生命科学研究LIFE SCIENCE RESEARCH1999年 第3卷 第3期 Vol.3 No.3 1999基质金属蛋白酶吴二喜 王凤飞 Norman McKIE摘 要：基质金属蛋白酶是一类分解细胞外基质组分的锌蛋白酶。
它们在有机体生长发育中的细胞外基质逆转与重塑以及疾病中的病理损害起着极为重要的作用。
基质金属蛋白酶的表达和活性在不同细胞水平受到严密调控，如细胞因子、生长因子以及激素的调节。
基质金属蛋白酶以酶原形式分泌，随后被其它蛋白酶如胞浆素或非蛋白酶类化学物质如有机汞所激活。
所有基质金属蛋白酶都受到天然抑制剂金属蛋白酶组织抑制剂所抑制。
两者的不平衡导致许多疾病的发生，如肿瘤侵入及转移。
合成基质金属蛋白酶组织抑制剂所抑制，如Marimastat能控制肿瘤转移的发生及进一步扩散。
本文将对基质金属蛋白酶的特征、分子区域结构、底物特性、激活机制、调控方式等方面进行最新概述。
关键词：基质金属蛋白酶；金属蛋白酶组织抑制剂；胶原酶；明胶酶；基质酶；膜型基质金属蛋白酶中图分类号：Q51；Q55 文献标识码：AMatrix MetalloproteinasesWU Er-xi1*，WANG Feng-fei1*，Norman McKIE2（1．Department of Human Metabolism and Clinical Biochemistry，University of Sheffield Medical School，Beech Hill Road，Sheffield S10 2RX，UK； 2．Department of Rheumatology，University of Newcastle-upon-Tyne Medical School Framlington Place，Newcastle-upon-Tyne NE2 4HH，UK）Abstract：Matrix metalloproteinases (MMPs) are zinc proteinases that degrade compounds of the extracellular matrix (ECM). These enzymes play a pivotal role in turnover and remodelling of the ECM during organism growth and development and the pathological destruction of tissues in diseases. The activities of metalloproteinases are tightly controlled at several different cellular levels such as modulation by cytokines, growth factors and hormones. MMPs are secreted as zymogens which can be activated by other proteinases such as plasmin or non-proteolytic agents such as organomercurials. All MMPs are inhibited by their natural inhibitors,tissue inhibitors of matrix metalloproteinases(TIMPs). Imbalance between MMPs and TIMPs has been implicated in many diseases such as tumour invasion and metastasis. The synthetic MMP inhibitors such as Marimastat can prevent the growthand further spread of established metastases.Key words：MMPs；TIMPs；collagenases；gelatinases；stromelysins；membrane type MMPsT0 Introduction Matrix metalloproteinases (MMPs) are also called matrixins. Since the first MMP was discovered by Gross and Lapiere in 1962, numerous other MMPs have been described and characterized. To date at least 14 MMPs have been found (Table 1). According to their structural properties and substrate specificities, MMPs can be divided into sub-Table 1 The matrix metalloproteinase family*Enzymes MMP No．**EC No．Mr（kDa）latent/activeExtracellular matrixsubstratesCollagenases Interstitial collagenase MMP1EC 3．4．24．757/48Collagen Ⅰ，Ⅱ，Ⅲ，Ⅶ，Ⅹ；gelatin，entactin，tenascin，aggrecan，progelatinaseA，progelatinase BNeutrophil collagenase MMP8EC3．4．24．3485/65Collagen Ⅰ，Ⅱ，Ⅲ；aggrecanCollagenase 3MMP13 60/48Collagen Ⅰ，ⅡGelatinases Gelatinase A MMP2EC 3．4．24．2472/66Collagen Ⅰ，Ⅳ，Ⅴ，Ⅶ，Ⅹ；gelatin，fibronectin，laminin，aggrecan elastin，progelatinase BGelatinase B MMP9EC 3．4．24．3592/84Collagen Ⅳ，Ⅴ；gelatin，elastin，entactin，aggrecan，vitronectinStromelysins Stromelysin 1MMP3EC 3．4．24．1760/50Collagen Ⅱ，Ⅳ，Ⅸ，Ⅹ，Ⅺ；gelatin，laminin，fibronectin，elastin，tenascin，aggrecan，procollagenase，progelatinase B，neutrophil procollagenaseStromelysin 2MMP10EC 3．4．24．2253/47Collagen Ⅳ，laminin，fibronectin，elastin，aggrecan，procollagenaseStromelysin 3MMP11 65/45Serpins，α1-PI，α2-antiplasmin，insulin-like growth factor-bindingprotein-1Others Matrilysin MMP7EC 3．4．24．2328/21Collagen Ⅳ，gelatins，laminin，fibronectin，entactin，elastin，aggrecan，progelatinaseA，progelatinase B，procollagenaseMetalloelastase MMP12EC．3．4．24．6552/43ElastinMembrane type MMPs MT1-MMP MMP14 63/54Progelatinase A，procollagenase 3，collagen，proteoglycan，fibronectin，tenascinMT2-MMP MMP15 72/61Progelatinase A，procollagenase 3，collagen，proteoglycan，fibronectin，tenascinMT3-MMP MMP16 64/55Progelatinase A MT4-MMP MMP17 70/54Unknown*Compiled from sources including Sang and Douglas[2]；Shingleton et al．[3]；Nagase [4]；Pei et al．[19]；Mari et al．[44]；Murphy et al．[96]；Takino et al．[28，46]andCockett et al．[116]**Some MMPs have been omitted．MMP5 and MMP2 are the same enzyme[5]，MMP4 and MMP6 have been described in only one laboratory and there are no sequence data to date [5]．There are some novel MMPs which have been found recently，see text for details．