Prolyl Isomerase Pin1-mediated Conformational Change and Subnuclear Focal Accumulation of Runx2 Are Crucial for Fibroblast Growth Factor 2 ( FGF2)-induced Osteoblast Differentiation

Background: Genetic interaction between Runx2 and Pin1 is critical for embryonic bone formation. Results: Pin1 is a critical modifying enzyme promoting both subnuclear accumulation and protein acetylation of Runx2. Conclusion: Pin1 determines the fate of Runx2 protein in osteoblast differentiation. Significance: The modulation of Pin1 activity may be a clinical target for the regulation of bone formation. Fibroblast growth factor 2 (FGF2) signaling plays a pivotal role in bone growth/differentiation through the activation of osteogenic master transcription factor Runx2, which is mediated by the ERK/MAPK-dependent phosphorylation and the p300-dependent acetylation of Runx2. In this study, we found that Pin1-dependent isomerization of Runx2 is the critical step for FGF2-induced Runx2 transactivation function. We identified four serine or threonine residues in the C-terminal domain of Runx2 that are responsible for Pin1 binding and structural modification. Confocal imaging studies indicated that FGF2 treatment strongly stimulated the focal accumulation of Pin1 in the subnuclear area, which recruited Runx2. In addition, active forms of RNA polymerase-II also colocalized in the same subnuclear compartment. Dipentamethylene thiuram monosulfide, a Pin1 inhibitor, strongly attenuated their focal accumulation as well as Runx2 transactivation activity. The Pin1-mediated structural modification of Runx2 is an indispensable step connecting phosphorylation and acetylation and, consequently, transcriptional activation of Runx2 by FGF signaling. Thus, the modulation of Pin1 activity may be a target for the regulation of bone formation.