LOCALIZATION OF CATHEPSIN-K IN HUMAN OSTEOCLASTS BY IN-SITU HYBRIDIZATION AND IMMUNOHISTOCHEMISTRY

We have recently cloned cathepsin K from a human bone cDNA library, Si nce cathepsins are proposed to be involved in the degradation of miner alized bone matrix, we have investigated, by in situ hybridization and immunocytochemistry, the expression of the cathepsin K mRNA transcrip ts and protein in sections of bone and giant cell tumor to determine w hich cells express this enzyme, Within all tissues studied, cathepsin K was highly expressed in osteoclasts. Furthermore, the expression of cathepsin K mRNA in giant cell tumor tissue appeared to be confined to the periphery of the osteoclast indicating a compartmentalization of the mRNA, Immunohistochemistry confirmed the specific localization of cathepsin K to the osteoclast, In actively resorbing osteoclasts, the immunostaining was localized at the ruffled border, whereas in osteocl asts in sections of giant cell tumor, staining was observed in lysosom al vacuoles, which in some cases mere seen to fuse with the cell membr ane, Other cells within the bone, such as osteoblasts and osteocytes, did not express either the cathepsin K transcript or protein, However, there were very low levels of cathepsin K detected in a population of mononuclear cells, possibly representing osteoclast progenitor cells, within the marrow/stromal layer, The specific localization of catheps in K within osteoclasts would therefore indicate the potential role of this enzyme in the bone resorptive process. (C) 1997 by Elsevier Scie nce Inc.