The Cell-surface Form of Colony-stimulating Factor-1 Is Regulated by Osteotropic Agents and Supports Formation of Multinucleated Osteoclast-like Cells

Abstract

Colony-stimulating factor-1 (CSF-1) is a hematopoietic growth factor that is released by osteoblasts and is recognized to play a critical role in bone remodeling in vivo and in vitro. CSF-1 is synthesized as a soluble or cellsurface protein. It is unclear, however, whether human osteoblasts express both molecular forms of CSF-1, and whether these isoforms can independently mediate osteoclastogenesis. In the present study, using a combination of quantitative reverse transcriptase polymerase chain reaction, flow cytometry, and Western immunoblot analysis, we have demonstrated that human osteoblast-like cells as well as primary human osteoblasts express the cell-surface form of CSF-1 both constitutively and in response to parathyroid hormone and tumor necrosis factor. Furthermore, using an in vitro coculture system, we have shown that cell-surface CSF-1 alone is sufficient to support osteoclast formation. These findings may be especially significant in view of evidence that direct cell-to-cell contact is critical for osteoclast formation, and suggest that differential regulation of expression of the CSF-1 isoforms may influence osteoclast function modulated by osteotropic hormones.The precise mechanism whereby osteoblasts mediate osteoclastic bone resorption is unclear. One widely held hypothesis is that activated osteoblasts secrete cytokines that directly or indirectly influence osteoclast formation or function (1). Although the exact nature of all of these cytokines is unknown, compelling in vivo and in vitro data have emerged to support a role for colony-stimulating factor-1 (CSF-1) 1 as an osteoblastderived factor involved in osteoclast formation. Thus, in vivo, deficiency of CSF-1 in the op/op osteopetrotic mouse causes a failure of osteoclast formation and bone resorption (2-4), while in vitro studies have demonstrated that CSF-1 is critical for the proliferation and differentiation of osteoclast progenitors (5, 6), that CSF-1 stimulates bone resorption in the fetal mouse metacarpal assay (7), and that CSF-1 receptors are present on osteoclasts (8, 9). Additionally, we have reported that CSF-1 is the principal colony-stimulating activity released from osteoblasts constitutively and in response to PTH and parathyroid hormone-related protein (8). In support of a role for CSF-1 in bone remodeling in humans, Sarma et al. (10) have recently reported that, consistent with studies in mice, recombinant human CSF-1 induces osteoclastogenesis and bone resorption in human marrow cultures.Multiple human CSF-1 mRNA species (4.0, 3.0, 2.3, 1.9, and 1.6 kb) are expressed by the CSF-1 gene (11-15), and molecular cloning of cDNAs derived from these transcripts has demonstrated that the size differences are due to alternative splicing in exon 6 and the alternative use of the 3Ј-end exons 9 or 10 (11-13). A combination of nucleotide sequence analysis and transfection studies indicates that two distinct CSF-1 protein products are encoded by these transcripts. Both primary translation products are membrane-bound glycoproteins t...

c-Fms and the αvβ3 integrin collaborate during osteoclast differentiation

“…Importantly, the β 3 -/-mouse is markedly hypocalcemic (7), and thus almost certainly hyperparathyroid. This observation raises the possibility that the relative normalization of OC differentiation in vivo reflects the influence of excess parathyroid hormone, which mediates its osteoclastogenic effect by prompting expression of RANKL and MCSF (38)(39)(40)(41). Our data suggest that increased levels of the latter cytokine, in the bone microenvironment, may account for the observed in vitro versus in vivo differences.…”

Concerted Action of Smad and CREB-binding Protein Regulates Bone Morphogenetic Protein-2-stimulated Osteoblastic Colony-stimulating Factor-1 Expression

“…Previous studies have shown that interleukin-1 and tumor necrosis factor are potent stimulators of CSF-1 gene expression (49). CSF-1 is released from osteoblasts constitutively and in response to parathyroid hormone and parathyroid hormonerelated protein (50). Another interesting report shows the involvement of hydrogen peroxide in transforming growth factor-␤-induced CSF-1 expression (51).…”

Incomplete restoration of colony-stimulating factor 1 (CSF-1) function in CSF-1–deficient Csf1op/Csf1op mice by transgenic expression of cell surface CSF-1

“…18,27 csCSF-1 is expressed in all cell types examined that express soluble CSF-1, including fibroblasts and osteoblasts. 18,[24][25][26][27][28][29]36,37 It has been shown to support macrophage proliferation 25,38 and the formation of multinucleated osteoclastlike cells in vitro 37,39,40 and in vivo. 41 csCSF-1 was proposed to play adhesion molecule-like roles in its interaction with the CSF-1 receptor (CSF-1R) on leukemic cells in a culture system.…”