Osteoclast-Mediated Bone Resorption Is Stimulated During Short-Term Administration of Granulocyte Colony-Stimulating Factor But Is Not Responsible for Hematopoietic Progenitor Cell Mobilization

Abstract

The cellular and molecular mechanisms responsible for hematopoietic progenitor cell (HPC) mobilization from bone marrow (BM) into peripheral blood after administration of cytokines such as granulocyte colony-stimulating factor (G-CSF) are still unknown. In this study we show that high concentrations of soluble calcium induce the detachment of BM CD34+ HPC adherent on fibronectin, a major component of BM extracellular matrix. Because G-CSF has been shown to induce osteoporosis in patients with congenital neutropenia and in G-CSF–overexpressing transgenic mice, we hypothesized that short-term G-CSF administration may be sufficient to induce bone resorption, resulting in the release of soluble calcium in the endosteum leading in turn to the inhibition of attachment to fibronectin and the egress of HPC from the BM. We show herein that in humans, serum osteocalcin concentration, a specific marker of bone formation, is strongly reduced after 3 days of G-CSF administration. Furthermore, in patients mobilized with G-CSF either alone or in association with stem cell factor or interleukin-3, the reduction of serum osteocalcin is significantly correlated with the number of HPC mobilized in peripheral blood. Urine levels of deoxypyridinoline (DPyr), a specific marker of bone resorption, gradually elevated during the time course of G-CSF administration until day 7 after cessation of G-CSF, showing a simultaneous stimulation of bone degradation during G-CSF–induced HPC mobilization. In an in vivo murine model, we found that the number of osteoclasts was dramatically increased paralleling the elevation of DPyr after G-CSF administration. When pamidronate, an inhibitor of osteoclast-mediated bone resorption, was administered together with G-CSF in mice, the G-CSF–induced increase of DPyr levels was completely abolished whereas the numbers of colony-forming cells mobilized in peripheral blood were not decreased, but unexpectedly increased relative to the numbers elicited by G-CSF alone. Collectively, our data therefore show that short-term administration of G-CSF induces bone degradation by a simultaneous inhibition of bone formation and an enhanced osteoclast-mediated bone resorption. This increased bone resorption is inhibited by pamidronate without reducing G-CSF–induced HPC mobilization, suggesting that the activation of bone resorption after G-CSF administration is not the direct cause of HPC mobilization as initially hypothesized, but a parallel event. © 1998 by The American Society of Hematology.

Granulocyte Colony-Stimulating Factor Induces Osteoblast Apoptosis and Inhibits Osteoblast Differentiation

“…6A and 6B). (6,8) In fact, a significant increase in osteoclasts was not noted until after 14 days of G-CSF treatment.…”

“…Previous studies have established that chronic treatment with G-CSF leads to increased osteoclast number and activity. (4)(5)(6)23,24) Although there is evidence that G-CSF can directly activate the osteoclast lineage, (8) the potent suppressive effect of G-CSF on osteoblasts suggests another possibility. Namely, because osteoblasts contribute to the regulation of osteoclastogenesis, the reduced number of osteoblasts during G-CSF treatment may secondarily activate osteoclasts.…”

“…G-CSF treatment in-creases osteoclast number in treated mice and increases the level of urine deoxypyridinoline in humans. (6) Several potential mechanisms by which G-CSF stimulates osteoclastogenesis have been advanced. G-CSF increases the proliferation of myeloid progenitors, potentially increasing the pool of monocytic precursors from which osteoclasts derive.…”

“…Whereas long-term G-CSF treatment results in increased osteoclastogenesis, recent evidence suggests that short-term G-CSF administration decreases osteoblast number and activity. (6,9,10) Our laboratory has shown that administration of G-CSF for 5 days in mice results in a marked decrease in histologically identifiable osteoblasts along bone surfaces and a corresponding decrease in osteocalcin mRNA. (10) Whether this decrease in osteoblast number results from increased osteoblast turnover or from a defect in osteoblast development remains unclear.…”

Hematopoietic Stem Cell Trafficking

“…Osteoclasts, which participate in HSPC egress following stress, 162 represent a prime cellular candidate for additional G-CSF-mediated signals, although other studies have revealed a paradoxical increase of circulating HSPCs when mice were administered pamidronate, a potent inhibitor of bone resorption, together with G-CSF. 163 Using a knockin mouse model expressing green fluorescent protein (GFP) under the Cxcl12 promoter, recent studies have confirmed the presence of a large subset of HSCs adjacent to CXCL12-abundant reticular (CAR) cells, which are largely near BM vessels. 164 It is intriguing that most SNS innervation of the BM is associated with blood vessels and numerous gap junctions have been noted by electron microscopy between nerve fibers and reticular cells, 151 suggesting that stromal cells are neurally regulated.…”