Colony stimulating factor-1 plays a role in osteoclast formation and function in bone resorption induced by parathyroid hormone and parathyroid hormone-related protein

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Abstract
Although colony stimulating factor‐1 (CSF‐1) plays a key role in osteoclast recruitment, studies examining the effect of CSF‐1 on mature osteoclasts indicate that it may directly inhibit bone resorption by isolated rat osteoclasts. To define further CSF‐1's role in bone remodeling, we examined the effect of neutralizing antisera to CSF‐1 on basal and parathyroid hormone (PTH)‐induced bone resorption using two organ culture assays designed to examine the recruitment of osteoclast precursors and the activation of mature osteoclasts, respectively. We first assessed whether PTH increases CSF‐1 production from bone in organ culture by examining conditioned medium from 19‐day‐old fetal rat long bones in a mitogenesis assay employing a CSF‐1‐responsive cell line, CRX‐1. Conditioned medium from untreated bones induced a titratable increase in CRX‐1 cell proliferation, and treatment of bones with PTH for 72 h caused a significant increase in mitogenic activity. CSF‐1 antiserum caused a significant decrease in mitogenic activity in conditioned medium, indicating that bone in organ culture produces CSF‐1 constitutively and in response to PTH. To examine bone‐derived CSF‐1's role in bone resorption, we examined the effect of neutralizing antisera to CSF‐1 on basal and PTH‐induced bone resorption in the fetal rat long bone assay, which reflects activation of mature osteoclasts. Anti‐CSF‐1 caused a significant increase in unstimulated and PTH‐induced bone resorption compared with control. By contrast, in the fetal mouse metacarpal assay, which examines proliferation and recruitment of osteoclast progenitors and precursors, anti‐CSF‐1 caused significant inhibition of PTH related protein (PTHrP)‐induced bone resorption after 3 and 6 days of incubation. Consistent with these findings, histological examination of cultured 17‐day‐old fetal metacarpals demonstrated that anti‐CSF‐1 inhibits the formation of tartrate‐resistant acid phosphatase‐positive osteoclasts in PTHrP‐treated explants, whereas it has no effect on unstimulated bones. We conclude that bone‐derived CSF‐1 may have a dual role in PTH/PTHrP‐induced bone resorption by enhancing the appearance of osteoclast precursors while restraining the resorptive function of mature osteoclasts.

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