Matrix metalloproteinases: they're not just for matrix anymore!

The matrix metalloproteinases (MMPs) have been viewed as bulldozers, destroying the extracellular matrix to permit normal remodeling and contribute to pathological tissue destruction and tumor cell invasion. More recently, the identification of specific matrix and non-matrix substrates for MMPs and the elucidation of the biological consequence of cleavage indicates that perhaps MMPs should be viewed more as pruning shears, playing sophisticated roles in modulating normal cellular behavior, cell-cell communication and tumor progression.

Growth factors regulate transin gene expression by c-fos-dependent and c-fos-independent pathways.

The rapid induction of the proto-oncogene c-fos by growth factors and other bioactive agents, and the recent evidence that the c-fos protein (Fos) is associated with transcriptional complexes, suggests that Fos may represent an integral part of an intracellular messenger pathway that triggers changes in gene expression and ultimately phenotypic alterations. This report examines the role of c-fos in growth factor stimulation of transin, a matrix-degrading secreted metalloproteinase. Platelet-derived growth factor (PDGF) stimulation of transin RNA was blocked by a selective reduction in Fos synthesis with antisense c-fos mRNA, whereas epidermal growth factor (EGF) stimulation of transin occurred despite an equivalent inhibition of Fos levels. The stimulatory effect of both PDGF and EGF on transin transcription involved factors recognizing the sequence TGAGTCA, which is found in the transin promoter and is reported to be a binding site for the transcriptional factor Jun/AP-1 and for associated Fos and Fos-related complexes. Thus both Fos-dependent and Fos-independent pathways exist for growth factor regulation of gene expression, and both effects may be mediated through the same cis-acting transcription element.

Regulation of intestinal alpha-defensin activation by the metalloproteinase matrilysin in innate host defense.

Precursors of alpha-defensin peptides require activation for bactericidal activity. In mouse small intestine, matrilysin colocalized with alpha-defensins (cryptdins) in Paneth cell granules, and in vitro it cleaved the pro segment from cryptdin precursors. Matrilysin-deficient (MAT-/-) mice lacked mature cryptdins and accumulated precursor molecules. Intestinal peptide preparations from MAT-/- mice had decreased antimicrobial activity. Orally administered bacteria survived in greater numbers and were more virulent in MAT-/- mice than in MAT+/+ mice. Thus, matrilysin functions in intestinal mucosal defense by regulating the activity of defensins, which may be a common role for this metalloproteinase in its numerous epithelial sites of expression.

Cancer biology: extracellular proteinases in malignancy.

Secreted, matrix-degrading proteinases have been viewed as contributing to tumor metastasis. A recent study indicates that the gene for one of these enzymes, the matrix metalloproteinase stromelysin-1, can actually cause cancer when expressed in transgenic mice.

Tumor progression: defining the soil round the tumor seed.

The tumor microenvironment, or stroma, is known to contribute to tumor progression. Two recent studies have shown that the stromal protein matrix metalloproteinase MMP-9 has a role in the early stages of tumor growth and angiogenesis.

The metalloproteinase matrilysin proteolytically generates active soluble Fas ligand and potentiates epithelial cell apoptosis.

BACKGROUND: The Fas ligand/Fas receptor (FasL/Fas) system is an important mediator of apoptosis in the immune system where the juxtaposition of cells expressing the cell-surface ligand induces the apoptotic pathway in Fas-expressing lymphocytes. The FasL/Fas system has also been shown to be involved in apoptosis in epithelial tissues, including the involuting rodent prostate. FasL can be shed through the action of an hitherto unidentified metalloproteinase to yield soluble FasL (sFasL), although the biological activity of sFasL has been disputed. RESULTS: Here we report that the matrix metalloproteinase matrilysin can process recombinant and cell-associated FasL to sFasL, and that matrilysin-generated sFasL was effective at inducing apoptosis in a target epithelial cell population. In the involuting mouse prostate, FasL and matrilysin colocalized to the cell surface in a restricted population of epithelial cells. Mice deficient in matrilysin demonstrated a 67% reduction in the apoptotic index in the involuting prostate compared with wild-type animals, implicating matrilysin in this FasL-mediated process. CONCLUSIONS: The results show that a functional form of sFasL was generated by the action of the metalloproteinase matrilysin, and suggest that matrilysin cleavage of FasL is an important mediator of epithelial cell apoptosis.

Metalloproteinases and their inhibitors in matrix remodeling.

