Cystatin B as an intracellular modulator of bone resorption.
Degradation of organic bone matrix requires proteinase activity. Cathepsin K is a major osteoclast proteinase needed for bone resorption, although osteoclasts also express a variety of other cysteine- and matrix metalloproteinases that are involved in bone remodellation. Cystatin B, an intracellular cysteine proteinase inhibitor, exhibits a lysosomal distribution preferentially in osteoclasts but it's role in osteoclast physiology has remained unknown. The current paper describes a novel regulatory function for cystatin B in bone-resorbing osteoclasts in vitro. Rat osteoclasts were cultured on bovine bone and spleen-derived cystatin B was added to the cultures. Nuclear morphology was evaluated and the number of actively resorbing osteoclasts and resorption pits was counted. Intracellular cathepsin K and tartrate-resistant acid phosphatase (TRACP) activities were monitored using fluorescent enzyme substrates and immunohistology was used to evaluate distribution of cystatin B in rat metaphyseal bone. Microscopical evaluation showed that cystatin B inactivated osteoclasts, thus resulting in impaired bone resorption. Cathepsin K and TRACP positive vesicles disappeared dose-dependently from the cystatin B-treated osteoclasts, indicating a decreased intracellular trafficking of bone degradation products. At the same time, cystatin B protected osteoclasts from experimentally induced apoptosis. These data show for the first time that, in addition to regulating cysteine proteinase activity and promoting cell survival in the nervous system, cystatin B inhibits bone resorption by down-regulating intracellular cathepsin K activity despite increased osteoclast survival.
Laitala-Leinonen
Tiina
T
Bone Biology Research Consortium, Department of Anatomy, University of Turku, Kiinamyllynkatu 10, FIN-20520 Turku, Finland. tilale@utu.fi
Rinne
Riitta
R
Saukko
Pekka
P
Väänänen
H Kalervo
HK
Rinne
Ari
A
D000818:Animals; D001862:Bone Resorption; D056657:Cathepsin K; D002403:Cathepsins / Q000378:metabolism; D002417:Cattle; D002470:Cell Survival; D002478:Cells, Cultured; D055313:Cystatin B; D015891:Cystatins / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000494:pharmacology; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000378:metabolism* / Q000494:pharmacology; D003599:Cytoskeleton / Q000378:metabolism; D006801:Humans; D010010:Osteoclasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats
C524994:CSTB protein, human; D015891:Cystatins; D015853:Cysteine Proteinase Inhibitors; D055313:Cystatin B; D002403:Cathepsins; D003546:Cysteine Endopeptidases; C534130:CTSK protein, human; D056657:Cathepsin K; C534133:Ctsk protein, rat