groups: collagenases, gelatinases, stromelysins, membrane type MMPs (MT-MMP) andothers[1～4]. This review summarises the characteristics, domain structure, substratespecificity, activation mechanisms, regulation, functions of the MMPs.1 Main characteristics of the MMPs All MMPs have a number of common characteristics which are also helpful to identify new members. These properties[2, 5～7]are: 1）they share a common domain structure comprising signal, propeptide, catalytic, and C- terminal hemopexin-like domains (except MMP7) (Fig．1).Fig．1 The domain structure of the matrix metalloproteinases2）They are secreted as zymogens. 3）Their activation can be achieved by other proteinases or organomercurials. 4）Their proteinase activity is blocked by 1,10-phenanthroline and chelating agents. 5）Activation is accompanied by a loss of molecular weight. 6）The active site contains metal ion zinc. 7）Their activity is inhibited by tissue inhibitor of metalloproteinases (TIMPs). 8）The active enzymes cleave one or more components of the extracellular matrix. 9）The enzymes act in neutral pH and need calcium ions for stability.2 Domain structure of MMPs After comparing the primary amino acid sequences of the MMP members, it can be seen that these proteins are divided into several distinct domains that are conserved among family members[1]. The largest MMP member (MMP9) has 7 domains in order from N-terminal: signal, propeptide, catalytic, fibronectin-like,α2V collagen-like, hinge, and C-terminal hemopexin-like. The simplest member MMP7 has only signal, propeptide and catalytic domains. The newly discovered MT-MMPs have a transmembrane domain[8]. All MMPs produce a signal as a leader sequence which cells cleave prior to secretion. The propeptide is lost on activation. For example, the human fibroblast collagenase is synthesized as a preproenzyme of Mr 54 kDa (57 kDa in Table 1) with the signal peptide of19 amino acids[9]. The primary secretion products of human fibroblast collagenase consist ofa minor glycosylated form of Mr 57 kDa and a major unglycosylated polypeptide of Mr 52 kDa[9]. 81 amino acids are removed after proteolytic activation of human fibroblast collagenase[9]. The catalytic domain contains a conserved zinc binding site comprising the sequence HEXGHXXGXXH[2]. The zinc acts as an active site and is ligated by the three-histidine residues of the zinc binding consensus sequence. The glutamic acid residue in the conserved zinc binding site acts as the catalytic base and proton shuttle during proteolysis and is involved in the fixation of a zinc-bound water molecule[10]. The structural integrity of the zinc-binding active site is maintained by a conserved methionine that is called “Met-turn”[10]. The catalytic domain also contains a calcium-binding region where the calcium ion is believed to stabilize the enzyme[11]. With the exception of matrilysin (MMP7), the MMPs contain an additional feature, that is their hemopexin-like or vitronectin-like C-terminal domain[1]. This domain is thought to help determine substrate specificity[1,5]. Thiscould be true, since recently Gohlke et al.[12]have found that the topology and the side chain arrangements of gelatinase A (MMP2) and fibroblast collagenases are very similar, but there are significant differences in surface charge and contouring. They thought these differences may be a factor in allowing the MMP2 C-terminal domain to bind to TIMP2. Very recently, Brooks et al.[13,14]have found that MMP2 can bind αvβ3 through its hemopexin-like domain. Besides the prototype domain structure, the gelatinases contain three tandem domains with sequences similar to the collagen-binding domain of fibronectin. The fibronectin-like domain is thought to be involved in the binding of two gelatinases to their substrate[5]. The MT-MMPs contain a transmembrane domain and a furin recognition site which is also found in stromelysin 3[8,15](Fig．1).3 Substrate specificity of MMPs The collagenases mainly cleave interstitial collagens (type I, II and III), unlike other MMPs, their substrate specificity is well defined. Collagenase action on the α2-macroglobulin results in the cleavage of Gly-Leu Peptide bond[5]．