title: Functional heterogeneity of osteocytes in FGF23 production: the possible involvement of DMP1 as a direct negative regulator.
abstract: Fibroblast growth factor 23 (FGF23) and dentin matrix protein (DMP1) are hallmarks of osteocytes in bone. However, the mechanisms underlying the actions of DMP1 as a local factor regulating FGF23 and bone mineralization are not well understood. We first observed spatially distinct distributions of FGF23- and DMP1-positive osteocytic lacunae in rat femurs using immunohistochemistry. Three-dimensional immunofluorescence morphometry further demonstrated that the distribution and relative expression levels of these two proteins exhibited reciprocally reversed patterns especially in midshaft cortical bone. These in vivo findings suggest a direct role of DMP1 in FGF23 expression in osteocytes. We next observed that the inoculation of recombinant DMP1 in UMR-106 osteoblast/osteocyte-like cells and long-cultured MC3T3-E1 osteoblastic cells showed significant downregulation of FGF23 production. This effect was rescued by incubation with an focal adhesion kinase (FAK) inhibitor or MEK (mitogen-activated protein kinase (MAPK)/extracellular signal regulated kinase (ERK)) inhibitor but not inhibitors of phosphoinositide 3-kinase or Rho kinase. Consistently, the levels of phosphorylated FAK, ERK and p38 were significantly elevated, indicating that exogenous DMP1 is capable of activating FAK-mediated MAPK signaling. These findings suggest that DMP1 is a local, direct and negative regulator of FGF23 production in osteocytes involved in the FAK-mediated MAPK pathway, proposing a relevant pathway that coordinates the extracellular environment of osteocytic lacunae and bone metabolism.
authors.affiliation: Division of Bio-Imaging, Proteo-Science Center (PROS), Ehime University , Ehime, Japan ; Department of Oral Pathology, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University , Tokyo, Japan.
authors.forename: Ji-Won
authors.initials: JW
authors.lastname: Lee
authors.affiliation: Department of Oral Pathology, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University , Tokyo, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Yamaguchi
authors.affiliation: Division of Bio-Imaging, Proteo-Science Center (PROS), Ehime University , Ehime, Japan ; Department of Oral Pathology, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University , Tokyo, Japan ; Translational Research Center and Artificial Joint Integrated Center, Ehime University Hospital , Ehime, Japan.
authors.forename: Tadahiro
authors.initials: T
authors.lastname: Iimura