Cytoplasmic tail-dependent internalization of membrane-type 1 matrix metalloproteinase is important for its invasion-promoting activity.

Membrane-type 1 matrix metalloproteinase (MT1-MMP) is an integral membrane proteinase that degrades the pericellular extracellular matrix (ECM) and is expressed in many migratory cells, including invasive cancer cells. MT1-MMP has been shown to localize at the migration edge and to promote cell migration; however, it is not clear how the enzyme is regulated during the migration process. Here, we report that MT1-MMP is internalized from the surface and that this event depends on the sequence of its cytoplasmic tail. Di-leucine (Leu571-572 and Leu578-579) and tyrosine573 residues are important for the internalization, and the mu2 subunit of adaptor protein 2, a component of clathrin-coated pits for membrane protein internalization, was found to bind to the LLY573 sequence. MT1-MMP was internalized predominantly at the adherent edge and was found to colocalize with clathrin-coated vesicles. The mutations that disturb internalization caused accumulation of the enzyme at the adherent edge, though the net proteolytic activity was not affected much. Interestingly, whereas expression of MT1-MMP enhances cell migration and invasion, the internalization-defective mutants failed to promote either activity. These data indicate that dynamic turnover of MT1-MMP at the migration edge by internalization is important for proper enzyme function during cell migration and invasion.     

Invasive matrix degradation at focal adhesions occurs via protease recruitment by a FAK-p130Cas complex

Tumor cell migration and the concomitant degradation of extracellular matrix (ECM) are two essential steps in the metastatic process. It is well established that focal adhesions (FAs) play an important role in regulating migration; however, whether these structures contribute to matrix degradation is not clear. In this study, we report that multiple cancer cell lines display degradation of ECM at FA sites that requires the targeted action of MT1-MMP. Importantly, we have found that this MT1-MMP targeting is dependent on an association with a FAK-p130Cas complex situated at FAs and is regulated by Src-mediated phosphorylation of Tyr 573 at the cytoplasmic tail of MT1. Disrupting the FAK-p130Cas-MT1 complex significantly impairs FA-mediated degradation and tumor cell invasion yet does not appear to affect invadopodia formation or function. These findings demonstrate a novel function for FAs and also provide molecular insights into MT1-MMP targeting and function.     

Multi-step pericellular proteolysis controls the transition from individual to collective cancer cell invasion

Invasive cell migration through tissue barriers requires pericellular remodelling of extracellular matrix (ECM) executed by cell-surface proteases, particularly membrane-type-1 matrix metalloproteinase (MT1-MMP/MMP-14). Using time-resolved multimodal microscopy, we show how invasive HT-1080 fibrosarcoma and MDA-MB-231 breast cancer cells coordinate mechanotransduction and fibrillar collagen remodelling by segregating the anterior force-generating leading edge containing beta1 integrin, MT1-MMP and F-actin from a posterior proteolytic zone executing fibre breakdown. During forward movement, sterically impeding fibres are selectively realigned into microtracks of single-cell calibre. Microtracks become expanded by multiple following cells by means of the large-scale degradation of lateral ECM interfaces, ultimately prompting transition towards collective invasion similar to that in vivo. Both ECM track widening and transition to multicellular invasion are dependent on MT1-MMP-mediated collagenolysis, shown by broad-spectrum protease inhibition and RNA interference. Thus, invasive migration and proteolytic ECM remodelling are interdependent processes that control tissue micropatterning and macropatterning and, consequently, individual and collective cell migration.     

Tumor cell traffic through the extracellular matrix is controlled by the membrane-anchored collagenase MT1-MMP

As cancer cells traverse collagen-rich extracellular matrix (ECM) barriers and intravasate, they adopt a fibroblast-like phenotype and engage undefined proteolytic cascades that mediate invasive activity. Herein, we find that fibroblasts and cancer cells express an indistinguishable pericellular collagenolytic activity that allows them to traverse the ECM. Using fibroblasts isolated from gene-targeted mice, a matrix metalloproteinase (MMP)-dependent activity is identified that drives invasion independently of plasminogen, the gelatinase A/TIMP-2 axis, gelatinase B, collagenase-3, collagenase-2, or stromelysin-1. In contrast, deleting or suppressing expression of the membrane-tethered MMP, MT1-MMP, in fibroblasts or tumor cells results in a loss of collagenolytic and invasive activity in vitro or in vivo. Thus, MT1-MMP serves as the major cell-associated proteinase necessary to confer normal or neoplastic cells with invasive activity.     