The matrix metalloproteinases are a tightly regulated family of enzymes that degrade extracellular matrix and basement membrane components. Recent evidence suggests that these proteases and their specific inhibitors play important roles in normal developmental processes and in pathological conditions. Interestingly, experiments designed to improve our understanding of metalloproteinase regulation have also resulted in new insights into mechanisms by which growth factors and proto-oncogenes may regulate biological processes.

Suppression of matrix metalloproteinases inhibits establishment of ectopic lesions by human endometrium in nude mice.

Matrix metalloproteinases of the stromelysin family are expressed in the human endometrium as a consequence of cellular events during the menstrual cycle that require extracellular matrix remodeling. We have recently documented the presence of these enzymes in lesions of endometriosis, a benign disease that presents as persistent ectopic sites of endometrial tissue, usually within the peritoneal cavity. Endometriosis can develop after retrograde menstruation of endometrial tissue fragments, and establishment of ectopic sites within the peritoneal cavity requires breakdown of extracellular matrix. To examine whether matrix metalloproteinases might contribute to the steroid-dependent epidemiology and cellular pathophysiology of endometriosis, we have developed an experimental model of endometriosis using athymic nude mice as recipients of human endometrial tissue. Our results demonstrate that estrogen treatment of human endometrial tissue in organ culture maintains secretion of matrix metalloproteinases, and promotes establishment of ectopic peritoneal lesions when injected into recipient animals. In contrast, suppressing metalloproteinase secretion in vitro with progesterone treatment, or blocking enzyme activity with a natural inhibitor of metalloproteinases, inhibits the formation of ectopic lesions in this experimental model.

Matrix metalloproteinase-3-dependent generation of a macrophage chemoattractant in a model of herniated disc resorption.

Herniated disc (HD) is a common health problem that is resolved by surgery unless spontaneous resorption occurs. HD tissue contains abundant macrophage infiltration and high levels of matrix metalloproteinases (MMPs) MMP-3 and MMP-7. We developed a model system in which disc tissue or isolated chondrocytes from wild-type or MMP-null mice were cocultured with peritoneal macrophages and used this system to investigate the role of MMPs and chondrocyte/macrophage interactions in disc resorption. We observed a marked enhancement of MMP-3 protein and mRNA in chondrocytes after exposure to macrophages. Chondrocytic MMP-3, but not MMP-7, was required for disc resorption, as determined by assaying for a reduction in wet weight and proteoglycan content after 3 days of coculture. Surprisingly, chondrocyte MMP-3 was required for the generation of a macrophage chemoattractant and the subsequent infiltration of the disc tissue by proteolytically active macrophages. We conclude that macrophage induction of chondrocyte MMP-3 plays a major role in disc resorption by mechanisms that include the generation of a bioactive macrophage chemoattractant.

Matrix metalloproteinase-7-dependent release of tumor necrosis factor-alpha in a model of herniated disc resorption.

Herniated disc (HD), one of the major causes of low back pain, is often resolved spontaneously without surgical intervention. Resorption is associated with a marked increase in infiltrating macrophages, and the matrix metalloproteinases (MMP) MMP-3 and MMP-7 have been implicated in this phenomenon. We developed a murine organ culture model in which intact intervertebral discs were cocultured with peritoneal macrophages to investigate the role of MMPs in HD resorption. Using macrophages isolated from MMP-null mice, we report that macrophage-produced MMP-7 was required for proteoglycan degradation, loss of wet weight, and macrophage infiltration of cocultured discs. The inability of MMP-7-deficient macrophages to infiltrate discs could not be attributed to a defect in macrophage migration. MMP-7 was required for the release of the cytokine TNF-alpha from peritoneal macrophages. The generation of soluble TNF-alpha was essential for the induction of MMP-3 in disc cocultures, which in turn is required for the generation of a macrophage chemoattractant and subsequent macrophage infiltration. TNF-alpha release from macrophages was necessary but insufficient for disc resorption, which required macrophage infiltration. We conclude that there is extensive communication between macrophages and chondrocytes in HD resorption and that an essential component of this communication is the requirement for MMPs to release soluble bioactive factors.

Patterns of matrix metalloproteinase expression in cycling endometrium imply differential functions and regulation by steroid hormones.

