Fibrodysplasia ossificans progressiva (FOP) is a rare autosomal dominant disorder characterized by congenital malformation of the great toes and by progressive heterotopic bone formation in muscle tissue. Recently, a mutation involving a single amino acid substitution in a bone morphogenetic protein (BMP) type I receptor, ALK2, was identified in patients with FOP. We report here that the identical mutation, R206H, was observed in

Although transforming growth factor (TGF)-␤ enhances bone formation, it inhibits the differentiation of osteoblasts. To clarify the regulatory mechanism of osteoblastic differentiation and TGF-␤ actions, the relationship among differentiation, TGF-␤ actions, and matrix protein synthesis was examined using murine osteoblast-like MC3T3-E1 cells. Alkaline phosphatase (ALP) activity continued to increase during long-term cultures, and the increase was closely associated with a reduction in cell surface TGF-␤ receptors competent to bind TGF-␤. Both the stimulation of proteoglycan synthesis and the inhibition of ALP activity by TGF-␤ were also suppressed. Collagen synthesis inhibitors and an anti-␣2␤1 integrin blocking antibody blocked the changes in ALP activity and TGF-␤ receptors, and a DGEA peptide that interferes binding of collagen to ␣2␤1 integrin also blocked the increase in ALP activity. Furthermore, when MC3T3-E1 cells were cultured on extracellular matrix layers obtained from these cells, all the differentiation-associated changes could be observed without collagen production, and the extracellular matrix-induced differentiation was also blocked by an anti-␣2␤1 integrin antibody. These results demonstrate that the interaction of cell surface ␣2␤1 integrin with matrix collagen synthesized by osteoblasts themselves is involved in the osteoblastic differentiation and the reduction in cell surface receptors and actions of TGF-␤. It is suggested that matrix collagen synthesized under the stimulation by TGF-␤ plays an important role in the regulation of osteoblastic differentiation and TGF-␤ actions by differentiation-associated down-regulation of TGF-␤ receptors.Cells of osteoblast lineage exert various functions to maintain bone formation. After bone resorption, osteoblast precursors migrate and proliferate at the site of bone formation. They, then, synthesize type I collagen and other matrix proteins. Onto the newly formed unmineralized matrices, hydroxyapatite crystals are accumulated, and mineralization of bone is completed (1). In order to form lamellar bones that maintain structural integrity and physical strength, it appears to be of critical importance for osteoblastic cells to maintain sequentially ordered development of these multiple functions.Using cultured osteoblastic cells obtained from bone or of clonal origin, it has become clear that these cells express various functional properties in a differentiation-dependent manner from the early proliferation phase, via the matrix formation phase with active matrix protein synthesis, to the mineralization phase (1-3). Various hormones and cytokines are shown to affect the differentiation process and functional properties of these cells. However, the mechanism that controls the differentiation of osteoblastic cells is as yet unclear.Transforming growth factor-␤ (TGF-␤) 1 is stored in bone matrix as a latent form (4), is thought to be released and activated during osteoclastic bone resorption (5), and to play an important role in the regulation of bone meta...

Cytokines in interleukin (IL)-11 subfamily participate in the regulation of bone cell proliferation and differentiation. We report here positive effects of IL-11 on osteoblasts and bone formation. Overexpression of human IL-11 gene in transgenic mice resulted in the stimulation of bone formation to increase cortical thickness and strength of long bones, and in the prevention of cortical bone loss with advancing age. Bone resorption and osteoclastogenesis were not affected in IL-11 transgenic mice. In experiments in vitro, IL-11 stimulated transcription of the target gene for bone morphogenetic protein (BMP) via STAT3, leading to osteoblastic differentiation in the presence of BMP-2, but inhibited adipogenesis in bone marrow stromal cells. These results indicate that IL-11 is a stimulatory factor for osteoblastogenesis and bone formation to conserve cortical bone, possibly by enhancing BMP actions in bone. IL-11 may be a new therapeutic target for senile osteoporosis.

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Growth factors such as fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) that activate extracellular signal-regulated kinases (ERKs) through receptor tyrosine kinases (RTKs) stimulate proliferation but suppress differentiation of osteoblasts. To study the mechanism of this inhibitory action of these growth factors on osteoblastic differentiation, we evaluated Smad1 transactivity in MC3T3-E1 osteoblast-like cells by reporters of promoter activity of mouse Smad6, an early response gene to bone morphogenetic proteins (BMPs). FGF-2 and EGF inhibited alkaline phosphatase activity and Smad6 promoter activity stimulated by BMP-2. Overexpression of constitutively active MEK by adenovirus mimicked, but that of dominant negative Ras or treatment with a MEK1 inhibitor, PD098059, reversed, the inhibitory effects of these growth factors on both activities. These effects are mediated by BMP-responsive elements (BMPREs) on Smad6 promoter, because an artificial reporter driven by three tandem BMPREs gave similar results, and these effects were all abolished when the BMPREs were mutated. RTK-ERK activation inhibited the promoter activity even when