Initiation of Osteoclast Bone Resorption by Interstitial Collagenase

Abstract

Osteoclasts form an acidic compartment at their attachment site in which bone demineralization and matrix degradation occur. Although both the cysteine proteinases and neutral collagenases participate in bone resorption, their roles have remained unclear. Here we show that interstitial collagenase has an essential role in initiating bone resorption, distinct from that of the cysteine proteinases. Treatment of osteoclasts with cysteine proteinase inhibitors did not affect the number of resorption lacunae ("pits") formed on the surface of dentine slices, but it generated abnormal pits that were demineralized but filled with undegraded matrix. Treatment with metalloproteinase inhibitors did not alter the qualitative features of lacunae, but it greatly reduced the number of pits and surface area resorbed. Treatment of bone cells with an inhibitory anti-rat interstitial collagenase antiserum reduced bone resorption markedly. In the presence of collagenase inhibitors, resorption was restored by pretreatment of dentine slices with rat interstitial collagenase or by precoating the dentine slices with collagenase-derived gelatin peptides or heatgelatinized collagen. Immunostaining revealed that interstitial collagenase is produced at high levels by stromal cells and osteoblasts adjacent to osteoclasts. These results indicate that interstitial collagenase can function as a "coupling factor," allowing osteoblasts to initiate bone resorption by generating collagen fragments that activate osteoclasts.Normal bone turnover is highly regulated. Osteoclasts, the cells that degrade bone, require activation to trigger their boneresorptive capacity. Once activated, osteoclasts secrete both protons and proteinases at their attachment site, resulting in dissolution of bone mineral and degradation of the matrix (1). Osteoclasts produce several cysteine proteinases (2-4), enzymes with acidic pH optima, of which cathepsin K appears to be essential for normal bone resorption (5, 6). Studies indicate that the major function of secreted cysteine proteinases is matrix degradation (3, 7).The role of neutral metalloproteinases, a second class of proteinase produced in bone tissue (3,8), is less clear. Members of the metalloproteinase family have neutral pH optima, are secreted as proenzymes, and contain a zinc atom at the active site (9, 10). Although some prior studies suggested that neutral metalloproteinases contribute to osteoclast matrix degradation (11), recent evidence indicates that osteoclasts do not produce collagenase (12). Collagenase is produced by cells of osteoblastic lineage and may be required for resorption of intact bone tissue (13)(14)(15)(16). Observations that isolated osteoclasts that had no detectable collagenase activity were able to resorb bone prompted the suggestion that collagenase promotes resorption by removing unmineralized matrix from the bone surface, facilitating osteoclast attachment (8,17,18).In this study we have examined the qualitative and quantitative roles of acid cysteine proteinases and interstitial...

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