3-D collagen-dependent cell surface expression of MT1-MMP and MMP-2 activation regardless of integrin β1 function and matrix stiffness

Matrix metalloproteinases (MMPs) play roles in spatially dynamic processes, including morphogenesis, wound healing, and tumor invasion. Three-dimensional (3-D) type I collagen stimulates cellular activation of MMP-2, however, the mechanisms underlying this are controversial. The present study investigated mechanisms for 3-D collagen-induced MMP-2 activation in highly invasive human malignant mesothelioma cells. MMP-2 was effectively activated by cells cultured in 3-D collagen but not in 2-D collagen, whereas MMP-2 activation was not regulated by the flexibility of collagen. The 3-D collagen did not largely increase the gene expression of MMP-2 and MT1-MMP. However, MT1-MMP exposed to the cell surface was much increased by 3-D collagen, and loss of MT1-MMP abolished MMP-2 activation in response to 3-D collagen. MT1-MMP and integrin β1 translocated to pericellular regions interacting with collagen-coated microbeads, however their localization was different. Importantly, inhibition of integrin β1 function and expression did not affect 3-D collagen-induced cell surface localization of MT1-MMP and MMP-2 activation. Our results strongly suggest that 3-D collagen scaffolding may provide opportunity for direct and multivalent interaction with MT1-MMP, by which MMP-2 activation occur in abundant cell surface MT1-MMP-dependent manner, rather than a manner regulated by matrix stiffness and integrin β1 function.     

SNAP23, Syntaxin4, and vesicle-associated membrane protein 7 (VAMP7) mediate trafficking of membrane type 1–matrix metalloproteinase (MT1-MMP) during invadopodium formation and tumor cell invasion

Movement through the extracellular matrix (ECM) requires cells to degrade ECM components, primarily through the action of matrix metalloproteinases (MMPs). Membrane type 1–matrix metalloproteinase (MT1-MMP) has an essential role in matrix degradation and cell invasion and localizes to subcellular degradative structures termed invadopodia. Trafficking of MT1-MMP to invadopodia is required for the function of these structures, and here we examine the role of N-ethylmaleimide–sensitive factor–activating protein receptor (SNARE)–mediated membrane traffic in the transport of MT1-MMP to invadopodia. During invadopodium formation in MDA-MB-231 human breast cancer cells, increased association of SNAP23, Syntaxin4, and vesicle-associated membrane protein 7 (VAMP7) is detected by coimmunoprecipitation. Blocking the function of these SNAREs perturbs invadopodium-based ECM degradation and cell invasion. Increased level of SNAP23-Syntaxin4-VAMP7 interaction correlates with decreased Syntaxin4 phosphorylation. These results reveal an important role for SNARE-regulated trafficking of MT1-MMP to invadopodia during cellular invasion of ECM.     

Inhibition of MT1-MMP activity using functional antibody fragments selected against its hemopexin domain

The membrane associated MMP, MT1-MMP, is a critical pericellular protease involved in tumour cell invasion and angiogenesis and is highly up-regulated in numerous human cancers. It therefore represents an exciting new therapeutic cancer-specific target. We have generated recombinant human scFv antibodies against the non-catalytic, hemopexin domain of MT1-MMP that modulate its interactions with collagen. One of these is an effective inhibitor of the invasive capacity of cancer cells and of angiogenesis in model systems. This demonstrates that targeting sites outside the catalytic domain presents a potential novel approach to proteinase inhibition that could have applications in cancer therapeutics.     

Inhibition of β1 integrin induces its association with MT1-MMP and decreases MT1-MMP internalization and cellular invasiveness

