MMPs in Pathological Conditions and MMPs Regulation


MMPs in pathological conditions
Under normal physiological processes, MMPs are implicated in embryonic development, organ morphogenesis, and wound healing. They are also involved in pathogenic processes, which include arthritis, atherosclerosis, and carcinogenesis. In cancer, they are responsible for facilitating the proteolytic degradation of the extracellular matrix (ECM), promoting the extravasation of cancer cells from the primary site. The ECM is composed of structural proteins such as collagen, elastin, fibrillin, fibronectin, and laminin, as well as proteoglycans. There are diverse types of proteases that are responsible for remodeling of the ECM, thus driving the movement of cancer cells into adjacent tissues. Serine proteases are involved in breakdown of various ECM proteins during events such as invasion, angiogenesis, and metastasis. Matrix metalloproteinases have been implicated in inflammation and carcinogenesis. Additionally, MMPs are involved in cell signaling, and have the capability to activate specific cell receptors and growth factors, or to liberate them from the ECM. As a result, MMPs are capable of regulating various cell behaviors, including cell growth, differentiation, apoptosis, angiogenesis, and migration.
Alterations in the regulation of MMP activity are implicated in diseases such as cardiovascular diseases, neurological diseases, fibrosis, arthritis, and most importantly, cancer. The pathological effects of MMPs in cardiovascular diseases involve vascular remodeling, atherosclerotic plaque instability and cardiac remodeling in congestive heart failure or after myocardial infarction. Since excessive tissue remodeling and increased matrix metalloproteinases activity have been demonstrated during atherosclerotic lesion progression (including plaque disruption), MMPs represent a potential target for therapeutic intervention aimed at the modification of vascular pathology by restoring the physiological balance of MMPs. In the central nervous system (CNS), MMPs have been shown to degrade components of the basal lamina, leading to disruption of the blood brain barrier and to contribute to the neuroinflammatory responses in many neurological diseases.
Recent findings suggest that MMPs are also involved in cancer initiation, invasion, tumor angiogenesis and metastasis. Despite the general lack of genetic alterations in the MMP genes in cancer cells, polymorphisms in MMP promoters exist, which affect gene transcription and influence cancer susceptibility. Homozygotes for some of these polymorphisms are more likely to develop invasive tumors, Initially, it was believed that MMPs were important only in invasion and metastasis; however, recent studies support the involvement of MMPs in several steps of cancer development. Some MMPs, such as MMP-9 are able to generate growth-promoting signals by releasing cell membrane-bound precursors of growth factors. MMPs can also free peptide growth factors from the ECM and regulate proliferative signals through integrins. On the other hand, MMPs have both apoptotic and anti-apoptotic actions. MMP-3, MMP-7, MMP-9 and MMP-13 regulate apoptosis. Some of the mechanisms involved in the apoptosis control include the release of membrane-bound FAS ligand (FASL), a transmembrane stimulator of the death receptor FAS. Released FASL induces apoptosis of neighboring cells, or decreases cancer-cell apoptosis, depending on the system. MMPs are also involved in the escape from immunosurveillance in response to cancer. MMPs, including MMP-9, can suppress the proliferation of T-lymphocytes by cleaving the interleukin-2 receptor-a (IL-2Ra). MMPs also activate TGF-β, an important inhibitor of the T-lymphocyte response against tumors. In addition, several chemokines are known targets of MMPs. Processing of these chemokines by MMPs can result in increased or reduced infiltration and migration of leukocytes.
The relationship between MMP overproduction and tumor progression has encouraged the development of a variety of strategies aimed to block the proteolytic activities of these enzymes. However, several clinical trials with diverse MMP inhibitors have ended with disappointing results. The recent recognition of the complex roles of these enzymes during physiological and pathological condition may explain the lack of success of the first generation of MMP inhibitors. Increased knowledge of the function and regulation of this proteolytic system may lead to the development of new and, most importantly, more specific strategies to target these enzymes at different spatial and temporal points.

