expressing
MMP-9.11,14,17,18
There is some evidence of the potential
involvement of many of these transcription factors in tumor
progression. Saez et al found that treatment of the skin of mice
genetically deficient in c-fos with an initiator-promoter
protocol for induction of skin carcinogenesis resulted in papilloma
formation that failed to progress to invasion or
carcinoma.43
Since AP-1 is formed by heterodimers of
fos and jun or jun homodimers, the
absence of c-fos would disrupt many AP-1 complexes. This
result is more complicated, however, because transgenic mice
overexpressing a dominant negative inhibitor of jun in the
skin fail to develop even papillomas after initiation and
promotion.44
Ets-1, one of a large family of proteins
binding to the ets consensus binding site, has been associated with
transformation and is required for transformation. Mice with a single
targeted deficient allele of ets-2 show decreased progression in
MMTV-polyoma middle T-induced tumors.45
PEA-3,
another transcription factor that binds to the ets consensus binding
site, has been shown by Trimble et al to be overexpressed in the
mammary carcinomas that result from expression of the MMTV-polyoma
middle T transgene, making it an attractive candidate for involvement
in the induction of MMP-9 promoter activity during tumor
progression.46
There are several reports implicating
enhanced NF
B expression in carcinomas including breast carcinoma and
squamous cell carcinoma of the skin.47,48
In contrast,
there is evidence that AP-2 is down-regulated during tumor progression
in melanaoma and that the presence of AP-2 activity facilitates the
expression of E-cadherin, a gene whose down-regulation is frequently
required for invasion and metastasis. Furthermore, down-regulation of
E-cadherin results in up-regulation of MMP-9 in squamous carcinoma cell
lines.49
Interestingly, up-regulation of AP-2 has been
suggested to be the critical factor regulating transcriptional activity
of MMP-9 during wound healing in the cornea.21
Whether any
of these transcriptional elements are activated during tumor
progression and whether their enhanced activity leads to MMP-9
expression remain to be determined.
In addition to the elements indicated above, additional upstream regions have important roles in regulation of MMP-9 activity. The region we have used, 510 bp upstream from the start site of the rabbit gene, does not direct expression in alveolar macrophages, cells that frequently express MMP-9. Munaut et al obtained expression in macrophages with a 2.8kb upstream fragment, but not with a smaller fragment homologous to the promoter used here.12 This smaller fragment directs other normal expression, in macrophages in the spleen, during wound healing in murine corneas, and in the bone, cartilage, and nervous system during development.21
We have also observed expression in the cells at the junction of the sebaceous gland and the hair shaft in the hair follicles, recalling the interesting association between decreased MMP-9 expression and aberrant hair formation in mice engineered to have a genetic deficiency of ets-2.50
In this study, we found that the smaller fragment was activated coincident with tumors developing the ability to invade. This finding indicates that there is an alteration in the transcriptional regulation of cells as they become invasive and that the shorter MMP-9 promoter can monitor this change. Whether other elements further upstream will alter this expression pattern remains to be determined. Nonetheless, the requirement of MMP-9 for metastasis in other systems is consistent with the temporal pattern of MMP-9 expression during tumor progression.
| Footnotes |
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Supported by National Institutes of Health grant RO1 NCI CA-46830 (to R. J. M). W. J. M. is supported by a Medical Research Council of Canada Scientist Award.
Accepted for publication September 8, 2000.
| References |
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