Integrin signaling plays a fundamental role in the establishment of focal adhesions and the subsequent formation of invadopodia in malignant cancer cells. Invadopodia facilitate localized adhesion and degradation of the extracellular matrix (ECM), which promote tumour cell invasion and metastasis. Degradation of ECM components is often driven by membrane type-1 matrix metalloproteinase (MT1-MMP), and we have recently shown that regulation of enzyme internalization is dependent on signaling downstream of β1 integrin. Phosphorylation of the cytoplasmic tail of MT1-MMP is required for its internalization and delivery to Rab5-marked early endosomes, where it is then able to be recycled to new sites of invadopodia formation and promote invasion. Here we found that inhibition of β1 integrin, using the antibody AIIB2, inhibited the internalization and recycling of MT1-MMP that is necessary to support long-term cellular invasion. MT1-MMP and β1 integrin were sequestered at the cell surface when β1-integrin was inhibited, and their association under these conditions was detected using immunoprecipitation and mass spectrometry analyses. Sequestration of β1 integrin and MT1-MMP at the cell surface resulted in the formation of large invadopodia and local ECM degradation; however, the impaired internalization and recycling of MT1-MMP and β1 integrin ultimately led to a loss of invasive behaviour.     

Membrane type 1 matrix metalloproteinase (MT1-MMP) ubiquitination at Lys581 increases cellular invasion through type I collagen

Membrane type 1 matrix metalloproteinase (MT1-MMP/MMP14) is a zinc-dependent type I transmembrane metalloproteinase playing pivotal roles in the regulation of pericellular proteolysis and cellular migration. Elevated expression levels of MT1-MMP have been demonstrated to correlate with a poor prognosis in cancer. MT1-MMP has a short intracellular domain (ICD) that has been shown to play important roles in cellular migration and invasion, although these ICD-mediated mechanisms remain poorly understood. In this study, we report that MT1-MMP is mono-ubiquitinated at its unique lysine residue (Lys(581)) within the ICD. Our data suggest that this post-translational modification is involved in MT1-MMP trafficking as well as in modulating cellular invasion through type I collagen matrices. By using an MT1-MMP Y573A mutant or the Src family inhibitor PP2, we observed that the previously described Src-dependent MT1-MMP phosphorylation is a prerequisite for ubiquitination. Taken together, these findings show for the first time an additional post-translational modification of MT1-MMP that regulates its trafficking and cellular invasion, which further emphasizes the key role of the MT1-MMP ICD.     

MT1-MMP: an enzyme with multidimensional regulation

The activity of membrane-type 1 matrix metalloproteinase (MT1-MMP) is a double-edged sword--it is crucial for both physiological processes and disease progression. MT1-MMP modifies various cellular functions and it is, sthus, regulated precisely as a proteinase and as a membrane protein. Recent studies have further revealed that the function of MT1-MMP is modified and regulated by O-glycosylation, interaction with CD44, internalization and recycling. Such multidimensional mechanisms enable MT1-MMP to be regulated spatially and temporally, and are essential for its proper functioning on the cell surface.     

Activation-coupled membrane-type 1 matrix metalloproteinase membrane trafficking

The transmembrane collagenase MT1-MMP (membrane-type 1 matrix metalloproteinase), also known as MMP-14, has a critical function both in normal development and in cancer progression, and is subject to extensive controls at the post-translational level which affect proteinase activity. As zymogen activation is crucial for MT1-MMP activity, an alpha1-PI (alpha1-proteinase inhibitor)-based inhibitor was designed by incorporating the MT1-MMP propeptide cleavage sequence into the alpha1-PI reactive-site loop (designated alpha1-PI(MT1)) and this was compared with wild-type alpha1-PI (alpha1-PI(WT)) and the furin inhibitory mutant alpha1-PI(PDX). Alpha1-PI(MT1) formed an SDS-stable complex with furin and inhibited proMT1-MMP activation. A consequence of the loss of MT1-MMP activity was the activation of proMMP-2 and the inhibition of MT1-MMP-mediated collagen invasion. alpha1-PI(MT1) expression also resulted in the intracellular accumulation of a glycosylated species of proMT1-MMP that was retained in the perinuclear region, leading to significantly decreased cell-surface accumulation of proMT1-MMP. These observations suggest that both the subcellular localization and the activity of MT1-MMP are regulated in a coordinated fashion, such that proMT1-MMP is retained intracellularly until activation of its zymogen, then proMT1-MMP traffics to the cell surface in order to cleave extracellular substrates.     

