Matrix metalloproteinase 2 (gelatinase A) is related to migration of keratinocytes.

The role of matrix metalloproteinases (MMPs) in cell migration was studied by measuring cell growth, migration, and production of MMP-2 and -9 in oral mucosal and skin keratinocytes cultured in the presence of synthetic MMP inhibitors. MMP-2 was the major gelatinolytic MMP produced by these cells while MMP-9 was produced at a low basal level. Inhibitor effects on MMP-9 production were therefore studied in keratinocytes stimulated by tumor necrosis factor alpha (TNFalpha). Tetracycline analogues at concentrations that inhibited the production of MMP-2 but not MMP-9 were able to drastically inhibit migration of both mucosal and skin keratinocytes. Tetracycline analogues also inhibited keratinocyte growth, an effect not found for the other inhibitors tested. Heterocyclic carbonate-derived compounds (LWs) that inhibited MMP-9 but not MMP-2 production had no effect on cell migration. Batimastat, a potent MMP inhibitor, did not have any effect on MMP production or cell growth but did inhibit keratinocyte migration. Tumor growth factor beta (TGFbeta) increased keratinocyte migration as well as both cell-associated and secreted MMP-2 production in wounded cell cultures. The secreted enzyme was partially converted into an active form. In this model batimastat totally blocked TGFbeta-promoted keratinocyte migration. Immunostaining of keratinocytes advancing into the wound revealed that MMP-2 was localized in extracellular matrix contactlike structures against the endogenously produced laminin-5-rich matrix. MMP-9 was localized diffusely along the cell membranes. Using in situ hybridization we observed that in chronically inflamed human gingiva MMP-2 is expressed in epithelium extending into subepithelial connective tissue. These results suggest that MMP-2 plays a specific role in epithelial migration, possibly by detaching the advancing cells from the pericellular matrix or by activating other MMPs.     

MMP-14 mediated MMP-9 expression is involved in TGF-beta1-induced keratinocyte migration

The importance of expression of matrix metalloproteinase (MMP) in keratinocyte migration is well established, but its role remains unclear. Here we investigated the function of MMP-14 in TGF-beta1-induced keratinocyte migration. TGF-beta1 stimulated cell migration and the expression of MMP-2, -9 in HaCaT human keratinocyte cells. When we lowered MMP-14 mRNA with siRNA, cell migration, and MMP-9 expression decreased. Furthermore, the MMP-14 siRNA also reduced activation of JNK in response to TGF-beta1, and a JNK-specific inhibitor decreased both cell migration and MMP-9 expression. Taken together, these results suggest that TGF-beta1-induced HaCaT cell migration is mediated by MMP-14, which regulates MMP-9 expression via JNK signaling.     

Matrix metalloproteinases-2,-3,-7,-9 and-10, but not MMP-11, are differentially expressed in normal, benign tumorigenic and malignant human keratinocyte cell lines

In order to investigate the correlations between constitutive proteinase expression and the degree of tumorigenicity of cancer cells we have studied a model system of three keratinocyte cell lines. RT-PCR studies showed that the cell lines express the genes of matrix metalloproteinase-2, -3, -7, -9, -10 and -11, indicating that they are able to synthesize the corresponding enzymes. Actual MMP synthesis was proven by zymography and Western blotting. In conditioned media gelatinolytic activities or immunoreactive forms of MMP-2, -3, -7, -9, -10 and -11 were detected. The signal intensities showed that MMP secretion increases in the order HaCaT < A5 < or = II-4RT, whereas only MMP-11 is secreted by all cell lines in equal amounts. Intracellularly, enhanced levels of one or both of the tumorigenic variants were only found for MMP-3, -9 and -10, suggesting special functions of these intracellular MMP pools for the tumorigenic cell lines. For MMP-11 exclusive expression in stromal fibroblasts of tumor tissues is widely accepted; however, our results and three other recent reports demonstrate that this concept is not generally valid. In conclusion, the three keratinocyte cell lines investigated here represent an excellent model for studying constitutive expression and secretion of MMPs in correlation to the degree of in vivo tumorigenicity.     

