Multiple signaling pathways converge on the Cbfa1/Runx2 transcription factor to regulate osteoblast differentiation

The Cbfa1/Runx2 transcription factor is essential for osteoblast differentiation. However, levels of Runx2 are often not well correlated with its transcriptional activity suggesting that this factor must be activated either by covalent modification or through interactions with other nuclear components. Runx2 is phosphorylated and activated by the mitogen-activated protein kinase (MAPK) pathway. This pathway is stimulated in at least two ways: by binding of type I collagen to alpha(2)beta(1) integrins on the osteoblast surface and by treatment of cells with the osteogenic growth factor, FGF2. Protein kinase A (PKA) also may phosphorylate/activate Runx2 under certain conditions. Runx2 activity also is enhanced by factors known to stimulate specific signal transduction pathways such as PTH/PTHrP (signals through PKA and PKC pathways) and BMPs (Signal through Smad proteins). Interactions with Runx2 are complex involving both binding of distinct components such as AP-1 factors and Smads to separate sites on DNA, direct interactions between Runx2 and AP-1/Smad factors and MAPK or PKA-dependent Runx2 phosphorylation. These findings suggest that Runx2 plays a central role in coordinating multiple signals involved in osteoblast differentiation.