The Wnt/Planar Cell Polarity Protein-tyrosine Kinase-7 (PTK7) Is a Highly Efficient Proteolytic Target of Membrane Type-1 Matrix Metalloproteinase: IMPLICATIONS IN CANCER AND EMBRYOGENESIS*

PTK7 is an essential component of the Wnt/planar cell polarity (PCP) pathway. We provide evidence that the Wnt/PCP pathway converges with pericellular proteolysis in both normal development and cancer. Here, we demonstrate that membrane type-1 matrix metalloproteinase (MT1-MMP), a key proinvasive proteinase, functions as a principal sheddase of PTK7. MT1-MMP directly cleaves the exposed PKP⁶²¹↓LI sequence of the seventh Ig-like domain of the full-length membrane PTK7 and generates, as a result, an N-terminal, soluble PTK7 fragment (sPTK7). The enforced expression of membrane PTK7 in cancer cells leads to the actin cytoskeleton reorganization and the inhibition of cell invasion. MT1-MMP silencing and the analysis of the uncleavable L622D PTK7 mutant confirm the significance of MT1-MMP proteolysis of PTK7 in cell functions. Our data also demonstrate that a fine balance between the metalloproteinase activity and PTK7 levels is required for normal development of zebrafish (Danio rerio). Aberration of this balance by the proteinase inhibition or PTK7 silencing results in the PCP-dependent convergent extension defects in the zebrafish. Overall, our data suggest that the MT1-MMP-PTK7 axis plays an important role in both cancer cell invasion and normal embryogenesis in vertebrates. Further insight into these novel mechanisms may promote understanding of directional cell motility and lead to the identification of therapeutics to treat PCP-related developmental disorders and malignancy.     

Modulation of the Membrane Type 1 Matrix Metalloproteinase Cytoplasmic Tail Enhances Tumor Cell Invasion and Proliferation in Three-dimensional Collagen Matrices

Increasing evidence suggests that the cytoplasmic tail of membrane type 1 matrix metalloproteinase (MT1-MMP) is subject to phos pho ryl a tion and that this modification may influence its enzymatic activity at the cell surface. In this study, phos pho ryl a ted MT1-MMP is detected using a phospho-specific antibody recognizing a protein kinase C consensus sequence (phospho-TXR), and a MT1-MMP tail peptide is phos pho ryl a ted by exogenous protein kinase C. To characterize the potential role of cytoplasmic residue Thr⁵⁶⁷ in these processes, mutants that mimic a state of either constitutive (T567E) or defective phos pho ryl a tion (T567A) were expressed and analyzed for their functional effects on MT1-MMP activity and cellular behavior. Phospho-mimetic mutants of Thr⁵⁶⁷ exhibit enhanced matrix invasion as well as more extensive growth within a three-dimensional type I collagen matrix. Together, these findings suggest that MT1-MMP surface action is regulated by phos pho ryl a tion at cytoplasmic tail residue Thr⁵⁶⁷ and that this modification plays a critical role in processes that are linked to tumor progression.Largely composed of a mixture of collagens, laminins, and vitronectin, the extracellular matrix (ECM)² serves as both a physical scaffold and a barrier against cell invasion. It has become increasingly evident that the structural condition of the ECM plays a unique role in regulating cell behavior. Proteolysis of integral components of the basement membrane disturbs the barrier provided by the ECM. Without physical restriction, cells invade the surrounding environment in an unregulated manner. The ability of matrix metalloproteinases (MMPs) to collectively degrade nearly all ECM constituents allows this class of enzymes to function in a diverse range of physiological processes (1, 2). Of the anchored MMPs, membrane type 1 matrix metalloproteinase (MT1-MMP) was the first to be discovered and has been most thoroughly characterized. Unlike soluble MMPs, MT1-MMP has a stretch of hydrophobic amino acids that traverse the cell membrane, followed by a short cytoplasmic tail composed of 20 amino acids (3). The advantage of cell surface localization is 2-fold. Surface restriction allows MT1-MMP to modify the immediate pericellular environment, overcoming physical constraints imposed by the ECM (2). Localization at the cell surface also places tethered MMPs in an optimal position to function at invadapodia, highly specialized areas of the cell membrane that form during focalized cell invasion (4). Although information regarding the role of the cytoplasmic tail is relatively limited (5, 6), this domain may function as a bridge to the intracellular machinery.MT1-MMP has an essential role in matrix remodeling during physiological processes (7, 8). Conversely, its enzymatic activity is key to acquiring a metastatic phenotype in a variety of tumor cells, including lung, colon, breast, and cervical carcinomas (2, 9–11). The ability to alter the physical structure of the pericellular environment, while triggering the activation and modification of several cell surface proteins, identifies a central role for MT1-MMP in influencing cellular behavior (12). In return, stringent cellular regulation of MT1-MMP enzymatic activity is necessary to prevent aberrant proteolysis.Increasing evidence suggests that the cytoplasmic tail of MT1-MMP may regulate its activity at the cell surface. It has been demonstrated that MT1-MMP is internalized from the cell surface and that this process requires the presence of the cytoplasmic domain (5, 6). Tail truncation restricts MT1-MMP to the cell surface, suggesting that this domain contains sequence(s) that either mediate internalization or are required for physical interaction with another protein that facilitates its internalization (5, 6). The mechanism regulating this process has yet to be determined. Interestingly, both invasion and migration are down-regulated in cells where MT1-MMP is restricted to the cell surface (5, 6). These data suggest a correlation between internalization and matrix turnover, where MT1-MMP activity is either abrogated or enhanced under appropriate stimuli.Reversible phosphorylation is widely recognized as a key post-translational modification that regulates protein function. The cytoplasmic domain of MT1-MMP has three potential phosphorylation sites: Thr⁵⁶⁷, Tyr⁵⁷³, and Ser⁵⁷⁷. Recent work by Nyalendo et al. (13) indicates that MT1-MMP is phosphorylated at tyrosine residue Tyr⁵⁷³, and that this modification influences cell migration. Several surface proteins are regulated by phosphorylation at multiple residues. In the MT1-MMP cytoplasmic tail, Thr⁵⁶⁷ has homology with the consensus sequence for both protein kinase C (TXR) and ERK1/2 (XTP) (14), suggesting the possibility that active MT1-MMP might also be regulated through phosphorylation of this cytoplasmic tail residue. In the present study, we report that MT1-MMP bears a threonine phosphorylation site in its cytoplasmic tail and that this modification plays an important role in regulating several aspects of carcinoma cell behavior, including invasion and three-dimensional growth.     