Effects of ceramide on apoptosis, proteoglycan degradation, and matrix metalloproteinase expression in rabbit articular cartilage

Cartilage loss in osteoarthritis is characterized by matrix degradation and chondrocyte death. The lipid messenger ceramide is implicated in signal transduction of the catabolic cytokines tumor necrosis factor (TNF) and interleukin-1 (IL-1), as well as in apoptosis. The aim of this study was to examine the in vitro effects of ceramide on proteoglycan degradation, matrix-metalloproteinase (MMP) expression and activity, and chondrocyte apoptosis in rabbit articular cartilage. Cell-permeant ceramide C(2) stimulated proteoglycan degradation in cartilage explants starting from 3 x 10(-5) M, with 100% increase at the dose of 10(-4) M. This effect was probably due to MMPs since it was blocked by the MMP inhibitor batimastat. Furthermore, in isolated chondrocytes, C(2) stimulated the expression of MMP-1, 3, and 13 at the mRNA level, MMP activity, and MMP-3 production. Ceramide also caused chondrocyte apoptosis at doses ranging from 10(-5) to 10(-4) M. This study supports the hypothesis that ceramide might play a mediatory role in both matrix degradation and apoptosis in processes of cartilage loss such as those observed in osteoarthritis.     

Differential effects of pressure or volume overload on myocardial MMP levels and inhibitory control

Left ventricular (LV) pressure (PO) or volume (VO) overload is accompanied by myocardial remodeling, but mechanisms that contribute to this progressive remodeling process remain unclear. The matrix metalloproteinases (MMPs) contribute to tissue remodeling in a number of disease states. This study tested the hypothesis that increased MMP expression and activity occur after the induction of an LV overload, which is accompanied by a loss of endogenous MMP inhibitory control. LV MMP zymographic activity and species abundance were measured in dogs under the following conditions: acute PO induced by ascending aortic balloon inflation (6 h, n = 9), prolonged PO by aortic banding (10 days, n = 5), acute VO through mitral regurgitation secondary to chordal rupture (6 h, n = 6), prolonged VO due to mitral regurgitation (14 days, n = 7), and sham controls (n = 11). MMP zymographic activity in the 92-kDa region, indicative of MMP-9 activity, increased over threefold in acute PO and VO and fell to control levels in prolonged PO and VO. The MMP-9 activity-to-abundance ratio increased by over fourfold with acute VO and twofold in acute PO, suggesting a loss of inhibitory control. Endogenous MMP inhibitor content was unchanged with either PO or VO. Interstitial collagenase (MMP-1) content decreased by 50% with acute VO but not with acute PO. Stromelysin (MMP-3) levels increased by 40% with acute VO and increased by 80% with prolonged PO. Although changes in LV myocardial MMP activity and inhibitory control occurred in both acute and prolonged PO and VO states, these changes were not identical. These results suggest that the type of overload stimulus may selectively influence myocardial MMP activity and expression, which in turn would affect the overall LV myocardial remodeling process in LV overload.     

The role of gelatinases in colorectal cancer progression and metastasis

Various proteases are involved in cancer progression and metastasis. In particular, gelatinases, matrix metalloproteinase-2 (MMP-2) and MMP-9, have been implicated to play a role in colon cancer progression and metastasis in animal models and patients. In the present review, the clinical relevance and the prognostic value of messenger ribonucleic acid (mRNA) and protein expression and proenzyme activation of MMP-2 and MMP-9 are evaluated in relation to colorectal cancer. Expression of tissue inhibitors of MMPs (TIMPs) in relation with MMP expression in cancer tissues and the relevance of detection of plasma or serum levels of MMP-2 and/or MMP-9 and TIMPs for prognosis are also discussed. Furthermore, involvement of MMP-2 and MMP-9 in experimental models of colorectal cancer is reviewed. In vitro studies have suggested that gelatinase is expressed in cancer cells but animal models indicated that gelatinase expression in non-cancer cells in tumors contributes to cancer progression. In fact, interactions between cancer cells and host tissues have been shown to modulate gelatinase expression in host cells. Inhibition of gelatinases by synthetic MMP inhibitors has been considered to be an attractive approach to block cancer progression. However, despite promising results in animal models, clinical trials with MMP inhibitors have been disappointing so far. To obtain more insight in the (patho)physiological functions of gelatinases, regulation of MMP-2 and MMP-9 expression is discussed. Mitogen activated protein kinase (MAPK) signalling has been shown to be involved in regulation of gelatinase expression in both cancer cells and non-cancer cells. Expression can be triggered by a variety of stimuli including growth factors, cytokines and extracellular matrix (ECM) components. On the other hand, MMP-2 and MMP-9 activity regulates bioavailability and activity of growth factors and cytokines, affects the immune response and is involved in angiogenesis. Because of the multifunctionality of gelatinases, it is unpredictable at what stage of cancer development and in which processes gelatinase activity is involved. Therefore, it is concluded that the use of MMP inhibitors to treat cancer should be considered carefully.     

