Biosynthesis and processing of cathepsin K in cultured human osteoclasts

Cathepsin K (cat K) is the major cysteine protease expressed in osteoclasts and is thought to play a key role in matrix degradation during bone resorp tion, However, little is known regarding the synthesis, activation, or turn over of the endogenous enzyme in osteoclasts, In this study, we show that m ature cat K protein and enzyme activity are localized within osteoclasts, P ulse-chase experiments revealed that, following the synthesis of pro cat K, intracellular conversion to the mature enzyme occurred in a time-dependent manner. Subsequently, the level of mature enzyme decreased. Little or no c at K was observed in the culture media at any timepoint, Pretreatment of os teoclasts with either chloroquine or monensin resulted in complete inhibiti on of the processing of newly synthesized cat K, In addition, pro cat K dem onstrated susceptibility to treatment with N-glycosidase F, suggesting the presence of high-mannose-containing oligosaccharides. Treatment of osteocla sts with the PI3-kinase inhibitor, Wortmannin (WT), not only prevented the intracellular processing of cat K but also resulted in the secretion of pro enzyme into the culture media. Taken together, these results suggest that t he biosynthesis, processing, and turnover of cat K in human osteoclasts is constitutive and occurs in a manner similar to that of other known cysteine proteases, Furthermore, cat K is not secreted as a proenzyme, but is proce ssed intracellularly, presumably in lysosomal compartments prior to the rel ease of active enzyme into the resorption lacunae, (Bone 28:282-289; 2001) (C) 2001 by Elsevier Science Inc. All rights reserved.