| Abstract: | Normal human bone marrow stroma cells include stem cells for both hematopoietic and osteochondrogenic lineages and express both bone morphogenetic protein (BMP) type I and II receptors. As a member of the TGF-beta superfamily, BMP-6 binds to both BMP receptors and is involved in the developmental processes of renal and hepatic systems as well as of human fetal intestine. BMP-6 induces osteoblastic differentiation of pluripotent mesenchymal stem cells and is an autocrine stimulator of chondrocyte differentiation. The present study was carried out to investigate the effect of BMP-6 on human cobblestone-area-forming cells (CAFC), which represent the functional primitive repopulating hematopoietic stem cell in long-term marrow culture. The effect of BMP-6 on marrow stroma production of interleukin-6, interleukin-11 and their common transducing receptor gp130, which is expressed in primitive hematopoietic stem cells and is indispensable for the proliferation and tri-lineage differentiation of these hematopoietic stem cells, was examined. The effect of BMP-6 on marrow stroma release of soluble adhesion molecule VCAM-1 mediating the primitive hematopoietic stem cell adhesion to marrow stroma was examined. The number of CAFC was significantly reduced after BMP-6 treatment from 88 ± 10 in control cultures in a dose-dependent manner to only 48 ± 3 per 105 cells in 50 ng/ml BMP-6– treated marrow stroma cultures, p < 0.01. Quantitative ELISA measurement revealed 50 ng/ml BMP-6 to significantly reduce IL-6 and IL-11 production from human marrow stroma on day 5, p < 0.01. BMP-6 significantly increased soluble gp130 release by 7.4 fold in 50 ng/ml BMP-6-treated marrow stroma cultures. This profound rapid increase in the natural antagonist of human IL-6 cytokine family could abrogate the gp130 signalling required for proliferation and/or tri-lineage differentiation of human primitive hematopoietic stem cells and may lead to hematopoietic stem cell survival. This study revealed that in 50 ng/ml BMP-6-treated marrow stroma cultures, the released soluble VCAM-1 increased by 2-fold. Marked increases in the soluble adhesion molecule could exert a significant antagonist effect on the function of VCAM-1 (ligand for VLA4) and may allow the mobilization of human hematopoietic stem cells. Recently, blocking the VLA4/VCAM-1 adhesion pathway was shown to mobilize hematopoietic CD34+ cells in normal individuals. Also, we previously observed a significantly lower expression of VLA4 (CD49d) on G-CSF-mobilized CD34+ cells in the peripheral blood than on bone marrow CD34+ before mobilization in the same individuals. BMPs are currently being used in clinical trials for bone repair and fracture healing; the present results suggest a possible novel therapeutic role for BMP-6 in mobilizing CD34− cells for transplantation. However, the potential therapeutic use of BMP-6 as a mobilizing agent should only be applied in patients with myeloid leukemia and not other tumors due to the expression of BMP-6 in solid tumors and lack of BMP expression in myeloid leukemia cells. |