Regulation of MMPs
Despite the complexity of MMP regulation, four major levels of endogenous control can be recognized: proenzyme activation, inhibition of enzyme activity, transcriptional mechanisms and post-transcriptional regulation.
MMPs, like most proteolytic enzymes, are synthesized as inactive zymogens. MMP activation is achieved through the disturbance of the interaction between a cysteine-sulphydryl group in the propeptide domain and the zinc ion bound at the catalytic site. This mechanism is known as the cysteine-switch model. In vivo, MMP activation requires the participation of other proteases to remove the propeptide domain. In most cases, these activating proteases form part of proteolytic cascades that take place in the immediate pericellular space. In addition to this mechanism of pro-MMP activation, a set of MMPs such as the MT-MMPs, MMP-11, MMP-23 and MMP-28, has furin-like recognition sequence that allows intracellular activation by furin-like proprotein convertases. Alternative MMP activation mechanisms have been described involving the MMP binding to specific ligands or substrates, causing the formation of an S-nitrosylated derivative with the thiol group of the cysteine switch motif.
MMP activity may subsequently be regulated by the action of endogenous inhibitors. Some of them are general protease inhibitors such as 2-macroglobulin, which mainly block MMP activity in plasma and tissue fluids, while other inhibitors such as the tissue inhibitors of metalloproteinases (TIMPs) are more specific. Four TIMPs have been identified in vertebrates: TIMP-1 to TIMP-4, TIMP-1, TIMP-2 and TIMP-4 are secreted proteins while TIMP-3 is anchored in the ECM. The TIMPs are six-loop disulphide-bonded proteins containing two domains: the N-terminal and the C-terminal domains. The Nterminal domain is primarily responsible for the MMP inhibition through its binding to the catalytic domain in a substrate-like manner. The four TIMPs can inhibit the active forms of all MMPs tested so far. However, TIMP-1 is a poor inhibitor of MMP-9 and some MT-MMPs.
While TIMPs are the primary tissue inhibitors of MMPs, recently described proteins are also able to block MMP function. Some of these novel endogenous inhibitors contain sequences with similarity to the N-terminal domain of TIMPs. RECK (reversion-inducing cysteine-rich protein with kazak motifs) is an interesting inhibitor of MMPs. It is a glycosylphosphatidylinositol membrane-anchored glycoprotein widely express in human tissues.
expression of most of the MMPs is low in normal tissues and is strongly upregulated when ECM remodeling is required. MMP induction mechanisms appear to be different depending on the characteristics of the diverse cells with the ability to produce them. Spatial and temporal variations of MMP expression can be induced by cytokines, growth factors, chemical agents (such as tumor promoters), physical stress, activated oncogenes, and interactions with the ECM. Promoter regions of inducible MMPs contain multiple cis-acting elements including AP-1, PEA3 (ETS binding site), SP1, and NF-kB binding sites. It has been demonstrated that the ETS and AP-1 binding sites cooperate to enhance transcription, although upstream elements such us NF-kB are also necessary to precisely regulate MMP gene expression and tissue specificity. On the other hand, mRNA stabilization has also been described as a mechanism for MMP-1 and MMP-3 regulation.
A series of post-transcriptional mechanisms have been observed in the regulation of MMPs. For example, MMP-9 levels have been shown to be regulated at the level of translational efficiency, while translational repression can regulate protein levels of MMP-13. Also, post-translational modifications of some MMPs have been described. These include glycosylation, covalent linkage to lipocalin and chondroitin sulfate proteoglycans. Moreover, protein storage and trafficking are also major mechanisms for MMP regulation. Sequestration of the secreted MMPs in Golgi vesicles has been described for many stimulated cells, as has the storage of MMP-8 and MMP-9 in the secretory granules of PMN leucocytes. The MT-MMPs seem to have distinct trafficking pathways to specific sites at the cell surface. 

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