| The majority of HSPC reside in the bone marrow surrounded by specialized bone-shielded environment. The specialized microenvironment or niche not only provides a favorable habitat for HSPC maintenance and development but also governs stem cell function. Here we investigated the potential role of bone remodeling osteoblasts and osteoclasts in homeostasis and stress-induced mobilization of hematopoietic progenitors. Herein, our results shown that1. Short term G-CSF treatment leads to decreased number and activity of endosteal and trabecular osteoblasts in mice as well as in humam beings. Mouse osteocalcin mRNA expression was sharply reduced 27±6 fold (P＜0.01) after 3 days treatment of G-CSF, and returned to normal levels on the fifth day of G-CSF cessation. The number of CD45-Ter119-OPN+osteoblast was significantly reduced (control nonmobilized,4085±135 cells/femur mobilized, d3,1118±80 cells/femur; mobilized, d5,1032±55 cells/femur;P=0.02). A significant decrease in serum osteocalcin protein was detected in G-CSF-treated mice (control nonmobilized,59.44±3.16 ng/ml; mobilized, d3,39.21±4.49 ng/ml; mobilized, d5,42.36±2.23 ng/ml;P＜0.01)2. Diminished osteoblast leads to SDF-1, SCF, and OPN falling, mouse osteoblast SDF-1 mRNA expression decline 3.44±0.3 fold (P＜0.05) on the third day of G-CSF mobilization.3. G-CSF treatment induced osteoblasts retraction is partly due to osteoblast apoptosis but not inhibits osteoblast differentiation inhibition through Wnt pathway.4. G-CSF acts through an indirect pathway to suppress osteoblasts, osteocalcin levels were 1.53±0.02 ng/mL (with G-CSF) vs 1.47±0.08 ng/mL (without G-CSF), P=0.89.5. G-CSF administration stimulates osteoclast activity, serum TRAP-5b levels showed lightly drop down on day 3 both in human (5.04±0.43 U/L on day 0 vs 3.45±0.37 U/Lon day 3, P=0.03) and mouse (5.43±1.2 U/L on day 0 vs 4.04±0.86 U/L on day 3, P=0.47) while increased gradually after that day (5.04±0.43 U/L on day 0 vs 6.87±0.57 on day 5,P=0.04) in human and (5.43±1.2 U/L on day 0 vs 13.06±1.65 on day 5, P=0.02) in mouse.In conclusion:Mobilization-induced changes in the osteoblast compartment were apparent before changes in HSCs. The loss of mature osteoblast may be the underlying reason of stem cell mobilization. Above findings indicated that bone remodeling plays an important role during HS cell mobilization. We further tested the hypothesis that targeting the niche might improve stem cell-based therapies using six mouse models to mimic the multiple rounds of chemotherapy followed by autologous HSC transplantation in a clinical setting. Herein, we show that1. Multiple rounds treatment of cytotoxic drugs influence osteoblasts and HSPCs, osteocalcin mRNA expression was reduced 9.32±0.3 fold (P＜0.01) in CTLs group and 16.82±0.8 fold (P＜0.01) in Gs group compared with untreated mouse, circulation osteocalcin decreased in CTLs group (33.81±1.99 ng/ml) and Gs group (27.18±1.09 ng/ml) compared with untreated mice (59.44±3.16 ng/ml) P＜0.01. The number of bone marrow HSPCs reduced significantly in comparison to untreated mice (29.17±1.22U, untreated vs 21.16±1.35U, CTLs vs 13.00±1.71U, Gs, P=0.01)2. Pharmacologic use of PTH or RANKL increases the number of HS cells mobilized into the peripheral blood for stem cell harvests and protects stem cells from repeated exposure to cytotoxic chemotherapy.These data provide evidence that targeting the HSC niche may enhance stem cell-based therapies.In conclusion:Cytotoxic chemotherapy markedly depletes HS cells in bone marrow. Targeting the niche can protect and expand the resident HS cell pool in the bone marrow during chemotherapy rounds, especially if co-administered with G-CSF. |
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