Signal Transductions Induced by Bone Morphogenetic Protein-2 and Transforming Growth Factor-β in Normal Human Osteoblastic Cells

Transforming growth factor β (TGF-β) activates Ras/MAPK signaling in many cell types. Because TGF-β and BMP-2 exert similar effects, we examined if this signaling is stimulated by both factors and analyzed the relationship between this signaling and the Smads in osteoblasts. BMP-2 and TGF-β stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/ΔFosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/ΔFosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-β was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-β activate both Ras/MAPK/AP-1 and Smad signaling in osteoblasts with Smads modulating AP-1 activity. To determine the roles of MAPK in BMP-2 and TGF-β function, we analyzed the effect of ERK and p38 inhibitors on the regulation of bone matrix protein expression and JunB and JunD levels by these two factors. ERK and p38 mediated TGF-β suppression of osteocalcin and JunD as well as stimulation of JunB. p38 was essential in BMP-2 up-regulation of type I collagen, fibronectin, osteopontin, osteocalcin, and alkaline phosphatase activity whereas ERK mediated BMP-2 stimulation of fibronectin and osteopontin. Thus, ERK and p38 differentially mediate TGF-β and BMP-2 function in osteoblasts.