| Research Abstract |
Although the action of bone morphogenetic protein (BMP) on osteoblast differentiation has been extensively investigated, its effect on osteoclast differentiation remained unknown. In this study, in vitro effects of BMP-2 on osteoclast differentiation and bone resorption were examined. BMP-2 (1-100ng/ml) significantly stimulated bone resorption in mouse bone cell culture, whereas it did not affect the bone-resorbing activity of isolated rabbit osteoclasts. BMP-2 also stimulated osteoclast differentiation in bone cell culture. We next examined the direct effect of BMP-2 on osteoclast precursors in the absence of stromal cells, using hemopoietic blast cells derived from spleen cells. The mRNA for BMP type 1 receptor was detected in these osteoclast precursor cells. And BMP-2 stimulated osteoclast formation from these hemopoietic blast cells supported by GM-CSF.The present data were the first to indicate that BMP-2 stimulates bone resorption through both direct stimulation of osteoclast fo
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rmation and activation of mature osteoclasts, possibly via stromal cells, in vitro. Next, we present a cDNA encoding a sulfate transporter (designed as st-ob for sulfate transporter from osteoblast) that was cloned as an induced gene in osteoblasts in association with osteoblastic differentiation. Based on the fact that BMP-2 induces osteoblastic phenotypes in immature fibroblastic C3H10T1/2 cells, we performed a subtraction hybridization between BMP-2-treated and -untreated cells, and have isolated one clone induced by BMP-2 to be constantly expressed in osteoblastic cells. The deduced amino acid sequence and proposed structure of st-ob were mostly identical to human diastrophic dysplasia sulfate transporter gene product (DTDST). St-ob mRNA was abundantly expressed in the thymus, testis, calvaria and osteoblastic MC3T3-E1 cells, while its expression was faint in C3H10T1/2 cells. BMP-2 increased sulfate incorporation in C3H10T1/2 cells about two fold of the baseline level. Proteoglycan is one of major components of extracellular matrix of bone and cartilage, and they induce a large amount of sulfate. In bone, osteoblasts actively uptake sulfate to synthesize proteoglycans. The sulfate transporter system in osteoblasts is mediated partly by st-ob distinct from sat-1, and st-ob is expressed during differentiation as a fundamental feature of osteoblasts. Less
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