They cleave the Gly-Ile peptide bond of α1(I) chain of collagen and Gly-Leu peptide bond of α2(I) chain of collagen[5]. The substrate cleavage pattern for collagenases is that P1 "residue is invariably hydrophobic (Leu, Ile, Val) and that P1 is usually Gly or a hydrophobic residue[5,16]. The gelatinases cleave denatured collagens and type IV collagen. As shown in table 1, while gelatinase A (MMP2) can also cleave the fibronectin and laminin, major components of the basement membrane, gelatinase B (MMP9) can only cleave the basement membrane component entactin[2]. Stromelysin 1 and 2 have a broad pH optimum and more general activity and are able to degrade many ECM proteins including proteoglycans, gelatins, fibronectin, laminin, elastin, type IV collagen and type IX collagen[17]． Niedzwiecki et al.[18]tested the substrate specificity of the human stromelysin 1 and found the preferences at P3, P2, P1, P1 ", and P2 "are for the hydrophobic amino acids Pro, Leu, Ala, Nva, and Trp, respectively. The mature stromelysin 3 does not degrade any major ECM components. Up to now, the only known substrates for stromelysin 3 are α1-proteinase inhibitor, serine proteinase inhibitors, α2-antiplasmin, and insulin-like growth factor-binding protein-1 [19,20]. However, the substrate specificity overlaps among the MMP members. The gelatinase A (MMP2) can also cleave triple helical type I collagen generating the 3/4 and 1/4 length collagen fragments characteristic of interstitial collagenases[21]. The cleavage site is the same Gly-Ile/Leu bond as the interstitial collagenases[21]. In fact all of the MMPs cleave gelatin and fibronectin at some rate[1]. Almost every MMP degrades an octapeptide containing collagen sequence GPQGIAGQ[5].4 Activation mechanisms of MMPs The MMPs are secreted in a latent form that is subsequently converted into the mature enzymes. The inactive zymogen can be processed into active forms by numerous reagentsincluding proteinases such as trypsin and plasmin; conformational perturbants such as sodium dodecyl sulfate (SDS); heavy metals such as Au (I) compounds and organomercurials; oxidants such as NaSCN; disulfide reagents such as oxidized glutathione; and sulfhydryl alkylating agents such as N-ethylmaleimide[22]. The latent proform of MMPs contains the highly conserved PRCGVNPD sequence with an unpaired cysteine residue. The cysteine residue links to the active site zinc, which blocks the active site. A ‘cysteine switch’ activation mechanism has been proposed[22,23](Fig． 2). In other words, in the latent form of MMPs, the cysteine residue with the thiol group coordinates the catalytically essential zinc ion in the active site of enzyme. Some reagents such as SDS can dissociate the cysteine residue from the zinc ion[22]. From this hypothesis, each MMP should have an unpaired cysteine residue and a zinc-binding site. In fact, all MMPs contain the PRCGVNPD sequence with an unpaired cysteine residue in the propeptide domain and the HEXXHXXGXXH sequence in the catalytic domain. Very recently, it has been shown that he structure of human pro-MMP2 supports this hypothesis[24]. The loops within the propeptide domain act as bait for activating proteinases. The prodomain structure breaks down and its shielding of the catalytic cleft is withdrawn upon cleavage, which allow water to enter and hydrolyze the coordination of the cysteine residue to the zinc ion[24]. All MT-MMPs found to date and stromelysin 3 contain a consensus sequence RXR/KR, which has already been found to be essential in the activation of stromelysin 3 and MT1-MMP by furin [15,25,26]. MMPs also have the ability to activate one another[27]. MT1-MMP[8]and MT3-MMP[28]have been shown to activate MMP2. MMP7 can activate MMP1, MMP2, MMP3 and MMP9[29]. MMP3 activates MMP1[30], MMP8[31], MMP9[32]and MMP13[33]. MMP2 and MT1-MMP activate MMP13[34], while MMP10 activates MMP8[35]. It has also been well documented that MMPs and serine proteinases can act on the proforms of one another [36,37]. Plasmin can cleave the prodomains of MMPs such as collagenase and stromelysin and activate these enzymes[36,37]. MMP7 can catalyze the formation of low molecular weight pro-urokinase and urokinase[38].Fig．