Matrix metalloproteinases are a highly regulated family of enzymes, that together can degrade most components of the extracellular matrix. These proteins are active in normal and pathological processes involving tissue remodeling; however, their sites of synthesis and specific roles are poorly understood. Using in situ hybridization, we determined cellular distributions of matrix metalloproteinases and tissue inhibitor of metalloproteinase-1, an inhibitor of matrix metalloproteinases, in endometrium during the reproductive cycle. The mRNAs for all the metalloproteinases were detected in menstrual endometrium, but with different tissue distributions. The mRNA for matrilysin was localized to epithelium, while the others were detected in stromal cells. Only the transcripts for the 72-kD gelatinase and tissue inhibitor of metalloproteinases-1 were detected throughout the cycle. Transcripts for stromelysin-2 and the 92-kD gelatinase were only detected in late secretory and menstrual endometrium, while those for matrilysin, the 72-kD gelatinase, and stromelysin-3 were also consistently detected in proliferative endometrium. These data indicate that matrix metalloproteinases are expressed in cell-type, tissue, and reproductive cycle-specific patterns, consistent with regulation by steroid hormones, and with specific roles in the complex tissue growth and remodeling processes occurring in the endometrium during the reproductive cycle.

Matrilysin expression and function in airway epithelium.

We report that matrilysin, a matrix metalloproteinase, is constitutively expressed in the epithelium of peribronchial glands and conducting airways in normal lung. Matrilysin expression was increased in airway epithelial cells and was induced in alveolar type II cells in cystic fibrosis. Other metalloproteinases (collagenase-1, stromelysin-1, and 92-kD gelatinase) were not produced by normal or injured lung epithelium. These observations suggest that matrilysin functions in injury-mediated responses of the lung. Indeed, matrilysin expression was increased in migrating airway epithelial cells in wounded human and mouse trachea. In human tissue, epithelial migration was reduced by > 80% by a hydroxamate inhibitor, and in mouse tissue, reepithelialization in trachea from matrilysin-null mice was essentially blocked. In vivo observations and cell culture studies demonstrated that matrilysin was secreted lumenally by lung epithelium, but upon activation or while migrating over wounds, some matrilysin was released basally. The constitutive production of matrilysin in conducting airways, its upregulation after injury, its induction by alveolar epithelium, and its release into both lumenal and matrix compartments suggest that this metalloproteinase serves multiple functions in intact and injured lung, one of which is to facilitate reepithelialization.

Epidermal growth factor stimulation of stromelysin mRNA in rat fibroblasts requires induction of proto-oncogenes c-fos and c-jun and activation of protein kinase C.

Stromelysin (transin) is a secreted metalloprotease that is transcriptionally induced by a variety of growth factors and oncogenes. We examined the necessity of specific secondary (protein kinase C) and tertiary (c-fos and c-jun protein products) messengers in the transactivation of stromelysin gene expression by epidermal growth factor (EGF). Rat-1 fibroblasts exposed to antisense c-fos DNA or RNA demonstrated that c-fos expression was necessary for complete EGF induction of stromelysin expression. Similar results demonstrating the necessity of c-jun protein in the EGF induction of stromelysin were obtained. We also demonstrated that protein kinase C activation is required for the EGF induction of stromelysin, since phorbol ester desensitization of C kinase proteins abolished the ability of EGF to induce stromelysin mRNA, protein, and promoter activity. In reconstitution experiments, neither c-fos, c-jun, nor C kinase activation alone induced significant stromelysin expression. Overexpression of c-fos and c-jun was able to induce stromelysin to a level similar to that of the growth factor, and stimulation of protein kinase C activity augmented this induction. The data suggest that the EGF induction of stromelysin in rat fibroblasts procedes through a pathway involving c-fos, c-jun, and protein kinase C.

Loss of JunB activity enhances stromelysin 1 expression in a model of the epithelial-to-mesenchymal transition of mouse skin tumors.

Chemical carcinogenesis in mouse skin has been useful in delineating the molecular events that underlie squamous cell carcinoma progression. A late event in this progression, the epithelial-to-mesenchymal transition (EMT), is characterized by the loss of epithelial markers and the presence of mesenchymal markers. One mesenchymal marker associated with this transition is the matrix metalloproteinase stromelysin 1 (Str-1). To examine the molecular mechanisms regulating the expression of Str-1 during the EMT, genetically related mouse skin tumor cell lines representing the epithelial (B9(SQ)) and mesenchymal (A5(SP)) phenotypes were studied. As expected, B9(SQ) cells did not make Str-1, while A5(SP) cells did. B9(SQ)-A5(SP) somatic hybrids did not make Str-1, suggesting that a critical regulatory factor was a B9(SQ)-specific repressor. Str-1 promoter analysis revealed that a canonical AP-1 site was sufficient to maintain differential reporter gene activity. This result correlated with the observed loss of binding of the transcriptionally inactive JunB-Fra-2 AP-1 complex from B9(SQ) cells, being replaced primarily by the more active JunD-Fra-2 complex in A5(SP) cells. The higher level of JunB binding to both DNA and Fra-2 correlated with its hyperphosphorylation by Jun N-terminal kinase, an activity that was significantly higher in B9(SQ) cells. In the somatic hybrids, JunB gene expression was highly upregulated, a condition that also was sufficient to repress the expression of the endogenous Str-1 gene in A5(SP) cells. These data suggested that alterations in JunB activity, by changes in either phosphorylation or gene expression, contributed to the phenotypic differences that occur in this model of the EMT.

