MATRIX METALLOPROTEASESMatrix metalloproteases (MMPs) are a family of zinc-dependent secreted or transmembrane proteins which degrade extracellular matrix components under physiological and pathological conditions (1, 2). Their main structural and functional similarities are a) a catalytic domain characterized by an HEXGH motif and the presence of a zinc atom essential for the proteolytic activity; b) a propeptide in the aminoterminal region, characterized by the PRCGVPD sequence responsible for latency in zymogens. Additional domains are characteristic of the different subfamilies of MMPs and contribute to substrate specificity, inhibitor binding, matrix binding and cell-surface localization. MMPs can be divided into three major subgroups on the basis of their substrate target: collagenases, which degrade fibrillar collagen; gelatinases, which degrade denatured and basement membrane collagens and stromelysins, which degrade proteoglycans and glycoproteins. An additional group, the transmembrane domain family (MTMMPs), is the most recently discovered class of MMPs. The members of this family contain a sequence spanning the cellular membrane and are involved in the proteolytic activation of other MMPs (3).
Physiological inhibitors of MMPsThe catalytic activity of the MMPs is regulated at multiple levels including transcription, secretion, activation of the proenzyme and function of the activated enzyme. The messenger RNA of MMPs is transcriptionally regulated by growth factors, cytokines, hormones, oncogenes and tumor promoters (4-7). mRNA stability and translation are also regulated (8,9). MMPs are synthesized in a latent form which needs to be activated by proteolytic cleavage by other enzymes; activation is thus another way to regulate the amount of active MMPs. A final level of regulation is the inhibition of MMP activity by non-specific (α2-macroglobulin) or specific inhibitors, the tissue inhibitors of MMPs (TIMP-1, TIMP-2, TIMP-3 and TIMP-4) (10).TIMPs bind non-covalently to the catalytic domain of MMPs, thereby inhibiting their activity. It has also been shown that TIMP-2 forms a trimolecular complex with MT1-MMP and pro-MMP2 on the cell surface and regulates the formation of mature MMP-2 (11).The International Journal of Biological Markers / Vol. 14 no. 4, pp. 232-238 © Wichtig Editore, 1999