Localization of follistatin, an activin-binding protein, in bone tissues

We have previously reported that activin-A has a mitogenic effect on osteoblast-like MC3T3-E1 cells and that these cells produce an activin-binding protein, follistatin (Hashimoto, M., Shoda, A., Inoue, S., Yamada, R., Kondo, T., Sakurai, T., Ueno. N., and Muramatsu, M. (1992) J. Biol. Chem. 267, 4999-5004). Activin-A was also detected abundantly in bone matrix tissues and was found to act as an osteoclast differentiation factor. Here, we studied the expression of follistatin in bone tissues by in situ hybridization with a follistatin cRNA probe. Follistatin transcript was detected in the osteoblasts and a fraction of osteocyte population in the developing mouse mandible. Immunohistochemistry using anti-follistatin antibody supported this observation at the protein level. Follistatin mRNA was also detected in the callus of repairing bone after fracture. These findings suggest that activin and follistatin may play significant roles in the bone system in vivo.