Isolation of the oncogene and epidermal growth factor-induced transin gene: complex control in rat fibroblasts.

Various oncogenes or epidermal growth factor (EGF) induce transcription of a 1.9-kilobase RNA (transin RNA) in rat fibroblasts. The induction by EGF can be blocked by cycloheximide. Thus the response of the transin gene to EGF appears to require de novo protein synthesis. Transin RNA induction is specific to EGF, as neither insulin, platelet-derived growth factor, fibroblast growth factor, nor transforming growth factor beta could elicit the same response. However, transforming growth factor beta could block the EGF induction of transin RNA. Whereas the calcium ionophore A23187 and the tumor promoter TPA, either alone or administered together, did not increase transin RNA levels, TPA could synergise with a serum factor to effect such an increase. Dibutyryl cyclic AMP also induced transin RNA. Treatment of cells with the microfilament-disrupting agent cytochalasin B, but not the microtubule-disrupting agent colcemid, resulted in an increase in transin RNA levels, suggesting a role for the cytoskeleton in control of transin gene expression. The transin RNA does not contain repeated sequences and appears to be encoded by a single-copy gene. The protein sequence encoded by the last four exons of the transin gene shows some homology to two regions of the heme-binding protein hemopexin.

The PEA3 subfamily of Ets transcription factors synergizes with beta-catenin-LEF-1 to activate matrilysin transcription in intestinal tumors.

The matrix metalloproteinase matrilysin (MMP-7) is expressed in the tumor cells of a majority of mouse intestinal and human colonic adenomas. We showed previously that matrilysin is a target gene of beta-catenin-Tcf, the transcription factor complex whose activity is thought to play a crucial role in the initiation of intestinal tumorigenesis. Here we report that overexpression of a stable mutant form of beta-catenin alone was not sufficient to effect expression of luciferase from a matrilysin promoter-luciferase reporter plasmid. However, cotransfection of the reporter with an expression vector encoding the PEA3 Ets transcription factor, or its close relatives ER81 and ERM, increased luciferase expression and rendered the promoter responsive to beta-catenin-LEF-1 as well as to the AP-1 protein c-Jun. Other Ets proteins could not substitute for the PEA3 subfamily. Luciferase activity was induced up to 250-fold when PEA3, c-Jun, beta-catenin, and LEF-1 were coexpressed. This combination of transcription factors was also sufficient to induce expression of the endogenous matrilysin gene. Furthermore, all matrilysin-expressing benign intestinal tumors of the Min mouse expressed a member of the PEA3 subfamily, as did all human colon tumor cell lines examined. These data suggest that the expression of members of the PEA3 subfamily, in conjunction with the accumulation of beta-catenin in these tumors, leads to coordinate upregulation of matrilysin gene transcription, contributing to gastrointestinal tumorigenesis.

TEL, a putative tumor suppressor, modulates cell growth and cell morphology of ras-transformed cells while repressing the transcription of stromelysin-1.

TEL is a member of the ETS family of transcription factors that interacts with the mSin3 and SMRT corepressors to regulate transcription. TEL is biallelically disrupted in acute leukemia, and loss of heterozygosity at the TEL locus has been observed in various cancers. Here we show that expression of TEL in Ras-transformed NIH 3T3 cells inhibits cell growth in soft agar and in normal cultures. Unexpectedly, cells expressing both Ras and TEL grew as aggregates. To begin to explain the morphology of Ras-plus TEL-expressing cells, we demonstrated that the endogenous matrix metalloproteinase stromelysin-1 was repressed by TEL. TEL bound sequences in the stromelysin-1 promoter and repressed the promoter in transient-expression assays, suggesting that it is a direct target for TEL-mediated regulation. Mutants of TEL that removed a binding site for the mSin3A corepressor but retained the ETS domain failed to repress stromelysin-1. When BB-94, a matrix metalloproteinase inhibitor, was added to the culture medium of Ras-expressing cells, it caused a cell aggregation phenotype similar to that caused by TEL expression. In addition, TEL inhibited the invasiveness of Ras-transformed cells in vitro and in vivo. Our results suggest that TEL acts as a tumor suppressor, in part, by transcriptional repression of stromelysin-1.

Overexpression of the matrix metalloproteinase matrilysin results in premature mammary gland differentiation and male infertility.

