MOLECULAR-CLONING OF A NOVEL MEMBRANE-TYPE MATRIX METALLOPROTEINASE FROM A HUMAN BREAST-CARCINOMA

A new member of the matrix metalloproteinase (MMP) family of enzymes h as been cloned from a human breast carcinoma cDNA library. The isolate d cDNA contains an open reading frame 1554 bp long, encoding a polypep tide of 518 amino acids. The predicted amino acid sequence displays a similar domain organization as the remaining MMPs, including a prodoma in with the activation locus, the zinc-binding site, and the hemopexin domain. In addition, it contains a C-terminal extension, rich in hydr ophobic residues and similar in size to those present in the different membrane-type MMPs (MT-MMPs) identified to date. On the basis of thes e structural characteristics, this novel MMP has been tentatively call ed MT4-MMP, because it represents the fourth member of this subclass o f MMPs characterized mainly by the occurrence of putative transmembran e domain in their amino acid sequences. MT4-MMP also contains a nine-r esidue insertion between the propeptide and the catalytic domain, whic h is a common feature of MT-MMPs and stromelysin-3. This amino acid se quence insertion ends with the consensus sequence R-X-R/K-R, which see ms to be essential in the activation of these proteinases by furin. No rthern blot analysis of polyadenylated RNAs isolated from a variety of human tissues revealed that the MT4-MMP gene (MMP-17) is expressed ma inly in the brain, leukocytes, the colon, the ovary, and the testis. T he expression of MT4-MMP in leukocytes together with its putative memb rane localization suggest that this enzyme could be involved in the ac tivation of membrane-bound precursors of growth factors or inflammator y mediators such as tumor necrosis factor-alpha. In addition, MT4-MMP transcripts were detected in all breast carcinomas, as well as in all breast cancer cell lines analyzed in the present work. On the basis of these expression data in breast tumors, a potential role for human MT 4-MMP in the tumoral process is also suggested.