Complete degradation of type X collagen requires the combined action of interstitial collagenase and osteoclast-derived cathepsin-B.

We have studied the degradation of type X collagen by metalloproteinases, cathepsin B, and osteoclast-derived lysates. We had previously shown (Welgus, H. G., C. J. Fliszar, J. L. Seltzer, T. M. Schmid, and J. J. Jeffrey. 1990. J. Biol. Chem. 265:13521-13527) that interstitial collagenase rapidly attacks the native 59-kD type X molecule at two sites, rendering a final product of 32 kD. This 32-kD fragment, however, has a Tm of 43 degrees C due to a very high amino acid content, and thus remains helical at physiologic core temperature. We now report that the 32-kD product resists any further attack by several matrix metalloproteinases including interstitial collagenase, 92-kD gelatinase, and matrilysin. However, this collagenase-generated fragment can be readily degraded to completion by cathepsin B at 37 degrees C and pH 4.4. Interestingly, even under acidic conditions, cathepsin B cannot effectively attack the whole 59-kD type X molecule at 37 degrees C, but only the 32-kD collagenase-generated fragment. Most importantly, the 32-kD fragment was also degraded at acid pH by cell lysates isolated from murine osteoclasts. Degradation of the 32-kD type X collagen fragment by osteoclast lysates exhibited the following properties: (a) cleavage occurred only at acidic pH (4.4) and not at neutral pH; (b) the cysteine proteinase inhibitors E64 and leupeptin completely blocked degradation; and (c) specific antibody to cathepsin B was able to inhibit much of the lysate-derived activity. Based upon these data, we postulate that during in vivo endochondral bone formation type X collagen is first degraded at neutral pH by interstitial collagenase secreted by resorbing cartilage-derived cells. The resulting 32-kD fragment is stable at core temperature and further degradation requires osteoclast-derived cathepsin B supplied by invading bone.