Osteoclasts are involved in bone resorption, and its activation is considered one of the causes of osteoporosis. The pit assay is the principal method for evaluating osteoclast function by measuring hydroxyapatite resorption in vitro. However, the pit assay requires time and trained techniques, including the pit image analysis, and there is no other easy method for evaluating bone resorption. In this study, we developed a novel approach to quantify the bone resorption activity using a calcium phosphate (CaP) coating labeled with fluorescent polyanion. Fluoresceinamine-labeled chondroitin polysulfate or Hoechst 33258-labeled deoxyribonucleic acid was used for CaP labeling. When macrophage cell line RAW264 was cultured on the labeled CaP under the stimulation with the receptor activator of the NF-κB ligand (RANKL), RAW264 cells differentiated into osteoclastic cells and the fluorescence intensity of the culture supernatant and pit area increased in a time- and dose-dependent manner. Furthermore, drugs for osteoporosis treatment, such as pamidronate and β-estradiol, inhibited fluorescein release by the cells stimulated with RANKL. A positive correlation between the fluorescence intensity and pit area was observed (r = 0.917). These results indicated that this new method using fluorescent polyanion-labeled CaP is a standardized useful assay system for the evaluation of bone resorption activity.
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