To examine the role of matrilysin (MAT), an epithelial cell-specific matrix metalloproteinase, in the normal development and function of reproductive tissues, we generated transgenic animals that overexpress MAT in several reproductive organs. Three distinct forms of human MAT (wild-type, active, and inactive) were placed under the control of the murine mammary tumor virus promoter/enhancer. Although wild-type, active, and inactive forms of the human MAT protein could be produced in an in vitro culture system, mutations of the MAT cDNA significantly decreased the efficiency with which the MAT protein was produced in vivo. Therefore, animals carrying the wild-type MAT transgene that expressed high levels of human MAT in vivo were further examined. Mammary glands from female transgenic animals were morphologically normal throughout mammary development, but displayed an increased ability to produce beta-casein protein in virgin animals. In addition, beginning at approximately 8 mo of age, the testes of male transgenic animals became disorganized with apparent disintegration of interstitial tissue that normally surrounds the seminiferous tubules. The disruption of testis morphology was concurrent with the onset of infertility. These results suggest that overexpression of the matrix-degrading enzyme MAT alters the integrity of the extracellular matrix and thereby induces cellular differentiation and cellular destruction in a tissue-specific manner.

Matrix metalloproteinases are expressed during ductal and alveolar mammary morphogenesis, and misregulation of stromelysin-1 in transgenic mice induces unscheduled alveolar development.

The matrix-degrading metalloproteinases stromelysin-1, stromelysin-3, and gelatinase A are expressed during ductal branching morphogenesis of the murine mammary gland. Stromelysin-1 expression in particular correlates with ductal elongation, and in situ hybridization and three-dimensional reconstruction studies revealed that stromelysin-1 mRNA was concentrated in stromal fibroblasts along the length of advancing ducts. Transgenic mice expressing an activated form of stromelysin-1 under the control of the MMTV promoter/enhancer exhibited inappropriate alveolar development in virgin females. Ultrastructural analysis demonstrated that the basement membrane underlying epithelial and myoepithelial cells was amorphous and discontinuous compared with the highly ordered basal lamina in control mammary glands. Transgenic mammary glands had at least a twofold increase in the number of cells/unit area and a 1.4-fold increase in the percent of cycling cells by 13 wk of age compared with nontransgenic littermates. In addition, transgenic glands expressed beta-casein mRNA, but not protein, and resembled the proliferative and differentiated state of an animal between 8 and 10 days pregnant. An analysis of metalloproteinase expression in the glands of normal pregnant females demonstrated that the same matrix metalloproteinase family members, including stromelysin-1, were expressed in connective tissue cells surrounding epithelial clusters during the time of lobuloalveolar development. These results suggest that metalloproteinases may assist in remodeling ECM during normal ductal and alveolar branching morphogenesis, and that disruption of the basement membrane by an activated metalloproteinase can affect basic cellular processes of proliferation and differentiation.

The metalloproteinase matrilysin is preferentially expressed by epithelial cells in a tissue-restricted pattern in the mouse.

To explore the role of the matrix metalloproteinase matrilysin (MAT) in normal tissue remodeling, we cloned the murine homologue of MAT from postpartum uterus using RACE polymerase chain reaction and examined its pattern of expression in embryonic, neonatal, and adult mice. The murine coding sequence and the corresponding predicted protein sequence were found to be 75% and 70% identical to the human sequences, respectively, and organization of the six exons comprising the gene is similar to the human gene. Northern analysis and in situ hybridization revealed that MAT is expressed in the normal cycling, pregnant, and postpartum uterus, with levels of expression highest in the involuting uterus at early time points (6 h to 1.5 days postpartum). The mRNA was confined to epithelial cells lining the lumen and some glandular structures. High constitutive levels of MAT transcripts were also detected in the small intestine, where expression was localized to the epithelial Paneth cells at the base of the crypts. Similarly, MAT expression was found in epithelial cells of the efferent ducts, in the initial segment and cauda of the epididymis, and in an extra-hepatic branch of the bile duct. MAT transcripts were detectable only by reverse transcription-polymerase chain reaction in the colon, kidney, lung, skeletal muscle, skin, stomach, juvenile uterus, and normal, lactating, and involuting mammary gland, as was expression primarily late in embryogenesis. Analysis of MAT expression during postnatal development indicated that although MAT is expressed in the juvenile small intestine and reproductive organs, the accumulation of significant levels of MAT mRNA appears to correlate with organ maturation. These results show that MAT expression is restricted to specific organs in the mouse, where the mRNA is produced exclusively by epithelial cells, and suggest that in addition to matrix degradation and remodeling, MAT may play an important role in the differentiated function of these organs.