Spatial analysis of RECK,MT1-MMP,and TIMP-2 proteins during early Xenopus laevis development

Proper cell-cell and cell-ECM interactions are vital for cell migration and patterning of the vertebrate embryo. MMPs and their inhibitors, RECK and TIMPs, are all differentially expressed during embryogenesis to regulate such ECM remodeling and cell interactions. MT1-MMP, RECK, and TIMP-2 are unique amongst other ECM-regulating proteins as they act in the pericellular space. Past studies have shown that RECK and TIMP-2 interact with MT1-MMP on the cell surface, thereby influencing cell behaviour as well as the microenvironment immediately surrounding the cells. We investigated the localization of RECK, TIMP-2, and MT1-MMP proteins throughout early X. laevis development using immunohistochemistry. We found that during neural tube formation, axis elongation, and organogenesis, RECK, TIMP-2, and MT1-MMP proteins show highly similar localization patterns, particularly in the ectoderm and in the dorsal-ventral differentiation of the neural tube. Our data suggests they function together during patterning events in early Xenopus development.     

MT1-MMP-deficient mice develop dwarfism, osteopenia, arthritis, and connective tissue disease due to inadequate collagen turnover.

MT1-MMP is a membrane-bound matrix metalloproteinase (MT-MMP) capable of mediating pericellular proteolysis of extracellular matrix components. MT1-MMP is therefore thought to be an important molecular tool for cellular remodeling of the surrounding matrix. To establish the biological role of this membrane proteinase we generated MT1-MMP-deficient mice by gene targeting. MT1-MMP deficiency causes craniofacial dysmorphism, arthritis, osteopenia, dwarfism, and fibrosis of soft tissues due to ablation of a collagenolytic activity that is essential for modeling of skeletal and extraskeletal connective tissues. Our findings demonstrate the pivotal function of MT1-MMP in connective tissue metabolism, and illustrate that modeling of the soft connective tissue matrix by resident cells is essential for the development and maintenance of the hard tissues of the skeleton.     

The type I collagen induction of MT1-MMP-mediated MMP-2 activation is repressed by alphaVbeta3 integrin in human breast cancer cells.

