Parathyroid hormone-related protein down-regulates bone sialoprotein gene expression in cementoblasts: Role of the protein kinase A pathway

PTH-related protein (PTHrP) acts as a paracrine and/or autocrine regulator of cell proliferation, apoptosis, and differentiation and is implicated in tooth development. The current studies employed cementoblasts to determine the role(s) and mechanisms of PTHrP in regulating cementum formation. Resul ts demonstrated that PTHrP repressed gene expression and protein synthesis of bone sialoprotein (BSP) and abolished cementoblast-mediated biomineraliz ation in vitro. The BSP gene inhibition required protein synthesis. The PTH rP analog (1-31) and other activators of the PKA pathway (3-isobutyl-1-meth ylxathine (IBMX), forskolin (FSK) and Sp-Adenosine-3', 5'-cyclic monophosph orothioate (Sp-cAMPss) also down-regulated BSP gene expression and blocked cementoblast-mediated biomineralization. In contrast, the PTHrP analog (7-3 4), a PTHrP antagonist, and the activators of the PRC pathway [phorbol 12-m yristate 13-acetate (PMA) and phorbol 12, 13-dibutyrate (PDBu)] promoted BS P gene expression. In addition, the PKA pathway inhibitor (9-(2-tetrahydrof uryl) adenine (THFA) partially, but significantly reversed the PTHrP-mediat ed down-regulation of BSP gene expression. Furthermore, THFA alone signific antly increased BSP messenger RNA (mRNA) expression in cementoblasts. In co ntrast, the inhibitor of the PKC pathway (GF109203X) did not reverse the PT HrP inhibitory effect on BSP gene expression. Furthermore, GF109203X alone dramatically reduced the BSP transcript levels. These data indicate that th e cAMP/PKA pathway mediates the PTHrP-mediated down-regulation of BSP mRNA expression in cementoblasts; and furthermore, this pathway may, through an intrinsic inhibition mechanism, regulate the basal level of BSP mRNA expres sion. In contrast, the activation of PKC promotes BSP gene expression. Thes e data provide new insights into the molecular mechanisms involved in PTHrP regulation of cementogenesis.