2 Cysteine switch mechanism for activation of MMPsProteinases such as trypsin and plasmin cleave the propeptide，ahead of the cysteine to generate intermediate forms．Alternatively，nonproteolytic agents such as aminiphenyl-mercuric acetate （APMA） and sodium dodecy1 sulfate （SDS） will modify the cysteine．In a second step，these intermediate forms can be autoproteolytically cleaved to remove the propeptide and confer permanent activity．5 Regulation of MMPs The activities of metalloproteinases are tightly controlled at several different cellular levels. There are five points: (i) modulation of gene expression by cytokines, growth factors and hormones; (ii) synthesis and secretion of proMMPs; (iii) selective expression of MMP genes in specific tissue/cell types; (iv) activation of proenzymes; and (v) inhibition of the active enzymes[7,36,39,40]. It is known that the MMPs are not constitutively expressed in most tissue types but their mRNAs can be induced by treatment with many kinds of agents such as cytokines, growth factors, hormones, tumour promoter and oncogene products. In some cases the induction of more than one MMP is coordinately regulated, for example, MMP1 and MMP3 are often coordinately expressed[40]. The synthesis of MMP1 and MMP3 can be upregulated by 12-O-tetradecanoyl-phorbol-13-acetate (TPA), interleukin 1 (IL1), tumor necrosis factor α(TNFα), epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and platelet derived growth factor (PDGF) and be downregulated by transforming growth factor β (TGF-β), interferon γ (IFN-γ) retinoic acid and dexamethasone[40], and MMP1 is down regulated by IL4[41]. The selective expression of MMP genes in specific tissue/cell types is one method of regulation of MMPs. Interstitial collagenase (MMP1) arises from connective tissue fibroblasts and macrophages. Neutrophil collagenase (MMP8) is only produced by cells of the neutrophil lineage. Stromelysin 1 (MMP3) is not widely expressed normally, but can be readily induced by growth factors, cytokines, tumour promoters and oncogene products in cultured mesenchymal cells such as chondrocytes and connective tissue fibroblasts. Stromelysin 2 (MMP10) is usually expressed in macrophages, keratinocytes and tumour cells[40,42]. Stromelysin 3 (MMP11) is often expressed in tumour stromal cells[37,40,43,44]. Gelatinase A (MMP2) is the most widespead of all MMPs and is frequently elevated in malignancies as well as occurring in connective tissue cells[36,40]. Gelatinase B (MMP9) is expressed in transformed and tumour-derived cells, neutrophil, corneal epithelial cells, cytotrophoblasts and keratinocytes[36]. Matrilysin is expressed in immature monocytes[36]. Metalloelastase (MMP12) is found in macrophages. MT1-MMP is expressed in tumour stroma cells[45]. MT2-MMP is produced by a human oral malignant melanoma and a human placenta[46]. MT3-MMP is expressed in normal tissues such as lung and kidney and cultured cells such as the squamous cell carcinoma cell line OSC-19 and human embryonal lung fibroblasts[28]. MT4-MMP is expressed in primary breast carcinomas and breast cancer cell lines examined[47]. All MMPs are inhibited by natural inhibitors called tissue inhibitors of metalloproteinases (TIMPs) of which four have been described (Table 2). TIMPs 1, 2 and 4 are secreted extracellularly in soluble form whereas TIMP3 binds to the ECM. All TIMPs share 35%～40% sequence identity and considerable higher structural similarity. The TIMPs bind with high affinity and 1∶1 molar ratio to active MMPs resulting in the loss of proteinase activity. All TIMPs contain 12 cysteine residues that have been shown to form disulfide bonds generating 6 loops. The mechanism of inhibition of MMPs by TIMPs is widely felt to be unclear. Murphy[39]. pointed out that the TIMP C-terminal domain has several