title: Increased tartrate-resistant Acid phosphatase expression in osteoblasts and osteocytes in experimental osteoporosis in rats.
abstract: Tartrate-resistant acid phosphatase (TRAP) is known as an osteoclast marker, but osteoblasts and osteocytes in the vicinity of bone remodeling sites also express TRAP. Cell culture studies suggest that osteoblasts endocytose osteoclastic TRAP for inactivation. To evaluate whether changes in osteoclast activity could alter TRAP expression in osteoblasts and/or osteocytes in vivo, we studied the ovariectomized and vitamin D-deficient rat (Ovx-D) and rats healing from rickets. Bone sections were analyzed for TRAP gene expression by in situ hybridization, TRAP protein by immunogold labeling, and TRAP enzyme activity using the fluorescent substrate ELF97. Osteoblasts and osteocytes close to intracortical remodeling sites and bone surfaces demonstrated TRAP, most prominently in cancellous bone and osteocytes. Intracellular TRAP was located to electron-dense vesicles with similar morphology in both cell types. Ovx-D increased osteoclast activity (p < 0.001) and ELF97⁺ osteocytes (p < 0.05) in cancellous bone, but no corresponding increase was observed in the osteocyte lacunar area. The level of TRAP⁺ vesicles in cortical osteoblasts (p < 0.01) in Ovx-D rats was also increased. Enhanced osteoclast activity was noted in healing rickets after 72 h (p < 0.05), but no differences in TRAP expression were detected in osteoblasts or osteocytes. Thus, increased osteoclast activity does not affect TRAP expression in osteoblasts and osteocytes, favoring the notion that increased TRAP in these cells is rather due to increased synthesis. Although the role of TRAP in osteoblasts and osteocytes remains elusive, we speculate that the function is related to the capability of the enzyme to regulate the phosphorylation of proteins known to be expressed by these cells.
authors.affiliation: Department of Pathology, Oslo University Hospital, Rikshospitalet, Postbox 4950, Nydalen, 0424, Oslo, Norway, lene.b.solberg@rr-research.no.
authors.forename: Lene B
authors.initials: LB
authors.lastname: Solberg
authors.forename: Sverre-Henning
authors.initials: SH
authors.lastname: Brorson
authors.forename: Gunhild A
authors.initials: GA
authors.lastname: Stordalen
authors.forename: Espen S
authors.initials: ES
authors.lastname: Bækkevold
authors.forename: Göran
authors.initials: G
authors.lastname: Andersson
authors.forename: Finn P
authors.initials: FP
authors.lastname: Reinholt
mesh_terms: D000135:Acid Phosphatase / Q000378:metabolism*; D000818:Animals; D016723:Bone Remodeling / Q000502:physiology; D004195:Disease Models, Animal; D005260:Female; D005455:Fluorescent Antibody Technique; D006801:Humans; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007527:Isoenzymes / Q000378:metabolism*; D046529:Microscopy, Electron, Transmission; D010006:Osteoblasts / Q000201:enzymology; D010010:Osteoclasts / Q000201:enzymology; D010011:Osteocytes / Q000201:enzymology*; D015663:Osteoporosis, Postmenopausal / Q000201:enzymology*; D051381:Rats; D012279:Rickets / Q000201:enzymology*; D000071681:Tartrate-Resistant Acid Phosphatase
chemical_list: D007527:Isoenzymes; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase