Matrix metalloproteinases at a glance

Meng-Huee Lee and Gillian Murphy*

Dept of Oncology, University of Cambridge, Cambridge Institute for Medical Research, Hills Road, Cambridge CB2 2XY, UK

* Author for correspondence (e-mail: gm290{at}cam.ac.uk)

The matrix metalloproteinases (MMPs) are one of the major families of proteinases that play key roles in the responses of cells to their microenvironment. Most notably the MMPs have the combined capacity to degrade all the components of the extracellular matrix. Besides modulating tissue structure to facilitate remodeling, cell migration, etc. MMP activities can result in the generation of epitopes that act as cell effectors or the release of sequestered growth factors. Proteolysis of adhesion molecules, growth factors, cytokines, chemokines and receptors have all been documented (Sternlicht and Werb, 2001Go) and the potential effects on cell behaviour are multifarious.

There are 24 human MMPs, and homologues can be identified in birds, African clawed toad (Xenopus laevis), zebrafish (Danio rerio), fruit fly (Drosophila melanogaster), sea urchin (Paracentrotus lividus), nematode (Caenorhabditis elegans) and Hydra (Hydra vulgaris), as well as in plants and algae. Because of their recognized role in disease the MMPs have long been considered as pharmacological targets, but their multiplicity, associated with their variable expression in different tissues and their apparently overlapping substrate specificities, has presented considerable challenges to those hoping to design suitable therapeutic inhibitors. As a consequence of their apparent redundancy, the majority of studies in which MMP genes have been ablated in mice have produced no overt or very subtle phenotypes, with the exception of MMP-14, which, when knocked out, gave defects in endochondral and intramembranous bone development. However, specific `challenges' to individual knockouts are yielding a clearer picture of novel tissue functions of the MMPs, at the level of both cell-cell and cell-matrix interactions (Sternlicht and Werb, 2001Go).Go



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The MMPs are zinc-dependent endopeptidases of the superfamily Metzincins (MEROPS, the protease databaseGo). They have specific domain structures, minimally consisting of a propeptide and a catalytic domain (MMP-7 and MMP-26), commonly with the addition of a hemopexin-like, four-bladed ß propeller domain connected by a linker or hinge region (MMP-1, MMP-3, MMP-8, MMP-11, MMP-12, MMP-13, MMP-18, MMP-19, MMP-20, MMP-21, MMP-27 and MMP-28). Others have these features plus a fibronectin-like domain of three type II repeats (MMP-2 and MMP-9) or a transmembrane region and a short cytoplasmic `tail' (MMP-14, MMP-15, MMP-16 and MMP-24), or a glycosylphosphatidyl anchor (MMP-17 and MMP-25). MMP-23 is exceptional in that it has unique cysteine-rich, proline-rich and IL-1 receptor type II like domains and might initially be anchored by an N-terminal transmembrane domain prior to propeptide processing. The propeptide of the MMPs contains a `cysteine switch' motif, PRCGXPD, in which the cysteine residue interacts with the catalytic zinc domain in order to maintain inactivity until the propeptide has been removed by proteolysis. The catalytic domains have the zinc-binding motif HEXGHXXGXXH, in which the three histidine residues ligate the zinc ion. Activation of the MMPs by propeptide removal is a critical feature of their regulation and, in the case of MMPs with the requisite RX(R/K)R motif (MMP-11, MMP-14, MMP-15, MMP-16, MMP-21, MMP-23, MMP-24, MMP-25 and MMP-28), might be effected intracellularly by the action of trans-Golgi-localised proprotein convertases or, for the majority, by cleavage by plasmin, autolysis or the action of other MMPs at the cell surface.