different MMP binding sites that act to increase the rate of inhibition. Further studies such as site-directed mutagenesis and the crystal structure research of the complex of MMP and TIMP will facilitate understanding of the mechanism. TIMPs play a pivotal role in the regulation of ECM degradation/remodelling. Disruption of the balance between MMPs and TIMPs has been implicated in many diseases such as tumour invasion and metastasis. Besides their inhibitory activities, the TIMPs play a role in growth-promotion[48]. Some effective synthetic MMP inhibitors also can inhibit MMPs[49,50]. For example, the hydroxamate-based inhibitors such as BB94 (batimastat) and BB2516 (marimastat) can inhibit metastatic spread of tumoursand block the process of tumour neovasculariation[51～54]. The hydroxamate group in thesemolecules can combine with the zinc atom in the active site of MMPs[49,50].Table 2 Comparison of TIMP family members* TIMP1TIMP2TIMP3TIMP4 Molecular mass28kDa21kDa24kDa22kDa Glycosylation yes no no/yes no Binding properties proMMP9proMMP2ECM proMMP2Transcripts0.9kb 1.1 and 3.5kb 4.5,2.8,2.4and 1.2kb1.4，4.1，2.1，1.2，and 0.97kbModulation of gene expression by TGF-β1updown or noobvious effectup not determinedMajor sites ofexpressionOvary，bone Placenta Kidney，brain Heart Chromosome locationof human genexq1117q2522q12.1～13.22p25 *Compiled from sources including Stetler-Stevenson[117]；Greene et al．[118]；Wick et al．[119]；Chamber and Matrisian[120]and Olson et al．[121]6 MMPs in physiological processes and pathological destruction Normal expression of MMPs is associated with turnover and remodelling of the ECM during growth and development. Cawston[7]summarised the involvement of MMPs in the normal turnover of connective tissue matrix. The MMPs are involved in the normal physiological processes such as ovulation and embryo implantation, embryological development, angiogenesis, bone turnover, uterine resorption and cervical ripening[7]. The MMPs are associated with the pathological destruction of tissues in diseases[1,7]. They are implicated in the processes such as wound healing, corneal ulceration, tumour growth and metastasis, periodontal disease, rheumatoid arthritis, arteriosclerosis and aortic aneurism[1,7]. Considerable research has been directed toward understanding both the steps involved in tumour cell invasion and metastasis and the molecular mechanism of the process. MMPs are thought to be one of the main contributors to tumour invasion and metastasis since they can degrade all of the components of basement membranes which the tumour cellsmust traverse[55～58]. Now the gene-targeting experiments have facilitated the examinationsof the effects that their absence, mutation or overexpression, in various physiological and pathological processes[59]. In the following sections more detailed information is presented on several MMP family members.7 Collagenases I, II and III (MMP1, MMP8 and MMP13) The group of enzymes termed collagenases includes 3 members: MMP1 (interstitialcollagenase or fibroblast collagenase), MMP8 (neutrophil collagenase) and MMP13 (collagenase 3). Their sequences share around 50%[33]. They cleave all three α chains of native types I, II, III collagens at a single site resulting in fragments corresponding to three-quarters and one-quarter of their initial length by hydrolyzing the peptide bond Gly-[Ile orLeu][60～62]. They do not degrade collagen IV and V, which are cleaved by otherproteinases[63～66]. The collagenases can also cleave type X collagen[67]. The threecollagenases have their own preference in cleaving the collagens. The preferred substrate of collagenase 3 (MMP13) is collagen type II[33], fibroblast collagenase (MMP1) preferentially cleaves collagen type III[68]and neutrophil collagenase (MMP8) prefers to cleave type I collagen[69]. Also collagenase 3 has a much stronger gelatinolytic activity than its homologous counterparts MMP1 and MMP8[33]. To date MMP1 expression has not been found in rats or mice[33]. All collagenases are located in the human chromosome 