Possible involvement of protein kinases and Smad2 signaling pathways on osteoclast differentiation enhanced by activin A

Bone tissues reportedly contain considerable amounts of activin A and folli statin, an activin A-binding protein, in the present study, we found that f ollistatin strongly inhibited osteoclast formation in cocultures of mouse b one marrow cells and primary osteoblasts induced by 1 alpha ,25 dihydroxyvi tamin D3, prostaglandin E-2, and. interleukin-1 alpha. Antibody aganist act ivin A also inhibited the osteoclast formation. Furthermore, activin A syne rgistically stimulated osteoclast differentiation mediated by receptor acti vator NF-kappaB ligand (RANKL). RT-PCR analysis revealed that osteoblasts p roduced not only activin A but also follistatin. Western blot analysis of a panel of phosphorylated proteins revealed that activin A stimulated the ph osphorylation of p44/42 mitogen activated protein (MAP) kinase (ERK1/2) and p38 MAP kinase in macrophage colony-stimulating factor-dependent bone marr ow macrophages (M-BMM Phis). In addition, phosphorylation of Smad2 was obse rved in M BMM Phis stimulated with activin A. These findings indicate that the phosphorylation of p44/42 MAP kinase, p38 MAP kinase, and Smad. is invo lved in activin A-enhanced osteoclast differentiation induced by RANKL. Tak en together, these results suggest that both activin A and follistatin prod uced by osteoblasts may play an important role in osteoclast differentiatio n through MAP kinases and Smad2 signaling pathways. (C) 2001 Wiley-Liss, In c.