Expression, maturation, and rhodamine-based fluorescence assay of human cathepsin K expressed in CHO cells.
Cathepsin K is a cysteine protease that degrades type I human collagen during bone resorption. We have expressed the recombinant human cathepsin K in Chinese hamster ovary (CHO) cells as a pre-proenzyme and demonstrated that it is processed intracellularly to an active enzyme form and that only the proenzyme form is secreted. Immunofluorescence detection of cathepsin K in CHO cells resulted in discrete punctate distribution consistent with a lysosomal localization of the enzyme. With both extract and cell preparations of CHO cells expressing cathepsin K, [Z-Leu-Arg](2)-rhodamine was the best substrate for analyzing cathepsin K activity over background proteases. We have established a cellular-based assay to analyze cell-permeable inhibitors of cathepsin K and validated the assay with detection of intracellular versus extracellular activity, fluorescence-assisted cell sorter (FACS) analysis, and a selective cathepsin K inhibitor. The intracellular activity of cathepsin K was monitored by FACS analysis using the rhodamine substrate, which demonstrated an increased fluorescence over mock-transfected cells that was also inhibitable by (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester (E64d). A selective cathepsin K inhibitor, 1, 3-bis(CBZ-Leu-NH)-2-propanone, had an IC(50) of 134 nM in the CHO/Cat K cells, which is the same potency as that measured against a purified enzyme preparation of cathepsin K. Therefore, we have established a system to evaluate intracellular cathepsin K activity and inhibition by cell-permeable inhibitors of this thiol protease.
Department of Biochemistry and Molecular Biology, Merck Frosst Centre for Therapeutic Research, Kirkland, Quebec, Canada.
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D000818:Animals; D016466:CHO Cells; D002401:Cathepsin B / Q000378:metabolism; D056657:Cathepsin K; D002403:Cathepsins / Q000235:genetics / Q000378:metabolism*; D006224:Cricetinae; D005455:Fluorescent Antibody Technique; D006801:Humans; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012235:Rhodamines / Q000378:metabolism; D014162:Transfection
D011994:Recombinant Proteins; D012235:Rhodamines; D002403:Cathepsins; D002401:Cathepsin B; C534130:CTSK protein, human; D056657:Cathepsin K