11q22.3 cluster[70,71]and share a highly conserved gene structure[72,73]. The expression of MMP13 in cartilage and its preference to degrade type II collagen suggests that it plays a critical role in the arthritides[74]. X ray crystallographic analyses of MMP1 and MMP8 are available now. The structures show that their catalytic domains harbour two zinc ions and one or twocalcium ions[75～77]. The structure consists of a five-stranded β sheet and three α helices[75].8 Gelatinases A and B (MMP2 and MMP9) Gelatinases, also called type IV collagenases, contain two members: MMP2 and MMP9. They degrade denatured collagens, type IV, V, VII, X, and XII collagens,vitronectin, aggrecan, elastin, galectin 3 and laminin[78～83]. Recently, it has beendemonstrated that MMP2 degrades native interstitial collagens[21]. MMP2 is the most widely distributed MMP[1]. Overexpression of gelatinases has been demonstrated in many tumorsystems and has been linked to tumour invasion[84～87]. While the proMMP2 is often foundcomplexed with the TIMP2 and is activated by MT1-MMP[88,89], the proMMP9 is associated with the TIMP1[78,90]. The regulation of MMP2 and MMP9 gene expression is different[91]. The promoter of MMP9 gene has a TATA-like sequence and a TPA response element, while MMP2 gene has no TATA motif-like sequence in the vicinity of the start site for transcription and TPA response element in its promoter[91]. There is a structural difference between MMP2 and MMP9 that comprises an extended 54 amino acid hinge region sequence which shares some homology with the α2 chain of type V collagen[78]. Brooks et al.[13,14]have found that MMP2 can directly bind integrin αvβ3. They further demonstrated that MMP2 binds αvβ3 through its hemopexin-like domain. It is likely to be distinct from other αvβ3-directed ligands as MMP2 has no RGD sequence[13,14]. Brooks et al.[14]have also found that PEX, a fragment of MMP2, which contains the C-terminal hemopexin-like domain, prevents MMP2 binding to αvβ3 and blocks cells surfacecollagenolytic activity. PEX is likely to be a natural breakdown product of MMP2 since the active form (62 kDa) of MMP2 can be further processed to a smaller species (43 kDa) resulting from autocatalytic removal of the 29 kDa hemopexin-like domain without the presence of TIMP2[14,92]. Both MMP2 and MMP9 are located on human chromosome 16[93].9 Stromelysins 1, 2 and 3 (MMP3, MMP10 and MMP11) MMP3 (stromelysin 1), MMP10 (stromelysin 2) and MMP11 (stromelysin 3) belong to this group. Stromelysins have wide substrate specificity. Stromelysin 1 degrades aggrecan, fibronectin， gelatin， laminin， type II， IV， IX， X， XI collagens and elastin[2,94]. It also participates in activation of other proMMPs such as proMMP1, proMMP8 and proMMP9[31,32,80]. MMP3 is not readily expressed in tissue but can be induced by cytokines such as IL1 and TNFα, growth factors and tumour promoters such as PMA[1,5,95]. Stromelysin 2 cleaves aggrecan, laminin, fibronectin, elastin and type IV[2]. It is transcriptionally active in normal human cells such as keratinocytes and it encodes the secreted Stromelysin 2[42]. Stromelysin 3 is a newly characterized MMP. It can degrade serpin, α1-proteinase inhibitor (α1-PI), α2-antiplasmin, and insulin-like growth factor-binding protein-1[19,20]; the truncated stromelysin 3 can also degrade fibronectin, laminin, aggrecan and type IV[96]. Stromelysin 3 is often found expressed in stromal cells surrounding primary and metastatic carcinomas[43]and may be involved in promoting local tumor development. Recently it has been suggested that the tumor-specific processing of stromelysin 3 to the 35 kDa protein is likely to be an important regulatory mechanism, since the generation of 35 kDa stromelysin in tumour/stroma coculture requires basic fibroblast growth factor (bFGF) and an MMP-like enzyme[44]. Unlike other MMPs, prodomain of stromelysin 3 contains a furin cleavage site, and therefore stromelysin 3 can be processed directly to its 45 kDa active form by furin within the constitutive secretory pathway[15]. MMP3 and MMP10 are located in the human chromosome cluster 11q22.3[70,72], while MMP11 is mapped to the Q11.2 region of chromosome 22[97]．