title: Endocytosis of tartrate-resistant acid phosphatase by osteoblast-like cells is followed by inactivation of the enzyme.
abstract: Tartrate-resistant acid phosphatase (TRACP) is generally used as a marker of osteoclasts. Yet, other bone-associated cells, such as osteoblasts and osteocytes, may also express activity of this enzyme. Osteoblasts containing TRACP activity are seen particularly in the vicinity of areas of bone resorption, suggesting that osteoclasts somehow induce TRACP activity in osteoblasts. In a recent study, we found that osteoblast-like cells appeared to have the capacity to endocytose TRACP released by osteoclast precursors. In the present study, we investigated the endocytosis of TRACP in more detail as well as the fate of the endocytosed enzyme. We found that incubation of osteoblast-like cells with TRACP-coated beads resulted in attachment of a high number of beads to the cells. After culturing osteoblast-like cells with medium conditioned by blood monocytes that contain TRACP, activity of the enzyme was found in the cells. Following replacement of the medium by normal medium that did not contain TRACP, a decrease in the level of TRACP activity in osteoblast-like cells occurred. Our data strongly suggest that osteoblast-like cells recognize TRACP released by osteoclast precursors and that upon endocytosis inactivation of the enzyme occurs. We propose that uptake of the enzyme is important for the control of enzyme activity, thereby preventing degradation of matrix constituents.
authors.affiliation: Department of Periodontology, Academic Center for Dentistry Amsterdam, Universiteit van Amsterdam, Louwesweg 1, Amsterdam, 1066 EA, The Netherlands.
authors.forename: S
authors.initials: S
authors.lastname: Perez-Amodio
authors.forename: D C
authors.initials: DC
authors.lastname: Jansen
authors.forename: W
authors.initials: W
authors.lastname: Tigchelaar-Gutter
authors.forename: W
authors.initials: W
authors.lastname: Beertsen
authors.forename: V
authors.initials: V
authors.lastname: Everts
mesh_terms: D000135:Acid Phosphatase / Q000378:metabolism*; D001862:Bone Resorption / Q000378:metabolism*; D002478:Cells, Cultured; D018920:Coculture Techniques; D017077:Culture Media, Conditioned; D004705:Endocytosis / Q000502:physiology*; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D007527:Isoenzymes / Q000378:metabolism*; D008297:Male; D009000:Monocytes / Q000378:metabolism; D010006:Osteoblasts / Q000378:metabolism*; D010010:Osteoclasts / Q000378:metabolism; D000071681:Tartrate-Resistant Acid Phosphatase
chemical_list: D017077:Culture Media, Conditioned; D007527:Isoenzymes; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase