Identification of the membrane-type matrix metalloproteinase MT1-MMP in osteoclasts

The osteoclasts are the cells responsible for bone resorption. Matrix metalloproteinases (MMPs) appear crucial for this process. To identify possible MMP expression in osteoclasts, we amplified osteoclast cDNA fragments having homology with MMP genes, and used them as a probe to screen a rabbit osteoclast cDNA library. We obtained a cDNA of 1,972 bp encoding a polypeptide of 582 amino acids that showed more than 92% identity to human, mouse, and rat membrane-type 1 MMP (MT1-MMP), a cell surface proteinase believed to trigger cancer cell invasion. By northern blotting, MT1-MMP was found to be highly expressed in purified osteoclasts when compared with alveolar macrophages and bone stromal cells, as well as with various tissues. In situ hybridization on bone sections showed that MT1-MMP is expressed also in osteoclasts in vivo. Antibodies recognizing MT1-MMP reacted with specific plasma membrane areas corresponding to lamellipodia and podosomes involved, respectively, in migratory and attachment activities of the osteoclasts. These observations highlight how cells might bring MT1-MMP into contact with focal points of the extracellular matrix, and are compatible with a role of MT1-MMP in migratory and attachment activities of the osteoclast.