10 Membrane type-matrix metalloproteinases (MMP15, MMP16, MMP17 and MMP18) Membrane type matrix metalloproteinases (MT-MMPs), comprising MT1-MMP, MT2-MMP2, MT3-MMP and MT4-MMP (MMP14, MMP15, MMP16 and MMP17, respectively) are a novel subgroup of MMPs which contain a transmembrane domain and a short cytoplasmic domain in addition to the signal, pro-, catalytic, hemopexin-like and C-terminal domains which are common to other MMPs. To date, four MT-MMPs have been described [8,28,47,98]. From the alignment of amino acid sequences for MT-MMPs[47], it can be seen that they have at least 30% sequence homology to each other. Like other MMPs, they contain cysteine switch sequence, zinc binding site and Met-turn. However, they have an insertion between the propeptide and the catalytic domain in addition to the above mentionedmembrane-binding domain and short cytoplasmic domain. This insertion as in stromelysin 3 contains a potential cleavage site of furin or furin like convertase[8,15]. It has already been demonstrated that furin can cleave this sequence in pro-stromelysin 3 and proMT1-MMP [15,25,26]. However, the proMT1-MMP is proteolytically activated by human plasmin[99]. Okumura et al.[99]proposed that this is an extracellular proteinase activator of proMT1-MMP whereas the intracellular mechanism of activation is mediated by furin or a furin-like enzyme. The second insertion is found in proteinase domain; the function of these 8 amino acids is not known. The third insertion which contains the hydrophobic transmembrane domain is located in the C-terminal. The transmembrane domain of MT-MMPs plays an essential role in the progelatinase A (proMMP2) activation function of MT-MMPs, although some portions of the truncated form remain on the cell surface after the removal of this domain[100]. In MT1-MMP, the activation complex is a trimer comprising MT1-MMP, progelatinase A and TIMP2[88,89]. A recent study has suggested that MT1-MMP is a TIMP2 receptor[101]. This is further confirmed by another recent study[73]that TIMP2 and MT1-MMP form a complex for activation of progelatinase A. TIMP2 and TIMP3 can efficiently inhibit MT1-MMP whereas TIMP1 is a poor inhibitor for MT1-MMP[102]. Will et al.[102] also demonstrated that TIMP2 and TIMP3 can bind more rapidly to the catalytic domain of MT1-MMP than to the catalytic domain of MMP2. At the same time, two groups showed that MMP14, MMP15 and MMP16 locate in chromosomes 14, 16 and 8, respectively [103,104]. The MT-MMP gene loci are dispersed whereas most MMPs are clustered on chromosome 11q22.3[70,71], suggesting that the MT-MMPs may be a genetically distinct subgroup of MMPs. It has been suggested that the MT-MMPs play a critical role in tumor cell invasion and in ECM degradation[8,57,102]. MT-MMPs also can degrade ECM components[25,105,106].11 Others This group includes MMPs such as matrilysin (MMP7) and metalloelastase (MMP12). Matrilysin, also called putative metalloproteinase 1 (pump 1), is the smallest member of the MMP family. Unlike other MMPs, it lacks a C-terminal hemopexin-like domain. MMP7 can degrade various ECM components. It also can activate other proMMPs such as proMMP1, 3, 2 and 9[29]. Metalloelastase (MMP12) is also called macrophage elastase (ME). The expression of human macrophage elastase (HME) is mainly restricted to tissue macrophages [107]. Besides the substrate elastin listed in Table 1, the purified recombinant HME can degrade fibronectin, laminin, entactin, type IV collagen, chondroinan sulfate, and heparan sulfate[108]. Chandler et al.[109]also demonstrated that HME can degrade myelin basic protein and processes a TNF α fusion protein. Therefore Gronski et al.[108]suggested that HME may be essential for macrophages to penetrate basement membranes and remodel injured tissue during inflammation. MMP7 and MMP12 are allocated in the human chromosome cluster 11q22.3[70,72,107].。
