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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00223-013-9834-3) contains supplementary material, which is available to authorized users.
Highlights
Tartrate-resistant acid phosphatase (TRAP; ACP5, EC 3.1.3.2)— known as purple acid phosphatase, uteroferrin, or type 5 acid phosphatase [1]—has been an established marker for osteoclasts and bone resorption for more than 50 years
Increased osteoclast activity was observed in ovariectomized and vitamin D-deficient rat (Ovx-D) versus sham (Fig. 1a) as well as in healing rickets after 72 h compared to fulminant rickets and normal controls, reflecting the healing of the growth plate with enhanced resorption monitored by an increased CTX/TRAP 5b ratio in serum (Fig. 1b)
In both models osteocytes and osteoblasts in cancellous bone and in cortical bone close to intracortical remodeling sites and endosteal/periosteal surfaces demonstrated TRAP gene expression and translation to protein as well as TRAP enzyme activity, no farther away than 166 lm. This is in line with the observations made by Nakano et al [16], who demonstrated TRAP? osteocytes in rat bone in the range of 200 lm from the resorption surfaces and concluded that there is a close relation between TRAP? osteocytes and bone resorption
Summary
Tartrate-resistant acid phosphatase (TRAP; ACP5, EC 3.1.3.2)— known as purple acid phosphatase, uteroferrin, or type 5 acid phosphatase [1]—has been an established marker for osteoclasts and bone resorption for more than 50 years. The origin and function of TRAP in these cells have been debated; one hypothesis is that osteoclastic TRAP from the resorption lacunae is endocytosed by the osteoblasts and/or osteocytes This theory is supported by cell culture studies reporting that osteoblast-like cells are able to engulf osteoclastic TRAP and inactivate the enzyme, suggesting that this could control the enzyme activity and prevent further degradation of matrix constituents [17, 18]. Qing and coworkers [19] have demonstrated enlarged osteocyte lacunae and canaliculi and increased amounts of TRAP and cathepsin K in osteocytes in lactating mice, suggesting that osteocytes are able to remodel their own matrix environment through osteoclast-like mechanisms under specific conditions
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