Transactivation of core binding factor alpha 1 as a basic mechanism to trigger parathyroid hormone-induced osteogenesis

During 28-day culture of bone marrow- and calvaria-derived osteoblasts, the constant presence of parathyroid hormone (PTH)(1-34) retarded differentiat ion and nodule formation (NF) in a dose-dependent fashion (C-phase). In con trast, addition of PTH(1-34) in late stage cultures (from day 10 to 21) acc elerated NF (A-phase). The stable production of such an A-phase allowed us to study the mechanism of bone anabolic action of PTH(1-34). Subcellular lo calization studies of core binding factor alpha 1 (Cbfa 1) and reporter ass ays provided the results indicating that in the A-phase, PTH(1-34) triggers its bone anabolic action via enhancement of Cbfa1 transactivation. RT-PCR and Northern blot analyses revealed that alkaline phosphatase, osteocalcin and bone sialoprotein expression decreased in the C-phase and increased in the A-phase; however, expression of other bone proteins (Cbfa1, PTH/PTH-rel ated peptide-receptor, osteopontin, collagen I alpha1, collagen I alpha2, v itamin K-dependent gamma -glutamyl carboxylase) did not change in a phase t ransition-related manner. Ovariectomized osteopenic mice, treated with PTH( 1-34) (4 and 40 mug/kg, s.c., every other day, 4 or 6 weeks), recovered los t bone, displayed elevated nuclear localization of Cbfa1 in tibiae without alteration of its cytosolic level and exhibited upregulation of expressions of the same set of proteins (alkaline phosphatase, osteocalcin and bone si aloprotein) in femora. These results obtained by a concerted study in vitro and in vivo suggest that PTH triggers its osteogenic action via promotion of the transactivation of Cbfa1.