Selective modulation of matrix metalloproteinase 9 (MMP-9) functions via exosite inhibition

Unregulated activities of the matrix metalloproteinase (MMP) family have been implicated in primary and metastatic tumor growth, angiogenesis, and pathological degradation of extracellular matrix components, such as collagen and laminin. However, clinical trials with small molecule MMP inhibitors have been largely unsuccessful, with a lack of selectivity considered particularly problematic. Enhanced selectivity could be achieved by taking advantage of differences in substrate secondary binding sites (exosites) within the MMP family. In this study, triple-helical substrates and triple-helical transition state analog inhibitors have been utilized to dissect the roles of potential exosites in MMP-9 collagenolytic behavior. Substrate and inhibitor sequences were based on either the alpha1(V)436-450 collagen region, which is hydrolyzed at the Gly (downward arrow) Val bond selectively by MMP-2 and MMP-9, or the Gly (downward arrow) Leu cleavage site within the consensus interstitial collagen sequence alpha1(I-III)769-783, which is hydrolyzed by MMP-1, MMP-2, MMP-8, MMP-9, MMP-13, and MT1-MMP. Exosites within the MMP-9 fibronectin II inserts were found to be critical for interactions with type V collagen model substrates and inhibitors and to participate in interactions with an interstitial (types I-III) collagen model inhibitor. A triple-helical peptide incorporating a fibronectin II insert-binding sequence was constructed and found to selectively inhibit MMP-9 type V collagen-based activities compared with interstitial collagen-based activities. This represents the first example of differential inhibition of collagenolytic activities and was achieved via an exosite-binding triple-helical peptide.     

Differential regulation of MMP-1/9 and TIMP-1 secretion in human monocytic cells in response to Mycobacterium tuberculosis.

In tuberculosis, matrix metalloproteinase (MMP) secretion is involved in leukocyte migration to sites of infection but in excess may contribute to tissue destruction. We demonstrate that human monocytic THP-1 cells and primary monocytes secrete MMP-1 (52 kD collagenase) when phagocytosing live, virulent M. tuberculosis but not inert latex. The magnitude of MMP-1 secretion was approximately 10-fold less when compared to MMP-9 (92 kD gelatinase) secretion. MMP-1 secretion was also relatively delayed (detected at 24 h vs. 4 h). M. tuberculosis, zymosan or latex stimulate similar TIMP-1 secretion within 8 h and increasing over 24 h. MMP-1/9 secretion was decreased by inhibitors of protein kinase (PK) C, PKA or tyrosine kinases (PTK) in a concentration-dependent manner. In contrast, TIMP-1 secretion was not affected by PKC or PTK blockade and only somewhat reduced by high level PKA inhibition. In summary, M. tuberculosis-infected monocytes secrete MMP-1 at lower concentrations than MMP-9 and such MMP secretion is regulated by multiple upstream signalling pathways which do not control TIMP-1 secretion. Divergent effects of i on MMP and TIMP secretion from monocytes may be important in influencing matrix degradation in vivo.     

Selective hydrolysis of triple-helical substrates by matrix metalloproteinase-2 and -9

The role of proteases in the tumor cell invasion process is multifaceted. Members of the matrix metalloproteinase (MMP) family have been implicated in primary and metastatic tumor growth, angiogenesis, and degradation of extracellular matrix (ECM) components. Differentiating between the up-regulation of MMP production and the presence of activated MMPs can be difficult but may well dictate which MMPs are critical to invasion. Because the hydrolysis of collagens is one of the committed steps in ECM turnover, we have investigated selective MMP action on collagenous substrates as a means to evaluate active MMPs. Two triple-helical peptide (THP) models of the MMP-9 cleavage site in type V collagen, alpha1(V)436-450 THP and alpha1(V)436-447 fTHP, were hydrolyzed by MMP-2 and MMP-9 at the Gly-Val bond, analogous to the bond cleaved by MMP-9 in the corresponding native collagen. Kinetic analyses showed k(cat)/K(m) values of 14,002 and 5,449 s(-1)m(-1) for MMP-2 and -9 hydrolysis of alpha1(V)436-447 fTHP, respectively. These values, along with individual k(cat) and K(m) values, are comparable with collagen hydrolysis by MMP-2 and -9. Neither THP was hydrolyzed by MMP-1, -3, -13, or -14. alpha1(V)436-447 fTHP and a general fluorogenic THP were used to screen for triple-helical peptidase activity in alpha(2)beta(1) integrin-stimulated melanoma cells. Binding of the alpha(2)beta(1) integrin resulted in the production of substantial triple-helical peptidase activity, the majority (>95%) of which was non-MMP-2/-9. THPs were found to provide highly selective substrates for members of the MMP family and can be used to evaluate active MMP production in cellular systems.     

