Constitutive expression of osteoclast-stimulating activity by normal clonal osteoblast-like cells: effects of parathyroid hormone and 1,25-dihydroxyvitamin D3.

Osteoblasts are known to produce osteoclast-stimulating activity (OSA). The aim of the current study was to relate the expression of OSA to the osteoblastic phenotype and examine its regulation by calciotropic hormones. The study was performed with the normal osteoblastic cell clone CRP 10/30 and the preosteoblastic clone CRP 4/7. OSA was determined with the well described isolated osteoclast pit assay, using sperm whale dentine as substrate. In contrast to previous studies, the assay was carried out at pH 7.36, rather than at pH 6.4 or 6.9. The results indicate that over 24 h, CRP 10/30 cells produce constitutively OSA, which compared to controls corresponds to an about 7-fold increase in resorption pits. There was considerably less activity expressed by either CRP 4/7 cells or fibroblasts. OSA proved to be heat labile, and its mol wt was estimated to be over 10 kilodaltons. While PTH-(1-34) did not influence the synthesis of OSA, the number of pits formed by osteoclasts incubated with medium conditioned by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]-treated CRP 10/30 cells was increased 3-fold above baseline values. A similar increase was obtained with 1.25-(OH)2D3 added directly to CRP 10/30-conditioned medium. These results could not be duplicated with 1,25-(OH)2D3 added to either control medium or medium conditioned by CRP 4/7 cells or fibroblasts. The present study shows that normal clonal bone cells synthesize constitutively OSA, which is not regulated by PTH or 1,25-(OH)2D3. Furthermore, the results suggest that the synthesis of bone cell-derived OSA is limited to cells expressing the mature osteoblastic phenotype. Finally, CRP 10/30-conditioned medium appears to permit 1,25-(OH)2D3 to function on osteoclasts.