Identification of matrix metalloproteinase 9 in rabbit osteoclasts

Osteoclasts are multinucleate giant cells that play key roles in bone resorption. To identify genes predominantly expressed in osteoclasts, we screened a cDNA library of osteoclasts with cDNA probes of osteoclasts and alveolar macrophages. Clones specifically hybridizing to the osteoclast probe were isolated and sequenced. The nucleotide sequence of one such clone, F17, was found to share significant similarity with the sequences of human and mouse matrix metalloproteinase 9 (MMP-9) cDNA. By isolation and sequencing of the full-length cDNA, F17 was revealed to encode the rabbit counterpart of MMP-9. By Northern blotting, messenger RNA for MMP-9 was found to be highly and predominantly expressed in isolated osteoclasts when compared with its level in alveolar macrophages and other tissues. By gelatin zymography, gelatinase activity was detected in conditioned medium of isolated osteoclasts, suggesting that MMP-9 is secreted by isolated osteoclasts. Expression of MMP-9 was also observed in in vivo osteoclasts in metacarpal bones of newborn rabbits by in situ hybridization. These facts suggest that MMP-9 is one of the major proteases produced by osteoclasts and possibly plays a role in osteoclastic bone resorption.