Implications for matrix metalloproteinases as modulators of pediatric lung disease

Matrix metalloproteinases (MMPs) are a large family (>20) of cation-dependent proteinases believed to be important modulators of normal human lung development and potentially harmful mediators of lung damage. Little is known about MMP production and secretion by the lung during childhood or how alterations in MMP levels may be involved in lung damage. We examined endotracheal aspirates from children (<19 years) without lung disease for the presence of MMP activity. Only gelatinase activity was detectable, and inhibitor profiles suggest they represented one or more MMPs. Comparison of gelatinase activity, MMP expression, and MMP activity in children without pulmonary disease with children who required mechanical ventilation for respiratory failure show: 1) gelatinase activity was approximately five- to sixfold higher in respiratory failure; 2) MMP-7, MMP-8, and MMP-9 concentrations and MMP-8 and MMP-9 activities were markedly elevated in respiratory failure; and 3) MMP-7, MMP-8, and MMP-9 levels were significantly correlated in children with lung disease. These studies provide compelling evidence that specific MMPs are present in the diseased lung and may participate in the pathogenesis of pediatric respiratory failure.     

Role of ERK and JNK pathways in regulating cell motility and matrix metalloproteinase 9 production in growth factor-stimulated human epidermal keratinocytes

Invasion is an essential cellular response that plays an important role in a number of physiological and pathological processes. Matrix metalloproteinase (MMP) production and cell movement are diverse cellular responses integral to the process of invasion. The complexity of the invasive process suggests the necessity of coordinate activation of more than one signaling pathway in order to activate specific factors responsible for regulating these cellular responses. In this report, we demonstrate that cell movement and MMP-9 production are both directly dependent on the activation of endogenous ERK signaling in hepatocyte growth factor (HGF)-or epidermal growth factor (EGF)-stimulated human epidermal keratinocytes. The kinetic profiles of endogenous MEK and ERK activity suggest that prolonged activation of these signal transducers is an underlying mechanism involved in stimulating cell motility and MMP-9 production. In support of this finding, a transient MEK/ERK signal elicited by keratinocyte growth factor (KGF) or insulin-like growth factor-1 (IGF-1) fails to stimulate these invasion-related responses. Specific inhibition of MEK leads to suppression of ERK activation, marked reduction in steady-state levels of c-Fos, and inhibition of cell movement and MMP-9 production. This occurs despite continued activation of JNK and c-Jun signaling in the presence of MEK-specific inhibition. In contrast, when JNK activity is specifically inhibited in HGF-stimulated cells, AP-1 activity is suppressed but cell motility is not affected. This evidence suggests that while ERK and JNK activity are necessary for AP-1 activation, ERK but not JNK is sufficient in stimulating cell motility. J. Cell. Physiol. 180:271–284, 1999. © 1999 Wiley-Liss, Inc.     

Expression of gelatinases A and B and their endogenous regulators in immortal and transformed fibroblasts

