Tartrate-resistant acid phosphatase 5b is a marker of osteoclast number and volume in RAW 264.7 cells treated with receptor-activated nuclear κB ligand

Tartrate-resistant acid phosphatase 5b (TRACP 5b) has been used as a biomarker of bone resorption and cancer metastasis. TRACP 5b has also been suggested to be a reliable marker of osteoclast number. In this study, the correlation of TRACP 5b level and osteoclast-like cell number was investigated in RAW 264.7 cells treated with receptor-activated nuclear factor κB ligand (RANKL). RAW 264.7 cells were cultured with α-MEM containing RANKL (40 ng/ml) for 3, 5 and 7 days. Osteoclast formation and TRACP 5b levels were determined by TRACP staining, scanning electron microscopy and enzyme-linked immunosorbent assay. The RAW 264.7 cells that were not exposed to RANKL did not secrete TRACP 5b. RANKL induced the RAW 264.7 cells to differentiate into osteoclasts and to secrete TRACP 5b. The TRACP 5b level in the RAW 264.7 cells treated with RANKL was significantly correlated with the number and volume of osteoclasts (r=0.95 and r=0.92, respectively; P<0.0001). TRACP 5b is a good marker of RANKL-induced osteoclast formation in RAW 264.7 cells. TRACP 5b analysis may be used as an alternative to osteoclast counting in vitro.