The majority of hematopoietic stem/progenitor cells (HSPCs) reside in bone marrow (BM) surrounded by a specialized environment, which governs HSPC function. Here we investigated the potential role of bone remodeling cells (osteoblasts and osteoclasts) in homeostasis and stress-induced HSPC mobilization. Peripheral blood (PB) and BM in steady/mobilized state were collected from healthy donors undergoing allogeneic transplantation and from mice treated with granulocyte colony stimulating factor (G-CSF), parathyroid hormone (PTH), or receptor activator of nuclear factor kappa-B ligand (RANKL). The number and the functional markers of osteoblasts and osteoclasts were checked by a series of experiments. Our data showed that the number of CD45(-) Ter119(-) osteopontin (OPN)(+) osteoblasts was significantly reduced from 4,085 ± 135 cells/femur on Day 0 to 1,032 ± 55 cells/femur on Day 5 in mice (P = 0.02) and from 21.38 ± 0.66 on Day 0 to 14.78 ± 0.65 on Day 5 in healthy donors (P < 0.01). Decrease of osteoblast number leads to reduced level of HSPC mobilization regulators stromal cell-derived factor-1 (SDF-1), stem cell factor (SCF), and OPN. The osteoclast number at bone surface (OC.N/B.s) was significantly increased from 1.53 ± 0.12 on Day 0 to 4.42 ± 0.46 on Day 5 (P < 0.01) in G-CSF-treated mice and from 0.88 ± 0.20 on Day 0 to 3.24 ± 0.31 on Day 5 (P < 0.01) in human. Serum TRACP-5b level showed a biphasic trend during G-CSF treatment. The ratio of osteoblasts number per bone surface (OB.N/B.s) to OC.N/B.s was changed after adding PTH plus RANKL during G-CSF treatment. In conclusion, short term G-CSF treatment leads to reduction of osteoblasts and stimulation of osteoclasts, and interrupting bone remodeling balance may contribute to HSPC mobilization.
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