Signal transductions induced by bone morphogenetic protein-2 and transforming growth factor-beta in normal human osteoblastic cells

Transforming growth factor beta (TGF-beta) activates Ras/MAPK signaling in many cell types. Because TGF-beta 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-beta stimulated Ras, MAPK, and AP-1 activities. The DNA binding activities of c-Fos, FosB/Delta FosB, Fra-1, Fra-2, and JunB were up-regulated whereas JunD activity was decreased. c-Fos, FosB/Delta FosB, and JunB were associated with Smad4. The stimulation of AP-1 by BMP-2 and TGF-beta was dependent on Smad signaling, and anti-Smad4 antibody interfered with AP-1 activity. Thus, BMP-2 and TGF-beta 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-beta 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-beta 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-beta and BMP-2 function in osteoblasts.     

Heregulin-β promotes matrix metalloproteinase-7 expression via HER2-mediated AP-1 activation in MCF-7 cells

It has been reported that HER2 level is strongly correlated with the expression of MMP-7 in some carcinomas. HER2 is a preferred heterodimerization partner of EGFR, HER3, and HER4. HER2 overexpression is believed to enhance the signaling from these receptors in response to binding of their specific ligands. In this study, we show that heregulin-beta (HRG-beta) stimulation remarkably induced MMP-7 promoter activity and significantly enhanced the expression and activity of MMP-7 in MCF-7 cells overexpressing HER2. The expression of c-Jun and c-Fos and the level of the phosphorylated c-Jun were markedly increased after HRG-beta treatment in MCF-7/HER2 cells. Increased MMP-7 promoter activity was observed in MCF-7/c-Jun cells. The activity of the MMP-7 promoter induced by HRG-beta in MCF-7/HER2 cells could be inhibited by a dominant negative c-Jun mutant TAM67 and by the mutagenesis of the AP-1 site. c-Jun binding to MMP-7 promoter was confirmed by ChIP assays. The data indicate a close link among HRG-beta stimulation, HER signaling, and AP-1 activation. Our data suggest that HRG-beta-induced MMP-7 expression was regulated by HER2-mediated AP-1 activation in MCF-7 cells.