Metalloproteinase inhibitors

Metalloproteinases (MPs) play key roles in the responses of cells to their microenvironment. By effecting proteolytic degradation or activation of cell surface and extracellular matrix (ECM) proteins they can modulate both cell-cell and cell-ECM interactions, which influence cell differentiation, migration, proliferation and survival. Both secreted and membranebound forms of metalloproteinases have been implicated in pericellular proteolysis, including the matrix metalloproteinases (MMPs), the adamalysin-like proteinases with both metalloproteinase and disintegrin-like domains (ADAMs and their counterparts that have a thrombospondin-1-like domain, ADAM-TSs) and the astacins (Werb, 1997). Cells use various strategies to regulate extracellular proteinases: transcriptional regulation, trafficking of membrane-bound forms (secretion and endocytosis), activation of latent proenzyme forms, extracellular binding proteins and the action of endogenous inhibitors. Here we will discuss the role of metalloproteinase inhibitors, from the well-known tissue inhibitors of metalloproteinases (TIMPs) and α2-macroglobulin through to newer and less well-understood putative inhibitors (Fig. 1). We look at the available evidence that their other roles in cell biology do not all relate to their metalloproteinase inhibitory activity. Finally, we discuss the potential for use of such natural metalloproteinase inhibitors as therapeutic agents. Tissue inhibitors of metalloproteinases (TIMPs): basic structure and activity Four mammalian TIMPs have been cloned, purified and characterised. These secreted proteins are thought to regulate MMP activity during tisssue remodelling. One TIMP gene has been identified in Drosophila, and its ablation generates a phenotype similar to that of integrin mutants, which indicates that it has a role in ECM function (Godenschwege et al., 2000). All four mammalian TIMPs have many basic similarities, but they exhibit distinctive structural features, biochemical properties and expression patterns (Table 1). This suggests that each TIMP has specific roles in vivo. As in Drosophila, the mammalian TIMP genes are embedded intragenically in intron 5 of synapsin genes (Edwards, 2000). The TIMPs have molecular weights of ~21 kDa and are variably glycosylated (Table 1). They have six disulphide bonds and comprise a three-loop N-terminal domain and an interacting three-loop C-subdomain. Most of the biological functions of these proteins discovered thus far are attributable to sequences within the N-terminal domain, although the Csubdomains mediate interactions with the catalytic domains of some MMPs and with the hemopexin domains of MMP-2 and MMP-9 (Brew et al., 2000). The TIMPs are secreted proteins, but may be found at the cell surface in association with membrane-bound proteins; for example, TIMP-2, TIMP-3 and 3719