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

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Abstract
Bone tissues reportedly contain considerable amounts of activin A and follistatin, an activin A-binding protein. In the present study, we found that follistatin strongly inhibited osteoclast formation in cocultures of mouse bone marrow cells and primary osteoblasts induced by 1α,25 dihydroxyvitamin D3, prostaglandin E2, and interleukin-1α. Antibody aganist activin A also inhibited the osteoclast formation. Furthermore, activin A synergistically stimulated osteoclast differentiation mediated by receptor activator NF-κB ligand (RANKL). RT-PCR analysis revealed that osteoblasts produced not only activin A but also follistatin. Western blot analysis of a panel of phosphorylated proteins revealed that activin A stimulated the phosphorylation of p44/42 mitogen activated protein (MAP) kinase (ERK1/2) and p38 MAP kinase in macrophage colony-stimulating factor-dependent bone marrow macrophages (M-BMMΦs). In addition, phosphorylation of Smad2 was observed in M-BMMΦs stimulated with activin A. These findings indicate that the phosphorylation of p44/42 MAP kinase, p38 MAP kinase, and Smad2 is involved in activin A-enhanced osteoclast differentiation induced by RANKL. Taken together, these results suggest that both activin A and follistatin produced by osteoblasts may play an important role in osteoclast differentiation through MAP kinases and Smad2 signaling pathways.

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