Direct Delivery of Recombinant Pin1 Protein Rescued Osteoblast Differentiation of Pin1-Deficient Cells

Pin1 is a peptidyl prolyl cis-trans isomerase that specifically binds to the phosphoserine-proline or phosphothreonine-proline motifs of several proteins. We reported that Pin1 plays a critical role in the fate determination of Smad1/5, Runx2, and beta-catenin that are indispensable nuclear proteins for osteoblast differentiation. Though several chemical inhibitors has been discovered for Pin1, no activator has been reported as of yet. In this study, we directly introduced recombinant Pin1 protein successfully into the cytoplasm via fibroin nanoparticle encapsulated in cationic lipid. This nanoparticle-lipid complex delivered its cargo with a high efficiency and a low cytotoxicity. Direct delivery of Pin1 leads to increased Runx2 and Smad signaling and resulted in recovery of the osteogenic marker genes expression and the deposition of mineral in Pin1-deficient cells. These result indicated that a direct Pin1 protein delivery method could be a potential therapeutics for the osteopenic diseases.