Cyclophilins are a highly conserved family of peptidylprolyl cis-trans-isomerases (PPI) that are targets of the immunosuppressant drug cyclosporin A. These isomerases have been proposed to aid in protein folding. Cyclophilin B (PPIB) is an ER-localized chaperone protein that associates with type I procollagen, and facilitates procollagen sorting and transport through the secretory compartment. Mutations in the cyclophilin B gene lead to aberrant biosynthesis of type I procollagen, and osteogenesis imperfecta, a disorder characterized by bone fragility. Cyclophilin B has also been implicated as a marker in several cancers, including glioblastomas, neuroblastomas, and gastric cancers. In gastric cancer, Stat3 suppression of miR-520d-5p leads to upregulation of its downstream target cyclophilin B, and enables the growth and survival of these cancer cells.