Matrix metalloproteinases (MMP) play a critical role in tumor invasion and metastasis. The goal of this study was to elucidate peculiarities of expression of gelatinases A and B (MMP-2 and MMP-9), membrane type MMP (MT1-MMP) and tissue inhibitor of MMP (TIMP-2) in immortal (IF) and transformed fibroblasts (TF). The study was carried out using embryo rat fibroblasts, sequentially immortalized with the polyomavirus LT gene and transformed with the E7 gene of human papillomavirus (HPV-16). Papillomaviruses type16 and 18 are the etiological factor for cervical cancer. A primary fibroblast (PF) culture of Fisher rats was used as control. Analysis of TF and IF included determination of MMP-2 and MMP-9 activity by hydrolysis of the specific substrate, radioactive collagen type IV; analysis of MMP spectra by a zymographic assay, and estimation of the mRNA expression by RT-PCR. It was found that: (1) collagenolytic activity of MMP was increased only in TF and it depended on the degree of cell tumorigenicity; (2) the study of MMP spectra revealed the presence of MMP-9 only in TF, whereas MMP-2 was found in IF as well; (3) the mRNA expression of MMP-9, MT1-MMP and TIMP-2 increased in all TF while the MMP-2 expression increased in TF only after TF cell selection on rats; (4) the collagenolytic activity as well as the mRNA expression of MMP-2 and MMP-9 and endogenous regulators (MT1-MMP and TIMP-2) did not change in immortalized fibroblasts compared to the PF culture. The data obtained indicate changes in the ratio enzyme/activator/inhibitor and also suggest a significant increase in the TF destructive potential. MMP-9 is supposed to be a marker of fibroblasts transformed by E7 HPV16 gene in a cell culture.     

Matrix metalloproteinase 19 regulates insulin-like growth factor-mediated proliferation,migration, and adhesion in human keratinocytes through proteolysis of insulin-like growth factor binding protein-3

Unlike most other matrix metalloproteinases (MMPs) MMP-19 is expressed in undifferentiated basal keratinocytes of healthy human skin. The human keratinocyte cell line HaCaT, which like basal keratinocytes constitutively expresses MMP-19, down-regulated the expression of MMP-19 at high calcium concentrations. Calcium-regulation occurred through E-cadherin mediated cell-cell contacts because neutralizing anti-E-cadherin antibodies restored MMP-19 expression in high calcium. Overexpression of MMP-19 in HaCaT cells (HaCaT-WT) increased cellular proliferation, as well as migration and adhesion on type I collagen. This was due to proteolysis of the insulin-like growth factor (IGF) binding protein-3 by MMP-19, which augmented signaling through the IGF-I receptor, as evidenced by its increased autophosphorylation. Conversely, these effects were not observed in cells transfected with MMP-2 or a catalytically inactive MMP-19 mutant. As further proof that increased IGF-signaling promoted adhesion and migration in HaCaT-WT cells, we reproduced these effects by treating parental HaCaT with IGF-I. We observed dephosphorylation of the focal adhesion kinase in HaCaT-WT as well as IGF-I-treated HaCaT cells, suggesting that inactivating focal adhesion kinase is a mechanism by which IGF-I enhances adhesion. Furthermore, IGF-I-triggered motility on type I collagen was mediated by MMP activity, which, however, was distinct from MMP-19. Considering the coexpression of IGFBP-3 and MMP-19 in the skin, we conclude that MMP-19 is a likely candidate to be the major IGFBP-3 degrading MMP in the quiescent epidermis. This activity might have widespread consequences for the behavior of epidermal keratinocytes.     

Hypoxia induces epidermal keratinocyte matrix metalloproteinase-9 secretion via the protein kinase C pathway

Hypoxia promotes keratinocyte migration on wound bed connective tissues and is a profound biological signal that transforms a basal keratinocyte, destined to differentiate, into a motile cell that is essential for re-epithelialization. In this study, we examined the effect of hypoxia on keratinocyte-derived collagenases associated with keratinocyte migration. Cells plated on various connective tissue matrices under normoxic and hypoxic conditions, demonstrated a two-fold increase in the 92 kDa, type IV collagenase (MMP-9) when examined by quantitative zymography and ELISA. Western blotting and ELISA demonstrated a two-fold increase in tissue inhibitor of metalloproteinase (TIMP-1), an enzyme that binds to MMP-9 and inhibits its activity. The hypoxia-induced increase in cell motility could be inhibited by a neutralizing antibody to MMP-9. Northern blotting demonstrated that MMP-9 and TIMP-1 mRNA increased 2.5- to 4-fold, 2-12 h after the cells were made hypoxic. The hypoxia-induced changes in MMP-9 and TIMP-1 were inhibited by staurosporine and bisindolylmaleimide, inhibitors of protein kinase C (PKC), but not by inhibitors of tyrosine phosphorylation and the mitogen-activated protein kinase pathway. Inhibition of PKC also inhibited hypoxia-induced keratinocyte migration on type I collagen. These data provide evidence that hypoxia-induced keratinocyte migration is mediated by increased cellular secretion of MMP-9 via the PKC pathway.     

