CATHEPSIN-K, BUT NOT CATHEPSIN-B, CATHEPSIN-L, OR CATHEPSIN-S, IS ABUNDANTLY EXPRESSED IN HUMAN OSTEOCLASTS

Random high throughput sequencing of a human osteoclast cDNA library w as employed to identify novel osteoclast-expressed genes. Of the 5475 ESTs obtained, approximately 4% encoded cathepsin K, a novel cysteine protease homologous to cathepsins S and L; ESTs for other cathepsins w ere rare. In addition, ESTs for cathepsin K were absent or at low freq uency in cDNA libraries from numerous other tissues and cells. In situ hybridization in osteoclastoma and osteophyte confirmed that cathepsi n K mRNA was highly expressed selectively in osteoclasts; cathepsins S , L, and B were not detectable. Cathepsin K was not detected by in sit u hybridization in a panel of other tissues. Western blot of human ost eoclastoma or fetal rat humerus demonstrated bands of 38 and 27 kDa, c onsistent with sizes predicted for pro- and mature cathepsin K. Immuno localization in osteoclastoma and osteophyte showed intense punctate s taining of cathepsin K exclusively in osteoclasts, with a polar distri bution that was more intense at the bone surface. The abundant express ion of cathepsin K selectively in osteoclasts strongly suggests that i t plays a specialized role in bone resorption. Furthermore, the data s uggest that random sequencing of ESTs from cDNA libraries is a valuabl e approach for identifying novel cell-selective genes.