Protein kinase signalling pathways involved in the up-regulation of the rat α1(I) collagen gene by transforming growth factor β1 and bane morphogenetic protein 2 in osteoblastic cells

Transforming growth factor beta (TGF beta) family members are known for their important role in bone physiology. TGF beta(1) and, to a smaller extent, bone morphogenetic protein 2 (BMP-2) have been reported to regulate the gene expression of different osteoblast markers in vitro. However, little is known about the molecular mechanisms involved in these actions. Here we report that BMP-2 like TGF beta(1), up-regulated alpha 1(I) collagen mRNA expression in ROS 17/2.8 osteoblastic cells. This was mediated through an increase in the transcriptional rate of the gene rather than through the stabilization of alpha 1(I) collagen mRNA, and required new protein synthesis. In addition, TGF beta(1)- and BMP-2-induced increases in alpha 1(I) collagen mRNA levels were both dependent on protein kinase C and protein tyrosine kinase activities. Furthermore, the mitogen-activated protein kinase (MAPK) [MAPK/extracellular signal-regulated protein kinase kinase 1/extracellular signal-regulated protein kinase (MEK-1/ERK)] pathway participated in the up-regulation of alpha 1(I) collagen gene expression by TGF beta(1) and BMP-2. In response to either TGF beta(1) or BMP-2, the stimulation of alpha 1(I) collagen mRNA levels was paralleled by an early increase in extracellular signal-regulated kinase protein activity. Moreover, the effects of both TGF beta(1) and BMF2 on alpha 1(I) collagen gene expression were markedly decreased in transfected ROS 17/2.8 cells expressing a dominant-negative MEK-1. Our findings therefore show that TGF beta(1) and BMP-2, which signal through discrete cell-surface receptors, are able to trigger analogous, if not identical, protein-phosphorylation-transducing cascades leading to comparable actions on the transcription of the alpha 1(I) collagen gene in osteoblastic cells.