Cerebral ischaemia and matrix metalloproteinase‐9 modulate the angiogenic function of early and late outgrowth endothelial progenitor cells

The enhancement of endogenous angiogenesis after stroke will be critical in neurorepair therapies where endothelial progenitor cells (EPCs) might be key players. Our aim was to determine the influence of cerebral ischaemia and the role of matrix metalloproteinase-9 (MMP-9) on the angiogenic function of EPCs. Permanent focal cerebral ischaemia was induced by middle cerebral artery (MCA) occlusion in MMP-9/knockout (MMP-9/KO) and wild-type (WT) mice. EPCs were obtained for cell counting after ischaemia (6 and 24 hrs) and in control animals. Matrigel™ assays and time-lapse imaging were conducted to monitor angiogenic function of WT and MMP9-deficient EPCs or after treatment with MMP-9 inhibitors. Focal cerebral ischaemia increased the number of early EPCs, while MMP-9 deficiency decreased their number in non-ischaemic mice and delayed their release after ischaemia. Late outgrowth endothelial cells (OECs) from ischaemic mice shaped more vessel structures than controls, while MMP-9 deficiency reduced the angiogenic abilities of OECs to form vascular networks, in vitro. Treatment with the MMP inhibitor GM6001 and the specific MMP-9 inhibitor I also decreased the number of vessel structures shaped by both human and mouse WT OECs, while exogenous MMP-9 could not revert the impaired angiogenic function in MMP-9/KO OECs. Finally, time-lapse imaging showed that the extension of vascular networks was influenced by cerebral ischaemia and MMP-9 deficiency early during the vascular network formation followed by a dynamic vessel remodelling. We conclude that focal cerebral ischaemia triggers the angiogenic responses of EPCs, while MMP-9 plays a key role in the formation of vascular networks by EPCs.     

Urinary matrix metalloproteinase activities: biomarkers for plaque angiogenesis and nephropathy in diabetes

Diabetic complications of nephropathy and accelerated atherosclerosis are associated with vascular remodeling and dysregulated angiogenesis. Matrix metalloproteinases (MMP) modify extracellular matrix during vascular remodeling and are excreted in urine of patients with vascular malformation or tumor angiogenesis. We hypothesized that urinary MMP activities would be sensitive biomarkers for vascular remodeling in diabetic complications. Activities of MMP-2, MMP-9, and its complex with neutrophil gelatinase-associated lipocalin (NGAL/MMP-9) were measured by substrate gel zymography in urine from nondiabetic (ND) and type 1 diabetic (T1D) rodents that were susceptible to both T1D-induced plaque angiogenesis and nephropathy, or nephropathy alone. Additionally, these urine activities were measured in ND and T1D adolescents. Urinary MMP-9, MMP-2, and NGAL/MMP-9 activities were increased and more prevalent in T1D compared with ND controls. Urinary MMP-2 activity was detected in mice with T1D-induced plaque neovascularization. In nephropathy models, urinary NGAL/MMP-9 and MMP-9 activities appeared before onset of albuminuria, whereas MMP-2 was absent or delayed. Finally, urinary MMP activities were increased in adolescents with early stages of T1D. Urinary MMP activities may be sensitive, noninvasive, and clinically useful biomarkers for predicting vascular remodeling in diabetic renal and vascular complications.     

Matrix metalloproteinase-9 triggers the angiogenic switch during carcinogenesis

During carcinogenesis of pancreatic islets in transgenic mice, an angiogenic switch activates the quiescent vasculature. Paradoxically, vascular endothelial growth factor (VEGF) and its receptors are expressed constitutively. Nevertheless, a synthetic inhibitor (SU5416) of VEGF signalling impairs angiogenic switching and tumour growth. Two metalloproteinases, MMP-2/gelatinase-A and MMP-9/gelatinase-B, are upregulated in angiogenic lesions. MMP-9 can render normal islets angiogenic, releasing VEGF. MMP inhibitors reduce angiogenic switching, and tumour number and growth, as does genetic ablation of MMP-9. Absence of MMP-2 does not impair induction of angiogenesis, but retards tumour growth, whereas lack of urokinase has no effect. Our results show that MMP-9 is a component of the angiogenic switch.