MMP activity may subsequently be regulated by the action of inhibitors, notably the tissue inhibitors of MMPs (TIMPs) - TIMP-1, TIMP-2, TIMP-3 and TIMP-4 - and the serum panproteinase inhibitor {alpha}2 macroglobulin (Baker et al., 2002Go) The TIMPs are six-loop disulphide-bonded proteins forming two domains. They interact via their N-terminal three disulphide-bonded loops with the active site cleft of the catalytic domain, although significant interactions of the hemopexin-like domains of MMP-2 and MMP-9 with the C-terminal domains of TIMPs appear to have specific biological relevance. The other MMP domains have distinct functions, such as as exosites for substrate interactions, e.g. the hemopexin-like domains of MMP-1, MMP-8, MMP-13, MMP-14, MMP-16 and MMP-18 are essential for their ability to cleave fibrillar collagens and the fibronectin-like domains of MMP-2 and MMP-9 confer their binding to denatured collagen substrates. The hemopexin-like domain of MMP-14 can homodimerise in order to promote its clustering at the cell surface, a property that promotes its activity. The hemopexin-like domain confers the ability to interact with other extracellular matrix components and cell adhesion molecules and may be of significance in the determination of specific pericellular locations of individual MMPs.

The MMPs are regulated at the transcriptional and post-transcriptional levels, as well as by activation, inhibition and cell/ECM localization, which allows tissue-specific spatial and temporal patterns of functional activity. Expression levels may be modulated by different cytokines, growth factors, hormones, extracellular matrix interactions and cytoskeletal changes through specific elements in the MMP promoters governing transcriptional regulation. Sequestration of the secreted MMPs in Golgi vesicles has been described for many stimulated cells, as has storage of MMP-8 and MMP-9 in the secretory granules of PMN leucocytes. The membrane-associated MMPs appear to have distinct trafficking pathways to specific sites at the cell surface. Association of some MMPs with integrins and other cell surface receptors has been described, e.g. MMP-1-integrin-{alpha}2ß1, MMP-2-integrin-{alpha}Vß3, MMP-14-integrin-{alpha}2ß1/{alpha}Vß3, MMP-7-CD44 and MMP-9-CD44. Many MMPs bind to specific ECM components (see above). With the exception of very rapidly remodeling tissues, extracellular levels of MMPs tend to be quite low, and unambiguous immunohistochemical detection is challenging.

The four TIMPs act as a further level of extracellular regulation and also have specific patterns of gene regulation and tissue-specific expression. TIMP-3 is unusual in that it is largely sequestered into the extracellular matrix or at the cell surface via heparan sulphate proteoglycans. Individual TIMPs differ in their ability to inhibit different MMPs; TIMP-1 is a poor inhibitor of MMP-14, MMP-16 and MMP-19. In addition there are specific interactions of TIMP-1 with proMMP-9, of TIMP-2 with proMMP-2 and of TIMP-3 with both proMMP-2 and proMMP-9 by binding through their three C-terminal disulphide-bonded loops, which allows complexes of the inactive MMPs to be formed, as well as giving very tight-binding active enzyme complexes. The true significance of this has only been elucidated for proMMP-2, where the TIMP-2 complex allows binding of the MMP to MMP-14 at the cell surface, promoting its activation and potentially focusing proteolysis to specific sites. The activation of proMMPs in general is probably strictly pericellular, e.g. where plasmin, generated by the activity of urokinase-type plasminogen activator, is an initiator of activation cascades. If there is an excess of TIMPs and serine proteinase inhibitors in the environment, these may also confine activity to the local environment. There is a further level of regulation of the MMPs through clearance by endocytosis. Little is known of the fate of most MMP-TIMP complexes, but complexes with {alpha}2 macroglobulin are thought to be endocytosed after binding to the low density lipoprotein receptor related protein (LRP). Thrombospondin 2 modulates both MMP-9-TIMP-1 and MMP-2 internalisation via LRP. The membrane-associated proteinase MMP-14 is endocytosed via clathrin- and nonclathrin-mediated pathways and may recycle to the cell surface in some situations. The other MT-MMPs probably have similar properties.


    References
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 References
 

Sternlicht, M. D. and Werb, Z. (2001). How matrix metalloproteinases regulate cell behavior. Annu. Rev. Cell Dev. Biol. 17, 463-516.[CrossRef][Medline]

Baker, A. H., Edwards, D. R. and Murphy, G. (2002). Metalloproteinase inhibitors: biological actions and therapeutic opportunities. J. Cell Sci. 115, 3719-3727.[Abstract/Free Full Text]

MEROPS: the Protease Database [http://merops.sanger.ac.uk].


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