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

Abstract

Long-term treatment of mice or humans with granulocyte colony-stimulating factor (G-CSF) is associated with a clinically significant osteopenia characterized by increased osteoclast activity and number. In addition, recent reports have observed a decrease in number of mature osteoblasts during G-CSF administration. However, neither the extent of G-CSF's suppressive effect on the osteoblast compartment nor its mechanisms are well understood. Herein, we show that short-term G-CSF treatment in mice leads to decreased numbers of endosteal and trabecular osteoblasts. The effect is specific to mature osteoblasts, because bone-lining cells, osteocytes, and periosteal osteoblasts are unaffected. G-CSF treatment accelerates osteoblast turnover in the bone marrow by inducing osteoblast apoptosis. In addition, whereas G-CSF treatment sharply increases osteoprogenitor number, differentiation of mature osteoblasts is impaired. Bone marrow transplantation studies show that G-CSF acts through a hematopoietic intermediary to suppress osteoblasts. Finally, G-CSF treatment, through suppression of mature osteoblasts, also leads to a marked decrease in osteoprotegerin expression in the bone marrow, whereas expression of RANKL remains relatively constant, suggesting a novel mechanism contributing to the increased osteoclastogenesis seen with long-term G-CSF treatment. In sum, these findings suggest that the hematopoietic system may play a novel role in regulating osteoblast differentiation and apoptosis during G-CSF treatment.

Mechanisms of G-CSF-mediated hematopoietic stem and progenitor mobilization

“…56,59,61 Strikingly, G-CSF also results in a profound suppression of mature osteoblast number and function, 11,13,60,79 with the death of half of all osteoblasts because of apoptosis. 80 Moreover, the surviving osteoblasts reduce their expression of CXCL12 mRNA by approximately one-half. 60 Interestingly, osteoblast suppression is a common feature of HSPC mobilization by other cytokines, including Flt3 ligand and kitL.…”

Regulation of hematopoiesis in endosteal microenvironments

“…Although the mechanism of how G-CSF mobilizes these immature cells has been investigated intensely, it is still not fully understood. Osteoblasts at the endosteal surface play an essential role in this process [33][34][35]. Mobilization is finely tuned by complex signals from multiple cell types, such as the sympathetic nervous system (SNS) [34,36] and BM resident macrophages [37][38][39].…”

Hematopoietic stem cell mobilizing agents G-CSF, cyclophosphamide or AMD3100 have distinct mechanisms of action on bone marrow HSC niches and bone formation

“…Furthermore, genetically modified C57BL/6 mice lacking some of these proteases mobilize normally in response to G-CSF. 10 In the past 5 years, it has emerged that G-CSF mobilizes in part by (1) blocking proliferation and osteogenic differentiation of mesenchymal stem cells (MSC), 11 (2) inducing osteoblast apoptosis 12 and (3) blocking bone formation. 13,14 This results in a dramatic reduction in the expression of chemokines and cytokines necessary to maintain and retain HSC in their endosteal and perivascular niches such as CXCL12, kit ligand (KL) and angiopoietin-1.…”

“…The effect of G-CSF on osteoblasts, HSC niches and BM macrophages has recently been reported, 12,13,21,22 however, it is not known whether CYP and AMD3100 have similar effects. As AMD3100 is progressively replacing CYP as adjuvant to G-CSF to mobilize HSC in lymphoma and multiple myeloma patients who previously failed to mobilize in response to G-CSF alone, 2 we compared the effect of these three mobilizing agents on BM macrophages, bone formation, osteoblasts and HSC niche function.…”