Role of activator protein 1 transcriptional activity in the regulation of gene expression by transforming growth factor β1 and bone morphogenetic protein 2 in ROS 17/2.8 osteoblast-like cells

In osteoblastic cells, transforming growth factor β1 (TGF-β1) has been found to regulate the expression of a variety of proto-oncogenes including c-fos, c-jun, and junB. The c-fos in particular has been implicated in the mitogenic effect of TGF-β1. Here, we examined the role of these early response genes in the regulation of osteoblast (OB) gene expression by two members of the TGF-β superfamily, TGF-β1 and bone morphogenetic protein 2 (BMP-2). In ROS 17/2.8 cells, TGF-β1 as well as BMP-2 up-regulated the expression of junB and c-fos messenger RNAs (mRNAs), and this increase was correlated in both cases with an increase in activator protein 1 (AP-1) DNA-binding activity involving JunB and c-Fos proteins. Protein kinase C (PKC)- and protein tyrosine kinase (PTK)-dependent pathways have been implicated in both TGF-β1 signaling and AP-1 gene regulation. Therefore, using the kinase inhibitors chelerythrine chloride and genistein, we showed that PKC and PTK activities, respectively, participated in TGF-β1- and BMP-2-induced increases in junB mRNA levels. Similarly, these kinase activities were involved in the stimulatory effect of BMP-2 on c-fos mRNA expression. Using a natural dominant negative for AP-1 transcriptional activity in ROS 17/2.8 cells, we then showed that AP-1 transcription factors mediated TGF-β1- and BMP-2-regulated expression of the (α1) collagen I gene as well as TGF-β1-regulated expression of the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor. Our data emphasize the role of the AP-1 transcription factor in TGF-β1 and BMP-2 signaling and highlight the importance of this transcription factor family in the expression of OB genes.