Human MT6-matrix metalloproteinase: identification, progelatinase A activation, and expression in brain tumors

The localization of proteolytic enzymes at the cell surface is a widely used strategy for facilitating tumor invasion. In this study, we have cloned a new member of the membrane-type subfamily of matrix metalloproteinases (MT-MMPs), a group of enzymes associated with tumor progression. The cloned cDNA encodes a protein of 562 amino acids with a domain organization similar to that of other MT-MMPs, including a prodomain with a cysteine switch, a catalytic domain with the zinc-binding site, a hemopexin-like domain, and a COOH-terminal extension rich in hydrophobic residues. The predicted protein sequence also contains a short insertion of basic residues located between the propeptide and the catalytic domain and involved in the proteolytic activation of MT-MMPs by furin-like enzymes. Furthermore, immunofluorescence and Western blot analysis of COS-7 cells transfected with the isolated cDNA revealed that the encoded protein is localized at the cell surface. Based on these properties, this novel human matrix metalloproteinase has been called MT6-MMP because it is the sixth identified member of this subfamily of matrix metalloproteinase. Cotransfection of expression plasmids encoding MT6-MMP and progelatinase A resulted in activation of COS-7-secreted progelatinase A, as demonstrated by gelatin zymography. In contrast, transfection of progelatinase A cDNA alone did not lead to the activation of the proenzyme. Northern blot analysis of polyadenylated RNAs isolated from human tissues demonstrated that MT6-MMP is predominantly expressed in leukocytes, lung, and spleen. MT6-MMP was also detected at high levels in SW480 colon carcinoma cells as well as in some anaplastic astrocytomas and glioblastomas, but not in normal colon or brain or in meningiomas. On the basis of these results, we propose that MT6-MMP may facilitate tumor progression through its ability to activate progelatinase A at the membrane of cells from colon carcinomas or brain tumors.