Collagenolytic activity of cathepsin K is specifically modulated by cartilage-resident chondroitin sulfates

Cathepsin K is the predominant cysteine protease in osteoclast-mediated bon e remodeling, and the protease is thought to be involved in the pathogenesi s of diseases with excessive bone and cartilage resorption. Osteoclastic ma trix degradation occurs in the er;extracellular resorption lacuna and upon phagocytosis within the cell's lysosomal-endosomal compartment. Since glyco saminoglycans (GAGs) are abundant in extracellular matrixes of cartilage an d growing bone, we have analyzed the effect of GAGs on the activity of bone and cartilage-resident cathepsins K and L and MMP-1. GAGs, in particular c hondroitin sulfates, specifically and selectively increased the stability o f cathepsin K but had no effect on cathepsin L and MMP-1. GAGs strongly enh anced the stability and, to a lesser extent, the catalytic activity of cath epsin K. To combine the activity and stability parameters, we defined a nov el kinetic term, named cumulative activity (CA), which reflects the total s ubstrate turnover during the life span of the enzyme. In the presence of ch ondroitin-4-sulfate (C-4S), the CA value increased 200-fold for cathepsin K but only 25-fold with chondroitin-6-sulfate (C-6S). C-4S dramatically incr eased the hydrolysis of soluble as well insoluble type I and II collagens, whereas the effects of C-6S and hyaluronic acid were less pronounced. C-4S acts in a concentration-dependent manner but reaches saturation at approxim ately 0.1%, a concentration similar to that found in the synovial fluid of arthritis patients. C-4S increased the cathepsin K-mediated release of hydr oxyproline from insoluble type I collagen 10-fold but had only a less than 2-fold enhancing effect on the hydrolysis of intact cartilage. The relative ly small increase in the hydrolysis of cartilage by C-4S was attributed to the endogenous chondroitin sulfate content present in the cartilage. Althou gh C-4S increased the pH stability at neutral pH, a significant increase in the collagenolytic activity of cathepsin K at this pH was not observed, th us suggesting that the unique collagenolytic activity of cathepsin K at aci dic pH is mechanistically determined and not by the enzyme's instability at neutral pH, The selective and significant stabilization and activation of cathepsin K activity by C-4S may provide a rationale for a novel mechanism to regulate the enzyme's activity during bone growth and aging, two process es known for significant changes in the GAG content.