The influence of alphaVbeta3 integrin on MT1-MMP functionality was studied in human breast cancer cells of differing beta3 integrin status. Overexpression of beta3 integrin caused increased cell surface expression of alphaV integrin and increased cellular adhesion to extracellular matrix (ECM) substrates in BT-549, MDA-MB-231 and MCF-7 cells. beta3 integrin expression also enhanced the migration of breast cancer cells on ECM substrates and enhanced collagen gel contraction. In vivo, alphaVbeta3 cooperated with MT1-MMP to increase the growth of MCF-7 cells after orthotopic inoculation in immunocompromised mice, but had no influence on in vitro proliferation. Despite these stimulatory effects, overexpression of beta3 integrin suppressed the type I collagen (Col I) induced MMP-2 activation in all breast cancer cell lines analyzed. This was also evident in extracts from the MCF-7 tumors in vivo, where MMP-2 activation was stimulated by MT1-MMP transfection, but attenuated with beta3 integrin expression. Although our studies confirm important biological effects of alphaVbeta3 integrin on enhancing cell adhesion and migration, ECM remodeling and tumor growth, beta3 integrin caused reduced MMP-2 activation in response to Col I in vitro, which appears to be physiologically relevant, as it was also seen in tumor xenografts in vivo. The reduction of MMP-2 activation (and thus MT1-MMP activity) by alphaVbeta3 in response to Col I may be important in scenarios where cells which are activated for matrix degradation need to preserve some pericellular collagen, perhaps as a substrate for cell adhesion and migration, thus maintaining a balanced level of proteolysis required for efficient tumor growth.     

MT-LOOP-dependent Localization of Membrane Type I Matrix Metalloproteinase (MT1-MMP) to the Cell Adhesion Complexes Promotes Cancer Cell Invasion

Localization of membrane type I matrix metalloproteinase (MT1-MMP) to the leading edge is thought to be a crucial step during cancer cell invasion. However, its mechanisms and functional impact on cellular invasion have not been clearly defined. In this report, we have identified the MT-LOOP, a loop region in the catalytic domain of MT1-MMP (¹⁶³PYAYIREG¹⁷⁰), as an essential region for MT1-MMP to promote cellular invasion. Deletion of the MT-LOOP effectively inhibited functions of MT1-MMP on the cell surface, including proMMP-2 activation, degradation of gelatin and collagen films, and cellular invasion into a collagen matrix. This is not due to loss of the catalytic function of MT1-MMP but due to inefficient localization of the enzyme to β1-integrin-rich cell adhesion complexes at the plasma membrane. We also found that an antibody that specifically recognizes the MT-LOOP region of MT1-MMP (LOOP_Ab) inhibited MT1-MMP functions, fully mimicking the phenotype of the MT-LOOP deletion mutant. We therefore propose that the MT-LOOP region is an interface for molecular interactions that mediate enzyme localization to cell adhesion complexes and regulate MT1-MMP functions. Our findings have revealed a novel mechanism regulating MT1-MMP during cellular invasion and have identified the MT-LOOP as a potential exosite target region to develop selective MT1-MMP inhibitors.     

O-glycosylation regulates autolysis of cellular membrane type-1 matrix metalloproteinase (MT1-MMP)

MT1-MMP is a key enzyme in cancer cell invasion and metastasis. The activity of cellular MT1-MMP is regulated by furin-like proprotein convertases, TIMPs, shedding, autoproteolysis, dimerization, exocytosis, endocytosis, and recycling. Our data demonstrate that, in addition to these already known mechanisms, MT1-MMP is regulated by O-glycosylation of its hinge region. Insignificant autolytic degradation is characteristic for naturally expressed, glycosylated, MT1-MMP. In turn, extensive autolytic degradation, which leads to the inactivation of the protease and the generation of its C-terminal membrane-tethered degraded species, is a feature of overexpressed MT1-MMP. We have determined that incomplete glycosylation stimulates extensive autocatalytic degradation and self-inactivation of MT1-MMP. Self-proteolysis commences during the secretory process of MT1-MMP through the cell compartment to the plasma membrane. The strongly negatively charged sialic acid is the most important functional moiety of the glycopart of MT1-MMP. We hypothesize that sialic acid of the O-glycosylation cassette restricts the access of the catalytic domain to the hinge region and to the autolytic cleavage site and protects MT1-MMP from autolysis. Overall, our results point out that there is a delicate balance between glycosylation and self-proteolysis of MT1-MMP in cancer cells and that when this balance is upset the catalytically potent MT1-MMP pool is self-proteolyzed.